Stopping at an Intersection

With what you’ve learned in this module, you can now use proportional control to drive your robot quickly and accurately along a line using both reflectance sensors. However, smooth line following is not the only reason the XRP has two sensors. Having two sensors also allows us to detect intersections between two lines.

Consider this diagram from the last module:

Simplified diagram of the reflectance sensors centered on a line.

Notice how both reflectance sensors are mostly on the line, but not fully. Just like at the start of the module, you’ll need to do some measurements of what the line sensors read. This time, you’ll need to see what they read when the robot is centered on the line. This is how the robot would be positioned on the line if it was following it accurately.

Try it out

Write an infinite loop to log the values from both line sensors using the webserver. Place your robot centered on the line and see what the values are.

Use the values you measure to determine a new threshold value. Note that this threshold will probably not be close to halfway between the white and black values, as your robot’s sensors may not actually be centered perfectly on the edges of your line, as this depends on the width of the line you are using. A good value for this threshold would be about halfway between the value you just measured and the maximum value you measured back in the first module.

Tip

If you don’t remember your sensor’s maximum value, just place the robot sideways on the line so that both line sensors are over the line. Use the same code you just used to read the values again.

Consider what the line sensor would see when it crosses an intersection:

Simplified diagram of the reflectance sensors over an intersection.

When crossing an intersection, the sensor sees nothing but black. This means that when both sensors see completely black, we can assume we are at an intersection.

Try it out

Write a function is_at_intersection() which reads the values of both line sensors and returns True if the robot is at an intersection.

Now that you have a way to detect an intersection, it’s time to tie everything together.

Try it out

Write a function follow_line_until_intersection() which runs your line following algorithm until an intersection is detected, and then stops the robot.

Challenge

Try writing code to turn at an intersection. Pick a direction to turn, and then have your robot turn that direction until it is over the other line of the intersection, and then start following that line

Tip

You will need to have your robot’s wheels centered over the intersection for this to work. Use one of the drivetrain functions to do this after you arrive at the intersection.