A miniature 3D-printed Kibble balance for mass sensing applications

Abstract

Mass measurements are an essential part of every day life. Many transactions and tasks from paying a fair price for groceries to receiving a safe and effective dose of medicine rely upon accurate and traceable mass metrology. The redefinition of the International System of Units (SI) kilogram in terms of the Planck constant (h) in 2019 encouraged the implementation of directly traceable mass measurement at the point of need. One of the routes to traceable mass measurement is the Kibble balance principle which compares a mechanical force with an electromagnetic force. The updated mass definition presented an opportunity for improved measurement uncertainty at scales of less than a kilogram and also the introduction of traceable mass measurement at scales of less than a milligram. An analytical feasibility study of the electromagnetic Kibble principle suggested that it may be possible to achieve competitive mass measurement uncertainties at scales of 1mg and extend the traceable range to 1 μg with an uncertainty of 0.5%. Three-dimensional (3D) printing is an emerging field in the development and fabrication of micro-electromechanical system (MEMS) devices. There are a number of benefits to 3D printing including the ability to rapidly produce prototype designs and the creation of fully 3D structures. The main aim of this work was to create a 3D printed prototype Kibble balance for masses between 1 g and 10 g with a measurement uncertainty of 0.25% (at k=1) or less as a proof-of-concept device for miniaturisation to MEMS scale. Two options for generation of a 0.1 T radial magnetic field, anti-Helmholtz coils and opposing permanent magnets, were investigated through computer simulated models and laboratory experiment. The results showed that permanent magnetic material was a viable solution. A “low cost” prototype system at the gram-level was designed based upon a single ring magnet with co-wound tare and bifilar main coils. Bespoke coils and 3D printed mechanical parts were manufactured and combined with commercially available components and National Physical Laboratory (NPL) produced hardware and software from the existing NPL Demonstration Kibble balance. This design was presented and published at the Joint International Measurement Confederation (IMEKO) TC3|TC5|TC16|TC22 Conference in 2022 [1]. The characteristics of the prototype system were measured and compared with the design estimates during the setup phase. Many of the key subsystems were calibrated individually to provide traceable measurements and estimates of uncertainty. The initial mass measurement performance of the prototype was determined using artifacts in the range 3.5 g to 6.0 g. The calculated uncertainty on the mass results was 0.2% (at k = 1). These results were compared with a traditional mass calibration of the artifacts performed at NPL to International Organization for Standardization (ISO) 17025 standard. This comparison revealed a +3.6% systematic error in the prototype system. Once identified, the source of this error can be eliminated to allow the device to be scaled to the micro-gram level within a target uncertainty of 0.5% (at k=1).

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Identifiers

ORCID for Harold Chong: orcid.org/0000-0002-7110-5761

ORCID for Ruomeng Huang: orcid.org/0000-0003-1185-635X

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