Patent Publication Number: US-2011074444-A1

Title: Sensor for detection of conductive bodies

Description:
The present invention relates to capacitive detection of conductive bodies or targets, e.g. human beings. 
     BACKGROUND 
     Presence of bodies or objects may be detected by determining a change of capacitance between two plates. The presence of an object causes a change in the dielectric constant between the plates, which causes a change in the capacitance formed by said two plates. 
     A capacitive sensor may be used e.g. to detect movements of people e.g. in an anti-theft alarm system. 
     SUMMARY 
     An object of the present invention is to provide a sensor, a system, and a method for detection of conductive bodies. 
     The sensor comprises at least a first signal electrode, a second signal electrode, and a base electrode, which have been disposed in or on an electrically insulating substantially planar substrate. The base electrode is between the signal electrodes, wherein the distance between the first signal electrode and the second signal electrode is smaller than or equal to 20% of the width of the signal electrodes. 
     The sensor according to the invention may provide improved sensitivity when compared to a conventional sensor where the width of the signal electrode is substantially equal to the width of a ground electrode or when difference in the widths of the electrodes is smaller than according to the present invention. 
     The sensor according to the invention may detect the presence of conductive bodies which are farther away from the sensor than in case of conventional sensor where the width of the signal electrode is substantially equal to the width of a ground electrode. The sensor according to the invention has an extended reading distance for conductive objects. 
     The sensor according to the invention may be substantially insensitive to the alignment of the detectable body. The inactive area between the signal electrodes is small, and consequently it is virtually impossible to e.g. step on said inactive area. Blind spots may be avoided. The orientation of e.g. a foot of a person does not have a significant effect on the detectability. 
     The embodiments of the invention and their benefits will become more apparent to a person skilled in the art through the description and examples given herein below, and also through the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following examples, the embodiments of the invention will be described in more detail with reference to the appended drawings, in which 
         FIG. 1  shows a sensor in a three dimensional view, 
         FIG. 2  shows, in a three dimensional view, a person stepping on a sensor, 
         FIG. 3  shows, in a side view, a person&#39;s foot positioned over a signal electrode, 
         FIG. 4  shows, in a side view, a person&#39;s foot positioned over a signal and a base electrode, 
         FIG. 5  shows, in a side view, a person&#39;s foot positioned over a sensor according to prior art, 
         FIG. 6  shows an equivalent circuit of a system comprising a sensor and a body, 
         FIG. 7   a  shows an equivalent circuit of a sensor without the presence of a body, 
         FIG. 7   b  shows an equivalent circuit of a system comprising a sensor, a body, and ground, 
         FIG. 8   a  shows an equivalent circuit of a system comprising a sensor and a cover layer disposed over the sensor, 
         FIG. 8   b  shows an equivalent circuit of a system comprising a sensor, a body, and a cover layer between the sensor and the body. 
         FIG. 9   a  shows signal and base electrodes disposed over a substrate, 
         FIG. 9   b  shows signal and base electrodes disposed under a substrate, 
         FIG. 9   c  shows signal and base electrodes between two substrates, 
         FIG. 9   d  shows signal and base electrodes disposed on different sides of a substrate, 
         FIG. 10  shows a sensor comprising an array of substantially rectangular signal electrodes having a base electrode structure between them, 
         FIG. 11  shows a sensor comprising an array of signal electrode groups, wherein each group comprises several signal electrodes connected in series, 
         FIG. 12  shows a base electrode structure which surrounds signal electrodes only partially. 
         FIG. 13   a  shows a sensor comprising an array of triangular signal electrodes, 
         FIG. 13   b  shows a sensor comprising an array of hexagonal signal electrodes, 
         FIG. 13   c  shows a sensor an array of square signal electrodes having rounded corners, and star-shaped base electrode areas in the vicinity of the corners of the signal electrodes, 
         FIG. 14   a  shows a web comprising signal and base electrode structures, 
         FIG. 14   b  shows a sensor provided by cutting the web of  FIG. 14   a,    
         FIG. 15  shows a measuring system comprising an array of signal electrodes and multiplexing unit, 
         FIG. 16  shows a measuring system comprising an array of signal electrodes and an array of monitoring units, and 
         FIG. 17  shows a sensor comprising an array of substantially circular signal electrodes, 
     
    
    
     All drawings are schematic. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a capacitive sensor  100  comprises a first signal electrode  10   a , a second signal electrode  10   b , and a base electrode structure  20  between said signal electrodes  10   a ,  10   b . The base electrode structure  20  is herein called as a base electrode  20 . 
     The electrodes  10   a ,  10   b ,  20  have been implemented in or on an electrically insulating substantially planar substrate  7 . The sensor  100  may comprise e.g. metal foils  10   a ,  10   b ,  20  attached to a plastic foil  7 . The sensor  100  may be flexible to facilitate transportation and storage in rolls. The thickness of the sensor (in direction SZ) may be smaller than or equal to 1 mm. 
     SX, SY and SZ denote three orthogonal directions. The directions SZ and SY define the plane of the substrate  7 . 
     a 1  denotes the height of the signal electrode  10   a  (in direction SY). s 1  denotes the width of the signal electrode  10   a  (in direction SX). s 3  denotes the distance between the first signal electrode  10   a  and the second signal electrode  10   b . s 2  denotes the width of that part of the base electrode  20  which is between the signal electrodes  10   a ,  10   b . s 4  denotes the width of a gap between the signal electrode  10   a  and the base electrode  20 . 
     The distance s 3  between the first signal electrode  10   a  and the second signal electrode  10   b  may be e.g. in the range of 5 to 30 mm. 
     The width s 2  may be e.g. in the range of 0.3 to 15 mm, advantageously in the range of 1 to 7 mm, preferably in the range of 2 to 7 mm. The width s 4  may be e.g. in the range of 0.3 to 15 mm, advantageously in the range of 1 to 7 mm. 
     The widths s 2  and s 4  may be substantially equal. 
     The surface area of the second signal electrode  10   b  may be in the range of 70% to 150% of the surface area of the first signal electrode  10   b.    
     The surface area of the first signal electrode may be in the range of 0.02 to 0.2 m 2  to match e.g. with size of a foot of a person. 
     The presence of a body in the vicinity of the sensor is detected by monitoring a change in the capacitance of the first signal electrode  10   a  and the base electrode  20  by a monitoring unit  50  (see  FIGS. 3 and 7   b ). 
     The presence of a body is detected by varying the voltage of a signal electrode with respect to the base electrode, and by determining a value which depends on the current of said signal electrode caused by said voltage variations. For example, a signal electrode may be charged to a predetermined voltage value, and discharged via a resistor to the base electrode. The presence of an object may be detected based on the time constant of the voltage decay. The voltage all signal electrodes may be varied with a substantially similar waveform. 
     The base electrode  20  acts as a counter-electrode for capacitive measurement. In addition, the base electrode  20  acts as a noise shield, i.e. as a Faraday cage. 
     In addition, also a change in the capacitance of the second signal electrode  10   a  and the base electrode  20  may be detected by a monitoring unit  50 . 
     Base electrodes  20 , which at least partially surround each of the signal electrodes  10   a ,  10   b  individually, may be in contact with each other. Thus, a single base electrode structure  20  may surround the first  10   a  and the second  10   b  signal electrode. 
       FIG. 2  shows a person walking over a sensor  100 , which comprises several independent signal electrodes  10   a   1 ,  10   a   2 ,  10   b   1 ,  10   b   2 ,  10   c   1 ,  10   c   2 , and one or more base electrodes  20 . 
     The voltage of the signal electrode  10   b   1  is varied with respect to the base electrode  20  and the ground GND. The varying voltage of the signal electrode is capacitively coupled via the foot of the person to the body BOD 1  of the person. The voltage is varied at such a frequency that the body BOD 1  acts as an electrical conductor. Consequently, the whole body BOD 1  of the person has a varying (e.g. alternating) voltage V HG  with respect to the base electrode  20  and the ground GND. This causes a varying electric field E between the body BOD 1  and the base electrode  20 , as well as between the body BOD 1  and the ground GND. Thus, the person&#39;s body is effectively coupled as a part of a capacitive system formed by the electrodes  10   b   1 ,  20 , and the ground GND. 
     The capacitance of each signal electrodes  10   a   1 ,  10   a   2 ,  10   b   1 ,  10   b   2 ,  10   c   1 ,  10   c   2  with respect to the base electrode may be monitored substantially independently. Thus, the location of the person may be effectively tracked. 
     For optimum spatial resolution, the area of an individual signal electrode may be in the range of 0.02 m2 to 0.2 m2, i.e. comparable to the bottom are of the foot H 1 . 
     There may be cover layer  120  between the sensor  100  and the body BOD 1 . The cover layer may be e.g. a carpet or a layer of epoxy coating. d 1  denotes the thickness of the cover layer  120 . The thickness d 1  of the cover layer may be e.g. in the range of 2 to 10 mm. 
       FIG. 3  shows a side view of a person&#39;s foot stepping over a signal electrode  10   a . A monitoring unit  50  varies the voltage V 12  of the signal electrode  10   a  with respect to the base electrode  20  and the ground GND. 
     The measuring system  200  comprises the sensor  100  and a monitoring unit  50 . 
     The ground GND may also act as an electrode  800  having a very large area. 
     The width s 1  of the signal electrodes  10   a ,  10   b  may be selected to be e.g. in the range of 0.5 to 2 times the length S H  ( FIG. 4 ) of the foot H 1 . In order to provide optimum spatial resolution. The narrow distance s 3  between the signal electrodes  10   a ,  10   b  makes it nearly impossible to step onto an inactive grounded area, where the presence of the person would not be detected. 
     The monitoring unit  50  provides a varying voltage V 12  at least to the electrodes  10   a ,  20 , and it determines a value which depends on the current of said signal electrode caused by said voltage variations. The monitoring unit  50  may comprise a decision sub-unit (not shown) for generating a digital signal based on said value or based on the rate of change of said value. The digital signal may indicate the presence or absence of the body BOD 1  in the vicinity of the electrode  10   a.    
     The voltage V 12  coupled to the signal electrode  10   a  may vary at a frequency f 1  which is e.g. in the range of 20 kHz to 1 MHz, advantageously in the range of 50 kHz to 300 kHz. The voltage V 12  may have a complex waveform, and in that case at least 90% of the power of the spectral components of said varying voltage (V 12 ) may be within the frequency range of 20 kHz to 1 MHz, preferably within 50 kHz to 300 kHz 
     The use of a higher frequency f 1  may lead to increased power consumption. The conductivity of e.g. human body may decreases at high frequencies. The signal-to-noise ratio may be low at a lower operating frequency f 1 . The frequency f 1  may be selected so that the sensor  100  does not generate interference to other electric devices, e.g. to medical appliances. 
       FIG. 4  shows the foot H 1  of the person stepping over the base electrode  20 . The capacitance of a capacitor formed between the foot H 1  and the base electrode is substantially smaller than the capacitance of a capacitor formed between the foot H 1  and the signal electrode, because the width s 2  of the base electrode  20  is substantially smaller than the width s 1  of the signal electrode  10   a  (see  FIG. 1 ). Consequently, the voltage V HG  coupled to body BOD 1  may have nearly the same magnitude as the voltage V 12  provided by the monitoring unit  50 . 
     The second signal electrode  10   b  may be switched to a high-impedance floating state when the varying voltage V 12  is coupled to the first signal electrode  10   a . Thus, the second signal electrode  10   b  does not capacitively short-circuit the voltage V HG  coupled to the body BOD 1 , and a coupled voltage V HG  may be high although the foot H 1  is partially over the second signal electrode  10   b , in addition to being over the first signal electrode  10   a  and over the base electrode  20 . 
     A single monitoring unit  50  may be connected to the first and to the second signal electrode by time-based multiplexing, by using a multiplexing unit  55  ( FIG. 15 ). The multiplexing unit  55  may be arranged to disconnect the second signal electrode  10   b  from the monitoring unit  50  and to leave it in a high impedance state when the varying voltage V 12  is coupled to the first signal electrode  10   a.    
     In particular, substantially all signal electrodes adjacent to the first signal electrode  10   a , may be switched into the high impedance state when the detection is performed by using the first signal electrode  10   a.    
     Alternatively, varying voltages V 12  may be simultaneously connected to the first signal electrode  10   a  and to the second signal electrode  10   b . The varying voltages V 12  coupled to the first signal electrode  10   a  and to the second signal electrode  10   b  may be substantially in the same phase In order to provide a high coupled voltage V HG  also in a situation when the foot H 1  is partially over the second signal electrode  10   b , in addition to the first signal electrode  10   a  and the base electrode  20 . However, the spatial resolution may be worse than when switching the second signal electrode into the high-impedance state. 
       FIG. 5  shows a comparative example (Prior Art), where the width s 2  of a base electrode  20  is substantially equal to the width of the signal electrode  10   a . In that case the voltage V HG  coupled to the body BOD 1  is nearly 50% lower than in case of  FIGS. 3 and 4 , because, and the capacitance between the foot H 1  and the base electrode  20  is substantially equal to the capacitance between the foot H 1  and the signal electrode  10   a . The foot H 1  is partially short circuited to the base electrode  20  due to the large area of the base electrode  20 . 
     The voltage V HG  coupled to the body BOD 1  in case of  FIGS. 3 and 4  is approximately 50-100% higher than in case of  FIG. 5 . Thanks to the large signal electrode  10   a , the body BOD 1  is effectively coupled to it. Simulations and experimental measurements indicate a signal to noise ratio (S/N) which is increased by 50% to 100% when compared to the situation of  FIG. 5 . The improved signal to noise ratio enables a more sensitive measurement and/or a longer reading distance. 
     The sensor according to  FIG. 5  does not utilize effectively the electrical conductivity of the body BOD 1 . It merely detects a change of permittivity caused by the presence of the foot H 1 . This leads to a limited detection performance when compared with the present invention. 
     The sensor  100  of  FIGS. 3 and 4  according to the present invention is optimized for detecting the presence of conductive bodies BOD 1  which substantially extend from the level of the substrate, e.g. upwards. 
     The sensor  100  according to  FIGS. 3 and 4  take advantage of the electrical conductivity of the body BOD 1 , thus providing improved sensitivity when compared with the prior art solutions ( FIG. 5 ). Almost the whole surface area of the body BOD 1  is coupled act as a capacitive electrode (not the bottom area of the foot H 1 ) which creates an electric field E together with the base electrode  20  and possibly also with the earth GND,  800 . 
     The sensor  100  is optimized to detect the presence of large conductive objects. A conductive object may be considered to be a “large” if its vertical dimension z 1  (in the direction SZ) is greater than the dimensional and the dimension s 1  of the signal electrode  10   a  ( FIG. 1 ). 
     The sensor  100  has a reduced sensitivity for smaller objects which are positioned at a low level. This is an advantage when the aim is e.g. to distinguish the presence of a human being from the presence of a smaller non-conductive object such as a wooden chair, for example. 
     For example, it was experimentally noticed that a glass of water positioned on the signal electrode  10   a  provided a rather low signal, wherein the signal level increased drastically when a person toughed the water in the glass with his finger. 
     For conventional sensors having signal and ground electrodes of equal size ( FIG. 5 ), and having the gap width between said electrodes substantially equal to size of said electrodes, it has been noticed that the effective reading distance of such sensors is approximately only 1.33 times the gap between the electrodes. Thus, for the sensor  100  according to the present invention, the sensitivity for low objects may be reduced by selecting the gap width s 4  between the signal electrode  10  and the base electrode  20  to be smaller than the thickness d 1  of the cover layer  120 . The gap width s 4  advantageously smaller than 0.75 times the thickness d 1  of the cover layer. 
       FIG. 6  shows a simplified equivalent circuit of system comprising a sensor  100  and a body BOD 1 . A varying voltage V 12  is coupled between terminals T 1  and T 2 . The terminal T 2  is coupled to a signal electrode  10  and the terminal T 1  is coupled to a base electrode  20 . The signal electrode  10  and the base electrode  20  form a capacitor C VG1  even when a body BOD 1  is not present. 
     When a body BOD 1  is positioned in the vicinity of the electrodes  10   a ,  20 , an impedance Z H  formed by the body is capasitively coupled between the electrodes  10 ,  20 . The body BOD 1  and the signal electrode  10  form together a capacitor C VH . The body BOD 1  and the base electrode  20  form together a capacitor C HG1 . 
       FIG. 7   a  shows a more detailed equivalent circuit of a measuring system where the base electrode  20  is also connected via a terminal T 0  to the ground GND. The ground GND forms an additional, very large capacitor plate  800 . The signal electrode  10  and the ground GND form together a further capacitor C VG2 , even when a body BND is not present. 
     The base electrode may be connected to the ground, e.g. to the ground of the mains network in a building, to the metallic water pipelines of a building or to a special ground electrode buried into the soil. This helps to provide a very large electrode surface. Alternatively, or in addition to, the ground GND may also be established by those parts of the base electrode structure which are relatively far away from the body BOD 1  or which are far away from the foot H 1  of a person. The base electrode may be mesh structure which covers substantially the entire area of a room. Thus, it may represent a relatively large surface area. 
     The surface area of the base electrode structure  20  may be greater than or equal to the surface area of the first signal electrode  10   a.    
     Referring to  FIG. 7   b , the surface of an electrically conductive body BOD 1  has surfaces H 1 , H 2  and H 3 , by which the impedance Z H  of the body BOD 1  is capacitively coupled to the signal electrode  10 , to the base electrode  20 , and to the ground GND. The body BOD 1  forms a capacitor C VH  together with the signal electrode  10 . The body BOD 1  forms a capacitor C HG1  together with those parts of the base electrode  20  which are in the vicinity of the body BOD 1 . The body BOD 1  forms a capacitor C HG2  together with the ground GND,  800 . 
     Referring to  FIGS. 8   a  and  8   b , a cover layer  120  may be positioned over the electrodes  10 ,  20 .  FIG. 8   a  shows the equivalent circuit without the presence of a body BOD 1 , and  FIG. 8   b  shows the equivalent circuit with the impedance Z H  of the body. The dielectric permittivity of the cover layer  120  deviates from the permittivity of air. Thus, the capacitance of the capacitors C VG1 , C VH , C HG1 , C HG2  is different from the values of  FIGS. 8   a  and  8   b.    
       FIG. 9   a  shows a sensor wherein the signal electrodes  10   a ,  10   b  and the base electrode have been implemented on an electrically insulating substrate  7  substantially in the same plane. 
       FIG. 9   b  shows the sensor  100  of  FIG. 9   a  upside down. Now the substrate  7  protects the electrodes from wear and prevents a galvanic contact between the electrodes and conductive bodies BOD 1 . However, the surface below the sensor  100  should be electrically insulating. The sensor  100  may be e.g. glued into a floor. In that case the glue and the floor should be electrically insulating. 
       FIG. 9   c  shows a sensor  100  where the signal electrodes  10   a ,  10   b  and the base electrode  20  have been implemented between two substrates  7   a ,  7   b . In that case the electrodes  10   a ,  10   b ,  20  are well protected from both sides. 
       FIG. 9   d  shows a sensor where the signal electrodes  10   a ,  10   b  are at a different level than the base electrode  20 . This may be more complex to manufacture than the examples shown in  FIGS. 9   a  to  9   c.    
     The upper and/or lower side of sensor  100  may be coated with an adhesive (not shown) in order to facilitate easier installation e.g. on a floor. E.g. a pressure sensitive adhesive (pressure-activated adhesive) may be used. The adhesive layer may be protected by a removable release layer (not shown). Installation is also possible by using normal gluing methods known in the art. 
     Referring to  FIG. 10 , the sensor  100  may comprise an array of substantially rectangular signal electrodes  10 , which have at least one base electrode structure  20  between them. 
     Referring to  FIG. 11 , two or more signal electrodes may be coupled electrically in series and/or in parallel in order to increase an individually monitored area. 
     Referring to  FIG. 12 , at least 70% of the perimeter of a signal electrode  10   a  may be surrounded by the base electrode  20 . Advantageously, at least 95% of the perimeter of the signal electrode  10   b  may be surrounded by the base electrode  20  as shown also in  FIGS. 11 and 14   b . The base electrode  20  may also completely surround the signal electrode, as shown e.g. in  FIG. 10 . 
     Referring to  FIG. 13   a , the sensor  100  may comprise a substantially triangular array of signal electrodes  10 . 
     Referring to  FIG. 13   b , the sensor  100  may comprise a substantially hexagonal array of signal electrodes  10 . 
     Referring to  FIG. 13   c , the sensor  100  may comprise e.g. rectangular signal electrodes  10  having rounded corners. The base electrode  20  may have star-shaped areas. 
     The sensors  100  of  FIGS. 10 ,  13   a  or  13   b  may comprise electrical feedthroughs (vias) in order to couple connectors to the signal electrodes which are in the middle of the array. The sensors  100  of  FIGS. 10 ,  13   a  or  13   b  may also be modified in a similar way as in  FIG. 11  so as to implement the conductive parts in a single plane. 
     The signal electrodes  10  may also have other forms, e.g. octagonal or circular shape. Adjacent signal electrodes may have a different shape. 
     However, it is advantageous to select the shape(s) of the signal electrodes  10  such that the distance between adjacent signal electrodes is kept substantially at the predetermined value s 3  ( FIG. 1 ). Thus, the signal electrodes may have mutually matching contours. 
     Referring to  FIG. 14   a , a plurality of signal electrodes  10  and at least one base electrode structure  20  may be implemented on a sensor web  77 , e.g. on a continuous band comprising electrode structures. A substantially similar electrode pattern may be periodically copied along the web in the direction SX, i.e. in the longitudinal direction of the web  77 . The electrode pattern has a period, which has a length L 1 . Thus, the consecutive periods PRD k+0 , PRD k+1 , PRD k+2 , PRD k+3 , PRD k+4  have substantially the same electrode pattern and substantially the same length L 1 . In other words, the web  77  may exhibit periodicity. 
     The signal electrodes  10  of successive periods may be electrically isolated from each other. Each of the electrodes  10 ,  20  is connected to a conductor W. The conductors W of at least three periods may be arranged to cross a transverse line LIN 2 , wherein conductors from farther periods may be arranged to terminate without crossing the line LIN 2 . 
     The electrodes and the conductors are advantageously implemented in the same plane in order to simplify the manufacturing of the web  77 . 
     The web  77  may manufactured e.g. by using a roll-to-roll process. 
     The sensor  100  shown in  FIG. 14   b  may be obtained by cutting along the lines LIN 1 , LIN 2  of the continuous web  77  of  FIG. 14   a . The conductors Wa 1 , Wa 2 , Wa 3 , Wb 1 , Wb 2 , Wb 3 , Wc 1 , Wc 2 , Wc 3 , and Wd 3  terminate in the vicinity of the cut edge of the sensor  100 . This facilitates coupling of a connector CON 1  to said conductors, in order to individually monitor the presence of objects in the vicinity of the signal electrodes  10   a   1 ,  10   a   2 ,  10   b   1 ,  10   b   2 ,  10   c   1 ,  10   c   2 . The base electrodes  20   a   3 ,  20   b   3  and  20   c   3  are shown to be connected together. However, they may also be galvanically separate. 
     The sensor comprises conductors Wd 1 , Wd 2 , We 3 , which terminate before reaching said cut edge. These conductors were connected to electrodes, which were cut away from the sensor  100 , or which will be inactive. 
     Referring to  FIG. 15 , the measuring system  200  may comprise the sensor  100 , a multiplexing unit  55 , a monitoring unit  50 , and a data processor  60 . The multiplexing unit  55  may be arranged to couple each independent signal electrode  10   a ,  10   b ,  10   c ,  10   d ,  10   f ,  10   e  to the monitoring unit  50 , each at a time. The multiplexing unit  55  may be arranged may be arranged to switch all other signal electrodes to the high impedance state. 
     The data processor  60  be arranged to provide information on the location of a body BOD 1  based on signal or signals provided by said monitoring unit. The system  200  may provide information on the movement of the body BOD 1  based on said signal or signals. 
     The data processor  60  may also communicate with the multiplexing unit  55  so as to control the order and/or the rate in which the varying voltage V 12  is coupled to the different signal electrodes. The multiplexing unit  55  may be arranged to send a synchronization signal and/or information regarding the identity of the electrode(s) which are activated at a given time. 
     Referring to  FIG. 16 , the measuring system  200  may comprise the sensor  100 , one or more measuring units  50   a ,  50   b ,  50   c ,  50   d ,  50   e ,  50   f , and a data processor  60 . Each independent signal electrode  10   a ,  10   b ,  10   c ,  10   d ,  10   f ,  10   e  may be connected to a respective monitoring unit. 
     The, the system  200  may comprise an array of monitoring units  50   a ,  50   b ,  50   c ,  50   d ,  50   e ,  50   f  coupled to an array of signal electrodes  10   a ,  10   b ,  10   c ,  10   d ,  10   e ,  10   f , and a data processor  60  arranged to provide information on the location of a body BOD 1  based on a plurality of signals provided by said monitoring units. The system  200  may provide information on the movement of the body BOD 1  based on said signals. 
     Yet, referring to  FIG. 17 , the sensor  100  may comprise e.g. an array of substantially circular signal electrodes  10  having e.g. star-shaped base electrode areas between them. In this example the distance s 3  between the diagonally adjacent signal electrodes is greater than 20% of the width s 1  of the signal electrodes. Thus, the blind spot between signal electrodes is rather large. However, because the width s 2  of the base electrode structure between the signal electrodes is still smaller than or equal to 20% (preferably smaller than or equal to 10%) of the width s 1  of the signal electrode  10 , the varying voltage is still effectively coupled to the body BOD 1 . 
     The surface area of that part of the base electrode structure  20  which is between the adjacent first and second signal electrodes may be smaller than 20% of the surface area of the first signal electrode, and preferably smaller than or equal to 10% of the surface area of the first signal electrode. 
     The terminals of the conductors W are formed by cutting the sensor web across its longitudinal direction to a desired length, and thus the ends of the conductors are exposed and are ready for forming an electrical contact. The attachment method of the sensor web in contact can be, but is not limited to, crimp connector, spring connector, welded contact, soldered contact, isotropic or anisotropic adhesive contact. However, a standard connector used in common electronic applications (e.g. Crimpflex®, Nicomatic SA, France) can be attached to the ends of the conductors W. 
     The surface area of a conductor W connected to a signal electrode  10   a ,  10   b ,  20  may be smaller than 10% of the surface area of said electrode, in order to guarantee spatial resolution and in order to minimize power consumption. 
     The sensor  100  may comprise at least six electrically separate signal electrodes, which together cover at least 70% of the surface area of the substrate  7 . 
     The sensor  100  according to the invention may be used e.g. to monitor the presence and/or movements of people in private houses, banks or factories in order to implement an anti-theft alarm system. A network of sensors  100  may be used to monitor the presence and/or movements of people in department stores e.g. in order to optimize layout of the shelves. The sensor may be used e.g. in hospitals or old people&#39;s homes to detect patient activity and their vital functions. The sensor may be used in prisons to monitor forbidden areas. The sensor may be used for detecting movement of other large conductive bodies, such as wheelchairs or aluminum ladders. The sensor may be used for detecting movement of animals. 
     The sensor  100  may be installed e.g. on or in a floor structure. 
     The substrate  7  may comprise plastic material, or fibrous material in the form of a nonwoven fabric, fabric, paper, or board. Suitable plastics are, for example, plastics comprising polyethylene terephtalate (PET), polypropylene (PP), or polyethylene (PE). The substrate is preferably substantially flexible in order to conform to other surfaces on which it is placed. Besides one layer structure, the substrate can comprise more layers attached to each other. The substrate may comprise layers that are laminated to each other, extruded layers, coated or printed layers, or mixtures of these. Usually, there is a protective layer on the surface of the substrate so that the protective layer covers the electrically conductive areas and the conductors. The protective layer may consist of any flexible material, for example paper, board, or plastic, such as PET, PP, or PE. The protective layer may be in the form of a nonwoven, a fabric, or a foil. A protective dielectric coating, for example an acrylic based coating, is possible. 
     The electrically conductive areas comprise electrically conductive material, and the electrically conductive areas can be, for example, but are not limited to, printed layers, coated layers, evaporated layers, electrodeposited layers, sputtered layers, laminated foils, etched layers, foils or fibrous layers. The electrically conductive area may comprise conductive carbon, metallic layers, metallic particles, or fibers, or electrically conductive polymers, such as polyacetylene, polyaniline, or polypyrrole. Metals that are used for forming the electrically conductive areas include for example aluminum, copper and silver. Electrically conductive carbon may be mixed in a medium in order to manufacture an ink or a coating. When a transparent sensor product is desired, electrically conductive materials, such as ITO (indium tin oxide), PEDOT (poly-(3,4-ethylenedioxythiophene)), or carbon nanotubes, can be used. For example, carbon nanotubes can be used in coatings which comprise the nanotubes and polymers. The same electrically conductive materials also apply to the conductors. Suitable techniques for forming the electrically conductive areas include, for example, etching or screen printing (flat bed or rotation), gravure, offset, flexography, inkjet printing, electrostatography, electroplating, and chemical plating. 
     E.g. the following manufacturing method may be used. A metal foil, such as an aluminum foil, is laminated on a release web. The electrically conductive areas and the conductors are die-cut off the metal foil, and the remaining waste matrix is wound onto a roll. After that, a first protective film is laminated on the electrically conductive areas and the conductors. Next, the release web is removed and a backing film is laminated to replace the release web. 
     Benefits of the above-mentioned manufacturing method include:
         the raw material is cheaper,   the manufacturing method is cheaper compared to e.g. etching,   the manufacturing method requires only one production line, and   the resulting sensor web is thinner; the thickness of the sensor web may be less than 50 μm.       

     Electrically conductive areas and conductors may be die-cut from a metal foil, and they may be laminated between two substrates, i.e. between two superimposed webs. 
     Electrically conductive areas and their conductors may be located in one layer, and optional RF loops and their conductors may be located in another layer. In principle, it is possible to use different techniques, e.g. etching, printing, or die-cutting, in the same product. For example, the electrically conductive areas may be die-cut from a metal foil, but their conductors may be etched. The electrically conductive areas and their conductors may be connected to each other through vias. 
     The monitoring unit  50  may be arranged to provide a signal which depends on the capacitance formed by the electrodes  10   a ,  20 . Said signal may be provided e.g. by a time constant measurement, by measuring an impedance by using the varying voltage V 12 , by connecting the electrodes as a part of a tuned oscillation circuit, or by comparing said unknown capacitance of the electrodes with a known capacitance. 
     The time constant may be determined e.g. by charging the capacitor formed by the electrodes to a predetermined voltage, discharging said capacitor through a known resistor or inductor, and by measuring the rate of decrease of voltage of said capacitor. 
     The impedance may be measured by varying the voltage of said capacitor, by measuring the respective the current, and by determining the ratio of the change of current to the change of voltage. 
     The unknown capacitance of said capacitor may be determined by coupling them as a part of a resonating circuit comprising and inductance and said capacitor. 
     The unknown capacitance of said capacitor may be determined by charging or discharging the unknown capacitance by transferring a charge to it several times by means of a known capacitor unit a predetermined voltage is reached. The unknown capacitance may be determined based on the number of charge transfer cycles needed to reach the predetermined voltage. 
     EXAMPLES 
     1. A sensor ( 100 ) for detecting presence of conductive objects (BOD 1 ), said sensor ( 100 ) comprising a first signal electrode ( 10   a ), a second signal electrode ( 10   b ), and a base electrode structure ( 20 ) implemented in or on an electrically insulating substrate ( 7 ), wherein the distance (s 3 ) between said first signal electrode ( 10   a ) and said second signal electrode ( 10   b ) is smaller than or equal to 0.2 times the width (s 1 ) of said first signal electrode ( 10   a ), and wherein at least a part of said base electrode structure ( 20 ) is between said first signal electrode ( 10   a ) and said second signal electrode ( 10   b ), and wherein said base electrode structure surrounds at least 70% of the perimeter of said first signal electrode ( 10   a ). 
     2. A sensor ( 100 ) for detecting presence of conductive objects (BOD 1 ), said sensor ( 100 ) comprising a first signal electrode ( 10   a ), a second signal electrode ( 10   b ), and a base electrode structure ( 20 ) implemented in or on an electrically insulating substrate ( 7 ), wherein the surface area of that part of said base electrode structure ( 20 ) which is between said first signal electrode ( 10   a ) and said second signal electrode ( 10   b ) is smaller than or equal to 20% of the area of said first signal electrode ( 10   a ), and wherein said base electrode structure surrounds at least 70% of the perimeter of said first signal electrode ( 10   a ). 
     3. The sensor ( 100 ) of example 1 or 2 wherein the surface area of said second signal electrode ( 10   b ) is in the range of 70% to 150% of the surface area of said first signal electrode ( 10   b ). 
     4. The sensor ( 100 ) according to any of the examples 1 to 3 wherein the surface area of said first signal electrode is in the range of 0.02 to 0.2 m 2 . 
     5. The sensor ( 100 ) according to any of the examples 1 to 4 wherein the distance (s 3 ) between said first signal electrode ( 10   a ) and said second signal electrode ( 10   b ) is in the range of 5 to 30 mm. 
     6. The sensor ( 100 ) according to any of the examples 1 to 5 wherein the width (s 2 ) of a part of said base electrode structure ( 20 ) between said signal electrodes is in the range of 0.3 to 15 mm. 
     7. The sensor ( 100 ) according to any of the examples 1 to 6 wherein the surface area of said base electrode structure ( 20 ) is greater than or equal to the surface area of said first signal electrode ( 10   a ). 
     8. The sensor ( 100 ) according to any of the examples 1 to 7 wherein said signal electrodes ( 10   a ,  10   b ) and said base electrode structure ( 20 ) are substantially in the same plane, and conductive parts of said sensor ( 100 ) have been implemented on a flexible substrate ( 7 ). 
     9. A monitoring system for detecting a conductive body (BOD 1 ), said system comprising a sensor ( 100 ) according to any of the examples 1 to 7, said system further comprising a monitoring unit ( 50 ), which is arranged to couple a varying voltage (V 12 ) between said first signal electrode ( 10   a ) and said base electrode structure ( 20 ), and which is arranged to provide a value which depends on the current of said signal electrode ( 10   a ) caused by said voltage variations. 
     10. The system of example 9 wherein said signal electrodes ( 10   a ,  10   b ) are covered with an electrically insulating layer ( 120 ), the thickness (d 1 ) of said layer being greater than a gap (s 4 ) between said first measuring electrode ( 10   a ) and said base electrode structure ( 20 ). 
     11. The system of example 9 or 10 wherein said sensor ( 100 ) has been installed on a floor and covered with a cover layer ( 120 ), wherein the thickness (d 1 ) of the cover layer over the electrodes is greater than or equal to a gap (s( 4 ) between the first signal electrode and the base electrode structure ( 20 ). 
     12. The system according to any of the examples 9 to 11 wherein said base electrode structure ( 20 ) connected to the earth (GND,  800 ). 
     13. The system according to any of the examples 9 to 12 wherein at least 90% of the power of the spectral components of said varying voltage (V 12 ) is within the frequency range of 20 kHz to 1 MHz. 
     14. The system according to any of the examples 9 to 13 wherein the second signal electrode  10   b  is switched to a high impedance state when the varying voltage (V 12 ) is coupled to said first signal electrode ( 10   a ). 
     15. The system according to any of the examples 9 to 14 comprising an array of monitoring units ( 50 ) coupled to an array of signal electrodes, and a data processor arranged to provide information on the location of said body (BOD 1 ) based on a plurality of signals provided by said monitoring units ( 50 ). 
     16. The system according to any of the examples 9 to 15 system comprising an array of monitoring units ( 50 ) coupled to an array of signal electrodes, and a data processor arranged to provide information on the movement of a body (BOD 1 ) based on a plurality of signals provided by said monitoring units ( 50 ). 
     17. A method of detecting a conductive body (BOD 1 ) by using a sensor ( 100 ) according to any of the examples 1 to 8 or a system according to any of the examples 9 to 16, said method comprising coupling a varying voltage (V 12 ) between said first signal electrode ( 10   a ) and said base electrode structure ( 20 ), and determining a value which depends on the current of said signal electrode ( 10   a ) caused by said voltage variations. 
     18. The method of example 17 wherein the vertical dimension (z 1 ) of said body (BOD 1 ) is greater than or equal to the height (a 1 ) and the width (s 1 ) of said first signal electrode ( 10   a ). 
     19. A sensor web ( 77 ) comprising a plurality of sensors ( 100 ) according to any of the examples 1 to 8, wherein a substantially similar electrode pattern has been copied along the longitudinal dimension (direction SX) of said web ( 77 ) so that the electrode pattern has a longitudinal period. 
     20. The sensor web ( 77 ) of example 19 wherein conductors W of at least N successive periods cross a transverse line LIN 2 , wherein at least one conductor connected to a signal electrode which does not belong to said N periods terminates without crossing said transverse LIN 2 , N being an integer greater than or equal to three. 
     21. A sensor ( 100 ) obtainable by cutting the sensor web ( 77 ) of example 20 along two transverse lines (LIN 1 , LIN 2 ). 
     22. The sensor ( 100 ) of example 21 wherein conductors (We 3 , Wd 1 , Wd 2 ), which terminate without crossing said line LIN 1  are not connected to any signal electrodes. 
     The word “comprising” is to be interpreted in the open-ended meaning, i.e. a sensor which comprises a first electrode and a second electrode may also comprise further electrodes and/or further parts. 
     For a person skilled in the art, it will be clear that modifications and variations of the devices and the method according to the present invention are perceivable. The particular embodiments and examples described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.