Inspection device

Abstract

A method for inspecting an object using an ultrasonic inspection device carried by a cooperating coordinate positioning device is provided. The ultrasonic inspection device includes an ultrasonic transducer and a coupling element that contacts and acoustically couples with the object being inspected, the coupling element being a self-lubricating material such as hydrogel. The method includes using the cooperating coordinate positioning device to scan the coupling module of the ultrasonic inspection device along a path on the surface of the object while acquiring ultrasonic measurements.

Description

Technical Field

[0001] The present invention relates to an ultrasonic inspection apparatus, and more particularly, to a modular ultrasonic inspection apparatus for use with a coordinate positioning apparatus such as a coordinate measuring machine (CMM).

Background Art

[0002] It is known to measure the dimensions of manufactured objects to ensure that they conform to tolerances. In the case of expensive parts such as aerospace turbine blades, the outer shape of the object can be measured with sub-micron accuracy using a surface contact probe attached to a CMM. Examples of techniques for measuring the positions of a plurality of points on the surface of an object using a CMM equipped with a surface contact (e.g., scanning) probe are described in Patent Document 1 (US5189806) and Patent Document 2 (WO2009 / 024783).

[0003] In addition to surface measurement, it is often necessary to measure internal features of an object. For example, turbine blades are typically hollow in order to be lightweight and strong for operation at extreme temperatures and pressures. Internal inspection of such hollow turbine blades is typically performed using an ultrasonic inspection apparatus, such as an ultrasonic immersion system or an ultrasonic thickness measurement probe.

[0004] An ultrasonic immersion system generally involves completely immersing a test piece in a water bath. A single pulse-echo transducer or a pair of transmit / receive transducers are appropriately positioned with respect to the part using a computer-controlled robotic arm. Water provides good acoustic coupling with the part, but the arrangement is expensive and complex, especially for large parts. An example of an ultrasonic immersion system is described in Patent Document 3 (GB2440959).

[0005] An ultrasonic thickness measurement probe does not require the part to be immersed in water. Instead, it typically relies on the local application of an adhesive material (e.g., an adhesive gel or liquid) to the part. Such probes tend to be handheld, and how such probes are attached to the quill of a CMM has been described previously. For example, Patent Document 4 (US2009 / 0178482) describes an ultrasonic probe attached to the quill of a CMM. The ultrasonic probe of Patent Document 4 (US2009 / 0178482) includes a gimbal mount that allows the sensor to align with the surface normal of the object when contact with the object is established. As described in paragraph 19 of Patent Document 4 (US2009 / 0178482), in order to ensure sufficient acoustic coupling between the ultrasonic probe and the object, a binding substance such as gel or grease must be applied to the relevant area of the object before inspection.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0007] Since it is necessary to apply the localized binder material to a limited angular range of the gimbal mount, ultrasonic inspection of the internal characteristics of components using such a system is a time-consuming and complex task. Handheld ultrasonic thickness measurement probes using a dry binder layer are also known, but the bonding efficiency and performance of such devices are poor, especially when operation at higher ultrasonic frequencies is required.

Means for Solving the Problem

[0008] According to a first aspect of the present invention, there is provided an ultrasonic inspection device for use with a coordinate positioning device, the ultrasonic inspection device comprising: a base module attachable to a movable member of the coordinate positioning device, the base module including an ultrasonic transducer and a first connector portion, and a plurality of coupling modules, each coupling module including a second connector portion releasably attachable to the first connector portion of the base module and a coupling element that acoustically couples in contact with the inspection target.

[0009] Accordingly, the present invention provides a modular ultrasonic inspection apparatus for use with a coordinate positioning apparatus such as a CMM. The modular apparatus includes a base module and a plurality of coupling modules attachable thereto. The base module of the modular ultrasonic apparatus itself can be attached to a movable member of the coordinate positioning apparatus. For example, the base module can comprise an attachment feature or mechanism that enables it to be attached to the column or rotating head of a CMM. The base module also includes an ultrasonic transducer, for example, a pulse-echo ultrasonic transducer comprising a piezoelectric element for transmitting high-frequency time-discrete longitudinal waveforms (hereinafter referred to as "L-waves"). A first connector portion is provided on the base module to enable any one of the plurality of coupling modules to be attached to the base module via a second connector portion provided on each coupling module. Each of the plurality of coupling modules also includes a coupling element (e.g., a hydrophilic sphere or other tip described below) designed to contact and acoustically couple to the object being inspected.

[0010] The modular ultrasonic inspection apparatus of the present invention has the advantage that different coupling modules can be attached to the base module as needed. The attachment and removal of the coupling modules are preferably performed in an automated manner by appropriate programming of the coordinate positioning apparatus. The plurality of coupling modules can include a range of different coupling modules for measuring different internal features of the part (e.g., in different directions and / or directing different amounts of divergent sound). The plurality of coupling modules can alternatively or additionally include a range of similar coupling modules having a limited lifespan (e.g., due to wear or damage of a soft coupling element) and can thus be replaced upon damage, wear, or after a period of use. Accordingly, the plurality of coupling modules can be consumables with a shorter lifespan compared to the base module.

[0011] The ability to exchange coupling modules means that a range of ultrasonic measurements (e.g., ultrasonic thickness measurements) can be pre-programmed and performed in an automated manner. The ability to perform such automated measurements is provided by the modular nature of the ultrasonic device of the present invention and also enables thickness measurements to be obtained along surface contact measurements of the same part (e.g., using a conventional touch trigger probe). Thus, the automated inspection process used for surface contact measurements can also be extended to obtain internal thickness measurements, thereby significantly improving the inspection capabilities provided by coordinate positioning devices such as CMMs.

[0012] Accordingly, the present invention enables a number of different coupling modules to be used with a common base module. Thus, one or more coupling modules may be provided that comprise a coupling element having a dry bonding agent material such as silicone rubber. One or more coupling modules may be provided that comprise a coupling element that requires the application of a bonding material (e.g., water or gel) prior to measurement. The one or more coupling elements can be an oil-based thermoplastic that is compressible and easily bendable and has low acoustic attenuation characteristics.

[0013] Advantageously, at least one of the plurality of coupling modules includes a coupling element provided with a self-lubricating material. The self-lubricating material preferably releases a lubricant such as water and / or oil in a controlled manner from its outer surface. The self-lubricating material may be a lipophilic elastomer. Advantageously, the self-lubricating material consists of a hydrophilic elastomer. For example, the hydrophilic elastomer can comprise a non-compressible gelatinous hydrophilic elastomer material such as a lightly crosslinked hydrophilic vinyl elastomer or a superabsorbent polymer hydrogel. An example of a hydrophilic polymer chain compound with a high water content is MMA:VP (i.e., a copolymer of N-vinyl pyrrolidone and methyl methacrylate). In this compound, the water content varies from about 35% to 95%, and the tear strength decreases as the water content increases, but excellent acoustic properties are exhibited. Conveniently, the self-lubricating material is provided as spheres. Thus, a preferred embodiment comprises a coupling element provided with hydrophilic elastomer spheres.

[0014] Providing a coupling module with hydrophilic elastomer spheres has many advantages. For example, water-swellable hydrophilic polymer spheres release a limited amount of water from their surface (i.e., "leach" water). This released water provides improved acoustic coupling with the object by filling pockets of air between the ultrasonic probe and the rough surface of the object. The amount of water released from the hydrophilic elastomer spheres can be controlled by an appropriate selection of the chemical properties of the polymer material. For example, the amount of water released can be minimal and thus configured to evaporate very easily into the atmosphere without leaving residual contaminants. Further, such hydrophilic elastomer spheres may be soft and elastic in order to provide a high degree of conformity to a curved inspection surface. Such elasticity is also beneficial as it allows for repeatable and accurate touch-contact measurements, as will be described in more detail below. The release of water can also act as a lubricant that allows such spheres to be scanned along a path on the surface of the object. Further, the hydrophilic material has inherently low acoustic attenuation and an acoustic impedance value well-suited for ultrasonic transmission between a ceramic transducer wear plate and a metal component.

[0015] Advantageously, at least one of the plurality of coupling modules comprises a delay line. For example, one or more of the coupling modules may include a coupling element that also functions as a delay line. In view of the above embodiments, at least one coupling module may be provided that comprises a hydrophilic elastomer sphere having a protrusion for contacting the object to be inspected. The hydrophilic elastomer sphere can also acoustically contact the base module directly when the associated coupling module is attached in the base mode. For example, the sphere can engage a wear plate of a transducer within the base module. The hydrophilic elastomer sphere can then act as both a coupling element and a delay line.

[0016] Conveniently, at least one of the plurality of coupling modules comprises a delay line coupled to the coupling element. In other words, a delay line (e.g., a solid plastic delay line) separate from the coupling element may be provided. Such a delay line is preferably rigid and may be formed, for example, from polystyrene or polycarbonate. The hydrophilic elastomer spheres may be coupled to the distal end of the delay line. Thus, the delay line can be disposed within the acoustic path between the transducer and the hydrophilic elastomer spheres.

[0017] Advantageously, at least one of the plurality of coupling modules comprises an ultrasonic beam control element. The ultrasonic beam control element preferably manipulates (e.g., refracts, manipulates, or focuses) the ultrasonic wavefronts transmitted to and / or received from the object being inspected. Preferably, a delay line that also functions as an ultrasonic beam control element is provided. For example, the coupling module can comprise a tapered delay line that results in a more divergent beam within the component. One or more of the coupling modules can comprise an ultrasonic beam control element in the form of an acoustic lens. For example, a spherically converging plano-concave lens may be provided. The hydrophilic elastomer spheres may be coupled to the concave lens (e.g., the lens cups around the sphere) to provide refractive focusing and increase the back wall reflection. One or more of the coupling modules can comprise an ultrasonic beam control element in the form of a refractive wedge. For example, the coupling module can comprise an asymmetric rigid wedge that refracts the ultrasonic beam projected onto the inspection surface at a set angle from the normal direction as determined by the relative speed of sound within the coupled materials. This can be useful for more complex shaped internal metrology where the front and back walls are not parallel. Acoustic mirrors and other acoustic components may be included in one or more of the coupling modules as needed.

[0018] Advantageously, at least one of the plurality of coupling modules comprises an ultrasonic absorption shell. The ultrasonic absorption shell suppresses or attenuates unwanted acoustic reflections from within the shell wall that could otherwise interfere with the reflected waveform of interest. The ultrasonic absorption shell can be formed, for example, from Teflon® or glass-filled PTFE.

[0019] Advantageously, a holder (e.g., a storage tray) for holding the plurality of coupling modules is provided. The holder can be attached to the bed of the coordinate positioning device. For example, the holder can include one or more features (e.g., threaded holes, magnets, etc.) that allow the holder to be fixed in a position and orientation where the holder is fixed to the bed of the CMM. The holder can include one or more ports or receptacles, each port or receptacle being arranged to hold a coupling module. Thus, the holder can store coupling modules that are not currently in use. That is, coupling modules that are not attached to the base module for measurement purposes can be stored within the holder. The holder can accommodate 5 or more, 10 or more, or 15 or more coupling modules. Thus, the apparatus can comprise more than 5, more than 10, or more than 15 coupling modules. The plurality of coupling modules can comprise a plurality of different designs or types of coupling modules. The plurality of coupling modules can comprise a plurality of substantially identical coupling modules.

[0020] As described above, a contact module can be provided where the coupling element is self-lubricating (e.g., gradually releases water). Thus, the holder may be hermetically sealed prior to use. This prevents the coupling element (e.g., a hydrophilic sphere) from drying out prior to use. The holder may be resealable. For example, the holder may be opened and then closed after each module is removed, or opened and then closed after a series of measurements have been taken. Alternatively, the holder (and optionally the coupling module having a self-lubricating coupling element) may be provided as a disposable or consumable item that is opened, used, and then discarded or recycled. The holder may initially contain a coupling module with dehydrated hydrophilic spheres that are hydrated prior to use. The new holder may be opened when needed.

[0021] In a preferred embodiment, the holder comprises a plurality of recesses for receiving a plurality of coupling modules. Advantageously, the recesses and the coupling modules are arranged to prevent rotation of the coupling module when disposed within the recess. For example, the coupling module can include a central hub with one or more radially projecting wings. The holder may then comprise complementary recesses. Thus, the coupling module can be inserted into and withdrawn from the recess using relative movement in the vertical (linear) direction. Once inserted, rotation of the coupling module relative to the holder is prevented.

[0022] The first and second connector portions may be provided by any suitable linkage. Advantageously, a magnetic connection device may be provided. The magnetic connection between the base module and the coupling module may be realized by at least one of the first connector portion and the second connector portion comprising one or more magnets (e.g., permanent magnets). In this way, any selected one of the plurality of coupling modules may be attached to the base module and magnetically held by the base module. The first and second connector portions may also include complementary physical features (e.g., mating parts, kinematic features, etc.) that engage to position the attached coupling module in a specific fixed position and / or orientation relative to the base module. The first and second connector portions preferably enable repeatable positioning of the coupling module relative to the base module. This allows the coupling module to be repeatedly attached to the base module in the same position and orientation. The one or more magnets are preferably strong enough to hold the coupling module on the base module, yet still allow the coupling module to be removed from the base module if necessary.

[0023] Alternatively, the first and second connector portions comprise complementary threads. For example, the first connector portion of the base module may comprise threads (e.g., male thread connector) provided on the outer surface of the distal end of the base module housing. The second connector portion of the coupling module may then include a recess having a surface with threads formed therein (e.g., female thread connector). The base module and the coupling module may be arranged such that they can engage and disengage from each other by imparting a relative rotational movement. This allows the base module (e.g., held by a rotating head) to be screwed into and out of engagement with the coupling module. The coupling module may be conveniently stored within a holder of the type described above that prevents rotation during the attachment process.

[0024] Attaching one of the plurality of coupling modules to the base module preferably establishes reliable and repeatable acoustic coupling between the modules. In particular, the mating of the first and second connector portions provides an acoustic connection between the transducer of the base module and the coupling element of the coupling module. The transducer within the base module comprises a wear plate conveniently arranged to acoustically couple with the coupling module attached to the base module. For example, the wear plate can engage the delay line of the coupling module or directly engage the hydrophilic elastomer spheres of the coupling module. In such embodiments, by attaching one of the plurality of coupling modules to the base module, the associated components of the coupling module (e.g., delay line, spheres, etc.) are firmly held against the wear plate of the base module.

[0025] The ultrasonic inspection device can excite and receive ultrasonic waves by any known method. The ultrasonic inspection device can operate at a high frequency. For example, the operating frequency can be greater than 5 MHz, greater than 10 MHz, and more preferably greater than 15 MHz. In a preferred embodiment, the operating frequency is about 20 MHz. The transducer comprising the piezoelectric element preferably excites longitudinal acoustic waves (L-waves).

[0026] Advantageously, the ultrasonic transducer is configured to operate in a pulse echo mode. Conveniently, the ultrasonic return signal is analyzed to enable the thickness measurement of the object. Such an analysis can be based on a "mode-3" method that analyzes the continuous back wall reflections present in an "A-scan", which is described in more detail below. However, alternative analysis techniques (e.g., so-called "mode-1" or "mode-2" techniques) can be used if necessary. The ultrasonic inspection device may also be configured to operate in any one of a plurality of different ultrasonic measurement modes. The thickness measurement may require a calibration step (e.g., to measure the speed of sound in the material of the part being inspected). The base module preferably includes a processor for analyzing the ultrasonic signals received by the ultrasonic transducer. Alternatively, the ultrasonic signals can be analyzed by an off-probe processor (e.g., at an external interface or using an offline computer).

[0027] The present invention also relates to a coordinate positioning device equipped with the above ultrasonic inspection device. The base module of the ultrasonic inspection device can be attached to or is attached to a movable member of the coordinate positioning device. The coordinate positioning device can include a machine tool, an industrial robot, an arm, an x-y scanner, or a crawler. In a preferred embodiment, the coordinate positioning device consists of a coordinate measuring device. The CMM can be of a Cartesian (e.g., bridge type) or non-Cartesian (e.g., hexapod) type of the CMM. The CMM preferably includes a rotating head that provides a movable member to which the base module is attached. The rotating head can include a single rotation axis, two rotation axes, or three rotation axes. Advantageously, the rotating head includes at least two rotation axes. The rotating head can include at least three rotation axes.

[0028] According to a further aspect of the present invention, there is provided an ultrasonic inspection apparatus for a coordinate positioning device. The apparatus comprises an ultrasonic transducer and a coupling element for acoustically coupling in contact with the object to be inspected, and the coupling element is made of a self-lubricating material. The speed of sound in the coupling element can be measured by analyzing reflections from within the coupling element when the coupling element undergoes a plurality of different deformations. The ultrasonic inspection apparatus may be of the modular design as described above, or may be of a single (single or non-modular) configuration. The apparatus can comprise any of the features referred to herein. Advantageously, the self-lubricating material is made of a hydrophilic elastomer. Conveniently, the self-lubricating material comprises a superabsorbent polymer hydrogel. Preferably, the coupling element consists of spheres of self-lubricating material. The ultrasonic inspection apparatus can be used in a method for measuring the porosity and / or density of components, such as components made using additive manufacturing techniques.

[0029] The present invention also extends to a method for measuring the thickness of an object and / or a point on the surface of the object using the apparatus described above.

Brief Description of the Drawings

[0030] The present invention will be described below by way of example only with reference to the accompanying drawings.

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Mode for Carrying Out the Invention

[0031] Referring to FIGS. 1(a)-(c), various ultrasonic probes equipped with longitudinal wave (L-wave) transducers for internal metrology measurements are shown. Such probes have been previously used typically as hand-held inspection devices for inspection purposes.

[0032] FIG. 1(a) shows an ultrasonic probe 2 having an outer body 4. An L-wave transducer having an active piezoelectric element 6 is provided. The relatively thin piezoelectric element 6 is arranged to have a thickness equal to approximately half of the wavelength of the generated ultrasonic wave, which enables high-frequency excitation necessary for accurate rise-time excitation. The piezoelectric element 6 is backed by a thick damping backing material 8 that absorbs energy from the piezoelectric element 6, thereby generating a desired strongly damped response in the forward direction. Thereby, optimal range resolution is obtained.

[0033] The delay line 10 is acoustically coupled to the piezoelectric element 6 via a wear plate 12. The wear plate 12 protects the piezoelectric element 6. The wear plate 12 has a thickness equal to 1 / 4 of the ultrasonic wavelength to enable it to act as a matching layer. This thickness of the wear plate is preferred because it ensures that the wave generated in the piezoelectric element 6 is in phase with the wave reverberating within the wear plate 12. This means that the amplitudes of the ultrasonic waves in the wear plate 12 and the piezoelectric element 6 are additive, and thus, maximum energy enters the delay line 10 coupled to the wear plate 12. At the distal end of the delay line 10, a liquid coupling agent layer (not shown) is provided to provide acoustic coupling with the object 14 to be inspected. In the example shown in FIG. 1(a), the delay line 10 comprises a tapered propagation medium. The propagation medium may be a polycarbonate resin or a cross-linked polystyrene. To suppress internal reflections from within the propagation medium, thin axial grooves 16 are machined on the side surface of the propagation medium.

[0034] (b) of FIG. 1 shows an ultrasonic probe 20 having many common features with the probe 2 described above with reference to FIG. 1. However, the probe 20 has a non-tapering delay line 22 having a circumferential waveform feature 24.

[0035] The main function of delay lines such as the delay lines 10 and 22 described with reference to FIGS. 1(a) and 1(b) is to physically remove ultrasonic excitation sufficiently far from the inspection surface in order to temporally resolve the excitation response from the initial reflection from the back wall 26 of the object 14 being inspected. Preferably, this is achieved without temporal interference from the ring-down of the finite bandwidth excitation pulse. Thus, it can be seen that the delay line serves to control the time when ultrasonic waves (i.e., longitudinal waves) enter the inspected part. Different degrees of tapering can be employed to generate a higher contact pressure and accommodate more curved portions. Such tapering also affects the natural focal length and beam divergence (i.e., diffraction effect) of the device.

[0036] The ultrasonic probes described with reference to FIGS. 1(a) and 1(b) provide normal incidence, non-contact (near-field) or natural divergence beam inspection. It is also possible to project ultrasonic waves at an angle away from the surface normal (e.g., for the detection and sizing of internal defects). FIG. 1(c) shows an alternative ultrasonic probe equipped with a refraction angle beam wedge 30 (sometimes also called an ultrasonic shoe). The beam wedge 30 is coupled to the piezoelectric element 6 via the wear plate 12 as in the embodiments described with reference to FIGS. 1(a) and 1(b). The beam wedge 30 projects the ultrasonic waveform at an off-axis beam angle from the surface normal of the inspection object. In the case of such a wedge-shaped transducer, refraction at the interface between materials with different acoustic impedances causes a shear wave mode (S-wave) to occur at the interface due to the phenomenon of mode conversion according to the refractive index (i.e., Snell's law).

[0037] Figure 1(c) also shows the ultrasonic waves projected from the beam wedge 30 of the ultrasonic probe onto the metal part 32. The slower S-wave has less refraction from the surface normal N than the faster L-wave mode. Further, it is emphasized that the relative ratio of the refracted L-wave and the subsequent S-wave mainly depends on the incident angle with respect to the S-wave. It should also be noted that a significant reflection mode is generated at the interface (i.e., an R-wave is generated) that redirects spurious (false) acoustic energy within the wedge. The absorption shell 34 is thus bonded around the propagating wedge material to attenuate this reflected energy. This shell 34 would otherwise reflect within the beam wedge 30 and prevent reflections that would interfere with the reflections of interest from the bonded metal part 32.

[0038] The ultrasonic probe described above with reference to FIGS. 1(a) - (c) is an example, and it should be recalled that different designs of ultrasonic delay lines, wedges, and lenses have been previously developed to enable optimal acoustic coupling to various solid inspection parts. In the embodiment outlined above with reference to FIGS. 1(a) - (c), an explanation is given of how an additional coupling layer (e.g., layer gel or grease) is provided between the ultrasonic probe and the object to be inspected. This is because all actual inspection parts exhibit some surface irregularities within the microstructures that cause air pockets to be trapped at the interface between the probe and the inspection surface. The presence of such air pockets reduces the acoustic coupling efficiency mainly due to the large impedance mismatch between the solid and air. Therefore, the use of a coupling layer such as gel can ensure that the required acoustic coupling efficiency is obtained.

[0039] When liquid water or gel cannot be freely applied to the object to be inspected (e.g., in automotive applications), a bonding layer in the form of a dry-bonded solid has been previously used. Some hydrophilic elastomers (e.g., Aqualene from Olympus) and silicone rubber-based materials (e.g., Ultracouple from Sonemat) are commercially available for dry ultrasonic non-destructive testing (DT) applications. However, their bonding performance is not always suitable for high-frequency precision thickness measurement probes due to the increase in L-wave attenuation at higher probe operating frequencies (15 - 20 MHz). The excessive material stiffness of such materials may also limit close conformity to more curved inspection surfaces. Additionally, silicone-based binder materials are considered unacceptable contaminants in some manufacturing environments (e.g., the aerospace industry).

[0040] Referring to FIGS. 2(a) and (b), the operating principle of a pulse-echo thickness measurement transducer is outlined. In particular, FIG. 2(a) shows an ultrasonic probe 40 similar to the ultrasonic probe 20 described above with reference to FIG. 1(b). The ultrasonic probe 40 includes a single-element ultrasonic delay line 22 having a dry-bonding polymer pad 42 for bonding to the object 14 to be inspected.

[0041] FIG. 2(b) is an example of an acoustic waveform received by the active piezoelectric element of the transducer in response to the application of a transient high-voltage excitation pulse to the piezoelectric element 6. This time-domain waveform, called an "A-scan" plot, is more often the time-averaged response from a train of such excitation pulses (e.g., a sequence of N pulses where N ranges from 16 to 32) to suppress random uncorrelated electronic noise.

[0042] The initial excitation pulse generated by the piezoelectric element 6 is labeled as the "Tx - pulse" in Fig. 2(b). This first excitation pulse propagates the inspection L - wave to the delay line 22 and moves along the delay line 22 at the speed of sound (CL). The first reflected peak (DL1) received back at the piezoelectric element 6 results from the reflection of sound from the tip of the delay line (i.e., from the interface between the delay line 22 and the polymer pad 42). It can be seen that this reflection from the distal end of the delay line interface (i.e., the DL1 pulse) occurs after the first transmission pulse (Tx - pulse) has completely decayed.

[0043] Most of the acoustic energy is reflected at the delay line interface and does not enter the inspection part, but a sufficient proportion of the acoustic energy is transmitted to the coupling part 14 as a measurable inspection pulse that enables subsequent thickness measurement. Due to the acoustic impedance mismatch between the metal part 14 (e.g., Z - 46MRayl) and the surrounding air (i.e., Z = 0.000429MRayl), the inspection L - wave propagates very efficiently within the part with only a very gradual attenuation due to acoustic leakage over several reflections at the back - wall interface. Since the speed of sound in the delay line 22 is lower than the speed of sound in the thin metal inspection part 14, multiple reflections between the back - walls of the part 14 can occur before the second reflected peak (DL2) from the delay line is registered by the transducer. Thus, these back - wall reflections result in the pulses BW1, BW2, and BW3 that can be seen in the "A - scan" plot of Fig. 2(b). Therefore, the time window observed within the A - scan between the first and second delay - line reflected peaks (DL1 and DL2) is the main measurement window of the probe.

[0044] The thickness of component 14 can be calculated in several ways from A-scan data of the type shown in Fig. 2(b). In practice, such thickness measurements typically involve one of three operating modes from which time-delay information can be extracted from the measured A-scan. These different modes are typically referred to as Mode-1, Mode-2, and Mode-3, respectively. In Mode-1 measurements, a time-delay measurement is made between the excitation pulse (t = 0) and the first back-wall reflection from the component under inspection, i.e., the primary echo. Mode-1 is usually associated with a direct-contact transducer. In Mode-2 measurements, a time-delay measurement is made between the interface echo representing near the surface of the test part and the first back-wall reflection. Mode-2 is typically used with a delay line or an immersion transducer. In Mode-3 measurements, a time-delay measurement is made between two or more consecutive back-wall reflections. Mode-3 is typically used with a delay line or an immersion transducer. Mode-3 is most effective when clean high-SNR multiple back-wall echoes are observed and is suggested to be most practical in low-attenuation high-acoustic impedance components such as fine-grained metals, glass, or ceramics. Mode-3 also has the advantage that it can negate the effects of coupling in different coupling modules and variability in the delay line since it does not depend on the absolute time of arrival of the back-wall reflection or delay-line reflection. Mode-3 also enables the measurement of components with an external coating layer. Any suitable mode (e.g., Mode-1, Mode-2, or Mode-3) can be used as required. Using different modes, and in some cases different coupling modules, can also be implemented during the measurement process.

[0045] Referring to FIG. 3, a modular ultrasonic inspection apparatus of the present invention is shown attached to a coordinate measuring machine (CMM) 50. The CMM 50 includes a quill 52 that is movable along three linear axes (X, Y, and Z) that are orthogonal to each other. A two-axis rotary head 54, such as a REVO (registered trademark) active head manufactured by Renishaw, is attached to the quill 52 of the CMM 50. A modular ultrasonic probe 56 is sequentially conveyed by the rotary head 54. In the inset to FIG. 3, the ultrasonic probe 56 shown in an enlarged view includes a base module 58 and an attached coupling module 60. The base module 58 is attached to the rotary head 54 by a standard probe joint that can detachably attach the ultrasonic probe 56 to the two-axis rotary head 54 as needed. An additional probe, for example, a conventional surface contact (scan) probe 72 having a stylus with a ruby tip, may be stored in a probe rack 74 for exchange with the ultrasonic probe 56. A calibration artifact 76 is also provided on the bed of the CMM. In this example, a turbine blade 62 held by a fixture 64 provides the measurement object.

[0046] As will be described in more detail below, the ultrasonic probe 56 has a modular configuration. The base module 58 includes a piezoelectric transducer and a wear plate, while the coupling module 60 includes an acoustic delay line and a coupling element for contacting the object to be measured. The modular ultrasonic probe 56 is shown in FIG. 3, with the coupling module 60 attached to the base module 58. The ultrasonic inspection apparatus also includes a plurality of additional coupling modules 66 held within a storage tray 68 placed on the bed of the CMM. During use, any one of the additional coupling modules 66 can be exchanged with the coupling module 60. In other words, any of the additional coupling modules 66 can be attached to the base module 58 and used to measure the internal characteristics of the object. The process of exchanging the coupling module attached to the base module 58 is performed in an automated manner. For example, a magnet-based connection or a threaded connection can be employed. The CMM 50 includes a computer 70 that controls the CMM operation and also controls the automatic exchange of the coupling modules.

[0047] Figures 4(a) - 4(d) show two examples of modular ultrasonic probes that can be used with the CMM described above with reference to FIG. 3.

[0048] Figures 4(a) and 4(b) show the modular ultrasonic probe 56 schematically shown in FIG. 3. The probe 56 includes a base module 58 having a proximal end 90 that can be attached to the rotating head 54 of the CMM 50 via a standard probe joint. The base module 58 also includes an elongated shaft 94 and a piezoelectric transducer 92 having a wear plate 93 disposed near the distal end of the elongated shaft 94.

[0049] Figure 4(b) provides an enlarged view of the distal end 96 of the modular ultrasonic probe 56 shown in Figure 4(a). It can be seen that the distal end of the elongated shaft 94 also includes a first connector portion 98. The coupling module 60 includes a second connector portion 100. The first connector portion 98 and the second connector portion 100 are arranged to enable the attachment (and subsequent removal) of the coupling module 60 and the base module 58. In other words, the first connector portion 98 and the second connector portion 100 are complementary connectors that can be releasably connected to each other. As will be described below, this connection can be achieved using a threaded arrangement or by various alternative methods (such as via magnetic coupling, etc.). The coupling module 60 includes a delay line 102 and a tip 104 for contacting the object to be measured. By attaching the coupling module 60 to the base module 58 via the first connector portion 98 and the second connector portion 100, the delay line 102 is caused to engage with the wear plate 93, whereby ultrasonic waves are coupled from the piezoelectric transducer 92 to the delay line 102 and then allowed to enter the object via the tip 104.

[0050] Figures 4(c) and 4(d) show a variant of the modular ultrasonic probe 56 described above. Instead of a base module having a substantially straight elongated shaft 94, the modular ultrasonic probe 109 includes a cranked shaft 110. This provides a different angular orientation of the tip 104 of the coupling module 60 with respect to the rotating head 54, which is advantageous for certain inspection processes. The cranked modular ultrasonic probe 109 can be stored in a CMM rack (such as the rack 74 described with reference to Figure 3) and used as an alternative to the non-cranked modular ultrasonic probe 56 as required.

[0051] The provision of the modular ultrasonic probe described in this specification has the advantage that different ranges of coupling modules can be attached to the base module. These coupling modules can provide, for example, a range of different coupling characteristics that can be used for different ultrasonic measurements. Figures 5(a) to 5(c) show examples of how a plurality of coupling modules are constructed and stored.

[0052] Figure 5(a) shows the storage tray 68 described above with reference to Figure 3 in more detail. The storage tray 68 comprises a 5×5 array of storage slots 142 (although any arrangement or number of slots can be provided). Each storage slot 142 comprises a central hole having two slots extending radially. Figure 5(b) shows the winged outer shell 144 of a coupling module that can be placed in and held by one of the storage slots 142. The inner surface of the coupling module comprises a thread 146 that provides a connector portion that can be screwed with a complementary thread of a base module of an ultrasonic probe (not shown). The complementary shapes of the storage slot 142 and the outer shell 144 limit the translational movement of any inserted coupling module within the plane of the tray (e.g., the XY plane). Further, the wings provided on the outer shell 144 allow the coupling module to be freely inserted into the storage tray 68 in a direction perpendicular to the plane of the storage tray 68 (e.g., via the Z-direction movement of a CMM quill) and limit the rotational movement of the inserted coupling module. In this way, the coupling module can be attached to or removed from the complementary base module of the ultrasonic probe using a rotational (screwing) operation. The use of such a screw-fastening fixture for providing a modular ultrasonic probe is preferred because it provides a consistent high-tension clamp attachment between the delay line of an individual contact module and the wear plate of the base module. Such a screw-fastening attachment is relatively inexpensive in terms of implementation cost.

[0053] In use, the storage tray 68 is positioned at a known location and orientation on the bed of the CMM as shown in FIG. 3. The coupling module is also positioned at a known location within the tray 68 (i.e., a predetermined slot 142). In use, the CMM points the base module (e.g., base module 58 of FIG. 3) downwardly towards the tray 68 and moves it directly above the coupling module attached to the base module. The base module is then lowered slowly until the base module engages the coupling module, and then the base module is rotated (using, for example, the rotational movement provided by the rotary head 54 of FIG. 3) such that as the two coupling screws engage and are screwed into the base module, the coupling module begins to be lifted out of the tray. The exact point at which the coupling module is fully attached and firmly secured to the base module can be determined, for example, by evaluating the ultrasonic response generated during the operating process or by continuously monitoring the torque load placed on the associated axis of rotation of the rotary head 54. This torque load is directly related to the current demand placed on the rotary motion servo motor within the rotary head 54. The rotational angle at which each coupling module is interpreted as being firmly attached to the base module can also be stored. The rotary head 54 can be returned to this rotational angle to enable it to be removed (unscrewed) by a rotational movement opposite to that used for attachment after the coupling module has been inserted back into the slot 142 of the storage tray 68.

[0054] It should be noted that the above-described screw - mounting method simply represents one possible way of enabling a coupling module to be attached to a base module. There are a number of alternative types of connectors available. For example, the connection can be provided by a Luer joint, a snap - fit mechanism, an embedded magnetic fixture, etc. A magnetic clamping device can, for example, comprise an assembly of three powerful and compact magnets arranged equidistantly around the circumference of the probe tip with a polarity of “++ - ”, and a matching distribution of three magnets with a polarity of “-- + ” around the periphery of each coupling module stored in the storage tray. This magnetic attachment provides an easy attachment for coupling variants of the module and provides only a single possible angle of rotation for the attachment. Attaching the coupling module to the base module can be carried out in an automated way as described above, but it should be noted that such attachment can also be carried out manually by an operator (for example, by scheduling it within a number of set breaks in an inspection procedure).

[0055] Next, turning to FIG. 5(c), various different coupling modules 180 - 191 are depicted. Each of the coupling modules is housed within an outer PTFE shell having the outer shape shown in FIG. 5(b) so that it can be stored within the tray 68 of FIG. 5(a). Also shown is the wear plate 194 of the base module that physically and acoustically couples to the delay line of each attached coupling module.

[0056] The coupling module 180 includes a Rexolite delay line and an outer PTFE shell with a hydrophilic vinyl elastomer tip. The coupling module 181 includes an outer PTFE shell having a Rexolite delay line and a spherical thermoplastic tip. The coupling module 182 includes a tapered outer PTFE shell having a Rexolite delay line and a curved thermoplastic tip. The coupling module 183 includes an outer PTFE shell having a Rexolite delay line and a thin latex rubber tip. The coupling module 184 includes an outer PTFE shell having a hydrophilic - vinyl - elastomer delay line and a tip. The advantages of using a hydrophilic vinyl elastomer ball as the tip are described in more detail below. The coupling module 185 includes a tapered outer PTFE shell having a Rexolite delay line and a hydrophilic vinyl elastomer tip. The coupling module 186 includes an outer PTFE shell having a Rexolite delay line with an angled distal end to which a hydrophilic vinyl elastomer tip is attached. The coupling module 187 includes an outer PTFE shell having a Rexolite delay line with an angled distal end to which a thermoplastic tip is attached. The coupling module 188 includes a tapered outer PTFE shell having a Rexolite delay line with an angled distal end having a thermoplastic tip. The coupling module 189 includes an outer PTFE shell having a thermoplastic tip. The coupling module 190 includes an outer PTFE shell having a stepped tip for holding the tip of a distorted hydrophilic vinyl elastomer ball that also functions as a delay line. The coupling module 191 includes an outer PTFE shell, a Rexolite delay line, and a thermoplastic material providing an object - contact tip.

[0057] Figure 6 schematically shows how a preferred embodiment of an ultrasonic probe with a single hydrophilic elastomer ball can be implemented. As will be described in more detail below, such an arrangement is advantageously provided as part of a modular ultrasonic probe.

[0058] The ultrasonic probe 200 shown in FIG. 6 includes a single hydrophilic elastomer sphere 208. The hydrophilic elastomer sphere 208 can be produced, for example, by the synthesis and hydration of a cross-linked hydrophilic vinyl elastomer, a superabsorbent polymer, or a hydrogel. The hydrophilic elastomer sphere 208 is housed within an acoustically absorbent shell 212 (machined, for example, from PTFE). The sphere 208 is uniformly deformed between the wear plate 214 and the surface 216 of the object 218 being inspected when the probe is loaded onto the surface.

[0059] The transducer 210, which includes an active piezoelectric element, generates L-waves when driven by a train of high-voltage impulse excitation pulses, for example, negative-going transition (NGT) pulses having a duration of 1 / 2f between 50 and 150 V. The characteristic acoustic impedance of the sphere 208 (i.e., the “coupling element”) is such that there is sufficient transmission of acoustic energy thereto from the transducer wear plate 214, which acts as a matching layer. The relative impedance between the contacting media (i.e., the sphere 208, the part being inspected 218, and the surrounding air) determines the acoustic transmission ratio (T) and the amount of reflection (R) at the interface between the probe and the object being inspected according to equations (1a), (1b). R = ((Z 2 - Z 1 ) / (Z 2 + Z 1 )) 2 (1a) T = 1 - R (1b)

[0060] In most thin metal parts, such acoustic impedance matching problems have a greater effect on the amplitude of the actual feedback echo signal than the inherent acoustic attenuation of the combined coupling medium and part being inspected. This is because the inherent acoustic absorption that attenuates ultrasonic waves while propagating through any medium is frequency-dependent and depends on a number of factors such as the temperature of the medium and its inherent particle structure.

[0061] In the example shown in FIG. 6, when the interaction between the hydrophilic sphere 208 and the object 218 is shown in the form of a steel plate, most of the first acoustic energy (e.g., > 80%) is due to the significant difference in acoustic impedance between the hydrogel material of the sphere 208 (e.g., 1 - 3.5 MRayls) and the steel (e.g., about 46 MRayls) of the object 218 adjacent thereto, and is reflected without entering the inspection part 218. However, with respect to the proportion of energy propagating to the part 218, without significant acoustic leakage (i.e., only about 1.3% of the transmitted energy from the first back wall reflection is returned to the transducer), and with a limited decrease in the signal amplitude level of the repeated back wall reflections, multiple reflections occur at the interface between the front wall and the back wall.

[0062] This example shows that the selection of the coupling member (i.e., the sphere 208 in this example) for mode - 3 ultrasonic inspection causes a compromise to be reached. In the compromise state, the coupling is very efficient and sufficient energy is transmitted to the part so that the reflected energy from the first back wall reflection does not escape too much from the part. This is because it results in a low SNR for subsequent back wall echoes. Also, most of the acoustic energy that does not enter the part (i.e., the ultrasonic energy that just bounces back inside the hydrophilic sphere) is also measured, and it has been found that it can be interpreted to infer something about the physical state of the sphere within its surroundings, or how it interacts with other solid, gelatinous or liquid bodies. As will be described in more detail below, the analysis of the delay - line peaks of the acoustic spectrum related to the reflections inside the sphere can also be used to establish contact between the sphere and the object. This can also be effectively utilized during an automatic scan using a coordinate positioning device to obtain surface position information. This is in contrast to the use of A - scan signals from conventional ultrasonic delay - line transducers, and such internal delay - line reflections are more generally either ignored or completely eliminated in time.

[0063] The arrangement of the hydrophilic spheres 208 shown in FIG. 6 has many performance advantages such as enabling both efficient and flexible inspection measurements, allowing for continuous scanning over complex shaped inspection surfaces using a platform with high precision and limited mechanical power. For example, the aqueous (hydrophilic) spheres 208 exhibit negligible acoustic attenuation, suggesting that they can be of any size. Furthermore, their acoustic impedance values are well-suited for ultrasonic transmission to metal parts. Additionally, the incompressible, deformable, and nearly gelatinous spheres are very soft and elastic and are adapted to naturally conform to reasonably curved inspection surfaces. Also, a perfect sphere is ideal for achieving sufficient coupling while maintaining the positional accuracy of point-contact measurements for a local planar inspection surface.

[0064] The hydrophilic sphere arrangement can also be adapted to provide a certain useful tapering or focusing effect to the contact tip that potentially changes the effective natural acoustic focal distance of the probe. In other words, the elastic hydrophilic sphere can provide a certain level of control over the beam divergence and direction of the ultrasonic waves projected onto the part through the accurate manipulation of the loading and orientation of the probe onto the inspection surface. Furthermore, the spherical element provides an optimal structural shape for allowing such brittle solid materials to undergo repeated elastic deformation due to loading on the inspection surface while maintaining mechanical integrity during short-term use due to the uniform distribution of the load stress when the ellipsoid is compressed between the planar wear surface and the inspection surface. Note that the incompressible spheres preferably exist rather loosely within the shell and can undergo elastic deformation many times under load, each time returning perfectly to a sphere when the load is removed. The element only breaks down when some crack or fissure occurs on the sphere surface.

[0065] The arrangement of the hydrophilic spheres 208 shown in FIG. 6 also has the important advantage that the water-swellable chemical properties of the hydrophilic polymer spheres enable them to release a controlled amount of water from their outer surface. The amount of water released is typically an amount that readily evaporates in the atmosphere. This subtle water release moves unwanted air pockets in which water is trapped between the probe and the rough microstructure of the surface, thus significantly improving acoustic coupling across all inspection surfaces. This is done without leaving any obvious residual liquid or contaminating substances, eliminating the need to apply a gelling agent to the part. Further, this water drainage can be controlled by known organic synthesis methods, thus providing the further advantage that it is possible to continuously scan the ultrasonic probe across the inspection surface without losing contact. More specifically, the tangential force introduced by any lateral movement of the scanning probe across the inspection surface (F) could potentially compromise the positional accuracy of the moving probe if the coupling member were completely dry, or induce a significant frictional resistance force (N) in the contact sphere that could cause premature mechanical failure. However, the controlled water-sweating characteristics of the hydrophilic spheres result in sufficient liquid being released from the spheres to act as a natural and effective lubricant, facilitating smooth continuous scanning movement in all directions across most surfaces.

[0066] Accordingly, it can be seen that the hydrophilic sphere-based arrangement described with reference to FIG. 6 offers many advantages with respect to efficient inspection and continuous automated scanning applications. For certain geometries (e.g., those including non-parallel front and rear walls and / or locations with restricted access), it is not always practical, i.e., even if it is physically possible to measure such portions by inducing L-wave beam refraction on the inspection surface solely by the refraction angles required by reorienting the probe with respect to a surface. Further, regardless of the natural focal lengths and refracted beam angles generated by the orientation of the aperture and probe with respect to the surface, the projection beam is inherently divergent and, as a result of the aperture size and operating frequency, is only naturally focused without a distinct near-field refraction focusing. Further, the release of water from these fixtures can be controlled and is extremely minimal, but in some applications (e.g., inspection of automotive assemblies), it may be necessary to have no liquid residues containing water. Accordingly, it is advantageous to alternatively or additionally provide a coupling module comprising a rigid plastic refractive lens or angle beam wedge material (e.g., acrylic or polystyrene) bonded to a suitable soft bonding layer forming the object contact tip. This "compound class" of coupling modules enables a selectively and firmly fixed acoustic beam pattern to be generated by an ultrasonic probe.

[0067] Referring to FIG. 7, an example of the composite design of the coupling module is shown. Again, this example shows the basic operating principle and can be implemented with the modular ultrasonic inspection device described above. The ultrasonic probe of FIG. 7 includes a piezoelectric element 248 coupled to a tapered delay line 250 that is incident vertically, and the piezoelectric element 248 is joined or loosely coupled to a thin soft coupling layer 252 that protrudes to provide an object contact tip. The coupling layer 252 in this example is a thin layer of latex rubber used, for example, to make surgical gloves or similar articles. Alternatively, the coupling layer 252 may be provided by a compressible oil-based thermoplastic. Both latex rubber and oil-based thermoplastics do not produce residues from deformation and thus do not cause liquid contamination during inspection. When firmly loaded (loaded) and acoustically coupled to the inspection surface, the normal incidence composite ultrasonic probe generates a known natural beam divergence 256 fixed within the component 254. However, in order to accommodate more complex internal shapes, a rigid refraction element can be specially shaped asymmetrically to refract (i.e., steer) the L-wave at a set angle from the normal. This follows Snell's law of refraction and the method can also be used to filter out slower shear wave modes.

[0068] FIG. 8 shows a further example of the composite design of the coupling module. Again, this example shows the basic operating principle and can be implemented with the modular ultrasonic inspection device described above. The ultrasonic probe of FIG. 8 includes a piezoelectric element 260 and a plastic plano-concave lens 262 that forms a rigid refractive element. The flat surface 264 of the lens 262 is coupled to the transducer wear plate 266, and the spherical concave surface 268 is coupled to the hydrophilic elastomeric sphere 270 (i.e., "cup-shaped around"). The refractive element (i.e., the plano-concave lens 262) may alternatively be shaped as any required type of acoustic lens to concentrate or focus the L-wave acoustic wavefront at a point within its component with a near field. The ultrasonic probe of FIG. 8 enables the L-wave to be focused within the component 272 when the relative speed of sound in each medium is given. For example, the L-wave can be focused at point P on the rear wall of the component 272. This arrangement provides an A-scan response similar to that obtained using a spherically focused probe in an immersion system, with the hydrophilic elastomeric sphere 270 replacing the water in which the probe is immersed.

[0069] FIGS. 9(a), 9(b), 9(c), 10(a) and 10(b) are various design images and photographs of components of the above-described type of modular ultrasonic probe having a coupling element in the form of a hydrophilic elastomeric sphere.

[0070] As described above, the above-described modular ultrasonic inspection device includes a plurality of coupling modules that can be attached to a common base module. FIG. 9(a) shows a design image of the distal end of the base module 290. The cylindrical body of the base module includes a threaded connector portion 292 on its outer surface. As will be described below, the threaded connector portion 292 enables a suitably arranged complementary coupling module (e.g., as shown in FIGS. 9(b) and 9(c)) to be screwed onto the base module.

[0071] Turning to the coupling module, the coupling element (e.g., a hydrophilic elastomer sphere) that contacts the part to be inspected and any necessary delay line (e.g., a normal delay medium or a refractive delay medium) is preferably held within an acoustically absorbent shell. Providing such a strongly absorbent shell means that the projected L-wave used for thickness measurement can dominate other waves (e.g., reflected waves from the side of the coupling element). This enables, for example, a more compact refractive wedge design. An example of an absorbent shell for housing hydrophilic elastomer spheres is described with reference to FIGS. 9(b) and 9(c), but it should also be noted that similar absorbent shells can also be used for various types of coupling elements.

[0072] FIG. 9(b) shows a base module 293 attached to a coupling module 295 made in the design shown in FIG. 9(a) and having an acoustically absorbent shell 294. The shell 294 suppresses internal acoustic reflections and holds the hydrophilic elastomer spheres 296. In this example, the shell 294 is glass-filled PTFE (e.g., PTFE sold under the Teflon® brand). Alternatively, pure PTFE or other suitable acoustically quiet polymers can also be used. It should also be noted that a range of acoustic polymers specially designed for the acoustic absorption of high-frequency acoustic reflections are commercially available. For example, Aptflex F28 from Precision Acoustics is a high-frequency acoustically quiet sound absorber used for test tank lining in immersion systems and is a material suitable for an absorbent shell that provides very good acoustic attenuation characteristics against unwanted internal ultrasonic echoes. However, PTFE has the advantage of not tending to adhere to the inner surface of the restraint shell and being an ideal low-friction material that allows the hydrophilic spheres to move freely within the shell when compressed under the surface load. The internal threads (not visible in FIG. 9(b)) form a second connector portion that enables the attachment of the coupling module 295.

[0073] Figure 9(c) shows a composite bonding module 300 formed from a shell 302 that encloses a delay line 304 and holds hydrophilic elastomer spheres 306. The shell 302 is formed from precision-machined glass-filled PTFE, similar to the shell 294 shown in Figure 9(b). The internal thread 308 forms a second connector portion that enables the bonding module 300 to be attached to a first connector portion having complementary threads 292 formed on the base module 290.

[0074] As shown in Figures 9(a) and 9(b), the PTFE shells 294 and 302 enclose only a portion of the hydrophilic elastomer spheres 296 and 306, which provide bonding elements that protrude from the ends of the shells to bring most of each bonding module into direct contact with the inspection surface. In the case of the composite bonding module 300 of Figure 9(c), the detailed shape of the PTFE shell 302 in the vicinity and periphery of the protruding soft, superelastic bonding material of the delay line 304 affects the bonding performance achieved by effective compressive restraint when the soft bonding element 306 is confined within the structure. Also, if this restraint creates a stress concentration profile across the soft bonding material, the likelihood of tearing the hydrophilic elastomer sphere 306 increases, affecting the possible lifespan of the consumable. It should also be noted that the PTFE shell provides some useful general protection for the soft and vulnerable bonding material.

[0075] The use of glass-filled PTFE to form the shells 294 and 302 of FIGS. 9(b) and 9(c) also helps to attach each of the coupling modules to the associated base module. In particular, the PTFE enables a smooth and automated screw fastening between each coupling module and the base module. Thus, a mechanical screw fastening assembly can be designed using materials that promote smooth interaction between the connecting components (e.g., a PTFE shell for the coupling module and steel for the base module). Further, the dimensions of the coupling module can be set such that when the threaded assembly is tightened, a flat wear surface or wear plate within the base module provides consistent contact with the appropriate clamping force with the inner coupling material of the coupling module. The use of such PTFE shells makes it possible to form delay lines without the need for micromachined grooves to suppress reflections, and enables the adoption of less expensive mass injection molding or vacuum casting manufacturing methods.

[0076] Referring to FIGS. 10(a) and 10(b), a photograph of one embodiment of the ultrasonic probe 330 is provided. The ultrasonic probe 330 shown in FIGS. 10(a) and 10(b) is configured to be attached to a rotating head that is in turn attached to the movable quill of a CMM. In particular, the modular ultrasonic probe shown in FIGS. 10(a) and 10(b) is arranged to be attached to a two-axis rotating head (e.g., a REVO™ head of the type described above with reference to FIG. 3). Of course, it is possible to attach such an ultrasonic probe to other measurement systems.

[0077] The ultrasonic probe 330 includes a base module that drives a piezoelectric ultrasonic transducer and includes a body portion 321 that encompasses all the transmit-receive (Tx-Rx) electronics necessary to digitally record the acoustic response to such excitation. The body portion 321 provided at the proximal end of the probe 330 that is attached to the CMM has optional electromagnetic shielding to protect the transmit-receive electronic circuits. The body 321 may also include all the electronics necessary to power the probe and convey control data and activation commands to the probe (e.g., schedule ultrasonic measurements). Power and / or control data, including ultrasonic data and thickness measurement results, may be passed through a rotary head communication channel.

[0078] The body 321 also includes a thin and elongated rigid carbon fiber tube 323 that extends along the axial length of the probe. The distal end of the tube 323 carries an ultrasonic transducer and a first connector portion 322 for attachment to the coupling module. FIG. 10(a) shows the base module without the coupling module attached, while the enlarged view 10(b) shows the coupling module 332 attached (i.e., screwed) to the first connector portion 322. A high-frequency and shielded coaxial cable (not shown) extends internally along the carbon fiber tube 323 to electrically connect the Tx-Rx electronics within the body 321 to the transducer provided near the tip. This transmits high-voltage pulses from the Tx pulsar electronics to the transducer to generate an acoustic waveform and also sends back the analog voltage signal measured from the transducer to the Rx electronics to be digitized and recorded. The physical form of the probe is advantageously selected such that the electronic module is compact and housed within the body close to the measurement head of the CMM, but the overall length can be specifically selected by varying the length of the carbon fiber tube and / or the crank angle so that the transducer module and the probe tip can access difficult-to-reach component shapes.

[0079] Referring to FIG. 11, the transceiver (Tx-Rx) electronics included within the body 321 of the ultrasonic probe 330 described with reference to FIGS. 10(a) and 10(b) are described.

[0080] FIG. 11 shows an embodiment of analog and digital electronic modules that may be provided within an ultrasonic probe. An analog “pulsar” circuit 350 is provided that can generate a repeating sequence of high voltage (50 - 150V) alternating current analog signals (e.g., NGT pulses). Although the pulsar 350 is provided, alternatively, a more sophisticated digital waveform synthesizer may be used to generate frequency or amplitude modulated waveforms to drive the piezo in a more attenuating environment. The high voltage pulses generated by the pulsar 350 effectively drive the piezoelectric active element 356 within the probe's transducer and output the required ultrasonic waveform 358 without exceeding the maximum voltage of such thin and fragile piezoelectric elements. The activation of each pulse may be initiated by an enable signal sent from the FPGA 352 or equivalent processor to the “pulsar” circuit 350 and may be precisely controlled in time. For all activations, the high-speed T / R switch 354 allows the device to instantaneously switch between the transmit mode and a long receive mode, during which the system acquires and digitally records the acoustic response to the transmitted pulse measured by the reciprocating piezoelectric element 356.

[0081] Since the amplitude levels of the received signals of interest can vary significantly, a variable gain amplifier (VGA) 360 is provided as an option to induce an SNR gain over the acquired A-scan responses in order to amplify the signals before digital acquisition. Further, in order to equalize the variability within each A-scan response due to propagation loss or attenuation by some materials, a form of automatic gain control (AGC), known as distance amplitude correction (DAC), may be implemented. The amplified A-scan is digitized using an analog-to-digital converter (ADC) 362 with an appropriately wide dynamic range (e.g., 12 bits). Here, the sampling rate basically affects the temporal resolution of the measurement system, and thus the accuracy of thickness measurement. For example, a sampling rate of 125 MHz or higher may be suitable for a 20 MHz transducer, so a sufficient oversampling rate above the Nyquist rate is provided. The encoded digital waveform from the ADC 362 may also require bandpass filtering using a digital filter, e.g., a low-order FIR having a passband that matches the operating frequency of the transducer. The Tx-Rx electronics are designed to minimize all sources of electronic noise that may be observed within an individual A-scan. Such uncorrected noise is most effectively suppressed by averaging over N consecutive A-scan measurements (i.e., providing a theoretical N SNR gain).

[0082] Referring to FIG. 12, an example of the input data requirements for inspection planning software used to compile a series of automatic movement commands for the CMM and ultrasonic inspection apparatus described with reference to FIG. 3 will be described.

[0083] In surface contact measurement using a surface contact (scan or touch trigger) probe, it is known to use software that automatically generates a part program for planning and executing measurement movements using the nominal CAD data model of the part being inspected. For example, high-resolution continuous sweep scan measurements of turbine blades can be performed using ApexBlade software sold by Renishaw that generates a part program in the industry standard DMIS language for controlling a CMM. Similar CNC software, for example, can also automatically plan and schedule ultrasonic probe inspections using an accepted high-level CMM control software language (e.g., DMIS). Such inspection plans preferably have some detailed part-specific inspection plan or scheduling, whether implemented automatically or manually.

[0084] The first requirement is to define where ultrasonic measurements are needed. This can be achieved by defining an inspection plan that defines the position of all measurement nodes, straight sections (B-scan lines), or inspection areas defined over the target part being inspected where ultrasonic inspection measurements are to be made, prior to the ultrasonic inspection. This process can use the external shape measurement of the part being inspected performed using a known type of surface contact measurement probe. After defining the measurement nodes, knowing the detailed shape of the part and the available mechanical degrees of freedom provided by the automated platform carrying the ultrasonic probe, the type of ultrasonic probe required for the measurement can be determined. For example, it may be possible to use only a normal axis ultrasonic probe (as described with reference to FIGS. 4(a) and 4(b)), or a crank angle ultrasonic probe (as described with reference to FIGS. 4(c) and 4(d), for example) may be required for some or all of the measurements. If more than one ultrasonic probe is required, an automatic probe exchange routine will be needed.

[0085] As described above, the modular ultrasonic probe comprises an interchangeable coupling module. As shown in FIGS. 3 and 5(a), the coupling module is stored in a storage tray and is thus automatically attached to and removable from the base module of the ultrasonic probe as required. Thus, the inspection plan can include selecting one or more coupling modules that are most beneficially used for measurements over different geometries within the inspection part. Each coupling module may also have a limited life (e.g., may be a consumable or limited-life item), so the planning process can include an exchange strategy for refreshing such coupling modules. For example, a purely scheduled change strategy would likely involve determining the optimal coupling module for each section of the part with respect to coverage and scan performance and scheduling the set number of exchanges within the inspection to eliminate the possibility of using a damaged coupling module. A predictable replacement strategy involves replacing the coupling module only when damage or suboptimal performance is detected, which preferably includes determining the optimal design for a particular geometry and ensuring that each design is sufficiently available to cover the required number of possibilities. A mix of scheduled and predicted replacements can also be employed.

[0086] After determining the measurement nodes, the changes to the ultrasonic probe and the changes to the coupling module for the planned inspection, an optimal movement path can be generated. This process preferably ensures that the movement of the probe is properly blended so that the probe does not collide with any obstacles (e.g., parts, fixtures or granite bed). In the case of a predicted coupling module change strategy, it is important to know where the probe is located within the measurement volume when damage is detected in the current coupling module and whether a safe movement path sequence can be called to return the probe to the storage tray. Next, a list of inner wall thickness measurement nodes on the inspection surface is compiled and the ultrasonic paths are defined.

[0087] After attaching the base module of the ultrasonic probe to the movable member (e.g., a two-axis rotating head) of the CMM, tests can be performed to ensure that the piezoelectric probe functions accurately. It can be assumed that the axial alignment and position of the base module when attached to the rotating head are consistently fixed with sufficient accuracy to automatically attach and detach the coupling module from the storage tray. This is because the base module of the modular ultrasonic probe is substantially a rigid body and can be attached to the measuring head using established kinematic joints. However, the calibration of the position of the coupling element (i.e., the tip) of the coupling module is preferably performed after attaching the coupling module to the base module. This is to accurately determine the position of the sensing tip (i.e., the coupling element) within the coordinate system of the CMM.

[0088] Figures 13(a) and 13(b) show an exemplary general calibration artifact that can be used for sound velocity calibration, XY position calibration, and other calibration operations. It should be remembered that this is only an example of a suitable calibration artifact, and other calibration artifacts and techniques can also be used instead.

[0089] Figure 13(a) shows a two-dimensional cross-sectional view of the calibration block 400 that is also shown in three dimensions in Figure 13(b). The calibration block 400 is a precision-machined part that incorporates a plane orthogonal surface 402 and is placed on the bed of the CMM so that its position is accurately measured (i.e., calibrated) with respect to the XYZ position and orientation using a surface measurement (scan or touch trigger) probe. This block 400 also has a flat top surface 404 with a central dimple feature 406 that can be placed within the CMM volume using a surface contact (e.g., touch trigger or scan) probe. The calibration block 400 is hollow and has an internal conical surface 408 with a shallow oblique angle (e.g., 5 - 10 degrees) with respect to the flat top surface 404. The apex of the cone defined by the conical surface 408 is concentric with the XY coordinates of the central dimple feature 406.

[0090] In use, the position and orientation of the calibration block 400 in the coordinate system of the CMM are determined by conventional metrology data processing. For example, the reference points and the spindle are determined from the mutually orthogonal plane sections of the block by using a conventional touch trigger probe to obtain at least six contact points (e.g., three points defining the Z-plane, two points defining the x-line, and one point defining the Y-point). Once the position of the calibration block has been found in this way, the position of the ultrasonic probe tip within the CMM volume can be determined.

[0091] In particular, the position of the block 400 within the CMM volume can also be determined from two sets of measurements obtained using an acoustic probe with a bonding element in the form of a hydrophilic elastomer sphere. In the first measurement, the position of the tip of the acoustic probe (and thus the position of the point on the surface of the block) is determined on the z-axis by moving the acoustic probe downwards to a point above the top plane of the calibration block 400. In other words, the probe is moved in the [0 0 -1] direction by zeroing the head so that it faces the top surface normal vector [0 0 1]. The acoustic probe is then loaded onto the top plane 404 of the block 400 at a known Z height by slowly moving the CMM quill in the Z direction. By repeatedly placing the probe on the top plane 404, the Z coordinate at which the probe makes tangential contact with this surface can be estimated. This is achieved by analysis of the acoustic signals generated from the reflections from the hydrophilic elastomer sphere, as will be explained in more detail below. This first measurement enables the Z position of the tip within the CMM volume to be accurately determined.

[0092] Second, to estimate the XY position of the tip of the acoustic probe within the CMM coordinate system, a sequence of ultrasonic thickness measurements (e.g., at least six for a unique solution) is made across the top surface 404 of the calibration block 400 above the internal conical feature. Again, the probe is positioned to point downward (i.e., by zeroing the probe head), and the XY position of the probe is recorded at each measurement node. Then, the block thickness is calculated at each measurement point, and the set of thickness measurements acquired in 3D is mathematically fitted to a conical shape using, for example, the Levenberg-Marquardt (LM) algorithm or any linear or non-linear least squares conical fitting algorithm. This fitting process reveals the offset between the XY estimate of the apex of the fitted cone and the actual XY position of the dimple.

[0093] In a preferred embodiment, the calibration block 400 shown in FIGS. 13(a) and 13(b) may be machined using the same type and grade of metallic material found in the part being inspected. The block 400 may be used to perform a sound speed calibration to estimate the wall thickness of subsequent part measurements. This can be accomplished by measuring the time delay for a known thickness portion around the calibration block. Alternatively, it should be noted that the sound speed may be measured directly from a known solid portion of the part being inspected (e.g., near the root of a blade or wing) to minimize a variable speed source. This is because sound speed calibration is likely to be the largest source of measurement error in thickness measurement calculations due to, for example, temperature differences within the environment, crystallographic direction, or differences in the fine structure of density / porosity.

[0094] Mode-3 measurement is a preferred method for thickness calculation using the modular acoustic probe described above because it is substantially unaffected by the variability of the coupling element (e.g., changes in the propagation path through the hydrophilic elastomer spheres). However, the calibration techniques described above can also be used for Mode-1 or Mode-2 measurements. In such measurement techniques, the absolute propagation time delay for the first backwall reflection over a range of thicknesses can be used to directly obtain any subsequent thickness measurement value within the measurement range by linear interpolation. This can have a limited number of applications, such as measuring thicker and more attenuating components with a coupling module having a rigid constant delay line and a very thin coupling element (e.g., latex rubber) to reduce variability, for example.

[0095] The calibration block 400, which can accurately represent the component to be inspected, can have additional applications in the context of highly automated point measurements and continuous scans. As described below, these additional applications include probe contact detection, normal probe load estimation, thickness measurement, and wiggle movement adaptation.

[0096] For the detection of probe contact, the calibration block 400 provides a convenient known geometry and the same planar target where a probe with any attached coupling module can measure and calibrate the relevant variations in the A-scan waveform of the probe at the point when its tip contacts the surface. As will be described in more detail below, this technique utilizes the changes in the amplitude, phase, fine shape, frequency, and / or time of arrival (TOA) of the internal reflection echoes from within the coupling module, which are extracted from successive repeated sequences of the measured A-scan, to infer contact of the probe tip with any solid. For example, the amplitudes of the second and third delay line reflections (DL2 and DL3) in the rigid delay line have been found to be extremely sensitive to probe tip contact from a solid.

[0097] For normal probe loading estimation, using internal reflections from a coupling module extracted from a series of repeated A-scans that infer such probe tip contacts, such calibration blocks can be used to extrapolate to normal loading measurements and calibration of the probe on a flat surface. By slowly loading the probe onto the calibration block while continuously recording the A-scans at high speed, the waveform characteristics from the internal reflection echoes of the coupling module when the flexible coupling element deforms relative to the inspection surface can be extracted and stored along with the tip and surface position information. These characteristics appropriately define how the physical state of the soft coupling element within the coupling module changes under normal loading conditions, and thus can infer the loading conditions when making subsequent measurements across the inspection surface. Such normal loading calibration for estimating displacement or deflection within the soft coupling element of the coupling module is extremely useful for ensuring accurate operation of the probe with respect to the inspection surface (e.g., for changing the effective aperture or incident angle of the probe), or for continuous scanning of the probe across complex and / or unknown geometry surfaces. Thus, by using such estimated measurements at high speed as direct feedback for alignment within a CMM and / or an active head controller, substantially constant loading conditions can be achieved across unknown topography.

[0098] For thickness measurement, it is possible to estimate accurately and computationally efficiently the time delay between backwall echoes. In a preferred embodiment, this time delay estimation process can include an implementation in the form of a generalized cross-correlation (GCC) algorithm that convolves a stored or extracted replica of the backwall echo over the measured A-scan in order to sharpen precisely the time delay estimation between successive backwall reflections. This spectral technique utilizes the overall shape of the backwall reflection waveform, including amplitude and most notably phase (e.g., using phase-transform pre-whitening), in order to measure the arrival time of the echo and thus the exact time difference between successive echoes. In this way, calibration block 400 can be used to measure and store an extended set of representative replica waveforms of the backwall echo that can be used during inspection.

[0099] It should also be noted that the same method for measuring the time delay between successive backwall reflections in a mode-3 measurement can be used for precise measurement of internal reflection echoes from a deformable coupling module. More specifically, it is beneficial to store in calibration the template signature of the internal reflection peak that can be used during subsequent inspection in order to extract the exact time of arrival of the internal reflection peak.

[0100] In the case of wiggle motion adaptation, ultrasonic coupling to any surface is not completely determinative in that the best SNR is not always obtained simply by applying a load perpendicular to the surface with the maximum force available for a normal incident L-wave transducer. The probabilistic process can also affect the SNR achieved for the probe loaded on the inspection surface. For example, dominant microstructures, humidity, and temperature conditions affect how air is trapped between the probe and the inspection surface, introducing significant potential variability in ultrasonic transmission. For these reasons, in both manual and automated ultrasonic NDT measurements (e.g., using the Marietta-NDT 5-550 system), it is known to apply a fine applied force in the orientation of the probe (e.g., rolling and / or twisting) while holding the probe tip stationary on the surface to optimize the received signal level. Furthermore, an accurate and infinitely programmable automated platform (e.g., a CMM with a 5-axis active head) enables determination of a favorable sequence of such fine probe movements (e.g., roll and twist) for specific inspection conditions. Thus, the calibration block 400 is further emphasized to provide such a known representative surface that can measure a continuous sequence of optimal and fine wiggle motions or can be algorithmically learned (e.g., using optimization, clustering, or artificial neural classifiers) for a modular acoustic probe that can be employed during subsequent component inspection.

[0101] Figure 14 shows a process of measuring an aerospace fan blade disk / hub 450 using a 5-axis CMM device of the type described with reference to FIG. 3 and a variation of the crank angle of the modular acoustic probe 109 shown in FIGS. 4(c) and 4(d).

[0102] The modular acoustic probe is attached to the two-axis rotating head of the CMM, and after the necessary calibration procedures are completed, measurements can be taken across the part using probe 109. Probe 109 can perform both point measurements 441 and continuous scan measurements 442 during inspection of the blade 450, if desired. For example, measurements can be taken at spatially distinct sets of nodes (e.g., 20 positions distributed across each blade), and / or continuous scan measurements can be obtained (e.g., by moving the probe along a path on the surface of the blade while collecting measurements at a 1 mm pitch).

[0103] As shown in FIG. 14, the crank angle allows the ultrasonic transducer (and thus the projected L-wave) to be oriented at a fixed angle away from the longitudinal axis of probe 109. Despite being placed in close proximity to adjacent blades, this array of angled probes allows the ultrasonic energy to be directed perpendicular to the surface of blade 450 using the 5 degrees of motion (three translational axes X, Y, Z and two rotational axes A, B) provided by the CMM and the rotating head. However, for seamless continuous scans, some geometries and scan patterns may benefit from the addition of a further rotational axis (C) around the main probe axis.

[0104] Referring to FIG. 15, the A-scan waveforms generated during thickness measurement of a flat part taken using a modular ultrasonic probe having a coupling module with hydrophilic elastomer spheres are shown.

[0105] Figure 15(a) shows the hydrophilic elastomer spherical tip 462 of the modular ultrasonic probe 464 moving vertically towards the flat inspection surface 466 mounted thereon by the CMM. The A-scan shown in the graph of Figure 15(a) shows the amplitude of the received (returned) ultrasonic pulse echo as a function of time before the probe contacts the surface. The first peak 470A corresponds to the excitation pulse generated by the transducer of the probe. The subsequent peaks 470B, 470C are peaks of time-delayed internal reflections from within the non-compressed hydrophilic sphere. Thus, these consistent A-scan waveforms in Figure 15(a) show the "rest state" from within the unsteered coupling module when surrounded only by air. That is, since the tip of the probe (i.e., the hydrophilic elastomer sphere 462) is not yet in contact with the inspection surface, there is no external mechanical force acting on the sphere. Such a series of A-scans can be performed at a high repetition rate (e.g., 1000 - 2000 Hz). It should be noted that, similar to the case of the A-scan of the delay-line transducer shown in Figure 2, the time window defined between the first peak 470A and the second peak 470B provides the main measurement window of the probe. However, the repetition rate is not so high as to cause significant interference between consecutive pulses.

[0106] (b) of FIG. 15 shows the exact point where the hydrophilic spherical tip 462 of the probe 464 first contacts the planar inspection surface 466. At the very initial moment of contact, there is no significant distortion of the shape of the sphere, but there is an obvious and immediate change in the measured A-scan waveform. First, the reflection peaks 480B and 480C (i.e., the time-delay internal reflection peaks from within the non-compressed hydrophilic sphere) show a decrease in peak amplitude. This is more prominent for the second reflection peak 480C. Second, as the probe starts to make a more significant contact, the peaks begin to shift slightly to the left (i.e., towards the excitation pulse 480A at t = 0). Third, even with a light contact between the hydrophilic sphere 462 and the hard inspection surface 466, within the main measurement window of the A-scan, multiple measurable reflection peaks 482A, 482B, and 482C from the successive back-wall reflections of the component can be observed.

[0107] In (b) of FIG. 15, for clarity, only three reflection peaks 482A, 482B, and 482C are shown (i.e., it is highly likely that there are more than three such reflection peaks), and it should be noted that these back-wall reflections are caused by the bonding characteristics of the hydrophilic sphere. In particular, an important advantage of the ultrasonic probe based on the hydrophilic sphere is that it only requires an appropriate probe contact with the inspection surface, but can provide sufficient delay for thin component measurement. This is a direct result of the ability to fill the air gap between the probe and the surface due to the soft isotropic contact characteristics of the hydrophilic sphere and the partially wet contact characteristics with respect to the contact feel.

[0108] Therefore, the changes in the A-scan due to contact with the surface enable the ultrasonic probe to perform surface contact measurements, which will be described in more detail below. The backwall reflections 482A - 482C may be suitable for providing mode-3 thickness estimations, but it is preferred to further load the ultrasonic probe onto the inspection surface to establish an increased acoustic coupling with the component. In particular, the further loading enables an optimal coupling contact (also referred to herein as the coupling "sweet spot") to be obtained, which is revealed by a combination of a decreased reflection peak from within the coupling module and an increased backwall reflection peak.

[0109] Figure 15(c) shows the result of further loading the hydrophilic sphere 462 of the modular ultrasonic probe 464 onto the inspection surface 466. As can be seen from the A-scan plot, the decrease in the amplitude of the reflection peaks 490B and 490C of the coupling module (i.e., the time-delayed internal reflection peaks from within the non-compressed hydrophilic sphere) is more prominent. This is accompanied by a significant increase in the SNR of the backwall reflection, which is the main concern for thickness measurement (i.e., peaks 492A, 492B, and 492C). Also, as the probe is further loaded onto the surface and the sphere is gradually more deformed, it can be seen that both the internal coupling module reflections (i.e., peaks 490B and 490C) and the backwall reflections (i.e., peaks 492A, 492B, and 492C) within the measurement window are further shifted towards the transmit pulse 490A at time t = 0. However, the delay between successive backwall reflections (i.e., peaks 492A, 492B, and 492C) remains invariant.

[0110] Figure 15(d) shows applying an additional load of the probe to the surface past the above-mentioned "coupling suite spot". A further decrease in the amplitudes of the coupling module reflection peaks 500B and 500C (i.e., the time-delay internal reflection peaks from within the non-compressed hydrophilic sphere) is observed, but no substantial change in the back wall reflection signals (i.e., peaks 502A, 502B, and 502C) is observed. It can also be seen that the peaks of interest are further shifted towards the initial transmission pulse at T = 0 (i.e., the main excitation peak 500A). Applying an additional load of the probe onto the surface past the "coupling suite spot" thus does not bring about any further improvement in the SNR of the back wall reflection signal. In addition, such an additional load means that the deformation of the sphere can approach a state where a temporal overlap between the transmission waveform and the reception waveform of interest within the A-scan is observed, or the hydrophilic sphere is damaged.

[0111] The A-scan data shown in FIGS. 15(a) to 15(d) can follow various signal or data processing methods to enable automatic detection of changes in a continuous series of A-scans while the probe is being continuously manipulated on the inspection surface. The waveform information extracted from these A-scans, particularly the transient waveforms from the internal reflection echoes from the hydrophilic sphere, provides a sensitive and reliable method for accurately detecting when the tip of the ultrasonic probe comes into contact with another body. This surface contact information has several applications.

[0112] The first detection method involves capturing a single reference A-scan with the probe positioned at some "null" position within the CMM volume when it is known that the tip of the ultrasonic probe is not in contact with a solid. This reference waveform (such as the waveform shown in Fig. 15(a)) contains only internal reflection peaks from an uncompressed sphere and represents a state defined without tip contact. Importantly, it has been observed that this A-scan waveform shape is consistently restored after the contact load at the tip is removed from any solid, mainly due to the very high elasticity of the soft coupling sphere. The segment of the A-scan that includes the coupling module reflection peak (i.e., the internal reflection from the hydrophilic sphere) is extracted from the segment passed at the same time from a continuous series of A-scans measured when the probe is manipulated before surface contact, and can be repeatedly compared or differentiated. Monitoring the differentiation in this method can be used to automatically detect when tip contact with the object occurs.

[0113] The automatic detection determination can be directed based on any suitable detection criteria. In some scenarios of the probe, advanced or adaptive detectors (e.g., CFAR, Bayesian detectors) can be described, but in many scenarios, a simple square-law energy detector with an absolute predetermined hardness detection threshold is sufficient. This approach is effective because the continuous series of measured A-scan waveforms has measurement / measurement variations that are small enough to be negligible while the probe is operating in free space at any speed or due to the complex movements that the CMM and / or head can induce. Further, the contact between the probe tip and the solid induces an instantaneous and very large change across the entire observed internal reflection echo from the hydrophilic sphere. The automatic contact detection algorithm within the probe can also analyze any number of reflection echoes beyond the first echo return. For example, the second and third reflection waveforms often have significantly different amplitudes than the first echo return on such a contact (e.g., as seen in FIG. 15(b)), and thus these waveforms can provide a sensitive indication of the touch contact event. The repetition rate of pulse generation is preferably selected such that interference from the previous pulse is minimized.

[0114] The characteristics of the signal that define the internal reflection echo of the combined module, extracted from each A-scan and used as input data in the detector, may be simply related to the differences in the accumulated waveform energy. However, it should be noted that the signal metrics used may have different abilities to affect the detection of the smooth but sensitive real-time tip contact. Other waveform metrics, including peak voltage, signal kurtosis (i.e., the fourth statistical moment), RMS, FFT, and AR coefficients, may also be extracted for use in the detector for any signal feature, but will likely be used well enough as well. Such algorithms for detecting significant changes in the A-scan waveform to infer tip contact events actually require minimal calculations, as the comparison or detection determination based on the differential process is calculated only over short time-gated segmented windows extracted from each A-scan that includes the internal echo peak of the combined module. Therefore, in practice, the speed at which the probe can report the tip contact state is more fundamentally limited by the frequency at which A-scans can be generated rather than the calculations of the detector. It should be noted that the A-scan generation speed depends on the thickness of the medium in the combined module and the L-wave sound speed, and the transit time required to record at least the first two reflections from the hydrophilic elastomer tip. Since it is relatively simple and requires few calculations for the detection task, the frequency at which the contact state information is reported by the probe and transmitted to peripheral devices (e.g., the controller of a CMM or measurement head) can be relatively high (e.g., up to 2000 Hz). However, if the repetition rate is increased such that a new transmission pulse is induced before the previous reflection decays more significantly, a set of additional unexpected / pseudo-noise peaks of decreasing amplitude will rapidly increase in successive A-scans. These can be effectively filtered within the primary measurement window using primary signal processing methods for extracting the required time delay between successive backwall reflections, but these are more important in terms of the amplitude between the transmitted pulse and the first internal reflection.

[0115] Generating such high-speed tip contact state data allows the automated inspection system to respond to any unexpected tip contact events that the probe may encounter unexpectedly during any type of movement induced by the automation platform (e.g., the movement of the CMM and / or the measurement head), i.e., to respond mechanically relatively quickly. For example, if tip contact is detected while the probe is moving along a linear trajectory at a typical scan speed (e.g., 100 mm / sec), the minimum possible movement to the object is equivalent to about 50 microns of deflection in the soft coupling element of the coupling module, assuming no latency in sending an interrupt command to the CMM and the head to stop the movement. When affecting the commands for the CMM and / or the measurement head to stop such movement, even allowing a reasonable amount of latency time, the amount of deflection within the soft coupling element of the coupling module will be of the order of the nominal maximum allowable deformation magnitude within which damage to the tip of the hydrophilic sphere or the hard probe would be induced. The combination of high temporal resolution contact state data and the positional tolerance provided by the soft elastic tip reduces the likelihood of significant undetected impact damage to the probe tip.

[0116] As described above, the sensitive touch contact ability is very useful for the navigation of the probe within the CMM space. This is especially so since the probe is a rigid body and does not have other sensing modalities that can be easily damaged. However, the ability of the ultrasonic probe to generate very fast and useful surface interaction data goes beyond simple binary contact detection. As will be described below, a signal and data processing method has been devised that allows the probe to be used within any inspection as a simple and sensitive touch probe that can generate Cartesian point cloud measurements that describe the outer shape of the inspected part. This basic touch point function has a direct benefit for a wider range of applications (e.g., sensitive and accurate measurement of soft gelatinous parts with difficult optical properties that could not be easily measured with conventional touch probes or optical scan probes) in addition to the metrology inspections (e.g., time savings) performed by the ultrasonic probe. Further, the load state of the probe on the inspection surface can be continuously estimated by directly utilizing the internal coupling module delay echoes within the measured A-scan. This has a direct and important advantage for both more controllable point measurements and continuous movement scan inspections performed using the ultrasonic probe.

[0117] Note that the touch ability can be further improved by exciting the piezoelectric active element of the probe with a continuous sine wave signal during operation. For example, the tip of a hydrophilic sphere can be driven with continuous sine wave excitation at a resonant frequency, e.g., 20 MHz. When the sphere is contacted by a solid, the dampening detected at resonance can be detected.

[0118] A processing method that allows a modular ultrasonic probe to be used as a basic touch trigger probe capable of performing useful point cloud measurements over the external form of an inspection part will be described with reference to FIGS. 16 to 19. For example, this will enable the probe to properly investigate the surface orientation for the next thickness measurement at a point by performing three touch measurements in proximity around the required measurement nodes to estimate the surface normal. As described above, the touch contact can be detected by the probe by continuously monitoring the internal reflection from the hydrophilic sphere of the coupling module, so that a significant change (e.g., peak amplitude, phase, arrival time change) in these echo waveforms from the calibrated non-contact reference state can be detected.

[0119] FIG. 16 shows a modular ultrasonic probe 546 moving at a constant speed towards a solid block 547 by linear movement in both the X and Y coordinates with the probe tip 548 maintained at a constant Z height. The ultrasonic probe 546 has a tip 548 with a hydrophilic sphere. The modular ultrasonic probe 546 is attached to the CMM for movement as described above with reference to FIG. 3.

[0120] The XYZ position of the probe tip in the CMM coordinate system (i.e., the position of the center of the hydrophilic sphere) is collected at a high data rate (e.g., 1000 - 2000 Hz) via the CMM controller. This tip position data is combined with an appropriate signal generated at the same speed by the ultrasonic probe indicating whether there is a large perturbation in the internal reflection echo of the hydrophilic sphere, thereby indicating that the sphere has contacted the object. This signal, similar to the trigger signal generated by the touch trigger probe, can be generated by monitoring the absolute difference between the second internal reflection peak voltage (Vp) in each measured A-scan and the stored "non-contact" reference A-scan as described above with reference to FIG. 15.

[0121] The touch event detected by the ultrasonic probe thus causes the probe to issue an immediate instruction to the CMM (e.g., via a change in the state of the trigger signal line) to stop the CMM operation and store the point measurement value. However, there are always some latency times associated with the delivery of the command from the probe to the CMM, and the period of CMM deceleration cannot be avoided. The delay in stopping the movement of the CMM causes the soft tip of the ultrasonic probe to deform into the solid block, so that its position at full stop may be quite far from the point P on the surface where the contact was first detected.

[0122] Figures 17(a) and 17(b) illustrate the above-described action. Figure 17(a) shows the point P where the contact is first detected, and Figure 17(b) shows the further movement to the surface (i.e., point O) that occurs before the ultrasonic probe stops. By interpolating the position and the time series of the trigger signal acquired up to this point, it is possible to approximate the point P. Alternatively, if the probe is moving relatively fast when the touch event occurs, more accurate results can be achieved by involving a backing-off movement at a slower speed. This would involve removing the probe from the surface at a slower speed in a direction opposite to the approach vector.

[0123] FIG. 18 shows plots of the X position 550, Y position 552, Z position 554, and state data 556 of the trigger signal (Vp) as a function of time while moving the probe to the surface and then away from the surface using the method described above. The time series plot indicates the contact time 558 and the time when contact is lost (560) with dashed lines. The probe is thus moved to contact the surface and stops at point O. Thereafter, there is a short dwell period (ending at point D) during which the probe is stationary before the slower back-off movement is initiated. This slower reverse movement allows for time quantization errors since a higher density of spatial measurement points is recorded and the time when Vp returns to the reference level is detected, thereby indicating that contact has been broken, which has little impact on the time quantization error when estimating the XYZ position of P (i.e., because the gradient is shallow). During the slower linear back-off movement, it is also possible to generate a trigger signal (e.g., the Vp signal) from the A-scan data at a faster rate than the probe position is reported within the hardware of the probe. This enables a more accurate estimate of P to be obtained by interpolation. A more sophisticated interpolation method can also be employed that corresponds to the subtle differences in the reflected peak variations when the contact deformation of the sphere occurs at different positions on the spherical surface and at different ball release angles.

[0124] The elastic hydrophilic spheres of the above-described coupling module may be synthesized to release different amounts of water during inspection. Releasing a greater amount of water (e.g., for lubrication over a rougher surface) has been found to reduce the accuracy of the touch contact measurements performed during the back-off movement. This is because small water droplets of various sizes accumulate around the position of the deformed sphere, creating a temporary physical bridge between the sphere and the inspection surface during the back-off movement. These water droplets can introduce variability when the ultrasonic data indicating contact with the object is lost. Such variability can be easily overcome by not utilizing the first back-off operation for making touch measurements and instead incorporating a second operation on the surface (e.g., along the same vector at a slower speed immediately after the first back-off operation) to acquire the touch position data.

[0125] The use of an ultrasonic probe to also obtain surface contact measurement values has the further advantage that it is quicker than swapping out the ultrasonic probe for a conventional surface contact (e.g., scan or touch trigger) probe.

[0126] Referring to FIG. 19, it should also be noted that due to the symmetry of the sphere, only differences across the latitudes (e.g., α and β) need to be considered. This is as long as the relationship between the deformation of the probe and the probe position in X, Y, or Z is linear or can be calibrated via an intentional surface contact touch.

[0127] In addition to analyzing the A-scan of the ultrasonic probe to establish when surface contact is first achieved, signals and data processing algorithms for estimating the loading of the probe, and thus the coupling state during inspection, can also be included in the probe. For simplicity, and because it is most relevant to how the probe is generally used, a scenario where the probe is nominally loaded onto the inspection surface from the normal direction will be described. However, the same principles and methods can be applied when loading the probe at an angle away from the propagation direction of the L-wave (i.e., the axial direction of the transducer).

[0128] As already explained with reference to FIG. 15, loading an ultrasonic probe with a hydrophilic spherical tip onto the surface induces a measurable change in the internal reflection echoes within the A-scan with respect to normal deformation or Z-displacement within the sphere. As described above, the loading can be evaluated by monitoring the peak amplitude (Vp) and / or time of arrival (TOA) of the first and second internal sphere echoes. It should be noted that in some cases, a single combined metric related to the internal reflection echoes can be sensitive and robust for both contact detection from calibration data and load deformation estimation (e.g., using the ratio of higher order reflection peaks).

[0129] Figures 20(a) and 20(b) show an example of how the TOA and Vp of the first and second internal reflection peaks, respectively, from the hydrophilic sphere of the ultrasonic probe can change as the probe is gradually vertically loaded (i.e., the normal deformation of the hydrophilic sphere gradually increases) onto the inspection surface at a constant slow speed. From the graphs of Figures 20(a) and 20(b), it can be seen that the relationship between the Z-deformation (or Z-deflection) of the soft sphere and the TOA and Vp of the first and second reflection peaks is substantially linear.

[0130] This consistent and reproducible relationship between the metrics that define the shape and / or position of the internal sphere reflection echo in the A-scan and the amount of sphere deformation induced by probe loading (loading) is effectively compiled during calibration. In other words, the data as shown in the plots of Figures 20(a) and 20(b) can be generated by appropriate calibration artifacts, for example, probe load measurements obtained from the artifacts described above with reference to Figure 13. Such a known (i.e., calibration-based) relationship can be directly used during any subsequent inspection to estimate the deformation or load state of the probe by extracting the same signal features from the relevant peaks (e.g., TOA, peak amplitude) in each measured A-scan. For example, a comprehensive calibration load measurement can be performed using a calibration block with the tip of the probe set at a range of angles with respect to the surface and a range of linear arrival angles on the surface (i.e., different grazing angles). Such a comprehensive load calibration is practical for symmetric probes because the compiled relationship between the load vector and the selected internal reflection peak features is the same regardless of the initial axial rotation of the probe. Having a set of calibration data related to sphere deformation, any new set of reflection peaks from the A-scan measurement can be classified either by interpolation (linear or non-linear curve fitting) or simply by a Euclidean nearest neighbor classifier to estimate the deformation displacement.

[0131] It should be noted that the accuracy achieved when measuring such load conditions can vary. The most robust and accurate estimation of the probe's load with respect to the deformation of a soft, isosceles tip (in mm) is typically achieved where the L-wave is projected perpendicular to the inspection surface along the probe axis. Fortunately, this is the most typical scenario for thickness measurements when the part has parallel front and back walls.

[0132] In addition to obtaining individual measurements, continuous acoustic scans are also possible, for example, over a simple shape such as a continuous solid with parallel front and back walls. Such continuous scans are preferably performed using an ultrasonic probe equipped with a hydrophilic elastomer sphere that is loaded against the inspection surface in a direction perpendicular to the surface. Such continuous scans are possible due to the self-lubricating action of the hydrophilic sphere and the ability to use the load evaluation provided by the analysis of internal reflection echoes.

[0133] Figure 21 shows a scan scenario in which a modular ultrasonic probe 600 having a tip consisting of the tip of a hydrophilic sphere 602 is scanned over an unknown wavy inspection surface 604. The probe continuously acquires A-scan measurements and estimates the Z deformation (Zd) in microns from each A-scan. This is performed relatively quickly so that any sudden changes in the load conditions become immediately apparent in the Zd-time plot, as shown.

[0134] As shown in FIG. 21, the probe is moved laterally across the surface from a starting point 606 to an ending point 608. The probe is initially at a constant height on the horizontal plane (i.e., a constant z-height) and is loaded at a constant level within a “bond sweet spot” region corresponding to a constant Z deformation (Zd) or a constant displacement of the tip of the sphere. When the probe first reaches the undulating region 610 having an increased Z height, the Zd estimate first increases without any deviation at the Z position of the probe. However, it is emphasized that the Zd estimate data can be used directly within the control loop of the CMM system to change the height of the probe in response to the measured change in Zd. As shown in the lower graph of FIG. 21, the CMM may be adapted to provide real-time adjustment of the height (i.e., Z position) of the probe in response to the Zd measurement. In this example, this is done by moving the probe upward in proportion to the increase in Zd. This retraction (Z direction) of the probe results in the Zd value quickly returning to its average (optimal) loading state. Similarly, when the probe reaches a surface of decreased height, the decrease in Zd can be immediately compensated for by the CMM to move the probe back downward toward the surface.

[0135] Therefore, this technique uses the internal reflection echoes directly from within the soft bonding layer of the hydrophilic spheres, almost in real time, to ensure an optimal acoustic coupling state. Such direct real-time estimation, and thus the interpretation of the reflected L-waves within the coupling module, to control the loading state of the probe with respect to the inspection surface (i.e., using feedback control on an automated platform) not only benefits the positioning and scanning of the probe, but also fundamentally affects the transmitted L-waves for which useful thickness measurements are made. It should also be noted that the highly elastic and conformable coupling element also provides the inherent ability to modify and / or precisely control the projected L-wave beam entering the part, either by a controlled change in the normal loading displacement or by reorientation of the axial probe vector away from the normal of the inspection surface, according to the basic laws of both acoustic diffraction and refraction. This positive beam manipulation is most practical and effective when the probe is attached to a high-precision automated platform such as a CMM described with reference to FIG. 3.

[0136] Figures 22(a) - 22(c) show some of the ways in which an ultrasonic probe having a hydrophilic spherical tip is used to induce more accurate control of the L-waves projected onto an object. In particular, such a probe enables the exploration of more complex internal shapes by ultrasound, either by controlling the divergence of the diffracted beam by applying a more precisely calibrated load in the normal direction of the probe (i.e., changing the aperture size), or by steering the refracted beam by precise reorientation of the transducer axis.

[0137] Figures 22(a) and 22(b) show how an increased load on an ultrasonic probe 620 with a hydrophilic spherical tip 622 can induce a larger diameter aperture on the inspection surface that produces a collimating effect that reduces the naturally diverging beam width. As shown in FIG. 22(b), the narrower beam is beneficial as it avoids internal features 624 of the object under inspection that could otherwise produce false reflection echoes that interfere with the measurement of interest.

[0138] Referring to FIG. 22(c), an object having non-parallel front and rear walls can be measured by reorienting the probe 620 away from the surface normal. Such beam steering may be limited to a small refraction angle to reduce the mode conversion effect. For such small angles, the slower shear wave mode may not be very important or may be time-gated from the A-scan.

[0139] A variety of modifications to the ultrasonic probe described above are possible. For example, a plurality (e.g., 15 - 20) of hydrophilic spheres can be connected in cascade to form a continuous chain of contact spheres within a corresponding long absorbent shell. The first sphere can be placed within the shell to contact the transducer wear plate, and the final sphere in the chain can protrude from the shell to contact the inspection surface. Such a probe design enables a more remote inspection situation when it is not desirable or physically impossible to place the probe tip of the transducer near the measurement node of the inspection part. Such a useful application of the modified design is possible due to the extremely low L-wave attenuation characteristics observed with such hydrophilic media components. As a result, the propagation loss from the transducer to the tip of the coupling module becomes negligible.

[0140] In addition to such a structure that provides a very efficient acoustic waveguide for the inspection L-wave, it is also possible to manipulate the L-wave projected within the coupling module. In particular, there may be a need to perform L-wave inspection along an axis other than the normal probe axis (e.g., for measurements in a confined space). For example, it may be possible to embed an acoustic mirror within a chain of hydrophilic spheres that simply redirects the L-wave in some known direction along the law of acoustic reflection (i.e., the angle of incidence equals the angle of reflection). Such a mirror is a flat acoustic reflecting surface that can be mounted at a set angle (e.g., having a high acoustic impedance).

[0141] Figures 23 to 25 show the selection of various scenarios in which the cascade chain of hydrophilic elastomer spheres can be effectively applied.

[0142] Figure 23 shows a method of inspecting the bottom of an elongated hole using the cascade chain of hydrophilic elastomer spheres 640. Note that, as shown by the illustrated A-scan, the primary measurement window of a probe equipped with such a chain of spheres is shifted according to the number of spheres in the chain. However, the resulting reflection of the back wall within the A-scan is measurable and has an SNR approaching that achieved with a single sphere.

[0143] Figure 24 shows that the spheres can be sized to form a tapered chain of spheres 650. The chain of spheres 660 can also be cumulatively bent off-axis using a weak diffraction effect. Again, the resulting reflection of the back wall within the A-scan is measurable and has an SNR approaching that achieved with a single sphere.

[0144] Figure 25 shows a chain of hydrophilic spheres 670 with a reflective mirror 672 for inspection perpendicular to the axis of the probe / transducer. Such an ultrasonic probe is useful for comprehensive measurement inspections within a tube and / or container, and the probe can rotate around the enclosure.

[0145] The ultrasonic probe may be provided with different shapes (non-spherical) of hydrated hydrophilic elastomers within a conforming absorbent shell. In addition to the described basic spherical shape, superabsorbent polymers or lightly cross-linked vinyl elastomers with a high water content (e.g., typically 75 - 95%) may be synthesized to grow into substantially any closed-form continuous shape that, when hydrated, is required to fit entirely inside the outer absorbent shell. The various custom shapes (e.g., longer and / or thinner prismatic shapes) of the continuous hydrophilic elastomer material within the conforming PTFE shell can be designed to accommodate complex geometric shaped parts. By observing the A-scan from such a coupling element, it is clear that the back wall reflection is present in a much wider first measurement window. It should also be noted that the various methods by which the internal reflection echoes within the coupling module are processed to estimate the load displacement or contact state are also retained for such alternative designs.

[0146] As described above, a coupling module of a compound class without a hydrophilic sphere may be provided. FIG. 26 shows, for example, an A-scan generated from an ultrasonic probe comprising a coupling module of a compound class as described above with reference to FIG. 7. As can be seen by comparing FIG. 26 with FIG. 15, there are differences in the signals generated by the coupling module of the compound class, and thus there are different processes that can be used to interpret the A-scan and extract information useful for the detection of contact, load, scan, and accurate thickness measurement processes.

[0147] FIG. 26 shows an ultrasonic probe 700 comprising a normal beam compound coupling module with a latex rubber tip 702 that is loaded onto a simple inspection surface.

[0148] Figure 26(a) shows the probe 700 approaching the surface. Before contact with the surface, it can be seen that the A-scan incorporates only the evenly spaced repetitive reflections (i.e., the first, second, and third delay line reflection peaks 701, 703, and 705) from the hard plastic delay line element 704 after the first Tx-pulse (not shown). These reflections exhibit measurement values that can be ignored with respect to measurement variations while the probe is moving in the free space within the CMM volume.

[0149] Referring to Figure 26(b), when the latex rubber probe tip 702 contacts the surface, there is an immediate change in the measured A-scan response. This first tangential contact does not induce an apparent shift in the arrival time of the internal reflection echo, but induces an obvious decrease in the amplitudes of the second and third reflection echoes (i.e., peaks 710 and 712). Depending on the coupling performance (e.g., defined by the soft coupling surface finish and the geometry of the part), this decrease in reflected energy occurs in relation to an increase in the back wall reflection waveform 714 from the energy transmitted to the part.

[0150] Referring to Figure 26(c), as the probe 700 is further loaded onto the surface, the soft coupling layer (i.e., the latex rubber tip) deforms and the rigid planar delay element contacts the surface more isogonally. The amplitudes of the second and third reflection echoes (i.e., peaks 720 and 722) further decrease, and the amplitude of the successive back wall reflection (i.e., peak 724) increases. However, this decrease in the peak amplitude of the delay line peak signals (i.e., peaks 720 and 722) is not accompanied by a change in the arrival time (TOA) or the phase of these reflection peaks. Further, as shown in Figure 26(d), when the probe is completely withdrawn from the surface, the peaks of the delay line return to the same level and assume the same shape as before surface contact.

[0151] FIG. 27 shows how the arrival times and peak amplitudes of the first and second delay line reflection peaks of the probe evolve as the probe is linearly loaded onto the inspection surface, as described with reference to FIG. 26. In particular, FIG. 27 shows that the reflection peak from the rigid delay medium remains fixed in time with respect to the T = 0 excitation in the A-scan. Thus, load calibration is more effective using waveform features that quantify the proportion of acoustic energy that can leak from the rigid delay medium with increased load conditions (e.g., the ratio of the second or third peak voltage Vp). However, it is emphasized that such plots can be effectively compiled and used during the calibration procedure to automatically detect contact (e.g., via a hard threshold energy detector) or to classify load conditions (e.g., via linear or polynomial interpolation from the plot, or other computationally efficient classifiers).

[0152] Next, a signal processing method for generating a thickness measurement from the measured A-scan within the measurement window will be described. Such a signal processing algorithm is preferably robust and can be based on a form of generalized cross-correlation, or replica correlation, to extract the exact time delays between the first, second, and optionally third backwall reflections. It should be noted that beyond the third backwall reflection, waveform dispersion can begin to affect the time difference estimation accuracy due to perturbations in the fine shape of the return.

[0153] A preferred signal processing approach uses a form of replica correlation processing. This technique is robust, computationally efficient and enables accurate time delay estimation. In particular, the cross-correlation algorithm using spectral pre-whitening retains accuracy better than the conventional amplitude threshold arrival time method. Note that while the replica correlation process is preferred, other techniques can also be used. For example, a square-law amplitude threshold detector can be used, assuming that the reflection peak is detected at the point where the waveform intensity or amplitude exceeds a certain set threshold. One-dimensional edge detectors or wavelet decomposition techniques can also be used as they allow the necessary time accuracy to be maintained while smoothing out noise. However, the cross-correlation algorithm is more suitable for high-frequency real-time implementation.

[0154] Referring to FIG. 28, the function of the replica correlator is schematically shown, and the absolute time delay between successive back wall reflections can be estimated with an accuracy approximating the basic measurement resolution provided by the digital acquisition system (e.g., the time equal to the reciprocal of the ADC sample rate). The replica correlator is a form of matched filter, the output of which is calculated from the cross-correlation between the measured A-scan and the delayed replica of the back wall reflection.

[0155] In particular, FIG. 28 shows how the replica correlation process involves correlating the time-windowed A-scan response y(n) with one or a bank of the stored or extracted backwall reflection waveforms x(n). This correlation process is implemented by using first and second DFT algorithms 750 and 752, respectively, to transform the input waveforms x(n) and y(n) into the frequency domain. The first and second DFT algorithms 750 and 752 can comprise any suitable form of the well-known FFT algorithm. Next, multiplier 754 performs a sequential multiplication operation in the frequency domain on the transformed signals. The output of multiplier 754 is returned to the time domain by a third DFT algorithm 758, and peak detector 760 outputs the data to delay estimator 762. When using a preferred phase-transformed version of the Generalized Cross-Correlation (GCC-PHAT) algorithm, only the signal phase information is preserved after the cross-spectrum is divided by its magnitude. Ideally, this processor approaches a delta function centered on an accurate time delay estimate with no additional noise.

[0156] Such a fast convolution process can suffer inaccuracies when returning the result to the time domain due to spectral smearing and leakage. Therefore, a pre-whitening filter 756 is implemented to improve the temporal accuracy and SNR robustness of the time delay estimation process. Pre-whitening of the phase transformation has the effect of equalizing the cross-spectrum (Pxy) phase to maximize the SNR and temporal accuracy of the dominant delay with respect to multipath reverberation. It can be the most effective form of pre-whitening for the A-scan measured by the probe, but any such correlation method (e.g., Knapp and Carter) can be used. For example, the phase transformation method may be less effective in low SNR environments.

[0157] Referring now to FIG. 29, the principle of the phase-transformed replica correlator algorithm is shown. In particular, this figure shows the measured backwall response from the measurement window presented to the signal processing stage.

[0158] The repeated backwall reflections within the measurement window (i.e., peak 780) show a strong correlation with respect to the repeated shape, especially in terms of its phase. The signal level attenuates from the first reflected echo 780 to the third reflected echo 782, but the SNR is still relatively high. As shown, the measurement window response can be correlated with the stored replica of the backwall reflection using the phase-transformed version of the cross-correlator (GCC-PHAT). This produces a correlation response 784 with some noise suppression (i.e., the correlation process induces some SNR gain) and effective sharpening of the waveform representing each echo at the exact time sample with maximum phase correlation. From this, a simple maximum peak detector can measure the time samples of each echo, and the time differences (t1 and t2) between these peaks represent the time delays from which thickness estimation can be performed. The replica waveform can be measured during calibration, but it has also been shown that the same algorithm is effective by extracting the backwall waveform directly from the A-scan itself or by the autocorrelation version of the algorithm.

[0159] In general, the data processing used to evaluate all input A-scans measured to simply detect tip contact (e.g., by a hard threshold detector) and / or to quantify the deformation (e.g., Zd) in the soft coupling module to ensure optimal coupling requires less computational effort than the subsequent signal processing required to measure the thickness of the coupled parts. Thus, a practical feature of the probe is that it can function in one or more automated operating modes.

[0160] Figure 30 shows a flowchart depicting the possible operating modes, control, and data flow between the probe and the peripheral hardware. As shown, while ultrasonic A-scans are being generated simultaneously at high speed, a probe movement command can be induced from the probe control software via the CMM / head controller. Naturally, the basic process would be to extract the ultrasonic reflection peak features for all the measured A-scans and evaluate whether the tip is in contact with any object via a hard threshold detector. If no contact is detected, the movement induced by the probe controller via the CMM / head controller can continue with more A-scans being acquired, but there is no other processing being executed on the probe.

[0161] If contact is detected, the CMM / head controller may be immediately alerted to report the probe position at the time of contact and / or induce some interrupt movement operation. This contact detection also enables the probe to start processing the ultrasonic reflection echoes in order to quantify the coupling deformation of the ultrasonic probe with respect to the inspection surface (i.e., relate to the "coupling sweet spot"). Even if intensive computational thickness measurement signal processing is not yet activated, more A-scans can be generated continuously at a higher speed until the coupling contact (i.e., coupling Z-deformation) is considered to be within the "coupling sweet spot", i.e., within the coupling deformation tolerance. This acceptable coupling state is mainly related to the evaluation of soft coupling elements (e.g., hydrophilic balls) and is mainly measured by the method described, but may include a basic evaluation of the back wall reflection in the measurement window (e.g., simple metrics such as kurtosis or peak amplitude).

[0162] Following an automated determination that probe coupling is sufficient or optimized, a more computationally intensive time processing signal processing algorithm is operative within the probe to extract successive back wall reflections within the primary measurement window of the A-scan and measure their time delays. For many scenarios, such as a continuous scan across a component, in practice the probe may need to continuously execute a thickness measurement processing algorithm for each measured A-scan. In this way, a computationally efficient and accurate time delay estimation method provides important operational benefits.

[0163] Accordingly, FIG. 30 depicts one architecture of a probe in which most of the processing is accomplished by a processing unit (e.g., FPGA, DSP, CISP) within the ultrasonic probe itself. This distributed processing architecture has the advantage of being deployed on a CMM where the communication channel bandwidth through the measurement head and scope of the device can be limited due to local memory storage on the probe. However, it should be noted that there are other possible architectures and methods for processing A-scan data. For example, in some cases, all of the basic (rudimentary) processing for performing contact detection and optimal coupling "on the fly" is affected, but it may be possible to locally record an extended batch of A-scans in memory for post-processing of thickness measurements with a batch transfer and other processors (e.g., laptop, PC, etc.). Regardless of the processing architecture, the basic signal processing presented here for estimating time delay via mode-3 measurements remains robust and accurate.

[0164] To measure accurate time delay information that can directly estimate the thickness of the contact component, it should be noted that the measurement resolution of the probe or device when applying the spectral correlation method (e.g., GCC) is typically limited by the digitization sampling frequency within the receiver electronics (i.e., ADC). In situations where the reflected waveform of interest is a smooth and predictable high SNR signal without significant probabilistic transient components (e.g., band-limited), it has been found that it is possible to artificially increase the effective system resolution by regular interpolative up-sampling of the raw measured A-scan response and the replica before applying the correlator. Furthermore, depending on the SNR and noise statistics of the measured A-scan, different pre-whitening filters (i.e., weighting functions in the frequency domain) can be applied within the signal processing to perform more accurate thickness measurements. Such pre-whitening filters include the smoothed coherence transform, the Roth filter, or the Hannah and Thompson filter.

[0165] It should also be noted that the phase conversion version of the GCC algorithm described above is not always optimal for lower SNR situations when used alone. For example, an A-scan measured with a device that includes a back wall reflection and / or an internal delay line reflection signal of interest may have a higher level of electronic noise from the measurement system (e.g., when averaging over the entire A-scan is not applied, i.e., when lower-cost equipment is used), or a higher acoustic background noise within the measurement band of the detection system (e.g., when measurements are made using a high-noise automated platform such as a high acoustic noise asset or the crawler platform described below). Therefore, those skilled in the art will understand how different the most appropriate signal analysis techniques are for various applications.

[0166] The embodiments of the ultrasonic system and related concepts of automated inspection described herein are useful for internal metrology measurements in metals and non-metals that are more highly attenuating and much thicker than those described above. In particular, the ultrasonic device may be used to measure certain high-value, high-security metal parts (e.g., medical implants) manufactured using an additive manufacturing (AM) method (e.g., using a selective laser melting device). Such parts may require both internal metrology measurements and non-destructive measurements to ensure that the porosity exhibited throughout the part is within the required tolerances. This type of porosity measurement is becoming more important as such AM techniques can specifically design various porosities throughout the part. It is emphasized that such porosity distribution estimation across an AM solid part of known or measured geometric shape can be suitably achieved by direct estimation of the sound speed distribution across the part. This is because there is a strong linear and easily calibrated relationship between the longitudinal sound speed and porosity. There is also a linear relationship between the shear wave speed and porosity. This may also be used in some cases.

[0167] The modular ultrasonic device described herein can also focus ultrasonic waves projected at a known depth or angle within a part (e.g., using a coupling module having a spherically converging plano-concave lens). Thus, this device can be used for the automatic detection, location, and sizing of internal defects and voids within the inspected part.

[0168] The above-described modular ultrasonic probe may be advantageously used for measuring the dimensional shape of soft material parts. For example, soft solid gelatinous, organic or non-metallic parts are such that surface interactions with such softer parts cannot mechanically deflect the probe stylus in a consistent manner (e.g., due to mechanical hysteresis effects) and / or such contact may be considered to induce undesirable dents in the soft parts during inspection, and thus are not inherently suitable for conventional contact probes. Such parts include soft elastic or plastic polymer films, fabrics and leathers, food products, and even organic films and human tissues.

[0169] Such soft measurement challenges involve soft coupling elements within the probe that are specifically selected to be softer than the object or material being inspected. This is to allow the coupling element to deflect by a measurable amount before inducing deflection within the part being inspected. Thus, a very soft hydrophilic vinyl polymer having a very high moisture content (e.g., 95%) can be provided as a coupling element of the ultrasonic probe. Such a coupling module can be provided as one of a set of coupling modules.

[0170] The above-described modular ultrasonic probe can be advantageously used for measuring the dimensional shape of high-quality surface-finished parts. In particular, it should be further noted that some other precision machined rigid parts having complex shapes but extremely high-quality smooth polished surface finishes may require automatic dimensional measurement. However, the contact interaction between a hard ruby ball stylus probe and such a polished inspection surface may not be ideal as such interactions may potentially induce some surface scratches or impact damage. Furthermore, alternative non-contact optical measurement probes (e.g., laser scan probes) may not be suitable due to non-conductive optical properties of the inspection surface (e.g., optically transparent acoustic concave or convex lenses or parabolic mirrors).

[0171] The modularity of the ultrasonic inspection device described above, specifically, the inherent ability to automatically change and adjust different coupling module designs to conform to specific inspection conditions, is particularly beneficial during the measurement of complex geometric shaped parts. Further, the selection of a particular coupling module often dictates the specific measurement method used to generate the output measurement values (e.g., the thickness values at both ends of each measurement node).

[0172] For example, during the inspection of a typical hollow aerospace wing, different coupling module designs and associated measurement method selections can be successively employed for a more comprehensive range of inspection. Specifically, most of the bulk outer surface of the part may be parallel to the inner back wall surface. For the measurement of such bulk "skin thickness", a coupling module equipped with a hydrophilic sphere is attached to the ultrasonic probe, and the probe can be continuously scanned laterally across the blade while maintaining a substantially perpendicular orientation to the inspection surface. Such a vertical incidence continuous scan where the probe does not leave the inspection surface utilizes the self-lubricating properties of this type of soft isotropic and elastic hydrophilic coupling module as described above. Thus, a very high density of measurement points across the inspection surface is facilitated by the mode-3 measurement method using the replica correlation method described above for a robust time delay estimation at each node.

[0173] However, it should be noted that the method of measuring the thickness of the bulk skin is not necessarily suitable for inspections across the entire blade. For example, near the leading and trailing edges of the blade wing, the outer front wall and the inner rear wall often deviate from such parallel geometries. In this case, a refractive coupling module can be used to project ultrasonic L-waves in the required direction towards the inner rear wall. This involves using a coupling module with a fixed rigid delay line having an appropriate refractive wedge angle. Alternatively, with appropriate calibration, a coupling module having a hydrophilic sphere but with the probe oriented at an appropriate angle from the surface normal can be used to achieve the refractive inspection challenges. In either case, mode-3 inspection is problematic because non-parallel front and rear wall surfaces do not return a continuous rear wall reflection to the probe. Mode-2 measurements where the time delay between the internal reflection peak from the first delay line or coupling module and the rear wall is estimated are also not appropriate because there is no strong internal reflection peak at such refractive angles. Therefore, in this case, mode-1 measurements (where the absolute time delay between the first excitation pulse and the subsequent rear wall reflection is estimated) can be advantageously performed instead.

[0174] To obtain the highest possible mode-1 thickness measurement accuracy, a range of refractive measurements may be made across a refractive calibration block machined from the same material as the part being inspected, and an additional calibration procedure may be required that incorporates one or more rear walls in the same front and rear wall orientation as the part being inspected. Similar to such mode-1 calibration procedures, using the same material as the part being inspected for the calibration block allows the sound speed calibration to be integrated into this refractive angle calibration. That is, another sound speed calibration procedure may not be necessary because the range of rear wall time delays for a known refractive thickness taken during calibration means that additional time delays measured with the probe during inspection can directly infer the unknown thickness by linear interpolation.

[0175] It is also possible to measure the speed of sound within the soft coupling element of the coupling module. Such speed of sound measurements are not necessarily essential (for example, not required for accurate thickness inspection of parallel eccentric components such as the fuselage skin and parallel hollow blades using the mode-3 method), but have several advantages. For example, the speed of sound in the bonding layer within the attached coupling module is slightly affected by changes in atmospheric temperature, and for certain probe functions such as off-normal inspection of inspection parts with non-parallel front and rear walls, it is beneficial to calibrate (i.e., measure) the speed of sound of the individual coupling modules during inspection. More specifically, the speed of sound of the coupling medium directly affects the projection angle of the ultrasonic waveform onto the inspection part according to Snell's Law of Refraction. Therefore, a more accurate and calibrated measure of this absolute speed of sound within a uniform isotropic coupling medium can be beneficial in constructing and projecting the accurate position and orientation of internal reflective surfaces.

[0176] There can also be alternative uses for measuring the sonic velocity of the bonding element. For example, it is to classify an unknown liquid sample that interacts with the bonding element. The sonic velocity (CL) of the bonding module is derived from the direct measurement of the longitudinal dimension (d) of the bonding medium and the estimation of the round-trip time of flight (t) (i.e., using the relationship CL = 2 * d / t) via mode-1, mode-2, or mode-3 methods. Alternatively, a method can be used where the probe is linearly loaded (loaded) in a highly controlled and accurate manner onto a known flat surface. More specifically, measuring the change in the arrival time (TOA) of the first internal reflection waveform from the bonding layer when the probe is loaded onto the flat surface has been found to enable the compilation of a linear relationship between this TOA and the deformation of the bonding layer (i.e., linear load displacement). From the plot of this linear relationship (i.e., for an incompressible bonding layer), the sonic velocity can be calculated directly as the gradient. In other words, the absolute gradient between TOA (t) and Z-deformation or load (r) is equal to half of the sonic velocity (CL) from the relationship r = (CL * t) / 2. This method has been found to be accurate and ensures that it does not require a potentially inaccurate estimation of the exact heuristic longitudinal dimension of the bonding layer to calculate the sonic velocity of the bonding layer.

[0177] Although it was described above how an ultrasonic device can be installed in a bridge-type CMM, it should also be noted that it can be used in combination with other devices.

[0178] Figure 31 shows how the ultrasonic probe 802 described above can be attached to an x-y scanner 800.

[0179] Figure 32 shows how the ultrasonic probe 802 described above can be mounted on an ultrasonic crawler system 810 for measuring internal cracks and corrosion within a thin aerospace structure (e.g., the outer skin of a fuselage). In such an embodiment, strong reflection echoes that occur in the A-scan between the back-wall reflections can be detected as additional unwanted interfaces within the component volume that can be classified as internal voids or cracks.

[0180] In such an embodiment, the crawler vehicle 810 can perform a continuous scan over a large curved structure to measure the internal thickness of the skin of a structure (e.g., an aerospace structure or a wind turbine blade). However, the curvature of the part and its interaction with the crawler's wheels can cause some variation in the exact clearance between the movable platform to which the ultrasonic probe is attached and the inspection surface. The high-resolution estimated value of the current deflection (Zd) at the soft coupling tip allows the Z position of the probe relative to the platform to be adjusted as the probe moves along the part to compensate for the deformed clearance, thus keeping the deformation of the probe within the range of a set tolerance (i.e., the "coupling sweet spot"). In its simplest form, the adaptation of the height of the probe relative to the platform in response to changes in the deformation of the soft coupling element can be implemented using a simple linear encoder attached to the platform and a linear stage motor that allows the Z height of the probe to vary in real time during inspection. In a more sophisticated embodiment, a second rotary motor and encoder may be incorporated and the probe may be allowed to rotate while remaining in its plane of motion so as to change the angle of incidence with respect to the surface in response to changes in the surface normal.

Claims

1. A method for inspecting an object using an ultrasonic inspection device (56, 330) conveyed by a cooperating coordinate positioning device (50), comprising: the ultrasonic inspection device (56, 330) includes an ultrasonic transducer (92) and a coupling element (104) that contacts and acoustically couples to the object to be inspected, and the coupling element (104) is made of a self-lubricating material; the method includes a scanning step in which the coupling element (104) of the ultrasonic inspection device (56, 330) is continuously scanned along a path on the surface of the object by the cooperating coordinate positioning device (50) while acquiring ultrasonic measurements, and during the scanning step, a deformation amount (Zd) of the coupling element (104) is estimated from internal reflections of the coupling element (104) obtained from the ultrasonic measurements, and the height of the ultrasonic inspection device (56, 330) is adjusted in response to the estimated deformation amount (Zd). A method characterized by the above.

2. The method according to claim 1, wherein the cooperating coordinate positioning device (50) is configured to move the ultrasonic inspection device (56, 330) toward or away from the surface of the object during the scanning step in response to any change in the estimated deformation amount (Zd).

3. The method according to claim 2, wherein the estimated deformation amount (Zd) is used within a control loop of the cooperating coordinate positioning device (50) to provide real-time adjustment of the height of the ultrasonic inspection device (56, 330) on the surface of the object.

4. The method according to any one of claims 1 to 3, further comprising using the ultrasonic inspection device (56, 330) to measure the thickness of the object.

5. The method according to any one of claims 1 to 4, further comprising using the ultrasonic inspection device (56, 330) in pulse-echo mode and measuring the speed of sound in the coupling element (104) by analyzing reflections from within the coupling element (104) when the coupling element (104) is subjected to a plurality of different deformations.

6. The method according to claim 5, further comprising a calibration step of measuring the speed of sound in the material of the object.

7. The method according to any one of claims 1 to 6, comprising the step of using the acoustic energy reflected within the coupling element (104) to obtain the physical state of the coupling element (104).

8. The method according to any one of claims 1 to 7, comprising the step of analyzing internal reflections from the coupling element (104) to determine when the coupling element (104) contacts an object.

9. The method according to any one of claims 1 to 8, wherein the cooperating coordinate positioning device (50) comprises a rotating head (54) for mounting the ultrasonic inspection device (56, 330), and the rotating head (54) includes at least two rotation axes.

10. The method according to any one of claims 1 to 9, wherein the self-lubricating material of the coupling element (104) comprises a hydrophilic elastomer.

11. The method according to any one of claims 1 to 10, wherein the self-lubricating material of the coupling element (104) comprises a superabsorbent polymer hydrogel.

12. The method according to any one of claims 1 to 11, wherein the coupling element (104) comprises spheres of a self-lubricating material.

13. The method according to any one of claims 1 to 12, wherein the ultrasonic inspection device (56, 330) has a modular configuration comprising a base module (58, 621) including the ultrasonic transducer (92) and a coupling module (60, 66, 180 - 191, 332) including the coupling element (104).

14. The method according to claim 13, including the step of using the cooperating coordinate positioning device (50) to connect the coupling module (60, 66, 180 - 191, 332) to the base module (58, 621) or to remove the coupling module (60, 66, 180 - 191, 332) from the base module (58, 621).

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