Ziqiang Cui

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.

A Specific Data Acquisition Scheme for Electrical Tomography

Electrical tomography techniques have been developed for monitoring the internal behavior of industrial processes. The electrical tomography offers some advantages over other tomography modalities, such as low cost, no radiation and being non-intrusive. In the past, individual data acquisition systems have been used for tomographic measurement, e.g., for ECT, ERT and EMT, most of which are in analog hardware. Generally, it is difficult to transfer these systems to other modalities. This paper presents a FPGA based data acquisition scheme, which can be used to facilitate the hardware design of electrical tomography. Improvements in system performance are achieved by implementing most functions in digital rather than analog hardware. Currently, individual ECT and ERT data acquisition systems have been used for achieving dual-modality tomography, which will result in poor hardware integration and data combination. With this scheme, multi-modality measurement techniques could be integrated into a system based on a single FPGA chip, providing effective integration. Preliminary results show that the signal to noise ratio is greater than 74 dB.

An integrated ECT/ERT dual modality sensor

Capacitance and resistance tomography systems have been developed for visualizing the distributions of materials in an industrial process. They offer certain advantages over other tomography modalities, such as low cost, rapid response, no radiation and being non-intrusive. Single modal tomography could provide a satisfactory solution in most process applications. However, in some more complex applications, additional information is required for comprehensive understanding of the process under investigation. ECT and ERT are two most researched electrical tomography techniques. Individual ECT and ERT systems has been combined for dual modal measurement in recent years, however, the two sensors were located at two different cross-sections of the process, resulting in poor sensor integration and data fusion. As a result of the present research an integrated tomography system with capacitive and resistive sensors located at the same cross-section of the process has been developed. In the integrated system the use of both resistive and capacitive electrodes help eliminate mutual interferences between the two modalities. Experimental work with water-gas two-phase flow on vertical and horizontal pipes shows that the integrated system can provide effective dual modal measurements. The reconstructed images together with the stacked images are used to deduce such process parameters as gas holdup and flow velocity.

Reconstruction of electrical impedance tomography (EIT) images based on the expectation maximum (EM) method.

Electrical impedance tomography (EIT) calculates the internal conductivity distribution within a body using electrical contact measurements. The image reconstruction for EIT is an inverse problem, which is both non-linear and ill-posed. The traditional regularization method cannot avoid introducing negative values in the solution. The negativity of the solution produces artifacts in reconstructed images in presence of noise. A statistical method, namely, the expectation maximization (EM) method, is used to solve the inverse problem for EIT in this paper. The mathematical model of EIT is transformed to the non-negatively constrained likelihood minimization problem. The solution is obtained by the gradient projection-reduced Newton (GPRN) iteration method. This paper also discusses the strategies of choosing parameters. Simulation and experimental results indicate that the reconstructed images with higher quality can be obtained by the EM method, compared with the traditional Tikhonov and conjugate gradient (CG) methods, even with non-negative processing.

Liquid Film Thickness Estimation using Electrical Capacitance Tomography

Abstract In air/oil lubrication systems, the flow parameters, e.g., flow pattern, liquid film thickness, and air/oil flow rate, are of great importance to the transportation efficiency. In most cases, the on-going two-phase flow is annular flow with the oil moving along the tube wall and the air travelling at high speed in the center. This usually results in the formation of a thin oil film, the thickness of which is a key parameter determining the efficiency of the lubrication system. As the oil film thickness of the on-going air/oil flow varies dynamically, there is actually no applicable method for a non-intrusive test. In this paper, the use of electrical capacitance tomography (ECT) to investigate the air/oil flow has been studied. Capacitance measurements are made from an externally mounted electrode array in a non-invasive and non-intrusive manner. Both average and distributed oil film thicknesses can be calculated from the reconstructed ECT images. Simulation and experimental results show that the ECT technique can provide satisfactory results of online oil film thickness estimation

Separation of Gas–Liquid Two-Phase Flow Through Independent Component Analysis

Two-phase flow measurement has attracted a major interest in the past four decades due to its wide range of applications in industry. This paper introduces a new method to separate the gas phase from the liquid phase through a blind source separation algorithm, without a separate device, based on the assumption that the two phases are separated and their independence is reflected in the statistical relation between the electrical signals generated by the process. Experimental data are obtained from a gas-liquid two-phase flow rig through electrical resistance tomography (ERT). An independent component analysis (ICA) method is applied to separate the gas phase from the liquid phase. The efficiency of the ICA method with the ERT data is assessed through experiments. The independent components (ICs) are interpreted by comparing them with the reconstructed images by ERT. The comparative studies show that ICA is effective in extracting phase information of gas-liquid two-phase flow, particularly for stratified, slug, and wave flows. Based on the extracted ICs, the cross-correlation technique is adopted to estimate the mean velocity of the liquid phase in the central area, the gas phase at the interface, and the liquid phase around the pipe wall and the liquid slug. Through correlating ICs representing different spatially independent processes from the upstream and downstream planes after the elimination of cyclostationary characteristics of ICs, the mean velocity of different spatially processes is obtained.

Electrical Capacitance Tomography With Differential Sensor

In electrical capacitance tomography, capacitance changes are used to determine the permittivity distribution in the imaging area. However, the changes are small compared with the standing capacitances, e.g., usually in the order of 10%-30%. For a single channel capacitance sensor, a differential configuration having a redundant pair of electrodes can be used to cancel the standing capacitance. However, there has been no report so far introducing such differential configuration into electrical capacitance tomography (ECT) sensors. This is mainly due to the fact that an ECT sensor is composed of an array of electrodes, e.g., 8, 12, and 16 and the capacitance measurements in ECT are required to interrogate capacitances of all electrode pair combinations, which introduce significant difficulty for a differential configuration. In this paper, a novel differential ECT sensor is proposed and designed, which consists of concentrically arranged dual array electrodes. It can also be thought as an additional array of electrodes being inserted between the measuring electrodes and the outer screen of a conventional ECT sensor. The new sensor design has been validated by the numerical simulation. A prototype sensor has been developed and evaluated with an field programmable gate arrays (FPGA)-based ECT measurement system, showing that with the proposed differential sensor, the dynamic range of the measured capacitance is reduced by

Application of electrical resistance tomography in bubble columns for volume fraction measurement

\sim 80

Image reconstruction for field-focusing capacitance imaging

% and an average improvement of 10.8 dB in signal-to-noise ratio is achieved.

A Novel ECT System Based on FPGA and DSP

Bubble columns have been widely used in chemical and pharmacy industries for its simplicity and efficiency. In bubble column, air distribution depends on various factors, e.g. sieve plate pattern, air and liquid volumes, temperatures, which in turn determines the efficiency of the reaction. Real-time measurement of air volume fraction distribution is of significant importance for process control and further optimization of the reactor. Therefore, great attentions have been paid to this problem in recent studies. Generally, conventional methods, e.g. differential pressure and conductivity probe method, can only obtain a lumped parameter or local information. Hence, electrical resistance tomography technique has been introduced to provide real-time measurement of air volume fraction distribution within its sensing region. With a dual-plane sensor, the electrical resistance tomography system could generate cross-sectional images as well as flow velocity map. Moreover, both global and local information of the flow can be obtained by data/image post-processing. Experimental results show that test results from electrical resistance tomography are in good accordance with those from conventional methods, i.e. only 5% deviation.