ATTITUDE CONTROL LAWS FOR GAS-JET SPACECRAFT

A new digital control algorithm is presented for attitude stabilization during maneuvering (thrust-vector control) of reaction-jet spacecraft, whose rotational dynamics may be closely approximated by two kinematic integrators about each control axis (negligible dynamic inter-axis coupling). The algorithm is called the Control Law for Orientation Using Thrusters (CLOUT). CLOUT is a good candidate for control of spacecraft continuously maneuvering under low thrust, although it is also appropriate for maneuvering units with shorter acceleration times. For the current design, the set of fixed jets is a box-shaped 24-thruster arrangement (an orthogonal jet triad at each corner of a cube), where four thrusters on one rectangular side provide translational acceleration. During a translational acceleration, the CLOUT algorithm modulates these four thrusters to maintain attitude. CLOUT uses noisy angle measurements and a third-order observer (in each axis) to estimate angle, angular rate, and the two components of center-of-gravity offset from the geometric center of the four translational jets. The control law uses these estimates to implement parabolic phase plane switching boundaries which adapt to the c.g. offset estimates, and achieve very tight thrust-vector control. Thus, this control scheme is appropriate for any mission with precision pointing requirements during low-thrust maneuvering. In view of the assumption of negligible gyroscopic coupling, the three axes could be treated separately for attitude hold, however, during a translational acceleration the rotational controls are coupled through the jet selection logic. The jet selection logic for a given spacecraft is known, and therefore, it is possible to exchange information between the two controlling axes, so that two-axis thrust-vector control is achieved. The logic is designed to properly drive the third-order observers and generate precision control requests within the coupled-control framework. The CLOUT scheme evolved from study of the implementation of ideal-limit-cycle control in the presence of a known environmental disturbing torque. During coasting flight (no translational acceleration), or for control about the unaffected axis during a translational acceleration, CLOUT is designed to revert to a single-axis ideal-limit-cycle control law. Thus, CLOUT provides both coasting flight attitude hold (in the presence of small environmental disturbing torques), and thrust-vector control (even when the c.g. is offset) to high accuracy, and is fuel conservative. Six degree-of-freedom simulation results verify the performance of the CLOUT system.