Simcenter Testing Solutions Simcenter Testlab MIMO Swept and Stepped Sine

While swept sine takes less time to acquire the data, the excitation energy is “spread out” compared to stepped sine because the excitation signal frequency is continuously varied.  Stepped sine testing takes longer, but each frequency is well excited because one frequency at a time is measured.

This article explains how to operate the Simcenter Testlab Swept and Stepped Sine software packages:

Contents:
1.    Getting Started
   1.1 Hardware Setup
   1.2 Software Start
2.    Channel Setup
3.    MIMO Sine Setup
   3.1 Control Panel
      3.1.1 Swept Sine Sweep Rate/Number of Periods
      3.1.2 Stepped Sine Frequency Ranges
   3.2 Number of Sweeps
   3.3 Safety
   3.4 Measure Panel
   3.5 Source Parameters
   3.6 Reference Profiles
4.    System Identification and Verification
   4.1 System Identification Worksheet
   4.2 System Verification Worksheet
   4.3 Load Last MIMO Acquisition
5.    Geometry
6.    MIMO Sine Acquisition

1. Getting Started

Setting up a swept or stepped sine test involves both hardware and software.

1.1  Hardware Setup

To setup for the test, do the following:

The connections and equipment are shown in Figure 2.
 

During the system identification step the hardware will be checked further to ensure that all connections are secure and transducers are selected properly for the expected test levels.

1.2    Software Start

Start the software by clicking on the “MIMO Swept & Stepped Sine Testing” icon in the “Testlab Structure Acquisition” folder as shown in Figure 3.
 

After starting the software, a primarily grey screen appears as shown in the figure below.  Click on the white icon in the upper left to start a new project (Figure 4).
 

By default, a new project called “Project1.lms” will be created (similar to Microsoft Powerpoint and “Presentation1” or Microsoft Word “Document1”).  It is highly recommended to save to a desired name for the project (File -> Save As) for later retrieval.  The “Project1” is a temporary and located in the Windows temporary folder.  It is hard to find if the project were to crash.

Under the “File” menu entry, previous projects can be opened if a new project is not needed.  The last four projects opened are listed at the bottom of the menu, or “File -> Open” can be used to open a previous project.

2.  Channel Setup

After opening a project, information about the measurement channels needs to be entered into the “Channel Setup” worksheet.  An example channel setup using two force load cells is shown in Figure 5.
 

For a closed loop stepped sine test with FRF measurements, at least one control channel and at least one reference channel must be defined in the Channel Setup.

Control channels can be of any engineering unit.  Typical control channels for structural dynamics testing are either force or acceleration:

If calibration needs to be performed on some of the sensors, this worksheet can be added under “Tools -> Add-ins -> Calibration”.  It is off by default.

For the PointId field:

More information on test setup in the following knowledge articles:

3.  MIMO Sine Setup

After entering channel/transducer information, go to the MIMO Sine Setup worksheet as shown in Figure 6 below:
 

In the Setup worksheet, important test parameters (frequency range, control modes, test profile, etc) can be entered.

3.1 Control Panel

Start entering the test setup in the upper right area of the MIMO Sine Setup as shown in Figure 7:
 

The following can be set in the control panel area in the upper right:

3.1.1 Swept Sine Sweep Rate/Number of Periods

In Swept Sine, the sweep rate (number of Hertz per second) can be set as either “Fixed” or “Tabulated” on the right side of the setup screen (Figure 8):
 

If varying the sweep rate over the frequency range of the test, click on the “Define…” button next to the Tabulated option on the right side of the menu as shown in Figure 9.
 

In the “Sweep Rate / Number of Periods Editor” menu, click on the “Tabulated” checkboxes to be able to change the “Number of Periods” or “Sweep Rate” as a function of frequency.  Decreasing the sweep rate helps improve the ability of the match the target amplitudes, especially by slowing the sweep near resonant frequencies.

Increasing the “Number of Periods” increases the number of cycles used to estimate the amplitude.  This improves the amplitude estimation but also increases the amount of time to perform the test.

3.1.2 Stepped Sine Frequency Ranges

To set the frequency spacing (distance between frequency steps) in Stepped Sine mode, click on the “Define…” button to open Frequency Ranges menu as shown in Figure 10:
 

In stepped sine, it is possible to have different frequency steps over different frequency ranges.  This can be helpful to have better frequency resolution near resonances, and less resolution between resonances where finer resolution is not needed. The Polymax modal curvefitter works with variable frequency resolution Frequency Response Functions (FRFs).

The “Number of Periods” can be tabulated in the same manner as described in the previous section.

3.2 Number of Sweeps

By setting the number of sweeps to two or higher (Figure 11), the sweep will be repeated the number of times specified.  
 

Setting the number of sweeps to two or higher ensures that meaningful Frequency Response Functions (FRFs) are calculated via averaging.  The coherence will be a value between 0 and 1 to indicate the repeatability of the measurement.  The closer the coherence is to one, the more repeatable.  

If only one sweep is performed, there are no averages.  The coherence function for the FRF will be a perfect value of one at every frequency.  However, this is not meaningful as an indicator of repeatable whatsoever.

More about Frequency Response Functions and coherence in the knowledge article: What is a Frequency Response Function (FRF)?

3.3 Safety

The Safety section of the control panel has settings for the startup time and shutdown time of the sources.

Under the “Advanced…” button of Safety area, changing the action on overload setting to “Increase Range” helps ensure the test does not stop during the sweep (Figure 12).
 

When an overload is encountered, the software will increase the range of the acquisition channel to try and continue the test without an overload.

More information on overloads and ranges in these knowledge articles:

3.4 Measure Panel

Next go to the Measure panel in the lower right as shown in Figure 13:
 

Frequency Response Function (FRFs) can be collected for:

Optionally, the raw time history of the data acquisition can be recorded by activating the throughput recording.  The “Time Recording during MIMO Sine Testing” add-in will also need to be activated under “Tools -> Add-ins” menu.

3.5 Source Parameters

Just next to the Control Panel, in the upper right area, turn on the source output channels (Figure 14):
 

With the sources activated, the amplitude target at the control locations can be entered with the reference profile menu.

3.6 Reference Profiles

At the bottom of the “MIMO Sine Setup” worksheet a target amplitude profile can be entered for each exciter as shown in Figure 15:
 

After highlighting the exciter in the lower left area, click on the “Edit reference profile…” button.  In the table, enter the frequency and amplitude for the control channels.  Note the frequencies entered have to cover the “Frequency Ranges” identified previously.

Only the amplitude and corresponding frequency range are entered in this table.  Phasing and the frequency step/rate are entered in other menus.

Once the test information is entered, proceed to the System Identification step.

4. System Identification and Verification

Before the test is run, the complete test setup usually “checked over” by using the System Identification and System Verification (previously called Self Check prior to Simcenter Testlab version 2206).  At least one System Identification must be performed before the test can be run. In this step, a low-level random signal will be sent to the exciters to measure transfer functions (g/V typically) to the control channels.  This information is user to project if there will be any issues running the full test.
 
4.1 System Identification Worksheet

Go to the System Identification worksheet. Press the START button (middle, right side of screen) as shown in Figure 16: 

 
A low-level broadband random signal is sent to the shaker system:

The Simcenter Testlab Vibration Control software will then use this information to determine if the random control test can be run successfully given the parameters specified in the System Identification/Verification settings.  If all checks are passed, the software will allow the full level test to be run. 
 
Some possible error or concerns that might be flagged: 

4.2 System Verification Worksheet

If desired, the predicted functions calculated during the system identification can be reviewed further in the System Verification worksheet (Figure 17): 
 

 
Functions that can be displayed include coherence, transfer functions, responses, etc.  This can be helpful especially when testing high-value test articles. 

4.3 Load Last MIMO Acquisition

If a MIMO Sine Acquisition has already been performed once on the test object, it can be used as the system identification by selecting “Last MIMO Sine Acquisition” in the upper left of the System Verification as shown in Figure 18. 
 

Using the previous test can help ensure that the target vibration levels on the control channels are followed more close.  Unlike the low level broadband excitation of the normal system identification, the levels and responses of a previous test identify the response of the structure more accurately.  This helps ensure more accurate reproduction of the target response levels when the previous MIMO sine results are used.

For a complete description of the System Identification or SelfCheck: 
•    Vibration Control: Understanding SelfCheck (Version 2021 and lower) 
•    Vibration Control: System Identification and Verification (Version 2206 and higher) 
 
If all checks are passed, go to the MIMO Sine Acquisition worksheet to start the test. 

5.  Geometry

If desired, a three-dimensional wireframe (Figure 19) can be created to visualize the vibration patterns measured by the accelerometers while the test is in progress.
 

Nodes representing the accelerometer measurement locations connected with a wireframe can be created with the Geometry Add-in in Simcenter Testlab. This is done by selecting “Tools -> Add-ins” and checking on the Geometry selection (Figure 20).
 

The geometry add-in is only needed to create the geometry initially.  Once the geometry has been made, the add-in can be turned off.  The created geometry can be used to visualize the vibration patterns during the test.

In the Channel Setup worksheet of MIMO Swept and Stepped Sine, the each geometry node can be assigned to a specific channel as shown in Figure 21.
 

The assignment of the geometry node to the channel setup is done with the “Use Geometry” option in the upper right corner of Channel Setup worksheet.

Instructions for building a geometry in the knowledge articles:

5. MIMO Sine Acquisition

To start the measurement, select the “MIMO Sine Acquisition” worksheet at the bottom of the software interface (Figure 22).
 

Press “Arm” and then “Start” on the measurement control panel on the right side of the MIMO Sine Acquisition worksheet as shown in Figure 23.
 

During the test, there are several action buttons that can be used while the test is in progress as shown in Figure 24.
 

Some of the action buttons: