Mechanische trillingen

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Mechanische trillingen

• LES 9 MODALE ANALYSE

Patrick Guillaume E-mail patrick.guillaume_at_vub.
ac.be Tel. [phone]
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Random Excitation

• Random sequence with Gaussian distribution
• Random amplitudes and phases
• Averaging is needed
• Converge to flat amplitude spectrum
• Remark Force is not flat in general due to
  interaction with the structure
• Signal processing errors (leakage errors)
• Effect of nonlinearities is reduced by averaging

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Random Noise
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Uniform Window
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Hanning Window
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Burst Random
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Burst Random with Uniform Window
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Exponential Window
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Periodic Random / Pseudo-Random Excitation

• Periodic random
• No leakage errors (periodic signal) in
  steady-state conditions
• Averaging is required
• Pseudo random
• Constant amplitudes and random phases
• No leakage errors (periodic signal) in
  steady-state conditions
• Averaging is not required

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Impact Excitation

• Force transducer and tip
• Advantages
• Easy to use
• No interaction with structure
• Force is flat in useful frequency range
• Relatively inexpensive

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Impact Excitation

• Disadvantages
• Large crest factor
• Nonlinearities
• Limited control of amplitude spectrum

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Force Window

• Force window (or transient window)
• Remove noise
• Effect of Double Hits

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Response Window

• Lightly damped structure
• Leakage errors
• Heavily damped structure
• Remove noise

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Overzicht EMA

• EMA Experimentele Modale Analyse
• Stap 1 Experimentele opstelling
• Stap 2 Opmeten van de FRFs
• Stap 3 Bepalen van de modale parameters door
  bvb. curve fitting
• Stap 4 Validatie van de resultaten
• Stap 5 Toepassingen

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Step 1 Setting Up the Modal Test

• Choosing DOFs
• Suspension
• Choice of excitation
• Position/connection of force transducer
• Mounting the response transducers
• Transducer conditioning and calibration
• Setting up the analyzer
• Range setting
• Frequency band
• FRF estimator

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Step 2 Making the Measurements

• Checking the quality of the measurements
• Noise
• Nonlinearities?

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Rigid Body Modes
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Step 3 Modal Parameter Estimation

• SDOF
• Uncoupled modes
• MDOF
• Coupled modes

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Curve-Fitters for Modal Analysis MDOF

• Two step approach
• Poles are global parameters
• Time MDOF (LSCE)
• Freq. MDOF (PolyMAX)
• Mode shape vectors are local parameters
• LSFD

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Stabilization Diagram
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Least Squares Frequency Domain LSFD
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Residual terms LR, UR
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Step 4 Checking the Model

• Synthesized FRF
• Modal Assurance Criteria MAC

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PZL Mielec Skytruck (FLiTE Project)
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Stabilization Diagrams

• LSCF (PolyMAX)

• LSCE

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Mode Shapes (3.17 Hz, 1.62 )
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Mode Shapes (8.39 Hz, 1.93 )
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Computer Simulations What if?

• Modification simulation
• Design optimization
• Response simulation
• Acoustic noise calculations
• Fatigue analysis

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Dynamic Modelling Process
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FE Model Updating
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Structural Dynamic Modifications
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Slat Track Optimisation
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Forced Response Simulation