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The effectiveness of positive position feedback (PPF) controllers to selectively damp a particular mode or a cluster of modes is demonstrated by controlling the vibration of a rectangular flexible plate. The modes of vibration of this plate occur in adjacent pairs. The plate is actuated by two patches of piezoelectric PZT actuators at two corners. The strain at the vicinity of each actuator is measured with a PZT sensor mounted in a nearly collocated arrangement with the each actuator. In a series of experiments, PPF controllers were tuned to different single or multiple modes with different damping ratios. The resulting controlled plate had considerable damping in its first four modes of vibration.

A narrowband feedback control is developed to cancel periodic noise. The method consists of digital bandpass filters centered at a reference frequency and its harmonics. Each of these bandpass filters is then followed by a finite impulse response (FIR) filter which changes the phase of the control signal to achieve the 180° phase shift in the open loop system. A gain is applied to the control signal to adjust the overall noise reduction, considering reasonable stability margins in the system. In a set of experiments the proposed technique was compared with the filtered-x adaptive feedforward control method. The periodic noise, introduced by a noise speaker, inside a reverberant box was controlled (reduced) using both techniques. With the gain properly adjusted, the amount of noise reduction at the microphone was slightly less than that achieved by the filtered-x adaptive control, but the stability characteristics of the feedback control system was considerably better.