Abstract:
A gas sensor is provided with a detection element, a circuit board, and a housing that houses the detection element and the circuit board. The housing has a first fastening section, a second fastening section, and a gas introducing port. The center of the gas introducing port is positioned on a virtual line connecting the first fastening section and the second fastening section, and is disposed corresponding to a corner section of the circuit board, said corner section being close to the first fastening section side.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a gas sensor including a detection element configured to contact a gas as a detection target, a circuit board for supporting the detection element and obtaining information of the contacted gas, and a casing containing the detection element and the circuit board. 
       BACKGROUND ART 
       [0002]    For example, a solid polymer electrolyte fuel cell has a membrane electrode assembly (MEA). The membrane electrode assembly includes a polymer ion exchange membrane as an electrolyte membrane, and an anode and a cathode provided on both sides of the electrolyte membrane. The membrane electrode assembly is sandwiched between separators (bipolar plates) to form a power generation cell. In the fuel cell, normally, a predetermined number of the power generation cells are stacked together to form an in-vehicle fuel cell stack mounted in a fuel cell vehicle (fuel cell electric automobile), for example. 
         [0003]    In the fuel cell vehicle, in particular, it is required to monitor leakage of hydrogen as a fuel gas. For this purpose, for example, gas sensors for use of hydrogen detection have been used. As a gas sensor, normally, a contact combustion type hydrogen sensor or a heat conductivity type hydrogen sensor has been used. Normally, the heat conductivity type hydrogen sensor detects hydrogen concentration and humidity by electrically detecting a change in the temperature of the heat emitting element (resistive element) caused by the difference in hydrogen heat conductivity, as a change in the resistance value of the temperature detection element. 
         [0004]    As techniques of this type, for example, a gas sensor disclosed in Japanese Patent No. 4165300 and a gas detection apparatus disclosed in Japanese Laid-Open Patent Publication No. 2013-221862 are known. 
       SUMMARY OF INVENTION 
       [0005]    In this regard, a gas sensor used in a fuel cell vehicle needs to be placed in a limited, narrow and small space, and thus it is desirable to downsize such a gas sensor. However, in Japanese Patent No. 4165300 and Japanese Laid-Open Patent Publication No. 2013-221862, the size of the gas sensor cannot be reduced effectively. 
         [0006]    The present invention has been made to solve the problem of this type, and an object of the present invention is to provide a gas sensor having the desired detection function where the structure of the gas sensor is simplified easily, and size reduction of the gas sensor is achieved. 
         [0007]    The gas sensor according to the present invention includes a detection element configured to contact a gas as a detection target, a polygonal circuit board configured to support the detection element and obtain information of the contacted gas, and a casing containing the detection element and the circuit board. 
         [0008]    The casing includes a first fastening section and a second fastening section fastened to a gas sensor mounting position and a gas inlet port configured to allow the gas to flow into the gas inlet port to contact the detection element. A center of the gas inlet port is positioned on a virtual line connecting the first fastening section and the second fastening section, and the gas inlet port is provided at a corner of the circuit board adjacent to the first fastening section. 
         [0009]    In the present invention, since the gas contacts the detection element, the center of the gas inlet port for allowing entry of the gas is positioned on the virtual line connecting the center of the first fastening section and the center of the second fastening section. In the structure, it becomes possible to maintain the surface pressure of the gas inlet port at the desired level, and maintain the desired sealing function. 
         [0010]    Further, the center of the gas inlet port is positioned at the corner of the circuit board adjacent to the first fastening section. Therefore, since the gas inlet port is positioned in a dead space at the corner of the circuit board, the area of the dead space in the circuit board is reduced, and size reduction of the circuit board is achieved. 
         [0011]    Accordingly, the gas sensor has the desired function of detecting the hydrogen concentration, and it becomes possible to simplify the overall structure of the gas sensor and reduce the overall size of the gas sensor. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a perspective view showing a state where a gas sensor according to a first embodiment of the present invention is mounted; 
           [0013]      FIG. 2  is a cross sectional view showing the gas sensor, taken along a line II-II in  FIG. 1 ; 
           [0014]      FIG. 3  is a plan view showing the gas sensor; 
           [0015]      FIG. 4  is a plan view showing a circuit board and a detection element of the gas sensor; 
           [0016]      FIG. 5  is a cross sectional view showing a gas sensor according to a second embodiment of the present invention; 
           [0017]      FIG. 6  is a cross sectional view showing a gas sensor according to a third embodiment of the present invention; and 
           [0018]      FIG. 7  is a plan view showing a circuit board and a detection element of the gas sensor. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    A gas sensor  10  according to a first embodiment of the present invention shown in  FIGS. 1 to 3  is a heat conductivity type hydrogen sensor. For example, the gas sensor is mounted in a fuel cell electric vehicle (not shown), and used for detecting leakage of a hydrogen gas (fuel gas). 
         [0020]    The gas sensor  10  includes a casing  14  fixed to a mounting section (gas sensor mounting position)  12  of the vehicle. A detection element  16  and a circuit board  18  are embedded (contained) in the casing  14 . The detection element  16  contacts hydrogen (gas) as a target of detection, and the detection element  16  is supported on the circuit board  18 . The circuit board  18  obtains concentration (information) of the hydrogen which contacts the detection element  16 . 
         [0021]    For example, the detection element  16  is a heat emitting element (heat emitting resistor) having a circular disk shape. When the detection element  16  contacts the hydrogen, the temperature of the detection element  16  changes depending on heat conductivity of the hydrogen. A power supply (not shown) for supplying electric current and a voltmeter (not shown) for measuring the voltage applied to both terminals of the detection element  16  are connected to the detection element  16 . The resistance value of the detection element  16  changes according to the change in its temperature. The hydrogen concentration can be detected by measuring the voltage. 
         [0022]    As shown in  FIG. 4 , the circuit board  18  has a quadrangular (polygonal) shape, e.g., rectangular shape in a plan view. Illustration of the circuit pattern provided in the circuit board  18  is omitted. A detection element  16  is attached to (supported on) a position adjacent to one corner  18   a  of the circuit board  18  through a heat insulating case  20 . 
         [0023]    For example, the heat insulating case  20  is formed using a heat insulating member of ceramic, glass, etc. The heat insulating case  20  has a circular disk shape, and a recess  22  is formed at the center of the heat insulating case  20  for placing the detection element  16  in the recess  22 . The heat insulating case  20  is directly joined to the circuit board  18  by adhesion, welding, etc., and the detection element  16  and the circuit board  18  are electrically connected through a plurality of connecting wires  24 . 
         [0024]    As show in  FIGS. 1 to 3 , for example, parts of the casing  14  are formed integrally using a resin member. The detection element  16  and the circuit board  18  are embedded in the casing  14  by insert molding. The casing  14  has a quadrangular (polygonal) shape, e.g., rectangular shape in a plan view. The casing  14  has a first fastening section  26   a  and a second fastening section  26   b  fastened to the mounting section  12  of the vehicle, and a gas inlet port  28 . Hydrogen is supplied into the gas inlet port  28  for allowing the hydrogen to contact the detection element  16 . 
         [0025]    The first fastening section  26   a  has a hole  30   a  and the second fastening section  26   b  has a hole  30   b.  Bolts  32  are inserted into the holes  30   a,    30   b,  and screwed into the mounting section  12  of the vehicle for attaching the casing  14  to the mounting section  12  of the vehicle. A curved R-shaped portion  28   a  is formed in an end of an inner wall surface of the gas inlet port  28  that is positioned opposite to the detection element  16 . Instead of the R-shaped portion  28   a,  a chamfered (taper cut) portion may be adopted. 
         [0026]    As shown in  FIG. 3 , the center O 1  of the gas inlet port  28  is positioned on a virtual line L connecting the center O 2  of the first fastening section  26   a  and the center O 3  of the second fastening section  26   b,  and provided at a corner  18   a  of the circuit board  18  adjacent to the first fastening section  26   a.  Specifically, the first fastening section  26   a  and the second fastening section  26   b  are provided at diagonal positions on opposite sides, i.e., a first side (one short side)  34   a  and a second side (the other short side)  34   b  of the casing  14 , respectively. 
         [0027]    In the first embodiment, the first side  34   a  or the second side  34   b  has a connector insertion slot  36 , and a connector (not shown) for supplying electricity to the gas sensor  10  is inserted into the connector insertion slot  36 . The connector insertion slot  36  is formed adjacent to an end of the second side  34   b,  oppositely to an end of the second side  34   b  where the second fastening section  26   b  is provided. 
         [0028]    Operation of this gas sensor  10  will be described below. 
         [0029]    The gas sensor  10  is attached to the mounting section  12  of the vehicle, and when significant amount of the hydrogen is accumulated along the mounting section  12  of the vehicle as a ceiling plate, the position of the hydrogen border (hydrogen level) is lowered. When the hydrogen level goes down to a lower position of the casing  14 , the hydrogen enters the gas inlet port  28  of the casing  14 . Consequently, the hydrogen contacts the detection element  16 , and hydrogen concentration is detected. 
         [0030]    In the first embodiment, as shown in  FIG. 3 , since the hydrogen contacts the detection element  16 , the center O 1  of the gas inlet port  28  for allowing entry of the hydrogen is positioned on the virtual line L connecting the center O 2  of the first fastening section  26   a  and the center O 3  of the second fastening section  26   b.  In the structure, it becomes possible to maintain the surface pressure of the gas inlet port  28  at the desired level, and maintain the desired sealing function. 
         [0031]    Further, the center O 1  of the gas inlet port  28  is positioned at the corner  18   a  of the circuit board  18  adjacent to the first fastening section  26   a.  In the structure, since the gas inlet port  28  is positioned in a dead space at the corner  18   a  of the circuit board  18 , the area of the dead space of the circuit board  18  is reduced, and size reduction of the circuit board  18  is achieved. 
         [0032]    Accordingly, the gas sensor  10  has the desired function of detecting the hydrogen concentration, and it becomes possible to simplify the overall structure of the gas sensor  10  and reduce the overall size of the fuel gas sensor  10 . 
         [0033]    Further, in the first embodiment, the casing  14  has a quadrangular (polygonal) shape, e.g., rectangular shape in a plan view. The first fastening section  26   a  and the second fastening section  26   b  are provided on opposite sides, i.e., on the first side  34   a  and the second side  34   b  of the casing  14 , respectively. The connector insertion slot  36  is formed adjacent to the end of the second side  34   b,  oppositely to the end of the second side  34   b  where the second fastening section  26   b  is provided. 
         [0034]    In the structure, the second side  34   b  has the second fastening section  26   b  and the connector insertion slot  36 . Therefore, size reduction of the gas sensor  10  is achieved easily, and the mounting position of the gas sensor  10  can be determined freely. 
         [0035]    Moreover, the second fastening section  26   b  and the connector insertion slot  36  protrude in the same direction (indicated by an arrow t in  FIG. 3 ) from the second side  34   b.  Therefore, the dimension of the casing  14  in a direction indicated by an arrow h is reduced as much as possible, and size reduction of the casing  14  is achieved easily. Further, the first side  34   a  only has the first fastening section  26   a.  Thus, the gas sensor  10  has an asymmetrical shape, and it is possible to reliably prevent assembling mistakes. 
         [0036]    Further, the connector insertion slot  36  is provided on the second side  34   b  together with the second fastening section  26   b  which is provided at a relatively large distance from the gas inlet port  28 . In the structure, since the detection element  16  is provided in consideration of the flow of electric current on the circuit board  18  (see the flow direction of electric current in  FIG. 3 ), the circuit pattern is simplified, and size reduction of the circuit board  18  is achieved effectively. 
         [0037]    Moreover, as shown in  FIG. 2 , the casing  14  includes a flat fastening surface  14 S facing the mounting section  12  of the vehicle. Therefore, the distance between the fastening surface  14 S and the detection element  16  is reduced suitably, and detection of the hydrogen accumulated on the ceiling plate is performed rapidly. Further, the curved R-shaped portion  28   a  is formed at the inner end of the inner wall surface of the gas inlet port  28  opposite to the detection element  16 . In the structure, the water is not retained on the wall surface by its surface tension, and it becomes possible to suppress influence on the gas sensor  10  due to the humidity. 
         [0038]    Further, the detection element  16  is placed in the recess  22  of the heat insulating case  20 . Therefore, the heat of the detection element  16  is not emitted to the circuit board  18 , and it is possible to suppress decrease in the detection accuracy. Moreover, it becomes possible to reduce the distance between the detection element  16  and the circuit board  18 . Reduction in the overall thickness and size of the gas sensor  10  is achieved easily. 
         [0039]      FIG. 5  is a cross sectional view showing a gas sensor  40  according to a second embodiment of the present invention. The constituent elements that are identical to those of the gas sensor  10  according to the first embodiment are labeled with the same reference numerals and description thereof is omitted. Further, also in a third embodiment described later, description of the constituent elements that are identical to those of the gas sensor  10  according to the first embodiment is omitted. 
         [0040]    In the gas sensor  40 , an explosion proof filter is directly connected to the heat insulating case  20 . For example, the explosion proof filter  42  comprises a metal mesh or a porous body. Instead of the explosion proof filter  42 , or in combination with the explosion proof filter  42 , a water repellent filter (not shown) may be used. The water repellent filter is a hydrogen permeable filter, and is not a liquid (water droplet) permeable filter. 
         [0041]    In the second embodiment, the same advantages as in the case of the first embodiment are obtained, and additionally, an explosion proof function (and a water repellent function) can be provided. 
         [0042]      FIG. 6  is a cross sectional view showing a gas sensor  50  according to the third embodiment of the present invention. 
         [0043]    The gas sensor  50  includes a casing  52 , and a circuit board  54  is embedded (contained) in the casing  52 . As shown in  FIG. 7 , the circuit board  54  has a quadrangular (polygonal) shape, e.g., rectangular shape in a plan view. A detection element  56  is formed integrally with the circuit board  54 , at a position adjacent to one corner  54   a  of the circuit board  54 . 
         [0044]    The detection element  56  directly forms an element pattern in the circuit board  54 . A plurality of, e.g., four heat insulating holes  58  extend through the circuit board  54  around the detection element  56 . The heat insulating holes  58  are formed for preventing heat emission from the detection element  56  to the circuit board  54 . 
         [0045]    In the third embodiment, it is not required to provide a detection element and a heat insulating case separately from the circuit board  54 . Therefore, it becomes possible to reduce the thickness (size) of the gas sensor  50  as much as possible. Additionally, in the third embodiment, the same advantages as in the case of the first embodiment are obtained.