Active vibration isolation with integrated virtual balance mass for a motion stage

Many precision machines, such as wafer scanners, have an internal motion stage. These stages generate reaction forces that act directly on the machine frame. A common method to minimize the effect of these reaction forces is a balance mass. To eliminate the need for this balance mass, it is proposed to use an active vibration isolation system as a virtual balance mass. Such can be achieved by constructing a force that is based on the reference of the motion stage that cancels its correlated reaction force. This appends the primary objective of an active vibration isolation system, which is to reduce the sensitivity to direct and indirect disturbances. A self-tuning algorithm is implemented to deal with parameter uncertainty and variation. The performance of the proposed method is experimentally validated on a 6-DoF active vibration isolation system, which is appended with a flexure based, straight guided motion stage. A 300 Hz frame mode is introduced of which the displacement can be measured to assess the performance of the method. The proposed method improved the tracking accuracy of the motion stage by a factor of 3.8 and reduced the settling time of the frame mode by a factor of 91, compared to the system without a virtual balance mass.