INTRODUCTION

Stepping motors are attractive because they can be controlled directly by computers or microcontrollers. Their unique feature is that the output shaft rotates in a series of discrete angular intervals, or steps, one step being taken each time a command pulse is received. When a definite number of pulses has been supplied, the shaft will have turned through a known angle, and this makes the motor ideally suited for open-loop position control.

The idea of a shaft progressing in a series of steps might conjure up visions of a ponderous device laboriously indexing until the target number of steps has been reached, but this would be quite wrong. Each step is completed very quickly, usually in a few milliseconds; and when a large number of steps is called for the step command pulses can be delivered rapidly, sometimes as fast as several thousand steps per second. At these high stepping rates the shaft rotation becomes smooth, and the behaviour resembles that of an ordinary motor. Typical applications include disc head drives, and small numerically controlled machine tool slides, where the motor would drive a lead screw; and print feeds, where the motor might drive directly, or via a belt.

Most stepping motors look very much like conventional motors, and as a general guide we can assume that the torque and power of a stepping motor will be similar to the torque and power of a conventional totally enclosed motor of the same dimensions and speed range. Step angles are...