I am performing experiment on function generator and software TSWA. I got results for frequency response curve. In results, it is showing only absolute values(+ve) of displacement. I draw a graph on MATLAB, which shows graph greater than zero. Why my system is not showing only magnitude?
I am not an expert here on frequency response but your question popped up twice, so I thought about it. Could it be that the output is in fact positive always? If it is a frequency I would think so (what is a negative freq?) The reason I suggest this is that I doubt that there is something hijacking your output and taking the absolute value of it!
Dear Morton E. O'Kelly ,
I am saying about displacement as the positive displacement occurs when the beam is moving away from the laser displacement sensor and the negative displacement occurs when moving towards the laser. But in frequency displacement, it is always showing positive. So i am confused what to do and how to overcome this one?
In general, the FRF is a complex function of the excitation frequency. What you obtain your software is probably its magnitude only, which is the amplitude of the (harmonic) displacement. Such an amplitude is nonnegative by definition. It is just a factor by which you multiply your harmonic response. The response of the opposite sign is indicated rather by a phase shift (180 deg) than by a negative amplitude.
Hi Rizwan.
I confirm the answer of Jean and I add this.
The spectrum of a signal is a linear mathematical transformation which projects the signal on harmonic a functions basis, from a physical point of view the value of the answer corresponds to the elementary component of the signal or its amplitude which is conventionally taken positive by definition of the transform.
A negative value corresponds quite simply to an opposite phase.
Moreover the amplitude is in direct connection with the energy contained in this signal or this mechanism.
You can look for the theorem of Parseval in signal theory.
Dear Rizwan,
I think that you have derived the magnitude of the displacement and not the 'complex displacement response'.
If not, if you are using the complex displacement fuction in the program, so you may have used a code plot for the FRF that draws the magnitude. I mean that the built in code used may be done to draw the nagnitude, you can chech its details and use another one.
Thanks.
Hi
I attest to what the above authors are stating.
FRFs can be presented as amplitude-phase or Real-Imaginary data to show the motion you discuss when excited by the function (cos(wt) + j*sin(wt)), where w is angular frequency and j = sqrt(-1).
Perhaps these basics texts from Hewlet Packard can help clarify matters? They are old, but they are good and very clearly written.
http://cp.literature.agilent.com/litweb/pdf/5952-8898E.pdf
http://cp.literature.agilent.com/litweb/pdf/5954-7957E.pdf
Sincerely
Claes
Hi,
I think that your response is calculated as Amplitude= [(Real_part^2)+(Imaginary_part^2)]^(1/2). Because of that you just see the results as positive values.
Sincerely
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