Why is Modal Analysis So Essential

8/13/2019 Why is Modal Analysis So Essential

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Modal Analysis General Presentation

PRODERA

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PRODERA

MODAL ANALYSIS

Why do we have to do Modal Analysis?

Why is Modal Analysisso essential?


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MODAL ANALYSIS

for Structural Investigation

GROUND VIBRATION TESTS ON THE AIRBUS A340/600

Utilization of the Prodera shakers types EX 420 C (1000 N / 225 lbf) &EX 520 C50 (550 N / 123.64 lbf) to perform the vertical and lateral excitations of

the left Rolls Royce inner engine

Location: Airbus plant in Toulouse (France)

Date: February 2001

Project leader company: ONERA


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GENERAL DEVELOPMENT

DIAGRAM

The development of a newdesign (or the modificationof a product) until the final

commercial configurationconsists of a series of

simulations and theircorresponding tests withthe aim of final validationand certification.

The objective of thisiterative process is the

optimization of the model.

In the field of vibrationanalysis, the process iscarried out as follows:

First, the differentassemblies and sub-assemblies are tested

independently Secondly, the system

is tested oncecompletelyassembled, takinginto account theinteractions betweenthe different parts

PROJECT

DEVELOPMENT

SIMULATION

PROTOTYPE

TESTS

MODEL

TUNING

THEORETICAL MODEL

CERTIFICATION

OPTIMIZATION


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MECHANICAL VIBRATIONS

Among the different analyses to be carried out during the development of aproduct, the vibration tests represent a very important part.

Two types of non-destructive tests are performed:

Environmental tests aimed at studying the operation of thespecimen in its future environment

Modal analysis tests aimed at establishing the parametersnecessary to know the dynamic behavior of the specimen

PRODERA systems (as the result of our 40 yearexperience and expertise), are specially designed andadapted to perform modal analysistests from scalemodels to full scale structures. Equipment forenvironmental tests can be added.

100.00m 200.00m 300.00m 400.00m 500.00m 600.00m 700.00m

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MODAL ANALYSIS

Any structure, submitted to an external excitation will deform and vibrate in a characteristicmanner that should be known in advance.

In fact, vibration is a combination of different modes of vibration defined by the frequencyfrom which the vibration is generated, its shape, the fact that this mode is damped or not, the

properties of rigidity, and the inertia characteristic of this mode. All these values are knownas the modal parameters of the considered vibration mode and are the result of the originalmechanical design.

Modes when properly damped are notconsidered dangerous, as oscillations disappearwith time. However, should a structure beexcited by a force whose frequency correspondsto one of its modes, the vibrations, instead ofbeing damped down, will increase until the

destruction of the structure. It is thereforeessential to know the different modes ofvibration, especially those which, because oftheir proximity, influence each other.This step is performed by calculations fromtheoretical models and by experimentation.

Because of the approximations and suppositions inherent in any numerical calculation, theresults obtained will include some errors. Experimentation is therefore the only way toeliminate this dangerous uncertainty, by calculating the real modes from the measured data.

THEORETICALMODEL

SIMULATED

EXCITATIONMEASURED

EXCITATION

PROTOTYPE

CALCULATEDRESPONSES

THEORETICAL

MODES

MEASUREDRESPONSES

REAL MODES

NUMERICALLY EXPERIMENTALLY


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VIBRATION TEST SYSTEMS

Experimental modal analysis is based onthe measurement of the response of thestructure submitted to a defined excitation.The different modes of vibration areobtained from the measured values.

A vibration test system will therefore beable to generate, with extreme accuracy,different types of excitation in a controlledway without modifying the structure to betested. The method of acquisition and measurement of responses will also beoptimized.

The analysis system, as well as the exciters and transducers, must be precisely

calibrated and maintained so that large amounts of data can be processed in anaccurate, user-friendly manner. The integration of computerized systemsrepresents a significant advantage but the importance of the user should not beminimized.

Courtesy VZLU

USER

GENERATION & ANALYSIS

SYSTEM

EXCITATION

SYSTEMMEASURE-

MENT

SYSTEMSTRUCTURE


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FACILITIES FOR VIBRATION TESTS

PRODERA manufactures complete turnkey systems specially defined toperform modal vibrationtests.

PRODERA electrodynamics shakers deliver thenecessary force to the structure. In all the Proderashakers ranging from 3 to 5000 Newton, the movingassembly is designed with the aim of reducing its

influence on the structure.Long-stroke shakers have been recently developedto meet the new requirements as far as big flexibilitystructures are concerned.

PRODERA current controlled amplifiers, by theirdesign, limit the influence of the structure on theinput signal. The force delivered by a shaker cantherefore be measured at the input of the amplifier

without the need for force transducers.

Due to their light weight of 1.2 gram, the PRODERAaccelerometers type PAC/20/M have no effect onthe structure even though they are highly sensitive.

The PRODERA 8 channel power controlledamplifiers type MCAV01 are filter and gain

programmable from the data acquisition system.

The aim of the PRODERA suspension systems is toisolate the structure from any external influence thatcould disturb the results. With pneumatic or elasticsystems, good isolations are possible even in the low

frequency range.


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P-SYS-MODAL

The PRODERA multi-channel excitationand analysis system P-Sys-Modalis the

most recent formodal analysis

.

Housed in a standard 19" rack with built-in ventilation, this system includes onegeneration module and one acquisitionmodule, both controlled from a PCPENTIUM computer through twostandard acquisition boards.

The PRODERA systems use analysis methods based on the appropriation of the

excitation forces.

P-Win-Modal

Appropriation

board

Acquisition

boardsMultiplier

board

Generator

board

Excitation

system

Acquisitionsystem

Console

P-Win-M

odal

US

ERSTRUCTURE


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P-SYS-MODAL

The generator type GLC 950 constitutes the heart of theP-Sys-Modalsystem. It includes 16 (or 32 uponrequest) independently adjustable channels ofexcitation generatingharmonic (phase 0 or ) or

impulse signals. Multi-pointexcitations, with signalscompletely in phase or out of phase in each channeland modulated in amplitude, are made possible bythis generator due to its stability in frequency of10-7Hz. A special designed board performs this forceappropriation. A part of the force in quadrature inrelation to the normal excitation signal can be added to the excitation signal.A module "random excitation" including 4 non-correlated excitations completes

the generator module.

The acquisition of the 256 or 1024 measurement channelsis not performed simultaneously. Due to the stability andprecision characteristics of the generator module, theprecision in the calculations is guaranteed.During the sinus excitation, P-Sys-Modaldelivers thetemporal responses of the transducers and their real and

imaginary parts via the special designed multiplier board.The computer calculation time is therefore highlyreduced. In addition, visualizations in real time ofthe mode shapes are processed. Controlled by theP-Win-Modalsoftware, P-Sys-Modaliscompletely automated. However, the user canmanually control the system by operating theremote control panel. The computer processes onlythe repetitive tasks.


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P-WIN-MODAL

P-Win-Modalis the result of theexperience acquired during the last fourdecades by PRODERA and uses the latestcomputer evolutions. PRODERA isconstantly in contact with its customers

and further ready to listen to any specialrequirement any specific user might have.

P-Win-Modalis comprised of three parts, instrumentation, structureand

operations, which is a logical set-up of the modal tests. In the manual operationmode, with a minimum of parameters to be defined, it is possible to perform asimple test in order to get familiar with the structure.

All the information corresponding to the

different devices in the inventory of thelaboratory (shakers, amplifiers and transducers)is stocked in the P-Win-Modalinternaldatabase.

All the geometrical information about thestructure is used to generate the variousrepresentations and animations of thevibration modes.

P-WIN-MODAL

OperationsStructure

Instrumentation


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P-WIN-MODAL

Harmonic and impulse tests arecarried out as standard practice with

P-Sys-Modal. The random methodis performed by using thecalculation power of Dynaworks.

By calculating the frequencyresponse functions from the acquired temporary signals, the impulse method is

the simplest and quickest way to obtain (within seconds) the resonancefrequencies modal parameters and mode shapes. Multi-point excitationincreases the precision of this method.

By selecting the position and adjusting the force level of thedifferent shakers, a particular mode can be isolated. This is

the appropriation method. A vibration mode can beisolated when the responses from the different pick-ups arein phase or out of phase with theexcitation forces. Appropriationbased methods, the calculation of thecomplex power or the forces inquadrature, provide the differentmodal parameters.

P-Win-Modalalso allows, via its multiplier board, thereal-time calculation of the complex components of thepick-ups' responses. During harmonic excitation, theseresponses are proportional to thefrequency response functions and

therefore to the vibration modes. The vibration modes can bevisualized by P-Win-Modalin real time for any frequencyvalue. P-Win-Modalautomatically saves the different

results and includes a series of processing tools such as theMAC matrix, and other quality criteria.

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COMPLETE ANALYSIS SYSTEM

As an acquisition and data analysis system, P-Win-Modaloperates closelywith a series of other software packages, each specialized in a specific field to

supply a flexible, coherent and very efficient system.

Structural Dynamics Toolbox, library forMATLABTM,is specially developed for modalanalysis. Based on the calculation power ofMATLABTM, this library includes all the functionsnecessary to the analysis of the temporal signalsacquired from P-Win-Modalas well as a series of

functions for the tuning and analysis of its results.SDToolboxalso allows, by using the finite element method, calculation in 2Dand 3D.

As a post-processing software of the modalparameters, FEMToolsallows, thanks to thedifferent links created with the most common codesof digital calculation (MSC NASTRAN, I-DEAS,ANSYS,), the tuning of their dynamic model. It

also allows the study of the influence that themodification of one of the parameters will have onthe global behavior of the structure.

DynaWorks

FEMTools

P-WIN-MODAL

SD

Toolbox


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DYNAWORKS

DynaWorksis the international"standard package" used to manageand analyze the test measurementsand calculations of the majoraerospace and engineeringorganizations.

As a database, DynaWorksallows theoptimal storage of all kinds of tests,

independently from their origin or nature(vibratory, acoustics, thermal)

DynaWorksincludes a complete range ofmathematical tools used to process, printand compare the different data.

Depending on the requirements,

DynaWorks

includes modules necessaryto signal treatment, processing of"Wavelets", acquisition, display andprocessing of the signals in real timeaimed at exporting databases andmanaging, preparing and generating testspecifications.

DynaWorksalso includes all the tools to carry out modal analysis from the

signals directly transmitted from P-Win-Modal, including the estimation ofmodes and display of results.


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FLUTTER PHENOMENA

The study of vibrations, asdescribed before, is carried out toidentify the mechanical propertiesof a structure. The next step in the

certification process is to performtests in real operating conditions. Inaerospace, for example, theaerodynamic conditions due to theair flow around the structure mustbe considered.

These aerodynamics forces can influencethe global behavior of the structure,either the position of the modes or thedamping rate. Aerodynamic forces canmodify a normally damped modemaking it unstable. Its amplitude growsexponentially until the destruction of thestructure.

Before performing the tests on a structure, it is therefore essential to know atwhich velocity this flutter phenomenon appears. This velocity depends on boththe test conditions and the altitude. All the velocities calculated with thismethod are useful to establish the flight envelope of the plane.


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P-FLIGHT-MODAL

Based on well-established calculationmethods using latest algorithms, P-Flight-Modalallows to foresee the evolution of

the vibration modes on a structure facingflutter phenomena in the Sub andSupersonics ratings.

The information directly obtained from theground vibration tests and the geometricalmodel of the aircraft is calculated byP-Flight-Modalrunning in the open

environment of Linux.

P-Flight-Modalcan also calculate thetransonics rating by taking into account the

behavior of the air around the airfoils in thisrandom field where modelisation is a difficulttask.

Measuredmodal

parameters

Geometric Data

Aerodynamic

Coefficients

Flutter

calculation

Limit conditions

of use


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ON-BOARD TEST SYSTEMS

The on-board test systemscomplete the different ground

tests and allow the study of thebehavior of the structuresduring their operation.

A telemetry system is made onthe one hand of on-boardequipment such asaccelerometers, excitationsystems, recorders or codingsystems and on the other handof a ground system includingdecoding units and processingtools.

Due to the complexity of thetests to be carried out, it is ofmajor importance to study in

great detail the complete set-up in order to select the bestadapted system.

ENCODING

ANTENNA

GROUND ANTENNA

DECODING

EXCITATION/MEASUREMENT

ANALYSIS SYSTEM


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IN-FLIGHT EXCITATION

SYSTEMS

PRODERA manufactures two types of vibration devices for the in-flightsystems:

Inertial shakers Impulse thrusters

The Inertial shakers, including movingmasses, are solidly fixed to the structureand installed in the plane together withtheir associated current controlledamplifiers. The Inertial shakers can be

used at different levels of excitation.

In certain cases, the use of inertial shakers isnot possible because of their weight anddimensions. Harmonic test is timeconsuming and it is difficult to performshock excitation. Pyrotechnical impulsethrusters are therefore the ideal exciters formany applications. These smallpyrotechnical thrusters have been designed

to generate short-duration, synchronized, calibrated shocks. Simultaneousexcitations in different positions insure a very efficient isolation of the vibrationmodes. The synchronization ignition is organized by a 16-channel selection

matrix and a firing unit operated by the pilot. Synchronized pulses are sent bytelemetry to the ground station to trigger the data acquisition.


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TELEMETRY SYSTEMS

Essential for performing on-board tests, a telemetry system links the structureunder test and the control center.

The PCM coded signals can come from various sources:

Responses of the transducers, strain gages,

Temperature, pressure transducers,

Microphones or video images,

Other types of transducers

All channels are simultaneously transmitted.A complete telemetry system allows the recordingof all data as well as the selection of

the data to be transmitted by programmingthrough an RS 232 interface.

A GPS system coupled to thetelemetry system insures thelocalization at any moment of theprototype, thus guiding the groundreceiving antennas.A telemetry system is configuredaccording to the user's specificneeds with a series of smalldimensions on-board boxes and

processing PC.


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ADDITIONAL PRODUCTS

PRODERA also supplies additional products and services to meet other testrequirements.

STRUCSIM-3-Dis a calibration and

simulation electronic unit simulating acomplex structure.With 8 shaker inputs and 64 accelerometeroutputs (acceleration, velocity ordisplacement) STRUCSIM-3-Dsimulates thebehavior of a complex system at 8 vibrationmodes.

Our after-sales department can modernize theearlier delivered systems including shakers,amplifiers and analysis systems.

PRODERA also distributes small mobilemodal analysissystems, vibrator control

systems for environmental tests followingMIL STD 810 E, etc.

PRODERA's R & D department can design, according to the users' needs,

shakers, amplifiers or other products and write modal analysis software forspecific cases.


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PRODERA

PRODERAEnclos d'Esquerre

31380 VILLARIES FrancePhone : +33 5 61 84 36 98

Fax : +33 5 61 84 17 91E-mail : [email protected] site: www.prodera.com

The manufacturer reserves the right to change technical or mechanical specifications of its products at any time.Product and brand names mentioned in this document are trademarks or registered trademarks of their respective holders.

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Why is Modal Analysis So Essential