Re: mecanum vs. omni
 

I'd suggest reading some (or all) of the threads I posted earlier about vector based (holonomic) systems, as well as Ian MacKenzie's whitepaper (also posted on page 1 of thread).
A kiwi (killough) drive is really just another form of holonomic drive (just like a mecanum). A kiwi drive is a 3-wheeled variant, with wheels every 120º (as mentioned earlier). The vector math is basically the same, just with wheels every 120º instead of every 90º. A kiwi has a considerably smaller support structure, making it more prone to tipping if the bot is top heavy. The frame is also more difficult to construct if you don't have the proper tools/experience, as it requires non-right angles (although still not terribly challenging). However, with only three wheels, you don't suffer the problems of losing contact with a wheel (unless you bottom out, but that could happen to any drive), resulting in fluky driving. You also only have to invest the weight and space for 3 wheels/motors, as opposed to 4+ (you can really put any amount ≥3 of wheels in a holonomic system).
No solution is truly "better". A 4-wheeled holonomic, mecanum, and kiwi each have their advantages, but each have their disadvantages. The basic principles in them are all basically the same, and the real distinctions between these drives comes depending on how well a team can implement them.

Re: mecanum vs. omni
 

One final thing to consider: the driver. Can your driver handle the extra maneuverability? IF definitely not, then stick with what works. If he just needs practice, do you have the resources to build a second frame (robot would be better, though), weight it down, and practice for weeks on end? If he can handle it without extra practice, give it to him anyway if possible.

Note: during testing in 2005, we noticed that strafing ability was impaired by holding a tetra about 2' away from the drive base. We just couldn't go straight sideways. We'd arc towards the tetra.

And yes, mecanums do tend to be slower sideways than front-to-back. At least, the first iteration does. It has to do with the fact that instead of having all the rollers work together, half of them are "fighting" the other half, so the robot moves more slowly. It looks something like this:
straight: --> --> ....sideways: --> <-- ....diagonally: --> --
........... --> --> ................. <-- --> ................... -- -->

Re: mecanum vs. omni
 

hey guys im from team 1382 too and i want ask u something:

if a robot with mecanum wheels or omni wheels receive a stroke on his side of another robot , it will be pushed? (i hope u understand the question...)

thanks for the attention with us...

cyaa

Re: mecanum vs. omni
 

If M (the mecnaum robot) hits N (normal robot), chances are that M will have to hit very hard on a corner to even move N. If N hits M, M will move. M won't move very far on one hit, but pushing will send M into a corner unless M runs (and the point of a mecanum is to avoid pushing contests.)

Re: mecanum vs. omni
 

very good point.. the point of making a omni or mecanum is to Not get pushed.. aviod pushing matches at all cost, cause any other type of drivetrain will be you in a pushing match.. hands down. So IMO you should work to get the omni or mecanum as agile as possible.

Re: mecanum vs. omni
 

yeah... seeing all the things said, and related here.. it's clear that mechanum/omni robots must be designed to provide quick movements and chiefly to use offensive strategies that would avoid the defense (and so "pushing clashes")...

the point at using the wheels seems to be: will the drivers be able to deal with them? (the wheels and the defensive bots)
and... what strategy will be used? (ofensive or defensive)

am i wrong?? If so, please correct me..^^

the team is planning to build a drivetrain system during the off-season period, to practice to the '08 season and now we are trying to see what would be more useful... so we need to think about all these things..


thank you all again^^

and sorry if i've made some spelling/grammar mistakes...

Re: mecanum vs. omni
 

You should be planning on playing offense with a holonomic or mecanum drive.

The downside is that if your manipulator fails or is ineffective, you can't really play defense that well. With skid steer you still can.

The best advice I can give is use this kind of drive if it is required, not because it is cool.

Re: mecanum vs. omni
 

Omni can easiel play defense if the game is correct. This year we did a good job of defending robots from getting to the rank because they couldn't get between us and the rack because we were so fast and agile. I never got in a pushing match but they still didn't score. Of course this will only take you so far.

You should decide based on the game and if you can get your prototype working well during the off season.

Re: mecanum vs. omni
 

Adam is quite right... don't assume that mecanum or omni or any other holonomic drive system is going to give you an edge at a competition unless it suits the game, your strategies and the rest of your robot. You will spend more time designing and debugging a more sophisticated drive system than you will a simpler tank-style drive system, and that will take away from something else you could be doing to make your team/machine better.

On the OTHER hand.... YES!! Build a Mecanum because it is cool! Why the heck do we get into this whole robot-building stuff in the first place? Trophies and banners are nice enough, I suppose, but people have seen them before. How many people have seen a mecanum wheel?? Robots that can go sideways really capture people's attention, especially when you bring it back from the competition and are driving it around in the community.

If you base the success of your season on winning a regional, then chances are you are going to be disappointed. If you base the success of your season on building a good team and a cool robot then chances are good that you are going to be successful... and you might even find that you win a fair bit, too!

Jason

Re: mecanum vs. omni
 

i agree totally. yes, it is fun to win, but the robot is REALLY why we do all of this :p . also, just because i'm nice, i have omni code if someone needs help

Re: mecanum vs. omni
 

I'm not basing success on winning a regional. Success is learning by buildint the best system you can. In the engineering industries, the best system is the one chosen.

If a 6 wheel skid-steer can outperform a holonomic in a certain game. Then the skid-steer is the better system, although it is more likely less complex.


Now, for offseason things. Go ahead and build the "cool' stuff.

Re: mecanum vs. omni
 

This is getting kinda off-topic, but anyway.
Not every team has the resources, location, and member base to build new systems during the off-season, and so the only time they have to build something cool is during the build season.
In addition, performance isn't the only measurement of success, and how to determine the "best system" is more than performance on the field.

More on topic.
For any new system, particularly in a component of the robot as crucial as the drive-train, I would suggest doing some form of prototyping before the build season starts if possible. The added experience, ability to work out kinks, and test out your code are invaluable. Additionally, its a great way for drivers to practice on the new system before it is finished, and after it is shipped.

Re: mecanum vs. omni
 

for now we're going to start (maybe?) designing our concepts in Inventor. hopefully by the end of school and the summer we might build an omni drive. we already have the code for with and without a gyro, and i'm just itching to test it out.

Re: mecanum vs. omni
 

I agree to an extent. If you prototype something on limited resources and then realise after the game is announced that your prototype concept has little chance of success in the game, then you have 2 choices: 1. Come up with a creative way to implement your prototype or 2. (more common) Adapt your prototype so that it fits into the current strategy, therefore introducing (sometimes) very complex integration issues or unforseen behaviors --

e.g. Adapting mecanum to play pure defense will really put some wear on the treads of the wheels -- enough wear to be of concern if you attend 2+ competitions with success.

Ok, so this argument probably holds little weight in the case of mecanums, as it's more likely you can adapt your strategy to use mecanums with some sucess every year. However, if you're like most teams, you'd rather adapt your robot to your chosen strategy and not vice versa. This is also good engineering practice, whereas adapting your strategy to your robot is something that's good practice for learning how to find & fight problems (namely integration problems) after the fact.

Being a strategist and an engineer at heart, I personally choose the former. In the meantime, the "prototypes" I'm doing on mecanums extend into the VEX realm, by trying to make a 100% legal mecanum VEX wheel for $20 or less per wheel (very very tough to do atm unless they make the white nylon spacers come in the VEX kit -- easy to do with those). Another thing you can do, is take a look at the white papers, do that math, and create a VEX omni-wheel drive. The hardest part about holonomic drive is the physics, vectors, and programming, so getting started with developing an algorithm in programming or creating a full VEX omni-drive will get you headed in the right direction.

Save the purchase of the wheels for the big robot for after the game announcement so you know that you're getting your money's worth.

Re: mecanum vs. omni
 

that would be great! ^^