Mecanum or 6WD

[Ether]

Originally Posted by Steve_Alaniz

Not to start an argument...

Not to worry. I welcome polite challenges.

..but the white paper actually confirms what I was saying. In fact, it states in the body that "Therefore, assuming no roller bearing friction, the “pushing force” (and speed) of the vehicle is the same in the fore/aft and sideways directions."

The above statement is true, but it doesn't confirm what you said. You said:

in any given direction, mechanums are the equivalent of the torque of TWO motors driving TWO wheels with the rollers locked

The above statement is not true.

So I don't see how you got the 41% unless you are including friction from the bolt/axle but I would be happy to hear how you reached these conclusions.

The 41% increase is a factor of 1.41, which is 1/cos(45), 45 being the angle of the roller.

To summarize:

In the forward direction, the pushing force of a (4 motor) mecanum vehicle is the same whether the rollers are locked or not, assuming that the vehicle is not traction-limited.

If the mecanum vehicle is traction-limited, then the pushing force is reduced by a factor of 1.41. However, compared to TWO locked-roller wheels (which is the comparison you made), this would be 1.41 times larger:

(F/1.41)*2 = F*1.41

If this still isn't clear, I don't mind giving it another shot. Tell me which part doesn't make sense.

[Josh Goodman]

Originally Posted by Charmander

If deciding on 6WD, what type of 6WD is recommended? Omni Wheels in the middle, all omni, Middle wheel lowered, etc. I am currently leaning toward all omni 6WD because of the increased maneuverability. Any thoughts?

1511 has had much success with four omni wheels in the corners and 2 high traction wheels in the center. With the traction wheels, it is close to impossible to push you sideways and moderately difficult to push when parallel with the wheel. With the 4 omni wheels though, it allows you to be able to turn on a dime. We've had a lot of success with both the maneuverability and ability to push around other teams with this system. I'm assuming this year we're still planning on using 2 instead of 4 CIM motors so I'll let you know how that works out. We still have a few calculations to do and if they aren't needed anywhere else, we'll slap 'em back on the DT. All the wheels are on the same plane/ center is in line with the omnis.

This system was able to climb ramps in 2006/7 and was one of the faster regional level (and in some cases Championship level) DTs.

[ThirteenOfTwo]

I think the advantages of both types of drive train are getting a little overhyped.

What are you going to do with pushing power on offense, shove the opponent into your zone and let them run interference on you (because there aren't any rules that force them to leave)? If a 6WD meets a 6WD in the middle of the field, they crash head to head, they lose all forward momentum, and the one on offense has to back up, turn, try to go around, get hit again, and so on. If a 6WD meets a mecanum in the middle of the field, they crash head to head and then the mecanum goes around. And since pinning=death this year, teams will be more hesitant to push mecanum bots into walls.

On the other hand, strafing in the homezone isn't too terribly important either when you can just come from an angle to score. The lanes can be entered by well-driven bots of either type, though admittedly it's a tad easier with mecanum. Mecanums on defense are good at driving in front of 6WDs and then get shoved out of the way so fast it's not funny, and as such are not a good idea.

Assuming a field that's full of 6WDs (as regional tournaments, at least here in Hawaii, usually are), your 6WD has to be buff if you want to push people around and your mecanum has to be well driven if you want significant maneuverability gains.

[davidcone315]

My team feels that manuverability is better than speed in this game because there will be so many bots in the field, it will be nearly impossible to find a clear path to the other side. We also think that offense will be more important than defense in this so you can get a big enough point lead to hold your own before the endgame. Does anybody feel differently about this?

[Team 3266]

We are going with a 4WD Mecanum setup. We feel that the strafing is worth the lost in pushing power because if a defensive bot tries to push us away from our scoring rack we can drive around and get in our zone before the defensive bot can turn around

[davidcone315]

That's exactly what we think, but we also are thinking that a vacuum would be efficent in picking up and holding tubes.

[Chris is me]

Originally Posted by davidcone315

That's exactly what we think, but we also are thinking that a vacuum would be efficent in picking up and holding tubes.

The only way to find out is to prototype!

Never commit to a design you have no information on. Always prototype to see if it will work. This is especially true for "risky" ideas like a vacuum.

It should take you like 10 seconds to get a shop vac and test - let us know how it goes!

[apalrd]

Originally Posted by Team 3266

...before the defensive bot can turn around

Or it could just back up. It has two directions of motion (its "front" and its "back"), and if aligns itself correctly, it can drive fast along that warning line (probably faster than you can strafe).

[Charmander]

I'm currently leaning toward 6WD because of how i'm predicting the game to be. I feel that there is always going to be a team who plays defensive. There is no point of having a great mecanum drive train if we can't get to the scoring grid. If there is a bot that plays defense on us, and plays it decently well, a mecanum drive train will be stopped. Regardless of its maneuverability, the scoring grid is surrounded on both sides with opposing team zones which we are not allowed in. Of course, the same problem occurs with a 6WD. However, i'd much rather take my chances with ramming into the side of a defending robot and see whats happens than waste precious time trying to overcome a robot. Any thoughts?

[Alexa Stott]

6WD has always worked well for us.

[AlexBond]

If you drop your center wheel wont that lead to problems with trying to hang rings on the topmost row because of the rocking at the base?

[Stephen of REX]

Originally Posted by AlexBond

If you drop your center wheel wont that lead to problems with trying to hang rings on the topmost row because of the rocking at the base?

Depends how much you drop by. If you drop the center wheel by 1/8 of an inch (which I believe is enough to get the desired effect) then rocking should be minimal, especially if you have distributed your weight to prevent rocking while hanging.

[Alan Anderson]

Originally Posted by Ether

The only wasted power in a mecanum is due to the friction in the bearings in the rollers.

Even with a frictionless roller, you're wasting power by squeezing or stretching carpet in the sideways direction while trying to move forward or back. You could perhaps argue that this isn't being wasted "in a mecanum", but it's definitely happening because of features inherent in a mecanum wheel.

[Alexa Stott]

Originally Posted by AlexBond

If you drop your center wheel wont that lead to problems with trying to hang rings on the topmost row because of the rocking at the base?

Or you could just not drop it.

[Jared Russell]

Originally Posted by Alexa Stott

Or you could just not drop it.

Warning: Your mileage may vary. I posted here about why not dropping the center wheel works for team 25, but in order to go this route, you need some very specific properties for your drivebase geometry and wheels (and gearing/torque output).

On the topic of the rocker affecting positioning accuracy...

1/8" rocker means your front wheels will vary in height by +/- 1/4" (actually less due to the carpet and tread deforming). If you build a mechanism that requires this kind of precision to score a tube you may want to think of other options.