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Unread 01-21-2011, 10:10 AM
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Re: Another chapter to the 6WD vs. Mecanum debate

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Originally Posted by Ether View Post
Yes. But that's a power limit, not a power loss. This may seem like nitpicking, but it's an important distinction.
That makes sense, and I'm all about nitpicking distinctions, otherwise ambiguity would prevail.

Quote:
The "force vector cancellation" does not cause any power loss unless you are operating on a very resilient surface at high vehicle force levels and speeds. And then it's not a fixed 30% number but rather a function of the force and speed.

Once a mecanum's wheels break traction (start slipping), then any additional power supplied to the motors is wasted in scrubbing the wheels on the floor.
This makes sense too.

Thanks for the input!
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Unread 01-21-2011, 10:54 AM
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Re: Another chapter to the 6WD vs. Mecanum debate

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Originally Posted by AdamHeard
Well, there is "a 6wd", and then a 6wd with high traction wheels, 4 CIMs, and shifting.

One of them is anywhere from mediocre to good, the other is great. The great one will beat a mecanum any day of the week.
Depends what you mean by beat. A pushing match? Probably so. A test in maneuverability? I'd disagree with that. If your driver knows how to truly utilize the strengths of the mecanum drive, then you can juke 6 wheel drive robots all day long.

Picking one of them revolves entirely upon your team's strategy for this year's game.
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Unread 01-21-2011, 11:09 AM
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Re: Another chapter to the 6WD vs. Mecanum debate

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Originally Posted by Ryan Dognaux View Post
Depends what you mean by beat. A pushing match? Probably so. A test in maneuverability? I'd disagree with that. If your driver knows how to truly utilize the strengths of the mecanum drive, then you can juke 6 wheel drive robots all day long.
I've literally never seen a mecanum "juke 6 wheel drives all day long". Not saying it can't be done... just saying I really, really want to see video of it. Partly for "proof" and partly because it would be really cool to watch.
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Unread 01-21-2011, 11:23 AM
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Re: Another chapter to the 6WD vs. Mecanum debate

You will see a difference in pushing performance due to the traction capabilities of the wheels used, not so much the power available.

The mecanums are usually a hard roller material, which will not provide much "grip" into the carpeted surfaces. Assuming the 6WD is using some sort of treaded wheel it will have more traction onthe carpet.

Think about walking across the playing field in your slick soled dress shoes. You can move, you can run, but you can also push your foot out to the side and it will slide across the carpet. In a pair of softer soled athletic shoes, you can also move and run, but if you try to push your foot out to the side it will take more force to do so.

The mecanum is like the dress shoe, the treaded wheel is like the athletic shoe.
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Unread 01-21-2011, 12:33 PM
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Re: Another chapter to the 6WD vs. Mecanum debate

As previous leader of the 449 Drive Team for two years, I'd like to put in a few good words for mecanum.

First off, there is almost no power loss in forward/backward movement in a mecanum drive - the rollers do not move unless you are strafing. The fact that mecanum is "less powerful" is a common misconception - mecanum is *not* omni holonomic, and the only difference in power is the difference in coefficient of friction with the floor between the roller material and the material of your six wheel drive wheels. Mecanum's only real weakness with respect to being pushed is from the side, which in this game isn't all that important as the other robots are not allowed to interfere with your goal scoring. Last year during the DC regional we had some issues with ball manipulation that forced us to change our strategy during the quarterfinals to "go into the front zone and push the defensive bot out of the way," which we were able to do with no problems at all.

Secondly, strafing is *amazingly handy," especially in a game like this years in which staying lined up with a goal is crucial to success.

Thirdly, it is not particularly complex. One your team invests in writing/acquiring decent drive code, building a mecanum drive is as trivially easy as building a normal four wheel tank drive. Well, perhaps a wee bit more, as you need to have one gearbox per wheel.

Fourthly, mecanum is also fairly robust - we have lost rollers and drive belts and still been able to play the game fairly well, as even a mecanum missing the strafing capability can turn better than a standard 4wd. You really don't even have to adjust that much for it on the controls. We were able to traverse the bump last year as well as any other robot at the competition without mucking up our drive in the slightest.

And finally, I am amazed at how AndyMark's mecanum wheels have improved over the years. We had very few maintenance problems with our rollers last year, and the main issue we had (difficulty in adjusting the connection to each roller) seems to have been fixed on this years generation of 6'' mecanum wheels.
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Unread 01-21-2011, 01:18 PM
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Re: Another chapter to the 6WD vs. Mecanum debate

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Originally Posted by Oblarg View Post
...the rollers do not move unless you are strafing.
or rotating.

Quote:
The fact that mecanum is "less powerful" is a common misconception - mecanum is *not* omni holonomic,
Mecanum is fully holonomic if properly coded. All three degrees of freedom can be executed simultaneously with no skidding of the contact patch.

Quote:
and the only difference in power is the difference in coefficient of friction with the floor between the roller material and the material of your six wheel drive wheels.
If by "power" you mean "maximum force" this is not correct. Even with the same material on the mec and the 6WD, the mec will have less maximum pushing force, as explained in many posts here on CD.



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Unread 01-21-2011, 01:20 PM
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Re: Another chapter to the 6WD vs. Mecanum debate

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Originally Posted by Oblarg View Post
First off, there is almost no power loss in forward/backward movement in a mecanum drive - the rollers do not move unless you are strafing.
This is not true.

Quote:
The fact that mecanum is "less powerful" is a common misconception - mecanum is *not* omni holonomic, and the only difference in power is the difference in coefficient of friction with the floor between the roller material and the material of your six wheel drive wheels.
Also not entirely true - it's worth noting a mecanum drive more or less necessitates a lower traction roller.

Quote:
Mecanum's only real weakness with respect to being pushed is from the side, which in this game isn't all that important as the other robots are not allowed to interfere with your goal scoring.
It can also be pushed from the front more easily due to those spinning rollers.

Keep in mind just because there's no "at the goal" defense doesn't mean there is no defense.
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Unread 01-21-2011, 01:23 PM
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Re: Another chapter to the 6WD vs. Mecanum debate

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Originally Posted by Chris is me View Post
I've literally never seen a mecanum "juke 6 wheel drives all day long". Not saying it can't be done... just saying I really, really want to see video of it. Partly for "proof" and partly because it would be really cool to watch.
I'm with you Chris. If Mecanum Drives could outmaneuver traditional Traction Drives so well, wouldn't we see them at the higher levels of competition much more often? (Think Division Finals, Einstein)

Now a Swerve Drive can Juke 6WD's all day long - and that's cool to watch.
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Unread 01-21-2011, 01:24 PM
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Re: Another chapter to the 6WD vs. Mecanum debate

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Originally Posted by Chris is me View Post
it's worth noting a mecanum drive more or less necessitates a lower traction roller.
It does not necessitate a lower traction roller. A properly coded mec is fully holonomic and there is no skidding of the contact patch... only rotating of the patch, like swerve.


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Unread 01-21-2011, 01:32 PM
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Re: Another chapter to the 6WD vs. Mecanum debate

My mouth got ahead of my brain again - I meant that mecanum drives more or less will always have less max pushing force ("lower traction") than a 6wd wheel, even if both were coated with roughtop tread.
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Unread 01-21-2011, 01:42 PM
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Re: Another chapter to the 6WD vs. Mecanum debate

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Originally Posted by Chris is me View Post
My mouth got ahead of my brain again - I meant that mecanum drives more or less will always have less max pushing force ("lower traction") than a 6wd wheel, even if both were coated with roughtop tread.
Would mecanum even work with the kind of tread that AM sells for traction wheels?
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Unread 01-21-2011, 01:52 PM
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Re: Another chapter to the 6WD vs. Mecanum debate

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Would mecanum even work with the kind of tread that AM sells for traction wheels?
Quote:
Originally Posted by Ether View Post
It does not necessitate a lower traction roller. A properly coded mec is fully holonomic and there is no skidding of the contact patch... only rotating of the patch, like swerve.


Based on what Ether said two posts earlier I would say yes.
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Unread 01-21-2011, 02:05 PM
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Re: Another chapter to the 6WD vs. Mecanum debate

Making a roughtop mecanum wheel would be an award-worthy feat, indeed.

Still, in a roughtop vs. roughtop pushing battle, the 6WD would STILL always beat the mecanum drive...because maximum tractive force for the mecanum drive IS influenced by the fact that there are passive rollers. Read Ether's white papers - they explain it all.
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Unread 01-21-2011, 03:29 PM
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Re: Another chapter to the 6WD vs. Mecanum debate

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Originally Posted by Ether View Post
If by "power" you mean "maximum force" this is not correct. Even with the same material on the mec and the 6WD, the mec will have less maximum pushing force, as explained in many posts here on CD.
By "omni holonomic" I was referring to the drive system of four omni wheels placed in square or diamond configuration. If that's not what the term means, then I'm sorry, that's what it's always referred to in our team.

No, I don't mean maximum force. I know mecanum wheels do start skidding before 6wd drive, but honestly I *rarely* see a pushing match actually reach that point in competition, and with the pinning rules in place it's even more of a nonissue now than it has been in previous years.

And yeah, the rollers turn while rotating, but it's not as if someone hitting you from the side will cause you to start spinning.

Quote:
Originally Posted by Chris is me View Post
This is not true.



Also not entirely true - it's worth noting a mecanum drive more or less necessitates a lower traction roller.



It can also be pushed from the front more easily due to those spinning rollers.

Keep in mind just because there's no "at the goal" defense doesn't mean there is no defense.
I've had to push against 4wd and 6wd drive robots with mecanum before, and not once have I seen the rollers spin - try it yourself, you'll notice that the difference is not nearly as big as most people make it out to be. Yes, there's a lower cap before mecanum wheels start skidding, but how often have you seen wheels skid in a FIRST pushing match? It's not exactly a common occurrence - in my time in FIRST I really can't recall ever seeing our mecanum wheels skidding on the carpet. In my experience, the idea that "mecanum drive gets pushed around a lot" is almost entirely a myth.

People put way too much store in "If we end up exactly head to head and are pushing each other, my robot will win out." That's not a realistic situation. In the context of an actual competition, in which your goal is not to push head to head with other robots, mecanum is not at a significant disadvantage in terms of being pushed. Take into account the fact that defense isn't allowed to push you while you're scoring, and the best way to get into the scoring zone will probably be dodging the defensive robot rather than pushing it (in fact, are you even allowed to push another alliance's robot into your scoring zone?), and I really don't see what the benefit of a 6wd is.
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Unread 01-21-2011, 03:36 PM
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Re: Another chapter to the 6WD vs. Mecanum debate

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And yeah, the rollers turn while rotating, but it's not as if someone hitting you from the side will cause you to start spinning.



I've had to push against 4wd and 6wd drive robots with mecanum before, and not once have I seen the rollers spin - try it yourself, you'll notice that the difference is not nearly as big as most people make it out to be. Yes, there's a lower cap before mecanum wheels start skidding, but how often have you seen wheels skid in a FIRST pushing match? It's not exactly a common occurrence - in my time in FIRST I really can't recall ever seeing our mecanum wheels skidding on the carpet. In my experience, the idea that "mecanum drive gets pushed around a lot" is almost entirely a myth.

People put way too much store in "If we end up exactly head to head and are pushing each other, my robot will win out." That's not a realistic situation. In the context of an actual competition, in which your goal is not to push head to head with other robots, mecanum is not at a significant disadvantage in terms of being pushed. Take into account the fact that defense isn't allowed to push you while you're scoring, and the best way to get into the scoring zone will probably be dodging the defensive robot rather than pushing it (in fact, are you even allowed to push another alliance's robot into your scoring zone?), .
2006 Boston Regional Finals Matches 1 and 2 come immediatly to mind for me. Every thing you say you have never seen occur happens in those matches and the drive train on the defensive bot was not that good, it was a poorly executed 4wd design.

I have seen Mecanum drives pushed directly backwards by a 4 or 6 wheel drive despite trying to push forwards. There is definitely a disadvantage to mecanums whenever there is pushing.

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and I really don't see what the benefit of a 6wd is.
I think the advantage to 6wd is pretty obvious and pretty much common knowledge, it is simple, reliable and easy to manuever. There is a reason that the majority of teams on Einstein every year seem to be 6 or 8wd
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