Abstract:
A load detecting device includes a plurality of load sensors which are disposed in a matrix shape and correcting means for setting at least one of the plural load sensors as a standard load sensor and for correcting level of output values of each load sensors to level of output value of the standard load sensor on the basis of the output values of each load sensors.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on and claims priority under 35 U.S.C. § 119 with respect to a Japanese Patent Application 2001-125989, filed on Apr. 24, 2001, the entire content of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates to a load detecting device. More particularly, this invention pertains to a load detecting device having a plurality of load sensors which are disposed in a matrix shape. 
   BACKGROUND OF THE INVENTION 
   A load detecting device which has a plurality of load sensors disposed in a matrix shape and which detects a distribution of the loads applied on a face on the basis of outputted values of each load sensors is already known in, for example, U.S. Pat. No. 5,905,209. A sensor in which a resistor changes in response to the applied load is adopted as the load sensor. In this case, for example, a current value which passes the load sensor is changed on the basis of the resistor changing in response to the load and this current value is transformed into a voltage value. Then, the voltage value changing in response to the load is fed to a central processing unit of a controller as an A/D (analog/digital) value through an A/D converting circuit of the controller. In general, a relationship between the load value applied to the load sensor and the above A/D value is all-inclusively represented by a predetermined calculation formula which corresponds to a structure of a circuit and the central processing unit obtains the load value on the basis of the calculation formula. 
   In case of that a plurality of load sensors are disposed in a matrix shape, however, generally scattering is in existence in a characteristic of each load sensors. Thereby, a distribution of the loads which is detected by the load detecting device becomes inaccurate. 
   A need exists for a load detecting device which can absorb scattering of the characteristics of plural load sensors disposed in a matrix shape. 
   SUMMARY OF THE INVENTION 
   A load detecting device comprises a plurality of load sensors which are disposed in a matrix shape and correcting means for setting at least one of the plural load sensors as a standard load sensor and for correcting level of output values of each load sensors to level of output value of the standard load sensor on the basis of the output values of each load sensors. 
   Further, according to another aspect of the present invention, a load detecting device comprises a plurality of load sensors which are disposed in a matrix shape and in which the load sensors in a standard line are set and correcting means for correcting level of output values of each load sensors to level of output value of the load sensors in the standard line on the basis of the output values of each load sensors. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     The foregoing and additional features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawing figures wherein: 
       FIG. 1  is a block view schematically illustrating an electric structure of an embodiment of a load detecting device in accordance with the present invention; 
       FIG. 2  is a plan view of a load detecting part of an embodiment a load detecting device in accordance with the present invention; 
       FIG. 3  is an electric circuit of an embodiment of a load detecting device in accordance with the present invention; 
     FIG.  4  and  FIG. 5  are flow charts showing routines for determining whether a passenger is sitting on a seat or whether the seated passenger is an adult or a child according to an embodiment of the present invention; 
       FIG. 6  is a graph illustrating a relationship between a load value and A/D value according to an embodiment of the present invention; and 
       FIG. 7  is a graph illustrating a relationship between a load value and A/D value obtained experimentally according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Having generally described the present invention, a further understanding of the invention can be obtained now according to an embodiment of the present invention with reference to  FIGS. 1  to  7  in accompanying drawings. 
     FIG. 1  illustrates a block view of an electric structure of a seating condition determination device  1  to which the present invention is applied. As shown in  FIG. 1 , the seating condition determination device  1  includes a load detecting part  10  and a controller  11 . 
   As shown in  FIG. 2 , the load detecting part  10  is accommodated in a seat surface of a vehicular seat (seat cushion)  12 . The load detecting part  10  has a structure including cells  13  which are disposed as load sensors on each locations on a surface defined by lines (i line) extending in the width direction (X direction) of the vehicular seat  12  and rows (j row) extending in back and forth direction (Y direction) of the vehicular seat  12 . In this embodiment, 80 pieces of cell  13  are disposed in a matrix having 7 lines and 12 rows on the vehicular seat  12 . 4 pieces of cell  13  which are located at both rear ends are removed. The number of cell is not limited to this embodiment. 
   Each cells  13  constitute a well known load sensor in which the resistance value is changed in response to the applied load. The signals (output values) of these cells  13  which correspond to each resistance values are fed to the controller  11 . The controller  11  detects the load values applied to each location on the basis of each signals. 
   These cells  13  are formed by screen printing which prints, for example, in the printing direction (−Y direction) shown in FIG.  2 . According to the general characteristic of the screen printing, the thickness of the membrane of each cells  13  becomes thinner in turn from the starting side toward the ending side in the printing direction. Accordingly, scattering which depends on the location of each cells  13 , namely the location of the low generates in a changing characteristic of the resistance value with respect to the load value among each cells  13 . 
   As shown in  FIG. 1 , the controller  11  which is connected to the load detecting part  10  includes a CPU (central processing unit)  21 , a power supply circuit  22 , an input circuit  23 , a first switching circuit  24 , a second switching circuit  25 , an A/D (analog/digital) converting circuit  26  and an output circuit  27 . 
   The CPU  21  performs a judgment of the seating condition of the vehicular seat  12  in accordance with a control program and initial data memorized in a ROM (read only memory) in advance. The power supply circuit  22  transforms the power supply (for example, 12V) supplied from a battery (not shown) to a predetermined voltage (for example, 5V) and supplies this transformed power supply to the CPU  21  as a power supply. 
   The input circuit  23  is provided on each lines of the cells  13  of the load detecting part  10 , respectively. One end of the cell  13  in each lines is connected to the input circuit  23  and is connected to the first switching circuit  24  through the input circuit  23 . 
   The first switching circuit  24  is connected to the load detecting part  10  through each input circuits  23 . The first switching circuit  24  switches selectively the line of the cells  13  of the load detecting part  10  by a switching signal from the CPU  21  and connects the line of the cells  13  to the A/D converting circuit  26 . On the other hand, the second switching circuit  25  is connected to the load detecting part  10 . The second switching circuit  25  switches selectively the row of the cells  13  of the load detecting part  10  by a switching signal from the CPU  21  and connects the row of the cells  13  to the ground. Accordingly, only the signals from the cells  13  in the line and row which is selectively switched are fed to the A/D converting circuit  26 . 
   The A/D converting circuit  26  converts the above signals from the cells  13  from analog signal into digital signal and fed it as an A/D value to the CPU  21 .  FIG. 3  show a circuit illustrating equivalently with respect to the cell  13  of the line and row selected by the first and second switching circuits  24 ,  25 . In this embodiment, the resistance value of this selected cell  13  are shown by rs (i, j). As shown in  FIG. 3 , one end of the cell  13  is connected at a connecting point a to the A/D converting circuit  26  of the controller  11  through the input circuit  23  (and the first switching circuit  24 ). The other end of the cell  13  is connected to the ground through the second switching circuit  25 . 
   The input circuit  23  includes a constant voltage source  31 , current mirror circuit  32  and a sensitivity resistance  33 . The constant voltage source  31  maintains an electrical potential within a predetermined standard electrical potential Vref. The current mirror circuit  32  equalizes the current value I 1  passing in a transistor Tr 1  with the current value I 2  passing in a transistor Tr 2  approximately. Accordingly, since the electrical potential is always maintained within the standard electrical potential Vref by the constant voltage source  31 , each current values I 1  and I 2  passing in the transistors Tr 1  and Tr 2  change in response to the resistance value rs (i, j). Namely, the current values I 1  and I 2  are calculated by the following formula (1).
 
 I   1 = I   2 = V ref/ rs   (1)
 
   One end of the sensitivity resistance  33  is connected at a connecting point b to the transistor Tr 1  of the current mirror circuit  32  and the other end thereof is connected to the ground. If the resistance value of the sensitivity resistance  33  is a predetermined resistance value Rs, the electrical potential Vb at the connecting point b is calculated by the following formula (2).
 
 Vb=Rs×I   1   (2)
 
   Then, the following formula (3) is obtained by the formulas (1) and (2).
 
 Vb=Rs×I   1 = V ref× Rs/rs   (3)
 
   The electrical potential Vb at the connecting point b is fed to the CPU  21  as the A/D value Y′ (i, j). In this embodiment, the characteristic between the applied load and the A/D value y in all cells  13  disposed in the line i is shown approximately by the following formula (4) which is shown in  FIG. 6  by a solid line.
 
 y=ai ×(load value)− bi   (4)
 
   The inclination ai and the intercept bi of the formula (4) are set by the A/D value which is obtained experimentally with respect to the load value in each cells  13 . The inclination ai and the intercept bi are obtained by rectilinear approximation based on the minimum square method. Further, the CPU  21  memorizes A/D value y′ (i, j) which approximates the i line with respect to the inputted A/D value Y′ (i, j). In other words, the CPU  21  can calculate the A/D value y′ (i, j) on the straight line which approximates the i line on the basis of the A/D value Y′ (i, j) inputted from a predetermined cell  13  of the i line. For example, the CPU  21  calculates the A/D value y′ (i, j) by a map or a table based on the A/D value Y′ (i, j) of the cell  13 . The CPU  21  substitutes the calculated A/D y′ (i,j) for the formula (4) and obtains the load value applied to the cell  13 . 
   In this embodiment, it is regarded that the central line (forth line) has a standard characteristic. The characteristic between the applied load value and the A/D value y of the cells  13  which are disposed in the fourth line is expressed approximately by the following formula (5) shown in  FIG. 6  by a dotted line.
 
 y=a ×(load value)− b   (5)
 
   The inclination a and the intercept b of the formula (5) are also set by the A/D value which is obtained experimentally with respect to the load value in each cells  13  disposed in the fourth line. In the screen printing, the cells  13  of the central line which is located at the center with respect to the printing direction have most stable characteristic and are set as the standard cells. The CPU  21  substitutes the load value obtained based on the formula (4) for the formula (5) and obtains the A/D value y (i, j) on the straight line in which the cells  13  in the central line are standard. 
   In this embodiment, the A/D value Y′ (i, j) inputted from the predeterrmined cells  13  of the i line is corrected by the deviation between the A/D value y′ (i, j) located on a straight line approximating the i line and the A/D value y (i, j) located on a straight line approximating the standard line. Thereby, the scattering of the A/D value Y′ (i, j) of the predetermined cells  13  with respect to the A/D value of the standard line is absorbed. Namely, if the corrected A/D value of the predetermined cells  13  of the i line under the condition which the scattering is absorbed is Y (i, j), Y is shown by the following formula (6). 
                     Y   =       Y   ′     -     y   ′     +   y                 =       Y   ′     -     y   ′     +     a   ×     (     load   ⁢           ⁢   value     )       -   b                         (   6   )             
 
   Further, if the load value by the formula (4) is substituted for the formula (6), the Y is shown by the following formula (7). 
                     Y   =       Y   ′     -     y   ′     +   y                 =       Y   ′     -     y   ′     +     a   ×     (         (       y   ′     +     b   ⁢           ⁢   i       )     /   a     ⁢           ⁢   i     )       -   b                         (   7   )             
 
     FIG. 7  shows a relationship between the load value and the corrected A/D value Y (i, j) which corrected the A/D value Y′ (i, j) of all cells  13  on the basis of the formula (7). This relationship is obtained by the experimentation. In  FIG. 7 , as shown as a divergence w with respect to the predetermined A/D value, the scattering of the corrected A/D value with respect to the load value is decreased. 
   The CPU  21  performs a threshold value judgment and so on based on the corrected A/D value Y (i, j). The CPU  21  performs a judgment of the seating condition of the vehicular seat  12  on the basis of the result of the threshold value judgment. 
   The output circuit  27  is connected to the CPU  21  and signals showing the seating condition of the vehicular seat  12  and so on are fed from the CPU  21 . The output circuit  27  is connected to an air bag controller  30  and outputs the seating conditions and so on judged by the CPU  21  as a seating signal to the air bag controller  30 . The air bag controller  30  operates an air bag for a driver seat or a passenger seat on the basis of the seat signal and a signal from a collision sensor (not shown). Especially, there are various cases in the seating condition of the passenger seat of the vehicular seat  12 , for example, an adult is seated, a child is seated, a child seat is mounted, or nothing on the seat. The air bag controller  30  receives the seating signal corresponding to each cases and controls the operation of the air bag for passenger seat suitably. 
   A routine for determining a passenger in the seating condition determination device  1  will now be described with reference to a flow chart shown in  FIGS. 4 and 5 . This routine is performed with a predetermined interval as interrupt handling. When the routine begins to start, the CPU  21  reads in step  101  the A/D values Y′ (i, j) inputted from each cells  13  of the load detecting part  10  and proceeds to step  102 . 
   In step  102 , the CPU  21  calculates A/D values y′ (i, j) located on the straight line approximating the i line on the basis of the A/D values Y′ (i, j) of each cells  13 . Then, the CPU  21  substitutes the A/D values Y′ (i, j) and the A/D values y′ (i, j) for the above formula (7) and calculates the corrected A/D values Y (i, j) of each cells  13 . Then the CPU  21  stores the corrected A/D values Y (i, j) as substantially A/D values for various processing in step  103 . 
   Next, the CPU  21  proceeds to the step  104  and judges whether the calculation and the storing of the corrected A/D values Y (i, j) is completed for all cells  13 . Then, when it is judged that the calculation and the storing of the corrected A/D values Y (i, j) is not completed for all cells  13 , the CPU  21  repeats the processes in steps  102  and  103  until the calculation and the storing of the corrected A/D values Y (i, j) is completed. When it is judged that the calculation and the storing of the corrected A/D values Y (i, j) is completed for all cells  13 , the CPU  21  proceeds to step  105 . 
   In step  105 , the CPU  21  judges whether the corrected A/D values Y (i, j) in each cells  13  exceed the predetermined threshold value or not. Then, in case that the whether the corrected A/D values Y (i, j) exceed the predetermined threshold value, the CPU  21  proceeds to step  106  and increases the number of ON cell on_cel by adding ┌1┘. Then, the CPU  21  proceeds to the step  107 . The number of ON cell on_cel corresponds to the number of the cells in which the corrected A/D values Y (i, j) exceed the predetermined threshold value. On the other hand, when the corrected A/D values Y (i, j) do not exceed the predetermined threshold value, the step  107  is performed. 
   In step  107 , the CPU  21  judges whether the comparison between the corrected A/D values Y (i, j) and the threshold value is completed for all cells  13 . When it is judged that the comparison between the corrected A/D values Y (i, j) and the threshold value is not completed for all cells  13 , the CPU  21  repeats the processes of the steps  105  and  106  until the comparison is completed and renews the number of ON cell. Then, when it is judged that the comparison between the corrected A/D values Y (i, j) and the threshold value is completed for all cells  13 , the CPU  21  stores the number of ON cell renewed in step  106  as the last number of ON cell. This number of ON cell is used for the judgment of the seating condition and so on of the vehicular seat  12 . Namely, the number of ON cell shows characteristic which corresponds to the seating conditions, for example, an adult or a child is seated, a child seat is mounted, or nothing on the seat. 
   When it is judged in step  107  that whether the comparison between the corrected A/D values Y (i, j) and the threshold value is completed for all cells, the CPU  21  proceeds to step  111  in FIG.  5 . In step  111 , the CPU  21  calculates the sum of the corrected A/D values Y (i, j) of all cells  13  and stores the sum value. This sum value is also used for the judgment of the seating condition and so on of the vehicular seat  12 . Namely, the sum value also shows characteristic which corresponds to the seating conditions. 
   Next, the CPU  21  proceeds to the step  112  and judges whether the sum value is more than a predetermined value A 1  or the number of ON cell on_cel is more than a predetermined value A 2 . In general, when an adult is seated on the vehicular seat  12 , in comparison with the other cases (child, child seat, and son), the sum value or the number of ON cell on_cel shows a certain measure of value or number. The above predetermined values A 1  and A 2  are set to values which are suitable for sorting the various seating conditions. 
   When it is judged that the sum value is more than the predetermined value A 1  or the number of ON cell on_cel is more than the predetermined value A 2 , the CPU  21  proceeds to the step  113  and judges that an adult is seated. The CPU  21  stores this result of the judgment in the memory and terminates subsequent process once. On the other hand, when it is judged that the sum value is less than the predetermined value A 1  or the number of ON cell on_cel is less than the predetermined value A 2 , the CPU  21  proceeds to the step  114 . 
   In step  114 , the CPU  21  judges whether the sum value is more than a predetermined value B 1  or the number of ON cell on_cel is more than a predetermined value B 2 . The predetermined value B 1  is smaller than the value A 1  and the predetermined value B 2  is smaller than the value A 2 . In general, when an adult is seated on the vehicular seat  12  or a child seat is mounted, in comparison with the other cases (child, and son), the sum value or the number of ON cell on_cel shows a certain measure of value or number. The above predetermined values B 1  and B 2  are set to values which are suitable for sorting the various seating conditions. 
   When it is judged that the sum value is more than the predetermined value B 1  or the number of ON cell on_cel is more than the predetermined value B 2 , the CPU  21  proceeds to the step  115 . In step  115 , the CPU  21  judges whether an edge strength is small and the conformity with human is anticipated or whether an adult is seated on a front side of the vehicular seat  12 . 
   The meaning of the edge strength is described as follows. In general, in comparison with the case which the human is seated on the vehicular seat  12 , when the child seat is mounted on the vehicular seat  12 , sudden change generates between the corrected A/D values Y (i, j) of the predetermined cells  13  and the corrected A/D values Y (i, j) of the cells being adjacent to the predetermined cells  13 . Because the child seat is hard in comparison with the soft human body and the remarkable load variation generates at the contacting portion between the child seat and the vehicular seat by the tightening of the seat belt. Accordingly, the degree of this load variation gives suggestions that the child seat is mounted. The edge strength expresses the degree of the load variation numerically by well known method. When the edge strength is small, since the load variation is small, the CPU  21  judges that the human is seated. When the edge strength is large, since the load variation is large, the CPU  21  judges that the child seat is mounted. 
   Next, the meaning of the conformity with human is described as follows. In general, the characteristic of the distribution on the seat  12  of the corrected A/D values Y (i, j) of all cells  13  under the condition which the human is seated on the vehicular seat  12  is differ from that under the condition that the child seat is mounted on the vehicular seat  12 . For example, in case that the human is seated on the vehicular seat  12 , the cells  13  which are located at relatively central portion of the seat  12  show a certain level of corrected A/D values Y (i, j). On the other hand, in case that the child seat is mounted on the seat  12 , the cells  13  which are located at relatively periphery side of the seat  12  show a certain level of corrected A/D values Y (i, j). Accordingly, the tendency of the distribution of the corrected A/D values Y (i, j) on the seat  12  gives suggestions that the child seat is mounted. 
   Further, the characteristic of the distribution on the seat  12  of the corrected A/D values Y (i, j) of all cells  13  is remarkably changed by the location where the human is seated. For example, when the human is seated on a front side of the vehicular seat  12 , the cells  13  which are located at relatively front portion of the seat  12  show a certain level of corrected A/D values Y (i, j). Accordingly, the tendency of the distribution of the corrected AND values Y (i, j) on the seat  12  gives suggestions that the human is seated on a front side of the vehicular seat  12 . 
   In step  115 , when it is judged that the edge strength is small and the conformity with human is anticipated or that an adult is seated on a front side of the vehicular seat  12 , the CPU  21  proceeds to the step  116  and judges that the adult is seated. The CPU  21  stores this result of the judgment in the memory and terminates subsequent process once. On the other hand, when it is judged that that the edge strength is large and the conformity with human is not anticipated or that an adult is not seated on a front side of the vehicular seat  12 , the CPU  21  proceeds to the step  117  and judges that the child seat is mounted. The CPU  21  stores this result of the judgment in the memory and terminates subsequent process once. 
   Further, in step  114 , when it is judged that the sum value is less than the predetermined value B 1  or the number of ON cell on_cel is less than the predetermined value B 2 , the CPU  21  proceeds to the step  118 . 
   In step  118 , the CPU  21  judges whether the sum value is more than a predetermined value C 1  or the number of ON cell on_cel is more than a predetermined value C 2 . The predetermined value C 1 is smaller than the value B 1  and the predetermined value C 2  is smaller than the value B 2 . In general, when an adult or a child is seated on the vehicular seat  12  or a child seat is mounted, in comparison with the other case (nothing on the seat), the sum value or the number of ON cell on_cel shows a certain measure of value or number. The above predetermined values C 1  and C 2  are set to values which are suitable for sorting the various seating conditions. 
   When it is judged that the sum value is more than the predetermined value C 1  or the number of ON cell on_cel is more than the predetermined value C 2 , the CPU  21  proceeds to the step  119 . In step  119 , the CPU  21  judges whether an adult is seated on a front side of the vehicular seat  12 . When it is judged that an adult is seated on the front side of the vehicular seat  12 , the CPU  21  proceeds to the step  120  and judges that an adult is seated on the seat  12 . The CPU  21  stores this result of the judgment in the memory and terminates subsequent process once. 
   On the other hand, when it is judged that an adult is not seated on a front side of the vehicular seat  12 , the CPU  21  proceeds to the step  121  and judges whether the conformity with human is anticipated. When it is judged that the conformity with human is anticipated, the CPU  21  proceeds to the step  122  and judges that a child is seated on the seat  12 . The CPU  21  stores this result of the judgment in the memory and terminates subsequent process once. Further, when it is judged that the conformity with human is not anticipated, the CPU  21  proceeds to the step  123  and judges that the child seat is mounted. The CPU  21  stores this result of the judgment in the memory and terminates subsequent process once. 
   Further, in step  118 , when it is judged that the sum value is less than the predetermined value C 1  or the number of ON cell on_cel is less than the predetermined value C 2 , the CPU  21  proceeds to the step  124 . In step  124 , the CPU  21  judges that the seat  12  is in no load condition (nothing on the seat  12 ). The CPU  21  stores this result of the judgment in the memory and terminates subsequent process once. 
   The CPU  21  feeds the results of the judgments in steps  113 ,  116 ,  117 ,  120 ,  122  to  124  as seating signals to the air bag controller  30  through the output circuit  26 . The air bag controller  30  controls the operation of the air bag suitably in response to the seating signals. 
   As described above, according to this embodiment, the level of A/D value Y′ (i, j) of each cells  13  is corrected to the level of A/D value of the cells  13  of the standard line (the corrected A/D value Y (i, j)) on the basis of the A/D value Y′ (i, j) of each cells  13 . Thereby, it is able to absorb the scattering of the characteristic of each cells  13  and it is able to improve the accuracy of the detecting of the load detecting device. 
   Further, in this embodiment, the number of ON cell on_cel and the sum value are calculated based on the corrected A/D values Y (i, j). Therefore, it is able to perform the judgment of the seating conditions and so on suitably. 
   Further, it is possible to modify the above described embodiment. For example, it is able to obtain the corrected A/D value Y (i, j) by the calculation of the load value in the formula (4) from a table or a map on the basis of the A/D value Y′ (i, j) from the predetermined cells  13 . Further, it is able to obtain the corrected A/D value Y (i, j) directly from a table or a map on the basis of the A/D value Y′ (i, j) from the predetermined cells  13 . 
   Further, the A/D value Y′ (i, j) inputted from the predeterrmined cells  13  may be corrected by the deviation between the A/D value Y′ (i, j) and the A/D value of the predetermined standard cells. Thereby, it is able to absorb the scattering between the A/D value Y′ (i, j) and the A/D value of the predetermined standard cells. In this case, the cells which are located at approximately central portion of the seat may be use as the standard cells. 
   Further, the relationship between the load value and the A/D value may be shown by the other formulas. In this embodiment, the load sensor is formed by the cells which formed by screen printing. However, it is able to use another sensor which is formed by another method. Further, it is able to use a sensor which an electrostatic capacity changes in response to the load. 
   The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiment disclosed. Further, the embodiment described herein is to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.