Mecanum or Swerve?

[sparrowkc]

Originally Posted by Jeffy

For a game like breakaway where pushing isn't a factor unless your playing a defensive strategy...

I would disagree with that. A great offensive robot can be easily shut down if it can't push back.

[thefro526]

Originally Posted by Chris is me

Mecanum drives are overrated, heavily.

Darn, I don't think I can give you anymore rep for a while.

Originally Posted by Nick Lawrence

Another drive system you could research is a kicker drive system, also known as a slide drive system. It's basically a 4WD setup with omni wheels, with a fifth powered omni wheel mounted in the center of the chassis, that allows the robot to strafe.

We've never built one for FRC, but many for VRC. I'd love to prototype one that would be FRC-ready though.

-Nick

This is basically what the 148/217 drive is based on. I'd love to play with one at the FRC scale aswell.

Originally Posted by davidthefat

But you can strife with regular omni.

While this is true, a "Slide Drive" has a few advantages over a regular omni drive. Theoretically, it's easier to drive, and because you aren't always relying on the omni wheels slipping it'll have more pushing power.

[XaulZan11]

Originally Posted by Chris is me

Mecanum drives are overrated, heavily. Often, people decide that they need the ability to strafe, or they decide "maneuverability" is important, so they jump to the conclusion that they should build a mecanum drive. Mecanum drive is a very specific tradeoff.

I completely agree. Along those lines, one thing I simply don't understand is why so many teams build a mecanum drive but never or rarely strafe. I just don't get it. Why would a team spend the time, money and weight on a mecanum drive and basically have a 4 wheel drive that can't push and can be pushed around? It is one very quick way to get on my 'no pick list'. Not only is it not effective at all but it raises a lot of questions of a team's competency.

If you decide you need mecanum for the game (2008 is the only game I could consider using one), but sure take full advantage to all of its capibilities.

[Aren_Hill]

Originally Posted by Jeffy

If you want an omnidirectional drivetrain, it appears that swerve would be the best because it gives omnidirectional motion with better traction and without the power loss of mecanum.
However, our meccanum drivetrain has served us very well this year. When we decided we wanted to have omni directional capabilites they were our best option because our team is not capable of building a successful swerve in 2 weeks.
The mecanum does have (atleast) one advantage, swerve has a lag while the pods turn. One other might be weight. If you direct drive 6" mecanums then it should surely be lighter than 4 (or 6) swerve pods, chain, and sterring assemblies. This did not prove to be the case for us this year because we are chain driving 4 8" wheels with 4 toughboxes.

For a game like breakaway where pushing isn't a factor unless your playing a defensive strategy, you really can't go wrong with either.

weight of an 8" AM mecanum = weight of one of our swerve modules, without making them anorexic

The weight of a swerve system will come down iteration by iteration, but the same goes for a solid tank system which can go much lower.

[Tom Line]

Originally Posted by Chris is me

Here's my thoughts on the matter. Full disclosure being that I have yet to assemble a swerve drive in anything other than concept sketches, and my mecanum experience is also somewhat limited.

Mecanum drives are overrated, heavily. Often, people decide that they need the ability to strafe, or they decide "maneuverability" is important, so they jump to the conclusion that they should build a mecanum drive. Mecanum drive is a very specific tradeoff. You exchange drive efficiency, resistance to defense, and a bit of speed for strafing. If teams spent more time prototyping bases to determine how well a 6WD does what they aim for, and how much better a mecanum does the same job, I imagine there would be a few less mecanums around. Well driven mecanum robots have seen success in FRC, though, especially this year. 2008 had several, this year there's teams like 190, 230, 188 rocking the mecanum.

Swerve takes away some of the disadvantages of mecanum, while adding extreme complexity in design, build, and driving. Extensive preseason testing should be done with a swerve base before most teams consider the option of building one. From what little experience I have, it's a completely different ball game.

So basically, The question shouldn't be "mecanum or swerve", it should be "what traits of a drivetrain are most important this year?", and you should pick based on what is most important.

Extensive prototyping will always help you make more informed decisions on any part of the robot, including drivetrains. I would encourage any team considering anything new to do it!



A question: Do you remove half and half teams from the list (2 traction 2 omni)? Personally I rate them lower but not completely off.

(I also generally avoid mecanums with any pick, personal preference, but there are exceptions to every rule)

Chris - that's exactly what we did. With the proliferation of robots using omni this year we didn't eliminate the traction / omni combos. They simply got lowered in the ratings.

[big1boom]

This year we built both drive systems.

During the build season we manufactured a coaxial swerve very similar to our drivetrain in 2009. This took most of the build season, and used up a significant amount of weight. (final robot weight of 120.0lbs) We had many problems with this drive at midwest due to major design flaws that would have been caught with preseason testing.

After Midwest, we took a step back to look at the options we had for 10000 lakes. Turns out, since the withholding allowance was upped to 65 pounds, we built an entirely new robot in 5 days. In those 5 days, we managed to build a fully functioning mecanum robot that had more functionality that out build season robot. We were actually able to get the entire robot built and wired in 3.5 days, with the rest devoted to driver practice and programming. With this 5 day build season we managed to built the most reliable robot our team has ever built.


So, now onto the pro's and con's
SWERVE
PRO:
Able to direct 100% of the force into the direction of travel
Not able to be pushed sideways

CON:
Very difficult to design
Multiple points of failure
Requires high quality sensor feedback
Heavy
Time intensive

MECANUM
PRO:
Very simple build
Reliable
Easy to program
Doesn't require sensors

CON:
Can be pushed easily
Not able to put 100% of force into any direction.

For an offseason project, I would say go ahead and try to build a functional swerve. It will be a great learning opportunity.

For build season though, I would recommend against doing a swerve without fairly successful offseason prototyping.


If you have any questions about either system, or about the specific failures we discovered, go ahead and ask.

EDIT: Woah this post got long quickly

[CalTran]

Well, I've been doing some research, and from the looks of it, I think what you use is really dependent on what you're trying to do with the drive. As Jeffy said before, Metal Mustang Robotics are using mecanums this year and they're working like a boss for us up in offensive. The only (slight) problem is probably just getting "bullied" by other robots. We got some faster sprockets for them and we can normally out-maneuver the defending robot. So far, we don't have a swerve yet, but it's definitely an off-season project our build team, or at least the build team captain, is looking in to.

[buildmaster5000]

Originally Posted by big1boom

For an offseason project, I would say go ahead and try to build a functional swerve. It will be a great learning opportunity.

For build season though, I would recommend against doing a swerve without fairly successful offseason prototyping.


If you have any questions about either system, or about the specific failures we discovered, go ahead and ask.

EDIT: Woah this post got long quickly

Now I am curious...you said sensors for swerve, but I'm not sure how we would implement said sensors (ie what types and where would we need them)

As far as programming, would you reccomend a joystick devoted to moving the robot in any direction and another axis (x axis on a 2nd stick comes to mind) devoted to rotating the robot around its CoG?

[Aren_Hill]

Originally Posted by buildmaster5000

Now I am curious...you said sensors for swerve, but I'm not sure how we would implement said sensors (ie what types and where would we need them)

As far as programming, would you reccomend a joystick devoted to moving the robot in any direction and another axis (x axis on a 2nd stick comes to mind) devoted to rotating the robot around its CoG?

the typical minimum sensors needed for a swerve is at least one form or rotary position sensor to alert you as to the direction of your wheels.

[big1boom]

Originally Posted by buildmaster5000

Now I am curious...you said sensors for swerve, but I'm not sure how we would implement said sensors (ie what types and where would we need them)

As far as programming, would you reccomend a joystick devoted to moving the robot in any direction and another axis (x axis on a 2nd stick comes to mind) devoted to rotating the robot around its CoG?

You absolutely need to have a position sensor for each rotating wheel. The sensors are necessary so that your program knows what direction the wheels are pointing. We used these

As for control, it is really up to your driver. Last year our driver wanted to have one 3axis joystick to point the wheels, and another joystick to power the wheels. This year our driver wanted everything on one three axis joystick for the swerve, while for the mecanum he wanted everything on the two joysticks of an Xbox 360 controller.

[davidthefat]

http://www.societyofrobots.com/robot_taurus2.shtml

Looks pretty cool, but is it really worth it?

[artdutra04]

Originally Posted by davidthefat

What about a 2 wheel drive? It can turn very tight and stuff

Unless you are balanced on those two wheels with no other part of the robot touching the playing surface, it's a bad idea. Any non-driven wheel or robot component that touches the carpet is robbing you of potential traction.

If a team builds a four-wheel drive robot, but with only two powered wheels, (assuming that weight is distributed evenly across all four wheels) you only have half the traction possible compared to if all four wheels were driven.

[Jeffy]

Originally Posted by sparrowkc

I would disagree with that. A great offensive robot can be easily shut down if it can't push back.

There are two goals, and hopefully more than one ball in your zone, so I don't think that you would be needing to push another robot our of the way.
However, there is one situation that I see you needing to push another robot while in your home zone: If they have hearded all of the balls in the zone into one corner and are guarding them. In which case, the only way to really push them would be into the wall. And it leaves the rest of the zone open to the offensive robot.

I would love to hear why you think what you do. Our team had a large discussion about the need for an omni drive vs. a tank drive at the beggining of the year.

[Laaba 80]

Originally Posted by big1boom

MECANUM
PRO:
Very simple build
Reliable
Easy to program
Doesn't require sensors

I admit that I have never programmed a mecanum drive, but I dont see it being easy to program. Sure, maybe the basic movements are simple, however I have seen few mechanum drive robots that are controlled effectively. This tells me that either the drivers dont know how to reap the benefits of a mecanum drive, or that the drive code is not up to par.

I programmed an omni drive robot in 08, and by no means was it easy, and it was nowhere near as controllable as I hoped.

[big1boom]

Originally Posted by Laaba 80

I admit that I have never programmed a mecanum drive, but I dont see it being easy to program. Sure, maybe the basic movements are simple, however I have seen few mechanum drive robots that are controlled effectively. This tells me that either the drivers dont know how to reap the benefits of a mecanum drive, or that the drive code is not up to par.

I programmed an omni drive robot in 08, and by no means was it easy, and it was nowhere near as controllable as I hoped.

I am not a programmer, so this is based off of what I have heard from the rest of my team.

Easy to program is relative to swerve drive programming. But still, programming a successful mecanum drive is fairly simple with a few modifications to the default code.