New mechanical concepts and strategies for adjusting of precision balances

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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.