15 Spectrometer Scan Mechanism for Encountering Jovian Orbit Trojan Asteroids  

Kenneth A. Blumenstock*, Alex ander K.  Cramer *, Joseph C. Church* , Jason A.  Niemeyer*,   Fil A. Parong*, Sam Zhao*, Nerses V. Armani **, and Kenneth Y. Lee* * 

Abstract  

This paper describes the design, testing, and lessons learned during the development of the Lucy Ralph  (L’Ralph) Scan Mirror System (SMS), composed of the Scan M irror Mechanism (SMM), Differential Position  Sensor System (DPSS) and Mechanism Control E lectronics  (MCE) . The L’Ralph SMS evolved from the  Advanced Topographic Laser Altimeter System (ATLAS)  Beam Steering Mechanism (BSM) , so design  comparisons will be made. Lucy is scheduled to launch in October 2021, embarking upon a 12- year mission  to make close range encounters  in 2025 and 2033 with seven Trojan asteroids  and one main belt asteroid  that are within  the Jovian orbit. The L’Ralph instrument is based upon the New Horizons Ralph instrument,  which is a panchromatic and color visible imager and infrared spectroscopic mapper  that slewed the  spacecraft for imaging. T he L’Ralph SMM  is to provide scanning  for imaging  to eliminate the need  to slew  the spacecraft.  One purpose of this paper is to gain understanding of the reasoning behind some of the  design features  as compared with the ATLAS BSM.  We will identify similarities and differences between  the ATLAS BSM and the L’Ralph SMM that resulted from the latter’s unique requirements.  Another  purpose  of this paper is to focus upon “Lessons Learned” that came about during the development of the L’Ralph  SMM and its MCE , both mechanism engineering issues and solutions as well as  Ground Support  Equipment issues and solutions  that came about  during  the validation of requirements  process . At the time  of this writing, the L’Ralph S MM has been flight  qualified and delivered to the project .   

Evolution of the L’Ralph Scan Mirror Mechanism  

Let us consider the ATLAS Beam Steering Mechanism ( BSM ) depicted in Figure 1 as our basis of  comparison [ 1]. The ATLAS BSM ’s purpose is to point rather than to scan. It ha s two degrees of freedom  provided by a custom flexure design, locating orthogonal  axes of rotation behind a relatively heavy  glass  mirror with a dielectric coating.  With the flexure axes location well behind the mirror , the actuators provide  some counterbalancing,  but the tungsten counterweight  attached by its titanium shaft  is the predominant  means for balancing. Four custom voice coil actuators with redundant windings provide  two-axis actuation  and damping.  A non-redundant inductive sensing system with two axes incorporate s two pairs of differential  sensors which view aluminum target areas at four locations of the moving plate behind the mirror.  The  angular range of each axis is ± 5 milliradians ( mrad) . On-orbit operating temperature is 10 °C to 35 °C. 

The L’Ralph SMM depicted in Figure 2 requires only a single axis of rotation with an angular range of  ±36 mrad , more than seven  times the BSM angular range. The mirror is lightweighted aluminum rather than  glass  with the sensor targets incorporated into  the backside of the mi rror. The i nductive displacement  sensor  technology  has a full range linear displacement resolution compatible with both BSM and SMM,  though in terms of angular resolution, BSM and SMM are quite different since their angular ranges are quite  different . Due to SMM required redundancy, t wo sensor pairs were moved as close together as t he target  diameters would allow.  This achieved the needed angular range and resolution, such that t he inductive  sensor pairs  operated over their full linear displacement  range of ±0.25 mm. With a single axis system, it  was convenient and beneficial to locate conventional flex pivots  on a rotation axis that passes through the  mirror. This allowed locating the axis such that mass was balanced without the need for a shaft  and  counterweight, saving significant mass and eliminating additional structural dynamics.  

* NASA Goddard Space Flight Center, Greenbelt, MD   ** ATA Aerospace LLC, Greenbelt, MD  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 2020