The grating projection phase measurement method can obtain 3D surface profile of an object with a grating projection device and two cameras, which is widely used in reverse engineering, industrial detection, cultural relic digitization and human body measurement due to its simple measuring principle, convenient operation, fast and high-precision data acquisition of 3D point clouds. As one of its calibration processes, binocular vision calibration plays a key role in getting high-precision measurement results, and the accuracy of extracting feature points on the calibration target directly affects the binocular calibration accuracy. In this paper, a sub-pixel corner extraction method based on Shi-Tomasi algorithm was proposed to extract corner points of checkerboard. An experiment is done to compare the extraction effect of the proposed algorithm with the traditional Harris method. The experimental results show that the proposed algorithm can locate the position of corner points more effectively with higher extraction accuracy. The mean square value of the reprojection error is about 0.008 pixels, the rate of corner extraction is 74.6%, and the processing time is shorter, about 0.78 seconds. Therefore, the method used in this paper is reliable and feasible for the feature point extraction in 3D reconstruction of measured objects.
Catenary is an important part of electrified railway, and its geometric parameters are important parameters reflecting the safe and stable operation of locomotives. With the improvement of its speed, there are higher requirements for high-accuracy and real-time detection of geometric parameters of catenary. The existing systems have problems of long sampling interval, low real-time performance, and light-sensitive. Aiming at the actual requirement of dynamic measurement of catenary geometric parameters, a non-contact catenary geometric parameter detection system based on machine vision was developed. Firstly, a measurement model based on high-power line lasers and high-resolution area cameras was established to meet the application requirements. The measurement principle of the system was analyzed and the detailed formulas were deduced. Secondly, image difference, laser spot roundness analysis and other image processing algorithms were used to quickly and accurately detect the characteristics of laser points on the contact line with complex background. Based on the measurement model and algorithms mentioned above, the hardware and software platform of the system were built, and fast image acquisition and processing was realized by using multi-thread programming technology on high-performance industrial computer, which solved the problems of long sampling interval and low real-time performance during the measurement. Real-time image storage and display and preservation of detection results were realized in the software. Finally, a preliminary experiment was performed on the prototype, and the accuracy of the measurement results was analyzed. Experiment results showed that the system works stably and has high accuracy, which meets the practical application requirements.
The three-dimensional measuring system based on structured light has the advantages of propermeasuring range, high speed and non-contact measuring mode，and it is widely used in medical and manufacturing fields. The strip center extraction is one of the key steps in the line structured light vision measurement system. According to the line structure laser measurement model, the essence of the line structure measurement is to extract the pixel coordinates of the laser beam and substitute them into the measurement mathematical model formula to calculate the spatial coordinates of the measured points. Thus the accuracy and precision of laser strip pixel coordinate extraction directly affect the final calculation results. When the laser is projected onto the surface of the object to be measured, it is susceptible by the surface material of the object, ambient light and other factors, which may lead to speckle, uneven brightness of the light, wide range of light width variation and other problems. In this paper, an adaptive method for extracting light strips is proposed to solve the problems of speckle and wide width range of the light strips.
3D profile measurement is widely used in many areas such as manufacturing, computer-aided design, virtual reality and medical diagnostics. As one of the core technologies in 3D profile measurement, digital fringe pattern projection is a highly sensitive noncontact technique for obtaining the 3D shape of an object. Then the grating pattern deformed by the measured object is captured by CCD cameras and decoded using appropriate algorithms so that the shape of the object can be deduced. In this paper, three sets of phase shift fringe patterns with different frequencies are projected on the surface of the measured object by a DLP projector and the deformed patterns are captured by two cameras. Then the four-step phase shift method is used to obtain the three groups of fringe patterns phases, and the three-frequency heterodyne method is adopted to unwrap the phase and obtain the absolute phase. The causes of the phase errors are analyzed and the subsequent compensation method of gamma correction of grating pattern is proposed to eliminate the main errors. Experiments are carried out and the results verify the accuracy and effectiveness of the proposed methods.
The absorption coefficient and refractive index of the tourmaline in different directions have been firstly characterized by the terahertz time-domain spectroscopy. Results show that the absorption and refractive index of terahertz are much related with the tourmaline structure. And the absorption along the optical axis direction is more sensitive than that of the vertical direction. Such a result indicates that the identification and characterization of crystals as well as minerals can be realized by the THz method.
An automatic large-scale 3D coordinate measurement system based on vision guidance is presented. With a high-accuracy total station accomplishing the basic coordinate measurement, a camera mounted on the total station is used to scan the measuring field. The camera can identify the target in the viewing field and provide its azimuth information for the total station to aim at it automatically. Thus high-accuracy non-contact measurement can be accomplished without additional effort for targeting. The results showed that the measurement system can realize automatic large-scale measurement precisely and efficiently which provides an efficient approach for solving automatic large-scale measurement problems.
In order to achieve automatic measurement function, laser scanning measurement system need to ensure that measurement system of the cross and the target center overlapped. To develop the obtaining accuracy of the cross central coordinate, a novel algorithm based on the combination of two-step Hough Transform and Sub-pixel curve fitting was presented. Firstly, Hough Transform was used to detect the straight line of long axis of the cross, and then the central coordinate value of the cross can be deduced roughly through straight line equations. Secondly, take this coordinate as a center, and a certain number of pixel values as the edge length, a region of interest (ROI) can be defined. In this ROI, the two short axes of the cross were obtained to detect the straight line roughly using Hough Transform again. Then, the coordinate value of short axis was acquired at the pixel level with Canny edge detection algorithm. According to the equations of the straight line, rough errors were removed through 3σ rule, and the coordinate values of short axis at the sub-pixel level with curve fitting algorithm is obtained. Finally, based at the sub-pixel level of coordinate values, straight line equations of two short axes was presented with LSM, and thus the coordinate of cross center would be accurately detected with the intersection of two short axes. Experimental results showed that the accuracy of the central coordinates value of cross approaches to sub-pixel level and can satisfy the measurement system requirements of high precision.
A new type femtosecond laser tracker is one high precision measurement instrument with urgent need in science research region and industrial manufacture field. This paper focuses on the operational principle and the structure development of the femtosecond laser tracer, and the method of error compensation as well. The system modules were studied and constructed. The femtosecond frequency comb module was firstly analyzed and developed. The femtosecond laser frequency comb performed perfectly high precise distance measurement for laser tracker. The experimental result showed that the stability of repetition rate reached 3.0×10-12@1s and the stability of carrier envelop offset reached 1.0×10-10@1s. The initial experiment showed that measurement error was less than 1ppm. Later the error compensation module was introduced, and the optoelectronic aiming and tracking control module was built. The actual test result showed that the stability of miss distance was better than 2.0 μm, the tracking speed could reach 2m/s.
Geometric errors in laser trackers such as light offset and transit tilt have essential influence on the system measurement errors. Thus error detection and calibration are very important for producers and customers to execute error compensation. Different methods are developed to detect and calibrate errors. However, the commonly used methods such as length measurement and two-face measurement are sensitive to several misalignments which cannot calibrate errors directly and separately. In this paper a series of methods for detecting and calibrating geometric errors such as mirror tilt, beam tilt and transit tilt were presented which can calibrate geometric errors individually and precisely. The mirror tilt could be detected with the help of two autocollimators and one polygon. Then the beam tilt and offset errors were calibrated using a CCD camera and condenser lenses. Finally the transit tilt error was calibrated using a gradient and a vertical plane. Experiments and error assessment were executed to show that the accuracy of the calibration methods can meet the user’s demand.
A novel method was proposed to measure the tilt error between the transit axis and standing axis of the laser tracker. A gradienter was first used to make the standing axis of the laser tracker perpendicular to the horizontal plane. The laser beam of the tracker was then projected onto a vertical plane set at a certain distance from the tracker with equal horizontal angles and diverse vertical angles in two-face mode. The contrail of the laser beam was recorded while the simulation was manipulated to estimate the beam trail under the same circumstance. The tilt error was thus obtained according to the comparison of the actual result against the simulated one. Experimental results showed that the accuracy of the tilt measuring method could meet the user’s demand.
The working principle and system design of the laser tracker measurement system are introduced, as well as the key technologies and solutions in the implementation of the system. The design and implementation of the hardware and configuration of the software are mainly researched. The components of the hardware include distance measuring unit, angle measuring unit, tracking and servo control unit and electronic control unit. The distance measuring devices include the relative distance measuring device (IFM) and the absolute distance measuring device (ADM). The main component of the angle measuring device, the precision rotating stage, is mainly comprised of the precision axis and the encoders which are both set in the tracking head. The data processing unit, tracking and control unit and power supply unit are all set in the control box. The software module is comprised of the communication module, calibration and error compensation module, data analysis module, database management module, 3D display module and the man-machine interface module. The prototype of the laser tracker system has been accomplished and experiments have been carried out to verify the proposed strategies of the hardware and software modules. The experiments showed that the IFM distance measuring error is within 0.15mm, the ADM distance measuring error is within 3.5mm and the angle measuring error is within 3〞which demonstrates that the preliminary prototype can realize fundamental measurement tasks.
Electronic theodolites are widely used for spatial coordinate measurement in assembly manufacturing industry, but the
traditional operation of theodolites is time-consuming, which can not meet the application of online measurement. So in
the automatic guided laser theodolite system including two electronic motored theodolites, a method for improving
resolution of the electronic theodolite based on subdivided locating of laser spot image was expounded to eliminate the
errors. On the basis of a brief introduction of the automatic guided laser theodolite system, factors related to the
measuring errors were analyzed, with the conclusion that angle resolution of theodolite had an effect impact on the
phenomenon of no coincidence between the laser spot and the target point. Then the image including the message of the
laser spot adopted by the external camera was processed to subdivide in pixel level. Sequently comparison of the
measuring results was made between the original measuring data and the data corrected by the subdivided locating
method in the same theodolite measuring model to evaluate the feasibility of the operation and to find the most suitable
interpolation method. The experimental data of the system verified the high precision and efficiency of the method.
Spatial coordinate measurement systems such as theodolites, laser trackers and total stations have wide application in
manufacturing and certification processes. The traditional operation of theodolites is manual and time-consuming which
does not meet the need of online industrial measurement, also laser trackers and total stations need reflective targets
which can not realize noncontact and automatic measurement. A new automatic guided laser theodolite system is
presented to achieve automatic and noncontact measurement with high precision and efficiency which is comprised of
two sub-systems: the basic measurement system and the control and guidance system. The former system is formed by
two laser motorized theodolites to accomplish the fundamental measurement tasks while the latter one consists of a
camera and vision system unit mounted on a mechanical displacement unit to provide azimuth information of the
measured points. The mechanical displacement unit can rotate horizontally and vertically to direct the camera to the
desired orientation so that the camera can scan every measured point in the measuring field, then the azimuth of the
corresponding point is calculated for the laser motorized theodolites to move accordingly to aim at it. In this paper the
whole system composition and measuring principle are analyzed, and then the emphasis is laid on the guidance
methodology for the laser points from the theodolites to move towards the measured points. The guidance process is
implemented based on the coordinate transformation between the basic measurement system and the control and
guidance system. With the view field angle of the vision system unit and the world coordinate of the control and
guidance system through coordinate transformation, the azimuth information of the measurement area that the camera
points at can be attained. The momentary horizontal and vertical changes of the mechanical displacement movement are
also considered and calculated to provide real time azimuth information of the pointed measurement area by which the
motorized theodolite will move accordingly. This methodology realizes the predetermined location of the laser points
which is within the camera-pointed scope so that it accelerates the measuring process and implements the approximate
guidance instead of manual operations. The simulation results show that the proposed method of automatic guidance is
effective and feasible which provides good tracking performance of the predetermined location of laser points.