During a sine control shaker test, due to the energy input at a single frequency, unrealistic resonances can cause the vibration at certain locations to become quite high. This can potentially cause unnecessary damage to a one-of-a-kind or expensive test objects.
Sine notching can help protect the Device Under Test (DUT) by automatically limiting the vibration levels at key locations on the test object or in the shaker system.
This article covers notching and how to use it in Simcenter Testlab Vibration Control: 1. What is Notching? 2. Primary Notching (Force and Moment Limiting) 3. Secondary Notching Example and Simcenter Testlab Settings 4. Control Parameters and Notching 5. Predictive Notching
1. What is 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 spacecraft qualification testing there are two possible notching strategies (one does not exclude the other)
Primary notching: pre-defined maximum level are applied to the forces and the moment of forces at the shaker control interface. The original name of this technique, introduced in the 80’s by NASA, was force limiting. It was named force limiting to highlight the action of putting a limit on the forces (versus the effect of generating a notch on the control acceleration spectrum).
Secondary notching: pre-defined maximum levels are applied to the responses on critical locations on the spacecraft.
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’s
With spacecraft, the notch limit profiles are typically coming from design considerations and negotiations with the designated launcher (or launchers).
The difference between the spacecraft-launcher interaction and the spacecraft-shaker interaction is what eventually justifies the use of notching. The connection between the launcher and the spacecraft is in fact much more flexible than a rigid shaker. The launcher interface elasticity acts as a vibration absorber, causing vibration levels and forces much smaller than the ones that will be generated on the shaker.
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 test
In summary, notching is a technique used in spacecraft shaker testing to remove conservatism (overtesting) due to mismatched mechanical impedance between flight and laboratory conditions.
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 test
Force limiting is typically done with a sandwich of force cells placed between the test object adaptor 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.
Figure 4: Three degree of freedom force cell
Moment limiting can be added in conjunction with force limiting.
With tall objects like a satellite, the Center of Gravity (CG) is high above the shaker table interface. When a satellite with a high CG is shaken horizontally, it acts like a long lever arm. The shaker table is extremely stiff, and as it pushes the base of the satellite, the mass at the top wants to stay put. This creates a overturning moment (torque) at the shaker interface to where the satellite is mounted
To perform moment limiting, in Simcenter Testlab vibration control software "virtual channels" are created. The virtual channels take the input from multiple force sensors (typically 4 or more) and use the physical distance between them to calculate the moment (force times the distance to the center). Just as limits are set in acceleration or force units in traditional notching, limits can be set for the moments with units of Newton-meters. If the calculated moment exceeds the safety threshold, the shaker automatically notches the input.
3. Secondary Notching Example and Simcenter Testlab Settings
To illustrate secondary notching (limiting the acceleration response of the test object) in Simcenter Testlab Sine Control (formerly called LMS Test.Lab), consider a shaker test with one control accelerometer and one response accelerometer as shown in Figure 5.
Figure 5: Shaker with one control and one response channel
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’ 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)
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 Simcenter Testlab ‘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”
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 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 Hertz
With 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)
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.
4. 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 factor
Notching is done to prevent exceeding vibration levels on the test object. Unfortunately, not all objects respond in a linear fashion, which can make notching a challenge.
"Predictive notching" is an option in the Simcenter Testlab Vibration Control software that is used to overcome non-linear response in the test object.
This is especially possible for test objects that are lightly damped (ie, have a high Q-factor) where the structure might change faster than the controller can react. This might cause a notch limit of 10 g's to be exceeded which is not desirable (Figure 13).
Figure 13: The 10 g limit is exceeded during a test due to non-linear behavior of test object.
In predictive notching, transfer functions, along with a "prediction margin" are used to ensure the test limit is not exceeded. Examples are shown in the video above.
In the Simcenter Testlab Documentation, there is extensive documentation in the “Simcenter Testlab Sine Control” manual. The documentation is located in “Start->Programs-> Simcenter {rev}->Documentation”.
Predictive notching capability can be added to sine control by turning it on "Notching and Limiting Prediction" under Tools -> Add-ins.
