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CI BLOG




MODAL ANALYSIS OF SATELLITE MODEL USING EDM MODAL

May 3, 2024 Aakash Umesh Mange - Senior Application Engineer


DOWNLOAD TEST CASE

Modal analysis of a unit under test yields important information about the
natural frequencies, damping coefficients and mode shapes to optimize its design
and improve its structural behavior. The modal parameters of a test unit and its
mechanical properties will help users understand the vibration characteristics
during operating conditions.

The case described in this article examines the modal characteristics of a
satellite model acquired from performing experimental modal analysis. A hammer
impact test was carried out with two teardrop uni-axial accelerometers (mounted
in different directions) to study the modal behavior. The roving excitation
method assists in completely avoiding the potential mass loading side effect
produced by a roving response procedure. A hard tip was selected to excite the
higher frequency modes.

Figure 1. Hammer Impact Modal Test of the Satellite Model

A geometry mesh configuration of 18 measurement points is uniformly distributed
throughout the satellite model to achieve good spatial resolution for the mode
shapes. Flexible bands and bungee cords were used to suspend the satellite model
vertically to imitate a free-free boundary condition (as shown in the
experimental setup). The tiny uni-axial teardrop accelerometers were fixed to
two locations (in X and Y directions) and the impact hammer was roved through
the points to acquire 108 FRFs at the 18 desired measurement locations. The
excitation force and response accelerations in the X, Y and Z directions were
measured to obtain the 3D mode shapes.

Figure 2. Satellite Model Geometry

A sampling rate of 6.4 kHz was set since this satellite model has modes in a
higher frequency range. A block size of 4096 was selected to ensure the response
decays naturally and windowing is not required. A fine frequency resolution of
1.5625 Hz is produced with these configuration settings. Measurements of higher
accuracy and reduced noise are obtained by linearly averaging 3 blocks of data
at each measurement DOF.

The hammer impact excitation imparts energy across a broad frequency range of
2.9 kHz. With this setup, there will be no leakage and a uniform window can be
selected.

Figure 3. Hammer Impact Measurement of the Satellite Model

The coherence plot helps validate the measurement results. The preceding
screenshot illustrates a good example of the results. The valleys in the
coherence plot occur at the anti-resonances which indicates that the response
level is relatively lower at these corresponding frequencies. Overall, the
inputs and outputs are well correlated within the desirable frequency range.

The FRF measurement displays several dominant peaks in the 0-2900 Hz frequency
band. Several modes can be identified when overlapping the 108 measured FRFs.
The peaks are well aligned which indicates good measurement results and the lack
of a mass loading effect.

Figure 4. Modal Data Selection tab showing the magnitude and phase part of all
overlapped FRFs

The Complex Mode Indicator Function (CMIF) was used to locate the modes in the
desired frequency range. (2 CMIFs were produced for 2 references.) In addition,
the summed FRF was also observed to identify the modes. The Poly-X method was
used to curve-fit the FRFs and extract thirteen flexible modes within the
desired frequency range.

Figure 5. Stability Diagram for the thirteen flexible modes

The Auto-MAC matrix assisted in validating the results. The following Auto-MAC
matrix shows that the modes are orthogonal to each other (low off-diagonal
elements) and are uniquely identified (high diagonal elements). Mode 10 and 12
have some similarity and increasing the number of measurement points (better
spatial resolution) will help users to understand the higher order modes.

Figure 6. Auto MAC chart for the Satellite Model Hammer Impact Modal Test

The animation of the thirteen mode shapes associated with the stable physical
poles is shown below. Since the satellite model is asymmetric (right side of the
model has more thickness), some local modes (right side of the model having
relatively more stiffness) can also be observed along with the global modes.
Modes that animate the torsion and bending deformations of the satellite model
in-phase and out-of-phase along the different axes can also be observed.

Modes 1 and 2 shows the torsional deformation (about Z axis) of the satellite
model.



Modes 3 and 4 animates the bending motion of the satellite model along the
sideway (Y axis) and forward directions (X axis) respectively.



Mode 5 shows local bending of the bottom panel of the satellite model. Mode 6
shows the global bending deformation of the panel along the vertical direction
(Z axis).



Mode 7 animates the higher order torsional motion of the satellite model. Mode 8
shows the local deformation of the satellite model.



Mode 9 shows the model bending along the Z and Y axes. Mode 10 shows the model
bending along the X and Y axes.



Mode 11 animates the bending of the model along the Y axis. Mode 12 shows a
similar deformational motion like mode 10.



Mode 13 shows some local bending and torsional deformation of the satellite
model.

 

 
In Modal Testing & Analysis Tags Structural Dynamics Testing, Environmental
Testing

← Fatigue Damage SpectrumDigital Filters & Resampling in Post Analyzer →


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