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Completed TRL 5 (started at 3, targeting 5)
The goal of this project is to develop an imagery system and algorithms that can be used to determine the mode shapes and frequencies of a structure during vibration testing. Traditional structural testing relies on accelerometers and strain gauges to measure motion from an applied input force. These sensors are accurate and sensitive to small motions, but they have some downsides:
One method of gathering the full-field motion data of the structure without changing the dynamic response is through the use of video cameras. A new technique called Motion Magnification allows for quick and easy visualization of the dynamic structural response. The goals of this project are a) maturation of the MSFC software tools that enable the measurement and easy visualization of the motion and b) use Motion Magnification for dynamic tests of space structures.
This project uses phase-based Motion Magnification to identify the modal frequencies of the system and to amplify the motion associated with the model frequencies. Phase-based Motion Magnification uses a complex steerable pyramid to determine the local spatial phase change between images. The local spatial phase is used to determine the motion of the structure, and it can be amplified and added back into the video so that the structural motion is easily visualized.
Motion Magnification has been used in the laboratory setting under ideal conditions, but has not been used on large space structures prior to this project. This project aims to use phase-based Motion Magnification to identify the modal frequencies and mode shapes of a structure.
Toward the goal of using Motion Magnification on large space structures, Motion Magnification was used on two space structure dynamic tests: the SLS Core stage LOX dome modal test and the Imaging X-Ray Polarimetry Explorer (IXPE) mirror module assembly (MMA) base excitation test.
For the LOX dome test, an impact hammer was used to excite the structure to collect modal data of the forward dome, particularly in the region surrounding the manhole cover as is seen in Figure 1. Motion Magnification predicted the first dome mode frequency to within 0.2% and the second dome mode frequency to within 7.0% of the accelerometer data. The increased error in the second mode is likely because the type of excitation (impact hammer) does not provide adequate excitation across the frequency band of interest for Motion Magnification, therefore, outside noise has an oversized influence on the mode frequency prediction.
During the IXPE test the MMA was excited through base excitation using a sine sweep approach. Figure 2a-b show the response of the structure during two sine sweeps: one from 155-240Hz and one from 285-315Hz. The peaks show a large structural response during the sine sweep, which is indicative of modal resonances of the structure. The focus of the test was the modes of the mirror shells, which are thin metallic cylinders and their response would change if accelerometers were added to the structure. Therefore, no accelerometer data was available to compare with the Motion Magnification results, but the frequencies and mode shapes identified were consistent with those from the pre-test finite element model.
Motion Magnification has been successfully implemented at MSFC and has been used on two large-scale modal tests: the LOX dome modal test and the IXPE MMA base shake test. Motion Magnification was able to accurately identify the first two modes of the LOX dome and many modes of the IXPE structure.
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