9 counterbalance shaft horizontal, with vect ors in plus /minus X, Y, and Z (Figure 8). Once balanced, changing  the gravity vector direction by 180 degrees should not  result in mirror positional changes. However, when  checking balanc e in the X vector, the technician was observing changes in Y, and when checking balance   in the Y vector, he was seeing changes  in X.  

Our technician  was advised to not pay attention to changes in the ax es he was not balancing since it would  only create confusion in the balancing process . That instruction allowed the technician to complete the  balancing, but we should have paid attention to the resulting magnitude of the non -gravity -aligned axis. We  realized that changing the gravity vector by 180 degrees of what we believed to be a balanced BSM resulted in significant motion in the orthogonal non- gravity aligned axis . This was a very puzzling behavior.    If mirror motions were constrained to flex either one flexure or its orthogonal flexure, axes A and B , that  motion would be rotated from the sensor axes X and Y by 45 degrees. The counterweight, its position  location Z at the end of the counterbalance shaft, is adjus ted by shims to achieve balance.  However, flexure  rotation locations on Z need to be identical . If flexure rotat ion locations are different, it is only possible to  achieve balance upon one flexure axis or the other. A  compromise  could be made, such that one flexure  axis would be mirror light  and the oth er axis c ould be equally mirror heavy . The lack of co- alignment of the  flexure axes with the sensor axes is important to understanding the behavior observed.   

Refer to Figure 9 for the imbalance phenomena explanation that follows. Assume the balance is perfect  and the Y axis is aligned with the gravity direction.  The mirror will point to position 0 at the origin. Let us   move one of the flexure axes in +Z to make the moving assembly  mirror light , and then move the other  flexure axis in - Z to make the moving assembly  mirror heavy. Remember that the flexure axes A and B are  such that mirror motion on either  flexure axis  A and B  is constrained to be in a direction that is 45 degrees  from the sensor axes . If looking from the back of the mirror along the counterbalance shaft, assume that  the mirror light flexure axis  A point ed the mirror up and to the right , following the blue arrow to position 1.  Motion on the mirror heavy axis  B would then be down and to the right , following the red arrow to position  2. The net result is that imbalance did not affect the Y sensor axis in th e gravity direction, but rather , the  net motion was to the r ight in the X horizontal , non- gravity -aligned axis, following the purple arrow from  position 0 to position 2. It does seem counterintuitive that changing the gravity vector  180 degrees  result s  in motion on an axis that is orthogonal to the gravity vector , yet such motion is an indication of imbalance.  Had the flexure axes been aligned with the sensor axes, it would have been apparent that balancing one axis would result in the other axis not be ing balanced.    The root cause turned out to be that flexure manufacturing tolerances  were not met, with the two flexure  rotation locations  separated in Z by about 0.009 inch (0.23 mm) . Inspec tion reports were incorrect , due to  improper fixturing and confusing geometric tolerances . New flexures needed to be manufactured,  unfortunately resulting in significant schedule slip.  Had there been better awareness of the behavior  anomaly, we could have caught the issue sooner.   

Unknown Pole   An unknown pole of 150 Hz was present in the frequency response, but was accommodated with filtering.   The pole is unknown in that a MATLAB/Simulink model of the BSM, based upon the mechanical and electrical specification s, did not reveal such a pole. S imilar behavior was observed on the  Cassini  Composite Infrared Spectrometer (CIRS) Scan Mechanism developed  in the early 1990s, which also had  damping designed into its custom actuator.  Since in both cases, we needed to deliver hardware on a tight  schedule and we were able to meet performance requirements with the extra pole, we moved forward. In  both cases the project ended without the opportunity  to spend the time to fully understand the physics  behind the behavior. However, we believe the  unknown pole to be the result of coupling between the  actuator wi ndings and the damping sleeve, which were arranged coaxial ly in both projects (Figure 10).