Simcenter Testing Solutions Getting Started with Random Control

Components include:

A note on control sensors: Accelerometers are one of many sensors that can be used for control.  Force, strain, LVDT, laser vibrometers, and microphones are just a few examples of sensors that could be used to control a test.   

The Simcenter SCADAS hardware must be equipped with a control card ending with “-V”.  The “-V” option (short for vibration control) indicates that the sources are equipped with an extra safety that will always shutdown gradually as not to damage the shaker or test object.

There is a “STOP” connector that must be in place for the Simcenter SCADAS to be able to output an analog signal from the source.  The stop is shown in Figure 2 below:
 

Figure 2: STOP must be in place for SCADAS to function.
 

The stop can be hooked up to a DAC shutdown as shown in Figure 3.
 

The DAC Shutdown Control Unit has a large yellow or red button that can be used to quickly stop a test in progress.  

2. Starting Simcenter Testlab Random Control

To start the Simcenter Testlab Random Control software, look in the “Testlab Environmental” folder.  With the Simcenter SCADAS connected and turned on, double click on the “Random Control” icon as shown in Figure 4:
 

After the control software is started, choose “Tools -> Options -> Shaker” to make sure the correct shaker is selected for the test as shown in Figure 5.
 

The shaker configuration menu lets important physical limits of the shaker be entered (maximum displacement, maximum acceleration, frequency range, etc).  The random vibration control software will check any vibration profiles to ensure that no shaker limits will be exceeded during a vibration test.

If the appropriate shaker is not listed, it will need to be setup.  Press the “Configure” button and enter the appropriate information for the shaker being utilized.

After starting the software, choose “File -> Save As” to create a project file as shown in Figure 6:

The project (*.lms) can be stored in any directory.

While using the Simcenter Testlab software, the test setup parameters (frequency range, settings, vibration profile, etc) and any data collected will be stored in the project file.

The bottom ribbon of the Random Control Workbook interface has several worksheet tabs.  The worksheets can be utilized from left to right to setup, perform, and then document a test. 

3. Channel Setup

In the “Channel Setup” worksheet, all sensor information can be entered as shown in Figure 7.
 

Each row in the table corresponds to one sensor input:

Some additional channel setup resources:

4. Random Setup Worksheet

Once the Channel Setup is complete, the Random Setup worksheet is the next step in the Random Control workflow (Figure 8). 
 

 

There are seven main steps to complete to setup a Random test: 

4.1 Control Parameters Pane

Starting from the top right, in the "Control" parameter area, enter the desired frequency range and frequency resolution for the test as shown in Figure 9.
 

A typical frequency resolution used in a random control test is between two and four hertz.  This is to keep the total control loop time at a reasonable interval. While both higher and lower frequency resolutions are possible, the minimum loop time must be considered when programming a random control test. Simceter Testlab calculates the minimum loop time and displays this in the Advanced Control Parameters pop-up shown in Figure 10.
 

• Sigma Limiting: The maximum target crest factor (see equation below). It is important to note that Sigma limiting does not guarantee that there will be no excursions above the sigma limit, but does certainly reduce the dynamic range of the output signal to the shaker.
• Sigma Iteration: The number of times the sigma limiting algorithm is applied in order to maximize the reduction of the output amplitudes. More than 5 iterations is not usually necessary, as this will only marginally improve the results at the cost of more calculations.  

The desired amplitude levels across the frequency range (entered in the Control pane) are entered in “Edit Reference Profile…” button shown in  Figure 15:
 

The reference profile is constructed as a series of breakpoints, each defined by a frequency and corresponding amplitude value. Each point is represented by a row in the table. The points must cover the complete frequency range defined for the test. The frequencies entered must cover the minimum and maximum frequencies specified in the Control Pane (upper right of the “Random Setup” worksheet). 

Breakpoints can be entered as individual breakpoint pairs that cover this frequency range, or by using left and right slopes from any frequency value to create a profile that extends to the minimum and maximum frequencies defined in the Control Pane. Separate upper and lower abort levels can be specified for each section between points (Figure 16). 
 

The functions of the Reference Profile Editor, as seen in Figure 16, are explained in detail below. 

4.2.1 Actions Pane 

The following is available in the Actions Pane:

4.2.2 Breakpoint Table 

Break points are defined by their amplitude and frequency. Slopes are normally expressed in terms of dB/octave. In creating a reference profile, sufficient information must be provided to ensure that the function is defined for the complete test frequency range. In the example shown above in Figure 16, defining two points and two slopes is sufficient. The middle point is defined through the intersection of the other parameters. 

Alternatively, profiles can be entered via breakpoints only. The same profile defined with breakpoints only is seen in Figure 17. 
 

 
Figure 17: Profile defined with frequency/amplitude breakpoints only.

In addition to defining the test level of the reference profile, upper and lower abort and alarm limits can be altered to fit the test specification. These are defined as differences in dBs above or below the reference profile level. A specific difference can be defined for each section of the reference profile, as illustrated in Figure 17 above. Typical limits are ± 3 dB for alarm limits and ± 6 dB for abort limits, but other limits are used depending on the DUT. 

The engineering units for the target are typically in g^2/Hz (although Simcenter Testlab can use any type of measurement unit for control).  The g^2/Hz is the target vibration level in RMS amplitude, squared, divided by the frequency resolution. This is a Power Spectral Density (PSD) format for specifying vibration. 

Target profiles are sometimes provided via standards for certain industries. Some examples:

Alternatively, for some products a customized profile based on specific customer usage may be desired. In this case, a test tailoring or Mission Synthesis approach would be helpful.

4.2.3 Reference Profile Display Window 

This window shows a graphical representation of the defined reference profile as well as abort and alarm levels as defined. All the Simcenter Testlab display window functions can be accessed from right click menus within the display (e.g., cursors, zoom functions, etc.). 

4.2.4 Scaling Pane 

In the Scaling Pane, following options are available:

4.2.5 Other Functions 

Other functions include:

More information in the following knowledge articles:

4.3 Safety

The Safety pane (Figure 18) is where alarm and abort conditions tolerances are entered.
 

Figure 18: The Safety Pane.
 

If test conditions exceed the alarm limits, Simcenter Testlab will inform the operator what parameters have been exceeded, and in some cases, prevent the software from increasing the drive signal until there are no alarms.  If test conditions exceed the abort limits, Simcenter Testlab will stop the test. The following parameters are entered in the Safety pane: 

The Advanced Safety parameters are accessed with the “Advanced…” button in the Safety pane as shown in Figure 19. 
 

 

The following can be defined: 

4.4 Schedule

There are two options for setting the random test schedule (how the test starts up): the standard Schedule Panel and the Advanced time level table. 

4.4.1 Standard Schedule Pane

On the middle of the right-hand side of the Random Setup worksheet, the test schedule can be entered (Figure 21).
 

Figure 21: Test Schedule.
 

The schedule area defines:

The Time Level Editor provides a more sophisticated way to schedule measurements and other operations like pauses or external processes.  The Time Level Editor dialog is shown in Figure 23. 
 

The time level table is a sequence of actions that will be executed sequentially. Each row of the table is one action. The Command column dictates which action the controller will take and in what order.  The next command line is only executed when the current command line is completed. Other columns in the table will become active depending on the selected command.  

Commands available in the “Command” of the time level table are: 

Other columns:

Action Buttons: 

4.5 Kurtosis

Kurtosis control is an option that is sometimes used to increase the severity of the test. This is done by increasing the number of peaks in the time history while keeping the overall g rms of the random vibration signals the same.

When using Simcenter Testlab Random Control, the generated vibration signal has a gaussian random amplitude distribution (red curve) by default as shown in Figure 24 below:
 

The vibration experienced by an object in the real world does not always have a gaussian random amplitude distribution. Often the real-life vibration might have some “spikey” behavior (Figure 24 above, green curve). 

A signal that has a gaussian random distribution of amplitude has a kurtosis value equal to 3 in the Simcenter Testlab Random control application.  The higher the kurtosis value, the more “spikey” the signal will become, while still maintaining the target PSD RMS level and frequency content. 

If desired, the random signal normally generated by Simcenter Testlab Random Control can be augmented with spikey behavior using the “Kurtosis Control” add-in located under “Tools -> Add-ins” as shown in Figure 25:
 

After turning on the Kurtosis control add-in, a new tab called “Kurtosis” is available under the advanced settings as shown in Figure 26.
 

Turning on the checkbox on “Use Kurtosis control” in the Kurtosis tab adds a “Desired Kurtosis value” field in the Control Pane in the Random Setup worksheet. The available values vary between 3 and 12. Default value is 3 corresponding to a Gaussian distribution signal generation. 

By enabling the Kurtosis control, the sigma limiting and the sigma iteration parameters are no longer used to calculate the drives. Instead, the sigma clipping value can be edited in the Kurtosis tab. By default, it is set to 20 (peaks higher than 20 times the RMS value are clipped). When the Kurtosis Control method is used, limiting (both RMS and spectral), defined in the Channel Parameters pane of the setup sheet, is not available either. 

Note: When performing analysis on throughput data with throughput processing in Simcenter Testlab, the values for kurtosis are excess kurtosis values, which is the kurtosis value minus 3. This leads to a Gaussian Random signal having an excess kurtosis value of zero. More background on Kurtosis and Kurtosis Excess in the knowledge article: Kurtosis. 

5. System Identification and System Verification Worksheets

In a typical random control test, the complete vibration control system is usually “checked over” by using the System Identification and System Verification (previously called SelfCheck prior to Simcenter Testlab version 2206).  Once the system identification is successfully performed, then the test can be run. 

Go to the System Identification worksheet. The System Identification worksheet is used to make sure the test setup is controllable and that the shaker system can perform the test. Press the START button (middle, right side of screen) as shown in Figure 27:

 

During the system identification:

Some possible error or concerns that might be flagged: 

If desired, the functions calculated during the system identification can be reviewed further in the System Verification workbook (Figure 28):
 

Functions that can be displayed include coherence, transfer functions, responses, etc.  Predicted responses can be evaluated versus simulation model predictions to validate the test setup.  As always, if something does not make sense, do not run the test!

For a complete description of the system check or self check:

If all checks are passed, go to the Random Control worksheet to start the test.

6. Random Control Worksheet

After passing the System Identification and Verification steps, go to the Random Control worksheet to run the test.  In this section of the article, the Random Control sheet functions are explained in depth.  Skip this section and go to the next section of the article to learn how to start and run the test if desired.

The Random Control Worksheet is used to initiate a test run, monitor it and take action outside of the programmed test schedule, such as taking manual measurements, increasing or decreasing the test level, or holding the test at the current level for an additional period of time. The Random Control worksheet is shown in Figure 29 below.
 

As noted above, before entering this worksheet a selfcheck procedure should have been satisfactorily completed.

Each section of the Random Control worksheet is explained in further details below:

6.1 Random Control Pane Header Options

In the header at the top right of the screen (Figure 30) are control pane options that can be turned on and off:
 

Available options are:

6.2 Random Display Area

Displays take up the majority of the screen (starting on left side):

Both the Control Signals Display and the User Defined Layout allow for additional signals to be displayed. While the test is running, other live or previously recorded measurement channels can be displayed besides the control channel as shown in Figure 31.

• Responses Display Panel: This panel offers a means to scroll through the response data. Much less flexible than the user defined layout, it has the advantage that it is preconfigured, offers Channel overview information and automatically adapts some display properties to the displayed signal.
• Channel Overview:  Checking this "on" brings up an overview display (below the responses display) of the levels on all channels. What is shown in these bar displays will depend on the current function on display. A tool tip display with the channel number, point id, direction and function will appear by hovering with the mouse over a particular bar display. Unselecting the "Channel overview" option can slightly increase the screen refresh rate for systems with a very large number of channels.
• Bar Displays: The area below the Responses display contains a set of bar displays that show the input level of each channel in terms of the full scale value of the currently selected function.
• Function Selection: This dropdown allows the selection of which function to display in the Response display windows. The offered selection is dependent on the workbook and the activated Online Data functions.
• Display Windows: The display windows show the selected function on the selected range of the active channels. Some display properties are set based on the selected function and possible user interactions will be limited.

6.3 Random Control Panel

The Random Control Panel (right side of Random Control screen) contains information and action buttons important to the execution of the programmed test.  The Control Panel may have more or less of the following panels depending on what features are enabled.  

The default Control Panel versus the Control Panel with full features enabled is shown in Figure 32.
 
The panel displays key information about the test in progress (duration, level, etc).  The measurement run name can also be set in addition to starting or ending the test.

6.3.1 Run Name Panel

The run name is shown in the upper right (Figure 33).  This name is given to the measurement to be performed:
 

• Run name: When the Random Control worksheet is entered, a new run name (e.g. Random_X) is automatically propagated, and the system needs to be armed to enable test start functionality. To change the name of the run, delete the current name and enter a new one before arming the system. 
• Arm button: This button initializes the system using the current settings. It is possible to change some parameter settings after “Arming” and before starting the next run, but these changes will not be used as settings for this run. They will only be valid for the run that follows the next “Arming” of the system. This button has a green background when it is sensitive and active. During arming and test run, the background turns grey and becomes insensitive and inactive.

Recommended best practice to avoid confusion is to avoid changing settings after the system is armed and before the test is stopped.

6.3.2 Status Panel

The status panel provides information about the progress of the current run (Figure 34).
  The status panel contain the following information:

• Status: This field displays the current state of the run. When the frontend is armed it will indicate that the system is Ready. Once the run is underway, it will display a series of messages indicating the current process or which action button has been hit.
• Drive RMS: This indicates the RMS level of the SCADAS output in Volts.
• OpenLoop: This field is grey when the test is in closed loop mode and highlighted yellow when the controller is set manually to open loop mode.
• Level: This shows the current level in dB as defined in the schedule or activated via the action buttons.
• Hold Level: This field is grey when the test is leveling automatically and highlighted in yellow when the manual HoldLevel is activated in the test schedule or via the action buttons.
• RMS: The first field shows the RMS level of the measured control PSD (averaged depending on the control mode defined) on the left-hand side and the reference RMS level for that level on the right-hand side.
• Control: This field displays the Control method. It shows "Average Control" when no Limiting is currently active on any channel. It shows "Spec. limit: {pointname}" when an algorithm is limiting the spectral response of any channel, as defined in the Limit profile editor. If the software is limiting the response of several channels, the channel creating the most restrictive (i.e. deepest) notch will be displayed.
• RMS lim: The RMS level can be limited on any channel. This is done by a global scaling down of the drive. In the RMS lim field, this scaling factor is displayed in dB. If RMS limiting is not active, 0.0 dB will be displayed (scale factor of 1). If RMS limiting is active, the scaling factor and the {pointname} will be displayed in red.
• Time: The time that has elapsed at the current level in hh:mm:ss format is displayed in the field. The progress indicator below shows the elapsed time proportionally to the total level time as defined in the schedule. 

6.3.3 The Alarm/Abort Indicator Panel

Alarms and aborts are shown in the lower right of the Random Control worksheet (Figure 35):
 

• Alarm lines: Shows the number of frequency lines that exceed the alarm levels.
• Repeats: Shows the number of frequency lines that are repetitively in abort every time the control PSD is updated.
• Abort lines: Shows the number of frequency lines that exceed the abort levels.
• Averages: Shows the number of averages taken as defined in the "automatic measurements" parameter of the Random setup.

6.3.4 Recording Panel
 
This Recording panel appears when the “Time Data Recording During Random Control” add-in is loaded (Figure 36).
 
This panel displays the actual state of the throughput recording during the test.

• Status: This field displays the current status of the throughput recording. Possible Status states are:
• Throughput not available: The “Time Recording During Random Control” add-in must be activated using the Tools Add-ins menu.
• Throughput deactivated: The corresponding setting is switched off in the setup worksheet.
• Waiting for start: Time recording is not yet running; it will start together with the start of the test.
• Active: Time recording is running.
• Stopped: Time recording is stopped.
• Time: The time field shows the elapsed time of throughput recording measurement for the current test. An estimate of how much recording time is available based on the current amount of free disk space is also provided.

7. Runtime Procedure: Random Control

To run the test, go to the “Random Control” worksheet.  Press "Arm" in the upper right corner, and then press "Start" in the middle right (Figure 37).
 

After starting the test, a control PSD will be displayed against abort and alarm limits.

In the lower right, the test will start at a lower level (usually -9 dB from full test level) and will step up to full level (0 dB).  The test will run at the specified “equalization time” at each lower level and the specified “full level” time 0 dB (full level) is attained.

Want to run a series of random, sine, and/or shock tests with no user interaction required?  See the knowledge article "Vibration Control: Test Sequencing".

8. Post Test: Batch Printing

After the test is finished, a report can be made quickly using the Batch Reporting worksheet (Figure 38):
 

After highlighting the test, click on the Print button to make the report.  Use “File -> Print Options” to select the desired printer or a Powerpoint/Word file for the report.

There are two ways to change the Siemens logo to a different logo in Batch Printing:

Logo.bmp is the Siemens logo in the top right corner. LmsHeading1.bmp is the Header you see in the top left corner in some versions of Simcenter Testlab.

Questions?  Email chris.sensor@siemens.com.

Related Links:

• Index of Simcenter Testing Knowledge Articles
• What is Vibration Control Testing?
• Vibration Control FAQs
• Vibration Control: Understanding Selfcheck
• Vibration Control: System Identification and Verification
• Vibration Control: Test Sequencing
• Random Control: Averages per Loop, Frequency Resolution, Weighting, and DOFs
• Combined Modes: Sine on Random (SoR), Random on Random (RoR), Sine on Random on Random (SoRoR)
• Random Control: Measurement Channels
• Power Spectral Density
• Simcenter Testlab: Breakpoint Generator
• Sine Control: Estimation Methods
• Sine Control: Closed Loop Control Parameters
• Sine Control: Notching
• Sine Data Reduction
• Creating COLA Channel from a Control Signal
• Tracked Sine Dwell
• Multi-Point Response Control Averaging
• Shock Response Spectrum (SRS)
• Classical Shock Pulses
• How to Calculate a Shock Response Spectrum with Testlab?
• Shock Response Analysis and Synthesis
• Single Axis Waveform Replication (SAWR)
• Fatigue Damage Spectrum (FDS)
• Direct Field Acoustic Noise Testing
• What is Total Harmonic Distortion (THD)?
• Kurtosis
• "Excessive DC Level" message
• Inverse Transfer Function (ITF) in Random Vibration Control
• Vibration Control YouTube Playlist