Introduction
MIMO SRS vibration testing is challenging to execute due to stringent hardware and software control requirements.
An SRS spectrum up to 10 kHz is commonly requested. In reality, many electromagnetic shakers can only perform up to 2.5 or 3 kHz.
One method to meet this test type is to control the SRS up to 2.5 kHz while computing the SRS spectrum up to 10 kHz.
The following photo depicts a test set up on a Sentek Dynamics dual shaker system with a vertical setup. A U-channel beam was rigidly fixed on both shaker tables. The accelerometers were attached on top of the shaker armatures as controls.
Shock Response Spectrum
Shock Response Spectrum (SRS) measures the damage potential of a pulse. It assumes that the pulse is applied to each SDOF structure (SRS filters) and computes the envelope of peak responses. The following diagram illustrates the process.
The time pulse for SRS control requires a synthesis process to determine the required wavelets, so that the resulting SRS will follow the reference response spectrum.
To control the SRS up to a certain frequency range, it is obvious there should be a drive component up to that frequency range. Otherwise, the SRS spectrum will simply attenuate naturally. Since the extended frequency range is defined higher than the shaker’s capability, the extended range will have a naturally decaying SRS.
Test Setup
A MIMO dual shaker SRS test was configured for this test. The shaker configuration was vertical.
Based on the shaker parameters, the profiles are set with an upper frequency of 2.5 kHz and the following reference spectrums.
100 Hz, 0.8g
650 Hz, 18g
2500 Hz, 18g
Crystal Instruments Shock Response Spectrum (SRS) provides a unique feature to allow a higher upper frequency of the SRS spectrum compared to a controlled SRS. This feature is enabled in the Advanced Settings window along with other test parameters.
The preceding set up parameters have an SRS control frequency up to 2500 Hz while the analysis SRS spectrum is set as up to 10,000 Hz.
SRS synthesis and compensation are necessary for this test. Identical profiles are required since this is a rigidly mounted test setup on a dual shaker configuration. Users can select a check box to “use one profile for all control channels” for this type of testing scenario.
Running the Vibration Test
The test is scheduled with several low-level pulses and then enters manual mode to manually send out pulses one by one.
The following screenshot depicts a full level pulse.
The time pulse is observed as well under control, despite the low-level noise at the beginning and ending portions. The noise could be caused by the fixture. There was an attempt to add additional bolts to fixate the u-channel to the shaker head. However, the hole pattern would not match perfectly. This synthesized time waveform with a peak of about 5 g likely has some hitting occur for each pulse.
The control SRS signals also follows the profile very well, up to 2500 Hz. The correction between pulses could keep the SRS well between the tolerance if more pulses are sent out.
The SRS of each enabled channel can have a frequency range up to 10 kHz, as shown in the lower left corner graph. It is obvious that there is no drive above 2500 Hz, so, the corresponding shock spectrum would be lowered.
Please visit, “Environmental & Structural Testing (crystalinstruments.com)” to learn more about CI vibration testing functionalities.