# Swerve Drive Rotation Control

So our team has undertaken the task of building a swerve drive this off season and we have a question. Say you have 4 modules, 1,2,3, and 4.

1------2
-…-
-…-
3------4

If 1 and 3 are driven (wheel rotation) together, and 2 and 4 are driven together should we chain 1 and 2 together for steering (module rotation) and 3+4. Or should the same wheels driven together for drive be chained for steering?

If we chain 1+2 and 3+4 it would make the chain and everything less cluttered. While if we chain 1+3 and 2+4 we would save the two sides for movement of game pieces.

if you have the space to do it steering 1 +4 and 2+3 is even better.

If 1 and 3 are driven (wheel rotation) together, and 2 and 4 are driven together should we chain 1 and 2 together for steering (module rotation) and 3+4. Or should the same wheels driven together for drive be chained for steering?

If that’s your plan be aware that because you essentially have a long wheel base configuration you will have a much harder time steering. If you do that you would have to do either a 1+2 and 3+4 (and then turn the wheels such to act as a wide base when turning) or steered 1+4 and 2+3 (turning such that they are tangent to a circle centered in the center of the robot) or independently. I would suggest that you drive the modules together as 1+2 and 3+4 instead.

If we were to do 1+4 &2+3 steering, would you suggest 1+3 &2+4 Drive? I cant really think of too much of a difference with which pair is driven. I would probably go with 1+3&2+4 because it frees up the front for game piece manipulation and such.

You pretty much need to have each wheel be powered independently if you want to get decent benefits out of using a swerve drive. Steering is somewhat less important (a small amount of slippage is fine) so you can usually tie pairs of wheels together for steering. Diagonal is best.

Crab Drive involves tying all wheels together for steering (i.e. all wheels will face the same direction). 118 did this in 2008 and all wheels shared power. They way they dealt with rotation was by putting their manipulator on a rotating turret. The chassis of the robot didn’t rotate at all during a match if I remember correctly.

I don’t really understand why you need to power each wheel independently if you want to get decent benefits. Could you please elaborate?

The reason we were thinking of driving two wheels together is because our team has very little access to cnc and advanced manufacturing techniques. If we were to drive all wheels independently we would have to use 4 pre-made gear boxes. By chaining wheels togeather we have half the weight and half the complexity.

Well let me ask, what kind of benefits does your team want to get out of a swerve drive as opposed to a normal tank-style drivetrain?

What we really want to get out of a swerve drive is the ability to have omni-directional movement. Our team used mecanum wheels for many years before switching over to a wcd this past year. Although we really like the traction and pushing power we gained, we still would like the ability to have omni-directional movement. With the swerve drive having both aspects it seemed like a next step for our team.

I guess what I was trying to ask was why would you drive each wheel individually if you steer two together?

If all you’re looking to do is have a robot which can drive back and forth and strafe left/right, then you don’t need individual power on each wheel. I just suggested that if you wanted to do more complex things like move and turn at the same time.

Steer 1+2 together, and 3+4 together.

It just makes more sense that way (to me), because if 1+3 and 2+4 are steered, and you steer them outwards and drive them forward, …well, it won’t do much good.

Here is a good example: http://team221.com/upload/818-layout.jpg

You may also want to study images of 118’s drive systems…

I see a lot of people mention this as a factor for going swerve over say mecanums or an omni drive but some of the real reasons with going WCD

is finishing early, having a more simple drivebase to program/drive on, and overall having a more reliable system. Don’t fall into the swerve trap to get “more traction”

Most of the time, pushing matches can be avoided with just better driving, which boils down to finishing earlier and giving more stick time to the drivers.

This is exactly right. Also if anybody is thinking of going independent swerve there is a chance that some of your wheels leave the ground in pushing matches causing you to lose 25% - 50% of your power.

1+2 and 3+4 vs 1+3 and 2+4 will get you the exact same thing. Consider the game though. If you have to bring a game piece up the middle of your robot then 1+2 and 3+4 is probably the wrong choice. However I highly recommend 1+4 and 2+3: 1717 used if for 2 years on their swerve and it allows you to turn very efficiently.

EH,
To respond to your original question, the simplest solution is to tie all four modules together for steering. You are going to have to provide some method of getting all the wheels aligned but then you should be good. We have made independent steering when we wanted to achieve something special. For instance, strafing in an arc for Rack and Roll. When discussing power, most teams will use some form of transmission built into the wheel module as external drives can be less efficient. The downside of crab steering designs is the weight and space needed for the system and the complexity of programming and feedback. Above all else, the best crab is only as good as it’s drivers so practice is essential. Lot’s and lot’s of practice.

Although simple, you will lose the ability to change your robots orientation in relation to the field, which may pose a problem depending on the game and how the rest of your robot is built.

I’m not going to tell you what is best for your team, but think about the different pros and cons of connecting the halves of the robot and how that effects your steering/turning/driving etc. Also, as said before, consider how your chain paths effects your robots design/ability to play the game. What you prototype now might be modified for your competition robot because its not optimal for the game.

Quasi-related: Good for you and your team! Building a swerve is a cool thing. They are a big learning experience for all involved regardless to implementation on a competition robot.

Not necessarily. If the drive power is tied 2-and-2 on the sides, or each module is individually powered, it shouldn’t be terribly difficult to program a mode where the swerve drive turns like a tank drive. As a matter of fact, the OP was asking about this very power setup.

I was referring to Al’s statement about chaining the steering of all the wheels together. Unless you’re saying they don’t power one half and do a skid-ish turn of some sort, which is something I’ve never heard of. Could you please elaborate?

EDIT: As I stated in my post after this, I was over-thinking the concept and making it out to be something far more complicated. I am now embarrassed.

Exactly what I’m talking about. You can use a 2-and-2 power arrangement to simulate a skid-steer. You’d only do it as one mode of driving; normal operation would be to drive like a swerve. But for a quick reorientation, a skid motion can help. (Turrets are also an option, but that’s another dose of mechanical complexity.)

EDIT: It was pointed out to me that I knew exactly what you mean, and was instead imagining some completely different method of turning. Original content removed for pride’s sake.

It’d be nice to list exactly which maneuvers you give up as you tie more and more degrees of freedom together.

[EDIT: The paragraph that was originally here was all kinds of wrong-- I’m going home.]

Katie,
Thanks for reminding me. We have used a descending foot, pneumatically operated to lift two wheels off the floor to make orientation changes. In some designs the foot became an AndyMark omni wheel(s).