Design of Attitude Control System of a Double CubeSat
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The NTNU Test Satellite, NUTS, is a satellite being build in a student CubeSat project at the Norwegian University of Science and Technology. The project was started in September 2010 as a part of the Norwegian student satellite program run by NAROM (Norwegian Centre for Space-related Education). The NUTS project goals are to design, manufacture and launch a double CubeSat by 2014. As payload an IR-camera observing waves in the air-glow layer is planned, as well as a short-range RF experiment. The satellite will fly two transceivers in the amateur radio bands. Final year master students from several departments are the main contributors in the project and most of the system components are designed and built by students. As the main payload is an IR-camera and one of the main goals once in orbit is to take pictures of the gravity waves in the atmosphere, a reliable Attitude Determination and Control System (ADCS) is important. In order to take reliable pictures, an absolute requirement is that the camera does not miss the Earth, and more accurately be able to point towards the Earth in such a matter that unwanted disturbances, such as stars and other light sources in space, is not in the camera's field of view. In this paper, a realistic simulation environment have been created in order to assess and design an Attitude Control System (ACS) to be implemented on the satellite. Magnetorquers as the satellite's actuators have been designed. Further, a detumbling control algorithm using an estimated derivative of the geomagnetic field measured with a magnetometer, is shown to be working satisfactory, even when being subjected to heavy environmental disturbances. Linear and nonlinear reference control algorithms subjected to realistic disturbances in the space environment have been tested and compared as well. Comparisons between a proportional derivative (PD) controller and a linear quadratic regulator (LQR) are presented, and the nonlinear controller proved to give best results and pointing control good to take pictures of the Earth atmposphere is achieved.