Re: How Do mecanum wheels handle the bumps?
 

Are you sure it's not 71.03% ?

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


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Re: How Do mecanum wheels handle the bumps?
 

We've got mecanums and have no problem going over the bump. We've found that if the robot is mis-aligned with the bump simply driving into it the mecanum wheels will slide around to let everything line up, then we climb over the bump without a problem.

Re: How Do mecanum wheels handle the bumps?
 

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.

Re: How Do mecanum wheels handle the bumps?
 

sqrt(2)/2

The derivation is left as an exercise to the reader. Think of the angle of the rollers.

Re: How Do mecanum wheels handle the bumps?
 

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


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).


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


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Re: How Do mecanum wheels handle the bumps?
 

See this link for a perfectly legal active suspension, fully controlled by sensors on the robot. Follow the link to the youtube video.

http://www.chiefdelphi.com/forums/sh...ad.php?t=84187

As for mechanums, there is always a question as to the loss of power vs manueverability issue. You have to trade it carefully based on how you think the game will play out. There is no straightforward answer. You need to decide what the strategy your team wants to execute is and whether or not that system fits into your strategy. This is why in the first part of build season should set criteria your robot needs before you build anything figure out how to play the game to win and develop a strategy before you build your robot and these decisions become easy now that you can buy mechanum and swerve as COTS.

Re: How Do mecanum wheels handle the bumps?
 

Thanks Joe. I know that 71% is half the square root of 2 :-)

The question is, does this number have anything to do with the claim that "Only ~71% of the output torque is transmitted in the direction of travel", when the bot is being commanded straight forward or backward. I say no. I say all the torque being applied to the wheel gets reacted by the carpet. I am challenging what appears to be the conventional wisdom on this topic. Think about it.



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Re: How Do mecanum wheels handle the bumps?
 

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.

Re: How Do mecanum wheels handle the bumps?
 

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

Re: How Do mecanum wheels handle the bumps?
 

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.

Re: How Do mecanum wheels handle the bumps?
 

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

Re: How Do mecanum wheels handle the bumps?
 

My post assumes the rollers of a mecanum wheel spin. If they don't, the number is greater than 71%, but then you don't have a mecanum wheel anymore, at least one as efficient in any other direction.

190's an exception to the rule. That's not exactly an easy to design feature.

Re: How Do mecanum wheels handle the bumps?
 

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 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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Re: How Do mecanum wheels handle the bumps?
 

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.

Re: How Do mecanum wheels handle the bumps?
 

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.