international journal of conceptions on mechanical...

International Journal of Conceptions on Mechanical and Civil Engineering Vol. 1, Issue. 1, Dec’ 2013; ISSN: 2357 – 2760

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A novel thermal compensation device using asimilar materials for a multiplexer of

communication space payload Animesh Kumar Jha, Dr. D P Vakharia

Dept. of Mechanical Engineering, SVNIT,

Surat, India. [email protected]

Piyush Shukla, A R Srinivas, and Dr. B S Munjal

Space Application Centre, ISRO,

Ahmedabad, India.

Prof. N Ramakrishnan IIT Gandhinagar,

Gandhinagar, India.

Abstract— Temperature compensation methods using dissimilar materials of varying coefficient of thermal expansion (CTE) is a known technology for decades now. Uses of such methods/mechanisms for compensation of thermal excursions have shown some unsolvable problems like thermal stress due to dissimilar expansions and contractions. Invar, a material mostly used for compensation has its own disadvantages of very low thermal conductivity (K), High density (ρ), and is difficult to machine. Additionally Invar’s low CTE which is a useful property for the space applications is temperature dependent. These features make the existing compensation mechanisms inappropriate to many applications, particularly for a very high power filter of multiplexers. The work presented here proposes a viable solution introducing a novel design of compensation mechanism using asimilar materials for cavity filter, diaphragm and the compensators. Extensive FEA simulations have been performed to evaluate and optimize the designs to maximize the compensatory effect. The Optimized design is realized and the measurements are performed to demonstrate and validate the design. Close confirmation of the simulated and the measured results has been observed. This paper presents the details of the design and the development of such devices using asimilar material.

Keywords- Diaphragm, Compensation Mechanism, Multiplexer, Cavity, Plunger.

I. INTRODUCTION There has been a tremendous growth in the field of

communication in the past few decades. The communication industries are providing their services in the areas of telecommunications to space science and earth observations via Satellite which is looking at transmitting more value added information through high quality video and audio in real time.

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. This paper deals with the thermal compensation technique for a sub-system called multiplexer.

Multiplexer as shown in fig 1 is one of the sub-systems used in the communication satellite for filtering and amplification of high power RF microwaves signals.

It consists of two or more channels connected together by manifold. Each channel contains cavity filters, diaphragm, input and output adapter, and manifold. All these components are connected together in sequential manner to meet the desired functional performance. These cavity filters are subjected to severe thermal excursions during the life cycle. The function of these filters depends on the dimensions of the components. Each cavity filter is designed to filter a particular wavelength signals, any change in the dimension of the component will allow the unwanted signal to pass through it which causes the shift in the frequency of that cavity filter. So it is very important to maintain the dimension of the components same even under the extreme thermal excursions [2, 3].

Temperature compensation methods using dissimilar materials of varying CTE have been known solution for decades now [4, 5]. Most of them use Invar material to overcome these problems. Invar is an alloy of Iron, Cobalt and Nickel with almost an invariable coefficient of thermal expansion (CTE) of the order of 1 to 1.5 ppm. Though invar is having Low CTE, its properties like high density, poor machinability, low thermal conductivity and dependence of its CTE on temperature makes Invar based multiplexer an improper selection for the given problem. Also the

Figure 1 Single channel Multiplexer Assembly


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combinations of different materials with different CTE “α”, when subjected to predominant thermal excursions tend to develop complex thermal stress fields. This in combination with elastic constraints like bolting connection will call for a structural stress field. Thus the problem now becomes very complex case of the thermo-structural stress fields, which gets more complicated when imposed by constraints like heat transfer, availability of space, manufacturing of such components, and finally the assembly. 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.

Aluminum alloys being light weight, possessing good strength to weight ratio, high thermal conductivity and easy to machine and is a proven material for space applications found to be the most appropriate material for the given application. This material also meets the requirements of multiplexers provided an innovative design modification is attempted. Use of aluminum alloy as material for cavity filter not only overcome the drawbacks of invar filters but also solves the problem of using dissimilar materials such as generation of thermal stress due to different CTE of various materials used.

The principal disadvantage of using aluminum alloy is its high CTE (α=24 ppm) which causes more frequency drift than conventional Invar filters when subjected temperature excursion, which severely affect the functional performance of system. But this problem of higher thermal expansion can be solved by introducing the compensating mechanisms. The work presented here deals with the design and development of one such mechanism to provide the compensation when the multiplexer undergoes temperature excursions.

II. PROBLEM DESCRIPTION The cavity filter carries high power RF microwave energy

and a part of energy is dissipated in the form of heat in the cavity which results in rise in temperature of cavity that causes expansion of cavity material. This leads to change in volume of cavity and will in-turn change the performance of the filter in the form of frequency drift which is un-desirable for the RF functional requirements. Therefore it is required to maintain a constant volume of cavity by employing a compensation mechanism. Many efforts have been made in analyzing the various options for compensation mechanisms using dissimilar material for cavity and compensation mechanism which are widely reported in literature in the form of papers and patents [4, 5]. But use of dissimilar material for a cavity, diaphragm and compensation mechanism induces thermal stress due the different CTE’s of materials. The work presented here on design and the development of compensation mechanism has evolved new possibilities of disseminating the compensating effect using asimliar materials for the cavity, diaphragm and compensation mechanism. This paper deals with a design, optimization, simulation and realization of a compensation mechanism which will provide the compensatory effect with minimal thermal stresses which are significantly lower compared to conventional methods published in the literature [1]. Various design approaches using asimilar material have

been proposed, and developed by the authors, one of such potential design practiced in the development asimilar based compensation filters is presented in the following sessions.

III. FINITE ELEMENT ANALYSIS OF CROSS COUPLED CAVITY FILTER SYSTEM

FEA of the cross coupled cavity is carried out using the CAE tools in the thermo structural environment with the following boundary condition to simulate the real life test condition. Firstly the system is simulated without any compensation mechanism and then with the compensation mechanism in order to see the effect of the compensation mechanism. The detailed multi-physics based simulations are presented by the author [1]. Similar procedures for thermo-structural analysis is followed and hence not discussed in detail here.

A. BOUNDARY CONDITIONS Thermal boundary condition Thermal contact conductance between metal to

metal=3000 W/m2 °C Heat flow on the inner surface of cavity=45 W. Ambient temperature =25 °C All surface exposed to atmosphere are given

convection at 25 °C. Structural Boundary conditions All parts are constraint as per the assembly

sequence. The system is grounded on the fixed base plate by

means of lugs which acts as fixed supports.

B. Thermo-structural simulation of the system without compensation mechanism.

The FEA of cross coupled cavity filter without compensation mechanism is shown in the Fig. 2; the meshing of the model is done using FEA software. Steady state thermal analysis is carried out by applying the thermal boundary conditions and resultant temperature distribution profile is as shown in Fig. 3. Then the static structural analysis is followed with the increased temperature as loading condition in addition to the structural boundary conditions. The deformation and the stress profile of uncompensated system are as shown in Fig.4 and Fig 5 respectively [1].

Figure. 2 Meshed model of system without compensation

Figure. 3 Temperature profile of system without compensation


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C. Result summary: Maximum temperature on the system: 75 ° C (on cavity) Maximum Deflection on cavity flange: 144µm Maximum deflection on at centre of diaphragm: 144µm Maximum stress on the diaphragm is 0.76 Mpa The simulated result of the system without compensation

mechanism shows that there is no deflection of the diaphragm indicating no compensation to the thermally induced volume change of the cavity filter. Also the maximum stress on the diaphragm is only 0.76 Mpa. So in order to achieve the compensation for the change in the volume of the cavity there is a need of compensation mechanism.

IV. COMPENSATION MECHANISM Various design options have been tried to achieve required

compensation using CAE tool. CAD based designs were developed and assembled. The thermo-structural analysis of the assemblies is then simulated under the given boundary condition. The results of the analysis carried out with the different options of compensation mechanism are listed in the table I. The working principle of the best compensation mechanism, simulation in thermo-structural environment and mathematical modelling is presented here.

TABLE I Results of analysis of various option of

Compensation Mechanism for cross coupled cavity filter

Sr. No.

Design option for compensation

mechanism

Diaphragm deflection

Stress on diaphragm

1

05 μ 3Mpa

2

10 μ 5Mpa

3

12 μ 7Mpa

4

15μ 9Mpa

5

20 μ 10 Mpa

A. Working principle of compensation mechanism The concept behind the given design is to use the radial

expansion of the cavity flange for creating the compensatory

effect by making the diaphragm to deflect into the cavity when the system shown in figure 6a is subjected to thermal excursions. Design details of externally mounted temperature compensation mechanism for cross coupled cavity is shown in Fig. 6b. The compensator is mounted on the cavity flange and is thermally isolated from flange with the separator so that the compensator does not expand. The present design makes use of the radial expansion of the cavity flange to compensate for the change in the volume of the cavity filter due to the thermal expansion. When the temperature of the cavity increases the cavity expands in longitudinal as well as in radial direction. The temperature on the flange is more than that on the compensator. So the radial expansion of the flange is more as compared to the compensator. Due to the expansion of the flange the compensator will be stretched along with the cavity flange. This will make the lower strip of the compensator to get straightened which in turn will force the diaphragm with the protruded plunger of the compensator towards the cavity and thus compensate change in the volume of cavity.

B. Thermo-structural simulation of the cross coupled cavity filter with compensation mechanism

Thermo structural analysis of the cross coupled cavity filter with the compensation mechanism is carried out using the same boundary condition as above. The FE meshed model of the filter system with compensation mechanism is shown in Fig.7. The thermal analysis of filter system with thermal boundary condition gives the temperature distribution profile for the system as shown in Fig.8,

Figure. 5 stress profile of system w/o compensation

Figure. 4 Deformation profile of system w/o compensation

Upper strip

Lower strip

Compensation mechanism

Isolator

Diaphragm

Filter body Mounting lugs

Figure. 7 Mesh model of Un-compensated system

Figure. 8 Temperature profile of Un-Compensated system

Figure. 6a. Cross coupled Cavity filter with compensation mechanism

Figure.6b.Compensation mechanism/Compensator

Plunger

Filter flange


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In addition to the given thermal boundary condition when structural constraints are applied, the total deformation and the deflection profile of the diaphragm of the cavity filter will be obtained shown in the figure 9 and 10 respectively.

C. Result summary: Maximum temperature on the system: 75° C (on cavity) Maximum deformation of cavity: 144µm Maximum deformation at centre of diaphragm: 124µm Maximum stress on the diaphragm is 10 Mpa The simulated result for the filter cavity with the

compensation mechanism has shown that the compensator mechanism which is mounted on the cavity flange is helpful in making the diaphragm to deflect in opposite direction to that of the cavity expansion when the system is subjected to the temperature rise. The deflection of the diaphragm into the cavity result into the compensation of the change of volume of the cavity.

The deflection of the diaphragm can be calculated by the equation given below [6]:-

2223

2

)(16

)1(3 raEh

vpy

…… (1)

Y = Deflection of the diaphragm at a distance r from the

centre of diaphragm of radius ‘a’ P= pressure exerted on the diaphragm by the compensator V= poisson ratio of the material E= Young’s Modulus of the material h=thickness of the material a=radius of the diaphragm r =distance from the centre From the equation (1) the maximum deflection of

diaphragm occurs at the centre of the diaphragm i.e. (r=0)

ymax= )(

16)1(3 4

3

2

aEh

vp …… (2)

The volume swept by the diaphragm for the given deflection is obtained by integrating the equation (1)

V= 6

3

2

48)1(3 a

EhvP

……. (3)

The simulated results for the deflections of diaphragm with

and without compensation mechanism are given in the table II TABLE II Simulated results of the analysis for the system with and without compensation mechanism Results

Without compensation

With compensation

Absolute deflection (µ) 144-144=0 144-124=20 Volume compensated 0 25 mm³ Percentage of Compensation (%)

0 30

V. EXPERIMENTAL TESTING Experimental set up for the cross coupled cavity filter as

shown in the fig 11 is established for measuring the deflection of the diaphragm with the applied thermo structural boundary conditions on the system

The heat is supplied to the system by means of three

heaters having 150 Ω resistance attached to the base plate through conduction. The multiplexer system is fastened to the base plate. A temperature controller is used to control the heat input to maintain the constant range of temperature gradient on the cavity to simulate the required boundary conditions. Temperature sensors are mounted on the outer surface of cavity flange to measure the temperatures of the filter system. Three dial gauges are used on the diaphragm and flange to measure deflection on the cavity and the diaphragm.

A. Measurement for the deflection of the diaphragm without and with compensation mechanism

The experimental set up for the measurement of deflection without and with compensation mechanism is shown in the figure 12 and 13 respectively. It has been already cleared from the simulated results that there is no compensation to the changed volume of filter body without compensation mechanism.

Figure. 10 Deflection profile of diaphragm

Figure. 9 Total Deformation of the cavity filter

Figure. 11 Experimental set up for the Cross Coupled filter system


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The deflection of the diaphragm and the deflection of the filter flange are measured using the dial gauges shown in the Figure 12 & 13. The difference between the two is the relative deflection of the diaphragm into the cavity. Experimental measurements for the system without compensation mechanism and with compensation mechanisms confirm the simulated results with close match.

B. Results Experimental measurements and Time Temperature and

Deflection graph is shown below in the table III and figure 14 respectively. The blue line indicates the rise and fall of temperature in the heating and the cooling cycles used to measure the temperature compensation. The trend of the red curve shows the actual compensation achieved with respect to the green curve which shows zero compensation.

Table III Experimental results of the filter system with and without compensation

Results Without

compensation

With compensation

Absolute deflection (µ) 2 20

Volume compensated 1.2mm³ 25 mm³ Percentage of Compensation (%)

1.5 30

VI. CONCLUSION AND DISCUSSIONS The work presented here has successfully shown that

compensation to the changed volume of the Microwave RF filters can be achieved by using compensation mechanism made up of asimilar materials. The same is simulated using FE tool and demonstrated by experimentation. The maximum volume compensation measured is 25 mm³. The compensation realized in present approach can be increased by optimization of various design parameters to suit various heat dissipations in the cavity filter and for different environmental conditions.

Aluminum alloy based compensation devices presented in the paper can be adopted for Microwave filters of communication payloads replacing heavy and complicated to realize Invar filters to get the advantage of reduced mass and the stable RF performance over operating temperature ranges. In addition the problems associated with dissimilar metallic expansions in the conventional invar based filters or conventional temperature compensated filters presented in the literature can be eliminated with the incorporation of proposed aluminum alloy based approaches using asimilar material compensating devices.

REFERENCES [1] A.R.Srinivas B.S Munjal and D. Modi, “Multiphysics based numerical

techniques for an optimum design of a space payload compensating mechanism” International Journal of Applied Engineering Research,vol.6, pp 813-821, november 2011

[2] C. Kudsia, R. Cameron, and W.-C. Tang, “Innovations in microwave filters and multiplexing networks for communications satellite systems,” IEEE Digest on Microwave Theory and Techniques, vol. 40, pp. 1133–1149, June 1992.

[3] 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. 794-805, March 2002

[4] D. J. Small and J. A. Lunn, "Temperature compensated high power band pass filter," U.S. Patent 6 529 104, Mar. 4, 2003.

[5] S.Lundquist, M. Yu et. al. “Ku-Band Temperature Compensated high Power Multiplexers”, dated May 15, 2002

[5] [6] J. Mc. Entee, L. Bowman, “ Ossilating diaphragm” nanotech 99 the nanotechnology conference and trade show , April 21, 1999.

Figure.12 Experiment setup for system w/o compensation

Figure.13 Experiment setup for compensated system

Fig 14. Deflection Vs Temperature graph with and without compensation mechanism.

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