Removing Spikes from RPM Signals
When measuring the rotating speed, usually in revolutions per minute (RPM), of a shaft, often a laser or other type of optical device is aimed at a striped surface as shown in Figure 1. The time between crossings of stripes is counted and timed to calculate the RPM.
In the ideal world, the RPM comes out perfectly. In practice, these measurements are often performed in “hostile” environments with interfering dirt, shaft surface imperfections, and other issues that can cause RPM anomalies.
These problems can manifest themselves in different ways. For example, there can be an occasional dropouts or spikes in the RPM signal. Or there can be a slight imperfections in the spacing of the stripes which cause a glitch in the RPM at the same spot in each rotation. Some of these RPM anomalies are shown in Figure 3.
There are tools available in LMS Test.Lab to remove these RPM anomalies. These tools can be accessed in the ‘Time Data Selection’ worksheet in conjunction with some specific add-ins.
Removing an Occasional Spike/Dropout in RPM
To remove an occasional spike, use “Time Data Editor - Standard” from “Tools -> Add-ins” as shown in Figure 4.
An editing toolbar is added above the strip chart display in the ‘Time Data Selection’ worksheet as shown in Figure 5.
The editing toolbar has several icons, each which corresponds to a particular editing operation.
To remove the spike, first highlight the appropriate time trace in the data set area on the left of the screen. Then in the Overview display, click and drag the segment around the spike. This can be seen in Figure 6.
Now the editing operation can be performed. Press the [R] button and the spike is replaced by a line as shown in Figure 7.
There are a few things to note during this operation. There is a settings button in the upper right of the screen. Pressing the settings button will toggle it on and off:
The settings button and dialog box are shown in Figure 8.
There are some other useful and important features when editing:
When replacing a spike by a line, any torsional RPM fluctuations that were in the data are removed and replaced by a line. So while this edit operation is good for preserving the overall RPM for tracked data processing, it would not be recommended for torsional vibration analysis.
Removing RPM Spike at Same Location in Each Revolution
Another type of spike anomaly in RPM occurs at the same angular position within each rotation. This can be caused by slight imperfections in a particular stripe. This type of RPM spike is shown in Figure 9.
In the ‘Time Signal Calculator’ there is a dedicated function called ‘TACHO_MOMENTS_SPIKEREMOVAL_TO_RPM’ to eliminate spikes in RPM on a per revolution basis.
To activate this function, select “Time Signal Calculator” under “Tools -> Add-ins” from the main LMS Test.Lab menu as shown in Figure 10.
After the Time Signal Calculator appears in the worksheet, press the ‘f(x)’ button in the Time Signal Calculator interface as shown in Figure 11. Select the ‘Tacho’ group of functions from the left side of the dialog box and find the function TACHO_MOMENTS_SPIKEREMOVAL_TO_RPM, and then press “OK”.
Several settings can be made in TACHO_MOMENTS_SPIKEREMOVAL_TO_RPM dialog box as shown in Figure 12. These settings are explained further at the end of this article.
After pressing OK, the spike removal is performed and a new trace is generated as shown in Figure 13.
The spikes detection is based on statistical properties of the average speed between pulses inside the running window. The running window is defined as a specific number of pulses, which by default corresponds to one revolution, but can be user adjusted. A Median Absolute Deviation (MAD) of the pulses within the window is calculated and used to evaluate if any of the pulses are outliers. This process is outlined in Figure 12.
Some more explanation on the settings used for the TACHO_MOMENTS_SPIKEREMOVAL_TO_RPM function shown in Figure 10:
The RPM spike tool is intended to remove errant spikes, but not to do zebra butt joint correction. In a zebra butt joint correction, all the pulses must be redistributed evenly around the shaft. In the case of a RPM spike correction, there are only a few stripes or pulses with errors, so a localized correction is needed. In some cases, both correction methods may need to be used.
This RPM spike removal tool was introduced in LMS Test.Lab Revision 17.
Measuring RPM accurately is not always straightforward, but many common RPM errors can be corrected using the appropriate post analysis tools.
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