Abstract:
A capacitive sensing system for being connected to a heating element comprises a capacitive detector connectable to the heating element and a common mode choke for essentially preventing alternating current from flowing from the heating element to the heating current supply. The capacitive detector is configured for driving an alternating current into the heating element and for producing an output indicative of capacitance based upon the alternating current. The choke has a first and a second winding for connecting the heating element with the heating current supply. The choke comprises a third winding connected in parallel of the first and/or second winding. The capacitive detector is configured for measuring a portion of the alternating current flowing across the third winding and for taking into account the measured portion of alternating current when producing the output.

Description:
TECHNICAL FIELD  
       [0001]    The present invention generally relates to a capacitive sensing system that can use a heating element as its antenna electrode. A capacitive sensing system as proposed herein may e.g. be used for detecting the absence or presence of an occupant on an occupiable item, which might be a vehicle seat, a hospital bed etc., or for detecting a driver&#39;s hand on the steering wheel. 
       BACKGROUND ART 
       [0002]    A capacitive sensor, called by some electric field sensor or proximity sensor, designates a sensor, which generates a signal responsive to the influence of what is being sensed (a person, a part of a person&#39;s body, a pet, an object, etc.) upon an electric field. A capacitive sensor generally comprises at least one antenna electrode, to which is applied an oscillating electric signal and which thereupon emits an electric field into a region of space proximate to the antenna electrode, while the sensor is operating. The sensor comprises at least one sensing electrode 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. 
         [0003]    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 ground. 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”. 
         [0004]    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 on 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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    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. 
         [0008]    U.S. 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. 
         [0009]    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. 
         [0010]    A disadvantage of the system disclosed in U.S. Pat. No. 6,703,845 is that the inductors used as AC-decoupling elements have to support the full heating current (up to 10 A DC and more) and present high AC impedance to the capacitive measurement circuit and the seat heater at the same time. High inductance and high operating DC current implies that the inductor have to be wound on large cores, which are expensive. Depending on the application chosen from U.S. Pat. No. 6,703,845, either two or four of these inductors have to be used. 
         [0011]    U.S. 2011/121618 discloses yet another variant of an occupant detection system including a heating element adjacent the seating surface of a seat. A capacitive occupant detection circuit is electrically coupled to the heating element. A common mode choke is interposed between the heating circuit and the heating element as an isolation circuit that prevents the heating circuit from influencing the occupant detection circuit. 
         [0012]    An ideal common mode choke would totally prevent flow of an AC current. In practice, however, an ideal common mode choke does not exist and there will be a residual alternating current across the common mode choke. In order to reduce this residual alternating current to an extent that the isolation circuit can be considered to prevent the heating circuit from influencing the occupant detection circuit, i.e. to an extent that the residual alternating current can be neglected in comparison to the alternating current flowing across the capacitance to be measured, one would have to choose (among the existing common mode chokes) a common mode choke whose resistance to the heating current would result in a considerable loss of heating power in the common mode choke. 
       BRIEF SUMMARY 
       [0013]    An improved capacitive sensing system is provided that can use a heating element as an antenna electrode. 
         [0014]    A capacitive sensing system for being connected to a heating element producing heat upon electrical current being caused to flow there across comprises a capacitive detector connectable to the heating element and a common mode choke for essentially preventing alternating current from flowing from the heating element to the heating current supply. The capacitive detector is configured for driving an alternating current into the heating element and for producing an output indicative of capacitance based upon the alternating current. The common mode choke has a first winding for connecting a first terminal of the heating element with a first terminal of the heating current supply and a second winding for connecting a second terminal of the heating element with a second terminal of the heating current supply. According to an aspect of the invention, the common mode choke comprises a third winding (wound on the same core as the first and second windings) connected in parallel of at least one of the first and second windings and the capacitive detector is further configured for measuring a portion of the alternating current flowing across the third winding and for taking into account the measured portion of alternating current when producing the output. 
         [0015]    The present invention uses a common mode choke with a coupling factor close to unity to achieve AC-decoupling of the heating element from its power supply. The common mode choke produces high impedance against common mode currents from the heating element into the first and second windings. The capacitive sensing system according to the invention furthermore takes into account the fact that an actual common mode choke cannot perfectly block alternating current. The total residual alternating current across the common mode choke (i.e. the part that cannot be blocked) stands in a known relationship to the portion of the alternating current that flows across the third winding. By measuring this portion of the alternating current, the capacitive sensing system thus can determine the alternating current actually flowing across the capacitance of the heating element. 
         [0016]    The capacitive sensing system as described above may be configured as a module so as to be easily connectable between to an existing heating element and its heating current supply. A preferred alternative aspect of the invention concerns a combined (integrated) heating and capacitive sensing system. Such a combined system comprises a heating element for producing heat when electrical current is caused to flow across it, a heating current supply having a first terminal connected to a first terminal of the heating element and a second terminal connected to a second terminal of the heating element so as to form a heating circuit, and a capacitive detector connected to the heating element, configured for driving an alternating current into the heating element and for producing an output indicative of capacitance based upon the alternating current. A common mode choke is provided for essentially preventing the alternating current from flowing from the heating element to the heating current supply. The common mode choke has a first winding connected between the first terminal of the heating element and the first terminal of the heating current supply and a second winding connected between the second terminal of the heating element and the second terminal of the heating current supply. The common mode choke comprises a third winding connected in parallel of at least one of the first and second windings. The capacitive detector is further configured for measuring a portion of the alternating current flowing across the third winding and for taking into account the measured portion of alternating current when producing the output. 
         [0017]    The combined heating and capacitive sensing system could e.g. be used in a steering wheel and/or a vehicle seat. 
         [0018]    The combined heating and capacitive sensing system may be implemented as a combined seat heating and capacitive occupancy sensing system, the output produced by the capacitive detector being in this case indicative of a seat occupancy state. 
         [0019]    In the following, it will be assumed that the heating current is direct current (DC) and that the capacitive measurement uses alternating current (AC) at a certain frequency. 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 also 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 current used for the capacitive measurement. For sake of simplicity, we will use “DC” to designate slowly varying or constant signals. The capacitance measurement network preferably operates at frequency selected in the range from about 50 kHz to about 10 GHz, more preferably in the range from about 50 kHz to about 30 MHz. 
         [0020]    Preferably, the capacitive detector comprises a first current meter configured and arranged for measuring the alternating current driven into the heating element. 
         [0021]    Preferably, the capacitive detector comprises a second current meter configured and arranged for measuring the portion of the alternating current flowing across the third winding. 
         [0022]    It follows from Kirchhoff&#39;s junction rule that the alternating current dissipated via the heating element can be calculated by withdrawing the total alternating current across the common mode choke from the alternating current driven into the heating element. The capacitive detector thus preferably comprises an evaluation circuit operatively connected to the mentioned current meters, the evaluation circuit being configured for producing the output indicative of the capacitance using the measurement signals of the current meters. 
         [0023]    According to a preferred embodiment of the invention, the third winding is AC-coupled (e.g. via a coupling capacitor) to a reference node, e.g. circuit ground. 
         [0024]    As indicated above, the portion of alternating current flowing across the third winding is indicative of the alternating current flowing across the common mode choke altogether. Assuming that the first, second and third windings are identical and AC-coupled to the same node on the side opposite the heating element, the alternating currents across the windings will be identical. The total alternating current I choke  across the common mode choke may in this case be calculated as I choke =I 1 +I 2 +I 3 =3·I 3 , where I 1 , I 2  and I 3  designates the alternating current across the first, second and third winding, respectively. On the side opposite the heating element, the first and second windings may be connected to the reference node with a low AC-impedance (e.g. by coupling capacitors). In order to avoid that the second current meter introduces to high an AC impedance between the third winding and the reference node, a step-up transformer may be used. The step-up transformer preferably has its primary winding connected between the third winding and the reference node and its secondary winding connected between the reference node and the second current meter. As indicated by the term step-up transformer, the number of turns of the primary winding is lower than the number of turns of the secondary winding. Using this configuration, the AC impedance caused by the second current meter will be reduced by the turn ratio of the step-up transformer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
           [0026]      FIG. 1  is a schematic circuit diagram of a combined heating and capacitive sensing system according to a first example of the invention; 
           [0027]      FIG. 2  is a schematic circuit diagram of a combined heating and capacitive sensing system according to a second example of the invention; 
           [0028]      FIG. 3  is a schematic circuit diagram of a combined heating and capacitive sensing system according to a third example of the invention; 
           [0029]      FIG. 4  is a schematic representation of a vehicle seat equipped with a combined heating and capacitive sensing system. 
           [0030]      FIG. 5  is a schematic representation of a steering wheel equipped with a combined heating and capacitive sensing system. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0031]      FIG. 1  shows a combined heating and capacitive sensing system  10  according to a first example of the invention. The combined system  10  comprises a heating element  12  that produces heat when electrical current is caused to flow across it. The heating element  12  may comprise a conductive wire, cable, fibre, bundle of fibres or a conductive track (e.g. made of a PTC material) printed on a flexible support. The heating element  12  has a first  14  and a second  16  terminal connected to a first  18  and a second  20  terminal of a heating current supply  22 , respectively. The heating current supply  22  (e.g. a direct current source and control electronics) and the heating element  12  form together the heating circuit of a heater, e.g. for a vehicle seat. The terminals of the heating current supply  22  typically have low AC impedance to AC-ground (e.g. due to coupling capacitors  23 ) in order to avoid that AC signals disturb the heating current supply  22 . The heating current supply preferably comprises a user-actuatable master switch (not shown) allowing the user to activate or deactivate the heating circuit as a whole and a temperature controller (not shown; including e.g. a thermostat) that regulates the temperature to a comfortable level. When the heater is operating, the temperature controller opens and closes the heating circuit (low-frequency 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). 
         [0032]    The combined heating and capacitive sensing system  10  further comprises a capacitive detector  24  connected to the heating element  12 . The capacitive detector  24  is configured for using the heating element  12  as its antenna electrode. In particular, during operation, the capacitive detector  24  drives an alternating current into the heating element  12  and produces an output indicative of the capacitance between the heating element  12  and AC-ground. based upon the alternating current. In  FIG. 1 , block  26  symbolically represents the capacitive coupling (the complex impedance) of the heating element  12  to a grounded electrode (typically the vehicle frame). The complex impedance  26  between the heating element  12  and the grounded electrode depends on whether the space between the heating element  12  and the grounded electrode is occupied by a conductive body or not. The capacitive detector  24  illustrated in  FIG. 1  operates in so-called loading mode. An oscillator  40  (e.g. an NCO) and a first current meter  42  are connected in series to the heating element  12 . 
         [0033]    The capacitive detector  24  comprises a common mode choke  28  with a coupling factor close to unity for essentially preventing the alternating current from flowing from the heating element  12  to the heating current supply  22 . The common mode choke comprises three windings  30 ,  32 ,  34  arranged on the same magnetic core (and having the same number of windings). The first winding  30  is connected between the first terminal  14  of the heating element  12  and the first terminal  18  of the heating current supply  22 . The second winding  32  is connected between the second terminal  16  of the heating element  12  and the second terminal  20  of the heating current supply  22 . The third winding  34  is AC-coupled, via coupling capacitor  36  and current meter  38 , to AC-ground. 
         [0034]    During operation of the capacitive detector, the oscillator  40  applies an alternating voltage, causing an alternating current I IN  to flow into the heating element  12 . Part of the current flows across impedance  26  to AC-ground (current I X ). Since the common mode choke  28  cannot perfectly block alternating current, another part of the applied current flows to AC-ground via the windings  30 ,  32  and  34  of the common mode choke  28  (current I choke =I 1 +I 2 +I 3 , where I 1 , I 2  and I 3  designates the alternating current across the first, second and third winding, respectively). From Kirchhoff&#39;s junction rule: I IN =I X +I choke . Since I X  contains the information about impedance  26 , it is not sufficient to measure I IN  if I choke  cannot be neglected. As indicated above, it may be difficult, in practice, to design a common mode choke that presents both sufficient impedance to the alternating current of the capacitive sensor and low resistance to the direct heating current. 
         [0035]    In accordance with the principles of the invention, in the illustrated example, the capacitive detector  24 , in particular its second current meter  38 , measures the part of the alternating current flowing across the third winding  34  of the common mode choke  28 . Evaluation circuit  44  is connected to the first  42  and second current  38  meter so as to receive their measurement signals, indicative of I IN  and I 3 , respectively. The AC impedance of the series connection of the coupling capacitor  36  and the second current meter  38  is adjusted in such a way that it is substantially equal to the AC impedance between the terminals  18  and  20  of the heating current supply  22  and AC-ground. That way, the alternating currents I 1 , I 2  and I 3 across the three windings  30 ,  32  and  34  of the common mode choke  28  are substantially equal in amplitude and phase. The evaluation circuit may thus calculate the total alternating current across the common mode choke  28  as I choke =3·I 3 . The subtraction of I choke  from I IN  yields I X . The multiplier  46  carries out the multiplication of the measurement value of I 3  by −3. The result of the multiplication is added to the measurement value of I N  in adder  48 , whereby the influence of the common mode choke is essentially compensated. The value of I X  thus obtained is fed to decision circuit  50 , which outputs the value of the complex impedance  26  and/or an occupancy status depending on I X . 
         [0036]      FIG. 2  shows a combined heating and capacitive sensing system  10  according to a second example of the invention. The only difference from the first example is that the decision circuit  50  of the evaluation circuit is directly connected to the current meters  42  and  38 . The decision circuit  50  in this example takes its decision based on the two measurement signals as inputs. In all other respects, the example illustrated in  FIG. 2  is identical to the example illustrated in and explained with respect to  FIG. 1 . 
         [0037]      FIG. 3  shows a combined heating and capacitive sensing system  10  according to a third example of the invention. The third example differs from the first example ( FIG. 1 ) in that the capacitive detector  24  comprises a step-up transformer  52  arranged between the third winding  34  and the second current meter  38 . The primary winding of the step-up transformer  52  is connected between the third winding  34  of the common mode choke  28  and AC-ground and the secondary winding of the step-up transformer  52  is connected between AC-ground the input node of the second current meter  38 . The number of turns N 1  of the primary winding is less than the number of turns N 2  of the secondary winding. The step-up transformer lowers the impedance “seen” by the alternating current across the third winding by a factor of about N 1 /N 2 . Accordingly, the configuration of  FIG. 3  may be used if the second current meter  38  has too high input impedance compared to the impedances between the terminals  18 ,  20  and AC-ground. It should be noted that the current I 3 ′ input to the second current meter  38  amounts only to N 1 /N 2 ·I 3 . This has to be taken into account in the further calculations, e.g. in the multiplier  46 . In this example, I X  can finally be calculated by the formula: I X =I IN −3·N 2 /N 1 ·I 3 ′. 
         [0038]      FIG. 4  is a schematic representation of a vehicle seat  54  equipped with a combined heating and capacitive sensing system  10  in accordance with the invention, e.g. as illustrated in any one of  FIGS. 1 to 3 . 
         [0039]      FIG. 5  is a schematic representation of a steering wheel  56  equipped with a combined heating and capacitive sensing system  10  in accordance with the invention, e.g. as illustrated in any one of  FIGS. 1 to 3 . 
         [0040]    While specific embodiments have been described in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.