SHAPE CONTROL OF FLEXIBLE SPACE REFLECTORS

Abstract

In space agencies nowadays, light weight structures are popular because of its cost of launching from ground station to space. Inflated structures mean literally "can be inflated by internal gas pressure". The inflatable adaptive membrane is flexible, high storage capacity and ultra light material. It posses good durability, strength because of an internal gas pressure. Keviar, Mylar & Kapton are different types of polyamides and polyesters are used as inflated structure for space application. Kapton and Kevlar have good strength and thermal properties. Application of gossamer structures has greatly attracted the interest of the scientist as a scientific platform in outer space exploration mission these advantages make it more special over other structural conventional form, especially when extreme limitations are placed upon the weight and volume for the application in space aeronautics and architectures. This research on inflatable structures for space application is given to more attention to the researches. The inflatable space structure consists of an assembly of torus, strut and reflector. These are the main inflatable parts of the antenna. Strut and torus are the supporting part of the reflector. In space, atmosphere and challenges for surviving is different than earth. Inilatable - antenna faces thermal expansion, bombarding particles stone randomly and vibration challenges while launching by launch vehicle's high frequency. One possible way to minimize the vibration of gossamer structure is the use of smart materials. Smart material posses the intelligent properties, e.g. PVI)F piezo patches, shape memory alloy etc. Due to harsh environmental condition, space based antenna faces the shape distortion, which influence the desired shape and surface accuracy where the reflector is the main part of the antenna, for reflecting the signal from one ground station to the desired ground station. Its surface accuracy should be adequate and sharp. Due to the motivation of this fact, vibration analysis of highly flexible reflector is carried out with and without piezo patches to control the shape. The focus of the research study is to develop an understanding of inflatable structures and control its shape. In this research work, numerical work on shape control of Kapton membrane is carried out in Matlab. Shape control is minimization of Root mean square error of surface to maintain it in flat mode. Matlab coding is done based on finite element formulation to analyze its dynamic effect and shape control action with the help of piezo coupled sensor actuator materials. In this dissertation, experimental work on Kapton membrane is carried out for the measurements of out plane displacement of the Kapton membrane with the help of a vibrator and Lab view software connected to a computer through a DAQ Data Acquisition card. Data acquisition card which acquires the response of membrane with the help of PZT patches. Piezo sensing and actuation system is used in the experiment for sensing the strain generated in the PZT patch and for amplifying the signal so that it can be used for further processing. This system has four channel sensing and 4 channel actuation capability. The output of the PZT sensor is connected to this system and output of this sensing device is connected to the NI card and also connected to the actuation system. Piezo actuation system is a voltage amplifier which amplifies the voltage sensed by the sensing system so that it can actuate the PZT for vibration suppression. This system has the capability that operates in both in and out of phase of the signal. First the signal is inverted and amplified by the system, then it is supplied the PZT actuator. If the sensed signal is in phase, then output signal can be made out of phase for the suppression of the out plane displacement of the structure. The output gain may be adjusted by the knob on the actuation and sensing system. The shape control can be made more effective by increasing the gain value of the piezo actuation system. Three positional arrangements are made to control the shape. Positions of patches make more effect on the amplitude of the membrane. It reduces the RMS errors which create the surface of the membrane more flat to transmit the signal accurately from one ground station to another station.