Electrical Capacitance tomography
Electrical capacitance tomography (ECT) is a method for determination of the dielectric permittivity distribution in the interior of an object from external capacitance measurements. The principle is based on Capacitance measurements which are done by placing electrodes around the known dielectric medium which is made of PVC and the unknown dielectric specimen is put inside the cylinder which is a virtual capacitor and capacitance value is measured. The capacitance will vary based on the area of the plates, distance between plates and dielectric material which will vary according to the object and other factors remain constant. Since the capacitance depends on the permittivity value of the material located between the electrodes, substances of differing dielectric properties can be distinguished by means of this method. The measured capacitance values are converted into voltage and it is fed into the computer where reconstruction is done by using Linear Back Projection Algorithm.
1 Electrical Capacitance Tomography Ajeesh Sunny , Bify Baby , Jackson Ouseph , Manjunarayanan.N.S , Sampreeth John and Mr. Jis Paul Abstract— Electrical capacitance tomography (ECT) is a Tomography can be classified into two types: direct and method for determination of the dielectric permittivity indirect. In the former, a method of visual recording not distribution in the interior of an object from external visible to the human eye is used e.g. X-ray or infrared capacitance measurements. The principle is based on imaging. Whereas in the later, boundary measurements Capacitance measurements which are done by placing related to the internal characteristics of the object of electrodes around the known dielectric medium which is interest are used for image reconstruction. In indirect made of PVC and the unknown dielectric specimen is put inside the cylinder which is a virtual capacitor and tomography, many physical quantities implemented capacitance value is measured. The capacitance will vary through different tomography systems can be used as the based on the area of the plates, distance between plates and boundary measurement quantity. However, from dielectric material which will vary according to the object engineering point of view, an acceptable tomography and other factors remain constant. Since the capacitance technique is one which is (1) Non-invasive: it requires no depends on the permittivity value of the material located direct contact between the sensor and the object or between the electrodes, substances of differing dielectric domain of interest and (2) Non-intrusive: it does not properties can be distinguished by means of this method. The change or disturb the nature of the object being explored. measured capacitance values are converted into voltage and it is fed into the computer where reconstruction is done by using Linear Back Projection Algorithm. II. BACKGROUND Research in tomography systems can be classified Index Terms — Tomography, capacitance measurement, into three categories: (1) sensor, (2) data acquisition and electrodes, LBP, sensitivity matrix hardware and (3) reconstruction techniques. Sensor design, performance, and associated problems depend on the tomography modality being used. In electrical I. INTRODUCTION tomography sensors, encountered problems are usually the soft field nature of the sensor, and ill posed response of the sensor to different location in the imaging domain. T omography by definition refers to the process of exploring the internal characteristics of a specified region through integral measurements related to the Problems in the data acquisition of electrical tomography sensors are mainly the low level power of sensed signal internal characteristics of the specified domain. and low signal to noise ratio (SNR). The low level of Traditionally, the term tomography has been used for the acquired signal is usually reflected on the sensor process of obtaining 2D cross sections called tomograms. dimensions. Increased sensor area provides higher SNR. However, recent developments in different tomography However, a trade off of lower spatial resolution of discipline have expanded the tomography concept to reconstructed images is usually associated with increasing include 3D and 4D (with time domain included) imaging. the sensor dimensions. Reconstruction techniques involve the process of solving the inverse problem for finding the electrical property distribution from the measured Manuscript was prepared on April 28, 2010 Ajeesh Sunny , Bify Baby, Jackson Ouseph , Manjunarayanan.N.S , capacitance data. The reconstruction process highly and Sampreeth John Student with Department of Biomedical depends on the sensor under consideration. There is Engineering, Sahrdaya College of Engineering and Technology, various kind of techniques or methods of tomographic Kodakara, P B No.17, Thrissur- 680 684, Kerala, India( E-mail measurement used in industrial processes. Each method :firstname.lastname@example.org) has its particular advantages and disadvantages. Various Mr. Jis Paul is Sr.Lecturer with Department of Biomedical Engineering, Sahrdaya College of Engineering and Technology, forms of tomography are being investigated around the Kodakara, P B No.17, Thrissur- 680 684, Kerala (E-mail: world, of which includes: Electrical Resistance email@example.com) Tomography, Electrical Capacitance Tomography, 2 Electrical Impedance Tomography, Optical Sensor measurements of the concentration distributions at two Tomography and Ultrasound or Ultrasonic Tomography. axial planes permit the velocity profile and the overall flow rate to be found in some cases. Electrical Capacitance Tomography has mainly 3 units Sensing III. TECHNOLOGY OVERVIEW unit, which will capture capacitance values from the specimen under electrodes, Secondly, Acquisition and Pulse generation unit, which will generate a fixed duration ECT is used to obtain information about the pulse and acquire capacitance values which will be further spatial distribution of a mixture of dielectric materials digitized for image processing and finally Reconstruction inside a vessel, by measuring the electrical capacitances and displaying unit will reconstruct image from the between sets of electrodes placed around its periphery and digitized readings using appropriate algorithm and converting these measurements into an image showing the displays it. Basic modules of ECT is shown in Fig.1 distribution of permittivity as a pixel-based plot or image averaged over a volume whose length is equal to that of the measurement electrodes. The images are approximate IV. PRINCIPLE and of relatively low resolution, but they can be generated at relatively high speeds. Although it is possible to image The principle is based on the measurement of the vessels of any cross section, most of the work to-date has capacitances between electrodes located on the exterior of been carried out on circular vessels. the region of interest, e.g. of a pipe. For instance, in the figure above, two capacitances (C12, 15 and C1, 4) as ECT can be used with any arbitrary mixture of shown in Fig 2.Since the capacitance depends on the different non-conducting dielectric materials such as permittivity value of the material located between the plastics, hydrocarbons, sand or glass. However, an electrodes, substances of differing dielectric properties important application of ECT is viewing and measuring can be distinguished by means of this method. To obtain a the spatial distribution of a mixture of two different spatially resolved image of the permittivity distribution dielectric materials (a two-phase mixture), as in this case, inside the pipe, multiple electrodes are arranged on the the concentration distribution of the two components over tube and all inter-electrode capacitances are measured. the cross-section of the vessel can be obtained from the Capacitance mainly dependents on area of the plate, permittivity distribution. The permittivity image resolution distance between plates and dielectric constant. Here achievable depends on the number of independent dielectric constant only changes which result in capacitance measurements, but is generally low. However, capacitance change. images can be generated at high frame rates, typically up to 5000 frames per second (fps). Fig.2 Capacitance Measurement Pattern Fig.1 Basic Modules of ECT System V. SENSING UNIT Successful applications of ECT include imaging Sensing unit consist of an eight electrode system 2-phase liquid/gas mixtures in oil pipelines and solids/gas which is excited using a pulse generator and capacitance mixtures in fluidized beds and pneumatic conveying reading along each electrode is taken by switching it using systems. Where the mixture is flowing along the vessel, a multiplexer and these values are sent for image processing . Capacitance measurements are made between 3 each electrode and every other electrode. For a sensor Reconstruction complexities vary depending on the type containing E measurement electrodes, there are E (E-1)/2 of input data and algorithm used. After reconstruction the unique capacitance measurements. This corresponds to 28 image is displayed on the screen for analysis of the individual measurements for our 8 electrode sensor. The specimen under sensing unit. number of electrodes used will depend on the A. Basic Steps in Image Reconstruction measurement priorities. There is a trade-off between the axial and radial resolutions of ECT sensors, the The basic steps involved in image reconstruction and capacitance measurement sensitivity and the maximum displaying operation are as follows: data capture rate .The sensor is developed by etching designed Pcb layout in a flexible copper pcb and it is 1) The properties of the sensor are measured or mounted on a 50mm PVC pipe. calculated to produce a sensitivity matrix of the sensor. This is a set of sub-matrices whose elements correspond to the individual pixels in a rectangular grid which is used to define the sensor cross-section. The sub-matrices are known as sensitivity maps and database is prepared. 2) The sensor is normally calibrated at each end of the range of permittivities to be measured by filling the sensor with the lower permittivity material initially and measuring all of the individual inter-electrode capacitances. This operation is then repeated using the higher permittivity material.. 3) Once the system has been calibrated, the Fig.3 Electrode design for an 8-electrode sensor. capacitances between all unique pairs of sensor electrodes are measured continuously at high speed, giving E.(E-1) values per measurement or image frame, where E is the number of sensor VI. ACQUISITION UNIT electrodes. Primary objective of Acquisition unit is measure the capacitances for all possible electrode combinations 4) An image reconstruction algorithm is used to and send it to computer in digital format. This unit consist compute the cross sectional distribution of the mainly 4 subunits namely – Pulse Generator which is permittivity of the material inside the pipe. coded in a PIC16F877A which will produce continuous Images can be constructed from the capacitance pulse finite number of time as we are taking measurement. measurements at the time of measurement (on- We use 8 ports for delivering pulse to 8 electrodes in one line). We use Linear Back-Projection (LBP) at a time fashion, Multiplexer which is used to switch each algorithm  because it is fast but electrode for giving pulse input and measure each approximate algorithm which uses the electrode pair. There are total 28 measurement pairs and capacitance measurements, together with the these switching is controlled by PIC 16F877A. sensitivity map to produce the image. Capacitance measurement which circuit measure capacitance and convert into equivalent voltage which can be used for reconstruction in digitized format by an B. Linear Back Projection Analog-to-Digital Converter and Microcontroller for controlling all these subunits. In this algorithm, the transpose of the sensitivity matrix, ST, is used. To apply the LBP, both sensitivity matrix and the measured capacitance values are normalized. The permittivity distribution can be obtained VII. RECONSTRUCTION UNIT as follows. Image Reconstruction is the final and complex K = ST.C (1) process in the development of ECT system. 4 VIII. WORKING capabilities in Process Tomography. We have achieved goal of our project of imaging test material i.e. water for displaying our system’s capability. ECT system comprises of mainly 3 units-Sensing unit, Signal conversion and processing unit and Image A. Testing of the ECT system Reconstruction unit. Sensing unit basically is a 8-electrode system which is arranged cylindrically with a diameter of Our ECT system is tested by following steps: 5cm. A pulse generator will generate 400ns pulse to one 1) The system is switched on and checks whether the of the electrodes say 1 and capacitance is measured electrodes are properly connected by giving test pulse. between other electrodes say 1-2,1-3,1-4,1-5,1-6,1-7 and 2) Then system is calibrated with surrounding air which 1-8 electrodes. Each detector and source is selected using has dielectric permittivity = 1 which is represented by a high speed channel selection by the micro controller. color blue. Data from the ports is transmitted to the computer by 3) Water is used as test medium since it has high dielectric using the serial communication. These values are constant (~80) .When water is filled at different levels it measured using a capacitance measurement circuit which will show equivalent permittivity distribution images. works on the principle charging and discharging period of 4) Image will be displayed on the front panel of the a capacitor and will vary according to various dielectric Software developed using MATLAB 7.9 version. constants material present inside the sensor. First excitation pulse is applied to electrode 1 while keeping all B. Result other at zero potential and measure charging voltage We have developed sensitivity maps of our between the electrode 1 and 2 for a given fixed duration. system which is necessary for reconstruction and using It is then passed to the 10-bit inbuilt ADC converter in the that we have displayed image of the water filled inside the PIC16f877a which takes 20 micro sec times to convert pipe cross-section and our system is capable to image digital signal for one measurement and stored into a buffer continuously the change occurring inside the pipe. The defined inside MATLAB for storing it as a matrix. ASCII image was good and further research must be done to characters are used to determine starting and ending of improve the quality of image. each frame. The matrix obtained is multiplied along with sensitivity matrix for image reconstruction.PIC will be programmed to control all the data flow and switching operation. The reconstructed image will be permittivity distribution inside the sensor. Fig.5 Primary Sensitivity Maps for an 8-electrode sensor Fig.4 Block Diagram of ECT System IX. TESTING AND RESULT The project on Electrical Capacitance Fig 6.2 Three-forth water is present inside the pipe and is represented by Tomography has been completed successfully. The bright color border prototype developed by us will help in proving its 5 In our project we had initialized the ADC development of this instrument and we are proud in module of the PIC16F877A instead of using the separate transferring our effort for the use of common man. The ADC modules such as ADS7805P/AD676JD which we project also gave more exposure to the various electronic couldn’t use in our project due to its limited circuits, software programs and different classes of its availability.PIC16F877A requires a few steps for components. The project has provided a chance for initializing its ADC module. It’s about 20µs .This has complete evaluation of difficulties in designing and become a major problem in our capacitance measurement development of electronic devices thereby enhancing the and makes our system response slow. We adopted the practical and technical knowledge of the team. The project method of using high value resistance 2.2k in series with has also helped us to see the importance of innovative the capacitance there by increasing the charging time. This ideas for serving humanity by linking the human life with method gave us a chance to read and take out the values advanced and sophisticated technologies. from the electrodes to the ADC module of the PIC. Even though we use this basic technique we prefer to use a ACKNOWLEDGMENT separate ADC module that will reduce the risk and also helps us to get values more accurately and faster. I owe my whole-hearted thanks to Almighty God for helping us through this venture. I would like to thank our In the reconstruction side we used LBP method. Executive Director Rev. Fr. Jose D Irimpan and Principal In the LBP method forward and inverse transforms are Dr. Sudha George Vallavi for their encouragement and defined by two equations given by C=SK and K=STC support. This work would not have been possible without ,where S is the sensor sensitivity matrix and ST is the the help and inspiration from our HOD Mrs. Asha, Project transpose sensitivity matrix and forward equation is coordinator Mr. Jibin Jose and Project guide Mr. Jis Paul. assumed to be accurate, inverse equation to be inaccurate. I would also like to express my sincere gratitude to all the It is possible to calculate enhanced inverse transforms staff members in Biomedical Engineering Department and which give better quality images than those produced by my friends who helped me in my endeavor. Above all I the LBP. There are two different matrices are there for express my deep sense of gratitude and profound thanks to these enhancements. They are Tikhonov transform and my loving parents who gave me support in all means Landweber method. throughout the work. During course of our project we also came to conclusion that broader electrodes provide enhanced REFERENCE images. In addition, better images are obtained when using shorter electrodes; whereas longer electrodes are  Christopher Edward Harford, Flat Panel Sensor for less sensitive to elements far from the electrode cross- 3D Electrical Capacitance Tomography, The Department section. The acceptability of the quality of image is of Electrical Engineering at the University of Cape subjective. The capacitance data might be with large Town,2006 noise, thus giving rise to wide variation in the image quality. We also understood a suitable method should be  Process Tomography Limited Homepage - used to filter this noise and to obtain good quality www.tomography.com capacitance measurements.  S. Sathyamoorthy, J. Saratchandrababu, Design of high speed pulse input based capacitance X. FUTURE measurementfor electrical capacitance tomography Research on feasibility of Electrical Capacitance Sensors and Transducers Journal, Vol. 75-1, 2007, pp. Tomography for 4-D imaging and enhance its field of 896-902., application to other branches of engineering. Electrical capacitance tomography will be useful a lot if it can be  W. Q. Yang, Modeling of capacitance combined with existing imaging modalities which will tomography sensors, Proc. Inst .Elect. Eng. Sci. Meas. give more details of the scanned specimen and enhance Technol., Vol.144, No. 5, 1997, pp. 203–208. quality of imaging. XI .CONCLUSION Electrical Capacitance Tomography is one of our dream projects and we have put lots of pain in the