1. 1. Lee Chia Chun (KEM100017) session 2013/2014 Supervised by: Dr. Ong Zhi Chao 28-May 2014 (Wed)
2. 2. EMA (benchmark) Auto Impact Excitation Device To study the effects of impact force sensor of model 200C20 with and without rubber tip To study the effect of the use of a more sensitive force sensor of model 208C04 To study the effects of boundary condition Flow of Thesis
3. 3. Modal Identification using MEScope Obtain natural frequency, mode shapes and damping frequencies Results Comparison with EMA Finish OK NO Control of Auto Impact Excitation Device Modal Testing using DASYLab Obtain impact profiles Analyze parameters such as duty cycle, shape of impact period, impact contact time, impact magnitude Combination of block size, sampling rate and frequency
4. 4. Resonant vibration is the root cause of many mechanical failures Dynamic characteristics of a structure must be extracted to better understand structural vibrational problem Existing modal extraction techniques are: (1) EMA, (2) OMA and (3) ISMA In this study, the excitation of a structure is made using impulse excitation technique Introduction
5. 5. Current EMA Practice #experimental condition #Labor-intensive #Time-consuming #Incur machine downtime cost Practice OMA Curren #Lacks of input force informatio
6. 6. EMA OMA ISMA Presence of Ambient Force Cannot be conducted Can be conducted Can be conducted Input Contains Input data from excitation Does not contain input data from excitation Contains Input data from excitation Output Response Response Response Averaging Frequency domain Time domain Time/frequency domain Averaging technique Perform Frequency averaging after FFT Perform Time Averaging before FFT Perform Impact- Synchronous Time Averaging before FFT Comparison of Existing Methods
7. 7. utilizes ISTA before performing Fast Fourier Transform (FFT) to obtain its corresponding Frequency Response Function (FRF) Non-synchronous components like noises and other unaccounted signals are averaged out in the time domain before performing FFT, after few random repetitive impacts Waveforms that are synchronous with the reference tend to be reinforced Hence, ISMA can be performed in the presence of ambient forces while having the input force information Why ISMA
8. 8. Importance of Averages slowly diminish non- synchronous components reinforce structures response synchronous to the repetitive impact force due to the trigger (impact hammer) Impact force slightly higher than cyclic load could determine the dynamic characteristics successfully Too low impact force with reference to the operating cyclic loads will not excite the structure whereas too high impacts may result in non-linearity Importance of Impact Level Importance of Impact Frequency is the inverse of impact contact time contact time should be as small as possible
9. 9. has difficulty in extracting dynamic characteristics of a structure which is closer to the operating speed for high speed machines perform badly if the impact frequency in ISMA is synchronous with the running speed performs random impacts using manually operated impact hammer which is labour-intensive and time-consuming Manual procedures result in inconsistency in terms of impact contact time, impact period and impact level, as well as human errors e.g. double impact This gives rise to the need of automating ISMA Limitation of ISMA
10. 10. To control and synchronize the portable calibrated auto impact excitation device with virtual instruments To study the impact profiles generated by the auto impact excitation device which facilitates ISMA To compare and verify the dynamic characteristics obtained by auto impact excitation device to that obtained by EMA during non-rotating condition (benchmark) Objectives
11. 11. Obtain modal parameters under experimental conditions Conducted in complete shutdown mode Excitation force applied in the time domain, but the system responses are auto-correlated with the measured input (Peter Avitabile, 2001; Peres & Bono, 2011) The correlated functions are transformed into frequency domain to obtain the transfer functions (FRF) 2.1 Experimental Modal Analysis (EMA)
12. 12. = where and are n x 1 frequency vectors of accelerations and forces respectively. is an n x n square matrix of FRF of the system. It also regarded as accelerance (Hosseini, Arzanpour, Golnaraghi, & Parameswaran, 2013)
13. 13. Alternatively, can be written as, = =1 (Chao, 2013), in page 30, describes above constitutes a reciprocal theorem for dynamic loads that is similar to Maxwells reciprocal theorem for static loads So, it is OK to rove or fix any of the impact hammer or the force transducer
14. 14. By performing FRF on the continuous system in EMA, the formula to obtain the FRF = =1 2 + 2 + 2 mode shape coefficient, the undamped natural frequency and damping can be obtained by selecting a band of frequency around the region curve-fitting the FRF through best-fit methods such as Least Square method
15. 15. Linear superimposition of unaccounted responses with response due to trigger = 1 1 + 2 2 + 3 3 + 2.2 Conducting EMA during Operation
16. 16. Time Enhanced Time Enhanced Spectrum Auto- spectrum Input Trigger Averaging Analysis Squaring **Requires trigger signal to be synchronous with the periodic signal of interest (A G A Rahman, 2013) Auto-spectrum, = + 1 deterministic component, noise/unaccounted component, Importance of average number = 1 =0 1 ( + ) 2.3 Impact-Synchronous Time Domain Averaging Method
17. 17. create an impact through virtual instrument, at a shortest possible impact contact time that would automatically on and off periodically at constant and shortest possible impact period and constant impact level s 0 .00 0 .25 0 .50 0 .75 1 .00 1 .25 1 .50 1 .75 Y /tCha rt0 5 .0 2 .5 0 .0 -2 .5 -5 .0 2.4 Control of Auto Impact Excitation Device
18. 18. time response block is defined as the block size, BS over the sampling rate, SR: tblock = BS SR The period of square wave, T is defined as the inverse of frequency, f of the square wave: T = 1 f The number of cycle of square wave within the response time block is: n = tblock T (n = integers; otherwise, truncate decimals) Duty cycle is the percentage of one period in which a signal is active and is given by: tON = DC T The square wave signals can only moves along the time axis provided that there is a time difference, t. Hence, the condition of t 0 nT, must be met. Hence, is given by: t = tblock nT
19. 19. Next, the number of block which is On, N can be evaluated as: N = tON t (N = integers; otherwise, truncate decimals) The impact contact time, is evaluated by taking the number of blocks which is On, to multiply by the time response block, : Tpulse = tON t tblock = N tblock The impact period is determined by using: Tinterval = t tblock The inverse of Tinterval gives the impact frequency: fimpact = 1 Tinterval a heuristic method is adopted to determine a range of accepted combination of sampling rate-block size-frequency-duty cycle
20. 20. quantitative technique to compare the closeness between two families of mode shapes x(1) and x(2) (Allemang, 2003; Allemang & Brown, 1998; Peter Avitabile, 2001) MAC (() (1) , () (2) ) = () (1) () (2) () (1) () (2) 2 indicates whether there are enough measurement points for the modal analysis (Gaetan Kerschen, 2006) MAC Value Interpretation = 1.0 Two mode shapes are identical > 0.9 Two mode shapes are similar < 0.9 Two mode shapes are different 2.5 Modal Assurance Criteria (MAC)
21. 21. Goals: To acquire impact profiles of the test structure obtained by using the auto impact excitation to compare with EMA To obtain the dynamic characteristics of the test structure to compare and validated with EMA 3.1 Methodology
22. 22. Setup FRF = Curve Fit ISTA before FFT Overview Modal validation Data synthesis Mode shape visualization
23. 23. Setup 20 Degree of Freedoms (DOFs); 5 averages/DOF Non-rotating condition Fix auto impact excitation device Rove tri-axial accelerometer Set Description 1 EMA 2 Device w/o rubber Tip 3 Device with rubber Tip 4 Device with built-in force sensor (208C04) 5 Improved device isolated from test rigs boundary condition
24. 24. Setup Auto Impact connected to Channel 1 and 9, supplied with voltage d.c 24V Change sensitivity at Measurement & Automation Explorer by National Institute of version 3.1.1 Pre-Setting on DasyLAB Open DASYLab Pre-Setting (see here) Collect Data Collect vibration data at all 20 points Post- Processing MEScope to get FRF and animate mode shape and Cross MAC
25. 25. Pre-Setting Interface
26. 26. Too low/high Trigger level Cabling Avoid double impact Set a pre- trigger delay
27. 27. To control and synchronize the portable calibrated auto impact excitation device with virtual instruments To study the impact profiles generated by the auto impact excitation device which facilitates ISMA To compare and verify the dynamic characteristics obtained by auto impact excitation device to that obtained by EMA during non-rotating condition (benchmark) Objectives
28. 28. m s 0 .0 2 .5 5 .0 7 .5 10 .0 12 .5 15 .0 17 .5 20 .0 Y /tChart0 5 .0 2 .5 0 .0 -2 .5 -5 .0 Duty Cycle of 0.0050 (0.5%)
29. 29. h :m in :s 12 :39 :30 12 :39 :40 12 :39 :50 12 :40 :00 12 :40 :10 12 :40 :20 50 45 40 35 30 25 20 15 10 5 0 -5 R eco rde r0 6.5024 s35N
30. 30. Set 1 (EMA) Set 5 h :m in :s 13 :23 :10 .520 13 :23 :10 .530 13 :23 :10 .540 13 :23 :10 .550 60 50 40 30 20 10 0 -10 R eco rde r0 h:m in:s 12:40:07.250 12:40:07.255 12:40:07.260 12:40:07.265 12:40:07.270 50 45 40 35 30 25 20 15 10 5 0 -5 Recorder0 0.00342 s 0.00586 s Duty Cycle of 0.0050 (0.5%)
31. 31. Data acquisition time = 2.0 s Auto impact sampling Rate: 10,000 100,000 Auto impact block Size: < 2,048 Experimentally found that 2 10 s Sampling rate < 50,000 blocks/s yields a stable impact level number of block ON should be above 2 to get a stable impact level Display delay &/or bog down of DASYLab program due to: speed, memory & limited video capability of the computer complexity of the worksheet Taking duty cycle = 0.0050, a heuristic method is adopted
32. 32. Auto Impact Sampling Rate Auto Impact Frequency (Hz) n block ON Min. Impact Time (s) Min. Impact Period (s) 20,000 39.25 1 0.0256 5.3248 78.32 1 0.0256 5.12 30,000 58.88 1 0.017067 3.4816 58.74 2 0.034133 6.82667 40,000 78.51 1 0.0128 2.5856 78.32 2 0.0256 5.12 78.25 3 0.0384 8.00 50,000 98.14 1 0.01024 2.05824 97.9 2 0.02048 4.096 97.81 3 0.03072 6.5024 97.77 4 0.04096 8.78582 Least Impact Contact Time and Impact Period correspond to Block Size 512 and Duty Cycle of 0.005
33. 33. Start-up Parameters
34. 34. To control and synchronize the portable calibrated auto impact excitation device with virtual instruments To study the impact profiles generated by the auto impact excitation device which facilitates ISMA To compare and verify the dynamic characteristics obtained by auto impact excitation device to that obtained by EMA during non-rotating condition (benchmark) Objectives
35. 35. Qualitative comparison: Overlaid Frequency Response Function (FRF) Spectral Mode Shape Quantitative comparison: Difference in Natural Frequencies Modal Assurance Criteria (MAC)
36. 36. Comparison between Set 1 & Set 5
37. 37. Set 1 (EMA) Set 5 (Device Isolated from the Boundary Condition of the Test Rig) Comparison of Overlaid FRF between Set 1 &Set 5
38. 38. Set 1 Set 5 Set 1 Set 5 Natural Frequency (Hz) 10.5 10.5 Damping (Hz) 3.22 2.88 MAC 1.000 0.981 Comparison of Mode Shapes between Set 1 & Set 5 at Mode 1
39. 39. Set 1 Set 5 Set 1 Set 5 Natural Frequency (Hz) 16.5 16.4 Damping (Hz) 1.45 1.64 MAC 1.000 0.966 Comparison of Mode Shapes between Set 1 & Set 5 at Mode 2
40. 40. Set 1 Set 5 Set 1 Set 5 Natural Frequency (Hz) 28.6 28.4 Damping (Hz) 2.20 2.50 MAC 1.000 0.864 Comparison of Mode Shapes between Set 1 & Set 5 at Mode 3
41. 41. Mode (Hz) (Hz) (%) MAC 1 10.50 10.50 0.00 0.981 2 16.50 16.40 0.61 0.966 3 28.60 28.40 0.70 0.864 Summary of Natural Frequencies and Mode Shapes Comparison between Set 1 & Set 5 under Non-rotating Condition
42. 42. Result Summary from Set 2 - 5
43. 43. Comparison of Percentage Difference in Natural Frequencies between Set 1 and Auto Impact Sets (Set 2 5) at Three Natural Modes
44. 44. Comparison of Percentage Difference in Cross MAC between Set 1 and Auto Impact Sets (Set 2 5) at Three Natural Modes
45. 45. Enhanced ISMA that uses ISTA technique has successfully automated the conventional modal testing methods by utilizing to replace for operational modal testing purpose 5.1 Conclusion
46. 46. The enhanced ISMA can automatically deliver impact onto a structure at a consistent impact level over constant impact period, at a very small impact contact time to accurately and effortlessly acquire the dynamic characteristics of a test structure under non- rotating condition The impact profile can be changed by the auto impact sampling rate, block size, frequency and duty cycle readily with the use of auto impact excitation device
47. 47. Auto impact excitation device with the built-in of high sensitivity that is covered with rubber tip and is isolated from the boundary condition of the test structure is developed for operational modal testing purpose as its dynamic characteristics are highly comparable to the EMA (benchmark set)
48. 48. Perform the enhanced ISMA technique on a rotating structure for verification purpose Create a programming algorithm in the virtual instrument (DASYLab) to immediately stop the data acquisition process after the running components and noises are successfully filtered out Devise a practical way to isolate the auto impact excitation device from the boundary condition of a test structure 5.2 Recommendation
49. 49. Questions?
50. 50. Comparison between Different Values of Duty Cycle
51. 51. m s 0 .0 2 .5 5 .0 7 .5 10 .0 12 .5 15 .0 17 .5 20 .0 22 .5 25 .0 27 .5 30 .0 32 .5 35 .0 37 .5 40 .0 Y /tChart0 5 .0 2 .5 0 .0 -2 .5 -5 .0 Impact Profile when Duty Cycle = 0.50
52. 52. Impact Response Zoomed Impact Response h :m in :s 12 :39 :35 12 :39 :45 12 :39 :55 12 :40 :05 12 :40 :15 12 :40 :25 12 :40 :35 60 50 40 30 20 10 0 -10 R eco rde r0 h :m in :s 12 :40 :09 .70 12 :40 :09 .85 12 :40 :10 .00 12 :40 :10 .15 12 :40 :10 .30 12 :40 :10 .45 12 :40 :10 .60 60 50 40 30 20 10 0 -10 R eco rde r0
53. 53. m s 0 .0 2 .5 5 .0 7 .5 10 .0 15 .0 20 .0 25 .0 30 .0 35 .0 40 .0 Y /tChart0 5 .0 2 .5 0 .0 -2 .5 -5 .0 Impact Profile when Duty Cycle = 0.01
54. 54. Impact Response Zoomed Impact Response h :m in :s 12 :39 :25 12 :39 :35 12 :39 :45 12 :39 :55 12 :40 :05 12 :40 :15 12 :40 :25 60 50 40 30 20 10 0 -10 R eco rde r0 h :m in :s 12 :39 :59 .0 12 :39 :59 .5 12 :40 :00 .0 12 :40 :00 .5 12 :40 :01 .0 60 50 40 30 20 10 0 -10 R eco rde r0
55. 55. Comparison between Set 1 & Set 2
56. 56. h :m in :s 1 :46 :45 11 :46 :50 11 :46 :55 11 :47 :00 11 :47 :05 11 :47 :10 11 :47 :15 11 :47 :20 11 :47 :25 11 :47 :30 11 :47 :35 11 :47 :40 11 :47 :45 100 75 50 25 0 -25 Recorder0 h :m in :s 11 :46 :59 .85 11 :46 :59 .90 11 :46 :59 .95 11 :47 :00 .00 11 :47 :00 .05 11 :47 :00 .10 11 :47 :00 .15 11 :47 :00 .20 11 :47 :00 .25 100 75 50 25 0 -25 Reco rde r0 6.5024 s Impact Profile of Set 2 Presence of double impact
57. 57. Set 1 (EMA) Set 2 (without Rubber Tip) Comparison of Overlaid FRF between Set 1 & Set 2
58. 58. Set 1 Set 2 Set 1 Set 2 Natural Frequency (Hz) 10.5 9.92 Damping (Hz) 3.22 5.00 MAC 1.000 0.434 Comparison of Mode Shapes between Set 1 & Set 2 at Mode 1
59. 59. Set 1 Set 2 Set 1 Set 2 Natural Frequency (Hz) 16.5 15.6 Damping (Hz) 1.45 2.00 MAC 1.000 0.772 Comparison of Mode Shapes between Set 1 & Set 2 at Mode 2
60. 60. Set 1 Set 2 Set 1 Set 2 Natural Frequency (Hz) 28.6 24.1 Damping (Hz) 2.20 2.55 MAC 1.000 0.094 Comparison of Mode Shapes between Set 1 & Set 2 at Mode 3
61. 61. Mode (Hz) 2 (Hz) (%) MAC 1 10.50 9.92 5.52 0.434 2 16.50 15.6 5.45 0.772 3 28.60 24.1 15.73 0.094 Summary of Natural Frequencies and Mode Shapes Comparison between Set 1 & Set 2 under Non-rotating Condition
62. 62. Comparison between Set 1 & Set 3
63. 63. Impact Profile of Set 3 h :m in :s 12 :05 :10 12 :05 :15 12 :05 :20 12 :05 :25 12 :05 :30 12 :05 :35 12 :05 :40 100 75 50 25 0 -25 s 30 .00 30 .25 30 .50 30 .75 31 .00 31 .25 31 .50 31 .75 Y /tChart0 30 25 20 15 10 5 0 -5 6.5024 s Obvious impact spectrum is seen
64. 64. Set 1 (EMA) Set 3 (with Rubber Tip) Comparison of Overlaid FRF between Set 1 & Set 3
65. 65. Set 1 Set 3 Set 1 Set 3 Natural Frequency (Hz) 10.5 9.99 Damping (Hz) 3.22 4.56 MAC 1.000 0.959 Comparison of Mode Shapes between Set 1 & Set 3 at Mode 1
66. 66. Set 1 Set 3 Set 1 Set 3 Natural Frequency (Hz) 16.5 16.0 Damping (Hz) 1.45 1.66 MAC 1.000 0.942 Comparison of Mode Shapes between Set 1 & Set 3 at Mode 2
67. 67. Set 1 Set 3 Set 1 Set 3 Natural Frequency (Hz) 28.6 24.2 Damping (Hz) 2.20 1.54 MAC 1.000 0.336 Comparison of Mode Shapes between Set 1 & Set 3 at Mode 3
68. 68. Mode (Hz) 3 (Hz) (%) MAC 1 10.50 9.99 4.86 0.959 2 16.50 16.00 3.03 0.942 3 28.60 24.20 15.35 0.336 Summary of Natural Frequencies and Mode Shapes Comparison between Set 1 & Set 3 under Non-rotating Condition
69. 69. Comparison between Set 1 & Set 4
70. 70. Set 1 (EMA) Set 4 (Device uses Force Sensor of model 208C04) Comparison of Overlaid FRF between Set 1 & Set 4
71. 71. Set 1 Set 4 Set 1 Set 4 Natural Frequency (Hz) 10.5 10.2 Damping (Hz) 3.22 3.69 MAC 1.000 0.986 Comparison of Mode Shapes between Set 1 & Set 4 at Mode 1
72. 72. Set 1 Set 4 Set 1 Set 4 Natural Frequency (Hz) 16.5 16.0 Damping (Hz) 1.45 1.87 MAC 1.000 0.912 Comparison of Mode Shapes between Set 1 & Set 4 at Mode 2
73. 73. Set 1 Set 4 Set 1 Set 4 Natural Frequency (Hz) 28.6 24.6 Damping (Hz) 2.20 1.50 MAC 1.000 0.379 Comparison of Mode Shapes between Set 1 & Set 4 at Mode 3
74. 74. Mode (Hz) 4 (Hz) (%) MAC 1 10.50 10.20 2.86 0.986 2 16.50 16.00 3.03 0.912 3 28.60 24.60 13.99 0.379 Summary of Natural Frequencies and Mode Shapes Comparison between Set 1 & Set 4 under Non-rotating Condition
75. 75. Gantt Chart (Sem 1)
76. 76. Gantt Chart (Sem 2)