1.11: Crosswalks and Line Following

Cars need to share the road with pedestrians and cyclists. This is why there are safe distances for cars to stop at an intersection. In these activities, you will use the bottom IR sensors to stop at a crosswalk and follow lines.

In the previous lesson, you used Zumi’s front IR sensors to avoid obstacles. Now you will use the bottom IR sensors to sense a white or black line on the road and stop.

Pseudocode

Like you did in the obstacle avoidance lesson, write each line of pseudocode as a comment. You will be translating this to code later.

Test!

Once you have your pseudocode, translate each line of code into Python and fill it in the for loop below. Remember that the index for the bottom right IR is 1 and the bottom left IR is 3.

In [ ]:

for x in range(300):
    # Write your code here

zumi.stop() # Don't forget to stop at the end!
print("Done!")

Use electrical tape on a light surface or the worksheet included with the lesson guide to test your code. If you don’t want your code to keep running even after Zumi has stopped, replace zumi.stop() in the if statement with break. For example:

if left_ir < 50 and right_ir < 50:
    break

Using a break statement will stop your for loop, even if all of the iterations haven’t finished. This will allow you to move on to the rest of your code (if there is any) without waiting for the for loop to finish. Test it in your code above!

Now instead of stopping when Zumi sees the line, she is going to follow it. On the road, Zumi uses the camera to detect lanes and stay inside of them. In this case, you will make Zumi stay on the line by calling a function. First, let’s learn how it works.

Since this lesson is all about bottom IR sensors, this code uses both of them to check the line. Pretend you are walking on a piece of tape on the floor. If your right foot steps off the line, which way do you turn? What about your left foot? The IR sensors use the same logic.

Let’s test the function! Use electrical tape or the driving school mat (if you have one) and line Zumi up on the starting black line. The function zumi.line_follower() needs at least one parameter: duration. For example, let’s have the program run for 3 seconds.

In [ ]:

zumi.line_follower(3)

If you think that Zumi is not detecting the lines very well, or that the difference between dark and light is not big enough, you might need to adjust the thresholds, or limits, that the IR sensors will detect. Their default is set to 100. Refer to the IR Sensor lesson to test your IR sensors and figure out your thresholds.

zumi.line_follower(3, left_thresh=100,right_thresh=100)

Note: This function calls a while loop. If both of the IR sensors see white (the end of the line), the code will automatically stop, even if the time was not completed.