Sensor mat comprising two switching levels

<i>introduction</i>

0001The present invention relates generally to a sensor mat for determining a seat occupancy situation in a vehicle seat.

0002Seat occupancy sensors have been used for some time to control the airbag in vehicles. On the basis of these seat occupancy sensors, a occupancy state of a vehicle seat is determined and the airbag (s) associated with the seat are activated only if the current occupancy condition necessitates such a deployment. These seat occupancy sensors generally have a plurality of switching elements, for example pressure sensors, which are arranged distributed in the seat surface of the vehicle seat. A control unit connected to the seat occupancy sensor queries the switching state of the individual switching elements and determines a state of occupancy of the seat from the respective switching states. If the seat is occupied by a person, a plurality of the shift elements are triggered by the weight force exerted by the person on the seat, a state which is recognized by the connected evaluation circuit as occupancy state of the seat and passed on to the airbag control.

0003In order to enable a safe triggering of the airbags assigned to the vehicle seat, such a seat occupancy sensor must be designed in such a way that a small triggering of the sensor mat is already recognized by the evaluation unit as occupancy of the seat so that the corresponding airbags are activated in the event of a crash. For this purpose, the sensor mat has to have a high dynamic, so that a distinction can be made between different seat occupancy situations.

0004For non-safety-critical applications such as, for example, in a seat belt warning system, such sensitive detection of the seat occupancy is not necessary or undesirable. A warning system of this kind, for example, outputs an acoustic or optical signal when the vehicle seat is occupied without the belt buckle of the assigned seat belt being snapped. The seat occupancy sensor in such a system must essentially allow a seat occupancy to be distinguished by a passenger and an occupancy by a light object, such as a handbag. For this purpose, the seat occupancy sensor is intended, preferably from a specific triggering, to produce a clear signal change, which applies as a switching threshold for the connected evaluation unit.

0005The requirements for a seat occupancy sensor for a safety belt warning system thus differ significantly from the requirements for a sensor mat for airbag control. If a single sensor mat is to be used both for the airbag control and in a seat belt warning system, these different requirements represent a considerable problem in the design of the sensor mat.

<i>Object of the invention</i>

0006It is therefore an object of the present invention to propose a sensor mat which allows a combined insert.

<i>General Description of the Invention</i>

0007This object is achieved by a sensor mat according to claim 1. Such a sensor mat for determining a seat occupancy situation in a vehicle seat comprises a plurality of sensor cells which can be assigned to a seat surface of the vehicle seat, the sensor cells being configured and interconnected with one another in such a way that an output signal of the seat sensor, Sensor mat varies depending on the seat occupancy situation. A plurality of first sensor cells and a plurality of second sensor cells are interconnected to form a sensor cell group such that, when the sensor mat is used as intended, an output signal of the sensor mat exceeds or falls below a predefined threshold value independently of the triggering state of the other sensor cells only if at least one sensor cell of the sensor sensor is connected A plurality of first sensor cells and at least one sensor cell of the plurality of second sensor cells.

0008The first sensor cells and the second sensor cells of the sensor mat are designed as pressure sensors with a pressure-dependent electrical resistance. Suitable pressure sensors are, in particular, film pressure sensors which can be produced cost-effectively in the form of sensor mats. In such an embodiment, the output signal of the sensor mat is, in principle, dependent on the weight force acting on the sensor mat, whereby the overall resistance of the sensor cells or of the sensor mat decreases with increasing weight force. In the case of the present sensor mat, at least two individual pressure sensors are connected to a pair of sensor cells in such a way that the resistance value of the sensor mat drops below a certain threshold value, independently of the triggering of the other pressure sensors, when the two pressure sensors are simultaneously triggered. The two pressure sensors connected to a pair of sensor cells are preferably configured in such a way that they are connected to one another by the simultaneous triggering of the two pressure sensors so that the resistance value of the sensor mat drops to a value close to zero when the sensor cell pair is triggered.

0009The first sensor cell and the second sensor cells configured as foil pressure sensors each comprise a first and a second electrode, which are arranged at a certain distance from one another on a first carrier film, and a triggering layer, which is arranged opposite the respective first and second electrodes on a second carrier film In that the release layer, when the first and second carrier film is pressed together, contacts the first and second electrode in a pressure-dependent manner.

0010According to the invention, the first sensor cell and the second sensor cell each have a third electrode, the third electrode of the first electrode being arranged opposite the second carrier film in the first sensor cell, and the third electrode of the second electrode being arranged in the second sensor cell on the second carrier film And wherein the third electrode of the first sensor cell is connected to the third electrode of the second sensor cell.

0011Due to this arrangement of the third electrodes in the individual sensor cells and the described connection of the third electrodes to one another, the two first and second electrodes of the sensor cells are short-circuited via the respective third electrodes and their interconnection in the event of triggering of the two pressure sensors An abrupt drop in the resistance value of the sensor mat is detectable below a predetermined threshold value.

0012The proposed sensor mat thus has two different switching thresholds, which can be adjusted essentially independently of each other depending on the requirements of the individual applications of the sensor mat. As long as no or only one of the two first and second sensor cells of the sensor cell pair is triggered by an occupancy situation, the output signal of the sensor mat can vary sensitively depending on the size of the occupied seat surface or depending on the weight force acting on the seat surface. This sensitive variation of the output signal can be evaluated in the connected evaluation circuit in order to precisely control activation of the restraint systems.

0013However, as soon as both a first sensor cell and a second sensor cell are triggered, the output signal of the sensor mat changes abruptly and passes through the predetermined threshold value. In the connected evaluation circuit, this behavior of the output signal can unambiguously be assigned to a specific occupancy situation in which, in addition to the activation of the restraint systems, a safety belt warning device, for example, is also activated. It is to be noted that the switching thresholds of the sensor mat can be adjusted in such a way that the predetermined threshold value of the output signal is only passed when the two sensor cells of the cell pair are activated. In this way, for example, it can be effectively excluded that a seat belt warning device is activated without a clear seat occupancy situation.

0014It should be noted that, according to the invention, a larger number of sensor cells can be connected to one another in such a way that a predetermined threshold value of the output signal of the sensor mat is exceeded or fallen below when several of these cells are triggered at the same time. For example, a parallel connection of a plurality of first sensor cells can be interconnected with a corresponding parallel connection of a plurality of second sensor cells in the manner described above. In this case, the predetermined threshold value of the output signal can, for example, already be reached during the simultaneous triggering of one of the first and one of the second sensor cells.

0015In a possible embodiment of the invention, the first and second sensor cells are arranged in immediate proximity to one another. In such an arrangement of the sensor cells, the predetermined threshold value is only passed if a seat occupancy in the region of the sensor cell pair locally exceeds a certain activation threshold. In another possible embodiment, the first sensor cell is arranged in a first region of the sensor mat and the second sensor cell is arranged in a second region of the sensor mat. The two sensor cells of the sensor cell pair can, for example, be arranged at least approximately the same distance with respect to a center line of the seat extending in the longitudinal direction of the vehicle and at a certain distance from one another. As a result of this symmetrical arrangement of the shifting elements with respect to the center line of the seat, a seat occupancy relevant to the seat belt warning system is only recognized as such when the occupancy of the vehicle seat extends over a predetermined distance and if the occupancy is at least approximately symmetrical with respect to the median plane of the seat. A local triggering of the seat occupancy sensor, for example by the presence of an object such as, for example, by a handbag placed on the seat, is not recognized as seat occupancy, and the belt warning system accordingly does not respond.

0016In a particularly simple embodiment of the invention, the first and second sensor cells are connected to one another via an individual connecting path. Such a direct interconnection via an individual connection path is particularly easy to implement if the two sensor cells of the pair of sensor cells are arranged at a small distance from one another. If, on the other hand, the sensor cells of the sensor cell pair are arranged in two regions of the sensor mat remote from one another, it may possibly be advantageous if the first and second sensor cells are connected to one another via a connected evaluation circuit. In such an embodiment, in which the individual sensor cells of the sensor cell pair are connected individually, for example in a matrix connection, to the evaluation circuit, no individual connection path need be laid over wide areas of the sensor mat. The integration of such a connecting path can, in fact, lead to unsolvable problems with regard to crossings of conductor paths which are to be avoided, in particular when additional functional elements such as, for example, antennas of a child-occupant identification are integrated into the sensor mat.

0017In an advantageous variant of the invention, the first and second sensor cells are connected in each case between a first and a second connecting conductor, and the first and second sensor cells are configured and interconnected in such a way that a short circuit occurs between the first and second sensor cells when the first and second sensor cells are triggered simultaneously The second connection conductor. The short circuit produced during the triggering of the two sensor cells can easily be detected by the connected evaluation circuit as a sign of a low resistance threshold value.

<i>DETAILED DESCRIPTION OF THE DRAWINGS FIG</i>

0018In the following, an embodiment of the invention is described with reference to the appended figures. Show it:<dl id="dl0001" compact="compact"><dt>FIG. 1:</dt><dd>2 shows a schematic representation of a first embodiment of a sensor mat according to the present invention;</dd><dt>FIG. 2:</dt><dd>2 shows a schematic representation of a second embodiment of a sensor mat according to the present invention; </dd><dt>FIG. 3:</dt><dd>2 shows a section through a possible embodiment of a foil pressure sensor pair.</dd></dl>

0019A first embodiment of a sensor mat 10 is shown schematically in FIG <figref idrefs="f0001">FIG</figref> Respectively. Such a sensor mat 10 comprises, for example, a plurality of individual sensor cells 12, which are arranged in an arrangement which is predetermined by a specific seat geometry, for example. The individual sensor cells are, for example, designed as foil pressure sensors, which are applied between common carrier films. Depending on the configuration of the sensor mat, the individual sensor cells 12 are contacted either individually or in groups via connecting lines running between the carrier films and a connecting lug 14 of the sensor mat 10. By means of this connecting lug 14, the sensor mat 10 can be connected to an evaluation circuit (not shown) in such a way that the switching states of the individual sensors or sensor groups can be determined.

0020In the embodiment shown in FIG <figref idrefs="f0001">FIG</figref> , A plurality of first sensor cells 16 and a plurality of second sensor cells 18 are connected to one another in such a way that an output signal of sensor block 10, with simultaneous triggering of at least one first sensor cell 16 and a second sensor cell 18 independently of the triggering state of the other sensor cells, Threshold. In the case of an embodiment with foil pressure sensors, ie with pressure-dependent resistors, such an interconnection can be carried out, for example, in such a way that the resistance value of the sensor mat or a group of sensor cells of the sensor mat drops to a resistance value close to zero.

0021A possible embodiment of such a interconnected foil pressure sensor pair is shown in FIG <figref idrefs="f0003">FIG</figref> Respectively. Each of the film pressure sensors 16 and 18 comprises a first electrode 22 and a second electrode 24 which are applied to a first carrier film 26 in a certain manner. Furthermore, each of the film pressure sensors 16 and 18 comprises a release layer 28 which is arranged on the second carrier film 30 opposite the respective first and second electrodes 22 and 24. The first carrier film 26 and the second carrier film 30 are arranged with respect to one another by means of a spacer 32 such that, when the switching cell is not triggered, the triggering layer 30 does not produce any electrical contact between the first electrode and the second electrode, while the release layer 28 compresses the first and second carrier films First and second electrodes contact each other in a pressure-dependent manner (represented by the current path 34).

0022Each of the two film pressure sensors 16 and 18 also has a third electrode 36, which is respectively applied to the second carrier film 30. In the first sensor cell 16, the third electrode 36 is arranged opposite the first electrode 22, while the third electrode 36 is arranged in the second sensor cell 18 opposite the second electrode 24. For the interconnection of the two sensor cells according to the invention, the respective third electrodes are contacted with one another by means of a connecting path. When at the same time the first sensor cell 16 and the second sensor cell 18 are triggered, the first electrode 22 in the first cell 16 and the second electrode 24 of the second cell 18 are contacted with one another via the opposing third electrodes 36 and the connecting path 38 thereof so that the resistance value Between the two electrodes 22 and 24 drops to a value close to zero.

0023In the embodiment shown in FIG <figref idrefs="f0001">FIG</figref> The different first and second sensor cells 16 and 18 are connected to one another via their respective third electrodes by means of a continuous individual connecting path 38. It should be noted that a sensor mat can have several sensor cell groups connected to one another. In the embodiment shown in FIG<figref idrefs="f0001">FIG</figref> A further group of switching cells with first and second switching cells 40 and 42 is connected to one another by means of a connecting path 44. The two switching cell groups are arranged symmetrically on either side of a central line of the sensor mat.

0024A further embodiment of a sensor mat 100 is shown in FIG <figref idrefs="f0002">FIG</figref> Respectively. In this embodiment, the first sensor cells 116 and 140 r and the second sensor cells 118 and 142, respectively, are not directly adjacent to one another but are arranged in different regions of the sensor mat 100, respectively. The first and second sensor cells of each sensor cell group can, for example, be arranged at least approximately the same distance with respect to a center line of the seat running in the longitudinal direction of the vehicle and at a certain distance from one another. Due to this symmetrical arrangement of the switching elements with respect to the center line of the seat, a seat occupancy relevant to the seat belt warning system is only recognized as such when the occupancy of the vehicle seat extends over a predetermined distance and if the occupancy is at least approximately symmetrical with respect to the median plane of the seat. A local triggering of the seat occupancy sensor, for example by the presence of an object such as, for example, by a handbag placed on the seat, is not recognized as seat occupancy, and the belt warning system accordingly does not respond.