Density Asymmetry Measurement Method and Apparatus

Abstract

The present invention relates to a non-destructive method for measuring density distribution deviations in solid objects. The non-destructive method includes: a) applying 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 natural frequency of a vibration mode in a first direction from the first vibration response; c) applying a second excitation pulse, preferably a mechanical shock, to the solid object; d) obtaining a second vibration response of the solid object to the second excitation pulse and deriving a second natural frequency of the vibration mode in a second direction from the second vibration response. The first excitation pulse is supplied to the solid object at a first position, and the second excitation pulse is supplied to the solid object at a second position different from the first position such that the first direction is different from the second direction. The method further includes e) obtaining a measure of the density distribution deviation in the solid object by comparing the first natural frequency with the second natural frequency.

Description

Technical Field

[0001] Technical Field The present invention relates to a method and an apparatus for measuring the density asymmetry of a solid object. Here, an excitation pulse is supplied to the solid object, the vibration response is recorded, and the deviation from the expected density distribution, in particular the deviation from a homogeneous density distribution, is analyzed. Accordingly, the present invention relates to the field of non-destructive testing by impact excitation technology and vibration response analysis.

Background Art

[0002] Background The high-precision manufacturing of solid objects is an important industrial activity. In many applications, it is important that the solid object has the intended shape and size. However, in many applications, it is also important that the object has a uniform density distribution or the expected density distribution.

[0003] For example, a gear is preferably symmetric when rotating over an integer multiple of an angle corresponding to the value obtained by dividing 360° by the number of teeth of the gear. However, even if the shape of the gear is completely symmetric, there remains a possibility that the density of the gear varies. In such a case, the balance of the gear is not properly maintained, which may cause problems, for example, in the machine in which the gear is used. Another example relates to a ring that is preferably rotationally symmetric in both shape and density distribution. This applies to rings for industrial 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 purposes.

[0004] The non-destructive measurement of the shape and size of a solid object can be regarded as a standard practice. However, the non-destructive measurement of the density distribution within a solid object can be very cumbersome and costly.

Summary of the Invention

Means for Solving the Problems

[0005] The present invention aims to overcome this problem and provides a method and an apparatus that enable measurement of the density distribution of a solid object in consideration of testing the deviation of the density distribution from an intended density distribution. In many cases, the intended density distribution is a uniform density distribution, and in that case, the present invention provides a method and an apparatus for measuring density non-uniformity. Such density non-uniformity can be caused by defects, microcracks, local voids, and the like.

[0006] One method for non-destructively measuring density variations, particularly porosity variations, 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 and enabling 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 solve quality degradation structural characteristics of all kinds. For example, X-ray micro-CT does not have good sensitivity for detecting microcracks. The present invention aims to provide a method that can obtain a quality assessment of a solid object regarding both porosity and microcracks in a single measurement or at least in a single measurement setting.

[0007] The present invention can be applied to any solid object, but it is most advantageous to apply it to solid objects manufactured by an additive manufacturing (AM) process, also called 3D printing. Such a manufacturing process can actually result in a deviation of the density of the solid object from the intended density distribution, particularly a deviation of the density from a uniform density distribution due to, for example, incomplete sintering or other causes.

[0008] The present invention is applicable to any shape of solid objects, but it is preferably applied to solid objects having substantially geometric symmetry.

[0009] The present invention aims to overcome the above and other problems. The present invention provides a method and an apparatus for measuring density fluctuations of a solid object, particularly for measuring density inhomogeneities, and more specifically for measuring density asymmetry.

[0010] Summary of the Invention The present invention relates to the method according to claim 1 and the apparatus according to claim 11.

[0011] The present invention is based on the following insights of the inventors. - A geometrically symmetric solid object may have density asymmetry, i.e., the density distribution of the solid object deviates from the geometric symmetry of the solid object.

[0012] - Such density asymmetry results in a shift in the natural frequency of the solid object when excited along directions with different natural frequencies, even if the directions are related to each other through the geometric symmetry of the solid object.

[0013] - The difference in the natural frequencies measured along different directions for the same vibration mode signal density asymmetry is a measure of the asymmetry, i.e., the greater the difference, the higher the asymmetry, i.e., the difference in the natural frequencies is a measure of the density asymmetry.

[0014] - For solid objects without substantial geometric symmetry, similar observations hold, but in order to measure density inhomogeneity, the measured difference in natural frequencies must be compared with a reference value.

[0015] - The method can be performed on objects intended to have a uniform density distribution, but can also be performed on objects intended to have a density distribution with a non-uniform reference density distribution, i.e., the method enables measuring a density distribution deviation, i.e., the deviation of the density distribution from a reference density distribution that can be uniform or non-uniform.

[0016] The present invention utilizes these insights by performing impact excitation (IE) measurement techniques, enabling the acquisition of the vibration response to an excitation pulse supplied to a solid object. The vibration response can be analyzed, and a set of natural frequencies can be derived from the response. By repeating the IE measurement at least once, preferably multiple times such as 2, 3, 4, 5, 6, 7, 8, 9, 10 times or more, at different collision positions on the object, multiple sets of natural frequencies can be obtained and compared. By selecting different collision positions such that the directions of the excited natural frequencies are different from each other, the difference in the natural frequencies of each vibration mode in different directions is a direct measure of the density distribution.

[0017] In a preferred embodiment, the object includes substantially geometric symmetry, and different collision positions can be selected substantially corresponding to the symmetry of the solid object. The difference in the natural frequencies of each vibration mode when excited by an excitation pulse at different positions is a direct measure of the density asymmetry. Further, the present invention enables the detection of density asymmetry in a solid object without requiring further input regarding the characteristics of the solid object. For example, there is no need to set a reference value for the natural frequencies to measure the asymmetry. Also, there is no need to compare the results of the object with the results of a reference object to measure the density asymmetry.

[0018] It should be noted that the shift in the natural frequency can be more prominent in some vibration modes than in other vibration modes. Thus, in a preferred embodiment, the vibration mode is selected to optimize the sensitivity of the method and / or apparatus of the present invention.

[0019] The IE measurement technique is relatively inexpensive, easy to perform, fast, and provides highly reliable information regarding density asymmetry, being much easier than X-ray micro-CT.

[0020] Summary of the Drawings

Brief Description of the Drawings

[0021]

Figure 1

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Figure 2C

Figure 2D

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Figure 7

Mode for Carrying Out the Invention

[0022] Detailed Description of the Invention The present invention relates to the method according to claim 1 and the apparatus according to claim 11.

[0023] The method according to the present invention relates to a non-destructive method for measuring density distribution deviation in a solid object.

[0024] This method a) applying a first excitation pulse, preferably a mechanical impact, to the solid object; b) obtaining a first vibration response of the solid object to the first excitation pulse and deriving a first natural frequency of the first vibration mode in a first direction from the first vibration response; c) applying a second excitation pulse, preferably a mechanical impact, to the solid object; d) obtaining a second vibration response of the solid object to the second excitation pulse and deriving a second natural frequency of the vibration mode in the second direction from the second vibration response; comprising the first excitation pulse is supplied to the solid object at a first position, the second excitation pulse is supplied to the solid object at a second position different from the first position such that the first direction is different from the second direction, and the method e) obtaining a measure of the density distribution deviation in the solid object by comparing the first natural frequency with a second frequency further comprising.

[0025] In a preferred embodiment, the solid object substantially has geometric symmetry, and the density distribution deviation is substantially density asymmetry. Here, substantial geometric symmetry refers to a transformation of the object that substantially reproduces the shape and size of the object. The most common geometric symmetries are two-dimensional and three-dimensional rotations, mirror symmetries with respect to axes or points, and combinations thereof. The geometric symmetry may be a continuous symmetry, a discrete symmetry, or a combination thereof. For example, a ring can have continuous two-dimensional rotational symmetry around a central axis and discrete mirror symmetries with respect to a plane perpendicular to the central axis and each plane containing the central axis. A gear can have discrete two-dimensional rotational symmetry and a set of discrete mirror symmetries. The discreteness of the rotational symmetry is caused by the teeth of the gear. Cubic and beam-shaped objects can also have a set of discrete rotational and mirror symmetries. In one embodiment of the present invention, the solid object may substantially include geometric symmetry. Thereby, the solid object can have a shape that slightly deviates from perfect geometric symmetry. Such a deviation from perfect geometric symmetry can be accidental and / or intentional. For example, a ring may have a small local thickness around its circumference, breaking the perfect geometric symmetry, or there may be an imprint along its circumference. These small deviations can result in a natural frequency difference offset that can be considered when the natural frequencies measured along different directions are compared, i.e., in step e or e' of the method according to the present invention. Such a natural frequency difference offset can be determined relatively easily by a calibration measurement or a computer simulation.

[0026] As described above, the present invention includes obtaining the natural frequencies of a solid object. These natural frequencies are the frequencies at which the object can resonate naturally. As a result, for example, when the object is vibrated by an impact, it continues to vibrate longer at its natural frequency than at other frequencies. Further, a solid object can have many natural frequencies, each of which can be due to a vibration mode. In a preferred embodiment of the present invention, the vibration mode may be a longitudinal extension mode, a bending mode, or a torsional mode. The choice may preferably depend on the shape of the object and, preferably, the geometric symmetry of the object in embodiments where the object substantially has geometric symmetry. The vibration mode may preferably be a fundamental mode or a harmonic mode such as a first harmonic mode, a second harmonic mode, a third harmonic mode, or a harmonic mode of the fourth order or higher. In a particularly preferred embodiment, the vibration mode is or is selected to be the first harmonic mode or the second harmonic mode. The inventors have found that the harmonic modes appear to be more sensitive to the natural frequency difference due to density distribution deviation and / or density asymmetry than the fundamental mode. As a result, the first harmonic mode, the second harmonic mode, or the n’-th harmonic mode is preferred, where n is 3, 4, 5, 6, 7, 8, 9, 10 or more.

[0027] The natural frequencies of the vibration mode are measured and derived by first performing steps a and b and second by performing steps c and d. The difference between the measured values is the impact point, i.e., the first and second positions that ensure excitation of the vibration mode along different directions.

[0028] The geometric symmetry of the object enables the excitation pulses provided at different positions to vibrate the object in essentially the same way when the density is uniformly distributed and when the different positions correspond to positions that are substantially transformed into each other by a geometric symmetry transformation.

[0029] In a preferred embodiment, the method comprises c’) applying a i’-th excitation pulse, preferably a mechanical impact, to the solid object; d’) Obtain the i’-th vibration response of the solid object to the i’-th excitation pulse, and derive the i’-th natural frequency of the vibration mode in the i’-th direction from the i’-th vibration response; including repeating; The i’-th excitation pulse is supplied to the solid object at an i’-th position different from the first position, the second position, and the j’-th position, whereby the i’-th direction is different from the first direction, the second direction, and the j’-th direction, where j < i; For i, this is at least 3, for example 3, 4, 5, 6, 7, 8, 9, 10 or more.

[0030] The method is more preferably f) obtaining a measure of the density distribution deviation in the solid object by comparing the first natural frequency with the second frequency further including.

[0031] According to the above embodiment, the method preferably e’) comparing the i’-th frequency with the first natural frequency, the second frequency, and / or the j’-th frequency, where j < i, thereby repeating obtaining a measure of the density asymmetry in the solid object including.

[0032] In a preferred embodiment, the solid object substantially has geometric symmetry, the density distribution deviation is substantially density asymmetry, the first position and the second position are substantially related by the geometric symmetry of the solid object, and step e) includes taking the difference between the first natural frequency and the second natural frequency to obtain the measure of the density asymmetry. Preferably, thereby, steps c’, d’, and e’ are performed n times, where n is selected based on the substantial geometric symmetry of the solid object. Also preferably, thereby, the i’-th position is associated by the geometric symmetry with respect to the first position, the second position, and the j’-th position of the solid object, where j < i.

[0033] From the above discussion focusing on the preferred embodiments of solid objects substantially having geometric symmetry, the present invention may seem to be related only to solid objects having geometric symmetry. However, the present invention can also be applied to objects having no geometric symmetry. In such cases, the methodology can be applied to many objects of the same type, typically objects having the same intended shape and size and manufactured in large numbers using the same manufacturing method. Then, by comparing the results obtained from multiple objects, it becomes possible to identify which objects contain a density distribution deviating from the intended density distribution.

[0034] Accordingly, in one embodiment of the present invention, step e) includes taking the difference between the first natural frequency and the second natural frequency and comparing the difference with the value of the reference natural frequency difference in order to obtain the measure of the density distribution deviation, and / or step e’) includes taking a set of differences between the i’-th natural frequency and the j’-th natural frequency, where j < i, and comparing the difference with the value of the reference natural frequency difference in order to obtain the measure of the density distribution deviation.

[0035] The impact excitation technique for characterizing solid objects can be known from the following documents. European Patent No. 3658868 discloses an apparatus for analyzing the mechanical vibration response of a solid material sample. The apparatus includes an array of impactors configured to apply an impact to each distinct point on the surface of the solid material sample, a sensor configured to capture the mechanical vibration response as a time-varying signal following the impact of the at least one impactor, and processing means configured to analyze the time-varying signal to determine the frequency and attenuation 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.

[0036] Document WO2020254698 discloses a method for acoustically measuring the material properties of a test piece at high temperature. The method includes: a. heating the test piece within a test temperature range; b. performing background measurements within the test temperature range by capturing vibration signals from the test piece during a calibration cycle, thereby obtaining a noise signal; c. performing acoustic measurements on the test piece within the test temperature range and during a test period, including: c1. applying a vibration excitation to the test piece; c2. capturing the vibration signal of the test piece during the 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 temperature.

[0037] Both prior art documents in the name of the applicant of the present application relate to methods and apparatuses for performing impact excitation (IE) measurements.

[0038] The IE technique basically consists of supplying 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 via several methods, including using a microphone, a piezoelectric displacement sensor, and / or a laser interferometer.

[0039] The solid object can basically be any kind of solid object, including but not limited to the following.

[0040] - Preferably, a metallic solid object including or made of a metal or alloy, where the metal may include iron, aluminum, zinc, copper, steel, etc. - Plastics such as polymeric materials - 3D printing materials, preferably: · Metals such as titanium, stainless steel, etc. · Ceramics such as aluminum oxide, or · Polymers such as polyetheretherketone (PEEK) or polyetherimide (such as ULTEM (trademark)), - Composite materials such as composites of plastic and metal.

[0041] Most preferably, the solid object is a 3D printed solid object, and / or the method of the present invention includes 3D printing a solid object.

[0042] Devices for measuring natural frequencies, such as Grindosonic (registered trademark) Mk7, SA, HT and IL devices, are commercially available.

[0043] The present invention also relates to an impact excitation (IE) measuring device for measuring the density distribution deviation of a solid object, and the impact excitation (IE) measuring device includes - An object holder for holding the solid object in a fixed position during IE measurement, - An impact device for supplying an excitation pulse to the solid object, preferably a mechanical impact device for applying a mechanical impact to the solid object, - A sensor for obtaining the vibration response of the solid object to the excitation pulse, - Processing means, · Processing means configured to derive the natural frequency of a vibration mode selected from the vibration response obtained by the sensor, and is provided with, Here, i) The impact device is configured to apply a first excitation pulse, preferably a mechanical impact, to the solid object, ii) The sensor is configured to obtain a first vibration response of the solid object to the first excitation pulse, and the processing means is configured to derive the first natural frequency of the vibration mode in the first direction from the vibration response, iii) The impact device is configured to apply a second excitation pulse, preferably a mechanical impact, to the solid object, iv) The sensor is configured to obtain a second vibration response of the solid object to the second excitation pulse, and the processing means is configured to derive a second natural frequency of the vibration mode in the second direction from the second vibration response. The object holder is configured to rotate and / or displace the object relative to the impact device, and / or the impact device is configured to rotate and / or displace relative to the object holder between steps b and c, whereby the first excitation pulse is supplied to the solid object at a first position, and the second excitation pulse is supplied to the solid object at a second position different from the first position such that the second direction is different from the first direction. Here, the processing means is further configured to compare the first natural frequency with the second frequency, thereby obtaining a measure of the density asymmetry in the solid object.

[0044] In embodiments where the solid object substantially has geometric symmetry, the first position and the second position are preferably associated by the geometric symmetry of the solid object. Note that it is obvious that the first position and the second position are associated by the geometric symmetry of the solid object if the object is transformed according to its geometric symmetry and the first position is substantially transformed into the second position.

[0045] The results of performing the method according to the present invention using the system according to the present invention are shown in FIG. 6. Thus, four rings were tested using the method of the present invention. The results are shown in four columns (20A, 20B, 20C, 20D) in FIG. 6. Each column shows the vibration responses to four mechanical impacts at four different positions along the ring. The different positions correspond to positions on the ring rotated in 45° increments. The frequency is shown on the vertical axis (f) in the frequency interval of the second harmonic for the bending mode of the ring.

[0046] The results of the second sample (20B) and the fourth sample (20D) show continuous bands at frequencies f2 and f4 respectively, indicating that the frequency spectrum is essentially independent of the position of the impact, and thus showing that the second and fourth sample rings have a uniform density distribution. This is different for the first sample ring (20A), especially the third sample ring (20C). The first sample ring shows that four impacts result in two clearly different vibration responses with natural frequencies f1A and f1B respectively. The first impact (21A) and the third impact (21C) result in peaks in the vibration response at the natural frequency f1B, and the second impact (21B) and the fourth impact (21D) result in peaks in the vibration response at the natural frequency f1A. Note that the result of the first impact is shown on both the left and right sides, indicating the rotational geometric symmetry of the ring. Although the difference between the two natural frequencies f1A and f1B is clearly significant but small, it shows that the density asymmetry of sample 20A is small but significant. In the third sample (20C), the difference between the two natural frequency peaks f3A, f3B is very large. Again, the first impact (22A) and the third impact (22C) result in a vibration spectrum with the peak natural frequency f3B, and the second impact (22B) and the fourth impact (22D) result in a vibration spectrum with the peak natural frequency f3A. The difference between f3A and f3B is large, indicating a large density asymmetry.

[0047] Also, for the first and third samples, the natural frequencies are lower than those of the second and fourth samples, which is consistent with previous results showing that solid objects with high porosity, which tend to have a high degree of defects, have lower natural frequencies than well - manufactured low - porosity samples such as the second and fourth samples.

[0048] The results obtained by the present invention correspond to those obtained by X-ray micro-CT, but are obtained by a much simpler procedure that is much faster and less expensive than X-ray micro-CT. In fact, the present method and system enable results to be obtained within seconds for each sample, while X-ray micro-CT requires minutes or more.

[0049] Figure 7 shows an IE measuring device (30) according to the present invention, which is applied to measure the density asymmetry within a ring (31) that substantially has geometric symmetry, in this case rotational symmetry. The device (30) comprises an object holder (34) for holding a solid object in a fixed position during IE measurement. The object holder preferably comprises a soft bedding to allow the solid object, i.e., the ring, to vibrate essentially freely. The device also comprises a shock device (1) for supplying an excitation pulse to the solid object. The shock device shown in the figure corresponds essentially to the shock devices shown in FIGS. 1, 2A - 2D and 3, which will be described later in this specification and the same reference numerals are used. The illustrated device also comprises a table (32) with a sensor (33) disposed therein configured to obtain the vibration response of the solid object to the excitation pulse. Although the processing means of the device are not shown, they can be functionally connected to a wired or wireless sensor to receive the vibration response of the solid object to the excitation pulse. The processing means are then configured to derive the natural frequency of a selected vibration mode from the vibration response obtained by the sensor. Further, the shock device (1) is configured to provide a first excitation pulse, in this case a mechanical shock by a tip (6) on a bendable arm (2), to the solid object (31). Also, the sensor is configured to obtain a first vibration response of the solid object to the first excitation pulse, and the processing means are configured to derive a first natural frequency of the vibration mode in a first direction from said vibration response.

[0050] In the illustrated figure, the object holder (34) is configured to rotate the object (31) with respect to the impact device (1). In the illustrated embodiment, the object holder (34) is connected to the rotation axis (35) via the table (32), and the rotation axis (35) can be rotated by the actuator (36), and thus the object holder and the object can be rotated as desired.

[0051] The impact device (1) is further configured to provide a second excitation pulse to the solid object, the sensor is configured to obtain a second vibration response of the solid object to the second excitation pulse, and the processing means is configured to derive a second natural frequency of the vibration mode from the second vibration response, similar to that shown in FIG. 6.

[0052] Preferably, the excitation pulse supplied to the solid object is as clean as possible to reduce the noise signal. In principle, it is desirable to use an impact device that does not generate noise itself to impart impulse excitation to the solid object. The impact device typically comprises an impactor that mechanically imparts an excitation pulse to the solid object and an impactor actuator that supplies kinetic energy to the impactor to impart the excitation pulse. In the prior art, ballistic impactors have been used, and such ballistic impactors are given momentum by the impactor actuator, and then the impactor follows a ballistic trajectory for at least a short period before colliding with the solid object. After the collision, the ballistic impactor can follow the ballistic trajectory again before being recaptured. Using a ballistic impactor has the advantage of enabling an essentially impulse impact, i.e., an impact with a very short contact period during which the kinetic energy of the impactor is mechanically transmitted to the solid object. In practice, noise may occur due to the following reasons.

[0053] -Multiple impacts: For example, the inertia of the impactor causes the impactor to contact the solid object surface more quickly than it pulls away from the solid object, so the impactor contacts the solid object more than once. In the case of a ballistic impactor, this also means that the impact is preferably applied from below the solid object so that the impactor does not bounce off the solid object more than once.

[0054] -The impactor may generate noise during operation, typically due to friction between the impactor and the impactor actuator. In the case of a ballistic impactor, the impactor actuator can be provided with a barrel or guide to ensure that the ballistic impactor is provided with a velocity in the correct direction. Friction with the barrel or guide can cause noise.

[0055] -The impactor may generate noise after the impact, for example by touching other parts of the device or when recaptured in the case of a ballistic impactor.

[0056] In the prior art, a ballistic impactor is disclosed as a preferred impactor that enables an impulse impact. However, the inventors have found that the ballistic impactor is not always the best solution. In particular, the ballistic impactor and the impactor actuator are not always easily adaptable to be used with different types of solid objects. Furthermore, the ballistic impactor is most suitably used from below to reduce the possibility of multiple collisions, thereby prohibiting collisions with the solid object from different sides and / or directions. It should be noted that this may be required when it is desired to excite or enhance different vibration nodes within the solid object, or different vibration nodes at different positions as in the present invention, in order to measure density asymmetry.

[0057] Therefore, the present invention is -easily adaptable to impart an impulse impact to solid objects of any type of material, geometric shape, size, etc., - can be easily positioned or repositioned to impact a solid object along different positions and / or in different directions, - In particular, it is also an object to provide an impact device that reduces noise, by essentially eliminating the possibility of multiple impacts and reducing noise due to friction or recapture of the impactor.

[0058] In addition, the present invention relates to an improved impact device for performing IE measurements on solid objects, as well as a method for operating the impact device. The impact device is preferably used in an impulse excitation measurement device, and thus the present invention also relates to an IE measurement device comprising the impact device.

[0059] In a preferred embodiment, an impact device for imparting an impulse impact to a solid object during impact excitation measurement comprises - an elongated elastically bendable arm (2) that is elastically deformable about a bending direction (3), the bendable arm (2) having a proximal longitudinal end (4) and a distal longitudinal end (5); - a hammer tip (6) for mechanically impacting the solid object (7), the hammer tip being attached to the distal end (5) of the arm; - a programmable actuator (8) attached to the proximal longitudinal end (4) of the arm (2); and The actuator (8) is programmed to provide an angular velocity to the proximal longitudinal end (4) of the arm by rotating the proximal longitudinal end (4) about an actuator axis (9) that is essentially parallel to the bending direction (3) of the arm. Preferably, the angular velocity follows a predetermined angular velocity profile, whereby the angular velocity profile is configured to impart an impulse impact to the solid object.

[0060] The system of the present invention can preferably comprise an impact device kit for attaching the impact device described above herein, the kit comprising - A set of elongated elastically bendable arms, each being elastically deformable about the bending direction and having a proximal longitudinal end and a distal longitudinal end, and a set of bendable arms; - A set of hammer tips for mechanically impacting a solid object, each being attachable, preferably attached, to the distal end of an arm of a set of elongated elastically bendable arms; - A programmable actuator that is removably attachable to the proximal longitudinal end of each arm; Comprising; The actuator is preferably programmed to provide an angular velocity to the proximal longitudinal end of the actuating arm by rotating the proximal longitudinal end about an actuator axis that is essentially parallel to the bending direction of the actuating arm when the actuator is attached to the proximal longitudinal end of the actuating arm, and the angular velocity preferably follows a predetermined angular velocity profile, whereby the angular velocity profile is configured to provide an impulse impact to the solid object.

[0061] In a preferred embodiment, the first excitation pulse and / or the second excitation pulse are provided using an impact method for providing an impulse impact to a solid object during impact excitation measurement using an impact device, and the impact device - An elongated elastically bendable arm that is elastically deformable about the bending direction and has a proximal longitudinal end and a distal longitudinal end, and a bendable arm; - A hammer tip for mechanically applying an excitation pulse to a solid object, the hammer tip being attached to the distal end of the arm; Comprising; A suitable impact application method includes providing an angular velocity to the proximal longitudinal end of the arm by rotating the proximal longitudinal end about an actuator axis that is essentially parallel to the bending direction of the arm. Preferably, the angular velocity follows a predetermined angular velocity profile, whereby the angular velocity profile is configured to provide an impulse impact to the solid object.

[0062] As used herein, the term "impulse shock" refers to a single elastic bounce provided to a solid object in which energy is transferred during a short contact period between the hammer tip and the solid object.

[0063] As a result of the angular velocity profile, the hammer tip follows a hammer tip trajectory configured such that the hammer tip imparts an impulse shock to the solid object. Preferably, the trajectory of the hammer tip starts with a position offset relative to the solid object. Thus, the trajectory of the hammer tip depends on, and preferably only depends on, the position offset, the angular velocity profile of the bendable arm, and the bending characteristics.

[0064] Note that in FIGS. 1 and 2A - 2D, the bending direction and the actuator axis are shown perpendicular to the plane of the paper. Also, the bending direction refers to the direction in which the arm can be bent around it.

[0065] The flexible arm preferably has a strip - like shape with a length L, a width W, and a thickness T along the longitudinal direction, where 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 monolithic. 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 monolithic and 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 the bending direction perpendicular to the longitudinal direction, more preferably the width direction. 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., the arm is bent only around the width direction during the execution of the angular velocity profile.

[0066] 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 such that the angular velocity profile of the proximal end of the bendable arm can be determined by the rotor of the electric motor.

[0067] In a preferred embodiment, the angular velocity profile includes a turning point at which the angular velocity provided at the distal longitudinal end of the arm changes sign, and the turning point occurs earlier than the impact on the solid object. This is particularly preferred to ensure that an impulse impact is imparted to the solid object by the hammer tip's flail-like movement. This operation is shown in FIGS. 1 and 2A - 2D. FIG. 1 shows the situation at the start of the method, where the flexible arm (2) is essentially stationary and straight or slightly bent (e.g., due to the weight of the flexible arm and the hammer tip). Then, the actuator starts to execute the angular velocity profile. In FIG. 2A, the angular velocity (10) is such that the proximal end of the arm rotates towards the solid object (7). Due to the inertia of the arm and the hammer tip, the bendable arm bends around the bending direction (3), thereby curving the flexible arm. Next, the angular velocity profile includes a turning point at which the angular velocity (11) applied to the proximal end (4) of the flexible arm is essentially zero. At the turning point (see FIG. 2B), the actuator stops the rotation of the longitudinal end (4) of the arm (2). However, due to the curvature of the arm and its elasticity, the hammer tip continues to move towards the solid object. At the moment of impact on the solid object (see FIG. 2C), the actuator has already imposed an angular velocity (12) on the proximal end (4) of the arm that has moved away from the solid object, ensuring that the hammer tip does not collide with the solid object multiple times after the impact. This is shown in FIG. 2D, where the actuator has stopped the rotation (13) of the proximal end of the arm. As a result, the hammer tip has moved away from the solid object and can return to a stationary state according to the situation in FIG. 1, and is ready to start again for follow-up measurements.

[0068] 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 an electric motor, preferably a rotary electric motor, and can be programmed to operate the electric motor to essentially follow an angular velocity profile.

[0069] Figure 4 shows an angular velocity profile (16) according to the present invention. The angular velocity profile preferably starts from a stationary state (ω = 0 at t = 0) and continuously increases to a maximum angular velocity (ω max ) at time t1. In the figure, a positive angular velocity indicates that the proximal longitudinal end of the arm rotates towards the solid object, and a negative angular velocity indicates that the proximal longitudinal end of the arm rotates away from the solid object. The angular velocity profile includes an inflection point (14) at time t2. At an instant t3 later than t2, the hammer tip collides with the solid object with a single elastic bounce. At this instant t3, the angular velocity is negative, indicating rotation of the proximal end of the arm away from the solid object. At instant t4, the actuator returns to the stationary state (ω = 0).

[0070] It should be noted that the inflection point (14) in the angular velocity profile refers to the instant when the sign of the angular velocity changes from positive to negative, i.e., the instant when the rotation towards the solid object changes to rotation away from the solid object.

[0071] Figure 5 shows another angular velocity profile (17) according to the present invention. The angular velocity profile preferably starts from a stationary state (ω = 0 at t = 0). In contrast to the embodiment shown in Figure 4, the angular velocity is negative in a first period (18) that ends at a second inflection point (19), and the second inflection point (19) refers to the point where the angular velocity changes from negative to positive, i.e., the rotation away from the solid object changes to rotation towards the solid object. After the first period (18), the angular velocity essentially follows a profile similar to that shown in Figure 4. That is, the angular velocity reaches a maximum angular velocity (ω at time t1max ) It continuously increases until. The angular velocity profile includes an inflection point (14) at time t2. At an instant t3 later than t2, the hammer tip collides with the solid object with a single elastic bounce. At this instant t3, the angular velocity is negative, indicating a rotation away from the solid object at the proximal end of the arm. At instant t4, the actuator returns to a stationary state (ω = 0).

[0072] Preferably, the arm is attached to the actuator in an interchangeable manner, enabling easy replacement of the arm and / or the hammer tip, for example, in case of tip degradation or when another type of arm and / or hammer tip is required to perform IE measurements. For example, the arm and / or the tip can be exchanged with another arm and / or tip having different elastic properties, particularly different bending properties. Another reason for exchanging the arm and / or the hammer tip is when the solid object needs to be impacted by a different type of hammer tip, for example, when a harder or softer tip is required, or when a tip of a different material (e.g., metal instead of plastic) is required for optimal IE measurement of a particular solid object. In this regard, it should be noted that in the case of a metal solid object, a hard hammer tip may be desired, while in the case of a solid object made of a 3D printing material, a plastic hammer tip may be preferred. Thus, the present invention can be used to test different solid objects. Here, in one embodiment, the present invention preferably includes the aforementioned impact device kit.

[0073] In one embodiment, the device preferably comprises one or more control units for controlling the actuator of the impact device and / or the sensor.

[0074] Here, the sensor can preferably comprise a microphone, a piezoelectric displacement sensor, and / or a laser interferometer for capturing the vibration response. The sensor can also preferably comprise 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, a) applying 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 natural frequency of a first vibration mode in a first direction from the first vibration response; c) applying a second excitation pulse, preferably a mechanical shock, to the solid object; d) obtaining a second vibration response of the solid object to the second excitation pulse, and deriving a second natural frequency of the vibration mode in a second direction from the second vibration response; comprising: the first excitation pulse is applied to the solid object at a first position, and the second excitation pulse is applied to the solid object at a second position different from the first position such that the first direction is different from the second direction; the method further comprises: e) obtaining a measure of the density distribution deviation in the solid object by comparing the first natural frequency with the second natural frequency. A non-destructive method further comprising the above.

2. c’) applying a pulse of the i’th excitation, preferably a mechanical shock, to the solid object; d’) obtaining an i’th vibration response of the solid object to the i’th excitation pulse, and deriving an i’th natural frequency of the vibration mode in the i’th direction from the i’th vibration response; repeating the above, wherein the i’th excitation pulse is applied 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 direction is different from the first direction, the second direction and the j’th direction, where j < i, and i is at least 3, for example 3, 4, 5, 6, 7, 8, 9, 10 or more; the method further comprises: e’) comparing the i’th frequency with the first natural frequency, the second natural frequency, and / or the j’th frequency, where j < i, and repeating to obtain a measure of the density distribution deviation in the solid object. The method according to claim 1, further comprising the above.

3. The solid object substantially has geometric symmetry, and the density distribution deviation is substantially density asymmetry, the first position and the second position are substantially related by the geometric symmetry of the solid object. The method according to any one of claims 1 or 2, wherein step e) comprises taking the difference between the first natural frequency and the second natural frequency to obtain the scale of the density asymmetry.

4. The method according to claim 3, wherein steps c’, d’ and e’ are performed n times, and n is selected based on the substantial geometric symmetry of the solid object.

5. The method according to any one of claims 3 or 4, wherein the i’-th position is associated by the geometric symmetry with the first position, the second position and the j’-th position of the solid object, where j < i.

6. The method according to any one of claims 1 to 5, wherein step e) comprises taking the difference between the first natural frequency and the second natural frequency, and comparing the difference with the value of the reference natural frequency difference to obtain the scale of the density distribution deviation; and / or step e’) comprises taking a set of the differences between the i’-th natural frequency and the j’-th natural frequency, where j < i, and comparing the difference with the value of the reference natural frequency difference to obtain the scale of the density distribution deviation, as claimed in claim 2.

7. The method according to any one of the preceding claims, wherein the vibration mode is a longitudinal extension mode, a bending mode or a torsion mode.

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

9. The solid object is - preferably a metal or alloy, wherein the metal may include iron, aluminum, zinc, copper, steel, etc. - a plastic such as a polymeric material - a 3D printing material, preferably: · a metal such as titanium, stainless steel, etc. · a ceramic such as aluminum oxide, or · a polymer such as polyetheretherketone (PEEK) or polyetherimide (ULTEM (trademark), etc.) - a composite material such as a composite of plastic and metal The method according to any one of the preceding claims, comprising any one or any combination thereof.

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

11. An impact excitation (IE) measuring device for measuring the density distribution deviation of a solid object, - An object holder for holding the solid object in a fixed position during IE measurement, - An impact device for supplying an excitation pulse to the solid object, preferably a mechanical impact device for applying a mechanical impact to the solid object, - A sensor for obtaining the vibration response of the solid object to the excitation pulse, - A processing means, - A processing means configured to derive the natural frequency of a vibration mode selected from the vibration response obtained by the sensor, Here, i) The impact device is configured to apply a first excitation pulse, preferably a mechanical impact, to the solid object, ii) The sensor is configured to obtain a first vibration response of the solid object to the first excitation pulse, and the processing means is configured to derive a first natural frequency of a vibration mode in a first direction from the vibration response, iii) The impact device is configured to apply a second excitation pulse, preferably a mechanical impact, to the solid object, iv) The sensor is configured to obtain a second vibration response of the solid object to the second excitation pulse, and the processing means is configured to derive a second natural frequency of the vibration mode in a second direction from the second vibration response, The object holder is configured to rotate and / or displace the object relative to the impact device, and / or the impact device is configured to rotate and / or displace relative to the object holder between steps b and c, whereby the first excitation pulse is supplied to the solid object at a first position, and the second excitation pulse is supplied to the solid object at a second position different from the first position such that the first direction is different from the second direction, where the processing means is further configured to compare the first natural frequency with the second natural frequency, thereby obtaining a measure of the density distribution deviation in the solid object. An impact excitation (IE) measuring device.

12. The impact device for applying an impulse impact to a solid object during impact excitation measurement is - An elongated elastic bendable arm that is elastically deformable around the bending direction, the bendable arm having a proximal longitudinal end and a distal longitudinal end, - A hammer tip for mechanically supplying an excitation pulse to the solid object, the hammer tip being attached to the distal end of the arm; - A programmable actuator attached to the proximal longitudinal end of the arm; comprising; The apparatus according to claim 11, wherein the actuator is programmed to provide an angular velocity to the proximal longitudinal end of the arm by rotating the proximal longitudinal end about an actuator axis that is essentially parallel to the bending direction of the arm. **Claim 13** The apparatus according to claim 12, wherein the angular velocity follows a predetermined angular velocity profile, whereby the angular velocity profile is configured to impart an impulse shock to the solid object. **Claim 14** The apparatus according to any one of claims 11 to 13, wherein the sensor comprises a microphone, a piezoelectric displacement sensor and / or a laser interferometer for capturing the vibration response. **Claim 15** The apparatus according to any one of claims 11 to 14, wherein the sensor comprises a processing unit and / or a memory unit for recording and / or analyzing the vibration response.

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