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.

https://iopscience.iop.org/article/10.1088/0957-0233/21/4/042001/pdf

Design of electrical capacitance tomography sensors

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...

Planar capacitive sensors – designs and applications

Real-time quantitative imaging is becoming highly desirable to study nondestructively the biological behavior of 3-D cell culture systems. In this paper, we investigate the feasibility of quantitative imaging/monitoring of 3-D cell culture processes via electrical impedance tomography (EIT), which is capable of generating conductivity images in a non-destructive manner with high temporal resolution. To this end, a planar miniature EIT sensor amenable to standard cell culture format is designed, and a 3-D forward model for the sensor is developed for 3-D imaging. Furthermore, a novel 3-D-Laplacian and sparsity joint regularization algorithm is proposed for enhanced 3-D image reconstruction. Simulation phantoms with spheres at various vertical and horizontal positions were imaged for 3-D performance evaluation. In addition, experiments on human breast cancer cell spheroid and a triangular breast cancer cell pellet were carried out for experimental verification. The results have shown that the stable measurement on high conductive cell culture medium and the significant improvement of image quality based on the proposed regularization method are achieved. It demonstrates the feasibility of using the miniature EIT sensor and 3-D image reconstruction algorithm to visualize 3-D cell cultures, such as spheroids or artificial tissues and organs. The established work would expedite real-time quantitative imaging of 3-D cell culture for assessment of cellular dynamics.

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A Miniature Electrical Impedance Tomography Sensor and 3-D Image Reconstruction for Cell Imaging

The paper highlights some of the significant works done in the field of medical and biomedical sensing using silicon-based technology. The use of silicon sensors is one of the pivotal and prolonged techniques employed in a range of healthcare, industrial and environmental applications by virtue of its distinct advantages over other counterparts in Microelectromechanical systems (MEMS) technology. Among them, the sensors for biomedical applications are one of the most significant ones, which not only assist in improving the quality of human life but also help in the field of microfabrication by imparting knowledge about how to develop enhanced multifunctional sensing prototypes. The paper emphasises the use of silicon, in different forms, to fabricate electrodes and substrates for the sensors that are to be used for biomedical sensing. The electrical conductivity and the mechanical flexibility of silicon vary to a large extent depending on its use in developing prototypes. The article also explains some of the bottlenecks that need to be dealt with in the current scenario, along with some possible remedies. Finally, a brief market survey is given to estimate a probable increase in the usage of silicon in developing a variety of biomedical prototypes in the upcoming years.

https://www.mdpi.com/1424-8220/19/13/2908/pdf

Silicon-Based Sensors for Biomedical Applications: A Review.

Capacitive proximity sensors (CPSs) are ubiquitous because of their simple design, low cost and low consumption. Capacitive displacement sensing, as one of the three sensing modalities, works for long distance and can be unitized to measure more physical quantities compared with capacitive volume and deformation sensing. In this paper, we firstly introduce the concept of capacitive displacement sensing. After that, we present applications of capacitive displacement sensing under three broad categories: distance measurements, indirect measurements, and the applications applied in smart environments. Finally, we discuss the challenges and possible solutions for CPSs development. We show that both the detection range and accuracy of CPS can be improved by multi-sensor fusion, and the application scenarios can be extensive through machine/deep learning approaches. We aim to provide a comprehensive, and state-of-the-art review of the capacitive displacement sensing, and inspire more researchers and developers to find wide application perspectives.

/pdf/a-review-on-applications-of-capacitive-displacement-sensing-2exh6zirht.pdf

A Review on Applications of Capacitive Displacement Sensing for Capacitive Proximity Sensor

A planar capacitance sensor matrix may be used to detect dangerous objects for security screening because different materials under test (MUT) have different permittivities. Usually, each capacitance sensing element consists of two electrodes, one for excitation and the other for detection. A weakness of such a two-electrode structure is that its sensing depth is limited. To increase the sensing depth, a planar capacitance sensor matrix operating in single-electrode mode has been designed for a security screening system. In this paper, the design of the planar single-electrode capacitance sensor matrix is described and its performance is analysed. A model for the single-electrode capacitance sensor and its equivalent circuit is discussed. The sensor response to the properties of MUT is studied by simulation to determine the design parameters. A single-electrode capacitance measuring circuit is designed to measure capacitance from the planar single-electrode capacitance sensor matrix. Preliminary experimental results show that a steel knife can be imaged by the screening system with the planar single-electrode capacitance sensor matrix.

http://iopscience.iop.org/article/10.1088/0957-0233/22/11/114026/pdf

Design of a security screening system with a capacitance sensor matrix operating in single-electrode mode

This note presents a low-cost data acquisition and function generation unit with a USB interface in such a way that it helps others to design a USB-based system without knowing details of the USB itself. The unit integrates most functions commonly used in the laboratory, such as analogue-to-digital converter (ADC), digital-to-analogue converter (DAC), synchronous direct digital synthesizers (DDSs) and digital in/digital out, into a single unit. Dedicated software with a user-friendly interface has been developed in Visual C++ 6.0, to control the hardware operation and to log the sampled data into a PC or a laptop. Preliminary tests show that all the designed functions are working. Due to the USB latency, however, the ADC sampling rate is slower than expected. An alternative design is suggested to increase the sampling rate.

https://iopscience.iop.org/article/10.1088/0957-0233/17/4/N03/pdf

Design of a data acquisition and function generation unit with USB

Imaging systems based on impedance measurement are widely used in medical, process monitoring and non-destructive testing, where their hardware designs, measurement range and accuracy are often application-specific. Impedance analysers can deliver excellent performance in terms of reliability, resolution and accuracy. A multi-channel imaging system has been developed using an impedance analyser with a specifically designed multiplexer box, hardware interface and corresponding software to provide flexibility in sensor connection. With a GUI written in Matlab, various algorithms have been implemented and data can be further analysed. We show here that it is a useful tool for a wide range of applications involving impedance measurement, single- or multi-modality 2D or 3D imaging. This system has been used for (a) imaging flow in a porous medium, (b) 3D ECT imaging and (c) security imaging with dual-modality planar capacitive and magnetic sensor arrays.

An impedance-analyser-based multi-channel imaging system and its applications

To detect hidden metallic objects, an imaging system with a planar PCB-coil sensor array has been developed. The sensor array consists of a driving board and a pickup board, with square spiral coil elements connected in series. Fast scan can be achieved by row-excitation and column-detection. Mutual inductance between the driving-pickup coil pairs is measured by an HP4192-based multi-channel measurement system. Using sensitivity coefficients, an image presenting conductivity/ permeability distribution can be obtained by interpolation and pixel-averaging from the normalised measurements. Initial tests show that the system can image both conductive and magnetic objects with an imaging depth of 10 mm and a spatial resolution of one coil element. The issues with cross-talk and image errors due to object covering multiple sensing elements should be compensated to improve image fidelity.

Xiaohui Hu

Papers

Planar capacitive sensors – designs and applications

Design of a security screening system with a capacitance sensor matrix operating in single-electrode mode

Design of a data acquisition and function generation unit with USB

An impedance-analyser-based multi-channel imaging system and its applications

An imaging system with planar PCB-coil sensor array