Abstract:
A device for detecting a dielectric object includes a first electrode and a second electrode. A first unit is configured to determine a first capacitance that exists between the first electrode and a common reference point and that is influenced by the object. A second unit is configured to determine a second capacitance that exists between the second electrode and the reference point and that is influenced by the object. The device further includes a control unit for actuating the first and second units and an evaluating unit that is configured to detect the object when the determined capacitances differ by more than a predetermined amount. The control unit is configured to actuate the units in such a manner that the capacitance determinations are carried out in succession. A switching unit is provided so as to electrically connect the electrode of the respective non-actuated unit to the common reference point.

Description:
PRIOR ART 
       [0001]    For the capacitive location of a dielectric object, an electric field can be generated and it can then be determined whether the electric field is influenced by the object. In a variant, two electric fields are generated by means of two adjacent electrodes and a comparison of the capacitances at the two electrodes is carried out. If the capacitances differ by more than a predefined amount, then the presence of the object in the range of the electric fields can be inferred. This procedure can be used for any kind of object which has dielectric properties, for example a wooden beam in a lightweight wall. 
         [0002]    The arrangement of adjacent electrodes is sensitive to all kinds of electrical conductors in their environment. The electrode arrangement is therefore set up at a certain spatial distance from an electric circuit which carries out the determination of the capacitance. For example, the electrode arrangement can be arranged on the same printed circuit board as the switching circuit, wherein there is a horizontal distance of a few centimeters between the electrodes and the switching circuit. As an alternative to this, the electrode arrangement can be mounted on a separate board so that there is a certain distance between the switching circuit and the electrodes in the vertical direction. 
         [0003]    It is the object of the invention to specify a device and a method for detecting a dielectric object which allow better use of space. 
       DISCLOSURE OF THE INVENTION 
       [0004]    The invention achieves this object by means of a device with the characteristics of claim  1  and by a method with the characteristics of claim  7 . Dependent claims describe preferred embodiments. 
         [0005]    A device according to the invention for detecting a dielectric object comprises a first and a second electrode, a first device for determining a first capacitance which exists between the first electrode and a common reference point and which can be influenced by the object, and a second device for determining a second capacitance which exists between the second electrode and the ground point and which can be influenced by the object. Furthermore, a control unit for actuating the devices and an evaluation unit for detecting the object are provided in the event that the determined capacitances differ from one another by more than a predefined amount. At the same time, the control unit is set up to actuate the device in such a way that the determinations are carried out successively, and a switching device is provided to electrically connect the electrode of the device which is not actuated at any one time to the common reference point. Advantageously, this enables parasitic capacitances between the first and the second electrode to be connected so that they do not influence the difference between the determinations. 
         [0006]    The invention enables the influence of parasitic capacitances to be minimized when differentially determining the capacitance. The procedure according to the invention of sequential measurement while at the same time short-circuiting parasitic capacitances can be extended to any number of electrodes or electrode pairs. 
         [0007]    Preferably, a third electrode is arranged in the region of the first and the second electrode. The third electrode can be used to achieve a shielding with respect to conductor tracks or electronic components in the region of the first two electrodes. Coupling capacitances resulting between the third and the first two electrodes are connected in the same way as the parasitic capacitances described above so that they do not influence the determination of the capacitance. This enables a measuring circuit to be designed in a compact manner; in particular, a distance between an evaluation circuit and the electrodes can be reducible to less than ten millimeters, preferably approx. 1.6 millimeters, which corresponds to the thickness of a normal printed circuit board, without detracting from the accuracy. 
         [0008]    The third electrode can be arranged on a side of the first and the second electrode which faces away from the dielectric object. This enables a space-saving construction of the electrodes and of the assemblies for evaluating the electrode signals which are connected to the electrodes to be achieved. It is not necessary to distribute the assemblies and electrodes over a plurality of circuit boards or to maintain a large distance between the electrodes and the assemblies. 
         [0009]    The devices for determining the capacitances can be set up to output time signals, the lengths of which in each case depend on the determined capacitances, and the evaluation unit can be set up to provide a time signal, the length of which depends on the difference between the capacitances. The time signals can be determined based on a charging or discharging of the capacitances. In this way, the capacitances can be determined with relatively low frequencies which can increase an accuracy of the determination. The time signal provided by the evaluation unit can be integrated and compared with a threshold value. An evaluation of this kind can be easily and reliably constructed in a known manner. 
         [0010]    The control unit can be set up to periodically actuate the first and the second device for determining a capacitance with the same frequency, wherein a phase relationship between the periodic actuations can be varied in order to compensate for different capacitances in the absence of the object. In this way a simple and efficient synchronization of the procedures for determining the capacitances can be combined with an ability of the device to be easily calibrated. 
         [0011]    A multiplicity of devices for determining capacitances between an electrode and the common reference point can be provided, wherein the control unit is set up to actuate only one of the devices at each point in time and the switching device is set up to electrically connect the electrodes of all devices which are not actuated to the common reference point. 
         [0012]    A series of a multiplicity of capacitance determinations can therefore be carried out by means of spatially distributed electrodes. 
         [0013]    A method according to the invention for detecting a dielectric object comprises steps of the determination of a first capacitance which exists between a first electrode and a ground point and which can be influenced by the object, the determination of a second capacitance which exists between a second electrode and the common reference point and which can be influenced by the object, and the detection of the object in the event that the determined capacitances differ from one another by more than a predefined amount. In doing so, the capacitances are determined successively and, during the determination by means of one of the electrodes, the respective other electrode is electrically connected to the common reference point. 
         [0014]    Finally, a computer program product includes program code means for carrying out the method and can run on a processing device or be stored on a computer-readable data medium. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]    The invention is now described in more detail with reference to the attached figures, in which: 
           [0016]      FIG. 1  shows an electrode arrangement; 
           [0017]      FIG. 2  shows a measuring circuit based on the electrode arrangement from  FIG. 1 ; 
           [0018]      FIG. 3  shows a beam finder for actuating the electrode arrangement from  FIG. 2 ; 
           [0019]      FIG. 4  shows a time diagram with characteristics at the beam finder from  FIG. 3 ; and 
           [0020]      FIG. 5  shows a flow diagram of a method for beam finding. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0021]      FIG. 1  shows an electrode arrangement  100 . With the electrode arrangement  100  shown in  FIG. 1 , a differential capacitance measurement is possible, thus enabling the object  120  to be found or determined. For example, the electrode arrangement  100  can be used as a beam finder for detecting a wooden beam hidden in a lightweight wall. Alternatively, the electrode arrangement  100  can also be used with a dielectric liquid in order to determine the level, wherein the electrode arrangement shown or a modified electrode arrangement can be used. Further applications which are based on a differential capacitance determination are likewise possible. 
         [0022]    Electrodes E 0 -E 3  are attached to both sides of a transparent printed circuit board (board)  110  shown. 
         [0023]    The electrode E 2 , which is encompassed in a U-shape by the electrode E 1 , lies on the top side of the printed circuit board  110 . In turn, the electrode E 1  is encompassed in a U-shape by the electrode E 0 . The flat electrode E 3  lies opposite the electrodes E 0  to E 2  on the bottom side of the printed circuit board  110 . The electrodes E 0  to E 3  are in the form of copper surfaces which are stuck to the printed circuit board  110 . The electrodes E 0  to E 3  can be formed on the printed circuit board  110  by an etching process, for example, with which further connecting elements for connecting electrical components can also be formed on the printed circuit board  110 . A dielectric object  120  is situated above the printed circuit board  110  and the electrode E 0 . 
         [0024]    While the electrodes E 0  to E 2  are connected to electrical components, the electrode E 3  is either not further connected or can be connected with a high impedance to a circuit, for example by means of a controlled switch such as a transistor. The electrode E 3  serves to shield the electrodes E 0  to E 2  from below. An influence of a measuring circuit attached here or of a measuring person on the electrodes E 0  to E 2  is minimized by the electrode E 3 . In some embodiments, the electrode E 3  can also be omitted. 
         [0025]    Capacitances C 1 , C 2 , C 12  and C 3 , which in each case occur between the electrodes E 0  to E 3 , are shown in the form of equivalent circuits. The capacitance C 1  is formed between the electrodes E 0  and E 1 , wherein the electrode E 0  is connected to ground; correspondingly, the capacitance is formed between the electrodes E 0  and E 2 . In other embodiments, the electrode E 0  can be connected to any potential other than ground, as long as this potential can be used as an invariable reference point for determining the capacitances C 1  and C 2 . The parasitic capacitance C 12  occurs between the electrodes E 1  and E 2 . A further parasitic capacitance C 3  consists in a series circuit of partial capacitances between the electrodes E 1  and E 3  and E 3  and E 2  respectively. 
         [0026]    In order to differentially detect the dielectric object  120 , the capacitances C 1  and C 2  are normally charged or discharged simultaneously and a difference in time between the ends of the charging or discharging processes is measured. If this time difference exceeds a predefined time threshold, then the dielectric object  120  is inferred. 
         [0027]    The parasitic capacitances C 12  and C 3  effect a coupling of the capacitances C 1  and C 2  to one another so that crosstalk occurs and the accuracy of the measurement, particularly for only small differences between the capacitances C 1  and C 2 , is reduced. The parasitic capacitance C 3  can also occur when, instead of the electrode E 3 , another conductive structure is arranged in the region of the electrodes E 1  and E 2 , for example an electrical component or an operating element. 
         [0028]      FIG. 2  shows a measuring circuit  200  based on the electrode arrangement  100  from  FIG. 1 . The measuring circuit  200  shown is an equivalent circuit for explanation purposes; with an actual measuring circuit, the parasitic capacitances C 12  and C 3 , for example, would be minimized. The electrode E 0  is electrically connected to ground so that the bottom connection of the capacitances C 1  and C 2  in each case is connected to ground. The top connections of the capacitances C 1  and C 2  are connected to one another by means of the parasitic capacitances C 12  and C 3 . The top connection of the capacitance C 1  is referred to in the following as test point A. Test point A is connected to a constant operating voltage by means of a resistor R 1 , as symbolized by the arrow on the top connection of the resistor R 1 . In a corresponding manner, the top connection of the capacitance C 2  is referred to in the following as test point B. Test point B is connected to the operating voltage by means of a resistor R 2 . A switch S 1  is arranged parallel to the capacitance C 2  and a switch S 2  parallel to the capacitance C 2 . 
         [0029]    The capacitance C 1  is determined in that the capacitance C 1  is charged via the resistor R 1  and a time until the voltage at test point A has exceeded a predefined threshold value is determined. This threshold value is usually ⅔ of the operating voltage when charging and ⅓ of the operating voltage when discharging. The time determined is proportional to the capacitance C 1 . If a dielectric object  120  is located in the region of the electrodes E 1  and E 0 , which form the capacitance C 1 , then the value of the capacitance C 1  changes, which can be detected by a change in the time until the voltage at test point A rises above the threshold value. The capacitance C 2  is determined in a similar manner in that the capacitance C 2  is charged by means of the resistor R 2  and the voltage at test point B is compared with a threshold value. 
         [0030]    The parasitic capacitances C 3  and C 12  electrically couple the capacitances C 1  and C 2  so that, with the described procedure, an actual difference between the capacitances C 1  and C 2  is greater than a verifiable difference. 
         [0031]    If the capacitances C 1  and C 2  are not determined simultaneously but successively, then the switch S 2  can be closed while the capacitance C 1  is determined, and the switch S 1  can be closed while the capacitance C 2  is determined. If the switch S 2  is closed, then the capacitance C 2  is short-circuited and the parasitic capacitances C 12  and C 3  lie parallel to the capacitance C 1 . The capacitance values are summed so that the capacitance C1+C12+C3 is determined. The switch S 2  is then opened and the switch S 1  closed, thus enabling the capacitance C2+C12+C3 to be determined. As C 12  and C 3  are independent of the effect of a dielectric object  120 , they affect the two capacitance determinations to an equal extent. A comparison of the time which is required to charge the capacitance C1+C12+C3 to a predefined voltage with the time which is necessary for a corresponding charging of the capacitance C2+C12+C3 allows the constant portion resulting from the parasitic capacitances C 12 , C 3  to be eliminated. The resulting time difference is therefore dependent on the capacitance C 1  and C 2 , and the parasitic capacitances C 12  and C 3  do not affect the measurement. 
         [0032]      FIG. 3  shows a beam finder  300  for actuating the electrode arrangement  200  from  FIG. 2 . A time element F 1  is connected to the test point A, and a time element F 2  to the test point B. When a measurement is carried out, the appropriate connection of the time element to the test point A or B respectively has a high impedance. Otherwise, the connection is connected to ground, thus implementing the functionality of the switch S 1  or S 2  respectively. 
         [0033]    A clock generator PWM provides a rectangular signal, with which the ratio between a high output signal (High) and a low output signal (Low) can be influenced during each clock period. A rising edge of the rectangular signal provided by the clock generator PWM triggers the time element FF 1 , and a falling edge triggers the time element FF 2 . Unused connections of the time element FF 1  and FF 2  are connected to ground or to the supply voltage respectively. 
         [0034]    A non-inverting output Q of the time element FF 2  is connected to the R-input of an RS flip-flop FF 3 . In other embodiments, any other state memory can also be used, for example an appropriately connected T flip-flop. The inverting output  Q  of the time element FF 1  is connected to the S-input of FF 3 . A non-inverting output Q of FF 3  is connected to an integrator which comprises a transistor T 1 , a resistor R 3  and a capacitor C 4 . The output of the integrator is connected to a low-pass filter, which is formed by a resistor R 5  and a capacitor C 5 . 
         [0035]    The output of the low-pass filter is connected to the non-inverting input of an operational amplifier OV 1 , to the inverting input of which is applied a constant voltage which is provided by a resistor R 6  and a Zener diode ZD 1 . The operational amplifier OV acts as a comparator. If the voltage applied to the non-inverting input exceeds the voltage applied to the inverting input, then the output of the operational amplifier OV 1  is set to a positive value (High). As an alternative to the simple comparator shown, a window comparator can also be used, the output of which outputs a signal which indicates whether or not the voltage provided by the low-pass filter lies between two predefined threshold values. The output of the operational amplifier OV 1  is connected to a terminal K. 
         [0036]    The components connected to the output Q of the RS flip-flop FF 3  serve to provide a positive signal at the terminal K when a pulse which appears periodically at the output Q of FF 3  exceeds a predefined length. This corresponds to a predefined difference between the capacitances C 1  and C 2  which is brought about by the dielectric object  120  in the region of the electrodes E 0 , E 1  and E 2 . The signal at the terminal K corresponds to the determination of the dielectric object  120 . 
         [0037]    The flip-flops FF 1  to FF 3  serve to alternately determine the capacitances C 1  and C 2  and to compare them with one another. The chosen circuit arrangement enables the temporary storage of a value which refers to the capacitance of one of the capacitances C 1 , C 2  while the other capacitance C 2 , C 1  is determined to be avoided. 
         [0038]    As an alternative to the diagram of  FIG. 3 , the capacitances C 1  and C 2  can also be determined and the pulses shown above evaluated in a number of other ways. For example, the pulses provided by the time elements FF 1  and FF 2  can first be integrated and only then compared with one another. Alternatively, one of the pulses can be inverted and shifted down by the voltage difference (High−Low) to then be fed to an integrator. Both the comparison and the evaluation can be carried out by a digital microcomputer, in which an analog-digital conversion and/or a digital-analog conversion can be carried out. The capacitances C 1  and C 2  can also be converted into digital values by means of a determination method different from by means of the time elements FF 1  and FF 2 . In a further embodiment, each of the capacitances C 1  and C 2  can be determined by means of an oscillator and frequencies of the oscillators can be subtracted from one another. 
         [0039]    The principle of operation of the interconnected flip-flops FF 1  to FF 3  of  FIG. 3  is now explained in more detail with reference to  FIG. 4 . 
         [0040]      FIG. 4  shows a time diagram  400  with characteristics at the beam finder  300  from  FIG. 3 . A time is plotted in the horizontal direction. Four characteristics are plotted from top to bottom. The top characteristic  410  corresponds to the output Q of the clock generator PWM. The following characteristics  420  and  430  correspond to the outputs Q of the time elements FF 1  and FF 2  respectively. It must be noted that, although the output Q of the time element FF 1  in  FIG. 3  is not wired, the characteristic  420  refers to this output and not to the wired output  Q . The fourth characteristic  440  corresponds to the output Q of the RS flip-flop FF 3 . 
         [0041]    Within a cycle T, the clock generator PWM generates the symmetrical rectangular signal shown in characteristic  410 . A positive portion Tp and a negative portion Tn are of equal length. In other embodiments, an asymmetrical signal can also be generated by the characteristic  410  in which Tp and Tn are of unequal length. At time t 0 , the first time element FF 1  is triggered by the rising edge of the first characteristic  410  in order to start a determination of the capacitance of C 1 . The output Q of the first time element FF 1  is set to “High” and the capacitance C 1  is charged via the resistor R 1 . At this time, the output Q of the second time element FF 2  is “Low” which corresponds to a closed switch S 2  in  FIG. 2 . 
         [0042]    At time t 1 , the voltage at test point A has exceeded a predefined threshold value and the measurement is complete. The characteristic  420  switches back to “Low”. The pulse duration Pw 1  in the characteristic  420  between t 0  and t 1  depends on the determined capacitance of C 1 . 
         [0043]    A corresponding determination of the capacitance C 2  starts at time C 2  with the falling edge of the characteristic  410 . At time t 3 , the determination is complete and the pulse duration Pw 2  of the characteristic  430  depends on the determined capacitance of C 2 . 
         [0044]    In order to compare the pulse durations Pw 1  and Pw 2  with one another, the RS flip-flop FF 3  is in each case set by the falling edge of the characteristic  430  and reset by the falling edge of the characteristic  420 . Setting occurs at times t 0  and t 3 ; resetting at times t 1  and t 4  respectively. If the capacitances of C 1  and C 2  are equal, then the pulse lengths Pw 1  and Pw 2  are of the same length and the characteristic  440  is a symmetrical rectangular signal. In other words, in this case, Pw 3  in characteristic  440  is the same length as Tp or Tn in characteristic  410 . 
         [0045]    By integrating the characteristic  440 , a voltage which corresponds to the ratio between High and Low time of the characteristic  440  can be provided and, after passing through a low-pass filter, this voltage can be compared with a constant voltage. If the voltage provided by the low-pass differs from the constant voltage by more than a predefined amount, then the signal of the characteristic  440  has a mark-space ratio which implies the presence of a dielectric object  120  in the region of the electrodes E 0  to E 2  in  FIG. 1 . 
         [0046]    Asymmetries, which may be caused by component spread or by parasitic effects between the components, can be compensated for in that the rectangular signal of the fourth characteristic  440  is made asymmetrical to a suitable extent. In this way, the beam finder  100  can be designed to be adjustable. 
         [0047]      FIG. 5  shows a method  500  for detecting a dielectric object  120 . The method  500  comprises steps  510  to  560 . In a first step  510 , a clock signal is generated for controlling the capacitance measurements of C 1  and C 2 . The first capacitance C 1  is determined in step  520  and the second capacitance C 2  in step  530 . While the method  500  returns to the beginning and runs through once more, the difference between the two determined capacitances is determined in step  540 . The determined difference is then compared with a threshold value in step  550 . If the determined difference deviates from the threshold value by more than a predefined amount, then the presence of the dielectric object  120  in the region of the electrodes E 0  to E 2  is inferred. This result is output in step  560 . It can be output, for example in a visual and/or audible manner, to a user of the beam finder  300 .