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2. Measurements and analysis of diagnostic signals.

Question 2.6.

What methods of analysis that are used in vibroacoustical monitoring and diagnostics can be considered as most perspective ones? The question was asked by the students of the Saint Petersburg State Marine Technical University, chair "The reliability and technical diagnostics".

The question is answered by Natalia A. Barkova:

According to our experience some methods of signal analysis can occur to be perspective in monitoring, but not in diagnostics. And vice versa. So in my answer I shall separate the monitoring from diagnostics.

1. In monitoring the most efficient are the methods of signal energetic parameters analysis. If necessary the signal is preliminary divided on frequency or spatial components.

The main frequency method of separation the signals by components is the spectral analysis. In this method the energetic evaluation of each component is given (its amplitude, power).

For spatial separation of signals the spatial separation of vibration (noise) measurement points, or signal correlation measurements in two or more points are used. The second approach is used more often in acoustical diagnostics of extended uniform medium, for example, to detect the leakage in the pipe-lines. It enables to define not only their energetic characteristics of the signal components, but also their spatial coordinates, and also to extract the useful signals from the interference, increasing the accuracy of the energetic parameters evaluation.

2. In diagnostic problems, especially in detection of incipient defects, the energetic parameters have the secondary value. The main value has the form of the signal. Just by the form of certain components it is easier to detect the defects. An example is the use of shock pulse method to detect certain types of incipient defects in the bearing.

Unfortunately, any deviation of the signal form from the simplest one, i.e. the sinusoidal one, expands its frequency band. In real machines many vibration (noise) signals of complicated forms are summarized, but as their frequency bands can overlap it is impossible to analyze the signal form by some certain general method. In this case it is necessary to use some specialized methods of extracting the signals of a certain form from the sum of many other types of signals. An example is the cepstrum (double spectrum) signal analysis that responses on appearance in the summarized signal the harmonic components, modulated by the frequency or amplitude by periodical process.

This method showed its comparatively good efficiency in diagnostics of the rotating machine's units with many teeth or blades as it has higher sensitivity to certain defects of teeth (blades), impellers, gears, magnetic part of the electric motors.

For already three decades we design the most general methods of analysis of complicated form real signals that are necessary to be preliminary extracted from a sum of several signals, part of which have complicated form. It occurred that the most universal approach is the analysis not of the signal itself, but the variations of its root-mean-square value or power. So the analysis of such type of signal is reduced to the analysis of the form of its envelope. In this case naturally all the problems of the preliminary extraction of analyzed signal from the sum of many others do not disappear and have to be solved. The simplest way to solve these problems is to measure only the high frequency vibration (noise) components in certain frequency band and in the immediate vicinity of the source.

When you transfer to the enveloped signal its form became significantly simplified. It enables to use more general methods of analysis. Three most general methods can be used in this case. The first one - you can compare the form of the signal just with the form of the standard for different types of defects. The second - the spectral signal analysis, if the signals are periodical and have not very complicated form. The third - the statistical analysis of the signals.

Nowadays the spectral methods of enveloped signal analysis are used most widely. But we cannot exclude that the other two methods in the nearest future will also find their application in solving the diagnostic problems.

 

 

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