Mechanum Wheel Sizes

[slijin]

Originally Posted by nxtsoccer

we should plan on using a semi-flexible frame that allows for the wheels to raise/lower a little based on terrain

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).

[nxtsoccer]

Originally Posted by slijin

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).

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?

[IndySam]

Originally Posted by nxtsoccer

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?

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.

[slijin]

Originally Posted by nxtsoccer

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?

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.

Originally Posted by IndySam

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.

Reiterated just to agree.

[nxtsoccer]

Originally Posted by IndySam

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.

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!

[nxtsoccer]

Originally Posted by slijin

Assuming you have a flat practice floor, work more on a sturdy frame, and just hope it doesn't warp.

Thanks!

Originally Posted by slijin

Look at it this way - when you discover that your chassis' been warped, you can get someone to jump up and down on it!

LOL - I agree!

[Garret]

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.

[Dad1279]

Originally Posted by slijin

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.
......

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.

[rocknthehawk]

Originally Posted by Dad1279

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

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.

[nxtsoccer]

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?

[Nemo]

Originally Posted by nxtsoccer

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?

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.

[Taylor]

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.

[Ryan Dognaux]

Originally Posted by nxtsoccer

So, I suppose all that remains is to figure out how to turn these wheels. Any suggestions for the drive train?

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.

[Kris Verdeyen]

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.

[pfreivald]

Originally Posted by Nemo

1) Mecanum drives are not difficult to build.

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? )

Originally Posted by Nemo

2) Mecanum drives are not difficult to program.

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

Originally Posted by Nemo

3) Mecanum drives are not difficult to drive.

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.

Originally Posted by slijin

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.

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.