A novel method for the measurement of mechanical mobility

Abstract

We describe a novel technique for direct measurement of the unloaded driving-point mechanical mobility of a structure. The method uses an electrically-excited inertial motor/actuator attached directly to the structure under test (SUT) to induce sinusoidal motion in the frequency range 7.5–750 Hz. Two accelerometers, one attached to the actuator frame and the other to the vibrating actuator mass, separately measure the induced motion of the SUT and the vibrating mass. We develop an expression for the driving-point mobility of the SUT using these two measurements and show analytically that any mass-loading effect of the actuator on the measured mobility can be removed mathematically so that the unloaded mobility of the SUT can be determined. In this study we validate the method using a cantilever as a mobility standard. Measured mobility, mode shape and resonant frequency of cantilevers with various lengths were consistent with theoretical predictions for an unloaded cantilever. We conclude that the method can be used to conveniently determine the mobility of a range of structures, without the limitations associated with some of the traditional methods of mobility measurement.