Patent Publication Number: US-2007102220-A1

Title: Vehicle collision detecting device

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      The present application is based on and claims priority from Japanese Patent Applications: 2005-325095, filed Nov. 9, 2005 and 2006-235903, filed Aug. 31, 2006, the contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a collision detecting device for detecting a collision with an object or a pedestrian at a portion of a vehicle that deforms when the collision takes place.  
      2. Description of the Related Art  
      In order to protect a pedestrian, it has been proposed. that a collision detecting device is mounted on a bumper of a vehicle to actuate a pedestrian protecting device such as an air bag. JP-A-2000-177514 or U.S. Pat. No. 6,561,301 B1, a counterpart U.S. patent, proposes a capacitor type collision detecting device that has a capacitor fixed to a surface portion of the bumper. The capacitor has a pair of electrodes and an elastic dielectric member disposed between the electrodes. When a collision takes place at the surface portion, the elastic dielectric member is crushed, so that the capacitance of the capacitor changes. The change in capacitance is detected by a capacitance detecting circuit, which generates an output signal of a specific frequency as a signal of the collision.  
      If the connection between the capacitance detecting circuit and the collision detecting device breaks down in the disclosed collision detecting device, the frequency of the output signal fluctuates, so that it is difficult to know which of a collision and a wire breakdown takes place,  
     SUMMARY OF THE INVENTION  
      Therefore, an object of the invention is to provide an improved capacitor type collision detecting device.  
      Another object of the invention is to provide a capacitor type collision detecting device of a simple structure that can detect both the collision and the breakdown without an erroneous detection.  
      According to a feature of the invention, a vehicle collision detecting device includes a capacitor type sensor mounted on a portion of the vehicle, a driving circuit including an oscillator generating an ac signal of a fixed frequency and a resonant circuit constituted of a coil and a diagnostic capacitor and a microcomputer for judging a collision with an object and a breakdown of a lead wire based on the voltage appears on the lead wire. The resonant circuit is connected with the capacitor type sensor so as to compose a portion of the capacitance of the resonant circuit and with the oscillator so as to drive the resonant circuit at the fixed frequency.  
      In the above collision detecting device, the following features can be added: the diagnostic capacitor is connected either in parallel with or in series with the sensor; the judging means judges a collision if the voltage on the lead wire at the fixed frequency is lower than a first threshold voltage; the judging means judges a wire breakdown if the voltage on the lead wire at the fixed frequency is higher than a second threshold voltage; and the judging means includes a peak voltage holding circuit that holds a peak value of the voltage of the lead wire so as to either judge a collision if the peak value of voltage of the lead wire at the fixed frequency is lower than a first threshold voltage or to judge a wire breakdown if the peak value of voltage of the lead wire at the fixed frequency is higher than a second threshold voltage.  
      According to another feature of the invention, a vehicle collision detecting device includes a capacitor type sensor mounted on a portion of the vehicle, a resonant circuit connected to one of the plate electrodes of the sensor by a lead wire and constituted of an oscillator generating an ac signal of a fixed frequency, a coil and a diagnostic capacitor and means for judging a collision with an object and a breakdown of the lead wire according to a change in resonant frequency of that resonant circuit.  
      This collision detecting device may further include an oscillator oscillating the resonant circuit at a fixed frequency, and the judging means judges a collision and a wire breakdown by comparison of the voltage of the lead wire. The judging means may judge a collision with an object if the voltage of the lead wire is lower than a first reference voltage and judges a wire breakdown if the voltage of the lead wire is higher than a second reference voltage. 
    
    
     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 schematic diagram illustrating a collision detecting device according to the first embodiment of the invention;  
       FIG. 2  is a schematic diagram illustrating a collision sensor of the collision detecting device shown in  FIG. 1  mounted in a bumper of a vehicle;  
       FIG. 3  is a circuit diagram of the collision detecting device according to the first embodiment of the invention;  
       FIG. 4  is a graph showing a change in the impedance of a resonant circuit of the collision detecting device according to the invention relative to the frequency of an oscillator when a collision takes place;  
       FIG. 5  is a graph showing a change in the impedance of a resonant circuit of the collision detecting device according to the invention relative to the frequency of an oscillator when a wire breakdown takes place;  
       FIG. 6  is a flow diagram of a process for detecting a collision and a wire breakdown;  
       FIG. 7A  is a graph showing an output signal of the resonant circuit when a collision takes place;  
       FIG. 7B  is a graph showing an output signal. of the resonant circuit when a wire breakdown takes place; and  
       FIG. 8  is a circuit diagram of the collision detecting device according to the second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Some preferred embodiments according to the present invention will be described with reference to the appended drawings.  
      A vehicle collision detecting device according to the first embodiment of the invention will be described with reference to  FIGS. 1-7A  and  7 B.  
      The vehicle collision detecting device includes a pair of sensors  11  and a collision detecting ECU  15 , which is connected by a wire  21  to an airbag ECU  20 . The sensors  11  and the collision detecting ECU  15  are connected by lead wires  14 . The sensors  11  may be directly connected to the airbag ECU  20  if the collision detecting ECU  15  is included in the airbag ECU  20 .  
      The sensors  11  are mounted in the front bumper  1  of a vehicle. Of course, the sensor  11  may be mounted in a rear bumper or side bumpers. The bumper  1  includes a bumper cover  2 , a bumper reinforcement  3 , an absorber  4 , a pair of side members  5 , etc. The bumper cover  2  is made of a resin such as polypropylene and extends laterally from one side of the vehicle to the other to cover the bumper reinforcement  3  and the absorber  4 . The bumper reinforcement  3  is a metal beam that extends along the bumper cover  2 . The side members  5  are made of a metal and respectively extend longitudinally at opposite sides of the vehicle. The reinforcement  3  is fixed to the front end of the side members  5  by bolts  3   a  via the sensors  11  and spacers  3   b , which are made of an insulating material. The absorber  4  is disposed between the bumper cover  2  and the reinforcement  3  to absorb a shock applied to the front bumper  1 .  
      Each of the sensors  11  is a capacitance type collision sensor that is constituted of a pair of plate electrodes  12   a ,  12   b  and a dielectric member  13  disposed between the plate electrodes  12   a ,  12   b . The plate electrodes  12   a ,  12   b  are insulated from each other by the spacers  3   b . The plate electrode  12   a  on the side of the bumper reinforcement  3  is connected to the body or ground of the vehicle, and the other electrode  12   b  is connected by the lead wires  14  to a driving circuit  16  of the collision detecting ECU  15 .  
      As sown in  FIG. 3 , the collision detecting ECU  15  is constituted of the driving circuit  16 , a peak hold circuit  17  and a microcomputer  18 . The driving circuit  16  is constituted of an oscillator  16   a , a coil  16   b  and a diagnostic capacitor  16   c . The oscillator  16   a  generates a sinusoidal ac voltage signal of a fixed frequency f. The diagnostic capacitor  16   c  and the sensor  11  form a parallel circuit having one end being grounded. The coil  16   b  is connected in series with the oscillator  16   a  and the parallel circuit of the sensor  11  and the diagnostic capacitor  16   c  to form an L-C resonant circuit. Therefore, the normal resonant frequency f 0  of the L-C resonant circuit under the normal condition is expressed as follows: f 0  =1/(2π√{square root over (L(Ca+Cb)}), wherein L is the inductance of the coil  16   b , Ca is the capacitance of the sensor  11  and Cb is the capacitance of the diagnostic capacitor  16   c . The capacitance Cb of the diagnostic capacitor  16   c  is set to be much smaller than the capacitance Ca of the sensor  11 .  
      The peak hold circuit  17  is connected with a joint  16   d  of the lead wire  14  that is connected with the driving circuit  16  to hold a peak value of the output voltage of the driving circuit  16  and to send it to the microcomputer  18 . The microcomputer  18 , which includes an A-D converter, converts the peak voltage of analog value to a digital value and judges whether a collision takes place or not or whether a break down of a lead wire takes place.  
      If a collision takes place, the sensor  11  is crushed. Accordingly, the distance between the pair of plate electrodes  12   a ,  12   b  reduces, thereby increasing the capacitance Ca. As a result, the resonant frequency f 1  becomes lower than the normal resonant frequency f 0 , and the impedance of the resonant circuit at the fixed frequency f changes from Z 0  to Z 1  as shown in  FIG. 4 . Accordingly, the peak voltage at the joint  16   d  changes from V 0  to V  1 , which is much lower than V 0 . The microcomputer  18  compares the peak voltage V 1  with a threshold voltage V th1 , and judges that a collision takes place if the peak voltage V 1  is lower than the threshold voltage V th1 . Subsequently, the microcomputer  18  sends a collision signal to the airbag ECU  20  via the lead wire  21  so that the airbag ECU  20  can operate a pedestrian protecting air bag based on the collision signal and a vehicle speed, which is detected by a vehicle speed sensor (not shown).  
      If the lead wire  14  breaks down between the sensor  11  and the driving circuit  16 , the capacitance Ca of the L-C resonant circuit drops out. Accordingly, the resonant frequency f 2  becomes higher than the normal resonant frequency f 0 , and the impedance of the resonant circuit at the fixed frequency f changes from Z 0  to Z 2  as shown in  FIG. 5 . Accordingly, the peak voltage at the joint  16   d  changes from V 0  to V 2 , which is much higher than V 0 . The microcomputer  18  compares the peak voltage V 2  with a threshold voltage V th2 , and judges that a wire breakdown takes place if the peak voltage V 2  is higher than the threshold voltage V th2 . As a result, the microcomputer  18  sends a breakdown signal to the airbag ECU  20  via the lead wire  21  so that the airbag ECU  20  can display an alarm of wire breakdown.  
      The judgment process of the microcomputer  18  is described in more detail with reference to a flow diagram shown in  FIG. 6 .  
      At first step S 1 , the peak hold circuit  17  takes in a digital peak voltage V x  of the joint  16   d . Then, whether or not the peak voltage V x  is equal to or lower than the first threshold voltage V th1 , is examined at S 2 , as shown in  FIG. 7A . If the result of the examination is Yes, a collision signal is sent to the airbag ECU  20  at S 3 , and the judgment process returns to start again. Incidentally, the first threshold voltage V th1  is set to be lower than the normal peak voltage V 0 .  
      On the other hand, whether or not the peak voltage V x  is equal to or higher than the second threshold voltage V th2  is examined at S 4 , as shown in  FIG. 7B , if the result of the examination at S 2  is No. If the result of the examination at S 4  is Yes, a breakdown signal is sent to the airbag ECU  20  at S 5 , and the judgment process returns to start again. On the other hand, the judgment process directly returns to start again, if the result of the examination at S 4  is No. Incidentally, the second threshold voltage V th2  is set to be higher than the normal peak voltage V 0 .  
      A vehicle collision detecting device according to the second embodiment of the invention will be described with reference to  FIG. 8 . Incidentally, the same reference numeral shown in  FIG. 8  represents the same or substantially the same portion, part or component as the first embodiment.  
      The driving circuit  16  of the collision detecting device according to the second embodiment includes a coil  16   b  connected in parallel with the sensor  11  and a diagnostic capacitor  16   c  connected in series with the sensor  11 , the oscillator  16   a  and the coil  16   b.    
      If a collision takes place, the sensor  11  is crushed to increase the capacitance Ca. Accordingly, the resonant frequency becomes lower than the normal resonant frequency, and the impedance of the resonant circuit lowers. Accordingly, the peak voltage at the joint  16   d  lowers in the same manner as the first embodiment. If the lead wire  14  breaks down between the sensor  11  and the driving circuit  16 , the capacitance Ca of the L-C resonant circuit drops out. Accordingly, the resonant frequency becomes higher than the normal resonant frequency, and the impedance of the resonant circuit increases. Accordingly, the peak voltage at the joint  16   d  increases in the same manner as the first embodiment.  
      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.