Mecanum or Swerve?
 

During the offseason this year, my team has discussed developing swerve drive, but after seeing a number of teams use mecanum with great sucess, I am torn between the two. Mecanum seems simpler and lighter, while swerve seems to give better control and more pushing power/resistance to being pushed. What are the experiences of the rest of the FRC world?

Re: Mecanum or Swerve?
 

I think the choice between mecanum and swerve lies in where your robot is best suited to play (offense/midfield/defense).

Offense: Swerve
From watching quite a few matches, swerve seems alot better suited to collect balls and maneuver them into the goal.

Midfielf: Swerve
When your playing the midfield balls constantly keep entering, so you need to be able to quickly "pick up" balls from your zone. And if your in the midfield, theres a higher chance of you needing to go over the bump. So if you use mecanum it seems you need to drive straight to go over on mecanum, but you can go also sideways with swerve over the bump

Defense: Mecanum
Mecanum seems better here because its alot easier to get from goal to goal and just block the goal from other robots scoring.

Re: Mecanum or Swerve?
 

I say go for the 4 or 6 legged robot. First because with 6 legs, 3 legs are ALWAYS touching the ground (Of course unless its falling off a cliff) minimum of 3 legs needs to be on the ground to be stable. Yes I do mean LEGS... Its more adaptable then wheeled vehicles. I mean look at the Protoss Colossus

Re: Mecanum or Swerve?
 

In My personal opinion, Swerve > Mecanum > Holonomic, and this goes for just about any game.

If you've got the resources, both Manufacturing and Programming, go with a swerve. Swerve drives have the traction advantage over all of the other omni-drives out there. Swerve drives can be complicated to build and drive though, so be careful (Though, you could look into the team 221 swerve modules). Personally, if I ever built and omni-drive for any game I would build a swerve.

Mecanum Drives are probably the most accessible form of omni-drive out there. You can use the kit frame, AM Mecanums, and some off the shelf transmission and have a reliable Mecanum drive. Mecanums lack the traction of a swerve or traditional drive so watch out. Mecanums can climb decently though, so this is an advantage over a Holonomic.

Holonomic Drives are interesting. They're somewhere between a swerve and a mecanum in terms of build difficulty, due to the fact that the wheels must be mounted 90* from one another. (or 120*) They also can be a bit of a handful to drive, and they REALLY don't like un-level playing fields. Holonomic drives also don't push well.

Re: Mecanum or Swerve?
 

We had looked into the 221 modules, but had decided that with our small team (12 members) it would be better to have an operable robot with a 6wd than a swerve that did nothing. We also only have 1 programmer and build in a basement, so our manufacturing abilities are very limited.

This was my primary reason for wanting to explore Mecanum...but it seems as though swerve offers better performance if it works right

Re: Mecanum or Swerve?
 

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

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

Re: Mecanum or Swerve?
 

No - at least not by our scouting methods. Mecanum robots are immediately removed from our defensive pic list, as are robots with slick wheels / omni wheels. A decently geared robot with traction wheels will have no problem moving a robot with slick/omni or mechanum out of their way and scoring.

Mechanum are a neat idea (as are omni wheels), but once you bring robot to robot contact into the equation, I'd much rather have robots with traction that won't get pushed out of the way easily.

Regarding any lag time with swerve modules: If you have it programmed correctly, your swerve modules should never have to turn more than 90 degrees from any given point. When you keep in mind that a tank drive has to "turn" before it drives forward, there really is no lag in a well-done swerve drive when compared to a tank drive. When you take into account acceleration time, there really isn't any lag in a swerve compared to a mechanum either.

You may want to note, however, that except for a few teams, most teams do NOT do swerve every year. Even teams that have done swerve, generally don't repeat it much. That's because it takes so much time, machining, programming etc to make it work well.

Swerve is that thing that every team has to try at least once. They try it, they may win a couple engineering awards with it: then most teams rarely do it again.

Re: Mecanum or Swerve?
 

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

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

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

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

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

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

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

Re: Mecanum or Swerve?
 

In my six years of competing in FIRST I don't remember a single mecanum robot ever dominating a competition.

The powerhouse teams have built swerves, tank-drives, and more unusual designs like 71's shuffler and the 148/217 nonadrives. But I can't remember any of the top teams EVER using mecanum.

I can only think of a few cases in which a mecanum robot even won a competition.

Why is this?

Is it simply two difficult to write controllable code?
Are they just too inefficient?
Not enough lateral traction?
Or has the mecanum drive simply never been perfected?

I've seen those mecanum forklifts, I know what the technology is capable of...

Re: Mecanum or Swerve?
 

I have to disagree. This is the third year my team has done mecanum drive, and we were able to play defense well. Defense doesn't necessarily have anything to do with pushing if you're maneuverable enough.

If you want to find an holonomic drive that also has pushing power, you might try over the summer looking into mecanums that have a mechanism which locks all the mech rollers.

Re: Mecanum or Swerve?
 

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

Re: Mecanum or Swerve?
 

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

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

-Nick

Re: Mecanum or Swerve?
 

But you can strife with regular omni.

Re: Mecanum or Swerve?
 

While true, a robot that can push is almost always more defensively useful than a robot that basically can't. I'm a pretty strong advocate of "defense != pushing", but I'd still rather do it with an XWD.

Except you lose significant efficiency in most directions, when a slide drive lets you make the primary direction of travel more efficient than the others. For example, if you have 5 CIMs, you can have a 100% efficient 4 cim forward drivetrain, while only using CIM 5 to go sideways. With a holonomic drive, you are stuck with 4 cims and the inherent efficiency losses that come with them.

Re: Mecanum or Swerve?
 

Mecanum and Holonomic involve great loses of power that drives like Swerve tend to provide for you. But swerve drives require weight and a certain skill level. A lot of teams this year saw that they had the need for the maneuverability, but not necessarily the experience/resources for a swerve, and mecanums offered an easy solution.

Re: Mecanum or Swerve?
 

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

Re: Mecanum or Swerve?
 

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

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

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

Re: Mecanum or Swerve?
 

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

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

Re: Mecanum or Swerve?
 

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

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

Re: Mecanum or Swerve?
 

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

Re: Mecanum or Swerve?
 

This year we built both drive systems.

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

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


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

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

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

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

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

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


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

EDIT: Woah this post got long quickly

Re: Mecanum or Swerve?
 

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

Re: Mecanum or Swerve?
 

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

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

Re: Mecanum or Swerve?
 

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

Re: Mecanum or Swerve?
 

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

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

Re: Mecanum or Swerve?
 

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

Looks pretty cool, but is it really worth it?

Re: Mecanum or Swerve?
 

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

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

Re: Mecanum or Swerve?
 

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

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

Re: Mecanum or Swerve?
 

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

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

Re: Mecanum or Swerve?
 

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

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

Re: Mecanum or Swerve?
 

I'm not so much saying that an offensive robot needs to push other robots, just that a slippery bot is more susceptible to being pushed into a corner or into an unfavorable position. Anyways, I think our ideas of offense are different. We play mostly from the middle zone, and that means competing for the space under the ball return and fighting for individual balls and clear shots. We're a 10:1 plaction wheel swerve drive, btw.

Re: Mecanum or Swerve?
 

Easy answer. We knew from our testing and from past experience that a robot defending you was going to try to push you all over the place.

Exactly that happened in the Michigan State Championships to us. Thunderchickens tried to play defense on us. They have traction when they want to, and can push very well. We out-pushed them somewhat and managed to wedge them in the goal for a short time, but they still kept our robot down to scoring only 5 balls.

Their drivers understood that to keep the other guy from scoring, you simply push his back corner so he can't aim. Or push him into the wall so he can't turn. Or pull in front of the goal if he can't push at all, then he's done.

Mechanum and omni look great as concepts, but when the rubber hits the road all it takes is one good traction bot to completely shut them down.

Re: Mecanum or Swerve?
 

It's easy to program "relative" absolute mecanum/holonomic code, as in you push the joystick left and the robot moves to the left relative to the robot. This method does not need any math functions other than elementary operators (like +-*/).

Programming "absolute" mecanum/swerve, as in you push the joystick left and no matter what way the robot is oriented it moves left relative to how the drivers are facing, is more difficult. This latter case involves the use of a gyro to track current robot heading, and a lot of trig functions for each of the wheel outputs.

Re: Mecanum or Swerve?
 

Control wise, swerve drive is extremely intuitive to me having play a LOT of FPS video games. The past 2 years we have had 2 2-axis joysticks for the driver (2 more for the co-driver), left hand controls all x/y translational movement, right hand controls rotation. Anyone who has played Halo before should be able to drive our robot.

Having done swerve 2 years in a row I might be a little biased, but the programming for a swerve drive seems quite simple to me if you approach position control the right way. We encapsulate a potentiometer and a motor into an object called a "steering motor", which is completely self contained with all of its own control and PID code; you have a goToAngle(angle) function that you call, and then the steering motor object takes care of everything else. We used 2 steering motor objects for our drive train this year and last, only changing port and PID constants for the most part.

The great advantage about encapsulating a motor and potentiometer for position control is that its usable in many places on robots(turrets, swerve modules, kickers, arms, etc) and the code is completely plug-n-play (aside from tuning PID constants and maybe limiting the possible angles). We used the exact same code to control the angle of our turret and drive train last year, with the desired angle for the PID code coming from the camera(or joystick under manual control) and joystick respectively.

The is the main reason why I made this post is to encourage teams who might be afraid of complicated PID implementations everywhere. If you do it once correctly, you won't have to do it again. Simplifying a few things slightly, you can always change the steering motor object around a little bit, and abstract out its desired angle so you can give it desired encoder ticks instead and use a driving motor instead of turning, and now you have PID distance control.

Re: Mecanum or Swerve?
 

After our experience this year, I fully agree - our mechanums were not that reliable (the 6" andymarks weren't strong enough to get bashed sideways against the ramp repeatedly), we didn't strafe that often, and we were easy to push around.

Also, we had 1114/2056's 8WD tank drives at both our regionals demonstrating that maneuverability is more about a well-controlled high speed drivetrain and less about how many directions you can move.

Re: Mecanum or Swerve?
 

Maybe our is too over-the-top but we use a few trig functions and things as well in our algorithm. In 08 on the old control system we even had to develop our own angular unit (b-rads!) and define trig lookup tables to provide a semblance of efficiency :)

It's all how you look at it, I think. I was the student developing our first ever algorithm in 07 that split the joystick area into 16 "control zones" with hardcoded values that just got scaled based on how hard the joystick was pushed. Our current algorithm is completely dynamic based on the positions of our translation and rotation sticks.

1675 has used mecanum in 07, 08, and now this year. We are happy with our home-brewed algorithm except for the fact that we never quite get the chance to slap encoders on. (And yes, our drivers do strafe :) )

I would like to try a dropped 6-wheel one year though...

Re: Mecanum or Swerve?
 

TANK DRIVE MECANUM SEPARATION OF VARIABLES
http://www.chiefdelphi.com/forums/sh...94&postcount=1

3-AXIS JOYSTICK MECANUM ALGORITHM
http://www.chiefdelphi.com/forums/sh...383#post916383



~

Re: Mecanum or Swerve?
 

More difficult, but not all that much so for a mecanum/holonomic. 2077's field-relative stick code looks like:
It's interesting to listen to the emerging consensus (with which I don't disagree) that a swerve drive wins virtually all the pure physics comparisons, but comes off worst or near-worst in:

• Hardware Complexity
• Weight
• Cost
• Reliability
• Development Time
• Software Complexity

In other words, practically every engineering factor other than pure physics. Don't forget making a robot is engineering, of which theoretical physics is a part, but only a part.

Re: Mecanum or Swerve?
 

Well, if you make a simple numeric list of upsides and downsides, swerve will look bad. I think it's unanimous that the "physics" advantages are HUGE, not just theoretical. All of those other difficulties can be worked through, but it's rather difficult to "work through" mecanum's on field tradeoffs versus a swerve's.

Re: Mecanum or Swerve?
 

I thought a bit more on this Subject last night, and came to the realization that Swerve Drives are going to become more and more common in FRC. One of the biggest issues with Swerve Drives is that they're difficult to design and manufacture, and many teams could spend a whole build season doing this and not get it right. But, now that two types of swerve module are available as COTS items these two issues are almost non-existent, or no more so prevalent than in a Mecanum or Holonomic Drive.

If a team were to use the Commercially Available Team 221 Swerve Modules, then they'd really only have to build a frame in which to house them, and a system to steer them. From there, it's pretty much just a control issue, and I bet most teams could figure out how to get them working relatively easily. We've already seen a handful of teams use the Team 221 "Wildswerve" modules with some degree of success (11, 20, 234) and I've yet to hear of any failures.

The only downfall to using an off the shelf Swerve Solution like the Team 221 Modules is cost. A set of 4 Swerve Modules will set you back about $1k, whereas a set of Mecanum Wheels and 4 Transmissions should run you in the ball park of $700 or so. IMO, the cost premium is worth it, especially if you're a team that plans on using an Omni-Directional Drive to it's fullest potential.

Strangely enough, all of this thinking is leading me towards possibly pursuing a swerve drive for the 2011 season, should the game call for it, and I was always one of those "Why not just use a skid steer" kind of guys.

In conclusion, building a reliable and effective swerve drive, seems to be getting easier by the year.
(Sorry for the long post, I had a lot of thoughts to get out.)

Re: Mecanum or Swerve?
 

Both 11 and 20, despite building practice robots, had code glitches at their first regionals, and didn't really get their swerve drive code up to par until their second event. These are two teams with excellent programming teams as well. I'd be hesitant to extrapolate from there that "anyone" can do it; apparently control is harder than the build.

I'll let you know after I prototype something this summer ;)

Re: Mecanum or Swerve?
 

This is the reason we wanted to introduce commercial swerve components to the market. There is always lots of discussion amongst the community about how to keep FIRST competitive....

I disagree with making the game easier or artificially leveling the playing field through other means.

I'd much rather see a large portion of teams building high quality robots by using off-the-shelf items when necessary to bolster an area of their team where they may not have the expertise.

That said, swerve is tough to pull off even if you start with pre-made transmissions. Cyber Blue is hosting a swerve discussion this week in Atlanta about the months they spent leading up to competition developing the technology. :)

Re: Mecanum or Swerve?
 

I'd like to ask this question on swerve drives: From what I've read, they typically don't have completely independent control of all 4 wheels, and perhaps there are choices and trade-offs on how exactly to control them and how many motors to devote to steering. How smoothly do they end up turning in place or making a tight turn for example? Does this involve wheels slipping? What is the experience on what level of control is helpful?

My experience with Mecanum is that it can turn an average-resource team into a competent scorer and move them up to seeded range, at least for Breakaway. With the included holonomic, gyro, and PID VIs programming is easy, and implementing field-oriented steering and gyro stabilization is also easy.

Re: Mecanum or Swerve?
 

but, most tank drives use only 2 drive motors, whereas omni and mechanum ALWAYS use 4, so there is pore power being delivered, despite the 30% decrease, we have mechanum, and were able to push around tank drives like it was notheing

Re: Mecanum or Swerve?
 

Shedding some light on programming of mecanums. 2337's uses a simple version of linear interpolation. The 4 cardinal directions were declared in an array. We then look up which two to use based on which angle we want to move at and interpolate between the two. Pseudo-code below:

Turning is done by specifying a rate of turn and then multiplying the left motors by that percent and the right motors by the negative of the percent. (This part could be backwards, I can't recall)

This was written after we looked at the ugly mess of sines and cosines that did the same thing. We may have more than the 4 cardinal directions and may divide by less than 90 but I can't recall.

Not hard and allows for field centric controls given a good way of determining orientation.

EDIT: Yes, I left out the fact that you need to add the matrices but really it isn't hard.

Re: Mecanum or Swerve?
 

The hard part is not the hardware. The devil is in the software. I admit, after 7 years of FRC and countless off season projects I am still intimidated by the thought of having to code a functional swerve drive during the build season.

What Tank Drive bots are you looking at? Ive seen some using 6 drive motors and most using 4.

Re: Mecanum or Swerve?
 

While I haven't seen any first omni robots using less than 4 wheels. There are omni-wheeled robots with 3 wheels. In the past we've considered using a 3 wheel setup.

Re: Mecanum or Swerve?
 

I would tend to disagree. Full mechanical designs for swerves (or the pictures to allow you to easily reverse engineer them) have been around for years. I believe 217 posted a cutaway of their serve from 5 or more years ago. Those teams are more than willing to lend help to anyone who needs it as well.

However, you still see a lot of teams who have already DONE swerve not using them going forward. Nearly every big name team has done swerve several times, yet most do not pick it as their drivetrain of choice.

Why?

1. Complexity. Both mechanical and programmatically.

2. Advantage gained. I would put forward that last year and this year are two of the biggest swerve-advantaged games so far. Yet if you look at the top tier of teams in OPR, the proportion of swerve teams comes no where near 50%. Swerve has a huge number of trade offs, and the advantages are actually questionable nearly every year. You'd have a hard time telling me that 67, 469, and 1114's lack of swerve this year is hurting them. I bet the choice not to do swerve HELPED them in a big way during the build season - it was that much more time to work on their ball handling systems.

3. Cost. Whether you purchase a turnkey system like the 221 one, or whether you build your own, there is no small cost in engineering, materials, and machining.

I keep pointing out and I will continue to point out that Swerve is cool, but most teams that build it realize they'd rather be spending their time solving the game rather than solving a drivetrain, and a 6-wheeled or 8-wheeled drivetrain will perform nearly as well in most applications, and better in many.

Re: Mecanum or Swerve?
 

Very few respectable tank drives use only 2 motors, and very few use relatively low traction wheels like KoP wheels that your mecanum drive could push. I guess if they used the wheels in the KoP you could push them around, but that's hardly the level of competition you have to play defense against.

A mecanum drive is limited by traction faster than a tank drive, thus, it will have less "pushing power" in most situations.

Re: Mecanum or Swerve?
 

What is of most interest to me:

217: Swerved in 2003. Hasn't done it again. (Nonadrive is the closest they've come)
67: Swerved in 2005. Hasn't gone back. (This was a cool flop bot swerve though)
1114: Swerved in 2004. Hasn't gone back.
33: Swerved in 2005, switched to different drive train halfway through the season. Again in 2009: Maybe?
71: has been using swerve since 2005. (Of particular interest: Hasn't won a Championship since 2004)
111: even I don't know how long.
68: 2008,2009,2010. This year their swerve drive was too much of a technical undertaking for them.
148: Swerved in 2008. Hasn't gone back since.

So many veteran teams have tried swerve and gone back to traditional 6wd/8wd machines. 2009 was not a FULL swerve, only 2 wheels actuated as far as I can recall.

67, 6wd the last couple years. 8wd this year.
1114, 6wd. 8wd this year
33, 6wd.
217, 6wd (excepting this year)

I know correlation does not imply causation but I have a hunch that there is a reason why none of these teams have gone swerving again. It may be a cool thing, and definitely a design every team should have in their arsenal, but in most cases it is not the most efficient design.

Re: Mecanum or Swerve?
 

Even though full designs for swerves had been posted for years, most teams lacked the resources to build one. The same thing was true for Mecanum wheels until 2007. There were a few teams who made their own in 2005 and 2006 and posted designs and research on them, but most teams lacked the ability to make the wheels and chassis. Then, in 2007 AM came out with their own line of Mecanum wheels and a huge number of teams began using them. I think the same thing will happen with Swerve Drives in the next few years with the introduction of the Team 221 Swerve Modules.

Re: Mecanum or Swerve?
 

Yes! In the pure forward and reverse directions, a mecanum wheel's pushing force is limited by traction, not by a vector reduction in the amount of available force.

Even if a mecanum wheel's rollers were made of the exact same material as a standard wheel of the same wheel diameter, the mecanum would lose traction before the standard wheel would.

This is because the reaction force of the floor (carpet) on the mecanum wheel's roller is larger than the reaction force on a standard wheel, given the same driving torque on the wheel.


~

Re: Mecanum or Swerve?
 

One very slight correction. 67's 2008 overdrive bot actually had rear-wheel-only pseudo swerve. If you watch their matches that year, it will suddenly dawn on your how they can turn their corners so quickly.

Re: Mecanum or Swerve?
 

IIRC from reading an old thread, the back wheels were unpowered, just turreted.

Re: Mecanum or Swerve?
 

Ah yes. I forgot that. This is outlined Here

Re: Mecanum or Swerve?
 

i was talking about true omni drive, the holonomic kind

Re: Mecanum or Swerve?
 

then they must be using Fisher Price motors to drive, because we're only allowed 5 cims

Re: Mecanum or Swerve?
 

Last time 111 didn't build a swerve they won both their regionals.

The same is true for 1625.

hmmmmm...

Re: Mecanum or Swerve?
 

We're using 8wd tank drive with 6 motors this year (4cims and 2 FP's)


Any team only using 2 cims in the drive this year is seriously hurting themselves

Re: Mecanum or Swerve?
 

Wow! Did you ever get a lot of responses, quickly, on this one thread. You'd think you had posted a game hint for next year, or something!

But you asked two questions... one was:

"What should we build?"

and the other was "What does your team prefer?"

Those are two entirely different questions.

Frankly, I really don't think that the "What does your team prefer?" question is relevant to the choice that you have to make. First of all, very, VERY few teams or people posting here will have built and worked with BOTH mecanum and swerve drive systems. Many people will have SEEN both systems in action... and a few will have built and worked with both... but very few will be able to give you a first hand opinion on what they prefer.

What you need to consider is WHY you want to build a robot in the off season. Do you want to build it to work on design and machining skills? Do you want to improve programming skills? Do you just want to get first hand experience with at least one form of omni-directional drive? Do you want to have a cool demo robot? All of the above?

We have built a mecanum... but not a swerve. It was pretty easy to build... but took a bit of work to program (at least if you want 4 wheel PID speed control on an IFI control system... the cRio should make it a bit easier.) Unless you choose to build your own wheels, or develop a fancy suspension system, a mecanum drive is a very simple build challenge... particularly if you use a direct drive from either a Banebots or AM gearbox.

Outside of the discussions surrounding FRC competition robots... which are really kind of irrelevant to an off-season build project, the #1 advantage of a mecanum drive is that 99% of the people on this planet have no idea what a mecanum wheel is... and aside from a brief shot of a forklift on the recent Star Trek movie... have never seen one, either. Think about that... these wheels fit in a STAR TREK movie! Honestly, there is not much that is cooler from a teacher's viewpoint than watching a grade 10 explain to a P.Eng how your wheels work.

If you're looking for a bigger machining challenge, however, a swerve has all sorts of intricate parts that need to fit together just so. Sure, you can buy some COTS parts now to make that easier... but you are still working on a more mechanically complex system.

That is the reason we have avoided swerve up until now... we just don't have the manufacturing resources (mostly human resources... we've got the machines...) to confidently put together a good working swerve during build season. It would certainly be less daunting a task if we had built one as an off-season project.

But your team needs to think about why you want to build this thing... what you want it to be able to do... how much you want it to cost (in terms of money AND time invested in it) and then go with the machine that will make your team a better team.

Who knows... we might be back on regolith next year... maybe we'll have to climb stairs, or maybe the field will be made of corrugated iron. Or maybe wheels will be outlawed entirely.

Focus on the team, not the machine, and you can't go wrong.

Jason

Re: Mecanum or Swerve?
 

The closest we've come to swerve was last year when our rear wheels were mounted on a small powered turret.

Re: Mecanum or Swerve?
 

I disagree. The first year 228 did swerve was the first year we won a regional.

Re: Mecanum or Swerve?
 

correlation != causation.

Just because your team won their first regional with a swerve design, doesn't mean that it's a good system. There are way to many variables that also have just as major effect on your regional win. The quality of drivers, your opponents, other mechanisms on your robot, the game itself, luck. There is simply too much room for other explanations.

That said, I've very little interest in swerve or mecanum drives. Sure, they're cool to prototype, but unless a team has many years of experience driving one (read: practicing with a swerve bot in the off-season), has the machine shop and hands to build it near perfectly, the coders to ensure it's working flawlessly, the pit crew to ensure it's maintained constantly, and a host of other things, I just don't see them as that great an asset. Sure the occasional team like 111 or 71 will make swerves work, but honestly, is it really worth the upkeep? Some may say yes. I say go with what's solid, can be easily maintained, can be easily adapted, and doesn't require two joysticks to control. And that, is a 4 wheel, 6 wheel, or 8 wheel drive. Let the creativeness show in how you play the rest of the game with your manipulator.

Re: Mecanum or Swerve?
 

That's certainly one point of view, and perhaps held by many teams and individuals.

There is another valid point of view however, a point of view which puts less emphasis on the "game" and puts more focus on learning. There is so much math and physics and engineering to be learned and so much creativeness and discovery can result from striving to understand and build a swerve or mecanum drive. Torques and force vectors, vector addition, trigonometry, bevel gears, software algorithms (closed-loop position control for the steering, closed-loop speed control for the wheels, how to properly adjust each of the wheel speeds - and directions for swerve - to reduce scubbing and maximize efficiency), the list goes on and on. So even if you don't win the game, you may come out ahead :-)


~

Re: Mecanum or Swerve?
 

Correct me if I'm wrong, but from what I understood you locked your swerve modules in place and skidded in elims at WPI.


I did the math last year about swerve percentages, but there have been 4 teams to win the championships with a swerve. (111 twice). In total 42 teams have won nationals. That's 9 percent to win with swerve. People often question "more teams compete with drivetrains other than swerve, so the odds are swerve won't win as much" but in reality, the best teams win, or at least really good teams do. If swerve teams consistently dominated skid teams, it would reflect in the stats, and they don't reflect that.

I'm not saying swerve is bad, I'm just suggesting that the advantages it appears it have on paper aren't quite the same as on the field.

Re: Mecanum or Swerve?
 

Yes, many teams use the FP through an AM planetary to mesh with a CIM motor.

There are exceptions, for some teams swerve works really REALLY well. Their drivers just get the hang of it.

Just to share a bit of a conversation I was having the other day, the problem with swerve drives is that the wheels are never in the direction you want to go. There is a slight delay. If the driver is aware of this and does not try to correct for it you will be fine. Otherwise you end up going in a bit of a circle. It takes some getting used to. Not a downside just a fact. If you decide to go with a swerve drive robot you need to build a practice bot so your driver can get the hang of it.

Re: Mecanum or Swerve?
 

Have you not seen the number of Mecanum drive bots this year with unbelievable pushing power? Back in Detroit the only bot with more push than us was the Juggernauts.

Re: Mecanum or Swerve?
 

That's an interesting and one hundred percent wrong rumor.

Actually, in this case our swerve drive was the reason for our success. Our alliance had two strong offensive robots (230, 20) and we were the strong defensive robot in the opponent's home zone. Without a swerve drive, we would never have been able to defend two robots simultaneously by constantly strafing, out-maneuvering, and out-pushing them.

Re: Mecanum or Swerve?
 

Hi Tom,

Is this true only for swerves with unlimited steering rotation, or is it also true for swerves with limited steering rotation?


Could you please explain what you meant by the above?


~

Re: Mecanum or Swerve?
 

For us, the reason we wanted to build a swerve drive is that we had trouble with our 6wd drop center wheel this year, so we wanted something that would offer more mobility without suffering in traction. We also had considered getting the 221 modules at the beginning of the year, but decided that our ONE programmer was going to be busy enough so we would wait for the offseason. I am personally more and more convinced that we should do swerve, and that we should be able to mount our modules on the bottom of our frame from this year after we take the kit wheel brackets off. It is interesting how different teams use swerve then never use it agian, but I agree with whoever said that driver practice, and lots of it, is how to get comfortable, and therefore good, with swerve.

Re: Mecanum or Swerve?
 

I really like the swerve drives over mecanum drives.
I am working on a cad of an offseason swerve system right now.
It should be posted soon.

Re: Mecanum or Swerve?
 

If you could PM me when its up, that would be great!! We have been stuck with the 221 modules because we lack the resources to design one ourselves

Re: Mecanum or Swerve?
 

I'm not exactly sure how my team did this year but we dominated FIRST Overdrive in 2008 with a mecanum robot, before FIRST Nationals we had a 30-1-0 record and two regional victories. It was an incredible design for turning around that track as we could essentially rotate around are front left wheel to make incredibly sharp turns. It also helped that we had a driver that also flew RC planes at a national level.

I understand that Lone Star and Bayou are considered to be "easier" regionals but we controlled that competition from our very first match. At nationals we had some code and design issues that prevented us from success... We gave in to the temptation to fix something that was already working.


But overall I love the mecanum drive train and watching one work is a beautiful demonstration of force vectors :)

Re: Mecanum or Swerve?
 

This year, we had a tank drive with 2 omni's in the back, so the robot would turn around its front (where the ball is), and we only used 2 cims. We could have added two more, but two seemed to be working fine; we could push well, and we could move around okay. We didn't end up adding two more because of the added weight (which would slow our hanging down).