National Instruments Order Analysis Toolset - Manual (Page 47)

© National Instruments Corporation 5-1 LabVIEW Order Analysis Toolset User Manual
5
Calculating Rotational Speed
This chapter describes the digital differentiator method used by the
LabVIEW Order Analysis Toolset to calculate rotational speed and
describes averaging pulses to smooth noisy calculated rotational speed
results.
Digital Differentiator Method
To successfully extract order components, create spectral maps, and
calculate waveform magnitudes, you need rotational speed as a function of
time during the data acquisition process. The LabVIEW Order Analysis
Toolset uses a digital differentiator to calculate the rotational speed. The
tachometer processing VIs buffer the input tachometer pulses. Because the
tachometer-processing VIs buffer the input tachometer pulses, they can use
tachometer pulses occurring before and after each time instance when
calculating rotational speed. Using tachometer pulses occurring before and
after each time instance improves the accuracy of the calculated rotational
speed.
Figure 5-1 illustrates a typical tachometer pulse train and the cumulative
rotation angle
θ of the shaft as a function of time t.