Attitude control of a satellite using fuzzy controllers

Abstract

This paper proposes the design of fuzzy controllers for the attitude stabilization of the Republic of China Satellite (ROCSAT-1). In the first step, the original satellite controllers, two classical linearized controllers, are superseded by two fuzzy controllers. The objective is to obtain faster convergent time and lower steady-state error. The two fuzzy controllers will be consolidated to form one fuzzy controller in the next step, to obtain the same effectiveness of control. To demonstrate a fuzzy controller’s performance, interference to disturb the attitude stabilization is added during the simulation.

Introduction

ROCSAT-1 was the first satellite of the Republic of China. It was launched on January 27, 1999, into a circular orbit with an altitude near 600 km. Its orbital inclination was 35°, its orbital period was 97 min, and its weight was 402 kg. The mission life of the satellite was at least 2 years, with a design life of 4 years. Reliability for the spacecraft was 0.9 at the end of its 4-year design life. Although its design life is over, ROCSAT-1 is still in motion in space.

A satellite must maintain a certain attitude while in orbit to allow precise pointing of an antenna toward the Earth, to allow the accurate orientation of observation instruments toward the object being observed, and to direct solar panels toward the Sun. In the short-term, the satellite receives interference from phenomena such as the Earth’s gravitation, airflow, magnetic fields, and the solar wind. These phenomena can disturb the satellite’s attitude at any time or place. This makes it necessary to control attitude to maintain the satellite’s stability. In the long-term, the satellite receives interference from phenomena such as the Sun and Moon’s gravitation, aerodynamic drag, plus other factors which will make the altitude of the satellite lower gradually.

The motion of the satellite can be influenced in its attitude by internal and external interferences. The factors that affect a low orbit satellite are the residual magnetic field, the disturbance torques caused by interacting with Earth’s magnetic fields, and gravity gradient torques affected by Earth’s gravitation. Recently many research studies have been done concerning about these factors.

Fuzzy theory was initiated in 1965 by Zadeh who had also worked in developing the concept of state, which forms the basis of modern control theory (Wang, 1997). As the control methods evolved, Zadeh thought that classical control theory put too much emphasis on precision to handle complex systems in a practical manner. Fuzzy theory generated many debates and there are still ongoing discussions regarding its excellent way of handling complicated nonlinear systems. Since the fuzzy logic controller successfully controlled the simple dynamic plant in the laboratory, it was applied to the controlling of commercial mercantile products in Japan (Akahoshi, 1991, Tersai et al., 1991).

Many applications of the fuzzy logic controller on attitude controlling of satellites have been proposed successfully. Three MISO fuzzy controllers are used for attitude stabilization of a small satellite in a low earth orbit, proving that the fuzzy controller solves the control constrain problem by choosing the best magneto-torque, polarity and switching instances. The fuzzy controller is also computationally less demanding than the adaptive MIMO LQR controller (Steyn, 1994). Fuzzy logic is incorporated in the robust LQG/LTR method and showed that the proposed method achieved precise attitude control (Nam & Zhang, 1997). Application of the fuzzy logic controller to attitude control of a satellite with bigger solar panels is proved that can have a low settling time, plus it can work in a very low force working environment (Zadeh & Wood, 1999). The Takagi–Sugeno type of fuzzy model and combined H_∞ output feedback is used for attitude control of a flexible satellite to achieve precise attitude stabilization (Nam & Kim, 2001). An adaptive fuzzy mixed H₂/H_∞ attitude control is used for nonlinear spacecraft systems with unknown or uncertain inertia matrix and external disturbances (Chen, Wu, & Jan, 2000). The controller is tuned by means of reinforcement learning without using any model of the sensors or the satellite is proposed (Buijtenen, Schram, Babuška, & Verbruggen, 1998).

Since the applications of fuzzy logic attitude controllers on satellites in the cited references are mature, introducing an interference to disturb the attitude stabilization during the simulation demonstrates the performance of a fuzzy controller in restraining the disturbance. As previously stated two fuzzy controllers supersede the classical controllers to obtain faster convergent time and lower steady-state error. Two fuzzy controllers are consolidated to form one fuzzy controller and it is expected that this one will obtain the same effectiveness of control.