Fuzzy Model Predictive Controller for Attitude Control of Satellite

Abstract

The technological advancement in satellite has tremendously improved in recent years. Generally, satellite applications have been classified into four categories i.e., navigation, communication, weather and earth observation missions. The successful execution of these applications heavily relies on the performance of attitude determination and control subsystem of satellite. Therefore, researchers have been continuously working to improve the satellite attitude regulation under challenging space environment by applying various control strategies i.e., PD, PID, LQR, MPC. MPC is a famous control technique, as it can handle the operational constraints of the system effectively. In satellite, actuator torque has limited capability and it can result in the saturation problem. MPC can regulate the satellite attitude without violating the actuator constraints hence ensuring system stability. However, the mathematical model of satellite is highly nonlinear and for nadir pointing satellite, it can be treated as simple linear system for a constrained operating range. Although, it impacts the accuracy of the nonlinear model when a model-based controller is used. This results in a compromise between simplicity and accuracy that is faced by every control engineer. Therefore, in this work a systematic approach to improve the attitude accuracy has been explored while deriving linear subsystems of the nonlinear satellite model for the controller design. In this approach, the mathematical model of the plant is represented by various linear subsystems in terms of Takagi-Sugeno Fuzzy rules which are independent of each other and a control law is designed for each rule using MPC. The output of each MPC is accumulated based on Parallel Distribution technique hence providing command to the actuator. Eventually, the torque produced by the actuator is applied on nonlinear plant. Comparison of the response specifications (transient and steady state response) between PID, MPC and Fuzzy-MPC controllers under different cases such as external disturbance, actuator constraints, large reference angles and initial angles has been discussed. The simulation results indicated that proposed Fuzzy-MPC exhibits consistent and improved performance in all four cases in comparison to other controllers. The pointing accuracy is successfully achieved while catering to the challenge of actuator constraints under external disturbances of LEO.