How to tune a line follower's proportional steering

To make a line follower steer smoothly, steer by how far it's off the line, not just which side. Subtract the two sensor readings, diff = right - left, and move each wheel away from a base speed in proportion: the left wheel slower and the right faster when the line drifts left, and more the further it drifts. A small drift gives a gentle lean, a big one a hard swing, and the zigzag disappears.

Why on-off rules zigzag

The classic follower has three rules: line under the left eye, stop the left wheel; under the right eye, stop the right; otherwise straight. Every correction is all or nothing, so the robot swings hard, crosses the line, swings hard back. It works, slowly and wobbly, and on a fast curve it falls off.

How far, not which side

Reflectance sensors read about 937 over the floor, 633 half over the line, and 251 right over it. The difference between the eyes measures how far off centre the robot is:

• Centred, both on floor: diff = 0
• Left eye half over the line: 937 − 633 = 304
• Left eye fully over: 937 − 251 = 686

A positive diff means the line is under the left eye; a negative one, under the right.

Steer in proportion

int diff = analogRead(A1) - analogRead(A0);   // right minus left
analogWrite(9,  constrain(180 - diff / 8, 0, 255));
analogWrite(10, constrain(180 + diff / 8, 0, 255));

Half over the line, 304 ÷ 8 is 38: left 142, right 218, a lean. Fully over, 686 ÷ 8 is 85: left 95, right 265, which constrain holds at 255, a swing. Centred, both 180.

Choosing the gain

The 8 sets how hard it steers. Divide by 2 and half a line becomes 152: left 28, nearly stopped, and the lurching comes back. Divide by 32 and it's only 9, too gentle to get round a bend. Tune it on the floor: start gentle, and make the divisor smaller until it holds the line on the tightest bend without weaving on the straights.

This is proportional control, the P in PID, with the gain written as a divisor. Race-level followers add more sensors for a finer position, and a D term that steers into a bend before the error grows.