Line Follower: help me to find Constants KP,KI,KD

Greetings I want to build a line follower
I purchased the materials are:
sensors QTR-8A (pololu)
10:1 Micro Metal Gearmotor MP (pololu)
2 Wheels (pololu)
1 ATMEGA 328
L293D motor driver
I have programmed a PID control but I need help finding the constants KP, KI, KD.
Actually the line follower is out of line.
The track is a black line white background.
Can someone help me and tell me why I can not control the line follower?

PD: The code is not mine and sorry my English, I from Ecuador.

[code]#include <QTRSensors.h>

#define NUM_SENSORS             6  // numero de sensores Usados
#define NUM_SAMPLES_PER_SENSOR  4  // promedio de 4 lecturas por sensor
#define EMITTER_PIN             2  // pin emisor

// Sensores del 0-5 conectados en las entradas analogicas del 0-5 
QTRSensorsAnalog qtra((unsigned char[]) {0, 1, 2, 3, 4, 5},
unsigned int sensorValues[NUM_SENSORS];

int MOTOR1_PIN1 = 5;//5
int MOTOR1_PIN2 = 6;//6
int MOTOR2_PIN1 = 11;//11
int MOTOR2_PIN2 = 12;//12

unsigned int anterior_proporcional = 0;
long integral = 0;

void setup() {
  pinMode(MOTOR1_PIN1, OUTPUT);
  pinMode(MOTOR1_PIN2, OUTPUT);
  pinMode(MOTOR2_PIN1, OUTPUT);
  pinMode(MOTOR2_PIN2, OUTPUT);
  int i;
  pinMode(13, OUTPUT);
  //pinMode(2, OUTPUT);
  digitalWrite(13, HIGH);    // Led Indica que se esta Calibrando el sistema
  for (i = 0; i < 400; i++)  
  digitalWrite(13, LOW);     // Apagado del Led indica terminado de calibrar


void loop() {

unsigned int position = qtra.readLine(sensorValues);

// El termino proporcional nos da el error lo restamos de 2500 para hacer el setpoint el centro
int proporcional = ((int)position) - 2500;

// Calcula la derivada (resta) y la integral (suma) de la prosicion porporcional
int derivativo = proporcional - anterior_proporcional;
integral += proporcional;

// Guardamos la ultima posicion
anterior_proporcional = proporcional;

int error = proporcional*0.305 + derivativo*350;// + integral*1/1000;// ; // + derivativo*1.5;

//Como valor maximo (max) le damos 255 que es nuestro pwm maximo y hacemos que despues de calcular el error
//No se sobrepase este valor
const int max = 255;
if(error > max)
    error = max;
if(error < -max)
    error = -max;
//finalmente seteamos los valores en la funcion creada para los motores (go)
if(error < 0)
//Aumenta la velocidad del Motor
    go(max+error, max);
//Disminuye la velocidad del motor
    go(max, max-error);

void go(int speedLeft, int speedRight) {

  if (speedLeft > 0) {
    analogWrite(MOTOR1_PIN1, speedLeft);
    analogWrite(MOTOR1_PIN2, 0);

  else {
    analogWrite(MOTOR1_PIN1, 0);
    analogWrite(MOTOR1_PIN2, -speedLeft);

  if (speedRight > 0) {
    analogWrite(MOTOR2_PIN1, speedRight);
    analogWrite(MOTOR2_PIN2, 0);
  else {
    analogWrite(MOTOR2_PIN1, 0);
    analogWrite(MOTOR2_PIN2, -speedRight);

There are lots of tutorials on line. Search for “PID tuning”.

To summarize you start by setting KI = 0 and KD = 0 and then experiment with different values of KP until your robot stays more or less on the line. It may oscillate back and forth, which you then control by adjusting KD and possibly then re-adjust KP. This is called “tuning”.

You don’t need KI for a line follower, so leave that at zero, or take KI out of the code completely.

A common problem with coding is to have the wrong sign for one or more of the K constants, the error term or the derivative, so you should print out lots of values to see whether the calculations are making sense.