Torque control of sinusoidal PMAC motors for direct-drive robots

A control approach for direct-drive robots with sinusoidal permanent magnet AC (PMAC) motors is developed. The conventional vector control for PMAC motors has been simplified by specifying the motor stator phase so that the rotating d-axis current is zero. This makes the motor work in maximum torque-per-amp operation. The proposed torque control, in the form of torque feedforward plus PI-type torque feedback, utilizes the feedback of nominal torque signal only, a signal that can be readily calculated, on-line. No torque sensor is required. Through proper design of the desired nominal torque with adaptive control technology, the proposed torque control can overcome the disturbances due to torque estimation error and model uncertainties. Experiments conducted on a commercial motion control system demonstrates the validity of the proposed approach in high-speed motions.