Re: pic: FRC294 Prototype base almost done
 

I think those wheels are the same as they used last year, which would be about 6" (also, the sideplates are listed as 5" and aren't that much smaller than the wheel).And they do have a lot of traction; they've lost tread playing defense with that blue material. And a four-speed of this type would be easier to design and build than a three-speed.

Re: pic: FRC294 Prototype base almost done
 

I'm kind of concerned about some things you are saying....

I'm not sure how we don't have a "huge" amount of traction. The coefficient of friction of that tread was measured to be 1.1-1.2. If we wanted to max it out we could go for natural rubber that 254/968 used this year and get about 1.3. I'm not sure how this isn't the maximum amount of traction teams have been able to get. Tank treads have been experimentally shown to only get a minimal increase (about 3% according to Andy Baker and testing he did with 45), if any (according to most. Paul Copioli will probably fight to the death on this one) at all.

Wheels are 6" btw, and yes #35 chain.


In terms of the transmission, it is an iteration of team 33's 4 speed; which is two crash shifters in a row. This Drivetrain is designed around the AndyMark Gen 3's (or a slightly modified 3 motor AndyMark single speed). We wanted to do something fun and different for for prototype though, and it didn't hurt that the material for the 4-speed was significantly less than the cost of two Gen 3's. Also, we figured the 4 speeds would give a better spread of gear ratios to show in person for prototyping next season (I think most people on the team who aren't into drivetrains would benefit from seeing 6fps vs 9fps vs 12 vs 16 [those aren't the actual speeds, just arbitrary numbers]).

I believe we are traction limited in the bottom two gears, and the speeds depend on the size of the output sprocket on the transmission. I think they have a 10 T on there. I have the excel at home with all the speeds (I'm going home tomorrow, and will post then). Even though the lowest gear is way past traction limited, that isn't necessarily a bad thing; it will allow for amazingly precise alignment and will be able to hold position better while drawing less current. I'm referring to a PID loop that will hold the robot's heading (even stationary, I think the triplets did that amazingly in 2006) very accurately.

EDIT: EricH; thanks for referencing that. The wheels are very similar to last year, just bit wider now. We were losing tread because we had two bolts side by side that almost cut the tread and have, so it was very easy to tear off. We fixed it before champs by changing it to single bolts like in the picture. Haven't lost a tread since. Too bad you won't be seeing it at San Diego again :(... but there is allways LA! (although, I'd much rather see 294 on the field with 330 than against.... I think 294 would have a better chance of winning that match with you instead of against)

Re: pic: FRC294 Prototype base almost done
 

Two more questions to bother you with.

Can we get a close up shot of your tensioner?

What do you use for axles? They don't appear to go through the plates or be driven, meaning you could simply use a grade 8 bolt through one side and threaded into the other.

Like this...

(Will post pic from my home computer tonight)

Re: pic: FRC294 Prototype base almost done
 

Close up shots will be posted this weekend by me (or sooner if borisdamole hears the request and goes to his livingroom).

It's almost identical to last year's. Pretty much it's just a block that sits in a piece of tubing. Screws pull the black away from the chain to tension, and there is a shoulder bolt in the block that the idler sits on. It was inspired by our drive in 2005/2006 where we used pillow blocks that were tensioned that way (same way the west coast drive has been tensioned sometimes... like 968 in '06. In fact, we originally got the idea from 60).

The axles are 3/8" 7075 hex we already had laying around. Just threaded one end 3/8-16. We considered bolts, but we wanted this base to be as light as possible.

Re: pic: FRC294 Prototype base almost done
 

Sorry Adam--Despite Mines's spring break ending the weekend of LA, I'm not yet sure whether or not I'll be leaving SD over break. Maybe I should drop by during Week 1???

Re: pic: FRC294 Prototype base almost done
 

As far as traction I was mostly referring to the width of the wheels. In my mind I was comparing them with wide tank treads or a 3" inch wide natural rubber wheel that we had custom wrapped with 60 durometer natural rubber a couple of years back. I realize that the normal force will not change because the weight of the robot is limited so widening the wheels only spreads out the normal force on a wider surface area. I haven't really experimented with this so I don't know what the trade off is. I know that there would be a point of dimenishing return as far as increasing the tread contact area to normal force ratio. I just know what I have observed which is I have seen tank tread and wide wheeled robots winning the pushing battle against the narrower wheeled robots. My imppresion was that the wheels on your frame were of the narrower variety. This is purely subjective. I would be interested in knowing the width of said wheels.

I would also love to do some testing with comparing different traction device and finding out what the return is on increasing the traction surface area realtive to the normal force. If my team does this experiment we will post the results of our findings.

As far as the different gear ratios being traction limited I agree that a slower ratio would definately give you more accurate control. I would love to see the spreadsheet of speeds when you get back home and have a chance to post them.

I agree with you EricH that the four speed in this type of shifting is easier to design then a three speed. I would love to see someone design and build a squential shifting three speed ball shifter I think that would be sweet. If I ever get the time to actually sit down and dore more inventor work I will make one and post the pics. One of the down sides of having a full time job and other demands on my time other than robotics.

Re: pic: FRC294 Prototype base almost done
 

I imagine that is more an effect of the wide wheels wearing down slower than thin wheels; since many tank tracks and wheel treads have a 'cleat-like' pattern, tread wear probably significantly reduces traction. Plus, if the tank treaded robot is running something like Brecoflex Red Linatex (sp?) treads with a CoF of 1.6, I am not suprised that they beat robots running traction wheels with a CoF of 1.3-1.4.

Its actually already been done.

Re: pic: FRC294 Prototype base almost done
 

Actually the custom wrapped rubber wheels never wore down because they did not have a tread pattern in them and the tank treads that we have used are solid with no tread either. So that is not really a problem. In my opnion the tread pattern doesn't really help but I have nothing to substantiate it and this thread is not rellay the place to discuss it. Also I was referring mostly to the surface contact area difference between the differnt wheels and treads not really to the coefficent of friction.

Coefficent of friction is not the end all be all comparisson to tell if someone has more traction it is a factor inthe equation which includes in addition the normal force per square inch and the square inches in contact with the ground. However you do want to maximize the coefficent of friction if you are looking for the best traction you can get.

Sorry if I'm contributing to pulling the thread away from the original intent which is to discuss 294's prototype drive system. I don't really want to do that and I don't really like when it happens.

Back on topic!

I really like the design of the drive sytem, using the pins on the corners really makes a lot of sense.

Another question what bearings are you using on the drive wheels?

I have found that a standard bronze bearing can wear down an aluminum shaft over time. Just something to think about. We had really good luck with rulon and peek bearings which are high perfomance plastic bearings. They work well because they are designed to be compatible with softer shaft materials. The rulon bearings are a reinforced PTFE bearing and I know that you can get them and the peek bearings at mc-master carr.

You could probably save some weight by hollowing out the axles some if you wanted. We have run aluminum tube axles and have great sucess with them. There is minimal shear on them and no unspported length so it hasn't been an issue.

Re: pic: FRC294 Prototype base almost done
 

correct... for the wheels, the drive chain is #35 & yes we will be placing idler sprockets to decrease noise/improve efficiency

6" wheels with approx. 1" tread width

We are using a design based on The KillerBees FRC33 4-speed pneumatic shifter... there are 2 sliding geared axles between 3 fixed geared axles..
The shifting is done by 2 pnuematic cylinders....



gear ratios are:
1.00:1
1.78:1
2.78:1
4.94:1

Adam Heard, I think in a later post answers that question..

This off-season project was done more for "the fun and challenge" in preparation for 2008.. We developed some new machining methods.. learned what will work and what needs to be corrected in V2.0... By putting this out on CD we also get feedback from other teams who can speak from experience.. invaluble to the process...
The side modules with wheels were if I remember correctly are 7ish pounds each.. the transmission assembly I don't know.. but I will find out soon and post that when we have them off for a bench break-in period..

Also some one was interested in the tensioner assemblies for the drive chain..





The axle block slides up and down in the cage and is held/drawn into tension by the overhead thumbscrews...

Hope this answers some of those questions...

Re: pic: FRC294 Prototype base almost done
 

Thanks for reminding me about the speeds John, I forgot about them :).

Well, we think an output sprocket of 12-16 is appropriate depending on the game situation. Which gives us 5 reasonable sets of speeds. Now, keep in mind, not all 4 speeds of each set are the ones that would be useful for a game; We already know 4-speeds are not the most practical and we already know that really high speeds can not always be used. Also, these speeds are with estimated inefficiency. I think I tended to overestimate the inefficiencies, so I don't know if they are above or below.

12; 3.4, 6.0, 9.2, 16.4
13; 3.6, 6.5, 10.0, 17.7
14; 3.9, 6.9, 10.8, 19.0
15; 4.2, 7.4, 11.6, 20.3
16; 4.5, 7.9, 12.3, 21.6 (that's the field length in 3.3 seconds from a standstill. That's with only two small CIMs as well, it's faster with the Big CIM and/or FP added)

Okay, once again, I know the high speeds there are high.... really high. But, that is just the available range. The lowest gear is well below traction limited in each, so it didn't hurt to bump up the speed much. (3.4 vs. 4.5 fps in low is such a small difference).

Like Uncle Waldo said; this was more for an all around fun/learning experience. I got a lot of feedback on my CADing (apparently things don't allways work like in inventor...:D ), and they learned a whole bunch of machining and mechanical things. Also, it's being handed off the programmers shortly to give it a nice gyro and encoder based closed loop control system with an automated shift scheduler. Now, considering this was cheaper than the AM Gen3s, and they can still just bolt right on; I think it was very worthwhile to use the 4-speeds on the prototype.

EDIT: and yes, those are spinners that ride on ball bearings in the last picture.... The students asked for them, how could I say no?