[Bryce Paputa]

you might be able to do something like an optical mouse (or multiple ones), I wouldn't be sure how though, and it might not work if the field isn't flat. you could even use something like a mechanical mouse, you could just have some omniwheels with encoders, you should be able to figure it out with 4? you might need more and it would probably take a lot of DIO channels, but i think it would be possible.