A study on dynamic performance of closed-loop commuted stepping motor to improve precision motion control

To achieve precision movements, a motor with very low speed ripple and torque ripple is required. A motor with more poles will normally give higher ripple frequency and hence lower ripple magnitude. The hybrid stepping motor when commuted in closed-loop mode will behave in the form of a multi-pole brushless DC servo motor with each step similar to one pole. To reduce torque ripple further, the commutation of the coils requires careful adjustment of the lead angle in order to compensate for the time constant of the coils at different speeds. This article establishes an accurate dynamic model for the closed-loop commuted hybrid-type stepping motor, which takes into account the drive circuit and the effect of magnetic saturation. Simulations have been performed on the model using the stiff system numerical method for higher accuracy. The results show a good match between the numerical solution and the actual measurement. The dynamic model developed can be used to predict speed ripple of the motor system under various commutation methods. This provides a means of minimizing speed ripple by using software to improve the accuracy of motion control systems on high-precision machines.