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  #46   Spotlight this post!  
Unread 08-01-2014, 12:43
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Re: Mecanum wheels

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Originally Posted by TheMadCADer View Post
Hitting a moving target is added complexity you don't need. I'd pick a catcher that has no wheels at all before I'd pick one with mecanums.
So ... you'd rely on the 100% accuracy of the throwing robot, rather than provide some flexibility by allowing the catcher to adjust?

Put another way, if the ball is a mere 6" off target, you'd rather have the ball bounce off you (and then have to go chase it) than see a robot with the ability to take a half step to the right?
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Unread 08-01-2014, 12:58
TheMadCADer TheMadCADer is offline
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Re: Mecanum wheels

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Originally Posted by Mr_I View Post
So ... you'd rely on the 100% accuracy of the throwing robot, rather than provide some flexibility by allowing the catcher to adjust?

Put another way, if the ball is a mere 6" off target, you'd rather have the ball bounce off you (and then have to go chase it) than see a robot with the ability to take a half step to the right?
Based on initial ball launching prototypes, yes I would.

My point is that the ball will never be anywhere close to 6" from being caught, it will be much farther away and completely un-catchable because a mecanum robot can not hold field position. If your catching robot uses mecanum wheels, pushing it out of the way will be a common and effective defensive strategy. In a game like this where field position is everything, a mecanum robot is at an extreme disadvantage.

The only role I can see a mecanum robot playing effectively is a robot that cycles the ball by itself with no assists. As soon as it tries to play a role where it has to stop and wait for its alliance to do something it would lose its field position and ability to add assist points.
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Unread 08-01-2014, 13:15
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Re: Mecanum wheels

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Originally Posted by TheMadCADer View Post
... a mecanum robot can not hold field position. If your catching robot uses mecanum wheels, pushing it out of the way will be a common and effective defensive strategy. In a game like this where field position is everything, a mecanum robot is at an extreme disadvantage.
Does the same argument apply to giving a healthy bump to the lighter corner of a 6WD drop-center bot?


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Unread 08-01-2014, 15:54
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Re: Mecanum wheels

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Originally Posted by Ether View Post
Does the same argument apply to giving a healthy bump to the lighter corner of a 6WD drop-center bot?


To a degree. It requires a more directed bump/push than defending a mecanum robot, and a 6/8 wheel drive train can better resist the imparted turning motion by turning against it, allowing you to hold a stalemate. Trying to drive a mecanum robot that is already slipping is an exercise in futility. The bump also causes rotation more than sliding, which would keep general field position close to static. In this case a good catcher would be orientation-agnostic.

Having a robot that can be pushed in any way is disadvantageous to being a good target for catching, so I'd say the best pure catcher is a robot without wheels. However, making the trade-off of allowing some pushing in exchange for the ability to move and complete other tasks could be a successful strategy. Toggling between a drivetrain and a traction device would give the "best of both worlds" at the expense of complexity (independent swerve being another solution, albeit even more complex).
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Unread 08-01-2014, 16:04
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Re: Mecanum wheels

Designing to be defended against is a really good way to put a ton of work that may or may not see a benefit in competition.

Remember 2006, when 25 had a robot that was solely human fed, and basically shot from the floor? And went undefeated right up through division elimination rounds?

Ultimately the benefit of mecanum ends up coming down to what your game strategy is. If it's to be a flexible assist bot, it may very well be worth it-- if it's being a thrower bot, it very well may not be. Ultimately it's all just speculation until we get to competition regardless. Just like any other year, there will be successful 6WD robots and successful mecanum robots.

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Unread 08-01-2014, 16:19
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Re: Mecanum wheels

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Originally Posted by TheMadCADer View Post
...a 6/8 wheel drive train can better resist the imparted turning motion by turning against it, allowing you to hold a stalemate.
I wonder if anyone has ever actually conducted such a test under controlled conditions. The mec bot would have a larger effective wheelbase than 6W drop center. And mec wheels (at least the affordable ones commonly used for FRC) are somewhat harder to push sideways than fwd/rev. Might make an interesting off-season science project to verify this claim or lay it to rest.

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Unread 08-01-2014, 16:33
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Re: Mecanum wheels

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Originally Posted by Qcom View Post
Question: You mentioned the Vex Mechanums cracking in an earlier post. Where did they break, and was the break catestrophic enough to warrant immediate disabling of robot movement (i.e. could the robot finish a match if this occured)?
Someone mentioned that they had a VEXpro mecanum wheel crack, but did not provide any additional information. We've done a lot of testing here and not seen any durability issues. We've also not heard very many complaints from users about Mecanum wheels breaking.

We're very happy with how that design turned out, and are confident in the durability, especially in a game with a flat field like Aerial Assist.

If you jump a robot off a loading dock, will the wheels crack? Yes: Based on our testing... not until the 3rd jump.

If you slam the wheels into a wall with a rolling robot, will they crack? Not in our testing.

I'm confident that VEXpro Mecanum Wheels will stand-up to normal wear and tear during an FRC season. If a wheel breaks, it is likely that the wheel wasn't damaged due to extended match play, but instead the robot was dropped when being loaded into a mini-van.

Also, I hope it goes without saying for VEX products at this point:
If you have a problem or break, please contact our tech support folks so they can help make it right. prosupport@vex.com

I'm especially excited to see teams using our new 4" Mecanum Wheels. This game lends itself perfectly to small wheels, and I believe there are some strategies which would benefit from holonomic motion.

-John
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Unread 08-01-2014, 16:43
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Re: Mecanum wheels

Is there any information on mecanums wheels and drivetrains? Live Axle vs Dead Axle?
I have never use them and am looking for information. Transmission? Does and one have some good information to share. Thanks
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Unread 08-01-2014, 16:47
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Re: Mecanum wheels

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I have never use them and am looking for information.
Where have you been looking?


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Unread 08-01-2014, 16:56
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Re: Mecanum wheels

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Originally Posted by SMHS View Post
Is there any information on mecanums wheels and drivetrains? ...
Sure, there's lots of info. See that search link at the top of this page? Click it, enter "mecanum", and you'll find lots.
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Unread 08-01-2014, 18:33
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Re: Mecanum wheels

Apologies to JesseK and bEdhEd. I was not attempting to say either of you said there was power loss for mecanum moving forward/backward. I was not trying to counter what either of you said, but using your correct statements as an example of the types that are very commonly misinterpreted.

my post was attempting to be pro-active about a future reader's misinterpretation, and was instead itself misinterpreted. I think there is a word for that (other than failure). I guess we can agree that I clearly didn't say it very well.
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Unread 08-01-2014, 18:38
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Re: Mecanum wheels

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Originally Posted by lcoreyl View Post
Apologies to JesseK and bEdhEd. I was not attempting to say either of you said there was power loss for mecanum moving forward/backward. I was not trying to counter what either of you said, but using your correct statements as an example of the types that are very commonly misinterpreted.

my post was attempting to be pro-active about a future reader's misinterpretation, and was instead itself misinterpreted. I think there is a word for that (other than failure). I guess we can agree that I clearly didn't say it very well.
Ah, I understand what you were trying to do. I guess your motive just wasn't clear enough for us to get. Part of it may have been our bad too.
As for your apology, team bEdhEd graciously accepts!
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Unread 08-01-2014, 19:37
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Re: Mecanum wheels

Quote:
Originally Posted by JVN View Post
Someone mentioned that they had a VEXpro mecanum wheel crack, but did not provide any additional information. We've done a lot of testing here and not seen any durability issues. We've also not heard very many complaints from users about Mecanum wheels breaking.

We're very happy with how that design turned out, and are confident in the durability, especially in a game with a flat field like Aerial Assist.

If you jump a robot off a loading dock, will the wheels crack? Yes: Based on our testing... not until the 3rd jump.

If you slam the wheels into a wall with a rolling robot, will they crack? Not in our testing.

I'm confident that VEXpro Mecanum Wheels will stand-up to normal wear and tear during an FRC season. If a wheel breaks, it is likely that the wheel wasn't damaged due to extended match play, but instead the robot was dropped when being loaded into a mini-van.

Also, I hope it goes without saying for VEX products at this point:
If you have a problem or break, please contact our tech support folks so they can help make it right. prosupport@vex.com

I'm especially excited to see teams using our new 4" Mecanum Wheels. This game lends itself perfectly to small wheels, and I believe there are some strategies which would benefit from holonomic motion.

-John
Wow! I was not expecting a VEX engineer to reply. Thank you, I think that may seal the deal.
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Unread 08-01-2014, 23:53
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Re: Mecanum wheels

So Mecanum vs 6WD and all things being equal I understand most of the pros and cons.

But if you look at the Robot in 3 Days team Boom Done they have 8 motors on their drivetrain - 4 cim and 4 minicim. What if instead of having a straight gear reduction, a team were to use a direct drive shifting transmission such as the Vex Pro ball shifter on each Mecanum wheel with a Cim and Mini-Cim as the input to each wheel? And you took that against a 6WD drivetrain with the three CIM Vex Pro Shifting transmission per side?

Does that muddy the water a bit?
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Unread 09-01-2014, 00:30
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Re: Mecanum wheels

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Originally Posted by MrRiedemanJACC View Post
So Mecanum vs 6WD and all things being equal I understand most of the pros and cons.

But if you look at the Robot in 3 Days team Boom Done they have 8 motors on their drivetrain - 4 cim and 4 minicim. What if instead of having a straight gear reduction, a team were to use a direct drive shifting transmission such as the Vex Pro ball shifter on each Mecanum wheel with a Cim and Mini-Cim as the input to each wheel? And you took that against a 6WD drivetrain with the three CIM Vex Pro Shifting transmission per side?

Does that muddy the water a bit?
What would lead you to believe that it would muddy the water? Can you provide reasons why, in your estimation, the mecanum drive implementation you're suggesting here would fare better against the 6WD robot you're describing than a more typical mecanum drive?

Do the math and you'll have your answer. This stuff is pretty cut and dry.
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