Analysis of a passive microforce sensor based on magnetic springs and upthrust buoyancy

This paper is focused on the design and modeling of a new micro and nano force sensor using magnetic springs. The force sensitive part is a macroscopic horizontal rigid platform used as a floating seismic mass. This platform presents a naturally stable equilibrium state for its six degrees of freedom (dof) thanks to the combination of upthrust buoyancy and magnetic forces. This force sensor allows the measurement of the external horizontal force and the vertical torque applied to the platform. Thanks to the magnetic springs configuration used, the seismic mass presents a 0.02 N/m horizontal mechanical stiffness (similar to the stiffness of a thin AFM micro-cantilever). The measurement range typically varies between ±50 μN. The resolution depends on the displacement sensors used to measure the seismic mass displacement and on the environmental conditions (ground, liquid and air vibrations). In steady state, this displacement is proportional to the applied force. Resolution of less than 10 nN can be reached with the use of an anti-vibration table.