Active Narrow-Band Vibration Isolation Of Machinery Noise From Resonant Substructures

Abstract Machinery noise and vibration is an important issue in many applications, including spacecraft, aircraft and naval vessels. A classic approach to ameliorating the effect of such disturbances is isolation at a vibration transmission bottleneck, the machinery mounts. Passive approaches are widespread, and introduce no risk of instability. Active techniques promise increased isolation performance, but introduce the risk of unstable interaction with system dynamics, particularly resonant vibrations of the machine or of the supporting structure. This paper focuses upon a specialization of active isolation, isolation of nearly time-periodic disturbances with feedback control. In this situation one can achieve significant isolation performance with minimal risk of instability. The paper demonstrates, with the aid of a simple case study, that the safely achievable performance is limited by three important factors. (1) The level of passive damping present in the unmodeled resonant response of the mechanical system (supporting structure and machine): the importance of passive damping is most concisely quantified by the modal overlap, the ratio of modal bandwidth to modal spacing. (2) The pole-zero structure of the transfer function from the actuators to sensors at frequencies near the disturbance frequency: for rigid machines, the single most important parameter is machine mass relative to effective modal mass. (3) The spectral bandwidth of the offending disturbance: slowly time-varying quasi-periodic disturbances are easiest to isolate by the techniques studied. For the case study, attention is restricted to a rigid "machine" and single axis motion. The paper develops modeling analyses, discusses sensor and actuator choices, discusses control algorithms, and presents the results of an experimental study.