High performance motion control of the METIS Cold Chopper Mechanism

We present the main results of the performance test campaign of the METIS Cold Chopper Demonstrator (MCCD). This tip/tilt mirror, which operates at a temperature of 77K, is one of the critical components in the Mid-Infrared E-ELT Imager and Spectrograph (METIS) for the European Extremely Large Telescope (E-ELT). The performance requirements of the MCCD relate to the field of fast and very accurate reference tracking. We discuss the applicability of different high performance motion control strategies and describe the control synthesis of a repetitive and of a novel hybrid controller. We identified the presence of non linearities in the plant, which limits the performance of the hybrid controller. The repetitive controller shows very promising results and can handle the non linearities in the system. This experimental phase concludes the MCCD program, which was initiated to verify the feasibility of a high performance cryogenic tip/tilt mirror at an early stage in the METIS development. Because of the very promising test results, no significant changes to the hardware will be implemented. We believe that minor adjustments will suffice to meet all requirements of the final hardware after integration with the METIS instrument.