ABSTRACT
Two necessary elements of any DC servo system are an amplifier and a feedback transducer. In this chapter we will take a look at the most common form of each element as used in fractional-horsepower DC servos today. These are (1) the pulse width modulated (PWM) switch-mode amplifier and (2) the optical incremental shaft encoder. We present these two elements together in the same chapter because the encoder provides digital (TTL-compatible) input to a microcontroller, and the PWM amplifier can take a fixed-frequency digital pulse train of varying duty cycle from a microcontroller and provide any average voltage between zero and 100 percent of rating to a brushed DC motor. These two innovations, used together in the 1980s by HewlettPackard in their high-performance plotter [Baron, 1981] and printer [Jackson, 1988] products, made microprocessor control of DC motors possible.
