MAGNETIC TORQUING SCHEME

Abstract

T note presents a simple scheme to use the earth's magnetic field as an angular momentum reversal device in the attitude control of satellites. The basic control concept involved is the use of momentum storage devices supplemented on a discontinuous basis by magnetic torques for desaturation purposes. Reaction wheels are used to attitude control the vehicle by storing angular momentum, and the magnetic torques are used to remove angular momentum from the system. The system presented here performs only logical (not arithmetic) computations based upon angular momentum and magnetic field measurements and, therefore, is a simplification over the control techniques discussed in Refs. 2 and 3. This magnetic momentum removal system requires only a simple threshold measurement of both the reaction wheel speeds and the body axis components of the earth's magnetic field. These signals are combined using some simple logical operations to produce constant control moments. The concept discussed below is for three-axis attitude stabilization as opposed to those which control a spin stabilized vehicle. A block diagram of the attitude control system under consideration is shown in Fig. 1. The block diagram shows the primary control loop, consisting of attitude sensors, compensation, and reaction wheels to control the body rotational dynamics, and the magnetic torquing system, consisting of magnetometers, logic, and magnetic moment generators. Because of disturbance torques acting on the orbiting satellite, the reaction wheel angular momentum storage will be fluctuating periodically with a component of secular angular momentum buildup. The function of the magnetic system is to remove this secular component of angular momentum. A magnetic dipole moment in a magnetic field experiences a torque given by the cross product of the magnetic moment and the magnetic field vector. Written out in body coordinates, this relationship becomes