439 Development and Testing of a High-Precision  Position and Attitude Measuring  System for a Space Mechanism  

Nikolay  Khanen ya, Gabriel  Paciotti*, Eugenio Forzani* and Luc Blecha * 

Abstract  

This paper describes a high- precision optical metrology system  – a unique ground test equipment  which  was designed and implemented for simultaneous precise contactless  measurements of 6 degrees -offreedom (3 translational  + 3 rotational ) of a space mechanism  end-effector  [1] in a thermally controlled  ISO 5 clean environment .    The developed contactless method reconstructs both position and attitude of the specimen from three  cross- sections measured by 2D distanc e sensors [2]. The cleanliness is preserved by the hermetic test  chamber filled with high purity nitrogen. The specimen’s temperature is controlled by the thermostat  [7].  The developed method excludes  errors caused by the thermal deformations  and manufacturing  inaccuracies of the test jig.    Tests  and simulations show  that the measurement accuracy of an object absolute position is of 20 micron  in in-plane  measurement (XY) and about 50 micron out of plane  (Z). The typical absolute attitude is  determined with an accuracy better than 3 arcmin in rotation around X and Y and better than 10 arcmin in  Z. The metrology system is able to determine relative position and movement with an accuracy one order  of magnitude lower than the absolute accuracy. Typical relative displacement measurement accuracies  are better than 1 micron in X and Y and about 2 micron in Z.  Finally, the relative rotation can be  measured with accuracy better than 20 arcsec in any direction.  

Introduction  

The m etrology system  was developed by Almatech (Switzerland) within the framework of the Solar  Orbiter  project. The mission overall goal is  to produce images of the Sun at an unprecedented resolution  and perform closest ever in -situ measurements.  The Slit Change Mechanism (SCM) is part of the SPICE   (SPectral Imaging of the Coronal Environment)  instrument  which provides spectral imaging of the  solar  disk, corona and characterizes plasma properties of  the Sun.  The SCM itself was previously presented at  the 42 nd Aerospace Mechanisms Symposium [ 1]. The SCM  prototype is shown in Figure 1, the metrology  system is shown in Figure  2. 

The SCM end effector shall move with a high accuracy and repeatability. These performances shall be verified by test during qualification campaign. No off-the-shelf  measurement equipment could be found  that fulfils all requirements such as geometrical limitations, measurement accuracy,  cleanliness, specimen  surface properties, gravity direction, test repeatability, temperature stability , and temperature range.  During the early stage of the project, a wide range of measurement means were studied. All contact measurement devices were ruled out as the specimen surface is delicate and the displacement introduced by any mechanical contact woul d bias the measurement. Indeed, the SCM end effector is only  supported by leaf spring elements with low stiffness and thus its position is very sensitive to any external  forces. Laser 3D scanners (e.g. , [3]) have insufficient accuracy. Confocal 3D microscopes (e.g. , [4]) and  digital holographic microscopes (e.g. , [5]) have a too small observation distance and a small field of view,  which makes them incompatible with the test setup. X -ray 3D  scanners (e.g. , [6 ]) cannot be combined  with an environment control system. Therefore, a unique high- precision measurement system was                                                    * Almatech, Lausanne, Switzerland  

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016