Application of a pure moment exciter for measurement of moment-rotational DOF frequency response functions

A variety of experimental methods to measure rotational responses of structures have evolved. Most applications have measured frequency response functions between a translational excitation and the corresponding rotational response. The measurement of the rotational response to a moment excitation has mostly been ignored, due to the difficulty of applying a moment excitation to the test system. The work in this paper introduces a technique that can produce a pure time varying moment, without a translational force, to a structure. The technique utilizes a set of geared eccentric masses, driven by a DC motor. The theory behind the design is first described. Then, a series of tests are performed to determine the capabilities of the pure moment exciter. The results show the analytical predictions are within 5% of the experimental values. The pure moment shaker is then applied to a laboratory test structure to measure the rotational degree of freedom with a moment excitation. The paper concludes with a discussion and assessment of the ability of the technique to effectively measure moment-rotational frequency response functions.