Wheel slippage w/ positioning systems

[Jay Lundy]

I'm working on a positioning system for our robot, and I would really like to account for wheel slippage as well. Here are some ideas I've come up with or heard from other people:

• Someone mentioned a non-driven wheel in another thread. I really liked that idea, but I think I'd have a hard time convincing my team we need another wheel tacked onto the robot, plus with uneven surfaces it could become a problem.
• An accelerometer. They are a little expensive and some people have mentioned noise, but I think if you see fast movement in your wheels (slippage) and very little movment (caused by noise) you could interpret that as slippage. If you are moving and slipping at the same time, the noise would affect your reading though. A 2-axis accelerometer could also detect side-movement if you are hit, so it might be beneficial to have one on there anyway.

By the way I'm working on a system with the gyro and optical encoders. With accelerometers this isn't an issue, but other problems come into play.

So, are there any other ideas?