WO2011154469A1

WO2011154469A1 - Capacitive measurement system with seat heater influence compensation

Info

Publication number: WO2011154469A1
Application number: PCT/EP2011/059541
Authority: WO
Inventors: Laurent Lamesch; Michael Virnich; David Hoyer
Original assignee: Iee International Electronics & Engineering S.A.
Priority date: 2010-06-08
Filing date: 2011-06-08
Publication date: 2011-12-15

Abstract

The present invention proposes to compensate the effect of the seat heater by either direct measurement of the current coupled into the seat heater or by indirect compensation by introducing an additional electrode into the system. This additional electrode, subsequently referred to as 'compensation electrode', may be integrated into the capacitive measurement system in such a way, that the capacitance between the sense electrode and this compensation electrode is subject to the same relative variations as the capacitance between the sense electrode and the seat heater.

Description

Capacitive Measurement System with Seat Heater Influence

Compensation

Field of the Invention

[0001 ] The present invention generally relates to the technical field of capacitive measurement circuits and more specifically to a capacitive measurement system having one or more electrodes, in which the characteristics of a conductive body such as shape and location are determined by means of capacitive coupling via the electrically conductive body.

Background of the Invention

[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 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). 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.

[0006] 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).

Object of the invention

[0007] The object of the present invention is therefore to propose a capacitive sensing system with seat heater influence compensation.

General description of the invention

[0008] In order to overcome the abovementioned problems, the present invention proposes to compensate the effect of the seat heater by either direct measurement of the current coupled into the seat heater or by indirect compensation by introducing an additional electrode into the system. This additional electrode, subsequently referred to as 'compensation electrode', may be integrated into the capacitive measurement system in such a way, that the capacitance between the sense electrode and this compensation electrode is subject to the same relative variations as the capacitance between the sense electrode and the seat heater. In doing so, it is possible to compensate the capacitance influence of the seat heater structure: • by evaluating the capacitance and variations in capacitance between said compensation electrode and the sense electrode (which are a measure for the sense electrode-seat heater capacitance) and by applying a suitable HW or SW compensation, or

• by actively driving a compensation signal on said compensation electrode to directly compensate the effect of the seat heater structure on the sense electrode by injecting a current into the sense electrode thereby cancelling out the effect of the seat heater.

[0009] A capacitive detection system, in an arrangement of a capacitive detection system and a conductive element in a vehicle seat comprises at least one sensing electrode and an electronics module. In accordance with an aspect of the invention, said electronics module comprises at least one compensation circuit for compensating the capacitive coupling between the conductive element and said at least one sensing electrode.

[0010] In a possible embodiment, said compensation circuit is configured for a direct measurement of a current coupled into the conductive element.

[001 1 ] In another possible embodiment, said compensation circuit comprises at least one compensation electrode, said compensation electrode being arranged in such a way with respect to said at least one sensing electrode, that a capacitance between said at least one sensing electrode and said compensation electrode is subject to the same relative variations as a capacitance between said at least one sensing electrode and said conductive element.

[0012] The compensation circuit comprises preferably an AC signal generator operatively coupled to said at least one compensation electrode for injecting a compensation current into said at least one compensation electrode. It will be appreciated, that in this embodiment the compensation current is preferably adjustable so as to compensate a parasitic current induced in said conductive element by said capacitive coupling between the conductive element and said at least one sensing electrode. [0013] In yet another embodiment, said compensation circuit comprises a current measuring circuit for determining the amount of current induced in said at least one compensation electrode by capacitive coupling between said at least one compensation electrode and said at least one sensing electrode. In this case said electronics module comprises a module for determining the capacitive coupling between the at least one sensing electrode and a reference electrode by subtracting the amount of current induced in said at least one compensation electrode from the total amount of current flowing trough said at least one sensing electrode.

Brief Description of the Drawings

[0014] Further details and advantages of the present invention will be apparent from the following detailed description of not limiting embodiments with reference to the attached drawings, wherein:

Fig. 1 schematically shows an electrode arrangement of a capacitive sensing system and a seat heater element in a vehicle seat;

Fig. 2 shows a simplified model based on basic circuit elements of the arrangement of Fig. 1 ;

Fig. 3 shows the situation, if a conductive object is positioned in the space above the sense electrode;

Fig. 4 shows a simplified model based on basic circuit elements of the arrangement of Fig. 3;

Fig. 5 shows an electrode arrangement of a first embodiment of a sensor system with compensation electrode;

Fig. 6 shows an electrode arrangement of a different embodiment of a sensor system with compensation electrode;

Fig. 7 shows an electrode arrangement of a different embodiment of a sensor system, in which the seat heater is used as sensing element. Description of Preferred Embodiments

[0015] The present invention generally relates to an arrangement of a vehicle seat with a seat heater and capacitive occupant detection system. The seat heater comprises a conductive heating element. Instead of a seat heater, the invention might also be applicable to any other conductive structure in the seat.

[0016] The capacitive occupant detection system comprises an electrode arrangement and an electronics module. The electrode arrangement comprises one or more sensing electrode and as an aspect of the invention one or more compensation electrodes. These compensation electrodes are according to a preferred aspect of this invention installed close to the heating element of the seat heater. In another potential implementation the compensation electrodes might be part of the seat heater itself or the seat heater element itself might be used directly for compensation.

[0017] In contrast to other implementations of such systems, there is no guard/ shield electrode required to mask the seat heater.

[0018] Figure 1 shows an electrode arrangement with a sense electrode (1 ) and a seat heater element (2) installed in a seat (3). The electrode (4) represents the reference electrode, which is the car body ground (5) electrode in a preferred implementation. There is none of said compensation electrodes. The impedance (6) between the seat heater element (2) and the car body ground (5) is shown as another relevant element of the arrangement.

[0019] It is obvious, that in said arrangement the capacitance between the sense electrode (1 ) and the car body reference (4)(5) is significantly increased, if a seat heater element (2) with a low-impedance (6) connection to the reference electrode (4) is positioned in close vicinity of the sense electrode (1 ). For a common implementation of a seat heater and a capacitive occupant detection system, which is referenced to the car body (5), this is usually the case.

[0020] It is also obvious, that the contribution of the seat heater element to the total sensing capacitance might easily supersede that of the car body ground electrode itself, taking into account that the distance between said sense electrode and the seat heater element is typically around 10 mm, whereas the distance to the car body electrode is in the range of several decimetres. Typical values for the capacitance between the sense electrode and the reference electrode in an occupant detection application are in the range of 10pF, for the capacitance between the sense electrode and a seat heater in the range of 0.5nF to 2nF.

[0021 ] Figure 2 shows a simplified model of the arrangement based on basic circuit elements.

[0022] For further clarification, the sense electrode (1 ) is split into two segments (1A, 1 B) to emphasize the two different portions of the total capacitance of the sense electrode (1 ) towards the car body reference (5). One portion is formed by the capacitance between the top side of the sense electrode (1A) and the reference electrode (4), which is the car body ground (5), and the other portion represents the capacitance between the bottom side of the sense electrode (1 B) and the seat heater element (2), which is connected to the car body ground reference (5) via an impedance (6).

[0023] The skilled person will easily note, that - if the impedance (6) is low, which is usually the case for a seat heater installed in a car - the total capacitance between the sense electrode (1 ) and the car body reference (5) can be approximated as the sum of said two capacitor portions formed by (1A,4) and (1 B,2).

[0024] Figure 3 shows the situation, if a conductive object (7) is positioned in the space above the sense electrode (1 ). In the preferred implementation in a car the conductive object (7) might be an occupant positioned on the surface of the seat foam (3). It is obvious, that the object (7) will alter the field distribution on the top side of the sense electrode (1 ) towards the car body ground reference electrode (4) (5) and hence change that portion of the total capacitance between the sense electrode (1 ) and the car body ground (5).

[0025] The skilled person will easily understand, that the capacitance portion from the sense electrode (1 ) through the object (7) towards the car body reference electrode (4)(5) will increase. In the preferred implementation in a car, this effect is the fundamental principle for the detection of an occupant.

[0026] The capacitance portion between the seat heater (2) and the sense electrode (1 ) is not altered. A typical value for the capacitance portion from the sense electrode (1 ) through the object (7) towards the car body reference electrode (4)(5) is in the range of 40pF for an occupant detection system.

[0027] For further clarification Fig. 4 shows the situation in an arrangement with an object on top of the seat surface in form of simple model based on basic circuit elements.

[0028] The sense electrode (1 ) is again split into two segments (1A)(1 B) to emphasize the two different portions of the total capacitance of the sense electrode (1 ) towards the car body reference (5). One portion is formed by the capacitance from the bottom side segment of the sense electrode (1 B) to the seat heater element (2), which is connected to the car body reference (5) via the impedance (6). The other portion is formed by the series connection of the capacitance between the top side of the sense electrode (1A) and the object (7B) and the capacitance between the object (7A) and the car body reference electrode (4)(5).

[0029] The skilled person will easily note, that - if the impedance (6) is low, which is usually the case for a seat heater installed in a car - the total capacitance between the sense electrode (1 ) and the car body reference (5) can be approximated as the sum of said portions formed by the capacitor (1 B,2) and the series connection of the capacitors (1A,7B) and (7A,4).

[0030] Comparing the modelled situations in Fig. 2 and Fig. 4 and taking into account the typical values for the different capacitance portions, it becomes obvious that the major part of the total capacitance between the sense electrode (1 ) and the car body reference (4)(5) is caused by the direct coupling from the sense electrode (1 ) to the seat heater element (2). Only a small portion results from the direct coupling from the sense electrode (1 ) via the object (7) to the car body reference (4)(5). [0031 ] For the preferred implementation and with the typical values given, e.g. 1 nF for the capacitance portion between the sense electrode (1 ) and the seat heater element (2), 10pF for the capacitance portion between the sense electrode and the car body reference (4)(5) for the arrangement without occupant and 40pF with occupant, the total capacitance results in values of 1 .01 nF without occupant and 1 .04nF with occupant.

[0032] The skilled person will thus easily realize, that due to the vicinity of the seat heater element the information containing portion of the capacitance is relatively small compared to the total sensing capacitance, which is dominated by the large offset capacitance portion due to the coupling of the sense electrode and the seat heater.

[0033] It is also obvious, that a capacitance measurement system connected to such an electrode arrangement will have to measure a small variation in capacitance in presence of a large offset capacitance, which conflicts at a given budget with the common balance between measurement resolution and dynamic range.

[0034] A further deteriorating effect arises from potential variations/tolerances of the capacitance portion between the sense electrode (1 ) and the seat heater element (2). In the preferred application in an occupant detection system, this capacitance portion is subject to variations caused by:

1 . Tolerances in capacitor geometry. These are mainly assembly related variations in distance between the sense electrode (1 ) and the seat heater element (2). A typical value for tolerance in distance might be +/- 1 mm at a typical value for the total distance of said 10mm.

2. Tolerances in electrical material properties of the seat material between the sense electrode (1 ) and the seat heater element (2), e.g. the relative electrical permittivity. A typical value for the tolerance in relative permittivity might be +/-TBD at a total relative permittivity of typically 2.

[0035] The skilled person will easily understand, that e.g. said variation in distance might cause a variation of the capacitance portion from the typical value of 1 nF by +/- 100pF. It is also obvious, that this variation will clearly cause a change in total capacitance, which is in the range of said total capacitance value resulting from the presence of an occupant in the preferred implementation. This will conflict with the robustness requirements for such a system and will impact on the definition of a fixed detection threshold for the occupant detection.

[0036] In order to alleviate the above effects, a further electrode is now introduced in different functions, which in combination with specific preferred implementations into capacitive measurement systems, reduces or neutralizes said negative effects which arise from the vicinity of the seat heater and which result in said offset capacitance value and said tolerance effects. This further electrode will be referred to as compensation electrode.

[0037] As a further step, a system implementation is developed, which even cancels the demand for an additional electrode by adding functionality to the seat heater module itself.

[0038] The following explanations will only given for the electrode arrangement without the presence of an object as said negative effects are caused by the capacitance portion on the bottom side of the sense electrode only. For further simplification the impedance between the seat heater and the car body ground will be considered as negligibly small at the operating frequency of the capacitive measurement system and will be replaced by a direct connection to the car body ground.

[0039] The seat material will not be shown.

Implementation 1 - Compensation Electrode as active Element

[0040] Fig. 5 shows an electrode arrangement with a sense electrode (1 ), a seat heater element (2) and a compensation electrode (13). The sense electrode is connected to a typical capacitive measurement system consisting of a first sine wave voltage generator (1 1 ) and current meter (10).

[0041 ] The skilled person will easily understand that an impedance connected to said measurement system can be evaluated by amplitude and phase comparison of the voltage and current. [0042] The compensation electrode (13) itself is in accordance with an aspect of the invention connected to a second sine wave generator (12). The voltage generators (1 1 )(12) and the seat heater element are referenced to the car body ground (5).

[0043] It is obvious, that if now the first sine wave voltage generator (1 1 ) is activated, a specific portion of the current through the current meter (10) results from the capacitive coupling between the sense electrode (1 ) and the seat heater element (2) and another portion from the coupling of the sense electrode (1 ) through the detection space on top of the electrode towards the car body reference (4)(5).

[0044] As explained earlier in this document said capacitive coupling of the seat heater (2) and the resulting current (8) respectively can be regarded as parasitic effect, which has a deteriorating effect on the measurement of the capacitance portion caused by the coupling on the top side of the sense electrode (1 ), which is represented by the detection current (14).

[0045] The compensation electrode (13) in combination with the second sine wave voltage generator (12) can now be used to compensate this deteriorating effect by injecting a compensation current (9) into the sense electrode (1 ) via capacitive coupling.

[0046] It can be derived that l_Current_Meter (10) = l_Seat_Heater (8) + l_Detection (14) - l_Compensation (9).

[0047] If said compensation current (9) - flowing into the sense electrode (1 ) - is equal to the parasitic current (8) - flowing out of the sense electrode -, the seat heater's (8) contribution to the total capacitance sensed by the measurement system (10)(1 1 ) is completely compensated. The current sensed by the current meter (10) is only the detection current (14), which is caused by capacitance portion on top of the sense electrode (14).

[0048] It follows (l_Current_Meter (10) = l_Detection (14)) , if (l_Seat_Heater (8) = l_Compensation (9)). [0049] The skilled person will easily note that such a system is advantageous, as it does not have to cope with deteriorating influence of an offset capacitance, which has been laid out in one of the previous paragraphs.

[0050] A further advantage compared to a system based on a guard electrode becomes obvious, if it is taken into account, that the compensation current is adjustable by modifications of the capacitive coupling between sense electrode (1 ) and compensation electrode (2) and by the voltage level of the second sine wave voltage generator (12).

[0051 ] It can be easily understood, that a guard electrode has to have at least the same size as the projection of seat heater (2) and sense electrode (1 ), whereas a compensation electrode (13) can be designed significantly smaller by increasing the voltage level of the second sine wave voltage generator (12). This might result in a positive effect on system cost.

[0052] Also regarding the negative influence of assembly-related distance variations between the sense electrode (1 ) and the seat heater element (2) or variations in seat material properties, which have been detailed in a previous paragraph, the system with compensation electrode is an improvement.

[0053] If the compensation electrode (13) is strongly mechanically coupled to the seat heater element (2) and if the compensation electrodes' (13) coupling to the sense electrode (1 ) is subject to the same assembly tolerances and variations in material properties as the coupling of the seat heater element (2), it can be noticed that the relative variation k of the capacitances is identical and has a self-compensating effect, which cancels said negative influence.

[0054] This can be derived from the following equation:

l_Current_Meter (10) = l_Seat_Heater (8) + l_Detection (14) - l_Compensation (9) l_Current_Meter (10) being the total current fed by the capacitive measurement system (10) into the sense electrode, l_Seat_Heater (8) being the current fed by the sense electrode (1 ) into the seat heater (2), ^Detection (14) being the current fed by the sense electrode (1 ) into the detection area and l_Compensation (9) being the current fed by the compensation electrode (13) into the sense electrode.

[0055] With

I_Seat_Heater = wC_Seat_Heater_Sense ^* L)_Sense,

i_Compensation = wC_Compensation_Sense ^* (L)_Compensation-L)_Sense),

I_Seat_Heater = I_Compensation

and introducing a variation k of C_Seat_Heater_Sense and C_Compensation_Sense

this can be rearranged to

i_Current_Meter (10) = k ^* wC_Seat_Heater_Sense ^* L)_Sense + l_Detection (14) - k ^* wC_Compensation_Sense ^* (L)_Compensation - L)_Sense)

[0056] This can be solved to i_Current_Meter (10) = l_Detection (14)

[0057] Hence the variation k does not influence the measurement.

[0058] As an example, the compensation electrode (13) might be placed directly aside the seat heater element (2) in the same distance from the sense electrode (1 ) with the same size as the seat heater element (2). In this case, both sine wave generators might be equal in frequency and phase, the amplitude of the second sine wave generator (12) being twice the amplitude of the first sine wave generator (1 1 ).

[0059] In a preferred implementation, the compensation electrode (13) might be part of the seat heater element (2) itself.

Implementation 2 - Compensation Electrode as Sensing Element

[0060] Fig. 6 shows an electrode arrangement with a sense electrode (1 ), a seat heater element (2) and a compensation electrode (13). The sense electrode is connected to a typical capacitive measurement system consisting of a first sine wave voltage generator (1 1 ) and current meter (10). [0061 ] The skilled person will easily understand that an impedance connected to said measurement system can be evaluated by amplitude and phase comparison of the voltage and current.

[0062] The compensation electrode (13) itself is as an aspect of the invention connected to a second current meter (15).

[0063] It is obvious, that - if now the sine wave voltage generator (1 1 ) is activated - a specific portion of the current through the current meter (10) results from the capacitive coupling between the sense electrode (1 ) and the seat heater element (2), another portion from the coupling of the sense electrode (1 ) through the detection space towards the car body reference (4)(5) and a last portion from the coupling between the sense electrode and the compensation electrode (13).

l_Current_Meter1 (10) = l_Seat_Heater (8) + l_Detection (14) + l_Current_Meter2 (9)

[0064] As explained earlier in this document said capacitive coupling of the seat heater (2) and the resulting parasitic current (8) respectively can be regarded as parasitic effect and they have a deteriorating effect on the measurement of the capacitance portion caused by the coupling on the top side of the sense electrode (1 ), which is represented by the detection current (14).

[0065] The compensation electrode (13) in combination with the second current meter (12) can now be used to compensate this deteriorating effect by calculation.

[0066] It is obvious, that at a known ratio L between the capacitance from the sense electrode (1 ) to the compensation electrode (13) and the capacitance from the sense electrode (1 ) to the seat heater element (2), the parasitic current (8) can be calculated by use of the current measured by the current meter (14).

[0067] It can be derived that

l_Seat_Heater (8) = L ^* l_Current_Meter2 (9), L = C_Seat_Heater_Sense / C_Compensation_Sense

[0068] Rearranging the formula for the total current (10) into the sense electrode (1 ) we can get: l_Detection (14) = l_Current_Meter1 (10) - L ^* l_Current_Meter2 (9) - l_Current_Meter2 (9) = l_Current_Meter1 (10) - (1 +L) ^* l_Current_Meter2 (9)

[0069] In a preferred implementation, this calculation might be performed by HW or SW. In doing so any further processing step will only have to evaluate the actual portion of the detection current (14) in phase and amplitude.

[0070] The skilled person will easily note that such a system is advantageous, as it reduces the deteriorating influence of an offset capacitance, which has been laid out in one of the previous paragraphs.

[0071 ] A further advantage compared to a system based on a guard electrode becomes obvious, if it is understood, that a guard electrode has to have at least the same size as the projection of seat heater (2) and sense electrode (1 ), whereas a compensation electrode (13) can be designed significantly smaller by adjusting the compensation factor L with a positive effect on system cost.

[0072] Also referring to the negative influence of assembly-related tolerances in distance between the sense electrode (1 ) and the seat heater element (2) or variations in seat material properties, which have been detailed in a previous paragraph, the system with compensation electrode is an improvement.

[0073] If the compensation electrode (13) is strongly mechanically coupled to the seat heater element (2) and if the compensation electrodes' (13) coupling to the sense electrode (1 ) is subject to the same assembly tolerances and variations in material properties as the seat heater element (2), it can be noticed that the relative variation k of the capacitances is identical and has a self- compensating effect, which cancels out said negative influence.

[0074] This be derived from the following equation

L^' = (k ^* C_Seat_Heater_Sense) / (k ^* C_Compensation_Sense) = C_Seat_Heater_Sense / C_Compensation_Sense = L

[0075] In a preferred implementation, the compensation electrode (13) might be part of the seat heater element (2) itself. Implementation 3 - Seat Heater as Sensing Element

[0076] Fig. 7 shows an electrode arrangement with a sense electrode (1 ) and a seat heater element (2). The sense electrode is connected to a typical capacitive measurement system consisting of a sine wave voltage generator (1 1 ) and current meter (10).

[0077] The skilled person will easily understand that an impedance connected to said measurement system can be evaluated by amplitude and phase comparison of the voltage and current.

[0078] The heater element (2) itself is according to an aspect of the invention connected to a second current meter (16).

[0079] It is obvious, that - if now the sine wave voltage generator (1 1 ) is activated - a specific first portion of the current through the current meter (10) results from the capacitive coupling between the sense electrode (1 ) and the seat heater element (2) and second portion from the coupling of the sense electrode (1 ) through the detection space towards the car body reference (4)(5). l_Current_Meter1 (10) = l_Detection (14) + l_Current_Meter2 (16)

[0080] As explained earlier in this document said capacitive coupling of the seat heater (2) and the resulting parasitic current (8) respectively can be regarded as parasitic effect and they have a deteriorating effect on the measurement of the capacitance portion caused by the coupling on the top side of the sense electrode (1 ), which is represented by the detection current (14).

[0081 ] The second current can now be used to compensate this deteriorating effect by calculation. It can be derived that

l_Seat_Heater (8) = l_Current_Meter2 (16)

[0082] Rearranging the formula for the total current (10) into the sense electrode (1 ) we can get:

l_Detection (14) = l_Current_Meter1 (10) - l_Current_Meter2 (16) [0083] In a preferred implementation, this calculation might be performed by HW or SW. In doing so any further processing step will only have to evaluate the actual portion of the detection current (14) in phase and amplitude.

[0084] The skilled person will easily note that such a system is advantageous, as it reduces the deteriorating influence of an offset capacitance, which has been laid out in one of the previous paragraphs, as it does not introduce a further compensation electrode or guard electrode.

[0085] Also referring to the negative influence of assembly-related tolerances in distance between the sense electrode (1 ) and the seat heater element (2) or variations in seat material properties, which have been detailed in a previous paragraph, the system is an improvement as it directly measures the resulting variation in offset capacitance.

[0086] For the implementation of such a system, there might be two preferred implementations:

1 . In a first preferred implementation, the second current meter (16) might be part of the seat heater module itself. Synchronized by a communication interface, the seat heater module would measure the current coupled into the seat heater element, the seat heater element grounded, itself and would transmit the result to the capacitive measurement system via a standard communication interface, e.g. a CAN or LIN interface. The capacitive measurement system would measure the total current, calculate the detection current based on the information received from the seat heater module and evaluate the result.

2. In a second preferred implementation, the second current meter (16) might be part of capacitive measurement system module. The capacitive measurement system module might comprise a subunit inserted into the supply lines between the seat heater element and the seat heater module, in order to connect the seat heater element periodically to either the capacitive measurement system module or to the seat heater control module itself. Further Potential for Improvement:

1 . Instead of one compensation electrode use multitude of smaller compensation electrodes homogenously distributed to cover capacitance variations over the complete seat heater surface.

2. Use a small compensation electrode between the sense electrode and the seat heater to actively sense the capacitance towards the seat heater. This compensation electrode should be positioned as close to the seat heater as possible and should be driven with a copy of the sense signal. In doing so, the compensation electrode would be completely shielded by the sense electrode and the seat heater and would exclusively sense the coupling towards the seat heater. The effect between sense and guard could be interpolated/compensated by HW/SW afterwards.

Claims

1 . An arrangement of a capacitive detection system and a conductive element in a vehicle seat, wherein said capacitive detection system comprises at least one sensing electrode and an electronics module, characterised in that said electronics module comprises at least one compensation circuit for compensating the capacitive coupling between the conductive element and said at least one sensing electrode.

2. The arrangement according to claim 1 , wherein said compensation circuit is configured for a direct measurement of a current coupled into the conductive element.

3. The arrangement according to claim 1 or 2, wherein said compensation circuit comprises at least one compensation electrode, said compensation electrode being arranged in such a way with respect to said at least one sensing electrode, that a capacitance between said at least one sensing electrode and said compensation electrode is subject to the same relative variations as a capacitance between said at least one sensing electrode and said conductive element.

4. The arrangement according to claim 3, wherein said compensation circuit comprises an AC signal generator operatively coupled to said at least one compensation electrode for injecting a compensation current into said at least one compensation electrode.

5. The arrangement according to claim 4, wherein said compensation current being adjustable so as to compensate a parasitic current induced in said conductive element by said capacitive coupling between the conductive element and said at least one sensing electrode.

6. The arrangement according to claim 3, wherein said compensation circuit comprises a current measuring circuit for determining the amount of current induced in said at least one compensation electrode by capacitive coupling between said at least one compensation electrode and said at least one sensing electrode, and wherein said electronics module comprises a mod- ule for determining the capacitive coupling between the at least one sensing electrode and a reference electrode by subtracting the amount of current induced in said at least one compensation electrode from the total amount of current flowing trough said at least one sensing electrode.