This measurement column addresses frequently asked questions received by our customer support center and provides answers to those questions.
In the 35th installment, we introduced the procedure for performing offline FFT analysis of data recorded with the DS-3000 series real-time Sound and Vibration Analysis System software DS-0320, including the display of individual spectra and average analysis. This time, we will introduce the procedure for displaying the time trend of the power spectrum. Note that the DS-0322 tracking analysis function option is required to perform this operation.
DS-0320 and Record File Viewer screen
Figure 1 shows the screen of the real-time Sound and Vibration Analysis System software DS-0320, and Figure 2 shows the screen of RecordFile Viewer.
-
Figure 1: Screenshot of the DS-0320 real-time Sound and Vibration Analysis System software.
-
Figure 2: Record File Viewer screen
Procedure for opening a record file and performing offline FFT analysis
Open the record data from the "Open Offline Analysis Data" option in the DS-0320's File menu (Figure 3). Select "FFT (Offline)" as the mode.
-
Figure 3 File menu ⇒ Open offline analysis data dialog
If the record data contains multiple recording results (records), launch the Record FileViewer (Figure 1 ③) and select the record to be analyzed by switching the displayed record (Figure 2 ⑤). Also, if you want to analyze only a part of the time range of the record data rather than the entire data, specify the analysis range using Figure 2 ⑥ "Register display range as analysis range" or Figure 2 ⑦ "Analysis range", etc.
Pressing the DS-0320's first move button (Figure 1 ④) followed by the START button will perform analysis sequentially from the beginning of the entire record data or the specified analysis range, stopping at the end of the range.
Methods for analyzing the time trend of power spectra
To perform a time trend analysis of the power spectrum, use the tracking analysis function (scheduled analysis function).
Open the Schedule Settings dialog box by going to the Input/Output Settings menu ⇒ “Schedule Settings” (Figure 4). You cannot change settings even if you open this dialog box when the SCHED button (Figure 1 ②) is ON. Please turn the SCHED button (Figure 1 ②) OFF before proceeding.
In the dialog box, set the schedule mode to fixed-time schedule. In the fixed-time schedule, set the time interval for performing FFT calculations in "Interval" and the total analysis time in "Total Completion Time". If you set the interval to 0.1 seconds and the total time to 50 seconds, a total of 500 FFT calculations will be performed at a frequency of once every 0.1 seconds, and power spectra for 500 calculations will be obtained. Note that the upper limit for the number of calculations is 1000, so please set the interval and total time within this limit.
-
Figure 4 Input/Output Settings Menu ⇒ Schedule Settings Dialog
Select "Schedule" using the graph type selection button (Figure 1 ①). Specify the desired analysis range in the Record File Viewer, turn on the SCHED button (Figure 1 ②), press the move-to-first button on the DS-0320 (Figure 1 ④), and then press the START button to perform the scheduled analysis.
Open the Trace Line Settings dialog box by going to the Data Display Settings menu ⇒ “Trace Line Settings” (Figure 5). Turning Line1 to Line8 ON will display a graph showing how the frequency components specified by the trace frequency changed over the total time. Turning Overall ON will display the time change of the overall (sum of all frequency components). P.Overall (Partial Overall) is the sum of the components in the specified frequency range.
-
Figure 5 Data Display Settings Menu ⇒ Trace Line Settings Dialog
Figure 6 shows the results of the time trend analysis of the power spectrum.
-
Figure 6: Example of time trend analysis of power spectrum
Figure 6 shows the results obtained with a frequency range of 20 kHz, 2048 sample points, and a frequency resolution (Δf) of 25 Hz. The length of the time waveform used in one FFT calculation is 40 ms, which is the reciprocal of the frequency resolution. Since the interval of the fixed time schedule was set to 0.1 seconds (100 ms), 60 ms of the time data was not included in the FFT calculation, resulting in gaps in the analysis data. If you want to include all time data in the calculation, please perform the analysis by setting a smaller frequency resolution or shortening the interval of the fixed time schedule.
summary
This time, we introduced the procedure for analyzing the time trend of the power spectrum using FFT offline analysis with the DS-3000 series.
For information on offline analysis with the DS-3000 series, please also refer to Measurement Column Nos. 221 and 223.
(Excerpt from the email newsletter issued on June 17, 2020)
