Mecanum vs. Tank Drive?

[JohnFogarty]

Originally Posted by Taylor

While your initiative is appreciated, and the results sure to be interesting, I think that sort of misses the point of mecanums.

I do understand that the mobility is the advantage, but I have suspicions that people on this forum seriously overstate the difference in torque in between the two drive systems. I don't dismiss the experience people have here off hand, I just want some real world data specific to this argument.

[Kevin Sevcik]

Originally Posted by JohnFogarty

I do understand that the mobility is the advantage, but I have suspicions that people on this forum seriously overstate the difference in torque in between the two drive systems. I don't dismiss the experience people have here off hand, I just want some real world data specific to this argument.

Toss in some sideways pushing tests while you're at it, if you have time.

A real A-B test could be run at a scrimmage or off-season event. Make up an alliance with 2-3 butterfly drives vs. a tank drive alliance and do 2-3 matches with the butterflies all mecanum vs all sticky. Although yes, there may need to be some adjustments for the typically lower gearing of the sticky wheels.

[Darkseer54]

We used octocanum in 2014 and had a good amount of success. However, we tried to be strategic about switching modes to conserve air so we only deployed it when we absolutely needed it. If I were able to count the teams that we were able to push on mecanums, even at IRI against some of the best of the best tank drives in the world, there would be a lot of people who would be covering their faces in shame.

[silverD]

Originally Posted by JohnFogarty

I do understand that the mobility is the advantage, but I have suspicions that people on this forum seriously overstate the difference in torque in between the two drive systems. I don't dismiss the experience people have here off hand, I just want some real world data specific to this argument.

While I do not have data, here is a video showing our mecanum from 2013 which we feel was our best implementation of this drivetrain. This was posted in another recent thread and now added here at John's recommendation. One thing to note, the video clips are more or less in order as we progressed through the season.

[efoote868]

Originally Posted by MrForbes

heh...our best regional year was 2009, with those hard plastic wheels.

Anecdotes aren't real helpful, some times.

It's funny, all the teams on Einstein that year used those wheels too.


4 out of the 6 years since mecanum wheels were commercially available (2007-2012), there was an element of the game discouraging or prohibiting use. I think 2011 was the first year there could've been wide use. 2013, for some robots, drive train didn't matter a whole lot (full court shooters). 2014, single game piece and ferocious defense, wasn't a good idea. 2015 never happened, 2016 favored 8" pneumatic wheels.

This year I'm thinking will be more favorable to mecanum wheels.

[Type]

As it looks like most people, I would recommend tank. Using the KoP, it is very simple to do and does't really take much thinking or programming to do. Mecanum from my understanding isn't overly complex but it takes more work. A downside to Mecanum is that it can't really hold it's ground that well, and you could be pushed a lot. A tank drive may not maneuver as sharp or nice but with the defense ability this year, I would highly not recommend mecanum, but that's for the strategy my team wants. My team did mecanum in 2015, but we also didn't have robot-to-robot contact.

[MrForbes]

I think a small 6 wheel tank drive robot, in the hands of a well practiced driver, will be able to do amazing things.....

[Jared Russell]

Originally Posted by JohnFogarty

I do understand that the mobility is the advantage, but I have suspicions that people on this forum seriously overstate the difference in torque in between the two drive systems. I don't dismiss the experience people have here off hand, I just want some real world data specific to this argument.

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

TL;DR, given the same coefficients of friction for a standard and ideal mecanum wheel, the mecanum wheel can exert 30% less force on the floor before breaking static friction in the forward direction. (Non-ideal mecanum wheels - with friction in the rollers - suffer less in the forward/backward direction, but more when strafing). Moreover, in practice you can find standard wheels that are far more tractive than you can get in a mecanum roller due to the size and geometry, which amplifies the difference, potentially to 50% or more.

Static friction provides an upper bound on your ability to accelerate, change direction, push, and resist pushing. Even on an open floor without obstacles and defenders, the traction-limited tank drive matches or exceeds the maneuverability of a traction-limited mecanum drive in many cases. For example, if you want to move 10 feet to the left, a mecanum drive can strafe for 10 feet, but a tank drive can reorient, drive, and reorient again (or drive any number of smoother maneuvers that accomplishes the same thing) and be able to execute each segment of the motion more quickly because of its superior ability to exert force against the floor.

Which one gets you to a given arbitrary goal location first? There are going to be cases where a traction-limited mecanum clearly wins (e.g. move 1 inch to the left) and cases where a traction-limited tank drive clearly wins (e.g. turning in place or moving purely forward). For hybrid motions, it depends on the details of the respective robot dynamics. The further away the goal, the more likely it is that the tank drive can drive a path that gets it there more quickly.

(The actual study of the optimal path/time to get from one spot to another for different types of drivetrains is fascinating and complex...see http://planning.cs.uiuc.edu/ch15.pdf for a good [graduate level] introduction).

So it may make sense to use a mecanum drivetrain if you are optimizing around short, controlled sideways movements in tight spaces. But most FRC games (2015 being an exception) are not dominated by these types of maneuvers. Once you add in the presence of "adversarial dynamic obstacles" (defense), things skew in the tank drive robot's favor in most cases.

[efoote868]

Originally Posted by Jared Russell

if you want to move 10 feet to the left, a mecanum drive can strafe for 10 feet, but a tank drive can reorient, drive, and reorient again (or drive any number of smoother maneuvers that accomplishes the same thing) and be able to execute each segment of the motion more quickly because of its superior ability to exert force against the floor.

Nothing prevents the mecanum drive from translating and reorienting all at once. With field-centric driving, this is a very easy maneuver.

Also I think Ether had some math that described turning with mecanums, the force vectors from the wheels naturally align (there's little to no wheel scrubbing while turning, this is also a feature of dropped center 6wd). I'll edit this post if I can find it.

EDIT:
https://www.chiefdelphi.com/forums/s...3&postcount=18

[IronicDeadBird]

I love how the beginning of my year is always punctuated by this discussion.
Anyway mecanun and tank are tools to execute a strategy, and you always use the right tool for the job.
Also omnidirectional capabilities on a robot should be only put on when it is absolutely needed cause if it isn't crucial you aren't going to use it. If your strategy might need it go back and refine the strategy and come back with a yes or no.

[Jared Russell]

Originally Posted by efoote868

Nothing prevents the mecanum drive from translating and reorienting all at once. With field-centric driving, this is a very easy maneuver.

Absolutely, this is a main benefit of mecanum drive. It still doesn't mean it's a quicker way to get from point A to all possible points B.

Originally Posted by efoote868

Also I think Ether had some math that described turning with mecanums, the force vectors from the wheels naturally align (there's little to no wheel scrubbing while turning, this is also a feature of dropped center 6wd). I'll edit this post if I can find it.

EDIT:
https://www.chiefdelphi.com/forums/s...3&postcount=18

It won't slip, but a similar well-balanced 6wd will still turn more quickly (assuming we are limited by traction) by virtue of a narrower effective wheelbase.

[waialua359]

Originally Posted by efoote868

It's funny, all the teams on Einstein that year used those wheels too.