Mechanum Wheel Sizes
 

Hello, my team (4087) recently completed our first FIRST season at FRC. We had a blast and are talking a lot about expanding the size of our team, budget, and quality of our robot for the 2013 season. Anyway, one major topic that my team has been talking about is holonomic drives, specifically mecanum ones for next season's robot. I've been doing some research into these wheels and have found many mixed success stories. At any rate, I found much info online as well as on Chief Delphi, however, I could not find a good thread discussing which sizes of mecanum wheels generally work best. I read somewhere that teams had problems with the 6" AndyMark mecanum wheels deforming and the 10" AndyMark wheels seem very expensive, too large, and too heavy for practical use in FRC. Does anyone have any first-hand experience as to which size(s) work well (movement not too bumpy, resistant to damage, ease of mounting, etc.)? Any recommendations and/or comments are welcome! Thanks in advance!

Re: Mechanum Wheel Sizes
 

Not to be a downer but in most FRC applications a standard 6 wheel "drop-center" tank drive with grippy wheels like Colsons will bring you far better performance than a mecanum drive train.

However, if your team is dead-set on using an omnidirectional drive train, I cannot stress enough how important it is to build the system you want to use in the OFFSEASON. This way you can work out all the kinks and learn from your mistakes. Building an omnidirectional drive system for the first time during build season will bring unexpected challenges that will require much more time than you think in such a short 6 week build window.

As for wheel size... I'm by no means an expert but the better applications of mecanum I've seen tend to be using 6" wheels.

Re: Mechanum Wheel Sizes
 

I beg to differ on this point. The simple reason why tank usually outperforms mecanum drives is because drivers usually don't use mecanum to it's fullest potential. Mecanum drive is an unconventional drivetrain, thus it takes practice more than anything to be able to use the strafing regularly in your driving. People generally are not used to having that ability when driving, nor are they used to being able to spin on a point. The biggest thing you need if you're going mecanum is practice using it.

Aside from that, everything else Eric said is true about mecanum, though our team managed it in season. More complicated omnidirectional drives (ie swerve or octacanum) would require much more time.

Re: Mechanum Wheel Sizes
 

I wouldn't be so quick to dismiss mecanums, but there are better ways to achieve maneuverability - of course, that's another discussion itself.

We used 6" mecanums in 2011 with great success; we had absolutely no problem whatsoever with wheel damage. We did come away from that season with two particularly important lessons, though:

1 - make sure your wheels are always in contact with the ground. Although it seems obvious as the kinematics of mecanum drive rely on all 4 wheels, it's not obvious when your chassis is warped. Following unaccountably erratic driving control halfway through qualifications at CMP last year, we discovered slight chassis warp, on the order of 1/8" to 3/8". It ended up with two mentors jumping up and down on our chassis to fix the warp (we students weren't heavy enough :( ). After that, driving control was much more precise.

2 - precise speed control is a must. Because the nature of mecanum requires that each wheel be individually controlled to a relatively high level of precision to enable accurate driving, it's imperative that the software is capable of precisely controlling wheel speeds relative to one another. Unfortunately, it wasn't as easy as tossing CIMs into Nanos and bolting them to our chassis. Developing effective speed control took our software engineering team a good amount of time, and it was only thanks to that that we were able to achieve the level of driver control that we did.

Re: Mechanum Wheel Sizes
 

AM sells 6" 8" and 10" mecanum wheels.

The 6" and 8" are geared towards FRC use. We have used 6" HD wheels and loved them. We never had a single failure.

10" are pretty cool but a bit of overkill for FRC.

Re: Mechanum Wheel Sizes
 

Even at its fullest potential, I'm not convinced you will get better performance our of a mecanum drive.

I can't speak about mecanum wheels specifically, but we've come to like using the smallest wheels we can. They have a lower moment, they're lighter, and you need less gearing.

Re: Mechanum Wheel Sizes
 

This thread has quite a long discussion about mecanum wheels. Take a look at it.

Re: Mechanum Wheel Sizes
 

It's perfectly natural for a team your age to feel things like this. Many of us have experimented with holonomic drive trains at one time or another. It's nothing to be ashamed of, and it's good that you've come to us for help. With hard work, focus and determination, you'll come through this, better than ever.

The thing to remember is this: every single person on every single team, including the best teams in FIRST, have, when first presented with them, looked at mechanum drive trains, and said, "Whoa, cool!", and not one robot built with them has made it to Einstein in at least 2 years (because that's how far back I've checked).

It's not because they suck inherently, it's because it's hard to make them run to their full potential. So, because you'll ignore this warning and build it anyway (because, "Whoa, cool!"), the best advice I can give you is this:

1. Encoders on all 4 wheels
2. Only run it closed loop (typically velocity mode)
3. Make sure each one of the (48) little rollers is free spinning before each run. Bent wheels can cause these to bind.
4. Understand what the code needs to make the math work, especially any code you don't write yourself.
5. It still probably won't work great on anything but a flat floor.

People below me will contradict points 1,2,5, and maybe 4. They will have counterexamples. They also probably have a grandfather who smoked 4 packs a day and lived to be 100. Ignore them, believe me.

Re: Mechanum Wheel Sizes
 

I would suggest waiting until the game comes out before you decide whether you want to do a mecanum drive. Some games are going to be better suited than others for that drive. In my opinion, 2011 was a pretty good year to use mecanum, and 2012 was less suited for it.

2011: flat field, pretty big protected offensive zone where defenders can't touch you, scoring pegs are easier to score on if you can strafe, human players can throw the game pieces most or all of the way to the scoring zone.

2012: Harder to balance on bridges with mecanum, strafing less of a benefit for lining up to goals (but admittedly useful for gathering balls). Mecanums can get over the bump, but I don't feel warm and fuzzy about ramming that type of wheel into a steel barrier (seems like rollers would get bent and start to stick)

To address your question, though, here are my thoughts on 6" vs 8" wheels:

6": cheaper, lighter, smaller, less gearing required
8": tougher (except now there is a heavy duty version of the 6" wheels)

Even though we've used 8" twice in the past, I think we'd start by looking at 6" if we were going to switch back to mecanum for some reason. I'd probably be looking at direct driving them with a set of Toughbox Nano gearboxes with one of the lower optional gear ratios.

Re: Mechanum Wheel Sizes
 

FYI, Mecanums ALMOST made it to Einstein. 2171 was a Finalist on Curie in 2008 and they were rocking AM 8" mecanums (IMHO the best COTS mecanums for FRC use).

Agreed that it is best done initially off-season and that competition usage should be decided on a game by game basis. Also agreed that part of the battle it is knowing how to use all 3 dimensions to the drivetrain.

Re: Mechanum Wheel Sizes
 

Woah! First off, I'd like to thank you all for the amazingly immediate responses! Second, based on recent posts, I'd like to put something out here: we're a one-season team with only two or so of our members having any previous robotic experiences (myself and one or two others). Additionally, this is the first time I (and I believe my teammates) have gone beyond the LEGO Mindstorms NXT (r) level of robotics. In a nutshell: I''m really not sure what to think of some of these posts, much less what they mean! I know what an encoder is (measures/counts rotations), but I don't see how that would be beneficial to a holonomic drive.:confused: As for code, if we decide to go with mecanum wheels, they'll almost certainly be driven using four jaguars and the code for holonomic drive found in the WPI Robotics Library in LabView, without any changes.

Thanks for understanding our current lack of experience in pretty much everything at this point! :o

Re: Mechanum Wheel Sizes
 

357 doesn't have a big presence on Chief Delphi, but if anyone is going to get to Einstein with mecanums, it will be them. I'd suggest trying to get in contact with them if you're looking for help designing driver practice for this or any other holomic drivetrain.

Re: Mechanum Wheel Sizes
 

What bad things should I expect to happen when I run mecanum with open loop control?

Re: Mechanum Wheel Sizes
 

Encoders are beneficial to a holonomic drive because these drives largely rely on the kinematics involved with the wheels going at very precise speeds. Since an encoder can measure the revolution (in this case of the drive axle or an axle coming off of the gearbox), it is the proper sensor to close that loop and accurately control the speeds of all of your wheels.

If a mecanum drive is coded for voltage control (no closed loop, just set the motors to a given voltage), you could have a situation where your driver commands the robot to drive straight forward, but only 3 of the 4 wheels are driving close enough to the same speed. That fourth wheel driving at a different speed than the other three will cause a slight rotation/strafe in the driving, resulting in a robot that will not be able to drive in a straight line. This is why speed control of the wheels is very important and encoders can be used to achieve this.

Good luck, and I hope you guys have success with your off season projects! It's this kind of work that will prepare your team for a successful season.

Re: Mechanum Wheel Sizes
 

If it's something your team wants to do, got for it! There will be naysayers that will give every reason not to try it, but what fun is that? Pushing yourselves to try something new, and learning in the process, can be hugely beneficial.

This was our (Team 126's) first year using mecanum wheels. I don't think anyone would have changed this decision. Our camera alignment was fantastic, with the ability to strafe. We spent a lot of time calculating gearing. We ended up with a final drive of about 17.5:1, CIM to wheel speed. Secondly, lots of drivers practice. Usually 4 days a week for 2 hours or more.

We did not run encoders on our robot. We initially made custom mounts for encoders, but ended up finding we did not need them. I don't know the programming side well enough to understand why, or what we did differently. Just that it didn't operate any differently for us not using them.

We used 8" AM wheels. They are heavy. I don't think the washer and brass bushing for the rollers is the best design possible, but it's simple and cost effective.

Re: Mechanum Wheel Sizes
 

first: I (like others who have posted before) would recommend at least a decent amount of experience on your team before dealing with more then 2 axis (x linear and z rotational [or equivalents]) in your drive train. At the very least, make sure someone who has done it before is helping you as it can get very confusing very quickly if u don't.

on topic: what is better depends on the intended use

mechanum is poor at climbing due to the rollers so it has to be significantly taller then whatever u intend to climb. It also stinks at countering gravity. general robustness is well below average for the weight involved. The only real advantage of the system is that it is highly mobile (capable of motion in x linear, y linear, and z rotational [or equivalents] all at once or independent).

finally: if u use it at a comp, bring at least one full set of spares since it takes time to repair and time isn't usually on your side (Murphy's Law is ever so true).

Re: Mechanum Wheel Sizes
 

Speaking from experience of 3 years of robots that used Mecanum wheels (and observing 1 year with swerve).

Mecanum wheels are the most forgiving holonomic drive. If all else fails, slap some skyway wheels on your robot and drive like you were never able to strafe. You can't say the same for kiwi, and you'll be pretty heavily invested in swerve.

Swerve drives have many more points of failure than mecanum wheels. If this is your first season with a non traditional drive train, mecanum will be the easiest to adjust to. Swerve drives have that nasty part where you have to turn your wheels before you go in the direction you want to go, which is another control challenge (having said all that, mad props to the guys on 111 for making it look easy every year).

As far as what you need for a mecanum wheel setup, at the bare minimum get a gyro and learn how to use it. Also learn about trigonometry and the math behind field centric drive.

Design your mecanum drive with either a bit of flex in the frame, or spring loaded wheels. It is very important that each wheel touches the ground, and the carpet at FIRST events typically is not perfectly level.

Last but not least. Buy your mecanum wheels now, along with four sets of gearboxes. Make a drive train with a wood frame and start playing around with it. Don't modify the wheels or the gearboxes, and you'll be able to use them once the next season starts. You'll then have learned valuable things about mecanum wheels that we may have missed here.

Good luck!

Re: Mechanum Wheel Sizes
 

As I said, the team is very new to this stuff... how would a gyroscope help with a mecanum wheel drive? :confused:

Re: Mechanum Wheel Sizes
 

that's not the Kind of gyro that was being referred to.

A gyro in this case is a sensor that outputs a rate of rotation. There are many types of gyro out there with various benefits and drawbacks, as some are shock-sensitive and others require calibration.

Re: Mechanum Wheel Sizes
 

Be very careful when choosing mechanums. Many teams have regreted the decision. On the other hand, there are some that have liked it. Just some things to consider:

a) mechanums while strafing will have much less speed than normal. I believe(anybody correct me if I am wrong) that they have around 60% speed when strafing

b) There is a noticeable decrease in performance in pushing matches compared to a standard traction drive(4, 6, 8, 10+ wheels). While it is true that in some cases you will be able to outmaneuver a traction drive, and avoid the pushing match due to strafing, in 99% of cases you will not be able to, due to the fact that you will not be able to move as fast as the traction drive while strafing(look above).

c) along with the lack of speed and traction, there is a noticeable increase in the amount of time needed for practice.

Therefor, many teams will recommend against mechanums. Though if you are really set on them, then just know what you are getting yourself into.

Re: Mechanum Wheel Sizes
 

Assuming 45 degree rollers, it would be 50% slower. It acts like a gear reduction.

Re: Mechanum Wheel Sizes
 

It's not analogous to a gear reduction.

Neglecting friction, axial free play in the rollers, and compliance of the floor surface, vehicle speed in the strafing direction with 45 degree rollers is exactly the same as the forward direction, for any given wheel speed.

The reason the inexpensive mecanums used in FRC are slower in the strafing direction in actual practice for a given throttle setting is due to friction and axial free play in the roller bearings, friction in the drivetrain, compliance in the flooring (carpet), and open-loop control of speed.

Re: Mechanum Wheel Sizes
 

While I still think your team's curiosity in mecanum would be a GREAT offseason project to learn and grow as a team, I firmly believe that THIS drive train will bring you a much more successful robot during the FRC competition season. It is simple, low-cost, and easy to build and maintain. This will let you spend far more time on your game piece manipulator and allow you to have a much better competition robot.

Re: Mechanum Wheel Sizes
 

When it comes to Mechanum Drive some of the things you would think and have been stated are issues can be overcome easily as we learned the last two years since we have used the drive for both Logomotion and Rebound Rumble.

Size argument for the wheels: 10 inchers are way to big for FRC we bought a set of 8 and 6 inch mechanums for testing purposes this year and discovered that the cost of the 8 inch were not worth the increase of skill they gave us

Below are some of the standard issues that have been brought up

Speed: We never experienced any issues with speed in the standard directions and in the strafing this year we had to use controls to slow down the speed because of fear of tipping (the wheels have less surface area in that direction of movement so they aren't as stable)

Climbing: We thought me might have issues this year with climbing the ramps but we were pleasantly surprised, we didn't have enough traction/force to push someone up the ramp but we were able to balance on our own several times with literally seconds left on the clock without issue. The wheels also make it very very easy to be pushed up the ramp.

Pushing: Last year we figured that our maneuverability would be more then enough that it wouldn't matter but this year since we had to score at the fender and were worried so we ran some tests with our tank drive bots and discovered that in the standard direction (as in both robots wheels in the same direction) we could hold our own for about 10 seconds before eventually succumbing and losing the contest. However once we came at the tank drive in the strafing direction (the mechanum robots wheels perpendicular to the tank) we were evenly matched

Driving: The best thing for this type of drive is to give the driver a video game controller and program it to run like a standard FPS game does, but nothing will replace practice for any person driving a robot.

"Einstein" Argument: The amount of teams utilizing Mechanum or any omnidirectional drive is less then the number of teams utilizing a more traditional drive, so they have less representatives competing for a place on Einstein, statistics and probability show that the smaller group has a smaller chance of being selected (Also in judging the amount of time passed since someone with omnidirectional drive got on Einstein try to ignor 2009 since lunacy didn't allow for mechanum or oni wheels).

The most important thing to consider though is what the game is asking you to accomplish and if the wheels will help you to do it. These last two years we felt it was worth it so we did just my two cents

Re: Mechanum Wheel Sizes
 

You seem to be saying that you were able to achieve speed and quite a bit of pushing power. I don't doubt that you may have been able to achieve this in quals. But in elims and if you get to championship, you will see a lot more robots that have 2 speeds, which you will never be able to push and outrun. A 2 speed traction bot done right with the same amount of practice against the best mechanum you will see, will be driven better(more driving practice), will take less time to build and program, be able to outrun the mechanum while strafing and sometimes while driving regularly(due to the fact that you gan gear for speed in the first gear), and outpush the mechanum anytime(due to the fact they have 50% power while strafing, and you have the second speed)

Re: Mechanum Wheel Sizes
 

Well, bomb squad did pretty well at Einstein this year with an omni directional drive train, however, it wasn't meccanum.

On a side note: In my engineering drawing class we were told to make bikes. (using inventor), so I decided it would be an interesting idea to make a mecanum bike. Originally I planned to use 26" diameter wheels, but accidentally plugged 26" in as the radius, but I've decided to roll with 52" diameter wheels. While I wouldn't be confident enough to actually use mecanum on a competition bot, especially considering that my team now has experience with swerve, I do think that mecanum is an interesting challenge and learning experience.

Re: Mechanum Wheel Sizes
 

Steps to having a great season as a newer team.

Step 1: Build this
Step 2: Use all the extra build season time you now have to build something awesome on top of it.
Step 3: Have a great competition season.

Regards, Bryan

Re: Mechanum Wheel Sizes
 

Some people always make mecanum sound harder than it is. Let's assume that a team has already figured out how to build, program, and drive a basic 2WD/4WD/6WD FRC robot. For such a team, I would claim the following:

1) Mecanum drives are not difficult to build. You can get preassembled wheels and gearboxes from AndyMark, and hubs to connect them. All you have to do is find a way to fasten the gearbox to your frame. That is about as easy as a drive can get. No chains required. It is not very heavy, either.

2) Mecanum drives are not difficult to program. You don't even have to program it, because holonomic drive code is included in the libraries with our software. All you have to do to turn an arcade drive into a holonomic drive is add "+ x" or "- x" to the motors for the strafing axis joystick input.
Front left motor: y + z + x
Front right motor: y - z - x
Back left motor: y + z - x
Back right motor: y - z + x

Adding gyro feedback prevents the robot from driving crookedly or rotating when you don't want it to. That admittedly adds a bit of complexity, but only a bit. I am not convinced that encoder feedback is necessary if you have a gyro.

3) Mecanum drives are not difficult to drive. I'd like to know how many of the people claiming otherwise have actually driven one. I thought the whole point of a mecanum drive was to make it EASIER to get where you're trying to go, pointing the direction you want to point. A 6WD is less forgiving to the driver than a mecanum drive, because it's faster and easier to correct your slight driving mistakes when you can strafe.

Mecanum can be a reasonable choice for some teams. It is not going to work well for all games, but it was fine for 2011 (flat field, protected offensive zone, and peg goals that were easier to score on with strafing ability).

Mecanum is also not going to be as good as 6WD at the highest level of competition, because a 6WD robot (which has inherently better traction) with a really good driver does not need strafing to quickly get the robot exactly where it needs to go. However, it is okay to focus on getting more competitive at the regional level. The forgiving nature of a Mecanum drive (item 3 above) might, in fact, make some teams more competitive at that level than they would be with 6WD.

Re: Mechanum Wheel Sizes
 

If you're dead set on using mecanum wheels, but still want to be in the "small wheel" category of offensive teams, these may be if use. If not, then you can probably use a 3D printer to print out some 4" ones. I've heard 3D printer wheels can actually be quite strong if you use the right material.

All in all, a good 6wd, drop center wheel will be the best way to get your team headed in the right direction. Why? 1 word: Traction. You don't know how useful it is until you're without it.

Re: Mechanum Wheel Sizes
 

Emphasis mine. We use plenty of ABS plastic on our bot, and I can't imagine that printing our own rollers would hold up for the season.

One thing we considered last season was making our own mecanum wheels; a plan we had to integrate our 3D Printer into it was making a mold to make "polysomething"(I don't remember what...) rollers.

Re: Mechanum Wheel Sizes
 

1) I agree with mechanum being easy to build(relatively). Our team from last year built one, and last year is a year that is not fun to remember. That being said, you do have reliability problems. If one wheel goes down, you aren't going to be able to strafe.

2) Mechanums are relatively easy to program. But, once you start putting gyros, encoders, etc. on(and you should, you will have problems driving straight otherwise), you then get a lot more complicated, vs the traction drive, where you really don't NEED any of those.

3) While mechanum may be easy to drive(it is basically Halo), you have to make a lot of compromises in strategy and build. You have to give up playing defense, and have to make sure you don't get into a pushing match while playing offense. Therefor, you must be a very good offensive robot, and it is STILL harder to drive, due to the fact that you have to avoid those situations. Furthermore, you must make some compromises in build. If you look at many of the successful swerve bots(just because better teams usually use swerve when they are looking for holonomic robots), all of them have made compromises in build. 148 in 2008 built a robot that was completely round. 118 built a robot in the same year that had a turret. 1717 from 2011 was very effective in the home zone, because they didn't have to turn much, but outside of the home zone, they were much less effective.

Re: Mechanum Wheel Sizes
 

Allright, from what I read (assuming we chose mecanum), we should plan on using a semi-flexible frame that allows for the wheels to raise/lower a little based on terrain, should do most of our prototyping work during the off-season, and use 6" mecanum wheels (AM 6" HD?). Any other suggestions or things I forgot?

So, I suppose all that remains is to figure out how to turn these wheels. Any suggestions for the drive train? We currently only have the frame kit included with the 2012 Rookie KOP, which means 2 CIMs, chain, 4 sprockets, and two CIMple boxes. Would purchasing two more CIMple boxes with accompanying CIMs to use with the existing lengths of chain and sprockets work, or will we need to purchase entirely new drive train parts to make the robot move? (The rookie kit came with four jaguars, so we do have enough motor controllers for the four necessary motors.)

And thanks to everyone for all of these awesome suggestions!:D

EDIT: And plenty of practice to go with the new axis of travel!

Re: Mechanum Wheel Sizes
 

Look here for our 2011 drive base CAD for one way to set it up.

Re: Mechanum Wheel Sizes
 

One thing you're missing is that for mecanum, you need to power each motor seperately, so 2 CIMple boxes won't work. I've seen teams use both these:
http://www.andymark.com/product-p/am-0688.htm
http://www.andymark.com/product-p/am-0482.htm

The 2nd one probably fits your idea of having a flexible frame for going over terrain.

Re: Mechanum Wheel Sizes
 

I know ;), that's why I was wondering in the next sentence if we should go with two more CIMple boxes, or scrap the two we have for entirely new drive train parts (hopefully keeping the CIMs).

Re: Mechanum Wheel Sizes
 

There is nothing wrong with using the CIMple boxes to save a few bucks. The kit sprocket reduction (26:12) plus the CIMple box gives about a 10:1 gear ratio, which is a nice ratio for 6" mecanum wheels. If we're starting from a 2012 kit of parts, that move saves about $180 as compared to using Toughbox Nano and AndyMark hubs to direct drive. If you want to use the same equipment on a competition robot next year, though, I think the extra $180 is a good move. The Nano boxes have the following advantages over CIMple boxes: takes up less space, no chains to mess with (easier to build + more reliable), Nano is a nice box that runs really smoothly and is probably more efficient than CIMple + chain.

Having said all of this, and everything in my previous post... honestly, I agree with the people who suggest doing a kitbot on steroids (a nice 6WD robot) instead. It sucks when you can't climb an incline very well and other robots can shove you around at will.

Re: Mechanum Wheel Sizes
 

Guys,

There is no "h" in mecanum.

Re: Mechanum Wheel Sizes
 

Yeah.... I noticed that after about my second post, sorry :o

Re: Mechanum Wheel Sizes
 

I still don't understand why everyone is interjecting about why NOT to use mecanum wheels, especially in the offseason. Just answer his questions, and move on. Too many opinions, not enough facts.

We used the kit frame, with cimple boxes to test 8" AM Mecanum wheels this year. It was too fast, the wheels spun a lot before actually gripping. Also, the robot would drift more because of the slipping. As I said, our final ratio was around 17.5:1, which gave us a great mix of speed and maneuverability. I would suggest going with the toughboxes, then a 2:1 sprocket or higher. Something like a 12 tooth on the box and 28 or 32 on the wheel. We used CIM's with Dewalt gearboxes.

My suggestion is to just use the kit frame. Our test frame we just moved the inside rails to fit the wider mecanum wheels, then made new spacers. It's quick, easy, and readily available. It definetely flexes. While yes, it is necessary to have all 4 wheels on the ground, practice will help you the most. We didn't have trouble climbing and balances bridges, or running over the key. And I know there were teams crossing the bump with mecanum wheels.

Someone else mentioned the programming. That's where we started with ours.

Re: Mechanum Wheel Sizes
 

Using a gyro allows you to do something called "field centric drive". Basically, if the robot is in front of you and you push your joystick forward, the robot moves away from you regardless of how it is spun.
In effect, the robot has no front.

This is probably the most useful sensor to use for mecanum wheels. I'd strongly recommend it ahead of encoders and accelerometers.

Also, as far as ramps and inclines go, mecanum wheels perform just fine. In 2006, our robot could climb the diamond plate ramp and get on the platform.
In 2010, our robot could climb over the carpeted bump.
In 2012, our robot could climb on the bridge.


Right now is the time to experiment with drive trains. If you have the time and the money, try building a 6 wheel drive and build a mecanum drive. Learn how to use gyros, accelerometers, and optical encoders. Figure out how to do a good job building each, and experience the strengths and weaknesses of both.

Then, when it comes time to pick a drive train, you won't be second guessing yourself.

Re: Mechanum Wheel Sizes
 

Any ideas on how to program this? I have some previous experience with LabView, but I haven't the faintest idea of where to even start with this. Not to mention we don't have a gyro to work with...

Although, we may just end up having a more traditional robot with a front and back. The field centric drive and encoder-based correction sounds more like something to do when we have had more experience with mecanum. I think that four motors with mecanum wheels would be a good starting point, and simply adjust for the lack of a perfect straight line by hand.

Re: Mechanum Wheel Sizes
 

Convert your joystick inputs into an angle and a magnitude. Add or subtract the gyro angle. Convert to wheel speeds.

About as simple as it gets.

Re: Mechanum Wheel Sizes
 

Hmm... not sure how to convert like that yet::rtm::... but it does sound rather simple:). Any idea where to get a decent, cheap gyro compatible with the circuits that come with the KOP? As a rookie team, we don't have any custom circuits or boards to work with that would allow different types of connections.

Re: Mechanum Wheel Sizes
 

Even simpler: use the WPILib robotdrive. It has an input for gyro, and will do all the math for you.

Your 2012 KoP came with a gyro.

Re: Mechanum Wheel Sizes
 

:o..... Nothing else to say... just embarrassed face....

Thanks, though!!

Re: Mechanum Wheel Sizes
 

In my opinion, this is a rather critical design decision that you may want to consider more thoroughly; I would avoid going with a semi-flexible frame. Although I did point out earlier that you want to make sure all wheels are always in contact with the ground, the reason I did so is to ease troubleshooting when driving becomes erratic.

A semi-flexible frame, as its name implies, has less structural rigidity, something you very rarely want to sacrifice in FRC due to head-on robot collisions.

The other problem with using a flexible frame to ensure constant 4-wheel contact is that this has the potential to similarly hinder effective driving. If the frame somehow warps such that all your wheels are touching the ground, but at different angles, your ability to drive straight will probably be seriously hindered. (If your frame flexes such that the wheels tilt out or tilt in, there will probably be minimal loss of control, given the roller design, but if your frame somehow spins your wheels with respect to the vertical, you're definitely going to lose some of your control). There's also the fact that mecanum wheels want to be mounted to a rigid frame, because they rely on that rigidity to achieve the effect that they do (the angled vectors that allow you to drive straight or strafe only do so because they have a constant angle, and the rigidity of the frame is part of what allows that angle to stay constant).

Re: Mechanum Wheel Sizes
 

Oh... I didn't think of those. Thanks! I think we'll be using 6" AM mecanum HD wheels, 4 tough box nanos, and the matching AM wheel hubs. Gyro is currently questionable, but will definitely be looked into! Do you think that having a way of keeping all wheels on the ground at once is very important, or should we concentrate more on a very sturdy frame (that has all four wheels on solid ground) and hope it doesn't warp too much?

Re: Mechanum Wheel Sizes
 

If you are going to order Nanos and hubs (a great choice by the way) go a head and order hex hubs and Nano's with hex shafts. SO much easier and stronger than using keys.

Re: Mechanum Wheel Sizes
 

The best way to keep all 4 wheels on the ground, imo, would be to use a suspension system, or some other mechanism that allows only vertical translational play, and that requires much more complex physics to effectively design.

Assuming you have a flat practice floor, work more on a sturdy frame, and just hope it doesn't warp. We went with a variation of the kitbot frame for our 2011 robot, and it held up fine for quite a while. Look at it this way - when you discover that your chassis' been warped, you can get someone to jump up and down on it!

The flat practice floor is really surprisingly important though. While calibrating speed control, we kept running into problems with the robot being unable to drive straight over certain areas of the floor, despite that we maintained much more control everywhere else. When we took a level to the floor, lo and behold, it wasn't flat, and when we dragged the robot over, we could visibly tell that one wheel wasn't touching the floor while it drove over that spot.

Reiterated just to agree.

Re: Mechanum Wheel Sizes
 

:eek: Oh, didn't know they had different versions. The nano I saw was a hex, and I also found the matching hex hubs.

Thanks! We'll be careful when we order them so we get right ones and don't make it any harder for ourselves! :D

Re: Mechanum Wheel Sizes
 

Thanks!

LOL - I agree! :D

Re: Mechanum Wheel Sizes
 

If you want to implement a successful mecanum drive I would really recommend that you use some sort of suspension. My team puts springs between the frame and the pillow blocks or wheel modules. Its incredibly easy to implement and it completely eliminates the whole rigid frame problem.

Here is the link to a picture of our suspension in 2011 and 2012 (you can see the springs right above the wheels).

http://www.chiefdelphi.com/media/photos/36399
https://www.facebook.com/photo.php?fbid=208337722601334&set=a.2064448461239 55.33843.136209953147445&type=3&theater

Edit: I just wanted to add, in 2011 our suspension had around half an inch of travel. In 2012 the suspension had closer to an inch of travel and we used bushings to make the movement smoother.

Re: Mechanum Wheel Sizes
 

YMMV, but in 2010 we used a modified kitbot, and it had enough 'flex' to work fine with the 8" mecanums.

In 2011 we again used a modified kitbot, but the mecanums were pushed down with an air cylinder, our version of octo-mecanums. The air provided a suspension, and again, it worked fine, frankly one of my favorite drivetrains that our team built.

Having said that, this year we went with 8wd, because it was simple, lighter, and less expensive.(4 transmissions and mecanums=$760)

Our testbed for mecanums was 4 transmissions bolted to a piece of 3/4"plywood, direct driving the wheels. We were surprised how well it drove without a suspension.

Re: Mechanum Wheel Sizes
 

Quoting this again, because it doesn't get any easier.

Use the kitbot frame. Figuring out a suspension and anything else can come after. My focus would be first to make it work.

Re: Mechanum Wheel Sizes
 

Hey, do you guys think we'll need this: http://www.andymark.com/ProductDetai...Code=am%2D0279

It's a hex-bore ball bearing to fit the shaft of the toughbox nano, probably would be mounted on the other side of the shaft. On that note, are the toughbox nano's built-in bearings strong enough to support the weight of the robot, or would they need to be supported by bearings on both sides?

Eh, I think it's a confusing post.... Basically, how many (if any) bearings will we need other than the built-in ones to support the robot's weight without breaking something or unnecessary wear and tear? Any ideas?

Re: Mechanum Wheel Sizes
 

You can go either way. In 2011 we bought the long axle versions and supported the opposite end of the shaft with simple blocks of plastic with 0.5" holes bored in them (we used the keyed versions). We liked that a lot. The disadvantage of doing that is that it's harder to get a wheel off if you need to replace it or repair it. Plus it is extra work to build.

The Toughbox Nano is strong enough to support the robot's weight. Of course, you want to put the wheel as close to the gearbox as is reasonably possible to minimize forces that would try to bend your axles.

Re: Mechanum Wheel Sizes
 

In 2011, we used a drive base very similar to what Scott has described. We then mounted a modified C-Base to the top. We had a nice surface to mount our electronics, and we were careful to leave them easy to reach.

http://www.chiefdelphi.com/media/photos/36120

It was the best, most reliable drive train we've had in 8 seasons, and if the game calls for it again, we'll certainly look at it again in the future. It was built in just a couple hours.

I don't know if this exact setup would work on a non-flat playing field.

Re: Mechanum Wheel Sizes
 

357 has used the Banebots P80 gearboxes now for several seasons with great success - http://banebots.com/c/P80K-S4 They work well if you're looking to keep your gearboxes inline with your mecanum wheels. See a photo below of our 2011 drive train showing how we implemented our gearboxes -



I tried to find a decent picture of our entire drive package mounted to this past year's robot. Our back wheel package pivots to ensure that our wheels are always in contact with the floor, which is an important thing to factor in when designing your mecanum drive. You never know what is under the carpet you'll be driving on.

Re: Mechanum Wheel Sizes
 

I meant to say, "except for 357" above when I wrote "believe me, ignore them". IF you're going to build mecanum drive, and you need a role model, those guys are the ones to look at.

Re: Mechanum Wheel Sizes
 

For several years we've used 12:1 or 9:1 Banebots p80 gear boxes to direct drive four Andymark 6" HD mecanum wheels on a kitbot frame. (The axle should be supported on the other side by some kind of pillow block.) This year the "canum" part of our octocanum is... 9:1 Banebots p80 gear boxes direct driving 6" HD mecanum wheels.

It's easy to assemble and almost foolproof in terms of the mechanics. (Mecanics? :p )

If you're not using sensor feedback, this is indeed true.

In fact, they drive exactly like a 1st person shooter -- most kids these days find driving a mecanum robot extremely intuitive. You can slap together a drivetrain in a day and spend hours and hours practicing on it... Though I would of course recommend that any new drivetrain be done in the off season!

...all that said, there's a reason we upgraded to octocanum, too.

Looks like added complexity to me. It's not like the robot just dies the moment one wheel is in the air -- this is what practice is for! A centered CG and a rugged, non-racked frame is more important than a flexible frame.

Re: Mechanum Wheel Sizes
 

If I could like this, I would.

You can see our base here: http://www.chiefdelphi.com/media/img...933a08b7_l.jpg

It was rigid, many times we'd lift a corner getting onto the bridge. The robot isn't going to instantly spin out if one wheels loses contact. Plenty of practice will be your biggest help. Build it with parts available, practice, and then decide if it's worth continuing to build upon.

Re: Mechanum Wheel Sizes
 

Exactly. It will still handle (almost) exactly like a tank drive if you aren't trying to strafe.

Re: Mechanum Wheel Sizes
 

Suddenly, I think I'm going to convince my buddy programmer to program an xbox controller for our demo bot...(It's our 2010 robot, Trigger, for those familiar with us)

Anyone maybe want to PM me simple code and instructions?