Servo - December 2016

sensors. First, if the robot was placed on the line in its expected setup position, then all line sensors should be ON initially (the state of each line sensor is represented by single bit in the line sensor data). If we rotate the robot a reasonable amount (perhaps 45° to the left), all of the sensors should be OFF. If either of these conditions is not met, then one or more of the line sensors are not working.

If the above conditions are found to be true, we can assume the line sensors are probably working properly — at least we know that each sensor will turn both ON and OFF. We can further verify the operability of the line sensors by using them to return the robot to its original starting position. As long as the wheel encoders are somewhat accurate, at least one of the line sensors should be back on the line if the robot turns right 45°. If none of the sensors see the line, the program should simply report the situation and advise the user to fix the problem (which is probably related to the wheel encoders).

If some of the sensors are ON after the initial rotation, the robot should use the sensors to center itself on the line again. If the robot is unable to position all three sensors on the line with a reasonable amount of tries, then we must assume the robot has problems with small movements (perhaps, for example, the motors are running too fast or too slow) and the user should correct the problem before testing can continue.

Of course, as mentioned earlier, if problems are detected when testing the first sensory subsystem, the program cannot be absolutely certain if the problem is with the wheel encoders, the motors, or the line sensors. As long as the robot is capable of rudimentary maneuverability though, the described algorithm for testing the line sensors should be able to ascertain if the sensors are working correctly. If any portion of the test fails, the user should be informed of the findings and advised to diagnose and fix the problem before testing can continue. After all, if the robot is not capable of basic maneuverability, we cannot expect it to perform a self-test. For that reason, a successful test can validate that the line sensors are working and that there is reasonable maneuverability. If the test fails, the program can only provide speculation on what might be wrong.

The code fragment in Figure 2 implements the line sensor testing algorithm described above. As you can see, the actual testing is very simple, and most of the code is just providing the user with the test results. If you implement this algorithm on an embedded processor with a limited ability to display text, it could simply report error codes to indicate each of the possible outcomes.

If the line sensors all turn ON and OFF, and if the robot can use them to position itself on the line again, then we can be confident that the sensors are working properly and that the robot is capable of reasonable maneuverability. Knowing that the robot has these minimal abilities allows us to easily test other sensors.

The User Interface

Before we look at additional testing though, let’s examine the GUI (graphical user interface) used in the demonstration program to make the simulator testing more friendly. Figure 3 shows a sample screen from the test program. Notice it includes the simulator test environment depicted by Figure 1.

The current test (in this case, the line sensors) is displayed in the upper left corner of the screen. The lower center of the figure describes the test and gives the user any special instructions that are needed. For the line sensor test, it explains that the robot will rotate to test the sensors and informs the user that they can click the PERFORM TEST button to run the current test, or the FULL AUTO button to perform all the tests in sequence automatically.

If the tests are performed individually, the description of each test will be displayed while the program waits for the user to click one of the buttons. The test program should return the robot to its original position at the end of each test. Notice also that the program displays the current status of all the sensors being tested.

Regardless of whether individual or automated testing is used, the test results will be displayed as shown in Figure 4. A great feature of the RobotBASIC simulator is that you can force it to add a specified amount of error to its movements and sensor readings to make it operate more