Method for forming a via in a pressure sensor having a glass-to-metal seal - Patent Application 20070122997

Abstract

A method for manufacturing a pressure sensor (56) for sensing the pressure of a process fluid includes obtaining a sensor body (114, 116) having a sensor cavity formed therein. A metal tube (94) is disposed within the sensor cavity (132, 134) through an opening in the sensor body. A rod (170) is disposed within the sensor cavity through the metal tube (94). The sensor cavity (132, 134) is at least partially filled with a dielectric material (105), the dielectric material (105) completely covering the metal tube (94) and a portion of the rod carried within the sensor cavity. The rod (170) is removed, thereby forming a dielectric passage fluidly coupled to the metal tube (94). The sensor cavity (132, 134) is sealed with a deflectable diaphragm (106) configured to deflect in response to pressure exerted by the process fluid. A differential pressure sensor (56) for sensing a differential pressure of a process fluid also includes a sensor body (114, 116) having a sensor cavity (132, 134) formed therein.

Description

[Technical Field]

[0001] The present invention relates to pressure sensors of the type used to measure the pressure of process fluids, and more particularly to providing a pathway through a glass-to-metal seal in pressure sensors of the type used to measure the pressure of process fluids. [Background technology]

[0002] Transmitters are used in process monitoring and control systems to measure various process variables in industrial processes. One type of transmitter measures the pressure of the process fluid within the process. Pressure sensors used in such transmitters utilize a variety of technologies. One well-known capacitance measurement technique is the use of a deflectable diaphragm. Capacitance is measured between two opposing surfaces; the diaphragm forms one capacitive plate of a capacitor, and another fixed electrode is typically attached to the body of the sensor, forming the second capacitive plate. As the diaphragm deflects due to applied pressure, the measured capacitance changes. Summary of the Invention [Problem to be solved by the invention]

[0003] In one configuration of such a pressure sensor, the sensor body is formed of metal and filled with a dielectric material, such as glass. The dielectric material carries a fixed electrode that provides the second capacitive plate. An open path is provided through the sensor body and the dielectric material into a cavity formed in the sensor body that carries the diaphragm. A metal tube transmits pressure to the open path. A seal must be provided between the metal tube and the dielectric material. [Means for solving the problem]

[0004] A method for manufacturing a pressure sensor for sensing the pressure of a process fluid includes obtaining a sensor body having a sensor cavity formed therein. A metal tube is disposed within the sensor cavity through an opening in the sensor body. A rod is disposed within the sensor cavity through the metal tube. The sensor cavity is at least partially filled with a dielectric material, the dielectric material completely covering the metal tube and a portion of the rod carried within the sensor cavity. The rod is removed, thereby forming a dielectric passage fluidly coupled to the metal tube. The sensor cavity is sealed with a deflectable diaphragm configured to deflect in response to pressure from the process fluid.

[0005] A differential pressure sensor for detecting a differential pressure of a process fluid includes a sensor body having a sensor cavity formed therein. A diaphragm within the sensor cavity is configured to deflect in response to an applied differential pressure. A dielectric material is deposited on the sensor body within the sensor cavity, and a capacitive plate is carried on the dielectric material to form a sensing capacitor with the deflectable diaphragm having a capacitance related to the applied differential pressure. This capacitance changes in response to deflection of the deflectable diaphragm. A metal tube is coupled to the process pressure and has a distal end extending from outside the sensor body into the sensor cavity and a proximal end located within the dielectric material. An opening in the dielectric material fluidly couples the distal end of the metal tube to the diaphragm cavity via a dielectric passage that carries a fill fluid that contacts the dielectric passage. In one method, the dielectric passage is formed using an elongated rod.

[0006] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended for use as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all of the disadvantages noted in the Background. [Brief explanation of the drawings]

[0007] [Figure 1] 1 illustrates a process measurement system having a process transmitter constructed in accordance with the present invention. [Figure 2] FIG. 2 is a schematic diagram of the transmitter of FIG. 1; [Figure 3] FIG. 2 illustrates a cross-sectional view of a portion of the process transmitter of FIG. 1. [Figure 4] FIG. 1 is a cross-sectional view of a half-cell in a conventional pressure sensor configuration. [Figure 5] 1 is a cross-sectional view of a half-cell of a pressure sensor configuration during fabrication according to a method of one embodiment of the present invention. [Figure 6] 6 is a cross-sectional view of the pressure sensor half-cell of FIG. 5 after removal of a rod used during the manufacturing process. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of the present invention will be described in more detail with reference to the accompanying drawings. Elements identified with the same or similar reference numerals represent the same or similar elements. For clarity of illustration, some components shown in each figure may be omitted. Various embodiments of the present disclosure may be embodied in many different forms and should not be construed as limited to the specific embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0009] As discussed in the background section, one technique for measuring differential pressure is to measure the deflection of a diaphragm based on a change in capacitance. The diaphragm is supported within the pressure sensor body, and the metal tube must be electrically isolated from contact with the capacitive plates of the pressure sensor. Thermal mismatch issues can be particularly pronounced when applying lead-free glass construction to pressure sensors. The present invention includes a method for providing an open path through the glass-to-metal seal of a pressure sensor.

[0010] FIG. 1 illustrates a schematic of the environment of an industrial process measurement system 32. FIG. 1 shows process piping 30 containing a process fluid under pressure coupled to process measurement system 32 for measuring the process pressure. Process measurement system 32 has impulse piping 34 connected to process piping 30. Impulse piping 34 is connected to a process pressure transmitter 36. A primary element 33, such as an orifice plate, Venturi tube, or flow nozzle, contacts the process fluid within process piping 30 at a location between the pipes of impulse piping 34. Primary element 33 causes a pressure change in the fluid as it flows past primary element 33, and this pressure difference can be related to the fluid flow.

[0011] Transmitter 36 is a process measurement device that receives the process pressure through impulse piping 34. Transmitter 36 senses and converts the process differential pressure into a standardized transmitted signal that is a function of the process differential pressure.

[0012] The process control loop 38 provides both power and bidirectional communication from the control room 40 to the transmitter 36 and can be constructed according to many 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 and communications to and from the transmitter 36 during normal operation using a 4-20 mA signal. A computer 42 or other information processing system via a network interface 44 is used for communication with the transmitter 36. A remote voltage power supply 46 provides power to the transmitter 36. In addition to the loop configurations described above, the process control loop 38 can include any suitable process control loop. Examples include the HART® communication protocol, Foundation Fieldbus, or Profibus communication protocols, in which digital information is modulated onto a 4-20 mA current. The process control loop 38 can also be implemented using wireless communication technology. One example of a wireless communication technology is the WirelessHART® communication protocol compliant with IEC 62591. Other technologies, including those implemented using Ethernet or optical fiber, can also be used.

[0013] FIG. 2 is a simplified block diagram of pressure transmitter 36. Pressure transmitter 36 includes sensor module 52 and electronics board 72 coupled together via data bus 66. Sensor module electronics 60 connects to pressure sensor 56, which receives applied differential pressure 54. Data connection 58 couples sensor 56 to analog-to-digital converter 62. An optional temperature sensor 63 is also shown, along with sensor module memory 64. Electronics board 72 includes a microcomputer system 74, an electronics memory module 76, a digital-to-analog signal converter 78, and a digital communications block 80. Pressure transmitter 36 measures differential pressure according to techniques described, for example, in U.S. Patent No. 6,295,875 to Frick et al. However, the present invention is not limited to this configuration.

[0014] 3 is a simplified cross-sectional view of one embodiment of pressure sensor module 52 showing pressure sensor 56. Pressure sensor 56 connects to the process fluid through an isolation diaphragm 90, which isolates the process fluid from a cavity 92. Cavity 92 is connected to pressure sensor module 56 through metal tubing 94. A substantially incompressible fill fluid fills cavity 92 and tubing 94. When pressure from the process fluid is applied to diaphragm 90, the pressure is transferred by the fill fluid through tubing 94 to pressure sensor 56.

[0015] The pressure sensor 56 is formed from two pressure sensor half-cells 114, 116, filled with a substantially incompressible, electrically insulating dielectric (ceramic) material 105, such as glass. The two sensor halves 114, 116 form the sensor body. A diaphragm 106 is suspended within diaphragm cavities 132, 134 formed within the pressure sensor 56. The outer walls of the cavities 132, 134 carry electrodes 144, 148 on the glass 105.

[0016] As shown in FIG. 3 , the various electrodes of sensor 56 are connected to analog-to-digital converter 62 via electrical connections 104, 108. Additionally, deflectable diaphragm 106 is connected to analog-to-digital converter 62 via connection 109. As described in U.S. Pat. No. 6,295,875, a differential pressure applied to sensor 56 can be measured using electrodes 144, 148, 106. As used herein, the "sensor body" is formed by sensor halves 114, 116. The "diaphragm cavity" is formed by regions 132 and 134. The "sensor cavity" is partially filled with glass 105 and includes the diaphragm cavity.

[0017] In a typical construction, the tube 94 comprises a metal such as stainless steel, and the dielectric material 105 comprises glass. The process pressure applied to the internal cavities 132, 134 of the pressure sensor 56 must be sealed from the external environment. Providing a seal between the glass and metal and electrically isolating the metal tube 94 from the electrodes 144, 148 is often difficult.

[0018] FIG. 4 shows a cross-sectional view of a conventional configuration of a sensor half-cell 114. The sensor half-cell 114 receives pressure related to the applied process pressure through a tube 94. The metal tube 94 extends into the glass 105. Therefore, the tube 94 must be sealed to the glass 105. A glass-to-metal seal requires that the materials of the assembly have selected thermal properties to avoid cracking of the glass due to differences in their thermal expansion. During manufacturing, temperature changes occur during the glass-slumping process, in which heated glass fills the cavity, forms into the desired shape, and is maintained within the cavity by a sleeve insert 206. A ceramic insert 160 is placed on top of the metal tube 94 in the glass 105 and has a passage 162 that aligns with the passage 164 in the tube 94. The ceramic insert 160 is ground away during the manufacturing process to expose the passage 162. The ceramic insert 160 may have a thermal mismatch with the glass 105, which can cause cracks at the interface of the materials 105. Furthermore, the metal tube 94 is typically connected to electrical ground and cannot extend completely into the cavity 132 and contact plate 144 because there must be electrical insulation between the metal tube 94 and the capacitive plate 144 (see FIG. 3).

[0019] FIG. 5 illustrates the construction of pressure sensor assembly 56 during manufacturing according to one embodiment of the present invention. Similar elements within the figures are numbered identically. In FIG. 5, the ceramic insert 160 of FIG. 4 is not used. During manufacturing, an elongated rod 170 extends through metal tube 94 and glass 105. In one configuration, rod 170 comprises a graphite rod. A sleeve 206 seals the lower portion of the sensor cavity. Glass 105 is heated and shaped according to known glass slumping processes. Material 105 may also be shaped using other processes. Once the glass 105 is formed, graphite rod 170 is removed using, for example, a drilling process. Other material removal processes may also be used.

[0020] As shown in FIG. 6, once the graphite rod 170 is removed, the passage 164 of the metal tube 94 aligns with the dielectric (glass) passage 180 extending through the ceramic 105. Thus, the metal tube 94 has a distal end 200 extending from the exterior of the sensor body 56 and a proximal end 202 located within the glass 105 and spaced from the inner surface 182 of the glass 105. The glass passage 180 extends through the glass 105, providing an open passage from the interior cavity 132 that connects with the passage 164. This passage is filled with a fill fluid that contacts the glass 105 along the glass passage 180. The metal tube 94 is spaced from the surface 182 of the glass 105, thereby electrically isolating it from the capacitor plate 144 (shown in FIG. 3). FIGS. 5 and 6 illustrate the configuration of one sensor half-cell 114. However, sensor half-cell 116 is fabricated in a similar manner and bonded to sensor half-cell 114, as shown in Figure 3. Any residual graphite remaining after the removal process can be cleaned using pressure washing and precision cleaning.

[0021] In one particular configuration, the ceramic material 105 comprises glass. In another particular configuration, the graphite rod 170 comprises a pencil lead or the like. The glass passage 180 forms a tube that carries a filler fluid. The filler fluid contacts the walls of the passage 180. The metal tube 94 is spaced from the interior cavity 132 by the glass 105.

[0022] While the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the present invention. While a graphite rod 170 has been described, any suitable material can be used for the rod 170. For example, an insulating rod may be used. In one configuration, the rod 170 can be removed using a chemical reaction in which a chemical is applied to dissolve the rod 170. As described above, in the method of manufacturing the pressure sensor 56, the sensor half-cell 114 is filled with a liquid glass material 105. During this filling process, the metal tube 94 is placed into the glass 105 from outside the sensor half-cell 114. The graphite rod 170 is placed through a hole in the tube 94 and extends beyond the top surface 182 of the glass 105. A sleeve insert 206 is positioned to maintain the glass 105 within the half-cell 114. The electrode 104 is positioned to extend through the glass 105 to the surface 182. Once the glass 105 solidifies, the graphite rod 170 is removed. This can be by any suitable material removal process, such as drilling holes through the graphite rod 170. In another configuration, chemical etching is used to remove material from the rod 170. Additionally, the surface 182 can be shaped to the desired shape using machining techniques. After the surface 182 has the desired shape, the electrodes 144, 148 can be deposited and electrically coupled to the electrodes 104, 106. A similar process is used to fabricate the sensor half-cell 116, where two half-cells 114, 116 are joined to create the sensor assembly 56, as shown in FIG. 3.

Claims

1. 1. A method of manufacturing a pressure sensor for sensing a pressure of a process fluid, comprising: obtaining a sensor body having a sensor cavity formed therein; placing a metal tube into the sensor cavity through the opening in the sensor body; placing a rod through the metal tube and into the sensor cavity; at least partially filling the sensor cavity with a dielectric material, the dielectric material completely covering the metal tube and a portion of the rod carried within the sensor cavity; removing the rod to form a dielectric passage fluidly coupled to the metal tube; The method further comprises sealing the sensor cavity with a deflectable diaphragm configured to deflect in response to pressure applied by the process fluid.

2. The method of claim 1 , wherein the rod comprises a graphite rod.

3. The method of claim 1 , wherein removing the rod comprises drilling the rod.

4. The method of claim 1 , further comprising forming the dielectric material into a desired shape within the sensor cavity.

5. The method of claim 1 , further comprising depositing a capacitive plate on the dielectric material within the sensor cavity to form a sensing capacitor with the deflectable diaphragm.

6. The method of claim 5 , further comprising measuring the capacitance of the sensed capacitor.

7. The method of claim 5 , wherein the capacitive plate is insulated from the metal tube by the dielectric material.

8. The method of claim 1 , wherein the dielectric material comprises glass.

9. The method of claim 8 , comprising forming the glass using a glass slumping process.

10. The method of claim 1 , including filling a metal tube and the dielectric passage with a fill fluid.

11. The method of claim 10 , wherein the fill fluid contacts the dielectric passage.

12. A differential pressure sensor for detecting a differential pressure of a process fluid, a sensor body having a sensor cavity formed therein; a diaphragm within the sensor cavity configured to deflect in response to an applied differential pressure; a dielectric material deposited on the sensor body within the sensor cavity, the dielectric material forming a diaphragm cavity within the sensor cavity between the dielectric material and the diaphragm; a capacitive plate carried on the dielectric material and having a capacitance related to the applied differential pressure that varies with deflection of the deflectable diaphragm, the capacitive plate forming a sensing capacitor with the deflectable diaphragm; a metal tube coupled to process pressure, the metal tube having a distal end and extending from outside the sensor body into the sensor cavity to a proximal end located within the dielectric material; and an opening in a dielectric material fluidly coupling the distal end of the metal tube to the diaphragm cavity via the dielectric passage forming a tube carrying a fill fluid in contact with the dielectric passage.

13. The differential pressure sensor of claim 12 , wherein the sensor cavity is annular.

14. The differential pressure sensor of claim 12 including a second sensor cavity.

15. The differential pressure sensor of claim 12 , wherein the diaphragm is made of metal.

16. The differential pressure sensor of claim 12 , wherein the sensor body is made of metal.

17. The differential pressure sensor of claim 12 , wherein the proximal end is spaced from the diaphragm cavity.

18. The differential pressure sensor of claim 12 , wherein the dielectric material electrically insulates the metal tube from the capacitive plate.

19. The differential pressure sensor of claim 12 , wherein the sensor body includes a second sensor cavity.

20. The differential pressure sensor of claim 19 including a dielectric material deposited within the second sensor cavity.

21. 21. The differential pressure sensor of claim 20, including a second metal tube extending into the dielectric material in the second sensor cavity.

22. 22. The differential pressure sensor of claim 21 including a second capacitive plate on the dielectric material in the second sensor cavity, the second capacitive plate forming a second sensing capacitor having a capacitance related to pressure that varies with deflection of the deflectable diaphragm.

23. The differential pressure sensor of claim 12 , wherein the dielectric material comprises glass.

24. The differential pressure sensor of claim 12 , wherein the metal tube and the dielectric passage are filled with the fill fluid.

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