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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: Thu Feb 21 00:10:01 CST 2013

Thanks Dick.  I was really hoping we could use physical limit switches as
well.  Too bad.  We can at least during testing - though it's best to test
under tournament conditions.  Actually the season is over for us so we have
more flexibility - though we'd like to develop subsystems that could be
reused next year if applicable.

Yes, we plan to monitor expected movement vs applied power to detect stall
conditions.  Of course for some stall conditions there could be degrees of
movement.  Determining the right hysteresis without burning up more motors
might be challenging for us.

Yes we are able to salvage the encoders.  Took us a while to find this video
that showed how to do it safely:

Interesting idea on bypassing Pitsco on the motors - though I guess that's
illegal for competition? 

Looks identical except for the sticker and the degree of chrome plating.  At
least they mention a load limit at 12v and have a warning that damage will
result if exceeded. The price is about the same as for the TETRIX with a
team discount.

I also found these that (except for shaft length) seem to be identical in
size - except they are lower in torque (too low for us):
Upside is that they seem to have measured stall currents at both 12v and

-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On Behalf
Of Dick Swan
Sent: Tuesday, February 19, 2013 6:57 PM
To: DPRG List
Subject: Re: [DPRG] How to protect TETRIX motors

Several suggestions.

Run the motors at 25 to 50% of max power if that is still sufficient to move
your arm. Or at least use this level for testing until you've got your code

Probably not legal but run the motor power outlets through a limit switch
that mechanically disconnects the motor power when reached.

Create a separate task that is monitoring the encoders and power level. If
motor encoder does not change for NN millisecond and high power is being
applied then motor is stalled. Have the task stop the motor!

There may be an alternative source for the motors? The original motors were
same manufacturer part number as a motor that I think Lynxmotion (now part
of Robot Shop) used to sell at a lower price. In any case, the encoders
should be re-usable on a different motor.

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