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Issue 11 ， 2020
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Automatic compensation method for beam drift in long-distance laser measurement
Abstract：To improve the stability of LDs and reduce the laser beam drift in long-distance measurement, a BPNN-PID-based dual-mirror laser-beam drift compensation method was proposed. Two four-quadrant photodetectors, which were placed at the near and far ends of the measurement position, were employed to measure the laser beam drifts. Two 2D precision-angle mirror mounts integrated with two piezoelectric actuators were used to adjust the propagation direction of the laser beam. A model relating the laser beam drift and the angular variation of the angle mirror mounts was established. The angular variation was then fed back with the proposed BPNN-PID method based on the output of the laser beam drift detector, from which the laser beam drift can be automatically compensated. It was verified that the signal stabilities of a laser measurement system that was used for measuring the straightness and angular errors of linear stages improved from ±9 μm and ±5" without laser-beam drift compensation to ±3μm and ±1.5" with laser-beam drift compensation over 15 minutes.Keywords：laser measurement;long-distance measurements;laser diode;beam drift;BPNN-PID- 342
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Published：2020-12-09 -
Angle measurement accuracy evaluation of rotating laser measuring system based on instantaneous rotation speed
Abstract：Based on the timing method of photoelectric scanning, a rotating-laser measuring system leverages the space-time conversion idea of measuring position differences by using time differences. The angle is measured using a uniform rotating platform equipped with linear lasers acting as a rotating measuring ruler. The angle measurement accuracy of the system is related to the uniform velocity of the rotating platform. To research the stability of the instantaneous rotation speed (IRS) of the system and its influence on the angle measurement accuracy, the angle measurement error, speed evaluation index, and instantaneous speed algorithm were investigated. An angle measurement model was established and the influence of the rotation speed on the angle measurement error was first analyzed. Then, we introduced the IRS stability (IRSS) as an index of the stability of the system's IRS. An optimized rotation speed algorithm is proposed to compensate the width error of the encoder pulse for low-precision encoders. Finally, experimental verification was conducted. The results show that the IRS fluctuation of a transmitter equipped with an air bearing decreased from ±4.5 r/min to ±0.25 r/min and the IRSS decreased to ±4" compared to the M/T method with the compensation algorithm. The measurement accuracy of the compensation algorithm was better than that of the traditional algorithm, and the accuracy of angle measurement was consistent with the IRSS. Interestingly, the angle measurement accuracy of the system could be quickly evaluated from the IRSS.Keywords：rotating laser scanning;space-time conversion;angle measurement accuracy;instantaneous rotation speed measurement;instantaneous rotation speed stability- 166
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Published：2020-12-09 -
Cerebral blood flow analysis based on laser speckle contrast imaging technology
Abstract：During cerebrovascular surgery, it is necessary to monitor the flow velocity and the flow in the target blood vessel. In this study, a method of cerebral blood flow analysis based on laser speckle contrast imaging (LSCI) technology was investigated to evaluate the blood flow conditions and cerebral blood flow perfusion after vascular anastomosis. First, the LSCI system was developed to acquire speckle images, and the blood flow velocity image was captured through improved time contrast analysis and pseudocolor processing to monitor the change in flow velocity. Next, the reference points of the region of interest (ROI) were manually selected to intercept the ROI. After the image preprocessing of the ROI was carried out, the improved Canny algorithm was used to extract the edge line of the blood vessel and calculate the blood vessel diameter. Finally, reliability for blood-flow velocity-change monitoring and blood-vessel diameter measurement functions of the LSCI system were verified through animal experiments. The experimental results showed that the improved Canny algorithm decreased the relative error of the vessel diameter to 5.1%. The cerebral blood flow analysis method based on LSCI technology is suitable for monitoring variations in blood flow velocity and measuring the effective flow diameter of blood vessels. When the proposed method is combined with a laser Doppler blood flow meter, blood flow velocity and flow during surgery can be monitored accurately.Keywords：laser technique;laser speckle;blood flow;blood vessel diameter;time contrast- 194
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Published：2020-12-09 -
Characteristics comparison of photoacoustic signal of glucose solution in time domain and frequency domain
Abstract：To overcome the shortcomings of the time-domain signal, which is easily affected by environmental interference, and improve the stability of photoacoustic measurements of the liquid concentrations, this study investigated the frequency-domain characteristics of a photoacoustic signal based on the time-domain peak-to-peak characteristics of the photoacoustic signal. The power spectrum characteristics and maximum entropy spectrum characteristics of the photoacoustic signal of glucose solution were evaluated. Different methods, such as peak detection and logarithmic fitting, were used to analyze the spectral characteristics of the photoacoustic signal. The nonlinear error and repeatability error of the time-domain peak detection, power spectrum, and maximum entropy spectrum methods were compared. The experimental results show that by using the intercept to characterize the photoacoustic signal after the logarithmic fitting of the maximum entropy spectrum curve, the nonlinear error and repeatability error can be reduced by 28% and 71%, respectively, compared with the traditional time-domain peak-to-peak detection method. The frequency-domain method can be used to reduce the nonlinear and repeatability errors of glucose concentration detection significantly. Furthermore, the frequency-domain method could improve the accuracy and stability of noninvasive blood glucose concentration detection.Keywords：photoacoustic detection;blood glucose measurement;photoacoustic signal;time domain;frequency domain;power spectrum;maximum entropy spectrum- 130
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Published：2020-12-09 -
Ultrahigh-precision measurement timing jitter based on self-reference source
Abstract：Timing jitter is a key parameter of low-noise systems, including optical frequency combs and low-noise laser microwave photon radar systems. Consequently, precisely measuring its values is quite important. The traditional direct detection method is limited by the floor noise of the microwave oscillator or photodetector noise, and its measurement accuracy is relatively low. Optical measuring methods, such as optical heterodyne and optical cross-correlation methods, are very complicated and have relatively high requirements on both reference and measured sources. This study presents a method to measure timing jitter with high precision without using a reference source, thereby overcoming some deficiencies of traditional methods. Based on long fiber delay line technology and optical carrier frequency interference, an attempt is made to realize ultra-high precision for measuring timing jitter. Results from the simulated system show that the noise base of a 10-MHz laser is 3.29×10 -13 fs2/Hz (equivalent to -211 dBc/Hz) when the frequency deviation is 100 MHz at its 100th power harmonic point 10 GHz, and the total root mean square timing jitter from 10 kHz to 10 MHz is 535 as, which has an obvious advantage for ultra-low timing jitter measurement. This measurement method is a convenient, high-efficiency method that can be applied to different measured sources, such as passively mode-locked lasers, optical frequency combs, and super-continuum spectra.Keywords：timing jitter;mode-locked laser;optical fiber;optical heterodyne;Frequency domain analysis- 152
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Published：2020-12-09 -
Generalized space-time pulse position modulation over Gamma-Gamma turbulence channel
Abstract：To overcome the low space resource utilization and limited transmission rate of traditional optical spatial modulation, a Generalized Space-time Pulse Position Modulation (GSTPPM) scheme was proposed by combining a linear dispersion code with optical spatial modulation. The principles of generalized space-time dispersion matrix mapping and pulse position modulation mapping were introduced in detail. In addition, Signal detection was performed by using a sphere decoding algorithm. The theoretical bit error rate (BER) expression for the GSTPPM scheme was derived, and the performance of the proposed scheme was compared with existing optical spatial modulation schemes by using the Monte Carlo method. When the number of lasers and the modulation order are fixed, the results show that the transmission rate of the GSTPPM scheme is significantly higher than that of spatial multiplexing, spatial pulse position modulation (SPPM), and generalized spatial pulse position modulation (GSPPM). At the same transmission rate, the BER of (2, 4, 4)-GSTPPM is clearly superior than that of (32, 4, 4)-SPPM and (5, 4, 4)-GSPPM. When BER=10 -3, GSTPPM scheme improves the signal-to-noise ratio by approximately 7 dB and approximately 5.5 dB, respectively.Keywords：wireless optical communication;optical spatial modulation;generalized space-time pulse position modulation;bit error rate- 115
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Published：2020-12-09 -
Long-gauge-length and high-accuracy optical extensometer based on field-of-view splitting
Abstract：Limited by the resolution of the camera and the effect of rigid-body out-of-plane displacement, the strain measurement accuracy of an optical extensometer based on two-dimensional digital image correlation (2D-DIC) is often not high. To address the problem of insufficient camera resolution, a field-of-view (FOV) splitting device was proposed. The device used a right-angled optical prism and two mirrors to split the camera's continuous FOV into two independent fields separated by a certain distance. This device considerably increased the gauge length of the optical extensometer, thus effectively reducing the strain error caused by insufficient camera resolution. Moreover, a telecentric lens was adopted to reduce the effect of out-of-plane motion on the 2D-DIC extensometer. Static tests and uniaxial tensile tests were conducted to verify the feasibility of the proposed extensometer. Experimental results demonstrate that the root mean square errors of corresponding strain results are all within 6 με. The proposed optical extensometer based on FOV splitting is a high-accuracy strain measurement technique.Keywords：digital image correlation;strain measurement accuracy;optical extensometer;FOV splitting device;gauge length- 133
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Published：2020-12-09
Modern Applied Optics
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Active compensation of aberration for large ground-based telescope based on Hexapod platform
Abstract：As the diameter of a ground-based telescope increases, the image quality will significantly degrade because of optical misalignment, which is caused by gravity and thermal deformations of the telescope structure. To accurately estimate the misalignment error, a mechanical model of a telescope was established, and the computation method and compensation process were investigated. According the structural components of the telescope, the structural model was simplified, and then the mechanical model was established using the finite element method. Based on nonlinear least square fitting, a method was presented to compute the misalignment error between the primary and secondary mirrors in which the inputs were the current node position of the primary and secondary mirror surfaces. Thereafter, a method in which the optical axis of the primary mirror used as the benchmark were adopted to determine the length of the hexapod leg with the objective of compensating for the misalignment error. Finally, numerical examples of a ground-based telescope with a diameter of 2 m were presented to verify the presented methods and the corresponding theories. The simulation results show that there are obvious misalignment errors because of gravity and thermal deformations, with maximum root mean square (RMS) radii of the optical spots being 1 473 and 557 μm, respectively. After the secondary mirror compensation, the RMS radii of the spots are decreased significantly, with all of them being < 32 μm. The results verify the misalignment error and the hexapod leg length computation methods.Keywords：ground-based telescope;Misalignment Error;active compensation;Nonlinear Least Square Fitting;finite element- 181
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Published：2020-12-09 -
Demolding deformation of surface micro-prism array structure part based on injection molding
Abstract：To study the influence factors of surface microstructure quality, three micro quadrangular structures with different aspect ratios were designed and prepared using an injection molding experiment. The factors influencing the demolding quality of this experiment, such as polymer materials, microstructure aspect ratios, demolding processing parameters, and if the nickel inserts were subjected to anti-sticking treatment, were studied. First, the difficulty of obtaining the target aspect ratio microstructure was examined for polypropylene (PP), cycloolefin copolymer (COC), and polymethyl methacrylate (PMMA). Next, using the PP material as an example, the forming quality with different aspect ratios was studied. Then, the influence on demolding deformation was discussed with respect to packing pressure and demolding temperature. Finally, the microstructure shape and size of the nickel inserts before and after undergoing anti-sticking treatment were compared. It was possible to obtain PP microstructures that approximated the target aspect ratio successfully. However, the COC microstructures were broken and deformed after demolding, with an average height of approximately 17.0μm. Meanwhile, the PMMA microstructures were also broken and adhered to the nickel cavities. The demolding deformation tended to be worse with the increase of the aspect ratio, and the microstructure was prone to inclined deformation when it was large. The average height of the microstructure increased from 25.0 μm to 87.0 μm when the packing pressure increased, but the microstructure warped when the packing pressure was too high. As the demolding temperature increased, the rate of retained micro prism was 55%, 60%, 30%, and 10%. The size of the nickel insert microstructure was closest to the target size after the anti-sticking treatment. The COC was brittle due to its low elongation at break, while the PMMA showed strong adhesion due to its high surface energy. With the increase of the aspect ratio, the adhesion force at the top of the microstructure and the friction force at the later stage increased. Increasing the packing pressure, reducing the demolding temperature properly, and the anti-sticking treatment could improve the demolding quality, but too high of a packing pressure would cause internal stress increase and warpage of the microstructure.Keywords：surface microstructure;Injection Molding;demolding deformation;polymer materials;aspect ratio;demolding processing parameter;anti-sticking treatment- 141
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Published：2020-12-09 -
Influence of micro vibration on measurement and pointing control system of high-performance spacecraft from Hubble Space Telescope
Abstract：The micro vibration study of the famous Hubble Space Telescope (HST), which is known for its superior in-orbit performance, was reviewed, the composition and characteristics of its pointing control system (PCS) were explained, and the key technical indexes were listed. The study analyzed the application of the advanced technology and concept of HST in the research of micro vibrations and high-performance PCS technology. The disturbance characteristics of the HST Reaction wheel assemblies (RWAs), Solar Array (SA), and the phenomenon of optical element jitter caused by micro vibrations were expounded, based on which five advanced pointing control technologies of modern spacecrafts were summarized, including Reduced-Order Model-Based Control Design, Linear Quadratic Gaussian-Based Controller Design, Analytically and Numerically Derived H∞ Controller Designs, Covariance Control Design, and Dual-Mode Disturbance-Accommodating Controller Design. The data gathered from this analysis and the experience can provide a valuable experience for high-performance spacecrafts that use RWA, CMG, SA, and other potential disturbance sources. The HST micro vibration technologies and analysis methods can provide effective references for the development of high-precision optical instruments, such as high-performance spacecrafts, millisecond optical sensors, and Guangxuecang of space station, ground test, and in-orbit interference environment quantitative assessment, which are involved in the Gaofeng project, Deep Space Exploration, Manned Spaceflight, and Gravitational Wave Detection in China.Keywords：Hubble Space Telescope(HST);micro vibration;high-performance spacecraft;pointing control system- 147
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Published：2020-12-09 -
Drop driving on digital microfluidic chip
Abstract：To improve the driving capability and efficiency of droplets on a digital microfluidic (DMF) chip, a DMF system was designed and platform building research was conducted. The system includes three parts: upper computer control software, a lower computer hardware system, and a DMF chip. A curved quadrilateral combined electrode is proposed. The edge of the electrode pattern can maintain a greater degree of coincidence with the droplet and can provide a greater initial driving force. Drive control of the droplets in air and oil baths on the chip was tested, and the average velocities of propylene carbonate droplets were measured to be 25 and 260 μm/s in air and oil baths, respectively. The experimental results demonstrate that the curved quadrilateral electrode designed in the study can effectively enhance drive control of the droplet.Keywords：digital microfluidics;droplet manipulation;electrode pattern;droplet drive- 154
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Published：2020-12-09 -
Thermal design and verification of micro remote-sensing satellite in low inclination orbit
Abstract：There is a need to satisfy the thermal control requirements of satellite platforms, achieve precise temperature control of space camera trusses, and minimize the power consumption of thermal control systems. In this study, the heat dissipation channel of the electronic equipment was planned reasonably, and the high-precision temperature control of a space camera was designed. First, mission analysis was performed based on the satellite structure layout, the power consumption of the electronic equipment, and the heat flow in low inclination orbit, and thus, the key and difficult points of the thermal design were identified. Next, a detailed design of the satellite thermal control system was carried out. A calibration method for the temperature measurement circuit was proposed, a multilayer-surface temperature equalization approach was adopted, and heat exchange channels between different components were opened. Hence, the entire satellite resources were reasonably used for integrated thermal control design. Finally, satellite thermal balance tests were performed to verify the thermal design. The temperature of the satellite in orbit indicates that the electronic equipment temperature ranges from -0.5 to 28.8 ℃, and the temperature fluctuation and uniformity of the camera truss are lower than ±0.15 ℃. In addition, the average power consumption of the thermal control system in orbit is 9.3 W, which satisfies the temperature control index conditions of the platform and the focusing requirements of the camera. The weight of the thermal control subsystem is 1.5 kg, which accounted for only 3% of the total satellite weight. This study lays a good foundation for the thermal design of low-cost commercial remote-sensing satellites for future investigations.Keywords：commercial remote-sensing satellite;low inclination orbit;thermal design;thermal test;flying in orbit- 149
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Published：2020-12-09 -
Vibration suppression for payload low-order modes using a Stewart platform
Abstract：In this study, an active control approach was applied using active and passive payload vibration isolation platforms to suppress the vibration induced by low-order modes of space precision payloads. This system was designed based on the classical two-parameter isolator and the soft Stewart platform. A Skyhook damper was combined with a notch filter control in the vibration isolation system to improve the vibration attenuation capability of the conventional platform. First, the developed laboratory-scale vibration isolation platform for the space payload was developed, and a theoretical rigid-body dynamic model of the payload in the space microgravity environment was established to assess the isolation performance of the apparatus and to design the controllers. Next, the controllers were designed. Finally, the effectiveness of the combined control was verified experimentally. The experimental results showed that in the conventional vibration isolation platform controlled using only the Skyhook damper, the flexible modes of the upper plate near 17.4 and 34.8 Hz were excited easily, which generated resonances near these frequencies in the vibration transmissibility curve of the vibration isolation platform along the vertical direction. The combined control successfully reduced the corresponding amplitudes by approximately 12.2 and 10.19 dB, respectively. The proposed method can be used to enhance the mitigation of disturbances from the spacecraft bus and to significantly reduce the interference near the target flexible modal frequencies, thus achieving high-performance vibration suppression.Keywords：space precision payloads;Vibration suppression;Stewart platform;Skyhook damper;notch filter- 202
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Published：2020-12-09
Micro/Nano Technology and Fine Mechanics
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Dim moving target detection based on fluctuation analysis
Abstract：To detect moving targets with very low SNRs for monitoring small objects near the ground and providing early warnings regarding such objects, a dim moving target detection method based on fluctuation analysis was proposed. Furthermore, the principle of a correlation feature extraction method based on the fluctuation analysis of a light field in a time interval was expounded. Strong background signals were distributed and changed steadily in the time dimension. Transient fluctuation in the time dimension, caused by the movement of a target, reflected the existence of a weak moving target. First, a one-dimensional signal model based on single pixel gray-scale time series was established. Then, the time window was divided in the time domain, and fluctuation analysis was used to extract related features. Finally, the target signal was detected based on changes in the correlation feature. The method was applied to one-dimensional time-series signal simulation, dim moving target simulation, and a near-Earth asteroid detection experiment. The experimental results show that the detection rate and false alarm rate of the proposed method are significantly better than those of several other commonly used methods. This method provides good detection using simulated and real data.Keywords：image sequence;fluctuation analysis;High frame rate;dim moving target;near earth asteroids- 188
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Published：2020-12-09 -
A method of real-time data reduction of asteroid optical observation image based on GPU
Abstract：Ground-based optical observation is an important means of asteroid detection and property research. In recent years, asteroid survey projects have developed toward larger apertures and larger fields of view. While the detection capability has increased, the amount of data has also increased greatly. To improve the speed of asteroid data reduction, a graphics-processing-unit-based real-time processing method for asteroid optical observation images was proposed. Based on a previous target extraction algorithm, we realized the parallelization of target extraction, researched the Source Extractor algorithm, simplified and parallelized the Source Extractor code, and improved the Match algorithm to improve both applicability and accuracy. The experimental results show that the simplified, parallelized algorithm implemented in an experimental platform built based on the NVIDA GeForce GTX 2080Ti graphics card offers an increase in speed by a factor of up to 17 (with the signal-to-noise threshold set to 3) compared with that of a serial algorithm running on only a central processing unit. With the further development of high-performance graphics cards, there is room for improvement in speedup ratio, and this method is also suitable for the processing of other optical survey observations.Keywords：asteroid optical observation;target extraction;GPU;code parallelization- 160
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Published：2020-12-09 -
Trajectory detection of small targets based on convolutional long short-term memory with attention mechanisms
Abstract：Trajectory detection of small infrared targets is important for infrared guidance and is widely used in aerospace for functions such as detection of small celestial bodies, missile guidance, and battlefield reconnaissance. Aiming at the traditional trajectory-detection algorithm based on track-before-detect technology, which requires prior knowledge of the target, such as the gray distribution or target speed, this paper proposes a convolutional long short-term memory method based on the attention mechanism in neural networks to detect weak target trajectories. This method involves the use of 3D convolutional kernels to extract short-term temporal and spatial information and combines the convolutional long short-term memory network to extract long-term spatial-temporal information of 15 consecutive infrared image sequences. It also uses the attention mechanism to focus on the key information related to the trajectory and discard irrelevant background information. This method enables the network to realize end-to-end prediction. Four objective indicators such as root mean square error, mean absolute error, peak signal-to-noise ratio, and structural similarity index were applied to three infrared sequences for an experimental evaluation. The experimental results indicated that, compared with 3DCNN, 3D-ConvLSTM, and 3D-AIConvLSTM, 3D-AOConvLSTM achieved an average reduction of 32.8% and 46.3% in the root mean square error and average absolute error, respectively, and an average increase of 18.3% and 4.3% in peak signal-to-noise and structural similarity, respectively. It can detect the trajectory of an infrared target with fewer than six pixels. The predicted trajectory is highly consistent with the actual trajectory. The proposed method provides the least background clutter and the best detection result.Keywords：trajectory detection of small target;infrared multiframe image;convolutional long short-term memory;deep learning;attention mechanisms- 166
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Published：2020-12-09 -
Photometry studies of physical properties for Near-Earth asteroids
Abstract：Near-Earth asteroids (NEAs), orbiting around the sun in the proximal space of the Earth, and they have become crucial for two reasons.Some of them, identified as potentially hazardous asteroids, pose the danger of collision with the Earth.They are adopted as the targets for investigating the origin of the planets and small bodies of the solar system, and for sampling and mining in future space missions.However, we couldnot "see" them in detail directly, even with the aid of the largest ground-based telescopes, owing to their small size.Hence, it is difficult to determine the size and shape of NEAs and more challenge to determine their mass and internal structure.Currently, some inversion techniques are applied to determine the basic physical properties (spin parameters, size, shape, scattering parameters, and composition) of asteroids.Among these techniques, the photometry inversion method is widespread used, by which the spin parameters, shape, and scattering parameters of asteroids are subtracted from the photometric data of asteroids.Assuming cartain an experimental scattering law and certain a shape model a series of brightness models of asteroids, and the models were used to inverse the spin parameters, shape, and scattering parameters of asteroids using the least-square and/or Markov chain Monte Carlo approaches.We used the Lommel-Seeliger ellipsoid model to estimate the physical parameters of the near-Earth asteroid (155140)2005 UD based on its photometric data.Further, the photometric data of (106) Dione were analyzed using the Cellinoid shape model.Based on the convex shape model, we analyzed the photometric data of (103) Hera and (346) Hermentaria.Furthermore, a hierarchical analysis platform was built to determine the basic physical parameters of near-Earth asteroids efficiently, which contained a series of brightness models developed based on an assumed scattering law and a selected shape model.For the targets with sufficient data (photometric data obtained for at least four apparitions), the best values of the parameters involved in the convex inversion and their uncertainties can be estimated using a Monte Carlo simulation procedure.Keywords：Near-Earth asteroids;brightness models;shape;spin parameters;photometric phase function- 140
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Published：2020-12-09 -
Study on observation simulation for space-based potentially hazardous asteroids warning mission on Earth trailing heliocentric orbit
Abstract：Monitoring and early warning of asteroids are the premise of planetary defense. To monitor and provide early warning of asteroids from the Sun direction, an Earth trailing heliocentric orbiting optical telescope mission was proposed. Furthermore, considering the third-body gravitational perturbation of the Earth, an orbit maintenance model was established for the telescope. Considering apparent visual magnitude constraints of asteroids and field of view constraints of the telescope, an observation model for potentially hazardous asteroids was established. The observation and early warning efficiency of the telescope were evaluated. Simulation results showed that with delta-v of approximately 580 m/s, the orbit drift distance of the telescope did not exceed the initial state by 6% within 6 years. The telescope, under the condition of limited apparent visual magnitude of 24, could observe 84.5% of potentially hazardous asteroids with a diameter of more than 140 m. It could also provide effective early warning for potentially hazardous asteroids from the Sun direction. Thus, the optical telescope on an Earth trailing heliocentric orbit exhibited good observation and early warning performance.Keywords：planetary defense;potentially hazardous asteroids;space-based optical monitoring;Earth trailing heliocentric orbit;orbit maintenance- 190
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Published：2020-12-09
Monitoring and Early Warning of NEOs
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