功率天平准直误差软补偿关键技术研究

The watt balance is one of the most focused experiments that can realize a new definition of the kilogram by precision measurement of the Planck constant. But due to the misalignment error, the accuracy of the watt balance is greatly limited. In order to overcome this problem, in this application, a novel misalignment error compensation method is proposed, in which the misalignment error would be reduced by introducing a correction term from related measurements. The difficulty for the compensation method is that the physical model of the misalignment error should be established with well determined quantities in the compensation model. Accordingly, we firstly aim to establish a mathematical model for the misalignment error. The three-dimensional contribution of the magnetic field in the watt balance is planned to be determined by partial measurement of the vertical profiles via a polynomial fitting. The horizontal velocity of the coil would be measured by a twin-Kelvin capacitor sensing system and the angular velocity of the coil will be determined by measurements of the vertical displacement of the coil at three different positions every 120 degrees. Based on a combination of the measurement and calculation, the misalignment error of the watt balance experiment can be determined and real-timely corrected, which would supply very useful information for further improving such a difficult experiment in the future.

功率天平是有望实现质量量子基准暨精确测量普朗克常数最重要的实验之一，但受准直误差制约，该实验的准确度提升遇到较大困难。为克服准直难题，申请项目提出一种全新的软补偿方法，以期通过引入修正量，减小准直测量误差并大幅降低实验系统对准直误差的敏感性。所提出的软补偿法难点在于：须通过对准直误差产生的机理进行深入分析，找到准确表征准直误差的数学模型，并能通过直接或间接测量，确定出准直误差表达式中的各个量。针对上述难点，申请项目拟：建立准确的准直误差数学模型，采用基于梯度线圈法测量功率天平磁体局部磁场，进而通过解耦拟合或保角变换确定整个磁体三维磁场分布；用双Kelvin电容传感器测定线圈的侧向运动速度；用三束相隔120度角的激光测定线圈沿水平轴旋转的角速度。上述测量量配合准直误差修正模型，有望最终实现对准直误差的实时软补偿，对推动质量量子基准这一世界性难题的进展具有重要意义。

本项目提出了通过对功率天平准直误差的软补偿来减低功率天平实验测量普朗克常数不确定度的方案：1）建立了准直误差和相关物理量之间的数学模型，明确了线圈附加感应电动势和线圈运动之间的数学物理关系，为功率天平准直误差补偿技术提供了理论依据；2）解决了如何对功率天平磁体的磁场进行准确3D测量的难题，提出了两种由局部已知磁场获取整个气隙区域磁场分布的算法（低磁导率拟合法和高磁导率分离变量法），实现了较高准确度的三维磁场反演；3）理论上证明了所建立准直误差补偿模型在磁体装配中出现误差时的适用性，即功率天平磁体在气隙中的磁场为二维场的话，通电线圈在磁场所产生的电磁力是守恒的，该结论为进一步减小或消除准直误差提供了重要的线索；4）提出了保持国际功率天平磁体基本性能的基础上使得磁场更接近二维场的新方案，揭示了功率天平磁体磁场产生边缘效应的原因，通过消除边界不对称或引入补偿磁场分量可将功率天平磁场竖直分量减小10倍； 5）提出并实现了在避免干扰主磁场下实现准直误差的最小化方案，提出了一种独立于主磁场的五维度主动阻尼系统，通过在三个维度的独立线圈中加载电流，可将线圈附加运动幅度降低约一个数量级。