Servo - November 2008

GETTING CONTROL

by KEVIN MCCULLOGH

WITH THE

Propeller

PART 3: Stepper Motors

Introduction

In the last article of this series, David Carrier covered how hobby servos work and an appropriate control scheme using the Propeller™ microcontroller. By taking advantage of the Propeller’s multiple onboard processors, the control board was designed to be easy to use with a minimum number of components. In this part of the series, we will take a “step” (bad pun intended) into the world of stepper motors. We will look at bipolar steppers in particular and some methods to improve their performance.

Stepper Motor General Description/Comparison

In general, stepper motors are brushless motors with many poles per rotation. In a standard DC motor, an applied voltage causes the motor to spin continuously because the windings are automatically commutated (switched) internally as the rotor travels through a full rotation. In a stepper motor, windings are commutated externally, usually with either discrete transistor switches or H-bridges. The main advantage they have over DC motors is their ability to be controlled to precise angles without using closed loop feedback. This is at the expense of increased motor weight, and lower efficiency and speed. However, open loop control systems are generally much simpler and cheaper than closed loop servo systems.

Types of Stepper Motors

Stepper motors come in a variety of configurations, as shown in Figures 1-3, but are generally categorized as

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unipolar or bipolar. From a controller’s point of view, the main difference is that unipolar motors only require current flow in one direction, while bipolar motors require the current to reverse direction. This makes unipolar motors easier to control electrically since current can simply be switched on or off in each winding. Bipolar motors generally require more complex circuitry, such as H-bridges to reverse current through the windings. This can increase the cost of the controller.

On the positive side, the power-to-weight ratio of bipolar motors surpasses unipolar motors since the entire winding is utilized at any given time on each pole. In unipolar motors, only half the coil is driven at a time. This reduces the size and usually cost of bipolar motors over unipolar motors with comparable specifications.

For this article, we will implement a control scheme for a bipolar stepper motor using the Propeller microcontroller and some external circuitry. And, since there is always something satisfying about improving hardware performance, we will also go over some ways to optimize the stepper motor drive system.