Not necessarily more maneuverable, But I bet it is nice to have the 'bot spin around it’s center set of wheels instead of whichever four happen to be touching the ground :rolleyes:
Wow, looks light! Do you have an estimated weight for the entire drive base?
And on the subject of omniwheels: It looks like with the setup you have it would be very easy to swap them for traction wheels if they are needed. I guess that would be decided in testing.
Can you explain why you chose not to have a cross brace? I’m not a mech-e but I would guess you need one.
The wheels are all in line – there’s no ‘rock’ at all. The chain is #25.
Because the center wheel is not lowered at all, at least one outside set of wheels should be omniwheels to alleviate friction while turning. I chose to put a set on each end because there’s no penalty to in line pushing force, generally, and I like to subtley encourage our drivers to avoid conflict. 
I’m now looking at a mounting scheme that will accept bumpers and protect the wheels from impact. I’ve never put bumpers on a robot before, so I’m looking at examples of others’ work to get an idea how to best accomodate the additional weight of the bumper assembly.
Our 2005 robot had a lowered center wheel and the tipping action drove me and our drivers crazy. It’s an impact to the system each time the robot’s inertia shifts and that starts to lead to annoying things like loosened bolts and the like. Practically, it’s cheaper and easier for me to make omniwheels than it is to make traction wheels and it’s certainly cheaper than buying them.
As shown – which includes everything but stiffening gussets and a base upon which to mount electronics, it’s 29.5 lbs.
The omniwheels are riding on 5/8" keyed shaft, while the AndyMark performance wheel is broached for a 1/2" hex shaft. It’d be relatively easy to change out a set of omniwheels for a set of traction wheels with a little bit of forethought. Namely, I’d need to make some shafts that are 5/8" diameter with keyway on one end and 1/2" hex on the other.
I’m waiting on the game before adding cross-bracing since I’m not sure if I’ll need a scoop or something on one or both ends. I’d like to, if weight allows, put an aluminum plate (1/16" or so) across the bottom to add rigidity.
I noticed that the front and rear c channels are notched for the drive rails on top and bottom. This could be a weak point. Shouldn’t the drive rails fit inside the c channel leaving the channel uncut ?
M -
Looks quite nice, and you have obviously put a lot of work and thought into the design. The efforts to minimize the total weight are obvious. The biggest concern you may have is strength in the event of an impact or collision. You might consider making the frame members out of rectangular tube instead of channel. Given the amount of material you are removing, the additional weight from the fourth side of the tube will be minimal, but there will a considerable increase in stiffness of the frame assembly.
Just out of curiosity, what is the spacing between the wheel sets? Is it sufficient to cross a 20-inch wide, 16-inch deep, water-filled moat, should that capability ever be necessary?
-dave
Oh goodness.
Here we go again…
Any crocs?
CRIKEY!
WATCH OUT!!! WE’VE GOT A DAVE ON THE LOOSE!!! SOS…SOS…SOS
Who keeps letting him in? Security!!!
I love the simple approach to the 6-wheel drive. I’ll be follwing up later if you use this to see what additional stiffening was used/needed. Good luck.
I took Dave’s advice and added the fourth wall to the front and rear cross members as a start. Because these drivetrains are almost never operating absent some additional mechanism or superstructure, I try to incorporate those future additions into the design and allow for them to be used as support for the drive in the same way that the drive supports the mechanisms, if that makes sense. I try to design such that the finished robot appears organic – not as if it was designed in pieces that were smashed together later.
Because of the holidays primarly, we haven’t really done much of anything with our mecanum drive prototype. It still has no feedback system installed and remains difficult to drive. Most of the team agrees that, unless it appears to be a perfect solution for the new game, we’ll stick with something more traditional like this drive, so there’s a good chance you’ll be seeing it somewhere.
Madison, I know that you have no broach capability, so I’m curious- where do you buy your #25 sprockets? I can’t find any finished ones online, or in the Browning catalog.
Wouldn’t the spinning of the robot automatically lift the front and back wheels up and equal distance from the floor? Kind of like when your arms raise up when you spin around.
Maybe on a perfectly 4-way symmetrical robot, but in My team’s experience, It stays on 4 wheels and rocks back and forth… Whichever four wheels happen to be touching are the ones the bot spins around. Kinda messes up high-speed maneuvering.
For this, we’ll be ordering from McMaster-Carr – part no. 2737T123. 5/8" bore was the smallest I found with a keyway for this particular sprocket. I didn’t take a comprehensive look at many teeth counts, though, so I’m not sure how consistent the keyways are in 5/8" bore sprockets – but I can’t imagine why they all don’t have one.
Ah, I see. We were going to be using 1/2" bore, and for some reason, no one on earth carries broached 1/2" bore sprockets. Thanks though!
The last two years I worked on center-drive, 6-wheel robots and both times we purposely loaded one end of the 'bot with a LOT more weight for game reasons. This also had the effect of having the same four wheels on the ground most of the time. We had little fore-and-aft rocking and the robot wasn’t tricky to steer at all. Having said that, Madison makes nice chassises – er, chasses – er, chassisses – what IS the plural of chassis anyway? I’d hire her to consult on our chassis, but since I’m now on a VEX-only team, I think we’d be wasting her skills.
I agree wiht what dave said about strenght, but on the contrary our team used aluminum channel for our frame and we didnt seem to have and problem with strength, and our frame took quite a beating during aim high this year