First of all thanks everyone for your replies. I think when I get access to
the internet I'll check out the servo motors. In the meantime though this
method of pwm then sampling the generated voltage intrigues me.
I want to deliberately control the speed, so if I'm generating say a 50%
duty cycle pwm, and I want to slow the motor down, I just shorten the duty
cycle then read off the inferred speed (generated voltage) and if its not
slow enough shorten the pwm again. Is this correct?

Is there a easy way of knowing what the generated voltage would be?

Thanks
Justin

-----Original Message-----
From: David Minkler [mailto:Mink@LUXTRON.COM]
Sent: Tuesday, 27 August 2002 3:14
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: DC motor control


Justin,

Without feedback you will not be able to control motor speed without
losing torque.  What Scott is describing is using the EMF from the motor
(operating as a generator / tachometer) at the end of the PWM off time
as his feedback signal.  With good filtering, sampling at the right
time, and not attempting to operate at very low speeds this technique
works well.

Regards,
Dave

"Scott M. Thomas" wrote:
>
> I bit banged a PWM routine (several bits of sample code available on
> piclist.com under I/O routines) then immediately before turning on the
> output transistor I measured the voltage at the motor using the A/D
module.
> I used this measured value to adjust the duty cycle up or down.  Worked
> pretty well.  All I did for a feedback circuit was a 10k resistor from the
> drain of the transistor to the RA0 input with a 6.8k to ground to divide
the
> voltage down and a .33uF cap to ground to smooth out the waveform.  I was
> running at about 150 Hz so I had lots of time to measure and do
> calculations.
>
> -----Original Message-----
> From: Justin Grimm [mailto:Justin.Grimm@SOUTHCORP.COM.AU]
> Sent: Monday, August 26, 2002 2:03 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE]: DC motor control
>
> 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?
>
> Thanks
> Justin
>
> --
> http://www.piclist.com hint: The list server can filter out subtopics
> (like ads or off topics) for you. See http://www.piclist.com/#topics
>
> --
> http://www.piclist.com hint: The list server can filter out subtopics
> (like ads or off topics) for you. See http://www.piclist.com/#topics

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.