Characteristics of Force Frequency Shifting for low frequency excitation

A novel approach know as Force Frequency Shifting (FFS) has been previously proposed by Koss for low frequency (less that 1 Hz) vibration excitation. The technique requires a time varying forcing function oscillates along an axis (i.e., x-axis) perpendicular to the force axis (i.e., z-axis). The implementation is based a traditional uniaxial shaker to apply a force to a structure in a spatially time variant fashion. The nonlinear dynamic behavior of the system produces excitation forces and moments at sum and difference frequencies of the shaker and motion operational frequencies. Several hardware configurations have been tried to achieve the necessary combination of shaker force and motion. Previous experimental work has been directed at exploring the feasibility of the method for structural excitation. The purpose of this paper is to explore the fundamental behavior of Force Frequency Shifting concept in more depth. A series of experiments have been performed on a laboratory prototype to quantify the character of the excitation output obtainable from the technique in relation to the models used for the hardware.