Abstract:
The invention relates to a flow sensor having a measuring chamber to which a fluid whose volume and/or rate of flow is to be measured is supplied and then drawn off. Inside the measuring chamber elements of a measuring mechanism are mounted so as to freely rotate. The sensor is also provided with a magnet which produces a magnetic field inside the measuring chamber and in close proximity thereto. At least one sensor device measures the magnetic field and/or changes in the magnetic field. The sensor device for measuring the magnetic field is provided with at least one giant magnetoresistance sensor.

Description:
TECHNICAL FIELD 
     The invention relates to a flow rate sensor having a measuring chamber, into which a fluid, the volume and/or flow rate of which is to be measured, can be fed and then discharged, having measuring mechanism elements disposed in the measuring chamber and mounted in a freely rotatable manner, having a magnet for generating a magnetic field in the measuring chamber and in the immediate vicinity thereof, and having at least one sensor device for measuring the magnetic field and/or variations of the magnetic field. 
     BACKGROUND OF THE INVENTION 
     Flow rate sensors are also known as volume sensors. Generally, they take the form of displacement meters. Examples of these are gear sensors, screw spindle meters, oval-wheel meters, cylindrical-piston meters or alternatively measuring turbines or proportioning gear pumps. They are used to measure a volume, a throughflow quantity or the rate at which a medium, here therefore a fluid, passes through the measuring instrument. The fluids may be liquids, pastes or gases. 
     In practice, flow rate sensors are often not measuring instruments in the narrower sense because the evaluation electronics are not part of the instrument but situated externally. Nevertheless, the term “flow rate measuring instrument” is often used and reference is also made to measuring chambers and measuring mechanism elements etc. The flow rate sensors are frequently also described as volume sensors, throughflow sensors, flow rate measuring instruments etc. 
     The volume sensors or flow rate sensors merely sense the flow or a volume that has flown through and transmit a signal to the evaluation unit or evaluation electronics, which only then produce a measured value therefrom. The expression “flow rate sensor” is used below. A confusion with specific structural elements in the instrument that are detectors or sensors in the narrower sense is avoided by use of the full designation. 
     Flow rate sensors or volume sensors in the form of gear sensors are known for example from EP 0 053 575 B1, EP 0 393 294 A1 or DE 40 42 397 C2 as well as EP 0 741 279 B1. They have a housing comprising two halves. In the one housing half, a pair of circular gear wheels are mounted in a freely rotatable manner in a measuring chamber on fixed axles by means of ball bearings and without wall contact. The two gear wheels mesh with one another. The medium, the displaced volume or flow rate of which is to be determined, is fed through a first bore to the two gear wheels, namely into the region where these gear wheels mesh with one another. The medium therefore passes into the chambers that are mutually formed in the tooth spaces of the two gear wheels. As a result of the following flow of the medium, the quantities situated in the chambers of the gear wheel are conveyed from the inlet side to the outlet side and by means of the movement of the teeth then set the gear wheels in rotation. The two gear wheels in said case rotate in opposite directions. At the other side of the gear wheels in flow direction downstream of the meshing region, the medium is discharged through a second bore. 
     The other housing half serves as a top cover for the region of the measuring chamber having the two gear wheels plus the medium flowing here. It therefore tightly closes the measuring chamber and prevents fluid streams from being able to pass through the measuring chamber outside of the meshing region of the gear wheels. The two housing halves lie one flat on top of the other and between them lies a, so to speak, virtual parting plane. 
     Permanent magnets such as for example in DE 40 42 397 C2 or carrier frequency sensors as in EP 0 741 279 B1 are provided in the housing adjacent to the meshing region of the gear wheels and build up an electromagnetic field. This field is varied by the teeth of the gear wheels and/or by the movement thereof. 
     The second housing half in the known arrangements mentioned above moreover accommodates a magnetoresistive differential sensor. The magnetoresistive differential sensor senses the variations of the fields caused by the movement of the teeth of the rotating gear wheels. 
     The sensor in these known instruments is separated from the medium or fluid to be measured by a non-magnetic insert, which protects the sensor in particular from the physical and chemical stresses imposed by the medium. The medium may not only have a very different temperature or consistency but may also be chemically aggressive. 
     This protection leads disadvantageously to a spacing of the sensor from the teeth of the rotating gear wheels that makes measurement more difficult and limits the accuracy and reliability of measurement. By means of suitable pole pins or other measures, the movement of each tooth flank during rotation of the associated gear wheel relative to the magnetoresistive differential sensor is then detected and communicated externally to suitable evaluation units. 
     Particularly with larger piece numbers of flow rate sensors, the economic aspect gains in importance. Nevertheless, even with larger piece numbers the accuracy and precision remains important. In many cases, the flow rate of a fluid is also adjusted in closed-loop control circuits depending on the measurement. 
     It would therefore be desirable to carry out as precise as possible a measurement of the quantity and/or rate of flow of a fluid by means of flow rate sensors that entail the least possible outlay. 
     The object of the invention is therefore to propose flow rate sensors that combine a particularly economical design with nevertheless accurate measurement results. 
     SUMMARY OF THE INVENTION 
     This object is achieved by flow rate sensors having a measuring chamber, into which a medium (fluid F), the volume and/or flow rate of which is to be measured, can be fed and then discharged, having measuring mechanism elements disposed in the measuring chamber and mounted in a freely rotatable manner, having a magnet for generating a magnetic field in the measuring chamber and in the immediate vicinity thereof, and having at least one sensor device for measuring the magnetic field and/or variations of the magnetic field, wherein the sensor device is disposed axially offset relative to the freely rotatably mounted measuring mechanism elements, and wherein the sensor device for measuring the magnetic field comprises at least one giant magnetoresistance sensor. 
     Giant magnetoresistance, mostly known as GMR effect, is a highly sensitive way of detecting magnetic fields and the changes thereof in a magnetoresistive manner. In connection with flow rate sensors, however, giant magnetoresistance has previously not yet been used. 
     The use of GMR sensors is known for the scanning of gear wheels in the different connection, say, from DE 296 12 946 U1. The GMR sensor from this known arrangement radially scans a tooth of a rotating gear wheel, alternatively the mutually offset teeth of a gear wheel pair disposed on the same axis. Such arrangements are however unsuitable for volume sensors because in these there always has to be two mutually meshing gear wheels mounted in a narrow housing with close tolerances. Given an arrangement of the GMR sensors as proposed, only the tooth tips are scanned, this leading to an inaccurate and asymmetrical electrical signal and hence to corresponding and accurate measurements of the flow quantities and flow rates. Furthermore, there is no room laterally alongside the gear wheels, and/or retro-fitting of the entire system in an installation is considerably impeded. 
     In an angular resolver according to DE 100 02 331 A1 it is proposed to carry out as precise as possible a measurement of the angle of a rotating part by means of a GMR sensor and a multi-stage gear. The gear arrangement alone prevents the use of these ideas in flow rate sensors and volume sensors. 
     All of the proposals to fit GMR sensors relate to conventional ambient conditions, i.e. substantially room temperature and normal pressure conditions. Comprehensive application examples in “GMR-Sensors Data Book”, published by the NVE Corporation, Eden Prairie, Minn., USA in April 2003, likewise relate to normal ambient conditions, possibly with increased temperatures. 
     A use of GMR sensors in volume sensors has therefore not yet been considered before because, here, pressures and pressure peaks of 60 Mpa to 80 Mpa (600 to 800 bar) may arise and under these extreme loads a highly precise working method still has to be ensured. What is more, the fluids to be measured may be electrically conductive and aggressive liquids and this should not impair the serviceability of the entire flow rate sensor. 
     Surprisingly, by virtue of the concept according to the invention it is possible to achieve a substantial improvement of conventional flow rate sensors by using GMR sensors in a suitable form. The GMR sensors are now fitted, not radially outside of the gear wheels as for example in DE 296 12 946 U1, but axially directly alongside the gear wheels and/or comparable elements. In contrast to the background art for flow rate sensors, they may however be disposed there much closer and nearer to the interior of the measuring chamber adjacent to the teeth. 
     As the signals of the sensor are markedly stronger and of greater magnitude than in conventional instruments, they may be processed better and with less trouble in downstream devices, such as pre-amplifiers or evaluation devices. 
     It is particularly advantageous that these GMR sensors may be constructed in the form of integrated circuits (ICs). 
     In a preferred manner, an integrated circuit of a GMR sensor that is surface-mounted onto an electronic printed circuit board is used. The printed circuit board is then on the one hand the electrical connection of the sensor to the evaluation unit and at the same time seals off the instrument and/or the measuring chamber mechanically from the pressurized fluids, i.e. liquids or gases, situated in the interior. 
     As it is possible to dispose the sensor practically in the interior of the measuring chamber and/or immediately adjacent thereto, then, unlike in the background art, there is no longer any need for an additional non-magnetic shielding, which increases the spatial requirement, and the sensor may therefore be fitted very much closer in towards the region of the gear wheels, measuring spindles and the like that are to be scanned. Only the thickness of a casting compound then separates the sensor from the measuring mechanism elements that are to be measured. The thickness of this casting compound is, on the one hand, very low and, on the other hand, is to be kept very small in dependence upon the external boundary conditions. 
     At the same time, the printed circuit board may be used to fix and position the magnet or magnets. One or more magnets were of course also required already in the background art to build up a magnetic field. The changes of the respective magnetic field were, as already mentioned, caused by the movements of the teeth of the gear wheels across the magnetic field. These changes were then conventionally measured by the magnetoresistive differential sensor and evaluated. 
     A magnetic field is also required according to invention. It is established, in the present case too, by means of one or more magnets. However, whereas previously this magnet had to be disposed separately likewise in a protected manner simultaneously in the housing, according to the invention the printed circuit board, which also carries the integrated circuits, may likewise receive, fix and/or position the magnet or magnets. The magnetic field built up by the magnet or magnets is likewise varied by the teeth of the gear wheels and/or by other measuring mechanism elements in other forms of flow rate sensors. The GMR sensor however now detects a different effect, namely an influencing of the giant magnetoresistance by the changing magnetic field. 
     By virtue of simultaneously using the printed circuit board to fix and position the magnet or magnets, however, it is possible to dispense with additional components, thereby allowing an even simpler design of the flow rate sensor. 
     The printed circuit board may be externally sealed by means of an O-ring. 
     The printed circuit board itself has plated-through holes, into which any cables that are required may then be soldered. 
     This then dispenses with practically all of the structural elements that are to be additionally installed in the background art for example according to DE 40 42 397 C2, i.e. the terminal posts or the holding plate for receiving the posts as well as the no longer required non-magnetic insert. The use of the integrated circuit (IC) with the printed circuit board practically in the pressure chamber and the said procedure during installation are extremely economical. Besides the integrated circuit of the magnet and the printed circuit board, practically no further components are now required. 
     A further advantage arises in that the Wheatstone bridge that is advantageously used for evaluation may now be disposed entirely in the instrument, namely unified with both branches of the Wheatstone bridge. This means that the two branches lie at practically the same temperature level and so it is no longer necessary computationally to effect compensations at the measured values for differing temperatures in the two branches. 
     For the magnet, in a preferred manner a samarium-cobalt magnet is used because it operates independently of temperature and therefore offers advantages over magnets constructed on a neodymium base. 
     The invention may be used not only in gear sensors but also in other flow rate sensors and/or volume sensors, for example in screw spindle meters, oval-wheel meters, cylindrical-piston meters or alternatively in measuring turbines or in proportioning gear pumps. There, instead of the teeth of gear wheels, measurement is effected by other measuring mechanism elements behaving in an equivalent form, mostly rotating. 
     Giant magnetoresistance is a specific effect that arises because of a change of the electrical resistance of a multi-layered structure of ferromagnetic and non-ferromagnetic layers, which are in each case only a few nanometres thick. This change of the electrical resistance arises upon the approach of a magnetic field. Giant magnetoresistance may therefore actually be utilized in the present case in that, here, this change is measured in dependence upon the change of the magnetic field resulting from the movement of the measuring mechanism elements, i.e. for example from the movement of the teeth of a gear wheel. 
     The concept according to the invention leads to a high degree of sensitivity and a large measuring range. 
     It is i.a. also highly advantageous that by virtue of the invention a very uniform sinusoidal signal may be generated as an output signal. Uniform sinusoidal signals allow particularly reliable evaluation and moreover a more detailed analysis of the individual sub-regions of the sinusoidal signal, thereby offering further-potential applications. 
     It is also advantageous that the corresponding sensors for the flow rate sensors according to the invention are very practicable and economical to manufacture. The printed circuit boards are manufactured in panels and their components are automatically fitted. 
     The electrical connection out of the region of the measuring chamber, i.e. out of the pressure chamber, may be effected by means of soldering operations in the plated-through holes of the printed circuit board. Such a connection is extremely secure and also withstands the very high pressures and thermal loads in the region of the fluids in the measuring chamber. 
     In said case, it is preferred when the sensor is connected by means of a flat ribbon cable electrically to a circuit device and the flat ribbon cable is run from the sensor through a drill hole in the housing of the flow rate sensor to the circuit device. 
     Said circuit device is in a preferred manner a pre-amplifier. The flat ribbon cable then forms the connection from the sensor to the pre-amplifier. The pre-amplifier in turn is a component part of the instrument according to the invention and transfers the data, which it processes, externally to an evaluation unit. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention is described in detail below with reference to the drawings. The drawings show: 
         FIG. 1  a vertical section through a flow rate sensor according to the invention along the line direction A-A of  FIG. 2 ; 
         FIG. 2  a plan view, partially broken away, of the flow rate sensor of  FIG. 1 ; 
         FIG. 3  an enlarged cutout of the detail X of  FIG. 1 ; 
         FIG. 4  a section through a sub-region of a flow rate sensor according to the invention; and 
         FIG. 5  a plan view of the sub-region of  FIG. 1 ; 
         FIG. 6  a diagrammatic representation of a Wheatstone bridge used in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows as an overview a section through a flow rate instrument according to the invention. The section does not extend in an exact plane but is offset forward and backward several times in order better to show the details of the flow rate instrument. The more exact line direction of the forward and backward offsets may be gathered from  FIG. 2 . 
     Particularly evident is a housing  10  of the flow rate sensor. The housing  10  comprises in particular three plate-like elements laid one on top of the other, namely a base plate  11 , a middle piece  12  and a cover  13 . In this case, the base plate  11  and the middle piece  12 , taken together, are roughly comparable to the first housing half mentioned in the background art in DE 40 42 397 C2. 
     The base plate  11 , the middle piece  12  and the cover  13  are connected to one another by fit bolts  16  as well as fastening screws  17 . This connection has to be very precise, on the one hand, and very strong, on the other hand, because a measuring chamber  20  is situated in the inner region of the middle piece  12 . 
     A short look at  FIG. 2  shows the arrangement viewed, in  FIG. 1 , from above. At the right side, the heads of the fit bolts  16  and the fastening screws  17  may be seen. As may be seen, around the top of the cover  13  eight heads of fit bolts and four heads of fastening screws are visible in order to allow a uniform, strong and sealed tightening of these bolts and screws. 
     Returning to  FIG. 1 , this reveals that both the fastening screws  17  and the fit bolts  16  pass right through the housing  10  comprising the base plate  11 , the middle piece  12  and the cover  13 . 
     The measuring chamber  20  is formed only in the middle piece  12  and is accordingly closed below and above by the base plate  11  and the cover  13  respectively, which therefore form the end walls of the measuring chamber  20 . 
     A connection bore  21  in the base plate  11  leads into the measuring chamber  20 . Through this connection bore  21  a fluid F, i.e. the medium in question here, may be fed. A second connection bore in the base plate  11  is not visible in  FIGS. 1 and 2 ; through this second connection bore the fluid F is then discharged after running through the measuring chamber  20 . 
     Besides the end walls which, as already mentioned, are formed by the base plate  11  and the cover  13 , the measuring chamber  20  is encircled by the walls of the middle piece  12 , as is also shown in the dashed representation on the left half of  FIG. 2 . Furthermore, (cf.  FIG. 1  again) O-rings  22  are provided for sealing the gaps between the underside and top of the middle piece  12 , on the one hand, and the underside of the cover  13  and the top of the base plate  11 . 
     Two measuring mechanism elements  30  and  40  are situated in the measuring chamber  20 . In the illustrated embodiment, these are in each case gear wheels. 
     In said case, the first gear wheel and/or the first measuring mechanism element  30  is clearly visible from above in  FIG. 2  on the left side. Situated in its centre is the axis  31 , about which the gear wheel and/or first measuring mechanism element  30  may rotate freely, and a series of teeth  32  project outwards from the axis  31  of the measuring mechanism element  30 . 
     In  FIG. 1  the first measuring mechanism element  30  may be seen only diagrammatically on the left side as the line direction A of the section passes through the measuring chamber  20  only in the edge region of the first measuring mechanism element  30 . 
     To make up for this, the second measuring mechanism element  40 , here therefore the second gear wheel, is shown in full section in  FIG. 1 . In the section, the axis  41  and in addition two flanks of teeth  42  may be seen. Also evident are various elements of a bearing  43 , which ensures the freedom to rotate of the second measuring mechanism element  40  too. 
     Both measuring mechanism elements  30 ,  40  are made of a ferromagnetic material, by means of which magnetic fields may be markedly influenced when the measuring mechanism elements  30 ,  40  rotate about the axes  31 ,  41 . 
     As may be seen, the two measuring mechanism elements  30 ,  40  mesh with one another and the fluid F fed through the connection bore  21  gives rise to a rotation of the two measuring mechanism elements  30 ,  40  in opposite directions. 
     A sensor device  50  is represented relatively small in  FIG. 1 . From  FIG. 2  it is evident that in the concrete embodiment two sensor devices  50  of similar design are provided. Both are situated above the measuring chamber  20  in a region, below and across which the teeth  32  of the first measuring mechanism element  30  rotate. Because of the ferromagnetic properties of the first measuring mechanism element  30 , a magnetic field  56  situated below the sensor device  50  is influenced and changes. Details of this are additionally indicated below. 
     One of the two sensor devices  50  is shown to a slightly enlarged scale in  FIG. 3 .  FIG. 3  therefore shows a sub-region of the cover  13  above the measuring chamber  20 . Cut out in the cover  13  is a channel, in which the sensor device  50  is fitted. 
     Central element of the sensor device  50  is a magnet  55 , here a round magnet. It builds up the magnetic field  56  that is varied by the ferromagnetic properties of the measuring mechanism element  30 . 
     The changes of the magnetic field are picked up and acquired by a sensor  52  that operates on the basis of the physical effect of giant magnetoresistance. This sensor  52  is disposed almost directly above the bottom edge of the cover  13  and therefore lies almost without clearance above the measuring chamber  20 , in which the first measuring mechanism element  30  rotates. The changes of the magnetic field  56  therefore occur practically immediately next to the sensor  52  and may be picked up in a highly precise and exact manner. 
     The sensor  52  and the magnet  55  are both disposed on a printed circuit board  60  and connected thereto. Also situated on this printed circuit board  60  is an integrated circuit (not shown). The printed circuit board is simultaneously a pressure plate. It is sealed on all sides inside the cover  13  by an O-ring  61  because in the measuring chamber  20  situated immediately below the sensor  52 , as already mentioned, there are fluids that may have very high temperatures. There, moreover, a high pressure may prevail and the fluids F may be chemically or physically aggressive. The pressure plate property of the printed circuit board  60  together with the sealing by the O-ring  61  prevents the fluid F from penetrating into the cover  13  behind the printed circuit board  60 , viewed in  FIG. 3  or in an upward direction in  FIG. 1 . 
     An electrical connection of the printed circuit board  60 , the magnet  55  and the sensor  52  having the GMR-measuring properties is effected by means of a flat ribbon cable  62 , which is not diagrammatically represented here. 
     The region around the flat ribbon cable  62  is filled by a casting compound  65  in order to keep the cable completely stable and prevent the penetration of foreign bodies from outside of the housing into this region. 
     A second casting compound  66  entirely fills the region between the printed circuit board  60  having the pressure plate properties and the measuring chamber  20  and therefore completely embeds the sensor  52 . Here, a smooth surface is desired in order to rule out any flow behaviour of the fluid F that might interfere with the measurement. 
     From  FIG. 1  it is evident that the connection by means of the flat ribbon cable  62  leads into an intermediate plate  71 , through which there is a connection to a pre-amplifier  72  and, from there, out of the flow rate sensor to an evaluation unit  73 . These are represented here purely diagrammatically. They may optionally be exchanged and adapted to the concrete external conditions of the flow rate sensor. 
     The magnet  55 , in a preferred form of construction a samarium-cobalt magnet, generates the magnetic field  56 . This magnetic field  56  extends into the measuring chamber  20  and the surrounding regions adjacent thereto. The magnetic field  56  penetrates in particular the sensor  52 , which here according to the invention is a GMR sensor. As a result of the rotation, the magnetic field  56  is perturbed by the adjacent tooth  32  running past just below the magnet  55  and the sensor  52  and by the associated tooth space of the gear wheel  30 . This varying magnetic field  56  generates in the GMR sensor  52  an electrical signal, which in the switching device, thus here the pre-amplifier  72 , is amplified and digitized. The digital signal is then transmitted via a further cable (not shown) to the evaluation electronics outside of the flow rate sensor and is evaluated there. 
     In  FIG. 4  the illustration of  FIG. 3  is repeated once more in a similar form. Here, for illustrative reasons, the view has practically been turned upside down so that the measuring chamber  20  of the flow rate sensor in the housing  10  is situated at the top. Also indicated there is that in this region the magnetic field  56  that is regularly changed by the movement of the teeth  32  (not shown) of the first measuring mechanism element  30  is situated. Here, it should moreover be taken into consideration that, should the measuring mechanism element not be a gear wheel, other elements instead of teeth are conceivable. 
     The magnet  55  and the sensor  52 , which utilizes the giant magneto-resistance effect, are illustrated once more to an enlarged scale. 
     The magnet  55  and the further elements are electrically connected by a flat ribbon cable  62 , running in a downward direction in the illustration in  FIG. 4 , to the intermediate plate  71  and the further elements described in connection with  FIG. 3 . 
       FIG. 5  once more shows the view of  FIG. 4 , namely in this case viewed from above. The view is therefore onto the casting compound  66 . Additionally indicated is a tooth  32  of the measuring mechanism element  30  that is situated precisely below the sensor device  50 , i.e. is situated in a movement, in which it sweeps past this region. 
     Notionally in the illustration the casting compound  66  is transparent, which in practice naturally need not be the case. It is therefore possible in the present case to see the sensor  52  through the casting compound  66 , and moreover run connections so that the printed circuit board  60  is partially visible. 
       FIG. 6  is a diagrammatic representation of the structure of a Wheatstone bridge, which as part of the integrated circuit on the printed circuit board  60  includes the GMR sensor  52 . 
     What may be seen is the conventional circuit of a Wheatstone bride having four resistors, of which three are known and the fourth is correspondingly influenced by the magnetic field  56 . 
     REFERENCE CHARACTERS 
     
         
           10  housing 
           11  base plate 
           12  middle piece 
           13  cover 
           16  fit bolts 
           17  fastening screws 
           20  measuring chamber 
           21  connection bore 
           22  O-rings for the measuring chamber 
           30  first measuring mechanism element, in particular first gear wheel 
           31  axis of first measuring mechanism element  30   
           32  tooth of first measuring mechanism element  30   
           40  second measuring mechanism element, in particular second gear wheel 
           41  axis of second measuring mechanism element  40   
           42  tooth of second measuring mechanism element  40   
           43  bearing arrangement of second measuring mechanism element  40   
           50  sensor device 
           52  sensor 
           55  magnet 
           56  magnetic field 
           60  printed circuit board 
           61  O-ring 
           62  flat ribbon cable 
           65  first casting compound 
           66  second casting compound 
           71  intermediate plate 
           72  pre-amplifier 
           73  evaluation unit 
         F fluid