Hi. I’m trying various CAD designs for 6 wheel driven robots. This is a fun project for myself. Currently I have the design roughly CADed up with a slight center wheel offset. I was wondering whether the center of mass should be in line with the center wheels, or shifted behind the center wheel and what the pro’s and con’s of each would be.
Thanks.
By offset, do you mean that the center wheel is dropped down lower than the other wheels? If so, then the center off mass should be above the center wheels, so that the chassis can turn with only two wheels on the floor. If you moved the center of mass back, then it would favor the middle and back four wheels, meaning the front wheels wouldn’t touch the ground as much.
I would respectfully have to disagree and say that it would probably have to depend on the game and your objectives. Our 2006 robot was 6WD (6 IFI 2x6 traction wheels) with the center pair dropped and built on an IFI KitBot chassis so whatever the standard KitBot drop is (IIRC 3/16”). We put as much mass down low over the back half of the robot including the battery, 2 AM shifters, 4 CIMs and every bit of electronics we could squeeze in there. The ball collector/ejector was in the front between the front two wheels so it was very light in the front. The result was a dropped center 6WD with very little rock. We were driving on the back 4 wheels 99% of the time which made driving more “predictable”. Basically the front wheels were only in play when we were on the ramps. The whole thing worked great and the drivers didn’t complain about any annoying rocking that I remember. Point is design for what you want depending on the game.
Look up 6WD, as there are a ton of great posts related to this material. From a robot dynamics standpoint, it depends on the behaviour you want out of your robot, the amount of drop, and the rigidity of the chassis.
Read through the other posts and see if they help.
What DeepWater was basically saying is that due to manipulator’s built to play certain games, you want a certain CoG for the entire frame. Depending on the placement, weight, and maneuvers (rotational, lateral,etc.) manipulators require, you would want the robot to essentially “tip” a certain way. The physics of the manipulator, height, and weight of the entire machine all come into play.
Oh, I didn’t mean that you always want center off mass over the center wheels. Yours is a perfect example of when you’d want weight towards the back.
DeepWater hit on exactly why you’d want the CG in the back (or front, depending on your objectives). Now, here is why you probably don’t want it in the middle:
The rock is a lot harder to control. You have, in effect, an inverted pendulum with a barrier on each side. Balance it right, you’re fine. Tap it lightly, or blow air on it, and it goes crazy. FRC robots get that kind of tapping from normal handling. Now, let’s say that you want to pick up an object and put it somewhere with a reasonable degree of precision. You’re rocking probably about 1/8" in either direction from dead horizontal, due to the “height” of the drop with relation to the end wheels. So you have to allow for a 1/4" variation in location, just in normal rocking–and that’s not including the multiple other places that you could get an error.
So you really don’t have a huge amount of precision, while by moving the CG back about a foot, you still have that amount of variation, but you know that it will be momentary at worst, and you’ll end up back in your “normal” position.
Yup, by offset I meant dropped/lowered relative to the outer wheels. So the design would be game dependent in an FRC situation, but if it was say for a slightly more general purpose, would adding more weight towards the rear wheels, as a few of you have suggested, make more sense?
Thanks for the quick responses!
I think so, for the same reasons as stated before about maneuvering items around with some precision. There are probably some other reasons, too, like shock loading (which can be a killer if it hits in the wrong way).
Another thing to consider…
Dropping the center wheel 1/8" - 3/18" works very well. However, there may be designs that are more efficient if all 3 wheels on a side are at the same height (same bearing plate is being used, etc.). If this is the situation, instead of dropping the center wheel, you can make the center wheel have a slightly larger diameter and still achieve this needed “rock” dynamic that enables a robot to turn on the center pivot.
In order to do this, you can make your center wheel have a slightly thicker tread… or make your outer wheels have slightly thinner tread thicknesses.
Have fun,
Andy B.
Or you can throw some nice Omni wheels on one end to help it turn better. We did this in 2008 because handling and lack of rock were more important than holding position that year. Now if only I could remember the name of a company that makes some cool ones???
Images would help a lot and we could offer suggestions from there.
-RC
I do like the omni wheel set-up especially if building a robot for non-FRC purposes, the best examples that come to mind are from 488.