Excitation Techniques Do s and Don ts

Transcription

1 Peter Avitabile UMASS Lowell Excitation Techniques Do s and Don ts Marco Peres The Modal Shop 1 Dr. Peter Avitabile

2 Excitation Considerations Objectives of this lecture: Overview impact testing considerations part 1 Overview shaker testing considerations part 2 Identify some typical concerns Provide some examples IMAC 27 presentation covered shaker excitation techniques IMAC 29 presentation covered shaker testing considerations 2 Dr. Peter Avitabile

3 MODAL SPACE In Our Own Little World Could you explain modal analysis and how is it used for solving dynamic problems? Illustration by Mike Avitabile Illustration by Mike Avitabile Illustration by Mike Avitabile Series of articles on various aspects of modal analysis currently in its 15th continuous year of publication 3 Dr. Peter Avitabile

4 Measurement Definitions - Refresher INPUT INPUT INPUT LINEAR INPUT SPECTRUM ANALOG SIGNALS ANTIALIASING FILTERS AUTORANGE ANALYZER ADC DIGITIZES SIGNALS APPLY WINDOWS COMPUTE FFT LINEAR SPECTRA OUTPUT OUTPUT OUTPUT LINEAR OUTPUT SPECTRUM Actual time signals Analog anti-alias filter Digitized time signals Windowed time signals Compute FFT of signal AVERAGING OF SAMPLES INPUT POWER SPECTRUM COMPUTATION OF AVERAGED INPUT/OUTPUT/CROSS POWER SPECTRA CROSS POWER SPECTRUM OUTPUT POWER SPECTRUM Average auto/cross spectra COMPUTATION OF FRF AND COHERENCE Compute FRF and Coherence FREQUENCY RESPONSE FUNCTION COHERENCE FUNCTION 4 Dr. Peter Avitabile

5 Measurements - Linear Spectra - Refresher x(t) h(t) y(t) TIME INPUT SYSTEM OUTPUT FFT & IFT Sx(f) H(f) Sy(f) FREQUENCY x(t) y(t) Sx(f) Sy(f) H(f) h(t) - time domain input to the system - time domain output to the system - linear Fourier spectrum of x(t) - linear Fourier spectrum of y(t) - system transfer function - system impulse response 5 Dr. Peter Avitabile

6 Measurements - Power Spectra - Refresher Rxx(t) Ryx(t) Ryy(t) INPUT SYSTEM OUTPUT Gxx(f) Gxy(f) Gyy(f) TIME FFT & IFT FREQUENCY Rxx(t) Ryy(t) Ryx(t) - autocorrelation of the input signal x(t) - autocorrelation of the output signal y(t) - cross correlation of y(t) and x(t) * xx x x Gxx(f) - autopower spectrum of x(t) G ( f) S ( f) S ( f) * yy y y Gyy(f) - autopower spectrum of y(t) G ( f) S ( f) S ( f) * yx y x Gyx(f) - cross power spectrum of y(t) and x(t) G ( f) S ( f) S ( f) 6 Dr. Peter Avitabile

7 Measurements - Derived Relationships - Refresher Sy HS x H1 formulation - susceptible to noise on the input - underestimates the actual H of the system S y S * x HS x S * x S H S y x S S H2 formulation - susceptible to noise on the output - overestimates the actual H of the system S y S * y COHERENCE 2 xy HS (S (S x y x S * y S S * x * x )(S )(S x y S S * y * y ) ) G G S H S yx yy y x / G / G S S xx xy * x * x * y * y G G G G H H yx xx yy xy 1 2 Other formulations for H exist 7 Dr. Peter Avitabile

8 Typical Measurements - Refresher Measurements - Auto Power Spectrum Measurements - Cross Power Spectrum x(t) y(t) AVERAGED INPUT POWER SPECTRUM AVERAGED OUTPUT POWER SPECTRUM G (f) xx G (f) yy INPUT FORCE OUTPUT RESPONSE G (f) xx G (f) yy AVERAGED INPUT POWER SPECTRUM AVERAGED OUTPUT POWER SPECTRUM AVERAGED CROSS POWER SPECTRUM G (f) yx Measurement Definitions 12 Dr. Peter Avitabile Modal Analysis & Controls Laboratory Measurement Definitions 13 Dr. Peter Avitabile Modal Analysis & Controls Laboratory Measurements - Frequency Response Function Measurements - FRF & Coherence Coherence 1 Real AVERAGED INPUT AVERAGED CROSS AVERAGED OUTPUT POWER SPECTRUM POWER SPECTRUM POWER SPECTRUM 0 G (f) xx G (f) yx G (f) yy 0Hz AVG: 5 COHERENCE 200Hz Freq Resp 40 db Mag -60 0Hz AVG: 5 200Hz FREQUENCY RESPONSE FUNCTION FREQUENCY RESPONSE FUNCTION H(f) Measurement Definitions 14 Dr. Peter Avitabile Modal Analysis & Controls Laboratory Measurement Definitions 15 Dr. Peter Avitabile Modal Analysis & Controls Laboratory 8 Dr. Peter Avitabile

9 IMAC 30 - Jacksonville, FL Dr. Peter Avitabile

10 IMAC 30 - Jacksonville, FL Dr. Peter Avitabile

11 Impact Excitation Objectives of this lecture: Overview impact excitation techniques Review hammer/tip characteristics Review special DSP considerations Identify areas of concern and things to consider 11 Dr. Peter Avitabile

12 Impact Excitation An impulsive excitation which is very short in the time window usually lasting less than 5% of the sample interval. ADVANTAGES - easy setup - fast measurement time - minimum of equipment - low cost CONSIDERATIONS - poor rms to peak levels - poor for nonlinear structures - force/response windows needed - pretrigger delay needed - double impacts may occur - high potential for signal overload and underload of ADC 12 Dr. Peter Avitabile

13 Practical Modal Impact Test Checklist General Range settings for channels Frequency range bandwidth BW Hammer Pre-trigger settings Hammer tip selection Windows Response Windows FRF / Coherence Measurement considerations 13 Dr. Peter Avitabile

14 Impact Excitation - Hammer Tip Selection The force spectrum can be customized to some extent through the use of hammer tips with various hardnesses. A hard tip has a very short pulse and will excite a wide frequency range. A soft tip has a long pulse and will excite a narrow frequency range. However, the hammer tip alone does not totally determine the frequency range excited. The local flexibility of the structure must also be considered. 14 Dr. Peter Avitabile

15 Impact Excitation - Hammer Tip Selection METAL TIP HARD PLASTIC TIP Real Real us TIME PULSE ms us TIME PULSE ms db Mag db Mag 0Hz FREQUENCY SPECTRUM 6.4kHz 0Hz FREQUENCY SPECTRUM 6.4kHz SOFT PLASTIC TIP RUBBER TIP Real Real us TIME PULSE ms us TIME PULSE ms db Mag db Mag 0Hz FREQUENCY SPECTRUM 6.4kHz 0Hz FREQUENCY SPECTRUM 6.4kHz 15 Dr. Peter Avitabile

16 Impact Test Pretrigger Delay Sometimes Confusing If the leading portion of the time pulse is not captured then there will be a distortion of the measured input spectrum t = 0 NO PRETRIGGER USED t = 0 PRETRIGGER SPECIFIED 16 Dr. Peter Avitabile

17 Impact Test Double Impact Double impacts can occur due to a sloppy hammer swing or many times due to the responsive nature of many structures. They should be avoided wherever possible. DOUBLE IMPACT DOUBLE IMPACT Real Real us TIME PULSE ms us TIME PULSE ms db Mag db Mag 0Hz FREQUENCY SPECTRUM 800Hz 0Hz FREQUENCY SPECTRUM 800Hz 17 Dr. Peter Avitabile

18 Impact Excitation - Windows May Be Necessary If response does not die out then a window may be required ACTUAL TIME SIGNAL SAMPLED SIGNAL WINDOW WEIGHTING WINDOWED TIME SIGNAL 18 Dr. Peter Avitabile

19 Impact Excitation - Exponential Window If the signal does not naturally decay within the sample interval, then an exponentially decaying window may be necessary. However, many times changing the signal processing parameters such as bandwidth and number of spectral lines may produce a signal which requires less window weighting T = N D t T = N D t 19 Dr. Peter Avitabile

20 Impact Excitation Force & Exponential Window 20 Dr. Peter Avitabile

21 Impact Excitation - Right Hammer for the Test Measurement adequacy depends on what is required 40 COHERENCE db Mag FRF INPUT POWER SPECTRUM -60 0Hz 800Hz 40 COHERENCE FRF db Mag INPUT POWER SPECTRUM -60 0Hz 200Hz 21 Dr. Peter Avitabile

22 Impact at One Point Listen at Another What FRF? H out/in = H row/col 22 Dr. Peter Avitabile

23 Impact at One Point Listen at Another What FRF? Dr. Peter Avitabile

24 Drive Point Measurements Drive point measurement Same input and output location in the same direction Dr. Peter Avitabile

25 Reciprocity - H out/in = H ij Reciprocity is an underlying necessity for modal theory Dr. Peter Avitabile

26 Reciprocity - H out/in = H ij Reciprocity is an underlying necessity for modal theory H out/in H ij 26 Dr. Peter Avitabile

27 Reciprocity - H out/in = H ij - What can go wrong? Dr. Peter Avitabile

28 Impact Test Multiple Reference Impact Test Either a row or column of the FRF matrix is needed to estimate mode shapes Ref#1 Ref#2 Ref#1 Ref#2 Ref#3 Ref#3 28 Dr. Peter Avitabile

29 Shaker Test vs. Impact Test What is the difference? Typical Shaker Test Typical Impact Test h h 23 3 h 31 h 33 h 33 h Dr. Peter Avitabile

30 Measurement Definitions June 1998 Modal Space Articles 30 Dr. Peter Avitabile

31 FRF from Impact or Shaker Data Impact Data Shaker Data ANALOG SIGNALS ANALOG SIGNALS INPUT OUTPUT INPUT OUTPUT ANTIALIASING FILTERS ANTIALIASING FILTERS AUTORANGE ANALYZER ADC DIGITIZES SIGNALS AUTORANGE ANALYZER ADC DIGITIZES SIGNALS INPUT OUTPUT INPUT OUTPUT APPLY WINDOWS APPLY WINDOWS INPUT OUTPUT INPUT OUTPUT COMPUTE FFT LINEAR SPECTRA COMPUTE FFT LINEAR SPECTRA LINEAR INPUT SPECTRUM LINEAR OUTPUT SPECTRUM LINEAR INPUT SPECTRUM LINEAR OUTPUT SPECTRUM AVERAGING OF SAMPLES AVERAGING OF SAMPLES COMPUTATION OF AVERAGED INPUT/OUTPUT/CROSS POWER SPECTRA COMPUTATION OF AVERAGED INPUT/OUTPUT/CROSS POWER SPECTRA INPUT POWER SPECTRUM CROSS POWER SPECTRUM OUTPUT POWER SPECTRUM INPUT POWER SPECTRUM CROSS POWER SPECTRUM OUTPUT POWER SPECTRUM COMPUTATION OF FRF AND COHERENCE COMPUTATION OF FRF AND COHERENCE FREQUENCY RESPONSE FUNCTION COHERENCE FUNCTION FREQUENCY RESPONSE FUNCTION COHERENCE FUNCTION 31 Dr. Peter Avitabile

32 IMAC 30 - Jacksonville, FL Dr. Peter Avitabile

33 FFT Reference vs. Modal Reference Confusing Nomenclature SHAKER TEST ROVING IMPACT TEST Reference means different things to different people That is why there is a swap HP35665 FFT Analyzer 002Z006Z.DAT OUT PUT USB Accelerometer FILTER IN OUT DYNAMIC SIGNAL ANALYZER Impact Hammer y 5 z 6 x H ij 33 Dr. Peter Avitabile

34 Why Do Initial Conditions Need to be Zero? Laplace Domain Equation of Motion 2 ( ms cs k) x(s) f (s) (ms c)x mx 0 0 Characteristic Portion Applied Force Initial Displacement Initial Velocity Assuming that initial conditions are zero (ms 2 cs k) x(s) f (s) 34 Dr. Peter Avitabile

35 Why Do Initial Conditions Need to be Zero? ACTUAL IMPACT RESPONSE USER PERCEPTION SAMPLE CAPTURED RESPONSE SAMPLE CAPTURED WINDOWED SAMPLE CAPTURED 35 Dr. Peter Avitabile

36 Why Do Initial Conditions Need to be Zero? SAMPLE CAPTURED SAMPLE CAPTURED WITH RINGING OF FIRST SAMPLE SAMPLE CAPTURED WITH RINGING OF FIRST AND SECOND SAMPLE 36 Dr. Peter Avitabile

37 Too Hard a Hammer Tip Can Cause Problems Energy is imparted to the structure beyond the frequency range of interest and may overload or saturate the response 40 db Mag 128 HZ BW Hz 800Hz db Mag 50 HAMMER TIP db Mag Hz EXCITES MODES OUTSIDE BAND OF INTEREST 400Hz 0Hz 200Hz db Mag 40 db Mag -50 0Hz 400Hz -60 0Hz 200Hz 37 Dr. Peter Avitabile

38 Impact Spectrum Considerations Selecting the right impact tip to excite the right frequency range is critical to optimizing the measured response 128 HZ BW INFORMATION BEYOND BW -30 STRONG RESPONSE WEAK RESPONSE db Mag VOLT ENERGY 1.5 VOLT ENERGY db Mag 4.0 VOLT ENERGY 0.1 VOLT ENERGY 0Hz 128Hz Hz 800Hz db Mag VOLT ENERGY 3.0 VOLT ENERGY db Mag 0Hz 128Hz VOLT ENERGY 0.5 VOLT ENERGY 0Hz 800Hz 38 Dr. Peter Avitabile

39 Exponential Window Can It Be a Problem? While a window may be ultimately required, never start with the window applied before the raw measurement is reviewed V Real 2.5 V Real V ms TIME PULSE WINDOWED RESPONSE ms How many peaks are observed in the measured FRF V ms 50 db Mag ms -50 0Hz FREQUENCY RESPONSE FUNCTION 400Hz 39 Dr. Peter Avitabile

40 Exponential Window Can It Be a Problem? Here is a measurement where a significant amount of damping is applied to the measurement V Real 1.2 V Real V ms RAW TIME RESPONSE WINDOWED RESPONSE ms How many peaks are observed in the measured FRF mv ms ms db Mag -25 0Hz FREQUENCY RESPONSE FUNCTION 400Hz 40 Dr. Peter Avitabile

41 Exponential Window Can It Be a Problem? Picking a longer time block allows the response to naturally decay and lessens the need of the exponential window V Real V Real RAW TIME RESPONSE V ms s WINDOWED RESPONSE V ms s How many peaks are observed in the measured FRF. 50 db Mag -50 FREQUENCY RESPONSE FUNCTION 0Hz 400Hz TWO CLOSELY SPACED MODES 41 Dr. Peter Avitabile

42 Exponential Window Can It Be a Problem? Window should only be applied once it is deemed necessary 3.5 V Real TIME PULSE 2.5 V Real RAW TIME RESPONSE 2.5 V Real RAW TIME RESPONSE -1.5 V ms ms -2.5 V ms ms -2.5 V ms s 2.5 V Real WINDOWED RESPONSE 1.2 V Real WINDOWED RESPONSE 2.5 V Real WINDOWED RESPONSE -2.5 V ms ms -800 mv ms ms -2.5 V ms s db Mag db Mag db Mag -50 FREQUENCY RESPONSE FUNCTION -25 FREQUENCY RESPONSE FUNCTION -50 FREQUENCY RESPONSE FUNCTION 0Hz 400Hz 0Hz 400Hz 0Hz 400Hz TWO CLOSELY SPACED MODES 42 Dr. Peter Avitabile

43 Double Impacts A Problem Or is it??? Picking a poor measurement location avoids the double impact but does the measurement look better? 43 Dr. Peter Avitabile

44 Double Impact Common Difficulty October 2008 Modal Space Articles 44 Dr. Peter Avitabile

45 If you can t avoid double impact what about multiple impacts October 2008 Modal Space Articles 45 Dr. Peter Avitabile

46 So if you can t avoid double impact what about multiple October 2008 Modal Space Articles 46 Dr. Peter Avitabile

47 So if you can t avoid double impact what about multiple October 2008 Modal Space Articles 47 Dr. Peter Avitabile

48 Multiple Impacts A Possibility!!! Single Impact Multiple Impacts October 2008 Modal Space Articles 48 Dr. Peter Avitabile

49 Should I look at all the measurements? This measurement looks fine but do all look this good? FORCE SPECTRUM COHERENCE IMPACT EXCITATION TIME RESPONSE ACCELEROMETER RESPONSE DRIVE POINT FREQUENCY RESPONSE FUNCTION 49 Dr. Peter Avitabile

50 Should I look at all the measurements? Here s a measurement that doesn t look as good as the rest. COHERENCE IMPACT EXCITATION FORCE SPECTRUM TIME RESPONSE ACCELEROMETER RESPONSE FREQUENCY RESPONSE FUNCTION 50 Dr. Peter Avitabile

51 All measurements should be reviewed IMPACT EXCITATION TIME RESPONSE ACCELEROMETER RESPONSE FORCE SPECTRUM COHERENCE COHERENCE FORCE SPECTRUM DRIVE POINT FREQUENCY RESPONSE FUNCTION FREQUENCY RESPONSE FUNCTION 51 Dr. Peter Avitabile

52 Filter Ring Sometimes there can be some ringing on the impact input. This is referred to as filter ring Depending on the bandwidth and impact spectrum, this may or may not appear on the measured data The following slide shows the effects of this phenomena 52 Dr. Peter Avitabile

53 Filter Ring 400 HZ BANDWIDTH SETTING 1600 HZ BANDWIDTH SETTING RED AIR CAPSULE RED AIR CAPSULE BLUE PLASTIC BLUE PLASTIC WHITE PLASTIC WHITE PLASTIC BLACK METAL BLACK METAL 53 Dr. Peter Avitabile

54 2KHz excitation for 500 Hz BW Sometimes data may be collected for multiple purposes. One group wants data to 500Hz and another needs 2KHz. Can a test be constructed with one set of accelerometers to acquire the data for both test ranges? Difficult to achieve unless you have infinite resolution and infinite spectral resolution. 54 Dr. Peter Avitabile

55 2KHz excitation for 500 Hz BW AVERAGED INPUT POWER SPECTRUM AVERAGED CROSS POWER SPECTRUM AVERAGED INPUT POWER SPECTRUM AVERAGED CROSS POWER SPECTRUM COHERENCE FUNCTION COHERENCE FUNCTION ALLOWS MORE SENSITIVE LOW FREQUENCY ACCELEROMETER TO BE USED TO MEASURE SYSTEM FREQUENCY RESPONSE FUNCTION HIGH FREQUENCY ACCELEROMETER REQUIRED TO MEASURE SYSTEM FREQUENCY RESPONSE FUNCTION 1 KHz 2 KHz 1 KHz 2 KHz COHERENCE FUNCTION FREQUENCY RESPONSE FUNCTION 55 Dr. Peter Avitabile 1 KHz Structural Dynamics 2 & KHz Acoustic Systems Lab

56 Impact Location Effects Skewed and Same Point When performing impact testing it is important to impact the same point in the same direction for all averages. One case will be presented to show the effects of having a skewed input, that is different for each average of the measurement. Another case is presented to show the effects of impacting close to the same point, but not exactly the same point, for all averages. 56 Dr. Peter Avitabile

57 Impact Location Effects Notice that the coherence for the skewed input is not as good as the measurement with consistent input excitation Good Measurement Skewed/Angle Impact 57 Dr. Peter Avitabile

58 Impact Location Effects Notice that the coherence for the impact around point is not as good as the measurement with consistent input excitation Good Measurement Impact Around Point 58 Dr. Peter Avitabile

59 Control the Location of Excitation STRAW SLEEVE HAMMER SWIVEL JOINT CLAMP Adaptor for small impact hammer enables easy orientation of hammer to impact structure using swivel joint on small tripod fixture. Current design uses a straw sleeve adapted to connector to swivel joint on tripod. 59 Dr. Peter Avitabile

60 Impact Location Control of input point and direction is very important 60 Dr. Peter Avitabile

61 Accelerometer Saturation But No Overload Sometimes the response transducer may be too sensitive which generally may cause an overload. But there are times when the accelerometer and the signal conditioner may not overload the data acquisition system BUT may be distorted due to saturation of the signal conditioner. 61 Dr. Peter Avitabile

62 Accelerometer Saturation But No Overload Accelerometer too sensitive Accelerometer with proper sensitivity 62 Dr. Peter Avitabile

63 Analyzer ICP / External ICP / DC Accelerometer Comparison DC 200mv/g DC 1V/g ICP 1V/g 63 Dr. Peter Avitabile

64 Analyzer ICP / External ICP / DC Accelerometer Comparison F F F FRF 200 mv DC:+Z/Force - 75%:+Z FRF 1 V ICP:+Z/Force - 75%:+Z FRF 1 V DC:+Z/Force - 75%:+Z 1.00 g/lbf db Amplitude F F F FRF 200 mv DC:+Z/Force - 75%:+Z FRF 1 V ICP:+Z/Force - 75%:+Z FRF 1 V DC:+Z/Force - 75%:+Z Hz g/lbf db Amplitude Hz Dr. Peter Avitabile

65 How Hard Should I Hit Air Capsules The hammer kits normally have the ability to use a variety of different tips to customize the input spectrum. But what happens if some impacts are harder and some are softer? Does this affect the input excitation spectrum? Depending on the hammer tip, this can be significant. 65 Dr. Peter Avitabile

66 Air Capsule Plastic Cap on Hard White Tip Hard White Tip How Hard Should I Hit February2010 Time Pulse Time Pulse Time Pulse H A R D H I T M E D I U M H I T S O F T H I T db 240 Hz db 300 Hz db 20 db Time Pulse 20 db Time Pulse 10 db db 220 Hz db 340 Hz db 20 db Time Pulse 20 db Time Pulse 8 db db 2500 Hz db 2700 Hz db 20 db Time Pulse 20 db Time Pulse 20 db I N C R E A S I N G I M P A C T H A M M E R F O R C E L E V E L February 400 Hz 2010 Modal Space Articles 400 Hz 3000 Hz 66 Dr. Peter Avitabile I N C R E A S I N G H A M M E R T I P H A R D N E S S

67 Selection of Measurement Locations So what are the chances that you would pick 9 of the worst possible measurement locations for a plate??? August 1998 Modal Space Articles 67 Dr. Peter Avitabile

68 Selection of Measurement Locations August 1998 Modal Space Articles 68 Dr. Peter Avitabile

69 The Modal Question Do these two test yield the same modal information? SETUP 1 STATIONARY TRI-AX AT ROVING IMPACT IN Z ONLY STATIONARY IMPACT AT 9 IN Z ONLY 9 6 SETUP ROVING TRI-AX 3 August 1998 Modal Space Articles 69 Dr. Peter Avitabile

70 The Modal Question SETUP 1 SETUP 2 x STATIONARY TRI-AX AT 9 z 9 y ROVING IMPACT IN Z ONLY 2 4 1x 1y 1z 1 9 STATIONARY IMPACT AT 9 IN Z ONLY ROVING TRI-AX 4 x 2 1x 1y 1 z y 2x 1z THREE PARTIAL ROWS OF FRFS 1x 1y 1z 2x 2y 2z 3x 3y 3z 9x 9y 9z 2y 2z 3x 3y 3z 9x 9y 9z ONE FULL COLUMN OF FRFS 2x 2y 2z 3x 3y 3z 9x 9y 9z 1x 1y 1z 2x 2y 2z 3x 3y 3z 9x 9y 9z August 1998 Modal Space Articles 70 Dr. Peter Avitabile

71 IMAC 30 - Jacksonville, FL Dr. Peter Avitabile

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73 73 Dr. Peter Avitabile

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75 Reference Selection Where Should Reference Be Located? Random Point Selection Organized Point Selection 75 Dr. Peter Avitabile

76 Composite Plate Pseudo-Repeated Root Example A plate structure with suspected pseudo-repeated roots was tested to determine the appropriate reference locations 13 Z 15 Z 3Z 76 Dr. Peter Avitabile

77 Composite Plate Pseudo-Repeated Root Example A summation plot and typical drive point FRFS are shown 77 Dr. Peter Avitabile

78 Composite Plate Pseudo-Repeated Root Example Using all 3 references, TRIP identifies a repeated root Note: plot only to 500 Hz 78 Dr. Peter Avitabile

79 Composite Plate Pseudo-Repeated Root Example Using references 3 Z & 15 Z, TRIP identifies repeated root 79 Dr. Peter Avitabile

80 Composite Plate Pseudo-Repeated Root Example Using references 13 Z & 15 Z, TRIP identifies repeated root 80 Dr. Peter Avitabile

81 Composite Plate Pseudo-Repeated Root Example Using references 3 Z & 13 Z, does not!!!!!!! 81 Dr. Peter Avitabile

82 IMAC 30 - Jacksonville, FL Dr. Peter Avitabile

83 Shaker Excitation Objectives of this lecture: Overview shaker testing considerations Identify some typical set up concerns Provide some examples IMAC 27 presentation covered excitation techniques IMAC 29 presentation covered shaker testing considerations 83 Dr. Peter Avitabile

84 Excitation Configuration Shaker Test Signal -random -burst Random -pseudo-random -periodic-random -Chirp Stinger force sensor AUTORANGING structure AVERAGING Power Amplifier AUTORANGING AVERAGING WITH WINDOW AUTORANGING AVERAGING Dr. Peter Avitabile

85 Reason for Stinger Purpose of Stinger Decouple shaker from test structure Force transducer between stinger and structure decouple forces acting in the axial direction only Forces acting in any other direction will be unaccounted for creating error in the measurements Modal Shaker Axial Bending Force Gage Stinger Structure 85 Dr. Peter Avitabile

86 Possible Problems with Stinger Location of stinger on structure may be affected by the local stiffness and/or structure deformation Axial stiffness Axial and bending stiffness 86 Dr. Peter Avitabile

87 Stinger Configuration with Through Hole Shaker 2-part chuck assembly Force sensor Modal Exciter collet Test Structure armature stinger 87 Dr. Peter Avitabile

88 Multiple Input Shaker Excitation Objectives of this part of lecture: Identify some basics of MIMO testing Discuss several practical aspects of multiple input multiple output shaker testing 88 Dr. Peter Avitabile

89 Multiple Input Shaker Excitation Provide a more even distribution of energy Simultaneously excite all modes of interest Multiple columns of FRF matrix acquired More consistent data is collected Same test time as SISO case 89 Dr. Peter Avitabile

90 Excitation Considerations - MIMO Multiple referenced FRFs are obtained from MIMO test Energy is distributed better throughout the structure making better measurements possible Ref#1 Ref#2 Ref#3 90 Dr. Peter Avitabile

91 Excitation Considerations - MIMO Large or complicated structures require special attention 91 Dr. Peter Avitabile

92 Excitation Considerations - MIMO Multiple shakers are needed in order to adequately shake the structure with sufficient energy to be able to make good measurements for FRF estimation 92 Dr. Peter Avitabile

93 Frequently Asked Questions Objectives of this part of lecture: Provide some measurements to illustrate issues Revisit reciprocity Compare impedance head vs force/accelerometer Compare MIMO measurements 93 Dr. Peter Avitabile

94 SISO vs MIMO Excitation technique is one necessary step to acquire better measurements (random/hann vs burst random). But using MIMO instead of SISO is another important consideration. And mass loading effects are also important (a) (b) (c) (d) S I S O S I S O M I M O M I M O RANDOM WITH HANNING BURST RANDOM RANDOM WITH HANNING BURST RANDOM 94 Dr. Peter Avitabile

95 Shaker Mass Loading Effects MIF MIF SUM BLOCKS & STABILITY DIAGRAM SUM BLOCKS & STABILITY DIAGRAM 95 Dr. Peter Avitabile

96 Shaker Mass Loading Effects Three Measurement Setups Compare Repeated Root: No Mass Compensation Mass Compensation All Accels Mounted Accelerometer and Mounting Cube Equivalent Mass 96 Dr. Peter Avitabile

97 What s an impedance head? Why use it? Where does it go? An impedance head is a transducer that measures both force and response in one device. This is a critical measurement for the structure and it is strongly advised that impedance heads be used in all cases. A combination of a separate force gage and accelerometer is often used but time and time again this measurement has been seen to never be better than that obtained with an impedance head. The force gage or impedance head needs to be mounted on the structure side of the stinger arrangement. 2-part chuck assembly Force sensor collet armature stinger 97 Dr. Peter Avitabile

98 Test Set Up Measurements taken to show difference in set up Incorrect Correct X Shaker Impedance Quill Structure Shaker Quill Impedance Structure 98 Dr. Peter Avitabile

99 What is the proper mounting technique for the force transducer? X Drive-point FRFs g/n db Amplitude g/n db Amplitude X Hz Hz Distinct difference in drive point FRF based on force configuration! 99 Dr. Peter Avitabile

100 Drive Point FRF Stinger Effects No sleeves With Sleeves 100 Dr. Peter Avitabile

101 Drive Point FRF Stinger Effects No sleeves With Sleeves 101 Dr. Peter Avitabile

102 Differences in Reciprocal Measurements Impedance vs Accel Top View Offset Accelerometer Accelerometer on Other Face of Structure Impedance Head Pt. 2 Bottom View Pt. 1 Measurement locations All reciprocity measurements are between points 1 & 2 with respect to force from impedance heads 102 Dr. Peter Avitabile

103 Reciprocal Measurements Offset Accelerometer What if I can only put the accelerometer next to the force gage? Accel Force 103 Dr. Peter Avitabile

104 Reciprocal Measurements Accelerometer on Other Face of Structure What if I can only put the accelerometer on the face of the structure that is opposite the force gage? Accel Force 104 Dr. Peter Avitabile

105 Reciprocal Measurements Impedance Head What if I have an impedance head that measures force and acceleration at the same place? Force & Accel 105 Dr. Peter Avitabile

106 What is the correct amplitude level for modal testing applications? The excitation levels for modal testing are usually very low. There is no need to provide large force levels for conducting a modal test especially if appropriate response transducers (accelerometers) are selected with good sensitivity. The level only need be sufficient to make good measurements. 106 Dr. Peter Avitabile

107 What is the correct amplitude level for modal testing applications? The excitation levels for modal testing are usually very low. There is no need to provide large force levels for conducting a modal test especially if appropriate response transducers (accelerometers) are selected with good sensitivity. The level only need be sufficient to make good measurements. In fact large force levels tend to overdrive the structure and can excite nonlinear characteristics of the structure and provide overall poorer measurements than with lower level force tests. 107 Dr. Peter Avitabile

108 30.00 (m/s 2 )/N db FRF 2:+Z/2:+Z MIMO FRF 2:+Z/2:+Z SIMO Amplitude / (m/s 2 )/N db FRF 2:+Z/2:+Z MIMO FRF 2:+Z/2:+Z SIMO Amplitude / What is the correct amplitude level for modal testing applications? Hz Hz High excitation level degrades drive point FRF quality!!! 108 Dr. Peter Avitabile

109 What is the correct amplitude level for modal testing applications? (m/s 2 )/N db FRF 4:-Z/2:+Z MIMO FRF 4:-Z/2:+Z SIMO Amplitude / Hz High excitation level degrades drive point FRF quality and measurements across components may be worse!!! 109 Dr. Peter Avitabile

110 How many shakers should I use in my modal test? The number of shakers is often a difficult one to answer. Basically there are never enough shakers when conducting a large modal test. Often we are limited by the total number of shakers available in the test lab for modal testing. Usually two shakers are sufficient for many tests. Sometimes three or four shakers are needed for much larger structures. But generally more than five shakers are rarely used. The main point is that there needs to be enough shakers acting as reference locations that are positioned so that all of the modes of the structure are adequately excited and good frequency response measurements are obtained. 110 Dr. Peter Avitabile

111 How many shakers should I use in my modal test? (m/s 2 )/N db Amplitude (m/s 2 )/N db db SISO MIMO Hz Hz Single input may not be able to provide accurate FRFs 111 Dr. Peter Avitabile

112 Why bother with MIMO testing? Why not run a SISO instead? Single shaker testing is adequate providing all the modes of the structure can be sufficiently excited and measured. In component testing this can often times be sufficient. However, when structures have several components, then the ability to provide sufficient excitation to acquire good measurements across the whole structure may be difficult. Tests can be conducted with a single shaker that is moved to different reference locations but generally this does not provide consistently related measurements. When this is the case (as it often is), then MIMO is needed. 112 Dr. Peter Avitabile

113 Why bother with MIMO testing? Why not run a SISO instead? FRFs Using SIMO vs FRFs Using MIMO Blue Shaker is Reference for SIMO & MIMO FRF 1:+Z/2:+Z MIMO FRF 1:+Z/2:+Z SIMO (m/s 2 )/N db (m/s 2 )/N db (m/s 2 )/N db FRF 2:+Z/2:+Z MIMO FRF 2:+Z/2:+Z SIMO FRF 3:+Z/2:+Z MIMO FRF 3:+Z/2:+Z SIMO Am Am Am 113 Dr. Peter Avitabile

114 Why bother with MIMO testing? Why not run a SISO instead? SIMO 0.96 (m/s 2 )/N db Amplitude Hz Dr. Peter Avitabile

115 Why bother with MIMO testing? Why not run 3 SISO instead? 115 Dr. Peter Avitabile

116 Why bother with MIMO testing? Are modal results better? SIMO Tests Combined Overall Stabilization Plot 100 th order polynomial 116 Dr. Peter Avitabile

117 Why bother with MIMO testing? Are modal results better? SIMO Tests Combined Stabilization Plot (Close-up) 100 th order polynomial 117 Dr. Peter Avitabile

118 Why bother with MIMO testing? Are modal results better? Individual SIMO Tests Stabilization Plots (Close-up) Reference Point 1 Reference Point th order polynomial Reference Point Dr. Peter Avitabile

119 Why bother with MIMO testing? Are modal results better? SIMO Tests Combined Overall Stabilization Plot 100 th order polynomial 119 Dr. Peter Avitabile

120 Why bother with MIMO testing? Are modal results better? SIMO MIMO 1 Volt 0.1 Volt 0.1 Volt 0.1 Volt 120 Dr. Peter Avitabile

121 Why bother with MIMO testing? Are modal results better? MIMO Test Overall Stabilization Plot 100 th order polynomial 121 Dr. Peter Avitabile

122 Why bother with MIMO testing? Are modal results better? MIMO Test Stabilization Plot (Close-up) 100 th order polynomial 122 Dr. Peter Avitabile

123 Experimental Modal - Considerations and Use Several items are very important Test Setup boundary conditions Excitation Methods accurate measurements Parameter Extraction accurate parameters Dynamic Model Development (some personal notes from experience) 123 Dr. Peter Avitabile

124 Things to Consider Test Setup Pre-Test helps in so many ways but be careful to not fully rely on the model to be correlated Do everything possible to make the best possible measurements (or the original sin results) Make sure that all measurements are consistent Be aware of all boundary conditions such as support structure, shaker stinger interaction, instrumentation effects on structure 124 Dr. Peter Avitabile

125 Things to Consider Test Setup Check for overloads and underloads of transducer Check for saturation of signal conditioning Check linearity of structure Check mass loading effects of transducers Check frequency shifts due to support condition Check frequency resolution for measurements 125 Dr. Peter Avitabile

126 Things to Consider Measurements Check every measurement including input/output time traces, power spectrum, frequency response function and coherence Check reciprocity where possible Repeat drive point measurements on test that require multiple sets of data to completely describe all points on the structure 126 Dr. Peter Avitabile

127 Things to Consider Impact Technique Check FRF with different tips, over different frequency ranges with different resolutions Maintain consistent force level for measurements Impact the same point in the same direction for each measurement Compare different number of averages to determine convergence to FRF 127 Dr. Peter Avitabile

128 Things to Consider Shaker Excitation Technique Check force/quill alignment to prevent any overturning moments on force gage Check reciprocity on MIMO tests Try multiple excitation techniques to determine what technique works best Check drive point FRFs when multiple banks of data are collected 128 Dr. Peter Avitabile

129 IMAC 30 - Jacksonville, FL Peter Avitabile UMASS Lowell Excitation Techniques Do s and Don ts Marco Peres The Modal Shop Impact Animation 129 Dr. Peter Avitabile