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Vibration Response Analyzer

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The Challenge:

To develop a portable control, test and analysis system that helps in performing structural dynamics tests.

The Solution:

Using National Instruments PXI platform, NI DAQmx and LabVIEW Development environment along with supporting toolkits like NI Sound and Vibration and NI Report Generation create a powerful and efficient Vibration Response Analyzer which is also flexible, scalable and user friendly.

Introduction:

Understanding the dynamic behavior of structures and structural components is becoming an increasingly important part of the design process for any mechanical system. Economic and environmental considerations have advanced to the state where over¬-design and less than optimum performance and reliability are not readily tolerated. Customers are demanding products that are cost effective, last longer, are less expensive to operate, while at the same time they must carry more payloads, run quieter, vibrate less, and fail less frequently. These demands, for improved product performance have caused many industries to turn to advanced structural dynamics testing technology.

Our clients, a leading R&D establishment specializes in various structural dynamics analyses. They use experimental structural dynamics as an integral part of their product development cycle.

System Architecture:

The setup consisted of a shaker table on which test specimen is mounted. Accelerometers are mounted on the shaker table as well as on the specimen under test. The stimulus signal is generated from a PXI 5421 arbitrary waveform generator card to excite the shaker. All the response and feedback signals from the shaker table and test specimen are connected to the PXI 4472 cards. Control, analysis and logging of data are done in the PXI. The PXI communicates the analyzed data to the host system through Ethernet. The host system displays the analyzed data.

A broad overview of the system is shown in fig 1 below:

Fig. 1

System Features:

The system developed by us helps to perform the following types of analyses:

Swept Sine:

This type of analysis, also called the “Modal Analysis” determines the resonance characteristics of a structure. It helps to locate the structural weak points.

The structure to analyze is mounted on a shaker table. The stimulus signal is generated through arbitrary waveform generator PXI 5421. The stimulus is a COLA (Constant Output Level Amplitude) signal. The signal frequency is swept over a particular range say, 5Hz to 2000 Hz with a specific sweep rate and resolution. Both the stimulus and the response signals are acquired using NI PXI 4472 module.

The amplitude level of the stimulus signal determines the vibration level of the shaker table. It is required that the over all vibration level of the shaker table remain uniform. Therefore, feedbacks are tapped from multiple points on the shaker table. The control algorithm changes the amplitude level of the stimulus signal output by the PXI 5421 arbitrary waveform generator based on the multiple feedbacks, ensuring that the structure under test is subject to uniform vibration levels through out the test.

After the completion of the test the, peaks at all the resonances are automatically calculated by the software and cursors are positioned at those points. It is also possible to manually move the cursors to other peaks of interest.

The swept sine analysis screen looks like fig 2 as shown below

Fig. 2

Random:

This type of analysis helps determine the amount of vibration/shock the structure can withstand.

The structure is mounted on a shaker table. The stimulus signal is random signal generated using the arbitrary waveform generator. The stimulus and the response signals are acquired by the 4472 modules and the power spectral density of the response is plotted. The analysis of data is done in blocks whose size is determined by the user. Along with the PSD plot other statistics like mean, RMS, skew and kurtosis of every block is also calculated and displayed.

Limit checking on the above statistics is also done. If value of any statistics exceeds the specified limit, the user has the option to reject that particular block of data from the analysis.

Finally a summary of the analysis shows the over all mean, RMS, skew and kurtosis of every analyzed channel. The summary also displays the total number of blocks analyzed and the number of rejected blocks during analysis.

The PSD analysis screen looks like fig 3 as shown below

Fig. 3

The summary of the PSD Analysis is as shown in Fig.4 below:

Other features provided by the system:

	3.	Compatible for ICP and charge accelerometers The PXI 4472 cards provide for acquiring from both ICP (require external current excitation) and charge accelerometers (do not require external current excitation).
	4.	Streaming data at the rate of 15 K Samples/sec/ch to the disk The PXI - RT architecture helped to achieve this without any performance bottlenecks.
	5.	Automatic data file transfer from PXI to Host through FTP The FTP toolkit from NI helped to implement this without any problems for files larger that 700 MB
	6.	Processing forty channels simultaneously The PXI - RT architecture helped to achieve this without any performance bottlenecks.
	7.	Generating reports The report generation toolkit from NI considerably reduced the development time to generate complex analyses reports.

Conclusion:

Using the PXI architecture, it was possible to develop a system that is efficient, accurate and flexible as per our client’s requirement. The biggest advantage of the system is that we are able to develop a multi-channel swept sine frequency response analyzer at a price which is much lesser than a desktop type instrument. Also, we are able to replace an existing setup which was bulky, with a compact solution. The development environment of Lab VIEW 8.0 helped in making the system very user friendly. The Lab VIEW Sound and Vibration Toolkits made it easier to implement the complex algorithms required for various analyses. The Report Generation Modules helped in analyzing the data as soon as tests were done and thus reduced the client’s valuable time.