Adaptive deformable mirror : based on electromagnetic actuators

Refractive index variations in the earth's atmosphere cause wavefront aberrations and limit thereby the resolution in ground-based telescopes. With Adaptive Optics (AO) the temporally and spatially varying wavefront distortions can be corrected in real time. Most implementations in a ground based telescope include a WaveFront Sensor, a Deformable Mirror and a real time wavefront control system. The largest optical telescopes built today have a ~ 1 Om primary mirror. Telescopes with more collecting area and higher resolution are desired. ELTs are currently designed with apertures up to 42m. For these telescopes serious challenges for all parts of the AO system exist. This thesis addresses the challenges for the DM. An 8m class telescope on a representative astronomical site is the starting point. The atmosphere is characterized by the spatial and temporal spectra of Kolmogorov turbulence and the frozen flow assumption. The wavefront fitting error, caused by a limited number of actuators and the temporal error, caused by a limited control bandwidth, are the most important for the DM design. It is shown that ~5000 actuators and 200Hz closed loop bandwidth form a balanced choice between the errors and correct an 8m wavefront in the visible to nearly diffraction limited. An actuator stroke of ~5.6J.!m and ~0.36J.!m inter actuator stroke is thereby needed. Together with the nm's resolution, low power dissipation, no hysteresis and drift, these form the main DM requirements. The design, realization and tests of a new DM that meets these requirements and is extendable and scalable in mechanics, electronics and control to suit further Extremely Large Telescopes (ELTs) is presented. In the DM a few layers are distinguished: a continuous mirror facesheet, the actuator grid and the base frame. In the underlying layer - the actuator grid - low voltage electromagnetic push-pull actuators are located. Identical actuator modules, each with 61 actuators, hexagonally arranged on a 6mm pitch can be placed adjacent to form large grids. The base frame provides a stable and stiff reference. A thin facesheet is needed for low actuator forces and power dissipation, whereby its lower limit is set by the facesheets inter actuator deflection determined by gravity or wind pressure. For both scaling laws for force and dissipation are derived. Minimum power dissipation is achieved when beryllium is used for the mirror facesheet. Pyrex facesheets with 100J.!m thickness are chosen as a good practical, alternative in the prototype development. Struts (00.1 x 8mm) connect the facesheet to the actuators and ensure a smooth surface over the imposed heights and allow relative lateral movement of the facesheet and the actuator grid. Measurements show 3nm RMS surface unflattness from the glued attachment. The stiffness of the actuators form the out-of-plane constraints for the mirror facesheet and determine the mirrors first resonance frequency. and is chosen such that the resonance frequency is high enough to allow the high control bandwidth but not higher that needed to avoid excessive power dissipation and fix points in the surface in case of failure. The electromagnetic variable reluctance actuators designed, are efficient, have low moving mass and have suitable stiffness. Other advantages are the low costs, low driving voltages and negligible hysteresis and drift. The actuators consist of a closed magnetic circuit in which a PM provides static magnetic force on a ferromagnetic core that is suspended in a membrane. This attraction force is increased of decreased by a current through a coil. The actuators are free from mechanical hysteresis, friction and play and therefore have a high positioning resolution with high reproducibility. The actuator modules are build in layers to reduces the number of parts and the complexity of assembly and to improve the uniformity in properties. Dedicated communication and driver electronics are designed. FPGA implemented PWM based voltage drivers are chosen because of their high efficiency and capability to be implemented in large numbers with only a few electronic components. A multidrop LVDS based serial communication is chosen for its low power consumption, high bandwidth and consequently low latency, low communication overhead and extensive possibilities for customization. A flat-cable connects up to 32 electronics modules to a custom communications bridge, which translates the ethernet packages from the control PC into LVDS. Two DMs prototypes were successfully assembled: a 050mm DM with 61 actuators and a 0l50mm DM with 427 actuators. In the second prototype modularity is shown by the assembly of seven identical grids on a common base. The dynamic performance of each actuator is measured, including its dedicated driver and communication. All actuators were found to be functional, indicating that the manufacturing and assembly process is reliable. A nonlinear mathematical model of the actuator was derived describing both its static and dynamic behavior based on equations from the magnetic, mechanic and electric domains. The actuator model was linearized, leading to expressions for the actuator transfer function and properties such as motor constant, coil inductance, actuator stiffness and resonance frequency. From frequency response function measurements these properties showed slight deviations from the values derived from the model, but the statistical spread for the properties was small, stressing the reliability of the manufacturing and assembly process. The mean actuator stiffness and resonance frequency were 0.47kN/m and 1.8kHz respectively, which is close to their design values of 500N/m and 1.9kHz. The time domain response of an actuator to a 4Hz sine voltage was used to determine hysteresis and semi-static nonlinear response of the actuator. This showed the first to be negligible and the second to remain below 5% for ±10J.!m stroke. Measurements showed that in the expected operating range, the total power dissipation is dominated by indirect losses in FPGAs. The static DM performance is validated using interferometric measurements. The measured influence matrix is used to shape the mirror facesheet into the first 28 Zernike modes, which includes the piston term that represents the best flat mirror. The total RMS error is ~25nm for all modes. The dynamic behavior of the DM is validated by measurements. A laser vibrometer is used to measure the displacement of the mirror facesheet, while the actuators are driven by zero-mean, bandlimited, white noise voltage sequence. Using the MOESP system identification algorithm, high-order black-box models are identified with VAF values around 95%. The first resonance frequency identified is 725Hz, and lower than the 974Hz expected from the analytical model. This is attributed to the variations in actuator properties, such as actuator stiffness. The power dissipation in each actuator of the 050mm mirror to correct a typical Von Karmann turbulence spectrum is ~ 1.5m W

Flexure-based alignment mechanisms : design, development and application

For high accuracy alignment of optical components in optical instruments TNO TPD has developed dedicated,monolithic, flexure-based alignment mechanisms, which provide accuracies below 0.1 µm and 0.1 µrad as well asstabilities down to subnanometer stabilty per minute.High resolution, high stability alignment mechanisms consist of an adjustment mechanism and a locking device.Complex monolithic flexure-based mechanisms were designed to align specific degrees of freedom. They are realizedby means of spark erosion. The benefits of these mechanisms are no play, no hysteresis, high stiffness, a simplifiedthermal design and easy assemblage. The overall system can remain a passive system, which yields simplicity.An actuator is used for positioning. Locking after alignment is mandatory to guarantee sub-nanometer stability perminute. A proper design of the locking device is important to minimize drift during locking.The dedicated alignment mechanisms presented here are based on: (a) the results of an internal ongoing researchprogram on alignment and locking and (b) experience with mechanisms developed at TNO TPD for high precisionoptical instruments, which are used in e.g. a white light interferometer breadboard (Nulling) and an interferometer withpicometer resolution for ESA’s future cornerstone missions "DARWIN" and "GAIA"

Large adaptive deformable membrane mirror with high actuator density: design and first prototypes

A large adaptive deformable mirror with high actuator density is presented. The DM consists of a thin continuous membrane which acts as the correcting element. A grid of low voltage electro-magnetical push-pull actuators, - located in an actuator plate -, impose out-of-plane displacements in the mirror’s membrane. To provide a stable and stiff reference plane for the actuators, a mechanically stable and thermally decoupled honeycomb support structure is added. The design is suited for mirrors up to several hundred mm with an actuator pitch of a few mm.One of the key elements in the design is the actuator grid. Each actuator consists of a closed magnetic circuit in which a strong permanent magnet (PM) attracts a ferromagnetic core. Movement of this core is provided by a low stiffness elastic guiding. A coil surrounds the PM. Both the coil and the PM are connected to the fixed world. By applying a current through the coil, the magnetic force acting on the core can be influenced. This force variation will lead to translation of the ferromagnetic core. This movement is transferred to the reflective mirror surface in a piston-free manner. The design allows for a long total stroke and a large inter actuator stroke. The actuators are produced in arrays which make the design modular and easily extendable.The first actuators and an actuator grid are produced and tested in a dedicated test set-up. This paper describes how relevant actuator properties, such as stiffness and efficiency, can be influenced by the design. The power dissipation in the actuator grid is optimized to a few milliwatts per actuator, thereby avoiding active cooling

Meeting highest performance requirements for lowest price and mass for the M1 segment support unit for E-ELT

The largest optical telescope in the world will be the E-ELT. Its primary mirror will be 42m in diameter. This mirror will consist of 984 hexagonal segments that are all individually supported. Each mirror will be controlled in six DOF while local shaping of the segments is provided by so called warping harnesses. These will correct for focus, astigmatism and trefoil. Hence a mirror with an extreme diameter to thickness ratio of almost 30 is obtained. Its support structure must guarantee a maximum surface form error of 30 nm rms independent of the segment attitude. Furthermore its stiffness to mass ratio must allow natural frequencies of 50Hz or higher to obtain sufficient bandwidth for the actuators that control the piston and tip/tilt of the segment. Designing such structure is a challenge that has been successfully completed. Three prototypes have been built and are about to be delivered to ESO. This paper discusses the main performance requirements and how they could be transferred into an elegant structure design. Furthermore an overview will be given on the main performance parameters in order to see whether the present design can be further optimized. © 2010 SPIE

Deformable mirrors: design fundamentals for force actuation of continuous facesheets

Adaptive Optics is an essential technology in today's large telescopes to compensate for atmospheric turbulence. Deformable Mirrors have many actuators(>10k), large stroke(>10mu), small spacing(100Hz).The well known use of piezoelectric ceramics has led to inappropriately stiff displacement actuators. However using force actuation, piezo's are superseded in performance, longevity and cost (factor 10-20 per channel).Regardless of the actuator type used, a model is presented for actuation of continuous facesheet mirrors to study its parameters. The model is validated using finite element simulations and is used to derive design fundamentals for optimization

Large adaptive deformable membrane mirror with high actuator density

With the future growing size of telescopes, new, high-resolution, affordable wavefront corrector technology with low power dissipation is needed. A new adaptive deformable mirror concept is presented, to meet such requirements. The adaptive mirror consists of a thin (30-50 µm), highly reflective, deformable membrane. An actuator grid with thousands of actuators is designed which push and pull at the membrane"s surface, free from pinning and piston effects. The membrane and the actuator grid are supported by an optimized light and stiff honeycomb sandwich structure. This mechanically stable and thermally insensitive support structure provides a stiff reference plane for the actuators. The design is extendable up to several hundreds of mm's. Low-voltage electro-magnetic actuators have been designed. These highly linear actuators can provide a stroke of 15 micrometers. The design allows for a stroke difference between adjacent actuators larger than 1 micron. The actuator grid has a layer-based design; these layers extend over a large numbers of actuators. The current actuator design allows for actuator pitches of 3 mm or more. Actuation is free from play, friction and mechanical hysteresis and therefore has a high positioning resolution and is highly repeatable. The lowest mechanical resonance frequency is in the range of kHz so a high control bandwidth can be achieved. The power dissipation in the actuator grid is in the order of milliwatts per actuator. Because of this low power dissipation active cooling is not required. A first prototype is currently being developed. Prototypes will be developed with increasing number of actuators

Large adaptive deformable membrane mirror with high actuator density

With the future growing size of telescopes, new, high-resolution, affordable wavefront corrector technology with low power dissipation is needed. A new adaptive deformable mirror concept is presented, to meet such requirements. The adaptive mirror consists of a thin (30-50 µm), highly reflective, deformable membrane. An actuator grid with thousands of actuators is designed which push and pull at the membrane"s surface, free from pinning and piston effects. The membrane and the actuator grid are supported by an optimized light and stiff honeycomb sandwich structure. This mechanically stable and thermally insensitive support structure provides a stiff reference plane for the actuators. The design is extendable up to several hundreds of mm's. Low-voltage electro-magnetic actuators have been designed. These highly linear actuators can provide a stroke of 15 micrometers. The design allows for a stroke difference between adjacent actuators larger than 1 micron. The actuator grid has a layer-based design; these layers extend over a large numbers of actuators. The current actuator design allows for actuator pitches of 3 mm or more. Actuation is free from play, friction and mechanical hysteresis and therefore has a high positioning resolution and is highly repeatable. The lowest mechanical resonance frequency is in the range of kHz so a high control bandwidth can be achieved. The power dissipation in the actuator grid is in the order of milliwatts per actuator. Because of this low power dissipation active cooling is not required. A first prototype is currently being developed. Prototypes will be developed with increasing number of actuators

Validation of a new adaptive deformale mirror concept

A new prototype adaptive deformable mirror for future AO-systems is presented that consists of a thin continuous membrane on which push-pull actuators impose out-of-plane displacements. Each actuator has =/-10mum stroke, nanometer resolution and only mW's heat dissipation. The mirror's modular design makes the mechanics, electronics and control system extendable towards large numbers of actuators. Models of the mirror are derived that are validated using in°uence and transfer function measurements. First results of a prototype with 427 actuators are also presented

Actuator tests for a large deformable membrane mirror

In the design of a large adaptive deformable membrane mirror, variable reluctance actuators are used. These consist of a closed magnetic circuit in which a strong permanent magnet provides a static magnetic force on a ferromagnetic core which is suspended in a membrane. By applying a current through the coil which is situated around the magnet, this force is influenced, providing movement of the ferromagnetic core. This movement is transferred via a rod imposing the out-of-plane displacements in the reflective deformable membrane. In the actuator design a match is made between the negative stiffness of the magnet and the positive stiffness of the membrane suspension. If the locality of the influence functions, mirror modes as well as force and power dissipation are taken into account, a resonance frequency of 1500 Hz and an overall stiffness of 1000 N/m for the actuators is needed. The actuators are fabricated and the dynamic response tested in a dedicated setup. The Bode diagram shows a first eigenfrequency of 950 Hz. This is due to a lower magnetic force than expected. A Helmholtz coil setup was designed to measure the differences in a large set of permanent magnets. With the same setup the 2nd quadrant of the B-H curve is reconstructed by stacking of the magnets and using the demagnetization factor. It is shown that the values for Hc and Br of the magnets are indeed lower than the values used for the initial design. New actuators, with increased magnet thickness, are designed and currently fabricated

Deformable membrane mirror with high actuator density and distributed control

Progress on the construction of a deformable mirror with 427 actuators is presented. The mirror consists of a thin continuous membrane on which actuators impose the out-of-plane displacements. Low voltage electro-magnetical push pull actuators are used. The stiffness of the actuator is chosen high enough to avoid mechanical resonances below 1 kHz and to avoid large coupling, but still low enough to be called a soft actuator and to limit the power dissipation to a few mWs per actuator. The actuators are arranged in 7 hexagonal grid modules of 61 actuators. With a pitch of 6 mm, this results in a 48 mm wide hexagonal plate. Each grid has its own 61 channel, 14 bits driving electronics. The drive electronics are designed for low power dissipation and make use of Pulse Width Modulation. A multi-drop LVDS connection provides up to 16 modules with a 1 kHz update rate. Both communication and power is provided through this single cable. In the prototype 7 of these modules are placed on a stable and stiff reference support structure. The modular design makes it possible to extend the mirror to very large numbers of actuators. The deformable mirror by its local influence functions suits the use of distributed control algorithms. A companion paper will report on the control algorithm developments. The 427-channel prototype is to be implemented in the existing breadboard setup at TNO Science and Industry together with a wavefront emulator, wavefront sensor and distributed control algorithm