Abstract:
A method of calibrating sensitivities of a plurality of pressure sensing cells are disposed at a passenger seat to detect presence of a vehicle passenger. The method is comprised of the following steps: pressing a presser against the passenger seat; detecting output signal levels of the sensing cells; and  
     adjusting sensitivity of the pressure sensing cells according to the output signal levels while the presser is pressed against the seat.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    The present application is based on and claims priority from the following Japanese Patent Applications: 2000-203502, filed Jul. 5, 2000; 2000-203550, filed Jul. 5, 2000; 2000-255288, filed Aug. 25, 2000; 2000-255498, filed Aug. 25, 2000; and 2000-256267, filed Aug. 25, 2000; the contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a vehicle passenger sensing system that includes a plurality of pressure sensing cells disposed in a vehicle passenger seat to sense presence of a passenger.  
           [0004]    2. Description of the Related Art  
           [0005]    In a vehicle that is equipped with a passenger&#39;s air bag, it is necessary to sense presence of a passenger seated on a passenger seat in order to prevent a passenger&#39;s air bag from inflating when no passenger is seated on the seat.  
           [0006]    Even if a passenger system senses the presence of a passenger and the passenger&#39;s air bag operates properly at a certain accident, such an air bag may harm the passenger if the passenger is a child or a baby.  
           [0007]    Therefore, it is necessary to sense whether the passenger is a child or not. Although, weight sensing is generally useful, mere weight sensing cannot tell whether the passenger is a child or not if a seat is reclined to a laid down position. If a child seated on a child seat is fastened to the passenger seat, the sensed weight is heavier than the child&#39;s weight.  
         SUMMARY OF THE INVENTION  
         [0008]    Therefore, a main object of the invention is to provide an improved passenger sensing system that can correctly tell whether a passenger is a child or not.  
           [0009]    Another object of the invention is to provide a passenger sensing system that can correctly sense the weight of the passenger from a plurality of weight sensing cells regardless of deviation in the sensitivities of the sensing cells.  
           [0010]    A method of calibrating sensitivities of a plurality of pressure sensing cells disposed at a passenger seat to detect presence of a vehicle passenger according to a main aspect of the invention is comprised of the following steps: pressing a presser against the passenger seat; detecting output signal levels of the sensing cells; and adjusting sensitivity of the pressure sensing cells according to the output signal levels while the presser is pressed against the seat.  
           [0011]    The step of pressing may be comprised of a step of applying equal pressures on all the sensing cells and a step of removing the pressures. The presser may be a flexible bag filled with liquid. The flexible bag may have a surface area sufficient to apply the pressure on all the sensing cells concurrently. The flexible bag may be comprised of a plurality of portions for separately applying pressures on a middle of the seat and sides of the seat.  
           [0012]    The method according to the main aspect of the invention may be comprised of a step of estimating final output signal levels before the step of adjusting. The step of estimating may be comprised of a step of estimating a change in output signal level of the sensing cells when the passenger seat is pressed by the presser. The step of estimating may be comprised of a step of estimating a final output signal level according to a response relationship between pressure applied to the sensors and output signal levels of the sensor. The final output level may be estimated according to a decreasing rate of output signal level of the sensing cell.  
           [0013]    In the method according to the main aspect of the invention, the step of adjusting may be comprised of separate steps of adjusting respective sensing cells.  
           [0014]    In the method according to the main aspect of the invention, the steps of pressing, detecting and adjusting may be carried out for those of the sensing cells disposed at predetermined sections of the passenger seat.  
           [0015]    In the method according to the main aspect of the invention, the step of adjusting may be comprised of a step of dividing the passenger seat to a plurality of sections, and a step of applying different weight of importance to each different section to provide a value to correct sensitivity of the each section.  
           [0016]    In the method according to the main aspect of the invention, the step of adjusting may be comprised of a step of dividing the passenger seat to a plurality of sections, and a step of correcting sensitivity of the sensing cells of each section.  
           [0017]    In the method according to the main aspect of the invention, the presser may be comprised of a bag having a contact surface fitted to the surface of the passenger seat. The presser may be formed of a plurality of bags having a contact surface fitted to the surface of each section of the passenger seat.  
           [0018]    In the method according to the main aspect of the invention, the step of detecting may be comprised of a step of detecting a shift of a distribution pattern of output signal levels of the sensing cells, and the step of adjusting may be comprised of a step of correcting the shift according to the output signal levels.  
           [0019]    In the method according to the main aspect of the invention, the presser may be comprised of means for profiling a surface of passenger&#39;s buttocks. The means may be comprised of a plurality of pistons having edges, each of which is controlled to follow a portion of the surface of passenger&#39;s buttocks. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    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:  
         [0021]    [0021]FIG. 1 is a schematic diagram of a method of calibrating sensitivities of a plurality of weight sensing cells according to a first embodiment of the invention;  
         [0022]    [0022]FIGS. 2A, 2B and  2 C are schematic diagrams illustrating alternative steps of the method of calibrating sensitivities of a plurality of weight sensing cells according to the first embodiment;  
         [0023]    [0023]FIGS. 3A and 3B are perspective and cross-sectional views of a passenger seat;  
         [0024]    [0024]FIG. 4 is an exploded perspective view of the passenger seat;  
         [0025]    [0025]FIGS. 5A and 5B are plan and cross-sectional views of a seat cushion that includes a sensor mat;  
         [0026]    [0026]FIG. 6 is a schematic diagram illustrating a step of fixing a seat skin;  
         [0027]    [0027]FIG. 7 is a schematic diagram illustrating a step of fixing a seat skin;  
         [0028]    [0028]FIGS. 8A and 8B are graphs showing a method of calibrating signal levels of sensing cells according to a second embodiment of the invention;  
         [0029]    [0029]FIG. 9 is a graph showing an idea of calibration of a sensing cell according to the second embodiment;  
         [0030]    [0030]FIGS. 10A, 10B and  10 C are schematic diagrams of a seat for illustrating a method of calibrating the sensitivities according to a third embodiment of the invention;  
         [0031]    [0031]FIGS. 11A and 11C are schematic diagrams of a method of calibrating signal levels of sensing cells according to a fourth embodiment of the invention, and FIGS. 11B and 11D are graph showing signal levels of the sensing cells;  
         [0032]    [0032]FIG. 12 is a schematic diagram of a method of calibrating signal levels of sensing cells according to a fifth embodiment of the invention;  
         [0033]    [0033]FIG. 13 is a schematic diagram of a step of the method of calibrating the signal levels of sensing cells according to the fifth embodiment;  
         [0034]    FIGS.  14  is a schematic diagram of a step of the method of calibrating the signal levels of sensing cells according to the fifth embodiment;  
         [0035]    [0035]FIG. 15 is a schematic diagram of a step of the method of calibrating the signal levels of sensing cells according to the fifth embodiment;  
         [0036]    [0036]FIG. 16 is a schematic diagram of a step of the method of calibrating the signal levels of sensing cells according to the fifth embodiment; and  
         [0037]    [0037]FIG. 17 is a schematic diagram of a step of the method of calibrating the signal levels of sensing cells according to the fifth embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    A vehicle passenger sensing system is described with reference to FIGS.  1 - 9 .  
         [0039]    As shown in FIGS. 4 and 5, a passenger seat  1  is comprised of a pair of rails  2 , an anchor member  3  anchored by the pair of rails  2 , a seat cushion  4  made of polyurethane foam, a seat skin  5  that covers the seat cushion and a seat mat  6  disposed between the seat cushion  4  and the seat skin  5 .  
         [0040]    Trenches  7  are formed on the upper side of the cushion  4  to wring or hold portions of the seat skin  5  therein to stretch the seat skin  5  over the upper surface of the seat cushion  4 . For this purpose, a pair of pocket-like folds  8  is formed at the portions of the seat skin  5 .  
         [0041]    The sensor mat  6  has a plurality of sensing cells  9 , each of which converts a pressure to an amount of electric resistance. The sensor mat  6  is comprised of a first sensor section  6   a  disposed at the central portion of the seat cushion  4  between a pair of the parallel trenches  7  and a pair of second sensor sections  6   b  disposed outside the parallel trenches  7 . A plurality of sensing cells  9  is uniformly distributed at the first and second sensor sections  6   a  and  6   b  in both longitudinal and lateral directions. The sensor mat  6  is connected to an ECU  11  via a printed circuit board  10  to give it electric signals relative to the pressure.  
         [0042]    The ECU  11  is an electronic control unit that includes a microcomputer, which is disposed at the center of the rear portion of the cushion  4 , as shown in FIG. 5A. The ECU  11  calculates a total weight applied to the cushion and a pattern of the weight distribution according to the signals from the respective sensing cells and detects whether a passenger is present or not and whether the passenger is a child or not in the following manner:  
         [0043]    a) it is judged that a passenger is present if the total weight is larger than a predetermined value, and otherwise, it is judged that no one is present.  
         [0044]    b) it is judged that a child seat is used if the weight distribution pattern comes under a predetermined pattern.  
         [0045]    c) whether the passenger is a child or not is judged according to the total weight and the pattern of the weight distribution pattern.  
         [0046]    A passenger seat is manufactured in the following manner.  
         [0047]    (1) The cushion  4  is mounted on the anchor member  3 .  
         [0048]    (2) Wires  13  shown in FIG. 6 are fixed to the bottom of the trenches  7 , and the sensor mat  6  is put on the upper surface of the cushion  4  and temporarily fixed thereto.  
         [0049]    (3) Then, wires  12  are inserted into the pocket-like folds  8  and thrusted into the pair of trenches  7 .  
         [0050]    The seat skin  5  is fixed to the seat cushion in the following manner.  
         [0051]    (4) The seat skin  5  is stretched over the sensor mat  6  and the seat cushion  4  so that the middle portion of the seat skin  5  can be positioned at the middle of the cushion  4 . Then, the wires  12  and  13  in each trench  7  are clamped by C-shaped clamp  14 .  
         [0052]    (5) The sides of the seat skin 5  are respectively extended to cover the sides of the cushion  4  so that hooks  5   a  formed at the edges of the seat skins  5  can engage projections  3   a  formed at edges of the anchor member  3 . In order to prevent wrinkles of the seat skin  5 , the cushion  4  is pressed from both upper and lower sides, as shown in FIG. 7 before the hooks  5   a  are engaged with the projections  3   a.    
         [0053]    (6) Finally, the sensitivity of the sensing cells  9  of the sensor mat  6  is calibrated in the following manner.  
         [0054]    Step  1 : A fluid-contained flexible bag  15  is put on a portion of the upper surface of the seat  1  and an amount of pressure is applied to the seat via the bag  15 , as shown in FIG. 1. The flexible bag  15  is made of rubber or resinous material and contains a kind of fluid such as gas, liquid, or gel. The pressure can be applied to a single large area, as shown in FIG. 2A or divided areas, as shown in FIG. 2B. If the pressure is applied to a plurality of areas, a plurality of flexible bags may be used, as shown in FIG. 2C.  
         [0055]    Step  2 : The output signal of each sensing cell  9  is read by a controller of the ECU  11  and is calibrated in the following manner.  
         [0056]    Even if a pressure Pa is applied to one of the sensing cells  9  aiming that the sensing cell  9  provides an output signal O 1 , the sensor actually provides the same output signal O 1  only when pressure Pi is applied:  
         [0057]    a) C 1 =Pi/Pa, by which the actual pressure is multiplied, or  
         [0058]    b) C 2 =Pi−Pa, which is added to the actual pressure.  
         [0059]    Even if pressure Pi is applied to a sensor aiming that the sensor provides an output signal Oa, the sensor actually provides an output signal O 1 :  
         [0060]    c) C 3 =Oa/O 1 , by which the actual output signal is multiplied, or  
         [0061]    d) C 4 =Oa−O 1 , which is added to the actual output signal.  
         [0062]    Step  3 : The output signal of each sensor is estimated in the following way before it is finally detected in order to shorten the calibration time.  
         [0063]    The final output signal is estimated from a change in the output signal of each sensing cell  9  after the calibrated pressure is applied. For example, a response characteristic of the sensing cell  9  to the actual calibration, as shown in FIG. 8A, is recorded, and is compared with a change in the output signals between two or more points.  
         [0064]    For example, if one of the sensing cells  9  is applied a step input signal that corresponds to a pressure to be applied, the output signal thereof becomes as high as 63% at time Δt after the input signal is applied. In the meantime, the time constant Δt of the sensing cell  9  is detected beforehand. Therefore, the final output signal can be estimated from a value at point c in FIG. 8A if the value is multiplied by 1/0.63. Points b or d can be added to increase the accuracy of the estimation.  
         [0065]    Variation: Before the final output signal is estimated, an initial pressure that is larger than a calibration pressure is applied to the sensing cells  9 . The final output signal is estimated after the initial pressure is reduced to the calibration pressure, as shown in FIG. 8B. For example, a broken line A shows an output signal when the initial pressure that is larger than the calibration pressure is applied to the sensing cell  9 . A broken line B shows the same output signal as that shown in FIG. 8A when the calibration pressure is applied to the sensing cell  9 . If the initial pressure is 1/0.63 times as much as the calibration pressure, the output signal at point b that appears at time Δt after the input signal is applied can be substituted for the final output signal at point d. The output signal at point c appears after the initial pressure is reduced to the calibration pressure and can be also substituted for the final output signal.  
         [0066]    A method of calibrating the sensitivities of the sensing cells according to a second embodiment of the invention is described with reference to FIGS.  10 A- 10 C.  
         [0067]    A plurality of sensing cells are grouped into two or more sections G 1 -G 4  as shown in FIGS.  10 A- 10 C, and the sensor sells  9  of a section that is disposed at more center of the seat are given more weight to be calibrated. For example, a mean value the output signals of the sensors in each section is used for the calibration.  
         [0068]    A method of calibrating the sensitivities of the sensing cells according to a third embodiment of the invention is described with reference to FIGS. 11A, 11B,  11 C and  11 D.  
         [0069]    At first, a pair of sensing cells  9  disposed to be approximately symmetrical with each other is selected, and a presser  15  is pressed against the portion of the seat skin  5  between the pair of sensing cells, as shown in FIG. 11A.  
         [0070]    Then, the output signal levels of the pair of sensing cells  9  and sensing cells adjacent to the pair are detected, as shown in FIG. 11B.  
         [0071]    Subsequently, the detected output signals are compared with a normal distribution pattern of the sensing cells to estimate an amount of difference in pattern. For example, it is estimated from the pattern shown in FIG. 11B that the sensor mat  6  is shifted from the normal position shown in FIGS. 11C and 11D by an interval between two adjacent sensing cells  9 .  
         [0072]    A method of measuring the output signal levels of the sensing cells is described with reference to FIGS.  12 - 17 .  
         [0073]    Firstly, a presser  15 , which is comprised of a presser block  17  and a drive member  18 , is pressed against the seat  1  to detect the output signal level of each of the sensing cells  9 .  
         [0074]    The presser block  17  has a plurality of lattice-shape compartments  17   a , in which presser fingers  16  are respectively held so that the edges thereof, as a whole, form a predetermined weight distribution pattern. The presser fingers  16  are moved in the compartments so that the edges thereof can form the weight distribution pattern and, thereafter, fixed by screws  19  or the like. The drive member  18  includes an actuator (not shown) for moving the presser block  17  up or down via a drive shaft  20 .  
         [0075]    Copy boards  21  of the weight distribution patters of a man, woman and a baby on a child seat are prepared. For example, all the presser fingers  16  are pressed against a copy board  21  of the weight distribution pattern, which corresponds to a 6-year child model seated on a passenger seat, so that the edges of the presser fingers can follow the surface profile  21   a  of the copy board  21 , as shown in FIG. 14. The number of the presser fingers  16  can be reduced if the edges thereof are formed to imitate the profile of the buttocks, as shown in FIG. 15.  
         [0076]    Secondly, each of the output signals of the sensing cells  9  is compared with a reference level that corresponds to the same position of a model weight distribution pattern.  
         [0077]    If an output signal of one of the sensing cells  9  is not correct, such a sensing cell is calibrated in the manner described above.  
         [0078]    The presser  15  may be comprised of a plurality of pairs of a pressure cylinder  22  and a piston finger  16 , as shown in FIG. 16. Each pair of the pressure cylinder  22  and the piston finger  16  is electronically controlled according to a map that corresponds to a pressure isobar pattern shown in FIG. 17.  
         [0079]    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.