From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


Hi Joe
 This wouldn't work because the LM317 is unity gain
for the feedback with 1.25V offset. I can't think of a way to get the IR
feedback mixed with the overall motor voltage feedback together at the same
time. With another op-amp, it could be done. One might even be able to do
it with two LM317's. One would provide the negative feedback for the motor
voltage and one would provide the positive feedback for current feedback.
I'm still not sure how I'd wire that up. Dwight


>From: "Joe Curran" 
>
>
>Why not just use a Radio Shack LM317T variable voltage regulator?
>Does this motor require PWM to avoid overheating?
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>From: "Jerry B. Hillman" 
>>I finally found a simple circuit to control the motor. Pulse Width
>>Modulation.
>
> Pulse modulating improves low speed action but doesn't effect
>the torque all that much. It overcomes some of the issues with
>stiction at low speed.
> For DC motors, you need to include a feedback of the back EMF
>of the motor to the controller in order to maintain speed.
>In doing this, you need to subtract out the voltage drop caused
>by the IR drop of the motor. You can put a small resistor in
>series with the motor and measure the current to the motor.
>For small motors you can just measure the resistance of the
>motor with an Ohm meter and use that to determine the voltage
>that would have been added because of the IR drop of the motor.
>You sum this feedback to the control voltage as a positive
>feedback for the IR and a negative feedback for the entire
>voltage across the motor. The trick is that the back EMF is
>proportional to the motor speed. The total voltage across the
>motor leads is the back EMF plus the IR loss. If you make the
>controller so that the control strength is controlling only the
>back EMF, you have a nice constant speed. The pulse width is then
>only related to keeping the control voltage equal to the
>back EMF.
> I hope this makes some sense.
> This method is used in many high quality cassettes drives and
>some of the direct drive turn tables. These usually use pure
>analog drive signals but one can use PWM drive with the right
>amount of filtering.
> This is also used on Orion's small battery Accu-Track drives.
>I use these drives on my smaller scopes ( some of the early
>units were not correctly made and had an incorrect resistor
>in them ).
>Dwight
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