Abstract:
In a flow sensor ( 1 ) comprising a flow channel ( 14 ) embedded in a base body ( 1′ ), a flow sensor element ( 13 ) adjacent to the flow channel ( 14 ) and a cover plate ( 12 ) covering the flow channel ( 14 ) and arranged on the base body ( 1′ ), the flow channel ( 14 ) is formed by an elastic sealing lip which delimits said channel ( 14 ), running on and around an upper side of the base body ( 1′ ) lying opposite the cover plate ( 12 ) and is pressed against the cover plate ( 12 ) such that a seal is formed. The design of the flow channel ( 14 ) by means of the sealing lip ( 5 ) allows the flow channel ( 14 ) to be sealed off from possible gaps between the base body ( 1′ ) and the cover plate ( 12 ) and in relation to the cover plate ( 12 ), such that a flow channel with a level channel bed ( 14 ) is formed by the sealing lip ( 15 ) and the cover plate ( 12 ) which channel bed has smooth peripheral areas and a uniform cross-section, and which if possible avoids the depositing and collection of dirt particles and thus prevents undesirable turbulence in the flow channel ( 14 ) and guarantees a laminar current through the flow channel ( 14 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a flow sensor and to a method for the production of the flow sensor, in particular a flow sensor for measuring the flow of fluids, in particular for measuring the flow of air and other gases. The present invention relates in particular to a flow sensor, and to a method for the production of the flow sensor, which comprises a base member with a flow channel, a flow sensor element adjoining the flow channel, and a cover plate arranged on the base member and covering the flow channel. 
       BACKGROUND OF THE INVENTION 
       [0002]    EP 1 182 432 describes a flow sensor consisting of two housing parts for measuring the flow of fluids, which flow sensor comprises a semiconductor chip with sensor arrangement arranged between the housing parts. The semiconductor chip is located at a measuring channel configured as a groove in one of the housing parts. A sealing ring is additionally arranged between the housing parts. A strip conductor foil is guided outwards from the semiconductor chip between the sealing ring and the other one of the housing parts. Although the measuring channel is sealed by the sealing ring relative to the outside in the region of the gap between the housing parts, the measuring channel is not sealed relative to the gap between the housing parts. 
         [0003]    A further flow sensor is described by Gruner A G, Bürglestrasse 15-17, 78564 Wehingen, Germany, this one consisting of two parts and a semiconductor sensor element. A measuring channel is again formed between the two parts, in which the sensor element is inserted. 
         [0004]    The two parts are adhesively bonded directly together, such that the measuring channel is for the most part sealed off from the gap between the two parts. It is however scarcely possible to achieve perfect sealing off of the measuring channel from the gap between the parts and at the same time completely to prevent penetration of adhesive into the measuring channel. 
         [0005]    As a result of connections between the measuring channel and the gap between the housing parts and as a result of glue deposits in the measuring channel, dirt may accumulate, which may cause turbulent flow through the measuring channel and thus impair flow measurement quality. 
         [0006]    US 2006/210445 A1 discloses a sensor comprising microfluidic channels, having lateral sealing lips made of resilient material. By arranging different microfluidic channels next to one another a high aspect ratio may be achieved. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the present invention to provide a flow sensor and a method for the production of the flow sensor which do not display at least certain of the disadvantages of the prior art. It is in particular an object of the present invention to provide a flow sensor and a method for the production of the flow sensor which allow the flow rate to be measured by means of a flow sensor element in a flow channel, which latter is less susceptible to the deposition of dirt particles in the flow channel. 
         [0008]    According to the present invention these objects are achieved in particular by the elements of the independent claims. Further advantageous embodiments are additionally provided in the dependent claims and the description. 
         [0009]    The flow sensor comprises a base member with a flow channel, a flow sensor element adjoining the flow channel, and a cover plate arranged on the base member and covering the flow channel. 
         [0010]    The above-stated aims are achieved by the present invention in particular in that the flow channel is formed by a sealing lip, which surrounds and defines the flow channel on the top of the base member and is pressed sealingly against the cover plate. Unlike the prior art, a gap between the base member and the cover plate is accordingly not sealed off from the outside world, but rather by configuring the flow channel by means of the sealing lip the flow channel itself is sealed off from possible gaps between the base member and the cover plate and thus also from the outside world. On the top of the base member, the sealing lip preferably forms a flow channel taking a repeatedly meandering path, whereby a larger length of flow channel can be obtained over a small surface, which is conducive to laminar flow. The sealing lip additionally seals the flow channel relative to the cover plate, such that the sealing lip and cover plate form a flow channel with a flat channel bed on the top of the base member, which flow channel comprises smooth defining surfaces and a constant cross-section and which as far as possible prevents deposition and accumulation of dirt particles and, if applicable, prevents the penetration of adhesives into the flow channel when the cover plate is attached to the base member. In this way, undesired turbulence may be prevented in the flow channel and laminar flow through the flow channel may be brought about. 
         [0011]    In this case, the base member is formed of a first and a second region, which consist in each case of a different material and are joined together, wherein the flow channel is formed in the first region, which consists of a significantly more resilient material than the second region. The two regions are preferably produced by the two component injection molding method and preferably form a material composite. By forming the flow channel in the resilient material region, the sealing lip is resilient and formed in one piece with the channel bed. The second region is configured in such a way that a surface plane is obtained which is defined relative to a channel bed of the flow channel and which lies between the plane of the channel bed and the sealing lips projecting therebeyond, such that at all points of the sealing face between cover plate and sealing lip a predetermined sealing force arises, which may be achieved in a simple, well-defined manner. 
         [0012]    In one variant embodiment the base member comprises ports formed in the second, nonresilient region for a flow medium, preferably air or other gases. The ports are in each case connected to one of the ends of the flow channel via a bore. 
         [0013]    In a further preferred variant embodiment the flow sensor element is set into the cover plate over the flow channel and, with the cover plate, forms a substantially flat ceiling for the flow channel. Because the flow sensor element is set flush into the cover plate and, with the cover plate, forms a flat ceiling for the flow channel, turbulence and thus the deposition and accumulation of dirt particles are in turn prevented at the transition between cover plate and flow sensor element. 
         [0014]    In a further variant embodiment, when the cover plate is removed the sealing lip is tapered at the end remote from the top of the base member. That is to say, the sealing lip is tapered at the end which is pressed resiliently against the cover plate when the cover plate is attached to the base member. As a result of the taper of the sealing lip, the resilient deformation of the sealing lip brought about by the pressing on of the cover plate can be compensated in such a way that, when the cover plate is attached, a substantially rectangular cross-section of the flow channel is formed, which does not display any significant deformation at the side walls of the flow channel formed by the sealing lip. 
         [0015]    In a further variant embodiment, the flow channel is formed on a channel bed on the top of the base member in such a way that the flow channel is separated in cross-section in each case laterally from an outer channel surrounding the flow channel by the sealing lips projecting upwards from the channel bed. In other words the sealing lip is arranged in such a way on a channel bed that the channel bed is separated into three regions by the sealing lip when viewed in cross-section: the flow channel arranged centrally on the channel bed and two outer channels, which are separated in each case from the flow channel by the sealing lip. The outer channels for example allow the accommodation of a surplus of adhesives optionally used to attach the cover plate to the base member without interfering with the flow channel, or of surplus sealing lip material. It should be noted that elastomers cannot be made smaller in volume, but rather can merely be displaced to another location. 
         [0016]    The present invention additionally relates to a method for the production of the flow sensor, in which method a base member with a flow channel is provided, a flow sensor element is arranged adjacent the flow channel, the flow channel is covered by attaching a cover plate to the base member, and the base member is provided by means of injection molding in such a way that a sealing lip is formed on the top of the base member opposite the cover plate, which sealing lip surrounds and defines the flow channel on the top of the base member and presses sealingly against the cover plate when the cover plate is attached to the base member. The flow sensor with the above-cited advantages of low susceptibility to soiling and laminar flow through the flow channel may thus be efficiently and inexpensively produced, by simply attaching the cover plate with flow sensor element set in flush to the base member produced by injection molding. In this case, the base member is formed by two-component injection molding with a first and a second region, which consist in each case of a different material and are joined together, wherein the flow channel is formed in the first region, which consists of a significantly more resilient material than the second region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    An embodiment of the present invention is described below by way of example. The exemplary embodiment is illustrated by the following appended figures: 
           [0018]      FIG. 1  shows a cross-sectional diagram, which schematically represents a cross-section through a flow sensor, which comprises a base member with ports and flow channel and a cover plate with a flow sensor element. 
           [0019]      FIG. 2  shows a plan view diagram, which schematically represents a plan view of the base member without cover plate attached thereto, comprising ports and a flow channel joined thereto and formed by sealing lips. 
           [0020]      FIGS. 3   a ,  3   b ,  3   c  and  3   d  show different variant embodiments of the flow sensor with differently arranged ports. 
           [0021]      FIG. 4   a  shows a cross-sectional diagram, which schematically represents a cross-section through a region of the flow channel across the flow direction, with the cover plate not as yet attached to the base member. 
           [0022]      FIG. 4   b  shows a cross-sectional diagram, which schematically represents a cross-section through the region of the flow channel across the flow direction, with the cover plate mounted on the base member. 
           [0023]      FIG. 4   c  shows a cross-sectional diagram, which schematically represents a cross-section through a region of the flow channel according to a further variant embodiment across the flow direction, with the cover plate not as yet attached to the base member. 
           [0024]      FIG. 4   d  shows a cross-sectional diagram, which schematically represents a cross-section through the region of the flow channel according to the further variant embodiment across the flow direction, with the cover plate mounted on the base member. 
           [0025]      FIG. 5  shows a plan view diagram, which schematically represents a further plan view of the base member without cover plate attached thereto, comprising ports and a flow channel joined thereto and formed by sealing lips. 
           [0026]      FIG. 6  shows a cross-sectional diagram, which schematically represents a cross-section through a region of the flow channel in the flow direction, in which region the flow sensor element is arranged. 
           [0027]      FIG. 7   a  shows a cross-sectional diagram, which schematically represents a cross-section through a region of the flow channel according to a further variant embodiment across the flow direction, with the cover plate not as yet attached to the base member. 
           [0028]      FIG. 7   b  shows a cross-sectional diagram, which schematically represents a cross-section through the region of the flow channel according to a further variant embodiment across the flow direction, with the cover plate mounted on the base member. 
           [0029]      FIG. 8  shows a cross-sectional diagram, which schematically shows a cross-section through the region of a cover plate with set-in flow sensor element and with a signal leadthrough passing through the cover plate. 
           [0030]      FIG. 9  shows a view from below, which schematically shows a cover plate from below in the region of a flow channel with sealing lips shown in cross-section and with set-in flow sensor element with signal leadthroughs leading laterally to a further region of the cover plate. 
           [0031]      FIG. 10  shows a view from below, which schematically shows a cover plate from below in the region of a flow channel with sealing lips shown in cross-section and with set-in flow sensor element with active sensor surface and with signal leadthroughs leading laterally to a further region of the cover plate. 
           [0032]      FIG. 11  shows a cross-sectional diagram, which schematically shows a cross-section through the region of the cover plate with set-in flow sensor element with signal leadthrough passing through the cover plate, according to  FIG. 10 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    In  FIGS. 1 ,  3   a ,  3   b ,  3   c  and  3   d  reference numeral  1  designates a flow sensor for measuring the flow of fluids, preferably of air or other gases. As is shown schematically in  FIG. 1 , the flow sensor  1  comprises a cover plate  12  and a base member  1 ′, which is also represented in various variant embodiments in  FIGS. 2 ,  5 ,  7   a  and  7   b . The cover plate  12  is mounted on the base member  1 ′ and attached to the base member  1 ′, for example by means of rivets  103 , screws, adhesive or by welding. The cover plate  12  is for example a flat electronic epoxy printed circuit board. 
         [0034]    The base member  1 ′ comprises a nonresilient region  10  and a resilient region  11  joined thereto. The resilient region  11  is arranged lying on the nonresilient region  10 . The nonresilient, or at least substantially less resilient, harder region  10  compared with resilient region  11  consists for example of a plastics such as nylon polyamide. The resilient (softer) region  11  consists for example of a thermoplastic material, for example a polyethylene-based material. The base member  1 ′ is preferably produced using the two-component injection molding method. In the two-component injection molding method, the resilient and nonresilient regions  10 ,  11  are joined together for example via a material bond or at least interlockingly. In a less preferable variant embodiment the resilient region  11  and the nonresilient region  10  are produced as separate parts, for example as base plate and channel plate, which are joined together, for example by adhesive bonding. 
         [0035]    As shown in  FIGS. 1 ,  2  and  5 , the nonresilient region (the base plate) comprises two ports  100  for supplying and removing the flow medium. The ports  100  are preferably constructed for the attachment of hoses, which have an internal diameter of 6 mm for example. The ports  100  are of conical configuration and are provided with radial ribs, for example. The ports  100  comprise an internal bore  102  with a diameter of approx. 0.5 mm. 
         [0036]      FIGS. 3   a ,  3   b ,  3   c  and  3   d  show examples with various arrangements of the ports  100 . In the preferred embodiment according to  FIG. 3   a  the flow sensor  1  is of cuboidal configuration and the ports  100  are arranged next to one another on the same side (for example end face) of the cuboid. In the embodiment according to  FIG. 3   b  the flow sensor  1  is of cuboidal configuration and the ports  100  are arranged on different sides of the cuboid, for example on opposing sides along a common axis. In the embodiment according to  FIG. 3   c  the flow sensor  1  is of cylindrical configuration and the ports  100  are arranged opposite one another along a common axis. In the embodiment according to  FIG. 3   d  the flow sensor  1  is of U-shaped configuration and the ports  100  are in each case arranged at one of the two ends of the U shape (horseshoe). 
         [0037]    As is shown in  FIGS. 1 ,  2 ,  5 ,  7   a  and  7   b , the resilient region  11  (the channel plate) comprises a flow channel  14 . The flow channel  14  comprises a flat channel bed  141  with a smooth surface and is covered by the cover plate  12  arranged parallel to the channel bed  141 . As is apparent in  FIGS. 2 and 5 , the flow channel  14  meanders repeatedly between its two ends. At its two ends the flow channel is in each case joined by a bore  101  to the internal bore  102  of one of the ports  100 . The bores  101  extend from the channel bed  141  through the resilient region  11  and the nonresilient region  10  to the internal bores  102  of the ports  100  and are arranged for example perpendicularly to the channel bed  141  and to the internal bores  102 . 
         [0038]    As is shown in  FIGS. 2 ,  4   a ,  4   b ,  4   c ,  4   d ,  5 ,  7   a  and  7   b , the resilient region  11  comprises a sealing lip  15 , which is arranged upright on the channel bed  141  and extends from the channel bed  141  to the cover plate  12 . The flow channel  14  is formed by the sealing lip  15 , which surrounds the channel bed  141  of the flow channel  14  and defines the sides of the flow channel  14  on the channel bed  141  in the flow direction. As is shown in  FIGS. 4   a  and  4   c  in cross-section across the flow direction of the flow channel  14 , the sealing lip  15  is tapered at the end remote from the channel bed  141 , when the cover plate  12  has not as yet been mounted on the base member  1 ′. Once the cover plate  12  has been mounted on the base member  1 ′, the sealing lip  15  is compressed, as illustrated schematically in  FIGS. 4   b ,  4   d , and the sealing lip  15  seals the flow channel  14  relative to the cover plate  12 . The cover plate  12 , the channel bed  141  and the sealing lip  15  in each case have smooth surfaces and form boundary surfaces, which form the flow channel  14  and define a constant, preferably rectangular cross-section. The cross-section of the flow channel  14  has a depth of 0.3 mm and a width of 0.8 mm, for example. The length of the flow channel  14  amounts for example to 100 mm. Thus, the flow channel  14  has a length which is large relative to its cross-section, for example the ratio of length to width or length to depth of the flow channel  14  ranges from approximately 100:1 to 400:1. The Reynolds number of the flow channel  14  amounts to approximately  50 , which brings about turbulence- and eddy-free laminar flow. 
         [0039]    In the variant embodiments according to  FIGS. 4   a ,  4   b ,  4   c ,  4   d  and  5 , the flow channel  14  formed by the sealing lip  15  is surrounded by an outer channel  18 , which is defined by the edges  17  of the resilient region  11  and by the sealing lips  15 . 
         [0040]    In the variant embodiments according to  FIGS. 1 ,  5 ,  7   a  and  7   b , the flow channel  14  formed by the sealing lip  15  is surrounded by an outer channel  18 , which is defined, over the entire sensor base member  1 ′, by the edges  117  of the nonresilient region  10  and by the sealing lips  15 . These edges  117  of the nonresilient region  10  may also be denoted spacers  117  or reference plane webs  117 . In  FIG. 1  these spacers  117  are the webs projecting at two opposing ends of the nonresilient member  10  into the region of the resilient region  11 , i.e. of the base member  1 ′. With their peripheral housing surface they form a reference plane  217 . In  FIG. 2  these spacers are shown in the form of surrounding housing edge webs  117 . The spacers  117  do not have to be arranged in surrounding manner. It is sufficient for them to be configured so as to produce a surface plane  217  defined relative to the channel bed  141 , which surface plane lies lower than the sealing lips  15  projecting therebeyond. 
         [0041]    In  FIG. 5  there is shown by way of example in the vicinity of a port  101  at two opposing points the resilient web of the resilient region  11  provided in the other figures with reference numeral  17  and the nonresilient web of the nonresilient housing part  10  provided in the other figures with reference numeral  117 . 
         [0042]    As may in particular be seen in  FIGS. 7   a  and  7   b , the nonresilient spacers  117  define a reference plane  217  which is lower, relative to the channel bed  141 , than the sealing lips  15  projecting therebeyond. It is thus clearly apparent that, when the cover plate  12  is lowered, the sealing lips  15  are compressed very simply in a defined manner, such that a defined sealing surface is produced which bounds the channel  14  on all sides. To this end it is advantageous with regard to the sealing lips  15  for the cavities  18  to remain, so that the sealing lips  15  have the necessary space to adopt their final position. By using two nonresilient sheet materials  10  and  12 , which meet in a defined manner in the region of the surface  217 , it is straightforwardly possible to obtain a very long channel route  14 , i.e. a high aspect ratio, by a meandering arrangement. The curved shape of the channel implied by the term “meandering” improves laminar flow. It would however also be possible to arrange a plurality of channels  14  directly next to one another and to bring about deflection at one side by way of a more or less U-shaped 180° deflection. 
         [0043]    As is illustrated schematically in  FIGS. 1 and 6 , the flow sensor  1  comprises a flow sensor element  13 , which is set into the cover plate  12  and adjoins the flow channel  14 . The flow sensor element  13  is set flush into the cover plate  12 , such that the cover plate  12  and the flow sensor element  13  form a substantially flat surface for covering the flow channel  14 , in order to prevent eddy formation and turbulence. Preferably, the flow sensor element  13  is adhesively bonded to the cover plate  12 , but other attaching means are possible, for example screws or snap closures.  FIG. 6  shows a cross-section through a part of the flow channel  14  in the flow direction, illustrating an example of a flush arrangement of the flow sensor element  13  in the cover plate  12 . The flow sensor element  13  comprises a thinned area, which forms a cavity  16  between the flow sensor element  13  and the cover plate  12 . The flow sensor element  13  is an element produced by thin or thick film technology, which comprises a plurality of sensor elements, for example at least one or two temperature sensors  131 ,  133 , or a differential temperature sensor and a heating element  133 . It is also possible to use one of the temperature sensors  131 ,  133  as a heating element. It should be added at this point that the medium temperature may be detected both in the flow channel  14  and on the electronic printed circuit board of the cover plate  12 , since the medium adopts the housing temperature of the flow sensor  1  due to the very slight flow in the flow channel  14 , i.e. in one variant embodiment the temperature sensor for determining the medium temperature is arranged outside the flow channel  14  on the cover plate  12 . The sensor elements  131 ,  132 ,  133  of the flow sensor element  13  are connected via strip conductors to an electronic circuit fixed to the cover plate  12 . For evaluation of the sensor signals supplied by the sensor elements  131 ,  132 ,  133 , the electronic circuit preferably comprises a programmable processor, in particular a PSoC (Programmable System on Chip) with integral programmable analog components (amplifier, A-D converter, etc). Either the heat transfer from the heating element  132  to one of the temperature sensors  133  or cooling of the heating element  132  by the medium flowing past is measured, in order on the basis thereof to determine flow velocity and thereby the flow in the flow channel  14 . 
         [0044]    The signal leadthrough may be embodied for example in the manner illustrated in  FIG. 8 . The cover plate  12  has a cavity  130 , into which the flow sensor element  13  with one or more sensors is set, as explained with reference to  FIG. 6 . A signal leadthrough  132  passing through the cover plate  12  in a corresponding channel  131  is shown. Since the flow sensor element  13  is preferably enclosed in the cavity  130  by means of sealing filler material, the flow channel  14 , which is defined, as is indicated schematically, at the sides by the sealing lips  15  and (not shown) by the channel bed  141 , is sealed off very effectively, so as to be able to measure even small pressure differences effectively, for example. 
         [0045]      FIG. 9  shows a view from below of a different signal leadthrough embodiment, which is particularly advantageous in the case of a single-sided printed circuit board  12 . The flow sensor element  13  is set into a cavity in the printed circuit board  12 . At least one, and advantageously two separate signal leadthrough grooves  231  are provided in the printed circuit board, in which signal leadthrough grooves signal conductors are guided in the form of cables or foils into a region  230  of the printed circuit board  12  for further processing in electronic components provided therein. The cavities  130  and  231  are here filled with a sealing material, such that the leadthrough does not impair functioning of the sensors below the sealing lip  15 . 
         [0046]    It is thus clear that, with a nonresilient base member with spacers  117  and an inserted or injection-molded-in resilient region  11 , a plurality of fluid channels  14  may be produced for a flow sensor, since these fluid channels are in each case defined on three sides (reference numerals  15 ,  141  and  15 ) and sealing may be ensured in a very simple manner by a nonresilient cover plate  12  to be placed thereon. Particularly advantageous is the use of a printed circuit board as cover plate  12 , since this is nonresilient and may additionally be provided directly with electronic units for evaluation, resulting in a very compact component. Use of a smooth printed circuit board  12  as a counterpart to the resilient sealing lips  15  allows any desired defined curving and meandering form of the fluid channel, such that a high aspect ratio may be produced with the smallest possible space, without jeopardizing laminar flow. 
         [0047]    In principle it is also possible to ensure, by corresponding surface configuration of cover plate  12  and webs  117 , that a defined spatial position of cover plate  12  and base member  1 / 1 ′is achieved, such that the sensor element  13  and electrical terminals come to lie at predetermined locations. 
         [0048]    Although  FIGS. 7   a  and  7   b  in each case show two sealing lips  15 ,  FIG. 6  shows that, when the resilient region  11  is appropriately configured, with fluid ports through the bottom, in actual fact only a single toroidal sealing lip  15  is present, which fully defines the sides of the fluid channel  14 . 
         [0049]      FIG. 10  shows a further exemplary embodiment of the signal leadthrough in a view from below, which is advantageous in particular in the case of a single-sided printed circuit board  12 . The flow sensor element  13  is set into a cavity in the printed circuit board  12 . The active sensor surface has been given reference numeral  134 , in order to refer to individual sensors, as in  FIG. 6 . The flow sensor element  13  thus comprises its own printed circuit board, on which at least two, here four separate signal lines  331  are provided, which end in contact surfaces  332  of the sensor element  13 . These contact surfaces  332  of the sensor element  13  lie opposite contact surfaces  333  of the cover plate  12 , from which signal lines which are not shown are conventionally continued. 
         [0050]    The connections between the contact surfaces  332  and  333  are provided by bonding wires  334 , which are protected in the side view of  FIG. 11  with a casting compound  335 . In the exemplary embodiment of  FIGS. 10 and 11  the sensor element  13  is thus larger than the width of the channel  14 , but the surface  134  active as a sensor is only a part thereof. In a further part of the sensor element  13  the signal lines are passed through under one of the sealing lips  15 . 
         [0051]    It would also be possible to arrange the active surface  134  more centrally and to pass a further part, inactive with regard to the circuit, of the sensor element  13  through under the other sealing lip. Greater symmetry is traded off against the additional sealing channels of the filling compound  130 . 
         [0052]    The junction with the cover plate or printed circuit board  12  is here obtained by bonding. The bonding wires  334  are finally also protected by a casting compound  335 .