LabVIEW has an extensive library of order analysis functions available in the Sound and Vibration Measurement Suite. These tools are very useful for performing online analyses of vibration data. The term online essentially means that the vibration analysis is done at the time of acquisition. DIAdem's order analysis capabilities are a great solution for offline, interactive vibration data management. Offline analysis is done after the acquisition takes place, and the raw vibration data has been stored in a file or a database. DIAdem is a great solution for accessing test data saved in files and databases while also providing an excellent environment for creating sophisticated waterfall plots and 3D surface graphs that are required to interpret noise or vibration data. DIAdem, along with the Joint Time Frequency and Order Analysis tool, delivers an easy to use, non-programming approach to manage, inspect, analyze, and report your vibration data.
There are three primary types of noise or vibration spectra that are examined when doing analysis on rotating machinery: Joint Time Frequency Spectrum, Order Analysis Frequency Spectrum, and Order Analysis Order Spectrum.
Joint-Time Frequency analysis divides a data set into time intervals and performs a Fast-Fourier Transform (FFT) on the data in each time interval separately. This enables you to inspect how the frequency content of a signal develops over time. In the below figure Frequency is displayed on the 'X' axis, Time on the 'Y' axis, and the frequency magnitudes on the 'Z,' or intensity, axis. You can easily identify that the frequency of this signal peaked at about the 1.5 second mark.
Figure 1. Joint Time Frequency Spectrum Graph
For the Order Analysis Frequency Spectrum case, the magnitude of the frequency content is examined slightly differently. In the figure below, Frequency is displayed on the 'X' axis, Rotational Speed (RPM) on the 'Y' axis, and frequency magnitudes on the 'Z,' or intensity, axis. This enables you to inspect how the vibration magnitudes track with Rotational Speed. For example, as an engine is run from startup (0 RPM) to idle speed (1000 RPM), the vibration the engine experiences changes at different RPM levels. Order analysis enables you to identify the critical rotational speeds that drive the noise or vibration of the UUT, perhaps to examine natural frequencies or other design problems.
Figure 2. Order Analysis Frequency Spectrum Graph
The Order Analysis Order Spectrum case is very similar to the Order Analysis Frequency Spectrum case. Its only fundamental difference is that instead of having frequency on the 'X' axis, a quantity called order is placed there. Order simply represents the frequency of the vibration divided by the instantaneous rotational frequency of the UUT. This enables you to easily identify relative overtone amplitudes of the rotational speed as the rotation speed develops.
Figure 3. Order Analysis Order Spectrum Graph
All of these graphs were generated by the Joint Time Frequency and Order Analysis tool in DIAdem. The tool is essentially a collection of DIAdem resource files (VBScript files, VAS files, and SUDialog source files), which combine to form a simple, interactive user interface that resides in the DIAdem environment. The tool enables you to change graph axis limits, analysis types, report templates, and other analysis parameters very quickly and easily.
Figure 4. The Joint Time Frequency and Order Analysis Tool User Interface
The Joint Time Frequency and Order Analysis tool consists of three main components: Joint-Time Frequency Spectrum, Order Analysis Frequency Spectrum, and Order Analysis Order Spectrum. One way to visualize the difference in these analyses is to look at what data is generated and displayed.
Figure 5. Main Analyses Provided by the Tool: Joint-Time Frequency Spectrum (Left), Order Analysis Frequency Spectrum (Center), and Order Analysis Order Spectrum (Right)
To switch between these components, simply select the type of analysis in the Analysis Type pull-down box. Switching between these components will, as appropriate, make certain components visible/invisible on the dialog. There is one additional function that is provided by the tool: the RPMs Channel from Tach. Channel selection. This function converts a tachometer pulse or rotary encoder channel into an RPM channel. For more information, see the Additional Tool Functions section of this document.There are many Automatic Axis Value Checking routines that are built into the tool. Due to the nature of the analysis algorithms, there are many limitations on what values can be put into the axis high, low, and mesh line input boxes. There are several routines built into the tool that will automatically check and prevent illegal values from being placed into the axis value boxes.
As previously mentioned, the Joint Time Frequency and Order Analysis tool is essentially a collection of DIAdem resource files (VBScript files, VAS files, and SU Dialog source files). There is no actual 'installation' for the tool, all that is necessary is to have the DIAdem software. If you do not have DIAdem currently, the tool is fully functional with the Evaluation Version. All that is necessary is to download the example program to your computer and extract the files to any directory. The tool will then be ready to use.To open and run the tool:
The tool will now be running on your screen. If there was not any data loaded into DIAdem when the tool was launched, it will automatically prompt you to load a data set. To load a new data set at any time, simply click the New Data File button. You can now perform the three vibration analyses on your data. Any registered DIAdem file type, whether it’s a built in format (*.DAT, *.TDM, *.LVM) or a custom format, will be able to be loaded and used.To follow along with this tutorial, load the Vibration.DAT data set, located in the Sample Data sub-directory of the tool.
A screen shot of the dialog when Joint Time Frequency Spectrum is selected is shown below:
Figure 6. Joint-Time Frequency Spectrum Dialog
To perform this analysis, set the Time Channel and Signal Channel boxes to the Time and Vibration channels. When you select these channels, the frequency and time scale boxes should be automatically populated with suggested values.
A screen shot of the dialog when Order Analysis Frequency Spectrum is selected is shown below:
Figure 7. Order Analysis Frequency Spectrum Dialog
To perform this analysis, set the Time Channel, RPMs Channel, and Signal Channel boxes to the Time, Revolutions, and Vibration channels respectively. If you do not have an RPM channel in your data set, but rather have a tachometer pulse or rotary encoder channel, you can convert that channel into an RPM channel. See the Additional Tool Functions section for how to use this routine. When you select these channels, the Frequency and Revolutions scale boxes will automatically populate with suggested values.
A screen shot of the dialog when Order Analysis Order Spectrum is selected is shown below:
Figure 8. Order Analysis Order Spectrum Dialog
To perform the analysis, set the Time Channel, RPMs Channel, and Signal Channel boxes to the Time, Revolutions, and Vibration channels respectively. If you do not have an RPM channel in your data set, but rather have a tachometer pulse or rotary encoder channel, you will can convert that channel into an RPM channel. See the Additional Tool Functions section for how to use this routine. When you select these channels, the Order and Revolutions scale boxes will automatically populate with suggested values.
Along with the three main functions of the Joint Time Frequency and Order Analysis tool, there are two additional features added for ease of use and visualization. the RPMs Channel from Tach. Channel function, and the Toggle Time Channel DateTime function.
When doing order analysis, it is necessary to have an RPM channel that corresponds with your time data. There must be a one-to-one correlation between each point in time and an RPM value at that time in order for the order analysis functions to work. Frequently, when doing vibrational data acquisition with a Dynamic Signal Acquisition device, an RPM channel is not available. Instead, an analog waveform of a tachometer pulse signal or a rotary encoder signal is acquired, which can then later be converted to an RPM channel. The Joint Time Frequency and Order Analysis tool provides this functionality in an easy to use, interactive interface.A screen shot of the dialog when RPMs Channel from Tach. Channel is selected is shown below:
Figure 9. RPMs Channel from Tach. Channel Dialog
To use this function:
This channel can now be used in the RPMs Channel pull down box when performing the order analysis functions.
For visualization, sometimes it might be necessary to convert your time channel into a DIAdem Date/Time channel. The Joint Time Frequency and Order Analysis tool provides a quick method for converting your time channel.
To use this function, simply click the button to convert in the Time Channel to and from a DIAdem Date/Time channel. Click the button again to switch back to relative seconds display.Conclusion
This tool takes advantage of DIAdem's technology: the ability to interactively manage, inspect, analyze, and report data. By using DIAdem VBScripts, we can perform complicated frequency analyses on very large data sets, and in seconds view and inspect our results. The tool provides an easy to use, non-programming interface to performing three common types of frequency analysis: joint time frequency analysis, order analysis frequency spectrum, and order analysis order spectrum. The tool also provides additional functionality to convert tachometer data into RPM data and to convert back and forth between time channels and DIAdem Date/Time channels.
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