When a man-made satellite flies in space, it will be affected by the residual aerodynamic force, the impact force of micro meteors, the uneven gravity caused by the oblateness of the earth, the pressure of solar radiation, and the interference forces such as the moving mechanism (such as spring and engine) inside the satellite, resulting in the change of satellite attitude and even orbit. In addition, each satellite has its own specific mission, and there are certain requirements for its flight attitude during flight. For example, the paraboloid antenna on the communication satellite and the camera on the earth observation satellite should always aim at the ground, and the telescope on the solar observation satellite should always aim at the sun. Therefore, the satellite is equipped with attitude control and orbit control subsystems.
The so-called satellite attitude control is to control the flight attitude of the satellite, maintain the stability of the attitude axis, and change the direction of the attitude axis as needed. Due to various interferences, the attitude angle and attitude angular velocity of the satellite in space often deviate from the design value. At this time, it is necessary to control and adjust.
There are two kinds of satellite attitude control subsystem: passive and active. Among them, the control force of the passive control system does not need to consume the energy on the satellite, but is provided by the dynamic characteristics of the satellite or space environmental torque, mainly in the form of spin stability. The active control system forms the control command according to the attitude error (the difference between the measured value and the nominal value), generates the control torque and realizes the attitude control. It mainly adopts flywheel control and jet control, which can control the three-axis stability of the satellite. This method is adopted by most satellites at present.
The spin stabilization method is to keep the satellite stable by spinning around an axis. In short, its principle is similar to that of a rotating gyro: high-speed rotation can keep the direction of the rotation axis of the object unchanged. Most of the early satellites used this simple control method. When the launch vehicle rotates symmetrically along the tangent direction of the last stage, the engine can also ignite the satellite. For a satellite rotating at high speed, the direction of its rotation axis in space will remain unchanged.
The three-axis stabilization mode is to control the three axes perpendicular to each other of the satellite, and no axis is allowed to rotate and swing beyond the specified value. The system of satellite three-axis attitude control generally includes three parts: attitude sensor, attitude controller and attitude actuator. Attitude sensors include inertial sensor, earth sensor, sun sensor, star sensor, etc., which are used to detect and measure the attitude change of the satellite, that is, the rotation angle and rotation angular velocity of the satellite along each axis, and whether they exceed the specified range.
The attitude controller is used to transmit the satellite attitude angle change signal sent by the attitude sensor to the attitude actuator after a series of comparison and processing. The attitude actuator generates torque according to the control signal sent by the attitude controller to restore the satellite attitude to the correct position. The commonly used actuators are reaction flywheel and thruster. When the attitude of the satellite is in the required attitude, the flywheel keeps rotating at a uniform speed; If the satellite deviates from a certain position, the flywheel accelerates or decelerates, producing a torque in the opposite direction to return the satellite to the required attitude position. Such a flywheel is set in each of the three axes of the satellite, which can control the attitude of the satellite in the three axes. Several small thrusters can also be placed in the direction of the three axes of the satellite. Once the satellite deviates from the required attitude, the thrusters in the corresponding direction will eject gas and generate thrust to return the satellite to the required attitude position.