The coherence is a quality indicator that allows you to check the quality of your signals. It checks how repeatable measurements of a particular measurement point were performed.
You should check the coherence of your data after measuring the excitations of one VP. This allows to spot and correct problems before moving to the next VP. Integrating this check into your measurement process ensures high-quality test-based modelling. (link)
In Analyze, there are some tools to investigate the coherence of the test-based models in every level of detail. The coherence matrix of the measured data is the easiest way to have an overview of the entire dataset to quickly verify its coherence.
To read the coherence matrix correctly to evaluate the quality of your FRFs, you have to:
The white areas correspond to the FRFs with the lowest signal quality. If the whole matrix turns white at the same time, you have very similar signal quality in your data. If you’re satisfied with the minimum coherence, you can move on to the next quality checks.
A good test-based model has no clear white patterns in the coherence matrix. The minimum coherence varies according to the component and the chosen setup, but is usually above 90%.
However, if there are clear white patterns in the matrix (as in the pictures above), you have signal problems in the data that you need to react to.
If you spot a white row when zooming in, this means that there’s a problem with that sensor channel. You have to identify the sensor, and check if there’s a broken part in its measurement chain. If the matrix row is only partly white, this would be an indicator that the sensor fell off at some point during the tests. To go, you have to fix the sensor and measure all impacts of this VP again.
If you spot a white row when zooming in, this means that there is a problem with that sensor channel.
If you identify white columns while zooming in into the matrix, it means that the repeatability of an impact was relatively bad. You have to identify the impact and make some more measurements. If you cannot improve the coherence, you should consider moving it to a position that you can excite more precisely. It is also important to use the Automatic Measurement Selection, which will choose the most similar impacts for you.
If you spot a white column when zooming in, this means that the repeatability of an excitation point was bad.
Symmetric VPs on a component should have similar coherence values. If the blocks in the matrix corresponding to these symmetrical VPs have different coherence, improve the impacts of the worse one.
If the blocks in the matrix corresponding to these symmetrical VPs have different coherence, improve the impacts of the worse one.
In the end, you should check whether the coherence is good for the entire frequency range you are interested in. If the coherence is low for the high or low frequencies, you should consider adding an input source with a different hammer tip.
When you use multiple input sources, also remember to check the coherence in the merging range. If the coherence drops in this frequency range, either the impacts with the two hammers are not similar and need to be repeated, or the cut-off and roll-off frequencies need to be adjusted.
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