Disturbance observer and feedforward design for a high-speed direct-drive positioning table

Design and implementation of a discrete-time tracking controller for a precision positioning table actuated by direct-drive motors is considered. The table has acceleration capabilities in excess of 5 G, positioning accuracy at the micron level, and is used in applications such as semiconductor packaging, Unlike a ballscrew driven system, the controller in a direct-drive system must provide a high level of disturbance rejection while avoiding problems due to the relatively slow electrical dynamics of the motor and power amplifier. The stiff mechanical elements in a direct-drive system allow generous use of feedforward, but complete inversion of the closed-loop dynamics at high frequencies does not necessarily give the best performance. The controller proposed here uses a disturbance observer and proportional derivative (PD) compensation in the feedback path and a zero phase error tracking controller and zero phase low-pass filter in the feedforward path. The focus of this work is in two areas. First, existing disturbance observer design techniques are extended to account for time delay in the plant. Second, practical difficulties with excessive feedforward gains are examined and a low-order filter design method is proposed. Experimental results for quantized low-order position reference trajectories, which are commonly used in industrial systems, demonstrate the effectiveness of the approach.