Jérôme Lucas

Analytical capacitive sensor sensitivity distribution and applications

Robustness, versatility, high level of performance and low cost are some of the characteristics that make capacitive sensors well suited for industrial applications. They consist only in electrodes and measurement circuits but give access to position information and/or material properties. The design of capacitive sensors is however not so obvious so that simple structures are generally used to avoid time-consuming calculations and developments. We propose an analytical method to determine the sensitivity distribution of any capacitive sensor structure. This method makes it possible to rapidly optimize structures in order to increase the sensor sensitivity to one parameter or to render it less sensitive to another. Comparisons between the sensitivity map of known sensors and those obtained with the analytical method proposed in this paper show a great accordance.

Simple and direct calculation of capacitive sensor sensitivity map

Purpose – The purpose of this paper is to introduce a rapid method for calculating capacitive sensor signal variations for any small permittivity or electrode modifications at any position in space.Design/methodology/approach – When a capacitive sensor is probing its surrounding, the modification of the permittivity, the displacement or the deformation of one or more electrodes induce a signal variation which depends on the position of the modification. Seeing that modification as a small perturbation and using Gauss identity, it is possible to find integral expressions of the sensor sensitivity map.Findings – Capacitive sensor sensitivity map expressions depend only on the perturbation to measure, on the electric field before the perturbation, and on a sensitivity field which is the electric field produced by the sensor when the measuring electrode is held to 1 V while the others, except the floating ones, are grounded. The sensitivity field is a kind of Greens function for the capacitive sensor. The kn...

Distance Corrected Capacitive Sensors for Advanced Air Bag Applications

The Federal Motor Vehicle Safety Standards No 208 now includes directives requiring the morphological estimation of passengers mandatory for Advanced Airbag systems. The Dynamic Automatic Suppression System, which is part of the advanced airbag system uses both morphological and positional information about the passenger to allow or prevent airbag deployment. Various solutions have been proposed to obtain this information by using for instance capacitive sensors. The response of this kind of sensors depends drastically on their distance from the passenger. This paper presents a method now implemented in the BIOVOLUME technology developed by Faurecia in partnership with Hitachi computer products to create the sensors independent from this distance.

Physical limitations on spatial resolution in electrical capacitance tomography

Electrical capacitance tomography (ECT) is an imaging technique providing the distribution of permittivity in a medium by means of electrodes. As for any imaging systems, the accessible spatial resolution is a key parameter. In this paper the physical limitations on the spatial resolution of ECT sensors are analysed in terms of the accuracy of an objects position and of the ability to distinguish between two close objects for any sensor geometry. Cylindrical geometry sensors are particularly studied and the example of a square geometry sensor is used to show how to apply the calculations to any other geometries. In cylindrical geometries, it is shown that a 50% gap between electrodes is a good compromise and that increasing the number of electrodes improves the spatial resolution near the electrodes but decreases the spatial resolution in the centre. The best spatial resolution at the centre of the sensor is obtained with 3 or 4 electrodes. In the square geometry studied, it is shown that a better distribution of the spatial resolution is obtained when there are electrodes in the corners.

A fast Fourier transform implementation of the Kramers-Kronig relations: Application to anomalous and left handed propagation

In this this paper we quickly derive the Kramers-Kronig relations from simple causality considerations and propose a simple way to implement them using the Fast Fourier Transform. This work focuses on how to make these relations a usable tool even when their conditions of validity are not strictly respected. In this respect we emphasize on their application to the constant low level loss approximation at microwave frequencies. The method presented is demonstrated on various typical cases of fancy propagation: high velocity, negative phase velocity and evanescent waves.

Morphological capacitive sensors for air bag applications

The Federal Motor Vehicle Safety Standards No. 208 includes directives rendering the morphological estimation of passengers mandatory for advanced air bag systems. The dynamic automatic suppression system uses both the morphological and positional information about the passenger to allow or prevent air bag deployment. Various solutions have been proposed to obtain these information by using capacitive sensors. This paper presents a method, that makes possible use of such sensors in the car industry by correcting their responses from these perturbing parameters.

Fast circular shapes detection in cylindrical ECT sensors by design selection and nonlinear black-box modeling

Purpose Within the framework of image reconstruction in cylindrical electrical capacitance tomography (ECT) sensors, the purpose of this study is to select the structure of a sensor in terms of number and size of the electrodes, to predict the radius and the position of a single circular shape lying in the cross-section defined by the sensor electrodes. Design/methodology/approach Nonlinear black-box models using a set of physically independent capacitances and least-square support vector machines models selected with a sophisticated validation method are implemented. Findings The coordinates of circular shapes are well estimated in fixed and variable permittivity environments even with noisy data. Various numerical experiments are presented and discussed. Sensors formed by three or four electrodes covering 50 per cent of the sensor perimeter provide the best prediction performances. Research limitations/implications The proposed method is limited to the detection of a single circular shape in a cylindrical ECT sensor. Practical implications This method can be advantageously implemented in real-time applications, as it is numerically cost-effective and necessitates a small amount of measurements. Originality/value The contribution is two-fold: a fast computation of a circular shape position and radius with a satisfactory precision compared to the sensor size, and the determination of a cylindrical ECT sensor architecture that allows the most efficient predictions.