# COMSOL _ frequency response of piezoelectric device

1. Apr 8, 2009

### luis_cam

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!

2. Apr 8, 2009

### Hootenanny

Staff Emeritus
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: Apr 8, 2009
3. Apr 9, 2009

### luis_cam

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?

4. Apr 9, 2009

### Hootenanny

Staff Emeritus
If you have set up your model correctly V/I should give you the impedance.

How have you defined the potential difference?

5. Apr 9, 2009

### luis_cam

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!!

6. Dec 10, 2009

### krgoutham

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