矩形电容层析成像传感器仿真模拟和实验测量

常规电容层析成像传感器为规则对称方形或圆形结构,其测量和成像算法已相对成熟.由于受循环流化床旋风分离器入口结构段的限制,上述传感器结构已不能应用在旋风分离器入口段的测量,因此为了研究大型化循环流化床多旋风分离器并联布置入口段固体颗粒浓度分布,提出和设计了一种矩形非常规电容传感器,该传感器为12电极结构,在每个入口段的短边布置2个电极,长边布置4个电极,成像敏感场仿真计算基于Ansoft-Maxwell软件.对此传感器进行了静态仿真模拟研究和静态实验测量,对不同激励电压和频率测量结果进行分析,并对测量结果进行误差分析,得到矩形不规则电容传感器最佳测量参数(激励电压和频率),并对循环流化床分离器入口段进行了动态测量.结果表明,这种矩形非常规电容传感器可以用于循环流化床冷态试验台的旋风分离器入口段的直接测量,测量结果对改进旋风分离器的布置方式有参考意义.     

A novel capacitive system for the concentration measurement of pneumatically conveyed pulverized fuel at power stations

A novel capacitive system for the concentration measurement of gas–solid flow in pneumatically conveyed pulverized fuel at power stations has been developed. The capacitance sensor for measuring the concentration uses source-grid sensing electrodes. An active sensor and a dummy sensor are used to form a differential configuration. In order to eliminate baseline drift, to be immune to stray capacitance, and to ensure accurate measurement under very low solids/air mass flux ratio conditions, a correlated double sample (CDS) technique and lock-in detector with closed loop are used in the interface circuit. A laboratory scale pneumatically conveyed pulverized fuel loop system incorporating facilities for calibration has been fabricated for this purpose. The test section is a vertical ceramic tube of diameter 100 mm with an upward flow direction and pulverized fuel has been used as the solid phase. Results demonstrate that the system is capable of achieving linearity within ±5% at different temperatures or moisture content.     

同心圆环区域电容层析成像传感器数目优化

针对同心圆环区域,设计了不同数目的组合电容层析成像传感器,以提高传感器在测量区域的空间分辨率。基于数值仿真,对内外径之比为0.2、0.3、0.4、0.5的4种同心圆环区域,采用了传统外层12电极和12-4、12-6、12-8 3种内外双层结构电极(IEE),研究了其成像特性。其中IEE传感器测量过程施行外部-对立-内部(EOI)和外部-对立(EO)两种激励策略,对几种典型的介质分布进行测量,并采用LBP、Tikhonov、Landweber算法开展图像重建。结合数值仿真和实验验证,结果显示内外双层电极测量随着同心圆环形区域内外径比增大,内层电极数目存在最优值,12-4型电极结构在不同环形区域的重建图像质量和稳定性更好。采用LBP算法进行成像时,EOI激励策略相比于EO激励策略在成像精度上略有优势;采用Tikhonov算法和Landweber迭代算法时,EO激励策略能有效避免异常电容值的影响,与IEE电极组合时能在不同内外径比的同心圆环区域测量中提高成像质量。     

基于ECT的复合材料构件胶层缺陷检测

航空用复合材料构件在制造和服役过程中不可避免会产生各种缺陷及损伤。通常复合材料构件是粘贴在基体上使用,针对其粘接胶层缺陷和损伤,提出了一种基于同面阵列电极传感器ECT技术的无损检测方法。基于电容边缘效应,利用同面阵列电极传感器,以循环激励方式工作,根据复合材料构件胶层内部缺陷/损伤对材料介电特性的影响,进行缺陷/损伤检测。通过仿真和实验研究了复合材料构件典型胶层缺陷与阵列电极传感器电容变化量的关系,并基于线性反投影算法对其成像。研究结果表明基于电容敏感机理的同面阵列电极传感器对复合材料构件胶层缺陷检测的可行性和有效性。     

Imaging conductive materials with high frequency electrical capacitance tomography

Electrical capacitance tomography (ECT) is known as an imaging technique for dielectric permittivity imaging. A novel ECT sensor model at a high excitation frequency is proposed to examine the capability of the ECT system to image both conductivity and permittivity contrasts. The proposed model uses a complex impedance forward model for the ECT system. This new model indicates that in higher excitation frequency both conductive and dielectric imaging may be feasible. Normally, capacitance tomography is designed for the measurements of imaginary part and resistance tomography is used to take the measurements of real part. The drawback of a typical capacitance tomography at a low excitation frequency, such as 200 kHz is that it cannot be used to measure the conductive phase of a conductive/dielectric mixed fluid, e.g. the gas/water flow. By increasing the excitation frequency, the capacitive impedance of the conductive material decreases and dielectric phenomena of the conductive fluid dominates so that it is possible to use capacitance tomography to characterise the dielectric/conductive flows. This paper presents a development of capacitance tomography with a high excitation frequency in measuring the gas/liquid mixture i.e. gas/water and gas/oil multiphase distributions. Both theoretical and experimental results are presented to verify this feasibility study.     

Improved estimation of contact compliance via atomic force microscopy using a calibrated cantilever as a reference sample

A method to improve accuracy of surface compliance determination by atomic force microscopy is presented, based on using calibrated cantilevers as the reference samples. During each work session, a 1-D compliance map of a reference cantilever is calculated from force–indentation curves along its axis, by the standard ‘indentation mode’. An independent measurement of local compliance on the reference cantilever is obtained by 2-D imaging in constant deflection and using analytical equations based on its known geometry and material properties, called ‘imaging mode’. A re-mapping of the apparent (‘indentation mode’) to the true (‘imaging mode’) compliance is thus obtained, which is applied on ‘indentation mode’ measurements of an unknown sample. This method demonstrates correction in the right direction for a polystyrene plate and a Teflon foil reference samples. The method is then applied on an unknown sample of flat agarose gel patterned with spots of polylysine protein.     

Determination of the optimal axial length of the electrode in an electrical capacitance tomography sensor

Electrical capacitance tomography (ECT) is one of the most successful process tomography techniques, which has been developed for more than a decade. Thus far, most of the research on ECT has been based on two-dimensional modeling, which simplifies the ECT sensor structure as two dimensional. However, certain key issues such as the three-dimensional effect of the ECT sensor are not considered at all. In this paper, the finite element method is used to solve the partial differential equation governing the ECT sensing zone, which is actually three dimensional. The sensitivity distributions between different electrode pairs are calculated and compared with those based on the two-dimensional modeling. The impacts of the length of both the measurement electrode and the guard electrode on the sensitivity distribution are investigated. The effect of the length of the measurement electrode on the normalized capacitance is also discussed. According to the numerical results presented in this paper, it is suggested that the optimal length of the measurement electrode for electrical capacitance tomography is the same as the width or diameter of the sensor.     

基于加权奇异值分解截断共轭梯度的电容层析图像重建

针对电容层析成像技术(ECT)中的"软场"效应和病态问题,提出了一种基于加权奇异值分解(SVD)截断共轭梯度的电容层析(ECT)图像重建算法。阐述了电容层析成像工作原理,提出了12电极ECT系统的测量方法。在分析灵敏度矩阵的奇异值分解理论的基础上,推导出了加权SVD截断共轭梯度的数学模型,并利用Tikhonov方法进行正则化加权处理。最后,分析了算法的收敛性,并将其应用于电容层析成像系统的图像重建中。实验结果表明,对于层流,截断共轭梯度算法的平均误差能达到27.54%,全部流型平均迭代步数达到13步,与LBP、Landweber和CG算法比较,该算法具有成像效果好,成像速度快,易于实现等特点。     

电容层析成象系统中微小电容测量法

电容层析成象技术（ＥＣＴ）是一种最早发展起来的过程成象技术。由于为重建图象所需测量的电容变化量相对于ＥＣＴ传感器的固有电容非常小,总的杂散电容值又远大于待测电容,因此ＥＣＴ对电容检测提出了极为严格地要求。本文介绍了ＥＣＴ系统中最常用的３种微小电容检测电路—充电／放电电路、带反馈补偿的交流测量电路以及自平衡电容测量电路,并指出了它们的优缺点。     

Decoupling the joint effect of moisture and mass in dense phase pneumatic conveying

Electric capacitance tomography (ECT) is advantageous for online imaging of pneumatic conveying of solids. From the ECT images, the spatial distribution of the mass of the material can be displayed at a very fast speed. However, as the mass distribution and the permittivity of the solids simultaneously affect the values of the measured capacitance arrays, they cause a coupled effect on the ECT images. In situations where the permittivity of the solids is influenced by the moisture, and this influence cannot be separated from the influence of the mass, or cannot be accounted for by an appropriate means, the measurement will be erroneous, as ECT images are usually interpreted as the distribution of the mass alone. This study aims to tackle the problem by an attempt to decouple the combined influence of the mass and the permittivity. A sensor of narrow electrodes is designed for the purpose of detecting only the variations in the permittivity. Simulation and experimental results both demonstrated that the influence of the permittivity and the mass of the solids can be decoupled under certain conditions, which provide a mechanism to correct the possible errors due to the unknown variation of the solids permittivity, or moisture.     

OPTIMAL DESIGN AND FABRICATION STEPS OF ELECTRICAL CAPACITANCE TOMOGRAPHY SENSORS

Electrical capacitance tomography (ECT) is a non destructive technique for obtaining the distribution of some physical quantity in a closed pipe by measuring the permittivity distribution. Several conducting plates are fixed around the circumference of the pipe so they do not disturb the flow or movement of materials. The imaging quality of an electrical capacitance tomography system depends upon many co-influential parameters such as number of electrodes, length and width of electrodes, axial guard electrodes, radial guard electrodes, and screen guard. Optimum selection of these variables coupled with advancements in sensing electronics and imaging algorithm provides the best inside view of a closed vessel/pipe. This, in turn, provides better process information for control. This article is intended to summarize such design rules for ECT electrodes and sensors. Numerical simulations were performed using commercially available finite element method (FEM) software to help verify the optimality of the design parameters chosen. Experimental verification was conducted using a fabricated capacitance sensing circuit that could measure capacitance in the range of 1 fF to 7999 femtofarads with a root means square noise level of 0.07 fF. Some parameters such as the thickness of the pipe are chosen as demanded by the process. This article will be useful for researchers to design the ECT Sensors with ease.     

流化床物料密度分布可视化研究

文中针对流化床物料密度分布的特点和要求,分析了电容层析成像（ECT）系统多层电容极板的性能,传感器阵列的各结构参数进行选取,最后给出流化床密度分布三维成像结果及误差。     

基于LS-SVM的油气两相流空隙率测量方法的研究

利用12电极电容层析成像系统电容传感器获取的66个电容测量值,基于最小二乘支持向量机（LS-SVM）算法,提出了两相流空隙率在线测量的新方法。该方法用LS-SVM来建立空隙率测量模型。在实际测量时,首先归一化ECT获取的电容测量值,然后将归一化电容值输入已经建立的空隙率模型即可计算出空隙率。实验结果表明：该方法是有效的,避免了复杂耗时的图像重建过程,测量误差在6%以内,测量时间小于0.08 s。     

油气润滑ECT传感器的轴向屏蔽电极及其影响*

建立UMIST型（带轴向接地屏蔽电极）和METC型（带轴向驱动屏蔽电极）油气润滑ECT传感器的三维物理模型,根据三维物理模型推导电容值和灵敏度数学表达式,并分析轴向屏蔽电极对传感器的敏感性和灵敏度均匀性的影响;采用线性反投影算法（Linear Back Projection,LBP）进行图像重建,并比较重建图像与真实图像的相对误差和相关系数。结果表明：2种轴向屏蔽电极均可消除边缘电容并提高传感器的敏感性;带轴向接地屏蔽电极使得传感器的灵敏度更加均匀,而带轴向驱动屏蔽电极的传感器则相反。带轴向接地屏蔽电极的传感器更适用于油气润滑中环状流的识别,其重建图像与真实图像的相对误差降低了28.79%,相关系数提高了14.05%。     

Capacitance sensor for void fraction measurement in water/steam flows

A capacitance sensor operating at RF range for void fraction measurements was developed. Two electrodes of the capacitor are mounted on the outer side of pipe walls. The variations in the percentage of phases in two-phase flow cause changes of the equivalent permittivity of the dielectric between the electrodes. The capacitor is connected in a resonant circuit of an oscillator tuned to high frequency of 80 MHz. The changes of frequency generated by the oscillator are the measure of the void fraction in the two-phase flow. An eight-channel system with capacitance sensors of this type was used for determination of the phase conversion along a steam injector.     

Nonlinear image reconstruction using a GA-ECT technique in electrical capacitance tomography

Electrical capacitance tomography (ECT) is a non-invasive measurement technique that estimates the dielectric permittivity distribution of an inhomogeneous object from the boundary potentials at floating electrodes or mutual capacitances. In this paper, a stochastic inverse technique based on genetic algorithm (GA-ECT) is developed, which is adapted to the two different methods, i.e. potential measurement and capacitance measurement. Numerical simulation results are presented to evaluate the inverse technique both for noise free and noisy data and the results show that quantitative image can be reconstructed not only with the low permittivity contrast but also with the high contrast. Furthermore, the influence of a priori knowledge to image reconstruction is discussed.     

非闭合电极电容层析成像传感器在冻土测试中的应用

为了实现电容层析成像技术对冻土冻结冰峰面的在线、非侵入测试,研制出了满足冻土测试要求的非闭合电极电容层析成像传感器,并对该种传感器的电容分布特性进行了实测;确定出了适合冻土测试的的高、低介电常数的标定物质;搭建了冻土一维冻结实验系统.对含水量8%的湿土样冻结过程的冰峰面迁移进行了电容层析成像测试,并利用IMNSNOF图像重建算法重建出了冻结过程各时刻的冻结截面物质分布图像,由图像可确定出冻土中已冻土、未冻区以及冰峰面的位置.电容层析成像测试结果与温度测试结果相吻合.     

Visualisation of gas–oil two-phase flows in pressurised pipes using electrical capacitance tomography

Electrical capacitance tomography (ECT) was used to image various two-phase gas–oil horizontal flows on a 7.62 cm (3 in.) pressurised test loop. ECT is a novel non-invasive technique for imaging mixtures of electrically non-conducting substances. One of its most promising applications is the visualisation of gas–oil flows. This work presents a series of imaging experiments using a pressure-resistant ECT sensor. By varying the oil and gas flow rates, different flow regimes were established in the test loop. ECT images were obtained for each case and compared with (a) the flow observed through a transparent section in the loop and (b) the prediction of the Taitel–Duckler flow map. The results confirm the suitability of ECT for imaging gas–oil flows.     

基于ECT传感器和模式识别的气液两相流空隙率测量新方法研究

基于电容层析成像(ECT)传感器和模式识别理论,提出了一种新型的气液两相流空隙率测量方法。根据流型几何特征分别建立了空隙率的三个模板库。测量时先对流型进行分类,再调用对应的模板库利用距离测度进行模板匹配,从而得到空隙率。实验结果表明该方法是可行的,有助于克服测量结果易受流型影响的问题,同时速度优于传统ECT技术方法,获得一个管截面空隙率值所花费的测量时间小于50ms,最大测量误差可小于5%,满足工业在线运行需要。     

Analysis of crude palm oil composition in a chemical process conveyor using Electrical Capacitance Tomography

In this work, we aim to analyse the capability of using a 16-segmented Electrical Capacitance Tomography (ECT) sensor system to monitor the internal composition of a chemical process conveyor that carries crude palm oil (CPO) multiphase flow. The source used to excite the electrodes is a differential potential, instead of the conventional single potential source, in order to obtain an improved sensitivity of the sensor, especially in the central area of the pipe. This system aims to recognise the phase concentration of the flow. The attained concentration profile that is received from the capacitance measurements is capable of providing an image of the liquid and liquid mixture in the pipeline, making the separation process (between oil and liquid waste) much easier and the CPO's quality can be dependably monitored. Experimental results and analysis are presented, and the new excitation technique is shown to provide better sensor sensitivity in the central pipe area. The visualisation results deliver information regarding the flow regime and concentration distribution in a two-phase flow-rate measurement system incorporating a liquid flow-measuring device. The information obtained will assist the design of process equipment, and the verification of existing computational modelling and simulation techniques.