WO1997000453A1 - Mesure de capacite electrique - Google Patents

Mesure de capacite electrique Download PDF

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Publication number
WO1997000453A1
WO1997000453A1 PCT/GB1996/001438 GB9601438W WO9700453A1 WO 1997000453 A1 WO1997000453 A1 WO 1997000453A1 GB 9601438 W GB9601438 W GB 9601438W WO 9700453 A1 WO9700453 A1 WO 9700453A1
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WO
WIPO (PCT)
Prior art keywords
electrodes
electrode
capacitance
excitation signal
excitation
Prior art date
Application number
PCT/GB1996/001438
Other languages
English (en)
Inventor
Wuqiang Yang
Original Assignee
Process Tomography Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Process Tomography Limited filed Critical Process Tomography Limited
Priority to AU62316/96A priority Critical patent/AU6231696A/en
Publication of WO1997000453A1 publication Critical patent/WO1997000453A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/22—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance
    • G01N27/24—Investigating the presence of flaws

Definitions

  • the present invention relates to a method and apparatus for measuring the capacitance between combinations of two or more electrodes.
  • ECT Electrical capacitance tomography
  • ECT systems are well known and are used to produce a visual representation of a physical process, for example a visual representation of the distribution of a two component fluid flow in an oil pipeline.
  • ECT systems produce cross-sectional images of a flow based on variations in the permittivity of a material flowing through or located within a sensing assembly.
  • Conventional sensing assemblies consist of an array of electrodes mounted either inside or outside an insulating pipe. For example in one known system twelve equally spaced electrodes are distributed around the circumference of a
  • ECT systems exhibit relatively low detection sensitivity at the centre of the electrode array which corresponds to the centre of the vessel within which measurements are being made.
  • all of the electrodes other than the single excitation electrode are held at a fixed potential and as a result field lines fan out from the excited electrode.
  • the field density at the centre of the array is relatively weak and hence the sensitivity in this region is low.
  • the signal to noise ratio and hence image quality in the central area could be improved by changing the excitation arrangement to produce a modified electric field inside the sensing array. This could be achieved for example by exciting half of the electrodes with an appropriate potential and by using the remaining electrodes as detection electrodes with each detection electrode being maintained at an appropriate complimentary potential to that ofthe opposite excitation electrodes.
  • the solution of the image calculation problem is therefore highly undetermined. If more measurements can be obtained, a more accurate solution for the image will be obtained.
  • a method for measuring the capacitance between two or more electrodes wherein an excitation signal is applied to at least one of the electrodes, and a measurement signal representative of the capacitance is derived from the electrode to which the excitation signal is applied.
  • the invention also provides an apparatus for measuring the capacitance between two or more electrodes, comprising means for applying an excitation signal to at least one of the electrodes, and means for deriving a measurement signal representative of the capacitance from the electrode to which the excitation signal is applied.
  • the present invention enables the number of measurements to be taken for a given number of electrodes to be substantially increased, thereby enabling greater measurement accuracy. Furthermore the present invention also enables the pattern of fields between the electrodes to be varied as between one measurement and another which allows an increase in accuracy and sensitivity. Given that each electrode can be used as a source of a measurement signal as well as a point to which an excitation signal can be applied, it is no longer necessary to operate most ofthe electrodes simply as passive detection electrodes. Hence the number of measurements which can be made for a given electrode configuration can be substantially increased.
  • excitation signals are applied to each electrode of an array and measurement signals are derived from each of those electrodes.
  • a guard electrode is associated with each capacitance electrode, the signal applied to each capacitance electrode also being applied to the associated guard electrode.
  • each electrode to which an excitation signal is applied is connected to one input of a first operational amplifier to the other input of which an excitation signal source is connected, and the or each measurement signal is derived from the output of the second operational amplifier one input of which is connected to the output of the first operational amplifier and the other input of which is connected to the excitation signal source.
  • the one input of the first operational amplifier is preferably directly connected to the electrode to which the excitation signal is applied to avoid any risk of spurious signal injections.
  • a guard ring may be provided which is connected to the excitation signal source.
  • An array of equally spaced electrodes may be provided with each electrode in the array being connected to a respective circuit capable of applying an excitation signal and deriving a measurement signal from the electrode to which it is connected.
  • a series of measurement signals may be derived from each of the electrodes with the excitation signals applied to the electrodes being varied during the series to alter the pattern and the direction of the field between the electrodes.
  • Figure 1 illustrates schematically the structure of the conventional 12-electrode ECT sensor
  • Figure 2 shows a simplified circuit diagram of a known ECT system which is used to monitor the capacitance between any two ofthe electrodes shown in figure 1;
  • Figure 3 illustrates a circuit which can be used to put into effect the method in accordance with the present invention
  • Figure 4 illustrates a circuit similar to that of figure 3 but inco ⁇ orating guard electrodes to prevent stray capacitance affecting the system;
  • Figure 5 illustrates an equivalent circuit to the structure shown in figure 4
  • Figure 6 illustrates the overall structure of an ECT 12-electrode sensor utilising circuits of the type illustrated in figures 3 to 5;
  • Figure 7 illustrates one arrangement for varying the excitation signals to a circuit of the type illustrated in figure 6;
  • Figures 8, 9, 10 and 11 illustrate alternative electrode connections to indicate some of the wide range of electric field distributions which can be established in accordance with the invention inside a sensor ofthe general structure shown in figure 1.
  • FIG. 1 twelve electrodes indicated by reference numerals 1 to 12 are arranged around the outside of a pipe 13 the interior 14 of which will in use carry for example a 2-component fluid flow.
  • one of the electrodes is connected to an excitation source and each of the other electrodes is connected to a detector.
  • Figure 2 illustrates part of the resultant circuit in schematic form.
  • a terminal 15 is connected to a signal source and a terminal 16 is connected to a measurement circuit.
  • a capacitor 17 represents the capacitance between two of the electrodes shown in figure 1.
  • the upper half of the capacitor 17 could represent the electrode 3 and the lower could represent the electrode 9.
  • the upper electrode of the capacitor is initially selected as an excitation electrode by closing switch 18 and opening switch 19 and the other electrode is connected to the detector by closing switch 20 and opening switch 21. This connection can be reversed by appropriate control of the switches 18 to 21.
  • This known arrangement involving electronic switches can be vulnerable to measurement errors because of problems associated with leakage, including charge injection effects and spurious coupling capacitance. As the measured capacitance in an ECT system, and especially the capacitance change which must be accurately detected, is very small, of the order of 10 " 3 F, and the switch equivalent capacitance is relatively large, of the order of 10 " F, any leakage of charge through the switch causes measurement inaccuracies.
  • FIG 3 shows a circuit which can be used to implement the present invention.
  • the illustrated circuit is AC -based, that is it relies upon applying an alternating signal, for example a sinusoidal signal.
  • Terminals 22 and 23 are connected to sine-wave sources with the same amplitudes but in anti-phase, and these terminals are connected to the positive terminals of two operational amplifiers 24 and 25.
  • sinusoidal signals are applied to the electrodes, any appropriate waveform could be applied, for example a square wave.
  • the gain-setting feedback components for the operational amplifiers are shown as capacitors C f , although resistive elements will also be needed in practice to set an appropriate DC gain at a finite value.
  • a capacitor 26 represents the capacitance C x defined between for example electrodes 3 and 9 of figure 1.
  • the left hand electrode of capacitor 26 is held at the potential applied to the terminal 22 and the right hand electrode of capacitor 26 is similarly held at the potential applied to terminal 23.
  • the two electrodes of the capacitor are thus excited simultaneously.
  • the output ofthe amplifier 25 is given by:
  • the outputs V 2 and V 3 are proportional to the capacitance between the two electrodes to which the circuits are connected.
  • the capacitance "seen" from one electrode will be different from the capacitance
  • An alternative arrangement can be provided which allows the detection electrodes to remain at ground potential. This can be achieved by offsetting the potentials of all the electrodes by the same voltage such that the detection electrodes are at ground potential. This is at the expense however of not being able to measure the capacitances between all the electrodes simultaneously.
  • FIG 3 shows an arrangement corresponding to that of figure 3 but inco ⁇ orating guard electrodes 29 and 30.
  • the left hand guard electrode 29 is directly connected to the terminal 22 and the right hand guard electrode 30 is directly connected to the terminal 23.
  • the guard electrodes are themselves located inside an earthed screen 31.
  • Figure 5 schematically represents the equivalent circuit for the structure of figure 4. Capacitances C sl and C s2 represent stray capacitances between the measurement electrode and the guard electrode, and capacitances C s3 and C s4 representing the stray capacitances between the guard electrodes and the earth screen.
  • C sl and C s2 are driven by the same sine-wave sources as those used to set the potentials on the electrodes of the unknown capacitor C x , the potentials across C sl and C s2 are zero. Therefore there are no currents through C sI and C s and they do not affect the capacitance measurement. Furthermore, as C s3 and C s are directly driven by the sine-wave sources, they draw current from the sources only and hence do not affect the measurement of C x . Therefore the illustrated circuit is stray- immune.
  • FIG. 6 illustrates an ECT sensor based on the principles described with reference to figures 3, 4 and 5.
  • Each of the twelve electrodes has its own associated guard electrode located radially outside the measurement electrode.
  • Twelve independent measuring channels are connected to the electrode array, each of the channels corresponding to one of the two circuits shown in figures 3, 4 and 5.
  • Each channel is connected to a respective measurement electrode and a respective driven guard electrode.
  • Figure 7 illustrates one circuit for varying the excitation signal applied to one electrode by one pair of operational amplifiers.
  • a micro-processor 31 controls a digital to analogue (DAC) converter 32 which in turn controls operational amplifiers 33 and 34.
  • DAC digital to analogue
  • a sine-wave source is connected to terminal 35, and thus the output of the amplifier 34 corresponds to the signal applied to for example electrode 23 of figure 3.
  • the magnitude of that signal is a function of the output of the DAC 32 which in turn can be controlled by the micro-processor 31.
  • Figures 8 to 1 1 illustrate some of the excitation signal patterns which are possible using the circuitry of figure 6 and 7. Each pattern results in a different field distribution in the sensor.
  • Figure 8 shows a pattern corresponding to a conventional ECT excitation arrangement where only one electrode at a time is set to be an excitation electrode and all of the other electrodes are held at the same zero potential and used for detection pu ⁇ oses only. Such an excitation arrangement can produce 66 independent measurements given that there are 12-electrodes.
  • Figure 9 shows how two electrodes at a time are in effect combined by applying to them the same excitation signal, all the other electrodes being held at zero potential. With such an arrangement 120 extra measurements can be obtained.
  • Figure 10 shows how a substantially parallel field can be set up inside the sensor by applying different potentials to pairs of electrodes. Such an arrangement can produce 72 further measurements, that is 12 individual capacitance measurements for each of six rotations.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Measurement Of Resistance Or Impedance (AREA)

Abstract

Procédé et appareil de mesure de capacité entre des combinaisons d'électrodes faisant partie d'une structure d'électrodes. Les signaux d'excitation sont appliqués à toutes les électrodes et les signaux de mesure représentatifs des capactités inter-électrodes dérivés de toutes les électrodes. Chaque électrode à laquelle est appliqué un signal d'excitation est reliée à l'une des entrées d'un premier amplificateur opérationnel à l'autre entrée duquel est reliée une source de signaux d'excitation. Le signal de mesure est dérivé de la sortie d'un second amplificateur opérationnel dont l'une des entrées est reliée à la sortie du premier amplificateur opérationnel et l'autre à la source de signaux d'excitation.
PCT/GB1996/001438 1995-06-16 1996-06-17 Mesure de capacite electrique WO1997000453A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU62316/96A AU6231696A (en) 1995-06-16 1996-06-17 Capacitance measurement

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GBGB9512322.0A GB9512322D0 (en) 1995-06-16 1995-06-16 A flexible field excitation and measurement technique for electrical capacitance tomography systems
GB9512322.0 1995-06-16

Publications (1)

Publication Number Publication Date
WO1997000453A1 true WO1997000453A1 (fr) 1997-01-03

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Application Number Title Priority Date Filing Date
PCT/GB1996/001438 WO1997000453A1 (fr) 1995-06-16 1996-06-17 Mesure de capacite electrique

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AU (1) AU6231696A (fr)
GB (1) GB9512322D0 (fr)
WO (1) WO1997000453A1 (fr)

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1267158A2 (fr) * 2001-06-13 2002-12-18 Laboratoire Central Des Ponts Et Chaussees Méthode de diagnostic de câbles de précontrainte externe contenus dans des gaines
EP1391751A2 (fr) * 2002-08-23 2004-02-25 Forschungszentrum Jülich Gmbh Procédé et dispositif pour la mesure tomographique rapide de la distribution de la conductivité électrique dans un échantillon
CN100362341C (zh) * 2005-12-22 2008-01-16 天津大学 Ert/ect双模态成像系统复合阵列传感器
CN102620855A (zh) * 2012-03-30 2012-08-01 华南理工大学 一种基于电容层析成像的聚合物熔体温度场测量方法及系统
EP2784494A1 (fr) 2013-03-26 2014-10-01 Rechner Industrie-Elektronik GmbH Système de reconnaissance et/ou de détermination de corps ou de substances
FR3062211A1 (fr) * 2017-01-24 2018-07-27 Technip France Procede de controle non destructif d'une ligne flexible et dispositif de controle non destructif associe
CN111579604A (zh) * 2020-05-20 2020-08-25 中国民航大学 一种可旋转的平面电容层析成像传感器

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3668523A (en) * 1969-05-07 1972-06-06 Bell Telephone Labor Inc Electrical testing of dielectric layers, exhibiting voltage dependent capacitance, with linear ramp voltages
FR2543687A1 (fr) * 1983-03-31 1984-10-05 Raffinage Cie Francaise Procede et dispositif pour la determination, en continu, de la teneur en l'un de ses constituants, d'un melange eventuellement heterogene
US4918376A (en) * 1989-03-07 1990-04-17 Ade Corporation A.C. capacitive gauging system

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3668523A (en) * 1969-05-07 1972-06-06 Bell Telephone Labor Inc Electrical testing of dielectric layers, exhibiting voltage dependent capacitance, with linear ramp voltages
FR2543687A1 (fr) * 1983-03-31 1984-10-05 Raffinage Cie Francaise Procede et dispositif pour la determination, en continu, de la teneur en l'un de ses constituants, d'un melange eventuellement heterogene
US4918376A (en) * 1989-03-07 1990-04-17 Ade Corporation A.C. capacitive gauging system

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1267158A2 (fr) * 2001-06-13 2002-12-18 Laboratoire Central Des Ponts Et Chaussees Méthode de diagnostic de câbles de précontrainte externe contenus dans des gaines
FR2826122A1 (fr) * 2001-06-13 2002-12-20 France Etat Ponts Chaussees Methode de diagnostic de cables de precontrainte externe contenus dans des gaines
EP1267158A3 (fr) * 2001-06-13 2005-11-09 Laboratoire Central Des Ponts Et Chaussees Méthode de diagnostic de câbles de précontrainte externe contenus dans des gaines
EP1391751A2 (fr) * 2002-08-23 2004-02-25 Forschungszentrum Jülich Gmbh Procédé et dispositif pour la mesure tomographique rapide de la distribution de la conductivité électrique dans un échantillon
EP1391751A3 (fr) * 2002-08-23 2004-04-07 Forschungszentrum Jülich Gmbh Procédé et dispositif pour la mesure tomographique rapide de la distribution de la conductivité électrique dans un échantillon
CN100362341C (zh) * 2005-12-22 2008-01-16 天津大学 Ert/ect双模态成像系统复合阵列传感器
CN102620855A (zh) * 2012-03-30 2012-08-01 华南理工大学 一种基于电容层析成像的聚合物熔体温度场测量方法及系统
EP2784494A1 (fr) 2013-03-26 2014-10-01 Rechner Industrie-Elektronik GmbH Système de reconnaissance et/ou de détermination de corps ou de substances
FR3062211A1 (fr) * 2017-01-24 2018-07-27 Technip France Procede de controle non destructif d'une ligne flexible et dispositif de controle non destructif associe
WO2018138151A1 (fr) * 2017-01-24 2018-08-02 Technip France Procede de controle d'une ligne flexible et dispositif de controle associe
US11169106B2 (en) 2017-01-24 2021-11-09 Technip France Device and method for nondestructive inspection of a flexible underwater pipe
AU2018211384B2 (en) * 2017-01-24 2022-10-20 Technip France Method for controlling a flexible line and associated control device
CN111579604A (zh) * 2020-05-20 2020-08-25 中国民航大学 一种可旋转的平面电容层析成像传感器

Also Published As

Publication number Publication date
AU6231696A (en) 1997-01-15
GB9512322D0 (en) 1995-08-16

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