Passive Damping Characterization of Parabolic Composite Reflectors with Hybrid PZT-Coated Layers
Authors: Munjal B.S. , Trivedi H.V. , Sarma P.V.B.A.S. 
Published: April 2008

Passive vibration damping of smart structural systems can be obtained using thin hybrid layers of visco-elastic Magnetostrictive powders as per the literature survey. Although these materials can be used for damping of thin flexible structural systems, they have some practical limitations in the domain of microwave antenna applications. This is due to, Electro-Magnetic Interference (EMI) and Electro-Magnetic Coupling (EMC) issues at higher transmit and receive radio frequencies. In this article, an attempt has been made to investigate the use of thin layers of highly sensitive ferroelectrically soft piezo electric ceramic material powders in lieu of Magnetostrictive powders for passive vibration damping effects. This investigation is on Carbon Fiber Reinforced Plastic (CFRP) and Glass Fiber Reinforced Plastic (GFRP) composite materials at ambient temperature for microwave antenna applications. Piezo ceramic hybrid polymeric coatings on parabolic reflectors have been envisaged to prevent EMI interference for antenna applications. At the same time these coatings are investigated experimentally and numerically in the passive vibration domain for energy dissipation due to their viscoelastic behavior and their electro-elastic coupling phenomena. Strain dependent material damping in the thin piezoceramic passive layers is a nonlinear behavior. To facilitate a simple proof of concept exercise at present, the nonlinear effects and hysteresis have been ignored in the Finite Element Analysis (FEA). FE model with piezo layer has been generated in ATILA, a software for modeling smart structures and structural systems for piezo coated reflector for undamped free vibration analysis. However, the material hysteresis losses and nonlinearities of the smart materials have not been considered in the model to simplify the proof of concept exercise. Moreover, damping increase due to epoxy and interactions between epoxy and PZT powder have not been investigated presently in this article. A 0.3 m diameter CFRP parabolic reflector along with a 0.7 m diameter. GFRP reflector has been developed using thin hybrid piezoceramic coating on the convex side of the reflector. As a corollary to the investigation, from the practical utility point of view, for piezo-coated structural elements in microwave antenna domain; efforts have also been put in the direction of studying the EMI/EMC by experimental investigations. The piezo-coated antenna feed support (spars) made up of Plasma activated Teflon (which is dielectric and Radio Frequency (RF) transparent) along with piezo-coated reflectors have been electrically tested with an intention of investigating the improvement of antenna efficiency due to reduction of RF blockage. Usually, conventional composite/metal spars are used and they cause RF blockage and reduce efficiency for some typical Radio Frequencies. The thin hybrid layers of these ceramic materials when coated on composite substrates have yielded benefits in passive vibration damping domain. Experimental damping investigations have shown an effective increase in modal damping at resonant frequencies with an added advantage of marginal change in stiffness for the coated composite specimens with a negligible weight penalty at ambient temperature. The 0.7 m diameter parabolic reflector made up of GFRP with thin hybrid piezoceramic coating has been tested at Compact Antenna Test Facility (CATF) at SAC/ISRO Ahmedabad, India and practically no EMI/EMC interface issues have been reported at 11.6 GHz transmit (Ku band) frequencies for Prime Fed (PF) type reflectors. The encouraging results have been observed w.r.t. the increased passive damping by putting thin hybrid piezo layer on one side of composite reflector substrates at resonant frequencies. Additional advantage observed is that it is without EMI interference and results in improved efficiency of microwave reflectors due to reduced blockage by using piezo powder-coated almost RF transparent Teflon spars. This may make the present investigation a possible candidate for RF applications in future for microwave antennas. It may also help in micro-vibrations control of futuristic spacecraft reflectors in orbit for today's sophisticated Earth observation satellites with improved pointing accuracies and resolutions.