[archiver]

Posted by Patrick Dingle at 04/21/2001 3:20 PM EST


College Student on team #639, Red B^2, from Ithaca High School and Cornell University.


In Reply to: Drill motor Demagnetization and lowered-torque woes
Posted by Joe Balint on 04/19/2001 12:06 AM EST:



I am not an engineer [yet] and can't answer this question but was wondering if anyone who reads this also knows any good resources about learning about how motors work. I would be interested in learning.

thanks
Patrick

: Joe or someone else who knows.-
: We had an issue with our drill motors at the National competition. I'll explain the scenario and our conclusions and I'd like to hear some expert opinions on the matter. First off I lead team #537 and I co-led team #128 for three years while at Ohio State University and I have worked with these drill motors quite a bit (more than I ever would've imagined possible.) We made the Delphi-mod to the drill motors which replaced the output shaft and got rid of the clutch and anti-backdrive hardware. (We did it last year on #128 and it was a god-send) The motors were behaving fine until we attempted to double balance. Our robot was designed to do this while attached to the goals and is a manual manuever. There is alot of quick changes of direction. After trying this in our second practice session on Thursday, we didn't notice anything amiss. Friday morning at our first match, we went into low-profile mode and found that we couldn't get through the barrier (a manuever that hadn't failed us at any time up at this point). In trouble -shooting the robot, we were able to rule out possible software limiting, low battey, bad speed controllers and bad hardware as our drivertrain problem. We could definitely hear that the motor was going different speeds forward and backwards on the left side. After switching that motor with a new one, we found that we could once again get through the barrier while in low profile mode and that was working well. Saturday morning I brought the drivers to the pits early to practice double balancing before the matches started. After 15 minutes of practice, we tested the robot and found that we had the same failure mode (difficulty pivoting to the right) After some more motor swapping, we figured oout it was the motors again (both sides this time (one was probably lingering from the day before)) and we were short on motors at this point. We got through the barrier in our last match, but just barely. We took apart one of the bad motors and could not find any thing mechanically or electrically wrong (heck, the brushes had barely worn in!) So I was looking at some an old DC Motor Design binder (put together by Ed Yahne) I had from a class I took while I was working at ITT Automotive in Kettering doing competitive analysis (where they make the seat, and windowlift motors) and looked at the section on motor demagnetiztion. It discusses demagging the magnets at very high loads (previous to this I always thought this occurred only at low tempertures (-40) and didn't think it could happen at operating or hot temperatures) But there it is. In the 2nd quadrant of the demagnetization curve where the line of the permeance coeficient intersects the curve below the knee, we get a degradation of performance that can not be recovered without remagnitzing the magnets. Now that is all well and good in theory, But my real question is this: Since we were probably operating well past the operating load of this motor, and double balancing was by far our highest torque manuever (full changes of direction while loaded with two goals), is it likely that our theory of drill motor demagnetiztion is indeed the cause of our drive train lowered-torque woes? What say you Engineers? Also, does anyone have the facilities to test this theory (not just dyno-ing the motor; but actually measuring the demagnetization of the drill motor magnets?) Any help or expert opinion would be appreciated.