Abstract:
A sensor assembly for a motor vehicle adapted for sensing impacts including pedestrian impacts. The sensor assembly includes first and second energy absorbing elements formed of differing materials which couple an applied force to the vehicle to a compressive force acting on a compressive sensor element. The first and second energy absorbers are combined in a manner to tune the response between the applied force and forces acting on the compressive sensor to provide desired response characteristics. The first and second energy absorbers can be configured to produce force flow paths which further aid in response tuning. Another embodiment utilizes an energy absorber having a shaped cross section which focuses and balances impact force is applied to the compressive sensor.

Description:
FIELD OF THE INVENTION 
     This invention relates to a motor vehicle mounted sensor system and, in particular, to one adapted to be mounted to the front end of a motor vehicle for detecting impacts including pedestrian-involved impacts like pedestrian and bicyclists impacts, and activating appropriate impact mitigation countermeasures. 
     BACKGROUND OF THE INVENTION 
     Motor vehicle collisions with pedestrians and bicyclists are a significant concern. While significant advancements have been made in protecting motor vehicle occupants from injury due to impacts, there remain significant opportunities to reduce injuries, particularly head injuries to pedestrians struck by motor vehicles. Various countermeasure systems have been devised for this purpose and are in use. Hood lifter mechanisms pop the engine compartment hood to an upward displaced position where it can absorb energy as a struck pedestrian hinges about their lower torso and strikes the hood area during an impact. The lifted hood provides energy absorption. Other measures such as external airbags have further been conceived and implemented. In this description, reference to pedestrian impacts is intended to include other types of impacts including those with bicyclists or animals and other low-energy (as compared with striking other vehicles or fixed objects) impacts. 
     For any deployable pedestrian impact countermeasure to be operative, some means of detecting the impact is required. Numerous systems are available for detecting such impacts. One approach uses an elongated flexible hollow tube which defines an enclosed volume of gas, typically air. Upon an impact, the soft fascia of the vehicle front end is deformed and the sensor tube is compressed, generating a gas pressure pulse in the tube which is transmitted to a pressure sensor, thereby detecting the impact. For these systems to be operative, a supporting structure behind the pressure based sensor is necessary. This structure enables the necessary compression to occur for generating the pressure pulse. Numerous other sensor technologies may be implemented which measure strain or compression exerted by deformation of the vehicle front end fascia. For example, other types of low energy impact sensing systems include switch arrays, peizo cable, fiber optic, etc. All such sensing techniques based on compression or deformation will be referred herein as compressive or compression sensors. 
     A particular design challenge is posed in extending the sensitive area of the vehicle front end to low energy impacts to include the outer corners or edges of the front end (referred in this description also as the boundaries of the front end). Typical passenger car and light truck vehicles feature rounded front end corners which create a glancing or oblique impact if the pedestrian strikes the vehicle in these areas. The glancing impact may not provide sufficient compression for a compression sensor to be operative as well as acting as part of the vehicle&#39;s high energy impact system. Moreover, typical vehicle front ends feature an energy absorbing cross beam in the front end needed for meeting low speed impact requirements. The structure of the energy absorbing beam may not extend laterally to these outer front corners. Accordingly, it is often the case that an underlying structure necessary for creating a reaction force to the impact resulting in compression of the sensing system in these outer corner areas is absent. 
     With the increasing demand for implementation of active pedestrian protection systems and improved frontal sensing capabilities, the packaging and detection capabilities are becoming more complex. Sensors required to detect events such as pedestrian impacts are packaged close to the front of the vehicle, and require accommodations for vehicle styling as well as bumper sensing area coverage. As compressive sensing technologies are introduced into the front end system of the vehicle, integration concepts to support the sensing technology are evolving. Body components such as fascia, energy absorber, and bumper beams are becoming key components in the impact energy transfer function. 
     Vehicle front end components are designed to meet damageability and injury criteria requirements, but generally do not consider requirements for pedestrian impact sensor integration or applications as a primary design objective. To meet the damageability and injury criteria requirements, the component suppliers incorporate a design balance of component stiffness versus compressibility. This balance can result in non-linear load transfer characteristics that make the integration of a compressive sensor technology complicated. It is critical that a compressive sensor assembly, in its installed condition in a motor vehicle structure, be properly tuned to respond to impacts of prescribed characteristics. Although it is possible to design compressive sensors having inherent sensitivity characteristics, such a sensor may not be adaptable for use over multiple vehicle product lines. In addition, it is often necessary to adjust the sensitivity and response of a compressive sensor along its extended length due to changes in the types of impact occurring at various areas of the vehicle and the characteristics of underlying and supporting structure. 
     In view of the aforementioned, there is a need in the art for improved pedestrian impact system which addresses the previously mentioned shortcomings in prior art systems. In particular, the need exists to enable flexibility in adjusting the sensitivity or tuning of a compressive sensor which is highly adaptable, and provides repeatable characteristics. 
     In any volume produced automotive application, cost concerns are significant. The increased sophistication and capabilities of motor vehicles must be provided in an efficient and low cost manner in order that the features become commercially viable. Accordingly, systems provided to meet the design objectives mentioned above need to be manufacturable and capable of being assembled in a cost effective manner. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a compressive sensor system is provided incorporating features for adjusting its response. In one embodiment, adjustability is provided by using a composite sensor system using energy absorbers coupled with the compressive sensor having different compressibility characteristics which are combined in a series arrangement with respect to acting on the compressive sensor element which, as a system, provides desired response characteristics. In another embodiment, a balanced compressive material is used for reacting against impact forces in a parallel force flow arrangement. Hybrid arrangements for providing combined parallel and series force flow relationships involving the various energy absorbing components are also contemplated and described with multiple energy absorbing materials combined in a manner to provide adjustability and response flexibility. A still further embodiment provides load transfer management in a sensor system by removing material from an energy absorbing component to provide desired response. 
     Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front pictorial view of a motor vehicle incorporating a compressive sensor system in accordance with this invention; 
         FIG. 2  is overhead schematic drawing of the a vehicle front and incorporating a compressive sensor in accordance with this invention; 
         FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 1  through a sensor assembly in accordance with a first embodiment of the invention; 
         FIG. 4  is a cross-sectional view through a sensor assembly in accordance with a second embodiment of this invention; and 
         FIG. 5  is a cross-sectional view through a sensor assembly in accordance with a third embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , a representative motor vehicle  10  is shown with its front end  11  which includes front fascia  12 , hood  13 , and bumper  15 , which joins front fenders  17  at front boundary (corner) areas  19 . In the lower portion of front end  11 , and typically behind front fascia  12  or bumper  15  is provided sensor assembly  14  in accordance with the present invention. Sensor assembly  14  is optimally placed behind a motor vehicle component at a position such that it that receives the best or first contact with a pedestrian during an impact and also high in terms of integration of the components. In the illustrated embodiment, sensor assembly  14  is mounted behind front fascia  12 , but is shown in  FIG. 1  in broken lines to show its positioning in an exemplary implementation. 
       FIG. 2  is an overhead view of the principal components of vehicle front end  11 . As shown, cross body bumper beam  18  is shown with energy absorbing structure  23  with elongated compressive sensor assembly  14  sandwiched between the bumper beam and the energy absorbing structure. Front fascia  12  (not shown in  FIG. 2 ) would cover the face of energy absorbing structure  23 . Sensor assembly  14  may be of various types including a gas filled hollow tube coupled with a pressure sensing element, or may use various other technologies for measuring compression or deformation along its length, including but not limited to peizo arrays, switch arrays, peizo cable, fiber optic, or another type which provides a signal responsive to compression, stress, or strain and which extends laterally along a vehicle body region. Sensor assembly  14  may comprise one elongated element or may be provided in the form of a linear array of discrete compression sensitive elements extending laterally across the vehicle front end  11 . 
     Now with reference to  FIG. 3 , a first embodiment of the present invention is illustrated as a balanced sensor assembly  20 . As shown in the figure, assembly  20  is mounted in contact with or coupled to bumper beam  18 . Sensor element  22 , as mentioned previously, may be of various types. In one embodiment, sensor element  22  is a hollow gas filled tube. Energy absorber  24  is elongated and extends laterally across the vehicle front end  11  and may be formed of various materials such as open or closed cell foam which is highly compressible, semi-rigid, or rigid, and may have a variety of Durometer characteristics. Gel type materials may also be provided. As shown, energy absorber  24  forms a rectangular pocket  26  having balanced energy absorber  28  therein. While energy absorber  24  may be a generic component, balanced compressive energy absorber  28  may be provided in a range of different materials having compressive characteristics differing from that of energy absorber  24 . A similar choice of materials is available for balanced compressive material  28 . Compressive energy absorber  28  in addition to its static deformation characteristics, and may also have rate dependent characteristics. 
     In an impact condition in which deformation of energy absorber  24  occurs, compressive forces are transmitted to sensor element  22  through energy absorber  24  and balanced energy absorber  28  (which may be termed as first and second energy absorbers, respectively). In this way, energy absorbers  24  and  28  act in series in that compressive forces are transmitted through both elements before acting upon sensor element  22 . For a given vehicle application, energy absorber  24  may be of a standardized design configuration for broad application whereas a variety of materials may be selected for balanced energy absorber  28  provided for the tuning function mentioned previously. In addition, the composition and characteristics of balanced energy absorber  28  may be varied along the lateral extent of sensor assembly  14 . For example, a first material composition or characteristic may be provided for balanced energy absorber  28  in the center regions of the vehicle front end  11 , with a different material composition used at or near corner areas  19 . As mentioned previously, this may be due to the differing impact conditions encountered at these areas such as a glancing or oblique contact which occurs at the corner areas  19 , or due to the differing structural characteristics of front fascia  12  or other integrated elements which affect transfer of impact forces to compressive sensor  14 . 
     Energy absorber  24  may as mentioned previously be formed of an open cell type foam material and accordingly the foam acts primarily as a structural member subject to deformation of the material forming the open cells but does not primarily react in terms of compressing of trapped gases or air. In such an application, balanced energy absorber  28  could be formed of a closed cell foam material or could be formed of a foam material with an outer skin which is gas impermeable. In this way, compression of balanced energy absorber  28  is a function both of inherent material compression characteristics as well as gas pressure which develops due to its compression. Open cell type foam materials may also act partially to absorb energy by compressing gas while deforming partially enclosed cells of gas and as such can offer rate dependent compression characteristics. 
     In the embodiment shown in  FIG. 3  the impact force is transmitted from the front surface  30  of energy absorber  24 , which forces act upon sensor element  22 . A uniformly applied impact force F a  shown by the vector arrows in  FIG. 3  is transmitted to bumper beam  18  along several force flow paths. Force flow path F 1  acts directly on bumper beam  18  with a flow path strictly through energy absorber  24  which exist at the outer boundaries of absorber pocket  26 . Compressive forces F 2  act on the planar surface  30  interface between energy absorbers  24  and  28 . Forces F 2  are in turn resolved, in part, into forces F 3  which couple directly to bumper beam  18 . Compressive forces F 4  act on sensor element  22  to compress it, which in turn activates a sensor for the detection of the vehicle impact. In the design of the system, the manner of the application of applied force F a  as it is resolved into compressive forces F 4  acting on sensor element  22  can be adjusted through the selection of the materials forming energy absorbers  24  and  28 . As is evident in  FIG. 3 , the design of balanced sensor assembly  20  provides numerous opportunities for precise tuning of impact sensing response. In addition to the selection of materials of energy absorber  24  and balanced energy absorber  28 , the size and shape of absorber pocket  26  can be adjusted as well as the extent to which energy absorber  24  overlaps pocket  26  at its edges for the direct coupling of forces F a  to result in force F 1  acting on bumper beam  18 . In other words, the contact width W 1  of the legs  27  of energy absorber  24  which bear directly on bumper beam  18  can be adjusted to control the proportion of the applied force F A  which, to a degree, bypasses acting on balanced energy absorber  28  and consequently sensor element  22 . 
     Now with reference to  FIG. 4 , a second embodiment of balanced sensor system  34  is illustrated. In this case, first energy absorber  36  is formed of a rigid thin-walled structure having front surface  38 , side walls  40  and  42 , with pads  44  and  46 , respectively. Below each of the pads  44  and  46  is mounted a compressive sensor element  22 , with the sensor elements supported at their sides by balanced energy absorber  48  in the form of columns or walls. The arrangement of  FIG. 4  is a cross-section taken in the same direction as that of  FIG. 3  and as such, one of sensor elements  22  shown in  FIG. 4  is positioned above the other (with respect to the ground and its installed position in the vehicle). Balanced energy absorber  48  is formed into four strips  50 ,  52 ,  54 , and  56 . 
     With continued reference to  FIG. 4 , an applied force F a  acting on the front surface  38  of first energy absorber  36  is transmitted through side walls  40  and  42  to pads  44  and  46 . First energy absorber  36  would preferably be designed to be compressible to reduce the duration and peak forces applied to pads  44  and  46  and distribute localized applied forces over a larger area. Force F 5  acts on the assembly of the sensor elements  22  and their associated strips  50 ,  52 ,  54 , and  56 . In this application, forces F 6  acting through strips  50 - 56  do not act directly on sensor element  22  but instead reduce the force F 7  acting on the sensor element  22 . In this way the material and dimensional characteristics of material strips  50 - 56  forming the balanced energy absorber  48  including their heights and widths can be selected to provide desired compressive sensor response characteristics. 
     In an alternate variation of sensor system  34 , the hollow cavity  43  of first energy absorber  36  may be filled with another material such as a foam-like material, or a honeycomb type structure as a means of controlling its impact response and as part of a design in tuning the response of the system. 
     Now with reference to  FIG. 5 , a third embodiment of a balance compressive sensor  60  in accordance with this invention is illustrated. In this case, the tuning characteristics of force transfer from an applied impact force to forces acting on sensor element  22  are managed by the removal of material from energy absorber  62  (or by shaping it in a desired manner during its production). Energy absorber  62  is similar in external configuration to energy absorber  24  shown in  FIG. 3 . However, energy absorber  62  adjusts force transfer to sensor element  22  through the absence of material in its cross-section. For example, as illustrated in  FIG. 5 , energy absorber  62  forms a pair of generally trapezium shaped cutouts areas  64  and  66 , leaving a trapezoid shaped central leg or rib  68 . Areas  64  and  66  may be left as voids or they may be filled with a second energy absorber material for force balancing. For balanced compressive sensor  60 , the applied force F A  is resolved into force F 1  which, as in the first embodiment, is coupled directly into bumper beam  18 , and forces F 8  which are focused toward and interact with sensor element  22 . Due to the width W 2  of the central rib  68 , some components of the force F 8  act directly against bumper beam  18 , designated by force vectors F 9 . Accordingly, one approach toward tuning the response of energy absorber  62  is to adjust the widths W 3  of the outer legs of the energy absorber, and width W 2  of the central leg. These variations affect the force transfer acting on sensor element  22 . As is evident from  FIG. 5 , the response of the system may be tuned by the configuration and material forming energy absorber  62  as well as the configuration shape and size of cutout areas  64  and  66 . The configuration of energy absorber  62  provides a force concentration feature, evident from the converging directions of force vectors F 8  shown in  FIG. 5 , which may enhance the sensitivity of sensor element  22  to low-energy frontal impact. Since the impact characteristics and response may vary for a sensor extending across the vehicle front end  11 , it may be desirable to change the configuration of cutout areas  64  and  66  as a function of lateral position along the vehicle front end. 
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation, and change without departing from the proper scope and fair meaning of the accompanying claims.