Views: 9 Author: Site Editor Publish Time: 2020-02-28 Origin: Site
The selection of a base station needs to be comprehensively considered from the aspects of performance, matching, compatibility, and use requirements. Among them, it is particularly important to note that the base station equipment must be compatible or compatible with the mobile switching center in order to achieve better communication results.
To maintain stable high bandwidth in long-distance transmission and maintain a reliable network environment, the most effective method at present is to combine two wireless network propagation media (such as light and millimeter waves), which can compensate each other's shortcomings. And do not interfere with each other's signals. Compared with those wireless communication systems that use one type of signal as the main force and the other type as backup, based on this set of networks, the functions of the two types of signals need to be in the same position to ensure even over long distances. During transmission, it can provide fiber-level network transmission speed, quality and reliability. Then to solve the problem of the displacement of the air base station, it is necessary to tether the UAV platform as the high-altitude base station to achieve it.
Tethered drones implement several core technologies for high-altitude base stations:
1. Power-to-weight ratio of drive motor. The power-to-weight ratio refers to the ratio of the output power of the motor to the weight of the motor itself, and the unit is kilowatt / kg. According to the empirical value of the lift / power ratio of rotorcraft, about 5 to 10 kilograms of lift are generated per kilowatt of power. It is more difficult to design a system with a power-to-weight ratio of less than 1 kilowatt / kg. It is emphasized here that the motor power refers to the rated power, not the maximum power; the weight of the motor includes the motor and ESC and the cooling equipment for its normal operation. As a high-altitude base station, a tethered drone must work continuously for a long time. Unlike a multi-rotor drone, which can work intermittently for a short period of time, the drive motor is the core component of power and must work within the rated power range.
2. High-voltage power supply system. The biggest difference between a tethered drone as a high-altitude base station and a multi-rotor drone is its power supply. The power supply of the mooring rotor platform from the ground to the air must pass a certain transmission distance. The use of high voltage power supply can reduce the transmission current, thereby reducing power loss, at the same time reducing the cross-sectional area of the transmission wire, and reducing the weight of the power cable. The high-voltage power supply is not the higher the voltage, the better, but it must be comprehensively considered according to the actual system requirements. One way is that the transmission voltage can directly match the high-voltage motor, and the voltage directly drives the motor without conversion, and the structure is relatively simple. However, the design of the high-voltage motor in this way is limited by the high-voltage and high-voltage control devices of the motor ESC, and it is particularly difficult to accept the cost. Another method is to design a switching power supply for step-down on the platform, which is driven by the existing low-voltage motor of the multi-rotor. However, the switching power supply also has the requirements of power-to-weight ratio and the constraints of high-power high-voltage control devices. It also has to pay a certain price. of.
3. Tether the composite optical fiber cable. In addition to the motive power transmission, the mooring cable is also designed with optical fiber to transmit the optoelectronic signals of the electronic equipment carried on the platform. Therefore, the mooring cable is a power / signal composite cable.
In order to reduce the weight of the lifted equipment on the platform, equipment other than antennas and transmitters are generally placed on the ground, and the electronic signal connection between the platform and the ground equipment is ensured by a cable optical fiber. This can save platform lift, increase lift height, and lift gain. The tether cable itself also needs to reduce weight. Currently, aluminum alloy material can be used as the power conductor, which is much lighter than copper wire.
4. Flight control function. The tethered UAV platform is mainly hovering around a fixed point, without too many complicated flight movements. This looks simpler than a multi-rotor flight control, but it's not. The flight control of the tethered rotor needs to fully consider the impact of the tether on its flight, especially under various wind field conditions, the swing of the tether with the wind will cause the platform to lose control. The civilian tethered rotary wing drone must meet the requirements of Hengfeng 6 and Gust 8 at least, so that it can have practical significance. In the 6 to 8 wind field, the mooring line does not have a fixed law of movement. It is difficult to adjust it by the robustness of the flight control to the rotor platform control.
5. Aerodynamic design of rotor platform. Considering that the system rotor platform must operate in strong wind fields, the aerodynamic design of the platform is particularly important. The system rotor platform hovering in the wind field is actually equivalent to the platform flying in the plane along the direction of the incoming wind. The 6 to 8 wind speed is 10.8 to 20.7 m / s, which is equivalent to the ability of the rotor platform to have a maximum speed of not less than 75 kilometers per hour. To achieve this speed, the aerodynamic design of the fixed wing is more reasonable. Recently, a vertical take-off and landing fixed-wing UAV has been evolved. In fact, it is a simple combination of two rotor multi-rotor fixed-wing power systems. It is not economical from the perspective of the UAV itself, but it can solve the problem of fixed-wing takeoff and landing in specific situations .
The aerodynamic design of the vertical takeoff and landing fixed wing has great advantages when applied to the system rotor. The captive rotor is powered by the ground and has sufficient energy. The two power systems have their own uses in strong wind fields. The vertical power system maintains the platform hovering attitude. The horizontal power system enables the platform to fly upwind; the fixed-wing shape can generate lift for the platform, reducing the burden on the vertical power system. The tethered rotor UAV has already faced the stage of widespread application, but there is still some distance from being able to be used to the actual use. It is expected that the industry colleagues will work together to innovate.