Active damping strategies for control of the E-ELT field stabilization mirror

The fifth mirror unit (M5) of the E-ELT is a field stabilization unit responsible to correct for the dynamical tip and tilt caused mainly due to the wind load on the telescope. The unit is composed of: i) an electromechanical subunit, and ii) an elliptical mirror with a size of approximately 2.4 by 3-m. The M5 unit has been designed and prototyped using a three point support for the mirror actuated by piezo actuators without the need of a counter weight system. To be able to meet the requirements of the telescope, i.e. sufficient wavefront rejection capability, the unit shall exhibit a sufficient bandwidth for tip/tilt reference commands. In the presence of the low damped mechanical resonant modes, such a bandwidth can be guaranteed thanks to an active damping loop. In this paper, different active damping strategies for the M5 unit are presented. The efficiency of the approaches are analyzed using a detailed model of the unit. On a scale-one prototype active damping was implemented and the efficiency was demonstrated.