Abstract:
A vehicle seat suspension mat ( 10 ) comprises one or more suspension springs ( 16 ) for supporting a seat cushion ( 22 ) and an occupancy sensor module ( 24 ) supported by at least one suspension spring. The module includes a first plate ( 26 ) having a switch element ( 38 ) arranged thereon and a second plate ( 28 ) mounted in a cantilevered manner to the first plate, in such a way that the first and second plates form together a pair of jaws, which the switch element is arranged between. The first and second plates are configured so as to move closer together against their resiliency in response to compressive forces acting on the first and second plates.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to the field of vehicle seats. In particular, the invention relates to a vehicle seat suspension mat. 
       BACKGROUND ART 
       [0002]    Seat occupancy sensors are nowadays widely used in automotive vehicles to provide a seat occupancy signal for various appliances, such as, e.g. a seat belt reminder, an auxiliary restraint system (airbag), etc. The possibility of customization and personalization of the vehicle by the customer is a key selling factor of modern cars. This leads to many different variants of car interiors being offered for one car model. With the increasing number of available options, severe constraints arise concerning the implementation of technical equipment in the vehicle. With seat occupancy sensors arranged between the foam body of the seat cushion and the seat cover, every seat design (leather, cloth, sport, comfort, . . . ) requires specific development effort for the occupant detection system. That induces high development costs and therefore is an unattractive solution for the automotive industry. A problem to be solved is, therefore, to find a sensor solution, which is less influenced by seat design and thus can be used for a greater variety of car seats or even car platforms. 
         [0003]    Document DE 197 52 976 A1 discloses vehicle seat suspension mat, comprising an occupancy sensor in the shape of a film-type pressure sensor. The pressure sensor includes a first carrier film, a spacer and a second carrier film, which are disposed on one another in the manner of a sandwich. Contact elements are arranged on the inner surfaces of the carrier films. An opening in the spacer allows the contact elements to get into contact with each other when pressure is applied on the sensor. The pressure sensor is arranged inside a cavity on the bottom side of the foam cushion of the vehicle seat. The pressure sensor is supported by a foam block, which closes the cavity and which rests on the seat pan. 
         [0004]    Document DE 20 2010 003 563 U1 discloses a vehicle seat suspension mat, comprising a film-type pressure sensor. The pressure sensor includes two films maintained at a distance by a spacer material arranged there between. The pressure sensor is disposed on a compressible intermediate layer, which is, in turn arranged on a base plate. 
         [0005]    A drawback of the vehicle seat suspension mats of the documents DE 197 52 976 A1 and DE 20 2010 003 563 U1 resides in the fact that it is necessary to adapt the characteristics of the pressure sensor for each reference of seat. 
       BRIEF SUMMARY 
       [0006]    The present invention provides a vehicle seat suspension mat that is adaptable to different reference of seat without need to modify the pressure sensor. 
         [0007]    A vehicle seat suspension mat in accordance with an aspect of the invention comprises one or more suspension springs for supporting a seat cushion and an occupancy sensor module supported by at least one of the one or more suspension springs. The occupancy sensor module includes a first plate having a switch element arranged thereon and a second plate mounted in a cantilevered manner to the first plate, in such a way that the first and second plates form together a pair of jaws, which the switch element is arranged between. The first and second plates are configured so as to move closer together against their resiliency in response to compressive forces acting on the first and second plates. 
         [0008]    The mechanical properties (such as e.g. thickness, modulus of elasticity, dimensions etc.) of the first and second plates are selected in such a way that the switch element is activated when the compressive forces exceeds a predefined threshold. The threshold corresponds to the weight exerted on the seat above which the seat is detected occupied, i.e. the occupancy state is “occupied”. The threshold may e.g. be selected such that a 5 th  percentile female or heavier occupant of the seat is reliably detected. A different threshold may be chosen in accordance with the needs of the application (e.g. seat belt reminder, airbag control system, etc.) 
         [0009]    As will be appreciated, the suspension mat is little or not influenced by the design of seat parts that can be readily perceived by the user (such as e.g. the shape of the cushion and the upholstery of the seat). For a particular car model, the suspension mat is normally defined only once, namely at the moment of the seat design. The suspension mat being a part of a car&#39;s passive safety concept, it will not be changed after the passive safety concept of a vehicle has been defined. As this happens at an early stage in the development of a car model, the car manufacturer has more time for making adjustments in the outer design of the seat, if necessary, than with an occupancy sensor arranged between the seat cushion and the trim. 
         [0010]    The first and second plates are preferably made of a single piece of material, e.g. plastic, in particular injection-molded plastic. 
         [0011]    According to a preferred embodiment of the invention, the switch element is part of a pressure-responsive membrane switch. Such pressure-responsive membrane switch preferably comprises a first carrier film and a second carrier film spaced from each other by a spacer film, the spacer film having therein an opening defining a pressure-sensitive cell (representing the switch element), the pressure-responsive membrane switch comprising at least two electrodes arranged in facing relationship with each other in the cell on the first and the second carrier film, respectively, in such a way that they are brought closer together, possibly into contact with each other, when pressure is applied on the pressure-responsive membrane switch. 
         [0012]    The occupancy sensor module preferably comprises one or more fixation elements (such as, e.g. clips or clamps) by means of which the occupancy sensor module is attached to the at least one of the one or more suspension springs. The fixation element(s) may be integrally formed with the first and second plates. The one or more fixation elements could be arranged on the first plate or the second plate. 
         [0013]    According to a preferred embodiment of the invention, a detection circuit is connected to the switch element for detecting whether the switch element is compressed by the first and second plates. 
         [0014]    The occupancy sensor module may comprise a third plate mounted in a cantilevered manner to the first plate and a second switch element arranged between the first and third plates. In this case, the first and third plates are configured to form together a second pair of jaws and to move closer together against their resiliency to compressive forces acting on the first and third plates. The second switch element may be of the same configuration as the first switch element. 
         [0015]    The third plate may have arranged thereon one or more fixation elements by means of which the occupancy sensor module is attached to the at least one of the one or more suspension springs. 
         [0016]    An aspect of the invention concerns a vehicle seat comprising a seat frame, a vehicle seat suspension mat as described hereinbefore, taut on the seat frame and a seat cushion supported by the vehicle seat suspension mat. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0018]      FIG. 1  is a transversal cross sectional view of the seating portion of a vehicle seat including a seat suspension mat according to a preferred embodiment of the invention; 
           [0019]      FIG. 2  shows a detail of  FIG. 1 ; 
           [0020]      FIG. 3  is a perspective view of a variant of a vehicle seat suspension mat according to the invention; 
           [0021]      FIG. 4  is a perspective view of another variant of a vehicle seat suspension mat according to the invention. 
       
    
    
       [0022]    For sake of clarity, the drawings are nor to scale. The different elements of each figure are thus not necessarily shown in proper proportion. 
       DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    A vehicle seat suspension mat  10  according to a first preferred embodiment of the invention is shown in  FIG. 1 . The suspension mat  10  is part of a seating portion  12  of a vehicle seat  14 . The suspension mat comprises a plurality of suspension springs  16  (e.g. made of steel wire) taut between fixation sites (not shown) of the structural frame  18  of the vehicle seat  14 . The suspension springs  16  are flexibly linked to each other by plastic cross-ties  20 . The suspension mat  10  supports the seat cushion  22  of the vehicle seat  14 . The suspension mat  10  further includes a seat occupancy detector  24  that is responsive to the weight of a seat occupant. 
         [0024]    The seat occupancy sensor module  24  is clipped on a suspension spring  16  by a fixation element or clip  17 . As best illustrated in  FIG. 2 , the seat occupancy sensor module  24  comprises a first  26  and a second plate  28  mounted in cantilevered manner on the first plate  26 . The first and second plates  26 ,  28  are made of injection-molded plastic and form together a pair of jaws. A pressure-responsive membrane switch  30 , is arranged opposite the second plate  28  on the first plate  26 . 
         [0025]    The pressure-responsive membrane switch  30  comprises a first carrier film  32  and a second carrier film  34  spaced from each other by a spacer film  36 . The spacer film  36  has therein an opening defining a pressure-sensitive cell  38  (switch element). In the cell  36 , a first electrode  40  and a second electrode  42  are arranged on the first carrier film, in facing relationship with a third electrode  44  on the second carrier film  34 . 
         [0026]    The first and second electrodes  40 ,  42  are connected to a detection circuit  46  that detects whether the first and second electrodes  40 ,  42  are shunted by the third electrode  44 . The detection circuit may e.g. be a microcontroller or an application-specific integrated circuit (ASIC). The detection circuit  46  comprises an output  48  for providing a signal indicative of whether the electrodes  40 ,  42 ,  44  are in contact or not. The detection circuit  46  detects a contact between the three electrodes  40 ,  42 ,  44  as a drop in resistance. The drop of resistance may be determined from an increase of current or a drop in voltage, depending on the configuration of the detection circuit  46 . 
         [0027]    As shown in  FIG. 1 , the seat occupancy sensor module  24  is maintained between the seat cushion  22  and at least one of the suspension springs  16 . When the seat is occupied, the seat occupancy sensor module  24  experiences part of the weight of the occupant and part of the reaction force of the suspension springs  16 . These compressive forces push the first and second plates  26 ,  28  toward each other. When sufficient load is applied on the seating portion  12 , the first and second plates squeeze the pressure-responsive membrane switch  30 , whereby the electrodes  40 ,  42 ,  44  therein are brought into electrical contact. The mechanical properties of the seat occupancy sensor module  24  are selected such that a predetermined minimum load on the seating portion  12  reliably results in the activation of the membrane switch  30  (i.e. in the electrodes  40 ,  42 ,  44  being brought into electrical contact with one another). The signal output by the evaluation circuit  46  thus indicates whether the seat is occupied or empty. 
         [0028]    The sensitivity of the seat occupancy sensor module  24  may be adjusted in numerous ways in order to achieve the desired activation threshold. As illustrated in  FIG. 2 , the second plate  28  may comprise a stud  50  arranged facing the pressure-responsive membrane switch  30 . The stud  50  serves to concentrate the application of force on the pressure-sensitive cell  38 . The dimensions (height, diameter etc.) of the stud  50  may be chosen in accordance with the desired activation threshold.  FIG. 2  also shows that the pressure-responsive membrane switch  30  may be arranged on a base  52 , whereby the distance between the pressure-responsive membrane switch  30  and the second plate  28  is reduced. The base  52  may be integrally formed with the first plate  26 . Alternatively, the base  52  could comprise a layer of compressible material (e.g. polyurethane or other foam). 
         [0029]      FIG. 3  shows a vehicle seat suspension mat  110  according to a further preferred variant of the invention. The seat suspension mat  110  comprises a plurality of suspension springs  116  and an occupancy sensor module  124  fixed to a pair of neighboring suspension spring  116  in the manner of a cross-tie. The occupancy sensor module  124  comprises a first plate  126  carrying a film-type pressure-sensor (pressure sensitive membrane switch)  130 . A second plate  128  and a third plate  154  are mounted respectively in cantilevered manner on the first plate  126 , each in facing a respective pressure-sensitive cell  136  of the film-type pressure-sensor  130 . The configuration of the first and second plates  126 ,  128  is the same as that of the first and third plates  126 ,  154 . The function of the occupancy sensor module  124  is otherwise the same as that of the occupancy sensor module described above with reference to  FIGS. 1 and 2 . 
         [0030]      FIG. 4  shows yet a further variant of a vehicle seat suspension mat according to the invention. The pressure sensor mat  210  comprises a plurality of suspension springs  216  and an occupancy sensor module  224  fixed to a pair of neighboring suspension spring  216  in the manner of a cross-tie. The occupancy sensor module  224  comprises a first plate  226  having a film-type pressure-sensor  230  applied against its bottom. A second plate  228  and a third plate  254  are mounted respectively in cantilevered manner on the first plate  226 , each in facing a respective pressure-sensitive cell  238  of the film-type pressure-sensor  230 . The configuration of the first and second plates  226 ,  228  is the same as that of the first and third plates  226 ,  254 . Fixation elements  217  for fixing the occupancy sensor module  224  to the suspension springs.  216  are arranged on the bottom surfaces of the second and third plates  228 ,  254 . When integrated into a vehicle seat, the top surface of the first plate  226  is applied against the bottom surface of the seat cushion. The function of the occupancy sensor module  224  is otherwise the same as that of the occupancy sensor module described above with reference to  FIGS. 1 and 2 . 
         [0031]    While specific embodiments have been described in detail, those with ordinary skill in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.