What drives accuracy for video vibration?

Abstract

Vibration detection and display using magnification, or amplification, of high- speed video (Vibration Video Amplification, VVA) has been demonstrated to be a very useful tool. VVA has been challenged to produce useful results, especially when lower quality cameras are used, or if the component being tested involves relatively high frequencies (e.g. 200 Hz or higher). For these cases, detection of very low displacement levels is required for VVA to give useful results. This is because video methods detect displacement, not velocity, and vibration standards, such as ISO 20816 or HI 9.6.4, are based on velocity. For a given velocity acceptance level, the displacement associated with that velocity decreases in proportion to the vibration frequency. At relatively low frequencies such as 50 or 60 Hz, the resulting displacements are easily detectable. However, at higher frequencies such as gas turbine generator (“high spool”) operating speeds, or pump or compressor blade passing frequencies, it is much more challenging. At 500 Hz for example, the detection capability required for vibration detection is roughly equal in terms of mils versus in/sec rms. Therefore, to detect 0.2 in/sec rms (5.1 mm/s), a displacement detection accuracy of 0.2 mil p-p (about 5 microns p-p) is required. This is achievable, but challenging for most cameras and scene evaluation software. Therefore, this paper will discuss methods to improve and evaluate the displacement resolution of VVA. The encouraging degree of precision that has been able to be achieved is reported.