Swerve Drive vs Mecanum Wheel drive?
 

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

Re: Swerve Drive vs Mecanum Wheel drive?
 

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

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

A quick note: you probably should have used the search function, it seems like this debate comes up every other week. At any rate...

I have seen mecanum drives that, at the very least, don't get pushed around. They won't really push much, but they can certainly hold their ground. Mecanum can be about equal in terms of agility, although you're right in that you can always build a swerve drive that's faster and more efficient. I can't comment much on the traction.


That said, as I noted above, this debate happens all the time and it basically boils down to this:

Both mecanum and swerve:
-aren't worth the effort most of the time, especially since most teams can't even build a decent tank drive
-heavy!
-complicated
-difficult and time consuming to implement
-have numerous points of failure due to the extra complexity, although it can be true that their failures are less severe/impactful than a tank drive failing depending on the implementation

Mecanum:
-easier to program (pretty sure they have this in the library nowadays)
-easier to design (pretty standard)
-as noted above, generally inferior in every way to swerve, although I have seen some implementations that are just "sometimes inferior, sometimes on-par" as opposed to "always inferior"

Swerve:
-very difficult to program for full control if you're going with independent modules, although getting a basic crab drive up and running is simple
-harder to design, lots of different implementations and things to worry about
-Theoretically as fast and powerful as a tank drive but with far superior maneuverability


I've personally programmed and driven a swerve drive with four independent swerve modules, though I've only driven against mecanum robots.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I've seen veteran teams (my old team, for instance) use swerve drive trains and sit an entire match out for something stupid related to swerve drive trains (such as the wheels not aligned properly at the start of the match).

If you want a holonomic drive, and you haven't been tuning a swerve drive train for the past 4 years, go with mecanum. A word of wisdom: before you pick a drive train, pick a strategy on how you want to play the game. Then, build that drive train as quickly as you can during the build season and let your drivers practice all the way until the end, build a second if you have to and practice before the competition.

Doesn't matter what you use if you can't drive it.

Re: Swerve Drive vs Mecanum Wheel drive?
 

As some one who has design, built, driven both of the two drives.

Swerve if done right is something that is amazing. It is something amazing to see it drive for the first time. Little lone ours is so much more reactive then the mecanum. I have by far learned the most when building out swerve then i have about anything else in robotics.Our swerve even though it is easily 50%-100% faster then last years mecanum drive can still plow it around all day long.

We had dome mecanum several times and after learning to drive on of those, our swerve is driven in exactly the same way so it was not that much getting use to.

I still do not know if i would chose to do swerve again. We spend 100s of hours just making the parts. That time could have been spend making a good manipulator. The swerve this year is our teams greatest strength and our greatest weakness.

Its all about risk V reward. Even though on the field a swerve should always trump a mecanum.

Re: Swerve Drive vs Mecanum Wheel drive?
 

To add to this statement, there has never been a robot on Einstein with Mecanum wheels. This isn't to say that it will never happen, but just that it has not happened yet.
As a note about Championship-winning teams with swerve drives, the most recent Championship winner with swerve drive was Team 16 in 2012.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Swerve is a huge resource draw that takes years to develop. Even if it is really awesome to design and work with. Mecanum loses far too much power to make up for its benefits. You can always easily come back to the logic that swerves, 6wd, 8wd, butterfly, ect have made it to Einstien, but never a mecanum wheel. In my opinion 95% of teams are better off just buying or manufacturing a west coast drive and practicing the hell out of it. Just my two cents though.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Another thing to keep in mind is your strategy for playing each game. To give an example, our team excelled last year with a 6-wheel tank drive because our strategy was cycling, and our drivers got very good at this. Speed was much more important than maneuverability for us, so it wouldn't have made sense to spend more time on a more complicated drive train. This year though, the heavy defense makes strategy a little different. The argument could definitely be made for maneuverability over speed in this game. In any case, your game play strategy should always determine your robot's design, not the other way around.

Re: Swerve Drive vs Mecanum Wheel drive?
 

We have done both but we keep coming back to a basic strong shifting six wheel drive. It is really a resource allocation issue.

I will say that a performing swerve is a wonderful demo robot... It consistently amazes everyone in how it can do what it does.

If you embark on swerve I would encourage you to make it a summer / fall project with no expectations of using it for that year's game. Refine your idea and your programming... do prototyping.... work out your bugs and put it in your arsenal of mechanisms you have refined.

It does come down to resources though... if you have a machine shop you can rely on, your own or a sponsor's, good student designers and good mentor designers for them to work with and the financial resources to do the prototyping by all means go for it.

I liken a swerve drive to the Queen on the chessboard...
A good one is hard to beat... its two weaknesses are reliability because of the complexity and software AND the relative difficulty of making a shifting one.

It can be done but it is tough....

It could be a great achievement for your team though, and something for your students to come together with and be proud of ....

That, alone is a good reason to do anything....

The workhorse of premier drivetrains is the six wheel drop center shifting drive base.
I guess it would be the Rook of the chessboard...or the Bishop...
A well driven one can be devastating in offense and defense..... and they are relatively easy to maintain and keep in the game.

We did a mechanum drive once....... once.... (Does that make it the Knight of the chessboard?....lol not really ... if it could jump over other robots maybe...)

your mileage can certainly vary.....

Most of all find something your students can be inspired by participating in the design and fabrication....

Re: Swerve Drive vs Mecanum Wheel drive?
 

Also on a risk-reward perspective, if mecanum stops working out, you can swap the wheels for traction and/or omnis with hopefully minimal problems. Swerve is generally "go big or go home", IMO.

Re: Swerve Drive vs Mecanum Wheel drive?
 

This statement has always irritated me. Mecanum drive trains were extremely rare before AndyMark manufactured them, while swerve drive trains were (relatively) much more popular. The first year I remember AM mecanum wheels was in 2007; before that it took teams significant resources to manufacture them. If I recall correctly, two teams had them in 2005, maybe a half dozen in 2006, maybe a few more in 2007.

2008 didn't lend itself to mecanum drive trains (successful robots were geared 18fps+ with 2 stage transmissions). 2009 mecanum wheels were illegal, 2010 a large segment of the teams automatically ruled them out ("can't traverse the bump!"), similarly with 2012 ("can't balance on the bridge!").

Even now they're still taboo due to all the misinformation floating around.


In my opinion, "never made it to Einstein" has no place in this discussion because it ignores the wheel's history, as well as game designs and strategies. Until there is a year in which a large segment (say 20% or more of teams) in the FRC population uses mecanum wheels, I'm going to give no credit to that statement. After all, 100% of the robots on Einstein in 2009 had hard plastic wheels for their drive train.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Mecanum if done well, geared for agility, and absolutely programmed FIELD CENTRIC and then practice the hell out of it can be a very competitive drive train. Anyone that does not put in the effort to code and set up a mecanum drive for field centric is wasting their time. I have seen many teams drive mecanum like a tank drive with only the occasional strafe/rotation and this is a waste of time. You can dance across the field dodging defense all day if you can just move the stick and your robot moves that way on the field no matter where it is pointed. But if defense catches up to you that's it, you must be faster! Mecanum shouldn't really even have a tank style drive mode.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Though this is the practice ramp (only pic I have on me) we could quite easily

Also 2012 bump? no problem

I loved our mecanum robots, they worked really well.

They work an unbelievably better when controlled by encoders and PID as well, the only reason we stopped using them is because we hate getting ruled off scouting lists for no reason but them, even though in 2011/2012 we pushed so many robots you would not even imagine.

And mecanum is really light. in 2012 and 2011 we used ToughBox nanos with the output direct driving the AM 6" wheel. Any 4 CIM chain tank drive is going to weigh just as much. Also they take up almost no space (note our 4 sided intake!)

Re: Swerve Drive vs Mecanum Wheel drive?
 

This is another reason mecanum can suck, because of the number of teams that don't utilize the full benefits of the drive most competitive teams will cross you right off of pick lists. You have to SHOW that you can rock it well with some epic driving to avoid that fate(this is one of the things 1678 looks for when scouting).

Re: Swerve Drive vs Mecanum Wheel drive?
 

I think that you are focusing on the wrong issue here. The issue with most team's mecanum drives is not whether or not it is field-centric, but whether or not teams have practiced driving it like a mecanum drive.

I agree that field-centric drive is awesome (and fun to code:)), and I might even agree that it is easier for a new driver to learn. However, if, on the last day of build season, I can give the programmers the robot to make field-centric code, or I can give the drivers the robot to practice, I will choose the later without hesitation.

For 95% of teams, the limiting factor for their drive system is the driver, not the motors/wheels/gearboxes/code. I would argue that a well driven standard mecanum drive could easily hold its own against a well driven field-centric drive.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I don't think mecanum drive needs to be field centric. We don't and we are still a very competitive team. If you can build it and drive it well, mecanum can be effective.
On the other hand, this year's Hawaii Regional was the first time I'd seen a swerve drive (or one that works well) and I was thoroughly impressed. 368's drive was able to maneuver super good. Our team will hopefully do an off-season project with swerve. If we get it right we may try it next year.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Popular belief was wrong, and there weren't many teams to challenge assumptions.

Even the belief that swerve drive done correctly is always better than mecanum drive done correctly is misguided. The teams that use swerve drive year in and year out effectively tend to have great organizations along with better than average resources. I'm not convinced these team's competitiveness entirely derives from their drive train. Likely they'd be just as successful with a 4/6/8 wheel tank drive, a mecanum drive train, or some other drive train.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Personally, I like both Mecanum and Swerve. However, my team decided that we weren't really looking to make a swerve drive due to the resource and experience concerns. Instead, we went with a modified octanum drive. With butterfly modules comprised of mecanum and colson, we feel as if we have achieved a similar performance to Swerve, but without several of the sacrifices.

When considering drive trains, you must consider everything. Do not make a hasty decision whilst either scouting or building. I suggest looking up 1114's drive train documentation. Implementing a system that allows for decisions based off of quantitative data enables a team to make intelligent choices.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Team 174 Arctic Warriors do belt driven swerve drives every year. It's honestly ridiculous if you get a chance to see it but it works well and it's really cool.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I have never personally seen a swerve drive in person, but only in videos. This is the first year that my team has used mecanems. We decided that we also wanted to develop a new drivetrain to go with it. It is sort of an octanum drive base. We used 4 4" mecanem wheels and 2 six inch traction wheels. We have gone through 2 district events and gotten second in one. The first event we got pushed around a decent amount, but in the second, we got pushed around less as we got better driving. The traction wheels also allow us to play some pretty good defense too. The choice that teams make between tank, mechanems, and swerve should be based on their desired strategy that year. I have found mechanems to be the greatest wheels i have seen when lining up for the one point goal. Since they can "drift" it makes it easy to drive up quickly, and then move side to side to put the ball in quickly. I have seen numerous teams have to line up multiple times when they try to get the ball in.

Re: Swerve Drive vs Mecanum Wheel drive?
 

To be fair, one of the finalists on curie in 2008, 2171, was running mecanums. And IIRC they were actually using them as well (to weave through traffic).

Re: Swerve Drive vs Mecanum Wheel drive?
 

I disagree. The most competitive mecanum robot I believe I have seen is 2052, who had a tank mode. the tank mode helps because it keeps your smallest side going through defensive holes:

https://www.youtube.com/watch?v=Ogy65hEPIXk

Re: Swerve Drive vs Mecanum Wheel drive?
 

Team 308 chose mecanums over swerve this year because it was easy to install, probably lighter, and less points of failure.

We chose mecanums because we wanted to avoid defense. And, at least I think so, we have done this correctly. It takes 2 and sometimes 3 robots to keep us from moving. We have incredible power on our drive train and incredible programming though.

On each wheel, we have a toughbiox mini with 1 big CIM and 1 min CIM. (Yes technically we have an 8 CIM drive)

This allows us to accelerate crazy fast, which helps with all the defense,

Programming wise, we have 5 PID loops on our drivetrain. I'm not a programmer, but how I think we set up the PIDs is the based on rpm from each wheel, us telling the PID how fast it should be going. We have 1 PID on each wheel, and one for rotation. It's cool because you can push our robot and it will push back with independent power on each wheel. And speaking of pushing, that's how we play defense (sometimes). We sit in front of a robot and step away from the controls and it does the work (mostly) by itself.

We spin around each robot when trying to be defended, and when defending, we push corners and even head on. We can normally stop robots from moving at that point.

We don't get pushed around easy, although it may look like it. We have adapted to rolling with defense and spinning off whenever we have the chance.

We have shut down power house teams from scoring with mecanums.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Do you have any video of the automated push-back? It sounds epic.

Re: Swerve Drive vs Mecanum Wheel drive?
 

As a mecanum lover, it really annoys me when people automatically rule out mecanums when the really don't know what they are capable of. Our robots in 2010 (http://youtu.be/rdAKiuimaY4?t=58s) and 2012 (http://youtu.be/tdAncHeHOEQ?t=1m36s) had mecanums and they could traverse the bump and balance on the bridge.

I especially agree with this. In past years we have driven our mecanums mostly as arcade drive and with crabbing at the press of a button. You can see the ineffectiveness of this in our video from 2012. This year though, I implemented a really cool (in my and the judges' opinions [we won the Innovation in Control Award for it at two districts]) field oriented drive mode that we use exclusively. I read about the idea on CD and decided to implement it early in the build season, so we had a lot of time to practice with it. Even though we aren't extremely powerful, we are agile.

That sounds really cool. I was thinking of using PID loops and encoders this year but I decided it was just easier to use a gyro to maintain our orientation if we were pushed and to correct for traction or weight imbalance problems, mostly because we have had bad experiences with encoders failing in the past. I might try that along with extra miniCIMs in the offseason.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Is there something unique about the way you did it?

Field oriented control has been around for a number of years. It's built-in to LabVIEW and WPILib for Java and C.

Re: Swerve Drive vs Mecanum Wheel drive?
 

We have actually never had any problems with our encoders, even when one broke, the rest of the PIDs made up for it.

As for Field Centric, we never used it. It is way faster to do robot centric.

85% of the time, we don't use the strafing feature of the mecanums. But, the most important 15% we use them. (Like lining up the 1pt goal and lining up for inbounding)

And that has made it worth while

Re: Swerve Drive vs Mecanum Wheel drive?
 

It's most unique aspect is that it could automatically correct its orientation when we were pushed or lost traction on a wheel, which made our system much easier to control than it would have been without it and it allowed us to travel in an absolutely straight line, unlike it the past where going sideways often went in an arc instead. This prevented us from needing to be so obsessive about weight distribution, because we could just let software fix our problems.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Share the technology: write a paper!

... and give credit where credit is due :)

Re: Swerve Drive vs Mecanum Wheel drive?
 

I don't have time to write up about it at the moment but I probably will during the offseason. For now, here is the code for it if anyone is interested in looking at or adapting it for their own use: https://github.com/RobotsByTheC/CMonster2014.

I would tell where specifically I got the idea from on CD, but I don't really remember anymore. :o

Re: Swerve Drive vs Mecanum Wheel drive?
 

Yep. Same with my old team, and we were very competitive those years.

This year is our first year using mecanum wheels, and I have to tell you, I love them. I'm not a big fan of trying to switch to swerve drive next year because we still have not mastered mecanum wheels.

John M

Re: Swerve Drive vs Mecanum Wheel drive?
 

this year was our first time of swerve, that said it saved our robot, it defenetly has its benifits, and if you where just given both drives on the feild without the need to make or program them, swerve will usually win over machanum. but there are businesses that sell machanum (get them from andymark), but there are none that sell full swerve drives, even though you could buy the lower end of the andymark crab drive, so it will come to desighn, if you biuld a good working swerve, it will beat mach, but if your swerve isn't well made, like how our encoders would slip on our's (locktite fixed that) it will not even move. i would recommend adding a third option of a dual drive/octanum, for a well made swerve will nearly always win a machanum.

Re: Swerve Drive vs Mecanum Wheel drive?
 

The problem with mecanum IMO is that they are basically the end. Apart from octacanum drives, mecanums don't have many places to go to design-wise. I think they will remain the same for a long time.

Swerve and tank drives, on the other hand, have a room for improvement. 221's Revolution Swerve weighs only a couple pounds per wheel, and their in-wheel Wild Swerve is fantastic. However, I still think that there is room for improvement- nobody's just found it yet.
Tank drives have changed little over the past few years, but gearboxes get more interesting every year.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I can think of several directions for improvement of mecanum-style wheels. Figuring out a way for the rollers to never stick would be at the top of my list, followed by a simple roller lock to improve the maximum forward torque before losing traction, and eventually adding powered rollers to give better sideways power.

Optimizing mecanum material selection and mechanical design isn't done at all. Just increasing robustness while decreasing weight would go a long way toward making mecanum wheels better for FRC team use.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I'd agree with this 100%. But, at least in 2012, I think you were competitive because of your accurate shooter and probably not your drive train (I wonder if you would have been better on bridges with a traction drive). Like you mentioned earlier with the good teams that build swerve probably are the teams with good scoring mechanisms and good team organization. These teams will be competitive no matter what drive train they select. Drive train selection is even more important for the fringe competitive teams; teams who may or may not have consistent scoring mechanisms or who may or may not have ample time to practice. The competitive teams that will have consistent scoring mechanisms and gave their drivers enough practice will be competitive with a swerve, tank or mecanum drive.

Re: Swerve Drive vs Mecanum Wheel drive?
 

My experience with mecanum is that it causes a serious power drain when strafing (going sideways). Our battery is usually at 11.5v when going forward (using 4 CIM motors & Toughboxes), but it drops to 8v when strafing.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Too much friction in the rollers.

It can be quite tricky to obtain an acceptably small level of roller friction in the affordable COTS mecanum wheels used by most FRC teams.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I've heard that PID control begins acting unusually when you have a constantly moving setpoint because the tuning parameters were only set for one particular speed. Is that not so, or has your team found a way to work around this (such as a lookup table for different setpoints)? My team never really got around to putting encoders on our wheels, and as a result wound up tediously multiplying each wheel by a particular constant depending on its direction (because our motors have a 20-30% speed bias in a particular direction), which was a pain to figure out through trial and error.

I've been tentatively skimming through this code for around half an hour and can't seem to find where the magic happens... I'm not particularly accustomed to the command based model :P Could someone point me in the right direction? Thanks!

Re: Swerve Drive vs Mecanum Wheel drive?
 

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).

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel 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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

src/org/usfirst/frc2084/CMonster2014/commands/FieldCentricMecanumDriveCommand.java

Re: Swerve Drive vs Mecanum Wheel drive?
 

I think its all about the drivers... I never drove a robot on swerves but I know how mecanums feel... If you are able to drive correctly, you don't need power. You only need to know your robot and how to counter the other team.

It's all about strategy and logic, it's not material...

Re: Swerve Drive vs Mecanum Wheel drive?
 

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

Re: Swerve Drive vs Mecanum Wheel drive?
 

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

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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

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

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.

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.

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.

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Horizontal pushing "transfers some of the weight" to the rear wheels, even if the front wheels do not leave the ground.

I believe the point being made was that with a chained drive, all motors on each side are still providing force... And that force will be distributed to the wheels with the greater traction (the rear wheels), once the wheels with the lesser traction (the front wheels) have reached their traction limit.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Right. If fox46 had worded it that way, I wouldn't have a problem with it.

As soon as any of the wheels leave the ground, though, you've lost at least some of your ~150 lb weight, making it more likely that you are limited by the traction of your wheels, not the power you can deliver to them. And if your wheels are off the ground, you've likely lost the pushing match already.
I guess the larger point here is: the primary advantage of an omnidirectional drivebase is you can avoid pushing matches all together by beating the opponent to the position. At that point, if you don't want to move, you lock the wheels in a position so that the robot won't roll in any direction. If you do want to move, you move sideways away from the opponent, a move the tank drive can't make.

Re: Swerve Drive vs Mecanum Wheel drive?
 

6 CIM, 2 speed, 2" wide natural rubber roughtop wheels? If you're going to champs I would love to conduct an experiment to prove or disprove this statement.

Where did your weight go then?? Weight doesn't just disappear- it is transferred. Furthermore, Ff = Fn * u + Fhys. So even if you are transferring your weight to only two rear wheels, your tractive force will remain the same.

Not true- my post infers that if you a) have capable drivers and b) have a control system capable of making their task intuitive then you can indeed utilize the maneuverability in such a way as to overcome the defences of a skidsteer/tank drive.

So how do you differentiate between the performance of the robot and ease in driving- do they not go hand in hand? I think you just proved my point.

It takes far less time to master a skid steer drive train than an Omni or swerve. The telepresence aspect of it is ^2 in any Omni/swerve vs skid steer.

Re: Swerve Drive vs Mecanum Wheel drive?
 

In my experience, it takes almost no time to "master" field-centric control. It doesn't get much easier than point your joystick where you want the robot to go. While Ether has shown field-centric control can be adapted to tank drives, it isn't as natural a control as it is for omnidirectional drives.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I don't know the detailed specs on the opposing robots.
We will be in St. Louis, but I have no interest in setting up an "experiment" where you attempt to buldoze our robot.

At least some of the weight is transferred to the opposing robot.


Additional functionality.
The robot operates in a robot-centric mode normally, When the driver initiates crab control, the gyro is reset and the robot is "field-centric" from the robot's orientation at that point. This gives the driver a twist input while in crab. If the twist input isn't used, it is the same drive code that we used for years before. If anything, it gives the driver one more thing to think about. It is, however extremely effective once mastered.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I put field-centric "Gyro Guided Skid Steer" on a practice bot back in 2011.

It was a hoot to drive. Try it some time.


efoote868 is right, though: the mecanum/omni "Halo Auto-Rotate" (Halo-AR) version is noticeably smoother.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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!

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I don't follow you...

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

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

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I agree 100%. My point was to provide an example of how teams can maximize their drivetrain. I wasn't providing an overall assessment of omni drives.

Compared to a team like 469 that can plow through teams to get to their smaller sweet spot near the goal, 33 needs the large sweet spot as they cannot depend on getting to a specific location and staying there under defense. 33 was smart enough to understand this and put a lot of time and effort in ensuring their shooter had a huge sweet spot and could shoot fade aways or while being pushed sideways. The same applies to human player loading: 33 cannot depend on getting right next to the human player (and staying there) so they went with the largest possible catch radius. The biggest weakness to an omni drive, the inability to resist being pushed, was down played by 33 because they can still accomplish the game's task without needing to be at one small, specific location.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I see, you're completely right.

Re: Swerve Drive vs Mecanum Wheel drive?
 

The lag time on getting a swerve drive to start strafing is insignificant. It's 1/2 of a second to 1/8 of a second.(shortest path algorithms help a lot) The drive motors are already ramping up at this point. By the time any other omni directional drive gets to speed the swerve will be their the extra acceleration that a swerve has strafing over mechanum is amazing.

On another note if you have a swerve drive, who spends their time pushing a west coast drive around. Most good teams with swerve know that the whole goal of a pushing match is to waste time. You can role out of pushing matches really easily. We have some really cool code that changes our center of rotation making doing this really easy. If done right you can use the other robots pushing power to help you accelerate past their top speed. (if you have a higher top speed)
If a swerve is playing defense just turn your wheels perpendicular to their thrust, Pushing a robot sideways is much much harder then trying to push one head on. With a swerve their is no "foward" making pushing agenst it is hard if the drivers know how the thing works well. If you do try pushing a swerve side ways the lifting motion on the wheels you are next to will cause the outhes to dig in to the carpet making the while thing much more difficult.

Re: Swerve Drive vs Mecanum Wheel drive?
 

You mention the fact that you have easily plowed through holonomic drivetrains as the proof that ALL holonomic drivetrains will be easy to plow through. I’m sure you have plowed through another bot with tank drive too, so why doesn’t it follow that ALL tank drivetrains will be easy to plow through?
My point is you are overstating your point to the degree of being misinformation. tank is better at pushing than mec or swerve, but let’s not make someone who is looking to get information think that any team with a thrown together tank will be able to easily push around any holonomic drive.


I was not arguing which is better. I was making the point that I think it is bad advice to say:
and was providing evidence to support my claim that you gave bad advice. I myself would advise to try mecanum, and also try to get sensor feedback to make it better, but that’s different advice.

I am not disregarding weight transfer, I am trying to make you see the difference between your 2 statements:

1:2:I have no problem with statement 1. If you had originally stated it this way you would have been the first to mention that in this thread and it would have been helpful, and we could have talked about torque limited vs traction limited
statement 2 means that 2 wheels are off the ground. You are saying a typical pushing match ends up with 2 swerve wheels off the ground, and I find that to be misleading

Re: Swerve Drive vs Mecanum Wheel drive?
 

From what I have seen mechanum has 3 problems.

1: Almost every time I see a team with mechanum I see the having issues with being pushed around and playing bad defense because they can be easily moved.

2: Drifting, I guess drifting can be good sometimes but when you are drifting every time you do a turn at high speed then I can see it being a problem.

3: From what I have heard from people on my team, (maybe true maybe not) they cannot go as fast because their speed is limited.

the third problem may not exist, but I think it does because I trust my teammates.

Re: Swerve Drive vs Mecanum Wheel drive?
 

The third one is both true and false :) The speed is equal to the speed of any other wheel of the same diameter when traveling forwards/reverse, but the speed when just strafing is only sqrt(2)/2 times the maximum forward speed (I believe... its definitely less than 1). Tank doesn't have this option in the first place, and swerve has 100% of its forward speed when traveling sideways.

Re: Swerve Drive vs Mecanum Wheel drive?
 

A lot of misinformation in this thread.

Mecanum drivetrains have a fraction of the traction that skid steers does. This really isn't debatable. There is no difference in traction between a good and a bad mecanum drive, they have the same 4 wheels touching the ground, if they are static there is nothing that makes them any different. The only difference is in gearing but that doesn't matter when the robot is sitting still.

Most drivetrains in FRC (especially with the advent of the 6 cim drivetrain) are traction limited, not limited by the power output of the motors. Our drivetrain is geared for ~24 fps in high gear and it's still traction limited.

A well designed swerve drive can have just as much pushing power as any tank drive assuming it uses the same tread material.

The maneuverability of mecanum or omni wheel drivetrains can be very advantageous for offense, however it's been proven time and again that they really can't do anything on defense. After watching over 400 matches just this season I can't even count the number of times I've seen a mecanum bot just get brushed aside by a tank or swerve drive as if they weren't even there.

Robots rarely have their wheels lifted off the ground. Nearly all pushing matches have both robots with all 4 wheels planted on the ground. Despite their weight shifting to the back wheels their force applied on the ground is relatively constant and it marginally affects a robots traction.

Mecanum wheels never behave the same as traction wheels. The rollers on mecanums make it so there is always a slight loss in efficiency since those rollers will always spin slightly unless you are going perfectly perpendicular to their direction of rotation. Also the rollers have a relatively smooth surface so even if the wheels are traveling perpendicular to direction of the rollers they still slip and cause a loss in efficiency.

It's not any easier to push a robot (on traction wheels) backwards than is to push it sideways assuming that it has enough power to at least keep it's wheels from spinning. It's the same amount of friction in every direction as long as the wheels are locked.

Do not take this to mean that mecanums are bad. Just stop saying that they can do things that they can't.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Here are two somewhat minor advantages to mecanum drives vs tank drives that I have never seen discussed.

1. I have never ever seen a mecanum robot tip over because they braked too fast, but I have seen numerous tank drive bots do this. Given, this can be corrected fairly simply with some ramp-up code, but most teams don't have anything like this in their code.

2. It is not possible to (for lack of a better word) pin a good mecanum bot in the center of the field with nothing around. Almost every tank drive I have seen gets trapped when a robot side loads them. They can still spin in circles, and maybe manage to break free this way, but a good mecanum robot can spin off quickly in this situation.

Value these things as you will, but keep them in mind.

Re: Swerve Drive vs Mecanum Wheel drive?
 

There is a world of difference in traction between a good and a bad mecanum drive. Proper design and craftsmanship are crucial to traction for a mecanum.

When the vehicle is going at a 45 degree diagonal (i.e. perfectly perpendicular to the direction of rotation of the rollers), the rollers do spin.

The only time the rollers do not spin¹ is when going forward and backward, is which case the vehicle is not going perfectly perpendicular to the direction of rotation of the rollers.


This is not true if the coefficient of friction of the wheels is different in different directions.


¹In theory, with no roller axial free play and a non-compliant floor surface.

Re: Swerve Drive vs Mecanum Wheel drive?
 

The theoretical strafing speed is the same as the forward/reverse speed. There is no sqrt(2) factor.

The reason most mecanums have noticeably slower strafing speed is due to roller friction, roller axial free play, and floor surface compliance.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I don't understand how the same 4 wheels touching the ground in a static state could possibly have different amounts of traction.

How can a free wheel be forced to spin when going perfectly perpendicular to it's direction of rotation?

In a standard mecanum drive set up if you drive the robot straight forward the rollers on all 4 wheels will slip slightly due to the fact the ground is applying force at a 45 degree angle to their direction of rotation.


In the case of normal traction wheels commonly used in FRC I think it's pretty safe to say the only difference in traction would be the slight change in the profile of your contacting surfaces depending on the direction of travel. Aside from that the materials on our wheels probably have a pretty uniform friction in every direction assuming the same profile.

I'm not trying to stir up an argument, I'd just like to see something the supports these claims.

Re: Swerve Drive vs Mecanum Wheel drive?
 

We were discussing a "good" and a "bad" mecanum drive. Who said the wheels on the "good" one were the same part number as those on the "bad" one?

And even if they were the same part number, who said they were all assembled and adjusted with the same care and attention to detail?

Differing amounts of friction in the rollers (due to wheel design, manufacturing tolerances, and care in assembly and adjustment) affects the wheel-to-floor traction of the wheels. A detailed explanation of the physics can be found here.


We were talking about the rollers, not the wheels. The rollers need to spin in order for the bot to translate at 45 degrees.


Read Note 1 at the bottom of the post to which you were responding.


It sounds like you are saying that the tread pattern on commonly-used FRC wheels results in isotropic friction. Was that your intent?


Understood, and appreciated.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Well that's what I was assuming; that both teams had the same wheels and simply assembled their drivetrains differently (e.g. different tolerances, different gearing, etc...). Aside from some play in the mounting of the wheels they shouldn't behave any differently. I think we're debating different situations.

I was assuming that the wheels didn't have differing amounts of internal friction in the rollers or on the axles. If they did then I would definitely agree with you that that would make a difference.
I'm not talking about the motion of the bot, just the motion of the wheels. Mecanum wheels can obviously only spin in two directions. Both of which are applying force at a 45 degree angle to the rollers. In practice it's impossible to make your robot move while keeping the rollers on all 4 mecanum wheels static, which is why I pointed that out. You will always have a slight loss in efficiency.

I'm making all of my statements assuming this is done in practice, not in theory.

Yes. Blue nitrile tread(what I have the most experience with and what is generally accepted to be the best tread material) interacting with carpet will have a relatively isotropic interaction resulting in fairly uniform friction in all directions assuming the same surface area and profile in the direction of motion each time.


I think this debate is a largely a difference in interpretation of the conditions under which these tests are done. I'm assuming that two teams with either a bad or good mecanum drive assembled their wheels and axles identically. Also I'm comparing mecanum specifically to well built 6 wheel WCD on 1-2" blue nitrile traction wheels.

Hope that clears things up.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Jake,

I agree that if you assume there are no differences between "good" and "bad" mecanum bots then there will be no differences between them. I just think that's a bad assumption, in practice.

When you said "The rollers on mecanums ... will always spin slightly unless you are going perfectly perpendicular to their direction of rotation" I read that to mean that the only time the rollers on a mec wheel on a mec vehicle would not spin is when said wheel was going in a direction perfectly perpendicular to the direction of rotation of the roller presently touching the ground (i.e., going in the direction of that roller's axis). If that is what you meant, it is not correct. If that is not what you meant, could you please clarify what you did mean?

In practice it is possible to build a mecanum vehicle which, when operated in the forward (or reverse) direction on a hard surface, will exhibit virtually no motion of the rollers. In other words, spinning rollers in the forward/reverse direction is not an inherent property of mecanum wheels: it is a mostly due to roller axial free play and operating on a compliant surface.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I'm not assuming there's no difference, I'm assuming that every team has the capability to at least assemble their wheels correctly.

That is exactly what I meant. When a wheel is traveling on a surface perfectly perpendicular to it's axis of rotation there will be no net force causing it to spin. Since a mecanum wheel only has two directions (one axis) of rotation and neither of those directions is perfectly perpendicular to the axis of rotation of the rollers; it is not possible to drive a mecanum wheel on a surface with friction in a manner that will cause a net zero force on it's rollers in their axis of rotation.

We're not talking about a hard surface, we're talking about robots on a soft carpet. Even so however, I don't buy that. I don't think that you can build a vehicle on mecanum wheels that will exhibit near-zero roller motion under hard acceleration forwards or backwards. With very slow acceleration of course it's possible have very little roller spin, however with the acceleration we see in FRC, I don't believe that.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Then what difference(s) were you assuming? What did you mean by "a good and a bad mecanum drive" in your post #81 when you claimed that "There is no difference in traction between a good and a bad mecanum drive" ?

Re: Swerve Drive vs Mecanum Wheel drive?
 

From what I've seen in competition the difference between a bad an good mecanum drive train largely has to do with gearing. Placement locations of the wheels, wiring work, and programming. I've never seen a team in person that didn't at least mount the wheels correctly to their chassis (not to say there aren't teams that have made this error, but it's not something I commonly see).

Re: Swerve Drive vs Mecanum Wheel drive?
 

Here are some of the factors that could cause a "good" and a "bad" mecanum drive to have different traction:

- type of wheel (model number)

- manufacturing tolerances of wheel

- manufacturing defects of wheel

- improper adjustment of wheel rollers

- improper alignment of wheels on vehicle

- frame warped or too rigid

- poor weight distribution

When it comes to traction, all mecanums are not created equal

Re: Swerve Drive vs Mecanum Wheel drive?
 

Of course these are going to make a difference. If you're not using the same wheels, I'd assume that they're not the same.

Quote:

- improper adjustment of wheel rollers - improper alignment of wheels on vehicle

This would affect your performance, however I doubt it would be to the degree that would make the difference between getting pushed and not getting pushed by an opposing robot with a strong drivetrain.

Quote:

- frame warped or too rigid - poor weight distribution

These are the most likely differences I see appearing and having the largest affect. However for the purpose of discussing pushing in mecanum vs skid steer, I don't think they'd turn the tables.
Exactly. As I said before, if they're not equal, they are not equal. Of course there's a difference. If I built a tank drive on lunacy wheels, and another on 2" blue nitrile traction wheels I'd expect them to be different.

The point I was initially trying to make in my original post was that a well built skid steer drivetrain can push through any mecanum drive train. I would love to see a drive train on four mecanum wheels that could outpush the tank drive on any first pick robot at CMP. If I saw one of those, I would convert to mecanum immediately, no doubt.

Re: Swerve Drive vs Mecanum Wheel drive?
 

One problem I see in this conversation is the assumption that most tank drives any given mecanum drive may be up against are all always well done. People seem to also judge said mecanum poorly based on pushing matches, why? Whoever said a mecanum was for pushing?

The advantage is strafing and maneuverability for a team that wants more dynamic offensive play and less predictable maneuvering. If you want that plus pushing power then you go for swerve if you have time, money, people for that. If your strategy doesn't need any of that then tank.

And for all you who denounce mecanum without ever driving one in fall 2011 before our team had started its' first season we were allowed to drive anther teams robot from that year in an off season competition. Despite no field-centric or PID nonsense we absolutely were able to push and deflect other robots, TANK drive robots. So in the right circumstances a rookie driver and overall FIRST rookie has actually done what you are all arguing about.

Re: Swerve Drive vs Mecanum Wheel drive?
 

Jake, it seems we've been talking past each other. None of my posts have been about the "tank pushing mecanum" hot-button issue. I've intentionally stayed out of that debate.

Ever since my post #83, I've been responding specifically to the following statement in your post #81:

I think we now agree that the above statement is false as it stands. Gearing is not the only factor which makes a difference.

The reason this is important is because not being aware of (or not paying due attention to) the factors¹ that affect the driveability (including traction) of a mecanum is a common pitfall for teams using mecanum.


¹Here's a list of some of them: http://www.chiefdelphi.com/forums/sh....php?p=1373392

Re: Swerve Drive vs Mecanum Wheel drive?
 

One problem I see in this conversation is the OP specifically asked for input from people who had direct experience with swerve AND mecanum drives, and the majority of posts here are primarily assumptions, generalizations, and misinformation.
Often from people who can't even spell mecanum correctly.
I hope the OP is able to sift through the clutter and pick out the reasoned, evidence-based data and ignore the baseless, rumor-based opinions.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I think there are very few teams that have used both Swerve and Mecanum, and it is probably a good idea to get info from teams that have used one or the other.

Re: Swerve Drive vs Mecanum Wheel drive?
 

I know I haven't built or driven a swerve but in the comparisons people are making I want to make it clear that the generalization that pushing tank with mecanum is not viable is incorrect. By that metric it is not universally always as inferior to swerve as many seem to think. And I want to make it clear I know this from actually haven driven one in a competition against other tank drive robots.

Re: Swerve Drive vs Mecanum Wheel drive?
 

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