Misurare per decidere. Misure e statistica di base.

Nel campo tecnico-scientifico molte decisioni sono supportate da misurazioni. Ma per poter decidere correttamente è importante assegnare ai risultati di misura il loro effettivo significato. Ciò è soprattutto importante, ed espressamente richiesto, quando si opera in Sistemi Qualità. In tal caso la gestione delle misure e prove deve essere rigorosa, e può trovare un concreto supporto negli argomenti qui trattati, per l’attenzione posta a curare insieme la correttezza sostanziale e l’eliminazione di vincoli inutili. Giulio Barbato, Alessandro Germak e Gianfranco Genta sono docenti di “Statistica sperimentale e Misure Meccaniche” ed “Experimental Statistics and Mechanical Measurement” presso il Politecnico di Torino. Giulio Barbato, professore ordinario di Misure Meccaniche e Termiche presso il Politecnico di Torino, ha lavorato per oltre vent’anni presso l’Istituto di Metrologia “G. Colonnetti” del C.N.R. (ora confluito a formare l’INRiM) ove si è occupato sia dei campioni primari nazionali di forza e durezza, sia degli accreditamenti dei Centri di taratura SIT (ora LAT-ACCREDIA). Dal 1997 è titolare di corsi di Misure Meccaniche e Statistica Applicata alla Sperimentazione. Alessandro Germak, primo tecnologo all’Istituto Nazionale di Ricerca Metrologica dove svolge attività di ricerca da oltre trent’anni, è responsabile dei campioni primari di forza e durezza e dei metodi primari per la misura dell’accelerazione di gravità locale. È esperto tecnico per gli accreditamenti dei Centri di taratura LAT-ACCREDIA ed è membro dei Comitati Consultivi del CIPM e dei comitati tecnici EURAMET per le grandezze di interesse. Gianfranco Genta è ricercatore in “Tecnologie e Sistemi di Lavorazione” presso il Dipartimento di Ingegneria Gestionale e della Produzione del Politecnico di Torino, dove ha conseguito nel 2010 il titolo dottore di ricerca in “Metrologia: Scienza e Tecnica delle Misure”. Si occupa, principalmente, di metrologia industriale, ingegneria della qualità e applicazione di metodi statistici in ambito tecnologico

Traceability of gravity acceleration measurement in calibration laboratories

Primary measurements of force, torque and pressureare directly influenced by local gravityacceleration,g. At present, gmeasurementin calibration laboratoriesisevaluated through absolute or relative measurementsor through theoretical/empiricalrelations. Except forsome absolute measurements, other methods are not traceable. As consequence, measurementsofthesequantities could behighly impacted by this lackin terms of accuracy and uncertainty.The Consultative Committee for Mass and related quantities (CCM) of CIPM and International Association of Geodesy (IAG) arecurrently working ona resolutionstrategy to ensure traceability to the SI for gravity measurementsthrough the establishment of a global absolute gravity reference system. Thisnetwork could becomea useful tool to transferthe gmeasurement to calibration laboratories at international level. However, as well any other measurements, thispossible solution should be established under the Quality Management system of accredited laboratories

Correction of squareness measurements of Vickers indenters due to the tilt of the pyramid axis

In Vickers hardness measurements, ISO 6507-2 and 6507-3 Standards require to verify that the quadrilateral of the pyramid indenter base has angles of 90° ± 0.2°. Such measurement is usuallyperformed through optical measuring systems, which, rotating the diamond indenter, allowsto evaluate the angles between two consecutive faces with high accuracy. These angles correspond to the angles of the quadrilateral base when the axis of the pyramidis perfectly perpendicularto the seating surface. Nevertheless, when the pyramid axis is tilted, the angles between two consecutive faces are different from the corresponding angles on the quadrilateral base, thus a correction is required. In this work, a method to correct squareness measurements, based on a geometrical model, is presented

Effects due to the misalignment of build-up systems for force measurements in the Meganewton range

Calibration of force transducers in the Meganewton range is typically performed by comparison with reference build-up systems (BUS) under hydraulic presses for high loads. The centring of a BUS isa difficultoperation due to itsweight and dimension, andpossible misalignmentsand the resultingeffects are usually neglected.In this work the effect on force measurements due toa 3 mmmisalignment of a 3 MN BUS wasevaluated.Measurements were performed at INRiM and at LNE in hydraulic presses.Itis shown that the relative measurement errorsdue to misalignment were lowerthan the declared CMC uncertainty,thus the shiftof the BUSdid not influence the measurements

Multicomponent force transducer calibration procedure using tilted plates

The calibration of a multicomponent force transducers (MFTs)represents a challenge in the meganewtonrange. In fact,the generation of transversal forces and moments iscomplex since a force standard machine (FSM) is only able to apply an uniaxial force. Furthermore since MFTs are composed of multi-transducers, each one dedicated to a particular component, correlations between force and moment componentsare possible. Therefore, acalibration systemthat could simultaneously generate all force/moment components and could be suitableinevery FSMis needed. For this purpose, a coupleof tilted plates was designed. Calibration measurements were performed on a 2 MN MFT at INRiM, LNE and PTB. Exploitation matrixes and performance indicators showed good results, unless small but not negligible correlations between MFT outputs. In particularsome spurious valuesdue to the uncertainty inthe vertical force application pointinfluencedthe moment components

Design and metrological evaluation of the new 5 MN hexapod-shaped multicomponent build-up system

The new 5 MN hexapod-shaped multicomponent build-up system (HSM-BUS) represents significant progress in the field of reference transducers in the high force range. As with any build-up system, the presented hexapod-shaped multicomponent force transducer can lead, not only to measure forces 5 times higher than the capacity of a each single uniaxial force transducer (UFT), but gives also information about the other components of the force vector and of the moment vector. Furthermore, the calibration of such types of multicomponent force transducer regards only the calibration of the signal outputs coming from each UFT and the calibration of the geometry of the system. In this work, an a priori evaluation of the expected uncertainty is performed. As a first approximation, the effects of the calibration uncertainties of UFTs and of the geometrical tolerances given on the construction drawing were considered. Subsequently, with a finite element simulation of the mechanical behavior of the 5 MN HSM-BUS under load, a mathematical model of elastic deformations has been evaluated and applied for evaluating and correcting the systematic errors due to the deformation of the geometry under load

Final report on the force key comparison CCM.F-K3

In the Force Key Comparison CCM.F-K3 the measurand force was compared at the two force steps 500 kN and 1 MN. 12 laboratories participated in this comparison which was organised by PTB as the pilot laboratory in two laboratory groups (group A and B). In group A, the comparison was carried out with two 1 MN compression force transducers at the two force steps 500 kN and 1 MN (CCM.F-K3a) and with 6 participating laboratories. In group B, the comparison was carried out with two 500 kN compression force transducers at one force step of 500 kN (CCM.F-K3b) and with 9 participating laboratories. The key comparison reference values were determined as the weighted mean of all results for the two force steps and set to 500 kN and 1 MN, respectively, with the associated uncertainties. The degrees of equivalence were determined for all 12 laboratories for 500 kN compression force and for 6 laboratories for 1 MN compression force

Final report on the force key comparison CCM.F-K3

In the Force Key Comparison CCM.F-K3 the measurand force was compared at the two force steps 500 kN and 1 MN. 12 laboratories participated in this comparison which was organised by PTB as the pilot laboratory in two laboratory groups (group A and B). In group A, the comparison was carried out with two 1 MN compression force transducers at the two force steps 500 kN and 1 MN (CCM.F-K3a) and with 6 participating laboratories. In group B, the comparison was carried out with two 500 kN compression force transducers at one force step of 500 kN (CCM.F-K3b) and with 9 participating laboratories. The key comparison reference values were determined as the weighted mean of all results for the two force steps and set to 500 kN and 1 MN, respectively, with the associated uncertainties. The degrees of equivalence were determined for all 12 laboratories for 500 kN compression force and for 6 laboratories for 1 MN compression force. KEY WORDS FOR SEARCH Force Key Comparison, high force, 500 kN, 1 MN Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA)