Flight Stability And Automatic Control Of Multirotor UAVs In A Wide Range Task

Multi-rotor UAV has the features of simple mechanical structure, low-cost maintenance, vertical taking-off and landing ability. So it is more widely used in recent years. Compared to miniature quadrotor aircrafts, large-size multi-rotor UAVs have better load capacity, flight endurance and wind resistance, therefore have more practical value in the industrial field application. This paper’s background is about using a big eight-rotor aircraft to complete a special task which the flight height is up to 1000 meters in a windy and deserted gobi. Since the multi-rotor aircraft is originally used in the near-ground space, this special task for multi-rotor can be called high-altitude mission. Research on multi-rotor’s stability and automatic control in this wide range flight can be very meaningful for expanding the multi-rotor UAVs application fields.Integrating model-based analysis and flight experiment, this paper discusses the most important obstacles which need to be solved in the aspects of automatic control in this task. The main contents and contributions of this dissertation are as follows:1. Referring to other people’s researches on rotor dynamics modeling, this paper describes a multi-rotor modeling method which has taken into account the windy environment and variable air density. It can be used in real-time simulation for project-developing.2. Based on the work of the eight-rotor UAV modeling and Hardware-in-loop simulation environment, this paper discusses the air density’s influence on the aerodynamic characteristic of the rotor. When multi-rotor UAV climbs to a higher altitude, the average speed of the rotor will arise, which can lead to a reducing of the rotor speed margin for attitude control. And also rotor’s speed-lift curve can be influenced by air density which can cause aircraft deviating from the original linear operating point. Controller gain should be adjusted to compensate the changes of altitude.3. In Chapter IV, we propose a method to evaluate the wind-resistance capacity of multi-rotor UAV, and validate the method by conducting an eight-rotor UAV hovering test in a measurable wind. Using the linearization tool in Matlab, we get the wind-disturbance channel’s transfer function under the closed-loop control. Based on that, we verify the controller’s robustness and attitude tracking ability in the wind disturbance environment in simulation.4. The real phenomenon of multi-rotor UAVs in vortex-ring state(VRS) is described in this paper by conducting the vertical descending experiment on a smaller hex-rotor UAV. Then we discuss the VRS boundary criterion and propose a VRS-avoiding control strategy to meet the demand of a faster descending speed. Also a VRS alarm and recovery strategy is put forward. Another problem that occurs in descending experiment is the inner-loop(attitude) set value will sometimes go into a cyclical fluctuation. In response to the problem, this paper finds out the reason and gives the controller’s improvement direction by using the frequency domain analysis method.5. As an extended discussion of stability and automatic control issues for multi-rotor UAV, we conclude the most typical abnormal cases in a wide range flight and the corresponding measurement for these emegencies.From the usage of the eight-rotor UAV after the project delivery, it has been proved that the proposed control method for multi-rotor UAV in this paper has better stability and flight quality in a wide range task.