Removing undesired periodic data from random vibration data
Conference Proceedings of the Society for Experimental Mechanics Series
When measuring the structural dynamic response of test objects, the desired data is sometimes combined with some type of undesired periodic data. This can occur due to N-per-revolution excitation in systems with rotating components or when dither excitation is used. The response due to these (typically unmeasured) periodic excitations causes spikes in system frequency response functions (FRFs) and poor coherence. This paper describes a technique to remove these periodic components from the measured data. The data must be measured as a continuous time history which is initially processed as a single, long record. Given an initial guess for the periodic signal's fundamental frequency, an automated search will identify the actual fundamental frequency to very high accuracy. Then the fundamental and a user-specified number of harmonics are removed from the acquired data to create new time histories. These resulting time histories can then be processed using standard signal processing techniques. An example of this technique will be presented from a test where a vehicle is dithered with a fixed-frequency, sinusoidal force to linearize the behavior of the shock absorbers, while measuring the acceleration responses due to a random force applied elsewhere on the vehicle.