Abstract:
A capacitive occupant detection system comprises at least one sensing electrode to be arranged on the bottom side of a seat cushion, a seat heater like structure to be arranged on a top side of said seat cushion, and an evaluation unit operatively coupled to said at least one sensing electrode for determining a value representative of the capacitance between the at least one sensing electrode and the seat heater like structure.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to the field of capacitive occupant detection systems e.g. to be used in the control of the deployment of secondary restraint systems of an automotive vehicle such as airbags, seat belt pretensioners and the like or in seat belt reminder systems. 
       BACKGROUND 
       [0002]    Capacitive measurement and/or detection systems have a wide range of applications, and are among others widely used for the detection of the presence and/or the position of conductive body in the vicinity of an electrode of the system. 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—which could comprise the one or more antenna electrodes themselves—at which the influence of an object or living being on the electric field is detected. 
         [0003]    The technical paper entitled “Electric Field Sensing for Graphical Interfaces” by J. R. Smith, published in Computer Graphics I/O Devices, Issue May/June 1998, pp 54-60 describes the concept of electric field sensing as used for making non-contact three-dimensional position measurements, and more particularly for sensing the position of a human hand for purposes of providing three dimensional positional inputs to a computer. Within the general concept of capacitive sensing, the author distinguishes between distinct mechanisms he refers to as “loading mode”, “shunt mode”, and “transmit mode” which correspond to various possible electric current pathways. In the “loading mode”, an oscillating voltage signal is applied to a transmit electrode, which builds up an oscillating electric field to ground. The object to be sensed modifies the capacitance between the transmit electrode and ground. In the “shunt mode”, which is alternatively referred to as “coupling mode”, an oscillating voltage signal is applied to the 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. 
         [0004]    The capacitive coupling is generally determined by applying an alternative voltage signal to a capacitive antenna electrode and by measuring the current flowing from said antenna electrode either towards ground (in the loading mode) or into the second electrode (receiving electrode) in case of the coupling mode. This current is usually measured by means of a transimpedance amplifier, which is connected to the sensing electrode and which converts a current flowing into said sensing electrode into a voltage, which is proportional to the current flowing into the electrode. 
         [0005]    While the above measurement principle generally leads to very useful results, it is clear that problems may arise in the vicinity of grounded structures (e.g. seat heaters in a vehicle seat). A seat heater comprises a heating element, which is typically a low-resistance conductor (in the form of a wire, cable, conductive trace printed on an insulating substrate, or the like) for being arranged under the seat cover. 
         [0006]    The base capacitance measured between the sensing electrode of a capacitive occupant detection system and the reference electrode (car body) increases in close vicinity of a seat heater structure especially for systems without guard/shield electrode. This is due to the additional capacitive coupling between the sensing electrode and the seat heater structure, which by itself has a low impedance connection to the reference electrode (car body ground). Accordingly it is normally not possible to clearly separate the capacitance influence of the seat heater structure from the influence of the remaining car body since for example the seat heater structure is usually a separate unit. 
         [0007]    In the current state of the art, the seat heater influence and the seat frame influence might be reduced by arranging the capacitive sensor electrodes above the seat heater in the seat any by the use of a guard/shield electrode between the sense electrode and the seat heater. These current implementations however increase the system cost by increasing the complexity of the integration into the seat. 
       BRIEF SUMMARY 
       [0008]    The present invention proposes a different capacitive sensing system, which is not negatively affected by seat heater interference. 
         [0009]    The capacitive occupant detection system according to the present invention comprises at least one sensing electrode to be arranged on the bottom side of a seat foam cushion. The sensing electrode being arranged at the bottom side of the seat cushion here means in the region between the cushion and seat frame or seat pan. The sensing electrode may be directly arranged on the bottom side of the foam cushion or placed directly on the seat frame, with the provision that the sensing electrode is electrically insulated from the seat fame. Alternatively there might be intermediate layers placed or arranged between the cushion and the electrode (fleece of the like) or the electrode and the seat frame. It will be noted that the sensing electrode may comprise a sheet like conductive material and/or a conductive wire applied on a carrier material and/or a conductive trace applied on a carrier material. 
         [0010]    The capacitive occupant detection system further comprises a seat heater like structure to be arranged on a top side of said seat cushion and an evaluation unit operatively coupled to said at least one sensing electrode for determining a value representative of the capacitance between the at least one sensing electrode and the seat heater like structure. The seat heater like structure may be an actual seat heater structure such as a seat heater mat to be connected to a seat heater power supply. Alternatively, if the actual seat heater function is not desired by the customer, the seat heater like structure may comprise a seat heater dummy, i.e. an electrical conductor structure which may e.g. be similar to the one of an actual seat heater without being connected to a seat heater control unit for supplying a heating current to the electrical conductor. 
         [0011]    The seat heater like structure is arranged on the top side of the seat foam cushion, i.e. on the so-called A-surface of the seat, between the cushion and seat trim. The seat heater like structure may be arranged directly on the seat foam or there may be intermediate layers e.g. of a non-woven material. Likewise there may be further layers arranged between the seat heater like structure and the seat trim. 
         [0012]    The arrangement of the seat heater like structure and the sensing electrode is such that if a passenger is sitting in the vehicle seat, the seat foam is compressed under the influence of the weight of the passenger sitting in the seat and thus the seat heater like structure is brought closer to the sensing electrode on the bottom of the seat foam. The capacitance between seat heater or seat heater dummy and the sensing electrode is thereby increased and this increase in capacitance is detected by the evaluation unit which is operatively coupled to the at least one sensing electrode. The presence of a passenger in the seat may e.g. be determined if the value representative of the capacitance between the at least one sensing electrode and the seat heater like structure passes a specific predetermined threshold value. 
         [0013]    Instead of directly determining the influence of an electrically conductive body on an electrical field, the capacitive sensing system of the present system indirectly determines the presence of a passenger by determining an amount of compression of the seat foam. The capacitive sensing system takes advantage of the presence of a seat heater present in the seat respectively relies on a seat heater dummy arranged in the seat. 
         [0014]    In a possible embodiment of the invention, said evaluation unit comprises an AC signal source coupled to said seat heater like structure for impinging an AC voltage signal on said seat heater like structure and a transimpedance amplifier coupled to said sensing electrode for converting an AC current flowing into the sensing electrode into an AC output voltage. 
         [0015]    If the seat heater like structure is a dummy seat heater, there is no specific technical problem to solve in order to supply the seat heater like structure with the appropriate AC signal. 
         [0016]    If however the seat heater like structure is an actual seat heater mat, which is to be supplied with DC power by a DC power source, one must make sure to decouple the DC power supply from the AC signal source. In one possible embodiment, the DC power source therefore supplies a DC power to the seat heater mat via a common mode choke. Alternatively the DC power source supplies a DC power to the seat heater mat via power amplifiers and said AC voltage signal of said AC signal source is coupled to an input of said power amplifiers. 
         [0017]    In a further embodiment, the capacitive occupant detection system comprises a guard electrode to be arranged between said sensing electrode and a lower seat frame. In this case, the evaluation unit may comprise an AC signal source coupled to said guard electrode for impinging an AC voltage signal on said guard electrode and a transimpedance amplifier, a reference input of which is connected to said AC voltage signal of said AC signal source and a signal input of which is connected to said sensing electrode for converting an AC current flowing into the sensing electrode into an AC output voltage. 
         [0018]    The seat heater like structure is in this case preferably coupled to AC ground by means of a grounding capacitor. 
         [0019]    It will be appreciated that the present invention also relates to a vehicle seat with a capacitive occupant detection system as described hereinabove. The vehicle seat comprises a lower seat frame, a seat foam cushion arranged on said lower seat frame and a seat trim arranged on top of said seat foam cushion. The seat heater like structure is preferably arranged between said seat foam cushion and said seat trim and said at least one sensing electrode is preferably arranged between said seat foam cushion and said lower sear frame so as to be being electrically insulated from said seat frame. 
         [0020]    Finally, if the capacitive occupant detection system comprises a guard electrode, said at least one sensing electrode and said guard electrode are preferably arranged between said seat foam cushion and said lower sear frame, said guard electrode being arranged between said sensing electrode and a lower seat frame. Advantageously the arrangement will be such that said guard electrode is electrically insulated from said seat frame and that said sensing electrode is electrically insulated from said seat frame and said guard electrode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
           [0022]      FIG. 1  shows a possible electrode arrangement of a capacitive sensing system in a vehicle seat; 
           [0023]      FIG. 2  shows a first embodiment of a capacitance measurement circuit of an evaluation unit of a capacitive sensing system operating in the so-called coupling mode; 
           [0024]      FIG. 3  shows a second embodiment of a capacitance measurement circuit operating in the so-called coupling mode; 
           [0025]      FIG. 4  shows a third embodiment of a capacitance measurement circuit operating in the so-called loading mode. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  shows the mechanical implementation of the measurement system in a passenger seat  1  of a car. The seat heater or seat heater dummy  3  is mounted below the seat trim  2  and on top of the seat foam  4 . A dummy seat heater or equivalent electrode is required to be installed when seat-heating function is not desired by the customer. Such a seat heater dummy may comprise an electrical conductor structure similar to the one of an actual seat heater without being connected to a seat heater control unit for supplying a heating current to the electrical conductor. 
         [0027]    A sensing electrode  5  is mounted below the seat foam  4  and on top of a guard electrode  6 . Guard electrode  6  is mounted on top of the seat frame or seat pan  7 . Electrodes  5  and  6  are electrically isolated from the seat frame  7 . Guard electrode  6  shields the sensing electrode  5  from the seat frame  7 , which is typically connected to ground or AC ground. Guard electrode  6  is not required for all possible implementation options, as described below. 
         [0028]    When a passenger sits down on the seat, the weight of the passenger compresses the seat foam  4  in the vertical direction, thereby approaching the seat heater or seat heater dummy  3  towards the sensing electrode  6 . The capacitance between seat heater or seat heater dummy  3  and sensing electrode  5  is thereby increased. This increase in capacitance is measured with an electronic circuit not shown in  FIG. 1  but detailed below. 
         [0029]      FIG. 2  shows a first implementation of a capacitive measurement circuit for an evaluation unit, which can be used in conjunction with the setup shown in  FIG. 1 . The circuit is measuring the unknown capacitance in the so-called coupling mode, that is, the capacitance between two electrodes. In this case, the first electrode is the seat heater  3  and the second electrode is the sensing electrode  5 . 
         [0030]    A DC power source  10  supplies the electrical power to the seat heater  3  through switch  11  and common mode choke  13 . Switch  11  can alternatively also be placed between common mode choke  13  and ground. 
         [0031]    An AC voltage source  14  impinges an AC voltage on the seat heater  3 . The common mode impedance of the common mode choke  13  is chosen to be substantially larger than the output impedance of AC voltage generator  14  in order to avoid loading the AC voltage source excessively. 
         [0032]    The capacitances  18  and  19  represent the capacitances respectively impedances to be determined. They are located between seat heater  3  and sensing electrode  5 . A transimpedance amplifier comprising a coupling capacitor  21 , feedback impedance including a capacitor  22  and a resistor  23 , and an operational amplifier  24  converts the AC current flowing from the seat heater  3  through the capacitances respectively impedances to be measured  18  and  19  into the sensing electrode  5  into an AC output voltage on output node  25 . 
         [0033]    The AC output voltage on output node  25  is thereby responsive to the unknown capacitances respectively impedances  18  and  19 . Obviously, the measurement circuit is also able to measure an unknown complex impedance instead of the unknown capacitances  18  and  19 , as the phase between the output voltage on output node  25  and the voltage of AC voltage source  14 , respectively the in-phase and quadrature parts of the output voltage on output node  25  compared to the voltage of AC voltage source  14  are responsive to the real and reactive parts of an unknown complex impedance used instead of unknown capacitances  18  and  19 . 
         [0034]    On order to avoid erroneous classification of the seat occupancy status in case of a rupture of the seat heater conductor, a capacitor  16  is connected in parallel the seat heater  3  electrical conductor. Capacitor  16  has an impedance which is substantially smaller than the impedance of the total capacitance or impedance to be measured  18  and  19 . 
         [0035]    Without capacitor  16 , if the seat heater  3  should break, then the capacitive measurement would only measure one part of the capacitance or impedance to be measured  18  and  19 , for example in  FIG. 2 , if heater  3  breaks in the middle, only capacitance or impedance to be measured  19  would be measured by the measurement circuit. Additionally, a break in the seat heater  3  can be detected by measuring the current heating current when the heater  3  is switched on and a warning message can then be issued to the car user. 
         [0036]    Capacitor  12 , typically having a small capacitance, avoids that any AC current coming from the AC signal source  14  is fed back into the power source  10  and thereby into the car power network. As can also be seen, the capacitive measurement circuit is able to operate independent whether the seat heater is heating, that is supplied with DC power or not. The guard electrode  6  in  FIG. 1  is not required for the so-called coupling mode as a capacitance between sensing electrode  5  and ground is shunted by the input impedance of said transimpedance amplifier. 
         [0037]      FIG. 3  shows a second implementation of a capacitive measurement circuit, which can be used in conjunction with the setup shown in  FIG. 1 . The circuit is also measuring the unknown capacitance in the so-called coupling mode, that is, the capacitance between two electrodes. In this case, the first electrode is the seat heater  3  and the second electrode is the sensing electrode  5 . 
         [0038]    The circuit shown in  FIG. 3  is almost identically to the circuit shown in  FIG. 2 , except that the AC voltage source  14  now drives two power amplifiers  35  and  36  in order to impinge an AC voltage on the DC heating supply voltage. Power amplifiers  35  and  36  are supplied with DC power from the DC source  10  through switch  11 . Power amplifiers  35  and  36  have preferably a voltage gain of one. 
         [0039]    A resistive voltage divider comprising resistors  30 ,  31  and  32  sets the DC bias points of power amplifiers  35  and  36 , for example for a DC supply voltage of 12 V, amplifier  35  DC output voltage is set to 11.5 V, and amplifier  36  DC output voltage is set to 0.5 V. AC voltage source  14  supplies an AC voltage with an amplitude of for example 0.2 V for the bias voltages described above. This AC voltage is AC coupled through coupling capacitors  33  and  34  onto the inputs of amplifiers  35  and  36 , and thereby on the outputs of amplifiers  35  and  36 . 
         [0040]    Output voltage of amplifier  35  is therefore a DC voltage of 11.5 V superimposed with an AC voltage with an amplitude of 0.2 V, and output voltage of amplifier  36  is therefore a DC voltage of 0.5 V superimposed with an AC voltage with an amplitude of 0.2 V. The AC voltage impinged on the seat heater is therefore the same AC voltage than the AC voltage of AC voltage source  14 . 
         [0041]    Power amplifiers  35  and  36  can for example be Mosfet transistors operated in conjunction with a feedback loop using an operational amplifier as error amplifier. The rest of the operation of the circuit is similar to the operation of the circuit in  FIG. 2 . Capacitor  16  in  FIG. 2  is not required as both amplifiers  35  and  36  output the same AC voltage. 
         [0042]    As can also be seen, the capacitive measurement circuit is able to operate independent whether the seat heater is heating, that is supplied with DC power or not. The circuit in  FIG. 3  is also able to measure complex unknown impedances  18  and  19 . 
         [0043]      FIG. 4  shows a third implementation of a capacitive measurement circuit, which can be used in conjunction with the setup shown in  FIG. 1 . The circuit is measuring the unknown capacitance in the so-called loading mode, that is, the capacitance between a sensing electrode  5  and AC ground. AC ground is supplied by the seat heater in this case. 
         [0044]    A DC power source  10  supplies the electrical power to the seat heater  3  through switch  11 . Switch  11  can alternatively also be placed between seat heater  3  and ground. Capacitor  12  ensures that the seat heater  3  is always fully connected to AC ground, even when switch  11  is in the off state or if the seat heater has a single wire break. 
         [0045]    AC voltage source  40  generates an AC voltage, which is fed into the reference input of the transimpedance amplifier, which is formed by the coupling capacitor  21 , the feedback impedance made of capacitor  22  and resistor  23 , and the operational amplifier  24 . The signal input of the transimpedance amplifier is connected to sense electrode  5 . 
         [0046]    As the AC voltage of the signal input of the transimpedance amplifier tracks the AC voltage of the reference input, which is the AC voltage of signal source  40 , the sense electrode  5 , and the guard electrode  6  connected to the signal source  40 , are maintained at the same AC voltage. Thereby, the guard electrode shields the sense electrode from unwanted coupling to the AC ground, typically the seat frame  7  in  FIG. 1 . 
         [0047]    The AC current flowing out of the sense electrode  5  through the unknown capacitances or impedances  18  and  19 , through the seat heater into the AC ground is measured by said transimpedance amplifier and converted by said transimpedance amplifier into an AC voltage on its output  25 . The AC output voltage on output node  25  is thereby responsive to the unknown capacitances respectively impedances  18  and  19 . 
         [0048]    Obviously, the measurement circuit is also able to measure an unknown complex impedance instead of the unknown capacitances  18  and  19 , as the phase between the output voltage on output node  25  and the voltage of AC voltage source  40 , respectively the in-phase and quadrature parts of the output voltage on output node  25  compared to the voltage of AC voltage source  40  are responsive to the real and reactive parts of an unknown complex impedance used instead of unknown capacitances  18  and  19 . As can be seen, the capacitive measurement circuit is able to operate independent whether the seat heater is heating, that is supplied with DC power or not. 
         [0049]    For all the cases described above, measuring the real part of the unknown impedances  18  and  19  may be used to detect if the seat foam or seat trim has become wet. Said wetness typically increases the measured capacitive part. Using the real part of the measurement allows the adaptation of the detection threshold against which the measured capacitance is compared in order to detect the presence of a passenger, in case of a wet seat.