Patent ID: 7289858

Claim:
A lithographic apparatus, comprising: a control system configured to control a parameter of the lithographic apparatus, said control system comprising: an actuator configured to adjust said parameter, said actuator having an actuator transfer function that comprises a stable inverse part and a remainder comprising a potentially unstable inverse part; a feed-forward path comprising a feed-forward controller having a feed-forward controller transfer function, said feed-forward controller transfer function comprising a first part and a second part, said feed-forward controller transfer function first part comprising an inverse of said actuator transfer function stable inverse part; a feed-back loop comprising a feed-back controller, said actuator, and a difference determinator having a difference determinator setpoint input and a difference determinator feed-back input, said difference determinator configured to determine a difference between respective signals at the difference determinator setpoint input and the difference determinator feed-back input; and a setpoint path from a setpoint input to the difference determinator setpoint input, said setpoint path comprising a setpoint delay function, wherein a difference in delay between the setpoint delay function and said feed-forward controller transfer function second part is equal to a delay of said potentially unstable remainder of the actuator transfer function.