System and method for determining operating deflection shapes of a structure using optical techniques

A system for measuring total operating deflection shapes of a structure includes one or more imagers, each including two cameras spaced apart from one another and each oriented and positioned to have corresponding fields of view of a different corresponding section of the structure, with the corresponding sections that may include overlap area of the structure within each of the different sections of the structure. Each of the cameras generates a corresponding data stream, which is communicated to a controller, which is configured to measure the response of the structure to an excitation, such as a vibration or an impulse. The system is configured to convert time-domain data from each of the data streams to the frequency-domain data using a Fourier Transform algorithm and stitching the shapes to obtain the total operating deflection shapes of the structure by scaling and stitching together the frequency-domain data.

FIELD

The present disclosure relates generally to optical measurement to determine vibration characteristics of a structure in the form of operating deflection shapes and mode shapes.

BACKGROUND

Vibration characteristics of a structure are conventionally obtained by exciting the structure using an impact hammer or a mechanical shaker and measuring the response using accelerometers. However, using accelerometers for vibration measurement may induce mass loading effects and does not provide the full-field response of the structure. Also, sometimes it is challenging to excite measurement points in all the three x, y and z directions using an impact hammer and obtaining 3D mode shapes becomes nearly impossible. Digital Image Correlation (DIC) and other optical techniques have provided a solution to these problems because they provide the full-field response of the structure, are non-contacting, and do not induce mass loading effects. However, the DIC technique (similar to other optical methods) is limited by the field of view of the cameras and can only measure the response on the parts of the structure that cameras have a line of sight. Thus, this technique has not traditionally been used to obtain the mode shapes and operating deflection shapes of large and complex structures.

As described in LeBlanc et al.,Damage detection and full surface characterization of a wind turbine blade using three-dimensional digital image correlation, Structural Health Monitoring(2013), images of overlapping portions of a structure may be combined for digital image correlation (DIC). For example, a single stereo camera may be moved around the structure. Measured deformations are then stitched together to obtain the deformations of the entire structure. This technique is similar to known stitching techniques used for creating panoramic photographs from multiple smaller images. However, this technique cannot be applied for a dynamic measurement because the response is transient, and the deformation of the structure may change when the camera system moves (unless for a steady state phenomenon). This occurs because the input force may vary during each measurement. Thus, deformations in each view are from a different state of the structure (e.g., when the first view is recorded, the first and second vibration modes of the structure are excited while the second view is recorded when the third and fourth modes are dominant).

As described in P. Poozesh, et al.,Large-area photogrammetry based testing of wind turbine blades, Mech. Syst. Signal Process. (2016), multiple cameras may be used in a system to measure the entire shape of an object and to obtain operating deflection shapes of a structure.

SUMMARY

A system for measuring a total operating deflection shape of a structure is provided. The system includes a first imager including one or more cameras. Each of the cameras of the first imager has a corresponding field of view of a first section of the structure, and each of the cameras of the first imager generates a corresponding first data stream including first time-domain data. The system also includes a second imager including one or more cameras. In some embodiments, the second imager can be the first imager moved to capture another section of structure. Each of the cameras of the second imager has a corresponding field of view of a second section of the structure, and each of the cameras of the second imager generates a corresponding second data stream including second time-domain data. Each of the first section and the second section includes an overlap area of the structure. The system also includes a controller in communication with each of the cameras and configured to measure a response of the structure to an excitation by converting the first time-domain data and the second time-domain data to frequency-domain data to obtain the total operating deflection shape of the structure.

A method for measuring a total operating deflection shape of a structure is also provided. The method includes the steps of: exciting the structure by an excitation source; imaging a first section of the structure by one or more cameras of a first imager to generate a first data stream including first time-domain data; imaging a second section of the structure different from and overlapping the first section by one or more cameras of a second imager to generate a second data stream including second time-domain data; converting the first time-domain data from the first imager to the frequency-domain data to obtain a first operating deflection shape of the first section of the structure, and converting the second time-domain data from the second imager to frequency-domain data to obtain a second operating deflection shape of the second section of the structure; and combining the operating deflection shapes for each of the sections of the structure to obtain the total operating deflection shape for an area of the structure larger than either of the first section or the second section.

DETAILED DESCRIPTION

Recurring features are marked with identical reference numerals in the figures, in which an example embodiment of a system20for measuring a total operating deflection shape11of a structure10is disclosed. The structure10may be a stand-alone object or a component of a larger assembly or apparatus. The present disclosure is particularly useful for measuring the total operating deflection shapes11of structures10that are configured to move or to be regularly subjected to vibrations, such as a vehicle body component an object that is regularly subjected to moving fluids or other sources of vibratory energy.

A new approach to obtain a uniform scaling factor that enables operating deflection shapes (unsealed mode shapes) to be stitched together from different views of cameras is provided. An operating deflection shape corresponding to a resonant frequency of a structure and shows the deformation that occurs (or a shape that a structure will show) when it is excited at that specific resonant frequency. This shape can be scaled using the exciting force to obtain a mode shape. The technique of the present disclosure enables the operating deflection shapes (or mode shapes) of complex structures to be extracted using only a single imager (e.g. a single stereo camera system). To show the merit of the proposed technique, the operating deflection shapes of an exhaust muffler are extracted using this optical based technique. The muffler is excited with a known or unknown force using an impact hammer. Alternatively or additionally, the excitation can be provided by normal operation of the component. A pair of high-speed cameras is used to measure the response of the structure limited to its field of view. The operating deflection shape of every field of view is then obtained based on a measured response using the digital signal processing theory. Further, the operating deflection shapes of individual fields of view are stitched using a minimum of three reference points in the region common to the adjacent field of view to obtain the operating deflection shapes of the entire structure. In some embodiments, the stitching can be done using a global coordinate system. The subject approach stitches the shapes in the frequency domain rather than the commonly used stitching technique in the time domain. This technique expands the applications of digital image correlation (DIC) and optical techniques in the field of structural dynamics and enables us to extract operating deflection shapes of a complex structure using optical techniques.

FIG. 1is a schematic diagram of an example system20for measuring operating deflection shapes of a structure10in the form of an elongated and cantilevered beam. The structure10shown in the example system20is attached to a fixed member6, such as a foundational structure at an attachment point8. In other embodiments, the structure10may be measured while being held loosely or while resting upon a surface. In other embodiments, the structure10may be installed as part of a larger assembly, such as where the structure10is a part of a machine, such as a vehicle, or as part of a constructed assembly, such as a building, a bridge, a crane, or a windmill. It should be appreciated that these are merely illustrative examples and that the structure10may be any physical object or assembly.

In some embodiments, and as shown inFIG. 1, the system20includes a first imager22having two cameras24spaced apart from one another and each oriented to have corresponding fields of view26of a first section12of the structure10. The first imager22may have one or more independent cameras24. Each of the cameras24of the first imager22generates a corresponding first data stream28. The system20also includes a second imager34having two cameras24spaced apart from one another and each oriented to have corresponding fields of view26of a second section14of the structure10different from the first section12. Each of the cameras24of the second imager34generates a corresponding second data stream38. Each of the two imagers22,34are oriented and positioned to view an overlap area16of the structure10within each of the first section12and the second section14. In some embodiments, the first and second imagers22,34may be synchronized. Alternatively, the imagers22,34may be not synchronized to one another and/or to any reference signal.

The system20also includes a controller44in communication with each of the cameras24and configured to measure the response of the structure10to an excitation, such as a vibration or an impulse, which may be applied to the structure10, for example, using an impact hammer or a shaker. The system20may also measure vibrations originating in the structure10or which are from an unknown source. For example, the system20may be used in troubleshooting noise and/or vibration in a structure10to determine the source and/or to aid in design changes to mitigate such noise and/or vibration.

The system20is configured to convert time-domain data from each of the data streams28,38from the cameras24to frequency-domain data. In some embodiments, the controller44may use a discrete Fourier transform (DFT) algorithm to convert the time-domain data to the frequency-domain data. In some embodiments, the controller44uses a Fast Fourier Transform (FFT) algorithm to convert the time-domain data to frequency-domain data. In some embodiments, the system20is also configured to stitch together the frequency-domain data to obtain the total operating deflection shape11for an area of the structure10larger than either of the first section12or the second section14. In some embodiments, the area of the structure10larger than either of the first section12or the second section14includes the entirety of the structure10, and thus the total operating deflection shape11covers the entire structure10. In some embodiments, the system is also configured to scale the frequency-domain data associated with one or both of the sections12,14prior to stitching the frequency-domain data to obtain the total operating deflection shape11. Scaling may be used, for example, to align the overlap areas16within each of the sections12,14of the structure10.

In the example embodiments shown inFIGS. 1-3, each of the imagers22,34is a stereoscopic device, which includes two or more cameras24, each spaced apart from one another and positioned to have a corresponding field of view26of a same section12,14of the structure10. Such stereoscopic imagers22,34can, therefore, measure the 3-dimensional shape of the structure10as well as the displacement of the corresponding section12,14of the structure10in 3-dimensions. However, the system20may include one camera24in each of the imagers22,34, which would measure only 2-dimensional (or 3-dimensional) displacement, and could therefore obtain 2-dimensional operating deflection shapes. The imagers22,34could also include three or more cameras. Combining the 2-dimensional images from the two or more cameras24into a stereoscopic image could be performed by the imagers22,34themselves. Alternatively, the controller44could combine 2-dimensional images from each of the two or more cameras in each of the imagers22,34to generate the stereoscopic image and data regarding the displacement of the structure10in 3-dimensions.

In some embodiments, and as shown inFIG. 1, the controller44includes a processor46and a machine readable storage medium48, such as a non-transitory memory, storing a program in the form of a series of instructions49for execution by the processor46. In some embodiments, the instructions49may be executed by the processor46to cause the processor46to convert the first and second time-domain data from each of the data streams28,38to frequency-domain data and to obtain the total operating deflection shapes11of the structure10. The processor46may include one or more microprocessors, microcontrollers, field programmable gate arrays (FPGAs) and/or special purpose hardware, such as an application specific integrated circuit (ASIC). The machine readable storage medium48may include one or more different types of storage media such as, for example, magnetic media, optical storage media, EEPROM, FLASH memory, DRAM, SRAM, cache memory, etc. The Fast Fourier Transform (FFT), the scaling, and/or the stitching functions may be performed by one or more software modules within the series of instructions49. Some or all of the functions performed by the system20to obtain the total operating deflection shapes11of the structure10may be performed on hardware and/or software that is located in or remotely from the controller44, for example, in a remote server.

FIG. 3is a schematic block diagram illustrating an example system20and method for measuring operating deflection shapes of a structure10. The system20may include any number (n) of the imagers22,34, where n is a number of 1 or more. As illustrated in the example ofFIG. 3, in which the structure10is a muffler for a vehicle, each of the imagers22,34measures a corresponding deformation profile32,42for a corresponding section12,14of the structure10. By using several different imagers22,34, the total operating deflection shapes11of a structure10that is very large and/or complex may be determined. Each of the imagers22,34includes one or more cameras24.

In some embodiments, the imagers22,34could be a single device that is moved with respect to the structure10, and which successively images the different sections12,14of the structure10at different times to capture different views of the structure10. The imager22,34could be stationary while the structure10is moved or the structure10could be stationary while the imager22,34is moved. Alternatively, both the structure10and the imager22,34may be moved relative to one-another. In some embodiments, and as shown inFIGS. 1-2, each of the first imager22and the second imager34may be separate and independent from one another, with the first imager22and the second imager22each configured to simultaneously generate a corresponding one of the data streams28,38representing the corresponding one of the first section12and the second section14of the structure10.

As described in the flow chart ofFIG. 4, a method100for measuring total operating deflection shapes11of a structure10is also provided. The method100includes exciting the structure10by an excitation source56at step102. This step102may include hitting the structure10at a predetermined position with an impact source, such as an impact hammer. Alternatively or additionally, this step102may include vibrating the structure10using a vibration source. This step102may include using a vibration source separate from the structure10or a vibration source in contact with the structure10, such as by activating a mechanized shaker device in physical contact with the structure10. In some embodiments, this step102may include inducing mechanical excitation such as vibration by the operation of the structure10. For example, this may include passing actual or simulated exhaust gasses through a structure10that is a vehicle muffler. This step may be performed by an unknown or unintentional excitation source56, such as in cases where a structure10is observed to be vibrating due to an unknown cause.

The method100also includes imaging a first section12of the structure10by one or more cameras24of a first imager22to generate one or more first data streams28at step104. In an example embodiment, step104includes imaging the first section12by a stereoscopic pair of cameras24that are spaced-apart from one another by a predetermined distance. The first data streams28may include, for example, a high speed video signal from each of the cameras24in the first imager22, and which may take the form of a digital or analog signal.

The method100also includes communicating images of the first section12of the structure10from the first imager22to a controller44as the first data streams28at step106. The first data streams28may be communicated via wired or wireless channels, and may be accomplished via electrical or optical signals. The first data streams28may be communicated in real-time or with some delay due, e.g. due to processing and/or bandwidth limitations. Alternatively, the controller44may be integrated with the first imager22and may obtain data directly therefrom.

The method100also includes tracing a first deformation profile32as a 2 or 3-dimensional displacement of the first section12of the structure10as time-domain data using the first data streams28from the first imager22and in response to the exciting of the structure10by the vibration source at step108. In the example embodiment, the first imager22is a stereoscopic device including two or more cameras24, the first deformation profile32includes the 3-dimensional displacement. This step108may be performed using a photogrammetry technique such as digital image correlation (DIC), and corresponds to the box labeled “DIC measurement 1” onFIG. 2.

The method100also includes imaging a second section14of the structure10different from and overlapping the first section12by one or more cameras24of a second imager34to generate a second data stream38at step110. In an example embodiment, step110includes imaging the second section14by a stereoscopic pair of cameras24that are spaced-apart from one another by a predetermined distance. The second imager34may be an independent device from the first imager22. Alternatively, the second imager34may be the same device as the first imager22, which is moved relative to the structure10in order to image the different sections12,14. The second data streams38may be, for example, high speed video signal from each of the cameras24in the second imager34, and which may take the form of a digital or analog signal.

The method100also includes communicating images of the second section14of the structure10from the second imager34to a controller44as the second data streams38at step112. The second data streams38may be communicated via wired or wireless channels, and may be accomplished via electrical or optical signals. The second data streams38may be communicated in real-time or with some delay due, e.g. due to processing and/or bandwidth limitations. Alternatively, the controller44may be integrated with the second imager34and may obtain data directly therefrom.

The method100also includes tracing a second deformation profile42as a 2 or 3-dimensional displacement of the second section14of the structure10as time-domain data using the second data streams38from the second imager34in response to the exciting of the structure10by the vibration source at step114. In the example embodiment, the second imager34is a stereoscopic device including two or more cameras24, the second deformation profile42includes the 3-dimensional displacement. This step114may also be called digital image correlation (DIC), and corresponds to the box labeled “DIC measurement 2” onFIG. 2.

The method100also includes converting the deformation profiles32,42from time-domain data to frequency-domain data to obtain operating deflection shapes50,52for each of the sections12,14of the structure10at step116. In some embodiments, this step116includes using a Fast Fourier Transform (FFT) algorithm. This step116corresponds to the boxes labeled “FFT” onFIG. 2.

The method100also includes combining the operating deflection shapes50,52together for each of the sections12,14to determine a total operating deflection shape11for a larger area including each of the sections12,14at step118. In some cases, the larger area may be the entire structure10.

In some embodiments, the step118of combining the operating deflection shapes50,52together for each of the sections12,14may include scaling the operating deflection shapes50,52for each of the sections12,14at sub-step118A. This sub-step118A may include increasing or decreasing one or more components of one of the operating deflection shapes50,52in order to match the value or values of associated components of the other one of the operating deflection shapes50,52in the overlapping area16.

In some embodiments, the step118of combining the operating deflection shapes50,52together for each of the sections12,14may include stitching together the operating deflection shapes50,52for each of the sections12,14at sub-step118B. This sub-step118B may include associating three or more reference points in the overlapping area16that is common to each of the sections12,14. The method100may therefore allow for the total operating deflection shape11of the entire structure10to be determined. The stitching can also be performed using a global coordinate system that is used for each section.

The method100also includes moving an imager22,34between a first position and a second position at step120, where the second position is spaced apart from the first position relative to the structure10. The imager22,34may, therefore image (i.e. capture a series of images of) the first section12of the structure10from the first position, and subsequently image (i.e. capture a series of images of) the second section14of the structure10from the second position. For example, the imager22,34could be stationary while the structure10is moved or the structure10could be stationary while the imager22,34is moved. Alternatively, both the structure10and the imager22,34may be moved relative to one-another.

In some embodiments, the method100may include including measuring a plurality of different total operating deflection shapes11of the structure10, with each of the different total operating deflection shapes11corresponding to a different excitation of the structure10. For example, the different excitations of the structure10may include exciting the structure10at different frequencies, and/or at different locations.