High-quality test-based models require good setup and signal quality of the measured data (link). During a measurement, you want the quality of your FRFs to be good in the entire frequency range of interest. In some occasions, one hammer tip is not enough to excite the full frequency range with the necessary energy. For this reason, in DIRAC you can do multi-hammer measurements, where you use multiple hammer tips to excite the component. To understand how to set up a multi-hammer measurement, check this article.
You have to check the quality of the hammer merge at the beginning of your measurement campaign, after performing a couple of impacts with each hammer. We recommend performing 3 to 5 impacts with each hammer at three different excitation locations (for example one in each direction).
After doing a couple of impacts with all the hammers you want to use, you need to check whether the merge is set up correctly. This includes checking the filters applied to the hammers and their cut-off and roll-off frequencies. To learn how to do this, read this article, in particular from paragraph “Merging the hammers signal”.
After the merge settings, you need to check the coherence of the signal from the two hammers at low and high frequencies, to verify if there is enough repeatability. You do this in Analyze.
If you see different levels of coherence when going from low to high frequencies, your impacts with one of the hammer tips were not repeatable. Check if you have selected the correct hammer tips, by following the instructions in this article.
You must also check the coherence at the merge, in the roll-off frequency range. This tells if the cut-off and roll-off frequencies are set properly. In Analyze, set the matrix frequency range as the roll-off frequency range. If the coherence drops, either the impacts with the two hammers are not similar (so you impacted at two different locations or with an angle) or the cut-off and roll-off frequencies are not set properly. You can also visualize the FRFs of the two different hammers in the Graphing area. Here, you can see the mismatch between the curves measured with the different hammers.
When the coherence drops in the roll-off range, we recommend deleting the measured data and doing some more impacts with both hammers. If the coherence does not improve, the hammer merge is not performed at the correct frequency. You can try to improve it by increasing the roll-off frequency. If this does not help, you need to change the hammer merge settings fully, following the instructions provided in this article.
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