The current flow modelling in a sensor with the active shield

Abstract

The concept of an active shield has been originally introduced for constant current sources. The main issue of an application of the active shield is a delimitation of the current flow to an object or its part situated exactly between current electrodes. Therefore more reliable impedance measurement results could be obtained. The active shield is an electrode that surrounds the current electrode and adopts its electric potential. Modelling of the current flow was carried out using the Finite Element Method in 2D space. The electric potential of the active shield was set in an iterative process. Initially, the electric potential of the active shield was set to zero. After a simulation, the potential of the active shield was changed to the potential of the current electrode and the simulation was repeated until convergence was obtained. Current flow modelling has been performed with and without the active shield in two different set-ups: in homogeneous environment and with large heterogeneity outside a shielded area. The distribution of the current density and electric potential gradient confirm theoretical considerations. Using the sensor with the active shield, current is injected perpendicularly to the current electrode surface due to equal electric potential on a parallel plane. © Springer-Verlag 2007.