Patent Publication Number: US-7915887-B2

Title: Device for generating a magnetic field in a goal area for taking a goal decision

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 60/894,554, which was filed on Mar. 13, 2007, and is incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a concept for generating a magnetic field in a goal area for determining the position of a movable object, as it can, for example, be used in soccer for taking a decision about whether a goal has been scored. 
     BACKGROUND 
     A number of tasks, such as ball localization in a football or soccer match, presuppose knowledge of the position and/or orientation of objects. In soccer matches, one of the most controversial topics is whether or not in critical situations the ball has crossed the goal line. To this end, it is necessary that the position of the ball can be measured with an accuracy of approx. +/−1.5 cm within a limited goal area around the goal line. Also, it is necessary for any influences exerted by persons who are moving close to the ball and/or are covering the ball to be irrelevant. 
     There are a number of localization methods based, for example, on optical 2D or 3D sensors having an evaluation system, or an exploitation of the known radar principle or of a principle of radio localization. 
     A principle of radio localization is the localization of objects by means of electromagnetic wave propagation. In this context, a receiver is integrated into an object to be localized, or is attached to an object to be localized, respectively, the receiver transmitting data to a central transceiver upon request. A position of the object may thereafter be calculated from signal traveling times and/or from differences between at least two signals received at different antennas. 
     Radio localization of objects may be performed, for example, by means of the so-called RFID technology (RFID=radio frequency identification). For spatial resolution methods, wherein a relatively precise position of an RFID transponder is to be determined in space, battery-powered, i.e. active RFID transponders, are most often used. A disadvantage of radio localization exists in a shadowing and/or a reflection of electromagnetic waves by certain obstacles, for example. As a result, systems based on a radio localization will not achieve the accuracy necessitated for taking goal decisions in football, or soccer, for example. 
     As has already been described, current localization methods are based, for example, on optical 2D or 3D sensors comprising an evaluation system, or they are based on the use of battery-powered, i.e. active, RFID transponders. Such localization methods entail high investment and maintenance costs, sensitivity towards environmental conditions and a high effort necessitated for adapting the evaluation algorithms. Systems exploiting radio localization are not suitable for local area localization, i.e. for determining the positions of objects within a small area, since with a small geometric expansion differences of different signal traveling times can hardly be measured. Thus, the requirements placed upon systems for localizing objects are not met, or are only met to an insufficient degree, by these methods with regard to economic efficiency, robustness, clock time and object independence for an exact position determination, for example within a range of a few centimeters. 
     SUMMARY 
     According to an embodiment, the present invention provides a device for generating a magnetic field in a goal area, comprising at least two coils arranged in parallel to a goal area defined and bounded by a goal, wherein a first coil is attached in an area behind the goal and a second coil is attached closer to the goal than the first coil or attached identically to the goal, wherein the first coil and the second coil respectively comprise a coil impedance, wherein the coil impedance of the second coil is set so that a magnetic field of the second coil generated due to a magnetic field of the first coil reduces the magnetic field of the first coil at a location within the second coil by at least 20%. 
     According to another embodiment, the present invention provides a system for determining information about a position of a movable object in a goal area, in which at least two coils are attached in parallel to a goal area bounded and defined by a goal, wherein a first coil is attached in an area behind the goal and a second coil is attached closer to the goal than the first coil or identical to the goal, wherein the first coil and the second coil respectively comprise a coil impedance, wherein the coil impedance of the second coil is set so that a field of the second coil generated due to a magnetic field of the first coil reduces the magnetic field of the first coil at a location within the second coil by at least 20%, which may have a device for providing information about a magnetic field which the movable object experiences at the position in the goal area; and a device for evaluating information about the magnetic field to obtain information about the position of the movable object in the goal area. 
     According to another embodiment, the present invention provides a method for determining information about a position of a movable object in a goal area in which at least two coils are attached in parallel to a goal area defined and bounded by a goal, wherein the first coil is attached in an area behind the goal and a second coil is attached closer to the goal than the first coil or identical to the goal, wherein the first coil and the second coil respectively comprise a coil impedance, wherein the coil impedance of the second coil is set so that a field of the second coil generated due to a magnetic field of the first coil reduces the magnetic field of the first coil at a location within the second coil by at least 20%, which may have the steps of generating a magnetic alternating field using the first coil; providing information about the magnetic alternating field which the movable object experiences at the position in the goal area; evaluating the information about the magnetic alternating field to obtain information about the position of the movable object in the goal area. 
     According to another embodiment, the present invention provides a computer program comprising a program code for performing the above-mentioned method, when the computer program runs on a computer or a microcontroller. 
     The findings of the present invention consist in that a position, direction and/or movement of a movable object and/or a ball may be determined by measuring the strength and/or orientation of a changing magnetic field at the location of the movable object. A system for determining the position of a ball includes, according to embodiments of the present invention, basically two coils arranged in parallel to a goal area bounded and defined by a goal, wherein a first coil is attached in an area behind the goal and a second coil is attached closer to the goal than the first coil or attached identically to the goal, i.e. for example runs within the goal frame. The first coil and the second coil respectively comprise a coil impedance, wherein the coil impedance of the second coil is set such that a magnetic field of the second coil generated due to a magnetic field of the first coil reduces the magnetic field of the first coil at a location within the second coil by at least 20%. In other words, a part of the magnetic alternating field caused by the first coil may cause an induction in the second coil which, due to its low overall impedance, may generate an opposing field to the magnetic field generated by the first coil. Thus, a magnetic field strength within the goal area spanned by the second coil is reduced. 
     According to one aspect, the present invention provides a system for determining information about a position of a movable object in a goal area, in which at least two coils are attached in parallel to a goal area defined and bounded by a goal, wherein a first coil is attached in an area behind the goal and a second coil is attached closer to the goal than the first coil or identical to the goal, wherein the first coil and the second coil respectively comprise a coil impedance, wherein the coil impedance of the second coil is set such that a field of the second coil generated due to a magnetic field of the first coil reduces the magnetic field of the first coil at a location within the second coil by at least 20%, having a device for providing information about a magnetic field which the movable object encounters at the position in the goal area, and a device for evaluating the information about the magnetic field to obtain information about the position of the movable object in the goal area. 
     According to embodiments of the present invention, the device for providing information about the magnetic field is located within the movable object or ball itself. Thus, using the information or a magnetic field strength, respectively, of the magnetic alternating field generated by the first coil it may be determined whether the ball has crossed the goal line or not. For this purpose, the ball, according to implementations, includes a chip on which a three-dimensional magnetic field sensor, a microcontroller, a transmit unit and a current supply are located. The ball permanently measures the magnetic field surrounding the same and transmits the strength of the field of all three space coordinates (x, y, z) to the device for evaluating the information about the magnetic field which is, for example, located in a central computer. The closer the ball gets to the first coil behind the goal, the higher the magnetic field strength measured by the ball of the magnetic field generated by the first coil, in the following also called first magnetic field. The field strength of the first magnetic field is not constant in the goal plane. By this, with regard to a determination of the location of the ball, ambiguities result which have to be corrected. For this purpose, further information is needed about a point of penetrating the goal plane. 
     According to embodiments this may be achieved by measuring a field strength and direction of the magnetic field generated by the second coil, wherein the magnetic field generated by the second coil is also referred to as the second magnetic field in the following. The first and second magnetic fields are here generated in a frequency-division multiplexing, i.e. with different frequencies, or in a time-division multiplexing, i.e. alternating in time. 
     A difference of the directions or orientations, respectively, of the first and the second magnetic field leads to an angle which gets larger, the further the ball is situated outside the center of the goal. In addition, the measured field strength of the magnetic field generated by the second coil changes, depending on whether the ball is located in the center of the goal or at the edge of the goal. The angle and the field strength of the second coil are, according to embodiments, used as parameters for correcting the field strength of the magnetic alternating field generated by the first coil. Now, the field strength of the first coil may be used to determine a distance of the ball to the goal plane. 
     According to embodiments of the present invention, the second coil is attached in or at the goal frame, respectively, and comprises an impedance which is as low as possible. Also a control electronics of the second coil comprises, according to embodiments, a very low impedance regarding alternating current. Thus, the second coil may act as a short-circuited secondary winding of the primary first coil, i.e. may be set into a short-circuit operation. A part of the magnetic alternating field caused by the first coil may thus cause an induction in the second coil. Due to the low overall impedance of the second coil, the current induced in the second coil may generate an opposing field to the magnetic field generated by the first coil. By this, field lines of the magnetic field of the first coil are attenuated in the area spanned by the second coil, i.e. the goal area. Outside the area spanned by the second coil, the opposing field and the magnetic field of the first coil may add up. By this, the overall magnetic field strength outside the second coil is increased. This strong field difference caused by the opposing field of the second coil at the outer boundaries of the second coil enables to determine extremely accurately whether the ball is inside or outside the goal. 
     According to a further aspect of the present invention, the second coil may be operated in a short-circuit operation or in an open-circuit operation by a switch. When the ball is in the proximity of the goal plane or in the goal area around the goal line, respectively, it may, depending on whether the switch is open or closed, measure a different magnetic field which is generated by the first coil behind the goal. If the switch of the second coil is open, the ball will measure an undisturbed magnetic alternating field of the first coil, which is strongest in the center of the goal and decreases in a characteristic way towards the goal edges. When the switch of the second coil is closed, the ball will measure a low magnetic field within the goal area bounded by the second coil using its electronics, outside the goal area a stronger magnetic field may be measured. In addition, a change of magnetic field directions may be measured when closing or opening the switch, respectively. With every measurement cycle, three measurement values are available, using which, according to embodiments, it may be calculated very accurately whether and where the ball crossed the goal plane. According to embodiments, a device for evaluating is thus implemented to provide an indication from a sequence of measurement values whether the movable object crossed the goal plane, wherein first information is information about a magnetic field of the first coil in an open-circuit or idle operation of the second coil, second information is information about a reduced magnetic field of the first coil in a short-circuited second coil and third information is information about a change between first information and second information. 
     In the direction perpendicular to the goal plane, the inventive system enables a relatively exact measurement of the field strength of the first coil. Parallel to the goal plane, without an influence of the second coil, a very low field change of the magnetic field generated by the first coil is measured. Thus, it would only partly be possible to see whether the ball flew past a goalpost or over the crossbar. 
     It is the advantage of the present invention that by the low impedance of the second coil a very strong signal difference may be embossed on an overall magnetic field generated by the first magnetic field and by the opposing field of the second coil, wherein this signal difference is located in particular at important locations like goalposts or the crossbar, respectively. Thus, it may be enabled that decisions regarding those critical positions are taken without occurring errors. 
     Thus, using the inventive concept there is the possibility of determining the position of a ball in a goal area or a goal plane, respectively, very precisely and thus be able to take a decision whether a goal has been scored without interrupting play. 
     Further, the inventive concept for deciding whether a goal has been scored is tolerant in view of persons, i.e. the influence of persons moving close to the movable object or the ball, respectively, or covering the movable object does not play any role. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, embodiments of the present invention are explained in more detail with reference to the accompanying drawings, in which: 
         FIG. 1   a  shows a schematical illustration of magnetic field lines around a current-carrying cylinder coil for explaining the inventive concept; 
         FIG. 1   b  shows a schematical illustration of a course of a magnetic field strength in a short range around a cylinder coil plotted over a distance from the cylinder coil; 
         FIG. 2  shows a schematical illustration of a football goal with a first coil wound around an area bounded by the net suspension posts and the net suspension crossbar and a second coil wound around an area bounded by the goalposts and the goal crossbar, wherein the coils are coupled to a device for controlling the coils, according to one embodiment of the present invention; 
         FIG. 3  shows a front view of a first coil behind a football goal and a second coil in a football goal and a field line course generated by the first coil within the area spanned by the second coil; 
         FIG. 4  shows a side view of a first and a second coil with a field line course of an undisturbed magnetic field generated by the first coil; 
         FIG. 5  shows a schematical illustration of a magnetic field strength distribution within a rectangular coil; 
         FIG. 6  shows a side view of a first coil generating a magnetic field and an idle or open-circuited second coil with a resulting field line course, according to one embodiment of the present invention; 
         FIG. 7  shows a side view of a first coil generating a magnetic field and a short-circuited second coil with a resulting field line course according to one embodiment of the present invention; 
         FIG. 8  shows a schematical illustration of a field strength course plotted over a distance from the goal line; 
         FIG. 9  shows a flowchart for illustrating a method for taking a goal decision according to one embodiment of the present invention; 
         FIG. 10  shows a schematical illustration of an inventive system for generating a magnetic field in a goal area; 
         FIG. 11  shows a side view of a first coil and a second coil in a goal frame according to one embodiment of the present invention; and 
         FIG. 12  shows a cross-section through a goalpost having cable channels. 
     
    
    
     DETAILED DESCRIPTION 
     Regarding the subsequent description it should be noted, that in the different embodiments like or similar functional elements comprise the same reference numerals and thus descriptions of those functional elements are exchangeable in the different embodiments illustrated in the following. 
     To explain the inventive concept for determining the position of a movable object using magnetic fields in more detail,  FIG. 1   a  shows a schematical illustration of magnetic field lines around a cylindrical coil  100  carrying a current I. 
     As it is known, a magnetic field is connected to each moving charge (electrons in lines or in the vacuum), i.e. a current flow. The field quantity associated with the cause of the magnetic field is the magnetic field strength H, independent of the material characteristics of the area. For generating a static magnetic field, for example short cylinder coils or conductor loops may serve as magnetic antennas. In general, the magnetic field strength H decreases with an increasing distance from a current-carrying conductor or the current-carrying cylinder coil  100 , respectively. If, for example, a measurement point is removed from the center of the coil  100  into the direction of the coil axis (x axis), then the field strength H of the magnetic field continuously decreases with an increasing distance x. This connection is shown as an example in  FIG. 1   b.    
     In a logarithmic illustration  FIG. 1   b  schematically shows a course of the magnetic field strength H in a short range of the current-carrying cylinder coil  100  with an increasing distance in the x direction, i.e. in the direction of the longitudinal coil axis. Here, the curve with the reference numeral  110  exemplarily designates a field strength course with a relatively large radius R of the windings of the coil  100 . The curve with the reference numeral  120  accordingly designates a course of the magnetic field strength H with a medium winding radius R. Accordingly, the curve with the reference numeral  130  designates a schematical field strength course with a small winding radius R of the coil  100 . 
     In the free space, the drop in the field strength in the so-called near field of the coil is first of all approx. 60 dB per decade, which then levels off to 20 dB per decade in the far field with an electromagnetic wave forming. At a closer look, it may be seen that the field strength H is almost constant depending on the radius (or the area) of the coil  100  up to a certain distance x, but then drops off. The magnetic field strength curves illustrated in  FIG. 1   b  refer to a short range of the cylinder coil  100 , i.e. an area around the cylinder coil  100  of a few meters. Thus it is possible to associate a distance x from the cylinder coil  100  to each magnetic field strength H. For example, as everybody knows, for a field strength course along the longitudinal coil axis x of a round coil carrying a current I the following relation results: 
                   H   =       I   ·   N   ·     R   2         2   ·         (       R   2     +     x   2       )     3                   (   1   )               
wherein N is the number of coil windings, R is the winding radius and x is the distance to the center of the coil in the x direction. As a boundary condition for the validity of the relation (1) h&lt;&lt;R holds true, i.e. a coil height h has to be much smaller than the coil radius, and x&lt;λ/2π (λ=wave length), wherein in a distance x&gt;2π a transition into the electromagnetic far field of the coil  100  begins.
 
     The above-mentioned equation (1) only serves for illustrating the dependence of the magnetic field strength on the distance from a magnetic antenna or coil, respectively. Likewise, equations may be set up which describe a field strength course around a coil in the three-dimensional space. In addition to a magnitude of the magnetic field strength H, there is also an alignment or orientation, respectively, of a magnetic field vector {right arrow over (H)}. If the three components (B x , B y , B z ) of the magnetic field vector {right arrow over (H)} are measured at the location of the movable object, according to one embodiment of the invention, using a system of equations, the space coordinates (x, y, z) of the location may be determined where the magnetic field vector {right arrow over (H)} was measured. To be able to exclude ambiguities, generally measurement values of the magnetic field vector {right arrow over (H)} of several coils are needed. 
     According to a further embodiment of the present invention it is possible to measure the magnetic field generated by the coil  100  three-dimensionally with a desired accuracy in a location determination area around the coil  100  and to store the measurement values or the components (H x , H y , H z ), respectively, of the field vector {right arrow over (H)} for each relevant point in space, for example in a so-called lookup table and associate the same with the respective space coordinates (x, y, z) of the space points. Likewise, it is, of course, possible that the field strengths and field directions are, according to a further embodiment of the present invention, calculated in an interesting area around the coil using mathematical formulae to subsequently be associated to the corresponding coordinates (x, y, z) in a lookup table. If subsequently a field strength and the associated field direction are measured at a random location of the location determination area or the goal area, respectively, around the coil, then the measurement values may thereupon be compared to the previously measured or calculated and stored values from the lookup table. The data set which has the best matches finally designates the location of the measurement. 
     If the movable object or a three-dimensional magnetic field sensor integrated in the movable object, respectively, rotates, then it is generally not possible to associate the components (Hx, Hy, Hz) of a magnetic field measured by the magnetic field sensor to a point in space in the location determination area. In this case, however, according to one embodiment of the present invention, the magnitude of the measured magnetic field vector |H|=(H x 2+H y 2+H z 2) 1/2  may provide information about the position of the movable object or the ball, respectively, in the location determination area or goal area, respectively. When only using one coil or only one magnetic field, respectively, ambiguities result with regard to the position, as curves or areas, respectively, exist around the coils on which the magnitude of the measured magnetic field vector |H| is respectively the same. If, however, at least two coils are used, which are arranged at different positions regarding the location determination area, then these ambiguities may be reduced or completely omitted, respectively. 
     This principle is now used according to embodiments of the present invention, for example to be able to determine a position of a ball by means of magnetic fields. As already mentioned above, for example in a football match one of the most controversial topics is whether in critical situations the ball crossed the goal line or not. For this purpose it is necessary that the position of the ball at the goal line may be measured with an accuracy of approx. +/−1.5 cm. An arrangement which enables a the determination of the position of a football by means of a magnetic field is schematically illustrated in  FIG. 2 . 
       FIG. 2  shows a football goal  200  comprising a first post  200   a , a second post  200   b  and a crossbar  200   c . Further, the goal  200  comprises a net suspension with a first net suspension post  200   d , a second net suspension post  200   e  and a net suspension crossbar  200   f . The goal  200  is positioned on a goal line  210  with its posts  200   a ,  200   b.    
     The net suspension posts  200   d,e , the net suspension crossbar  200   f  and an area  220  below the surface of the earth form a frame of a first rectangular coil  100   a  behind the goal  200  which is wound around the net suspension opening area according to one embodiment of the present invention in the net suspension posts  200   d,e , in the net suspension crossbar  200   f  and in the area  220  below the surface of the earth. 
     The goalposts  200   a,b , the crossbar  200   c  and the goal line  210  form a frame of a second rectangular coil  100   b , which is, according to one embodiment of the present invention, wound around the goal opening area within the goalposts  200   a,b , within the crossbar  200   c  and in an area below the goal line  210 . 
     According to one embodiment of the present invention, the two coils  100   a,b  thus form a coil pair similar to a Helmholtz coil pair. Further,  FIG. 2  shows, in an area in front of the goal  200 , a movable object or a ball  230 , respectively, whose position is to be determined. Further,  FIG. 2  shows a device  240  for controlling the two coils  100   a,b.    
     The two coils  100   a,b  are arranged at least approximately in parallel to a goal area defined and bounded by the goal  200 . The first coil  100   a  is attached in an area behind the goal  200  and the second coil  100   b  is, according to the invention, attached closer to the goal  200  than the first coil or attached identically to the goal  200 . A coil impedance of the second coil  100   b  is, according to an embodiment of the present invention, set such that a magnetic field of the second coil  100   b  generated due to a magnetic field of the first coil  100   a  reduces the magnetic field of the first coil  100   a  at a location within the second coil  100   b  at least by 20%. 
     According to embodiments, the first coil  100   a  may be attached behind the goal  200 , for example at a net suspension of the goal  200 , as it is exemplarily shown in  FIG. 2 . The coil opening area of the first coil  100   a  may be equal to or larger than the goal area defined by the goal  200 . The center or center of gravity, respectively, of the first coil  100   a  is advantageously at least approximately identical to the center or the center of gravity, respectively of the goal  200 , i.e. an axis passing perpendicular to a center of gravity of the bounded goal area passes at least approximately through the center of gravity of the coil opening area of the first coil. Apart from that, the first coil  100   a  is advantageously aligned absolutely in parallel to the goal  200 . According to embodiments, the second coil  100   b  is attached in the goal frame as illustrated in  FIG. 2 . The second coil  100   b  may, however, also be mounted outside the goal frame, for example at a net fixation at the goal frame. A coil part of the second coil  100   b  which passes along the goal line  210  is advantageously buried a few centimeters below the goal line  210 . 
     For determining whether the ball  230  crossed the goal plane, according to the invention the field strength of the magnetic field generated by the first coil  100   a  is determined at the location of the ball  230 . For this purpose, the ball  230  for example comprises a chip on which a three-dimensional magnetic field sensor, a microcontroller, a transmit unit and a current supply are accommodated. The ball  230  or the three-dimensional magnetic field sensor, respectively, continually measures the magnetic field surrounding the same and transmits the strength of the field of all three space coordinates (x, y, z), for example to a central computer (not shown). In order to be able to reliably differentiate the magnetic fields of the coils  100   a,b  from the magnetic field of the earth and other magnetic fields, an alternating field is generated in each of the coils  100   a,b.    
     According to embodiments, both coils  100   a,b  may emit an alternating field with a respectively different frequency, i.e. they are operated in a frequency division multiplex operation. For this purpose, a frequency of a magnetic alternating field of one of the two coils  100   a,b  may, according to embodiments, for example be in a range from 500 Hz to 5 kHz. The magnetic field sensor integrated in the ball  230  is, according to embodiments, connected to an electric filter, whereby the different frequencies may be separated and passed on to a field strength measurement. Thus, the ball  230  is able to separately detect the magnetic field strength of both magnetic fields generated by the coils  100   a,b , each in the three directions of space, and transmit the same to a central control unit, like, for example, a personal computer. 
     The closer the ball  230  gets to the first coil  100   a , the higher the magnetic field strength measured by the ball  230  or the magnetic field sensor, respectively. The magnetic field strength of the magnetic field generated by the first coil  100   a  or of the first magnetic field, respectively, is not constant in the goal plane, i.e. within the second coil  100   b . This connection is schematically illustrated in  FIG. 3 . 
       FIG. 3  schematically shows a front view of a football goal  200  with a first coil  100   a  arranged behind the goal  200  and a second coil  100   b  attached identically to the goal. 
     In the embodiment of the present invention illustrated in  FIG. 3 , the first coil  100   a  comprises a coil opening area bounded by its coil windings behind the goal  200  which is larger than the goal area of the goal  200  bounded by the goalposts  200   a,b  and the crossbar  200   c . Due to the fact that the second coil  100   b  is attached identically to the goal  200 , its opening area corresponds at least approximately to the goal area of the goal  200 . 
     If only the first coil  100   a  is controlled by a device for controlling in order to generate a magnetic alternating field, and if the second coil  100   b  is operated in an open-circuit operation, then a distribution of magnetic field lines of the magnetic field generated by the first coil  100   a  results, as it is schematically indicated by reference numerals  300  in  FIG. 3 . Within the goal opening area of the goal  200 , a magnetic field course of the first magnetic field results such that the magnitude of the first magnetic field within the goal opening area decreases from the center outwards. The magnitude of the magnetic field strength of the first magnetic field is indicated in  FIG. 3  by the density of the magnetic field lines  300  directed out of the drawing plane. The denser the magnetic field lines  300 , the higher the magnitude of the magnetic field strength and vice versa. 
     If a goal decision is to be taken, i.e. a decision whether the ball  230  crossed the goal line  210 , ambiguities may result by this magnetic field strength distribution within the goal opening area which is not constant. Without further information, a device for evaluating can, for example, not assess whether the ball  230  crossed the goal plane close to a side post  200   a,b , or whether the ball is located in front of the goal line  210  towards the field in an area close to an axis perpendicular to the center of gravity (middle) of the goal area. In order to clear up these ambiguities, further information is needed about the point of crossing the goal plane. This may be achieved by the fact that a field strength and a direction of the magnetic field of the second coil  100   b  is measured at the location of the movable object or the ball  230 , respectively. The difference between the directions of the magnetic field of the first coil  100   a  and the magnetic field of the second coil  100   b  results in an angle α which becomes larger the further the ball  230  is located out of the center of the goal. This connection is schematically illustrated in  FIG. 4 . 
       FIG. 4  shows a side view of a first coil  100   a  and a second coil  100   b  arranged in parallel of the same and a first field line course  300   a  of the magnetic field generated by the first coil  100   a  and a second field line course  300   b  of the magnetic field generated by the second coil  100   b . In the example illustrated in  FIG. 4 , the orientations of the field lines  300   a  and  300   b  within the coil  100   b  are opposing. 
     As it may be seen in  FIG. 4 , in the center of the second coil  100   b  or the goal  200 , respectively, an angle between the field lines  300   a  and  300   b  of approximately 0° results. If you go from the coil center of the second coil  100   b  outwards, then the angle α, designated by the reference numeral  310 , increases with an increasing distance from the coil center, as it is illustrated in  FIG. 4 . 
     In addition to the angle α between the orientations of the magnetic fields, the field strength of the magnetic field generated by the second coil  100   b  measured by the ball  230  changes depending on whether the ball is located in the center of the goal or at the edge of the goal. This connection is schematically illustrated in  FIG. 5 . 
       FIG. 5  shows a front view of the goal or coil opening area, respectively, of the second coil  100   b . The lines designated by the reference numeral  500  schematically designate a field strength distribution within the rectangular coil  100   b . Here, a small distance of two neighboring lines  500  designates a comparatively high field strength of the magnetic field at the respective location, wherein a large distance between two lines  500  designates a comparatively low magnitude of the magnetic field strength within the coil  100   b . It may be seen from  FIG. 5  that, in particular in the corner areas of the second coil  100   b , there are higher magnetic field strengths than in the center of the coil  100   b . This fact may, for example, also be shown analytically by the law of Biot-Savart. In general, a contribution d{right arrow over (H)} of an infinitesimal line piece d{right arrow over (l)} through which a current I flows with respect to the magnetic field {right arrow over (H)} may be calculated in a point P according to 
     
       
         
           
             
               
                 
                   
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     Here, {right arrow over (r)} designates a connection vector from the line piece to the point P where the magnetic field is to be calculated. For any (not necessarily closed) conductor, the magnetic field H is obtained as an integral over the conductor according to 
     
       
         
           
             
               
                 
                   
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     Due to the inhomogeneous distribution of the magnetic field within the second coil  100   b  illustrated in  FIG. 5  ambiguities result with regard to the position of the ball  230 . If only the first coil  100   a  would be used for determining the position of the ball  230 , for example using a measured magnitude of the magnetic field, it would not be possible to differentiate whether the ball is located close to a corner of the second coil  100   b  or the goal  200 , respectively, just before or behind the goal line  210 , respectively, or, for example, in the center of the second coil  100   b  or the goal  200 , respectively, on a level with the goal line  210 . There will be a point close to the corner area of the coil  100   b  in front of the goal line at which the magnitude of the magnetic field strength is at least approximately as high as at a point in the center of the coil  100   b  on a level with the goal line  210  or in the plane, respectively, spanned by the goal line  210  and the football goal  200 . 
     To be able to eliminate these ambiguities, the first coil  100   a  behind the goal  200  within the net suspension is advantageous. By separately measuring the magnetic fields generated by the first coil  100   a  and the second coil  100   b  at the location of the ball or the movable object  230 , respectively, sufficient information may be obtained to be able to determine the precise position of the ball  230  within the goal area. 
     By adding the measurement values of the second magnetic field to the measurement values of the first magnetic field, it may now be determined whether the ball  230  is located close to a corner area of the coil  100   b  in front of the goal line  210 , behind the goal line  210  or in the center of the coil  100   b  or the goal  200 , respectively, on a level with the goal line  210 . The angle α described with reference to  FIG. 4  between the field lines  300   a  of the first and the second magnetic field  300   b  and the field strength of the second coil  100   b  described with reference to  FIG. 5  are, according to embodiments of the present invention, also used as parameters for correcting the field strength of the first magnetic field. Thus, the field strength of the first magnetic field and the parameters may be used to determine a distance of the ball  230  from the goal plane. 
     According to further embodiments, both coils  100   a,b  may radiate a magnetic alternating field having the same frequency offset in time, i.e. they are operated in a time-division multiplex operation. Here, in a first time interval the first coil  100   a  is activated, while the second coil  100   b  is switched off, and the magnetic field generated by the first coil  100   a  is measured by the ball  230  or the magnetic field sensor in the ball, respectively. In a second time interval, the second coil  100   b  is activated, while the first coil is switched off, whereupon the ball  230  measures the second magnetic field. Also here, the angle α between the field line  300   a  of the first and the second magnetic field  300   b  and the field strength of the second coil  100   b  are used as parameters for correcting the field strength of the first magnetic field. 
     It is an advantage of this embodiment that only one frequency is needed and thus the number of components in the ball may be reduced. Apart from that, measurement errors by a possible mutual influence of the filters in the ball may be prevented. 
     In certain situations, in a football match a football may reach velocities of up to 140 km/h, i.e. approx. 40 m/s. If a measurement accuracy of approx. +/−1.5 cm is requested, advantageously the overall measurement cycle of the two coils  100   a,b  should happen within a timeframe of approx. 375 μs. The time interval of the measurements of the first and the second magnetic field should not be selected too large in this implementation. A time interval which is too high would have negative effects on the accuracy of the determination of the position. 
     It is important for a goal decision that it may be seen whether the ball  230  is within or just about outside the goal, i.e. for example just about outside the goal  200  at a side post  200   a,b  or the crossbar  200   c . In order to be able to easily and reliably decide about this, the second coil  100   b  which is applied at or in the goal frame, respectively, comprises an impedance which is as low as possible according to an embodiment. According to embodiments, also control electronics of the second coil comprise a very low impedance regarding alternating currents. For this reason, the second coil  100   b  may act as a short-circuited secondary winding of the primary first coil  100   a . This has the consequence that a part of the magnetic alternating field caused by the first coil  100   a  causes an induction in the second coil  100   b  in or at the goal frame, respectively. Due to the low overall impedance of the second coil  100   b  and its control electronics, the current induced in the coil  100   b  generates an opposing field to the magnetic field of the first coil  100   a . By this, the magnitude of the field strength of the first magnetic field in the area spanned by the second coil  100   b , i.e. the goal opening area, is reduced. Outside the area spanned by the second coil  100   b  or the goal opening area, respectively, the opposing field may be summed up to the alternating field of the first coil  100   a . By this, the field strength outside the second coil  100   b  may be increased, whereas the field strength within the area spanned by the second coil  100   b  is reduced. This connection is explained in the following with reference to  FIGS. 6 and 7 . 
       FIG. 6  shows the scenario already described with reference to  FIG. 3  in a side view.  FIG. 6  shows a first coil  100   a  and a second coil  100   b , wherein the first coil  100   a  is attached in an area behind the goal  200  and the second coil  100   b  is attached closer to the goal  200  than the first coil  100   a  or identically to the goal  200 . In the scenario illustrated in  FIG. 6 , the second coil  100   b  is in an open-circuit operation, i.e. no current may flow through the coil windings. The first coil  100   a  behind the goal is controlled such that it generates a magnetic alternating field with a predetermined frequency. The resulting field lines are indicated by the reference numeral  300  in  FIG. 6 . 
     If the second coil  100   b  is in an open-circuit operation, the magnetic field of the first coil  100   a  may propagate undisturbedly and an “undisturbed” field line course results, as it illustrated exemplarily in  FIG. 6 . 
     If the second coil  100   b  attached at or in the goal frame, respectively, of the goal  200  is used in a short-circuit operation, however, then, as already described above, an opposing field to the first magnetic field of the first coil  100   a  is generated by the second coil  100   b . By this, a field line course in the close proximity of the second coil  100   b  results, as it is schematically shown in  FIG. 7 . 
     Due to the low impedance of the second coil  100   b  in the goal frame, by the induced current, an opposing field is generated which ideally makes the area spanned by the second coil  100   b  field-free. The ideal case results exactly when the second coil  100   b  comprises a coil impedance Z sp =0. This ideal case, will, however, not be realizable in practice, which is why in the area spanned by the second coil  100   b  only an attenuation of the first magnetic field may be achieved, for example by at least 20%. The compensation or the attenuation, respectively, of the magnetic field generated by the first coil  100   a  is strongest close to the coil windings, i.e. to the goalpost, the crossbar and the goal line, and thus enables to determine extremely accurately whether the ball  230  is inside or outside the goal  200 . 
     According to an embodiment of the present invention, only the first coil  100   a  behind the goal  200  is used for generating a field. As already described above, the magnetic alternating field of the first coil  100   a  may generate a current in the second coil  100   b  which acts against its cause, the first magnetic field. According to embodiments, the current is only generated sufficiently when the second coil  100   b  is short-circuited and comprises a low impedance. According to embodiments of the present invention the coil impedance of the second coil is in a relevant frequency range (500 Hz to 5 kHz) in an impedance range between 0 and 100 Ohms. 
     According to further embodiments, an electronic switch may be used for periodically short-circuiting the second coil  100   b.    
     When the ball  230  is located in the goal plane, it will measure a different magnetic field depending on whether the switch is closed or not. If the switch is open, the magnetic field sensor in the ball  230  will measure an undisturbed magnetic field of the first coil  100   a  which is strongest in the center of the goal  200  and decreases towards the goal edges in a characteristic way, as was already described above. If the switch is closed, the magnetic field sensor of the ball  230  will measure a low field in the center of the goal, and outside the area spanned by the second coil  100   b  it will measure a stronger field. Additionally, a change of field direction may be measured when closing or opening the switch, respectively. After every measurement cycle, according to embodiments, three measurement values are available, using which it may be calculated very accurately whether and where the ball crossed the goal plane. It may, for example, be determined using the field strengths whether the ball is located in front of or behind the goal line  210 , respectively, and using the change of the field direction a statement may be made whether the ball crossed the goal opening area close to one of the side posts  200   a,b , close to the crossbar  200   c  or close to the goal line  210 . Further, due to the signal difference or field strength difference, respectively, which is very large at the edge of the goal  200 , it may be determined, when the switch is closed, whether the ball  230  only just passed the goal  200  or not. 
     It is one advantage of this embodiment that electronics within the ball  230  may be implemented relatively simply as here only one frequency, i.e. the frequency of the first coil  100   a , is measured and no frequency differentiation between the first and the second magnetic field is necessary. 
     If only one piece of information is needed about whether the ball  230  crossed the goal line  210  within the area spanned by the second coil  100   b  or not, then the inventive concept may be used for providing this piece of information. For this purpose, according to embodiments, a device for evaluating is implemented to provide an indication whether the movable object  230  crossed the goal plane from information about a time course of a magnetic field experienced from the movable object  230 . Here, the device for evaluating may be located within the ball  230  or outside the same, for example in a personal computer. The device for evaluating is further implemented to provide the goal statement by means of a derivation of the time course of the magnetic field over time, wherein the derivation of the time course of the magnetic field over time is approximately zero at the point of time of crossing the goal line. This connection is illustrated in  FIG. 8 . 
       FIG. 8  shows a course of the magnetic field strength in the proximity of the goal  200  with a short-circuited secondary coil  100   b  and a primary coil  100   a  generating a magnetic alternating field. As already described above, within the area spanned by the second coil  100   b  by the opposing magnetic field a reduction of the magnetic field of the first coil  100   a  is achieved. Accordingly, a movable object  230  moving towards the goal  200  in the goal area will experience a field strength time course as it is exemplarily shown in  FIG. 8 . 
     Coming from the positive x direction, the ball will first experience an increasing course of the field strength  800  which decreases when the ball crosses the area spanned by the second coil  100   b , i.e. the goal opening area. At this moment, the field strength time course comprises a local minimum  810 . After crossing the goal opening area in the negative x direction, the field strength course increases again, as illustrated in  FIG. 8 , to finally drop off again behind the first coil  100   a.    
     Thus, according to embodiments of the present invention, a decision about whether a goal has been scored (goal decision) may be brought about based on a detection of a minimum of the magnetic field time course. The conditions for a minimum of the time course of the magnetic field strength are d|H|/dt=0 and d 2 |H|/dt 2 &gt;0, wherein the absolute value |H| of the magnetic field strength may be calculated from the components (H x , H y , H z ) of a magnetic field measured by the magnetic field sensor in a point of space according to |H|=(H x   2 +H y   2 +H z   2 ) 1/2 . Using a sequence of magnetic field measurement values sent by the ball  230  and a corresponding logic the two above-mentioned conditions may thus be continually checked. 
     According to further embodiments of the present invention, a criterion for a decision about a goal may also be a change of sign of the first derivation d|H|/dt. When crossing the maximum of the magnetic field course, in general a change of sign from “−” to “+” takes place, as the magnetic field strength, with an approximation to the goal line  210 , first decreases to then increase again after crossing the same. 
     In addition, further events may be inferred from the course of the first derivation d|H|/dt of the time course of the magnetic field strength. If the first derivation comprises a discontinuity at a certain point in time, it may be assumed that the ball, for example, touched a side post or the crossbar, respectively. 
     To be able to clear up ambiguities, for example a Doppler frequency may additionally be evaluated which occurs due to a movement of the movable object  230  towards the goal  200  or away from the same. 
     Here, the device for evaluating may be implemented to obtain the goal statement by comparing the measurement values of the sequence of measurement values to predetermined values which are, for example, stored in a lookup table. 
     A method for taking a goal decision based on information about a position of a movable object in a goal area according to one embodiment of the present invention is illustrated in summary in  FIG. 9 . 
     In a first step S 1  the switch of the second coil  100   b  is open (open-circuit operation), wherein a measurement of an undisturbed magnetic field of the first coil  100   a  is performed in the goal area by the movable object or the ball  230 , respectively. In a second step S 2  the switch of the second coil  100   b  is closed (short-circuit operation) to perform a measurement of the overall magnetic field from the magnetic field of the first coil  100   a  and the opposing field of the second coil  100   b . In a third step S 3  a goal decision may be taken based on the measurement values from the steps S 1  and S 2 . 
     Finally,  FIG. 10  and  FIG. 11  again give an overview over a system for determining information about a position of a movable object  230  in a goal area, wherein  FIG. 10  illustrates a front view and  FIG. 11  a side view. 
       FIG. 10  illustrates a front view of a goal  200 , behind which a first coil  100   a  is located. A second coil  100   b  is attached closer to the goal  200  than the first coil  100   a  or attached identically to the goal  200 . The two coils  100   a,b  are connected to a means  240  for generating coil activation signals for the two coils  100   a,b  using a frequency-division multiplex operation. A coil activation signal is here a current or a voltage.  FIG. 10  further shows a movable object  230  or a ball, respectively, which is connected, via a radio link  1020 , to a device  1030  for evaluating the information about the magnetic field. 
       FIG. 11  shows another side view of a goal  200 , a first coil  100   a  attached behind the goal  200  and a second coil  100   b  attached identically to the goal  200 . A ball  230  may detect a magnetic alternating field  1100  of the first coil  100   a  and/or the second coil  100   b  when crossing the goal plane, as was already described above. 
     The device  1030  for evaluating is implemented, according to embodiments, to provide an indication from information about a time course of a magnetic field experienced by the movable object  230  whether the movable object  230  crossed the goal plane. 
     According to embodiments, the ball  230  includes a device for providing information about the magnetic field in which the movable object  230  is located which includes a magnetic field sensor. The device for providing is implemented to provide both information about the magnetic field generated by the first coil  100   a  and also information about the magnetic field generated by the second coil  100   b . For this purpose, it comprises, for example, an electric filter to be able to separate the magnetic alternating fields regarding frequency. The ball  230  further includes a transmitter for transmitting at least one measurement value and a controller for controlling the magnetic field sensor or the transmitter, so that magnetic field measurement values may be sent. Here, the magnetic field sensor is a three-dimensional magnetic field sensor which may, for example, be assembled using Hall sensors or magneto-resistive elements. 
     The movable object or the ball  230 , respectively, further necessitates an energy supply means for energy supply. The energy supply may, for example, be guaranteed by a battery in the ball  230 . In order to guarantee a long lifetime of the energy supply of the ball, it is, for example, possible to be able to activate and deactivate the same. This should advantageously take place so that as few interruptions of play as possible are necessary. The ball  230  may be activated in the proximity of the goal  200  via a weak signal which is, for example, sent from a respectively implemented transmitter of a central control/evaluation means. For this purpose, the ball for example comprises a receiver which receives the activation signal and thereupon activates the measurement system in the ball in the proximity of the goal  200  via a processor. The processor, for example, switches on the receiver in the ball briefly every 100 milliseconds. As soon as the activation signal is detected by the ball, the ball enters steady-state operation. 
     Further, also the magnetic field generated by an inventive device may be used as the activation signal. When the ball  230  gets into the proximity of the goal  200 , then this is detected by the three-dimensional magnetic field sensor in the ball. As soon as this is the case, the measurement system in the ball switches on. Also here, for example, the sensors may only be put into operation briefly every 100 milliseconds. 
     In the two procedures described above a detection is only switched on briefly to save energy. If the ball  230  does not detect a signal any more over a long period of time, for example one day, a timer for detection is, for example, set to ten seconds. Thereby, the energy consumption may again be decreased drastically. As, for example, the state of a battery in the ball may be sensed, it is guaranteed that a timer in the ball is, for example, set to 100 milliseconds again at the beginning of the match. 
     If conductive objects (including persons) move in a magnetic field, then a magnetic field may be induced within these objects. This magnetic field might influence the field geometry of the generated magnetic field. In a football match the players do not move as fast, however, so that a noticeable induction could be caused. The ball  230 , however, may reach velocities of up to 140 km/h. Thus, it is to be considered in an implementation that the electronics within the ball  230  is as small as possible and comprises no large conductive areas. 
     An influence on the generated magnetic field by power cables close to the goal  200  is relatively low. A power cable comprises usually at least one go and return conductor so that the magnetic fields of the go and return conductor cancel each other out. Even with individual conductors, the influence would be relatively low, as with a network frequency of 50 Hz, the field influence would be equal to a slight change of the magnetic field of the earth. 
     The described system is very accurate in the direction perpendicular to the goal plane by measuring the field strength of the field generated by the first coil  100   a . Alongside the goal plane, only a very low field change of the magnetic field is measured from the first coil  100   a . By the low impedance of the second coil  100   b , the field of the first coil  100   a  is embossed with a very strong signal difference which is located exactly at important locations like the posts  200   a,b  or the crossbar  200   c , respectively. Therefore it is possible to decide about those critical positions error-free. If only one piece of information is needed about whether a movable object or a ball  230 , respectively, crossed the goal line  210  within the goal  200 , then by monitoring the time course of field strength measurements and by the detection of a minimum of the time course, a goal decision may be taken. 
       FIG. 12  shows a cross-section through a goalpost with cable channels. At the back part of the goalpost, further the goal net suspension is located which may, for example, be implemented like a curtain rail. The cable shafts are manufactured symmetrically so that each and the same goalpost may be manufactured for the left and the right post. Thus, it is advantageous to provide the outer shaft with a cable to obtain a coil, using which the magnetic field may be generated. The interior shaft remains empty in this case. The crossbar may only have one single shaft which is arranged at the top with reference to the goal. Advantageously, the shaft is dimensioned such that it is just big enough for a cable to be inserted, and that the position of the cable in the shaft, however, is predetermined as far as possible and will only deviate slightly from goal to goal. 
     In particular, it is noted that, depending on the circumstances, the inventive scheme may also be implemented in software. The implementation may be on a digital storage medium, in particular a floppy disc or a CD, having electronically readable control signals which may cooperate with a programmable computer system and/or a microcontroller so that the corresponding method is performed. In general, the invention thus also consists in a computer program product having a program code stored on a machine-readable carrier for performing the inventive method, when the computer program product runs on a computer and/or a microcontroller. In other words, the invention may thus also be realized as a computer program having a program code for performing the method when the computer program runs on a computer and/or a microcontroller. 
     While this invention has been described in terms of several embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations and equivalents as fall within the true spirit and scope of the present invention.