© 2016 Sierra Nevada Corporation   369 techniques considered. Ultimately , a plan was develop ed to utilize  force- limiting  to reduce  peak loads at  mechanism resonant frequencies. The Semi -Empirical method from NASA -HDBK_7004C (“ Force Limited  Vibration Testing”) was the approach implemented. This test method uses load  response data from force  gauges placed at  the component /fixture mounting interface for  closed- loop feedback  to the shaker control  system . The goal was to modulate shaker input into the test articl e and reduce the overall PSD at select  frequencies  while maintaining a pre- determined force level at  the input . 

As discussed earlier, FEM simulation  results from the refined model highlighted the SADA first lateral  mode around 650 Hz. The refined model was used to run f orce- limited simulation s to predict  the effects of  a deep notch in the shaker input around the SADA resonant frequency shown in Figure 10 . Simulation  results match data recovered from successful  SADA re -tests . For reference, this result is compared to  GEVS minimum workmanship levels.  

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     Figure 10.  Profile Comparisons - Force Limiting FEM Predictions (Lateral Axis)    While this force- limited test method worked to address the immediate concerns of the ongoing program  (i.e., the re- tested SADA did successfully survive the force- limited tests), the method is not preferred. The  modified process and resulting profile did not adequately address the high brush loads at the second  mode where as little a s 3.2- sigma  event s could  cause slip ring brushes to gap. Additionally, this type of  testing is extremely time consumi ng and resource intensive. High- level engineering oversight by skilled  dynamists prior to and during each run is required to ensure the control system and feedback process yield precisely correct test ar ticle input loads. Numerous low -level tests are requ ired and run prior to each  full-level test for each axis. Wit h each run, response data must be scrutinized to verify that control set  points are properly configured to achieve desired input response at full level. This intensive oversight and test complexity combined with the inherent risk and increased costs motivated the team to find a different  approach to address the over -test problem for subsequent SADA tests.  

Figure 11 is an example of key output from the improved test -correlated FEM and related analytical  process. It shows the FEM prediction for brush force during the force- limited test shown in Figure 10 and  clearly identifies both the first and second modes of the SADA that were believed to be responsible for the brush excitation during test. These results depict a peak brush/rotor contact load of 1.25 grams with  contributions of 0.5 gram  at SADA resonant frequencies up to 650 Hz and the remaining 0.75 gram  at  frequencies from 650 Hz up to the second mode near 1600 Hz. Results from simulations like these  helped the team realize the importance of secondary resonant effects in any planned so lution intended to  mitigate brush movement during test. As the plot illustrates, energy in the brushes was significantly larger due to the second higher frequency peak. This was a very important finding that highlighted a need to seriously consider the imp act of higher as well as lower frequency content when defining future test  profiles.   Acceptance Levels w/  transition to Protoflight  between 500 -600Hz   GEVS Minimum  Workmanship Level   Force Limited Notch near  650Hz (SADA First Mode)