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Part 3.3: Vibration Tests

I decided on a vibration test for the flight prototype SCM that will allow it to fly on 'just about anything'. The testing method and vibration levels used were recommended by the Air Force Research Lab. The SCM was shaken at 14.5Gs (random) for three minutes on each orthogonal axis. A sin survey of .5G's was done before and after each 14.5G shake.

Figure 3.3.01: Vibration test G's at varying frequencies

Figure 3.3.02: SCM vibration test mount

Figure 3.3.03: SCM on JHU/APL 'small shaker' for vibration tests

The SCM vibration tests were performed at JHU/APL under a Goembel Instruments subcontract between March and November 2005. The first test was performed on an SCM with 'dummy' electronics. It proved the mechanical components of the SCM could withstand the rigorous 14.5 Gs test without failure. SCM failed to work after its first vibration test with fully functioning electronics. Subsequent analysis showed the 5MHz timing oscillator for the FPGA had failed. The failed part was rated to withstand "20 G's for 120 minutes" so a replacement oscillator with similar specifications from a different manufacturer was tried. During a subsequent vibration test the replacement oscillator failed. A completely solid state oscillator is now used in the SCM. The oscillator was modified to fit into real estate on the digital PCB so that a minimal work was required to make the change.

Figure 3.3.04: 5MHz crystal oscillator disassembled

Figures 3.3.05 and 3.3.06: Solid state retrofit in old package (left) and Potted unit (right)

During what became the 'next to last' vibration test the improved oscillator survived, but the FPGA shook loose. This had been the third nine-minute shake for the FPGA. After a new FPGA was soldered on the board, I staked it in place and coated the boards with epoxy. All SCM boards are now staked and coated with low outgassing epoxy.

Figures 3.3.07 and 3.3.08: The FPGA shook loose during its third 9-minute shake at 14.5 Gs (left); Board conformally coated (right)

The final vibration tests of the SCM took place on November 18, 2005. SN004 was vibrated at the full 14.5Gs 'qualification' level and SN001 was vibrated at about half that level, the 'acceptance' level. Both units passed the tests without any ill effects.

Figure 3.3.09: Example of the data collected during vibration tests of the SCM

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