Precision low-speed digital control of a permanent-magnet synchronous motor

A digital controller is designed and built for a 3-phase 4-pole permanent-magnet synchronous motor that is the prime mover in a mechanically novel kinematically-coupled friction-drive linear motion system. The goal is to provide constant low-speed linear motion at 3.2 mm/sec with an easily assembled system using a low-cost rotary optical shaft encoder as the only feedback element. The control task is made difficult by the low rotational speed of the motor, 4.4 RPM. This paper investigates different velocity estimation schemes, and a disturbance mapping and cancellation technique is used to reduce repeatable errors. The present system achieves a performance of 14% or less peak-to-peak velocity error with an RMS error of 0.93%. Performance limiting factors, both electrical and mechanical, that could potentially affect any low-speed low-cost system are identified and addressed