This is the place where you’ll always find the newest version of my red PID line follower. All the coding (every version of it) is available on the code downloads page.
Hardware
This line follower is equipped with three sensors: one light (port 3), one magnetic field (port 2) and one IR link, the last of which are by HiTechnic. The light sensor is used for the robot’s main purpose: line following, while the magnetic field sensor detects whether the robot needs to pause or keep going. The IR Link doesn’t have any function; it’s just there to keep the whole thing symmetrical.
The robot moves using two independently moving motors connected to ports B and C. They form the follower’s back and sides. At the front, next to the light sensor is a caster wheel.
Programming
The robot was programmed in RobotC and runs on PID control.The motors’ built-in PID is off. When the code starts, it takes the black (line) and white (background) light values and averages them to get an offset. It then sets the bias of the HiTechnic magnetic field sensor to 0, while the magnet is in front of it. That way, the sensor’s value will change when the magnet is (re)moved.
Next, after a short wait, it starts driving around the NXT test paper, guided by the PID. It keeps on doing so until the magnet is removed, in which case it pauses the program and turns off the motors. Once the magnet is back in place, the robot keeps going, even if it’s been moved to somewhere else on the line. If the magnet stays away for too long (more than four seconds), the program shuts down.
You can download the latest version of the code on the downloads page or download version 2.1 (which was the latest version when this page was last updated) below.
Setup and Performance
To start the robot, place it above the middle of the black line you want to follow right after starting the program on the NXT. Then, when it bleeps, move the robot to the left of the line. Make sure you neither touch nor move the magnet.
After a split second, the robot will start to follow the line. To pause it, lift the “tail”, moving the magnet. To get it back on line, let go of the tail. To abort the entire program, hold the tail for four seconds, or until you see the light sensor turn off (it’s in active mode, so the LED will be on all the time when it’s operative).
The robot follows the black line pretty quickly and very smoothly. I’ve had to remove the video I had of it because of copyrighted music – maybe I’ll re-upload it if anyone’s interested.
More info:
- PID Line follower 2.1 coding
- HiTechnic website
- I’d Rather be Building Robot (magnetic field sensor driver creator’s blog)
- Posts about this line follower
- RobotC website
