An adaptive anechoic termination for active vibration control

The active broadband control of the flexural vibration of a slender structure, in particular a beam, is obtained by the use of an adaptive anechoic termination. The anechoic termination, which absorbs any energy incident upon it, is implemented by applying a force close to one end of the structure. The force is determined by a feed-forward adaptive control that uses estimates of the incident and reflected waves as reference and error signals. Digital filters are implemented to estimate, in real-time, the amplitudes of these waves by filtering the outputs of an array of sensors. The reflected wave is used as the cost function in a filtered-X LMS adaptive control. The use of the propagating waves as reference and error signals also allows the method to be effective for resonant structures, a situation in which conventional approaches fail to be reliable. In order to compare the method with a conventional approach an anechoic termination that uses the primary excitation as reference is also considered. Numerical and experimental results demonstrate the method applied to semi-infinite and finite resonant structures. A broadband reduction of up to 20 dB in the ratio of the reflected and incident powers is demonstrated both numerically and experimentally. The use of the adaptive anechoic termination to reduce the vibration levels in structures is shown to be more effective than other typical feed-forward active control systems. Furthermore, it can be applied to cases where no reference signal, such as the primary excitation, is directly available.