Validation of Light Weight Thermal Compensating

Mechanism for Space Payload System Bipin D. Patel

#1, A. R. Srinivas

*2, Prof. D. A. Patel

#3

#1Mechanical Engineering Department, Sankalchand Patel College of Engineering- Visnagar,

#3Mechanical Engineering Department, Sankalchand Patel College of Engineering- Visnagar

Gujarat Technological University-Ahmedabad

North Gujarat, India. 1bdpatel7129@yahoo.com

3mech.dhaval@gmail.com

*2 Space Application Centre, ISRO-Ahmedabad, India.

2arsrinivasu@yahoo.com

Abstract Development of kuband output multiplexer for

communication satellite requires handling of high power levels of

the order of 140 watts per channel and yet providing stable RF

performance over operating its temperature ranges. The

development activities are multidisciplinary in nature ranging

from electrical, mechanical, designs, fabrication, assembly, test

etc which make the system highly complicated everything serially

adding up to the limitations in the process of realizing the

multiplexer. Such high power multiplexers were

traditionally/conventionally built and relied on a technology

based on thin walled invar cavities. But all this at a very pretty

cost of living with Invar material which is a very poor conductor,

very hard to machine, involving various stress relief cycles for

machining to maintain the dimensional accuracy and its

temperature dependency of the coefficient of thermal expansion.

Aim of this paper is to replace conventional Invar cavity filter

with aluminum and to develop temperature compensation

mechanism. Thermally compensated high power aluminum

cavity filter introduced due to large CTE (coefficient of thermal

expansion) of aluminum provide major optimization in cost and

mass. Development of the compensating technique is made

possible with the use of CAE tool like CAD and FEA for

multidisciplinary cases and experimental methods to validate the

concepts. These developments are presented in the paper.

Keywords- diaphragm, compensation mechanism, multiplexer, cavity filter, performance.

I. INTRODUCTION AND NEED FOR COMPENSATION

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. This paper focuses on one of the channel filter of

the Multiplexer which is used for the high power application

in the space payload system.

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.

To develop a solution for these problems aluminum alloy is

used. This gives advantage of light weight, higher strength to

weight ratio 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

effect 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 compensation mechanism will

eliminate the effect of high CTE of aluminum [1,6].

II. DESCRIPTION OF CONVENTIONAL CAVITY FILTER

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 satellite transponders is shown in

Fig.1 has six channels connected by single manifold. All

channels are consisting of cavity filters made up of invar

material. The multiplexer is one type of microwave filter

which segregates different radio frequencies (RF) of

microwave energy to different channels according to the band

width allocation.

The single channel of the Multiplexer contains of circular

cavity filter, irises, input adapters, output adapters, manifold,

rigid bracket, flexible bracket and base plates as shown in Fig.

2.

Fig. 1 Multiplexer Assembly

Fig. 2 Single channel of Multiplexer

All these components are assembled to meet a defined

functional performance. These channels are made of invar

material. Invar having higher density so that Multiplexer

assembly is too much bulky and overall mass of satellite will

increase which is in turn increase the lunching cost also. Here

attempt is made to replace the Invar filter with aluminum

equivalent so that mass, cost and realization of the filter are

significantly reduced [2].

III. OPERATIONAL COMPLICATION OF RF FILTER

The cavity filter carries high power RF microwave 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 performance. Therefore it is required to maintain a

constant volume of cavity by employing a compensation

mechanism.

When the existing cavity filter made of invar is replace

with aluminum alloys having higher CTE () will expand more and volume of cavity will increase more as compared to

the invar cavity filter which will shift the input resonator

frequency which is undesirable for the system. Various

solutions for the thermal compensating mechanism have been

developed by the SAC among one of the plate and rod

compensating mechanism is presented in detail.

IV. PRINCIPAL OF PREPOSED COMPENSATION MECHANISM

Design of plate and rod mechanism for coaxial cavity as

shown in Fig. 3 works on principle that when aluminum cavity

expands, the diaphragm attached to cavity expands and the

plunger attached to cavity also expands, which push top plate

but top plate being invar and rigid will restrict the expansion

and create a counter effect on the diaphragm applying

retracting force on the plunger which in turn pushes

diaphragm into the cavity and thus changes the volume of

cavity to its original volume. To design such system is

proposed this mechanism will compensate change in

volume.[3,4]

Fig. 3 Coaxial Cavity filter with plate and rod mechanism

V. FINITE ELEMENT ANALYSIS

Thermo structural analysis of the coaxial cavity filter is

carried out by simulating the system under free-free condition

as well as by compensation mechanism considering following

thermal and structural boundary conditions.

Boundary conditions:

Thermal contact conductance between metal to metal=3000 W/m

2 C

Heat flow on the inner surface of cavity=12 W.

Ambient temperature =25 C

All surface exposed to atmosphere are given convection at 25 C.

All parts are constraint as per the assembly sequence.

Grounding all four holes is provided by using fixed supports.

A. Simulating under free-free condition with thermo-structural

analysis

The finite element model of coaxial cavity and diaphragm

meshed with FEA software with tetrahedron element by patch

conforming sweeping method as shown in Fig. 4.

Total no. of Nodes= 24683

Total no. of Elements = 11459

Element type: Tetrahedron

1 cavity

2 cavity flange 3 diaphragm

4 plunger

5 Invar plate 6 bracket 7 Invar rod

Fig. 4 Mesh model under free-free condition

Steady state thermal analysis carried out by implementing

boundary conditions mentioned above and resultant

temperature distribution profile is as shown in Fig. 5.

Fig. 5 Temperature profile under free condition

Later the Thermo structural analysis carried out considering

previously achieved temperature as thermal loading condition.

Deformation profile of system and diaphragm under free

condition are shown in Fig.6 and Fig.7 respectively.

Fig.6 Deformation profile of system under free condition

Fig.7 Deformation profile of diaphragm under free condition

Result:

Maximum temperature on the system: 73 C

Maximum deformation of cavity: 50m

Maximum deformation at centre of diaphragm: 39 m

Maximum stress on system: 45 Mpa

B. Simulating under compensation with thermo-structural

analysis

Thermo structural analysis of the coaxial cavity filter is

carried out with compensation mechanism of plate and rod by

using the above mentioned same boundary condition. The

finite element model of plate and rod mechanism and

diaphragm meshed with FEA software with tetrahedron

element by patch conforming sweeping method is shown in

Fig.8.

Total no. of Nodes= 48959

Total no. of Elements = 19777

Element type: Tetrahedron

Fig.8 Mesh model with compensation mechanism

The applied thermal boundary condition with compensation

that gives the temperature distribution profile for the system as

shown in Fig.9. Later the structure load are applied which

gives the deformation profile of system and also deformation

of the diaphragm as shown in Fig.10 and Fig.11respectively.

Fig.9 Temperature profile with compensation mechanism

Fig.10 Deformation profile with compensation mechanism

Fig.11 Deformation profile of diaphragm with compensation mechanism

Result:

Maximum temperature on the system: 72 C

Maximum deformation of cavity: 80 m

Maximum deformation at centre of diaphragm: -45 m

Maximum stress on system: 67 Mpa

VI. EXPERIMENTAL TESTING

The Experimental set up for the coaxial cavity filter in both

conditions of free-free and compensating is established for

measuring the deformation by simulating a thermo structural

environment. The following are the assumption for the

experimental setup;

The condition of the thermal loading is assumed to be constant through out the cycle of operation of the

filter whereas in actual environment dissipation of

microwave energy in the filter could be random and

therefore the generation could be of unsteady nature.

More over in actual environment the heat transfer from the system is through the base plate by

conduction.. The practical set up consists of

supplying heat by means of three heaters mounted on

the inner circular surface of the cavity.

The temperature sensor is mounted on the outer surface of cavity to measure the temperatures of the

system.

The dial gauges are used for the measurement of deformation on cavity flange and diaphragm under

the condition of free as well as with compensation

mechanism.

A. Experimentation under the free-free Condition For comparison and the evaluation of different designs of

compensation for coaxial cavity, one reference condition has

to be set the best suited condition is without any

compensation. The Fig.12 shows snapshots of the set up for

measuring the practical deflection for the coaxial cavity filter

made of aluminum without any compensation mechanism.

The configuration is used for reference condition. Parts are

expanding due to the heat supply and expansion measure with

help of two dial gauges.

Fig.12 Experiment setup under free-free condition

Results achieved by experimental measurement of coaxial

cavity without compensation are plotted in the graph of

deformation and temperature versus time of practical

measurement of coaxial cavity without compensation shown

in Fig. 13.

Deflection,Temperature vs Time

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70

Time (miniutes)

Defl

ecti

on

(

m)

Tem

pera

ture

(C

)

Diaphragm(m)

Cavity

Temperature(C)

Fig.13 Deformation and temperature vs. time graph of experimental

measurement of coaxial cavity filter under free condition

B. Experimentation under the compensation condition

The experiment setup shown in Fig.14 established for

deflection measurement on to the cavity flange and diaphragm

with help of dial gauges under compensation mechanism.

When input heat supply start thin wall diaphragm is start to

expanding but at particular when cavity expands due to rise in

temperature, the diaphragm attached to cavity expands and the

plunger attached to cavity also expands outside cavity, which

try to push top plate, but top plate being of invar and rigid

tries to restrict the expansion and also create a counter effect

on the diaphragm by applying retracting force on the plunger

which in turn pushes diaphragm into the cavity as diaphragm

is weaker and thus changes the volume of cavity to its original

volume.

Fig.14 Experiment setup under compensation

Results achieved by experimental measurement of

deflection of coaxial cavity under compensation are plotted in

the graph of deformation, temperature versus time of practical

measurement of coaxial cavity as shown in Fig.15.

Deflection,Temperature vs Time

0

10

20

30

40

50

60

70

0 10 20 30 40 50

Time(min.)

Defl

ecti

on

(m

)

Tem

pera

ture

(C

)

Diaphragm

Cavity

Temperature

Fig.15 Deformation and temperature vs. time graph of experimental

measurement of coaxial cavity filter with compensation mechanism

VII. DISCUSSION & CONCLUSIONS

The following are the results of the experimentation;

TABLE I EXPERIMENTAL RESULTS

Condition

Cavity

Temp

(m )

Diaphragm

deflection Absolute

(m )

Cavity

deflection Absolute

(m )

Compensation (m )

Free (without

compensation mechanism)

27 27 30

+28 70 55 57

28 30 30

Under compensation

mechanism

26 40 40

-38 55 2 60

28 38 38

TABLE II

COMPARISON OF RESULTS

Condition Experimentation Simulation

Free (without compensation

mechanism)

+28 +39

With compensation mechanism

-38 -45

Volume Compensation of the Microwave RF filters is now

successfully simulated and demonstrated by experimentation.

The maximum compensation measured is 38 microns. The

amounts of compensation realized in present approaches are

tunable/changeable by controlling the design parameters to

suit various heat dissipations in the cavity filter and for

different environmental conditions. Aluminum alloy

Microwave filters for Communication purpose can adopt these

mechanisms to replace the heavy and complicated to machine

and realize Invar filters to get the advantage of mass and the

stable RF performance over operating temperature ranges. The

problems associated with dissimilar metallic expansions in the

conventional invar based filters (presently used) will become

null and void with the incorporation of Compensated filter.

The proposed solution is now a potential application for the

space payload.

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] 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.

[2] A.R. Srinivas, Thermoplastic behavior and analysis of dissimilar joints, Report No.SAC/SPG/INS/TR03, SAC, ISRO, Ahmadabad, 1993. [3] D. Rosowsky and D.Wolk, A 450-W output multiplexer for direct

broadcasting satellites,IEEE Digest on Microwave Theory and Techniques Symposium, vol. 82, pp. 13171323, September 1982, issue9.

[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 [6] Small et. al, Temperature compensated high power band pass filter,US Patent No.6529104B1, dated March 4, 2003.