Designing Embedded Hardware, 2nd Edition

Motor Control with a PIC

Now let's look at using a PIC in a completely different sort of application: motor control via user input. While the design presented in this section is targeted at a specific application, it is just as easily adapted to any task where small DC motors need to be controlled, from tools to robotics.

My young nephews are into model trains, and the standard controller that came with the railroad was a fairly simple device. The speed of the locomotives is controlled by simply varying the voltage on the track. Turn up the voltage, and the trains go faster; turn down the voltage, and the trains stall on dirty rails. Fine control and realistic operation just wasn't possible. I decided to solve this problem by throwing a little high-tech at it and designed for them a microprocessor-based controller using a PIC. It's this design that I will share with you.

Tip

I've designed literally many dozens of embedded systems over the years, from tiny controllers to small parallel supercomputers, but I have to say that I've never had so much fun debugging as when I was debugging the train controller.

The design uses pulse-width modulation (PWM), discussed in Chapter 13, to control the motors inside the locomotives. As PWM uses pulses of fixed amplitude to drive the motor, the problems of low-voltage stalls disappear. Further, since the PWM is controlled by software, very slow speeds can be achieved, giving very realistic operation. The PIC processor has two PWM generation modules ...