Input value constantly

Question

The code is for a DC motor position control PID system. I need the angle I need to move to "setpoint" and change it throughout the process. Though it is only changing once and then nothing happens if I insert a new value unless I upload the code again. I need help in finding the problem, thank you.

Code:

double count = 0; //set the counts of the encoder
double angle = 0;//set the angles
boolean A,B;
byte state, statep;

double pwm = 9;// this is the PWM pin for the motor for how much we move it to correct for its error
const int dir1 = 6;//these pins are to control the direction of the motor (clockwise/counter-clockwise)
const int dir2 = 7;

double    setpoint = 0;
double Kp = 0.2;// you can set these constants however you like depending on trial & error
double Ki = 0.25;
double Kd = 1;

float last_error = 0;
float error = 0;
float changeError = 0;
float totalError = 0;
float pidTerm = 0;
float pidTerm_scaled = 0;// if the total gain we get is not in the PWM range we scale it down so that it's not bigger than |255|
String readString;

void setup() {
   Serial.begin(9600);
  pinMode(2, INPUT);//encoder pins
  pinMode(3, INPUT);
  attachInterrupt(0,Achange,CHANGE);//interrupt pins for encoder
  attachInterrupt(1,Bchange,CHANGE); 

  pinMode(pwm, OUTPUT);
  pinMode(dir1, OUTPUT);
  pinMode(dir2, OUTPUT);



}

void loop(){



   if (Serial.available()) { //Check if the serial data is available.
    delay(3);                  // a small delay
    char c = Serial.read();  // storing input data
    readString += c;         // accumulate each of the characters in readString
  }

  if (readString.length() >0) { //Verify that the variable contains information

   Serial.println(readString.toInt());  //printing the input data in integer form
    setpoint = readString.toInt();   // here input data is store in integer form

  }
   PIDcalculation();// find PID value  

  Serial.print("         ");
Serial.print(setpoint);
Serial.print("         ");

  if (angle > setpoint+0.5) {
    digitalWrite(dir1, LOW);// Forward motion
    digitalWrite(dir2, HIGH);
  } 
else if  (angle < (setpoint-0.5)) {
    digitalWrite(dir1, HIGH);//Reverse motion
    digitalWrite(dir2, LOW);
  }
  else{
      digitalWrite(dir1, LOW);//Reverse motion
    digitalWrite(dir2, LOW);
  }
  analogWrite(pwm, pidTerm_scaled);

  Serial.println("WHEEL ANGLE:");
  Serial.print(angle);  // here input data is store in integer form;
  delay(100);

}

void PIDcalculation(){


  angle = (0.9 * count);//count to angle conversion
  error = setpoint - angle;

  changeError = error - last_error; // derivative term
  totalError += error; //accumalate errors to find integral term
  pidTerm = (Kp * error) + (Ki * totalError) + (Kd * changeError);//total gain
  pidTerm = constrain(pidTerm, -155, 155);//constraining to appropriate value
  pidTerm_scaled = abs(pidTerm);//make sure it's a positive value

  last_error = error;
}

void Achange() //these functions are for finding the encoder counts
{
  A = digitalRead(2);
  B = digitalRead(3);

  if ((A==HIGH)&&(B==HIGH)) state = 1;
  if ((A==HIGH)&&(B==LOW)) state = 2;
  if ((A==LOW)&&(B==LOW)) state = 3;
  if((A==LOW)&&(B==HIGH)) state = 4;
  switch (state)
  {
    case 1:
    {
      if (statep == 2) count++;
      if (statep == 4) count--;
      break;
    }
    case 2:
    {
      if (statep == 1) count--;
      if (statep == 3) count++;
      break;
    }
    case 3:
    {
      if (statep == 2) count --;
      if (statep == 4) count ++;
      break;
    }
    default:
    {
      if (statep == 1) count++;
      if (statep == 3) count--;
    }
  }
  statep = state;

}

void Bchange()
{
  A = digitalRead(2);
  B = digitalRead(3);

  if ((A==HIGH)&&(B==HIGH)) state = 1;
  if ((A==HIGH)&&(B==LOW)) state = 2;
  if ((A==LOW)&&(B==LOW)) state = 3;
  if((A==LOW)&&(B==HIGH)) state = 4;
  switch (state)
  {
    case 1:
    {
      if (statep == 2) count++;
      if (statep == 4) count--;
      break;
    }
    case 2:
    {
      if (statep == 1) count--;
      if (statep == 3) count++;
      break;
    }
    case 3:
    {
      if (statep == 2) count --;
      if (statep == 4) count ++;
      break;
    }
    default:
    {
      if (statep == 1) count++;
      if (statep == 3) count--;
    }
  }
  statep = state;

}