MINOR-CLOSED-LOOP CONTROL OF A SINGLE-STACK VARIABLE-RELUCTANCE STEPPING MOTOR

Although stepping motors are readily controlled by digital signals, they are frequently rejected for specific applications using open-loop control systems, because of the possibility of missing steps. To overcome this potential problem, as well as to more fully exploit a motor's capabilities, a minor-closed-loop control approach is adopted.;To avoid limiting the maximum motor speed because of controller characteristics, software implementation is rejected in favour of a digital hardware system.;On the basis of motor speed and rotor orientation, the controller selects the switching angle which maximizes the average output torque magnitude, using a look-up table. This table contains an approximation to the experimental data at discrete intervals. The data are stored in read-only memories and the contents are keyed to the motor speed. For a large class of stepping motors, the needs of specific types can be accommodated by modifying the contents of the read-only memories.;Rotor orientation is evaluated by a combination of position sensing and a time-based interpolation technique. In addition to the rotor orientation, major-closed-loop position information is provided.;It is necessary to establish a relationship between motor speed (stepping frequency) and the switching angle which yields the maximum average output torque. The feasibility of obtaining the desired relationship using non-linear stepping motor models incorporating various combinations of theoretical consideration and supplementary experimental data is examined. These approaches fail due primarily to the lack of suitable techniques for evaluating the motor's magnetic field characteristics under various operating conditions. Hence, an experimental technique is adopted to evaluate the switching angle relationship.;The procedure to maximize the average output torque magnitude is in effect during deceleration as well as acceleration. As a result, the controller is oriented toward high speed positioning. In addition, speed control is available.;Although the minor-closed-loop control system is intended to operate in conjunction with a main control system, the design includes test facilities for evaluation of the switching angle relationships of specific stepping motors.