Newsgroups: comp.robotics
Path: brunix!news.Brown.EDU!noc.near.net!howland.reston.ans.net!pipex!uunet!utcsri!psych.toronto.edu!colin
From: colin@psych.toronto.edu (Colin Hinz)
Subject: Re: DC motor current sensor
Message-ID: <CFHnFH.Doq@psych.toronto.edu>
Followup-To: comp.robotics
Keywords: DC motor, current i
Sender: colin@psych.toronto.edu
Organization: Department of Psychology, University of Toronto
References: <jnicol.751591641@sfu.ca>
Distribution: na
Date: Tue, 26 Oct 1993 05:04:29 GMT
Lines: 23

In article <jnicol.751591641@sfu.ca> jnicol@malibu.sfu.ca (Jordan James Nicol) writes:
>
>Does anyone know a good way of measuring the current drawn by a DC
>motor (aside from measuring the voltage drop across a resistor) for the
>purpose of determining the load torque? I want my 6811 to monitor the
>current for a stall condition but not be fooled by the transient
>current when the motor first turns on. I've heard of some IC
>mentioned here a few months back that does this quite nicely. Anybody
>know about this? Also, I've thought of just having the micro wait for
>the initial current rush to subside before monitoring begins--any comments
>about this approach?
>
Use a current-sensing FET. If the load current is low enough then you can
drive the FET gate directly with the micro. You can get a reasonably
accurate current reading by doing an A/D with a minimum of components
inbetween. As for not measuring the inrush, this may be taken care of by
the cycle time of the micro. In the application that i'm experienced with
(10-20A motors, 24V) the measurement "dead time" should be under 4 us
from the start of the motor turn-on or PWM cycle.

- Colin


