Abstract:
A capacitive sensor configured for connection between a heating element and a heating current supply comprises a common mode choke (CMC) for AC-decoupling the heating element from the heating current supply. The CMC comprises first and second inductively coupled windings, the first winding for connection between a first terminal of the heating current supply and a first terminal of the heating element and the second winding for connection between a second terminal of the heating element and a second terminal of the heating current supply. The capacitive sensor further comprises a control and evaluation circuit for injecting an AC signal into the heating element via a measurement node, measuring a voltage on and/or a current across the measurement node, and to derive an impedance between the heating element and a counter electrode from the measurement. The CMC comprises a third winding inductively coupled with the first and second windings, the third winding being operatively coupled with the measurement node.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to capacitive sensing, 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&#39;s hand on the steering wheel of a car (hands-off or hands-on detection.) 
       BACKGROUND ART 
       [0002]    Capacitive sensors have a wide range of applications, and are among others used for the detection of the presence and/or the position of conductive body in the vicinity of an antenna electrode. As used herein, the term “capacitive sensor” designates a sensor, which generates a signal responsive to the influence of what is being sensed (a person, a part of a person&#39;s body, a pet, an object, etc.) upon an electric field. A capacitive sensor generally comprises at least one antenna electrode, to which is applied an oscillating electric signal and which thereupon emits an electric field into a region of space proximate to the antenna electrode, while the sensor is operating. The sensor comprises at least one sensing electrode—which may be identical with 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, Issue May/June 1998, pp 54-60 describes the concept of electric field sensing as used for making non-contact three-dimensional position measurements, and more particularly for sensing the position of a human hand for purposes of providing three dimensional positional inputs to a computer. Within the general concept of capacitive sensing, the author distinguishes between distinct mechanisms he refers to as “loading mode”, “shunt mode”, and “transmit mode” which correspond to various possible electric current pathways. In the “loading mode”, an oscillating voltage signal is applied to a transmit electrode, which builds up an oscillating electric field to ground. The object to be sensed modifies the capacitance between the transmit electrode and ground. In the “shunt mode”, which is alternatively referred to as “coupling mode”, an oscillating voltage signal is applied to the transmitting electrode, building up an electric field to a receiving electrode, and the displacement current induced at the receiving electrode is measured. The measured displacement current depends on the body being sensed. In the “transmit mode”, the transmit electrode is put in contact with the user&#39;s body, which then becomes a transmitter relative to a receiver, either by direct electrical connection or via capacitive coupling. 
         [0004]    The capacitive coupling strength may e.g. be determined by applying an alternating voltage signal to an antenna electrode and by measuring the current flowing from that antenna electrode either towards ground (in the loading mode) or into a second antenna electrode (in coupling mode). This current may be measured by a transimpedance amplifier, which is connected to the sensing electrode and which converts the current flowing into the sensing electrode into a voltage proportional to the current. 
         [0005]    Capacitive sensors, which use a heating element as antenna electrode are known in the patent literature. US 2011/0148648 A1 discloses a capacitive occupant sensing system for a vehicle seat, using a seat heating element  12  as antenna electrode.  FIG. 1  schematically shows an illustration of this prior art. Voltage source  2  represents the power supply for the heater, for example a seat heater control unit. Electronic control module (ECM)  1  is configured as a capacitive measurement 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 AC voltage source  9  into the node  11 . The heating element  12  has complex impedance  13  towards ground. The complex impedance  13  includes a capacitive component as well as a resistive component, which depend on the occupancy state of the vehicle seat. Complex impedance  13  is thus hereinafter also referred to as “unknown impedance” or “impedance to be determined”. The capacitor  8  forms together with the unknown impedance  13  a voltage divider. The complex voltage U meas  between node  11  and circuit ground  10  can be used to calculate the unknown complex impedance  13 . The common mode choke  5  decouples the AC voltage on node  11  from AC ground due to its large impedance. The heating element  12  may at the same time be traversed by the DC current supplied by voltage source  2  and driven with the AC voltage by the capacitive measurement circuit. Capacitors  6  and  7  ensure that a defined AC ground is present on the side of the common mode choke  5  that is connected to the DC power supply of the seat heater. Ground  3  is the reference ground. The connections of the common mode coke  5  are numbered  5 . 1  through  5 . 4 : connection  5 . 1  connects the first winding to the high potential side of the voltage source  2 ; connection  5 . 2  connects the first winding to the high potential side of the heating element  12 ; connection  5 . 3  connects the second winding to the low potential side of the heating element  12  and connection  5 . 4  connects the second winding to the low potential side of the voltage source  2 . 
         [0006]    Resistor  4  represents the wiring resistance of the wiring between the low potential side of voltage source  2  and the fourth connection  5 . 4  of common mode choke  5 . There is a similar wiring resistance for the upper wiring between the high potential side of voltage source  2  and common mode choke  5 , but this can be neglected for the explanation that follows. Typically, the voltage source  2 , which represents the seat heater control unit, is switched on and off periodically to control the heating power of seat heater  12  according to a pulse-width-modulation scheme. A typical switching frequency is 25 Hz, for example. Each time voltage source  2  is switched on, the current through wiring resistance  4  rises from substantially 0 A to the operating current of the seat heater, which is, for example, for a voltage source  2  voltage of 12 V, a seat heater resistance of 1Ω and a wiring resistance of 0.1 Ω equal to about 10.9 A. This current of 10.9 A generates a voltage drop of 1.09 V across wiring resistance  4  each time the voltage source  2  is switched on. This implies that the voltage on the fourth connection  5 . 4  of the common mode choke  5  will rise to 1.09 V, and consequently also the voltage on node  11  will rise to 1.09 V. The resistance of the second winding of common mode choke  5  is neglected here, but it will also contribute to an additional voltage drop due to its finite conductance. The voltage step of 1.09 V on the sense node may disturb the measurement of the signal voltage on sense node  11 , since the step function has a wide frequency bandwidth. The situation is even worse if the seat heater control unit connected to the electronic control module  1  does not interrupt the heating circuit on the high potential side but on the low potential side. This means indeed that the node  11  experiences a voltage drop of about 12 V−1.09 V=10.91 V, which is worse than the 1.09 V step mentioned above. This situation may arise if one type of electronic control module  1  for the capacitive sensing must be usable for different types of seat heater control units e.g. for cost reasons. 
         [0007]    U.S. Pat. No. 6,703,845 B2 discloses an occupant sensor for a vehicle seat, wherein the heating element is used either as the sensing electrode or as a driven shield electrode. In some of the described embodiments, the heating element AC-decoupled from the heating current source by inductors.  FIG. 2  is a schematic illustration of the occupant sensor of U.S. Pat. No. 6,703,845 B2. The most substantial difference is that the system of U.S. Pat. No. 6,703,845 B2 uses separate inductors  14  and  15  instead of a common mode choke  5 . The system of U.S. Pat. No. 6,703,845 B2 suffers from the same disadvantage discussed with respect to  FIG. 1 . Furthermore, experiments and simulations show that inductors exhibiting the necessary impedance to alternating current below 1 MHz will be so expensive that the solution of U.S. Pat. No. 6,703,845 B2 is unrealistic in an automotive vehicle. 
         [0008]    WO2011/117237 discloses an occupancy sensor for a vehicle seat, which measures the complex current flowing into the heating element in response to an AC voltage applied thereto. The circuit configuration is schematically illustrated in  FIG. 3 . The transimpedance amplifier  17  keeps node  11  at the same AC voltage as the output of AC signal source  9 . The reference input  17 . 1  of the transimpedance amplifier  17  is connected to the AC signal source  9 . Transimpedance amplifier  17  converts the current flowing into its signal input  17 . 2  into a voltage on its output  18 , which is indicative of the input current. Since the voltage on node  11  is known, the complex current flowing into the node  11  and, hence, the complex voltage at the output  18  of the transimpedance amplifier is indicative of the complex impedance  13 . The capacitive sensing system of  FIG. 3  suffers from the same problems as those of  FIGS. 1 and 2 , when the voltage source  2  of the heating circuit is switched on and off. 
       BRIEF SUMMARY 
       [0009]    The disclosure provides a capacitive sensing system that may use a heating element as antenna electrode, wherein the above-mentioned problem of the heating current steps is mitigated. 
         [0010]    The capacitive sensor according to the present invention is configured for being connected between a heating element and a heating current supply and for using the heating element as antenna electrode. The capacitive sensor comprises a common mode choke for AC-decoupling the heating element from the heating current supply. The common mode choke comprises first and second inductively coupled windings, the first winding for being connected between a first terminal (e.g. the high potential side) of the heating current supply and a first terminal (e.g. the high potential side) of the heating element and the second winding for being connected between a second terminal (e.g. the low potential side) of the heating element and a second terminal (e.g. the low potential side) of the heating current supply. The capacitive sensor further comprises a control and evaluation circuit (implemented e.g. as an application-specific integrated circuit) configured to inject an AC signal into the heating element via a measurement node, to measure a voltage on and/or a current across the measurement 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 with the first and second windings, the third winding being operatively coupled with the measurement node for inductively injecting the AC signal into the heating element. In other words, the measurement node is operatively connected to the third winding, e.g. by galvanic connection or AC-coupled, so that the AC signal is applied to the third winding and via said third winding inductively injected into the heating element. 
         [0011]    The AC signal used for the capacitive sensing may thus be inductively injected into the heating element through the common mode choke, which acts as a transformer. As a consequence, no galvanic connection between the control and evaluation circuit and the heater circuit is required. As will be appreciated by those skilled in the art, when the heating current supply switches on or off, substantially the same step (steep increase or decrease) in heating current will occur in the first and the second winding of the common mode choke but with opposite direction. Therefore, the magnetic fields generated in the core of the common mode choke by the first and second windings substantially cancel each other out. The net magnetic flux thus remains substantially constant, such that the voltage on the measurement node does not change substantially. 
         [0012]    Preferably, the common mode choke comprises a first connection for DC-coupling the first winding to the first terminal of the heating current supply, a second connection for DC-coupling the first winding to the first terminal of the heating element, a third connection for DC-coupling the second winding to the second terminal of the heating element and a fourth connection for DC-coupling the second winding to the second terminal of the heating current supply, at least one of the first and fourth connections being AC-coupled (e.g. by means of a coupling capacitor) to a ground conductor. Preferably, the first and fourth connections are both AC-coupled to ground, e.g. by a first coupling capacitor between ground and one of the first and fourth connections and by a second coupling capacitor between ground and the other of the first and fourth connections or between the first and fourth connections or between the second and third connections. The AC coupling to ground guarantees that the AC potential on the first and fourth connections of the common mode choke are on a defined AC potential, irrespective of the precise configuration of the heating current supply. 
         [0013]    According to a first possible variant of the invention, the control and evaluation circuit comprises an oscillator coupled to the measurement node via an impedor. As used herein, the term “impedor” designates a capacitor, a resistor, an inductor or a combination of two or more of these components. Those skilled will appreciate that the impedor appears in series with the impedance to be measured, which results in a voltage divider configuration. The unknown impedance may thus be derived from measurements of the voltage on the measurement node. Accordingly, the control and evaluation circuit is preferably configured to measure the voltage on the measurement node and to derive the impedance between the heating element and a counter electrode (typically on ground potential) from the measured voltage. It is worthwhile noting that a current source could be used instead of a voltage source and an impedor. 
         [0014]    According to a second possible variant of the invention, the control and evaluation circuit comprises an oscillator and a transimpedance amplifier having a reference input, a current signal input and an output, the current signal input being AC-coupled to the measurement node and the oscillator being operatively connected to the reference input for applying an alternating voltage thereto as the AC signal. The transimpedance amplifier is configured to drive a current into the current signal input such that the voltage difference between the reference input and the current signal input is substantially cancelled and to output a voltage on the output indicative of the current. The ratio of the voltage on the output to the voltage of the oscillator thus indicates the impedance between the heating element and ground. 
         [0015]    Preferably, the capacitive sensor comprises a bulk current protection network for protecting the control and evaluation circuit against bulk current injection (BCI). The bulk current protection network preferably includes a first bulk current protection capacitor connected between the reference input and a ground conductor and a second bulk current protection capacitor connected between the reference input and a connection of the first or second winding provided for being connected with the heating element. The so-called BCI test is used in the automotive industry and simulates the influence of electromagnetic fields on the behavior or electronic appliances integrated in cars. The BCI test comprises the injection of a high-frequency current into the wiring of the appliance under test using a so-called current clamp. To pass the test, the appliance must not (significantly) deviate from its intended behavior. 
         [0016]    According to a preferred embodiment of the invention, the third winding has a number of turns equal to a number of turns of the first winding and the second winding, respectively. As a consequence, the AC signal arriving in the heating element thanks to the inductive coupling will have substantially the same amplitude as the original AC signal in the measurement node. There may be a phase reversal between these signals depending on the sense of winding of the third winding and the direction of connection but any phase reversal may be corrected or compensated for by the control and evaluation circuit. 
         [0017]    If 1-to-1 transformation between the third winding and each of the first and second windings is not required, the third winding may have a number of turns different from a number of turns of the first winding and the second winding, respectively. 
         [0018]    Preferably, the capacitive sensor comprises a shielded cable with a first core conductor connected to the first winding for connecting the first winding to the first terminal of the heating element, a second core conductor connected to the second winding for connecting the second winding to the second terminal of the heating element, and a shield conductor surrounding the first and second core 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 a same AC potential as the first and second core conductors during the measurement of the unknown impedance. It should be noted that the control and evaluation circuit might be configured to operate in different modes (e.g. for calibration.) When operating in a mode different from the measurement mode, the control and evaluation circuit may keep the shield conductor at a different potential, e.g. at a floating potential. 
         [0019]    During the measurement of the unknown impedance (measurement mode), the shield conductor may be operatively connected to the oscillator via an amplifier and/or a transformer, the amplifier and/or transformer having a gain corresponding to the ratio of the number of turns of the first or second winding to the number of turns of the third winding. This configuration is especially useful if the number of turns of the third winding is not equal to the number of turns of the first or the second winding. 
         [0020]    A preferred aspect of the present invention relates to an occupant sensor for a vehicle seat comprising a capacitive sensor as described hereinabove. 
         [0021]    Another preferred aspect of the present invention relates to a hands-on or hands-off detector for a steering wheel, comprising a capacitive sensor as described hereinabove. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
           [0023]      FIG. 1  is a schematic circuit diagram of a first combined heating and capacitive sensing system according to related art; 
           [0024]      FIG. 2  is a schematic circuit diagram of a second combined heating and capacitive sensing system according to related art; 
           [0025]      FIG. 3  is a schematic circuit diagram of a third combined heating and capacitive sensing system according to related art; 
           [0026]      FIG. 4  is a schematic circuit diagram of a combined heating and capacitive sensing system according to a first preferred embodiment of the invention; 
           [0027]      FIG. 5  is a schematic circuit diagram of a combined heating and capacitive sensing system according to a second preferred embodiment of the invention; 
           [0028]      FIG. 6  is a schematic circuit diagram of a combined heating and capacitive sensing system according to a comparative example; 
           [0029]      FIG. 7  is a schematic circuit diagram of a combined heating and capacitive sensing system according to a third preferred embodiment of the invention; 
           [0030]      FIG. 8  is a schematic circuit diagram of a combined heating and capacitive sensing system according to a fourth preferred embodiment of the invention; 
           [0031]      FIG. 9  is a schematic circuit diagram of a combined heating and capacitive sensing system according to a fifth preferred embodiment of the invention; 
           [0032]      FIG. 10  is a schematic circuit diagram of a combined heating and capacitive sensing system according to a sixth preferred embodiment of the invention; 
           [0033]      FIG. 11  is a schematic circuit diagram of a combined heating and capacitive sensing system according to a seventh preferred embodiment of the invention; 
           [0034]      FIG. 12  is a schematic circuit diagram of a combined heating and capacitive sensing system according to an eighth preferred embodiment of the invention; 
           [0035]      FIG. 13  is a schematic drawing of a vehicle seat comprising a combined heating and capacitive sensing according to the invention; 
           [0036]      FIG. 14  is a schematic drawing of a steering wheel comprising a combined heating and capacitive sensing according to the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0037]      FIG. 4  schematically illustrates a combined heater and 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 includes a heating current supply, represented in  FIG. 4  as DC voltage source  2  and a heating element  12 , which produces heat when traversed by current. The system also comprises 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 AC voltage source  9  into the measurement node  24 . Capacitors  6  and  7  ensure that a defined AC ground is present on the side of the common mode choke  5  that is connected to the DC power supply of the seat heater. Ground  3  is the reference ground. The connections of the common mode coke  5  that are part of the heating circuit are numbered  5 . 1  through  5 . 4  as in  FIG. 1 : connection  5 . 1  connects the first winding to the high potential side of the voltage source  2 ; connection  5 . 2  connects the first winding to the high potential side of the heating element  12 ; connection  5 . 3  connects the second winding to the low potential side of the heating element  12  and connection  5 . 4  connects the second winding to the low potential side of the voltage source  2 . The common mode choke  5  comprises a third winding with connections  5 . 5 . and  5 . 6  of the common mode choke  5 . AC signal source  9  couples an AC voltage through capacitor  8  into the third winding of common mode choke  5 . Assuming that the numbers of turns of all three windings are equal, through the transformer action of the 3-winding common mode choke, the same AC voltage as on node  24  also appears on node  11 , as the connections  5 . 1  and  5 . 4  of common mode choke  5  are connected to AC ground via capacitors  6  and  7 . There is possibly a phase reversal between the voltages on nodes  19  and  11 , depending on the sense of winding and the direction of connection of the third winding relative to the first and second windings. This is, however, uncritical, as any phase reversal can be corrected by the control and evaluation circuit, e.g. in the measurement evaluation software. The fact that the sense of winding and the direction of connection of the third winding are not important is indicated by the missing winding start dot next to the third winding. (According to the dot convention, when the current increases in the direction from the dot to the respective winding, then positive voltage is induced at the dots of all the coupled windings). Since the relationship between the AC voltages on nodes  11  and  24  is known, it may be assumed for the remainder of the explanations in relation with  FIG. 4  that these voltages are the same. The control and evaluation circuit thus works analogously to the circuit in  FIG. 1 . In particular the capacitor  8  and the unknown impedance  13  form together a voltage divider and the complex AC voltage on node  24  is thus indicative of the complex impedance  13 . To determine the complex impedance  13  using the voltage on node  24 , the measurement principle of EP 2 368 771, which is herewith incorporated by reference in its entirety with effect for the jurisdictions permitting incorporation by reference, may be used. 
         [0038]    The advantage of the capacitive sensor of  FIG. 4  is that any voltage step resulting from the switching of the heating current supply on the node  11  does not substantially influence the measured signal on node  24  anymore. Indeed, there is no galvanic connection between nodes  24  and  11 . Whenever the voltage source  2  switches on, the resulting current step through the first winding of common mode choke  5  is substantially equal to the current step through the second winding of common mode choke  5  but of opposite direction as both windings are connected in series but in reverse order. Therefore, the magnetic fields generated by the first and second windings substantially cancel each other in the core of the common mode choke. It follows that the change of magnetic flux seen by the third winding is substantially zero and that the voltage on node  24  is thus not affected. 
         [0039]      FIG. 5  shows a second embodiment of a combined heater and capacitive sensor in accordance with the invention. The capacitive sensor  1  uses basically the same measurement principle as the capacitive sensor of  FIG. 3 . However, the measurement node (signal input  17 . 2  of transimpedance amplifier  17 ) is inductively coupled to the node  11  and thus to the heating element  12  via the third winding of the common mode choke  5 . The advantages of the galvanic separation between the measurement node and the heating element are those explained already with respect to  FIG. 4 . 
         [0040]    The inductive coupling through the third winding of the common mode choke yields an additional advantage. As briefly indicated hereinabove, in an automotive application, an electronic control module is required to be immune against so-called ‘bulk 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 with a so-called current clamp, and the electronic control module must not substantially deviate from its intended behavior during the test. For the circuit in  FIG. 3 , this means that a common mode current is for example injected into the connections between voltage source  2  and common mode choke  5 . This current will mainly flow through capacitors  6  and  7 , through the electronic control module ground into the signal source  9 , through transimpedance amplifier  17 , capacitor  16  and finally through unknown impedance  13 . The problem is that the high frequency current flows out of the signal input  17 . 2  of the transimpedance amplifier  17  and is thereby superimposed on the measurement current. As the high frequency current has high amplitude (50 mA at 10 MHz for example) and the measurement current is in the order of magnitude of 100 μA, this can lead to a saturation of the transimpedance amplifier  17 . One option to mitigate this issue is shown in the comparative example of  FIG. 6 . Compared to the system of  FIG. 3 , capacitors  19  and  20  are added in parallel to the AC signal source  9  and the transimpedance amplifier  17 , respectively. Instead of totally flowing into the transimpedance amplifier  17 , the high frequency test current is divided between capacitor  20  and transimpedance amplifier  17 , depending on the ratio of the impedances of capacitor  20  and series connection of capacitor  16  and transimpedance amplifier input impedance. Furthermore, instead of totally flowing into the AC signal source  9 , the high frequency test current is divided between capacitor  19  and signal source  9 , depending on the ratio of the impedance of capacitor  9  to the output impedance of AC signal source  9 . It shall be noted that capacitance  20  cannot be made arbitrarily large, since capacitor  20  will also deviate the current to be measured away from the transimpedance amplifier  17  and thereby reduce the signal to noise ratio of the system. 
         [0041]    The capacitive sensor according to a third embodiment of the invention illustrated in  FIG. 7  shows how the third winding described above can alleviate this problem. The circuit is substantially the same as the circuit in  FIG. 6 , except for the third winding of the common mode choke  5 , that inductively couples the measurement node (signal input  17 . 2 ) to the heating element  12 . The capacitive sensor of  FIG. 7  combines the features of the circuits shown in  FIGS. 5 and 6 . The unknown impedance is determined in the same way as in the circuit of  FIG. 5 . In the embodiment of  FIG. 7 , the transformer ratio is 1:1:1 and the senses of winding as well as the directions of connection of the windings are indicated by the winding start dots, in order to obtain identical voltages in amplitude and phase on nodes  11  and  17 . 1 . Indeed, any voltage difference or phase difference between nodes  11  and  17 . 1 , would cause measurement current to be deviated across capacitor  20  (away from the transimpedance amplifier  17 .) Contrary to the circuit in  FIG. 6 , any high frequency test current injected into the wiring between DC voltage source  2  and common mode choke  5  has no direct path into the transimpedance amplifier  17 . Substantially all of the high frequency current flows through capacitors  6  and  7 , through the electronic control module ground into the paralleled signal source  9  and capacitor  19 , through capacitor  20  and finally through unknown impedance  13 . There is still a small high frequency current that flows through the first and second common mode choke windings, which causes a current through the third winding due to inductive coupling. For example, assuming a high frequency current having an amplitude of 50 mA at a frequency of 10 MHz, the common mode choke having an inductance of 20 mH, the heating element having a resistance of 1Ω, capacitor  19  having a capacitance of 1 nF, capacitor  20  having capacitance of 10 nF, capacitors  6  and  7  having capacitances of 100 nF, AC signal source  9  having an output impedance of 1Ω, the transimpedance amplifier  17  having an input impedance of 50Ω, the coupling capacitor  16  having a capacitance of 1 pF, the unknown impedance  13  being a capacitance of 100 pF, the common mode choke having a coupling factor of 0.999, and neglecting the capacitive coupling between the common mode windings, the circuit of  FIG. 6  yields a high frequency current injection into the transimpedance amplifier  17  of 26 mA, whereas the circuit of  FIG. 7  only yields about 430 μA. It follows that the capacitive sensor of  FIG. 7  is significantly less prone to fail due to saturation of the transimpedance amplifier  17  than the capacitive sensor according to the comparative example of  FIG. 6 . 
         [0042]      FIG. 8  illustrates a fourth preferred embodiment of the invention. The capacitive sensor  1  corresponds largely to the one of  FIG. 4 . In addition, the capacitive sensor of  FIG. 8  comprises a shielded cable  22  with a first core conductor  22 . 1  connected to the first winding and connecting the first winding to the high potential side of said heating element  12 , a second core conductor  22 . 2  connected to the second winding and connecting the second winding to the low potential side of the heating element  12 , and a shield conductor  22 . 3  insulated from and surrounding the first and second core conductors  22 . 1  and  22 . 2 . The shield conductor  22 . 3  is connected to an amplifier  21 , which receives the AC voltage on the measurement node  24  on its input. The amplifier is adjusted in such a way as to maintain the shield conductor  22 . 3  at a same AC potential as the first and second core conductors  22 . 1  and  22 . 2 , whereby the shield conductor  22 . 3  is operated as a driven shield, which neutralizes the capacitance to ground of at least part of the wiring from the common mode choke  5  to the heating element  12 . Indeed, since the shield conductor  22 . 3  is maintained at the same AC voltage as the core conductors  22 . 1  and  22 . 2  both in amplitude and phase, the alternating electric field between the core conductors  22 . 1  and  22 . 2  and the shield conductor is substantially cancelled. 
         [0043]    If the common mode choke has a transformer ratio of 1:1:1, then the amplifier  21  has unity gain, i.e. it merely buffers the AC voltage onto the shield conductor. However, if the number of turns of the third winding is different from the number of turns of the first and second winding, respectively, then the gain or attenuation (gain &lt;1) of the amplifier  21  has to be chosen different from unity in order to correspond to the transformer ratio between the third and one of the first and the second windings. 
         [0044]    A fifth preferred embodiment of a capacitive sensor according to the invention is shown in  FIG. 9 . The capacitive sensor according to the fifth embodiment corresponds to the capacitive sensor according to the second embodiment ( FIG. 5 ), except for the presence 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 such that it has a transformer ratio of 1:1:1. The sense of winding and the direction of connection of the third winding is furthermore selected such that the AC voltage on the core conductors  22 . 1  and  22 . 2  is in phase with the voltage output by the AC voltage source  9 . 
         [0045]    A sixth preferred embodiment of a capacitive sensor according to the invention is shown in  FIG. 10 . The embodiment of  FIG. 10  largely corresponds to the embodiment of  FIG. 9  but additionally comprises a bulk current protection network (including capacitors  19  and  20 .) The advantages of the bulk current protection network have been discussed hereinabove with respect to  FIG. 7  and need not be repeated here. 
         [0046]    A seventh preferred embodiment of a capacitive sensor according to the invention is shown in  FIG. 11 . The embodiment of  FIG. 11  largely corresponds to the embodiment of  FIG. 9  but additionally comprises an amplifier  21  in order to adjust the amplitude of the AC voltage on the shield conductor  22 . 3 . It will thus be appreciated that in the seventh embodiment of the invention the transformer ratio between the third and the first or second winding may be different from one. 
         [0047]    The eighth preferred embodiment of a capacitive sensor according to the invention shown in  FIG. 11  combines all the features of the embodiments of  FIGS. 10 and 11 . 
         [0048]    It is worthwhile noting that in the embodiments of  FIGS. 11 and 12 , the amplifier  10  could be replaced by an appropriate transformer. 
         [0049]      FIG. 13  shows a vehicle seat  25  equipped with a combined seat heating and capacitive occupant sensing system  26 .  FIG. 14  shows a steering wheel  28  equipped with a combined heating and capacitive hands-on or hands-off sensing system  27 . 
         [0050]    While specific embodiments have been described in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.