Real-time measurement of wave components and intensity in a beam in the presence of a near field

A method is described by which the individual flexural wave components in a beam can be measured in real time. Attention is focussed on the case in which two propagating waves and a single near-field wave exist, although the case of two near fields is also considered. Because the presence of the near field is included, the measurements can be taken close to the force, boundary or discontinuity from which the near field arises. Potential applications include intensity measurement, active control and adaptive-passive vibration control.The wave components are measured by digitally filtering and combining the outputs of an array of sensors, with an array of three, equally spaced sensors being considered in detail. The filters are designed in the frequency domain using a wave decomposition approach, and implemented in the time domain as FIR filters. Design, implementation and performance issues are discussed and an experimental implementation described. It is seen that accurate estimates of the amplitudes of the wave components can be obtained using FIR filters of moderate order, and that the method is relatively insensitive to sensor miscalibration and measurement noise.