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[DPRG] Encoderless Speed Control

Subject: [DPRG] Encoderless Speed Control
From: Chuck McManis cmcmanis at mcmanis.com
Date: Fri Jan 2 12:52:00 CST 2004

You'll notice that the motor has "smooth 12V" in to it, this is because 
trying to PWM that motor would generate some false readings. It does 
however make for a good tachometer, and it might work for a linear control 
system where you were modulating the motor voltage linearly.

I worked on getting a similar technique to work, however I used a 
transformer in-line with the  motor sort of like this:

     V motor -----+
                  |  +-----+
                  )||(    / \      ||
                  )||(  +<   >-----||-->
                  )||(  | \ /      ||
                  |  +-----+             brush pulses
                  |     |
               +--+--+  +-------------->
     Motor Gnd ---+

That's a full wave rectifier in there and a capacitor. Anyway the theory 
was that the motor current would cover the magnetizing current of the 
transformer core, then the brush spikes could be amplified out to an 
isolated input of a comparator or something to be picked up. The trick was 
the one could only look for spikes during an "on" PWM time.

Anyway, it was never very accurate and I couldn't get it to work reliably. 
If someone does manage it, please write it up!


At 09:29 AM 1/2/2004, Kipton Moravec wrote:
>I have heard that you are able to get speed information from a DC motor 
>without using encoders. Here is an article that tells how to do it.  I 
>have not tried it, but what he says makes sense.
>You can not use this for precise position, because he says the pulses are 
>not even around the motor.  But there is a consistent number per revolution.
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