New mechanical concepts and strategies for adjusting of precision balances

现代高精度天平和质量比较仪都是基于电磁力补偿（EMFC）原理。在测量过程中，检测到由重物重力引起的平衡梁的偏转，然后通过音圈致动器进行平衡。通过致动器线圈的电流是称量盘上质量的量度。为了克服带有悬挂称量盘的经典等臂梁天平的某些缺点，现代称重传感器配备了柔性平行导向机构，以抑制平衡梁上的寄生力和扭矩。与等臂梁秤不同，EMFC称重传感器使用非对称传动杠杆和固定配重，将补偿力限制在所需范围内。在质量比较仪和精密天平的运行过程中，不可避免会产生对称重过程的残余误差影响，例如温度漂移、因砝码偏心放置而产生的角载荷以及因各种原因（称重系统内外的移动载荷、风载荷或地震效应导致的建筑物倾斜）而产生的倾斜。称重传感器的稳定性取决于其对倾斜和角载荷的敏感性，通过有针对性的调整可以将其降至最低。另一个相关参数是称重单元的弹簧常数，因为该参数与位置控制相结合，决定了天平的力分辨率。该项目的第一个资助期为基于先进机械模型的真空质量比较器中的电磁燃料电池称重传感器的可测量性的大幅增强奠定了基础。从这一点出发，开发了一种全面的调整概念，可以提高灵敏度，同时降低测量不确定性。这一点已被广泛的机械建模所证实。在商业称重传感器的实验研究中，测量了滞后、非线性刚度特性和随时间变化的弹性后效等效应。这些影响尚未包括在力学模型中。如果通过扩展的力学模型和进一步的措施可以降低由于这些影响而导致的测量不确定性，则可以进一步提高精密称重系统和质量比较器的已调整称重传感器的性能。要做到这一点，必须实现称重传感器的自动调节、先进的控制策略以及盘摆动的阻尼。

Modern high precision balances and mass comparators are based on the principle of electromagnetic force compensation (EMFC). During the measurement, the deflection of a balance beam, caused by the gravitational force of a weight, is detected and then counterbalanced by a voice coil actuator. The electrical current passing through the coil of the actuator is a measure for the mass on the weighing pan. To overcome certain disadvantages of classical equal arm beam balances with hanging weighing pans, modern weighing cells are equipped with a compliant parallel guide mechanism to suppress parasitic forces and torques on the balance beam. Unlike equal arm beam balances, EMFC weighing cells use an asymmetrical transmission lever and a fixed counterweight, to restrict the compensational force to a required range. Residual error influences on the weighing process, such as temperature drifts, corner loads due to eccentric placing of the weight and tilt due to various reasons (moving loads inside and outside the weighing system, tilt of the building caused by wind loads or seismic effects) are unavoidable during the operation of mass comparators as well as precision balances. The metrological performance of a weighing cell depends on its sensitivities to tilt and to corner loading which can be minimized by targeted adjustments. A further relevant parameter is the resulting spring constant of the weighing cell, since this parameter, in combination with the position control, determines the force resolution of the balance. The first funding period of the project provided the basis for a substantial enhancement of the metrological properties of EMFC weighing cells in vacuum mass comparators based on advanced mechanical models. Starting from this, a comprehensive adjustment concept was developed that allows the enhancement of the sensitivity with a simultaneous reduction of the measurement uncertainty. This was proven by extensive mechanical modeling. During experimental investigations with commercial weighing cells, effects like hysteresis, nonlinear stiffness characteristics and time-dependent elastic aftereffects were measured. These effects are not yet included in the mechanical models. If the measurement uncertainty due to these effects can be reduced by an extended mechanical model and further measures, the performance of the adjusted weighing cells for precision weighing systems and mass comparators can be further improved. To do so, an automated adjustment of the weighing cell, advanced control strategies as well as damping of pan swing has to be implemented.