Mecanum vs Treads

[Dumper FTW 435]

Originally Posted by commodoredl

Mecanums?
Treads?
Why not both?

(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

[wireties]

Originally Posted by Ido_Wolf

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

[Kevin Sevcik]

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]

Originally Posted by Kevin Sevcik

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]

Originally Posted by Peter Matteson

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

Originally Posted by Bryscus

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.

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]

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.

[efoote868]

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]

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!)

[PVCpirate]

Originally Posted by Kevin Sevcik

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]

I would say neither. Both options are way toheavy if you are planning to climb higher than level one.

[Ether]

Originally Posted by PVCpirate

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]

Originally Posted by lcoreyl

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

Originally Posted by Ether

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]

Originally Posted by Ether

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]

Originally Posted by buchanan

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.

[karomata]

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.