TESTING AND LESSONS LEARNT OF LISA GPRM

The Caging Mechanism (CM) is one of the core devices of the LISA Technology Package (LTP) to be flown on the LISA Pathfinder Mission, a technology demonstrator for the LISA mission. As part of the Inertial Sensor System it shall cage and constrain a Test Mass (TM) during launch and subsequently release it within an Electrode Housing (EH) ready to be controlled and measured via electrostatic forces and an interferometer using a Drag Free Actuation and Control System, when the TM is placed in a perfect free-fall or Geodesic Orbit. The two main functions of the CM are to cage the TM during launch which is done by a mechanism called CMSS (Caging Mechanism Sub-System) and to release it into zero-g condition which is performed by the GPRM (Grabbing, Positioning and Release Mechanism), a piezo actuated walking mechanism which has been presented at the ESMATS 2007. Figure 1: LISA GPRM flight model A number of unexpected challenging issues revealed during testing. Testing activities, design recovery solutions and lessons learnt of the GPRM are described in detail within this paper. 1. DESIGN OVERVIEW 1.1. Caging Mechanism Figure 2: Caging mechanism The GPRM is part of the CM which itself consists of two parts, one (+z) located above the TM and one (-z) located below the TM. Each part of the CM contains two separate and independent mechanisms. The CMSS (one at +z and one at -z) cages the TM at its 8 corners with fingers during launch as well as during on ground operation and storage. The loading system of the CMSS is a hydraulic system where 4 actuators apply the preload to the TM by the 4 fingers. Once in the final orbit the CMSS releases the preload and hands over the TM to the grabbing fingers of the upper and lower GPRM. Now the GPRM has to position the TM with a high accuracy and release it into zero-g condition within the cavity of the electrode housing which is a few mm larger than the TM. Should the TM be out of control of the forces of the electrostatic suspension system the GPRM grabs the TM again, positions and releases it again. 1.2. Key Requirements Following key requirements are defined for the CM: __________________________________________________________________________________________ Proc. ‘13th European Space Mechanisms and Tribology Symposium – ESMATS 2009’, Vienna, Austria 23–25 September 2009 (ESA SP-670, July 2009) Table 1: Key requirements for caging mechanism Requirement Value No damage to the constrained TM (especially gold surfaces) TM linear release velocity < 5 μm/sec TM angular release velocity < 100 μrad/sec TM linear position accuracy < 60 μm in all axes TM angular position accuracy < 60 μrad about 2 axes Quasi static design load ± 75 g for all axis Random loads ~25 grms in all axes Static launch acceleration (superposed to random loads) 10 g Tiny envelope for the CM ø125 x 65 per CM half Knowledge of TM position < 5 μm over 2 mm range Grab TM from all possible position Gold coated constraining elements and gold coated TM No use of magnets nor ferromagnetic materials Extremely high out-gassing and cleanliness requirements From Table 1 it is obvious that not only the accuracy requirements are challenging but also unusual requirements using two gold coated, separable I/F surfaces, high preloads at launch for the CMSS and no use of ferromagnetic materials. 1.3. GPRM Design The GPRM consists of 2 main sub-assemblies:  Actuator unit with the Nexline walking mode piezo actuator and all none moving items Figure 3: Actuator unit  Grabbing finger unit (GF) containing all elements that are moving Figure 4: Grabbing finger unit Within the GF the preloaded release tip and 2 piezo stack actuators are housed. The stack actuator retracts the release pin very fast which induces an inertial release of the TM. Figure 5: Inner parts of grabbing finger Piezo stack actuator Grabbing finger Gold cap Release tip Centring ring Disc spring Positioning surface Release pin (gold) Grabbing surface The GF is preloaded by the Nexline and guided in the lateral direction by a spring preloaded side guiding system consisting of sliders on the fix and rollers on the flexure side. Figure 6: Side guiding system Roller Fix bracket