At 04:16 PM 11/24/99 +1300, you wrote:
>Hi, has anyone got, or can they point me to, info on torque control
>of DC motors ?
>
>I've got a 1/2HP 12V DC motor, PWM (10kHz) speed-controlled by
>an IRF151 to ground. The F84 has 8 pushbuttons for Stop, Full and
>6 fixed speeds, which works OK, but the customer's needs have
>changed and would like better speed regulation under a now variable
>load.

You can use a sample-hold to read the back EMF from the motor during the
time the IRF151 is off. This voltage is proportional to the motor speed -
essentially the motor acts as a generator during this time. I'm assuming
the motor is a permanent magnet type.

The sample-hold can be a 4066 analog switch, a capacitor and an opamp
configured as a voltage follower or non-inverting amplifier. Be sure to use
a series resistor and a 15V zener across the input to the 4066 or it won't
last long.

Load the voltage with 1K or so and sample a few microseconds after the
transistor turns off to reduce pickup of transients. Then lowpass filter
the voltage and amplify it so you can read it with a DAC. Use that to
control the PWM. The lowpass filter can just be a resistor and a cap, 10K
and .1uF might do for a start.

I used this with a 12V 1/4HP motor with 1KHz PWM and the results were quite
good, the motor speed stayed constant around 2% over the full range of loads.

A possibly simpler way would be to use a toothed disk mounted to the motor
shaft with a slotted photosensor on the edge, you could read the time
between pulses and use that to adjust the PWM.

Or if that's not practical, you could mount a tiny magnet to some rotating
part such as a pulley (often nonmagnetic) and use a hall effect sensor to
pick up the time between rotations. This will work up to around 15,000 rpm.
Radio Shack sells small cobalt-samarium magnets (about 1/8" diameter),
these should work OK. Digi-Key has a variety of Hall-effect sensors, pick a
sensitive one so you don't have to place the sensor too close to the
rotating part.

Maris