Change In Complex Impedance Research Articles

Complex conductivity reconstruction in multiple frequency electrical impedance tomography for fabric-based pressure sensor

Purpose – The aim of this paper is to introduce and to evaluate the performance of a multiple frequency complex impedance reconstruction for fabric-based EIT pressure sensor. Pressure mapping is an important and challenging area of modern sensing technology. It has many applications in areas such as artificial skins in Robotics and pressure monitoring on soft tissue in biomechanics. Fabric-based sensors are being developed in conjunction with electrical impedance tomography (EIT) for pressure mapping imaging. This is potentially a very cost-effective pressure mapping imaging solution in particular for imaging large areas. Fabric-based EIT pressure sensors aim to provide a pressure mapping image using current carrying and voltage sensing electrodes attached on the boundary of the fabric patch. Design/methodology/approach – Recently, promising results are being achieved in conductivity imaging for these sensors. However, the fabric structure presents capacitive behaviour that could also be exploited for pressure mapping imaging. Complex impedance reconstructions with multiple frequencies are implemented to observe both conductivity and permittivity changes due to the pressure applied to the fabric sensor. Findings – Experimental studies on detecting changes of complex impedance on fabric-based sensor are performed. First, electrical impedance spectroscopy on a fabric-based sensor is performed. Secondly, the complex impedance tomography is carried out on fabric and compared with traditional EIT tank phantoms. Quantitative image quality measures are used to evaluate the performance of a fabric-based sensor at various frequencies and against the tank phantom. Originality/value – The paper demonstrates for the first time the useful information on pressure mapping imaging from the permittivity component of fabric EIT. Multiple frequency EIT reconstruction reveals spectral behaviour of the fabric-based EIT, which opens up new opportunities in exploration of these sensors.

The Change in Impedance of a Single-Turn Coil Due to a Flaw in a Coaxial Conducting Cylinder

The problem of detection and location of a small flaw inside a conducting cylinder using an eddy current coil coaxial with the cylinder has been addressed. The electric field at an arbitrary axial and radial position inside the conductor has been obtained from a previous solution of the boundary value problem. An expression for the change in complex impedance due to a small flaw located within a conducting body has been derived and is shown to be a function of the electric field at the position of the flaw. For the case of a degenerate point flaw, this expression is further simplified by using just the value of the electric field at the position of the centroid of the flaw. The overall impedance is shown to be a function of the ratio of the radii of the loop and cylinder and of the conductivity of the material. The expression for the change in complex impedance has been factored into two terms, one dependent on the axial location of the flaw, and the other on the depth of the flaw. The axial location of the flaw is seen to affect only the magnitude and phase of the change in impedance; whereas the depth of the flaw is seen to affect both the magnitude and phase of the change in impedance. Plots of the complex change in impedance as a function of the axial location and depth of the flaw have been provided to illustrate its functional dependence on these parameters.

Change in impedance of a single-turn coil due to a flaw in a conducting half space

The problem of detection and characterization of a flaw in a conducting half-space using an eddy-current coil oriented parallel to the interface is examined. An expression is derived for a first order approximation for the change in complex impedance due to a flaw located within the conducting medium. The overall impedance is a function of the radius and lift-off distance of the test coil and the conductivity of the material. An analytical expression is derived for the change in impedance as a function of the electric fields at the position of the flaw. It is found to be an integral over the volume of the flaw of the electric fields found with and without the flaw being present. The limiting case of a degenerate point flaw may be examined in greater detail by allowing the field in the presence of the flaw to be approximated by the unperturbed field. For flaws small enough that the field does not vary much over its volume, the field may be even further approximated by using just the value of the field at the position of the centroid of the flaw. Plots are shown to illustrate the behavior of the change in impedance as a function of the radial range of the flaw and the depth of the flaw centroid, using previously derived expressions for the fields for the unflawed case.

Impedance of a loop surrounding a conducting cylinder

The change in complex impedance between an ideal one-turn coil surrounding and coaxial with an infinitely long circular cylinder of conductivity δ and permeability μ and a similar coil without the core has been calculated. From the exact expression, a power series in the quantity δ/b (δ=skin depth; b=radius of core) has been developed. From this result, the change in impedance of a physically realistic multiturn coil can be estimated. The theory permits a rational approach to optimization of the design of eddy-current test coils and provides a possible means of detecting changes in the radius and conductivity of the cylinder.

The impedance of a single-turn coil near a conducting half space

The change in complex impedance between an ideal one-turn circular coil located above and parallel to a conducting half space with respect to a similar isolated coil has been calculated. From this result, a series expansion of the integrand allows the solution to be approximated by terms expressed as complete elliptic integrals. Results have been calculated for the change in impedance as a function of the liftoff distance and the conductivity of the half space for a coil of representative radius.