Development of Force-Frequency Shifting for Low Frequency Structural Vibration Testing

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Research AbstractPROPOSAL NO.: 0218668INSTITUTION: PA St U University ParkNSF PROGRAM: DYNAMIC SYSTEMS AND CONTROLPRINCIPAL INVESTIGATOR: Trethewey, Martin W.TITLE: Development of Force-Frequency Shifting for Low Frequency Structural Vibration TestingExperimental vibration testing of civil and architectural systems is useful for addressing a number of issues including; assessment of structural health (degradation of bridges), and reduction of troublesome oscillatory motion in buildings for sensitive installations such as medical MRIs in hospitals and electron microscopes. The testing of these systems requires that a high amplitude force excitation be applied at very low frequencies (0.1 - 10 Hz), which is inherently very difficult to accomplish. The research program is focused on developing a new technique, referred to as Force Frequency Shifting (FFS), to provide such an excitation. The FFS technique is based on the application of a high amplitude, high frequency force (at frequency fz) to a structure in a spatially time variant fashion (at frequency fx). The nonlinear dynamic behavior of the system produces an excitation at a sum and difference (fz+fx, and fz-fx) of the force and spatial motion frequencies. Of particular interest for low frequency vibration testing, is the difference frequency component (fz-fx). It is the innate ability of the FFS technique to create this low frequency component that is the focus of the project. The research will develop an FFS exciter with a computer-controlled array of variable stiffness actuators. This development will increase the low frequency force performance and control making the FFS suitable for vibration testing of the targeted systems.