Bi-modal vibration analysis with stroboscopic heterodyned ESPI

An ESPI system, utilizing stroboscopic illumination, was developed for bi-modal vibration analysis, in which the timing of the illumination pulses is controlled to isolate the individual modes. An object vibrating simultaneously at two frequencies, that are related by an irrational number, was examined experimentally. It was observed that when the frequency of the illumination pulses corresponded to one of the vibration frequencies and the pulse train was phased with the vibration, the fringe pattern observed only showed information on the other component frequency. In this way, the fringe pattern associated with either of the component frequencies could be made to disappear, and each mode could be investigated independently. The vibration amplitude map of the remaining component was obtained using heterodyned fringe analysis techniques.