Methods for measuring and controlling the movable coil position of the joule balance

A system based on laser heterodyne interferometer is proposed in this paper to measure and control the movable coil position of a joule balance. A damping system is used to suppress the movement of movable coil in the horizontal direction while a piezoelectric ceramic control unit with PID controller is used to inhibit the vibration and long term drift of movable coil in the vertical direction. The effectiveness of the proposed method in measuring and controlling the movable coil position of a joule balance is proved through experiments. Experiment results indicate that the displacement of movable coil in the vertical direction can be reduced from 400 nm to 50 nm while the drift of movable coil is successfully inhibited.