Watt balance with lever transmission based on commercial EMFC load cell

In balances based on the principle of electromagnetic force compensation (EMFC-balances), the weight force on the balance pan is compensated by an electromagnetic counterforce. According to the state of the art, the relation of the current in the actuator that produces the counterforce to the weight force of the goods to be weighed is determined by calibration using standard mass. After the redefinition of the unit mass based on a fixed value of the Planck-constant, that is expected for 2018, there will be a direct connection of mechanical and electrical quantities. Thus the Watt-balance-principle known from fundamental experiments for the redefinition of the unit, can be used for mass calibrations. In the present publication a method for determination of the calibration factor BI of a commercial EMFC-load cell, based on these principles is presented. The calibration is done by simultaneous measurements of the voltage on the actuator coil while it is moving in a stationary magnetic field and its velocity. During the calibration a harmonic AC-current through the same coil is used for generating a sinusoidal coil motion, which is specific for this particular method. The frequency of this harmonic motion has a significant influence on the uncertainty of the resulting calibration factor Bl. A separation of the different portions of the resulting measured voltage is done by simultaneous, frequency depended measurements of current and voltages across the coil and data evaluation with respect to amplitudes and phases. The coil of the balance is coupled to the load carrier via a lever system, a configuration which is common in commercial EMFC-balances. By using such a lever system it is possible to achieve a high value for the effective BI (force coupling factor to load carrier) with a compact electrodynamic actuator (combination of coil and permanent magnet system). As a result the system can be calibrated for force measurements and generation of reference forces in arbitrary direction in space. For mass measurements, which are a special case of the above mentioned, a value of the local gravity acceleration has to be known. Furthermore there is the possibility for calibrating the EMFC-load cell as a position sensor using the here introduced formalism by exchanging certain input and output parameters. In this case a reference mass is used and in combination with the output signal of the integrated position sensor of the load cell it is possible to calibrate the position of the load carrier. For both applications based on the method presented here the aim is rather to achieve uncertainties relevant to industrial applications than those required in fundamental experiments. Measurements for mass and force calibration and also for position calibration were carried out using a commercial EMFC-load cell and also commercial electrical measurement devices. The results are presented and the achieved uncertainties and their composition are discussed.