Design and manufacturing of a precision cryogenic actuator

The CEA CryoMechanism (CM35) was created in the late nineties from the association of basic industrial components. At this stage, the goal was to design a very robust and highly repeatable rotating actuator that could operate from room temperature, down to cryogenic temperatures (20K) with a very low power dissipation. This first model was designed according rules of thumb for ground instruments. Manufactured in 12 units series in 2004 (including two spare units), ten CM are operating once every hour in the mid-infrared imager/spectrometer VISIR, on the Very Large Telescope in Chile, without any major failure reported. From 2010 to 2012, within the framework of the Mid InfraRed Instrument (MIRI) on the James Webb Space Telescope (JWST), a significant evolution of the CM35 was done in order to become a space mechanism. The new CM35 was designed, manufactured and tested according to the flight qualification test program. For this test campaign, thermal cycles (293K-20K), vibrations and life-test (150,000 actuations) were carried out. This test phases allowed to highlight some upgrades to be implemented such as the vibrations behavior improvement. From 2008 to 2012, a smaller CM, called CM21, was developed with a major evolution in the mechanical architecture that reduced the number of part, so minimize the manufacturing cost and time to integration. This model was implanted in CAMISTIC instrument in Antarctic. Today, within the framework of EUCLID mission (launch expected by 2021), the CM35 development, based on the same basic industrial components from the beginning but with the optimized CM21 design, is complete and reaches the status of a flight model mechanism. This is one of the first flight components delivered in the Euclid space mission. The story of CM is not finished yet as this actuator is going to be dispatched into the ELT-METIS instrument by 2021 (Extremely Large Telescope, Mid Infrared ELT Imager and Spectrograph) to rotate around 20 optical wheels. To achieve this goal, the CM design has been reconsidered with a goal of cost optimization. This paper explains the system constraints that had an impact on the conception of the CryoMechanism keeping in mind the links with the cryogenic environment constraints. A focus is done on the mechanical simulations developed for the modeling of the bearing and the correlation from dynamic analysis and vibration test measures. The paper will highlight the manufacturing challenges that have led to a highly repeatable actuator, capable of operations in the range from 300K down to 10K.