Good Modal Practices

Transcription

1 Good Modal Practices Introduction Transducer Considerations Proper Excitation Ensuring Data Gathered is Good Post Processing Tips and Tricks Wrap Up Dr. C. Novak University of Windsor Good Modal Practices 1

2 Sources of Machine Vibration Vibration level 1 st 2 nd 3 rd 4 th 5 th 6 th 7 th 8 th 9 th 12 th Order Frequency, Hz (Frequency, Hz) Good Modal Practices 2

3 Forcing Functions and Structural Resonances Forcing Functions x Structural Characteristics = Real World Response F(f) H(f) X(f) x = Frequency Forced Motors Gearboxes Torsional Vibrations Frequency Structural Resonances of structure Frequency Real World Forced Vibration Structural Vibration External Noise Good Modal Practices 3

4 Good Modal Practices Introduction Transducer Considerations Proper Excitation Ensuring Data Gathered is Good Post Processing Tips and Tricks Wrap Up Good Modal Practices 4

5 Accelerometers Considerations: Type Weight Sensitivity Mounting Triax vs. Uniax Good Modal Practices 5

6 Types of Piezoelectric Accelerometers Centre-mounted Compression S Planar Shear P Theta Shear M P B M R B M R P B Delta Shear P R M B Annular Shear E M P B OrthoShear P M R E B P: Piezoelectric Elements E: Built-in Electronics S: Spring R: Clamping Ring B: Base M: Seismic Mass Good Modal Practices 6

7 Weight and Mass Loading 0,1 pc/ms g M > 7 g Dynamic Mass < 1 10 M 10 pc/ms g M > 600 g M 1000 pc/ms g M > 5 kg Remember! Mass Loading is cumulative Good Modal Practices 7

8 Choosing an Accelerometer Smaller Accel = Higher Freq. range, less sensitive Larger Accel = Lower Freq. range, more sensitive Acceleration ms ,000 20, , mv/ms -2 Weight: gram mv/ms -2 Weight: g Frequency ~0.1 ~ k 15-30k Hz Good Modal Practices 8

9 Mounting Techniques Thin double adhesive tape Cementing stud Stud Mounting Level db Beeswax Max.40 C k 2k 5k 10k 20k 30k 50 khz Frequency Good Modal Practices 9

10 Good and Bad Mounting Responses Properly Mounted Response Off Axis: Too much Glue Mounted with a human hair! Good Modal Practices 10

11 Mounting Clip Mounting Clip UA 1407 Weight 0.4g Good Modal Practices 11

12 Mounting Clip Mounting Clip UA 1475 Weight 0.7g Upper limit frequency 10% 4507 with grease 3 khz 4507 dry 1.5kHz 4508 with grease 4 khz 4508 dry 2 khz Good Modal Practices 12

13 Swivel Base Mounting Clip Mounting clip with Swivel Base UA 1478 Weight 0.8g Upper limit frequency 10% with grease khz khz Good Modal Practices 13

14 Triax vs. Single Axis Single Axis Easier to work with Come in a variety of weights and sensitivities Only measures one axis Triaxial Measures X, Y, Z simultaneously Balance between size and sensitivity More bookkeeping involved Good Modal Practices 14

15 Environmental Effects Good Modal Practices 15

16 Triboelectric Noise Charge Accelerometers AO 0122: Industry benchmark for low triboelectric noise Good Modal Practices 16

17 Modal Testing - Accelerometers Information about the structural dynamic properties of the test object Low frequency transfer function measurements (excitation* and response) Low frequency response measurements* Performed at all stages from component level up to full dressed body Requirements High sensitivity Low noise Flat phase response Low weight to minimise mass loading Easy 3 D mounting possibilities (fast) Clear marking for orientation Convenient for use with high channel counts (TEDs) Good Modal Practices 17

18 Modal Testing - Accelerometers Current industry benchmarks 4506 Series Series 4507 & 4508 Series and introducing the future industry benchmarks (TEDS version to come) 4517 family 4 Types Best selection on the planet, you ll have to leave earth to do better Good Modal Practices 18

19 Good Modal Practices Introduction Transducer Considerations Proper Excitation Ensuring Data Gathered is Good Post Processing Tips and Tricks Wrap Up Good Modal Practices 19

20 Two Ways to Excite a Structure a F shaker F hammer Good Modal Practices 20

21 Impact Hammer Excitation Advantages: Speed No fixturing No variable mass loading Portable and highly suitable for field work relatively inexpensive Conclusion Use for poor man s modal Best suited for field work Useful for determining shaker and support locations Disadvantages High crest factor means possibility of driving structure into non-linear behavior Tip Performance is often overlooked Not good for very large structures Repeatable hammer strikes require calibrated elbow Good Modal Practices 21

22 Shaker Excitation Advantages: Repeatable due to electronic control Wide range of excitations Can excite very small thru very large structures More automated test Conclusion Best suited for lab work Good on very light, very large, and difficult objects Tests requiring high degree of repeatability Disadvantages More equipment required than hammer Skilled operators necessary to supervise test A lot of setup time required Shaker issues such as: stingers, mounting of shaker, proper shaker selection Good Modal Practices 22

23 Practical Advice Excite Structure Evenly» Excite structure equally at every location Do Not Overexcite Structure» Choose a more sensitive accelerometer Plan and Practice» Plan where you will excite the structure FIRST!» Practice setting up excitation Pay Attention» Even though many points are measured, they are all equally important! Remember» There is no way to process out badly taken data» Garbage In = Garbage Out Result Will Be the Same» If excited correctly, both techniques will yield the same result! Good Modal Practices 23

24 Good Modal Practices Introduction Transducer Considerations Proper Excitation Ensuring Data Gathered is Good Post Processing Tips and Tricks Wrap Up Good Modal Practices 24

25 System Analysis F( ) H( ) X i ( ) System Analysis H(f) Use Frequency Response Function Measure Input and resultant Output Phase and magnitude retained Describes structural response Use FFT Analyser Frequency Good Modal Practices 25

26 FFT Set Up Parameters T = 1 s Time Lines = resolution Span = upper freq. range dt = s df = Span/Lines T = 1/df dt = 1/(Span * 2.56) df = 1 Hz Freq. Span Frequency (Hz) Good Modal Practices 26

27 Improper Setup Can Result In Ensure Data Fits Time Block! Good Modal Practices 27

28 FRF and Coherence FRF measures system response Output / Input Units are not intuitive Coherence is a quality indicator Interpret as how much force that went in caused the response that came out? 0 to 1 scale think in terms of 0% to 100% 90% or better is desirable Good Modal Practices 28

29 Observe the FRF and Coherence Frequency Response H1(Vib 10,Impact) - Input (Magnitude) \ FFT Analyze Coherence(Vib 10,Impact) - Input (Real) \ FFT Analyzer [db/1u (m/s^2)/n] Cursor values X: Hz Y: db/1u (m/ Look for high Coherence Look for smooth FRF functions [] k 1.2k 1.4k 1.6k [Hz] Good Modal Practices 29

30 Good Modal Practices Introduction Transducer Considerations Proper Excitation Ensuring Data Gathered is Good Post Processing Tips and Tricks Wrap Up Good Modal Practices 30

31 Mode Characterizations All Modes Can Be Characterized By: 1. Resonant Frequency 2. Modal Damping 3. Mode Shape Good Modal Practices 31

32 Residues are Directly Related to Mode Shapes! H ( ) ij r H ijr r Rijr j p r R * ijr j p * r Residues express the strength of a mode for each measured FRF Amplitude First Mode Second Mode Therefore they are related to mode shape at each measured point! Third Mode Beam Acceleration Force Force Force Force Force Force Force Force Force Force Force Force Good Modal Practices 32

33 How Do We Get There? Curve Fitting is the process of estimating the Modal Parameters from the measurements H 2 R/ Find the resonant frequency Frequency where small excitation causes a large response d Find the damping What is the Q of the peak? Phase Frequency Find the residue Essentially the area under the curve Good Modal Practices 33

34 Modal Analysis Step by Step Process 1. Visually Inspect Data Look for obvious modes in FRF Inspect ALL FRFs sometimes modes will show up in one FRF but not another (nodes) Use Imaginary part and coherence for verification Sum magnitudes of all measurements for clues 2. Select Curve Fitter Lightly coupled modes: SDOF techniques Heavily coupled modes: MDOF techniques Stable measurements: Global technique Unstable measurements: Local technique MIMO measurement: Poly reference techniques 3. Analysis Use more than 1 curve fitter to see if they agree Do mode shapes make sense? Good Modal Practices 34

35 Good Modal Practices Introduction Transducer Considerations Proper Excitation Ensuring Data Gathered is Good Post Processing Tips and Tricks Wrap Up Good Modal Practices 35

36 Practical Advice Proper Transducer Use» Mass loading can be a problem» Mount the accelerometer correctly Proper Excitation» Hammers not repeatable but easy to use» Shakers more repeatable but more setup required Analyzer Setting» Resolution is important, but remember the inverse relationship, set windowing according to excitation type.» Watch your coherence Post Processing» Use multiple curve fitters to verify one another» Don t be in a rush! Plan Your Test BEFORE Measuring» How many points? Triax or Uniaxial?» What Excitation Technique?» What frequency and resolution is necessary? Good Modal Practices 36

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