Mecanum vs. Swerve Drive

[Kernaghan]

1114 has really good information on drivetrain designs and advantages here.

[apples000]

If you're considering swerve, here's some advice. Before you decide to go ahead, figure out some basic stuff about the drive train. Make a simple CAD drawing and figure out where the big stuff is going to go and how the modules will be steered and driven. Make sure you spend time to figure out how the control system will work with the swerve. You need to figure out how the steering feedback will work, and if you are doing independent steering/drive, you need to do some math(see Ether's swerve kinematics whitepaper). Finally, try to find a good machining mentor. It helps a TON to have a machinist help make the modules. If you get an experienced machinist to help out, you will avoid making silly mistakes, and you'll get your modules done faster, cheaper, and with stricter tolerances. But if swerve is something your team is really interested in, it will pay off HUGE! My old team attempted to develop a swerve drive during the build season, and we ended up in 45th place in our regional. We qualified for cmp through the chairman's award, refined our drive train during the extra 5 weeks, and once we had all of our problems solved, we ended up on Einstein that year!

[Nemo]

I disagree that programming a mecanum drive takes a significant amount of extra time.

Mecanum pseudo code:
M1 = x + y + z
M2 = x - y - z
M3 = x - y + z
M4 = x + y - z

6WD pseudo code:
M1 = y + x
M2 = y + x
M3 = y - x
M4 = y - x

Mecanum drives are nice and simple, both mechanically and in terms of software. I disagree with claims that in order to drive well, it needs suspension, closed loop control, or other extras.

[SoftwareBug2.0]

Originally Posted by Nemo

6WD pseudo code:
M1 = y + x
M2 = y + x
M3 = y - x
M4 = y - x

Or you can use this pseudo code:
M1=y1
M2=y2

[Gdeaver]

Over the years I have only seen a few Mecanum bots that have true omnidirectional movement on the field. They could strafe at best. We love to have our way with them. There have been many times where we had to defend a good scoring Mecanum on our alliance from the opposing alliance defense. I have yet to see a Mecanum on the field have control of chassis orientation and drive direction. I would choose a well developed 6 or 8 wheel tank over Mecanum any day.

[efoote868]

Originally Posted by Gdeaver

I have yet to see a Mecanum on the field have control of chassis orientation and drive direction.

How is this not a problem with swerve drives?

[Siri]

Originally Posted by efoote868

There are many adjectives I'd use to describe swerve drive trains, but easy is not one of them.

Yeah, that'd be a matter of "they're easy provided you build them right" (true) vs "building them right is easy" (false).

Originally Posted by efoote868

How is this not a problem with swerve drives?

Chassis orientation is controlled independently of translational direction. e.g, Just push in the defender (because they can't push you), spin around them, and then spin around again until you're lined up the way you want. Like this or this. Or after a brief pin. Or before not being pushed at the unprotected feeder. Or after rocking an FCS blocker.

Taylor - absolutely agree. It's not a theoretical issue, it's a game play one. For instance, there's no way we'd be putting all the swerve work in at this point (nor will we continue to), if it wasn't providing such a huge in-game advantage. Still, I'd think that after 7 years I'd have seen someone riding mecanum that really embraced this. Is coaching and practice really bringing this to (near? video?) zero? I've seen some really excellent drive teams on them, but they just don't seem to weigh this heavily. I know a lot of the biggest benefits for chassis orientation rely on holding traction, but I'd venture others do not. Is the default (or otherwise reasonably doable) code amenable to it?

[efoote868]

Originally Posted by Siri

Chassis orientation is controlled independently of translational direction. e.g, Just push in the defender (because they can't push you), spin around them, and then spin around again until you're lined up the way you want. Like this or this. Or after a brief pin. Or before not being pushed at the unprotected feeder. Or after rocking an FCS blocker.

Mecanums can move in exactly the same way as swerve drives, and can do it without having to wait for their wheels to turn. In any holonomic drivetrain, teams can use a gyro throughout the match to keep track of their change in angular position and implement something called field centric drive.

I don't understand why this is a challenge exclusive to mecanum drivetrains, or maybe I'm not understanding your point.

[Siri]

Originally Posted by efoote868

Mecanums can move in exactly the same way as swerve drives, and can do it without having to wait for their wheels to turn. In any holonomic drivetrain, teams can use a gyro throughout the match to keep track of their change in angular position and implement something called field centric drive.

I don't understand why this is a challenge exclusive to mecanum drivetrains, or maybe I'm not understanding your point.

My point is the same as Taylor's. Swerves can and do control chassis orientation simultaneously with translational path. I have to say, it pays off big. There's nothing physical that limits mecanum drivetrains' ability to do the same. (On an open field) The phenomenon is that they just plain don't do it. I'm wondering why.

For instance, Madison's videos are very cool. I bet that really worked out for them in Logomotion matches. My question is why, with so many out there, do so few mecanums even attempt to drive this way. Is it the standard control strategy? (Madison mentioned good programming) Is it practice? Is it coaching? (As Taylor said) Do they see less benefit in it for some other reason?

[Taylor]

Originally Posted by Gdeaver

Over the years I have only seen a few Mecanum bots that have true omnidirectional movement on the field. They could strafe at best. We love to have our way with them. There have been many times where we had to defend a good scoring Mecanum on our alliance from the opposing alliance defense. I have yet to see a Mecanum on the field have control of chassis orientation and drive direction. I would choose a well developed 6 or 8 wheel tank over Mecanum any day.

The problem is not with the drive train. The problem is with teams that don't properly know how to use it.

Mecanum is a very easy system to implement and use. Code is readily available; we had ours built and programmed within a day when we used it in 2011. This makes it a viable option for low-budget, low-experience teams looking for a quick fix.
In my experience (and this is a broad generalization - I'm not talking about any particular team), teams that use mecanum drive aren't as advanced regarding gameplay strategies. Driving is all about anticipation. Most mecanum-based offensive strategies are all about avoiding defense, not getting into pushing matches.
Mecanum systems aren't as good at pushing because they don't need to be.
Also, if a team does find itself in a pushing contest and is outmatched, it needs to find a way to disengage without pushing back. Mecanums, if used properly, can do this well. Unfortunately, most teams don't utilize this*.

On the other hand, most teams that use swerve drive, and have iterated it to the point of usefulness on the field (I've seen more bad swerves than I have bad mecanums), have a degree of experience and wiliness about them. They're usually well-practiced and well-coached so that they don't get into bad situations in the first place, and if they do, they can get out of them with ease.

*Watching matches, I'm often reminded of the cartoons where Wile E. Coyote is trying to outrun a rocket. He could simply dive to the side and the rocket would fly by, but that never occurs to him.

[Madison]

Originally Posted by Gdeaver

Over the years I have only seen a few Mecanum bots that have true omnidirectional movement on the field. They could strafe at best. We love to have our way with them. There have been many times where we had to defend a good scoring Mecanum on our alliance from the opposing alliance defense. I have yet to see a Mecanum on the field have control of chassis orientation and drive direction. I would choose a well developed 6 or 8 wheel tank over Mecanum any day.

This, in my mind, is a fundamental component in the debate about the effectiveness of mecanum drive in FRC. It's difficult to make a determination about how effective they can be when a significant number of teams that implement them do so very, very poorly.

The conventional wisdom has been that implementing mecanum has become easy. The wheels are available as COTS parts, the code is shared freely, etc. The reality has been, though, that there's a significant performance discrepancy between the few good implementations and the piles and piles of really terrible ones. That suggests that mecanum really isn't that easy to get right and we should probably be putting it into the same category as WCD or swerve -- perhaps still not in terms of effectiveness, but certainly in terms of knowledge required to do it right.

[avanboekel]

Originally Posted by Madison

This, in my mind, is a fundamental component in the debate about the effectiveness of mecanum drive in FRC. It's difficult to make a determination about how effective they can be when a significant number of teams that implement them do so very, very poorly.

The conventional wisdom has been that implementing mecanum has become easy. The wheels are available as COTS parts, the code is shared freely, etc. The reality has been, though, that there's a significant performance discrepancy between the few good implementations and the piles and piles of really terrible ones. That suggests that mecanum really isn't that easy to get right and we should probably be putting it into the same category as WCD or swerve -- perhaps still not in terms of effectiveness, but certainly in terms of knowledge required to do it right.

I completely agree. But why? On paper, it should be an easy drive to execute.

Edit: What I'm trying to say, is what's the difference between a good and bad mecanum drive?

[Boe]

Originally Posted by avanboekel

I completely agree. But why? On paper, it should be an easy drive to execute.

Edit: What I'm trying to say, is what's the difference between a good and bad mecanum drive?

Driver Practice.

[bbradf44]

Originally Posted by avanboekel

I completely agree. But why? On paper, it should be an easy drive to execute.

Edit: What I'm trying to say, is what's the difference between a good and bad mecanum drive?

In my experience, a good mecanum drive tikes time to perfect, not nearly as much time as swerve though. My team used them for the first time in 2013, we ended up having to re balance our wheels and straighten them out and fix programing on several occasions. In the end we did have very very good but un traditional mecanum drive (un traditional on the control end). At our regional we saw some good examples of bad mecanum drives where they were basically tank drive, they could go forward, reverse, and turn but couldn't strafe to save them.

[Madison]

I think it might be an interesting exercise for people to share video of their idea of a good mecanum drive. I suspect most FRC participants have never seen an above-average implementation of mecanum drive in action.

A mecanum drive is, in some ways, a lot like fly-by-wire controls in an aircraft. It requires closed-loop software assistance or it doesn't really work at all.