Abstract:
A locator for capacitively sensing an object includes a transmission electrode to which an excitation signal can be applied, a reception electrode, a sensing region in the region of the transmission electrode and the reception electrode, and a measuring device for sensing a capacitance between the transmission electrode and the reception electrode. The locator further includes a processing device for determining the presence of the object in the sensing region if the sensed capacitance differs from a reference capacitance, and a screening electrode that is arranged in the region of the transmission electrode and the reception electrode. The screening electrode is connected to a potential so as to reduce the base capacitance between the transmission electrode and the reception electrode.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2012/061732, filed on Jun. 19, 2012, which claims the benefit of priority to Serial No. DE 10 2011 079 704.1, filed on Jul. 25, 2011 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The present disclosure relates to a locator. In particular, the present disclosure relates to a locator for a dielectric or metallic object using a capacitive coupling. 
     For locating an object concealed in a wall of a building, for example, a wooden beam, a steel reinforcement, a water or gas line, or a power cable, various so-called stud detectors are known, which detect the aforementioned objects based on their influence on an electric field. 
     DE 10 2008 054 445 A1 discloses a stud finder that includes a two-dimensional arrangement of multiple electrodes for providing various sensor signals. 
     DE 10 2008 054 460 A1 discloses a similar stud finder having a matrix-like arrangement of a plurality of sensor elements. 
     DE 10 2007 058 088 A1 discloses yet another stud finder and a geometrical arrangement of a transmitting electrode system relative to a receiving electrode system. 
     Other stud finders are presented in U.S. Pat. No. 4,099,118 and DE 20 2009 017 337. 
     Known stud finders generally function using a transmitting electrode and a receiving electrode, an excitation signal acting upon the transmitting electrode, and a capacitance between the transmitting electrode and the receiving electrode then being determined. If a dielectric or metallic object approaches the arrangement of the transmitting electrode and the receiving electrode, the determined capacitance changes, so that the object may be detected. 
     The object of the present disclosure is to specify a locator having an arrangement of electrodes that enables a more sensitive and more selective determination of the object. 
     SUMMARY 
     The present disclosure achieves this object using a locator having the features of the disclosure. Subclaims disclose preferred specific embodiments. 
     A locator according to the present disclosure for the capacitive detection of an object comprises a transmitting electrode for being acted upon by an excitation signal, a receiving electrode, a detection area in the area of the transmitting electrode and the receiving electrode, a measuring device for detecting a capacitance between the transmitting electrode and the receiving electrode, a processing device for determining the presence of the object in the detection area if the detected capacitance differs from a reference capacitance, and a shielding electrode that is arranged in the area of the transmitting electrode and the receiving electrode and is connected to a potential, in order to reduce the basic capacitance between the transmitting electrode and the receiving electrode. 
     In contrast to the related art, an additional shielding electrode is used in the locator according to the present disclosure. This shielding electrode reduces the intended capacitive coupling between the transmitting electrode and the receiving electrode in an area close to the sensor without essentially influencing the capacitive coupling in the detection area of the sensor. 
     The shielding electrode thus differs from both a guard electrode, which shields the non-homogeneous edge region of the electric field of a capacitive sensor from the measuring electrode, thus ensuring a homogeneous field up to the edge of the sensor surface, and from a shield electrode, by which parasitic capacitive couplings are eliminated. 
     The transmitting electrode, the receiving electrode, and the shielding electrode are preferably arranged in a plane. The shielding electrode is preferably is provided between the other two electrodes. Generally, the shielding electrode is preferably to be placed in such a way relative to the transmitting and receiving electrodes that the portion of the electric field of the transmitting electrode that does not run through a detection area of the locator is established to the greatest possible extent between the transmitting electrode and the shielding electrode, instead of between the transmitting electrode and the receiving electrode. This is what is meant when it is subsequently stated in particular that the shielding electrode is situated in the area of the transmitting electrode and the receiving electrode. 
     The dynamics of a capacitive measuring system are in principle limited by a ratio of the change in capacitance under the influence or without the influence of the object, to a basic capacitance between the transmitting electrode and the receiving electrode without the influence of objects. This ratio directly influences the signal-to-noise ratio or SNR. By reducing the basic capacitance according to the present disclosure, the specified ratio is increased, thereby making possible a direct improvement of the signal-to-noise ratio. As a result, a sensitivity, a detection distance, and/or a selectivity may be improved when detecting the object. 
     In one preferred specific embodiment, the shielding electrode is arranged and connected to the potential in such a way that a difference in the capacitances between the transmitting electrode and the receiving electrode caused by the object is minimized, ideally being unchanged. It is thus possible to reduce parasitic capacitances without reducing the desired difference in capacitance that is used for identifying the object. 
     The detection area also advantageously lies outside an area of influence of the object on the capacitance between the transmitting electrode and the shielding electrode. It is thus possible to achieve the goal of reducing only the basic capacitance while the difference in the capacitances caused by the object simultaneously remains the same. The exact arrangement of the shielding electrode relative to the transmitting electrode or receiving electrode may thus be a function of the position of the detection area. As a result, it may be possible to choose the shape and size of the shielding electrode as a function of a predetermined detection area such that the described improvement of the ratio of the change in capacitance and the basic capacitance is optimized. 
     In one preferred specific embodiment, the transmitting electrode, the receiving electrode, and the shielding electrode are arranged in a plane. This arrangement may be advantageous in particular for detecting an object that is concealed in a building wall. A sensitivity of such a planar sensor arrangement with respect to tilting relative to the wall may be minimized. The transmitting electrode, the shielding electrode, and the receiving electrode are also preferably arranged radially symmetrically. As a result, independence of a determination result from a position of the object relative to the electrode arrangement may be achievable. 
     In another preferred specific embodiment, a second shielding electrode may be provided, which surrounds the other electrodes or is situated radially outside the other electrodes. 
     As a result, a sensitivity of the sensor arrangement toward its edge may be reduced, so that it is possible to limit the detection area more sharply. The second shielding electrode may be connected to the same potential as the first shielding electrode, or to a different potential. 
     In order to limit the detection area, a shielding electrode may attached to a side of the transmitting electrode, the receiving electrode, and the shielding electrode facing away from the detection area. 
     In a particularly preferred specific embodiment, the measuring device comprises a push-pull measurement bridge. The push-pull measurement bridge may include two transmitting electrodes and one receiving electrodes, with each of the transmitting electrodes, including the receiving electrode, forming one of the arrangements described above. In one alternative specific embodiment, two receiving electrodes may also be provided, in which each of the transmitting electrodes, each including one of the receiving electrodes, forms one of the arrangements described above. In one preferred specific embodiment, the two receiving electrodes may be interconnected with low impedance. In another specific embodiment, the two receiving electrodes may also be connected with high impedance, for example, separated by impedance transformers. In yet another alternative specific embodiment, only one transmitting electrode may also be provided, and the capacitance between the transmitting electrode and the receiving electrode in comparison to a reference capacitance may be determined, which, for example, may be formed via a compensation network at the push-pull measurement bridge. 
     In the case of the push-pull measurement bridge, the shielding electrode is preferably connected to a potential that essentially corresponds to that of the receiving electrode. In an alternative specific embodiment, a low-impedance amplifier may also be used instead of the push-pull measurement bridge. 
     In yet another alternative specific embodiment, a high-impedance amplifier may also be used instead of the push-pull measurement bridge, the shielding electrode preferably being connected to a potential that essentially corresponds to that of the transmitting electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The present disclosure will now be described in detail with reference to the accompanying figures. 
         FIG. 1  shows an arrangement of a transmitting electrode and a receiving electrode including a shielding electrode; 
         FIG. 2  shows a circuit diagram of a locator; and  FIGS. 3 to 6  show additional arrangements of electrodes. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a longitudinal sectional view through an electrode arrangement  100  at an object  105 . The electrode arrangement  100  includes a transmitting electrode  110 , a receiving electrode  115 , and a shielding electrode  120 . In the non-visible dimension, the electrodes  110  to  120  are designed, for example, rectangularly having a constant thickness, other configurations also being conceivable. The transmitting electrode  110  is connected to a potential Φ 1 , the receiving electrode  15  is connected to a potential Φ 2 , and the shielding electrode  120  is connected to a potential Φ 3 . In other specific embodiments, the electrodes  110  to  115  do not have to be arranged in a plane. For example, the shielding electrode may extend vertically further upwards in the direction of the detection area  130 . 
     Field lines  125  are shown between the electrodes  110  to  120 , which arise from an electric field that exists between the ends of the field lines  125 . The indicated direction of the field lines  125  is purely exemplary. The field lines  125  run such that, at each point of a field line  125 , the direction of the electrostatic force is specified by a tangent to the field line into the point. The electric potential at this point is determined by the difference in potential between the start point and the end point of the field line  125  and the ratio of the lengths of the field line  125  to the start point and the end point. 
     The object  105  is located in a detection area  130 . The detection area is only depicted above the electrodes  110  to  120 . A detection area located symmetrically beneath the electrodes  110  to  120  is not discussed further here. In addition, an area  132  close to the sensor is defined between the electrodes  110  to  120  and the detection area  130 . The shielding electrode  120  lies inside the area  132  close to the sensor, preferably horizontally between the transmitting electrode  110  and the receiving electrode  115 . 
     Field lines  125 , which run on the upper side of the electrode arrangement  100  from the transmitting electrode  110  to the receiving electrode  115 , pass through the detection area  130 . On the other hand, field lines  125  that start at the transmitting electrode  110  and remain within the area  132  close to the sensor, i.e., do not pass through the detection area  130 , end at the shielding electrode  120 . Thus, the portion of the electric field starting at the transmitting electrode  110  that cannot be influenced by the object  105  is guided to the shielding electrode  120 , so that it does not contribute to a basic capacitance that exists between the transmitting electrode  110  and the receiving electrode  115 . 
     The basic capacitance between the transmitting electrode  110  and the receiving electrode  115  is the capacitance that arises if the object  105  is not present in the detection area  130 . However, if the object  105  is situated in the detection area  130 , it is detected by a field line  125 . Depending on whether or not the object  105  as shown in  FIG. 1  is connected to reference potential or ground via a resistor  135 , and depending on whether the object  105  has metallic or dielectric properties, the capacitance between the transmitting electrode  110  and the receiving electrode  115  may be decreased or increased relative to the basic capacitance by the presence of the object  105 . The field line  125  that contacts the object  105  may end at the object  105  or continue to the receiving electrode  115 . 
     In order to shape the field lines  125  in the described manner, the shielding electrode  120  must not only assume a suitable position relative to the transmitting electrode  110  and the receiving electrode  115 , but must also be connected to a corresponding potential. This potential is among other things a function of the design of a measuring circuit by which the capacitance between the electrodes  110  and  115  is determined. In the illustrated case, the potential Φ 3  of the shielding electrode  120  essentially corresponds to the potential Φ 2  of the receiving electrode  115 . This arrangement may advantageously be used in determining the capacitance via a low-impedance amplifier or a push-pull measurement bridge. If a high-impedance amplifier is used, the potential Φ 3  may essentially correspond to the potential Φ 2  of the receiving electrode  115 . However, in this case, the potential Φ 3  must be actively tracked to the potential Φ 2 . For this purpose, for example, the potential Φ 2  at the receiving electrode  115  is detected with high impedance using an impedance transformer and applied to the shielding electrode  120  with low impedance. 
       FIG. 2  shows a block diagram of a push-pull measurement bridge  200 . The push-pull measurement bridge  200  is part of a locator  205  for detecting the object  105  in  FIG. 1 . The object  105  may in particular comprise a dielectric object  105 , a grounded metallic object  105 , or an ungrounded metallic object  105 . In other specific embodiments of the locator  205 , an electrode arrangement like that in  FIG. 1  may also be connected to another measuring device, for example, an oscillating circuit or a sigma-delta modulator. 
     A clock generator  210  of the push-pull measurement bridge  200  has two outputs, at which it provides phase-shifted periodic alternating signals, preferably phase-shifted by 180°. The alternating signals may in particular include rectangular, triangular, or sinusoidal signals. The outputs of the clock generator are connected to a first controllable amplifier  215  or a second controllable amplifier  220 . Each of the controllable amplifiers  215 ,  220  has a control input via which it accepts a signal that controls an amplification factor of the respective controllable amplifier  215 ,  220 . An output of the first controllable amplifier  215  is connected to a first transmitting electrode  225 , and an output of the second controllable amplifier  220  is connected to a second transmitting electrode  230 . A receiving electrode  235  serves as a potential probe. The transmitting electrodes  225  and  230  correspond to the transmitting electrode  110  in  FIG. 1 , and the receiving electrode  235  corresponds to the receiving electrode  115  in  FIG. 1 . 
     The receiving electrode  235  is connected to an input amplifier  240 . A compensation network  265  depicted in the region of the first receiving electrode  225  will not be examined at this stage. The input amplifier  240  is represented having a constant amplification factor; however, in other specific embodiments, an amplification factor of the input amplifier  240  may also be controllable. As a result, for example, a spatial resolution and/or sensitivity of the push-pull measurement bridge  200  may be influenceable and may, for example, be controllable as a function of a measurement signal at the input amplifier  240 . 
     The output of the input amplifier  240  is connected to a synchronous demodulator  245 . The synchronous demodulator  245  is furthermore connected to the clock generator  210  and receives a clock signal from it that references the phase position of the signals provided at the outputs of the clock generator  210 . In a simple specific embodiment in which the signals provided by the clock generator  210  are symmetrical rectangular signals, one of the output signals fed to the controllable amplifiers  215 ,  220  may be used as a clock signal. The synchronous demodulator  245  connects the measurement signal received by the input amplifier  240  to its upper or lower output in an alternating manner essentially based on the clock signal provided by the clock generator  210 . 
     Both outputs of the synchronous demodulator  245  are connected to an integrator (integrating comparator)  250 , which is represented here as an operational amplifier connected to two resistors and two capacitors. Other specific embodiments are also possible, for example, in the form of an active low-pass filter. A digital design in connection with the synchronous demodulator  245  is also conceivable, in which the signal at the outputs of the synchronous demodulator  245  is converted from analog to digital at one or multiple points in time within one half-cycle and then compared to the corresponding value from a following or previous half-cycle. The difference is integrated and, for example, converted again to an analog signal and used for controlling the amplifiers  215 ,  220 . While the synchronous demodulator  245  provides the measurement signal received from the input amplifier  240  at its lower output, the integrator  250  integrates this signal over time and provides the result at its output. While the synchronous demodulator  245  provides the measurement signal received from the input amplifier  240  at its upper output, this signal is inverted by the integrator  250  over time, and the result is provided at the output of the integrator  250 . The voltage at the output of the integrator  250  is the integral of the difference of the low-pass-filtered outputs of the synchronous demodulator  245 . 
     If the capacitance of the receiving electrode  235  relative to the first transmitting electrode  225  is exactly equal to the capacitance of the receiving electrode  235  relative to the second transmitting electrode  230 , then the signals provided at the outputs of the synchronous demodulator  245  are equal on average over time, and a signal is provided at the output of the integrator  250  that approaches zero (reference potential). However, if the specified capacitances are not equal, for example, because the object  105  influences the capacitance between the receiving electrode  235  and the first transmitting electrode  225  differently than the capacitance between the receiving electrode  235  and the second transmitting electrode  230 , then the signals provided at the outputs of the synchronous demodulator  245  are no longer equal on average over time, and a positive or negative signal is provided at the output of the integrator  250 . The sign and magnitude of the signal indicate the ratio of the specified capacitances, a signal of zero corresponding to a ratio of the capacitances of one. 
     The signal provided by the integrator  250  is provided via a connection  255  to an evaluation and output device of the locator  205 , which is not shown. The evaluation device may, for example, carry out a comparison with a threshold value, so that a user of the locator  205  receives a visual, audible, and/or haptic output if the signal provided by the integrator  250  exceeds a predetermined threshold. The entire signal or a magnitude of the signal may be compared to the threshold value. 
     The signal provided by the integrator  250  is also used for controlling the gains of the controllable amplifiers  215  and  220 , the second controllable amplifier  220  being directly connected to the output of the integrator  250 , and the first controllable amplifier  215  being connected to the output of the integrator  250  via an inverter  260 . The inverter  260  causes an inversion of the signal provided to it in such a way that the gain of the first controllable amplifier  215  increases as a function of the output signal of the integrator  250  to the extent that the gain of the second controllable amplifier  220  decreases, or vice versa. It is also conceivable that only the gain of one of the controllable amplifiers  215 ,  220  is controlled, while the gain of the second controllable amplifier  220 ,  215  is held to a fixed value. 
     The gains of the controllable amplifiers  215  and  220  increase or decrease until an AC component, which is synchronous with the alternating voltage present at the transmitting electrodes  225  and  230  and is present at the receiving electrode, is minimized in magnitude. 
     The push-pull measurement bridge  200  is a control loop that is equipped to sustain a predetermined ratio of differences in potential at each of the transmitting electrodes  225  and  230  to the receiving electrode  235  by applying signals to the transmitting electrodes  223  and  230  at a suitable ratio. 
     The signal provided by the integrator  250  is a control signal for compensating for an asymmetrical influence of the variable capacitances on the differences in potential caused, for example, by the dielectric object. In other specific embodiments, the variable ratio at the electrodes is determined based on currents or voltages at the electrodes. 
     In another specific embodiment, the first transmitting electrode  225  is replaced by the compensation network  265 . The compensation network  265  is made up of a voltage divider made of two impedances, whose output is coupled to the input of the input amplifier  240  via a third impedance. Thus, the alternating voltages of the controllable amplifiers  215 ,  220  are balanced out between the capacitance that exists between the at the second (and only) transmitting electrode  230  and the receiving electrode  235 , and a reference capacitance formed by the compensation network  265 . The reference capacitance is invariant relative to the dielectric object  105 . Only the first transmitting electrode  225  and the receiving electrode  235  are required for the measurement. 
     Depending on the measuring method, the reference capacitance does not have to be implemented as a physical capacitance. It may also be a “virtual” reference capacitance. In one specific embodiment, for example, a time constant of a charging curve of a capacitor may be determined and compared to a reference time. This reference time is then a measure of the reference capacitance, without the reference capacitance being physically present in the push-pull measurement bridge  200 . 
     The arrangement of each of the transmitting electrodes  225 ,  230  relative to the receiving electrode is preferably provided with a shielding electrode  120 , as described above in detail with reference to  FIG. 1 .  FIG. 3  shows an arrangement of electrodes that is suitable for use with the push-pull measurement bridge  200  in  FIG. 2 .  FIG. 3A  shows a longitudinal sectional view, and  FIG. 3B  shows a top view. 
     The transmitting electrodes  225  and  230 , the receiving electrode  235 , and two shielding electrodes  120  are arranged in a plane. The shielding electrodes  120  are electrically interconnected and set to a potential Φ 3 , as described above in detail with reference to  FIG. 1 . The width and position of the shielding electrodes  120  in the intermediate spaces between the receiving electrode  235  and the transmitting electrodes  225  and  230  may be used to influence the amount by which the basic capacitance between the transmitting electrodes  225 ,  230  and the receiving electrode  235  is decreased. 
     As a result, the position and size of the detection area  130  (not illustrated) may simultaneously be influenced, as described above with reference to  FIG. 1 . 
     In one specific embodiment, a shield electrode is used, which extends parallel to the plane in which the electrodes  225  to  230  and  120  lie. The shield electrode is connected to a suitable potential, for example, reference potential or ground, and limits the detection area  130  to one side relative to the plane of the electrodes  225  to  235  and  120 . 
     Based on the specific embodiments shown in  FIG. 3 , a number of variants is possible. For example, instead of being rectangular, the electrodes  225  to  235  and  120  may also be designed in any other shape, for example, rhomboid, triangular, elliptical, circular, or polygonal. It is also possible to break down the transmitting electrodes  225  and  230  and/or the receiving electrode  235  into multiple topologically disconnected areas that are, however, electrically interconnected. It is also possible to combine multiple transmitting electrodes  225 ,  230  with multiple receiving electrodes  235 . The electrodes  225  to  235  and  120  do not necessarily have to be arranged in a plane, but may assume different distances from a plane or generally form any three-dimensional shape. 
       FIG. 4  shows another arrangement of electrodes, which is suitable for use at the push-pull measurement bridge  200  in  FIG. 2 . The views in  FIGS. 4A and 4B  correspond to those in  FIGS. 3A and 3B . 
     Unlike the specific embodiment shown in  FIG. 3 , the shielding electrodes  120  surround the transmitting electrodes  225  and  230  in the plane of the transmitting electrodes  225 ,  230 . All electrodes  225  to  235  and  120  are again situated in a plane. Two shield electrodes  405  are shown in a parallel plane, each having a surface that covers one of the transmitting electrodes  225 ,  230  and the associated shielding electrode  120 . In the depicted specific embodiment, no shield electrode  405  exists in the area of the receiving electrode  235 . However, in other specific embodiments, a shield electrode  405  may also be provided here. Only one shield electrode  405  may also be provided, which covers the surface of all electrodes  225  to  235  and  120 . 
     The specific embodiment shown in  FIG. 4  may be varied, as described above with reference to  FIG. 3  with respect to the shape and arrangement of the electrodes  225  to  235  and  120 . 
       FIG. 5  shows an arrangement of electrodes that is rotationally symmetrical relative to an angle of rotation of 90°. Four transmitting electrodes  505 ,  510 ,  515 , and  520  are arranged around a receiving electrode  525 , each offset at equal distances by 90°. The shielding electrode  110  surrounds the receiving electrode  525  and also extends outwardly between adjacent transmitting electrodes  505  to  520 . When using the push-pull measurement bridge  200  in  FIG. 2 , the transmitting electrodes  505  to  520  may be associated with the transmitting electrodes  225  to  230  in any manner. The receiving electrode  525  corresponds to the receiving electrode  235 . A more complex measuring circuit may also be used, in which the four transmitting electrodes  505  to  520  are activated in four different phases. 
     The size, shape, and position of the electrodes  505  to  525  may be varied, as described above with reference to  FIGS. 3 and 4 . In addition, one or multiple shielding electrodes may be arranged on one side at a predetermined distance from the electrodes  505  to  525 . 
       FIG. 6  shows another arrangement of electrodes, which is also suitable for use at the push-pull measurement bridge  200 . 
     Four circular electrodes  605 ,  610 ,  615 , and  620  are arranged concentrically in a plane. In one specific embodiment, the innermost electrode  605  corresponds to the receiving electrode  235 , the next outer electrode  610  corresponds to the shielding electrode  620 , and the in turn next outer electrode  615  corresponds to the transmitting electrode  230 . In another specific embodiment, the arrangements of the electrodes  605  and  615  are exchanged. In both specific embodiments, the electrode  620  situated farther outside may optionally be provided as a second shielding electrode  120 , which is connected to the same potential as, or to a potential other than that of, the shielding electrode  120  situated farther inside. 
     The electrodes  605  to  620  do not have to be situated in a plane and may also form any other closed curves other than circles. As already described above with reference to  FIGS. 3 to 5 , one or multiple shielding electrodes may also be used here on one side of the electrodes  605  to  620 .