An iterative method for electrostatic object reconstruction in a half space

Abstract

Abstract. Sensing electrodes arranged in or around a display can provide input function for interactive displays. Commercially this is interesting because the sensing electrodes and electronics can be made in the same manufacturing process as that of the display itself thus reducing cost. In engineering terms the electrodes measure capacitance changes resulting from the presence and movement of objects such as hands and fingers in front of the display. At the quasi static frequencies used (100kHz) the human body is conductive and the hands or fingers provide a screen between the capacitive electrodes. There is no need to touch the actual display and the overall system constitutes a touchless gesture input system.Determining the shape of the hand or fingers is a boundary condition reconstruction problem of finding the boundary of an earthed conductive object D from electrostatic measurements. This is the ill-posed problem of recovering the zero-surface of a solution to Laplace's equation from Cauchy data on part of the boundary of a domain. The problem has similarities with object reconstruction in EIT or inverse scattering but is complicated because only a partial Dirichelet-Neumannn map is available as experimental data.We suggest an algorithm where at each iteration we have an approximation ∂D k to ∂D on which we calculate approximate Cauchy data by solving a Tikhonov regularized linear system. This data is used to modify ∂D k by extrapolation towards the zero-surface giving the next approximation ∂D k+1 .We implemented the algorithm in two and three space dimensions using the Boundary Element Method for discretization. Numerical results using simulated data with added noise show that simply connected but not necessarily convex objects can be reconstructed with reasonable positional accuracy and approximate shape, but as might be expected the shape is more accurately determined near the plane of measurements.

Electrical capacitance volume tomography with high contrast dielectrics using a cuboid sensor geometry

“…According to equation ( 8), the four conductors will produce six independent measurements which are insufficient to yield nine dielectric values; however, by limiting each pixel to one of two dielectric constants, higher resolutions are achievable. Since the nine pixels are limited to two values, 2 9 = 512 distributions are possible. Calculating the capacitance matrices for each of these distributions, it was found that there is no degeneracy when considering the full capacitance matrix.…”

“…An open faced configuration [8] would have been ideal, but the change in capacitance due to material above it falls off rapidly with the distance from the sensing electrodes. Additionally, only partial boundary condition information is available, making image reconstruction much more complex [9]. The parallel arrays provide a more sensitive closed system from which it was easier to model and develop a new image reconstruction algorithm.…”