29 Point Ahead Mechanism for Deep Space Optical Communication  Development of a N ew Piezo-Based Fine Steering Mirror 

Adrien Guignabert *, Thomas Maillard* , Francois B arillot *, Olivier Sosnicki * and Frank Claeyssen*  

Abstract  

The purpose of this paper is to present the development of a novel tip- tilt mechanism, with integrated optics,  designed for the  JPL Deep Space Optical Communication (DSOC) module of the up coming Psyche mission  (2022  launch). This paper presents the design,  assembly and tests of the produced models. Regarding the  design phase, an emphasis was put on the mirror calculations to ensure that the required flatness would  be maintained after integration,  and that the part would withstand the thermal/mechanical envi ronment. The  actual optical measurement s performed after assembly are also presented. The qualification results for a  new alpha- case removal process for titanium parts are presented. Tests results are especially interesting  regarding the temperature behavi or of the mechanism,  impact on the strok e, and strain gage sensor  feedback.  

Introduction  

In the up coming NASA Psyche mission  (2022  launch) , JPL is planning the assessment of a first Deep  Space Optical Communication (DSOC) module. In this module, a Point Ahead Mechanism (PAM)  aims at  steer ing the optical downlink signal towards anticipated earth position during DSOC communication  phases.  

As a background, f or 20 years, Cedrat Technologies ( CTEC ) has provided various piezoelectrically - actuated Beam Steering Mirrors as well as  Fast Steering Mirrors for space missions (PHARAO  for CNES ,  ATLID  for A irbus DS) as well as  for optronic  equipment in defense  [1-5]. More recently, CTEC has also  been active in Free Space Optical  Communication with a new large- stroke Fast Steering Mirror  [4]. 

In this context , CTEC  was subcontracted by L3Harris, to design, manufacture and test the performance of  the PAM  engineering and flight models  for JPL PSYCHE DSOC . The developed PAM is a new tip- tilt  mechanism based on low -voltage Amplified P iezoelectric Actuators  (APA®), exploiting its space heritage.  

This paper presents the design, assembly and tests of the produced PAM models, covering the involved  technologies and failure modes: piezo materials for actuation, strain gages for indirect angular position  sensing, mechanical parts treatments, tip- tilt mechanical structure, mirror flatness , etc.  

Mechanism Design  

Specifications and T imeline   The main specifications for this mechanism were to ensure  an angular stroke of ± 2,8 mrad throughout the  full operational temperature range of the mission ( -25/+50°C  full perf, - 40/+65°C reduced perf ) and a mirror  surface flatness under 63 nm while remaining inside a very limited volume  and surviving launch vibrations . 

The project really started in summer 2018  with a preliminary feasibility study  aimed at validating these  specifications , which ended positively in Fall 2018. The initial design was fi rst based on ATLID mechanism  but specific requirements made it progressively , noticeably different in the end. The required schedule for  the final mechanism development, i.e, . delivering Flight models less than 1 year after the actual project  start, was ve ry unusual and challenging.  

* Cedrat Technologies, Meylan, France; Adrien.guignabert@cedrat -tec.com  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 20 20