Abstract:
A pressure sensor assembly for sensing a pressure of a process fluid includes a sensor body having a cavity formed therein to couple to a process fluid pressure. A deflectable diaphragm in the cavity deflects in response to the first and second process fluid pressures. A first primary electrode couples to a wall of the cavity and forms a first primary capacitor between the first primary electrode and the deflectable diaphragm. A first secondary electrode couples to the wall of the cavity to form a first secondary capacitor between the first secondary electrode and the deflectable diaphragm. A second primary electrode and second secondary electrode are preferably coupled to a wall of the cavity opposite the first. Line pressure of the process fluid is determined based upon variation in the secondary capacitors relative to the primary capacitors.

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. 
   Transmitters are used in process monitoring and control systems to measure various process variables of industrial processes. One type of transmitter measures pressure of process fluid in the process. 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 an absolute (line or gauge) pressure in addition to a differential pressure measurement. In such an application, an additional pressure sensor is typically required to measure the line pressure. 
   SUMMARY 
   A pressure sensor assembly for sensing a pressure of a process fluid includes a sensor body having a cavity formed therein. The cavity is configured to couple to a first process fluid pressure. A deflectable diaphragm in the cavity deflects in response to the first process fluid pressure. A first primary electrode is coupled to a wall of the cavity and forms a first primary capacitor between the first primary electrode and the deflectable diaphragm. A first secondary electrode is coupled to the wall of the cavity to form a first secondary capacitor between the first secondary electrode and the deflectable diaphragm. Line pressure of the process fluid is calculated as a function of variation in the first primary capacitor and the first secondary capacitor due to changes in the size of the cavity from the first process fluid pressure. A method is also provided. 

   
     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  56  for use in illustrating operation of the present invention. 
       FIG. 5A  is a graph of line pressure versus primary sum effective gap and  FIG. 5B  is a graph of line pressure versus ring sum effective gap. 
       FIGS. 6A and 6B  are graphs of line pressure versus ring sum/primary sum and (ring gap/primary gap) X +(ring gap/primary gap) Y . 
       FIGS. 7A and 7B  are three dimensional graphs of line pressure versus primary transfer function versus ring sum/primary sum. 
       FIG. 8A  is a graph of line pressure versus ring sum/primary sum at various transfer function values. 
       FIG. 8B  is a graph of slope ′ versus nominal primary transfer function. 
   

   DETAILED DESCRIPTION 
   The present invention provides an apparatus and method for determining line pressure for a multi-electrode 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 line pressure of the process fluid can be determined. 
     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 passes past the primary element  33 . 
   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 pressure. 
   A process loop  38  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 . 
     FIG. 2  is a simplified block diagram 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 received 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 . 
   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 impulse piping  94 . A substantially incompressible fill fluid fills cavities  92  and impulse piping  94 . When a pressure from the process fluid is applied to diaphragms  90 , it is transferred to the pressure sensor  56 . 
   Pressure sensor  56  is formed from two pressure sensor halves  114  and  116  and filled with a preferably brittle, substantially incompressible material  105 . A diaphragm  106  is suspended within a cavity  132 , 134  formed within the sensor  56 . An outer wall of the cavity  132 ,  134  carries electrodes  146 , 144 , 148  and  150 . These can, generally, be referred to as primary electrodes  144  and  148 , and secondary or secondary electrodes  146  and  150 . These electrodes form capacitors with respect to the moveable diaphragm  106 . The capacitors, again, can be referred to as primary and secondary capacitors. 
   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 - 150 . As discussed below, the capacitance measured using these electrodes can also be used to determine the line pressure of the process fluid applied to the pressure sensor  56 . 
     FIG. 4  is a simplified cross-sectional view of sensor  56  used to illustrate operation of the present invention.  FIG. 4  illustrates various capacitive values, M X  between electrode  144  and diaphragm  106 , M Y  between electrode  148  and diaphragm  106 , R X  between electrode  146  and diaphragm  106  and capacitor R Y  between electrode  150  and diaphragm  106 . 
   It has been discovered that during an operation of pressure sensor  56 , the line pressure applied to the pressure sensor through the capillary tubes  94  causes a deformation in the body  220  of pressure sensor  56 . 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 ′. As the body deforms, the interior walls  126  and  128  of cavity  132 ,  134  also expand outward to the position shown in phantom  126 ′ and  128 ′, respectively. As the walls  126  and  128  move outwardly, the electrodes  144 ,  146 ,  148  and  150  also move in an outwardly position as illustrated in phantom at  144 ′,  146 ′,  148 ′ and  150 ′, respectively. This change in position of the electrodes  144 ,  146 ,  148  and  150  results in a change in the capacitance values as measured at M X , M Y , R X  and R Y . In accordance with the present invention, this change in capacitance is used to measure the line pressure applied to the pressure sensor  56 . 
   As used herein, the capacitance between electrodes  144  and  148  and the diaphragm  106  is referred to as a “primary capacitance” and the capacitance between electrodes  146  and  150  and the center diaphragm  106  is referred to as a secondary capacitance. In accordance with the present invention, the line pressure is determined as a function of the capacitance of a primary capacitor and the capacitance of a secondary capacitor. These capacitance values can be used in a ratio of sums, or sum of ratios, in a configuration to reduce errors in the line pressure measurements. 
   A line pressure (P) signal can be derived from the multi-electrode capacitance based differential pressure sensor  56  described above. This determination can be made by computing a ratio of sums, or a sum of ratios of the appropriate inverse active capacitance signals. As used herein, active capacitance is that capacitance which responds to movement of the center diaphragm (CD) relative to the sensor cavity and excludes any stray capacitance. The inverse of active capacitance is proportional to the separation or gap between the two large area (primary) electrodes separated by a relatively small distance. The configuration illustrated above with a center primary electrode and the ring secondary electrodes can be used to determine line pressure when the inverse active ring capacitances are divided by the inverse active primary capacitances. More specifically, line pressure can be determined as follows:
 
 LP=k *(1 /Rx +1 /Ry )/(1 /Mx +1 /My )  EQ. 1
 
A different, equally useful formulation can be written as follows:
 
 LP=j *(1 /Rx )/(1 /Mx )+(1 /Ry )/(1 /My )= j *( Mx/Rx+My/Ry )  EQ. 2
 
Where M is the active capacitance of the primary electrode, R is the active capacitance of the ring electrode, x and y refer to the low and high sides of the differential pressure sensor as illustrated in  FIG. 4 . The constants k and j are proportionality constants. Line pressure can also be determined using a single active capacitance value. However, in such a configuration, the capacitance is particularly sensitive to errors, for example, due to temperature variations. In contrast, using the ratios discussed above, a much greater signal to noise ratio can be obtained, for example a factor  100  improvement over the use of single active capacitance.
 
     FIG. 5A  is a graph of line pressure (PSI) versus primary sum-effective gap (μm) showing the temperature effect on the sum of the gaps between the primary electrodes  144 ,  148  and the diaphragm  106  utilizing inverse capacitance.  FIG. 5B  is a similar graph utilizing inverse ring capacitance. As illustrated in  FIGS. 5A and 5B , measuring the line pressure with inverse capacitance provides a very steep slope or low gauge factor with large changes in apparent line pressure (y-intercept) for small changes in temperature.  FIG. 6A  is a graph of line pressure versus ring sum/primary sum in accordance with equation 1 at various temperatures and  FIG. 6B  is a graph of line pressure versus (ring gap/primary gap) X +(ring gap/primary gap) Y  in accordance with equation 2. As illustrated in  FIGS. 6A and 6B , and in contrast to the graphs  FIGS. 5A and 5B , the slope of the LP signal is greatly reduced (higher gauge factor) and the y-intercept offsets caused by temperature are small relative to the LP span. The raw temperature error in  FIGS. 6A and 6B  is similar to comparable sensors and is at least partially correctable. In the graphs of  FIGS. 5A ,  5 B,  6 A and  6 B, the data was collected at a differential pressure of 0. 
   A line pressure signal can also be obtained when the differential pressure and line pressure are superimposed by combining either of the ratios illustrated in equations 1 or 2 with a standard differential pressure transfer function. In the case of equation 1, such a combination results in a fit of the data to a surface in three dimensional space with line pressure being a function of a ratio from equation 1 and the standard transfer function 
               (       M   ⁢           ⁢   x     -   My     )       (       M   ⁢           ⁢   x     +   My     )       .         
. For example,  FIGS. 7A and 7B  are the views of a graph of line pressure versus primary transfer function versus ring sum/primary sum. In this example, 192 data points are fitted based upon using a standard pressure transmitter at various line pressure and differential pressure values. The reorientation of the axes in  FIG. 7B  illustrates the close fit of the data into a plane.
 
     FIG. 8A  is a graph of the data from  FIGS. 7A ,  7 B of line pressure versus ring sum/primary sum.  FIG. 8B  is a graph of the slope of each line in  FIG. 8A  versus nominal primary TF. As illustrated in these figures, the data is easily segregated with great consistency by the transfer function. The plot in  FIG. 8B  illustrates that there is no higher order effect that warps or twists the plane and confirms the simplicity of the ratio/transfer function/line pressure relationship. 
   With the present invention, the instability of the dielectric constant of the fill fluid caused by heating, cooling, compression, decompression and transients cancels out to a relatively large degree. This is achieved by using more than one electrode on either side of the diaphragm in the pressure cell to obtain the line pressure signal. 
   In specific experiments, the precision of the data provides an error band of +1-70 PSI line pressure at a 95% confidence level. This level of precision is sufficient for reducing zero and span line pressure errors by a factor of 10 over a standard configuration by correcting the differential pressure transmitter output based upon the line-pressure signal. Manipulation of the capacitance data alone provides this improvement and no additional line-pressure sensor is required. Further, the present invention can be used to extract a useable line pressure signal based upon the measured capacitance values, again without the use of an additional line pressure sensor. In more advanced configurations, the line pressure signal can be used in combination with differential pressure and, in some configurations, combined with temperature to provide a mass flow calculation. 
   In one example, the capacitance of the primary capacitor changes by about 0.2% as the line pressure changes from 0 to 2000 PSI. Similarly, the capacitance of the ring capacitor changes by about 0.7% over such a range. The change in capacitance is substantially linearly relative to the change in applied line pressure. The two capacitances are used to accurately measure the applied line pressure. 
   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. In some embodiments the present invention includes compensating the calculated line pressure based upon the applied differential pressure. As used herein, the “primary” electrodes and capacitors and “secondary” electrodes and capacitors can be alternatively referred to as “primary” and “secondary”, respectively. 
   The measured or calculated line pressure can be used independently, or can be used to, for example, compensate for errors in the measured differential pressure. It is contemplated that the measured line pressure can be used for other purposes.