New mechanical concepts and strategies for adjusting of precision balances

现代高精度天平和质量比较仪是基于电磁力补偿(EMFC)原理的。在测量过程中，由重量的重力引起的平衡梁的挠度被检测到，然后由音圈致动器来平衡。通过执行器线圈的电流是称量盘上质量的量度。为了克服传统等臂式天平和悬挂式称重盘的某些缺点，现代称重单元配备了柔顺的平行导轨机构，以抑制平衡梁上的寄生力和扭矩。与同等的臂梁天平不同，EMFC称重单元使用不对称的传动杆和固定的配重，将补偿力限制在所需的范围内。在质量比较仪和精密天平的操作过程中，由于各种原因(称重系统内外移动的荷载、风荷载或地震作用引起的建筑物倾斜)造成的残余误差对称重过程的影响是不可避免的，如温度漂移、称重偏心引起的拐角荷载和倾斜。称重单元的计量性能取决于其对倾斜和转角加载的敏感度，这可以通过有针对性的调整来最小化。另一个相关参数是称重单元的最终弹簧常数，因为该参数与位置控制相结合，确定天平的力分辨率。该项目的第一个资助期为根据先进的机械模型大幅提高真空质量比较仪中EMFC称重单元的计量性能奠定了基础。从这一点出发，提出了一种综合调整概念，在提高灵敏度的同时降低了测量不确定度。广泛的机械建模证明了这一点。在商业称重单元的实验研究中，测量了迟滞、非线性刚度特性和随时间变化的弹性后效等效应。这些影响还没有包括在力学模型中。如果能通过扩展的力学模型和进一步的措施来减小由这些影响引起的测量不确定度，则可以进一步提高精密称重系统和质量比较器中调整后的称重单元的性能。要做到这一点，必须实现称重单元的自动调整、先进的控制策略以及平底盘摆动的衰减。

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.