Structural modal excitation using travelling impulses—force frequency shifting

A review of existing hardware and methods for vibration testing of large structures is given by Koss and has shown that the size of inertial vibration shakers, to achieve a specific displacement, has to increase, as a structure becomes larger. In previous papers the concept of “force frequency shifting (ffs) for structural excitation”, was introduced to develop a more compact structural vibration exciter than is presently available for low frequencies. An ffs shaker operates at a frequency much greater than the natural frequency of the structure under test but generates a modal force at the lower frequency of the structure. This effect is accomplished by moving a vibrating force back and forth across the structure while the force is applied normally to its surface. For example, the generalized force generated by an ffs shaker at the fundamental structural frequency for a simply supported beam is given by 1.65Pr/l where P is the high frequency out of balance force, r is the throw amplitude and l is the beam length. The term that reduces the efficiency of force transfer from high to low frequencies is “r/l” as, usually, the length of a structure is much greater than the throw of the force. This paper introduces another force frequency shifting approach that allows r/l to be large. This is accomplished by placing force exciters along a structure-spatial array, spaced a distance ΔX apart, and each force exciter is activated for a short period of time to simulate a travelling force traversing the structure forwards and backwards. The “force throw r “can thus be made large. Results of simulations and experiments verify that force frequency shifting can be accomplished using travelling impulses and modal identification can be achieved.