Abstract:
An electrostatic sensor generally includes a sensor head with at least one passive sensing electrode responsive to an electric field and a high-impedance amplification stage associated with the sensing electrode. The high-impedance amplification stage is configured for outputting at least one output signal in response to an electric signal induced on the at least one sensing electrode by the electric field. The sensor head further includes a screen of electrically insulating material, which is associated with the at least one sensing electrode. In an operational mode of the electrostatic sensor, the screen is electrically charged and induces an electric field in the surroundings of the sensing electrode.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to the sensing of electric fields in the regime commonly referred to as “electrostatics” and in particular to an extremely sensitive electrostatic sensor. 
       BRIEF DESCRIPTION OF RELATED ART 
       [0002]    The field of electrostatics, which concerns itself with electrical charges, potentials and forces, has first been studied in the 18 th  and 19 th  centuries. The most important instruments for exploring the world of electrostatics are electroscope or electrometer. 
         [0003]    An electroscope usually comprises two thin gold leaves suspended from an electrical conductor inside an electrically insulating container. The electrical conductor is connected to an electrode outside the container. The electroscope indicates the presence of a charged body by the gold leaves standing apart at a certain angle. A charged body, which is brought close to or in contact with the electrode induces or transfers a like electric charge to each gold leaf, which in consequence repel each other. 
         [0004]    An electrometer is usually an elaborate variant of a voltmeter with a very high input impedance (up to the order of 10 15  Ohms). Such an electrometer can be used for remotely sensing any electrically charged object. It is not possible, however, to sense uncharged, i.e. electrically neutral bodies. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The invention to provides an electrostatic sensor capable of remotely sensing uncharged electrically conductive bodies. 
         [0006]    An electrostatic sensor generally comprises a sensor head with at least one passive sensing electrode responsive to an electric field and a high-impedance amplification stage associated with the sensing electrode. The high-impedance amplification stage is configured for outputting at least one output signal in response to an electric signal induced on the at least one sensing electrode by the electric field. According to an important aspect of the invention, the sensor head comprises a screen of electrically insulating material, which is associated with the at least one sensing electrode. In an operational mode of the electrostatic sensor, the screen is electrically charged and induces an electric field in the surroundings of the sensing electrode. It has been surprisingly found that by the presence of the charged screen, the electrostatic sensor can be used to detect conductive, uncharged objects, which are in movement with respect to the sensor head or the sensing electrode. The charged screen electrostatically induces a separation of positive and negative charges in the conductive object, which has an effect on the surrounding electric field. When the sensor head is moved with respect to the conductive object, this effect can be detected. As shall be noticed, the sensor is passive in the sense that it does not include an excitation electrode, which applies an alternative electromagnetic field to be sensed by a receiving electrode. 
         [0007]    It will be appreciated that the electrostatic sensor can be used for detecting an electrically uncharged conductive body at rest in a target region. To this effect, the sensor head is moved with respect to the target region and spatial variations of the electric field are sensed so as to locate the electrically uncharged conductive body, e.g. a metal landmine. Similarly, the electrostatic sensor can be used for detecting an electrically uncharged conductive body moving in a target region. In this case, one preferably keeps the sensor head at rest with respect to the target region and senses the spatial variations of the electric field so as to locate the electrically uncharged conductive body. 
         [0008]    The impedance and the amplification factor of the electrostatic sensor can be chosen such that currents in the sensing electrode of the order of 10 −17  Amperes can be measured. Preferably, the gain and/or the input impedance can be adjusted, e.g. by means of a rotary-type switch. As shall be noted, the sensitivity of the system also increases if the electric charge of the screen of electrically insulating material increases. 
         [0009]    Preferably, the electrostatic sensor comprises a grounded reference electrode connected to the amplification stage. 
         [0010]    The sensing electrode can have a variety of forms, e.g. rectangular, circular, cylindrical, etc. Preferably, however, the sensing electrode comprises a stick electrode or a plate electrode. The material of the sensing electrodes may be any good conductor, e.g. copper, gold, silver, aluminium, nickel, etc. It will be appreciated that the sensor&#39;s sensitivity increases with the size of the sensing electrode. In the case of a stick electrode, the insulating screen advantageously comprises a tubular screen arranged coaxially around the electrode, e.g. a plastic drinking-straw. It will be appreciated that the charged layer can be movably or removably mounted on the sensor head. The electrostatic sensor can hence easily be used in two different modes: first, for the extremely sensitive detection of uncharged conductors and second, for the extremely sensitive detection of charged objects. 
         [0011]    According to a preferred embodiment of the invention, the electrostatic sensor further includes a processing unit operationally connected to the amplification stage for analysing the sensed electric field. In particular, the amplification stage or the processing unit can comprise an analog-to-digital converter unit for digitizing the amplified signals. 
         [0012]    According to a further embodiment of the invention, the electrostatic sensor comprises a plurality of passive sensing electrodes. The amplification stage is configured so as to output a plurality of output signals, each one of these output signals being in response to an electric signal induced on a respective sensing electrode. Such an electrostatic sensor can, for instance, be used for tracking the movement of an uncharged, conductive object. The movement of the conductive object can be determined by triangulation methods. The distance from the object to each electrode can be obtained by comparing the amplitudes of the signals induced in the electrodes. The number of electrodes required for following the movement may depend on the degrees of freedom of the object in movement. 
         [0013]    According to yet another embodiment of the invention, the sensing electrodes are arranged in a matrix-like configuration, wherein the distance between the electrodes is substantially smaller than the objects to be detected/and or imaged. With such a sensor, a two-dimensional image of an electric field can be produced. Advantageously, it includes a processing unit connected to the amplification stage for producing the 2D-image of the electric field and/or means for displaying information related to said electric field. In some embodiments of the invention, the matrix is rectangular but it could also be hexagonal. 
         [0014]    An application of an electrostatic sensor is, for instance, the contactless sensing of vibrations. By means of a sensing electrode matrix, two-dimensional images of vibrational modes can be contactlessly obtained. 
         [0015]    It will furthermore be highly appreciated that the electrostatic sensor can be used for recording an electroencephalogram or an electrocardiogram. This is done without applying electrodes on the patient&#39;s skin, which constitutes a considerable advantage over the traditional technique. The sensor head can be configured as a hood with the sensing electrodes distributed over its inner surface. Consequently, a map of the patient&#39;s cerebral activity can be provided. 
         [0016]    Another useful application of the electrostatic sensor is the contactless detection of an electric signal in a cable or wire. As will be appreciated, even a shielded cable or wire can be eavesdropped with a sufficiently sensitive electrostatic sensor. 
         [0017]    The skilled person will appreciate that the electrostatic sensor can be used for detecting landmines, especially low-metal landmines, e.g. by applying the methods above. Today, the most widely used tool for humanitarian demining is the metal detector. The principal drawbacks of metal detectors are the high false alarm rate and the difficulty of finding low-metal mines, e.g. mines composed of less than 0.5% of metal. In this context one may note that for about 20 years, almost all antipersonnel mines produced have been low-metal mines. Since mines are mostly composed of metal and plastic (besides of explosives) a plastic detector constitutes a good alternative or complementary detector for finding mines. Indeed, one can also integrate both metal and plastic detectors in a single mine detector. A landmine detector may for instance comprise an electrostatic field imager (i.e. an electrostatic sensor having the sensing electrodes arranged as a matrix) with a movable or removable plastic screen, which can be electrostatically charged and brought in front of the sensing electrodes. When the plastic screen is moved aside or completely removed, plastic objects can be detected, when it is in place, metal objects can be detected. It will highly be appreciated that the electrostatic field imager provides at least a coarse image of the object sensed, thus allowing determination of size and shape of the object. 
         [0018]    Another interesting application of an electrostatic sensor is the in-vivo detection of a ruminal bolus ingested by a living being. Ruminal boluses are currently used for electronically identifying ruminants. A ruminal bolus is usually constituted by a body having an electronic device for storing and interchanging data, such as a passive RFID transponder unit, which is encapsulated in a capsule presenting a high resistance to the digestive juices and to the processes that take place in the pre-stomachs of ruminants. Materials used for fabricating the capsule include resins, high-density glasses, or materials based on alumina or silica. For identification of the animal, a reading device sends a query signal to the RFID transponder, which in turn emits a response signal containing some information about the ruminant, e.g. an identification code. In some cases, however, there is no response from the RFID transponder unit. Authorities my have an interest in determining if the bolus has intentionally not been put into place or if it is malfunctioning. To find out whether the RFID transponder has a defect or the bolus is not in place, there are presently two options, namely radiography with X-rays or post-mortal examination. Both methods involve prohibitive costs and are not suited for systematic testing. Detecting a bolus with an electrostatic sensor is a viable alternative, as it is non-lethal and involves reasonable costs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
           [0020]      FIG. 1 : is a perspective view of a setup for the detection of the movement of an uncharged conductive object; 
           [0021]      FIG. 2 : is a perspective view of a setup for the detection of a movement in three dimensions of an uncharged conductive object; 
           [0022]      FIG. 3 : is an illustration of the use of an electrostatic sensor for recording an electroencephalogram; 
           [0023]      FIG. 4 : is a perspective view of an experimental setup with an electrostatic sensor adapted for spatially resolved detection of uncharged conductive objects; 
           [0024]      FIG. 5 : is a perspective view of an experimental setup with an alternative electrostatic sensor adapted for spatially resolved detection of unconductive objects; 
           [0025]      FIG. 6 : is a simplified block diagram of the electrical circuits of the electrostatic sensors of  FIGS. 4 and 5 ; 
           [0026]      FIG. 7 : is a block diagram illustrating the contactless detection of signals in a shielded cable. 
           [0027]      FIG. 8 : is a perspective view of an alternative embodiment of a sensor head for an electrostatic sensor; 
           [0028]      FIG. 9 : is an illustration of an electrostatic imager used for ruminal bolus detection; 
           [0029]      FIG. 10 : is a perspective view of a landmine detector comprising an electrostatic field imager. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]      FIG. 1  shows an experimental setup illustrating the detection of a movement of a conductive object by means of an electrostatic sensor  10 . The electrostatic sensor comprises a sensor head  12  with a passive cylindrical metal sensing electrode  14 , which is at rest with respect to a reference system (corresponding in this case to the lab room). The sensor head  12  also comprises an electrically charged plastic screen  16 , which is tangent to the cylindrical sensing electrode  14 . In this case, the plastic screen  16  is arranged between the object to be sensed and the sensing electrode  14 . The sensing electrode  14  is electrically connected to an electrometer  18  integrating a high-impedance amplification stage, preferably with variable amplification. Alternatively, the sensing electrode can also be connected to a computer, via the amplification stage and an analog-to-digital converter. 
         [0031]    A metal body  20  is suspended from the lab room ceiling and may freely swing. When the metal body  20  moves with respect to the electrode  14 , small currents are electrostatically induced in the latter, which can be detected by the electrometer  18 . Experiments under lab conditions have shown that currents of 10 −17  Amperes can be measured. The sensitivity of the system is extraordinarily high and the achieved precision is comparable to interferometric measurement techniques, with the distance from the electrode  14  and the metal body  20  up to three metres. 
         [0032]    The electrostatic sensor of  FIG. 1  can also be used for detecting vibrations of any conductive structure, e.g. an engine or a wall in proximity of an engine. In contrast to acceleration sensors, the electrostatic sensor needs not being in contact with the object that vibrates. This constitutes a considerable advantage, as any additional mass on the object alters its resonance frequencies changes the measurement. 
         [0033]      FIG. 2  shows another experimental setup illustrating the tracking of an object in three dimensions. The electrostatic sensor  10  comprises in this case three passive metal-plate sensing electrodes  14 , each one covered with a plastic screen  16 . The sensing electrodes are electrically connected to a high-impedance amplification stage  22 , which converts the electrical currents electrostatically induced on in the electrodes  14  to output signals. The output signals are passed on to a computer  24 , which is equipped with an analog-to-digital converter. The computer  24  analyses the output signals of the amplification stage  22  and determines the position of the metal body  20  by triangulation, i.e. by calculating the distance of the metal body to each sensing electrode  14 . The movement of the metal body is displayed in real time on the computer screen and stored in memory for later analysis. 
         [0034]      FIG. 3  illustrates the use of an electrostatic sensor  10  for recording an electroencephalogram. The sensor head  12  is shown in a cross-sectional view. The sensor head  12  comprises a stick-like sensing electrode  14 , which is arranged on the axis of a grounded paraboloidal reference electrode  26 . The sensing electrode  14  is fixed to the reference electrode  26  with an insulating mounting  28 . The sensing electrode  14  is covered with a tubular plastic screen  16 , which has been electrically charged prior to the measurement. The sensor head  12  is oriented towards a patient&#39;s head  30 . It should be noted that other sensor head configurations, especially regarding the form of the sensing electrode or the reference electrode can be used. 
         [0035]    The sensing electrode  14  is connected to a high-impedance amplification stage  22  with a shielded cable  32 . The amplification stage amplifies the signals received on the sensing electrode and outputs corresponding output signals. A computer  24 , which includes an analog-to-digital converter, records and analyses the output signals of the amplification unit  22  and visualizes the recorded data  34 . The electrostatic sensor remotely senses the surface electric potentials caused by the currents flowing in the patient&#39;s head. 
         [0036]    The setup illustrated in  FIG. 3  allows the recording of a patient&#39;s electroencephalogram but it will be appreciated that the sensor head could also be directed to other regions of the patient&#39;s body, e.g. the chest for recording an electrocardiogram. A plurality of sensing electrodes may also be used. The measurement method does not require contacting the patient with paste-on electrodes. It shall be emphasised that the sensitivity of the system is greatly enhanced by the screen of electrically charged, insulating material. Furthermore, if the electric charge of the screen increases, the sensitivity of the system increases. By increasing the electric charge of the screen, one may reduce the amplification factor of the high-impedance amplification stage or increase the distance between the patient and the sensing electrode(s). 
         [0037]      FIGS. 4 and 5  show an experimental setup with an electrostatic sensor  10  adapted for spatially resolved detection of uncharged conductive objects. A metal body  20  is suspended from the ceiling and its movements are to be detected by the electrostatic sensor  10 . The sensor head  12  comprises a 10×10 array of sensing electrodes  14 . In the embodiment of the electrostatic sensor shown in  FIG. 4 , each sensing electrode  14  is covered with a charged electrically insulating tubular plastic screen. In the alternative embodiments of  FIG. 5 , a charged plane plastic screen  16 , common to all the sensing electrodes  14 , is arranged between the sensing electrodes and the object to be detected. The plastic screen  16  can be moved from its operational position in front of the sensing electrodes to an inactive position. In its inactive position, the plastic screen  16  is not arranged in front of the sensing electrodes. Switching between operational and inactive positions can be achieved by rotating the plastic screen  16  around an axis outside the matrix of the sensing electrodes  14 . With the plastic screen  16  in its inactive position, the electrostatic sensor  10  can be used for detection of electrostatically charged objects. Grounded reference electrodes  26  are arranged laterally around the sensing electrodes  14 . The sensing electrodes  14  are electrically connected to amplification circuits inside the amplification unit  22 . The signals of sensing electrodes  14  are separately provided to the amplification unit by shielded cables  32  (not all of them shown in the figures) and amplified by an adjustable factor. The input impedance of the amplification circuits is extremely high (up to 10 15  Ohms), so that virtually no current is drawn from the sensing electrodes  14 . The amplified signals are provided to a multiplexer  42  (see  FIG. 6 ), which produces a multiplexed output signal. The multiplexed output signal is provided to a computer  24 , which analyses the received signals. Depending on the application, the computer can display an image of the received signal amplitudes, store the amplitudes in memory and/or identify certain patterns in the image. 
         [0038]    The sensor head  12  may comprise an electric motor, which drives the plastic screen  16  from its operational to its inactive position. The plastic screen  16  can also be achieved as a curtain (see  FIG. 8 ), which is rolled up on a cylinder  52  in its inactive position and which can be moved, manually or automatically, over the sensing electrodes  14  along the direction indicated by arrow  54 . The plastic may be chosen such that the rolling off from the cylinder  52  creates the electrostatic charges on the plastic screen  16 . An additional charging step could then be omitted. 
         [0039]    For detecting the uncharged metal body  20 , the sensor head  12  is in movement with respect to the metal body  20 . The skilled person will appreciate that it can actually be the metal body  20  that moves while the sensor head  12  is at rest. 
         [0040]    Electrostatic sensors like those of  FIGS. 4 and 5  can for instance be used for imaging the modes of a vibrating object, e.g. an engine. With its extremely high impedance, displacements of conductive structures can be remotely detected in the sub-micrometer range. 
         [0041]    It shall further be noted that the electrostatic sensor matrix may be used for recording a spatially resolved electroencephalogram or electrocardiogram. A two-dimensional map of the brain or heart activity may thus be obtained. 
         [0042]    In certain cases, it may prove useful if the sensing electrodes are arranged on a curved surface, for example on the inner side of a hood, which is put over a patient&#39;s head for taking an electroencephalogram at several points of the head. As the sensing electrodes need not being in contact with the patient&#39;s skin, there can be a spacing structure, which keeps them at a defined distance from the head. Air may thus circulate between the sensing electrodes and the head, which greatly enhances the patient&#39;s comfort during the measurement as sweating may for instance be reduced. 
         [0043]    A simplified block diagram of the electrical circuits of an electrostatic sensor as in  FIGS. 4 and 5  is shown in  FIG. 6 . A plurality of passive sensing electrodes  14 . 1 ,  14 . 2 , . . . ,  14 . n  (n being a positive integer) are connected to an amplification stage  22 , which comprise at least one first low-noise operational amplifier  36 . 1 ,  36 . 2 , . . . ,  36 . n  associated with each sensing electrode  14 . 1 ,  14 . 2 , . . . ,  14 . n . In certain embodiments, the output of the first low-noise operational amplifier is connected to an input of a second low-noise operational amplifier. It will be appreciated that the signals on the sensing electrodes  14 . 1 ,  14 . 2 , . . . ,  14 . n  are amplified individually. The ultrahigh impedance of the amplification stage is achieved by the feedback loops  38 . 1 ,  38 . 2 , . . . ,  38 . n . The gain can be adjusted by changing the resistance  40 . 1 ,  40 . 2 , . . . ,  40 . n  of the feedback loops  38 . 1 ,  38 . 2 , . . . ,  38 . n ; preferably, the system comprises a switch or an automated system for adjusting the gain to an optimal value, depending on the amplitude of the sensed signal. After amplification, the signals are fed to a multiplexer  42 , which preferably operates at a rate above 30 Hz, still more preferably between 50 to 100 Hz. Advantageously, the circuits comprise a filtering stage, which eliminates undesired frequency components, like for instance the 50-Hz- or 60-Hz-peak caused by mains. Such a filtering stage may be integrated into the multiplexer  42 . The multiplexed signal is fed to a computer  24 , which is equipped with an analog-to-digital converter and wherein the signal is demultiplexed. The individual signals of the sensing electrodes can thus be retrieved, analysed, displayed and/or stored in memory. 
         [0044]      FIG. 7  illustrates the contactless detection of signals in a shielded communication cable. In a video surveillance system  44 , a digital camera  46  is connected to an input port (e.g. RS 485 serial port) of a control computer  48  via a shielded communication cable  50 . An electrostatic sensor  10  is provided for contactlessly eavesdropping the communication between the camera  46  and the control computer  48 . The sensor head  12  is brought into proximity of the communication cable  50 . The sensor head  12  may e.g. be a smaller version of the sensor head shown in  FIG. 3  and will not be described in detail again. It shall be noted, however, that other sensor head configurations could also used for the present purpose. The sensor head  12  is connected to the high-impedance amplification stage  22 , which feeds the amplified signals to the computer  24 . 
         [0045]    In the present case, the communication signal transmitted between the camera  46  and the control computer  48  is assumed to be of square-wave type. The signals measured by the electrostatic sensor  10 , which are shown in an exemplary fashion on the screen of the computer  24 , are usually not of square-wave type. The intervals between the detected electrostatic peaks correspond to those of the original communication signal. By convolution of the electrostatic signal with a square-wave function, it is possible to retrieve the original communication signal. The electrostatic sensor thus can detect the communication signals either emitted by the camera to the computer or vice versa. 
         [0046]    The electrostatic sensor  10  can also be used to detect the electric signals inside an electronic appliance, e.g. a computer or a camera. For instance, if the sensor head  12  is brought into proximity of the camera  46 , electric activity of the latter can remotely be detected. From the signal detected by the electrostatic sensor  10 , one can draw certain conclusions, for instance, it is possible to determine the recording interval of the camera  46  or to eavesdrop on data exchanges inside the camera by using e.g. Fourier or wavelet analysis methods. 
         [0047]      FIG. 9  illustrates the use of an electrostatic field imager (e.g. as in  FIG. 4 ) for detecting a ruminal bolus. In order to detect the presence of a dysfunctional ruminal bolus  56  in the digestive tract  58  of a ruminant  60 , a sensor head  62  of an electrostatic field imager  64  is arranged next to the ruminant&#39;s body, and an electric field is generated at or from behind the ruminant&#39;s body, e.g. by creating a small electric discharge behind the ruminant or on the ruminant as shown at  63 . The term “behind” is used here with respect to the electrostatic field imager. As the bolus  56  contains a certain amount of electrically insulating material, it alters the electric field caused by the discharge, which can be detected by the electrostatic field imager  64 . The bolus normally consists an elongated substantially cylindrical capsule of about 7 cm long and about 2 cm in diameter. When an insulating object is detected inside the ruminant  60 , the sensor head  62  can be moved in order to determine the shape of the detected object under different angles. From these observations, it can be easily concluded with high certainty whether the detected object is a ruminal bolus or not. 
         [0048]      FIG. 10  illustrates the use of an electrostatic imager for detecting landmines. First, one has to understand that electrostatic charges remain a long time on the plastic parts of a mine, especially if the soil is dry. The electrostatic field created by these charges can be detected by an electrostatic field imager as described above. 
         [0049]    The situation may nevertheless occur that the plastic parts of a mine wear less than a detectable amount of electrostatic charges. It is therefore recommended, especially for humid soil, to first apply an electrostatic discharge to the area that is to be scanned. This can be done by approaching to the ground an electrode at a high electric potential or by using a stun gun (delivering electric discharges of the order of 10 5  V). 
         [0050]    Experiments with dummy mines have shown that even mines, which had been covered with a metal plate can be reliably detected by means of the electrostatic field imager. As a matter of fact, the field created by the electrostatic charges on the mine is not completely stopped at the metal plate due to imperfect grounding. One thus observes an attenuation of the signals on the sensing electrodes, but detection is still possible. 
         [0051]    The landmine detector  66  comprises an integrated electrostatic field imager. The shaft of the battery-powered detector  66  has an armrest  68  on its first end and a sensor head  62  on its second end, which is opposed to the first end. The sensor head comprises a sensing electrode matrix, which faces the ground when the detector is in use. In this case, the matrix is rectangular with ten rows and ten columns, but these numbers and the shape of the matrix may vary. A grounded reference  26  electrode is arranged laterally around each sensing electrode  14 . The landmine detector  66  further comprises an amplification stage for amplifying the signals of the sensing electrodes and an A/D converter for digitizing the amplified signals. A processing unit is integrated into the detector  66 , which analyses the digitised signals. A display  70  is included, which provides in real time a 2D-image of the sensed electric field. As shown in  FIG. 6 , the display  70  can be an LCD integrated into a control unit  72  on the detector handle  74 , by which the detector  66  can be carried. Preferably, the most used control buttons  76  are located on the handle or next to it on the control unit  72  in such a way that the user can actuate them with only one hand, e.g. with the thumb. Those skilled will appreciate that the display  70  could also comprise an a matrix of LEDs, which probably makes the mine detector  66  more affordable and lighter. Moreover, the display  70  can be arranged on the upper side of the sensor head  62 . 
         [0052]    The sensor head  62  comprises an additional plastic screen  16  rotatably mounted thereon. The plastic screen  16  can be brought into an active position, where it is located between the sensing electrodes and the ground  78  or in an inactive position. In its active position, the plastic screen can be electrostatically charged, which enables the landmine detector  66  to detect buried conductive bodies, in particular the metal parts of a mine. 
         [0053]    The handling of the present landmine detector  66  is very similar to metal detectors commonly used for de-mining. The user swings the sensor head  62  at more or less constant speed in small arcs over the track he intends to take. The detection principle is the same as above: when the sensing electrodes move with respect to the electrostatically charged plastic parts of a mine  80 , currents are induced in the sensing electrodes  14 , which can be measured and used for providing an image of the electric field. This image can be directly displayed so that the user may immediately decide whether the detected electric field is caused by a mine  80 . In order to facilitate the user&#39;s task, the detector  66  preferably comprises a discriminator, which analyses the structures of the detected electric field, for instance by comparing these structures with stored ones in a database. In case one of the stored structures matches an actually detected structure, the detector can emit an audible and/or visible alarm. Preferably, the discriminator takes into account environmental conditions, such as humidity, temperature, soil consistency, etc. 
         [0054]    The user can perform a second sweep over the area in front of him, with the plastic screen  16  in its active position. During the second sweep, metal parts are detected. The combined results of the two sweeps constitute an improved basis for evaluating the situation. In elaborate versions of the mine detector  66 , the processing unit may be able to automatically combine the images of the two sweeps. 
         [0055]    It will be appreciated that the electrostatic field imager can be combined with other mine detection devices for increasing their reliability.