Vertical Vibration Isolation Using Displacement Feedback Control

A vertical active vibration isolation system using the displacement feedback force,a method widely used in gravitational and other experiments,is reanalyzed in detail.We find that more effective vibration isolation could be obtained if the feedback variable is proportional to the relative displacement between the platform and the foundation other than the absolute displacement of the platform itself The physical meaning is first recognized by introducing the concept of quasi-fixed reference system,this is significant for improving the isolation ratio of an active isolation system.