Patent GB-1294362-A (GB)
Title: Artificial satellite attitude control systems
IPC: B64G1/34
Filed: 1969-12-16
Abstract: 1294362 Artificial satellite NATIONAL AERONAUTICS & SPACE ADMINISTRATION 16 Dec 1969 [20 Dec 1968] 61208/69 Heading B7W  An artificial satellite comprises a body 12 and an attitude control system, the system comprising a gravity gradient member 17 mounted to the body, with two degrees of rotational freedom, means for causing the mounting means to tend to move the member towards alignment with a local vertical, and inertia means for damping vibrations of the gravity gradient member. The satellite is provided with a radio antenna 13 directed along its yaw axis, and the attitude of this axis referred to x, y and z axes defined with respect to the instantaneous direction of travel is determined by an infra-red horizon sensor 15 and a startracking detector 16. The gravity gradient member is mounted in a gimbal system (21), Figs. 2 and 3 (not shown), with shafts (24), (25) which are respectively aligned with the y and x axes when the satellite yaw axis is vertical, i.e. aligned with the z axis. To alter the attitude of the capsule in a given direction the member 17 is rotated in the opposite direction, the angular momentum of the satellite remaining constant. When the capsule has achieved its desired attitude the member 17 is returned to the vertical in order to stabilize the new attitude. This is achieved by damping the vibrations of the boom 17 in response to its angular displacement from the vertical, i.e. by absorbing its angular momentum by momentum wheels carried by the gimbal system (21). The control system associated with movement of the member 17 about the shaft 25 of the gimbal device is shown diagrammatically in Figs. 4, 5. The shaft is turned by a gimbal torquer 27 and a momentum wheel 41, for damping vibrations of the gravity gradient member 17, driven by a motor 44, and an optical encoder 29 for determining the angle turned by the boom 17. This angle p, is the angle between the member 17 and the yaw axis, and p = # - #, where #, # are the angles between the boom 17 and the axis respectively and the local vertical, (cf. Fig. 7, not shown). When the boom is vertical p = - #. To alter the attitude angle # in the yz plane of the capsule to a desired value # c  an r.f. signal is transmitted from a ground station, and a resulting signal # c  is compared in subtractor 56 with a signal # derived from the sensor 15 to give a signal (# c -#) to which is added, in summing amplifier 57, a signal dp/dt, derived by differentiating network 58 from a signal p received from the encoder 29. (# c -#) represents the angle by which the capsule must be turned to take up its desired attitude, and dp/dt the torque needed to damp vibrations of the member 17, and their sum is applied to the gimbal torquer 27, which together with network 58 and amplifier 57 constitutes gimbal torquer assembly 70. Action of the torquer 27 causes movement of the member 17 and of the capsule, represented by boxes 71 and 72 respectively, and the respective "outputs" from these boxes are values A, #. The gravity gradient torque resulting from the inclinations # of the member 17 and represented by box 73 is physically added to the action of the torquer assembly 70 at node 74. The received signal # c  is phase inverted and the resulting signal - # c  is compared with the signal p in subtractor 53, whose output controls the motor 44 which drives the momentum wheel 41, so that the member 17 will be damped until p= -# c , i.e. the member is vertical, and this part of the system together with subtractor 56 and node 74 constitutes a boom control loop. The feedback of signal # from the "output" of box 72 to subtractor 56 forms part of a capsule control loop, which has a much faster response than the boom control loop and can be considered independent of it. There is also provided compensation (orbital rate coupling torque) for the fact that the x-axis is continuously changing in direction because of the orbiting of the satellite.  The system for controlling the shaft 24 of the gimbal system is similar except that the lastmentioned feature is not necessary, as the y axis does not change in direction during orbit. However as the z axis does change it is necessary to provide a feedback control for that axis (Fig. 6, not shown) even though the gravity gradient member 17 does not have freedom of movement orthogonal to the plane of the gimbal shafts 24 and 25. The actual z axis rotation # of the capsule as derived by the detector 16 is compared with the desired value # c , which is generally zero, in a subtractor (76), and the output controls motor (46) driving yaw axis momentum wheel (43), and the resulting torque is subtracted physically from the orbital rate coupling torque. Reference has been directed by the Comptroller to Specification 1,210,582.