Robotc color sensor error

Question

I am trying to write a simple program where the robot(Lego NXT2) will follow a blue line.

#pragma config(Sensor, S1,     ,               sensorCOLORBLUE)
//!!Code automatically generated by 'ROBOTC' configuration wizard               !!//

task main()
{
    while(true)
    {
        if(SensorValue[S1] == sensorCOLORBLUE)
        {
            motor[motorB] = 0;
            motor[motorC] = -50;
        }
        else
        {
            motor[motorB] = -50;
            motor[motorC] = 0;
        }
    }
    wait1Msec(1);
}

I am using an nxt color sensor and the problem is that only 1 motor is moving. I know that none of the motors are broken either because I tested them out. Can somebody help me diagnose my problem?

Answer 1

Someone who owns a Lego NXT2 system may be able to check your code and provide a good answer.

Meanwhile, load some of the color sensor programs from the following links, run them with the sensor pointed at known colors, and edit your question to report whether your color sensor is working properly.

Color Range Block v1.0 from teamhassenplug.org

G1-FindColor, G2-SumoColor, etc from nxtprograms.com

Also see philohome.com's “Color Sensors Showdown” page.

Answer 2

According to this page the sensor basically gives you three integers corresponding to the intensity of red/green/blue light. The larger the number, the brighter the incoming light.

The error I suspect is that SensorValue[S1] actually is a value from 0 to 255 and the chance that you get exactly the number sensorCOLORBLUE is very small.

However, such an output means, that you can adjust the thresholds you want to be recognized as blue very nicely in software: for example by saying if the blue value is the highest of those 3, you have a blue line and act accordingly. Example (pseudo) code would be something like this:

if ( (sensor_output_blue > sensor_output_red) && (sensor_output_blue > sensor_output_green) {
    //react to blue
    //for example by turning left
} else {
    //react to red or green
    //for example by turning right (ie. in opposite direction)
}

If using all three colors at the same time is inconvenient, you can do something similar using only the brighness of blue:

if (sensor_output_blue > threshold_blue) {
    //react to blue
} else {
    //react to the other colors
}

Answer 3

Your comparison is wrong - sensorColorBLUE is a sensor setting, not a color. To detect a color, you need your sensor in RGB mode, and your comparison needs to be against a TColors type. Change the code as follows:

#pragma config(Sensor, S1,     colorSensor,    sensorColorNxtFULL)
//*!!Code automatically generated by 'ROBOTC' configuration wizard               !!*//

task main()
{
    while(true)
    {
        if((TColors)SensorValue[colorSensor] == BLUECOLOR)
        {
            motor[motorB] = 0;
            motor[motorC] = -50;
        }
        else
        {
            motor[motorB] = -50;
            motor[motorC] = 0;
        }
    }
    wait1Msec(1);
}

I kept the rest of your code [almost] as-is, however I suggest naming the sensor, as shown in my code. I also suggest naming your motors in "Motors and Sensors setup"; if you're applying -50 power because the motor is reversed, you should reverse the motors in setup.

Finally, you don't need the wait1Msec(1) command - that doesn't do anything at all here - the NXT color sensor will take about 3ms to scan, this additional delay is not at all necessary.