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[DPRG] Re: Overload protection on a motor not working

Subject: [DPRG] Re: Overload protection on a motor not working
From: Kipton Moravec kip at kdream.com
Date: Sun Jul 22 18:20:19 CDT 2001

My guess is that you have to synchronize the A/D reading with the PWM so
that you read the current when the current is on, not when it is off and not
when it is transitioning.  I think the hard part is finding the delay after
turn on when the current settles down, prior to turning it off.  As the on
portion gets shorter (to reduce speed, or with faster PWM frequency) you may
never have a steady current to measure.

On the other side, based on my information theory, signal processing, and
stochastic processes background, there should be enough information
available to derive it, it is just not as easy as measuring the voltage drop
across a resistor and using Ohm's Law.

Kip


----- Original Message -----
>From: "David P. Anderson" <dpa at io.isem.smu.edu>
To: <dprglist at dprg.org>
Sent: Sunday, July 22, 2001 10:32 AM
Subject: [DPRG] Re: Overload protection on a motor not working


> Hi DPRG and Benoit
>
> Concerning current monitoring circuit on your robot not
> working when driven by an h-bridge, Benoit wrote:
>
> <snip>
> > One possibility that I suspect is that I use PWM
> >
>
> I believe this is correct, but I'm not sure why.  I have
> several M.I.T. 6.270 robot controller boards.  Four of
> the 8 A/D channels on it's HC11 microcontroller are
> hardwired to four onboard h-bridge (L293) chips.  When
> driven directly these A/D channels provide useful
> feedback about how much current the connected motors are
> drawing.
>
> However, when driven with PWM signals the current feedback
> is not useful.  The M.I.T. 6.270 manual says as much in
> the section covering the H-bridge design.  That this feedback
> is not valid when driven with PWM signals.  I'm not quite
> sure why, but I have observed the same thing.  Perhaps one
> of our DPRG theorists can explain.
>
> dpa
>
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