Active vibration isolation using an inertial actuator with local displacement feedback control

The design of inertial actuators with local displacement feedback control and their use in active vibration isolation systems is considered. Unlike reactive actuators, inertial actuators do not need to react off a base structure and can therefore be directly installed on a vibrating structure. However in order to guarantee good stability margins in the active isolation controller, the actuator resonance must have a low natural frequency and it must be well damped. However, the need to have an inertial actuator with a low resonance frequency leads to unwanted static deflections of the actuator proof-mass. The use of integral displacement feedback as a local loop within the actuator provides self-levelling capabilities for the inertial actuator proof-mass, thus overcoming the static deflection problem. A novel device for active vibration control, based on an inertial actuator with displacement sensor and local PID controller, is described and its performance is demonstrated experimentally. It is found that the natural frequency and damping of the actuator can also be changed substantially with such a controller, thus allowing an inertial actuator to be customised for a specific application. A frequency-domain formulation is then used to analyse the stability and performance of an active isolation system using the modified inertial actuator and an outer velocity feedback control loop. The plant response, from force actuator input to sensor output, is derived in terms of the mechanical mobilities of the equipment structure being isolated and the vibrating base structure, and the mechanical impedance of the intervening mount. The results of an experimental study of active vibration isolation using a modified inertial actuator are then described. Theory and experiments agree well, demonstrating the effectiveness of the modified inertial actuator. (C) 2004 Elsevier Ltd. All rights reserved.