(offlist)

Hi Justin! I'm in Aust too and do some hobby CNC stuff,
with steppers and servos etc. I have two machines nearing
completion, both 3 axis machines, and am trying to start
a local hobby robot/CNC/PIC club sydney/wollongong area.

Are you in NSW?

here's my web page:
www.romanblack.com
:o)
-Roman



Justin Grimm wrote:
>
> 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
> >
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