Speed control of the 12-step sensorless drive for a brushless dc motor

The 12-step sensorless drive scheme requires an interval to detect the zero crossing of the back emf. The voltage regulating pulse-width modulation (PWM) is commonly used for the speed control of a motor. However, it induces the fluctuation of the back emf and makes the zero-crossing detection difficult. This paper proposes two digital approaches to the zero-crossing detection for the presence of the PWM. One approach is to vary the zero-crossing detecting period linearly with respect to the speed of the motor. The other is to detect the back emf only on the positive pulses of the PWM in order to avoid the fluctuation of the back emf. A digital filter is also incorporated in order to eliminate the noise generated by the PWM. Experiments show that both approaches have good performance for the speed control of the brushless dc motor.