Rotor Oscillation Damping of a Stepping Motor by Sliding Mode Control

Abstract

Stepping motors are used mainly in OA and FA systems due to the merit of their digital driving behavior using switching devices. They are driven by closed-loop control in the systems where special performance such as prevention of out of synchronism or highspeed drive is required.
It is well known that rotor oscillation is one of the principal problems in switched drive of a stepping motor, and nowadays several methods for damping of oscillation in which switching sequence is changed in respective manners have been suggested. In such method, the exciting time of stator windings must be tuned appropriately, or effect of damping becomes insufficient and oscillation may be even amplified in some circumstances. To resolve this problem, some adaptive methods for tuning of the exciting time have been developed. However, they have the disadvantage that there must be a period to tune the exciting time to the optimal value at the beginning of control or when the machine parameter is varied by changing of driving condition.
The authors developed a new method for rotor oscillation damping of a stepping motor in a closed-loop system. It is based on the technique of sliding mode control and designed to be robust to variation of rotor inertia which affects significantly to the oscillatory characteristic. In this method, a lower-order dynamical model, obtained by reaching to a sliding mode where the angle-torque characteristic of the motor is bound to a certain linear function, is made to be non-oscillatory by pole assignment over a certain region of varying rotor inertia. Applying this method to the experimental system, rotor oscillation is damped excellently in the cases of single-step and low-speed-multi-step drive. On the other hand, it is shown that deceleration must be needed near the last step in the case of high-speed drive.