Experimental Validation of the Kinematic Design of 3PRS Compliant Parallel Mechanisms

In this paper, a procedure for the kinematic design of a 3-PRS compliant parallel manipulator of 3 degrees of freedom is proposed. First, under the assumption of small displacements, the solid body kinematics of the 3-PRS has been studied, performing a comprehensive analysis of the inverse and forward kinematic problem, and calculating the rotations that the revolute and spherical flexure joints must perform. Then, after defining some design requirements and therefore the necessary displacements to fulfill, a design process based on the finite element calculations has been stablished, giving the necessary guidelines to reach the optimal solution on a 3-PRS compliant mechanism. Also, a prototype has been tested, using a coordinate measuring machine to verify its dimensions and the resulting displacements in the end effector and the actuated joints. Finally, those measurements have been compared with the FEM and the rigid body kinematics predictions, contrasting the validity of those two modelling approaches for the kinematic design of compliant mechanisms.The authors of this paper wish to acknowledge the financial support received from the Spanish Government via the Ministerio de Educación y Ciencia (Project DPI2011-22955) and Ministerio de Economía y Competitividad (Project DPI2015-64450-R), the ERDF of the European Union, the Government of the Basque Country (Project GIC07/78, IT445-10 and SAIOTEK 2013 SAI13/245, SPC13UN011), and the University of the Basque Country (Project EHUA13/30 and Zabalduz-2012). Thanks are also addressed to Dr. Jorge Presa and Alfonso Urzainki from Egile Corporation XXI for their valuable contributions

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

Traceability of on-machine tool measurement: a review

Nowadays, errors during the manufacturing process of high value components are not acceptable in driving industries such as energy and transportation. Sectors such as aerospace, automotive, shipbuilding, nuclear power, large science facilities or wind power need complex and accurate components that demand close measurements and fast feedback into their manufacturing processes. New measuring technologies are already available in machine tools, including integrated touch probes and fast interface capabilities. They provide the possibility to measure the workpiece in-machine during or after its manufacture, maintaining the original setup of the workpiece and avoiding the manufacturing process from being interrupted to transport the workpiece to a measuring position. However, the traceability of the measurement process on a machine tool is not ensured yet and measurement data is still not fully reliable enough for process control or product validation. The scientific objective is to determine the uncertainty on a machine tool measurement and, therefore, convert it into a machine integrated traceable measuring process. For that purpose, an error budget should consider error sources such as the machine tools, components under measurement and the interactions between both of them. This paper reviews all those uncertainty sources, being mainly focused on those related to the machine tool, either on the process of geometric error assessment of the machine or on the technology employed to probe the measurand

A concept for actuating and controlling a leg of a novel walking parallel kinematic machine tool

The scope of this paper is to present a novel method of actuating the legs of a walking parallel kinematic machine tool (WalkingHex) such that the upper spherical joint can be actively driven while walking and remain a free, passive joint while performing machining operations. Different concepts for the number of Degrees of Freedom (DoF) and methods for actuating the chosen concept are presented, leading to a description of a three-wire actuated spherical joint arrangement. The inverse kinematics for the actuation mechanism is defined and a control methodology that accounts for the redundantly actuated nature of the mechanism is explored. It is demonstrated that a prototype of the system is capable of achieving a motion position accuracy within 5.64% RMS. Utilising the concept presented in this paper, it is possible to develop a walking robot that is capable of manoeuvring into location and performing precision machining or inspection operations

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

Traceable onboard metrology for machine tools and large-scale systems

Esta tesis doctoral persigue la mejora de las funcionalidades de las máquinas herramienta para la fabricación de componentes de alto valor añadido. En concreto, la tesis se centra en mejorar la precisión de las máquinas herramienta en todo su volumen de trabajo y en desarrollar el conocimiento para realizar la medición por coordenadas trazable con este medio productivo. En realidad, la tecnología para realizar mediciones en máquina herramienta ya está disponible, como son los palpadores de contacto y los softwares de medición, sin embargo, hay varios factores que limitan la trazabilidad de la medición realizada en condiciones de taller, que no permiten emplear estas medidas para controlar el proceso de fabricación o validar la pieza en la propia máquina-herramienta, asegurando un proceso de fabricación de cero-defectos. Aquí, se propone el empleo del documento técnico ISO 15530-3 para piezas de tamaño medio. Para las piezas de gran tamaño se presenta una nueva metodología basada en la guía VDI 2617-11, que no está limitada por el empleo de una pieza patrón para caracterizar el error sistemático de la medición por coordenadas en la máquina-herramienta. De esta forma, se propone una calibración previa de la máquina-herramienta mediante una solución de multilateración integrada en máquina, que se traduce en la automatización del proceso de verificación y permite reducir el tiempo y la incertidumbre de medida. En paralelo, con el conocimiento generado en la integración de esta solución en la máquina-herramienta, se propone un nuevo procedimiento para la caracterización de la precisión de apunte del telescopio LSST en todo su rango de trabajo. Este nuevo procedimiento presenta una solución automática e integrada con tecnología láser tracker para aplicaciones de gran tamaño donde la precisión del sistema es un requerimiento clave para su buen funcionamiento.<br /

Contribution à l’amélioration de la précision absolue des robots parallèles

Le but de la présente étude est de contribuer à l’amélioration de la précision absolue des robots parallèles, en ayant recours aux méthodes d’étalonnage géométrique. Ces méthodes consistent à identifier les valeurs des paramètres géométriques du robot, en vue d’améliorer la correspondance entre le robot réel et le modèle mathématique utilisé par son contrôleur. En plus de la compensation des erreurs géométriques, les opérations d’étalonnage proposées permettent d’identifier précisément le référentiel de base de chaque robot étudié. Les méthodes développées sont appliquées à deux robots parallèles à moins de six degrés de liberté (ddl) : une table de positionnement précis à trois ddl (PreXYT) et un robot plan cinqbarres (DexTAR) à deux ddl. Pour le premier robot, l’étalonnage est effectué en utilisant d’abord une méthode d’identification directe. Le deuxième travail destinée à améliorer la précision absolue du PreXYT résulte de la méthode géométrique directe d’étalonnage. En ce qui concerne le robot DexTAR, sa précision est améliorée en utilisant une approche d’autoétalonnage qui exploite les modes de fonctionnement et les modes d’assemblage, pour réduire le nombre de positions d’étalonnage. Cette approche est particulièrement intéressante pour sa simplicité : à chaque position d’étalonnage une sphère de précision est installée en permanence pour servir de cible de mesure. Les positions de ces billes, placées sur une plateforme amovible, n’est mesurée qu’une seule fois, en utilisant une machine de mesure tridimensionnel (MMT). Après la réinstallation de la plateforme sur la base du robot, l’étalonnage peut se faire n’importe quand en n’utilisant que les informations provenant des encodeurs des actionneurs. Les données d’étalonnage et de validation des résultats sont récoltées en utilisant deux appareils mesurant par palpage. Le premier appareil est un bras articulé de mesure de coordonnées, de la compagnie FARO Technologies ; le second est une MMT de la compagnie Mitutoyo. Les incertitudes de mesures de ces machines sont respectivement ±18 μm et ±2,7 μm (niveau de confiance de 95%). Sachant que la qualité de l’étalonnage est inversement proportionnelle aux incertitudes de mesures, l’utilisation d’instruments précis avec des modèles géométriques d’étalonnage quasi-complet nous a permis d’atteindre ces résultats : les erreurs maximales en position et en orientation ont été réduites respectivement à 0,044 mm et 0,009° pour le PreXYT, à l’intérieur d’un cercle de 170 mm de diamètre. Pour le robot DexTAR, l’erreur maximale de position a été réduite à 0,080 mm dans l’ensemble de son espace de travail, soit une zone d’environ 600 mm × 600 mm. Améliorer la précision des robots au-delà de ces valeurs, en utilisant juste les approches géométriques, pourrait s’avérer peu probable. En ce sens, l’ajout de la modélisation et la compensation des erreurs non géométriques serait utile pour obtenir des résultats meilleurs

Noncollocated proprioceptive sensing for lightweight flexible robotic manipulators

This article presents the design of a noncollocated feedback system for flexible serial manipulators. The device is a passive serial chain of encoders and lightweight links, mounted in parallel with the manipulator. This measuring arm effectively decouples the manipulator's proprioception from its actuators by providing information on the actual end effector pose, accounting for both joint and link flexibility. The kinematic redundancy of the measuring chain allows for safe operation in the context of human–robot interaction. A simple yet effective error model is introduced to assess the suitability of the proposed sensor system in the context of robotic control. The practicality of the device is first demonstrated by building a physical joint-encoder assembly and a simplified planar measuring arm prototype. With this additional feedback, a task-space position controller is devised and tested in simulation. Finally, the simulation results are validated with an experimental 3-DoF lightweight manipulator prototype equipped with a five-joint measuring arm