Abstract:
A pressure sensor assembly for sensing a pressure of a process fluid includes a sensor body having a cavity formed therein and first and second openings to the cavity configured to apply first and second pressures. A diaphragm in the cavity separates the first opening from the second opening and is configured to deflect in response to a differential pressure between the first pressure and the second pressure. A capacitance based deformation sensor is provided and configured to sense deformation of the sensor body in response to a line pressure applied to the sensor body.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to pressure sensors of the type used to measure the pressure of a process fluid. More specifically, the present invention relates to a pressure sensor configured to measure both a differential pressure as well as a line pressure in a process fluid. 
     Transmitters are used in process monitoring and control systems to measure various process variables of industrial processes. One type of transmitter measures differential pressure of process fluid in the process. This differential pressure measurement can then be used to calculate the flow rate of the process fluid. Various techniques have been used in the pressure sensors used in such transmitters. One well known technique is to use a deflectable diaphragm. A capacitance is measured with respect to the diaphragm, with the diaphragm forming one of the capacitive plates of the capacitor. As the diaphragm is deflected due to applied pressure, the measured capacitance changes. In such a configuration, there are a number of sources of inaccuracies in pressure measurements. 
     One technique which addresses these inaccuracies is set forth in U.S. Pat. No. 6,295,875 entitled, “PROCESS PRESSURE MEASUREMENT DEVICES WITH IMPROVED ERROR COMPENSATION” issued Oct. 2, 2001 to Frick et al., which is incorporated herein by reference in its entirety. This patent describes a differential pressure sensor that includes an additional electrode for use in reducing measurement inaccuracies. However, in some installations it is desirable to measure a line pressure of the process fluid (absolute or gauge), in addition to a differential pressure measurement. 
     SUMMARY 
     A pressure sensor assembly for sensing a pressure of a process fluid includes a sensor body having a cavity formed therein and first and second openings to the cavity configured to apply first and second pressures. A diaphragm in the cavity separates the first opening from the second opening and is configured to deflect in response to a differential pressure between the first pressure and the second pressure. A capacitance based deformation sensor is provided and configured to sense deformation of the sensor body in response to a line pressure applied to the sensor body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a process measurement system with a process transmitter constructed in accordance with the present invention. 
         FIG. 2  is schematic view of a transmitter of  FIG. 1 . 
         FIG. 3  shows a cross sectional view of a portion of the process transmitter of  FIG. 1 . 
         FIG. 4  is a simplified cross sectional view of pressure sensor for use in illustrating operation of the present invention. 
         FIG. 5  is a cross sectional view of pressure sensor including electrodes used to measure line pressure. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides an apparatus and method for determining line pressure and differential pressure for a capacitance-based pressure sensor. By computing the ratios of sums, or sums of ratios, of appropriate capacitances in a multi-capacitance based pressure sensor, the differential pressure of the process fluid can be determined. As discussed in the Background section, in some installations it may be desirable to measure a line pressure (absolute or gauge) in addition to a differential pressure measurement. One such technique to measure line pressure is shown and described in co-pending U.S. patent application Ser. No. 11/140,681 entitled “LINE PRESSURE MEASUREMENT USING DIFFERENTIAL PRESSURE SENSOR,” filed on May 27, 2005 to Donald E. Harasyn et al., and U.S. patent application Ser. No. 11/138,977 entitled “PRESSURE SENSOR USING COMPRESSIBLE SENSOR BODY”, filed on May 26, 2005 to David A. Broden et al., which is commonly assigned with the present application, and whose contents are incorporated herein by reference in their entirety. 
       FIG. 1  shows generally the environment of a process measurement system  32 .  FIG. 1  shows process piping  30  containing a fluid under pressure coupled to the process measurement system  32  for measuring a process pressure. The process measurement system  32  includes impulse piping  34  connected to the piping  30 . The impulse piping  34  is connected to a process pressure transmitter  36 . A primary element  33 , such as an orifice plate, venturi tube, flow nozzle, and so on, contacts the process fluid at a location in the process piping  30  between the pipes of the impulse piping  34 . The primary element  33  causes a pressure change in the fluid as it flows past the primary element  33 . This pressure change (differential pressure change) is related to the flow of process fluid. A differential pressure sensor can be used to measure this pressure change and measurement circuitry used to provide an output related to the flow of process fluid. 
     Transmitter  36  is a process measurement device that receives process pressures through the impulse piping  34 . The transmitter  36  senses a differential process pressure and converts it to a standardized transmission signal that is a function of the process flow. 
     A process loop  38  preferably provides both a power signal to the transmitter  36  from control room  40  and bidirectional communication, and can be constructed in accordance with a number of process communication protocols. In the illustrated example, the process loop  38  is a two-wire loop. The two-wire loop is used to transmit all power to and all communications to and from the transmitter  36  during normal operations with a 4-20 mA signal. A computer  42  or other information handling system through modem  44 , or other network interface, is used for communication with the transmitter  36 . A remote voltage power supply  46  powers the transmitter  36 . Another example of a process control loop is a wireless communication in which data is transmitted wirelessly either directly to a central location, or a to mesh network type configuration or using other techniques. 
       FIG. 2  is a simplified block diagram of one embodiment of pressure transmitter  36 . Pressure transmitter  36  includes a sensor module  52  and an electronics board  72  coupled together through a databus  66 . Sensor module electronics  60  couples to pressure sensor  56  which receives an applied differential pressure  54 . The data connection  58  couples sensor  56  to an analog to digital converter  62 . An optional temperature sensor  63  is also illustrated along with sensor module memory  64 . The electronics board  72  includes a microcomputer system  74 , electronics memory module  76 , digital to analog signal conversion  78  and digital communication block  80 . An output is provided on loop  38  related to the sensed pressure.  FIG. 2  also schematically illustrates an external capacitance based deformation sensor  59  which is located externally to the body of pressure sensor  56  and arranged to provide a capacitance value. The sensor  59  is arranged to have a capacitance value which changes in response to deformation of the body of pressure sensor  56  due to an applied pressure. As illustrated schematically in  FIG. 2 , a line pressure is applied to the body of pressure sensor  56  due to the application of pressure  54 . 
     In accordance with techniques set forth in U.S. Pat. No. 6,295,875 to Frick et al., pressure transmitter  36  senses differential pressure. However, the present invention is not limited to such a configuration. 
       FIG. 3  is a simplified cross-sectional view of one embodiment of a sensor module  52  showing pressure sensor  56 . Pressure sensor  56  couples to a process fluid through isolation diaphragms  90  which isolate the process fluid from cavities  92 . Cavities  92  couple to the pressure sensor module  56  through capillary tubes  94 . A substantially incompressible fill fluid fills cavities  92  and capillary tubes  94 . When a pressure from the process fluid is applied to diaphragms  90 , it is transferred to the pressure sensor  56 . 
     According to one embodiment, pressure sensor  56  is formed from two pressure sensor halves  114  and  116  and filled with a substantially incompressible solid material  105  such as glass or ceramic. A center diaphragm  106  is disposed within a cavity  132 , 134  formed within the sensor  56 . An outer wall of the cavity  132 ,  134  carries electrodes  144 , 146 , 148  and  150 . These electrodes are generally referred to as primary electrodes  144  and  148 , and secondary electrodes  146  and  150 . These electrodes form capacitors with respect to the moveable diaphragm  106 . The capacitors, again, are referred to as primary and secondary capacitors, respectively. 
     As illustrated in  FIG. 3 , the various electrodes in sensor  56  are coupled to analog to digital converter  62  over electrical connection  103 ,  104 ,  108  and  110 . Additionally, the deflectable diaphragm  106  couples to analog to digital converter  62  through connection  109 . 
     As discussed in U.S. Pat. No. 6,295,875, the differential pressure applied to the sensor  56  can be measured using the electrodes  144 , 146 , 148  and  150 . As discussed below,  FIG. 3  schematically illustrates the capacitance based differential pressure  56 , which is described below in more detail. 
     In operation, pressures P 1  and P 2  press against isolation diaphragm  90  thereby pressing on a substantially incompressible fill fluid which fills the cavity between the center diaphragm  106  and the isolation diaphragm  90 . This causes center diaphragm  106  to deflect resulting in a change in capacitance between diaphragm  106  and electrodes  146 ,  144 ,  148 , and  150 . Using known techniques, changes in these capacitances can be measured and used to determine differential pressure. 
       FIG. 4  is a simplified cross-sectional view of sensor  56  used to illustrate operation of the present invention.  FIG. 4  illustrates various electrical connections to electrodes  144 ,  146 ,  148 , and  150 . 
     During operation of pressure sensor  56 , the line pressure applied to the pressure sensor through the capillary tubes  94  (see  FIG. 3 ) causes a deformation in the body  220  of pressure sensor  56 . While both pressures P 1  and P 2  cause a deformation of the sensor. The sensor will be based upon three different conditions. A high upstream pressure and a low downstream pressure, and a low upstream pressure and a high downstream pressure and a high upstream pressure with a high downstream pressure. The sensor will measure a line pressure defined as the maximum of the upstream or downstream pressure. The applied line pressure causes a pressure difference between the pressure within body  220  and the internal environment of the pressure transmitter. This pressure differential causes the deformation in the body  220 . In the example shown in  FIG. 4 , a greatly exaggerated deformation is shown. Specifically, the applied line pressure causes exterior walls  200  and  202  of body  220  to expand outward to the positions shown in phantom at  200 ′ and  202 ′. 
     The present invention provides a technique for measuring line pressure based upon the distortion, or bending, along the edge of the pressure sensor  56 . This bending is illustrated by the dashed lines labeled  200 ′ and  202 ′. Near the center end of the sensor  56 , the rate of displacement is illustrated as Δd 1 . As illustrated in  FIG. 4 , the displacement near the center of sensor  56  Δd 1  is greater than the displacement near the edge Δd 2 . The line pressure is related to both Δd 1  and Δd 2 , as well as relative measurement such as Δd 1 −Δd 2  or Δd 1 /Δd 2 . 
       FIG. 5  is a simplified cross-sectional view of sensor  56  illustrating one technique to measure displacement Δd 1  or Δd 2 . In the example embodiment of  FIG. 5 , the displacements are monitored by placing annular capacitive electrodes  240  and  242  proximate one end of sensor  56 . The electrodes  240 ,  242  are carried on an insulated backing plate  244  supported by support  248 . In one configuration, support  248  comprises a tube or the like. Support  248  can be attached continuously, or at points, to the sensor  56  and the insulated backing plate  244 . In another example configuration, support  248  comprises multiple supports, or has a configuration which does not continuously extend along the outer circumference of sensor  56 . In another example configuration, insulated backing plate  244  is mounted to capillary tube  94  using a bond  250 . Such a configuration may optionally include support  248 . In such a configuration, insulating backing plate  244  may or may not be attached to support  248 . Preferably, backing plate  244  is configured to either experience little distortion in response to line pressure or distort in a manner which contributes to Δd 1  and/or Δd 2  to thereby increase the sensitivity of the device. 
     Electrical connections are provided to electrodes  240  and  242  and can be used to measure capacitances C 1  and C 2  which are formed with respect to the sensor body  220 . 
     The nominal distance d 0  between electrodes  240  and  242  and the sensor can be controlled when the insulated backing plate  244  is mounted with respect to the sensor body  220 . The dielectric material between electrodes  240 ,  242  and the sensor body can be the ambient gas that surrounds the sensor  56 , for example nitrogen. In one preferred embodiment, the capacitors C 1  and C 2  have the same value (i.e. C 1 =C 2 ) in a rest condition. In such a configuration, nominal changes in the spacing d 0 , or in the dielectric constant of the gas, do not affect the difference between C 1  and C 2  since at rest C 1 −C 2 =0. Somewhat improved redundancy can be obtained by placing capacitor sensors on both sides of the sensor  56 . In one configuration, a temperature sensor is also provided and used to provide temperature compensation to line pressure measurements due to variations in the capacitance C 1 , C 2  based upon temperature. The deformation sensor  59  illustrated in  FIG. 2  as thus formed by the electrodes shown in  FIG. 5  which are mounted externally to the sensor body in a manner whereby their capacitance varies in response to deformation of the sensor body. 
     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. For example, the sensor body and the insulated backing plate do not need to be circular. Various attachment techniques can be used to reduce the stress which is applied to the backing plate. As used herein, “fluid” includes liquids and gasses or mixtures that may include solids.