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

Subject: [DPRG] Re: Overload protection on a motor not working
From: David P. Anderson dpa at io.isem.smu.edu
Date: Sun Jul 22 10:32:39 CDT 2001

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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