Abstract:
An impact sensing apparatus includes a laterally extending air channel within a block of foam or other crushable medium disposed between a bumper fascia and a rigid bumper frame element, and one or more airflow sensors for detecting airflow within the air channel. The air channel may be defined by a flexible hollow tube embedded within the crushable medium or by a molded-in air cavity in cases where the crushable medium is formed by molding. The measured airflow provides a measure of crush rate, and two or more airflow sensors may be distributed within the air channel to optimize impact detection response time and to identify the impact location.

Description:
TECHNICAL FIELD 
   The present invention relates to pedestrian impact detection for a vehicle, and more particularly to an airflow sensing apparatus incorporated into a vehicle bumper. 
   BACKGROUND OF THE INVENTION 
   A vehicle can be equipped with deployable safety devices designed to reduce injury to a pedestrian struck by the vehicle. For example, the vehicle may be equipped with one or more pedestrian air bags and/or a device for changing the inclination angle of the hood. Since the initial point of impact is nearly always the bumper, many deployment systems include one or more pressure or deformation responsive strips disposed in or on the front and/or rear bumpers. See, for example, the U.S. Pat. No. 6,784,792 to Mattes et al., which suggests the use of wire strain, piezoelectric film, magneto-electric, magneto-resistive or optical sensor elements on the bumper fascia. In another approach disclosed in the U.S. Pat. No. 6,607,212 to Reimer et al., light energy scattered within a block of translucent polymeric foam disposed between the bumper fascia and a rigid bumper frame element is detected to form a measure of crush experienced in a collision. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to an improved impact sensing apparatus including a laterally extending air channel within a block of foam or other crushable medium disposed between the bumper fascia and a rigid bumper frame element, and one or more airflow sensors for detecting air displacement within the air channel. The air channel may be defined by a flexible hollow tube embedded within the crushable medium or by a molded-in air cavity in cases where the crushable medium is formed by molding. The measured airflow provides a measure of crush rate, and two or more airflow sensors may be distributed within the air channel to optimize impact detection response time and to identify the impact location. Additionally, the size and placement of the air channel within the compressible medium can be configured to control the sensitivity of the sensing apparatus. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a vehicle bumper equipped with a pedestrian impact sensing apparatus according to this invention; 
       FIG. 2  is a diagram of an alternative configuration of the sensing apparatus of  FIG. 1 , and further illustrates deformation due to a pedestrian impact; 
       FIG. 3A  is a diagram of a heated element airflow sensor for the sensing apparatus of  FIGS. 1-2 ; 
       FIG. 3B  is a diagram of a venturi airflow sensor for the sensing apparatus of  FIGS. 1-2 ; 
       FIG. 3C  is a diagram of a Pitot tube airflow sensor for the sensing apparatus of  FIGS. 1-2 ; and 
       FIG. 4 , Graphs A and B, graphically depict test data for the sensing apparatus of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , the reference numeral  10  designates a vehicle that is equipped with one or more pedestrian safety devices (PSDs) and a sensing system for deploying the safety devices when a pedestrian impact is detected. The PSDs are designated by a single block  12 , and may include one or more pedestrian air bags and a mechanism for changing the inclination angle of the vehicle hood. The PSDs are selectively activated by a microprocessor-based electronic control unit (ECU)  14 , which issues a deployment command on line  16  when a pedestrian impact is detected. The ECU  14  detects pedestrian impacts based on airflow signals produced by one or more airflow sensors  18 ,  20 . The airflow sensors  18 ,  20  are disposed in an air channel  22  that extends laterally across the vehicle; that is, perpendicular to the direction of vehicle travel. The air channel  22  resides within a block  24  of foam or other crushable energy-absorbent material disposed between a bumper fascia  26  at the leading edge of vehicle  10  and a rigid bumper frame element  28  that extends substantially parallel to the bumper fascia  26 . The air channel  22  may be defined by an open-ended flexible tube  30  passing through the foam block  24  as depicted in  FIG. 2 , or simply by an air-filled void or cavity in the foam block  24  as depicted in  FIG. 1 . In either case, the airflow sensors  18 ,  20  measure airflow without substantially impeding airflow within the air channel  22 . 
     FIG. 2  depicts the fascia  26  and foam block  24  when the vehicle  10  impacts a soft-bodied object  32  such as a pedestrian leg form. The impact locally crushes the foam block  24  and proportionally crushes the air passage  22  as indicated by the reference numeral  22 ′. The localized crushing of air passage  22  produces transient airflows away from the crush zone as indicated by the arrows  34   a  and  34   b.  The airflows are respectively detected by the sensors  18  and  20 , and the airflow signals produced by sensors  18  and  20  provide a measure of the crush rate and impact location to ECU  14 . 
   The embodiment of  FIG. 2  depicts an additional pair of airflow sensors  36 ,  38  disposed approximately at the midpoint of the air passage  22 . The addition of sensors  36  and  38  improves the response time of the sensing apparatus by shortening the distance between an impact and a pair of airflow sensors. Furthermore, the difference between the airflow signals produced by sensors  36  and  38  provides an early indication of the airflow direction within air passage  22 ; since the sensors  36  and  38  are located in the mid-section of the air passage  22 , the airflow direction tells ECU  14  which side of the vehicle  10  was impacted by the object  32 . 
     FIGS. 3A-3C  depict three examples of suitable airflow sensors.  FIG. 3A  depicts a heated element sensor  40 ;  FIG. 3B  depicts a venturi sensor  50 ; and  FIG. 3C  depicts a Pitot tube sensor  60 . 
   Referring to  FIG. 3A , the heated element sensor  40  comprises four resistors  41 ,  42 ,  43 ,  44  configured in a conventional Wheatstone bridge arrangement and a differential amplifier  45  responsive to the potential difference between the bridge nodes  46  and  47 . The amplifier  45  adjusts the bridge voltage (Vout) as required to balance the bridge. The resistors  41 - 44  are selected so that when the bridge is balanced, the resistor  42  (which may be a wire, for example) is maintained at an elevated temperature such as 250° C. The resistor  42  is positioned within the air channel  22  so that transient airflow (as represented by the arrows  48 ) due to a pedestrian impact displaces the heated air surrounding the resistor  42  with air at essentially ambient temperature. This cools the resistor  42  and the amplifier  45  responds by increasing the bridge voltage. In this way, the amplifier output voltage Vout provides a measure of the magnitude of the airflow across resistor  42 . 
   Referring to  FIG. 3B , the venturi sensor  50  has a sensor body  51  and a differential pressure sensor  52 , such as a silicon diaphragm sensor. The sensor body  51  is located within the air channel  22  and is configured to define restricted and unrestricted airflow ports  53 ,  54  that are in-line with the transient air airflow (designated by arrows  48 ) produced by deformation of the air channel  22  during a pedestrian impact. The pressure sensor  52  is disposed in a passage  57  extending between the airflow ports  53 ,  54 , and the difference between the airflow in restricted airflow port  53  (designated by arrow  55 ) and the airflow in unrestricted airflow port  54  (designated by arrows  56 ) produces a corresponding pressure difference across the sensor  52 . The sensor  52  produces a signal corresponding to the pressure difference, which is also an indication of the magnitude of the impact-related transient airflow. 
   Referring to  FIG. 3C , the Pitot tube sensor  60  has a sensor body  61 , first and second pressure chambers  62 ,  63  and a differential pressure sensor  64  separating the pressure chambers  62  and  63 . The sensor body  61  is located within the air channel  22  and defines a central air passage  65  having an inlet  66  that is in-line with the transient air airflow (designated by arrows  48 ) produced by deformation of the air channel  22  during a pedestrian impact, and one or more static air passages  66 ,  67  having inlets  68 ,  69  that are perpendicular to the impact-related airflow. The central air passage  65  is coupled to the first pressure chamber  62 , while the static air passages  66 ,  67  are coupled to the second pressure chamber  63 . The sensor  64  is responsive to the difference in pressures between the first and second chambers  62 ,  63 , and such difference provides a measure of velocity of the impact-related transient airflow. 
     FIG. 4 , Graphs A-B, depict data collected in a mechanization of the present invention during a 13.7 MPH collision of a human leg form with the center of a stationary vehicle bumper constructed substantially as depicted in  FIGS. 1-2 . The leg form was instrumented with an accelerometer, and  FIG. 4A  depicts the measured deceleration during the impact. In the test apparatus, two Pitot tube airflow sensors were installed in the outboard ends of a flexible tube within a foam block substantially as depicted in  FIG. 2 , and Graph B depicts the air velocity measured by one of the airflow sensors. In general, the magnitude of the airflow signal provides a predictable and reliable measure of impact severity. The decision as to whether deployment of one or more PSDs is warranted for a given impact can be made by calibrating a fixed or time-variant threshold and deploying the restraint(s) if the measured airflow exceeds the threshold. The depicted data additionally demonstrates that the severity of an impact can be determined very quickly, enabling timely deployment of supplemental restraints for virtually any crash event. In particular, the test illustrates the worst-case time response for the sensing apparatus because the airflow sensors are equally displaced from the point of impact. Even so, the data shows that the impact detection time is only about 5 milliseconds for an air channel that is six feet in length. About one-half of the response time is required for the airflow pulse to reach either airflow sensor, with the remaining time required for signal processing in ECU  14 . 
   In summary, the present invention provides a novel sensing apparatus capable of detecting pedestrian impacts both quickly and reliably by responding to a transient airflow in an air channel within a crushable medium. Since the sensors are responsive to transient airflow, the air channel does not need to be closed or sealed. Additionally, the slope of the measured airflow signal can be used to infer crush rate, for purposes of discriminating between pedestrian impacts and other impacts. 
   While the present invention has been described with respect to the illustrated embodiments, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. For example, the air channel  22  may be equipped with more or fewer airflow sensors than shown, and the apparatus may be applied to a rear bumper or to a vehicle body panel such as a fender or side-door. Also, the size and placement of the air channel  22  within the foam block  24  can be configured to control the sensitivity of the sensing apparatus. Placing the air channel closer to the fascia  26  increases the detection sensitivity, while placing the air channel closer to the bumper frame element  28  reduces the detection sensitivity. Such placement of the air channel  22  can be used, for example, to provide maximum sensitivity in the central area of the bumper and reduced sensitivity near the ends of the bumper. Alternatively or additionally, the shape of the air channel  22  may be configured to provide increased or decreased sensitivity; for example the air channel  22  may be larger in diameter in the center than near the ends to heighten the detection sensitivity for impacts near the center of the bumper. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.