Strength Analysis - Finite Element Analysis for Satellite Structures - page 118

response analysis and stress fatigue analysis for Small Sat structure. Therefore,
modal analysis can be considered as intermediate calculations to specify the
natural frequencies and mode shapes of the satellite structure, and then sort them to
define which ones have the greatest contribution to forced vibration response.
4.10.1 Definition of Modal Analysis
Modal analysis [ 1 ] is used to determine the vibration characteristics (natural fre-
quencies and mode shapes) of a structure while it is being designed. The natural
frequencies and mode shapes are important parameters in the design of a structure for
dynamic loading conditions. Modal analysis is also a starting point for more detailed
dynamic analyses, such as transient dynamic analysis, harmonic response analysis,
or spectral analysis. When frequencies of loading vibrations match one of these
natural frequencies, then resonance takes place. For any dynamic structure, reso-
nance is one of the most critical problems that control its design.
4.10.2 Performing a Modal Analysis
The procedure of modal analysis of Small Sat structure consists of the following
steps:
1. Build the model: The entire finite-element model of the satellite based on the
substructural superelement modules is used during modal analysis. This model
is built in Sect. 4.1 .
2. Apply the loads and define boundary conditions: The only ''loads'' valid in a
typical modal analysis are zero displacement constraints. If a nonzero dis-
placement constraint is specified, the solution will assign a zero constraint to
some DOF instead. Other loads—forces, pressures, temperatures, accelerations,
… etc.—can be specified in a modal analysis, but they are ignored in the mode-
extraction procedure. For directions in which no constraints are specified, the
program calculates rigid body (zero frequency) as well as higher (nonzero
frequency) free body modes.
3. Set solution controls and solve the analysis: A modal analysis type is defined
during performing the analysis on ANSYS. The default settings that will work
well for many structural modal analyses are selected. Modal analysis in the
ANSYS is a linear analysis. There are several mode-extraction methods. The
default method on ANSYS which is used during analysis is Block Lanczos.
The number of modes to be extracted during modal analysis is selected to cover
all frequency bands affecting the satellite during transportation and launch.
From the satellite mechanical loads listed in Appendix A, launch has the largest
band of random vibrations in the three mutually perpendicular directions which