Crab drive steering

[Aren Siekmeier]

To start, you had a really slick drive base this year that was a good start with swerve/crab as well as with the machining resources you've gotten nailed down. And its even better that you're focusing on improving it.

Which wheels you steer together depends on the motion you want. If you want to turn around a point in front or in back, you need the sides chained together, and vice versa. However, the "unicorn" drive as it has been dubbed here is certainly preferable in its unlimited ability. This is where you steer all four (or three) independently. For either case, you will need to work through some more advanced kinematics/dynamics to figure out the heading and speed for each wheel.

With four wheels in a rectangle and the center of mass in the geometric center of your wheel base, you get the same torque canceling as from a square base. Lots of teams do this with no extra math involved to drive correctly. Also, now that you have one dimensions longer than the other, you have the ability to turn if the wheels are in line with the short side, because the wheel base is wider than it is long. I believe you had some issues turning at first with your square base at first so you put in two omnis, which gives you less traction. Square bases are usually just for true crab where you have no need to turn the wheel base at all (manipulator is turreted or nonexistent).

The kit optical encoders (and all others) are incremental encoders, meaning they only measure change. This is not terribly useful if you have two separate steering motors you need to point in the same direction, say, at the beginning of a match. Absolute encoders are the way to go here, since they give an absolute heading of the module(s). Potentiometers and magnetic encoders both work for this. Mag encoders have no gap in readout.

Many teams have also used belts for steering because, as you said, they don't stretch, and they also give more precise motion transfer, useful for steering modules precisely. They are however more expensive and probably heavier.