Jorge Santolaria

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

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 LTSs reference system to the CMMs 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.

3D Geometrical Inspection of Complex Geometry Parts Using a Novel Laser Triangulation Sensor and a Robot

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.

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

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.

Articulated Arm Coordinate Measuring Machine Calibration by Laser Tracker Multilateration

A new procedure for the calibration of an articulated arm coordinate measuring machine (AACMM) is presented in this paper. First, a self-calibration algorithm of four laser trackers (LTs) is developed. The spatial localization of a retroreflector target, placed in different positions within the workspace, is determined by means of a geometric multilateration system constructed from the four LTs. Next, a nonlinear optimization algorithm for the identification procedure of the AACMM is explained. An objective function based on Euclidean distances and standard deviations is developed. This function is obtained from the captured nominal data (given by the LTs used as a gauge instrument) and the data obtained by the AACMM and compares the measured and calculated coordinates of the target to obtain the identified model parameters that minimize this difference. Finally, results show that the procedure presented, using the measurements of the LTs as a gauge instrument, is very effective by improving the AACMM precision.

Towards an effective identification strategy in volumetric error compensation of machine tools

The influence of technical parameters for volumetric error compensation in large-volume machine tools (MTs) is presented in this paper. The techniques presented are based on characterization models using nonlinear optimization procedures. The parameters presented allow for the characterization of different errors in the MT studied and depend on the kinematics and geometry of the system, regardless of the optimization methodology. The kinematics is affected by the MT errors on the number and type of axes and movements. To relate the coordinates of the tool to the coordinates of a laser tracker, a kinematic model of the MT that includes the measurement system must be defined. Kinematic models can be realized by using homogeneous transformation matrices or independent rotation and translation arrays according to the type of machine. Chebyshev, simple or Legendre polynomial regression functions can be used to characterize the geometric errors of the MT and are presented and compared. The distribution of measurement points, mesh or cloud, and optimization constraints of polynomial regressions are factors that also affect volumetric error compensation. Therefore, these parameters were studied and presented as well. In addition to the parameters discussed above, another parameter that affects the accuracy of data capture is the measurement noise. To improve the measurement accuracy, multilateration techniques need to be applied. Each of the aforementioned parameters has been studied by using a synthetic test generated by a parametric synthetic data generator. The selected parameters constitute a package of optimization improvement regardless of the optimization methodology, which have improved the nonlinear optimization from 60–70% to 98%.

An Overview of Kinematic and Calibration Models Using Internal/External Sensors or Constraints to Improve the Behavior of Spatial Parallel Mechanisms

This paper presents an overview of the literature on kinematic and calibration models of parallel mechanisms, the influence of sensors in the mechanism accuracy and parallel mechanisms used as sensors. The most relevant classifications to obtain and solve kinematic models and to identify geometric and non-geometric parameters in the calibration of parallel robots are discussed, examining the advantages and disadvantages of each method, presenting new trends and identifying unsolved problems. This overview tries to answer and show the solutions developed by the most up-to-date research to some of the most frequent questions that appear in the modelling of a parallel mechanism, such as how to measure, the number of sensors and necessary configurations, the type and influence of errors or the number of necessary parameters.

A self-centering active probing technique for kinematic parameter identification and verification of articulated arm coordinate measuring machines

A crucial task in the procedure of identifying the parameters of a kinematic model of an articulated arm coordinate measuring machine (AACMM) or robot arm is the process of capturing data. In this paper a capturing data method is analyzed using a self-centering active probe, which drastically reduces the capture time and the required number of positions of the gauge as compared to the usual standard and manufacturer methods. The mathematical models of the self-centering active probe and AACMM are explained, as well as the mathematical model that links the AACMM global reference system to the probe reference system. We present a self-calibration method that will allow us to determine a homogeneous transformation matrix that relates the probes reference system to the AACMM last reference system from the probing of a single sphere. In addition, a comparison between a self-centering passive probe and self-centering active probe is carried out to show the advantages of the latter in the procedures of kinematic parameter identification and verification of the AACMM.

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.

Modelling and calibration of parallel mechanisms using linear optical sensors and a coordinate measuring machine

This paper presents a new procedure for the modelling and calibration of a parallel mechanism by using linear optical sensors and a coordinate measuring machine. Three standard spheres, fixed to the moving platform, were measured by means of a coordinate measuring machine. Additionally, a control algorithm was developed to store sensor readings in each analysed position. These readings and the kinematic model allow us to obtain the calculated sphere coordinates. The use of high-accuracy linear optical sensors allows us to correct actuator backlash, thereby increasing the mechanism accuracy. The developed method defines an objective function that compares the measured and calculated coordinates of the three-sphere centres in order to obtain the identified model parameters that minimize this difference. This procedure combines both inverse and forward kinematics, and solves the nonlinear system loop of the kinematic model inside a second loop that optimizes the geometric parameters of the model. Numerical optimization techniques based on Levenberg–Marquardt algorithm are used to solve both optimization loops. Results show that the platform position and orientation errors are improved by more than one order of magnitude.

Development and calibration of self-centring probes for assessing geometrical errors of machines

A new type of probe for calibration, verification or interim checking of machine tools as well as robots or parallel-kinematics machine tools is presented in this paper. This probe is part of a ball-artefact-based method to assess the geometrical errors of linear and angular axes in a quick and reliable way. The discussion about the best design concept for the self-centring probe, the mathematical modelling and design process of one of those concepts and the development of three different prototypes are shown. Different sensors and design options were developed to optimize the size and cost of the probe. The calibration of the probe prototypes by way of a kinematically coupled reference device to allow on-machine tests is shown. The uncertainties obtained were around 1 µm for one of the prototypes and below 5 µm for the other two. Finally, results of a geometrical verification of a machine tool are demonstrated, with values similar to the ones achieved using other methods, such as laser interferometry. Compared to those other methods, the new technique was shown to be less time consuming.