Drivetrains in Ultimate Ascent
 

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?

Re: Drivetrains in Ultimate Ascent
 

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.

Re: Drivetrains in Ultimate Ascent
 

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.

Re: Drivetrains in Ultimate Ascent
 

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.

Re: Drivetrains in Ultimate Ascent
 

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).

Re: Drivetrains in Ultimate Ascent
 

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! :D

Re: Drivetrains in Ultimate Ascent
 

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 :cool:

Re: Drivetrains in Ultimate Ascent
 

Our team's strategy relies on being able to move omnidirectionally, and mecanum wheels weigh quite a bit less than swerve. I think that our robot goes as fast as this game allows. We've built a few scary fast robots(15+ fps) that could cross the whole field in <5 seconds, and when driving those around, we still found that the mecanum wheeled robot driven by an experienced driver could pass by them on the way to the feeder station. We're not trying to get perfection, and we don't plan on even trying to push another robot around. Ideally, we would have swerve, but we can't afford the weight/time to manufacture, so we've compromised. I think that mecanum wheels are a great idea for average teams looking to get a quick/easy omnidirectional drive.

Re: Drivetrains in Ultimate Ascent
 

To add to this:

With mecanums, your weight needs to be equally balanced across the robot. The reason for this is that if the center of gravity is shifted to any one side, the forces will no longer balance properly and you will experience "drift." Unless your robot design minimizes this as much as possible, it will need to be accounted for.

This can be rectified by using gyros, accelerometers, and encoders to adjust the output to the wheels... but at the cost of mechanical, electrical, and programming complexity and decreased robustness.

Properly implemented mecanums can be great in games where their use is justified (high agility). However, it's something that teams should decide early on and take into consideration when deciding every other aspect of the design. It may be a better use of your team's time to optimize a simpler, more standard drivetrain instead.

Just my $.02

Re: Drivetrains in Ultimate Ascent
 

Our mecanum wheels have encoders for speed PID control, and a gyro to correct for any rotational drift. Also, we have "suspension" modules on our wheels so that they have equal force pushing down. What we've seen is that if somebody is pushing us, we have enough power to slide away, and there's enough room for our experienced driver to drive around them. We've gotten pretty good at avoiding defense.

Re: Drivetrains in Ultimate Ascent
 

You have enough power to push out of pins? ;)

Re: Drivetrains in Ultimate Ascent
 

We don't have enough power to push out of pins, but from what we've seen, for our level of competitiveness, we don't really need to. We've built shifting swerve modules (and spent way too much time designing the shifting part), and found that when we were playing, our drivers never really had to worry about being pinned down, because they could drive quickly away from pushing teams. Our goal is to be a fast robot. Our strategy is to drive over to the other side of the field, get in the loading station, fire feeder station shots until we get defended, the zip over the the pyramid and climb for 30 pts/dump. In order to get a great climber that we felt was "safe" enough, we made some parts out of steel. To line up with the pyramid, we need to have onmidirectional drive, so mecanum wheels were our choice.

Re: Drivetrains in Ultimate Ascent
 

They have about G31 power to push out of pins.

Re: Drivetrains in Ultimate Ascent
 

We are using an 8 wheel drop with VEXpro Versa Wheels. We have custom built dual output AM SuperShifters Picture here We have a low gear speed of about 5 FPS and a high gear of around 12. 6 wheel drop has always served us well, but after trying out 8 we like it better. We used mechanums once and liked them for the game (2007), but not for anything else. In other games where mechanums would have been good we opted for crab and swerve.

Re: Drivetrains in Ultimate Ascent
 

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!

Re: Drivetrains in Ultimate Ascent
 

How did you like VEXpro's wheels? We considered buying some wheels from them (not these), but decided not to because they are a new design and were never used before.
To be honest I really like some of their designs but we didn't have the courage to use them.

Re: Drivetrains in Ultimate Ascent
 

1014 is using six (4") wheel drive, with two omni wheels at the front (as determined by where discs are released) and two pairs of traction wheels, one pair in the middle and one in the back back. With tougbox mini at a 12.75:1 ratio. It is not fast but is very maneuverable and has good pushing power. No drop center. The omni wheels allow the robot to turn almost in place.

Re: Drivetrains in Ultimate Ascent
 

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

Re: Drivetrains in Ultimate Ascent
 

We're running 4WD WCD, direct-driven via VexPro single speed gearboxes and a 2nd VexPro gear stage to 4" AM Performance wheels. Can't remember what the final ratio is, but the speed is roughly 11.6ft/s before losses. The wheelbase is 30" wide x 18.5" long. Center of Gravity is biased 4" forward of front/rear center, and is nearly perfectly balanced left/right. It took us 1.5 weeks to fab it, and most of that was waiting to figure out what the frame would look like due to climbing and/or floor pickup. It weighs only 24.5 lbs and cost less than the kitbot because we made everything in-house.

Man does it fly too.

Re: Drivetrains in Ultimate Ascent
 

They're actually quite nice and are very, very grippy. Versa hubs are great too because they are very light weight and small. I encourage you to try the ones you considered for next season or later this season.

Re: Drivetrains in Ultimate Ascent
 

I will have a second look. Thanks

Re: Drivetrains in Ultimate Ascent
 

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!

Re: Drivetrains in Ultimate Ascent
 

This is the second year in a row we are using Brecoflex treads. We are still debating whether to use treaded belts or smooth belts during competition, though.

Re: Drivetrains in Ultimate Ascent
 

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.

Re: Drivetrains in Ultimate Ascent
 

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

Re: Drivetrains in Ultimate Ascent
 

What product are you using from them? A belt with a certain type of backing? or something else? I am quite intrigued by this brecoflex brand and its products.

Re: Drivetrains in Ultimate Ascent
 

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.

Re: Drivetrains in Ultimate Ascent
 

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.

Re: Drivetrains in Ultimate Ascent
 

The term "friction-limited" refers to the fact that the wheels will slip before the motors will stall, due to the amount of torque available.

Re: Drivetrains in Ultimate Ascent
 

We're using VexPro mecanums direct driven off of toughbox minis, with drop-down, geared-down 2" Colsons for anticipated pushing matches.

Re: Drivetrains in Ultimate Ascent
 

As build lead and drive coach for Team 422, I can confirm that if we initiate the contact, a bad time is assured.

Re: Drivetrains in Ultimate Ascent
 

We bought two pairs of belts, one with no backing and one with this backing.

Even with a 1/4 drop, the backing was giving our power supply a hard time.

Re: Drivetrains in Ultimate Ascent
 

We're using 4 Mini-CIMs on a pair of 31 inch Toughbox Nanotubes (gear drive) with outboard 4 inch performance wheels and roughtop tread. I believe our top speed is around 12fps (it's within a sixteenth).

Re: Drivetrains in Ultimate Ascent
 

Any particular reason for using Mini-CIMs over CIMs?

Re: Drivetrains in Ultimate Ascent
 

2 Cim 4 Wheel Drive.

Re: Drivetrains in Ultimate Ascent
 

4 CIM, 4WD wide configuration Kitbot, KoP wheels.

Re: Drivetrains in Ultimate Ascent
 

6 x 4" VersaWheels with 3/32" center drop in a long configuration. Laser-cut sheetmetal. 4 CIM VexPro Ball Shifters. Like everyone else we anticipate a lot of pushing matches this year moving back and forth to and from the feeder station.

Re: Drivetrains in Ultimate Ascent
 

Vex Pro Drive in a Day, 6 x 4" Vex Pro Performance Wheels, Roughtop Tread, 4 CIMs in VexPro Ball Shifters, 14/6(ish) ft/s.

Re: Drivetrains in Ultimate Ascent
 

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.

Re: Drivetrains in Ultimate Ascent
 

4 CIMs in 3 stage VEX Pro ball shifters. We might switch to 3 CIMS in 1 stage gearboxes after our first event, based on how well these work.

The more interesting thing is a completely custom tread drive system. See video for some footage of it driving. The tread is a Brecoflex conveyor with the drive cog run by chain from the gearboxes. Two 3/16 side plates hold each drive module together; the idlers are made from round delron stock and the last one can move to tension the tread.

Why treads this year? It is part of our team credo never to lose a pushing match, even with Trucktown (we were on an alliance with them last year, we never got a chance to test their mettle unfortunately). Speed is reasonably good and they are quite stable.

Re: Drivetrains in Ultimate Ascent
 

We're using one CIM for our shooter and two for our climber, so we couldn't put four of them in drive. We didn't find any significant decrease in drive performance between the full sized CIMs and the Minis, so we stuck with it.

Re: Drivetrains in Ultimate Ascent
 

We went with Andymark Mechanums. Our driver is quite adept with tem and has been practicing nonstop from week three til now with the practice bot to get even better before FLR.