Analysis and design of PI-based controllers for the axes of European Solar Telescope

Abstract

European Solar Telescope (EST) represents the largest European research infrastructure projected in the field of solar astrophysics. It is a 4-meter class solar telescope currently in the design phase. Within this phase, an essential objective is to analyze the control strategy of the telescope axes to achieve an accurate sun trajectory tracking. This paper presents a cascade control scheme based on proportional-integral (PI) controllers specifically designed for the elevation and azimuth axes of EST. The control strategy, which employs nested position-velocity loops, is devised to adeptly track the sun’s trajectory while mitigating wind impact. The controller tuning adheres to criteria of relative stability margins and bandwidth constraints using the state-space representation derived from Finite Element Model of the EST structure. An end-to-end dynamics model has been proposed to evaluate the performance of the control strategy during a ten-minute simulation. Results evidenced the convenience of the proposed controllers, highlighting wind as the main source of the control error (0.78 arcsec and 0.14 arcsec in azimuth and elevation axes, respectively) compared to the sun tracking error. Future control strategy enhancements should target the reduction of wind-induced effects to meet stringent image stability requirements.