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Sine Control: Notching

Siemens Theorist Siemens Theorist
Siemens Theorist

 

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Sine Control: Notching

 

During a sine control shaker test, due to the energy input at a single frequency, resonances can cause the vibration at certain locations to become quite high.  This can potentially cause damage to a one-of-a-kind or expensive test objects.

 

Sine notching can help protect test objects by limiting the vibration levels at key locations on the test object or in the shaker system.

 

What is sine notching?

 

Using the notching technique, one or more control or measurement channels can have individual frequency ranges where each response (in whatever dimension/units it is being measured in) must not exceed a pre-determined level.

 

For example, the control accelerometer at the base of a test object may have a target vibration level of 2 g’s. The rest of the test object may have an imposed acceleration response limit of 3 g’s as shown in Figure 1.

Figure 1: Control accelerometer is vibration target is set to 2 g’s, while measurement response channels should not exceed 3 g’sFigure 1: Control accelerometer is vibration target is set to 2 g’s, while measurement response channels should not exceed 3 g’s

This 3 g vibration limit might come from a field test, where the vibration level was monitored at the same location.  If in the field test the vibration never exceeded 3 g, than it may be desirable not to exceed 3 g’s on the laboratory shaker test.

 

Notching prevents the vibration on the measurement channel from reaching 3 g’s by reducing the control below the 2 g control target if necessary.

 

Any channel can be defined as a notch channel. Whenever a limit is about to be exceeded during the test, that channel takes over complete control and the drive voltage is reduced (notched) until the response is within its required limit.  Each active notch channel can have defined frequency bands or a complete frequency range notch profile that limits that channel’s response.

 

Notching must be monitored constantly during a closed loop sine control test as shown in Figure 2.

 

Figure 2: Notching is checked constantly during closed loop sine control testFigure 2: Notching is checked constantly during closed loop sine control test

Notching is sometimes referred to as “limiting”.

 

Force and Moment Limiting

 

The notch level does not need to be g’s of acceleration, but can also be specified in terms of force or any other quantity being measured during the test. When using force, notching is referred to as “force limiting”. An example of a shaker test which employs force limiting is shown in Figure 3.

 

Figure 3: Force cell used for force limiting during shaker testFigure 3: Force cell used for force limiting during shaker test

Force limiting is typically done with a force cell placed between the test object and the shaker. The force levels for the test have a fixed limit, and these force levels should not be exceeded during the shaker test. With the mass of a large satellite suspended on the shaker system, there can be reaction forces that are very large, which can cause force levels to exceed the desired levels.  Force limiting is used to prevent this from occuring.  

Figure 4: Three degree of freedom force cellFigure 4: Three degree of freedom force cell

Moment limiting is required in conjunction with force limiting when the center of gravity of the test object is not exactly centered with the shaker support interface. There are certain overturning limits on the shaker, and the force cell must be used to ensure these are not exceeded. For large structures such as satellites, this becomes even more important.

 

Notching Example and LMS Test.Lab settings

 

To illustrate how sine notching is setup and used in LMS Test.Lab Sine Control, consider a shaker test with one control accelerometer and one response accelerometer as shown in Figure 5.

 

 

The measurement and control channels are defined in the ‘Channel Setup’ worksheet as shown in Figure 6. The channel group is set to “Control” for the control accelerometer channel and “Measure” for the response accelerometer channel.

 

Figure 6: One control channel and one measured response channel defined in ‘Channel Setup’ worksheetFigure 6: One control channel and one measured response channel defined in ‘Channel Setup’ worksheet

After running a sine test, the response accelerometer shows a peak acceleration of 6.5 g, which is over 3 times the control vibration amplitude of 2 g (Figure 7).

 

Figure 7: The response accelerometer peak amplitude (bottom) is over 3 times the control amplitude (top)Figure 7: The response accelerometer peak amplitude (bottom) is over 3 times the control amplitude (top)

Because the response channel is so high, it might be desirable to limit the vibration level.  This could be especially important if the peak vibration on the response location during actual field testing never reached 6.5 g’s of vibration. For this example, a notch limit of 3 g’s will be used on the response accelerometer.

 

To create a notch limit on the response channel, in the LMS Test.Lab ‘Sine Setup’ worksheet, turn ON the ‘Notching’ checkbox next to the desired notch channel as shown in Figure 8.  Press “Edit notch profile” to set the actual notch limits.

 

Figure 8: To define notching for a channel, turn ON the Notching checkbox and press “Edit notch profile”Figure 8: To define notching for a channel, turn ON the Notching checkbox and press “Edit notch profile”

The ‘Notch profile’ dialog box will appear. In the ‘Notch profile’ dialog box, the vibration limits can be defined as shown in Figure 9.  Notching levels can be set independently on each channel, and can be set as a function of frequency.

 

Figure 9: In the Sine Setup worksheet, choose “Edit notch profile” from the channel list to define notch limits in the ‘Notching Profile’ dialog boxFigure 9: In the Sine Setup worksheet, choose “Edit notch profile” from the channel list to define notch limits in the ‘Notching Profile’ dialog box

In Figure 10  below, a notch profile is defined from 20 to 3000 Hertz, with a limiting acceleration of 3 g’s. In this case, as the test sweeps from 20 to 3000 Hertz, the response on the measurement channel will be limited to 3 g’s, which is less than the 6.5 g levels seen during the original sine shaker test.  The drive amplitude will be reduced when the control frequency amplitude approaches 3 g’s at the response location.

 

Figure 10: Vibration limit of 3 g’s over 20 to 3000 HertzFigure 10: Vibration limit of 3 g’s over 20 to 3000 HertzWith the notching definition in place, when the sine test is re-run, the vibration level on the response accelerometer is limited to 3 g’s as shown in Figure 11.

 

Figure 11: Notch on control channel (upper display) keeps the response channel below 3 g (lower display)Figure 11: Notch on control channel (upper display) keeps the response channel below 3 g (lower display)

In the example shown in Figure 11, the control amplitude was still within the abort limits of the control.  In practice, the control accelerometer vibration can be reduced beyond the defined abort limits without interrupting the test.

 

Control Parameters and Notching

 

Whichever response channel requires the most notching takes over as the control channel. When notching occurs, other control parameters can be changed.  For example, the compression factor of the notching channel could be set to a different value than the original compression factor used on the control accelerometer.

 

To change the compression factor used during notching, in the ‘Sine Setup’ worksheet, press the “Tabulated…” button. Turn OFF the “Same as Control” checkbox under the “Notch Compression Factor”. Now a different compression factor can be defined and used when notching occurs (Figure 12).

 

Figure 12: In the Sweep Rate menu, the compression factor used during notching can be set differently than the test compression factorFigure 12: In the Sweep Rate menu, the compression factor used during notching can be set differently than the test compression factor

Please refer to the Community Article: Sine Control: Closed Loop Control Parameters for a description of how compression factor works.

 

In the LMS Test.Lab Documentation, there is extensive documentation in the “LMS Test.Lab Sine Control” manual.  The documentation is located in “Start->Programs-> LMS Test.Lab {rev}->Documentation”.

 

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