Experimental Modal Analysis (EMA)

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An example mode shape animation.

Click to view an example mode shape animation.

MSI performs Experimental Modal Analysis (EMA) to determine the natural frequencies and characterize the vibration mode shapes of structures and mechanical systems.  Knowing system natural frequencies and mode shapes is key to avoiding and/or diagnosing resonance.  Resonance is the excitation of a system at one of its natural frequencies causing vibration amplitudes to increase unbounded until failure occurs, whether due to fracture in the short term, or fatigue in the long term.

EMA involves mechanically exciting the system under test via impulse, then measuring the resulting vibratory response.  The impulse is most commonly achieved by striking the system with an instrumented hammer, while the vibratory response is measured with accelerometers.  The size of the instrumented hammer is selected based on the system under test.  Smaller systems require lighter hammers, which better excite the higher natural frequencies commonly associated with small or lightweight structures.  Large systems, such as industrial turbomachinery, require heavier hammers capable of transferring the energy required to achieve a measurable response.

An MSI Engineer Performing an Experimental Modal Analysis Test.

An MSI engineer strikes a machine with an instrumented hammer.

The resulting vibration ring-downs from several repeated impacts are measured for each response measurement location, with the responses from each strike averaged together to reduce noise in the data.  Impacts are commonly applied in all three directions.  The response accelerometers are typically placed at many locations around the system under test to create a comprehensive mapping of the vibratory motion.  EMA can be performed on both stationary systems and on running systems to capture the effects of the forces and masses associated with operation on the natural frequencies.  When performing EMA on operating equipment, MSI’s Time Averaged Pulse (TAP™) technique enables the extraction of natural frequency and mode shape information from the vibration signatures associated with normal operation.

A Frequency Response Function Resulting from a Modal Test.

A frequency response function calculated from a modal test. The peaks identify the natural frequencies of the system under test.

The relative magnitude and phase lag between the response acceleration and the input excitation impulse are computed for all frequencies to produce Frequency Response Function (FRF) spectra.  The peaks in the magnitude spectra identify the natural frequencies of the system.  These spectra captured at all the various locations around the system can be mapped onto a CAD model of the system that can be animated at a particular natural frequency to characterize the associated mode shape of that frequency.

When analyzing the mode shapes of a system, it’s important to note the locations of zero motion (the nodes), the locations of maximum deflection (the anti-nodes), and the locations of known force inputs to the system.  When a force input to the system is located near a particular mode shape’s anti-node, and the frequency of that force input is sufficiently close to that mode shape’s corresponding natural frequency, resonance is likely to occur.

Animation of a lateral rocking mode shape of a pump created from an impact test.

Click to view an animation of a lateral rocking mode shape of a pump created from an impact test.

Experimentally determining natural frequencies and mode shapes is an important tool used in engineering with a wide variety of use cases.  Similar methods exist for other applications such as acoustic systems, fluid systems, and electrical systems.  If you’re interested in learning more about this testing method, or wish to discuss your needs with an engineer, please contact us.