A step-versus-short-pulse input technique to demonstrate acceleration response using an LDV

Abstract

This technique for extracting acceleration-like signals from a laser Doppler vibrometer (LDV) depends on comparing the responses of structures to specific impulsive inputs. What is compared here is the derivative-like response of a sufficiently short pulse to that of a step input. This is derivative-like because the pulse itself consists of an abrupt up-thrust followed shortly by an identical, but inverted down-thrust. The short pulse response emulates the negative numerator of the derivative definition in analog fashion. Certain structures excited by long pulses with equal widely-spaced rising and falling step inputs, respond such that the up and down- thrusts responses are exact inverted copies of each other regardless of the structural modes excited. Adjusting this pulse-width to be much smaller than the period of the highest frequency response component forms this derivative responses. Three validation tests are discussed. A consistency check shows the LDV pulse response is close to that of the finite difference scan of the LDV step response. The next test compares LDV pulse responses relative to differing input pulse-widths. The last test correlates the step input response of a mounted accelerometer on a test fixture to that of the pulse input response of an LDV focused on top. This technique should be helpful in comparing the LDV-to-accelerometer responses in mechanical setups where this derivative character of impulse inputs is valid. Since this comparison encompasses multiple excited modes, one can check or validate device calibrations at these different frequencies. It may be useful in some tests as an analog means of simulating acceleration using the velocity output of a no-mass-load LDV.