Abstract:
A positioning device for capacitively detecting an object enclosed in a medium includes a measuring electrode, a reference electrode, and a receiving electrode. The measuring electrode with the receiving electrode forms a measuring capacitance, which can be influenced by the object. The reference electrode with the receiving electrode forms a reference capacitance, which cannot be influenced by the object. The positioning device further includes an oscillator configured to supply the measuring capacitance and the reference capacitance with phase-shifted AC voltages and a control device configured to control amplitudes of at least one of the AC voltages, in order to adapt effects of electrical fields of the measuring electrode or of the reference electrode on the receiving electrode to one another. The measuring, reference, and receiving electrodes are planarly formed. The measuring electrode has a larger surface area than the reference electrode.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2013/052929, filed on Feb. 14, 2013, which claims the benefit of priority to Serial No. DE 10 2012 205 122.8, filed on Mar. 29, 2012 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The disclosure relates to a locating appliance. In particular, the disclosure relates to a locating appliance for the capacitive detection of an object enclosed in a medium. 
     BACKGROUND 
     In order to sense an article concealed in a wall, for example a beam in a wall of lightweight construction, capacitive detectors are known. Such detectors use an electrode that has its charging or discharge behavior determined in order to infer the dielectric object. Detectors having a plurality of electrodes are also known, which involve determining a change in the capacitance of a pair of electrodes. Usually, it is necessary for such detectors to be calibrated manually on the wall, since the appliances cannot detect wall contact themselves and the capacitance of the electrodes or electrode pairs is dependent on ambient conditions, such as a temperature, a humidity, an object averted from the sensor, grounding via a user or electrical or dielectric properties of the wall material. In order to take account of these variable influencing factors, it is necessary for known appliances to be calibrated on the wall, which requires either appropriate control by a user or a complex sensor system. 
     DE 10 2007 058 088 A1 shows a sensor for locating dielectric objects in a medium. The sensor shown determines a ratio between a reference capacitance and a measurement capacitance, the latter being dependent on the position of the object in relation to electrodes of the two capacitances. 
     DE 10 2008 005 783 B4 shows a capacitive detector as a crash protection system that uses a push-pull measurement bridge to compare the capacitance of two capacitances with one another. One of the capacitances is formed by two electrodes that can be positioned relative to one another, so that a change in their relative interval can be used to generate a signal that warns of crashing. 
     The disclosure is based on the object of specifying a locating appliance for capacitive detection that does not require calibration in order to attain a high level of measurement accuracy. 
     SUMMARY 
     The disclosure achieves this object by means of a locating appliance having the features described herein. 
     There are essentially two reasons for requiring calibration of the locating appliance. Firstly, uncontrollable influences, such as an ambient temperature, an ambient humidity, an object averted from the sensor or grounding of the locating appliance via a user, can influence the output signal. Secondly, the output signal differs, regardless of the object against a medium, from an output signal in air, with a material and a material thickness of the medium and also electrical wall properties, such as a dielectric constant or a conductivity, being able to be included in the output signal. 
     A locating appliance for the capacitive detection of an object enclosed in a medium comprises a measurement electrode, a reference electrode and a reception electrode, wherein the measurement electrode forms, with the reception electrode, a measurement capacitance that can be influenced by the object, and the reference electrode forms, with the reception electrode, a reference capacitance that cannot be influenced by the object, and also an oscillator for supplying the measurement capacitance and the reference capacitance with phase-shifted AC voltages and a control device for controlling amplitudes of at least one of the AC voltages in order to match the influences of electrical fields from the measurement electrode or the reference electrode on the reception electrode to one another. In this case, the electrodes are of two-dimensional design by virtue of their having only small thicknesses in relation to their surfaces. In a preferred embodiment, the surfaces are furthermore flat. In addition, the measurement electrode has a greater area content than the reference electrode. 
     The use of the described electrode arrangement on a push-pull measurement bridge means that there can be compensation for interference factors that can influence the capacitances beteen the electrodes. Such influences may be independent of the object and the medium, such as an ambient temperature, a humidity, an object that is averted from the electrodes or grounding of a locating appliance via a user who is holding the appliance in his hand. 
     In a preferred embodiment, the interval between the measurement electrode and the reception electrode is greater than the interval between the reference electrode and the reception electrode. 
     The described push-pull measurement bridge takes a presence of the object as a basis for determining the alteration of a quotient that is formed by a difference between the measurement capacitance and the reference capacitance and a sum of these two capacitances. Influences that influence both capacitances in the same way therefore do not alter the measured variable. The specified geometric form of the individual electrodes means that the influenceability of the reference capacitance by the object may be reduced. 
     At least to some extent and particularly in the region between the measurement electrode and the reception electrode, the measurement electrode may be surrounded by a guard electrode that is connected to a constant potential. In this and other embodiments, it is appropriate for the measurement electrode, the reception electrode and the reference electrode to be arranged in the same plane. In this case, the guard electrode can likewise run in this plane. The constant potential may correspond to the appliance ground, in particular. In the plane, the guard electrode provides shielding that is adjusted for a predetermined potential, particularly one that is constant over time. Hence, no additional involvement is required in order to implement the shielding, and passive shielding of this kind is effective particularly when the potential on the reception electrode is adjusted, on the basis of the regulatory condition, such that AC voltage components that are in sync with the AC voltages described above disappear. The guard electrode can therefore improve the determination quality of the locating appliance in conjunction with the described push-pull measurement bridge. 
     The measurement electrode, the reference electrode and the reception electrode may be situated in one plane, wherein a shielding electrode that is connected to a constant potential and at least partially or, preferably, completely covers the electrodes situated in the plane is arranged on a side that is averted from the object. An influence of an object that is not intended to be detected, particularly of a user of the locating appliance, may be reduced as a result. In one variant, the coverage may also provide just partial coverage, for example the reception electrode or the reference electrode may be excluded from the coverage. 
     In another embodiment, which can be combined with the aforementioned embodiment, the support material has a recess between the measurement electrode and the reception electrode. 
     In yet another embodiment, which can be combined with the aforementioned embodiment, the reference electrode and the reception electrode are arranged on a surface of a flat support material, wherein the support material has a recess between the reference electrode and the reception electrode. 
     The aim of both measures is for a number of electrical field lines to be minimized by the support material between the measurement electrode and the reception electrode and between the reference electrode and the reception electrode. An influence on the measurement capacitance or on the reference capacitance as a result of properties of the support material may be reduced as a result, and a dependency of the capacitances on temperature and moisture may be simplified. 
     The guard electrode may be electrically connected to the shielding electrode. The two different types of shielding may thus be connected to one another in a simple manner. This can contribute to the locating appliance being of simple design, which allows production and development costs to be saved. 
     A multiplicity of conductor pieces for electrically connecting the shielding electrode to the guard electrode may be situated between the measurement electrode and the reception electrode. 
     In another embodiment, which can be combined with the aforementioned embodiment, the measurement electrode and the reception electrode are arranged on a surface of a flat support material, wherein the support material has a recess between the measurement electrode and the reception electrode, at least some of the delimitation of said recess being provided with a conductive layer that is electrically connected to the shielding electrode and the guard electrode. 
     The aim of both variants of the vertical contact-connection is for the electrical connection between the guard electrode and the shielding electrode to be designed, in a region between the measurement electrode and the reception electrode, such that electrical field lines, particularly those through the support material, are shielded between the measurement electrode and the reception electrode. A fundamental capacitance between the measurement electrode and the reception electrode may be reduced as a result. This means that the sensitivity of the measurement circuit toward an influence of the object may be increased. An influence on the fundamental capacity by properties of the support material may be reduced as a result and a dependency of the fundamental capacitance on temperature and moisture may be simplified. 
     The measurement electrode, the reference electrode and the shielding electrode may each be coated with an insulating layer. An electrical influence by the medium, for example, on the reference or measurement capacitance may be reduced as a result. In particular, the insulating layer can be used as a moisture barrier, so that humidity cannot penetrate the support material and influence the capacitances. 
     The measurement electrode or the reference electrode may comprise a plurality of sections that are at intervals from one another and that are connected to one another with low electrical impedance. The electrodes may thus be arranged such that two-dimensional determinability of the object is made possible. 
     The measurement electrode or the reference electrode may comprise a plurality of sections that are electrically insulated from one another and that have unequal but mutually proportional signals applied to them. A sensitivity of the electrode arrangement may therefore have different magnitudes in different directions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention disclosure is now described more precisely with reference to the appended figures, in which: 
         FIG. 1  shows a locating appliance; 
         FIGS. 2A and 2B  show an arrangement of electrodes for the locating appliance in  FIG. 1 ; 
         FIG. 3  shows an electrical connection between electrodes on the arrangement in  FIGS. 2A and 2B ; and 
         FIG. 4  shows an alternative electrical connection between electrodes on the arrangement in  FIGS. 2A and 2B . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a locating appliance  100  for the capacitive detection of an object  110  enclosed in a medium  105 . 
     The locating appliance  100  comprises a push-pull measurement bridge  115  and an arrangement  120  of electrodes. 
     An oscillator  125  provides two phase-shifted AC voltages, preferably in antiphase, at the same frequency on the measurement bridge  115 . The two AC voltages are routed to two amplifiers  130  and  135 , at least one of which can have its gain factor controlled by means of a signal. The output of the first amplifier  130  is connected to a measurement electrode  140  and the output of the second amplifier  135  is connected to a reference electrode  145 . 
     The arrangement  120  comprises at least the electrodes  140  and  145  and also a ground-free reception electrode  150 . The electrodes  140 ,  145  and  150  are arranged relative to one another such that a measurement capacitance C 1  becomes established between the measurement electrode  140  and the reception electrode  150  and a reference capacitance C 2  becomes established between the reference electrode  145  and the reception electrode  150 . In this case, the electrodes  140 ,  145  and  150  are designed such that the measurement capacitance C 1  can be influenced by the object  110 , whereas the reference capacitance C 2  cannot, or can to a negligibly small extent. 
     The reception electrode  150  is connected to a measurement amplifier  155 , the output of which is connected to a synchronous demodulator  160 . On the basis of a clock signal that is provided by the oscillator  125  and the frequency of which corresponds to that of the AC voltages that are provided for the amplifiers  130  and  135 , the influences of the measurement electrode  140  and the reference electrode  145  on the reception electrode  150  are determined at alternate times and provided for an integrator  165 , which may be in the form of an integrating comparator, for example. An output of the integrator  165  is connected to an interface  170  at which a measurement signal is provided. Furthermore, the measurement signal is used to control the gain factors of at least one of the amplifiers  130  and  135 . If both amplifiers  130 ,  135  are controllable, an inverter  175  is provided in order to control the gain factors in opposite directions. 
     The push-pull measurement bridge  115  is set up to apply AC voltages to the measurement electrode  140  and the reference electrode of the arrangement  120  such that the effect of a dielectric influence of the object  110  on the capacitances C 1  and C 2  at the reception electrode  150  is of equal magnitude. In this case, the reference capacitance C 2  is of a physical design such that it cannot or practically cannot be influenced by the object  110 . If the object  110  is situated asymmetrically in the region of the electrodes  140 ,  145 , for example, so that the capacitances C 1  and C 2  are influenced by the object  110  dielectrically to different degrees, the AC voltages have unequally high amplitudes, so that the influences of the measurement electrode  140  and the reference electrode  145  on the reception electrode  150  are the same on average over time. The measurement signal provided at the interface  170  reflects the modulation of the amplifiers  130 ,  135 . If the measurement signal is higher or lower than a predetermined value that corresponds to a nonexistent object  110 , it is possible to infer the object  110  from the measurement signal. 
       FIGS. 2A and 2B  show the arrangement  120  of electrodes for the locating appliance  100  from  FIG. 1 . In this case,  FIG. 2A  shows electrodes in a first plane, which faces the object  110 , and  FIG. 2B  shows an arrangement of electrodes in a second plane, which is averted from the object  110  in relation to the first plane. In practice, the arrangement shown may be in the form of a printed circuit on different layers of a board made of insulating material, for example. 
     In  FIG. 2A , the first plane contains a first measurement electrode  205  and a second measurement electrode  210 , which each correspond to the measurement electrode  140  in  FIG. 1 , a first reference electrode  215  and a second reference electrode  220 , which each correspond to the reference electrode  145  from  FIG. 1 , and a reception electrode  225 , which corresponds to the reception electrode  115  from  FIG. 1 , and a guard electrode  242 . Mutually corresponding electrodes  205  and  210 ,  215  and  220  may be electrically connected to one another at low impedance. In another embodiment, mutually corresponding electrodes  205 - 220  have signals applied to them that are the same or not the same but proportional to one another and that may come from different sources. For this purpose, a dedicated amplifier  130  may be provided in the measurement bridge  115  from  FIG. 1  for each of the measurement electrodes  205  and  210 , for example. Each of the duplicate electrodes  205  and  210 ,  215  and  220  may also be in single form. 
     The reference electrodes  215 ,  220  are smaller than the measurement electrodes  205  and  210 , specifically preferably much smaller, so that the area content of a measurement electrode  205 ,  210  is a multiple of the area content of a reference electrode  215 ,  220 . Preferably, the measurement electrodes  205  and  210  are of equal magnitude. Likewise preferably, the reference electrodes  215 ,  220  are of equal magnitude. 
     Preferably, the measurement electrodes  205  and  210  are at a greater distance from the reception electrode  225  than the reference electrodes  215  and  220 , specifically at a much greater distance, so that the distances from the measurement electrodes  205  and  210  to the reception electrode  225  are each a multiple of the distances from the reference electrodes  215  and  220  to the reception electrode  225 . Preferably, the interval between the first measurement electrode  205  and the reception electrode  225  corresponds to the interval between the second measurement electrode and the reception electrode  225 . Likewise preferably, the interval between the first reference electrode  215  and the reception electrode  225  corresponds to the interval between the second reference electrode  220  and the reception electrode  225 . 
     Optionally, the arrangement  120  furthermore contains a first opposing electrode  235  and possibly also a second opposing electrode  240 . The measurement electrodes  205 ,  210  and the opposing electrodes  235 ,  240  are preferably at the same magnitude and are arranged horizontally and vertically at intervals of the same magnitude from one another. The measurement electrodes  205  and  210  and also the opposing electrodes  235  and  240  may each be surrounded by a guard electrode  242 . 
     Approximately in the center of  FIG. 2 a    there runs a guard electrode  232  in a horizontal direction, isolating the measurement electrodes  205  and  210  arranged at the top, the respective associated guard electrodes  242 , the reference electrodes  215  and  220  and the first reception electrode  225  from the opposing electrodes  235  and  240  arranged at the bottom with their associated guard electrodes  242  and the further guard electrode  230  and reducing the capacitive coupling between the opposing electrodes  235 ,  240  and the reception electrode  225 . That portion of the arrangement  120  that is situated below the horizontal guard electrode  232  in  FIG. 2A  can also be omitted in other embodiments. 
     All of the guard electrodes  230 ,  232 ,  242  are optional. The guard electrodes  242  are used to interrupt capacitive couplings between electrodes  205 - 225 ,  235 ,  240  situated in the first plane. The guard electrode  230  corresponds to the reception electrode  225  and increases the symmetry of the electrode arrangement and hence of the field line distribution. The guard electrodes  230 ,  232 ,  242  are connected to a predetermined potential φ 1 , particularly one that is constant over time, for example to an appliance ground of the locating appliance  100  from  FIG. 1 . This approach differs from known active shielding in that the potential of the guard electrodes is constant over time and is not tracked to another potential. The guard electrodes  242  are particularly suitable when the push-pull measurement bridge  115  shown in  FIG. 1  is used, since the measurement bridge  115  is set up to adjust the potential on the reception electrode  150  such that AC voltage components that are in sync with the clock of the AC voltages on the measurement electrode  140  and the reference electrode  145  disappear. 
     Insulation between adjacent electrodes in the first plane can also be provided by means of air by virtue of a recess  244  being introduced between the electrodes, as shown by way of example between the first reference electrode  215  and the reception electrode  225  and between the second reference electrode  220  and the reception electrode  225 . 
     In the preferred embodiment shown, all of the electrodes  205 - 242  of the arrangement  120  are covered by an insulating layer  246  in order to hamper resistive coupling to the medium  105  of the ambient air or to another object. The insulating layer is also used as a moisture barrier, so that moisture, for example from the air, cannot get into the support material and influence the capacitances. 
       FIG. 2B  shows four shielding electrodes  250 , which are each proportioned and positioned such that they cover one of the measurement electrodes  205 ,  210  or one of the opposing electrodes  235 ,  240  together with the possibly associated guard electrode  242 . The shielding electrodes  250  are connected at the locating appliance  100  to a potential φ 2   that is constant over time and that may correspond to an appliance ground of the locating appliance  100 . In addition or alternatively, the shielding electrodes  250  may be connected to the guard electrodes  242 . The shielding electrodes  250  may also be protected from external influences by means of an insulating layer  246 -not shown. 
       FIG. 3  shows an electrical connection between different planes of the arrangement  120  in  FIGS. 2A and 2B . 
     The recess  244  is made in a board  305  that carries the first plane from  FIG. 2A  on its top and the second plane from  FIG. 2B  on its bottom. The recess  244  is made in the board  305  between the reference electrode  145  and the reception electrode  150 . The recess  244  is optionally provided with a conductive layer  310  on at least one side, said conductive layer being electrically connected at the top to one of the electrodes of the first plane, in this case the guard electrode  242 , for example, and at the bottom to one of the electrodes of the second plane, in this case the shielding electrode  250 , for example. In a further example, the recess  244  may be made in the board  305  between the measurement electrode  140  and the reception electrode  150 . In this case, the conductive layer  210  preferably makes electrical contact between the guard electrode  242  on the top and the shielding electrode  250  on the bottom. 
     An alternative mechanical form of the electrical connection is shown in  FIG. 4 . Instead of the recess  244 , a number of vertical holes are made in the board  305  and have conductor pieces  315  passing through them. The conductor pieces  315  are preferably produced as plated-through holes (vias), for example by means of electroplating or riveting. 
     The two variants shown in  FIGS. 3 and 4  are used for producing an air-filled region and optionally a conductor piece in the vertical direction in order to shield an electrical field between electrodes that are situated opposite one another in respect of the conductor piece in the plane. Both embodiments may be provided in addition or as an alternative to a guard electrode  242  that can be used for the same purpose.