Abstract:
A field sensor ( 1 ) comprising means for measuring a magnetic field and an electric field, wherein at least six capacitor plates ( 5, 6, 7, 15, 16, 17, 25, 26, 27, 35, 36, 37 )are provided which are arranged in a cube and in that in the space thus formed the means for measuring the magnetic field is provided, which means is three-dimensionally aligned and for example consists of coils ( 2, 3, 4 ), Hall-effect sensors, magnetoresistors or the like.

Description:
This application is the national phase under 35 U.S.C. §371 of prior PCT International Application No. PCT/EP97/0153 which has an International filing date of Mar. 21, 1997 which designated the United States of America, the entire contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a field sensor comprising means for measuring a magnetic field and/or an electric field as well as a corresponding method. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is based on the object of providing a field sensor of the aforementioned type with which electric or magnetic fields may be measured simultaneously or successively in any direction, and a respective device as well as a corresponding method. 
     A field sensor is provided with at least six capacitor plates which are arranged in a cube and in the space thus formed a means for measuring the magnetic field is provided which means is three-dimensionally aligned and consists for example of coils, Hall-effect sensors, magnetoresistors or the like. 
     The capacitor plates serve in this connection for measuring the electric field three-dimensionally and simultaneously generate in the interior of their spatial arrangement a field-free space. The latter is provided with the measuring means which enables a three-dimensional detection of the magnetic field without interference by the electric field. 
     Thus, an electric and a magnetic field can simultaneously be measured three-dimensionally, however, without affecting each other. 
     In an advantageous embodiment of the invention, three annular and/or cylindrical coils are provided which are arranged on the x-, y- or z-axis and the coils are surrounded by the capacitor plates every two of which are arranged in parallel to each other. 
     According to an alternative embodiment, the capacitor plates are partially or completely arranged within a space which is formed by the components of a magnetometer. In a magnetometer consisting of coils, e.g., the coils extend partially or completely beyond the capacitor plates. The size of the capacitor plates and their distance from each other is selected as a function of the size of the magnetometer components such that an electric and a magnetic field may simultaneously be measured three-dimensionally, their mutual interference being preferably low. Preferably, the edge length of the capacitor plates is low with respect to the outer dimensions of the magnetometer components. The ratio between the edge length of the capacitor plates and for example the diameter of a magnetic field coil is in the range of 1 to 100 up to 100 to 1. The capacitor plates may be displaced up to 30% of the coil diameter to the inside. 
     In other words, in the alternative embodiments, the space formed by the capacitor plates and the space formed by the means for measuring the magnetic field are aligned with each other and both systems intersect with each other three-dimensionally while having a common reference point. 
     In a further preferred embodiment of the invention, the coils are fitted into each other annularly with an almost identical diameter. 
     Thus, the magnetic field may uniformly be measured in all three directions. 
     It has also turned out to be quite advantageous if, according to a further embodiment of the invention, each coil has a couple of capacitor plates which are provided in parallel to the respective coil outside the coil arrangement. 
     Thus, the coils are perfectly screened by the capacitor plates. 
     According to an embodiment of the invention it is also very advantageous to provide the capacitor plates with a high-resistance coating. 
     It is, however, also possible according to a further embodiment of the invention that the capacitor plates consist of a high-resistance material. 
     In both embodiments, any interference with the magnetic field to be measured is excluded. 
     A further embodiment of the invention is characterized in that the capacitor plates are arranged in couples and each of the respective inner plates is electronically connected to a common earth terminal. 
     Thus, any interference of the coils, Hall-effect sensors or magnetoresistors with the electric field is excluded. 
     According to a further embodiment of the invention, it has turned out to be very advantageous to provide the electric terminals of the capacitor plates at their edge. Thus, any influence of the current within the lines on the measuring result is avoided. 
     The present invention further relates to a device for measuring electric fields which comprises a field sensor with at least one capacitor-plate couple whose output voltage is applied to the non-inverting and the inverting input of a differential amplifier, a capacitor being connected between the two inputs of the differential amplifier. This measuring device may in particular be used with the field sensor described above. The measuring device works on the following basic principle. An arrangement of two capacitor plates leads to a voltage drop in the electric field at the capacitor; this voltage drop is decoupled by earthing via a differential amplifier preferably with a high resistance and preferably with a low input capacity. The relatively high current through the capacitor minimizes the coupling of interference voltages by the measuring amplifier as well as the field intensity between the capacitor plates. The measuring device may preferably be used for measuring electric and/or magnetic fields with a frequency of 1 Hz to 30 GHz, particularly preferred are 5 Hz to 400 kHz. The measuring device is in particular advantageous in that it may be designed as a hand tool which takes measurements both as a stationary tool and when moved with electrically conducting lines and the resulting measuring errors are minimized. 
     A further preferred feature is the use of thin wires as connections between the measuring device and the capacitor-plate couple. The advantage thereof is that on account of the small cross-section of the wires the interference with the field is low as compared with the plate size of the capacitor-plate couple. Preferably, two wires each are twisted for being connected with one capacitor-plate couple. Thus, magnetic fields generated within the wires on account of a current counterbalance each other such that the resulting magnetic field is kept low and a reverse effect is reduced. 
     Preferably, the distance between the field plates and the measuring amplifier is so large that the interference of the measuring device with the electric field to be measured is low. 
     It is particularly preferred that the area of the capacitor and the differential amplifier is small as compared with the area of the capacitor plates in order to further avoid the interference of the measuring device with the electric field to be measured. The method according to the present invention for measuring electric and/or magnetic fields is preferably carried out with a combination of the above-described field sensor and the respective measuring device. In particular, measuring errors caused by the person taking the measurement or the electric lines can be kept within tolerable limits with the present method depending on the measurement set-up. This is in particular achieved by measuring the electric and the magnetic field simultaneously at one point. Moreover, the smallest possible sensors are used to avoid errors on account of field inhomogeneities. 
     Furthermore, the smallest possible sensors have the advantage that averaging is avoided when measuring an electric or magnetic field. Moreover, the object to be measured may be appreciated. 
     In particular by using sensitive sensors, the present invention fulfils the electromagnetic compalitrility (EMC) standards. 
     A simultaneous three-dimensional measurement of electric and magnetic fields moreover avoids possible measuring errors which may occur in one-dimensional measurements when searching the maximum field. 
     On account of a larger distance between the person taking the measurement and the measuring device as well as technical measures at the measuring device for providing a high-resistance set-up as well as a small capacity between sensor and display unit, the measuring accuracy in particular of the electric field is improved. 
     With the method according to the present invention, electric and magnetic fields may be measured simultaneously. Preferably, the person taking the measurement holds the measuring device in his/her hand when measuring electric fields, in particular three-dimensional electric fields. More preferably, for measuring electric and/or magnetic fields the device is kept at a distance from the body of the person taking the measurement by means of a short handle, such as a rod. It is an advantage of the device and the method according to the invention that the person taking the measurement may move with the device within a room or in the open and that accurate measurements are even possible if there are highly static fields of several kV/m of the electric field. Preferably, the sensor is moved steadily and uniformly so that static fields so not affect the measurement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be illustrated by means of working examples in the drawings, which: 
     FIG. 1 shows a schematic view of a three-dimensional field sensor comprising three coils and six capacitor plates; 
     FIG. 2 shows a schematic arrangement of six capacitor plates; 
     FIG. 3 shows an arrangement of twelve capacitor plates; and 
     FIG. 4 shows a block diagram of a device for measuring electric and/or magnetic fields. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference sign  1  in FIG. 1 designates a field sensor comprising three intermeshing annular coils  2 ,  3  and  4 . Coil  2  is perpendicular to the x-axis, coil  3  is perpendicular to the y-axis and coil  4  is perpendicular to the z-axis. Outside the spherical structure formed by the three coils, there are six capacitor plates  5 ,  15 ;  6 ,  16  and  7 ,  17 , every two of which are arranged in parallel and form a capacitor. The capacitor plates  5 ,  15  are in this connection perpendicular to the x-axis, the capacitor plates  6 ,  16  are perpendicular to the y-axis and the capacitor plates  7 ,  17  are perpendicular to the z-axis. 
     The three coils  2 ,  3 ,  4  serve for measuring a magnetic field three-dimensionally, whereas the three capacitors  5 ,  15 ;  6 ,  16  and  7 ,  17  serve for measuring the electric field. 
     The capacitor plates are advantgeously so big that they form an almost closed cube, as depicted in FIG.  2 . The capacitor plates have a circular shape and completely cover the coils  2 ,  3 ,  4  inside. 
     In the embodiment according to FIG. 3, pairs of superimposed capacitor plates  5 ,  25 ;  6 ,  26 ;  7 ,  27 ;  15 ,  35 ;  16 ,  36 ;  17 ,  37  are provided which have a small distance from each other. In each case, the respective inner plate  25 ,  26 ,  27 ,  35 ,  36 ,  37  is connected to the common earth. Both the earth terminals and the terminals of the other plates are at the outer edge of the plates in order to avoid interference on account of the current within the lines. 
     The surfaces of the capacitor plates have a high-resistance coating. The plates may, however, also be made of a material having a high-resistance surface. 
     FIG. 4 shows a preferred embodiment of a device for measuring electric and magnetic fields as a basic circuit. It schematically shows a capacitor-plate couple  5 ,  15  for unidimensionally measuring an electric field (E-field) and a coil  2  for unidimensionally measuring a magnetic field (H-field). 
     The device  50  for measuring an electric field comprises a differential amplifier  51  whose non-inverting input is connected to the capacitor plate  15  and whose inverting input is connected to the capacitor plate  5 . A capacitor C 1  is connected between the input terminals of the differential amplifier. The capacitor-plate couple  5 ,  15  and the measuring device  50  are connected via thin wires  53 ,  55 , which are preferably twisted with each other. Both input terminals of the differential amplifier  51  have a low input stray capacity (Cstray), which is preferably in a range between 0.5 pF and 1 μF with respect to earth. The input resistance of both inputs Rin is connected to earth with a high resistance; the resistance ranges preferably from 1 MΩ to 100 TΩ. The capacity of the capacitor C 1  ranges preferably from 0.5 pF to 10 μF. Preferably, the capacity may be switched from for example 1 nF to 10 nF while the sensitivity is changed for different measuring ranges. The output signal of the differential amplifier  51  is rectified by a rectifier D 1  and supplied to an evaluation unit  60 . 
     In the depicted example, the capacitor plates are square and preferably have an edge length of 5 mm to 300 mm. The distance between the capacitor plates approximately corresponds to the edge length of the capacitor plates. The distance between the field sensor  1  and the measuring device is preferably 5 mm to 1000 mm. 
     When a field sensor as depicted in FIGS. 1 to  3  is used, each capacitor-plate couple is connected with a respective measuring device  50 . Thus, the simultaneous three-dimensional detection of the electric field is possible. 
     In the shown example, the capacitor plates consist of a high-resistance material, such as printed circuit material, FR4, Pertinax or the like. Each of the capacitor plates facing the outside is provided with a conductive layer, such as a graphite layer. The wires are connected with the respective capacitor plates by means of a conductive lacquer/varnish. 
     In an alternative embodiment (not shown), a cube of plastics is injection-moulded. Each of the 6 surfaces forms a capacitor plate. This injection-moulded cube is hollow and for example forms a box with a hinged lid. An arrangement of three perpendicular coils  2 ,  3  and  4  (cf. FIGS. 1,  2  and  3 ) is inserted into this box. Preferably, the coils are fixed to the capacitor plates by gluing. 
     FIG. 4 further shows a basic circuit of a device for measuring the magnetic field comprising an integrator  40  connected with a coil  2 . The integrator  40  consists in the shown embodiment of a capacitor C 2  which is connected to earth and a diode D 2 , which are both connected with one end of the coil  2 . The other end of the coil  2  is earthed. In the shown example, the coil has an annular or cylindrical winding of wires and the number of windings is between 1 to 50,000. In the shown example, the coil has a diameter of approximately 30 mm corresponding to the preferred edge length of the capacitor plates  5 ,  15 . 
     The measuring device further comprises an evaluation means  60  to which the output signal of the differential amplifier  51  and the output signal of the integrator  40  is applied. The output signals are processed by the evaluation means  60  and outputted to a display  70  in a scaled form. The display preferably comprises a bar chart display and additionally outputs a digital numerical value. The display further comprises an add-on acoustic display which preferably changes frequency and/or volume as a function of the signal strength and converts a start value and an end value into a corresponding tone. Simultaneously, a parallel evaluation of the two measuring values is carried out. 
     If an electric field is three-dimensionally measured, the signals of the three capacitor-plate couples  5 ,  15 ;  6 ,  16  and  7 ,  17  are collected by three measuring devices  50  and the signals of the coils  2 ,  3 ,  4  are collected by three measuring devices  40  and applied to the evaluation means  60 . The evaluation means comprises analog-to-digital converters and a microprocessor which further processes the digital signals. Preferably, the output signals are squared, subsequently the squares are added and the square root of the sum is calculated. Moreover, scaling is accomplished. The result thus gained is supplied to the display  70 . 
     Furthermore, the measuring unit may be floatingly connected to a distant evaluation unit via optical waveguides and is preferably remotely controllable. Thus, a completely floating measurement is in particular achieved and measurements in highly electric or magnetic fields are made possible without exposing the person taking the measurements to these fields.