Quadcopter oscillation using PID

Question

I am building an Arduino based flight controller for a quadcopter and have got it flying but it is not performing how I want it to. The drone is flying but when I release the sticks the drone does not stay perfectly still. There is some random and small oscillations that occur when it is just hovering.

Right now I am using a single PID controller for each axis though functions like this.

dt=0.005;
error = *in - *set;

P = (error * kp);
I = I + (error * ki)*dt;
D_M = ((*in - in1)*kd)/dt;
D_E = ((error - error1)*kd)/dt;
D=D_M*(1-jerk) + D_E*(jerk);

I = constrain(I, LOWER_BOUND*0.75, UPPER_BOUND*0.75);
in1 = *in;
error1 = error;
pid = P + I + D;

pid = constrain(pid, LOWER_BOUND, UPPER_BOUND);
*out = pid;

The error is the Euler angle and the setpoint is the angle I want it to go to.

I then output the PID controller values to the motors using the Arudino Servo library.

  motor1 = throttle;
  motor2 = throttle;
  motor3 = throttle;
  motor4 = throttle;

  motor1 += rollOUT;
  motor2 += rollOUT;
  motor3 -= rollOUT;
  motor4 -= rollOUT;

  motor1 -= pitchOUT;
  motor2 += pitchOUT;
  motor3 -= pitchOUT;
  motor4 += pitchOUT;

  motor1 += yawOUT;
  motor2 -= yawOUT;
  motor3 -= yawOUT;
  motor4 += yawOUT;

Is there something I am missing about quadcopter PID controllers? Thank you for the help!

Answer 1

I'd suggest setting Kd to zero, Kp to a minimal value and letting the integral alone function as the controller. Any response to an error might be slow but the integrator alone will control a process with zero error. After observing the response of the integral for a period of time, hopefully without the problem you referenced, gradually introduce proportional control and watch for the periodic instability. If increasing the proportional gain results in oscillation, reduce the proportional back to a stable value and gradually introduce more derivative gain. I have worked with PID throughout my career and the instability you described sounds like something I'd expect to see originating from the derivative control action. No sensor is perfectly noise free and even if the noise is low amplitude, the rate of change may still be fairly large which will result in a very obvious periodic instability in the flight of your drone.

If this is the case, finding the source of the induced noise may not be easy. Perhaps reducing the derivative gain and using mostly proportional and integral control will effectively filter and attenuate the instability you're noticing. Good luck.