Abstract:
A combination absolute and differential pressure sensing device including a plurality of absolute pressure transducers, each transducer including a plurality of half bridge piezoresistive structures and a device for selectively coupling at least one of the plurality of half bridge piezoresistive structures of a first one of the absolute pressure transducers to at least one resistor to form a half-active full bridge structure adapted to measure an absolute pressure and at least one other of the plurality of half bridge piezoresistive structures of the first one of the plurality of absolute pressure transducers to at least one of the half bridge piezoresistive structure of a second one of the plurality of absolute pressure transducers to form a full bridge structure adapted to measure a differential pressure.

Description:
FIELD OF INVENTION 
     The instant invention relates generally to pressure transducers and more particularly to a piezoresistive assembly adapted to simultaneously measure both absolute and differential pressures. 
     BACKGROUND OF INVENTION 
     Piezoresistive sensor structures are widely used in pressure or force measuring. Generally, the prior art is replete with a number of patents which describe various configurations and methods of fabricating piezoresistive pressure sensing devices. 
     Generally, a piezoresistive device includes a bridge pattern of resistors which are mounted or otherwise diffused on one side of a relatively thin diaphragm member. The diaphragm which may be fabricated from silicon, and deflects upon application of a pressure thereto causes the piezoresistors to vary their magnitude according to the deflection of the diaphragm. 
     Differential pressure measurements can be accomplished using a differential transducer which provides an output which is the difference between two pressures. In the particular case of a gage sensor one of these pressures is atmospheric pressure and the other pressure is the pressure to be measured. In the case of the absolute pressure transducer, the output is solely indicative of a pressure applied. 
     Presently there are many implementations that require the measurement of both absolute and differential pressures. It is unfortunately necessary to often duplicate sensors and/or complicate associated circuitry thus resulting in elevated costs of manufacture and maintenance. 
     Further, the demand for pressure measuring assemblies adapted for use in hostile (high temperature and/or highly corrosive for example) environments has grown in recent years. 
     Commonly assigned U.S. Pat. No. 4,222,277, filed Aug. 13, 1979, entitled “Media Compatible Pressure Transducer”, teaches an absolute pressure transducer which is adaptable for use in various deleterious mediums. Therein, a single wafer contains a gage sensor configuration on one portion and an absolute sensor configuration on another adjacent portion. 
     However, both the absolute and gage sensors of the &#39;277 are exposed to a single pressure. Accordingly, it is desirable, and the object of the present invention to provide a relatively inexpensive structure capable of measuring multiple absolute pressures and a differential pressure associated therewith. It is another object of the instant invention to provide a relatively inexpensive structure capable of measuring both absolute and differential pressures which is further adapted for prolonged use in a hostile environment. 
     SUMMARY OF INVENTION 
     A combination absolute and differential pressure sensing device including: two pressure transducers each respectively including two electrically separated half bridge piezoresistive structures; two resistors; and, a device for selectively coupling one of the half bridge piezoresistive structures of a first one of the absolute pressure transducers to two resistors to form a half-active full bridge structure adapted to measure an absolute pressure; and, one of the plurality of half bridge piezoresistive structures of the first one of the two absolute pressure transducers to one of the half bridge piezoresistive structures of a second one of the absolute pressure transducers to form a full bridge structure adapted to measure a differential pressure. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 illustrates a perspective view of a first embodiment of the present invention. 
     FIG. 2 illustrates a cross-section view of the embodiment of FIG.  1 . 
     FIG. 3 illustrates a circuit incorporated into a first pressure sensor according to the present invention. 
     FIG. 4 illustrates a circuit incorporated into a second pressure sensor according to the present invention. 
     FIG. 5 illustrates an electronic interconnection of portions of the circuits of FIGS. 3 and 4 which can be utilized to determine a difference between pressures applied to the first and second pressure sensors. 
     FIG. 6 illustrates an electronic interconnection of portions of the circuits of FIGS. 3 and 4 which can be utilized to determine a sum of pressures applied to the first and second pressure sensors. 
     FIG. 7 illustrates an electronic interconnection of a portion of the circuit of FIG. 4 which can be utilized to determine a pressure applied to the second pressure sensor. 
     FIG. 8 illustrates an electronic interconnection of a portion of the circuits of FIG. 3 which can be utilized to determine a pressure applied to the first pressure sensor. 
     FIG. 9 illustrates a cross-section of a preferred configuration of a sensor chip, the isolation diaphragm and the oil filled cavity utilized according to the present invention. 
     FIG. 10 illustrates a cross-section of a preferred configuration of a sensor chip, the isolation diaphragm, oil filled cavity and pressure port utilized according to the present invention. 
     FIG. 11 illustrates an alternative embodiment of a pressure sensor and header assembly utilized according to the present invention. 
     FIG. 12 illustrates the alternative embodiment of FIG. 11 further including an isolation diaphragm and oil filled cavity. 
     FIG. 13 illustrates a side view of the embodiment of FIG. 12 further including a port. 
     FIG. 14 illustrates a cross-sectional view of a preferred form of a sensor utilized according to the present invention. 
     FIG. 15 illustrates an overview of a circuitry configuration which can be utilized according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the numerous figures wherein like references refer to like element of the invention, FIGS. 1 and 2 illustrate a preferred form of the invention including a set of two substantially identical piezoresistive absolute pressure sensors  10 ,  20  mounted in a common housing  30  with means of applying pressures separately to respective sensor housings  40 , 50 . Alternatively, it should be recognized the sensors  10 ,  20  could of course be mounted in separate housings without effecting the operation of the present invention. The critical feature being that different pressures to be measured can be respectively applied to the two sensors  10 ,  20 . 
     Referring now also to FIGS. 3 and 4, each sensor  10 ,  20  preferably includes four resistive elements ( 60 ,  70 ,  80 ,  90 , and  100 ,  110 ,  120 ,  130 ) arranged in an open set of two uncoupled half-bridges. Preferably each resistive element in each sensor  10 ,  20  exhibits as close as possible a same percentage change of resistance when exposed to an identical pressure. 
     Each sensor  10 ,  20  preferably also includes six contacts or pins ( 1 ,  2 ,  3 ,  4 ,  5 ,  6 , and  7 ,  8 ,  9 ,  10 ,  11 ,  12 ) such that each resistive element ( 60 ,  70 ,  80 ,  90  and  100 ,  110 ,  120 ,  130 ) of each half-bridge can be connected to a contact (see for example FIG.  15 ). 
     It should be recognized that depending on the particular configuration of each sensor  10 ,  20  different resistive element ( 60 ,  70 ,  80 ,  90  and  100 ,  110 ,  120 ,  130 ) will either be subjected to compression or tension forces. The arrows associated with a resistive element designate associated forces for each particular resistive element. In other words, resistive elements having arrows pointed in a same direction are subject to a same compressive or tensile force and resistive elements having arrows pointed in opposite directions are each subjected to opposite forces (i.e. one tensile and one compressive). Thus, each half-bridge from each sensor  10 ,  20  can be interconnected to a half-bridge of the same sensor  10 ,  20  giving a fully active full bridge or interconnected to a half-bridge from the other sensor giving a full bridge which depending on which half-bridge of the second housing is used, either represents the sum or the difference of the pressures applied to each housing. 
     If a half-bridge from each sensor  10 ,  20  is interconnected to form a full-bridge, then the remaining half-bridge of either sensor  10 ,  20  can be interconnected with two fixed resistors to form a half-active full bridge. Thus, each remaining half-bridge can be coupled independently in this way to provide an output from each sensor  10 ,  20  that is proportional to the specific pressure applied to that particular sensor. Thus, by proper interconnection, if P 1  represents a pressure applied to the first pressure sensor  10  using the first port  40  and P 2  represents a pressure applied to the second pressure sensor  20  using the second port  50 , the following outputs can be obtained by proper connections: 
     
       
         P 2 −P 1 ,  (1) 
       
     
     
       
         P 2 +P 1 ,  (2) 
       
     
     
       
         P 2 ,  (3) 
       
     
     and 
     
       
         P 1   (4) 
       
     
     These connections can be made by any suitable means, for example by electronic switching controlled by a micro-controller, integrated circuit or any other suitable configuration such as a mechanical switch or timer circuit. This can be accomplished for example using circuit  200  and leads  210 . 
     Referring now also to FIG. 5, P 2 −P 1  (1) is ascertained using a half-bridge from each sensor (resistive elements  60 ,  70  from sensor  20  and elements  100 ,  110  from sensor  10 ) which are connected to give an output representing the difference in pressure between P 2  and P 1 . 
     Referring now also to FIG. 6, P 2 +P 1  (2) is ascertained using the same half-bridge as in P 2 −P 1  from the sensor  20  but using the half-bridge not used in P 2 −P 1  from the P 1  sensor. (resistive elements  60 ,  70  from sensor  20  and elements  120 ,  130  from sensor  10 ). 
     Referring now also to FIG. 7, P 2  is ascertained by interconnecting the half-bridge from the P 2  housing not used in P 2 −P 1  and P 2 +P 1  (resistive elements  80 ,  90  from sensor  20 ) to set, fixed resistors  140 ,  150 . 
     Finally, referring now also to FIG. 8, P 1  is found by interconnecting the half-bridge from the P 1  housing not used in P 2 −P 1  (resistive elements  100 ,  110  from sensor  10 ) connected to the set of fixed resistors  140 ,  150 . Alternatively, each sensor  10 ,  20  could of course have its own set of fixed resistors associated with it. 
     Referring now also to FIGS. 9 and 10 in one preferred embodiment each sensor  10 ,  20  has a header  160  with eight (8) dielectrically isolated pins  170  on which the open bridge semiconductor piezoresistive sensor  10 ,  20  is mounted together with a port  40  suitably configured for attachment to a pressure source. The header  160  is preferably further made suitable for providing an isolation diaphragm  180  and oil-filled enclosure cavity  190 , wherein the isolation diaphragm  180  acts on oil within the cavity  190  to impart to the semiconductor piezoresistive sensor  10 ,  20  the applied pressure while isolating the semiconductor sensor  10 ,  20  from a media associated with the pressure to be measured. 
     In any event, the semiconductor piezoresistive sensor  10 ,  20  is in and of itself an absolute sensor with its own internal reference cavity. Such a preferred configuration together with the sensor chip, the isolation diaphragm and the oil filled cavity is shown in FIG.  9  and is shown in FIG. 10 with a pressure port attached. 
     Referring now also to FIGS. 11 and 12 in a second preferred embodiment, a leadless sensor as described in copending U.S. patent application Ser. No. 09/160,976, entitled “Hermetically Sealed Ultra High Temperature Silicon Carbide Pressure Transducers and Method for Fabricating Same” and copending U.S. patent application Ser. No. 09/245,158, entitled “High Pressure Piezoresistive Transducer Suitable for Use in Hostile Environments and Method for Making the Same”, the entire disclosures of which are hereby incorporated by reference, can be welded or otherwise affixed to the housing such that only the non-active side of the sensor is exposed to the pressure media. For severe environments it is clear that one can also provide means in the housing to provide for an isolation diaphragm and an oil cavity. This is shown in FIG.  11  and with a second header attached in FIG. 12 and a port  140 ′ attached thereto in FIG.  13 . 
     It should be recognized that the semiconductor sensors  10 ,  20  for this application preferably meet stringent requirements. Not only does each of the two sensors  10 ,  20 , one adapted to receive P 2  and the other adapted to receive P 1 , need to demonstrate a nearly identical relative resistance change as a function of applied pressure but moreover the linearity of change of output voltage with applied pressure must be as close to zero as possible (preferably&lt;0.2% Full Scale). This requirement results from the realization that if P 2  is near the top of its range, one must get the same output for a given value of P 2 −P 1  as when P 2  is nearer to the bottom of its range when P 2 −P 1  are of the same value. Such a sensor is shown in FIG. 4 where it may be noted that an open bridge configuration having six contact points (1-6)is shown (see FIG. 15 also). 
     The sensor as shown in a side view of FIG. 14 preferably includes a bossed deflecting portion  200 , the presence of the boss  200  and the various widths of the thin deflecting portions serve to insure an ultra-linear voltage versus pressure relationship. 
     Additionally, by properly controlling the distance between the boss  200  and the glass  210 , the boss  200  will stop against the glass  210  preventing an overpressure from fracturing the sensor  10 , for example at point  220 . 
     Referring now also to FIG. 15, therein is illustrated a top view of a circuitry structure which can be utilized to realize the four resistive elements (i.e.  60 ,  70 ,  80 ,  90 ) arranged in an open set of two uncoupled half-bridges which is utilized according to the present invention. Sensor  10  can of course be realized using an identical structure. 
     The present invention yields a number of unanticipated advantages, because both sensors are absolute there is no associated zero error in the differential measurement as the zero errors cancel out. Further, there is no media exposure to either side of the sensors since the front side of the sensor is protected by a metal isolation diaphragm, resulting in longer anticipated life, and internal stoppage is inherently provided for over-pressuring of the differential pressure transducer, as it is formed from two absolute pressure transducers. 
     Although the invention has been described in a preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example, and that numerous changes in the details of construction and combination and arrangement of parts may be made without departing from the spirit and scope of the invention as hereinafter claimed. It is intended that the patent shall cover by suitable expression in the appended claims, whatever features of patentable novelty exist in the invention disclosed.