Patent ID: 8091694

Claim:
Actuator arrangement comprising: an actuator, a reference mass, a first suspension with at least a predetermined spring characteristic in at least one degree of freedom, and a first sensor, a second sensor, an airmount, a summing device, the actuator being arranged to apply a force between a first object and a second object, the reference mass being arranged in parallel, to said actuator, and supported by a third object by means of said first suspension, the airmount being arranged to support the first object, the airmount being supported on said second object by a fourth object by means of a second suspension, said reference mass and said first suspension being decoupled via an air gap from, said first object and having a response characteristic independent of said first object, and said first sensor being arranged to generate a first distance signal that depends on a first distance between said reference mass and said first object and to provide said first distance signal to a first controller, said first controller producing a first control signal in dependence on the received first distance signal, said second sensor being arranged to generate a second distance signal that depends on a displacement of the first object and to provide a second distance signal to a second controller for producing a second control signal in dependence on the received second distance signal, said summing device for summing the first and second control signals received from the respective first and second controllers thereby producing a summed control signal for actuating said actuator to enable the first object to substantially follow the position of the reference mass, said third object not being one and the same as said second object, said fourth object not being one and the same as said second and third objects.