PeakVue+Theory

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CAT IV Part 1 – Signal Processing slidebook

Autocorrelation 

Correlation Statistics Processing

The autocorrelation function is used to reveal periodicities in waveform data. • The FFT is also used to reveal periodicities, but autocorrelation keeps the data in time waveform format • We will also see that autocorrelation is effective at revealing signals with low duty cycles

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The autocorrelation function can be considered “a measure of the similarity a signal has with a time shifted version of itself*” 

Autocorrelation can be calculated by performing the inverse Fourier transform of the power spectrum.

* Noise and vibration analysis – Brandt – page 71 Copyright © 2005-13 Mobius Institute  – All rights reserve reserved d

Autocorrelation

Correlation Statistics Processing

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CAT IV Part 1 – Signal Processing slidebook

132

Correlation Statistics Processing

Autocorrelation 

The remaining waveform contains the correlated signals. Uncorrelated (random) data is removed. • Synchronous and non-synchronous periodic data survives, unlike time synchronous averaged data

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It is therefore useful for rolling element bearing analysis and other applications where the periodic patterns are difficult to see in the waveform. It is very useful for identifying faults at a low periodic rate relative to analysis bandwidth* • For example, faults in the cage of a bearing • Signals with higher periodic rates and higher duty cycles will be revealed via the FFT

* Autocorrelation as an analysis tool – James C. Robinson Copyright © 2005-13 Mobius Institute  – All rights reserved

Example

Correlation Statistics Processing

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133

CAT IV Part 1 – Signal Processing slidebook

Example

Correlation Statistics Processing

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Example

Correlation Statistics Processing

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CAT IV Part 1 – Signal Processing slidebook

Example

134

Correlation Statistics Processing

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Example

Correlation Statistics Processing

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135

CAT IV Part 1 – Signal Processing slidebook

Example

Correlation Statistics Processing

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Example

Correlation Statistics Processing

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CAT IV Part 1 – Signal Processing slidebook

Example

136

Correlation Statistics Processing

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Example

Correlation Statistics Processing

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137

CAT IV Part 1 – Signal Processing slidebook

Correlation Statistics Processing

Example: One side of the roller

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Example: The other side of the roller

Correlation Statistics Processing

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CAT IV Part 1 – Signal Processing slidebook

138

Correlation Statistics Processing

Example

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That concludes this section

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