Here is a project that I was in charge of. The idea was to build a drive similar to the one we built a 5 months ago. The purpose of this drive was to include shifting as well shed 20 lbs from our drive train. We were also dissatisfied with our chain tensioners, so one of the freshman’s jobs we to design a better one. (We used an andymark previously)
This design differs from what most seem to favor here on CD, but other iterations of this drive have served us will in the past. She weighs just under 45 lbs.
Thoughts?
Edit* I have the images embedded but the are not appearing, until I fix it I have posted the raw url.
Back wheel is in the output shaft of the gearbox. Traction is connected to both omnis.
We had a REALLY bad experience with them this year,we did everything right, lowered the middle wheel and all, eventually remove the plactions farthest from the front, but no mater what we did it was slow forward/back and could not turn. Especially because we ar losing over a third of are team this year due to graduation I wanted to leave the other students with a good, simple, working concept for next year.
Upper runs have our electronic mounted on plywood, though it may be redone with pegboard to make wiring neater. I did not include it in the renders because it covered up most of the interesting mechanical stuff.
Battery and pnuematic stuff (compressor, cylinders and anything else heavy we can stick down in there) are all mounted just in front of the motors underneath the electronics. Due to the pit we built this year it is quite easy to access the underside of the bot.
We didn’t include bumper mounts because we have no good guess as to what their height would be and we would like to see what the game is before we really design the entire DT. Likely the vertical portions of flexmation will be extended and a second level to mount the bumpers will be built, but it depends on the game.
1/2 aluminum. Ya, ya it’s supposed to be heavy, but its dirt cheep, like 1.20 a foot and we have a CNC mill to mill the squares out easily. Each aluminum panel weighs 1.6 lbs so the weight isn’t horrible, and they are fairly robust, similar but larger ones were used this year and we had no problems with durability.
It is preference, it seems to lave a bit more room for other things like electronics and a manipulator.
Omnis in the corners plactions in the middle. We would mount heavy components in the middle (battery, compressor) and that combined would the low side to side friction omnis should make us pivot around our center, because you are absolutely right to suggest to make the robot pivot around its COA.
Faster then 14 fps? 14 is a jump up from what we did last year, but if other people feel the same I will certainly look at implementing it.
A question then? what kind of fps do people aim for when they are designing a DT?
Well having only driven in two years of FRC, I can only speak limitedly on speed. However, when designing a robot you have to look at what you want it to do so the speed you want may be determined by the game… but in my experience, 4 CIMs can easily drive a 6 wheel drive train at about 16 fps, and if your shifting you really don’t need to worry about lots of torque in high gear. Especially with the four omni’s giving you almost no resistance to turning. In one of our off season bots we had it so that we limited speed through software and we could pull both triggers to open up the full 100% of the speed. Your sacrificing weight for Supershifters, use them to their complete advantage.
Also, Servo or pneumatic shifting?
Pneumatics, servos suck. With the gearbox being a direct drive I don’t think I will try to squeeze out the extra foot or two per second, the best i could do would be change the gearbox from a 9 to 1 to a 7.5 to 1, and that is to little of a reduction.
45 pounds is quite a lot of weight. I understand the structural reasons for going with 1/2 inch plating, but we have gone with 3/16 inch and never had any problems, even with lightening.
Ya, ideally I would use 5/16 or 1/4, but we don’t have it and off season projects are on a shoestring budget. Dropping the plate to 1/4 would save me 3.2 lbs if I left it with as many holes in it as I do currently, but that would be to weak. Likely I could shave off 2 lbs by doing thinner material and triangular cutouts.
If 45 lbs is considered heavy the women of our shop are quite obese :o
Are you sure it would be too weak? I recommend running stress analysis on one side of the frame, using the wheels for your fixed constraints. Put 300lbf on the outer plate and see what happens.
Since this is for off season, go ahead and use what you have, I understand completely. We’re currently working on an old bot using the IFI control system because its what we have around. As previously stated, switch the 80-20 out for 1"x1" with 1/16 wall square tubing. You can mount it to the sides with threaded inserts pressed into the tube. My team usually uses 1/8" thick wall and to mount things on to the tube we just use a bolt. I’m not sure how well 1/16" wall would hold up to this though. Again, I wouldn’t be too concerned about implementing these suggestions if it’s only a off season robot.
I did run it through a stress analysis already, it crumpled like a wet rag. ( I don’t really know what I’m doing in the stress analysis, I will give the tutorials another shot tonight however) A large portion of the strength comes from the flexmation attached to the sides and I do not have the skill to run a compound system like that. I anyone else knows how I would be happy to post the cad files. They are for Inventor 2010.
With the flexmation tubing I will look into it, but flexmation is one of our primary sponsors, we build build most of our bot out of the stuff because they give us anything we need for free.
What do people shoot for for chassis weight? 30 to 40 lbs?
There are several sources for the 1" nylon inserts if you want to go that way. Mcmaster.com has them. We’ve been using the 80/20 Quickframe series and so far have never had a failure of an insert. http://www.8020.net/Quick-Frame-5.asp
These are designed to be used with their 1" aluminum box that have little groves inside to to hold in the inserts. We always drive in a sheet metal screw or 10-32 bolt just to make sure they don’t come apart. The profiles are at http://www.8020.net/Quick-Frame-4.asp.
Compared to normal 80/20 extrusion it’s more of a pain to work with but it’s almost exactly half the weight.
Frame should easily be in the 10-20lb range. For a complete drivetrain 45lbs isn’t great but its also not entirely horrible. You still have a decent amount of weight to play with. It really just depends on what you’re going to add on the robot.
looks like our frame weighs 22 lbs and the gearboxes/motors/wheels weigh 23 lbs
I like the sound of these, I will suggest them to the other students.
With the women comment, that is just what we have always called our bots, mainly due to the fact that our whole design and fab team is male… I can see how the comment could easily be misconstrued.
Actually my mentor showed me how to run the side panels and their supports as a single part, which apparently is a good enough approximation. I will post the files though if anyone wants to look at them.
