Designing Embedded Hardware, 2nd Edition

Motor Control

One of the fun things you can do with an embedded computer is get it to actually move something, whether it be an external system or the embedded computer itself. Motion implies motor, and this section will look at how you interface an embedded computer to an electric motor. The possible applications could range from controlling locomotives on your model railroad layout to experiments in robotics, and anything in between. A note of caution, though: if your hardware and software are responsible for moving a physical object, then a bug can easily cause physical damage too. So be careful.

Let's say that we have an electric motor than operates from a 12 V supply. Applying 12 V across the motor will cause it to turn at full speed. Similarly, by applying 6 V, we can get the motor spinning at half speed. By varying the applied voltage, we can vary the speed at which the motor turns.

There are several ways to generate this voltage to drive the electric motor. The most obvious may seem to be to use a DAC to generate an analog output voltage and then use an amplifier to boost the signal to the voltage and current required to turn the motor. The speed of the motor is proportional to the output voltage. However, this technique has a major drawback. For very low-speed operation, the required output voltage may be too low to actually cause the motor to turn.

A better way is to use PWM. Consider the PWM signal in Figure 13-28, with an amplitude of 12 V.

Figure 13-28. PWM signal with ...