×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Getting model from frequency response

Getting model from frequency response

Getting model from frequency response

(OP)
Hi All,

I need to know how can we get a mathematical model(state space or transfer function) from frequency response?
The frequency response is resulted from FFT analysis (Force vs. Acceleration.
Thank you in advance for any reply.

sweetday

RE: Getting model from frequency response

Is this for a class?

TTFN



RE: Getting model from frequency response

SWEETDAY:  The FFT results in "amplitude verses frequency", not force vs. frequency, where amplitude is typically acceleration.  This is obtained hy doing the FFT on a time domain plot of acceleration.  If you truely have the FFT you can do an inverse FFT and get back to the time domain out to the filter frequency.  The amplitude of the higher frequencies past the filter will be reduced and will probably be aliased.  If you have a PSD you can do an inverse PSD and get a time domain plot that may or may not be what you want.  

Regards
Dave

RE: Getting model from frequency response

In the experimenatl modal analysis world a frequency response function, or FRF, is usually a transfer function of acceleration/force vs frequency. (velocity/force, diplacement/force and all their inverses are also used)

It is obtained from the complex spectra of the two signals in a variety of ways, known as H1 H2 H3 and HV (the latter being an engineer's joke by a Mr Harvard Vold).

H1 = (BxA*)/(AxA*)

where B and A are the original complex spectra of each channel, and * means conjugate.

Each of these H formulations has its uses, as they are snsitive in different ways to the noise in each of the data channels. Obviously H1 is more sensitive to errors in A than B for example.

 

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Getting model from frequency response

Perhaps you are asking how to get a set of eigenvectors (modeshapes) froma set of FRFs.

There are many ways, get Ewins book.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources