Design and Implementation of Automatic Airport Inspection System of Vehicle-mounted Rotor UAV

A distributed UAV automatic airport inspection system, with the advantages of unmanned autonomous inspection operation, is widely used in power patrol, highway inspection, forest fire monitoring, and other fields. The disadvantage is that the operating radius distance is limited, with the multi-fixed site layout and high cost. In this paper, a vehicle-mounted rotor UAV automatic airport inspection system is designed, which can flexibly move the operation site and increase the operation radius. This inspection system is based on 5G wireless communication control technology and can remotely control the UAV. After the operation is completed, the UAV is automatically stored, charged, and maintained, and other functions have a good application prospect.


Introduction
Multi-rotor UAVs have the advantages of simple structure, small size, lightweight, low cost, and flexible flight attitude control, which are suitable for multi-platform and multi-space operation background applications, such as the application in the power line condition detection system [1] , the application of urban firefighting and rescue [2] , the application in the field of agricultural plant protection [3] , and the application of forest fire monitoring [4] .
With the development and innovation of artificial intelligence, the unmanned operation and application of multi-rotor UAVs have gradually become a trend.The more mature application mode is the unmanned control system integrating UAV and unmanned automatic airport.Unmanned automatic airports are mostly fixed hangar or nest forms, which cannot be moved, restricting the operating radius of UAVs, and requiring the deployment of multiple sets of UAV automatic airport systems for special application scenarios with a high hard cost [5][6] .To solve this problem, this paper designs an automatic airport inspection system for vehicle-mounted rotor UAV, which increases the emergency operation range of UAV through the vehicle-mounted maneuverability and can also control the unmanned system operation process such as autonomous recovery, charging, and maintenance of UAV.

System scheme design
The automatic airport inspection system of vehicle-mounted rotor UAV designed in this paper is based on the pickup truck chassis for the layout design of the UAV automatic airport system.The overall design of the system is shown in Figure 1.The automatic airport inspection system of the vehicle-mounted rotor UAV is mainly composed of four parts: vehicle-mounted pickup vehicle, UAV automatic airport, rotor UAV, and remote cloud control platform.Among them, the vehicle-mounted UAV automatic airport is mainly composed of an airport cabin, cabin door mechanism, collection institution, lifting mechanism, charging module, 5G communication module, vehicle-mounted GPS module, vehicle-mounted attitude detection module, microcontroller module, etc.The pickup truck's second-row seating layout is a driver's seat, with a small table with a folding function for the operator to unfold the laptop control.Block diagram of the automatic inspection system of vehicle-mounted rotor UAV.The automatic inspection system of the actual vehicle rotor UAV is shown in Figure 2. When the work vehicle drives to the work site, the operator opens the laptop in the operation seat, connects to the wireless network, opens the cloud operation control platform, and detects the working status of the UAV.After the status of the drone is normal, the automatic inspection task of the drone is planned on the cloud operation platform.After the mission starts, the vehicle-mounted UAV automatic airport raises the UAV to the take-off platform and performs flight operations according to the set flight track.
When the operation is completed, the drone autonomously returns home and lands on the take-off platform, and the vehicle-mounted UAV automatic airport collects the drone and lowers it to the cabin for charging and maintenance for the next operation.

Circuit design of system power supply
The embedded control system of the vehicle-mounted automatic airport requires 5 V and 3.3 V DC voltage input, and the servo system requires 24 V DC voltage input, but the pickup vehicle system only has a 220 V/50 Hz AC voltage interface, so it is necessary to convert the 220 V/50 Hz AC voltage step-down into the DC voltage required by each system.The system power supply circuit design is shown in Figure 3.The first stage of circuit conversion adopts the LM7824CT three-terminal voltage-regulated IC module, which integrates overcurrent, overtemperature, and other protection circuits, and requires fewer peripheral components.The AC voltage of 220 V/50 Hz is stepped down by the T1 transformer, rectified by the rectifier bridge circuit of D1~D4, and then input to the LM7824CT and converted to 24 V DC voltage as the power supply input of the servo system.The second stage adopts LM2596NDH step-down switching-type integrated voltage regulator chip, which contains a reference regulator and a fixed-frequency oscillator, which has good linearity and load regulation characteristics, converts the 24 V DC voltage output by the first stage to a 5 V DC voltage, and supplies power to the interface circuit of the embedded system.The third stage adopts LDO power conversion chip MIC5219-3.3BM5,the output voltage accuracy is better than 1%, the output noise is ultra-low, and the 5 V DC voltage output by the second stage can be converted into a 3.3 V DC voltage, ensuring the stable and reliable power supply input of the embedded minimum system.

Circuit design of servo motor drive interface
The vehicle-mounted UAV automatic airport can realize the flight preparation and autonomous storage of UAV through the motion control of servo mechanisms such as drive hatch mechanism, lifting mechanism, and storage mechanism.The motor drive interface circuit of the servo system is shown in Figure 4. ADG3308BRUD chip is an 8-way bidirectional logic-level translation IC device.The microcontroller outputs a 3.3 V PWM square wave signal, which is converted to a 5 V PWM level signal by the chip to the driver end, enhancing the driving capability of the PWM square wave signal.The encoder inputs a 5 V data level signal, which is converted into a 3.3 V level signal by the chip to the microcontroller, thus playing a buffer protection role for the microcontroller's input acquisition.

Circuit design of 5G communication interface
Most of the industrial-grade 5G communication application modules are USB interfaces, and the system selects CH340G, a USB bus adapter chip, which is compatible with the USB V2.0 protocol to meet the interface needs of ordinary USB devices.The 5G communication interface circuit is shown in Figure 5.When the CH340G chip works normally, the externality needs to match the 12 MHZ clock oscillation circuit, and the interface circuit is designed with a power supply voltage of 5 V.The chip has built-in independent transmit and receives buffers to support simplex, half-duplex, or full-duplex asynchronous serial communication.Through the 5G communication module, the remote cloud control platform can realize the interactive communication of command data and status data with the automatic airport microcontroller module of the vehicle-mounted UAV.

Circuit design of GPS communication
The vehicle-mounted GPS module provides the position coordinate information of the vehicle for the UAV, which is convenient for the UAV to accurately identify the target position of the unmanned automatic airport when landing.The communication interface circuit design of the GPS module and the automatic airport microcontroller of the vehicle-mounted UAV are shown in Figure 6.The GPS module interface is standard RS232 level logic, defined as logic "1" when the data level is -12 V, and logic "0" when the data level is 12 V, which is very different from TTL level logic.To achieve the conversion matching of the RS232 logic level and TTL logic level, the SP3232EU chip is selected.

Automatic inspection task planning
The vehicle-mounted UAV automatic airport is the end node of the UAV for automated operations.
The operator needs to check the status of the UAV, plan the operation task, collect the autonomous landing after the operation, charge control, and analyze the cloud platform of operation data through the cloud control platform [7][8] on the Internet computer.The task planning process of the automatic airport inspection system of the vehicle-mounted rotor UAV is shown in Figure 7.
In the case of an uncontrolled drone, it relies on satellite navigation and obstacle avoidance systems to automatically complete the task and return home.Escape flights cannot receive RTK data, and flight deviation is large due to the use of GPS data alone, which is prone to collision risk.Therefore, we should set up a safety strategy of de-control and return.In the fixed route, the time threshold of the disconnection alarm can be set according to experience, and the manual timely disposal will be guided if the threshold is exceeded.In the case of task interruption, the interruption node is recorded, and the control center automatically generates supplementary routes to complete the breakpoint resumption.

UAV landing navigation strategy
The precise landing of drones into the hangar platform is one of the key technologies to achieve the fully autonomous operation of equipment.The traditional UAV landing method is based on INS and GNSS navigation, but the landing accuracy and landing safety are far from meeting the requirements of the automatic inspection system [9] .In this paper, a UAV landing navigation method based on GPS and image visual fusion is proposed, as shown in Figure 8.The attitude angle and GPS information of the vehicle-mounted automatic airport are sent to the flight control system of the UAV through the cloud control platform, and after a period of recording statistics, the location radius of the vehicle-mounted unmanned automatic airport in the ground coordinates is fitted.The UAV visually detects the cooperative identification at the ground end through the camera [10][11] , and obtains the horizontal offset between the UAV and the ground cooperative identification center, the height of the airframe, the angle between the positive direction of the ground sign and the UAV course, and the target deviation from the UAV heading angle information in real-time.When the deviation between the real-time GPS beacon value and the ground position is greater than a certain threshold, the drone adopts the original value of the GPS sensor on the airborne end.When the real-time GPS beacon value of the airborne terminal converges at the ground position, the fusion GPS beacon value is calculated through the perspective transformation of the image identification.Through double correction and complementary correction, the navigation accuracy of the drone when landing is greatly improved.

Experimental verification and analysis
To verify the effectiveness and reliability of the design, the system function test of the automatic inspection system of the vehicle-mounted rotor UAV field is carried out under different wind speed environments, as shown in Figure 9.In the test, the working reliability of the cabin door, the task planning success rate of the automatic inspection system, the deviation of the aircraft landing accuracy, the success rate of the completion of the aircraft's autonomous collection, and the success rate of the aircraft charging were tested, as shown in Table 1.show that within the wind speed of Level 5, the automatic inspection system of the vehicle-mounted rotor is in a stable operating state.When the system detects that the wind speed is greater than Level 5, the system stops the inspection operation task, which meets the expected design indicators and is stable and reliable.The rotor UAV adopts the landing navigation method based on GPS and image visual fusion, greatly improves the landing accuracy of the UAV through double correction and complementary correction, and adopts the operating conditions with complex environments in the field.
The application scenario of the automatic airport inspection system of the vehicle-mounted UAV is shown in Figure 10.In operation scenarios such as power line inspection, forest fire monitoring, and highway inspection, the inspection environment is complex and the inspection operation volume is large.Through the data sharing of the cloud control platform, the remote command hall can monitor and analyze the inspection operation data in real-time, and take timely and effective measures to deal with and solve abnormal problems once they are found.This inspection system not only ensures the unmanned, standardized, and intelligent inspection operation, but also greatly improves the efficiency of the inspection operation.

Conclusion
Aiming at the problems existing in the actual use of existing distributed airports, this paper designs the automatic airport system of a vehicle-mounted rotor UAV.This paper introduces in detail the system composition, hardware circuit design of the system, and software control strategy of the vehicle-mounted rotor UAV automatic airport.Experimental results show that the landing navigation method based on GPS and image visual fusion can make the UAV work stably and reliably in the complex environment of the field.At the same time, remote autonomous planning and control based on a cloud platform can make the operation process of UAVs unmanned and intelligent.The vehicle-mounted automatic airport system can make the UAV carry out independent collection, charging, and maintenance, etc., with strong mobility, greatly expanding the operation range of the UAV.It also has high engineering application value in power line patrol, highway inspection, and forest fire monitoring.

Figure 1 .
Figure 1.Block diagram of the automatic inspection system of vehicle-mounted rotor UAV.The automatic inspection system of the actual vehicle rotor UAV is shown in Figure2.When the work vehicle drives to the work site, the operator opens the laptop in the operation seat, connects to the wireless network, opens the cloud operation control platform, and detects the working status of the UAV.After the status of the drone is normal, the automatic inspection task of the drone is planned on the cloud operation platform.After the mission starts, the vehicle-mounted UAV automatic airport raises the UAV to the take-off platform and performs flight operations according to the set flight track.When the operation is completed, the drone autonomously returns home and lands on the take-off platform, and the vehicle-mounted UAV automatic airport collects the drone and lowers it to the cabin for charging and maintenance for the next operation.

Figure 2 .
Figure 2. Physical diagram of the automatic inspection system of vehicle-mounted rotor UAV.

Figure 8 .
Figure 8. GPS-based and vision-based navigation system.The attitude angle and GPS information of the vehicle-mounted automatic airport are sent to the flight control system of the UAV through the cloud control platform, and after a period of recording statistics, the location radius of the vehicle-mounted unmanned automatic airport in the ground coordinates is fitted.The UAV visually detects the cooperative identification at the ground end through the camera[10][11] , and obtains the horizontal offset between the UAV and the ground cooperative identification center, the height of the airframe, the angle between the positive direction of the ground sign and the UAV course, and the target deviation from the UAV heading angle information in real-time.When the deviation between the real-time GPS beacon value and the ground position is greater than a certain threshold, the drone adopts the original value of the GPS sensor on the airborne end.When the real-time GPS beacon value of the airborne terminal converges at the ground position, the fusion GPS beacon value is calculated through the perspective transformation of the image identification.Through double correction and complementary correction, the navigation accuracy of the drone when landing is greatly improved.

Figure 9 .
Figure 9. Flight test of inspection system.Figure 10.Application scenarios of the inspection system.

Figure 10 .
Figure 9. Flight test of inspection system.Figure 10.Application scenarios of the inspection system.

Table 1 .
Test record table of the automatic inspection system of vehicle-mounted rotor UAV field.