Patent Publication Number: US-9428080-B2

Title: Seat occupancy sensor unit for seat with spring suspension or seat pan

Description:
TECHNICAL FIELD 
     The present invention generally relates to sensing the occupancy state of a vehicle seat. Specifically, the invention relates to a pressure-responsive seat occupancy sensor unit for being placed on the bottom side of a seat cushion; i.e. between the seat cushion and the cushion-supporting means such as, e.g. a seat pan, springs etc. 
     BACKGROUND ART 
     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. Seat occupancy sensors exist in a number of variants, e.g. based on capacitive sensing, deformation sensing or pressure (force) sensing. Pressure-sensitive seat occupancy sensors have typically been arranged between the foam body of the seat cushion and the seat cover. 
     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. 
     Document DE 197 52 976 A1 discloses a vehicle seat 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. 
     Document DE 20 2010 003 563 U1 discloses a pressure-sensor unit, 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. Document JP 2011 105278 discloses a seat occupancy sensor comprising a support plate with bearing elements for fixing the support plate on seat cushion suspension springs, wherein the bearing elements define an upper surface from which the top surface of the support plate is recessed. A foam pad carrying a pressure-responsive detector is arranged between the bearing elements on the support plate. 
     With pressure sensor units arranged on the B-surface of the seat cushion (i.e. on the side facing away from that on which an occupant may seat himself, between the seat cushion and the support thereof), the seat cushion transfers the pressure from the seating surface to the pressure sensor unit. As a matter of fact, the padding of the seat cushion becomes part of the measurement unit. Whereas the production tolerances of film-type pressure sensors may be controlled such that a uniform activation pressure threshold (i.e. the pressure, at which the films of the pressure sensor get into contact with each other) is achieved for the whole production, important investments would be necessary in the seat manufacturers&#39; production in order to guarantee that the foam of every seat cushion has exactly the same thickness and behaves the same way under pressure, in particular, transfers the same amount of pressure to the pressure-sensor unit. 
     BRIEF SUMMARY 
     The disclosure provides a pressure-responsive B-surface seat occupancy sensor unit compatible with the production tolerances of vehicle seats. 
     A pressure-responsive seat occupancy sensor unit for detecting an occupancy state of a seat, e.g. a vehicle seat, according to a first aspect of the invention is configured for seats wherein the cushion is supported by springs. Such pressure-responsive seat occupancy sensor unit comprises a support plate having a top surface and a bottom surface, a plurality of bearing elements for fixation of the support plate on seat cushion suspension springs, the bearing elements defining an upper surface of the pressure-responsive seat occupancy sensor unit, which upper surface the top surface of the support plate is arranged recessed from, a foam pad disposed between the bearing elements on the support plate, a recess formed in the top surface of the support plate, underneath the foam pad, and a pressure-responsive membrane switch arranged in the recess. The recess has a depth exceeding the thickness of the pressure-responsive membrane switch. The foam pad is configured such that, upon it being compressed by application of pressure exceeding a certain threshold, it deforms so as to penetrate into the recess and activates the pressure-responsive membrane switch. 
     As will be appreciated, due to the depth of the recess greater than the thickness (height) of the pressure-responsive membrane switch, there is an air gap between the pressure-responsive membrane switch and the bottom of the foam pad when the seat is in unloaded condition. The air gap thus prevents a so-called pre-loading of the pressure-responsive membrane switch. Furthermore, if the foam pad deforms due to ageing and begins sinking into the gap, it will not immediately apply a pre-load on the membrane switch. Accordingly, a longer lifetime of the unit may be obtained. 
     Preferably, the bearing elements are integrally formed with the support plate. The bearing elements and the support plate can be made from any suitable material. Preferably, however, they are made from injection-molded plastic. 
     According to a second aspect of the invention, the pressure-responsive seat occupancy sensor unit is configured for seats wherein the cushion is supported by a seat pan (e.g. made from sheet metal or from plastic). The pressure-responsive seat occupancy sensor unit for detecting an occupancy state of a seat according to the second aspect of the invention comprises a support plate having a top surface and a bottom surface, a plurality of spacer elements defining an upper surface of the pressure-responsive seat occupancy sensor unit from which upper surface the top surface of the support plate is arranged recessed from, a foam pad disposed between the spacer elements, carried by the support plate, a recess formed in the top surface of the support plate, underneath the foam pad, and a pressure-responsive membrane switch arranged in the recess. The recess has a depth exceeding the thickness of the pressure-responsive membrane switch. The foam pad is configured such that, upon it being compressed by application of pressure exceeding a certain threshold, it deforms so as to penetrate into the recess and activates the pressure-responsive membrane switch. 
     Preferably, the spacer elements are integrally formed with the support plate. The support plate and the spacer elements are preferably made from injection-molded plastic, although other materials may prove suitable. 
     The pressure-responsive membrane switch used in accordance with the first or second aspect of the invention advantageously 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 cell, 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. When a contact between the electrodes on the first and second carrier film is established, one speaks of “activation” of the membrane switch. The minimum amount of pressure at which the contact is established is called the “pressure threshold” or the “activation threshold”. 
     A further aspect of the invention relates to a vehicle seat, comprising a seat cushion supported by cushion-supporting springs and a pressure-responsive seat occupancy sensor unit according to the first aspect of the invention. The pressure-responsive seat occupancy sensor unit rests on the cushion-supporting springs and is applied by the cushion-supporting springs against the seat cushion. 
     Yet a further aspect of the invention relates to a vehicle seat, comprising a seat cushion supported by a cushion-supporting pan and a pressure-responsive seat occupancy sensor unit in accordance with the second aspect of the invention. The pressure-responsive seat occupancy sensor unit in this case rests on the cushion-supporting pan and is applied by the cushion-supporting pan against the seat cushion. 
     When the seat is loaded (by an occupant), the seat foam is compressed and transfers a part of the pressure to the foam pad of the seat occupancy sensor unit. In turn, the foam pad deforms and penetrates into the recess. If the pressure transferred to the pressure-responsive membrane switch exceeds the pressure threshold, the latter is activated and the occupancy of the seat is recognized. 
     As will be appreciated, the sensitivity of the seat occupancy sensor unit depends on and may thus be adjusted by at least the following parameters:
         Height and hardness of the foam pad;   Height of the gap between the foam pad and the pressure-responsive membrane switch;   Activation threshold of the membrane switch;   The size (diameter) of the recess in relation to the supported area of the foam pad.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
         FIG. 1  is a cross sectional view of a pressure-responsive seat occupancy sensor unit according to the first aspect of the invention; 
         FIG. 2  shows the pressure-responsive seat occupancy sensor unit of  FIG. 1  when the seat, which it is arranged in, is occupied; 
         FIG. 3  is a perspective view of the pressure-responsive seat occupancy sensor unit of  FIG. 1  without the foam pad; 
         FIG. 4  is a perspective view of the pressure-responsive seat occupancy sensor unit of  FIG. 1  wherein the foam pad is shown; 
         FIG. 5  is a cross sectional view of a pressure-responsive seat occupancy sensor unit according to the second aspect of the invention; 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIGS. 1 to 4  show a pressure-responsive seat occupancy sensor unit  10  for a vehicle seat according to a preferred embodiment of the first aspect of invention. The occupancy sensor unit  10  comprises a support plate  12  which is integrally formed with lateral bearing elements  14 , by which the support plate is attached to the cushion-supporting springs  16  of the seat. The bearing elements  14  define an upper surface  18  of the occupancy sensor unit, from which upper surface  18  the top surface  20  of the support plate  12  is arranged recessed. A foam pad  22  is disposed between the bearing elements  14  on the top surface  20  of the support plate  12 . A recess  24  formed in the top surface  20  of the support plate  12 , underneath the foam pad  22 , accommodates a pressure-responsive membrane switch  26 . The depth of the recess  24  exceeds the thickness (height) of the membrane switch  26 . 
     The upper surface  28  of the foam pad  22  is substantially in alignment with the upper surface  18  of the occupancy sensor unit  10 . Both surfaces are thus applied against the bottom side (B-surface) of the seat cushion  30 . When the seat is loaded (illustrated in  FIG. 2  by arrow  32 ), the seat cushion transfers part of the resulting pressure onto the occupancy sensing unit  10 , in particular onto the foam pad  22 . The mechanical properties of the foam pad  22  are chosen such that, when the foam pad  22  is compressed by application of pressure exceeding a certain threshold, it deforms so as to penetrate into the recess  24  in the top surface of the support plate  12  and activates the pressure-responsive membrane switch  26 . 
       FIG. 3  shows the occupancy sensing unit  10  from a different perspective. The foam pad  22  and the seat cushion are not shown in  FIG. 3  for sake of clarity. The lateral bearing elements  14  rest upon the cushion-suspension mat formed by the springs  16  and the cross-ties  34 . The support plate  12 , which is in recess from the upper surface  18  of the occupancy sensing unit  10  carries a sensor device with two active areas  36 , each of which represents a pressure-responsive membrane switch  26 . Both switches are electrically linked to a connection tail, which is protected by a hot-melt casing  38 . The connection tail serves to interface the sensor device with read-out electronics (such as e.g. a car&#39;s onboard computer).  FIG. 4  shows the same perspective as  FIG. 3 , with the foam pad  22  in place. 
       FIG. 5  illustrates a preferred embodiment of a seat occupancy sensor unit  10 ′ according to a second variant of the invention, integrated into the seating portion of a vehicle seat  40 . The vehicle seat comprises a seat pan  42 , which supports the seat cushion  30 ′. The occupancy sensor unit  10 ′ rests upon the seat pan  42 . 
     The pressure-responsive seat occupancy sensor unit  10 ′ comprises a support plate  12 ′ a plurality of spacer elements  14 ′ defining an upper surface  18 ′ of the pressure-responsive seat occupancy sensor unit from which upper surface  18 ′ the top surface of the support plate is arranged recessed from. A foam pad  22 ′ is disposed between the spacer elements  14 , carried by the support plate  12 ′. A recess  24 ′ formed in the top surface of the support plate  12 ′, underneath the foam pad  22 ′, accommodates a pressure-responsive membrane switch  26 ′. The recess  24 ′ has a depth exceeding the thickness of the membrane switch  26 ′. The foam pad  22 ′ is configured such that, upon it being compressed by application of pressure exceeding a certain threshold, it deforms so as to penetrate into the recess  24 ′ and activates the membrane switch  26 ′. 
     While specific embodiments have been described in detail, those skilled 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.