Measuring and Adjusting the Stiffness and Tilt Sensitivity of a Novel 2D Monolithic High Precision Electromagnetic Force Compensated Weighing Cell

Abstract

Further improvements in high precision mass comparison are a recent issue in the dissemination chain of the mass standard. One of the most precise methods of mass comparison is achieved by the use of high precision electromagnetic force compensated (EMFC) weighing cells as part of mass comparators. The mechanics of EMFC weighing cells are based on compliant mechanisms with concentrated compliances in form of flexure hinges. Total mechanical stiffness and tilt sensitivity are limiting factors with regard to the resolution of EMFC weighing cells. In order to optimize their performance, the stiffness and the tilt sensitivity of the systems need to be minimized. Due to manufacturing restrictions and robustness requirements, a further reduction of the thickness of the pivots is not desirable. In this paper, an alternative to reduce stiffness and tilt sensitivity by adding trim weights in combination with an astasizing adjustment is presented. Based on the results of the investigations, a new planar monolithic mechanism for an EMFC weighing cell is designed, providing the possibility to adjust trim masses. The new mechanism is set up and adjusted according to the developed mechanical model. A parameter combination for a total stiffness slightly above zero and a tilt sensitivity close to zero is found. For the evaluation of the adjustment success and the vacuum compatibility, the system is tested under high vacuum conditions.