Drill motor Demagnetization and lowered-torque woes
 

Posted by Joe Balint at 04/19/2001 12:06 AM EST


Engineer on team #537, Domo ARIGATO, from Sussex Hamilton HS and GE Medical Systems.



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.

Re: Drill motor Demagnetization and lowered-torque woes
 

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.

Re: Drill motor Demagnetization and lowered-torque woes
 

Posted by Dan at 04/21/2001 5:31 PM EST


Other on team - from Carnegie Mellon sponsored by -.


In Reply to: Re: Drill motor Demagnetization and lowered-torque woes
Posted by Patrick Dingle on 04/21/2001 3:20 PM EST:



Try the Electric Motor Handbook, you can buy it at Team Delta's website linked below.
Dan

Re: Drill motor Demagnetization and lowered-torque woes
 

Posted by Bill Beatty at 04/21/2001 5:10 PM EST


Other on team #71, Team Hammond, from Team Hammond.


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



We had some similar symptoms with our drive motors this year when pushing them very hard, such as practicing with a loaded stretcher. We came to the conclusion we were just suffering from a warm, tired circuit breaker, usually on the backwards drill side. Letting things cool down and changing breakers usually solved the problem.

Never seen this before
 

Posted by Joe Johnson at 04/21/2001 6:42 PM EST


Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.


In Reply to: Re: Drill motor Demagnetization and lowered-torque woes
Posted by Bill Beatty on 04/21/2001 5:10 PM EST:



I have never seen this. The closest I have seen is
when we toast motors from simply overheating them. I
am not sure of the failure mode actually. I assume
that it is from either high resistance in the brushes
or form some of the windings of the motor being shorted
out of the current path by the insulation breaking
down. This will make effectively reduce the number of
turns of the armature which will make the motor
performance less.

I will keep this in mind. Maybe your explanation is
the correct one. If so, I don't know exactly what the
prevention scheme is.

Maybe I need to do a little research or maybe I will
call up one of my motor suppliers to pick their brains
on this matter. It is SO nice to have your day job and
FIRST overlap some times ;-)

I will let you know if I find out anything.

By the way, did you keep the motors that were possibly
demagnitized? I am sure Johnson Electric (the motor
mfg.) would be interested in them if they are handy.

Joe J.

I'd never seen it either, that's why I posted
 

Posted by Joe Balint at 04/25/2001 12:25 AM EST


Engineer on team #537, Domo ARIGATO, from Sussex Hamilton HS and GE Medical Systems.


In Reply to: Never seen this before
Posted by Joe Johnson on 04/21/2001 6:42 PM EST:



Joe-
I'd neither seen it nor heard of it before it happened to us. I don't believe it was a circuit breaker (swapping motors fixed our problem) but I will certainly look into this option (Although why it would only affect the motor in one direction, I can not fathom) And yes we have the motors and we are sending them to the manufacturer to find out if there are any shorted windings, or if our theory on demagnetizing the magnets is indeed the root cause. I'll keep you posted and then post the final analysis on the sharingFIRST site. Our temporary fix is to software limit the motor speeds by ramping the the speed instead of allowing instantaneous full power direction changes.

a couple things
 

Posted by Ken Patton at 04/23/2001 12:22 AM EST


Engineer on team #65, The Huskie Brigade, from Pontiac Northern High School and GM Powertrain.


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



You should rule out the 30A circuit breakers as a source of the problem. Check to see if they are getting hot (a sign that they might be tripping), or even replace them with fuses (just to test). If they are tripping, you will see the speed controller lose power.

What is your gear ratio, and how bad is your drivetrain friction under load? These are primary sources of loading for the motors.

Ken

Re: a couple things
 

Posted by Joe Balint at 04/25/2001 1:03 AM EST


Engineer on team #537, Domo ARIGATO, from Sussex Hamilton HS and GE Medical Systems.


In Reply to: a couple things
Posted by Ken Patton on 04/23/2001 12:22 AM EST:



: You should rule out the 30A circuit breakers as a source of the problem. Check to see if they are getting hot (a sign that they might be tripping), or even replace them with fuses (just to test). If they are tripping, you will see the speed controller lose power.
: What is your gear ratio, and how bad is your drivetrain friction under load? These are primary sources of loading for the motors.

: Ken

Ken-
From what we observed the motors were not getting hot and the circuit breakers were not tripping. That's one of the things that was bugging me. If we were generating enough of a Magnetic Field to de-mag the motors, wouldn't that amount of current trip the breakers? But my educated guesstimate is that the answer lies in the fact that when we are commanding these direction changes, we are on the ramp with two goals. our gear ratio is around 22:48 (I remember ordering these sprockets, but I wasn't on Drivetrain this year so I'm not positive about this ratio), each drill motor is driving both front and rear wheel assemblies. Each assembly is actually 2 8" wheel chair wheels bolted together and grooved for traction. So this is the most I've ever loaded these drill motors (by design) and I think it is feasible that this load is too much and falls into that part of the demagnetization curve that the design notes warn about causing permanent torque loss by demagnetiztion. We'll see what we find during the analysis. -joe

Re: a couple things
 

Posted by Ken Patton at 04/26/2001 12:39 AM EST


Engineer on team #65, The Huskie Brigade, from Pontiac Northern High School and GM Powertrain.


In Reply to: Re: a couple things
Posted by Joe Balint on 04/25/2001 1:03 AM EST:



Joe-
I don't know enough about demagnetization to know the answer, but I think it still might be worthwhile to *for sure* rule out the breakers tripping. Ours tripped this year while at a gear ratio from the drill motors to 8" wheels of 95:1 with two goals plus robot plus solid traction on the ramp AND cool motors. We ended up going lower in ratio and adding motors (f-p's) to finally eliminate this problem. From what you say I assume you were in low gear on the drills plus your 2:1 chain ratio for a motor-to-8" wheel ratio of about 130:1

Ken

Re: Drill motor Demagnetization and lowered-torque woes
 

Posted by Al Skierkiewicz at 04/23/2001 8:05 AM EST


Engineer on team #111, Wildstang, from Wheeling High & Rolling Meadows High and Motorola.


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



Joe,
We have run across motor demag on motors we use here at work. They are a pancake/printed circuit armature design used in Ampex tape recorders. For some reason, the design of the magnet structure allows field to be drained off when the motors are disassembled for cleaning, armature replacement or brush replacement. It was astounding how much the field deteriorated after a few rebuilds. Since you are not disturbing the magnet structure, I don't think this is a problem. I would test bed the motors, out of the robot, with known good ones. This will eliminate bad electrical interface(circuit breakers, wire, connections). Under the load conditions you describe, it is possible to short a few windings (see Joe Johnson's post) in each of the many poles of the motor. We know that the circuit breakers reset very quickly and could be tripping on over current each time the bad pole comes around, thereby reducing the overall power output of the motor. Two ways to check the current on the motor is to scope the voltage across a small resistor in series with the motor, (1 ohm) or a current probe for a scope.(Hard to come by and expensive) The current should be fairly constant with no large peaks. (there will be spikes as the brushes move on and off the commutator) A repetive large peak would be a suspect short winding.
If the magnet structure is still suspect, a gaussmeter is the instrument of choice to check.
On our Ampex motors, when the magnet went bad we could stall it by hand, there was no doubt of the defect.
Al

Re: Drill motor Demagnetization and lowered-torque woes
 

Posted by Joe Balint at 04/25/2001 12:36 AM EST


Engineer on team #537, Domo ARIGATO, from Sussex Hamilton HS and GE Medical Systems.


In Reply to: Re: Drill motor Demagnetization and lowered-torque woes
Posted by Al Skierkiewicz on 04/23/2001 8:05 AM EST:



Al-
We are working on your solution: testbedding both affected and gold standard motors and comparing them. We are also sending affected motors back to the manufacturer (we ended up with 2 pair of lowered-torque drillmotors) We are just waiting on getting new ones in.
on a side note: We are working on putting on a friendly competition/exhibition during the Wisconsin state fair. It's in August in Milwaukee and we are currently inviting all the WI teams and any other teams close by. (Including northern Illinois) Would Wildstang like to make an appearance? We have buy-in from the State Fair, so it's definitely a "Go". We are currently working on a date and the logistics. We did something similar for Open house at the HS and it was quite fun. Please let me know if you think your team would be available. Note: Your team may also be getting contacted by us through other channels as well.
Thanks for your time. -Joe