Patent Publication Number: US-2013234736-A1

Title: Occupant detection device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims the benefit of priority of Japanese Patent Application No. 2012-52743, filed on Mar. 9, 2012, the disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure generally relates to an occupant detection device for detecting an occupant in a vehicle. 
     BACKGROUND 
     An electrostatic sensor disposed in a seat of a vehicle, detects an occupant seated in the seat by detecting capacitance. However, in the event a liquid spill condition in which a liquid, such as water, is spilled onto the seat and absorbed through the seat fabric, the electrostatic sensor ability to detect the occupant may be affected. In Japanese Patent Laid-Open No H07-270541 (JP &#39;541) detection of the occupant in the vehicle is enabled by providing dedicated electrodes for the electrostatic sensor, and by calculating the capacitance between the dedicated electrodes. 
     However, since the detection device of JP &#39;541 has dedicated electrodes, the cost of the device increases and the device configuration becomes more complex. In addition, the device may not provide sufficient detection accuracy when the humidity around the electrostatic sensor changes. Further, such technique is not capable of detecting a liquid spill condition. 
     SUMMARY 
     In an aspect of the present disclosure, an occupant detection device includes an electrostatic sensor, an occupant determination unit, a reference sensor device, and a determination standard change unit. The electrostatic sensor is disposed in a seat of a vehicle, and has a detection electrode that generates a capacitance with a vehicle body. The reference sensor device is also disposed in the seat, and is arranged such that it is not affected by a liquid. The reference sensor device has the same detection characteristics as the electrostatic sensor, and detects humidity. 
     The occupant determination unit determines the presence of an occupant at a seat based on an output of the electrostatic sensor. The determination standard change unit changes an occupant determination threshold based on an output of the reference sensor device, where the occupant determination threshold is used by the occupant determination unit for determining the presence of the occupant. 
     According to the above configuration, the humidity around the occupant detection device is detected by the reference sensor device, which has the same detection characteristics and level of detection as the electrostatic sensor. The reference sensor device is disposed in the seat in a manner that prevents the reference sensor device from being affected by a liquid. Therefore, the humidity is accurately detected when the electrostatic sensor does and does not have a liquid spread thereon. 
     The occupant determination threshold that is used in the occupant determination unit is changed based on the output of the reference sensor device. Thus, the occupant detection device provides an accurate occupant determination as well as a liquid spill alert to the occupant of the vehicle when a liquid has affected the output of the electrostatic sensor. Accordingly, the occupant detection device accurately detects an occupant of the vehicle without increasing the cost of the device and detects an occupant regardless of the condition of the seat. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present disclosure will become more apparent from the following detailed description disposed with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram of an occupant detection device in an embodiment of the present disclosure; 
         FIG. 2  is an illustration of an electrostatic sensor and a sensor circuit connected thereto of the occupant detection device; 
         FIG. 3  is an illustration of an equivalent circuit that is representative of a detection object; 
         FIG. 4  is an illustration of a phase of an electric current that flows in each electrode of the electrostatic sensor; 
         FIG. 5  is a graph of a sine wave of a signal source and a wave form of an electric current that flows in each of the electrodes of the electrostatic sensor; 
         FIG. 6  is graph of a conductance and a susceptance according to a seat condition; 
         FIG. 7  is a graph of a conductance and a susceptance for an adult, a child, and a vacant seat with a control threshold superposed thereon; 
         FIG. 8  is an illustrative schematic diagram of a sensor circuit at a time of measuring an impedance of the electrostatic sensor; 
         FIG. 9  is an illustrative schematic diagram of the sensor circuit at a time of measuring a conductance of a reference sensor device; 
         FIG. 10  is an illustration of the reference sensor device; 
         FIG. 11  is a side view of the reference sensor device when the reference sensor device is implemented on a wiring board; 
         FIG. 12  is a partial cross-sectional view of the reference sensor device of  FIG. 10  along line XII-XII in  FIG. 10 ; and 
         FIG. 13  is a flowchart of a detection and control method performed by the occupant detection device. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present disclosure is described with reference to the drawings. With reference to  FIG. 1 , an occupant detection device  1  includes an electrostatic sensor  2 , an occupant detection ECU  3 , an airbag ECU  4 , and a passenger seat airbag  5  (P-seat airbag). 
     With reference to  FIG. 2 , the electrostatic sensor  2  includes a seat  42 , a sensor mat  25 , a seat frame  21 , and a back frame  23 . The seat  42  includes a seat bed  20  for accommodating a hip of the occupant and a back portion  22  for accommodating a back of the occupant. The sensor mat  25  is disposed between a seat surface and a seat cushion of the seat bed  20 , and has a guard electrode  26  and a main electrode  27  inserted therein. 
     The seat frame  21  is disposed on a bottom surface of the seat bed  20 . The back frame  23  is disposed at a center of the back portion  22 . The seat frame  21  and the back frame  23  that is connected to the seat frame  21  are conductive to a vehicle body  24 , and are grounded to such body. The guard electrode  26  and the main electrode  27  are respectively connected to a sensor circuit  28  by a connection wiring such as a wire harness, and the sensor circuit  28  forms a part of the occupant detection ECU  3 . 
     Capacitance is formed between the main electrode  27  and the seat frame  21  and between the main electrode  27  and the back frame  22 . An electric line of force is formed based on a sine wave  37  supplied by a signal source Vsg of the sensor circuit  28 . The guard electrode  26  is attached on a lower side of the main electrode  27 , and the guard electrode  26  has the sine wave  37  directly applied thereto with the same phase as a sine wave applied to the main electrode  27 . Therefore, the electric line of force is only generated on an upper side of the main electrode  27  (i.e., the side of the main electrode  27  facing the seat surface on which the occupant sits), and is not generated on the lower side of the main electrode  27 . The capacitance is thus formed between the vehicle body  24  and the main electrode  27 , which serves as a detection electrode of the electrostatic sensor  2 . 
     With reference to  FIGS. 3-5 , an equivalent circuit of the detection object, such as a person or a cup, which is detected by the electrostatic sensor  2 , may be represented by a parallel circuit of a resistor RMX (a real part: a conductance) and a capacitance CMX (i.e., an imaginary part: a susceptance). Therefore, the detection of the detection object by using the electrostatic sensor  2  is actually a detection of an impedance having a real part and an imaginary part. 
     When a signal having the sine wave  37  is applied from the signal source Vsg of the sensor circuit  28  to the electrostatic sensor  2 , an electric current detection resistor Rs in the sensor circuit  28  has a voltage difference generated therein, according to an impedance of the detection object. In such a case, when the impedance of the detection object has the real part only, the generated voltage difference in the electric current detection resistor Rs does not include a phase-advance factor relative to the sine wave  37  of the signal source Vsg. Therefore, when the generated voltage difference in the electric current detection resistor Rs is sampled at a real-part sampling timing  38 , which has the same phase as the sine wave  37 , the result is an output  40  that is solely proportional to the size of the real part. 
     Further, when the impedance of the detection object has the imaginary part only, the generated voltage difference in the electric current detection resistor Rs has a phase-advance factor relative to the sine wave  37  of the signal source Vsg. Therefore, when the generated voltage difference in the electric current detection resistor Rs is sampled at an imaginary-part sampling timing  39  that has a phase advance of  90  degrees relative to the sine wave  37  of the signal source Vsg, the result is an output  41  that is solely proportional to the size of the imaginary part. Since the actual detection object may have both of the real part and the imaginary part, the actually-measured impedance has a phase illustrated in  FIG. 4 . 
     The sensor circuit  28 , which is part of a sensor characteristic measurement unit  12  of the occupant detection ECU  3 , detects an electric line of force of the capacitance that is generated by the detection object sitting on the seat  42  of the electrostatic sensor  2 . For the purpose of measuring an impedance of the electrostatic sensor  2 , the electric line is detected as a voltage difference based on an electric current flowing from the signal source Vsg to the electric current detection resistor Rs of the sine wave  37 . The sensor characteristic measurement unit  12  outputs the measured impedance as an analog signal that has the real part of the measured impedance and the imaginary part of the measured impedance separated from each other. A CPU  13  then converts such analog signal into a digital signal and processes the converted digital signal. In such case, the sensor characteristic measurement unit  12  may also be configured to output the digital signal instead of the analog signal. 
     With continuing reference to  FIG. 1 , the occupant detection device  1  includes a vehicle power source  6  that supplies electric power to a power source  15  of the occupant detection ECU  3 , and a switch  7 . A passenger seat buckle switch  9  is connected to the CPU  13  of the occupant detection ECU  3  through a power supply &amp; detection controller  18 . The passenger seat buckle switch  9  provides information related to the fastening or non-fastening of the buckle by the occupant (i.e., fastening state information). 
     A passenger seat&#39;s position sensor  10  is connected to the CPU  3  of the occupant detection ECU  3  through a power supply &amp; detection controller  19 . The passenger seat position sensor  10  provides position information regarding a front-rear positioning (e.g., a sliding position) of the passenger seat to the airbag ECU  4 . A deployment speed of the passenger seat airbag  5  may be controlled based on the position information. 
     The occupant determination is performed based on a relationship illustrated in  FIG. 6 . The relationship of  FIG. 6  is measured by using a circuit shown in  FIG. 8 , and shows a general load characteristic of an occupant, who is a detection object of the device. The value of the imaginary part and the value of the real part respectively change according to a seat condition regarding dryness, such as (i) a dry condition, (ii) a moisture absorbed and highly humid condition, and (iii) a liquid spill condition. 
     The measurement results by using the sensor characteristic measurement unit  12  are shown in  FIG. 7 . Data  30  shows a sample of an occupant who is an adult female having a small stature. Data  31  shows a sample of an occupant who is a child sitting on a child seat. Data  32  shows a sample of vacancy, i.e., when the passenger seat is not occupied. 
     The CPU  13  performs an occupant determination based on the comparison between the data  30 ,  31 ,  32  and an occupant determination threshold  29  that is stored in a nonvolatile memory  14 . The result of the occupant determination is output to the airbag ECU  4  through communication interface (I/F)  16  or to a breakdown diagnosis  8  through communication interface (I/F)  17 . 
     When the measurement data exceeds the threshold  29 , such as the data  30 , the CPU  13  outputs an airbag deployment signal for the deployment of the passenger seat airbag  5  from the airbag ECU  4 , which is indicated as “A/B ON” in  FIG. 7 . When the measurement data does not exceed the threshold  29 , such as data  31  and data  32 , the CPU  13  does not output the airbag deployment signal for the deployment of the passenger seat airbag  5  from the airbag ECU  4  which is indicated as “A/B OFF” in  FIG. 7 . 
     As described above, the occupant determination relying on one axis, i.e., only on the imaginary part component, is improved by relying on two axes, i.e., on the imaginary part and the real part, which may be further improved by using a reference sensor device  11 . That is, in a specific situation, the occupant determination relying on the two axes may still be difficult, and such a difficulty may be resolved by an improved accuracy that is realized by the use of the reference sensor device  11  under control of the occupant detection ECU  3 . 
     With reference to  FIGS. 10 and 11 , the reference sensor device  11  is implemented on a wiring board  49  of the occupant detection ECU  3 . The reference sensor device  11  has substantially the same configuration and humidity detection characteristic (i.e., the same real part characteristic) as the main electrode  27 . 
     Since the reference sensor device  11  should be in the same humidity environment as the main electrode  27 , the device  11  is disposed in the seat  42 . Therefore, the occupant detection ECU  3  accommodating the reference sensor device  11  does not have a sealed structure, that is, the reference sensor device  11  has a structure that is susceptible to the ambient humidity. However, the ECU  3  has a position setting of the reference sensor device  11 , which does not allow liquid to soak the reference sensor device  11 . 
     Thus, the reference sensor device  11  is easily disposed due to its disposal in the occupant detection ECU  3 , in a space-saving manner. However, the reference sensor device  11  may be disposed separately from the ECU  3  at a position in the seat  42 , which is not susceptible to a liquid spill, and can be connected to the occupant detection ECU  3  through wiring. 
     The reference sensor device  11  has a lead  46  that is inserted into the wiring board  49  via a terminal  45  to which the lead  46  is affixed to. The reference sensor device  11  also has the electrode  47  that is disposed on a tip of the lead  46 , which has an L-bent shape, and a main body  48  that is disposed between two leads  46 . 
     The terminal  45  is made of a material having (C2600+Sn+Au) or the like. With reference to  FIG. 12 , the main body  48  is formed to have a gap G interposed between a silver film  52  and a carbon film  53  that are attached on a main film  50  by using an adhesive  51 , with a cover film  54  covering the carbon film  53 . The size of the gap G is calculated based on the impedance detection characteristic of the electrostatic sensor  2 , and may preferably be a value of 0.5 to 2 mm. The humidity of the environment is detected by having water deposited on the gap G, which leads to the change of a resistance value of the sensor device  11 . Such a structure of the electrode having the carbon film  53  and the gap G is same as the structure of the main electrode  27 . 
     With continuing reference to  FIGS. 10 and 11 , the width W of the electrode is calculated from the impedance detection characteristic of the electrostatic sensor  2 , and may preferably be a value of about 2 mm. The implementation height H may preferably be equal to or greater than 5 to 10 mm, for decreasing the influence of a parasitic capacitance, which may be formed with the wiring board  49 . The parasitic capacitance may also be formed with an object above or beside the main body  48 . Therefore, such object above/beside the main body  48  should be disposed at a distance of at least 5 to 10 mm from the body  48 , if such object is made of metal. The implementation pitch P may preferably be a value of about 20 mm, in consideration of the element implementation density of the wiring board and the deterioration of the implemented parts due to the warpage of such parts. The cover film  53  may be, for example, a PET film having a thickness of 40 micron. 
     With reference to the drawings, the operation of the vehicular occupant detection device that uses the reference sensor device  11  is described. The process of  FIG. 13  is performed by the occupant detection ECU  3 . 
     Firstly, in S 1 , a switch Sm is closed to apply the sine wave  37  to the main electrode  27  through the electric current detection resistor Rs ( FIG. 2 ), thereby enabling the impedance measurement of the electrostatic sensor  2 . At S 2 , a switch Sgn is closed to directly apply the sine wave  37  to the guard electrode  26 , thus enabling the impedance measurement of the electrostatic sensor  2 . 
     Subsequently, at S 3 , the sensor circuit  28  is put in a state shown in  FIG. 8 , and the impedance of the electrostatic sensor  2  is measured. At such moment, the sine wave  37  of the signal source Vsg is directly applied to the guard electrode  26 . In such manner, the main electrode  27  and the guard electrode  26  have the same voltage, and the impedance of the lower side of the main electrode  27  is cancelled. In other words, the impedance of the main electrode  27  towards the guard electrode  26  and seat cushion of the seat bed  20  is cancelled. Therefore, the impedance measurement is enabled only towards the seat surface of the seat bed  20  (i.e., the upper side of the main electrode  2 ), thereby allowing detection of an occupant on the seat  42 . 
     At S 4 , the switch Sm is opened to stop the impedance measurement of the electrostatic sensor  2 . The switch Sgn is opened, at S 5 , to stop the impedance measurement of the electrostatic sensor  2 . 
     The conductance measurement of the reference sensor device  11  is then enabled by closing a switch Ss to apply the sine wave  37  to the reference sensor device  11  through the electric current detection resistor Rs at S 6 . The conductance measurement of the reference sensor device  11  is enabled by closing a switch Esg at S 7 . Subsequently, the sensor circuit  28  is put in a state shown in  FIG. 9 , and the conductance of the reference sensor device  11  is measured at S 8 . The switch Ss is then opened to stop the conductance measurement of the reference sensor device  11  at S 9 . The switch Esg is opened, at S 10 , to stop the conductance measurement of the reference sensor device  11 . 
     The real part of the impedance value (i.e., the conductance) of the electrostatic sensor  2  measured at S 3  and the conductance value of the reference sensor device  11  measured at S 8  are compared with each other by the CPU  13  at S 11 . When the difference between both values is comparatively small or when the conductance value of the reference sensor device  11  measured at S 8  is less than a first predetermined value, the electrostatic sensor  2  is determined to be in a dry condition. The occupant determination is then performed based on the threshold  29  in  FIG. 7  at S 13 , and the occupant determination result is provided to the airbag ECU  4  at S 14 . 
     When the conductance value of the reference sensor device  11  measured at S 8  is greater than a second predetermined value, which is greater than the first predetermined value, the electrostatic sensor  2  and its environment is determined to have an increased humidity level (i.e., highly humid). Accordingly, the imaginary part of the detection value from the electrostatic sensor  2  is corrected, or the threshold  29  is corrected at S 12 . A corrected threshold that is derived by correcting the threshold  29  of  FIG. 7  is used to perform the occupant determination at S 13 , and the occupant determination result is provided to the airbag ECU  4  (S 14 ). Such a determination may be performed by, for example, correcting (e.g., increasing or decreasing) the value of the threshold  29  to be closer to one of data values among the data  30 ,  31  of  FIG. 7 , which is higher than the other, since the higher one of the data  30 ,  31  is distant from the threshold  29 . By performing such a correction, the occupant determination in a high humidity condition has an improved accuracy. 
     Also, if the conductance value of the reference sensor device  11  measured at S 8  is less than the second predetermined value but greater than the first predetermined value, the electrostatic sensor  2  and its environment is determined to be humid, and the threshold  29  or the imaginary part of the detection value from the electrostatic sensor  2  is corrected. When the electrostatic sensor  2  has a liquid spill thereon, the conductance value of the electrostatic sensor  2  takes an extremely high value, thereby resulting in an extremely great difference between the conductance of the electrostatic sensor  2  measured at S 3  and the conductance of the reference sensor device  11  measured at S 8 . When the difference between the conductance of the electrostatic sensor  2  and the conductance of the reference sensor device  11  exceeds a third predetermined value, the electrostatic sensor  2  is determined to have a liquid spilled thereon (i.e., a liquid spill condition) and S 12  is performed. At S 12 , the imaginary part of the detection value of the electrostatic sensor  2  is corrected (e.g., increasing or decreasing per  FIG. 7 ), or the threshold  29  is corrected. At such moment, a spill alert may be provided to the occupant from the device. 
     The corrected threshold that is derived by correcting the threshold  29  of  FIG. 7  is used to perform the occupant determination at S 13 , and the occupant determination result is provided to the airbag ECU  4  (S 14 ). In such manner, as readily understood from  FIG. 6  and  FIG. 7 , the liquid spill is clearly and unambiguously determined, and the accuracy of the occupant determination is improved, as well as enabling the abnormality warning. 
     The first, second, and third predetermined values serve as a determination setting standard for determining the correct setting of the threshold  29 , which is to be used in the occupant determination performed at S 13 . In addition, the first, second, and third predetermined values may be appropriately determined according to the characteristics of the sensor mat  25  and the reference sensor device  11 , and may be stored in the nonvolatile memory  14 . 
     The sensor circuit  28  includes the switch Ss, a switch Sg, the switch Sm, a switch Ssn, the switch Sgn, a switch Smn, and the switch Esg. These switches are used to switch the object electrodes to be measured, and are also used to perform the breakdown diagnosis of the occupant detection device. 
     From the above description, the occupant detection device  1  in the present embodiment enables the detection of the humidity of the environment by the reference sensor device  11 , which is performed with the same level of detection characteristics as the detection by the electrostatic sensor  2 , and the reference sensor device  11  is disposed in the seat  42  in a manner that avoids the liquid spill. Therefore, the occupant detection device  1  can accurately detect the humidity of the environment when the electrostatic sensor  2  does not have the liquid spill condition, and can change the occupant determination threshold  29  that is used by an occupant determination (S 13 ) based on the output of the reference sensor device  11 . Thus, the occupant detection device  1  enables an accurate occupant determination, as well as providing a liquid spill alert for alerting the occupant of the vehicle about the spill of liquid over the electrostatic sensor  2 . The occupant determination performed at S 13  may be referred to as an occupant determination unit. 
     Further, since the reference sensor device  11  is used for the measurement of only the real part, which is different from how the main electrode  27  is used, the reference sensor device  11  has a simple structure, with fewer design restrictions. Therefore, the production cost of the vehicular occupant detection device  1  is decreased. 
     Since the reference sensor device  11  can be disposed in the occupant detection ECU  3 , such configuration leads to a space saving of the vehicular occupant detection device  1 . In addition, since the reference sensor device  11  has two electrodes facing each other with the gap G interposed therebetween, and outputs the capacitance between those electrodes, the reference sensor device  11  can perform the humidity detection with the same level of detection characteristics as the detection performed by the electrostatic sensor  2 . 
     Further, since the reference sensor device  11  upholds the film having the above electrodes formed thereon by using the lead  46  above the wiring board  49  at a predetermined height from the surface of the board  49  in a gap-reserving manner, i.e., separately from the surface of the board  49 , the influence of the parasitic capacitance, which may be formed with the wiring board  49 , is avoided. 
     Further, a determination standard change unit (i.e., provided as the process of S 11  and S 12  of  FIG. 13 ) changes the occupant determination threshold when the conductance value of the reference sensor device  11  exceeds a predetermined conductance value, the occupant determination threshold can securely be changed for the accurate occupant determination when the humidity of the environment is increased. 
     Further, since the determination standard change unit changes the occupant determination threshold when the difference between the conductance value of the electrostatic sensor  2  and the conductance value of the reference sensor device  11  exceeds a predetermined value, the occupant determination standard can securely be changed for the accurate occupant determination when the humidity of the environment is increased. 
     Further, since the determination standard change unit can correct the imaginary part of the output of the electrostatic sensor  2 , the accurate occupant determination can be performed without changing the occupant determination threshold  29  even when the humidity of the environment is increased. 
     Further, since (i) the occupant determination unit  3  (i.e., S 13 ) is configured to determine an occupant on the seat  42  based on a comparison between the output of the electrostatic sensor  2  and the occupant determination threshold  29  and (ii) the determination standard change unit corrects the occupant determination threshold  29 , an accurate occupant determination can be performed without changing the imaginary part of the output of the electrostatic sensor  2  even when the humidity of the environment is increased. 
     Although the present disclosure has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art, and such changes and modifications are to be understood as being within the scope of the present disclosure as defined by the appended claims.