How do you plan to do this? Are you going to use a finite state machine in your VHDL? In any case, perhaps you should post the logic that you plan to use -- Such as, if my right sensor hits the black line, I turn left -- and so on. If you are going to use one sensor, it is a bit more complex, though a good example of an FSM. For instance, with a single sensor, you will run over the black line and into the white. The question is "Which way to turn?" You can imagine in such a system the vehicle will zig-zag back and forth over the line as it moves forward. So if you make the black->white transistion, you can look at the last turn that you made and turn in the opposite direction on this iteration.