Pulse width modulation is a technique for reducing the amount of power delivered to a DC motor. Instead of reducing the voltage operating the motor (which would reduce its power), the motor s power supply is rapidly switched on and off. The percentage of time that the power is on determines the percentage of full operating power that is accomplished. This type of motor speed control is easier to implement with digital circuitry. It is typically used in mechanical systems that will not need to be operated at full power all of the time. For an ELEC 201 robot, this would often be a system other than the main drivetrain or when the main drivetrain is steered.

FIGURE 3.62   Example of several pluse width modulation waveforms.

Figure 3.62 illustrates this concept, showing pulse width modulation signals to operate a motor at 75%, 50%, and 25% of the full power potential.

A wide range of frequencies could be used for the pulse width modulation signal. The ELEC 201 system software used to control the motors operates at 1,000 Hertz.

A PWM waveform consisting of eight bits, each of which may be on or off, is used to control the motor. Every 1/1000 of a second, a control bit determines whether the motor is enabled or disabled. Every 1/125 of a second the waveform is repeated. Therefore, the control bit make 8 checks per cycle, meaning the PWM waveform may be adjusted to eight power levels between off and full on. This provides the RoboBoard with eight motor speeds.