Trying to overcome 2 design desireables:
So what would happen if I just drive the back wheels (4 cim 2 boxes) and used omins in front ? Basically this would look like a commercial lawn mower.
The weight of the stacks would be toward the center of the bot.
Anyone done something like this ?
I think you meant “accomplish”?
So what would happen if I just drive the back wheels (4 cim 2 boxes) and used omins in front ?
Anyone done something like this ?
Search for the phrase “shopping cart” in these two threads:
http://www.chiefdelphi.com/forums/showthread.php?t=132293
http://www.chiefdelphi.com/forums/showthread.php?t=131862
1444 is considering a variation of this drive for Recycle Rush.
We are thinking about doing a very similar thing with a wide chassis. I don’t see the need to have power driven to the front two wheels because that just takes up extra weight and you don’t have to push against other robots this year, just the totes. If you have time why not test both ideas? You could have a small piece of plywood angled at 16 degrees if you didn’t build the scoring platform and push a tote that weighed 50 pounds up the ramp. If you are happy with how it preforms then you have found your solution.
On a personal note chain can be a pain some times to get just right and add a lot of weight and precision machining to the chassis. Just remember to always keep it simple! Our design over all at the moment uses 2 CIM motors, a compressor and 2 pneumatic pistons. We might bump it up to 4 CIMs but we aren’t sure yet.
FRC 1625 ran a massively successful drive in this fashion last year. I remember from speaking with them that weight distribution was important, but I don’t remember in what manner.
The OP stated a two wheel drive bot, with the rear wheels powered, but the omnis in front not powered at all. With scoring platforms and stray noodles, it seems any time you are on uneven ground (except when driving over a bump orthogonally) you’ll have one wheel off the ground and this may reduce to a one wheel drive. With a four wheel drive, loss of wheel contact will only drop you from 4 wheel drive to 3 wheel drive.
Wouldn’t this be a major issue?
Good point. What if you tried three wheels? That way you’ll always have the drive wheels on the ground (unless, of course, you bottom out). One unpowered omni wheel in the back and two powered wheels up front may work.
Our Ultimate Ascent drive was like this. You should put the drive wheels on the end which is nearer the center of gravity for best traction and general performance. We had ours on the front, because we were front-heavy that year. Even then, when we tried to do a sprint, we found that we had much better control in reverse because the acceleration moves the weight towards the rear tires (as you’re moving). So if your weight is centered, you will have better acceleration and control as you move forward. Unfortunately, you then give up braking traction, unless you figure out a way to brake the free-wheeling omnis.
Two wheel drive was my team’s second option behind the drive train we’re using this year. What ultimately decided it for us was the fact that unless you put the motorized wheels in the front, the weight from the totes will make the acceleration slow. The solution to that would be to put the motorized wheels in the front but that affects a driver’s ability to drive. These two major downfalls made us ultimately decide two wheel drive was not for us.
Just direct drive the rear tractions wheels and belt drive the front omnis
im confused what preventing you from doing a normal 4 or 6 wheel drive?
Might want to re-read the section on transport configuration, those aren’t the right limits for your dimensions.
The drive you are talking about can be very effective, we used it for 2 years and it worked well for a highly maneuverable robot. You might want to see how it handles the platforms, when we used it the field was flat and it might have issues driving straight onto the platform.
Sorry those are my desired dimensions, with the key problem being either dimension over 28. We may end up 38x38 or something else square.
Q: Why not six wheels ?
A: Because if we drop the stack while perpendicular to the platform bot isnt level and the stack might might tip over. Also want to straddle platform for precision.
Q: Why not 4 wheel tank ?
A: To get around the 28’ limit the wheels need to deploy out and dealing with
drive belts adds risk and complexity.
Q: What about traction/stablity issues
A: If center of gravity is rear of center wont that minimize those problems ?
I’m not sure why your drivetrain would need to exceed the transport configuration in-game. It seems like an extra engineering challenge for no good reason (I know we’ve already got plenty of those to deal with).
A 2-powered-traction/2-unpowered-omni drive should work fine for this game. Make sure you understand the trade-offs involved, especially when making a drivetrain decision.
If any system on your robot works, make sure it’s that one (That also means that if any system on your robot might compromise the effectiveness of your drivetrain, the other system should generally be the one changed).
I believe that they are planning to transport their robot in a different orientation than that which they deploy on the field - a dreadfully simple solution, if you can figure out how to be no more than 28" tall.
I can certainly imagine some elegant “large base, bottom stacker” solutions which ship under 28" tall (and some which never get taller) yet deliver a stack of six totes to the backstop for efficient stowage. Alas, our design is pretty much locked in by the orders we’ve placed at AndyMark. I have to remember that perfect is the enemy of good enough…
We ran power to our front wheels as well as back in 2014. Having your weight biased toward the back of your bot will help with turning if you have omnis in the front.
Sorry those are my desired dimensions, with the key problem being either dimension over 28. We may end up 38x38 or something else square.
Q: Why not six wheels ?
A: Because if we drop the stack while perpendicular to the platform bot isnt level and the stack might might tip over. Also want to straddle platform for precision.
Q: Why not 4 wheel tank ?
A: To get around the 28’ limit the wheels need to deploy out and dealing with
drive belts adds risk and complexity.
Q: What about traction/stablity issues
A: If center of gravity is rear of center wont that minimize those problems ?