Chapter 4 - Theory III: The Stepper Motor and Its Control

Abstract

The stepper motor is a very popular low-cost electric motor whose drive shaft rotates in discrete angular steps, usually a basic standard 1.8 degrees/step, that is, 200steps/rev. It derives its torque from electromagnetic energy. Depending on the coil driving sequence, it is possible to increase the step resolution in integer multiples of its standard 200steps/rev resolution, for example, 400, 800, up to 25,600steps/rev, which is called “microstepping.” Stepper motors give resolute and accurate position, velocity, and acceleration control of, for example, mechanical links, levers, cams, and linear slideways. It is an open-loop actuator which has certain advantages over a closed-loop servomechanism but the advantages have certain conditions and limitations which must be followed for its effective operation. Such conditions and limitation relate to its “pull-in speed,” “pull-out speed,” and the lack of knowledge of the position of the output shaft. Such disadvantages relate to it not being a closed-loop servomechanism device and that it can “lose lock” such that its output angle is unknown. This chapter describes the motor and two types of electrical power circuitry, the unipolar and bipolar drivers, which energize the motor to provide the mechanical power to propel the mechanism, which, in turn, hits or throws the ball to its target. The electrical circuitry that drives the stepper motor to a given position, or at a given velocity and acceleration is the responsibility of power H-bridges that are sequenced by a microcomputer. The chapter describes the software algorithms that accomplish this sequencing.