Corner Omni Wheels in West Coast Drivetrain

Hello,

We are planning to design a west coast drivetrain as an off-season project. We are currently discussing if we should put omni wheels in the corners to ease turns and increase maneuverability.

We were concerned that using omni wheels will affect encoder data and our autonomous program, does anybody have any experience using omni wheels in their autonomous programs?

Another concern is that using omni wheels will obviously decrease traction, as we will have only two traction wheels instead of six, thus we wonder how corner omni wheels will perform under heavy defense. Will two traction wheels be enough to resist pushes from other teams?

We would like to hear experiences of teams that used west coast drivetrains in previous seasons. What were the advantages and disadvantages of using omni wheels on the corners in your opinion?

Also we are quite open to any advices from your team in west coast drive as it will be our first time doing it.

Thanks a lot.

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If your wheels are linked together, such as through chain, the omni wheels won’t be an issue. If you drivetrain has a center drop, you can always trust that the middle wheel will be contacting the ground and driving the encoder. I recommend you make the encoder live to the middle wheel shaft so as to reduce slop.

With regards to traction: It depends. Some teams put traction wheels in only the middle, which does have a measurable effect on traction. In a few years, we’ve only put omnis on the lighter side of the drivetrain. This lets you reap the benefits of reduced turning current while also keeping almost all of the weight on the traction wheels. In addition, having a center of gravity closer to the middle wheels will mean that very little friction is lost by switching.

The omni wheels decision is one that we typically make pretty late in the season. The beauty of a WCD is that it’s extremely easy to swap wheels - if you’re tired of being pushed around, you can quickly switch back to traction. I recommend you try a lot of things and collect as much data as you can with a fully-built drivetrain.

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I’m just gonna tell you our team’s experience. For the past three years we have run 2 traction and one omni per side (the one omni being in one of the corners not the center). This year we initially started out with omnis in all four corners but with NEOs and our gearing it was very difficult to control in autonomous so we went back to our old method.

As long as your encoder is on one of the traction wheel shafts it shouldn’t matter, in fact this year we had our encoder on one of our omni wheels and the data wasn’t affected enough for us to notice a lot.

The rollers on the omni wheels have plenty of forwards/backwards traction so that shouldn’t be an issue. The only place where you’ll see a noticeable loss of traction is turning (which is what you’re going for anyways with ease of turning.

I would highly highly recommend having both a high gear and low gear option for your drivetrain. It will make playing defense, resisting counter defense, and scoring so much easier.

A side note is that typically if you are planning to use omni wheels then teams don’t use a center drop, but then again some do. It’s really up to your team.

Let me know if you have any questions

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Four of the wheels in a 6WD carry the robot at any instant of time; which four depends on where the robot’s center of gravity is at that instant. If there’s no drop, the unloaded wheels act as stabilizers, resisting steering and the tendency to over steer. Omnis on one end will provide less of that stabilization, making steering and over-steering easier. You’ll get more stabilization if you use the omnis without a center drop.

The math behind this has been presented many times in the history of FRC. The earliest I am aware of is Chris Hibner’s white paper called “Drive Train Basics”. That paper may have been lost when CD moved to Discourse. @Brandon_Martus might be able to help find it.

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Thread still exists, but the paper seems to be gone

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I updated the thread with the restored file.

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My team used a west coast drivetrain with corner omnis this year. We had similar reservations about how it would effect our ability to play and resist defense, but decided to go for it anyway. But we ended up playing very effective defense and counter-defense at our first regional - I think our biggest takeaway was that defense is not just pushing matches, agility is a HUGE benefit when you’re trying to get around a defender, or get in a scorer’s way. And we did still have a fair amount of pushing power - we certainly couldn’t push everyone we played against across the room, but we were pleasantly surprised by how many we could.

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Regarding the autonomous, it should not be not a problem. Regarding getting pushed, forwards and backwards should not be much different than, lets say, a 6 colson drive. Side pushing you will be more vulnerable to being pushed, though that may be a pro or con depending on what you are trying to do, and what game it is.

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On auto, totally concur with @StephenR. In fact, I’ll double down and assert that having some omni wheels may improve your auto repeatability.

On defense resistance, I agree again, with amplification. You will be more vulnerable to pushing at the end of your drive train with the omnis, other things being equal. However, there are situations where this is less important - e.g. this year* if you designed your robot to score while square against the rocket or cargo ship, you could have put the omnis there, applied a bit of drive force to keep you against the ship, and not had to worry very much. On the other hand, if you had put the omnis away from the scoring side of your robot, you would have been quite vulnerable to being sideswiped on your flank.

* 2019 Destination Deep Space in case you’re reading this in the not-too-immediate future

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My knowledge of corner omnis is as follows. If you have omni wheels on the corners of a 6 or 8 wheel robot, it is almost pointless to have a drop center, as you’d find in a West Coast Drivetrain. However, comparing a WCD with tractions to a tank drive with omnis, there are a few notable differences. The WCD will be slightly more effective at pushing much harder to push. The omni bot will have better encoder data, and turn more reliably.

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When 1710 ran corner omnis in 2018 we didn’t add a drop center. We didn’t notice reduction in our ability to play defense or avoid defense. If we turned sideways no one was able to push through. Without the omnis and a drop center the robot was noticeably less smooth at driving.

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