Step 2: Design the Circuit
Motor control is accomplished through pulse width modulation. Although PWM amps are slightly more complicated in both hardware and control, they are much more power efficient than linear amplifiers. A PWM amp operates by very quickly switching the current or voltage to a load between on and off states. The power supplied to the load is determined by the duty cycle of the switching waveform. Provided that the dynamics of the load are slower than the frequency of switching, the load sees the time average.
In this design, the switching frequency is approximately 87 kHz, which was tuned to the motors on the rover. The duty cycle is voltage controlled by setting the threshold of monostable oscillators driven by an astable oscillator. A digital to analog converter on the rover's computer controls the threshold voltage and thus the duty cycle of the amplifiers.
The PWM wave forms are generated by seven timers (each of the four 556's has two timers, and the eighth timer is unused). The first timer is set for astable oscillation, and switches between an on and an off state at 87 kHz. This 87 kHz clock signal is fed into the triggers of the other six timers, which are set to operate in monostable mode. When a monostable timer receives a trigger signal, it changes state from off (0 volts) to on (5 volts) for an amount of time set by the input voltage. The maximum time is approximately 75% the period of the astable clock signal and the minimum time is zero. By varying the input voltages, each monostable timer will generate a 87 kHz square wave with duty cycle between 0 and 75%. The LMD18200 chips act simply as digital switches controlled by the output of the timers and by the brake and direction digital inputs from the computer.