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Four-axis stability frame, four-axis aircraft flight principle and double closed-loop PID control

  Four-axis stability frame, four-axis aircraft flight principle and double closed-loop PID control

The key technology of the quadcopter is the control strategy. Because intelligent control algorithms run complex floating-point operations and matrix operations, the microprocessor's computational power is limited and it is difficult to meet the requirements for real-time flight control. PID control is simple, easy to implement, and the technology is mature, so the current mainstream The control strategy is mainly around the traditional PID control deployment.
1 Four-axle aircraft structure and basic flight principle
The structure of the four-axis aircraft is mainly composed of a main control board and four electronic governors, a motor, and a rotating pulp that are in a criss-cross configuration. The motor is controlled by an electronic governor. The main control board is mainly responsible for solving the current flight attitude and controlling the ESC. Other functions.
Take the cross flight mode as an example. Rotor No. 1 is the head, No. 1 and No. 3 rotors rotate counterclockwise, and No. 2 and No. 4 rotors rotate clockwise, as shown in Figure 1.


  Referring to the flight state table 1, the motor speed is changed. Since the four motors rotate at different speeds, they are inclined at a certain angle from the horizontal plane, as shown in FIG. 1. The resultant force generated by the four motors is decomposed into upward lift and forward force components. When gravity is equal to lift, the forward component forces the quadcopter to fly horizontally in the direction of the tilt angle. The space Euler angles of three-axis angle are divided into elevation angle, roll angle, and heading angle: when the inclination angle is the elevation depression angle, it will fly forward and backward; when the inclination angle is the roll angle, it will fly to the left and right; At the heading angle, the left and right rotations are performed, and the left (right) rotation is due to the sum of the counter-torque generated by the two clockwise motors and the counter-clockwise counter-torque generated by the two counter-clockwise motors, that is, they cannot cancel each other out. Self-rotation around the z-axis under anti-torque.

  2 PID control

Double closed loop PID control
When the four-axle aircraft normally flies, it encounters external forces (wind, etc.) or magnetic field disturbances, distorts the acceleration sensor or magnetic sensor acquisition data, causes Euler angle errors solved by the attitude, and only uses the angle single ring to make the system It is difficult to operate stably, so the angular velocity can be added as the inner ring, and the angular velocity is output by the gyroscope. The collected value is generally not affected by external influences, and the anti-interference ability is strong, and the angular velocity is sensitive. When the external interference, the response is rapid; Similarly, the air pressure sensor in the high-level ring is also affected by external interference. The introduction of the z-axis acceleration loop can effectively avoid the influence of external interference and enhance the robustness of the system.

Four-axis aircraft double closed-loop PID control, as shown in Figure 3, Figure 4. The angle is used as the outer ring and the angular velocity is used as the inner ring to perform the attitude PID control. When the height needs to be set, the height is used as the outer ring and the z-axis acceleration is used as the inner ring to perform high-level PID control. Among them, the PID output is the throttle value, the throttle is given the value of the electronic governor, and the electronic governor controls the motor to change the spatial three-axis Euler angle and height.