Moment excitation of structures using two synchronized impact hammers

Moment mobilities are difficult to measure because there is no simple way to excite a structure with a well-controlled moment. In this paper, a technique to apply a nearly pure moment is presented. It is based on a simple approach that uses two impact forces applied, respectively, by two impact hammers. The limits and the validity of this approach are first examined theoretically on a simply supported plate. Comparisons between the excitation by two distant opposite forces and by a pure moment are performed. Then, the coupling between the two hammers and the structure during the impact phase is considered in order to analyze practical limits of obtaining identical forces impacting at different locations. Finally, a detailed description of the experimental system developed to obtain identical synchronized forces is presented. Two hammers driven by electric motors controlled by a Digital Signal Processor are used. Experimental results are compared to theoretical predictions. It is shown that the technique yields accurate measurements of moment frequency response functions.