COMSOL _ frequency response of piezoelectric device

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Discussion Overview

The discussion revolves around modeling the frequency response of a thin PZT disc using COMSOL, specifically focusing on calculating the impedance (Z) and phase in relation to the current density and voltage. Participants explore various aspects of the modeling process, including boundary conditions and the implications of symmetry in the calculations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions how to calculate impedance Z versus frequency, expressing uncertainty about the relationship between V/I and Z.
  • Another participant inquires whether the solution was computed for the entire disc or just a segment, suggesting that symmetry may affect the results.
  • A participant describes their approach of using symmetry and adjusting the current density variable but still seeks clarification on calculating Z and phase.
  • There is a discussion about the definition of current density, with one participant questioning whether to use the imaginary part of the current density.
  • Concerns are raised about the impedance being flat and near zero at frequencies below resonance, suggesting that the definition of current density may not be yielding accurate impedance calculations.
  • Another participant suggests that the absence of damping in the model could explain the zero impedance at resonance, indicating that including damping might correct the impedance values.

Areas of Agreement / Disagreement

Participants express differing views on the correct approach to defining current density and calculating impedance. There is no consensus on the best method to achieve accurate results, and the discussion remains unresolved regarding the correct definitions and implications for the model.

Contextual Notes

Limitations include potential missing assumptions regarding the definition of current density and the impact of damping on impedance calculations. The scope of the discussion is confined to the specific modeling setup in COMSOL and does not address broader implications or alternative modeling approaches.

luis_cam
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Hi,

I'm trying to model the frequency response of a thin PZT disc using COMSOL, but I am not sure how to calculate the impedance Z vs frequency. I defined the current density at the top electrode as I=-imag(nJ_smpz3d) and then plotted V/I using the domain plot parameters in the postproc menu. This gives me a V/I vs freq plot, but V/I is not equal to Z!

Does anybody know how to calculate Z? And the Phase?

Thanks!
 
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Hmm...

Did you compute the solution for the entire disc or did you make use of the symmetry and only compute the solution for a segment?
 
Last edited:
In fact I did both cases, the main difference is that when I used symmetries then defined the variable I as I=-imag(nJ_smpz3d)*N, where N is the number of segments contained in the whole disc, i.e., if a 10 deg segment then N=36. I obtained in both ases the same solution.

However the question still remains, and is: how do we, in either case, calculate Z? Is V/I = Z using the difinition above?? Or not? How do I use the current density to calculate Z? and the Phase? Is there a way of calculating Z without defining I?
 
luis_cam said:
In fact I did both cases, the main difference is that when I used symmetries then defined the variable I as I=-imag(nJ_smpz3d)*N, where N is the number of segments contained in the whole disc, i.e., if a 10 deg segment then N=36. I obtained in both ases the same solution.

However the question still remains, and is: how do we, in either case, calculate Z? Is V/I = Z using the difinition above?? Or not? How do I use the current density to calculate Z? and the Phase? Is there a way of calculating Z without defining I?
If you have set up your model correctly V/I should give you the impedance.

How have you defined the potential difference?
 
From the boundary settings dialogue box, I grounded the base of the disc and applied 1V to the top. The rest of boundaries are set at "zero charge/symmetry"
I am using the "frequency response" analysis, and I am varying the frequency from 3 to 4.5 MHz in steps of 0.02MHz (I expect the disc to resonate at around 3.8MHz since it is PZT 0.6 mm thick. I defined I at the top electrode as I=-imag(nJ_smpz3d)

Should I define I=nJ_smpz3d, rather than the imaginary part?
Are you sure Z=V/I, i.e., V/(-imag(nJ_smpz3d))? Do you know how do I find the phase?

Looking at the plot of the results I obtain, there is a resonance at around the right frequency (plus another one at 4.2MHz I don't know where it comes from...), but somehow does not look totally right: Z is flat and near 0 at frequencies lower that the resonance. Then I assume that my definition of I is giving me information about the resonance but Z is not well calculated...

Thanks!
 
if your imedance is zero at resonance, then it means you have not defined damping in your model. By defining damping you lower the Q *quality factor) and hence will provide you with the right resistance (impedance).

Good luck
 

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