Abstract:
A capacitive occupant detection system comprises at least one antenna electrode to be arranged in a seat and an evaluation unit operatively coupled to said at least one antenna electrode, said evaluation unit being configured for applying, during operation, an alternating voltage signal to said antenna electrode and for detecting an amplitude and/or phase of a displacement current flowing from said antenna electrode towards ground. According to the invention said antenna electrode comprises an antenna electrode conductor and at least one dedicated ground electrode, said ground electrode being arranged at a predetermined distance of said antenna electrode conductor and extending along said antenna electrode conductor.

Description:
TECHNICAL FIELD 
     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. The invention more particularly relates to capacitive occupant detection systems operating in the so called loading mode and for which a sensing electrode is arranged or assembled in close proximity to a seat heating unit in a vehicle seat. 
     BACKGROUND ART 
     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. 
     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. 
     The capacitive coupling is generally determined by applying an alternating 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. 
     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. 
     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. As a further effect, this capacitance is subject to tolerances due to (assembly-related) variations in distance between sensing electrode and seat heater structure, due to ageing effects of the composite material between the sense electrode and the seat heater, as well as modifications of physical parameters of said composite material (e.g. permittivity). This increased capacitance and the related tolerances affect the useful dynamics of a connected measurement system and limit the accuracy of decisional thresholds and the robustness of these thresholds. Finally as there is also a capacitive coupling between the sensing electrode and the seat frame, said seat frame&#39;s grounding state needs to be also controlled to avoid variance in the capacitive load. 
     In the current state of the art, the seat heater influence and the seat frame influence might be reduced by the use of a guard/shield electrode between the sense electrode and the seat heater or by active functional coupling with a seat heater electronics module which is controlled by the capacitive measurement system. These current implementations increase system cost (additional large-area guard electrode) and/or complexity (interaction with seat heater module). 
     BRIEF SUMMARY 
     The invention proposes a capacitive sensing system with seat heater and seat frame influence compensation. 
     A capacitive occupant detection system comprises at least one antenna electrode to be arranged in a seat and an evaluation unit operatively coupled to said at least one antenna electrode, said evaluation unit being configured for applying, during operation, an alternating voltage signal to said antenna electrode and for detecting an amplitude and/or phase of a displacement current flowing from said antenna electrode towards ground. According to the invention said antenna electrode comprises an antenna electrode conductor and at least one dedicated ground electrode, said ground electrode being arranged at a predetermined distance of said antenna electrode conductor and extending along said antenna electrode conductor. 
     Existing combinations of capacitive sensing systems and seat heaters often feature parallel or quasi-parallel sensing electrodes/seat heater wire design. This, on the one hand, generates a relatively high constant capacitive offset in the capacitive sensing system and reduces thus the relative measurement dynamics. On the other hand, as this capacitive offset depends on the distance between seat heater and sensing electrode, parallel or quasi-parallel sensing electrodes/seat heater wire design is very sensitive against assembly/production tolerances and needs to be costly controlled. According to the present invention, the influence of the seat frame and the seat heater on the capacitive offset in the sensing system is reduced by establishing a known and defined capacitive coupling between the sensing electrode and vehicle GND, which, in parallel, causes the coupling between sensing electrode and seat frame to decrease. 
     It should be noted that in the context of the present invention, the expression parallelism should be understood largely to cover not only straight conductors extending in a parallel manner but also differently shaped conductors, which extend one along the other. In fact the heater conductor and the antenna electrode conductor have preferably meandering shapes extending over a major part of the seating surface of the seat. In the context of such meandering conductors, the expression parallelism or quasi-parallelism is meant to cover two meander-shaped conductors extending along each other. Likewise the basic orientation of the heater conductor and the basic orientation of the antenna electrode conductor have to be understood to designate the global orientation of the conductors, which in a meandering shape would normally be understood to be equal to the direction of the main segments or regions forming the meandering shape. 
     Adding a defined capacitive coupling path to GND according to  FIG. 5  has the benefit that the capacitive offset in the sensing system remains virtually constant. A drawback is that the capacitive offset usually increases, leading to a reduced measurement dynamics. In a preferred embodiment of the invention, the evaluation unit therefore further comprises circuitry for compensating the capacitive current flowing from said antenna electrode conductor to said at least one dedicated ground electrode. As the current flowing from the sensing electrode to the dedicated ground electrode represents a constant offset in the overall current flowing into the capacitive electrode and can be determined by calculation without measuring it, it can be subtracted from the overall current which results in a differential current still carrying the information about the change in occupancy status. Said differential current can be taken for evaluation in the sensing system. 
     It will be appreciated that the present invention is especially useful in an assembly of a seat heater and a capacitive occupant detection system. The seat heater comprises at least one heater conductor to be arranged in a seat. In this case, the said seat heater and a capacitive occupant detection system are preferably arranged in said seat in such a way, that said dedicated ground electrode is arranged between said heater conductor and said antenna electrode conductor. In fact, if one or more GND electrodes are added in close proximity to the sensing electrode, e. g. between sensing electrode and seat heater wiring, the electrical field which develops around the sensing electrode is of completely different shape than without the ground electrode, leading to a smaller capacitive coupling between the sensing electrode and the seat heater. In that case, there is virtually no impact of a seat heater interruption or assembly tolerances between seat heater and the sensing electrode on the classification performance of the sensing system. 
     It will further be noted that said heater conductor and/or said antenna electrode conductor and/or said ground electrode may comprise a conductive wire and/or a conductive trace applied on a carrier material. The skilled person will further appreciate that the heater conductor and the antenna electrode conductor may be arranged on a common carrier material or on individual carrier materials, and that the sensing electrode may be arranged above the heater conductor of vice versa. Finally it will be noted that the antenna electrode conductor is preferably loop shaped with both the first and the second end of said loop being connected to the evaluation unit so that the integrity of the electrode conductor is checkable by known means. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
         FIG. 1  shows a possible embodiment of a capacitive sensing system; 
         FIG. 2  shows a top view of a vehicle seat equipped with a seat heater; 
         FIG. 3  shows a schematic layout of a seat heater electrical connection; 
         FIG. 4  schematically illustrates the capacitive coupling between sensing system and seat frame; 
         FIG. 5  shows the effect of a dedicated ground electrode to the electrical field; 
       FIG.  6 . 1 - 6 . 3  shows a schematic layout of a seat heater ( 6 . 1 ), a schematic layout of a sensing and GND electrode ( 6 . 2 ) and the assembled layout of the seat heater and the sensing and GND electrode ( 6 . 3 ); 
         FIG. 7  shows the changed coupling situation due to added GND electrode; 
         FIG. 8  shows a schematic layout of a constant capacitive offset compensation circuit in the sensing system. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows an exemplified seat heater connection circuitry and capacitive measurement system. The different reference numerals denote the following features:  1  is the vehicle&#39;s power supply (battery voltage);  2  is a high side power switch;  3  is the low side power switch;  4  is the seat heater;  5  is the parasitic capacitance of the high side/low side power switch;  6  is the seat heater control unit;  7  is the vehicle&#39;s GND;  8  capacitive sensing system;  9  capacitive coupling between seat heater and sensing electrode;  10  capacitive coupling between sensing electrode and passenger;  11  is the passenger;  12  is the seat frame;  13  is the passenger&#39;s capacitance to GND;  14  capacitive coupling between sensing electrode and seat frame;  15  is the grounding connection of the seat frame. 
     Today&#39;s automotive capacitive sensing systems used to activate/de-activate the airbag in case of a crash use either the seat heater, a separate sensing electrode or a shielded electrode as sensing element. In any case, the seat heater has to be AC-decoupled from vehicle&#39;s ground GND, since, even if the seat heater is not switched ON, it is coupled to GND from an AC point of view via the power switches parasitic capacitances.  FIG. 1  features a typical seat heater connection circuitry together with a separate capacitive sensing system usually assembled in the passenger seat of a vehicle. Typically, the grounding state  15  of the metallic seat frame  12  is not well defined. 
     The seat heater  4  is connected to the vehicle&#39;s power via either a high side switch  2  or a low side switch  3  or both. These switches are controlled by the seat heater control unit  6  in order to adjust the correct seat temperature. Normally, the switches are field effect transistors, but they can also be mechanical or electro-mechanical switches. 
     The parasitic capacitances  5  cause the seat heater to be coupled to the vehicle&#39;s GND  7 , even if the seat heater is not switched ON. For this reason, via the coupling capacitance  9 , the capacitive sensing unit  8  measures a capacitive offset depending on said coupling capacitance  9  and the parasitic capacitances  5 . In addition, the capacitive offset is increased by the coupling  14  between sensing electrode and seat frame  12 . 
     If this capacitive offset varies due to e. g. an interrupted seat heater  4  or a undefined grounding connection  15  of the seat frame  12 , the ability of the system to detect the passenger  11  sitting on the passenger seat by determination of the series capacitance formed by capacitor  10  and  13  degrades. 
     Said series capacitance is used to differentiate between persons sitting on the seat and child seat, for which the airbag deployment shall be suppressed in case of a crash. Low values of the series capacitance cause the sensing system to decide that the passenger seat is empty or occupied by a CRS, which will lead to a deactivation of the airbag in both cases. High values of the series capacitance cause the sensing system to decide that a person is sitting on the passenger seat, which leads to an activation of the airbag to deploy in case of a crash and, thus, protecting the passenger. 
       FIG. 2  shows a top view of a vehicle seat equipped with a seat heater. Reference numeral  16  denotes the vehicle seat and reference numeral  17  a (wire based) seat heater. Today&#39;s capacitive sensing systems use either the seat heater as sense electrode or a separate shielded or un-shielded sensing electrode, placed above the seat heater or integrated together with the seat heater on the same carrier material. Especially the performance of realizations with un-shielded separate electrodes degrades due to the high capacitive coupling between sensing electrode and seat heater. 
       FIG. 3  shows a schematic layout of a seat heater electrical connection. The different reference numerals in  FIG. 3  denote the following features:  5  is the parasitic coupling of the power switches inside the seat heater control unit;  6  is the seat heater control unit;  7  is the vehicle&#39;s GND;  17  is the (wire based) seat heater;  18  is the capacitive system&#39;s sensing electrode;  19  is the seat heater connector;  20  is the socket for the seat heater connector.  FIG. 3  shows an example of an embodiment of a capacitive sensing system with an un-shielded electrode. Both seat heater  17  and sensing electrode  18  can be, but must not be, integrated on one carrier. Since the sensing electrode&#39;s wiring is basically hold in parallel to the seat heater&#39;s wiring, there is a high capacitive coupling between seat heater and sensing electrode, which causes a high capacitive offset in the sensing system. If this offset varies due to e. g. assembly tolerances between sensing electrode and seat heater or an unplugged seat heater connector  19 , misclassification might be possible. 
     In addition to the coupling to the seat heater, there is a capacitive coupling  22  between sensing electrode  18  and the seat frame  21  of the seat  16 , see  FIG. 4 . Depending on the grounding state of said seat frame  21 , the capacitive offset in the sensing system might vary, increasing the probability for a misclassification. 
     The influence of the seat frame  21  or the seat heater  17  on the capacitive offset in the sensing system can be reduced by establishing a known and defined capacitive coupling between the sensing electrode and vehicle GND, which, in parallel, causes the coupling between sensing electrode and seat frame to decrease. 
       FIG. 5  shows the effect of a dedicated ground electrode to the electrical field. Without the GND electrode  28 , there is a capacitive coupling  25  and  26  between sensing electrode  23  and seat frame  21  and sensing electrode  23  and seat heater  17 . The grounding states of the seat heater  17  and the seat pan  21  are subject to variations, increasing the risk for misclassification by the sensing system. 
     If one or more GND electrodes  28  are added in close proximity to the sensing electrode  23 , e. g. between sensing electrode and seat heater wiring, the electrical field which develops is of completely different shape, leading to a smaller capacitive coupling  25  between sensing electrode  23  and seat frame  21  and a smaller capacitive coupling  26  between sensing electrode  23  and seat heater  17 . 
     In that case, variations of the seat frame&#39;s grounding state can be virtually neglected. In addition, there is virtually no impact of a seat heater interruption or assembly tolerances between seat heater and the sensing electrode on the classification performance of the sensing system. 
     Since the remaining capacitive offset in the sensing system is caused by the capacitive coupling  27  between sensing electrode  23  and GND electrode  28 , there is virtually no change in said offset, as said coupling  27  can be well controlled, e. g. when sensing electrode  23  and GND electrode  28  are realized on the same carrier material. 
       FIG. 6  shows a schematic layout of a seat heater ( 6 . 1 ), a schematic layout of a sensing and GND electrode ( 6 . 2 ) and the assembled layout of  6 . 1  and  6 . 2  (in  6 . 3 ). The GND electrode  28  is designed in such a way that, if assembled together with a seat heater  15  in a passenger seat, it virtually prevents capacitive coupling between sensing electrode  23  and seat heater  15 . At the same time, the coupling between sensing electrode and seat frame can be virtually neglected. As a consequence, there is no need to diagnose the seat heater for interruption or to add a galvanic GND connection to the seat frame to realize a defined grounding status. Seat heater  15 , sensing electrode  23  and GND electrode  28  can be, but most not be, arranged on one common carrier material. 
     Adding a defined capacitive coupling path to GND according to  FIG. 5  has the benefit that the capacitive offset in the sensing system remains virtually constant. A drawback is that the capacitive offset usually increases, leading to a reduced measurement dynamics. In the following, a measure to avoid said drawback is explained with reference to  FIG. 7 , which shows the changed coupling situation due to added GND electrode, and  FIG. 8 , which illustrates a schematic layout of a constant capacitive offset compensation circuit in the sensing system. 
     Referring to  FIG. 7 , the different reference numerals denote the following features or elements:  7  is the vehicle&#39;s GND;  23  is the sensing electrode (vertical cut);  28  is a GND electrode (vertical cut, GND potential);  29  is the null capacitance between sensing electrode and GND electrode;  30  is the null current flowing from sensing to GND electrode;  31  is the AC voltage applied to the sensing electrode;  32  is the current flowing into the sensing electrode;  33  is the null capacitance between sensing electrode and vehicle chassis/seat frame;  34  is the null current flowing from sensing electrode to vehicle chassis/seat frame;  35  is the capacitance between sensing electrode and vehicle chassis caused by occupancy/environmental influences;  36  is the capacitive current through  35 ;  37  is the capacitance between sensing electrode and GND electrode caused by occupancy/environmental influences;  38  is the capacitive current through  37 ;  39  is the vehicle chassis on GND potential. 
     The capacitive sensing system applies the voltage  31  to the sensing electrode  23 . The capacitive current  32  is evaluated by the sensing system in amplitude and/or phase angle in relation to the amplitude/phase angle of the applied AC voltage  31 . 
     In an ‘empty seat’ condition, i.e. where the seat is neither occupied by a person nor by a Child Restraint Seat, said current  32  flowing into the sensing electrode splits up into a null current  30  flowing to the GND electrode  28  and a null current  34  flowing to the vehicle chassis/seat frame. Especially the null current  30  can reach high values and can be calculated out of the known AC voltage  32 , the frequency of the AC voltage and the capacitance  29  between sensing electrode  23  and GND electrode  28 , since latter capacitance is known and well defined by the geometry of sensing electrode  23  and GND electrode  28 . 
     Due to a change in the occupancy situation or due to environmental influences like water on the passenger seat, current  32  changes in amplitude and phase angle in relation to the applied AC voltage  31 . In  FIG. 7 , this is indicated by the capacitances  35  and  37 . The currents  36  and  38  through said capacitances contain all necessary information about the changed occupancy situation and should be taken for evaluation. 
     Thus, as current  30  represents a constant offset in current  32  and can be determined by calculation without measuring it, it can be subtracted from current  32  which results in a differential current still carrying the information about the change in occupancy status. Said differential current can be taken for evaluation in the sensing system. Since only the remaining virtually constant offset in the differential current is due to the very low current  34  flowing through capacitance  33  between sensing electrode  23  and vehicle chassis  39 , the measurement dynamics of the sensing system has increased. 
     Said current  34  can be compensated using the same principle if capacitance  33  is virtually constant, leading to a further increase of the measurement offset. 
     Referring now to  FIG. 8 , the different reference numerals denote the following features or elements:  7  is the vehicle&#39;s GND;  23  is the sensing electrode;  28  is a GND electrode;  29  is the null capacitance between sensing electrode and GND electrode;  40  amplitude control signal;  41  frequency control signal;  42  is the sine voltage generator;  43  is the sine voltage;  44  sensing electrode driver;  45  is multiplier;  46  is the complex transfer function of the null capacitance  29 ;  47  is an equivalent for the calculated capacitive current through  29 ;  48  is an equivalent for the capacitive current flowing in  23 ;  49  is a difference building block;  50  is the difference between  48  and  47 ;  51  is a control circuitry;  52  is the current necessary to drive  23 ;  53  is the current flowing through  29 ;  54  is current flowing towards vehicle GND, e, g, through a passenger. 
     Via the amplitude and frequency control signals  40  and  41 , the control circuitry  51  defines a since wave  43  which is generated by the sine wave generator  42 . The sensing electrode driver  44  generates the necessary current  52  to drive to the voltage  43  on the sensing electrode  23 . A signal  48  representing said current is fed into the input of a difference building block  49 . The current  53 , which flows from sensing electrode  23  through capacitance  29  to the GND electrode  28 , can be calculated by multiplying voltage  43  with the complex transfer function  46  of the known value of the null capacitance  29 . This step respectively the generation of a signal  47  representing said current  53  through  29  is done by block  45 . It outputs said signal  47  to the second input of difference building block  49 . The output signal  50  of difference building block  49  represents the remaining current  54  which carries the needed information about the occupancy status of the passenger seat and/or environmental influences. This information is extracted by the control unit  51  by e. g. evaluating said signal&#39;s amplitude and phase angle in relation to the voltage  43  applied to the sense electrode  23 .