Fast Line Follower (PID), Please Help


I’m currently working on a fast line follower but I can’t get it to follow the line correctly.
It feels like it is too slow to respond. It comes to a turn and it starts turning after all sensors have passed the line (then it starts turning(fast) till it sees the line again and usually loses control). The line nearly never stays on the middle of the sensors (except for the straight parts).
It follows the line ok if I hold it and slow it down (there’s a little wobble, I believe it’s because of un-calibrated Kp/Kd).
I’ve been trying to calibrate Kp and Kd values for a month now but no luck (of course some values give better results). I also tried playing with Default_Speed, Max_Speed values (code below).

It’s components are:
Arduino Uno R3
QTR-8RC (8 sensors, all being used)
4x 3000rpm 12V motors
11.1v 1A Li-Po
Old Adafruit Motor Shield
(Motors are 10-15 cm appart from the sensors.)


#include <QTRSensors.h>
#include <AFMotor.h>

AF_DCMotor motor1(1, MOTOR12_8KHZ ); // create motor #1 pwm
AF_DCMotor motor2(2, MOTOR12_8KHZ ); // create motor #2 pwm
AF_DCMotor motor3(3, MOTOR12_8KHZ ); // create motor #3 pwm
AF_DCMotor motor4(4, MOTOR12_8KHZ ); // create motor #4 pwm
//motor 1,4 are right side motors. motor 2,3 are left side motors
#define KP 4 //no idea about Kp and Kd values. tried Kp= 0.2 and Kd=5 to Kp= 1 and Kd= 20 and many possibilities inbetween
#define KD 60
#define M1_DEFAULT_SPEED 80 //also no idea about motor speeds
#define M4_DEFAULT_SPEED 80
#define M2_DEFAULT_SPEED 80
#define M3_DEFAULT_SPEED 80
#define M1_MAX_SPEED 95
#define M4_MAX_SPEED 95
#define M2_MAX_SPEED 95
#define M3_MAX_SPEED 95
//#define MIDDLE_SENSOR 4 //comemmented this because it doesn't look like that it makes a difference and since I have 8 sensors I don't know how to get the middle one
#define NUM_SENSORS  8      // number of sensors used
#define TIMEOUT       2500  // waits for 2500 us for sensor outputs to go low
#define EMITTER_PIN   QTR_NO_EMITTER_PIN     //not sure if I need this
#define DEBUG 0 // set to 1 if serial debug output needed

QTRSensorsRC qtrrc((unsigned char[]) {13,2,19,18,17,16,15,14} ,NUM_SENSORS, TIMEOUT, EMITTER_PIN); //sensor pins defined

unsigned int sensorValues[NUM_SENSORS];

void setup()

int lastError = 0;
int  last_proportional = 0;
int integral = 0; //Ki isn't used at all

void loop()
  unsigned int sensors[8];
  int position = qtrrc.readLine(sensors);
  int error = position - 3500; //line stays on the middle of the sensors if I hold the robot (and slow it down) and on straight paths, I think there is no problem here
  int motorSpeed = KP * error + KD * (error - lastError); 
  lastError = error;

  int rightMotorSpeed = M1_DEFAULT_SPEED - motorSpeed; //right motors </v
  int rightMotorSpeedy = M4_DEFAULT_SPEED - motorSpeed; //added "-y" to make variable name difference
  int leftMotorSpeed = M2_DEFAULT_SPEED + motorSpeed; //left motors </v
  int leftMotorSpeedy = M3_DEFAULT_SPEED + motorSpeed;  //added "-y" to make variable name difference
  // set motor speeds using the two motor speed variables above
  set_motors(rightMotorSpeed,leftMotorSpeed, leftMotorSpeedy, rightMotorSpeedy);

void set_motors(int motor1speed, int motor2speed, int motor3speed, int motor4speed)
  if (motor1speed > M1_MAX_SPEED ) motor1speed = M1_MAX_SPEED;// limit top speed
  if (motor4speed > M4_MAX_SPEED ) motor4speed = M4_MAX_SPEED;// limit top speed
  if (motor2speed > M2_MAX_SPEED ) motor2speed = M2_MAX_SPEED;// limit top speed
  if (motor3speed > M3_MAX_SPEED ) motor3speed = M3_MAX_SPEED;// limit top speed
  if (motor1speed < 0) motor1speed = 0;// keep motor speed above 0
  if (motor4speed < 0) motor4speed = 0;// keep motor speed above 0
  if (motor2speed < 0) motor2speed = 0;// keep motor speed above 0
  if (motor3speed < 0) motor3speed = 0;// keep motor speed above 0
  motor1.setSpeed(motor1speed);// set motor speed
  motor4.setSpeed(motor1speed);// set motor speed
  motor2.setSpeed(motor2speed);// set motor speed
  motor3.setSpeed(motor2speed);// set motor speed;;;;

void manual_calibration() {
  int i;
  for (i = 0; i < 250; i++)  // the calibration will take a few seconds

  if (DEBUG) { // if true, generate sensor dats via serial output
    for (int i = 0; i < NUM_SENSORS; i++)
      Serial.print(' ');

    for (int i = 0; i < NUM_SENSORS; i++)
      Serial.print(' ');


It sounds like your robot is overshooting its target, which is an indication of too large of a proportional term. However, instead of just lowering Kp to fix this, I think you should start by using only a proportional term (i.e. set Kd to 0) while running your robot at a low speed to get something that behaves reasonably. Then, once you have a good sense for the proportional term, you can try adding back in a differential term. You can read this post by Ben, which goes into more detail about how to get started tuning your constants.

If you continue to have trouble, can you post what constants you are using and include a link to a video that shows the behavior of your robot and the kind of turns you are having trouble with?