A SIMPLE PICO-SATELLITE 3-AXIS EARTH POINTING CONTROLLER

摘要

Nano or pico class satellites do not have the luxury of having a full 3-axis Attitude Determination and Control subsystem (ADCS) as other medium and large class satellites due to the limitations of mass, power and volume. In the small satellite industry, there have been advances in the field of Micro Electrical Mechanical subsystem (MEMS) for attitude sensors but MEM's actuators are still lagging, such as a full 3-axis micro reaction wheel configuration. Conventional actuator solutions include utilizing just magnetic torque coils or combining with a small gravity gradient boom. These solutions however only provide attitude stability with no 3-axis slew capabilities. To address the issue for a full 3-axis capable Earth pointing boomless pico-satellite, a simple Proportional Derivative (PD) control is proposed. The controller exploits the availability of conventional 3-axis magnetic torque coils and a single axis actuator such as a miniature variable thruster. The controller is not new as it simply utilizes a generic magnetic cross product law for the roll and yaw compensation and a standard PD for the pitch compensation, but it is designed specifically for a boomless pico satellite in a low Earth equatorial orbit. The satellite dynamics were derived and simplified to analytically obtain the PD controller gains for each axis. Using a full satellite simulator with space disturbance torques, the controller was able to provide Earth pointing capability to any desired roll, pitch and yaw within the expected accuracy and settling time. Although the controller does not eliminate the need for a single axis miniature actuator, it does provide 3-axis pointing capability utilizing only miniature and limited ADCS hardware. The controller is expected to open up more mission capabilities for future pico satellites in a low Earth equatorial orbit.