Curve Fitting & Excitation Techniques
During modal testing, an instrumented hammer, electromagnetic shaker, or piezoelectric actuator is used to excite vibration of the test article.
The vibration amplitude and phase in response to the excitation is measured at multiple locations using sensors including accelerometers and laser vibrometers. Frequency response functions are calculated with a dynamic signal analyzer.
Curve fitting techniques are used to extract the natural frequencies, mode shapes and damping ratios of a structure.
Versatility of Modal Testing
These modal parameters may be used for finite element analysis (FEA) model validation (correlation) and refinement, failure investigation, vibration troubleshooting and diagnostics, and quality assurance.
Due to the versatility of modal testing it can go by several names such as resonance test, natural frequency measurement, frequency response test (FRF), modal survey, or modal vibration test to name a few.
Modal Testing Services Include:
- Natural frequencies, mode shapes and damping ratios
- Hammer, shaker and piezoelectric excitation
- Laboratory and field (on-site) testing available
- Finite Element Model Validation
- Modal Assurance Criterion (MAC)
- Generator vibration testing
Modal Testing Applications
Recent modal test applications include flight qualification of helicopter modifications, mixed flow impellers, gas turbine blades and an optical table.
The modal assurance criterion (MAC) is a technique for quantifying the comparison of mode shapes from two sources. A MAC value greater than 90% generally indicates highly correlated modes. SimuTech engineers often use MAC to compare the mode shapes from two test articles or to correlate the results from a modal test with those from a corresponding finite element analysis.
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