Understanding the line sensor

In the last module, you used the distance sensor to measure distances and make the XRP follow along a wall. The XRP has another sensor that allows it to navigate: the line following sensor.

The line following sensor consists of two “reflectance” sensors. Simply put, the reflectance sensor shines a light at the ground and measures how much of the light is reflected back. The darker an object is, the less light it reflects. The sensor uses infrared light, just like a TV remote, so the light is not visible to the human eye.

This sensor is perfect for sensing dark lines on a light background! If the sensor is on top of a dark line, less light will be reflected back, and if it is not on a line, more light will be reflected back. You can use this information in your code to let the robot decide what to do in these situations.

The XRP has two reflectance sensors, a left sensor and a right sensor. If you look at the bottom of your XRP on the sensor board, you will see the two sensors. S1 is the left sensor and S2 is the right sensor. Later in the module you will learn a way to use both sensors to follow lines very smoothly, For this lesson, we will only use the right sensor.

../../_images/line_sensors.png — The two reflectance sensors on the XRP.

XRPLib provides functions to read the values of the reflectance sensors:

from XRPLib.defaults import *

# Reads the left sensor and stores the value in the variable "left"
left = reflectance.get_left()

# Reads the right sensor and stores the value in the variable "right"
right = reflectance.get_right()

Before doing anything with a new sensor, you need to have a good understanding of the values it will give you in different conditions. For the reflectance sensor, it would be good to know what the sensor reports when it is completely off of the line (seeing a white surface), completely on the line (seeing a black surface), and some “middle of the road” values, when the sensor is half on the line and half off the line.

Tip

Remember that for this exercise you should only be using the right line sensor. Make sure that you center the correct part of the sensor board over the line when taking your measurements.

Try it out

Write code to read the value of the right reflectance sensor and use the webserver to log and graph the values in an infinite loop. Move the XRP around on a white surface with a line, and take note of the values the sensor reads in the different conditions above.

What do you notice from the values you measured? The documentation for the reflectance module in XRPLib states that the get_left() and get_right() functions return a number between 0 and 1. Did your values ever reach exactly 0 or exactly 1? Can you tell which range of numbers corresponds to seeing white and which range of numbers corresponds to seeing black?

../../_images/reflectance_sensor_graph.png — Example graph of reflectance sensor data.

Above is an example graph of some data from the reflectance sensor. At around 50 units on the X axis, the reflectance sensor was moved over a line (this data was recorded while the robot was driving across a line) and at around 60 units, the reflectance sensor was moved back off of the line. Does your graph look similar to the one above?

Note

Your graph will not look exactly the same as ours. No two reflectance sensors are exactly the same, so it is important for you to take your own measurements with your own robot.

It’s good to experiment with the reflectance sensor to see what it does, but you took this data for a reason. The line following sensor reports back a number, but what we’d really like it to tell us is whether it sees a line or not. To do this, you’ll need to select a “threshold” value, where if the sensor reports a value greater than the threshold, we can confidently assume the sensor is seeing a line, and if the sensor reports a value below the threshold, we can assume it is not seeing a line.

Try it out

Look at your graph and select a threshold value that makes sense to you. A number around halfway between the minimum and the maximum value you measured is a good starting point.

Write a function called is_over_line() which reads the value of the right reflectance sensor and returns True if the sensor sees a line (value above the threshold) or False if it does not. Don’t delete this function when you’re done, because you’ll use it for the rest of the module!

Use the webserver to log the result of calling your function in an infinite loop. Move your robot around a surface with lines on it to make sure it always returns the correct value based on what the sensor is seeing. If you are getting incorrect values, adjust your threshold value.