20      

Figure 7. Liquid Pin Joint Shear Load Measurement Test Setup and Coupons after Joint Failures  

Scan Mirror System Performance Verification   The SMM Engineering Test Unit ( ETU) vibration tests , which included 22 g sine burst,  swept sine, and  random,  were deemed very successful with no evidence of damage to the structure, no degradation of  mirror figure, and no damage to the flex pivots . Post-test inspections did however suggest that the neutral  angle of the mirror had changed slightly with respect to the fixed housing as a result of flex  pivot shift despite  being clamped.  To correct this issue in the flight  design, alignment and preload- angle clocking features  were milled into the flex pivots  to engage with set  screws in the cl amps.  Non-destructive X -ray cross- section  evaluations of the pivots were performed before and after these modifications to ensure that the brazed  joints were not disturbed. Figure 8  is an X -ray cross- section of the flex  pivot with the clocking features .  The major challenge of performance verification for this system was to accurately measure commanded  mirror position across the on -orbit operational temperature range of - 120°C to - 89°C. A primary and  redundant  DPSS is used in L’Ralph to provide mirror position feedback for closed- loop control. Each DPSS  was tested by the vendor at various temperatures within the operating temperature range to validate  performance, but that testing was performed with a flat double- sided  aluminum target  on the moving portion  of a linear stage placed between an opposing differential sensor pair  on the fixed portion of that stage. The  sensor arrangement in L’Ralph is different, with each sensor of a pair arranged side-by-side and its  relatively  large angular rotation of ±36 mrad might  add some non- linearity  since the target becomes less  orthogonal to each sensor as the mirror moves away from mid- range . Furthermore, each sensor target  at  the back of the mirror is at the bottom of a counterbore, s o there is possibility of  a non-linearity contribution  by the cylindrical conductive surface surrounding each  sensor.  Any non- axial motion of the mirror resulting  from flex  pivot behavior could be yet another contributor to sensor non- linearity. Therefore, validation of  commanded mirror position  was a necessity at the mechanism level.   The ATLAS BSM also required mirror position validation while in a thermal vacuum ( TVAC ) test chamber.  The solution was t o use a n Inter-target Differential Electronic Autocollimator (IDEA)  developed by Leviton  Metrology Solutions, Inc . This is a very compact optical instrument  as compared with a typical  autocollimator . One of its features is the ability to measure the mirror angle of interest while calibrating  against a reference mirror, which we located on the fixed portion of the mechanism under test . With IDEA  looking through a window on the chamber, motion of the mec hanism mount  within the chamber is calibrated  out by the fixed reference measurements . The IDEA system that was custom developed by the vendor  for  BSM was limited to measuring an angle somewhat beyond that of  the BSM range of motion of ±5 mrad. A  new versi on of IDEA was developed by the vendor  with the capability to measure with some margin beyond  that of the L’Ralph angular range of ± 36 mrad .