Do you need to do modal analysis without controlling the input?
You might wonder what Operational Modal Analysis, OMA, is and how it differs from the traditional Experimental Modal Analysis (EMA) that has been around for the last decades. Operational Modal Analysis is also called output-only modal analysis, ambient response analysis, ambient modal analysis, in-operation modal analysis, and natural input modal analysis. No matter which name that is used the idea is the same: To do modal analisys without knowing and/or controlling the input excitation. This modal technology is capable of estimating the same modal paramters as the traditional known techniques. The modal parameters are the mode shape, the natural frequency and the damping ratio. Some thinks that Operational Modal Analysis just is another name for Operating Deflection Shapes, ODS. This is not the case. Operational Modal Analysis separates noise and input and returns the modal information only.
There are a number of benefits in using the Operational Modal Analysis compared to the more traditional techniques.
Multiple Input Multiple Output Modal Technology
The Operational Modal Analysis are Multiple Input Multiple Output, MIMO, techniques. This means that the techniques are capable of estimating closely space modes and even repeated modes with a high degree of accuracy. Traditional modal anlysis techniques are typically Single Input Multiple Output, SIMO, or Multiple Input Single Output, MISO, or in rare cases even Single Input Single Output, SISO. Such testing procedures will not be able to find repeated poles due to the lack of mode seperation.
Easier Laboratory Modal Testing
There is no need for vibration shaker or impact hammer anymore. If you are in your lab doing modal testing in a test rig on some structural component, just do some random tapping on the structure while you are measuring the vibration response in multiple locations. The tapping must be random in time but also spatially. The excitation produced in this way will be a good approximation of a multivariate white noise stochastic process.
Winning Technology in In-situ Modal Testing
Vibration shakers and impact hammers are impossible as excitation sources when it comes to insitu testing of structures, such as buildings or rotating machinery. In cases like this the traditional modalanalyse fails, because there are a number of unknown inputs acting on the structures. What is a problem for traditional modalanalysis is a strength for Operational Modal Analysis. The more random input sources there are the better the modal results gets. Since the real strength of the technology really lies in the in-situ testing it is no wonder why the technology is called Operational Modal Analysis. Another important features that comes for free are that the estimated modes are based on true boundary conditions, and the actual ambient excitation sources.
Please have a look at the Technical Review for a technical introduction OMA and its similarities and differences to EMA.