Following the Line: On/Off Control

Now, let’s turn our attention towards one of the core challenges in the final project - following a line. In the project, the robot will need to drive to multiple different locations, but doing this just based on distance can result in the robot not getting to exactly the right place. What if the wheels slip while driving? What if the robot needs to drive along a complex curve? It’s easier to follow a line than it is to exactly measure out the course the robot needs to follow and program it.

How do we follow a line?

Consider using one of the reflectance sensors. As a refresher, gives a reading from 0 (black) to 1 (white). Assuming that the reflectance sensor is approximately at the center of the robot, it will at least partially reading the black line when the robot is centered on the line. What type of logic would we need if we wanted to follow the center of the line?

Well, if the reflectance sensor reads black, it means the robot is perfectly on the line, and we’d want to go straight, setting the motors at the same speed. But if the reflectance sensor reads grey or white, it would mean that the robot is partially or completely off the line. We’d want to correct this by steering it back to the center, but does it turn left or right?

Unfortunately, there’s no way to tell. The robot has no way of knowing which direction it is drifting off the line. Instead, try following an edge of the line. If we try to follow the left edge, then there’s two possible states in which the robot reacts.

• If the sensor reads closer to white, that means we’re too far to the left, so we need to turn slightly to the right.

• If the sensor reads closer to black, that means we’re too far to the right, so we need to turn slightly to the left.

And that’s it! We want to keep polling (getting the value of) the reflectance sensor quickly, and at each time determine whether it’s closer to white (with a value less than 0.5) or closer to black (with a value greater than 0.5), and depending on the result, either set the motor to turn right (set left motor speed to be faster than right) or turn left (set right motor speed to be faster than left).

This seems like a solution involving an if-else statement. Our condition would be related to whether the value is greater or less than 0.5.

An if / else statement allows you to run different blocks of code based on a condition (the same kind of condition you used in a while loop)

Consider the following example code:

Python

from XRPLib.defaults import *

while True:
    if drivetrain.get_left_encoder_position() > 20:
        print("Left encoder is greater than 20 cm")
    else:
        print("Left encoder is less than 20 cm")

Blockly

In this example code we just show different messages on the computer based on the value of the left encoder, but you could put whatever code you want in the blocks instead. For example, you could have the robot turn clockwise or counterclockwise depending on a condition using an if / else statement.

Actions your robot should take based on what the sensor sees.

Above is an illustration of how we’d want the robot to act based on the reading of the sensor.

Try it out

Write some code which uses an if / else statement to turn the robot one direction or another based on the reflectance sensor. To do this, you’ll use the drivetrain.set_speed function using different speeds for the left and right wheels. Remember, using a higher speed on the left wheel will make the robot turn right, and vice versa. You can use your is_over_line() function to check if the sensor sees a line.

You will need to experiment with different speed values for each wheel; too high and your robot will drive off the line before it gets a chance to correct for it, too low and your robot will not correct in time and will spin in circles. Try to get your robot to follow the line as fast as you can!