Load detecting device for a vehicular seat

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.

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.

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 toFIGS. 1to7in accompanying drawings.

FIG. 1illustrates a block view of an electric structure of a seating condition determination device1to which the present invention is applied. As shown inFIG. 1, the seating condition determination device1includes a load detecting part10and a controller11.

As shown inFIG. 2, the load detecting part10is accommodated in a seat surface of a vehicular seat (seat cushion)12. The load detecting part10has a structure including cells13which 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 seat12and rows (j row) extending in back and forth direction (Y direction) of the vehicular seat12. In this embodiment, 80 pieces of cell13are disposed in a matrix having 7 lines and 12 rows on the vehicular seat12. 4 pieces of cell13which are located at both rear ends are removed. The number of cell is not limited to this embodiment.

Each cells13constitute a well known load sensor in which the resistance value is changed in response to the applied load. The signals (output values) of these cells13which correspond to each resistance values are fed to the controller11. The controller11detects the load values applied to each location on the basis of each signals.

These cells13are 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 cells13becomes thinner in turn from the starting side toward the ending side in the printing direction. Accordingly, scattering which depends on the location of each cells13, namely the location of the low generates in a changing characteristic of the resistance value with respect to the load value among each cells13.

As shown inFIG. 1, the controller11which is connected to the load detecting part10includes a CPU (central processing unit)21, a power supply circuit22, an input circuit23, a first switching circuit24, a second switching circuit25, an A/D (analog/digital) converting circuit26and an output circuit27.

The CPU21performs a judgment of the seating condition of the vehicular seat12in accordance with a control program and initial data memorized in a ROM (read only memory) in advance. The power supply circuit22transforms 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 CPU21as a power supply.

The input circuit23is provided on each lines of the cells13of the load detecting part10, respectively. One end of the cell13in each lines is connected to the input circuit23and is connected to the first switching circuit24through the input circuit23.

The first switching circuit24is connected to the load detecting part10through each input circuits23. The first switching circuit24switches selectively the line of the cells13of the load detecting part10by a switching signal from the CPU21and connects the line of the cells13to the A/D converting circuit26. On the other hand, the second switching circuit25is connected to the load detecting part10. The second switching circuit25switches selectively the row of the cells13of the load detecting part10by a switching signal from the CPU21and connects the row of the cells13to the ground. Accordingly, only the signals from the cells13in the line and row which is selectively switched are fed to the A/D converting circuit26.

The A/D converting circuit26converts the above signals from the cells13from analog signal into digital signal and fed it as an A/D value to the CPU21.FIG. 3show a circuit illustrating equivalently with respect to the cell13of the line and row selected by the first and second switching circuits24,25. In this embodiment, the resistance value of this selected cell13are shown by rs (i, j). As shown inFIG. 3, one end of the cell13is connected at a connecting point a to the A/D converting circuit26of the controller11through the input circuit23(and the first switching circuit24). The other end of the cell13is connected to the ground through the second switching circuit25.

The input circuit23includes a constant voltage source31, current mirror circuit32and a sensitivity resistance33. The constant voltage source31maintains an electrical potential within a predetermined standard electrical potential Vref. The current mirror circuit32equalizes the current value I1passing in a transistor Tr1with the current value I2passing in a transistor Tr2approximately. Accordingly, since the electrical potential is always maintained within the standard electrical potential Vref by the constant voltage source31, each current values I1and I2passing in the transistors Tr1and Tr2change in response to the resistance value rs (i, j). Namely, the current values I1and I2are calculated by the following formula (1).
I1=I2=Vref/rs(1)

One end of the sensitivity resistance33is connected at a connecting point b to the transistor Tr1of the current mirror circuit32and the other end thereof is connected to the ground. If the resistance value of the sensitivity resistance33is a predetermined resistance value Rs, the electrical potential Vb at the connecting point b is calculated by the following formula (2).
Vb=Rs×I1(2)

Then, the following formula (3) is obtained by the formulas (1) and (2).
Vb=Rs×I1=Vref×Rs/rs(3)

The electrical potential Vb at the connecting point b is fed to the CPU21as the A/D value Y′ (i, j). In this embodiment, the characteristic between the applied load and the A/D value y in all cells13disposed in the line i is shown approximately by the following formula (4) which is shown inFIG. 6by 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 cells13. The inclination ai and the intercept bi are obtained by rectilinear approximation based on the minimum square method. Further, the CPU21memorizes 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 CPU21can 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 cell13of the i line. For example, the CPU21calculates the A/D value y′ (i, j) by a map or a table based on the A/D value Y′ (i, j) of the cell13. The CPU21substitutes the calculated A/D y′ (i,j) for the formula (4) and obtains the load value applied to the cell13.

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 cells13which are disposed in the fourth line is expressed approximately by the following formula (5) shown inFIG. 6by 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 cells13disposed in the fourth line. In the screen printing, the cells13of 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 CPU21substitutes 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 cells13in the central line are standard.

In this embodiment, the A/D value Y′ (i, j) inputted from the predeterrmined cells13of 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 cells13with respect to the A/D value of the standard line is absorbed. Namely, if the corrected A/D value of the predetermined cells13of 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. 7shows 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 cells13on the basis of the formula (7). This relationship is obtained by the experimentation. InFIG. 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 CPU21performs a threshold value judgment and so on based on the corrected A/D value Y (i, j). The CPU21performs a judgment of the seating condition of the vehicular seat12on the basis of the result of the threshold value judgment.

The output circuit27is connected to the CPU21and signals showing the seating condition of the vehicular seat12and so on are fed from the CPU21. The output circuit27is connected to an air bag controller30and outputs the seating conditions and so on judged by the CPU21as a seating signal to the air bag controller30. The air bag controller30operates 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 seat12, for example, an adult is seated, a child is seated, a child seat is mounted, or nothing on the seat. The air bag controller30receives 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 device1will now be described with reference to a flow chart shown inFIGS. 4 and 5. This routine is performed with a predetermined interval as interrupt handling. When the routine begins to start, the CPU21reads in step101the A/D values Y′ (i, j) inputted from each cells13of the load detecting part10and proceeds to step102.

In step102, the CPU21calculates 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 cells13. Then, the CPU21substitutes 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 cells13. Then the CPU21stores the corrected A/D values Y (i, j) as substantially A/D values for various processing in step103.

Next, the CPU21proceeds to the step104and judges whether the calculation and the storing of the corrected A/D values Y (i, j) is completed for all cells13. 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 cells13, the CPU21repeats the processes in steps102and103until 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 cells13, the CPU21proceeds to step105.

In step105, the CPU21judges whether the corrected A/D values Y (i, j) in each cells13exceed 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 CPU21proceeds to step106and increases the number of ON cell on_cel by adding ┌1┘. Then, the CPU21proceeds to the step107. 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 step107is performed.

In step107, the CPU21judges whether the comparison between the corrected A/D values Y (i, j) and the threshold value is completed for all cells13. 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 cells13, the CPU21repeats the processes of the steps105and106until 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 cells13, the CPU21stores the number of ON cell renewed in step106as 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 seat12. 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 step107that whether the comparison between the corrected A/D values Y (i, j) and the threshold value is completed for all cells, the CPU21proceeds to step111in FIG.5. In step111, the CPU21calculates the sum of the corrected A/D values Y (i, j) of all cells13and stores the sum value. This sum value is also used for the judgment of the seating condition and so on of the vehicular seat12. Namely, the sum value also shows characteristic which corresponds to the seating conditions.

Next, the CPU21proceeds to the step112and judges whether the sum value is more than a predetermined value A1or the number of ON cell on_cel is more than a predetermined value A2. In general, when an adult is seated on the vehicular seat12, 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 A1and A2are 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 A1or the number of ON cell on_cel is more than the predetermined value A2, the CPU21proceeds to the step113and judges that an adult is seated. The CPU21stores 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 A1or the number of ON cell on_cel is less than the predetermined value A2, the CPU21proceeds to the step114.

In step114, the CPU21judges whether the sum value is more than a predetermined value B1or the number of ON cell on_cel is more than a predetermined value B2. The predetermined value B1is smaller than the value A1and the predetermined value B2is smaller than the value A2. In general, when an adult is seated on the vehicular seat12or 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 B1and B2are 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 B1or the number of ON cell on_cel is more than the predetermined value B2, the CPU21proceeds to the step115. In step115, the CPU21judges 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 seat12.

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 seat12, when the child seat is mounted on the vehicular seat12, sudden change generates between the corrected A/D values Y (i, j) of the predetermined cells13and the corrected A/D values Y (i, j) of the cells being adjacent to the predetermined cells13. 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 CPU21judges that the human is seated. When the edge strength is large, since the load variation is large, the CPU21judges 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 seat12of the corrected A/D values Y (i, j) of all cells13under the condition which the human is seated on the vehicular seat12is differ from that under the condition that the child seat is mounted on the vehicular seat12. For example, in case that the human is seated on the vehicular seat12, the cells13which are located at relatively central portion of the seat12show 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 seat12, the cells13which are located at relatively periphery side of the seat12show 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 seat12gives suggestions that the child seat is mounted.

Further, the characteristic of the distribution on the seat12of the corrected A/D values Y (i, j) of all cells13is remarkably changed by the location where the human is seated. For example, when the human is seated on a front side of the vehicular seat12, the cells13which are located at relatively front portion of the seat12show 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 seat12gives suggestions that the human is seated on a front side of the vehicular seat12.

In step115, 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 seat12, the CPU21proceeds to the step116and judges that the adult is seated. The CPU21stores 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 seat12, the CPU21proceeds to the step117and judges that the child seat is mounted. The CPU21stores this result of the judgment in the memory and terminates subsequent process once.

Further, in step114, when it is judged that the sum value is less than the predetermined value B1or the number of ON cell on_cel is less than the predetermined value B2, the CPU21proceeds to the step118.

In step118, the CPU21judges whether the sum value is more than a predetermined value C1or the number of ON cell on_cel is more than a predetermined value C2. The predetermined value C1is smaller than the value B1and the predetermined value C2is smaller than the value B2. In general, when an adult or a child is seated on the vehicular seat12or 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 C1and C2are 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 C1or the number of ON cell on_cel is more than the predetermined value C2, the CPU21proceeds to the step119. In step119, the CPU21judges whether an adult is seated on a front side of the vehicular seat12. When it is judged that an adult is seated on the front side of the vehicular seat12, the CPU21proceeds to the step120and judges that an adult is seated on the seat12. The CPU21stores 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 seat12, the CPU21proceeds to the step121and judges whether the conformity with human is anticipated. When it is judged that the conformity with human is anticipated, the CPU21proceeds to the step122and judges that a child is seated on the seat12. The CPU21stores 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 CPU21proceeds to the step123and judges that the child seat is mounted. The CPU21stores this result of the judgment in the memory and terminates subsequent process once.

Further, in step118, when it is judged that the sum value is less than the predetermined value C1or the number of ON cell on_cel is less than the predetermined value C2, the CPU21proceeds to the step124. In step124, the CPU21judges that the seat12is in no load condition (nothing on the seat12). The CPU21stores this result of the judgment in the memory and terminates subsequent process once.

The CPU21feeds the results of the judgments in steps113,116,117,120,122to124as seating signals to the air bag controller30through the output circuit26. The air bag controller30controls 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 cells13is corrected to the level of A/D value of the cells13of the standard line (the corrected A/D value Y (i, j)) on the basis of the A/D value Y′ (i, j) of each cells13. Thereby, it is able to absorb the scattering of the characteristic of each cells13and 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 cells13. 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 cells13.

Further, the A/D value Y′ (i, j) inputted from the predeterrmined cells13may 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.