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introduction
Today, drones have achieved great development with their own unique advantages. They have a wide range of applications in military, police, homeland security, disaster warning, line inspection and film aerial photography, communication, agriculture, environmental protection and many other fields. Especially in the current era of artificial intelligence, the research results are also changing with each passing day, and logistics transportation has become one of the important application directions of drones. As an ideal working platform, the drone is one of its missions. It has flexible maneuverability, can quickly and efficiently carry out the transportation of materials in places where other means of transportation are difficult to reach; it does not need to consider the geographical environment, is not subject to the control of land transportation, and is easy to use. The transport in the form of hanging does not have to worry about the influence of the shape of the hanging object. Because of this, hanging drones have been used more and more widely in the military and civilian fields. However, when the drone is flying, the system stability will be affected by the swing of the hanging object, which is also the focus of attention of universities and research teams at home and abroad.
1. Hanging drone flight control features
The hanging drone is a multi-variable, nonlinear, strongly coupled, time-varying, under-actuated high-order coupling system. It is a complex controlled object. Its main features are reflected in the following aspects:
Difficult to model
The control of the multi-degree-of-freedom composite structure such as the unmanned platform flexible suspension has its own special technical difficulties, and cannot be solved by the simple combination of the modeling, control and planning methods of the drone and the hanging object, and it is very It is difficult to establish an accurate full-machine dynamics model, which poses a great challenge to the robustness of the drone control system.
Serious coupling characteristics
The drone itself has severe coupling, and the introduction of slings and hangs changes the overall aerodynamic layout of the system, which in turn exacerbates coupling.
Complex machine dynamics
During the task-oriented operation, the relative movement of the drone, the hanging object, and the landing target, combined with the random environmental disturbance, make it generate complex full-machine dynamics. The force/torque and random external force/torque disturbance between the hanging object and the landing ground will cause the system dynamics model to present more uncertain structures and parameters.
2. Status of development and technology research at home and abroad
Hanging drones is a new concept that has emerged in recent years. As far as the relevant literature is published, its research results are still relatively few. Especially in the model drones, because it has always been the equipment that the military in the world respects, the related research is classified, and some research results of developed international countries have not been published yet.
In the research of large-scale hanging drones, it is currently the United States and Israel that are relatively mature in technology. For example, the Grumman Company of the United States and the Malate Division of the Israel Aircraft Industry Corporation are all models of drones. Has quite mature technical achievements. Secondly, the more mature technologies are developed countries such as the United Kingdom, Germany, Italy and France. In addition, Japan's Yamaha company is also in the forefront of research in this area. However, there are relatively few research results for unmanned helicopters that perform external hanging transport tasks, and there are few published articles. What we can see, such as K-MAX of the United States Kaman and the MQ-8C of Lockheed Martin are all unmanned helicopters dedicated to the execution of off-board suspension transport missions. The K-MAX unmanned helicopter is now able to reach the technical level of autonomous flight and successfully carried out the hanging transport replenishment mission on the battlefield in Afghanistan. In June 2015, the Aerosystems division of the Kaman company resumed the production of the K-MAX truck. K-MAX helicopters are used throughout the world for fire fighting, logging and other tasks requiring high loads. In May 2017, K-MAX conducted its first flight test since production resumed.
Figure 1 K-MAX hanging unmanned helicopter
Figure 2 MQ-8C hanging unmanned helicopter
The domestic development of unmanned helicopters began during the "Eighth Five-Year Plan" period. Although it has achieved certain results, the gap between the key technologies such as flight control and power is still large compared with foreign countries. Most of the core components need to be imported, and the development model is basically for mature models abroad. Imitation or unmanned transformation of manned aircraft. "Evaluation Standards for Equipment Pre-Research Technology Maturity", the technical maturity of China's industrial unmanned helicopters is ranked 7 (a total of 9). The relatively mature technology is 602 of AVIC, followed by 60 general staff, and finally is a private enterprise headed by Beijing Zhonghangzhi Technology Co., Ltd. However, the research results on the implementation of the drone transport mission drone have not yet appeared in China.
From the mid-20th century to the 1990s, it was the initial stage of research and development using helicopters to carry out hanging transportation operations. However, due to the key technical problems, the development process was slow. With the breakthrough of flight control technology and the rapid development and wide application of core technologies such as composite materials, power and sensors, and the need for military reconnaissance in high-tech battlefields, China has paid more and more attention to the research of unmanned helicopters to ensure the gradual realization of intelligence. Diversification. However, the control of the multi-degree-of-freedom composite structure such as the unmanned platform flexible suspension has its own special technical difficulties, and cannot be solved by the simple combination of the respective modeling, control and planning methods of the drone and the hanging object. This poses a great challenge to the robustness of the drone control system. Therefore, the existing control technology for drones cannot be fully applied to hanging drones.
Although there are few research results on hanging drones, the domestic and foreign scholars have conducted extensive research on the flight control of non-suspended drones, which is still useful for reference. From classical PID control to modern control theory to artificial intelligence control, both theoretical and practical model applications have achieved certain results, as shown in Table 1. At present, the main control methods are: robust control, LQR control, feature structure configuration, variable structure control, MPC control, dynamic inverse control, neural network, fuzzy control and explicit model tracking control.
Table 1 UAV flight control method and application
Table 1 UAV flight co ntrol method and its application
Control Method
Characteristics
Application and case
PID control
Engineering practicability, weak processing uncertainty
Most domestic unmanned helicopters
Fuzzy control
Do not rely on the mathematical model of the object, rely on experience to reason
Gesture, hover, position, etc.
Adaptive Control
Strong ability to deal with uncertainty, less dependence on the object itself, weak processing ability under strong disturbance
Unmanned helicopter all-inclusive line control
Neural network control
In the reasoning, control parameter optimization, fault diagnosis has a strong advantage, but real-time is difficult to guarantee and requires accurate mathematical model of the object
Unmanned helicopter all-inclusive line control
Robust control
Robust and difficult to design and calculate
Attitude control, hovering, position control, etc.
Dynamic inverse control
The accuracy of the mathematical model of the object is high, the inverse model is difficult to solve, and the robustness is not strong.
Attitude control, all-inclusive flight control
MPC control
Related to the model
Gesture, height, navigation, etc.
LQR control
Ability to handle system dynamic and noise problems, requiring accurate models of objects
Attitude decoupling control
Explicit model tracking control
Simple design, good response, and low robustness
All-inclusive flight control, applied to ADOCS projects and RASCAL projects
Feature structure configuration
Closed-loop system damping, decoupling, good stability, multi-input multi-output system
Currently applied on the BO-105 helicopter
Variable structure control
Robust and not applicable in engineering
All-inclusive flight control and attitude control
The concept of small hanging drones has been initially verified. Drexel University's autonomous system laboratory has completed and completed the autonomous tracking of the rotorcraft, load hoisting, and vehicle deployment operations. The entire system was verified by a lifting device suspended under the frame. The Tübingen Institute of Biological Control in Germany used the simulated rotor drone to study the relative trajectory tracking between the UAV and the load, and achieved the capture of the moving target in the simulation environment. The actual flight test of the prototype was not carried out in the above verification cases.
The Department of Computer Science of the University of New Mexico designed and implemented autonomous flight based on the disturbance observer-based stratified control of the rotor unmanned aerial vehicle. A motion planning method was proposed. The rotor flying drone used to generate the suspended load has the smallest residual oscillation (none The trajectory of the oscillating) was initially verified by computer simulations and indoor demonstration experiments.
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