Abstract:
A measuring apparatus for detecting a dielectric object comprises a potential probe for determining an electric potential in an electric field, a first capacitance device, a second capacitance device, and a control device configured to supply alternating voltages to the first and the second capacitance devices. The control device is configured to amplify the alternating voltages in opposite directions to one another in order to minimize the magnitude of an AC voltage component, which is clock-synchronous with the alternating voltages, of a voltage which is recorded by means of the potential probe. The dielectric object is detected when a ratio of the alternating voltages does not correspond to a ratio of (i) a first distance of the potential probe from the first capacitance device to (ii) a second distance of the potential probe from the second capacitance device.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2011/053617, filed on Mar. 10, 2011, which claims the benefit of priority to Serial No. DE 10 2010 028 718.0, filed on May 7, 2010 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The disclosure relates to a detector for a dielectric object. In particular, the disclosure relates to a measuring device for detecting a dielectric object. 
     BACKGROUND 
     Capacitive detectors for detecting an object concealed in a wall, such as a beam concealed in a wall, have been disclosed in the prior art. These frequently use an electrode, the charging behavior of which is observed in order to home in on the dielectric object. Detectors with a plurality of electrodes, wherein a change in the capacitance of electrode pairs is determined, have also been disclosed. It is usually necessary to calibrate these detectors manually on the wall, as the units themselves are not able to detect contact with the wall, and the capacitance depends on ambient conditions such as a dampness of the wall, a humidity or an ambient temperature. It is therefore not possible to calibrate detectors of this kind once during manufacture; instead the calibration must be carried out by a user relatively shortly before a measurement. In doing so, the result of a measurement depends on the quality of the calibration. If the detector is calibrated with wall contact at a point where a beam is located, a later measurement result can be ambiguous, which can confuse the user. 
     Furthermore, the detectors described are prone to tilting on the wall. As a result of tilting, the distances of a plurality of electrodes from the wall are unequal, so that measured capacitances are subject to error. In addition, the capacitance measured at the electrodes when the detectors described approach the wall does not increase steadily, so that the distance from the wall and an approach or distancing of the detector from the wall can be difficult to reproduce. 
     DE 10 2008 005 783 A1 describes a moisture-independent capacitive device for protecting against trapping. The trapping sensor comprises two electrodes with variable spacing. The trapping sensor and a circuit with constant capacitance are supplied with alternating voltages, wherein the voltages are amplified or attenuated so that a voltage at one of the electrodes of the trapping sensor is minimized. If the relative spacing of the electrodes is changed, then trapping is detected based on an increased voltage at the electrode. 
     The disclosure is based on the object of providing a simple and accurate detector for a dielectric object, e.g. a wooden beam in a wall. 
     SUMMARY 
     The disclosure achieves this object by means of a measuring device and by a method as described herein. 
     According to the disclosure, a measuring device for detecting a dielectric object comprises a potential probe for determining an electrical potential in an electric field, a first and a second capacitance device and a control device for supplying the first and second capacitance device with alternating voltages. The control device is designed to amplify the alternating voltages in opposite directions to one another in order to minimize the magnitude of an alternating voltage component, which is clock-synchronous with the alternating voltages, of a voltage which is recorded by means of the potential probe. The dielectric object is detected when the alternating voltages are of unequal magnitude. 
     The measuring device according to the disclosure determines a difference between two capacitances, so that interference effects which affect both capacitances do not affect the result of the measurement. Here, each of the capacitance devices can comprise an electrode which forms a capacitance together with an electrode of the potential probe. Calibration of the measuring device by a user can be dispensed with, and the calibration can be carried out once, for example during the manufacture of the measuring device. As calibration on the wall is unnecessary, an object can also be directly and reliably detected when the device is placed on the wall in the vicinity of the object. 
     Preferably, the electrodes of the first and second capacitance device are arranged substantially in one plane and the electrode of the potential probe is arranged outside this plane. The relative change in distance of the electrodes during tilting is small compared with the overall distance of the electrodes from the wall compared with an arrangement with which the transmitting electrodes rest directly on the wall. The relative change in capacitance of the capacitance devices when the measuring device is tilted with respect to the wall can be minimized, thus minimizing a measuring error. 
     The potential probe can comprise two receiving electrodes connected to a differential amplifier, wherein the receiving electrodes are arranged on different sides of the plane with distances, preferably equal distances, from the first and second capacitance device. The differential amplifier can additionally include a high-pass filter. As a result of the differential measurement, the signal provided by the differential amplifier is insensitive to outside influences, such as a wall connected to a certain potential or a conducting wall. In addition, the field-compensated measurement carried out by this means is very accurate. The sign of the signal indicates the relative position of the dielectric object, enabling the dielectric object to be located more easily. A change in sign (zero transition) indicates the center of the beam. The effect of tilting can be minimized as in the design described above. 
     A screening electrode, which is connected to a potential which lies centrally between the alternating voltages, can be arranged on a side of at least one of the capacitance devices which faces away from the dielectric object. This screening electrode can also be a further board or display, for example, and therefore may not be immediately recognizable as a screening electrode. In general, this potential will be ground. A user of the measuring device is therefore shielded from the measuring device, thus enabling interference with the measurement to be minimized. 
     One of the capacitance devices can be designed in such a way that the capacitance which exists between this capacitance device and the potential probe is independent of the dielectric object. By this means, the measurement can be carried out on the basis of just one capacitance which is dependent on the dielectric object, as a result of which the design of the measuring device can be simplified. In particular, such a measuring device can be insensitive to tilting, as the potential probe can assume the same potential as the electrode, so that the resulting electric field does not change due to the electric field of the potential probe. 
     A compensation network for changing at least one of the capacitances provided by the capacitance devices by an amount which is independent of the dielectric object can be provided. As a result, a sensitivity of the measuring device can be adjustable, and capacitances of the capacitance devices can be matched to geometrical circumstances for example. 
     Further, a multiplicity of electrode pairs of the capacitance devices, which for example are located in pairs on different sides of the potential probe, can be provided, wherein the control device is designed to determine at least one of a magnitude, an extension, a distance and an orientation of the dielectric object based on differences of the alternating voltages when different electrode pairs are supplied with the alternating voltages. A spatial resolution of the measuring device can be increased by miniaturizing the electrodes, in one embodiment up to an imaging range. Preferably, the electrodes of the electrode pairs are arranged in the form of a matrix in one plane around the potential probe. 
     A method for detecting a dielectric object comprises steps which include the determination of an electrical potential in an electric field by means of a potential probe, the supply of two capacitance devices with alternating voltages which are amplified in opposite directions in such a way that the magnitude of an alternating voltage component, which is clock-synchronous with the alternating voltages, of the determined electrical potential is minimized, and the detection of the dielectric object when the alternating voltages are of unequal magnitude. 
     The method can run on a program-controlled processing device, for example a microcomputer or microcontroller, or be stored on a computer-readable storage medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is described in more detail below with reference to the attached figures, in which: 
         FIG. 1  shows a block circuit diagram of a measuring device; 
         FIGS. 2 a -2 c    show different arrangements of electrodes for the measuring device from  FIG. 1 ; 
         FIGS. 3 a , 3 b    show a potential probe for the measuring device from  FIG. 1 ; 
         FIG. 4  shows a further arrangement of electrodes for the measuring device from  FIG. 1 ; and 
         FIG. 5  shows a flow diagram of a method for detecting a dielectric object. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a block circuit diagram of a measuring device  100 . The measuring device  100  is part of a beam locator  105  for detecting dielectric objects, for example made of wood. 
     A clock generator  110  has two outputs, at which it provides phase-shifted, preferably phase-shifted by 180°, periodic alternating signals. In particular, the alternating signals can be rectangular, triangular or sinusoidal signals. The outputs of the clock generator are connected to a first controllable amplifier  115  and a second controllable amplifier  120  respectively. Each of the controllable amplifiers  115 ,  120  has a control input, by means of which it receives a signal which controls a gain factor of the controllable amplifier  115 ,  120 . An output of the first controllable amplifier  115  is connected to a first transmitting electrode  125 , and an output of the second controllable amplifier  120  is connected to a second transmitting electrode  130 . 
     A receiving electrode  135  serves as a potential probe and is connected to an input amplifier  140 ; a compensation network  165  shown in the region of the electrodes  125 - 135  is not considered at this stage and an impedance  170  is omitted. The input amplifier  140  is shown with a constant gain factor; in other embodiments, however, a gain factor of the input amplifier  140  can also be controllable. As a result, a spatial resolution and/or sensitivity of the measuring device  100 , for example, can be influenced and controlled as a function of a measured variable for example. 
     The output of the input amplifier  140  is connected to a synchronous demodulator  145 . Furthermore, the synchronous demodulator  145  is connected to the clock generator  110 , from which it receives a clock signal which indicates the phase relationship of the signals provided at the outputs of the clock generator  110 . In a simple embodiment, in which the signals provided by the clock generator  110  are symmetrical rectangular signals, one of the output signals can be used as a clock signal. The synchronous demodulator  145  essentially switches the measured signal received from the input amplifier  140  alternately to its upper or lower output based on the clock signal provided by the clock generator  110 . 
     The two outputs of the synchronous demodulator  145  are connected to an integrator (integrating comparator)  150  which is shown here by an operational amplifier wired with two resistors and two capacitors. Other embodiments are also possible, for example in the form of active low-pass filters. A digital configuration following the synchronous demodulator  145 , in which the signal at the outputs of the synchronous demodulator  145  is converted from analog to digital at one or more points in time within a half wave and then compared with the corresponding value from the next half wave, is also conceivable. The difference is integrated and, for example, reconverted to an analog signal and used to control the amplifier. While the synchronous demodulator  145  provides the measured signal received from the input amplifier  140  at the lower of its outputs, the integrator  150  integrates this signal with respect to time and makes the result available at its output. While the synchronous demodulator  145  provides the measured signal received from the input amplifier  140  at its upper output, this is inverted and integrated by the integrator  150  with respect to time and the result made available at the output of the integrator  150 . The voltage at the output of the integrator is the integral of the difference of the low-pass-filtered outputs of the synchronous demodulator. 
     If the capacitance of the first transmitting electrode  125  is exactly the same as the capacitance of the second transmitting electrode  130 , then the mean value with respect to time of the signals provided at the outputs of the synchronous demodulator  145  is the same and a signal which tends to zero (ground) is provided at the output of the integrator  150 . If, however, the capacitances are unequal, possibly because a dielectric object is arranged in the region of one of the transmitting electrodes  125 ,  130 , then the mean values of the signals provided at the outputs of the synchronous demodulator  145  are no longer equal and a positive or negative signal is provided at the output of the integrator  150 . 
     The signal provided by the integrator  150  is made available for further processing via a terminal  155 . In addition, a microcomputer  165  is connected to the control inputs of the controllable amplifiers  115 ,  120 . The microcomputer  165  carries out a comparison of the provided signal with a threshold value and outputs a signal at an output  170  which indicates the dielectric object. The signal can be presented to a user of the beam detector  105  in optical and/or acoustic form. 
     In addition, the microcomputer  165  can carry out a further processing of the signals taken from the control inputs of the controllable amplifiers  115 ,  120 , and can control parameters of the measuring device  100  as a function thereof. For example, a frequency or signal form of the alternating voltages at the outputs of the clock generator  110  can be varied, or a sensitivity of the receiving amplifier  140  can be changed. In a further embodiment, further elements of the measuring device  100  shown are implemented by the microcomputer  165 , for example the clock generator  110 , the synchronous demodulator  145  or the integrator  150 . 
     The same signal of the integrator  150  is also used for controlling the gain factors of the controllable amplifiers  115  and  120 , wherein the second controllable amplifier  120  is connected directly to the output of the integrator  150 , and the first controllable amplifier  115  is connected to the output of the integrator  150  by means of an inverter  160 . The inverter  160  effects a reversal of the signal provided thereto in such a way that, depending on the output signal of the integrator  150 , the gain factor of the first controllable amplifier  115  increases to the same extent as the gain factor of the second controllable amplifier  120  decreases and vice versa. It is also conceivable that only the gain factor of one of the controllable amplifiers is controlled, while the gain factor of the second controllable amplifier is kept at a fixed value. 
     The compensation network  165  comprises a voltage divider consisting of two impedances at each of the transmitting electrodes  125 ,  130 . The divided voltages are fed to the input amplifier  140  by means of a further impedance in each case. The receiving electrode  135  is not fed directly to the input amplifier  140  but by means of the impedance  170 . The effective impedances at the outputs of the controllable amplifiers  115 ,  120  can be varied by appropriate choice of the individual impedances mentioned. As an example, this enables an asymmetrical arrangement of the electrodes  125 - 135  to be compensated for. 
     In a further embodiment, in contrast with the diagram of the compensation network  165  in  FIG. 1 , the impedances in the region of the first transmitting electrode  125  and also the second transmitting electrode  130  are omitted. The alternating voltages of the controllable amplifiers  115 ,  120  are therefore balanced out between a capacitance connected to the first (only) transmitting electrode  125  and a reference capacitance formed by the compensation network  165 . The reference capacitance is invariant compared with a dielectric object. Only the first transmitting electrode  125  and the receiving electrode  135  are required for the measurement. 
     A reverse embodiment, in which, in contrast with the diagram of the compensation network  165  in  FIG. 1 , the impedances in the region of the second transmitting electrode  130  and also the first transmitting electrode  125  are omitted, is also possible. 
     By providing switches, the measuring device  100  can be operated in accordance with the described embodiments in a three-electrode measuring mode using both transmitting electrodes  125  and  130 , a first two-electrode measuring mode using the first transmitting electrode  125  and the receiving electrode  135 , and a second two-electrode measuring mode using the second transmitting electrode  130  and the receiving electrode  135 . Switching between the different measuring modes can take place cyclically or be controlled by a user. 
     While, in the two-electrode measuring mode, a voltage applied to the terminal  155  of the measuring device  100  in  FIG. 1  is greatest when the dielectric object is closest to the receiving electrode  135 , in the three-electrode measuring mode, the magnitude of this voltage is maximum when the dielectric object is closest to one of the transmitting electrodes  125  or  130 , wherein the sign of the voltage indicates the nearest transmitting electrode in each case. If the object is moved past the electrodes, in the three-electrode measuring mode, this results in a signal with a sign change and, in the two-electrode measuring mode, a signal with a local maximum at the moment of passing. 
       FIGS. 2 a -2 c    show different arrangements  200  of the transmitting electrodes  125 ,  130  and the receiving electrode  135  from  FIG. 1 . The viewing direction is towards the floor. A plan view on a horizontal section through a wall  210  is shown in the top part of  FIGS. 2 a -2 c    in each case. Wooden beams  220  are concealed in the wall  210 . The arrangements  200  shown, in particular those in  FIGS. 2 b  and 2 c   , can be used universally for capacitive beam locators with a plurality of electrodes and are not restricted to use with the beam locator  105  from  FIG. 1 . 
     In  FIG. 2 a   , the electrodes  125 ,  130 ,  135  are arranged next to one another in one plane; the distances of the receiving electrode  135  from each of the transmitting electrodes  125 ,  130  are the same. To screen against influences which come from a different direction from the direction of the wall  210 , a screening electrode  230 , which is arranged below the electrodes  125 ,  130 ,  135 , can be provided as an option. 
     In  FIG. 2 b   , unlike the diagram in  FIG. 2 a   , the receiving electrode  135  is arranged outside the plane in which the transmitting electrodes  125 ,  130  lie. In the diagram, the receiving electrode  135  is shown above the plane; in an alternative embodiment it can also lie below the plane. If the measuring device  100  with the electrodes  125 ,  130 ,  135  and optionally the screening electrode  230  is tilted about a vertical axis parallel to the wall  210 , the relative field change at the position of the receiving electrode  135  is less than with the arrangement  200  in  FIG. 2 a   , as the field strengths of the electric fields generated by means of the transmitting electrodes  125 ,  130  become smaller as the distance increases. The reduced effect of tilting also results from the fact that the distance of the receiving electrode  135  from the wall  210  increases to a smaller extent during tilting than the distance of the transmitting electrodes  125 ,  130  as a whole from the wall  210 . 
       FIG. 2 c    shows an arrangement  200  which is suitable for the two-electrode measuring modes described above with reference to  FIG. 1 . The receiving electrode  135  is arranged above one of the transmitting electrodes  125  or  130 , wherein the transmitting electrode  125  or  130  is wider than the receiving electrode  135 . The optional screening electrode  230  is preferably even wider than the transmitting electrode  125  or  130 . The transmitting electrode can also be formed by electrically connecting two discrete transmitting electrodes  125  and  130 . By means of the described arrangement  200 , the influence of tilting in the two-electrode measuring mode can be further reduced. 
       FIGS. 3 a  and 3 b    show a potential probe  300  for the measuring device from  FIG. 1 . An arrangement  310  of electrodes of the potential probe  300  is shown in  FIG. 3 a   , and a circuit of the arrangement  310  with further elements of the potential probe  300  is shown in  FIG. 3   b . The potential probe  300  can replace the receiving electrode  135  of the measuring device  100  in  FIGS. 1 and 2   a - 2   c.    
     The diagram in  FIG. 3   a  corresponds to the viewing direction in  FIGS. 2 a -2 c   . The transmitting electrodes  125  and  130  are arranged in one plane above the screening electrode  230 . A first receiving electrode  320  and a second receiving electrode  330  are arranged on a vertical line of symmetry  340  which lies between the transmitting electrodes  125  and  130 . The distances of the receiving electrodes  320 ,  330  from the plane of the transmitting electrodes  125 ,  130  are preferably equal. Lines of equal electrical potential are shown in the region of the transmitting electrodes  125 ,  130 . The receiving electrodes  320 ,  330  lie on the zero potential line; however, it is already sufficient if the receiving electrodes  320 ,  330  lie on the same potential line. A field-compensated measurement can be carried out by means of the measuring device  100  in the arrangement shown of the receiving electrodes  320 ,  330  on the line of symmetry  340 . 
     As shown in  FIG. 3 b   , the receiving electrodes  320 ,  330  are connected to inputs of a differential amplifier  360  by means of resistors  340  and  350  respectively. The differential amplifier  360  forms a difference of the voltages applied to its inputs (differential measurement). The output of the differential amplifier  360  leads via a high-pass filter  370  to the input amplifier  140  in  FIG. 1  or to the impedance  170  connected before the input amplifier  140  respectively. The high-pass filter  370  removes low-frequency interference which can be caused, for example, by a wall connected to a certain potential or a conducting wall  210 . 
     A dielectric object  220  mainly affects the first receiving electrode  330  so that the second receiving electrode  320  serves as a reference potential. The screening electrode  230  is connected to a zero potential (ground). The screening electrode  230  can also be connected to any other potential, as equal components in the fed-in signals are cancelled out due to the subtraction which takes place in the differential amplifier  360 . The potential probe  300  itself is therefore potential-free. In addition, the potential probe shown is insensitive to tilting on account of the electrodes  125 ,  130 ,  230 ,  330 ,  340  being distributed between a total of three planes, as explained above with reference to  FIGS. 2   a - 2   c.    
     The output signal of the potential probe  300  depends on a lateral position of a dielectric object such as the beam  220  from  FIGS. 2 a -2 c   . If the beam  220  is moved from right to left past the potential probe  300 , then an output signal applied to the terminal  155  of the measuring device  100  in  FIG. 1  is negative as long as the beam  220  lies to the left of the axis of symmetry  340 , and positive as soon as the beam  220  lies to the right of the axis of symmetry  340 . An edge of a large beam  220  can therefore easily be located based on extremes of the output signal by moving the arrangement  310  on the wall  220 . Likewise, the center of a beam  220  can easily be located based on a change in sign by moving the arrangement  310  on the wall  220 . 
       FIG. 4  shows a further arrangement  400  of electrodes for use with the measuring device  100  from  FIG. 1 . The arrangement  400  is shown in plan view. First transmitting electrodes  125  and second transmitting electrodes  130  lie opposite one another in pairs with respect to a receiving electrode  135 . The overall structure has the form of a 3×3 matrix, wherein the central element is formed by the receiving electrode  135 . 
     Different pairs of mutually opposing transmitting electrodes  125 ,  130  can be successively connected to the measuring device  100  from  FIG. 1  in the three-electrode measuring mode. Since, in the three-electrode measuring mode, a polarity of the output signal of the measuring device at the terminal  155  depends on a lateral orientation of the dielectric object with respect to the receiving electrode, the orientation of the dielectric object can be accurately determined by the plurality of successive measurements. 
     In a further embodiment, the electrodes  125 ,  130  can lie on the diagonals of the arrangement  400 , so that the receiving electrodes  135  are arranged in the form of a plus sign (not shown). As described above with reference to  FIG. 2 , the arrangement  400  can be used in conjunction with a screening electrode  230  on a side of the electrodes  125 ,  130  which faces away from the dielectric object. 
       FIG. 5  shows a flow diagram of a method  500  for detecting the dielectric object  220  by means of the device of  FIG. 1 . The method  500  comprises steps  510  to  550 . 
     In step  510 , the capacitance devices  125  and  130  are supplied with alternating voltages. In step  520 , an electrical potential, which is established at the potential probe  135 ,  300  in the region of the capacitance devices  125  and  130 , is determined. In step  530 , an alternating voltage component of the electrical potential, which is clock-synchronous with the alternating voltages at the capacitance devices  125  and  130 , is determined. The supply to the capacitance devices  125  and  130  in step  510  is controlled as a function of the determined alternating voltage component, wherein it may be necessary for the method  500  to run through again for this purpose. The magnitude of the alternating voltage component of the electrical potential determined at the potential probe  135 ,  300 , which is clock-synchronous with the alternating voltages, is minimized by controlling the supply of the capacitance devices  125  and  130 . Subsequently, the alternating voltages of the capacitance devices  125  and  130  are compared with one another in step  540 . If the voltages deviate from one another by more than a predetermined amount, then the dielectric object  220  is detected in step  550 . The method  500  then returns to the beginning and runs through again.