System Identification Missing Frequency Component

Hi,

I am trying to identify a vibration system consisting of a beam and an accelerometer using System Identification Toolbox. I hit the beam with an impulse hammer and recorded force and acceleration data. I did FFT on the acceleration data and there were two peaks at 13 Hz and 89 Hz.

When I try to identify the system using time domain data, I cannot get a good result. The result has only the 13 Hz component. When I try with 4 poles, I get the same poles as before and the other poles and zeros cancelling out each other.

My problem is that I have no idea how to get both 13 Hz and 89 Hz components in my model. Should I be using frequency domain data?

Appreciate your help,

Gulay

[Rajiv Singh]

What is the sampling frequency of your data? It should be around 10 times the highest frequency you want to identify.