i’ve also heard 1/8"
Here are some pics of the drive system of the team 25 recent six wheeled modular monsters. The whole robot pic is the 2002 robot Silver Scorpion- that made it to the final 4 in Nats. The modular drive pic is from Evil Machine 1- the 2003 finalist. As we’ve learned we have made the drives modular so they can be detached as a unit and replaced quickly. The kids strip them down and reassemble them like a soldier does with a rifle. The drives have never failed- never. They are now obsolete and the next generation will be a little faster and a little more powerful, at least according to prototypes. We have some new ideas up our sleeves. ; )
WC
John-
Although it seems to make sense and would make design interesting we have never moved any of the center wheels from our machines out of line from the rest. We want the deep grab traction for the motors and pushing power. And I think any who play us will agree that the machines are speedy and maneuverable. We use two joystick tank drive on the machines and they can spin in place with no problem at all.
Perhaps the answer to this is the fact that we have such wide wheels (custom made from Skyway wheels) and a gearbox with great power. We are probably horsing the machine around so much when we turn that the coefficient of friction on the center wheel is negligible. We usually give the machines the “Big Mike” test- if they can shove him away they are strong enough. If we lowered the center wheel we negate the traction we are looking for back to 4 wheel drive.
WC
we dropped our middle wheels just .1", and it worked fine for us.
I’ve always imagined and wanted the opportunity to design and build, for anyone who’s interesested (hint, hint, nudge, nudge), a six-wheel drivetrain that keeps them all at a single height, but uses omniwheels on the four corners. It seems like that’s a simple solution for enhancing traction in the robot’s direction of travel while minimizing additional friction in turning.
I’d be surprised if another team has not yet tried this method, though nothing springs to mind.
We lowered ours 3mm (close to 1/8") and we could spin like a top.
To the point of traction: if all the motors are geared together per side and then run to the wheels, the lowered center wheel does not reduce your tractive effort (pushing ability). A six wheel drive concept is like using powered castors. The design is such that the center wheels take most of the load and the outside wheels balance load when the CG is moved on either side of center. If the drive is designed correctly, you pushing ability is not reduced as long as all the items touching the floor are powered.
Team 40 used that method this year.
I seem to remember them turning pretty well.
Now for some editorializing:
In order to push competitively in this competition, one needs as much tractive force available as possible.
Tractive Force = Wheel Coeff. x Normal Force
Normal Force is limited to the weight of the robot (and whatever the robot is able to pick up. In order to maximize Tractive Force, we want our wheels/treads/whatever to have the MAXIMUM coefficient of friction possible. (Many teams use high traction belting, or pneumatic tires).
Here is the kicker – I have NEVER seen an omni wheel that can compete with straight up, high traction wheels when it comes to coefficient of friction. So… if you use omni-wheels on your robot, you are “wasting” normal force. You have weight sitting on “low” traction wheels, and have decreased your tractive force.
The GREAT thing about the 6WD configuration, is that it allows the robot to turn like crazy, but ALL the robot’s normal force is sitting on driven, high traction wheels. It’s the best of both worlds! Turning + Pushing!
Note: Casters pose the same problem. If you have weight resting on “non-driven wheels” you’re shooting yourself in the foot again, when it comes to pushing matches.
For more information about WHY the 6WD, lowered middle wheel configuration turns better… check out Chris Hibner’s turning whitepaper. He’s got GOLD in there: http://www.chiefdelphi.com/forums/papers.php?s=&action=single&paperid=222
It’s physics,
John
Wayne,
First off, I’d like to say I’m a huge fan of your team’s machines. You guys are great.
The amazing turning ability of your machines is DEFINITELY linked to your “wide” wheel configuration. It’s all physics (you can see it in Chris Hibner’s whitepaper, I linked in my previous post).
I disagree with you on one point:
I would argue that your 6WD configuration isn’t helping you get more traction. For the most part, surface area isn’t a factor in tractive force. (F = mu x N) <– No “A”
For our applications (rubber on carpet) these are NOT perfect surfaces, and cannot be expressed perfectly with the above equation. They are intertwining meshes. This would help explain getting “more” traction using 6WD.
However, I would argue that you’re not getting very much more.
Is there any chance you’d be willing to help with a little experiment?
It is for the mutual sharing of knowledge to benefit the entire communitty. 
Wayne, do a straight up pull test with one of your robots. Record some values for it’s maximum linear force. THEN, have one of your kids remove the 2 middle wheels from your drive modules (put them on top of the robot somewhere, so the normal force doesn’t change). Now try the pull test again.
I hypothesize that your second test will be within 10 lbs of the first test (if not closer).
If you participate in this little experiment, I know I’d be (along with many others) very interested in the results.
Thanks,
John
PS - Your gearbox isn’t any more over-powered than the rest of ours! 2 Drills + 2 Chips = 1552W = 2.08 Hp. How you guys manage to gear it so fast, yet still push so hard is beyond me. Up here on 229, we’re genuinely convinced you guys are defying physics somehow. Are you sure you don’t have 2 extra drills tucked inside that thing somewhere?
Running into something at 15-18 fps probably helps explain that to some degree :ahh:
Momentum is one explanation but…
I watched them (From a standstill) win pushing matches against teams they shouldn’t have! It was incredible…
John, on a purely non-scientific note, one needs to watch very closely to see how 25 gets that “little extra” in a pushing match. In the past two years, for part of (nearly) every pushing match they are in they are actually not moving at all. Once they are in a desired position they stand still with almost no exertion on the motors or battery at all. As to how they get that done, you’ll have to ask 25, but I do know, that at the end of a match they generally have happier and cooler drive motors than most opponents not to mention more left in the battery.
against who or what matches? so i can see it on soap (Very interested in this machine)
New 6wd Question : IF the Robot Keeps Teterring on The Center Wheels then doesn’t the Center Of Turning Keep Changing From a Point in the Middle of the 4 wheels Touching the ground, then another center when the other two wheels make contact and the previous two rise off the ground?
2nd New 6wd Question: i Noticed Several teams who executed a 6wd, didn’t use all 6 wheels of one type (8’ 12’ 6’ 3’ there were all over the place, 233 comes to mind) How does this effect the 6wd?
3rd New 6wd Question: Also Noticed like in the previous Question that some teams went out of there way to have a lower traction wheel in the middle, why and how important is it?
-Osc-
i Like the 2003 25 Bot Alot more , flew an like eagle and look cool too (Threatening)
yeah we have 4 drill and 4 chips, can’t believe it took someone so long to figure that out on CD. just kidding…
Wanna know a big secret? Yes, we are powerful and all that, but in a pushing match, we use BRAKES(used very less this year, but in last years game it was the thing). I am just starting to learn more about drive trains, torque etc etc from you guys, so maybe I can figure out how on earth we’re so fast and powerful at the same time. I think I will ask one of our tradesmen…
-Bharat
John- you’re probably right on the physics of it all. One thing that I do notice is that the rubber of the tires is quite soft and wears quickly- possibly just enough to naturally create the surface needed. In any case it works. And when the wheels get some pressure pushing back on them they probably flatten a little and grab better. I’m not sure I want to test it however although all this discussion gave me an idea about recycling some of the old tires we have layiing in the shop.
As for experiments- it IS more fun to just have the machine throw Big Mike around…
WC
Of course now every robot in the world is going to try to shove us around- thanks Rich !!!
As Bharat posted- we use brakes. At the flip of a switch we can lock up the entire drive train. It is very simple. Of course knowing when to use the brake is the key and that’s what the drive team works out. Since we have six, wide wheel surfaces in contact with the ground I believe we get a little edge on robots with normal wheels.
And no, we dont defy the laws of physics but we do have a genius tech guy on the team named Mike Lubniewski who devotes a lot of time to thinking about improving the drives each year. (hence Bharat’s avatar)
WC
And its probably the only thing that can throw him around…
As for the wheels, the custom made Skyway wheels with diamond cuts on them makes them stick to the carpet like crazy(traction). Then, Straight-cut spur gears along with needle and ball bearings ensure all the motor torque gets to the wheels with minimal parasitic loss…
Thats the explanation I got from our tradesmen.
My team is thinking about 6 wheels. Has anyone experienced any problems with it? Does it go straight when its supposed to? Is turning smooth? Those are our main concerns right now. Thanks.
-gavin-
6-wheel drive is probably the best way to go. It will drive straight if your chains and everything is tensioned properly. The only problem with driving straight is in the gearbox of the Drill motors. But thats only in the gearbox and is easily fixable. For turning the best to do that is to lower the middle set of wheels so that when you turning you are basically turning on those two set of wheels. We haven’t had any sort of problem with our 6-wheel drive systems yet.
if you’re talking about the directional bias that the drills have (i.e., they spin slower in reverse), its actually built into the drill motors themselves.
I thought that was already true … oops 