Servo - December 2013

robot to travel, we explained the benefit of using subroutines. Students learned how subroutines shorten the length of their programs while making it much easier to understand.

Now that they had robots that could drive, they could control the direction each wheel spun and could understand the effects of the PAUSE command. So, we gave students the task of programming their robot to drive in a square path. We explained that this is called dead reckoning and that sailing ships used this method for navigation in the 15th century.

Dead reckoning is a method for driving a robot that does not make use of feedback. As a result, robots using this technique eventually deviate from their programmed path. The reason is that the wheels do not turn at exactly the same speed. There are ways to address this issue; in fact, one of our families corrected for this behavior by occasionally halting the faster spinning wheel.

Using Feedback For Line FoLLowing

We asked students to imagine trying to walk to the door with their eyes closed. Even if they aimed themselves accurately towards the door before closing their eyes, each step was likely to take them off course. This mental exercise and observing the behavior of their robot made our students aware of the need for feedback in their program.

Because this was a beginner’s class, the method of feedback we chose was line following. Students found themselves soldering a second but simpler PCB. The line follower consisted of a left side and right side detector.

Each detector consisted of a phototransistor and neighboring IR LED. If the light from the IR LED reflected off the surface below it, then there was no black tape beneath the detector, so it produced a signal of zero volts (a logic low). The opposite was true when the black tape was beneath the sensor.

After completing their line follower circuit, students bolted it to the front of their robot and plugged its cable into the robot controller’s GPIO. Then, we explained how to use the IF-THEN command to control the driving of the robot’s wheels. At this point, we were running out of class time. Several of our students needed extra time to catch up with the rest of the class, so they began programming their robot at home.