Measuring torsional vibration with black and white striped (i.e., zebra) tape? Be careful about the "Butt Joint" where the tape ends overlap on the shaft! (Figure 1)
Figure 1: The orange arrow points to a "butt joint" where black and white tape wraps around a shaft and comes together. This uneven spacing causes an incorrect estimate of the rpm.
As the black and white striped tape is wrapped around the shaft, the overlap where the ends meet causes a uneven spacing in the stripes. This leads to an incorrect reading of the rpm at the overlap.
This article explains how to correct the rpm spike from a zebra tape butt joint. Contents include: 1. Background 2. Simcenter Testlab Neo: Zebra Moments Correction 3. Simcenter Testlab Classic: Zebra Moments to RPM 4. Results
1. Background
When measuring the torsional vibration of a rotating shaft, a optical pickup is often used in conjunction with “zebra tape” as shown in Figure 2.
Figure 2: Zebra tape is wrapped around the rotating shaft.
“Zebra tape” has black and white stripes, and is wrapped around the shaft. The optical sensor is pointed directly at the surface of the tape.
As the shaft rotates, the optical sensor “sees” each stripe as it passes, and the time between stripes in used to determine the rpm of the rotating shaft.
By using many stripes, very fine fluctuations in the rpm can be measured, including fluctuations within one revolution of the shaft.
Despite taking great care, it is possible that the stripes are not evenly distributed over the radius of the shaft. When this happens, a “butt joint” is created where the two ends of the tape overlap (Figure 3).
Figure 3: A "butt joint" can occur when the ends of the zebra tape do not wrap evenly around the rotating shaft. This causes erroneous readings in the rpm.
If a gap occurs at the overlapping ends, the shaft will appear to slow down momentarily once per revolution. This instantaneous slow down appears as a sudden dip in the rpm value as shown in Figure 4.
Figure 4: Uneven overlap in the zebra tape causes artificial fluctuations in the rpm of the rotating system.
This momentary changes in rpm are not real, but are artificial readings caused by the zebra tape overlap.
It is also possible that two stripes are closer together at the overlapping “butt joint”, causing a momentary increase in rpm speed (ie, a “rpm spike”).
From the Method Library, insert the "Zebra Moment Correction" method in the process (Figure 5):
Figure 5: Simcenter Testlab Neo Process Designer with "Zebra Moments Correction" method.
A breakdown of the "Zebra Moments Correction" name:
Zebra: Refers to the black and white striped tape.
Moments: Refers to the time stamps (i.e., "moments in time") of the pulse crossings in the recorded tachometer signal. These are needed to perform the correction. When measuring with Simcenter SCADAS hardware, these time stamps are automatically recorded in the "raw" time trace with the tachometer.
Correction: Software will correct for the rpm error caused by the overlap at the butt joint.
Click on the "Include for Processing" button in the Process ribbon.
Figure 6: Turn on the checkbox for Angular Velocity in the "Include for Processing" menu.
This ensures that any rpm based trace is analyzed (spectrums, etc) in the process. Normally rpm channels would be used as "annotation". Annotation channels are used to label spectral data with rpm values, but are have no spectrums calculated on themselves.
Double click on the "Zebra Moments Correction" method to change the Properties as shown in Figure 7.
Pulse per revolution: Set the number of pulses per revolution in the original signal.
Number of Butt Joints: If multiple pieces of tape are used to wrap around the shaft the correction can be used to correct more than one butt joint.
Output Attributes Point: Give a new name to the correct rpm data if desired.
Press the "Run" button (lower left of screen) to apply the correction as shown in Figure 8:
Figure 8: To apply the correction, execute the process by clicking on the "Run" button in the lower left corner of the Simcenter Testlab Neo Process Designer.
Note that the pulses per revolution only have to be close to the actual number of pulses per revolution in the original signal. The software will automatically determine the correct number of pulses per revolution. This is a feature in Simcenter Testlab Neo.
The number of pulses per revolution that the software determined automatically can be seen by hovering the mouse over the "Zebra Moments Correction" method as shown in Figure 9:
Figure 9: After running the process, hover the mouse over the "Zebra Moments Correction" method to see if the software corrected the given "Pulses per revolution".
This correction only works if the pulses per revolution entered are close to the actual number. It would not work if "1" was entered for example.
In Simcenter Testlab Time Signal Calculator, there is a function called “ZEBRA_MOMENTS_TO_RPM”. This function is used to correct for the rpm spikes or dips introduced by the butt joint.
“ZEBRA_MOMENTS_TO_RPM” is part of the "Tacho group" of functions in Time Signal Calculator (Figure 10).
Figure 10: The ZEBRA_MOMENTS_TO_RPM function in Simcenter Testlab Classic Time Signal Calculator can be used to reduce artificial fluctuations in rpm due to zebra tape butt joints.
A breakdown of the "ZEBRA_MOMENTS_TO_RPM" function:
Zebra: Refers to the black and white striped tape.
Moments: Refers to the time stamps (i.e., "moments in time") of the pulse crossings in the recorded tachometer signal. These are needed to perform the correction. When measuring with Simcenter SCADAS hardware, these time stamps are automatically recorded in the "raw" time trace with the tachometer.
RPM: Software will correct for the rpm error caused by the overlap at the butt joint.
“Function1” is used to point to rpm data with the butt joint error (Channel number or Channel name can be used). The intended number of stripes is entered into the “Pulses_per_rev” field.
In the Time Signal Calculator itself, set the "PointId" to a new name (like "Corrected RPM") if desired. Set the Group to "Other".
Press the "Calculate" button (lower left) to apply the correction as shown in Figure 11:
Figure 11: Press "Calculate" button in lower left of screen to apply the correction.
More information on using using Time Signal Calculator in Simcenter Testlab Classic in the knowledge article: Time Signal Calculator Tips!
4. Results
After applying the zebra moment correction function, a new data trace is created that has the butt joint effects removed as shown in Figure 12.
Figure 12: Corrected torsional rpm (Green) with artificial fluctuations removed versus original butt joint affected data (Red).
