Steering Mirror Research Articles

An all-pass based internal model principle controller for galvanometer mirror steering

Galvanometer is a critical component for beam steering in additive manufacturing, profilometry, and medical imaging. In these applications, the quality of the process relies on the precision motion control of the galvanometer to either track or reject narrow-band signals. From the internal model principle, this can be accomplished by incorporating the dynamic model of the reference or disturbance signal in the feedback loop. This paper proposes an innovative internal model principle controller based on numerically robust all-pass filters. The main concept revolves around converting the controller design into a phase response design problem with all-pass filters. For the proposed approach, the targeted frequencies of the internal model can be arbitrarily placed without sacrificing the performance. Further, it produces lower-order controller and is robust against quantization effect. When combining with frequency estimation algorithm and adaptive filter, it can be readily applied to unknown and time-varying disturbance rejection. The advantages of the proposed method are demonstrated by comparing with conventional approaches through analysis, simulation, and experimentation.

Design and control of a fast steering mirror based on flexible supports and piezoelectric ceramic actuators.

The use of the fast steering mirror in an optical path requires strict volume control, and traditional structures have low space-utilization efficiency, resulting in traditional actuators having limited output in narrow spaces. The design in this paper adopts a combination of flexible universal supports and piezoelectric ceramic actuators, greatly reducing the layout space of the rotating-shaft system. We accurately model the design structure and develop closed-loop control methods to further improve the closed-loop control accuracy of the system. The experimental results indicate that the developed control method effectively improves the response speed and bandwidth and thus has good potential for use in engineering applications.

Thermo-mechanical and quasi-optical analysis of the ECRH equatorial launcher steerable mirror towards the CFETR

This study reports a prototype design of the front steerable mirror (SM) in the Electron Cyclotron Resonance Heating (ECRH) system, which is used to validate the manufacturing technology of the large-scale quasi-optical mirror in the China Fusion Engineering Test Reactor (CFETR). As a plasma-oriented component, the SM will experience more neutron heat and plasma radiation heat than the remote focusing mirrors, and the simulation results indicate that the peak temperature of the SM would then reach 292.5 °C, the maximum deformation and the maximum equivalent stress would be 1.16 mm and 446 MPa, respectively. The beam exhibits little distortion because the thermal deformation gradient around the wave center is small, and the quasi-optical analysis shows that the position of the wave heating plasma should be shifted by -6 mm along the local Z-axis, while the amplitude of the central electrical field would increase by 0.3%, from 294.7 kV/m to 295.5 kV/m.

Pointing stabilization of 140 mJ, 10 Hz UV laser for Laser-Assisted Charge Exchange

This paper details a laser beam delivery and pointing stabilization system designed for the Laser-Assisted Charge Exchange (LACE) experiment at the Spallation Neutron Source (SNS) accelerator. The UV (355 nm) laser beam of 140 mJ pulses at 10 Hz must be transported over 65 m through a retrofitted laser transport line in a high-radiation area of the accelerator, causing the alignment of the beam to jitter and drift significantly on timescales of 0.1–1 s and >1 min, respectively, due to thermal effects and other disturbances. The laser pointing stabilization system is based on a novel technique which uses active feedback between CMOS cameras and a piezoelectric steering mirror controlled by feedback software to make corrections to the horizontal angle and vertical position of the laser beam at 10 Hz. Low frequency drift is easily controlled, and the long-term stability is limited by the amplitude of high frequency components of the spectrum near the Nyquist limit (5 Hz). With feedback turned on, the root-mean-square (RMS) deviations of the horizontal angle and vertical position over 20 min fell from 314 to 157 μrad and 309 to 167μm, respectively, which is below the pulse-to-pulse jitter at 10 Hz of 189 μrad RMS and 180μm RMS.

Modelling of fast steering mirror assembly: A review of the various methodologies

The interest towards using Fast Steering Mirror (FSM) assembly in optical communication is mainly fueled by the fact that this device has the capability of precise pointing and stabilization of the beam in optical systems. Scientists across the world are working on modelling and control of FSM that will not only perform efficiently but also guarantee stability. Computationally efficient model of the FSM is one basic demand from scientists, for developing a control strategy of the FSM. Attention is focused on works which involves building a feedback control design to model a FSM. The survey results are not only used to identify the differences and similarities of the approaches, but also used to identify the modelling methodology suitable for a specific application.

Calibration of the sub-aperture backscatter system on OMEGA EP

The sub-aperture backscatter (SABS) diagnostic on the OMEGA EP Laser System [Waxer etal., Opt. Photonics News 16, 30 (2005)] is a diagnostic that is used to measure the backscattered and sidescattered light during laser-plasma interaction experiments [W. L. Kruer, The Physics of Laser Plasma Interactions, Frontiers in Physics Vol. 73, edited by D. Pines (Addison-Wesley, Redwood City, CA, 1988) and Myatt etal., Phys. Plasmas 21, 055501 (2014)] that are relevant to high-energy-density physics and inertial confinement fusion. The diagnostic collects stimulated Brillouin scattering (SBS) UV light at around 351nm and stimulated Raman scattering (SRS) in the visible-light regime in the 420-720-nm-wavelength range and provides spectrally and temporally resolved information. Five 1-in. light collectors, composed of a lens, ground glass diffuser, and coupling into a 300-μm fiber, are positioned behind the last steering mirror on one of the four beamlines to catch a portion of the beam cross section (∼1.5%) of the emission that is scattered into the beamline. The SRS light is collected in two light collectors, combined, and transported via graded index fibers to a streaked spectrometer. The SABS-SRS streak spectrometer has a temporal and spectral resolution of 100ps and 1nm, respectively. Three other light collectors collect, combine, and transport the SBS signal to a Hamamatsu high-voltage photodiode, where an oscilloscope digitizes the data, providing a time resolution of better than 1ns. To obtain an absolute energy calibration of SRS measurements, light signals of known energy and wavelength were injected into the light collectors one at a time. The resulting counts on the streak camera charge-coupled device for SRS are then correlated with the incident fluence of scattered light at the light collector in order to allow a quantitative assessment of streak camera sensitivity to determine the energy of the scattered light during experiments. The measurements were performed in situ from the light collectors to the detectors. Additional offline measurements provided the transmission of the optics between the target chamber center and the light collectors.

Preliminary design and thermo-mechanical analysis of the ECRH equatorial launcher focusing mirror (M1) towards the CFETR

The Electron Cyclotron Resonance Heating (ECRH) Equatorial Launcher (EL) is under research towards the China Fusion Experimental Tokamak Reactor (CFETR). In the latest pre-research model, 6MW of electromagnetic waves would be guided to the plasma by two focusing mirrors (M1 & M2) and one front plate steering mirror (SM). Since the M1 will experience intense heat generation from ohmic dissipation and neutronic irradiation, a specific water-cooling structure has been designed for it to reduce the thermal deformation, allowing the M1 to experience less mechanical pressure during long pulses. The simulation demonstrates that water can efficiently dissipate ∼0.3 kW of neutronic heat and ∼30 kW of ohmic loss heat. As a result, the maximum deformation and equivalent stress are less than 0.4 mm and 160 MPa, respectively, which satisfactorily meets the system requirements.

Design of optical transmission line of ITER poloidal polarimeter

ITER Poloidal Polarimeter is laser-aided diagnostics that measures the change of polarization of injected far-infrared laser beam in order to identify plasma current profile. PoPola consists of thirteen measurement chords, and each chord is comprised of a set of optical components. This study shows the design of optics in the ITER vacuum vessel and those in high radiation area outside the vacuum vessel. The design takes into account zero-offset configuration, the minimization of the neutron streaming, remote alignment, accessibility, manufacturability and radiation hardness. The laser power loss due to vignetting loss and absorption is evaluated, and the signal-to-noise ratio at a detector is estimated at 50,000. This study evaluates the acceptable errors of manufacturing and assembly of the optical transmission line on the condition that the demonstrated alignment method using the two steering mirrors outside the vacuum vessel is applied. The evaluation results show that the medium tolerance class of ISO 2768 is basically enough for manufacturing ITER poloidal polarimeter. Only for assembling the in-vessel mirrors to the mirror module, the mirror angles need to be carefully adjusted with accuracy of 0.1° Finally, radiation hardness of a steering mirror is presented. The steering mirror consists of an angle sensor and a piezo actuator. The angle sensors are developed by the authors and free from semiconductor and soft resin. The prototype test results show that angle sensor can measure the mirror angle in the range of ±2.3° with accuracy of 0.37%. As for the piezo actuator, the irradiation test was carried out. The test results show that the piezo actuator is able to operate after the neutron fluence of 3.5 × 1015 n/cm2 and the gamma dose of 0.99 MGy.

Laser Beam Jitter Control Based on a LabVIEW FPGA Control System

The thermal blooming effect, platform jitter, and other effects of laser beam propagation cause serious deviation, which will have a negative impact on laser tracking systems. It is important to ensure that the laser beam does not deviate. Based on a fast steering mirror, a CMOS camera, and a Flex RIO system, a laser beam jitter control system is designed and implemented. The error is detected by camera and compensated for by the fast steering mirror (FSM), which is controlled by the Flex RIO device. By using LabVIEW and MATLAB, a closed loop model is realized. The results show that the system response is highly stabilized and has a short rise time, providing a reference for engineering applications.

Study of magnetic effects on DTT ECRH front-steering mirror

In the final configuration of the Divertor Tokamak Test (DTT) facility, 32 front-steering launchers of the Electron Cyclotron Resonance Heating (ECRH) system will be distributed in 4 sectors of the machine. Two different antennas are hosted in the equatorial and in the upper port of each ECRH sector, with 6 and 2 single launchers respectively. This setup will deliver up to 35.2 MW installed power to the plasma, making such ECRH system the most powerful ever implemented at the time of its completion. To comply with compactness and performance requirements, a fully in-vessel driving system has been proposed for the steering mirrors. The system relies on UHV-compliant piezoelectric walking drives to provide biaxial steering capability. The main drawback of piezoelectric drives is their low driving force, in an environment where magnetic torques can be very high because of variable magnetic fields, as those induced by the non-axisymmetric in-vessel coils for plasma control, and especially in case of disruption events. Therefore, the materials of the water-cooled steering mirrors must be chosen in such a way as to minimize magnetic torques while guaranteeing adequate heat conduction. Numerical and analytical models of the magnetic torques acting on the steering mirror have been developed. The models have been applied to the steering mirror to quantify magnetic torques due to various sources and dynamic regimes. The currently proposed Copper–Chromium–Zirconium mirror was proven to be critical. Therefore, compliance requirements for the mirror have been computed, and different solutions have been numerically evaluated for magnetic torque mitigation and heat conduction capability, including combination of different materials and implementation of a tungsten-coated dielectric mirror.

Probabilistic model to spatially acquire optical links in space under the influence of band-limited beam jitter.

An analytical model is derived for the probability of failure (P-fail) to spatially acquire an optical link with a jittering search beam. The analytical model accounts for an arbitrary jitter spectrum and considers the associated correlations between jitter excursions on adjacent tracks of the search spiral. An expression of P-fail in terms of basic transcendental functions is found by linearizing the exact analytical model with respect to the correlation strength. Predictions from the models indicate a strong decrease of P-fail with increasing correlation strength, which is found to be in excellent agreement with results from Monte Carlo simulations. The dependency of P-fail on the track width and scan speed is investigated, confirming previous assumptions about the impact of correlations. Expressions and applicable constraints are derived for the limits of full and no correlations, and the optimal track width to minimize the acquisition time is computed for a range of scan speeds. The model is applicable to optical terminals equipped with a fast beam steering mirror, as often found for optical communications missions in space.

High-precision dynamic pointing method for improving the acquisition performance of laser communication between high-altitude platform stations

Fast and stable acquisition is the prerequisite for establishing a space laser communication links. In this paper, the acquisition technology of establishing a space laser communication link between high-altitude platform stations (HAPS) is studied, a dynamic pointing and scanning model is established, and the acquisition performance of the laser communication system under line of sight (LOS) disturbance is analyzed based on this model. To improve the acquisition performance, we propose a high-precision composite pointing method to reduce the pointing error of the line of sight. A composite pointing controller with a fast steering mirror (FSM) is designed to reduce pointing errors, and the aiming accuracy of line of sight is improved by predicting the target with improved unscented Kalman filter (IUKF). A dynamic laser communication experiment based on HAPS is built to verify the method. The results show that the proposed method reduces the initial pointing error by 45%, and the stability of the line of sight is increased by a factor of 5. By conducting several acquisition tests, the average acquisition time is 1.57 s with 100% acquisition probability. The acquisition performance is significantly improved compared with that before the improvement, in which the acquisition probability is increased by 8% and the acquisition time is reduced by 44.3%, which verifies the effectiveness of the proposed method and lays the foundation for the establishment of a fast communication link for laser communication between HAPS.

Prototype development of multi-target tracking system for space multi-node laser communication network

Laser communication network is the future trend, but the disadvantage of a traditional laser communication system is that it can only transmit with a single node. To solve this problem, we propose a multi-target tracking technology to simultaneously establish multiple laser communication links. This study also develops a multi-target detection method based on beacon modulation and code division multiple access, proposes an optical antenna central stabilization method, and enhances the controller of the fast steering mirror (FSM) to improve the tracking performance. To verify the feasibility of our proposed method, we build a prototype system for multi-target laser communication and conduct the corresponding experiment. The experiment simulates the multi-target tracking situation by three beam emitting subsystems, and the dynamic tracking performance of the prototype system is verified by applying perturbations through a shaking table. Results show that the proposed method can effectively extract the position of the target in the mixed beam, with an average detection deviation of 2.95 μrad compared with when only a single beam is incident. The introduction of central stabilization enhances the dynamic tracking performance of the optical antenna. Tracking accuracy of the optical antenna is 24.63 μrad when attitude disturbance is applied. The improved controller enhances the performance of FSM, composite tracking accuracy is 5.69 μrad, and tracking error is reduced by 22.16% compared with the traditional PI controller. Performance also improved by 45.42% compared with when there was no improvement. Experimental results verify the effectiveness of the multi-target tracking system. On the basis of the proposed method, we achieve simultaneous tracking of multiple targets in different directions and significantly improve the dynamic tracking performance.

Operational domain for the new 3MW/1000s ECRH System on WEST

The ECRH system formerly used in Tore Supra is being upgraded to start on WEST in 2023, at a power level of 1MW and frequency of 105 GHz. Its ultimate 3MW/1000s capability is expected to enlarge the WEST operational domain by increasing margins with respect to H-mode access, and by providing additional flexibility in terms of achievable scenarios using impurity and/or MHD control. This flexibility is made possible using an antenna based on three steerable mirrors for controlled power injection. In order to determine an appropriate range of EC wave injection angles for WEST scenarios, the fast and reliable ray-tracing code REMA has been interfaced with the WEST IMAS database. This allows the EC power damping rate to be quickly assessed, as well as deposition profiles to be predicted in realistic plasma conditions. Based on a typical WEST discharge at central magnetic field B0~3.6 T, central line-averaged electron density nl~4 × 1019 m−3 and central electron temperature Te0~3keV, ray-tracing calculations have been performed. Comprehensive poloidal and toroidal angle scans, as well as variations of Bt, nl and Te0 with respect to the reference parameters have allowed an adequate range of injection angles to be determined for efficient use of ECRH and/or ECCD in typical WEST scenarios, and compared with the mechanical limits set by the antenna mechanical characteristics. In order to further characterize the effect of this new power source in WEST scenarios, EC wave deposition and current profiles from ray-tracing calculations have been included in integrated simulation codes. It has been shown that this additional power source could allow central electron heating to be achieved, potentially alleviating the issue of radiative collapse caused by impurities observed in some situations.

Control of a MEMS Fast Steering Mirror With Improved Quasi-Static Performance

Control of a MEMS Fast Steering Mirror With Improved Quasi-Static Performance

Theoretical modeling and simulation analysis for fast steering mirror

为有效指导快速反射镜设计，开展了快速反射镜理论建模与仿真分析研究。相比以往单自由度运动模型，该文基于动力学分析，建立了两轴快速反射镜的多自由度运动微分方程，推导出被控对象传递函数和对基座扰动的隔离传递函数；仿真分析了结构谐振频率对被控对象特性的影响，指出转动固有频率较大时，被控对象为欠阻尼系统，传递函数曲线存在谐振峰；建立了包含结构特性的快速反射镜控制系统仿真模型，分析了负载质心偏离柔性铰链支承中心、结构谐振频率等对其扰动隔离性能的影响，指出非工作轴固有频率越大，对扰动隔离性能越好，一般选择在伺服系统增益交界频率2倍以上，工作轴固有频率需权衡扰动隔离性能和电机力矩约束等进行选择。最后结合实物振动试验，验证了仿真模型的正确性。

Application of Nonlinear PID Active Disturbance Rejection Control in the Temperature Control System of Fast Steering Mirror

Application of Nonlinear PID Active Disturbance Rejection Control in the Temperature Control System of Fast Steering Mirror

Modeling-free Learning Control of Cross-coupled Fast Steering Mirror for 2-D Trajectory

This paper proposes modeling-free inversion-based iterative control (MF-IIC) for a fast steering mirror, which is a dual-input dual-output system to generate 2-D optical scanning trajectories. To explicitly compensate for the cross-coupling motions, the MF-IIC iteratively updates the control inputs by learning from the previous trials based on a 2 × 2 Jacobian matrix. To operate the MF-IIC without system identification in advance, it estimates the Jacobian matrix including the non-diagonal elements for the cross-coupling dynamics simultaneously during learning. For experiments, 19 Hz and 20 Hz sine waves with an amplitude of 0.65° are used for a Lissajous pattern as a reference trajectory. Experimental results show that a tracking error of 320 × 10−3 degrees when feedback control is used for stability. By additionally combing the proposed MF-IIC, the error is decreased by a factor of 68 to 4.64 × 10−3 degrees. It is smaller than MF-IIC without considering the cross coupling explicitly, which achieves a tracking error of 12.1 × 10-3 deg. These two MF-IIC algorithms are also compared by the resulting sampled trajectories in the spatial domain. For that purpose, maximum dead zone diameter is defined as a diameter of the largest circle without sampled points in a scanning area in this paper. While the MF-IIC without considering the cross coupling explicitly for comparison creates a Lissajous pattern with a maximum dead zone diameter of 74.8 × 10−3 degrees, the proposed MF-IIC achieves a Lissajous pattern with a smaller maximum dead zone diameter of 72.9 × 10−3 degrees, demonstrating its effectiveness for high-quality scanning trajectories.

Research progress of reliability of MEMS fast steering mirror for satellite laser communication

Research progress of reliability of MEMS fast steering mirror for satellite laser communication

Improved Active Disturbance Rejection Control Algorithm Based on Fast Steering Mirror for Optical Communication

Improved Active Disturbance Rejection Control Algorithm Based on Fast Steering Mirror for Optical Communication