Swerve Drive vs Mecanum Wheel drive?

[JohnM]

If you are a team that has used both, which one did you like better? What are the major differences between the two? Is swerve drive any harder to control?
Just give me your overall experience with the two drive system.

Thanks,
John

[Joe195]

Overall, swerve drive has much better control, but is very challenging to design and program. Mecanum is easier to program, but doesn't give you the same power. I would say swerve drive is better, but only if your team can use it to its fullest potential

[Niezrecki]

Originally Posted by Joe195

Overall, swerve drive has much better control, but is very challenging to design and program. Mecanum is easier to program, but doesn't give you the same power. I would say swerve drive is better, but only if your team can use it to its fullest potential

I am in full agreement. We have done swerve drive before and it has the ability to keep control and deliver some amount of force as well. Swerve also commonly consists of one CIM per wheel which delivers a nice amount of power. Mecanum has a history of being pushed quite easily in FIRST and tends to impress other teams less. Swerve however definitely delivers a better turning capacity than Mecanum. It's also beautiful to watch. I am looking at you Apple Pi (2014).

[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.

Offensive robots with swerve are amazing. 368 was amazing on our alliance at SVR, and of course 1717 has produced some of the best robots possible.

My assessment: if you're not ready to contend for being one of the very best robots at your regionals, then stick with tank drive, but if you're going to make that plunge, choose swerve over mecanum.

[Alan Anderson]

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.

Prejudice like that shouldn't be part of scouting. I think the appropriate thing would be to make your assessment based on what each robot actually does, and how well, rather than what you think its design is capable of.

[Joe Ross]

Originally Posted by Alan Anderson

Prejudice like that shouldn't be part of scouting. I think the appropriate thing would be to make your assessment based on what each robot actually does, and how well, rather than what you think its design is capable of.

I think 1678 probably has the scouting thing figured out pretty well. They won Curie last year, after 4 teams declined them.

While I wouldn't recommend going beyond objective evidence for most teams, it's clear that additional information in the hands of informed individuals can make for better decisions. If a team passes over picking a awesome robot with mecanum drive, it's their potential loss.

[Michael Corsetto]

Originally Posted by Joe Ross

I think 1678 probably has the scouting thing figured out pretty well. They won Curie last year, after 4 teams declined them.

Thanks Joe.

Obviously, there will always be good robots with a whole spectrum of drivetrain configurations (great example is 1425 this year, what a machine!)

In many cases, selecting a first pick robot is a straightforward offensive scouting exercise. Keep in mind, we picked 368 at SVR, a fantastic offensive machine, which also happened to feature one of the meanest swerve drives I've ever seen. Seriously, check it out at Champs, it may surprise you to see how many COTS components play a key role in that design.

However, our second pick preferences are very specific, and mecanum does not fit our desired robot configuration for our second pick. This is obviously our team bias, and is shaped by how we want to build our alliance, but it is a guideline we have followed with decent success over the past few years.

I won't say mecanum robots aren't good, because that's not true, there are plenty of great robots with mecanum drivetrains. However, they probably won't make our 2nd pick list.

But that's just us, we're still figuring this whole scouting thing out. I'd like to think we're making progress, but there's still so much we can be doing better!

-Mike

[Chadfrom308]

For defense, I say we are no slouch this year.

Yes we do have mecanums, but as said above, its a different style of defense. It is turning and ramming that will get the opponent annoyed. We have played defense quite effectively, even against tank. We can normally stop tanks in their track when we go up against them. I have played better defense than some teams do with the kit of parts.

Defense is not about drive train, it's about the driver skill

[Keegbot]

Originally Posted by Chadfrom308

Defense is not about drive train, it's about the driver skill

I agree. Different drive trains can all play effective defense (the style will vary), but a good driver makes all the difference. If the driver doesn't know how to use his/her drive to the best of its abilities, the robot will be less effective regardless of the type of drive train.

[fox46]

I like mechanum, Omni and swerve drives- it makes it real easy to bulldoze through them and get to where you want to go on the field!

Keep in mind, this is coming from someone who helped pioneer the design and iteration of several omnidirectional drives back in my days with 854 so it's not that I don't like them, I'm just honest-

Unless you have an awesome driver- preferably one who has experience with RC helicopters, don't even bother with omnidirectional drive unless you're building a practice machine and can allow piles of stick time and endless drills to get their skill level up. Far too many teams build omnidirectional drives and just don't use them to their full capability and if you aren't using them to their full capability then you WILL be outgunned by a skid steer/tank drive.

Secondly, don't even bother with Omni if you aren't using a gyro and accelerometer to take some of the work away from the driver. DC motors always run better in one direction than another and to try and pull off one of these drivetrains without some sort of logic to make sure it is doing what the driver tells it is just setting yourself up for an extremely difficult to drive robot.

Swerve drive- People think that with a swerve drive you have the tractive capabilities of a tank with the maneuverability of Omni- you do not. Each wheel has (at least) one motor. When you start pushing against something, all your weight transfers to your rear wheels leaving your front wheels spinning in the air. Hence, in a shoving match, typically you are only using half your available drive power. Furthermore, the lag time associated with steering your modules is painful. Any lag time slows down your ability to utilize your maneuverability to its maximum advantage and if you don't utilize it to its maximum advantage you WILL be outgunned by a skid steer/tank.

[Jefferson]

Well... I've successfully avoided this thread until now, but I'm not going to walk away from this one...

Originally Posted by fox46

I like mechanum, Omni and swerve drives- it makes it real easy to bulldoze through them and get to where you want to go on the field!

We've not been buldozed much this season, even by very well built 6 CIM tank drives.

Originally Posted by fox46

Keep in mind, this is coming from someone who helped pioneer the design and iteration of several omnidirectional drives back in my days with 854 so it's not that I don't like them, I'm just honest-

For someone who doesn't not like omnidirectional drives, your post has a very different tone. Everything you say in here is stated as absolutes, and it's just not that clear cut.

Originally Posted by fox46

Unless you have an awesome driver- preferably one who has experience with RC helicopters, don't even bother with omnidirectional drive unless you're building a practice machine and can allow piles of stick time and endless drills to get their skill level up. Far too many teams build omnidirectional drives and just don't use them to their full capability and if you aren't using them to their full capability then you WILL be outgunned by a skid steer/tank drive.

This is true of any drivebase, not just omni drives. A well built drivebase does nothing if the drivers don't have practice time on the sticks.

Originally Posted by fox46

Secondly, don't even bother with Omni if you aren't using a gyro and accelerometer to take some of the work away from the driver. DC motors always run better in one direction than another and to try and pull off one of these drivetrains without some sort of logic to make sure it is doing what the driver tells it is just setting yourself up for an extremely difficult to drive robot.

We've never used an accelerometer in our drive code and only late last year incorporated a gyro into the driver-controlled code. The gyro improved the performance of the robot, not the ease of driving.

Originally Posted by fox46

Swerve drive- People think that with a swerve drive you have the tractive capabilities of a tank with the maneuverability of Omni- you do not. Each wheel has (at least) one motor. When you start pushing against something, all your weight transfers to your rear wheels leaving your front wheels spinning in the air. Hence, in a shoving match, typically you are only using half your available drive power. Furthermore, the lag time associated with steering your modules is painful. Any lag time slows down your ability to utilize your maneuverability to its maximum advantage and if you don't utilize it to its maximum advantage you WILL be outgunned by a skid steer/tank.

Pushing doesn't make the wheels leave the ground, getting under the robot (bumper) does. ANY robot that has weight taken away will have less traction and lack pushing force.

[Citrus Dad]

Originally Posted by Alan Anderson

Prejudice like that shouldn't be part of scouting. I think the appropriate thing would be to make your assessment based on what each robot actually does, and how well, rather than what you think its design is capable of.

The assessment is based in large part on a fairly quantitative measure of scouting assessment data. (In fact we are probably the most empirically data-driven scouting team (for good or bad) in the the state now that I've seen scouting data from most of the top teams.) There are probably exceptional robots, and as Mike mentioned, we focus on offensive ability regardless of drive for our first choice. However, we observe in California that successful implementation of mecanum drives by third tier robotic teams is such a rarity that we focus on tank drives. A team that successfully uses mecanum drive probably will be an alliance captain and therefore not available as a 2nd pick.

[lcoreyl]

Let me address some of the "mecanum has power loss" type comments:

in the forward/backward direction, assuming the same motors/gearbox efficiency & ratio, same wheel diameter, weight, etc: swerve, mecanum, and tank have the same power/acceleration/speed.
Assuming they all have the same wheel material, CofM, bumper configuration, etc, then in a pushing contest tank wins, followed by swerve (which gets closer to even as CofM/weight transfer issues are minimized). mecanum is a definite 3rd; however, this has nothing to do with the fact there are rollers (since they don't roll in this case), it is that they reach max force of friction before the others. Also, this does not equate to tank or swerve always having an "easy" time pushing mecanum around, nor does it mean that mecanum must suck at pushing, it just means that mecanum shouldn't be selected if pushing was a main criterion of your design.

moving at any other angle relative to the robot mecanum will have power loss due to turning motor power down/off in the code (such as in 45 degree when only 2 motors are driving), and roller friction losses. So in this ranking it goes swerve, mecanum 2nd, then definitely last tank (since it can't strafe at all). the significance of the differences matters on what the game is, and what your goals within the game are.

[lcoreyl]

Originally Posted by 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.

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.

Originally Posted by fox46

I like mechanum, Omni and swerve drives- it makes it real easy to bulldoze through them and get to where you want to go on the field!

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.

Originally Posted by fox46

Secondly, don't even bother with Omni if you aren't using a gyro and accelerometer to take some of the work away from the driver. DC motors always run better in one direction than another and to try and pull off one of these drivetrains without some sort of logic to make sure it is doing what the driver tells it is just setting yourself up for an extremely difficult to drive robot.

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.

Originally Posted by fox46

Swerve drive- People think that with a swerve drive you have the tractive capabilities of a tank with the maneuverability of Omni- you do not. Each wheel has (at least) one motor. When you start pushing against something, all your weight transfers to your rear wheels leaving your front wheels spinning in the air. Hence, in a shoving match, typically you are only using half your available drive power.

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?

Originally Posted by fox46

Furthermore, the lag time associated with steering your modules is painful. Any lag time slows down your ability to utilize your maneuverability to its maximum advantage and if you don't utilize it to its maximum advantage you WILL be outgunned by a skid steer/tank.

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.

[lcoreyl]

Originally Posted by Alan Anderson

a simple roller lock to improve the maximum forward torque before losing traction

I'm assuming you mean a lock to keep the roller from rolling? In that case, if you were to lock mecanum wheels (not rollers) from turning then try to push the robot forward/backward, or left/right, the rollers already won't roll* so the lock won't change anything*. worst case would be 45 degrees, but still with no lock you have 2 wheels resisting the push, and this would be difficult for another robot to capitalize on.
The decrease in traction comes from the fact that the mecanum wheel translates torque to a force at an angle to the plane of the wheel, which means it must necessarily create more force than a regular wheel (with the same input torque) and therefore would reach the maximum force of static friction before a regular wheel (with the same wheel material).

Somewhere on CD awhile ago, I saw someone put forth the idea of a mecanum wheel that is 20 or 30 degrees between roller axle and the plane of the wheel. This would give a larger maximum friction force with all other things being equal at the cost of some strafe performance. I wish I had the resources and manufacturing skills to create wheels like this to test...
The other option would be to change the roller angle on the fly, but at that point, it's probably easier and more effective to just build an octocanum or swerve.

*OK, roller compliance and axial free play have some effect on this, but IMO these effects are not FRC significant enough that I would spend time designing a locking mechanism.

EDIT: since I just spent a bunch of posts addressing other posts that seemed to blur fact and opinion, I'll add that this is just my hypothesis for how a lock would behave, and it includes a couple untested assumptions about locked rollers, and I'm open to being disproved!