Computer Science 014 LEGO Robot Engineering Williams College Winter 2007

Today, you will teach your robot how to follow a (possibly curved) line. Using only light sensors, your robot will follow a path marked out as a black line on a white surface. With minor modifications, it should also follow a light-colored line on a dark surface.

In addition, your robot will need to follow the path politely. If it bumps into anything along the way (another robot, for example), it should wait a bit, allowing the other robot to move out of the way.

Preparing Your Robot

You will need one or two light sensors for this lab. (While either option can work, robots using the two-sensor implementation usually stay on track much better than those using the one-sensor implementation.) The light sensors should be mounted on the front of your robot, centered and pointing down toward the ground. You can put them close together, or you may separate them a little bit (so that they straddle the line, for example).

Implementing Line Following

• A simple algorithm for two light sensors The path your robot needs to follow will not be a straight line. Even if it were a straight line, following it would be tricky. (Previous labs probably convinced you of this.) You will need to monitor the light sensor(s) constantly and make adjustments to the motors to keep the robot on track. If you have two sensors, for example, you can continuously compare their readings. If one is sensing brightness while the other is sensing darkness, you can adjust the motors to turn the robot in the direction of the darkness (if the line is black). If the robot has moved off the line to the right, the right light sensor will see a brighter surface, and the adjustment should be made to the left; if the robot has moved off the line to the left, the left light sensor will see a brighter surface, and the adjustment should be made to the right.
• Brightness and darkness are subjective How can you measure brightness and darkness? What we humans see as "bright" or "dark" can vary from person to person. Similarly, you have discovered that your sensors don't all perceive light in exactly the same way. Some sensors will provide light readings in the entire range of 0 to 255; others will have a smaller range. You will want to normalize the values produced, perhaps so they always fall within the range 0-100. This should allow you to compare the relative light/darkness detected by each of your sensors.

Implementing the Complete Program

As described above, your robot should do line following, but it should do so politely. That is, the robot should wait a bit if it bumps into another robot, allowing the other one to move out of the way.

A nice way to go about implementing this is to think of each of these two behaviors separately. First, write a function that implements the line following behavior. Then write another function that implements bumper sensing behavior. Once those work properly, you can make modifications so that they work together.

• Bumper sensing The bumper sensing function should continuously monitor the left and right touch sensors on the front of your robot. If either of these is pressed, the robot should stop, wait for some short amount of time (a couple of seconds), perhaps back up just a bit, and then start to move forward again.
• Coordinating two behaviors We have seen how to start up functions as separate processes, thereby allowing them to execute at the same time. Sometimes such separate processes are truly independent. That is, they don't affect each other. Here the behaviors aren't completely separate. If a bump is detected by the bumper sensing process, line following should be suspended temporarily. The best way to handle this is through a shared (i.e., global) variable. This variable should take on one of two values. One of those values (say, 1) should signify that the robot is waiting; the other value (say, 0) should signify that the robot is not waiting. The bumper sensing function should be responsible for setting the value of this variable (to 1 when it is time to wait; to 0 when it is time to go). The line following function should continuously check the value of this variable, doing light sensing and motor adjustment only when the robot is not waiting.

Testing and Demonstration

In lab, you will find an oval track using black tape on a white surface and a figure eight track using black tape on a white surface. All robots should be tested on these tracks. In addition, with just a minor modification to your program, your robot should be able to follow a light-colored line on a dark surface. Make the necessary changes to your program, and test the robot on the track set out on the carpet in the lab. Demonstrate each of these behaviors and submit a printout of your program.