Transfer Function

[Carlos Gutierrez]

I've the following enviroment: I'm generating a stimulus signal with
LabWindows/CVI(Frecuency Swept) and acquiring a response.

I would like generate the function transfer with this data and display it as
a Bode Diagram (Frecuency Domain).

My questions are:

How can I generate the Frecuency swept?
How can I generate the Bode Diagram?

Thanks for your attention.

[Jason F.]

You can use the TransferFunction() function found in the Advanced Analysis library to generate the data for a Bode plot (assuming you have a properly acquired signal to input to the function), and use a regular graph set to log scale for plotting your transfer function on. To understand the proper use of TransferFunction just right-click on the function panel and look at the help. To set up the graph for log scale just double-click it in the UIR editor and then click the axis button for the axis you want to plot against and check the "log scale" box in the axis properties dialog.

Jason F.
Applications Engineer
National Instruments
www.ni.com/ask

[RPM]

Generating a rotating machine Bode/polar plot from a Transfer Function feature is fine as long as the parameters used to obtain the Bode plot reflect actual machine inputs. How? Use the tachometer signal normalized to unity-gain-vector as the denominator of the TF [Amplitude-vibration/normalized-unity-tacho-vector]. Complex division of any frequency swept amplitude by a unity vector yields the magnitude back with a phase angle component related to the one per rev rotation. For more in depth information about this process go to rpmengr.com and look up the UNITY VECTOR BODE PLOT paper. It gives the reference as to where to seek the full paper publication. Bode's amplitude and phase and polar plots generated by the UNITY VECTOR Method compare very well to the same pl
ots derived from a tracking filter as long as the acceleration decelaration rate of the shaft is slower than the recursion rate of the software doing the TF. Tachometer uUnity vector process is greatly simplified by using an external box that latches onto the tach signal and delivers an instantaneous unity vector phase-locked to the tachometer. The latter usually has a variable amplitude and/or waveform shape (non-contact-probe or TTLpulse etc.) as a function of speed thus a need to have a dedicated circuit to phaselock the any tachometer signal. RPM & Predictive Engr. designed this UNITY VECTOR BOX as an external box fully dedicated with analog circuitry to catch elusive (AC?DC coupled) tachometer signals. rpmengr@cts.com