Team 2052's Mecanum Showcase!

[pntbll1313]

Originally Posted by Siri

I don't see how to make this a 'cancel out' situation: how does it overcome the phenomenon that mecanums break traction before any tank drive with at least the same* CoF and weight? If the issue is slipping (which is why I asked), it's not CoF vs CoF issue; it's the geometry of the wheels. The fact that mecanum CoFs are then 40-75% that of most tanks only serves to compound this.


Roller slip will vary by specific DT, but not nearly as much as between mecanum and tank in general, and it's not helpful in strafing. It's also (thanks to Ether), not overly difficult to model the description, at least to the point of illustrating its negative effect. As the paper explains, the friction that keeps rollers from slipping adversely effects mecanum's strafing traction. If this is what we're talking about, I don't understand the implication that driving the wheels produces more tank-like results.

*Apparently also lower, to a factor of sec(α)--Ether's paper is great.

All very good points! I don't claim to be a physics expert. I am not. That's why I asked Ether to give me some numbers

Again we were never trying to push in the sideways direction. Our video does not show us winning too many big pushing matches. Mecanum allowed us to avoid those situations. We did not choose mecanum for its pushing power haha, nor do we generally use it for such things. The video is just meant to show some examples of how we used mecanum to our benefit this year, and to hopefully show it's not as weak of a drivetrain choice as it is in most CD'ers minds. No one (hopefully) chooses mecanum for traction purposes. We just wanted to highlight some of our match videos because people cited us as a good example of mecanum.

[Siri]

Originally Posted by pntbll1313

All very good points! I don't claim to be a physics expert. I am not. That's why I asked Ether to give me some numbers

Again we were never trying to push in the sideways direction. Our video does not show us winning too many big pushing matches. Mecanum allowed us to avoid those situations. We did not choose mecanum for its pushing power haha, nor do we generally use it for such things. The video is just meant to show some examples of how we used mecanum to our benefit this year, and to hopefully show it's not as weak of a drivetrain choice as it is in most CD'ers minds. No one (hopefully) chooses mecanum for traction purposes. We just wanted to highlight some of our match videos because people cited us as a good example of mecanum.

No problem. I think the discussion just got sidetracked because I (perhaps also Ether) was worried that teams--who would otherwise be influenced by the video in very positive and correct ways--might take these statements the wrong way:

Originally Posted by pntbll1313

...I would bet we are at least as hard if not harder to push sideways when we are strafing into the hit than a tank drive...What we have found is that when you push a tank robot sideways the only thing stopping them is their static friction with the carpet. When we get pushed sideways we can provide a force in the opposite direction to the push...Our force we apply with the motors I feel cancels out the slip of the rollers, leaving still static friction with the carpet. Which makes us just as good as tank if they have the same CoF.

Particularly the clarity of the last one, as it's contrary to the actual geometry of mecanum wheels.


Also, I seem to have distracted the thread from Mr. Elston's very good questions:

Originally Posted by Chris_Elston

How much did your robot weigh?

Do you know the ratio of the gearboxes you were using with the 8 inch mecanums?

Is there any special software or sensors you guys used in conjunction with the drive system such as a gyro or field oriented control?

Sorry

[Ryan Dognaux]

Probably the best example I've seen of answering this challenge. Great job!

[silverD]

Great discussion here and thanks for everyone's input.
KnightKrawler used 2 AndyMark Nano Tube 20's with hex shafts in the stock 12.75:1 ratio. One CIM per wheel and each wheel driven directly from the gearbox. Since we need a drive motor per wheel, we used this as an opportunity to leave out chain. As noted before, the wheels are AndyMark 8" HD mecanums that come with side rollers which we do think help to smooth out the movement of "Silverfish" as it transitions between mecanum rollers. The robot weights in just under the 120 lbs. limit with the 84" blocker and GoPro installed. I bet "Silverfish" was just over 118 lbs. for its fighting weight.
While we did develop field oriented drive code this year, it was not utilized. The robot is driven in open-loop as we do not use any sensors on the drivetrain. We did have a gyro on board and tinkered with auto robot orientation for loading and hanging, but it was never used in the competition code.
Here is a picture of the drivetrain:

I hope this clears up some of the questions, let us know if you have any more.
Thanks,
Nate
2052 KnightKrawler Mentor

[Ether]

Originally Posted by silverD

AndyMark 8" HD mecanums that come with side rollers

Does anyone know if there is an engineering drawing available -- or even just a concept sketch -- which shows how the axial forces on the rollers are reacted in this design?

[Nate Laverdure]

Originally Posted by Ether

Does anyone know if there is an engineering drawing available -- or even just a concept sketch -- which shows how the axial forces on the rollers are reacted in this design?

See attached sketch from page 2 of this document.

[Ether]

Originally Posted by Nate Laverdure

See attached sketch from page 2 of this document.

Thanks Nate.

It looks like

1) The Inner Roller is rubbing against the 1/4-20 SS washer which is in turn rubbing against the Mecanum Plate.

2) The Outer Roller is rubbing directly against the Mecanum Plate

Am I reading that correctly?

[Navid Shafa]

Originally Posted by Ether

Am I reading that correctly?

I see it the same way.

[Chris is me]

Originally Posted by Ryan Dognaux

Probably the best example I've seen of answering this challenge. Great job!

Yup, this is definitely the closest anyone has ever gotten. I don't think many defenders were playing particularly well, but you did quite literally "run circles around" defenders at many points. I'm particularly impressed with your driver's seamless use of the mecanum features (spinning around a point outside the robot's center, strafing) as techniques to avoid defense.

Keep in mind there is far more going on in a pushing match than the type of wheels on each robot, and that a well-driven anything will almost always beat a poorly-driven anything. Also, if you're under 30" tall and your defender is over 30", it doesn't matter what drive base you have, you can still get past them.

[Ether]

Originally Posted by Chris is me

if you're under 30" tall and your defender is over 30", it doesn't matter what drive base you have, you can still get past them.

Did you mean to use the same number (30) in two places?

[Chris is me]

Originally Posted by Ether

Did you mean to use the same number (30) in two places?

Yes, 30" is the height of the lowest bar of the pyramid. 2052 often escaped from defenders by slipping underneath of it when the defender could not.

[Ether]

Originally Posted by Chris is me

30" is the height of the lowest bar of the pyramid. 2052 often escaped from defenders by slipping underneath of it when the defender could not.

Cannot a wise defender choose to draw his line in the sand away from the pyramid?

[Chris is me]

Originally Posted by Ether

Cannot a wise defender choose to draw his line in the sand away from the pyramid?

I agree that this is the correct way to play defense with a tall robot, but a lot of 2052's defenders did not do so. The 30" robot still has somewhat of an advantage even against a defender playing the "correct" strategy - if the 30" robot goes straight, it has less distance to "push through". If the 30" robot goes through the pyramid, then a blocking defender is also obstructing the primary scoring location of many robots, and thus the defense is a bit of a wash.

[pntbll1313]

Originally Posted by Ether

Cannot a wise defender choose to draw his line in the sand away from the pyramid?

Although if they don't defend us near the pyramid we shoot full court, if you haven't already seen our video

http://www.chiefdelphi.com/forums/sh...hreadid=117022