Juan José Aguilar Martín

Design and Mechanical Evaluation of a Capacitive Sensor-Based Indexed Platform for Verification of Portable Coordinate Measuring Instruments

During the last years, the use of Portable Coordinate Measuring Machines (PCMMs) in industry has increased considerably, mostly due to their flexibility for accomplishing in-line measuring tasks as well as their reduced costs and operational advantages as compared to traditional coordinate measuring machines (CMMs). However, their operation has a significant drawback derived from the techniques applied in the verification and optimization procedures of their kinematic parameters. These techniques are based on the capture of data with the measuring instrument from a calibrated gauge object, fixed successively in various positions so that most of the instrument measuring volume is covered, which results in time-consuming, tedious and expensive verification procedures. In this work the mechanical design of an indexed metrology platform (IMP) is presented. The aim of the IMP is to increase the final accuracy and to radically simplify the calibration, identification and verification of geometrical parameter procedures of PCMMs. The IMP allows us to fix the calibrated gauge object and move the measuring instrument in such a way that it is possible to cover most of the instrument working volume, reducing the time and operator fatigue to carry out these types of procedures.

Development of a New Calibration Procedure and Its Experimental Validation Applied to a Human Motion Capture System

Motion capture systems are often used for checking and analyzing human motion in biomechanical applications. It is important, in this context, that the systems provide the best possible accuracy. Among existing capture systems, optical systems are those with the highest accuracy. In this paper, the development of a new calibration procedure for optical human motion capture systems is presented. The performance and effectiveness of that new calibration procedure are also checked by experimental validation. The new calibration procedure consists of two stages. In the first stage, initial estimators of intrinsic and extrinsic parameters are sought. The camera calibration method used in this stage is the one proposed by Tsai. These parameters are determined from the camera characteristics, the spatial position of the camera, and the center of the capture volume. In the second stage, a simultaneous nonlinear optimization of all parameters is performed to identify the optimal values, which minimize the objective function. The objective function, in this case, minimizes two errors. The first error is the distance error between two markers placed in a wand. The second error is the error of position and orientation of the retroreflective markers of a static calibration object. The real co-ordinates of the two objects are calibrated in a co-ordinate measuring machine (CMM). The OrthoBio system is used to validate the new calibration procedure. Results are 90% lower than those from the previous calibration software and broadly comparable with results from a similarly configured Vicon system.Motion capture systems are often used for checking and analyzing human motion in biomechanical applications. It is important, in this context, that the systems provide the best possible accuracy. Among existing capture systems, optical systems are those with the highest accuracy. In this paper, the development of a new calibration procedure for optical human motion capture systems is presented. The performance and effectiveness of that new calibration procedure are also checked by experimental validation. The new calibration procedure consists of two stages. In the first stage, initial estimators of intrinsic and extrinsic parameters are sought. The camera calibration method used in this stage is the one proposed by Tsai. These parameters are determined from the camera characteristics, the spatial position of the camera, and the center of the capture volume. In the second stage, a simultaneous nonlinear optimization of all parameters is performed to identify the optimal values, which minimize the objective function. The objective function, in this case, minimizes two errors. The first error is the distance error between two markers placed in a wand. The second error is the error of position and orientation of the retroreflective markers of a static calibration object. The real co-ordinates of the two objects are calibrated in a co-ordinate measuring machine (CMM). The OrthoBio system is used to validate the new calibration procedure. Results are 90% lower than those from the previous calibration software and broadly comparable with results from a similarly configured Vicon system.

Mechanical design of an indexed metrology platform for verification of portable coordinate measuring machines

The demand of faster and reliable measuring tasks for the control and quality assurance of modern production systems creates new challenges for the field of coordinate metrology. Thus, search for new solutions in coordinate metrology systems and development of existing ones that respond to industry’s demands still persists. An example of such coordinate metrology systems are the portable coordinate measuring machines (PCMMs), which use in industry has considerably increased during the last years, mostly due to their flexibility for accomplishing in-line measuring tasks as well as their reduced costs and operational advantage as compared to traditional coordinate measuring machines (CMMs). In this work, the mechanical design of an indexed metrology platform (IMP) is presented. The aim of the IMP is to increase the final accuracy and radical simplification of the calibration, identification and verification of geometrical parameters procedures of PCMMs.

A Strategy for Geometric Error Characterization in Multi-Axis Machine Tool by Use of a Laser Tracker

This paper aims to present different methods of volumetric verification in long range machine toll with lineal and rotary axes using a commercial laser tracker as measurement system. This method allows characterizing machine tool geometric errors depending on the kinematic of the machine and the work space available during the measurement time. The kinematic of the machine toll is affected by their geometric errors, which are different depending on the number and type of movement axes. The relationship between the various geometrical errors is different from relationship obtained in machine tool whit only lineal axes. Therefore, the identification strategy should be different. In the same way, the kinematic chain of the machine tool determines determines the position of the laser tracker and available space for data capture. This paper presents the kinematic model of several machine tools with different kinematic chains use to improve the machine tool accuracy of each one by volumetric verification. Likewise, the paper thus presents a study of: the adequacy of different nonlinear optimization strategies depending on the type of axis and the usable space available.

Homing Sensor System Design for a 2D Long Range Nanopositioning Moving Platform

The nanotechnology field has been developing strongly in recent years and ultra-precision measuring systems are nowadays required. A new two-dimensional moving platform with 50x50 mm range of travel, nanometer resolution and sub micrometer accuracy is being designed by the authors in order to be integrated with an Atomic Force Microscope (AFM). In this work the definition, design and experimental characterization of a homing sensor system for this 2D moving platform is presented. The homing sensor system will allow the generation of an absolute 2D reference for the platform (X-Y axis and θz rotation), defining an initial cero for the measuring system, which is based on laser encoders.

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.

Teaching Mechanism Calibration to Engineering Students

A new practical application for teaching mechanism calibration to engineering students in senior undergraduate courses is presented in this paper. The modelling, characterization and calibration of kinematic mechanisms is performed. This application presents a special interest in kinematic mechanisms in areas such as industry and research. The application interface allows the students to carry out the different steps of the calibration procedure in an easy way, to change different variables and to analyze results. This application therefore presents an important utility as a teaching tool for the learning process and analysis of the different steps in the mechanism optimization. The application developed allows students: to design their own mechanisms and to analyze the mechanism workspace by changing some design parameters, to test accuracy and repeatability of mechanism components, to introduce the mechanism kinematic model, to evaluate the mechanism performance before calibration, to calibrate the mechanism and, finally, to analyze the improvement in the system accuracy.

Análisis y mejoras en el diseño de un mecanismo cinemático de alta precisión

A new design for reducing the kinematic mechanism error is presented in this paper. Firstly, a prototype having a universal joint is presented and the kinematic model in the Simmechanics environment of Matlab is described, studying the system behaviour by means of simulation. Then, the manufacturing and assembly of the platform is performed and the mechanism performance is evaluated. To do this, different tests are carried out in which the mechanism backlash and the influence of the elevation angle and the misalignment in every axis due to backlash are analyzed. Finally, the mechanism design is improved by replacing the universal joint with a new element that consists on bearings and precision locknuts. This element allows us to obtain the two desired degrees of freedom. The replacement of the commercial universal joint by the developed element implies a considerably increment of the mechanism accuracy. The developed platform has many applications in the industry, such as, machining tool processes, high range metrology, holder base for measurement tools such as topography instruments, mirror orientation in telescopes, etc.

ESTUDIO DEL ÁNGULO DE GIRO DE ELEMENTOS FLEXIBLES APLICADO A MECANISMOS PARALELOS

ABSTRACT This paper analyses the flexible hinge parasitic error in function of the rotation angle in the 0-45o range. First, an analysis of the different geometries -taking into account considerations such as the stiffness and the rotation angle- is carried out, and a new kinematic model of two degrees of freedom parallel mechanism is developed. This mechanism consists of a exible system that allows us to obtain the azimuth and elevation angles. The model developed is based on the Denavit-Hartenberg method. The kinematic chain containing flexible elements is therefore modelled by adding ransformation matrices that consider the angle and translation error of those elements with respect to the initial reference systems. Subsequently, he flexible hinge geometric parameters are determined, by means of a simulation software based on the finite element method in order to obtain a system that allows us to perform the desired rotations. Finally, the element has been experimentally characterized, for two selected materials: aluminium EN AW-7075 and steel F-1430 and the rotation axis error in function of the rotated angle is obtained. This characterization let us observe which material is more suitable in function of the work range, and know for a particular application, the rotation magnitude that can be achieved with the flexible element in function of the desired accuracy.

ANALISIS DEL JUEGO, REPETIBILIDAD DEL POSICIONAMENTO Y PRECARGA DE UN MECANISMO PARALELO.

complejidad existente en el desarrollo de los modelos matematicos de los mecanismos paralelos, es necesario realizar un analisis previo de los errores caracteristicos del sistema. En este trabajo se determina el juego y la repetibilidad de posicionamiento de un mecanismo paralelo de dos grados de libertad giratorios, y se analiza la precarga necesaria para aumentar la precision del sistema. Inicialmente se revisa la bibliografia especializada en el diseno y caracterizacion de mecanismos basados en cinematica paralela. En la mayoria de los estudios analizados, los errores como el juego son absorbidos por el modelo matematico sin caracterizar los componentes ni analizar cuales de los errores del sistema son los mas determinantes. Posteriormente se presenta la metodologia utilizada para la caracterizacion de los componentes del sistema de posicionamiento. Para ello, se describen los procedimientos y los utillajes utilizados para cada equipo de medida y se comparan los diferentes sistemas de medicion utilizados como sensores opticos lineales, interferometro laser y palpadores inductivos. Los resultados obtenidos muestran errores de juego del actuador en tomo a los 45 11m con una desviacion del error de 311m. La aplicacion de una precarga a contraccion sobre la rotula esferica permite obtener un juego menor de 3 i.1m y, de esta manera, mejorar la precision del sistema de posicionamiento. Finalmente se ha caracterizado el mecanismo paralelo de dos grados de libertad, disenado para aplicaciones de metrologia y procesos de maquina-herramienta, utilizando una maquina de medir por coordenadas. La caracterizacion desarrollada del mecanismo permitira determinar los parametros geometricos en la fase de calibracion del mismo, asi como desarrollar, posteriormente, procedimientos de correccion en funcion de la precarga necesaria para aumentar la precision del sistema.