Vibration Measurement and Analysis

Transcription

1 Measurement and Analysis Why Analysis Spectrum or Overall Level Filters Linear vs. Log Scaling Amplitude Scales Parameters The Detector/Averager Signal vs. System analysis

2 The Measurement Chain Transducer Preamplifier Filter(s) Detector/ Averager Output

3 Why Make a Analysis B C A Amplitude Time Amplitude A B C D E D E

4 Analysis Acc. Level

5 Spectrum or Overall Level Transducer Preamplifier Filter(s) Detector/ Output Averager Overall Level Spectrum

6 Spectrum or Overall Level Spectrum Overall Level Fan Date Gearbox Date

7 Presenting the Data Linear vs. Log Scaling Amplitude in db? Linear and Logarithmic Scales Decades Octaves

8 Linear vs Logarithmic Scales 0 1/2 1 Empty Full /10 1/5 1/2 1 Full

9 Linear vs Logarithmic Scales Decade 1 Decade Linear Decade 1 Decade 1 Decade 1 Decade Logarithmic

10 Linear vs Logarithmic Scales Level 120 Hz 50 Hz K 1,2K 1,4K 1,6K 1,8K 2K Hz Linear Level K 2K 5K 10K 20K Logarithmic

11 Bandpass Filters and Bandwidth 0 B Ideal filter Bandwidth = f 2 f 1 Centre = f 0 f 1 f 0 f 2 0 Ripple Area = Area - 3 db Real filter and definition of 3 db Bandwidth Real filter and definition of Noise Bandwidth f 1 f 0 f 2 f 1 f 0 f 2

12 Filter Types Constant Bandwidth B = x Hz Constant Percentage Bandwidth or Relative Bandwidth B = y% = y f Linear B = 31,6 Hz B = 10 Hz B = 3,16 Hz Logarithmic B = 1 octave B = 1/3 octave B = 3%

13 Constant Bandwidth Filtering Bandwidth = 400 Hz Linear Axis 0 1k 2k 3k 4k 5k 6k 7k 8k 9k 10k Logarithmic Axis k 2k 5k 10k

14 Constant Percentage Bandwidth Filters Bandwidth = 1/1 octave = 70% of Centre Linear Axis 0 1k 2k 3k 4k 5k 6k 7k 8k 9k 10k, Hz Logarithmic Axis k 2k 5k 10k 125 8k, Hz

15 Linear vs Logarithmic Scales Level 120 Hz 50 Hz K 1,2K 1,4K 1,6K 1,8K 2K Hz Linear Level K 2K 5K 10K 20K Logarithmic

16 Selecting Bandwidth Filter width Level Spectrum Level

17 Most important in Analysis B = bandwidth T = time BT 1 (often called the Uncertainty Principle)

18 Linear scale Linear vs Logarithmic Amplitude Scales Linear Logarithmic amplitude amplitude Advantages of logarithmic amplitude scale Constant factor changes are equally displayed for all levels Optimal way of displaying a large dynamic range Logarithmic scale

19 The db Scale Acceleration db re m/s 2 60 Acceleration m/s a ( db) = 10log 10 = 20log 2 a N 10 a ref ref a = 40 db 3.16 = 10 db Logarithmic amplitude 3.16 = 10 db = 20 db 3.16 = 10 db 1 1

20 Transmission of Input Forces System + Response = (Mobility) 8 db + = 8 db Forces caused by Imbalance Shock Friction Acoustic Structural Parameters: Mass Stiffness Damping Parameters: Acceleration Velocity Displacement

21 Real World Levels ms -2 db

22 Parameters Relative Amplitude k 10 khz Acceleration Velocity Displacement

23 Which Parameter to Choose Measurement A Acc. Vel. Disp. Measurement B Acc. Vel. Disp. Measurement C Acc. Vel. Disp. Choose Displacement Choose Velocity Choose Acceleration

24 The Detector/Averager RMS Peak Peak- Peak Level Peak-Peak Hold Peak-Peak Time Peak RMS Time

25 Averaging Time Level (Peak) Averaging Time = 10 s Averaging Time = 1 s Time Time

26 Signal vs. System Analysis Signal Analysis System Analysis signal Excitation (Input) Response (Output)

27 Conclusion This lecture should provide you with sufficient information to: Choose the right vibration parameters to measure Present the measured data in a suitable way Understand the basic filter and analysis parameters and limitations Understand the difference between signal and system analysis

28 Literature for Further Reading Shock and Handbook (Harris and Crede, McGraw-Hill 1976) Analysis (Brüel & Kjær Handbook BT ) Structural Testing Part 1 and 2 (Brüel & Kjær Booklets BR and BR ) Brüel & Kjær Technical Review No (BV ) No (BV ) No (BV ) No (BV ) No (BV ) No (BV )