Fast Steering Research Articles

A Fast Piezo Actuated Tip/Tilt Mirror for raster scan applications

This paper presents the system and control design of a novel high performance piezo-actuated fast steering mirror (FSM) for high speed optical scanning which is centered around an easy-to-manufacture membrane-like mechanical flexure and uses an optical sensor system for position measurement. With the membrane flexure the first fundamental resonance mode of the actuator is placed as high as 6.7 kHz, while still enabling an optical angular range of ±4.8 mrad. Using a PI controller together with three notch filters designed in a loop shaping approach for maximizing the system bandwidth and tracking of high resolution raster trajectories, a closed-loop bandwidth as high as 2.7 kHz and a positioning uncertainty of 3.8 µrad rms is obtained. The obtained tracking error when tracking a 250 Hz raster scanning trajectory with a scan amplitude of 2.2 mrad is as small as 73.8 µrad rms (3.6% of scan amplitude).

Research progress and development trend of fast steering mirror

Research progress and development trend of fast steering mirror

Selective embolization with magnetized microbeads using magnetic resonance navigation in a controlled-flow liver model.

The purpose of this study was to demonstrate the feasibility of using a custom gradient sequence on an unmodified 3T magnetic resonance imaging (MRI) scanner to perform magnetic resonance navigation (MRN) by investigating the blood flow control method invivo, reproducing the obtained rheology in a phantom mimicking porcine hepatic arterial anatomy, injecting magnetized microbead aggregates through an implantable catheter, and steering the aggregates across arterial bifurcations for selective tumor embolization. In the first phase, arterial hepatic velocity was measured using cine phase-contrast imaging in seven pigs under free-flow conditions and controlled-flow conditions, whereby a balloon catheter is used to occlude arterial flow and saline is injected at different rates. Three of the seven pigs previously underwent selective lobe embolization to simulate a chemoembolization procedure. In the second phase, the measured invivo controlled-flow velocities were approximately reproduced in a Y-shaped vascular bifurcation phantom by injecting saline at an average rate of 0.6mL/s with a pulsatile component. Aggregates of 200-μm magnetized particles were steered toward the right or left hepatic branch using a 20-mT/m MRN gradient. The phantom was oriented at 0°, 45°, and 90° with respect to the B0 magnetic field. The steering differences between left-right gradient and baseline were calculated using Fisher's exact test. A theoretical model of the trajectory of the aggregate within the main phantom branch taking into account gravity, magnetic force, and hydrodynamic drag was also designed, solved, and validated against the experimental results to characterize the physical limitations of the method. At an injection rate of 0.5mL/s, the average flow velocity decreased from 20±15 to 8.4±5.0cm/s after occlusion in nonembolized pigs and from 13.6±2.0 to 5.4±3.0cm/s in previously embolized pigs. The pulsatility index measured to be 1.7±1.8 and 1.1±0.1 for nonembolized and embolized pigs, respectively, decreased to 0.6±0.4 and 0.7±0.3 after occlusion. For MRN performed at each orientation, the left-right distribution of aggregates was 55%, 25%, and 75% on baseline and 100%, 100%, and 100% (P<0.001, P=0.003, P=0.003) after the application of MRN, respectively. According to the theoretical model, the aggregate reaches a stable transverse position located toward the direction of the gradient at a distance equal to 5.8% of the radius away from the centerline within 0.11s, at which point the aggregate will have transited through a longitudinal distance of 1.0mm from its release position. In this study, we showed that the use of a balloon catheter reduces arterial hepatic flow magnitude and variation with the aim to reduce steering failures caused by fast blood flow rates and low magnetic steering forces. A mathematical model confirmed that the reduced flow rate is low enough to maximize steering ratio. After reproducing the flow rate in a vascular bifurcation phantom, we demonstrated the feasibility of MRN after injection of microparticle aggregates through a dedicated injector. This work is an important step leading to MRN-based selective embolization techniques in humans.

A New Disturbance Feedforward Control Method for Electro-Optical Tracking System Line-Of-Sight Stabilization on Moving Platform.

A feedforward control was proposed based on the decoupling of target movement and disturbance from gyro signals to improve the stabilization precision of line-of-sight (LOS) for an electro-optical tracking system (EOTS) on a moving platform. Signals measured by gyros mounted on gimbal consist of target movement and disturbance. To remove target movement and obtain middle and high frequency disturbance velocity, the gyro signals were filtered by a high pass filter. The disturbance velocity was integrated into the position signal and fed forward to the inner position loop of the fast steering mirror. A detailed analysis was provided to show the proposed approach, to improve disturbance suppression performance with only slight weakening of target tracking ability. The proposed feedforward control was effectively verified through a series of comparative simulations and experiments. Besides, the method was applied in a real ship-based project.

The Optimization Design of Sub-Regions Scanning and Vibration Analysis for Beaconless Spatial Acquisition in the Inter-Satellite Laser Communication System

We mainly report an efficient approach to optimize the design of beaconless spatial laser acquisition. In order to analyze the influence of vibration environment on the beaconless spatial acquisition, a demonstration of an efficient method will be presented to simulate the level of satellite platform vibration. An important power spectral density (PSD) model of satellite platform vibration used by the European Space Agency is introduced. Then through LABVIEW, a simulated vibration waveform is generated, and the simulated PSD has a basic consistency with the in-orbit measurement spectral line, referring to the design of the filter, generating the Gaussian white noise sequence, filtering the pseudorandom signal and the whole processing procedure of parameter matching. The variation interval of the vibration amplitude is from −100 to 40 μrad. Then, the model of beaconless acquisition is proposed, which uses the spiral scan pattern, laser with a divergence angle of 80 μrad scan in an uncertainty cone (UC), and also the scan coupled with the simulated vibration is given. Through the beaconless acquisition program, the vibration is suppressed between −10.7 and 5.9 μrad; when the overlap factor is selected as 10%, it does not produce any leakage scanning area. It can be used for a wide variety of platforms. This new method will be good for analysis and test of beaconless spatial acquisition algorithm. Finally, the description of acquisition duration, which mainly includes the fast steering mirror spiral scan and the mechanical rotary table step scan, is introduced. And the corresponding optimal selection method of sub-region is proposed; the design of 1.6-mrad scanning pattern with 7 × 7 sub-regions in the 8-mrad acquisition UC is given. Through the actual measurement, the acquisition time is 18.21 s, the tracking error is better than 0.4 μrad.

Increasing the SOFIA Secondary Mirror Mechanism’s Fast Steering Capability by Identification of a Structural Resonance and Its Subsequent Elimination Through Mass Re-Distribution

The Stratospheric Observatory for Infrared Astronomy (SOFIA) consists of a 2.7[Formula: see text]m infrared telescope integrated into a Boeing 747 SP. One of the most complex subsystems of the observatory is the secondary mirror assembly (SMA). This active steering mechanism is used for image stabilization and infrared chopping. Since its integration in 2002, the performance of the mechanism is limited by a structural resonance. Based on Finite Element (FE) simulations and experimental modal surveys, a ring shaped reaction mass was identified to be the causing element of this structural mode. Attenuating the resonance on the hardware level would result in a larger actuation bandwidth for faster chopping and image stabilization. Concentrating mass at the suspension points while keeping the inertia of the ring structure is expected to take strain energy out of the mode. An end-to-end simulation, including a FE model of the mechanism and a controller model was set up to predict the in-flight performance of this concept. A segmented ring made from tungsten and AlSiC (i.e. strong mass redistribution) mounted on the original suspension was selected for the design of a prototype. The prototype was manufactured and thoroughly tested on a full-scale mockup of the mechanism confirming the predicted performance. An actuation bandwidth improvement of 80% was achieved. The settling time for infrared chopping was reduced from 10 to 7[Formula: see text]ms providing about 3.3% higher efficiency for observations with 5[Formula: see text]Hz chopping.

On-orbit beam pointing calibration for nanosatellite laser communications

We describe techniques developed to optimize beam pointing control for a CubeSat laser downlink demonstration mission being developed at the MIT Space Telecommunications, Astronomy, and Radiation Laboratory. To fine-point its downlink beam, the mission utilizes an uplink beacon signal at 976 nm captured by an on-board ±5-deg field-of-view detector and tracked by a 3.6-mm commercial, off-the-shelf MEMS fast steering mirror. As these miniature actuators lack feedback sensors, the system design is augmented with an optical calibration signal to provide the mirror’s pointing feedback. We describe the system and introduce calibration algorithms utilizing the feedback signal to achieve higher fidelity beam pointing control. A demonstration in the laboratory is conducted to obtain a quantitative performance analysis using quasi-flight hardware with simulated spacecraft body pointing disturbances. Experimental results show that beacon tracking errors of only 16 μrad root-mean-square are feasible for both axes, significantly exceeding the mission pointing requirement of 0.65 mrad and indicating the feasibility of narrower beams and higher data throughputs for next-generation downlink demonstration missions.

Composite modeling and parameter identification of broad bandwidth hysteretic dynamics in piezoelectric fast steering platform

Abstract Over a broadband range of frequencies, it is difficult to develop precise model of piezoelectric fast steering platforms (FSPs) due to the complex hysteretic and dynamic behaviors. This paper thus presents one approach of composite modeling and parameter identification by employing a composite hysteretic model. The proposed composite model consists of the Bouc-Wen hysteresis, electro-mechanical dynamics, creep dynamics and the high frequency mirror dynamics. Physical meanings can be found in the linear component which is beneficial to controller design and other further developments using the proposed model. A step-by-step approach is proposed to identify the composite hysteretic model. Finally, the FSP experiment is provided to validate the effectiveness of the proposed modeling and parameter identification approaches.

Design and Characterisation of a Fast Steering Mirror Compensation System Based on Double Porro Prisms by a Screw-Ray Tracing Method.

This study proposes a novel FSM compensation system for four degrees of freedom (DOF) laser errors compensation, which has the advantage of shorter optical path length, fewer elements and an easier set-up process, meaning that it can be used at different locations. A commercial software, Zemax, is used to evaluate the function of the proposed FSM compensation system and the mathematical modelling of the proposed FSM compensation system is established by using a skew-ray tracing method. Finally, the proposed FSM compensation system is then verified experimentally using a laboratory-built prototype and the result shows that the proposed FSM compensation system achieves the ability to compensate the 4 DOF of the laser source.

Space-qualified fast steering mirror for an image stabilization system of space astronomical telescopes.

A space-qualified fast steering mirror (SQ-FSM) was designed, built, and tested at the National Astronomical Observatories of the Chinese Academy of Sciences for an image stabilization system of space astronomical telescopes, which is used for the tip-tilt correction of small jitter of the satellite platform; this achieved image stability in a closed-loop manner. Its design primarily faces four challenges involving (1)sustaining the specified sine and random vibration without launch lock, as well as shock response spectrum experiments; (2)surface form error of a clear aperture of ϕ120 mm less than 1/50λ root mean square (RMS, λ=632.8 nm) with a relatively rigid mirror support; (3)resonance frequency of at least 800Hz and as high as possible; (4)minimum reaction force and torque in order to decrease its unfavorable influence on the satellite platform. To achieve these goals, the global optimizations and compromises have to be made throughout the design process. The study reviews the detailed design of the SQ-FSM with respect to the four challenges, mainly by keeping the mirror and its support lightweight, mirror bonding and solidification, actuator and its stiffness, flexure support of the mirror and its holder, material optimization for weight, stiffness, and coefficient of thermal expansion, as well as finite element analysis on statics and dynamics. The performances are also measured and expatiated, including the surface form, resonant frequency, tip-tilt stroke, vibration and shock response spectrum experiments, etc., which validate the performances of the SQ-FSM.

High-Performance Hybrid-Reluctance-Force-Based Tip/Tilt System: Design, Control, and Evaluation

This paper presents the design, assembly, and evaluation of a novel integrated two degree of freedom (2DoF) actuator design for tip/tilt motion and its integration into a fast steering mirror with comparably large range. The system is designed for scanning applications, such as in optical metrology systems, and comprises an integrated symmetric 2DoF hybrid-reluctance-force actuator and moverflexure-design. The actuator design employs a permanent magnet for biasing the actuator with a constant flux and two actuator coils per axis to generate a steering flux for positioning the mover. The system prototype has an angular range of ±3° in tip and tilt and achieves a closed-loop control bandwidth of 1 kHz based on the signal of eddy current sensors. Compared to the only so far realized state-of-theart reluctance force actuated fast steering mirror system with integrated sensors, the presented design enables to improve the product of range times' bandwidth, representing a measure for the system performance, by more than 85%.

Improved ADRC With ILC Control of a CCD-Based Tracking Loop for Fast Steering Mirror System

In order to improve the tracking accuracy of the fine tracking system of free-space optical communications, a proposed active disturbance rejection control (ADRC) with iterative learning control (ILC) strategy is designed to regulate the fast steering mirror (FSM) system. First, based on the open-loop frequency response results of the system, we can approximately confirm the system transfer function and prepare for the design of the controller. Then, an improved ADRC that using sliding mode control instead of the traditional nonlinear state error feedback method is proposed to attenuate the influence of the unknown and unmolded parts. In addition, the ILC method is used to compensate the hysteresis of piezoelectric actuators and improve the overall system performance. The experimental results indicate that the tracking accuracy of the system is 1 μrad at 1k sampling frequency. In addition, the recommended method shows the better disturbance suppression performance than traditional ADRC in simulation and experimental investigations. This method has a very high application value in the field of visual axis stability, adaptive optics, and other fields.

Study on application of model reference adaptive control in fast steering mirror system

The control accuracy of traditional controllers for fast steering mirror system depends largely on the accuracy of the system model, which limits the performance of the controllers. Model reference adaptive controller (MRAC) is suggested for further improving the control accuracy of the fast steering mirror. Before designing the controller, we first introduced rate feedback to reduce the resonance peak of the fast steering mirror. Then a reference model which has good dynamic performance is selected. The MRAC controller is designed base on Lyapunov theory, which guarantees the asymptotic stability of the whole system and makes the output of the FSM system evolves asymptotically to the selected model. Experimental results show that FSM system with MRAC controller shows excellent dynamic performance.

A Super-responsive Tip-Tilt Motion Control of a Mirror Driven by Electromagnetic Force and Supported by an Elastic Rod Made of Super Elastic Alloy

Fast steering mirrors (FSMs), which are mounted on observation satellites to realize a shake correcting function, are required to have large aperture, high responsivity and multi control axes. This paper presents a tip-tilt motion controlled FSM having an 80 mm diameter mirror driven by four compact voice coil motors and supported by an elastic rod made of super elastic alloy called GUMMETAL®. Four rubber dampers are also attached to the mirror to suppress an unexpected low-frequency vibrational mode. To evaluate the effect of the elastic rod and rubber dampers on the static and dynamic performance, tip-tilt range and open-loop frequency responses of the FSM were measured with or without dampers. Each PID controller for tip-tilt and axial motion was designed based on the FSM dynamic model. The control result showed that the bandwidth of more than 2,500 Hz in the tip-tilt direction was achieved suppressing the low-frequency vibrational mode.

Two-degrees-of-freedom piezo-driven fast steering mirror with cross-axis decoupling capability.

Because mechanical cross coupling between its axes would lead to degradation of the scanning precision of a piezo-driven fast steering mirror (PFSM), a two-degrees-of-freedom (2-DoF) PFSM with a cross-axis decoupling capability, in which 2-DoF flexure hinges are used, is proposed in this work. The overall structure of the proposed PFSM is first introduced and then both static and dynamic models are established analytically; in addition, the decoupling mechanism is described in detail and the low dynamic cross coupling ratios that occur between the two DoFs are shown. Because of the decoupling property of the PFSM, the 2-DoF motion is treated as a combination of two independent one-degree-of-freedom (1-DoF) motions and two independent proportional-integral-derivative controllers are thus used separately in the control of the two DoFs. Based on this control strategy, experiments involving both 1-DoF trajectory tracking and 2-DoF trajectory tracking are implemented. The test results show that the proposed PFSM can achieve the tilt range of ±7 mrad for both axes with the low coupling ratios that are less than 2% (-34 dB), and the bandwidths of both axes are higher than 810 Hz; in addition, the maximal tracking full scale range errors for 1-DoF trajectory tracking and 2-DoF trajectory tracking are less than 0.2% and 1%, respectively, where the larger error of 2-DoF trajectory tracking is mainly caused by the remaining cross coupling between axes.

Decision making improves sperm chemotaxis in the presence of noise.

To navigate their surroundings, cells rely on sensory input that is corrupted by noise. In cells performing chemotaxis, such noise arises from the stochastic binding of signalling molecules at low chemoattractant concentrations. We reveal a fundamental relationship between the speed of chemotactic steering and the strength of directional fluctuations that result from the amplification of noise in a chemical input signal. This relation implies a trade-off between steering that is slow and reliable, and steering that is fast but less reliable. We show that dynamic switching between these two modes of steering can substantially increase the probability to find a target, such as an egg to be found by sperm cells. This decision making confers no advantage in the absence of noise, but is beneficial when chemical signals are detectable, yet characterized by low signal-to-noise ratios. The latter applies at intermediate distances from a target, where signalling molecules are diluted, thus defining a ‘noise zone’ that cells have to cross. Our results explain decision making observed in recent experiments on sea urchin sperm chemotaxis. More generally, our theory demonstrates how decision making enables chemotactic agents to cope with high levels of noise in gradient sensing by dynamically adjusting the persistence length of a biased random walk.

A visually guided beamformer to aid listening in complex acoustic environments

The purpose of this paper is to provide an overview of recent work on the visually guided hearing aid (VGHA; Kidd et al. JASA 133, EL202). The latest prototype VGHA consists of a lightweight, head-mounted, 18-microphone array combined with PC-based software to create a highly-tuned acoustic beamformer steered in real time by an integrated binocular eye tracker. Recent work on the two main functional sub-components of the VGHA (beamforming and eye-gaze control) focused on the benefits that may be obtained by listeners with hearing loss (and in some cases by listeners with normal hearing) attempting to understand speech masked by one or more competing sounds (such as Gaussian noise or additional speakers). These benefits include improved signal-to-noise ratio under different masked conditions and faster steering (compared to fixed directional amplification steered by head turns). The VGHA may be simulated over headphones by PC-based implementations using HRTFs and an optional external eye-tracker. This has allowed testing a number of variations of beamformer properties and the evaluation of multiple types of sound processing strategies including the incorporation of natural binaural cues. Overall, the VGHA holds considerable promise for improving selective listening in a variety of complex listening conditions.

Deflection angle detecting system for the large-angle and high-linearity fast steering mirror using quadrant detector

A deflection angle detecting system (DADS) using a quadrant detector (QD) is developed to achieve the large deflection angle and high linearity for the fast steering mirror (FSM). The mathematical model of the DADS is established by analyzing the principle of position detecting and error characteristics of the QD. Based on this mathematical model, the method of optimizing deflection angle and linearity of FSM is demonstrated, which is proved feasible by simulation and experimental results. Finally, a QD-based FSM is designed and tested. The results show that it achieves 0.72% nonlinearity, ±2.0 deg deflection angle, and 1.11-μrad resolution. Therefore, the application of this method will be beneficial to design the FSM.

快反镜在高功率连续激光辐照下的热性能分析

快反镜在高功率连续激光辐照下的热性能分析

Optimization design of the angle detecting system used in the fast steering mirror

In this paper, in order to design a fast steering mirror (FSM) with large deflection angle and high linearity, a deflection angle detecting system (DADS) using quadrant detector (QD) is developed. And the mathematical model describing DADS is established by analyzing the principle of position detecting and error characteristics of QD. Based on this mathematical model, the variation tendencies of deflection angle and linearity of FSM are simulated. Then, by changing the parameters of the DADS, the optimization of deflection angle and linearity of FSM is demonstrated. Finally, a QD-based FSM is designed based on this method, which achieves ±2° deflection angle and 0.72% and 0.68% linearity along x and y axis, respectively. Moreover, this method will be beneficial to the design of large deflection angle and high linearity FSM.