pic: Team XBot Mecanum Practice Chassis

[Cody Carey]

That appears to be the max size of a drive base according to last years rules, right? For a competition bot, would you scale that design down somewhat in order to add a protective structure around the outside? or would you just rely on the fabric bumpers? It would seem to me that the channel used on the outside would bend/dent easily, and no amount of suspension will keep a Mecanum drive operating correctly if one or two of the wheels are suddenly bent at some odd-angle. The video looks very nice indeed, and I can't wait to see what the drivers can do with a bit of practice

[Fred Sayre]

Originally Posted by AdamHeard

How durable is that setup?

It hasn't broken yet

The frame seems plenty rigid and sturdy. We have not really ran into anything yet, but with bumpers and such it should be fairly protected. How it deals with the strains of the rest of the weight of the robot we still have to evaluate.

[Madison]

Originally Posted by Cody C

That appears to be the max size of a drive base according to last years rules, right? For a competition bot, would you scale that design down somewhat in order to add a protective structure around the outside? or would you just rely on the fabric bumpers? It would seem to me that the channel used on the outside would bend/dent easily, and no amount of suspension will keep a Mecanum drive operating correctly if one or two of the wheels are suddenly bent at some odd-angle. The video looks very nice indeed, and I can't wait to see what the drivers can do with a bit of practice

The chassis is about an inch short of last year's maximums in all directions. If you look at some of our past work, you'll see that I don't typically add structure to things for protection, but since these wheels are quite expensive, this chassis was kept simple to facilitate accepting bumpers. Also consider that it's for practice and won't be seeing competition, so there was no use in wasting time and resources on making it stronger than it needs to be for learning, practice and demonstrations.

[Gabe]

I wanna see a Mecanum drive at Portland this year! I could spend hours looking over this robot in the pits.

[GMKlenklen]

alright, some things to try:

See what effect tightnening the rollers has compared to haveing them loose.
Drive up some ramps.
Drive up some stairs!

Oh, and did you incorperate the suspention system? I can't tell from the picture.

[Madison]

Originally Posted by GMKlenklen

okok, now you gotta try driving it up some stairs like the airtrax did!!! Please? oh, and ramps too...

Btw, I can't quite tell, do you have suspension on it? or did that get left out?

If we have a chance, we'll try running it across all sorts of weird surfaces and objects to see how it behaves. We have our ramp from last season around somewhere and I'm already considering mechanisms that'll allow it to climb curbs and stairs and the like.

Worse comes to worst, we have a reliable design for a simple 6WD drive train waiting in the wings. I can post CAD models of that as well, if folks are interested.

The suspension is implemented, but not quite as we'd originally intended. Because we wanted to get this thing running, we improvised a bit and used both tennis and racquet balls as suspension springs. You can see the bright green and dark blue balls in the video and photos. The tennis balls give a better ride height, but don't compress as easily as the racquet balls, so the jury's still out on which we'd use for competition. More testing is required.

[GMKlenklen]

oh, that's cool!

[Crazy Ivan]

Looks awesome! That was cool idea using the racket/tennis balls for the suspension.

[Kingofl337]

If you want to be able to rotate the base has to be square. Your current base will not rotate well when you load another 100lb on it.

[Madison]

Originally Posted by Kingofl337

If you want to be able to rotate the base has to be square. Your current base will not rotate well when you load another 100lb on it.

When I'm back after the holiday, we'll add weight and do some more playing around with things to see how it goes. Bringing the wheels closer together so the footprint is square is an easy change, if it's needed, but it's by no means a given method of achieving that discussed result.

[Alan Anderson]

Originally Posted by Kingofl337

If you want to be able to rotate the base has to be square. Your current base will not rotate well when you load another 100lb on it.

The required torque to get a Mecanum wheel to move sideways certainly increases with additional weight. I think that can be addressed somewhat by making the rollers spin a little freer.

Why do you think squareness is an issue? The center of mass should probably be as close as practical to the center of the 'bot in order to keep weight equalized on all the wheels, but I think a long or wide rectangle ought to work fine.

[viking1902]

Originally Posted by Alan Anderson

Why do you think squareness is an issue? The center of mass should probably be as close as practical to the center of the 'bot in order to keep weight equalized on all the wheels, but I think a long or wide rectangle ought to work fine.

I can verify that squareness is not an issue. I have built two holonomic drives on team 1083 in 2004 and 2005 (couldn't find a pic). Both drivetrains were rectangular and worked just fine.

[Lil' Lavery]

Originally Posted by viking1902

I can verify that squareness is not an issue. I have built two holonomic drives on team 1083 in 2004 and 2005 (couldn't find a pic). Both drivetrains were rectangular and worked just fine.

116 built a rectangular holonomic drive in 2005 as well, rotation was definately NOT a problem.

[Kingofl337]

Let me update my post. It doesn't matter if the robot is square but the wheels should be equal distances from each other.

Our test wheelbase was inadvertently square and drove fine. Our final bot was rectangular and had an extremal difficult time rotating. The weight load was similar to our test platform, also WPI had a square robot 2005, and a few other teams with holonomic drives had square robots. None had no problem rotating, leading us to conclude the squareness of the robot effects the rotational ability.

I would recommend making sure you load the system with at least 140lbs throughly testing it before you commit. Rotation is a big part of this drive system.