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Electrical Capacitance tomography

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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.

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                           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
:jackson.ouseph@gmail.com)
                                                                       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
jisspaul79@gmail.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 [4].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 [4][3] 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       [1]   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       [2]    Process Tomography Limited Homepage -
used to filter this noise and to obtain good quality          www.tomography.com
capacitance measurements.
                                                              [3]     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      [4]      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

								
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