From the viewpoint of the utilization of space environment technology, the much more enlargement of space structures, such as a space antenna and a solar cell of satellite, is expected to improve the performance of these structures. Up to now, various large-scale space structures are being planned, since, unlike the ground environment, the space environment is in the state of micro gravity and high vacuum without many spatial restrictions. However, as for the construction of space structures using the materials transported from the earth, very severe weight saving is required due to the weight limit of rocket launch. To achieve the limited weight, lightweight materials, such as aluminum or composites, are widely employed, which makes the space structures possess the characteristics of high flexibility and low stiffness. As a consequence, in these flexible space structures, such as solar power satellites or large-scale space antennas, static deformations and vibrations occur easily and frequently under the external forces, which becomes a serious problem.
For a space antenna structures, for instance, the required surface accuracy depends on its frequency band of operation. Because the frequency used currently tends to be higher and higher, the demand on the surface accuracy becomes severer. The structural thermal deformation induced by temperature change ranging from -150ºC to +200ºC on orbit and the in-process member length errors are cited as the main causes of deterioration of surface accuracy. In general, the precise measurement of antenna configuration is required for high precision shape control of antenna structure. However, since it is difficult to place sensors and actuators to all structural components, the techniques of highly precise measurement by a small number of sensors become more and more important. Furthermore, as stated previously, low frequency vibrations tend to occur easily in these space structures due to the rapid temperature change of surrounding environment or the attitude control of structures. Since those vibrations do not decrease in the environment of micro gravity and high vacuum, the performance of a satellite main part or an electric device may drop remarkably. Therefore, it is crucial to suppress those structural vibrations by some active or passive control techniques on real time.
Up to now, many previous researches on the static shape control and the vibration control for application in space structures have been proposed. First, for the static shape control, highly precise shape control becomes possible under the condition of a large number of sensors and actuators. However, accurate shape estimation and shape control using a small number of sensors and actuators are still difficult for a deformed structure with arbitrary disturbance acted. For the realization of next generation space structures, such as a space antenna with higher performance, the development of highly precise shape estimation/control techniques only employing a limited number of sensors and actuators is indispensable. Second, for vibration control, many researches using the conception of modal sensor and modal actuator to control the dominant low-order modes have been carried out based on the modal analysis of structural vibration. These kinds of traditional approaches can only be applied to some simple structures, like a beam structure. Also, they usually need to attach a sensor, such as a PVDF film, on the whole surface of a beam and the computational cost is comparatively high. Therefore, the techniques for realizing highly precise vibration measurement and control using limited number of sensors and actuators have not been developed yet for large-scale and complicated structures.
In this research, based on the optimal placement of sensors and actuators, the static shape control of a truss antenna by partial measurement is performed. Moreover, the vibration control by using a system of modal sensor and modal actuator with a small number of sensors and actuators is realized for a plate structure.
In the static shape control, for a truss antenna structure, the techniques for shape estimation and the shape control are developed, which use a small number of sensors and actuators placed on the optimally designed locations. Two kinds of sensors, i.e., the displacement sensor and strain sensor, and the strain actuator are employed. Moreover, to achieve the highly precise shape estimation using a limited number of strain sensors, a shape presumption technique based on a polynomial approximation of thermal distribution acting on an antenna structure is proposed.
In the vibration control, the modal sensor consisting of strain sensors or accelerometers as well as the modal actuator of PZT is built up for a plate structure. The structural vibration control is realized by the modal control for each mode independently. Sensors/actuators are optimally placed based on a criterion for the minimization of observation/control spillover so that the best accuracy of measurement of modal displacement and the maximum control effect can be acquired.
edited by Nariyuki KAWABATA