18-05-2010 07:18

ttldomination

Re: pic: Drivetrain Prototype

Hm...

What's the diameter of the wheels?

And what is the effective wheel base?

A lot of robotics in '09 were wide based, so this stance might be alright. But some teams took *this* design even further and turned it into a 6WD.

And also, you have a lot of supports, or spacers, running along the top of the chassis, it might also be benefitial to put some support along the bottom.

- Sunny

18-05-2010 07:28

sgreco

Re: pic: Drivetrain Prototype

Make sure you add some cross supports to the bottom of the two plates. Right now if that thing takes a hit it's going to bend inward.

Your sprockets look large, if you reduce the size of the sprockets both off the gearbox and on the wheel you can save some weight (and space) and keep the same ratio.

18-05-2010 09:55

Ian Curtis

Re: pic: Drivetrain Prototype

I'm not particularly qualified to talk about drive chassis durability, so I don't have much to add.

However, I think I can save you some weight on this, or another drive train that you build. Having a pair of sprockets with built in hubs is heavy, and having those extra couple of inches of steel shaft is heavier. What you could do is take an AM hub and attach a pair of sprockets to it, spacing the sprockets with bits of plastic. Then just run enough shaft to cantilever the sprockets and hub.

A top down view would look like = []: () : ()

Where = is shaft, : are spacers, () are sprockets, and [] is the hub. Spaces are only there to prevent them from being turned into smilies.

Does that make sense?

18-05-2010 12:32

EricH

Re: pic: Drivetrain Prototype

You could save a bit more weight without losing much strength by changing all the X patterns into pairs of triangles. Your machinist will also thank you for doing that; fewer cuts means fewer opportunities for something to go haywire and less time before you get the plate back.

Also, I might ramp up the plate thickness a bit around high-stress areas--mounting points and axle attachment areas--or put a flange on, like you would do for sheet metal.

18-05-2010 18:22

Hawiian Cadder

Re: pic: Drivetrain Prototype

i would recommend against dually P80s. our team has used them in the past and found that they are hard to assemble, as well as grease migrating towards the motors and out of the planetary part of the gearbox. two single P80s would be better in my opinion.

18-05-2010 18:24

548swimmer

Re: pic: Drivetrain Prototype

Two single p80's per side? Or two total?

Also, we have never had issues with our p80's, though is the first year we have used them.

18-05-2010 18:33

sdcantrell56

Re: pic: Drivetrain Prototype

I would shy away from the p80's simply for the fact that the planetary transmissions are less efficient by design and arguably more fragile than standard toughboxes. You can easily buy all the parts necessary for direct drive from andymark as well and direct driving one wheel and chaining to the other would be the most reliable and lightest weight solution.

18-05-2010 18:46

PAR_WIG1350

Re: pic: Drivetrain Prototype

yup, as long as it's properly tensioned 35 chain.

18-05-2010 19:08

548swimmer

Re: pic: Drivetrain Prototype

I'm not sure why, but we always use 25 chain

18-05-2010 19:16

EricH

Re: pic: Drivetrain Prototype

25 chain is lighter, but less forgiving than 35 chain.

18-05-2010 19:27

548swimmer

Re: pic: Drivetrain Prototype

Yeah, thats probably why. We also invest a lot of time in tensioning our chains perfectly, so we can get away with it.

18-05-2010 20:16

PAR_WIG1350

Re: pic: Drivetrain Prototype

The issue is reliability, you can tension 25 chain properly, but then it stretches and falls off, with 35 chain, some stretching is more acceptable and doesn't need to be adjusted as much. If weight is your focus, and you have no issues getting things tensioned properly, than timing belts may be a better option as they don't stretch (depending on the material), but they aren't always as strong or reliable either.

18-05-2010 20:25

NickE

Re: pic: Drivetrain Prototype

I have never seen a properly tensioned #25 chain break or fall off. Team 254 has been using 25 chain for years with no reliability issues.

18-05-2010 21:04

Akash Rastogi

Re: pic: Drivetrain Prototype

I agree with Nick here. I've seen many people complain about reliability of 25 chain, yet I have yet to see my team have any problems with it. There has to be something you aren't doing properly.

18-05-2010 21:18

sdcantrell56

Re: pic: Drivetrain Prototype

I would say for a team that is designing everything in CAD and understands the need for tensioning, 25 chain should absolutely be used. We have only ever used 25 chain and this past year used timing belt. The belt worked flawlessly but does require even more planning beforehand. I would definitely put emphasis on really determining what size of belt you need for the given power. All things considered, 25 chain isnt much heavier than belt and probably easier for you guys so I'd say stick with that but move to direct driving one of the wheels and a spur gear transmission ala toughbox

19-05-2010 02:55

Akash Rastogi

Re: pic: Drivetrain Prototype

If you're using 1/8" aluminum without flanges, you might as well leave it unpocketed.

19-05-2010 08:28

Mop Iii Top

Re: pic: Drivetrain Prototype

Hold the phone here. Why are you using 4 CIMs? I can see using 4 CIMs for Mecanum wheel, besides you have to use 1 CIM per mecanum wheel. This years limit on CIM Motors was only 5. You are using up most of them and not leaving yourself more for other things. If you used only 2 CIM Motors, one for each side, you would give yourself 3 CIM Motors to play around with on the rest of the robot.

19-05-2010 08:53

artdutra04

Re: pic: Drivetrain Prototype

Only using 2 CIM motors in a drive train will have a noticeable performance drop on regular carpet, as the robot will accelerate slower and in most cases will be torque limited (which means it'll lose almost every pushing match). Make Mark Leon happy and do the math. The numbers won't lie.

In 2009, the FRP flooring and acetal wheels had a much lower coefficient of friction, thus making a 2-CIM drivetrain not only feasible, but much better than a 4-CIM drivetrain at maximizing the potential use of each motor.

Edit: If you really want to have three CIMs for elsewhere on the robot, use CIM+FP combo for each gearbox. It'll provide better performance than a lone CIM would, and depending on how it's utilized (here's where math comes back in) might just very well be able to pull off a friction limited drivetrain, especially if it's use in conjunction with a shifting transmission.

19-05-2010 10:03

Jared Russell

Re: pic: Drivetrain Prototype

As 254's 2004 & 2010 robots and 1114's 2010 robot (among others) demonstrated, just because there are 4 CIMs in the drivetrain doesn't necessarily mean that they can't *also* be used for other functions...

19-05-2010 10:14

Ether

Re: pic: Drivetrain Prototype

Hey Art,

You might be interested in the discussion going on here:

http://www.chiefdelphi.com/forums/sh...ad.php?t=85837

(mixing Victor and Jag controllers for 2 CIMs on one toughbox)


~

19-05-2010 10:34

Andrew Schreiber

Re: pic: Drivetrain Prototype

There are also another, less obvious benefit of having your CIMs in your drive. It will help keep your CG lower. 10lbs of CIM motors is a pretty decent amount of weight that you want to keep as low as possible.


Op, my only suggestion to you is to use the Toughboxes in the KOP. Not knocking the P80s at all but why spend money when you don't have to? Also, a word of caution, a wide stance bot like this will be susceptible to tipping if your CG is too high. In a game where you have to have a large arm lifting a heavy object (2008, 2005) I would be cautious about using a wide stance bot like this without outriggers (see 118 in 2005 or 33 in 2008 for a good example).

19-05-2010 11:06

Rion Atkinson

Re: pic: Drivetrain Prototype

On the "wide stance tipping" comment I'll back that up, all the way up until the point where the arm is required to reach right to left of the robot. Then wide stance would probably be the better option. (In my opinion)

I'll echo what Akash said. If you aren't going to flange the metal, don't put holes in it. You need that strength. (If you don't know what flanges are, I'll be more than happy to explain.

Also, your bars going all the way across. You really only need them to be 1/8" wall. I would put three on top, two one bottom. Or even two on two, and two on bottom. I would also only take them across the center of the chassis and then use stand-offs to support the outer cover, done right you will be plenty strong enough.

My $0.02
-Rion

19-05-2010 13:17

jspatz1

Re: pic: Drivetrain Prototype

Looks good, but here's a suggestion: Your front-to-back wheelbase dimension looks pretty short. Looks to be quite unstable in terms of tipping forward or backward. For any wide format chassis, this is always an issue, and the wheel centers usually need to be pushed as far apart as the frame design allows. Most FRC games require the robot CG to be high enough that tipping can be a problem if the wheelbase gets too short. Perhaps you are trying to shorten this distance to enhance turning. That is a factor, but is usually not as important as staying upright.

19-05-2010 14:09

PAR_WIG1350

Re: pic: Drivetrain Prototype

Now that I think about it, you're right. this years robot had many issues with 35 chain, but the 25 chain never failed.

but still, if you take the time to tension the sprockets properly, timing belt is another option

19-05-2010 15:23

sdcantrell56

Re: pic: Drivetrain Prototype

There is no question that 35 chain is more forgiving than 25. It allows for much more carelessness in design and misalignment albeit at a huge weight penalty. I would look at the systems in question on your design and try to identify the cause of the failure as it is not from the chain itself but rather the implementation.

19-05-2010 18:02

Chris is me

Re: pic: Drivetrain Prototype

Using a drivetrain with less than 4 CIMs in FRC is something that should be approached carefully (I'm trying to get as close to saying "never" as possible without saying it). If you don't shift, I would only use it in the extremely rare case that your drivetrain is only used for a short while in a match, like 125's robot this year which cannot function on the ground basically. Traction limited driving with 1 CIM is a painfully slow speed, acceleration will take a hit, and you basically just lag behind. Using KOP wheels to lower traction in order to rectify this makes you lose in pushing. Adding a shifter can help then, but you're still getting up to 7-8fps rather slowly and with a high current draw. 4 motors can give you a nice 6.5 fps or so with a good balance of current draw, acceleration, and traction-limited-ness. Your drivetrain is the absolute most important mechanical system on the robot. Before you shortchange it, make sure you fully understand the tradeoffs and do the math.

19-05-2010 20:29

Garret

Re: pic: Drivetrain Prototype

My team has used drivetrains that use only two CIMs for as long as I can remember (exception 2004 when we used four gearboxes) and we know that being torque limited is a major problem. In general our only problem was overheating. In the past we dealt with this by making heatsinks for the CIMs so as to prevent them from over heating. In my opinion how you choose to distribute you CIMs really dependes on the challenge.

19-05-2010 21:53

PAR_WIG1350

Re: pic: Drivetrain Prototype

I blame frame warping. Boston was no issue for the drive-train, Atlanta was a totally different story. But the complexity of the chain runs probably helped
(double sprocket on tough-box to double sprocket on bearing blocks to wheel to wheel [times 2]) + (double sprocket on tough-box to wheel to wheel [times 2])
{we had a double sided drive-train and needed to get power to both sides.}