Below is the program which makes use of LeJOS DifferentialPilot and PIDController classes to make a line follower. The underlying PID Controller concept is the same as explained in the article Line Follower with PID Controller. The appropriate values of Kp, Ki and Kd are set in the program. Higher values of Ki makes the robot unstable in line following (makes it go round and round around itself). Lower values of Kd gives a delayed response when following sharp runs (like a right/left/U turn). Higher values of Kp makes the robot follow the line in a shaky manner. The video clip shows the performance of the program.
Let me explain the Cruiser class of the LeJOS program below. The core classes, their attributes and methods used in the Cruiser class are explained in the LeJOS APIs at the below links:
The program starts with setting the parameters - wheel diameter, track width, left motor, right motor and reverse setting which are required for DifferentialPilot to function. The OdometryPoseProvider is then initialized with DifferentialPilot class object passed to its constructor, in order to track the pose of the robot in the 2D-space. The pose represents the current location and the heading of the robot. For the pose, the distance travelled by the robot is measured in Pilot units (that means if we have set the wheel diameter and track width in inches, we will get the distance travelled in inches. If we have set them in centimeters, we will get the distance in centimeters). The heading angle is measured in degrees. The pose readings are required if we want to plot the trajectory of the robot. An example of expected trajectory is given below.
After feeding the instantaneous pose readings into a graph plotter (like plotly), we get to see the actual trajectory as follows:
To take a look at good in-place U-Turns at the dead ends, please take a look at below video.
In our example of Line Follower, the pose readings do not exactly match with the actuals after the robot performs U-turns at the dead-ends. If we apply certain correction for in-place U-Turns, we may be able to get the exact trajectory. However, I have not applied any correction for this plot but we do get an idea of the trajectory that the robot is following. More concrete use of the pose, will be explained in the Maze Solver article. For now, in the Cruiser, we need to set the travel speed and the rotate speed of the robot. Travel speed is in the wheel-diameter unit per second and the rotate speed is in degrees per second. Further, we initialize the PIDController constructor with the set-point (in our case, the threshold value), set the required PID parameters and pass the instantaneous value of the light intensity to the doPID () method. The Kp, Ki and Kd needs to be tuned appropriately in order for the robot to follow the black line over white surface.
NOTE: It would serve great help if you read the below article before going through code given in current article. That will make you familiar with the calibration concept.
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