Departure Device Design With Dual Redundancy for the Assistant Landing Gear of Long-endurance UAV

In modern wars and regional conflicts, in order to reduce casualties, more and more unmanned equipment and weapons are utilized. Among them, the UAV is the new favorite in the sky. Application fields of the military UAV are wide and varied. It is found to be used for intelligence reconnaissance, battlefield surveillance, as well as strikes. As for the scout UAV, long endurance is of utmost importance. There is no denying that less weight can result in longer endurance. A departure device for the assistant landing gear of a long-endurance UAV is designed in this paper. The system constitution and working principles are introduced in the beginning. Then, detailed system designs are presented, including the hardware design, software design, and other parts of the departure device. Several static and dynamic experiments are conducted to prove its feasibility. At last, the conclusion part summarizes its applications.


Introduction UAV (Unmanned Aerial Vehicle
) is an aircraft that can complete flying tasks without a manipulator [1].With the advance of science and technology, UAVs are developing more and more rapidly, which can be applied in different application areas, especially in the military domain [2].The main military application is intelligence reconnaissance.The UAV can be equipped with a variety of sensors, which help to track different targets and scan topographical maps, offering important intelligence information for the modern war [3].The key feature of the scout UAV is long endurance, which needs to reduce weight as much as possible [4][5].Sometimes, there is a pair of assistant landing gears for the longendurance UAV which mainly helps the UAV to take off and then drop at a certain height above ground [6].The departure device for the assistant landing gear is mainly used to disengage the assistant landing gear from the UAV during the take-off stage, which executes the departure action according to the height information acquired by the sensor.As long as the preset height is reached, the assistant landing gear is unlocked and dropped, thus reducing the weight of the UAV platform and enhancing long-endurance performance [7].In this context, a departure device is designed.Dual redundancy technology is adopted for higher reliability and better performance.Features of the departure device are power-on BIT function, status monitoring, status feedback, as well as configurable height or time trigger mode [8].

System constitution
The departure device is mainly composed of a battery, departure controller, height sensor, degaussing electromagnet, and mechanical structure.The battery offers a power supply for the whole system to ensure normal operation.The departure controller mainly conducts digital control of departure operation and is designed with a human-machine interaction interface which facilitates parameter configuration and departure operation.The height sensor is a liar sensor and it measures the relative height of the assistant landing gear above ground.The degaussing electromagnet serves as the actuator of the system.It connects the departure device and the assistant landing gear when there is no power supply.Once the departure operation is conducted, the electromagnet degausses to ensure it drops smoothly.The mechanical structure mainly functions to fix and seal the departure device and it is equipped with a protective shell to absorb shock when hitting the ground.

Working principles
As shown in Figure 1, the departure device adopts a dual-redundancy design with the height control unit and the time control unit.Once the power switch is turned on, the departure device begins to work.After the UAV takes off, the height sensor sends the relative height information of the assistant landing gear back to the controller in real time.If the preset height is reached for a certain time, the corresponding relay is closed and the electromagnet is powered and degaussed.The connecting force between the assistant landing gear and the UAV platform vanishes.In this way, the assistant landing gear is dropped.There is also a timing switch in the departure device.No matter whether the height control unit is available or not, the timer counts down to wait for the trigger of departure operation with the timing switch on.The height control unit and the time control unit work independently from each other.Both units can complete the departure task.The human-machine interaction interface comprises the status indicator lamp, power switch, and timing switch.At the same time, there is an isolated RS422 serial port for communication with external devices.In addition, an external charging interface is designed for battery charge.

Battery design
The battery pack of the departure device utilizes an 18650 lithium-ion monomeric cell in the form of a cylindrical steel shell.The cell's rated voltage is 3.6 V with its rated capacity of 3.0 Ah and its working voltage ranges from 2.5 V to 4.2 V.The battery pack is designed with 7-series and 1-parallel cells with a rated voltage of 25.2 V and a voltage range of 17.5 V to 29.4 V.The total capacity is 75.6 Wh, which is enough for system application.A stable 28 V power supply for the departure device can be obtained via a power regulator.

Controller design
The departure controller adopts a dual-redundancy design, including a power supply unit, height control unit, time control unit, and communication unit, as well as a status indication and storage unit.

Power supply circuit.
The internal conversion of the power source is shown in Figure 2.After the battery power source, there is a Transient Voltage Suppressor to suppress transient peak voltage and conduct surge protection.There is also an EMI filter to improve Electromagnetic Compatibility.A regulator exists to produce a stable 28 V supply from the battery.Various DC-DC converters are used to generate different secondary power supplies for different units in the departure controller.+5VD and +5VD_2 are the power sources of the height control unit and time control unit separately.+12VD offers a power source for the lidar sensor, while +5VP provides a power supply for the drive circuit of the degaussing electromagnet.

Height control unit.
As shown in Figure 3, the height control unit mainly accomplishes heighttriggered departure operation, which includes the communication circuit of the height sensor, the control core, and the drive circuit.For the simple reason that the output data of the height sensor is transmitted via the RS485 interface, an RS485 bus transceiver is designed to receive real-time height data from the height sensor.The height control core serves as the main control core of the device.Besides the heighttriggered departure control, it collects the monitoring information on system status and records the necessary working information.
Figure 3. Height Control Circuit.In addition, the main control core needs to communicate with the backup control core, namely, the timing control core.There are various digital peripheral interfaces such as UART, IIC, AD, and TIMER.The drive circuit exerts on-off control on the dis degaussing electromagnet.It can be divided into three parts, which are the signal isolation circuit, the relay drive circuit, as well as the relay.The signal isolation circuit features an Optocoupler to realize signal isolation and transformation.

Time control unit.
The timing control unit mainly accomplishes time-triggered departure operations, which include the timing control core and the drive circuit.The timing control core is mainly responsible for the time-triggered departure control.With the timing switch on, the counting down begins.When the preset time is met, it conducts departure operation immediately.The timing control core also needs to communicate with the height control core to monitor each other's working status.The timing control core adopts the same design scheme as the height control core.The drive circuit design of the timing control unit is also the same as that of the height control unit.

Communication unit.
The height control core and the timing control core communicate with each other through the internal serial port.As shown in Figure 4, to communicate between the departure device and the external device, there is an isolated RS422 communication interface in the height control unit.The RS422 interface can serve as a medium for the bootloader to update the embedded program.As for the timing control unit, a UART serial interface (TTL level) is reserved for the serial bootloader.

Status indication and storage unit. Various LED lights are designed to indicate the working status.
There is an EEPROM chip to record flight information as needed.As shown in Figure 5, the status monitoring circuit mainly acquires battery voltage, battery current, voltage and current of stable 28 V power supply, electromagnet current, as well as voltage of sensor power supply.Based on the current and voltage information mentioned above, the fault diagnosis can be performed to judge whether the departure device works normally or not.The current is acquired via the Hall-effect current sensor, whose output is voltage, thus realizing isolated collection.The voltage is acquired by the isolation amplifier to generate differential voltages.Then it is converted into unipolar voltage with the signal processing circuit.All the current and voltage information is sent to the internal ADC peripheral of the height control core.

Software design
As for the embedded software, there are two kinds of software.The height control software for the height control core serves as the main software and the timing control software for the timing control core serves as the backup software.The control software is designed based on the foreground-background idea [9].As shown in Figure 6, the height control software consists of the main program and the interrupt subprogram.In the beginning, the whole system and its peripherals such as ADC, UART, DMA, and IIC, as well as the watchdog are initiated [10].The TIMER is then initiated and the time reference is provided for different peripherals and different interrupt vectors.Following these initiations, there is an infinite loop running forever in which the watchdog is reset periodically to prevent the program from entering "dead circulation".There are three interrupt subprograms.Once a certain interrupt occurs, the corresponding subprogram works and conducts its expected functions.The timing control software adopts a similar design as the height control software.The main difference is the departure-operationtriggered mode.

Mechanical structure
The battery and control board are installed in the control box.The departure device is installed on the pillar of the assistant landing gear via the shock absorber and the collar.In the meanwhile, the shockabsorbing shell is added outside the departure device.It can not only protect the battery, sensor, control board, and other components from corrosion by the external environment effectively but also reduce damages in the impact environment.As shown in Figure 7, there are various indicating LED lights and switches on the control panel.

Barycenter calibration test
When the barycenter is calibrated, the departure device is worked in the time-triggered mode, with a countdown time of 100 seconds.At first, the departure device is mounted in a certain position of the assistant landing gear and opens the timing switch.A specific lifter brings it 1 meter above the ground.When the time is reached and the assistant landing gear is dropped, it is necessary to check carefully whether it is dropped vertically or not.The position is adjusted until the assistant landing gear can be dropped vertically 1 meter above the ground.The calibration test is repeated at the height of 2 meters, 3 meters, and 4 meters above the ground and the mounting position is adjusted correspondingly.At last, the appropriate mounting position is determined.

Static test
The static test is conducted at a height of 5 meters above the ground and the trigger source is the height.The departure device is mounted to the assistant landing gear in the position determined by the barycenter calibration test.The lifter brings the assistant landing gear slowly above the ground until it reaches a height of 5 meters.It is recommended to check carefully whether the assistant landing gear can be dropped or not.After that, the departure device must be checked again with a power-up built-in test.Only by satisfying both of the following criteria that the assistant landing gear can be dropped successfully and the departure device can pass the built-in test without any fault, the static test can be regarded as passed.
The static tests are conducted 5 times successfully to prove the departure device's static performance.

Dynamic test
The specific lifter is put in a truck that runs at a speed of 10 m/s to conduct the dynamic tests, which can be divided into three sets with two times in each set.The first set of tests is stability tests.The assistant landing gear is brought to 3 meters above the ground and the truck begins to run at a speed of 10 m/s for 5 kilometers.Then, it is recommended to check carefully whether the landing gear is dropped or not.Theoretically, the assistant landing gear will not be dropped with the departure device working in the mode of 5-meter height-triggered control.In practical tests, the landing gear is held stably in the lifter two times.The second set of tests is the height control test.The assistant landing gear is brought to 4 meters above the ground.The truck begins to run at a speed of 10 m/s while the lifter brings the assistant landing gear higher and higher slowly.When the height reaches 5 meters, we need to check whether the assistant landing gear is dropped or not.The third set of tests is conducted similarly to the second set.The difference lies in that the tests are time-triggered.When the preset time is met, we need to check whether the assistant landing gear is dropped or not.The latter two sets of tests are conducted successfully.

Conclusion
Scout UAVs for military applications are becoming more and more prevalent in the modern world.To enhance its long endurance, weight should be reduced as much as possible to achieve the goal of being lightweight.Sometimes there is a pair of assistant landing gears, which are only available in the takeoff stage and dropped from the UAV platform once reaching a certain height above the ground.In this background, the dual-redundant departure device is designed, with the introduction of detailed hardware design and software design.Several ground tests are conducted, and the device can accomplish the control task itself only by turning on certain switches.Both static and dynamic tests are conducted successfully.It is proved in grounds tests that the departure device designed in this paper is feasible and flexible.What's more, the departure device's performance will be tested further in the following flight tests.

Figure 1 .
Figure 1.Block Diagram of Departure Device.The height control unit and the time control unit work independently from each other.Both units can complete the departure task.The human-machine interaction interface comprises the status indicator lamp, power switch, and timing switch.At the same time, there is an isolated RS422 serial port for communication with external devices.In addition, an external charging interface is designed for battery charge.

Figure 5 .
Figure 5. Status Indication and Storage Circuit.

Figure 7 .Figure 8 .
Figure 7. Mechanical Structure.4. Experimental test Before the experimental tests, the departure device has to be verified as feasible.Normal temperature tests of 100 hours, high temperature (+60℃) tests of 4 hours, low temperature (-45℃) tests of 4 hours, and vibration tests with 3 hours are conducted with the automatic trigger of degaussing electromagnet actuation every 5 minutes.In total, 1332 times actuation of degaussing electromagnet is successful.In this way, the feasibility of the hardware design and the effectiveness of the software design are verified.As for the experimental test, the departure operation is triggered by 5 m height or 200 seconds at the