Modelling and Control of Piezoactuators for High Precision Positioning Systems Used in Radioactive Environments

Abstract

The future High Luminosity upgrade (HL-LHC) of the Large Hadron Collider (LHC) at CERN requires technical improvements in a wide range of areas such as the collimation system. The Crystal Collimation techniques, which are based on the use of bent crystals to deflect the halo particles, are under study as a possible upgrade due to their improved cleaning performance, particularly promising with ion beams. However, the success of this technique requires a high positioning performance of the crystal with respect to the beam, which combined with the limitations imposed by the harsh operating environment in particle accelerators (ultra-high vacuum, high mixed-field radiation, high temperature resistance, potential external disturbances, etc.) constitutes a multidisciplinary technological challenge. The piezo goniometer system developed at CERN for Crystal Collimation is presented and the technological solutions involved are analysed. Its performance and limitations are studied, motivating the three main contributions of the thesis: the study of the radiation effects on the piezoelectric actuator, the proposal of novel absolute angle recovery method for the angular measurement system and the proposal of a phase-locked loop disturbance rejection algorithm for the improvement of the positioning performance. The radiation effects on the piezoactuator are evaluate with three irradiation tests (gamma and proton irradiation) to simulate similar mixed-field conditions of LHC tunnel. The results show the suitability, under some considerations, of this type of actuator in this specific application and for mixed-field highly radioactive environments in general. In the field of angular measurement, the dual-interferometer based setup used in the piezo goniometers is presented. A novel Absolute Angle Recovery Method is proposed to overcome the relative angle measurement limitation of the interferometric system. This technique allows the use of a device-specific curve as an absolute reference system to recover the angle in case of interferometer interruption or error, with no need of additional elements such as home switches. The method is simulated (Monte Carlo) and experimentally applied in the rotational stage with excellent results. In the field of closed loop control, a phase-locked loop disturbance rejection algorithm is proposed with the capability of attenuating disturbances at frequencies beyond the bandwidth of the original controller. The proposal is simulated including the possibility to update the disturbance observer and the various control techniques are benchmarked. The novel technique is successfully implemented experimentally in a rotational stage used for Crystal Collimation at CERN.