7.3 Measurement Matrix

Matrix settings

Below the matrix, there are options to edit the data displayed in the matrix; see the following figure. This includes: frequency range, averaging, and quality mode (coherence, consistency, or reciprocity).

The frequency range over which the statistics are calculated can be edited using the slider bars or manually entering values. Some presets are included for Low (0 – 100 Hz), High (0 – 3000 Hz), and Focus (20 – 1000 Hz) frequencies. The presets can be edited by right-clicking on them with a specific frequency range already selected. Averaging can be set to Mean, Minimum and Maximum: this defines the averaging over the selected frequency range.

The range for the values in the matrix can be edited by dragging the color bar on the right. Right-click presets are available for 0–100%, 50–100%, and 90–100%.

Quality modes

There are several modes available for reviewing the quality of the measurement matrix. These options are accessed using the Mode drop-down just below the matrix, as shown in the figure above. The available quality modes include: Coherence, Magnitude, Response/Force Consistencies and Reciprocity. The active mode can be easily recognized by the color displayed in the matrix.

Coherence (green)

The diagram shows the coherence over frequency. For measured FRFs, this is intuitive. For the various VP transformed FRFs, coherence is propagated from the measured FRFs. The VP transformation matrices are used here to calculate weighted averaged values of the “input” coherence.

Magnitude (blue)

The diagram shows the magnitude of the FRF over frequency. This is intuitive for any measured or VP-transformed FRFs.

Response Consistency (red)

The diagram shows the specific response consistency over frequency for those response channels that are associated with a Virtual Point. For measured data, an additional curve is added to the graph in purple, which is the VP-filtered FRF for the selected FRF element.

For the various VP transformed FRFs, the consistencies are propagated from the measured FRF. The VP Transformation matrices are used here to calculate weighted averaged values of the “input” consistencies.

Force Consistency (red)

This mode is similar to the response consistency mode, as described above, but for specific force consistency.

Reciprocity (green)

In Reciprocity mode, only available for VP FRFs, the diagram shows the reciprocity between different VP DoF. Since the inputs and outputs of VP FRFs are truly collocated (some would say, “vectorially associated”), reciprocity can strictly be required on the matrix. Thus, the measurement matrix value shows the level of reciprocity between reciprocal entries. As a result, the diagonal is always 100%. Matrices with 100%-score throughout all entries are rarely seen; rather it is advised to focus on directions that have significant “coupling” as a consequence of the structural nature of the component at hand. For diagonal entries, it is interesting to assess the “driving-point” FRFs’ passivity by verifying if the phase angle is bound by 0–180 degrees.

In this mode, the graphing card shows the selected FRF (blue) together with reciprocal FRF (red), which is mirrored along the symmetry axis of the matrix. When blue is not seen, it means it is hidden by orange, and you have in fact reached 100% reciprocity.

Matrix options

There are additional options that can be used to customize the display of the measurement matrix. These options are available using the buttons in the upper right corner of the measurement matrix card, as shown in the following figure.

Changing the axis labels

It is possible to change the labels that are displayed for the response and reference channels. This is done by clicking the settings icon and choosing the desired label type.

Hiding empty cells

There are several scenarios when it may be desired to hide empty columns or rows. For example, if not all excitations have been measured, or if additional VPs are included in the project but have not been measured. The empty cells are hidden by clicking on the Settings icon and selecting Hide empty cells.

Filtering desired channels

For very large datasets, it may be desired only to display a subset of the channels in the matrix. This action is done using the filter icon and toggling on or off the channels that should be shown or hidden, respectively.

Note that filtering is only used to preview data in the matrix, but a custom preset is required to export a subset of the matrix.

Using the frequency lock

The Graphing card has a toggle button FL, which stands for “frequency lock”. When activated, the frequency range of the graphing card and the frequency range selection of the measurement matrix stay synchronized.