Abstract:
The purpose of the present invention is to provide a flow sensor that makes it possible to detect temperature abnormalities that the flow sensor has been exposed to and the time history of the flow sensor in a high-temperature environment from an externally visible cover material and that uses a cover material for which the laser welding quality can be guaranteed through visual inspection when the sensor is delivered as a product and even if the sensor is used in an abnormal state. A flow sensor provided with a housing, a cover, a circuit chamber that houses a wiring portion sealed between the housing and the cover, and an auxiliary passage through which the liquid to be sensed flows, wherein: a joint portion formed on the housing and a joint portion formed on the inner surface of the cover are joined together through the laser welding; the main material of the cover is a crystalline resin; the cover includes an amorphous alloy; and the cover is made to have a natural color.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a flow sensor constructed of a housing and a cover formed from a resin material suitable for laser welding. The present invention also relates to a method for producing a resin structure. 
       BACKGROUND ART 
       [0002]    A thermal flow sensor that measures the flow rate of a gas includes a flow rate detector to measure the flow rate and is configured to measure the flow rate of the gas by allowing thermal conduction between the flow rate detector and the gas to be measured. The flow rate measured by the thermal flow sensor is widely used as an important control parameter of various devices. The thermal flow sensor is characterized in that when compared with other types of flow sensors, the flow rate of a gas can be measured with relatively high accuracy, but in recent years, further improvements of measurement accuracy of the gas flow rate are desired. In vehicles mounted with an internal combustion engine, for example, requirements for fuel saving and requirements for exhaust gas purification are high. To meet such requirements, measurements of an intake air flow as a principal parameter of the internal combustion engine with high accuracy are demanded. A thermal flow sensor that measures an intake air flow introduced into an internal combustion engine includes a sub-passage that takes in a portion of the intake air flow and a flow rate detector arranged in the sub-passage and measures the state of a measured gas flowing through the sub-passage by heat being conducted to the measured gas by the flow rate detector to output an electric signal representing the intake air flow introduced into the internal combustion engine. Such a technology is disclosed by, for example, JP-A-2011-252796 (Patent Literature 1). 
         [0003]    Also, a technology to weld a housing and a cover by using laser is disclosed by, for example, JP-A-2007-210165 (Patent Literature 2). Also, as an inspection method of a laser welded portion, a method of determining welding in an expanded state of the welded portion by providing an opening on a transparent resin side is disclosed by, for example, JP-A-2009-056722 (Patent Literature 3). Natural materials containing no carbon material originating from coloring are frequently used for laser welding and depending on the material of resin material or the usage environment, discoloring may occur over time and the appearance may also be important depending on the type of product. Thus, in the Munsell color system, if the lightness is V, the chroma is C, and the hue when a color circle is divided into one hundred portions and a hue 10RP is set as “0” or “100” is H, discoloring over time can be blocked by tinting in color satisfying relationships of “V≦0.229H+3.714, V≧−0.8H+24, V≧3” and “C≧−0.075H2+1.936H+1.267, C≧2”, which is disclosed by JP-A-2010-090234 (Patent Literature 4). 
       CITATION LIST 
     Patent Literatures 
       [0004]    PATENT LITERATURE 1: JP-A-2011-252796 
         [0005]    PATENT LITERATURE 2: JP-A-2007-210165 
         [0006]    PATENT LITERATURE 3: JP-A-2009-056722 
         [0007]    PATENT LITERATURE 4: JP-A-2010-090234 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    A flow sensor includes a flow rate detector and a temperature detector and these detectors are arranged on a housing (cabinet). In addition, various electronic components are mounted inside a circuit chamber of the housing and sealing is needed to prevent short-circuits and corrosion of wiring portions. Thus, a method of sealing and fixing the housing and a cover using a plurality of adhesives is normally used for the purpose of sealing. Many curable types of adhesives are known, but in consideration of reliability for the use of vehicles, heat-hardening adhesives are mainly used. However, when a heat-hardening adhesive is used, a time of 10 min or longer is needed for heat hardening, posing a problem of low productivity. In addition, a large quantity of adhesive is used for sealing, also posing a problem of higher costs. Further, an unnecessary area is needed for control of a protruding adhesive, constraining the flexibility of design. Moreover, difficulty of controlling a suitable amount of adhesive to be filled to seal electronic components poses a challenge. As a means for solving such problems, as mentioned in Patent Literature 2, the laser welding method capable of directly joining the cover and the housing with accuracy without damaging electronic components can be cited. 
         [0009]    For laser welding, when an absorbent resin is melted to weld to a transmission resin, the transmission resin is required to have a certain transmittance or higher for a light source of the wavelength of 800 nm or more used for laser welding to improve productivity or inhibit poor carbonization on the surface. In addition, the method of directly observing a welded portion in visual inspection is considered to the most accurate method as a method of determining a laser welding state. However, challenges posed include necessity of a high transmittance in the visible light in a lower wavelength region than the wavelength of 800 nm or more of the light source used for laser welding. In addition, if the method described in Patent Literature 3 is used as an inspection method other than the visual inspection, a large amount of pyrolytic components (gas) is generated from the portion and pressing materials become dirty, leading to constant cleaning and substantially lower productivity and making the adoption thereof difficult. 
         [0010]    It is known that when an internal combustion engine control system of a vehicle into which a flow sensor is incorporated operates normally, the service temperature is in many cases 100° C. or less as an actual usage environment of the flow sensor itself and the flow sensor is not used for a long time at temperature exceeding 100° C. However, if the internal combustion engine control system fails somewhere in the system, the flow sensor itself may be exposed to a high temperature for a long time and it may become difficult to guarantee the quality of the flow sensor alone. A cover of resin material is used for the flow sensor and the resin material as such may be discolored over time. In contrast, Patent Literature 4 describes coloring before product shipment so as to block discoloring, but the flow sensor is used in an internal combustion engine control system and as long as mechanical characteristics of the resin is not significantly degraded, the change of appearance itself due to discoloring of the resin poses no problem. 
         [0011]    The present inventors found that a history of the actual usage environment of a flow sensor can be determined by using discoloring of a resin cover material. 
         [0012]    From the above, an object of the present invention is to provide a flow sensor using, when selecting a cover material on the outer circumference of a flow sensor, the cover material allowing to recognize an abnormal temperature for the flow sensor in a high temperature environment and a time history based on discoloring of the cover material and capable of guaranteeing laser welding quality by visual inspection when the product is shipped or even after the product is used in an abnormal state. 
       Solution to Problem 
       [0013]    To solve the above challenge, for example, the configuration described in CLAIMS is adopted. The present application includes a plurality of solutions to the problems, and examples thereof include configuring a flow sensor including a sub-passage constituting a measurement channel by taking in a measured fluid from an opening, a flow rate detector arranged inside the sub-passage to measure a flow rate of the measured fluid, a housing isolated from the sub-passage in a circuit chamber to house electronic components that drive the flow rate detector, and resin cover joined to the housing to airtightly seal the sub-passage and the circuit chamber from an outside air, so that a joining portion formed on the housing and a joining portion formed on an inner surface of the cover are joined by laser welding, and a main material of the cover is a crystalline resin and contains at least one non-crystalline alloy material and the cover has a natural color. 
         [0014]    A resin material of a region corresponding to the joining portion of the cover has an average transmittance of 35% or more for light of a wavelength of 450 nm to 1100 nm. 
         [0015]    A color of the resin material of the region corresponding to the joining portion of the cover to the housing and the neighboring portion satisfies lightness L*&lt;75 and chroma C*&lt;10 in an L*a*b color system. 
         [0016]    A glass transition temperature of the alloy material contained in the resin material of the cover is 80° C. to 120° C. 
         [0017]    Also in the present invention, to solve the above challenge, a method for producing a resin structure includes positioning a joining portion formed on an inner surface of a cover made of resin opposite to a joining portion formed on a housing made of resin and placing the cover on the housing, welding the joining portion of the housing and the joining portion of the cover by applying pressure such that the joining portion of the housing and the joining portion of the cover are brought into close contact and irradiating the joining portions with laser through the cover, and carrying out visual inspection of a state of a welded portion through the cover, wherein the cover is formed by using a crystalline thermoplastic resin as a main material and containing by 10% to 30% an alloy material of a non-crystalline resin whose glass transition temperature is 80° C. to 120° C. 
       Advantageous Effects of Invention 
       [0018]    By adopting the present invention, an abnormal temperature and a time history of a flow sensor can be grasped from the appearance of a cover material. Further, visual inspection of welding quality after welding and an actual usage environment can be carried out. Therefore, a flow sensor that detects an abnormal temperature and a time history and realizes high quality capable of guaranteeing quality of laser welding and lower costs in the actual usage environment can be provided. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a system configuration diagram showing an embodiment in which a flow sensor according to the present invention is used in an internal combustion engine control system. 
           [0020]      FIG. 2  is a diagram showing the appearance of the flow sensor,  FIG. 2(A)  is a left side view, and  FIG. 2(B)  is a front view. 
           [0021]      FIG. 3  is a diagram showing the appearance of the flow sensor,  FIG. 3(A)  is a right side view, and  FIG. 3(B)  is a rear view. 
           [0022]      FIG. 4  is a diagram showing a housing when a cover of the flow sensor is removed,  FIG. 4(A)  is a left side view of the housing, and  FIG. 4(B)  is a front view of the housing. 
           [0023]      FIG. 5  is a diagram showing the housing when the cover of the flow sensor is removed,  FIG. 5(A)  is a right side view of the housing, and  FIG. 5(B)  is a rear view of the housing. 
           [0024]      FIG. 6  is a diagram showing the appearance of a front cover,  FIG. 6(A)  is a left side view,  FIG. 6(B)  is a front view, and  FIG. 6(C)  is a plan view. 
           [0025]      FIG. 7  is a diagram showing the appearance of a rear cover,  FIG. 7(A)  is a left side view,  FIG. 7(B)  is a front view, and  FIG. 7(C)  is a plan view. 
           [0026]      FIG. 8  is a diagram showing a welded portion of the front cover and the housing by laser welding. 
           [0027]      FIG. 9  is a diagram showing a welded portion of the rear cover and the housing by the laser welding. 
           [0028]      FIG. 10  is an example of a sectional view showing a laser welded structure of the cover and the housing. 
           [0029]      FIG. 11  is an example of the sectional view showing the laser welded structure of the cover and the housing. 
           [0030]      FIG. 12  is an example of the sectional view showing the laser welded structure of the cover and the housing. 
           [0031]      FIG. 13  is a result of evaluating color differences when using the cover of polystyrene from a polybutylene terephthalate resin and an alloy material and left standing for a long time at each temperature. 
           [0032]      FIG. 14  is a result of evaluating color differences when using the cover of polycarbonate from the polybutylene terephthalate resin and the alloy material and left standing for a long time at each temperature. 
           [0033]      FIG. 15  is a diagram showing a color difference result when using the cover of polystyrene or polycarbonate from the polybutylene terephthalate resin and the alloy material and left standing for 500 h at each temperature. 
           [0034]      FIG. 16  is a result showing the relationship between ΔT (standing temperature−glass transition temperature of the alloy material)×standing time and color difference when using the cover of polystyrene from the polybutylene terephthalate resin and the alloy material. 
           [0035]      FIG. 17  is a result showing a decreasing rate of flexural strength when using the cover of polystyrene or polycarbonate from the polybutylene terephthalate resin and the alloy material and left standing at 140° C. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0036]    An embodiment described below to carry out the invention solves various challenges desired as an actual product and in particular, solves various challenges desirable for use as a measuring device to measure the intake air flow of vehicles to achieve various effects. One of various challenges solved by the embodiment below is the effect described in ADVANTAGEOUS EFFECTS OF INVENTION. Other various challenges solved by the embodiment below and other various effects achieved by the embodiment below will be mentioned in the description of the embodiment below if recognized to be necessary to describe means for solving challenges of the present invention. 
         [0037]    In the embodiment described below, the same reference sign indicates the same component even if drawing numbers are different and achieves the same operation and effect. The description of a component already described may be omitted by attaching only the reference sign. 
       First Embodiment 
     1. Embodiment in which a Flow Sensor According to the Present Invention is Used in an Internal Combustion Engine Control System 
       [0038]      FIG. 1  is a system diagram showing an embodiment in which a flow sensor according to the present invention is used in an internal combustion engine control system of an electronic fuel injection system. Based on an operation of an internal combustion engine  110  including an engine cylinder  112  and an engine piston  114 , an intake air is taken in from an air cleaner  122  as a measured gas  30  and introduced into a combustion chamber  112  of the engine cylinder via, for example, an intake body, a throttle body  126 , and an intake manifold  128  as a main passage  124 . The flow rate of the measured gas  30  as an intake air introduced into the combustion chamber is measured by a thermal flow sensor  300  according to the present invention and based on the measured flow rate, fuel is supplied from a fuel injection valve  152  and introduced into the combustion chamber together with the measured gas  30  as an intake air in a fuel-air mixture state. In the present embodiment, the fuel injection valve  152  is provided in an intake port of the internal combustion engine and fuel injected into the intake port forms a fuel-air mixture together with the measured gas  30  as an intake air before being introduced into the combustion chamber for combustion via an intake valve  116  to generate mechanical energy. 
         [0039]    In recent years, a system of directly injecting fuel from the fuel injection valve  152  into each combustion chamber by mounting the fuel injection valve  152  in a cylinder head of the internal combustion engine as a system superior in exhaust gas purification and fuel saving in many vehicles. The thermal flow sensor  300  according to the present invention can be used, in addition to the system of injecting fuel in the intake port of the internal combustion engine shown in  FIG. 1 , similarly for the system of directly injecting fuel into each combustion chamber. Basic concepts of the method of measuring control parameters including the method of using the thermal flow sensor  300  and the method of controlling the internal combustion engine including the fuel supply amount and ignition time are substantially the same for both systems and a system in which fuel is injected into the intake port is shown in  FIG. 1  as a representative example of both systems. 
         [0040]    The fuel and the air introduced into the combustion chamber form a fuel-air mixture state and are explosively burned by spark ignition of an ignition plug  154  to generate mechanical energy. The gas after the combustion is led from an exhaust valve  118  to an exhaust pipe and then discharged out of the vehicle from the exhaust pipe as an exhaust gas  24 . The flow rate of the measured gas  30  as an intake air introduced into the combustion chamber is controlled by a throttle valve  132  whose opening is changed based on the operation of an accelerator pedal. The fuel supply amount is controlled based on the flow rate of the intake air introduced into the combustion chamber. The driver can control mechanical energy generated by the internal combustion engine by controlling the flow rate of the intake air introduced into the combustion chamber by controlling the opening of the throttle valve  132 . 
         [0041]    The flow rate and the temperature of the measured gas  30  as an intake air flowing through the main passage  124  after being taken in from the air cleaner  122  are measured by the thermal flow sensor  300  and an electric signal representing the flow rate and the temperature of the intake air is input into a control apparatus  200  from the thermal flow sensor  300 . Also, the output of a throttle angle sensor  144  measuring the opening of the throttle valve  132  is input into the control apparatus  200  and further, the output of a rotation angle sensor  146  is input into the control apparatus  200  to measure the positions and states of the engine piston  114 , the intake valve  116 , and the exhaust valve  118  of the internal combustion engine and further the rotational speed of the internal combustion engine. The output of an oxygen sensor  148  is input into the control apparatus  200  to measure the state of a mixing ratio of the fuel amount and the air amount from the state of the exhaust gas  24 . 
         [0042]    The control apparatus  200  operates the fuel injection amount and the ignition time based on the flow rate of an intake air as an output of the thermal flow sensor  300  and the rotation speed of the internal combustion engine measured based on the output of the rotation angle sensor  146 . Based on these operation results, the amount of fuel supplied from the fuel injection valve  152  and the ignition time ignited by the ignition plug  154  are controlled. The fuel injection amount and the ignition time are actually controlled more finely based on the intake air temperature measured by the thermal flow sensor  300 , a change state of the throttle angle, a change state of the engine rotational speed, and an air-fuel ratio state measured by the oxygen sensor  148 . The control apparatus  200  further controls the rotational speed of the internal combustion engine in an idle operation state by controlling the amount of air bypassing the throttle valve  132  by an idle air control valve  156  in the idle operation state of the internal combustion engine. 
       2. Configuration of the Thermal Flow Sensor  300   
       [0043]      FIGS. 2 and 3  are diagrams showing the appearance of the thermal flow sensor  300  and  FIG. 2(A)  is a left side view of the thermal flow sensor  300 ,  FIG. 2(B)  is a front view thereof,  FIG. 3(A)  is a right side view thereof, and  FIG. 3(B)  is a rear view thereof. The thermal flow sensor  300  includes a housing  302 , a front cover  303 , and a rear cover  304 . The housing  302  includes a flange  312  to fix the thermal flow sensor  300  to the intake body as the main passage  124 , an external connection unit  305  having an external terminal to electrically connect to an external device, and a measuring unit  310  to measure the flow rate and the like. A sub-passage groove to create a sub-passage is provided inside the measuring unit  310  and further inside the measuring unit  310 , a circuit package  400  including a flow rate detector  436  to measure the flow rate of the measured gas  30  flowing through the main passage  124  and a temperature detector  452  to measure the temperature of the measured gas  30  flowing through the main passage  124 . 
         [0044]    An entrance  350  of the thermal flow sensor  300  is provided on a tip side of the measuring unit  310  extending from the flange  312  toward the center direction of the main passage  124  and thus, a gas flowing through a portion close to the center away from the inner wall surface, instead of near the inner wall surface, can be taken into the sub-passage. Therefore, the thermal flow sensor  300  can measure the flow rate and the temperature of the gas in a portion away from the inner wall surface of the main passage  124  and the degradation of measurement accuracy due to the influence of heat or the like can be inhibited. Measurements are more likely to be affected by the temperature of the main passage  124  near the inner wall surface of the main passage  124  and the measured gas  30  is in a different state from an original state of the gas and so is different from the average state of the main gas in the main passage  124 . Particularly, if the main passage  124  is an intake body of the engine, the main passage  124  is affected by heat from the engine and frequently maintained at a high temperature. Therefore, the gas near the inner wall surface of the main passage  124  is frequently higher than the original temperature of the gas flowing inside the main passage  124 . 
         [0045]    The fluid resistance near the inner wall surface of the main passage  124  is large and the flow velocity there is slower than the average flow velocity of the main passage  124 . Thus, if the gas near the inner wall surface of the main passage  124  is taken into the sub-passage as the measured gas  30 , a lower measured flow velocity than the average flow velocity in the main passage  124  could lead to a measurement error. In the thermal flow sensor  300  shown in  FIGS. 2 and 3 , the entrance  350  is provided in a tip portion of the measuring unit  310  in a thin and long shape extending from the flange  312  toward the center of the main passage  124  and thus, a measurement error related to a lower flow velocity near the inner wall surface of the main passage  124  can be reduced. Also in the thermal flow sensor  300  shown in  FIGS. 2 and 3 , not only the entrance  350  is provided in the tip portion of the measuring unit  310  extending from the flange  312  toward the center of the main passage  124 , but also the exit of the sub-passage is provided in the tip portion of the measuring unit  310  so that a measurement error can further be reduced. 
         [0046]    The measuring unit  310  of the thermal flow sensor  300  forms a shape extending long from the flange  312  toward the center direction of the main passage  124  and the tip portion thereof is provided with the entrance  350  to take a portion of the measured gas  30  such as an intake air into the sub-passage and also an exit  352  to return the measured gas  30  from the sub-passage to the main passage  124 . The measuring unit  310  has a shape long extending along the axis from an outer wall of the main passage  124  toward the center, but the width thereof has, as shown in  FIGS. 2(A) and 3(A) , a narrow shape. That is, the measuring unit  310  of the thermal flow sensor  300  has a thin width of the side face and the front thereof forms a substantially rectangular shape. Accordingly, the thermal flow sensor  300  can include a sufficiently long sub-passage and limit the fluid resistance to the measured gas  30  to a small value. 
         [0047]      FIGS. 4 and 5  show a state of the housing  302  in which the front cover  303  and the rear cover  304  are removed from the thermal flow sensor  300 .  FIG. 4(A)  is a left side view of the housing  302 ,  FIG. 4(B)  is a front view of the housing  302 ,  FIG. 5(A)  is a right side view of the housing  302 , and  FIG. 5(B)  is a rear view of the housing  302 . The housing  302  has a structure in which the measuring unit  310  extends in the center direction of the main passage  124  from the flange  312  and a sub-passage groove  306  to form the sub-passage is provided on the tip side thereof. Because an entrance groove  351  to form the entrance  350  of the sub-passage and an exit groove  353  to form the exit  352  are provided in the tip portion of the housing  302 , the gas in a portion away from the inner wall surface of the main passage  124 , in other words, the gas flowing through a portion neat the center of the main passage  124  can be taken in as the measured gas  30  from the entrance  350 . 
         [0048]    When the housing  302  is molded, a connection terminal  412  of the circuit package  400  and an external terminal inner end  361  of the external connection unit  305  are molded. Thus, after the molding, these wires are joined by welding or soldering to form a terminal connection unit  320  before the housing  302  is completed. In the present embodiment, the housing  302  is provided with the sub-passage groove  306  to form the sub-passage and is configured such that the sub-passage is completed by arranging a cover on the front and rear sides of the housing  302  and welding a protruding portion  307  arranged near the sub-passage groove  306  and the front cover  303  and the rear cover  304  by laser. In the present embodiment, the joining portion to the front cover  303  or the rear cover  304  on the housing  302  is the protruding portion  307 , but the joining portion does not necessarily need to be formed in a protruding shape and the joining portion may have a plane shape. Also by providing the front cover  303  and the rear cover  304  on both sides of the housing  302 , sub-passages on both sides of the housing  302  can be completed. 
         [0049]      FIG. 6  is a diagram showing the appearance of the front cover  303 ,  FIG. 6(A)  is a left side view thereof,  FIG. 6(B)  is a front view thereof, and  FIG. 6(C)  is a plan view thereof.  FIG. 7  is a diagram showing the appearance of the rear cover  304 ,  FIG. 7(A)  is a left side view thereof,  FIG. 7(B)  is a front view thereof, and  FIG. 7(C)  is a plan view thereof. In  FIGS. 2 and 3 , the front cover  303  and the rear cover  304  are used to create the sub-passage by blocking the sub-passage groove  306  of the housing  302 . In addition, a protruding portion  356  is included, which is used to provide a throttle in the channel. In addition, the front cover  303  and the rear cover  304  have a protruding portion  380  and a protruding portion  381  formed thereon and are configured, when welded to the housing  302 , to fill a gap of a cavity portion  382  on the tip side of the circuit package  400  shown in  FIGS. 4(B) and 5(B)  and at the same time, to cover the tip portion of the circuit package  400 . 
         [0050]    The front cover  303  and the rear cover  304  shown in  FIGS. 6 and 7  have a protective portion  322  formed thereon. As shown in  FIGS. 2 and 3 , the protective portion  322  on the front side provided on the front cover  303  is arranged on a side face on the front side of an entrance  343  into the temperature detector  452  of the measured gas  30  and the protective portion  322  on the rear side provided on the rear cover  304  is arranged on the side face on the rear side of the entrance  343 . The temperature detector  452  arranged inside the entrance  343  is protected by the protective portion  322  and mechanical damage of the temperature detector  452  caused by collision of the temperature detector  452  with something during production or while mounted on a vehicle can be prevented. 
         [0051]    The protruding portion  356  is provided on the inside surface of the front cover  303  and is arranged, as shown in the example shown in  FIG. 6 , opposite to a measurement channel surface  430  and has shape long extending in a direction along the axis of the channel of the sub-passage. A throttle is formed in the channel by the measurement channel surface  430  and the protruding portion  356  to reduce eddies in the measured gas  30  and also to act to generate a laminar flow. The front cover  303  and the rear cover  304  are provided with an insertion hole  326  for initial positioning during laser welding. The initial positioning can be performed by using the insertion hole  326  for reference and setting to an ejecting pin  324  formed on the housing  302  shown in  FIGS. 4 and 5 . 
       3. Material Composition of the Housing  302  and the Covers  303 ,  304  and a Laser Welding Structure 
       [0052]    The thermal flow sensor  300  in the present invention is mainly characterized in that the material composition of the housing  302  and the covers  303 ,  304  suitable for visual inspection of a welded state of the housing  302  and the covers  303 ,  304  after laser welding is selected. 
         [0053]    For the housing  302  and the covers  303 ,  304 , polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), polyamide 6 (PA6), polyamide 66 (PA66), polyamide 6T (PA6T), and polyamide 9T (PA9T) as heat-resistant crystalline resins are frequently used as main materials. 
         [0054]    Laser welding is a method of joining resins by irradiating a joining portion with laser through a light transmission resin while the light transmission resin and a light absorbent resin are put together and melting the other light absorbent resin and then, melting the light transmission resin. Laser having a wavelength of the infrared region of 800 to 1100 nm including the semiconductor laser, YAG laser, and fiber laser is effective in terms of cost as a light source used for laser welding, but laser having different wavelengths may also be used in accordance with the absorption of resin. The intensity distribution of a laser light source can be changed to various intensity distributions by an attached lens such as the Gaussian type, top hat type, and ring type and more uniform welding can be achieved by using the top hat type or ring type. For laser irradiation, the laser light source or a product may physically be moved by a stage for welding or laser light itself may be controlled using a galvanometer mirror for irradiation. 
         [0055]      FIGS. 8 and 9  show an example of a laser welded portion  390  on the housing  302  of the thermal flow sensor  300  according to the present embodiment (representing a diagram viewing the laser welded portion  390  and the inside by seeing through the front cover  303  and the rear cover  304  after laser welding by putting the front cover  303  and the rear cover  304  on the housing  302 ). First, an overview of laser welding will be provided. First, the housing  302  is set in a predetermined position and positioning adjustments are made on the ejecting pin  324  formed on the housing  302  using the insertion hole  326  formed in the covers  303 ,  304  for reference to arrange the covers  303 ,  304  precisely on the housing. In this case, it is desirable to fit the ejecting pin  324  formed on the housing  302  and the insertion hole  326  formed in the covers  303 ,  304  such that the gap of portions to be laser-welded of the covers and the housing does not become too large. Then, the covers  303 ,  304  and the housing  302  are pressed by a transparent pressing material such as glass or acrylic resin. The pressing force in this case is desirably 0.1 MPa or more to reduce the gap. 
         [0056]    The protruding portion (joining portion)  307  on the housing  302  and the joining portion formed inside the cover are close to each other so as to be almost in contact. 
         [0057]    Next, the surroundings of a circuit chamber  500  including the connection terminal  412  are laser-welded while the pressing state is maintained and further laser-welded such that a sub-passage  501  is formed. As the order of laser-irradiation, the sub-passage  501  may first be welded and then the circuit chamber  500 . However, the start point and the end point of laser irradiation tend to be unstable and particularly for the circuit chamber, the start point and end point are desirably positioned by shifting from the welded portion to form the circuit chamber  500 . Laser irradiation may be repeated a plurality of times and the influence of a gap between the covers  303 ,  304  and the housing  302  can be reduced by repeating the welding. The circuit chamber and the sub-passage  501  of the thermal flow sensor  300  are formed by performing laser welding according to the above procedure, but in view of productivity and yields of laser welding, a high transmittance of resin materials of the covers  303 ,  304  is required. 
         [0058]    Thus, to improve the transmittance of resin materials of the covers  303 ,  304 , it is effective to add a transparent non-crystalline thermoplastic resin as an alloy material and the color needs to be a natural color. Normally, near infrared laser light is frequently used for laser welding itself and so attention is frequently paid to only the transmittance for near infrared laser light, but when the visual inspection is applied to check the quality of laser welding, it turns out that a high transmittance in the visible light region is also needed. Because an alloy material contained in the covers  303 ,  304  is close to transparent, a high transmittance in the visible light region is obtained, effectively contributing to improvements of the transmittance in the visible light region. 
         [0059]    Thus, a test of whether visual inspection after laser welding can be carried out by using a material obtained by including polystyrene (PS) in a PBT resin by 20% as a laser welding resin material and changing the thickness of the covers  303 ,  304  before laser welding reveals that the welding width can correctly be recognized by setting the average transmittance in the visible light wavelength region of 450 nm or more to 35%. The test also reveals that by setting the transmittance to 45% or more, up to voids on the order of a few hundred ?m remaining in the welded portion can be detected. Therefore, it turns out that the visual inspection can be carried out by setting the average transmittance to 35% or more for visible light of 450 nm or more. This time, a detection system in which a CCD camera is used for detection is used and in such a case, having a high transmittance in the wavelength region of 450 nm to 750 nm is effective in terms of sensitivity of the CCD camera, but detection at a wavelength close to infrared becomes possible by using an infrared CCD camera or the like. In general, a crystalline resin has a higher transmittance with a wavelength closer to infrared and if this relationship holds, the transmittance for the wavelength of laser light needed for laser welding can be said to be a sufficiently unproblematical level. 
         [0060]    In addition, as an index other than the transmittance, the lightness L* and the chroma C*=[(a*) 2 +(b*) 2 ] 1/2  are measured according to the L*a*b color system defined in JISZ8729 to convert object colors into numbers. Verification under the assumption that whether the inspection can be carried out can be determined from the lightness L* and the chroma C* of the resin material of the covers  303 ,  304  reveals that if the relationships of the lightness L*&lt;75 and the chroma C*&lt;10 are satisfied by the color of a portion to be laser-welded, the visual inspection can be carried out. 
         [0061]    It also turns out that the accuracy of inspection is significantly affected in the visual inspection by increasing the difference of contrast between the laser welded portion  390  and other portions than the laser welded portion  390 . When, for example, as shown in  FIG. 10 , the visual inspection of the welded state of the laser welded portion  390  is carried out from above through the covers  303 ,  304 , if the covers  303 ,  304  in a portion  391  adjacent to the laser welded portion  390  and the housing  302  are in close contact, the portion  391  is detected similarly due to a small contrast difference between the portion  391  and the welded portion  390 . As a means for avoiding such circumstances, it is effective to make the transmittance of the covers  303 ,  304  corresponding to a region of the neighboring portion  391  smaller and the lightness L* thereof larger than those of the covers  303 ,  304  corresponding to a region of the portion  390  to be laser-welded and the lightness L*. Thus, as shown in  FIGS. 10 and 11 , it is effective to increase the contrast difference by lowering the transmittance by making the neighboring portion  391  thicker than the laser welded portion  390  of the covers  303 ,  304  and increasing the lightness L*. 
         [0062]    Also, as shown in  FIG. 12 , fine irregularities  392  may be formed on the laser irradiation surface of the covers  303 ,  304  in the neighboring portion  391  other than the laser welded portion  390 . By lowering the transmittance of light by causing scattering through the formation of the fine irregularities  392 , a contrast difference between the laser welded portion  390  and the neighboring portion  391 . However, if fine irregularities are too large, adhesiveness to the pressing material deteriorates and thus it is desirable to limit the roughness Rz to 5 μm or less. It is also effective to make a gap between the covers  303 ,  304  and the housing  302  large in a region of the neighboring portion  391  of the laser welded portion  390  between the covers  303 ,  304  and the housing  302 . 
         [0063]    In terms of the transmittance, the application of a transparent material can be considered to satisfy the above relationships, but the transparent material is basically a non-crystalline resin and does not have sufficient heat resistance and environmental resistance for use as vehicle components and if the material is close to transparent, no difference of contrast is obtained between the laser welded portion  390  and the color of the housing  302  and therefore, in terms of visual inspectability and necessary contrast, it is effective to have a color of the natural material of a crystalline resin as the main material. However, if an increase in cost can be permitted, the color can be changed from the natural color a little by adding pigment or dye after the transmittance and the color being satisfied. In addition to transparency, colors meeting the above range include black, but if black is adopted, laser welding and visual inspection cannot be combined. From the above, it is desirable to set the thickness of the covers  303 ,  304  in the laser welded portion  390  to 0.5 mm to 0.8 mm. It is also desirable to set the amount of the non-crystalline alloy material used to improve transparency to 10 to 30%. Even if, for example, molding conditions are appropriate, the transmittance may deteriorate in accordance with the shape of the covers  303 ,  304  or constraints of the balance with other components. In such a case, the quality may be guaranteed by, in addition to the visual inspection, determining sinking of the laser welded portion  390  based on the height thereof. 
       4. Method of Evaluating a Usage Environment History of the Flow Sensor 
       [0064]    It is known that when a flow sensor is used in the engine room and the internal combustion engine control system operates normally, the service temperature is in many cases 100° C. or less as an actual usage environment of the flow sensor itself and the flow sensor is not used for a long time at temperature exceeding 100° C. However, if the internal combustion engine control system fails somewhere in the system, the flow sensor itself may be exposed to a high temperature for a long time and it may become difficult to guarantee the quality of the flow sensor alone. In such a case, grasping the environment in which the flow sensor is used frequently leads to a solution. A flow sensor according to the present invention assumes the use of the natural color for the material of the covers  303 ,  304  made of resin and we found using discoloring of the material of the covers  303 ,  304  of the resin used for the flow sensor as a method of evaluating a usage environment at a high temperature for a long period at low cost without using additional components. 
         [0065]    The present invention is characterized in that the main material of the covers  303 ,  304  used for the flow sensor is a crystalline resin and the natural color containing an alloy material in which the non-crystalline glass transition temperature is 80° C. to 120° C. is used. 
         [0066]    Here, discussion results leading to the present invention will be described. Color difference changes when the PBT resin is used as the main material of the covers  303 ,  304 , polystyrene (PS) whose glass transition temperature is about 100° C. or polycarbonate (PC) whose glass transition temperature is about 150° C. is contained by about 20% as an alloy material, and left standing at each temperature are shown in  FIGS. 13 and 14 . The color difference change is calculated by converting colors of the covers  303 ,  304  into numbers by a chromatometer conforming to the standard of JISZ8729. A color difference between the initial color of the covers  303 ,  304  and the color after being left standing is calculated by a color difference ΔE=[(ΔL*) 2 +(Δa*) 2 +(Δb*) 2 ] 1/2  between two colors defined by ΔL*, Δa*, and Δb* as differences of coordinates L*, a*, and b* in the L*a*b color system. 
         [0067]    From the above result, discoloring increases with a rising temperature in each resin and it turns out that the amount of discoloring is decreased by adding PC with a high glass transition temperature.  FIG. 15  shows a result of summarizing the relationship between ΔT (standing temperature−glass transition temperature Tg) when left standing for 500 h and the color difference ΔE. The result newly reveals that when ΔT is 0, that is, the difference between the standing temperature and the glass transition temperature is larger than 0, discoloring quickly increases. 
         [0068]    Thermoplastic resins including the PBT resin is discolored by oxidation at a high temperature and when ΔT is smaller than 0, the discoloring is affected by an antioxidant or PBT resin elements more than the alloy material contained. In contrast, it turns out that if the standing temperature is higher than the glass transition temperature of the alloy material as a non-crystalline resin, oxidation of the alloy material itself clearly affects discoloring significantly. If the relationship is summarized in terms of the product of ΔT and the standing time and discoloring (color difference ΔE), as shown in  FIG. 16 , the relationship of the standing temperature and time can be estimated from discoloring. Therefore, the usage environment when left standing at an abnormal temperature can be grasped according to the degree of discoloring of the covers  303 ,  304  in accordance with discoloring of the alloy material contained. 
         [0069]    On the other hand, the flow sensor is not used at 100° C. or higher for a long time as an actual usage environment, but it is necessary to guarantee the operation at a high temperature like 130° C. for 2000 h.  FIG. 17  shows a result of evaluating flexural strength of materials when the standing time is increased to 2000 h at 140° C. using the materials of the covers  303 ,  304  described above. In  FIG. 17 , like the above case, a sample containing polystyrene (PS) whose glass transition temperature is about 100° C. or polycarbonate (PC) whose glass transition temperature is about 150° C. by about 20 wt % as an alloy material. As a result, even if polystyrene (PS) whose glass transition temperature is about 100° C. is contained, the flexural strength hardly deteriorates at 140° C. after 2000 h when extreme discoloring occurs, revealing that the material can safely be used for the flow sensor. That is, even if discoloring occurs, the discoloring is caused by oxidation only near the surface of the alloy material as a non-crystalline resin and physical properties are considered not significantly degraded. From the above, in consideration of the fact that the usage environment of the flow sensor is at most 100° C. even if the temperature rises, the glass transition temperature of the non-crystalline resin to be added needs to be between 80° C. and 120° C. 
         [0070]    In addition to the tests of leaving the flow sensor standing, various tests of the discoloring at high temperature/high humidity (85° C., 85%) are performed and results thereof show a little lower level of discoloring than that when left standing at high temperature of 90° C. and the influence of humidity is not recognized. 
         [0071]    By satisfying conditions of the transmittance and color shown above for the material of the covers  303 ,  304 , even if discoloring occurs, whether large peeling occurs in the welded portion can be determined by visual inspection of the laser welded portion  390 . Therefore, by adopting the conditions of the transmittance and color and the configuration of the alloy material, not only the quality before shipment after laser welding, but also the quality of laser welding itself can be guaranteed even if used at an abnormally high temperature for a long time. 
         [0072]    The present flow sensor is configured, as shown in  FIGS. 2 and 3 , to use the front cover  303  and the rear cover  304 . Thus, the non-crystalline alloy material contained in the front cover  303  and the rear cover  304  may be changed. Thus, by changing the glass transition temperature of the non-crystalline alloy material contained in the front cover  303  and the rear cover  304 , the temperature or the time based on discoloring of the covers  303 ,  304  can be identified more accurately. 
         [0073]    If laser welding is performed while the gap between the covers  303 ,  304  and the housing  302  is large by laser irradiation, large voids may be generated in the welded portion or an irradiated portion of the covers  303 ,  304  may be carbonized, leading to poor welding. Particularly in a flow sensor, the covers  303 ,  304  are thin and priority is given to the transmittance and thus, the rigidity of a joining portion is low and the covers  303 ,  304  are known to follow the shape of the housing  302 . However, if the warping of the housing  302  is large, the occurrence of a portion where the gap becomes large poses a challenge. If the housing  302  integrated with the circuit package  400  and having high rigidity is pressed to the extent that the warping is forced, problems such as deterioration of characteristics of the flow rate detector  436  made of Si elements and LSI contained in the circuit package  400  and the occurrence of cracks in a portion of the housing  302  are dispersed. Thus, to limit the warping when the material of the housing  302  is molded, the material of the housing  302  is desirably configured to contain a non-crystal alloy. In addition to improvements of dimensional stability after molding, containing an alloy material is also effective in improving impact resistance. 
         [0074]    If the housing  302  or the covers  303 ,  304  are deformed during operation of the flow sensor in a long-term environment, it is known that channel characteristics change and detection variations of the flow rate increase. In the present invention, an alloy material whose glass transition temperature is 80° C. to 120° C. is used for the covers  303 ,  304  and thus, in a temperature environment of, for example, about 100° C., a decrease of the elastic modulus becomes conspicuous. However, as has been described, the covers  303 ,  304  and the housing  302  are joined and the influence of the housing  302  having high rigidity on long-term deformation is large. Thus, heat resistance of the alloy material contained in the covers  303 ,  304  needs to be higher than that of the material of the housing  302  and it is desirable that the relationship of the glass transition temperature of the alloy material contained in the housing  302 ≧glass transition temperature of the alloy material contained in the covers  303 ,  304  holds. The same can be said about the elastic modulus and it is desirable that the relationship of the elastic modulus of thermoplastic resin constituting the housing  302 ≧the elastic modulus of thermoplastic resin constituting the covers  303 ,  304  holds. 
         [0075]    Regarding the covers  303 ,  304 , though the influence is small when compared with the housing  302 , it is known that a long-term dimensional change affects characteristic variations. Among others, particularly the cover  303  arranged on the side opposed to the flow rate detector made of Si elements has a more influence than the cover  304  arranged on the opposite side. Thus, while taking the transmittance into account, it is desirable that the relationship of the degree of crystallinity {the ratio of a crystalline portion in a state divided into a crystalline state in which macromolecules are regularly arranged and a non-crystalline state in which macromolecules are present like a clew or entangled and is defined like (degree of crystallinity)=(crystalline region portion)/(sum of the crystalline region portion and the non-crystalline region portion)} of the cover  303 &gt;degree of crystallinity of the cover  304  on the rear side holds. 
         [0076]    In some cases, the relationship of the glass transition temperature of the alloy material contained in the cover  303  arranged on the side opposite to the flow rate detector made of Si elements&gt;glass transition temperature of the alloy material contained in the cover  304  arranged on the opposite side may hold. 
         [0077]    An inorganic substance, for example, glass fiber, glass flakes, or glass in a special shape may be added and a glass material of about 20% to 50% may be added as a means for improving dimensional stability during molding and over a long period. However, the addition of the glass material deteriorates laser transmission of the covers  303 ,  304  and thus, in view of compatibility with long-term dimensional stability, the glass material contained in the material of the covers  303 ,  304  may be set to 20% to 30%. 
         [0078]    A coloring agent such as carbon black is added to the material of the housing  302  for laser welding and thus, there is no need to consider discoloring, the transmittance, and the color thereof, but the housing  302  needs to have less deformation than the covers  303 ,  304 . From the above, the relationship of the addition ratio of the glass material of thermoplastic resin constituting the housing  302 ≧addition ratio of the glass material of thermoplastic resin constituting the covers  303 ,  304  needs to hold. 
         [0079]    From the viewpoint of inhibiting long-term deformation, it is known that the degree of crystallinity of crystalline thermoplastic resin decreases and the transmittance thereof increases as the die temperature during molding decreases. Thus, a low die temperature satisfying conditions of the transmittance and color may be applied to the covers  303 ,  304 . On the other hand, dimensional stability in a usage environment increases with an increasing degree of crystallinity and thus, a higher degree of crystallinity is desirable for the housing  302 . Therefore, for the flow sensor, it is desirable that the relationship of the degree of crystallinity of thermoplastic resin constituting the housing  302 &gt;degree of crystallinity of thermoplastic resin constituting the covers  303 ,  304  holds. 
         [0080]    Various additives, for example, stabilizers (antioxidants, ultraviolet absorbers, and heat stabilizers), crystallizing nuclear materials, fire retardants and the like may be added to a crystalline resin forming the covers  303 ,  304  and the housing  302  of the flow sensor. However, the covers  303 ,  304  need to avoid containing any material contributing to the fall of the transmittance if possible. On the other hand, the housing  302  desirably contains a coloring agent that absorbs laser light, for example, carbon black. In view of laser welding of the covers  303 ,  304  and the housing  302 , materials of the cover  303 ,  304  and the housing  302  desirably does not contain any mold releasing agent. Alloy materials of non-crystalline resin to be added to a crystalline resin constituting the covers  303 ,  304  are materials whose glass transition temperature is 80° C. to 120° C. such as polystyrene (PS), acrylonitrile styrene (AS), acrylonitrile butadiene styrene copolymer (ABS), polymethyl methacrylate (PMMA), cycloolefin polymer (COP), cycloolefin copolymer (COC), and polyvinyl chloride (PVC) and alloy materials to be added to a resin constituting the housing  302  include, in addition to alloy materials to be added to the covers  303 ,  304 , polycarbonate (PC), modified polyphenylene ether (mPPE), polyether-imide (PEI), polyarylate (PAR), polysulfone (PSF), and polyether sulfone (PES). While at least one of alloy materials of non-crystalline resin to be contained in the covers  303 ,  304  is desirably set to 80° C. to 120° C., not only one alloy material, but also other alloy materials having crystallinity may also be contained. This also applies to the housing  302 . 
         [0081]    In the present invention described above, laser welding of the covers  303 ,  304  and the housing  302  has been described in detail, but similar effects such as quality guarantee based on the appearance and identification of the usage environment based on discoloring can be obtained by using, in addition to laser welding, joining using an adhesive or other welding (thermal welding, vibration welding, and ultrasonic welding). 
         [0082]    Also, the present invention can be used for application of products other than the thermal flow sensor  300  and can be adopted for laser welding of general thermoplastic resins. Thermoplastic resins include, in addition to those mentioned above, polyethylene (PE), polypropylene (PP), polyoxymethylene (POM), polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), polyether ether ketone (PEEK), liquid crystalline polymer (LCP), and polytetrafluoroethylene (PTFE). Non-crystalline resins contained as alloy materials include the above materials. Thermoplastic resins containing inorganic substances such as glass fiber or special additives are also included. The present invention can be applied not only to thermoplastic resins as main materials, but also to epoxy or acrylic thermoset resins having a relatively high transmittance. 
         [0083]    In the foregoing embodiments, the divided embodiments are described, but each divided embodiment is not unrelated to each other and one divided embodiment is related to a portion or all of modifications of another divided portion. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           24  Exhaust gas 
           30  Measured 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 apparatus 
           300  Thermal flow sensor 
           302  Housing 
           303  Front cover 
           304  Rear cover 
           305  External connection unit 
           306  Sub-passage groove 
           307  Housing protruding portion for laser welding 
           310  Measuring unit 
           312  Flange 
           320  Terminal connection unit 
           322  Protective portion 
           324  Ejecting pin 
           326  Insertion hole 
           343  Entrance 
           350  Entrance 
           351  Entrance groove 
           352  Exit 
           353  Exit groove 
           356  Protruding portion 
           361  External terminal inner end 
           380  Protruding portion 
           381  Protruding portion 
           382  Cavity portion 
           390  Laser welded portion 
           391  Neighboring portion of the laser welded portion 
           392  Fine irregularities 
           400  Circuit package 
           412  Connection terminal 
           436  Flow rate detector 
           452  Temperature detector 
           500  Circuit chamber 
           501  Sub-passage