Patent ID: 7661627
Filing Date: 2010-02-16
Classification: B64G

Abstract:
1. A method for controlling the attitude of a satellite equipped with an attitude control system in a reference coordinate system (X, Y, Z) fixed with respect to the satellite for positioning the satellite, the method comprising: providing the attitude control system that comprises: only three primary actuators including two control moment gyros and one Z-axis actuator, the two control moment gyros each comprising: a rotor connected to a steerable gimbal, each rotor having a fixed axis with respect to the steerable gimbal, each gimbal having a fixed axis with respect to the satellite, the fixed axis of each rotor being perpendicular with respect to the fixed axis of its connected gimbal, the gimbal axes being parallel to each other and the Z-axis, each rotor driven to rotate about its fixed axis to orient its connected steerable gimbal about the fixed axis of said gimbal, the Z-axis actuator delivering torque in at least one direction not lying in a (X, Y) plane, the angular momentum vectors (H wherein the method further comprises the following steps: providing a set of secondary actuators used to achieve the offloading of the primary actuators; the secondary actuator being chosen among magnetic torquers, jet actuators, steerable reflecting ailerons or tabs; the method comprises an initialization phase during which the secondary actuators are operated in order to generate an angular momentum in at least one direction in the (X,Y) plane for bringing the pair of control moment gyros into a configuration in which the angle (α) has a value different from 0° and 180°; the method further comprising the steps of: using the Z-axis actuator for delivering torques in at least one direction not lying in the (X, Y) plane; imparting nonzero skew angle (ε) between the angular momentum vectors (H estimating kinematic and dynamic variables of the satellite along the X, Y and Z axes from measurements provided by sensors disposed on board the satellite; calculating setpoint variables for controlling a desired attitude of the satellite with respect to the three axes of the (X, Y, Z) coordinate system; and calculating control commands from differences between said estimated kinematic and dynamic variables and said setpoint variables, the control commands comprising commands to vary the orientation of the gimbal axes of the control moment gyros; sending the calculated control commands to the three primary actuators; sending commands to the secondary actuators to modify the angle (α) between the angular momentum vectors (H1 and H2) of the control moment gyros so that said angle (α) remains within a specified range that includes neither 180° nor 0°.