Features of UAV flight control system
UAV is the abbreviation of unmanned aircraft (Unmanned Aerial Vehicle), which is an unmanned aircraft using radio remote control equipment and self-provided program control devices, including unmanned helicopters, fixed-wing aircraft, multi-rotor aircraft, and unmanned airships. , Unmanned paraglider. In a broad sense, it also includes near-space vehicles (20-100 km airspace), such as stratospheric airships, high-altitude balloons, and solar-powered drones. From a certain point of view, UAVs can complete complex aerial flight tasks and various load tasks under unmanned conditions, and can be regarded as "air robots".
The flight control subsystem is the core system for the UAV to complete the entire flight process of take-off, air flight, mission execution and return to the field. One of the most core technologies. Flight control generally includes three major parts: sensors, airborne computers and servo actuation equipment. The functions realized mainly include UAV attitude stabilization and control, UAV mission equipment management and emergency control.
The so-called flight control of the UAV is the flight control system of the UAV.
There are mainly gyroscope (flight attitude perception), accelerometer, geomagnetic induction, air pressure sensor (rough control of hovering height), ultrasonic sensor (precise control of low altitude or obstacle avoidance), optical flow sensor (accurate determination of hovering horizontal position), GPS module (horizontal position height rough positioning), and control circuit composition. The main function is to automatically maintain the normal flight attitude of the aircraft.
UAV flight control refers to a control system that can stabilize the flight attitude of the UAV and control the autonomous or semi-autonomous flight of the UAV. It is the brain of the UAV.
With the development of intelligence, today's UAVs are not limited to fixed-wing and traditional helicopters, and have emerged in four-axis, six-axis, single-axis, vector control and other forms.
The control of fixed-wing UAV flight usually includes control surfaces such as direction, ailerons, lifts, throttles, flaps, etc. The wing surface of the aircraft is changed through the steering gear to generate corresponding torque, and control the aircraft to turn, climb, dive, and roll. and so on.
UAVs in the form of traditional helicopters control the aircraft's turn, climb, dive, roll and other actions by controlling the swashplate, throttle, tail rudder, etc. of the helicopter.
UAVs in the form of multi-axis generally control the attitude of the UAV by controlling the rotational speed of the blades of each axis to realize actions such as turning, climbing, diving, and rolling.
For fixed-wing UAVs, generally speaking, when the attitude is stable, controlling the rudder will change the heading of the aircraft, which usually causes a certain angle of roll. On a stable aircraft, it looks like a car turning on the ground. , which can be called a slip test. Rudder is the most commonly used method for automatic control turning. The disadvantage of rudder turning is that the turning radius is relatively large, which is slightly less maneuverable than aileron turning. The function of the ailerons is to control the roll of the aircraft. When a fixed-wing aircraft rolls, it will turn in the direction of the roll and drop a certain height at the same time. The function of the elevator is to control the pitch of the aircraft, the pull rod is raised, and the push rod is lowered. When the stick is pulled, the aircraft lifts up and climbs, and the conversion of kinetic energy to potential energy will reduce the speed. Therefore, the airspeed should be monitored during control to avoid stalling due to excessive stick pull. The function of the throttle rudder is to control the speed of the aircraft engine. Increasing the throttle amount will increase the power of the aircraft, accelerate or climb, and vice versa.
After understanding the control function of each rudder, we start to discuss the control of elevator and throttle. Fixed-wing aircraft have a minimum speed per hour called stall speed. When the speed is lower than this speed, the rudder effect will fail because the aircraft cannot obtain enough lift, and the aircraft will lose control. Through the airspeed sensor of the aircraft, we can know the current airspeed of the aircraft in real time. When the airspeed decreases, the aircraft must lose altitude by increasing the throttle or push rod in exchange for the increase in airspeed. When the airspeed is too high, reduce the throttle or pull the rod to make the aircraft Gain altitude in exchange for a reduction in airspeed. Therefore, the fixed-wing aircraft has two different control modes, which can be selected by the user according to the actual situation: The first control mode is, according to the set target airspeed, when the actual airspeed is higher than the target airspeed, control the The elevator pull rod, and vice versa; the airspeed affects the altitude, so the throttle is used to control the altitude of the aircraft. When the flight altitude is higher than the target altitude, reduce the throttle, otherwise increase the throttle. From this, we can analyze that when the aircraft is flying, if it is lower than the target altitude, the flight control throttle increases, resulting in an increase in airspeed, which in turn causes the flight control control lever, so the aircraft rises; when the aircraft altitude is higher than the target altitude, the flight control The control throttle is reduced, resulting in a reduction in airspeed, so the flight control controls the push rod to reduce the altitude. The advantage of this control method is that the aircraft is always controlled with airspeed as the first factor, thus ensuring the safety of flight, especially when abnormal conditions such as engine stall occur, the aircraft can continue to maintain safety until the altitude is reduced to ground. The disadvantage of this method is that the height control is indirect control, so the height control may have a certain lag or fluctuation. The second control method is: set the angle of attack when the aircraft is in level flight. When the flight altitude is higher or lower than the target altitude, on the basis of the level flight angle of attack, set a PID based on the difference between the altitude and the target altitude. The climb angle of the limited amplitude output by the controller is controlled by the deviation of the current pitch angle and climb angle of the aircraft to control the elevator surface, so that the aircraft can quickly reach this climb angle and complete the elimination of the altitude deviation as soon as possible. However, when the altitude of the aircraft increases or decreases, it will inevitably cause changes in airspeed. Therefore, the throttle is used to control the airspeed of the aircraft, that is, when the airspeed is lower than the target airspeed, the throttle is increased on the basis of the current throttle, and the current airspeed is high. After the target airspeed, reduce the throttle based on the current throttle. The advantage of this control method is that it can respond immediately to the change in altitude, so the altitude control is better. the maximum elevation angle, and eventually stall due to lack of power. Therefore, the two control modes are selected according to the actual situation. We chose the second control mode, and added that when the airspeed is lower than a certain speed, it is considered that an abnormality occurs, and it immediately switches to the first control mode to ensure the safety of the aircraft.
The structure of the five-hole probe pitot tube (differential pressure ratio) angle of attack sensor and the side-slip angle sensor is relatively simple, mainly including a hemispherical or conical five-hole probe used to measure various pressures, a sensor support part installed on the fuselage, and Connect the probe to the connector part of the support part. Several pneumatic pipelines conduct pressure through the probe, the connecting part and the supporting part. The differential pressure sensor converts the pressure signal into an electrical signal, and calculates the angle of attack (angle of attack) and sideslip angle through formulas.
The future development direction of UAVs: intelligent, systematic and clustered.