Abstract:
A capacitive sensor includes a sensing antenna electrode for capacitively coupling to a counterelectrode to form a capacitance, this capacitance being responsive to an electric-field-influencing property of an object or person proximate to the antenna electrode. The counterelectrode may be part of the capacitive sensor. The capacitive sensor also includes a capacitive sensing network connected to the antenna electrode to apply an oscillating signal thereto and to determine the capacitance based upon characteristics of the oscillating signal. The capacitive sensing network includes at least one inductor and a plurality of reactive components arranged to form a resonant network together with the capacitance, the plurality of reactive components being activatable and deactivatable in such a way as to modify a resonance frequency of the resonant network.

Description:
TECHNICAL FIELD 
     The present invention generally relates to capacitive sensing, in particular to capacitive sensing using a resonant network. An aspect of the invention relates to a combined seat heating and capacitively occupancy sensing device. 
     BACKGROUND ART 
     A capacitive sensor, called by some electric field sensor or proximity sensor, is 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 at which the influence of an object or living being on the electric field is detected. In some (so-called “loading mode”) capacitive occupancy sensors, the one or more antenna electrodes serve at the same time as sensing electrodes. In this case, the measurement circuit determines the current flowing into the one or more antenna electrodes in response to an oscillating voltage being applied to them. The relationship of voltage to current yields the complex impedance of the one or more antenna electrodes. In an alternative version of capacitive sensors (“coupling mode” capacitive sensors), the transmitting antenna electrode(s) and the sensing electrode(s) are separate from one another. In this case, the measurement circuit determines the current or voltage that is induced in the sensing electrode when the transmitting antenna electrode is operating. 
     The different capacitive sensing mechanisms are explained in 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. The paper 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 a counterelectrode, which is typically at ground potential. The object to be sensed modifies the capacitance between the transmit electrode and ground. In the “shunt mode”, an oscillating voltage signal is applied to the transmit electrode, building up an electric field to a receive electrode, and the displacement current induced at the receive electrode is measured, whereby the displacement current may be modified by 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. “Shunt mode” is alternatively referred to as the above-mentioned “coupling mode”. 
     Capacitive occupant sensing systems have been proposed in great variety, e.g. for controlling the deployment of one or more airbags, such as e.g. a driver airbag, a passenger airbag and/or a side airbag. U.S. Pat. No. 6,161,070, to Jinno et al., relates to a passenger detection system including a single antenna electrode mounted on a surface of a passenger seat in an automobile. An oscillator applies an oscillating voltage signal to the antenna electrode, whereby a minute electric field is produced around the antenna electrode. Jinno proposes detecting the presence or absence of a passenger in the seat based on the amplitude and the phase of the current flowing to the antenna electrode. U.S. Pat. No. 6,392,542, to Stanley, teaches an electric field sensor comprising an electrode mountable within a seat and operatively coupled to a sensing circuit, which applies to the electrode an oscillating or pulsed signal “at most weakly responsive” to wetness of the seat. Stanley proposes to measure phase and amplitude of the current flowing to the electrode to detect an occupied or an empty seat and to compensate for seat wetness. 
     The idea of using the heating element of a seat heater as an antenna electrode of a capacitive occupancy sensing system has been known for a long time. WO 92/17344 A1 discloses a an electrically heated vehicle seat with a conductor, which can be heated by the passage of electrical current, located in the seating surface, wherein the conductor also forms one electrode of a two-electrode seat occupancy sensor. 
     WO 95/13204 discloses a similar system, in which the oscillation frequency of an oscillator connected to the heating element is measured to derive the occupancy state of the vehicle seat. 
     U.S. Pat. No. 7,521,940 relates to a combined seat heater and capacitive sensor capable of operating, at a time, either in heating mode or in occupant-sensing mode. The device includes a sensor/heat pad for transmitting a sensing signal, a first diode coupled to a first node of the sensor/heat pad, a second diode coupled to a second node of the sensor/heat pad, a first transistor coupled to the first diode and a second transistor coupled to the second diode. During sensing mode, the first and second transistors are opened and the nodes between the first transistor and the first diode, as well as between the second transistor and the second diode are reverse-biased to isolate the sensor/heat pad from the power supply of the heating circuit. 
     US 2009/0295199 discloses a combined seat heater and capacitive sensor, wherein each of the two terminals of the heating element is connected to the heating power supply via two transistors in series. The device may not operate in sensing mode and in heating mode at a time. When the device is in sensing mode, the nodes between each pair of transistors are actively kept at the same potential as the heating element by means of respective voltage followers in order to neutralize any open-switch impedance of the transistors. 
     The very same idea has already been disclosed in U.S. Pat. No. 6,703,845. As an alternative to transistors, that document discloses inductors to achieve a high impedance at the frequency of the oscillating signal between the heating element and the power source of the heating circuit. As in the previously discussed document, a voltage follower maintains the intermediate nodes substantially at the same potential as the heating element in order to effectively isolate, at the frequency of the oscillating signal, the power supply of the heating circuit from the heating element. 
     Document DE 43 38 285 A1 discloses a combined seat heater and capacitive occupancy sensor wherein the heating element, together with the vehicle body as a counterelectrode, constitutes a capacitor. The capacitor is connected to an oscillating circuit, the frequency of which depends on the capacitance between the electrodes of the capacitor. The capacitance is dependent on the dielectric constant of the material, which is present between the electrodes. Thus, when the seat is unoccupied, a low dielectric constant exists, thereby providing low capacitance. This implies that the oscillator circuit oscillates at a relatively high frequency. Conversely, when the seat is occupied by a passenger, a higher dielectric constant is present and consequently the oscillator circuit oscillates at a relatively low frequency. By providing a control circuit that is activated by the presence of a frequency of certain magnitude, an arming signal can be transmitted to the airbag sensor when the seat is occupied. 
     A system of a similar type is described in document DE 41 10 702 A1. In this system the capacitor, whose frequency varies depending on the occupancy state, is formed by the heating element and electrode wires arranged in the vicinity of the heating element. A central control device measures the oscillation frequency to determine the occupancy state. 
     Document U.S. Pat. No. 5,525,843 also relates to a combined seat heater and capacitive occupancy sensor, wherein the change of the resonance frequency of the oscillator is used to determine whether the seat is occupied or empty. 
     What one tries to measure with such a capacitive sensing system is the overall impedance between the heating element and a counterelectrode (typically a grounded conductive surface or structure). The behaviour of the overall impedance is that of an a priori unknown complex network of resistors, capacitors and inductors. For a given, single, frequency, that complex network is electrically equivalent to a simple parallel network of a capacitive and a resistive component. The values of these components are frequency-dependent, which means that in a given situation (e.g. for a given occupancy state), measurements at different frequencies will yield different capacitance values and different resistance values. Therefore, measurements of the capacitance and the resistance carried out at different frequencies cannot directly be compared with one another. This represents a difficulty when the capacitive sensing network is allowed to oscillate at different resonance frequencies. 
     The variation of the resonance frequency over a large frequency range has the additional disadvantage that electromagnetic radiation is generated over this large frequency range. This poses a problem when radiation levels defined by automotive standards for example must not be exceeded by the capacitive measurement, for example to exclude interference in the AM bands of a radio receiver located in the car where the measurement circuit is installed. It is therefore preferred to restrain the frequency range to a defined range which does not overlap a critical frequency band where only low allowed radiation levels are defined, for example the AM radio frequency bands. 
     Finally, if variation of the resonance frequency is allowed over a large frequency range, there is also a non-negligible risk of receiving electromagnetic interference from other electronic appliances. 
     BRIEF SUMMARY 
     The disclosure provides a capacitive sensor wherein the above-mentioned drawbacks are eliminated or at least reduced. 
     A capacitive sensor comprises an antenna electrode (sensing antenna electrode) for capacitively coupling to a counterelectrode to form a capacitance, this capacitance being responsive to an electric-field-influencing property of an object or person proximate to the antenna electrode (i.e. between the antenna electrode and the counterelectrode). The counterelectrode may be or may not be part of the capacitive sensor. The capacitive sensor further comprises a capacitive sensing network connected to the antenna electrode to apply an oscillating signal (current or voltage) thereto and to determine the capacitance based upon characteristics (e.g. amplitude, phase, frequency, attenuation etc.) of the oscillating signal. According to the invention, the capacitive sensing network includes at least one inductor and a plurality of reactive components arranged to form a resonant network together with the capacitance, the plurality of reactive components being activatable and deactivatable in such a way as to modify a resonance frequency of the resonant network. 
     According to a preferred aspect of the invention, the capacitive sensor is implemented in a combined seat heater and capacitive occupancy sensor, e.g. for a vehicle seat. Such a combined seat heater and capacitive occupancy sensor comprises a heater network including a heating element connected between a first node and a second node to dissipate heat when a heating current is caused to flow between the first and second nodes, across the heating element, and a capacitive sensing network connected to the heating element to use the heating element as a sensing antenna electrode. The heating element is arranged for forming a capacitance with a counterelectrode, the capacitance being responsive to an electric-field-influencing property of an object or person proximate to the heating element. The capacitive sensing network is configured to apply an oscillating signal to the heating element and to determine the capacitance based upon characteristics of the oscillating signal. The heater network comprises a common mode choke with at least two windings, the heating element being connected in series between a first and a second winding of the at least two windings so as to be operatively connectable to a power source via the common mode choke. The capacitive sensing network includes a plurality of reactive components, arranged to form a resonant network with the first and/or the second winding and the capacitance, the plurality of reactive components being activatable and deactivatable in such a way as to modify a resonance frequency of the resonant network. 
     As those skilled will appreciate, thanks to the invention, the resonance frequency of the resonant network may be adjusted, in particular depending on the capacitance between the sensing antenna electrode (or the heating element) and the counterelectrode. The oscillating voltage preferably has a frequency in the range from about 50 kHz to about 10 GHz, more preferably in the range from about 50 kHz to about 30 MHz. 
     By activating or deactivating different groups of the reactive components, the capacitive sensor may perform a multitude of measurements at different resonance frequency. The combination of activated or deactivated reactive components may in particular be selected in such a way that the resonance frequencies of the measurements lie within a narrow frequency band, e.g. between 120 kHz and 150 kHz. As a consequence, the capacitance values obtained from these measurements can be directly compared to one another. The same is true for the resistance values obtained. Preferably, the capacitive sensing network comprises a control loop to confine the resonance frequency within a predefined target frequency band. 
     Since the resonance frequency of the capacitive sensing network may be confined to a narrow frequency band, noisy frequency bands can be avoided. 
     Preferably, the reactive components comprise capacitors. Alternatively or additionally, the reactive components may comprise further inductors. The reactive components may be arranged electrically in parallel to the unknown capacitance (between the antenna electrode or the heating element and the counterelectrode). The reactive components may have mutually different reactance values. 
     Preferably, the combined seat heater and capacitive occupancy sensor or the capacitive sensor comprises an electronically controlled switching arrangement configured to individually activate and deactivate the reactive components. The switching arrangement may e.g. comprise electronically controlled switches (e.g. transistors) arranged each electrically in parallel to or in series with a respective reactive component. 
     The capacitive sensing network preferably includes a controller, such as e.g. a microcontroller, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a digital signal processor (DSP) or the like, operatively connected with the electronically controlled switching arrangement to control said resonance frequency by activating or deactivating the electronically controlled switches or groups thereof. 
     The capacitive sensing network preferably comprises means to sustain the oscillating signal in or to drive the oscillating signal into the antenna electrode or the heating element as well as a high-impedance amplifier having an input node operatively connected to the antenna electrode or the heating element to probe the oscillating signal, and an output node to provide an output signal indicative of the oscillating signal. Preferably, the capacitive sensing network derives not only the capacitive load of the heating element but also the resistive part of the complex impedance between the heating element and ground. 
     Generally speaking, the output signal of the high-impedance amplifier allows measuring the voltage present on the antenna electrode or the heating element substantially without disturbing the measurement by its presence. The output voltage of the high-impedance amplifier permits to derive the complex impedance and thus the capacitance between the antenna electrode or heating element and ground. As the capacitance between the antenna electrode or heating element and ground depends on whether there is or not a conductive body (e.g. an occupant) in proximity of the antenna electrode or heating element, the occupancy state of an occupiable item (e.g. hospital bed, vehicle seat, office chair, etc.) containing the antenna electrode or the heating element can be derived from the output voltage of the high-impedance amplifier. As used herein, the term “impedance” designates the modulus (absolute value) of the complex impedance, which is itself defined as the ratio between (complex) voltage and (complex) current. When reference is made to the (complex) impedance to be measured or the capacitance to be measured, these terms designate the (complex) impedance or the capacitance between the heating element and the (typically grounded) counterelectrode (e.g. the vehicle frame). In the context of the present, the term “high-impedance amplifier” designates an amplifier, the complex impedance of which has a reactive part that is substantially higher (e.g. at least five times higher) than the reactive part of the complex impedance to be measured and a resistive part that is substantially higher (e.g. at least five times higher) than the resistive part of the complex impedance to be measured. 
     In the case of a combined seat heater and capacitive occupancy sensor, we will in the following assume that the heating current is direct current (DC) and that the oscillating signal sustained or driven into the heating element is an AC signal within a frequency region well above DC level. This is insofar a simplification that transient states (e.g. switching on/or off of the heating current), noise and parasitic currents are not taken into account. It should be noted that the heating current need not be direct current in the strictest sense: it may be variable, but on a long time-scale, so as not to interfere with the oscillating signal used for the capacitive measurement. For sake of simplicity, we will use “DC” to designate slowly varying or constant signals. 
     The means to sustain an oscillating signal in or to drive an oscillating signal into the antenna electrode or the heating element preferably comprises a negative resistance device (e.g. the “active” or power-supplying part of an oscillator circuit) to sustain the oscillating signal (at the resonance frequency) in the resonant network and to compensate for resistive losses and power extracted from the resonant network. The negative resistance device and the resonant network form together an oscillator, the resonance frequency of which depends on the inductance of the resonant network, and therefore, in particular, on the capacitance to be measured. 
     Preferably, the capacitive sensing network comprises a feedback branch from the output node of the high-impedance amplifier to the negative resistance device to regulate the amplitude of the oscillating signal to a reference amplitude. 
     The means to sustain an oscillating signal in or to drive an oscillating signal into the antenna electrode or the heating element may comprise an AC source operatively connected to the heating element to drive an alternative current into the resonant network and a frequency control unit for controlling the frequency of the alternative current. In this case, the oscillation of the resonant network is constrained to oscillation at the frequency determined by the frequency control unit. Preferably, the latter frequency is equal to or close to the resonance frequency of the resonant network (preferably within a narrow range around the resonance frequency). The complex impedance to be measured can then be obtained from the complex impedance of the resonant network, which is given by the ratio of the complex voltage probed by the high-impedance amplifier and the complex current driven into the resonant network by the AC source. The frequency control unit is preferably configured to vary the frequency of the alternative current within a frequency window. More preferably, the capacitive sensing network comprises a feedback branch from the output node of the high-impedance amplifier to the frequency control unit to regulate a phase difference of the output signal and the alternative current to a reference phase difference value. The reference phase difference value is preferably set to 0°, so that the feedback branch in fact regulates the frequency control unit to the resonance frequency of the resonant network. 
     Preferably, the extremities of the heating element are AC-coupled with one another, e.g. with a coupling capacitor. Such coupling capacitor is chosen to have an impedance that is substantially less than the impedance of the capacitance to be measured. The coupling capacitor thus represents a short for the AC component of the current but isolates the DC component thereof. A coupling capacitor between the extremities of the heating element ascertains that the capacitive occupancy sensor remains operational even if the heating element should break. 
     Preferably, the capacitive sensing network comprises a driven shield electrode. As used herein, the term driven shield electrode designates a further antenna electrode that is kept at substantially the same AC potential as the sensing antenna electrode or the heating element. As a consequence, the oscillating electric field substantially cancels between the driven shield electrode and the sensing antenna electrode or the heating element. It follows that a driven shield electrode substantially prevents the sensing antenna electrode or the heating element from capacitively coupling to objects, which, as seen from the sensing antenna electrode or the heating element, lie behind the driven shield electrode. One or more driven shield electrodes may thus be used to focus the sensitivity of the sensing antenna electrode or the heating element towards a region of interest, e.g. the part of space above a vehicle seat that is occupied by a normally seated occupant. To keep the driven shield electrode the same AC potential as the sensing antenna electrode or the heating element, an amplifier with high input impedance and gain substantially equal to 1, commonly known as a voltage follower or buffer amplifier, may be connected between the sensing antenna electrode or the heating element and the driven shield electrode to keep the driven shield electrode at the same AC potential as the sensing antenna electrode or the heating element. 
     A preferred aspect of the present invention concerns a vehicle seat equipped with a capacitive sensor or a combined seat heater and capacitive occupancy sensor. 
     Yet another aspect of the present invention concerns a capacitive sensing network configured to apply an oscillating signal to an antenna electrode forming a capacitance with a counterelectrode, the capacitance being responsive to an electric-field-influencing property of an object or person proximate to the antenna electrode, and to determine the capacitance based upon characteristics of the oscillating signal. The capacitive sensing network according to this aspect of the invention comprises an interface for connecting the capacitive sensing network to a seat heater including a heating element for dissipating heat when a heating current is caused to flow across the heating element, the interface being configured for operating the heating element as the antenna electrode. The interface comprises a common mode choke including a first winding for connecting a first node of the heating element to a first terminal of a power supply, a second winding for connecting a second node of the heating element to a second terminal of the power supply. The capacitive sensing network further includes a plurality of reactive components, arranged to form a resonant network with the first and/or the second winding of the common mode choke and the capacitance when the heating element is connected between the first and second windings, the plurality of reactive components being activatable and deactivatable in such a way as to modify a resonance frequency of the resonant network. 
     A capacitive sensing network according to this aspect of the invention may be used in combination with seat heaters known as such. This will be highly appreciated by the automotive industry, since it may be possible to use the same type of seat heater both in a configuration without capacitive occupancy sensing ability and in a configuration with capacitive occupancy sensing ability. In a vehicle seat without occupancy sensor, the seat heater may be directly plugged to the seat heater ECU including the power supply and the temperature controller, whereas in a vehicle seat with an occupancy sensor, the capacitive sensing network as described above may be connected between the seat heater ECU and the heating element as well as the temperature sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details and advantages of the present invention will be apparent from the following detailed description of limiting embodiments with reference to the attached drawings, wherein: 
         FIG. 1  is a schematic circuit diagram of a combined seat heater and capacitive occupancy sensor according to a preferred embodiment of the invention; 
         FIG. 2  is a schematic diagram of a first embodiment of the plurality of reactive components shown in  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a second embodiment of the plurality of reactive components shown in  FIG. 1 ; 
         FIG. 4  is a schematic diagram of a third embodiment of the plurality of reactive components shown in  FIG. 1 ; 
         FIG. 5  is a schematic circuit diagram of a preferred implementation of the combined seat heater and capacitive occupancy sensor of  FIG. 1 ; 
         FIG. 6  is a schematic illustration of a vehicle seat equipped with a combined seat heater and capacitive occupancy sensor substantially as in  FIG. 1 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a block schematic diagram of a combined seat heater and capacitive occupancy sensor according to an embodiment of the invention. The seat heater comprises a heating element  10 , which is used by the capacitive occupancy sensor as an antenna electrode that capacitively couples to ground. The strength of the capacitive coupling between the heating element  10  and ground depends on whether an occupant is present in the zone between the heating element  10  and the grounded counter-electrode. In a loading-mode capacitive occupancy sensor for a vehicle seat, the grounded counter-electrode normally corresponds to the vehicle chassis. 
     Turning first to the seat heater, the heater network includes power source  12  supplying the required DC heating current to the heating element  10  to perform the heating function. The heater network comprises temperature controller  14 , which turns the DC heating current on and off, depending on the actual and required temperature of the seat heater. 
     The heating element  10  is connected between a first  21  and a second  22  node. When a potential difference is applied by the power supply between the first and the second nodes  21 ,  22 , the heating current flows across the heating element  10 , which is thus caused to dissipate heat. The heating element  10  is operatively connected to the power source  12  with a common mode choke  16 . A first  16 . 1  and a second  16 . 2  winding thereof connects the first  21  and the second  22  node to a third  23  and a fourth  24  node, respectively. In  FIG. 1 , the third node  23  corresponds to ground, whereas the fourth node  24  is operatively connected to the high potential terminal of the power source  12  via the temperature controller  14 . The common mode choke  16  exhibits low impedance to DC but substantial impedance to AC at the operating frequency of the capacitive occupancy sensor. 
     The temperature controller  14  is operatively connected with a temperature sensor (not shown), which is arranged in vicinity of the heating element  10 . The temperature controller  14  may comprise a user-actuatable master switch (not shown) allowing the user to activate or deactivate the seat heater as a whole and control electronics (including e.g. a thermostat) that regulate the temperature to ascertain comfortable seating. When the seat heater is operating, the temperature controller  14  opens and closes the heating circuit (pulse-width modulation of the heating current) in such a way as to achieve a preset target temperature. Preferably, the target temperature may be selected by the user using a temperature control interface (e.g. a knob, a slider, a wheel or the like). The master switch and the temperature control interface are preferably integrated in the same control element. 
     When the seat heater is supplied with DC heating current (i.e. when temperature controller  14  closes the heating circuit), current flows from power source  12  though the controller  14 , the node  24  herein designated as fourth node, the second winding  16 . 2  of common mode choke  16 , the node  22  herein designated as second node, the heating element  10 , the node  21  herein designated as first node, the first winding  16 . 1  of common mode choke  16 , the node  23  herein designated as the third node, which is tied to ground potential. The heating circuit is completed via the ground connection between the third node  23  and power source  12 . 
     The capacitive sensing network (indicated in  FIG. 1  by the dotted line) comprises a high-impedance amplifier  32 , the input node  34  of which is connected to the heating element  10  at the first node  21 , an active component (in this case the negative resistance device  52 ) operatively connected to the heating element  10  at the first node  21 , a plurality of reactive components (generally indicated by reference number  36 , detailed hereinafter) and a microcontroller  90  operatively connected to receive the output signal of the high-impedance amplifier and to control the negative resistance device as well as to activate or deactivate the reactive components  36 . 
     Capacitors  40  and  42  symbolically represent the capacitive coupling of the heating element  10  to a grounded electrode (typically the vehicle frame). The capacitance (and hence the impedance) of these capacitors  40 ,  42  depends on whether the space between the heating element  10  and the grounded electrode is occupied by a conductive body (e.g. an occupant) or not. Capacitances  40  and  42  together represent the capacitance or impedance to be measured. It should be noted that the impedance to be measured behaves in practice like a distributed network comprising of resistive, capacitive and inductive parts. It is modelled for the purpose of this application by capacitors  40 ,  42 , which are paralleled by a single resistance (not shown in the drawings). However, this simplified model is valid only for a single frequency, which means that the resistance and capacitance measured at a first frequency and the resistance and capacitance measured at a second frequency cannot be compared directly, i.e. without any compensation for the difference in frequency. Such compensation may, however, be omitted if measurement errors introduced by this effect are negligibly small. This is achieved by keeping the resonance frequency within a narrow frequency band (such that variation of the resonance frequency can be neglected). 
     Capacitances  40  and  42 , as well as the reactive components  36  are electrically in parallel to the common mode choke  16  between the heating element  10  and ground. Accordingly, the common mode choke  16 , the reactive components  36  and the capacitance to be measured form a parallel resonant network, the resonance frequency of which depends among others on the capacitance to be measured. The reactive components  36  may be individually switched active or inactive by the microcontroller  90  in such a way as to shift the resonance frequency into a desired frequency band. 
     Negative resistance device  52  is preferably the active, oscillation-sustaining part of an oscillator. It sustains an oscillating current in the resonant network by compensating for resistive losses, in such a way that the resonant network operates at its resonance frequency. 
     The high input impedance amplifier  32  probes the AC voltage on the first node  21  and outputs a corresponding output signal on output node  44 , which is then processed further by the microcontroller  90  to derive the capacitance to be measured. 
     The complex impedance to be measured (and thus the capacitance to be measured) may be determined based on the frequency and the amplitude of the output signal, together with the known complex impedances of the common mode choke  16  and the reactive components  36 . 
     The capacitive sensing network shown in  FIG. 1  further comprises a coupling capacitor  46 , which represents an AC shunt of the heating element  10 . The impedance of capacitor  46  is chosen substantially smaller than the impedance of the total capacitance to be measured. In the absence of capacitor  46 , an interruption (break) of the heating element  10  would result in a substantially smaller antenna electrode: this, in turn, would reduce the measurable capacitance. For instance, if heating element  10  shown in  FIG. 1  broke in the middle, the measurement circuit would measure capacitance  40  (but not capacitance  42 ). Coupling capacitor  46  achieves an AC short between the first and second nodes  21 ,  22 , i.e. the terminals of the heating element  10 . If a (single) break occurs in heating element  10 , then the capacitive sensing network remains substantially unaffected and still measures the total capacitance between the heating element  10  and ground due to the AC shunt provided by capacitor  46 . 
     Coupling capacitor  48  provides an AC short between the third node  23  and the fourth node  24 . Capacitor  48  avoids that any AC current is fed into the DC power source  12  and thereby possibly into the car power network. 
       FIG. 2  shows a first possible embodiment of the plurality of activatable or deactivatable reactive components  36 . The plurality of activatable or deactivatable reactive components  36  comprises capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  arranged electrically in parallel. Each of the capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  is connected in series with an electronic switch  137 . 1 ,  137 . 2 ,  137 . 3  or  137 . 4 , respectively. Electronic switches  137 . 1 ,  137 . 2 ,  137 . 3  and  137 . 4  are individually controllable by the microcontroller  90  (see  FIG. 1 ) in order to activate or deactivate the corresponding capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4 . The capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  have known capacitances and are selectively connectable in parallel to the capacitance  40 ,  42  (see  FIG. 1 ) to be measured. The switches  137 . 1 ,  137 . 2 ,  137 . 3  and  137 . 4  can for example be MOSFETs. 
     A problem which may arise when the inductance of a common mode choke is used as inductance of the parallel resonant LC tank together with the capacitance to be measured, is that the drift or temperature dependence or part tolerance of the inductance will lead to a measurement error of the unknown capacitance. The computation of the capacitance to be measured may be made independent on the complex impedance of the common mode choke  16  using the capacitors  136 . 1 ,  136 . 2 ,  136 . 3  or  136 . 4  or any combination thereof. 
     Each of the capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  has a known capacitance (C 136.1 , C 136.2 , C 136.3  and C 136.4 , respectively). We will assume that the open-switch capacitances of switches  137 . 1 ,  137 . 2 ,  137 . 3  and  137 . 4  can be neglected compared to the capacitance of the associated capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4 . 
     To eliminate the potentially variable impedance, the following procedure may e.g. be executed under control of the microcontroller. A first measurement of the resonance frequency of the parallel resonant LC tank is made with a first combination of capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  activated (the corresponding switches  137 . 1 ,  137 . 2 ,  137 . 3  and  137 . 4  are closed). This frequency value is stored (here as fa). A second measurement of the resonance frequency is made with a second combination of the capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  activated (the corresponding switches  137 . 1 ,  137 . 2 ,  137 . 3  and  137 . 4  are closed), i.e. connected in parallel to the capacitance to be measured. The so-obtained frequency value is stored (here as fb). The first and second combinations of the capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  have to be chosen such that the resulting frequencies fa and fb are different. The relations between the resonance frequencies and the inductive and capacitive components of the circuit may be expressed through: 
             fa   =     1     2   ⁢     π   ·       L   ·     (     Cx   +     C   1       )                           fb   =     1     2   ⁢     π   ·       L   ·     (     Cx   +     C   2       )                     
where L is the inductance of the common mode choke, Cx is the capacitance to be measured, C 1  is the total capacitance of the activated one(s) of capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  according to the first combination and C 2  is the total capacitance of the activated one(s) of capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  according to the second combination.
 
     The two equations can be combined to yield Cx as a function of the measured frequencies fa and fb: 
     
       
         
           
             Cx 
             = 
             
               
                 
                   
                     fa 
                     2 
                   
                   ⁢ 
                   
                     C 
                     1 
                   
                 
                 - 
                 
                   
                     fb 
                     2 
                   
                   ⁢ 
                   
                     C 
                     2 
                   
                 
               
               
                 
                   fb 
                   2 
                 
                 - 
                 
                   fa 
                   2 
                 
               
             
           
         
       
     
     In the latter equation, the inductance L has been eliminated and thus does not influence the capacitance measurement. 
     Since the inductance L and the unknown capacitance Cx may not vary much between the measurements of the resonance frequencies fa and fb, these measurements have to be carried out sufficiently shortly one after the other. 
     At the resonance frequency, current and voltage of the parallel resonant network are in phase and the resistive part of the impedance to be measured thus corresponds to the ratio of the voltage to the current. The microcontroller may thus determine the resistive part of the impedance to be measured by measuring the voltage and the current across the resonant network. 
     Another advantage of the individually activatable capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  is that the microcontroller  90  can shift the resonance frequency to a predefined (narrow) target frequency band whether the seat is occupied or free. That predefined frequency band is preferably chosen such that it does not overlap with frequency bands occupied for transmission or reception by other devices in the vicinity of the combined seat heater and capacitive occupancy sensor or reserved frequency bands (e.g. AM radio frequency bands). By appropriately choosing the target frequency band wherein the combined seat heater and capacitive occupancy sensor may operate and selecting the reactive components in such a way that by activating or deactivating combinations thereof the resonance frequency of the resonant network may be shifted into the target frequency band for any unknown capacitance within a certain specified capacitance range, it will thus be possible to prevent or reduce electromagnetic interference with other electronic devices. This ascertains that other electronic devices may operate without being disturbed by the combined seat heater and capacitive occupancy sensor and that the combined seat heater and capacitive occupancy sensor may also operate without being disturbed by the other electronic devices. Automotive standards for example define levels of electromagnetic radiation, which must be tolerated by a measurement circuit without generating a measurement error. These levels depend on the frequency. Thanks to the present invention it is thus possible to avoid frequency bands wherein the tolerable radiation levels are large. 
     When the capacitive sensing network is powered up, the microcontroller  90  preferably controls the capacitive sensing network to perform one or more measurements of the unknown capacitance with a power level that is lower than during the normal measurements. This is because at start-up, the capacitance to be measured is completely unknown (it may be completely different from the capacitance at the previous shut-down of the system) and, hence, it is not known which will be the resulting resonance frequency. By keeping the amplitude of the LC tank at a lower level during start-up than during normal operation, it is avoided that the system generates significant interference outside its target frequency band. During this phase, the microcontroller  90  preferably dynamically adjusts the paralleled reactance  36  in such a way as to shift the resonance frequency into to the target frequency band. The microcontroller may achieve this using a feedback loop or by the calculating the amount by which the paralleled reactance must be increased or decreased and adjusting the paralleled reactance  36  in consequence. 
     The microcontroller  90  preferably maintains a safety margin between the current resonance frequency and the bounds of the target frequency band so as to be able to react in case the capacitance to be measured changes between the last and the next measurement. Thus, if the resonance frequency is too close to the upper or the lower bound of the target frequency band, the microcontroller  90  activates a different combination of the capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  such that the thus resulting resonance frequency is shifted towards the centre of the target frequency band. The repetition rate of the impedance measurements is selected depending on the safety margin of the target frequency band and the maximum expected rate of change of the capacitance to be measured. In particular, the repetition rate is chosen sufficiently high and the safety margin of the target frequency band sufficiently large to allow the microcontroller to deal with any impedance change that does not exceed a certain predefined rate of change. Preferably, the microcontroller is configured to reduce the power level of the LC tank in case the resonance frequency should accidentally leave the target frequency (e.g. due to an abrupt change of the capacitance to be measured). Once the paralleled reactance has been adjusted to the new situation and the resonance frequency has been shifted back to the target frequency band, the power level may again be raised. 
     It could happen that one or more of the reactive components break partly or completely, thereby changing their value, implying that the assumption of known value is not true anymore. This problem can be solved by adding in parallel to the capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  an additional set of capacitors having substantially the same capacitance values C 136.1 , C 136.2 , C 136.3  and C 136.4 . In order to check the capacitance of e.g. capacitor  136 . 1 , a measurement of the unknown capacitance may be performed a first time with capacitor  136 . 1  and a second time with the capacitor having the same nominal capacitance as capacitor  136 . 1 . If these measurements yield different resonance frequencies, it can be deduced that the capacitor  136 . 1  or its duplicate is defective. 
     Numerical Example 
     FIG.  2   
     The target frequency band of the capacitive sensing network is assumed to range from 120 kHz to 150 kHz. 
     In this example, L (common mode choke inductance)=10 mH, C 136.1 =10 pF, C 136.2 =20 pF, C 136.3 =40 pF, C 136.4 =80 pF and switches  137 . 1 ,  137 . 2 ,  137 . 3  and  137 . 4  have negligible open-switch capacitances. 
     All the possible combinations of the paralleled capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  yield  16  different known capacitances, ranging from 0 pF (all switches  137 . 1 ,  137 . 2 ,  137 . 3  and  137 . 4  are open) to 150 pF (all switches  137 . 1 ,  137 . 2 ,  137 . 3  and  137 . 4  are closed). Assuming that 10 pF≦Cx≦100 pF, the total capacitance C total , which is the sum of the unknown capacitance and the known capacitance, will range from 10 pF to 250 pF depending on the combination of activated capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4 . The resulting resonance frequencies will range from 100.66 kHz to 503.29 kHz. 
     In particular, the following resonance frequencies are obtainable if the unknown capacitance Cx amounts to 10 pF: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Activated capacitances 
                 C known /pF 
                 C total /pF 
                 F res /kHz 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 none 
                 0 
                 10 
                 503.29 
               
               
                   
                 C 136.1   
                 10 
                 20 
                 355.88 
               
               
                   
                 C 136.2   
                 20 
                 30 
                 290.58 
               
               
                   
                 C 136.1  and C 136.2   
                 30 
                 40 
                 251.65 
               
               
                   
                 C 136.3   
                 40 
                 50 
                 225.08 
               
               
                   
                 C 136.3  and C 136.1   
                 50 
                 60 
                 205.47 
               
               
                   
                 C 136.3  and C 136.2   
                 60 
                 70 
                 190.23 
               
               
                   
                 C 136.3  and C 136.1  and C 136.2   
                 70 
                 80 
                 177.94 
               
               
                   
                 C 136.4   
                 80 
                 90 
                 167.76 
               
               
                   
                 C 136.4  and C 136.1   
                 90 
                 100 
                 159.15 
               
               
                   
                 C 136.4  and C 136.2   
                 100 
                 110 
                 151.75 
               
               
                   
                 
                   C 
                   136.4  
                   and C 
                   136.1  
                   and C 
                   136.2 
                 
                 110 
                 120 
                 
                   145.29 
                 
               
               
                   
                 
                   C 
                   136.4  
                   and C 
                   136.3 
                 
                 120 
                 130 
                 
                   139.59 
                 
               
               
                   
                 
                   C 
                   136.4  
                   and C 
                   136.1  
                   and C 
                   136.3 
                 
                 130 
                 140 
                 
                   134.51 
                 
               
               
                   
                 
                   C 
                   136.4  
                   and C 
                   136.2  
                   and C 
                   136.3 
                 
                 140 
                 150 
                 
                   129.95 
                 
               
               
                   
                 
                   C 
                   136.1  
                   to C 
                   136.4 
                 
                 150 
                 160 
                 
                   125.82 
                 
               
               
                   
                   
               
             
          
         
       
     
     The usable (allowed) combinations of capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  and the corresponding frequencies in the target frequency band, with Cx=10 pF, are shown in bold. 
     If the unknown capacitance Cx amounts to 100 pF, the obtained resonance frequencies are: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Activated capacitances 
                 C known /pF 
                 C total /pF 
                 F res /kHz 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 none 
                 0 
                 100 
                 159.15 
               
               
                   
                 C 136.1   
                 10 
                 110 
                 151.75 
               
               
                   
                 
                   C 
                   136.2 
                 
                 20 
                 120 
                 
                   145.29 
                 
               
               
                   
                 
                   C 
                   136.1  
                   and C 
                   136.2 
                 
                 30 
                 130 
                 
                   139.59 
                 
               
               
                   
                 
                   C 
                   136.3 
                 
                 40 
                 140 
                 
                   134.51 
                 
               
               
                   
                 
                   C 
                   136.3  
                   and C 
                   136.1 
                 
                 50 
                 150 
                 
                   129.95 
                 
               
               
                   
                 
                   C 
                   136.3  
                   and C 
                   136.2 
                 
                 60 
                 160 
                 
                   125.82 
                 
               
               
                   
                 
                   C 
                   136.3  
                   and C 
                   136.1  
                   and C 
                   136.2 
                 
                 70 
                 170 
                 
                   122.07 
                 
               
               
                   
                 C 136.4   
                 80 
                 180 
                 118.63 
               
               
                   
                 C 136.4  and C 136.1   
                 90 
                 190 
                 115.46 
               
               
                   
                 C 136.4  and C 136.2   
                 100 
                 200 
                 112.54 
               
               
                   
                 C 136.4  and C 136.1  and C 136.2   
                 110 
                 210 
                 109.83 
               
               
                   
                 C 136.4  and C 136.3   
                 120 
                 220 
                 107.3 
               
               
                   
                 C 136.4  and C 136.1  and C 136.3   
                 130 
                 230 
                 104.94 
               
               
                   
                 C 136.4  and C 136.2  and C 136.3   
                 140 
                 240 
                 102.73 
               
               
                   
                 C 136.1  to C 136.4   
                 150 
                 250 
                 100.66 
               
               
                   
                   
               
             
          
         
       
     
     The usable combinations of capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  and the corresponding frequencies in the target frequency band, with Cx=100 pF, are shown in bold. 
     If the inductance of the common mode choke is not precisely known (e.g. due to temperature variations, ageing, etc.), the unknown capacitance may be determined as described above, by using different pairs of combinations of capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4  selected among the usable combinations. Designating by Cx i  the capacitance value obtained by using a first combination (capacitance C 1,i , resonance frequency fa i ) and a second combination (capacitance C 2,i ≠C 1,i , resonance frequency fb i ≠fa i ), one obtains: 
     
       
         
           
             
               
                 
                   
                     Cx 
                     i 
                   
                   = 
                   
                     
                       
                         
                           fa 
                           i 
                           2 
                         
                         ⁢ 
                         
                           C 
                           
                             1 
                             , 
                             i 
                           
                         
                       
                       - 
                       
                         
                           fb 
                           i 
                           2 
                         
                         ⁢ 
                         
                           C 
                           
                             2 
                             , 
                             i 
                           
                         
                       
                     
                     
                       
                         fb 
                         i 
                         2 
                       
                       - 
                       
                         fa 
                         i 
                         2 
                       
                     
                   
                 
               
               
                 
                   
                     ( 
                     * 
                   
                   ) 
                 
               
             
           
         
       
     
     With n usable combinations, there are 
               (         n           2         )     =     n   ⁡     (     n   -   1     )             
possible ways of calculating Cx using the above formula (*). Preferably, the microcontroller is configured to carry out a plurality of resonance frequency measurements using different allowed combinations of the capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4 , to calculate values Cx i  (i=1 . . . n(n−1)) of the unknown capacitance from a plurality of combination pairs and to compute the final value Cx of the unknown capacitance as the average or the median of the individual measurements Cx i . The resistive part of the impedance is determined as the average or median of the calculated resistive parts for the different combination pairs. It should be noted that the averaging of the individual measurements of the capacitance and the individual measurements of the resistance is possible because the intervening resonance frequencies are all contained in a narrow frequency band (here: from 120 kHz to 150 kHz).
 
     In case of interference with another electronic appliance in the neighbourhood of the capacitive sensing network, one or more of the calculated capacitance values Cx i  may be invalid. If these invalid measurements were taken into account for the calculation of the average capacitance, this could give rise to a significant measurement error. Therefore, the capacitance values Cx i  (i=1 . . . n(n−1)) obtained with the different capacitor combinations are preferably analysed for outliers. Any method suitable for outlier detection in a population of measurement values can a priori be used in this context. For instance, one could calculate the difference ΔCx i  between each value Cx i  and the average or median value Cx (ΔCx i =Cx−Cx i ) and discard those values Cx i  that are more distant from the calculated average value Cx than a predetermined threshold value. 
     For example, taking the numerical values from the second table above, the measured resonance frequencies with C known =30 pF, 40 pF, 50 pF, 60 pF and 70 pF are located within the target frequency band. The values Cx i  are calculated using different combinations of the retained resonance frequencies. For this example, it is also assumed that an interference creates a measurement error of 1% of the measured resonance frequency measured with C known =30 pF, that is, a resonance frequency of 140.99 kHz instead of the 139.59 kHz (as shown in the second table above) is measured. 
     The following table shows the calculated unknown capacitances Cx i  (in pF) obtained by all the possible combinations of the measured resonance frequencies. The C known -values in bold characters in the left column and the top row indicate the known capacitances in pF that have been used to calculate the unknown capacitances Cx i . 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                 C known 
                 
                   30 
                 
                 
                   40 
                 
                 
                   50 
                 
                 
                   60 
                 
               
               
                   
               
             
             
               
                 
                   40 
                 
                 71.5 
                   
                   
                   
               
               
                 
                   50 
                 
                 83.0 
                 100.0 
               
               
                 
                   60 
                 
                 87.4 
                 100.0 
                 100.0 
               
               
                 
                   70 
                 
                 89.8 
                 100.0 
                 100.0 
                 100.0 
               
               
                   
               
             
          
         
       
     
     The median value of all the calculated values in this example is 100 pF. A threshold is defined which determines which unknown capacitances are considered to be valid. For this example, the threshold is defined to be 10%, that is, all the values that are lower than 90% of the median value and all the values that are above 110% of the median value are discarded. From the table above, all the unknown capacitances Cx i  measured with an applied known capacitance of 30 pF are therefore discarded. 
     As an alternative to the detection of outliers among the values Cx i  (i=1 . . . n(n−1)), the microcontroller may proceed as follows. For each of the allowed combinations of capacitors  136 . 1 ,  136 . 2 ,  136 . 3  and  136 . 4 , several measurements of the resonance frequency and of the parallel resistance are carried out. The standard deviation of each measured resonance frequency and of the equivalent parallel resistance is calculated. If the standard deviation of the frequency and/or the parallel resistance exceeds a predetermined threshold, the corresponding measured capacitance and resistance of that resonance frequency are discarded. 
     The capacitors of known value can also be replaced individually or altogether with inductances, or any complex impedances, i.e. combinations of a reactive and a resistive part. 
       FIG. 3  shows a second possible embodiment of the plurality of activatable or deactivatable reactive components  36  (see also  FIG. 1 ). According to this embodiment, the plurality of activatable or deactivatable reactive components  36  comprises a capacitor  236   c , arranged in parallel with inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4 . Each of the inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4  is connected in series with an electronic switch  237 . 1 ,  237 . 2 ,  237 . 3  or  237 . 4 , respectively. Electronic switches  237 . 1 ,  237 . 2 ,  237 . 3  and  237 . 4  are individually controllable by the microcontroller  90  (see  FIG. 1 ) in order to activate or deactivate the corresponding inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4 . The inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4  have known inductance each and are selectively connectable in parallel to the capacitor  236   c  and the capacitance  40 ,  42  (see  FIG. 1 ) to be measured. The switches  237 . 1 ,  237 . 2 ,  237 . 3  and  237 . 4  can for example be MOSFETs. The parallel capacitor  236   c  has a known capacitance and is provided to keep the resonance frequency of the resonant network in an acceptable range while using practical inductance values for the known inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4 . 
     To eliminate the potentially variable impedance of the common mode choke, the following procedure may e.g. be executed under control of the microcontroller  90  (see  FIG. 1 ). A first measurement of the resonance frequency of the parallel resonant LC tank is made with a first combination of inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4  activated (the corresponding switches  237 . 1 ,  237 . 2 ,  237 . 3  and  237 . 4  are closed). This frequency value is stored (here as fa). A second measurement of the resonance frequency is made with a second combination of the capacitors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4  activated (the corresponding switches  237 . 1 ,  237 . 2 ,  237 . 3  and  237 . 4  are closed), i.e. connected in parallel to the capacitance to be measured. The so-obtained frequency value is stored (here as fb). The first and second combinations of the inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4  have to be chosen such that the resulting frequencies fa and fb are different. The relations between the resonance frequencies and the inductive and capacitive components of the circuit may be expressed through: 
             fa   =     1     2   ⁢     π   ·           L   ·     L   1           L   1     +   L       ·     (     Cx   +     C     236   ⁢   c         )                           fb   =     1     2   ⁢     π   ·           L   ·     L   2           L   2     +   L       ·     (     Cx   +     C     236   ⁢   c         )                     
where L is the inductance of the common mode choke, Cx is the capacitance to be measured, L 1  is the total inductance of the activated one(s) of inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4  according to the first combination, L 2  is the total capacitance of the activated one(s) of capacitors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4  according to the second combination and C 236c  is the capacitance of capacitor  236   c . These equations can be combined to yield Cx as a function of the measured frequencies fa and fb:
 
     
       
         
           
             Cx 
             = 
             
               
                 
                   
                     L 
                     1 
                   
                   - 
                   
                     L 
                     2 
                   
                 
                 
                   
                     
                       ( 
                       
                         2 
                         ⁢ 
                         π 
                       
                       ) 
                     
                     2 
                   
                   · 
                   
                     L 
                     1 
                   
                   · 
                   
                     L 
                     2 
                   
                   · 
                   
                     ( 
                     
                       
                         fa 
                         2 
                       
                       - 
                       
                         fb 
                         2 
                       
                     
                     ) 
                   
                 
               
               - 
               
                 
                   C 
                   
                     236 
                     ⁢ 
                     c 
                   
                 
                 . 
               
             
           
         
       
     
     In the latter equation, the inductance L has been eliminated and thus does not influence the capacitance measurement. 
     Since the inductance L and the unknown capacitance Cx may not vary much between the measurements of the resonance frequencies fa and fb, these measurements have to be carried out sufficiently shortly one after the other. 
     At the resonance frequency, current and voltage of the parallel resonant network are in phase and the resistive part of the impedance to be measured thus corresponds to the ratio of the voltage to the current. The microcontroller  90  ( FIG. 1 ) may thus determine the resistive part of the impedance to be measured by measuring the voltage and the current across the resonant network. 
     As for the embodiment of  FIG. 2 , the microcontroller  90  may shift the resonance frequency of the capacitive sensing network by activating or deactivating the reactive components, i.e. in this case the inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4 . The microcontroller may in particular be configured to run a similar start-up procedure and take similar measures to ascertain low or no electromagnetic interference with other appliances as described with respect to the embodiment of  FIG. 1 . 
     Numerical Example 
     FIG.  3   
     In this example, capacitor  236   c  is assumed to have a capacitance of 300 pF, L (common mode choke inductance)=10 mH, and the inductances L 236.1 , L 236.2 , L 236.3  and L 236.4  of the inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4  are 20 mH, 10 mH, 5 mH and 2.5 mH, respectively. It will be assumed that the open-switch capacitances of switches  237 . 1 ,  237 . 2 ,  237 . 3  and  237 . 4  can be neglected. 
     The target frequency band of the capacitive sensing network is assumed to range from 120 kHz to 150 kHz. 
     By activating or deactivating different groups of the inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4 , the following resonance frequencies (Fres) may be obtained for an unknown capacitance of 0 pF: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Activated inductances 
                 L known /mH 
                 L total /mH 
                 F res /kHz 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 none 
                 / 
                 10 
                 91.89 
               
               
                 
                   L 
                   236.1 
                 
                 20 
                 6.67 
                 
                   112.54 
                 
               
               
                 
                   L 
                   236.2 
                 
                 10 
                 5 
                 
                   129.95 
                 
               
               
                 
                   L 
                   236.1  
                   and L 
                   236.2 
                 
                 6.67 
                 4 
                 
                   145.29 
                 
               
               
                 L 236.3   
                 5 
                 3.33 
                 159.15 
               
               
                 L 236.3  and L 236.1   
                 4 
                 2.86 
                 171.91 
               
               
                 L 236.3  and L 236.2   
                 3.33 
                 2.5 
                 183.78 
               
               
                 L 236.3  and L 236.1  and L 236.2   
                 2.86 
                 2.22 
                 194.92 
               
               
                 L 236.4   
                 2.5 
                 2 
                 205.47 
               
               
                 L 236.4  and L 236.1   
                 2.22 
                 1.82 
                 215.5 
               
               
                 L 236.4  and L 236.2   
                 2 
                 1.67 
                 225.08 
               
               
                 L 236.4  and L 236.1  and L 236.2   
                 1.82 
                 1.54 
                 234.27 
               
               
                 L 236.4  and L 236.3   
                 1.67 
                 1.43 
                 243.11 
               
               
                 L 236.4  and L 236.1  and L 236.3   
                 1.54 
                 1.33 
                 251.65 
               
               
                 L 236.4  and L 236.2  and L 236.3   
                 1.43 
                 1.25 
                 259.9 
               
               
                 L 236.1  to L 236.4   
                 1.33 
                 1.18 
                 267.9 
               
               
                   
               
             
          
         
       
     
     Resonance frequencies that lie inside the target frequency band are again in bold characters. 
     The following table shows the same results with an unknown capacitance of 100 pF: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Activated inductances 
                 L known /mH 
                 L total /mH 
                 F res /kHz 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 none 
                 / 
                 10 
                 79.58 
               
               
                 L 236.1   
                 20 
                 6.67 
                 97.46 
               
               
                 L 236.2   
                 10 
                 5 
                 112.54 
               
               
                 
                   L 
                   236.1  
                   and L 
                   236.2 
                 
                 6.67 
                 4 
                 
                   125.82 
                 
               
               
                 
                   L 
                   236.3 
                 
                 5 
                 3.33 
                 
                   137.83 
                 
               
               
                 
                   L 
                   236.3  
                   and L 
                   236.1 
                 
                 4 
                 2.86 
                 
                   148.88 
                 
               
               
                 L 236.3  and L 236.2   
                 3.33 
                 2.5 
                 159.15 
               
               
                 L 236.3  and L 236.1  and L 236.2   
                 2.86 
                 2.22 
                 168.81 
               
               
                 L 236.4   
                 2.5 
                 2 
                 177.94 
               
               
                 L 236.4  and L 236.1   
                 2.22 
                 1.82 
                 186.63 
               
               
                 L 236.4  and L 236.2   
                 2 
                 1.67 
                 194.92 
               
               
                 L 236.4  and L 236.1  and L 236.2   
                 1.82 
                 1.54 
                 202.88 
               
               
                 L 236.4  and L 236.3   
                 1.67 
                 1.43 
                 210.54 
               
               
                 L 236.4  and L 236.1  and L 236.3   
                 1.54 
                 1.33 
                 217.93 
               
               
                 L 236.4  and L 236.2  and L 236.3   
                 1.43 
                 1.25 
                 225.08 
               
               
                 L 236.1  to L 236.4   
                 1.33 
                 1.18 
                 232.01 
               
               
                   
               
             
          
         
       
     
     The usable combinations of inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4  and the corresponding frequencies in the target frequency band, with Cx=100 pF, are shown in bold. 
     The microcontroller  90  ( FIG. 1 ) may thus control the switches  237 . 1 ,  237 . 2 ,  237 . 3  and  237 . 4  in such a way as to keep the resonance frequency within the target frequency band when the capacitance to be measured varies. 
     As for the previous example, the microcontroller may advantageously be configured such that it uses different pairs of combinations of inductors  236 . 1 ,  236 . 2 ,  236 . 3  and  236 . 4  selected among the usable combinations in order to measure a plurality of capacitance values Cx i . The microcontroller may then compute the final value Cx of the unknown capacitance as the average or the median of the individual measurements Cx i . The resistive part of the impedance is determined as the average or median of the calculated resistive parts for the different combination pairs. 
       FIG. 4  shows a third possible embodiment of the plurality of activatable or deactivatable reactive components  36  (see  FIG. 1 ). According to this embodiment, the plurality of activatable or deactivatable reactive components  36  comprises a capacitor  336   c , arranged in parallel with a series of inductors  336 . 1 ,  336 . 2 ,  336 . 3  and  336 . 4 . The microcontroller  90  control the overall impedance of the network of inductors  336 . 1 ,  336 . 2 ,  336 . 3  and  336 . 4  by activating different groups of the inductors  336 . 1 ,  336 . 2 ,  336 . 3  and  336 . 4  at a time. Each of the inductors  336 . 1 ,  336 . 2 ,  336 . 3  and  336 . 4  is connected parallel to an electronic switch  337 . 1 ,  337 . 2 ,  337 . 3  or  337 . 4 , respectively. Electronic switches  337 . 1 ,  337 . 2 ,  337 . 3  and  337 . 4  are individually controllable by the microcontroller  90  (see  FIG. 1 ) in order to activate or deactivate the corresponding inductors  336 . 1 ,  336 . 2 ,  336 . 3  and  336 . 4 . The inductors  336 . 1 ,  336 . 2 ,  336 . 3  and  336 . 4  have known inductance each may and be selectively deactivated by closing the corresponding switch  337 . 1 ,  337 . 2 ,  337 . 3  or  337 . 4 , respectively. The switches  337 . 1 ,  337 . 2 ,  337 . 3  and  337 . 4  can for example be MOSFETs. The parallel capacitor  336   c  has a known capacitance and is provided to keep the resonance frequency of the resonant network in an acceptable range while using practical inductance values for the known inductors  336 . 1 ,  336 . 2 ,  336 . 3  and  336 . 4 . 
     Numerical Example 
     FIG.  4   
     In this example, capacitor  336   c  is assumed to have a capacitance of 300 pF, L (common mode choke inductance)=10 mH, and the inductances L 336.1 , L 336.2 , L 336.3  and L 336.4  of the inductors  336 . 1 ,  336 . 2 ,  336 . 3  and  336 . 4  are 1.25 mH, 2.5 mH, 5 mH and 10 mH, respectively. It will be assumed that the open-switch capacitances of switches  337 . 1 ,  337 . 2 ,  337 . 3  and  237 . 4  can be neglected. 
     The target frequency band of the capacitive sensing network is assumed to range from 120 kHz to 150 kHz. 
     By activating or deactivating different groups of the inductors  336 . 1 ,  336 . 2 ,  336 . 3  and  336 . 4 , the following resonance frequencies (Fres) may be obtained for an unknown capacitance of 0 pF: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Activated inductances 
                 L known /mH 
                 L total /mH 
                 F res /kHz 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 L 336.1   
                 1.25 
                 1.11 
                 275.66 
               
               
                 L 336.2   
                 2.5 
                 2 
                 205.47 
               
               
                 L 336.1  and L 336.2   
                 3.75 
                 2.73 
                 175.95 
               
               
                 L 336.3   
                 5 
                 3.33 
                 159.15 
               
               
                 L 336.3  and L 336.1   
                 6.25 
                 3.85 
                 
                   148.17 
                 
               
               
                 L 336.3  and L 336.2   
                 7.5 
                 4.29 
                 
                   140.36 
                 
               
               
                 L 336.3  and L 336.1  and L 336.2   
                 8.75 
                 4.67 
                 
                   134.51 
                 
               
               
                 L 336.4   
                 10 
                 5 
                 
                   129.95 
                 
               
               
                 L 336.4  and L 336.1   
                 11.25 
                 5.29 
                 
                   126.29 
                 
               
               
                 L 336.4  and L 336.2   
                 12.5 
                 5.56 
                 
                   123.28 
                 
               
               
                 L 336.4  and L 336.1  and L 336.2   
                 13.75 
                 5.79 
                 
                   120.76 
                 
               
               
                 L 336.4  and L 336.3   
                 15 
                 6 
                 118.63 
               
               
                 L 336.4  and L 336.1  and L 336.3   
                 16.25 
                 6.19 
                 116.79 
               
               
                 L 336.4  and L 336.2  and L 336.3   
                 17.5 
                 6.36 
                 115.19 
               
               
                 L 336.1  to L 336.4   
                 18.75 
                 6.52 
                 113.78 
               
               
                   
               
             
          
         
       
     
     Resonance frequencies that lie inside the target frequency band are in bold characters. 
     The following table shows the same results with an unknown capacitance of 100 pF: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Activated inductances 
                 L known /mH 
                 L total /mH 
                 F res /kHz 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 L 336.1   
                 1.25 
                 1.11 
                 238.73 
               
               
                 L 336.2   
                 2.5 
                 2 
                 177.94 
               
               
                 L 336.1  and L 336.2   
                 3.75 
                 2.73 
                 152.38 
               
               
                 L 336.3   
                 5 
                 3.33 
                 
                   137.83 
                 
               
               
                 L 336.3  and L 336.1   
                 6.25 
                 3.85 
                 
                   128.31 
                 
               
               
                 L 336.3  and L 336.2   
                 7.5 
                 4.29 
                 
                   121.56 
                 
               
               
                 L 336.3  and L 336.1  and L 336.2   
                 8.75 
                 4.67 
                 116.49 
               
               
                 L 336.4   
                 10 
                 5 
                 112.54 
               
               
                 L 336.4  and L 336.1   
                 11.25 
                 5.29 
                 109.37 
               
               
                 L 336.4  and L 336.2   
                 12.5 
                 5.56 
                 106.76 
               
               
                 L 336.4  and L 336.1  and L 336.2   
                 13.75 
                 5.79 
                 104.59 
               
               
                 L 336.4  and L 336.3   
                 15 
                 6 
                 102.73 
               
               
                 L 336.4  and L 336.1  and L 336.3   
                 16.25 
                 6.19 
                 101.14 
               
               
                 L 336.4  and L 336.2  and L 336.3   
                 17.5 
                 6.36 
                 99.76 
               
               
                 L 336.1  to L 336.4   
                 18.75 
                 6.52 
                 98.54 
               
               
                   
               
             
          
         
       
     
     The usable combinations of inductors  336 . 1 ,  336 . 2 ,  336 . 3  and  336 . 4  and the corresponding frequencies in the target frequency band, with Cx=100 pF, are shown in bold. 
     As another option, instead of using only either switchable capacitors or inductors as in the examples of  FIGS. 2-4 , switchable paralleled inductors and capacitors can be used as reactive components. 
       FIG. 5  shows a practical implementation of the circuit in  FIG. 1 . In particular,  FIG. 5  illustrates a possible way to implement the negative resistance device  52  of  FIG. 1 .  FIG. 5  thus uses the same reference numbers as  FIG. 1  where appropriate. Elements that have already been discussed with reference to  FIG. 1  will not be discussed again for sake of conciseness. The microcontroller  90  is not shown in  FIG. 5 . 
     The negative resistance device  52  is the active, oscillation-sustaining part of an oscillator. It is the active part of an emitter-coupled LC oscillator and is comprised of transistors  54  and  56  and a current sink (transistor  68 , resistor  70  and bias voltage source  72 ). The same circuit is implemented as oscillator core in the Motorola MC1648 ‘Voltage controlled oscillator’ integrated circuit. Transistor  54  samples the voltage across the parallel resonant network, and steers the current through transistor  56  via the common emitter connection. Current through transistor  56  is itself fed back via its collector into the parallel resonant network, thereby sustaining the oscillation of the oscillator. The current sink supplies the operating current to the circuit. A distinction is sometimes made between a current source and current sink. The former term then designates a device having a positive current flowing out of it, whereas “current sink” designates a device having a positive current flowing into it (or, likewise, a negative current flowing out of it). It the context of the present, taking into account that current is generally considered an algebraic quantity that can be positive and negative, the term “current sink” may also be a “current source”. 
     The high-impedance amplifier probes the AC voltage on the first node  21  and outputs a corresponding output signal on its output node  44 . If the supply current generated by the current sink is set to an appropriate value, the amplitude of the AC voltage on node  21  depends essentially only on the resistive component of the resonant network. The capacitance to be measured may then be calculated based on the frequency of the output signal of high-impedance amplifier  32  as described hereinbefore. In addition, the resistive part of the complex impedance to be measured can be determined by measuring the amplitude of the output signal on node  44  and/or the DC power drawn by the current sink from its power supply. The resonance frequency of the resonant network may be adjusted as described hereinabove. 
     The embodiment shown in  FIG. 5  implements an ‘automatic levelling loop’ (e.g. as implemented in the Motorola MC1648 ‘Voltage controlled oscillator’ integrated circuit mentioned above). Rectifier  60  converts the peak amplitude of the output signal of high-impedance amplifier, which is proportional to the amplitude of the AC voltage at node  21  into a proportional DC voltage. An error amplifier  62  compares this DC voltage with a reference value defined by voltage source  64 , and outputs a control voltage on its output node  66 . That control voltage controls the current sink comprised of transistor  68 , resistor  70  and bias voltage source  72  in such a way that the resonant network amplitude (the amplitude of the AC voltage on node  21 ) remains substantially constant. The magnitude of the current through the current sink around transistor  68  is then inversely responsive to the parallel resistive component of the resonant network. Since the control voltage of node  66  is substantially proportional to the current through the current sink, the control voltage of node  66  can be used to calculate the resistive value of the impedance to be determined. 
       FIG. 6  schematically shows a vehicle seat  86  equipped with a combined seat heater and capacitive occupancy sensor, which essentially corresponds to the one shown in  FIG. 1 , except for the driven shield electrode (or guard electrode)  88  connected to the first node  21  via a voltage follower  91 . The combined seat heater and capacitive occupancy sensor of  FIG. 6  comprises a plurality of activatable or deactivatable reactive components  36  (also referred to as paralleled reactance) that may e.g. be implemented as shown in  FIGS. 2-4  and described hereinabove. 
     Heating element  10  is arranged in seat  86 , more specifically underneath the seating surface. In addition to the capacitance or impedance to be measured (illustrated again by capacitors  40  and  42 ), there is an additional capacitance between the heating element  10  and the seat frame  92 . The additional capacitance is in parallel to the capacitance to be measured and may introduce considerable measurement errors, because it is not well known and may vary during the lifetime of the application. In order to suppress the influence of the additional capacitance, a guard electrode  88  is arranged between the seat heater  10  and the seat frame  92 . The guard electrode  88  may e.g. be a conductive foil or textile, which covers at least the area spanned by the heating element  10 . Preferably the guard electrode  88  is larger than the area spanned by the heating element  10  for better shielding. As indicated above, the guard electrode  88  is electrically connected to via voltage follower  91 . Voltage follower  91  has high input impedance in order not to disturb the measurement. The voltage follower  91  keeps the voltage on the guard electrode  88  substantially equal to the voltage on the heating element  10 . Therefore, when the capacitive measurement is carried out, there is no or only a very small AC voltage difference between the heating element  10  and the guard electrode  88 . As a result, substantially no AC current flows between the heating element  10  and the guard electrode  88 . The guard electrode  88  being arranged between the heating element  10  and the seat frame  92 , substantially no AC current flows between the heating element  10  and the seat frame  92 . 
     While specific embodiments have been described in detail, those with ordinary skill 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.