Abstract:
The invention is concerned with a pressure sensor which avoids temperature hysteresis effects caused on account of heat-induced expansions as a result of stresses between a ceramic pressure-measuring cell and a metallic housing. 
     The inventive pressure sensor ( 100 ) for determining the pressure of a process medium comprises a metallic housing ( 110 ), which is open to a process medium and has a continuous hole ( 111 ) for accommodating the ceramic pressure-measuring cell ( 120 ), the pressure-measuring cell ( 120 ) comprising a ceramic base body ( 122 ) and a ceramic diaphragm ( 121 ), which is fitted thereon and is exposed to the process medium, and also suitable means ( 124   a,    124   b,    130 ), which supply an electrical signal which can be picked off and corresponds to the process pressure acting on the diaphragm ( 121 ). The pressure-measuring cell ( 120 ) is fixed axially in a resilient manner in the housing ( 110 ) by means of a metallic fixing device ( 150 ) which acts on a non-metallic intermediate body ( 140 ), which is provided between the pressure-measuring cell ( 120 ) and the fixing device ( 150 ) and can be moved radially at least with respect to the fixing device ( 150 ).

Description:
This application claims benefit of provisional application Ser. No. 60/109,817 filed Nov. 25, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a pressure sensor for determining the pressure of a process medium. The pressure sensor has a ceramic pressure-measuring cell with a diaphragm which is exposed to the process medium, is axially fixed in a metallic housing and generates an electrical signal which can be picked off and corresponds to a deflection of the diaphragm caused by the pressure. 
     Usually, with pressure sensors of this type the ceramic pressure-measuring cell is pressed against a stop by means of a metallic fixing device in the housing, said fixing device being directly in contact with said pressure-measuring cell. Temperature changes result in undesirable mechanical stresses between the pressure-measuring cell and the fixing device (and also the housing), said stresses originating from different thermal expansion coefficients of the materials used. For example, rising temperatures cause the metal housing and the metallic fixing device, serving to fix the pressure-measuring cell, to expand to a greater extent than the ceramic pressure-measuring cell. Displacements of individual parts relative to one another can likewise occur, said parts changing their relative position with respect to one another. In some cases these displacements have not been fully reversible in the course of a subsequent temperature decrease, and permanent deformations have remained as a result. Such deformations result in an undesirable temperature hysteresis which falsifies the measured pressure value or values. 
     DE-A 42 34 290 describes a ceramic pressure-measuring cell which is exposed directly to a process medium and is fixed in a metallic housing by means of a metallic fixing device, a so-called fixing sleeve. The subject matter of DE-A 42 34 290 serves to support the ceramic pressure-measuring cell radially and to relieve it to the greatest possible extent of axial stresses due to the fixing in the housing. For this purpose, DE-A 42 34 290 also proposes (inter alia) a ceramic disk (called “ceramic supporting body” therein) which is arranged in the axial direction between the ceramic pressure-measuring cell and the metallic fixing device (called “fixing sleeve” therein) and whose thermal expansion coefficient corresponds to that of the pressure-measuring cell. 
     The subject matter of DE-A 42 34 290 has the disadvantage, however, that the ceramic disk mentioned is fixedly joined to the pressure-measuring cell by means of active braze or glass solder. Different heat-induced expansions of the materials that adjoin one another cause, however, as outlined above, undesirable radial stresses between the metallic fixing device and the ceramic disk. Owing to its strong brazed fixture connection to the pressure-measuring cell, the ceramic disk passes the thermally induced radial stresses on to the pressure-measuring cell, which leads to the abovementioned falsifications of the measured pressure values and/or to temperature hysteresis effects. 
     Therefore, it is an object of the invention to provide a pressure sensor which avoids temperature hysteresis effects caused by stresses between the pressure-measuring cell and the housing generated by heat-induced expansions. 
     SUMMARY OF THE INVENTION 
     In order to achieve this object, a first variant of the invention consists in a pressure sensor for determining the pressure of a process medium, 
     having a metallic housing, 
     which is open to the process medium and 
     which has a throughhole for inserting a ceramic pressure-measuring cell therein, 
     which comprises a ceramic substrate and 
     a ceramic diaphragm, which is fitted thereon and is exposed to the process medium, and also 
     means, which supply an electrical signal which can be picked off and corresponds to a deflection of the diaphragm caused by the pressure acting thereon, whereby 
     the diaphragm is supported axially in a resilient manner on a first stop in the housing and 
     the substrate is supported on a non-metallic intermediate body, which 
     is retained by a metallic fixing device in the housing, and whereby 
     the intermediate body is movable radially with respect to the metallic fixing device. 
     A preferred embodiment of the first variant of the invention provides a pressure sensor in which the metallic fixing device is a metallic ring or a disk which is fixed in a releasable manner in the housing. 
     In order further to achieve the object, a second variant of the invention consists in a pressure sensor for determining the pressure of a process medium, 
     having a metallic housing, 
     which is open to the process medium and 
     which has a throughhole for inserting a ceramic pressure-measuring cell therein, which comprises 
     a ceramic substrate and 
     a ceramic diaphragm, which is fitted thereon and is exposed to the process medium, and also 
     means, which supply an electrical signal which can be picked off and corresponds to a deflection of the diaphragm caused by the pressure acting thereon, whereby 
     the diaphragm is supported axially in the housing in a resilient manner on a process connection and 
     the substrate is supported on a non-metallic intermediate body, 
     which is retained by a metallic fixing device in the housing, and 
     the intermediate body is movable radially with respect to the metallic fixing device. 
     In a preferred embodiment of the first or second variant of the invention, the intermediate body of the pressure sensor consists of a material whose thermal expansion coefficient corresponds to that of the ceramic substrate of the pressure-measuring cell. 
     In a particular embodiment of this embodiment, the intermediate body is composed of ceramic material. 
     According to a further preferred embodiment of the first or second variant of the invention, the intermediate body has a centering part, which at least partially encompasses the pressure-measuring cell radially for the purpose of centering the pressure-measuring cell and the intermediate body in the housing. 
     According to still another preferred embodiment of the first or second variant of the invention, recesses are provided in a first end face of the intermediate body, said first end face resting on the pressure-measuring cell, through which recesses electrically conductive connections are routed. The advantage is that shields against interfering electromagnetic irradiation of an electronic portion and of the pressure-measuring cell are thereby connected conductively to one another in a simple manner. 
     According to still another preferred embodiment of the first or second variant of the invention, a film made of cold-flowing material is arranged between the intermediate body and the fixing device. This embodiment minimizes the friction between the fixing device and the intermediate body of the pressure sensor, said friction being caused by heat-induced expansions. 
     A further advantage of the invention is that it not only enables stresses which occur in the radial direction and act on the pressure-measuring cell to be relieved but also permits, in a simple manner, the control of stresses which act on the pressure-measuring cell in the axial direction caused by different heat-induced expansions. By a suitable choice of the axial extent of the intermediate body (height and/or thickness of the intermediate body) taking account of resilient and/or elastic properties of a seal with respect to the process medium, axial loading on the pressure-measuring cell can also be kept as low as possible and mostly constant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described and explained using preferred embodiments which are illustrated in the figures of the drawing. 
     FIG. 1 schematically shows, in vertical section, a first variant of the pressure sensor, with a partly broken-open pressure-measuring cell, and a first variant of an intermediate body; 
     FIG. 2 shows, in section and in an enlarged manner, a second variant of the intermediate body in accordance with the detail designated by “E” in FIG. 1; 
     FIG. 3 shows, in an enlarged manner, the intermediate body according to FIG. 2 with friction-reducing films provided at interfaces; 
     FIG. 4 shows, in an enlarged manner, a third variant of the intermediate body in accordance with the detail designated by “E” in FIG. 1; 
     FIG. 5 shows, in an enlarged manner, a particular embodiment of the intermediate body according to FIG. 4; 
     FIG. 6 shows, in an enlarged manner, a fourth variant of the intermediate body in accordance with the detail designated by “E” in FIG. 1; 
     FIG. 7 shows schematically a partial view of a surface of the intermediate body according to FIG. 6, said surface being in contact with the pressure-measuring cell, as seen from the direction designated by “VII” in FIG. 6, and 
     FIG. 8 shows schematically and in a simplified manner in vertical section, a second variant of the pressure sensor with a process connection. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a pressure sensor  100 , which comprises a metallic housing  110  with a throughhole  111 , a ceramic pressure-measuring cell  120  being retained therein. This pressure-measuring cell is exposed directly to a process medium, which is not illustrated here for the sake of simplicity and acts on a diaphragm  121  of the pressure-measuring cell  120  via an inlet opening  112  in the front region  113  of the hole  111 . The pressure-measuring cell  120  is preferably a capacitive pressure-measuring cell, this being illustrated by the schematically shown electrodes  124   a,    124   b  on the diaphragm  121  and on substrate  122 . At its edge, the diaphragm  121  is brazed to substrate  122 , preferably by means of an active braze  123 . However, it is equally possible to use a piezoresistive pressure-measuring cell instead of the capacitive measuring cell for the pressure sensor  100 . 
     A measurement signal which can be picked off on the output side of the pressure-measuring cell  120  and corresponds to the pressure of the process medium acting on the diaphragm  121  is conditioned in an electronic portion  130 , which is illustrated only schematically in this case. 
     The pressure-measuring cell  120  is resiliently mounted in the housing  110  on a first stop  114 , which faces the process medium and is formed by a reduction of the free cross section of the throughhole  111  in the housing  110 . The interior of the housing  110  is sealed off by means of a seal  115 , which is arranged between the pressure-measuring cell  120  and the first stop  114  and simultaneously ensures the resilient properties of the support of the pressure-measuring cell  120 . As illustrated in FIG. 1, the seal  115  is, for example, an elastic O-ring seal, but this is not absolutely necessary for the realization of the invention, with the result that it is also possible to use any other resilient seal (and any form of seal). 
     An intermediate body  140  rests on that side of the substrate  122  of the pressure-measuring cell  120  which is remote from the diaphragm, said intermediate body in turn being retained by a metallic fixing device  150  in the housing  110 . The intermediate body  140  consists of non-metallic material having a thermal expansion coefficient which preferably corresponds to that of the ceramic substrate  122  of the pressure-measuring cell  120 . 
     Although various non-metallic materials for the intermediate body are possible for the invention, provided that their thermal expansion coefficients correspond to that of the substrate  122 , an intermediate body  140  made of a ceramic material is provided in the preferred embodiment. This material may, but does not absolutely have to, correspond to that of the substrate  122 . 
     The metallic fixing device  150 , which acts axially on the intermediate body  140  and thus the pressure-measuring cell  120 , is fixed in a releasable manner to or in the housing  110 . The fixing device  150  is preferably a ring, as illustrated in FIG.  1 . However, it may also be a disk which, like the ring, is composed of brass, for example, and is provided with an external thread which can be screwed into a corresponding internal thread in the housing  110 , to be more precise, in a region of the hole  111  which is remote from the process medium. The two threads are not illustrated, in order to simplify FIG.  1 . 
     A designated center line CL illustrates that the housing  110 , the hole  111 , the pressure-measuring cell  120 , the intermediate body  140  and the fixing device  150  are of rotationally symmetrical design in the preferred embodiment of FIG.  1 . If electrical supply leads (not illustrated here) to the electronic section  130  are used, it is advantageous, as illustrated in FIG. 1, for the intermediate body  140  and the fixing device  150  to be of annular design. 
     FIG. 2 diagrammatically illustrates the detail E from FIG. 1, which represents, in a simplified form and on an enlarged scale with respect to FIG. 1, a second variant of an intermediate body  140 ′ arranged between the pressure-measuring cell  120  and the fixing device  150 . This second variant of the intermediate body  140 ′ differs from the first variant of the intermediate body  140  according to FIG. 1 by a bevel  141  facing the housing  110 . A transition from the diameter of the pressure-measuring cell  120  to the larger internal diameter of the hole  111  in the housing  110  is realized in a simple manner by means of this bevel. 
     FIG. 3 illustrates a particular development of the arrangement according to FIG. 2. A thin layer made of cold-flowing material, for example a fluoroplastic, is fitted between a first end face  142  of the intermediate body  140 ′ and a corresponding surface  125  of the pressure-measuring cell  120 . This layer is preferably a first film  143  made of polyfluoroethylene (PTFE) which is advantageously applied on the intermediate body  140 ′, in particular on the first end face  142  thereof. 
     It is more important, however, to provide a second film  144  made of cold-flowing material, for example a fluoroplastic, on a second end face  145  of the intermediate body  140 ′ in order there, too, to minimize the friction between the fixing device  150  and the intermediate body  140 ′ in the event of heat-induced expansions that occur, and to ensure the displaceability of intermediate body  140 ′ and fixing device  150  relative to one another under all circumstances and to avoid irreversible changes in position. The second film  144  preferably cosists of polyfluoroethylene (PTFE), like the first film  143 . 
     FIG. 4 shows, likewise schematically, the detail E from FIG. 1 and, in a simplified form and on an enlarged scale with respect to FIG. 1, a third variant of an intermediate body  140 ″. For the purpose of simplification, here, too, the housing  110 , the housing wall illustrated in the detail E of FIG.  1  and also that part of the electronic section  130  which is situated in the detail are not illustrated. This third variant of the intermediate body  140 ″ differs from the other two variants represented in FIGS. 1 to  3  by a centering part  146 , which is provided on the intermediate body  140 ″ and at least partially encloses or encompasses the pressure-measuring cell  120  radially. The advantage of such an intermediate body  140 ″ with centering part  146  is that the pressure-measuring cell  120  can thereby be better centered in the housing  110  during assembly. In addition, the non-metallic centering part  146  can ensure electrical insulation with respect to the metallic housing  110  if the diaphragm  121  of the pressure-measuring cell  120  is fixed on the substrate  122  by means of a (metallic) active brazed joint  123 . 
     FIG. 5 schematically illustrates a particular embodiment of the inventive intermediate body  140 ″ according to FIG.  4 . This embodiment of the intermediate body  140 ″ differs from that illustrated in FIG. 4 by the fact that the centering part is designed as a separate centering part  147 . It may consist of a plastic which is less expensive than ceramic. 
     FIG. 6 is a schematic illustration of a fourth variant of an intermediate body  140 ′″ in connection with the detail E of FIG.  1 . For the sake of simplicity, the basic cross-sectional form of the intermediate body  140 ′ illustrated in FIG. 2 has been chosen for FIG. 6, but this does not signify any restriction. In principle, any other desired variant of the intermediate body is also suitable for the fourth variant explained below. 
     The special feature of the fourth variant of the intermediate body  140 ′″ of FIG. 6 is that it is designed for a pressure sensor  100  with an internal electromagnetic shield  161 , which surrounds the electronic section  130  and is essentially in the form of a cover. In order to be able to connect such a shield  161  of the electronic section  130  conductively to a corresponding shield  162  for the pressure-measuring cell  120 , continuous recesses  163  are provided, as shown by FIG. 7, in the first end face  142  of the intermediate body  140 ′″, which recesses are as shallow as possible and through which recesses electrical contact leads  164  are routed which connect the shields  161  and  162  to one another. If desired, it is also possible, as mentioned with respect to FIG. 3 that a first film  143  made of friction-reducing material is applied to the raised parts of the first end face  142  which are in direct contact with the pressure-measuring cell  120 . 
     FIG. 8 schematically illustrates a preferred embodiment of a second variant of the invention. A pressure sensor  200  is designed particularly for a simple change or replacement of a pressure-measuring cell  220  to be performed from the process side. In FIG. 8, a hole  211  in a housing  210  has a diameter which is greater than the external diameter of the pressure-measuring cell  220  only in that part of said hole  211  which is on the process side. This part  212  of the hole  211  on the front side ends within the housing  210  at a rear, second stop  213 , which in turn serves to support an intermediate body  240  for which, for the sake of simplicity, the cross-sectional form of the embodiment according to FIG. 2 has been chosen and which is arranged between the second stop  213 , serving as fixing device, and the pressure-measuring cell  220 . The latter is retained axially and resiliently in the housing  210  of the pressure sensor  200  by means of a process connection  250 , which is fixed to the housing  210  by releasable fixing means, preferably screws  251 . 
     The pressure-measuring cell  220  is preferably a capacitive pressure-measuring cell, which is illustrated by the electrodes  224   a,    224   b  shown schematically on the diaphragm  221  and on the substrate  222 . At its rim, the diaphragm  221  is brazed to a substrate  222 , preferably by means of an active braze  223 . However, it is equally well possible to use a piezoresistive pressure-measuring cell instead of the capacitive pressure-measuring cell for the pressure sensor  200 . 
     A measurement signal which can be picked off on the output side of the pressure-measuring cell  220  and corresponds to the pressure of the process medium acting on the diaphragm  221  is processed in an electronic section, which is not illustrated here for the sake of simplicity. 
     As an alternative to the second stop  213  which is represented in FIG.  8  and is formed by a sudden diameter change of the hole  211  in the housing  210 , it is alternatively possible, if only a single diameter is provided for reasons of production technique, to use the annular fixing device  150  which is illustrated in FIG.  1  and is screwed into the hole in a corresponding manner. 
     The sealing of the interior of the housing  210  against the process medium is ensured by a resilient and/or elastic seal  215  arranged between the pressure-measuring cell  220  and the process connection  250 . This seal  215  may be, for example, an O-ring or another resilient, annular seal which ensures axial and resilient clamping of the pressure-measuring cell  220  in the housing  210 . 
     For the embodiment of the invention which is illustrated in FIG.  8  and is particularly suitable for process-side mounting of the pressure-measuring cell  220 , it is also possible to use the intermediate body  140 ″ represented in FIG. 5 with a separate centering part  147 , for example made of plastic. It is likewise possible to use an intermediate body according to FIG. 3 or FIG. 6 for the second variant of the invention as illustrated in FIG.  8 . 
     The invention is not restricted to the embodiments illustrated in FIGS. 1 to  8 . It is conceivable and possible with no great effort for a person skilled in the art to combine the different embodiments mentioned above and illustrated in the drawing with one another in an expedient, yet free manner.