A New Sinusoidal Wave Synchronous Drive of a Brushless DC Servomotor Only Using Hall Element Signals

Abstract

This paper describes about new method of low torque ripple operation, and wide range velocity control from 10 to 40000rpm of a brushless motor. The proposed method uses only three position signals of Hall elements arranged in the motor and does not need a position-sensor with high resolution or a velocity-sensor which used conventional control system. A sinusoidal wave drive or a square wave drive is selected according to the motor speed and the switch-over is implemented at an appropriate timing to keep out the transitional oscillation. In low speed range, the motor is operated as a plain synchronous motor using sinusoidal wave. The other hand, in high speed range, the motor is driven by square wave generated by position signals of Hall elements, and rotational speed is controlled using PI scheme. If the brushless motor is driven as a synchronous motor, the motor can be made to turn smoothly at low speed range. However, this drive method of the motor is inefficient and also the motor itself heats up at low load. In order to get a high efficiency drive method of the motor, the apply voltage to the winding is adjusted by the torque angle detected by the phase difference between the Hall element signal and the excitation voltage. A technique of switch-over without transitional oscillation which causes when the synchronous speed and the real motor speed are not the same at switch-over point, is also described here. This proposal method will be useful for the application of brushless motor which can be not attached a position-sensor with high resolution or a velocity sensor for small size or environment of operation such as dental hand-piece.