9.2 Creating an analysis

New analyses are added within the Analyses card. To add a new one, click on the “+ Add” button. A menu will appear, showing all types of TPA analyses implemented in SOURCE. Depending on the types of data that have been loaded and the types of channels that have been specified in the data, SOURCE will recognize the type of TPA that can be performed and highlighting the available analyses.

Adding a new Analysis

After selecting your desired analysis, it will appear in the Analyses card. The name of the analysis can be changed by right-clicking and selecting “Rename”. By default, the analysis will include all of the loaded operational and FRF data; this information is shown below the analysis name.

Defining analysis settings

By default, the analysis will include the full time series, with FFT settings as specified in the preferences page. If a matrix inverse is being performed, the full matrix will be included in the inverse by default.

However, it is possible to change these settings in the Settings card, as shown in the figure below. In particular, it may be necessary to truncate the matrix inverse to ensure noise is not propagated through the analysis. Currently these settings are defined manually, although future SOURCE releases will provide smart suggestions for doing the matrix inversion.

FFT and matrix inverse settings for the analysis

Defining analysis inputs

As mentioned, the analysis will include all previously selected operational and FRF data by default. To modify this: first select the desired analysis in the Analyses card, and then in the Inputs card select or deselect the desired operational data, FRF data, and channels. For example, the channel selection is showed in the figure below.

Channel inputs for the analysis

Time blocks and tracking channels

Here, the settings are defined for cutting the time blocks from the complete time series, which is then used in the FFT calculation.

By default, the entire time series will be cut into blocks using a block length such that the resulting frequency spacing of the FFT will match that of the loaded FRF data (e.g. 1 sec blocks for 1 Hz frequency spacing), along with 50% overlap. One can also manually set the start time, end time, block length and overlap of the time blocks.

It is also possible to set a tracking channel as block method. A tracking channel could be, for example, a bench speed, a channel measuring rotation, a strain gauge, etc. One channel should be set as “tracking” in the Mapping channel card. When selecting a tracking channel as block method, it is necessary to define:

  • Block length: the length of the block (see the figure below).
  • Tracking channel: if multiple channels are set as tracking, the correct one must be selected.
  • Tracking values: start and end value of the tracking channel. E.g. in a run-up, it might be interesting to select the range 80-120 kph. In this case, the user should enter “80:120”, or “80:0.5:120” to specify steps different than 1. This field uses a syntax that is similar to MATLAB. If the analysis rather requires a few discrete values, they can be specified as comma-separated input. e.g. “100, 250, 500, 600”.
  • Tolerance: the tolerance for the signal fluctuation (see the figure below). For example, a tolerance of 5 kph on a tracking value of 100 kph will mean that the analysis would accept the data in the range 100 ± 5 kph, so 95-105 kph.
  • Tracking alignment: the user should decide if the alignment is done on the left, center or right (see the figure below).

Tracking channels parameters

Fourier transform

Here, the settings are defined for converting the time blocks to frequency blocks. By default, a Hann window is used, and the FFT will be calculated up to the Nyquist frequency of the time signal or the frequency limit of the loaded FRF, whichever is lower. It is also possible to change the window type and set a different (lower) frequency limit for the FFT calculation. The defaults can be changed in the Preferences page.

Matrix inversion

Here, the settings are defined for (truncation of) the matrix inverse. By default, the complete matrix including all singular values (and no truncation) will be used. It is also possible to truncate the inverse by setting a relative or absolute threshold or setting the number of singular values to retain in the inverse. If this method is used, a Singular Value Decomposition (SVD) is performed on the FRF matrix at each frequency line, and only the “n” most prominent singular values are retained, where ‘n’ is the number entered in the “Nr of singular values” box. For setting the thresholds, it is useful to review the CMIF and condition number of the FRF matrix, as described in 7.3 Plotting FRF data.

Partial Contributions

Here, we are given the option to also calculate the partial contributions from each of the individual forces. By default, the partial contributions are not computed.

If the “Include partials” box is checked, partial contribution will be computed. If nothing is entered in the “Partial groups” box, the contribution from each of the individual forces will be computed. However within this box, one can also combine e.g. the X/Y/Z contributions at each of several interfaces. In this box, enter the indices of the forces which should be combined, separated by commas. For example, entering “1-3, 4-6” or “1 2 3, 4 5 6” will compute the partial contributions for the first three forces combined, and for the following three forces combined.

Time Domain conversion

Blocked forces and component TPA syntheses can be transformed back to the time domain. This conversion can be activated in the settings of the Analyze card, selecting “Time Domain conversion”. To perform this step, it is necessary to have the “Auralization” license in SOURCE.

The time domain conversion takes place after the blocked forces and TPA computation. The steps that SOURCE does to perform the conversion are the following:

  1. Invert the FFT to obtain time blocks from the spectral results;
  2. Apply a Hann window over the time blocks to ensure a smooth transition between the blocks;
  3. Merge the blocks to generate a continuous time signal;
  4. Compensate for any windowing that was applied in the spectral analysis.

The recommended settings for the purpose of auralization are the following:

  • Rectangular window
  • 50% overlap

Duplicating analyses

One of the strong features of SOURCE is the ability to perform several analyses on any combination of datasets. By right-clicking on an analysis in the Analyses card and selecting “Duplicate”, we create a duplicate analysis. We can then change, for example, the matrix inverse settings or the forces used in the TPA, and easily see what effect this has on the results.

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