Coil-current effect in Kibble balances: analysis, measurement, and optimization

Li, Shisong; Bielsa, Franck; Stöck, M; Kiss, A.; Fang, Hao

doi:10.1088/1681-7575/aa9a8e

Cited by 30 publications

(47 citation statements)

References 19 publications

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“…Tolerances in the machining process will have less effect on the field uniformity for a wide air gap. Scientifically, several systematic effects, e.g., the current effect [16] of the coil on the result, are smaller for magnets with wider air gaps. A possible solution to increase the trajectory length of the coil sweep in the velocity phase in a uniform field, is to increase the total height of the air gap (and, hence, the magnet).…”

Section: Profile Flatness Dependence On the Fringe Fieldmentioning

confidence: 99%

“…The current in the coil during weighing mode slopes the magnetic profile. This is caused by the change in the coil inductance energy change at different vertical positions [16]. This effect yields a bias in force mode when the vertical coil position is different for the two measurements in the weighing phase, mass-on and massoff.…”

Section: Change Of the Coil-current Effectmentioning

confidence: 99%

“…The size of the bias is proportional to the slope k in the magnetic profile slope produced by the coil current. The slope depends on the air gap size (δ, d) [16]. Since the suggested modification of the inner yoke alters the air gap (at least at the top and bottom of the air gap), it is prudent to check the magnitude of the coil current effect.…”

Section: Change Of the Coil-current Effectmentioning

confidence: 99%

“…Interestingly, the change in profile is more linear over larger scales after the compensation is applied. For a Kibble balance operating with a single mode [10], [16], better linearity of the measurement trajectory simplifies the velocity profile fit in the data processing.…”

Section: Change Of the Coil-current Effectmentioning

confidence: 99%

“…(3) It is harder to magnetically center a coil in a narrow gap to avoid magnetic torques on the coil. (4) Systematic effects, for example, the coil-current effect are large [16]. The aspect ratio can also be changed by making the magnet system taller.…”

Section: Introductionmentioning

confidence: 99%

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A Simple Improvement for Permanent Magnet Systems for Kibble Balances: More Flat Field at Almost No Cost

Schlamminger

Wang

2020

IEEE Trans. Instrum. Meas.

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Permanent magnets together with yokes to concentrate the magnetic flux into a cylindrical airgap are widely employed in Kibble balances. These experiments require a uniform magnetic flux density along a vertical path, typically a substantial fraction of the length of the air-gap. Fringe fields that are present at both ends of the air-gap limit the region where the flux density does not change more than a certain relative fraction (here: 5 × 10 −4 ) of the flux density in the center of the magnet system. By simply adding an iron ring with a rectangular cross-section to the inner yoke at each end of the air gap, the effects of the fringe fields can be counteracted, and, hence, the length of the region, where the flux density remains within a given tolerance band is increased. Compared to the alternative, employing a taller magnet, the proposed method yields a magnet system with an extended region of a uniform field without significantly increasing the mass of the magnet system. Potential applications include compact and table-top Kibble balances. We investigate possible adverse effects on the performance of the magnet system caused by the additional rings: magnetic field strength, coil-current effect, and a dependence of the radial field on the radial position in the field. No substantial disadvantage was found. Instead, the method presented here outperformed previously suggested methods to improve the radial dependence of the radial field, e.g., shorter outer yoke. In summary, adding rings to the inner yoke improves the uniformity of the field without a detrimental effect to function, cost, and form factor of the magnet system.

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Section: Profile Flatness Dependence On the Fringe Fieldmentioning

confidence: 99%

Section: Change Of the Coil-current Effectmentioning

confidence: 99%

Section: Change Of the Coil-current Effectmentioning

confidence: 99%

Section: Change Of the Coil-current Effectmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

A Simple Improvement for Permanent Magnet Systems for Kibble Balances: More Flat Field at Almost No Cost

Schlamminger

Wang

2020

IEEE Trans. Instrum. Meas.

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The Kibble balance and the kilogram

Schlamminger¹,

Haddad²

2019

Comptes Rendus. Physique

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Resolution of the paradox of the diamagnetic effect on the Kibble coil

Schlamminger

Marangoni

et al. 2021

Sci Rep

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Employing very simple electro-mechanical principles known from classical physics, the Kibble balance establishes a very precise and absolute link between quantum electrical standards and macroscopic mass or force measurements. The success of the Kibble balance, in both determining fundamental constants (h, $$N_A$$ N A , e) and realizing a quasi-quantum mass in the 2019 newly revised International System of Units, relies on the perfection of Maxwell’s equations and the symmetry they describe between Lorentz’s force and Faraday’s induction, a principle and a symmetry stunningly demonstrated in the weighing and velocity modes of Kibble balances to within $$1\times 10^{-8}$$ 1 × 10 - 8 , with nothing but imperfect wires and magnets. However, recent advances in the understanding of the current effect in Kibble balances reveal a troubling paradox. A diamagnetic effect, a force that does not cancel between mass-on and mass-off measurement, is challenging balance maker’s assumptions of symmetry at levels that are almost two orders of magnitude larger than the reported uncertainties. The diamagnetic effect, if it exists, shows up in weighing mode without a readily apparent reciprocal effect in the velocity mode, begging questions about systematic errors at the very foundation of the new measurement system. The hypothetical force is caused by the coil current changing the magnetic field, producing an unaccounted force that is systematically modulated with the weighing current. Here we show that this diamagnetic force exists, but the additional force does not change the equivalence between weighing and velocity measurements. We reveal the unexpected way that symmetry is preserved and show that for typical materials and geometries the total relative effect on the measurement is $$\approx 1\times 10^{-9}$$ ≈ 1 × 10 - 9 .

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Coil-current effect in Kibble balances: analysis, measurement, and optimization

Cited by 30 publications

References 19 publications

A Simple Improvement for Permanent Magnet Systems for Kibble Balances: More Flat Field at Almost No Cost

A Simple Improvement for Permanent Magnet Systems for Kibble Balances: More Flat Field at Almost No Cost

The Kibble balance and the kilogram

Resolution of the paradox of the diamagnetic effect on the Kibble coil

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