California Drive

[Jon Stratis]

Originally Posted by LondonBoy29

In a California or West Coast drive system, how much lower should the middle two wheels be relative to the two in the front and the two in the back? What are the pros and cons of using California vs. Tank?

Using standard tank drive, where all wheels are the same height, leads to trouble turning. Picture your robot from the top. Now draw two circles representing the direction you want to turn - one will hit the middle of the left side of the drive train, the other will hit the middle of the right. As the robot turns, you'll realize that the wheels have to move sideways (called scrub). This motion is high friction, and reduces your ability to turn quickly or sharply. See the attached example for details.

Now, change this to a West Coast drive train with 3 wheels and the center dropped lower than the others. If you imagine adding a center wheel to the tank example, it now rests entirely on the circle it's traveling on. That means it can travel along the circle without any scrub*. By dropping the center wheel, you can essentially pick up the outer wheels. If your center of gravity is balanced over your center wheels, you'll probably see the robot rock back and forth on it a little as it turns - that's the small amount of friction when the outer wheels hit the ground causing them to pop back up.

If your center of gravity is NOT balanced over the middle wheels, then two of your wheels (either the front or the back two) will essentially be off the ground the entire time - giving you a standard tank drive between the middle wheels and the other two outer wheels. The advantage here is the distance between those 4 wheels is much shorter - in effect, they are closer to the ideal spot for turning, giving you less scrub. Having those two wheels off the ground all the time isn't a bad thing - there are still situations where they'll be used, like when you hit another robot, or hit an uneven surface of the field (like around the towers this year).


I've seen high traction tank drive robots that couldn't turn. West Coast robots can easily spin in place, giving you a turning radius of 0. I've also seen normal tank drive robots that do just fine... but they do require more power to turn than West Coast systems, power that can often be used elsewhere on the robot.


* In an idealized example where the contact surface area between wheel and floor are points. In reality, it's more of a rectangle and you have a small amount of scrub from one side of the rectangle to the other... but it's really not worth considering.