[bobcroucher]

My son Bryce, has dreamed of trying to implement what he refers to as "caster drive" for FRC for a couple of years now. It took me a bit of pondering to understand the differences.

It's not that swerves can't move in the same directions, but if you don't want wiggling or squirming in unintentional directions at first, you really need to wait until the steering motors find their correct directions before powering the drive motors. In real use, the steering motors can get oriented up to the maximum required 90 degrees in less than 0.25 seconds, so we don't actually wait. For an FRC weight robot running on carpet, it's just not necessary.

The magic of the caster, is that by gradually mixing the steering rotation and the driving rotation into a cross blended function, you can achieve any motion direction instantly and with zero slip other than that due to the width of the tire. The tire will scrub only slightly due to differing path lengths for the inside and outside of the tire. Think of pushing a cart with 4 casters, not 2 like a shopping cart, you can make it move in precisely any direction with no wheel slippage, regardless of wheel direction. A powered robot is the same mechanically, except that the forces come from the wheels to move the cart, instead of the cart moving the wheels. The key is that any desired vector of motion can be expressed as the weighted sum of the two perpendicular directions created by the steering and drive wheel motors. The downside is that it takes a lot of simultaneous accurate control of both steering and driving motors. Is it worth it for FRC? I kind of doubt it, but I hope someone tries it in the off season some year, and decides to unveil it during build season anyway.

The 1996 paper referred to above has some nice drawings and graphs of the kinematic problem, which I've never seen before. Thanks for sharing.