Chengyi Yang

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

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

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.

Dynamical lag correlation exponent based method for gas–solid flow velocity measurement using twin-plane electrical capacitance tomography

In a twin-plane electrical capacitance tomography (ECT) system, velocity measurement of two-phase flow is transformed into the time delay estimation problem, while the nongaussianity and nonstationarity of two-phase flow signals have put the validity of the conventional cross-correlation algorithm in jeopardy. To improve the robustness and reliability of flow velocity measurement, an alternative method is proposed based on the dynamical lag correlation exponent and applied to coal ash measurement in a pneumatic pipeline. Different from the cross-correlation method which picks the peak point of the cross-correlation function as the delayed frames between the upstream and downstream signals, the proposed method determines the delayed frames by finding the minimum point of the dynamical lag correlation exponent. The preliminary results of flow velocity measurement indicate that the proposed method is capable of detecting various velocities (8–25 m s−1), which is useful for monitoring and predicting flow instability.

Image reconstruction based on L1 regularization and projection methods for electrical impedance tomography

Electrical impedance tomography (EIT) is a technique for reconstructing the conductivity distribution by injecting currents at the boundary of a subject and measuring the resulting changes in voltage. Image reconstruction in EIT is a nonlinear and ill-posed inverse problem. The Tikhonov method with L(2) regularization is always used to solve the EIT problem. However, the L(2) method always smoothes the sharp changes or discontinue areas of the reconstruction. Image reconstruction using the L(1) regularization allows addressing this difficulty. In this paper, a sum of absolute values is substituted for the sum of squares used in the L(2) regularization to form the L(1) regularization, the solution is obtained by the barrier method. However, the L(1) method often involves repeatedly solving large-dimensional matrix equations, which are computationally expensive. In this paper, the projection method is combined with the L(1) regularization method to reduce the computational cost. The L(1) problem is mainly solved in the coarse subspace. This paper also discusses the strategies of choosing parameters. Both simulation and experimental results of the L(1) regularization method were compared with the L(2) regularization method, indicating that the L(1) regularization method can improve the quality of image reconstruction and tolerate a relatively high level of noise in the measured voltages. Furthermore, the projected L(1) method can also effectively reduce the computational time without affecting the quality of reconstructed images.

Development and application of ECT digital system for online flow measurement

The paper presents the latest development of a digital-based measurement system for electrical capacitance tomography (ECT), that consists of a high-capacity FPGA and fast digital converters. In this system, most functions, e.g. signal generation, phase-sensitive demodulator and low-pass filters, are implemented digitally in FPGA. Tomographic software in Visual C++ has been developed for real-time data acquisition, image re-construction and visualization and parameter calculation. Having been specially optimized for ECT measurement, the custom-built hardware and software are capable of achieving acquisition rate of >; 1000fps. The developed ECT system has been applied in online monitoring of a pneumatic conveying system that is to be used for mine out area refilling. Regional flow parameters could be obtained according to specific subregion division. In addition, having been tested on the pneumatic conveying system for several days, the ECT system is proved to be reliable and durable.

Visualization imaging software design for Electrical Tomography

As a non-invasive detection method, Electrical Tomography (ET) can reach a much higher imaging speed than γ-ray or X-ray CT, etc. To archive an even higher speed, there are several factors which should be considered thoroughly, such as excitation/measurement strategy, data acquisition system design, communication protocol, reconstruction algorithm, imaging software, etc. To apply the wooden bucket theory to this situation, the slowest section determines the systems actual speed. And imaging software becomes the lowest board in some cases. Based on Tianjin University Electrical Tomography (TJU-ET) hardware, this paper introduced optimized multithread program architecture, standard BLAS (Basic Linear Algebra Subprograms) and LAPACK (Linear Algebra PACKage) numerical library, optimized OpenGL rendering engine and many other improvements into imaging software. As a result, the brand new software can fully releases TJU-ET systems power, which means it can perform 750fps on TJU dual plane 12-electrode ECT system.

Experimental research on two-phase flow visualization using a low-energy gamma CT system with sparse projections

This paper describes the application of a low-energy γ-ray computed tomography (CT) system to two-phase pipe flow measurement. Because this γ-ray CT system offers only three to nine projections, and the detector elements are sparsely arranged, the conventional reconstruction algorithms basically cannot meet the accuracy requirement, thus improvement based on the matrix completion method has been developed to improve the spatial resolution. Reconstructed results from limited measurement data of several typical flow regimes demonstrate the feasibility of this CT system. Comparisons between the proposed method and three other reconstruction algorithms show that the former has improved the spatial resolution with acceptable reconstruction speed.

Estimation of in-tube oil film thickness using electrical capacitance tomography

In air/oil lubrication system, the flow pattern of intube air/oil flow is of great impact to the transportation efficiency. Among all the flow parameters, flow pattern and oil film thickness are of most interested to the researchers and operators. In the transportation tube, small amount lubrication oil is blew by the high-speed compressed air, which results in the formation of a small thickness oil film along the circumference. Therefore, online estimation of oil film thickness is a challenging job. In this paper, the application of electrical capacitance tomography to monitor the in-tube air/oil flow has been introduced. The measurements are made from an externally mounted electrode array that made of FPC board, which will not interfering the on-going air/oil flow. Both average and distributed oil film thickness can be calculated from the capacitance images. Experimental results show that the ECT system can provide satisfactory on-line flow pattern monitoring and oil film thickness estimation.

Change-point-detection-based method for solid velocity measurement using twin-plane electrical capacitance tomography

This paper proposes a novel method for flow velocity measurement in twin-plane electrical capacitance tomography (ECT) system, and applies it to coal ash measurement in a pneumatic pipeline. The proposed method is base on the Mann-Kendall method, which has been frequently used in abrupt climate change detection. First, the change-point of cross-sectional solid concentration values at each plane is detected separately using the M-K method; thereafter the transit time can be estimated as the difference between the change-points detected at the upstream and downstream planes. The change-point-detection-based method has released the basic assumption required by the classical cross-correlation method, and thus can increase the scope of velocity measurements. Experimental results indicate that the ECT system is capable of detecting various velocity profiles and solids distributions, which can be useful for the monitoring and predicting of flow instability and solid blockages.