Vibration cancellation apparatus

DETAILED DESCRIPTION It is therefore an object of the present invention to provide a compact vibration cancellation apparatus based on a combination of air springs and electromagnetic actuators with smaller power capacity to enable the anti-vibration table to be maintained in a level position even when the center of gravity of the anti-vibration table moves a large distance because of a movement of the objective equipment on the table.
According to the present invention, there is provided a vibration cancellation apparatus which includes at least three displacement sensors for detecting vertical positions and outputting displacement signals, acceleration sensors for detecting vertical accelerations of the anti-vibration table and outputting acceleration signals, a levitation position control means for controlling the electromagnetic actuators according to output signals of a first compensation circuit which are based on displacement signals from the displacement sensors.
The vibration cancellation apparatus also includes a vibration canceling control means for controlling the electromagnetic actuators according to output signals of a second compensation circuit which are based on acceleration signals from the acceleration sensors, and air spring actuators for supporting four corners of the anti-vibration table by air pressure.
The air pressure of each air spring actuator is independently adjustable by a respective air pressure control device.
The air pressure in each air spring actuator is controlled in response to a change of load which corresponds to a movement of the center of gravity caused by movement of the objective equipment on the anti-vibration table.
Accordingly, actions for maintaining the table in a level position are provided by two separate control actions: one of which controls the electromagnetic actuators for the normal levitated position control of the table sensed by the displacement sensors; and another which controls the air spring actuators for the air pressure to maintain the table level