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动态测试技术

动态测试技术. 余征跃 上海交通大学 工程力学实验中心 yuzy@sjtu.edu.cn 54743053 13341763417 闵行校区电工力学楼 103 室. 6.Experimental Modal Analysis. 2 – LASER DOPPLER VIBROMETER. Outline. Basics of LDV Measurement & Analysis Applications. Contactless vibration measurements High spatial resolution High frequency range.

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动态测试技术

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  1. 动态测试技术 余征跃 上海交通大学 工程力学实验中心 yuzy@sjtu.edu.cn 54743053 13341763417 闵行校区电工力学楼103室

  2. 6.Experimental Modal Analysis 2 – LASER DOPPLER VIBROMETER

  3. Outline Basics of LDV Measurement & Analysis Applications 3

  4. Contactless vibration measurements High spatial resolution High frequency range Laser Doppler Vibrometry 4

  5. Laser Doppler Vibrometry 5

  6. 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” 6

  7. Laser Basics • LASER = Light Amplification by Stimulated Emission of Radiation 7

  8. Laser Basics • Optical cavity 8

  9. 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 9

  10. The Doppler Effect • Doppler’s experiment • Doppler used two sets of trumpeters: one set stationary at a train station and one set on a train 10

  11. The Doppler Effect • Beat frequency 11

  12. Doppler Effect in Optics 12

  13. Doppler Effect in Optics • Frequency shift (Doppler frequency) is of the order of 1 to 100 MHz • Frequency of laser light is 4.7E+14 Hz • Direct detection of the frequency shift is not possible • To detect the Doppler frequency shift an interferometer is used 13

  14. Interferometer 14

  15. Interferometer • M = Measured (moving) object • R = Reference 15

  16. Interferometer 16

  17. Bragg Cell 17

  18. Bragg Cell 18

  19. Surface Effects • Highly reflecting surfaces 19

  20. Surface Effects • Diffuse surfaces scatter the incident light over a large angular area 20

  21. Vibrometers • “Out-of-Plane” Single-Point Vibration • “Out-of-Plane” Differential Vibration 21

  22. Vibrometers • “In-Plane” Vibration • “Rotational” Vibration • “3D” Vibration 22

  23. Vibrometers • “Out-of-Plane Scanning” LDV • “3D Scanning” LDV 23

  24. “Rotational” Vibration 24

  25. “In-Plane” Vibration 25

  26. “3D” Vibration 26

  27. “3D Scanning” LDV 27

  28. 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) 28

  29. Measurement Mode 29

  30. Setting up the Scanning Head • Focussing the Laser Beam • Positioning the Laser Beam • Zooming and Focussing the Video Camera 30

  31. Alignment of Laser Beam to Video Image 31

  32. Definition of Measurement Points 32

  33. General Channels Frequency Window SE Vibrometer Generator Measurement mode FFT Fast Scan Averaging Acquisition Settings 33

  34. Averaging • None • Magnitude • No reference signal and no trigger • Complex • Reference signal or trigger 34

  35. Averaging • No reference signal and no trigger • Trigger • Reference signal 35

  36. Acquisition Settings • Channels • Reference • Range • Coupling • Quantity • Sensitivity • Frequency • Bandwidth • FFT lines • Overlap 36

  37. Acquisition Settings • Window • Rectangular • Hanning • … • Trigger • External (TTL) • Analog • Pre-trigger 37

  38. Acquisition Settings • Signal Enhancement • Speckle tracking • Vibrometer • Sensitivity • Tracking filter • Low-pass filter 38

  39. Acquisition Settings • Waveform • Sine • Periodic Chirp • Pseudo Random • Random • Sweep • User Defined • … 39

  40. Zero phase CF = 16 Schroeder phase CF = 1.70 Multisines • L-infinity algorithm • CF = 1.40 40

  41. Nonlinearities • Excitation level 1 • Amplitude (dB) • Coherence • Excitation level 2 • Amplitude (dB) • Coherence 41

  42. X Frequency Xodd Xeven Nx (Odd-odd) Multisines 42

  43. Choice of Excitation Signal • Periodic chirp • Avrg. = 5 • Uniform window • Random noise • Avrg. = 20 • Hanning window 43

  44. Presentation Mode 44

  45. Metal Plate 45

  46. Metal Plate • Resonant frequencies 46

  47. Metal Plate • Mode shape @ 314 Hz 47

  48. Metal Plate with CLD 48

  49. Metal Plate with CLD • Resonant frequencies 49

  50. Metal Plate with CLD • Mode shape @ 314 Hz 50

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