Accuracy enhancement and joint calibration method of multi-station triangulation network based on absolute ranging.

Precision measurement methods and technologies for large-scale three-dimensional coordinates are in high demand in advanced equipment manufacturing. The multi-station triangulation network represented by the rotary-laser scanning measurement system has the advantages of having high precision, having multitask parallel measurement capability, and having a high degree of automation. It is widely used in the docking of large components, quality control of key points, and collaborative positioning of production equipment. Nevertheless, due to the limitations in the measurement principle, the positioning accuracy along the depth direction is notably lower when compared to other directions. This difference becomes more pronounced with increasing distance. This paper proposes a method to address this issue by integrating a distance measurement station into the network. A novel, to the best of our knowledge, cooperative target, coupled with a high-dynamic beam guidance mechanism, is designed to achieve fast absolute distance measurement to the target. The weighted fusion of the distance and angle observations effectively enhances the measurement accuracy while preserving the advantages of highly automated measurement. Additionally, we introduce a joint calibration method for extrinsic parameters of multi-type stations. High-precision absolute distances are utilized to establish optical scale bars, complemented by the incorporation of physical scale bars, thereby obviating the necessity for using external reference instruments such as laser trackers. Finally, a series of experimental verifications demonstrate the effectiveness of calibration and measurement methods. The root mean square error of all measured points drop to 42.6% of that the triangulation method measures.

Absolute distance meter without dead zone based on free-running dual femtosecond lasers.

Absolute distance measurements based on femtosecond lasers have been extensively studied for precision metrology and advanced manufacturing, with the advantages of traceability, high speed, and nanometer precision. However, in previous studies, the dual femtosecond laser ranging system showed limitations such as system complexity, lower integration, dead zone problems in single optical path detection, and high requirements for laser coherence. It is challenging to achieve a high degree of integration and large-scale continuous measurements using femtosecond lasers, ineluctably limiting practical applications in engineering fields. Here, based on the free-running dual femtosecond lasers and the nonlinear asynchronous optical sampling method, we design a highly integrated absolute distance meter. In particular, the dead zone problem is solved by the polarization multiplexing technique, and the digital control system and signal processing system are completed by the Field Programmable Gate Array (FPGA). The absolute distance meter enables rapid, continuous, and accurate measurements over a considerable range without dead zones, which paves a promising way for the integration, instrumentation, and industrial applications of femtosecond laser ranging systems.

Triple line-scan camera measurement for efficient and accurate narrow-space 3-D scanning with motion estimation.

Vision-based three-dimensional (3-D) shape measurement plays a crucial role in the inspection of vehicles and trains in the field of transportation. Consequently, the demand for more comprehensive narrow-space inspection has become an inevitable necessity and presents a great challenge to conventional vision methods. We propose the following efficient and accurate narrow-space 3-D scanning method based on triple line-scan cameras. First, the structure of coplanar dual line-scan cameras with a narrow reflector is proposed, making it easy for the optical path to enter a narrow space and obtain 3-D shape information. Efficient in-motion measurement with a large field of view is thereby achieved without the worries that often accompany narrow-space constraints. Secondly, an additional line-scan camera is attached to the coplanar dual cameras and creates a time-space constraint in point cloud stitching direction through the triple line-scan camera structure. With an efficient algorithm template including matching and motion estimation, accurate point cloud stitching is ensured. Lastly, point cloud texture mapping and light source optimization are realized. Our experimental results prove that the method realizes low-distortion in-motion reconstruction in narrow space despite motion variation.

Multi-target automatic positioning based on angle and distance parallel measurement.

Precisely and efficiently measuring three-dimensional coordinates of key points on large-scale components in the manufacturing process of aircraft and ships is critically essential. This study presents a multi-target automatic positioning method based on rapid angle and distance measurement in parallel. The measurement processes for angles and distances are decoupled and, when executed simultaneously, aims to enhance the measurement efficiency and automation compared with conventional metrology systems. A cooperative target is devised to realize the rotary-laser scanning angle measurement and absolute distance measurement in parallel. The method of multi-target rough positioning based on rotary-laser scanning and then the precise coordinate measurement method introducing absolute distance constraint are detailed. Especially for the distance measurement, we propose a method to determine the internal zero length and compensate for the distance error caused by mirror offset. A real-site experiment is implemented to verify the method's feasibility and demonstrate that the 3D coordinate measurement accuracy is better than 0.17 mm compared with laser tracker.

Tightly-coupled fusion of iGPS measurements in optimization-based visual SLAM.

The monocular visual Simultaneous Localization and Mapping (SLAM) can achieve accurate and robust pose estimation with excellent perceptual ability. However, accumulated image error over time brings out excessive trajectory drift in a GPS-denied indoor environment lacking global positioning constraints. In this paper, we propose a novel optimization-based SLAM fusing rich visual features and indoor GPS (iGPS) measurements, obtained by workshop Measurement Position System, (wMPS), to tackle the problem of trajectory drift associated with visual SLAM. Here, we first calibrate the spatial shift and temporal offset of two types of sensors using multi-view alignment and pose optimization bundle adjustment (BA) algorithms, respectively. Then, we initialize camera poses and map points in a unified world frame by iGPS-aided monocular initialization and PnP algorithms. Finally, we employ a tightly-coupled fusion of iGPS measurements and visual observations using a pose optimization strategy for high-accuracy global localization and mapping. In experiments, public datasets and self-collected sequences are used to evaluate the performance of our approach. The proposed system improves the result of absolute trajectory error from the current state-of-the-art 19.16mm (ORB-SLAM3) to 5.87mm in the public dataset and from 31.20mm to 5.85mm in the real-world experiment. Furthermore, the proposed system also shows good robustness in the evaluations.

In-motion 3D reconstruction of high dynamic range surfaces.

Efficient and refined three-dimensional (3D) reconstruction of industrial parts has become an urgent need in the field of advanced manufacturing, and it's a great challenge when facing in-motion and online inspection requirements of high dynamic range (HDR) surfaces that have large reflectivity variations. This paper proposes a method using RGB line-scan cameras to realize in-motion multiple-shot 3D shape measurements with RGB channel fusion to increase the measurement dynamic range. First, multi-channel one-dimensional background-normalized Fourier transform profilometry (MC-1DBNFTP) is proposed as an effective in-motion HDR method. Second, for HDR surfaces with strongly overexposed areas, we propose a solution that obtains 6 results of different dynamic ranges for fusion with only 5 projected patterns, which further extends the measurement dynamic range while ensuring the small projection period. Third, we develop a fusion method based on reliability evaluation, which is more reliable than the existing methods in fringe projection systems. In addition, colored textures can be mapped to the reconstructed surfaces. Experimental results prove that the proposed method realizes accurate and reliable in-motion 3D reconstruction of HDR surfaces.

In-motion continuous point cloud measurement based on bundle adjustment fused with motion information of triple line-scan images.

The point cloud continuous measurements in some in-motion circumstances, such as quality inspection of products on assembly lines or rail traffic, have been requiring higher measurement speed, accuracy, and point cloud density. With the advantages of high acquisition rates and ultrahigh resolution, line-scan cameras have been developing gradually for dynamic measurements. However, because of non-coplanar installation and unidimensional images, the measurement based on line-scan cameras is affected by movement. In this article, a dynamic scanning point cloud measurement based on triple line-scan images is present. The point cloud optimization is based on bundle adjustment fused with motion information. The epipolar constraint of line-scan images in dynamic conditions is researched for matching. The effect of motion on matching error is analyzed. A triple line-scan cameras experimental setup validates the proposed method.

Proof-of-concept study of the virtual optical scale bar by the pulse-to-pulse interferometry.

The optical scale bar with calibrated or measured internal point-to-point length has many applications in coordinate measurements. In this paper, the virtual optical scale bar with two retroreflectors is constructed by the absolute distance measurement based on pulse-to-pulse interferometry. The temporal and dispersive coherence could be utilized to determine the adjustable internal length of multiple pulse-to-pulse intervals with high precision. The proposed scheme was combined with a pellicle beamsplitter to minimize systematic error. The influence of its thickness on precision is also discussed and calibrated in detail. Besides, a femtosecond mode-locked pulse laser with 100-MHz repetition rates was employed in our system to develop an optical scale bar and verify the feasibility of the proposed method. The sub-micron precision could be realized by temporal coherence with a piezo-driven stage or a simplified non-polarized scheme of dispersed coherence. It shows that this method could achieve a flexible and high-precision virtual optical scale bar for further practical applications.

Photoelectric scanning-based resection method for mobile robot localization.

Indoor localization is a key enabling technology for mobile robot navigation in industrial manufacturing. As a distributed metrology system based on multi-station intersection measurement, the workshop measurement positioning system (wMPS) is gaining increasing attention in mobile robot localization. In this paper, a new, to the best of our knowledge, wMPS-based resection localization method is proposed using a single onmidirectional transmitter mounted on a mobile robot with scanning photoelectric receivers distributed in the work space. Compared to the traditional method that requires multiple stationary transmitters, our new method provides higher flexibility and cost-effectiveness. The position and orientation of the mobile robot are then iteratively optimized with respect to the constraint equations. In order to obtain the optimal solution rapidly, two methods of initial value determination are presented for different numbers of effective receivers. The propagation of the localization uncertainty is also investigated using Monte-Carlo simulations. Moreover, two experiments of automated guided vehicle localization are conducted, and the results demonstrate the high accuracy of the proposed method.

Analytical solution of uncertainty with the GUM method for a dynamic stereo vision measurement system.

Directed at the strong correlation among the input parameters and long measurement chain, which are difficult for uncertainty analysis with the guide of the expression of uncertainty in the measurement (GUM) method, a novel dynamic stereo vision measurement system based on the quaternion theory is presented to reduce the orthogonality restrictions of shafting manufacturing and application. According to the quaternion theory in the kinematic model of the cameras and the analytical solution of uncertainty with the GUM method, the complete, detailed, and continuous uncertainty results of the full-scale measurement space can be obtained. Firstly, one-dimensional turntables and rigid connections are utilized to form the motion cores and the automatic control carriers in the system. Secondly, the novel measurement model is used in the measurement process to shorten the calibration and measurement chains. Once the system based on the novel measurement model is set up, the analytical solution of uncertainty is utilized in the accuracy process. During the analysis process, the strong correlation among the extrinsic parameters is decoupled by introducing virtual circles and the measurement strategy with the GUM method. Through analyzing the relationship among the attitude angles, the major factors which influence the uncertainties in each axis and the final uncertainty are clarified. Moreover, the analytical continuous uncertainty maps for the uncertainties along each axis, combined standard uncertainty, and the expanded uncertainty are illustrated and the uncertainty variation tendency is declared. Finally, the analytical solution of uncertainty with the GUM method proposed in this paper predicts the uncertainty in the full-scale space and provides a new idea of the uncertainty analysis for the complicated combined measurement system.

Absolute angular measurement with optical frequency comb using a dispersive interferometry.

We have demonstrated a simple method to measure high-precision absolute angular displacement using an optical frequency comb (OFC). The dispersive interferometry with parallel configuration can take advantage of its large non-ambiguity range and achieve absolute angular measurement in a large range. The influence factors of the angle accuracy, including the accuracy of optical path difference, the determination of absolute zero position and the correction of sine arm have been analyzed in detail. The angle comparison is performed with the autocollimator and multi-tooth indexing table. The angle accuracy can reach ±2 arcsec (k=2) in the range of 5°, which represents a good agreement with the Monte Carlo simulation. The proposed approach has potential to be extended to multi-degree-of-freedom measurement with a simple structure in future.

Precision improvement in frequency scanning interferometry based on suppression of the magnification effect.

Frequency scanning interferometry (FSI) is a promising technique for absolute distance measurement and has been demonstrated in many industrial applications. However, in practice, the measurement precision is limited and sensitive to the variations of the measured distance while sweeping the optical frequency of the laser. The induced errors would be amplified by hundreds of times due to the magnification effect. In this paper, an incremental interferometer was established on the basic scheme of the FSI system for monitoring the variations of distance. The compensation could be achieved by multiplying the heterodyne signals from monitor and measurement interferometer without complex and time-costing data processing. The system performance has been verified by experiments for different kinds of vibrating targets. Finally, after compensation by suppression of the magnification effects, a measurement precision of 4.26 µm has been achieved in a range of 10 m.

Flexible calibration method for visual measurement using an improved target with vanishing constraints.

In this paper, an improved calibration method based on vanishing constraints is proposed for calculating the extrinsic parameters of cameras. First, we come up with a improved target based on the conventional target with two groups of orthogonal parallel lines. The novel target is composed of two groups of parallel lines with a certain angle range from 80° to 90°, which can reduce the difficulty of target production and the manufacturing cost. Furthermore, in the optimization process, we design a new function with a more robust penalty factor instead of using the experienced values to get the extrinsic parameters for the binocular vision sensors. Finally, on account of using the improved target and the novel optimiazation function, the proposed method is more flexible and robust compared with Zhang's method.

Long-range, high-precision, and high-speed absolute distance measurement based on alternately oscillating optoelectronic oscillators.

We propose an absolute distance measurement method using alternately oscillating optoelectronic oscillators (OEOs) with high speed, high precision, and long range, and describe the dynamic characteristics of the measurement system. Measurement and reference OEOs are oscillated using a 2×2 optical switch, and rough and fine measurements are achieved by low- and high-order-mode oscillation. The distance is determined by the loop length difference between the two OEOs. OEO length control is not necessary, so the system is simple and the time per measurement is only 40 ms. The maximum measurement error is 3.4 µm with an emulated distance of 7.5 km, and the relative measurement accuracy reaches 4.5×10-10.

A Sub-Regional Calibration Method That Can Accomplish Error Compensation for Photoelectric Scanning Measurement Network.

In the measurement process of photoelectric scanning measurement network, the laser surface edge area has lower measurement accuracy than the middle area due to the geometrical distortions of the laser surface of the transmitter. This paper presents a sub-regional calibration method that can accomplish error compensation for the measurement system. Unlike the camera sub-regional calibration, the regional division and identification of the laser surface are more difficult. In this paper, the pitch angle in the transmitter coordinate frame of the spatial point was used as the basis for the division and identification of the laser surface. In the calibration process, the laser surface of the transmitter was divided into different regions and each region was calibrated independently, so that an intrinsic parameters database containing the intrinsic parameters of different regions could be established. Based on the database, the region identification and error compensation algorithm were designed, and comparison experiments were carried out. With the novel calibration method, the measurement accuracy of the system had an obvious upgrade, especially at the edges of the laser surface within a certain measurement area, which could enlarge the effective measurement area of the transmitter and would broaden and deepen the application fields of photoelectric scanning measurement network.

A Multi-Camera Rig with Non-Overlapping Views for Dynamic Six-Degree-of-Freedom Measurement.

Large-scale measurement plays an increasingly important role in intelligent manufacturing. However, existing instruments have problems with immersive experiences. In this paper, an immersive positioning and measuring method based on augmented reality is introduced. An inside-out vision measurement approach using a multi-camera rig with non-overlapping views is presented for dynamic six-degree-of-freedom measurement. By using active LED markers, a flexible and robust solution is delivered to deal with complex manufacturing sites. The space resection adjustment principle is addressed and measurement errors are simulated. The improved Nearest Neighbor method is employed for feature correspondence. The proposed tracking method is verified by experiments and results with good performance are obtained.

Construction of traceable absolute distances network for multilateration with a femtosecond pulse laser.

The traceable absolute distances network with multiple global targets for multilateration is developed with a femtosecond pulse laser. It is aiming to enhance the ability and flexibility of the coordinate measurement, especially to monitor the positions of distributed stations in real time for some critical industrial environments. Here, multi-target absolute distances are determined by the temporal coherence method simultaneously with the pulse-to-pulse interferometer. Besides, the performance of the proposed system is evaluated in detail by comparing with a conventional interferometer. The experimental results indicate that the accuracy of distances measurement could all reach the sub-micron level and could be traceable to the length standard. Furthermore, a simple scheme of multilateration is presented based on the developed network. The coordinate of the initial point of multiple beams is measured by cooperation with a laser tracker. The results of coordinate measurement show that these methods have the potential for further industrial applications.

A Novel Semi-Supervised Feature Extraction Method and Its Application in Automotive Assembly Fault Diagnosis Based on Vision Sensor Data.

The fault diagnosis of dimensional variation plays an essential role in the production of an automotive body. However, it is difficult to identify faults based on small labeled sample data using traditional supervised learning methods. The present study proposed a novel feature extraction method named, semi-supervised complete kernel Fisher discriminant (SS-CKFDA), and a new fault diagnosis flow for automotive assembly was introduced based on this method. SS-CKFDA is a combination of traditional complete kernel Fisher discriminant (CKFDA) and semi-supervised learning. It adjusts the Fisher criterion with the data global structure extracted from large unlabeled samples. When the number of labeled samples is small, the global structure that exists in the measured data can effectively improve the extraction effects of the projected vector. The experimental results on Tennessee Eastman Process (TEP) data demonstrated that the proposed method can improve diagnostic performance, when compared to other Fisher discriminant algorithms. Finally, the experimental results on the optical coordinate data proves that the method can be applied in the automotive assembly process, and achieve a better performance.

Stereo line-scan sensor calibration for 3D shape measurement.

The stereo line-scan sensor opens up new potentialities for 3D measurement owing to the ultra-high resolution and acquisition rate. Calibration is a crucial key technology for a stereo line-scan sensor. This paper presents a precise calibration method for the stereo line-scan sensor. Several reference points are installed onto the sensor's body as an intermediary. The calibration turns into a two-step process: calibrating the cameras in the laboratory prior to measurement and locating the sensor in an actual measurement field. A mobile apparatus that comprises a planar pattern and extra reference points is designed. By incorporating the apparatus in combination with an auxiliary instrument, an optimal calibration configuration is created by placing the apparatus into multiple positions. A robust algorithm is proposed to enhance the stability of the parameter estimation. The quality of the calibration method is experimentally tested, and the performance is further investigated. Experimental results demonstrate that the proposed method offers a practical solution to calibrate a stereo line-scan sensor for 3D shape measurement.

Rapid Global Calibration Technology for Hybrid Visual Inspection System.

Vision-based methods for product quality inspection are playing an increasingly important role in modern industries for their good performance and high efficiency. A hybrid visual inspection system, which consists of an industrial robot with a flexible sensor and several stationary sensors, has been widely applied in mass production, especially in automobile manufacturing. In this paper, a rapid global calibration method for the hybrid visual inspection system is proposed. Global calibration of a flexible sensor is performed first based on the robot kinematic. Then, with the aid of the calibrated flexible sensor, stationary sensors are calibrated globally one by one based on homography. Only a standard sphere and an auxiliary target with a 2D planar pattern are applied during the system global calibration, and the calibration process can be easily re-performed during the system's periodical maintenance. An error compensation method is proposed for the hybrid inspection system, and the final accuracy of the hybrid system is evaluated with the deviation and correlation coefficient between the measured results of the hybrid system and Coordinate Measuring Machine (CMM). An accuracy verification experiment shows that deviation of over 95% of featured points are less than ±0.3 mm, and the correlation coefficients of over 85% of points are larger than 0.7.