Design of an enhanced mechanism for a new Kibble balance directly traceable to the quantum SI

Abstract

The “Quantum Electro-Mechanical Metrology Suite” (QEMMS) is being designed and built at the National Institute of Standards and Technology. It includes a Kibble balance, a graphene quantum Hall resistance array and a Josephson voltage system, so that it is a new primary standard for the unit of mass, the kilogram, directly traceable to the International System of Units (SI) based on quantum constants. We are targeting a measurement range of 10 g to 200 g and optimize the design for a relative combined uncertainty of $2\times 10^{-8}$ 2 × 10 − 8 for masses of 100 g. QEMMS will be developed as an open hardware and software design. In this article, we focus on the design of an enhanced moving and weighing mechanism for the QEMMS based on flexure pivots.

Preliminary characterization of anelastic effects in the flexure mechanism for a new Kibble balance at NIST

“…The new balance kinematics [15] is based on two submechanisms as shown in figure 3. One is to ensure the tare functionality in weighing mode-a truncated wheel is used here-and the other one is to realize a dedicated guiding feature-a folded parallelogram linkage is used to provide an in principle linear motion.…”

“…This disturbance is not static and usually the maximum amplitude is < ±20 µm, which yields θ 0 < ±0.1 mrad for the present mechanism. And (2) the velocity mode requires large excursions where the flexures in the new mechanism need to pivot about θ 0 = ±7 • to ensure sufficient travel for precise information about the magnetic profile [15]. This induces large amounts of stress around a mean position/stress in an oscillatory manner and the phase-delay of the anelastic response is expected to yield a characteristic, anelastic drift.…”

“…The balance mechanism designed in [15] was machined, assembled, and aligned so that a movement is free of buckling, and the output stages move aligned with the vertical to within better than ±10 µm for displacements of the final stage of approximately ±25 mm. The apparatus, see figure 4, is housed in a vacuum chamber in a laboratory 12 m underground and the vacuum pressure is <0.001 Pa.…”

Flexures for Kibble balances: minimizing the effects of anelastic relaxation

“…Hence, it is not surprising that there is renewed interest in this topic. Researchers at the Physikalische Technische Bundesanstalt [14], NPL [15], and NIST [16] are actively working on such systems, and the BIPM Kibble balance [17] is switching to a single flexure mechanism for moving and weighing, as well. Hence, it is time to revisit the anelastic relaxation of flexures and the applicability and limitations in this use case.…”

“…In the following three sections, we present fundamental investigations into the anelasticity of the flexure mechanism for the new Kibble balance at NIST [16]. The theoretical work can be seen as the relevant extension of existing models of anelasticity at low frequencies to describe the modes of operation in the Kibble balance.…”