Rigid Body Mode

061108 – Pre and Post sales training Webex

Mieke Robijns – Product Development Engineer

What’s new in Structural Testing 7B

2 copyright LMS International - 2006

Overview

Analysis: Rigid Body Calculator

Why are inertia properties needed ?

How to determine inertia properties ?

LMS Test.Lab Rigid body properties calculation

� How does it work ?

� Application

Practical Examples

Test case

� Demo: Frame

Conclusion

Acquisition: “From Geometry” in

Modal Impact


Overview






� Application

Practical Examples

Test case

� Demo: Frame

Conclusion


Modal Impact



� Verification of CoG & MoI values

� Input for simulation models

� Kinematic and dynamic prediction

(multibody dynamics calculation)

� Coupling of an FE model with smaller

“rigid” components

� Accurate Modal based modification or

Substructuring requires flexible modes +

rigid body modes

?


Overview






� Application

Practical Examples

Test case

� Demo: Frame

Conclusion


Modal Impact



� Pendulum test� Based on measured Frequency response

functions

� Typical modal test with hammer or shaker

excitation

� At least 6 excitation locations (SDOF)

� 8 – 12 response locations (3 DOF)

� Time consuming

� Requires multiple suspensions - difficult

for complex structures

� No extra equipment is needed

� Limited measurement effort

� Highly accurate alternative to

conventional pendulum test


Overview






� Application

Practical Examples

Test case

� Demo: Frame

Conclusion


Modal Impact


Rigid Body Properties Calculation

How does it work – Test setup

� Weigh the test item to obtain mass [kg]

� Suspend Test item (once) in free-free conditions

� Create geometry wire-frame model in global or local coordinates

� Measure FRF matrix with hammer or shaker(s)

Test Setup:



How does it work - Mass line methods

� Unchanged FRFs

� Rigid body modes and first deformation modes are sufficiently spaced

� Measured FRFs are used

� Corrected FRFs

� Rigid body modes and first deformation modes are not sufficiently spaced

� Estimate first set of flexible modes from measured FRFs

� Correct measured FRFs by subtraction of contribution of flexible modes

� Lower Residual

� No accurate FRFs are measured in the frequency range directly above rigid body modes

� Lower residuals represent the influence of the modes below the deformation modes, and are

therefore representative of the rigid body modes.

Extract mass line:



How does it work - Calculation and results

� Coordinates of centre of gravity

� Moments and products of inertia about CoG and any user defined reference

point

� Principal moments of inertia and their direction

� Synthesis of 6 scaled rigid body modes with user defined frequency and

damping for use in simulation models

� Least square solution over all measured DOFLeast squares over selected frequency band of mass-line

� Validation through animation of rigid body motion

Calculate Rigid Body Properties:

Results:



Application

Calculate Rigid body properties and

synthesis of rigid body modes

Available as Add-in in

Test.Lab Modal



Application – Data Selection

� Easy FRF selection and visualization

� Calculate sum of FRFs

� Select band with double cursor

� 3 mass line methods

� Unchanged FRFs

� Corrected FRFs

� Lower residual

� Validate FRFs selection with

� Rigid body correlation

� ODS animate at selected

frequency band

� Animate Lower Residual

� Coloring and text feedback on

validation

Solid data validation before analysis



Application - Calculate

� Extract rigid body properties

� Coordinates of CoG

� Moments and products of inertia about a reference

� Principal moments of inertia and their direction

� User defined frequency and damping for rigid body modes

� Synthesized rigid body modes

� 3 translational

� 3 rotational

� Animate rigid body modes immediately

� Automatically add CoG with principal axes orientation on geometry

Rigid body properties and rigid body modes in 1 click



Application - Results


Overview






� Application

Practical Examples

Test case

� Demo: Frame

Conclusion


Modal Impact




Overview






� Application

Practical Examples

Test case

� Demo: Frame

Conclusion


Modal Impact



Test Case / Demo - Frame

� Frame structure

� ANSYS finite element model

� 2272 elements

� 2347 nodes

� Test data 408 FRFs

� 34 response DOFs

� 12 excitation DOFs

� Demo



Test Case - Frame

� Rigid body modes and the first

flexible mode are sufficiently

spaced

� Comparison of the 3 mass line

methods with the FE results

� 12 references

� 34 responses

� Excellent prediction for all

the methods



Test Case - Frame

Use the inertia properties to synthesize 6 rigid body modes


Overview






� Application

Practical Examples

Test case

� Demo: Frame

Conclusion


Modal Impact


Conclusion

�Based on classical FRF measurement

�Limited measurement effort

�Fast and accurate analysis, using least squares solution

�Compensate the effect of the first flexible mode

�Fast and easy data validation

�6 rigid body modes for modal-based substructuring

�For a wide range of applications


Overview






� Application

Practical Examples

Test case

� Demo: Frame

Conclusion


Modal Impact


Test.Lab Modal Impact 7B: From Geometry

� Allows easy editing of Point Id’s based on defined geometry

� As a side effect roving accelerometer measurements are also better supported

061108 – Pre and Post sales training Webex

Mieke Robijns – Product Development Engineer

Thank you

Rigid Body Mode

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