Drivetrains in Ultimate Ascent

[apples000]

Our team has been curious as to what drives teams are using on their robots. Our team's performance has never actually been limited by our drive system so this year we agreed to sacrifice a complex drive system for a more refined climber/shooter. During week 1 we played around with older robots, and determined that we would benefit from an omnidirectional drive. Several years ago our team made a shifting swerve drive that had shifters/CIM's inside the rotating wheel modules. After our full season (2 regionals/Atlanta), we never shifted to the low gear to push other teams around. This year we've done testing with 6wd with vex pro ball shifters, or mecanum wheels. We've actually found that our 8" vex pro wheels, when geared properly, let us easily outmaneuver our 6wd robot. After reading online that mecanum wheels are so bad, we were reluctant to put them on, but we've only been impressed by them. Why do people say that mecanum wheels are so bad?

[protoserge]

Our official release video will be Monday, but we used the KoP chassis and gearboxes (not KoP ratio) to maximize our schedule for more important objectives.

[bardd]

Kitbot. Simple and elegant.
I have to admit, before recieving the kit, I thought the belts won't cut it and wanted to use a frame with chains like past years, but AndyMark and Gates did a wonderful job and created the best kitbot I got to work with. This is the first time I liked working with belts.

For me, recognizing AndyMark and Gates's amazing job with this kitbot was almost enough reason for me to vote for it, even before examining its advantages.

[apples000]

I agree that the kitbot frame is a great way to go. We used almost exactly what came in the KoP! Our only modifications were some cutouts/reinforcements, and we replaced the belts with chains. Also, we've found that using well designed products from andymark is really the way to go if you need all the time you can get.

[sgreco]

Originally Posted by apples000

Why do people say that mecanum wheels are so bad?

Mecanum are "bad" because they are inefficient. The theory of a mecanum wheel is that it behave in the same manner as having omni wheels at 45 degree angles (with respect to the front of the bot) on each corner of the robot.

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Sort of like that. When all the wheels spin "forward" they are generating force vectors in both the x and y direction (if you're looking down at the page, y up, x left and right). So you have a force vector from each wheel in the forward direction of Ftotal * sin45, all the x forces are lost because they cancel out. Sin45 (or cos45) is about .707. So you're only going to get about 70.7% efficiency with respect to what you could get if all the wheels were straight.

Also in order to drive "sideways" mecanum wheels have to be geared pretty low in order to have enough torque to actually move sideways. This means they will end up being reasonably slow in the forward direction in an FRC application.

Because mecanum wheels have rollers they don't have a lot of friction resistance to being pushed sideways, so they're easy to be defended. They also have trouble playing effective defense because they can be pushed out of the way sideways. In head on contact, they are limited by the 70.7% relative (and theoretical) efficiency referenced above.

Mecanum wheels also all have to be powered separately. This is not a huge deal, but it means 4 gear boxes. Then if you want to fix the low gear ratio problem and you put on 4 shifters, your robot gets really heavy really quickly.

Also, Mecanum wheels are heavy, and they come no smaller than 6 inches (unless you make you own). This means they have a high moment of inertia. A high moment of inertia in your wheels will restrict your acceleration.



(Note: I don't have any physical proof for the "70.7%", so maybe somebody has run a test and they can give you better information. Also, note that the 70.7% number is not the overall efficiency, it's a theoretical number relative to what you would theoretically get from normal wheels, all pointing straight, that have the coefficient of friction, the same weight, size and same moment of inertia).

[ice.berg]

Originally Posted by sgreco

Mecanum are "bad" because they are inefficient. The theory of a mecanum wheel is that it behave in the same manner as having omni wheels at 45 degree angles (with respect to the front of the bot) on each corner of the robot.

/ \

\ /

Sort of like that. When all the wheels spin "forward" they are generating force vectors in both the x and y direction (if you're looking down at the page, y up, x left and right). So you have a force vector from each wheel in the forward direction of Ftotal * sin45, all the x forces are lost because they cancel out. Sin45 (or cos45) is about .707. So you're only going to get about 70.7% efficiency with respect to what you could get if all the wheels were straight.

Also in order to drive "sideways" mecanum wheels have to be geared pretty low in order to have enough torque to actually move sideways. This means they will end up being reasonably slow in the forward direction in an FRC application.

Because mecanum wheels have rollers they don't have a lot of friction resistance to being pushed sideways, so they're easy to be defended. They also have trouble playing effective defense because they can be pushed out of the way sideways. In head on contact, they are limited by the 70.7% relative (and theoretical) efficiency referenced above.

Mecanum wheels also all have to be powered separately. This is not a huge deal, but it means 4 gear boxes. Then if you want to fix the low gear ratio problem and you put on 4 shifters, your robot gets really heavy really quickly.

Also, Mecanum wheels are heavy, and they come no smaller than 6 inches (unless you make you own). This means they have a high moment of inertia. A high moment of inertia in your wheels will restrict your acceleration.



(Note: I don't have any physical proof for the "70.7%", so maybe somebody has run a test and they can give you better information. Also, note that the 70.7% number is not the overall efficiency, it's a theoretical number relative to what you would theoretically get from normal wheels, all pointing straight, that have the coefficient of friction, the same weight, size and same moment of inertia).

OOOFTA!! That was a long analysis but it included all the important disadvantages of mechanums.
Well our team went with mech wheels this year for the third year in a row. But every year we change something up a little bit. Check out the live stream tonight to see what we did!

[protoserge]

Mecanum wheels are pretty awesome. They have their place, and for our strategy, this was not the year for them. Logo Motion, on the other hand, was fantastic with them.

I'm curious how VexPro Mecanums are... I might get some this summer and create a 12-CIM Mecbot

[ice.berg]

Originally Posted by stinglikeabee

I'm curious how VexPro Mecanums are... I might get some this summer and create a 12-CIM Mecbot

We are running the Vex mecanums and have been very pleased with the results so far. The weight advantage over the AM mec. wheels is HUUUGE. Only problem is you can not replace any of the parts on the vex unlike the AM wheels. Overall though I think it is a great product!

[JVN]

Originally Posted by ice.berg

We are running the Vex mecanums and have been very pleased with the results so far. The weight advantage over the AM mec. wheels is HUUUGE. Only problem is you can not replace any of the parts on the vex unlike the AM wheels. Overall though I think it is a great product!

We now sell mecanum roller kits for those teams who wish to swap out used rollers:
http://www.vexrobotics.com/vexpro/wh...um-wheels.html

Are there any other parts you wanted to be able to replace? I think everything else is pretty modular.

-John

[ice.berg]

Originally Posted by JVN

We now sell mecanum roller kits for those teams who wish to swap out used rollers:
http://www.vexrobotics.com/vexpro/wh...um-wheels.html

Are there any other parts you wanted to be able to replace? I think everything else is pretty modular.

-John

John, This is exactly what we wanted, nice and cheap to be able to replace the rollers like on the AM versions. Your team at vex has put out some great products and a see a bright future for you guys. Keep it up!

[Nemo]

We're running VersaWheels with Vex single speed / single stage gearboxes. Gearbox is chained to the wheels (32:16). That gives us a theoretical speed of 8.7 ft/s, which is the slowest we've geared since I started.

We decided that we had enough on our plates without doing anything fancy on the drive. With the chain reduction, we were able to to keep gearing it slower until we could spin the tires.

VersaWheels + single stage is just about as simple, cheap, and as light as it gets. We like it.

[ehfeinberg]

449 is running AM mecanums directly driven off of Toughbox Minis. Our entire robot doesn't have a spec of chains or belts.

[Anupam Goli]

1648 is using a wide 4wd with the nice Vexpro traction wheels and custom black roughtop tread. I love these wheels, they have so much traction that we ended up pulling carpet fibers out of our wheels. The traction was too much for our old carpet (talk about burnin' rubber)! We're also using the Toughbox mini's with a custom ratio (can't remember off the top of my head) with some sprocket work (15T on gearboxes, 22T on wheels) to get ~9 fps using JVN's calculator.

[PayneTrain]

We'll see if the build lead for 422 comes out of the woodwork for this discussion, but I'll give you the spiel that you'll probably hear if you stop by the pit:

The drive train is quite the venture for 422, being the first non direct kit or kit-modified drive train system in years. There are a lot of things that were learned ("Hey, maybe we should be a little more gentle when welding" or "Hey, maybe our bearing blocks shouldn't be 1/12 of the robot weight") It's a 6WD "West Coast Inspired" drive system with 6-inch AndyMark performance Wheels lovingly covered in Blue Nitrile, feeding into a full VEXpro system of sprocket, chain, the single reduction gearbox, and 2 CIMs and 1 mini CIM on each side.

The numbers you're probably looking for:
15.17 fps, operating at 90% system efficiency on a total gear reduction of 8.25:1, creating what he calls a "friction limited system," a term I can't remember because I haven't slept in 30 hours. I think the idea is that as long as 422 initiates the contact, the other team is probably in for a bad time.

Now obviously a 6 motor drive train is not a battery's best friend (our new battery chargers are!), but 422 can rev up their shooter while driving (taking shots while moving isn't an ideal situation strategically or mechanically). I imagine the team will learn a lot this year, as they already have in building it. The drive team is just itching for Week 3 to come so they can pull it out of the bag and drive it around some more.

I didn't put a hand on a tool for this, that's just the readout they gave me to make the video.

[AllenGregoryIV]

Originally Posted by Nemo

We're running VersaWheels with Vex single speed / single stage gearboxes. Gearbox is chained to the wheels (32:16). That gives us a theoretical speed of 8.7 ft/s, which is the slowest we've geared since I started.

We decided that we had enough on our plates without doing anything fancy on the drive. With the chain reduction, we were able to to keep gearing it slower until we could spin the tires.

VersaWheels + single stage is just about as simple, cheap, and as light as it gets. We like it.

We're using 8 VersaWheels + 1:5.33 single stage + 15/26 chain reduction. All of this on the VEXPro Chassis. So far it has been one of the best drive trains I have worked with in FRC. It's cheap and works well.