QTR-8RC Reflectance Sensor Array with servo

Hi, I am using QTR-8RC Reflectance Sensor Array and used the QTRRCExample.ino to obtain the values.

#include <QTRSensors.h>

// This example is designed for use with eight QTR-1RC sensors or the eight sensors of a
// QTR-8RC module.  These reflectance sensors should be connected to digital inputs 3 to 10.
// The emitter control pin can optionally be connected to digital pin 2, or you can leave
// it disconnected and change the EMITTER_PIN #define below from 2 to QTR_NO_EMITTER_PIN.

// The setup phase of this example calibrates the sensor for ten seconds and turns on
// the pin 13 LED while calibration is going on.  During this phase, you should expose each
// reflectance sensor to the lightest and darkest readings they will encounter.  For
// example, if you are making a line follower, you should slide the sensors across the
// line during the calibration phase so that each sensor can get a reading of how dark the
// line is and how light the ground is.  Improper calibration will result in poor readings.
// If you want to skip the calibration phase, you can get the raw sensor readings
// (pulse times from 0 to 2500 us) by calling qtra.read(sensorValues) instead of
// qtra.readLine(sensorValues).

// The main loop of the example reads the calibrated sensor values and uses them to
// estimate the position of a line.  You can test this by taping a piece of 3/4" black
// electrical tape to a piece of white paper and sliding the sensor across it.  It
// prints the sensor values to the serial monitor as numbers from 0 (maximum reflectance) 
// to 9 (minimum reflectance) followed by the estimated location of the line as a number
// from 0 to 5000.  1000 means the line is directly under sensor 1, 2000 means directly
// under sensor 2, etc.  0 means the line is directly under sensor 0 or was last seen by
// sensor 0 before being lost.  5000 means the line is directly under sensor 5 or was
// last seen by sensor 5 before being lost.

#define NUM_SENSORS   8     // number of sensors used
#define TIMEOUT       2500  // waits for 2500 us for sensor outputs to go low
#define EMITTER_PIN   2     // emitter is controlled by digital pin 2

// sensors 0 through 7 are connected to digital pins 3 through 10, respectively
QTRSensorsRC qtrrc((unsigned char[]) {3, 4, 5, 6, 7, 8, 9, 10},
unsigned int sensorValues[NUM_SENSORS];

void setup()
  int i;
  pinMode(13, OUTPUT);
  digitalWrite(13, HIGH);    // turn on LED to indicate we are in calibration mode
  for (i = 0; i < 400; i++)  // make the calibration take about 10 seconds
    qtrrc.calibrate();       // reads all sensors 10 times at 2500 us per read (i.e. ~25 ms per call)
  digitalWrite(13, LOW);     // turn off LED to indicate we are through with calibration

  // print the calibration minimum values measured when emitters were on
  for (i = 0; i < NUM_SENSORS; i++)
    Serial.print(' ');
  // print the calibration maximum values measured when emitters were on
  for (i = 0; i < NUM_SENSORS; i++)
    Serial.print(' ');

void loop()
  // read calibrated sensor values and obtain a measure of the line position
  // from 0 to 5000, where 0 means directly under sensor 0 or the line was lost
  // past sensor 0, 1000 means directly under sensor 1, 200 means directly under sensor 2, etc.
  // Note: the values returned will be incorrect if the sensors have not been properly
  // calibrated during the calibration phase.  To get raw sensor values, call:
  //  qtra.read(sensorValues);
  unsigned int position = qtrrc.readLine(sensorValues);

  // print the sensor values as numbers from 0 to 9, where 0 means maximum reflectance and
  // 9 means minimum reflectance, followed by the line position
  unsigned char i;
  for (i = 0; i < NUM_SENSORS; i++)
    Serial.print(sensorValues[i] * 10 / 1001);
    Serial.print(' ');
  Serial.print("    ");

I calibrated it and it showed different values when it detected black and white.
My goal is that my robot can turn left or right using a servo when it detects a black line.

My problem is how can I obtain the values (for example from [0 0 9 9 9 9 0 0]) and use the values to control the servo?
The servo I am using is similar to this: [url]http://www.hooked-on-rc-airplanes.com/images/servo.jpg[/url]
Thank you!

Your question is really two questions:

  1. How can you take the readings from the input, and turn those into an estimate of the relative location of the line compared to the robot?

  2. How can you take the relative line position estimation, and turn that into control signals for a servo?

To answer 1, easiest is to let the sensors “vote.” The leftmost sensor gets the strength -4; the rightmost gets the strength 4; the others get -3, -2, -1, 1, 2, 3. Then you multiply the sensor position strength by the actual read value, and sum them all together. The more positive the number is, the more to the right the line is; the more negative it is, the more to the left the line is; at 0, the line is dead center.

Second, how to control a servo. That depends on what your controller is! If you’re using an Arduino, use the Servo library.