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An active damping system to dampen a vibration of at least a part of the structure, including a device to determine a position quantity of the structure and an actuator to exert a force on the structure in dependency of the determined position quantity, wherein the device is a calculation device con

An active damping system to dampen a vibration of at least a part of the structure, including a device to determine a position quantity of the structure and an actuator to exert a force on the structure in dependency of the determined position quantity, wherein the device is a calculation device configured to calculate the position quantity of the structure on the basis of actuator signals.

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1. An active damping system configured to dampen a vibration of at least part of a structure, the active damping system comprising: a calculator configured to determine a position quantity of the structure; andan actuator configured to exert a force on the structure in dependency of the determined p

1. An active damping system configured to dampen a vibration of at least part of a structure, the active damping system comprising: a calculator configured to determine a position quantity of the structure; andan actuator configured to exert a force on the structure in dependency of the determined position quantity,wherein the calculator is configured to calculate the position quantity of the structure based on actuator signals, andwherein the actuator signals used for calculation of the position quantity are clamp voltage and current. 2. The active damping system of claim 1, further comprising a controller unit and wherein the calculator is at least partly comprised in the controller unit. 3. The active damping system of claim 1, wherein the structure is a projection system. 4. The active damping system of claim 1, wherein the actuator is a piezo-electric actuator or a Lorentz actuator. 5. The active damping system of claim 1, wherein the calculator is in communication with the actuator so as to receive the actuator signals from the actuator. 6. The active damping system of claim 1, wherein the calculator is configured to calculate the position quantity of the structure without using a sensor signal representative of a vibration of on an object to which said actuator is connected. 7. The active damping system of claim 6, wherein said object is an interface mass mounted on the structure. 8. An active damping system configured to dampen a vibration of at least part of a structure, the active damping system comprising: a calculator configured to determine a position quantity of the structure; andan actuator configured to exert a force on the structure in dependency of the determined position quantity,wherein the calculator is configured to calculate the position quantity of the structure based on actuator signals, andwherein an interface mass is mounted on the structure, and the actuator is connected to the interface mass without directly contacting the structure so that, in use, the force is exerted on the structure via the interface mass. 9. The active damping system of claim 8, wherein the actuator is connected to an inertial reaction mass on which a reaction force is exerted. 10. The active damping system of claim 9, wherein said inertial reaction mass is arranged without directly contacting the structure. 11. An active damping system of configured to dampen a vibration of at least part of a structure, the active damping system comprising: a calculator configured to determine a position quantity of the structure; andan actuator configured to exert a force on the structure in dependency of the determined position quantity,wherein the calculator is configured to calculate the position quantity of the structure based on actuator signals, andwherein the actuator is connected to an inertial reaction mass on which a reaction force is exerted, said inertial reaction mass being arranged without directly contacting the structure. 12. An active damping system, configured to dampen a vibration of at least part of a structure, the active damping system comprising: a calculator configured to determine a position quantity of the structure; andan actuator configured to exert a force on the structure in dependency of the determined position quantity,wherein the calculator is configured to calculate the position quantity of the structure based on actuator signals,wherein the actuator is connected to an inertial reaction mass on which a reaction force is exerted, andwherein the position quantity is a velocity of the structure which is determined by the velocity of the reaction mass and a difference between the relative velocity between the structure and the reaction mass. 13. The active damping system of claim 12, wherein the relative velocity is based on a difference between a clamping voltage of the actuator and a current times the resistance of the actuator. 14. The active damping system of claim 12, wherein the velocity of the reaction mass is determined from the integrated exerted force. 15. A lithographic apparatus comprising: a support constructed to support a patterning device, the patterning device being capable of imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam;a substrate table constructed to hold a substrate;a projection system configured to project the patterned radiation beam onto a target portion of the substrate; andan active damping system configured to dampen a vibration of at least part of a structure, the active damping system comprising a calculator configured to determine a position quantity of the structure; andan actuator configured to exert a force on the structure in dependency of the determined position quantity,wherein the calculator is configured to calculate the position quantity of the structure based on actuator signals, and wherein the actuator signals used for calculation of the position quantity are clamp voltage and current. 16. The lithographic apparatus of claim 15, wherein the structure is the projection system. 17. The lithographic apparatus of claim 15, wherein the actuator is a piezo-electric actuator or a Lorentz actuator. 18. The lithographic apparatus of claim 15, wherein the calculator is in communication with the actuator so as to receive the actuator signals from the actuator. 19. The lithographic apparatus of claim 15, wherein the calculator is configured to calculate the position quantity of the structure without using a sensor signal representative of a vibration of on an object to which said actuator is connected. 20. The lithographic apparatus of claim 19, wherein said object is an interface mass mounted on the structure.