4 Damping also proved very useful for m inimizing loads during launch. The mirror is free to move until hard  stops are contacted. Much of the energy is absorbed by the passive damping.  This prevented the need for  implementation of a launch lock  mechanism .  

Position Sensors   Differential Impedance Transducer  position sensors were chosen from Blue Line E ngineering. A pair of  sensors was  used to sense the angle of each axis. Sensor targets were located on the rotation axes  as one  function of the aluminum “moving plate, ” to which  the m irror mount assembly attaches.  The surface was   machined  to be planar . Keeping the targets close to the rotation axis minimizes  target translation, which is   a source of sensor error.  Ensuring a planar surface minimizes sensor error  that would result from  target  translation.  

Modal Considerations   Precision pointing  mechanisms require closed- loop control  for fast response and required disturbance  rejection, which results in the need for high controller bandwidth.  It is of critical importance to design the  mechanism such that its structural modes are compatible with t he controller bandwidth--typically the  minimum structural mode must  be several times the c ontroller bandwidth frequency. However, high Q  modes  (high amplitude modal peak)  may need to be farther from the controller bandwidth frequency, and  lower Q modes could be closer. Also, modes that are in a different direction than the controlled directi on  might be less of a problem. It is difficult to accurately assess how the structural modes will affect  performance until after the hardware is built.  

All mechanism components were analyzed for modal frequencies , with the goal of  keeping those  frequencies to at least four times  the controller bandwidth of 70 Hz. In some cases , it was not practical  to  achieve modal frequencies of four times the controller bandwidth.  Most  exceptions were  modes that  analysis showed to be occurring in directions other than in directions that the mirror is controlled.  

Thermal Stability   Absolute position knowledge required tight thermal stability, dict ating the mechanism materials. The BSM  Engineering Model had an aluminum structure that mounted onto  an aluminum housing . The housing  interfaced with a composit e bench with titanium inserts. Analysis found thermal in stability to be greater than  acceptable, so the housing and BSM structure material was changed to titanium  for the BSM Flight  Model .  So, all is titanium u p to and including the flexure. The moving side of the flexure comprises the moving plate   and the mirror mount , all of which are aluminum. The titanium  flexure to aluminum moving plate interface,  with its symmetrical and small diameter connection,  did not pose any thermal interface issues . 

Lessons Learned  

Overview   Various issues came about during the development of the BSM, as happens with any engineering development . Most issues were resolved in short order . Some of these issues and their lessons learned  will be discussed. For this project, verification of requirements was a major f ocus throughout the  development, which will be touched upon.  

Performance Verification Challenge   Performance verification required a high data sample rate along with highly  accurate dynamic  measurements over the operating environment, so it was determined that a lab interferometer would be a necessary  verification  instrument . However, due some measurement non-linearity  error s resulting from  BSM mirror properties  being incompatible with the laboratory interferometer wavelength as well as  temperature stability issues of the set -up, static measurements were employed t o compensate for these   shortcomings. Thus, utilization of multiple measurement instruments along with a data acquisition approach  was devised and successful  requirement s verification was achieved. Pre-planning of the  data acquisition  approach and the proposed data reduction methods  were approved by the project science team early on ,  paving the way for a very successful outcome.