Video-Bbased Methods for Measurement of Vibration Mode Shapes

Non-contact methods for the measurement of vibrations and displacements of aerospace structures are convenient because of their ease of setup and ability to measure hard-to-access regions. They typically only require line of sight to desired locations of measurement. Standard methods for testing of aerospace structures that involve the use of strain gauges or accelerometers that need to be physically attached may be less convenient due to access issues in large structures, the cost of dense instrumentation, and the complicated logistics of running wiring. By contrast, the resolution and processing of measurements from these standard sensors is simpler, while non-contact methods may have more complicated data processing and resolution limitations. This paper will provide a general overview of the current state of the art of computer vision and photogrammetry methods for measuring operational resonant frequencies and vibration mode shapes of aerospace structures. We will do a deep dive into video-based methods that use optical flow and related algorithms collectively called “motion magnification”, that provide visualizations of vibrations and displacements. We will describe the theory behind these computer vision algorithms, review recent work by researchers and companies in the field, and discuss capabilities and limitations of these methods, specifically the impacts of camera quality, noise, and video compression algorithms on measurements. We will give examples of measurements made in both laboratory settings and real-world vibration testing scenarios with a wide variety of camera hardware ranging from expensive high-speed cameras to consumer action cameras. Examples include an airplane wing recorded by a cell phone camera, the Space Shuttle, and the Space Launch System modal testing campaign. We will finally discuss potential future developments in the field. We hope to convey a thorough understanding of the theory and capabilities of these techniques, to guide experimentalists and testing engineers in using these methods accurately and effectively.