阅读说明：本技术 确定多个换能器相对于目标物体的空间配置的方法 (Method for determining the spatial configuration of a plurality of transducers relative to a target object ) 是由 德米特里·彻尼亚克 马丁·克维斯特·奥尔森 于 2019-05-22 设计创作，主要内容包括：本公开的一个方面涉及确定附接到目标物体的多个换能器的相应空间配置(包括相应位置)的计算机实现的方法；该方法包括：从多个换能器中的每一个接收传感器信号,该传感器信号指示在目标物体的诱导运动期间相应换能器的相应运动；确定多个换能器的空间配置；其中,基于所接收的传感器信号来确定换能器的至少一个子集的空间配置(包括相对于目标物体的位置)。(One aspect of the present disclosure relates to a computer-implemented method of determining respective spatial configurations (including respective locations) of a plurality of transducers attached to a target object; the method comprises the following steps: receiving a sensor signal from each of the plurality of transducers, the sensor signal being indicative of a respective motion of the respective transducer during the induced motion of the target object; determining a spatial configuration of the plurality of transducers; wherein the spatial configuration (including the position relative to the target object) of at least a subset of the transducers is determined based on the received sensor signals.)

1. A computer-implemented method of determining respective spatial configurations of a plurality of transducers attached to a target object, the spatial configurations being indicative of respective positions and orientations; the method comprises the following steps:

-receive a sensor signal from each of the plurality of transducers, the sensor signal being indicative of a respective motion of the respective transducer during an induced motion of the target object;

-determining the spatial configuration of the plurality of transducers; wherein the spatial configuration of at least a subset of the transducers indicative of the position and orientation relative to the target object is determined based on the received sensor signals.

2. The method of claim 1; wherein the transducers comprise respective accelerometers, in particular triaxial accelerometers.

3. The method of any one of the preceding claims; the method comprises the following steps: receiving as input information representing only a portion of the spatial configuration of the plurality of transducers; and wherein determining the spatial configuration of the plurality of transducers comprises determining a remainder of the spatial configuration in order to establish a complete spatial configuration of all transducers of the plurality of transducers.

4. The method of any one of the preceding claims; wherein the plurality of transducers comprises a first subset of transducers and a second subset of transducers; and wherein determining the spatial configuration of the plurality of transducers comprises:

-obtaining a spatial configuration of the transducers of the first subset; and

-determining the spatial configuration of the transducers of the second subset from the received sensor signals and from the obtained spatial configuration of the transducers of the first subset.

5. The method of claim 4: wherein the first subset includes three tri-axis accelerometers.

6. The method of any of claims 4 to 5, wherein the first subset comprises a plurality of transducers sufficient to measure translational motion of the target object along three orthogonal directions and rotational motion about three orthogonal axes.

7. The method of claim 6; including verifying whether the first subset includes a sufficient number of transducers.

8. The method of claim 7: wherein the transducers comprise respective three-axis accelerometers, and wherein verifying comprises verifying whether a transformation matrix composed of respective transducer transformation matrices of the transducers of the first subset has a full rank; wherein each transducer transformation matrix represents a transformation between an acceleration of the corresponding transducer relative to a local coordinate system of the target object and an acceleration measured by the transducer along an axis of a measurement coordinate system of the transducer.

9. The method of any one of claims 4 to 8; including determining whether the induced motion is sufficient to determine the spatial configuration of the transducers of the second subset of transducers.

10. The method of claim 9; wherein the transducers comprise respective three-axis accelerometers; wherein the received sensor signal is indicative of an acceleration of the transducer measured during the induced motion; and wherein determining whether the induced motion is sufficient to determine the spatial configuration of the transducers of the second subset comprises:

-optionally performing a low-pass filtering of the sensor signal;

-determining whether a local acceleration matrix representing the measured acceleration has a predetermined number of dominant singular values.

11. The method of any one of claims 4 to 10; wherein determining the spatial configuration of the transducers of the second subset comprises:

-calculating an object acceleration indicative of an acceleration of the target object relative to a reference coordinate system from the received sensor signals of the transducers of the first subset and the received spatial configuration;

-calculating the spatial configuration of the transducers of the second subset from the calculated object acceleration and from the received sensor signals of the respective transducers of the second subset.

12. The method of any one of claims 4 to 11; wherein obtaining the spatial configuration of the transducers of the first subset comprises:

-receiving information indicative of a set of spatial relationships between the transducers of the first subset;

-calculating the spatial configuration of the transducers of the first subset from the received information and from the received sensor signals.

13. The method of any one of claims 4 to 12; wherein the target object is a deformable object whose movement is limited by an object support; and wherein the method comprises:

-calculating a time-dependent acceleration coefficient of the target object from the spatial configuration of the transducers of the first subset and from the received sensor signals;

-calculating the spatial configuration of the transducers of the second subset from the calculated time-dependent acceleration coefficients and from the received sensor signals.

14. The method of any one of the preceding claims; including filtering the recorded signals to suppress frequencies associated with bending modes of the induced motion.

15. A computer-implemented measurement process, comprising:

-determining a spatial configuration of a plurality of transducers attached to a target object by performing the steps of the method according to any one of claims 1 to 14;

-receive further sensor signals from the plurality of transducers, the received further sensor signals being indicative of respective movements of the respective transducers during the induced movement of the target object;

-performing structural analysis calculations based on the received further sensor signals.

16. A data processing system configured to perform the steps of the method according to any one of claims 1 to 15.

17. A computer program configured to cause a data processing system to perform the steps of the method according to any one of claims 1 to 15 when the computer program is executed by the data processing system.

18. A measurement system, comprising:

-the data processing system of claim 17:

-a plurality of input channels configured to be coupled to respective transducer output signals of a plurality of transducers attached to a target object via respective wireless or wired signal connections;

a data input interface configured to receive respective spatial coordinate data of the first subset of transducers from another apparatus, e.g. by manual data input or via a wireless or wired data communication link.

19. A transducer assembly comprising:

-a measurement system according to claim 18; and

a plurality of transducers mountable at a plurality of predetermined measurement positions distributed over the target object.

Technical Field

In one aspect, the present disclosure relates to a method of determining a spatial configuration of a plurality of transducers (in particular accelerometers) relative to a target object to which the plurality of transducers are attached.

Background

In many applications, such as structural analysis, vibration measurement, etc., multiple transducers, such as accelerometers, are typically attached to a target object, for example, using an adhesive or a suitable mounting device. It is often desirable to establish the position and spatial orientation of the individual transducers relative to the target object with reasonable accuracy in order to facilitate useful measurements. In particular, it may be desirable to establish a spatial configuration of the transducers relative to a digital model (e.g., a CAD model, an FE model, etc.) of the target object.

For the purposes of this specification, the term "spatial configuration" of a body (particularly a transducer) refers to the position and orientation of the body relative to a suitable reference coordinate system.

In many types of applications (such as in structural analysis, vibration measurement, etc.), checking and verifying the spatial configuration (i.e., position and orientation) of multiple transducers of a large measurement setup remains a significant challenge. As the size of the target object increases, the number of measurement positions on the target object also increases. This is usually followed by a corresponding increase in the number of transducers included in the measurement setup. Furthermore, many target objects have complex geometries including complex surface structures, which makes it difficult to determine the spatial configuration of the transducers.

Thus, the chance of errors or inaccuracies in the measurement setup (such as incorrectly placed transducers, wrong spatial configuration, etc.) may increase even faster. Recent progress has been made in so-called "smart sensors" comprising non-volatile semiconductor memory. The semiconductor memory is capable of electronically storing various types of useful transducer information, such as serial number, calibration values, and location, in a standardized format defined within the IEEE-P1451.4 standard. The latter format is now designated as transducer spreadsheet (TEDS). Each TEDS-compliant transducer is capable of transmitting its stored transducer information to a remote measurement system or device via a standardized communication protocol. The remote measurement system may automatically load the transducer information directly into the measurement system's setup specification or file. Thus, this feature may reduce human error associated with manually inputting transducer data into a measurement system.

However, checking and verifying the position and/or spatial orientation of each transducer in such a measurement setup remains a significant challenge. A suitable spatial configuration of the transducer is important in many types of vibration measurement to ensure that an expected or at least known component of acceleration is measured, for example by the transducer at an expected or at least known position.

WO 2016/135198 discloses a method of detecting the spatial orientation of a transducer by means of a handheld optical scanning device. To this end, this prior art approach uses spatial orientation features on the outer housing surface of the transducer.

Although the above prior art method significantly reduces the time and effort required to determine the spatial orientation of the transducers, the method still requires manual determination of the individual transducers. Furthermore, it is often desirable to obtain both position and orientation coordinates. Still further, it may be desirable to position the transducer at a location on the target object that is difficult to reach, such as in a pinhole that is difficult to scan spatially oriented features.

Therefore, there is still a need for an improved method that allows determining the spatial configuration (i.e. position and orientation) of even a large number of transducers relative to a target object to which the transducers are attached in a time and cost efficient manner and with sufficiently high accuracy.

Disclosure of Invention

According to a first aspect, disclosed herein are embodiments of a computer-implemented method of determining respective spatial configurations (indicative of respective positions and orientations) of a plurality of transducers attached to a target object; the method comprises the following steps:

-receive a sensor signal from each of the plurality of transducers, the sensor signal being indicative of a respective motion of the respective transducer during the induced motion of the target object;

-determining a spatial configuration of the plurality of transducers; wherein the spatial configuration (indicative of the position and orientation relative to the target object) of at least a subset of the transducers is determined based on the received sensor signals.

The inventors have realized that the spatial configuration of multiple transducers (including their position and orientation relative to a target object to which they are attached) can be efficiently determined with sufficiently high accuracy based on sensor signals from the multiple transducers and possibly other transducers attached to the target object. Thus, the need for manually measuring the position and orientation of all transducers attached to the body is eliminated or at least greatly reduced. Thus, the time and effort required to establish a spatial configuration of all the transducers is greatly reduced, especially when a subset of transducers includes a large number of transducers.

In a preferred embodiment, the method receives as input information representing only a portion of the spatial configuration of the plurality of transducers. Thus, the process only requires the determination of the remaining part of the spatial configuration in order to establish a complete spatial configuration of all the transducers of the plurality of transducers. The information may comprise a spatial configuration of the first subset of transducers, in particular information defining both a position and an orientation of the first subset of transducers. To this end, the information may include information representing coordinates of six degrees of freedom (three positional degrees of freedom and three rotational degrees of freedom) along each transducer of the first subset of transducers. Alternatively or additionally, the information may include a spatial relationship between at least a first subset of the transducers. The spatial relationship may be a relative distance between pairs of transducers, partial information about the orientation of the transducers (e.g., the direction of one axis of the transducer's measured coordinate system), or other partial spatial configuration representing coordinates along less than six degrees of freedom (e.g., coordinates along one or two or three degrees of freedom). Thus, the received information representing only a portion of the spatial configuration of the plurality of transducers may represent the full spatial configuration of only the first subset of transducers and/or the partial spatial configuration of at least the first subset of transducers.

Thus, the plurality of transducers may be divided into a first subset and a second subset of transducers, the second subset being different from the first subset; in particular, the subsets may be non-overlapping. According to some embodiments, determining the spatial configuration of the plurality of transducers may therefore comprise:

-obtaining a spatial configuration of the transducers of the first subset; and

-determining the spatial configuration of the transducers of the second subset from the received sensor signals and from the obtained spatial configuration of the transducers of the first subset.

Thus, once the spatial configuration of the first subset of transducers is known, the spatial configuration of the second subset of transducers may be automatically determined from their respective sensor signals and from the known spatial configuration of the first subset. Thus, the time and effort required to establish a spatial configuration of all the transducers is greatly reduced, especially since the first subset of transducers may be much smaller than the second subset of transducers. For example, if one hundred or more transducers are attached to the target object, it may be sufficient to include only a few of them (e.g., only three transducers) into the first subset. Once the spatial relationship of the transducers of the first subset has been established manually, for example in a conventional manner, or calculated from the known spatial relationships of the transducers and from the received sensor signals, the spatial configuration of all the remaining transducers may be determined automatically. For the purposes of this specification, the transducers of the first subset will also be referred to as reference transducers. In general, the spatial configuration of a transducer attached to a target object may be expressed as a transformation between the measurement coordinate system of the transducer and the local coordinate system of the target object. Alternatively, the spatial configuration of the transducer attached to the target object may be expressed as a transformation between the measurement coordinate system and the local coordinate system of the transducer. The measurement coordinate system of the transducer may define an axis along which acceleration is measured by the transducer.

Each transducer may be or include a single or multi-axis (e.g., three-axis) accelerometer. The sensor signal from the transducer may represent one or more series of measurements of the transducer. For example, each measurement may indicate acceleration in one or more directions (e.g., in a third direction). A three-axis accelerometer may provide a time series of measurements, where each measurement represents an acceleration vector relative to a measurement coordinate system of the accelerometer. Other examples of transducers include strain gauges, proximity probes, and inclinometers or other measurement devices operable to measure motion, acceleration, force, or other quantities from which the motion of the target object may be derived.

The spatial configuration of the transducers of the first subset may be obtained in any suitable manner (e.g., manually determined and input). For example, the transducers of the first subset may be mounted to the target object at easily identifiable locations on the surface of the target object, for example at certain different features of the geometry of the target object, which may be used as "anchor points" for placement of the transducers. The transducer may be mounted in the position using a level and/or a rotating base to align the transducer with respect to the direction of gravity. The thus identified position and orientation may be manually entered into a suitable measurement system, for example using a suitable digital 3D model (e.g. a CAD model) of the target object. Thus, in some embodiments, obtaining the spatial configuration of the first subset of transducers comprises receiving as input the spatial configuration of the first subset of transducers. In other embodiments, the process does not receive the exact spatial configuration of the first subset of transducers, but only receives information indicative of a set of spatial relationships between the first subset of transducers. The process then calculates the spatial configuration of the first subset of transducers from the received information and from the received sensor signals.

Preferably, the first subset comprises a plurality of transducers sufficient to measure the motion of the target object in all its degrees of freedom. For a target object supported to be able to perform translational motion along three orthogonal directions and rotational motion about three orthogonal axes, the number of transducers may be selected to be sufficient to measure the motion of the target object in six degrees of freedom (i.e. three translational degrees of freedom and three rotational degrees of freedom). The exact number of transducers required depends on the capabilities of the transducers and their relative positions with respect to the target object as well as the number of degrees of freedom of movement of the target object. For example, when the transducers are tri-axial accelerometers, three transducers are sufficient to form the first subset when they are not positioned along a straight line. Preferably, the three transducers should also be positioned sufficiently far from each other or at least as far from each other as the geometry of the target object allows.

In some embodiments, the method includes verifying whether the first subset includes a sufficient number of transducers and/or whether the transducers are sufficiently located. For example, in some embodiments in which each transducer comprises a three-axis accelerometer, each transducer may be associated with a transducer transfer matrix representing the transformation between the acceleration of the transducer relative to the local coordinate system of the target object (in particular translational and rotational accelerations) and the acceleration measured by the transducer along the axes of the transducer's measurement coordinate system. Typically, the transducer transfer matrix of a transducer attached to the target object represents the position and orientation of the measurement coordinate system of the transducer relative to the local coordinate system of the target object. Thus, for a known position and orientation of the transducer relative to the target object, the transducer transfer matrix may be determined from the known position and orientation. Verifying whether the first subset includes a sufficient number of transducers may then comprise verifying whether a transformation matrix composed of transducer transformation matrices of the transducers of the first subset has a full rank. If the transform matrix has a full rank, the process may determine that the first subset includes a sufficient number of transducers.

In some implementations, the process includes determining whether the induced motion is sufficient to determine a spatial configuration of the transducers (e.g., the transducers of the second subset of transducers). To this end, in some embodiments in which each transducer includes a dual-axis accelerometer, determining whether the induced motion is sufficient to determine the spatial configuration of the transducers of the second subset of transducers comprises:

-receiving a sensor signal indicative of an acceleration of the transducer measured during the induced motion;

-optionally performing a low-pass filtering of the sensor signal;

-determining whether the local acceleration matrix representing the measured acceleration has a predetermined number of dominant singular values, for example six dominant singular values corresponding to an unrestricted motion. This determination may be made by comparing the largest singular value (e.g., the largest six dominant values) to the largest one of the remaining singular values.

If the local acceleration matrix includes a sufficient number (e.g., six) of dominant singular values, then determining the resulting motion is sufficient to determine the spatial configuration of the transducers of the second subset of transducers.

In general, in some embodiments, particularly in certain embodiments where the target object may move along all six degrees of freedom, the method includes filtering the recorded signals to suppress frequencies associated with bending modes that induce motion. In particular, in some embodiments, the method includes low pass filtering of the sensor signal. To this end, the process may include detecting the natural frequency of the lowest bending mode and the natural frequency of the highest rigid body mode; the cut-off frequency of the low-pass filter is selected so as to suppress at least most of the flexural modes and to preserve at least most of the rigid body modes. To this end, a cutoff frequency may be selected that is greater than the natural frequency of the highest detected rigid body mode and less than the natural frequency of the lowest detected bending mode.

Determining the spatial configuration of the transducers of the second subset of transducers may comprise:

-calculating an object acceleration indicative of an acceleration of the target object relative to a reference coordinate system from the received sensor signals of the transducers of the first subset and the obtained spatial configuration of the transducers of the first subset;

-calculating the spatial configuration of the transducers of the second subset from the calculated acceleration of the object and from the received sensor signals of the respective transducers of the second subset.

In particular, the object acceleration may represent the spatial configuration (twice differentiated with respect to time) of the local coordinate system of the target object with respect to the inertial reference coordinate system.

Calculating the object acceleration may include calculating a least squares solution of a system of linear equations, each linear equation representing a transformation between a measured acceleration measured by one of the reference accelerometers and the unknown object acceleration.

As described above, in some embodiments, the process receives information indicative of a set of spatial relationships between the transducers of the first subset; and the process calculates the spatial configuration of the first subset of transducers from the received information and from the received sensor signals. The information about the spatial relationship between the transducers of the first subset may comprise distances between respective pairs of transducers of the first subset. Alternatively or additionally, the information about the spatial relationship between the transducers of the first subset may comprise a combination of relative distance and orientation or another combination of spatial parameters.

Calculating the spatial configuration of the first subset of transducers may comprise:

-defining a first set of sensor signals from the received sensor signals from the first subset of transducers and a second set of sensor signals from the received sensor signals from the first subset of transducers;

-calculating an estimate of the sensor signals in the second set of sensor signals from the sensor signals in the first sensor signals and from the received information indicative of the spatial relationship between the transducers of the first subset, the estimate of the sensor signals depending on the estimated spatial configuration of the transducers of the first subset; and

-adjusting the estimated spatial configuration to reduce (in particular minimize) the error between the calculated estimate and the first set of sensor signals.

Thus, the spatial configuration of the first set of transducers may be calculated according to a minimization problem indicating the degree to which the calculated spatial configuration interprets the second set of received sensor signals given the first set of received sensor signals. In some embodiments, the calculation of the spatial configuration of the transducers of the first subset may be further refined by mounting the transducers of the first subset to have one axis pointing in a known direction (e.g., in the direction of gravity).

In general, some embodiments of the methods described herein may be performed with respect to a target object that is suspended or otherwise supported to allow the target object to move in all six degrees of freedom.

Other embodiments of the methods disclosed herein may be performed with respect to a deformable target object whose motion is constrained by an object support (e.g., along one or more degrees of freedom). To this end, some embodiments of the method include:

-calculating a time-dependent acceleration coefficient of the target object from the spatial configuration of the transducers of the first subset and from the received sensor signals.

Calculating the spatial configuration of the transducers of the second subset from the calculated time-dependent acceleration coefficients and from the received sensor signals, e.g. by minimizing a fitting error indicative of a difference between the estimated sensor signals and the received sensor signals.

The time-dependent acceleration coefficients may be coefficients associated with the modal coordinates of the target object. In this case, the process of calculating the coefficients may be similar to obtaining the modal coordinates, for example, in a modal decomposition. Alternatively, the time-dependent acceleration coefficient may be a coefficient based on another extension of a suitable set of basis functions. A suitable set of basis functions should preferably satisfy a set of boundary conditions (e.g. so-called base boundary conditions) and be mutually orthogonal.

It should be noted that the features of the computer-implemented methods described above and below may be implemented at least partly in software or firmware and may be implemented on a data processing system or other processing means resulting from the execution of program code means such as computer executable instructions. Here and in the following, the term processing means comprises any circuitry and/or means adapted to perform the above functions. In particular, the above terms include general or special purpose programmable microprocessors, Digital Signal Processors (DSPs), Application Specific Integrated Circuits (ASICs), Programmable Logic Arrays (PLAs), Field Programmable Gate Arrays (FPGAs), Graphics Processing Units (GPUs), special purpose electronic circuits, and the like, or combinations thereof.

The present disclosure is directed to various aspects, including the methods, further methods, systems, devices, and product means described above and below, each yielding one or more of the benefits and advantages described in connection with one or more other aspects and each having one or more embodiments corresponding to the embodiments described in connection with one or more other aspects described herein and/or as disclosed in the appended claims.

In particular, another aspect disclosed herein relates to embodiments of a computer-implemented measurement process comprising:

-determining a spatial configuration of a plurality of transducers attached to the target object by performing steps of a method according to the first aspect described above and below;

-receiving further sensor signals from the plurality of transducers, the received further sensor signals being indicative of a respective motion of the respective transducer during the induced motion of the target object;

-performing structural analysis calculations based on the received further sensor signals.

Further sensor signals may be obtained based on movements suitable for performing the intended structural analysis calculations. Typically, the further sensor signals may be indicative of the motion of the respective transducer during a further induced motion of the target object (different from the induced motion for which the spatial configuration of the transducer has been determined). In addition to or instead of rigid body motion, further induced motion may include bending motion. Further induced motion may or may not include rigid motion in all six degrees of freedom, depending on the desired structural analysis to be performed.

Yet another aspect disclosed herein relates to an embodiment of a method of determining a spatial configuration of a plurality of transducers attached to a target object; the plurality of transducers comprises a first subset of transducers and a second subset of transducers; the method comprises the following steps:

-inducing a motion of the target object;

-receiving sensor signals from the first and second subsets of transducers, the received sensor signals being indicative of respective movements of the respective transducers;

-obtaining a spatial configuration of the transducers of the first subset;

-determining the spatial configuration of the transducers of the second subset from the received sensor signals and the obtained spatial configuration of the transducers of the first subset.

In some embodiments, the spatial configuration of the first subset of transducers may be obtained prior to inducing motion of the target object and/or prior to receiving the sensor signals. However, it should be understood that in embodiments where the spatial configuration of the first subset of transducers is determined based on the received sensor signals, the spatial configuration of the first subset of transducers is obtained after receiving the sensor signals.

The induced motion of the target object may include applying one or more forces to the target object, for example, by manually swinging the target object and/or striking the target object with an energizing hammer. In some embodiments, the motion should include components along all available degrees of freedom. For a target object that can move in all six degrees of freedom, the induced motion should include components in three orthogonal directions and rotational components about three linearly independent axes. To this end, the target object may be suspended from an elastic band, supported on an air cushion or otherwise supported such that the object may perform at least small movements in three orthogonal directions and at least small rotations about three linearly independent axes. In other embodiments, the target object may be supported such that its motion is constrained along one or more degrees of freedom, as will be described in more detail below.

Yet another aspect disclosed herein relates to an embodiment of a measurement process, the measurement process comprising:

-determining the spatial configuration of the plurality of transducers attached to the target object by performing the steps of the method of determining the spatial configuration of the plurality of transducers attached to the target object described above and below.

-inducing further motion of the target object;

-receive further sensor signals from the first and second subsets of transducers, the received further sensor signals being indicative of a respective motion of the respective transducer during the induced further motion;

-performing structural analysis calculations based on the received further sensor signals.

In particular, although it may be advantageous to avoid or at least suppress bending modes for the purpose of determining the spatial configuration of the transducer, the induced further motion on which the structural analysis is performed may also include significant bending motions which need not be suppressed, but may in fact be of interest for the purpose of structural analysis.

Yet another aspect disclosed herein relates to embodiments of a computer program configured to cause a data processing system to perform the steps of the computer-implemented method described above and below. The computer program may comprise program code means adapted to cause a data processing system to perform the steps of the computer implemented method described above and below when executed on the data processing system. The computer program may be stored on a computer readable storage medium or embodied as a data signal. The storage medium may include any suitable circuitry or device for storing data, such as RAM, ROM, EPROM, EEPROM, flash memory, magnetic or optical storage devices (such as CD ROM, DVD, hard disk), and so forth.

Yet another aspect disclosed herein relates to embodiments of a measurement system comprising a data processing system configured to perform the steps of the computer-implemented method described herein. The measurement system may further include:

-a plurality of input channels configured to be coupled to respective transducer output signals of a plurality of transducers attached to a target object via respective wireless or wired signal connections;

a data input interface configured to receive respective spatial coordinate data of the first subset of transducers or at least information about spatial relationships between the first subset of transducers, e.g. by manual data input or from another apparatus via a wireless or wired data communication link.

The measurement system may include a display showing the determined spatial configuration of the plurality of transducers coupled to the plurality of signal input channels.

Yet another aspect disclosed herein relates to an embodiment of a measurement assembly that includes a plurality of transducers mountable at a plurality of predetermined measurement locations distributed on a target object. The measurement assembly further comprises a measurement system as described above and below.

Drawings

The above and other aspects will be apparent from and elucidated with reference to the embodiments described hereinafter with reference to the accompanying drawings, in which:

fig. 1A-1C schematically illustrate examples of measurement assemblies disclosed herein. In particular, fig. 1A shows a schematic block diagram of a measurement assembly, fig. 1B shows a schematic diagram of a transducer of the measurement assembly, and fig. 1C shows an example of a coordinate system for the purposes of this specification.

Fig. 2 shows a flow chart of an example of a measurement process disclosed herein.

Fig. 3 shows a flow diagram of an example of a computer-implemented process for determining a spatial configuration of a transducer as disclosed herein.

Fig. 4A-4B illustrate measured frequency response functions of an example target object.

Fig. 5 shows an example of normalized singular values determined for an example motion of a target object.

Fig. 6A-6B schematically show displayed 3D digital models of target objects indicative of the determined spatial configuration of the respective accelerometers.

Fig. 7 shows a flowchart of an example of a computer-implemented process for determining the spatial configuration of a second subset of transducers (particularly a three-axis accelerometer) as disclosed herein.

Fig. 8 shows an example of a deformable target object whose motion is constrained.

FIG. 9 shows a diagram of an example of a target object with a transducer mounted thereon.

Fig. 10A-10B illustrate position and orientation errors of a determined spatial configuration as determined by an example of the process disclosed herein.

Detailed Description

Fig. 1A-1C schematically illustrate examples of measurement assemblies disclosed herein. Specifically, for purposes of this description, FIG. 1A shows a schematic block diagram of a measurement assembly, FIG. 1B shows a schematic diagram of a transducer of the measurement assembly, and FIG. 1C shows an example of a coordinate system.

The measurement assembly includes a measurement system 110 and a plurality of tri-axial accelerometers 100 attached to a target object 120 and communicatively coupled to the measurement system 110.

The tri-axial accelerometer 100 may comprise, for example, a tri-axial piezoelectric accelerometer of the type 4524, 4524-B or 4504, all manufactured by briel and Kjaer sound and vibration measurement corporation of Naerum, denmark. The skilled person will appreciate that other types of transducers, for example other types of accelerometers, may be used in alternative embodiments. For example, other types of single or multi-axis accelerometers from many other manufacturers may of course alternatively be used to implement the invention. Depending on certain structural details of the accelerometer, one or more piezoelectric transducer elements 101 may be mounted inside the transducer housing 107, which respond to acceleration in a particular direction (designated for purposes of this specification as the i, j, k directions) relative to the three orthogonal spatial directions of the accelerometer by generating a sensor signal (e.g., a voltage or current proportional to the acceleration in that particular direction). The directions i, j, k thus define the measurement coordinate system of the accelerometer. Various types of electronic signal conditioning circuitry and/or storage devices may likewise be mounted on suitable carriers within the transducer housing 107, such as low noise preamplifiers, filters, a/D converters, power supplies, and the like. Electronic signal conditioning circuitry may be coupled to respective output terminals of one or more piezoelectric transducer elements to provide a low impedance and possibly frequency shaped output signal of the accelerometer 100 for each sensitivity direction. The accelerometer housing 107 may include a metallic composition or material, such as titanium, to protect the transducer elements from various harmful environmental contaminants (e.g., humidity, mechanical shock, dust, light, heat, etc.). The metal accelerometer housing 107 may also be used for EMI shielding purposes. The accelerometer housing 107 includes a plurality of optional slots 109a, 109b, the optional slots 109a, 109b supporting mounting clips for attaching or fitting the accelerometer 100 to a plurality of different target objects. The lower, generally planar outer housing surface of the accelerometer housing 107 (hidden from view-disposed opposite the outer housing surface 101) is in physical contact with the target object, either directly or indirectly (e.g., via a suitable adhesive or adhesive layer). Thus, the lower, generally planar outer housing surface of the accelerometer housing 107 functions as an engagement or coupling surface with the target object.

The tri-axial accelerometer 100 further comprises an electrical connector 102, which electrical connector 102 may comprise a 4-pin connector comprising a common ground terminal and three separate output signal terminals carrying respective accelerometer i, j, k component output signals representing respective first, second and third orthogonal axes of sensitivity of the transducer elements of the tri-axial accelerometer 100.

The i-component output signal is indicative of the acceleration of the accelerometer 100 in a predetermined i-direction of the accelerometer projecting perpendicularly through the first planar outer housing surface 101. The same applies to j and k component output signals indicative of acceleration along the j and k axes, respectively, to provide true three-axis acceleration measurements of the vibration of the target object. The target object 120 may, for example, include an automobile body, an aircraft structure, a train structure, a wind turbine blade, a satellite structure, an engine, a transmission, or the like, or a component thereof. The electrical connector 102 may be used to couple the i, j, and k component output signals to the measurement system 110 via a suitable cable (e.g., a low noise shielded cable). The skilled person will appreciate that the actual measurement setup may comprise a plurality (e.g. more than 20 or even more than 100) of individual tri-axial accelerometers 100 coupled to the measurement system via suitable cables.

The measurement system 110 may include a suitable accelerometer instrumentation system in conjunction with various types of data acquisition software executing on a personal computer or other computing hardware platform. In particular, the measurement system may include a data acquisition interface 113, a data processing unit 111, a memory 112 or other data storage device, and a user interface 114.

The data acquisition interface 113 may include a connector to allow each of the plurality of tri-axial accelerometers to be coupled to a particular measurement channel of the measurement system.

The data processing unit 111 may for example be a suitably programmed central processing unit of a computer or other data processing system. The data processing unit 111 may also be configured to execute a structural analysis software application that performs structural analysis calculations with respect to the target object and based on the sensor signals received from the accelerometer 100.

The user interface 114 may include a display configured to graphically depict the status and identifier of each measurement channel and the identification and status information of each accelerometer, for example, by serial number. The display may also allow for displaying a digital 3D model of the target object, the digital 3D model indicating the determined position and orientation of the accelerometer.

The measurement system associates or links the determined spatial 6-dimensional position and orientation data of each tri-axial accelerometer 100, and optionally their serial number and/or with other information about the respective accelerometer, with a particular measurement channel connected (via the 4-pin connector 102) to a respective output signal of the i, j, k output signals of the respective accelerometer, which represents the i, j, k components of the acceleration of the tri-axial accelerometer 100 on the target object, respectively. It should be understood that the measurement system may comprise a plurality of individual components. The components may be communicatively coupled to each other to allow data transfer. For example, the measurement system may include a data acquisition system configured to record sensor signals from the transducer and a data processing system configured to analyze the recorded sensor signals.

For purposes of the following description of some embodiments of a process for determining the spatial configuration of a transducer relative to a target object, the target object 120 may be considered a rigid body Ω that may move unrestricted in 3D space. The position and orientation of the subject is defined in an inertial Global Coordinate System (GCS). Let us define a Local Coordinate System (LCS) fixed to the body. Let us consider point P_nN triaxial accelerometers 100 attached to the body, and theyCoordinates in LCS ofFor ease of illustration, only a single accelerometer is explicitly shown in FIG. 1C; it will be appreciated that the number N of accelerometers will typically be greater than 1, in particular greater than 3, and typically greater than 10 or even greater than 100.

Each accelerometer 100 measures the appropriate acceleration relative to the (inertial) GCS; the acceleration vector is provided via three scalar components measured along the measurement axis of the accelerometer. Let us then define N Measurement Coordinate Systems (MCS) associated with the measurement axis of each accelerometer. The position and orientation of the MCS is fixed in the LCS.

Since the LCS is fixed to the target object, the position and orientation of the LCS is fully indicative of the position and orientation of the target object when the target object appears to be a rigid body. At any time t, the given vector in GCS { C (t) }_G∈R^6×1，

The position and orientation of the LCS in the GCS is fully described, i.e. the vector describes the spatial configuration of the local coordinate system associated with the target object. Here, theIs the radius vector of the LCS origin, θ₁(t)；θ₂(t)；θ₃(t) are the three Euler angles which define the continuous rotation that brings the GCS to the LCS.

There do exist different euler angle equations in which we use the tympan (Tait-Bryan) z-y' -x "convention (also known as the seagoing or kadan angles): yaw, pitch and roll). Let the coordinate of point P in GCS be { r }_GThe coordinates of point P in the LCS are { r }_LAnd the origins of the GCS and LCS coincide. Then, the coordinates { r }_LCan be obtained as three successive rotations, first rotation θ around the z-axis of the GCS₁Then, thenRotation of theta about a new y-axis₂And finally rotated by theta about the new x-axis₃：

Where each rotation is represented by a pre-multiplication of the radius vector and the rotation matrix. The combination of the three rotation matrices produces a rotation matrix [ R ]_GL]. The backward coordinate transformation from LCS to GCS can be expressed as { r }_G＝[R_LG]{r}_LWherein, in the step (A),

[R_LG]＝[R_GL]^-1＝[R_GL]^T (3)

the second equation here is because the rotation matrix is an orthogonal matrix. Using a rotation matrix, a point P in the GCS_nThe coordinates of the accelerometer in (a) can be expressed as its translation in the GCSAnd the sum of the rotations, both of which are time dependent,

{r_n(t)}_G＝{r₀(t)}_G+[R_LG(t)]{r_n}_L (4)

the acceleration vector measured at this time will be

Attached to point P_nThe three-axis accelerometer provides three signals a_n，i(t)、a_n，j(t) and a_n，k(t) which correspond to the acceleration vector (5) projected onto the MCS axis associated with the accelerometer

Wherein, { i }_n(t)}_G、{j_n(t)}_GAnd k_n(t)}_GIs the position (orts) of the MCS presented in the GCS.

The position coordinates in the GCS can be obtained by a series of successive rotations of the MCS first to the LCS and then to the GCS:

wherein [ R ]_ML，n]＝[R_LM，n]^-1＝[R_LM，n]^TAnd [ R ] is_LM，n]Is to indicate and install at point P_nA matrix of LCS to MCS rotations associated with the accelerometer at.

A set of euler angles phi may be used_n，1、φ_n，2And phi_n3It is configured in a similar manner to (2):

wherein, c_l，n＝cosφ_l，nAnd s and_l，n＝sinφ_l，n，l＝1...3。

substituting (5) and (7) into (6) and rewriting in matrix form,

without loss of generality, we can assume that initially, when the subject is stationary, the LCS coincides with the GCS (otherwise additional translations and rotations can be applied to move the GCS to its initial position, and then a backward transformation of the result is applied): [ R ]_LG(0)]＝[E]. Let us now assume that the movement of the body, w.r.t., is small in its initial position:

here, epsilon is used as a book-keeping device (book-keeping device) to indicate the order of magnitude of the appended addition.

Substituting (2) into (9) and carrying out differentiation twice, wherein the result can be expanded into Taylor series, thereby obtaining

Neglecting orders of magnitude greater than or equal to ∈²And rearrange the terms of

(12) Two matrices in (2) can be combined into a time-invariant 3 x 6 matrix beta_n]＝[β_n(r_n，x，r_n，y，r_n，z，φ_n，1，φ_n，2，φ_n，3)]The element of which is a given point P in the LCS presented using three Euler angles providing a rotation from the LCS to the MCS_nAnd a non-linear function of the orientation of the accelerometer.

Note that none of these parameters are assumed to be small.

In summary, in the case of small movements, the signal measured by the nth triaxial accelerometer can be approximated as a matrix [ β [ ]_n]The product with vector c (t), differentiated twice with w.r.t. time:

to simplify the representation, let us introduce the vector a_n(t)}＝{a_n，i(t)，a_n，j(t)，a_n，k(t)}^TExpression (14) is thus compressed as:

thus, in general, the acceleration measured by the accelerator can be calculated from the transducer transfer matrix associated with the transducer and from the spatial configuration of the local coordinate system of the target object (differentiated twice with respect to time). The transducer transfer matrix represents the spatial configuration of the accelerometer's measurement coordinate system relative to the target object's local coordinate system.

Fig. 2 shows a flow chart of an example of a measurement process disclosed herein. The measurement process may be performed, for example, using the components described in connection with fig. 1A-1C.

In an initial step S10, a target object (e.g., the object 120 shown in fig. 1A and 1C) on which structural analysis is performed is installed at a test site. It will be appreciated that the type of installation may depend on the characteristics of the target object (such as its volume, mass and structural integrity) and on the purpose of the measurement to be made, for example the type of structural analysis to be performed. For example, when the target object is mounted on a rubber band and/or air cushion, its rigid body motion in any direction remains substantially unrestricted. However, it will be appreciated that in some embodiments it may be desirable to partially constrain the mounting of rigid body motion of the target object.

In a subsequent step S20, a plurality of transducers (such as accelerometers, for example the three-axis accelerometer shown in fig. 1B) are mounted to the target object so that they can record the movement of the target object. For example, the transducer may be distributed and attached on the surface of the target object, e.g. by suitable mounting elements, adhesives or the like. The transducers are mounted such that the spatial configuration (i.e. position and orientation) of the first subset of transducers relative to the target object is known or determined manually, e.g. measured. For example, the transducers of the first subset may be positioned at easily identifiable locations of the target object and aligned with easily identifiable directional features of the target object, such as at corresponding edges, corners, or similar surface features. The position of the reference transducer may be provided as its coordinates w.r.t.lcs. The orientation of the reference transducer relative to the LCS may be provided using any suitable convention for defining the orientation. For example, they may be provided as three Tetbutyne angles according to the z-y' -x "convention. This angle corresponds to a continuous rotation from the LCS to the MCS of the reference transducer. In general, it may be preferable to position the reference transducer such that the area surrounded by the reference transducer covers most of the target object. As will be described in more detail below, in some embodiments, only information about the spatial relationship between the reference transducers (e.g., their mutual distance from each other) need be input, rather than their full spatial configuration.

When the transducer comprises a three-axis accelerometer, the first subset preferably comprises at least three-axis accelerometers positioned non-along a straight line. For the purposes of this specification, the first subset of accelerometers will also be referred to as reference accelerometers.

In step S30, a measurement system, such as the measurement system 110 of the embodiment of fig. 1, is initialized. To this end, the transducer is communicatively connected to a data acquisition interface of the measurement system. Furthermore, known spatial configurations of the reference transducers (or at least information about their spatial relationship) may be input into the measurement system. Also, digital models (e.g., CAD models and FE models, etc.) of the target object may be loaded into the memory of the measurement system. Finally, one or more operating parameters of the signal processing to be performed by the measurement system may be set, such as adjustable attenuation, filter parameters, etc. For example, the value of the high-pass filter to be applied to the acquired sensor signal may first be set to a small value, e.g. less than 1Hz, such as 0.7 Hz. It will be appreciated that suitable values may depend on the particular data acquisition system and/or the characteristics of the target object to be tested.

In some embodiments, some parameters may be set based on initial measurements. In particular, for the purpose of determining the spatial configuration of the transducers other than the reference transducer, it may be desirable to filter the sensor signals in order to suppress frequencies associated with bending modes of motion of the target object while maintaining frequencies associated with rigid body modes of motion of the target object.

To this end, the target object may be excited, for example, using a modal hammer, a hammer-like object, or in another suitable manner. The measurement system may acquire sensor signals from the transducer in response to an excitation (e.g., in response to a modal hammer or impact of a hammer-like object). This step may be repeated for a plurality of excitations, e.g. at respective excitation points. The process may calculate a frequency spectrum or frequency response function for all transducers and estimate the natural frequencies of the lowest bending mode and the highest rigid body mode. The process may then estimate the cut-off frequency of a low-pass filter to be applied to subsequent sensor signals for the purpose of determining the spatial configuration of the transducer. In particular, the cut-off frequency may be chosen such that the low-pass filter will retain information about rigid body motion, but not bending motion of the target object. Further, the data acquisition portion of the measurement system may then be configured to record at a sampling frequency greater than a predetermined threshold. The threshold may be selected according to the determined cut-off frequency of the low-pass filter (e.g., 2.56 times the cut-off frequency of the low-pass filter).

In step S40, the process acquires a sensor signal from the transducer. To this end, a motion (in particular a rigid body motion) of the target object may be induced, and a sensor signal responsive to the induced motion may be acquired.

In particular, data acquisition may be initiated and the target object may be oscillated in such a way that it experiences all 6 degrees of freedom (3 translational degrees of freedom and 3 rotational degrees of freedom). This can be done, for example, by swinging the suspended object by hand or manually by using a heavy hammer with a very soft tip. Since the resonant frequency of rigid body modes is typically very low, it may be difficult to excite rigid body modes to have sufficient signal-to-noise ratio in the acceleration signal. Therefore, it may be advantageous to record sufficient data based on multiple wobbles in each of the 6 degrees of freedom (e.g., based on at least 20 to 40 wobbles in each of the 6 directions). The acquired sensor signals may be represented as a time series of measurements. In some embodiments, where only the spatial relationship of the first subset of transducers is initially input, this step may be followed by calculating the complete spatial configuration of the first set of transducers.

In a subsequent step S50, the process calculates the spatial configuration of the transducers other than the reference transducer from the known spatial configuration of the reference transducer and from the acquired sensor signals. An example of a calculation process for calculating the spatial configuration of the transducers will be described in detail below with reference to fig. 3. As will be apparent from the following description, as part of the calculation, the process may detect to what extent the acquired data is suitable or sufficient to calculate the spatial configuration of all transducers with the desired accuracy. If such a calculation is not possible, the process may alert the user. The user may then repeat the data acquisition, for example by acquiring more data to improve the signal-to-noise ratio, by readjusting certain parameters (e.g. the cut-off frequency of the low-pass filter) or by moving the accelerometer to another location (keeping the reference accelerometer in place) and repeating the measurement.

Once the spatial configuration of all the transducers is determined, the process may proceed with the actual data acquisition to perform the desired structural analysis. For this, additional data may be acquired in step S60. In particular, the target object may be moved to excite both bending and rigid body modes, and further sensor signals from all transducers may be acquired. During this step, the low-pass filtering previously used to suppress the bending mode can be omitted.

In step S70, the process may perform the desired structural analysis calculations based on the acquired additional data and based on the known and determined spatial configurations of all the transducers. In general, the term structural analysis is intended to cover any computational method based on recorded transducer signals that yield information about the behavior of a target object when subjected to a force and in particular to a dynamic load. Examples of structural analysis may include analysis of dynamic displacements, time history and modal analysis, vibration analysis, and the like. In some embodiments, the structural analysis may include finite element analysis, for example, to calculate mode shape and frequency.

Fig. 3 shows a flow diagram of an example of a computer-implemented process for determining the spatial configuration of a transducer, in particular a three-axis accelerometer, as disclosed herein. This process may be performed, for example, by measurement system 110 described in conjunction with FIG. 1A.

In an initial step S51, the process receives input data. Specifically, the process receives the position and orientation of the respective reference accelerometer. For example, the location may be received as coordinates of a reference accelerometer w.r.t.lcs. The orientation of each reference accelerometer may be received as three tynblien angles following the z-y' -x convention. The angle corresponds to a continuous rotation of the reference accelerometer from LCS to MCS. The position and orientation may have been manually entered, determined by another automated process, or calculated based on input information referencing the spatial relationship between the transducers and based on the sensor signals.

The process also receives sensor signals of all accelerometers that have been recorded in response to the induced motion of the target object, e.g., the record acquired in step S40 of the process of fig. 2. The process may also receive a table of channels of the data acquisition interface to which the accelerometer is connected during the measurement, and an indication of which channels correspond to the reference accelerometer, i.e., which channels correspond to the accelerometers of the first subset and which channels correspond to the accelerometers of the second subset.

The process may also receive additional information, such as the cut-off frequency of the low-pass filter described in connection with step S30 of the process of fig. 2.

In a subsequent step, the process calculates the spatial configuration of the second subset of accelerometers. Before discussing the various steps, the overall method of calculating the position and orientation of the accelerometers of the second subset will be described in more detail:

let us assume that the first subset of transducers comprises data from the setR reference accelerometers, we call setIs a reference set. Reference setThe position and orientation of all accelerometers in the stack are known. Superimposing expressions for all accelerometers in the set (15)

Wherein, the matrix [ beta ]_r]R1.. R is also referred to as a transducer transfer matrix; the transducer transfer matrix of the reference accelerometer is known from (13). Each transducer transfer matrix associates the sensor signals of the corresponding transducer with a second derivative vector representing the position and orientation (i.e. acceleration) of a rigid body of the target object (i.e. the local coordinate system LCS associated with the target object). Accelerometer signal { a_r(t) } may be used for the reference accelerometer. The vector may then be transformedThe estimate is the least squares mean of the measured accelerations:

here, the symbolsRepresenting the pseudo-inverse of the matrix.

Matrix arrayFormed by a transducer transfer matrix of a reference accelerometer; it has 3R rows and 6 columns. In order to make equation (17) solvable, the matrixShould be full rank, so it should have at least 6 rows. However, two triaxial reference accelerometers (R ═ 2) are not sufficient to constitute a full rankA matrix and need forAt least three reference accelerometers (R ═ 3) (and they should not be placed on a straight line).

Let us assume that the above conditions are satisfied, and can be obtained using (17)Let us consider another set of accelerometersAttached to the same body and from the collectionThe accelerometers of (1) measure synchronously. Accelerometer readings are available; however, their position and orientation are unknown. For each accelerometerIt reads and(15) cf. Linear correlation

From (12), the matrix [ beta ]_q]Is r_q，x，r_q，y，r_q，z，φ_q，1，φ_q，2，φ_q，3And an analytical (non-linear) expression may be used for each element of the 3 x 6 matrix.

By solving for the unknowns r in (18)_q，x，r_q，y，r_q，z，φ_q，1，φ_q，2，φ_q，3Can be selected from the groupThe position and orientation of any accelerometer.

A solution to this (non-linear) problem is possible. Two examples of methods for solving the non-linear problem are disclosed below:

the method comprises the following steps:

as described above, the matrix [ β ] in (18) may be expressed_q]Regarding as a transfer matrix; one of the known estimators (i.e., H) from multiple-input multiple-output (MIMO) modal analysis may be used (i.e., H)₁、H_vOr H_s) (see, e.g., J.S.Bendat, A.G.Piersol, "Engineering applications of correlation and spectral analysis", John Wiley&Sons, inc., 1980), based on measured { a }_q(t) } and estimatedTo estimate. For at t_iM sampled signals, let us present accelerometer readings and will do soAs a matrix:

thus (17) becomes

Obey the simplest H₁An estimator for estimating the time of the measurement,

equivalent to element

18 non-linear equations of 6 unknowns can be solved. Since accelerometer readings are contaminated by noise, this can be expressed as finding the production matrix equation (22)) Of the best fit of the elements in (1)_q，x，r_q，y，r_q，z，φ_q，1，φ_q，2，φ_q，3So that [ epsilon ] is as follows]Or some norm thereof.

The second method comprises the following steps:

instead of fitting 18 elements of the transfer matrix, the measured time histories may be fitted. Let us consider:

wherein the content of the first and second substances,is a vector for a variable v_q}＝{r_q，x，r_q，y，r_q，z，φ_q，1，φ_q，2，φ_q，3}^TThe calculated estimated reading of the accelerometer q. The difference between the measured signal and the estimate is

And the rows of the matrix on the left can be considered as prediction errors for the x, y and z measurement axes of the accelerometer. The scalar fit error for each measurement axis can be expressed as the mean square:

where to the left of expression (26) is a positive scalar function of the vector of variables { v }, which can be combined into a positive scalar function:

E({v_q})²＝E_x({v_q})²+E_y({v_q})²+E_z({v_q})². (27)

then the minimization problem can be expressed as:

compliance

{v}_min≤{v_q}≤{v}_max

The latter proposes possible constraints, e.g. the position of the accelerometer should not be outside the target object, and the euler angles are generally obeyed

Each of the above two methods can be used alone as an independent method. Alternatively, both methods may be used in combination. Although method 1 above may converge faster, method 2 may result in a more accurate estimate. Thus, in some embodiments, method 1 may be used to calculate an initial estimate, while method 2 may be used to refine the output of method 1.

Generally, the above methods make many assumptions. Specifically, the above method assumes that the target object moves as a rigid body, the displacement of induced motion is small, and the target object moves in all six degrees of freedom.

The accuracy of embodiments of the method disclosed herein depends on how well the above assumptions are met in actual measurement situations.

This will be discussed in more detail below.

In typical measurement situations, the target object does not move as an ideal, unconstrained rigid body. However, there are many measurement scenarios when the target object is supported by e.g. a soft rubber band and/or an air cushion in order to simulate the dynamics of the object in free conditions. This is a typical scenario for Experimental Modal Analysis (EMA) for validating/adapting FE models of objects. The selection of the stiffness of the support may be based on the optimal separation of the frequency of the rigid body mode (defined by the inertia of the object and the stiffness of the support) and the flexible mode resonance (dependent on the stiffness of the object); a sampling Frequency Response Function (FRF) or accelerometer auto-spectrum may be used to check this, for example, as shown in fig. 4A-4B. Fig. 4A shows the FRF of the target object in the frequency range between 0Hz and 900Hz, and fig. 4B shows an enlarged view of the FRF in the frequency range of 0Hz to 50 Hz. As can be seen from fig. 4A-4B, the lowest bending mode occurs at about 462HZ, while the highest rigid body mode occurs at about 6.5 HZ. Thus, a good separation between the bending mode and the rigid mode is indeed possible. Thus, by applying a low pass filter and setting its cutoff frequency above the highest rigid body mode and below the lowest bending mode, it is possible to significantly attenuate the flexible component of the response and satisfy the assumption of rigid body motion.

With respect to the motion of the target object in all six degrees of freedom, to find all six unknowns to solve for (18), the matrixShould be full rank (i.e., its rank should be equal to six). Rewriting expression (17) for the sampled accelerometer signal, e.g. (19)

According to the Servicester inequality (Matrix Cookbook)

Has already discussedRank of (d): the full rank of the matrix can be easily achieved by positioning the reference accelerometers in the appropriate way (recall that it should be three or more reference accelerometers, not mounted along a straight line). And ifFor full rank, its pseudo-inverse is also full rank:

therefore, the temperature of the molten metal is controlled,is thatA requirement for full rank. To this end, during data acquisition, the target object should undergo motion in all six degrees of freedom, and particular care should be taken to excite the target object so that it undergoes motion in all six directions.

By examining the singular values after application of a low-pass filterIt can be checked how this assumption is fulfilled, which will be discussed below.

To obtain expression (11), we assume that the translational displacement and the rotational displacement are small. In practice, it has been found that this assumption is satisfied when the rotational displacement θ is small enough to satisfy sin θ ≈ θ, cos θ ≈ 1; at most two decimal values, if | theta | is less than or equal to 10⁰Then the situation is satisfied. To have the same amplitude, the translational displacement d should be in the range of | d/l | ≦ 0.17, where l is some characteristic length of the target object.

On the other hand, it should be noted that some reasonable displacement should be excited to ensure a good signal-to-noise ratio of the accelerometer signal, which may be difficult to achieve at very low frequencies.

The inventors have found that it is sufficient to excite the target object when the target object is moved by hand in all six directions, or excited with a relatively heavy hammer with a soft tip. It has been found that when using a hammer, it is helpful to hit the object at many points in random directions. In this way, the structure will experience oscillations around its initial position, which is considered to be a very useful way of implementing this assumption.

Referring again to fig. 3, in step S52, the process performs low pass filtering on the received sensor signal using the determined cutoff frequency.

In step S53, the process verifies whether the induced motion of the target object can be expressed as a rigid body motion and whether the motion occurs in all six degrees of freedom. To this end, the process may calculate singular values [ a (t) for all accelerometer signals_i)]And determines whether the difference and/or ratio between the singular values #6 and #7 is greater than a predetermined threshold. Fig. 5 shows an example of normalized singular values determined for an example motion of a target object. The gap between singular values #6 and #7 is indicated by arrow 551.

If the induced motion satisfies the above criteria, the process proceeds to step S54. Otherwise, the process returns an appropriate error message, e.g., instructing the user to repeat the step of inducing motion (e.g., returning to step S40 of the process of fig. 2).

At step S54, the process removes data corresponding to singular values greater than 6 (e.g., by performing singular value decomposition and then truncation), scales the coordinates so that the maximum distance between references is 1, and calculates a matrix using expression (16)

In step S55, the process compares the calculated matrixIs compared to a predetermined threshold. If the condition number is greater than the threshold, the process returns an appropriate message instructing the user to select a different position of the reference accelerometer, i.e., a position spaced further from the line (e.g., returning to step S20 of the process of FIG. 2), and repeat the data acquisition. Otherwise, the process advances to step S56.

In step S56, the process calculates the acceleration of the local coordinate system of the target object using expression (17), i.e., the process calculatesAs described above, this step may include calculating a least squares solution to the system of linear equations.

In step S57, the process calculates the spatial configuration of each accelerometer of the second subset, i.e., the position and orientation of each non-reference accelerometer. An example of a method for calculating the spatial configuration of the second subset of accelerometers will be described in more detail below with reference to figure 7.

When the spatial configuration of all reference accelerations has been calculated, the process proceeds to step S58, where all coordinates are rescaled back to the original scale in step S58.

In step S59, the process stores the determined spatial configuration for use during subsequent measurements. 6A-6B, the process may also present the results to the user, for example, via a graphical user interface. Figure 6A schematically shows a displayed 3D digital model of a target object indicative of the determined spatial configuration of the respective accelerometer. Fig. 6B shows an enlarged view of a portion of the displayed 3D digital model.

Fig. 7 shows a flowchart of an example of a computer-implemented process for determining the spatial configuration of a second subset of transducers (particularly a three-axis accelerometer) as disclosed herein. This process may be performed, for example, by measurement system 110 described in conjunction with FIG. 1A.

In initial step S571, the process calculates a transfer matrix using expression (21)

In step S572, the process calculates a starting point, for example as an average of the positions of the reference accelerometers.

In step S573, the process calculates an initial estimate of the unknown position and orientation, for example using Newton' S method.

If Newton' S method converges, the process proceeds to step S575; otherwise, if the method does not converge, the process proceeds to step S574, where the process adds a random displacement to the starting point, and returns to step S573. If the algorithm does not converge after a predetermined number of attempts (e.g., after 5 to 10 attempts), the process flags the current accelerator as erroneous and proceeds to the next accelerometer. The accelerometers marked with errors may have to be repositioned or their positions have to be determined manually.

In step S575, the process stores the calculated position and orientation of the current accelerometer.

Alternatively, it has been found that when the process performs the additional minimization step S576, the accuracy of the result can be further improved. In particular, the additional minimization step may use the results of newton's method as a starting point, generate a set of constraints using, for example, expression (29), and run a minimization algorithm that minimizes a suitable objective function, for example, as defined by expression (28). The optimization problem (28) attempts to make the function E ({ v })_q})²Minimization, the function is 6 variables v_q}＝{r_q，x，r_q，y，r_q，z，φ_q，1，φ_q，2，φ_q，3}^TAnd periodic over the last three variables. Minimization is subject to linear constraints. The minimization may be performed by any suitable routine (e.g., FMINCON using MATLAB). The routine automatically selects an optimization method based on the type of function and constraints. For this particular optimization problem, there is no need to provide a Jacobian matrix (Jacobian) and a Hessian matrix (first derivative w.r.t. variables and second derivative w.r.t. variables). However, they can be easily obtained in case another optimization algorithm requiring sensitivity is used. If the value of the objective function becomes smaller after minimization, the process may store the optimization results as a final estimate of position and orientation. The process may further store a quality indicator of the results, such as the relative error of the fit.

It should be understood that many variations of the above method are possible. In the following, alternative embodiments of a process for determining the spatial configuration of the transducers will be discussed. The above embodiments rely on knowledge of the position and orientation of the reference accelerometer: at least three accelerometers must be mounted in positions with known coordinates and carefully aligned with the GCS, for example. The accuracy of the position/orientation of the reference accelerometer directly affects the final position/orientation of the other accelerometers. In the embodiments of the process presented below, the requirement to determine the position and orientation of the reference accelerometer is significantly relaxed, replacing the need for an accurate reference accelerometer position and orientation with the distance between them. Thus, in this embodiment, in step S20 of the process of fig. 2, only the relative distance between the reference accelerometers mounted on the target object is determined. This embodiment will also be referred to as the no reference embodiment. It should be understood that instead of relative distances, other spatial relationships between transducers may be used as inputs to other non-referenced embodiments.

Assuming the distances between the three reference accelerometers, the method outputs the positions and orientations of the three reference accelerometers in an LCS attached to one of the three reference accelerometers. The output of this method can be considered as an input to the previously described method, so the position and orientation of all other accelerometers can be obtained based on the same dataset.

For this purpose, it is considered to mount three triaxial accelerometers on a non-deformable target object that can move unrestricted in 3D space. Let us assume that the target object experiences small displacements in all six degrees of freedom. Let us further use lettersAndthe accelerometer is labeled. Using the same convention as before, let us define the position and orientation of each accelerometer using its coordinates in the LCS and three tynbraine angles (using the z-y' -x "convention), which rotates the LCS of the accelerometer to the MCS. Let us use vectors for convenienceWherein the content of the first and second substances,is thatOrLet us now choose an accelerometerAs the origin of the LCS, and let us select the accelerometerThe MCS at its initial orientation acts as the orientation of the LCS, and therefore

The accelerometer readings are available: { a (t) } e R^9×1And we can divide the set of nine signals into two sets:

wherein, { a_r(t)}∈R^6×1And { a_q(t)}∈R^3×1For the purposes of this specification, we refer to the first set as the reference set and the second set as the test binding.

According to (29), can be according toThe vector estimates the acceleration in two sets:

will be provided withSeparating from the second equation and substituting into the first equation to obtain

{a_q(t)}＝[β_q][β_r]^-1{a_r(t)}. (34)

Matrix [ beta ]_r]And [ beta ]_q]Depends on the position and orientation of the accelerometer (13) and can be written asAnd [ beta ]_q]＝[β_q({v_b}，{v_c})]. For any { v_bAnd { v } and_cthe test set signal can be estimated from the reference set signal as:

wherein, the matrix

Which may be interpreted as a transfer matrix between signals from the reference set and signals from the test set.

The difference between the jth predicted signal from the test set and the corresponding measured signal is

In the form of a sample, the sample is,wherein the time sample t_iM, (i-1) Δ t, i ═ 1. The difference can then be characterized by its mean square error:

and for all three of the test channels,

the minimization problem can now be expressed as

Compliance

(40) The constraint in (1) is a geometric constraint-a constraint that does not allow finding the accelerometer position and euler angle outside the target object dimensions (29).

The inventors believe that the product isIs invariant to linear scaling, i.e. ifAndall linear dimensions in (a) are multiplied by the same scalar, the resulting matrix product is the same. Intuitively, it may be understood that different linear units are used when measuring linear distances (e.g., inches or millimeters). This means that the solution will contain a distance up to a multiplier; in the case of minimization, the minimization routine will have difficulty converging. A method of avoiding such a situationBy introducing additional constraints, e.g. providing three accelerometers for the optimization routineAndthe distance between them. These constraints would be:

wherein the content of the first and second substances,is an accelerometerAndfor estimated distance between, forAndthe allowable linearity tolerance is calculated.

Note that the reference set consists of six signals from all three accelerometers. A convenient (but not mandatory) method is by accelerometersAll three signals fromAnd one or two signals fromOf the remaining signal components, e.g. possible parametersThe test set can beWhere the subscript indicates the measurement direction of a given accelerometer. The test set should then include the remaining signals, i.e.The strategy for selecting the preferred signal will be discussed later.

The structure is as follows [ beta ]_r]And [ beta ]_q]The matrix is convenient: first, construct a full 9 × 6 matrix

And then extracts the rows corresponding to the reference set and the test set:

as does the time history. If it is notIs the set of all time histories from the three accelerometers, then

Since expression (35) relates to [ beta ]_r]Is therefore probably preferred to be { a_rThe six time histories in (t) } are as independent as possible. If written in sample form { a_r(t)}→[a_r(t_i)]M, i-1.. M, then it may be preferred to be a matrix [ a ]_r(t_i)]The content of the set R is selected in such a manner that the condition value of (b) is minimum. Since the number of possible combinations of signals in R is 6! (3! (6-3)!) 20, and SVD operations are very fast, so all possible combinations can be iterated and the combination given the minimum condition number is selected.

In summary, solving the minimization problem (40) with a set of constraints extended by (41) would provide an accelerometerAnd andin the accelerometerThe position and orientation in the LCS defined by the MCS of (a).

(40) An objective function ofIs a strong non-linear function of its 12 parameters, where the optimization problem is affected by a non-linear constraint (41). In some cases, the convergence of the optimization problem may be sensitive to choosing a good starting point.

To solve this problem, it may be beneficial to seek a relaxation formula (delayed formulation) that optimizes the problem. It is important to note that such relaxation formulas should still be practical.

Below, we propose a possible relaxation formula, in which the number of variables is reduced from 12 to 8. This may be achieved by defining the LCS in such a way that its XOY plane is horizontal. Because LCS is composed of accelerometerIs defined, thus in practice means that its Z-axis should be vertical. AccelerometerAndthe same should be true. If this requirement is met, the rotation of the latter two accelerometers from the LCS to the MCS involves only one rotation, i.e. the rotation angle around the Z-axis of the LCSAndand the other four angles are set to 0:

the practical implementation of this requirement is not particularly difficult to achieve: using a level, it is relatively easy to align the three accelerometers so that their XOY plane (or another plane of the MCS) is horizontal. Note that for the orientation of the accelerometers in the XOY plane (i.e., their orientation)Andangle) is not a special requirement, but in practice it is advantageous to know the approximate orientation of the accelerometer, using these values as a starting point for the minimization routine.

In fact, the non-relaxation optimization may follow the relaxation optimization as follows:

1. mounting three reference accelerometers according to relaxation formula

2. Approximate position and orientation using a reference accelerometerAndthe relaxation optimization is performed as a starting point.

3. After the relaxation formula converges, the resulting value can be used as a starting point for non-relaxation optimization to obtain a precise position and orientation.

4. Finally, the coordinates and orientation of the reference accelerometer are used to obtain the coordinates and orientation of all other accelerometers based on the same dataset.

This may improve the accuracy of the result, for example, in the case where the accelerometer is not perfectly aligned with the direction of gravity.

The optimization problem (40) seeks a real scalar positive objective functionIs a strongly non-linear function of 12 variables (8 variables in the relaxed case) and is periodic over some of them. The minimization is subject to a set of non-linear constraints (41).

MATLAB FMINCON, which is part of the MATLAB's optimization toolkit, is a convenient program to solve the minimization problem. In this particular formula, FMINCON requires sensitivity that provides an objective function and constrains a w.r.t. variable. It requires both the first derivative (jacobian matrix) and the second sensitivity (hessian matrix) of the objective function and the constraints. FMINCON may automatically calculate the derivative using finite differences.

However, where possible, it may be preferable to provide analytical (accurate) sensitivity. The derivation of the sensitivity is explained below.

The objective function (39) is a function of 8 or 12 variables depending on whether a relaxation or non-relaxation formula is considered. If all variables are combined into a single vectorThe Jacobian matrix of the objective function is a vectorThe ith element is

According to the definition of the objective function (39),

by means of the results of (35) to (38),

wherein the difference between the predicted signal and the measured signalIs composed of

Using a sampled representation of the signal: { a_r(t)}→[a_r(t_i)]And [ a ]_r(t_i)]∈R^6×M. Note that in { } or { [ 2 ]]The subscript j after the parenthesis indicates the jth element of the vector or jth row of the matrix, respectively. Here, the difference between the prediction signal and the measurement signal is

Differentiation of

The result is substituted into (48),

and using (36) we obtain

Although it is used forThe derivatives of the matrix are rather simple, but the derivatives of the inverse are more involved:

combining all the above and substituting (45) can obtain the analytical expression of the Jacobian matrix (except for matrix inversion, which is more convenient to calculate by using a numerical method).

The Hessian matrix is a square matrix with elements of

The elements of the hessian matrix may be calculated using the same considerations as the jacobian matrix.

Sensitivity of the constraint:

the constraint (41) can be restated as a standard constraint representation of the optimization problem

Wherein the value of the variable is givenIs estimated as

And the gradient of the constraint (54) is very simple. The constrained gradient is a 3 × 8 or 3 × 12 matrix; by way of example, the elements provided below are few.

The derivative w.r.t. euler angle is zero.

The embodiments of the above method assume that the target object is rigid and its motion is not constrained. In the following, embodiments will be described that can be applied to a flexible target object whose motion can be constrained. In many applications, such objects represent a significant part of the target object, wherein the proposed method may be helpful. Examples include wind turbine blades mounted on test benches, civil structures like bridges, towers, chimneys, etc.

Fig. 8 shows an example of a deformable target object whose motion is constrained, the latter being described by some boundary conditions. In the undeformed state of the target object 810, each point P of the target object has coordinates described by a radius vector, which is { r } in the GCS_GE Ω, where Ω is the domain that defines the target object. At any time t, the displacement of point P is represented by the vector d ({ r }_G，t)}_GDefine then its acceleration as

Here and further, herein, subscripts representing GCS are omitted.

Consider a target object in a non-deformed state 810. The biaxial accelerometer 811 is mounted on the object with coordinates GCS { r }_nAt a point of ∈ Ω and oriented such that it can be rotated by the matrix [ R ∈ ]_GM，n]Its MCS is obtained from the GCS.

When the target object vibrates, the three (scalar) signals sensed by the accelerometer are

Wherein, { i }_n(t)}，{j_n(t) } and { k }_n(t) } is the position of the MCS displayed in the GCS.

The position coordinates in the GCS can be obtained by:

wherein the content of the first and second substances,

[R_MG，n(t)]＝[R_GM，n(t)]^-1＝[R_GM，n(t)]^T (60)

is the rotation matrix from MCS to GCS. Using (59), (60) and combining the scalars in (58) into a vector { a }_n(t)}＝{a_n，i(t)，a_n，j(t)，a_n，k(t)}^TIn (b) can obtain

{a_n(t)}＝[R_GM，n(t)]{a({r_n}，t)}. (61)

Using the Carler gold method, it is possible to extend

Wherein { Ψ_k({ r }) }, k ═ 1.∞ is a set of mutually orthogonal time-dependent vectors that satisfy boundary conditions (so-called basic boundary conditions). D_k(t) is a time-dependent scalar. { Ψ_kA convenient (but not required) choice for ({ r }) } is the mode shape of the object, then D_k(t) is the so-called modal coordinate.

Within any given limited frequency range, (62) may be expressed as

Wherein the first sum contributes significantly to the overall displacement of the target object, whereas the latter is absent and can be neglected. Differentiates the w.r.t. time twice,

wherein the content of the first and second substances,

substituting (64) into (61) to obtain

Time dependent rotation matrix R_GM，n(t)]Can be expressed as a rotation matrix R of a non-deformed state_GM，n]And a rotation matrix [ R_MM′，n(t)]Wherein the subscript M' represents the MCS of the object in a deformed state:

[R_GM，n(t)]＝[R_GM，n][R_MM，n(t)]. (66)

the latter depending on three euler angles theta₁(t)，θ₂(t) and θ₃(t) which define the order of rotation from the non-deformed to the deformed MCS.

Let us assume that the vibration amplitude is small, and use e as a small term in the bookkeeper's labeling equation,

A_k(t)＝∈A_k(t)， (67)

and theta₁(t)＝∈θ₁(t)，θ₂(t)＝∈θ₂(t) and θ₃(t)＝∈θ₃(t) of (d). The construction of the rotation matrix is similar to (8); for smaller euler angles cos (∈ θ (t)) + 1+ O (∈ θ (t)))²) And sin (∈ θ (t)) ═ O (∈), the matrix becomes

[R_MM′，n(t)]＝[E]+[O(∈)]. (68)

Substituting (68) into (66), and then substituting (65) with (67), yields

Omitting orders of magnitude e²And higher terms and remove bookkeeper, weTo obtain

The latter sum can be rewritten as a product

Where k is the setIs indexed, andis its size. Therefore, the temperature of the molten metal is controlled,

now let us assume from the setIs known, we call the setIs a reference set. Thus forIs/are as followsAnd [ R ]_GM，n]Are known. The expressions for all accelerometers in the reference set are stacked together,

we also assume that the readings (time histories) of these accelerometers are available at any time t. Therefore, the time correlation coefficient on the right side of (73) can be estimated as

(74) A necessary condition for the solution is that the matrix in (73) is square or vertical, i.e., has a number of rows greater than or equal to the number of columns, or K.ltoreq.3R.

Described for obtaining the coefficient A_kThe process of (t) is similar to that of calculating Modal coordinates, which is detailed in, for example, (D.J. ewins, "Modal testing: Theory, practice and applications," Research students Press Ltd., 2000). In this case, the vector { Ψ }_i({ r }) } is the mode shape of the target object.

From the calculated coefficient A_k(t), the positions and orientations of the accelerometers not in the reference set (i.e. the second subset of accelerometers) may now be determined as follows:

for convenience, the expression (72) rewritten below indicates that, in a certain frequency range, it is located at the point { r }_nAre oriented such that their rotation matrix is [ R ]_GM，n]Can be estimated as

Here, we consider the accelerometer reading { a } as described in the previous section_n(t) } (three time histories) is available, vector { Ψ_k({ r }) } is known (or can be approximated) at any point { r }. epsilon.omega, and a time-dependent weight A can be found_k(t)。

For at t_iM, accelerometer reading { a ═ 1.. M_n(t) } and { A }₁(t)，...，A_K(t)}^TCan be represented as a matrix:

thus (75) becomes

Expression (77) is similar to (20), which is formulated for the unconstrained rigid body case, and shows the similarity between the two cases.

There are six unknowns: from three Euler angles phi_n，1，φ_n，2，φ_n，3Accelerometer coordinates described as a rotation matrix r_nAre Euler angles [ R ] and their orientation_GM，n]＝[R_GM(φ_n，1，φ_n，2，φ_n，3)]As a function of (c). Combining all six unknowns into one vector v_n}＝{r_n，x，r_n，y，r_n，z，φ_n，1，φ_n，2，φ_n，3}^TIs very convenient.

In the vector v_qAt the characteristic trial position/orientation, the estimated acceleration is

And at each sampling the error between the measured acceleration and the estimated acceleration is

The error for each measurement axis can be expressed in root mean square:

wherein, to the left of expression (80) is a variable vector { v }_qPositive scalar functions of which can be combined into a positive scalar function

E({v_q})²＝E_x({v_q})²+E_y({v_q})²+E_z({v_q})². (81)

Then, the minimization problem can be expressed as:

[R_GM，n]＝[R_GM(φ_n，1，φ_n，2，φ_n，3)]，_q})². (82)

satisfy the requirement of

{v}_min≤{v_q}≤{v}_max

The latter offers possible constraints, e.g. the location of the accelerometer should not be located at the target object ({ r })_n，x，r_n，y，r_n，z}^TE Ω) and euler angles are generally satisfied

The embodiments presented above illustrate how the method for finding the position/orientation of an accelerometer is applied to a deformable target object whose movement is constrained by some (sufficiently rigid) support. Examples of such target objects include many "in situ" tested objects, which are typically large civil structures, such as wind turbines, towers, bridges, etc.

The method assumes that a set of spatial functions { Ψ ] can be found_k({ r }) }, r ∈ Ω, which (i) satisfy the basic boundary conditions of the target object and (ii) are mutually orthogonal. A preferred set of such functions is for example a known set of pattern shapes from FE analysis of the target object. Any other set that satisfies the above conditions is valid, but they may require more shapes to be considered and, therefore, more reference accelerometers.

The method comprises the following steps:

-calculating a time correlationCoefficient A_k(t), K ═ 1.. K. The process is similar to acquiring modality coordinates, for example, in modality decomposition.

-calculating the position and orientation of the accelerometer by minimizing the fitting error.

Example (c):

FIG. 9 shows different views of an example of a target object with a transducer mounted thereon. As shown in fig. 9, the transmission case 901 is supported by a rubber band 902. Three accelerometers 903 type 4506B, available from briel & Kjaer sound and vibration measurements a/S of Naerum, denmark, were selected as reference accelerometers and mounted as shown in fig. 9. The accelerometer is aligned with its Y axis pointing vertically and upwards using a level UA-140 and a rotating base UA-1473, both available from briel & Kjaer sound and vibration measurements a/S of Naerum, denmark. The coordinates of the reference accelerometer were measured using a BIG FP 55003D Creator available from Boulder Innovation Group, inc. of Boulder, colorado, with a sub-millimeter accuracy of:

reference(red): 227.7; 248.5; 665.8 mm

Reference(blue): 179.2; 380.6, respectively; 22.9 mm

Reference(green): 376.9; 86.4 of the total weight of the mixture; 59.8 mm

The coordinates are given in GCS, measured when the transmission is standing vertically, with the Z-axis of the GCS pointing upwards. The eight corners of each sensor are digitized and the coordinates provided correspond to the geometric centers of the eight corners.

The main features of the structure (most pronounced edges and ribs) were digitized point-by-point by BIG FP 55003D Creator using ATC software that provides a high precision wire frame model. To improve visualization, the transmission case was also digitized using the sony 3D Creator application running on a sony Xperia XZ1 cell phone, which was exported to MATLAB via a Wavefront OBJ file. The latter consists of nodes and triangular faces.

More specifically, the following procedure is employed:

1. the reference accelerometer is mounted and aligned as described above.

2. Structure digitization using BIG FP5500

a. Obtaining coordinates of a center of a reference accelerometer (based on its corners)

b.31 points were structurally selected, indicated by crosses, numbered, and indicated by cross coordinates obtained from BIG FP 5500.

c. 12 out of 31 points were selected for checking the orientation accuracy: i.e. placed near a feature of the structure (e.g. a corner) to allow visual alignment of the accelerometer with the structure.

3. Measurement of six settings was done (named Setup [1, 2, 4, 5, 6, 7])

a. Each arrangement includes three reference accelerometers, an

b. Five further accelerometers 904 attached to the transmission housing at known locations.

c. Settings #4, 5, 6 are 4 specifically designed for measuring orientation accuracy, for each setting,

a. the position and orientation of all 8 accelerometers are calculated using the "no reference" embodiment described herein.

b. The obtained position of the accelerometer is compared to its known coordinates. This is done by "projecting" the center of the accelerometer to the surface of the structure using a representative measurement axis perpendicular to the surface.

c. For settings # #4, 5, 6, the positions of the MCSs are obtained, and the angles between the positions are calculated. This allows statistics to be made about the spread of locations, allowing conclusions to be drawn about orientation errors.

5. Based on 35 estimates, the mean positioning error is 11.2mm (σ ═ 4.8)

Fig. 10A shows a histogram of the positioning error. The histogram is based on a non-reference accelerometer. Since the reference accelerometers can also be considered as belonging to the test accelerometers, their positions can also be compared with the known positions, but they result in smaller errors. Therefore, they are not included in the statistical information. Examples of sensor positioning are shown in fig. 6A to 6B. In particular, fig. 6A shows a diagram of a digitized 3D model of the transmission housing 901, wherein the determined position and orientation of the accelerometer, e.g., setting #2, is shown. In fig. 6B, an enlarged portion of the transmission housing and two accelerometer positions are shown, including the known positions of the accelerometers represented by the black dots. As shown by the red arrow in fig. 6B, the positioning error is estimated as the distance between the black dot (obtained with the 3D Creator) and the second point, which is the estimated projection of the accelerometer center to the mounting surface.

Fig. 10B shows a histogram of the orientation error. For each setup, there are 5 test accelerometers, and thus 5 positions per orientation. For each pair of positions, calculating the angle between them (expressed as the inverse cosine of the scalar product of the positions); this results in 10 angles per direction, i.e. 30 angles for the three directions. For the three settings (#4, 5, 6), the statistical information is:

i. setting # 4: mean 4.3 °, 2.3 °

Setting # 5: mean 4.4 °, 2.0 °

Setting # 6: mean 6.0 °, 2.5 °

For setting #4, a histogram is shown in fig. 10B.

Based on the above, it can therefore be concluded that the method disclosed herein provides a reasonable estimation quality that is comparable to that achievable by current techniques and sufficient for typical applications.

Embodiments of the methods described herein may be employed during a test setup, particularly for determining the position and orientation of a transducer on a target object.

Embodiments of the methods described herein may also be employed to verify the mounting of the accelerometer, for example, in order to verify that the position/orientation of the accelerometer has not changed during testing.

Similarly, embodiments of the methods described herein may also be employed in order to create a digital representation, such as a digital 3D model, of the simplified geometry of the target object. In some measurement scenarios, it is convenient to make a simplified geometry of the target object, for example to visualize the obtained frequency response function or mode shape. Since the proposed technique allows to determine the position of the accelerometer, it can be used for a fast and rough "digitization" of the target object by connecting the obtained nodes where the accelerometer is placed with lines.

Embodiments of the methods described herein may be implemented by means of hardware comprising several distinct elements, and/or by means of a suitably programmed microprocessor, at least in part. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware, component or item. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, elements, steps or components, but does not preclude the presence or addition of one or more other features, elements, steps, components or groups thereof.