The modal testing experiment presented here is performed by a manufacture plant using ModalVIEW software and National Instruments DSA hardware as the modal testing and analysis tool. The analysis result provides instructive guidance on fault diagnosis and NVH analysis.

Experiment scenario

The testing object is a air compressor and pipes system:

To identify modes of this air compressor system from its vibration data, operational modal analysis method of stochastic subspace identification (SSI) is used. ModalVIEW modal analysis software provides advanced SSI method based on mode stabilization diagram to extract modes from time records. National Instruments 24-bit portable module NI -9233 is used to guarantee the accurate measurement of vibration signal of measured points. The portable NI DSA hardware is suitable for field modal testing experiment.  

System configuration

In this modal testing experiment, the following software and hardware are used：

1．ModalVIEW 2008 Software.

2．NI DSA System (Module carrier NI-9162, 4 channel DSA module NI-9233)

3．Tri-axial accelerometer.

 Analysis results

Modal analysis for the structure is finished with ModalVIEW’s advanced mode estimation method based on mode stabilization diagram。The following figure shows the clear mode stabilization diagram from measured vibration data.

The stabilization diagram is compared with time-frequency spectrogram from vibration data. The postions of identified modes on stabilization chart are consistent with resonance bars in spectrogram.  

According to the verification from spectrogram, the modes can be selected from stabilization diagram. The real modes are kept with mathematical or faked modes being discarded. Part of the picked meaningful modes are listed in following table :

Some of the mode shapes is displayed as following :

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