Abstract:
A load sensor includes a optical fiber sensor that provides output analog signals in response to shocks of various magnitudes caused by hits by a vehicle on various objects, two or more amplifiers each of which approximates a non linear function of the analog signals and a unit that converts the linear functions into digital data. Therefore, each analog signal either in a lower magnitude range or a higher magnitude range can be amplified by one of the amplifiers that has a more suitable gain, so that the magnitude of the shock can be accurately converted into digital data for a pedestrian protection airbag system.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority from Japanese Patent Application 2004-314239, filed Oct. 28, 2004, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a load sensor, a method of detecting a load and a pedestrian protection system of a vehicle that employs a load sensor and a method of detecting a load. 
     2. Description of the Related Art 
     A pedestrian protection system for a vehicle usually includes an optical fiber sensor and an object examining ECU (electronic control unit). The optical fiber sensor includes an LED (light emitting diode), a photo diode and an optical fiber. The optical fiber is disposed at a front bumper of a vehicle to conduct the light of the LED to the photo diode. 
     If the front bumper hits an object, the optical fiber is crushed. Consequently, the rate of the light conduction loss in the optical fiber drawn from the LED to the photo diode changes, and the output current of the photo diode changes. The output current is converted into a voltage signal, which is converted into digital data. The digital data are inputted into the object examining ECU, which examines if the hit object is a pedestrian or not. If it is a pedestrian, the object examining ECU operates a pedestrian protection system, such as a pedestrian protecting airbag system. 
     However, the rate of the light conduction loss of the optical fiber is not proportional to the magnitude of the load that crushes the optical fiber. In other words, the magnitude of the load is not a linear function of the rate of the light conduction loss. Accordingly, it is difficult to detect an accurate magnitude of load or to discriminate a pedestrian from others with a single amplifier that has only one gain for voltage signals inputted thereto. 
     U.S. Pat. No. 6,739,177B2 discloses a gas sensor in which an arrangement for accurately amplifying non-linear voltage using a map. However, it is necessary to have extra means and software for map matching, which raises the cost of the load sensor. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the invention is to provide an inexpensive and accurate load sensor. 
     According to a main aspect of the invention, a load sensor includes first means for providing one of output signals in response to a shock caused by a hit on an object, second means for providing a plurality of linear functions by approximating various output analog signals of said first means corresponding to shocks of various magnitudes and third means for providing digital data that correspond to the one of output signals from one of the linear functions. In the above load sensor, the second means is preferably a plurality of amplifiers each of which has a prescribed gain. The first means may include a light emitting element, a light sensing element and an optical fiber connecting the light emitting element and the light sensing element. The first means may also include fourth means for defomring the optical fiber in response to the shocks. 
     Another object of the invention is to provide a pedestrian protection system. 
     This system includes the above described load sensor, an object examining unit for providing an output signal when discriminating a pedestrian according to the digital data of the load sensor and an air bag control unit for providing an airbag operating signal when receiving the output signal from the object examining unit. 
     In the above pedestrian protection system, the second means may include a lower load amplifier and a higher load amplifier. 
     Another object of the invention is to provide a method of accurately detecting a load by an inexpensive load sensor. 
     The method of detecting a load caused by hitting an object includes a first step of providing output analog signals in response to shocks of various magnitudes caused by hits on various objects, a second step of approximating a non linear function of the analog signals of the magnitudes of shocks by a plurality of linear functions and a third step of providing digital data converted from the linear function. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
         FIG. 1  is a perspective view illustrating a vehicle front portion in which a pedestrian protection system according to a preferred embodiment of the invention is mounted; 
         FIG. 2  is a schematic cross-sectional side view of a front bumper of the vehicle shown in  FIG. 1  with a load sensor of the pedestrian protection system being mounted therein; 
         FIG. 3  is a block diagram of the pedestrian protection system; 
         FIG. 4  is a graph showing a relationship between magnitude of load and output voltage of the load sensor; 
         FIG. 5  is a signal processing diagram of the pedestrian protection system; 
         FIG. 6  is a graph showing a relationship between the magnitude of load and the output voltage signal of a lower-load amplifier; and 
         FIG. 7  is a graph showing a relationship between the magnitude of load and the output voltage signal of a higher-load amplifier. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A pedestrian protection system according to a preferred embodiment of the present invention with a load sensor and a method for detecting load using the load sensor will be described with reference to the appended drawings. 
     As shown in  FIG. 1 , the pedestrian protection system  1  includes a load sensor  2 , a touch sensor (not shown), an air bag ECU  3 . The air bag ECU  3  includes an object examining ECU. 
     The load sensor  2  includes a sensor ECU  20  and an optical fiber  21 . The load sensor  2  is embedded into a front bumper  90  of a vehicle  9 . The optical fiber  21  is made of a light conductive resin and put around the front portion of the front bumper  90  from one side thereof to the other side. 
     The airbag ECU  3  is accommodated in a place above a floor tunnel of the vehicle  9  below the transversely central portion of an instrument panel  91 . The airbag ECU  3  and the sensor ECU  20  are connected by a wire harness. 
     As shown in  FIG. 2 , the front bumper  90  includes a bumper absorber  900 , a load transmission plate  901 , a bumper reinforcement member  902  and a bumper cover (not shown). The bumper cover is exposed to the outside in the front of vehicle  9 . The bumper absorber  900  is a long plate made of foamed resin and disposed at the back of the bumper cover to extend in the transversal direction of the vehicle. 
     The load transmission plate  901  is a long plate made of metal and is disposed at the back of the bumper absorber  900  to extend in the transversal direction of the vehicle. A columnar touch sensor  4  is mounted in the space between the load transmission plate  901  and the bumper absorber  900  to extend in the transversal direction of the vehicle. The touch sensor  4  has a long tube that contains four spiral lead wires that are connected to each other like a line drawn with a single stroke. When two lead wires are brought in contact, the touch sensor  4  provides a turn-on signal. 
     The bumper reinforcement member  902  is also a member made of metal that extends in the transversal direction of the vehicle. The bumper reinforcement member  902  is supported by a front side member (not shown) at the back thereof. The optical fiber  21  is disposed between the reinforcement member  902  and the load transmission plate  901 . The sensor ECU  20  is disposed at the left corner of the bumper reinforcement member  92 , as shown in  FIG. 1 . 
     As shown in  FIG. 3 , the pedestrian protection system  1  includes the load sensor  2 , the touch sensor  4  and the airbag ECU  3 . The sensor ECU  20  includes an LED  200 , a photo diode  201 , a buffer  202 , a 5-volt-power source  203 , a control unit  204 , a processing unit  205 , an amplifying unit  206 , an adjusting/correcting unit  207  and a communication I/F (interface)  208 . 
     The 5-volt-power source  203  is converted from battery voltage to apply 5-volt constant voltage to the LED  200 , the photo diode  201  and the buffer  202 . The LED  200  is disposed at one end of the optical fiber  21 . The LED  200  supplies the optical fiber  21  with a light of a prescribed wave length (e.g. 650 nm). The photo diode  201  is disposed at the other end of the optical fiber  21 . The photo diode  201  receives the light of the LED  200  conducted by the optical fiber  21 . The buffer  202  is an operational amplifier. The buffer  202  receives the output voltage of the photo diode  201  and prevents interference of the circuits on the input side with the circuits on the output side. The processing unit  205  is a low-pass filter that has a comparatively large time constant. The amplifying unit  206  amplifies the output voltage of the processing unit  205 . The control unit  204  controls the current of the LED  200  so that the output voltage of the photo diode  201  can be kept constant. The adjusting/correcting unit  207  adjusts or corrects the variation of the output signal of the load sensor  2  that is caused in the manufacturing process. 
     The output voltage of the amplifying unit  206 , which is an analog signal, is converted into digital data by the communication I/F  208 . These digital data are sent to the airbag ECU  3  via a single communication line S 1 . The turn-on signal of the touch sensor  4  is also converted into digital data, which are sent to the airbag ECU  3 . 
     If the vehicle  9  hits an object at the bumper cover of the front bumper  90 , a shock caused by the hit is transmitted from the bumper cover via the bumper absorber  900  to the touch sensor  4 . Accordingly, the touch sensor  4  is crushed so that its round section becomes a flat elliptic section. Consequently, the lead wires of the touch sensor  4  are brought in contact with each other to provide a turn-on signal. 
     The shock is also transmitted from the bumper absorber  900  via the load transmission plate  901  to the optical fiber  21 . Incidentally, the rigidity of the load transmission plate  901  and the bumper reinforcement member  902  is higher than optical fiber  21 . Therefore, the optical fiber  21  is compressed between the load transmission plate  901  and the bumper reinforcement member  902 . Accordingly, the section of the optical fiber  21  is deformed from a round shape to a flat elliptic shape, so that the quantity of the light conducted from the LED  200  to the photo diode  201  changes. Consequently, the current passing through the photo diode  201  changes, and the output voltage of the photo diode  201  changes, as shown in  FIG. 4 . 
     That is, the output voltage of the photo diode  201  draws a curve with change in the magnitude of the input load (shock) that is applied to the bumper cover. This curve is approximated by two or more straight lines LL and LH as shown in  FIG. 4 . Incidentally, line LL represents the output voltage of the photo diode  201  at the lower magnitude range of the input load, and line LH represents the output voltage of the photo diode  201  at the higher magnitude range of the input load. 
     The amplifying unit  206  of the load sensor  2  includes a lower load amplifier  206 L and a higher load amplifier  206 H, as shown in  FIG. 5 . The lower load amplifier  206 L amplifies the input load signal at a higher gain than the higher load amplifier  206 H as shown in  FIGS. 6 and 7 . The amplifying unit  206  amplifies the input load in the lower magnitude range from zero to X by the lower load amplifier  206 L and the input load in the magnitude range higher than X by the higher load amplifier  206 H. The amplified voltage of each of the amplifiers  206 L,  206 H is separately converted into digital data by the communication I/F  208  and sent to the airbag ECU  3  via the communication line S 1 . 
     The airbag ECU  3  includes an object examining unit and an airbag driving unit, which are common in this technical field. The object examining unit examines an object by comparing the magnitude of the input load with a threshold voltage. 
     If the object is presumed to be a pedestrian as a result of the examination while the touch sensor  4  provides a turn-on signal, the driving unit drives an airbag system to inflate the pedestrian protection airbag, which is mounted in a front portion of the vehicle  9 . 
     Thus, the magnitude of load that is detected by the load sensor can be accurately converted into digital data without using additional hardware such as a logarithmic amplifier or soft ware. 
     In the above described pedestrian protection system, the amplifying unit  206  may be included into the airbag ECU  3  instead of the sensor ECU  20 , and the object examining ECU may be separated from the airbag ECU  3 . 
     In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention is to be regarded in an illustrative, rather than a restrictive, sense.