Laser Tracker Measurement Research Articles

视觉引导激光跟踪测量系统的Cayley变换校准方法

视觉引导激光跟踪测量系统的Cayley变换校准方法

COMPARISON OF TARGET- AND MUTUAL INFORMATON BASED CALIBRATION OF TERRESTRIAL LASER SCANNER AND DIGITAL CAMERA FOR DEFORMATION MONITORING

Abstract. In the current state-of-the-art, geodetic deformation analysis of natural and artificial objects (e.g. dams, bridges,...) is an ongoing research in both static and kinematic mode and has received considerable interest by researchers and geodetic engineers. In this work, due to increasing the accuracy of geodetic deformation analysis, a terrestrial laser scanner (TLS; here the Zoller+Fröhlich IMAGER 5006) and a high resolution digital camera (Nikon D750) are integrated to complementarily benefit from each other. In order to optimally combine the acquired data of the hybrid sensor system, a highly accurate estimation of the extrinsic calibration parameters between TLS and digital camera is a vital preliminary step. Thus, the calibration of the aforementioned hybrid sensor system can be separated into three single calibrations: calibration of the camera, calibration of the TLS and extrinsic calibration between TLS and digital camera. In this research, we focus on highly accurate estimating extrinsic parameters between fused sensors and target- and targetless (mutual information) based methods are applied. In target-based calibration, different types of observations (image coordinates, TLS measurements and laser tracker measurements for validation) are utilized and variance component estimation is applied to optimally assign adequate weights to the observations. Space resection bundle adjustment based on the collinearity equations is solved using Gauss-Markov and Gauss-Helmert model. Statistical tests are performed to discard outliers and large residuals in the adjustment procedure. At the end, the two aforementioned approaches are compared and advantages and disadvantages of them are investigated and numerical results are presented and discussed.

Calibration method for a vision guiding-based laser-tracking measurement system

Laser-tracking measurement systems (laser trackers) based on a vision-guiding device are widely used in industrial fields, and their calibration is important. As conventional methods typically have many disadvantages, such as difficult machining of the target and overdependence on the retroreflector, a novel calibration method is presented in this paper. The retroreflector, which is necessary in the normal calibration method, is unnecessary in our approach. As the laser beam is linear, points on the beam can be obtained with the help of a normal planar target. In this way, we can determine the function of a laser beam under the camera coordinate system, while its corresponding function under the laser-tracker coordinate system can be obtained from the encoder of the laser tracker. Clearly, when several groups of functions are confirmed, the rotation matrix can be solved from the direction vectors of the laser beams in different coordinate systems. As the intersection of the laser beams is the origin of the laser-tracker coordinate system, the translation matrix can also be determined. Our proposed method not only achieves the calibration of a single laser-tracking measurement system but also provides a reference for the calibration of a multistation system. Simulations to evaluate the effects of some critical factors were conducted. These simulations show the robustness and accuracy of our method. In real experiments, the root mean square error of the calibration result reached 1.46 mm within a range of 10 m, even though the vision-guiding device focuses on a point approximately 5 m away from the origin of its coordinate system, with a field of view of approximately 200 mm × 200 mm.

Verification of an articulated arm coordinate measuring machine using a laser tracker as reference equipment and an indexed metrology platform

In this work the analysis of the use of a laser tracker as a reference instrument in calibration and verification procedures for articulated arm coordinate measuring machines (AACMM) with an indexed metrology platform is presented. In the case of AACMM verification procedures, where it is necessary to evaluate its maximum working volume, this technique represents an alternative to conventional one-dimensional gauges and avoids the need of materializing the lengths required for a conventional gauge by increasing the flexibility for defining test positions and broadening the definition of reference test lengths depending on equipment calibration or verification requirements. First a procedure for the AACMM verification with the indexed metrology platform (IMP) and the laser tracker (LT) used as a reference instrument was defined and the required accuracy of the equipment was assessed. The experimental testing to validate its proper application was carried out by means of laser tracker measurements of six retroreflector targets located in a mesh that simulated the AACMM working volume. Finally, the results obtained with both equipment, laser tracker and AACMM with indexed metrology platform, showed that the procedure presented is suitable and that the laser tracker could be considered as a reference instrument for AACMM verification processes when the nominal accuracy of the laser tracker is assured.

Large-scale compact range on-site alignment based on laser tracker measurement network

Large-scale compact range generally consists of multi reflectors, how to align these numerous small reflectors into a high precision surface, this has to be carefully taken into account following two key issues: measurement system and alignment approach. Firstly, aim to solve the large scale, high precision and longtime stability measurement system issues, a measurement network based on laser tracker metrology was established, key technology for the metrology including: deducing the error in point measurement and uncertainty ellipsoid of laser tracker; optimizing the measuring stations and carrying out weighted measurement network regression. Secondly, a practical alignment approach was proposed; this approach divides the entire process into three key phases including: marked points positioning phase, gaps equalizing phase and constrained surfaces smoothing phase; due to gradually improve the adjustment precision step-by-step, it effectively reduces the adjustment iterations and avoids mutual physical contact, and ensure the surface accuracy and the gaps widths. Finally, the on-site alignment of a large compact range with a width of 23m and height of 16m which will generate a Φ15m quiet zone at 40Ghz is presented; The uncertainty of the measurement network is controlled to 0.025mm, and the final surface root mean square accuracy was up to 0.054mm and the gaps widths achieved 0.4±0.2mm.

Efficient Methods for Evaluating Task-Specific Uncertainty in Laser-Tracking Measurement

For task-specific coordinate metrology, it is necessary to associate with task-specific parameters an estimate of their uncertainty at a specified level of confidence. In recent years, various sources of uncertainty for laser trackers themselves have been studied. However, methods for determining the uncertainty in laser-tracker-based task-specific measurement must be further studied. In this paper, the guide to the expression of uncertainty in measurement (GUM) uncertainty framework (GUF), the Monte Carlo method and a hybrid method combining grey evaluation and neural network technologies are used to evaluate the task-specific uncertainty in laser-tracking measurement. First, this article discusses the contributions to measurement uncertainty in specific laser-tracking measurement, including instruments themselves, data fusion, measurement strategies, measurement environment and task-specific data processing. Second, the principles of GUF, Monte Carlo and the aforementioned hybrid method are presented. Finally, a case study involving the uncertainty evaluation of a cylindricity measurement process using the above-mentioned methods is illustrated. The results demonstrate that these methods have different characteristics in task-specific uncertainty evaluation.

Comparison of GUF and Monte Carlo methods to evaluate task-specific uncertainty in laser tracker measurement

Measurement uncertainty plays an important role in laser tracking measurement analyses. In the present work, the guides to the expression of uncertainty in measurement (GUM) uncertainty framework (GUF) and its supplement, the Monte Carlo method, were used to estimate the uncertainty of task-specific laser tracker measurements. First, the sources of error in laser tracker measurement were analyzed in detail, including instruments, measuring network fusion, measurement strategies, measurement process factors (such as the operator), measurement environment, and task-specific data processing. Second, the GUM and Monte Carlo methods and their application to laser tracker measurement were presented. Finally, a case study involving the uncertainty estimation of a cylindricity measurement process using the GUF and Monte Carlo methods was illustrated. The expanded uncertainty results (at 95% confidence levels) obtained with the Monte Carlo method are 0.069 mm (least-squares criterion) and 0.062 mm (minimum zone criterion), respectively, while with the GUM uncertainty framework, none but the result of least-squares criterion can be got, which is 0.071 mm. Thus, the GUM uncertainty framework slightly underestimates the overall uncertainty by 10%. The results demonstrate that the two methods have different characteristics in task-specific uncertainty evaluations of laser tracker measurements. The results indicate that the Monte Carlo method is a practical tool for applying the principle of propagation of distributions and does not depend on the assumptions and limitations required by the law of propagation of uncertainties (GUF). These features of the Monte Carlo method reduce the risk of an unreliable measurement of uncertainty estimation, particularly in cases of complicated measurement models, without the need to evaluate partial derivatives. In addition, the impact of sampling strategy and evaluation method on the uncertainty of the measurement results can also be taken into account with Monte Carlo method, which plays a guiding role in measurement planning.

Laser Tracker Error Modeling and Kinematic Calibration Strategy

Calibration of Laser Tracker systems is based most times in the determination of its geometrical errors. Some standards as the ASME B89.4.19 [1] and the VDI 2617-10 [2] describe different tests to calculate the geometric misalignments that cause systematic errors in Laser Tracker measurements. These errors are caused not only because of geometrical misalignments and other sources of error must also be taken in count. In this work we want to state the errors in a kinematic form. Errors will be split in two different components, geometric and kinematic errors. The first ones depend on the offsets, tilts and eccentricity of the mechanical and optical components of the system. Kinematic errors are different for every position of the Laser tracker, so they will be formulated as functions of three system variables: distance (R), vertical angle (V) and horizontal angle (H) usually called d, φ and θ. The goal of this work is to set up an evaluation procedure to determine geometric and kinematic errors of Laser Trackers.

Study of self-calibration and multilateration in machine tool volumetric verification for laser tracker error reduction

This article aims to present the influence of the measurement uncertainty of a commercial laser tracker on the volumetric verification of a machine tool through the study of verification procedures that are affected by measurement uncertainty, multilateration and laser tracker self-calibration. Self-calibration provides relative positioning between measuring coordinate systems (laser trackers) and the reference system from the measured points of the same mesh. The measured points are affected by the noise of each laser tracker; therefore, they provide positions that are different from the real positions of the laser trackers. By applying the technique of multilateration and by knowing the positions of the laser trackers, the measurement noise can be reduced. The range of the measurement noise reduction is influenced by the radial measurement noise of the laser tracker, the distance between the laser tracker and the measured point and the techniques that multilateration and laser tracker self-calibration employs. This article presents different laser tracker self-calibration procedures, a least squares adjustment, trilateration and quadrilateration as well as the scope and appropriateness of each method relative to the laser tracker measurement noise. Moreover, the influences of radial laser tracker noise on the trilateration and quadrilateration techniques are described as well as the influence of the distance between the laser tracker and the measured point on multilateration.

Measuring rough optical surfaces using scanning long-wave optical test system 1 Principle and implementation

Current metrology tools have limitations when measuring rough aspherical surfaces with 1-2 μm root mean square roughness; thus, the surface cannot be shaped accurately by grinding. To improve the accuracy of grinding, the scanning long-wave optical test system (SLOTS) has been developed to measure rough aspherical surfaces quickly and accurately with high spatial resolution and low cost. It is a long-wave infrared deflectometry device consisting of a heated metal ribbon and an uncooled thermal imaging camera. A slope repeatability of 13.6 μrad and a root-mean-square surface accuracy of 31 nm have been achieved in the measurements of two 4 inch spherical surfaces. The shape of a rough surface ground with 44 μm grits was also measured, and the result matches that from a laser tracker measurement. With further calibration, SLOTS promises to provide robust guidance through the grinding of aspherics.

Researching Metrology System on the Basis of Laser Tracker

A digital flexible assembly system is developed which is used to assemble one fuselage large component of a large aircraft. The laser tracking metrology system is the important part of the flexible assembly system. The metrology system is composed of three modules: laser tracking measurement module, data processing module, and real-time motion simulation module. The laser tracking measurement module is mainly completing the measurement functions of the laser tracker. Data processing module is the core module of the metrology system, and is responsible for all data computation and transmission in assembling and positioning the aircraft component. The real-time motion simulation module provides the three-dimensional scene simulation of the assembly to the users and gives the users a more intuitive understanding of the assembly process. This paper introduces the system structure and working principle of the developed metrology system which based on the laser tracker. The application of the metrology system and the flexible assembly system can greatly improve the assembly accuracy.

A Laser Tracking-Vision Guiding Measurement System for Large-Scale Parts Assembly

To address the problems of low efficiency, high cost and low automatic level existing in traditional laser tracking measurement systems, a laser tracking-vision guiding measurement system for large-scale parts assembly is introduced in this paper. The system is composed of mono-camera and a laser tracker, can fulfill real-time tracking and automatic measurement throughout the whole assembly process. A global calibration method based on public planes and a method for finding the 3D positions of the target reflectors based on monocular vision measuring are expounded. Mathematical model is established, measurement system is built, and the experiment is accomplished. The experiment result indicates that the proposed system features relative high degree of automation and high measuring speed, has future in practical application.

Uncertainty estimation and multi sensor fusion for kinematic laser tracker measurements

Laser trackers are widely used to measure kinematic tasks such as tracking robot movements.Common methods to evaluate the uncertainty in the kinematic measurement include approximations specified by the manufacturers, various analytical adjustment methods and the Kalman filter. In this paper a new, real-time technique is proposed, which estimates the 4D-path (3D-position + time) uncertainty of an arbitrary path in space. Here a hybrid system estimator is applied in conjunction with the kinematic measurement model. This method can be applied to processes, which include various types of kinematic behaviour, constant velocity, variable acceleration or variable turn rates. The new approach is compared with the Kalman filter and a manufacturer's approximations. The comparison was made using data obtained by tracking an industrial robot's tool centre point with a Leica laser tracker AT901 and a Leica laser tracker LTD500. It shows that the new approach is more appropriate to analysing kinematic processes than the Kalman filter, as it reduces overshoots and decreases the estimated variance. In comparison with the manufacturer's approximations, the new approach takes account of kinematic behaviour with an improved description of the real measurement process and a reduction in estimated variance. This approach is therefore well suited to the analysis of kinematic processes with unknown changes in kinematic behaviour as well as the fusion among laser trackers.

Identification and Kinematic Calculation of Laser Tracker Errors

Calibration of Laser Tracker systems is based most times in the determination of its geometrical errors. Some standards as the ASME B89.4.19 (2006) and the VDI 2617-10 (2011) describe different tests to calculate the geometric misalignments that cause systematic errors in Laser Tracker measurements. These errors are caused not only because of geometrical misalignments and other sources of error must also be taken in count. In this work we want to express the errors in a kinematic form. Errors will be split in two different components, geometric and kinematic errors. The first ones depend on the offsets, tilts and eccentricity of the mechanical and optical components of the system. Kinematic errors are different for every position of the Laser tracker, so they must be formulated as functions of three system variables: range (R), vertical angle (V) and horizontal angle (H). The goal of this work is to set up an evaluation procedure to determine geometric and kinematic errors of Laser Trackers.

Research on volumetric error compensation for NC machine tool based on laser tracker measurement

Error compensation is an effective method to improve the accuracy of NC machine tool. The compensation results are greatly affected by the error modeling and error measurement, so how to establish an accurate error model and accurately detect the geometric errors of machine tool are particularly important. In the paper, the volumetric error model of a multi-axis NC machine tool is established based on the multi-body system theory. The relative position deviations between the rotary axis and linear axis of the machine tool are considered in the modeling. To achieve quick and accurate detection, a laser tracker is based on the multi-station and time-sharing measurement principle to detect the geometric errors of machine tool, and each error of linear axis and rotary axis can be identified by this method. With the error model, error compensation can be carried out by modifying the machining process (G code), and the machining accuracy can be improved with the new G code. Experiment results show that the motion errors of a milling machine are significantly reduced after compensation, which verifies the effectiveness of compensation method.

Automated Fiducial Localization in CT Images Based on Surface Processing and Geometrical Prior Knowledge for Radiotherapy Applications

We propose a novel method for radio-opaque external marker localization in CT scans for infrared (IR) patient set-up in radiotherapy. Efforts were focused on the quantification of uncertainties in marker localization in the CT dataset as a function of algorithm performance. We implemented a 3-D approach to fiducial localization based on surface extraction and marker recognition according to geometrical prior knowledge. The algorithm parameters were optimized on a clinical CT dataset coming from 35 cranial and extra-cranial patients; the localization accuracy was benchmarked at variable image resolution versus laser tracker measurements. The applicability of conventional IR optical tracking systems for localizing external surrogates in daily patient set-up procedures was also investigated in 121 proton therapy treatment sessions. Our study shows that the implemented algorithm features surrogates localization with uncertainties lower than 0.3 mm and with a true positive rate of 90.1%, being this latter mainly influenced by fiducial homogeneity in the CT images. The reported clinical validation in proton therapy confirmed the submillimetric accuracy and the expected algorithm sensitivity. Geometrical prior knowledge allows judging the reliability of the extracted fiducial coordinates, ensuring the highest accuracy in patient set-up.

Coordinate Transformation of Laser Tracking in-Site Measurement System for Big Gears

An in-site measurement method for big gears based on laser tracking and coordinate measurement technology is proposed in this paper. The coordinate transformation method, in which direction cosine are used as parameters, is suitable for large rotation angle transformation. The coordinate transformation with large rotation angle in the in-site measurement, hence, can be realized. The experimental results show that the proposed coordinate transformation method has the advantages of high accuracy and stability, and can meet the requirements of the laser tracking in-site measurement system for big gears.

Verification procedure for isocentric alignment of proton beams

We present a technique—based on the Lutz, Winston, and Maleki test used in stereotactic linear accelerator radiosurgery—for verifying whether proton beams are being delivered within the required spatial coincidence with the gantry mechanical isocenter. Our procedure uses a proton beam that is collimated by a circular aperture at its central axis and is then intercepted by a small steel sphere rigidly supported by the patient couch. A laser tracker measurement system and a correction algorithm for couch position assures precise positioning of the steel sphere at the mechanical isocenter of the gantry. A film‐based radiation dosimetry technique, chosen for the good spatial resolution it achieves, records the proton dose distribution for optical image analysis. The optical image obtained presents a circular high‐dose region surrounding a lower‐dose area corresponding to the proton beam absorption by the steel sphere, thereby providing a measure of the beam alignment with the mechanical isocenter. We found the self‐developing Gafchromic EBT film (International Specialty Products, Wayne, NJ) and commercial Epson 10000 XL flatbed scanner (Epson America, Long Beach, CA) to be accurate and efficient tools. The positions of the gantry mechanical and proton beam isocenters, as recorded on film, were clearly identifiable within the scanning resolution used for routine alignment testing (0.17 mm per pixel). The mean displacement of the collimated proton beam from the gantry mechanical isocenter was 0.22±0.1 mm for the gantry positions tested, which was well within the maximum deviation of 0.50 mm accepted at the Proton Therapy Center in Houston.PACS numbers: 87.53.Xd, 87.53.Oc, 87.56.‐v, 87.66.‐a, 87.56.Fc

An Application of the Kalman Filter in a Laser Tracking Measurement System

Flexible coordinate measuring system based on laser tracking measurement system (LTS) is an effective method to detect 3D coordinates of moving target. However, the system suffers from various interferences resulting in low accuracy. This paper extends the application of a Kalman filtering algorithm in LTS to solve the problem. The laser tracking system is introduced. The state model of the laser tracking measurement system is developed and the linearization method of the model is analyzed. A Kalman filtering algorithm is used for the system. The result of the simulation shows that the proposed Kalman filter method works well for the improvement of accuracy of LTS.

Pulse-Width Control of the Position of the End-Effector of an Industrial Robot

The design of control laws to precisely control the position of the end-effector of an industrial robot using real-time measurements of the position of the end-effector, in addition to the usual measurements of the joint positions, is considered. The end-effector position measurements are obtained with a laser tracker sensor. The control laws are pulse width control laws, which utilize rectangular force pulses to overcome stiction and other nonlinear friction forces. An approach to designing piecewise-linear-gain pulse width control laws is introduced. It is shown that, when this approach is used to design a control law for the industrial robot, control of the position of the robot’s end-effector to the tens-of-micrometers accuracy of the laser tracker measurements of the position of the end-effector is achieved.