Vibration Testing  Figure 11 illustrates the test configuration and the test instrumentation, with the environments previously  summarized in Table 1. The diagonal strut first bending mode (Y-Axis Input, Sine) and the first global mode  of the mechanism (X-Axis Input, Random) drive the mechanism design minimum stress margins. The four  single-axis accelerometers mounted on the LAA in 45o increments measure the responses of the diagonal  strut first bending mode. The force transducers at the base of the mechanism measure the mechanism  global mode responses and they are used to force limit during random vibration testing.   The mass simulator was designed to be a rigid body, which simulates the mass properties of hardware  mounted to the AM in the flight configuration during launch. All test predicts used a coarse mesh density  for the mass simulator FEM. As mechanical environments evolve late in the project, the coarse mesh  density allowed the team to complete the dynamic pre-test analysis in a timelier manner. Table 6 illustrates  the modal mass comparison between the two FEMs. In general, the mode frequencies are higher with the  finer meshed GSE FEM, supported by the test results in Figure 12. As input decreases at higher  frequencies, typically higher stiffness results in lower stresses. For Random-X, force (RMS) was measured  15% higher than predicts, which is acceptable based on the stress margins of safety.   As previously discussed, the LAA initially transfers small bending loads through the leadscrew and then, as  the loads are further increased, transitions to the cylindrical bearing housings being the primary bending  load path. For X-axis and Y-axis inputs, the transition from soft to stiff bending load path appears clearly in  the single-axis accelerometer responses around 45 Hz, see Figure 13. Above 80 Hz, the response appears  to deviate from the predicts for the Y-Axis Sine input, which seems at least partially due to the input not  ramping down entirely. Figure 14 shows the diagonal strut first bending mode frequency came close to  predicts. Interestingly, the diagonal strut first rigid bending mode does not appear for the Z-axis input, which  is likely due to the input levels being insufficient to engage the cylindrical bearing housing load path.   The AM vibration test envelopes the stresses within the AM flight hardware during all higher level of  assembly vibration testing. During vibration testing, the hall sensors monitored the motor rotor position,  which verified the motor did not back-drive. While the mechanism does still move during vibration testing  due to ACME thread resettling and 0.2% plastic deformation, the motion is small relative to the mechanism  range of motion (ROM). Random vibration signature runs pre- and post- 0dB level test runs verify no change  greater than 5%. As a mechanism functional health check, the mechanism stepped through its ROM after  each test axis, with the hall sensor data used to verify the step integrity. After all vibration testing, an autocollimator measures the mechanism stepping through its ROM, with the results previously discussed.  Figure 11. Alignment Mechanism Vibration Test Configuration  NASA/CP—2018-219887 35