Its for a plotter Im making. I'm going to use an encoder for feedback
(position) and maybe a PI loop. Its pretty heavy duty, Id use steppers but I
cant find any powerful enough. I want to slow the dc motor down as it
approaches the encoder position I want. I was thinking of using power window
winder motors.

Justin

-----Original Message-----
From: Spehro Pefhany [mailto:speff@INTERLOG.COM]
Sent: Monday, 26 August 2002 23:37
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: DC motor control


At 07:03 AM 8/26/02 +0100, you wrote:
>Does anyone know the best way to control the speed of a 12v DC motor
without
>losing too much torque? Would pwm work or just varying the voltage?

Open-loop PWM works pretty well on PM DC motors (or DC motors with a
separately energized field winding that you leave at full voltage).
Depends what you need, of course. No-load RPM will be approximately
proportional to the average voltage, IOW  Vin* (ton/(ton+toff)).
You can't compare it to AC motors under similar circumstances (much better).

If you *do* need feedback, there are a couple of sensorless ways of doing
it (each with its own limitations and advantages), or you could just
hang a tacho off the motor and code up a PI controller.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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