Mecanum or 6WD
 

So my team has had a big debate between Mecanum and 6WD. 6WD is ideal for transporting tubes from one side of the field to the other because of the potential speed, while Mecanum is ideal for placing tubes on the scoring grid due to the lateral movement. We are allowed to "throw" the tubes to the center. This ability supports Mecanum because it shortens the distance of travel. However, this will cause an increase in robot to robot contact to which mecanum wheels are not well equipped for due to low traction. We have heard a lot of hype about the 6WD on the forums.

Questions:

6WD or Mecanum? Which do you prefer and why?

What is the turn radius of 6WD?

Do you predict the game to be high contact or not? Why?

Re: Mecanum or 6WD
 

I think Mecanum seeing as it brought success for us last year, the turn radius can't be that good for 6WD not to mention the extra power expended in turning those two extra wheels. Then again Mecanum requires four motors. I predict we will see robots shatter. I liked that at the kickoff.

Re: Mecanum or 6WD
 

6WD - higher traction, less weight, simplicity, comes in the KoP!

Turning radius - of a properly designed 6WD should be zero, it should turn around the center point of the robot the weight should be relatively centered on the robot.

Yes - after seeing 2007

Edit:

Simplicity because of less gearboxes, another plus is less weight because of that same reason.

Turning radius - see above

Extra power - this is a misnomer. The same amount of power would be going to the ground, in fact in mecanum, there is wasted power because the force vectors work to cancel another wheel's. A 6WD should also require 4 CIMs.

Re: Mecanum or 6WD
 

The rules state that four motors is max just fyi.

Re: Mecanum or 6WD
 

It would be helpful if you could explain some of your assumptions:

Why do you assume Mecanum slower? Atleast, it isn't slower in forward and reverse.

Also, why would "throwing tubes" to the center give the advantage to mecanum? Assuming that you would go pick them up again, which is what I think you mean, how is it quicker?

To answer your question, a dropped center 6wd can turn with 0 turn radius.

To those above, can you cite a rule that says you can only use four motors?

Re: Mecanum or 6WD
 

Mecanum has less traction, which will be key in this game. A good defender should be able to lock down the scoring zones--unless there are two robots attacking at once.

What would you say is a properly designed 6WD? With the middle wheel lower than the others in West Coast style?

Re: Mecanum or 6WD
 

"throwing tubes to the center" is an advantage to mecanum because it is a shorter distance from where the tubes are picked up to where the grid is. Thus, more time is spent placing tubes on pegs and less time is spent traveling. The forums have stated again and again that 6WD is faster in a forward, reverse direction than mecanum.

Re: Mecanum or 6WD
 

This does not waste any power.

The only wasted power in a mecanum is due to the friction in the bearings in the rollers.

Re: Mecanum or 6WD
 

I think he means in the sense that part of the force "cancels out" as it's applied in a direction the robot is not travelling in.

Re: Mecanum or 6WD
 

You can have a max of 4 cims plus the motors included in the kop.


I would go with 6wd over mecanum in a heartbeat. Mecanums as mentioned have no pushing power and you won't be driving as fast around the field either. One problem with throwing the tubes to the center is you are increasing the chance for your opponents to grab it especially if you are the weaker drive. Yes there is some advantages with the lateral movement in front of the rack, but I'd take speed and pushing power over that.

A properly built 6wd and 8wd will have a turn radius of zero and waste no power.

Re: Mecanum or 6WD
 

A little definition: West coast drive means that the wheels are cantilevered (unsupported on one end). 6WD is usually implied that the middle wheel is lowered (usually between 1/8" and 3/16"). A way around this is to have equal diameter omni wheels on the outer wheels.

You can use 4 CIMs <R45c> plus the rest of the motors in the kit of parts <R45a>, there's a special section about the RS motors (you can use up to four in any combo, even duplicates) <R45d)

My bad, you're right.

Re: Mecanum or 6WD
 

If i were on the other team, I would be stealing those tubes that are "in the open".

As per speed, in the forward direction, mecanums, (for the most part) act as any other wheel with lower traction would. Therefore, if it were geared the same way and the wheels are the same size, and your wheels do not slip on acceleration, a 6wd, 8wd and mecanum drive will be the same speed forwards and back.

Re: Mecanum or 6WD
 

6wd is the best because, not only is it faster and has more power, it can even be considered more manuverable than a mecanum. We tested this with our previous robots it takes 4 seconds with 6wd and 11 seconds with mecanum.

Re: Mecanum or 6WD
 

to do what?

Re: Mecanum or 6WD
 

As per the posts so far....

Speed: Equal
Maneuverability: Mecanum
Traction: 6WD
Difficulty in Design: Mecanum
Power Consumption: equal

Is that it? Do mecanum and 6WD have different acceleration rates?

Re: Mecanum or 6WD
 

I disagree with difficulty in design. The programming, as well as driver training is a much greater requirement for a good mecanum system (I have built an omnidirectional system before, and I say this even considering the availability of example code). The maneouvrability is also a lot tighter than you would think (strafing is not as much of an advantage as most people think). In the end, with a well designed system, it is driver skill, either way.

If I recall, 80%+ of championship Einstein robots have been tank drive over the years.

Re: Mecanum or 6WD
 

First: I think he was saying difficulty in design is greater for mecanums.

Second, I think that when the safe zone is only seven feet long (minus the 16" pegs for <6 feet), strafing could actually be really useful, especially since an effective defender will just body block in front of where they know you're going to put the tube. With a mecanum you can sort of get in from anywhere and sidle over, with tank that takes quite a bit longer.

Third, I agree that while each system is stronger than the other in certain respects neither is inherently superior.

Re: Mecanum or 6WD
 

6WD for 2 reasons: First is pushing power, second is "we've never done Mecanum before".

Re: Mecanum or 6WD
 

The rules say:

Re: Mecanum or 6WD
 

More reason to try it. This will be our team's 7th FRC season and we have done 6wd for each year, but we are heavily debationg 6wd vs. Mecanum.

Re: Mecanum or 6WD
 

It's a pretty board statement, but from my experience I'd have to say 6WD because a macanum bot will get pushed around easily.

Re: Mecanum or 6WD
 

This drive system team 2404 used is interesting... it has many of the characteristics of the mecanum drive system.

http://www.youtube.com/watch?v=pZf1-...eature=related

Re: Mecanum or 6WD
 

This drive system 2404 is testing is interesting. It has many of the pro's that mecanum wheels have.

http://www.youtube.com/watch?v=JqJK0E9Jpmk

Re: Mecanum or 6WD
 

I'm going to disagree with teams that go "mecanum = agility". Every year I see so many teams who say "oh we should be agile so we need a mecanum drive" and it drives me nuts.

First, agile is a very vague term. Mecanum has a specific kind of motion it can do that 6wd can't. The real question is do you need that kind of motion?

Second, it is very easy to underrate the "agility" of a 6wd system. It can turn easily on a dime and drive very well in that fashion. The only thing it can't do is strafe.

Essentially, the trade off is strafing sideways or resistance to pushing / pushing power. Which is more important?

Re: Mecanum or 6WD
 

What part of the programming of mecanum did you struggle with?

Re: Mecanum or 6WD
 

We have done 6WD and mecanum, both have advantages. I do think that given the depth of the scoring zone, a mecanum drive could be a big advantage. Furthermore, while mecanum drive robots are easier to push around, I have found them to be more difficult to block. The ability to always be able to move directly away from a blocker is very nice. It makes it much more difficult to continuously block the robot.

All that said, the best drive train we have produced was probably for Rack 'N' Roll, a 6WD with omni wheels on the "front" (read: the end where the tube is dangling) and the other four wheels (aft and a little aft of center) were traction wheels. That year we geared for torque over speed, which served us pretty well. The base of our arm gave the back end a lot of traction, but the whole thing spun really close to in place.

Side note: If you choose to do a one speed mecanum system you have to give yourself enough torque. We had quite a few problems in Overdrive with motors overheating because we didn't have enough torque when strafing.

Re: Mecanum or 6WD
 

I see the same exact thing! It makes me confused. I would not consider mecanum more maneuverable also. The only trade-off is pushing power to strafing.

Well stated.

Re: Mecanum or 6WD
 

How about Mechanum6? It could be like this:

\\\ ||| ///

\\\ ||| ///

So that there are six wheels, where the front and back pairs are Mechanum, and the center pair is straight Omnis. It seems like it could be possible, so it may not work.

Just an idea.


Thaine

Re: Mecanum or 6WD
 

like this?

I assume it would allow you enough traction to effectively push other mecanum bots, but thats about it.

Re: Mecanum or 6WD
 

Maybe they needed the omni's to assist rotation since the mecanums are mounted incorrectly.

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? ;)

Re: Mecanum or 6WD
 

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?

Re: Mecanum or 6WD
 

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.

Re: Mecanum or 6WD
 

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.

Re: Mecanum or 6WD
 

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.

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

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.

Re: Mecanum or 6WD
 

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

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

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

Re: Mecanum or 6WD
 

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.

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

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

Re: Mecanum or 6WD
 

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.

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

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

Honest question.

Re: Mecanum or 6WD
 

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

Re: Mecanum or 6WD
 

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!"

Re: Mecanum or 6WD
 

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.

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

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.

Re: Mecanum or 6WD
 

It's not obvious. If you have 4 CIMs, you can gear it down less and go much faster.

Re: Mecanum or 6WD
 

From a builder's standpoint, 6WD can be built from the KOP, but macanum is also very easy to build as the mounts mimic that of a 4WD.

From a driver's standpoint, I would have to say that both drivetrains present an equal amount of challenge to drive. The most important thing to remember is to tailor the controls to your liking and don't be afraid to practice with multiple layouts.

From a programming standpoint, 6WD is as simple as it gets. While mecanum can be challenging at first, there are many example programs available that basically brings the need to understand the physics behind mecanum down significantly.

From a strategist's standpoint, I feel traversing the feild quickly will benifit your team much more than being able to strafe in the scoring zone. No one ever said that the tubes needed to be perpendicular with the racks before they are placed. Placing on a slight angle should not be a big deal for a good arm. Also, simply rotating to place may be faster than strafing to place in some situations.

Our team used mecanum last year and I definately felt like it was a strong drivetrain, but seriously lacked in the area of bullying robots. Since i feel like crossing the feild this year is going to be rigged with other robots getting in your way, I feel speed and power are going to be an advantage over omnidirectional movement.

6WD for us this year.

Re: Mecanum or 6WD
 

Is anybody else anticipating robots getting stuck and/or having a hard time in the lanes (including leaving the lanes around the towers)?

If you have a defense bot preventing you from zipping right out of the lane and past the tower, you want to be able to go around the tower as fast as possible to get by that robot -- or be able to just manhandle it out of the way.

I'm seeing defensive bottlenecks as very possible in this area of the field, but it still doesn't inform the decision between 6WD or mecanum, because each has a 'solution' to the problem.

Re: Mecanum or 6WD
 

In this context, what is octocanum? Four mecanum wheels along with four other wheels?

Re: Mecanum or 6WD
 

I'm pretty sure Octocanum is a Nonadrive-style setup using mecanum wheels instead of omni wheels.

Re: Mecanum or 6WD
 

It sounds like 8 omnis since some team did a 6 omni drive last year.

Re: Mecanum or 6WD
 

Yeah, I was thinking this too. One strategy I was thinking about is the human player popping a tube through the feeder so it lands by the safe zone line (still within the boundaries). This would allow for the robot not going all the way in the safezone, therefore harder to get blocked (Can back up or pull out around the tower) and the opposition alliance still can't get the tube.

Just one thought.

Re: Mecanum or 6WD
 

Main reason (as of day 2 of the build) for not doing a Mecanum drive, the work best when each wheel has an equal force on it (weight) the fact that we will be moving arms (or something) around to score will be shifting that weight, admittedly that will be in a "safe" zone. But while on the field these shifts can lead to chaotic motions from a Mecanum drive.

But the drive isn't *completely* off the list yet , but 6 wheel is easy, light, inexpensive, easy to program and drive, predictable, robust, able to run while damaged and we tend to have better years when we use 6WD

Re: Mecanum or 6WD
 

Valid point. My use of the AM numbers wasn't supposed to be an end-all-be-all commentary on mecanum but rather, on the mecanum that teams asking this question on the Sunday after kickoff will more than likely use. Locking rollers are a beast in and of themselves, no doubt, but they're not a beast you first debate during build season. ;)

That said, I wasn't aiming to dismiss the trade-off either. It's a team choice. Personally, I've never seen the appeal of mecanum, but that's my team/drive team and the games. Simulate it, watch '07 videos*, work with your drive team and builders, weigh your options. Either may be right. What I find odd is the view that this is the year of mecanum simply because of the scoring challenges. I've never seen a game that necessitated a certain drivetrain, and certainly not mecanum. It may be true, but it's a heck of a statement make without testing, especially considering the results in 2007.

*PatrickS has got this down, by the way. Consider that you [general pronoun] may be over-stating the alignment necessary. I dropped many a misaligned tube on the rack in 2007 with no issue. Make some pegs and test it out.

Re: Mecanum or 6WD
 

Just to be clear: the force changes themselves do not affect the behavior of the mecanum, unless the force on a wheel (or wheels) becomes so small that the wheel(s) loses traction and begins to slip. Then you'll get the odd behavior mentioned.

In other words, as long as the wheels don't slip, the kinematics remain unchanged.

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

Not to start an argument but the white paper actually confirms what I was saying. In fact, it states in the body that "Therefore, assuming no roller bearing friction, the “pushing force” (and speed) of the vehicle is the same in the fore/aft and sideways directions." So I don't see how you got the 41% unless you are including friction from the bolt/axle but I would be happy to hear how you reached these conclusions.


Steve

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

If deciding on 6WD, what type of 6WD is recommended? Omni Wheels in the middle, all omni, Middle wheel lowered, etc. I am currently leaning toward all omni 6WD because of the increased maneuverability. Any thoughts?

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

Don't do it. Omniwheels in all 6 wheel locations will allow you to be pushed sideways with practically no resistance.

I'd look at 6 traction wheels with a dropped center, 4 traction wheels with 2 omniwheels on one end, or 2 traction wheels in the center with omniwheels on the four corners.

Re: Mecanum or 6WD
 

Madison,

Octocanum is nonadrive with mechanums replacing omni wheels and the kicker drive.

Paul

Re: Mecanum or 6WD
 

Got it. Thanks, Paul.

Your prediction that at least one team will build it will come true. :)

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

Not to worry. I welcome polite challenges.

The above statement is true, but it doesn't confirm what you said. You said:

The above statement is not true.

The 41% increase is a factor of 1.41, which is 1/cos(45), 45 being the angle of the roller.

To summarize:

In the forward direction, the pushing force of a (4 motor) mecanum vehicle is the same whether the rollers are locked or not, assuming that the vehicle is not traction-limited.

If the mecanum vehicle is traction-limited, then the pushing force is reduced by a factor of 1.41. However, compared to TWO locked-roller wheels (which is the comparison you made), this would be 1.41 times larger:

(F/1.41)*2 = F*1.41

If this still isn't clear, I don't mind giving it another shot. Tell me which part doesn't make sense.

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

1511 has had much success with four omni wheels in the corners and 2 high traction wheels in the center. With the traction wheels, it is close to impossible to push you sideways and moderately difficult to push when parallel with the wheel. With the 4 omni wheels though, it allows you to be able to turn on a dime. We've had a lot of success with both the maneuverability and ability to push around other teams with this system. I'm assuming this year we're still planning on using 2 instead of 4 CIM motors so I'll let you know how that works out. We still have a few calculations to do and if they aren't needed anywhere else, we'll slap 'em back on the DT. All the wheels are on the same plane/ center is in line with the omnis.

This system was able to climb ramps in 2006/7 and was one of the faster regional level (and in some cases Championship level) DTs.

Re: Mecanum or 6WD
 

I think the advantages of both types of drive train are getting a little overhyped.

What are you going to do with pushing power on offense, shove the opponent into your zone and let them run interference on you (because there aren't any rules that force them to leave)? If a 6WD meets a 6WD in the middle of the field, they crash head to head, they lose all forward momentum, and the one on offense has to back up, turn, try to go around, get hit again, and so on. If a 6WD meets a mecanum in the middle of the field, they crash head to head and then the mecanum goes around. And since pinning=death this year, teams will be more hesitant to push mecanum bots into walls.

On the other hand, strafing in the homezone isn't too terribly important either when you can just come from an angle to score. The lanes can be entered by well-driven bots of either type, though admittedly it's a tad easier with mecanum. Mecanums on defense are good at driving in front of 6WDs and then get shoved out of the way so fast it's not funny, and as such are not a good idea.

Assuming a field that's full of 6WDs (as regional tournaments, at least here in Hawaii, usually are), your 6WD has to be buff if you want to push people around and your mecanum has to be well driven if you want significant maneuverability gains.

Re: Mecanum or 6WD
 

My team feels that manuverability is better than speed in this game because there will be so many bots in the field, it will be nearly impossible to find a clear path to the other side. We also think that offense will be more important than defense in this so you can get a big enough point lead to hold your own before the endgame. Does anybody feel differently about this?

Re: Mecanum or 6WD
 

We are going with a 4WD Mecanum setup. We feel that the strafing is worth the lost in pushing power because if a defensive bot tries to push us away from our scoring rack we can drive around and get in our zone before the defensive bot can turn around

Re: Mecanum or 6WD
 

That's exactly what we think, but we also are thinking that a vacuum would be efficent in picking up and holding tubes.

Re: Mecanum or 6WD
 

The only way to find out is to prototype!

Never commit to a design you have no information on. Always prototype to see if it will work. This is especially true for "risky" ideas like a vacuum.

It should take you like 10 seconds to get a shop vac and test - let us know how it goes! :D

Re: Mecanum or 6WD
 

Or it could just back up. It has two directions of motion (its "front" and its "back"), and if aligns itself correctly, it can drive fast along that warning line (probably faster than you can strafe).

Re: Mecanum or 6WD
 

I'm currently leaning toward 6WD because of how i'm predicting the game to be. I feel that there is always going to be a team who plays defensive. There is no point of having a great mecanum drive train if we can't get to the scoring grid. If there is a bot that plays defense on us, and plays it decently well, a mecanum drive train will be stopped. Regardless of its maneuverability, the scoring grid is surrounded on both sides with opposing team zones which we are not allowed in. Of course, the same problem occurs with a 6WD. However, i'd much rather take my chances with ramming into the side of a defending robot and see whats happens than waste precious time trying to overcome a robot. Any thoughts?

Re: Mecanum or 6WD
 

6WD has always worked well for us. :rolleyes:

Re: Mecanum or 6WD
 

If you drop your center wheel wont that lead to problems with trying to hang rings on the topmost row because of the rocking at the base?

Re: Mecanum or 6WD
 

Depends how much you drop by. If you drop the center wheel by 1/8 of an inch (which I believe is enough to get the desired effect) then rocking should be minimal, especially if you have distributed your weight to prevent rocking while hanging.

Re: Mecanum or 6WD
 

Even with a frictionless roller, you're wasting power by squeezing or stretching carpet in the sideways direction while trying to move forward or back. You could perhaps argue that this isn't being wasted "in a mecanum", but it's definitely happening because of features inherent in a mecanum wheel.

Re: Mecanum or 6WD
 

Or you could just not drop it.

Re: Mecanum or 6WD
 

Warning: Your mileage may vary. I posted here about why not dropping the center wheel works for team 25, but in order to go this route, you need some very specific properties for your drivebase geometry and wheels (and gearing/torque output).

On the topic of the rocker affecting positioning accuracy...

1/8" rocker means your front wheels will vary in height by +/- 1/4" (actually less due to the carpet and tread deforming). If you build a mechanism that requires this kind of precision to score a tube you may want to think of other options.

Re: Mecanum or 6WD
 

To get across the field.

Re: Mecanum or 6WD
 

I really think whatever drive you choose is less important than it being designed properly, the people programming it, and the person driving it. Our team will be using mecanum again this year, not because it is inherently superior to all other drive platforms, but because we have experience with it, it works well with our strategy, we know how to program it for our purposes, and we have a driver who is crazy good with it. It isn't so much about the drivetrain, but how you use it.

Re: Mecanum or 6WD
 

6WD but have 2WD and 4W Floaters...either with Omni's or the Lunacy wheels

Re: Mecanum or 6WD
 

Now my team captain said we are doing the 6 wheel drive with the middle wheel slightly lowered. We are extremely tight on budget this year, so any other would be asking for too much (except for 4 wheel drive)

Re: Mecanum or 6WD
 

The rocking is the reason why we want to use a mecanum drive

Re: Mecanum or 6WD
 

Mecanum isn't going to help much if you can't get to the scoring area. If a team on the other alliance plays defense, mecanum loses its value since it can't push

Re: Mecanum or 6WD
 

Rocking has at most maybe 1/2" effect on an extended manipulator. If you need that much precision to score, you won't be scoring frequently.

Re: Mecanum or 6WD
 

Agreed! Last year was the year that rock was bad when dealing with ball manipulators and lifting off the ground or losing your ball. That is what made 8wd so popular. We almost went with 8wd to eliminate rock, but choose 6wd since it was faster to make, cheaper, easier, and the rock wasn't going to affect our design.

Re: Mecanum or 6WD
 

may not be as able to push as well but if it dances around the defense it doesn't need too.

Re: Mecanum or 6WD
 

Then I can argue that the defense is doing its job; the more time you take away from the enemy, the less they score. That is an argument why my team should go defensive this year, but everyone wants to be quarterback and score.