Dynamics and control of a large space antenna

Abstract

Large space antennae will play an important role in space utilization in the coming decades. In this paper, a control design study of a large space-deployable antenna is presented. For comparative analysis, three controller-estimators were designed and optimized based on requirements of focused communications missions, disturbances, and hardware configurations. One of the major issues investigated is the effect of model error sensitivity. Stability margins, surface accuracy, and pointing errors have been obtained over a wide range of model parameter variations. The results show that structural uncertainties and model errors can cause serious control deterioration. Performance can be improved by strengthening certain structural elements. Flight experiments and in-flight system identification are imperative measures for reducing mission risk and enhancing performance. ARGE space antennae and other large space structures will play an important role in the coming decades as :ommercial applications of space become feasible, especially n the area of communication s. Structures of 10-120 m and arger have been considered by NASA and other government igencies for future missions.1 As the structural size and mass distribution change drastically from those of conventional spacecraft, many difficult control problems arise. The basic Droblem, however, comes in modeling highly flexible structures. Structures of this type are known to have a large lumber of packed modes at very low frequencies. Mode shapes and frequencies cannot be accurately predicted or measured for even a small number of modes at preflight time. This means model uncertainties. Model uncertainties in the :otnrol loop can cause serious consequences, including the Dossibility of making the system unstable. Model order is mother problem having great impact on control design. Using :oday's in-flight computer capability, one can expect to have Dnly a modest-order controller—which often means model ;runcation and which will further performance deterioration. Dynamics and control problems for specific configurations nust be characterized and evaluated in terms of incomplete knowledge of the system dynamics so that the required performance can be insured and the risk reduced. In this paper, the structural dynamics and the control properties for a 64-m-diam mesh deployable antenna are investigated. Some interesting results are presented on the effects of model parameter uncertainties to the system stability, surface accuracy, and pointing accuracy. Critical :ontrol problems are identified and potential solutions recommended. In Sec. II the antenna configuration and the structural dynamic properties are briefly described. The :ontrol design, disturbance assessment, hardware sizing, and instruction of weighting matrices are summarized in Sec. [II. Numerical results are presented in Sec. IV and control design issues relevant to rf (radio frequency) beam pointing in Sec. V. Conclusions are drawn in Sec. VI.