Speed controllers have long exploited the reverse-EMF characteristics of dc motors to control their speed. These linear driver circuits use power op amps to create a negative resistance drive to the motor that counteracts the voltage drop in the motor’s series resistance (Reference 1). The circuit in Figure 1 shows an implementation of this type of speed control using a PWM drive, which reduces power dissipation in the drive circuitry.
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| Figure 1. | Positive feedback derived from current-sense resistor RS increases the duty-cycle drive from PWM controller IC2 to compensate motor speed with varying loads. |
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Control voltage VIN sets the speed, and IC1, Q1, and R3 convert VIN to a 0- to 200-µA current. The current source controls the duty cycle of the PWM driver, IC2, at Pin 3. D1, D2, and R5 prevent the circuit from pulling the control input too low, which can cause an inversion in the control loop.
Motor current flows through the internal switching transistor in IC2 out the common terminal and through the current-sense resistor, RS. The circuit filters and scales the voltage across the sense resistor to provide positive feedback to the input circuitry through R2. With the proper amount of positive feedback, an increase in motor load increases motor current, which increases duty-cycle drive to maintain constant speed.
The equation of speed balance is
This model neglects losses in the switching circuitry of IC2. Depending on supply voltage, such losses may significantly affect the required feedback. Unlike the power op amp implementation of the scheme, the absolute supply voltage affects this compensated PWM controller. The voltage lost in switching is not truly resistive and can be tricky to model. In practice, you can optimize the speed control by adjusting the positive feedback control at R4. With proper adjustment, motor speed remains relatively constant with substantial changes in load. Although less accurate than true closed-loop tachometer-feedback controllers, this low-cost technique provides a dramatic improvement over simple voltage drive.
