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'[PIC]: 10 PWM outputs on PIC16F874'
2000\08\23@074942 by Ramana

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

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

rgds
ramana

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2000\08\23@080809 by Alan B. Pearce

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

This depends on the core of the motor. If the motor has an iron core, then a
reasonably low frequency needs to be used. If the motor is a coreless type
(Falhauber or similar, also known as a bell wound armature) then the frequency
needs to be quite high, of the order of 20kHz. If you really want to chase the
differences , try a model railroading list which has details of DCC decoders for
electronic control.

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2000\08\23@080813 by Bob Ammerman

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----- Original Message -----
From: Ramana <spam_OUTramanabvTakeThisOuTspamDANLAWINC.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Wednesday, August 23, 2000 7:47 AM
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.

Sure.  Try using 'vertical math'.

> 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.

This means you can probably use a relatively poor resolution in your PWM. I
would _not_ expect to get away with a resolution of just 4 equal-sized
intervals however.

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

The lowest you can get away with given the fact that you want to bit bang
ten of these.

> rgds
> ramana
>
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>

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2000\08\23@090211 by Ramana

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Hello,
May I know what "Try using 'vertical math'" is.
thanks
ramana

-----Original Message-----
From: Bob Ammerman [RAMMERMANspamKILLspamPRODIGY.NET]
Sent: Wednesday, August 23, 2000 5:36 PM
To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
Subject: Re: [PIC]: 10 PWM outputs on PIC16F874


----- Original Message -----
From: Ramana <EraseMEramanabvspam_OUTspamTakeThisOuTDANLAWINC.COM>
To: <PICLISTspamspam_OUTMITVMA.MIT.EDU>
Sent: Wednesday, August 23, 2000 7:47 AM
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.

Sure.  Try using 'vertical math'.

> 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.

This means you can probably use a relatively poor resolution in your
PWM. I
would _not_ expect to get away with a resolution of just 4 equal-sized
intervals however.

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

The lowest you can get away with given the fact that you want to bit
bang
ten of these.

> rgds
> ramana
>
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> (like ads or off topics) for you. See http://www.piclist.com/#topics
>
>

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2000\08\23@160803 by Lance Allen

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On 23 Aug 2000, at 17:17, Ramana wrote:

> Hi, I need to control 10 DC motors using DC Voltage control. Can this be
> done in Software PWM Only. Any Ideas.
> 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.
>
> 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 will find numerous references on the PICLIST archive covering
multiple PWM outputs in software under the "servo" keyword
(including code listings).
If the motor is a standard iron core (like most model railway motors)
then I personally found 400Hz an excellent frequency, i.e. the motor
started best at this frequency under heavy load.

_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

http://www.psych.auckland.ac.nz

_____________________________

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2000\08\23@210443 by 772-3129

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> 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, @spam@olinKILLspamspamcognivis.com, http://www.cognivis.com

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2000\08\23@233552 by Ramana

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

{Original Message removed}

2000\08\25@024251 by Scott Dattalo

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On Wed, 23 Aug 2000, Ramana wrote:

> Hi, I need to control 10 DC motors using DC Voltage control. Can this be
> done in Software PWM Only. Any Ideas.
> 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.
>
> What frequency of PWM would be good for DC Motor control The motors are
> small motors taking about 100ma at 13.6 Volts DC


Well, how 'bout 8?

http://www.dattalo.com/technical/software/pic/pwm8.asm

Of course, you'll still need the low pass filter and power amplifier.

Scott

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