After finding the natural frequency of a system, what could be done to stop or reduce the systems resonance being excited? Basically put, how do we avoid resonance
This is simple in theory, but not always so simple in practice.
If the natural frequency is
And where the undamped natural frequency is ,
Where k is stiffness and m is mass.
Therefore to avoid resonance being excited we must change either k or m or both. In general, the fundamental consideration for an example SDOF system is to make the system as stiff as possible, increase k, but keeping the mass as low as possible, decrease m. This will have the effect of raising the natural frequency, the objective is to raise it enough that it is outside of the working range or out of the excitation range.
So, in practice how could this be carried out?
As a guide the general rules of thumb are
- Stiffening without adding mass raises the natural frequency.
- Adding mass without stiffening lowers the natural frequency.
- Increasing damping lowers the response, but widens the range of the response.
- Decreasing damping raises the response, but in a narrower range.
- Reducing the forcing function reduces the response.
You may be dealing with one or any combination of the above list with an initial design. However, modifying a design after can be more complex. For example, if the stiffness is increased, but the change adds mass, it is possible the resonance would not have changed as the two changes could have cancelled each other out.
When working with new designs, or modifying existing designs, simulations can be left wanting. In all situations it is best practice to test the system before and after changes.
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James Wren was Sales & Marketing Manager for Prosig Ltd until 2019. James graduated from Portsmouth University in 2001, with a Masters degree in Electronic Engineering. He is a Chartered Engineer and a registered Eur Ing. He has been involved with motorsport from a very early age with a special interest in data acquisition. James is a founder member of the Dalmeny Racing team.