Feedforward control of a vibration isolation system for disturbance suppression

For precision machines, it is very important to attenuate microvibrations caused by various sources. In order to suppress the transmission of vibration from the floor, it is standard to employ a rigid platform supported by air springs and install a precision machine on it. However, because of high compliance, such a vibration isolation system is easily disturbed by movements of the machine on the platform. To suppress this direct disturbances, feedback control such as PID and LQR has been extensively used, which however cannot satisfy severe specifications required, for example, in recent semiconductor manufacturing processes. This paper proposes a new feedforward control scheme for suppression of the direct disturbance. The feedforward control signal is not generated online, but computed in advance from the data of a response to the direct disturbance and the transfer function from the control input to the controlled output, which are obtained by a test. An experimental result shows that the proposed feedforward control works very well to improve the response to the direct disturbance.