Swerve Drive vs Mecanum Wheel drive?

[Jefferson]

Originally Posted by lcoreyl

Never built or driven a swerve, but wouldn’t lag depend on how it’s built and programmed? Are you saying it’s impossible to build one with acceptably small lag? In watching swerve used in competitions, I can’t say I’ve noticed lag as a “painful” problem. Hopefully someone with good swerve experience can speak on this.

It's not a problem at all. We actually used the Denso window motors to turn the modules for years until we were limited to two. We have since switched to the Andy Mark 9015. We actually limit these motors to a max 75% output and the hardware ramp on the Jags to limit battery drain.

[Abhishek R]

Originally Posted by lcoreyl

zero appealing aspects? How about the tiny fraction of time to design and implement compared to the other holonomic drives (some with “unappealing” rollers) which receive your blessing? I’m not sure how giving such a strong opinion without mentioning any characteristics or tradeoffs is useful.

Okay, let me preface this with a few things. First, I don't consider the benefits of mecanum over other holonomic drives, I dislike them equally. Second, I currently drive an H-drive 50% of the time (we're using a version of nonadrive this year), so I feel I have a bit of credibility to what I was saying. I stated a few reasons earlier here and in the omni v mecanum thread as well.

The *consensus* is that mecanum and omni wheels can be pushed around because they have rollers on them as well as already being a wheel in the inherent nature of a ... wheel. This is typically considered their drawback. My favorite drive train are the slide drives because they seem to do just that, they make the field your ice rink and you bounce effortlessly of of other robots and the field. This has been great in previous years due to the existence of safe zones, but now we forward to 2014. No safe zones, and now you find yourself unable to get to your preferred shooting location because you're getting knocked around. That's when the stability of a traction system found on many butterfly and octacanum drives comes in. I have seen brakes on more teams this year than I have before (2848, 1523, 118, etc.).

At this point, people like to continue to make the claim that if you have a mecanum drive train, you shouldn't be in any pushing matches anyway, you should just outmaneuver your tank drive opponent. This is true up to a point. You will meet teams that will match your every step with a regular skid steer system, and then not only will you not be able to get around them, you won't be able to do much until the aggressor basically decides to leave you alone.

There are times for omnidirectional drives, but I never see a use for a pure mecanum or omni system.

[XaulZan11]

Originally Posted by Abhishek R

No safe zones, and now you find yourself unable to get to your preferred shooting location because you're getting knocked around. That's when the stability of a traction system found on many butterfly and octacanum drives comes in.

Or just have a huge sweet spot, the ability to shoot while moving (or being moved) in any direction and fantastic human player loading. This year 33 has provided a text book example of how to take advantages of your drive while masking the weakness. I'm not sure if the pure omni drive was worth it but they certainly maximized it.

[Abhishek R]

Originally Posted by XaulZan11

Or just have a huge sweet spot, the ability to shoot while moving (or being moved) in any direction and fantastic human player loading. This year 33 has provided a text book example of how to take advantages of your drive while masking the weakness. I'm not sure if the pure omni drive was worth it but they certainly maximized it.

The majority of teams are unable to score in the high goal while being stationary. Scoring while moving is a more difficult task, and even being able to inbound reliably is surprisingly hard for a lot of teams from what I've seen. The Killer Bees are one of the best, and it's hard to justify something by using one elite team as an example. Also, those features such as being able to shoot while moving, or having a large spot to inbound effectively do not show a certain drive train to be better than another, they just show the robot's manipulative capabilities to be better.

[fox46]

by this logic, if robot A has 6WD and bulldozes robot B with 6WD, then robot A has just proven their own drivetrain sucks. There’s more to it than this.

I don't follow you...

They still have a shiny blue banner they brought home, however.

How many blue banners have been won with skidsteer and how many with Omni/mechanum?

How about discussing the actual tradeoffs of swerve vs. tank tractive capabilities instead of saying tipping to 2 wheels is somehow inherent in swerves design? “typically” you are using only half power? Really?

There are enough other people discussing the tradeoffs on CD, I shouldn't have to regurgitate them.

Yes REALLY! Tipping to two wheels is inherent in every design! This is what allows for the wonders of wheelies! This includes cars during acceleration, ATVs, bobcats- most any wheeled or tracked vehicle exerting a force above the level of the surface it is driving on transfers weight to the rear. I would think this is common knowledge. If you have four motors, one driving each wheel of a swerve drive, as you reduce the normal force on your front two wheels during a pushing contest, then YES you are reducing the tractive power of your machine as the front wheels and hence two of your drive motors lose their effectiveness. In a skidsteer/tank- your wheels and drive motors are all daisy chained together so whatever power is sent to one wheel on one side is sent to all the wheels on that side- if the front wheels are in the air, all your tractive power is sent to your rear wheels thus maintaining the use of all your drive motors and 100% of your tractive power.

http://en.wikipedia.org/wiki/Weight_transfer

Never built or driven a swerve, but wouldn’t lag depend on how it’s built and programmed? Are you saying it’s impossible to build one with acceptably small lag?

I'm not saying it's impossible- nothing is impossible. I'm saying your time and driver's cognitive limits would be better spent improving whatever goes on top of the robot and its use rather than sinking your time and resources on being able to move sideways when defensive play is a reality. I'll remember this discussion every time I see a swerve drive do its awkward little "wiggle" when it changes direction to avoid a defensive machine.

[lgphoneeric]

Originally Posted by lcoreyl

zero appealing aspects? How about the tiny fraction of time to design and implement compared to the other holonomic drives (some with “unappealing” rollers) which receive your blessing? I’m not sure how giving such a strong opinion without mentioning any characteristics or tradeoffs is useful.


by this logic, if robot A has 6WD and bulldozes robot B with 6WD, then robot A has just proven their own drivetrain sucks. There’s more to it than this.

Yes, sensors make driving mecanum easier, but you can be successful without them. For example, a mentor from 2996, Cougars gone wired, told me their mecanum drive encoders and gyro were not working in Utah. They still have a shiny blue banner they brought home, however.

How about discussing the actual tradeoffs of swerve vs. tank tractive capabilities instead of saying tipping to 2 wheels is somehow inherent in swerves design? “typically” you are using only half power? Really?


You make me want to create a verb form of hyperbole.

Never built or driven a swerve, but wouldn’t lag depend on how it’s built and programmed? Are you saying it’s impossible to build one with acceptably small lag? In watching swerve used in competitions, I can’t say I’ve noticed lag as a “painful” problem. Hopefully someone with good swerve experience can speak on this.

As a driver from 2996, I would have to say that I personally don't like the robot auto correcting for me, It slows me down. I prefer no sensors during driver control because at least for me, it isn't as responsive in a controlling sense. Accurate, yes. Responsively accurate, no. Just my opinion for the way I drive.

[Monochron]

Originally Posted by Citrus Dad

From a scouting perspective, we heavily discount robots with mecanum drive because they don't have the same defensive abilities as tank drive. To be honest, we haven't see a high-scoring mecanum drive robot, even at Worlds, so I don't have an opinion about the use there.

This is an incorrect assessment of mecanum wheels. See the RoboBees this year:
https://www.youtube.com/user/TheHive836

They were easily the top performing robot at the NC Regional, had the highest OPR by nearly 20 points, and missed out on taking the whole thing due solely to a Tech foul that their alliance got in the last match of finals. Yes, there were absolutely robots out there with more pushing power, but letting another team snatch up the robobees in selections would be a bad idea.
https://www.youtube.com/watch?v=A3dKBW8dbOI

[Keegbot]

Originally Posted by Citrus Dad

From a scouting perspective, we heavily discount robots with mecanum drive because they don't have the same defensive abilities as tank drive. To be honest, we haven't see a high-scoring mecanum drive robot, even at Worlds, so I don't have an opinion about the use there.

Mecanum can play defense, but it is a different style than defense with tank. While tank can dish out hits and stop an opponent in their tracks, mecanum can maneuver around the opponent and get in their way when when they shoot or try to pick up the ball. I know mecanum can play defense because I have done it.

As for the high-scoring part, I think that has more to do with type of shooter and intake that the robot has, not the drive.

[Tyler2517]

On the other side swerve drives can play some really mean defense too. The ability to lock your wheels in to a X or O. This way you cant get pushed but maintain a very high gear ratio. We played some mean defense by just out maneuvering robots and out running them then locking the wheels and that's a nice 1/2-1 foot a second across the field no matter how much pushing power you got.

[Abhishek R]

In my opinion only, I feel there will always be exceptions, but mecanum just doesn't have any appealing aspects. Whenever I see a competitive team with mecanum I just wonder what they could have done with tank, butterfly/nonadrive (I've heard this is called TexCoast Drive now...) or swerve. The idea of using rollers on wheels is very unappealing to me.

[Tyler2517]

I think that a drive should be chosen for what it's robots goal is. Yes you can push a mecanum drive. But a good mecanum driver would not be being pushed for to long.
On a different note mecanum is much funner to enplane with people then a tank drive is. Cool maths and some programming a a long talk on vector math beats explaining a tank drive. At least for marketing stuff.

[Botwoon]

I've used neither, but I can draw on enough of my experience to tell you that Mek drivetrains don't hold a candle to swerve in terms of speed, traction, efficiency, agility, etc.

This is of course assuming that you have the necessary resources to properly build and implement a swerve drive over the course of the season, and that you have enough of those resources to ensure that building a swerve drivetrain won't hurt the rest of your development. Keep in mind how many teams have won championships on swerve drives; it's a small number.

[Chris is me]

I would do neither... Swerve is best if you have the team set up for it (good mechanical, great software, practice space, good drivers), mecanum is theoretically "easier swerve" but being vulnerable to pushing defense is not worth the gains of going sideways. I would do tank drive.

If I absolutely had to pick one, I would pick swerve and be sure to have a wide base robot so it could tank steer just in case we never figured out the code.

[Ben Wolsieffer]

Both mecanum and swerve drive depend a lot on the implementation and especially on programming. We haven't used swerve but have used mecanum on most of our robots. Until this year we had to consciously switch modes with a button press so we didn't really take advantage to them fully. This year I implemented a field oriented drive system that has amazing maneuverability.

I know I am very biased towards mecanums, but I believe that their advantage of maneuverability outweighs their disadvantages and they can be very useful for teams (like ours) who don't have the resources to build effective swerve systems.

[Monochron]

I'm going to take a different approach to this and say that you are actually asking the wrong question. Rather than asking which is objectively better, you should be asking which drive is worth the risk vs. reward.

I think nearly everyone will tell you that swerve is going to perform better than mecanum in nearly every situation, assuming you can build and program it well. And I'm not just talking about precision here, using proper materials, being willing to hunt down ghosts in the code, and ensuring that your drivers have a huge amount of practice time is necessary to not lose control of the robot.

I have never been on a team that used one in competition, but have experimented with them in the off season. If your encoders telling you wheel position start to slip you end up with wheels at all angles. And thus, your robot may very well end up trying to drive itself in 4 different directions at once.

That being said, with enough practice at it, building a reliable swerve drive can be done as long as you can precision machine the parts for it.