Creating a spectral map

spectral map creation

The following model, Spectral_Map.ame, based on a second order lag, will illustrate the creation process for a spectral map.

Note

You can get this model through Help > Get demo, from the Platform/AnalysisTools/SpectralAnalysis directory.

The second order lag is excited with the sum of 4 sine waves of different amplitudes, with a variable frequency.

The top part of the model (green) simply converts a signal variable into a rotary velocity in rev/min. Such a physical quantity is useful to make the link with real life models of rotating machines.

Figure 18: Second order lag model

The generated frequency corresponds to a run up from 500 up to 7000 RPM. The run up will be simulated during 100 seconds. The print interval is set to 0.001 s in order to observe frequency effects up to 500 Hz.

The discontinuity printout is disabled: FFT requires equally spaced time points. Note that the discontinuity points are always automatically removed before computing an FFT on data.

The output of the second order lag is plotted with respect to the RPM. The spectral map requires the X axis unit be homogeneous with a frequency (rev/min, rev/s, rad/s,...).

Figure 19: Analyzed signal v RPM

The spectral map is set with 200 FFTs and 20% overlap. Note the detrend type set to constant, since the analyzed signal oscillates around an average value of 100.

Figure 20: Set up Spectral map parameters

The spectral map obtained is the shown in the figure below. The X axis is normally scaled up to 500 Hz since T=0.001 and fc=1/2T. The maximum value on X axis has been resized to 300 Hz to zoom on interesting phenomena occurring at lower frequencies.

Figure 21: Spectral map of the run up

The figure shows the 4 orders of the system, each reaching their maximum value around 50 Hz.