Implementation of floor vibration feedforward using an absolute displacement sensor combined with acceleration PI feedback

In the field of vibration control of semiconductor exposure equipments, commercial accelerometers have been used as feedforward sensors of floor vibrations. In order to suppress direct motion i.e., disturbances against an isolated table, the equipments have air springs in anti-vibration apparatuses. Therefore, once integration was required to provide the floor vibration feedforward, even when we utilized an integral characteristic of the air spring. In this paper, an absolute displacement sensor is utilized instead of the accelerometers to detect velocity and displacement signals of the floor vibration, and these signals are directly fed back to a voice coil motor as feedforward signals. Then, we can implement the floor vibration feedforward without amplifications of the pink-noise in low frequency. Although the effects of the feedforward are generally investigated in frequency responses of transmissibility, we evaluate time responses. As a result, transmissions of the floor vibrations are successfully suppressed on the isolated table.