In Figure 4, notice that there is high amplitude vibration at 60 Hertz and 180 Hertz. This is caused by electrical interference in the instrumentation and is not actual vibration.
Figure 4: Waterfall plot of run-up from 1200 RPM to 4000 RPM contaminated by 60 Hertz and 180 Hertz electrical noise
Electrical contamination noise is often caused by ground loops or electromagnetic interference when a power cable is laying too close to a transducer cable. Also, fluorescent lights could induce electrostatic interference on poorly isolated wires.
A Kalman filter can be used to eliminate the 60 Hertz and 180 Hertz electrical noise. Depending on the countries electrical power grid, the electrical contamination could be at 50 Hertz and multiples, rather than 60 Hertz.
The advantage of a Kalman filter is how narrow the filter is (see Figure 5) in comparison to conventional FIR and IIR digital filters.
Figure 5: Comparison of three different digital filters (FIR, IIR and Kalman)
The narrow filter shape helps ensure only the electrical component is removed from the signal while preserving the actual signal content.
To use the Kalman filter, turn on the following add-ins in Simcenter Testlab: Time Signal Calculator (26 tokens) and Harmonic Tracking (20 tokens). This is done from the main Simcenter Testlab menu under "Tools -> Options".
In the Time Data Selection worksheet, add the function KALMAN_FREQ_CUT to the Time Signal Calculator (Figure 6) using the fx button.
Figure 6: Left Side – ‘Harmonic Tracking’ Add-in, Right Side – Time Signal Calculator function selection
Each KALMAN_FREQ_CUT creates a new time trace of containing the time frequency of the electrical noise. It must be subtracted from the original time signal as shown in Figure 7.
Figure 7: Simcenter Testlab – Time Signal Calculator
Note increasing the Harmonic confidence parameter will make Kalman filter sharper or narrower. This may need to be adjusted interactively.
With the Kalman filter applied, everything looks great! (Figure 8)
Figure 8: Waterfall plot of run-up from 1200 RPM to 4000 RPM with electrical noise removed
For more information on Time Signal Calculator and how to avoid ground loops, see the following links:
The occasional spike/dropout in the RPM signal in Figure 9, are often related to imperfections in the Zebra tape or perhaps dirt on contamination on the reflecting surface.
Figure 9: RPM Time History with occasional RPM spikes
To remove an occasional spike/dropout, use “Time Data Editor”. After selecting “Time Data Editor” under “Tools -> Add-ins” an editing toolbar is added to the ‘Time Data Selection’ worksheet (Figure 10).
Figure 10: Time Signal Calculator – ‘Time Data Editor’ Toolbar
Next, 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. Then press the [R] button to edit the RPM trace and the spike is replaced by a line.
For more information on Removing RPM Spikes, see the following links:
Slight imperfections in a particular stripe of zebra tape can cause an RPM anomaly to occur at the same angular position within each rotation as shown in Figure 11 below.
Figure 11: A RPM-time history with a spike shown in every other rotation.
The "Time Signal Calculator" has a dedicated function called ‘TACHO_MOMENTS_SPIKEREMOVAL_TO_RPM’ to eliminate spikes in RPM on a per revolution basis. Add it to the time signal calculator as shown in Figure 12.
Figure 12: Time Signal Calculator – TACHO_MOMENTS_SPIKEREMOVAL _TO_RPM Function
The TACHO_MOMENTS_SPIKEREMOVAL_TO_RPM command is found
Click on the "fx" button in Time Signal Calculator
Highlight the "Tacho" group
Select the TACHO_MOMENTS_SPIKEREMOVAL_TO_RPM
Press "OK"
After applying the RPM spike removal, results in Figure 13 show comparison the corrected RPM time history and the original RPM time history.
Figure 13 – Overlay of original RPM-Time history (red) versus the original time history with spikes removed (green).
For more information on Removing RPM Spikes, see the following links:
Performed a modal test and entered some Point ID or directions incorrectly?
The point ID, direction or other properties of the data can be edited individually as well as on a group basis. This is done by right clicking on the data in the Simcenter Testlab Navigator worksheet and selecting “Edit Properties” as shown in Figure 14 below.
Figure 14 –Right Click on data to select “Edit Properties”
Note: Data must be in active project, not under “My Computer” to edit the properties. Before doing this, make sure a backup copy of the project is made. This operation will permanently alter the data headers.
Choose the property “Point Direction” and make sure it is the only property selected. Choose the correct direction from pulldown list (Figure 15).
Figure 15 – “Edit Properties” menu allows data properties to be edited and changed, including the Point Direction
Choose either “Apply to Current” to change one frequency function, or “Apply to All” to update multiple functions at once.
For directions on creating a geometry, see the following article:
An error message “No matching nodes found” (Figure 16) indicates that the measurements and geometry are not named the same.
Figure 16: Non-animating Golf Club Mode Shape Error Message
In Figure 17, the geometry and measurement names did not match. The measurements have labels “club” while the geometry is called “golfclub”.
Figure 17 – Left side – Frequency Response Function measurements have the name "club", Right side –Geometry with nodes named "golfclub"
As of Simcenter Testlab Version 17 and higher, an alias table (similar to a lookup table) has been introduced so that mapping can be done between the geometry and the modal data. This makes it possible to animate shapes without editing individual measurement names one at a time.
In the Navigator worksheet of Simcenter Testlab, open the alias mapping table by right clicking on the project name in the project tree and selecting "Add/Edit Alias Mapping" (Figure 18 -Left).
Figure 18: Left Side – Add/Edit Alias Map, Right Side – Alias Mapping Table
Fill in the table to match the geometry and measurement names. The geometry names are selected on the left, and the corresponding measurement names are on the right.
Figure 19: Properly animating golf club
For more information on Removing RPM Spikes, see the following article:
