Wuqiang Yang

Design of electrical capacitance tomography sensors

Electrical capacitance tomography (ECT) has been developed since the late 1980s for visualization and measurement of a permittivity distribution in a cross section using a multi-electrode capacitance sensor. While the hardware and image reconstruction algorithms for ECT have been published extensively and the topics have been reviewed, few papers have been published to discuss ECT sensors and the design issues, which are crucial for a specific application. This paper will briefly discuss the principles of ECT sensors, but mostly will address key issues for ECT sensor design, with reference to some existing ECT sensors as a good understanding of the key issues would help optimization of the design of ECT sensors. The key issues to be discussed include the number and length of electrodes, the use of external and internal electrodes, implications of wall thickness, earthed screens (including the outer screen, axial end screens and radial screens), driven guard electrodes, dealing with high temperature and high pressure, twin planes for velocity measurement by cross correlation and limitations in sensor diameter. While conventional ECT sensors are circular with the electrodes in a single plane or in twin planes, some non-conventional ECT sensors, such as square, conical and 3D sensors, will also be discussed. As a practical guidance, the procedure to fabricate an ECT sensor will be given. In the end are summary and discussion on future challenges, including re-engineering of ECT sensors.

Role of tomography in gas/solids flow measurement

Abstract Gas/solids flow measurement presents many challenges to academics and engineers. Although this subject has been investigated for many years, it still remains a notorious task. The main difficulty is that all existing multi-phase flow meters are flow-regime-dependent and suffer from severe non-linearity problems. Industrial process tomography is based on measuring a subject, such as a gas/solids flow, from many different viewing angles and reconstructing cross-sectional distributions, i.e. tomographic images. From the images the flow regime can be identified, the solids fraction and velocity profiles be derived, and the volumetric flow rate be measured. This paper discusses these possibilities, in particular with electrical capacitance tomography (ECT). Some new results are presented.

Planar capacitive sensors – designs and applications

Purpose – The purpose of this paper is to present the sensing mechanism, design issues, performance evaluation and applications for planar capacitive sensors. In the context of characterisation and imaging of a dielectric material under test (MUT), a systematic study of sensor modelling, features and design issues is needed. In addition, the influencing factors on sensitivity distribution, and the effect of conductivity on sensor performance need to be further studied for planar capacitive sensors.Design/methodology/approach – While analytical methods can provide accurate solutions to sensors of simple geometries, numerical modelling is preferred to obtain sensor response to different design parameters and properties of MUT, and to derive the sensitivity distributions of various electrode designs. Several important parameters have been used to evaluate the response of the sensors in different sensing modes. The designs of different planar capacitive sensor arrays are presented and experimentally evaluated...

Dynamic imaging in electrical capacitance tomography and electromagnetic induction tomography using a Kalman filter

Electrical capacitance tomography (ECT) and electromagnetic induction tomography (EMT) attempt to visualize the distributions of materials with different permittivity and conductivity/permeability, aiming to reveal electrical and magnetic characteristics of an object, by measuring electrical capacitance and electromagnetic inductance on the periphery of the object. In ECT, capacitances of pairs of electrodes placed around the periphery are measured and in EMT, mutual induction of pairs of coils is measured. In this paper, a dynamic imaging technique is developed for ECT and EMT with a linearized Kalman filter to improve the temporal resolution of images. The inverse problem is treated as a state estimate. A Kalman estimator is used to obtain the material distribution. Experimental results demonstrate that the dynamic imaging technique can improve the spatio-temporal resolution of both ECT and EMT.

Evaluation of Effect of Number of Electrodes in ECT Sensors on Image Quality

It is commonly assumed that the use of more electrodes in an electrical capacitance tomography (ECT) sensor would result in higher resolution images. In this paper, the issues with the number of electrodes will be discussed. To investigate the effect of the number of electrodes on the quality of reconstructed images, ECT sensors with different number of electrodes-4, 8, 12, 16, 20, 24, and 32-are investigated. The capacitance between different electrode pairs is calculated for some typical permittivity distributions using a finite element method. The obtained capacitance data are then used to reconstruct images using the projected Landweber iteration algorithm based on both linear and semi-linear ECT models. The sensitivity distributions for the ECT sensors with different number of electrodes are analyzed. The main conclusion is that limited new information can be obtained and little improvement in the quality of images be achieved if the number of electrodes is more than 12.

An intelligent cross correlator for pipeline flow velocity measurement

Abstract Cross correlation techniques are becoming widely used for pipeline flow velocity measurement. Usually a polarity cross correlation method is employed because of its simplicity, but at the expense of losing information compared with a direct cross correlation method. This paper describes a direct cross correlator based on a Transputer 1 which uses 8-bit data in the calculations. To reduce the discrimination error, a quadratic curve is fitted to three sample points around the cross correlation peak, and the delay time is interpolated. The cross correlation software is highly intelligent with automatic selection of transducer type, circuit gain, sampling frequency and pre-delay. Experiment results show that the cross correlation peak can be positioned in the middle of the correlation delay range by means of one of two automatic functions, and the measurement accuracy is 1%.

A high-performance digital system for electrical capacitance tomography

This paper describes a recently developed digital-based data acquisition system for electrical capacitance tomography (ECT). The system consists of high-capacity field-programmable gate arrays (FPGA) and fast data conversion circuits together with a specific signal processing method. In this system, digital phase-sensitive demodulation is implemented. A specific data acquisition scheme is employed to deal with residual charges in each measurement, resulting in a high signal-to-noise ratio (SNR) at high excitation frequency. A high-speed USB interface is employed between the FPGA and a host PC. Software in Visual C++ has been developed to accomplish operational functions. Various tests were performed to evaluate the system, e.g. frame rate, SNR, noise level, linearity, and static and dynamic imaging. The SNR is 60.3 dB at 1542 frames s−1 for a 12-electrode sensor. The mean absolute error between the measured capacitance and the linear fit value is 1.6 fF. The standard deviation of the measurements is in the order of 0.1 fF. The dynamic imaging test demonstrates the advantages of high temporal resolution of the system. The experimental results indicate that the digital signal processing devices can be used to construct a high-performance ECT system.

Scale-up of an electrical capacitance tomography sensor for imaging pharmaceutical fluidized beds and validation by computational fluid dynamics

The aim of this research is to apply electrical capacitance tomography (ECT) in pharmaceutical fluidized beds and scale up the application of ECT from a lab-scale fluidized bed to a production-scale fluidized bed. The objective is to optimize the design of the production-scale fluidized bed and to improve the operation efficiency of the fluidization processes. This is the first time that ECT has been scaled up to a production-scale fluidized bed of 1.0 m diameter and batch process capacity of 100 kg in a real industrial environment. With a large-scale fluidized bed in a real industrial environment, some key issues on the ECT sensor design must be addressed. To validate ECT measurement results, a two-phase flow model has been used to simulate the process in a lab-scale and pilot-scale fluidized bed. The key process parameters include solid concentration, average concentration profiles, the frequency spectrum of signal fluctuation obtained by the fast Fourier transfer (FFT) and multi-level wavelet decomposition in the time domain. The results show different hydrodynamic behaviour of fluidized beds of different scales. The time-averaged parameters from ECT and computational fluid dynamics are compared. Future work on the ECT sensor design for large-scale fluidized beds are given in the end of the paper.

Sensitivity distributions of capacitance tomography sensors with parallel field excitation

In conventional electrical capacitance tomography (ECT) systems the single-electrode excitation scheme is used in which an excitation signal is applied to each of the sensor electrodes in turn. This conventional ECT sensor is more sensitive in the wall area than the central area and consequently the signal-to-noise ratio (SNR) in the central area is poor. To improve this situation, a scheme called parallel field excitation has recently been proposed. This paper analyses the sensitivity distributions of the parallel field sensors. Analytical, simulation and experimental results show that the sensitivity maps of the parallel field sensors are simply the linear superimpositions of those of the conventional sensor, and give inferior image reconstructions.

Image Reconstruction for Electrical Capacitance Tomography Based on Sparse Representation

Image reconstruction for electrical capacitance tomography (ECT) is a nonlinear problem. A generalized inverse operator is usually ill-posed (unbounded) and ill-conditioned (with a large norm). Therefore, the solutions for ECT are not unique and highly sensitive to the measurement noise. To improve the image quality, a new image reconstruction algorithm for ECT based on sparse representation is proposed. An unconventional basis, i.e., an extended sensitivity matrix consisting of some normalized capacitance vectors corresponding to the base permittivity elements is designed as an expansion frame. The permittivity distributions to be reconstructed can have a natural sparse representation based on the new basis and can be represented as a linear combination of the base elements. Another sparsity regularization method-the standard Landweber iteration with a threshold is also conducted for comparison. The proposed algorithm has been evaluated by both simulation (with and without noise) and experimental results for different permittivity distributions.