Following the Line

Try It — Where Is the Line, Exactly?

Put your robot on the field with the sensor over a black line. Open the Sensor List, then push the robot sideways by hand, slowly, across the line and off the other side. Watch the number the whole way.

Now the question that matters

Your robot drifts off the line. Just from the number, can it work out which way it drifted?

Learn It — Follow the Edge, Not the Line

Put the sensor on one edge of the line and everything becomes answerable. Now dark and light mean two different directions, which you can write using two conditions.

• Reading is high: Drifted onto black, so steer away from the line.
• Reading is low: Drifted onto white so teer back toward the line.

Two choices, checked forever

This is a while loop with an if inside it. You have had both since Projects 10 and 13 — this is the first time they work together.

// Keep going until someone stops me
while (a_button() == 0)
{
	// I see black: I drifted onto the line
	if (analog(line) > threshold)
	{
		// Arc away from it
		motor(left,  20);
		motor(right, 60);
	}
	// I see white: I drifted off the line
	else
	{
		// Arc back toward it
		motor(left,  60);
		motor(right, 20);
	}
}
motor(0, 0);
motor(3, 0);
msleep(30);

That is the same rule you found in Project 5 — the further apart the two power numbers, the sharper the curve. It has been true this whole time.

Tuning it

Following until something else happens

Line following on its own goes forever. To be useful it has to stop for a reason — a tick count, a bump, or a second line crossing the first.

cmpc(left);

// Follow for this far, then move on
while (gmpc(left) < 5000)
{
	if (analog(line) > threshold)
	{
		// Arc away
	}
	else
	{
		// Arc back
	}
}
motor(0, 0);
motor(3, 0);
msleep(30);

Now the line keeps you straight and the counter tells you when you have arrived. Two sensors doing two different jobs in one loop.

Do It — Steer and Deliver

Step 1 — Build the follower

Create a new project called Line Follow. Start with the two-choice loop. Put the sensor on the left edge of a line and let it run until you press a button.

• My robot follows a straight line without losing it

Step 2 — Switch edges

Move the robot to the other edge of the same line and run the identical program.

What happened, and why?

Step 3 — Follow a curve

Straight lines are easy. Find a line on the field that bends and follow that.

• My robot stays on the line around a bend
• Neither motor ever runs backward in my loop

Step 4 — Follow, then stop for a reason

Wrap the follower in a tick count so it travels a set distance and then hands over to whatever comes next. This is the pattern every mission below uses.

Turn it into a function while you are at it — follow_line(int ticks).

• I have a follow_line() function that stops after a set distance

Step 5 — Mission 16 — Freight Shelving for 9 points

Robot support is permitted, but this is final position scored — so the cube has to still be up there at the end, which means letting go cleanly.

• A spilled cube is ON TOP OF the Large Green Cube and stays there

Step 6 — Mission 17 — Freight Racking for 9 points

• An unstraight cube is ON TOP OF the Large Brown Cube and stays there

Step 7 — Mission 7 — Hazard Containment for 11 points

• One blue and one orange pom are both FULLY WITHIN the same enclosure

Step 8 — Mission 6 — Pallet Builder · 22 pts

These are the same four green and yellow cubes you used for Mission 3 in Projects 8 and 13. What is your plan — Mission 3’s stacks or Mission 6’s pallet?

• All four cubes are on one pallet
• The pallet ends FULLY WITHIN a starting box, off every line

Step 9 — Run it five times

Score It — Checkpoint

My score

My line-following settings

Can you do it again?

• I can explain why one sensor follows an edge and not the middle
• I can put an if/else inside a while loop and say what each part does
• I can make a turn sharper or gentler without reversing a motor
• I know what to change to follow the other edge
• I can make a line follower stop for a reason instead of running forever
• My line follower is a function I can call with a distance
• I can work out which missions compete for the same cubes

Think about it

Your line follower checks and steers several times a second and never drives perfectly straight. Would a person following a line with their eyes shut, feeling for the edge with a foot, do anything different?

A second sensor, mounted a few inches away from the first, would let the robot see both edges at once. What could it do then that it cannot do now?

Missions 16 and 17 want two or more cubes for the bonus, and Mission 7 wants a second enclosure. Look at how you wrote these runs. What would it take to do each one a second time?

Next

You have written every skill this game needs. What you have not done is tidy up — your functions live in one long file, and half your missions have a bonus that just means “now do that again.”

In Project 16 — Building Your Toolbox, your functions move into a library you can carry between projects.

KIPR · Botball Explorer — Discovery Projects · © KISS Institute for Practical Robotics 1997–2026