1. What is a drone
UAV is a big concept. The introduction on Baidu Encyclopedia is "unmanned aircraft" ("UAV") for short. Aircraft. UAV is actually a general term for unmanned aerial vehicles. From a technical point of view, it can be divided into: unmanned fixed-wing aircraft, unmanned vertical take-off and landing aircraft, unmanned airship, unmanned helicopter, unmanned multi-rotor aircraft, Unmanned parawing aircraft, etc. Compared with manned aircraft, it has the advantages of small size, low cost, convenient use, low requirements for the combat environment, and strong battlefield survivability.” It can even be said that unmanned aircraft can Machines that continuously move in the air are considered drones, but let’s limit the scope of the concept. Here we only discuss multi-rotor and fixed-wing aircraft. In this tutorial, drones refer to multi-rotors (quadrotors, six-rotors) and fixed-wing aircraft.
At the same time, pay attention to the difference between drones and model airplanes. UAVs refer to aircraft that do not need to be driven by humans (automatic flight), and model airplanes refer to aircraft that require manual remote control. This is the main difference between the two.
2. What is flight control and airborne computer
1. Introduction to flight control
Flight control, also known as flight controller and autopilot, is a component used to assist or fully autonomously control other systems and components of the aircraft during take-off, cruise, landing and other stages. To put it simply, the flight control is what realizes the underlying control of the UAV. What is the underlying control? The underlying control is power output (how to distribute power to multiple rotors), stable flight (how to maintain stability when disturbed), etc., which can be compared to the human cerebellum. Because the analogy of human movement requires the cerebellum, their respective muscles, and various nerves to achieve specific movements, which are relatively independent of thinking. After all, the human brain mostly thinks about what to do, and does not specifically think about how each muscle moves. It also wants to achieve The complete intelligence of the UAV, one needs to rely on the stable movement of the UAV, and the other requires the perception and decision-making of the UAV. The former is equivalent to the human muscles and cerebellum, and the latter is analogous to the human senses and brain.
At present, the mainstream flight controllers include foreign Pixhawk, domestic Leixun CUAV, Anonymous Kechuang, ACFly, etc. The difference between different flight controllers is similar to the difference between different brands of mobile phones.
At the same time, please note that the flight control is essentially a specially designed 32 single-chip microcomputer (of course, some may be other single-chip microcomputers, but they are still within the range of single-chip microcomputers), with modules necessary for drone flight, such as IMU, barometer, etc., at the same time The flight control can be connected to other modules to achieve more functions, such as adding a GPS module to realize GPS cruise, adding an optical flow module to enhance stability under certain circumstances, etc.
However, students who have studied single-chip microcomputers know that single-chip microcomputers need to be programmed to run, and flight control is no exception as a single-chip microcomputer. The program it burns is called flight control program (also called flight control software). The control program is the program that processes the data and controls the drone during flight. Due to the complexity of the flight of the drone, the flight control program is also very complicated. Fortunately, we do not need to write it ourselves, and can directly use the open source flight control program or closed source.
Here is an explanation of the difference between open source and closed source. Open source means open source code, which can be browsed and modified by everyone. Closed source means that the source code is not open. DJI UAV is an example of not open source code. Although the performance of the UAV is good, it is not recommended for everyone to use because it is not open source and the price is relatively expensive. (PS: There seems to be tens of thousands of DJI drones with the function of secondary development)
At present, the mainstream flight control programs include PX4, FMT, APM, etc., but in order to facilitate our autonomous flight and obstacle avoidance, we choose PX4 or FMT flight control programs here. As for the flight control programs of Anonymous Kechuang and ACFly, they are only suitable for completion. It is difficult to achieve true autonomous flight and obstacle avoidance in UAV-related competitions at the undergraduate level, so we do not recommend it for everyone.
Here we explain why we say this, because the flight controller is the cerebellum, and the onboard computer is the brain. Communication between the brain and the cerebellum is required, but this process is a bit complicated, so it needs a flight controller that supports this complex communication protocol to complete (actually The above is the flight control and flight control software that supports mavros and mavlink. We will explain these terms later. You only need to understand it as a special communication method), but ACfly and anonymous flight control do not support this, and cannot complete the simulation. Function (simulation is simply the way to simulate the flight of drones in the real world, you can test whether your program is reasonable, otherwise, if you put it directly on the real drone, it may directly cause the plane to crash), so for a higher upper limit, We do not choose these flight controls, but choose the PX4/FMT flight control program and the matching flight control.
2. Introduction to Airborne Computer
As the name implies, the airborne computer is a computer installed on the UAV. It has the characteristics of small size, low power consumption, and powerful performance. Running programs on it can realize the intelligent control and independent decision-making of the UAV, which makes up for the inability of the flight control to perform advanced functions. Disadvantages of development, the current mainstream airborne computers include Raspberry Pi, Jetson, etc., equipped with Ubuntu system, using ROS and other frameworks and libraries, connected to the flight controller, for intelligent control and independent decision-making of the drone, here the airborne computer is unmanned The brain of the computer can also be called the host computer.
3. Introduction of UAV hardware composition and structure
First, we dissect the composition and structure of the drone from the real object of the drone
1. Rack
The rack is the skeleton of the UAV, which carries all the electronic equipment of the UAV. As shown in the figure, all electronic equipment can only operate normally when they are fixed on the rack. The size of the frame is distinguished by the wheelbase. The 450 model means that the distance between the two motor shafts on the diagonal is 450mm. The larger the frame, the greater the power of the motor and the larger the blades, otherwise it will not be able to take off; the current frame There are mainly two kinds of plastic and carbon fiber. Carbon fiber has good mechanical properties, but the price is more expensive.
2. Paddle
The paddle is a special model. According to the physical principle, the moment of inertia of the paddle (relative to the rotating shaft) is proportional to the fifth power of the radius (approximately). All large paddles respond slowly and are not easy to change the state of motion (that is, the speed). The blades are divided into positive and negative directions, because when a single propeller is driven by the motor, it will generate a force to make the drone rotate horizontally along the central axis, so it is necessary to set different propellers to different directions to offset, otherwise the multi-rotor drone will continue to fly after takeoff. Rotation, but based on this principle, the yaw movement of the drone can be controlled by the speed
Generally speaking, the high speed of small propellers is good for stability, the aircraft can fly more stably, but the endurance performance will be worse
3. Motor
The motor is a special three-phase brushless motor with three terminals (there is no distinction between positive and negative poles, just reverse any two terminals to achieve reverse rotation), one-way rotation, high power, low friction, and can provide Very high lift. The kv value of the motor refers to the speed per minute of the no-load motor under 1v voltage, and the 380kv motor refers to the 380 revolutions per minute of the motor under no-load voltage under 1v voltage.
Brushless motors of the same series with the same dimensions will show different KV characteristics according to the number of winding turns. If the number of winding turns is large, the KV value is low, the maximum output current is small, and the torque is large; if the number of winding turns is small, the KV value is high, the maximum output current is large, and the torque is small.
But the motor does not have a larger kv value, the better. Under the same power condition, the larger the kv value, the smaller the torque provided by the motor, so a motor with a high kv value is equipped with a small blade, and a motor with a low kv value is equipped with a large blade, such as 380kv 16-inch diameter blades are generally installed on the motors of 980kv motors, and 10-inch diameter blades are installed on 980kv motors, and 6-inch and 4-inch small blades are equipped with higher kv motors and even used as ducted motors (the duct is a fixed-wing aircraft that is All fighter-shaped aircraft, propulsion backwards)
4. ESC
Electronic Speed Controller (ESC for short) is an electronic device that accepts PWM control signals and controls the speed of the motor. It can receive PWM signals from Pixhawk or signal receivers, and then control the rotation of the motor. Currently, multi-rotor UAVs generally use the Hobbywing brand. , the multi-rotor mainly uses the Lotte model (specially designed for multi-rotor), as shown in the picture, the ESC has three types of wires, and three black round heads (called banana heads, as shown in the second picture above) are connected to none Brush the motor, regardless of positive and negative, any reverse connection of the two interfaces can make the motor reverse rotation; then there is a long thick black line and red line, connected to the power supply, black ground (negative pole), red connected to Vcc (positive pole), Do not reverse the connection, otherwise it will burn out the ESC when the power is turned on; the last is the black and white wires that are entangled together. This is the new wire, which accepts the control signal and controls the current. Signal line, connected to the signal receiver or the output of the flight control
5. Galvanometer
The galvanometer is not a sensor that displays the current, but a module that supplies power to the flight control. It is connected to a high-voltage lithium battery, and then outputs a stable voltage to drive the flight control. The galvanometer of Pixhawk4 (as shown in the figure) integrates more functions. It can be a hub for powering the ESC (equivalent to a breakout board), and an adapter board for the flight control (leading out multiple pins of the flight control helps reduce the size of the flight control)
6. Battery
The power source of drones is a special battery for drones, with high discharge power and high discharge efficiency. 1S means that there is a lithium battery connected in series (all lithium batteries used in drones are connected in series), for example, a 3s battery is three lithium batteries Batteries connected in series; the larger the drone, the higher the voltage generally required, such as 450 drones generally use 3S or 4S lithium batteries, 680 will use 6S lithium batteries, super large drones (for example, with a wheelbase of more than 1000 12s lithium battery may be used; the full voltage of each lithium battery is 4.2V, and the empty voltage is 3.7V, so when the battery reaches the maximum voltage (the number of cells is 4.2V), the charging is completed, and when it reaches the minimum voltage (Number of nodes 3.7V) should not be used