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Modal analysis in a few words

By 25 February 2021October 18th, 2021No Comments
Modal analysis

Introduction

Everything vibrates... Every object vibrates, and every object is characterised by its own vibration frequencies. Each frequency has its own corresponding deformation and a damping coefficient. These parameters are called "natural characteristics" or "dynamic properties". This is where modal analysis comes into play.

Modal analysis

Modal analysis is a process by which a structure is described by its "natural characteristics".

Example

Let us take a free rule. Imagine that an oscillating force (a force that varies sinusoidally, with constant amplitude) is applied to one end of the ruler and that the acceleration is measured at the other end. Let's imagine that we then increase the oscillation frequency of the applied force. Now, if we measure the response of the ruler, we will notice that the amplitude changes when we change the frequency of oscillation. " The response is amplified when we apply a force with an oscillation frequency that is close to certain particular frequencies. These particular frequencies are the natural frequencies of the system.

When you think about it, it's quite amazing since you apply the same amplitude all the time, only the oscillation frequency changes!

If we take the time data (accelerations for example) and apply the Fourier transform to it, we will see peaks at the resonance frequencies of the system... It is quite amazing to see that each structure has unique characteristic frequencies! Even more amazing is that the distortions of these natural frequencies also take different forms depending on the frequency of the excitation. Let's see what happens to each of these natural frequencies.

Let's place several sensors along the ruler and measure the amplitude of the ruler's response with different excitation frequencies. When we stop at the first natural frequency, we observe a bending deformation pattern. When we stop at the second natural frequency, we see a second torsional deformation pattern... and so on. These deformation patterns are called "modal deformations".

In conclusion

These natural frequencies and modal distortions describe each system in a unique way. These parameters help in the design of systems, in a better understanding of their responses in their functional environment and in the monitoring of their health status (their variations can be synonymous with anomalies) all thanks tomodal analysis.