3-DOF closed-loop control for planar linear motors

Planar linear motors (Sawyer motors) have been used in industry as open-loop stepping motors, but their robustness and versatility has been limited. Using a sensor integrated into such a motor, a closed-loop 3-DOF controller has been implemented. The software-based control system consists of a commutator for computing amplifier currents from actuator forces, a force resolution function for solving the redundant actuation and saturation problems, and an observer for producing a velocity estimate, together with a PID controller. Experiments are performed using a 2-axis laser interferometer to show that the controller has sub-micron resolution, 2 μm peak-to-peak repeatability, and settling times after trajectories of about 20 ms. Limitations of the PID controller are discussed and ideas for improvements are presented