Synthesis of approximate zonal controllers for MEMS DMs

Abstract

We present a novel technique for the design of DM controllers in high spatial resolution adaptive optics systems, operating in open-loop. It consists of a Shack-Hartmann (SH) figure sensor and multiple overlapping MIMO controllers based on the H∞ synthesis method. The controller synthesis can be carried out periodically using a linearized representation of a continuously adjusted model that accounts for varying physical or ambient conditions and incorporates the spatial geometry of the SH. The figure sensor uses a bright reference source and a fast CMOS detector to sample the DM surface sequentially with an optical arrangement that does not interfere with the main corrected beam. Taking full advantage of such robust techniques, the controller can successfully handle the dynamics and non-linearity of the DM, allowing one to decouple, from the main AO control loop standpoint, the turbulence estimation errors from those originating in the DM servo-loop. It can also implement noise and vibration rejection without compromising the loop stability, pushing the control bandwidth to the physical limits imposed by hardware and software components. By splitting the control function into several overlapping controllers, implementation complexity is reduced and continuous updating of the controller can be easily achieved. Simulations show its ability to successfully control the DM shape, in spite of partial and non-simultaneous sampling of the SH figure sensor due to detector speed limitations.