Experimentele Modale Analyse

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Experimentele Modale Analyse

• LES 2 LASER DOPPLER VIIBROMETRIE

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

1. Basics of LDV
2. Measurement Analysis
3. Applications

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Laser Doppler Vibrometry

• Contactless vibration measurements
• High spatial resolution
• High frequency range

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Laser Doppler Vibrometry
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Basics of LDV

• The fundamental basics of LDV can be deduced from
  its name
• LASER
• An LVD consists of a LASER source
• DOPPLER
• An LDV uses the Doppler Effect
• VIBROMETER
• An LDV measures Vibrations

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Laser Basics

• LASER Light Amplification by Stimulated
  Emission of Radiation

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Laser Basics

• Optical cavity

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Laser Basics

• Properties of laser light
• Nearly monochromatic (very sharp defined
  wavelength)
• Low divergence (the complete energy of the laser
  beam can be directed and focused)
• High coherence (prerequisite for interference)
• Typical coherence lengths
• Light bulb some µm
• Normal HeNe lasers 0.3 m
• Frequency stabilized mono mode HeNe lasers
  several km

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The Doppler Effect

• Dopplers experiment
• Doppler used two sets of trumpeters one set
  stationary at a train station and one set on a
  train

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The Doppler Effect

• Beat frequency

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Doppler Effect in Optics
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Doppler Effect in Optics

• Frequency shift (Doppler frequency) is of the
  order of 1 to 100 MHz
• Frequency of laser light is 4.7E14 Hz
• Direct detection of the frequency shift is not
  possible
• To detect the Doppler frequency shift an
  interferometer is used

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Interferometer
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Interferometer

• M Measured (moving) object
• R Reference

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Interferometer
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Bragg Cell
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Bragg Cell
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Surface Effects

• Highly reflecting surfaces

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Surface Effects

• Diffuse surfaces scatter the incident light over
  a large angular area

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Vibrometers

• Out-of-Plane Single-Point Vibration
• Out-of-Plane Differential Vibration

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Vibrometers

• In-Plane Vibration
• Rotational Vibration
• 3D Vibration

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Vibrometers

• Out-of-Plane Scanning LDV
• 3D Scanning LDV

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Rotational Vibration
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In-Plane Vibration
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3D Vibration
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3D Scanning LDV
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Measurement and Analysis

• Scanning LDV
• Measurement Mode
• Referenced measurements
• FRF, coherence,
• Non-referenced measurements
• Auto power spectrum,
• Presentation Mode
• Analysis of the measurements
• resonant frequencies
• mode shapes (ODS)

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Measurement Mode
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Setting up the Scanning Head

• Focussing the Laser Beam
• Positioning the Laser Beam
• Zooming and Focussing the Video Camera

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Alignment of Laser Beam to Video Image
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Definition of Measurement Points
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Acquisition Settings

• General
• Channels
• Frequency
• Window
• SE
• Vibrometer
• Generator

• Measurement mode
• FFT
• Fast Scan
• Averaging

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Averaging

• None
• Magnitude
• No reference signal and no trigger
• Complex
• Reference signal or trigger

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Averaging

• No reference signal and no trigger
• Trigger
• Reference signal

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Acquisition Settings

• Channels
• Reference
• Range
• Coupling
• Quantity
• Sensitivity
• Frequency
• Bandwidth
• FFT lines
• Overlap

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Acquisition Settings

• Window
• Rectangular
• Hanning
• Trigger
• External (TTL)
• Analog
• Pre-trigger

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Acquisition Settings

• Signal Enhancement
• Speckle tracking
• Vibrometer
• Sensitivity
• Tracking filter
• Low-pass filter

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Acquisition Settings

• Waveform
• Sine
• Periodic Chirp
• Pseudo Random
• Random
• Sweep
• User Defined

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Multisines

• L-infinity algorithm
• CF 1.40

• Zero phase
• CF 16

• Schroeder phase
• CF 1.70

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Nonlinearities

• Excitation level 1
• Amplitude (dB)
• Coherence
• Excitation level 2
• Amplitude (dB)
• Coherence

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(Odd-odd) Multisines
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Choice of Excitation Signal

• Periodic chirp
• Avrg. 5
• Uniform window

• Random noise
• Avrg. 20
• Hanning window

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Presentation Mode
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Metal Plate
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Metal Plate

• Resonant frequencies

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Metal Plate

• Mode shape _at_ 314 Hz

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Metal Plate with CLD
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Metal Plate with CLD

• Resonant frequencies

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Metal Plate with CLD

• Mode shape _at_ 314 Hz

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FRF Comparison
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Export to Universal File
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Modal Parameter Estimation
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LMS PolyMAX (MDOF)
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Comparison
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Estimated Mode Shape
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Estimated Mode Shape