|Title of Invention||
MICROWAVE ANTENNA REFLECTORS
|Abstract||This invention relates to microwave antenna reflectors made of flexible composite membranes such as carbon fibre reinforced plastic and glass fibre reinforced plastic composite body having a parabolic structure, the convex side thereof being coated with at least one layer of piezoelectric materials. The microwave antenna of this invention provides passive vibration damping effect.|
|Full Text||This invention relates to microwave antenna reflectors coated with hybrid piezoelectric material. Thin hybrid layers of high sensitivity ferroelectrically soft piezoelectric ceramic are capable of providing passive vibration damping effect on carbon fibre reinforced plastic and glass fibre reinforced plastic composite materials. These composites are hereinafter represented by CFRP and GFRP respectively. Parabolic antenna reflectors made of CFRP and GFRP coated with a layer of hybrid piezoceramic on the convex side of the reflector skin show an increase in overall efficiency and gain of antenna. Antenna reflectors thus coated also exhibit passive vibration damping benefits at ambient temperature with negligible weight penalty.
Prior art in vogue as on today is the use of baseline (without piezo coating) CFRP & Glass fibre composite materials with embedded Kevlar flexcore or Alumium honeycomb for the development of space segment & ground segment reflectors respectively.
Presently, these CFRP & GFRP composite materials are being used for developing reflectors without any thin hybrid piezoceramic material coatings made up of ferro-electrically soft or ferro-electrically hard materials on the convex side of the
Materials like CFRP have high specific stiffness, high specific strength & low CTE (Coefficient of Thermal Expansion). According to this invention, in order to make them suitable for handling micro, vibrations in space, their hydrophobic surface has been activated on one side through RF Plasma etching technique and then thin hybrid coating of ferroelectrically soft and hard piezo materials like SP4 & SP5A have been done on the activated convex side of the reflector skin. This approach helps generating futuristic ultra high precision spacecraft reflectors capable to handle micro vibrations in space domain along with on-orbit thermal loads.
Handling of micro vibrations is one of the key potential issues for thin ultra light weight high frequency spacecraft reflectors subjected to milli ‘g’ vibrations due to movement of three axis spacecraft gyros, altitude correction exercise of spacecrafts and differential thermal gradients which can effect their performance.
0.7 m dia. parabolic reflector made up of GFRP with thin hybrid piezoceramic coating on the convex side has been developed and electrically tested along with Piezo coated RF plasma etched RF transparent stiffened Teflon spars at SAC/ISRO Ahmedabad and practically no EMI/EMC interface issues have been reported at 11.6 GHz transmit/Receive frequencies.
Moreover, the thin hybrid layers of these piezoceramic materials when coated on composite parabolic reflectors, have given encouraging results in terms of increase in overall efficiency and Gain of antenna and has also given passive vibration damping benefits at ambient temperature with negligible weight penalty.
The approach suggested is hither to fore unexplored in literature. We subjected the substrate to plasma corona surface etching treatment (a few Angstroms deep) for better adhesion of the thin piezo coat with the composite substrate. Piezo coated CFRP & Glass fibre composites is capable of producing vibration control of reflectors.
Moreover, significant damping has been observed in the tests on piezo coated reflectors, vis-a-vis, baseline reflectors, at ambient temperatures.
Passive damping of structural systems using hybrid layer(s) of soft viscoelastic or hard ceramics having high damping characteristics has attracted attention of lot of researchers. A special class of materials elongate when placed in a suitable excitation field. Applications of smart materials in literature highlight the use of thin hybrid layers of visco-elastic magnetostrictive powders for obtaining damping of smart structural systems.
More recently, it has been observed in the literature that smart materials like piezoceramics, magnetostrictive materials & shape memory alloys have been investigated for active vibration suppression. Smart magnetostrictive materials like Terfinol-D rods have displayed good potential for active vibration damping purpose. Active vibration damping, per se, may produce system instability for thin flexible reflectors. In most of these applications, mini actuators built using Terfinol-D rods are used as point actuators on the host structure for vibration control. With the advent of particulate piezoceramic/magnetostrictive composites, it is shown that such materials could also be used as an intelligent distributed layer over the host substrate to introduce distributed control of vibration. However, in general, these materials do not possess good inherent structural damping. Hence for the space reflectors, active control using such materials may lead to instability, if there is an error in the feed back control. It is expected that a thin layer of passive damping coating could enhance the stability of the system in such circumstances without any significant weight penalty.
It has now been observed that some ceramic and polymeric coatings yield significant structural damping and there is a renewed interest in using them for passive vibration control. These materials dissipate vibrational energy mainly through magneto/electro-elastic coupling. During vibration, the coating of such materials would undergo a change of strain, initiating as movement of magnetic domains
thereby dissipating the mechanical energy through hysteresis. This damping capacity is dependent on the strain induced in the material. It is observed that the ferromagnetic material has high damping at low strain levels (around 50-100 (x strain). However, for structural vibration control, induced strains are often to the tune of at least one order higher in magnitude. Hence, if only ferromagnetic material is used, the extent of damping achieved may be negligible. Moreover, although these ferromagnetic materials can be used for damping of thin flexible structural systems, such materials cannot be used for microwave antenna applications due to EMI/EMC interference issues at higher transmit and receive frequencies. Keeping microwave reflector applications in mind, an attempt has been made to study the use of thin high sensitivity ferroelectrically soft piezoelectric layers & hybrid piezo ceramic powder coatings (which are independent of Electromagnetic issue) in lieu of magnetostrictive powder coatings for vibration damping effects on a wide gamut of composite materials at varying temperatures. A combined hybrid approach of active-passive piezo layers system on composites, however, can enhance the vibration damping.
Following approach synthesis has been adopted for the investigation of passive damping on small size reflectors:
Approach synthesis for this part of the study was to carry out experimental and theoretical work on a hither to fore not fully explored field of achieving passive vibration damping at ambient temperature due to a thin hybrid piezoelectric coating on one side of the high specific stiffness composites like CFRP & GFRP laminates. It was decided to first study the vibration damping behaviour on cantilever beams and then apply it to doubly curved parabolic reflector shells with coating on the convex side of the antenna shell.
It is envisaged that futuristic flexible spacecraft reflectors would need to be designed to damp out low frequency vibrations arising due to the rapid temperature change of surrounding space environment or due to the attitude control of spacecraft structures or due to three axis movement of spacecraft gyros. These low frequency perpetual vibrations would need to be damped out at reflector level itself as these may cause single point failures in spacecrafts due to sometimes dropping of electrical connectors. These issues become vulnerable due to perpetual disturbances continuing in micro gravity conditions due to post launch loads i.e. milli g vibrations along with thermal loads. The inventors explored this aspect by investigating the damping behaviour of piezoceramic materials on CFRP & GFRP composites. Passive damping approach is found to be the most practical and feasible way to damp out the micro vibrations of present & futuristic spacecraft reflectors made up of thin flexible composite membranes.
In this investigation , an attempt has been made to simplify & study the behavior of piezocearamic material coating by assuming a linear behavior and the hysteresis effect and non-linearities have not been considered in the Finite element analysis models.
The electromechanical constitutive equations (1) - (8) for linear material behavior are as follows :
The following equations provide constitutive equations for structural and electrical fields, respectively : Structural Elastic matrix in displacement domain:
[C] it is the elasticity matrix & is the usual |D) matrix Structural stiffness:
Piezoceramic material coating on the composite reflectors at ambient temperature has been found very effective and solves problems faced with at present. Properties of the piezo electric actuators are as follows in Table-1
Types of test specimens :
For this investigation following two types of test specimens are chosen :
• Carbon fiber reflector
• Glass fiber reflector
The specimens tested at ambient temperature were developed in-house by taking the Carbon M55J /Ml8 fabric (0/90 orientation ), Aluminum honeycomb & Kevlar honeycomb. Boat grade epoxy resin was used and specimens were cured at room temperature only for ambient temperature testing case.
An example of making microwave antenna reflectors according to this invention is described herein after.
With proper parabolic coordinates, wooden moulds were machined and fabricated for both test specimens as per the dimensions finalized by FEA.
A GFRP reflector of 0.7m dia. (F/D=0.4) & a CFRP reflector of 0.3 m dia. (F/D=0.4, where F = Focal length of parabolic reflector & D = dia. of the reflector) was fabricated respectively for a parametric study w.r.t the vibration damping using piezoelectric powder coating applications.
Vibration damping investigations have been carried out on both the specimens i.e. 0.7m dia. GFRP reflector with spars and 0.3 M diameter CFRP reflector without spars as the proof of concept exercise at ambient temperature only.
The CFRP reflector is having front & rear skin thickness of 0.3mm and is having Aluminum as honey comb for 5 mm thickness.
Epoxy mixed with piezoceramic powder of two types was sprayed in the form a hybrid layer of 300 microns thickness on the convex side of the CFRP reflector and was cured at room temperature for 48 hours. The two types coatings are made up of high power ferroelectrically-hard SP-4 & High sensitivity Ferroelectrically-soft SP-5A smart materials. Both the layers are of total thickness of 150 microns each.
Investigation of Damping Characteristics of a CFRP reflector:
A 4T shaker was used for testing both the reflectors by inputting 0.5 g forcing function as a resonance search (Sine loading) exercise.
Table 2 shows the results compiled for the experimental and theoretical analysis of the 0.3 m dia, CFRP reflector.
This improvement in damping was also studied & ratified on reflector of 0.7 m dia GFRP antenna with GFRP spars by keeping in view that this reflector will be used for electromechanical testing also at
The results for the FE and vibration investigations are as follows in Table 3.
The ATILA model predicted the frequency of piezo coated GFRP reflector (300 micron thickness
case) as 47.65 Hz without spars.
TheNISA model predicted the frequency of bare GFRP reflector as 53.1 Hz without spars.
Electrical Testing of the GFRP Reflector:
In the present investigation, the possibilities for improving the gain / efficiency of the antenna reflectors in the light of usage of smart materials have also been explored. Also as a part of the investigation, efforts have been put in the hither to fore unexplored field of identifying by practical & theoretical investigations, some unique materials for feed support spars for C / Ku band PFF type reflectors, which are dielectric, RF (Radio Frequency) transparent and exhibit the requisite stiffness. Electro-mechanical testing of prime feed fed type of parabolic reflectors made up of feed support spars of various piezo coated materials have been tried to investigate the reduction of the RF blockage and improvement in antenna gain and efficiency.
The main reason of this RF blockage is the shadow effect caused due to conventional
Carbon fiber spars which are structurally stiff but reduce the efficiency of the antenna due to the RF blockage caused by them up to Ku band RF frequencies.
An attempt has been made to practically propose a hither to fore unexplored concept of piezo coated spars made up of unconventional dielectric plasma etched Teflon tubes for Prime Feed Fed (PFF) type parabolic reflectors for two different transmit frequencies for checking the improvement of antennas efficiency and gain.
Teflon material is chosen from the list of other potential aerospace materials like Torlon, Noryl, Zylon etc. In this case, a proper chemical etching treatment has been used for obtaining hybrid piezo coating on the Teflon spars. Without any surface activation on Teflon the piezo powder peels off form the Teflon tubes as per experimental experience .
It was conceived that a thin hybrid layer of piezoceramic materials when coated on the tubular Teflon spars after surface preparation by specific chemical etching process / RF etching process may make the Teflon spars a right candidate to swap conventional CFRP spars for C / Ku Band RF signals. These spars were tested electrically in the SAC / ISRO Ahmedabad, on the 0.7 m diameter Ku-band GFRP reflector. For handling Ku-band frequency signals the copper coated Kapton sheet was impregnated on the concave side of the reflector. This copper coated polymide film had 50 microns thickness of the substrate and had 30 microns thickness of the copper sufficient for 99,9 % reflectivity for the Ku-band microwave signals. Electrical testing was carried out at CATF at SAC/ISRO for two different feeds. Due to global stiffness derived from the shell, during vibration testing of the GFRP reflector , the feed mounted on the piezo coated Teflon spars reported drop in the amplifications due to 0.5 g sine input given at the base of the reflector. In electrical testing at CATF, this antenna with piezo coated Teflon spars reported increase in efficiency to the tune of 2 % due to reduction in RF blockage for both the type of frequency bands (C & Ku band) as these spars remained more than 70 % RF transparent for the tested frequencies.
It was observed that the piezo ceramic material coated reflectors did not give any EMI / EMC issues . Moreover, the antenna efficiency improves by nearly 2 % with chemically treated piezo coated RF transparent Teflon spars for both the test frequencies of C- band & Ku-band. The Prime Feed Fed type antenna gain improves by about +0.2 dBi for both the test frequencies. This test was repeated for the following case to ratify the behaviour of the reflector.
1. Microwave antenna reflectors made of flexible composite membranes such as carbon fibre reinforced plastic and glass fibre reinforced plastic composite body having a parabolic structure, the convex side thereof being coated with at least one layer of piezoelectric materials.
2. The reflectors as claimed in claim 1, wherein said piezoelectric materials are ferroelectrically soft powders and hybrid piezoceramic powders.
3. The reflectors as claimed in claims 1 and 2, wherein carbon fibre reinforced plastic antenna body has 0.3 m dia and the glass fibre reinforced plastic body has 0.7 m dia.
|Indian Patent Application Number||1809/CHE/2006|
|PG Journal Number||22/2011|
|Date of Filing||28-Sep-2006|
|Name of Patentee||INDIAN SPACE RESEARCH ORGANISATION|
|Applicant Address||INDIAN SPACE RESEARCH ORGANISATION (ISRO) HEADQUARTERS AN INDIAN GOVERNMENT ORGANIZATION ANTARIKSH BHAVAN NEW B.E.L ROAD BANGALORE.|
|PCT International Classification Number||H01Q19/19|
|PCT International Application Number||N/A|
|PCT International Filing date|