A new stepping motor servo system for improved precision profiling performance

Stepping Motors are conventionally used for open loop position control purpose. On close study of the stepping structure of the Hybrid Stepping Motor, it was found to be similar to a multipole motor which has the effect of averaging out the torque ripple if the rotor position is fed back to control the commutation. This rotor position feedback for commutation was found to give an additional control dimension in the form of lead angle in addition to the voltage control for an ordinary d.c. servo system. Further study was carried out to examine how this additional dimension of control can be exploited to produce a low cost servo system.; Although experimentations have shown that the torque ripple with careful adjustment of the lead angle is comparable to that of servo motors, it was felt that there is room for further improvement on precision profile tracking where the requirement is for good control of not only the rotor position and velocity, but higher orders of the dynamics. Further studies into the torque ripple have shown that the permanent magnet in the hybrid stepping motor has made its torque-current-position characteristics highly nonlinear, especially under low operating speeds. The research was then switched to study the feasibility of developing a set of simple and efficient control algorithms to compensate for this dynamic characteristic so as to further improve its high-precision tracking control. The study then examined principles of several control schemes for minimizing the motor's torque ripple that is periodic and nonlinear in the system states especially at low-speed. A series of algorithms was proposed, that utilize the feedback controller to stabilize the transient dynamics of the servomotor and then a feedforward controller was used to compensate for the effect of the torque ripple and other disturbances to improve tracking accuracy. The stability and convergency of these control schemes are studied and their results presented. Simulation of the proposed algorithms have revealed that all the error signals in this control system are bounded, and the motion trajectory approaches the desired output within an arbitrarily designated zone in a finite time or converges to the desired value asymptotically. The results of these Simulations and experimentations are presented to demonstrate the effectiveness and performance of the proposed algorithms that could make the servo system better than systems using servo motors.