Application of a low frequency moment shaker for vibration testing

A novel device know as a Frequency Shifting Shaker (FSS) has been previously proposed by Koss for low frequency (less that 1 Hz) vibration excitation. The system consists of oscillating an eccentric mass shaker along an axis (i.e., y-axis) perpendicular to an imbalance forcing function axis (i.e., z-axis). The system produces a dynamic excitation moment, instead of a point force, at the operational sideband frequencies. The inherent operation of the device produces a `modal shaker' in that it only excites a mode when the sum or difference frequency coincides with a natural frequency. The device produces a controllable high amplitude excitation at a very low frequency (i.e., 0.5 Hz) making the technique well suited for vibration testing of large structures. Previous work has demonstrated the fundamental feasibility with a laboratory model. The objective of this paper is to report on experiences gained with a full scale prototype FSS system with a large test structure (inertial vibration isolation test bed). The results from two tests are compared; 1) a traditional modal analysis using an impact test and; 2) an Operating Deflection Shape (ODS) analysis using the FSS as the excitation. The comparison was excellent provided the FSS was capable of sufficiently exciting the mode.