Proceedings Volume 12282

2021 International Conference on Optical Instruments and Technology: Optoelectronic Measurement Technology and Systems

Jigui Zhu, Lijiang Zeng, Jie Jiang, et al.
Proceedings Volume 12282

2021 International Conference on Optical Instruments and Technology: Optoelectronic Measurement Technology and Systems

Jigui Zhu, Lijiang Zeng, Jie Jiang, et al.
Purchase the printed version of this volume at or access the digital version at SPIE Digital Library.

Volume Details

Date Published: 8 July 2022
Contents: 8 Sessions, 59 Papers, 0 Presentations
Conference: 2021 International Conference on Optical Instruments and Technology 2022
Volume Number: 12282

Table of Contents


Table of Contents

All links to SPIE Proceedings will open in the SPIE Digital Library. external link icon
View Session icon_mobile_dropdown
  • Front Matter: Volume 12282
  • Optoelectronic Measurement Technology and System I
  • Optoelectronic Measurement Technology and System II
  • Optoelectronic Measurement Technology and System III
  • Optoelectronic Measurement Technology and System IV
  • Optoelectronic Measurement Technology and System V
  • Optoelectronic Measurement Technology and System VI
  • Poster Session
Front Matter: Volume 12282
Front Matter: Volume 12282
This PDF file contains the front matter associated with SPIE Proceedings Volume 12282, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.
Optoelectronic Measurement Technology and System I
Three-dimensional measurement with stereo polarization digital image correlation method insensitive to ambient light
Xi Xu, Yao Hu, Shaohui Zhang, et al.
The measurement of the external topography and deformation of large industrial equipment is important. Photoelectric measurement technology can quickly obtain three-dimensional (3D) topography information of components and equipment. Among them, stereo digital image correlation (stereo-DIC) is a 3D measurement method which combines binocular stereo vision system and digital image cross-correlation matching technology. This method has many advantages such as non-contact, wide measuring range, high speed and single frame reconstruction. However, this passive optical measurement method is prone to the interference of ambient light and the impact of high reflective surface, resulting in data loss, which limits the application scenarios of this method. This paper proposes a method of 3D reconstruction against ambient light based on stereo polarization digital image correlation (stereo-PDIC). In this method, speckle is generated by a laser beam irradiating a ground glass and is then projected onto the surface of the target. The polarization speckle image in single frame and multiple polarization channels is collected by binocular polarizing cameras. The influence of the reflected stray light and ambient light is eliminated and the contrast of speckle image is improved. Stereo matching of the speckle images with the corresponding angle of polarization captured by the binocular polarizing cameras is carried out, and finally the 3D reconstruction of the target is completed. The feasibility of this method is verified by both simulation and experiment.
Development and application of line measurement Linnik-type white light dispersive interferometer
Semiconductor processing technology is the foundation of the development of semiconductor industry, and the manufacturing level will affect the key parameters of semiconductor products. Therefore, it is necessary to develop metrology technique that matches the current semiconductor processing accuracy. White light dispersive interferometry (WLDI) technology, as a highresolution optical metrological technique for measuring complicated surfaces, has been applied in the technical research in the semiconductor inspection. In this paper, the set-up of Linnik type line-scan WLDI system is developed, and the data processing algorithm is proposed to realize the simultaneous measurement of the surface profile and film thickness with high measurement efficiency. The experimental results indicate the current system has a good measurement performance.
Heterodyne three-degree-of-freedom grating interferometer for ultra-precision positioning of lithography machine
Junhao Zhu, Guochao Wang, Gaopeng Xue, et al.
In this paper, we proposed a new reflection type heterodyne three-degree-of-freedom (three-DOF) grating interferometer for ultra-precision positioning of the worktable of the lithography machine. The grating interferometer is based on a 780 nm wavelength dual-frequency laser that can generate a 40 kHz beat signal through two acoustic-optical modulators (AOM). The reflected lights from the grating and the mirror are used to obtain the heterodyne interference signal in the z-direction, and the four beams of diffracted light from the two-dimensional planar grating are used to obtain the heterodyne interference signal in the x- and y-directions. By comparing the phase of the reference signal, the phase changes in the x-, y-, and z- directions are calculated to derive the three-DOF displacement. Based on optical subdivision and electronic subdivision, the analysis result shows that the resolutions in the x- and y-directions are 0.069 nm, and in the z-direction is 0.108 nm, providing a nanometer resolution for all three axes. In the experiment, the z-direction measurement optical path was constructed to test the static stability of the zero-crossing point of the heterodyne interference signal and the straightness of the z-direction displacement. The preliminary experiment result verified that this interferometer can simultaneously provide the static stability of the zero-crossing point of ±0.1 nm and high straightness. Due to the common optical path design of x-, y- and z-directions, the heterodyne three-DOF grating interferometer structure can be compact and allow a high-performance uniformity of the three axes.
Simultaneous high-precision velocimetry and ranging based on beat frequency signal splitting via a FMCW lidar with Fabry-Pérot cavity
Junchen Liu, Xingrui Cheng, Fumin Zhang, et al.
In industrial sites, large-scale advanced manufacturing equipment such as optical diamond cutting lathes require stable operation during processing and are very sensitive to operating speed. However, the vibration generated by the environment or equipment will interfere with its operation and machining accuracy. A precise laser interference measurement system is required to monitor the running speed and position of the three-axis tool of the lathe in real time, providing accurate parameters for the closed-loop control system of the equipment, and improving the stability of lathe operation. Frequency-modulated continuous-wave (FMCW) lidar is widely used in the field of industrial measurement due to its non-contact, high accuracy, and fast dynamic response. The basic principle of FMCW lidars is to measure the velocity of a moving object through the Doppler frequency shift phenomenon. But the vibration generated by the moving object will cause the spectrum to broaden and the precision and repeatability of measurement to decrease. Therefore, we propose a large-bandwidth triangular wave-modulated lidar structure with Fabry-Pérot(F-P) cavity to achieve real-time high-precision measurement of the speed and distance of moving targets. This structure is based on the F–P resonance peaks generated by changing the length of the F-P cavity to accurately split the beat signal generated in the sweep frequency range of 1545-1565nm to obtain 40 sets of data with equal frequency intervals of 62GHz, effectively solve the problem of excessive data volume when measuring the continuous moving target speed and reduces the complexity of the algorithm. Splitting the measured beat signal based on the resonance peaks signal of the F-P cavity, which reduces the phase delay of the beat signal corresponding to the up- and down-scanning, thus reducing the signal spectrum broadening caused by frequency deviation and nonlinear, and increasing the target measurement resolution, accuracy and range. The experimental results show that for speeds of up to 250mm/s, the mean standard deviation was less than 152μm/s, the mean error was less than 183μm/s, the relative error of the mean value of speed measurement does not exceed 0.22%, and the standard deviation of the distance measurement results within a range of 4m under various speed conditions does not exceed 17μm, the error does not exceed 14μm, it has good accuracy and repeatability for the speed and distance measurement. The lidar architecture solution we proposed has important application value for large-scale industrial equipment measurement and operation monitoring.
Optoelectronic Measurement Technology and System II
Calibration method for frequency scanning interferometry distance measurement system using dispersive interferometry
Qiang Zhou, Tengfei Wu, Jiarui Lin, et al.
The long delay optical fiber is the length reference in the frequency scanning interferometry (FSI) distance measurement system, which is directly related to the stability of the system. This paper proposes a calibration method for the optical path difference (OPD) of long optical fiber. In this scheme, the optical frequency comb (OFC) is introduced into the measurement system, and the dispersive interferometry is used to complete the measurement of the OPD of the fiber. The light generated by the tunable laser and the light generated by OFC are led to the long optical fiber using a fiber coupler at the same time, which ensures the common path design of the calibration system and the measurement reference. The dispersive interferometry signal is detected by an optical spectrum analyzer. Experiments were designed to verify the repeatability of this calibration method. Experimental results show that using dispersion interferometry to calibrate the OPD of the fiber can achieve a relative accuracy lower than 10-7.
Vibration compensation method of frequency modulated continuous wave ranging based on the superposition of frequency scanning measurement signals in the opposite direction
Qihua Liu, Jindong Wang, Xinghua Qu, et al.
Frequency modulated continuous wave (FMCW) laser interferometry is very sensitive to optical path drift, and a small length drift will lead to a great measurement error in the ranging result. In order to improve the anti-interference ability of the system and realize the large-scale, high-precision absolute distance measurement, the influence of environmental vibration on the frequency-modulated continuous wave laser ranging is analyzed, and a vibration error compensation method based on the superposition of the reverse frequency sweep signal is proposed. Simulation and experimental results show that this method can effectively improve the spectrum offset, distortion and broadening of the measurement signal caused by the optical path drift caused by vibration. Within the measurement range of 43m, the system's ranging resolution and measurement stability have been significantly improved after vibration error compensation. The measurement resolution is very close to the theoretical value, and the measurement standard deviation is stable within 9μm. The method proposed in this paper can realize the compensation of optical path drift without real-time measurement of vibration, and improve the anti-interference ability of large-scale FMCW laser ranging system without increasing the system complexity.
Optoelectronic Measurement Technology and System III
Autonomous discrimination method of refracted stars based on projection error
Correctly distinguishing between refracted stars and nonrefracted stars is a prerequisite for a single field of view celestial navigation system. An autonomous discrimination method of refracted stars based on projection error is proposed that distinguishes between refracted and nonrefracted stars and estimates the attitude simultaneously depending only on knowledge of the star point centroid coordinates. Firstly, a rough attitude is calculated according to the body vectors and inertia vectors of all stars in the image. Next, for each of the stars, the reprojection vector is calculated and compared with its body vector. Stars with large residual errors are classified as refracted stars. Then the attitude is recomputed without these refracted stars until all the residual errors are small. The classification precision is 98% and the attitude estimation error can be reduced by 68% compared with the attitude estimated before refracted star discrimination.
A novel method for reducing the defocus error of a five-degree-of-freedom measurement system
Multi-degree-of-freedom measurement (MDFM) system is an effective way to measure multiple degrees of freedom errors simultaneously. In the MDFM system, the pitch angle and yaw angle are often measured based on the autocollimation measurement principle. Due to the lens fabrication and installation errors, the angle detector will deviate from the focal plane, causing defocus error. To address the problem, a defocus error model is established to analyze the influencing factors and solve the position of the focal plane. It is found that the straightness is the main factor of defocus error. When the defocus amount is present, the defocus error is larger with the increase of straightness. And the sensitivity of angle detector has a specific relationship with the intensity density of spot, which is related to the defocus amount. By using a precision displacement table, the position of the focal plane and the corresponding angle detector output can be accurately obtained. The experiment was carried out to verify the feasibility of this method. The angle detector and lens were installed precisely according to the angle detector output. When the vertical straightness is within ±400 μm, the influence of the defocus error on the pitch angle is less than 2 μrad. The results showed that the method can reduce the defocus error of auto-collimation measurement effectively, which can improve the accuracy of angle measurement in MDFM system.
An object tracking algorithm with adaptive template updating for visual tracking measurement system
Xiaoyun Chen, Jiarui Lin, Yanbiao Sun, et al.
This paper proposes a novel adaptive template updating method with continuous variational spatial-temporal characteristics, which is utilized as a part of visual object tracking in the measurement process of the vision measurement system. The measurement system is set up by cameras and turntables to catch the profile of a moving object in largescale space. Among all steps, the proposed method is applied to positioning the object. In order to enhance the spatial constraints of the tracking algorithm to make the measurement results more reliable, the Average Peak-to-Correlation Energy (APCE) whose property includes spatial information is used as a part of the learning rate in the template updating step. Experiments show that the results calculated by the proposed method have a smoother profile. With the comparison of the fixed distances measured by Leica T-Mac, the proposed method applied in the system performed higher accuracy.
Optoelectronic Measurement Technology and System IV
Spectral-spatial outlier filter for image matching
Junfu Zhou, Ting-Bing Xu, Zhenzhong Wei
Feature-based image matching is often contaminated by some mismatches due to the limited representation of descriptors. Existing two-stage mismatch removal filters usually select some seed points first and then remove outliers according to the consistency in neighborhoods. However, the filter's performance is directly influenced by the selection of effective seed points. In this paper, we design an elegant Spectral-spatial Outlier Filter (SOF) to harvest high-accuracy image matching. Specifically, we first calculate eigenvectors of Laplacian matrix from the joint image graph as feature descriptors in the spectral domain to select more reasonable seed points, and then these points are fed into the local a fine verification in the spatial domain in the second stage to effectively remove outliers. Experimental results on challenging datasets demonstrate that the proposed filter further improves the precision of image matching, and steadily outperforms other state-of-the-art methods.
Refined attention Siamese network for real-time object tracking
Jiaqi Xi, Yi Wang, Huaiyu Cai, et al.
The Siamese tracker shows great potential in achieving a balance between accuracy and speed, but the twin structure of the Siamese network makes the tracker vulnerable to background interference in the tracking scene. To deal with this problem, a tracking algorithm based on the attention mechanism is proposed. The algorithm introduces the channel attention module based on the Siamese network and dynamically enhances the robust channel feature response by modeling the context relationship between channels. This paper also verified the network performance on the OTB and VOT benchmark. Experimental results show that the proposed algorithm can achieve robust tracking results on challenging datasets, and achieves the goal of improving network performance with a slight increase in computational cost.
A visual/IMU system for head pose estimation based on non-cooperative targets
Changku Sun, Jingjing He, Peng Wang, et al.
The helmet-mounted sight is a sighting device fixed on the pilot’s helmet and becomes increasingly important in flight battles. An important premise for the helmet to perform correctly is obtaining the pilot’s head orientation. The traditional vision-based method to obtain the pilot’s head orientation relies on cooperative targets, which can be constituted by multiple LEDs embedded on the helmet. However, the installation of multiple LEDs will increase the weight of the helmet. In addition, the measurement accuracy will decline due to the complex environment interference, strong illumination, and uncertainty of the LED luminous center. In order to solve these problems, this paper proposes a tightly coupled visual/IMU system for head pose estimation based on non-cooperative targets. A camera and an inertial measurement unit (IMU) are installed on the helmet to track the head orientation. In the reconstruction phase, a binocular system is used to reconstruct the internal environment of the cockpit and build the feature points database based on structure from motion (SFM) and the scale-invariant feature transform (SIFT) feature descriptors. In the measurement phase, the feature points of the captured images are extracted and matched with the database to obtain the 3D world coordinates of feature points. The coordinates are directly fused with the inertial data through the cubature Kalman filter (CKF) to realize fast and accurate attitude measurement. The practical experimental platform is set up to simulate the measurement of the pilot’s head attitude. The experimental results effectively verify the feasibility of the proposed measurement system and scheme.
Optoelectronic Measurement Technology and System V
Accuracy evaluation method of laser tracking attitude measurement system based on homogeneous coordinate transformation
The Six-degree-of-freedom(6-DOF) measurement system based on laser tracking equipment provides a good solution for large-scale industrial measurement. However, there is no unified standard to evaluate the accuracy of attitude measurement system. At present, the accuracy evaluation method of attitude measurement mainly adopts the comparison method of standard parts, which is easily constrained by space dimensions. Aiming at this problems, a homogeneous coordinate transformation method based on spatial distance constraint is proposed to realize the on-site accuracy evaluation of attitude measurement. Firstly, a reasonable control field was arranged, and the distance constraints between the reference points and the measuring points were established. Secondly, the mathematical model which described the relationship between space distance and attitude was established by using homogeneous coordinate transformation matrix. Through the above evaluation method, the accuracy of attitude measurement can be evaluated by tracing the measurement results of attitude angle to the length measuring standards. The simulation results show that the accuracy of the evaluation model decreases linearly with the measurement distance of attitude measurement system. Assuming that the measurement accuracy of the distance constraint is 15μm+6μm/m, the size of the control field is 1.1m*6.4m*5.6m, and the attitude angle range is [-60°, 60°]. The accuracy of the evaluation model can be controlled within [0.34°,3.25°], when the working distance is 3 to 15 meters. This method provide an effective idea for the on-site evaluation of attitude measurement accuracy.
Online time-space calibration method of hybrid wMPS-IMU system
Limited by the principle of multi-angle intersection measurement, photoelectric scanning measurement systems such as workshop Measuring and Positioning System (wMPS) and indoor Global Positioning System (iGPS) have systematical positioning errors when measuring moving targets. Fusing target movement information can effectively compensate the positioning error introduced by the relative motion between the target and the transmitter. Inertial measurement systems are capable of high-frequency, all-weather and autonomous rotation and acceleration measurement. They have good measurement complementarity with photoelectric scanning measurement systems. Integrating movement information from inertial measurement unit (IMU) is an effective method to improve the dynamic measurement performance. Inertial information fusion needs two preconditions: the first one is clock synchronization and the second one is spatial alignment. In response to the two problems above, we research on wMPS and propose a wMPS-IMU integrated online calibration method. The time offset of the two systems is precisely calibrated by compensating the delay between the wMPS reference clock and the IMU clock. Due to the fact that wMPS receivers and the IMU are connected as a rigid body, the rotation R and the translation T between the wMPS and the IMU can be easily calibrated through a linear estimation method. Our novel method benefits to achieve real-time online temporal and spatial calibration without the help of any external measurement equipment and prior information and significantly improves the applicability of wMPS. Verification experiments were carried out. A three-axis turntable and a linear guide were utilized as the roundness and straightness references. The calibration experiment shows that the calibration accuracy of our method is at the same level as that achieved from the high accuracy coordinate measuring machine (CMM). The precision evaluation experiment shows that the hybrid wMPS-IMU system calibrated through the proposed method has slightly better dynamic performance than that calibrated on a CMM. This method is fully verified to be effective.
Research on automatic calibration system and measurement method of high precision photoelectric encoder
High precision photoelectric angle encoder is a key component of intelligent angle measuring with strategic significance. A special fixture and adjustment measurement method is proposed, which can effectively solve many problems such as eccentricity, tower difference error and rotation interference caused by the direct fixation of rotating shaft (mover) and driving shaft during measurement. An automatic calibration system is also setup, which can realize many measurement schemes, such as direct calibration of turntable, polyhedral prism calibration and permutation comparison calibration. The measurement accuracy of the system is verified by comparison experiments and repeatability experiments. The measurement uncertainty of the optimal angular division error can reach 0.15 "(k = 2).
Optoelectronic Measurement Technology and System VI
Research on a simple wide-angle optical modality based on computational imaging technology
Jiarui Ji, Lei Yang, Hongbo Xie
With the rapid development of information technology, people's demands for large-angle coverage and high-information acquisition of optical systems are increasing in industrial production and daily life. Therefore, the requirements for imaging quality of wide-angle lenses are improving. Aiming at solving the severe aberrations in the design of the wide-angle lens, traditional optical designs often introduce complex structures. However, a complex structure will bring several drawbacks such as large volume, heavyweight and high cost. Computational imaging technology(CIT) takes information-driven as the core, breaks the imaging mode of independent optimization of optical design and image restoration, realizes global integrated optimization design, and breaks the limit of traditional imaging. Based on the CIT theory, a simple wide-angle optical system design method is proposed and demonstrated in this paper. Firstly, in the optical design process of the wideangle lens, the requirements for suppressing aberrations are relaxed, and the image quality is worse. Then, the cross-channel image restoration algorithm is used to remove the residual aberrations to obtain a high-quality image. Finally, the system is simplified. This method can not only obtain high-quality images but also reduce the complexity of a wide-angle optical system.
A survey on stereo matching and semantic matching
Huaiyuan Xu, Siyu Ren, Shiyuan Yang, et al.
This paper reviews the development of stereo matching and semantic matching in the field of image correspondence. The existed matching methods of these two kinds of matching problems are discussed and summarized. Since 2014, technologies based on data-driven and deep learning have played an important role in these two types of matching problems, which accelerates the development of image correspondence technology. This paper discusses stereo matching from three perspectives: local stereo matching, global stereo matching, and stereo matching based on neural networks. Besides, this paper divides semantic matching methods into two categories: parametric semantic matching and nonparametric semantic matching. By reviewing and tracking the research development of these two matching problems, this paper provides good navigation for people who are new to the image correspondence field.
Deployment of road-garbage detection solution based on YOLOv5
Mandi Luo, Danhua Cao, Chenglong Lian, et al.
Because of changeable daylight and weather, the natural scenes of road are complicated. And the garbage on roads comes in polytropic shapes and sizes, especially small targets such as leaves. To detect the location and types of road-garbage in different scenarios quickly and accurately, we propose a road-garbage detection solution based on YOLOv5. The solution is finally applied to the garbage sweeper, where different deployment strategies are required for different modules. Firstly, to reduce the loss of image information caused by overexposure and underexposure, we design automatic exposure algorithm to adjust camera parameters in time and use OpenMP to accelerate it. Secondly, YOLOv5 algorithm based on object detection is trained for recognition and TensorRT framework is used for deployment. Taking into account the computing speed and accuracy, we choose FP16 computational precision for YOLOv5’s inference acceleration. Lastly, a low-cost computing platform named Jetson AGX Xavier is selected and the algorithm is optimized in combination with the characteristics of the hardware platform. Multithreading is also used to accelerate in software architecture. The results show that the average detection accuracy for various types of road-garbage reaches 70.4% under four different scenarios of self-built datasets. And the area of the smallest object detected accounts for 0.2‰ of the total area of the image. The speed of this road-garbage detection solution can reach 5fps when processing the 12-bit image of 2432*1226 size on the low-cost AGX Xavier computing platform.
Calibration and measurement space inconsistency analysis for workshop measurement positioning system (wMPS)
Rao Zhang, Shendong Shi, Jiarui Lin, et al.
This paper discusses the measurement precision loss problem introduced by the inconsistent between measurement space and calibration space and presents a hybrid calibration method to solve this problem. The proposed method establishes the hybrid adjustment model based on the ray intersection constraint, ruler length constraint, and sparse control points' rescan constraint. The Levenberg-Marquardt algorithm is employed to solve the objective function. The initial value is calculated in two steps, in which the ruler length constraint is used to calculate the relationship between each transmitter, and the global coordinate system's transformation parameter is calculated by space intersection. In addition, the part of the adjustment objective function introduced by the ray intersection constraint is extended by considering the ray intersection constraint of the measured point. About 78% improvement in measurement accuracy is achieved compared with the traditional way.
Poster Session
Design of disturbance suppression controller for optical inertial reference unit
The optical inertial reference unit (OIRU) can provide a reference beam for the line-of-sight stabilization system to achieve precise pointing and tracking. Inertial sensors are mounted to the OIRU platform to provide feedback of angular motions being experienced by the reference beam. However, disturbances and noises existing in the OIRU system will inevitably prevent the reference beam be stabilized within microradian. In this paper, an improved noise reduction disturbance observer (IMNRDOB) is proposed to enhance the disturbance suppression performance of the OIRU. Simulations results verify the effectiveness and experiments data show the disturbance is suppressed by 88.60% with the proposed method.
Adaptive loosely-tightly coupled algorithm for head attitude tracking
Aiming at the problem that the loosely coupled Kalman Filter algorithm cannot perform measurement updating when the feature points are few, a head attitude tracking algorithm based on adaptive loosely-tightly coupled Extended Kalman Filtering is proposed. Firstly, according to the angular velocity measurement data from an IMU mounted on the head, the algorithm realizes the time updating of the head attitude. Then the algorithm completes the adaptive loosely-tightly coupled measurement updating according to the number of available feature points. When there are more than 4 feature points, the PnP pose is solved firstly. Then the loosely coupled measurement updating is performed by using the pose measurement. Otherwise, the tightly coupled measurement updating is performed directly by using the image measurement data. Finally, the experimental results show that the proposed algorithm can significantly expand the updating range of the head pose measurement, and improve the accuracy and stability of the head attitude tracking.
Geometrical parameters evaluation method of aspheric micro-lens by using white light interferometer
Tong Guo, Xinyang Li
Aspheric micro-lenses provide excellent imaging performance and can effectively reduce the complexity of optical system implementation, but the current detection technology for aspheric micro-lens still faces problems with imperfect measurement parameters and implementation constraints. In this paper, a method for evaluating the geometric parameters of aspheric micro-lenses achieved by white light interferometry technology is proposed, which evaluates the geometric size, roughness, and surface error of aspheric micro-lenses. We have adopted a white light interferometer to measure the aspheric micro-lens and experimentally evaluated the geometrical parameters of the sample, the results proved the feasibility of our proposed method.
A high-accuracy non-contact online measurement method of the rotor-stator axial gap based on frequency scanning interferometry
Rotor-stator axial gap is a key design parameter that directly affects the efficiency and safety of large rotating machines. With optimum axial clearance value determined, the rotating machine can work with the highest efficiency. To realize active clearance control (ACC) of rotating machinery, high-precision non-contact online measurement of the axial gap must be carried out. However, traditional measurement methods are challenged by the extreme working environment, such as extreme temperature, high rotating speed, and narrow space conditions; no mature measurement method was reported to realize the high accuracy, non-contact, and online performance required by the axial gap measurement. In this paper, a rotor-stator axial gap measurement method is proposed based on frequency scanning interferometry, and the mathematical model of axial clearance measurement is established. The weak reflection signal of lens and time delay estimation algorithm is employed to undermine the impact of probe pigtail length drift caused by the temperature change under extreme conditions. The axial gap measurement prototype based on sweep frequency ranging was developed, and the measurement and system calibration was carried out. The experimental results verify the effectiveness and accuracy of the method. The drift of the system was less than 20μm in 30 minutes with millimeter level pigtail drift, and the measurement accuracy was better than 50μm.
Several improvements of traditional system calibration method for fringe projection profilometry
Yuanjun Zhang, Xinghua Qu, Fumin Zhang
Fringe projection profilometry (FPP) is widely used because of its many advantages, such as high accuracy, no-scanning properties, and full-field measurement. When fringe projection profilometry is employed, system calibration is one of the vital procedures. The accuracy of system calibration directly affects the quality of measurement results. The traditional calibration methods of measurement system include several calibration contents, like verticality, parallelism and the calibration of system geometric parameters. Aiming at the difficulties of them and the complexity of operation and adjustment process, this paper puts forward corresponding improvements. The calibration of verticality and parallelism uses checkerboard, which eliminates the requirements of hardware preparation for calibration such as standard parts. In the calibration of system parameters, it is not necessary to know the specific value of each parameter, and the parameters can be divided into two parts for calibration. The validity and practicability of the proposed methods have been proved by experiments. We take the ceramic step shape standard part as the tested object. After calibration by the above methods, the maximum measurement error of the fringe projection profilometry measurement system is -0.1400mm and the maximum standard deviation is 0.7251mm.
Research on helmet integrated positioning algorithm based on vision and dual MIMU
Yupeng Li, Peng Wang, Changku Sun, et al.
In order to overcome the shortcomings of Kalman filter algorithm based on Euler Angle and quaternion, a vision and inertial fusion filtering algorithm based on error quaternion is proposed. In this algorithm, error quaternion parameters are used to describe attitude, which not only avoids the singularity of Euler Angle description, but also eliminates the unit constraint of quaternion description. In addition to improving the positioning accuracy of helmet, it can also estimate and compensate the drift error of helmet MIMU online in real time.The effectiveness of the proposed algorithm has been verified by simulation experiments, and the main factors affecting the integrated positioning accuracy has been analyzed.
Introduction of general specifications of Raman spectrometers: a new national standardization progress and interpretation
With the rapid development and application of various Raman spectrometers, the lack of the general inspection or evaluation methods of the Raman spectrometers has become increasingly prominent. The author’s group has completed the compilation of National Standard “General Specification for Raman Spectrometers"(GB/T 40219-2021), which is the first Chinese national standard for Raman spectrometer and was just implemented in December 2021.This paper firstly introduces the background and latest progress of the standardization of Raman spectrometer. Then the drafting process and content of the first national standard are briefly introduced. Finally, an interpretation of key technical specifications for inspection is presented. This work therefore would provide a technical support for the smooth and effective implementation of the national standard, improving the breadth and depth of the application and exchange of Raman technology around the world.
Dual-comb velocity measurement
Laser measurement systems occupy an important role in many application fields, such as autonomous driving, robotics, precision manufacturing, and basic science. The appearance of the optical frequency comb provides a new solution for measurement and has gradually become a hot topic of research. This paper proposes a speed measurement scheme based on dual optical frequency combs (OFCs) to realize dynamic speed monitoring for moving objects. The dual-comb scheme has the advantages of high speed, high resolution and high precision. It can not only use the periodic pulse in time domain to realize large scale and high accuracy measurement, but also the interference spectrum in frequency domain can obtain interference information. The experiment uses two OFCs with a small repetition rate difference, one is the signal laser and the other as the local oscillation (LO) laser. The cavity length of the two OFCs is continuously adjusted to control the repetition rate difference of the two OFCs at 2 kHz. And use the frequency counter to monitor the repetition rate of the two OFCs during the measurement, and compensate the drift of the OFC in real time. Set the measurement target on the high-speed guide rail, control the guide rail to move at different speeds, and measure the movement speed of the object in real time, which can accurately measure the speed of objects.
A high-precision calibration method of PSD in long-distance laser auto-collimation system
Wenzheng Liu, Cong Zhang, Fajie Duan, et al.
Laser auto-collimation technology is an important method of precision micro-angle measurement. It has the advantages of high measurement accuracy and simple optical structure. However, the position-sensitive detector (PSD) has nonlinearity, and the uniformity of laser spot is affected by the working distance of the auto-collimation system, which seriously affects the accuracy of angle measurement based on laser auto-collimation technology. To improve the accuracy of laser auto-collimation technology in long-distance angle measurement, PSD is calibrated by linear interpolation to solve the error caused by its nonlinearity. In addition, due to the non-uniformity of the light spot, the PSD zero point is not collinear at different distances, and the measurement error can be corrected by a laser interferometer. Experimental results showed that the interpolation calibration method effectively improved the angle measurement accuracy of PSD. After the zero deviation was corrected, when the PSD moved within 5m, the maximum standard deviation (SD) of the pitch error was 0.12 ", and the SD of the yaw error was 0.09 ". The method proposed in this paper can achieve high precision angle measurement based on PSD at long working distances.
High precision real-time non-contact measurement of rotor-stator axial gap in narrow space based on swept-wavelength interferometry with 1kHz speed
Xiuming Li, Fajie Duan, Xiao Fu, et al.
In our work, a rotor-stator axial gap non-contact real-time measurement system based on SWI with an auxiliary interferometer for nonlinearity correction is established. The measurement speed is 1kHz and the principle is given. The data of 6-10mm distance is collected, and the distance of each position is calculated by windowed FFT and Hilbert transform respectively. The simulation results indicate that the measurement error of Hilbert algorithm is on the order of a fraction of nanometers, while that of FFT is micrometer level. However, FFT takes 36μs in one measurement which is three times faster than Hilbert algorithm. Both of the two meet the precision requirement of axial gap measurement, it helps us to choose the more appropriate algorithm according to the actual situation.
An ultrasound-guided diffuse fluorescence tomography for small animals
Xin Wang, Teng Pan, Yuhong Liu, et al.
Diffuse fluorescence tomography (DFT) is an emerging optical imaging tool for in-vivo observation of organisms and small-animal. Especially, dynamic diffuse fluorescence tomography can provide contrast-enhanced and comprehensive information for tumor diagnosis and staging with the pharmacokinetic image. However, the conventional reconstruction algorithms for DFT always suffers from low spatial resolution. Multi-modality imaging methods have been proposed to integrate DFT with other imaging modalities with in general intricate and costly experimental apparatus. We developed a dual-modality system that combines the ultrasound imaging and DFT which is simple and low-cost with no ionization damage. The results in phantom experiments demonstrate that with the a priori guidance of ultrasound imaging, the quantitativeness and spatial resolution of the fluorescence image can be considerably improved.
An improved method for accurately extracting feature information of robot environment modeling based on stereo vision
Robot 3D vision inspection plays a very important role in intelligent manufacturing process such as automated picking, obstacle avoidance, path planning and so on. Recently, there is a need for a fast detection method that has applicability to complex environments, strong anti-interference capabilities, and balances speed and accuracy to meet the above requirements. An environment feature detection method based on laser-assisted machine vision is proposed. By illuminating the grid structure to the target scene, the binocular camera is used to collect the grid image on the surface of the target scene. Then A two-step feature extraction method is proposed, which is locating the feature position quickly first, and then accurately obtaining the coordinate of the feature point. Firstly, an improved fast extraction method is proposed to realize the fast recognition of feature points. Secondly, in the aspect of accurate acquisition, a new improved steger fitting method is proposed to accurately extract the position of feature points. Finally, fast matching and reconstruction of the exact position of feature points on the two images collected by binocular camera are implemented to achieve fast and high precision 3D detection. This experiment has verified the rationality of the system scheme, the correctness, the precision and effectiveness of the relevant methods.
Long-range high-precision laser ranging using a joint measurement method
Bowen Qiu, Yawei Zhang, Zhongyang Xu, et al.
A joint measurement method for long-range and high-precision laser ranging is proposed, in which the frequencymodulated- continuous-wave (FMCW) laser ranging method is used for long-range coarse ranging, while the phase-shift laser ranging method is used for high-precision ranging. The key of the joint measurement method is to ensure that the resolution of the FMCW method is smaller than the unambiguous range of the phase-shift method. In this paper, a super resolved FMCW laser ranging method based on a low-duty-cycle linear-frequency-modulated (LFM) signal is used to achieve phase unwrapping with small bandwidth. A demonstrated experiment is carried out, in which a 2-4 GHz LFM signal with the duration of 1 μs and duty cycle of 10%, and a 5 GHz single-frequency signal are used to simultaneously modulate the lightwave in a dual-parallel Mach-Zehnder modulator (DPMZM). The theoretical measurement range and resolution are 300 m and 0.16 mm, respectively. The absolute length of a 140-m fiber spool and a variable optical delay line (VODL) varied with a step of 0.2 cm in a range from 7 to 20 cm is measured. The standard deviation of the displacements between the measured distances and the exact values is 0.377 mm. The proposed method promises a long range laser ranging with high resolution and measurement rate.
Research on a laser radar measuring station layout optimization method for large civil aircraft components
The measurement requirements for civil aircraft components when using laser radar are always large-scale, high precision, high efficiency and so on, so several measuring stations are needed to construct a large-scale measuring network. Thus how to set up the layout of the stations is very important as it has direct influence with the precision. In this paper we develop a layout optimization method to solve the question in order to meet the requirements for precision and efficiency. The math model of the optimization is constructed by using the precision as constraints, and the solution is given to get the exact number and division of the laser radar. The initial layout can be obtained by the region growing algorithm to carry on the characteristics of discrete information and extract the discrete point method. And then we use the measurement uncertainty to optimize the results and the division. The experimental results show that compared with the experience-based manual layout, this method is more feasible and effective in obtaining large range and small number of measurement area division results and reasonable stations measurement stations. This experiment has verified the rationality, the correctness, the precision and effectiveness of the relevant methods.
Generalized sidelobe canceler beamforming with an improved covariance matrix estimation for medical ultrasound imaging

For adaptive beamforming, the covariance matrix estimation is a key part of the adaptive algorithm. Previously, some preprocessing algorithms must be applied to obtain an accurate covariance matrix, such as the spatial smoothing and diagonal loading. In this paper, we propose the forward-cross-backward subaperture averaging method to estimate the covariance matrix and combine it with the generalized sidelobe canceler (GSC) beamforming for the medical ultrasound imaging. This method can obtain an accurate and robust covariance matrix and overcome shortcomings of traditional preprocessing algorithms. Since the covariance matrix is in a better state, the modified beamformer can improve the quality of the echo images.

We demonstrate the performance of the modified beamformer by resolving the point scatterers and cyst phantom, and compare it with the synthetic aperture beamforming (SA) and the traditional GSC beamforming. The FWHM and CR are calculated to describe the performance of the lateral resolution and contrast, respectively. The scattering points experiment shows that our method can decrease FWHM by 53.8% and 34.9% compared with SA and traditional GSC, respectively. Meanwhile, 10.2% improvement in CR is achieved compared with GSC beamforming according to the cyst phantom experiment. The results indicate that the proposed method can achieve better image quality of the system in lateral resolution and contrast.

Design of optical-correlator system based on wavefront phase modulation
Tong Yang, Lei Yang, Hongbo Xie
Optical phase correlation calculation can achieve high-speed, passive, high-signal-to-noise ratio target signal detection. This paper proposes and demonstrates a one-dimensional information parallel comparison system based on an optical correlator. The system uses 4-f optical architecture as the basic device and combines multiple optical modules such as beam shaping, wavefront phase modulation, and optical phase correlation calculation to achieve the parallel computing function with one-dimensional information as the main detection object is developed, and the feasibility of the comparison calculation is verified by simulation. The detection system based on optical correlation proposed in this paper has potential application value in the field of genetic detection.
Structure optimization for airborne multi-FOV star trackers
An airborne multiple-field-of-view (multi-FOV) star tracker operating inside the atmosphere has particular limitations in observation. It is necessary to optimize its structure parameters for improving its attitude determination and reliability in the working circumstances. In this paper, performance simulations for different multi-FOV structures are carried out. In the simulation design, the terrain occlusion, stellar atmospheric refraction, and atmospheric extinction are the main considerations. When conducting the simulation experiments, within the permitted attitude ranges, abundant random attitude of a star tracker was generated for performance testing, including three-axis attitude error and stellar detection probability. The results show that, with no refraction exists, when the tilt angle of the boresight of each FOV is at 40°~45°, regardless of which structural layout is adopted, both the attitude measurement accuracy and the stellar detect probability of single FOV is relatively high. With refraction exists, the tilt angles of the boresights are larger, the attitude measurement error is greater. For an airborne multi-FOV star tracker, a FOV with 0° tilt angle is necessary to promote its reliability.
Precision measurement method based on optimized combination of pixel-binning cameras
Traditional binocular stereo vision measurement system based on high resolution area array camera has many defects, such as limited acquisition speed, insufficient sensitivity under high frame rate and overlarge data volume. To overcome these defects, this paper proposes a precision vision measurement method which is based on the optimized combination of pixelbinning cameras. This method transforms the traditional area array camera into an equivalent narrow area array (ENAA) camera through single-dimensional binning. Then, multiple ENAA cameras are used to achieve precise 3D measurement of target points through reasonable combination. However, single-dimensional binning will reduce the image resolution in one direction, while the image resolution in another direction (maximum image resolution) remains unchanged. Therefore, this paper studies the typical spatial layouts of two even three ENAA cameras and analysis their measurement errors. Finally, an optimal spatial layout of three ENAA cameras was obtained, and it can reach the same level of measurement accuracy as the binocular stereo vision measurement system based on area array camera. Comparative experiments verify the effectiveness and measurement accuracy of the above method, which can reduce the data amount to 3/8 and increase the acquisition rate by 4 times while ensuring the measurement accuracy.
Similar model assisted phase unwrapping for three-dimensional measurement based on fringe projection
Yong Li, Xiaosong Zhang, Guanghui Zhang, et al.
A novel algorithm for phase unwrapping with the assistance of similar model of scene is proposed. The major steps of proposed algorithm are as follows. (1) Obtain wrapped phase and modulation of fringe by phase shift method. (2) Extract feature points of scene from background of fringe. (3) Obtain scaling and motion parameters of model from 2D and 3D coordinates of feature points. (4) Calculate reference phase from model by motion parameter and shape of model according to camera model. (5) Retrieve absolute phase by wrapped phase of scene and reference phase. The experimental results are demonstrated. If the similar model of scene is available, the proposed algorithm is efficient for phase unwrapping in phase measurement profilometry without additional patterns, even though the scene contains isolated parts.
A calibration method of structural model for multi-FOV star tracker based on theodolite crosshair imaging
Compared with the traditional single field of view (FOV) star tracker, the multi-FOV star tracker has the advantages of equal measurement accuracy on three axes and better dynamic performance. The measurement accuracy of a multi-FOV star tracker is directly determined by the accuracy of the structural model. However, existing structural model calibration methods cannot be applied to the high precision multi-FOV star trackers with large size and weight. To solve this issue, a calibration method of the structural model for the multi-FOV star tracker based on theodolite crosshair imaging is proposed. An imaging model of theodolite crosshair in star tracker is established which elaborates the relationship between the theodolite angles and the star tracker images. Multiple theodolites are utilized to pair each FOV of the star tracker. In each pair, through collecting the angles measured by the theodolite and the images captured by the star tracker, the rotation between the theodolite frame and the star tracker FOV frame could be solved. Additionally, the rotations between theodolite frames are obtained by mutual collimation of theodolites. Finally, the structural model which contains the rotations between the different FOV frames is acquired by merging the above rotations. Structural model calibration experiments of a multi-FOV star tracker with a large size and weight have been conducted. The experimental result indicated that the mean star angular distance error between FOVs was less than 10 arcseconds. The accuracy of the calibration result met the practical requirements. The proposed method is free from the influence of the size and weight of multi-FOV star trackers and maintains high calibration accuracy.
Fast and nanoscale-accuracy surface recovery of white-light scanning interferometry based on GPU-accelerated FFT-LMA-PSI algorithm
Yiting Duan, Xiaodong Zhang, Zexiao Li
White-light scanning interferometry (WLSI) has been widely in the field of Nano-Micro topography measurement. In this study , to improve accuracy and robust of surface recovery, the Fast Fourier Transform-Levenberg Marquardt Algorithm-Phase Shift Interference (FFT-LMA-PSI) algorithm is first proposed to position zero optical path difference (OPD), and the FFT-LMA-PSI algorithm is a cascaded algorithm that involves FFT and multiple iterative optimization steps. In order to improve the computational efficiency, the GPU-accelerated FFT-LMA-PSI is developed and designed via compute unified device architecture (CUDA) C language. The speedup ratio of the proposed method are 28.6756×, 36.6950×, 41.9732×, 33.1928× for CCD pixel sizes of 128 × 128, 256 × 256, 512 × 512, 1024 × 1024. The standard deviation (σ) error are 7.34 nm and 9.79 nm and the peak-valley (PV) error are 87.04 nm and 90.36 nm for the Step and Sphere, respectively. The experiment shows that the proposed method has good performance in calculation accuracy and efficiency.
FPGA displacement calculation platform and test of two-phase grating interference encoder
Ningning Shi, Junhao Zhu, Shengtong Wang, et al.
Compared with the four-phase optical structure, the grating encoder based on two-phase optical structure reduces the number of optical devices used in the system and makes the system more compact. Due to the high requirements for realtime and parallel processing of algorithm solution, the powerful parallel computing ability of Field Programmable Gate Array (FPGA) and customized hardware acceleration algorithm are needed to improve the real-time performance. In the previous research, the displacement signal generated by the grating encoder can be input into the FPGA through analog to digital converter (ADC) sampling, and then complete self-designed filter filtering, phase correction and displacement solution. In this paper, further, the ADC sampling rate adjustable interface is added to the FPGA, the global signal and the dc offset remove algorithm is added, and the displacement solution results in the form of fixed-point number are output to the host computer through the MicroBlaze (MB) soft core. MB core can realize process control and interface conversion on FPGA, and use a small amount of logic resources to replace the functions of MCU and DSP of traditional embedded measurement system, so as to further improve the integration of the instrument. A series of experiments are carried out on the two-phase FPGA platform. ADC sampling rate is 200ksps, 8-Channel synchronous parallel sampling, FPGA system clock frequency is 200MHz. The linear displacement table is set to drive the measurement grating at different displacement speeds, and the total stroke is set to 10mm. The FPGA real-time displacement solution platform is tested. The experimental results show that FPGA obtains accurate displacement solution results under different speed tests. In the test of 2 mm/s, the maximum cumulative displacement measurement error is 5um, which shows the real-time performance and accurate displacement solution performance of FPGA platform.
Research on the measuring method for position parameters of large rocket sled by using the fusion of multiple measurement systems
The rocket sled is a large-scale, high-precision ground dynamic simulation test equipment that uses a powerful rocket booster to push the test object at high speed on a special slide which is similar to a railroad track. It is mainly used for dynamic performance analysis of such as speed and acceleration. There are several targets along the sled and the ultra-long position parameters of the targets are very important for the dynamic test. Thus it is a worldwide problem to get the position parameters precisely. This paper states a measuring method for position parameters of large rocket sled by using the fusion of multiple measurement systems. At present, the methods of obtaining dynamic parameters mainly include high-speed photography and photoelectric detection position marking method. We use a laser tracker and a total station to set a collaborative network which can integrate the advantages of the both systems. The mathematic model is established by using the global coordinate control field as the constraint equations. In order to solve the equations we use the iterative optimization algorithm, and the weights for each value are given. The experiment is designed under field environment and The results show that the measurement error is kept at 3mm within the range of 3km by comparing with the standard length of the reference ruler. This experiment has verified the high-precision and high-efficiency of the ultra-long position measuring method.
Study of a static torque measurement device for composite variable pitch propeller blades

The propeller engine uses the rotation of the propeller to push the air behind the aircraft, which would produce the reaction force to propel the aircraft forward. The propeller is generally made up of multiple propeller blades and a propeller hub. The overall balance of the assembly needs to be measured to ensure that the unbalanced synthetic torque generated by the individual blades is within a reasonable range when the propeller engine is rotating. Therefore, it is essential to measure the static torque of each propeller blade after production and before assembly. Due to the unique irregular shape of the propeller, the radial torque varies at different rotation angles. Moreover, the variable pitch propellers with adjustable blade angles have been widely used, so the measurement of the radial static torque of the propeller at different working angles also needs to be addressed. The existing propeller blade static torque measurement devices are mainly based on the principle of torque balance, which has the advantages of low measurement error, high stability, and repeatability, and the results can be traced using weights.

In this paper, a propeller static torque measurement device is developed for the new composite variable pitch propeller, which can achieve the measurement of both propeller axial static torque and radial multi-angle static torque measurement. Besides, a specified gauge is developed for the device and the performance parameters of the device are tested by the gauge and the actual blades to verify the feasibility of the device.

Accuracy test method of multi-instrument cooperative measurement
A traceable accuracy test method of multi-instrument cooperative measurement system is studied. Here a precise angle dividing table and a linear guide with a laser interferometer are used as standards to test the cooperative measurement’s external parameters. And the coordinates of common points and uncommon points are tested by the scale bars. This testing method is illustrated with a cooperative measurement system composed of two laser trackers. It is obvious that this method can intuitively test the accuracy of the multi-instrument cooperative measurement’s external parameters and coordinates. This method is not restricted by the measurement principle, the data fusion algorithm and the measurement targets of the cooperative system, and it is suitable for various non-fixed layout cooperative measurement system.
A research on compact short-distance grating interferometer based on ridge prism
Baiqi Liao, Shengtong Wang, Jianjiu Lin, et al.
Grating interferometer has great application potential in precision measurement. In view of its miniaturization, a compact installation based on ridge prism is adopted in this paper, and the interference signal with 90° phase is obtained by numerical calculation. The specific scheme is to divide the laser beam through the splitting prism to make the two beams diffract at the reference two-dimensional grating and the measurement two-dimensional grating respectively. The four beams of diffracted light are collimated through the ridge prism. The collimated two groups of beams produce interference at the splitting prism, and the interference signal is received by the photodetector. Differing from the conventional method, the second group of 90° phase interference light data is obtained by derivation. Through the solution of the interference signal, the displacement in X and Z directions of the measurement grating can be deduced. Experiments were carried out to verify the measurement accuracy of the grating interferometer. The moving speed of scale grating is 1μm/s ,distance is 0.5 mm and the sampling frequency is 100 Hz. Repeatability and measurement accuracy of the grating interferometer are obtained. The repeatability measurement error is less than 0.5%. Specially, this method benefits for simple installation, high precision, high stability and low energy loss. It has high practical value in industrial production. Precision measurement of two degrees of freedom of short-distance displacement is realized and error of the measurement results is analyzed.
Photoacoustic microfluidics based on all-optical detection
Jianpeng Cai, Hui Fang, Sheng Yan
Photoacoustic microfluidics emerges as a promising method for particle or droplet analysis. Here we propose a microfluidic chip design based on laser polarization-dependent reflection from the interface of an optical prism and the liquid, which can detect photoacoustic signal without the need of the bulky ultrasonic transducer. This design will reduce the structure complexity and also the cost. As demonstration, we made such chips and detected moving microspheres by analyzing the photoacoustic signal from the black tape fixed underneath the flow channel.
Ultra-high resolution optoelectronic frequency responses measurement with doubled measurement range
Zongxin Xu, Min Xue, Yuqing Heng, et al.
Photodetectors (PDs) are optoelectronic (O/E) devices to achieve optical-to-electrical conversion, which are essential and of great importance in optical communication, optoelectronic oscillator, etc. Measuring O/E frequency responses, including magnitude response and phase response, is a fundamental measurement processing in their development and application. Microwave photonics (MWP) is a promising solution to achieve ultrahigh-resolution characterization. However, the frequency measurement range is restricted by the relatively small working bandwidth of modulators. To enlarge the measurement range, an approach to measure magnitude response of O/E devices is proposed and experimentally presented. In the approach, two optical double-sideband (ODSB) signals with the carrier suppression are generated. One ODSB signal filtered out +1st-order sideband is used as the frequency-shifted carrier. By coupling the frequency-shifted carrier and the other ODSB signal, an asymmetrical ODSB signal is thus achieved and served as a probe signal. After square-law detection of a PD under test, a photocurrent is produced. Detecting the frequency downconversion component in the produced photocurrent, the magnitude response in the low-frequency regime is obtained. Similarly, the magnitude response in the high-frequency regime is observed via extracting the magnitude information of the frequency up-conversion component. Thanks to the MWP-based frequency conversion, the measurement range is doubled, and the nonlinear error is suppressed. Furthermore, an ultrahigh-frequency resolution up to Hz or even sub-Hz is theoretically achievable. In an experiment, a 20-GHz commercial PD is accurately measured using a 200-kHz resolution. A measurement range as large as 67 GHz is enabled by 33.5 GHz RF frequency sweeping.
Method and system for phase measurement of the heterodyne interference system
Yi Yang, Guochao Wang, Junhao Zhu, et al.
Laser interferometers and grating interferometers are typical optical measurement devices. The measurement resolution and range of the two devices are generally nanometers and meters, so they can meet the needs of high-resolution, largerange measurement. Whether it is a laser interferometer or a grating interferometer, it can be implemented based on a technical route based on the principle of homodyne interference or heterodyne interference. Heterodyne interference is not sensitive to changes in signal amplitude and DC offset, and can effectively avoid measurement errors. To design a highprecision displacement measuring device based on the principle of heterodyne interferometry, the key is whether it can accurately measure the phase change of the measured signal relative to the reference signal. The accuracy of the phase measurement determines the accuracy of the displacement measurement. Phase measurement methods can be divided into two categories: analog method and digital method. In this paper, a high-precision phase measurement system based on FPGA is designed based on the automatic digital phase detection method. The hardware part of the system includes FPGA, high-speed ADC module, signal conditioning circuit, the phase detection algorithm selects the automatic phase detection algorithm, and finally realizes the output display of the phase measurement results. The experimental results show that the deviation between the experimental data of all measurement points and the true value does not exceed 0.1°. Therefore, the accuracy of the phase measurement system designed in this paper is 0.2° and 0.018° resolution.
A study on the axial strain characteristics of extrusion in fiber winding process based on BOTDA
The optical fiber winding structure is used to make the fiber optic guided missile (FOG-M). When the FOG-M is released, if the winding structure has extrusion, which will eventually lead to the missile out of control. Based on stress analysis, a theoretical model is established to study the axial strain characteristics of extrusion during the winding process. The axial strain difference between standard and extrusion structure is calculated, showing that the axial strain in extrusion is always greater than standard structure. The axial strain increases with the increasing angle of extrusion. Distributed brillouin optical time domain analysis (BOTDA) technology is used to measure the distributed strain of extrusion at different winding layers. Experimental results show that the extrusion appears during the winding process, and the axial strain at the extrusion increases rapidly with the increasing winding layers. Theoretical calculations agree with the experimental results. This technique provides an important means for the optimization of winding process and the identification of extrusion.
Measurement of surface shape and deformation of small-aperture mirrors based on coaxial normal incidence speckle deflectometry system
Yu Zhang, Yao Hu, Qun Hao, et al.
For small-aperture aspheric mirror surface measurement, the traditional measurement method phase measuring deflectometry (PMD) requires multi-step phase shifting, and the result is not high in accuracy and complicated in operation. The digital image correlation (DIC) that can be used for transient measurement is mostly used for diffuse reflection. surface. At present, a speckle deflection technique combining the former two methods has appeared, but its optical system is based on the traditional triangulation method, which has problems of angle occlusion and defects. This paper improves the speckle deflection algorithm and optical path system, replacing the traditional triangulation optical path system with Coaxial normal incidence speckle deflectometry system (CNISDS), adding a plate beam-splitter and a telecentric camera. On the one hand, it can avoid the changes in the shape of the captured image caused by perspective, and on the other hand, it can effectively avoid angle occlusion and missing information. Among them, the DIC method is used for image matching, the PMD method is used to calculate the displacement distribution. Then use the slope data to calculate to reconstruct the surface shape. The improved method can measure absolute surface shape and deformation at the same time, and has the advantages of simple measurement steps, fast response speed, and low cost. Finally, the correctness and feasibility are verified by simulation and actual measurement with deformable mirror. Compared with the phase-shifting operation which has complex steps or the low-precision mirror profile measurement results, the improved method is effective and superior, and provides a new idea for measuring small-aperture aspheric mirrors.
Preliminary research on calibration method of digital image correlation measurement system
Yong-xin Chen, Xiaochuan Gan, Xuejun He, et al.
Digital image correlation method, also known as digital speckle correlation method, is a non-contact optical measurement method based on modern computer technology and optical principle. The main purpose is to study the displacement and strain of the measured object. The digital image correlation method has the characteristics of full field, real-time, non-contact, high measurement accuracy and fast response speed, and has developed rapidly in recent years. At present, the composition and principle of typical digital image correlation systems have been basically agreed, but there is no more feasible and effective method for the calibration of displacement measurement systems based on digital image correlation technology. This paper first analyzed the factors affecting the accuracy of the system, among which the speckle image is an important information source affecting the calculation results of the digital image correlation system. Therefore, we generated the simulated speckle image by writing the program and simulated the displacement, and used the software of the measurement system to calculate the simulated displacement. Then, a verification experiment was designed to prove the effectiveness of using the speckle image to complete the calibration. We built a set of calibration experiment equipment, using the laser interferometer with the characteristics of high measurement accuracy and small numerical limit of the measurement displacement and the printed speckle target points to complete the calibration process. Finally, we completed the experiment and carried out the precision analysis, and realized the preliminary research on the calibration method of the digital image correlation measurement system.
Laser interference sensor for absolute shape and orthogonal vibration measurements in CNC machines
An in-situ coordinate measurement sensor based on a Mach-Zehnder interferometer is presented in this work. The sensor system allows to simultaneously measure distance and velocity at fast rotating objects for inspecting vibrations, surface profile and absolute diameter of rotors in CNC machines. With a novel signal processing technique, in-situ, synchronized bi-directional vibration measurements are achieved directly at tool tip by using a single sensor with a measurement rate of 50 kHz at rotational speeds up to 300 Hz. Moreover, a novel line camera based sensor system is proposed with an extended measurement volume for submicron non-scan 3D shape measurements.
Research on positioning and control method of high-precision lifting platform
As a high-precision liquid level measuring device, magnetostrictive liquid level gauge needs to ensure the reliability and accuracy of its calibration. In this paper, a precise measurement platform of liquid level gauge with precision grating ruler as measurement standard is designed. The device uses servo motor to drive the workbench to move along the vertical axis. The actual displacement value of the workbench and the output value of the liquid level gauge are analyzed and processed by computer to realize the calibration of the liquid level gauge. By analyzing the error source of the device, the measurement uncertainty of the device is determined. Based on the principle of inverse compensation method, the systematic error is separated from the error curve and superimposed with its inverse curve to eliminate the influence of systematic error. After compensation, the positioning error increases from 52 μ M improved to 6 μ m。 Theoretical analysis and experimental results show that the designed device can meet the calibration requirements of 0-2m float level gauge.
Phase diversity wavefront sensing based on modified sparrow search algorithm
Qiufeng Ye, Gang Liu, Xinqi Hu, et al.
Atmospheric turbulence, optical system aberrations and other factors will cause the wavefront of the incident light wave to be distorted, thereby causing the degradation of the optical system's imaging quality. Phase diversity (PD) is an effective approach to measure these wave-front distortions. It uses two or more degraded images to estimate the wavefront aberration in the pupil plane of the imaging system . The essential of the PD is to develop an appropriate optimization algorithm to minimize the evaluation function. Traditional gradient-based nonlinear optimization algorithms, such as conjugate gradient algorithm, and quasi-Newton algorithm, are easily trapped in local minimums, which greatly limits the dynamic range of the PD method. This paper proposes a Modified Sparrow Search Algorithm (MSSA) to solve this problem. Chaotic sequences, Elite Opposition-Based Learning strategy and mutation operators are introduced to enhance the global search ability. The simulation results show that, this algorithm has a dynamic range of larger than 9λ PV and an accuracy of λ/100 rms, while, compared with other swarm intelligence algorithms, it has the advantages of strong search ability, fast convergence speed, and high solution accuracy. Experiments are made, which shows the effectiveness of the algorithm.
Optical techniques for determining wax appearance temperature of waxy crude oil
Accurate determination of wax appearance temperature (WAT) is of great significance for the safe running of crude oil pipelines. At present, a variety of WAT measuring methods have been developed, among which optical techniques have attracted great attentions due to their advantages in convenient operation and non-contact measurement, etc. Here, we summarize optical methods for WAT measurement according to different light effects, including absorption, scattering, and birefringence, during light transmission in crude oil. We introduce the principles of light transmission method, near-infrared absorption spectroscopy, polarization microscopy, optical speckle method, and fluorescence imaging method, and discuss their advantages and disadvantages. Among them, the laser-scanning confocal fluorescence microscopy can be used to study factors such as cooling rates to nucleation and its subsequent wax growth morphology, which helps to enrich our understanding of paraffin nucleation mechanism. Finally, we also discuss other related methods for WAT measurement.
A piston detection method for segments via convolutional neural networks and its robustness analysis
Hao Wang, Gang Liu, Weirui Zhao, et al.
To achieve a diffraction-limited imaging, the piston errors between the segments of the segmented primary mirror telescope should be reduced to λ/40 RMS. The piston detection method using convolutional neural network (CNN) is an advanced technology with high precision and simplicity. However, such methods based on the deep learning strategy usually have generalization problems, that is, the network prediction precision will inevitably decrease if there is a certain difference between the test image and training set used in the network. This will directly affect the scope of application of the method. In this letter, we propose a CNN-based high-precision piston detection method and analyze its robustness. The point spread function (PSF) images acquired under the wide-spectrum light source are used to construct the dataset to overcome 2π ambiguity. In addition, a set of neural networks system including the classification CNN and the regression CNN with good generalization ability is designed to extract the piston value directly from the PSF image. Under the ideal condition, the piston detection precision can reach about 8.4 X 10-4 λοRMS in the capture range of the interference length of the operating light. Finally, we focus on testing the effect degree of the main disturbance factors in the actual system on the accuracy of the method, such as surface error, residual tip-tilt error, and CCD noise, so as to evaluate the robustness of the method. This method is robust and does not require complex hardware. It can be widely applied in segmented and deployable primary mirror telescopes. We believe that the study in this letter will contribute to the applications of the CNN-based technique for piston sensing.
High precision and fast calibration method of fish-eye camera based on two-dimensional turntable
Wenhao Wu, Ming Liu, Mei Hui, et al.
Camera calibration is a prerequisite for spatial position measurement. The fish-eye camera can increase the field of view, but at the same time, the distortion it brings is more serious. It is difficult to obtain high accuracy when using traditional methods to calibrate the fish-eye camera. Therefore, this paper proposes a fast calibration method for fisheye cameras, which makes the calibration process convenient and fast, and improves the calibration accuracy. The camera is fixed on a high-precision two-dimensional turntable, and the turntable rotates according to the set law through a fully automatic program. At the same time, the camera captures the bright spots in the collimator and extracts the pixel coordinates of its centroid. Collect several sets of data, use the bicubic interpolation method to expand the data, find the relationship between the pixel coordinates of the centroid of the bright spot and the rotation angle of the turntable. Finally, the three-dimensional space coordinates can be obtained by binocular vision triangulation to verify the calibration accuracy. The above method bypasses the traditional camera optical imaging model and distortion model, but uses traversal and interpolation. Experiments show that the accuracy of this method can reach the centimeter level in long-distance, large field of view, and severely distorted scenes.
Non-invasive blood glucose measurement based on visible light and embedded system
Blood glucose is an important physiological parameter. Regular evaluation of blood glucose level is of great significance for the monitoring and treatment of diabetes mellitus and its complications. Noninvasive detection is the ideal technology to achieve periodic blood glucose assessment, among which optical measurement method is the current research hotspot, but due to low SNR and low accuracy, optical noninvasive blood glucose detection method cannot be used in clinic. To solve the problems above, we designed a device for non-invasive blood glucose detection based on visible light and embedded system in this paper. We used the visible light source with a wavelength of 625nm and a high-quality camera to obtain the scattered image information of fingertips in a dark environment, and then feature vectors and dimensionality would be extracted from the original image by the method of Convolution Auto-Encode (CAE). Then, we used the theoryoriented method partial least squares regression (PLSR) and the data-oriented method gradient boosting regression (GBR) to establish the correlation model of the relationship between the scattering image feature vectors and blood glucose level, and the performance of the models were verified by the test set. Experiments show that the GBR performs better than PLSR, the accuracy of GBR in test set is up to 92.56 percent. Finally, the device is highly integrated centered on embedded system, and GBR has the advantages such as high precision, low cost, simple and convenient to use, which has great application value for the research of non-invasive blood glucose measurement.