Reference Data Processing (Sound and Vibration Measurement Suite)
December 2007
NI Part Number:
372416A-01
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Imperfections on a device under test can result in distortion or unwanted signals in order analysis applications. These imperfections affect the accuracy of proximity probe signals. A proximity probe measures the distance between the probe tip and the shaft surface. Because of the existence of shaft scratches, shaft bow, and variation in electrical properties, the shaft imperfections and resulting unwanted signals distort the vibration signals of interest. When the shaft rotational speed is low, shaft imperfections dominate the vibration signal that the proximity probes measures. In this case, you can perform slow-roll compensation. In slow-roll compensation, you collect a reference signal by setting the shaft to rotate at a low speed. This reference signal is called the slow-roll reference signal. You then subtract the reference signal from the signal you acquire during normal operation to remove imperfection errors. The slow-roll speed is typically less than 10 percent of the first resonance speed.
Complete the following steps to perform slow-roll compensation.
- Extract the slow-roll reference signal. Use the OAT Get Vector Reference VI to extract a vector reference signal or use the OAT Get Even Angle Reference VI to extract an even-angle reference signal from the slow-roll vibration signal and tachometer signal.
- Remove the reference signal from the vibration signal you acquire during normal operation. Use the OAT Compensate Vector Data VI to remove the slow-roll errors in a vector signal or use the OAT Compensate Even Angle Signal VI to remove the slow-roll errors from an even-angle signal. You typically perform even-angle signal compensation before displaying an unfiltered orbit plot or unfiltered timebase plot. You can compensate the even-angle signal with an even-angle reference or vector reference.
You need to compensate for DC gap offset when you compute the centerline plot of shafts. This compensation requires another type of reference signal, called DC gap reference. The DC gap reference contains the DC values of the probe when the shaft is at rest.
Complete the following steps to perform shaft centerline position compensation.
- Measure the DC gap voltage values of the proximity probes when the shaft is at rest. Use the OAT Get Gap Reference VI to compute the DC gap.
- Subtract the at-rest DC gap voltage values from the DC voltages measured during normal operation and combine the results to form the shaft centerline plot. Use the OAT DC Gap Estimator VI to subtract the at-rest DC gap voltage values from the measured DC gap voltage values.
Refer to the VIs in the labview\examples\Order Analysis\Reference Data Processing directory for examples of extracting and compensating for a reference signal.