MOTION CONTROL STRATEGIES FOR MODERN PM MOTORS

Abstract

The purpose of this research is to study motion control strategies for a permanent magnet motor driven in a brushless direct current drive configuration. The motor has high torque capabilities which makes it suitable for direct drive applications. In a direct drive system the parameter variations and load disturbances vary over a wide range, this imposes a challenging problem to the design of the controller. First, a conventional variable structure strategy for the position control of the experimental PM motor is presented. The use of sliding modes to achieve invariant motion control is discussed. Second, an alternative variable structure control strategy which reduces the control chattering inherent in the original variable structure strategy is proposed. This controller retains variable structure switching in the large, but near the equilibrium point introduces a form of linear PID control which removes the chattering. Third, a state space design using the pole placement method is discussed. The feedback arrangement incorporates an integral term for offset rejection. Fourth, a switched mode method of velocity estimation is proposed. This method makes use of the pulse counting method of speed estimation when the rotor speed is high. As the rotor speed drops, the velocity estimator switch to an interval counting method. Finally the performance of the drive under the three control schemes are investigated in simulation and experimentally. Experimental results are evaluated and a quantitative comparison of the three schemes is performed.