Abstract:
A wet-to-wet differential pressure sensing device including: a housing including first and second pressure receiving ports; a doubly stopped pressure sensor secured within the housing between the first and second ports; a first isolation diaphragm secured within the housing between the first port and the sensor; a second isolation diaphragm secured within the housing between the second port and the sensor; a first filling tube for introducing oil into a first cavity formed between the first diaphragm and the housing; and, a second filling tube for introducing oil into a second cavity formed between the second diaphragm and the housing; wherein the first and second filling tubes are accessible through a side wall of the housing being interposed between the first and second ports and the sensor and header are secured within the housing such that they are offset with respect to a center of the first diaphragm.

Description:
FIELD OF INVENTION 
     The present invention relates to pressure transducers, and more particularly to an improved wet-to-wet differential pressure transducer. 
     BACKGROUND OF INVENTION 
     There are many types of pressure transducers for which there are many well known uses. One particular type of transducer is that conventionally characterized as being wet-to-wet and which can be used to measure the differential pressure between two fluid or gaseous substances for example. Wet-to-wet differential transducers are used in automotive, medical and aerospace applications. 
     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. Typically, a piezoresistive device includes a bridge pattern of piezoresistive resistors which are formed on or within one side of a relatively thin deflecting member. The deflecting member, which may be fabricated from silicon, (as well as the resistors) deflects upon application of a pressure thereto which causes the piezoresistors to vary their magnitude as is well known. If a pressure P, is applied to one face of the deflecting member and a pressure P 2  is applied to the other face of the deflecting member, the resulting deflection will be determined by the difference in pressure P 1 ,-P 2 . Thus, differential pressure measurements can be accomplished using a differential transducer, which provides an output, which is the difference between two pressures. Examples of both absolute and differential piezoresistive bridge pressure transducers are illustrated in commonly assigned U.S. Pat. No. 4,222,227, entitled “MEDIA COMPATIBLE PRESSURE TRANSDUCER”, the entire disclosure of which is hereby incorporated by reference. 
     If oil-filled cavities are used in conjunction with media-isolation diaphragms, it is desirable to minimize the quantity of oil included in the oil-filled pressure communicating cavity to improve sensor performance. It is further desirable to miniaturize the pressure-sensing device as size and weight is often design considerations in many applications. 
     It is the object of the present invention to provide a miniaturized and hence improved wet-to-wet differential pressure-sensing device. 
     SUMMARY OF INVENTION 
     A pressure sensing device including: a housing including a first port and a second port; a pressure transducer secured within the housing between the first and second ports, said sensor having one face capable of being exposed to a pressure P 1  and the other face capable of being exposed to a pressure P 2  said sensors having overpressure stops in each direction, i.e. P 1  and P 2 ; a first isolation diaphragm, secured within the housing between the first port and the sensor; a second isolation diaphragm secured within the housing between the second port and the sensor, said isolation diaphragms serving to insure that the pressure media from the first and second ports do not directly touch the sensor; a first filling tube for introducing a fluid material into a first cavity located between the first isolation diaphragm and the sensor; and, a second filling tube for introducing fluid material into a second cavity located between the second isolation diaphragm and the sensor; wherein the first and second filling tubes are fastened to a side wall of the housing being interposed between the first and second ports, and each tube separately intersecting holes formed at right angles to the tubes, one hole going into the first cavity and connecting to the first tube and a second hole accessing the second cavity connected to the second tube. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 illustrates an end view of the transducer device according to the present invention. 
     FIG. 2 illustrates a cross section  2 — 2  of the embodiment of FIG.  1 . 
     FIG. 3 illustrates a cross section  3 — 3  of the embodiment of FIG.  1 . 
     FIG. 4A illustrates a partial perspective view of the embodiment of FIG.  1 . 
     FIG. 4B illustrates a cross-section of the embodiment of FIG.  4 A. 
     FIG. 5A illustrates a plan view of a preferred form of a piezoresistive closed bridge structure sensor utilized according to the present invention. 
     FIG. 5B illustrates a plan view of a preferred form of a piezoresistive open bridge structure sensor utilized according to the present invention. 
     FIG. 5C illustrates a cross-section of the pressure sensor utilized according to the present invention, 
     FIG. 6A illustrates a plan view of a preferred form of a header utilized according to the present invention. 
     FIG. 6B illustrates a cross-section of the header of FIG.  6 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Basically, according to the present invention a header and a piezoresistive pressure sensor are disposed within a housing of a device and electrically coupled together. Two ports are provided for supplying pressures to be measured, and a thin isolation diaphragm is supplied adjacent to each of the ports. Oil filled cavities are formed behind the diaphragms and serve to communicate pressures received using the ports and diaphragms to respective surfaces of the pressure sensor. These applied pressures serve to deflect the sensor (the deflection being caused by the differences of the oppositely applied pressures and hence being indicative of the differential pressure), and a signal indicative of this differential pressure can be supplied using the sensor, header and electrical connection there between. 
     It has been discovered that a miniaturized differential pressure transducer can be formed by offsetting the sensor and header with respect to a center axis of the device, and by placing oil filling tubes out of a side surface of the device different from the surface on which the ports are supplied. 
     Referring now to the numerous figures, it is noted that like references identify like elements of the invention. FIG. 1 illustrates an end view of a preferred embodiment of the present invention. The device  10  includes a sensor  20 , interconnection  30  and header  40  mounted below a pressure-communicating diaphragm  60 . In the preferred embodiment, interconnection  30  takes the form of a flexible circuit board and specifically Kapton interconnect flex circuit, although any suitable interconnection means can be effectively utilized. The interconnection  30  serves to electrically couple the sensor  20  to the header  40  which serves to further electrically couple the sensor  20  beyond the device  10 , i.e. to a monitoring system, display, microcontroller, etc., using wires  90  for example. The device  10  further includes a main housing  80  having a recessed portion  70  which has the pressure communicating diaphragm  60  secured therein. The diaphragm  60  serves to isolate the sensor  20  from the pressure transmitting media. The resultant device  10  is adapted for longer life as any deleterious effects which may be caused by direct exposure of sensor  20  to the media are avoided. Further, by using isolation diaphragm  60  contamination of the media due to direct exposure to the sensor  20  is also avoided. 
     Referring now also to FIG. 2, the diaphragm  60  and housing  80  form a cavity  150  in which the sensor  20  is secured and which cavity is filled with an appropriate oil. The oil selected is relatively incompressible so a pressure  130  applied to the diaphragm  60  is directly transferred to the sensor  20  through the oil. The pressure  130  applied to the sensor  20  through the oil filled cavity  150  effects the electrical characteristics of the sensor  20  as is well known. Accordingly, a signal is produced by the sensor  20  indicative of the applied pressure  130  using the interconnect  30  and header  40 . 
     The transducer to be formed is intended to be a wet-to-wet differential device. A recessed portion  70 ′ is preferably similarly formed on an oppositely disposed surface of the main housing  80  from the recessed portion  70 . A diaphragm  60 ′ is affixed within the recessed portion  70 ′ and forms a cavity  150 ′ in conjunction with housing  80 . It is noted that for operation the cavities  150  and  150 ′ should be of minimal depth such that the volume of oil on each side of the sensor is minimized. A channel  140  is provided through the main housing  80  to the sensor  20  and is also filled with the same oil type as the cavities  150  and  150 ′. Thus when a pressure  130 ′ is applied it is communicated to the sensor  20 . 
     One aspect of the present invention is to provide a device  10  of reduced size, and hence, an improved wet-to-wet differential pressure transducer. However, a major limitation in miniaturizing the device  10  is encountered in attempting to fill the cavities  150 ,  150 ′ and channel  140  with the suitable oil. Fill tubes, which respectively open into cavities  150 ,  150 ′ and extend into the recessed area  70 ′ can be employed. In turn these fill tubes can then be folded over and crimped to be sealed. However, this results in a device  10  which cannot be sufficiently miniaturized, as the fill tubes must be of sufficient length to reach the cavity  150  and channel  140 . The tubes when folded over, present an obstacle to miniaturization due to this required length. Thus, in order to fill the cavity  150 , the recessed portion  70 ′ must be of sufficient size to hold the oil fill tubes once they have been folded over and to enable the securing of the diaphragm resulting in a larger oil volume. Additionally, the recessed portions  70 ,  70 ′ are preferably adapted to be coupled to other tubes, ports or pipes to provide the pressures to be measured thereto (i.e., they also serve as ports). For example, an interior surface of the housing  80  in the recessed portion  70 ′, above the diaphragm  60 ′ can be threaded so as to accept a threaded port, for example. 
     Referring to FIGS. 1 and 2, it has been found by offsetting the position of the sensor  20  and header  40  from the center of the recessed portion  70 , in combination with a unique configuration for the tubes for filling the cavities  150  and  150 ′ and channel  140 , these limitations  10  can be overcome. 
     Referring now to FIG. 3, therein is illustrated a cross section  3 — 3  of the device  10 . Basically, oil fill tubes  120  are provided such that they do not extend into either recessed portion  70  or  70 ′ in order to be accessed, but rather are fed out one of the sides of the main housing  80  between the recessed portions  70 ,  70 ′ such that they can be accessed to fill the cavities  150 ,  150 ′ and channel  140  with suitable oil. As one can see, each of the holes to which the tubes are connected have the same minimum length and diameter. Thus, if the plane of the isolation diaphragm is vertical, there will be no pressure differences due to gravity because the tubes and the holes are the same dimension. Moreover, when the plane of the isolation diaphragm is horizontal, the minimal length of the holes and the fact that they are equal in length insures that there will be a minimum pressure column of oil on the sensor and that when the plane of the isolation diaphragm is rotated through 180° while still being horizontal, there will be no change in the net displacement of the active region of the sensor. Additionally, as illustrated in FIG. 1, wires  90  can further be fed through the same side of the main housing  80  as the oil fill tubes  120 . Again, the wires  90  are coupled to the sensor  20  using header  40  and interconnect  30 . 
     The housing is preferably provided with a port  110  and adapter  100  through which the oil fill tubes  120  and wires  90  are fed. The main housing  80 , port  110  and adapter  100  are formed of stainless steel in the preferred embodiment, although any suitable material for the intended application could of course be utilized. The port  110  is preferably welded, brazed or otherwise coupled to the main housing  80  at suitable locations, and the adapter  100  is preferably welded, brazed or otherwise suitably coupled to the port  110  using conventional means. Further connections can be made by threading an interior or exterior of the adapter  100  and then coupling it to a threaded piece of conduit for example, through which the wires  90  may be run. 
     Using this configuration, the oil fill tubes  120  can be used to adequately fill the cavities  150 ,  150 ′ and channel  140  with suitable oil without adversely affecting the final dimensions of the device  10 . The recessed portions  70  and/or  70 ′ no longer need to be of the necessary dimensions to allow the oil fill tubes  120  to be folded within the recess. 
     Referring now also to FIGS. 4A and 4B, therein is illustrated an enlarged perspective view of the sensor  20 , interconnect  30  and header  40 . As set forth, in combination with the novel oil fill tube configuration, by offsetting the header  40  and sensor  20  from the center of the diaphragm  60  it is possible to substantially miniaturize the device  10 . For example, using the configuration according to the instant invention a device  10  can be manufactured which is approximately 33%-50% smaller than using the approach of accessing the oil fill tubes through the recessed area  70 ′. 
     FIG. 5A illustrates a plan view of a sensor  20  particularly well suited for the preferred embodiment of the present invention. Such a sensor structure is described in commonly assigned U.S. Pat. No. 5,891,751, entitled “Hermetically Sealed Transducers and Methods for Producing the Same”, the entire disclosure of which is incorporated by reference herein. A piezoresistive bridge structure  160  is shown which can be used in accordance with the teaching of the present invention. The pressure sensitive structure  160  is of the type having serpentine or tortuous piezoresistors  170 ,  180 ,  190 ,  200  composed of highly doped (P+) silicon. Each (piezoresistor  170   180 ,  190 ,  200  is essentially a variable resistor in one of four legs of a Wheatstone bridge circuit with each of the respective resistances varying in response to an applied force or pressure to the transducer  20 . The portion of the bridge structure  160  defined within the dotted lines  210  is generally referred to as the “active area”  220  of the transducer since this area  220  overlays a thinner region that deflects upon the application of a pressure to the sensor (by the oil for example). The areas of the transducer  20  that are external to the active area  220  are termed the “non-active” areas. 
     For a closed bridge the four circuit nodes of the Wheatstone bridge consist of electrical contacts  230 ,  240 ,  250 ,  260 , and are located in the non-active areas of the transducer. Interconnecting the contacts  230 ,  240 ,  250 ,  260  with the piezoresistors  170 ,  180 ,  190 ,  200  are electrical interconnections  270 ,  280 ,  290 ,  300 , which are also formed of P+ silicon. It is noted that the contacts  230 ,  240 ,  250 ,  260  being doped P+ are conductive, as are the interconnections  270 ,  280 ,  290   300 , to allow ohmic contact between the piezoresistive array and the respective contacts. It should be understood, while the terms “electrical contacts” and “interconnections” are used for convenience, it is understood that these terms can be considered together to essentially consist of integral electrical contacts that electrically couple the piezoresistor elements with the interconnect  30 . The interconnections  270 ,  280 ,  290 ,  300  are wider than the piezoresistors  170 ,  180 ,  190 ,  200  to provide a low resistance path to the contacts  230 ,  240 ,  250 ,  260 , while the long, tortuous lengths and narrow widths of the piezoresistors are designed to prove a desired resistance for those elements. For an open bridge, as shown in FIG. 5B, six contact areas can be employed by adding two additional contact areas  261  and  262  and by adding interconnection  271 ,  281 ,  291 , and  301 . External leads (not shown) can be readily attached to each contact  230 - 260  to supply a bias voltage to two opposite nodes of the bridge (such as contacts  240 ,  260 ) and to the externally measure the voltage between the two other nodes (such as contacts  230  and  250 ). In the preferred form these leads take the form of flexible circuit  30  and more particularly a Kapton flex-circuit. One can readily determine the applied pressure from the measured voltage. The structure  160  is deposited or otherwise formed using a conventional technique onto a wafer to form a dialectically isolated silicon sensor  310 , such as that taught in commonly assigned U.S. Pat. No. 5,286,671 entitled “Fusion Bonding Technique for Use in Fabricating Semiconductor Devices.” The entire disclosure of U.S. Pat. No. 5,286,671 is also incorporated by reference herein. 
     Referring to FIG. 5C, therein is illustrated a cross-section of the preferred form of the sensor  20 . The silicon sensor  310  is bonded to glass portions  320  and  330 . The sensor itself is composed of three separate segments, a top piece of Pyrex glass having an aperture to allow pressure to act on one face of the intermediate piece of silicon, as well as additional apertures to access the terminal areas of the fingers on the silicon, the silicon itself which was previously described and a second piece of Pyrex glass which also contains an aperture which allows pressure to access the other face of the silicon. In the central section of each Pyrex piece is a central shallow depression of about the same size as the active portion of the silicon sensor. When pressure is applied to the silicon through either of the central apertures in the glass, the active portion of the silicon will deflect, and will continue to deflect until it makes contact with the Pyrex above the depression in the Pyrex, thus preventing further deflection of the active portion of the silicon and thereby acting as a “stop.” Since each Pyrex piece is similarly constructed there will be a “stop” for pressure applied in either direction with respect to the silicon surface. 
     Referring now to FIGS. 6A and 6B, therein is illustrated a header  40  particularly well suited for the preferred embodiment of the present invention. As set forth, the header  40  serves to electrically couple the interconnect flexible circuit  30 , and hence the sensor  20 , to the wires  90 . The header  40  includes pins  360  each respectively electrically coupled to interconnect flex-circuit  30  and wires  90  (see for example FIG. 2 also). The header  40  further preferably includes pins  370  which may be electrically coupled to the sensor  20 , in order to permit the electrical equivalent of an open bridge to be measured using the sensor  20  by coupling particular ones of wires  90  thereto. Each of the pins  360  and  370  are preferably surrounded by a glass seal  380  which serve to respectively electrically isolate the pins  360 ,  370  from portion  390  of the header  40  for example. Any suitable means of respectively electrically coupling the pins  360  to the associated wires  90  can be used, such as soldering for example (see FIG.  2 ). 
     Although the invention has been described and pictured 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 claim, whatever features of patentable novelty exist in the invention disclosed.