Mecanum or 6WD

[Charmander]

Siri: Then would you be in favor of Mecanum? The 360 directional maneuverability of the drive system will be "pwnage" when picking up tubes and placing them on pegs?

[Chris is me]

Originally Posted by Joe Schornak

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.

Have you actually tried to hang tubes with a 6wd base before? It is a lot more forgiving than you are implying; there's not much need for precision.

[apalrd]

Originally Posted by pfreivald

...though the lack of a 5th CIM makes Patrick a very sad boy...

A CIM does 340 watts at peak power.

A Fisher-Price from this year does 290 watts at peak power.

There is no reason to liberate a CIM from the drive if your mechanism needs ~300 peak watts of power, since the FP can do it.

The BaneBots RS550 motors do 250 watts, and there are up to FOUR of them. I would never, ever go with less than four motors in the drive, especially because there are FIVE motors that do more than 250+ watts available for mechanisms. So even if you used just TWO you would still have a lot more power than a CIM (or you could use all four and have a ton of power, aprox. 1 kw.)


If you want your mechanism to be fast, make it light. Lighter = less driving power at the same speed, and with the same driving power, you can be faster. We set our goal speed, from ground to top row, to be well under 2 seconds. We know we can do it without removing CIMs from our drivetrain.


Someone mentioned the Fat CIM's in 07. One RS550 motor has about the same output power (in watts) as a Fat CIM. The Fat CIM just provides its power at a higher torque and lower speed.

@Josh Goodman:
Having 4 CIMs in your drivetrain gives you much better acceleration then 2 CIMs to the same speed.


On the thread topic, I prefer a 6wd hands down over a mecanum. I have driven 6wd, 8wd, and our swerve. I prefer the skid-steer 6 wheel and 8 wheel drive handling over the swerve. Why? The control is really simple. It does exactly what I want it to do, and with practice I can do the tasks you assume a 6wd can't easily do much faster than you. (I also had some fun driving a ~40lb test chassis at about 9 ft/sec, and pushing Jim Zondag across the carpet with it)


Final though (sorry about the long post):
To all of you who say a 6wd can't line up:
http://thebluealliance.net/tbatv/match/2007new_qf1m1
Watch starting at about 00:50 or so. We score through another team while they are trying to block us and score at the same time.

[Siri]

Originally Posted by Charmander

Siri: Then would you be in favor of Mecanum? The 360 directional maneuverability of the drive system will be "pwnage" when picking up tubes and placing them on pegs?

I'm not sure I'd jump on that bandwagon immediately. As Chris said, 6wd is actually reasonably forgiving, and a lot of the precision pickup issues will likely be arm-related for most teams (especially if 2007 is any indication). Moreover, if I'm fighting someone to pickup a tube, I think I'd rather have traction than strafing. Especially if your robot has a definite "front" (ie where the pickup operates), you'll still have to turn your mecanum much like a 6wd anyway.

Consider simulating it in full. With "HumanBots" or even kitbots or VEX/etc if you have them. You may be surprised exactly what actually goes into placing a tube. (My team's leaning towards pivot drive again, so I honestly haven't put much thought into the traction-strafing trade-offs myself.)

apalrd: I'm with you. There's a reason so many tank drives end up in Championship and IRI elims, and it's not just because they're so common. (The teams that end up there tend to have thought real long and smart about drive choices.) That's certainly not to rule out mecanum or say everyone must go with the flow. Do understand though that, especially for young teams, there are a lot of things about game play you just haven't experienced yet. Make your own decisions, but try to learn from others as well.

[James Tonthat]

Originally Posted by Steve_Alaniz

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

I think the argument is a 4 CIM 6WD versus a 4 CIM mecanum drive. The 4 CIM wins every time. I don't think I've ever seen anyone build a 6WD with less than 4 CIMs (on a winning robot and occasionally I see 2 CIMs and 2 FP) last time I saw 2 CIMs on a 6WD was 254 in 2006 but they had shifters.

[Steve_Alaniz]

Originally Posted by Siri

T

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.

)

I was mostly on the theoretical plane. Still, not everyone uses the Andy Marks wheels. It IS possible to make your own Urethane Mechanums and come closer to the traction belt co-efficients. For that matter, replace the rollers on the AM mechanums with a urethane roller... but I'm too lazy. Even so, the mechanums are usually not intended for a pushing battle. I think it was mentioned earlier that the ability to escape and maneuver are more in line with the mechanum strategy. Wheels have been around since Roman times, Mechanums are relatively recent. I have a plan for a different mechanum but I think I'll skip on re-inventing the wheel. I just need an escape plan from the people who will tell me I'm crazy for building it.... I also have a plan for locking the wheels on a mechanum but have never bothered to build it. No one can have their cake and eat it too.

Such is life and then you die.

Steve

[Steve_Alaniz]

Originally Posted by James Tonthat

I think the argument is a 4 CIM 6WD versus a 4 CIM mecanum drive. The 4 CIM wins every time. I don't think I've ever seen anyone build a 6WD with less than 4 CIMs (on a winning robot and occasionally I see 2 CIMs and 2 FP) last time I saw 2 CIMs on a 6WD was 254 in 2006 but they had shifters.

OK I just never saw anyone specify. As with your experience, I have seen two motor 6WD and I think there needs to be clarity on that point.

Steve

[PatrickS]

From what I have seen with the 2007 challenge, 6WD really doesn't suffer from as large of a disadvantage as some people state. As said above you have a large margin of error when placing the tube just because of the width of the rack. Also, most of the maneuverability issues can be minimized with a well designed arm and a proficient driver.

[ratdude747]

my vote is for mecanum. here's why:

the pushing arugement is valid but overplayed. you can push but mecanum can just slide sideways out of the way.

mecanum would make hanging tubes so much better as all you have to do is get to the grid and slide into position.

lining up for minibot release is another spot where mecanum would pwn. getting lined up is critical and mecanum would allow one to back up and slide into position. with 6wd, one would have to either get it right the first time or pull forward and try again.

not to mention the tube loading areas are narrow. once again, being able to slide makes lining up w/o getting stuck a lot easier and quicker.

anybody who has parallel parked will know how being able to slide would be helpful...

i see it as this:

if you want to be a defensive dozer go ahead and use 6wd

if you want to score and do it well, mecanum or crab (if you know what you are doing).

my $0.02

[1086VEX]

our team talked about this for about 2 hours on Saturday. both are great choices for this game. as a former driver, including '07, i preferred meccanums. i like the overall driving capabilities along with the option for precise strafing movements when going to hang a tube. it doesn't fair too well with defensive play but it can hold its own.
6WD is also great with the traction and pushing power it delivers. it does lack the unique abilities for precise movement that meccanums have. but seeing that a huge number of successful teams used 6WD in '07 i'm sure they'll work great as well.

[Chris is me]

Originally Posted by ratdude747

if you want to be a defensive dozer go ahead and use 6wd

if you want to score and do it well, mecanum or crab (if you know what you are doing).

my $0.02

Mecanum has never been vital for FRC. What makes this year different?

Honest question.

[Paul Copioli]

All,

All of the drive systems mentioned have been successfully implemented by a wide range of FRC teams, but the only thing that matters is what your team can implement given all of your specific team constraints and talents.

We had a pretty decent debate between traditional swerve (think 111 or 71), mecanum, Nonadrive (148 & 217), Octocanum (I know someone will do it this year), and 6WD skid. To understand our final decision, here is some history of team 217:

2000 - 2WD using 2 drill motors no shifting
2001 - Started as 6WD using drill motors ended up 4WD no shifting
2002 - 4WD swerve with a CVT we called the Crazy Chicken Transmission using 2 CIMS, 2 drill motors, and 2 FP motors.
2003 - 4WD Forklift drive (2 rear wheels were swerve) pneumatic shifter with 2 CIMS and 2 drill motors
2004 - 6WD skid steer with shifting using 2 CIMs and 2 Drill motors
2005 - 6WD skid steer no shifting using 4 CIMs* and 2 FP.
*NOTE: 2005 was the first year of the kitbot as we know it and the first year of 4 CIMs in the kit. The drill motors were retired from the kit.
2006 - 6WD skid no shifting using 4 CIM motors
2007 - 6WD skid no shifting using 4 CIM motors
2008 - 6WD skid using 4 CIM motors with shifting
2009 - 6WD skid using 2 CIM motors no shifting
2010 - Nonadrive using 5 CIM motors no shifting

This year we have decided to use 6WD skid once again. Given all of our constraints we want to put our innovation into other parts of the robot(s). I can see arguments for the other drive systems, but for us we are going with 6WD this year.

Paul

[IndySam]

6WD, mecanum, swerve, and whatever can all be viable winning drives for this game.

The best drive will be the one that your team can build in time to give your programmers time to program and your drivers plenty of time to practice.

You could build the slickest unobtainium drive in the world but if your drivers can't practice with it you won't be a winner.

Like JVN said, "KNOW THYSELF!"

[pfreivald]

Originally Posted by IndySam

6WD, mecanum, swerve, and whatever can all be viable winning drives for this game.

The best drive will be the one that your team can build in time to give your programmers time to program and your drivers plenty of time to practice.

You could build the slickest unobtainium drive in the world but if your drivers can't practice with it you won't be a winner.

Like JVN said, "KNOW THYSELF!"

That's extremely true, and it's why we're leaning toward 6WD or mecanum.

If our main driver hadn't graduated last year, we'd do mecanum, no contest -- he was an absolute wizard with that thing!

...but now I'm not so sure. This will take some serious team discussions, and as usual the final decision will be up to the team, not the mentors.

[Ether]

Originally Posted by Steve_Alaniz

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.

The above statement is not correct.

http://www.chiefdelphi.com/media/papers/download/2739

In the forward direction for example, mecanum is the equivalent of 4 motors driving 4 wheels with the rollers locked, if the vehicle is not traction-limited.

If the vehicle is traction-limited, then mecanum is 41% more force than two motors driving two wheels with the rollers locked.