Torque Ripple Suppression of Brushless DC Motors with Non-Ideal Back EMF for EPS Applications

The torque ripple of brushless dc (BLDC) motors at low rotation speeds is prominent, while in the majority of working time of the automotive electric power steering (EPS) system, the steering rate is low and the assistant motor runs at low rotation speeds. Therefore, to develop a high-performance EPS system with a BLDC motor, the problem of torque ripple at low rotation speeds must be solved. Otherwise, the steering feel will not be smooth. This paper proposed an algorithm based on BLDC motors' essential torque formula combined with the back-EMF (Electromotive Force) waveform coefficient to modify the target current to suppress the torque pulsation effectively. Meanwhile, the commutation process of this type motor is very short. Therefore, the torque coefficient can be regarded as constant during the commutation intervals. A direct pulse-width modulation (PWM) control method considering phase resistances is analyzed with a unipolar PWM scheme to reduce the commutation torque ripple further. Finally, simulations and experiments are conducted to verify the effectiveness of this method.