10-02-2011 08:35 PM
Hi everyone
Is it possible to determine the resonance frequency (natural frequency) from signal acquired from an accelerometer when the source of excitation on the body which the accelerometer is placed is at constant frequency excitation. or let me explain more. I placed an accelerometer on a building and excites this building with a shaker set at a constant frequency. I can acquire the time response of this building but is there anyway I can determine the resonance frequency from the time response signal?
10-02-2011 08:45 PM
If you excite a linear resonant system with a continuous sinusoidal input at some frequency other than the resonant frequency, you will not get any response at the system resonance after the initial transient has died down. Look at the basic differential equations.
If you can excite the system with an impulse, you should be able to get the resonant response directly. However, a realistic impulse may not have much energy and the signal to noise ratio can be poor.
Can you turn the shaker on and off fairly quickly? For example after one or two cycles of its fixed frequency? Then do this periodically (or at random intervals) and see if this puts enough energy into the system to excite the resonance.
Lynn
10-02-2011 09:04 PM
If you can not excite the system with an impulse and read the frequencey response, your best bet is to run a frequency sweep on the system. With a constant power output to the shaker table, read from the accelerometer and the frequency response is quite marked. There is always the possibility that the fundamental frequency is below what you have read and the highest response you see is a harmonic.
A building should have a very low resonant frequency (typically, anyway).
Rob
10-02-2011 09:09 PM
Thanks for your response,
Rob pls how do I apply a frequency sweep on the building?
Thanks
10-02-2011 09:26 PM
The FFT still looks like a time response after I did FFT on the signals. can anyone help pls?
10-02-2011 09:31 PM
Please post your VI with some typical data saved as default.
To apply a sweep you would need to vary the frequency of the shaker.
Lynn
10-03-2011 06:45 AM
I'm not sure what you are using for a shaker or control system for it, but most vibration test systems can perform frequency sweep and random (broadband) tests. The random test does not always show you resonances, so that the frequency sweep is your best bet for finding the resonant frequency.
Rob