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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.
Ok, that makes sounds reasonable. It would be helpful to provide a description of the mechanics of your system. For instance, is it a platform with two independently driven wheels in the center of the left and right side? Or is it like a car with articulating front wheels and non-articulating rear wheels? The first seems easier to implement using your logic.