Electrical Capacitance tomography

Description

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, Image Reconstruction, Process Tomography, Electrical capacitance, industrial process, capacitance measurements, tomography system, inverse problem, inverse problems, reconstruction algorithm, sensor design, electrical tomography, sensitivity maps, flow measurement, Neural Network,

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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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