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Setting Up FE Harmonic Modal Analysis to Match Experimental Data

Analysis, Data Acquisition

Modal analysis helps test engineers identify a structure’s response to excitation. Often, the engineer uses a mechanical device to excite the test item and records the output. Then, the software divides the recording into simple mode shapes that describe the structure’s response. The overall structural response helps determine the natural vibration frequencies and validate the design.

Alternatively, the engineer might use the modal data extracted from computer-aided design (CAD) finite element (FE) models. These multi-dimensional models are a tool for visualization, estimation, and analysis. CAD is an efficient and accessible alternative to experimental modal testing, but the engineer must select the analysis parameters to reflect the observed response as closely as possible.

Setting Up FE Harmonic Modal Analysis

A VR engineer ran an experiment to confirm the best practices for CAD simulation for modal analysis. Specifically, they wanted to examine an unconstrained method of FE modal analysis.

To do so, they performed an experimental modal analysis and FE analysis of a shaker’s head expander. Then, they compared the frequency analyses to determine the accuracy of the FE analysis.

Consideration to Fixture Design

free hanging setup

A test item can be constrained or unconstrained, meaning it can be attached to one or more mounting points or freestanding. The fixture design may depend on the purpose of the modal analysis, although some methods are more accurate overall.

Constraints have one of the most significant impacts on the accuracy of a CAD simulation. An unconstrained method is a standard approach that can produce favorable results, but the engineer must consider the effect of translational and rotational degrees of freedom.

The first six vibrational modes identified in an unconstrained FE model (three translational and three rotational) are associated with the six degrees of freedom at 0Hz. These rigid body modes represent the structure’s movement in the six directions of motion. They must be anticipated for and removed to ensure an accurate analysis.

Experiment and Results

transfer function of head expander frequency response

The experimental paper describes the setup of the FE analysis in SolidWorks and compares the results to the experimental data.

The results confirm that an unconstrained FE model produces a frequency response reflective of the observed response, so long as the engineer understands that the first six modes are due to translation and rotation. If you’re interested in viewing the results and reading more, please visit the resources page.

Read the Paper


July 8, 2021


Cherie Stoll


Analysis, Data Acquisition

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