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From: Madhu on 3 Jun 2010 04:54
Hello Everybody,

I have the following data of the complex stiffness of a damper obtained from testing:

Frequency(in rad/sec)=[1.665,3.333,4.999,6.666,8.332,8.999,9.999,11.665,13.332,14.998,16.665,17.998];

Real=[0.001,0.002,0.000,0.000,0.000,0.000,0.001,0.001,0.001,0.001,0.001,0.001]

Imaginary=[12.906, 23.690, 38.625, 57.753, 79.373, 88.387, 103.227, 129.577, 151.260, 172.733, 197.908, 216.782];

I obtained the 3rd order transfer function for this data using invfreqs. But when I plot the freq Vs Response for this transfer function, I get a peak at 6.25 Hz. (As if the resonates at 6.25 Hz).. Can somebody suggest as to what is happening here.

Thanks
Madhu
From: Luca Zanotti Fragonara on 3 Jun 2010 05:15
"Madhu " <shurpalimv(a)yahoo.com> wrote in message <hu7qje$slb$1(a)fred.mathworks.com>...
> Hello Everybody,
>
> I have the following data of the complex stiffness of a damper obtained from testing:
>
> Frequency(in rad/sec)=[1.665,3.333,4.999,6.666,8.332,8.999,9.999,11.665,13.332,14.998,16.665,17.998];
>
> Real=[0.001,0.002,0.000,0.000,0.000,0.000,0.001,0.001,0.001,0.001,0.001,0.001]
>
> Imaginary=[12.906, 23.690, 38.625, 57.753, 79.373, 88.387, 103.227, 129.577, 151.260, 172.733, 197.908, 216.782];
>
> I obtained the 3rd order transfer function for this data using invfreqs. But when I plot the freq Vs Response for this transfer function, I get a peak at 6.25 Hz. (As if the resonates at 6.25 Hz).. Can somebody suggest as to what is happening here.
>
> Thanks
> Madhu

How can we possibly answer your question if we don't know the question? What's the problem. Why do you think that 6.25 Hz is wrong. What is the system.
From: Madhu on 3 Jun 2010 06:20
"Luca Zanotti Fragonara" <Luca_Zanotti(a)libero.it> wrote in message <hu7rrk$l0t$1(a)fred.mathworks.com>...
> "Madhu " <shurpalimv(a)yahoo.com> wrote in message <hu7qje$slb$1(a)fred.mathworks.com>...
> > Hello Everybody,
> >
> > I have the following data of the complex stiffness of a damper obtained from testing:
> >
> > Frequency(in rad/sec)=[1.665,3.333,4.999,6.666,8.332,8.999,9.999,11.665,13.332,14.998,16.665,17.998];
> >
> > Real=[0.001,0.002,0.000,0.000,0.000,0.000,0.001,0.001,0.001,0.001,0.001,0.001]
> >
> > Imaginary=[12.906, 23.690, 38.625, 57.753, 79.373, 88.387, 103.227, 129.577, 151.260, 172.733, 197.908, 216.782];
> >
> > I obtained the 3rd order transfer function for this data using invfreqs. But when I plot the freq Vs Response for this transfer function, I get a peak at 6.25 Hz. (As if the resonates at 6.25 Hz).. Can somebody suggest as to what is happening here.
> >
> > Thanks
> > Madhu
>
> How can we possibly answer your question if we don't know the question? What's the problem. Why do you think that 6.25 Hz is wrong. What is the system.

Ok.. Sorry for the short description. The data is obtained from different Force Defelction curves of the Shock absorber. The Shock absorber was excited with diff frequency upto 2.8 Hz (constant amplitude). At each frequency the force deflection curve was obtained and complex stifffness obtained from the curve. Now I have the complex stiffness at diff frequencies . I put this data into 'invfreqs' and get the tranfer function. When I plot the response of this transfer fucntion using freqs, I get a peak at 6.25 Hz. Since it is TF of a shock absorber, there shouldnt be any reason to get a peak at 6.25.

I know my data is only till 2.8 Hz and I am looking at response at 6.25, but that still doesnt explain why I get a peak only at 6.25 Hz.

I hope I have clarified my probelm..

Thanks for the help.

Madhu
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