paper review gtu rpf may 2011

Research Project Faire-2011, 13th May, GTU-Ahmedabad, Paper ID: 1238.

1

Abstract-Development of thermal compensating

mechanism based on material differential co-efficient of thermal expansion (CTE) for cavity filter is a part of spacecraft component. Most of time spacecraft component operate in thermally varying environment. When satellite components made of dissimilar materials and made to operate in thermally varying environment shows tendency to change in dimension of the component as well as develop stresses at joint and cause change in functional performance of sub-system. Conventionally cavity filter constructed from Invar having higher density cause increase the overall mass of the system. Aim of this review work is to replace invar cavity filter with aluminum and to develop temperature compensation mechanism. Thermally compensated aluminum high power cavity lter provide major optimization in cost and mass. Development of compensating technique is made possible with the use of various experimental methods. The development activities are multidisciplinary in nature from electrical, mechanical, designs, fabrication, assembly, test etc. which make the system complicated everything serially adding up to the limitations in the process of realizing the multiplexer. The work concept of development of compensation mechanism by using thermally stable material, temperature control design and net thermal stability with different shape and size of components are explained in detail. So here an attempt has been made to develop adaptable compensation mechanism using different materials and configuration which maintain the overall dimension of the system and minimize the weight also.

I INTRODUCTION There has been a tremendous growth in the use of satellite based applications for the betterment of human life on earth. Services like Meteorology, Disaster warning, Search and Rescue, Telemedicine, Urban Planning, Remote Sensing, Communications like broadcasting, telephony, internet, Navigation like GPS, Country defense, Digital video on [1] Bipin D. Patel, PG Student, Sankalchand Patel College of Engineering, Visnagar, Email: [email protected], phone: 091-09909468081. [2] A.R.Srinivas, Senior Scienst SE, Space Application Center, ISRO- Ahmedabad, Email: [email protected], phone: 079-26915184/89. [3] Prof. D.A.Patel, Associate Professor, Mechanical Engineering Department SPCE-Visnagar, Email: [email protected].

demand, etc. have been increasing in demand year after year. These and many more emerging scientific & planetary research requirements laid the basis and the need for more and more numbers, high quality and power channels of satellite transponders in space. To cater to these needs, one of the most challenging tasks is the design and development of high power multiplexers operating at Ku-band and beyond frequencies. Modern day multiplexers need to combine 12 to 24 channels to meet minimum needs of above mentioned applications. These channels are closely spaced to minimize the overall transmission bandwidth which is a premium entity in international Radio Frequency communication spectrum. A detail discussion of the multiplexer had from reference [1].

Fig. 1 MUX Assembly

Satellite is the communication medium from earth to Geo Synchronous orbit (GSO) which carries many transponders for varying purpose based on demand and need. Multiplexer is one of the component of the transponders is shown in Fig.1 has six channels connected by single manifold. All channels are consisting of different cavity filters made up of invar material. The multiplexer segregates radio frequencies (RF) of microwave energy to different channels according to the band width allocation. The Multiplexer contains of circular cavity filter, irises, input adapters, output adapters, manifold, rigid bracket, flexible bracket and base plates. All these components are assembled to meet a defined functional performance. MUX consists of mainly two types of brackets depending upon the functionality supporting brackets and flexible brackets. These channels are made of invar material. Invar having higher CTE () so that MUX assembly is too much bulky and overall mass of satellite will increase. Aim of the present research work is to replace the invar cavities with aluminum alloys results optimization of the overall mass and cost of system.

Critical Review of Thermal Compensation Techniques for Space Craft Component

Bipin D. Patel [1], A.R.Srinivas [2], Prof D.A. Patel [3].


2

II THE PROBLEM The market to cover applications ranging from Telecommunications to space science and earth observation has seen a steady increase. This has in turn increased the complexities in design, development and production of electronic components for space applications. Design and development of such space craft components operating for high power application involve combinations of different materials to satisfy the functionality. Such combinations of different materials are likely to undergo thermal excursions during the operational life or at the time of the hardware realization. Operating temperatures of satellite and it system does not remain constant when it is in space. When satellite is near the moon its temperature reduces, while temperature increases when it is near the sun. Due to working of various system of satellite, some heat will be continuously dissipated to the satellite environment. Closely spaced channels or minimized guard band channels because frequency drifts into adjacent channels which are an undesirable feature. This is due to expansion and contraction in the materials of the multiplexer filters which are excited either by resistive heating of the devices because of high power carrier signals or temperature excursions in the satellite operating environmental changes over a day. To keep these problems at bay many conventional methods use materials like Invar an alloy of Iron, Cobalt and Nickel having almost an invariable coefficient of expansion(CTE) of the order of 1 to 1.5 parts per million. While CTE of invar controls the dimensional stability of the filters, its high density, poor machinability, low thermal conductivity and dependence of its CTE on temperature makes Invar based multiplexers not only very heavy and cumbersome but consume long life cycle development time, reach very high temperature ultimately rendering them, incapable, of handling high carrier signal powers and thereby forming a highly cost ineffective methodology of producing multiplexers. Combinations of different materials with different linear coefficient of thermal expansion , when subjected to predominant thermal excursions tend to develop complex thermal stress fields. This in combination with the elastic constraints may be adhesive bonding or bolting connection will call for a structural stress field. Thus the problem becomes now very complex case of the thermo-structural stress fields, which gets more complicated when imposed by the constraints like heat transfer, availability of the space, method of production of such components, production difficulties for attaining desired flatness and finally assembly procedure. Under the influence of such thermo structural stress fields the component will tend to deform and deviate from its tolerable design limits, thereby affecting the design functionality and performance of system. Therefore the new generation multiplexers are looking at light weight, cost effective and high conductive materials for development of filters.

III APPROACH FOR MICROWAVE DEVICE

Research on the topic of temperature compensation for microwave devices can be split into three broad categories

A. Thermal stability and material science The simplest way to ensure low temperature drift in a microwave filter is to use materials with high thermal stability. Dielectrics must be both dimensionally stable, and have a stable dielectric constant. Early designs requiring high thermal stability used Invar conductor. Invar is an iron-nickel alloy that exhibits exceptionally high dimensional stability. Steven Barton Lundquist proposed Temperature compensated microwave filter as shown in Fig.2. The microwave resonator cavity made from Invar is separated by an iris into two wave guide cavities and serves as an end wall on one side for each wave guide cavity. The another end wall of each wave guide cavity is in the form of an end cap and these end caps provides a slot for an input and output for filtering the frequency as shown in figure. The cavity, iris and end caps are bolted together. The end caps and iris are made from a material has a more positive CTE then the side wall of the microwave resonator cavity. The end caps have a U shapes cross section with the base of the U projected towards the cavity. When cavity tends to expand due to increase in temperature, the end caps deflecting toward the resonator cavity and substantially compensate the volume of microwave resonator cavity.

Fig.2 Temperature Compensated Filter with single Iris

By, Using the above similar concept the author extended the above work by three, four and five cavities separated by three, four and five irises having same arrangement of the U shaped cross section irises and end caps referenced in above Fig. 2 except the slot for input and output for RF signal[2]. The coefficient of thermal expansion (CTE) of Invar is approximately 1.5 ppm/C (depending on the alloy) which provide thermal stability to the system. But when compared to other conductors such as aluminum and copper, Invar has a number of drawbacks. A major drawback in many applications is its mass; the density of Invar is 8050 kg/m3. By comparison, the density of aluminum is 2700 kg/m3, nearly 1/3 the weight by volume. This is a major penalty in space applications where mass reduction is critical. The poor machinability of Invar is another drawback. During machining, Invar can exhibit work hardening that can lead to warping. It also has a tendency to break up during machining. Resistive heating is a major source of heat in the high power


3

applications where waveguide filters are commonly used. Heat generated inside the cavity walls must be dissipated to the environment efficiently in order to minimize temperature drift. Since the thermal conductivity of Invar is 10 W/mK, it conducts heat less efficiently than other metals. The thermal conductivity of aluminum and copper are 200 W/mK and 400 W/mK respectively. Invar will act as an insulator when compared to these materials, trapping heat inside a waveguide. In the 1970s and 80s, composite materials began to replace Invar in some designs [3]. These composites consisted of a graphite structure, with a high conductivity layer of silver to serve as a conductor. These plastic fiber composites are lightweight, and exhibit dimensional stability that meets or exceeds that of Invar. Although graphite waveguides are lightweight and have high thermal stability, manufacturing issues hinder their implementation. Graphite waveguides require elaborate quality control, and are more difficult to manufacture than modern Invar waveguides. For these reasons, graphite filters are rarely used today.

B. Temperature Control in RF Design Attempting to control a components temperature by removing the root cause of temperature drift. For such temperature control RF design is proposed by, Rosowsky and Wolk for 450Watts Output multiplexer for direct broadcasting satellites multiplexer that dissipates heat by means of heat pipes. This multiplexer use Invar material as waveguide material to construct the filter portion. Heat pipes containing NH3 are attached to the Invar cavities. As the NH3 is heated, it evaporates and travels to the evaporation zone, near the perimeter of the satellite. Heat is dissipated by radiation because the temperature in space (sink) always less than the apparatus i.e. resonator cavity, and thus NH3 condenses, and the cycle continues. Since the medium in the heat pipes is transported by capillary action, no moving parts are required. Cross sections of the waveguide and heat pipe are shown in Fig.3. In order to reduce the temperature of the irises, they are constructed from pure silver. This allows for more efficient heat conduction when compared to Invar. The cavities are constructed from Invar so that the drift in resonant frequency is reduced [4].

Fig.3 Cross-section of heat pipes cooled waveguide

A large contact area is needed between the cavity and micro heat pipe for given application. This increased size adds mass to the filter. In order to reduce the temperature of the irises, they are constructed from pure silver. This allows for more efficient heat conduction when compared to Invar. The microwave resonator cavity is made from material of Invar having low CTE to minimize the large expansion due to increase in temperature but the weight of the Invar is very high which is not desirable in Space application.

C. Net Thermal Stability Approach A third category of thermal compensation attempts to maximize the net thermal stability, while using materials with low thermal stability. This involves choosing appropriate materials for parts of resonator geometry so that net temperature drift is reduced. Aluminum alloys is used to get advantage of light weight, strength to weight ratio is higher and easy machinability and high conductivity. But aluminum alloy has high CTE () 24*10-6 mm/mm/c these property of aluminum will cause higher expansion when subjected temperature excursion and these for very severely affect the functional performance of sub-system. It can handle very high RF powers with marginal temperature rise and thus enable construction of a low cost and low development cycle time filters for the above said multiplexers. Nevertheless, its high CTE (24ppm) is principal disadvantage causing more frequency drift than conventional Invar filters. So that development of thermal temperature compensation mechanism will help when expansion occurs at that time compensation mechanism produce contraction by country effect of expansion. So that an attempt has been made to replace the conventionally multiplexer by using by using light weight material require to develop efficient thermal compensating technique to maintain the dimensional change of the filter due to thermal excursion can be explained in the following work [5]. Teppo Matias Lukkarila, proposed a invention as shown in Fig.4 a conductive housing formed a microwave resonator cavity with resonator tap from high CTE material (aluminum) and assembled with stabilizer strip assembly with lower and upper strip. A conductive housing is secured by a conductive plate at the top of cavity. The stabilizer strip may be secured in retainer located along the top of the conductive housing by soldering, chemical adhesion snap fit or reverting.


4

Fig.4 Stabilizer Strip Temperature Compensation

The stabilizer strip consists of strip assembly with upper strip which is made from (aluminum) same material as the conductive hosing and lower strip made from (copper) material having different CTE than upper strip and conductive body. The lower strip is curved relative to the upper strip, such that the central portion of the lower strip is separated from upper strip by a distance. Because of the upper and lower strip made from a material having different CTE, this distance varies as a function of temperature. The topological parameter for this present invention is the stabilizer strip in which length, width and thickness of the lower strip and upper strip as well as the distance between the lower strip and upper strip at central portion which provide the desired temperature compensation for the microwave resonator cavity [6]. In the present stabilizer temperature compensation system two or three different materials are used of different CTE and fixed together with soldering or any chemical adhesion snap fit so chances of thermal stress is more which deviate the original position of the resonator cavity. Also the resonator tap on the bottom side of the cavity is also increase the weight of the whole structure. S. Lundquist, proposed Ku-Band Temperature Compensated high Power Multiplexers In this paper author developed a new generation of temperature compensated Ku-Band OMUXs which improving the performance of the system by changing the base material of the filter to aluminum by replacing the conventional Invar material of the existing system technology. To meet the high power requirement a lightweight aluminum temperature compensated channel filter is coupled with the traditionally waveguide manifold resulting in a multiplexer assembly whose RF performance, mass property, thermal conductivity and size are improved. The configuration of the Ku-band temperature compensation channel filter is shown in Fig.5 in which elliptical shaped parallel cavity filter is coupled with compensating bridge are positioned across each cavity end wall, top and bottom, causing the end wall to move inwards as the temperature increase in order to compensate the system.[7]

Fig.5 Temperature Compensated Ku-Band Channel Filter

The given cavity filter is constructed separately into two individual parts top and bottom cavity filter. In the given work Invar based Ku-Band channel filter is replaced by the aluminum for bottom part of the cavity which give the advantages in the terms of the RF performance, mass property, thermal conductivity and size but still it is not the completely desirable for space application, because the top portion of the filter also made from Invar material that of the base filter. A common approach to reducing temperature drift is to constrain cavity expansion. The first design, shown in Fig.6 uses a control-arm constructed from a low CTE material such as Invar to constrain the expansion of one or more of the cavity surfaces. The deflection of the cavity walls will reduce the electric length of the cavity which reduces its resonant frequency. The amount of temperature compensation depends on several factors: the size and shape of the cavity; the pre-tensioning of the control-arm; the CTE of the cavities and control-arm; the placement of the control-arm; and the thickness of the deformed walls [8].

Fig.6 A compensated resonator with control arms

The single parameter that can be easily modified for tuning is the tension in the control-arm. This design is sensitive to the thickness of the deflected walls. The appropriate thickness of the wall must be determined in the design phase. Finding an appropriate material for the control-arm is also an issue with this design. Invar can be used because of its extremely low CTE. But the problems associated with the Invar, that it is expensive and very difficult to machine should not be undermined.

IV CONCLUSION


5

The theoretical backgrounds of the above explained review suggest different approaches for compensating the volume of the resonator cavity filter. Various compensation mechanisms are studied from their pros and cons of manufacturing, realization, amount of compensation and range and bandwidth of compensation. To obtain the low temperature drift, cavity filter constructed from a high stability material like Invar but has many drawbacks, For these reasons, constructing components from Invar is time consuming, cumbersome and expensive. To control a components temperature by removing the root cause of temperature drift using micro heat pipe is also suggested but designing and manufacturing a micro heat pipe with large surface area is also costly as well as complex task. The best and viable compensation mechanism approach for the thermal compensating technique is developed by designing a compensating device using materials with specific dimensional and/or property drift, in such a way that net temperature drift is low. Aluminum alloy Microwave filters for Communication purpose can adopt these mechanisms to replace the heavy and complicated Invar filters to get the advantage of mass and the stable RF performance over operating temperature ranges. After studying the above different approach for microwave devices it is require to developing adaptable compensation mechanism by optimizing the different design parameters provide stable RF performance in a thermally varying environment.

ACKNOWLEDGMENT The authors are thankful to Space Application Center (SAC) for enabling them to work on the project. We deeply acknowledge the knowledge base bestowed on us by SAC official at various levels for generating the solutions proposed.

REFERENCES [1] I. C. Hunter, L. Billonet, B. Jarry, and P. Guillon, Microwave filters-applications and technology, IEEE Transactions on Microwave Theory and Techniques, vol. 50, pp. 794805, March 2002. [2] Steven Barton Lundquist, Temperature compensated microwave filter, US Patent No..5867077, dated February 2, 1999 [3] H.-W. Yao and A. E. Atia, Temperature characteristics of combline resonators and filters, IEEE Digest on Microwave Theory and Techniques, vol. 3, pp. 14751478, May 2001. [4] D. Rosowsky and D.Wolk, "A 450-Woutput multiplexer for direct broadcasting satellites," IEEE Digest on Microwave Theory and Techniques Symposium, vol. 82, pp. 13171323, September 1982, issue: 9. [5] C. Kudsia,et.al, Innovations in microwave filters and multiplexing networks for communications satellite systems, IEEE Digest on Microwave Theory and Techniques, vol. 40, pp. 11331149, June 1992. [6] Teppo Matias Lukkarila et. al. Temperature compensation for resonator cavity, US Patent No. 5905419, May 18, 1999. [7] S.Lundquist, M. Yu et. al. Ku-Band Temperature Compensated high Power Multiplexers, dated May 15, 2002. [8] Small et. al, Temperature compensated high power band pass filter, US Patent No.6529104B1, dated March 4, 2003.

About Author

Name: Bipin D. Patel Qualification:.B.E. in Mechanical Engineering from South Gujarat University, June-2003. Pursuing M.E.-CAD / CAM from Sankalchand Patel College of Engineering-Visnagar, Gujarat Technological University-Ahmedabad. Experience: Currently working as a ME Project Trainee at Space Application Center, ISRO-Ahmedabad on Thermal Compensation Mechanism for one of the Space Craft component from last six month. In addition to four year working in Chemical Industries as a maintenance engineer and also one year in academic field as a lecturer.

paper review gtu rpf may 2011

Documents