Density asymmetry measurement method and device

Abstract

The invention relates to a non-destructive method for measuring density distribution deviations in a solid object, comprising the following steps: a) providing a first excitation pulse, preferably a mechanical shock, to the solid object; b) obtaining a first vibration response of the solid object to the first excitation pulse and deriving a first eigenfrequency of the vibration mode in the first orientation from the first vibration response; c) providing a second excitation pulse, preferably a mechanical shock, to the solid object; d) obtaining a second vibration response of the solid object to a second excitation pulse, and deriving a second eigenfrequency of the vibration mode in a second orientation from the second vibration response, a first excitation pulse is provided to the solid object at a first position and a second excitation pulse is provided to the solid object at a second position different from the first position such that the first orientation is different from the second orientation, and wherein the method further comprises the steps of: e) comparing the first eigenfrequency with the second eigenfrequency, a measurement of density distribution deviation in the solid object is thus obtained.

Description

Technical Field

[0001] The present invention relates to a method and a device for measuring density asymmetry of a solid object. Hereto, an excitation pulse is supplied to the solid object and the vibration response is recorded and analyzed for determining deviations from an expected density distribution, in particular from a uniform density distribution. The present invention thus relates to the field of non-destructive testing by means of impact excitation techniques and vibration response analysis. Background Art

[0002] High-precision manufacturing of solid objects is an important industrial activity. For many applications, it is important that the solid object has the desired shape and size. However, in many applications it is also important that the object has a uniform density distribution or a density distribution as desired.

[0003] For example, a gear is preferably symmetrical when rotated over an angle corresponding to an integer multiple of 360° divided by the number of teeth of the gear. However, even if the shape of the gear is completely symmetrical, there is still the possibility of density fluctuations in the gear. In this case, the gear may not be properly balanced and may cause problems for, for example, the machinery in which it is used. Another example relates to a ring that is preferably rotationally symmetrical in both shape and density distribution. This applies to rings for industrial use or decorative use. Further examples relate to beams, rods, plungers, etc., each of which may need to have a specific shape and density distribution for their intended purpose.

[0004] Measuring the shape and size of solid objects non-destructively may be considered standard practice. However, non-destructively measuring the density distribution within a solid object can be very cumbersome and expensive.

[0005] The present invention aims to overcome this problem and aims to provide a method and an apparatus which allow measuring the density distribution of a solid object taking into account the deviation of said density distribution from an expected density distribution. In many cases, the expected density distribution is a uniform density distribution, in which case the present invention provides a method and an apparatus for measuring density inhomogeneities. Such density inhomogeneities may be due to defects, microcracks, local pores, etc.

[0006] One way to measure density changes, and in particular porosity changes, non-destructively is by using X-ray tomography, more specifically X-ray micro-computed tomography (CT). X-ray micro-CT has the advantage of reconstructing a complete 3D model of the scanned object, allowing non-destructive inspection of internal features or defects. However, X-ray micro-CT is an expensive and time-consuming measurement method. Furthermore, the inventors have found that X-ray micro-CT cannot resolve all types of quality-degrading structural characteristics. For example, X-ray micro-CT does not have a good sensitivity for detecting microcracks. The object of the present invention is to provide a method wherein a quality assessment of a solid object with respect to both porosity and microcracks can be obtained in a single measurement or at least with a single measurement setup.

[0007] The invention can be applied to all types of solid objects, but is most advantageous when applied to solid objects manufactured by additive manufacturing (AM), also known as 3D printing. This manufacturing process does result in the density of the solid object deviating from the expected density distribution, in particular the density deviating from a uniform density distribution, for example due to incomplete sintering or other reasons.

[0008] The invention can be applied to solid objects of all shapes, but preferably to solid objects which substantially comprise geometrical symmetry.

[0009] The present invention is directed to overcoming the above and other problems. To this end, the present invention provides a method and apparatus for measuring density variations of solid objects, and in particular for measuring density inhomogeneities, and more particularly for measuring density asymmetry. Summary of the invention

[0010] The invention relates to a method according to claim 1 and to an apparatus according to claim 11 .

[0011] The present invention is based on the following insights of the inventors:

[0012] - A geometrically symmetric solid object can have density asymmetry, i.e. the density distribution of the solid object deviates from the geometric symmetry of the solid object;

[0013] - this density asymmetry leads to a shift of the vibrational eigenfrequencies of the solid object when these eigenfrequencies are excited along different orientations, even if said orientations are related to each other via the geometrical symmetry of the solid object;

[0014] - For the same vibration mode signal density asymmetry, the eigenfrequencies measured along different orientations differ, and the greater the difference, the higher the asymmetry, i.e. the difference in eigenfrequencies is a measure of density asymmetry;

[0015] - For solid objects not comprising substantial geometrical symmetry, similar observations hold, but the differences in the measured eigenfrequencies must be compared with reference values, thus measuring the density inhomogeneities,

[0016] - The method can be performed on objects intended to comprise a uniform density distribution, but also on objects intended to comprise a density distribution according to an inhomogeneous reference density distribution, i.e. the method allows measuring a density distribution deviation, i.e. a deviation of the density distribution relative to a reference density distribution which may be uniform or inhomogeneous.

[0017] The present invention exploits these insights by performing an impact excitation (IE) measurement technique, allowing the vibration response to an excitation pulse provided to a solid object to be obtained. The vibration response can be analyzed, and sets of eigenfrequencies can be derived from the response. By repeating the IE measurement at least once, and preferably multiple times, such as 2, 3, 4, 5, 6, 7, 8, 9, 10 or more times, at different impact locations on the object, multiple sets of eigenfrequencies can be obtained, which can be compared. By choosing different impact locations so that the orientations of the eigenfrequencies excited are different from each other, the difference in the eigenfrequencies of each vibration mode at different orientations is a direct measure of the density distribution.

[0018] In a preferred embodiment, the object substantially comprises geometric symmetry, and the different impact positions can be selected substantially corresponding to the symmetry of the solid object, and the difference in the eigenfrequencies for each vibration mode when excited by the excitation pulses at the different positions is a direct measure of the density asymmetry. Furthermore, the present invention allows the detection of density asymmetry in the solid object without requiring further input on the properties of the solid object. For example, in order to measure the asymmetry, it is not necessary to provide a reference value for the eigenfrequencies. Moreover, in order to measure the density asymmetry, it is not necessary to compare the results of the object with the results from a reference object.

[0019] Note that for some vibration modes the shift in eigenfrequency may be more significant than for other vibration modes.Thus, in a preferred embodiment, the vibration mode is selected so as to optimize the sensitivity of the method and / or apparatus of the present invention.

[0020] The IE measurement technique is relatively cheap, easy and fast to perform, making it much easier than X-ray micro-CT while still providing highly reliable information about density asymmetry. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 An impact device according to a preferred embodiment of the present invention is shown.

[0022] Figure 2A -2D shows an impact method and an impact device according to a preferred embodiment of the present invention.

[0023] Figure 3 An embodiment of an arm according to a preferred embodiment of the invention is shown.

[0024] Figure 4 and Figure 5 An angular velocity curve according to a preferred embodiment of the present invention is shown.

[0025] Figure 6 The result of carrying out the method according to the invention using the system according to the invention is shown.

[0026] Figure 7 A device according to the invention is shown. DETAILED DESCRIPTION

[0027] The invention relates to a method according to claim 1 and to an apparatus according to claim 11 .

[0028] The method according to the invention relates to a non-destructive method for measuring density distribution deviations in solid objects.

[0029] The method comprises the following steps:

[0030] a) providing a first excitation pulse, preferably a mechanical shock, to the solid object;

[0031] b) obtaining a first vibration response of the solid object to a first excitation pulse, and deriving a first eigenfrequency of a vibration mode in a first orientation from the first vibration response;

[0032] c) providing a second excitation pulse, preferably a mechanical shock, to the solid object;

[0033] d) obtaining a second vibration response of the solid object to a second excitation pulse, and deriving a second eigenfrequency of the vibration mode at a second orientation from the second vibration response,

[0034] wherein a first excitation pulse is provided to the solid object at a first position and a second excitation pulse is provided to the solid object at a second position different from the first position, such that the first orientation is different from the second orientation, wherein the method further comprises the following steps:

[0035] e) comparing the first eigenfrequency to the second frequency, thereby obtaining a measure of the deviation of the density distribution in the solid object.

[0036] In a preferred embodiment, the solid object substantially comprises geometric symmetry, and the density distribution deviation is substantially density asymmetry. Here, the basic geometric symmetry refers to the transformation of the object that substantially reproduces the shape and size of the object. The most common geometric symmetries are rotations in two and three dimensions, mirror symmetry with respect to an axis or a point, and combinations thereof. The geometric symmetry may be continuous symmetry, discrete symmetry, or a combination thereof. For example, a ring may comprise a continuous two-dimensional rotational symmetry around a central axis and discrete mirror symmetries with respect to a plane perpendicular to the central axis and with respect to each plane including the central axis; a gear may comprise a discrete two-dimensional rotational symmetry and a set of discrete mirror symmetries, the discreteness of the rotational symmetry being caused by the teeth of the gear; a cube-shaped and beam-shaped object may also comprise a set of discrete rotational symmetries and mirror symmetries. In one embodiment of the present invention, the solid object may substantially comprise geometric symmetry. Thus, the solid object may have a shape that deviates slightly from perfect geometric symmetry. This deviation from perfect geometric symmetry may be accidental and / or intended. For example, a ring may have a small local thickening around its periphery that destroys the perfect geometric symmetry, or the ring may have a carving along its periphery. These small deviations may result in an eigenfrequency difference shift which can be taken into account when comparing the measured eigenfrequencies in different orientations, i.e. in step e or e' of the method according to the invention. Such an eigenfrequency difference shift can be determined relatively easily by calibration measurements or by computer simulations.

[0037] As previously discussed, the present invention relates to obtaining the eigenfrequencies of solid objects. These eigenfrequencies are the frequencies at which an object can resonate naturally. Thus, if an object is vibrated, for example, by an impact, the object continues to vibrate longer at its eigenfrequencies than at other frequencies. In addition, a solid object may have many eigenfrequencies, each of which may be attributed to a vibration mode. In a preferred embodiment of the present invention, the vibration mode may be a longitudinal tensile mode, a bending mode, or a torsion mode. In an embodiment in which the object substantially comprises geometric symmetry, the selection of the vibration mode may preferably depend on the shape of the object, and preferably on the geometric symmetry of the object. The vibration mode may preferably be a fundamental mode or a harmonic mode, such as a first (first) harmonic mode, a second (secondary) harmonic mode, a third (third) harmonic mode, or a harmonic mode having a fourth (fourth) or higher order. In a particularly preferred embodiment, the vibration mode is or is selected to be a first harmonic mode or a second harmonic mode. The inventors have found that, due to density distribution deviations and / or density asymmetry, harmonic modes appear to be more sensitive to eigenfrequency differences than fundamental modes. Therefore, the first harmonic mode, the second harmonic mode or the nth harmonic mode are preferred, wherein n is 3, 4, 5, 6, 7, 8, 9, 10 or even higher.

[0038] The eigenfrequency of the vibration mode is measured and derived for the first time by performing steps a and b, and for the second time by performing steps c and d. The difference between the measurements is the impact point, i.e., the first position and the second position, which ensures the excitation of the vibration mode in different orientations.

[0039] If the density is uniformly distributed and if different positions correspond to positions that are essentially transformed into each other by a geometric symmetry transformation, the geometric symmetry of the object allows excitation pulses provided at different positions to vibrate the object in substantially the same manner.

[0040] In a preferred embodiment, the method comprises repeating the following steps:

[0041] c') providing a ith excitation pulse, preferably a mechanical shock, to the solid object; and

[0042] d') obtaining a ith vibration response of the solid object to the ith excitation pulse and deriving a ith eigenfrequency of the vibration mode at the ith orientation from the ith vibration response,

[0043] wherein the ith excitation pulse is provided to the solid object at a ith position different from the first position, the second position and the jth position, such that the ith orientation is different from the first orientation, the second orientation and the jth orientation, where j < i,

[0044] and i is at least 3, such as 3, 4, 5, 6, 7, 8, 9, 10 or higher.

[0045] Preferably, the method further comprises the following steps:

[0046] f) comparing the first eigenfrequency with a second frequency so as to obtain a measurement of the deviation of the density distribution in the solid object.

[0047] According to the above embodiment, preferably, the method comprises repeating the following steps:

[0048] e') comparing the ith frequency with the first eigenfrequency, the second frequency and / or the jth frequency, where j < i, so as to obtain a measurement of the density asymmetry in the solid object.

[0049] In a preferred embodiment, the solid object substantially includes geometric symmetry, and the density distribution deviation is substantially density asymmetry, wherein the first position and the second position are substantially related to the geometric symmetry of the solid object, and wherein step e) includes: obtaining the difference between a first eigenfrequency and a second eigenfrequency to obtain said measurement of the density asymmetry. Thus, preferably, steps c', d' and e' are performed n times, where n is selected based on the basic geometric symmetry of the solid object. Also preferably, thus, the i-th position is related to the first position, the second position and the j-th position by the geometric symmetry of the solid object, where j < i.

[0050] From the above discussion of the preferred embodiment focused on a solid object substantially including geometric symmetry, it can be seen that the present invention only relates to solid objects having geometric symmetry. However, the present invention can also be applied to objects without geometric symmetry. In this case, the method can be applied to many objects of the same type, typically objects manufactured to have the same intended shape and size and using the same manufacturing method. Then, comparison of the results from multiple objects allows identification of which of the objects includes a density distribution deviating from the expected density distribution.

[0051] Thus, in an embodiment of the present invention, step e) includes: obtaining the difference between a first eigenfrequency and a second eigenfrequency and comparing said difference with a reference eigenfrequency difference value to obtain said measurement of the density distribution deviation, and / or step e′) includes: obtaining a grouped difference between an i-th eigenfrequency and a j-th eigenfrequency, where j < i, and comparing said difference with a reference eigenfrequency difference value to obtain said measurement of the density distribution deviation.

[0052] The shock excitation technique for characterizing solid objects can be known from the following documents:

[0053] Document EP3658868A1 discloses an apparatus for analyzing the mechanical vibration response of a solid material sample, the apparatus including: an array of impactors arranged to apply impacts at corresponding clearly defined points on the surface of the solid material sample; sensors configured to capture (acquire) the mechanical vibration response as a time-varying signal after the impact of at least one of the impactors; and a processor configured to analyze the time-varying signal to determine the frequency and decay constant of the sine waves constituting the time-varying signal. The present invention also relates to a corresponding method for characterizing a solid material sample.

[0054] Document WO2020254698A1 discloses a method for acoustically measuring the material properties of a test piece at high temperatures, comprising the following steps: a. heating the test piece to a test temperature range; b. performing background measurements within the test temperature range by capturing vibration signals from the test piece within a calibration period, thereby obtaining a noise signal; c. performing acoustic measurements on the test piece within the test temperature range and during the test, by: c1. applying vibration excitation to the test piece; c2. capturing (collecting) the vibration signal of the test piece within a test period, thereby obtaining a vibration response signal to the vibration excitation, and d. obtaining the material properties of the test piece by analyzing the vibration response signal, thereby taking into account the noise signal. In one embodiment, the present invention also relates to a system for acoustically measuring the material properties of a test piece at high temperatures.

[0055] Two prior art documents in the name of the present applicant relate to methods and devices for performing impulse excitation (IE) measurements.

[0056] IE technology basically involves providing an excitation pulse to a solid object and obtaining and analyzing a vibration response signal. This signal is also called an acoustic response signal. The response signal can be captured in a variety of ways, including by using a microphone, a piezoelectric displacement sensor, and / or a laser interferometer.

[0057] The solid object may be substantially any type of solid object, including but not limited to:

[0058] - a metallic solid object, comprising or made of a metal or an alloy, wherein preferably the metal may comprise iron, aluminum, zinc, copper, steel, etc.;

[0059] - plastics, such as polymer materials;

[0060] - 3D printing materials, preferably:

[0061] Metals, such as titanium and stainless steel,

[0062] ·Ceramic, such as alumina, or

[0063] Polymers, such as polyetheretherketone (PEEK) or polyetherimide (such as ULTEMTM),

[0064] - Composite materials, such as composites of plastic and metal.

[0065] Most preferably, the solid object is a 3D printed solid object and / or the method of the invention comprises the step of 3D printing a solid object.

[0066] Equipment for measuring eigenfrequency is commercially available, such as Mk7, SA, HT and IL equipment.

[0067] The invention also relates to an Impact Excitation (IE) measurement device for measuring density distribution deviations in a solid object, the device comprising:

[0068] - Object holder for holding solid objects in place during IE measurements;

[0069] - an impact device for providing an excitation pulse to a solid object, preferably a mechanical impact device for providing a mechanical impact to the solid object;

[0070] - a sensor for obtaining a vibration response of a solid object to an impact;

[0071] - a processing device, which is configured to:

[0072] The eigenfrequencies of the selected vibration mode are derived from the vibration response obtained by the sensor, where:

[0073] i) an impact device configured to provide a first excitation pulse, preferably a mechanical impact, to the solid object;

[0074] ii) the sensor is configured to obtain a first vibration response of the solid object to a first excitation pulse, and the processing device is configured to derive a first eigenfrequency of a vibration mode in a first orientation from the vibration response;

[0075] iii) the impact device is configured to provide a second excitation pulse, preferably a mechanical impact, to the solid object;

[0076] iv) the sensor is configured to obtain a second vibration response of the solid object to a second excitation pulse, and the processing device is configured to derive a second eigenfrequency of the vibration mode in a second orientation from the second vibration response,

[0077] wherein the object holder is configured to rotate and / or displace the object relative to the impact device between steps b and c, and / or the impact device is configured to rotate and / or displace relative to the object holder between steps b and c, such that the first excitation pulse is provided to the solid object at a first position and the second excitation pulse is provided to the solid object at a second position different from the first position, such that the second orientation is different from the first orientation, and

[0078] Wherein the processing device is further configured to compare the first eigenfrequency with the second frequency to obtain a measure of density asymmetry in the solid object.

[0079] In embodiments where the solid object substantially comprises a geometric symmetry, the first position and the second position are preferably related to the geometric symmetry of the solid object. Note that it should be clear that if the object is transformed according to its geometric symmetry, if the first position is substantially transformed into the second position, then the first position and the second position are related by the geometric symmetry of the solid object.

[0080] exist Figure 6 The results of carrying out the method according to the invention using the system according to the invention are shown in FIG. Thus, four rings were tested using the method according to the invention. The results are shown in FIG. Figure 6 The vibration responses of four mechanical shocks at four different positions along the ring are shown in four columns (20A, 20B, 20C, 20D). In each column, the vibration responses of four mechanical shocks at four different positions along the ring are shown. The different positions correspond to positions rotated on the ring in 45° steps. The frequencies are shown on the vertical axis (f) at the frequency intervals of the second harmonic of the bending mode of the ring.

[0081] The results for the second sample (20B) and the fourth sample (20D) show continuous bands at frequencies f2 and f4, respectively, indicating that the spectrum is substantially independent of the location of the impact, and therefore indicating that the second and fourth sample loops have a uniform density distribution. This is different for the first sample loop (20A), and especially for the third sample loop (20C). The first sample loop shows that the four impacts result in two distinct vibration responses, with eigenfrequencies at f1A and f1B, respectively. The first impact (21A) and the third impact (21C) result in a peak in the vibration response at eigenfrequencies f1B, and the second impact (21B) and the fourth impact (21D) result in a peak in the vibration response at eigenfrequencies f1A. Note that the results for the first impact are shown on both the left and right sides, illustrating the rotational geometric symmetry of the loop. The difference between the two eigenfrequencies f1A and f1B is clearly perceptible, but small, indicating that the density asymmetry of sample 20A is small, but significant. For the third sample (20C), a much larger difference can be noted between the two eigenfrequency peaks f3A and f3B. Here again, the first impact (22A) and the third impact (22C) result in a vibration spectrum with a peak eigenfrequency f3B, while the second impact (22B) and the fourth impact (22D) result in a vibration spectrum with a peak eigenfrequency f3A. The larger difference between f3A and f3B indicates a larger density asymmetry.

[0082] It is also noted that for the first and third samples, the eigenfrequencies are lower than those of the second and fourth samples, which is consistent with previous results showing that solid objects with high porosity tend to have a higher degree of defects, and that solid objects with high porosity have lower eigenfrequencies than well-manufactured, low porosity samples, such as the second and fourth samples.

[0083] The results obtained by the present invention correspond to those obtained with X-ray micro-CT, but are obtained with a simpler procedure, which is faster and cheaper than X-ray micro-CT. In fact, for each sample, the present method and system allow obtaining results in seconds, whereas X-ray micro-CT requires minutes or even more.

[0084] Figure 7 An IE measurement device (30) according to the invention is shown, which is applied to measure density asymmetry in a ring (31), the ring (31) substantially comprising a geometric symmetry, in this case a rotational symmetry. The device (30) comprises an object holder (34) for holding a solid object in place during the IE measurement. The object holder preferably comprises a soft padding material to allow the solid object, i.e. the ring, to vibrate substantially freely. The device also comprises an impact device (1) for providing an excitation pulse to the solid object. In the figure shown, an impact device according to a preferred embodiment of the invention is shown, which impact device substantially corresponds to Figure 1 , Figure 2A - Figure 2D and Figure 3 The impact devices shown in the accompanying drawings will be discussed later in this document and the same figure numbers are used for these impact devices. The device shown also includes a table (32) in which a sensor (33) is positioned to obtain a vibration response of the solid object to an excitation pulse. The processing device of the device is not shown, but can be functionally connected to the sensor by wire or wirelessly to receive the vibration response of the solid object to the excitation pulse. The processing device is then configured to derive the eigenfrequency of the selected vibration mode from the vibration response obtained by the sensor. In addition, the impact device (1) is configured to provide (3) a first excitation pulse to the solid object (31), in this case a mechanical impact using the end (6) on the bendable arm (2). In addition, the sensor is configured to obtain a first vibration response of the solid object to the first excitation pulse, and the processing device is configured to derive a first eigenfrequency of the vibration mode in a first orientation from the vibration response.

[0085] In the figures shown, the object holder (34) is configured to rotate the object (31) relative to the impact device (1). In the embodiment shown, the object holder (34) is connected to a rotation axis (35) passing through the table (32), and the rotation axis (35) can be rotated by an actuator (36), thus allowing the object holder and the object to be rotated as desired.

[0086] The impact device (1) is further configured to provide a second excitation pulse to the solid object, and

[0087] The sensor is configured to obtain a second vibration response of the solid object to a second excitation pulse, and the processing device is configured to derive a second eigenfrequency of the vibration mode from the second vibration response, similar to Figure 6 As shown in .

[0088] Preferably, the excitation pulse provided to the solid object is as pure as possible to reduce the noise signal. In principle, it is desirable to apply the pulse excitation to the solid object using an impact device that does not generate any noise of its own. The impact device generally comprises: an impactor that mechanically applies the excitation pulse to the solid object; and an impactor actuator that provides kinetic energy to the impactor for applying the excitation pulse. In the prior art, a ballistic impactor is used: such a ballistic impactor is provided with momentum by the impactor actuator, and the impactor follows a ballistic trajectory for at least a short period of time before impacting the solid object. After the impact, the ballistic impactor can follow the ballistic trajectory again before being recaptured. The advantage of using a ballistic impactor is that it allows such an impact that is essentially a pulse impact, i.e. a very short contact period during which the kinetic energy of the impactor is mechanically transferred to the solid object. In practice, noise can be generated due to the following reasons:

[0089] -Multiple impacts The impactor contacts the solid object two or more times, for example because the inertia of the impactor allows the surface of the solid object to vibrate faster than the impactor is retracted from the solid object. For a ballistic impactor this also means that the impact is preferably provided from below the solid object so that the impactor does not bounce off the solid object two or more times.

[0090] - The impactor may generate noise when actuated, usually due to friction between the impactor and the impactor actuator. For ballistic impactors, the impactor actuator may include a barrel or guide to ensure that the ballistic impactor is provided with velocity in the correct direction. Friction with the barrel or guide may generate noise.

[0091] -The impactor may produce noise after impact, for example by touching other parts of the device or when the ballistic impactor is recaptured.

[0092] In the prior art, ballistic impactors are disclosed as a preferred impactor, allowing for pulse impacts. However, the inventors have found that ballistic impactors may not always be the best solution. In particular, ballistic impactors and impactor actuators may not always be easily adapted to be used with different types of solid objects. Furthermore, ballistic impactors are best used from below to reduce the chance of multiple impacts, which may therefore prohibit impacting the solid object from different sides and / or directions. Note that this may be necessary if one wants to excite or enhance different vibration nodes in the solid object, or as in the present invention to excite or enhance vibration nodes at different locations in order to measure density asymmetry.

[0093] Therefore, the present invention also aims to provide an impact device, which:

[0094] - can be easily adapted to provide pulse impact to solid objects of any type of material, geometry, size, etc.,

[0095] - can be easily positioned or repositioned to apply impacts to solid objects at different locations and / or in different directions,

[0096] - Reduce noise, specifically by substantially eliminating the opportunity for multiple impacts and by reducing noise generated by friction or recapture of the impactor.

[0097] To this end, the present invention relates to an improved impact device for performing impact excitation (IE) measurements on solid objects, and a method for operating the impact device. The impact device is preferably used in a pulse excitation measurement apparatus, and the invention therefore also relates to a pulse excitation (IE) measurement apparatus comprising the impact device.

[0098] In a preferred embodiment, the impact device for applying a pulse impact to a solid object during an impact excitation measurement comprises:

[0099] - an elongated elastically bendable arm (2) which is elastically deformable about a bending direction (3), the bendable arm comprising a longitudinal proximal end (4) and a longitudinal distal end (5);

[0100] - a hammer tip (6) for providing a mechanical impact to a solid object (7), the hammer tip being attached to the distal end (5) of the arm, and

[0101] - a programmable actuator (8) attached to the longitudinal proximal end (4) of the arm (2),

[0102] wherein the actuator (8) is programmed to provide an angular velocity to the longitudinal proximal end (4) of the arm by rotating the longitudinal proximal end (4) about an actuator axis (9), the actuator axis (9) being substantially parallel to the bending direction (3) of the arm. Preferably, the angular velocity follows a predefined angular velocity profile, wherein the angular velocity profile is configured to provide a pulsed impact to a solid object.

[0103] The system of the invention may preferably comprise an impact device kit for mounting an impact device as specified herein above, the kit comprising:

[0104] - a set of elongated elastically bendable arms, each arm being elastically deformable about a bending direction, the bendable arms comprising a longitudinal proximal end and a longitudinal distal end;

[0105] - a set of hammer tips for mechanically providing an impact to a solid object, each hammer tip being attachable to, and preferably attached to, the distal end of one of the set of elongated resiliently bendable arms, and

[0106] - a programmable actuator which can be replaceably attached to the longitudinal proximal end of each arm,

[0107] Wherein, the actuator is preferably programmed to provide an angular velocity to the longitudinal proximal end of the operating arm when the actuator is attached to the longitudinal proximal end of the operating arm, by rotating the longitudinal proximal end around an actuator axis substantially parallel to the bending direction of the operating arm, and the angular velocity preferably follows a predefined angular velocity curve, wherein the angular velocity curve is configured to provide a pulse impact to a solid object.

[0108] In a preferred embodiment, the first excitation pulse and / or the second excitation pulse is provided using an impact method for applying a pulse impact to a solid object during an impact excitation measurement using an impact device, the impact device comprising:

[0109] - an elongated elastically bendable arm, said arm being elastically deformable about a bending direction, said bendable arm comprising a longitudinal proximal end and a longitudinal distal end;

[0110] a hammer tip for mechanically providing an excitation pulse to a solid object, the hammer tip being attached to the distal end of the arm, and

[0111] Wherein, the preferred impact method comprises the following steps: providing an angular velocity to the longitudinal proximal end of the arm by rotating the longitudinal proximal end around an actuator axis, wherein the actuator axis is substantially parallel to the bending direction of the arm. Preferably, the angular velocity follows a predefined angular velocity curve, wherein the angular velocity curve is configured to provide a pulse impact to the solid object.

[0112] The term "impulse impact" refers here to a single elastic rebound provided during a short contact period between the end of the hammer and the solid body of the object, in which energy is transferred.

[0113] Due to the angular velocity profile, the hammer tip follows a hammer tip trajectory which is configured such that the hammer tip provides an impulse impact to the object solid. Preferably, the hammer tip trajectory starts from an offset relative to the position of the object solid. Thus, the hammer tip trajectory depends on, and preferably only on, the offset of said position, the angular velocity profile and the bending characteristics of the bendable arm.

[0114] Note that in Figure 1 and Figures 2A-2DIn FIG. 1 , the bending direction and the actuator axis are shown perpendicular to the page. Note also that the bending direction refers to the direction about which the arm can bend.

[0115] Preferably, the flexible arm comprises a ribbon-like shape having a length L in the longitudinal direction, a width W and a thickness T, wherein L>W>T. The tip (6) is attached to the distal end (5) of the arm (2). Preferably, the arm and the hammer tip are integral. Preferably, the arm and / or the hammer tip comprise or are made of plastic, metal, alloy or a combination thereof. Most preferably, the arm and the hammer tip are integral and are made of plastic. The shape of the arm and the material properties of the arm are preferably selected to ensure the bendability of the arm around a bending direction perpendicular to the longitudinal direction, more preferably the width direction, and most preferably, the shape of the arm and the material properties of the arm are selected to ensure the bendability of the arm only around the width direction, i.e. so that the arm only bends around the width direction during the execution of the angular velocity profile.

[0116] In a preferred embodiment, the actuator comprises an electric motor, more preferably a rotary electric motor. The electric motor preferably comprises a rotor attached to the proximal end of the bendable arm, so that the angular velocity profile of the proximal end of the bendable arm can be determined by the rotor of the electric motor.

[0117] In a preferred embodiment, the angular velocity curve includes a turning point at which the angular velocity provided to the longitudinal distal end of the arm changes its sign, said turning point being earlier than the impact on the object solid. This is particularly preferred to ensure that the object solid is provided with an impulse impact by the whip-like motion of the hammer end. Figure 1 and Figure 2A -2D. Figure 1 In FIG. 1 , the method is shown at the beginning, wherein the flexible arm ( 2 ) is substantially at rest and is straight or only slightly bent (e.g. due to the weight of the flexible arm and the end of the hammer itself). Then, the actuator starts to execute the angular velocity profile. Figure 2A In the example of FIG. 1 , the angular velocity (10) causes the proximal end of the arm to rotate towards the solid object (7). Due to the inertia of the arm and the end of the hammer, the flexible arm bends about the bending direction (3), so that the flexible arm assumes a curved shape. The angular velocity curve then includes a turning point, where the angular velocity (11) applied to the proximal end (4) of the flexible arm is substantially zero. At the turning point (see FIG. 1 ), the angular velocity (11) applied to the proximal end (4) of the flexible arm is substantially zero. Figure 2B ), the actuator stops the rotation of the longitudinal proximal end (4) of the arm (2). However, due to the curvature of the arm and its elasticity, the end of the hammer keeps moving towards the solid object. At the moment of impact with the solid object (see Figure 2C ), the actuator has applied an angular velocity (12) away from the solid object to the proximal end (4) of the arm, ensuring that after impact, the end of the hammer does not strike the solid object multiple times. Figure 2D, where the actuator has stopped (13) the rotation of the proximal end of the arm. Thus, the hammer tip is moving away from the solid object and allowing the Figure 1 The situation in the middle returns to the static state, ready to start the subsequent measurement again.

[0118] Preferably, the programmable actuator comprises a programmable computing unit, more preferably an Arduino computing unit or a raspberry pi computing unit. The programmable computing unit can be configured to control the electric motor, preferably to rotate the electric motor, and can be programmed to steer the electric motor to substantially follow an angular velocity curve.

[0119] Figure 4 An angular velocity curve (16) according to the invention is shown. The angular velocity curve preferably starts from rest (ω=0 at t=0) and increases continuously to a maximum angular velocity (ω max ). Note that in the figure, a positive angular velocity indicates a rotation of the longitudinal proximal end of the arm toward the solid object, and a negative angular velocity indicates a rotation of the longitudinal proximal end of the arm away from the solid object. The angular velocity curve includes a turning point (14) at time t2. At time t3, which is later than t2, the end of the hammer impacts the solid object with a single elastic rebound. At this time t3, the angular velocity is negative, indicating a rotation of the proximal end of the arm away from the solid object. At time t4, the actuator returns to a stationary state (ω=0).

[0120] Note that the turning point (14) in the angular velocity curve refers to the moment when the angular velocity changes its sign, specifically from positive to negative, that is, from rotating toward the solid object to rotating away from the solid object.

[0121] Figure 5 Another angular velocity curve (17) according to the present invention is shown. Preferably, the angular velocity curve starts from rest (ω=0 at t=0). Figure 4 In contrast, the embodiment shown has a first time period (18) during which the angular velocity is negative, which ends at a second turning point (19), which is the point at which the angular velocity changes from negative to positive, i.e., from turning away from the solid object to turning towards the solid object. After the first time period (18), the angular velocity substantially follows the same Figure 4 A similar curve is shown in FIG. 1 , that is, the angular velocity increases continuously to the maximum angular velocity (ω) at time t1. max ). The angular velocity curve includes a turning point (14) at time t2. At time t3, which is later than t2, the end of the hammer strikes the solid object with a single elastic rebound. At this time t3, the angular velocity is negative, indicating that the proximal end of the arm is rotating away from the solid object. At time t4, the actuator returns to the stationary state (ω=0).

[0122] Preferably, the arm is attached to the actuator in a replaceable manner, allowing the arm and / or hammer tip to be easily replaced, for example in the event of end degradation or another type of arm and / or hammer tip is needed for performing IE measurements. For example, the arm and / or tip can be replaced with another arm and / or tip having different elastic properties, in particular other bendable properties. Another reason for replacing the arm and / or hammer tip is when it is necessary to impact on a solid object with another type of hammer tip, for example when a harder or softer tip is needed, or a tip of a different material (e.g. metal instead of plastic) is needed for optimal IE measurement of a particular solid object. In this regard it can be noted that for metal solid objects, one would like to use a hard hammer tip, while for solid objects made of 3D printed materials, a plastic hammer tip may be preferred. Therefore, the present invention can be used to test different solid objects. To this end, in one embodiment, the present invention preferably includes an impact device kit as previously discussed.

[0123] In one embodiment, the apparatus preferably comprises one or more control units for controlling the actuators and / or sensors of the impact device.

[0124] Here, the sensor may preferably include a microphone, a piezoelectric displacement sensor and / or a laser interferometer to capture the vibration response. The sensor may also preferably include a processing unit and / or a memory unit for recording and / or analyzing the vibration response.

Claims

1. A non-destructive method for measuring density distribution deviation in a solid object, comprising the following steps: a) Providing a first excitation pulse to the solid object, preferably a mechanical shock; b) Obtaining a first vibration response of the solid object to the first excitation pulse, and deriving a first eigenfrequency of the vibration mode in a first orientation from the first vibration response; c) Providing a second excitation pulse to the solid object, preferably a mechanical shock; d) Obtaining a second vibration response of the solid object to the second excitation pulse, and deriving a second eigenfrequency of the vibration mode in a second orientation from the second vibration response, wherein the first excitation pulse is provided to the solid object at a first position and the second excitation pulse is provided to the solid object at a second position different from the first position, such that the first orientation is different from the second orientation, and wherein the method further comprises the following steps: e) Comparing the first eigenfrequency with the second eigenfrequency to obtain a measurement of the density distribution deviation in the solid object.

2. The method according to claim 1, comprising repeating the following steps: c') Providing an i-th excitation pulse to the solid object, preferably a mechanical shock; and d') Obtaining an i-th vibration response of the solid object to the i-th excitation pulse, and deriving an i-th eigenfrequency of the vibration mode in an i-th orientation from the i-th vibration response, in, providing the i-th excitation pulse to the solid object at an i-th position different from the first position, the second position and the j-th position, such that the i-th orientation is different from the first orientation, the second orientation and the j-th orientation, where j < i, where i is at least 3, such as 3, 4, 5, 6, 7, 8, 9, 10 or higher, the method further comprising repeating the following steps: e') Comparing the i-th frequency with the first eigenfrequency, the second eigenfrequency and / or the j-th frequency, where j < i, to obtain a measurement of the density distribution deviation in the solid object.

3. The method according to any one of claims 1 or 2, characterized in that The solid object substantially comprises geometric symmetry, and wherein the density distribution deviation is substantially density asymmetry, wherein the first position and the second position are substantially related by the geometric symmetry of the solid object, and wherein step e) comprises obtaining the difference between the first eigenfrequency and the second eigenfrequency to obtain the measurement of the density asymmetry.

4. The method according to claim 3, characterized in that: Steps c', d' and e' are performed n times, where n is selected based on the basic geometric symmetry of the solid object.

5. The method according to any one of claims 3 or 4, characterized in that The i-th position is related to the first position, the second position and the j-th position by the geometric symmetry of the solid object, where j < i.

6. The method according to any one of claims 1 to 5, characterized in that Step e) comprises obtaining the difference between said first eigenfrequency and said second eigenfrequency and comparing said difference with a reference eigenfrequency difference value to obtain a measurement of said density distribution deviation, and / or according to the method of claim 2, wherein step e′) comprises taking a grouped difference between said i-th eigenfrequency and said j-th eigenfrequency, where j < i, and comparing said grouped difference with a reference eigenfrequency difference value to obtain said measurement of said density distribution deviation.

7. The method according to any one of the preceding claims, characterized in that The vibration mode is a longitudinal stretching mode, a bending mode or a torsional mode.

8. The method according to any one of the preceding claims, characterized in that The vibration mode is a first harmonic mode or a second harmonic mode.

9. The method according to any one of the preceding claims, characterized in that The solid object comprises any one or any combination of the following: - a metal or an alloy, preferably, wherein the metal may comprise iron, aluminum, zinc, copper, steel, etc.; - a plastic, such as a polymeric material; - a 3D printing material, preferably: · a metal, such as titanium or stainless steel, · a ceramic, such as alumina, or · a polymer, such as polyetheretherketone (PEEK) or polyetherimide (such as ULTEMTM)- a composite material, such as a composite of plastic and metal.

10. The method according to any one of the preceding claims, characterized in that The solid object is a 3D printed solid object, and / or wherein the method comprises the step of 3D printing the solid object.

11. An impact excitation (IE) measurement device for measuring a density distribution deviation in a solid object, the device comprising: - an object holder for holding the solid object in place during the IE measurement; - an impact device for providing an excitation pulse to the solid object, preferably a mechanical impact device for providing a mechanical impact to the solid object; - a sensor for obtaining a vibration response of the solid object to the excitation pulse; - a processor configured to: · derive an eigenfrequency of a selected vibration mode from the vibration response obtained by the sensor, wherein: i) the impact device is configured to provide a first excitation pulse to the solid object, preferably a mechanical impact; ii) the sensor is configured to obtain a first vibration response of the solid object to the first excitation pulse, and the processor is configured to derive a first eigenfrequency of the vibration mode in a first orientation from the vibration response; iii) the impact device is configured to provide a second excitation pulse to the solid object, preferably a mechanical impact; iv) the sensor is configured to obtain a second vibration response of the solid object to the second excitation pulse, and the processor is configured to derive a second eigenfrequency of the vibration mode in a second orientation from the second vibration response. wherein the object holder is configured to rotate and / or displace the object relative to the impact device between steps b and c, and / or the impact device is configured to rotate and / or displace relative to the object holder between steps b and c, such that the first excitation pulse is provided to the solid object at a first position and the second excitation pulse is provided to the solid object at a second position different from the first position, such that the first orientation is different from the second orientation, and Wherein, the processing device is further configured to compare the first eigenfrequency with the second eigenfrequency to obtain a measurement of the density distribution deviation in the solid object.

12. The device according to claim 11, characterized in that The impact device for applying a pulse impact to a solid object during an impact excitation measurement comprises: - an elongated elastically bendable arm, said arm being elastically deformable about a bending direction, said bendable arm comprising a longitudinal proximal end and a longitudinal distal end; a hammer tip for mechanically providing an excitation pulse to the solid object, the hammer tip being attached to the distal end of the arm, and - a programmable actuator attached to the proximal longitudinal end of the arm, wherein the actuator is programmed to provide an angular velocity to the longitudinal proximal end of the arm by rotating the longitudinal proximal end about an actuator axis, the actuator axis being substantially parallel to the bending direction of the arm.

13. The device according to claim 12, characterized in that The angular velocity follows a predefined angular velocity profile, wherein the angular velocity profile is configured to provide a pulse impact to the solid object.

14. The device according to any one of claims 11 to 13, characterized in that The sensor includes a microphone, a piezoelectric displacement sensor and / or a laser interferometer to capture the vibration response.

15. The device according to any one of claims 11 to 14, characterized in that The sensor comprises a processing unit and / or a memory unit for recording and / or analyzing the vibration response.

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