Variable orifice flow sensor - Patent Review 5970801

Claims

What is claimed is:

1. A variable orifice flow sensor, of the type including a flow conduit member defining a flow orifice and fluidly connecting first and second fluid flow ports, a flapper with a pressure sensing tap on either side thereof, said flapper made of a magnetizable metal and mounted in the conduit member by a hinge portion so that the flapper angularly deflects out of the plane of the orifice in response to fluid flow through the conduit member to vary the effective fluid flow cross-sectional area of the orifice in proportion to the flow rate of fluid through the conduit member, and wherein a zero flow gap is defined between the flapper and the conduit member when there is no fluid flow through the orifice, wherein the improvement comprises:

a) a deflection-limiting surface in the conduit member adjacent the hinge portion against which the hinge portion abuts when the flapper experiences an angular deflection at least equal to a predefined angle in response to a fluid flow rate that is at least equal to a predetermined value; and

b) a magnetic element in the conduit member adjacent the flapper, the magnetic element being located and configured so as to generate a magnetic field that forces the flapper into a position that tends to minimize the zero flow gap.

2. The flow sensor of claim 1, wherein the magnetic field is selected so that the threshold flow rate needed to deflect the flapper is less than a predetermined minimum flow rate.

3. The flow sensor of claim 1, wherein the magnetic element comprises a pair of permanent magnets.

4. The flow sensor of claim 1, wherein the conduit member includes an outer wall surface in which is formed a pocket, and wherein the magnetic element includes a permament magnet retained in the pocket.

5. The flow sensor of claim 1, wherein the conduit member includes an outer wall surface in which is formed a pair of pockets, and wherein the magnetic element comprises a permanent magnet retained in each of the pockets.

6. A variable orifice flow sensor, of the type including a flow conduit member defining a flow orifice and fluidly connecting first and second fluid flow ports, a magnetizable metal flapper that is mounted in the conduit member by a hinge portion so that a zero-flow gap is defined between the flapper and the conduit member when there is no fluid flow through the orifice and so that the flapper angularly deflects out of the plane of the orifice in response to fluid flow through the conduit member to vary the effective fluid flow cross-sectional area of the orifice in proportion to the flow rate of fluid through the conduit member, and a pressure sensing tap on either side of the flapper, wherein the improvement comprises:

a magnetic element in the conduit member adjacent the flapper, the magnetic element being located and configured so as to generate a magnetic field that forces the flapper into a position that tends to minimize the zero flow gap.

7. The flow sensor of claim 6, wherein the magnetic field is selected so that the threshold flow rate needed to deflect the flapper is less than a predetermined minimum flow rate.

8. The flow sensor of claim 6, wherein the magnetic element comprises a pair of permanent magnets.

9. The flow sensor of claim 6, wherein the conduit member includes an outer wall surface in which is formed a pocket, and wherein the magnetic element includes a permament magnet retained in the pocket.

10. The flow sensor of claim 6, wherein the conduit member includes an outer wall surface in which is formed a pair of pockets, and wherein the magnetic element comprises a permanent magnet retained in each of the pockets.

11. The flow sensor of claims 6, 7, 8, 9, or 10, wherein the improvement further comprises:

a deflection-limiting surface in the conduit member adjacent the hinge portion against which the hinge portion abuts when the flapper experiences an angular deflection at least equal to a predefined angle in response to a fluid flow rate that is at least equal to a predetermined value.

12. The flow sensor of claim 11, wherein the deflection-limiting surface is defined by a support plate depending from the conduit member adjacent the hinge portion.

13. The flow sensor of claim 12, wherein the deflection-limiting surface comprises a pair of deflection-limiting surfaces defined by a pair of support plates depending from the conduit member adjacent the hinge portion, the deflection-limiting surfaces being separated by an angled notch.

14. The flow sensor of claim 13, wherein the hinge portion is connected to the conduit member by a pivot member having an axial dimension, and wherein the notch has an apex along a line that is perpendicular to the axial dimension of the pivot member.

15. A variable orifice flow sensor, of the type including a flow conduit member defining a flow orifice and fluidly connecting first and second fluid flow ports, a flapper that is mounted in the conduit member by a hinge portion so that the flapper angularly deflects out of the plane of the orifice in response to fluid flow through the conduit member to vary the effective fluid flow cross-sectional area of the orifice in proportion to the flow rate of fluid through the conduit member, and a pressure sensing tap on either side of the flapper, wherein the improvement comprises:

a deflection-limiting surface in the conduit member adjacent the hinge portion against which the hinge portion abuts when the flapper experiences an angular deflection at least equal to a predefined angle in response to a fluid flow rate that is at least equal to a predetermined value, and further wherein a zero flow gap is defined between the flapper and the conduit member when there is no fluid flow through the orifice, wherein the flapper is made of a magnetizable metal, and wherein a magnetic element in the conduit member is adjacent the flapper, the magnetic element being located and configured so as to generate a magnetic field that forces the flapper into a position that tends to minimize the zero flow gap.

16. The flow sensor of claim 15, wherein the magnetic field is selected so that the threshold flow rate needed to deflect the flapper is less than a predetermined minimum flow rate.

17. The flow sensor of claim 15, wherein the magnetic element comprises a pair of permanent magnets.

18. The flow sensor of claim 15, wherein the conduit member includes an outer wall surface in which is formed a pocket, and wherein the magnetic element includes a permanent magnet retained in the pocket.

19. The flow sensor of claim 15, wherein the conduit member includes an outer wall surface in which is formed a pair of pockets, and wherein the magnetic element comprises a permanent magnet retained in each of the pockets.

20. A variable orifice flow sensor, of the type including a flow conduit member defining a flow orifice and fluidly connecting first and second fluid flow ports, a flapper that is mounted in the conduit member by a hinge portion so that the flapper angularly deflects out of the plane of the orifice in response to fluid flow through the conduit member to vary the effective fluid flow cross-sectional area of the orifice in proportion to the flow rate of fluid through the conduit member, and a pressure sensing tap on either side of the flapper, wherein the improvement comprises:

a deflection-limiting surface in the conduit member defined by a support plate depending from the conduit member adjacent the hinge portion against which the hinge portion abuts when the flapper experiences an angular deflection at least equal to a predefined angle in response to a fluid flow rate that is at least equal to a predetermined value, and further wherein a zero flow gap is defined between the flapper and the conduit member when there is no fluid flow through the orifice, wherein the flapper is made of a magnetizable metal, and wherein a magnetic element in the conduit member is adjacent the flapper, the magnetic element being located and configured so as to generate a magnetic field that forces the flapper into a position that tends to minimize the zero flow gap.

21. The flow sensor of claim 20, wherein the magnetic field is selected so that the threshold flow rate needed to deflect the flapper is less than a predetermined minimum flow rate.

22. The flow sensor of claim 20, wherein the magnetic element comprises a pair of permanent magnets.

23. The flow sensor of claim 20, wherein the conduit member includes an outer wall surface in which is formed a pocket, and wherein the magnetic element includes a permanent magnet retained in the pocket.

24. The flow sensor of claim 20, wherein the conduit member includes an outer wall surface in which is formed a pair of pockets, and wherein the magnetic element comprises a permanent magnet retained in each of the pockets.

25. A variable orifice flow sensor, of the type including a flow conduit member defining a flow orifice and fluidly connecting first and second fluid flow ports, a flapper that is mounted in the conduit member by a hinge portion so that the flapper angularly deflects out of the plane of the orifice in response to fluid flow through the conduit member to vary the effective fluid flow cross-sectional area of the orifice in proportion to the flow rate of fluid through the conduit member, and a pressure sensing tap on either side of the flapper, wherein the improvement comprises:

a deflection-limiting surface in the conduit member adjacent the hinge portion against which the hinge portion abuts when the flapper experiences an angular deflection at least equal to a predefined angle in response to a fluid flow rate that is at least equal to a predetermined value, with said deflection-limiting surface defined by a support plate depending from the conduit member adjacent the hinge portion and comprising a pair of deflection-limiting surfaces defined by a pair of support plates depending from the conduit member adjacent the hinge portion and being separated by an angled notch, and further wherein a zero flow gap is defined between the flapper and the conduit member when there is no fluid flow through the orifice, wherein the flapper is made of a magnetizable metal, and wherein a magnetic element in the conduit member is adjacent the flapper, the magnetic element being located and configured so as to generate a magnetic field that forces the flapper into a position that tends to minimize the zero flow gap.

26. The flow sensor of claim 25, wherein the magnetic field is selected so that the threshold flow rate needed to deflect the flapper is less than a predetermined minimum flow rate.

27. The flow sensor of claim 25, wherein the magnetic element comprises a pair of permanent magnets.

28. The flow sensor of claim 25, wherein the conduit member includes an outer wall surface in which is formed a pocket, and wherein the magnetic element includes a permanent magnet retained in the pocket.

29. The flow sensor of claim 25, wherein the conduit member includes an outer wall surface in which is formed a pair of pockets, and wherein the magnetic element comprises a permanent magnet retained in each of the pockets.

30. A variable orifice flow sensor, of the type including a flow conduit member defining a flow orifice and fluidly connecting first and second fluid flow ports, a flapper that is mounted in the conduit member by a hinge portion so that the flapper angularly deflects out of the plane of the orifice in response to fluid flow through the conduit member to vary the effective fluid flow cross-sectional area of the orifice in proportion to the flow rate of fluid through the conduit member, and a pressure sensing tap on either side of the flapper, wherein the improvement comprises:

a deflection-limiting surface in the conduit member adjacent the hinge portion against which the hinge portion abuts when the flapper experiences an angular deflection at least equal to a predefined angle in response to a fluid flow rate that is at least equal to a predetermined value, with said deflection-limiting surface defined by a support plate depending from the conduit member adjacent the hinge portion and comprising a pair of deflection-limiting surfaces defined by a pair of support plates depending from the conduit member adjacent the hinge portion and being separated by an angled notch, and with the hinge portion connected to the conduit member by a pivot member having an axial dimension and wherein the notch has an apex along a line that is perpendicular to the axial dimension of the pivot member, and further wherein a zero flow gap is defined between the flapper and the conduit member when there is no fluid flow through the orifice, wherein the flapper is made of a magnetizable metal, and wherein a magnetic element in the conduit member is adjacent the flapper, the magnetic element being located and configured so as to generate a magnetic field that forces the flapper into a position that tends to minimize the zero flow gap.

31. The flow sensor of claim 30, wherein the magnetic field is selected so that the threshold flow rate needed to deflect the flapper is less than a predetermined minimum flow rate.

32. The flow sensor of claim 30, wherein the magnetic element comprises a pair of permanent magnets.

33. The flow sensor of claim 30, wherein the conduit member includes an outer wall surface in which is formed a pocket, and wherein the magnetic element includes a permanent magnet retained in the pocket.

34. The flow sensor of claim 30, wherein the conduit member includes an outer wall surface in which is formed a pair of pockets, and wherein the magnetic element comprises a permanent magnet retained in each of the pockets.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

This invention relates generally to the field of fluid flow sensors. More specifically, it relates to improvements in fluid flow sensors of the variable orifice or variable obstruction area type, that are commonly used, in conjunction with a pressure transducer, to generate a differential pressure signal from fluid (especially gas) flow in a conduit, wherein the value of the differential pressure signal is correlated with a flow rate value. The flow rate value, in turn, may be integrated over time to yield a volumetric value.

Flow sensors of the class described above have become commonly used in medical applications, particularly for measuring the flow rate of respiratory gas in medical ventilators. Specific examples of such flow sensors are found in the following U.S. Pat. Nos. 4,989,456--Stupecky; 4,993,269--Guillaume et al.; and 5,038,621--Stupecky. The variable orifice flow sensors exemplified by these patents employ a hinged obstruction or flapper that is mounted within a flow orifice of known area. The flapper is mounted so that the portion of the total area of the orifice that it opens to fluid flow is proportional to the flow rate through the orifice. The pressure drop across the orifice is proportional to the open area through the orifice. Thus, the differential pressure across the orifice is directly related to the flow rate through the orifice. This pressure differential is sensed by pressure ports upstream and downstream from the orifice. The sensed upstream and downstream pressures are directed to a differential pressure transducer, which generates an analog electrical signal having a value representing the differential pressure value. The analog signal is digitized and input to a microprocessor that is programmed to compute a value representing the instantaneous volumetric fluid flow rate through the orifice.

While flow sensors of the type described above have exhibited acceptable levels of accuracy and reliability, further improvements have been sought. Specifically, the use of such flow sensors in respiratory therapy equipment, particularly medical ventilators, has led to their fabrication from materials, such as stainless steel, that can be sterilized in autoclaves. A flapper made of stainless steel, however, is susceptible to fatigue and failure (especially at its hinged attachment point) due to repeated deflections over long periods of use, and due to overstress in response to high flow rates. This problem could be overcome by strengthening the hinged attachment part of the flapper and by increasing the area of the flapper so that it is not overstressed at high flow rates, but this solution would result in a loss of resolution at low flow rates.

Furthermore, it has been found that the use of stainless steel flappers has, in itself, resulted in the loss of low-end resolution. This is because, during the fabrication process, the stainless steel sheet takes a "set" that results in a flapper that deviates somewhat from a true planar configuration. In other words, the flapper is often slightly curved, so that, when it is installed in the orifice, there is a gap between the flapper's peripheral edge and the annular surface that defines the orifice. This results in a non-zero orifice area at zero flow rate, which, in turn, results in the loss of resolution at low flow rates. Furthermore, to avoid deterioration in accuracy due to exposure to the high temperatures of an autoclave, the flapper is advantageously annealed. The annealing process, however, softens the stainless steel, and the resultant loss of stiffness or rigidity causes the flapper to lose a measurable degree of responsiveness, most noticeable at low flow rates. Thus, the annealing process, while addressing one cause of inaccuracy (high temperature exposure), introduces another cause (loss of flapper rigidity).

Thus, there has been a need for a variable orifice flow sensor with a metal (preferably stainless steel) flapper that is capable of withstanding repeated deflections over a long period of use without fatigue or failure, even with repeated exposure to high flow rates. There has been a further need for such a flow sensor that also yields good low end sensitivity and resolution. There has been a still further need for such a flow sensor that is capable of withstanding repeated autoclaving without deterioration of accuracy over time, and without sacrificing responsiveness.

SUMMARY OF THE INVENTION

Broadly, the present invention is a variable orifice flow sensor, of the type comprising a first fluid flow port, a second fluid flow port, a flow conduit member defining a flow orifice and fluidly connecting the first and second fluid flow ports, a flapper of magnetizable sheet metal that is mounted in the conduit member by a hinge portion so that the flapper angularly deflects out of the plane of the orifice in response to fluid flow through the conduit member to vary the effective fluid flow cross-sectional area of the orifice in proportion to the flow rate of fluid through the conduit member, and a pressure sensing tap on either side of the flapper, wherein the improvement comprises a deflection-limiting surface in the conduit member adjacent the hinge portion against which the hinge portion abuts when the flapper experiences an angular deflection at least equal to a predefined angle in response to a fluid flow rate that is at least equal to a predetermined value, whereby overstressing of the hinge portion is minimized. The improvement further comprises at least one magnet in the conduit member adjacent to the flapper, whereby the magnet generates a magnetic field that acts on the flapper so as to force it into a position that tends to minimize the zero flow gap that exists between the flapper and the portion of the conduit member that defines the fluid flow orifice when there is no fluid flow through the orifice.

In a specific preferred embodiment, the hinge portion of the flapper is attached to a pivot member, such as a pin, a rivet, or an equivalent element. The pivot member has an axis that is substantially parallel to the axis of the conduit member. Extending radially inwardly into the conduit from the pivot member is a pair of support plates separated by an angled notch. The notch has an apex along a line that is perpendicular to the axis of the pivot member, so that the notch defines a pair of opposed deflection-limiting surfaces on the respective support plates that angle away from each other as they extend radially inwardly from the pivot member. The deflection-limiting surfaces of the plates thus define angular limits of travel for the hinge portion of the flapper.

Also, in the specific preferred embodiment, first and second disc-shaped magnets are respectively contained in first and second recesses formed in the exterior wall of the conduit member adjacent the flapper. The flapper is cut, stamped, or chemically etched from a sheet of stainless steel that is cold rolled to give it good magnetic properties. The location and the field strength of the magnets are selected so that the magnet field they create forces or pulls the flapper into a position in which the gap that exists between the edge of the flapper and the orifice-defining portion of the conduit member is minimized at zero flow conditions. The field strength is strong enough to simulate and thus compensate for the stiffness or rigidity that is lost during the annealing process (as discussed above), but not so strong, however, as to create a significant bias on the flapper at higher flow rates. Thus, low end responsiveness is improved without sacrificing high end accuracy.

As will be better appreciated from the detailed description that follows, the present invention offers significant advantages over the prior art. Specifically, the deflection-limiting surfaces limit the angular flexing of the hinge portion of the flapper, so that once the angular limits of travel are reached, the hinge portion undergoes little or no increase in stress. Rather, the flapper resiliently bends over the edges of the support plates at high flow rates, thereby yielding a measurement without significantly causing stress-induced fatigue in the hinge portion. This increases the useful lifetime of the sensor, even when it is subjected to high flow rates, without significantly affecting low flow rate resolution or sensitivity. Furthermore, the gap minimization created by the magnet or magnets enhances resolution at low flow rates. Thus, the sensor according to the present invention yields high lifetime and good resolution at both the high and low extremes of the flow rate range to be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a fluid flow sensor in accordance with a preferred embodiment of the present invention;

FIG. 2 is an axial cross-sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is a detailed view taken within the area defined within the broken outline 3 of FIG. 2, showing the flapper in a zero flow position;

FIG. 4 is a view similar to that of FIG. 3, but showing the flapper at one angular extreme of travel in response to a high fluid flow rate;

FIG. 5 is a cross-sectional view taken along line 5--5 of FIG. 2;

FIG. 6 is a cross-sectional view taken along line 6--6 of FIG. 2;

FIG. 7 is a cross-sectional view taken along line 7--7 of FIG. 6;

FIG. 8 is a cross-sectional view taken along line 8--8 of FIG. 7;

FIG. 9 is a graph of pressure drop across the flow orifice as a function of flow rate, comparing the performance of the sensor at low flow rates with and without the improvement provided by the magnet or magnets of the present invention; and

FIG. 10 is a schematic representation of a flow rate measuring system incorporating a flow sensor in accordance with the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to FIGS. 1 through 8, a variable orifice fluid flow sensor 10, in accordance with a preferred embodiment of the invention, is shown. As best shown in FIGS. 1 and 2, the sensor 10 comprises a first tubular member 12 that defines a first fluid flow port 14 and a second tubular member 16 that defines a second fluid flow port 18. The second tubular member 16 has an internally-threaded opening 20 opposite the second fluid flow port 18 that receives an externally-threaded end 22 of the first tubular member 12. Thus, the first and second tubular members 12, 16 may be detachably coupled to each other by the mating screw threads of the threaded opening 20 and the threaded end 22. Because the sensor 10 is a bidirectional sensor, each of the fluid flow ports 14, 18 may function as either an input port or an output port.

Formed inside the second tubular member 16 a short distance inwardly from the threaded opening 20 is an annular shoulder 24. The shoulder 24 forms a seat for a short tubular conduit member 26, to be described more fully below. An annular washer 27 may advantageously be provided between the shoulder 24 and the conduit member 26. The second tubular member 16 is provided with a first internally-threaded radial bore 28 just inwardly from the threaded opening 20. A second internally-threaded radial bore 30 is provided in the second tubular member 16 a short distance inwardly from the shoulder 24, on the opposite side thereof from the first radial bore 28. The first and second radial bores 28, 30 respectively receive first and second pressure sensing taps 32, 34, each of which has an externally threaded portion that mates with the internal threads of its respective radial bore. The pressure sensing taps 32, 34 are respectively connected, by flexible conduits 35, to the input side of an electronic module 36, which contains an electrically-erasable programmable read-only memory (EEPROM) chip 39 (FIG. 10). The module 36 is made of an autoclavable plastic, and the EEPROM 39 is potted in a heat-resistant epoxy, so that the entire module 36 may be sterilized in an autoclave along with the sensor 10. The module 36 includes a plurality of conductive pins 41, as shown in FIG. 1, that are electrically connected to appropriate input and output terminals (not shown) in the EEPROM 39, so that data can be stored in and retrieved from the EEPROM 39, as described below.

The conduit member 26 is preferably formed of a corrosion-resistant, nonmagnetic material, such as aluminum or an autoclavable plastic. It provides fluid communication between the first and second fluid flow ports 14, 18. The conduit member 26 has an outside diameter that is slightly