The present application is based on and claims the benefit of U.S. provisional patent application Serial No. 60/181,866, filed Feb. 11, 2000, the content of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to pressure sensors. More specifically, the invention relates to pressure sensors which measure deflection using optical techniques. 
     Pressure sensors are used to measure pressures of various media and have a wide range of uses in industrial, commercial and consumer applications. For example, in industrial process control, a pressure sensor can be used to measure the pressure of a process fluid. The pressure measurement can then be used as an input to a formula which provides an indication of another process variable such as a fluid level or a flow rate. 
     There are a number of different techniques which are used to measure pressures. One basic technique involves the use of a deflectable diaphragm. In such a pressure sensor, a pressure is applied to the diaphragm, either directly or through an isolating medium, and the deflection of the diaphragm is measured. Various deflection measurement techniques can be used. For example, a strain gauge mounted to the diaphragm can provide an indication of deflection. In another technique, the deflection causes a change in capacitance which can be measured and correlated to the applied pressure. Preferably, pressure sensors are able to have long lives, provide high accuracy and are capable of withstanding environmental extremes, exposure to caustic fluids, vibrations, impacts and other potentially damaging inputs. 
     Typically, the techniques which are used to measure deflection require electrical contact to electrical components which are carried on the pressure sensor. Such contact can be difficult to achieve and can be a source of failure. Additionally, the additional processing as well as the electrical components themselves can be a source of errors in pressure measurements. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a pressure sensor which does not require electrical contact to the diaphragm of the sensor or the surrounding material. 
     The pressure sensor is configured to sense an applied pressure. A diaphragm support structure is coupled to a diaphragm which deflects in response to applied pressure. A moveable member is coupled to the diaphragm and moves in response to deflection of the diaphragm. An optical interference element moves with the moveable member and is configured to interfere with incident light. The interference is a function of position of the moveable member. In one aspect, the moveable member is coupled between opposed diaphragms. In this configuration, a pressure sensor is less susceptible to being damaged when exposed to high pressures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side cross-sectional view of a pressure sensor in accordance with one embodiment of the present invention. 
     FIG. 2 is a top plan view of one layer in the pressure sensor of FIG.  1 . 
     FIG. 3 is a top plan view of another layer in the pressure sensor of FIG.  1 . 
     FIG. 4 is a simplified electrical schematic diagram of a pressure transmitter which uses the pressure sensor of FIG.  1 . 
     FIG. 5 is a cross-sectional view of a pressure transmitter including the pressure sensor of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A pressure sensor using optical sensing is shown at  10  in FIG.  1 . Generally, the pressure sensor  10  includes a diaphragm support structure  12  having a bore  14 . Isolator diaphragms  16 A and  16 B are mounted to opposite sides of the diaphragm support structure  12 , while in the embodiment illustrated, portions  18 A and  18 B are secured together to form a rigid coupling member  20  extending within the bore  14 . In addition, the isolator diaphragms  16 A and  16 B are secured to the diaphragm support structure  12  on outer peripheries or rims  22 A and  22 B to form corresponding annular cavities  24 A and  24 B that open to and are about an axis  15  of the bore  14 . The annular cavities  24 A and  24 B provide space between opposed surfaces of each of the isolator diaphragms  16 A and  16 B, and the diaphragm support structure  12 . This, in turn, also allows the isolator diaphragms  16 A and  16 B to deflect relative to the diaphragm support structure  12  in response to a difference in pressure P 1  and P 2 , while providing inherent overtravel protection. The rigid coupling member  20  formed by portions  18 A and  18 B couples the isolator diaphragms  16 A and  16 B together and replaces an incompressible fluid commonly used in differential pressure sensors. 
     The diaphragm support structure  12  and the isolator diaphragms  16 A and  16 B define a cavity  26  comprising bore  14  and annular cavities  24 A and  24 B that can be completely isolated and sealed from the external environment. The inside of the cavity  26  can be evacuated or filled with an inert gas. However, the cavity  26  need not be evacuated and can be left at gauge pressure. Since the cavity  26  is substantially isolated, changes in environmental conditions will have less of an effect on sensing elements mounted within the cavity  26  to measure displacement of the isolator diaphragms  16 A and  16 B relative to the diaphragm support structure  12 . In addition, dust particles cannot easily enter the cavity  26 . 
     With the present invention, deflection of diaphragms  16 A,  16 B is detected using an optical technique. In the example illustrated in FIG. 1, an optical receiver  30  receives light from an optical source  32 . Source  32  and receiver  30  are positioned on opposite sides of sensor  10 . Movement of diaphragms  16 A and  16 B can cause distortion in the transmitted light. An optical member  34 , such as a defraction grading, can be coupled to diaphragms  16 A and  16 B at coupling member  20  to enhance the distortion and deflection of the light traveling between the source  32  and the receiver  30 . In one embodiment, light is reflected from sensor  10  and the source  32  and receiver  30  are positioned accordingly. 
     Preferably, at least the isolator diaphragms  16 A and  16 B are made of chemically resistant material that does not degrade in order that the isolator diaphragms  16 A and  16 B can directly receive the process fluids to be measured. For example, the isolator diaphragms  16 A and  16 B can be made from a single crystal corundum such as “sapphire” or “ruby” containing chromium. The material that is substantially transparent so that the sensor  10  can carry light from source  32 . The diaphragm support structure  12  can also be made from the same material as the isolator diaphragms  16 A and  16 B, and can be directly fusion bonded to the isolator diaphragms  16 A and  16 B on the rims  22 A and  22 B at a temperature lower than the melting point of the material used to form these components. When crystalline materials, such as sapphire are used, the resulting structure of the pressure sensor  10  behaves elastically without hysteresis. Furthermore, since the diaphragm support structure  12  and the isolator diaphragms  16 A and  16 B are formed of the same material, stress induced by different rates of thermal expansion is minimized. Other suitable materials include spinels, zirconia and silicon. When a material such as silicon is used, an oxide or other insulator may be required. 
     In one embodiment, direct bonding of the rims  22 A and  22 B to the corresponding isolator diaphragms  16 A and  16 B is used which typically requires that each of the bonding surfaces be atomically smooth. One alternative method of attachment includes depositing a glass or suitable metallic solder (preferably having a thermal expansion coefficient similar to the diaphragm support structure  12  and the isolator diaphragms  16 A and  16 B) on the rims  22 A and  22 B and/or the opposing surfaces of the isolator diaphragms  16 A and  16 B. By applying heat and pressure, such as in an evacuated press, a seal is formed between the rims  22 A and  22 B and the corresponding isolator diaphragms  16 A and  16 B. Since the seal forms an interface layer between the rims  22 A and  22 B and the isolator diaphragms  16 A and  16 B, atomically smooth surfaces are not required. 
     In the embodiment illustrated, the diaphragm support structure  12  includes substantially identical base members  40 A and  40 B secured together on planar surfaces  42 A and  42 B, respectively. Each base member  40 A and  40 B includes an aperture  44 A and  44 B respectively, aligned with each other to form the bore  14 . Annular cavities  24 A and  24 B are formed by providing recessed surfaces  46 A and  46 B on the base members  40 A and  40 B below the outer peripheries  22 A and  22 B and about the apertures  44 A and  44 B. 
     FIGS. 2 and 3 show top plan views of base member  40 A and isolator diaphragm  16 A, respectively. Base members  40 A and  40 B are identical as are isolator diaphragm  16 A and isolator diaphragm  16 B. In this manner, only two unique components (i.e., the isolator diaphragms  16 A and  16 B and the base members  40 A and  40 B) need be manufactured and assembled to form the pressure sensor  10 . Members  40 A and  40 B can also be integral components forming a single member in which case no surface bonds  42 A and  42 B are required. As appreciated by those skilled in the art, if desired, the base members  40 A and  40 B can be simple blocks of material, while the isolator diaphragms  16 A and  16 B have corresponding rims to form the annular cavities  24 A and  24 B. 
     If the pressure sensor  10  is formed from sapphire or other similar crystalline materials, a suitable method of fabrication includes first micro-machining the isolator diaphragms  16 A and  16 B, and the base members  40 A and  40 B (or the diaphragm support structure  12  if the base members  40 A and  40 B are integrally joined together). Suitable micro-machining techniques include wet or dry chemical etching, and ion or ultrasonic milling techniques. Grating  34  can be fabricated directly onto the member  20 , adhered or applied using any appropriate technique. 
     The pressure sensor  10  can then be assembled by first securing the isolator diaphragm  16 A to the base member  40 A, and then securing the isolator diaphragm  16 B to the base member  40 B. The base member  40 A and  40 B can then be secured along surfaces  42 A and  42 B, which would also form the coupling member  20  by securing the portion  18 A to the portion  18 B. Using separate base members  40 A and  40 B, which are later bonded together, is particularly advantageous because each of the components, the isolator diaphragms  16 A and  16 B and the base members  40 A and  40 B, need only be machined on one side thereof. 
     It should be understood that although isolator diaphragms  16 A and  16 B are preferably substantially identical for the reasons discussed above, if desired, the isolator diaphragms  16 A and  16 B can be machined differently. For example, the portions  18 A and  18 B can be of different length such that one of the portions  18 A and  18 B extends further within bore  14  or out of the bore  14 . 
     Source  32  is illustrated as an optical fiber, however, any type of optical source can be used including a light emitting diode, a laser diode, etc. Source  32  can also be placed close to member  20 . For example, an optical channel can extend between surfaces  42 A and  42 B to a location proximate member  20 . Similarly, receiver  30  can be placed near sensor  10  or light can be conducted for example through an optical fiber, to the receiver  30 . Further, appropriate optics such as polarizers or optics to provide coherent light can be placed between the source and pressure sensor  10 . The light entering pressure sensor  10  can be coherent or incoherent. 
     Movement of coupling member  20  is detected based upon variations in the light received by receiver  30 . The sensed variations in the light received can be used to determine the differential pressure applied to pressure sensor  10 . In general, the light will enter the side of the pressure sensor  10 , i.e., the light will have a vector component which is perpendicular to the deflection of member  20 . Further, the material used to fabricate sensor  10  should be at least partially transparent to the light provided by source  32 . Member  20  constitutes a moveable member, however, other configurations and orientations can be used with the invention. In one general aspect, the member  20  can be any structure which moves in response to applied pressure. In general, the member  20  simply needs to be either formed directly from the diaphragm or otherwise be coupled to the diaphragm in a manner such that the member  20  moves in response to diaphragm deflection. The member  20  can be formed integrally with the diaphragm or can be formed from a separate component coupled to the diaphragm by any appropriate technique. 
     A diffraction grating or other optical interference element  34  is carried on member  20 . Movement of the optical interference element  34  causes the detectable light variations that can be used to determine pressure. In one embodiment, member  20  or element  34  can reflect the light. In such an embodiment, sensor  30  need not be positioned opposite source  32 . For example, the light can be reflected back toward source  32  which, if source  32  is an optical fiber, can conduct light to a light sensor  30  located remotely. The movement of member can be detected based upon any phenomena which causes light variations. These can be, for example, interference patterns, intensity variations, phase shifts, polarization variations, etc. Further, interference element  34  can comprise a change in the material, such as a void, within member  20  which alters the speed of the light through the member  20 . 
     Multiple optical sensors can be used which sense more than one pressure. In one technique, the light is directed from a side of sensor  10  toward a diaphragm such as diaphragm surface that carries a reflective element. For example, diaphragm  16 A and  16 B in FIG. 1 can be a reflective surface. In such an embodiment, movement of the surface will cause displacement of the optical beam. The diaphragm itself comprises moveable member  20  and the interference element is either the diaphragm itself or an element carried on the diaphragm. The deflective element can form an interference pattern in the reflected light which will change in accordance with deflection. 
     FIG. 4 is a simplified schematic diagram of a process transmitter  60  employing pressure sensor  10 . Sensor  10  is shown in simplified form and receives two pressures, P 1  and P 2 . As discussed above, optical interference element  34  moves in response to a difference between pressures P 1  and P 2  Transmitter  60  is shown coupled to a two-wire process control loop  62 . Loop  62  shown for example purposes only and the sensor  10  or transmitter  60  can be used in other environments. Loop  62  can comprise, for example, a process control loop which carries both power and information related to pressures P 1  and P 2  measured by sensor  10 . Example loops includes loops in accordance with industry standards such as the HART® standard and FOUNDATION™ Fieldbus standard. Loop  62  couples to a remote location such as a control room  64 . Control room  64  is shown in electrical schematic form as a resistance  64 A and a voltage source  64 B. In one embodiment, loop  62  carries a current I which is controlled by I/O circuitry  66  in transmitter  60  to be related to pressures P 1  and P 2 . In some configurations, transmitter  60  is powered using power generated by I/O circuitry  66  from power which is completely received from loop  62 . This power is used to completely power transmitter  60 . A preprocessing circuit  68  receives an output from receiver  64  and responsively provides an input to microprocessor  70  which is related to deflection of a diaphragm in sensor  10  and the resultant movement of optical interference element  34 . Microprocessor  70  operates at a rate determined by a clock  72  and in accordance with instructions stored in a memory  74 . Preprocessing circuitry  68  can be any type of circuitry which is capable of detecting variations in the output from the receiver  30  due to the variations in the light received by receiver  30  in response to movement of optical interference element  34 . Some sensing techniques may use the output from source  32  as a reference. Additionally, source  32  can be controlled or modulated by microprocessor  70 . Microprocessor  70  can also perform the computations required to convert the received signal into a signal representative of diaphragm deflection, applied pressure, or more advanced process variables such as process fluid flow rate or process fluid level. The diagram shown for transmitter  60  in FIG. 4 is provided for explanatory purposes only and other embodiments can be implemented by those skilled in the art. In actuality, the various components may not be discrete components and may be implemented in hardware, software, or their combination. 
     FIG. 5 is a cross-sectional view of a transmitter  100  which includes a pressure sensor  10  in accordance with the present invention. Transmitter  10  is shown in simplified form to explain one possible configuration for coupling a pressure sensor  10  to process fluid. The pressure sensor of the present invention can be used in direct contact with process fluid or when isolated from process fluid using appropriate techniques. The invention can be used with any appropriate type of pressure sensor structure. Example structures are shown in U.S. patent application Ser. No. 09/780,033, entitled “OIL-LESS DIFFERENTIAL PRESSURE SENSOR”, filed Feb. 9, 2001, which is incorporated herein by reference. Transmitter  100  includes sensor  10  in accordance with an embodiment of the present invention. Additionally, transmitter  100  includes housing  82  which is a ruggedized enclosure suitable for protecting the contents of housing  82  from harsh environmental extremes. Process pressures P L  and P H  are coupled to diaphragms  84 ,  86 , respectively and such pressures are conveyed to sensor  10  through tubes  90  via fill fluid  88 . As illustrated, preprocessing circuitry  68  is coupled to sensor  10  and provides a signal to circuitry  80  that is indicative of differential pressure. Circuitry  80  can include any suitable circuitry such as microprocessor  70  (shown in FIG. 4) and I/O circuitry  66  (also shown in FIG.  4 ). Connectors  62  extend to an axis point through which transmitter  100  is coupled to a process control loop  62  as described above. Process control loop  62  can provide operating energy to transmitter  100 . Additionally, process control loop  62  can operate in accordance with suitable process control protocols such as the HART® protocol and the FOUNDATION™ Fieldbus protocols, for example. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. Although the terms “optical” and “light” have been used herein, these terms are intended to include appropriate wavelength including non-visible wavelengths. Further, the sensor can be used to measure differential, gauge or absolute pressure.
A pressure sensor configured to sense an applied pressure, comprising a diaphragm support structure, a diaphragm coupled to the diaphragm support structure and configured to deflect in response to applied pressure, a moveable member coupled to the diaphragm and configured to move in response to deflection of the diaphragm, and an optical interference element coupled to the moveable member and configured to interfere with incident light, wherein the interference is a function of position of the moveable member.