An investigation into the stability and robustness of two sensorless control techniques for brushless PM motors
Brushless PM motors are used in many applications because they have advantages of high torque, small size and low maintenance due to the lack of brushes. Rotor position information is required for current commutation in the windings of brushless PM motors, and for servo feedback control. However, the use of shaft position sensors is not always possible at a reasonable cost. Therefore, a large amount of work has been done on the so-called sensorless control in which rotor position is estimated from the terminal voltages and currents of the motor. However, there are very few papers on design issues such as parameter selection, system stability and robustness, i.e., sensitivity to motor parameter errors. This thesis investigates two existing sensorless techniques of position estimation for surface-magnet brushless motors. The first technique is the one proposed by Barinberg, which is a variant of the well-known Matsui's technique. The thesis presents a practical implementation of the technique using a DSP system. The thesis also presents experimental, analytical and simulation based investigations into the stability and robustness of this system. In the light of these investigations, the thesis recommends some guidelines for the design of the estimator and the selection of its parameters. The second sensorless control technique that is investigated is that proposed by lizuka for brushless DC PM motor with square wave current. The thesis studies the design of the filter circuit proposed!by lizuka and proposes design criteria and methodology for the filter circuit. The thesis also develops a robust FPGA based hardware implementation for the system that is tolerant of position estimate errors that can occur under severe load torque changes. The thesis investigates the stability and robustness of such a system under various load conditions using this hardware and software implementation. The developed system is exploited commercially by TSL Technology Ltd.