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Modal Analysis

 

Dig right into the root causes of your noise and vibration problems with modal analysis

  • Modal analysis will help you reduce the noise level emitted by your product
  • Modal analysis assist in pointing out the reasons of vibrations cause cracking issues of components
  • Modal analysis can improve the overall product performance in specific operating conditions

For a large number of noise, vibration or durability related issues, modal analysis essential in discovering the root causes of the problem. Stop acting on the symptoms only and gain a profound insight into the true nature of your product’s weakspots as a first step in tackling them.

Content:

A. What is modal analysis?
B. Why should you use modal analysis?
C. More information on modal analysis?
 
D. Attend free live web seminars on modal analysis and vibration measurement

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A. What is modal analysis?

Modal analysis studies the dynamic properties or “structural characteristics” of a mechanical structure under dynamic excitation:
  1. resonant frequency
  2. mode shapes
  3. damping
To explain this in a simple manner, we’ll take a plate as a theoretical example. We’ll apply a force that varies in a sinusoidal fashion on one corner. Then, we’ll change the rate of oscillation (frequency rate) of the sinusoidal force, but the peak force stays the same. And then, we’ll measure the response of the excitation with an accelerometer attached to the other corner of the plate.

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The measured amplitude can vary depending on the frequency rate of the input force. The response amplifies as we apply a force with a frequency rate that gets closer and closer to the system’s resonant or natural frequencies.

The resonant frequency is the frequency at which any excitation produces an exaggerated response. This is important to know since excitation close to a structure’s resonant frequency will often produce adverse effects. These generally involve excessive vibration leading to potential fatigue failures, damage to the more delicate parts of the structure or, in extreme cases, complete structural failure.


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Example: when spinning, the washing machine’s drum vibrations induce such a powerful resonant frequency that the machine begins to actually move causing the door to spring open.




If we take the time data and transform it to the frequency domain using a Fast Fourier Transform algorithm to compute something called the “frequency response function”, we see the functional peaks that occur at the resonant frequencies of the system.

 

MODAL ANALYSIS FRFMODAL ANALYSIS FRF IN TL

Left: example of a simple plate frequency response function

Right: example of online FRF curves in 
LMS Test.Lab during impact testing

 

Deformation patterns (bending, twisting …) at these resonant frequencies take on a variety of different shapes depending on the excitation force frequency. These deformation patterns are referred to as the structure’s mode shapes.

 


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Example of second mode animation: Noise and vibration engineers appreciate the animated resonance display of LMS Test.Lab




Structural damping provides information about how quickly the structure dissipates vibrational energy and returns to rest when the excitation force is removed.

Modal analysis 6 Modal analysis 2








Example: in the well-known case of the Tacoma bridge, the damping was not high enough to absorb all the excitation energy.
 

Modal analysis refers to a complete process including both an acquisition phase and an analysis phase. The structure is excited by external forces such as an impact hammer or shaker. In this case, we talk about experimental modal analysis.

Modal testing systems consist of transducers (typically accelerometers and force cells), an analog to digital converter or front-end to digitize the analog instrumentation signals and a host PC to review and analyze the data.

Operational Modal Analysis
complements traditional modal analysis methods. It only measures the response of test structures under actual operating conditions. It is used to test cars, airplanes, wind turbines and any other applications that are difficult or even impossible to excite by external force, owing to boundary conditions or sheer physical size. Modal testing results can also be used to correlate simulation analysis and create a ‘real-life’ simulation model.

To guarantee realistic high fidelity simulations, it is essential that simulation models meet stringent accuracy standards. Ensuring reliable simulation results requires component, subsystem and full-system models to be compared with experimental data, or alternatively validated models of similar structures. Building and validating system models from the bottom up is the only way to prevent accumulating inaccuracies. Besides more reliable what-if analyses, validated models provide a better understanding of assumptions made regarding material properties, connections, joints and boundary conditions.


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B. Why should you use modal analysis?

 

 

Modal analysis helps you understand how a structure vibrates (frequency, damping and mode shapes)

Modal analysis can be used for:
  • Troubleshooting
    • Direct insight into the root cause of vibration problems
  • Simulation and prediction
    • Find structural flexibility properties quickly
    • Monitor incremental structural changes
  • Design optimization
    • Design according to noise and vibration targets
    • Enhance performance and reduce component and overall vibration
    • Fast, test-based evaluation of redesign for dynamics
  • Diagnostics and health monitoring
    • Confirm product quality from the production line and in the field
Modal analysis benefits:

  • Competitive advantage with better performing products
  • Fewer prototypes
  • Less insulation and absorption material required
  • Shorter development cycles
  • Fewer product recalls
  • Faster intervention in the field
 
 
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C. More information on modal analysis?
 
2) Ask a LMS specialist to visit

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D. Attend free live web seminars on modal analysis and vibration measurement

 
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