Re: Mecanum or 6WD
 

A. Yes, like that.

B. I haven't build a Mechanum drive before, so humor me.

I was thinking you'd get the strafing ability as well as extra power and speed from six wheels.

Re: Mecanum or 6WD
 

Is there any method way to increase traction of a mecanum drive system?

Re: Mecanum or 6WD
 

Last year i think team 190 had a method in which they locked the rollers of their mecanum wheels effectively turning them into regular wheels. Can anyone from team 190 double check me on this?

I have also seen some examples of rovers which had the ability to switch from mecanum to regular wheels. They looked like an 8 wheeled robot that could lift its outer wheels.

Re: Mecanum or 6WD
 

My team will be using mecanum because of the lateral-motion factor, which will be critical in maneuvering to hang a logo piece, as detailed below.

Process of acquiring and scoring a tube:
6WD:
1. Get tube from feeder.
2. Zip across field to scoring zone, using superior speed and traction to avoid other robots.
3. Advance to hang tube. Miss.
4. Back up, adjust angle, try again. Miss.
5. Repeat. Miss. Waste 20+ seconds on a single tube before finally succeeding.

Mecanum:
1. Get tube from feeder.
2. Drive across field to scoring zone, strafing around opponents.
3. Advance to hang tube. Miss.
4. Strafe to quickly align tube. Succeed.
5. Repeat 1-4. Achieve massive victory.

Realigning a 6WD robot to hang a tube involves backing off less than 7 feet (so as to avoid opponents), turning a bit, driving forward a bit, turning back a bit, and then finally reattempting the hang. Whatever advantage you had in zooming across the field has been lost in your inability to quickly and accurately hang a piece.

While 6WD will be good at defense, mecanum, omni, and swerve will rule at offense.

Re: Mecanum or 6WD
 

If one can make a robot orientation independant hanging device, then there is no reason for mechanum. If not, then there is a compelling one, atleast for robots who take on a main scoring position of the team.

Re: Mecanum or 6WD
 

Exactly! I was replying to someone who stated that only 4 cims can be used on the robot and only the four no other motors.

Re: Mecanum or 6WD
 

I think good driving will rule at offense. Mecanum will certainly have advantages, but for a well-driven 6WD robot there will likely not be much backing off, turning, advancing, backing off...any more than mecanum will be strafe left, strafe back to the right, back to the left,... It will be more like get into approximate position and pivot left or right in place. We saw this with more than a few robots in 2007. In fact we saw a number of robots better at approaching the rack from other than a 90 degree angle.

Re: Mecanum or 6WD ...we're leaving something out
 

In the Andy Mark type you can tighten the roller bolts to put friction on the roller so you can have a bit more "traction" but you'll sacrifice ...probably a bit of speed in other directions. I also depends on the co-efficient of friction of the rollers.


OKAY... A PROBLEM WITH THIS DISCUSSION. Mechanum are NOT that easy to push around. If you mean a 6WD trying to push a mechanum bot.... it's a TIE... DEPENDING ON... something people have not mentioned. Do you mean a 2 motor 6WD or a 4 motor 6WD... A European Swallow or an African Swallow?
Mechanums are all about understanding the vectors of the wheels and without needing to put numbers in... in any given direction, mechanums are the equivalent of the torque of TWO motors driving TWO wheels with the rollers locked. (Remember all mechanum drives are 4 motors unless you have a REALLY elaborate nightmarish power distribution system!)
When you push a regular robot sideways, you are trying to overcome the sideways wheel contact with the floor so co-efficient of friction is the major factor. Anytime you contact a mechanum, you are contacting it "head on". The driver can elect to drive into you and push back until the wheels slip... co-efficient of friction again. I think the real issue is the number of motors you have to burn up with mechanums. You need 4 for a mechanum while you can have a nice 6wd for only 2.

In the end, either is a good choice.


Steve

Re: Mecanum or 6WD
 

True, but robots are just over 3' long (at most), so they've got essentially gobs of room to back up to readjust. And a good driver (potentially with tracking software help) will get "close enough" most of the time, considering the 9.5" - 12" opening of the tubes and the 2-3/4" size of the retention plate on the pegs.

Team 811 is just starting the 6WD vs. Mechanum (vs. Holonomic) debate; it's interesting to watch, considering we've never built either :)

Re: Mecanum or 6WD
 

Do any of you predict that there will be tubes on the ground? It seems unlikely because teams are not able to take tubes from opposing team robots. If each robot gets control of tubes from the feeding sites, virtually no tubes will be "up for grabs". This is a huge factor in deciding whether to go Mecanum or 6WD because if there is a lot of travel back and forth from feeders to pegs, 6WD would be preferred. However, if there are tubes in the middle of the field, i feel that mecanum will be more usefull since they are able to pick up tubes and travel a shorter distance to the pegs.

Re: Mecanum or 6WD
 

Our team just finished discussing this problem today. We went with 6WD. Why?

Motor Consumption.

There was talk about Maneuverability and Torque and Strafing and Speed. But here's what it came down to:


Now, speed is irrelevant because whether you have two CIMs or 4 CIMs running your drivetrain, you will go the same speed. Obviously.

Maneuverability IS what you are losing here. But consider this. You can teach a driver how to be more accurate, with more practice comes greater accuracy. No matter how much the robot practices, it will not get faster. Plus, the built "Safe Zones" in this game are so frequent it's like you're playing match mode on "Easy". If you can get into the safe zone...does it matter? Also, when you're positioning a tube in close range, does it really matter with no defense which is faster?

Torque really doesn't matter much this year (IMO, and for our strategy), so more motors aren't a necessity.

Now teams operate in different ways so there really isn't one answer for this. Our team has never done a mechanum drive before. As easy as it may be, we can build a 6WD with our eyes shut. Also, this year's game is screaming "Mechanum/Crab/Swerve". Might it be easier to outmaneuver a sea of multi-directional drivetrains with something you're more familiar with? In 2007 we had a problem with our arm motor and had to use the big CIM (thank god it was available) to compensate. I think you really need to prioritize what you REALLY want to accomplish in this game and go for that first.

There are VERY few teams that can play "God-Bot"

Re: Mecanum or 6WD
 

What is that compelling reason? I'm not seeing it.

Re: Mecanum or 6WD
 

Absolutely. Robots being jarred, or failing to score UBERTUBES, etc will result in tubes on the ground.

That said, doesn't a holonomic or swerve drive system incorporate all of the advantages of mecanum with all of the advantages of 6WD (strafe capability with no loss of pushing torque or speed)?

We used mecanum last year, and it was key to our success, hands-down. We're leaning toward some kind of modified holonomic/swerve this year, though the lack of a 5th CIM makes Patrick a very sad boy...

Re: Mecanum or 6WD
 

Swerve drive and crab drive are both, IMO, the TOUGHEST tasks to try to conquer during a build season. If you want to use them on your competition robot, you should first spend an offseason building one and learning how to keep it running. Crab and Swerve do combine qualites of Mecanum and 6wd: Mecanum's mobility but with the 6wd ability to push back.

Re: Mecanum or 6WD
 

That's interesting reasoning. How did you come up with this assessment? We haven't been able to test it with old bots yet, but by contrast tank drive was the norm for effective hangers in '07. And that was with a free-swinging rack with much larger end caps. The flat scoring grid does highlight lateral motion somewhat, but I wouldn't think it was that much at first estimate. In both cases, a lot of its down to drivers and sensors. Best case: do not let go unless you know it'll hang.

Steve: Good point; motors and frictional coefficients rule the game here. But correct me if I'm wrong, even if 6wd and mecanum tie the motor battle (i.e. a 2 motor 6wd), mecanum still loses on the coefficient front.
- AM Mecanum Wheel values (static forward/reverse, sideways): 0.6 and 0.5 (6"), 0.7 and 0.6 (8"), 0.54 and 0.41 (10").
- AM Traction Belt values: 1.2 (wedgetop), 1.3 (roughtop)
Even dropping the 6wd center, you're still looking at double or more the mu's.
Purely anecdotal: I've never seen a mecanum win a pushing war against a reasonable tank drive, 6 wheel or otherwise. In fact, in 4 years behind the alliance station glass, I can't think of (m)any that tied either.

Charmander: Expect tubes on the ground. There is 1 very important rule in the world of FIRST game pieces: no matter what they are, no matter where they start, no matter where they're going, significant numbers will end up pushed against the wall on the floor. This year, I'd predict lot of that happening from robots dropping and human players throwing. No all-star team is going to show up at competition without very well-exercised human players. Why go all the way to the store when you can get them air-mailed? ;)