Effect of Voltage Boundary Conditions on the Sensitivity and Design of Coplanar Capacitive Sensors

Coplanar capacitive sensors are widely employed in various applications due to their advantages in noninvasive evaluation, fabrication simplicity, and versatility. While there have been several experimental demonstrations, optimization of the boundary conditions and geometry is lacking. In this letter, we investigate the effect of both the voltage boundary conditions and the electrode geometry on the sensitivity with respect to an overlaid insulating layered structure. We perform electrostatic analysis using finite element simulations to study the distribution of electric fields between two coplanar electrodes, with various combinations of applied voltages and shielding electrodes beneath the sensing electrodes. By combining this analysis with variations in electrode geometry and permittivity distribution of layers, we demonstrate how specific geometries and material properties can amplify the effect of voltage boundary conditions. Our results indicate that incorporating conductive shielding electrodes can enhance the total capacitance while enabling increased sensitivity within regions close to the electrodes. Furthermore, our study reveals that conductive shielding electrodes are more effective in achieving higher sensitivity when the gap is of similar size to the electrode width. The outcomes of this research can facilitate the more efficient design of coplanar capacitive sensors, particularly for pressure sensing and proximity sensing applications where the overlaid material under test may undergo deformation.