According to the different working requirements of the satellite, the satellite attitude control methods are also different. Depending on whether a special control torque device and attitude measurement device are used, satellite attitude control can be divided into two categories: passive attitude control and active attitude control.
Passive attitude control is a method that uses the dynamic characteristics of the satellite itself and environmental moments to achieve attitude stability. Passive attitude control methods include spin stabilization, gravity gradient stabilization, etc.
Some satellites require one axis to always point in a fixed direction in space, and the satellite body is rotated around this axis to maintain stability. This attitude stabilization method is called spin stabilization. Its principle is to use the gyro-axis orientation obtained by the satellite’s rotation around its spin axis to orient the satellite’s spin axis in the inertial space. This control method is simple, and most early satellites adopted this control method. To make the satellite rotate, small rocket engines can be installed symmetrically on the surface of the satellite along the tangential direction. When necessary, the small engines are ignited to generate torque to spin the satellite or the final stage launch vehicle. my country’s Dongfanghong-1 satellite, Dongfanghong-2 communication satellite and Fengyun-2 meteorological satellite all use spin stabilization.
Gravity gradient stabilization is stabilized by using the torque (gravity gradient moment) generated by the unequal gravitational forces on parts of the satellite at different distances from the earth when the satellite is flying around the earth. For example, install an extension rod on the satellite. After the satellite enters the orbit, let it extend upward. After it is extended, its top end will be farther from the earth than the other parts of the satellite, so it will experience less gravity, while its other end will be farther away from the earth. When the satellite is close, the gravity it experiences is greater, and the resulting gravity difference forms a restoring moment on the center of mass of the satellite. If the satellite’s attitude (extent) deviates from the local plumb line, this moment can restore it to its original attitude. This control method is simple and practical, but the control accuracy is low.
Active attitude control is a method that forms control instructions based on the attitude error (the difference between the measured value and the nominal value) and generates control torque to achieve attitude control.
Many satellites need to control their three mutually perpendicular axes when flying, and do not allow any axis to rotate or swing beyond a specified value. This stabilization method is called the three-axis attitude stabilization of the satellite. At present, satellites are basically controlled using three-axis attitude stabilization, because it is suitable for satellites operating in various orbits and with various pointing requirements, and can also be used for satellite return, rendezvous, docking, and orbit changes. .
The system that realizes satellite three-axis attitude control generally consists of three parts: attitude sensor, attitude controller and attitude actuator. The function of the attitude sensor is to sense and measure the attitude changes of the satellite; the function of the attitude controller is to take the signal of the satellite attitude angle change value sent by the attitude sensor, and after a series of comparisons and processing, generate a control signal and send it to the attitude execution mechanism; the function of the attitude actuator is to generate torque according to the control signal sent by the attitude controller to restore the satellite attitude to the correct position.
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