ELT Telescope: control system dynamic simulations

The ELT Telescope defines new levels of performances for all the engineering fields involved in the design and realization of the telescope. This is true also for the Telescope axes Control System that has to meet the requirements while guaranteeing robustness of the control loops and avoiding the resonances excitation. The telescope requirements include slewing and offsetting time, trajectory rate limits, tracking accuracy, wind disturbances rejection and the avoidance of resonances excitation. To evaluate the Control System behavior, a State Space Matrix has been estimated from the modal analysis of a finite element model of the complete telescope, including also the pier, in the “free rotor” condition. The plant transfer functions of the Azimuth and Altitude axes have been analyzed to evaluate the more critical resonances that can affect the control loops bandwidth. The velocity and the position loops architectures have been designed and tuned to evaluate how the control bandwidth influences the structural resonances. Different loops architectures have been implemented to compare the results, also including feedforward control to enhance the tracking performance and low pass filters to minimize the structural modes excitation. The control design results are presented. A telescope model, including Azimuth and Altitude axes, frictions and motor torque disturbances, encoders quantization, loops sampling and latencies, has been created. The wind disturbance has been implemented as a time-varying force acting directly on the telescope structure, generated using a velocity time history with the requested PSD. Several simulations, here presented, with and without the wind disturbance, have been done to analyze the performances respect to all the requirements and to assess the structural behavior. The simulations consider the axes moving at the same time to evaluate the cross coupling effect following all the foreseen trajectories.