Abstract:
The purpose of this invention obtain a physical-quantity detection device, the external shape of the housing of which can be reduced in size. Said physical-quantity detection device, which detects a plurality of physical quantities of a gas being measured that flows through a main channel, is characterized by having a housing positioned inside said main channel, a circuit board insert-molded into said housing, and a plurality of detection sensors mounted on both sides of the circuit board.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a physical quantity detection device which detects a physical quantity of an intake air of an internal combustion engine. 
       BACKGROUND ART 
       [0002]    In an automobile field, a regulation on a fuel efficiency, an exhaust gas (mainly nitrogen oxides (NOx), and particulate matters (PM)) becomes tighter every year, and a number of new control schemes for satisfying the regulation are considered as a future control on an internal combustion engine. Among them, a physical quantity detection device used in various types of controls becomes diverse. In particular, physical quantities such as an air, temperature, humidity, and pressure in an intake pipe for the connection to a combustion chamber of the internal combustion engine are necessarily measured with accuracy because the quantities directly influence on the fuel efficiency and an exhaust gas. 
         [0003]    The regulation on the fuel efficiency, CO 2 , and the exhaust gas are calculated and regulated on the basis of a running cycle represented as NEDC of Europe. For a future regulation, not only the regulation values but also a running cycle condition and an on-vehicle diagnosis system (OBD) regulation value are changed. 
         [0004]    Presently, the physical quantity detection device to be inserted in the intake pipe generally measures mass flow rate, pressure, and temperature. However, a control using an absolute humidity (an amount of moisture in the air) is received a lot of attention as an internal combustion control in view of the above situation. 
         [0005]    Since the moisture in the air affects a flame spread time in a combustion control of the internal combustion engine, a gasoline engine is degraded in combustion efficiency for example. In addition, there is known an influence of an emission increase of PM in a diesel engine as the combustion temperature is lowered. 
         [0006]    Herein, the absolute humidity indicates an amount of moisture contained in the air (g gram/kg kilogram), and can be calculated from a temperature, a relative humidity, and a pressure in the air. On the other hand, the relative humidity indicates a ratio (% percent) of the amount of moisture in the air. 
         [0007]    As described above, temperature and pressure sensors are used in the automobile field for a long time, but a sensor for measuring the relative humidity in the air flowing in the intake pipe are not much known. Presently, there are disclosures that the humidity sensor is integrally configured to an air flow rate detection device in the automobile field (see PTLs 1 to 3). 
         [0008]    The air flow rate detection devices disclosed in PTLs 1 and 2 are integrated with an air flow rate sensor, a humidity sensor, and a pressure sensor. The air flow rate sensor is positioned in a bypass passage through which the air flowing in a main air passage (simply referred to as intake pipe) is taken, and is disposed in a terminal member formed of a metal material. The humidity sensor is positioned in a second bypass passage through which the air flowing in the bypass passage is taken, and mounted in an electronic printed-circuit board. Finally, the pressure sensor is disposed in a housing member. In other words, the respective physical quantity detection sensors are disposed in different members. 
       CITATION LIST 
     Patent Literature 
       [0009]    PTL 1: JP 2010-43883 A 
         [0010]    PTL 2: JP 2012-163505 A 
         [0011]    PTL 3: JP 2013-36892 A. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0012]    In recent years, various technical improvements are achieved in the automobile field in order to improve an exhaust gas regulation, safety, comfort, and convenience in addition to fundamental performances of the vehicle. Thus, a wide variety of sensors are used for the technical improvement. Therefore, the number of wire harnesses connecting the sensors and an engine control unit (hereinafter, referred to as ECU) is also increased and complicated. Thus, there is a problem in a cost viewpoint and a space viewpoint in the engine room. Therefore, there is currently increasing demand for the physical quantity detection device in which the plurality of sensors and the control machine are integrated. The number of wire harnesses and a miniaturization are expected through the integration. 
         [0013]    In the air flow rate detection device disclosed in PTLs 1 to 3, the air flow rate sensor, the pressure sensor, and the humidity sensor are disposed in different members and disposed in consideration of performance of each sensor, but there is room for improvement in size of a casing (hereinafter, referred to as housing). 
         [0014]    First, the air flow rate detection device is disposed in the intake pipe used for the connection to the combustion chamber of the internal combustion engine, and a measurement unit of the housing where the sensor is disposed is mounted to be exposed in the intake pipe. Therefore, the housing causes a pressure loss with respect to the air in the intake pipe. In other words, when the size of the housing is increased, the pressure loss is increased, the amount of air introduces to the combustion chamber is reduced. An engine output is obtained by converting heat energy generated by a chemical reaction between the fuel and the air into kinetic energy. Therefore, a reduction of a maximum air flow rate in the combustion chamber caused by the pressure loss results in a reduction of the engine output. An increase of the pressure loss together with the maximum air flow rate influences even on a minimum air flow rate which can flow in the combustion chamber. In other words, the measurement accuracy in an ultra-low flow rate will be required for the air flow rate detection device in the future as the engine is miniaturized and also a bore diameter of the intake pipe is reduced. 
         [0015]    In the air flow rate detection device, a flange and a connector which are formed integrally to the housing and fixedly supported to the intake pipe are exposed in the engine room while not being exposed in the intake pipe. The engine room is configured by an engine hood and a vehicle body, and various engine components are disposed therein. It is expected that the space is reduced still more in the future due to a miniaturization of the engine and a protection standard of a pedestrian head in recent years. Among them, the integration of the plurality of sensors in the air flow rate detection device is essentially considered for the size of the housing. 
         [0016]    The invention has been made in view of the above problems, and an object thereof is to provide a physical quantity detection device in which the exterior of the housing can be miniaturized. 
       Solution to Problem 
       [0017]    In the invention, a configuration described in claims will be employed for example in order to solve the problems. A physical quantity detection device according to the present invention detects a plurality of physical quantities of a measuring target gas flowing in a main passage, and includes: a housing that is disposed in the main passage; a printed circuit board that is formed to be inserted in the housing; and a plurality of detection sensors that are mounted on one surface and the other surface of the printed circuit board. 
       Advantageous Effects of Invention 
       [0018]    According to the invention, the printed circuit board can be miniaturized by disposing a plurality of physical quantity detection sensors using both surfaces of the electronic printed-circuit board. In other words, a casing part of the physical quantity detection device can also be miniaturized along the miniaturization of the printed circuit board, resulting in securing the space of the engine room and reducing the pressure loss in the intake pipe which have been the problems. Further, advantages, configurations, and effects other than the above description will be cleared through the descriptions of the following embodiments. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a system diagram illustrating an embodiment in which a physical quantity detection device according to the invention is used in an internal combustion engine control system. 
           [0020]      FIG. 2  is a front view of the physical quantity detection device. 
           [0021]      FIG. 3  is a rear view of the physical quantity detection device. 
           [0022]      FIG. 4  is a left side view of the physical quantity detection device. 
           [0023]      FIG. 5  a right side view of the physical quantity detection device. 
           [0024]      FIG. 6  is a bottom view of the physical quantity detection device. 
           [0025]      FIG. 7  is a front view illustrating a state where a front cover is removed from the physical quantity detection device. 
           [0026]      FIG. 8  is a rear view illustrating a state where a rear cover is removed from the physical quantity detection device. 
           [0027]      FIG. 9  is a cross-sectional view taken along arrow A-A of  FIG. 7 . 
           [0028]      FIGS. 10( a ) and 10( b )  are diagrams for describing a configuration of the front cover. 
           [0029]      FIGS. 11( a ) and 11( b )  are diagrams for describing a configuration of the rear cover. 
           [0030]      FIGS. 12( a ) and 12( b )  are diagrams for describing a structure of a sensor chamber, in which  FIG. 12( a )  is an enlarged view of the sensor chamber, and  FIG. 12( b )  is a cross-.sectional view taken along a line D-D of  FIG. 12( a ) . 
           [0031]      FIGS. 13( a ) and 13( b )  are diagrams for describing a structure of a sensor chamber according to another embodiment, in which.  FIG. 13( a )  is an enlarged view of the sensor chamber, and  FIG. 13( b )  is a cross-sectional view taken along a line E-E of  FIG. 13( a ) . 
           [0032]      FIGS. 14( a ) and 14( b )  are diagrams for describing a structure of a sensor chamber according to another embodiment, in which  FIG. 14( a )  is an enlarged view of the sensor chamber, and  FIG. 14( b )  is a cross-sectional view taken along a line F-F of  FIG. 14( a ) . 
           [0033]      FIG. 15  is a diagram for describing inputs/outputs of the physical quantity detection device. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]    Embodiments for implementing the invention (hereinafter, referred to as embodiments) to be described below solve various problems, which is desired for an actual product, and particularly solve various problems which should be required when a physical quantity of an intake air of a vehicle is used as a detection device, and various effects can be achieved. One of the problems to be solved by the following embodiments is the content shown in a column of “Technical Problem”. In addition, one of the effects to be achieved by the following embodiments is the effect shown in a column of “Advantageous Effects of Invention”. The various problems to be solved by the following embodiments and also the various effects to be achieved by the following embodiments will be described in the explanation of the embodiments. Therefore, the effects and the problems to be solved by the embodiments are described even in the content other than the content of the columns of “Technical Problem” and “Advantageous Effects of Invention”. 
         [0035]    In the following embodiments, the same symbols indicate the same configuration even in the different drawings, and draw the same operational effect. The configurations described already will be assigned only with the symbols in the drawings, and the descriptions thereof will not be repeated. 
         [0036]    1. Embodiment of Physical Quantity Detection Device According to Invention Used in Internal Combustion Engine Control System 
         [0037]      FIG. 1  is a system diagram illustrating an embodiment in which a physical quantity detection device according to the invention is used in an internal combustion engine control system in which the fuel is injected in an electronic manner. The intake air is sucked as a measuring target gas  30  from an air cleaner  122  on the basis of an operation of an internal combustion engine  110  which is provided with an engine cylinder  112  and an engine piston  114 , and is guided through a main passage  124  (for example, an intake body, a throttle body  126 , and an intake manifold  128 ) to a combustion chamber of the engine cylinder  112 . 
         [0038]    A physical quantity of the measuring target gas (intake air)  30  guided to the combustion chamber is detected by a physical quantity detection device  300  according to the invention. The fuel is supplied by a fuel injection valve  152  on the basis of the detected physical quantity, and is guided to the combustion chamber in a state of being mixed with the intake air  20 . Further, in this embodiment, the fuel injection valve  152  is provided in an intake port of the internal combustion engine. The fuel injected to the intake port forms a mixed gas together with the measuring target gas (intake air)  30 , guided through an intake valve  116  to the combustion chamber, and combusted to generate mechanical energy. 
         [0039]    In recent years, a method of using the fuel injection valve  152  mounted in a cylinder head of the internal combustion engine to directly injecting the fuel from the fuel injection valve  152  into each combustion chamber is employed in many vehicles as a method excellent in exhaust gas purification and in fuel efficiency. The physical quantity detection device  300  can use not only the method of injecting the fuel to the intake port of the internal combustion engine illustrated in  FIG. 1  but also similarly the method of directly injecting the fuel into each combustion chamber. Both methods are substantially common in basic concept of a method of measuring control parameters including a method of using the physical quantity detection device  300  and a method of controlling the internal combustion engine including a fuel supply amount and an ignition time. As a representative of both methods, a method of injecting the fuel to the intake port is illustrated in  FIG. 1 . 
         [0040]    The fuel and the air guided into the combustion chamber come into a fuel-air mixed state, and explosively combusted by spark ignition of an ignition plug  154 , so that mechanical energy is generated. The combusted gas is guided from an exhaust valve  118  to an exhaust pipe, and discharged as an exhaust gas  24  from the exhaust pipe to the outside of the vehicle. A flow rate of the measuring target gas (intake air)  30  guided into the combustion chamber is controlled by a throttle valve  132  of which the opening is changed on the basis of an operation of an accelerator pedal. The fuel supply amount is controlled on the basis of the flow rate of the intake air guided into the combustion chamber, and a driver controls the opening of the throttle valve  132  to control the flow rate of the intake air guided into the combustion chamber, so that the mechanical energy generated by the internal combustion engine can be controlled. 
         [0041]    1.1 Outline of Control of Internal Combustion Engine Control System 
         [0042]    The physical quantities such as a flow rate, a temperature, a humidity, and a pressure of the measuring target gas (intake air)  30  taken in from the air cleaner  122  and flowing in the main passage  124  are detected by the physical quantity detection device  300 . An electric signal indicating the physical quantity of the intake air is input from the physical quantity detection device  300  to a control device  200 . In addition, the output of the throttle angle sensor  144  for measuring the opening of the throttle valve  132  is input to the control device  200 , and also the output of a rotation angle sensor  146  is input to the control device  200  to measure positions and states of the engine piston  114 , the intake valve  116 , and the exhaust valve  118  of the internal combustion engine together with a rotation speed of the internal combustion engine. The output of an oxygen sensor  148  is input to the control device  200  to measure the state of a mixture ratio of the amounts of fuel and air from the state of the exhaust gas  24 . 
         [0043]    The control device  200  calculates a fuel injection amount and an ignition timing on the basis of a physical quantity of the intake air (the output) of the physical quantity detection device  300  and the rotation speed of the internal combustion. engine measured from the output of the rotation angle sensor  146 . On the basis of these calculation results, the fuel amount supplied from the fuel injection valve  152  and the ignition timing when the ignition plug  154  ignites are controlled. In practice, the fuel supply amount and the ignition timing are finely controlled on the basis of a state of changes in temperature and throttle angle detected by the physical quantity detection device  300 , a stage of change in rotation speed of the engine, and a state of fuel ratio measured by the oxygen sensor  148 . The control device  200  controls the amount of air bypassing the throttle valve  132  using an idle air control valve  156  in an idle operation mode of the internal combustion engine, and controls the rotation speed of the internal combustion engine in the idle operation mode. 
         [0044]    1.2 Importance of Detection Accuracy Improvement of Physical Quantity Detection Device and Installation Environment of Physical Quantity Detection Device 
         [0045]    Both the fuel supply amount and the ignition timing, which are primary control quantities in the internal combustion engine, are calculated on the basis of the output of the physical quantity detection device  300  as main parameters. Therefore, improvement of detection accuracy of the physical quantity detection device  300 , suppression of a change with time, and improvement of reliability are important for the improvement of control accuracy of the vehicle and for the securement of reliability. 
         [0046]    Particularly, in recent years, a request for fuel saving of the vehicle is significantly increased, and a request for the exhaust gas purification is sign significantly increased. In response to these requests, it is extremely important to increase the detection accuracy of the physical quantity of the intake air detected by the physical quantity detection device  300 . In addition, it is also important to keep a high reliability of the physical quantity detection device  300 . 
         [0047]    The vehicle equipped with the physical quantity detection device  300  is used under an environment having a large temperature variation, and also in the middle of rainy or snowy weather. In a case where an automobile runs on a snowy road, it becomes a running on a road sprinkled with an antifreezing agent. A countermeasure against the change in temperature in such an environment and against dust and contaminants are also desirably taken into consideration for the physical quantity detection device  300 . Furthermore, the physical quantity detection device  300  is installed under an environment where the internal combustion engine vibrates. A high level of reliability is also required with respect to the vibration. 
         [0048]    In addition, the physical quantity detection device  300  is mounted in an intake pipe which is affected by heating from the internal combustion engine. Therefore, the heat of the internal combustion engine is transferred onto the physical quantity detection device  300  through the intake pipe that is the main passage  124 . The physical quantity detection device  300  is to detect the flow rate of the measuring target gas  30  through the heat transmission with respect to the measuring target gas  30 , and thus it is important to suppress an influence of heat from the outside as much as possible. 
         [0049]    The physical quantity detection device  300  mounted in the vehicle is provided not only simply to solve the problem described in the column of “Technical Problem” and to achieve the effect described in the column of “Advantageous Effects of Invention” as described below, but also to solve various problems required to be solved as a product in sufficient consideration of the above-described various problems and to achieve various effects as described below. The specific problems to be solved and the specific effect to be achieved through the physical quantity detector device  300  will be described in the embodiments described below. 
         [0050]    2. Configuration of Physical Quantity Detection Device  300   
         [0051]    2.1 Exterior Structure of Physical Quantity Detection Device  300   
         [0052]      FIGS. 2 to 6  are diagrams illustrating the exterior of the physical quantity detection device  300 , in which  FIG. 2  is a front view of the physical quantity detection device  300 ,  FIG. 3  is a rear view,  FIG. 4  is a left side view,  FIG. 5  is a right side view, and  FIG. 6  is a bottom view. 
         [0053]    The physical quantity detection device  300  is provided with a housing  302 , a front cover  303 , and a rear cover  304  as components forming a casing part. The housing  302  includes a flange  311  for fixing the physical quantity detection device  300  to the intake body that is the main passage  124 , an external connecting portion  321  equipped with a connector which protrudes outward from the flange  311  for electrical connection with an external machine, and a measurement unit  331  which is extended to protrude from the flange  311  to the center of the main passage  124 . 
         [0054]    The measurement unit  331  is inserted and integrally provided with a printed circuit board  400  (see  FIGS. 7 and 8 ). The printed circuit board  400  includes a plurality of detection units for detecting various physical quantities of the measuring target gas  30  flowing in the main passage  124 , and a circuit unit for processing signals detected by the plurality of detection units. 
         [0055]    The measurement unit  331  is provided with bypass passage grooves in the front surface and the rear surface, and there are formed a first bypass passage  305  and a second bypass passage  306  in cooperation of the front cover  303  and the rear cover  304  (see  FIG. 7 or 8 ). In the distal end portion of the measurement unit  331 , there are provided a first bypass passage inlet  305   a  for taking part of the measuring target gas  30  such as the intake air into the first bypass passage  305 , and a first bypass passage outlet  305   b  for returning the measuring target gas  30  from the first bypass passage  305  to the main passage  124 . 
         [0056]    In the intermediate portion of the measurement unit  331  near the flange  311  from the first bypass passage  305 , there are provided a second bypass passage inlet  306   a  for taking part of the measuring target gas  30  such as the intake air into the second bypass passage  306 , and a second bypass passage outlet  306   b  for returning the measuring target gas  30  from the second bypass passage  306  to the main passage  124 . In the middle of the first bypass passage  305 , a flow rate detection unit  456  (see  FIG. 7 ) is provided to configure one of the detection units, and to detect the flow rate of the measuring target gas  30 . The second bypass passage  306  includes a passage portion which connects the second bypass passage inlet and the second bypass passage outlet in a straight line shape, and a sensor chamber  342  which communicates with the passage portion and includes a detection sensor (see  FIG. 8 ). In the sensor chamber  342 , pressure sensors  454  and  455  and a temperature and humidity sensor  452  are contained as the detection unit to detect the physical quantities different from the flow rate provided in the rear surface of the printed circuit board  400 . 
         [0057]    2.2 Effects Based on Exterior Structure of Physical Quantity Detection Device  300   
         [0058]    The physical quantity detection device  300  is provided with the second bypass passage inlet  306   a  an the middle of the measurement unit  331  which is extended from the flange  311  in a direction toward the center of the main passage  124 , and the first bypass passage inlet  305   a  in the distal end portion of the measurement unit  331 . Therefore, the gas near the center portion away from the internal wall surface can be taken into the first bypass passage  305  and the second bypass passage  306  instead of the vicincity of the internal wall surface of the main passage  124 . 
         [0059]    Therefore, the physical quantity detection device  300  can measure the physical quantity of the gas in a portion away from the internal wall surface of the main passage  124 , and it is possible to suppress a reduction in measurement accuracy due to an influence such as heat. The vicinity of the internal wall surface of the main passage  124  is easily influenced by the temperature of the main passage  124 , and comes to be in a state where the temperature of the measuring target gas  30  is different from the actual temperature of the gas, and thus becomes different from an average state of the main gas in the main passage  124 . In particular, in a case where the main passage  124  is the intake body of the engine, the main passage is influenced by the heat from the engine, and kept in a high temperature in many cases. Therefore, the gas in the vicinity of the internal wall surface of the main passage  124  is highly heated with respect to the original air temperature of the main passage  124  in many cases, which causes the measurement accuracy to be lowered. 
         [0060]    In the vicinity of the internal wail surface of the main passage  124 , a fluid resistance is large, and the flow rate comes to be lowered compared to an average flow rate of the main passage  124 . Therefore, when the gas in the vicinity of the internal wall surface of the main passage  124  is taken as the measuring target gas  30  into the first bypass passage  305  and the second bypass passage  306 , there is a concern that the reduction in flow rate with respect to the average flow rate of the main passage  124  results in a measurement error of the physical quantity. Therefore, the first bypass passage  305  where the flow rate detection unit is disposed is provided with the first bypass passage inlet  305   a  in the distal end portion of the measurement unit  331  which is extended thin and long toward the center of the main passage  124  from the flange  311 . 
         [0061]    On the other hand, the second bypass passage  306  is provided with the second bypass passage inlet  306   a  in the intermediate portion of the measurement unit  331 , and a humidity and pressure detection unit is disposed therein with which the physical quantity can be measured regardless of the reduction in flow rate in the vicinity of the internal wall surface. In addition, the first bypass passage  305  is provided with the first bypass passage outlet  305   b  in the distal end portion of the measurement unit  331 , and provided with the second bypass passage outlet  306   b  in the intermediate portion of the measurement unit  331 , both of which form the bypass passage independently from each other. Therefore, each detection unit can secure a necessary flow rate, and the measurement error can be reduced. 
         [0062]    The measurement unit  331  is formed in a long shape extending along an axis toward the center from an outer wall of the main passage  124 , and the thick width is formed in a narrow shape as illustrated in  FIGS. 4 and 5 . In other words, the measurement unit  331  of the physical quantity detection device  300  is formed such that the width of the side surface is thin and the front surface is in a substantially rectangular shape. With this configuration, the physical quantity detection device  300  can be provided with a sufficiently long bypass passage, and the fluid resistance against the measuring target gas  30  can be suppressed to a small value. Therefore, the physical quantity detection device  300  can measure the flow rate of the measuring target gas  30  with a high accuracy while suppressing the fluid resistance to a small value. 
         [0063]    2.3 Structure of Temperature Detection Unit  451   
         [0064]    A temperature detection unit  451  serves as one of the detection units for detecting the physical quantity of the measuring target gas  30  flowing in the main passage  124 , and is provided in the printed circuit board  400 . The printed circuit board  400  includes a protrusion portion  450  which protrudes toward the upstream of the measuring target gas  30  from the second bypass passage inlet  306   a  of the second bypass passage  306 . The temperature detection unit  451  is provided in the protrusion portion  450  and also in the rear surface of the printed circuit board  400 . The temperature detection unit  451  includes a chip type of temperature sensor  453 . The temperature sensor  453  and the wiring portion thereof are coated with a synthetic resin material, and it is prevented electrolytic corrosion caused when saltwater is adhered. The synthetic resin material is applied onto the rear surface of the protrusion portion  450  in a melted state, and cured after application to cover the temperature sensor  453 . 
         [0065]    For example, as illustrated in  FIG. 8 , an upstream outer wall  336  in the measurement unit  331  of the housing  302  is recessed toward the downstream side in the center portion of the measurement unit  331  provided with the second bypass passage inlet  306   a.  The protrusion portion  450  of the printed circuit board  400  protrudes toward the upstream side from the recessed upstream outer wall  336 . The distal end of the protrusion portion  450  is disposed at a position recessed from the surface on the most upstream side of the upstream outer wall  336 . The temperature detection unit  451  is provided on the rear surface side of the printed circuit board  400  (that is, on a side near the second bypass passage  306 ) and the upstream side thereof. 
         [0066]    Since the second bypass passage inlet  306   a  is formed continuously to the downstream side of the temperature detection unit  451 , the measuring target gas  30  flowing from the second bypass passage inlet  306   a  into the second bypass passage  306  flows into the second bypass passage inlet  306   a  after coming in contact with the temperature detection unit  451 , and the temperature is detected when coming in contact with the temperature detection unit  451 . The measuring target gas  30  coming in contact with the temperature detection unit  451  flows in this state from the second bypass passage inlet  306   a  into the second bypass passage  306 , and passes through the second bypass passage  306  so as to be discharged from the second bypass passage outlet  306   b  to the main passage  124 . 
         [0067]    2.4 Effects Related to Temperature Detection Unit  451   
         [0068]    The temperature of the gas flowing from the upstream side in a direction along the flowing of the measuring target gas  30  into the second bypass passage inlet  306   a  is measured by the temperature detection unit  451 . Furthermore, since the gas flows from the distal end portion of the protrusion portion  450  toward a proximal end portion, the temperature of the proximal end portion of the protrusion portion  450  is cooled down in a direction approaching to the temperature of the measuring target gas  30 . The temperature of the intake pipe (the main passage  124 ) is normally increased, and the heat is transferred to the proximal end portion of the protrusion portion  450  through the upstream outer wall  336  or the printed circuit board  400  in the measurement unit  331  from the flange  311  or an abutting portion  315 , and thus there is a concern that the accuracy of the temperature measurement of the temperature detection unit  451  is influenced. As described above, after the measuring target gas  30  is measured by the temperature detection unit  451 , the proximal end portion is cooled down when the gas flows to the proximal end portion of the protrusion portion  450 . Therefore, it is possible to suppress the heat from being transferred from the flange  311  or the abutting portion  315  to the proximal end portion of the protrusion portion  450  through the upstream outer wall  336  or the printed circuit board  400  in the measurement unit  310 . 
         [0069]    In particular, since the upstream outer wall  336  in the measurement unit  331  is formed in a recess shape (see  FIGS. 7 and 8 ) toward the downstream side in the proximal end portion of the protrusion portion  450 , the length of the upstream outer wall  336  from the flange  311  up to the proximal end portion of the protrusion portion  450  can be made long, a heat conduction distance from the flange  311  and the abutting portion  315  can be made long, and a distance of the portion cooled down by the measuring target gas  30  can be made long. Therefore, it is possible to reduce the influence of heat caused from the flange  311  or the abutting portion  315 . In addition, for example, when the measurement unit  331  is inserted from a mounting hole provided in the main passage  124  to the inside, the protrusion portion  450  does not hinder an operation of mounting the physical quantity detection device  300  in the main passage  124 . The protrusion portion  450  can be prevented from coming in conflict with the main passage  124 , and thus protected from damage. 
         [0070]    2.5 Structure and Effects of Flange  311   
         [0071]    In the flange  311 , a plurality of recesses  313  are provided in a lower surface  312  facing the main passage  124  to reduce a heat transfer surface with respect to the main passage  124 , so that the physical quantity detection device  300  is hardly influenced by the heat. The physical quantity detection device  300  is configured such that the measurement unit  331  is inserted to the inside from a mounting hole provided in the main passage  124 , and the lower surface  312  of the flange  311  faces the main passage  124 . The main passage  124  is the intake body for example. The main passage  124  is normally kept at a high temperature. On the contrary, upon activating in a cold region, it is considered that the main passage  124  is at an extremely low temperature. When such a high or low temperature of the main passage  124  has an influence on the temperature detection unit  451  or the flow rate measurement described below, the measurement accuracy is lowered. The flange  311  includes the recess  313  in the lower surface  312  to form a space between the lower surface  312  facing the main passage  124  and the main passage  124 . Therefore, the heat transfer from the main passage  124  to the physical quantity detection device  300  is reduced, and a reduction in measurement accuracy caused by the heat can be prevented. 
         [0072]    Since screw holes  314  of the flange  311  are used to fix the physical quantity detection device  300  to the main passage  124 , the space between the surface facing the main passage  124  surrounding the respective screw holes  314  and the main passage  124  is formed to separate the surface facing the main passage  124  surrounding these screw holes  314  from the main passage  124 . With such a configuration, the heat transfer from the main passage  124  with respect to the physical quantity detection device  300  is reduced, and the structure is made to enable to prevent the reduction in measurement accuracy due to the heat. 
         [0073]    Furthermore, the recess  313  operates to reduce an influence of shrinkage of the resin of the flange  311  at the time of forming the housing  302  not only the reduction effect of the heat transfer. The flange  311  is formed with a thick resin compared to the measurement unit  331 . At the time when the housing  302  is molded with resin, a volume is shrunk when the resin is cooled down from a high temperature to a low temperature and cured, and a distortion may occur due to stress. The volume shrinkage can be evenly made by forming the recess  313  in the flange  311 , and a stress concentration can be reduced. 
         [0074]    The measurement unit  331  is inserted to the inside from the mounting hole provided in the main passage  124 , and fixed to the main passage  124  by being screwed through the flange  311  of the physical quantity detection device  300 . The physical quantity detection device  300  is desirably fixed in a predetermined positional relation to the mounting hole provided in the main passage  124 . The recess  313  provided in the flange  311  can be used for positioning the main passage  124  and the physical quantity detection device  300 . A projection may be formed in the main passage  124  to form a shape such that the projection is fitted to the recess  313 , and the physical quantity detection device  300  can be fixed to the main passage  124  at an accurate position. 
         [0075]    2.6 Structure of External Connecting Portion  321   
         [0076]    The external connecting portion  321  includes a connector  322  which is provided in the upper surface of the flange  311  and protrudes from the flange  311  toward the downstream side in a flowing direction of the measuring target gas  30 . In the connector  322 , there is provided a plug-in hole  322   a  for plugging a communication cable which is connected to the control device  200 . Four external terminals  323  are provided in the plug-in hole  322   a  as illustrated in  FIG. 5 . The external terminal  323  serves as a terminal for outputting information of the physical quantity (measurement result) of the physical quantity detection device  300  and a power terminal for supplying a direct current power to operate the physical quantity detection device  300 . Further, the connector  322  in this embodiment has been described about a case where the connector protrudes from the flange  311  toward the downstream side in the flowing direction of the measuring target gas  30 , and has an insertion shape from the downstream side in the flowing direction toward the upstream side, but the invention is not limited to this shape. For example, the connector may vertically protrude from the upper surface of the flange  311  and have an insertion shape along the extending direction of the measurement unit  331 , and may be changed in various forms. 
         [0077]    3. Entire Structure and Effects of Housing  302   
         [0078]    3.1 Structures and Effects of Bypass Passage and Flow Rate Detection Unit 
         [0079]      FIGS. 7 and 9  illustrate a state of the housing  302  where the front cover  303  and the rear cover  304  are removed from the physical quantity detection device  300 .  FIG. 7  is a front view of the housing  302 ,  FIG. 8  is a rear view of the housing  302 , and  FIG. 9  is a cross-sectional view taken along a line A-A of  FIG. 7 . 
         [0080]    The housing  302  is structured such that the measurement unit  331  is extended from the flange  311  toward the center of the main passage  124 , the printed circuit board  400  is disposed on the proximal end side of the measurement unit  331 , and the bypass passage groove for forming the first bypass passage  305  is provided on the distal end side of the measurement unit  331 . 
         [0081]    The printed circuit board  400  has a fiat plate shape and includes a main body portion  433  which partitions the proximal end portion of the measurement unit  331  into the front surface side and the rear surface side and has a substantially rectangular shape in plan view, and a protrusion portion  432  which is disposed in the first bypass passage  305  to protrude from one side of the main body portion  433 . 
         [0082]    The printed circuit board  400  is provided along the plan of the measurement unit  331  as illustrated in  FIGS. 7 and 8 , and disposed in parallel along the surface of the measurement unit  331  to partition the proximal end portion of the measurement unit  331  into the front surface side and the rear surface side at the intermediate position between the front surface and the rear surface of the measurement unit  331  as illustrated in  FIG. 9 . 
         [0083]    In the printed circuit board  400 , the flow rate detection unit (air flow rate sensor)  456  is disposed in the same front surface (one surface) as the mounting surface where a circuit such as a microprocessor is mounted, and at least one or more physical quantity detection sensors (for example, a humidity sensor, a pressure sensor, etc.) are disposed in the rear surface (other surface). In other words, the printed circuit board  400  includes, in its front surface, a detection sensor surface region in which the flow rate detection unit (the physical quantity detection sensor)  456  is disposed, and a circuit component surface region in which the circuit component such as an LSI other than the physical quantity detection sensor is disposed. Then, a facing surface region facing the circuit component surface region is provided in the rear surface of the printed circuit board  400 , and at least a part of the facing surface region is exposed to the second bypass passage  306  in the rear surface of the printed circuit board. 
         [0084]    In this embodiment, the circuit component is disposed in the front surface of the printed circuit board  400  to be wire-bonded to the LSI or the air flow rate sensor, and the circuit component is disposed in the rear surface of the printed circuit board  400  to be soldered to the temperature and humidity sensor  452  or the pressure sensors  454  and  455 . In this way, the printed circuit board  400  can be easily manufactured by disposing the wire-bonding circuit component in one surface of the printed circuit board  400 . 
         [0085]    A circuit chamber  341  is formed on the front surface side of the measurement unit  331  to contain the circuit component such as the LSI and the microprocessor mounted in the front surface of the printed circuit board  400 . The circuit chamber  341  is sealed in cooperation with the front cover  303 , and completely isolated from the outside. 
         [0086]    Then, the second bypass passage  306  is formed on the rear surface side by the printed circuit board  400 . The second bypass passage  306  is formed in cooperation with the rear cover  304 . The second bypass passage  306  includes the passage portion which is extended in a straight line along a flowing direction of the measuring target gas  30  flowing in the main passage  124 , and the sensor chamber  342  which is formed at a position shifted to a direction orthogonal or perpendicular to the flowing direction of the measuring target gas  30  from the passage portion. The sensor chamber  342  is formed in a predetermined interior space in which the rear surface side is sealed by the rear cover  304 , but communicates to the outside through the second bypass passage  306  which is continuously formed on the distal end side of the measurement unit  331 . In the sensor chamber  342 , the pressure sensors  454  and  455  and the temperature and humidity sensor  452  mounted in the rear surface of the printed circuit board  400  are contained. 
         [0087]    The bypass passage groove for forming the first bypass passage  305  includes a front-side bypass passage groove  332  illustrated in  FIG. 7 , and a rear-side bypass passage groove  334  illustrated in  FIG. 8 . The front-side bypass passage groove  332  is gradually bent toward the flange  311  (on the proximal end side of the measurement unit  331 ) as it goes from the first bypass passage outlet  305   b  opened to a downstream external wall  338  of the measurement unit  331  toward the upstream outer wall  336 , and communicates with an opening  333  at a position near the upstream outer wall  336 . The opening  333  is formed to pass through the measurement unit  331  in a thickness direction. The opening  333  is formed along the flowing direction of the measuring target gas  30  of the main passage  124  to be extended along between the upstream outer wall  336  and the downstream external wall  338 . 
         [0088]    In the opening  333 , the protrusion portion  432  which is a part of the printed circuit board  400  is disposed. The protrusion portion  432  of the printed circuit board  400  passes through partition walls  361  and  362  which separate the circuit chamber  341  of the measurement unit  331  and the second bypass passage  306  to protrude to the opening  333 . The protrusion portion  432  includes a measurement flow-passage surface  430  and a rear surface  431  thereof which are extended in parallel along the flowing direction of the measuring target gas  30  in the opening  333 . 
         [0089]    The rear-side bypass passage groove  334  moves from the first bypass passage inlet  305   a  opened to the upstream outer wall  336  of the measurement unit  331  toward the downstream external wall  338 , and branches off into two parts at an intermediate position between the upstream outer wall  336  and the downstream external wall  338 . One of the branches is extended itself in a straight line shape as a discharge passage to communicate with a discharge port  305   c  opened to the downstream external wall  338 . The other one of the branches is gradually bent toward the flange  311  (the proximal end side of the measurement unit  331 ) as it goes to the downstream external wall  338 , and communicates with the opening  333  at a position near the downstream external wall  338 . 
         [0090]    The rear-side bypass passage groove  334  forms an inlet groove of the first bypass passage  305  through which the measuring target gas  30  flows in from the main passage  124 . The front-side bypass passage groove  332  forms an outlet groove of the first bypass passage  305  through which the measuring target gas  30  taken in from the rear-side bypass passage groove  334  returns to the main passage  124 . The front-side bypass passage groove  332  and the rear-side bypass passage groove  334  are provided on the distal end side of the measurement unit  331 . Therefore, the gas in a portion separated from the internal wall of the main passage  124  (that is, the gas flowing a portion near the center portion of the main passage  124 ) can be taken in as the measuring target gas  30 . The gas flowing in the vicinity of the internal wall of the main passage  124  is influenced by the temperature of the wall surface of the main passage  124 , and has a temperature different from an average temperature of the gas flowing in the main passage  124  such as the measuring target gas  30  in many cases. In addition, the gas flowing in the vicinity of the internal wall surface of the main passage  124  shows a flow speed delayed from an average flow speed of the gas flowing in the main passage  124  in many cases. Since the physical quantity detection device  300  according to the embodiment hardly receives such an influence, it is possible to suppress a reduction in measurement accuracy. 
         [0091]    In the embodiment, the bypass passage grooves  332  and  334  are provided to form the first bypass passage  305  in the housing  302 , the first bypass passage  305  is completely configured by the bypass passage grooves  332  and  334  and by the covers  303  and  304  by putting the covers  303  and  304  on the front surface and the rear surface of the housing  302 . With such a structure, it is possible to form all the bypass passage grooves as a part of the housing  302  in a resin mold process of the housing  302 . In addition, since molds are provided on both surfaces of the housing  302  at the time of forming the housing  302 , it is possible to form both of the from side bypass passage groove  332  and the rear-side bypass passage groove  334  as a part of the housing  302  by using the molds of the both sides. The bypass passages of the both surfaces of the housing  302  can be completely formed providing the front cover  303  and the rear cover  304  in the both surfaces of the housing  302 . Since the front-side bypass passage groove  332  and the rear-side bypass passage groove  334  are formed in the both surfaces of the housing  302  using the mold, the first bypass passage  305  can be formed with a high accuracy. In addition, a high productivity can be achieved. 
         [0092]    As illustrated in.  FIG. 8 , a part of the measuring target gas  30  flowing in the main passage  124  is taken into the rear-side bypass passage groove  334  from the first bypass passage inlet  305   a,  and flows in the rear-side bypass passage groove  334 . Then, a foreign object having a heavy mass in the measuring target gas  30  flows to the discharge passage extending in a straight line from the branch together with a part of the measuring target gas  30 , and is discharged from the discharge port  305   c  of the downstream external wall  338  to the main passage  124 . 
         [0093]    The rear-side bypass passage groove  334  has a shape deepening as it progresses. The measuring target gas  30  gradually moves to the front side of the measurement unit  331  as it goes along the rear-side bypass passage groove  334 . In particular, the rear-side bypass passage groove  334  is provided with a steep slope portion  334   a  which is steeply deepened before the opening  333 . Part of the air having a light mass moves along the steep slope portion  334   a,  and flows toward the measurement flow-passage surface  430  of the printed circuit board  400  in the opening  333 . On the other hand, since it is not easy for the foreign object having a heavy mass to abruptly change its route, the foreign object flows toward a measurement flow-passage rear surface  431 . 
         [0094]    As illustrated in  FIG. 7 , the measuring target gas  30  moved toward the surface side in the opening  333  flows along the measurement flow-passage surface  430  of the printed circuit board while performing the heat transfer with respect to the flow rate detection unit  456  to measure the flow rate through a heat transfer surface exposing portion  436  provided in the measurement flow-passage surface  430 , and the flow rate is measured. The air flowed from the opening  333  to the front-side bypass passage groove  332  flows also along the front-side bypass passage groove  332 , and discharged from the first bypass passage outlet  305   b  opened to the downstream external wall  338  toward the main passage  124 . 
         [0095]    Since a material having a heavy mass such as dust mixed in the measuring target gas  30  has large inertia, it is difficult to steeply change to the depth direction of the groove along the front surface of a portion of the steep slope portion  334   a  where the depth of the groove is steeply deepened along. Therefore, the foreign object having a heavy mass moves toward the measurement flow-passage rear surface  431 , and thus it is suppressed that the foreign object approaches the heat transfer surface exposing portion  436 . In this embodiment, a majority of foreign objects having a heavy mass other than the gas passes through the measurement flow-passage rear surface  431  (rear surface) of the measurement flow-passage surface  430 . Therefore, it is possible to reduce an influence of contamination due to the foreign objects such as oil, carbon, or dust, and the reduction in measurement accuracy can be suppressed. In other words, since the shape is formed such that the route of the measuring target gas  30  is abruptly changed along an axis traversing the flowing axis of the main passage  124 , the influence of the foreign object mixed in the measuring target gas  30  can be reduced. 
         [0096]    In this embodiment, the flow passage formed by the rear-side bypass passage groove  334  faces the flange  311  from the distal end portion of the housing  302  while drawing a curve, the gas flowing the bypass passage at the position nearest to the flange  311  flows in an opposite direction with respect to the flow in the main passage  124 , and the bypass passage on the rear surface side (one side) is connected to the bypass formed on the front surface side (the other side) in a portion of the flow of the opposite direction. With such a configuration, the printed circuit board  400  can be easily fixed to the bypass passage of the heat transfer surface exposing portion  436 . Furthermore, the measuring target gas  30  can be easily taken in at a position near the center portion of the main passage  124 . 
         [0097]    3.2 Structures and Effects of Second Bypass Passage and Humidity and Pressure Detection Unit 
         [0098]    The second bypass passage  306  is configured in cooperation with the housing  302 , the printed circuit board  400  illustrated in  FIGS. 8 and 9 , and the rear cover  304  bonded to the housing  302 . The printed circuit board  400  is provided along the surface of the measurement unit  331 , and is disposed in parallel along the surface of the measurement unit  331  to partition the proximal end portion of the measurement unit  331  into the front surface side and the rear surface side at the intermediate position between the front surface and the rear surface of the measurement unit  331 . 
         [0099]    On the upstream side of the housing  302 , there is provided a partition  307  which forms a part of the second bypass passage inlet  306   a,  is extended toward the flange  311  (the proximal end side of the measurement unit.  331 ), and blocks the measuring target gas  30  as illustrated in  FIG. 8 . Similarly, as illustrated in  FIG. 8 , on the downstream side of the housing  302 , there is provided a partition  308  which forms a part of the second bypass passage outlet  306   b,  and is extended toward the flange  311  (the proximal end side of the measurement unit  331 ). In addition, the partitions  307  and  308  on the upstream and downstream side of the housing are connected by a partition  309  which is extended in a direction in parallel with the flow of the measuring target gas  30  to surround the temperature and humidity sensor  452  and the pressure sensors  454  and  455  in the intermediate portion connected to the flange  311 . The partitions  307 ,  308 , and  309  have the same height in the thickness direction of the measurement unit  331 , and form the sensor chamber  342  by mounting the rear cover  304 . 
         [0100]    The second bypass passage  306  is extended in parallel with the flowing direction of the measuring target gas  30  flowing in the main passage  124 , and the temperature and humidity sensor  452  and the pressure sensors  454  and  455  (the physical quantity detection sensors) are disposed at positions separated in a direction intersecting with a straight line connecting the second bypass passage inlet  306   a  and the second bypass passage outlet  306   b.  The inlet  306   a  and the outlet  306   b  of the second bypass passage  306  are vertically opened with respect to the measuring target gas  30  flowing in the main passage  124 , and disposed on the same straight line in parallel with the flowing direction of the measuring target gas  30 . In addition, the temperature and humidity sensor  452  and the pressure sensors  454  and  455  are disposed in the sensor chamber  342  surrounded by the partitions  307 ,  308 , and  309  at positions shifted toward the flange  311  from a flowing line of the air in the passage portion connecting the second bypass passage inlet  306   a  and the second bypass passage outlet  306   b.    
         [0101]    In general, in a case where a plurality of sensors are configured by the same electronic circuit, the power consumption is simply increased according to the number of physical quantity detection sensors. It has been known that the power consumption (electric energy) is converted into heat (energy) through a resistor, the heating of the entire circuit is increased as the power consumption is increased. When the circuit is increased in its self-heating, the durability of the circuit component or the performance of the physical quantity detection sensor are adversely affected. A temperature range required for an automobile component becomes a wide range of −40° C. to 125° C. In particular, a semiconductor component such as a microprocessor is used in the electronic circuit of the physical quantity detection sensor. The semiconductor component is typically used in a range not exceeding a junction temperature of about 150° C. in accordance with a high humidity environment and the self-heating of the circuit. The junction temperature is a temperature of the junction portion between a semiconductor element and a lead line. When being used under an environment equal to or more than about 150° C., a durable life of the product is significantly lowered. Therefore, a radiation design is required to extremely suppress the self-heating of the circuit. In addition, from a viewpoint of the performance of the physical quantity detection device  100 , there is a concern that the temperature increase due to the heat transfer caused by the self-heating of the circuit results in deterioration of the measurement accuracy of the detection sensor since a change in properties due to a temperature influence always occurs at high and low temperatures. 
         [0102]    With regard to such a problem, the rear surface of the printed circuit board  400  in this embodiment forms a part of the second bypass passage  306 . Therefore, the rear surface of the printed circuit board  400  is exposed to the air flowing in the second bypass passage  306 . In other words, the self-heating occurring in the circuit component such as a microprocessor  605  mounted in the front surface of the printed circuit board  400  causes the heat transfer to the rear surface of the printed circuit board  400 . Furthermore, the heating of the entire printed circuit board  400  can be suppressed by transferring the heat to the air flowing in the second bypass passage  306 . 
         [0103]    In addition, since the pressure sensor  455  is disposed at the back of the partition  307  on the upstream side of the housing  302 , the measuring target gas  30  flowed to the second bypass passage  306  is prevented from coming into direct conflict with the pressure sensor  455 , and it is possible to suppress the air flow from directly influencing on the pressure sensor  455 . In other words, a dynamic pressure generated by the air flow is not detected by the pressure sensor  455  but can correctly measure a static pressure to be measured, so that the measurement accuracy can be secured. 
         [0104]    The inlet  306   a  and the outlet  306   b  of the second bypass passage are positioned on the same line, and the detection sensor (herein, an order of disposing the plurality of detection sensors is not limited to that illustrated in  FIG. 8 ) is shifted from the same line to be disposed in the intermediate portion of the partitions  307  and  308  on the upstream and the downstream sides of the housing  302 . Therefore, it is possible to suppress dust and water droplets mixed into the measuring target gas  30  from coming into direct conflict with the detection sensor. Further, staining/deterioration and variation of the output can be reduced. 
         [0105]    3.3 Structures and Effects of Rear Cover, and Humidity and Pressure Detection Unit 
         [0106]      FIGS. 10( a ) and 10( b )  and  FIGS. 11( a ) and 11( b )  are diagrams illustrating configurations of the front cover and the rear cover. In addition,  FIGS. 12( a ) to 14( b )  illustrate a plurality of embodiments of the second bypass passage configured by the rear cover. 
         [0107]    As described above, the bypass passage groove is configured in the rear surface of the housing  302  to form the second bypass passage  306 , and the rear cover  304  is disposed to separate the measuring target gas  30  from parts other than the inlet  306   a  and the outlet  306   b  of the second bypass passage of the bypass passage groove. 
         [0108]      FIGS. 12( a ) and 12( b )  illustrate an example in which the temperature and humidity sensor  452  and the pressure sensors  454  and  455  are mounted in the rear surface of the printed circuit board  400 . In  FIGS. 11( a ) and 12( a ) , a projection  350  on the upstream side formed in the rear cover  304  and a projection  351  on the downstream side are illustrated with a dotted line.  FIG. 12( b )  illustrates a cross section taken along a line D-D of  FIG. 12( a ) , and shows an example of disposing the projections  350  and  351 . 
         [0109]    The projections  350  and  351  form a partition wall which partitions the second bypass passage  306  of the printed circuit board  400  into the passage portion and the sensor chamber  342  by mounting the rear cover  304 . The projection  350  on the upstream side is formed to be extended along the flowing direction of the measuring target gas  30  along between the second bypass passage inlet  306   a  and the pressure sensor  455  on the upstream side. Then, the projection  351  on the downstream side is formed to be extended along the flowing direction of the measuring target gas  30  along between the pressure sensor  454  on the downstream side and the second bypass passage outlet  306   b.  The projections  350  and  351  both are formed integrally to the rear cover  304  by a thin protruding piece, protrude toward the printed circuit board  400  along the thickness direction of the measurement unit  331 , and are disposed on a straight line at the same height position with respect to the longitudinal direction of the measurement unit  331  in parallel with the flowing of the measuring target gas  30 . 
         [0110]    In this embodiment, when the measuring target gas  30  flows in through the second bypass passage inlet  306   a,  the flowing is corrected by the projections  350  and  351  on the upstream and downstream sides, and passes through a straight line connecting the second bypass inlet  306   a  and the outlet  306   b  and then discharged to the outside from the outlet  306   b.    
         [0111]    In other words, since the sensor chamber  342  is shifted to the proximal end side of the measurement, unit  331  (on a side near the flange  311 ) from the passage portion of the second bypass passage  306 , the measuring target gas  30  flowed from the second bypass passage inlet  306   a  into the second bypass passage  306  directly progresses through the passage portion of the second bypass passage  306 , and then discharged to the outside from the second bypass passage outlet  306   b,  but not directly flows into the sensor chamber  342 . Therefore, it is possible to suppress the measuring target gas  30  from coming into direct conflict with the physical quantity detection sensor such as the pressure sensors  454  and  455  an the sensor chamber  342  and the temperature and humidity sensor  452 . 
         [0112]    In general, a water droplet or a pollutant having a certain mass in the intake pipe is mixed to the measuring target gas  30  and passes through the second bypass passage  306 . Therefore, the measuring target gas  30  is suppressed from coming into direct conflict with the physical quantity detection sensor. Therefore, the staining and deterioration of the physical quantity detection sensor or the output variation due to the water droplet can be suppressed, and the measurement error can be reduced. Specifically, when the direct conflict of the measuring target gas  30  onto the pressure sensors  454  and  455  is prevented, the influence of the dynamic pressure is reduced, and the detection accuracy can be prevented from being deteriorated. Then, when the direct conflict of the measuring target gas  30  onto the temperature and humidity sensor  452  is prevented, it is possible to prevent the resistance from being lowered due to the attachment of the water droplet or the pollutant. 
         [0113]      FIGS. 13( a ) and 13( b )  illustrate an example in which the temperature and humidity sensor  452  and the pressure sensor  454  are mounted in the rear surface of the printed circuit board  400 .  FIG. 13( a )  is an enlarged view of the sensor chamber  342 , and  FIG. 13( b )  is a cross-sectional view taken along a line E-E of  FIG. 13( a ) . As illustrated in  FIGS. 13( a ) and 13( b ) , the projection  350  on the upstream side is provided between the second bypass passage inlet  306   a  and the pressure sensor  454 , and is formed by a thin plate to be extended in the flowing direction of the measuring target gas  30 . The symbols, configurations, and effects already described will be omitted herein. In this embodiment, since the number of pressure sensors is reduced by 1 compared to  FIGS. 12( a ) and 12( b ) , the length of the projection  350  on the upstream side is made longer to bury the corresponding space. 
         [0114]      FIGS. 14( a ) and 14( b )  show an example in which the temperature and humidity sensor  452  is mounted in the rear surface of the printed circuit board  400 .  FIG. 14( a )  is an enlarged view of the sensor chamber  342 , and  FIG. 14( b )  is a cross-sectional view taken along a line F-F of  FIG. 14( a ) . As illustrated in  FIGS. 14( a ) and 14( b ) , the projection  350  on the upstream side is provided between the second bypass passage inlet  306   a  and the temperature and humidity sensor  452 , and is configured by a thin plate which is extended in the flowing direction of the measuring target gas  30 , bent before the temperature and humidity sensor  452 , and extended in a direction intersecting with the flowing of the measuring target gas  30 . 
         [0115]    In this embodiment, the temperature and humidity sensor  452  is mounted at a position separated by a certain distance from the partition  307  on the upstream side of the housing  302 . Therefore, the projection  350  on the upstream side of the cover  304  is configured by a thin plate  350   a  of a shape intersecting with the flowing of the measuring target gas  30  in order to achieve the same effect as that of the partition  307 . Therefore, it is possible to suppress the water droplet or the pollutant mixed in the air passing through the second bypass passage  306  from coming into direct conflict with the sensor. The output variation of the sensor caused by the staining and deterioration or by the water droplet can be suppressed, so that the measurement error can be reduced. 
         [0116]    4. Signal Processing of Physical Quantity Detection Device  300   
         [0117]    An input-output relation of a signal of the physical quantity detection device  300  is illustrated in  FIG. 15 . In this embodiment, the front surface and the rear surface of one printed circuit board  400  are both mounted with the physical quantity detection sensor, and the substrate is miniaturized. Therefore, even in the signal processing, one microprocessor  605  is used to take all the signals from the respective physical quantity sensors in order to make an electronic circuit component small, and the signals readable by the control device  200  is generated and corrected. In addition, as illustrated in  FIGS. 5 and 7 , an electrical signal in the printed circuit board  400  is transferred to the control device  200  through an AL wire  324  and the external terminal  323 . 
         [0118]    5. Conclusion. 
         [0119]    According to the physical quantity detection device of this embodiment, the detection sensors  451  to  456  are mounted on one surface and the other surface of the printed circuit board  400 , so that the printed circuit board  400  can be miniaturized. With the miniaturization of the printed circuit board  400 , the casing part of the physical quantity detection device  300  can be also miniaturized. Therefore, a space is secured in the engine room, or a pressure loss in the intake pipe is reduced. 
         [0120]    In addition, in this embodiment, a part of the printed circuit board  400  forms a part of the second bypass passage  306 . Therefore, the other surface of the printed circuit board  400  is exposed to the air flowing in the second bypass passage  306 . In other words, the self-heating generated by the circuit component such as the microprocessor  605  mounted one surface of the printed circuit board  400  is transferred to the other surface of the printed circuit board  400 . Furthermore, since the heat is transferred to the air flowing in the second bypass passage  306 , it is possible to suppress the heating of the entire printed circuit board  400 . 
         [0121]    Hitherto, the description has been made about embodiments of the invention, but the invention is not limited to the embodiments. Various changes in design can be made within a scope not departing from the spirit of the invention described in the accompanying claims. For example, the embodiments are described in a clearly understandable way for the invention, and thus the invention is not necessarily to provide all the configurations described above. In addition, some configurations of a certain embodiment may be replaced with the configurations of another embodiment, and the configuration of the other embodiment may also be added to the configuration of a certain embodiment. Furthermore, additions, omissions, and substitutions may be made on some configurations of each embodiment using other configurations. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           24  exhaust gas 
           30  measuring target gas 
           110  internal combustion engine 
           112  engine cylinder 
           114  engine piston 
           116  intake valve 
           118  exhaust valve 
           122  air cleaner 
           124  main passage 
           126  throttle body 
           128  intake manifold 
           132  throttle valve 
           144  throttle angle sensor 
           146  rotation angle sensor 
           148  oxygen sensor 
           152  fuel injection valve 
           154  ignition plug 
           156  idle air control valve 
           200  control device 
           300  physical quantity detection sensor 
           302  housing 
           303  front cover 
           304  rear cover 
           305  first bypass passage 
           305   a  first bypass passage inlet 
           305   b  first bypass passage outlet 
           306  second bypass passage 
           306   a  second bypass passage inlet 
           306   b  second bypass passage outlet 
           307  partition on upstream side of housing 
           308  partition on downstream side of housing 
           309  partition 
           311  flange 
           312  lower surface facing main passage  124   
           313  recess 
           314  screw hole 
           315  abutting portion 
           321  external connecting portion 
           322  connector 
           322   a  plug-in hole 
           323  external terminal 
           324  AL wire 
           332  front-side bypass passage groove 
           333  opening 
           334  rear-side bypass passage groove 
           334   a  steep slope portion 
           336  upstream outer wall 
           338  downstream external wall 
           341  circuit chamber 
           342  sensor chamber 
           350  projection on upstream side of cover 
           351  projection on downstream side of cover 
           400  printed circuit board 
           430  measurement flow-passage surface 
           431  measurement flow-passage rear surface 
           436  heat transfer surface exposing portion 
           450  protrusion portion 
           451  temperature detection unit 
           452  temperature and humidity sensor 
           453  temperature sensor 
           454  pressure sensor 
           455  pressure sensor 
           456  flow rate detection unit 
           605  circuit component (microprocessor)