Abstract:
A positioning device for capacitively detecting an object enclosed in a medium includes a measuring electrode, a receiving electrode, and a reference capacitance. The measuring electrode and the receiving electrode form a measuring capacitance that can be influenced by the object and the reference capacitance cannot be influenced by the object. The electrodes are disposed in a plane, and the device includes a spacer that is configured to keep the electrodes at a predetermined minimum distance from the surface of the medium. The predetermined minimum distance is different from zero.

Description:
This application is a 35 U.S.C §371 National Stage Application of PCT/EP2013/052918, filed on Feb. 14, 2013, which claims the benefit of priority to Ser. No. DE 2012 205 126.0, filed on Mar. 29, 2012 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The disclosure realtes to a locating appliance. In particular, the disclosure relates to a locating appliance for the capacitive detection of an object encloses in a medium. 
     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 of the disclosure. Subclaims reproduce preferred embodiments. 
     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. 
     An inventive locating appliance for the capacitive detection of an object enclosed in a medium comprises a measurement electrode, a reception electrode and a reference capacitance, wherein the measurement electrode forms, with the reception electrode, a measurement capacitance that can be influenced by the object, and the reference capacitance cannot be influenced by the object. The electrodes are arranged in a plane and a spacer is provided in order to hold the electrodes at a predetermined minimum distance, other than zero, from the surface of the medium. 
     The reception electrode is preferably ungrounded. 
     In a preferred embodiment, the reference capacitance is formed from a reference electrode and the reception electrode. 
     The locating appliance provides an output signal which indicates the presence of the object. However, the output signal preferably depends not only on the presence or absence of an object but also on the distance between the locating appliance and the medium or on electrical and dielectric properties of the medium. 
     In one embodiment, the locating appliance comprises an evaluation circuit for providing an output signal on the basis of a ratio between the measurement capacitance and the reference capacitance. 
     In a first variant, the minimum distance is therefore chosen such that the output signal is at a minimum when the locating appliance is placed on the medium. The minimum distance is therefore chosen such that the output signal is smaller than for other distances of the locating appliance from the medium, in particular also such distances that are smaller than the minimum distance. In the mathematical sense, this is therefore a local minimum of the amount of the output signal. 
     The minimum distance defined in this way may depend primarily on a geometry of the electrodes used and be easily determinable empirically. By placing the electrodes at a distance from the medium at which the output signal is at a minimum, the locating appliance can be almost independent of electrical or dielectric properties of the medium. In particular, the locating appliance becomes independent of minor changes in the distance of the locating appliance from the medium, such as are virtually unavoidable when the locating appliance is moved on the medium by tilting or as a result of rough surfaces. 
     In a second variant, the output signal may qualitatively behave oppositely as a function of the distance, so that the output signal is at a maximum when the locating appliance is placed on the wall. Here, too, the spacer allows the dependence of the measured value on electrical or dielectric properties of the medium and minor changes in distance of the locating appliance from the medium to be avoided. 
     The locating appliance may be set up to determine the measurement signal on the basis of a quotient of a difference and a sum respectively of the measurement capacitance and the reference capacitance. Different approaches for such evaluation circuits are known and can be used for the locating appliance. These include, for example, a bridge measurement circuit with a feedback amplifier. 
     In a particularly preferred embodiment, the evaluation circuit is formed on the principle of the push-pull measurement bridge and comprises an oscillator for supplying the measurement capacitance and the reference capacitance with phase-shifted AC voltages, a control device for controlling the amplitude of at least one of the AC voltages and also a determining device for providing a control signal for the control device in order to match the influences of electrical fields from the measurement electrode and the reference electrode respectively on the reception electrode to one another. In this case, the output signal is provided on the basis of the control signal. 
     By combining the benefits described above with the advantages of a push-pull measurement bridge, a locating appliance for which the type of construction or the measuring principle already takes account of many of the usual disturbing influences can be provided. As a result, a simple, robust and accurate locating appliance can be provided. 
     In one embodiment, the measurement electrode, the reference electrode and the reception electrode are situated in one plane and a shielding electrode that at least partially covers the electrodes mentioned and is connected to a constant potential is arranged on a side that is averted from the object. This allows an influence of an object that is averted from the side of the medium, for example a user holding the locating appliance, to be reduced. 
     The locating appliance may comprise a housing, wherein the spacer is integrated in the housing. In particular, the electrodes may be situated within the housing at a predetermined distance from the housing. This allows reliable avoidance of an operating error being caused by too small a distance between the electrodes and the medium. 
     In a particularly preferred embodiment, the minimum distance between the electrodes and the medium that is provided by the spacer is at least 5 mm. Tests have shown that, with an arrangement of electrodes of the usual size and relative positioning, the minimum distance mentioned can to a great extent lead to the aforementioned advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is now described more precisely with reference to the appended figures, in which: 
         FIG. 1 a    shows a locating appliance with a first evaluation circuit; 
         FIG. 1 b    shows a locating appliance with a second evaluation circuit; 
         FIG. 2  shows an arrangement of electrodes for the locating appliances in  FIGS. 1 and 2 ; 
         FIG. 3  shows a characteristic curve of an output signal from one of the evaluation circuits in  FIGS. 1 a  and 1 b   ; and 
         FIG. 4  shows a locating appliance according to  FIG. 1 a    or  1   b  with different spacers. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  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. 
       FIG. 1B  shows a locating appliance  100  as shown in  FIG. 1A , but where the push-pull measurement bridge  115  has been replaced by a bridge measurement circuit  178  with a feedback amplifier. 
     The measurement electrode  140  is supplied with an AC voltage from a first AC voltage source  180  and the reference electrode  145  is supplied with a second AC voltage from a second AC voltage source  185 . The voltages provided by the AC voltage sources  180  and  185  are in antiphase with respect to one another and have the same amplitudes. 
     The AC voltages from the AC voltage sources  180  and  185  each have an output signal from an amplifier  195  mixed with them by means of a mixer  190 , the inverting input of said amplifier being connected to the ground-free reception electrode  150 . The output signal from the amplifier  195  and the AC voltage from the first AC voltage source  180  are both mixed together with positive arithmetic signs and forwarded to the measurement electrode  140 . For the reference electrode  145 , the lower mixer  190  likewise mixes the output signal from the amplifier  195  positively, but mixes the AC voltage from the second AC voltage source  185  negatively, and forwards them to the reference electrode  145 . 
     As a result, the measurement electrode  140  and the reference electrode  145  have AC voltages in antiphase applied to them, the amplitudes of which, in a similar manner to at the push-pull measurement bridge  115  shown in  FIG. 1 , are controlled such that the influences of electrical fields from the electrodes  140  and  145  on the object  110  correspond to one another. The interface  170  is provided with an AC voltage that indicates the object  110  when it exceeds a predetermined value. In this case, the signal applied to the interface  170  is proportional to a quotient of the difference and the sum of the capacitances C 1  and C 2 . The advantage of the circuit shown is that in the stabilized case the reception electrode  150  is at ground in terms of AC voltage and therefore no alternating currents flow between the reception electrode  150  and ground planes. 
       FIG. 2  shows 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. 2 , 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. 
     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 . 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  150  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 φ 1  of the guard electrodes is constant over time and is not tracked to another potential. The guard electrodes  230 ,  232 ,  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 first reception electrode  225  and between the second reference electrode  220  and the first 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 a graph  300  of an output signal of one of the evaluation circuits  115  and  178  of  FIGS. 1 a  and 1 b    with an arrangement  120  of electrodes such as that described above with reference to  FIG. 2 . The graph  300  applies generally to capacitive sensors with groundless electrodes. In a horizontal direction, a distance between the medium  105  and the arrangement  120  of electrodes is shown and in a vertical direction an output signal S provided at the interface  170  is shown. 
     A characteristic curve  305  qualitatively represents the relationship between the output signal S and the distance d independently of whether an object  110  is present in the region of the arrangement  120  and what influence the object  110  has on the output signal S as a result of its position, size and dielectric properties. If the distance d is zero, the output signal S is great. 
     With increasing distance d, the output signal S falls, initially steeply and later flatter, until at a distance d 1  it reaches a minimum. With further increasing distance d beyond the distance d 1 , the output signal S increases and in the further progression closely follows a predetermined value of the output signal S. 
     The characteristic curve  305  comes about by two effects that are dependent on the distance d acting oppositely on the output signal S. With reference to the evaluation circuits  115  and  178  of  FIGS. 1 a  and 1 b   , it is assumed that the following applies: 
     
       
         
           
             
               
                 
                   S 
                   ≈ 
                   
                     
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                       - 
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                       
                     
                     
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                       + 
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     Expressed in words, the output signal S is proportional to a quotient of the difference and the sum of the measurement capacitance C 1  and the reference capacitance C 2 . In this case, the measurement capacitance C 1  is formed by the measurement electrode  140  and the reception electrode  150  and the reference capacitance C 2  is formed by the reference electrode  145  and the reception electrode  150 . 
     A first effect, which acts in particular in the case of relatively small distances d, brings about a decrease in the sensor signal S by a decrease in the capacitance between the measurement electrode and the reception electrode or the reference electrode and the reception electrode. As a result, a current between the electrodes  140  or  145  and the reception electrode  150  is reduced and the sensor signal becomes smaller with increasing distance d. 
     By a second effect, which acts in particular in the case of relatively great distances d, a capacitance between the electrodes  140 ,  145  and  150  and the medium  105  increases with increasing distance d. This leads to an increase in the current between the measurement electrode  140  and the reception electrode  150  or the reference electrode  145  and the reception electrode  150 , whereby the output signal S becomes greater. 
     The two effects occur concurrently, so that qualitatively the characteristic curve  305  with a minimum sensor signal S at the distance d 1  is obtained. It is therefore of advantage to use a spacer to keep the arrangement  120  of electrodes at a distance d of the second portion  320 . Particularly preferably, the spacer is set up for fixing the distance d at d 1 , which is the case for electrodes  140  to  150  of usual dimensions and arrangements in the range of about 3-10 mm, in particular at about 5 mm. 
     The minimum distance may depend on several geometrical properties of the electrodes. The greater the distance between the measurement electrode and the reception electrode, the greater the minimum distance usually is. Similarly, the minimum distance may depend on the presence of a guard electrode between the measurement electrode and the reception electrode. The presence of a guard electrode may increase the minimum distance. In tests, the following values of the minimum distance were determined as a function of the electrode distance for electrode configurations with a guard electrode: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Distance of the measurement 
                 Minimum distance of 
               
               
                   
                 electrode from the reception 
                 the electrodes from 
               
               
                   
                 electrode [mm] 
                 the object [mm] 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 2.1 
                 5.5 
               
               
                   
                 5 
                 8.1 
               
               
                   
                 9 
                 9.9 
               
               
                   
                   
               
             
          
         
       
     
       FIGS. 4 a  and 4 b    show two different embodiments of a locating appliance  100  as shown in  FIGS. 1 a  and 1 b   , in each case with an arrangement  120  of electrodes as described above with reference to  FIG. 2 . 
       FIG. 4 a    shows a first embodiment of the locating appliance  100 , comprising a housing  405 , on the underside of which the arrangement  120  of electrodes is attached. The remaining components, in particular the evaluation circuit  115  or  178 , are not shown here. On the underside of the housing  405  there is/are one or more spacers  410 , in order to keep the housing  405 , and consequently also the arrangement  120 , at a predetermined distance d from the upper surface of the medium  105 . The distance d advantageously lies in the second region  320  of  FIG. 3 , ideally at the distance d 1 . 
       FIG. 4 b    shows an alternative embodiment, which is based on the embodiment from  FIG. 4 a   . Here, however, the arrangement  120  is not arranged directly on the underside of the housing  405 , but is attached to or provided in the housing  405  in such a way that the predetermined distance d is obtained when the underside of the housing  405  is placed onto the upper side of the medium  105 . In this case, the housing may be closed on its underside and the arrangement  120  of electrodes be fastened to the housing  405 , so that the housing  405  itself serves as the spacer  410 . In another embodiment, a dedicated spacer  410  may be provided within the housing  405  for fastening the arrangement  120  with respect to the housing  405 .