I. Introduction
The unmanned aerial vehicles (UAVs) is an aircraft without flight pilot. In early stage, it is usually used for military reconnaissance and some dangerous missions. In recent years, it has gradually developed into many fields, spraying pesticides and fertilizers, taking pictures and transporting stuff, etc. The quadrotor UAVs is a kind of multicoptor which has four fixed rotors to generate thrust to achieve the attitude, position and altitude that we want. Its characteristics are flexible, light, simple in structure and relatively low in cost, so it has been widely loved by the public in recent years [1] – [5] .