# Rapid Tracking Satellite Servo Control for Three-Axis Satcom-on-the-Move Antenna
## 3 Modified ADRC Servo Control Design  
Currently, PID control is widely used in the servo control of SOTM antennas. After long-term development, the advantages of PID control, which are widely applicable and easy to design, have been widely recognized. PID control does not require an exact mathematical model of the controlled object; adjusting only three tunable parameters--the proportional constant, integral constant and differential constant, it can achieve good control effects and performance in most applications. However, PID control has some inherent disadvantages: it relies on an error-based design concept to eliminate errors, creating a certain time delay between system control action and disturbances. To swiftly suppress errors, higher control efforts are necessary, increasing the likelihood of overshoot, or even significant overshoot; when solely using the proportional part for control, the system may exhibit steady-state error. To eliminate this steady-state error, the integral element is introduced, but it may cause phase lag in the system and the integral element cannot effectively suppress specific disturbances. The mathematical model of the differential element is non-regular, making it a non-causal system that cannot be directly implemented. In digital PID controllers, the differential element is often replaced with a difference element, which reduces the efficiency of the differential element and subsequently affects the control effects of the PID controller. While combining PID and feedforward control can enhance the disturbance rejection capability of the controller, it is generally effective only for specific disturbances and necessitates a highly accurate system model, limiting its practical engineering applications. A common scheme for servo control is to adopt a multi-loop design. The loops typically selected include the position loop, current loop, speed loop and others. For example, reference [30] proposed a design concept and method of adding an RC filtering network in the innermost loop to suppress high-frequency noise based on a multi-closed-loop design scheme. Each loop is designed using the PI control method, but the system's uncertainties and nonlinearities were not considered. Building upon this foundation, reference [31; 32] considered the nonlinear characteristics of the SOTM antenna system and introduced sliding mode control to enhance the stability of the control system and reduce steady-state errors based on the multi-loop control design concept. This method requires high modeling accuracy for the object and is unable to simultaneously meet the requirements of rapidity and overshoot. References [33; 34] proposed a design scheme combining feedback control structure with feedforward control. By predicting the trend of system disturbances and providing feedforward compensation, it partially overcomes the shortcomings of traditional PID controllers in suppressing system disturbances. However, the premise is that the disturbances and the system model are known, so the disturbance rejection effect of feedforward control is not significant in complex disturbance environments. Reference [35] introduced feedback linearization methods and nonlinear system switching systems methods from modern control theory to enhance the robust stability of the control system. This method involves complex system design and requires high modeling accuracy. The aforementioned approaches cannot guarantee stable satellite communication in SOTM antenna system. Therefore, in the engineering field, PID control remains the mainstream design scheme for SOTM antenna servo control. However, there is the contradiction between rapidity and overshoot in PID control, and it does not address disturbances specifically but passively eliminates errors based on error. Therefore, using PID control SOTM antenna servo control systems cannot fully harness the efficiency of SOTM antenna systems.