Laser Tracker Measurement Research Articles

Quantifying errors in GNSS antenna calibrations

We evaluated the performance of GNSS absolute antenna calibrations and its impact on accurate positioning with a new assessment method that combines inter-antenna differentials and laser tracker measurements. We thus separated the calibration method contributions from those attainable by various geometric constraints and produced corrections for the calibrations. We investigated antennas calibrated by two IGS-approved institutions and in the worst case found the calibration’s contribution to the vertical component being in excess of 1 cm on the ionosphere-free frequency combination L3. In relation to nearby objects, we gauge the 1sigma accuracies of our method to determine the antenna phase centers within pm ,0.38 mm on L1 and within pm ,0.62 mm on L3, the latter applicable to global frame determinations where atmospheric influence cannot be neglected. In addition to antenna calibration corrections, the results can be used with an equivalent tracker combination to determine the phase centers of as-installed individual receiver antennas at system critical sites to the same level without compromising the permanent installations.

Three-Dimensional Network Adjustment of Laser Tracker Measurements for Large-Scale Metrology Applications

AbstractThe adjustment of laser tracker network measurements for large-scale metrology applications has unique considerations not commonly encountered in the network adjustment approaches generally...

Automatic Guidance Method for Laser Tracker Based on Rotary-Laser Scanning Angle Measurement.

Laser-tracking measurement systems (laser tracker) have been playing a critical role in large-scale 3D high-precision coordinate measurement. However, the existing visual guidance of laser trackers is still limited by the disadvantages of operator-dependence, small-angle view field, time-consuming laser-guided process. This paper presents an automatic guidance method for laser trackers based on the rotary-laser scanning angle measurement technology. In this method, a special target consisting of six photoelectric receivers and a retroreflector is integrated into the rotary-laser scanning transmitter’ coordinate systems. Real-time constraints calculated by the proposed method would provide the coordinates of the target in a laser tracker coordinates system for guidance. Finally, the experimental results verified the automatic re-establish of sightline can be realized in horizontal 360° angle field within tens of arc-seconds, and this method is robust against the fast movement of the target.

Deformation Analysis Using B-Spline Surface with Correlated Terrestrial Laser Scanner Observations—A Bridge Under Load

The choice of an appropriate metric is mandatory to perform deformation analysis between two point clouds (PC)—the distance has to be trustworthy and, simultaneously, robust against measurement noise, which may be correlated and heteroscedastic. The Hausdorff distance (HD) or its averaged derivation (AHD) are widely used to compute local distances between two PC and are implemented in nearly all commercial software. Unfortunately, they are affected by measurement noise, particularly when correlations are present. In this contribution, we focus on terrestrial laser scanner (TLS) observations and assess the impact of neglecting correlations on the distance computation when a mathematical approximation is performed. The results of the simulations are extended to real observations from a bridge under load. Highly accurate laser tracker (LT) measurements were available for this experiment: they allow the comparison of the HD and AHD between two raw PC or between their mathematical approximations regarding reference values. Based on these results, we determine which distance is better suited in the case of heteroscedastic and correlated TLS observations for local deformation analysis. Finally, we set up a novel bootstrap testing procedure for this distance when the PC are approximated with B-spline surfaces.

Configuration Optimisation of Laser Tracker Location on Verification Process

Machine tools are verified and compensated periodically to improve accuracy. The main aim of machine tool verification is to reduce the influence of quasi-static errors, especially geometric errors. As these errors show systematic behavior, their influence can be compensated. However, verification itself is influenced by random uncertainty sources that are usually not considered but affect the results. Within these uncertainty sources, laser tracker measurement noise is a random error that should not be ignored and can be reduced through adequate location of the equipment. This paper presents an algorithm able to analyse the influence of laser tracker location based on nonlinear optimisation, taking into consideration its specifications and machine tool characteristics. The developed algorithm uses the Monte Carlo method to provide a zone around the machine tool where the measurement system should be located in order to improve verification results. To achieve this aim, different parameters were defined, such as the number of tests carried out, and the number and distribution of points, and their influence on the error due to the laser tracker location analysed.

A multilateral laser-tracking three-dimensional coordinate measuring system based on plane constraint

In a multilateral laser-tracking three-dimensional (3D) coordinate measuring system, the error caused by the laser tracker angle measurement is not an issue, while the 3D coordinates of space points are determined using only distance measurement data. Thus in order to improve the accuracy of the whole system it is crucial to calibrate the parameters of the multilateral system with precision. To this end, a parameter-calculating method based on plane constraint is proposed in this paper. A standard plane constraint is used to provide optimal system equations without increasing the number of unknowns. Moreover, while a traditional multilateral system requires at least four stations of laser-tracking measurement, a system based on plane constraint produces more equations, reducing the number of required stations to three. A method for obtaining the required standard plane at the measurement site is also presented in this paper, thus making the above-mentioned scheme feasible. In order to highlight the advantages of a multilateral system over a long distance, an experiment is set up where three scale bars are placed within the range of 25–38 m. The experiment shows that the measurement error of scale bars using a four-station plane constraint model is distributed within the range from 10 to 49 µm, with an average value of 29 µm, while the error using the three-station model is distributed within the range from 31 to 55 µm, with an average value of 39 µm. Using traditional four-station multilateral method the error is distributed from 33 to 79 µm, with an average of 56 µm. The accuracy of the system based on plane constraint appears to be improved compared with single-station measurement and traditional multilateral systems.

Variable FOV of Target Laser Tracking System

In this paper, based on the research of the basic principle of laser tracking measurement, a variable field of view laser radar tracking measurement system was designed. The system can be divided into three parts: laser scanning and ranging module, data processing module and double galvanometer field of view capture tracking module. According to the application requirements, the overall scheme of the optical system and mechanical structure was determined, and the three-dimensional model of the tracking measurement system was established. Based on the debugging of the system, tracking measurement experiment of targets field of view was completed. Through the experimental study of the variable field of view, the results show the feasibility of the variable field of view measurement target. Advantage analysis was performed by comparing the experiments of full field of view scanning. The variable point field system has a higher point cloud density than the full field of view. It can meet various tasks such as equipment manufacturing and installation, engineering measurement, and construction. demand.

Application and comparative analysis on three dimensional camera measurement system in full size inspection of batch production parts

Box-type parts are the basic parts of general machines, they assemble the shaft, sleeve, gear and other related parts of the machine into a whole, so as to maintain the correct mutual position accuracy between the parts. Their complex forming structure, large matching dimensions and high precision requirements directly affect the accuracy, performance and life of the machine. Aiming at the problems of large size and low efficiency of single piece inspection for box parts, this paper takes the spindle box of horizontal lathe as the research object, simulates through virtual testing platform, puts several parts of the same box parts in a certain order, determines the appropriate angle and shoots batch parts placement model in many directions, and adopts three-dimensional method. The camera measurement system detects the full size of the same batch of products and obtains the test results. On this basis, the traditional laser tracking measurement method is used to detect and compare the randomly selected single spindle box. The comparison results show that the three-dimensional camera measurement method can detect the forming accuracy of batch box parts well under the appropriate position and photographic angle, and the full-scale measurement error can be effectively controlled within the technical allowable range. This study provides a preliminary basis for the three-dimensional camera measurement technology in improving the efficiency and accuracy of full-scale measurement of batch box parts.

「オープンループ・レーザトラッカ測定」によるスカラロボットに対する２軸の関節の角度位置決め偏差の同定法

「オープンループ・レーザトラッカ測定」によるスカラロボットに対する２軸の関節の角度位置決め偏差の同定法

Algorithm to optimize measurement system location in a machine tool verification

Nowadays, machine tool accuracy is a competitive element. To improve it, machine tools are verified and compensate periodically reducing the influence of their geometric errors. As geometric errors have systematic behavior, their influence can be compensated after verification. However, verification itself is influenced by random uncertainty sources that affect verification results.Within all influences on machine tool volumetric verification, laser tracker measurement noise is a random uncertainty source that is not usually considered. However, it should not be ignored and can be reduced through an adequate location. This paper presents an algorithm able to analyze the influence of laser tracker location, taking into consideration its specifications and machine tool characteristics. To do that, the developed algorithm provides a zone around MT to locate the measurement system using the Monte Carlo Method. Moreover, it provides the probability distribution function of laser tracker influence related to LT location zone. Therefore, if MT is used as a traceable measurement system, its uncertainty cannot be smaller than LT location uncertainty.

Tarantula: Design, Modeling, and Kinematic Identification of a Quadruped Wheeled Robot

This paper firstly presents the design and modeling of a quadruped wheeled robot named Tarantula. It has four legs each having four degrees of freedom with a proximal end attached to the trunk and the wheels for locomotion connected at the distal end. The two legs in the front and two at the back are actuated using two motors which are placed inside the trunk for simultaneous abduction or adduction. It is designed to manually reconfigure its topology as per the cross-sections of the drainage system. The bi-directional suspension system is designed using a single damper to prevent the trunk and inside components from shock. Formulation for kinematics of the wheels that is coupled with the kinematics of each leg is presented. We proposed the cost-effective method which is also an on-site approach to estimate the kinematic parameters and the effective trunk dimension after assembly of the quadruped robot using the monocular camera and ArUco markers instead of high-end devices like a laser tracker or coordinate measurement machine. The measurement technique is evaluated experimentally and the same set up was used for trajectory tracking of the Tarantula. The experimental method for the kinematic identification presented here can be easily extended to the other mobile robots with serial architecture designed legs.

Influence of laser tracker noise on the uncertainty of machine tool volumetric verification using the Monte Carlo method

Verification of workpieces is typically performed in the post-process with coordinate measuring machines, thereby increasing the manufacturing cycle time. However, machine tools presently can perform contact measuring operations by using a probe. Moreover, there is a growing need for in-process inspection of workpieces. Therefore, using the machine tool itself for the verification whilst the workpiece remains clamped to the machine can lead to an improvement in manufacturing efficiency, cost reduction, higher energy saving and better equipment productivity. However, the use of touch probes as a measurement tool in manufacturing requires some preparatory works. Firstly, the accuracy of the machine tool should be improved to reduce the influence of its geometric errors. Secondly, the uncertainties in calibration and measuring procedure should be determined to obtain the measurement uncertainty. This study presents a new tool that can analyse the effect of different verification parameters in calibration uncertainty based on Monte Carlo method. On the basis of the actual tests performed on a milling machine and its geometric errors, the effect of laser tracker measurement noise in calibration uncertainty is investigated.

High-precision quality assurance of robotic couches with six degrees of freedom

A robotic couch capable of six degrees of freedom (6-DoF) of motion was introduced for state-of-the-art radiation therapy. Patient treatment requires precise quality assurance (QA) of 6-DoF. Unfortunately, conventional methods do not provide the requisite accuracy and precision. Therefore, we developed a high-precision automated QA system using a visual tracking system (VTS). The VTS comprises four motion-sensing cameras, a cube with infrared reflective markers. To acquire data in treatment room coordinates, a transformation matrix from VTS coordinates to treatment room coordinates was determined.The mean error and standard deviation of linear and rotational motions, as well as couch sagging were analyzed from continuously acquired images in the moving couch. The accuracy of VTS was 0.024 mm deviation for the sinusoidal motion, and the accuracy of the transformation matrix was 0.02 mm. In a cross-comparison, the difference between Laser Tracker (FARO) measurements was 0.14 ± 0.12 mm for translation and 0.032 ± 0.026° on average for yaw rotation. The new system provides QA of yaw, pitch and roll motion as well as sagging of the couch and sub-millimeter/degree accuracy together with precision.

New algorithms for motion error detection of numerical control machine tool by laser tracking measurement on the basis of GPS principle

As a three-dimensional measuring instrument, the laser tracker is widely used in industrial measurement. To avoid the influence of angle measurement error on the overall measurement accuracy, the multi-station and time-sharing measurement with a laser tracker is introduced on the basis of the global positioning system (GPS) principle in this paper. For the proposed method, how to accurately determine the coordinates of each measuring point by using a large amount of measured data is a critical issue. Taking detecting motion error of a numerical control machine tool, for example, the corresponding measurement algorithms are investigated thoroughly. By establishing the mathematical model of detecting motion error of a machine tool with this method, the analytical algorithm concerning on base station calibration and measuring point determination is deduced without selecting the initial iterative value in calculation. However, when the motion area of the machine tool is in a 2D plane, the coefficient matrix of base station calibration is singular, which generates a distortion result. In order to overcome the limitation of the original algorithm, an improved analytical algorithm is also derived. Meanwhile, the calibration accuracy of the base station with the improved algorithm is compared with that with the original analytical algorithm and some iterative algorithms, such as the Gauss-Newton algorithm and Levenberg-Marquardt algorithm. The experiment further verifies the feasibility and effectiveness of the improved algorithm. In addition, the different motion areas of the machine tool have certain influence on the calibration accuracy of the base station, and the corresponding influence of measurement error on the calibration result of the base station depending on the condition number of coefficient matrix are analyzed.

Об оптимизации опорных геодезических кольцевых сетей ускорителей при использовании лазерных трекеров

The features of laser tracker measurements and their accounting for designing geodetic networks on the accelerator complexes are considered. The key parameters for the optimization of the spatial geodetic network of the accelerator are proposed. The dependence of average mean square errors on the radial and altitudinal directions of orientation of the sighting beam for distances from 1 to 30 meters was defined. The influence of the choice of stations for mounting the laser tracker to determine the parameters of the ellipsoid of errors was studied. The measurements for three variants of geodetic networks in the tunnel with the adopted geometric parameters were simulated. The data of the deviations of the points of the network radius, altitude relative to the project was indicated. The obtained results can be useful both at the design stage of geodetic network accelerator, and at the stage of installation of technological equipment.

Online absolute pose compensation and steering control of industrial robot based on six degrees of freedom laser measurement

In-situ intelligent manufacturing for large-volume equipment requires industrial robots with absolute high-accuracy positioning and orientation steering control. Conventional robots mainly employ an offline calibration technology to identify and compensate key robotic parameters. However, the dynamic and static parameters of a robot change nonlinearly. It is not possible to acquire a robot’s actual parameters and control the absolute pose of the robot with a high accuracy within a large workspace by offline calibration in real-time. This study proposes a real-time online absolute pose steering control method for an industrial robot based on six degrees of freedom laser tracking measurement, which adopts comprehensive compensation and correction of differential movement variables. First, the pose steering control system and robot kinematics error model are constructed, and then the pose error compensation mechanism and algorithm are introduced in detail. By accurately achieving the position and orientation of the robot end-tool, mapping the computed Jacobian matrix of the joint variable and correcting the joint variable, the real-time online absolute pose compensation for an industrial robot is accurately implemented in simulations and experimental tests. The average positioning error is 0.048 mm and orientation accuracy is better than 0.01 deg. The results demonstrate that the proposed method is feasible, and the online absolute accuracy of a robot is sufficiently enhanced.

Monte Carlo method to machine tool uncertainty evaluation

Currently machine tools are not only a way to make different parts based on material removal processes. These ones can be used as a measurement system too. In this way, overall inspection time is reduced and equipment productivity is increased.Nevertheless, the use of machine tool probes as measurement tool in manufacturing parts required previous works. Firstly, the machine tool accuracy should be improved, in order to reduce the influence of its geometric errors. This way, volumetric verification based on laser tracker measurement has increased strongly in the last few years, especially in long range machine tools. Secondly, calibration uncertainty should be calculated to provide measurement uncertainty.This way, the paper presents a new tool able to analyze the effect of different influence verification parameters in calibration uncertainty based on Monte Carlo method. Using real tests carried out on a milling machine and its geometric errors, the influence or laser tracker measurement noise in calibration uncertainty is studied using Monte Carlo method.

Forecasting method in multilateration accuracy based on laser tracker measurement

Multilateration based on a laser tracker (LT) requires the measurement of a set of points from three or more positions. Although the LTs’ angular information is not used, multilateration produces a volume of measurement uncertainty. This paper presents two new coefficients from which to determine whether the measurement of a set of points, before performing the necessary measurements, will improve or worsen the accuracy of the multilateration results, avoiding unnecessary measurement, and reducing the time and economic cost required.The first specific coefficient measurement coefficient (MCLT) is unique for each laser tracker. It determines the relationship between the radial and angular laser tracker measurement noise. Similarly, the second coefficient is related with specific conditions of measurement β. It is related with the spatial angle between the laser tracker positions α and its effect on error reduction. Both parameters MCLT and β are linked in error reduction limits.Beside these, a new methodology to determine the multilateration reduction limit according to the multilateration technique of an ideal laser tracker distribution and a random one are presented. It provides general rules and advice from synthetic tests that are validated through a real test carried out in a coordinate measurement machine.

Analysis of the initial thermal stabilization and air turbulences effects on Laser Tracker measurements

During the several last years, Laser Trackers have become more common as a measurement tool in the manufacture and assembly of large components such as aircraft wings and ship hulls, as well as for error mapping in coordinate measuring machines and machine tools. Most of these processes cannot be developed in a controlled metrological laboratory but must be implemented directly on a shop floor. Therefore, the process of stabilization of the Laser Tracker has been studied in several experimental tests, and it has been observed that the warm-up time suggested by the manufacturer is not enough. During the first hours of the measurement process two types of thermal errors significantly affect the measurements, causing inaccuracies of between 20 and 80μm, depending on the equipment used and the positions of the measured points. These thermal errors are systematic and repeatable; therefore they can be estimated and compensated for each measurement system. Because environmental conditions on a shop floor cannot be controlled, once the Laser Tracker is stabilized, the effects of ambient air in measurements have also been studied, focusing on the effect of turbulent flows on the beam path. It has been observed that this turbulence may cause radial distance drifts on the order of micrometers, deflection of the beam trajectory and signal loss.

Empirical analysis of the efficient use of geometric error identification in a machine tool by tracking measurement techniques

Volumetric verification is becoming increasingly accepted as a suitable technique with which to improve machine tool accuracy. In the same way, the use of laser trackers to obtain machine error information using the new Active Target motorised retro-reflector allows the verification of all types of machine tool throughout their workspaces.Non-linear optimisation methods and machine tool kinematic models are the mainstays of this technique. Whereas the latter provide the relationship between the nominal coordinates, the geometric errors of the machine and laser tracker measurement, the former reduces the combined influence of geometric errors by obtaining their approximation functions. However, within these two procedures, several factors affect the scope of the produced verification results.The present paper focuses on the analysis of the adequacy of commercial measurement techniques using laser trackers and the new motorised retro-reflector in a real milling machine. An examination is also made regarding the influence of the optimisation sequence defined by the identification strategy, as well as the impact of the number of measured points in relation to the employed regression functions.