Abstract:
Provided is a welding structure which enables external visual checks and has improved stability of bonding properties at a laser weld between a cover and a housing of this flow sensor. This flow sensor is provided with a housing, a cover, a circuit chamber sealed between these and housing electronic components or wiring, and a subpassage through which the fluid flows that is to be sensed, and is characterized in that at least the part near the gate section of the cover that transmits the laser is thinner than the other parts that transmit the laser.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a flow sensor and a manufacturing method therefor. Further, the present invention relates to a joining structure of both plastics using laser welding and a method therefor. 
       BACKGROUND ART 
       [0002]    In Patent Literature 1 (JP-A-2002-67165), disclosed is a method for providing a concave part on a laser irradiation part on a transmitting resin side, shortening a transmission distance, and making welding effectively as a method for making laser welding of a housing for storing measurement devices and a cover for covering them in a measuring instrument such as a thermal type flow meter. Further, in Patent Literature 2 (JP-A-2009-056722), disclosed is a method for providing an opening on a transmitting resin side and determining welding in a state in which a welded part is evaginated as a method for inspecting a laser welded part. 
       CITATION LIST 
     Patent Literature 
       [0003]    PATENT LITERATURE 1: JP-A-2002-67165 
         [0004]    PATENT LITERATURE 2: JP-A-2009-056722 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    A flow sensor includes a flow rate detection unit and a temperature detection unit, and they are arranged on a housing (chassis). Further, various electronic components are mounted in a circuit chamber provided in the housing. In order to prevent short circuit, corrosion, or the like of a wiring unit etc, the housing and a cover need to be sealed. As a method for directly connect the cover and the housing precisely without giving damages to electronic components, a laser welding method is used. However, according to investigations of the inventor, as a problem, it is revealed that when a cover made of PBT resin is molded by a conventional laser welding method, since a transmittance near a gate is as low as a half or less of the transmittances of other portions, it is necessary to adjust laser power or suppress a speed and it is difficult to make stable welding because of complicated control. Further, as a problem, it is revealed that since a transmittance is low near the gate, an appearance inspection of the welding cannot be performed using images. In Patent Literature 1, disclosed is that the transmission distance is shortened, and the laser irradiation part is in a concave state by this method. When crystalline resins having lots of scattering like PBT are used, an effect of the scattering increases and a heat input distribution of laser also is unstable. Based on the above, it is revealed that there arises a problem that a welded state is unstable particularly in end portions of a laser spot. 
         [0006]    In the inspection method disclosed in Patent Literature 2, there arises a problem that since a large amount of pyrolytic component (gas) is generated from that portion, pressurizing materials become tainted, cleaning has to be always performed, and productivity is largely reduced. 
         [0007]    In view of the foregoing, it is an object of the present invention to provide a laser welding structure of a cover and a housing in which productivity is improved and a low cost is implemented while maintaining high quality and high reliability of a flow sensor without causing these new problems. 
       Solution to Problem 
       [0008]    To solve the above problems, for example, a configuration described in a scope of claims is adopted. The present invention includes a means for solving the above problems in plurality, and one example is taken. A flow sensor includes a housing, a cover, a circuit chamber that is sealed between the housing and the cover and has electronic components and wiring parts built-in, and a sub-passage part through which a fluid flows that is to be sensed, and is characterized in that at least a thickness of one portion that transmits laser light near a gate part of the cover is thinner than thicknesses of the other portions that transmit the laser light. 
       Advantageous Effects of Invention 
       [0009]    By an adoption of the present invention, provided is a low-cost flow sensor that has high quality and high reliability. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  An overall view of an internal combustion engine control system using a flow sensor of the present invention; 
           [0011]      FIG. 2(A)  A left side view illustrating an appearance of the flow sensor; 
           [0012]      FIG. 2(B)  A front view illustrating an appearance of the flow sensor; 
           [0013]      FIG. 3(A)  A right side view illustrating an appearance of the flow sensor; 
           [0014]      FIG. 3(B)  A back view illustrating an appearance of the flow sensor; 
           [0015]      FIG. 4(A)  A left side view of a housing of the flow sensor; 
           [0016]      FIG. 4(B)  A front view of the housing of the flow sensor; 
           [0017]      FIG. 5(A)  A right side view of the housing of the flow sensor; 
           [0018]      FIG. 5(B)  A back view of the housing of the flow sensor; 
           [0019]      FIG. 6(A)  A cross-section view of a welded part illustrated from a vertical direction to a laser scanning direction according to a first embodiment; 
           [0020]      FIG. 6(B)  A cross-section view of the welded part illustrated from a direction along the laser scanning direction according to the first embodiment; 
           [0021]      FIG. 7  A cross-section view of the welded part illustrated from a vertical direction to the laser scanning direction according to a second embodiment; 
           [0022]      FIG. 8  A front view of the housing of the flow sensor according to a third and a fourth embodiment; 
           [0023]      FIG. 9  A cross-section view of a concave part  501  illustrated from a vertical direction to the laser scanning direction according to the third embodiment; 
           [0024]      FIG. 10  A cross-section view of the concave part  501  illustrated from a vertical direction to the laser scanning direction according to the third embodiment; 
           [0025]      FIG. 11  A front view of the housing of the flow sensor according to a fifth embodiment; 
           [0026]      FIG. 12  A cross-section view of the welded part in which the cover and the housing are laser-welded, and a view of the welded part illustrated from a direction along the laser scanning direction according to the fifth embodiment; 
           [0027]      FIG. 13  A front view of the housing of the flow sensor according to a sixth embodiment; 
           [0028]      FIG. 14  A front view of the housing of the flow sensor according to a seventh embodiment; 
           [0029]      FIG. 15  A front view of the housing of the flow sensor according to an eighth embodiment; and 
           [0030]      FIG. 16  A front view of the housing of the flow sensor according to a ninth embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0031]    An internal combustion engine control system including a flow sensor of the present invention will be described with reference to  FIG. 1 . Based on operations of an internal combustion engine  110  including an engine cylinder  112  and an engine piston  114 , air is inhaled and it is gas to be measured  30  that is measured by a thermal type flow sensor  300  of the present invention. The inhaled gas to be measured  30  flows through an air cleaner  122  and is guided to a combustion chamber of the engine cylinder  112  via a main passage  124 , a throttle body  126 , and an intake manifold  128 . Based on a flow rate measured by the thermal type flow sensor  300 , fuel is supplied from a fuel injection valve  152  and is guided to the combustion chamber in a state of an air-fuel mixture together with the gas to be measured  30 . The present embodiment will be described by using a system in which fuel is injected to an intake port of the internal combustion engine as illustrated in  FIG. 1 , namely, a so-called engine of a premix type. The thermal type flow sensor  300  of the present invention is not limited thereto, and is applicable also to a direct injection type in which fuel is directly injected to each combustion chamber. 
         [0032]    The fuel and air guided to the combustion chamber form a mixing state of the fuel and air. By spark ignition of a spark plug  154 , the fuel and air burn explosively and generate mechanical energy. The gas after the combustion is guided to an exhaust pipe from an exhaust valve  118  and is exhausted as exhaust air  24  to an outside of a vehicle from the exhaust pipe. An amount of intake air guided to the combustion chamber is controlled by a throttle valve  132  in conjunction with an accelerator pedal. The amount of fuel to be supplied is controlled based on the intake air amount, and a driver controls the opening degree of the throttle valve  132  to control the intake air amount. The process permits mechanical energy generated by the internal combustion engine to be controlled. 
         [0033]    The flow rate and the temperature of the gas to be measured  30  that is taken from the air cleaner  122  and flows through the main passage  124  is measured by the thermal type flow sensor  300 , and measured values thereof are input to a control device  200 . Further, an output from a throttle angle sensor  144  that measures the opening degree of the throttle valve  132  is input to the control device  200 . Further, positions and states of the engine piston  114 , an intake valve  116 , and the exhaust valve  118  are input to the control device  200 . In addition, to measure a rotating speed of the internal combustion engine, an output from a rotation angle sensor  146  is input to the control device  200 . To measure a state of a mixing ratio between the amount of fuel and the amount of air based on a state of the exhaust air  24 , an output from an oxygen sensor  148  is input to the control device  200 . 
         [0034]    The control device  200  calculates a fuel injection amount and an ignition timing based on the intake air amount being an output from the thermal type flow sensor  300  and the rotating speed of the internal combustion engine. Based on the calculation results, the amount of fuel supplied from the fuel injection valve  152  and the ignition timing in which ignition is performed by the spark plug  154  are controlled. Further, the amount of fuel to be supplied and the ignition timing are minutely controlled in practice based on a changed state of the intake air temperature and the throttle angle measured by the thermal type flow sensor  300 , a changed state of an engine rotating speed, and a state of an air-fuel ratio measured by the oxygen sensor  148 . In an idle operating state of the internal combustion engine, the control device  200  further controls the amount of air that bypasses the throttle valve  132  by using an idle air control valve  156 , and controls a rotating speed of the internal combustion engine in the idle operating state. 
         [0035]    Next, an appearance structure of the thermal type flow sensor  300  will be described with reference to  FIGS. 2 and 3 .  FIG. 2(A) ,  FIG. 2(B) ,  FIG. 3(A) , and  FIG. 3(B)  illustrate a left side view, a front view, a right side view, and a back view of the thermal type flow sensor  300 , respectively. 
         [0036]    The thermal type flow sensor  300  includes a housing  302 , a front cover  303 , and a rear cover  304 . The housing  302  includes a flange  312  for fixing the thermal type flow sensor  300  on the main passage  124 , an external connection part  305  having an external terminal for providing electrical connection with an external device, and a measuring unit  310  that measures a flow rate or the like. In the measuring unit  310 , a sub-passage groove for making a sub-passage is provided. Further, as illustrated in  FIGS. 4 and 5 , in the measuring unit  310 , provided is a circuit package  400  including a flow rate detection unit that measures a flow rate of the gas to be measured  30  flowing through the main passage  124  and a temperature detection unit  452  that measures a temperature of the gas to be measured  30  flowing through the main passage  124 . 
         [0037]    Next, an internal structure of the thermal type flow sensor  300  will be described with reference to  FIGS. 4 and 5  illustrating states of the housing  302  from which the front cover  303  and the rear cover  304  are detached.  FIG. 4(A)  is a left side view of the housing of the flow sensor, and  FIG. 4(B)  is a front view thereof.  FIG. 5(A)  is a right side view of the housing of the flow sensor, and  FIG. 5(B)  is a back view thereof. In  FIGS. 4(A) and 4(B) , the sub-passage groove  306  for molding the sub-passage is provided on the housing  302 . A projecting part  307  in which the covers are disposed on a front surface and a rear surface of the housing  302  and that is disposed near the sub-passage groove  306 , the front cover  303 , and the rear cover  304  are welded by laser to thereby complete the sub-passage. 
         [0038]    In  FIGS. 5(A) and 5(B) , a part of the gas to be measured  30  flowing through the main passage  124  is taken in the rear sub-passage groove  306  from an inlet groove  351  for molding an inlet  350  and flows through the rear sub-passage groove  306 . The rear sub-passage groove  306  has a shape in which the groove thereof is deeper as advancing through the groove more. As flowing through the groove more, the gas to be measured  30  moves more gradually to a direction of the front side. Particularly, the rear sub-passage groove  306  is provided with a steep slope part that is drastically deepened in an upstream part  342  of the circuit package  400 . A part of air in which mass is small moves along the steep slope part and, in the upstream part  342  of the circuit package  400 , the air flows through a measuring flow passage surface  430  illustrated in  FIG. 4(B) . On the other hand, foreign materials in which mass is large move along a rear measuring flow passage surface  431  illustrated in  FIG. 5(B)  since it is difficult to rapidly change a course because of an inertia force. Thereafter, the foreign materials move along a downstream part  341  of the circuit package  400  and flow through the measuring flow passage surface  430  illustrated in  FIG. 4(B) . With that, the appearance structure and the internal structure of the thermal type flow sensor  300  are described. 
         [0039]    Next, a laser welding method for the housing and the covers according to the present invention will be described with reference to  FIGS. 2 to 7 . The laser welding method is a method for irradiating laser, in a state in which a light-transmitting resin and a light-absorbing resin are overlapped, through the light-transmitting resin, melting a portion in which the light-absorbing resin has contact with the light-transmitting resin, and further melting the light-transmitting resin by heat transmitted from the light-absorbing resin to be brought into contact with the light-absorbing resin. Because of the above-described welding principle, a natural material containing no coloring agent is preferably used as the light-transmitting resin for the covers  303  and  304 . On the other hand, preferably, into materials used as the light-absorbing resin for the housing  302 , carbon black is contained and the materials are colored into black. In addition, for the housing  302  and the covers  303  and  304  of the present invention, polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), nylon 6 (PA6), nylon 66 (PA66), nylon 6T (PA6T), or the like being crystalline resins having high heat resistance is assumed. 
         [0040]    Further, in the thermal type flow sensor  300  of the present invention, high dimensional precision and dimensional stability are particularly required for the housing  302  side, and therefore glass materials of approximately 20 to 40% are added in many cases. However, laser transmission tends to be deteriorated by the addition of the glass materials. Therefore, an addition rate of glass fiber of thermoplastic resins composing the housing  302  is preferably equal to or greater than that of the thermoplastic resins composing the covers  303  and  304 . 
         [0041]    Further, in crystalline thermoplastic resins, as a tool temperature during molding is lower, crystallinity is lower and a transmission factor is higher. Therefore, the crystallinity of the thermoplastic resins composing the housing  302  is preferably equal to or greater than that of the thermoplastic resins composing the covers  303  and  304 . 
         [0042]    Further, from the standpoint of the dimensional precision, not only the glass fiber to be added but also an alloy system containing amorphous resins is preferably used as resin materials of the housing  302 . 
         [0043]    As a light source used for the laser welding, laser having wavelengths of light in an infrared region including semiconductor laser, YAG laser, and fiber laser is effective in terms of costs. Further, laser having other wavelengths may be used in accordance with the absorption of resins. Further, an intensity distribution of laser light sources can be converted to various intensity distributions based on a lens belonging to Gaussian type, top-hat type, ring type, or the like. However, when the top hat type or the ring type is used, the welding can be uniformly made. When laser is irradiated, a laser light source or a product may be physically moved to be welded on a stage, or laser light itself may be controlled and irradiated using a galvanometer mirror. 
         [0044]    Next, the laser welding method of the present invention will be described. First, the housing  302  is set at a predetermined position, and the covers  303  and  304  are arranged on the housing  302  with high precision. Thereafter, the covers  303  and  304  and the housing  302  are pressurized by transparent pressurizing materials such as glass or acrylate resin. Laser welding is made around a circuit chamber in a state in which the pressurized state is maintained. Further, the laser welding is made so that the sub-passage is formed. 
         [0045]    Here, in conventional laser welding, a transmittance in a portion near a gate is reduced to half or less as compared to those in the other portions, and therefore it is necessary to adjust laser power or reduce a speed. Further, the control is complicated and stable welding also is difficult. Further, since the transmittance is low near the gate, a welding state cannot be understood by an appearance of an inspection after the welding. 
         [0046]    As a means for improving a transmittance of molded components, it is considered that a material system having a high transmittance is used, a thickness is thinned, and a tool temperature is lowered. In the above, from the standpoint of the fact that the material having the high transmittance is used, according to investigations of the inventor, a crystalline material such as PBT is used in many cases as a material for the covers  303  and  304  of the thermal type flow sensor  300 . Further, since a material for lowering a transmittance, such as glass fiber is also contained, it is difficult to raise up the transmittance by the material itself. 
         [0047]    Further, from the standpoint of the fact that a thickness is thinned while keeping a thickness of the covers  303  and  304  at a constant, according to the investigations of the inventor, molding conditions are rationalized in consideration of melt viscosity or flow characteristics of PBT resin. As a result, an improvement effect is recognized in the transmittances of portions other than the gate part, but the transmittance of the gate part is kept still low. 
         [0048]    Further, from the standpoint of the fact that the tool temperature is lowered by the PBT resin, according to the investigations of the inventor, it is revealed that even if the tool temperature is lowered up to 40° C., the improvement effect of the transmittance is small near the gate part through which molten resin flows. As described above, the improvement effect of the transmittance in a portion near the gate part is small by these methods. 
         [0049]    To cope with the problems, as a result of further investigations of the inventor, it is revealed that since a temperature of the resin is high near the gate part, crystallinity is high and the transmittance is low. Further, it is revealed that the resin temperature is high, and thereby sufficient flowability of the resin can be secured even in a portion in which a thickness is thin and the resin can be filled up into fine parts. Further, it is revealed that even if the thickness is slightly thin, the crystallinity is high, and thereby a problem is not fundamentally caused by strength itself. Therefore, it is revealed that in the case in which the PBT resin is used as a cover material, even if a thickness of a concave part is made to be as thin as approximately 0.5 mm only near the gate part, the molding can be performed. Further, it is revealed that a thickness of a portion near the gate part is made to be thinner than those of the other portions, and thereby transmittance variations can be largely reduced based on securement of the strength. 
         [0050]    To solve the problems, in the present invention, proposed is a flow sensor in which in  FIGS. 2(B) and 3(B) , concave parts  501  are provided near the gate of the covers  303  and  304 , respectively, and thicknesses of the covers  303  and  304  are made to be thinner than those of other portions. The present embodiment will be described with reference to  FIGS. 6(A) and 6(B) . 
         [0051]      FIGS. 6(A) and 6(B)  illustrate cross-section views of a laser welded part  390  of the concave part  501  of the front cover  303 .  FIG. 6(A)  illustrates a vertical direction to a laser welding line, and  FIG. 6(B)  illustrates a direction along the laser welding line. 
         [0052]    In  FIG. 6(A) , the concave part  501  is provided in the front cover  303 , and thereby stability of the laser welding in this portion can be improved and an appearance inspection can be performed. An inspection method using the laser welding includes a measuring method using a radiation temperature, a measuring method using optical interference, an inspection method using an appearance, and the like. As a method for directly determining quality of a welded state in a short tact, the appearance inspection is a most effective method. 
         [0053]    In addition, in general laser welding, a wavelength in an infrared region is used in many cases. In this case, the transmittance is set to 20% or more, and thereby the preferable welded part  390  can be formed. Further, in order to grasp a state of the welded part  390  by the appearance, a necessary wavelength region is a visible light region and a transmittance largely greater than that required for the laser welding is necessary. Particularly, in the case in which the appearance inspection is performed using a CCD, a necessary wavelength region is 450 to 750 nm in many cases. In the case in which the transmittance is set to 30%, detection cannot be performed as much as 20% or a large void cannot be detected. On the other hand, in the case in which the transmittance is set to 35% or more, a probability of the detection is 100% and a large void can be observed. 
         [0054]    Further, in the case in which the transmittance in a portion corresponding to the welded part  390  other than the gate part of the covers  303  and  304  also is made to be high, through the welded part  390  and the covers  303  and  304 , gradations of the housing  302  are hard to create and an image is hard to inspect in some cases. In such a case, the thicknesses of the covers  303  and  304  are set so that the transmittance in the covers  303  and  304  of a portion in which the welding is not made near the welded part  390  is set to 20% or less in which the appearance observation of the welded part  390  cannot be perfectly performed, and thereby an image of the welded part  390  is easy to inspect. That is, a ratio between the thicknesses of the covers  303  and  304  is set so that a difference between the transmittance in the covers  303  and  304  of the welded part  390  and the transmittance in the portion other than the welded part  390  is set to 15% or more. Thereby, it is possible to perform a preferable appearance inspection using the images. In the laser welding, it is sufficient to just consider the transmittance in only a wavelength region of the laser light source. In addition, in the appearance inspection, because of dependence on sensitivity of a CCD etc, not only the transmittance in an infrared wavelength region but also that in a visible light region is preferably high. 
         [0055]    Further, to realize stable welding, the concave portions  501  provided in the covers  303  and  304  on the laser irradiation side illustrated in  FIGS. 6(A) ,  6 (B), and  7  need to be made to be greater than the projecting part  307  formed on the housing  302 . The projecting part  307  formed on the housing  302  is preferably provided in all places of the welded part  390 . 
       Second Embodiment 
       [0056]    A second embodiment of the present invention will be described with reference to  FIG. 7 . In the present embodiment, not only the concave part  501  is proved in a laser irradiation surface of the front cover  303  but also the concave part  308  is provided in the front cover  303  on a joining surface side with the housing  302 . 
         [0057]    Also in that case, the concave part  308  provided in the joining surface of the front cover  303  is preferably provided in all places of the welded parts  390 . 
         [0058]    In the case in which a gate structure is set to a side gate in which a gate part is located on a side surface of a product, a gate position may be provided in any place in the longitudinal direction of the front cover  303  in order to make high a flatness of the front cover  303 . Therefore, the gate part may be arranged not on a passage side but on a circuit chamber side near the flange  312 . In that case, like  FIG. 8 , it is sufficient to just provide the concave part  501  in the front cover  303  on the circuit chamber side. 
       Third Embodiment 
       [0059]    A third embodiment of the present invention will be described with reference to  FIG. 9 . In the present embodiment, a burr is provided in the welded part  390 , and  FIG. 9  illustrates a cross-section view in the vertical direction to the welding line. The crystallinity is high near the gate part in which the concave part  501  is provided in the laser irradiation part of the front cover  303 . Therefore, the front cover  303  near the gate part is higher in the strength than the other welded parts, and a thickness of the front cover  303  is thinner than those of the other welded parts  390 . Therefore, in the case in which the thickness is reduced to half or less, this may cause problems in terms of the strength. Like the present embodiment, a burr is provided in the welded part  390 , and thereby a stress relaxation function and an improvement in the strength can be realized at the same time. 
       Fourth Embodiment 
       [0060]    A fourth embodiment of the present invention will be described with reference to  FIG. 10 . When the large burr is extended to a passage part in the third embodiment, characteristic variations of the thermal type flow sensor  300  are increased. Therefore, like  FIG. 10  in the present embodiment, the concave part  308  is provided in the front cover  303  on the joining surface side with the housing  302  to store the burr in the inside of the concave part  308 . In addition, the burr in the present embodiment may be formed in the laser welded part  390  other than a portion in which the concave part  501  is formed in the laser irradiation surface, and further the burr may be formed in all portions. 
         [0061]    A spot size of the laser light  550  is made to be greater than the projecting part  307  formed on the housing, and thereby the burr of the present embodiment can be formed. Like  FIG. 11 , a width of the laser welded part  390  in only a portion corresponding to the concave part  501  of the front cover  303  is made to be great to thereby improve the strength. 
       Fifth Embodiment 
       [0062]    A fifth embodiment of the present invention will be described with reference to  FIG. 12 .  FIG. 12  is a cross-section view in a direction along the laser welding line of the laser welded part  390  of the concave part  501  of the front cover  303 . In the case in which the concave part  501  is provided in a portion of the laser irradiation part of the front cover  303 , a portion in the concave part  501  largely differs in the transmittance from a laser irradiation portion other than the above portion. Further, a polished mirror surface needs to be used as much as possible as a portion on which laser is irradiated. However, it is difficult to change to a mirror surface a vicinity of a boundary between the concave part  501  of the front cover  303  and a thick portion other than the concave part  501 , and scattering increases also during the laser welding. Therefore, like  FIG. 12 , a boundary between the concave part  501  in the direction along the laser welding line and the portion other than the concave part  501  is changed to an inclination part, and thereby a difference between transmittances can be changed gradually. According to the present embodiment, the laser irradiation surface can be changed to a mirror surface, and an effect caused by the difference between the transmittances can be reduced also to the image inspection. An inclination angle is preferably 15 degrees or less. In addition, in the case in which a galvanometer mirror is used in a laser irradiation method, the concave part  501  is inclined also to the irradiation direction side so that the concave part  501  of the front cover  303  is prevented from scattering laser light. 
       Sixth Embodiment 
       [0063]    A sixth embodiment will be described with reference to  FIG. 13 .  FIG. 13  is a front view of the housing of the thermal type flow sensor  300 . In the first to fifth embodiments, the gate structure is set to the side gate. However, a finish work of a gate cut is required in the side gate, and therefore extra costs are required. To cope with the problem, in the present embodiment, the gate structure is set to a pin gate arranged on a top face of the product, and thereby costs can be reduced. However, when the gate structure is set to the pin gate, a molten resin is radially spread, and therefore the pin gage tends to be greater than the side gate in the region in which the transmittance is reduced. In the case of the side gate, the side gate is separated from the gate position by 5 mm, whereas in the case of the pin gate, the pin gate is separated from the gate position by 7 to 9 mm. In consideration of the above point, in the present embodiment, it is sufficient to just provide the gate position in the central part of the circuit chamber in which a shape is relatively isotropic as illustrated in  FIG. 13 . Further, it is sufficient to just provide a region in which a thickness of the concave part  501  of the front cover  303  is thin in all portions corresponding to the welded part  390  of the circuit chamber. In the case in which the gate structure is set to the pin gate, the gate part is automatically cut and a leftover of the gate cutting is generated. Therefore, in consideration of the gate cutting, a thickness of the gate position is preferably thinned. 
       Seventh Embodiment 
       [0064]    A seventh embodiment will be described with reference to  FIG. 14 .  FIG. 14  is a front view of the housing of the thermal type flow sensor  300 . Like a throttle part of the front cover  303 , a flow of resins during the molding is deteriorated in a portion in which a thickness is rapidly thickened. Therefore, a common portion of the passage part and the circuit chamber in which the thickness is rapidly thickened tends to be deteriorated more than a portion of the other circuit chamber in the transmittance. To cope with the problem, in the present embodiment, a second concave part  502  is provided in the common portion of the passage part and the circuit chamber of the front cover  303 . Further, a thickness of a flat part of the concave part  502  is made to be thinner than the thickness of the concave part  501  formed in the front cover  303  on the circuit chamber side other than the flat part. 
       Eighth Embodiment 
       [0065]    An eighth embodiment will be described with reference to  FIG. 15 .  FIG. 15  is a front view of the housing of the thermal type flow sensor  300 . In the case in which the gate position is arranged in the central part of the circuit chamber, the gate position and the welded part can be separated from each other, and therefore the gate position is not provided in all portions of the circuit chamber in some cases. In such a case, like  FIG. 15 , the concave part  501  may be provided in the laser irradiation surface of the front cover  303  only in the common portion of the circuit chamber and the passage part. 
       Ninth Embodiment 
       [0066]    A ninth embodiment will be described with reference to  FIG. 16 .  FIG. 16  is a front view of the housing of the thermal type flow sensor  300 . In  FIG. 16 , a position of the pin gate is provided on the passage part. In the case in which the gate position is set to the side gate, the gate position can be arranged at a portion of the inlet or outlet of the thermal type flow sensor  300  as a position that is relatively separated from the welded part. However, due to variations of a shape of the gate cut part  500 , variations may occur in a characteristic itself. However, in the case in which the gate position is set to the pin gate, the gate cut part  500  can be arranged at a position that is not related to a portion through which air flows, only in the thickness direction of the front cover  303 . Accordingly, in such a case, the gate position may be arranged on the passage on the outer peripheral side. The reason is that when resins flow in the longitudinal direction as much as possible, a dimensional precision of the front cover  303  is preferable. 
         [0067]    Above described above, in any structure, in the case in which the gate position is set to the pin gate, a region tends to increase in which a flat part is provided in which a thickness of the concave part  501  formed in the front cover  303  is thin. Therefore, the burr may be provided in the welded part and a welding area may be increased at the same time. Further, in this structure, any of the laser welded part  390  is formed by lines, and all portions are not necessarily welded relating to the sub-passage part in some cases. In such a case, it is sufficient to just use the concave part  501  formed in the covers  303  and  304  only in a portion in which the welding is partially made. In the invention, most of descriptions are heretofore made with reference to figures from the front cover  303  side, and further the same configuration is formed also relating to figures from the rear cover  304  side. Further, the gate position is described in the case of only one place, and further the gate position may be arranged in plurality. In this case, the number of the concave parts  501  formed in the covers  303  and  304  preferably corresponds to the number of the gate positions. 
         [0068]    In addition, the present invention can be used for applications of products in which problems are analogous other than the thermal type flow sensor and the present invention can be adopted for the laser welding of general thermoplastic resins. Amorphous resins of the thermoplastic resins include polystyrene (PS), acrylonitrile-styrene (AS), acrylonitrile-butadiene-styrene copolymer (ABS), polyetherimide (PEI), polycarbonate (PC), polyarylate (PAR), polymethylmethacrylate (PMMA), cycloolefin polymer (COP), cycloolefin copolymer (COC), polysulfone (PSF), polyether sulfone (PES), polyvinyl chloride (PVC), and polyvinylidene chloride (PVDC). Other than the above, the crystalline resins include polyethylene (PE), polypropylene (PP), polyoxymethylene (POM), polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), polyethylenenaphthalate (PEN), polyether ether ketone (PEEK), liquid crystal polymer (LCP), and polytetrafluoroethylene (PTFE). Further, the crystalline resins include their alloy materials, an inorganic material such as glass fiber, and a thermoplastic resin including particular addition agents. Generally, an amorphous resin is excellent in moldability or transparency whereas a crystalline resin is excellent in heat resistance or chemical resistance. Further, the present invention may be applied to not only a thermoplastic resin but also an epoxy-based thermosetting resin. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           24  Exhaust air 
           30  Gas to be measured 
           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  Angle sensor 
           146  Rotation angle sensor 
           148  Oxygen sensor 
           152  Fuel injection valve 
           154  Spark plug 
           156  Idle air control valve 
           200  Control device 
           300  Thermal type flow sensor 
           302  Housing 
           303  Front cover 
           304  Rear cover 
           305  External connection part 
           306  Sub-passage groove 
           307  Projecting part for laser welding 
           308  Concave part of joining surface 
           310  Measuring unit 
           312  Flange 
           315  Thermal insulating part 
           317  Upstream projection 
           318  Downstream projection 
           320  Terminal connection part 
           322  Protection part 
           324  Knockout pin 
           326  Insertion hole 
           328  Aligning part 
           341  Downstream part 
           342  Upstream part 
           343  Inlet 
           350  Inlet 
           351  Inlet groove 
           353  Outlet groove 
           356  Projecting part 
           361  External terminal inner edge 
           380  Projecting part 
           381  Projecting part 
           382  Hollow part 
           386  Front-side flow passage 
           387  Rear-side flow passage 
           390  Laser welded part 
           400  Circuit package 
           412  Connection terminal 
           430  Measuring flow passage surface 
           431  Rear measuring flow passage surface 
           436  Heat transfer surface exposed part 
           438  Opening 
           452  Temperature detection unit 
           500  Gate cut part 
           501  Concave part of laser irradiation surface 
           502  Second concave part of laser irradiation surface 
           550  Laser light 
           602  Flow rate detection unit