Concept, modeling, and performance prediction of a low-cost, large deformable mirror

While it is attractive to integrate a deformable mirror (DM) for adaptive optics (AO) into the telescope itself rather than using relay optics within an instrument, the resulting large DM can be expensive, particularly for extremely large telescopes. A low-cost approach for building a large DM is to use voice-coil actuators connected to the back of the DM through suction cups. Use of such inexpensive voice-coil actuators leads to a poorly damped system with many structural modes within the desired bandwidth. Control of the mirror dynamics using electro-mechanical sensors is thus required for integration within an AO system. We introduce a distributed control approach, and we show that the “inner” back sensor control loop does not need to function at low frequencies, leading to significant cost reduction for the sensors. Incorporating realistic models of low-cost actuators and sensors together with an atmospheric seeing model, we demonstrate that the low-cost mirror strategy is feasible within a closed-loop AO system

Suppressing low-order eigenmodes with local control for deformable mirrors

To improve the mechanical characteristics of actively controlled continuous faceplate deformable mirrors in adaptive optics, a strategy for reducing crosstalk between adjacent actuators and for suppressing low-order eigenmodes is proposed. The strategy can be seen as extending Saint-Venant’s principle beyond the static case, for small local families of actuators. An analytic model is presented, from which we show the feasibility of the local control. Also, we demonstrate how eigenmodes and eigenfrequencies are affected by mirror parameters, such as thickness, diameter, Young’s modulus, Poisson’s ratio, and density. This analysis is used to evaluate the design strategy for a large deformable mirror, and how many actuators are needed within a family

Distributed Control of Large Deformable Mirrors

While it is attractive to integrate a deformable mirror (DM) for adaptive optics (AO) into the telescope itself rather than using relay optics within an instrument, the resulting large DM can be expensive, particularly for extremely large telescopes. A low-cost approach for building a large DM is to use voice-coil actuators, and rely on feedback from mechanical sensors to improve the dynamic response of the mirror sufficiently so that it can be used in a standard AO control system. The use of inexpensive voice-coil actuators results in many lightly- damped structural resonances within the desired control bandwidth. We present a robust control approach for this problem, and demonstrate performance in a closed-loop AO simulation, incorporating realistic models of low-cost actuators and sensors. The first contribution is to demonstrate that high-bandwidth active damping can be robustly implemented even with non-collocated sensors, by relying on the "acoustic limit" of the structure where the modal bandwidth exceeds the modal spacing. Next we introduce a novel local control approach, which significantly improves the high spatial frequency performance relative to collocated position control, but without the robustness challenges associated with a global control approach. The combination of these "inner" control loops results in DM command response that is demonstrated to be sufficient for integration within an AO system

Progress in developing a low-cost deformable mirror

Large (>1m) deformable mirrors with hundreds or thousands of actuators are attractive for extremely large telescopes. Use of force actuators coupled to the mirror via suction cups, and electret microphones for position sensing, has the potential of substantially reducing costs. However, a mirror controlled with force actuators will have many structural resonances within the desired system bandwidth, shifting the emphasis somewhat of the control aspects. Local velocity and position loop for each actuator can add significant damping, but gives poor performance at high spatial frequencies. We therefore introduce a novel control strategy with many parallel "actuator families", each controlled by single-input-single-output controllers. This family approach provides performance close to that of global control, but without the accompanying robustness challenges. Using a complete simulation model of a representative large deformable mirror, we demonstrate feasibility of the approach. This paper describes the challenges of non-ideal actuators and sensors. The results presented give an understanding of the required actuator bandwidth and the effects of the sensors dynamics. The conclusion is that the introduction of actuator and sensor dynamics does not limit the control system of the deformable mirror

Study of a Large Deformable Mirror Concept

It is attractive to integrate a large deformable mirror for adaptive optics into an astronomical telescope rather than using relay optics within an auxiliary instrument. However, the resulting large deformable mirror can be expensive, particularly for extremely large telescopes. We have pursued a low-cost approach using force actuators connected to the back of the deformable mirror through suction cups. This innovative concept for attachment of force actuators does not require high mechanical tolerances. Use of inexpensive voice-coil actuators and a thin mirror leads to a poorly damped system with many structural eigenfrequencies within the desired bandwidth. A feedback signal (in addition to the one from the wavefront sensor) is introduced by electro-mechanical sensors placed at the back of the deformable mirror. Using these sensors, stiffness and damping are added to the mirror through feedback loops. We introduce a local control concept with actuator families that have predetermined force patterns. Use of actuator families reduces crosstalk between adjacent actuators and prevents excitation of a number of low-order eigenmodes. This strategy can be seen as extending Saint-Venant`s principle beyond the static case. Thus, low-order eigenmodes are only weakly excited by actuation, leading to significant cost reduction for the sensors. The suggested sensors are of the electret microphone type. We present an integrated model of our suggested deformable mirror concept, which we use to demonstrate the controllability of the proposed first experimental laboratory setup. The experimental setup encompasses a partially illuminated large deformable mirror, where some force actuators are replaced by dummy actuators. From the experiment, key features, such as local control performance, dynamic range, controllability and robustness of the deformable mirror can be evaluated

Distributed Force Control of Deformable Mirrors

Large (>1m) deformable mirrors are attractive for adaptive optics on ground-based telescopes; the mirrors typically have hundreds or thousands of actuators. The use of force actuators instead of position actuators has the potential to significantly reduce total system cost. However, the use of force actuators results in many lightly-damped structural resonances within the desired bandwidth of the control system. We present a robust control approach for this problem and demonstrate its performance in simulation. First, we demonstrate that high-bandwidth active damping using velocity feedback from mirror sensors that are not quite collocated with the actuators can be robustly implemented, because at sufficiently high frequencies the structural dynamics enter an “acoustic” limit, where the half power bandwidth of a mode exceeds the modal spacing. This is important, because the system can be made less expensive using sensors placed in between actuators rather than collocated with each actuator. Introduction of active damping leads to a much easier problem for subsequent position control. It is known that a position control system in which each of the actuators is controlled using feedback from a collocated sensor can be made robustly stable. However, the resulting performance at high spatial frequencies is poor because there is no shared information between neighbouring actuators. In contrast, global control gives excellent performance but lacks robustness to model uncertainty. We introduce an innovative local control approach, which significantly improves the high spatial frequency performance without the robustness challenges associated with a global control approach. The overall approach is demonstrated to provide excellent command response suitable for an adaptive optics outer loop

Progress in Developing a Low-Cost Large Deformable Mirror

Large (> 1m) deformable mirrors with hundreds or thousands of actuators are attractive for extremely large telescopes. Use of force actuators coupled to the mirror via suction cups, and electret microphones for position sensing, has the potential of substantially reducing costs. However, a mirror controlled with force actuators will have many structural resonances within the desired system bandwidth, shifting the emphasis somewhat of the control aspects. Local velocity and position loop for each actuator can add significant damping, but gives poor performance at high spatial frequencies. We therefore introduce a novel control strategy with many parallel "actuator families", each controlled by single-input-single-output controllers. This family approach provides performance close to that of global control, but without the accompanying robustness challenges. Using a complete simulation model of a representative large deformable mirror, we demonstrate feasibility of the approach. This paper describes the challenges of non-ideal actuators and sensors. The results presented give an understanding of the required actuator bandwidth and the effects of the sensors dynamics. The conclusion is that the introduction of actuator and sensor dynamics does not limit the control system of the deformable mirror

Modeling large deformable mirrors

Planned Extremely Large Telescopes will rely oil availability of large Deformable Mirror in the 2-3m class. Design and construction of such mirrors are challenging and call for powerful simulation tools. We present an evaluation model which is used to study performance of a large deformable mirror for three: actuator topologies. Back sensors topologies are discussed from the point of view of sensor noise propagation. Two methods for estimating the deflection at the actuator locations on the basis of sensor signal are presented and comapred regarding the computational power needed

Integrated Modeling of a Laboratory Setup for a Large Deformable Mirror

We study a concept for a low-cost, large deformable mirror for an Extremely Large Telescope. The use of inexpensive voice-coil actuators leads to a poorly damped faceplate, with many modes within the desired control bandwidth. A control architecture, including rate and position feedback to add damping and stiffness, for the faceplate has been presented in our previous papers. An innovative local control scheme which decouples adjacent actuators and suppresses low-order eigenmodes is a key feature in our controller. Here, we present an integrated model of a partially illuminated large deformable mirror in an experimental laboratory setup with a limited amount of actuators. From the model, conclusions are drawn regarding the number of actuators needed to identify the key features, such as local control performance, dynamic range, and controllability and robustness of the deformable mirror