[EricH]

OK, so you want the robot to travel in a straight line one way while spinning in circles in place??? Ouch.

I think the easiest way is, don't. Then again, I'm not a programmer.

From what I know of what a mecanum can do, I know they can arc while sliding in any direction (which may be what you want). They can spin in place. They can go straight in any direction. I don't think they're quite capable of doing what you apparently want. Then again, I can't think of any drive that can do that, unless you take a TechnoKat ball drive and put it on a freely rotating turret in the middle of an omnidrive base.

The only way I can think of is to have the direction and rotation separate sticks, then put something like a mouseball on the robot to tell which way you were going (set up something like the ball drive referenced above, but with a rotation sensor of some sort) and program the robot to reorient its own programming based on the feedback... I'm out of my depth now.