Design and Initial Test of the Weighing Unit for Tsinghua Tabletop Kibble Balance

Lisha Peng; Lushuai Qian; Nanjia Li; Shisong Li; Songling Huang; Weibo Liu; Wei Zhao

arxiv: 2605.19203 · v1 · pith:N5WKSWWFnew · submitted 2026-05-19 · ⚛️ physics.ins-det

Design and Initial Test of the Weighing Unit for Tsinghua Tabletop Kibble Balance

Weibo Liu , Nanjia Li , Lushuai Qian , Wei Zhao , Lisha Peng , Songling Huang , Shisong Li This is my paper

Pith reviewed 2026-05-20 03:12 UTC · model grok-4.3

classification ⚛️ physics.ins-det

keywords Kibble balanceweighing unitflexure hingemass metrologycapacitive sensorPID controlrepeatability

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The pith

A modified commercial weighing cell achieves repeatability better than 0.1 mg for 1 kg masses in air.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper describes the design of a weighing unit for a compact Kibble balance that starts from a commercial flexure hinge mechanism and adds a capacitive displacement sensor plus a new PID strategy in the control loop. This combination is intended to give both quick response and stable position holding during mass exchanges. Early tests show the unit can swap 1 kg masses with repeatability better than 0.1 mg in air. A sympathetic reader would care because Kibble balances are used to realize the kilogram from electrical measurements; a simpler, tabletop version could make high-accuracy mass metrology more accessible.

Core claim

The weighing unit is built on a flexure hinge mechanism taken from a commercial weighing cell. The feedback control loop is redesigned to include a capacitive displacement sensor for position detection and a novel PID control strategy that aims for fast dynamics together with high static stability. Initial characterization in air shows repeatability better than 0.1 mg during 1 kg mass exchanges, supporting use in high-accuracy Kibble balance metrology.

What carries the argument

The modified feedback control loop that adds a capacitive displacement sensor and a novel PID strategy to the commercial flexure hinge mechanism.

If this is right

The unit can be integrated into the Tsinghua tabletop Kibble balance for mass measurements at the 0.1 mg level.
Similar modifications could be applied to other commercial weighing cells to create compact metrology instruments.
The design supports the goal of realizing the kilogram through electrical measurements in a smaller apparatus.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

If the repeatability holds under vacuum or with electromagnetic forces applied, the unit could directly support Kibble balance operation without further hardware changes.
The approach might lower the cost and footprint of future Kibble balance experiments by reusing commercial flexure stages.
Long-term drift tests would be needed to confirm suitability for continuous operation in a national measurement institute setting.

Load-bearing premise

The changes to the control loop will keep fast response and static stability while avoiding new systematic errors or instabilities when the unit runs inside a full Kibble balance.

What would settle it

Repeated 1 kg mass exchanges performed in air that yield a standard deviation larger than 0.1 mg would show the claimed repeatability has not been reached.

Figures

Figures reproduced from arXiv: 2605.19203 by Lisha Peng, Lushuai Qian, Nanjia Li, Shisong Li, Songling Huang, Weibo Liu, Wei Zhao.

**Figure 2.** Figure 2: Using traditional PID strategy, the measured current ramping time [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗

**Figure 5.** Figure 5: The initial 8-hour repeatability result of 1 kg mass-on/off in air. [PITH_FULL_IMAGE:figures/full_fig_p002_5.png] view at source ↗

**Figure 4.** Figure 4: Mechanical design for the corner error adjustment. [PITH_FULL_IMAGE:figures/full_fig_p002_4.png] view at source ↗

read the original abstract

This paper presents a customized weighing unit developed for the Tsinghua tabletop Kibble balance. The system is based on a flexure hinge mechanism sourced from a commercial weighing cell, with a major modification to the feedback control loop. The redesigned loop incorporates a capacitive displacement sensor for high-resolution position detection and a novel PID control strategy that ensures both fast dynamic response and high static stability. Initial characterization results demonstrate a repeatability better than 0.1 mg in air for 1 kg mass exchanges, validating the system's potential for high-accuracy mass metrology in Kibble balances.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

This is a practical engineering report on tweaking a commercial flexure cell with a capacitive sensor and custom PID for a tabletop Kibble balance, showing static repeatability under 0.1 mg in air, but the validation stops short of the dynamic and integrated conditions the device will actually face.

read the letter

Colleague, the core of this paper is a hardware customization: they took an off-the-shelf weighing cell, added a capacitive displacement sensor, and reworked the feedback loop with what they call a novel PID strategy to get both quick response and steady position hold. The initial test swaps 1 kg masses in air and reports repeatability better than 0.1 mg. That number is the main concrete result they put forward. The description of the mechanical changes and the control tuning is clear enough that another group could try to copy the setup, which is the useful part here. They give credit to the commercial base and focus on the modifications rather than claiming a whole new principle. The soft spot is exactly what the stress-test note flags. The tests are static mass exchanges with no coil current and no velocity motion, so they do not check how the loop behaves when electromagnetic forces are present or when the balance runs in the combined weighing and velocity modes that define Kibble operation. Without bandwidth data under load or checks for cross-talk, it is hard to know whether the claimed fast response and stability will hold up in the real instrument. The abstract also skips error budgets, statistical details, and any mention of how many cycles or what exclusion rules were used, which makes the repeatability claim difficult to assess on its own. This kind of note is mainly for labs already building or planning tabletop Kibble balances who need a worked example of a weighing unit. It is not a broad metrology advance, but the specific implementation details could be worth seeing. I would bring it to a reading group for the instrumentation discussion and would not cite it in my own work unless we were replicating something similar. It deserves peer review because the work is concrete and the topic sits inside an active experimental area, even though the validation section will probably need expansion on the dynamic side.

Referee Report

2 major / 2 minor

Summary. The manuscript describes the design of a customized weighing unit for the Tsinghua tabletop Kibble balance, based on a modified commercial flexure-hinge weighing cell. A capacitive displacement sensor replaces the original transducer and a novel PID controller is implemented to achieve both fast dynamic response and high static stability. Initial air-based characterization reports repeatability better than 0.1 mg for 1 kg mass exchanges, which the authors interpret as validation of the unit’s potential for high-accuracy mass metrology within a Kibble balance.

Significance. If the reported static repeatability can be reproduced under the combined static and velocity-mode conditions of a full Kibble balance, the work would represent a useful incremental step toward compact, tabletop-scale realizations of the Kibble balance. The hardware modification itself is straightforward and the claimed sub-0.1 mg performance, if substantiated, would be competitive for the target application.

major comments (2)

[Initial characterization / test results] The experimental characterization (described in the section on initial test results) is performed exclusively in air with static mass on/off exchanges and no coil current. This protocol does not test closed-loop behavior under the velocity-mode conditions or electromagnetic force compensation that the weighing unit must sustain in the complete Kibble balance; therefore the central claim that the results validate suitability for high-accuracy metrology rests on an incomplete data set.
[Control system description and experimental results] No quantitative information is supplied on closed-loop bandwidth, position noise under load, or cross-talk between the weighing and velocity modes. Without these metrics it is impossible to assess whether the modified capacitive sensor and novel PID loop introduce velocity-dependent offsets or force-dependent stiffness changes that would appear only in the integrated system.

minor comments (2)

[Abstract] The abstract states positive repeatability results but supplies no details on test protocols, statistical methods, error budgets, or data exclusion criteria; these should be added for reproducibility.
[Figures] Figure captions and axis labels in the characterization plots should explicitly state the number of mass exchanges, the time scale of each cycle, and whether any data filtering was applied.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the thorough review and insightful comments on our manuscript. We address the major comments point by point below and have made revisions to the manuscript to improve clarity and address the concerns where possible.

read point-by-point responses

Referee: [Initial characterization / test results] The experimental characterization (described in the section on initial test results) is performed exclusively in air with static mass on/off exchanges and no coil current. This protocol does not test closed-loop behavior under the velocity-mode conditions or electromagnetic force compensation that the weighing unit must sustain in the complete Kibble balance; therefore the central claim that the results validate suitability for high-accuracy metrology rests on an incomplete data set.

Authors: We agree with the referee that the initial tests are conducted under static conditions in air without coil current or velocity-mode operation. This characterization serves as a foundational step to verify the basic functionality and repeatability of the modified weighing unit and control system. The manuscript has been revised to explicitly state that these results indicate the potential for use in a Kibble balance but do not fully validate performance under the combined static and dynamic conditions of the complete system. We plan to include such integrated tests in future work. revision: partial
Referee: [Control system description and experimental results] No quantitative information is supplied on closed-loop bandwidth, position noise under load, or cross-talk between the weighing and velocity modes. Without these metrics it is impossible to assess whether the modified capacitive sensor and novel PID loop introduce velocity-dependent offsets or force-dependent stiffness changes that would appear only in the integrated system.

Authors: We acknowledge the value of providing quantitative metrics such as closed-loop bandwidth and position noise. In the revised manuscript, we have included additional details on the PID controller parameters and estimated bandwidth based on the system design. However, direct measurements of position noise under load and cross-talk with velocity mode are not yet available because the velocity mode has not been implemented in the current setup. These aspects will be addressed in subsequent experiments with the full Kibble balance apparatus. revision: partial

Circularity Check

0 steps flagged

No circularity: experimental hardware characterization with direct measurements

full rationale

The paper describes a customized weighing unit derived from a commercial flexure hinge mechanism, details modifications to the feedback control loop including a capacitive sensor and PID strategy, and reports direct experimental results on repeatability for static 1 kg mass exchanges in air. No mathematical derivations, equations, fitted parameters presented as predictions, or self-citations are used to justify the central performance claims; the validation rests on measured data rather than any reduction to prior inputs or self-referential logic. This is a standard self-contained experimental report with no load-bearing steps that collapse by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

This is a pure experimental instrumentation report with no mathematical derivations, free parameters, axioms, or postulated entities; all elements derive from hardware adaptation and basic characterization testing.

pith-pipeline@v0.9.0 · 5637 in / 1082 out tokens · 49051 ms · 2026-05-20T03:12:40.569968+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

8 extracted references · 8 canonical work pages

[1]

A measurement of the gyromagnetic ratio of the proton by the strong field method,

B. P. Kibble, “A measurement of the gyromagnetic ratio of the proton by the strong field method,” inAtomic masses and fundamental constants

work page
[2]

Springer, 1976, pp. 545–551

work page 1976
[3]

Honey IShrank the Kibble balance: a second generation NIST table top balance,

K. Arumugam,et al, “Honey IShrank the Kibble balance: a second generation NIST table top balance,” in2024 Conference on Precision Electromagnetic Measurements (CPEM). IEEE, 2024, pp. 1–2

work page 2024
[4]

Progress on the NPL, NMISA, RISE Kibble bal- ance collaboration,

I. A. Robinson,et al, “Progress on the NPL, NMISA, RISE Kibble bal- ance collaboration,” in2024 Conference on Precision Electromagnetic Measurements (CPEM), 2024, pp. 1–2

work page 2024
[5]

Small mass value realization equivalent to accuracy class E1 using the Planck-Balance at PTB,

J. Konrad,et al, “Small mass value realization equivalent to accuracy class E1 using the Planck-Balance at PTB,”Metrologia, vol. 62, no. 2, p. 025014, 2025

work page 2025
[6]

Design of the Tsinghua tabletop Kibble balance,

S. Li,et al, “Design of the Tsinghua tabletop Kibble balance,”IEEE Trans. Instrum. Meas., vol. 72, pp. 1–8, 2023

work page 2023
[7]

Updates on the Tsinghua tabletop Kibble balance,

S. Li,et al, “Updates on the Tsinghua tabletop Kibble balance,”IEEE Trans. Instrum. Meas., vol. 74, pp. 1–9, 2025

work page 2025
[8]

A bi-polar current source with high short-term stability for Tsinghua tabletop Kibble balance,

K. Ma,et al, “A bi-polar current source with high short-term stability for Tsinghua tabletop Kibble balance,”IEEE Trans. Instrum. Meas., vol. 73, pp. 1–9, 2024

work page 2024

[1] [1]

A measurement of the gyromagnetic ratio of the proton by the strong field method,

B. P. Kibble, “A measurement of the gyromagnetic ratio of the proton by the strong field method,” inAtomic masses and fundamental constants

work page

[2] [2]

Springer, 1976, pp. 545–551

work page 1976

[3] [3]

Honey IShrank the Kibble balance: a second generation NIST table top balance,

K. Arumugam,et al, “Honey IShrank the Kibble balance: a second generation NIST table top balance,” in2024 Conference on Precision Electromagnetic Measurements (CPEM). IEEE, 2024, pp. 1–2

work page 2024

[4] [4]

Progress on the NPL, NMISA, RISE Kibble bal- ance collaboration,

I. A. Robinson,et al, “Progress on the NPL, NMISA, RISE Kibble bal- ance collaboration,” in2024 Conference on Precision Electromagnetic Measurements (CPEM), 2024, pp. 1–2

work page 2024

[5] [5]

Small mass value realization equivalent to accuracy class E1 using the Planck-Balance at PTB,

J. Konrad,et al, “Small mass value realization equivalent to accuracy class E1 using the Planck-Balance at PTB,”Metrologia, vol. 62, no. 2, p. 025014, 2025

work page 2025

[6] [6]

Design of the Tsinghua tabletop Kibble balance,

S. Li,et al, “Design of the Tsinghua tabletop Kibble balance,”IEEE Trans. Instrum. Meas., vol. 72, pp. 1–8, 2023

work page 2023

[7] [7]

Updates on the Tsinghua tabletop Kibble balance,

S. Li,et al, “Updates on the Tsinghua tabletop Kibble balance,”IEEE Trans. Instrum. Meas., vol. 74, pp. 1–9, 2025

work page 2025

[8] [8]

A bi-polar current source with high short-term stability for Tsinghua tabletop Kibble balance,

K. Ma,et al, “A bi-polar current source with high short-term stability for Tsinghua tabletop Kibble balance,”IEEE Trans. Instrum. Meas., vol. 73, pp. 1–9, 2024

work page 2024