Hello all, this year our team plans to design our very own West Coast Drive for the upcoming season. We plan to be using the Vex Pro Chassis and tubing to construct our drive train. My question being, is it possible to use a 6 wheel drive with the center being an omni-wheel that isn’t dropped off center? I would assume this is possible due to the functionality of the wheel, but I could be wrong. Please provide input and ideas.
Is it physically possible? Sure. Will it do what you want? Probably not.
The reason dropping the center wheel or using omni wheels on the ends makes it easier to turn is because you’re cutting your effective track length in half. It takes a robot that’s normally longer than it is wide and now makes it wider than it is long (which is generally good practice for designing skid-steer robots). If you put an omni wheel in the center you still have traction wheels on the ends, so your track length will still be the whole length of the drivetrain, which doesn’t help you to turn at all.
Are you putting the omni only on the center, and regular traction wheels on the corner? If so, then you will have trouble turning.
If you want a good amount of maneuverability, then you put omnis on the corner with a traction in the middle.
Yup, just reverse your initial idea and you will be golden. Omnis on the corners, traction in the middle.
In terms of doing this COTS: versablocks. Just don’t flip the middle one over and you’re golden.
Ok, so we will put the Omni Wheels on the corners and a normal traction wheel in the center, but will we need to drop the center wheel?
No, but it wouldn’t hurt
What would the benefit be of dropping the center wheel?
In case the middle traction wheel wears down you still have sideways traction so you can’t be pushed as easily
If you’re putting omnis in the corners there isn’t really any benefit to dropping the center wheel. It just wouldn’t do any damage either.
“What would the benefit be of dropping the center wheel?”
There would be benefits and drawbacks, and you’d need to decide what your priorities are. Benefit: ensuring that your center traction wheels are always in contact with the field, even as they wear. Drawback: rocking motion could mess with some autonomous routines. Neutral: difficulty of design and assembly, whether you are milling bearing holes or slots in the rails yourself, or using versablocks. There shouldn’t be any increased difficulty to design or manufacture either option in either configuration.
The drop will help ensure the center traction wheels stay in contact with the carpet when the floor is not perfectly flat like it typically would be in your CAD. In some years, the field elements have bases (2/4" thick?) that extended outside of the element and you have to drive over it.
We talk a ton about drop center, configuration, and specific examples in this video series we put together. A picture is worth 1000 words but a video series is hopefully more than helpful:
Keep in mind that with omni wheels on all 4 corners you will turn very easily. Sometimes this is a good thing, sometimes it’s bad. In a game like Deep Space where defense was so prevalent, it was very easy to defend against a robot that ran this setup because you could simply bump the corner of the robot’s frame and they’d turn.
I’d recommend experimenting with all traction and omni+traction combinations. Some teams have run 4 traction and 2 omni. You’ll find they all drive a little differently and it’s good to experiment to figure out which setup you like the most.
It is fine if your team is intending to learn how to build a WCD chassis for some future year however, you may want to wait until after Kickoff and your team has chosen a game strategy before committing to a particular drivetrain/chassis configuration for 2020.
Deciding on a drivetrain early can be a valuable tactic. By settling on a drivetrain earlier, you can have your robot driving earlier. Having a robot driving earlier means more time for your programmers, more time to work out driving paths (which influences your manipulator design), and clearing up some design resources (the students who designed the chassis/drivetrain) earlier to work on other problems. It also evens out your manufacturing workload. 1678 does it, and they do okay.
A 6WD tank with drop center omni wheels would pivot around the center of the front or back axle, depending on which 2 of the 4 tank wheels are touching the ground.
A little unconventional, but depending on needed rotation point, could be useful.
90% of all tank drive applications would be best served with 6 or 8 traction wheels and 0 omnis. You can’t prove me wrong.
They can treat it as a learning experience. Considering that the OP’s team is entering their third year, they may go down the same path as many teams and flirt with meccanum once the 2020 game is announced and use their design in some other year.
My team ran 4 traction and 2 omni (back of robot) this year with no drop center; it worked very well for us. Turning is fine, plus you get more traction than 4 omni/2 traction, and no rocking due to center drop.
My team already knows what we want to do for drive–same thing we did last year, and the year before that, and the year before that…
Unless the game calls for something different at Kickoff, the design team should be able to have a frame designed within a week with the tweaks for the year.