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Dumper FTW 435 09-01-2013 00:09

Mecanum vs Treads
 
One of the biggest issues with my team right now is the debate between mecanum wheels vs treads. I was wondering what you all thought.

The proponents for treads believe it is essential for preventing other teams from pushing us when we're trying to shoot, and allow us to push defensive robots trying to get in our way. We've had a lot of good luck with treads in the past, and it's proven to be very effective.

The proponents for mecanums believe that we need strafing and other fine control inorder to pick up disks (since our disk pick up mechanism will likely be finicky and difficult to aim. We have also used mecanums in the past to good effect.

I was originally supportive of treads, but as I realized how hard it would be to pick up disks, i became undecided.

Our strategy for this year is likely to be offensive, where we try to consistently shoot 2 or 3 pointers, but we also like having a defensive option in case the shooter doesn't work, or if we need to shove defenders out of our way.

What do you guys think?

z_beeblebrox 09-01-2013 00:14

Re: Mecanum vs Treads
 
Consider a simple 6-wheel dropped center tank drive. It allows for being fast and/ or pushy (depending on gears) and is very maneuverable. Treads are slow and have high friction, while mecanum wheels have low friction and efficiency.

(My $0.02)

Shane 2429 09-01-2013 00:19

Re: Mecanum vs Treads
 
use treads or a multiple wheel drive system this is not the year for mechanum yes its fast and maneuverable but has absolutely no torque a robot could push you using CIMple boxes for gear boxes

if your gonna shoot and stay still for long amounts of time use treads also probably a good idea to build your shooter with turret so your driver can focus on the keeping the defender from moving your robot and not aiming the robot

StevenB 09-01-2013 00:20

Re: Mecanum vs Treads
 
I suggest you count the number of Einstein robots which used treads and mecanums.

Don't count on mecanums to compensate for a poor harvester design, and don't count on treads to make you immune to defense. Both will disappoint. In this game, where the field is wide open, the maneuverability of mecanums is not as beneficial as it has been in some years. You'll probably need to make some full-field sprints, and both mecanums and treads sacrifice power that you'll desparately want.

nikeairmancurry 09-01-2013 00:25

Re: Mecanum vs Treads
 
Quote:

Originally Posted by StevenB (Post 1211216)
I suggest you count the number of Einstein robots which used treads and mecanums.

If I remember correctly that number is still 0 for mecanums.

Kevin Sevcik 09-01-2013 00:51

Re: Mecanum vs Treads
 
Also important, mecanums need all four wheels solidly on the ground to function correctly. See that drawing of that bump around the entire pyramid? That bump means mecanums are going to hate driving near the pyramids. Any precision driving you try to do near the pyramids is guaranteed to take twice as long on mecanums as it would on a 6 wheel drive.

MARS_James 09-01-2013 05:29

Re: Mecanum vs Treads
 
Quote:

Originally Posted by Kevin Sevcik (Post 1211236)
Also important, mecanums need all four wheels solidly on the ground to function correctly. See that drawing of that bump around the entire pyramid? That bump means mecanums are going to hate driving near the pyramids. Any precision driving you try to do near the pyramids is guaranteed to take twice as long on mecanums as it would on a 6 wheel drive.

I don't understand this at all, how does the bump make mecanums not work? We used them last year and got double or single bridge balances very easily

Chris C. 2410 09-01-2013 07:13

That may have depended on which mecanums you used.

What I would say is from a repair standpoint, mecanums will be quite a bit easier. I am a huge fan of treads, but replacing them is tricky enough that I wouldn't consider using them unless you had a really good reason. There are ways to do it quickly, for example slotting the frame, but you have to have confidence in your ability to fabricate it properly. Mecanums are nice because if you have a problem with one, it is a self contained module. You can pull one off and pop a replacement in much faster than with a tread system.

My question is if you were debating treads, why not reasses the merit of the standard 6WD system? It seems like you are debating highly mobile vs. heavy and slow.

Taylor 09-01-2013 08:00

Re: Mecanum vs Treads
 
Quote:

Originally Posted by nikeairmancurry (Post 1211220)
If I remember correctly that number is still 0 for mecanums.

If I remember correctly, 100% of 9-sided teams won Einstein. So you should do that.

Just because something has never been done, doesn't mean it's impossible. See: everything ever.

Anupam Goli 09-01-2013 08:03

Re: Mecanum vs Treads
 
Quote:

Originally Posted by MARS_James (Post 1211303)
I don't understand this at all, how does the bump make mecanums not work? We used them last year and got double or single bridge balances very easily

It has to deal with weight. Meccanum drive relies on an even weight distribution on each wheel for accurate strafing and turning. if you don't have that, the robot will strafe in a slightly offset angle. Introduce a different height for 2 wheels, and suddenly your back wheels have more weight than your front. It only affects strafing though, if I remember correctly.

Also, some non selling points for meccanum:

Actually learning to take advantage of strafe is harder than it looks. Have you ever played Halo and strafed at all? I don't think i've strafed at all when fighting in Halo 4. I'll bet when you play halo, the majority of the time you move the player's orientation and have him walking in that direction instead of strafing. The only times I strafe is to snipe and to avoid shots, both of which probably won't be done in this year's FRC game.

Meccanum also uses the vectors of motion on the wheel to produce strafing motion. Unless your PID is tuned to godly precision, you're always going to have some cancelled force, reducing your strafing accuracy and your power.

Also, the coefficient of friction on those wheels is low. Unless your control system has taken account for your driver
s tendencies, its going to be a lot easier to slip and waste even more drivetrain power.

See if these outweigh any advantage you think you may be getting. I don't know, they might for you.

JamesCH95 09-01-2013 09:06

Re: Mecanum vs Treads
 
I would questions two things in your thought process.

First do you really think picking up frisbees will be worth compromising your drivetrain decision? It may be an okay strategy for qualifications, but if you're picking up non-trivial amounts of discs in eliminations it means either 1) your alliance partners are missing lots of shots (very bad, you probably can't make up for them) or 2) your opponents are missing lots of shots (in which case they aren't a very serious opponent anyway) or 3) it's the last 30s of the match, human players are throwing discs in, and you can reliably score more points by hanging than a last-minute grab and shoot of a few discs.

Second, if your robot is touching your pyramid you're more or less protected from being interfered with while shooting. Additionally, your robot can be aligned against the pyramid to help your aim.

In my mind you don't have a really strong argument for either mecanum wheels or treads. To pick between the two I would say treads would make your more effective at breaking past defensive robots, provided they could be fast enough to effectively traverse the field end-to-end (10-12ft/s I'd guess). However, I think a 6WD drivetrain would be at least as effective as treads.

Kevin Sevcik 09-01-2013 09:12

Re: Mecanum vs Treads
 
Quote:

Originally Posted by MARS_James (Post 1211303)
I don't understand this at all, how does the bump make mecanums not work? We used them last year and got double or single bridge balances very easily

Take a look at the force vector diagram for the mecanum drivetrain. The wheels always apply force at 45 degrees from the direction of the wheel rim. The only reason you can drive straight with them is because opposite pairs of wheels push against each other, cancelling the sideways force and resulting in straight motion. When you take one of those wheels out of the equation or start shifting weights unevenly between wheels, things start getting wonky pretty quickly. The fact that the rollers aren't perfectly frictionless means driving straight isn't affected quite as much as strafing, but the effect is still there.

Ryan Dognaux 09-01-2013 09:27

Re: Mecanum vs Treads
 
I'll be the voice from the other side since so many seem to immediately dismiss mecanum wheels.

Quote:

Originally Posted by Kevin Sevcik (Post 1211236)
Any precision driving you try to do near the pyramids is guaranteed to take twice as long on mecanums as it would on a 6 wheel drive.

I don't think this is true, at all. Guaranteed? Would love to know why it's guaranteed.

One set of wheels, either front or back, should have some kind of suspension or adjust-ability built in to compensate for the variations in the floor around the pyramid in order to maintain driving as usual. However, I'd argue that even without it you would probably still be able to strafe with two front wheels on the raised part & two wheels off the raised part. Even weight distribution would be important but it would still work, perhaps with a little driver compensation involved.

If your team is considering mecanum wheels, it needs to be because your team thinks it gives you a competitive advantage and that it plays a part in your overall design strategy. Maybe your frisbee intake from the slot is on the side of your robot, so you can strafe into the wall and pick up frisbees but have your shooter on the front or back of your robot. That's just one example. A mecanum drive also means you're choosing to play pure offense, which isn't a bad thing. Give your driver enough time to learn how to really use the drive effectively.

My opinion is that you have a wide open field this year, fewer obstacles than last year to navigate around and a quick mecanum drive could be very effective if the driver is well-versed in using it.

I don't get this notion of dismissing mecanum drives just because they haven't 'been to Einstein.' The majority of teams in FIRST haven't been to Einstein, that doesn't mean the choices they made or will make for their robots are wrong.

Nemo 09-01-2013 09:37

Re: Mecanum vs Treads
 
If somebody forced me to choose between mecanum and treads, I'd choose mecanum. Not because I think it would be good this year (I don't), but because treads are a bunch of extra work while mecanum can be quickly bought and assembled without any fancy design and fabrication.

It seems like an obvious 6WD kind of year to me. The climbing and frisbee pickup is so complicated and difficult that I wouldn't want to spend any precious time designing a drive system, unless it somehow tied into the climbing ability.

Strafing doesn't seem like much of a benefit this year. Like last year, you can pivot to aim your shot. I liked it in 2011 when strafing made it easier to score tubes.

J.Warsoff 09-01-2013 09:47

Re: Mecanum vs Treads
 
Mecanum wheels will allow you high maneuverability options (e.g. strafing), which would indeed come in handy this year, since there are no humps or anything on the field. This can help get you around defending robots, or be a good defending robot yourself.

On the other hand, treads will give you much more traction, therefore giving you more pushing power, as you did find.

All of this is going to depend on what kind of robot you want to build. Defensive bot? Offensive bot? Maybe a little bit of both? Other things like how you shoot, acquire discs, and other factors will also be variables in this.

MamaSpoldi 09-01-2013 10:16

Re: Mecanum vs Treads
 
Quote:

Originally Posted by Wing (Post 1211325)
It has to deal with weight. Meccanum drive relies on an even weight distribution on each wheel for accurate strafing and turning. if you don't have that, the robot will strafe in a slightly offset angle. Introduce a different height for 2 wheels, and suddenly your back wheels have more weight than your front. It only affects strafing though, if I remember correctly.

In our experience, a properly implemented gyro-stabilization in the control loop for the mecanums make them very controllable and maneuverable. In addition you can certainly take advantage of the protection of touching the pyramid for shooting without fear of being pushed/bumped.

Peter Matteson 09-01-2013 10:22

Re: Mecanum vs Treads
 
Quote:

Originally Posted by Taylor (Post 1211324)
If I remember correctly, 100% of 9-wheeled teams won Einstein. So you should do that.

Just because something has never been done, doesn't mean it's impossible. See: everything ever.

I believe it's 100% of 9 sided robots, it was 3 wheeled.

No 9-wheeled robot has made it.

Also I believe 3 wheeled robots are 2 for 3.
67 -2005 Champion
16 - 2008 Finalist
148 -2008 Champion

buchanan 09-01-2013 10:44

Re: Mecanum vs Treads
 
Mecanum drivetrains, like every other, have advantages and disadvantages, but some of the disadvantages being discussed here are overstated and/or misinformation.

Even weight distribution is optimal, but not an absolute requirement for straight driving. So long as each wheel has sufficient loading for sufficient traction, it will go the way it's supposed to. The problem only arises when you're at the limit, trying to get more acceleration/deceleration/pushing force than the wheels can transmit. This is true for every drivetrain, though holonomic ones are generally to be more sensitive to it and have more interesting behavior when it happens. The better the weight distribution, the higher the limit, but until there's slippage behavior stays correct.

Concerns about non-flat surfaces tend to be exaggerated for two reasons. One, it's not quite true that loss of traction on one (or even two) wheels guarantees incorrect behavior. The effect of traction loss varies between nothing and complete failure to move, depending on which wheels lose traction and what motion is being attempted; in practice it's usually something in between. Two, this criticism assumes an infinitely rigid chassis. Even if you could make such a thing, it's a basic of holonomic drivetrain design with >3 wheels that you don't try. In practice, even without intentional flexibility/suspension, a basic kitbot type frame has all the flex necessary to keep things like tower base plates from being problems.

Anupam Goli 09-01-2013 11:35

Re: Mecanum vs Treads
 
Quote:

Originally Posted by buchanan (Post 1211392)
Mecanum drivetrains, like every other, have advantages and disadvantages, but some of the disadvantages being discussed here are overstated and/or misinformation.

I'm not sure if experience is misinformation though. In 2012, my old team (1002) used meccanum wheels, and the center of gravity was somewhere close to the back. This affected driving in more than one way. The robot strafed at offset angles, and sometimes, especially on the key, the robot wouldn't even go straight properly! It was not at all an optimal experience. I'm not trying to say don't do meccanums, I'm just trying to say that you should keep weight distribution in mind.

buchanan 09-01-2013 12:18

Re: Mecanum vs Treads
 
Experience is important, and often trumps theory. But it is possible to go wrong tracing back from practical experience to theoretical root causes.

There are challenges in getting a holonomic drive to go straight, but they probably have more to do with how computed wheel speeds map to actual wheel speeds than with traction under low/moderate force. In particular, motors tend to be non-linear, and since holonomic drives regularly run different motors at different speeds/loads, the final wheel speeds relative to one another end up different than the ratios the drive calculations ask for.

Closed loop motor controls can prevent this problem; gyro feedback can compensate for it. Without some such mechanism, it's hard to get these things to drive straight, no matter how good the weight distribution.

Weight distribution matters (for other reasons), and driving straight is a challenge (for other reasons), but they're not linked quite so directly as it might appear.

Dumper FTW 435 09-01-2013 12:24

Re: Mecanum vs Treads
 
I didn't know there was so much hate for mecanums :confused:

I would like to say that my team used mecanums quite effectively in 2011 and we were able to get it to work near perfectly without dealing with weight issues or most of the other issues people claim mecanums chronically suffer from. Of course, I think 2011 was a better year for mecanums for this year, but after seeing both treads and mecanums work well, I don't think we should just dissmiss one or the other without seriously considering them. I'm more curious about specific things this year that would suggest treads over mecanums or vice versa, I think the general pros and cons of treads vs mecanums are already well established.

Ether 09-01-2013 12:46

Re: Mecanum vs Treads
 
Quote:

Originally Posted by buchanan (Post 1211444)
Experience is important, and often trumps theory. But it is possible to go wrong tracing back from practical experience to theoretical root causes.

There are challenges in getting a holonomic drive to go straight, but they probably have more to do with how computed wheel speeds map to actual wheel speeds than with traction under low/moderate force. In particular, motors tend to be non-linear, and since holonomic drives regularly run different motors at different speeds/loads, the final wheel speeds relative to one another end up different than the ratios the drive calculations ask for.

Closed loop motor controls can prevent this problem; gyro feedback can compensate for it. Without some such mechanism, it's hard to get these things to drive straight, no matter how good the weight distribution.

Weight distribution matters (for other reasons), and driving straight is a challenge (for other reasons), but they're not linked quite so directly as it might appear.

^^Much wisdom here^^

One other factor to mention: precision mecanum wheels are expensive. The affordable ones most teams use can have significant variation in roller friction and roller axial free-play if not very carefully assembled. This can also affect the precision of the robot's motion.



JamesCH95 09-01-2013 12:46

Re: Mecanum vs Treads
 
Quote:

Originally Posted by Dumper FTW 435 (Post 1211447)
I didn't know there was so much hate for mecanums :confused:

I would like to say that my team used mecanums quite effectively in 2011 and we were able to get it to work near perfectly without dealing with weight issues or most of the other issues people claim mecanums chronically suffer from. Of course, I think 2011 was a better year for mecanums for this year, but after seeing both treads and mecanums work well, I don't think we should just dissmiss one or the other without seriously considering them. I'm more curious about specific things this year that would suggest treads over mecanums or vice versa, I think the general pros and cons of treads vs mecanums are already well established.

In my opinion yhey ought to be compared to the pluses and minuses of a wheeled tank drive and various wheeled tank drive configurations with omni and traction wheels. Treads and mecanum drives aren't the only drives out there.

In 2010 we used mecanum wheels with reasonable success without using any suspension other than a flexible chassis. Driving was reasonably consistent even getting onto and over the bumps. The point many posters are trying to make is that you should be a little careful when designing a mecanum drive.

Edit: there are some really good posts out there discussing the theory, implementation, and control of a mecanum drive by Ether (among others) a thread search might yield some great information.

Bob Steele 09-01-2013 12:47

Re: Mecanum vs Treads
 
During this discussion there seems to be some disagreement about what you meant by "treads". Did you mean regular treaded wheels or a tracked system?

This will be a game that is going to have a ton of contact. Mechanums have an inherent inefficiency because they can't use all of the force directly in one direction. They, therefore, are not by any stretch of the imagination a means to push around other robots or push through other robots.

They do have the ability to be agile and avoid some of the contact.

Because of the "pinch points" on either side of the pyramids this year... this agility might be compromised to a certain extent. They are great in the open field... because other robots can't strafe. As I said this is somewhat negated by the way the field is this year.

I think you should decide what you want to do and just be aware of the limitations AND the advantages of the mechanum (or any other drive).

You might look up a cool drive called the Octocanum drive system implemented by Team 488 in 2011. It has the advantages of both systems, albeit it is complicated... very cool though...great design work done by a great team (Go XBOT!!!)

No hate here...just opinions....why may vary.... (<understatement)

wireties 09-01-2013 14:07

Re: Mecanum vs Treads
 
FIRST Team 1296 used mecanum the last couple of years. As mentioned in this thread there advantages and disadvantages. To make effective use of a mecanum setup the frame & drive train requires precision design and assembly (so all 4 wheels are on the ground at all times), the weight distribution must be close to ideal (so the same amount of weight is over each wheel), the velocity of each wheel controlled (so encoders required at each wheel) and the software well done. If you do it correctly, mecanum is nice. We rarely met a robot that could block us or push us around, we were just too fast and too manoeuvrable. Our drivers could fake out defensive bots kinda like a running back juking a linebacker, spin around them etc. But we did not make it to St Louis, it might not have gone as well against better competition.

That said 1296 is going with a 6-wheel drive this year and building a climber.

philso 09-01-2013 14:13

Re: Mecanum vs Treads
 
Quote:

Originally Posted by Kevin Sevcik (Post 1211236)
See that drawing of that bump around the entire pyramid? That bump means mecanums are going to hate driving near the pyramids.

Kevin is wrong! The mechanums are innanimate objects so they are not capable of hating. It will be the pit crew who have to fix the mechanums after the drivers bump into the lip around the pyramids who will be doing the hating.

At Alamo last year, we learned that we had to check that all the rollers on all four mechanums were turning freely after each and every match. If a roller was binding, we removed it and bent the aluminum spider back out. Bumping into the edge of the bridge (lowered) was just one way to bend the aluminum spiders. Just a few (2-3) binding rollers caused the robot to deviate from the expected path in unpredictable ways. We could see the driving performance degrade throughout a match. I am sure this messes up your force vector diagrams.

We also found that the rollers on our mechanums would bind when new because the urethane had coated the inside of the brass bushings. It took a lot of work to disassemble them and clean them out so that they would spin freely.

www.youtube.com/watch?v=1mrNH0Czq0Il

Ido_Wolf 09-01-2013 14:29

Re: Mecanum vs Treads
 
I'd definitely leave mecanum (at least only mecanum wheels or just 4 wheels, as 6 wheels seem fine and are probably the way our team's gonna roll) out this year. While the ability to maneuver is important by all means, and throwing frisbees might be slightly more simple mechanically, compared to last year's balls (depending on the way you tackle that problem), you really don't want to be pushed way off your stance when trying to shoot. When a robot is so easy to push that every single tackle messes its shots up so bad that it doesn't even seem to be remotely close, it's a very significant issue for scouters who watch it.

Regarding your pick-up issue, it's only the 4th day so it's not too late to rethink your concept, or maybe scrap the pick-up system overall and just rely on the feeded; your chosen mechanisms should be modified so that they stand in line with your ideal strategy, not vice-versa.

commodoredl 09-01-2013 14:37

Re: Mecanum vs Treads
 
Mecanums?
Treads?
Why not both? :p

(Not actually endorsing mecanums, treads, or mecanum-treads)

wireties 09-01-2013 14:39

Re: Mecanum vs Treads
 
Quote:

Originally Posted by Ido_Wolf (Post 1211556)
you really don't want to be pushed way off your stance when trying to shoot.

just gather your 4 disks and park near the pyramid where you can't be bumped/pushed - it might make picking up disks difficult though

Bryscus 09-01-2013 14:41

Re: Mecanum vs Treads
 


That. is. AWESOME!

- Bryce

Kevin Sevcik 09-01-2013 14:49

Re: Mecanum vs Treads
 
If I might summarize the Mecanum side of things, for easy reference...

Pros: Highly maneuverable, Fast (if you gear it to be), Cheapest Holonomic drive available.

Cons: Less traction, More driver training to use effectively, more expensive than a 6-wheel drive, and prone to wonky driving on uneven surfaces unless you have a suspension, use encoder and gyro closed loop control, and check and maintain the rollers on a regular basis.

So, unless maneuverability is really that incredibly important to you, you probably want to use something other than Mecanums.

On the treads side of things, as far as I'm concerned, a tank tread drivetrain is almost identical to a good 6 wheel drive, only it's more expensive and complicated and less reliable.

Bryscus 09-01-2013 14:51

Re: Mecanum vs Treads
 
Quote:

Originally Posted by Kevin Sevcik (Post 1211575)
On the treads side of things, as far as I'm concerned, a tank tread drivetrain is almost identical to a good 6 wheel drive, only it's more expensive and complicated and less reliable.

It's even more similar to a treaded 6-wheel robot on level ground...

- Bryce

CalTran 09-01-2013 16:07

Re: Mecanum vs Treads
 
Quote:

Originally Posted by Peter Matteson (Post 1211381)
I believe it's 100% of 9 sided robots, it was 3 wheeled.

No 9-wheeled robot has made it.

Also I believe 3 wheeled robots are 2 for 3.
67 -2005 Champion
16 - 2008 Finalist
148 -2008 Champion

Quote:

Originally Posted by Bryscus (Post 1211577)
It's even more similar to a treaded 6-wheel robot on level ground...

- Bryce

Well, I'll argue that every single one of six wheel swerve robots have made it to Einstein. Therefore, you should look into six wheeled swerve. All the advantages of traction with the omnidirectional motion of mecanums. :rolleyes:

I have not had experience with Treads (of the tank variety) but one of our current mentors came over from Team 1987, Broncobots, and has informed me many many times that treads are simply not worth their trouble. Without a lot of background and prototyping on them, they are difficult to make, repair, and you're probably going to be traversing the field dead slow. Also, for this year, keep in mind that tank treads will be a heavy system.

I have had experience with mecanum wheels and, while not endorsing them over the other, will share what I know about them. They are somewhat difficult to service between matches, unless you bring a full spare wheel or two to just replace the questionable wheel and fix it at your leisure. While it is true that it is an omnidirectional system, it does require a fairly significant amount of driver practice (as compared to a standard drive) because you have to train yourself to utilize the onmidirectional motion availiable.

I, like those before, do submit that you look into a solid six or eight wheel drive system, as there are many many threads discussing the merits of mecanum vs (6 or 8) wheel drive. It is vastly simpler, lighter than both of the aforementioned drives, and it will be more intuitive for your driver.

sybert1ger 09-01-2013 16:19

Re: Mecanum vs Treads
 
You may want to step back and define what you're trying to accomplish with your drive train. You can't decide which is "better" unless you've defined some metrics you can use for comparison.

As explained in detail by many of these posts, your metrics should include more than just on field performance.

It's really easy to jump right into design comparison but you'll end up with better robot performance if you slow down a few minutes and define what you want your drive train to accomplish independent of implementation.

karomata 09-01-2013 16:56

Re: Mecanum vs Treads
 
If you think that you could pull it off, a teammate and I developed a drop center tread/mechanum drivetrain. If you want to know more and maybe see some image designs, let me know.

efoote868 09-01-2013 17:31

Re: Mecanum vs Treads
 
While AM mecanum wheels may not have as much grip as high traction wheels, that doesn't matter too much. You might not be able to push your opponent across the field, but defense with mecanum wheels in a shooting game is as simple as colliding at a medium speed with your opponents.

pfreivald 09-01-2013 17:55

Re: Mecanum vs Treads
 
The woes of mecanum are not nearly so great as many would like you to believe, but:

1551 has used mecanum for five years (not counting The Game That Shall Not Be Named), and there's a reason last year we switched to octocanum. (Well, several, not counting wanting to challenge ourselves with something new.) Given the experience we've had, I would not choose mecanum-only for this game (and it has nothing to do with wonky driving over the tiny bit of uneven carpet -- that wouldn't stop me for a second!)

CalTran 09-01-2013 18:51

Re: Mecanum vs Treads
 
Quote:

Originally Posted by karomata (Post 1211664)
If you think that you could pull it off, a teammate and I developed a drop center tread/mechanum drivetrain. If you want to know more and maybe see some image designs, let me know.

So like Octocanum with treads then.

Dumper FTW 435 09-01-2013 19:30

Re: Mecanum vs Treads
 
Quote:

Originally Posted by commodoredl (Post 1211562)
Mecanums?
Treads?
Why not both? :p

(Not actually endorsing mecanums, treads, or mecanum-treads)

Please tell me you have more pictures of this or video of it driving. This is the greatest thing ever :ahh:

cmrnpizzo14 09-01-2013 19:55

Re: Mecanum vs Treads
 
Quote:

Originally Posted by StevenB (Post 1211216)
I suggest you count the number of Einstein robots which used treads and mecanums.

I don't think that this is ever a good strategy......

341, 67, and 1717 did not make it to Einstein last year, ergo shooting 5+ balls in hybrid, utility arms, and unique end game strategies are a bad choice.

If this is how we think, there would be no new ideas ever.

Out team personally is using mechanums this year because we believe that being able to maneuver around the defense is more effective than withstanding the hits or pushing bots. I don't think that a bump less than .5" tall will be any sort of noticeable problem for the mechanums. I might be wrong, but I wouldn't be too worried about it.

Also, nice job, Eric, on those treads they are sweet. (and dreadfully heavy)

karomata 09-01-2013 20:24

Re: Mecanum vs Treads
 
Quote:

Originally Posted by CalTran (Post 1211739)
So like Octocanum with treads then.

Not quite, I will be sure to post it though.

dtengineering 09-01-2013 23:30

Re: Mecanum vs Treads
 
This is all very practical advice... and I agree that for winning this years' game, likely neither tank treads nor mecanums are the optimal choice.

Not to understimate the significance of competition, but allow me to point out that your robot will retire from competition just four months from now. Okay... maybe the odd off-season event, but really, once the championship is over, every design choice you made for "winning the game" suddenly becomes redundant.

It is totally okay to use treads or mecanums or swerve or hovercraft technology just beacause you think it is awesome.

We used mecanum wheels in 2007 (it didn't work nearly as well as we'd hoped) but those wheels have kept on being awesome. People still look at the machine and ask about the wheels... allowing me to give a brief lecture on vector mechanics. Our design choice might not have accomplished its mission on the field.... but as far as inspiring science and technology discussions it is still hard at work six years later.

Sure, if you have the time and resources you could build an off-season bot (especially if you are going for the hovercraft drive), or just demonstrate mecanums and treads using VEX... but that isn't quite as cool as having it on a "real" machine.

So kudos for asking advice... you've got plenty of good advice so far in this column... but honestly, if Orville and Wilbur Wright had taken good advice they would have packed up and gone back to their bicycle shop.

Of course, Orville did darn near kill himself flying one of those things. Ignore the advice at your own peril... but feel free to ignore it!

Jason

lcoreyl 20-01-2013 05:44

Re: Mecanum vs Treads
 
I had to revive this thread due to some great discussion found here. Our team has posted a whitepaper addressing a few mecanum misconceptions, and I think all of them are here in this thread: mecanum sacrifices torque and/or speed, is inherently inefficient, et al. The link is in my signature.

*while i'm about to address some points in this thread, i suggest that your responses specific to mecanum go in this thread

Some points more geared to a teams experience with mecanum:

*NOTE: I'm not advocating for mecanum this year (although we are using it... most likely) I've just decided for some reason that now after most teams have decided was a good time to start weeding out bad information. :rolleyes:

-We don't do a good job with weight distribution or frame flex and have NEVER even considered a suspension, and our bot operates just fine (i.e. very little deviation from expectation). We even balanced on the bridge last year. (would regular wheels have been easier? sure)

-our inexperienced programmers get gyro/encoder feedback working every year without much trouble and then it reallyoptimizes the driving.

-our drivers have found robot-centric drive vs. regular about the same, and strafing when needed was very easy and intuitive. Field-centric drive is REALLY intuitive and a major upgrade. (I guess you could do a 6wd field-centric program... just never heard of it)

-While we did get 2 sets of andymark 8" (because everyone scared us about them), we never had to do any maintenance during competition (on the wheels that is)

-I've seen over and over the "straw man" argument that if you want to be a block/push defense bot, mecanum is a terrible choice. I've never seen anyone advocate that, and can't recall a team asking input for that idea. Last year driving towards while also orienting the intake towards a ball was actually a defensive technique for which mecanum could be a good choice. The "keep away" defense. Playing "get in the way" style can also have success. Our alliance member last year actually did some heavyweight blocking/pushing (although not as their main strategy) with good success. would 6wd push better? sure. Does mecanum suck at it? no.

-Cool factor IS a valid consideration since we are looking to attract mentors and sponsors

PVCpirate 20-01-2013 17:12

Re: Mecanum vs Treads
 
Quote:

Originally Posted by Kevin Sevcik (Post 1211575)
Cons: Less traction, More driver training to use effectively, more expensive than a 6-wheel drive, and prone to wonky driving on uneven surfaces unless you have a suspension, use encoder and gyro closed loop control, and check and maintain the rollers on a regular basis.

I'm gonna have to disagree with you on one point here. If you use field-oriented driving controls, it probably makes it easier to learn to drive the bot, since it will always move in the direction the joystick is pointed. Combined with a spin control the driver is comfortable with, you've got an intuitively controlled omnidirectional drivetrain, and its driver doesn't need to think about which way it's facing to move somewhere.

Robofreak3130 20-01-2013 17:25

Re: Mecanum vs Treads
 
I would say neither. Both options are way toheavy if you are planning to climb higher than level one.

Ether 20-01-2013 17:32

Re: Mecanum vs Treads
 
Quote:

Originally Posted by PVCpirate (Post 1219161)
I'm gonna have to disagree with you on one point here. If you use field-oriented driving controls, it probably makes it easier to learn to drive the bot

Using a gyro you can do field-centric control of any skid-steer vehicle (which includes 6WD). I did this 2 years ago,and it's a hoot. Not sure if experienced 6WD drivers would like it though :-)



lcoreyl 20-01-2013 21:31

Re: Mecanum vs Treads
 
Quote:

Originally Posted by lcoreyl
(I guess you could do a 6wd field-centric program... just never heard of it)

Quote:

Originally Posted by Ether (Post 1219168)
I did this 2 years ago,and it's a hoot. Not sure if experienced 6WD drivers would like it though :-)

I should have known...

No more putting off doing a mecanum v 6WD off season test now! (for better understanding of both, not crowning the "best", so pls don't start...)

buchanan 21-01-2013 11:29

Re: Mecanum vs Treads
 
Quote:

Originally Posted by Ether (Post 1219168)
Using a gyro you can do field-centric control of any skid-steer vehicle (which includes 6WD). I did this 2 years ago,and it's a hoot. Not sure if experienced 6WD drivers would like it though :-)



I would love to see this done well in competition. It's certainly possible in theory, but it has some challenges apart from those you get doing it with a true holonomic drive.

Team 1988 tried it in the "Overdrive" game with my not-quite-sufficient assistance. The robot was optimized for easy turning, but at the cost of straight line stability. In the end, it was indeed "a hoot" when it worked, but there were issues with control of angular momentum and with gyro limits that made it hard to drive in game situations. You could never quite guess which way the rotation transform thought it was going, and the gyro reset got used nearly as much as the stick.

We learned a lot and it seems like many of the issues could be managed with better mechanical design, so it would be fun to try again with that plus the improved gyros and closed loop motor controls we have now. We haven't gotten around to it because our drivetrain focus has been on learning to do holonomic right.

Field-relative control software for non holonomic drive trains must make decisions in the software that aren't issues with omni/mecanum drive trains. In particular, the tradeoff between changing heading and changing position. Holonomic drives have the same issue, but they're entirely taken care of by physics - the software doesn't have to specify the tradeoffs. I'm guessing a properly tuned program could approach equivalent performance if the software were carefully matched to the hardware, but it's not trivial to do, and it's on top of software that's more complex to begin with.

Either is correct, it's absolutely possible. But unless/until it's successfully done in real life the ability to implement field-relative controls must number among the advantages of holonomic drivetrains over others.

efoote868 21-01-2013 13:33

Re: Mecanum vs Treads
 
Quote:

Originally Posted by buchanan (Post 1219450)
Team 1988 tried it in the "Overdrive" game with my not-quite-sufficient assistance. The robot was optimized for easy turning, but at the cost of straight line stability. In the end, it was indeed "a hoot" when it worked, but there were issues with control of angular momentum and with gyro limits that made it hard to drive in game situations. You could never quite guess which way the rotation transform thought it was going, and the gyro reset got used nearly as much as the stick.

IIRC the kit gyro in 2008 had a limit of about 80 degrees / second, which is wholly inadequate for anything teams do.


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