how to balance quadcopter

i bought gy-521 imu balance quadcopter.
now use value angle_x gyro control  motors symmetric each other pid algorithm.
i roll  , cant blance, me pls , thanks.

code: [select]

void loop()
{
  esc1.write(u1);
      esc2.write(u2);
      esc3.write(u3);
      esc4.write(u4);
      delay(200);
if (serial.available() > 0) {
  cmd = serial.read();
}

if (cmd == 'a')
  {
// value enough make quadcopter lift off ground
    u1=46;
    u2=110;
    u3=115;
    u4=113;
   
   

  }


if (cmd == 'b')

{
//// stop motors
  u1=0;
  u2=0;
  u3=0;
  u4=0;

}

///////////////////////////////////  pid, balance quadcopter

unsigned long = millis();
  double timechange = (double)(now - lasttime);
  if(timechange>=sampletime)
  {

errx= abs(0-angle_x);
err_del= errx - last_err;
if (angle_x < -9)
{
  u1= u1 + kp*errx + kd*err_del;
  u4= u4 + kp*errx + kd*err_del;
 
}

if (angle_x > 3)
{
  u2= u2 + kp*errx + kd*err_del;
  u3= u3 + kp*errx + kd*err_del;

}
last_err = errx;
//***********************************
   
  delay(5);
}
}



what changes angle_x ? interrupt routine ? nothing in code posted seems update value based on sensor input .

ps.
you need tidy code.

i've never seen complex code, worked, messy.

sort out indentation, remove swathes of adjacent newlines, etc.

also. why use both blocking , non-blocking in same code ie have delay(5);
and code

code: [select]

unsigned long = millis();
  double timechange = (double)(now - lasttime);
  if(timechange>=sampletime)

if code works @ all, suspect delay pointless

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