NXT/EV3: Colour sensors

Introduction to colour sensors

Colour sensors return red, green and blue (RGB) values. They're like light sensors but instead of shining red/white light and measuring the reflected light, they shine red, green and blue light and measure their reflected components.

This is a lot of data to work with (you will have six variables, 3 for left sensor, 3 for right). Sometimes we add them up to get a combined 'white' value (though you will notice some colours change slower than others). You will have to experiment with this.

Don't trust your eyes!

Our eyes also do not see the same way as a colour sensor - what we say is green might not be green. So, always trust your colour sensor. (If you print a piece of paper that has a rgb(255,0,0) red square, you won't actually get a reading of 255, 0, 0 - printers usually print brighter than actual).

Getting the values

To get the RGB values, it depends on what sensor/brick you're using and what RobotC version you're using. For NXT users, we download a driver/library so that we can communicate with these sensors - this is because there are no NXT colour sensors so no 'in-built' command. Drivers are pieces of C code that let our normal RobotC code communicate with the sensor. Libraries are a broader term for any group of C code that helps us code.

RobotCv3, NXT HiTechnic Colour V2

Most of you are using this.

Download all drivers https://github.com/botbench/rdpartyrobotcdr/archive/master.zip

To read values, you need to create variables and then pass them into a function. The driver will place values back into the variables for you to use. To include the driver, you need to add one line at the top (with a path to the colour .h file you downloaded above).

#pragma config(Sensor, S1, LeftColourSensor, sensorI2CCustom)

// Attach the colour sensor to port 1

#include "hitechnic-colour-v2.h"

task main() {
    // Create variables which we can store 
    int lRed, lGreen, lBlue;

    // Repeatedly get values
    while (1) {
        // Read currently detected RGB values
        // If this function returns true, the values are correct.
        if (HTCS2readRGB(LeftColourSensor, lRed, lGreen, lBlue)) {
            // Show the red, green and blue values on the screen
            nxtDisplayCenteredTextLine(3, "%3d %3d %3d", lRed, lGreen, lBlue);
        }
    }
}

RobotCv4, NXT HiTechnic Colour V2

Download all drivers https://github.com/botbench/robotcdriversuite/archive/master.zip

To read values, you need to create a struct (a collection of data) and then pass it into a function. The driver will place values back into the struct for you to use. To include the driver, you need to add one line at the top (with a path to the colour .h file you downloaded above).

#pragma config(Sensor, S1, LeftColourSensor, sensorI2CCustom)

// Attach the colour sensor to port 1

#include "robotcdriversuite/drivers/hitechnic-colour-v2.h"

task main() {
    // Create struct and some variables which we can store data
    tHTCS2 leftColourData;
    int lRed, lGreen, lBlue;
    // Initialise and configure the sensor
    initSensor(&leftColourData, S1);

    // Repeatedly get values
    while (1) {
        // Read currently detected RGB values
        // If this function returns true, the values are correct.
        if (readSensor(&leftColourData)) {
            // Move data from struct to ints
            lRed = leftColourData.red;
            lGreen = leftColourData.green;
            lBlue = leftColourData.blue;

            // Show the red, green and blue values on the screen
            nxtDisplayCenteredTextLine(3, "%3d %3d %3d", lRed, lGreen, lBlue);
        }
    }
}

RobotCv4, EV3 Colour Sensor

You don't need drivers for EV3 colour sensors - they're part of RobotCv4.

To read values, you need to create variables and then pass them into a function. The function will place values back into the variables for you to use.

#pragma config(Sensor, S1, LeftColourSensor, sensorEV3_Color, modeEV3Color_RGB_Raw)

// Attach the colour sensor to port 1

#include "hitechnic-colour-v2.h"

task main() {
    // Create variables which we can store 
    int lRed, lGreen, lBlue;

    // Repeatedly get values
    while (1) {
        // Read currently detected RGB values
        getColorRGB(LeftColourSensor, lRed, lGreen, lBlue);

        // Show the red, green and blue values on the screen
        displayCenteredTextLine(3, "%3d %3d %3d", lRed, lGreen, lBlue);
    }
}

Exercises

1. Write a program that will make a beep whenever the sensor sees something that is very red, e.g. red > (blue + green). Test it out on common red objects (e.g. your diary).
2. Write a program that will differentiate black, white and green (take a tile from the storeroom). Print these on the NXT/EV3 screen as 'black', 'white', 'green' and 'unknown'.
3. Try out line follow, see next page.