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[DPRG] How to protect TETRIX motors

Subject: [DPRG] How to protect TETRIX motors
From: Karim Virani karim at compuguru.com
Date: Mon Feb 18 16:44:38 CST 2013

So I've grown to love/hate TETRIX as a prototyping system for robotics. VEX
might be better - I'm not in a position to judge, but we went with TETRIX
because it continues our investment and experience in NXT hardware and
sensors.  Mostly, (expense aside) I think it's great.

My biggest beef with the platform, though, is the motors.  The standard DC
motor is called a 12 volt and is meant to run off a 12v nominal NIMH battery
pack, which usually runs 14v on a fresh charge.  

http://www.legoeducation.us/eng/product/tetrix_dc_drive_motor/1610

These motors burn out easily.  My robotics team has just burned up its 6th
motor. At 30 bucks a pop this feels like a racket.  They burn up within one
second of hitting a stall condition.  In my book, I don't think these motors
should be rated for 12 volts if they're going to burn up that fast.  I don't
think they'd ever be considered in the automotive industry.  Am I off target
with this opinion?  

None of the motors driving our wheels has had a problem - the wheels will
typically lose grip before stalling the motors.  The motors used on our arm,
though, are susceptible to stalling when the end of travel is hit.  We have
encoders on the motors and have profiled the arm's legal limits and have
those limits built into the software.  But remember that I'm working with
middle school kids who are still learning and far from cautious.  Limits
have been disabled by errors in coding and/or by invalid startup conditions.
And the limits don't help when getting stalled by other barriers in the
environment.

So we need to know how to protect these motors.  TETRIX produces an optional
thermally protected wire harness for these motors:
http://www.legoeducation.us/eng/product/tetrix_thermal_protected_dc_motor_po
wer_cable/2135

These are actually supposed to work more like a circuit breaker.  They are
supposed to reset automatically once the power goes to zero.  They did
indeed protect the motors.  But in our experience, after they've been
triggered once or twice they begin to progressively drop the voltage to
levels where the joints might barely move. This behavior, before it was
understood, cause havoc with our attempts to tune our PID constants.  So
they're actually more like expensive disposable fuses.  I'd rather figure
out a cheap and reliable fuse to try out.

So as a software guy, I'm pretty clueless with electronics.  I'm not sure
what we need.  I believe fast blow fuses aren't right for inductive loads,
but a slow blow might be too slow to protect these fragile motors.  I want
something that trips within half a second (or less) at stall but otherwise
permits the motors to drive well above no-load conditions. Are there medium
speed fuses?  Is that something Tanner's would carry?  What values should I
choose?  Is there some other kind of option?  AFAIK there is no current
sensing ability in the HiTechnic motor controllers - and it likely wouldn't
be legal in FTC to add such circuitry.

Here's a thread that has some specs for the motors:
http://www.chiefdelphi.com/forums/showthread.php?t=89072

Greg Needle participated in that thread and posted this link to the
semi-official specs for these motors:
http://www.chiefdelphi.com/forums/attachment.php?attachmentid=9675&d=1294973
675
note how they didn't measure but rather extrapolated the stall figures.  I
can't actually say what load will fry them.

Thanks for any and all advice,

Karim



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