Using encoders for traction control help

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Hi all,
We've been using encoders to measure the wheel velocity, and then encoders on some omni wheels to measure actual velocity. However, I've been unsuccessful in getting this to turn into good traction control.

My code currently finds the velocity according to the omnis and the encoders on the transmission, and also finds the current motor value and the desired motor value (according to the joystick). Then, using some magical function I have yet to get working, it computes what the new motor value should be.

The problem, of course, is said magical function. I've been trying a bunch of different ideas, and none of them seem to work well (or really improve it at all). However, I am able to tell when the robot is slipping by seeing if the difference between the velocity on the real wheels and the velocity on the omni wheels is above some threshold. Has anyone had any success programming traction-control drive code using encoders?

Thanks,
Kevin