How Do mecanum wheels handle the bumps?

[Ether]

Originally Posted by Chris is me

(Only ~71% of the output torque is transmitted in the direction of travel)

Are you sure it's not 71.03% ?

All seriousness aside, where did you get the 71% number?


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[sgreco]

Originally Posted by Ether

Are you sure it's not 71.03% ?

All seriousness aside, where did you get the 71% number?


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I can't answer for Chris, nor have I done the math, But if you think about a mechanum wheel, its rollers are angled at 45 degree angles in relation to the direction of the wheel. So when you input a speed to go forward, you lose efficiency in that you are transmitting power in a direction that you arent heading in.

Think about a free body diagram for physics. You may have 9 N of force in one direction and 2 N in the opposite direction, so you are letting off 11 N of force, but you only have a net of 7 N in the forward direction.

Mechanum wheels seem nice in theory, but if I'm not mistaken, a team has never won nationals with a mechanum drive. If they meet tough defense they can easily be out pushed. They're good for qual matches with low defense, but I'll be interested to see how they fair in elims in Atlanta.

[Ether]

Originally Posted by sgreco

Mechanum wheels seem nice in theory, but if I'm not mistaken, a team has never won nationals with a mechanum drive. If they meet tough defense they can easily be out pushed. They're good for qual matches with low defense, but I'll be interested to see how they fair in elims in Atlanta.

Please understand: I am not claiming that mecanum wheels are the superior design choice for Breakaway.

Originally Posted by sgreco

I can't answer for Chris, nor have I done the math, But if you think about a mechanum wheel, its rollers are angled at 45 degree angles in relation to the direction of the wheel. So when you input a speed to go forward, you lose efficiency in that you are transmitting power in a direction that you arent heading in.

I question your claim that "power is being transmitted" in a non-forward direction when the bot is going straight forward.

But that's beside the point.

The question on the table is whether or not "only 71% of the output torque is transmitted in the direction of travel" when the bot is going straight forward (or backward).

Think about a free body diagram for physics. You may have 9 N of force in one direction and 2 N in the opposite direction, so you are letting off 11 N of force, but you only have a net of 7 N in the forward direction.

This analogy doesn't appear to apply to the situation at hand.


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[efoote868]

Originally Posted by sgreco

Mechanum wheels seem nice in theory, but if I'm not mistaken, a team has never won nationals with a mechanum drive. If they meet tough defense they can easily be out pushed. They're good for qual matches with low defense, but I'll be interested to see how they fair in elims in Atlanta.

Considering the fact that only a handful of teams ever used mecanum wheels prior to when AndyMark introduced them (2007?), and then again the fact that last year they were illegal, and the game in 2008 was made for speed more than maneuverability, I'd say they haven't had their fair share of playing time.

My team uses the 6" version of AndyMark Mecanum wheels (the ones with the lower coeff of friction), and we have no problems going over the bump.


The only problem to date (that I'm aware of) is when our mentor was driving the drive train during the build season, and he drove the robot full speed into and over the bump. We lost one roller; considering the way he was driving it I'd say it did 3x better than expected.

[joeweber]

Originally Posted by sgreco

Mechanum wheels seem nice in theory, but if I'm not mistaken, a team has never won nationals with a mechanum drive. If they meet tough defense they can easily be out pushed. They're good for qual matches with low defense, but I'll be interested to see how they fair in elims in Atlanta.

2008 Team 2337 made it to the finals in Atlanta and I thought they were using Mecanums.

[Ty Tremblay]

If mecanum drive trains lack pushing power, why not have the opportunity to lock the rollers, effectively turning mecanum into 4 wheel tank drive?

While I was away in Thailand during build season, 190 did just that.

[alicen]

Ty -- it would be possible to pull that off, but would takea lot of research and testing (off-season project anyone?) where as, in my opinion, mecanums are still good for defense, but not the pushing kind. They're much better for the manueverable, get-in-the-way kind

[Ty Tremblay]

Originally Posted by alicen

...in my opinion, mecanums are still good for defense, but not the pushing kind. They're much better for the manueverable, get-in-the-way kind

IMO, where locking mecanums really shine is on offense. When going for a goal, one can lock the rollers in order to make it difficult for a defending robot to knock them off course. Traditional mecanum drive trains are not expected to push, and thus can be geared quite high to increase mobility. However, when you lock the rollers in order to push, you now have a highly geared robot (which isn't good for pushing anyway). So, I would agree that mecanum is not the drive train for pushing, but locking the rollers does increase a robots ability to stay on course when hit.

Edit: "Disclaimer"
I was in no way involved with 190's robot this year so, essentially, this is just me analyzing another cool robot I saw at a regional.

[Ether]

Originally Posted by Ty Tremblay

why not have the opportunity to lock the rollers, effectively turning mecanum into 4 wheel tank drive?

When you are pushing directly forward, the rollers are effectively locked.

All the forward torque applied to the wheel is transmitted to the carpet.

The torque is not reduced by cos(45).

If mecanum drive trains lack pushing power,

If mecanum wheels lack pushing power (in the forward direction), it is because they slip more easily. And they slip more easily even if they are using the same tread material as a conventional wheel.


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[Chris is me]

Originally Posted by Ether

When you are pushing directly forward, the rollers are effectively locked.

All the forward torque applied to the wheel is transmitted to the carpet.

The torque is not reduced by cos(45).

The torque is not being sent straight forward at each wheel. If that were the case, mecanum drives would not have the ability to strafe.

Mecanums have the same math / efficiency setups as normal "omni" drives, with the same vector math. You'll pretty quickly figure out where 71% comes from.

[Alan Anderson]

Originally Posted by Ether

When you are pushing directly forward, the rollers are effectively locked.

All the forward torque applied to the wheel is transmitted to the carpet.

The torque is not reduced by cos(45).

None of these assertions is true.

The rollers do spin noticeably when you're driving straight forward. This is where the obvious loss of pushing power manifests itself.

If you go through the vector analysis of how much torque is pushing in which directions for an ideal rectangular-layout mecanum or omniwheel drivebase, you find that ~70% goes to movement in the forward direction, and ~70% goes to trying to squash or stretch the frame in the sideways direction.

All the torque is available in the diagonal direction. The torque in the forward direction is only cos(45) of the maximum.

[NyCityKId]

Originally Posted by Ty Tremblay

If mecanum drive trains lack pushing power, why not have the opportunity to lock the rollers, effectively turning mecanum into 4 wheel tank drive?

While I was away in Thailand during build season, 190 did just that.

how would you be able to lock the rollers? please send me more information on this or point me towards any websites that might help me figure this out.

anyone can respond to this

[Joe Ross]

Originally Posted by NyCityKId

how would you be able to lock the rollers? please send me more information on this or point me towards any websites that might help me figure this out.

One way is presented in this paper: http://ftp.mi.fu-berlin.de/Rojas/omn...eter-Tlale.pdf

[Ty Tremblay]

Originally Posted by NyCityKId

how would you be able to lock the rollers? please send me more information on this or point me towards any websites that might help me figure this out.

anyone can respond to this

It's not simple by any means, but I will try my best to describe what 190 has done this year. I don't have any pictures or CAD models, so bear with me.

190 designed a mecanum wheel with double internal yokes (this means that the end pieces of the rollers are cantilevered, and the middle piece is supported on both ends) that is driven like any other mecanum wheel via sprocket on one side. On the other side of the wheel, however, is a friction plate with a face profile that matches the side of the rollers closest the the wheel's axis of rotation. When the friction disk is pressed into the rollers via pneumatic cylinder, the disk prevents the rollers from spinning while allowing the wheel to rotate. When they want to return to mecanum drive, they disengage the friction plate.

I will now attempt to illustrate this on my computer...

EDIT: Here's the pic.

[Clinton Bolinger]

Originally Posted by joeweber

2008 Team 2337 made it to the finals in Atlanta and I thought they were using Mecanums.

Yes in 2008 and this year in 2010 we used Mecanums. We did make it into the eliminations of our division but unfortunately we did not make it to Einstein.

This year as a mid fielder, I think we play very aggressive defense and can push people around with our mecanums. Also defense isn't always about having the ability to just push people, there is more to it.

-Clinton-