François Hennebelle

Methodology for the assessment of measuring uncertainties of articulated arm coordinate measuring machines

The Articulated Arm Coordinate Measuring Machines (AACMM) have gradually evolved and are increasingly used in mechanic industry. At present, measurement uncertainties relating to the use of these devices are not yet well-quantified. The work carried out consists on determining the measurement uncertainties of a mechanical part by an Articulated Arm Coordinate Measuring Machine. The studies aiming to develop a model of measurement uncertainties are based on the Monte Carlo method developed in Supplement 1 of the Guide to Expression of Uncertainty in Measurement [1] but also identifying and characterizing the main sources of uncertainty. A Multi-level Monte Carlo approach principle has been developed which allows characterizing the possible evolution of the Articulated Arm CMM during the measurement and quantifying in a second level the uncertainty on the considered measurand. The first Monte Carlo level is the most complex and is thus divided into 3 sub-levels, namely characterization on positioning error of a point, estimation of calibration errors and evaluation of fluctuations of the “localization point”. The global method is thus presented and results of the first sub-level are particularly developed. The main sources of uncertainty, including AACMM deformations are exposed.

Optimization of the straightness measurements on rough surfaces by Monte Carlo simulation

The straightness error of a coordinate measuring machine (CMM) is determined by measuring a rule standard. Thanks to a reversal technique, the straightness uncertainty of the CMM is theoretically dissociated from the straightness uncertainty of the rule. However, stochastic variations of the whole measurement system involve uncertainties of the CMM straightness error. To quantify these uncertainties, different sources of stochastic variations are listed with their associated probability density functions. Then Monte Carlo methods are performed first to quantify error and secondly to optimize measurement protocol. It is shown that a 5-measurement distance from 0.1 mm to each measurement coordinate allows a double reduction of uncertainties, principally due to the rule roughness amplitude (R(a) = 0.35 µm) and because this optimal distance of 0.1 mm is equal to the autocorrelation length of the rule roughness. With this optimal configuration, the final uncertainly on the straightness error of the CMM studied is less than 1 µm on the whole evaluated length of the rule (1 m). An algorithm, including Probe Tip Radius of the CMM and surface roughness of the piece, is finally proposed to increase CMM reliability by minimizing error measurements due to surface roughness of the measured piece.

Common Features on Damage Mechanisms Identified on Various Metal and Ceramic on Polyethylene Articulating Surfaces of Total Hip Prostheses

This paper presents a methodology for assessing the in-vivo degradation mechanisms of articular components of total hip replacement (THR) prostheses of Charnley type. The experimental procedure revealed that common features can be observed even if the clinical cases under investigation were quite different with regard to the demographic data. It particularly emphasises the detrimental effects of foreign bodies on the damage of the articulating surfaces. These foreign bodies can migrate into the joint space before embedding definitely into the Ultra High Molecular Weight Polyethylene (UHMWPE) acetabular cup surface where they further participate to a third body wear mechanism accelerated by a progressive increase of the femoral head roughness. Our experimental results underline, from a practical point of view, the need for careful manufacturing and clinical handling of porous surfaces, advocate for a systematic assessment of retrieved components, particularly when changed because of unexplained wear, and make questionable the clinical use of multifilament trochanteric cables.