Abstract:
A measuring instrument for measuring mass of a flowing medium in which a measuring body extends across the flowing medium and has a rectangular flow channel through which the medium flows. The flow channel has a constricted inlet, a central portion with parallel sides and a widened outlet. A measuring element is supported in the measuring body such that the medium flows thereon. The measuring element carries temperature and heating sensors.

Description:
FIELD OF THE INVENTION 
     The invention relates to a measuring instrument for measuring the mass of a flowing medium, and particularly for measuring the rate of air flow to the air intake of an internal combustion engine. 
     BACKGROUND AND PRIOR ART 
     A device for measuring the mass of a flowing medium of the above type is disclosed in DE 29 11 631 C2. Therein, a heating wire functioning as a temperature-dependent resistance is mounted together with another temperature-dependent resistance on a probe ring projecting in a tubular unit and over a cross-piece to an insert unit. The tubular unit forms the flow cross section, for example, of an intake pipe of a combustion engine. An electrical control unit, which is configured as a hybrid board circuit is adhered onto an assembly plate, in the insert unit. Conductive paths or tracks are formed on a carrier plate by thick-film technology. An increased current is passed to the heating wire after a specific measurement cycle, in order to remove any deposition formed on the heating wire. 
     DE 38 44 354 A1 disclosed another measuring instrument of the above type. Therein, a ceramic substrate with at least one temperature-dependent resistance layer is mounted on one side of a measuring unit and projects in an air flow passage. This measuring unit is introduced into a flow cross section, such that the flowing medium flows through the flow passage. 
     In order to avoid a disruption of the ceramic substrate, support elements and protective cross-pieces are provided in the flow passage. 
     DE 43 08 227 A1 discloses a measuring element for determining the mass of a flowing medium. Therein, a substrate is secured at one side, and a measuring resistance, a heating resistance, and a compensation resistance are disposed on this substrate. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a measuring instrument of the above type, which has a simple and robust structure, and an increased insensitivity to contamination. 
     In accordance with the invention, the measuring element is mounted in the flow passage of a housing such that it is fixed or clamped in the housing on all sides, whereby the flowing medium is guided to flow on one side at or above the measuring element. By embedding the measuring element in the housing, the measuring element is protected from breakage and consequent disruption of service. It also has a robust but simple structure. Because it is fixed in the housing on all sides, the measuring element is also relatively free of stress. The measuring element of this measuring instrument is also protected against the medium flow, since the measuring element is flush with the flow passage of the housing, so that the medium flow cannot attack the measuring element itself. 
     A cooling unit is conventionally mounted on the measuring element on which sensors are disposed and heat transfer to the ambient atmosphere can be obtained in the direct vicinity of the sensors. It is also advantageous that the measuring element and an electronic component for the necessary evaluation and control of the instrument can be introduced inside this measuring instrument. For this purpose, both the measuring element as well as the electronic component, which are both produced by hybrid board or silicon technology, are integrated adjacent to one another, i.e., in one plane, in the instrument. The sensors of the measuring element are protected from contamination, for example, by being embedded in membranes, which are mounted on a substrate of the measuring element. A measuring element with the utilization of membrane elements on a substrate is disclosed in copending application 09/258,865 filed concurrently herewith and claiming the priority of German Application No. 198 08 249.5. 
     Both the quantity of the flowing medium as well as its flow direction can be determined with only one measuring instrument, due to the arrangement of two heating sensors and two temperature sensors on one measuring element. The detection of small residual flows of medium is also possible. Both the heating sensors and temperature sensors are respectively accommodated in inside two separate membranes on one substrate. 
     By providing a frame around the sensor carrier, i.e., around the measuring element, an optimal flow channel geometry is produced, whereby a constriction is set up in the flow passage. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING 
     FIG. 1 diagrammatically illustrates a measuring instrument according to the invention; 
     FIG. 2 shows, on enlarged scale, a detail A of the measuring instrument of FIG. 1; 
     FIG. 2 a  shows another embodiment of detail A of the measuring instrument of FIG. 1; 
     FIG. 3 is a sectional view taken on line III—III in FIG. 2 a.    
     FIG. 4 is a top plan view of the measuring element. 
    
    
     DETAILED DESCRIPTION 
     The measuring instrument shown in FIG. 1 has a measuring unit  1  which is provided with a housing  2  for a measuring element  3  on which are mounted a heating sensor  4 , a temperature sensor  5 , and other elements (not shown) of a measuring circuit. A flow passage  6  for a flow  7  is incorporated in housing  2 . Measuring element  3  lies in a plane in housing  2  above or below flow  7  approximately centrally in flow passage  6 . A cooling unit  8 , for example, of aluminum, with fins  9  is integrated on measuring element  3  and preferably in housing  2 . 
     Electronic components  10  are accommodated in measuring unit  1  on a hybrid P.C. board  11 , and these components are joined electrically by means of a connector plug  12  in connection part  13  to another electronic unit or a current supply unit (not shown) for controlling the intake air as well as various indicators (not shown). Electronic components  10  may contain complete evaluation and control electronics, for example, a microprocessor, and it is installed with connections and conductive tracks optionally in the form of a printed circuit on hybrid board  11  as well as being connected to measuring element  3 . 
     Housing  2  is shown slightly enlarged in the region of flow passage  6  in FIG.  2 . In this embodiment, housing  2  is constructed in one piece and is produced, for example, by injection molding after introducing measuring element  3 . Measuring element  3 , which is shown in more detail in FIG. 4, is recessed in housing  2  and is flush with outer surface  3 . 1  of housing  2  in the lower region  2 . 2  of the housing. Cooling unit  8  with fins  9  is adhered to the lower surface  3 . 2  of measuring element  3 , preferably by a heat-conducting adhesive. 
     FIG. 2 a  shows a variation of the housing. Therein, housing  2  is made in two parts, such that upper housing part  2 . 1  is snapped onto lower housing part  2 . 2 . Also, measuring element  3  seals the surface of the lower housing part  2 . 2 . Flow passage  6  is formed by the two parts  2 . 1  and  2 . 2 . of housing  2 . 
     Flow passage  6  has a rectangular cross-section as shown in FIG.  2  and in FIG.  3 . This flow passage  6  forms a flow channel, which has a constriction in the inlet section  6 . 1 , and parallel sides  6 . 2  and  6 . 3  in the region of measuring element  3  and it again widens in the outlet cross section  6 . 4 , the arrangement being such that the flow passage can be reversed depending on flow direction  7 . This channel form of flow passage  6  is advantageous in order to reduce the contamination of sensors  4  and  5 , in order to configure the measuring instrument so that it is insensitive to angle, but particularly in order to detect reverse flow, for which purpose, the measuring element  3  preferably has two heating sensors  4  and two temperature sensors  5 . 
     Measuring element  3  is shown in a simplified manner in the top view in FIG.  4 . Therein, two separate membranes  14  and  15 , in which at least one heat sensor  4  or at least one temperature sensor  5  are embedded, are disposed on measuring element  3 . Sensors  4 ,  5  themselves are protected from contamination by membranes  14  or  15  disposed above sensors  4 ,  5 . Membranes  14 ,  15  are made of silicon dioxide layers, which are introduced on a substrate  16  of measuring element  3  and in which a conducting layer, for example, of nickel is introduced in order to create sensors  4  and  5 . Measuring element  3  is manufactured by semiconductor technology, whereby substrate  16  consists of silicon, so that small structures can be introduced on substrate  16 . 
     The measuring device is clamped to a part  17  of measuring unit  1  (see FIG. 1) in an intake channel (not shown) such that flow passage  6  points in flow direction  7 . The flowing mass is detected in the conventional way by sensors  4 ,  5 , for which purpose sensors  4 ,  5  are connected together with other passive resistances (not shown) to a conventional bridge circuit. These passive resistances can be disposed on the substrate of measuring element  3  or hybrid board  11 . 
     With the use of two heat sensors  4  and two temperature sensors  5  of the same type, flow direction  7  as well as small residual flows can be detected. When using two heating sensors  4  and two temperature sensors  5 , the two heating sensors  4  should lie as close to one another as possible, in order to determine the polarization, i.e., flow direction  7  as well as backflow, but without permitting thermal cross-talk from the front to the back sensor due to heat conduction. In normal operation, heat sensors  4  and temperature sensors  5  should not influence one another. 
     Modifications are also possible within the scope of the concept of the invention. Thus flow direction  7  and backflows can be determined with two measuring instruments of the described type and the utilization in. each of a heating sensor and a temperature sensor  5  on one measuring element  3 . For this purpose, flow passage  6  can have the shape of a venturi-type channel. It is also possible to use additional cooling units, which are preferably connected to cooling unit  8  on measuring unit  1  itself, for increased cooling of the measuring instrument. 
     Although the invention is disclosed with reference to particular embodiments thereof, it will become apparent to those skilled in the art that numerous modifications and variations can be made which will fall within the scope and spirit of the invention as defined by the attached claims.