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Automotive Modal Analysis
Accelerometers; Dynamic Force Sensors; Modally Tuned®, ICP®, Impact Hammers; Electrodynamic Modal Shakers; and Accessories
Download BrochureClassical vs. Operational Modal Analysis
Classical modal analysis is the process of extracting dynamic characteristics of a vibrating system from measured force inputs and vibratory responses, whereas operational modal analysis extracts the dynamic characteristics of a vibrating system in its operating environment solely from vibratory responses. Both of these methods offer distinct advantages and disadvantages in designing and developing today's automotive structures (e.g., automobiles, trucks, ATV, etc.) and their systems and components (e.g., body, engine, exhaust, etc.)

Why Classical Modal Analysis?
Classical modal analysis is a more mature technique, in comparison to operational modal analysis, and is extremely useful in the design of automotive structures. The understanding and visualization of scaled mode shapes is invaluable in the design process to identify areas of weakness and provide direction on structural improvements. Enhanced computing power and advances in finite element analysis (FEA) techniques have increased the fidelity of today's automotive analytical model and in several cases have reduced the need for classical modal analysis, especially with legacy structures. However, classical testing will continue to be required to give engineers the confidence they need to continue to bring new product into development in today's competitive automotive market. Common applications for classical modal analysis include:

  • Modal alignment
  • Analytical model correlation
  • Design studies
  • Force response simulation
  • Cascade target setting

Modal alignment is performed early in the design process to mitigate risk of structural resonance issues in the automotive structure. The desired resonant behavior of structures, systems, and components is mapped out prior to design and development and is predominately used as a constraint in the design process. Adherence to this requirement is performed analytically and experimentally with early development prototypes.

Four Primary Assumptions of Classical Modal Analysis
Whether it is quick troubleshooting or full model correlation, successful classical modal analysis relies heavily on adhering to the four primary assumptions: observability, linearity, time invariance and reciprocity.
Modes of interest are observable:
  • Response Degrees of Freedom (DOF) need to have adequate spatial resolution (both sensor location and orientation) to represent the modes of interest
  • The input location and forcing function need to adequately excite the modes of interest
Test structure behaves linearly:
  • The input and output characteristics need to remain proportional within the measurement range
  • This assumption is best confirmed using precisely controlled inputs from a shaker at a range of input force levels and comparing the resulting Frequency Response Function (FRF) measurements
Test article exhibits time invariance & stationarity:
  • Modal parameter estimation algorithms need to assume consistent global modal frequencies and vectors
  • Modal parameters need to remain consistent throughout the entire data set
  • Changes in the test environment (temperature, humidity, etc.) during the data acquisition process need to be minimal
Maxwell's theory of reciprocity must be followed:
  • The FRF matrix is symmetric; meaning the FRF between input A and output B is the same as the FRF between input B and output A
  • Excite with shakers and measure response with an array of accelerometers or rove the input with an impact hammer and fix a few reference accelerometers
Why Operational Modal Analysis?
Although the technique is still being refined, many of today's automotive engineers choose operational modal analysis over classical modal analysis because of its simplicity of test, in situ test configuration, and ability to separate closely coupled modes. Unlike classical modal analysis, there is no requirement for instrumented force applicators such as modal shakers or impact hammers, only that the excitation is random in time and that it is spatial. This can be accomplished either from operational forces and/or external inputs. The ability to test the structure in situ allows for efficiency and flexibility. Assuming adequate spatial resolution on the responses, closely coupled modes can be extracted due to the random nature of forces acting on the test structure. When done correctly, this technique will extract the same modal information as a classical modal test including natural frequencies, damping ratios, and mode shapes. Obtaining this real-world data allows automotive engineers to confirm dynamic properties of automotive structures based on true boundary conditions and actual excitation sources and levels.

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