The algorithm used by DIRAC is based on the Principal Response Function (PRF). The PRF helps to reduce the effect of noise and to fit modes on a large number of FRFs quickly. The LSCF algorithm (Guillaume et al., 2003, link) is used to find the poles of the system. Doing this over multiple iterations results in the stabilization diagram, from which the stable poles are selected. These are then used to find the mode shapes for each mode of the system.
The modes of your structure are calculated using the Principal Response Function (PRF) algorithm.
To calculate the modes in DIRAC, you have to:
You can create a new preset in the presets dropdown.
After calculating the modes, you can change their frequency, damping and name, according to your liking.
To edit a mode, complete the next steps:
You can decide whether a mode should be used or not for the calculation of the synthesized FRF. By default, all calculated modes are used.
To deactivate a mode, you have to:
To calculate the modes, you have to select the preset to use and define the desired settings in the Modal Fit Settings card. The settings are the following:
The stability diagram shows how the algorithm determines the true number of poles in the system. Poles are compared according to two metrics: their eigenfrequency and damping value. The stability of a pole is determined by comparing each pole to the poles from different iterations. Poles can be marked as:
In the stability diagram, you can visualize the poles and the iterations.
To visualize the stability diagram in the Stabilization card, you have to:
You can plot poles against damping by right-clicking on the stabilization diagram and selecting Plot poles against and selecting Damping.
You can manually select and deselect poles in the stability diagram.
To select a pole, you have to:
Similarly, you can deselect a pole by clicking on it in the stability diagram
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