mecanum vs. omni
 

hey everyone!
we (1565) are working on developing a drive system for next year. so far we're thinking of omni wheels (we almost did them this year). i don't know much about these drive systems, and i was wondering about the pros and cons of omni vs. macanum wheels.

Re: mecanum vs. omni
 

Omni wheels are cheaper, and allow for truly omni-directional motion. The programming is really not that difficult. My team built a holonomic drive train this year and it was one of the best decisions we've ever made.

Re: mecanum vs. omni
 

Assuming that you are using off the shelf components a Mecanum drive is easier to make. Omni-Drives require precise angles (60° for a Kiwi and 90° for a Holonomic) and if you are not near perfect your robot will have some strange driving characteristics. Also it is much easier to climb ramps with Mecanums. To my understanding Mecanums are also more efficient in forward and reverse directions (like a normal wheel) while still giving you lateral mobility. Omni-Drives have a much lower efficiency (~50%) in all directions but they will give you true omnidirectional motion. Also Omni-Drives require much more skill to drive than a Mecanum Drive. Still, you should test out both drives to see what works better for your application and to suit the game.

Re: mecanum vs. omni
 

I think the one major advantage that mecanum wheels have over omni-wheels is that they can fit in a normal frame set up. Omni wheels have to go on in a way that requires more fabrication while you can simply drop some mecanum wheels into a kitframe.

Re: mecanum vs. omni
 

The one real problem with these drives that we discovered was victor bias. The values that come out are not symmetric, linear, or normal by any means. We ended up using look up tables, ask Kelly for more information about how we did it.
And for the record, it's not hard to line up the wheels at angles that are close enough to 90 for all practical purposes.

Re: mecanum vs. omni
 

I have to disagree. Mecanum wheels tend to wander side-to-side when trying to negotiate a ramp, even one that is relatively small. One of our alliance partners at nationals struggled to get on our lifts which had 1.5" (very low) ramps. Their robot had difficulty driving in a straight line, something that was required to get up on our lifts.

Perhaps the problem was with some other component in the drive system of their robot but it has caused our team to shift our focus from mecanum to crab for experimentation.

Spend the 2 cents wisely! :)

Sean

Re: mecanum vs. omni
 

Actually, if you understand the math, it's okay if your angles are not perfect, as it's fairly easy to fix in code. The ease of climbing ramps is determined by the type of omni wheel you buy. Unfortunately, most of the off the shelf omni wheels our team found had lousy traction, although it would be possible (mabye not feasible?) to manufacture your own.

Re: mecanum vs. omni
 

also, programming wise, which is easier? i'm the new team programmer...
i don't know why, but for some reason i like the idea of mechaum wheels better than omni....maybe they look like they get better traction (i dont know if they do). we originally wanted a crab drive, but today we came to the conclusion that omni would be easier and better for our first time. i'm just wondering the differences so that we make the right choice...

Re: mecanum vs. omni
 

That's a lot of generalizations.
Mecanum drives require just a much precision to make as holonomic (omni/kiwi) drives, but instead of worrying about how to place your wheels radially, you have to place them in facing the same angle (if this is off, you will once again experience incorrect driving, unless you account for it in programming). In addition, like a mecanum drive, you can account for any imperfections in wheel placement in a holonomic drive with programming.
In general cases, Mecanums are capable of climbing inclines/steps/bumps better than most holonomic drives, but it is possible for holonomic drives to climb them given the right wheels and/or suspensions (and the size of the inclines/steps/bumps).
Mecanums and Holonomic systems aren't really any more efficient than eachother, they just function differently. Assuming a 45º roller placement on a Mecanum wheel (standard for FIRST purposes, used on the AM Mecanums), a Mecanum wheel delivers about 70% ([square root of 2]/2) of the force applied in both the "vertical" and "lateral" directions. Each omni-wheel delivers 100% of the force in the direction it faces, but the over-all efficiency of the drive depends on the number of wheels used, direction of travel, and rotational movement. Basically, a mecanum system uses angeled rollers and straight wheels to achieve the same goal a holonomic system uses straight rollers and angeled wheels for.
As for the driver skill, I fail to see how one would be harder to drive than the other (when programmed properly). A common control style is using a single joy-stick to control the lateral movements of the robot (using both the x and y axis) and either a z-axis or a knob (pot) to control the rotational motion. I have seen this system applied both to holonomic and mecanum drives (and have driven it with a holonomic....with ease).

Neither system is truly "better". They are very very similar systems, that function on the principle of vector translation. The programmers for either system should have a firm grasp on vectors and (preferably) kinematics in general. The efficiencies, torque, speed, traction, agility, cost, weight, size, and complexity vary greatly on the exact system you use (wheel choice, wheel placement, programming ability, etc.). Ideally both systems should have some form of suspension to ensure that all wheels remain in full contact with the driving surface, but teams have succeeded in the past with-out them.
I'd suggest you consult teams that have used each system for tips and lessons learned, as well as reading Ian Mackenzie's whitepaper on the kinematics of omni-directional systems. http://www.chiefdelphi.com/media/papers/1836

Some existing threads on both systems:
http://www.chiefdelphi.com/forums/sh...hlight=mecanum
http://www.chiefdelphi.com/forums/sh...hlight=mecanum
http://www.chiefdelphi.com/forums/sh...hlight=mecanum
http://www.chiefdelphi.com/forums/sh...hlight=mecanum
http://www.chiefdelphi.com/forums/sh...hlight=mecanum
http://www.chiefdelphi.com/forums/sh...hlight=mecanum
http://www.chiefdelphi.com/forums/sh...highlight=omni
http://www.chiefdelphi.com/forums/sh...highlight=omni
http://www.chiefdelphi.com/forums/sh...highlight=omni
Programming emphasis:
http://www.chiefdelphi.com/forums/sh...ad.php?t=36205
http://www.chiefdelphi.com/forums/sh...092#post421092

Re: mecanum vs. omni
 

one more thing: where can mecanum wheels be purchased and roughly how much do they cost?

Re: mecanum vs. omni
 

http://andymark.biz/am-0083.html
$375 per set (plus shipping)

Re: mecanum vs. omni
 

From the point of view of the software, a straightforward Mecanum drive is identical to a typical omniwheel setup with 45-degree-angled wheels at the corners. They're equally easy.

On the other hand, they're equally difficult. To be able to drive a holonomic platform easily requires tight feedback control of wheel speed. Giving the driver complete responsibility for motor power is simple, and works fine for demonstrations, but if you want to do something tricky like climb a ramp it's probably going to have to be a bit fancier.

Re: mecanum vs. omni
 

Our team has used a holomonic for the last 3 years. I would strongly suggest that you use some for of feedback is this system. We had no luck with it untill we put a gyro in the base. This would apply for both holomonic and mechanum because both depend on the motor speeds.

In terms of which to use I would suggest holomonic much more than mechanum. I don't like mechanum for two reasons: 1. Much more expensive 2. because of the roller set up on mechanums, even when driving forward or backward there is a side force placed on the bearings. This year we could find a washer to captivate our wheel (all of the lateral force on the wheel is placed on this piece). We decided to cut them on a laser engraver out of 16th inch acrylic (shatter prone) and we have had no problems with the wheels or anything to do with the drive train. I could not have done this on a mechanum because, in the most simplistic view, for every pound of forward force there would be one pound of sideways force and the washer would have shattered the first time the machine took off.

Just a couple of thoughts

Re: mecanum vs. omni
 

just surious, why did you use the gyro? i mean, why is it needed? can't you just do everything in the programming?

Re: mecanum vs. omni
 

It depends on the style on control you want really. For a robot-centric drive (push up on the joysticks, the robot moves to its "front"), a gyro isn't always necessary, but for a field-centric (push up on the joystick, the robot moves away from the driver), a gyro, accelerometer, and/or other sensor packages are almost required.

Re: mecanum vs. omni
 

o ok....now i get it....so when the robot isnt facing you it still steers like it is.....ya...that would be cool

Re: mecanum vs. omni
 

In addition, field-centric steering is virtually the only way to achieve both rotational and straight lateral motion at the same time ("frisbeeing" in a direction). If the controls are robot centric, the rotational motion will result in the translation to become an arcing motion, rather than a straight line.
See one of the programming related threads I posted earlier.

Re: mecanum vs. omni
 

on that note, it would probably be difficult to program....

Re: mecanum vs. omni
 

The "center" of the Victor 884 range is NOT 127. It is more like 132 or 133. Some example ranges are provided in this thread in the IFI Forum. I suggest testing your own speed controllers by giving them steady known values (from a pot or wheel and reading it on the dashboard) and looking at the lights.

This makes reversing the motor direction in software difficult, so I suggest just simply reversing the polarity at the Victor output. The motors will still have different power output in forward vs. reverse (that is inherent to the motor) but at least the input electric power will not be the problem. Wheel encoder and yaw rate sensor feedback can correct the former problem.

Here is another good programming link for Jester Drive

Re: mecanum vs. omni
 

Drive systems using Mecanum (note spelling) wheels are holonomic (note spelling). The only real difference between Mecanum and omniwheel systems is in the orientation of the axles. Nearly everything else can be analyzed identically. For example, I don't see how a Mecanum wheel sees greater side forces than a traditional omniwheel. Can you explain your reasoning more completely?

Re: mecanum vs. omni
 

I think I see where he's coming from.

Mecanum:

/// \\\ or \\\ ///
\\\ /// /// \\\

depending on how easily you want to turn.

Omni (45-degree setup):

----
l l
l l
l l
----

The mecanums are always pushing, but a lot of the force is directed sideways by the rollers. However, omni's are often off, so their rollers take the load. The pushing omni's are only pushing one direction. For a mecanum, they are pushing in two directions, unless the robot is moving diagonally.

Some other things to take into consideration: how easy is it to push you? Omni's are a little easier (two-wheel drive in any given direction save diagonal). Mecanums may be set up in two orientations ("easy-spin", which looks like an X, and "hard-spin", which looks like an O) and are harder to push because you have to fight all four wheels.

Ease of building: Mecanums are easier to build. Given four AM planetaries, a set of mecanums, and a kit of parts, it is possible to put one together other than programming in about an day. Omni's need a custom(ized) frame.

Maneuverability: there is a slight difference (I think). Mecanums may be programmed to allow turning while strafing, enabling large arcs. I'm not sure an omni-wheeled robot can do that without spinning.

Traction: Most teams that use mecanums have relatively high-traction rollers (often rubber). Most teams that use omnis do not (often plastic). It's a little harder to push a mecanum robot partly due to this and partly due to four-wheeled resistance.

Re: mecanum vs. omni
 

The gyro can also be convenient in robot centric drive. With gyro feedback you can help to ensure that the robot is actually driving straight when you want it to or that it is rotating when you want it to. The same thing can be accomplished with encoders (or gear tooth sensors on the wheels). As a general rule closed loop control is better than open loop.

Re: mecanum vs. omni
 

A mecanum is really a form of holonomic drive (as mentioned earlier), but instead of angeled wheels with straight rollers, a mecanum has angeled rollers on straight wheels. Instead of applying a large portion of the force from not a large number of motors, mecanums apply a smaller portion of force from more motors.

As for your notes:
Pushing:
Assuming both sets of wheels have identical traction, it is really the same. You can push on each from angels where you will get partial resistance from all the wheels, or locations where you will ignore 2 wheels (as you will be pushing along the rollers) and fight the full resistance of 2 wheels.

Ease of build:
Mecanums will adapt easier to a "traditional" (kit of parts) frame than a holonomic system. AM Planetaries would be a poor choice for a drive train though (given current rules). The AM CIM motor planetary is more for manipulator use, with a 181:1 reduction.

Maneuverability:
Both Mecanum and Holonomic drives can drive in arcs, spin in place, translate in any direction, and "frisbee" (rotate while translating in a straight line) given proper programming.

Traction:
In general, neither system gives a great amount of traction (because of the general principle of rollers), and each wheel has a direction it can be pushed by (in theory) only overcoming the rolling resistance of the rollers. AndyMark sells both plastic roller omni-wheels, and the "trick wheels" (which you can have rubber and neoprene rollers, with higher traction).

Re: mecanum vs. omni
 

I understand what a mecanum is; my team worked with them between 2003 and 2005 but decided they weren't worth it. My "sketch" indicates only roller direction, and not wheel direction (the axis is along the line)
With a traditional omni, those locations are much easier to hit. Also, you now have a 4v2 motor battle. 2 can't beat 4 very easily in pushing power, although it can be done. (Assuming that both robots have 4-motor drivetrains). With a mecanum, if you hit it just right (only four locations that are very small), you get less resistance, but it's easy for said mecanum to spin away.

I just threw the AM planetary in because I didn't want the BB. Any gearbox will do.

Then the question is, which is easier?

Traction is not very good, but most omnis that people use seem to not be AM, and of the AM's I've seen, very few are "trick". Most mecanums are a) custom built or b) AM, and both (tend to) use sticky material. The thing is, what role do vectors play? for each wheel, you have two vectors. One is in line with the wheel; one is at an angle. Those vectors govern how much traction/force is applied in any given direction. Mecanums can apply a lot of force going sideways, but a standard omni can too.

[joking, almost]I have an idea: let's settle this. 357, 1138, and 1540 (mecanum) in a series of tests against 116 (2005) and two other onmiwheeled robots (maybe 67 2005 for a three-wheeled version?). Tests include maneuverability, simplicity, and traction (old-fashioned shoving match, choose your direction of push). The better system is the one that wins the most tests.[/joking, almost]

Re: mecanum vs. omni
 

Word of advice: Build the drive train early on. Before kickoff, even.

Make a very basic drive before the season, train your drivers on it... and let the programmers work with it. This way, you'll know what you need to do, what improvements are necessary, and also you'll be able to make your drivetrain by the end of the first build week.

When the season does start, immediately design the new one, and build it... all the while you can prototype with the other, and continue to train with it. The earlier everyone gets their hands on your completed drivetrain, the better off you'll be.

Re: mecanum vs. omni
 

Eric and Lavery; Calm Down.

You both know what you're talking about, don't try to nit pick each others mistakes; overall, you're both right.

Also, there is a lot of misinformation in this thread, unless you know what you are talking about and have experience, please refrain from merely speculating.

In terms of straight omni-wheel vs. mecanum each have their pro's and cons. I have seen more succesful mecanum drives than omni wheeled ones, but that's because most of the omni wheeled bots I have seen have been poorly done.

Re: mecanum vs. omni
 

We built a mecanum this year... during the six week build period with only a VEX omnidrive holonomic as a practice chassis. It used the banebots encoders to do PID speed control to each wheel. It worked pretty well (thanks to copious amounts of advice available through CD and the FIRST workshop presentation archives), although by the time we had the drive worked out and the robot built there were approximately five minutes of practice for the drivers before we had to drop it into the shipping crate. A typical driver will need much more practice time to maximize the benefits of an omni-drive vs. the benefits of a tank drive. (It is far easier to figure out how to get INTO a pushing battle than to get AROUND one. If you've got an omni drive and you are in a pushing battle... you're probably losing.)

We would have probably won more matches with a tank drive and practice time than with the mecanum... but the mecanums provided a programming challenge for our programming team... the best programmers we have had... or are likely to have... for a few years. They also have inspired the "wow" factor amongst people who view the robot, and generated some interest in the community... and this is where I'll weigh in with two advantages of mecanum over omni...

Omni wheels are cool... but mecanum wheels are COOL!! They just look technologically menacing, and really make people stop to think about how they work. Mecanums are the FIRST equivalent of "spinners" or whatever the flashy shiny rim of the month is in cars these days.

The Andy Mark wheels have the ability to adjust tension in the rollers by tightening the roller screws... in Portland we almost lost an encoder when we fell off a ramp (mecanums can climb, but not like a tank). If we had lost the feedback we could have driven omni in open loop, OR simply tightened down the rollers on a pair of wheels so that they would not roll at all, which would have basically given us the equivalent of a tank drive... we could have easily reset the programming and been back on the field in 30-45 minutes. It would have cost us our omni maneuverability, but we would have still had control. Had EVERYTHING gone screwy with the mecanums, we could have dropped traction wheels on the front and omnis on the back and been a tank drive a few weeks later in Toronto. That is one unmentioned (so far) benefit of having your wheels aligned in a conventional manner. I think the mecanums offer a bit more redundancy and options in the event of a serious competition-season "oops."

Either choice will be a good one to pursue. Keep in mind that (IMHO) it is more difficult to build a competition-winning omni-drive robot (mecanum or omni wheels) than to build a winning tank drive robot. You will end up devoting time and resources to the drive that could otherwise go to programming, actuators, and practicing. That said, when you see your team finally get the drive figured out and go drifting past a defender and slam home a tube... or when your grade 12's get to explain the vector mechanics of the mecanum to an experienced adult engineer who has "never seen a wheel like that before"... that is something you won't get from a tank drive.

Jason

Re: mecanum vs. omni
 

This man gets it. It's not about the robots.

I think the point he makes is that even if it sacrifices time and competition success, you have to get your hands dirty or you'll never know for sure.

It is my personal opinion based on what I've seen that mechanum's are better due to traction -- and traction has alot to do with how well your bot drives.

To reduce weight on the mechanum wheel's, you could make custom wheels such as 357's Jester Drive.

Another technical piece that I'd like to know is what kind of power transfer systems do you put on each drive wheel? In Atlanta I saw robots with straight 1:1 drives, 1:5 ratios (or similar) and there was one team that had the 3-speed drill transmissions attached to each mechanum motor, and the Jester Drive CAD shows something like 1:3 connected to 1:3 again.

Also, generally I've seen large-diameter mechanum wheels (are the AM's 6"?), but what is gained or sacrificed in the power and control realm by going to smaller wheels?

Re: mecanum vs. omni
 

ya....for some reason i'm liking the mecanums just because they have more traction. plus, if something went wrong, you could use regular driving code and put regular wheels on the robot....

Re: mecanum vs. omni
 

I can't see them giving you much more traction. Omni and mecanums are fairly close when they come to lack or traction. mecanums may have a slight advantage, but not as much as you would think. As I pointed out at our meeting today, mecanums slow down a lot when strafing, which is one of the drawbacks, it won't be uniform no matter what direction you choose. Keep it in mind!!! :p

Re: mecanum vs. omni
 

Thank you for the kind comment. I think there are a lot of people who "get it"! And thank goodness... that's what makes FIRST so cool. As for the drive train, I don't really think there is much point in putting a lot of torque to the wheels in either a mecanum or omni system... after all, if you are in a pushing match with one of these drive systems, you are probably losing. So I expect you will see them geared higher... and, as there are four gearboxes, rather than two, you will be less likely to see them have (or need) multi-speed gearboxes. We used 12:1 banebots with a direct drive system. For the first year in four years we NEVER lost a chain! The Andymark mecanums are 8" diameter, so we're moving pretty fast at high speed, and don't have much poop to push with... but when you build omni-drive, you're building a wide receiver, not a linebacker. If you are pushing... you're losing!

As for control, on board feedback and programming is much more crucial, simply because you have an extra degree of freedom. We went with a PID speed control loop set by encoders on each axle... but I know teams have also had success with a gyro. So long as all the wheels are on the ground (almost all of the time on a carpeted floor) we have good pretty good control of what we are doing. So so long as you have good feedback on-board, and an intutive control system, and time to practice... you really don't give much up in the way of control.

There are a few examples on youtube... just search up mecanum or meccanum or mechanum. Wikipedia also has a link to some video from a company that builds mecanum forklifts... which are really rather cool.

Jason

Re: mecanum vs. omni
 

Can any one verify this? (Not that he said it at a meeting but that it is true) I have heard this a few times but Mecanum wheels with 45 degree rollers (Swiss 45 wheels) should have the same speed strafing as driving forward. If this isn't the case could some one explain why (my relatively basic understanding of vectors tells me same speed Forward and back wards as side to side, no different than traditional omnis in the 4 corners at 45* angles.)

James

Re: mecanum vs. omni
 

:cool: jamesbrown
what is ment is 4 wheel drive forward and back as only 2 wheel drive while strafing. As with an omni its 2 wheel drive in all directions.

can1

Re: mecanum vs. omni
 

It depends on how stiff the diagonal rollers are. If you have them nice and loose and freewheeling, a mecanum wheel will respond just like an omniwheel with similarly loose rollers. If you have them tightened down a little so they don't roll quite as easily, you'll get better forward traction but lose power to roller friction in sideways motion.

Re: mecanum vs. omni
 

I've enjoyed using the omni's for the past few years just because they are "cool". Admittedly not as cool as mecanums but still...

To clarify what I said earlier (why I don't like mecanums), each wheel creates a vector of force in the direction that it moves but regardless of what direction the robot moves in it also creates a vector perpendicular to the wheel. When driving forward two of the wheels have this vector to the left and two have it to the right so the net is no lateral vector. When strafing these vectors point all in the same direction and the forward/reverse vectors cancel. so no matter how you build or program you robot, by the nature of mecanums there is extra force on the frame and extra sidewards force on the bearings.

In regard to the gyro. I really like to gyro because our bot would be unusable without it. Since there can be errors in the gyro code we have a way to turn that function off and the machine becomes completely uncontrollable just because of differences in the motors. With the gyro on it is so stable that one of the drive motors can be unplugged but the machine is still very much drivable.

For me I have never even begun to try and delude myself that an omni or mecanum could compete with a tank drive in a pushing match. We tried once in 2005 and got pushed around the field, however, the team that won our regional didn't actually get to win or even score against us simply because they couldn't get around us since we were too maneuverable. Traction has never been a factor in my reasoning. However, I do like the idea of the ability to change a mecanum into something else more easily.

Re: mecanum vs. omni
 

I would love to try a mecanum drive sometime. The only problem I see is that unless there is a huge change in the current game dynamics, the robot might do well in qualifying, but would be completely dominated by a good defensive driver in the finals.

Re: mecanum vs. omni
 

What's missing in hardware can always be fixed in software. :D

Re: mecanum vs. omni
 

This is true (gee... funny how those basic physical concepts like vectors tend to be, eh?) and something we briefly worried about during the design stage this year, but it turned out to be only a minor issue with our mecanum drive. We machined our own hubs to do a direct drive of the AM mecanum off a BB tranny. We ended up putting set screws into each side of the hub, and actually mounted a small allen key right on the robot so that we didn't have to look for one when it was time to adjust the wheels and/or tighten up the set screws that had wiggled loose due, in part, to the changing lateral forces. This only needed to be done a few times each regional. If we were really concerned about it we could have simply pinned or loc-tited the hubs to the shaft and they would have stayed put permanently.

During build we did have to manufacture a couple of spacers to go between the bearings of two of the the BB trannies due to some issues with the location of the shoulder on the drive shaft... the drive shaft actually had about 1/16" of end play... but that would have been necessary with tank drive wheels, as well. After all, although mecanums put lateral forces onto the drive shaft, so do traction wheels when you are pushed from the side.

So yes, mecanums generate lateral forces on the drive axle, but it isn't really that big of a deal. Regular bearings can handle it, and if your axle mounts/drive system can handle the lateral forces imposed by traction wheels being pushed sideways, then your mecanums should not present a real challenge based on our experience.

Jason

P.S. It has just occured to me that traction wheels may actually generate higher peak lateral forces on the drive train (although I know your concern focuses mostly on the cycling of lateral forces whenever the mecanum changes speed as opposed to peak). When the robot is hit... hard... from the side the mecanums will translate some of that impact force into rotational force... or, at the very least... slide and skid... while the traction wheels will direct 100% of the impact as a lateral force (okay... with a bit of it coming in as torque, too).

Re: mecanum vs. omni
 

Sure, that part made sense from the beginning. What I'm trying to find out is why you believe this is any more of an issue with mecanums than with omniwheels.

(It's an issue with regular wheels too, by the way. There's a lot of lateral force involved when a tank-drive robot turns.)

Re: mecanum vs. omni
 

Maybe that would be a very stupid question... but I need to make it, so first of all i will explain the situation

Here in Brazil, no team has used Omni or Mechanum wheels at all... and i must say that my team has an old obsession about using it... but since no team here in brazil has used, is very hard to get them here, and we didn't talk so much about it before, some things are "dark" for us...

I would like to know the diference between Holonomic drive, Omnidrive and as i saw in some posts here... the Kiwi drive...
i mean about the movements of the bot... is there any difference or it's just the way the wheels and the system is designed??

We never heard about it before... for us using these kind of wheels would create just one more way to drive the bot... and now we see that we have 3 at least... we are confused...

Re: mecanum vs. omni
 

The difference between holonomic and kiwi is the number of wheels. A kiwi drive uses three wheels that are set up 120 degrees apart. A holonomic drive uses four wheels set 90 degrees apart. A kiwi drive is easier because of control and the physics of a holonomic but a kiwi inherently unstable because it is a tripod (how many three wheel cars do you see? They tend to be too tippy).There was another team that used a kiwi drive the first year that we used a holonomic but they where very tippy and didn't drive very well.

The reason that side forces aren't a problem with omni is that in an ideal omni wheel you can't have side force since there are rollers in that direction. People use them in tank drives to turn the robot easier because they don't create any lateral force in almost every situation. They only case that creates side force on the wheel is when the side of the wheel contacts something without contacting the roller. I could use 1/16th in acrylic to captivate my wheel because it didn't create side force. That is what I like about them, I only needed one strong piece for mounting my wheels (hub).

Re: mecanum vs. omni
 

Our team has been using Mecanum Wheels for the past three competition seasons. While we do not have the same experience with omni wheels, we have found Mecanums to be very advantageous. For starters, the myth that Mecanums cannot push or defend all depends on the specific style and type mecanum wheel being. In 2005 and 2007, we used a six inch mecanum wheel design, which ment we had to use a small roller. This gave us little contact with the floor at any given time. However, we were fast and maneuverable enough that we did not need to defend and we had very offensive robots. However, in 2006 when we used a larger wheel and large rollers, we were a very defensive bot. At our final competitions we could stop one or ever two robots at a time very easily from scoring in the high goal as well as push other robots when necissary. The amount of friction and pushing power you get from mecanum wheels depends on the size and material of the wheel as well as the roller design. We also found that we could climb the 30 degree rame at the end of the match without any problems.

Re: mecanum vs. omni
 

Tippiness is just a matter of where the center of gravity ends up. A Killough ("kiwi") base is actually a very good holonomic platform because it's a tripod. All three omniwheels are always contacting the ground, so it won't go off in odd directions because of losing traction at one corner like a four-wheeled platform can.
Ah, I think I understand now. When you say "side forces", you're apparently talking about thrust forces on the axle. Agreed, an ideal omniwheel won't try to push its axle in or out. And since you just now mentioned tank drive, I think we might have been talking past each other. With a pair of omniwheels instead of traction wheels on a tank-drive platform, the in/out force on the axle is indeed a side force on the robot. I've been focusing on holonomic drives, and I was tripped up by your mention of extra forces on the frame. Since a traditional four-omni drivebase has the axles at 45-degree angles, it puts just as much side force on the frame as a traditional mecanum drivebase.

Re: mecanum vs. omni
 

It sounds like there is an ideal opportunity for you to introduce some cool new wheels to Brazil. The Omni wheels are great for much more than just an omni directional drive... as mentioned above when you put omni wheels on one end of your traditional "tank" drive robot it becomes much, much easier to turn it.

However in the context of an introduction to the various omni-directional drive systems, you might find http://www.youtube.com/watch?v=r5WKgQJtToM helpful. If you go to http://www.andymark.biz/wheels.html you will see some good photos (and pricing) for some very effective wheel solutions.

There is much more available by searching the forums here and finding the FIRST workshop presentations at the USFIRST website. Hope that helps clear things up, and thanks to MVRT for a great video resource on youtube.

Jason

Re: mecanum vs. omni
 

yeah... that's our plan... but first of all we need to understand it consistently, after we need to convince our sponsor... and then go to the "dirty work"...

It seems that this year we'll finally test some new kind of drive system... and for just one reason it looks like mechanum whells are a little better than omniwheels (for a person that never worked with any of them):

If something goes wrong... it's easy to put a "normal" wheel in one of the end and go for a simple tank drive...

This year we experienced great problems with your drivetrain system... because our bot wasn't able to turn well... so we always lost much time turning... maybe that made us "far away" from greater success...

thanks... the video was very nice... it cleared many things here... now we are studying some topics lil' avery listed here... nice ones, by the way...

Good bye, at least for now :D

Re: mecanum vs. omni
 

I'd suggest reading some (or all) of the threads I posted earlier about vector based (holonomic) systems, as well as Ian MacKenzie's whitepaper (also posted on page 1 of thread).
A kiwi (killough) drive is really just another form of holonomic drive (just like a mecanum). A kiwi drive is a 3-wheeled variant, with wheels every 120º (as mentioned earlier). The vector math is basically the same, just with wheels every 120º instead of every 90º. A kiwi has a considerably smaller support structure, making it more prone to tipping if the bot is top heavy. The frame is also more difficult to construct if you don't have the proper tools/experience, as it requires non-right angles (although still not terribly challenging). However, with only three wheels, you don't suffer the problems of losing contact with a wheel (unless you bottom out, but that could happen to any drive), resulting in fluky driving. You also only have to invest the weight and space for 3 wheels/motors, as opposed to 4+ (you can really put any amount ≥3 of wheels in a holonomic system).
No solution is truly "better". A 4-wheeled holonomic, mecanum, and kiwi each have their advantages, but each have their disadvantages. The basic principles in them are all basically the same, and the real distinctions between these drives comes depending on how well a team can implement them.

Re: mecanum vs. omni
 

One final thing to consider: the driver. Can your driver handle the extra maneuverability? IF definitely not, then stick with what works. If he just needs practice, do you have the resources to build a second frame (robot would be better, though), weight it down, and practice for weeks on end? If he can handle it without extra practice, give it to him anyway if possible.

Note: during testing in 2005, we noticed that strafing ability was impaired by holding a tetra about 2' away from the drive base. We just couldn't go straight sideways. We'd arc towards the tetra.

And yes, mecanums do tend to be slower sideways than front-to-back. At least, the first iteration does. It has to do with the fact that instead of having all the rollers work together, half of them are "fighting" the other half, so the robot moves more slowly. It looks something like this:
straight: --> --> ....sideways: --> <-- ....diagonally: --> --
........... --> --> ................. <-- --> ................... -- -->

Re: mecanum vs. omni
 

hey guys im from team 1382 too and i want ask u something:

if a robot with mecanum wheels or omni wheels receive a stroke on his side of another robot , it will be pushed? (i hope u understand the question...)

thanks for the attention with us...

cyaa

Re: mecanum vs. omni
 

If M (the mecnaum robot) hits N (normal robot), chances are that M will have to hit very hard on a corner to even move N. If N hits M, M will move. M won't move very far on one hit, but pushing will send M into a corner unless M runs (and the point of a mecanum is to avoid pushing contests.)

Re: mecanum vs. omni
 

very good point.. the point of making a omni or mecanum is to Not get pushed.. aviod pushing matches at all cost, cause any other type of drivetrain will be you in a pushing match.. hands down. So IMO you should work to get the omni or mecanum as agile as possible.

Re: mecanum vs. omni
 

yeah... seeing all the things said, and related here.. it's clear that mechanum/omni robots must be designed to provide quick movements and chiefly to use offensive strategies that would avoid the defense (and so "pushing clashes")...

the point at using the wheels seems to be: will the drivers be able to deal with them? (the wheels and the defensive bots)
and... what strategy will be used? (ofensive or defensive)

am i wrong?? If so, please correct me..^^

the team is planning to build a drivetrain system during the off-season period, to practice to the '08 season and now we are trying to see what would be more useful... so we need to think about all these things..


thank you all again^^

and sorry if i've made some spelling/grammar mistakes...

Re: mecanum vs. omni
 

You should be planning on playing offense with a holonomic or mecanum drive.

The downside is that if your manipulator fails or is ineffective, you can't really play defense that well. With skid steer you still can.

The best advice I can give is use this kind of drive if it is required, not because it is cool.

Re: mecanum vs. omni
 

Omni can easiel play defense if the game is correct. This year we did a good job of defending robots from getting to the rank because they couldn't get between us and the rack because we were so fast and agile. I never got in a pushing match but they still didn't score. Of course this will only take you so far.

You should decide based on the game and if you can get your prototype working well during the off season.

Re: mecanum vs. omni
 

Adam is quite right... don't assume that mecanum or omni or any other holonomic drive system is going to give you an edge at a competition unless it suits the game, your strategies and the rest of your robot. You will spend more time designing and debugging a more sophisticated drive system than you will a simpler tank-style drive system, and that will take away from something else you could be doing to make your team/machine better.

On the OTHER hand.... YES!! Build a Mecanum because it is cool! Why the heck do we get into this whole robot-building stuff in the first place? Trophies and banners are nice enough, I suppose, but people have seen them before. How many people have seen a mecanum wheel?? Robots that can go sideways really capture people's attention, especially when you bring it back from the competition and are driving it around in the community.

If you base the success of your season on winning a regional, then chances are you are going to be disappointed. If you base the success of your season on building a good team and a cool robot then chances are good that you are going to be successful... and you might even find that you win a fair bit, too!

Jason

Re: mecanum vs. omni
 

i agree totally. yes, it is fun to win, but the robot is REALLY why we do all of this :p . also, just because i'm nice, i have omni code if someone needs help

Re: mecanum vs. omni
 

I'm not basing success on winning a regional. Success is learning by buildint the best system you can. In the engineering industries, the best system is the one chosen.

If a 6 wheel skid-steer can outperform a holonomic in a certain game. Then the skid-steer is the better system, although it is more likely less complex.


Now, for offseason things. Go ahead and build the "cool' stuff.

Re: mecanum vs. omni
 

This is getting kinda off-topic, but anyway.
Not every team has the resources, location, and member base to build new systems during the off-season, and so the only time they have to build something cool is during the build season.
In addition, performance isn't the only measurement of success, and how to determine the "best system" is more than performance on the field.

More on topic.
For any new system, particularly in a component of the robot as crucial as the drive-train, I would suggest doing some form of prototyping before the build season starts if possible. The added experience, ability to work out kinks, and test out your code are invaluable. Additionally, its a great way for drivers to practice on the new system before it is finished, and after it is shipped.

Re: mecanum vs. omni
 

for now we're going to start (maybe?) designing our concepts in Inventor. hopefully by the end of school and the summer we might build an omni drive. we already have the code for with and without a gyro, and i'm just itching to test it out.

Re: mecanum vs. omni
 

I agree to an extent. If you prototype something on limited resources and then realise after the game is announced that your prototype concept has little chance of success in the game, then you have 2 choices: 1. Come up with a creative way to implement your prototype or 2. (more common) Adapt your prototype so that it fits into the current strategy, therefore introducing (sometimes) very complex integration issues or unforseen behaviors --

e.g. Adapting mecanum to play pure defense will really put some wear on the treads of the wheels -- enough wear to be of concern if you attend 2+ competitions with success.

Ok, so this argument probably holds little weight in the case of mecanums, as it's more likely you can adapt your strategy to use mecanums with some sucess every year. However, if you're like most teams, you'd rather adapt your robot to your chosen strategy and not vice versa. This is also good engineering practice, whereas adapting your strategy to your robot is something that's good practice for learning how to find & fight problems (namely integration problems) after the fact.

Being a strategist and an engineer at heart, I personally choose the former. In the meantime, the "prototypes" I'm doing on mecanums extend into the VEX realm, by trying to make a 100% legal mecanum VEX wheel for $20 or less per wheel (very very tough to do atm unless they make the white nylon spacers come in the VEX kit -- easy to do with those). Another thing you can do, is take a look at the white papers, do that math, and create a VEX omni-wheel drive. The hardest part about holonomic drive is the physics, vectors, and programming, so getting started with developing an algorithm in programming or creating a full VEX omni-drive will get you headed in the right direction.

Save the purchase of the wheels for the big robot for after the game announcement so you know that you're getting your money's worth.

Re: mecanum vs. omni
 

that would be great! ^^

Re: mecanum vs. omni
 

Don't forget choice 3: set aside the prototype as inappropriate to the task, and build something else that is more appropriate.

Re: mecanum vs. omni
 

I agree. Just because you prototype something does not mean that you have to use it. Look at 488, who made considerable noise with their Mecanum prototype this fall, then promptly decided to use their 6 Wheel Drive design again when build season started.

Re: mecanum vs. omni
 

I finished my prototype VEX mecanum wheel this weekend. It's definitely under $20, not VEX competition legal (we can always hope the spacers come in the kit right?), and currently far from perfect since I need to file more metal down and figure out roller spacing that's off by minute amounts.

I'll post a pic tonight when I get home and put a link here.

Re: mecanum vs. omni
 

We made some a few years ago. Admittedly, they aren't VEX legal or cheap or extremely strong. Dad was running some SLS tests at his work and slipped a few wheels in, one of our sponsors cut the rollers and pins to hold them. They worked for testing with Edubot and so should work with VEX if we ever wanted to try.

Re: mecanum vs. omni
 

I'll make a full robot to run it around, but it seems sturdy enough:

http://www.chiefdelphi.com/media/photos/28608

Re: mecanum vs. omni
 

The forces are the same for omnis and mecanums. The programming is exactly identical, as an omni setup and a mecanum setup with the wheels in the same spot in the corners would turn the motors on and off exactly the same amount of time, and the same outputs. The vectors of mecanums in the same spot are diagonal just like the omnis, and are 100% of the omni's diagonal forces.

Theoretically, they're completely the same, just with different motor/wheel angles and thus different mounting. But like is stated earlier, mecanums' 45 degree wheel/motor angle causes lateral force on the bearings, but can be more easily switched with normal wheels.

The rollers themselves hit the ground at the same angle, push in the same direction, and with the same force. They're both just as efficient, neither moves slower. They both push with 71% force forward/backward and left/right, and 100% at a 45 degree angle. You only use two of your motors diagonally though (mecanum or omni) so you go 41% faster fwd/back/sideways.

Because the forces are the same, the two have theoretically the same traction if they use the same roller material and size and wheel diameter and number of rollers and both have circular profiles and all of that.