Abstract:
The invention relates to a sensor for detecting an object. The sensor includes a probe electrode for forming a measuring capacitance with the object to be detected, a charging generator for generating an alternating charging voltage, wherein the probe electrode is charged by means of the alternating charging voltage, an amplifier for amplifying a voltage across the measuring capacitance, wherein the amplifier includes a first supply connection and a second supply connection, electronic means for processing signals outputted at an output of the amplifier to form at least one output signal and at least one sensor output for outputting the at least one output signal. According to the invention, the sensor is characterized in that a DC voltage in the form of a second supply voltage is connectable to the second supply connection, and that a first supply voltage having an AC voltage portion is connectable to the first supply connection, wherein a supply voltage generator is available for generating the DC voltage portion. The invention also relates to a method for detecting an object.

Description:
TECHNICAL FIELD 
     The present invention relates, in a first aspect, to a sensor for detecting an object. In a second aspect, the invention relates to a method for detecting an object. 
     RELATED ART 
     A generic sensor comprises a probe electrode for forming a measuring capacitance with the object to be detected, a charging generator for generating an alternating charging voltage by means of which the probe electrode is charged, an amplifier for amplifying a voltage across the measuring capacitance, wherein the amplifier includes a first supply connection and a second supply connection, electronic means for processing signals that are outputted at an output of the amplifier to form at least one output signal, and at least one sensor output for outputting the at least one output signal. 
     In a generic method for detecting an object, a measuring capacitance is formed by means of a probe electrode and the object to be detected. The probe electrode is charged by means of an alternating charging voltage, a voltage across the measuring capacitance is amplified by an amplifier, and signals that are outputted at the output of the amplifier are processed to form at least one output signal and the at least one output signal is outputted. 
     Such a method and a generic sensor are disclosed in EP 1 214 787 B1. In this method, an AC voltage source is connected to both supply connections of the amplifier. The supply voltage of the amplifier thus remains constant. While both the measuring electrode and the supply connections of the amplifier are connected to the AC voltage source, it is possible to detect variations in the capacitance between the probe electrode and the target object. 
     SUMMARY 
     According to an aspect of the invention, a sensor for detecting an object is provided. The sensor, with its cost-effective design, operates with particular accuracy and resistance against interferences. 
     According to another aspect of the invention, a method for detecting an object is provided. The method achieves, at minimum expense, highly accurate measurements largely unaffected by interferences. 
     According to the invention, provision is made for a second supply voltage to be connectable in the form of a DC voltage to the second supply connection of a sensor of the aforementioned generic type and for a first supply voltage having an AC voltage portion to be connectable to the first supply connection, a supply voltage generator being available for generating the AC voltage portion. 
     In the method of the aforementioned generic type, provision is made, according to the invention, for a second supply voltage in the form of a DC voltage to be connected to a second supply connection of the amplifier, and for a first supply voltage having an AC voltage portion to be connected to a first supply connection of the amplifier, the AC voltage portion being generated by a supply voltage generator. 
     The method of the invention may be executed using the sensor of the invention. 
     The measuring principle of the sensor of the invention is based on the fact that the voltage across the measuring capacitance created by the alternating charging voltage does not immediately jump to a constant value, but instead a charging and a discharging take place, the time constants of which are a function of the level of the measuring capacitance. The greater the measuring capacitance, the longer the charging and the discharging take. Thus, it is possible to deduce the magnitude of the measuring capacitance from the voltage across the measuring capacitance or from a temporal mean value of this voltage. The magnitude of the measuring capacitance in turn depends on the distance from the object to be detected. Thus it is possible to detect an object by measuring the voltage across the measuring capacitance. 
     The invention is based on the cognition that it is possible to increase the measuring accuracy when there is a minimum of active parasitic capacitances that influence charging of the measuring capacitance. 
     The present invention is configured to deactivate a parasitic capacitance occurring between the first supply connection and the input of the amplifier. This is achieved, according to the present invention, by the application of a supply voltage having an AC voltage portion at the first supply connection. A parasitic capacitance retards the charging and discharging process at the measuring capacitance, thereby altering the temporal mean value of the voltage across the measuring capacitance. A voltage surge is generated across the parasitic capacitance at the amplifier input and therefore at the measuring capacitance by the AC voltage portion, which parasitic capacitance forms, together with the measuring capacitance, a voltage divider. The voltage surge alters the temporal mean value of the voltage across the measuring capacitance just sufficiently to compensate the effect of the parasitic capacitance. Thus, a temporal mean value of the voltage across the measuring capacity is advantageously not dependent on the magnitude of the parasitic capacitance. 
     In the present invention, the AC voltage portion is applied only at the first supply connection. However, only a DC voltage is applied at the second supply connection. With transistor amplifiers, in particular, there is no active parasitic capacitance between the second supply connection and the amplifier input, as a result of which an AC voltage portion is not required at this connection for compensating a parasitic capacitance. The use of a DC voltage at the second supply connection makes for a particularly cost effective design, with which very accurate measurements are nevertheless possible and which is not, or only slightly, affected by parasitic capacitances. Unlike known sensors therefore, the supply voltage of the amplifier, being the difference between the voltages at the two supply connections of the amplifier, is according to the present invention not constant, but instead is varied periodically, for example, by a square wave voltage. 
     In practical implementations it is possible only with comparatively great time and effort to prevent minor DC voltage fluctuations. Therefore, the terms “DC voltage” or “DC voltage potential” to be applied at the second supply connection should be taken to mean any potential, the value of which varies during operation by not more than 10% and preferably by not more than 5% from, in particular, the minimum supply voltage of the amplifier. 
     A voltage supply can be available for generating the DC voltage to be applied at the amplifier as the second voltage supply. The voltage supply means may differ from, or be independent of, the supply voltage generator used to generate the AC voltage portion. In principle, it is possible for one and the same voltage supply to deliver the second supply voltage and the AC voltage portion. 
     The object to be detected can be an object of basically any desirable material. For example, it can be a sheet of paper, a wooden palette, a metallic object, a liquid, or a bulk material disposed, for example, in the toner cartridge of a printer. For the method of the invention the object may be grounded, such that, in particular, the electrical potential of the object is equal to ground potential. 
     To render the parasitic capacitance between the first supply connection and the amplifier input harmless, the AC voltage portion of the first supply voltage should match as closely as possible in frequency and phase the alternating charging voltage used to charge the probe electrode. For this purpose, provision is made, in a variant of the sensor of the invention, for the charging generator and the supply voltage generator to be formed by one and the same generator, and for the AC voltage portion of the first supply voltage to be defined by the alternating charging voltage. In addition, the use of a single generator makes for a particular cost-effective sensor design. 
     In another embodiment of the sensor of the invention, the amplitude of the AC voltage portion of the charging voltage deviates at most by 20% from the amplitude of the AC voltage portion of the first supply voltage. When using a single generator, this voltage deviation can be caused, for example, due to additional diodes or resistors. 
     The supply voltage at the first supply connection may comprise a DC voltage portion. The latter differs sufficiently from ground potential for the voltage between the two supply connections to be high enough at any time to operate the amplifier within its operating range. 
     The DC voltage portion may have a value of, say, from 2V to 20V. In particular, the DC voltage portion can be selected such that the first supply voltage of the amplifier varies between 5V and 10V. 
     Particularly good measuring accuracy can be achieved when the amplitude of the AC voltage portion is as high as possible. 
     It is advantageous when the second supply voltage applied at the second supply connection is equal to ground potential. 
     In an embodiment of the sensor of the invention, in order to inhibit the occurrence of a parasitic capacitance between the second supply connection and the amplifier input of the amplifier, the amplifier is a transistor amplifier in common collector circuit, more particularly a common collector Darlington circuit or a complementary collector Darlington circuit. The second supply connection at which the DC voltage is applied can be either the positive or the negative supply connection, depending on the design of the amplifier. When using an npn transistor amplifier, the second supply voltage is a constant negative supply voltage, whereas the first supply voltage is a positive supply voltage having an AC voltage portion. Conversely, when using a pnp transistor amplifier, the second supply voltage is a constant positive supply voltage and the first supply voltage having the AC voltage portion is a negative supply voltage. Furthermore, it is alternatively possible to employ a normal or complementary Darlington circuit in npn or pnp design. 
     Bipolar transistors may be used, although it is basically possible to use field effect transistors. 
     In an embodiment, the amplifier has an AC voltage gain of 1,in order to prevent interference effects of a parasitic capacitance between the output and input of the amplifier, that is to say, it is operated as an impedance converter. In this embodiment, it is advantageous when all parasitic capacitances between the output and input of the amplifier are deactivated. 
     The voltage across the measuring capacitance can be evaluated by determining temporal mean values of the voltage during charging and discharging of the measuring capacitance. To accomplish this, provision is made, in a design variant of the sensor of the invention, for the electronic means to comprise a rectifier circuit for the purpose of rectifying a signal at the output of the amplifier, and for the first supply voltage of the amplifier to be used as the reference potential for the rectifier circuit. 
     Such a rectified signal is just equal to the temporal mean value of the voltage at the measuring capacitance. It is significant in this regard that rectification is performed separately for charging and for discharging. For this purpose, provision can be made for the rectifier circuit to be a synchronous rectifier circuit and for the synchronous rectifier circuit to be controlled by the charging voltage. In addition, it is advantageous when the voltage drop associated with rectification is particularly small in a synchronous rectifier having analog switches or field effect transistors. 
     Advantageously, provision may be made for the electronic means to be set for generating an analog output signal or a switching signal from voltage mean values that are outputted by the rectifier circuit, and for the analog output signal or the switching signal to be outputted at the sensor output. The analog output signal can, for example, be a difference of the two voltage mean values which correspond to the charging and discharging of the measuring capacitance. 
     The switching signal can be obtained, for example, by comparing the voltage mean values with threshold values or by comparing the difference between the two voltage mean values with the threshold value. To accomplish this, provision may be made for the electronic means to comprise a first comparator for comparing the output signal of the rectifier circuit with a threshold value, and for outputting, at the sensor output, a switching signal that is derived from the switching signal of the first comparator. 
     A cost-effective sensor design is achieved when the reference potential for the output of the first comparator is the first supply voltage, when a second comparator is present which uses the ground potential as the reference potential for its output, and when the switching signal of the first comparator is outputted to the second comparator such that the output of the second comparator does not alternate with the pulse of the charging voltage. For this purpose, a signal transmission circuit may be provided which transmits the switching signal of the first comparator to the second comparator. In this configuration, the signal transmission circuit relates the switching signal of the first comparator to a DC voltage potential, more particularly to ground potential, so that the output signal of the second comparator does not alternate with the frequency of the charging voltage. 
     In an embodiment of the method of the invention, each of the oscillation periods of the alternating charging voltage comprises a first and a second charge phase. During the first charge phase the measuring capacitance is charged, whereas it is discharged during the second charge phase. The alternating charging voltage can, for example, have a square wave shape with which the first and second charge phases are formed by the two different constant voltage levels of the square wave voltage. In an embodiment, provision can be made for a rectifier circuit to output a voltage mean value for one or both of the charge phases, or for a rectifier circuit to output an output signal which is derived from the voltage mean values of the two charge phases. Such an output signal is a function of the magnitude of the measuring capacitance and therefore provides information concerning the object to be detected. 
     In embodiments of the method of the invention, as described below, it is possible to further improve the interference immunity to external influences: When an external interfering component couples capacitively to the amplifier input, it changes the voltage at that input. As long as a parasitic frequency of this component is not synchronous with the operating pulse of the rectifier, there is on average no change in the rectifier output voltage thereof, so that the measurement is not disrupted. If, however, the parasitic frequency is synchronous with the operating pulse of the rectifier, the rectifier output voltage is changed, thereby distorting the measurement. These interfering signals can be masked by generating the alternating charging voltage so as to vary in frequency and/or phase. Then, when evaluating the mean values at varying frequencies or phases of the charging voltage, the effects of a parasitic component at a constant parasitic frequency and phase are advantageously masked. In this way, it is possible to discern with even greater reliability objects that are to be detected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and features of the invention are described below with reference to the appended schematic figures. 
         FIG. 1  is a schematic circuit diagram of a generic sensor. 
         FIG. 2  is a graph of the alternating charging voltage which supplies the probe electrode, in relation to time. 
         FIG. 3  is a graph of an idealized AC voltage between the probe electrode and an object to be detected when no parasitic capacitances occur. 
         FIG. 4  is a graph of an AC voltage between the probe electrode and the object, in which the curve of the AC voltage is affected by parasitic capacitances. 
         FIG. 5  is a graph of a first supply voltage, which is applied at a first supply connection of an amplifier of a sensor of the invention. 
         FIG. 6  is a graph of an AC voltage which is applied between the probe electrode and the object, a parasitic capacitance being present between the input of the amplifier and the first supply voltage having an AC voltage portion. 
         FIG. 7  is a schematic circuit diagram of an exemplary embodiment of a sensor of the invention. 
         FIG. 8  is a schematic circuit diagram of a second exemplary embodiment of a sensor of the invention. 
         FIG. 9  is a schematic circuit diagram of a third exemplary embodiment of a sensor of the invention. 
     
    
    
     In general, identical elements or elements acting equivalently are identified in the figures by the same reference numerals or characters. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic circuit diagram of a generic sensor  90 . The sensor  90  comprises a probe electrode  10  and an amplifier A. 
     The probe electrode  10  and an object  5  to be detected together form a measuring capacitance C 1 . The magnitude of the measuring capacitance C 1  is influenced by the position and type of object  5 , and it is thus possible to deduce from the measuring capacitance C 1  the presence of the object  5  as well as the distance from the object  5 . 
     The sensor  90  further comprises a charging generator  30 , which charges the probe electrode  10  with an alternating charging voltage U 3  through a charging resistor R 1 . The target object  5  is on the other hand at ground potential. 
     The effects of the alternating charging voltage U 3  and the resultant AC voltage between the probe electrode  10  and the target object  5  are explained below with reference to  FIGS. 2 and 3 . 
       FIG. 2  shows a graph in which the alternating charging voltage U 3  is diagrammatically plotted on the ordinate U against time t along the abscissa. The alternating charging voltage U 3  has a square wave shape, in which a period of the charging voltage U 3  comprises at least one first and one second charge phase P 1 , P 2  at different electrical potentials. 
     As a result of the alternating charging voltage U 3 , an AC voltage U 4  is present between the probe electrode  10  and the object  5 , which is shown schematically in  FIG. 3  as a function of time t. During the first charge phase P 1 , the probe electrode  10  is charged to a high voltage level relative to the object  5 , as indicated by the curve UP 1 . During this phase the temporal curve of the AC voltage U 4  is flattened, not square. The greater the measuring capacitance C 1 , the greater is the time constant of the charging process, that is to say, the flatter is the slope of the curve UP 1 . In the second charge phase P 2 , the AC voltage U 4  at the probe electrode  10  drops to a lower value, as indicated by the curve UP 2 . Here, too, the curve UP 2  is flattened as a function of the magnitude of the measuring capacitance C 1 . 
     In addition,  FIG. 3  shows the mean values MP 1  and MP 2  of the AC voltage U 4  during the charge phases P 1  and P 2 . The greater the measuring capacitance C 1 , the flatter are the curves UP 1  and UP 2 , and the shorter is the distance between the two mean values MP 1  and/or MP 2 . Thus it is possible to deduce the presence or the distance of an object  5  from the mean values MP 1  and MP 2 , or alternatively from the difference between MP 1  and MP 2 . 
     In order to determine as accurately as possible the AC voltage U 4  or its temporal mean value, the probe electrode  10  is connected to an input of the amplifier A. In conventional sensors  90  and methods of measurement, the amplifier A can cause parasitic effects, as is discussed with reference to  FIG. 1 . 
     The amplifier A has a first supply connection A 1  at which a first supply voltage U 1  is applied, and a second supply connection A 2  at which a second supply voltage U 2  is applied. 
     Depending on the design of the amplifier A, a parasitic capacitance CP 1  is present between the first supply connection A 1  and the input A 3  of the amplifier A and/or a parasitic capacitance CP 2  is present between the second supply connection A 2  and the input A 3 . In addition, another parasitic capacitance arises between the output of the amplifier A and the input A 3 . 
     If both the first supply voltage U 1  and the second supply voltage U 2  are each a DC voltage, then the parasitic capacitances are active. The probe electrode  10  is connected to the input A 3  of the amplifier, as are the parasitic capacitances CP 1 , CP 2 , and CP 3 . For that reason, charging of the measuring capacitance C 1  formed by the probe electrode  10  and the object  5  is affected by the parasitic capacitances CP 1 , CP 2 , and CP 3 . Consequently, this changes the temporal curve of the AC voltage U 4  across the measuring capacitance C 1 . 
       FIG. 4  shows a graph in which the AC voltage U 4  is plotted diagrammatically against time t. As a result of the effects of the parasitic capacitances CP 1 , CP 2  and CP 3 , the curve of the AC voltage U 4  is flattened to a greater extent than the drop shown in  FIG. 3 . Thus, the distance is shortened between the mean values MP 1  and MP 2  of the AC voltage U 4  during both charge phases P 1  and P 2 . The parasitic capacitances CP 1 , CP 2 , and CP 3  therefore influence the mean values MP 1  and MP 2 . Thus, any inference concerning the magnitude of the measuring capacitance  10 , and thus concerning the presence or distance of an object  5 , will be distorted. 
     According to the present invention, the effect of the parasitic capacitances CP 1 , CP 2 , and CP 3  is neutralized to the greatest possible extent, leaving the measuring results unaffected, or only barely affected, by the parasitic capacitances CP 1 , CP 2 , and CP 3 . 
     The effect of the parasitic capacitance CP 1  is neutralized by the following measure: 
     A supply voltage U 1  having an AC voltage portion is applied at the first supply connection A 1  of the amplifier A. A graph of such a supply voltage U 1  is shown in  FIG. 5 . The AC voltage portion of the supply voltage U 1  has a square wave shape and matches the charging voltage in terms of period length, amplitude and phase. In addition, the supply voltage U 1  comprises a DC voltage portion, the value of which is selected sufficiently high to operate the amplifier A. 
     This alternating first supply voltage U 1  changes the curve of the AC voltage U 4  at the probe electrode  10 .  FIG. 6  shows the curve of the AC voltage U 4 , in which again an ascending segment UP 1  of the AC voltage U 4  is present during the first charge phase P 1 , whereas a descending section UP 2  of the AC voltage U 4  is present during the second charge phase P 2 . A capacitive voltage divider is formed by the parasitic capacitance CP 1  and the measuring capacitance. At the transitions between the charge phases P 1  and P 2 , the first supply voltage of the amplifier U 1  initiates in each case a voltage surge. The voltage surge of U 1  is coupled to the measuring capacitance with the division factor of the capacitive voltage divider. 
     In this way, the mean value MP 1  is raised during the first charge phase P 1 . The lowering of the mean value MP 1  as a result of the parasitic capacitance CP 1 , as described with reference to  FIG. 4 , is thus sufficiently compensated to achieve that the parasitic capacitance CP 1  has no disruptive effect on the mean value MP 1 . 
     Similarly, the surge occurring during the second charge phase P 2  causes the mean value MP  2  to drop, which drop also compensates any effect of the parasitic capacitance CP 1  on the mean value MP 2  during the second charge phase P 2 . 
     In this way, it is advantageously possible to draw very accurate conclusions on the measuring capacitance and thus on the presence and/or position of an object. 
     The following measure is taken to inhibit the parasitic capacitance CP 2 : 
       FIG. 7  shows a schematic circuit diagram of a sensor  100  of the present invention. As described with reference to the sensor  90  shown in  FIG. 1 , a measuring capacitance is again formed by a probe electrode  10  and an object  5  to be detected. Likewise, a charging voltage U 3  is connected to the measuring capacitance C 1  through a charging resistor R 1 . 
     In this embodiment, the amplifier A is in the form of a transistor amplifier. It comprises a bipolar transistor TR, in this case an npn-transistor and a resistor R 10 . The probe electrode  10  is connected to the base of the transistor TR. The resistor R 10  is connected to the emitter of the transistor TR. The other side of the resistor R 10  and the object  5  to be detected are grounded. The voltage U 10  drops across the resistor R 10  to a value dependent on the AC voltage U 4  between the probe electrode  10  and the object  5 . 
     The first supply voltage U 1 , which in the present embodiment is a positive supply voltage, is applied at the collector of the transistor TR. This supply voltage has a DC voltage portion and an AC voltage portion, as shown in  FIG. 5 . 
     Each integrated amplifier has a parasitic capacitance CP 2  between the input and the negative supply voltage. That is not the case with the transistor amplifier described here. Therefore, it is not necessary to neutralize the effect of CP 2 . For that reason, the negative supply voltage can remain constant, i.e., in the present case, connected to ground potential. 
     In a further embodiment (not shown) the transistor is a pnp transistor. In this case, there is no parasitic capacitance between the positive supply voltage and the amplifier input, but there is one between the negative supply voltage and the amplifier input. In this case, therefore, the first supply voltage, which comprises the AC voltage portion, is the negative supply voltage of the transistor. The positive supply voltage is on the other hand a DC voltage. 
     In an alternative design variant, the amplifier A is formed by a Darlington circuit or complementary Darlington circuit, again using either an npn or a pnp design. 
     The effect of the parasitic capacitance CP 3  is neutralized by the following measure: 
     The amplifier A has an AC voltage gain of 1. No current is able to flow as a result of a parasitic capacitance CP 3  between the input and output of the amplifier. The capacitance CP 3  has no effect. 
       FIG. 8  shows another exemplary embodiment of a sensor  100  of the invention. 
     This sensor is again largely of the same design as the sensor  90  shown in  FIG. 1 . Unlike the sensor  90 , however, the first supply voltage U 1  has an AC voltage portion, whereas the second supply voltage U 2  is a DC voltage. 
     The embodiment shown in  FIG. 8  additionally includes a rectifier circuit  40  connected to the output of the amplifier A. The rectifier circuit  40  is in this case a synchronous rectifier controlled by the charging voltage via a control signal  41 . The mean voltage value across the measuring capacitance is outputted at the output of the rectifier circuit  40  for each of the first and second charge phases P 1  and P 2  of the charging voltage. Thus, the synchronous rectifier outputs the voltage mean values MP 1  and MP 2  shown in  FIG. 6 , or voltages proportional to these mean values. 
     Alternatively, the rectifier  40  can be configured such that only one of the voltage mean values MP 1  and MP 2  is outputted. 
     The negative or positive supply voltage of the amplifier or a reference potential derived therefrom can be selected as the reference potential for the rectifier circuit  40 . The rectifier voltage U 6  and rectifier voltage U 7  are shown in  FIG. 8  by way of example. 
     As an alternative to the rectifier circuit  40  described, a rectifier having two outputs may be present, at which outputs the two mean values MP 1  and MP 2  or signals corresponding thereto can be outputted. 
     In a further alternative, the difference between the mean values MP 1  and MP 2  or the difference between signals corresponding to these mean values is outputted at one output of the rectifier circuit  40 . 
     The output voltage of the rectifier circuit  40  can be evaluated in different ways. For example, by means of the amplifier it is possible to amplify, shift, scale and/or linearize the output voltage of the rectifier circuit  40  and subsequently output it at an output of the sensor  100 . 
     Alternatively, the output voltage of the rectifier circuit  40  can be supplied to an analog-digital converter and subsequently processed with the aid of a microprocessor to form an output signal, which can in turn be outputted as an analog signal or as a switching signal. 
     It is further possible to compare the output voltage of the rectifier circuit  40  with a threshold voltage using a comparator. A schematic circuit diagram of such an embodiment of a sensor  100  of the invention is shown in  FIG. 9 . 
       FIG. 9  depicts components of the sensor  100  of the invention, particularly the amplifier A, the probe electrode  10 , the rectifier circuit  40 , and a generator  30 . 
     The generator  30  generates an AC voltage which alternates, for example between 0V and 5V, in square waves. This voltage is used both for charging the probe electrode and for supplying voltage to the amplifier. Thus, the generator  30  simultaneously functions as a charge generator and a supply voltage generator. This generator or frequency generator  30  is connected through a hold capacitor C 2  to the first supply connection A 1  of the amplifier A. Also connected to the first supply connection A 1  through a diode D 2  is a DC voltage supply U 5 . 
     As a result, the first supply connection A 1  assumes the voltage level of the DC voltage supply U 5  during a negative half-wave of the AC voltage of the frequency generator  30 . This value can be lowered slightly by a drop in potential across the diode D 2 . 
     In addition, during the negative half-wave of the AC voltage of the frequency generator  30 , the hold capacitor is charged by the DC voltage supply U 5 . During a positive half-wave of the AC voltage of the frequency generator  30  a voltage is generated at point A 1  that corresponds to the sum of the DC voltage of the DC voltage supply U 5  and the AC voltage amplitude of the frequency generator  30 . The amplitude of the AC voltage of the frequency generator  30  may be equal to the DC voltage level of the DC voltage supply U 5 , with the result that the voltage is doubled at point A 1 . If, for example, the DC voltage supply U 5  supplies 5V and the AC voltage of the frequency generator  30  alternates between 0V and 5V, then a supply voltage that alternates between 5V and 10V will be present at the first supply connection A 1 , cf.  FIG. 6 . 
     The second supply connection A 2  is grounded and is therefore at 0V. An AC voltage does not have to be applied at the second supply connection A 2 , since, on account of the design of the amplifier A, there is no parasitic capacitance to be compensated between the amplifier input A 3  and the second supply connection A 2 . 
     An embodiment of the amplifier A is shown in detail in  FIG. 9 . It comprises a complementary Darlington circuit having a pnp transistor TR 1  and a npn transistor TR 2 . The supply voltage having the AC voltage portion is applied at the emitter of the pnp transistor TR 1 . A resistor R 2  is disposed parallel to the emitter and base of the pnp transistor TR 1 . The collector of the npn transistor TR 2  is connected to the base of the pnp transistor TR 1 . The emitter of the npn transistor TR 2  is connected to the collector of the pnp transistor TR 1 . 
     The probe electrode  10  is connected to the base of the npn transistor TR 2 . This connection is designated by A 3  in  FIG. 9  as the amplifier input. 
     At this point the charging voltage of the probe electrode  10  is derived from the first supply voltage of the amplifier. Thus, it is advantageously possible to achieve a match between the charging voltage and the first supply voltage in terms of frequency and phase, in a cost-effective design. 
     To this end, the first supply connection of the amplifier at which the first supply voltage is present, is connected through a Zener diode D 1  and a resistor R 1  to the probe electrode  10  and to the amplifier input A 3 . Connected between the Zener diode D 1  and the resistor R 1  is a further resistor R 3  which is connected to ground. 
     The voltage outputted by the amplifier A is passed to the rectifier circuit  40 , in which case the amplifier&#39;s first supply voltage having the AC voltage portion or a reference potential derived therefrom serves as the reference potential for the output of the amplifier. 
     Furthermore, the amplifier&#39;s first supply voltage having the AC voltage portion is the reference potential for the rectified output voltage outputted by the rectifier circuit  40 . 
     This rectified output voltage is passed to a comparator K 1  which compares the rectified output voltage with an adjustable reference voltage. If the output voltage is lower than the reference voltage, then the voltage supplied by the frequency generator  30  will be outputted as a switching signal, that is to say, in the example, a voltage of 0V or 5V depending on the charge phase P 1  or P 2 . Conversely, if the rectified output voltage is higher than the reference voltage, then the voltage applied at the first supply connection of the amplifier will be outputted as a switching signal, i.e., in the present example, 5V or 10V. The operating direction of the comparator may be reversed, in which case the voltage supplied by the frequency generator  30  will be outputted as a switching signal when the reference voltage is lower than the output voltage of the rectifier. 
     Connected to the output of the comparator K 1  is a signal transmission circuit  45 , which relates the switching signal outputted by the comparator K 1  to ground, that is, 0V. 
     The signal transmission circuit  45  is followed by a second comparator K 2  which compares the switching signal coming from the signal transmission circuit  45  with a threshold value and, depending on the result of the comparison, outputs either a switching signal of 0V or a switching signal having the value of the DC voltage provided by the DC voltage supply U 5 , i.e., in the present case, 5V. This switching signal is passed to an output  50  of the sensor  100 . 
     One advantage of this arrangement is that this switching signal is generated as a function of extremely small changes in the capacitance at the probe electrode. This can be achieved by the sensor of the present invention through the fact that there are no parasitic capacitances acting at the amplifier. To this end, a supply voltage having an AC voltage portion is applied at one of the supply voltage connections of the amplifier, whereas a DC voltage is applied at the other supply voltage connection, since, owing to the design of the amplifier, no parasitic capacitance occurs between this supply voltage connection and the amplifier input. 
     Thus, it is possible to very reliably detect minimal changes of capacitance caused by an object to be detected.