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[DPRG] How to protect TETRIX motors

Subject: [DPRG] How to protect TETRIX motors
From: Greg Needel greg at robogreg.com
Date: Tue Feb 19 11:29:54 CST 2013

I hate these motors with a passion. Admittedly I have never used them in
FTC, but for the minibot associated with FRC in 2011 but I am directly
responsible for ~30 motor deaths. The only thing I can advise is design you
gearing correctly and avoid stalling the motors. Use surgical tubing to act
as counter weights to reduce the load on the motors for arms and lifts. In
the drivetrain, tell you driver to avoid defensive actions which required
locking up the wheels. It is possible to replace the inductor if it goes,
just make sure that you attach the cap back in the right orientation or you
can alter the performance of the motors.


On Tue, Feb 19, 2013 at 11:17 AM, David Jannke
<david at proboticsamerica.com>wrote:

>  Hey Karim, do you have the part number from the part on this:
> http://www.legoeducation.us/eng/product/tetrix_thermal_protected_dc_motor_power_cable/2135? It looks like cutting the heatshrink and replacing just the part that
> starts failing would be easy enough. They might be available from Mouser
> pretty cheap too. Admittedly you don't end up eliminating the problem
> entirely but cheap expendable fuses are better than expensive ones.
>   *From:* Karim Virani <karim at compuguru.com> <karim at compuguru.com>
> *To:* dprglist at dprg.org
> *Sent:* Monday, February 18, 2013 2:44 PM
> *Subject:* [DPRG] How to protect TETRIX motors
> 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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> --
> David Jannke
> Probotics America
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