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paper_Scholz_Romagnoli_Dachwald.doc

paper_Scholz_Romagnoli_Dachwald.doc - PERFORMANCE ANALYSIS...

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PERFORMANCE ANALYSIS OF AN ATTITUDE CONTROL SYSTEM FOR SOLAR SAILS USING SLIDING MASSES Christina Scholz 1 , Daniele Romagnoli², Bernd Dachwald 1 1 University of Applied Science Aachen, Germany ²German Aerospace Center – Institute of Aerospace Systems, Germany [email protected] , [email protected] , [email protected] ABSTRACT This paper deals with the attitude control performance analysis of a square solar sail with two sliding masses moving along the mast lanyards for pitch and yaw control. A robust nonlinear controller with a feedback and feed forward part is used to control the attitude of the sail. Numerical simulations have been carried out to investigate the system’s ability of performing precise and near-time-optimal reorientation maneuvers as well as the controller’s sensitivity with respect to the sail parameters, like the center of pressure to the center of mass offset or the sail’s geometry. Our simulation results are finally discussed and compared to previous results that have been obtained by others and are reported in the literature. INTRODUCTION The possibility of using only solar radiation pressure as thrust, as well as their unlimited propulsion capability, makes solar sails very attractive for high demanding missions [1]. In order to control a solar sail spacecraft in three axes, various solutions for the attitude control system have been designed. Most of them cause an offset between the center of pressure and the center of mass. For example, it is possible to control a solar sail with one gimbaled boom with two degrees of freedom, mounted at the center of the sail. Alternatively the change of attitude can be performed by articulated reflecting vanes at the masts end or by sail panel translation/warping [1,2]. In this paper another kind of attitude control actuator is considered. The so-called sliding masses, also known as trim control masses or ballast masses in the literature, are two small masses, which move inside and along the masts via lanyard lines and electric motors [3,4]. The sliding masses also provide a center of pressure to center of mass displacement. Assuming that the center of mass and the center of pressure are in the same plane, this displacement can be used for pitch and yaw attitude control. To control the movement around the roll axis a second controller is needed, for example, pulsed plasma thrusters mounted at the top of the booms could be used [4,5], or a rotating boom around the roll axis [6]. This paper, however, will not go into details about the control of the roll axis. Let us consider a 40 by 40m square sail with four carbon fiber- reinforced plastics booms, four triangular sail segments and two small sliding masses of 1kg each [3]. The sail area is assumed to be 1200m² and the satellites bus has a mass of 150kg. The total mass of the configuration is 185kg, taking into account the sail mass, the mass of the masts, the sliding masses, the central assembly and the payload mass. For more detailed information about the properties of the sail see Table 1 [3].
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