Abstract:
In a pressure sensor module according to the prior art, which is intended for detecting the pressure of a corrosive medium, the conventional sensor cell with a pressure sensor chip is modified in order to protect it from corrosion, which results in a large volume for a pressure-transmitting-fluid. This is disadvantageous for the calibration and for a high degree of measurement precision. 
     In a pressure sensor module ( 1 ) according to the invention, a convention sensor cell ( 5 ) is used that has an adapter ( 21 ) connected to it, which has a very small volume for a pressure-transmitting medium.

Description:
BACKGROUND OF THE INVENTION 
   A pressure sensor module contains, for example, piezoresistive sensor cells, which are used for a broad range of applications, for example a pressure measurement in an intake manifold of a motor vehicle. In this connection, a pressure sensor chip is soldered to a socket, which has glass feedthroughs for external electrical connections. Then electrical connections of the pressure sensor chip are electrically contacted with the external electrical connections by means of bonding wires. 
   The pressure of a medium acts, for example, on a back side of the pressure sensor chip by means of a metal tube, which is soldered to the socket. For absolute pressure measurements, a metal cap is welded to the socket under vacuum. For relative pressure measurements, the cap has an opening. The pressure sensor chip is protected, for example by means of a gel, against external influences. A sensor cell of this kind cannot be used with intensely corrosive mediums since the corrosive medium corrodes the sensor chip or the gel. 
   JP 2000 046 666 A1 has disclosed a pressure sensor module in which the sensor membrane is protected by a gel in the pressure fitting. However, this gel is also corroded by the corrosive medium and since as a gel, it does not have great chemical resistance or mechanical strength, it changes in shape over time, which has an influence on the measuring results. 
   To this end, according to the prior art, the pressure is indirectly transmitted to the pressure sensor chip by means of a separating membrane and a pressure transmitting medium, e.g. silicon oil. The design of such a pressure sensor module, with this oil seal is known, for example, from the AMA seminar tape, 1989, pp. 285 to 295. In this design, the oil volume around the pressure sensor chip is relatively large. Due to the high thermal volume expansion coefficient of the silicon oil in comparison to the volume expansion coefficient of a housing material, the separating membrane is deflected in the event of a temperature change. Due to a non-negligible rigidity of the separating membrane, a pressure then builds up on the pressure sensor chip in the pressure sensor module, which is due solely to the temperature change and distorts the measurement signal. 
   In order to reduce the oil volume in the known pressure sensor module, the oil volume around the pressure sensor chip is reduced, e.g. by means of a filling body made of ceramic. In this instance, the filling body requires a separate assembly step. In the known pressure sensor module, due to the relatively high manufacturing tolerances, there is a large oil volume for the cavity around the pressure sensor chip in comparison to the cavity underneath the separating membrane. 
   As a result, for greater measurement precision, it is necessary to calibrate the pressure sensor chip when it is filled with oil. In this connection, the amount of time required to complete the temperature steps during calibration is longer since there is a higher heat capacity. In addition, in the known pressure sensor module, a defect of the pressure sensor chip can only be detected once it is completely assembled. As a result, the costs arising from rejections are higher since the finished component has to be discarded. 
   DE 195 07 143 A1 and U.S. Pat. No. 5,595,939 have disclosed a pressure sensor module according to the principal of the oil seal, with a simpler assembly process. In this pressure sensor module, the filling body for reducing the oil volume is replaced by a recess for the pressure sensor chip in the plastic plug connector housing. In this instance, a sealing of the separating membrane in relation to the plug connector is produced by means of an O-ring or a crimped seal. With this type of seal, there is the danger, for example in the event of a damaged O-ring or when particles adhere to the sealing surface, of oil escaping from the interior of the sensor. In the pressure sensor module, when the module is being filled with oil, the exertion of a high pressure on the separating membrane forces the residual air enclosed in the sensor out from the plastic material, which allows the air molecules to pass through. In this connection, there is the danger of the pressure sensor chip, which is designed for low pressure measurement ranges, being damaged by the high pressure for displacing the residual air in the housing. 
   For the pressure sensor chip, further steps are required to improve the electromagnetic compatibility, by means of capacitors, for example. The integration of these capacitors into the housing in the known sensor module is complicated and increases the manufacturing costs for the pressure sensor module. 
   JP 2000 009 568 A1 has disclosed a pressure sensor module in which an adapter is connected to a pressure sensor cell in order to determine the pressure of a fluid. The design is very complex and includes a large dead volume of the transmitting medium. 
   JP 11 316 166 A1, DE 44 15 984 A1, and U.S. Pat. No. 5,629,538 have disclosed pressure sensor chips, which have a protective film on the sensor membrane in order to protect it from a corrosive environment. This protective layer changes the reaction behavior and measurement behavior of the membrane in a negative fashion and also does not offer sufficient protection over a long time since this protective layer must be thin enough to allow the sensor membrane to continue to flex in a favorable fashion. 
   JP 81 36 380 A1 has disclosed a pressure sensor module, which is connected to an adapter in order to protect the pressure sensor chip from the corrosive medium. In this design, an O-ring must once again be used to produce a seal, which brings about the O-ring-related problems described above. 
   SUMMARY OF THE INVENTION 
   The pressure sensor module according to the invention has the advantage over the prior art that a pressure sensor module can be simply produced inexpensively and with a high degree of measurement precision for use in a corrosive environment. The high precision Is achieved by sharply reducing the oil volume. As a result, In the pressure sensor module according to the invention, the influence of the thermal expansion of the oil volume on the measurement signal is very low. As a result, a rapid calibration of the pressure sensor chip, which Is built into the sensor cell, can be executed in order to avoid the complex and long temperature steps in the finished pressure sensor module. A balancing of the sensor coils reduces the rejection costs in the calibration of the pressure sensor module. Since the sensor cell is smaller than the pressure sensor module, several sensor cells can be calibrated at the same time. 
   With one embodiment of advantageous adapter of the pressure sensor module, the adapter has a hole through it, which is connected at one end to a pressure fitting of the sensor cell and at the other end, is closed by means of a separating membrane; the volume thus enclosed is advantageously filled with a fluid. The adapter has a simple design and is therefore inexpensive. 
   It is advantageous if the adapter has an additional opening through which the fluid can be introduced into the hole. 
   A printed circuit board, for example with components on it, can advantageously be connected to external electrical connections of the pressure sensor chip of the sensor cell. This allows additional components for improving the electromagnetic compatibility to advantageously be mounted onto the printed circuit board. 
   The pressure sensor module housing can advantageously contain the electrical plug connections. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are shown in a simplified fashion in the drawings and will be explained in detail in the subsequent description. 
       FIG. 1  shows an axial cross section through a sensor cell with a pressure sensor chip according to the prior art, 
       FIG. 2  shows a cross section through a pressure sensor module according to the prior art, 
       FIG. 3  shows an axial cross section through an adapter, 
       FIG. 4   a  shows a sensor cell with a printed circuit board and electrical connections and with an adapter, in the disassembled state, 
       FIG. 4   b  shows the components in  FIG. 4   a  when they are assembled, 
       FIG. 5   a  shows an exemplary embodiment of a pressure sensor module embodied according to the invention, in the disassembled state, 
       FIG. 5   b  shows an axial cross section through an exemplary embodiment of a pressure sensor module according to the invention from  FIG. 5   a,    
       FIG. 5   c  shows another exemplary embodiment of a pressure sensor module embodied according to the invention, without a printed circuit board, and 
       FIG. 6  shows an axial cross section through another pressure sensor module embodied according to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows an axial cross section through a sensor cell  5 , which contains a pressure sensor chip  7  and is used for example in the pressure sensor module  1  according to the invention (FIG.  5 ). The pressure sensor chip  7  is mounted on a glass support  11 , for example by means of anodic bonds. The sensor cell  5  has a for example two-piece sensor housing  9  in which the glass support  11  is affixed by means of a soldering process. Inside the sensor housing  9 , bonding wires  13  electrically connect the pressure sensor chip  7  to electrical sensor connection elements  15 , which protrude through the sensor housing  9 . For example, the pressure sensor chip  7  has a membrane  17 , whose underside  49  is acted on by a medium, which is conveyed by means of a pressure fitting  19 , for example an additional metal tube, which is shown with dashed lines. The membrane  17  can flex as a result of a pressure acting on it so that the deflection results in a measurement signal that is transmitted by means of the bonding wires  13 . A different pressure sensor chip  7  can also be used for the sensor cell  5 . 
   For example, a vacuum prevails inside the sensor housing  9  so that the sensor cell  5  can be used for absolute pressure measurement. 
   The membrane  17  has a top side  47 , which is disposed in approximately the same plane as the top of the pressure sensor chip  7 . The bonding wires  13  are attached to this top side  47  of the pressure sensor chip  7 . The membrane  17  also has an underside  49 , opposite from the top side of the membrane. 
     FIG. 2  shows a pressure sensor module according to the prior art. 
   In a pressure sensor module of this kind, a fluid  28  serving as a pressure-transmitting medium is disposed on the membrane top side  47  of the pressure sensor chip  7 , which is also where the electrical contacting with the bonding wires  13  takes place. A separating membrane  29  encloses the fluid  28  so that the pressure sensor chip  7  is protected from corrosive mediums in this case as well. The volume for the pressure-transmitting medium  28 , however, is relatively large because it encompasses the entire surface of the pressure sensor chip  7  and the sensor connection elements  15 . Furthermore, there must be a certain distance from the separating membrane  29  to the pressure sensor chip  7  so that the separating membrane  29  cannot touch the bonding wires  13  or the pressure sensor chip  7 . In particular, this large volume of the pressure-transmitting medium  28  results in the difficulties mentioned above. 
     FIG. 3  shows an axial cross section through an adapter  21 , which is embodied, for example, in the shape of a cylinder with a hole  23  passing through the center. On a sensor end  25 , the adapter  21  is connected to the pressure fitting  19  of the sensor cell  5  (FIG.  5 ). On the opposite end, a medium end  27 , a separating membrane  29  closes the hole  23 . There is a small distance between the separating membrane  29  and the medium end  27 . 
   Lateral to the hole  23 , the adapter  21  has a filling opening  31  and a filling hole  32  through which a fluid can be introduced into the hole  23  when the adapter  21  is assembled with the sensor cell  5 . The volume of the hole  23  and of the filling hole  32  is deliberately selected as small in order to minimize an influence of the fluid on the measurement behavior of the pressure sensor chip  7 . The fluid that is introduced into the hole  23  and the filling hole  32  serves to transmit a pressure, which is exerted against the separating membrane  29 , to the membrane  17 . Preferably, a silicon oil is used for this fluid. After being filled with oil, the filling hole  32  is tightly closed, for example by having a metal ball press-fitted into it. On its outer surface, the adapter  21  has a sealing ring  33 , which seals the adapter  21  in relation to an opening or a receptacle that contains the medium to be measured. 
     FIG. 4   a  shows the sequence in which the sensor cell  5  is assembled with the adapter  21  and a printed circuit board  36 . The printed circuit board  36  is disposed, for example, between the sensor cell  5  and the adapter  21 , and is electrically connected, for example, to at least one contact pin  37 , which is in turn connected, for example, to plug connections  39  (FIG.  5 ). 
     FIG. 4   b  shows the components in  FIG. 4   a  when they are assembled. The pressure fitting  19  is tightly connected to the hole  23  at the sensor end  25  of the adapter  21 . The adapter  21  protects the membrane  17  from a corrosive medium. The printed circuit board  36  is connected in an electrically conductive fashion to the sensor connection elements  15  of the sensor cell  5  and is electrically connected to the contact pins  37 , for example by means of lines on the printed circuit board  36 , or is connected for example directly to plug connections  39  ( FIG. 5   c ) so that an external plug connector  43  ( FIG. 5   a, b, c ) produces an electrical connection of the sensor cell  5  with the outside. The printed circuit board  36  is not a required component of the pressure sensor module  1  according to the invention. In lieu of the printed circuit board  36 , for example, a pressed screen can also be used. 
     FIG. 5   a  shows a pressure sensor module  1  according to the invention, once again in the partially assembled state. The apparatus shown in  FIG. 4   b  now has a pressure sensor module housing  45  mounted onto it and together with it, is connected to the adapter  21 , for example in a mechanical fashion. The sensor cell  5  is held by the pressure sensor module housing  45 , e.g. by means of glue between the sensor cell  5  and the housing  45 . 
   For example, the plug connections  39  are injection molded into the housing  45  and secured there, for example. Furthermore, a part of the housing  45  with the plug connections  39  forms of plug connector  43  in which the plug connections  39  can be accessed from the outside. 
   For example, the printed circuit board  36  has at least one electrical component  41  disposed on it, which serves, for example, to improve the electromagnetic compatibility. For example, these components are capacitors. 
     FIG. 5   b  shows an axial cross section through a pressure sensor module  1  according to the invention. 
   The contact pins  37  are connected in an electrically conductive fashion to the plug connection elements  39 , for example by being soldered at a contact point  57 . The contact point  57  can be additionally protected from external corrosion by means of a gel or glue. The separating membrane  29 , which is clamped continuously around its edge, flexes when an external pressure is exerted on it. 
     FIG. 5   c  shows another exemplary embodiment of a pressure sensor module  1  according to the invention, without a printed circuit board  36 . For example, a direct electrical contact has been provided between the plug connections  39  and the sensor contact elements  15 . The housing  45  can, for example, be injection molded or cast around the sensor cell  5  and the plug connections  39 . 
   The advantage of the pressure sensor module  1  according to the invention is comprised in that a commercially available sensor cell  5  can be used to measure a pressure of a corrosive medium. 
   The separating membrane  29  is embodied as flexible and transmits a pressure to the pressure sensor chip  7  by means of the fluid, which is contained in the volume between the separating membrane  29  and the membrane  17 . The separating membrane  29  is comprised of a corrosive-resistant material. 
     FIG. 6  shows an axial cross section through another pressure sensor module  1  according to the invention, which for example has a fastening flange  51 , by means of which it is fastened for example to a wall  53 . The pressure sensor module  1  is inserted into an insertion opening  69  of the wall  53 . The wall  53  is part of a line  61 , which a for example corrosive medium  59  flows through, or is part of a reservoir  61 , which is used to store a corrosive medium  59 . 
   The medium  59  can freeze and since it is in direct contact with the separating membrane  29 , the expansion of the freezing medium  59 , by means of the separating membrane  29 , can cause the pressure on the membrane  17  to become so great that the membrane  17  breaks. In this connection, it makes sense to keep the volume of the medium  59  in the vicinity of the separating membrane  29  as low as possible in order to likewise keep the volume increase of the freezing medium  59  low. In order to avoid a membrane breakage, a homogeneous freezing is advantageous. This is achieved, for example, by means of a lateral pressure supply ( 72 ) and a uniform distance between the membrane  29  and the opposing blind hole bottom ( 74 ). 
   In addition, in order to avoid membrane breakage, the pressure sensor module  1  is fastened to the wall  53  in a flexible fashion. This is achieved, for example, in that the fastening flange  51  is embodied as flexible. For example, the fastening flange  51  is comprised of an elastic material, for example a spring steel, and it  51  is affixed in at least two fastening points  63  between which the fastening flange  51  can flex. 
     FIG. 6  shows another possibility. The pressure sensor module  1  is fastened to the wall  53  in a flexible fashion by means of a spring  55  and a T-piece  66 , for example a screw  66 . The screw  66  is inserted through an opening of the fastening flange  51  and is screwed into the wall  53 . The spring  55  is disposed between the rigid fastening flange  51  and the screw  66 . In this instance, the spring  55  engages the screw  66  and the fastening flange  51  so that the pressure sensor module  1  can be pushed out from the insertion opening  69  by exerting a force that compresses the spring  55 . 
   The spring force of the spring  55  is set so that the spring  55  only flexes upon expansion of the freezing medium  59 . The same is true of a fastening flange  51  that is embodied as flexible. 
   The sealing ring  33  is disposed just above the separating membrane  29  in the axial direction in order to reduce a friction between the adapter  21  and an inner wall of the wall  53  caused by the freezing medium. 
   It is not necessary to the function of the pressure sensor module  1  that the medium be corrosive.