Shock Simulation of the Optics Mirror Assembly By Numerical Method

Mr. Brijeshkumar Patel et al. Int. Journal of Engineering Research and Applications www.ijera.com
ISSN: 2248-9622, Vol. 5, Issue 9, (Part - 3) September 2015, pp.119-123
www.ijera.com 119 | P a g e
Shock Simulation of the Optics Mirror Assembly By
Numerical Method
Mr. Brijeshkumar Patel1
, Mr. Vipulkumar Vasava2
, Dr. Mayur Sutaria3
1,2
Assistant Professor, Ipcowala Institute of Engineering & Technology- Dharmaj, Gujarat, India.
3
Associate Professor, Charotar University of Science and Technology (CHARUSAT)-Changa, India
ABSTRACT
Satellite faces many extreme types of loading throughout their life time from the harsh launch environment to
the critical space environment. Launch load mainly dynamic is the main design concern for space structure.
Shocks are the one of the most critical dynamic load occurs in spacecraft. Optics Mirror Assembly (OMA) is
used in the telescope of the satellite. The telescope performance relies on dimensional control & the geometric
positioning of the mirror, pointing accuracy and controlled surface deformation of the mirror; Mirror fixation
device (MFD) is used for controlling all these factors. It should not distort due to launch loads mainly shocks as
well as loads during operation of the telescope. In the present work an attempt has been made to perform
experimental and computational analysis of the shock load on Optics Mirror Assembly.
The FE modal for Shock Analysis purpose has been analysed with a specific Linear Transient Response
Analysis in order to obtain the time history of acceleration in several output points. The analysis has been
conducted over the time interval 0 to 62 ms and frequency band between 10 - 10 KHz. In order to verify the
feasibility and reliability of the numerical (Implicit Finite Element Code, Nastran) analysis, the numerical
results obtained by Nastran have been compared with those obtained experimentally in the form of SRS. The
overall outcome of the simulation method has proven its reliability in simulating Satellite payloads subjected to
shocks.
KEYWORDS: Shock; Implicit Transient Response analysis; Optics Mirror Assembly;
I. INTRODUCTION
Shock loads are generally not simple time base
pulse such as half sine pulse and square waves.
Rather, they are complex very short duration wave
form as a result of the rapid transfer of energy from
the pyrotechnic in the surrounding structure. Shock
Response Spectrum (SRS) [1-14] is the tool for the
representation of the shock pulse from time domain
in to the frequency domain. It is the most commonly
used and main standard shock environmental
qualification requirements used in the space industry.
Shock response spectrum is a measure of the
maximum response of an array of single degree of
freedom systems when their base is excited by a
certain transient pulse as presented in figure 1.
The amplitude of the maximum acceleration
response can be calculated by considering all the
independent systems separately with constant
damping value (5% critical damping) [1-4].
Figure 1: Concepts of SRS
OMA is one of the on-board Payload for
Imaging Satellite as shown in figure 2. The OMA
consists of top mirror, MFDs and mounting ring.
MFD is having two blades and one pad, pad is glued
to the mirror. Blade’s Geometry decides the
directional stiffness of MFD. The main functional
objective of the MFD is to provide flexibility in the
radial dimension of the mirror at fixing, which
permits in-plane thermal expansion of the mirror.
MFD is stiffer in the other two directions which
avoids mirror movement in those two directions, and
in turn improve the natural frequency as well as
reduce dynamic strain at the most critical location of
the MFDs. This behavior provides Isostatic
RESEARCH ARTICLE OPEN ACCESS

mounting. This peculiar behavior is obtained by
providing notch (thin section) in two perpendicular
directions as shown in figure 3.
The OMA must withstand a series of shocks at
those critical locations such as optical element mirror
and MFD notch section. These high frequency pulses
result from pyrotechnics devices during their lifetime
like the separation of structural subsystems (e.g.
booster, fairing or stage separation, clamp band
release), the release of deployable appendages (e.g.
solar panels or antenna) and the activation of on-
board operational subsystems (e.g. propellant valves).
These devices produce high frequency and high
amplitude shock energy as a by-product. The payload
must survive these shock pulses and thus it is very
decisive to know the dynamic behavior of the OMA
for these shocks.
Figure 2: CAD modal of the optics mirror
assembly
Figure 3: Detail view of notch section of the
MFD
The CAD modal of OMA was generated in the
.iges file format. Finite Element Mesh was generated
using Hypermesh; properties, loads and constrains
were applied in MSC Patran; and finally it is solved
using MSC Nastran.
II. SHOCK RESPONSE SPECTRUM
SPECIFICATION
The SRS specification for OMA payload as per
Environmental Test Level Specifications is given in
table-1 (-6dB of Qualification level).
Table 1: Shock response spectrum
specification
Frequency (Hz) SRS (g)
100-300 15dB/Octave
300-5000 150 (X and Y axis)
5000-10000 -6dB/Octave
SRS is converted into a time domain signal by
synthesizing using vibration controller software.
Synthesized time domain signal corresponding to
above SRS is shown in figure. 4, used as input in the
numerical analysis as well as shock test.
Figure-4: Time domain input corresponding
to SRS
III. NUMERICAL SHOCK ANALYSIS
Numerical Shock Analysis of the OMA is
carried out using Implicit Finite Element Method [15-
20].
3.1LINEAR TRANSIENT RESPONSE
ANALYSIS
Far field structure shock problems are based on
structural response and wave propagation theory and
can be solved by an implicit finite element method.
The OMA payload is made up of Invar and
Zerodur materials. Details of material properties for
different components of OMA are shown in table.2.
FE modal of OMA consists Tetrahedral and
Hexahedral elements, as shown in figure.5.

Figure 5: FE modal of OMA
Table-2: Material properties of the OMA
Material
Comp.
Young
Modulus
(N/m2
)
Poisson
ration
Density
(N/m2
)
Invar
MFDs
Ring
1.45
E+11
0.3 8100
Zerodur
Mirror
0.91
E+11
0.24 2530
3.2 SELECTION OF PARAMETERS
The following parameters need to consider in the
implicit transient response analysis using Nastran are
as follows:
= Analysis time step
C = Velocity of the sound through structure
= Minimum element length
= Duration of the pulse
= Fundamental period of the structure
= Length of the finite element in the direction of
shock load
= Dynamic magnification factor
𝜻 = Critical damping factor
1. Time steps are decided based on a beloved criteria.
C……………………………. (1)
It can be seen from above equation (1) that the
time step depends on minimum element
length , modulus of elasticity and
density . Total calculation time (CPU time)
directly depends on the time step size for each
iteration. To have faster calculation time steps must
be larger, but also has to satisfy the equation (1)
condition.
2. should be small enough to satisfy the
requirements of finite element analysis. These
requirements differ from element to element. The
finer the finite element mesh gives good results of
capturing all natural modes of the structure.
3. This is subject to the definition of input
shock pulse.
4. 𝜻 and (𝜻=1/2 ) are the most critical
parameters and direct related to responses at
resonance frequencies. The actual values of them can
only know after experimental.
Using all above parameters, Shock response
analysis performed for X and Y axis, which are the
critical axis of the OMA structure using implicit
transient response analysis and output from MSC
Nastran converted into the time domain to SRS by
using Kelly-Richman algorithm and Smallwood
algorithm (MATLAB code) [23-25].
IV. EXPERIMENTAL SHOCK TEST
Shock test was performed on OMA with
application of the electrodynamic shaker with time
domain input. Shock pulse is given at the base of the
OMA which is mounted on the shaker fixture with
six bolted joints and shock will go through that joint.
The response measured in terms of acceleration in
time domain measured at critical locations such as
MFD thin section and on top of mirror. Piezoelectric
accelerometer location during the shock test is shown
in figure.6.
Figure 6 Shock test setup with accelerometer
locations
It is possible to compare the numerical results
with experimental data obtained from test performed
on the OMA in the form of Shock Response
Spectrum (maximum acceleration values versus
frequency). The results obtained are reported in the
figures 7-10 in the form of SRS. The two diagrams
(Figs. 7-8) show the experimental and calculated

shock response spectrum (SRS) response based on
accelerometer placement on MFD thin section
during X and Y testing and transient response
analysis axis respectively.
Figure 7: Results SRS along the x-axis on
the MFD thin section
Figure 8: Results SRS along the y-axis on
the MFD thin section
Figure 9: Results SRS along the x-axis on
the mirror top
Figure 10: Results SRS along the y-axis on
the mirror top
The SRS diagrams (Figs. 9-10) are referred to
the accelerometer placed at mirror top during X and
Y testing and analysis axis respectively.
V. CONCLUSIONS
It can be noted the numerical and experimental
results in some cases reach a sufficiently good
agreement.
It can be assessed that by taking into account
critical parameters such as minimum size of the
element, time steps for analysis and damping in the
numerical analysis of optics mirror assembly can
produce results with a sufficiently good
approximation with experimental results, allowing
one to conclude that the proposed numerical analysis
can be considered a reliable, fast and economical
method to obtain realistic predictions of the structural
behaviour subject to the shocks.
VI. ACKNOWLEDGEMENTS
The authors would like to acknowledge the
contribution of the vibration test facility team, for
their contribution during an experimental shock test
of OMA and the useful suggestions from vibration
team.
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Shock Simulation of the Optics Mirror Assembly By Numerical Method

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