Attitude Control of UWE-4 for Orbit Correction during Formation Flying

UWE-4 is the fourth pico-satellite in the University of Wuerzburg Experimental satellite series. After the successful demonstration of on-board attitude determination and control with UWE-3 in 2013, the mission of UWE-4 is to validate electric propulsion technology for formation flying missions at the pico-satellite level of miniaturization. The challenge in this project is to demonstrate precise orbit and attitude control given the extremely limited resources available to the satellite. In this paper, attitude control of a CubeSat is studied and a simulation framework for the attitude and orbit control is presented. Different control philosophies are developed, compared and implemented to fly on-board UWE-4. The absence of a requirement for precise attitude control in the thrust axis together with having insufficient action in that axis has motivated the development of a quaternion based dynamic target attitude solution. This result helps in achieving precise 2D attitude control while also providing complete stabilization on the third axis. A non-linear controller has found to be advantageous over a linear controller for this particular problem of attitude control where frequent large angle manoeuvres are desired for changing the direction of thrust. Therefore, a sliding mode control has been implemented and evaluated for its performance.