The National Physical Laboratory (NPL) provides realisation of measurement units for the UK, including the metre which is the SI unit of length. To disseminate the realisation of the metre to users across society, a suite of bespoke instruments is used to perform dimensional calibration of various forms of length artefacts. These instruments are approaching end-of-life and a new generation of equipment is required. The uncertainty of measurement associated with the calibrations performed at NPL acts as the limit of achievable uncertainty for all length measurements which claim traceability from NPL. The enabling of further advances in science and industry depends on better measurement capability, so the new generation of calibration instrumentation must surpass the previous in terms of measurement uncertainty. This makes the measurement uncertainty the driving design requirement for such new instruments. The aim of this research is to develop the methodology that will enable the design of any new low measurement uncertainty instrumentation, allowing uncertainty performance requirements to be met in the most efficient or cost effective way. The main research objectives include: developing an uncertainty led design process for metrological instrumentation; applying this process to the design of a new one-dimensional (1D) calibration instrument and demonstrating how it can be used to drive both design decisions and innovation including the creation of novel strategies to reduce uncertainties associated with tactile probing of artefacts; and of a new approach to the automatic alignment of measurement artefacts. After reviewing existing design methodologies and the requirements for the next generation of calibration instrument, it was determined that a means of developing technical specifications based on measurement uncertainty contributions would bring significant benefit to the project. No such approach was found in literature, so a new methodology, termed Design for Measurement Uncertainty, was created. In a case study of the use of this methodology, a technology agnostic uncertainty model (TAUM) of a generic length measurement system was developed. This general model was then specialised towards the particular requirements of the 1D instrument by identifying high level technology choices dictated by other requirements. For example, the requirement for primary traceability to the definition of the SI metre, coupled with the operating range of the instrument constrained the selection of the main length measurement technology to a laser interferometer. The specialised model was then used to carry out sub-system selection by populating the model with the uncertainty contributions of the different options and finding combinations of systems that achieved the target uncertainty. These combinations could be further down-selected by consideration of other parameters, such as cost, complexity of build/procurement or ease of integration to select a final overall system design. The individual sub-systems selected are then discussed with reference to why those options were selected. A detailed discussion of the probing system (a critical part of the design) is included which shows how using two carriages and a novel probing strategy can remove the need to determine the contact probe effective diameter for all measurements. The proposed novel approach for aligning the artefacts under measurement is detailed. Further, an explanation of how the design uncertainty model can be developed into a real-time uncertainty evaluation tool for the final system is presented. The novel Design for Measurement Uncertainty process followed has both aided the selection of sub-systems for the new one-dimensional calibration instrument and enabled the identification of new ways to reduce measurement uncertainties, for example those associated with probing. In the future it will also form the basis of a "live" measurement-specific uncertainty evaluation method reducing uncertainties for the majority of the time where uncertainty contributions are not at the extremes of their potential ranges.
| Date of Award | 14 May 2024 |
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| Original language | English |
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| Sponsors | National Physical Laboratory |
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| Supervisor | Andrew Longstaff (Main Supervisor) & Simon Fletcher (Co-Supervisor) |
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