From: Joe Curran 
To: atm{at}shore.net
Reply-To: 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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