Difference in forward/reverse drill motor speed

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Posted by Patrick Dingle at 2/5/2001 11:38 PM EST


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



Has anyone noticed that the drill motors have a different angular velocity when in forward than in reverse? If so, how have teams overcome this problem if they use the motors for the drive system?

Patrick

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Posted by Matt Leese at 2/6/2001 12:10 AM EST


Other on team #73, Tigerbolt, from Edison Technical HS and Alstom & Rochester Institute of Technology.


In Reply to: Difference in forward/reverse drill motor speed
Posted by Patrick Dingle on 2/5/2001 11:38 PM EST:



Yep, noticed this one awhile ago. You can work around it several ways. The more correct solution (i.e. the mechanical one) is to either mount the drill motors turning the same direction or put an extra gear in one side so they both turn the same direction. The less correct solution (i.e. the programming one -- Matt's Law: never fix something in programming if it can be done mechanically) is to wire a potentiameter onto the control system that will scale down one of the drive PWM outputs. You then drive the robot over a distance and fiddle with the pot until you get the robot driving in a straight line. This does lower the overall top speed of the robot however (you have to scale down the motor turning the forward direction). This takes a little bit of finesse to get right and careful drivers who don't bump the pot. It worked fairly well for us last year but there are a few caveats that I'll let you figure out on your own (hint: it has to do with driving in reverse).

Matt

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Posted by Anton Abaya at 2/6/2001 12:15 AM EST


Coach on team #419, Rambots, from UMass Boston / BC High and NONE AT THE MOMENT! .


In Reply to: Difference in forward/reverse drill motor speed
Posted by Patrick Dingle on 2/5/2001 11:38 PM EST:



: Has anyone noticed that the drill motors have a different angular velocity when in forward than in reverse? If so, how have teams overcome this problem if they use the motors for the drive system?

: Patrick

yes the drill motors are not in sync when they are running on opposite directions. this is because one motor is running in forward and the other one is going in reverse.

apparently there are two solutions, one is mechanical and the other is software. the mechanical solution is whats called a "right angle drive" or so they call it. basically what this does is it makes both run at the same direction therefore making the drive more efficient and not having to make the driver compensate for the drag occurring on one side.

check this link out. http://sharingfirst.mit.edu/users/os...n/FRC2000page/

the software solution, is to sync the right to the left via ...software. this will take trial and error as it would vary from robot to robot. but the basic idea is "if right joystick is X, then left should be x+y (y=2-10?)..." with this, it's the left joystick (motor) that drags.

i hope i answered ur question.

-anton
"I think I did."

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Posted by Joe Ross at 2/6/2001 3:28 AM EST


Engineer on team #330, Beach Bot, from Hope Chapel Academy and NASA/JPL , J&F Machine, and Raytheon.


In Reply to: Difference in forward/reverse drill motor speed
Posted by Patrick Dingle on 2/5/2001 11:38 PM EST:



: Has anyone noticed that the drill motors have a different angular velocity when in forward than in reverse? If so, how have teams overcome this problem if they use the motors for the drive system?

We have always used a tank drive system with one joystick for each side. I don't think the drivers have even noticed that they need to slightly compensate. Even if they do, it should become a habit and never be a problem.

Using a mechanical system to compensate may very well be a worse idea than leaving it alone. Even if it means that you slightly increase the efficiency in one direction, you very well may lose that advantage when you consider the complexity of the mechanical system, as well as the extra friction that it introduces.

Using software to compensate is almost as bad. You lose some small range of motion that may come in very usefull later on in competition.

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Posted by Jim Meyer at 2/6/2001 9:05 AM EST


Engineer on team #67, HOT Team, from Huron Valley Schools and GM Milford Proving Ground.


In Reply to: don't bother
Posted by Joe Ross on 2/6/2001 3:28 AM EST:



: Using a mechanical system to compensate may very well be a worse idea than leaving it alone. Even if it means that you slightly increase the efficiency in one direction, you very well may lose that advantage when you consider the complexity of the mechanical system, as well as the extra friction that it introduces.

Mechanical solutions do not have to be complicated or less efficient. Right angle drivetrains with bevel gears are not the only solution either. For the last 2 years the HOT team has always spun our motors the same direction and will continue to do so. This year we are using a single stage gear reduction from the drill motors using only spur gears.

It's probably a little late this year for this type of change but stop by our pits and check out how easily this can be done. I would explain it but I don't feel I can do an adequate job without a picture. I'll work on getting a picture to post.

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Posted by Matt Leese at 2/6/2001 9:35 AM EST


Other on team #73, Tigerbolt, from Edison Technical HS and Alstom & Rochester Institute of Technology.


In Reply to: don't bother
Posted by Joe Ross on 2/6/2001 3:28 AM EST:



I'm not sure how you had your robot setup last year but ours running tank drive with two powered wheel chair wheels in the back and two swivel casters in the front would track to the left rather badly. Our solution was the software one rather than a mechanical one because we discovered it too later. Our solution wasn't perfect and it had to be recalibrated rather regularly (you might've seen me in FL trying to find a long space to drive the robot...) but it did work.

Matt

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Posted by Joe Johnson at 2/6/2001 1:16 PM EST


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


In Reply to: don't bother
Posted by Joe Ross on 2/6/2001 3:28 AM EST:



I am still convinced that this is a small problem that can be easily compensated for by the drivers when they have the time to practice.

Focus on getting your drivers more time behind the wheel and you will be farther ahead in the long run.

Just one man's opinion.

Joe J.

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Posted by Mike Gray at 2/7/2001 11:55 AM EST


Engineer on team HOT from Huron Valley Schools sponsored by GM Milford Proving Ground.


In Reply to: don't bother
Posted by Joe Ross on 2/6/2001 3:28 AM EST:




We discovered the speed difference in the drill motors in 1998 while trying to determine why the machine pulled to the left. After exhausting all the software/joystick/speed controller/mechanical possibilities, only the motors remained. We measured the forward and reverse speeds with a strobotach.

We found that that the speed difference is a huge 15-20% depending upon the applied voltage. Motor-to-motor variation is a low 2 percent.

With the drill motors rotating in the same direction, anyone can handle the machine well with a few minutes of practice. When the machine performs as expected, driver confidence soars.

The speed difference is caused by the motor, not the gearset.

After much thought, we found a pretty easy way to rotate them in the same direction. If my team members don't object, I'll post an AutoCAD rendering or photo.

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Posted by Joe Johnson at 2/7/2001 9:59 PM EST


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


In Reply to: Re: Make 'em go the same direction!
Posted by Mike Gray on 2/7/2001 11:55 AM EST:



We have NEVER seen such a difference as 15-20%

I know that many folks have very strong ideas about
this but I am serious when I say that we have never
done a thing to try to compensate for this reported
difference.

Just as another point of data, I called my contact at
Johnson Electric (I often use them in my day job). He
was able to look up the print of the motor for me (he
is sending me a pdf file of it soon). He tells me that
the motor should have the same performance in CW and
CCW directions. Yes, some motors do have differences,
but not this motor.

Now I am between a rock and a hard place because I have
conflicting reports from usually reliable sources
(Johnson has ZERO incentive to mislead me,
Raul-the-Magnificent is almost NEVER wrong, and who
argues with ANYONE from the Heros-Of-Tomorrow team?).

Believe it or not, I am thinking of having our lab do a
gov't job for me and test the motor on our dyno.

BOTTOM LINE:

I don't know what to believe. But again, my advice is
to do nothing, focus on getting your drivers more time
behind the joystick(s).

Joe J.

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Posted by Mike Gray at 2/8/2001 8:02 AM EST


Engineer on team HOT from Huron Valley Schools sponsored by GM Milford Proving Ground.


In Reply to: Beating a dead horse...
Posted by Joe Johnson on 2/7/2001 9:59 PM EST:



I know its hard to believe, and I can't explain why they do it, but turning them the same direction dramatically improves performance, especially when launching from a dead stop.

I would love to see some data from a real lab! Think you can do this? We got some EMI data on these motors also. Both radiated and conducted emissions are very high, but that is expected.

The drill motors are the only motors exibiting this odd behavior. The Delphi motors are especially quiet, both audibly and electrically. They rotate symetrically too!

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Posted by Joe Johnson at 2/8/2001 2:39 PM EST


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


In Reply to: Re: Might be sleeping...
Posted by Mike Gray on 2/8/2001 8:02 AM EST:



I just got a frantic message from my Johnson Electric contact. It seems that he was wrong yesterday when he told me the motor was not biased.

When he confirmed with his manufacturing sources in Hong Kong, they told him that the magnets are oreinted 15 degrees with respect to the brushes, making the motor performance asymmetrical.

So... I was wrong. Sorry for misleading folks.

But... I am still of the opinion that this is not such a big deal. Again, I believe driver skill can compensate very effectively.


Yet... I have to confess that because the design on Chief Delphi 6 is such that we can change the motor orientation without changing any parts, we will probably have them installed to run in the same direction.

Bottom line:

If it is easy, I say do it. If it is hard, I say don't bother.

Joe J.


P.S. I KNEW that I should never take sides against The Heros of Tomorrow (and especially Mike Gray & Jim Meyer) and/or Raul-the-Magnificent -- ah well, live and learn.

P.P.S. I am going to try to get Johnson to give me the CCW and CW motor curves. I am very curious to see the differences on paper.

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Posted by Mike Gray at 2/8/2001 3:07 PM EST


Engineer on team HOT from Huron Valley Schools sponsored by GM Milford Proving Ground.


In Reply to: I am wrong.
Posted by Joe Johnson on 2/8/2001 2:39 PM EST:




If its easy to alter the rotational direction of the drill motors in your machine, you might try a quick experiment:

Evaluate the quality of the launch with the motors running in opposite directions. Turn one around and do the same thing. I'm convinced that you will want them to run the same direction.

Please let us know how the operators respond to the A/B test.

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Posted by Bill Beatty at 2/9/2001 11:05 AM EST


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


In Reply to: Re: I am wrong.
Posted by Mike Gray on 2/8/2001 3:07 PM EST:



The difference for us last year was as high as 15 percent and for the first time we compensated with software. When we changed motors a quick check with the tach and we would change the percentage factor. Not a biggie, but it seemed to help the drivers somewhat.

Mr Bill

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Posted by Ken Patton at 2/9/2001 8:06 PM EST


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


In Reply to: We Compensated Last Year
Posted by Bill Beatty on 2/9/2001 11:05 AM EST:



sdkldfjkdf

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Posted by Alan Ing at 2/9/2001 9:31 PM EST


Engineer on team #368, Kika Mana, from McKinley High School.


In Reply to: I am wrong.
Posted by Joe Johnson on 2/8/2001 2:39 PM EST:



: I just got a frantic message from my Johnson Electric contact. It seems that he was wrong yesterday when he told me the motor was not biased.

: When he confirmed with his manufacturing sources in Hong Kong, they told him that the magnets are oreinted 15 degrees with respect to the brushes, making the motor performance asymmetrical.

: So... I was wrong. Sorry for misleading folks.

: But... I am still of the opinion that this is not such a big deal. Again, I believe driver skill can compensate very effectively.

:
: Yet... I have to confess that because the design on Chief Delphi 6 is such that we can change the motor orientation without changing any parts, we will probably have them installed to run in the same direction.

: Bottom line:

: If it is easy, I say do it. If it is hard, I say don't bother.

: Joe J.

:
: P.S. I KNEW that I should never take sides against The Heros of Tomorrow (and especially Mike Gray & Jim Meyer) and/or Raul-the-Magnificent -- ah well, live and learn.

: P.P.S. I am going to try to get Johnson to give me the CCW and CW motor curves. I am very curious to see the differences on paper.

Joe, thanks for the last minute motor seminar at the kickoff, it was really informative.

In regards to that drill motor. My guess is that the motor curves are going to be significantly different. Our robot uses one motor in the forward direction and one in reverse. We are pretty sure that friction is not an issue. Our robot definitely turns right faster than left. We put a meter on the motor and made some continuous turns in both directions. When turning one way, the motors are drawing 10 amps (both motors running forward), but in the other direction (both motors running backwards), it is drawing about 18 amps. This is the case for either motor. Seems pretty significant. Some robots might pop their relays while turning in one direction and controlling the stretcher but not in the other direction. Anyway, I would recommend measuring the current through both motors while turning to make sure your gearing is appropriate for both directions.