Experimental evaluation of force frequency shifting for low-frequency vibration excitation

A novel approach known as force frequency shifting (FFS) has been previously proposed for low-frequency (less that 1 Hz) vibration excitation. The technique requires a time-varying forcing function to be applied in a spatially varying fashion. The non-linear dynamic response produces excitation forces and moments at sum and difference frequencies of the force (i.e. f _z) and translational motion (i.e. f_x) frequencies. It is the difference frequency component (i.e. f_z - f_x) that produces the desired low-frequency excitation. The purpose of this paper is to examine the fundamental behaviour of the FFS concept in depth and explore the feasibility of the method for structural excitation of large-scale systems with low natural frequencies. First, a set of tests was performed on a laboratory prototype to quantify the character of the excitation output. The prototype system was instrumented at component interfaces allowing reaction forces to be measured and compared to analytical model results. The tests show that the model frequency predictions were very accurate. Force and moment amplitude predictions were nominally within 5% of the experimental values. However, some amplitude comparisons showed larger errors and can be attributed to a number of identified modelling and experimental issues. Next, a full-scale FFS prototype was used to perform vibration tests on a large test structure (inertial vibration isolation platform). The results from two tests were compared: (1) a traditional modal analysis using an impact excitation and (2) an operating deflection shapes analysis using the FSS as the excitation. The comparison was excellent demonstrating the potential for the FFS technology to be used for modal testing of large structures.