Subject:
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Re: moving in a straight line with two motors?
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Newsgroups:
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lugnet.robotics
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Date:
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Sat, 14 Nov 1998 14:40:40 GMT
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Original-From:
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Bob Wind <rwind@SPAMCAKEgfn.org>
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Viewed:
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2150 times
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> From: Kekoa Proudfoot <kekoa@Graphics.Stanford.EDU>
> To: Lego Robotics <lego-robotics@crynwr.com>
> Subject: Re: moving in a straight line with two motors?
> Date: Friday, November 13, 1998 3:20 PM
>
> > > Also, there is constant current leakage whenever there are motors • attached.
> > > Should be pretty low if the motor is not moving, but should climb a
> > > LOT if the motor is driven by some mechanical force.
> >
> > Please explain why there is necessarily leakage current when the motor • is
> > shorted. I can see there being very little leakage current if the CMOS • is
> > done right, but I haven't reviewed the circuit diagram on the data • sheet
> > recently, so I'm not sure if this is the case with the chip in the RCX.
>
> Hm. So I just reviewed the data sheet again, and it seems that there is
> indeed a significant current when the motor is shorted. That sucks.
>
> -Kekoa
I expect that the "motor is shorted" state is achieved by turning on
either both high-side drive FET's or both low-side drive FET's, and
niether of these combinations would provide a high current path
from supply to ground. The spec sheet quotes 240 mA as a
minimum rating probably to guarantee that up to that amount of
current will be drawn on the motor when it is moving and acting
as a generator. You're right that it wouldn't be very good
to draw that current from the battery whenever the motor is
stopped in "shorted" mode, but that's not how I interpret
the driver data sheet.
Bob Wind
rwind at gfn dot org
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