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The latest generation of astronomical telescopes is equipped by primary mirrors about 8 meter in diameter increasing demands not only of the general mechanical structure but also of the technical performances of the mirror support systems. The Large Binocular Telescope has two 8.4 meter primary mirrors supported on the same elevation mechanical structure and, each of them, located in a mirror cell enviroment. Into the latter structures hundredth of pneumatic actuators bear the weigth of the primary mirror and six positioning actuators find out the six degrees of freedom of the mirror itself, then a new control system is able to determine realtime the stiffness and the damping required by the primary mirror system. In this paper the authors describe the mechanical and the electronic active control system design and testing of the position actuator prototype that mechanically link the 8.4 m honeycomb mirror to six rigidly reinforced locations on each primary Mirror Cell structure. During telescope operation, the adjustable length of the actuators precisely control the six degrees of freedom of the mirror. Each actuator has a high mechanical axial stiffness and, as new feature, an active control system, based on piezoelectric elements and capacitive sensor, in order to control the axial stiffness versus damping, with a bandwidth from DC up to 50 Hz, assuring that the natural frequencies of the mirror do not degrade the optical performance of the telescope under external forces as the wind spectrum. Moreover, other requirements have been satisfied in the mechanic of the actuators: flexures are provided to minimize any moments applied to the attachment of the actuator to the mirror; one axial load cell for each actuator provides a precise realtime measurement of the external forces applied to the mirror, such as wind loads, to feedback the pneumatic force system that supports the weight of the mirror; a very sensitive and precise capacitive sensor measures the total length of the actuator to submicron resolution upon request. Last but not least each actuator has a reliable fail-safe system that limits the compressive and tensile forces that can be applied to the mirror. The mechanical and the electronic design DSP based and all the experimental tests of this actuator prototype have been performed in the Astrophysical Observatory of Arcetri laboratories under the supervision of the authors of this paper.
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