Mecanum wheels under varying loads

[dellagd]

Originally Posted by howe

I searched ad nauseum before posting my question with no result.

Please point me to the threads or at least give me some key words for my search.

Thanks

'Ad nauseum' is just a latin term for 'continued until its sickening' (until nausea)

[GeeTwo]

We haven't tried to climb onto platforms, but 3946 has experience with mecanums that were WAY out of balance. Our competition robot last year wasn't too bad, but our proto'bot, made mostly of 2"x4"s, only put about 10# of force on the left front wheel (total weight in excess of 100#). Our drivers learned to compensate well enough that a casual observer at practice would not have suspected how out-of-balance we were. I demo'd "Woodie" at my office with absolutely zero drive practice, and impressed my co-workers (mostly scientists and engineers) with the capabilities of mecanum - I could execute all sorts of translation as well as rotation.

[howe]

Originally Posted by dellagd

'Ad nauseum' is just a latin term for 'continued until its sickening' (until nausea)

Duh, I just don't know what possessed me to type "ad nauseum" into the search bar BEFORE I even posted the question, foolish me.

Now seriously, how about showing some gracious professionalism and post the urls of the threads where the topic of shifting load has been discussed previously? I have gone through several pages searching simply under mecanum in addition to other key words I thought appropriate. It would be nice to know how others have modified their code or driving style to compensate as mentioned in some of the responses.

Thanks,
howe

[Tom Line]

http://www.chiefdelphi.com/forums/sh...hreadid=133086

http://www.chiefdelphi.com/forums/sh...hreadid=132855

http://www.chiefdelphi.com/forums/sh...hreadid=121317

Google search terms:
mecanum wheel load uneven site:chiefdelphi.com

I usually have much better luck with Google's search than Chief Delphi's.

[dellagd]

Ive also been curious about 'perfecting' a mecanum drive system by taking the load on each wheel and programming the drive to account for the uneven loading. All papers I found only referenced the geometric center of the base, not the center of gravity. I have a pretty good understanding of how they work, and I feel like there should be a way to incorporate the different normal forces into the reverse kinematics, but unfortunately I've come up short on figuring out the calculation myself.

So I'll ask too: Has anyone ever done a write up on the reverse kinematics with uneven loading or even written some code to deal with it? This seems like something Ether would have addressed in the past, but I can't find anything for it.

[Ether]

Originally Posted by dellagd

Has anyone ever done a write up on the reverse kinematics with uneven loading or even written some code to deal with it? This seems like something Ether would have addressed in the past, but I can't find anything for it.

assuming each wheel is being driven at the kinematically correct speed, any deviation from the kinematically predicted behavior, due to uneven loading, would mean that the bot is no longer behaving "holonomically" -- there is wheel scrub -- and this would require an analysis of the forces involved.

[dellagd]

Interesting. I'm guessing doing the complete analysis would be pretty complicated if there aren't any papers about it. I wonder if a good estimation could be made that works well enough in practice?

[Ether]

Originally Posted by dellagd

I'm guessing doing the complete analysis would be pretty complicated

Yeah I think so. Alan's post#16 does a pretty good job explaining why load affects mec behavior and possible ways to deal with it.

Other options to deal with it involve using actual vehicle motion sensors (e.g. gyro, encoders on follower wheels, etc) to make corrections to the driver's commands.

[Alan Anderson]

Originally Posted by dellagd

Ive also been curious about 'perfecting' a mecanum drive system by taking the load on each wheel and programming the drive to account for the uneven loading.

Varying the normal force "load" on each wheel has two primary effects. One is a subtle change in the wheel speed vs motor voltage. That can be compensated for using closed-loop control of wheel speed. The other is a significant change in the maximum thrust that the wheel can apply before slipping. That one is not so easy to account for, though I suspect monitoring the motor current can provide a way to do traction control and turn the wheel slip problem into a maximum robot acceleration problem.

[howe]

Originally Posted by Tom Line

http://www.chiefdelphi.com/forums/sh...hreadid=133086

http://www.chiefdelphi.com/forums/sh...hreadid=132855

http://www.chiefdelphi.com/forums/sh...hreadid=121317

Google search terms:
mecanum wheel load uneven site:chiefdelphi.com

I usually have much better luck with Google's search than Chief Delphi's.

Thank you Tom, for the information I was looking for.
Thanks for the other tip too, sometimes I just can't help myself.
howe