Rookie team questions on chassis and drive train

[KleinKid]

Well we obviously had no idea what we have gotten ourselves into but are excited to be apart of the program. We received our KoP and categorized each item and placed it into a closet with drawers and cataloged where we put each item. Then the team just sat there for a while and stared at each other with a blank look on each of our faces because none of us know really what any of the parts are used for or do. After much reading we have a better idea at what we want to do.


I'll try to limit my questions to a manageable amount but i'm sure i'll ask too much.

Regarding our chassis would it be better to just use the KoP provided one for simplicities sake or CAD our own and weld it in our school mechanical shop? We were thinking of using 3/8ths inch aluminum cut to 1" width and basing the shape off of what we have seen very commonly done in the photos section.

We plan to use the KoP wheels for now in a 6 wheel drive format. Would it be better to buy omnidirectional wheels for the outer 4 or to drop the middle wheel? The KoP doesn't seem to have come with enough sprockets for a 6 wheel drive so i'm guessing we will need to buy more.

The KoP comes with two CIM motors but most designs i've seen have had 4 so i believe we will be purchasing an additional 2 but we are not sure if we need additional gearboxes for each CIM motor.


Hopefully we will have all of this in CAD tomorrow so that we can move on to our manipulator design but we are just so unsure on how to proceed so any addition tips or advice to a rookie team will be much appreciated.

[Stephen of REX]

The KOP CIMple gearboxes should have spaces for your additional CIMs.

Dropping the center wheel or using omnis on the outside is a bit of a tradeoff. Using omnis will keep your robot more stable and make it rock less, but aren't as good for pushing power. Dropping the center wheel will introduce some rocking, which could be detrimental trying to reach the high goals. However, you will gain more pushing power.

I would recommend sticking with the KOP chassis, but that's just my opinion. Alternate drivetrains can be lighter, but usually take more time to make and are more complicated.

[Zachary Pilarsk]

for the drive train it greatly depends on the type of robot you are making (fast, slow, manurvable, good grip)

as for the chassis I would recomend using the KOP chassis your first year to save you build build time, we used it last year and it worked out well letting us focus on other problems

[Chris is me]

I would HIGHLY recommend using smaller (6") wheels with the kit transmission this year. The provided gear ratio is VERY fast, faster than you might be able to control with 8" wheels.

[KleinKid]

Originally Posted by Chris is me

I would HIGHLY recommend using smaller (6") wheels with the kit transmission this year. The provided gear ratio is VERY fast, faster than you might be able to control with 8" wheels.

Rather than switch out for smaller wheels could we just buy larger sprockets to put on our current wheels? That would make the wheels turn slower but overdoing it might make them turn too slow.

My team was pretty set on building a custom frame but i don't think everyone was quite sure how much work that is actually going to take.

[EricH]

That would also work, to a point.

At this point, I'd go with the KOP frame, and then study the custom frames posted in CD-Media to get a better idea. It's not exactly easy, but it's not exactly hard. Might be a little much right now, but maybe next year it'll be easier.

[Zachary Pilarsk]

Originally Posted by KleinKid

My team was pretty set on building a custom frame but i don't think everyone was quite sure how much work that is actually going to take.

well first is a learning experience if your build team wants to custom make a chassis then let them learn it

just be sure to leave yourself enough time to do so

[KleinKid]

Ok so i'm going to assume that we choose to do a custom frame would the 3/8" aluminum be best or is there a lighter or stronger option? Maybe i'll do a fast sketch of what we lined out.

[EricH]

3/8" is either too big or too small.

The most common wall thickness is 1/8" on a 1x1 or larger tube. For a drivetrain, you probably want at least a 2"x1"x1/8" wall tube. Unless, of course, you have sheet metal thickness.

A 3/8" outside dimension tube will be too small for a drivetrain.

[Chris is me]

I would suggest you use the Kit Bot. It's pretty strong and not that heavy - there's no shame in using it! It's a very good option this year.

[team 3311]

My rookie year we made our own fraim from 80/20 it worked very well for us and we made it to worlds so my suggestion would be to buld a simple effective desin THAT IS VERY EASY TO FIX!!!!!!! and if you are planning to make your chassy i would go for a 4 weel driveshain we went with 6 weel drive and it proved a challange to get the middle weel just right

[joek]

80/20= HEAVY, and the kitbot frame is much simpler

[RoboMentorY]

Originally Posted by KleinKid

Ok so i'm going to assume that we choose to do a custom frame would the 3/8" aluminum be best or is there a lighter or stronger option? Maybe i'll do a fast sketch of what we lined out.

My study suggests that you need to be about 1/16 or 0.064" sheet Aluminum. 1/8" is too much. 3/8" is a tank for a chassis with no ability to to add anything.

Do a weight study. At most you should have 40 pounds allocated to chassis, gearboxes, motors, chain, and wheels.

Depending on your approach above 8" above the floor, you need a lot of stuff, motors, actuators, hinges, etc. to make an arm or whatever work for you.

Any comments on designing for weight are welcome.

[EricH]

For the drivetrain, unless you're actually using sheet, 1/8" wall tube can take everything thrown at it. I don't quite trust 1/16" in drive chassis applications. Even with the bumpers, the drivetrain takes a massive beating. From before bumpers were even allowed outside of the box, 1/8" has yet to fail on any robot I've known about. Welds have failed, but the structure has at best bent. And this is with full-speed, metal-on-metal, defense all over defense. Heck, falling from the 10' high bar in 2004 hurt my team's robot less than running our lift down when it was all the way down already!

Superstructure, yeah, I can see (and have used) 1/16" wall. But we aren't talking superstructure yet.

40 lb for a chassis is kind of light. A 50-60 lb chassis (with some of the superstructure power) is more like it, though I'd shoot for 50 when driveable. Heavy chassis=low CG. Low CG=less chance of tipping. As we all know, tipping == bad.

Designing for weight is not about making everything as light as possible. It's about tracking weight--where is it going, how much is going there, stuff like that. If you don't have one already, get a scale--you'll want both a platform type and a fish (or hook) type. Before a part goes onto the robot, weigh it and note the weight in the sheet. Try to get an estimate of how much the robot will weigh--most CAD programs have a way to do that. Then you know where you can remove a lot of weight quickly.

[Fred Sayre]

I don't recommend a custom chassis as rookies. I was on a team that was not experienced and we made heavily customized drivetrain components that failed completely during competition. There is nothing more demoralizing than not being able to drive. Focus on reliability and simplicity. If your machine performs consistently and accomplishes at least some scoring aspect of the game you will do very well as rookies. Consistency is a very hard thing to achieve in this competition. I can't tell you how many times veteran teams pick up solid rookies as the 3rd or even 2nd team in an elimination alliance because they are reliable and can put up some points, or run some interference in a pinch. Sometimes the rookies are even up in the top 8 picking, even with a kitbot chassis and reliable manipulator.

Additionally I second the thought about the gear ratio being extremely fast this year for the kit chassis. Consider going a bit slower, especially if you are planning on have an arm to reach the tall posts, as the speed may be just what causes your robot to tip over. Going with smaller wheels can allow you to lower your machine's center of gravity and add to your stability as well. As far as omni's, there is an important consideration to make here. Are you planning on playing defense? Are you expected to be heavily defended against? Since the scoring zone is protected, you shouldn't expect defense while scoring so having the added maneuverability of 2 or 4 of the 6 wheels being omnis could help your ability to turn and get situated to score the tubes. If you are worried about getting pushed around in the middle, consider only two omni's or all traction type wheels. If you use a dropped center wheel, and have a tall arm placing tubes, the effect of the slight tilt of the robot from one set of 4 wheels to the other set of 4 can be magnified and cause problems. Good luck!