Omni Wheels Vs. Mecanum Wheels

[EricH]

Originally Posted by Abhishek R

Our main drive is omni, and we've 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).

Perhaps you're taking on weak mecanum robots. By saying mecanums have the least traction of any wheel, you're making a pretty broad generalization--and I'm willing to bet there's 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 isn't exactly easy to build, particularly compared to a tank/mecanum setup.

[Skyehawk]

Originally Posted by RunZach

Do mecanum wheels have more traction then omni wheels, or KOP wheels for that matter?

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 "X" 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.

[Abhishek R]

Originally Posted by EricH

Perhaps you're taking on weak mecanum robots. By saying mecanums have the least traction of any wheel, you're making a pretty broad generalization--and I'm willing to bet there's 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 isn't exactly easy to build, particularly compared to a tank/mecanum setup.

You're right, it is a broad generalization, but I don't 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 I'm gonna fill you in on every bit of it, because you're 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 we're 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 y'all 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 aren't 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 we're 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 JVN's 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. It's essentially how the power from the transmission makes its way to the wheels. For what you're 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. We'll 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 doesn't mention omni or mecanum wheels? We'll get into that later on as well. There's a place for them in this world of FRC.

So first off we're gonna start with the staple: The Tank Drive. Yes I know tank drive is the control system used for these drives, but it's 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, it's wrong, and here's why: Because of the trackwidth and wheelbase ratio of a 4 wheel drive robot, unless it's in the wide configuration, that robot ain't turning. No turn = no win (unless you're 71 in 2002, but you're not (nobody is). So let's not hold on to that). I highly suggest reading Chris Hibner's 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. It's the standard for the kitbot, and for a good reason. I'm assuming you've already read through Chris's 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 don't 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 we've 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 I've never heard too many people complain about them, so I doubt that they'll be extremely pressing issues.

Now we've spoken about the chassis setups for the three main types of tank drive, now we're 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 it's 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 it's not like that's 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 that's just me. I don't 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 doesn't 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 that's been around for a while, but has become popular again with 829's recent gear drivetrain. Gear drives increase reliability by not having the liability of chain or belt snapping, and due to the nature of gears won't 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 isn't good.

Next up is wheels (wasn't 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) you're 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 won't 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 you're running a flat 8 wheel drive a la 1986 you're going to be easily spun around. A question I'm often asked is "why not just use omnis by themselves?", and the answer is similar to the previous sentence. All omnis by themselves are great when you're driving, but once you get defense played on you you're essentially dead in the water because your lack of traction puts you completely under your opponent's control. That being said, those previous few lines don't apply to 33 this year. They're magic, and an exception to the rule (just because they do it doesn't mean you should).

So that's 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 I'm 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 WCD's in various threads here on Chief. If you see any posts from me on them prior to 2013, ignore them - I didn't know anything back then (and still don't, to a point).


Phew. That was the big one. Everything from here on out should be a lot shorter.

Mecanum Drive:

I've seen mecanums go in any configuration, and from what I've seen they all perform terribly regardless of the setup, so it really doesn't 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, you'll surprisingly do better.

That's 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 don't lie.

Slide Drive or Holonomic Drive:
Don't use these.

Swerve Drive:

Now this is what I'm 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 it's too good to be true. There's a reason you don't see every team out there with a swerve drive. While they're amazing systems, they're incredibly complex and resource-intensive. Swerves require intense coding, custom setups, and a lot of driver practice. Now don't 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 it's 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 you're 16).

I'm not going to go into too much detail on swerve, because I'm 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. I'd suggest reading these threads if you want to learn more, and try talking to someone from Team 16. They are the kings of swerve.

Octocanum/Butterfly Drive:

The 'jump drives', or so they have come to been called, have been recently new designs that originated with 148's 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 I've come to like is where the CIM's 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 3928's 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 don't 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 I've 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. I'm 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 isn't getting you points, so don't spend too much time on it. If it takes you more than 2 weeks to make in build season, it's not worth your time.

[ferrari77]

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.

Originally Posted by ferrari77

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.

8973 isn't butterfly.

[sanelss]

Originally Posted by Andrew Lawrence

Slide Drive or Holonomic Drive: Don't use these.

I extremely disagree, we've used these for years and have liked them very, very much. We may get pushed around sure but then again so does almost every other team anyway, at least we have agility so we can move around the field very easily.
here are videos demonstrating how well our system works:
https://www.youtube.com/watch?v=Q5CDQrQYh8s
https://www.youtube.com/watch?v=Kr3jjJhHSsM

we're team 1658

[Oblarg]

Originally Posted by sanelss

I extremely disagree, we've used these for years and have liked them very, very much. We may get pushed around sure but then again so does almost every other team anyway, at least we have agility so we can move around the field very easily.
here are videos demonstrating how well our system works:
https://www.youtube.com/watch?v=Q5CDQrQYh8s
https://www.youtube.com/watch?v=Kr3jjJhHSsM

we're team 1658

There are uses for pushable holonomic drives, but there are also drawbacks. In neither of those videos were you on the receiving end of dedicated, forceful defense.

It is categorically false that you "should not use" a holonomic drive as a fundamental rule, though. I wouldn't pick one this game due to the huge prevalence of defense in competitive matches, but there have been many FRC games in which it could be a huge asset.

[Ernst]

Someone should build a drivetrain that fuses Octocanum and Butterfly. It can normally drive on omnis and articulate to mecanums.

We can call it Buttercanum.

[Ether]

Originally Posted by ZehP

We can call it Butterchanum.

What's the "h" for ?

[Oblarg]

Originally Posted by Ether

What's the "h" for ?

To annoy prescriptivists?

I kid, I kid.

[sanelss]

Originally Posted by Oblarg

There are uses for pushable holonomic drives, but there are also drawbacks. In neither of those videos were you on the receiving end of dedicated, forceful defense.

It is categorically false that you "should not use" a holonomic drive as a fundamental rule, though. I wouldn't pick one this game due to the huge prevalence of defense in competitive matches, but there have been many FRC games in which it could be a huge asset.

that's true but if that was the case than any other robot would be in same situation, it's a drawback for all designs other than the specific ones made to be able to push others around. With a bit more speed(using 9:1 instead of 12:1 transmissions) we would have been sufficiently fast to outrun most other bots. I looked through all of our matches and there were several where there was heavy defense on us but we were able to skate around them but there ins't any footage I could find. Though yes we did get pinned several times but they have to back off after a few seconds and we slither out.

[SoftwareBug2.0]

Originally Posted by Andrew Lawrence

Slide Drive or Holonomic Drive:
Don't use these.

This is the conventional wisdom. For example, if you look at 1114's drivetrain presentation and look at pages 17-22 you'll see an analysis of the strengths and weeknesses of different drivetrain types which says pretty clearly that holonomic drives are a terrible idea.

I'm not sure that the analysis is right though. Attached, I have reproduced the numbers from that presentation with an extra column for what I think are more realistic numbers for holonomic drives and some comments on why I have chosen the numbers that I have. The result is that it ends up being the best option besides traction drive for all three of the skill levels that were considered.

[chrisfl]

Originally Posted by EricH

Perhaps you're taking on weak mecanum robots. By saying mecanums have the least traction of any wheel, you're making a pretty broad generalization--and I'm willing to bet there's 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 isn't exactly easy to build, particularly compared to a tank/mecanum setup.

Challenge accepted. We have mecanum drive

[blaze8902]

This year Team 4265 used a modified Octanum drive, where the butterfly modules were 2" Colsons and Mecanum. If you have any questions I can answer them, and of course all the CAD stuff is online.