Finding a line again
An important thing to consider is what the robot should do if it has lost the line. Coming back to our examples of an industrial setting, this could be a safety measure.
Our current robot stops. That requires you to put it back on the line. However, when you do so, the robot immediately starts moving again. This behavior is fine for our little robot, but it could be a dangerous hazard for a large robot.
Another behavior that you could consider is to spin until the robot finds the line again. Losing a line can be because the robot has under/oversteered off the line and couldn't find it again, or it could be because the robot has gone past the end of a line. This behavior is suitable perhaps for small robot competitions.
We need to consider things like this carefully and where you would use the robot. Note that for competition-type robots, or industrial robots, they will have either multiple sensors at different angles or a wider angled sensor &...
