Piezoelectric pressure sensor

• US 5,209,125 A
• Filed: 03/27/1991
• Issued: 05/11/1993
• Est. Priority Date: 12/22/1989
• Status: Expired due to Term

• Current Assignee: Invensys Systems Incorporated (Invensys PLC)
• Original Assignee: The Foxboro Company (Schneider Electric SE)
• Inventors: Vignos, James H., Kalinoski, Richard W., Chitty, Gordon W.
• Primary Examiner(s): WOODIEL, DONALD
• Application Number:
  US07/676,038
• Time in Patent Office: 776 Days
• Field of Search: 73/861.24, 73/861.22, 73/DIG. 4, 73/726, 73/721, 73/754, 73/753, 73/720, 73/723, 73/756, 73/727, 73/119 A, 73/717, 310/338
• US Class Current: 73/861.24
• CPC Class Codes: G01F 1/3266 by sensing mechanical vibrations
• CPC Class Codes: G01L 9/08 by making use of piezoelectric devices , i.e. electric circuits therefor
• CPC Class Codes: Y10S 73/04 Piezoelectric

Abstract

A vortex sensor 14 measures a flow rate of a fluid flowing through a flow passage by detecting alternating pressure variations 26a and 28a generated by a shedding body 16 placed in the flow passage. The vortex sensor 14 includes a sensor housing 32 enclosing piezoelectric sensing elements 62, 64 which are acted on by a movable member 30. The lifetime of the sensing elements in the closed interior is enhanced by providing surface barriers, oxidizing exposed interior surfaces, or providing a gas reservoir. In applications where fluid leakage may be accepted, a controlled leak restrictor provides a limited rate diffusion path to the atmosphere allowing a minimum partial pressure of oxygen to be maintained.

Claims

1. A piezoelectric differential pressure sensor comprising:

a sensor housing having an interior including a first and a second cavity interconnected by a channel

a spool-like actuator having an axle slideably disposed in the channel for allowing each of two flange members, connected proximate to each end of the axle, to shuttle back and forth relative to the sensor housing in response to forces applied to the flange members

piezoelectric sensing means disposed in the interior between the flange members and the sensor housing, for generating signals in response to force mechanically applied to the sensing means by the flange members of the actuator, and

means for controlling migration of oxygen from the piezoelectric sensing means to retard aging thereof.

2. The sensor as claimed in claim 1, wherein the means for controlling migration includes a barrier film covering a substantial surface portion of the piezoelectric sensing means.

3. The sensor as claimed in claim 1, wherein the piezoelectric sensing means is sealed from a sensed fluid, and said means for controlling migration includes means for providing an oxygen partial pressure through a secondary seal located between the surrounding atmosphere and the housing interior forming a limited rate diffusion path for entry of oxygen into the interior.

4. The sensor as claimed in claim 1, wherein the piezoelectric sensing means is a lithium niobate crystal.

5. The sensor as claimed in claim 1, wherein the means for controlling migration includes a sintered porous plug between the interior and the surrounding atmosphere to allow limited gas flow therebetween and maintain a threshold oxygen partial pressure in the interior.

6. The sensor as claimed in claim 1, wherein the piezoelectric sensing means includes first and second sensing elements, and further comprising means for detecting signals generated by the sensing means, including

a charge amplifier interconnected to each sensing element for amplifying signals generated by each of the sensing elements, and

a summer circuit for combining output signals of each amplifier to reduce common mode noise.

7. A vortex sensor for a vortex-shedding flowmeter that measures a flow rate of a fluid flowing through a flow passage by detecting alternating pressure variations generated by a shedding body placed in the flow passage, the vortex sensor comprising

a sensor housing

two piezoelectric sensing means arranged within the sensor housing for generating signals in response to alternating pressure variations applied to each sensing means, the two piezoelectric sensing means being connected so that a common pressure variation applied equally to both produces substantially matched signals of opposite polarity which cancel each other,

means forming a controlled leak restrictor providing a diffusion path for atmospheric oxygen to reach the two piezoelectric sensing means, and

means interconnected with the piezoelectric sensing means, for detecting the signals generated by each of the piezoelectric sensing means to determine the flow rate of the fluid.

8. The vortex sensor of claim 7, wherein the piezoelectric sensing means comprises a lithium niobate piezoelectric crystal.

9. In pressure transducer apparatus of the type having a piezoelectric sensing element mounted in an interior of a housing for placement within a sensing environment to transduce pressure variations of the sensing environment and produce corresponding electrical signals, and wherein the piezoelectric sensing element is formed of a crystalline lithium niobate having an ordered lattice structure resulting in defined electrical characteristics for generating said signals, the improvement comprising

means in the housing for retarding chemical change of the order lattice structure by maintaining an oxygen partial pressure within said housing above a threshold value.

10. In transducer apparatus according to claim 9, the improvement wherein the means for maintaining includes a controlled leak restrictor.

11. In transducer apparatus according to claim 9, the further improvement wherein said means includes an oxidized element in the interior of the housing.

12. In transducer apparatus of the type having a piezoelectric sensing element mounted in an interior of a housing for placement within a sensing environment to transduce pressure variations of the sensing environment and corresponding electrical signals, wherein the piezoelectric sensing element is formed of a chemical compound having an ordered lattice structure resulting in defined electrical characteristics for forming said signals, said piezoelectric sensing element being isolated from said environment by a seal which is subject to failure in use, the improvement comprising means for providing a defined oxygen partial pressure in said interior by communicating with ambient atmosphere while restricting outflow therefrom so that in case of seal failure within said housing, the fluid of said sensing environment is restricted from escaping through said means for providing a defined oxygen partial pressure.

13. In transducer apparatus according to claim 12, the further improvement wherein said means for providing a defined oxygen partial pressure comprises a controlled leakage rate restrictor communicating with said interior and having a leakage in the range of 10^-6 -10^-4 scc/sec at 30 psi.

14. In transducer apparatus according to claim 13, the further improvement wherein said means for providing a defined oxygen partial pressure comprises a sintered microporous restrictor.

15. In transducer apparatus according to claim 12, the further improvement wherein said means for providing a defined oxygen partial pressure comprises pre-oxidized surface elements provided in the interior of the housing.

16. A pressure sensor apparatus comprising

a sensor housing for holding a sensing element

a lithium niobate sensing element mounted in an opening in the sensor housing and in mechanical communication with a sensed medium located exteriorly thereof

said sensor housing including sealing means interposed between the sensed medium and the sensing element for sealing the opening, and

means forming a controlled leakage restrictor constituting a diffusion path between the lithium niobate sensing element and the surrounding atmosphere, whereby sufficient oxygen enters the sensor housing to counteract or prevent deterioration of the sensor element, while only negligible quantities of the sensed medium reach the surrounding atmosphere in the event of failure of said sealing means.

17. Pressure sensor apparatus according to claim 16, wherein the controlled leakage restrictor includes a capillary tube venting the interior of the housing.

18. Pressure sensor apparatus according to claim 16, wherein the controlled leakage restrictor includes a porous sintered element having a strength to withstand a pressure level over 1000 psi.

19. A vortex sensor for a vortex-shedding flowmeter for measuring a flow rate of a fluid flowing along a flow passage by detecting alternating pressure variations generated by a shedding body placed in the flow passage, said vortex sensor comprising

a sensor housing for providing a fixed support extending into a flow passage and having first and second sides spaced apart along an axis transverse to fluid flow,

a coupling assembly extending between said first and second sides and movably supported in the sensor housing for receiving fluid pressure forces from said first and second sides and moving back and forth along said axis responsive thereto,

at least first and second piezoelectric sensing means mechanically coupled between said sensor housing and said coupling assembly for receiving the fluid pressure forces from said coupling assembly, each said piezoelectric sensing means having poling oriented along said axis thereby achieving substantial immunity to noise and producing vortex-indicating electrical output signals representative of fluid pressure applied to opposing sides of the coupling assembly.

20. A vortex sensor according to claim 19, wherein the first and second sensing means are arranged with opposite poling and are spaced along said axis and coupled between the coupling assembly and the housing such that increase pressure at the first side increases pressure applied to the first sensing means while reducing pressure applied to the second sensing means, and increased pressure applied to the second side applies increased pressure to the second sensing means while reducing pressure applied to the first sensing means.

21. A vortex sensor according to claim 20, further comprising a charge amplifier connected in a nulling feedback loop to provide a feedback signal to said first and second sensing means to cancel the vortex-indicating electrical output signals, frequency of the feedback signal equalling the frequency of sensed vortices in the fluid, thereby providing a measure of fluid flow.

22. A vortex sensor according to claim 19, further comprising means in the sensor housing for preventing loss of oxygen from the piezoelectric sensing means.

23. In pressure transducer apparatus of the type having a piezoelectric sensing element mounted in an interior of a housing for placement within a sensing environment to transduce pressure variations of the sensing environment and produce corresponding electrical signals, and wherein the piezoelectric sensing element is formed of a chemical compound having an ordered lattice structure resulting in defined electrical characteristics for generating said signals, the improvement comprising

means in the housing for retarding chemical change of the ordered lattice structure, said means for retarding including an oxygen-filled chamber in communication with the interior of said housing.

24. In pressure transducer apparatus of the type having a piezoelectric sensing element mounted in an interior of a housing for placement within a sensing environment to transduce pressure variations of the sensing environment and produce corresponding electrical signals, and wherein the piezoelectric sensing element is formed of a chemical compound having an ordered lattice structure resulting in defined electrical characteristics for generating said signals, the improvement comprising

means in the housing for retarding chemical change of the ordered lattice structure, said means for retarding including a barrier film placed over a major surface portion of the piezoelectric sensing element for providing a barrier to loss of oxygen from the surface portion.