Abstract:
An occupant detecting system includes a temperature sensor that sequentially detects temperatures at a fixed position, a plurality of weight sensors disposed at predetermined positions of a seat to respectively provide weight signals, an occupant ECU. The ECU estimates temperatures of the weight sensors based on the temperatures sequentially detected by the temperature sensor and temperature characteristic data of the weight sensors relative to temperatures detected by the temperature sensor and corrects the weight signals based on the estimated temperatures.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is based on and claims priority from Japanese Patent Application 2004-281352, filed Sep. 28, 2004, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a system that detects an occupant of a seat and sends a signal to an airbag system or a seatbelt pre-tensioning system. 
   2. Description of the Related Art 
   Such an occupant detecting system includes four weight sensors disposed at four corners of a seat and an ECU (electronic control unit). Because the output signals of the weight sensors are affected by temperature, it is necessary to remove temperature-affected-variation of the signals. US2002/0134167A1 discloses an occupant detecting system in which a temperature sensor is attached to each weight sensor to remove temperature-affected-variation. Therefore, four temperature sensors are necessary for four weight sensors, thereby resulting in a higher cost and a larger size of the occupant detecting system. 
   If the number of the weight sensors is reduced to reduce the cost and size, detection accuracy of the system may lower when the ambient temperature changes in a short time because the temperature of the four temperature sensors may differ one from another. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the invention is to provide an improved system for detecting an occupant at a higher accuracy with less number of temperature sensors. 
   Another object of the invention is to provide an improved method for detecting an occupant at a higher accuracy with less number of temperature sensors. 
   The inventor considered that irregular temperature distribution was caused by air-conditioned air passages and heat conductivities of seat components such as a seat frame and seat rails. The inventor found out a fact that temperatures of various positions of a seat are related to each other, so that a temperature of a certain position of the seat can be estimated by a temperature of another position of the seat. 
   According to an aspect of the invention, an occupant detecting system includes a temperature sensor that sequentially detects temperatures at a fixed position, a plurality of weight sensors for respectively providing weight signals, estimating means for estimating temperatures of the weight sensors based on the temperatures sequentially detected by the temperature sensor and previously stored temperature characteristic data of the weight sensors relative to temperatures detected by the temperature sensor and correcting means for correcting the weight signals based on the temperatures estimated by the estimating means. 
   In the above occupant detecting system, the estimating means may include first means for calculating a speed of changing temperature based on a temperature difference between two points of times and second means for calculating temperatures of the weight sensors based on the temperatures detected by the temperature sensor and the temperature characteristic data if the temperature difference is larger than a predetermined temperature, and the speed of changing temperature is higher than a predetermined speed. Further, the occupant detecting system may includes an occupant detecting ECU in which the estimating means and the correcting means are included. In the above occupant detecting system, the temperature sensor may be also included in the occupant detecting ECU. Preferably, the weight sensors are disposed at four corners of a seat, and the estimating means estimates a common temperature of the weight sensors disposed at two front corners of the seat and a common temperature of the weight sensors disposed at two rear corners of the seat. 
   Another aspect of the invention is a method of detecting an occupant of a seat. The method includes steps of sequentially detecting temperature of a fixed position, detecting weight of the seat at a plurality of positions of the seat by weight sensors respectively disposed at the plurality of positions, estimating temperatures of the weight sensors based on the temperatures sequentially detected by the temperature sensor and previously stored temperature characteristic data of the weight sensors relative to temperatures of the temperature sensor and correcting the weight signals based on the estimated temperatures. 

   
     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 perspective view illustrating an occupant detecting system according to a preferred embodiment of the invention mounted in a vehicle seat; 
       FIG. 2  is a block diagram showing components of the occupant detecting system; 
       FIG. 3  is a graph showing temperature characteristics relative to time at various positions of the occupant detecting system; 
       FIG. 4A  is a graph showing estimated temperature changes, and  FIG. 4B  is a graph showing a relationship between estimated temperatures and correction values of detected weights; and 
       FIG. 5  is a flow diagram showing an algorism of the occupant detecting system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An occupant detecting system  1  and a method for detecting an occupant according to a preferred embodiment of the invention will be described with reference to the appended drawings. 
   As shown in  FIG. 1 , the occupant detecting system is mounted in the front passenger&#39;s seat  96  of a vehicle, which is disposed on a pair of seat rails  8 . Each seat rail  8  is comprised of an upper rail  80  and a lower rail  81 . The pair of seat rails  8  is fixed to the floor of a vehicle to line up in the transverse or width direction of the vehicle body. The upper rail  80  is slidably disposed on the lower rail  81  and fixed to the seat  96  so that the seat  96  can slide back and forth along the lower rail  81 . The occupant detecting system  1  includes four weight sensors—a front right sensor  20 FR, a front left sensor  20 FL, a rear right sensor  20 RR and a rear left sensor  20 RL—and an occupant detecting ECU  3 , which are electrically connected by wire harness. 
   The seat  96  has a seat frame (not shown), and the four weight sensors  20 FR,  20 FL,  20 RR,  20 RL are disposed at four corners of the seat  96 —the front right corner, the front left corner, the rear right corner and the rear left corner—between the seat frame and the upper rail  80 . Each weight sensor is a strain gage type sensor that includes a bridge circuit of four strain gages and an amplifier. The occupant detecting ECU  3  is fixed to the bottom of the seat  96  in the middle of the width thereof. 
   As shown in  FIG. 2 , The occupant detecting ECU  3  includes a temperature sensor  30 , a CPU (central processing unit)  31 , an EEPROM  32  and a communication I/F (interface)  33 . The CPU  31  functions as a temperature estimation unit, a temperature change judging unit and a weight signal correction unit. For this purpose, the CPU  31  includes an A/D converter  310 , a RAM  311  and a ROM  312 . 
   The A/D converter  310  converts analog voltage signals sent from the weight sensors  20 FR,  20 FL,  20 RR,  20 RL into digital data. The RAM  311  temporarily stores the digital data. The ROM  312  has stored a program for detecting an occupant, a threshold value for judging whether the seat is occupied or not and respective temperature characteristics of the weight sensors  20 FR,  20 FL,  20 RR,  20 RL. The EEPROM  32  stores data of failure if one of the weight sensors  20 FR,  20 FL,  20 RR,  20 RL fails. The communication I/F  33  sends the judgment data of the CPU  31  to an airbag ECU  94  of an airbag system  95 . The airbag ECU  94  locks or unlocks an airbag  940  based on the judgment data. 
   The CPU  31  estimates temperature of the weight sensors  20 FR,  20 FL,  20 RR,  20 RL in the following manner. 
   If an air conditioner heats a passenger compartment in a short time, the temperatures of the occupant detecting ECU 3  and the weight sensors  20 FR,  20 FL,  20 RR,  20 RL rise as shown in  FIG. 3 . Although all the members have the same temperature Te 0  when the air conditioner starts heating at time ta, each member has a different temperature at time tb when time dt lapses. That is: the occupant detecting ECU  3  has a temperature Te (i.e. Teo+ΔTe); the weight sensor  20 FR has a temperature Tfr; the weight sensor  20 FL has a temperature Tfl; the weight sensor  20 RR has a temperature Trr; and the weight sensor  20 RL has a temperature Trl. 
   Although all the temperatures finally become the same temperature as the room temperature, there is the following relationship among the temperatures in a transient time: Te&gt;Tfl&gt;Tfr&gt;Trl&gt;Trr. Because warm air-conditioned air flows as indicated by an arrow in  FIG. 1 , the temperatures Tfl and Tfr of the front weight sensors  20 FR,  20 FL are almost the same, and the temperatures Trl and Trr of the rear weight sensors  20 RR,  20 RL are almost the same. 
   Accordingly, it is possible to set a common temperature Tf of the front sensors  20 FR,  20 FL and a common temperature Tr of the rear sensors  20  RR,  20 RL as follows: Tf=(Tfr+Tfl)/2; and Tr=(Trr+Trl)/2. Then, it is possible to estimate Tf and Tr by the following expressions.
 
 Tf=Te 0 +ΔTr=Te 0 +α·ΔTe   [E1]
 
in which α is a variable that is proportional to 1/dt.
 
 Tr=Te 0 +ΔTr=Te 0 +β·ΔTe   [E2]
 
in which β is a variable that is proportional to 1/dt.
 
   These expressions can be also applied to estimation of Tf and Tr in case the air conditioner starts cooling. In this case, the temperature changes in the opposite direction along the curves shown in  FIG. 3 . 
   Weight signal correction of the weight sensor  20 FR will be described below with reference to  FIGS. 4A and 4B . 
   The temperature characteristic data of the weight sensor  20 FR are stored beforehand in the ROM  312  as a matrix or an arithmetic expression. In  FIG. 4A , line a represents a temperature change characteristic in a short time from a high temperature to a target room temperature, and line b represents a temperature change characteristic in a short time from a low temperature to a target room temperature. 
   For example, a weight correction value FL 1  is calculated from an estimated temperature TL 1  via a temperature characteristic line c. In the same manner, a weight correction value FL 2  is calculated from an estimated temperature TL 2 , a weight correction value FH 1  is calculated from an estimated temperature TH 1 , and a weight correction value FH 2  is calculated from an estimated temperature TH 2 . Thus, the weight data are corrected. Weight data of the other weight sensors  20 FL,  20 RR and  20 RL are also calculated in the same manner as above. The occupant detection is carried out by comparing the total sum of the corrected weight data of the four weight sensors  20 FR,  20 FL,  20 RR,  20 RL and an occupant detection reference value that is stored in the ROM  312 . 
   The algorism of the occupant detecting system will be described below with reference to a flow diagram shown in  FIG. 5 . 
   When an ignition key is turned on at step Step S 1 , a timer t 1  is reset at step S 2 . Then the initial temperature signal Te 0  of the temperature sensor  30  is read at step S 3 . Subsequently, an initial weight data correction value to be used in case that the temperature of a passenger compartment (room temperature) is unchanged or out of high or low temperature range is set at step S 3 . 5 . This initial weight data correction value is set based on the initial temperature signal Te 0  and temperature characteristics of the four weight sensors  20 FR,  20 FL,  20 RR,  20 RL, as shown in  FIG. 4B . Thereafter, whether the initial temperature signal Te 0  is in a low temperature range such as a temperature range lower than 15° C. or in a high temperature range such as a temperature range higher than 35° C. is examined at step S 4 . If the result is YES, the timer t 1  is incremented at step S 5 . On the other hand, the timer t 1  is reset again at step S 2  if the result is NO. That is, either the temperature estimation at step S 10  or the weight data correction at step S 11  is not carried out if the initial temperature signal Te 0  does not come in the low temperature range or in the high temperature range. 
   After the timer t 1  is incremented at step S 5 , whether or not t 1 =t 2 ×n is examined at step S 6 . If the result is YES, the temperature signal Te of the occupant detecting ECU  3  is read to replace the initial temperature signal Te 0  at step S 7 . On the other hand, the timer t 1  is reset again at step S 2  if the result is NO. That is, the temperature signal Te is read and renewed each set time. 
   Thereafter, whether the difference between the renewed temperature signal Te and the initial temperature signal Te 0  is larger than a reference value A or not is examined at step S 8 . If the result is YES, whether the speed of the temperature change, which is |Te−Te 0 |/t 1 , is higher than a reference value B or not is examined. On the other hand, the timer t 1  is reset again at step S 2  if the result is NO. 
   If the speed of the temperature change is higher than the reference value B and the examination result of the step S 9  is YES, the temperature estimation is carried out at step S 10  and the weight data correction is carried out at step S 11 . On the other hand, the timer t 1  is reset again at step S 2  if the result is NO. 
   At step S 10 , the common temperatures Tf and Tr are calculated by means of the expressions E1 and E2. At step S 11 , the weight data correction is carried out in the manner described above with reference to  FIGS. 4A and 4B . The temperature Tf or Tr is substituted by TH 1  or TH 2  if it is higher than a target room temperature and by TL 1  or TL 2  if it is lower than the initial room temperature. 
   After the weight data correction is carried out, the timer t 1  is incremented at step S 5 . Thereafter, the above described steps S 5 -S 11  are repeatedly carried out. 
   The weight data correction may be made by changing the threshold value of the ROM  312  according to the temperature estimation. The temperature characteristics of the weight sensors  20 FR,  20 FL,  20 RR,  20 RL may be stored in the EEPROM  32  instead of ROM  312 . 
   Another temperature sensor of the same type may be added to the temperature sensor  30 , so that one of them detects temperature corresponding to the front weight sensors  20 FR,  20 FL and so that the other detects temperature corresponding to the rear weight sensors  20 RR,  20 RL. Further, the temperature sensor  30  may be disposed at a position that is separated from the occupant detecting ECU  3 . 
   The occupant detecting system may be located at the driver&#39;s seat or a rear seat instead of the front passenger&#39;s seat. 
   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.