Simcenter Testing Solutions Simcenter Testlab: Tracked Sine Dwell

Simcenter SCADAS Simcenter Testlab



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Exciting a structure on a shaker at resonance with a single sine frequency is a common way to induce High Cycle Fatigue (HCF). The resonance frequencies are typically identified with a swept sine test, but these frequencies often shift due to accumulated damage, temperature changes, or non-linear behavior of the test structure.  In order to stay on resonance during the test, it is important to track shifting resonance frequencies and adjust the dwell frequency accordingly.

This article outlines the importance of this kind of testing and how to use Simcenter Testlab Tracked Sine Dwell to perform a test. The Tracked Sine Dwell interface is shown in Figure 1:
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Figure 1: Simcenter Testlab Tracked Sine Dwell graphical interface
This article has the following contents:
   1.    Background 
   2.    Getting Started
   3.    Channel Setup
   4.    Sine Setup
   5.    Selfcheck or System Identification
   6.    Sine Control
   7.    Dwell Setup
   8.    Dwell Control

1. Background 

Tracked Sine Dwell is a software application used in conjunction with a shaker table to cycle a DUT (Device Under Test) at resonance.  A test article will generate higher levels of vibration when tested at resonance which leads to failure in a shorter amount of time by replicating a “worst case” vibration environment.

It is done in a “closed loop”, meaning the amplitude of vibration is monitored and controlled.  An example Simcenter Testlab system and shaker setup is shown in Figure 2.
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Figure 2: Setup for a Tracked Sine Dwell vibration control test.

A sine wave is matched to the natural frequency of the test article.  It is either tested for a specific time duration or number of cycles.

In “Tracked” Sine Dwell testing, both amplitude AND frequency of the sine wave excitation are controlled.  As the test article begins to fatigue or change temperature, its natural frequency will change, typically shifting down in frequency.

The Tracked Sine Dwell control system will adjust and follow the natural frequency, as shown in Figure 3:
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Figure 3: During a tracked sine dwell test, the resonance frequency of the device under test typically shifts down as damage accumulates, but phase angle remains constant until the part completely fatigues.

There are different methods for tracking the natural frequency, but the most common is monitor the phase difference in a  Frequency Response Function between the control accelerometer and a measurement accelerometer.  For a simple Single-Degree-of-Freedom (SDOF) system with base excitation, the natural frequency is defined in Equation 1:
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Equation 1: Natural frequency of mass-spring system

  • ωn is the natural frequency
  • k is the stiffness
  • m is the mass.

For a SDOF system at resonance, there is a 90 degree phase shift between input and response, as shown in Figure 4:
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Figure 4: Animation of three cases of base excitation: Below, at, and above resonance frequency.

Below the natural frequency, there is a 0 degree phase shift, and above the natural frequency, there is a 180 degree phase shift.  By tracking the phase angle at resonance, the Tracked Sine Dwell software can vary the frequency of sine excitation to keep the DUT at resonance.  This is possible because while resonance frequency can change, the phase angle at resonance remains constant.

It is worth noting that while the natural frequency of a SDOF system is mathematically defined as a 90 degree lag between excitation and response, complex structures rarely exhibit this exact change in phase angle. For example, while a SDOF system only has one resonance frequency, complex structures often have multiple resonances that can be close together in frequency that alters the phase shift at each resonance. 

The basic process for Tracked Sine Dwell is to:
  • Calculate a FRF to identify the phase angle at resonance of the DUT on the shaker.  This is commonly done by performing a sine sweep over a frequency range.
  • Build a Dwell Table with the identified resonance frequencies and phase angles.
  • Add amplitude and duration for each dwell in the table. 
  • An upper and lower frequency limit is also identified around the peak frequency outside of which the resonance will not be tracked.
  • The lower frequency limit is typically related to a value at which the DUT has fatigued sufficiently to meet the requirements of High Cycle Fatigue (HCF) type tests.

Some helpful knowledge articles:

2. Getting Started with Simcenter Testlab

In the Simcenter Testlab folder, find the “Tracked Sine Dwell” icon in Simcenter Testlab Environmental folder (Figure 5).
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Figure 5: The Tracked Sine Dwell icon is found in the Simcenter Testlab Environmental folder.
Be sure that a Simcenter SCADAS frontend is turned on and attached to the PC and structure per the schematic in Figure 2.

After the program starts, a primarily blank screen appears. Click on the white icon in the upper left corner (Figure 6) to open a new project.
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Figure 6: Clicking the white icon in the upper left of Simcenter Testlab starts a new project.
When a new project is created, communication to the SCADAS hardware is also performed.  The software will check the available hardware and configure the channel setup appropriately.

The Simcenter Testlab Tracked Sine Dwell program occupies 22 tokens while running.  For more information on token licensing, see the article: Simcenter Testlab Tokens: What are they, and how do they work?

The test is performed by following the steps outlined at the bottom of the Simcenter Testlab application from left to right.

3. Channel Setup

Define at least two accelerometers in the Channel Setup worksheet as shown in Figure 7:
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Figure 7: Tracked Sine Dwell requires at least two accelerometers to be defined in the Channel Setup worksheet In this setup, the control accelerometer in monitored in the Z direction, while the measurement accelerometer will be recorded in three directions (X, Y, Z).
One of the channels will be set to the control channel, the other as a measurement channel (Figure 8).  
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Figure 8: Example Tracked Sine Dwell set-up with control and measurement accelerometer on a cantilever beam.
In this case, the DUT is the cantilever beam upon which the control and measurement accelerometer are mounted. A Frequency Response Function (FRF) will be calculated between the two channels to identify and track on a natural frequency.  The measurement accelerometer should be placed on the DUT, far from the control accelerometer, to better pick up the natural frequencies of the DUT.

Normally, the control accelerometer is on the shaker table, not on the DUT.  In this demonstration with the simple cantilever beam, there is no separate shaker table expander head, so the control accelerometer is place close to where the beam is mounted to the shaker.

Additional channels can also be measured during the test, but at least two are required to capture the phase shift between excitation and response.

4. Sine Sweep and Transfer Function

To identify the dwell frequencies of the Device Under Test (DUT,) a sine sweep is executed over an identified frequency range.  This is an important step, even if the natural frequencies of the DUT are thought to be well known. The resonance of a Device Under Test (DUT) can be different than its natural frequency in standalone configuration due to the boundary conditions of the test environment.

For example, if a DUT is part of an assembly, and the shaker fixturing is made to mimic the DUT’s mounting within that structure, the resonance frequency of that coupled structure will be different than the natural frequency of the DUT in free-free boundary conditions or from a field test. The sine sweep is used to identify the natural frequencies of the DUT mounted on the shaker.

The sweep range can be determined based on the operating conditions of the DUT (e.g. 0 – 3000 RPM), the resonances identified by a computer model, or by simply sweeping over the entire frequency range of the shaker to identify all potential resonances (e.g. 5 – 2500 Hz).  These are just some examples and not an exhaustive list.

Swept Sine setup is done in the “Sine Setup” worksheet.  Some key parameters are shown in Figure 9:

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Figure 9: Key parameters to define in Sine Setup worksheet include frequency range and the sine estimator method.

Key settings:
  1. Control Frequencies: The minimum and maximum frequency to be swept over are defined, including the frequency step.  This needs to cover the frequency range of interest. The estimator on the control channel is also defined.
  2.  Measurement Channels: The estimator for the measurement channels as well as additional measurements are set.  Additional measurements included FRFs and Total Harmonic Distortion (THD).  It is also possible to record time data during the test by turning on the “Activate recording” checkbox. The “Time Recording during Sine Dwell” set ON under “Tools -> Add-ins”.  This requires an additional 19 tokens.
  3. Measure FRF: An optional suggested practice is to also check “Measure FRF” in the “Measurements” section.  The FRF will give the best estimate of the Phase and Sharpness of the resonance response of the DUT.
For more information on estimators, see the knowledge article: Sine Control: Amplitude Estimation Methods

The targeted amplitude levels for the sine sweep frequency range need to be entered in the “Edit reference profile…” button as shown in Figure 10.
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Figure 10: Set the target levels for the sine sweep under the “Edit reference profile…” button.
When working with a very lightly damped structure (i.e., a high Q-factor ), try adjusting the following (to slow the control):
  • Compression Factor: Try increasing from default of 4 to higher. For example, 7 or higher.
  • Number of Periods for Estimator (in "Tabulated" button): Try increasing from default of 1 to higher.  For example, 15 or higher.
After entering the parameters, a self check can be performed.

5. Self Check or System Identification

Before running the sweep, the test setup and expected levels can be checked for potential issues:

Self Check results are shown in Figure 11 below:
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Figure 11: Before running the test, the Self Check ensures all transducers can be measured properly.  The status should read “OK” if the self check is successfully passed.
The Self Check excites the shaker and test object with a low level random excitation.  Based on the measured responses, the Simcenter Testlab software checks the following:
  • Open: No “Open” or dead measurement channels.  For example, an accelerometer may have fallen off, or the wire connection is broken.
  • Overload: Identifies and flags any channels that might overload when the test is run at full level.
  • Excessive Noise: Determines if there is any undesirable noise on the system that might interfere with executing a successful test.
More in the knowledge articles: 

6. Sine Control

Move to the Sine Control workbook to run the sine sweep as shown in Figure 12.
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Figure 12: A sine sweep is performed in the Sine Control workbook over the frequency range of interest on the test article to aid in identifying frequencies of interest for the dwell test.

Perform the following to run the sine sweep:
  1. Arm: Press the “Arm” button to get the SCADAS hardware ready for executing the test.  The setup is locked after arming.
  2. Press the “Start” button to run the test.
  3. View: The control accelerometer and limits are shown by default.
It is also possible to monitor other measurement channels besides the control during the sweep as shown in Figure 13.
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Figure 13: Data from the “Online Data” folder (2) in the Data Explorer (1) are used to display additional channels during the sine sweep.

The magenta curve is one of the measurement channels which shows the resonance peaks.  The control channel in blue is relatively flat.

To view the additional channels:
  1. Data Explorer: Open the Data Explorer.  Choose either the icon in the toolbar with red/yellow/white squares or choose “Data -> Data Explorer” from the main menu. Find the “Online Data” folder, then drill into the “Sweep” find the Harmonic Spectrum.
  2. Once the measurement channel is located, drag and drop it into the display.
More information on sine sweep testing:

7. Dwell Setup

After measuring the Frequency Response Function of the system, the Dwell Setup is used to identify the resonant frequencies to be test as shown in Figure 14:
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Figure 14: In the Dwell Setup worksheet, the frequencies to be targeted for sine dwell are identified with their respective durations.

The following settings are made:
  1. Identify Potential Frequencies for Dwell: Choose the sine sweep FRF under "Data source", and the number of potential resonances to be tested.  A FRF and cursors should appear in the display to aid in identifying the possible peaks to test.
  2. Select Frequencies for Dwell test: Select the desired frequencies to be tested from the list on the left to the final list on the right using the arrow key.
  3. Dwell Overview: Set the amplitude amplification factor, dwell criterion (phase, amplitude,...), and frequency band for each dwell frequency. This is called the Dwell Table.
  4. Tracked Channels: Check the Tracked Channels to enable “over time” graphs for the selected channels.
  5. Between Dwells: Choose a method for transitioning between selected dwell frequencies. 
  • Sweep: Sweeps between frequencies just like a swept sine test
  • Step: Test ramps down from one resonant frequency and ramps back up near the next resonant frequency, thereby stepping between dwell frequencies.  This method is common for most Tracked Sine Dwell tests.

On the right side of the screen, the duration of the dwell can be shown in either time or number of cycles by changing the X-axis setting as shown in Figure 15.
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Figure 15: View the test in Time or Number of Cycles as desired.

Selection can be in either time or number of cycles depending on the specification.  For example, cycles are typically used for aircraft turbine blade dwell testing.

For a test object that is lightly damped, consider:
  • Dwell Delay: Increase from default of 8.0 cycles to something higher (300 to 400 cycles)
  • Dwell tracking step factor: Decrease from default of 20.0.  For example 5.0 or lower.

Optional: Dwell Amplitude Control

If desired, the dwell test amplitude can be based on the response of the measurement accelerometer instead of the control accelerometer used during the Swept Sine portion of the test. 

To set up control based on the measurement accelerometer, set the base amplitude in the Dwell Setup -> Dwell Overview>Ref Amplitude (%) to the a value higher than the desired level at the measurement point. The actual amplitude target is then set using the notch profile in the Sine Setup Worksheet.  Setting the Ref Amplitude higher than desired ensures that the notch profile level controls the amplitude during the test (Figure 16):
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Figure 16: Set target levels for the measurement channel in the Sine Setup worksheet.

 In the Sine Setup Worksheet:
  1. Check the box in the “Notching” column of the measurement channel that will be used to control amplitude.  Then click on the “Edit notch profile” button to bring up the Notching profile window.
  2. In the Notching profile window, set the appropriate breakpoints for the amplitude at the desired frequencies.  In this example, the 79.8 Hz resonance was set to a 0.5 mm amplitude and the 282 Hz resonance was set to 0.05 mm displacement. A good practice is to set the breakpoints to the same frequencies as the upper and lower frequency limits set in the “Dwell Overview” of the “Dwell Setup” Worksheet.

8. Dwell Control

Once the dwell and optional measurement point amplitude settings are complete, the dwell test is ready to run. Move to the “Dwell Control” worksheet as shown in Figure 17:
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Figure 17: The Dwell Control worksheet is used to run the test.

Complete the steps as follows:
  1. Arm: Press the “Arm” button to get the SCADAS hardware ready for executing the test.  The setup is locked after arming.
  2. Press the “Start” button to run the test.
  3. Amplitude, frequency and phase over time can be all be displayed and monitored in real-time during the test.
Figure 18 shows a detailed view of the results of the Dwell test.
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Figure 18: Top: Phase over time from 79.8 Hertz dwell (orange) and 282 Hertz dwell (green).  Middle: Frequency (red) and amplitude (blue) of vibration over time from 79.8 Hertz dwell. Bottom: Frequency (red) and amplitude (blue) of vibration over time from 282 Hertz dwell.

The first graph is the phase over time data for each test. The second graph is an overlay of the frequency and amplitude over time for the 79.8 Hz dwell. The third graph is and overlay of the frequency and amplitude over time for the 282 Hz dwell. The graphs all clearly show the initial “settling” work that the controller does.  Upon startup the controller recognizes that it is not on the dwell frequency does not have the correct phase to be on resonance.  It then adjusts dwell frequency to stay on resonance. After this brief “settling” period the software stays on resonance for the duration of the test. 

This is also an example of how DUTs exhibit non-linear behavior.  The sine sweep at 0.5 g amplitude indicated a dwell frequency of 79.8 Hz, but at the higher dwell amplitude of approximately 1.25 g, the frequency shifted down to about 78 Hz.  The 282 Hz dwell exhibits the same non-linear behavior. 

Why does this matter? Due to the sharpness of the resonance, if the Dwell testing were performed at a fixed frequency, the DUT would experience an undertest of about 50% less than if the test were performed at the tracked dwell frequency (Figure 19)!  

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Figure 19: Swept Sine data showing the sharpness of the resonance peak of the measurement response (red) at 79.8 Hertz versus the control (blue).

The reason that this happens all boils down to relative displacement.  When a DUT is exposed to vibration at the resonance frequency, there is a large relative displacement between the control and measurement points.  If the DUT is excited off resonance, the shaker will have to increase the level of base excitation to get the required displacement at the measurement point.  This reduces the relative displacement between the control and measurement points.  As strain is proportional to displacement, this would mean that lower relative displacement leads to lower strain on the DUT. 

The undertest would likely lead to a gross over-estimate of the life cycle of the DUT.  And that friends would be bad indeed. 

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