Abstract:
A differential pressure sensor includes a housing having first and second housing members. The first and second housing members define opposing sections of a pressure chamber. The differential pressure sensor also includes a multi-layer laminate forming a pressure sensing diaphragm. The diaphragm is positioned between the first and the second housing members such that it generally bisects the pressure chamber into first and second chamber sections such that a pressure differential between the chamber sections causes the diaphragm to deflect toward the chamber section having the lower pressure. The first and second housing members are positioned to provide an overpressure stop that limits deflection of the diaphragm beyond a predetermined deflection limit.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application is a non-provisional of, and claims the benefit of, co-pending, U.S. Provisional Application No. 60/500,252, entitled “SENSING DIAPHRAGM FOR A DIFFERENTIAL PRESSURE SENSOR WITH OVER-PRESSURE PROTECTION AND METHOD OF CONSTRUCTING SAME,” filed on Sep. 5, 2003, by Philip R. Couch, et al., the entire disclosure of which is herein incorporated by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Embodiments of the present invention relate generally to pressure sensing systems. More specifically, embodiments of the present invention provide differential pressure sensors having overpressure protection.  
         [0003]     Differential pressure sensors have many uses. On example is measuring fluid flow in a pipe. A pressure tap may be placed on either side of a flow restriction, and the pressure difference between the taps provides an indication of the rate fluid is flowing through the pipe.  
         [0004]     Pressure sensors for some applications (e.g., high pressure applications) often are large and heavy. This may be the case because the diaphragm whose deflection is used to measure pressure differentials must be able to withstand large differences without rupturing. Such devices, however, may not be suitable for certain applications that would benefit from lighter weight, lower cost, portable differential pressure sensors. Thus, improved differential pressure sensors are needed.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     Embodiments of the invention thus provide a differential pressure sensor. The differential pressure sensor includes a housing having first and second housing members. The first and second housing members define opposing sections of a pressure chamber. The differential pressure sensor also includes a multi-layer laminate forming a pressure sensing diaphragm. The diaphragm is positioned between the first and the second housing members such that it generally bisects the pressure chamber into first and second chamber sections such that a pressure differential between the chamber sections causes the diaphragm to deflect toward the chamber section having the lower pressure. The first and second housing members are positioned to provide an overpressure stop that limits deflection of the diaphragm beyond a predetermined deflection limit.  
         [0006]     In some embodiments of the differential pressure sensor, the diaphragm includes a deformable disk having first and second sides and a measurement device bonded to the first side of the disk configured to sense shape changes in the disk. The measurement device has a thickness and covers only a portion of the first side of the disk. The diaphragm also includes a first spacer having a thickness generally equal to the thickness of the measurement device and generally covering the portion of the first side of the disk not covered by the measurement device. The diaphragm also includes a second spacer bonded to the second side of the disk. The second spacer has a thickness generally equal to the thickness of the first spacer. The diaphragm also includes first and second cover plates bonded to opposing spacers. The measurement device may be a strain gauge. The strain gauge may include at least two differential resistor pairs, a first differential resistor pair positioned near a middle portion of the disk and a second differential resistor pair positioned near a periphery portion of the disk. The at least two differential resistor pairs may be wired together as a Wheatstone bridge network. The first and second spacers may be polyimide. The deformable disk may be stainless steel. The deformable disk may have a thickness of about 0.25 millimeters. The first and second cover plates may be stainless steel having a thickness of approximately 0.025 millimeters. The differential pressure sensor also may include a temperature sensing device configured to measure the temperature of the diaphragm.  
         [0007]     In further embodiments a pressure sensing diaphragm is adapted for insertion between opposing sections of a housing. The opposing sections define first and second pressure chambers and the diaphragm is positioned to deflect in response to pressure differences between the first and second champers. The housing sections further define deflection stops that prevent the diaphragm from deflecting beyond a pre-determined limit. The diaphragm includes a deformable disk having first and second sides and a measurement device bonded to the first side of the disk configured to sense shape changes in the disk. The measurement device has a thickness and covers a first portion of the first side of the disk. The diaphragm includes a first spacer having a thickness generally equal to the thickness of the measurement device and generally covering a second portion of the first side of the disk not covered by the measurement device. The diaphragm further includes a second spacer bonded to the second side of the disk. The second spacer has a thickness generally equal to the thickness of the first spacer. The diaphragm also includes first and second cover plates bonded to opposing spacers.  
         [0008]     In some embodiments the pressure sensing diaphragm may include a temperature sensor configured to measure the temperature of the diaphragm. The sensor may be a strain gauge. The strain gauge may include a plurality of sensors wired as a Wheatstone bridge network.  
         [0009]     In still other embodiments a method of assembling a differential pressure sensor includes bonding a sensor to a first side of a deformable disk, laminating spacers to a second side of the disk and a portion of the first side of the disk, thereby forming layers having substantially equal thickness on either side of the disk, and bonding first and second cover plates to either side of the disk, thereby forming a pressure sensing diaphragm. The cover plates comprise anti-corrosive material with respect to a fluid medium for which the pressure sensor is used. The method also includes installing the pressure sensing diaphragm between first and second housing members. The housing members, together with the diaphragm, form first and second pressure chambers on opposing sides of the pressure sensing diaphragm. At least one of the first and second housing members may be positioned to provide an overpressure stop that prevents the pressure sensing diaphragm from deflecting beyond a pre-determined distance. Bonding a sensor to a first side of a deformable disk may include bonding a multi-sensor strain gauge to the disk. The sensors may be wired as a Wheatstone bridge network. The method may include bonding a temperature sensor to the first side of the disk. Installing the pressure sensing diaphragm between first and second housing members may include clamping the members together with a force in excess of about 1,000 kg. The method may include cooling the housing members to below a predetermined temperature prior to the installing the diaphragm to thereby introduce a tension force in the diaphragm following the clamping step. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings wherein like reference numerals are used throughout the several drawings to refer to similar components. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.  
         [0011]      FIG. 1A  illustrates an exploded view of a pressure sensing diaphragm according to embodiments of the invention.  
         [0012]      FIG. 1B  illustrates the assembled pressure sensing diaphragm of  FIG. 1A .  
         [0013]      FIG. 1C  illustrates an embodiment of a strain gauge that may be used with the pressure sensing diaphragm of  FIG. 1A .  
         [0014]      FIG. 2  illustrates an exploded view of a pressure sensor using the pressure sensing diaphragm of  FIG. 1A .  
         [0015]      FIG. 3  illustrates an embodiment of a method of assembling a pressure sensor, such as the pressure sensor of  FIG. 2 .  
         [0016]      FIG. 4  illustrates a pressure sensing device using the pressure sensor of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     Embodiments of the invention relate to a pressure sensor having a multi-layer pressure sensing diaphragm. In some embodiments, the pressure sensing diaphragm converts a differential pressure between two chambers to an imbalance of a strain-gauge-based Wheatstone bridge. Some embodiments correct for temperature variations. Some embodiments prevent corrosive material from contacting the sensor elements by laminating the sensor elements between thin stainless steel films.  
         [0018]     In some embodiments, temperature effects are compensated using a balanced laminated buildup. Expansion forces that may give rise to distortion of the diaphragm are cancelled on each side of the plate.  
         [0019]     Some embodiments employ an unconventional film strain gauge. Conventional strain gauges are too small to be laminated into a pressure-sensing diaphragm with the electrical connections exposed for connection outside the laminations. Connecting to a smaller gauge within the laminations is not practical. The minimum diaphragm size is constrained by the measurement to be made. In one example, a diaphragm less than about 25 mm in diameter deflects too little to accurately and reliably detect the deflection at certain required pressures.  
         [0020]     Some embodiments of the invention may be used in many environments and under a variety of conditions including operation at relatively high static pressures, which is often the case when measuring fluid flow. In a high static pressure application, a partial restriction may be inserted into a fluid pipeline, and a pressure tapping made on either side of the tapping. The difference in pressure between the tappings is related to the flow rate of the fluid. The fluid is often at a high pressure in the pipeline. The pressure sensor is required to operate with this high absolute pressure on each inlet port and accurately measure a small differential pressure with minimal error introduced by the high absolute pressure or by other causes. Of course, embodiments of the invention find utility in many other applications with similar or other constraints. Further, embodiments of the invention also may be used in less demanding applications (e.g., low static pressure fluid flow) that do not require the full range of advantages provided by these embodiments. The invention is not limited to the specific details described and illustrated above.  
         [0021]     Attention is directed to  FIG. 1A , which illustrates an exploded view of an exemplary pressure sensing diaphragm  100  according to embodiments of the invention. The pressure sensing diaphragm  100  consists of a multi-layer laminate having a central disk  102 , a strain gauge  104 , a first spacer  106 , a second spacer  108 , a first cover plate  110 , and a second cover plate  112 . The central disk  102  includes a retention tab  114  configured to engage a cable  116  that caries sensing signals to a processor (not shown). The assembled pressure sensing diaphragm  100  is shown in  FIG. 1B .  
         [0022]     The central disk  102  may comprise any suitable deformable material, including metal, plastic, and the like. In a specific embodiment, the central disk  102  comprises stainless steel having a thickness of approximately 0.25 millimeters. The central disk  102  defines a generally circular active region  118  and an inactive region  120  generally concentric to the active region.  
         [0023]     The strain gauge  104  is also pictured in  FIG. 1C . In may be formed on, for example, polyimide or other suitable material. The strain gauge  102  includes two differential resistor pairs  122 ,  124 . The strain gauge  104  is bonded to a first side of the central disk  102  such that a first pair of differential resistors  122  is located near the center of the active region  118  and the other pair of differential resistors  122  is located near an edge of the active region  118 . The strain gauge  104  is bonded to the central disk  102  such that electrical contacts  126  are positioned in the inactive region  120 . The differential resistor pairs  122 ,  124  are wired as a Wheatstone bridge network. A temperature sensor  128 , configured to measure the temperature of the central disk  102 , also is positioned in the inactive region  120 .  
         [0024]     In some embodiments, the strain gauge  104  covers the entire active region  118  of the central disk  102 . In such embodiments, the first spacer  106  may not be necessary. In embodiments in which the strain gauge  104  does not cover the entire active region  118 , then the first spacer may be used. The first spacer  106  may be polyimide or other suitable material, and has a thickness substantially equal to the thickness of the strain gauge  102 . Thus, the strain gauge  102  and first spacer  106  form a layer of uniform thickness covering the active region  118  of the central disk  102 . The second spacer  108  is bonded to the opposite side of the central disk  102 . Its size and thickness are substantially similar to the strain gauge/first spacer layer on the opposite side. The second spacer  104  also may comprise polyimide or other suitable material.  
         [0025]     The first and second cover plates  110 ,  112  are bonded to opposing sides of the pressure sensing diaphragm  100 , forming the outer layers of the laminate. The cover plates  110 ,  112  may be stainless steel or other suitable material. The cover plates  110 ,  112  are sized to cover the active region  118  and, in some embodiments, have a thickness on the order of one-tenth the thickness of the central disk  102 .  
         [0026]     The cable  116  caries electrical signals from the differential resistor pairs  122 ,  124  and the temperature sensor  128  to a processor or other signal processor. The signals from the resistor pairs  122 ,  124  relate to the deflection of the central disk  102 . The signals from the temperature sensor  128  may be used in the pressure calculations for greater accuracy.  
         [0027]     Attention is directed to  FIG. 2 , which illustrates an exploded view of a pressure sensor  200  using the pressure sensing diaphragm  100  of  FIG. 1 . The pressure sensing diaphragm  100  is positioned between first and second housing members  202 ,  204 . The housing members  202 ,  204 , together with the cover plates  110 ,  112 , define opposing pressure chambers  206 ,  208 . In addition to providing a pressure chamber, the housing members  202 ,  204  serve as overpressure stops that prevent the diaphragm  100  from deflecting beyond a predetermined limit. O-rings  210 ,  212  provide a pressure-tight seal.  
         [0028]     The housing members  202 ,  204 , may be stainless steel or other appropriate material. The O-rings  210 ,  212  may be neoprene or other appropriate material and may be positioned in machined grooves in the housing members  202 ,  204 .  
         [0029]     Pressure ports  214 ,  216  extend into chambers  206 ,  208  on either side of the diaphragm  100 . The ports may be fitted with filters to prevent dirt or debris from entering the chambers  206 ,  208  and fouling the device. The chambers  206 ,  208 , in this exemplary embodiment, are dome-shaped, generally conforming to the shape of a sphere having a large radius with respect to the size of the device. The housing members  202 ,  204  prevent the diaphragm  100  from deforming beyond its elastic limit.  
         [0030]     Having described a pressure sensor according to embodiments of the invention, attention is directed to  FIG. 3 , which illustrates a method  300  of assembling a pressure sensor, such as the pressure sensor  200  of  FIG. 2 , according to embodiments of the invention. Those skilled in the art will appreciate that the method  300  is merely exemplary of a number of possible methods according to embodiments of the invention. Other methods may include more, fewer, or different steps than those illustrated and described here.  
         [0031]     The method  300  begins at block  302  at which point a strain gauge is bonded to a first side of a deformable disk. The sensor may comprise a strain gauge having a plurality of sensors wired as a Wheatstone bridge network as previously described. At block  304 , spacers are laminated to both sides of the disk such that the spacer and sensor on one side of the disk have roughly the same thickness as the spacer on the other side of the disk. Cover plates having roughly the same thickness are bonded to either side of the disk at block  306 , thereby forming a multi-layer laminate.  
         [0032]     At block  308 , housing members are cooled to below the temperature of the disk, and the disk is clamped between the housing members at block  310 . The clamping force exceeds a predetermined degree of force that, in one embodiment is about 1,000 kg. This prevents the edges of the disk from slipping from between the housing members when the device is exposed to pressure in operation. As a result of cooling the housing members, the disk is exposed to a tension force as the housing members expand upon warming. This may prevent the disk from buckling with temperature gradients. The pressure sensor thus assembled may be employed in a number of useful applications, as is apparent to those skilled in the art, some of which applications are described herein.  
         [0033]      FIG. 4  illustrates a portable pressure monitor  400  that employs the pressure sensor  200  according to embodiments of the invention. The pressure monitor  400  may be positioned such that pressure taps transmit pressure from either side of a restriction in a pipe through which a material is flowing. The differential pressure may be used to determine the flow rate of the material in the pipe. The pressure monitor  400  includes a processor  402 , a solar panel  404 , a battery  406 , and a transmitter  408 . The processor  402  receives signals from the strain gauge  104  and temperature sensor  128  and determines the differential pressure between the chambers of the pressure sensor  200 . The result may be transmitted to a central monitoring location. The solar power and battery allow the device to be deployed in a number of useful applications.  
         [0034]     Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. For example, those skilled in the art know how to manufacture and assemble electrical devices and components. Additionally, those skilled in the art will realize that the present invention is not limited to measuring fluid flow. Embodiments of the present invention may be configured to measure pressure differentials in a number of applications. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims.