Operational Modal Analysis

Operational Modal Analysis is based on measuring only the output of a structure and using the ambient and operating forces as unmeasured input. It is used instead of classical mobility- based modal analysis for accurate modal identification under actual operating conditions, and in situations where it is difficult or impossible to control an artificial excitation of the structure.

Many civil engineering and mechanical structures are difficult to excite artificially due to their physical size, shape or location. Also civil engineering structures are loaded by ambient forces, for example, waves (offshore structures), wind (buildings) or traffic (bridges), and operating machinery exhibits self-generated vibrations. These natural input forces, which cannot easily be controlled or correctly measured are used as unmeasured input for Operational Modal Analysis. In classical modal analysis, they would be superimposed as noise on the controlled artificial forces and would provide erroneous results.

For aircraft, vehicles and operating machinery there is a need to determine real-life modal parameters using actual operating conditions, that is to say, actual boundary conditions, actual spatial and frequency distributions of forces and actual force and response levels.

Advantages of Using Operational Modal Analysis

The main advantages of Operational Modal Analysis are:

• The measured response is representative of the real operating conditions of the structure
• The setup is simple, straightforward and fast, as only accelerometers are used
• The measurement procedure is simple and quite similar to Operating Deflection Shapes  (ODS) analysis
• Costly downtime can be reduced doing in-situ testing during normal operation. No interruption or interference with the operation of the structure are required
  

Core technology behind the Operational Modal Analysis software is protected by patents. This includes the efficient and powerful Frequency Domain Decomposition and Enhanced Frequency Domain Decomposition algorithms.  

Operational Modal Analysis Type 7760

Type 7760 is the effective analysis tool for operational modal analysis. It contains powerful algorithms for accurate modal identification and is yet very easy to use with its task oriented user interface, intuitive workflow and high degree of automation.

Type 7760 is available in three versions: Pro, Standard and Light. The versions differ only in the number of techniques available.

Modal Test Consultant Type 7753 is used for geometry-driven data acquisition, pre-analysis and validation. Data are then seamlessly transferred to OMA Type 7760 for modal parameter extraction and presentation. Together they form an integrated, easy-to-use modal test and analysis system.

Operational Modal Analysis systems from Brüel & Kjær are scalable and easy to upgrade in hardware and software.

	Modal Test Consultant Type 7753 is used for geometry creation, geometry-driven data acquisition, pre-analysis and transfer of data to Operational Modal Analysis Type 7760 for post-analysis
	The peak-picking method used in both the FDD and EFDD techniques lets you identify a mode by using a snap-to-peak facility. The identified modes can be selected and immediately animated. Automated mode estimation is also supported
	Using the EFDD technique deterministic signals (harmonic components) are automatically identified and their effect eliminated
	For the SSI techniques, a stability diagram is used to discriminate between stable, unstable and noise modes. Tools are available for optimal selection of model order
	A number of validation tools such as overlaid, difference and side-by-side animation, and MAC plots and tables are available for comparison of modes from different projects and techniques