Feedback stability limits for active isolation systems with reactive and inertial actuators

Some of the compromises inherent in using a passive system to isolate delic ate equipment from base vibration can be avoided using fully active skyhook damping. Ideally, a secondary force, which is made proportional to the abs olute equipment velocity by a feedback controller, acts only on the equipme nt and so the response of the system under control, between the secondary f orce input and the collocated velocity output, i.e., the plant response, is proportional to the driving point mobility of the mounted equipment. The f requency response of the plans is guaranteed to have a positive real part u nder these ideal conditions, and so the feedback system is unconditionally stable for any positive real feedback gain. In practice, the actuator gener ating the secondary force must either react off the base structure or an in ertial mass. in both of these cases the plans response is no longer guarant eed to be positive real and so the control system may become unstable at hi gh gains. Expressions for the overall plant responses are derived for both of these arrangements, in terms of the dynamic response of the individual p arts of the isolation system. When using a soft mount, the stability of the reactive system is found to be surprisingly tolerant of the additional con tributions to the plant response from the reactive force. In order for the inertial system to be stable with a high feedback gain, however, the natura l frequency of the actuator must be well below the natural frequency of the equipment on the mounts. Experimentally measured plant responses are compa red with those predicted from theory for both types of actuator and the per formance of practically implemented feedback controllers is discussed.