Electronic Design

PWM Motor Speed Control Uses AC Tachometer Feedback

Many dc servo motors provide an ac tachometer output for speed measurement or regulation. The circuit in Figure 1 uses such a tachometer output (output frequency proportional to motor speed) to create a speed controller with high-efficiency pulse-width-modulated (PWM) drive. The motor speed is precisely proportional to the input control voltage (VIN).

Q1 and the associated RC circuitry condition the tachometer’s sine-wave output to produce narrow negative-going pulses that trigger the frequency-to-voltage converter IC. The IC also includes an op amp that serves as the primary integrator for the speed control loop. The feedback loop is balanced when the integrator receives equal and opposite average currents from the input control voltage and the rate-controlled one-shot current pulses in the frequency-to-voltage converter IC. The balance condition and the relationship of the input control voltage to the motor speed is governed by the equation:

IIN = (FTACH) (TOS) (1 mA)

VIN/RIN = (N × RPM)/60 × (7500)(COS)(1 mA)

where N is the number of poles in the tachometer and TOS is IC1’s one-shot period (determined by COS).

IC2, a Burr-Brown PWM driver (DRV101), is intended primarily for driving valves and solenoid actuators, but its 24-kHz, duty-cycle-modulated output is suited for driving dc motors as well. Pin 3, a resistor-set duty-cycle-programming pin, also can be controlled with an analog input voltage. A 0.8- to 3.7-V input signal will vary the duty cycle of the motor drive from 95% to 0%.

The integrator output of IC1 supplies the control signal, servoing the motor speed to create a charge balance in the integrator. Resistor R6 and the output swing limitations of the integrator (it can’t swing lower than approximately 0.7 V) combine to limit the lowest voltage that can be applied to the control pin of IC2. Voltages lower than 0.2 V on pin 3 will shut off IC2—a phase reversal that will cause the loop to latch with the motor off.

The maximum output current from IC2 is approximately 1.9 A, enough to drive many common servo motors. An equivalent linear-pass motor control circuit would dissipate substantially more power and require larger heatsink.

See associated figure

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