Design and adjustment of weighing cells for vacuum mass comparators
Weighing cells based on compliant mechanisms are the backbone of mass metrology. The mechanical properties of the instruments and their adjustment define the metrological performance. The current work focuses on the design and adjustment of weighing cell mechanisms for a 1 kg vacuum mass comparator application. Three mechanical parameters of the compliant mechanisms define the metrological performance: stiffness, tilt sensitivity and off-center load sensitivity. An entire chapter is devoted to the ultra-thin flexure hinges used in the weighing cell mechanism. It covers their modeling, their manufacturing, and measurement. Starting from the concept level, two weighing cell prototypes were developed, assembled, and tested. Mechanical modeling, ranging from analytical models to finite element models, was used throughout the development. A quasi-independent adjustment of stiffness and tilt sensitivity based on the combination of trim masses was modeled and experimentally verified. A metrological model was used to define the requirements for the robust design of the final weighing cell. It allows the compensation of manufacturing deviations. The implemented adjustment methods were designed to eliminate the mechanical first-order error components of the weighing cell and thus enable a further reduction of measurement uncertainties in the mass comparison process.
Electromagnetic force-compensated weighing cells based on compliant mechanisms are crucial for mass metrology. The metrological performance of these instruments is determined by their mechanical properties, such as stiffness, tilt sensitivity, and off-center load sensitivity. This study aims to further develop the weighing cells utilized in vacuum mass comparators through design and adjustment. The ultra-thin flexure hinges were analyzed in detail as they are the most critical component of the compliant mechanism. The weighing cell's mechanical system was modeled and experimentally investigated using three prototypes. An adjustment method was developed and implemented to enable targeted adjustments under vacuum conditions. The adjustment system was designed to eliminate the first-order mechanical uncertainty contributions of the weighing process, which further reduces the measurement uncertainty for mass comparators.
Wägezellen auf Basis von nachgiebigen Mechanismen sind von entscheidender Bedeutung in der Massenmetrologie. Die Justierung der mechanischen Eigenschaften bestimmen die messtechnische Leistungsfähigkeit der Instrumente. Die vorliegende Dissertation leistet einen spezifischen Beitrag zu deren weiteren Steigerung. Die Konstruktion und Justierung von nachgiebigen Mechanismen für elektromagnetisch kraftkompensierte Wägezellen in Vakuum-Massekomparatoren werden behandelt. Wichtigster Bestandteil dieser Mechanismen sind ultradünne Festkörpergelenke, deren Modellierung, Fertigung und Messung betrachtet werden. Das mechanische Gesamtsystem wird hinsichtlich der mechanischen Eigenschaften Steifigkeit, Neigungsempfindlichkeit und Ecklastempfindlichkeit modellbasiert und ausgehend von drei Wägezellen-Prototypen experimentell untersucht. Die entwickelte Justiermethode und deren Umsetzung in Justiereinrichtungen erlauben eine zielgerichtete Justierung unter Vakuumbedingungen. Sie sind darauf ausgelegt, die mechanischen Unsicherheitsbeiträge erster Ordnung der Wägezelle zu eliminieren und ermöglichen so eine weitere Verringerung der Messunsicherheit für Massekomparatoren.
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