Capacitive proximity sensor for a motor vehicle

Abstract

Capacitive proximity sensor for a motor vehicle, comprising a sensor electrode (1), wherein changes in capacitance (3; C) S ) the sensor electrode (1) relative to a surface (4A) lying on a reference potential (4), wherein the capacitive proximity sensor has: a first circuit node (5) coupled to the sensor electrode (1), which can be coupled via a first switching device (8) either to a fixed voltage potential (14) or to the input (11) of a measuring circuit (10), wherein the first switching device (8; 8', 8") then, when it couples the first circuit node (5) to the input (11) of the measuring circuit (10), simultaneously an integrator capacitor (13; C) L ) couples in between the first circuit node (5) and the reference potential (4), wherein the measuring circuit (10) serves to determine a voltage applied across the integrator capacitance (13), a coupling capacity (7; C K ), which is coupled between the sensor electrode (1) and a second circuit node (6), wherein the second circuit node (6) can be coupled via a second switching device (9) either to the fixed voltage potential (14) or to the reference potential (4), a control circuit (15) that controls the first switching device (8; 8', 8") and the second switching device (9) at least approximately synchronously such that when the first switching device (8; 8', 8") couples the first circuit node (5) with the fixed voltage potential (14), the second switching device (9) does not couple the second circuit node (6) with the reference potential (4), and when the first switching device (8) couples the first circuit node (5) with the measuring circuit (10), the second switching device (9) does not couple the second circuit node (6) with the fixed voltage potential (14), wherein the control circuit (15) repeatedly switches the first and second switching devices (9, 8; 8', 8") until the voltage across the integrator capacitance (13) has at least approximately reached a steady state, and thereafter causes the measuring circuit (10) to measure the voltage across the integrator capacitance (13) or its change relative to a reference value as a measure of the capacitance (3; C). S ) of the sensor electrode (1) or its change.

Description

[0001] The invention relates to a capacitive proximity sensor for a motor vehicle with a sensor electrode, in which changes in the capacitance of the sensor electrode relative to a surface lying at a reference potential are to be detected.

[0002] Such capacitive proximity sensors are used particularly in automatic door or tailgate operating devices in motor vehicles, where the approach of a body part (hand or foot) of an operator is to be detected in order to trigger an opening or closing process depending on this.

[0003] In a known capacitive proximity sensor of this type, the sensor electrode is coupled, for example, to a first port of a microcontroller. Furthermore, a capacitor is connected to the sensor electrode and coupled to a second port of the microcontroller. The capacitance of the capacitor is chosen to be many times greater than the capacitance of the sensor electrode relative to the reference potential. The capacitance of the sensor electrode relative to the reference potential, and in particular its changes, are then measured by a charging and discharging process as follows. In the first part of a switching cycle, the sensor electrode is coupled to an operating voltage (or another fixed voltage) via the first port of the microcontroller, so that the capacitance of the sensor electrode charges up to the operating voltage.In this phase, the second port is in a high-impedance state, preventing any charge from flowing away from the coupled capacitor. In a subsequent second phase of the switching cycle, the first port is switched to a high-impedance state, preventing any charge from flowing through it; simultaneously, the second port is connected to ground. This discharges most of the charge accumulated on the sensor electrode's capacitance to the capacitor. This switching cycle is then repeated several thousand times, with increasing charge accumulating on the capacitor and the voltage across it gradually approaching the voltage to which the sensor electrode's capacitance was charged in the first phase of the cycle. To measure the capacitance, a threshold voltage value below the maximum voltage is set, and then the time or...The number of cycles is measured until the voltage across the capacitor exceeds this threshold. The voltage across the capacitor reaches the threshold sooner the greater the capacitance of the sensor electrode relative to the reference potential. To verify that the voltage threshold has been reached, the input of a comparator is connected to the first port of the microcontroller during or after the second part of the switching cycle (the input must, of course, have a very high impedance to prevent charge loss).

[0004] It has been shown that this known sensor arrangement can no longer detect changes in the capacitance of the sensor electrode with sufficient accuracy under certain operating conditions that can occur during vehicle operation. For example, if the sensor electrode is installed in the area of ​​a vehicle's rear bumper to detect the approach of an operator's foot and, based on this foot movement, trigger the operation of the tailgate, then, for instance, if the sensor's surroundings are heavily contaminated by moisture (water and ice) and dirt (especially salts), the approach of the foot and the associated change in capacitance can no longer be reliably detected.This means that the number of switching cycles until the threshold voltage value is reached when the operator's foot approaches does not vary sufficiently under these environmental conditions to reliably distinguish it from interference (noise).

[0005] Document WO 2010 / 045662 A2 discloses a capacitive measurement method based on repeated charge transfer. A sensor capacitor is periodically charged to a reference voltage; its charge is then transferred to a reference capacitor. The number of transfer cycles required to reach a predetermined reference voltage is proportional to the sensor capacitance. Parasitic capacitances are neutralized in each cycle by a pre-charged compensation capacitor of the same size but opposite polarity. Furthermore, the document describes a current mirror circuit that reflects the current generated during charge dissipation with a selectable mirror factor and directs it linearly away from the reference capacitor, thus enabling scalable and linearized evaluation in integrated circuits.

[0006] The object of the invention is to improve the capacitive proximity sensor so that even under difficult environmental conditions small changes in the capacitance of the sensor electrode can be detected more reliably.

[0007] This problem is solved according to the invention by a capacitive proximity sensor for a motor vehicle with the features of claim 1.

[0008] The capacitive proximity sensor according to the invention for a motor vehicle comprises a sensor electrode, wherein changes in the capacitance of the sensor electrode relative to a surface at a reference potential are to be detected. The reference potential is, for example, the earth potential or ground potential, in particular the potential of the vehicle body. The capacitive proximity sensor has a first circuit node coupled to the sensor electrode, which can be coupled via a switching device either to a fixed voltage potential (for example, the supply voltage potential) or to the input of a measuring circuit (which is, for example, arranged in a microcontroller).The first switching device, when it couples the first circuit node to the input of the measuring circuit, simultaneously couples an integrator capacitance between the first circuit node and the reference potential (for example, a capacitor forming the integrator capacitance can be coupled either between the first circuit node and a switch coupled to ground, or between a switch coupled to the first circuit node and ground). The measuring circuit serves to detect a voltage applied across the integrator capacitance. According to the invention, the capacitive proximity sensor further comprises a coupling capacitance that is coupled between the sensor electrode and a second circuit node, wherein the second circuit node can be coupled via a second switching device either to the fixed voltage potential or to the reference potential.Furthermore, a control circuit is provided that controls the first switching device and the second switching device at least approximately synchronously such that when the first switching device couples the first circuit node to the fixed voltage potential, the second switching device does not couple the second circuit node to the reference potential, and when the first switching device couples the first circuit node to the measuring circuit, the second switching device does not couple the second circuit node to the fixed voltage potential. The control circuit then switches the first and second switching devices repeatedly until the voltage across the integrator capacitance has reached at least an approximate steady state. Afterward, it causes the measuring circuit to measure the voltage across the integrator capacitance, or its change relative to a reference value, as a measure of the capacitance of the sensor electrode.to determine their modification.

[0009] The invention is based on the finding that when a coupling capacitance is connected to the sensor electrode and its switching between the reference potential and the fixed voltage potential in the steady state is at least approximately synchronous, a charge distribution takes place such that the voltage across the integrator capacitance, after several switching cycles, finally approaches a value that represents a measure of the capacitance of the sensor electrode.The term "approximately synchronous" control here refers not only to fully synchronous control but also to a slightly offset control of the two switching devices (e.g., due to technological tolerances), in which it is ensured that when the first switching device switches in such a way that it couples the first circuit node with the fixed voltage potential, the second switching device either keeps the second circuit node at high resistance (not coupled to any potential) or also couples it with the fixed voltage potential, but in any case does not couple it with the reference potential, because this would result in the coupling capacitance being charged to the full voltage difference.On the other hand, the slightly offset synchronous switching ensures that when the first switching device switches in such a way as to couple the first circuit node to the measuring circuit, the second switching device either still holds the second circuit node in isolation or already couples it to ground potential, and in any case does not couple it to the fixed voltage potential. The control circuit then switches repeatedly until the voltage across the integrator capacitance has at least approximately reached a steady state. Since the voltage curve asymptotically approaches a steady state, this state is—strictly speaking—not actually reached. Therefore, an approximately steady state should be understood as one in which further changes in the applied voltage remain within a permissible tolerance.The number of switching cycles required can be predetermined, for example, based on knowledge of the circuit's behavior. This can be achieved by selecting a number after which further voltage changes are known to be negligible within the desired measurement accuracy. Alternatively, the voltage changes between successive cycles can be monitored, and switching can be stopped and / or the last measured value can be used if a predetermined low threshold is undershot.

[0010] In one embodiment, the coupling capacitance could, for example, be formed by a capacitor element coupled between the first and second circuit nodes. However, a preferred embodiment is characterized in that the coupling capacitance is a capacitance that forms between the sensor electrode and a coupling electrode arranged at a fixed distance adjacent to the sensor electrode. The coupling electrode can, for example, be a shielding electrode. A further preferred embodiment is characterized in that the sensor electrode is an outer conductor of a coaxial line and the coupling electrode is an inner conductor of this coaxial line. This embodiment is used in particular in a capacitive proximity sensor for a tailgate opening mechanism and is arranged in the area of ​​the rear bumper below the tailgate, transversely across the entire width or part of the tailgate.Such a capacitive sensor arrangement is intended to detect the movement of an operator's foot in the longitudinal direction of the vehicle towards and below the rear apron.

[0011] One embodiment of the capacitive proximity sensor is characterized in that the integrator capacitance is formed by a capacitor, one terminal of which is connected to the first circuit node, and that the first switching device comprises a first switch and a second switch, wherein the first switch couples the first circuit node either to the fixed voltage potential or to the input of the measuring circuit, and wherein the second switch either leaves the other terminal of the capacitor open or couples it to the reference potential. In this embodiment, it is preferably provided that the second switching device, as well as the first switch and the second switch of the first switching device, are each formed by a port of a microcontroller containing the measuring circuit, in particular by electronic switches (for example, FETs) of these ports.Each of these ports is preferably capable of coupling the connected terminal either to the fixed voltage potential (e.g., operating voltage) or the reference potential (e.g., ground) and / or to the input of an evaluation circuit, or of leaving it open with a high impedance. This embodiment has the advantage of being inexpensive, since, in addition to the microcontroller, only an external capacitor (and further input protection circuits not described here) are required. Disadvantages of this embodiment include parasitic capacitances of the microcontroller ports and its switching behavior, such as the switching delays that must be considered when programming the switching functions, which can vary from port to port.

[0012] In a preferred embodiment, the integrator capacitance is formed by a capacitor coupled between the input of the measuring circuit (coupled to the first switching device) and the reference potential, or by an RC network configured as a low-pass filter and coupled between the first switching device and the input of the measuring circuit. In this embodiment, the first switching device connects the first circuit node either to the fixed voltage potential, to the input of the measuring circuit (and the capacitor coupled to ground), or to the input of the RC network. The first and second switching devices are then controlled approximately synchronously such that they connect either the two circuit nodes to the fixed voltage potential or the first circuit node to the input of the measuring circuit and the second circuit node to the reference potential (ground potential).In this embodiment, it is preferably provided that both the first and the second switching device are designed as analog switch components with a break-beforemake characteristic, meaning that they first decouple the respective circuit node from one potential before coupling it to the other potential. The approximately synchronous control of the two analog switch components then occurs such that both circuit nodes are always first decoupled from the preceding potential before one or both circuit nodes are coupled to the subsequent potential.

[0013] In a preferred embodiment of the proximity sensor according to the invention, the switching operations of the switching devices initiated by the control circuit are performed with a period that is at least 10 times longer than the time constant for charging the capacitance formed by the sensor electrode when coupled to the fixed voltage potential. In a typical arrangement, the sensor electrode has, for example, a capacitance relative to the reference potential on the order of 10 pF to 100 pF. This capacitance is coupled to the fixed voltage potential via the first switching device, for example, with a coupling resistor on the order of a few ohms. The time constant for charging the sensor capacitance is therefore extremely short (for example, in the nanosecond to picosecond range).The switching period is at least 10 times longer, preferably at least 100 times longer, so that in each sub-section of the switching cycle the charging and discharging processes of the sensor capacity are largely or completely completed before the next switching begins.

[0014] In a preferred embodiment, the switching operations of the switching devices, initiated by the control circuit, are performed at a frequency between 10 kHz and 100 MHz, preferably at a frequency between 100 kHz and 10 MHz. In a typical embodiment, switching occurs with a period of 1 MHz. At such switching frequencies, it can be assumed that the charging and discharging processes are completed well before half the period has elapsed.

[0015] Preferably, the capacitive proximity sensor is characterized in that the number of switching operations of the switching devices initiated by the control circuit before determining the voltage across the integrator capacitance depends on the ratio of the integrator capacitance to the capacitance of the sensor electrode to be measured relative to the reference potential, wherein the integrator capacitance is dimensioned such that the voltage across the integrator capacitance reaches approximately a steady state after about 10 to 1000 switching operations, preferably after a few hundred switching operations, i.e., further changes in the voltage are below a negligible tolerance threshold.

[0016] In the capacitive proximity sensor according to the invention, a significantly reduced measurement time can also be achieved by using an integrator capacitance that is lower than the capacitance of the capacitor used in the known circuit. The number of required switching cycles is no longer several thousand, but, for example, several hundred cycles (at comparable switching frequencies of, for example, 1 MHz).

[0017] Advantageous and / or preferred embodiments of the invention are characterized in the dependent claims.

[0018] The invention will now be explained in more detail with reference to preferred embodiments illustrated in the drawings. The drawings show: Fig. 1 a schematic diagram of a first embodiment of the capacitive proximity sensor according to the invention; Fig. 2 a schematic diagram of a second embodiment of the capacitive proximity sensor according to the invention; Fig. 3 a circuit diagram of an embodiment of the first embodiment of the capacitive proximity sensor according to the invention; and Fig. 4 A schematic representation to illustrate the voltage conditions at the input of the measuring circuit that occur during the charging interval.

[0019] Fig. Figure 1 shows a schematic diagram of a first embodiment of the capacitive proximity sensor according to the invention for a motor vehicle with a sensor electrode 1, whose capacitance C SThe capacitance 3 is to be detected relative to a surface 4A lying at a ground potential 4. The capacitance 3, shown with a dashed line, is not a real circuit element but merely illustrates the capacitance that develops between the sensor electrode 1 and the ground potential 4. A coupling electrode 2 is arranged adjacent to the sensor electrode 1. In one embodiment, the sensor electrode 1 can be a flat plate or metal layer, and the coupling electrode 2 can be a spaced-apart metal plate or metal layer arranged parallel to it. In a preferred embodiment, in which, for example, a commercially available coaxial cable can be used, the outer conductor of the coaxial cable serves as the sensor electrode 1, while the inner coaxial conductor serves as the coupling electrode 2. An approximately fixed coupling capacitance C forms between the sensor electrode 1 and the coupling electrode 2. K out. This coupling capacity CK is in Fig. 1 illustrated by the imaginary capacitor element 7 shown with a dashed line.

[0020] The sensor electrode 1 is coupled to a first circuit node 5. The circuit node 5 is connected to a first switching device 8, which can connect the circuit node 5 either to an operating voltage potential U0 14 or to the input 11 of a measuring circuit 10. The first switching device 8 is of a type that, when switching from one contact to the other, briefly decouples the first circuit node 5 from both contacts. The coupling electrode 2 is coupled to a second circuit node 6, which in turn is connected to a second switching device 9. The second switching device 9 is of the same type as the first switching device and can selectively connect the circuit node 6 to either the operating voltage potential 14 or to the ground potential 4.The first switching device 8 and the second switching device 9 are coupled to a control circuit 15 in such a way (the coupling is indicated by the dashed double line 16) that they can be switched essentially synchronously, wherein in a first switching state both the first circuit node 5 and the second circuit node 6 are connected to the operating voltage potential 14, while in a second switching state the first circuit node 5 is coupled to the input 11 of the measuring circuit 10, while the second circuit node 6 is connected to the ground potential 4.

[0021] A capacitor 13, referred to here as an integrator capacitance, is coupled between input 11 of the measuring circuit 10 and ground potential 4. Alternatively, it could also be called a collector capacitance, as will be explained in the following functional description.

[0022] The measuring circuit 10 serves to detect the voltage across the integrator capacitor 13 at the input 11. The measuring circuit 10 can, for example, include an analog-to-digital converter that converts the voltage measured at the input 11 into a digital value when a corresponding control command is transmitted by the control circuit 15. This digital value can then be output at the output 12. The coupling of the measuring circuit 10 with the control circuit 15 is illustrated by the double-dash line 17. In an alternative embodiment, the measuring circuit 10 could also simply detect whether a predetermined minimum change has occurred compared to a previous measurement of the voltage drop across the capacitor 13.For this purpose, the measuring circuit 10 could, for example, contain a comparator whose one input is coupled to the input 11 of the measuring circuit, while the other input is supplied with a reference voltage, which could, for example, be a voltage derived from the previous measurement. For example, both the control circuit 15 and the measuring circuit 10 are part of a microcontroller. Those skilled in the art will know how the above-mentioned different measurement sequences can be implemented using a programmable microcontroller, which is why this need not be described in more detail here.

[0023] The capacitive proximity sensor according to Fig. 1 operates as follows. In the first part of a switching cycle, both the first circuit node 5 and the second circuit node 6 are connected to the operating voltage U0 14. Therefore, the sensor electrode 1 is charged to the operating voltage U0 relative to ground 4. The resulting capacitance C between the sensor electrode 1 and the coupling electrode 2 K In contrast, it remains uncharged, since both terminals are at the same potential. Because all capacitors are discharged at the beginning of the entire measurement process, the voltage drop across the integrator capacitance 13, which is present at input 11 of the measuring circuit 10, is also zero.

[0024] After a predetermined time period, which corresponds, for example, to half the switching cycle, the first switching device 8 and the second switching device 9 are caused by the control circuit 15 to switch simultaneously. The second switching device 9 then connects the second circuit node 6, coupled to the coupling electrode 2, to ground 4. The first switching device 8 connects the first circuit node 5, coupled to the sensor electrode 1, to the input 11 of the measuring circuit 10 and simultaneously to a terminal of the capacitor 13. Since this capacitor 13 is uncharged at the beginning of the several switching cycles, a portion of the charge stored in the capacitance C is dissipated. SThe charge located at sensor electrode 1 is transferred via circuit node 5 and the first switching device 8 to capacitor 13. The capacitance of capacitor 13 is chosen to be many times larger than the maximum expected capacitance of sensor electrode 1 with respect to ground. For this reason, almost the entire charge of the sensor capacitance C flows during the charge transfer. S on capacitor 13. The voltage at circuit node 5 drops to a value between ground potential and operating voltage U0.

[0025] The control circuit 15 then controls a multitude of such switching cycles, so that the sensor electrode 1 is repeatedly charged to the operating voltage U0 and subsequently a portion of this charge is repeatedly transferred to the capacitor 13 via the circuit node 5. However, a portion of the charge accumulated on the sensor electrode 1 during connection to the operating voltage 14 is transferred to the coupling capacitance C. K 7 transmitted. Depending on the size of the sensor capacitance C S and the coupling capacity C K After a large number of switching cycles, a voltage is established across capacitor 13, which depends on the size of the sensor capacitance to be measured. This is to be demonstrated using the Fig. 4 will be explained in more detail.

[0026] Fig. Figure 4 illustrates the voltage conditions at the beginning and end of a charging interval (second section of the switching cycle) at a time when the charging of capacitor 13 (C) is progressing. L ) does not change further, i.e., the voltage U L does not change further, i.e., the voltage U L above capacitor 13 has reached a steady state.

[0027] The beginning of the charging interval is shown on the left. At this point, the two switching devices 8 and 9 separate the two capacitors C. S and C K from the operating voltage U0 14. At this point, the capacitance C S charged to the voltage U0, so that the charge Q S1 = U0C S The charge on capacity C K is zero (Q K1 = 0).

[0028] Then the transshipment interval begins, the end of which is on the right side of the Fig. Figure 4 is shown. This is due to the fact that there is a [missing information] above capacitor C. L has reached a steady state, i.e., its voltage U has changed. L If the situation no longer changes, it can be assumed that no charge is transferred from node 5 via switching device 8 to capacitor C. L 13 flows away. This is due to the equation Q = 0 in Fig. Figure 4 is shown. At the end of the charging interval, it can also be assumed that there is a voltage between the first circuit node 5 and ground, and thus across all three capacitors C. L , C S and C K the same voltage, namely U L , falls away. It follows that the charge of the capacity C S is calculated as follows: Q S2 = U L * C S The charge on the coupling capacitance C K is: Q K2 = U L * C K .

[0029] Since no charge flowed through the switching device 8 during the recharging interval, the charge on the capacitors C must be S and C K The available capacity at the beginning and end of the transshipment interval must be the same: Q S1 + Q K1 = Q S2 + Q K2 .

[0030] As in Fig. As shown below, this results in an equation for calculating the integrator capacity C. L of the capacitor 13 setting voltage U L : UL=U0*1 / (1+CK / CS).

[0031] Thus, the integrator capacity C is determined in the steady state. L Setting the voltage is a measure of the capacitance C to be measured. S the sensor electrode 1 opposite the mass 4.

[0032] The equation also shows that the coupling capacitance should be dimensioned so that it is within the range of the capacitance C to be measured. SIf the coupling capacitance is smaller than the capacitance being measured, the change in the measured voltage as a function of the change in capacitance being measured will be smaller; if the coupling capacitance is chosen to be significantly larger, the absolute value of the voltage being measured will be too low. For example, if the capacitance to be measured can range between 20 pF and 100 pF, the coupling capacitance should be chosen to be around 100 pF.

[0033] Fig. Figure 2 shows a second embodiment of the capacitive sensor according to the invention. Identical or comparable elements are designated with the same reference numerals as in Figure 2. Fig. 1 is marked. The sensor electrode 1 in turn has a capacitance C. S 3 relative to the ground potential 4. The coupling capacitance C that establishes itself between the sensor electrode 1 and the coupling electrode 2. KThis is again illustrated by a capacitor 7 shown with a dashed line. The coupling electrode 2 is again coupled to the second circuit node 6, which is connected to the second switching device 9, the second switching device 9 connecting the node 6 either to the operating voltage U0 14 or to the ground potential 4.

[0034] A first change compared to the circuit according to Fig. 1 results from the coupling of capacitor 13'. In the embodiment according to Fig. 2 is directly connected to the first circuit node 5, which is coupled to the sensor electrode 1. This necessitates the other terminal of the capacitor 13', which in the arrangement according to Fig. 1 was fixedly coupled to the ground potential 4, to be coupled to the ground potential 4 via a further switching device.

[0035] A second deviation from the embodiment according to Fig. This results from the fact that all switching devices are implemented by switching elements within the port circuits of a microcontroller 18. The first circuit node 5 is connected to a first port 19 of the microcontroller 18. The ground-side terminal of the capacitor 13' is connected to a second port 20 of the microcontroller 18. The circuit node 6 is connected to a third port 21 of the microcontroller 18. The microcontroller 18 is programmed to connect the first port 19 either to the operating voltage U0 or to the input 11 of a measurement circuit 10 contained within the microcontroller 18. The second port 20 is programmed to either decouple the terminal of the capacitor 13' with a high impedance or connect it to ground potential 4. The switching elements of the first port 19 are collectively designated 8' and the switching elements of the second port 20 are designated 8".The third port 21 is programmed to be connected either to the operating voltage U0 or to ground potential 4. The control circuit 15 is also located in the microcontroller 18 and is implemented here using the usual components of a microcontroller, namely a microprocessor, program and data memory, bus systems, and corresponding port circuits.

[0036] The following 18 processes then run program-controlled in the microcontroller, leading to a measurement of the sensor capacitance C SInitially, capacitor 13' is discharged, and ports 19 and 21 are connected to the operating voltage U0. Simultaneously, the lower terminal of capacitor 13' is decoupled, meaning port 20 is switched to high impedance (it is absolutely essential to ensure that when port 19 is connected to operating voltage U0, port 20 must under no circumstances be connected to ground potential, as this would cause the relatively large capacitor 13' to charge immediately to the full operating voltage, and the resulting currents could destroy the controller circuit).

[0037] Subsequently, the control circuit 15 of the microcontroller 18, implemented by program execution, causes the ports in the Fig. 2 schematically represented configurations. Here, in a first section of each switching cycle, the sensor capacitance C is again Scharged at U0, while in a second section the recharging of the charges from C S on capacitor 13' or on the coupling capacitance C K takes place, as already indicated by Fig. As described in section 4. As a result, capacitor 13' is again charged to a voltage that is a measure of the sensor capacitance C. S represents.

[0038] Fig. Figure 3 shows a circuit diagram of an embodiment of the first embodiment of the capacitive proximity sensor according to the invention, which illustrates in particular the protection circuits present at the connections to the sensor electrodes 1, 2 and the input circuitry of the measuring circuit 10. The measuring circuit 10 includes an analog-to-digital converter, and the integrator capacitance results from a first capacitor and a downstream low-pass filter, for example with the capacitance and resistance values ​​shown.

[0039] The sensor electrode 1 and also the coupling electrode 2 are connected to the first circuit node 5 and the second circuit node 6 respectively via a relatively large capacitance compared to the capacitances to be measured (in Fig. 3, for example, 100 nF), are connected, with the capacitance bridged by a relatively large resistor (here 100 kΩ). Furthermore, the coupling includes two relatively small resistors of, for example, 10 Ω, between which a protection diode is connected to ground. This diode is intended to dissipate any overvoltages occurring at the electrodes due to external sources to ground. These protection circuits influence the charging time constant for the sensor electrode and the coupling electrode, but the resistance values ​​are chosen such that the resulting time constants are always well below half the switching time specified by the control circuit 15.

Claims

[1] Capacitive proximity sensor for a motor vehicle, comprising a sensor electrode (1), wherein changes in capacitance (3; C S ) the sensor electrode (1) relative to a surface (4A) lying on a reference potential (4), wherein the capacitive proximity sensor has: a first circuit node (5) coupled to the sensor electrode (1), which can be coupled via a first switching device (8) either to a fixed voltage potential (14) or to the input (11) of a measuring circuit (10), wherein the first switching device (8; 8', 8") then, when it couples the first circuit node (5) to the input (11) of the measuring circuit (10), simultaneously an integrator capacitor (13; C) L ) couples in between the first circuit node (5) and the reference potential (4), wherein the measuring circuit (10) serves to determine a voltage applied across the integrator capacitance (13), a coupling capacity (7; C K ), which is coupled between the sensor electrode (1) and a second circuit node (6), wherein the second circuit node (6) can be coupled via a second switching device (9) either to the fixed voltage potential (14) or to the reference potential (4), a control circuit (15) that controls the first switching device (8; 8', 8") and the second switching device (9) at least approximately synchronously such that when the first switching device (8; 8', 8") couples the first circuit node (5) with the fixed voltage potential (14), the second switching device (9) does not couple the second circuit node (6) with the reference potential (4), and when the first switching device (8) couples the first circuit node (5) with the measuring circuit (10), the second switching device (9) does not couple the second circuit node (6) with the fixed voltage potential (14), wherein the control circuit (15) repeatedly switches the first and second switching devices (9, 8; 8', 8") until the voltage across the integrator capacitance (13) has at least approximately reached a steady state, and thereafter causes the measuring circuit (10) to measure the voltage across the integrator capacitance (13) or its change relative to a reference value as a measure of the capacitance (3; C). S ) of the sensor electrode (1) or its change. [2] Capacitive proximity sensor according to claim 1, characterized by , that the reference potential is a ground potential (4) and the fixed voltage potential is a supply voltage potential (14). [3] Capacitive proximity sensor according to claim 1 or 2, characterized by , that the coupling capacitance (7) is a capacitance (C) forming between the sensor electrode (1) and a coupling electrode (2) arranged at a fixed distance adjacent to the sensor electrode (1). K ) is. [4] Capacitive proximity sensor according to claim 3, characterized by , that the sensor electrode (1) is an outer conductor of a coaxial line and the coupling electrode (2) is an inner conductor of the coaxial line. [5] Capacitive proximity sensor according to one of claims 1-4, characterized by , that the integrator capacity (C L ) is formed by a capacitor (13) coupled between the input (11) of the measuring circuit (10) and the reference potential (4) or by an RC network coupled between the first switching device (8) and the input (11) of the measuring circuit (10) and configured as a low-pass filter. [6] Capacitive proximity sensor according to one of claims 1-4, characterized by , that the integrator capacity (C L) is formed by a capacitor (13') whose one terminal is connected to the first node (5), and that the first switching device (8) comprises a first switch (8') and a second switch (8"), wherein the first switch (8') couples the first circuit node (5) either to the fixed voltage potential (14) or to the input (11) of the measuring circuit (10), and wherein the second switch (8') either leaves the other terminal of the capacitor (13') open or couples it to the reference potential (4). [7] Capacitive proximity sensor according to claim 6, characterized by , that the second switching device (9) as well as the first switch (8') and the second switch (8") of the first switching device (8) are each formed by a port (19 - 21) of a microcontroller (18) containing the measuring circuit (10). [8] Capacitive proximity sensor according to one of claims 1-7, characterized by, that the switching operations of the switching devices (9, 8; 8', 8") initiated by the control circuit (15) are carried out with a period that is at least 10 times greater than the time constant of charging the sensor capacity (3) when coupled with the fixed voltage potential (14). [9] Capacitive proximity sensor according to claim 8, characterized by , that the switching operations of the switching devices (9, 8; 8', 8") initiated by the control circuit (15) are carried out at a frequency between 10 kHz and 100 MHz, preferably at a frequency between 100 kHz and 10 MHz. [10] Capacitive proximity sensor according to claim 8 or 9, characterized by , that the number of switching operations of the switching devices (9, 8; 8', 8") caused by the control circuit (15) before determining the value across the integrator capacitance (13; C) L ) applied voltage of the ratio of the integrator capacitance (13; C L) to the capacity to be measured (3; C S ) of the sensor electrode (1) depends, wherein the integrator capacitance is dimensioned such that the voltage across the integrator capacitance reaches approximately the steady state after about 10 to 1000 switching operations. [11] Capacitive proximity sensor according to one of claims 7-10, characterized by , that the switching devices (9, 8; 8', 8") are analog switches that have a break-before-make characteristic.

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