Hi    guys
got a question here
is there a standard for the value of PWM frequency for motors and how you
determind it??

thanks
regards
----- Original Message -----
From: Maris <maris@TIAC.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, November 24, 1999 12:53 PM
Subject: Re: Torque control


> 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