Feedback Stability Limits for Active Isolation Systems with Reactive and Inertial Actuators

Abstract

Some of the compromises inherent in using a passive system to isolate delicate equipment from base vibration can be avoided using fully active skyhook damping. Ideally, a secondary force, which is made proportional to the absolute equipment velocity by a feedback controller, acts only on the equipment and so the response of the system under control, between the secondary force input and the collocated velocity output, i.e., the plant response, is proportional to the driving point mobility of the mounted equipment. The frequency response of the plant is guaranteed to have a positive real part under these ideal conditions, and so the feedback system is unconditionally stable for any positive real feedback gain. In practice, the actuator generating the secondary force must either react off the base structure or an inertial mass. In both of these cases the plant response is no longer guaranteed to be positive real and so the control system may become unstable at high gains. Expressions for the overall plant responses are derived for both of these arrangements, in terms of the dynamic response of the individual parts of the isolation system. When using a soft mount, the stability of the reactive system is found to be surprisingly tolerant of the additional contributions to the plant response from the reactive force. In order for the inertial system to be stable with a high feedback gain, however, the natural frequency of the actuator must be well below the natural frequency of the equipment on the mounts. Experimentally measured plant responses are compared with those predicted from theory for both types of actuator and the performance of practically implemented feedback controllers is discussed.

Multichannel Feedback Control for the Isolation of Base-Excited Vibration

“…The additional phase shift due to the #exible base thus does not appear to cause any noticeable stability problem or ampli"cation of the equipment motion. A further discussion of the stability of a single-channel active isolation system with either a reactive or inertial actuator is provided by Elliott et al [15].…”

Design of a stand-alone active damper for distributed control of vibration

“…The force F T transmitted to the ground by the inertial actuator, and therefore the force useful to control vibration is: 11 F…”

“…The feedback gain cannot then be increased beyond a certain limit without the danger of the feedback loop becoming unstable and the stability of this feedback loop will limit the magnitude of the skyhook damping which can be implemented. Stability limits due to the use of proof-mass devices have been deeply studied [10][11][12] and different control strategies have been implemented to increase the performance of these devices. [13][14][15][16][17][18][19][20][21][22][23] Despite the use of these actuators is characterized by limitations on stability, their use is still intensive due to the versatility of the devices and their ease of installation.…”

Optimisation of a velocity feedback controller to minimise kinetic energy and maximise power dissipation

“…In order to implement skyhook damping with a velocity feedback loop using an inertial actuator, the natural frequency of the actuator should be lower than the first natural frequency of the primary system and the resonance of the actuator has to be well damped to improve control stability [17,18,28]. However a low resonance is difficult to achieve in practice because a soft suspension will cause large static deflection and will increase the risk that the inertial mass could hit the ends-stop, potentially leading to instability [29].…”