david herrin university of kentucky -...
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David HerrinUniversity of Kentucky
Signal Processing Basics
Vibro-Acoustics Consortium
July 23, 2020
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Vibro-Acoustics Consortium
July 23, 2020
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https://community.plm.automation.siemens.com/t5/Testing-Knowledge-Base/What-is-a-Frequency-Response-Function-FRF/ta-p/354778
Frequency Response Function
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Vibro-Acoustics Consortium
July 23, 2020
3
Auto / Cross Spectra
WindowingDigitizationLP Filter
AnalogSignal
Signal Processing
Averaging FFT
Frequency Domain Data
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Vibro-Acoustics Consortium
July 23, 2020
Digitization Analog to Digital
Time signals are sampled using an analog-to-digital converter (a digital logic hardware device) to yield a digitized version of the waveform for further processing.
ADC
Analog voltagesignal fromtransducer
Time (s) Voltage (V)
0.0 0.0
0.001 0.15
0.002 0.18
0.003 0.28
etc.
Voltage values stored in computer memory
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Vibro-Acoustics Consortium
July 23, 2020
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A 24-bit ADC can capture up to 224 (~16.8 million) levels. Audio CD’s are 16-bit (~65.5 thousand levels).
From P. Avitabile (2017).
Quantization Error
Overload Error
Digitization Quantization Errors
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Vibro-Acoustics Consortium
July 23, 2020
Digitization Time Domain Terms
𝑦 𝑡𝑦 𝑟𝑡
𝑦 𝑡
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Δ𝑡 sample interval
𝑓 sampling rate 1 Δ𝑡⁄
𝑁 total number of data samples in one frame
𝑇 total sample period or frame size
𝑟 sample index number 1, 2, 3, … ,𝑁
𝑡 𝑟Δ𝑡 time of any given sample
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Vibro-Acoustics Consortium
July 23, 2020
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Digitization Sample Rate
https://community.plm.automation.siemens.com/t5/Testing-Knowledge-Base/Digital-Signal-Processing-Sampling-Rates-Bandwidth-Spectral/ta-p/402991
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Vibro-Acoustics Consortium
July 23, 2020
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Auto / Cross Spectra
WindowingDigitizationLP Filter
AnalogSignal
Signal Processing
Averaging FFT
Frequency Domain Data
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Vibro-Acoustics Consortium
July 23, 2020
Time and Frequency Domains
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Vibro-Acoustics Consortium
July 23, 2020
FFT Frequency Domain Terms
𝑓 1/𝑇 (Hz)
𝑓 1/𝑇 𝑓
FourierCoefficients
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Δ𝑓 1/𝑇 frequency resolution
number of spectral lines (total number of frequency samples)
bandwidth is the highest frequency captured in Fourier transform
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Vibro-Acoustics Consortium
July 23, 2020
Fourier Transform
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Used to determine the frequency spectrum of dynamic signals –spectrum analyzer hardware
𝑇 2𝜋/𝜔 Is the period of the signal, 𝜔 is the fundamentalfrequency (first harmonic), 2𝜔 is the second harmonic, etc.
𝑦 𝑡𝐴2 𝐴 cos 𝑛𝜔𝑡 𝐵 sin 𝑛𝜔𝑡
𝑦 𝑡𝐴2 𝐶 cos 𝑛𝜔𝑡 𝜙
𝐴2𝑇 𝑦 𝑡 𝑑𝑡 𝐴
2𝑇 𝑦 𝑡 cos 𝑛𝜔𝑡 𝑑𝑡 𝐵
2𝑇 𝑦 𝑡 sin 𝑛𝜔𝑡 𝑑𝑡
𝐶 𝐴 𝐵 𝜙 tan𝐵𝐴
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Vibro-Acoustics Consortium
July 23, 2020
FFT Fast Fourier Transform
Discrete (digital) form:
The DFT is a CPU intensive algorithm. 𝑁 operations are required. If we split the signal into parts and then take the DFT for each part, we can reduce the number of operations. The computation will take less than 1% of the original time if a smart algorithm referred to as the FFT is used. There are many different FFT recipes but all require a power of 2 number of samples i.e., 512, 1024, 2048, etc. (Bodén, Ahlin, and Carlsson, Signal Analysis)
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𝐴2𝑁Δ𝑡 𝑦 𝑟Δ𝑡 ⋅ cos
2𝜋𝑁Δ𝑡 ⋅ 𝑛 ⋅ 𝑟Δ𝑡 Δ𝑡
2𝑁 𝑦 𝑟Δ𝑡 ⋅ cos
2𝜋𝑁 ⋅ 𝑛 ⋅ 𝑟 𝑛 1,2, … ,
𝑁2
𝐵2𝑁 𝑦 𝑟Δ𝑡 ⋅ sin
2𝜋𝑁 ⋅ 𝑛 ⋅ 𝑟 𝑛 1,2, … ,
𝑁2
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Vibro-Acoustics Consortium
July 23, 2020
FFT Settings
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Pick Then AndΔ𝑡 𝑓 1 2Δ𝑡⁄ 𝑇 𝑁Δ𝑡
Δ𝑓 1 𝑁Δ𝑡⁄𝑓 Δ𝑡 1 2𝑓⁄Δ𝑓 𝑇 1 Δ𝑓⁄ Δ𝑡 𝑇 𝑁⁄
𝑓 𝑁Δ𝑓 2⁄𝑇 Δ𝑓 1 𝑇⁄
Example
Δ𝑓 5 Hz and 𝑁 1024
Then𝑇 1 Δ𝑓⁄ 1 5⁄ Hz 0.2 sec𝑓 𝑁Δ𝑓 1024 5 Hz 5210 Hz
𝑓 𝑓 5120 2⁄ Hz 2560 Hz
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Vibro-Acoustics Consortium
July 23, 2020
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Auto / Cross Spectra
WindowingDigitizationLP Filter
AnalogSignal
Signal Processing
Averaging FFT
Frequency Domain Data
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Vibro-Acoustics Consortium
July 23, 2020
LP Filter The Aliasing Phenomenon
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https://community.plm.automation.siemens.com/t5/Testing-Knowledge-Base/Aliasing/ta-p/367750
Nyquist frequency (𝑓𝑚𝑎𝑥) is the highest frequency resolved in a DFT is:
𝑓𝑓2
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Vibro-Acoustics Consortium
July 23, 2020
LP Filter Aliasing Phenomenon
True spectrum
fNyq
fMeasured spectrum
fNyq
f
Aliasing results in harmonics above the Nyquist frequency being “folded” back onto their neighbors.
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Vibro-Acoustics Consortium
July 23, 2020
LP Filter Preventing Aliasing
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Apply an analog low-pass filter https://community.plm.automation.siemens.com/t5/Testing-Knowledge-Base/Aliasing/ta-p/367750
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Vibro-Acoustics Consortium
July 23, 2020
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Auto / Cross Spectra
WindowingDigitizationLP Filter
AnalogSignal
Signal Processing
Averaging FFT
Frequency Domain Data
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Vibro-Acoustics Consortium
July 23, 2020
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Windowing What is Leakage?Ideally
From P. Avitabile (2017).
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Vibro-Acoustics Consortium
July 23, 2020
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Windowing What is Leakage?
From P. Avitabile (2017).
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Vibro-Acoustics Consortium
July 23, 2020
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Windowing What is Leakage?
From P. Avitabile (2017).
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Vibro-Acoustics Consortium
July 23, 2020
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Windowing Applying a Window
Capturing an integer number of periods (minimum error).
From P. Avitabile (2017).
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Vibro-Acoustics Consortium
July 23, 2020
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Windowing Applying a Window
Capturing an integer number of periods plus half a period (maximum error).
3.9 dB amplitude error 1.5 dB amplitude error 0.1 dB amplitude error
From P. Avitabile (2017).
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Vibro-Acoustics Consortium
July 23, 2020
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Auto / Cross Spectra
WindowingDigitizationLP Filter
AnalogSignal
Signal Processing
Averaging FFT
Frequency Domain Data
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Vibro-Acoustics Consortium
July 23, 2020
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Averaging Data Processing Speed
Time Record 1 Time Record 2 Time Record 3
FFT 1 FFT 2
Real Time Operation
Time Record 1
Time Record 2
Time Record 3
FFT 1 FFT 2
Non-Real Time Operation
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Vibro-Acoustics Consortium
July 23, 2020
26
Auto / Cross Spectra
WindowingDigitizationLP Filter
AnalogSignal
Signal Processing
Averaging FFT
Frequency Domain Data
![Page 27: David Herrin University of Kentucky - vac.engr.uky.eduvac.engr.uky.edu/sites/vac/files/Webinars/08_VAC_Signal_Processing… · David Herrin University of Kentucky Signal Processing](https://reader034.vdocument.in/reader034/viewer/2022051913/6004caf05d78b558a3103ce1/html5/thumbnails/27.jpg)
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July 23, 2020
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• There is a problem in averaging time displaced signals• We can plot correlation as a function of the time shift
Auto Correlation
Agilent Technologies, 1999
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Vibro-Acoustics Consortium
July 23, 2020
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Auto / Cross Correlation
Identical Signals (Autocorrelation) Different Signals (Cross Correlation)
Agilent Technologies, 1999
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Vibro-Acoustics Consortium
July 23, 2020
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𝑅 𝜏 lim→
1𝑇 𝑥 𝑡 𝑥 𝑡 𝜏 𝑑𝑡
𝜏 time shift
Agilent Technologies, 1999
Auto Correlation
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Vibro-Acoustics Consortium
July 23, 2020
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Auto Correlation Square Wave
1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0
1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.016𝑁 1.0
𝜏 0
𝜏 Δ𝑡1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0
-1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0
-1.0 1.0 1.0 1.0 -1.0 1.0 1.0 1.0 -1.0 1.0 1.0 1.0 -1.0 1.0 1.0 1.08𝑁 0.5
𝑁 16 samples
1 step
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Vibro-Acoustics Consortium
July 23, 2020
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Auto Correlation Square Wave
1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0
-1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0
-1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0 -1.0
𝜏 4Δ𝑡
𝜏 8Δ𝑡1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0
16𝑁 1.0
16𝑁 1.0
4 steps
8 steps
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Vibro-Acoustics Consortium
July 23, 2020
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Auto Correlation Square Wave
Δ𝜏
+1.0
-1.0
𝑇
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Vibro-Acoustics Consortium
July 23, 2020
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Auto Correlation
𝑦 𝑡4𝐴𝜋
sin 𝜔𝑡13 sin 3𝜔𝑡
15 sin 5𝜔𝑡
⋅⋅⋅1𝑛 sin 𝑛𝜔𝑡 ⋯
𝑦 𝑡8𝐴𝜋
cos 𝜔𝑡19 cos 3𝜔𝑡
125 cos 5𝜔𝑡
⋅⋅⋅1𝑛 cos 𝑛𝜔𝑡 ⋯
Agilent Technologies, 1999
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July 23, 2020
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Auto Correlation
𝑅 𝜏 lim→
1𝑇 𝑥 𝑡 𝑥 𝑡 𝜏 𝑑𝑡
Example 𝑥 𝑡 𝐴 ⋅ sin 𝜔𝑡 𝜑
𝑅 𝜏𝐴2 cos 𝜔𝑡
Autocorrelation of a sine wave
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Vibro-Acoustics Consortium
July 23, 2020
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Auto Correlation
𝐴2
𝐴2
𝐴
𝐴
Agilent Technologies, 1999
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Vibro-Acoustics Consortium
July 23, 2020
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Auto Correlation
Agilent Technologies, 1999
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July 23, 2020
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Auto Correlation
Autocorrelation of sine wave buried in noise after many averages.
Agilent Technologies, 1999
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Cross Correlation 2 Square Waves
1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.08𝑁 0.5
𝜏 0
𝜏 Δ𝑡1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0
-1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0
-1.0 -1.0 1.0 1.0 -1.0 -1.0 1.0 1.0 -1.0 -1.0 1.0 1.0 -1.0 -1.0 1.0 1.00𝑁 0.0
𝑁 16 samples
Wave x
Wave y
-1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 1.0 1.0 1.0 1.0 -1.0 -1.01 step
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Cross Correlation 2 Square Wave
Δ𝜏
+1.0
-1.0
𝑇
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Cross Correlation
𝑥 𝑡 𝐴 sin 𝜔𝑡 𝜑𝑦 𝑡 𝐴 sin 𝜔𝑡 𝜑 𝜗
𝑅 𝜏 lim→
1𝑇 𝑥 𝑡 𝑦 𝑡 𝜏 𝑑𝑡
⁄
⁄
𝑅 𝜏𝐴 𝐴
2 cos 𝜔𝜏 𝜗
Example𝑥 𝑡 2 sin 𝜔𝑡𝑦 𝑡 3 sin 𝜔𝑡 𝜋
𝑅 𝜏
𝜏/𝑇
Boden et al., 2010
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Auto / Cross Spectra
WindowingDigitizationLP Filter
AnalogSignal
Signal Processing
Averaging FFT
Frequency Domain Data
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Transfer Function Measurement
Siemens, 2020
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The Measurement Chain
43
From P. Avitabile (2017).
𝐺 𝐺𝐺
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Transfer Function Measurement
44
𝐻 𝑓𝑌 𝑓 𝑋 𝑓𝑋 𝑓 𝑋 𝑓
𝐺𝐺
𝐻 𝑓𝑌 𝑓 𝑌 𝑓𝑋 𝑓 𝑌 𝑓
𝐺𝐺
Use if output is more susceptible to noise
Use if input is more susceptible to noise
Transfer Function Measurement
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Transfer Function Measurement
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Coherence
𝛾 𝑓𝐺 𝑓 𝐺 𝑓𝐺 𝑓 𝐺 𝑓
https://community.plm.automation.siemens.com/t5/Testing-Knowledge-Base/What-is-a-Frequency-Response-Function-FRF/ta-p/354778
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References
• The Fundamentals of Signal Processing, Agilent Technologies, Application Note 243 (2000). http://www.modalshop.com/techlibrary/Fundamentals%20of%20DSP.pdf
• P. Avitabile, Modal Testing: A Practitioner’s Guide, Wiley / SEM, West Sussex (2018).
• H. Bodén, K. Ahlin, and U. Carlsson, Applied Signal Analysis, KTH (2014).• R. G. Lyons, Understanding Digital Signal Processing, 3rd Edition, Prentice
Hall (2011).• Siemens Simcenter Online Articles (2020).