Hi,
My team just finished the North Star Regional (#4687). This year we used standard KOP wheels, but we want to start Alpha testing new ones. What are the advantages of Mecanum Wheels versus Omni Wheels? Also is there another wheel type we should consider?
Thanks!
-Zach
Well, thatâs a pretty wide open question.
Lets start with mecanum wheels. You need 4, one in each corner of a ânormalâ rectangular robot. Power each one separately (4 motors, 4 gearboxes), and set them up so the rollers form an âXâ when looking from the top of the robot. With this setup, you can drive and turn like you do with a ânormalâ tank drive, but you can also strafe - move sideways or at an angle without rotating the robot.
With omni wheels, there are a number ways to set them up. Assuming you can keep them all level, you can set them up to get similar control as mecanum drive, by having a pair parallel to each other, and another pair perpendicular to the first (either one in each corner, turned 45 degrees, or one on each side: left, right, front, and back). You can also use them to make turning easier. For example, if you find it difficult to turn, or see that your robot bounces around when turning, having a ânormalâ traction wheel in the middle and an omni in each corner on each side will make the robot turn a lot smoother. Or if you have 4-wheel drive, having one pair of omniâs and one pair of traction (omniâs probably in the front) makes turning easy, but it puts your center of rotation towards the traction wheels.
There are other drive train configurations available, like swerve, octocanum, and apparently something my students were talking about today called âbutterflyâ⌠Iâm still not entirely clear on how that one works, but apparently they got it from another team at the regional. However, those are drastically more complex and difficult to do right.
This post goes into excellent detail about non-omni-directional wheels: http://www.chiefdelphi.com/forums/showpost.php?p=1316805&postcount=9 Though what form of traction wheels you use is less important than the kind of drivetrain, since most traction wheels are reasonably interchangable. (All AndyMark wheels use the same mount pattern.)
Mecanum wheels are only used in mecanum drivetrains (at least for driving).
Simple omni-wheels are used only to reduce scrub forces when turning tank-style drivetrains that also use traction wheels or in holonomic drivetrains where (typically) 4 omniwheels are set up in a square pattern.
To elaborate on butterfly drive as Team Neutrino has used it in the past two years:
Is this drive train each corner has a small traction wheel and a larger omni or traction wheel. By using pneumatic cylinders, a module can be moved to allow the robot to drive on either all the small traction wheels (slower, high pushing force, low maneuverability) or on the larger wheels (faster, lower force, high maneuverability).
Here is an older blog post on the modules used last year: http://www.teamneutrino.org/seasons/ultimate-ascent/robot/butterfly/
A butterfly drive is an articulating drivetrain with some sort of traction wheels paired with omni wheels (very similar to octocanum). In our configuration (which we kept off our robot for weight purposes and lack of necessity for our strategy) we have a 4" omni wheel paired with a custom made 2.5" traction wheel with rough top tread. Each module is actuated by a 1 1/16" bore 1" stroke pneumatic cylinder. We have found that butterfly drives are not overly complex but I wouldnât recommend using one without experimenting with it in the off season. The team that has really made this design famous is 3928 Team Neutrino. They had a butterfly drive for Ultimate Ascent and it worked extremely well for them. They have an even better version of it on their Aerial Assist bot. I believe Aren Hill a former mentor for Neutrino current mentor for 148 Robowranglers conceived the idea and could explain it much better than I can.
Edit: Trent B beat me to it
Didnât 148 conceive and run the first butterfly drive in 2011?
Sort of⌠but youâre off by a year.
148 and 217 built a ânonadriveâ in 2010âin fact, they collaborated on it. The butterfly was omnis and traction, and a 5th omni was mounted perpendicularly to the rest of the drivetrain (for sideways motion).
Iâd actually thought about (and tried to convince my team to do) one back in the mid-2000s I want to say '06 or so, and I know a team that did a traction/traction butterfly-type setup back in '03, so theyâve been around a while, in concept at any rate. These days, more teams have the resources to pursue 'em, and are more willing to share designs, so theyâve been getting more common.
Do mecanum wheels have more traction then omni wheels, or KOP wheels for that matter?
A favorite of ours is using traction wheels on a six wheel drop center drive. The tractions that you choose kind of varys on what you want to be doing more of. We have found that the 4 inch versawheel spike tread is really nice for good grip and still being low to the ground. The 3.25 inch diamond tread wheels are also good but wear down a bit quicker. so these are bit better for more offensive purposes.
Mecanum wheels have the least traction out of any wheel.
I was considering nonadrive as a different drive because of the 5th omni, they did not run the drive that everyone now calls butterfly drive until 2011.
For omnis, in general, yes, mecanums have more traction. General caveat that there are omni setups that would that statement would not apply to.
As far as KOP wheels, most mecanums wonât have quite as much traction. That tends to be due to the fact that mecanum traction surfaces happen to be rollers⌠However, a mecanum system can give a fair amount of pushing powerânot that you EVER want to have to USE that pushing power! In the individual wheel, the effect is more pronounced than in the whole mecanum system.
By the way, one thing that I should make clear: The above statements are for the whole drive system, not just one wheel. Thatâs something that you need to take into consideration.
Some other things to consider: Plaction wheels (or other wheels with conveyor-belt tread attached) are very good in a pushing match. Blue nitrile tread is favored, but wedgetop and roughtop tread are common. Colson wheels are reputed to give good tractionâIâve never dealt with 'em in FRC applications, so I canât comment on that.
(Side note: youâre asking about mecanums having more traction than KOP wheelsâyou do know what a mecanum wheel is, right?)
Sorry, thatâs omnis youâre talking about. Iâd take a mecanum drive over an omni drive any day.
Our main drive is omni, and weâve been able to push around mecanum bots all day, in both first and second gear (Orlando seems to be heavy on mecanum compared to Alamo).
On a side note: I have yet to see any bot that has ever made me want to use mecanum, even when we used it pretty effectively in 2010.
It is difficult to track the origins of an ideaâŚ
In the pre-season of 2010 148 build a drivetrain which combined an H-drive (5 omni wheels, 4 in ânormalâ configuration and 1 sideways) with 4 drop down traction wheels. We called this a Nonadrive, since it has 9 wheels and we really like the number 9.
In 2010 we ran this same configuration on our competition robot Armadillo.
In pre-season 2011 we prototyped an Octocanum drivetrain (an articulating drivetrain which switches between 4 mecanum wheels and 4 traction wheels). This is something which I believe several other teams had done in the past.
We didnât like the performance of the mecanum wheels we had at the time, and switched them out for omni-wheels as part of a new experiment. This configuration could not move sideways, but had some cool features we liked. We jokingly called it âButterflyâ drive since you could swat it aside like a butterfly when it was in omni-mode. I believe we were the first to experiment with this configuration.
In 2011 we ran this same configuration on our competition robot Raptor.
We ran an evolved configuration in 2013 on our robot Viper.
This year, weâre running another cool evolution of the Nonadrive which involves 2 center Omni-Wheels, which weâre calling the âDecadrive.â This is on our robot Vader.
We like articulating drivetrains. 
Weâve been playing with them since pre-season 2010, and we will probably continue to play with them. Weâre actually starting to work on a âHistory of the Robowrangler Driveâ which will hopefully turn into something cool for our website.
Perhaps youâre taking on weak mecanum robots. By saying mecanums have the least traction of any wheel, youâre making a pretty broad generalizationâand Iâm willing to bet thereâs at least one mecanum robot out there that can push you around all day and all night.
Of course, both mecanum and omni pale in comparison to a good swerve drive, but a good swerve drive isnât exactly easy to build, particularly compared to a tank/mecanum setup.
This question is loaded, I will explain the characteristics of the five common wheel types.
Mecanum wheels tend to have a grippy rubber coating on the ground contact points. They are made by having rollers mounted at a 45 degree angle surrounding the circumference of the wheel. This allows them (when rollers create in an âXâ pattern, and each wheel is powered individually) to provide a strafing motion.
Omni wheels tend to be hard plastic on the ground contact points. They are made by mounting small rollers 90 degrees to the detection of the wheels travel. They provide good forward motion (especially on carpet) but will roll sideways if force is applied ay 90 degrees to the direction of travel (not a good choice for more than 2 wheels on your robot, front wheels on a 6 wheel skid steer).
Traction wheels have a rubber tread along the circumference of the wheel. They have the second most contact with the ground of the five wheel types. They are the most commonly used type of wheel. They provide lots of traction. Need I say more?
Pneumatic wheels are larger than the others and have the most contact with the ground. Filled with air, they are similar to the wheels on your car, riding lawn mower, etc. (except smaller). As a result they are much heaver than other wheel types.
KOP wheels these are the wheels that come in the kit of parts. Created out of hard rubber, they are a balanced wheel, providing suitable traction, low weight, and the ability to be slid sideways when turning. They are a common choice for rookies.
The choice of Mecanum vs. Omni depends on your application. If you use 4 mecanums (one on each corner of the robot, you will be able to strafe. if You are going for a skid steer design I encourage you to put most of the weight in the back of the robot and lower the axil for the center wheel down 1/8" (drop center skid steer). If this is not possible then an Omni wheel in the front corners of a 6 wheel tank drive will serve you nicely. I hope this clarified everything.
Youâre right, it is a broad generalization, but I donât see the speed or responsiveness out of mecanum drives that seem to be in other drives.
Your original post specifically asks about wheels, but the wheels are only a small part of the whole - the subsystem that is the drivetrain - and Iâm gonna fill you in on every bit of it, because youâre a smart team who is going to learn as much as possible and test and iterate and design as much as possible this offseason to ensure success next year and make the most important subsystem of your robot not only the most reliable one you have, but also the one that you can assemble as quickly as possible (because time is money during the build season, and weâre gonna get you as much time as possible to work on your scoring mechanisms).
The offseason is the best time to design and try out new ideas, and those who work hard at it end up very successful in the build season. My goal after writing this is seeing yâall upload a video to chief week 1 of the 2015 build season of your chassis (whatever it may be) driving circles around your opponents.
But before we start talking about drivetrains and what they are, we gotta look at what they generally consist of. The drivetrain is a subsystem, which means it consists of many different parts. While those parts arenât always the same for each type of drive, there are a set of common elements that each drivetrain is going to share that we will be discussing: The chassis, the transmissions (which include the motors), the power transmission (method of powering wheels), and the wheels.
The chassis of a robot is most commonly one of three shapes: Rectangular long, rectangular wide, or square. What chassis shape you use is dependent on what your design needs are, but to simplify things weâre just going to say it depends on track width and wheelbase.
The transmissions are generally a collection of gears that slow down or speed up the rpm from the motors. The motors connect to the transmission input, and the transmission output is connected to the power transmission. A helpful tool for configuring what gears you should use is JVNâs Design Calculator. Also, you already know this, but Imma say it anyways - the CIM motors are to be used in the drive, and you are going to be required to have a minimum of 4 CIMs in your drive to even be competitive.
The power transmission is not necessarily a transmission in the sense of the word in the part above. Itâs essentially how the power from the transmission makes its way to the wheels. For what youâre working with, all wheels are going to be powered. The most common ways of transmitting power from the transmission are chains, belts, gears, and direct driving. Weâll go into detail on those a bit later.
Last but not least are the wheels. What wheels you use along with what diameter and width are going to be dependent on your design. This post by Alec Mataloni gives a lot of detail on the many kinds of traction wheels. Notice it doesnât mention omni or mecanum wheels? Weâll get into that later on as well. Thereâs a place for them in this world of FRC.
So first off weâre gonna start with the staple: The Tank Drive. Yes I know tank drive is the control system used for these drives, but itâs also a common way to describe drive setups with an even amount of wheel sets on each side.
Tank Drives usually consist of an even set of wheels greater than 2. Most common are 4, 6, and 8 wheel drives, though there have been more used in the past. Now the general idea of a tank drive is that half the wheels are on the left side, and half the wheels are on the right side. The left wheels all go in the same direction, and the right wheels all go in the same direction. To turn the left and right sets of wheels can go in opposite directions. I will be discussing the most common three types of tank drive: 4 wheel, 6 wheel, and 8 wheel.
4 wheel tank drive:
The 4 wheel tank drive is almost always seen in a rectangular wide chassis configuration. If you see it done any other way with all traction wheels, itâs wrong, and hereâs why: Because of the trackwidth and wheelbase ratio of a 4 wheel drive robot, unless itâs in the wide configuration, that robot ainât turning. No turn = no win (unless youâre 71 in 2002, but youâre not (nobody is). So letâs not hold on to that). I highly suggest reading Chris Hibnerâs white paper on drive train basics - he goes into great detail on the physics of turning.
6 wheel tank drive:
The 6 wheel tank drive is the most common drivetrain in FRC. Itâs the standard for the kitbot, and for a good reason. Iâm assuming youâve already read through Chrisâs paper I linked to above, so you should know about the reasoning for dropping the center wheel on each side, but in case you need a slight refresher, remember that by offsetting the inner wheels you essentially halve your effective wheelbase, greatly increasing maneuverability, yet keep the stability of a non-wide robot chassis. Now donât get me wrong, there are plenty of 6 wheel drive robots with a wide configuration chassis, but the beauty of the 6 wheel drive setup as compared to the 4 wheel drive is that you can choose any chassis configuration you want - rectangular wide, rectangular long, or square.
8 wheel tank drive:
The 8 wheel tank drive has been gaining popularity recently, and weâve been seeing more and more each year since they became extremely popular back in 2010. 8 wheel drives have the distinct advantage of making climbing obstacles easier (hence the widespread use in 2010, and later on 2012). Another favorable feature of them is that because the two center wheels on each side are dropped, the robot is able to be parallel with the ground if it needs to, something the 6 wheel drive usually cannot do. Even on a flat field, an 8 wheel drive is still a plausible solution to your drivetrain dilemma. Of course there is the obvious drawbacks of more wheels to power and more weight, but Iâve never heard too many people complain about them, so I doubt that theyâll be extremely pressing issues.
Now weâve spoken about the chassis setups for the three main types of tank drive, now weâre moving on to the transmissions. This is easy, since for the most part most transmissions will be the same. Generally the motors power some gears which reduce the speed the motors put in to the transmission to make driving easier and provide more torque (pushing power). Where transmissions really start to change is the style of tank drive, which we will discuss in a bit.
Power transmission is next, and itâs really simple. Generally the wheels are powered by either chain, belt, or gears. Chain is the most common and has been used the longest. There are two types mainly used in FRC: #25 and #35. #25 is smaller and lighter than #35, but also more prone to breaking and stretching. That being said itâs not like thatâs going to happen all the time, and I know many teams who use it with great success year after year. I personally prefer to use #35 chain just for peace of mind, but thatâs just me. I donât know everything - make your own decisions based on what you need. Belt has been growing in popularity ever since WCP and VEXPro came out with their COTS pulleys and belts. Belts rely on having a perfect center to center distance, and luckily WCP made a calculator just for that. Belt in general is more efficient than chain, and doesnât stretch nearly as much, which means a good belt drive system can be set up and never touched for the remainder of the season. Gear drive is something thatâs been around for a while, but has become popular again with 829âs recent gear drivetrain. Gear drives increase reliability by not having the liability of chain or belt snapping, and due to the nature of gears wonât require any tensioning, ever. The downside is it can get a bit heavier than most chain or belt systems, and you need to make sure your gears are correctly greased or else something could break, which usually isnât good.
Next up is wheels (wasnât that what you were asking about originally?). Tank drives only ever use two kinds of wheels: Traction wheels and omni wheels. If you use anything else in a tank drive setup (cough cough mecanum cough cough) youâre going to be hurting yourself more than helping yourself. All of the above descriptions assumed that all wheels were traction wheels, which is the general case for most tank drive setups, but some teams put omnis as the outside wheels to turn. General rule of thumb is if you have offset wheels somewhere in your drivetrain, you wonât need omnis. If you do, your drivetrain is bad. The best use of omnis in a tank drive setup is when all wheels are flat on the ground. I prefer doing this with a 6 wheel drive where only the front two wheels are omnis, and the center and back wheels are traction wheels. Some teams put both the front and back wheels as them, but unless youâre running a flat 8 wheel drive a la 1986 youâre going to be easily spun around. A question Iâm often asked is âwhy not just use omnis by themselves?â, and the answer is similar to the previous sentence. All omnis by themselves are great when youâre driving, but once you get defense played on you youâre essentially dead in the water because your lack of traction puts you completely under your opponentâs control. That being said, those previous few linesdonât apply to 33 this year. Theyâre magic, and an exception to the rule (just because they do it doesnât mean you should).
So thatâs everything on tank drives, right? Nope. There are still different styles of tank drive. Now I could go on all night about the different styles there are, but Iâm only going to focus on two: Kitbot (pre-2014) style and West Coast Drive style. The kitbot before 2014 was a c-channel chassis where the wheels were on bearings and chained (or belted) to the output shaft of the tranmission, which was usually mounted on top of the c-channel. In drivetrain terms, it was a dead axle system (axles the wheels rest on do not move) with fixed positions and wheels supported on either side. The West Coast Drive (WCD) is a tank drive system designed and popularized by Teams 60 and 254 where the center wheel is directly driven by the output shaft of the transmission, and that shaft is connected to chain or belts that drive shafts that the outer wheels sit in (live axle). All wheels are cantilevered (only supported on the inside), and are on tensioning systems to ensure each chain/belt has the best tension in it. You can read more about WCDâs in various threads here on Chief. If you see any posts from me on them prior to 2013, ignore them - I didnât know anything back then (and still donât, to a point).
Phew. That was the big one. Everything from here on out should be a lot shorter.
Iâve seen mecanums go in any configuration, and from what Iâve seen they all perform terribly regardless of the setup, so it really doesnât matter.
Mecanum drives require 4 separate transmissions - one for each wheel. Most of the time each transmission will have one motor in it. More weight than most teams need is a downside. Have yet to hear of an up side.
Power transmission is pretty much the same as usual. Direct drive, chained, belted, geared, live axle, dead axle. All are gonna make you wish you went with something else.
Wheels are mecanum wheels if your intent is omni directional ability. If you have a mecanum drive setup with non-mecanum wheels, youâll surprisingly do better.
Thatâs it for mecanums. I know there will be people who disagree with me on this, but hey - every other drive type here (excluding slide) has won world champs. The stats donât lie.
Slide Drive or Holonomic Drive:
Donât use these.
Now this is what Iâm talking about. Clearly a crowd favorite, the swerve drive combines omni directional motion and high pushing power into a beautiful system that - when used right - is essentially the king of all drive systems. And of course itâs too good to be true. Thereâs a reason you donât see every team out there with a swerve drive. While theyâre amazing systems, theyâre incredibly complex and resource-intensive. Swerves require intense coding, custom setups, and a lot of driver practice. Now donât let that deter you. I know a team who a long time ago started trying out swerve and they kept on improving it year in and year out - every season and every offseason - until they became famous for their amazing swerve system. Last I heard they were world champs in '12. Just goes to show if you put a lot of hard work and iteration into a swerve drive, it can be a great advantage to your team.
As expected, the nature of swerve allows for any shape chassis. Go nuts.
The transmission and power transmission change depending on if itâs a coaxial swerve or a traditional swerve (most commonly called the 111 swerve). In the coaxial model, the power flows to from the CIM and the gearbox to the wheel through a bevel gear. This means that the axle that powers the robot is inside the axle that turns the module. The traditional model has the CIM turn with the module, which may allow wires to get wrapped up (unless youâre 16).
Iâm not going to go into too much detail on swerve, because Iâm honestly not an expert on it and I think it would be better if you got your information from a more informed source on the subject. Iâd suggest reading these threads if you want to learn more, and try talking to someone from Team 16. They are the kings of swerve.
The âjump drivesâ, or so they have come to been called, have been recently new designs that originated with 148âs Nonadrive back in 2010. These systems work by using pneumatics (or another motion source) to change which wheels in the system touch the ground. The drives generally have 4 sets of wheels - each set containing a traction wheel, and either a mecanum wheel or an omni wheel, respectively. These drives are designed with the intent of having both high maneuverability and high pushing power, and being able to switch between the two at will. In an octocanum setup, there are 4 modules - one in each corner - each with a traction wheel and a mecanum wheel. Each module is powered by its own motor and transmission - if it has a transmission. The same is true with Butterfly, except replace the mecanums with omni wheels. The chassis can be any shape for these systems, but in my opinion the most useful setup is a rectangular long chassis, which lets you abuse the pushing power of the traction wheels. While the power transmission in each design varies, one that Iâve come to like is where the CIMâs pinion drives a gear that is directly attached to the omni/mecanum wheel on one side, and on the other side is a chain or belt that drives the traction wheel. Because of these multiple stages, you can run the two wheels in a module at different speeds. It is common to abuse this and make a built-in shifting mechanism, where your mecanum/omni stage is a high gear, and your traction wheel stage is a low gear. Since the idea is the same for both Butterfly and Octocanum, learning about any single one will help with knowledge on the other. I suggest reading 3928âs page on their Butterfly drive posted on their website. Also if you want to see the effects of butterfly in action, 973 prototyped one in the offseason of 2013 and played it at some offseason events. This match shows the features off pretty well (watch red robot 9973).
Before ending this post, I want to write a bit about shifting transmissions and 6 CIM transmissions. In case you donât know, shifting allows you to have a high and low gear that will let you move quickly across the field and push through defense if you need to. This versatility is always a useful tool to have at your disposal, but not always necessary. Likewise with the new rules on CIM motor and Mini CIM motor allotment, it is now possible to have drivetrains with more than 4 CIMs in them. This provides more available torque for pushing and slightly better acceleration. Be wary of tripping the main breaker, though, if you use 6 CIMs in your drive. It has happene dto many teams before. Also just a note that while these two transmission options can be used in any drivetrain setup, they made their best use in tank drives.
I hope this has been useful and informative. There is a whole bunch more Iâve left out and simplified to make this easy, so if you have any questions feel free to ask me or anyone else on this website. Iâm looking forward to seeing what you and your team make in the offseason.
So with that being said, remember: Simplicity wins matches, COTS items are your friend, and in the grand scheme of things, your drivetrain isnât getting you points, so donât spend too much time on it. If it takes you more than 2 weeks to make in build season, itâs not worth your time.
Unrelated, but in that video Andrew posted its cool to watch both 9973 and 8973. They seem to be the exact same two practice robots and for the first three cycles they are within a few seconds of each other. By the end they did the same number of cycles but 8973 missed out on the hang because they went for one more cycle. You usually dont get to see a comparison of the same robot with different drivers.
