Line-following robot car

// PID line follower: Arduino Nano + TB6612FNG + 5-channel IR array
const int AIN1 = 7, AIN2 = 8, PWMA = 5, BIN1 = 4, BIN2 = 12, PWMB = 6, STBY = 11, BTN = 2;
const int SENSORS[5] = {A0, A1, A2, A3, A6};

float KP = 22, KD = 140;
int BASE_SPEED = 150;
int lo[5], hi[5];
float lastError = 0;
bool running = false;

void motor(int in1, int in2, int pwm, int speed) {
  digitalWrite(in1, speed >= 0); digitalWrite(in2, speed < 0);
  analogWrite(pwm, constrain(abs(speed), 0, 255));
}

float position() {                       // -2 (far left) .. +2 (far right)
  long sum = 0, weighted = 0;
  for (int i = 0; i < 5; i++) {
    int v = map(analogRead(SENSORS[i]), lo[i], hi[i], 0, 1000);
    v = constrain(v, 0, 1000);
    sum += v; weighted += (long)v * (i - 2) * 1000;
  }
  if (sum < 300) return lastError > 0 ? 2 : -2;   // line lost: keep turning the way we were
  return weighted / (float)sum / 1000.0;
}

void calibrate() {
  for (int i = 0; i < 5; i++) { lo[i] = 1023; hi[i] = 0; }
  unsigned long t = millis();
  while (millis() - t < 3000)
    for (int i = 0; i < 5; i++) { int v = analogRead(SENSORS[i]); lo[i] = min(lo[i], v); hi[i] = max(hi[i], v); }
}

void setup() {
  for (int p : {AIN1, AIN2, PWMA, BIN1, BIN2, PWMB, STBY}) pinMode(p, OUTPUT);
  pinMode(BTN, INPUT_PULLUP);
  digitalWrite(STBY, HIGH);
  if (digitalRead(BTN) == LOW) calibrate();
  else for (int i = 0; i < 5; i++) { lo[i] = 100; hi[i] = 900; }
}

void loop() {
  if (digitalRead(BTN) == LOW) { running = !running; delay(400); }
  if (!running) { motor(AIN1, AIN2, PWMA, 0); motor(BIN1, BIN2, PWMB, 0); return; }
  float error = position();
  float correction = KP * error + KD * (error - lastError);
  lastError = error;
  motor(AIN1, AIN2, PWMA, BASE_SPEED + correction);
  motor(BIN1, BIN2, PWMB, BASE_SPEED - correction);
}