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[DPRG] How to protect TETRIX motors

Subject: [DPRG] How to protect TETRIX motors
From: Karim Virani karim at bigthought.org
Date: Wed Feb 20 22:48:02 CST 2013

Wish I'd been able to attend RBNO last night.  Maybe next week.  It's weird that there are differing specs out for these motors.  Must have been an OEM change.  The motors are simply branded with a TETRIX label but otherwise have no markings on the outside.  I haven't tried opening one up yet.  So now I'm not sure which spec applies to our motors, though they were bought recently.  Either way, a 4 or 5A fuse would probably do the trick?  Assuming I can find one that will blow fast enough at that current without blowing on every quick ramp of up power.  I think I'll try the approach with the polyswitch devices first - at least during testing.  It did at least protect the motor while it was in use.

The motor does need to maintain power to maintain position, and the amount of power needed can vary widely depending on the set position with respect to gravity.  There are also perturbations caused by the movement of the chassis and varying loads on the arm.

From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On Behalf Of Doug Paradis
Sent: Monday, February 18, 2013 11:07 PM
To: Bob Jordan
Cc: Karim Virani; dprglist at dprg.org
Subject: Re: [DPRG] How to protect TETRIX motors

Karim,
     I looked at the motor link that you gave and found this spec: https://c10645061.ssl.cf2.rackcdn.com/resources/dcdrivemotorspecs.pdf
I would interpret this spec as the motor having a maximum output of 10.7W at 89 RPM with a rated current of 2.46A. Above 2.46A would be overloading the motor.
The "semi-official" spec you mentioned is significantly different and has a maximum output of 9.36 W at 78 RPM with a rated current of 3.3A.
Both of these specs are at a rated voltage of 12 VDC.

The performance measured by the individual who had access to a dynamometer, showed a maximum power of 10.8W at 75 RPM when run at 12 VDC. He didn't mention current, but in a later post he states that the motors used 39W of power from the 12V supply, or  39/12 or ~3.25A. This is similar to the "semi-official" spec.

With a fully charged 12V battery (voltage ~ 14V), the motor provides a higher max power, ~15.5W at 90 RPM. Since the voltage increased by 16.6%, the current should also go up 16.6% or to 3.8A. This amount of current overload, is probably ok (motor will be hotter), but stalling the motor with a current of  8A or more is badness.

The power lost to heat in a motor is current squared times armature winding resistance. So increasing current by 2 causes heat to go up by 4. It is a very bad idea for any dc motor to sit at stall.

As an electrical engineering type, I would consider adding a current limiting circuit, and if possible limit switches. I would also suggest adding a current control loop to your motor control system.

Bob's idea of a mechanical clutch/slip joint  is great. I see them a lot in toy cranes. If you want, I can bring you some at RBNO. They may give you some ideas.

You might like this link: http://www.micromo.com/Data/Sites/1/technical-library-pdf/dcmotor-howtoselectadcmicromotor_mme.pdf
or this one
http://www.ebay.com/itm/5A-range-Current-Sensor-Module-ACS712-Module-free-shipping-/190793664903?pt=LH_DefaultDomain_0&hash=item2c6c304987

Does the motor need to retain power to retain position in your application?

Regards,
Doug P.
On Mon, Feb 18, 2013 at 5:55 PM, Bob Jordan <rljordan at airmail.net<mailto:rljordan at airmail.net>> wrote:
Consider installing a mechanical clutch/slip joint in the mechanical
linkage, like a friction fit gear or a belt drive. Motor can turn rather
than stall.
Or you could install a current limiting circuit before the motor. Full power
until current goes too high, then power reduced until current load drops.
This could be done with a 12VDC 3 lead regulator and a current limiting
resistor.

Bob Jordan

-----Original Message-----
From: dprglist-bounces at dprg.org<mailto:dprglist-bounces at dprg.org> [mailto:dprglist-bounces at dprg.org<mailto:dprglist-bounces at dprg.org>] On Behalf
Of Karim Virani
Sent: Monday, February 18, 2013 4:45 PM
To: dprglist at dprg.org<mailto:dprglist at dprg.org>
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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