In vibration-based tests it is often desirable to use digital image correlation (DIC) to experimentally obtain the mode shape and strains over a specimen. Due to the high frequency of such tests, high-speed cameras are often employed to capture the oscillation. However, high speed cameras are typically resolution limited, thus reducing the effectiveness of DIC. To improve resolution, lower speed cameras can be paired with precise triggering in order to construct a representative period of oscillation. However, if the frame rate of the camera is less than the Nyquist rate of the system under investigation, then simple triggering of the cameras at their maximum frame rate will not yield usable results. To resolve this issue, a method known as phase locking is often employed.In the phase locking method, low-speed and high-resolution cameras are precisely triggered at either (a) the peak of the waveform, or (b) at incremental phase angles. In the case of (a), DIC can be used to to resolve the amplitude of the oscillation with the maximum possible signal-to-noise ratio and least possible motion blur. In (b) the cameras can be used to construct a representative period of oscillation. This allows for the better detection of non-zero mean amplitudes or distortions in the waveform. In either case, an accurate camera triggering algorithm is necessary to ensure images are captured at the desired location in the waveform.In this work, a new phase locking algorithm is presented for triggering the cameras. The new algorithm is both noise-tolerant, and can trigger accurately even in the presence of large waveform distortions (i.e. high THD+N[RB1]&nbsp;). Underlying this method is a linearized harmonic fit that allows for the fast, and accurate, analysis of a waveform. This algorithm can consistently analyze a 500,000 data point waveform with N harmonics in less than 1/20th of a second on a typical desktop computer, allowing for predictive triggers to be sent to the cameras shortly after. Due to the short time delay, this method can be used to phase lock dynamic signals with relatively fast changing frequency, amplitude, and phase components.The method is then applied to a vibration-based fatigue test. In this test an aluminum specimen was loaded in bending fatigue at a frequency near 1000 Hz. While the specimen was being fatigued, images were captured at the peak of the oscillation every two seconds for the duration of the test. By using DIC on these images, changes in the mode shape and specimen strain were tracked over the duration of the fatigue test.

A New Phase Locked Camera Triggering Method and Its Application to a High Frequency Vibration-Based Fatigue Test

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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