Optical Differential Delay Line

Optical Differential Delay Line: large scale nanometer positioning

Yvan Michellod, Philippe Müllhaupt, Denis Gillet


The strive for extrasolar planetary research has become increasingly popular over the last years. A new astrometric measurement method, the microsecond-astrometry, promises further discoveries of characteristics of other planetary systems. Furthermore, the atmosphere blurs the observations taken from earth and more precise and accurate instruments are being developed to compensate these disturbances to earthbound observation.

The Differential Delay Line (DDL) developed by a European consortium including the Ecole Polytechnique Fédérale de Lausanne (EPFL) and the Observatory of Geneva is one subsystem of an instrument to enable microsecond-astrometry in the Very Large Telescope Interferometer (VLTI). In addition, it is able to reduce the effect of atmospheric disturbance in other instruments and therefore improve the performance of earthbound observation with the VLTI. The VLTI is part of the Very Large Telescope, which is operated by the ESO and situated on Cerro Paranal in Chile.
Keywords : Nonlinear Control; Nano-Positionning; Overactuation; Dual-Stage; Piezoelectric Actuator; PM Stepper Motor;

                       Dual-stage DDL
          click for the DDL animation


The atmospheric disturbances can be described by a Kolmogorov signal. The purpose of the DDL is to track this signal in order to reject the disturbances from the observation. The bandwidth and amplitude of this signal requires the stage to be able to cover a full stroke of 70 mm at a precision of up to 5 nm. The currently developed solution includes two actuators combining their specific characteristics to achieve an efficient, high-performance tracking of the atmospheric disturbances. This is accomplished by a so called dual-stage structure where a coarse stage and a fine stage act in parallel on the same output. This output is the path length of the optics and is reflected in a cat’s eye mounted on top of the coarse stage. Varying the path length can therefore be realized by moving the overall cat’s eye or by altering the path difference in the cat’s eye.
The coarse actuator, a permanent-magnet stepper motor, moves over a leadscrew a blade-guiding structure on which the cat’s eye is mounted. The mirror in the center of the cat’s eye is attached to the top of a tripod piezoelectric and is able to be maneuvered at nanometer scale.

A prototype has been developed and is the base for research considering the accurate tracking with the DDL. As the DDL is overactuated, that is two actuators are used to obtain one output position, specific considerations are explored to guarantee the correct use of each stages advantages.

Various control approaches have been implemented to combine these two actuators and to analyze the behavior of the overactuation on the tracking performance. These approaches include decoupled SISO design techniques as well as multivariable regulator developments.

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