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Metrological performance verification of optical Coordinate Measuring Systems
Full text linkAt the state-of-the-art, the use of Coordinate Measuring Systems (CMS) for verification of geometrical and dimensional tolerances is becoming very wide-spread in industrial manufacturing. Traditional CMMs are generally equipped with mechanical probes, but recent trends are highlighting several limitations of tactile probes, due mainly to the increasing requirement of faster inspection and higher complexity in measurement tasks. In particular, due to the miniaturization of components and the employment of new delicate materials, the use of optical measurement systems is becoming more and more suited for industrial applications. Nevertheless, only a very small percentage of the potential applications for non contact measurement systems is established so far. The main obstacles to a large integration of optical sensors on CMMs can be found in the lack of international accepted specification and verification rules. International standards on performance verification of optical systems are still missing and the existing ones related to mechanical CMMs cannot be applied directly to non contact machines.
For these reasons, the main objective of the present work has been to contribute to the development of methods and artefacts for performance verification of non-contact measuring systems.
The thesis is composed of three main parts.
The first part deals with a study of the state-of-the-art in non contact Coordinate Metrology, including examples of measurements and test on own machines. After a first introductive Chapter related to the productive role of metrology in manufacturing processes, the second Chapter focuses on actual industrial requirements for quality assurance and related non-contact instruments review and classification.
The second part is committed to traceability of non contact Coordinate Systems, including experimental investigations and results on different optical systems. In particular the third Chapter is dedicated to methods, standards and guideline for performance verifications and traceability of non-contact CMS, while the fourth Chapter describes activities related to the development and testing of cooperative calibration artefacts.
The third part is finally dedicated to industrial applications. The newly developed cooperative artefacts, in particular, have been applied within the European Cooperative Research Project OP3MET. During the Project, an innovative optical measuring system for automated inspection of dimensional and geometrical tolerances, including free-form surfaces, has been developed. The contribution of the author has concerned metrological verification and traceability of the new developed system. Particular attention has been paid to the application of the guideline VDI/VDE 2617-6.2: 2005. On the basis of specific experimental results on a laser scanner, the main problems arising in the implementation of testing procedures were analyzed. The second industrial case reported in this work has been related to the integration of a chromatic sensor into a high precision circular grinding machine. The author of this Thesis participated to the integration-project working directly to the development of a software module for in-line measurement of roundness and automatic correction of systematic errors of the measurement system. After a first phase of modelling and simulation of the measuring process, the developed module has been validated by comparison with results obtained with dedicated roundness equipment and metrological software.
In the last Chapter of the present work, the main results from an industrial inter-laboratory comparison for Coordinate Measuring Machines equipped with optical sensors are presented. The comparison, named VideoAUDIT, has been organized and coordinated by the Laboratory of Industrial and Geometrical Metrology of the University of Padova, involving a total of 21 CMMs in Italy and other European countries. As one of the most important result, the comparison has proved that the quality of dimensional measurement results on real industrial workpieces is largely independent on the CMM length measurement performance, as well as the limited ability of most participants to properly evaluate task-specific measurement uncertainty.Allo stato dell’arte, l’uso di sistemi di misura a coordinate (CMS) per la verifica di tolleranze geometriche e dimensionali risulta essere sempre più diffuso in ambito manifatturiero. Tuttavia, l’esigenza relativa alla riduzione dei tempi di controllo, unita ad una maggiore complessità nel task di misura, sta mettendo in luce i limiti dei tradizionali sistemi di misura a contatto. In particolare la miniaturizzazione dei componenti e l’utilizzo di nuovi materiali facilmente danneggiabili rende l’impiego dei sistemi ottici sempre più indicato nell’ambiente produttivo industriale. Tuttavia, alcuni problemi permangono ancora a limitare la diffusione di strumenti di misura ottici per il controllo geometrico e dimensionale. Se da un lato, infatti, sono numerosi i vantaggi che essi presentano rispetto agli strumenti a contatto, dall’altro una maggiore sensibilità a fonti di errore addizionali e un panorama normativo carente rendono difficoltoso l’impiego di questi strumenti. In particolare, la mancanza di metodi standardizzati per la verifica delle prestazioni metrologiche e per la riferibilità delle misure impediscono il confronto con i risultati ottenuti mediante sistemi a contatto o tra sistemi ottici basati su principi di acquisizione diversi.
Il presente lavoro di Tesi ha avuto come obiettivo principale quello di contribuire allo sviluppo di metodi e campioni per la verifica di prestazioni di sistemi ottici, mediante lo studio accurato dell’attuale impiego in ambito manufatturiero e attraverso l’applicazione dei criteri proposti in casi di interesse industriale.
In particolare il presente elaborato risulta essere composto da tre parti.
La prima parte contiene lo studio dello stato dell’arte relativo alla Metrologia a Coordinate non a contatto, con particolare riferimento ai requisiti in ambito industriale, alla descrizione e alla classificazione dei principali strumenti ottici utilizzabili.
La seconda parte del lavoro di Tesi risulta essere invece interamente dedicata alla verifica di prestazioni dei sistemi non a contatto, comprendendo test e risultati ottenuti su diversi sistemi di misura. In dettaglio, dopo una descrizione di metodi, norme e linee guida relativi a criteri di accettazione e verifica di sistemi a coordinate ottici, particolare attenzione viene dedicata allo sviluppo di campioni di taratura.
I nuovi campioni sviluppati sono stati utilizzati dall’autore all’interno del Progetto di Ricerca OP3MET, il primo di tre casi aventi ricaduta industriale riportati nella terza parte del lavoro di tesi. All’interno del Progetto OP3MET, avente per obiettivo principale lo sviluppo di un nuovo sistema di misura 3D mediante scansione laser, l’attività dell’autore ha riguardato prevalentemente lo studio della riferibilità delle misure ottenute, l’applicazione di metodi per la verifica di prestazioni, il calcolo dell’incertezza di misura e lo sviluppo di test specifici per le verifica del nuovo software metrologico sviluppato.
Il secondo caso industriale affrontato durante il lavoro di Tesi ha riguardato l’integrazione di un sensore di misura cromatico in una macchina utensile per rettifica circolare. L’attività svolta dall’autore ha avuto come obiettivo lo sviluppo di un modulo software per la misura di rotondità in linea, in grado di effettuare la correzione automatica dei principali errori sistematici del sistema di misura stesso. Dopo una prima fase di modellazione e simulazione del processo di misura, il modulo software sviluppato è stato validato mediante il confronto con strumenti e software dedicati
Nell’ultimo capitolo del lavoro di tesi vengono riportati i risultati principali del Progetto VideoAUDIT, un confronto inter-aziendale tra macchine di misura a coordinate con sensori ottici ideato e coordinato dal Laboratorio di Metrologia Geometrica ed Industriale dell’Università di Padova. Dal confronto, comprendente 21 CMM in Italia e altri paesi europei, è emerso in particolare come la qualità delle misure effettuate su comuni componenti industriali sia alquanto indipendente dalle prestazioni metrologiche del sistema, così come sussistano evidenti problemi da parte degli utilizzatori nel valutare propriamente l’incertezza di misura
Il confronto interaziendale per CMM con sensori ottici 'VideoAUDIT' – risultati generali
No full textComparing optical coordinate measuring machines reveals the need for standardization
No full textOptical dimensional measuring techniques are increasingly used for quality control across many industries due to their high speed and other inherent advantages of contactless techniques. An industrial assessment of 21 devices verifies their actual, 3D metrological performance
An industrial comparison of coordinate measuring systems equipped with optical sensors: the VideoAUDIT Project
No full textThe main results of an industrial inter-laboratory comparison for CMMs equipped with optical sensors are presented in this paper. The comparison, named VideoAUDIT, was organized and coordinated by the Laboratory of Industrial and Geometrical Metrology - University of Padova - and carried out in Italy and Switzerland from August 2007 to June 2008. A total of 16 CMMs from different companies participated in the Project, using different kinds of optical sensors. The participants were asked to measure a set of calibrated artefacts, following detailed procedures. The Audit items have been chosen with the following criteria: (1) objects that can be measured with different types of optical sensors and (2) including both reference artefacts for performance verification and common industrial products. Special attention has been paid to the design of the comparison in order to respect the proficiency testing rules; in particular, the long term stability of the audit items was checked during the comparison as a main requirement. An important task of the comparison was to test the ability of the participants to determine the uncertainty of their measurements
Quantification of uncertainty contributors in coordinate measurements using video probes
No full textThis paper presents the results of an experimental investigation on the effects of imaging parameters in coordinate measurements using video probes. In particular, the results show the influence of illumination, objective magnification, measuring window size, use of autofocus and image filtering. The investigation is performed with a video-CMM on two artefacts commonly used for performance verification of optical CMMs: a linear glass scale and an optomechanical hole plate
International Video Audit: Metrological performance of optical coordinate measuring machines under industrial conditions
No full textIndustrial Comparison of Optical Coordinate Measuring Systems
No full textIn this paper the main results from the industrial comparison "VideoAUDIT" are presented and discussed. The comparison, designed and coordinated by the Laboratory of Industrial and Geometrical Metrology of the University of Padova, involved 21 optical CMMs from various companies in Europe (Italy, Switzerland, Spain and Denmark). The comparison was run between August 2007 and January 2009. The participants were given the opportunity to verify the metrological performances of their CMMs with suited artefacts and procedures. Based on the comparison results, the actual quality of industrial optical measurements was evaluated
Launch of an industrial inter-comparison for Coordinate Measuring Systems with optical sensors
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