WO2012038326A1

WO2012038326A1 - Capacitive occupant detection system

Info

Publication number: WO2012038326A1
Application number: PCT/EP2011/066056
Authority: WO
Inventors: Michael Virnich; David Hoyer; Martin Thinnes
Original assignee: Iee International Electronics & Engineering S.A.
Priority date: 2010-09-23
Filing date: 2011-09-16
Publication date: 2012-03-29
Also published as: LU91738B1

Abstract

An assembly of a seat heater and a capacitive occupant detection system is presented. The seat heater comprises at least one heater conductor to be arranged in the vicinity of a seating surface of a seat while the capacitive occupant detection system comprises at least one antenna electrode to be arranged in the vicinity of a seating surface of a seat and an evaluation unit operatively coupled to said at least one antenna electrode. The evaluation unit is 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, the antenna electrode comprises an antenna electrode conductor and the heater conductor and the antenna electrode conductor are arranged in such a way with respect to each other, that a basic orientation of said heater conductor and a basic orientation of said antenna electrode conductor regionally form an angle α of at least 45° between each other.

Description

DESCRIPTION

CAPACITIVE OCCUPANT DETECTION SYSTEM

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. 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

[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'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'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 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.

[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. 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.

[0007] In the current state of the art, the seat heater 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).

Technical problem

[0008] The object of the present invention is therefore to propose a capacitive sensing system with seat heater influence compensation. This object is achieved by an assembly as claimed in claim 1 .

General Description of the Invention

[0009] In the assembly of a seat heater and a capacitive occupant detection system in accordance with the invention, the seat heater comprises at least one heater conductor to be arranged in the vicinity of a seating surface of a seat while the capacitive occupant detection system comprises at least one antenna electrode to be arranged in the vicinity of a seating surface of a seat and an evaluation unit operatively coupled to said at least one antenna electrode. The evaluation unit is 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, the antenna electrode comprises an antenna electrode conductor and the heater conductor and the antenna electrode conductor are arranged in such a way with respect to each other, that a basic orientation of said heater conductor and a basic orientation of said antenna electrode conductor regionally form an angle a of at least 45° between each other. Due to the basic orientation of the antenna electrode conductor at an angle of at least 45° with respect to the basic orientation of the heater conductor, the present invention avoids first of all large laminar electrodes structures in close proximity to the seat heater's wiring and minimises parallelism between the heater conductor and the antenna electrode conductor. These measures result in a substantially lower capacitive coupling between said seat heater and the system's sensing electrode and accordingly to a substantially reduced influence of the coupling between the seat heater and the capacitive electrode on the capacitive detection result.

[0010] 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 arrangement of the heater conductor and the antenna electrode conductor is such that parallelism or quasi-parallelism between the two conductors is avoided and thus the above- mentioned problems are likewise avoided.

[001 1 ] It should be noted that in the context of the present invention, the expression parallelism should not 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.

[0012] In a preferred embodiment of the invention, the angle a is substantially equal to 90°, which means that at the locations where the conductive tracks of the sensing electrode virtually crosses (in different planes or layers) the seat heater conductive structures respectively vice versa, the conductors of the seat heater and the sensing electrode cross at an angle of approx. 90°. This arrangement ensures that the capacitive sensing system's offset caused by the coupling between seat heater and sensor is minimized and that accordingly the dynamics of the capacitive detection system is increased.

[0013] As stated above, the said heater conductor preferably comprises a meandering shape, the main segments of which extend in a first direction, and the antenna electrode conductor comprises a meandering shape, the main segments of which extend in a second direction. These first direction and second directions are preferably substantially at right angle.

[0014] In a preferred embodiment of the invention, the antenna electrode further comprises at least two extended structures of a conductive material, which are electrically connected to the antenna electrode conductor. The at least two extended structures are then preferably arranged on said antenna electrode conductor at either side of a crossing region between said heater conductor and said antenna electrode conductor at locations at which the capacitive coupling between an crossing portion of the heater conductor and each one of the extended structures is substantially equal. The extended structures increase the surface of the antenna electrode and thus the useful surface of the capacitor versus ground. Due to this increase in the active surface, the coupling between the sensing electrode and an occupant and therefore the useful signal produced by an occupant sitting on the seat can be increased. Due to the specific arrangement of the extended structures with respect to the heater conductor, this increase in useful signal is achieved substantially without increasing the coupling between sensing electrode and set heater. In fact, according to the invention, the coupling improving electrode structures of the sensing electrodes are arranged at locations where there is virtually no effect on the constant capacitive offset caused by the coupling between seat heater and sensing electrode.

[0015] In a possible variant of this embodiment, said at least two extended structures have a similar shape and size said at least two extended structures are arranged at locations which are substantially symmetrical with respect to said crossing region between said heater conductor and said antenna electrode conductor. This arrangement ensures that the capacitive offset caused by the coupling from the antenna electrode to the seat heater when averaged over the complete sensing electrode is virtually constant even in case of a translation of the antenna conductor in relation to the seat heater wring caused by e.g. assembly/production tolerances. The extended structures may for instance have a circular shape or a quadrilateral shape, wherein the at least two extended structures are similarly oriented with respect to said heater conductor and/or said crossing region between said heater conductor and said antenna electrode conductor.

[0016] Since the performance of the sensing system depends on a virtually constant capacitive offset to avoid misclassification, the seat heater has to be diagnosed for double interruption. For this reason, the capacitive occupant detection system preferably comprises at least one diagnosis electrode, wherein said at least one diagnosis electrode being operatively coupled to said evaluation unit and wherein said diagnosis electrode being arranged in the vicinity of a portion of the heater conductor so as to extend along said portion of said heater conductor. In contrast to the antenna electrode conductor, the diagnosis electrode is arranged so as to extend along a region of the heater conductor. This arrangement results in a relatively high capacitive coupling between the diagnosis electrode and the heater conductor, which enables the integrity of the seat heater conductor being checked without the need for a galvanic connection from the evaluation unit to the seat heater.

[0017] In order to increase the capacitive coupling between the diagnosis electrode and the heater conductor, said heater conductor may comprise a diagnosis section which is specifically configured to increase the capacitive coupling between said diagnosis section and said diagnosis electrode when said diagnosis electrode is arranged in a facing relationship to said diagnosis section of said heater conductor.

[0018] It will be noted that the influence of the seat frame 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. For this reason, said capacitive occupant detection system comprises in a preferred embodiment 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. Alternatively the capacitive occupant detection system comprises at least two dedicated ground electrodes, one of said at least two ground electrodes being arranged at either side of said antenna electrode conductor at a predetermined distance of said antenna electrode conductor and extending along said antenna electrode conductor.

[0019] 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

[0020] Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

Fig. 1 shows an exemplified seat heater connection circuitry and capacitive measurement system;

Fig. 2 shows a top view of a vehicle seat equipped with a seat heater;

Fig. 3 shows a first schematic layout of a non shielded sensing system;

Fig. 4 shows a second schematic layout of a non shielded sensing system (with basic electrical connection of the seat heater);

Fig. 5 shows a top view of an optimized sense electrode design to lower capacitive coupling to the seat heater;

Fig. 6 shows a view of an optimized sense electrode design with improved coupling to passenger;

Fig. 7 shows a possible seat heater and sensing electrode design, which is tolerant for assembly tolerance;

Fig. 8 & 9 show different embodiments of extended sensing electrode structures and their arrangement with respect to the seat heater conductor;

Fig. 10 & 1 1 show different embodiments of an assembly with integrity check of the heater conductor; Fig. 12 shows the effect of dedicated ground electrodes to the electrical field; and Fig. 13 shows a layout of a sensing electrode with ground electrodes.

Description of Preferred Embodiments

[0021 ] 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'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's GND; 8 capacitive sensing system; 9 capacitive coupling between seat heater and sensing electrode; 10 capacitive coupling between sensing electrode and passenger; 1 1 is the passenger; 12 is the passenger seat; 13 is the passenger's capacitance to GND.

[0022] Today'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 is AC- coupled to the vehicle'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.

[0023] The seat heater 4 is connected to the vehicle'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 electromechanical switches.

[0024] The parasitic capacitances 5 cause the seat heater to be coupled to the vehicle'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. This offset represents a constant capacitive load which negatively impacts the measurement dynamics of the sensing system and, thus, worsens the ability of the system to detect the passenger 1 1 sitting on the passenger seat 12 by determination of the series capacitance formed by capacitor 10 and 13.

[0025] 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.

[0026] Fig. 2 shows a top view of a vehicle seat equipped with a seat heater. Reference numeral 14 denotes the vehicle seat and reference numeral 15 a the (wire based) seat heater. Today'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.

[0027] Especially the performance of realizations with un-shielded separate electrodes degrades due to the high capacitive coupling between sensing electrode and seat heater.

[0028] Different schematic layouts of a non shielded sensing system are shown in Fig. 3 and 4. Fig. 3 shows a first example of a realization of a capacitive sensing system with an un-shielded electrode. Both seat heater 15 and sensing electrode 16 can be, but must not be, integrated on one carrier. Since the sensing electrode's wiring is basically hold in parallel to the seat heater's wiring, there is a high capacitive coupling between seat heater and sensing electrode, which results in high capacitive offset in the sensing system and degradation in the measurement dynamics.

[0029] Fig. 4 gives another example of a realization of a capacitive sensing system with an un-shielded laminar style electrode 17. Both seat heater 15 and laminar sensing electrode 17 can be, but must not be, integrated on one carrier. Since the sensing electrode's shape is of laminar style in close proximity to the seat heater's wires, there is a high capacitive coupling between seat heater and sensing electrode, which results in high capacitive offset in the sensing system and degradation in the measurement dynamics.

[0030] Fig. 4 further shows the typical electrical connection scheme relevant for the capacitive sensing system assembled into the vehicle seat together with the seat heater. Since the seat heater 15 is AC-coupled to vehicle's GND 7 on both terminals inside the seat heater control unit 6, a single interruption of the seat heater does not have any influence on the capacitive offset inside of the sensing system. In contrast, a double interruption of the seat heater, e. g. caused by a not plugged seat heater connector, can influence the capacitive offset in the sensing system, depending on the location of both interruptions.

[0031 ] It is essential that said capacitive offset remains virtually constant to avoid that the overall measured capacitance drops below the threshold for empty seat / child seat detection, in case where a person is sitting on the passenger seat. Therefore, the seat heater 15 has to be checked for double interruption.

[0032] The measurement dynamics of capacitive sensing systems with non shielded electrodes assembled in a passenger seat together with a wire-based seat heater can be increased by lowering the capacitive coupling between said seat heater and the system's sensing electrode. This can be achieved by avoiding large laminar electrodes structures in close proximity to the seat heater's wiring and realizing a virtually 90° angle between the basic orientation of sense electrode wiring and seat heater wiring. Fig. 5 shows a top view of such an optimized sense electrode design to lower capacitive coupling to the seat heater.

[0033] The design of the sensing electrode 20 indicated in Fig. 5 is reduced to a wire-like shape, which is the condition for a minimum capacitive coupling to the seat heater 15. Due to the virtually 90° angle in the crossings between sensing electrode and seat heater, parallelism between seat heater and sensing electrode is virtually avoided. The sensing electrode can be assembled above or below the seat heater in the passenger seat, as this does not affect the capacitive coupling between both, provided that that the distance between seat heater and sensing electrode remains the same.

[0034] The measurement dynamics of a capacitive system with non shielded electrodes assembled in a passenger seat together with a wire-based seat heater can be increased by increasing the useful signal produced by an occupant sitting on the seat. This can be achieved by increasing the coupling between sensing electrode an occupant virtually without increasing the coupling between sensing electrode and set heater. It can be realized by adding e. g. coupling improving electrode structures to the sensing electrodes arranged at locations where there is virtually no effect on the constant capacitive offset caused by the coupling between seat heater and sensing electrode.

[0035] Fig. 6 shows a view of an optimized sense electrode 21 design with low capacitive coupling to seat heater 15 and improved coupling to passenger. From this figure it is obvious that coupling improving electrode structures 22 are preferably arranged in a manner to avoid close proximity to the seat heater wiring. In the example of Fig. 6, the coupling improving structures 22 are realized as filled respectively meshed circles, but they can be of any shape as long as they improve the capacitive coupling to the passenger more than the coupling to the seat heater wiring.

[0036] Since the decision whether a vehicle seat is occupied by a person or a child seat is derived by the capacitance to GND which is, directly or indirectly via e. g. resonant frequency of an oscillating circuit, determined by the capacitive sensing system, it is important that the capacitive offset caused by the coupling to the seat heater remains virtually constant to avoid misclassification. As assembly tolerances of the seat heater and the sensing system's electrode can easily reach some millimeters without costly counter measures, the sensing electrode's proximity and the resulting capacitive coupling to the seat heater cannot be expected to be free of tolerance. Within limits, the influence of the assembly tolerance on the capacitive offset in the sensing system can be minimized by special design of the sensing electrode.

[0037] Fig. 7 shows the principle of how the influence of assembly tolerances on the capacitive coupling between sensing electrode and seat heater can be virtually neglected. Between the sensing electrode 24 in typical assembly position and the seat heater 23, there is a capacitive coupling that determines the capacitive offset in the sensing system. Due to the fact that the seat heater is crossed in a virtually 90° angle and that the sensing electrode 24 does not contain structures that are hold in parallel and in close proximity to the seat heater, a small translation of the electrode (reference numeral 25 shows the sensing electrode in a slightly shifted or translated position) in relation to the seat heater 23 do virtually not cause any change in the coupling capacitance to the seat heater.

[0038] It will be noted that the sensing electrode design 15 shown in Fig. 5 is already optimized to cope with assembly tolerances, as it crosses the seat heater in virtually 90° angles. In addition, it avoids large structures in parallel and in close proximity to the seat heater. If the sensing electrode contains e.g. laminar or meshed structures to increase the coupling to the passenger, these structures can be arranged in a manner so that, if averaged over the sensing electrode, the capacitive offset in the sensing system remains virtually constant over assembly tolerances. Likewise the electrode design Fig. 6 is already optimized to feature a certain robustness against assembly tolerances between seat heater and sensing electrode positioning.

[0039] Fig. 8 shows an optimized arrangement of laminar / meshed structures to reduce assembly tolerance impact on performance (top view). The different reference numerals denote the following features: 15 is the seat heater wire; 27 is the upper electrode structure; 28 is the lower electrode structure; 29 is the capacitance between the upper electrode structure and the seat heater; 30 is the capacitance between the lower electrode structure and the seat heater; 31 is the sum of 29 and 30.

[0040] Fig. 8 shows that although the coupling 29 and 30 of a single electrode structure 27 and 28 to the seat heater 15 varies a lot, the sum 31 of both couplings just changes very little over translation in x-direction. Thus, when arranged in a manner so that the average proximity between the electrical structures of the sensing electrode and the seat heater 15 remains substantially constant, the capacitive coupling between sensing electrode and seat heater doesn't substantially change and, hence, the capacitive offset remains substantially constant.

[0041 ] Since the coupling of each laminar electrode structure 26 increases non- linearly with decreasing proximity to the seat heater, there could be the need to use a special shape for the electrode structures to linearise the effect to further improve robustness against assembly tolerances between seat heater and sensing electrode. [0042] Fig. 9 shows an example of such an optimized shape with which the immunity against assembly tolerance can be further increased. The different reference numerals in Fig. 9 denote the following features: 15 is the seat heater wire; 32 is the upper electrode structure; 33 is the lower electrode structure; 34 is the capacitance between the upper electrode structure and the seat heater; 35 is the capacitance between the lower electrode structure and the seat heater; 36 is the sum of 29 and 30. Due to the shape of the laminar 1 meshed electrode 32 and 33, the capacitance to the seat heater 15 increases 1 decreases virtually linearly with x, see 34 and 35. As consequence, the sum 36 of both capacitances virtually remains constant.

[0043] Since the performance of the sensing system depends on a virtually constant capacitive offset to avoid misclassification, the seat heater has to be diagnosed for double interruption as stated above. As an additional galvanic connection to the seat heater necessary to run said diagnose generates additional costs in a seat heater independent sensing system, a capacitive measurement principle can be realized.

[0044] Figure 10 shows an example for an embodiment of a capacitive seat heater diagnosis concept. The capacitive sensing system 8 diagnoses the seat heater for double interruption using a seat heater diagnose electrode 37. Not any kind of double interruptions are detectable, but the most likely ones, means interruption close to the connector 18 or an unplugged connector itself. The seat heater diagnosis is performed by measuring the coupling between seat heater diagnose electrode 37 and vehicle GND 7, which is virtually equal to the coupling between seat heater diagnose electrode 37 and seat heater 15, since the seat heater is well AC-coupled to GND.

[0045] The coupling between diagnosis electrode 37 and seat heater 15 must be big enough to distinguish between a "double interruption & seat occupied by a person" condition and a "no interruption & seat not occupied" condition in order reliably detect said double interruption. If the coupling between diagnosis electrode 37 and seat heater 15 is too small to discriminate between "double interruption & seat occupied by a person" condition and a "no interruption & seat not occupied" condition, it can be increased by adding a separate electrode for diagnostic purposes to the seat heater as is shown is the embodiment of Fig. 1 1 . By realizing the shapes 38 and 39 for the seat heater part diagnose capacitor, a virtually ideal plate capacitor can be formed. [0046] A part of the capacitive offset in the sensing system has its origin in the capacitive coupling between sensing electrode and the metallic frame of the passenger seat. In addition, the grounding state of the seat structure is often not known, since a defined galvanic connection between seat frame and vehicle GND is usually missing. This can lead to a variable capacitance between the sensing electrode and vehicle GND via the seat frame, which negatively impacts the performance of the sensing system by increasing the probability for a misclassification.

[0047] The influence of the seat frame 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.

[0048] Fig. 12 shows the effect of different configurations of dedicated ground electrodes to the electrical field. Without the GND electrode 41 , the electrical field 43 between sensing electrode 40 and seat frame 41 is well developed in case where the seat frame is on GND potential, leading to a high capacitive coupling between both. When one or more GND electrodes 41 are added in close proximity to the sensing electrode 40, the electrical field which develops is of completely different shape, leading to a virtually bad capacitive coupling between sensing electrode and seat frame. In that case, variations of the seat frame's grounding state can be virtually neglected.

[0049] If the there is a seat heater assembled in the passenger seat together with the sensing system, the coupling between seat heater and the sensing electrode takes over the role of establishing a defined capacitive coupling between sensing electrode to GND (via the seat heater), if the seat heater is diagnosed for double interruption and coupling tolerances to the seat heater are kept under control.

[0050] If there is no seat heater assembled in the seat, adding a GND electrode in close proximity to the sensing electrode generates a defined coupling between sensing electrode and GND and reduces therefore the influence of the variation of the seat frame's grounding state on the performance of the sensing system.

[0051 ] Fig. 13 shows a layout of a sensing electrode with ground electrodes. The sensing system's electrode 44 is assembled into the passenger seat 14. It is surrounded by a GND electrode 45, which reduces the coupling between electrode 44 and the seat pan.

Claims

An assembly of a seat heater and a capacitive occupant detection system, said seat heater comprising at least one heater conductor to be arranged in the vicinity of a seating surface of a seat and said capacitive occupant detection system comprising at least one antenna electrode to be arranged in the vicinity of a seating surface of 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, characterized in that

said antenna electrode comprises an antenna electrode conductor and

said heater conductor and said antenna electrode conductor are arranged in such a way with respect to each other, that a basic orientation of said heater conductor and a basic orientation of said antenna electrode conductor regionally form an angle a of at least 45° between each other.

The assembly of a seat heater and a capacitive occupant detection system as claimed in claim 1 , wherein said angle a is substantially equal to 90°.

The assembly of a seat heater and a capacitive occupant detection system as claimed in any one of claims 1 or 2, wherein said heater conductor comprises a meandering shape, the main segments of which extend in a first direction, and said antenna electrode conductor comprises a meandering shape, the main segments of which extend in a second direction, and wherein said first direction and said second direction are substantially at right angle.

The assembly of a seat heater and a capacitive occupant detection system as claimed in any one of claims 1 or 3, wherein said antenna electrode further comprises at least two extended structures of a conductive material, said at least two extended structures being electrically connected to said antenna electrode conductor and said at least two extended structures being arranged on said antenna electrode conductor at either side of a crossing region between said heater conductor and said antenna electrode conductor at locations at which the capacitive coupling between an crossing portion of the heater conductor and each one of the extended structures is substantially equal.

5. The assembly of a seat heater and a capacitive occupant detection system as claimed in claim 4, wherein said at least two extended structures have a similar shape and size, and wherein said at least two extended structures are arranged at locations which are substantially symmetrical with respect to said crossing region between said heater conductor and said antenna electrode conductor.

6. The assembly of a seat heater and a capacitive occupant detection system as claimed in any one of claims 4 or 5, wherein said extended structures have a circular shape.

7. The assembly of a seat heater and a capacitive occupant detection system as claimed in any one of claims 4 or 5, wherein said at least two extended structures have a quadrilateral shape and wherein said at least two extended structures are similarly oriented with respect to said heater conductor and/or said crossing region between said heater conductor and said antenna electrode conductor.

8. The assembly of a seat heater and a capacitive occupant detection system as claimed in any one of claims 1 to 7, wherein said capacitive occupant detection system comprises at least one diagnosis electrode, said at least one diagnosis electrode being operatively coupled to said evaluation unit and said diagnosis electrode being arranged in the vicinity of a portion of the heater conductor so as to extend along said portion of said heater conductor.

9. The assembly of a seat heater and a capacitive occupant detection system as claimed in claim 8, wherein said heater conductor comprises a diagnosis section, wherein said diagnosis electrode is arranged in a facing relationship to said diagnosis section of said heater conductor, and wherein said diagnosis section is specifically configured to increase the capacitive coupling between said diagnosis section and said diagnosis electrode.

10. The assembly of a seat heater and a capacitive occupant detection system as claimed in any one of claims 1 to 9, wherein said capacitive occupant detection system comprises 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.

1 1 . The assembly of a seat heater and a capacitive occupant detection system as claimed in any one of claims 1 to 9, wherein said capacitive occupant detection system comprises at least two dedicated ground electrodes, one of said at least two ground electrodes being arranged at either side of said antenna electrode conductor at a predetermined distance of said antenna electrode conductor and extending along said antenna electrode conductor.

12. The assembly of a seat heater and a capacitive occupant detection system as claimed in any one of claims 1 to 1 1 , wherein said heater conductor comprises a conductive wire and/or a conductive trace applied on a carrier material.

13. The assembly of a seat heater and a capacitive occupant detection system as claimed in any one of claims 1 to 12, wherein said antenna electrode conductor comprises a conductive wire and/or a conductive trace applied on a carrier material.

14. The assembly of a seat heater and a capacitive occupant detection system as claimed in any one of claims 1 to 13, wherein said heater conductor and said antenna electrode conductor are arranged on a common carrier material.