title_body: What are controllers of Parrot AR drone 2?
upvoted_answer: After reading relevant materials, i think I have find the answers for this question.
  Q1:Since AR drone2 is a proprietary system,  this company didn't publish any documentation about the internal structure.
  Q2:According to my understanding and guess, the controllers will use the estimated states of AR drone2 and the estimated states may from EKF(I am not 100% sure)
  Q3:Since AR drone2 is a proprietary system, it seems there is no way you can control the motors directly by using your own controllers. But there is an alternative way:
  using Simulink and Optitrack to control AR Drone
  using Simulink and Vicon to control AR Drone