Thirty years since their invention, laser trackers are now recognized as the measurement tool of choice in the manufacture and assembly of large components. The general design of laser trackers, i.e., a ranging unit on a two-axis gimbal, has not changed significantly over the years. However, innovations in ranging technology, for example, the emergence of increasingly accurate absolute distance meters (ADMs), are providing users with an alternative to interferometers (IFMs). Hand-held accessories such as touch probes and line scanners are expanding the scope and applicability of laser trackers. In this paper, we survey the literature in all areas of laser trackers as applied to large-scale dimensional metrology (LSDM), with emphasis on error modeling, measurement uncertainty, performance evaluation and standardization.

Laser trackers for large-scale dimensional metrology: A review

This paper proposes a calibration method for a coordinate measuring machine (CMM) using a laser tracking system. The laser tracking system can measure three-dimensional coordinates based on the principle of trilateration with high accuracy and is easy to set up. The accuracy of length measurement of a single laser tracking interferometer (laser tracker) is about 0.3 µm over a length of 600 mm. In this study, we first measured 3D coordinates using the laser tracking system. Secondly, 21 geometric errors, namely, parametric errors of the CMM, were estimated by the comparison of the coordinates obtained by the laser tracking system and those obtained by the CMM. As a result, the estimated parametric errors agreed with those estimated by a ball plate measurement, which demonstrates the validity of the proposed calibration system.

https://iopscience.iop.org/article/10.1088/0957-0233/16/12/010/pdf

Geometric calibration of a coordinate measuring machine using a laser tracking system

A technique for intrinsic and extrinsic calibration of laser line scanners, also called laser triangulation sensors (LTSs), for integration in a coordinate measuring machine (CMM) is presented in this paper. Setting out from the modeling of a commercial LTS for use in a CMM and the algorithms implemented for image capture and processing, with the use of a gauge object, a one-step calibration procedure has been developed to obtain both intrinsic parameters?laser plane, CCD sensor and camera geometry?and extrinsic parameters which relate the LTS's reference system to the CMM's reference system, integrating both mathematical models. This method performs both calibrations in a single step, thus avoiding the digitalization of a reference sphere in order to obtain the extrinsic parameters, or optimization procedures subsequent to LTS calibration. The results obtained in accuracy and repeatability tests performed on gauge geometric primitives attest to the viability of this technique for the integration of LTSs in CMMs.

A one-step intrinsic and extrinsic calibration method for laser line scanner operation in coordinate measuring machines

This article discusses different non contact 3D measuring strategies and presents a model for measuring complex geometry parts, manipulated through a robot arm, using a novel vision system consisting of a laser triangulation sensor and a motorized linear stage. First, the geometric model incorporating an automatic simple module for long term stability improvement will be outlined in the article. The new method used in the automatic module allows the sensor set up, including the motorized linear stage, for the scanning avoiding external measurement devices. In the measurement model the robot is just a positioning of parts with high repeatability. Its position and orientation data are not used for the measurement and therefore it is not directly “coupled” as an active component in the model. The function of the robot is to present the various surfaces of the workpiece along the measurement range of the vision system, which is responsible for the measurement. Thus, the whole system is not affected by the robot own errors following a trajectory, except those due to the lack of static repeatability. For the indirect link between the vision system and the robot, the original model developed needs only one first piece measuring as a “zero” or master piece, known by its accurate measurement using, for example, a Coordinate Measurement Machine. The strategy proposed presents a different approach to traditional laser triangulation systems on board the robot in order to improve the measurement accuracy, and several important cues for self-recalibration are explored using only a master piece. Experimental results are also presented to demonstrate the technique and the final 3D measurement accuracy.

https://www.mdpi.com/1424-8220/11/1/90/pdf

3D geometrical inspection of complex geometry parts using a novel laser triangulation sensor and a robot.

A technique for intrinsic and extrinsic calibration of a laser triangulation sensor (LTS) integrated in an articulated arm coordinate measuring machine (AACMM) is presented in this paper. After applying a novel approach to the AACMM kinematic parameter identification problem, by means of a single calibration gauge object, a one-step calibration method to obtain both intrinsic―laser plane, CCD sensor and camera geometry―and extrinsic parameters related to the AACMM main frame has been developed. This allows the integration of LTS and AACMM mathematical models without the need of additional optimization methods after the prior sensor calibration, usually done in a coordinate measuring machine (CMM) before the assembly of the sensor in the arm. The experimental tests results for accuracy and repeatability show the suitable performance of this technique, resulting in a reliable, quick and friendly calibration method for the AACMM final user. The presented method is also valid for sensor integration in robot arms and CMMs.

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Modelling and Calibration Technique of Laser Triangulation Sensors for Integration in Robot Arms and Articulated Arm Coordinate Measuring Machines

As an optical retroreflector used in a laser interferometer system, a cat's eye is expected to provide high parallelism of the reflected beam, especially for large-range measurements. On the other hand, a cat's eye should be light and portable for the convenience of operation in taking the actual measurement. To solve these problems, a new method for optimizing the parameters of a cat's eye is proposed to obtain the least divergence angle of the reflected beam. Theoretical analysis and experiment show that the improved cat's eye not only has smaller volume and less weight, but also greatly improves the parallelism of the reflected beam.

An improved cat's-eye retroreflector used in a laser tracking interferometer system

Modeling and calibration of a structured-light-sensor-based five-axis scanning system

Historical position of coordinate measuring machines (CMMs) and problems needed to be solved are expounded. The working principle and key techniques of intelligent CMMs are discussed. The specific features of the non-orthogonal coordinate measuring systems including cylindrical CMMs, spherical CMMs, articulated arm CMMs, light pen CMMs, triangulation measuring systems and multi-lateration coordinate measuring systems are analyzed. The prospect of non-contact probes, in particular that of confocal probes, in coordinate metrology is demonstrated. The status of researches on nano-CMMs at home and abroad is introduced.

Development orientations of coordinate measuring techniques

Today several new kinds of laser beam sensors appear with high resolution and accuracy and find their applications in reverse engineering and quality control. Due to the incapability of changing their orientations continuously in response to the surface fluctuation of a part to be measured, they are not competent for measuring parts with complicated structures. In this paper a five-axis laser scanning system integrated with a CMM, a laser beam sensor and a PH10 rotary head is proposed, which can measure complicated parts by frequently indexing the laser sensor approximately consistent with the normal direction of the surface. As the laser value is a 1-D data and the measured data must be given in 3-D expression in the world coordinate system, a system model for coordinate transformation is established. An “equivalent probe” approach is presented for system verification, and an iterative verifying process is adopted to eliminate the verification error caused by the inclination error of the laser sensor. Experiment study shows that the system can measure a part from any direction with an accuracy of 30 μm.

Modeling and verification of a five-axis laser scanning system

D coordinate measuring machines have wide applications in advanced manufacturing technology and in scientific researches. The paper discussed the development tendency of coordinate measuring machines in enhancing their accuracy and efficiency, in probing techniques, software, construction materials, control systems, in entering the manufacturing system, in developing non orthogonal coordinate measuring systems, in solving the environmental problems, and in error calibration and error compensation.

Zhang Guo-xiong

Papers

An improved cat's-eye retroreflector used in a laser tracking interferometer system

Modeling and calibration of a structured-light-sensor-based five-axis scanning system

Development orientations of coordinate measuring techniques

Modeling and verification of a five-axis laser scanning system

The Development Tendency of Coordinate Measuring Machines