Taking torsional data with Simcenter Testlab (formerly called LMS Test.Lab)?
Here some tips that will save time and help understand the results better!: 1. Tacho versus Vibration Group 2. Displays: RPM versus Angle 3. Displays: RMS, Peak, or Peak-to-Peak 4. Torsional Vibration Animation 5. Multiple Pulse per Revolution Encoders 5.1 Magnetic Pickups 5.2 Zebra Tape 5.3 Resolvers in Electric Motors 5.4 Incremental Encoders
1. Tacho versus Vibration Group
In the Channel Setup of Simcenter Testlab, it is helpful to change the "ChannelGroupId" field from Tacho to Vibration when using a dedicated tachometer input on a Simcenter SCADAS hardware frontend.
Normally, the "Tacho" group is only used to calculate RPM to annotate the data. In the case of torsional vibration, frequency analysis needs to be performed on the rpm time trace to quantify the rpm fluctuations. No analysis is performed on channels in the "Tacho" group. Putting the rpm tachometer channel in the vibration group allows frequency analysis to be performed AND the channel can still be used for RPM annotation.
In Channel Setup worksheet, turn on a tachometer channel. Change the "ChannelGroupId" from "Tacho" to "Vibration" (Figure 1).
Figure 1: Change the ChannelGroupId from Tacho to Vibration in the drop-down menu.
With this trick, there is no need to split the tacho channel and route it into both a dynamic channel and a tacho channel. Both a tachometer and vibration channel are calculated simultaneously.
When the tachometer channel is set to vibration, it is possible to create order cuts and FFTs from the throughput data (Figure 2).
Figure 2: Change the ChannelGroupId to vibration and it will be possible to make colormaps and order cuts of the torsional vibration.
Switching the "ChannelGroupId" of the tachometer gives the flexibility to use the channel for both RPM calculations and for torsional analysis.
More about measuring rotating machinery dynamics in Simcenter Testlab:
Torsional vibration data can also be displayed in RPM (speed) or angle (angular distance).
An example of a torsional order is below. The x-axis represents the overall RPM level and the y-axis represents the fluctuation in RPM (torsional vibration).
Figure 3: Second torsional order from an engine run-up.
To view the torsional vibration in rotational displacement rather than rotational speed:
Right click on the y-axis and select "Processing. Under "Integrate/Differentiate" choose "Integrate (Single)" as shown in Figure 4.
Figure 4: The above menus will open when following the steps to convert the Y-axis to angle.
Instead of RPM, the graph will now show the angular deflection in degrees (Figure 5).
Figure 5: By integrating RPM once, the Y-axis is now represented in terms of angle instead of RPM.
It is sometime useful to understand torsional vibration fluctuations in degrees of travel rather than in terms of RPM.
These types of display changes can also be done in active picture reports.
3. Displays: RMS, Peak, or Peak-to-Peak
The amplitude of torsional data can be expressed in three different ways (RMS, Peak, or Peak-to-Peak) as shown in Figure 6:
Figure 6: Peak, RMS, and Peak-to-Peak are three different methods for expressing the amplitude of a torsional vibration.
Want to display the maximum angular displacement?
Right click on the y-axis and choose "Processing". Under "Section Scaling", choose “Peak-to-Peak” as shown in Figure 7 below.
Figure 7: Right click on the Y-axis, choose "Processing", and under "Section Scaling" select the method for expressing the amplitude (RMS, Peak, or Peak-to-Peak) of the torsional vibration.
When switching from RMS to Peak-to-Peak, the graph will then appear as shown in Figure 8 below.
Figure 8: The y-axis is now represented in Peak-to-Peak format.
Peak-to-peak is a common representation used for torsional data. In the case of angle, this is the maximum rotational fluctuation of the rotating system.
Using two encoders at either end of a shaft, the maximum angular twist can be found by taking the difference of the two ends.
Create an operational deflection shape of your torsional vibration in the Geometry workbook of Simcenter Testlab.
Go to the “Torsional node” sub-worksheet of the Geometry worksheet as shown in Figure 9.
Figure 9: In the Geometry worksheet, open the Torsional node sub-worksheet (red circle near the top).
Click on “Add Disc…”. The “Add disc” window will appear (Figure 10).
Figure 10: The “Add disc” window allows the user to enter the node name, shaft size, and orientation of the node.
Type in the node name, the radius of the rotating component, and the orientation in which you want to create the disc. Click apply and then click close.
The torsional node will appear in the Geometry Display as shown below (Figure 11).
Figure 11: The torsional node.
Once all the desired nodes and torsional nodes are created, it is possible to animate the nodes with spectrums, orders, and time histories of the nodes (Figure 12), just as you would animate any geometry.
Figure 12: An animation of the torsional nodes shows the relative fluctuations in a multiple pulley system.
Both translational and rotational vibration can be displayed simultaneously.
When measuring torsional vibration, it is important to use a multiple pulse per revolution encoder rather than a single pulse per revolution encoder as shown in Figure 13.
Figure 13: The lower PPR rate (black) did not capture the torsional vibration. The high PPR (red) captured the fluctuation of rotational speed within the rotation cycle.
Torsional vibration speed variations occur within one revolution of the rotating system being measured. A one pulse per revolution encode will never capture the speed variations within one revolution.
There are many different multiple pulse per revolution encoders and devices available.
5.1 Magnetic Pickups
Magnetic pickups are often used with multiple tooth gears to measure torsional vibration (Figure 14).
Figure 14: Magnetic pickups (left) can be used with metallic gears (right) to measure torsional vibration.
Magnetic pickups do not require external power to produce a signal. Gears are often available on rotating systems with multiple teeth to help facilitate torsional measurements.
Sometimes gears have one missing or two missing teeth for indexing purposes. In this case, the torsional vibration measurement is thrown off because the gear RPM will appear to momentarily slow down more than it actually does in real life. With magnetic pickups, the speed distortion may not be only be due to the missing teeth, but there can be residual effects on the adjacent (but present) teeth.
In the case of a gear (or any other encoder) with missing pulses/teeth, there are correction algorithms in Simcenter Testlab. See the knowledge article: Measuring RPM: Missing Pulses for more information.
5.2 Zebra Tape
When no gear is available to use with a magnetic pickup, another technique is to apply zebra tape to a shaft as shown in Figure 15.
Figure 15: A multiple stripe zebra tape can be wrapped around a shaft.
With the stripes placed around the shaft, a optical encoder can be used to count pulses and perform torsional analysis.
If the zebra tape dimensions are not matched exactly to the diameter of the shaft, a discontinuity in the stripes is created where the tape overlaps. This creates an artificial fluctuation in the RPM.
Many electric motors are equipped with resolvers that measure angular position with high precision. An example resolver is shown in Figure 16.
Figure 16: Resolver (center right) on an electric motor.
Resolvers produce three voltage signals that can be used to determine the angular position of the rotating shaft. The signals are sinusoidal in nature (see Figure 17):
Figure 17: The three signals from an electric motor resolver that indicate the angular position.
If an electric motor is equipped with a resolver, this saves having to install special instrumentation to measure the RPM torsional fluctuations of the motor.
There are specialized routines with Simcenter Testlab to convert the three voltage signals from a resolver into angular position. See the knowledge article: Simcenter Testlab: Calculating Angular Difference for more information.
5.4 Incremental Encoders
Most encoders cannot distinguish the direction of rotation of a shaft. The shaft could be spinning forward or backward and the encoder cannot tell when it produces only one series of pulses.
Incremental encoders can distinguish the direction of rotation. They are a single device that is fixed to the end of a shaft. They produce three output signals - two high speed pulse trains (used for sense of direction and precise measurement of fluctuations) and a single pulse per revolution (used as an index to mark 0 degrees if desired).
An incremental encoder is shown in Figure 18 below.
Figure 18: An incremental encoder outputs three signals. The two high speed signals precisely measure the RPM and sense of direction. The index signal is used to mark zero degrees if desired.
Simcenter SCADAS hardware equipped with a SCADAS-RV4 card can be used to measure with an incremental encoder.
Incremental encoders are often used in applications like combustion analysis. See the knowledge article: Simcenter Testlab Combustion Analysis for more information.
Enjoy these tips when working with torsional vibration!
