Stopping at a Line

In the last module, you wrote and tested a function which could accurately determine if the reflectance sensor was able to see a line. In this activity, you’ll use that function to make the robot stop when it sees a line.

../../_images/stop_at_line.gif — The XRP stopping when it sees a line.

Let’s consider a previous exercise - using a while loop to drive a certain distance:

Python

from XRPLib.defaults import *

while drivetrain.get_left_encoder_position() < 20:
    drivetrain.set_speed(5, 5)
drivetrain.stop()

In this code, the condition being checked is drivetrain.get_left_encoder_position() < 20 meaning that the robot will drive forward at 5 cm/s until the left encoder reads a distance of 20 cm. This code can be easily modified to replace the current condition with a condition that uses the function you wrote.

Blockly

In this code, the condition being checked is meaning that the robot will drive forward at 5 cm/s until the left encoder reads a distance of 20 cm. This code can be easily modified to replace the current condition with a condition that uses the function you wrote.

Try it out

Modify the example code to use your function (is_over_line()) as the condition for the loop.

If you need to “invert” the value of your function (convert False to True and True to False), you can use the not operator before calling your function like this: not is_over_line(). This code does exactly what it sounds like: returns True when the robot is not over the line.

Once you’ve tested your code and proved it to meet the challenge, make a new function called drive_until_line() and put your code in it. Don’t delete this function, as you’ll need it later!

Challenge activity

For an added challenge, try to write code which makes the robot capable of driving over and stopping at several lines. The robot should drive over a line, stop for some amount of time, say two seconds, and then start driving again until it sees another line. This cycle should repeat forever.

Tip

You’ll need to write some logic which handles the robot driving off of the line too! Your code from the main activity might not be enough to handle this! Think about what your code would do if it started out already on a line.