GB2401951A

GB2401951A - Electric field sensor

Info

Publication number: GB2401951A
Application number: GB0411398A
Authority: GB
Inventors: Makoto Kageyama
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 2003-05-22
Filing date: 2004-05-21
Publication date: 2004-11-24
Anticipated expiration: 2024-05-21
Also published as: JP4516724B2; JP2004347453A; GB0411398D0; GB2401951B

Abstract

An electric-field sensor has at least two electrode units. The electric-field sensor compares a voltage generated between the electrodes of the electrode units with a threshold voltage, and outputs a detected signal if the generated voltage is equal to, or higher than, the threshold. Each of the electrode units includes: a housing (10) having an electrode installation space (11) defined therein which is filled with an electrolytic solution, an electrode (12) disposed in the electrode installation space, and a dielectric lens (14) sealing the electrode installation space, for converging electric fluxes in the seawater toward the electrode. The dielectric lens has a dielectric constant higher than that of the seawater.

Description

ELECTRIC-FIELD SENSOR, AND

ELECTRODE UNIT FOR THE SENSOR

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to an electric-field sensor, and more particularly to an electric-field sensor that may be laid at the bottom of the sea or within the sea for detecting marine vessels including submarines travailing in the sea.

2. Description of the Related Art:

One conventional electric-field sensor is schematically shown in Fig. 1A of the accompanying drawings. The electric field sensor shown in Fig. 1A has two confronting electrode units 51 disposed in a case 50 in spacedapart relation to each other. As shown in Fig. 1 B of the accompanying drawings, each electrode unit 51 comprises tubular housing having one open end and electrode 61 disposed in housing 60. The space in housing 60 is divided by an ion-sensitive membrane or a porous membrane (referred to as an "ion-sensitive membrane") 62. Electrode installation space 64 is defined between bottom 63 of housing 60 and ion- sensitive membrane 62, and filled with artificial seawater 65. Electrode 61 is disposed inelectrode installation space 64, filled with artificial seawater À 65, or specifically, on bottom 63 of housing 60, and electrically connected to a differential amplifier (not shown). Housing 60 has an opening closed by cover 66 having an inlet hole (not shown) defined therein. When the electric-field sensor is laid on the bottom of the sea or within the sea, seawater flows through the inlet hole in cover 66 into seawater influx space 67 that is defined between cover 66 and ion-sensitive membrane 62.

The principles of a process of detecting a marine vessel with the above conventional electric-field sensor will be described below. General marine vessels carry an electric corrosion-resistant device for applying a voltage to the hull to cancel out a potential which tends to corrode the hull.

Therefore, when the electric corrosion-resistant device is in operation, an electric field is developed in seawater around the hull, passing an electric current through the seawater. The flow of the current caused by the electric field that is produced in the seawater (including artificial seawater) which serves as an electrolytic solution gives rise to an equivalent motion of ions. When a marine vessel moves near the electricfield sensor, therefore, an ion-quantity difference occurs between respective electrodes 61 of two electrode units 51, developing a voltage difference between the electrodes which is dependent on the distance between the electrodes. The voltage difference between the electrodes 61 is amplified by the differential amplifier, and an output voltage from the differential amplifier is compared with a threshold to determine whether a marine vessel exists in a sensing area of the electric-field sensor (for details, see Japanese Laid-open Patent Publication No. 26088/2003, No. 304533/2000).

However, the conventional electric-field sensor has suffered the following problems: (1) If the intensity of an electric field produced within the sea remains the same, the detecting sensitivity of the electric-field sensor is higher as the ion-quantity difference between the two electrodes is greater.

For increasing the ion-quantity difference between the two electrodes, it is necessary to increase the distance between the electrodes or increase the area of the electrodes which is crossed by electric fluxes. Either approach invites an increase in the size of the electric-field sensor.

(2) Since the concentration of the artificial seawater in the electric field sensor is adjusted to the concentration of general seawater, the concentration of the artificial seawater may not be the same as the concentration of seawater where the electric-field sensor is laid. If such a concentration difference exists, then an osmotic pressure is developed between the artificial seawater and the surrounding seawater that are separated from each other by the ion-sensitive membrane, causing the concentration of the artificial seawater to change and clogging the ionsensitive membrane, with the result that the ability of the artificial seawater to transfer ions is lowered. Therefore, the artificial seawater and the ion sensitive membrane need to be periodically replaced or cleaned.

(3) When an osmotic pressure is developed between the artificial seawater and the surrounding seawater, ions are caused to move in the artificial seawater, tending to generate a voltage between the electrodes.

As a result, the electric-field sensor may erroneously detect the presence of a marine vessel in its sensing area though no marine vessel actually exists in the sensing area. The threshold which is to be compared with the output voltage from the differential amplifier may be set to a higher level for preventing such an error from happening. However, the higher threshold is liable to make the electric-field sensor judge that no marine vessel exists in its sensing area though in fact a marine vessel exists in the sensing area.

SUMMARY OF THE INVENTION

It is an object of the preferred embodiment of the present invention to provide an electric-field sensor which is better in sensitivity and accuracy, while having a size equal to, or smaller than, conventional electric-field sensors.

In one aspect the invention provides an electrode unit for use in an electric-field sensor for detecting an electric field in seawater based on a voltage difference developed between two electrodes, the electrode unit comprising: a housing having an electrode installation space defined therein which is filled with an electrolytic solution; an electrode disposed in said electrode installation space; and, means for converging electric fluxes in the seawater toward said electrode.

An electrode unit for use in an electric-field sensor according to another aspect of the present invention has an electrode disposed in an electrode installation space defined in a housing which is filled with an electrolytic solution. The electrode installation space is sealed by a dielectric lens for converging electric fluxes in the seawater toward the electrode, the dielectric lens having a dielectric constant higher than the seawater. The electrode unit according to the present invention can offer one or more of the following advantages: (1) If the intensity of an electric field existing around the electrode units remains the same, then more electric fluxes cross the surface of the electrode. Therefore, using the electrode unit according to the present invention, it is possible to realize an electric-field sensor having a high

sensitivity for detecting a weak electric field.

(2) Since the electrolytic solution filled in the electrode installation space is isolated from the surrounding seawater by the dielectric lens, no osmotic pressure is developed between the electrolytic solution and the surrounding seawater, unlike the conventional electrode unit in which the electrolytic solution and the surrounding seawater are separated from each other by an ion-sensitive membrane. Consequently, the concentration of the electrolytic solution is not changed, and the ion transfer capability is not 1 0 lowered.

(3) As no osmotic pressure is developed between the electrolytic solution and the surrounding seawater, no ions are moved in the electrolytic solution.

(4) Inasmuch as at least seawater influx space 67 in the conventional electrode unit shown in Fig. 1 B is dispensed with, the electrode unit according to the present invention is of a small size, having its entire length smaller than the conventional electrode unit by about 10 mm.

An electric-field sensor according to the present invention has at least two to the above electrode units, compares a voltage generated between the electrodes of the electrode units with a threshold, and outputs a detected signal if the generated voltage is equal to, or higher than, the threshold. The electric-field sensor according to the present invention provides the following advantages: (1) The electric-field sensor can have a sufficient detection sensitivity without increasing the interval between the two electrodes (inter- electrode distance) or without increasing the area of the electrodes of the electrode units. If the electrode unit has its entire length reduced by about mm, then the inter-electrode distance can be increased by about 20 mm without changing the outer dimensions of the electric-field sensor. If the inter-electrode distance is increased by about 20 mm, then the sensitivity of the electric-field sensor is increased by 20 x log(300/280) = 0.6 dB. There- fore, the electric-field sensor according to the present invention has a high sensitivity even if it has the same size as, or a smaller size than, the

conventional electric-field sensor.

(2) The concentration of the electrolytic solution filled in the electrode installation space of each of the electrode units is not changed, and the ion transfer capability is not lowered. Therefore, the electric-field sensor does not need to be periodically collected for replacing or cleaning the electrode units in order to avoid the above drawbacks.

(3) No ions are moved in the electrolytic solution filled in the electrode installation space of each of the electrode units. Therefore, a voltage difference, which would otherwise occur due to the motion of ions in the electrolytic solution, is not generated between the two electrodes. The electric-field sensor is thus free of such a malfunction that it would output a detected signal regardless of no electric field being present in the seawater, and hence is of a high detecting accuracy. The detecting sensitivity of the electric-field sensor is not lowered because it is not necessary to set the threshold to a higher level for preventing the above malfunction.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1A is a block diagram of a conventional electric-field sensor; Fig. 1 B is a cross-sectional view of an electrode unit of the conventional electric-field sensor shown in Fig. 1A; Fig. 2 is a block diagram of an electric-field sensor according to the present invention; Fig. 3 is a cross-sectional view of an electrode unit of the electric

field sensor shown in Fig. 2;

Fig. 4 is a view showing the principles of convergence of electric fluxes with a dielectric lens; Fig. 5A is a view showing the manner in which electric fluxes are converged by the electrode unit according to the invention; Fig. 5B is a view showing the manner in which electric fluxes are converged by an electrode unit to be compared with the electrode unit according tot he present invention; and, Fig. 6 is a view showing the principles of a process of detecting a marine vessel with the electricfield sensor according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An electric-field sensor according to an embodiment of the present invention will be described in detail below. The electric-field sensor ac cording to the present embodiment is an electric-field sensor to be laid on the bottom of the sea or within the sea for detecting marine vessels which travel on the sea. As shown in Fig. 2, the electric-field sensor according to the present embodiment has a pressure-resistant case 5 housing therein two electrode units 1 a, 1 b, a differential amplifier 2, a comparator 3, and a reference-voltage generating circuit 4.

As shown in Fig. 3, each of the electrode units 1 a, 1 b has a tubular housing 10 and an electrode 12 disposed in an electrode installation space 11 in a housing 10. The electrode installation space 1 1 is defined by a dielectric lens 14 sealing an opening 13 of the housing 10 in a watertight manner, a bottom 15 of the housing 10, and a sidewall 16 of the housing 10. The electrode 12 is fixedly mounted on the bottom 15 of the housing 10 by a resin mount (not shown) in confronting relation to the dielectric lens 14.

The electrode installation space 11, which accommodates the electrode 12 therein, is filled with electrolytic solution (artificial water) 17, providing a pseudstate in which the electrode 12 is held in direct contact with seawater at all times. The dielectric lens 14 has a portion exposed to the exterior. When the electric-field sensor is laid on the bottom of the sea, the dielectric lens 14 is held in physical contact with the seawater, and, when the electric-field sensor is laid on the bottom of the sea or within the sea' the electrodes 12 of respective electrode units 1a, 1b are electrically conducted to the surrounding seawater through the dielectric lens 14 and electrolytic solution 17.

The dielectric lens 14 is made of a metal which is hardly corrodable by seawater (e.g. a titanium alloy) and is shaped into a form similar to a convex lens. The dielectric lens 14 has a dielectric constant c1 which is greater than dielectric constant c2 of the seawater. Since a property of electric fluxes is that they tend to concentrate on a region having a high dielectric constant, electric fluxes that are applied to a convex surface 18 of the dielectric lens 14, which is held in contact with the surrounding seawater, are converged by the dielectric lens 14 and are emitted from an opposite convex surface 19 (see Fig. 4). Specifically, as shown in Fig. 5A, when an electric field is developed in seawater in contact with the dielectric lens 14, electric fluxes applied to convex surface 18 of the dielectric lens 14 are converged toward the surface 20 of electrode 12 which faces the opposite convex surface 19. Stated otherwise, the electric fluxes are converged when they pass through the dielectric lens 14, and have a flux density higher on the convex surface 19 than on the convex surface 18.

The material of the dielectric lens 14 is not limited to a titanium alloy, but may be another material insofar as it has a dielectric constant higher than that of the seawater. If the metallic dielectric lens 14 is corrodes, it discharges ions which are liable to produce a voltage between the two electrodes 12. Therefore, the dielectric lens 14 should preferably be made of a material which is highly resistant to seawater. In general, the material of the dielectric lens 14 should be highly resistant to seawater and have a high dielectric constant. From this standpoint, at present a titanium- based metal is most preferable as the material for the dielectric lens 14.

As shown in Fig. 2, the electrodes 12 of the respective electrode units 1 a, 1 b are electrically connected to differential amplifier 2. A voltage corresponding to a voltage difference that is developed between electrodes 12 is amplified to a certain level by differential amplifier 2, and then output to a comparator 3. The comparator 3 compares the voltage input from the differential amplifier 2 with a reference (threshold) voltage input from a reference-voltage generating circuit 4, and outputs a detected signal if the voltage input from the differential amplifier 2 is equal to, or higher than, the reference voltage.

The electric-field sensor thus constructed detects a marine vessel travailing on the sea, as follows: As shown in Fig. 6, when a marine vessel carrying electric corrosion-resistant device 40 travels on the sea, an electric field is produced in the seawater around the marine vessel, as described above. When the electric field is produced in the seawater, an ion-quantity difference dependent on the inter-electrode distance is developed between the electrode 12 of the electrode unit 1 a and the electrode 12 of the electrode unit 1 b, producing a voltage difference between the electrodes 12. A voltage corresponding to the voltage difference between the electrodes 12 is amplified by the differential amplifier 2, and is compared by the comparator 3 with the reference voltage input from the reference- voltage generating circuit 4. If the voltage input to the comparator 3 is equal to, or higher than, the reference voltage, the comparator 3 outputs a detected signal. In this manner, the marine vessel travailing in the sensing area of the electric-field sensor is detected. Each of the electrode units 1a, 1 b has a respective dielectric lens 14 (see Fig. 3) for converging electric fluxes in the seawater toward the respective electrode 12. As can be understood from a comparison between Figs. 5A and 5B, more electric fluxes converged by the dielectric lens 14 cross the surface of each elec bode 12 than those fluxes applied to another electrode unit 71 (see Fig. 5B) without any dielectric lens, an electrode 70 of the electrode unit 71 having the same area as the electrode 12. Consequently, a voltage is produced between two electrodes 12 even under a weak electric field developed in the seawater. This means that the electric- field sensor according to the present invention has a sensitivity higher than the conventional electric-field sensor, which has electrodes having the same area as the electrodes of the electric-field sensor of the present invention. This also means that the electric-field sensor according to the present invention has a sensitivity equal to, or higher than, the conventional electric-field sensor even when the area of the electrodes is reduced.

While a preferred embodiment of the present invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the scope of the following claims.

The text of the abstract filed herewith is repeated here as part of

the specification.

An electric-field sensor has at least two electrode units. The electricfield sensor compares a voltage generated between the electrodes of the electrode units with a threshold voltage, and outputs a detected signal if the generated voltage is equal to, or higher than, the threshold.

Each of the electrode units includes: a housing having an electrode installation space defined therein which is filled with an electrolytic solution, an electrode disposed in the electrode installation space, and a dielectric lens sealing the electrode installation space, for converging electric fluxes in the seawater toward the electrode. The dielectric lens has a dielectric constant higher than that of seawater.

Claims

CLAIMS: 1. An electrode unit for use in an electric-field sensor for

detecting an electric field in seawater based on a voltage difference developed between two electrodes, the electrode unit comprising: a housing having an electrode installation space defined therein which is filled with an electrolytic solution; an electrode disposed in said electrode installation space; and, means for converging electric fluxes in the seawater toward said electrode.
2. The electrode unit according to claim 1, wherein said electric-flux converging means is a dielectric lens having a dielectric constant higher than that of the seawater.
3. The electrode unit according to claim 2, wherein said dielectric lens seals said electrode installation space.
4. An electrode unit for use in an electric field sensor for detecting an electric field in seawater based on a voltage difference developed between two electrodes, comprising: a housing having an electrode installation space defined therein which is filled with an electrolytic solution; an electrode disposed in said electrode installation space; and, r a dielectric lens sealing said electrode installation space, for converging electric fluxes in the seawater toward said electrode, said dielectric lens having a dielectric constant higher than that of the seawater.
5. The electrode unit according to any of claims 2 to 4, wherein said dielectric lens is made of a titanium-based metal.
6. An electric-field sensor for detecting an electric field in seawater based on a voltage differential developed between two electrodes, the sensor comprising: at least two electrode units disposed in spaced-apart relation to each other, each electrode unit being an electrode unit according to any of claims 1 to 5; a differential amplifier for amplifying a voltage differential developed between respective electrodes of said at least two electrode units, and for outputting an amplified voltage; and, a comparator for comparing the amplified voltage output from said differential amplifier with a threshold, and for outputting a detected signal if the amplified voltage output from said differential amplifier is equal to, or higher than, said threshold.
7. An electrode unit for use in an electric-field sensor for detecting an electric field in seawater, the electrode unit being substantially as herein described with reference to and as shown in Figures 2 to 6 of the accompanying drawings.
8. An electric-field sensor for detecting an electric field in seawater based on a voltage differential developed between two electrodes, the sensor being substantially as herein described with reference to and as shown in Figures 2 to 6 of the accompanying drawings.