CAPACITIVE PROXIMITY SENSOR

Description

Technical area

[0001] The invention relates to capacitive proximity sensors with a measuring capacitance which is measured in particular by cyclic charging and discharging using a changeover switch. Technical background

[0002] Capacitive proximity sensors feature an electrically conductive probe. A capacitance develops between the probe and the object to be detected, depending on the distance between the probe and the object. For simplicity, the object is assumed to be conductive and grounded. The capacitance, which develops with the distance of the object from the probe, can be measured to obtain an indication of the object's distance.

[0003] In the case of a capacitive proximity switch, a threshold comparison is also carried out with the output of the proximity sensor, in which a state of a switching output of the proximity switch indicates the presence or absence of the object within an area around the probe.

[0004] Capacitive proximity sensors are often operated in switched capacitor mode, where the measuring capacitance is cyclically charged to a defined voltage potential via a switch and then discharged to a further voltage potential. The charging and / or discharging current is taken as a measure of the size of the measuring capacitance. Using a converter, the charging and / or discharging current can be converted into a measuring voltage.

[0005] To switch between charging and discharging the measuring capacitance, a switch is required, which is usually implemented as an integrated component, a so-called analog switch. However, such active, switching components have parasitic capacitances between the switching contacts and the supply voltage terminals. Thus, when the measuring capacitance is recharged, a parasitic capacitance is also charged, increasing the charging and discharging current. This reduces the sensitivity of the proximity sensor, since the parasitic capacitance arranged parallel to the measuring capacitance significantly increases the total capacitance, but the sensitivity of the change in the total capacitance to a change in the distance of the object to be detected decreases.

[0006] The document EP 0 226 082 A1 discloses a capacitance measuring circuit with a switching arrangement which periodically alternately applies the measuring capacitance to a constant voltage for charging at a predetermined switching frequency and connects it to a storage capacitor for discharging, the capacitance of which is large compared to the measuring capacitance and the terminal voltage of which is kept substantially at a constant reference potential by a controlled discharge current, the magnitude of the discharge current being proportional to the measuring capacitance and representing the measured value, wherein a further switching arrangement is provided which periodically alternately applies a shield assigned to the measuring capacitance to potentials which substantially correspond to the constant voltage or the reference potential at the switching frequency.

[0007] The document EP 2 642 663 A1 discloses a sensor for detecting an object, comprising 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 has a first supply terminal and a second supply terminal, electronic means for processing signals output 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, wherein a direct voltage can be connected to the second supply terminal as a second supply voltage, and wherein a first supply voltage with an alternating voltage component can be connected to the first supply terminal.

[0008] Document EP 0 908 736 A2 discloses a circuit arrangement for detecting the capacitance or a change in capacitance of a capacitive circuit or component, comprising a clock generator, a changeover contact controlled by the clock generator, a storage capacitor, a voltage source, and an evaluation stage, in which one electrode of the capacitive circuit or component is connected to the input of the changeover contact. The first output of the changeover contact is connected to the first electrode of the storage capacitor, and the first electrode of the storage capacitor is connected, on the one hand, via a resistor network to the voltage source and, on the other hand, to the evaluation stage. The second electrode of the storage capacitor is connected to a reference potential. The evaluation stage is suitable for current evaluation, so that virtually no voltage swing occurs at the changeover contact.

[0009] The task is therefore to operate a proximity sensor designed for switched capacitor operation as independently as possible from the influences of parasitic capacitances and, in particular, to increase the sensitivity of a proximity sensor with regard to the measuring capacitance. Disclosure of the invention

[0010] This object is achieved by the capacitive proximity sensor according to claim 1.

[0011] Further embodiments are specified in the dependent claims.

[0012] According to a first aspect, a capacitive proximity sensor is provided for detecting an object within a detection range and for providing a sensor output voltage, comprising: an electrically conductive probe; a charging circuit and / or a discharging circuit for cyclically charging a measuring capacitance that develops on the probe during a charging phase to a charging potential or for discharging the measuring capacitance that develops on the probe during a discharging phase to a discharging potential, wherein the charging circuit and / or the discharging circuit are designed to provide a first output voltage in particular as a function of a charging current into the measuring capacitance and / or a second output voltage in particular as a function of a discharging current from the measuring capacitance, wherein the sensor output voltage depends on the orthe output voltages, in particular the first and / or the second output voltage; an active switching device for alternately electrically connecting the probe to the charging circuit and the discharging circuit; a switch supply voltage source designed to cyclically switch switch supply potentials for electrically supplying the active switching device synchronously with the switching between the charging phase and the discharging phase.

[0013] A common design for implementing switched capacitor operation involves switching the measuring capacitance between a charging phase and a discharging phase using an active switching device, particularly in the form of an analog switch. Analog switches are integrated components that have a control input that causes an input terminal to be connected to one of two output terminals, depending on a level at the control terminal. Such analog switches implement their function using transistors, particularly field-effect transistors, and require a supply voltage for their operation.

[0014] By using an analog switch, the measuring probe is connected not only to the input impedance of a downstream amplifier but also to parasitic capacitances in the analog switch. A significant portion of the parasitic capacitances is located between the low or high supply potential of the analog switch and the input terminal of the analog switch, to which the measuring capacitance is connected. Further parasitic capacitances are located between the low or high supply potential of the analog switch and the output terminals of the analog switch.

[0015] These parasitic capacitances reduce the sensitivity of the proximity sensor because they are arranged or act parallel to the measuring capacitance. This increases the charge flow during charging and discharging, so that the portion of the charge flow change attributable to the change in the measuring capacitance decreases with a change in distance. This leads to a reduction in the sensitivity of the proximity sensor to the object's distance.

[0016] For charging and discharging, a charging potential and a discharging potential are alternately applied to the measuring capacitance via the switching device. If the charging potential is applied to the measuring capacitance, the parasitic capacitance present between the input terminal and the low supply potential or the high supply potential of the analog switch must also be charged.

[0017] To prevent this, the potentials of the high and low supply potentials for the switching device are varied according to the switching operation of the switching device, thus reducing the charge-transfer currents. In particular, when the charging potential is applied to the input terminal to charge the measuring capacitance, the potentials of the supply voltage of the switching device can be changed (raised), preferably by the difference between the charging potential and the discharging potential. In contrast, when the discharging potential is applied to the input terminal to discharge the measuring capacitance, the supply potentials for the switching device can be changed (lowered) back to their original potential levels.

[0018] Thus, across the parasitic capacitance between the input terminal of the analog switch, i.e. the terminal connected to the measuring capacitance, and the low supply potential and / or the high supply potential of the switching device, regardless of the switching state, there is a reduced voltage which causes a reduced charge reversal current if the raising and lowering of the supply potentials of the analog switch occurs synchronously with the switching through the charge state and discharge state of the measuring capacitance.

[0019] By raising and lowering the low supply potential and the high supply potential of the switching device in synchronization with the switching between the charging and discharging phases, the effect of parasitic capacitances is largely eliminated, as any change in the voltage across the parasitic capacitances is reduced or completely prevented. This reduces or eliminates the current flow through the parasitic capacitance, and it is no longer included, or only to a reduced extent, when converted into an output voltage by the downstream converter, which converts the charging and / or discharging current from the measuring capacitance into a sensor output voltage.

[0020] Furthermore, the active switching device can comprise an active electronic circuit formed with transistors, in particular field-effect transistors, and in particular designed as an analog switch. The switching device can be implemented with two individual switches that are switched alternately or with a single analog switch.

[0021] According to one embodiment, one of the switch supply potentials or a signal derived from one of the switch supply potentials can be used to control the switching device.

[0022] It can be provided that the charging circuit has a charging capacitance and a first transimpedance amplifier which is designed to provide a charging potential for charging the measuring capacitance at least during the charging phase and to provide the corresponding output voltage depending on a charging current into the measuring capacitance and / or wherein the discharging circuit has a discharging capacitance and a second transimpedance amplifier which is designed to provide a discharging potential for discharging the measuring capacitance at least during the discharging phase and to provide the corresponding output voltage depending on a discharging current from the measuring capacitance.

[0023] In particular, the charging and discharging capacitances may each have a value that is greater than the typical capacitance values ​​of the measuring capacitance, in particular the charging and discharging capacitances have at least ten times the capacitance value of the measuring capacitance when the object is as close as possible to the probe.

[0024] Furthermore, the switch supply voltage source can be connected to the charging circuit and / or the discharging circuit in order to supply them with electrical energy using the switch supply potentials.

[0025] According to one embodiment, the charging capacitance may provide the charging potential with respect to a switch supply potential and / or the discharging capacitance may provide the discharging potential with respect to the switch supply potential.

[0026] Furthermore, a series resistor can be connected in series with the probe. This limits the current into or out of the measuring capacitance, thus increasing the charging time of the measuring capacitance during cyclic switching by the switching device. This has the advantage of avoiding current peaks during cyclic switching of the proximity sensor.

[0027] Furthermore, the charging potential and / or the discharging potential can be generated depending on the switch supply potential.

[0028] A reference potential source may be provided which cyclically changes between two potential levels, wherein the switch supply potential and / or the charging potential and / or the discharging potential is generated depending on the reference potential or with the aid of the reference potential.

[0029] In particular, the charging and discharging potential can be between the positive and negative switch supply potential.

[0030] According to one embodiment, a signal delay element may be provided to take into account a switching delay of the switching device.

[0031] According to a further aspect, a proximity switch is provided with the above proximity sensor and a comparator connected thereto, which specifies a switching threshold so that the presence or absence of the object can be signaled depending on whether the switching threshold is exceeded by the sensor output voltage. Brief description of the drawings

[0032] Embodiments are explained in more detail below with reference to the attached drawings. They show: Figure 1 shows a schematic representation of a sensor system with a proximity switch; Figure 2 shows a circuit diagram of a proximity sensor with an active switching device according to a first embodiment; Figure 3 shows a circuit diagram of a proximity sensor according to a further embodiment; Figure 4 shows a circuit diagram of a proximity sensor according to a further embodiment. Description of embodiments

[0033] Figure 1 shows a schematic representation of a proximity switch 1 for detecting a conductive, preferably grounded object 2 within a detection area E with an electrically conductive probe 3. The probe 3 is coupled to a proximity sensor 5, which outputs a sensor output voltage V Sens with respect to the object 2 located in the detection area E and its distance from the probe 3.

[0034] The output of the proximity sensor 5 can be coupled to a comparator 6, which specifies a switching threshold Vt. Depending on whether the sensor output voltage V Sens exceeds the switching threshold Vt, the presence or absence of the object 2 in an environmental area specified by the switching threshold Vt can be signaled by a switching signal S. The switching signal S serves to control a switch 7 to close or open an electrical circuit depending on the switching signal.

[0035] The proximity sensor 5 is shown as a circuit diagram in Figure 2This essentially comprises an active switching device 11 as well as a charging potential unit and a discharging potential unit. The proximity sensor 5 operates in switched capacitor mode, in which charging and / or discharging currents into and / or out of a measuring capacitance that is established between the probe 3 and the object 2 are measured. The resulting charging and / or discharging current corresponds to a measure of the size of the measuring capacitance, which is representative of the distance of the object from the probe.

[0036] The active switching device 11 is designed, for example, as an analog switch. An analog switch represents an integrated component that switchably connects an input terminal ES to one of several output terminals AS1, AS2. In the present case, the active switching device 11 is designed in the form of a changeover switch for connecting the input terminal ES to one of two output terminals AS1, AS2. Furthermore, the active switching device 11 has a first and a second supply potential terminal SV1, SV2 for applying a high and a low switch supply potential V vers1 , V vers2 , respectively. The switching position of the switching device 11 is set depending on a level at a control terminal SA.

[0037] The active switching device 11 has an active electronic circuit 11a formed with transistors, in particular field-effect transistors. The active circuit 11a is supplied with electrical energy via the supply potential terminals SV1, SV2 by the high switch supply potential V vers1 and the low switch supply potential V vers2 with a switch supply voltage. The high switch supply potential V vers1 and the low switch supply potential V vers2 are provided by a switch supply voltage source 20. The specific structure of the active circuit 11a of the switching device 11 will not be discussed in detail here.

[0038] However, the active circuit 11a of the switching device 11 is connected to the input terminal ES and the output terminals AS1, AS2 in such a way that parasitic capacitances CP1, CP2, CP3 act between these terminals ES, AS1, AS2 and the second supply potential terminal SV2. Further parasitic capacitances exist between the terminals ES, AS1, AS2 and the first supply potential terminal SV1. The effect of these further parasitic capacitances is identical to the above-mentioned parasitic capacitances with respect to the second supply potential terminal SV2. For simplicity, the further parasitic capacitances are not shown in the figures.

[0039] The first output terminal AS1 of the active switching device 11 is connected to a first terminal of a charging capacitor CH1, and a low supply potential V general2 is applied to a second terminal of the charging capacitor CH1. The second output terminal AS2 of the active switching device 11 is connected to a first terminal of a discharging capacitor CH2, and the low supply potential V general2 is applied to a second terminal of the discharging capacitor CH2.

[0040] The charge and discharge capacitances CH1, CH2 each have a value that is significantly larger than the typical values ​​of the measuring capacitance CM when the object is as close as possible to the probe 3. In particular, the charge and discharge capacitances CH1, CH2 have at least ten times the capacity of the measuring capacitance CM.

[0041] The proximity sensor 5 is operated cyclically. For this purpose, the active switching device 11 is alternately switched according to a cycle frequency f. The cycle frequency f is typically in the range of several tens of kHz, in particular 50 kHz, to several MHz, such as up to 10 MHz. For this purpose, a cyclically varying control potential V ST is applied to the control terminal of the switching device 11.

[0042] The first output terminal AS1 is further connected to a charging circuit 13, which is preferably configured with a first transimpedance amplifier 13a. The first transimpedance amplifier 13a enables the provision of a charging voltage V lad . The charging voltage V lad is applied to a charging capacitor CH1, which buffers the cyclic charging processes. Furthermore, when the measuring capacitor CM is switched to the first output terminal AS1, the measuring capacitor CM can be charged by the charging capacitor CH1. The first transimpedance amplifier 13a converts this charging current I1 into a corresponding output voltage dependent thereon.

[0043] The first transimpedance amplifier 13a can comprise an operational amplifier, to whose non-inverting terminal the setpoint V set1 of the charging voltage V charge is applied. This can be generated, for example, by a voltage divider. The inverting terminal of the operational amplifier is connected to the first output terminal AS1. A resistor R1 is connected between the output of the operational amplifier and the inverting terminal. R1 sets the factor by which the charging current is converted into the output voltage V(I1).

[0044] The second output terminal AS2 is further connected to a discharge circuit 14, which is preferably configured with a second transimpedance amplifier 14a that provides a discharge potential V discharge. As a result, when the measuring capacitance CM is connected to the second output terminal AS2, the measuring capacitance CM can be discharged into the discharge capacitance CH2, so that a voltage dependent on the discharge current I2 is present at the output of the second transimpedance amplifier 14a.

[0045] The second transimpedance amplifier 14a may comprise an operational amplifier, to whose non-inverting terminal a setpoint value V set2 of the discharge potential V discharge is applied. The inverting terminal of the operational amplifier is connected to the second output terminal AS1. A resistor R2 is connected between the output of the operational amplifier and the inverting terminal. The resistor R2 adjusts the factor by which the discharge current is converted into the output voltage V(I2).

[0046] The operational amplifiers of the transimpedance amplifiers 13a,14a are supplied with electrical energy via the high supply potential V allg1 and the low supply potential V allg2.

[0047] In the present proximity sensor 5, the sensor output voltage Vout (which can correspond to V Sens) results from a voltage difference between the outputs V(I1)-V(I2) of the transimpedance amplifiers 13a, 14a, depending on the magnitude of the charging current I1 and the discharging current I2. The charging current corresponds to I1=V charge x CM x f. The discharging current corresponds to I2=--V discharge x CM x f. By forming the difference between the output voltages of the transimpedance amplifiers 13a, 14a, the sensitivity to changes in the measuring capacitance can be improved.

[0048] Through the use of the transimpedance amplifiers 13a, 14a, the charging and discharging capacitances CH1, CH2 are each maintained at voltage potentials specified by the transimpedance amplifier. Thus, the first transimpedance amplifier 13a applies the charging voltage V charge to the charging capacitance CH1 and maintains it at this voltage level during the charging phase. Similarly, the second transimpedance amplifier 14a maintains the voltage level at the second discharging capacitance CH2 at the discharging potential V discharge and maintains it during the discharging phase.

[0049] The transimpedance amplifiers 13a, 14a are particularly suitable for use in such a proximity sensor 5 since, on the one hand, they can specify a charging and discharging potential and, on the other hand, can provide an output voltage that is proportional to a current flow into or out of the measuring capacitance.

[0050] According to the invention, the switch supply potentials V vers1 , V vers2 are generated using a square-wave signal V REF,f , so that the switch supply potentials V vers1 , V vers2 are raised or lowered according to the switching cycle. The switching cycle can be specified by a control signal. To control the switching device 11, the square-wave signal V REF,f can correspond to or deviate from the control potential V ST . Alternatively, the square-wave signal can also be generated depending on a specified control potential to control the switching device 11.

[0051] The input terminal ES can be connected to the first output terminal AS1 when the control potential V ST is high, and the input terminal ES can be connected to the second output terminal AS2 when the control potential is low. The low switch supply potential V vers2 is preferably raised and lowered cyclically between 0 V and V REF,f.

[0052] Raising and lowering the switch supply potentials V vers1 , V vers2 synchronously with the switching between the charging phase and discharging phase makes it possible to significantly reduce the influence of the parasitic capacitances in the switching device 11, since these do not need to be recharged, or only to a reduced extent, by the charging current I1 or discharging current I2. This has the advantage that the effective measuring capacitance is not increased, or only to a reduced extent, by the parasitic capacitances, so that the sensitivity of the proximity sensor 5 is only affected to a reduced extent by the parasitic capacitances CP1, CP2, CP3 of the switching device 11.

[0053] As in Figure 3 shown, in addition to the switch supply potentials V vers1 , V vers2 , the supply potentials V allg1 , V allg2 (reference symbol V allg1 therefore no longer in Figure 3The transimpedance amplifiers 13, 14 and the operational amplifiers (shown in the figure) are supplied with electrical energy via the cyclically switched supply potentials V vers1 , V vers2 . This ensures that the transimpedance amplifiers 13a, 14a output essentially DC voltages related to the supply potentials V vers1 , V vers2 .

[0054] Furthermore, the low or high switch supply potential can be applied to the terminals of the charging and discharging capacitances CH1, CH2 that are not connected to the output terminals of the switching device 11.

[0055] In addition, as in the embodiment of the Figure 3As shown, a series resistor RV may be provided in series with the measuring capacitance CM. This limits the current from the measuring capacitance CM, so that the recharging time of the measuring capacitance CM is increased during cyclic switching by the switching device 11. The resistor RV may be dimensioned with respect to the switching frequency of the switching device 11 such that the measuring capacitance CM is within T = 1 2 f is essentially recharged. This has the advantage of avoiding current peaks during cyclic switching of the proximity sensor 5, which has particular advantages with regard to the EMC behavior of the circuit of the proximity sensor 5.

[0056] The cyclical raising and lowering of the supply potentials V general1 , V general2 can be achieved by applying a square-wave voltage signal V REF,f from a reference voltage source 16 to a supply voltage source 15. This square-wave voltage signal V REF,f can be used as a control potential for the switching device 11 and, at the same time, raise and lower the supply potentials V general1 , V general2 accordingly to generate the switch supply potentials V vers1 , V vers2 . At the same time, the charging potential V charge and the discharging potential V discharge can be raised or lowered accordingly by applying the reference voltage source 16 to them. The charging potential V charge and the discharging potential V discharge can be identical or different from one another.Thus, the setpoint V set1 for the charging potential V charge is generated by raising the supply voltage potential V supply2 by a predetermined offset potential V offset, which in turn is raised in the charging phase by the reference voltage source 16 when the measuring capacitance is connected to the charging capacitance CH1. When the measuring capacitance CM is connected to the discharging capacitance CH2 in the discharging phase, the discharging potential at the non-inverting terminal of the operational amplifier of the second transimpedance amplifier 14a can correspond only to the offset potential V offset by lowering the square-wave voltage signal of the reference voltage source 16 to 0V. The offset potential V offset can be provided for both transimpedance amplifiers 13a, 14a using the voltage divider of resistors R3, R4. Thus, the setpoint V set2 for the discharging potential V discharge is generated in the same way as the setpoint V set1 for the charging potential V charge.

[0057] As the switch supply potentials V vers1 and V vers2 are raised and lowered, the potentials of the parasitic capacitances CP2 and CP3 are also raised and lowered. With each raise and lowering, a current flows between the parasitic capacitance CP2 and the discharge capacitance CH2, as well as between the parasitic capacitance CP3 and the discharge capacitance CH2. These currents can be avoided by relating the holding capacitances CH1 and CH2 to the rising and falling supply potentials of the switching device 11.

[0058] To ensure that the transimpedance amplifiers 13a, 14a can be controlled positively and negatively, the potentials at the non-inverting terminals should lie between their high and low switch supply potentials V vers1 , V vers2 . This can be achieved, for example, with the voltage divider formed by the resistors R3 and R4, which is connected between the high and low switch supply potentials of the transimpedance amplifiers 13a, 14a. If the high and low switch supply potentials are applied to the transimpedance amplifiers 13a, 14a, the corresponding charging potential V lad and discharging potential V entlad rise and fall accordingly by the voltage swing specified by the square wave signal V REF,f of the reference voltage source 16.

[0059] With reference to the embodiment shown in the circuit diagram of Figure 4As shown, a signal delay element 21 can be provided to take into account a switching delay of the switching device 11. As a result, a time delay can be provided between the signaling of a switchover between the charging phase and the discharging phase and the provision of the cyclic potential change of the switch supply potential V vers1 , V vers2 complete switching of the switching device 11. List of reference symbols

[0060] 1 Proximity switch 2 Object 3 Probe 5 Proximity sensor 6 Comparator 7 Switch 11 Active switching device 12 Active electronic circuit 13 Charging circuit 13a First transimpedance amplifier 14 Discharge circuit 14a Second transimpedance amplifier 15 Supply voltage source 16 Reference voltage source 20 Switch supply voltage source 21 Signal delay element Vt Switching threshold V Sens , V out Sensor output voltage ES Input terminal AS1, AS2 Output terminals SV1, SV2 First and second supply potential terminal V vers1 , V vers2 High and low switch supply potential SA Control terminal CP1, CP2, CP3 Parasitic capacitances CH1, CH2 Charging and discharging capacitance CM Measuring capacitance R1- R4 Resistors V REF,f Square wave signal V ST Control potential V general1 , V general2 High and low supply potential V offset Offset potential I1Charging current I2Discharging current

Claims

1. Capacitive proximity sensor (5) for detecting an object (2) within a detection range (E) and for providing a sensor output voltage (VSens, Vout), comprising: - an electrically conductive probe (3); - a charging circuit (13) for cyclically charging a measuring capacitance (CM) established at the probe (3) to a charging potential (Vlad) during a charging phase, and a discharging circuit (14) for cyclically discharging the measuring capacitance (CM) established at the probe (3) (CM ) during a discharge phase to a discharge potential (Ventlad), wherein the charging circuit (13) and the discharging circuit (14) are designed to provide a first output voltage ( V(I1) ) dependent on a charging current (11) in the measuring capacitance (CM) and a second output voltage ( V(I2) ) dependent on a discharge current (I2) from the measuring capacitance (CM), wherein the sensor output voltage (VSens) is provided dependent on the first and second output voltages ( V(I1), V(I2) ); - an active switching device (11) for alternately electrically connecting the probe (3) to the charging circuit (13) and the discharging circuit (14); - a switch supply voltage source (20) which is designed to switch switch supply potentials (Vvers1, Vvers2) for the electrical supply of the active switching device (11) cyclically in synchronism with the switching between the charging phase and the discharging phase.

2. Proximity sensor (5) according to claim 1, wherein the active switching device (11) has an active electronic circuit (12) which is designed with transistors, in particular field-effect transistors, and is designed in particular as an analog switch.

3. Proximity sensor (5) according to one of claims 1 to 2, wherein one of the switch supply potentials (Vvers1, Vvers2) or a signal (VST) derived from one of the switch supply potentials (Vvers1, Vvers2) is used to control the switching device (11).

4. Proximity sensor (5) according to one of claims 1 to 3, wherein the charging circuit (13) has a charging capacitance (CH1) and a first transimpedance amplifier (13a) which is designed to provide a charging potential (Vlad) for charging the measuring capacitance (CM) at least during the charging phase and to supply the corresponding first output voltage (V(I1)) to the measuring capacitance (CM) as a function of a charging current (I1), and wherein the discharge circuit (14) has a discharge capacitance (CH2) and a second transimpedance amplifier (14a) which is designed to provide a discharge potential (Ventlad) for discharging the measuring capacitance (CM) at least during the discharge phase and to provide the corresponding second output voltage ( V(I2) ) from the measuring capacitance (CM) as a function of a discharge current (I2).

5. Proximity sensor (5) according to claim 4, wherein the charging and discharging capacitances (CH1, CH2) each have a value that is greater than the typical capacitance values of the measuring capacitance (CM) when the probe is as close as possible to the object (2), wherein, in particular, the charging and discharging capacities (CH1, CH2) have at least ten times the capacity value of the measuring capacity (CM).

6. Proximity sensor (5) according to one of claims 3 to 5, wherein the switch supply voltage source (20) is connected to the charging circuit (13) and / or the discharging circuit (14) in order to supply these with electrical energy at the switch supply potentials (Vvers1, Vvers2).

7. Proximity sensor (5) according to one of claims 4 to 6, wherein the charging capacity (CH1) provides the charging potential (Vlad) relative to a switch supply potential (Vvers1, Vvers2).

8. Proximity sensor (5) according to one of claims 1 to 7, wherein a series resistor (RV) is connected in series with the probe (3).

9. Proximity sensor according to one of claims 1 to 8, wherein the charging potential (Vlad) and / or the discharging potential (Ventlad) is generated depending on the switch supply potential (Vvers1, Vvers2).

10. Proximity sensor according to one of claims 1 to 9, wherein a reference potential source (16) is provided which cyclically switches between two potential levels, wherein the switch supply potential (Vvers1, Vvers2) and / or the charging potential (Vlad) and / or the discharging potential (Ventlad) is generated depending on the potential of the reference potential source (16) or with the aid of the reference potential source (16).

11. Proximity sensor (5) according to claim 10, wherein the charging and discharging potentials (Vlad, Ventlad) lie between the positive and negative switch supply potentials (Vvers1, Vvers2).

12. Proximity sensor (5) according to one of claims 1 to 11, wherein a signal delay element is provided to take into account a switching delay of the switching device (11).

13. Proximity switch (1) with a proximity sensor according to one of claims 1 to 12 and a comparator connected thereto, which specifies a switching threshold (Vt) such that, depending on the switching threshold (Vt) being exceeded by the sensor output voltage (VSens), the presence or absence of the object (2) can be signaled.