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Stepper Motors

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Stepper motors work in a different way to the motors just discussed. Instead of rotating smoothly, they do so in 'steps'. A control pulse is needed to make the motor move to the next position. The advantage of this is that the rotation can be precisely controlled - ten pulses means a rotation through ten steps. With a conventional motor, it is pot-luck where it stops spinning.

The next diagram shows the principle of the stepper, though actual motors are much more sophisticated! The rotating part of the motor, the rotor, is a bar magnet. It is surrounded by four coils of wire, wound on magnetic material. When a current flows around one of these coils, it becomes an electromagnet. The direction in which the current flows around the coil determines which end of the electromagnet is a North pole and which a South pole. Once a current flows, the electromagnet produced interacts with the rotor's magnetism. 'Like poles repel, and unlike poles attract' was a lesson learned very early in school science! The rotor moves round to try to get unlike poles as close together, and like poles as far apart, as possible. It is held in that position by the opposing magnetic forces. This is another difference between stepper motors and 'normal' motors. The stepper has this built-in brake. When not moving, it is held in position by the magnetic forces. A 'normal' motor can be rotated with very little force.

By sending pulses of electric current in the correct direction to the correct coil, the rotor can be made to step from one position of balanced magnetic forces to the next. The rotor moves through 45° from one position to the next. For this simple stepper motor, the step angle is 45°. The next diagram illustrates this idea.

Stepper motors come with the same two problems we discussed for 'normal' motors - the need for driver circuits to deliver sufficient current for the motors, and the need for a diode to protect against the high voltage generated when the current switches off, or in this case, reverses. Stepper motors also need more circuitry to generate the pulses of current to the right coil, in the right direction, at the right time. This can be done by a software program directly, but an integrated circuit known as a SAA 1027 can make this job easier. More of that later!

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