This topic causes some confusion for many people. Like Experimental Modal Analysis (EMA) techniques, Operational Modal Analysis (OMA) techniques are very powerful and can have very good results, but there are several issues that need to be clearly identified when using OMA that often go unnoticed but can have consequences if not understood. Let's discuss some of the critical elements.
In Episode 5 we introduced the main difference between AME and AMO. This difference lies in the knowledge of the excitation force of the structure: AMO is often called Output only analysis, i.e. the analysis of the structure's response alone without knowledge of the excitation (also called background analysis). This is its greatest advantage, and also one of its disadvantages because :
1- there is no guarantee that the background noise actually causes all modes of the system under study to respond. This can lead to the definition of a system model that does not fully identify all dynamic features - only those features excited by the "unmeasured, unknown" force are estimated.
2- while the frequency can be estimated relatively easily, this is not the case for the damping. The most important point to note here is that, in general, AMO techniques tend to always predict much higher damping than actually exists - even in a linear invariant system.
As regards the continuous health monitoring of structures (the SHM), AMO methods are still the preferred solution. In some cases, a preliminary study phase is necessary. Here is an example implemented by AD-SIGNUM during the monitoring of the Ribou dam in Cholet to identify one of these correctly excited frequencies:
The black dots are the natural frequencies identified each day by our solution based on the transmission of 6 SMS each day for each sensor (4 for the structure status, 2 for the sensor status).
In green we have represented the variation of the temperature of the concrete (data Agglomération du Choletais/Direction de l'environnement)
and in orange the HST model (only S in our case).
It should be noted that the SHM measurements and processing are fully automated, there is no human intervention or filtering!
Do you have any questions?
Contact us