Hi,
Thanks everybody.
I did not imagine that audio problem is also involved. I have to check
this out.
Rgds
ramana

-----Original Message-----
From: Olin Lathrop, Cognvision, 978-772-3129
[mailto:olin_piclist@COGNIVIS.COM]
Sent: Thursday, August 24, 2000 4:45 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: 10 PWM outputs on PIC16F874


> Hi, I need to control 10 DC motors using DC Voltage control. Can this
be
> done in Software PWM Only. Any Ideas.

I've done similar things a few times doing the PWM in software.  There
are
different approaches to this, depending on how dedicated the processor
is
to just the PWM task.  For example, a single dedicated PIC can do fast
PWM
by jumping an adjustable amount into a list of PWM "on" instructions,
followed by a single PWM "off" instruction.

One project I'm currently working on needs to control a DC motor and
still
do quite a bit of other stuff, like floating point servo computations.
There I did the PWM from the interrupt routine.  It's a tradeoff between
how often you are willing to allow interrupts, how long a PWM period you
can tolerate, and how much effective resolution you need.  In this case
I
used a 20MHz PIC with a 500 Hz PWM frequency with the on time in steps
of
50uS.  This therefore required a regular interrupt every 50uS.  We are
still playing with how much granularity is really needed to drive the
motor.  I think we're overdoing it and could therefore decrease the PWM
period, but others are investigating that right now.  At 500Hz with this
motor, you can hear the PWM frequency, but the inertia completely damps
it
out from a drive point of view.

> Another point is that the motors will all be operated in 4 discrete
> steps
> off, low, medium, high. If this is so does it simplify things.

Generally yes, since most software PWM schemes have a finite number of
on/off slices per PWM interval.  This means you only need 4 slices,
which
allows shorter periods.

> What frequency of PWM would be good for DC Motor control The motors
are
> small motors taking about 100ma at 13.6 Volts DC

You just have to experiment.  The motor is a mechanical system that
probably can't even "see" 100Hz.  The whine at the PWM frequency may be
more of a problem than any angular vibration.  All things being equal,
it's nice to do PWM for mechanical systems above the audible frequency
range, but your not going to get that doing it in software.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@cognivis.com, http://www.cognivis.com

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