ASPECTS OF CONTROL OF LARGE FLEXIBLE SPACECRAFT.

摘要

In the last few years a large number of potential future spacecraft projects have been identified which require spacecraft of unprecedented size, and hence unprecedented flexibility. The attitude control problems for such spacecraft are best characterized as simultaneous pointing control and shape control of the vehicle. The shape control problem is a distributed parameter control problem requiring significant advances in the state of the art. It will be necessary to distribute control actuators throughout the flexible vehicles in order to control various modes or oscillations. How should the actuator locations be chosen in order to best control the flexible vehicle?; In this work a meaningful concept of the degree of controllability of a system is developed which takes into consideration all the pertinent factors such as controllability, total time of control, control effort, stability, and control objective which should have a bearing on the degree of controllability. An algorithm to generate an approximation to this degree of controllability is developed which satisfies the condition that the approximate measure goes to zero if and only if the system becomes uncontrollable. Using this concept it is then possible to compare a set of potential control system designs for a given space structure, each using a different distribution of actuator locations, and to decide which design is best.; A typical spacecraft model consisting of a rigid core with two symmetrically disposed flexible appendages is analyzed. The properties of the degree of controllability as a function of actuator location are examined for a single force actuator and a torque actuator, and it is shown that a significant knowledge can be obtained from the single actuator analysis so that the relative quality of the locations can be judged without actually computing the degree of controllability. This enhances the usefulness of this approach in two ways: (1) it allows us to have a lot more freedom with the control time T and the weighting factors one might wish to choose for the variables to be controlled, and (2) it gives a rational approach to distribute multiple actuators in the system. Also, some relatively simple controllability tests are derived as part of this analysis which are applicable to all linear time invariant systems.; The future spacecraft also include a class of spacecraft each of which is designed to perform several missions during its lifetime. Such spacecraft are essentially characterized by the interchangeability of certain of their components with other physically compatible components which may be designed to accomplish different tasks. A Shuttle-based Instrument Pointing System (IPS) is such a spacecraft which essentially consists of an Instrument Pointing Mount (IPM) to which one of a family of scientific instruments can be attached. These instruments are typically lightweight and very flexible. The flexibility limitations of such instruments which are stably controllable with a given control system design are analyzed in Part II.; The predesigned controllers of proportional-derivative-integral type are located one each in the Shuttle and the IPM. It is shown that when the IPM is locked to the Shuttle and only a single controller is operational the flexibility characteristics of the instruments can be elegantly portrayed using two parameters, thus allowing ample freedom in the design of these instruments. This freedom is considerably reduced when the IPM is allowed rotation relative to the Shuttle and both controllers are operational.