Principles of Vibration Measurement and Analysis Dr. Colin Novak, P.Eng. 92-315 July 29, 2015
Vibration Transducers and Signal Conditioning Types of Vibration Transducers The Piezoelectric Accelerometer Choosing an Accelerometer Using an Accelerometer Calibration Conditioning Amplifiers
The Measurement Chain Transducer Preamplifier Filter(s) Detector/ Averager Output
Early Methods of Vibration Measurements
Mechanical Lever Application: Obsolete, but still found in a few old power stations
Eddy Current Proximity Probes Applications: Relative motion Shaft eccentricity Oil film thickness Etc.
Velocity Pickup e = Blv Limited frequency range: 10 < f < 1000 Hz 800289
Piezoelectric Accelerometer Principles of operation F F F V mv F Q pc F F V mv F Q pc F 800290/2
Types of Piezoelectric Accelerometers Planar Shear P Centre-mounted Compression S ThetaShear M R B M P B M R P B Annular Shear E M P B Delta Shear P R M B OrthoShear P M R E B P: Piezoelectric Elements E: Built-in Electronics S: Spring R: Clamping Ring B: Base M: Seismic Mass 800284
Operational Range of Vibration Transducers 10 8 :1 Relative Amplitud e 10 6 :1 Piezoelectric Acceleromete r 10 000:1 100:1 1 0.2 2 20 200 2k 20kHz Frequency Velocity transducer Eddy current Proximity probe 930656
Choosing an Accelerometer or General Purpose, medium weight and sensitivity Small, light and high frequency Acceleration ms -2 250,000 20,000-100,000 1-10 pc/ms -2 Weight: 10-50 gram 0.1-0.3 pc/ms -2 Weight: 0.5-3 g 0.003-0.01 0.0001-0.001 Frequency ~0.1 ~ 1 5-12k 15-30k Hz 800299
Useful Frequency Range Output ~ 10% Input Acc. ms -2 (pc) 0.3 f 0 ~ 35 db f 1 f 0 Frequency 800298
Sensitivity and Frequency Range Sensitivity pc/ms -2 31.6 1 0.004 13 42 180 khz 800295
Accelerometer Mounting Fixed Thin double adhesive tape Cementing stud Stud Mounting 30 20 10 0 Level db Beeswax Max.40 C 200 500 1k 2k 5k 10k 20k 30k 50 khz Frequency 930616
Accelerometer Mounting Handheld Hand held probe Magnet Level db 30 20 10 0 200 500 1k 2k 5k 10k 20k 30k 50kHz Frequency 930617
Choosing a Mounting Position B A D C 800305
Loading the Test Object 0,1 pc/ms -2 0.65 g M > 7 g 10 pc/ms -2 54 g M > 600 g Dynamic Mass < 1 10 M M 1000 pc/ms -2 470 g M > 5 kg 800304
Transverse Sensitivity Axis of maximum sensitivity < 2.3 Accelerometer mounting axis Red dot 100 % < 4 % Max. transverse sensitivity < 4 %
Triboelectric Noise + + + + + + + ++ + ++ 800317
Environmental Effects Base Strain Corrosive substances Humidity Magnetic fields Acoustic noise Nuclear radiation 800314
Special Accelerometers Calibration Triaxial z High sensitivity (with built-in amplifier) x y High temperature 55 g 10 g Shock 316 mv/ms -2 T max. = 400 C a max. = 1000 km -2 a min. = 20 10-6 ms -2
Handle the Accelerometer Carefully 2 5 100 2 5 1k 2 5 10k 20 50 930616
The Calibration Chart
Force Transducer Force Transducer Piezoelectric material F1 a Force Transducer F2 930624
Overload Brüel & Kjær Conditioning Amplifier Vibration Transducers and Signal Conditioning (cont.) Laser Velocity Systems Accelerometer Calibration Conditioning Amplifiers NEXUS
Laser Velocity-Transducer Set- Ometron VH300+
Principle of Laser Velocity Transducer Object moving toward laser gives higher frequency Object moving away from laser gives lower frequency
Applications For vibration measurements on hot structures (engine exhausts) For non-contact measurement of vibration velocity and displacement For vibration measurements on light structures (e.g loudspeaker diaphragms) For lateral and axial vibration measurements on ro-tating components (e.g. on rotating machinery) For vibration measurements on small structures (e.g. on the tops of engine valves) For vibration measurements on delicate structures (e.g optical devices such as mirrors) For relative vibration measurements (e.g. on board ships, aircraft and cars) For vibration measurements on rubber and other soft materials
Example of Use: Loudspeaker Cone Vibration
Calibration Why calibrate To find the sensitivity Why recalibrate Legal obligation - QA requirement Good instrument practice Test for damage
Accelerometer Check In the field Sensitivity check Total system check In the lab Frequency Response Sensitivity Calibration Multianalyzer Frequency = 159.2 Hz = 1000 rad/sec Acceleration = 10 ms -2 Power Amplifier Calibration Exciter with built-in or external reference accelerometer
Vibration Measurement and Analysis
Why Make a Frequency Analysis B C A Amplitude Time Amplitude A B C D E D E Vibration Frequency
Frequency Spectrum or Overall Level Transducer Preamplifier Filter(s) Detector/ Output Averager Overall Level Frequency Spectrum
Frequency Spectrum or Overall Level Vibration Frequency Spectrum 5 4 3 2 1 Overall Level 1 2 3 4 5 Fan Vibration 5 4 3 2 1 Frequency Date 1 2 3 4 5 Gearbox Hz Date
Presenting the Data Linear vs. Log Scaling Amplitude in db? Linear and Logarithmic Frequency Scales Decades Octaves
Linear vs Logarithmic Scales 0 1/2 1 Empty Full 0 0.25 0.5 0.75 1 0 1/10 1/5 1/2 1 Full 0.05 0.1 0.2 0.5 1
Linear vs Logarithmic Frequency Scales Vibration Level 120 Hz 50 Hz 0 200 400 600 800 1K 1,2K 1,4K 1,6K 1,8K 2K Hz Linear Frequency Vibration Level 20 50 100 200 500 1K 2K 5K 10K 20K Logarithmic Frequency
Bandpass Filters and Bandwidth 0 B Ideal filter Bandwidth = f 2 f 1 Centre Frequency = f 0 f 1 f 0 f 2 Frequency 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 Frequency f 1 f 0 f 2 Frequency
Linear vs Logarithmic Frequency Scales Vibration Level 120 Hz 50 Hz 0 200 400 600 800 1K 1,2K 1,4K 1,6K 1,8K 2K Hz Linear Frequency Vibration Level 20 50 100 200 500 1K 2K 5K 10K 20K Logarithmic Frequency
Selecting Bandwidth Filter width Vibration Level Frequency Spectrum Frequency Vibration Level Frequency Frequency Frequency
Linear scale Linear vs Logarithmic Amplitude Scales 1000 1000 Linear Logarithmic 3.16 900 amplitude amplitude 316 800 3.16 3.16 700 100 600 3.16 500 400 300 200 100 0 Advantages of logarithmic amplitude scale Constant factor changes are equally displayed for all levels Optimal way of displaying a large dynamic range 3.16 Frequency 3.16 3.16 3.16 31.6 10 3.16 1 Logarithmic scale
Real World Vibration Levels ms -2 db 1 000 000 240 1000 180 1 120 0.001 60 0.000 001 0
Vibration Parameters Relative Amplitude 100 000 10 000 1000 100 10 1 10 100 1000 10 000 100 000 0.1 1 10 100 1 k 10 khz Frequency Acceleration Velocity Displacement
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