Capacitive sensor designed to use a heating element as an antenna electrode.

Abstract

Capacitive sensor (1) configured to be connected between a heating element (12) and a heating power supply (2) and to use the heating element (12) as an antenna electrode, comprising a common-mode choke (5) with a first and a second inductively coupled winding, wherein the first winding is provided for connection between a first connection point (5.1) of the heating current supply (2) and a first connection point (5.2) of the heating element (12) and the second winding is provided for connection between a second connection point (5.3) of the heating element (12) and a second connection point (5.4) of the heating current supply (2); a control and evaluation circuit configured to inject an alternating current signal into the heating element (12) via a measuring node (17.2; 24) in order to measure a voltage on and / or a current through the measuring node (17.2; 24) and to derive an impedance (13) between the heating element (12) and a counter electrode from the measured voltage and / or current, characterized by the fact that the common-mode choke (5) has a third winding which is inductively coupled to the first and second windings, wherein the measuring node (17.2; 24) is effectively coupled to the third winding to inductively feed the AC signal into the heating element (12).

Description

Technical field

[0001] The present invention relates generally to capacitive measurement, e.g. for detecting the presence or absence of a person on a seat (seat occupancy detection) or the presence or absence of a person's hand on the steering wheel of a car (hand touch detection). State of the art

[0002] Capacitive sensors have a wide range of applications and are used, among other things, to detect the presence and / or position of a conductive body near an antenna electrode. As used here, the term "capacitive sensor" refers to a sensor that generates a signal that responds to the influence of the detected object (a person, a part of a person's body, a pet, an object, etc.) on an electric field. A capacitive sensor generally includes at least one antenna electrode to which an oscillating electrical signal is applied, and which then emits an electric field into a region of space near the antenna electrode while the sensor is operating. The sensor has at least one measuring electrode, which may be identical to or different from transmitting antenna electrodes, at which the influence of an object or living being on the electric field is detected.

[0003] The technical paper entitled "Electric Field Sensing for Graphical Interfaces" by J.R. Smith, published in Computer Graphics I / O Devices, May / June 1998 issue, pages 54-60, describes the concept of electric field measurement as it is used to perform non-contact three-dimensional position measurements, and in particular to measure the position of a human hand for the purpose of inputting three-dimensional positions into a computer. Within the general concept of capacitive measurement, the author distinguishes between distinct mechanisms, which he refers to as "loading mode," "shunt mode," and "transmit mode," corresponding to different possible paths for the electric current. In "loading mode," an oscillating voltage signal is applied to a transmitting electrode, which generates an oscillating electric field relative to ground.The object being measured modifies the capacitance between the transmitting electrode and ground. In "parallel mode," also known as "coupling mode," an oscillating voltage signal is applied to the transmitting electrode, creating an electric field at a receiver electrode, and the displacement current measured at the receiver electrode is recorded. The measured displacement current depends on the object being measured. In "transmitting mode," the transmitting electrode is brought into contact with the user's body, who then becomes a transmitter relative to a receiver, either through a direct electrical connection or via capacitive coupling.

[0004] The capacitive coupling strength can be determined, for example, by applying an AC voltage signal to an antenna electrode and measuring the current flowing from this antenna electrode either to ground (in charging mode) or into a second antenna electrode (in coupling mode). This current can be measured by a transimpedance amplifier connected to the measuring electrode, which converts the current flowing into the measuring electrode into a voltage proportional to the current.

[0005] Capacitive sensors that use a heating element as an antenna electrode are known in patent literature. US 2011 / 0148648 A1 discloses a capacitive occupant detection system for a vehicle seat using a seat heating element 12 as an antenna electrode. Fig. Figure 1 schematically illustrates this state of the art. The voltage source 2 provides the power supply for the heating element, for example, a seat heating control unit. The electronic control module (ECM) 1 is designed as a capacitive measuring circuit. It comprises a common-mode choke 5, an AC voltage source 9, and capacitors 6, 7, and 8. Capacitor 8 couples the AC voltage generated by the AC voltage source 9 to the node 11. The heating element 12 has a complex impedance 13 to ground. The complex impedance 13 includes a capacitive component and a resistive component, which depend on the occupancy state of the vehicle seat. The complex impedance 13 is therefore also referred to below as the "unknown impedance" or "impedance to be determined." Capacitor 8, together with the unknown impedance 13, forms a voltage divider. The complex voltage U measThe connection between node 11 and ground 10 can be used to calculate the complex, unknown impedance 13. Due to its high impedance, the common-mode choke 5 decouples the AC voltage at node 11 from the AC ground. The heating element 12 can simultaneously be powered by the DC current supplied by voltage source 2 and by the AC voltage through the capacitive measuring circuit. Capacitors 6 and 7 ensure a defined AC ground on the side of the common-mode choke 5 connected to the DC supply of the seat heater. Ground 3 is the reference ground. The terminals of the common-mode choke 5 are numbered 5.1 to 5.4: terminal 5.1 connects the first winding to the high-voltage side of voltage source 2; terminal 5.2 connects the first winding to the high-voltage side of heating element 12; terminal 5.3 connects the second winding to the low-voltage side of the heating element 12, and terminal 5.4 connects the second winding to the low-voltage side of the voltage source 2.

[0006] Resistor 4 represents the wiring resistance of the connection between the low-voltage side of voltage source 2 and the fourth terminal 5.4 of common-mode choke 5. A similar wiring resistance exists for the upper connection between the high-voltage side of voltage source 2 and common-mode choke 5, but this can be disregarded for the following explanation. Typically, voltage source 2, which represents the seat heater control unit, is switched on and off periodically to control the heating energy of the seat heater 12 according to a pulse-width modulation schedule. A typical switching frequency is, for example, 25 Hz.Each time the voltage source 2 is switched on, the current through the wiring resistor 4 rises from essentially 0 A to the operating current of the seat heater, which, for example, with a voltage of 12 V from voltage source 2, a seat heater resistance of 1 Ω, and a wiring resistance of 0.1 Ω, is approximately 10.9 A. This current of 10.9 A produces a voltage drop of 1.09 V across the wiring resistor 4 each time the voltage source 2 is switched on. This implies that the voltage at the fourth terminal 5.4 of the common-mode choke 5 rises to 1.09 V, and consequently, the voltage at node 11 also rises to 1.09 V. The resistance of the second winding of the common-mode choke 5 is not considered here, but due to its finite conductance, it will also contribute to an additional voltage drop.The voltage step of 1.09 V at the measuring node can interfere with the measurement of the signal voltage at measuring node 11 because the step function has a wide frequency bandwidth. The situation worsens if the seat heating control unit connected to the electronic control module 1 interrupts the heating circuit not on the high-voltage side, but on the low-voltage side. This means that node 11 experiences a voltage drop of approximately 12 V - 1.09 V = 10.91 V, which is worse than the aforementioned 1.09 V step. This situation can occur if, for example, for cost reasons, one type of electronic control module 1 for capacitive detection must be usable for different types of seat heating control units.

[0007] US Patent 6,703,845 B2 discloses an occupancy sensor for a vehicle seat, wherein the heating element is used either as the measuring electrode or as a driven shielding electrode. In some of the described embodiments, the heating element is AC-decoupled from the heating current source by inductors. Fig. Figure 2 is a schematic illustration of the occupant sensor from US 6,703,845 B2. The most significant difference is that the US 6,703,845 B2 system uses separate inductors 14 and 15 instead of a common-mode choke 5. The US 6,703,845 B2 system has the same disadvantage, which is related to Fig. 1 was discussed. Furthermore, experiments and simulations show that inductors with the necessary impedance for an alternating current below 1 MHz are so expensive that the solution of US 6,703,845 B2 in a motor vehicle is unrealistic.

[0008] WO 2011 / 117237 discloses an occupancy sensor for a vehicle seat that measures the complex current flowing into the heating element in response to an applied alternating voltage. The circuit configuration is shown schematically in Fig. Figure 3 illustrates this. The transimpedance amplifier 17 holds node 11 at the same AC voltage as the output of the AC signal source 9. The reference input 17.1 of the transimpedance amplifier 17 is connected to the AC signal source 9. The transimpedance amplifier 17 converts the current flowing into its signal input 17.2 into a voltage at its output 18, which indicates the input current. Since the voltage at node 11 is known, the complex current flowing into node 11, and thus the complex voltage at the output 18 of the transimpedance amplifier, indicates the complex impedance 13. The capacitive measurement system consists of Fig. 3 has the same problems as those in the Fig. 1 and Fig. 2, when the voltage source 2 of the heating circuit is switched on and off.

[0009] Other capacitive sensor elements, in which a heating element is used as a measuring electrode for occupant detection in a vehicle seat, are known from the documents WO 2012 / 080066 A1, EP 2 572 942 A1 and DE 11 2011 101 041 T5. Task

[0010] One object of the present invention is to provide a capacitive measuring system that can use a heating element as an antenna electrode, thereby mitigating the aforementioned problem of heating current steps. This object is achieved by a capacitive sensor according to claim 1. General description of the invention

[0011] The capacitive sensor according to the present invention is configured to be connected between a heating element and a heating power supply and to use the heating element as an antenna electrode. The capacitive sensor comprises a common-mode choke for AC decoupling of the heating element from the heating power supply. The common-mode choke comprises a first and a second inductively coupled winding, wherein the first winding is provided for connection between a first connection point (e.g., the high-voltage side) of the heating power supply and a first connection point (e.g., the high-voltage side) of the heating element, and the second winding is provided for connection between a second connection point (e.g., the low-voltage side) of the heating element and a second connection point (e.g., the low-voltage side) of the heating power supply. The capacitive sensor further comprises a control and evaluation circuit (which, for example,(implemented as an application-specific integrated circuit) configured to inject an AC signal into the heating element via a sensing node to measure a voltage across and / or current through the sensing node, and to derive an impedance between the heating element and a counter electrode from the measured voltage and / or current. The common-mode choke comprises a third winding inductively coupled to the first and second windings, with the third winding effectively coupled to the sensing node to inductively inject the AC signal into the heating element. In other words, the sensing node is effectively connected to the third winding, for example, by a galvanic connection or AC coupling, so that the AC signal is applied to the third winding and inductively fed into the heating element via the third winding.

[0012] The AC signal used for capacitive measurement can thus be inductively fed into the heating element through the common-mode choke, which acts as a transformer. Consequently, no galvanic connection is required between the control and evaluation circuit and the heating circuit. As will be clear to those skilled in the art, when the heating current supply switches on or off, essentially the same step (steep rise or fall) of the heating current will occur in the first and second windings of the common-mode choke, but in opposite directions. Therefore, the magnetic fields generated in the core of the common-mode choke by the first and second windings essentially cancel each other out. The net magnetic flux thus remains essentially constant, so the voltage at the measuring node does not change significantly.

[0013] Preferably, the common-mode choke comprises a first connection for DC coupling of the first winding to the first connection point of the heating current supply, a second connection for DC coupling of the first winding to the first connection point of the heating element, a third connection for DC coupling of the second winding to the second connection point of the heating element, and a fourth connection for DC coupling of the second winding to the second connection point of the heating current supply, wherein at least one of the first and fourth connections is AC-coupled to an earth conductor (e.g., by a coupling capacitor). Preferably, both the first and fourth connections are AC-coupled to earth, e.g.,This is achieved through a first coupling capacitor between ground and one of the first and fourth terminals, and through a second coupling capacitor between ground and the other of the first and fourth terminals, or between the first and fourth terminals, or between the second and third terminals. The AC coupling to ground ensures that the AC voltage at the first and fourth terminals of the common-mode choke is at a predetermined AC voltage, regardless of the exact configuration of the heater power supply.

[0014] According to a first possible embodiment of the invention, the control and evaluation circuit comprises an oscillator coupled to the measuring node via an impedance element. As used here, the term "impedance element" refers to a capacitor, a resistor, an inductor, or a combination of two or more of these components. Those skilled in the art will recognize that the impedance element appears in series with the impedance to be measured, resulting in a voltage divider configuration. The unknown impedance can thus be derived from measurements of the voltage at the measuring node. Accordingly, the control and evaluation circuit is preferably configured to measure the voltage at the measuring node and to derive the impedance between the heating element and a counter electrode (typically at ground potential) from the measured voltage. It should be noted that a current source could be used instead of a voltage source and an impedance element.

[0015] According to a second possible embodiment of the invention, the control and evaluation circuit comprises an oscillator and a transimpedance amplifier with a reference input, a current signal input, and an output. The current signal input is AC-coupled to the measuring node, and the oscillator is effectively connected to the reference input to apply an AC voltage to it as the AC current signal. The transimpedance amplifier is configured to inject a current into the current signal input such that the voltage difference between the reference input and the current signal input is substantially eliminated, and to output a voltage indicating the current. The ratio of the voltage at the output to the voltage of the oscillator thus indicates the impedance between the heating element and ground.

[0016] Preferably, the capacitive sensor includes a bulk current injection (BCI) protection network to protect the control and evaluation circuitry against bulk current injection. The BCI protection network preferably comprises a first BCI capacitor connected between the reference input and a ground conductor, and a second BCI capacitor connected between the reference input and a terminal of the first or second winding intended for connection to the heating element. The so-called BCI test is used in the automotive industry to simulate the influence of electromagnetic fields on the behavior of vehicles or on electronic devices integrated into cars. The BCI test involves injecting a high-frequency current into the wiring of the device under test using a current clamp.To pass the test, the device must not deviate (significantly) from its intended behavior.

[0017] According to a preferred embodiment of the invention, the third winding has a number of turns equal to the number of turns of the first or second winding. Consequently, the AC signal arriving at the heating element via inductive coupling has essentially the same amplitude as the original AC signal at the measuring node. A phase reversal between these signals can occur depending on the winding direction of the third winding and the connection direction, but any phase reversal can be corrected or compensated for by the control and evaluation circuitry.

[0018] If no 1-to-1 transformation is required between the third winding and each of the first and second windings, the third winding can have a number of turns that differs from the number of turns of the first winding or the second winding, respectively.

[0019] Preferably, the capacitive sensor comprises a shielded cable with a first inner conductor connected to the first winding to connect the first winding to the first connection point of the heating element, a second inner conductor connected to the second winding to connect the second winding to the second connection point of the heating element, and a shield conductor surrounding the first and second inner conductors, the shield conductor being connected to the control and evaluation circuit. The control and evaluation circuit is preferably configured to maintain the shield conductor at the same AC potential as the first and second inner conductors during the measurement of the unknown impedance. It should be noted that the control and evaluation circuit could be configured to operate in different modes (e.g., for calibration).) When operating in a mode other than measurement mode, the control and evaluation circuit can hold the shield conductor at a different voltage, e.g., a floating voltage.

[0020] During the measurement of the unknown impedance (measurement mode), the shield conductor can be effectively connected to the oscillator via an amplifier and / or a transformer, where the amplifier and / or transformer has a gain equal to the ratio of the number of turns in the first or second winding to the number of turns in the third winding. This configuration is particularly useful if the number of turns in the third winding is not equal to the number of turns in the first or second winding.

[0021] A preferred aspect of the present invention relates to an occupant sensor for a vehicle seat, which has a capacitive sensor as described above.

[0022] Another preferred aspect of the present invention relates to a detector for detecting a hand touching the steering wheel, which has a capacitive sensor as described above. Brief description of the drawings

[0023] Preferred embodiments of the invention will now be described by way of example with reference to the accompanying drawings. The drawings show: Fig. 1 a schematic circuit diagram of a first combination of a heating and a capacitive measuring system according to related technology; Fig. 2 a schematic circuit diagram of a second combination of a heating and a capacitive measuring system according to related technology; Fig. 3 a schematic circuit diagram of a third combination of a heating and a capacitive measuring system according to related technology; Fig. 4 a schematic circuit diagram of a combination of a heating and a capacitive measuring system according to a first preferred embodiment of the invention; Fig. 5 a schematic circuit diagram of a combination of a heating and a capacitive measuring system according to a second preferred embodiment of the invention; Fig. 6 a schematic circuit diagram of a combination of a heating and a capacitive measuring system according to a comparative example; Fig. 7 a schematic circuit diagram of a combination of a heating and a capacitive measuring system according to a third preferred embodiment of the invention; Fig. 8 a schematic circuit diagram of a combination of a heating and a capacitive measuring system according to a fourth preferred embodiment of the invention; Fig. 9 a schematic circuit diagram of a combination of a heating and a capacitive measuring system according to a fifth preferred embodiment of the invention; Fig. 10 a schematic circuit diagram of a combination of a heating and a capacitive measuring system according to a sixth preferred embodiment of the invention; Fig. 11 a schematic circuit diagram of a combination of a heating and a capacitive measuring system according to a seventh preferred embodiment of the invention; Fig. 12 a schematic circuit diagram of a combination of a heating and a capacitive measuring system according to an eighth preferred embodiment of the invention; Fig. 13 a schematic drawing of a vehicle seat comprising a combination of a heating and a capacitive measuring system according to the invention; Fig. 14 a schematic drawing of a steering wheel comprising a combination of a heating and a capacitive measuring system according to the invention. Description of preferred embodiments

[0024] Fig. Figure 4 schematically illustrates a combination of a heater and a capacitive sensor, e.g., for a car seat or a steering wheel, according to a first preferred embodiment of the invention. The system comprises a heating circuit that provides a heating current supply, as shown in Figure 4. Fig. 4 as a DC voltage source 2, and a heating element 12 which generates heat when current flows through it. The system also includes a capacitive sensor, represented as an electronic control module (ECM) 1. The capacitive sensor comprises a common-mode choke 5, an AC voltage source 9, and capacitors 6, 7, and 8. Capacitor 8 couples the AC voltage generated by the AC voltage source 9 to the sensing node 24. Capacitors 6 and 7 ensure a defined AC ground on the side of the common-mode choke 5 that is connected to the DC supply of the seat heater. Ground 3 is the reference ground. The terminals of the common-mode choke 5 that are part of the heating circuit are numbered 5.1 to 5.4, as shown in Fig. 1: Terminal 5.1 connects the first winding to the high-voltage side of the voltage source 2; terminal 5.2 connects the first winding to the high-voltage side of the heating element 12; terminal 5.3 connects the second winding to the low-voltage side of the heating element 12, and terminal 5.4 connects the second winding to the low-voltage side of the voltage source 2. The common-mode choke 5 comprises a third winding with terminals 5.5. and 5.6 of the common-mode choke 5. The AC signal source 9 couples an AC voltage through the capacitor 8 into the third winding of the common-mode choke 5. Assuming that the number of turns of all three windings is the same, the same AC voltage appears at node 11 as at node 24 due to the transformer effect of the common-mode choke with 3 windings, since the terminals 5.1 and 5.4 of the common-mode choke 5 are connected to AC ground via the capacitors 6 and 7.There may be a phase reversal between the voltages at nodes 19 and 11, depending on the winding direction and the direction of the third winding's connection relative to the first and second windings. However, this is not critical, as any phase reversal can be corrected by the control and evaluation circuitry, e.g., in the measurement evaluation software. The fact that the winding direction and the connection direction of the third winding are not important is demonstrated by the absence of a winding start point next to the third winding. (According to the dot convention, if the current increases in the direction from the point towards the respective winding, a positive voltage is induced at the points of all coupled windings.) Since the relationship between the AC voltages at nodes 11 and 24 is known, the remaining explanations regarding this relationship can be derived from it. Fig. 4. It is assumed that these voltages are equal. The control and evaluation circuit therefore operates analogously to the circuit in Fig. 1. In particular, the capacitor 8 and the unknown impedance 13 together form a voltage divider, and the complex AC voltage at node 24 thus indicates the complex impedance 13. To determine the complex impedance 13 using the voltage at node 24, the measurement principle of EP 2 368 771 can be used, which is hereby incorporated by reference in its entirety with effect for the legal system that permits incorporation by reference.

[0025] The advantage of the capacitive sensor from Fig. Point 4 consists in the fact that each voltage step resulting from switching the heating current supply at node 11 no longer significantly affects the measurement signal at node 24. This is because there is no galvanic connection between nodes 24 and 11. Each time the voltage source 2 is switched on, the resulting current step through the first winding of the common-mode choke 5 is essentially equal to the current step through the second winding of the common-mode choke 5, but in the opposite direction, since both windings are connected in series, but in reverse order. Therefore, the magnetic fields generated by the first and second windings in the core of the common-mode choke essentially cancel each other out. It follows that the change in magnetic flux experienced by the third winding is essentially zero, and thus the voltage at node 24 is not affected.

[0026] Fig. Figure 5 shows a second embodiment of a combination of a heater and a capacitive sensor according to the invention. The capacitive sensor 1 essentially uses the same measuring principle as the capacitive sensor from Fig. 3. However, the measuring node (signal input 17.2 of the transimpedance amplifier 17) is inductively coupled to node 11 and thus to the heating element 12 via the third winding of the common-mode choke 5. The advantages of the galvanic isolation between the measuring node and the heating element are those already discussed with reference to Fig. 4 were explained.

[0027] The inductive coupling through the third winding of the common-mode choke offers an additional advantage. As briefly mentioned above, in automotive applications, it is essential that an electronic control module be immune to so-called "current injection." The BCI test simulates the influence of electromagnetic fields on the behavior of the electronic control module. A high-frequency current is injected into the wiring using a current clamp, and the electronic control module must not deviate significantly from its intended behavior during the test. For the circuit in Fig. In this context, 3 means that a common-mode current is injected, for example, into the connections between the voltage source 2 and the common-mode choke 5. This current flows primarily through capacitors 6 and 7, through the electronic control module ground into the signal source 9, through the transimpedance amplifier 17, capacitor 16, and finally through the unknown impedance 13. The problem is that the high-frequency current flows from the signal input 17.2 of the transimpedance amplifier 17 and is thus superimposed on the measurement current. Since the high-frequency current has a high amplitude (for example, 50 mA at 10 MHz) and the measurement current is on the order of 100 µA, this can lead to saturation of the transimpedance amplifier 17. One way to mitigate this problem is shown in the comparative example of Fig. 6 shown. Compared to the system from Fig. In step 3, capacitors 19 and 20 are added in parallel to the AC signal source 9 and the transimpedance amplifier 17, respectively. Instead of flowing entirely into the transimpedance amplifier 17, the high-frequency test current is split between capacitor 20 and the transimpedance amplifier 17, depending on the ratio of the impedances of capacitor 20 and the series connection of capacitor 16 and the input impedance of the transimpedance amplifier. Furthermore, instead of flowing entirely into the AC signal source 9, the high-frequency test current is split between capacitor 19 and the signal source 9, depending on the ratio of the impedance of capacitor 9 to the output impedance of the AC signal source 9. It should be noted that the capacitance 20 cannot be made arbitrarily large, as capacitor 20 will also divert the current being measured away from the transimpedance amplifier 17 and therefore reduce the signal to a noise level of the system.

[0028] The capacitive sensor according to a third embodiment of the invention, which is described in Fig. Figure 7 illustrates how the third winding described above can mitigate this problem. The circuit is essentially the same as the circuit in Figure 7. Fig. 6, with the exception of the third winding of the common-mode choke 5, which inductively couples the measuring node (signal output 17.2) to the heating element 12. The capacitive sensor made of Fig. 7 combines the features of the Fig. 5 and Fig. The unknown impedance is determined in the same way as in the circuit shown in Figure 6. Fig. 5. In the embodiment from Fig. Figure 7 represents the turns ratio 1:1:1, and the winding directions and the directions of the winding connections are indicated by the winding start points to obtain identical voltages with respect to amplitude and phase at nodes 11 and 17.1. Indeed, any voltage or phase difference between nodes 11 and 17.1 would cause the measuring current to be deflected via capacitor 20 (away from the transimpedance amplifier 17). In contrast to the circuit in Fig. 6. Any high-frequency test current injected into the wiring between the DC voltage source 2 and the common-mode choke 5 has no direct path to the transimpedance amplifier 17. Essentially, the entire high-frequency current flows through capacitors 6 and 7, through the electronic control module ground to the signal source 9 and capacitor 19, which are connected in parallel, through capacitor 20, and finally through the unknown impedance 13. There is still a small high-frequency current flowing through the first and second windings of the common-mode choke, which causes a current to flow through the third winding due to inductive coupling.For example, assuming a high-frequency current with an amplitude of 50 mA at a frequency of 10 MHz, where the common-mode choke has an inductance of 20 mH, the heating element has a resistance of 1 Ω, capacitor 19 has a capacitance of 1 nF, capacitor 20 has a capacitance of 10 nF, capacitors 6 and 7 have capacitances of 100 nF, the AC signal source 9 has an output impedance of 1 Ω, the transimpedance amplifier 17 has an input impedance of 50 Ω, the coupling capacitor 16 has a capacitance of 1 µF, the unknown impedance 13 has a capacitance of 100 pF, the common-mode choke has a coupling factor of 0.999, and if the capacitive coupling between the common-mode windings is not taken into account, the circuit results in . Fig. 6. A high-frequency current of 26 mA is fed into the transimpedance amplifier 17, while the circuit consists of Fig. 7 only yields approximately 430 µA. It follows that the capacitive sensor made of Fig. 7 due to the saturation of the transimpedance amplifier 17 is considerably less prone to failure than the capacitive sensor according to the comparison example from Fig. 6.

[0029] Fig. Figure 8 illustrates a fourth preferred embodiment of the invention. The capacitive sensor 1 largely corresponds to that shown in Figure 8. Fig. 4. In addition, the capacitive sensor includes Fig. 8 A shielded cable 22 with a first inner conductor 22.1, which is connected to the first winding and connects the first winding to the high-voltage side of the heating element 12, a second inner conductor 22.2, which is connected to the second winding and connects the second winding to the low-voltage side of the heating element 12, and a shield conductor 22.3, which is insulated from and surrounds the first and second inner conductors 22.1 and 22.2. The shield conductor 22.3 is connected to an amplifier 21, which receives the AC voltage at the measuring node 24 at its input. The amplifier is set such that it keeps the shield conductor 22.3 at the same AC voltage as the first and second inner conductors 22.1 and 22.2, whereby the shield conductor 22.3 is operated as a driven shield that neutralizes the earth capacitance of at least part of the winding from the common-mode choke 5 to the heating element 12.In fact, since the shield conductor 22.3 is held at the same alternating voltage as the inner conductors 22.1 and 22.2 with respect to both amplitude and phase, the alternating electric field between the inner conductors 22.1 and 22.2 and the shield conductor is essentially canceled out.

[0030] If the common-mode choke has a turns ratio of 1:1:1, amplifier 21 has a gain of one, meaning it only buffers the AC voltage on the shield conductor. However, if the number of turns in the third winding differs from the number of turns in the first or second winding, the gain or attenuation (gain < 1) of amplifier 21 must be chosen other than one to match the turns ratio between the third and one of the first or second windings.

[0031] A fifth preferred embodiment of a capacitive sensor according to the invention is described in Fig. 9 shown. The capacitive sensor according to the fifth embodiment corresponds to the capacitive sensor according to the second embodiment ( Fig. 5), with the exception of the shielded cable 22 between the common-mode choke and the heating element 12. Since the shield conductor 22.3 of the shielded cable 22 is directly connected to the output of the AC voltage source 9, the common-mode choke 5 is configured to have a turns ratio of 1:1:1. Furthermore, the winding direction and the connection direction of the third winding are chosen such that the AC voltage at the inner conductors 22.1 and 22.2 is in phase with the voltage output from the AC voltage source 9.

[0032] A sixth preferred embodiment of a capacitive sensor according to the invention is described in Fig. 10 shown. The embodiment from Fig. 10 largely corresponds to the embodiment from Fig. 9, but additionally includes a current imprint protection network (including capacitors 19 and 20). The advantages of the current imprint protection network have been discussed above with reference to Fig. Points 7 have been discussed and do not need to be repeated here.

[0033] A seventh preferred embodiment of a capacitive sensor according to the invention is described in Fig. 11 shown. The embodiment from Fig. 11 largely corresponds to the embodiment from Fig. 9, but additionally includes an amplifier 21 to adjust the amplitude of the alternating voltage at the shield conductor 22.3. It will thus be clear that in the seventh embodiment of the invention, the turns ratio between the third and the first or second winding can be other than one.

[0034] The eighth preferred embodiment of a capacitive sensor according to the invention, which is described in Fig. Figure 11 combines all the features of the embodiments of the Fig. 10 and Fig. 11.

[0035] It should be noted that in the embodiments from the Fig. 11 and Fig. 12 of the amplifiers could be replaced by a suitable transformer.

[0036] Fig. Figure 13 shows a vehicle seat 25 equipped with a combination of a seat heater and a capacitive occupant detection system 26. Fig. Figure 14 shows a steering wheel 28 equipped with a combination of a heater and a capacitive system for detecting hand touch 27.

[0037] While specific embodiments have been described in detail, the person skilled in the art will recognize that, in light of the overall teaching of the disclosure, various modifications and alternatives to these details could be developed. Accordingly, the particular arrangements disclosed are intended only for illustration and not to limit the scope of the invention, for which the entire scope of the appended claims and all equivalents thereof applies. Legend: 1 Electronic control module implementing a capacitive sensor 2 Heating power supply 3 Reference masses 4. Resistance, which represents the wiring resistance 5 common-mode choke 5.1-5.6 Connections of the common-mode choke 6 Capacitor 7 Capacitor 8 Impedance element (shown here as a capacitor) 9 AC signal source 10 circuit earth conductors 11 nodes that are directly connected to the heating element 12 Heating element 13 Unknown Impedance 14 Inductor 15 Inductor 16 Coupling capacitor 17 Transimpedance Amplifiers 17.1 Reference input 17.2 Signal input (measuring node) 18 Voltage signal output 19 Capacitor (for deflecting the power supply) 20 Capacitor (for deflecting the power supply) 21 amplifiers 22 Shielded cable 22.1, 22.2 Inner conductor 22.3 Shielding conductor 24 measuring nodes 25 vehicle seats 26 Combination of seat heating and capacitive occupant detection system 27 Combination of heating and capacitive system for detecting a hand touch 28 Steering wheel

Claims

[1] Capacitive sensor (1) configured to be connected between a heating element (12) and a heating power supply (2) and to use the heating element (12) as an antenna electrode, comprising a common-mode choke (5) with a first and a second inductively coupled winding, wherein the first winding is provided for connection between a first connection point (5.1) of the heating current supply (2) and a first connection point (5.2) of the heating element (12) and the second winding is provided for connection between a second connection point (5.3) of the heating element (12) and a second connection point (5.4) of the heating current supply (2); a control and evaluation circuit configured to inject an alternating current signal into the heating element (12) via a measuring node (17.2; 24) in order to measure a voltage on and / or a current through the measuring node (17.2; 24) and to derive an impedance (13) between the heating element (12) and a counter electrode from the measured voltage and / or current, characterized by , that the common-mode choke (5) has a third winding which is inductively coupled to the first and second windings, wherein the measuring node (17.2; 24) is effectively coupled to the third winding to inductively feed the AC signal into the heating element (12). [2] Capacitive sensor (1) according to claim 1, wherein the common-mode choke (5) has a first connection (5.1) for DC coupling of the first winding to the first connection point of the heating current supply (2), a second connection (5.2) for DC coupling of the first winding to the first connection point of the heating element (12), a third connection (5.3) for DC coupling of the second winding to the second connection point of the heating element (12) and a fourth connection (5.4) for DC coupling of the second winding to the second connection point of the heating current supply (2), and wherein at least one of the first (5.1) and fourth connection (5.4) is AC-coupled to an earth conductor (10). [3] Capacitive sensor (1) according to claim 1 or 2, wherein the control and evaluation circuit comprises: a) a power source coupled to the measuring node (24), or b) an oscillator (9) coupled to the measuring node (24) via an impedance element (8), wherein the impedance element (8) comprises at least one capacitor, one resistor or one inductor. [4] Capacitive sensor (1) according to claim 3, wherein the control and evaluation circuit is configured to measure the voltage at the measuring node (17.2; 24) and to derive the impedance between the heating element (12) and a counter electrode from the measured voltage. [5] Capacitive sensor (1) according to claim 1 or 2, wherein the control and evaluation circuit comprises an oscillator (9) and a transimpedance amplifier (17) with a reference input (17.1), a current signal input (17.2) and an output (18), wherein the current signal input (17.2) is AC-coupled to the measuring node (17.2), wherein the oscillator (9) is connected to the reference input (17.1) to apply an AC voltage as the AC current signal to it, wherein the transimpedance amplifier (17) is configured to inject a current into the current signal input (17.2) such that an AC voltage difference between the reference input (17.1) and the current signal input (17.2) is substantially eliminated and a voltage indicating the current is output at the output (18). [6] Capacitive sensor (1) according to claim 5, comprising a current imprint protection network comprising a first current imprint protection capacitor (19) connected between the reference input (17.1) and an earth conductor (10), and a second current imprint protection capacitor (20) connected between the reference input (17.1) and a terminal (5.2 or 5.3) of the first or second winding to be connected to the heating element (12). [7] Capacitive sensor (1) according to any one of claims 1 to 6, wherein the third winding has a number of turns equal to the number of turns of the first winding or the second winding. [8] Capacitive sensor (1) according to any one of claims 1 to 6, wherein the third winding has a number of turns that differs from the number of turns of the first winding or the second winding. [9] Capacitive sensor (1) according to one of claims 1 to 8, comprising a shielded cable (22) with a first inner conductor (22.1) which is connected to the first winding in order to connect the first winding to the first connection point of the heating element (12), a second inner conductor (22.2) which is connected to the second winding in order to connect the second winding to the second connection point of the heating element (12), and a shield conductor (22.3) surrounding the first and second inner conductors (22.1, 22.2), wherein the shield conductor (22.3) is connected to the control and evaluation circuit, wherein the control and evaluation circuit is configured to keep the shield conductor (22.3) at the same AC potential as the first and second inner conductors (22.1, 22.2). [10] Capacitive sensor (1) according to claim 9 when this is related back to claim 3 or 5, wherein the shield conductor (22.3) is connected to the oscillator (9) via an amplifier (21) and / or a transformer, wherein the amplifier (21) and / or the transformer has a gain which corresponds to the ratio of the number of turns of the first or second winding to the number of turns of the third winding. [11] Occupancy sensor for a vehicle seat (25) comprising a capacitive sensor (1) according to any one of claims 1 to 10. [12] Detector for detecting a hand touch with a steering wheel (28), comprising a capacitive sensor (1) according to any one of claims 1 to 10.

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