Electrical Capacitance Tomography

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1 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 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. Index Terms Tomography, capacitance measurement, electrodes, LBP, sensitivity matrix T I. INTRODUCTION omography by definition refers to the process of exploring the internal characteristics of a specified region through integral measurements related to the internal characteristics of the specified domain. Traditionally, the term tomography has been used for the process of obtaining 2D cross sections called tomograms. However, recent developments in different tomography discipline have expanded the tomography concept to include 3D and 4D (with time domain included) imaging. Manuscript was prepared on April 28, 2010 Ajeesh Sunny, Bify Baby, Jackson Ouseph, Manjunarayanan.N.S, and Sampreeth John Student with Department of Biomedical Engineering, Sahrdaya College of Engineering and Technology, Kodakara, P B No.17, Thrissur , Kerala, India( jackson.ouseph@gmail.com) Mr. Jis Paul is Sr.Lecturer with Department of Biomedical Engineering, Sahrdaya College of Engineering and Technology, Kodakara, P B No.17, Thrissur , Kerala ( jisspaul79@gmail.com) Tomography can be classified into two types: direct and indirect. In the former, a method of visual recording not visible to the human eye is used e.g. X-ray or infrared imaging. Whereas in the later, boundary measurements related to the internal characteristics of the object of interest are used for image reconstruction. In indirect tomography, many physical quantities implemented through different tomography systems can be used as the boundary measurement quantity. However, from engineering point of view, an acceptable tomography technique is one which is (1) Non-invasive: it requires no direct contact between the sensor and the object or domain of interest and (2) Non-intrusive: it does not change or disturb the nature of the object being explored. II. BACKGROUND Research in tomography systems can be classified into three categories: (1) sensor, (2) data acquisition and hardware and (3) reconstruction techniques. Sensor design, performance, and associated problems depend on the tomography modality being used. In electrical 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. Problems in the data acquisition of electrical tomography sensors are mainly the low level power of sensed signal and low signal to noise ratio (SNR). The low level of acquired signal is usually reflected on the sensor dimensions. Increased sensor area provides higher SNR. However, a trade off of lower spatial resolution of reconstructed images is usually associated with increasing the sensor dimensions. Reconstruction techniques involve the process of solving the inverse problem for finding the electrical property distribution from the measured capacitance data. The reconstruction process highly depends on the sensor under consideration. There is various kind of techniques or methods of tomographic measurement used in industrial processes. Each method has its particular advantages and disadvantages. Various forms of tomography are being investigated around the world, of which includes: Electrical Resistance Tomography, Electrical Capacitance Tomography,

2 2 Electrical Impedance Tomography, Optical Sensor Tomography and Ultrasound or Ultrasonic Tomography. III. TECHNOLOGY OVERVIEW ECT is used to obtain information about the spatial distribution of a mixture of dielectric materials inside a vessel, by measuring the electrical capacitances between sets of electrodes placed around its periphery and converting these measurements into an image showing the 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 and of relatively low resolution, but they can be generated at relatively high speeds. Although it is possible to image vessels of any cross section, most of the work to-date has been carried out on circular vessels. ECT can be used with any arbitrary mixture of different non-conducting dielectric materials such as plastics, hydrocarbons, sand or glass. However, an important application of ECT is viewing and measuring the spatial distribution of a mixture of two different dielectric materials (a two-phase mixture), as in this case, the concentration distribution of the two components over the cross-section of the vessel can be obtained from the permittivity distribution. The permittivity image resolution achievable depends on the number of independent capacitance measurements, but is generally low. However, images can be generated at high frame rates, typically up to 5000 frames per second (fps). measurements of the concentration distributions at two 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 unit, which will capture capacitance values from the specimen under electrodes, Secondly, Acquisition and Pulse generation unit, which will generate a fixed duration pulse and acquire capacitance values which will be further digitized for image processing and finally Reconstruction and displaying unit will reconstruct image from the digitized readings using appropriate algorithm and displays it. Basic modules of ECT is shown in Fig.1 IV. PRINCIPLE The principle is based on the measurement of the capacitances between electrodes located on the exterior of the region of interest, e.g. of a pipe. For instance, in the figure above, two capacitances (C12, 15 and C1, 4) as shown in Fig 2.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. To obtain a spatially resolved image of the permittivity distribution inside the pipe, multiple electrodes are arranged on the tube and all inter-electrode capacitances are measured. Capacitance mainly dependents on area of the plate, distance between plates and dielectric constant. Here dielectric constant only changes which result in capacitance change. Fig.2 Capacitance Measurement Pattern Fig.1 Basic Modules of ECT System Successful applications of ECT include imaging 2-phase liquid/gas mixtures in oil pipelines and solids/gas mixtures in fluidized beds and pneumatic conveying systems. Where the mixture is flowing along the vessel, V. SENSING UNIT Sensing unit consist of an eight electrode system which is excited using a pulse generator and capacitance reading along each electrode is taken by switching it using a multiplexer and these values are sent for image processing. Capacitance measurements are made between

3 3 each electrode and every other electrode. For a sensor containing E measurement electrodes, there are E (E-1)/2 unique capacitance measurements. This corresponds to 28 individual measurements for our 8 electrode sensor. The number of electrodes used will depend on the measurement priorities. There is a trade-off between the axial and radial resolutions of ECT sensors, the capacitance measurement sensitivity and the maximum data capture rate [4].The sensor is developed by etching designed Pcb layout in a flexible copper pcb and it is mounted on a 50mm PVC pipe. Reconstruction complexities vary depending on the type of input data and algorithm used. After reconstruction the image is displayed on the screen for analysis of the specimen under sensing unit. A. Basic Steps in Image Reconstruction The basic steps involved in image reconstruction and displaying operation are as follows: 1) The properties of the sensor are measured or 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.. Fig.3 Electrode design for an 8-electrode sensor. VI. ACQUISITION UNIT Primary objective of Acquisition unit is measure the capacitances for all possible electrode combinations and send it to computer in digital format. This unit consist mainly 4 subunits namely Pulse Generator which is coded in a PIC16F877A which will produce continuous pulse finite number of time as we are taking measurement. We use 8 ports for delivering pulse to 8 electrodes in one at a time fashion, Multiplexer which is used to switch each electrode for giving pulse input and measure each electrode pair. There are total 28 measurement pairs and these switching is controlled by PIC 16F877A. Capacitance measurement which circuit measure capacitance and convert into equivalent voltage which can be used for reconstruction in digitized format by an Analog-to-Digital Converter and Microcontroller for controlling all these subunits. VII. RECONSTRUCTION UNIT Image Reconstruction is the final and complex process in the development of ECT system. 3) Once the system has been calibrated, the 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 electrodes. 4) An image reconstruction algorithm is used to compute the cross sectional distribution of the permittivity of the material inside the pipe. Images can be constructed from the capacitance measurements at the time of measurement (online). We use Linear Back-Projection (LBP) algorithm [4][3] because it is fast but approximate algorithm which uses the capacitance measurements, together with the sensitivity map to produce the image. B. Linear Back Projection In this algorithm, the transpose of the sensitivity matrix, S T, is used. To apply the LBP, both sensitivity matrix and the measured capacitance values are normalized. The permittivity distribution can be obtained as follows. K = S T.C (1)

A pulse generator will generate 400ns pulse to one of the electrodes say 1 and capacitance is measured between other electrodes say 1-2,1-3,1-4,1-5,1-6,1-7 and 1-8 electrodes.

4 4 VIII. WORKING ECT system comprises of mainly 3 units-sensing unit, Signal conversion and processing unit and Image Reconstruction unit. Sensing unit basically is a 8-electrode system which is arranged cylindrically with a diameter of 5cm. A pulse generator will generate 400ns pulse to one of the electrodes say 1 and capacitance is measured between other electrodes say 1-2,1-3,1-4,1-5,1-6,1-7 and 1-8 electrodes. Each detector and source is selected using a high speed channel selection by the micro controller. Data from the ports is transmitted to the computer by using the serial communication. These values are measured using a capacitance measurement circuit which works on the principle charging and discharging period of a capacitor and will vary according to various dielectric constants material present inside the sensor. First excitation pulse is applied to electrode 1 while keeping all other at zero potential and measure charging voltage between the electrode 1 and 2 for a given fixed duration. It is then passed to the 10-bit inbuilt ADC converter in the PIC16f877a which takes 20 micro sec times to convert digital signal for one measurement and stored into a buffer defined inside MATLAB for storing it as a matrix. ASCII characters are used to determine starting and ending of 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. capabilities in Process Tomography. We have achieved goal of our project of imaging test material i.e. water for displaying our system s capability. A. Testing of the ECT system Our ECT system is tested by following steps: 1) The system is switched on and checks whether the electrodes are properly connected by giving test pulse. 2) Then system is calibrated with surrounding air which has dielectric permittivity = 1 which is represented by color blue. 3) Water is used as test medium since it has high dielectric constant (~80).When water is filled at different levels it will show equivalent permittivity distribution images. 4) Image will be displayed on the front panel of the Software developed using MATLAB 7.9 version. B. Result We have developed sensitivity maps of our system which is necessary for reconstruction and using that we have displayed image of the water filled inside the pipe cross-section and our system is capable to image continuously the change occurring inside the pipe. The image was good and further research must be done to improve the quality of image. 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 Tomography has been completed successfully. The prototype developed by us will help in proving its Fig 6.2 Three-forth water is present inside the pipe and is represented by bright color border

5 5 In our project we had initialized the ADC module of the PIC16F877A instead of using the separate ADC modules such as ADS7805P/AD676JD which we couldn t use in our project due to its limited availability.pic16f877a requires a few steps for initializing its ADC module. It s about 20µs.This has become a major problem in our capacitance measurement and makes our system response slow. We adopted the method of using high value resistance 2.2k in series with the capacitance there by increasing the charging time. This method gave us a chance to read and take out the values from the electrodes to the ADC module of the PIC. Even though we use this basic technique we prefer to use a separate ADC module that will reduce the risk and also helps us to get values more accurately and faster. In the reconstruction side we used LBP method. In the LBP method forward and inverse transforms are defined by two equations given by C=SK and K=S T C,where S is the sensor sensitivity matrix and S T is the transpose sensitivity matrix and forward equation is assumed to be accurate, inverse equation to be inaccurate. It is possible to calculate enhanced inverse transforms which give better quality images than those produced by the LBP. There are two different matrices are there for these enhancements. They are Tikhonov transform and Landweber method. During course of our project we also came to conclusion that broader electrodes provide enhanced images. In addition, better images are obtained when using shorter electrodes; whereas longer electrodes are less sensitive to elements far from the electrode crosssection. The acceptability of the quality of image is subjective. The capacitance data might be with large noise, thus giving rise to wide variation in the image quality. We also understood a suitable method should be used to filter this noise and to obtain good quality capacitance measurements. X. FUTURE Research on feasibility of Electrical Capacitance Tomography for 4-D imaging and enhance its field of application to other branches of engineering. Electrical capacitance tomography will be useful a lot if it can be combined with existing imaging modalities which will give more details of the scanned specimen and enhance quality of imaging. development of this instrument and we are proud in transferring our effort for the use of common man. The project also gave more exposure to the various electronic circuits, software programs and different classes of its components. The project has provided a chance for complete evaluation of difficulties in designing and development of electronic devices thereby enhancing the practical and technical knowledge of the team. The project has also helped us to see the importance of innovative ideas for serving humanity by linking the human life with advanced and sophisticated technologies. ACKNOWLEDGMENT I owe my whole-hearted thanks to Almighty God for helping us through this venture. I would like to thank our Executive Director Rev. Fr. Jose D Irimpan and Principal Dr. Sudha George Vallavi for their encouragement and support. This work would not have been possible without the help and inspiration from our HOD Mrs. Asha, Project coordinator Mr. Jibin Jose and Project guide Mr. Jis Paul. I would also like to express my sincere gratitude to all the staff members in Biomedical Engineering Department and my friends who helped me in my endeavor. Above all I express my deep sense of gratitude and profound thanks to my loving parents who gave me support in all means throughout the work. REFERENCE [1] Christopher Edward Harford, Flat Panel Sensor for 3D Electrical Capacitance Tomography, The Department of Electrical Engineering at the University of Cape Town,2006 [2] Process Tomography Limited Homepage - [3] S. Sathyamoorthy, J. Saratchandrababu, Design of high speed pulse input based capacitance measurementfor electrical capacitance tomography Sensors and Transducers Journal, Vol. 75-1, 2007, pp , [4] W. Q. Yang, Modeling of capacitance tomography sensors, Proc. Inst.Elect. Eng. Sci. Meas. Technol., Vol.144, No. 5, 1997, pp XI.CONCLUSION Electrical Capacitance Tomography is one of our dream projects and we have put lots of pain in the

PROCESS TOMOGRAPHY Ltd. ELECTRICAL CAPACITANCE TOMOGRAPHY SYSTEM TYPE TFLR5000 OPERATING MANUAL VOLUME 1. FUNDAMENTALS OF ECT

PROCESS TOMOGRAPHY Ltd. ELECTRICAL CAPACITANCE TOMOGRAPHY SYSTEM TYPE TFLR5000 OPERATING MANUAL Issue 1 December 2009 VOLUME 1. FUNDAMENTALS OF ECT Software Version 659 Firmware version 2.2p1_48FSR System