WO1998036245A1

WO1998036245A1 - An apparatus for indicating the flow rate of a fluid through a conduit

Info

Publication number: WO1998036245A1
Application number: PCT/AU1998/000082
Authority: WO
Inventors: Barton John Kenyon
Original assignee: Resmed Limited
Priority date: 1997-02-14
Filing date: 1998-02-13
Publication date: 1998-08-20
Also published as: AUPO511497A0

Abstract

An apparatus (10) for indicating the flow rate of a fluid (F) through a conduit (12). The apparatus comprises a flap (14) mounted in the conduit (12), the flap being off-centre pivotally mounted to pivot in response to the flow of the fluid (F) through the conduit (12). Biasing means (24, 26) associated with the flap (14) rotationally urge the flap (12) against the action of the flow (F). Means (36) are provided to sense the position of the flap (14) and issue a signal indicative of the flap position. The flap position signal is indicative of the flow rate of the fluid (F) through the conduit (12).

Description

AN APPARATUS FOR INDICATING THE FLOW RATE OF A FLUID THROUGH A CONDUIT

FIELD OF THE INVENTION

The present invention relates to an apparatus for indicating the flow rate of a fluid through a conduit.

The invention has been developed primarily for use in indicating the flow rate of air (or breathable gas) supplied to a patient undergoing continuous positive airway pressure (CPAP) treatment for obstructive sleep apnea. In particular, when the pressure of the gas supplied to the patient is bi-level (in synchronism with patient inspiration and expiration) or auto setting in level, the flow rate of the supplied air is required by a control system for triggering purposes.

The invention is also suited for many other flow rate indicating or measuring applications. An example is the monitoring, for diagnostic purposes, of the respiratory flow rate of a patient. Also, by integrating the flow rate with respect to time the volume of fluid supplied over a particular time can be determined.

BACKGROUND OF THE INVENTION

A known method of measuring the flow rate of a fluid (liquid or gas) through a conduit involves measuring the pressure either side of a restriction of constant area in the conduit. If the pressure drop across the restriction is recorded for a variety of known flow rates then the function of the flow rate with respect to the pressure drop can be determined. The flow rate is basically related to the pressure drop by a quadratic function (ie. the pressure drop is proportional to the square of the flow rate). When the function is known then, by measuring the pressure drop, it is possible to calculate the corresponding flow rate. The pressure drop can be measured by an electronic differential pressure transducer which provides an electrical signal indicative of the pressure drop to a conversion unit in the control system for converting the signal into a flow rate.

Due to the quadratic relationship, small variations or errors in the measuring of the pressure drop at low flow rates produce large variations or errors in the calculated flow rate. Restrictions of constant area also produce a large pressure drop at high flow rates. Accordingly, a restriction of constant area is unsuitable when a large range of flow rates is required to be measured, especially where accuracy is required at the lower end of the range. In the CPAP application mentioned above, accurate flow rate readings of down to zero flow are required for triggering purposes by the control system. Also, the flow rate must also be able to be measured at peak flows of up to about 200 litres per minute without a large pressure drop being caused.

A restriction of variable area can ameliorate some of the above problems. A prior art variable area restriction includes a resilient plastic flap that, in an unstressed

5 state, almost occludes a restriction in the conduit. The flap deflects to enlarge the permitted flow area of the conduit under the influence of the fluid flowing through the restriction. The higher the air flow, the more the flap deflects, and the larger the restriction area becomes. The resilient flap can be configured to provide an almost linear relationship between pressure drop and flow rate over a useful range of flows. In ι o this way, the resilient flap provides the desired level of accuracy at both relatively low and high flows. Further, as the area of the restriction is increased at high flows, the resilient flap does not cause a large pressure drop at these high flows.

However, the resilient flap suffers from the disadvantage that, after continuous use, it can take on a permanently deflected set and therefore provide erroneous or

15 inaccurate readings at low flow rates. Further, the pressure drop across either the constant area orifice or resilient flap is generally measured by electronic differential pressure transducers which are expensive and often also require expensive amplifiers and other circuitry to provide a signal compatible with other components or systems.

It is an object of the present invention to substantially overcome or at least

20 ameliorate these prior art deficiencies.

SUMMARY OF THE INVENTION

Accordingly, the present invention discloses an apparatus for indicating the flow rate of a fluid through a conduit, the apparatus including: a flap mounted in said conduit, said flap being off-centre pivotally mounted to pivot in response to the flow of

25 the fluid through the conduit; biasing means associated with the flap to rotationally urge the flap against the action of the flow; and means to sense the position of the flap and issue a signal indicative of the flap position, wherein the flap position signal is indicative of the flow rate of the fluid through the conduit.

The biasing means preferably urges said flap to a rest position when no fluid is

30 flowing through the conduit. The flap preferably substantially occludes the conduit in the rest position. The flap is also preferably substantially perpendicular to the general direction of fluid flow through the conduit when in the rest position.

Desirably, the flap progressively pivots to progressively increase the flow area of the conduit in response to increasing fluid flow therethrough.

35 In a preferred embodiment, the biasing means takes the form of a magnet mounted on the flap which, in the rest position, is positioned between one or more, preferably two, magnets mounted on the conduit remote from the flap magnet. In another preferred embodiment, the flap is mounted on a pivotable shaft and the biasing means includes a flap magnet eccentrically mounted on the shaft, the flap magnet, in the rest position, being positioned between at least two magnets mounted on the conduit remote the flap magnet. In one form, the shaft passes through a wall of the conduit and the flap magnet is mounted exterior the conduit. The conduit magnets are preferably symmetrically mounted either side of the flap magnet. The flap magnet and the conduit magnets are preferably adapted to provide a repelling force therebetween to urge the flap to the rest position.

In another preferred embodiment, an attracting magnet is placed on the conduit adjacent the flap magnet, when the flap is in the rest position, to therefore attract the flap magnet and urge the flap into the rest position. In a variation of this embodiment, a ferro-magnetic metal, such as steel, is placed on the flap and is attracted by the conduit magnet to bias the flap to the rest position. Alternatively, a ferro-magnetic metal can be placed on the conduit and a flap magnet used to attract the flap into the rest position.

In a still further embodiment, a spring, for example, a clock spring, has one end attached to the flap and the other end attached to the conduit to urge the flap to the rest position.

In yet another embodiment, gravity can be used to urge the flap to the rest position. In one form of this embodiment, a weight is preferably placed on the portion of the flap below the hinge axis. In another form of this embodiment, the portion of the flap below the hinge axis is preferably configured to be larger and thereby heavier than that above. These embodiments are particularly suitable when the orientation of the apparatus is constant.

The flap position sensing means is preferably one or more continuous output Hall Effect sensors mounted on or adjacent the conduit and adapted to issue an output signal in response to their distance from a magnet disposed on the flap. In an embodiment, the magnet associated with the Hall Effect sensor(s) is desirably also used to provide the biasing force in combination with either conduit magnets or ferro- magnetic material mounted on the conduit. In another embodiment, the magnet associated with the Hall Effect sensors is disposed remote from any magnets or metal associated with the biasing means.

In a preferred form of this embodiment, two Hall Effect sensors are each arranged either side of the flap rest position and the difference in their signals used to indicate the flap position.

In yet another embodiment, the flap position sensing means is a linearly variable differential transformer having a first and second part adapted for relative rotation therebetween wherein the output signal of the transformer is indicative of flap position. In this embodiment, one of the first or second parts is non-rotatably mounted to the flap and the other of the first or second parts is non-rotatably mounted to the conduit. In still another embodiment, the flap position sensing means includes a light source and a photodetector each arranged either side of the flap, wherein flap rotation alters the amount of light passing the flap and incident on the photodetector which thereby issues a signal indicative of flap position. In a still further embodiment, an optically encoded disc is attached to the flap having for example a rotary Grey scale and an optical sensor is mounted on or adjacent the conduit and adapted to recognise regions of the scale and thereby indicate the amount of rotation or position of the flap.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a cross-sectional side view of a first embodiment; Fig. 2 is a cross-sectional end view along line 2-2 of the embodiment shown in Fig. 1 ; Fig. 3 is a side view of the embodiment shown in Fig. 1 under the influence of a relatively low flow rate;

Fig. 4 is a side view of the embodiment shown in Fig. 1 under the influence of a relatively high flow rate;

Fig. 5 is a plot of the relationship between the flow rate of the fluid in the conduit and the output voltage of the differential amplifier connected to the Hall Effect sensors of the embodiment shown in Fig. 1 ;

Fig. 6 is a top view of a second embodiment; Fig. 7 is a bottom view of the embodiment of Fig. 6; Fig. 8 is a rear perspective view of the embodiment of Fig. 6; Fig. 9 is a top perspective view of the embodiment of Fig. 6;

Fig. 10 is an exploded view of the embodiment shown in Fig. 6; Fig. 11 is a side view of the embodiment of Fig. 6;

Fig. 12 is a plot of the relationship between the flow rate of fluid (air) in the conduit and the output voltage of the differential amplifier connected to the Hall Effect sensors of the embodiment of Fig. 6;

Fig. 13 is a side view of a third embodiment; Fig. 14 is a side view of a fourth embodiment; Fig. 15 is a side view of a fifth embodiment;

Fig. 16 is a diagrammatical end view of the embodiment shown in Fig. 1 ; Fig. 17 is a diagrammatical end view of a sixth embodiment;

Fig. 18 is a diagrammatical end view of a seventh embodiment; Fig. 19 is a diagrammatical side view of an eighth embodiment; and Fig. 20 is an end view of the embodiment of Fig. 19. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring firstly to Figs. 1 and 2, there is shown a first embodiment of an apparatus 10 for indicating the flow rate of a fluid F through a conduit 12. The apparatus 10 includes a substantially rigid flap 14 which is mounted for off-centre

5 pivotal rotation about axis 16 in response to the flow of fluid F through the conduit 12. The flap has two pivot shafts 17 that each terminate in a relatively sharp cone 18. The pivot shafts 17 are received within two pivot bores 20 which each terminate in a relatively wide cone 22 to provide point contact of low friction between each of the pivot shafts 17 and the associated pivot bores 20. ι o The apparatus 10 also includes biasing means in the form of magnet 24 mounted on the flap 14 and magnets 26 mounted on the conduit 12 either side of the flap magnet 24. The magnets 24 and 26 are preferably fabricated from Neodymium Iron Boron or Samarium Cobalt type material. The conduit magnets 26 are oriented so that they each repel the flap magnet 24 and thereby urge the flap to the position shown

15 in Fig. 1 , being the position of the flap when there is no fluid flow through the conduit 12 (hereinafter referred to as the "rest position").

The flap 14 has a relatively large area 28 and a relatively small area 30 either side of the pivot axis 16. Accordingly, when fluid flow F is occurring in the conduit 12 the flap 14 is forced to pivot in the direction of arrow 32 (see Figs. 6 and 7) about

20 axis 16 because the force of the fluid flow F on the relatively large area 28 is higher than that on the relatively small area 30. The rotation of the flap 14 in the direction of arrow 32 is opposed and balanced by the repelling force, in the direction of arrow 34, generated between magnets 24 and 26.

The apparatus 10 also includes means to sense the position of the flap 14 and

25 issue a signal indicative of the flap position in the form of Hall Effect sensors 36 of the continuous output type. The Hall Effect sensors 36 are mounted on the conduit 12 and issue an output signal along connecting wires 38 indicative of the distance between the flap magnet 24 and each of the Hall Effect sensors 36. The output signals travel along the wires 38 to a differential amplifier (not shown) which issues a voltage indicative of

30 flap position, as will be explained below.

Fig. 3 shows the flap 14 rotated to a position Θ_L in response to the flow of fluid F_L. In this position, the force on the flap 14 in the direction of arrow 32 is balanced by the repelling force provided by the magnets 24 and 26 in the direction of the arrow 34.

35 Fig. 4 shows the flap 14 under the influence of a higher flow of fluid F_H where the flap 14 is rotated to position Θ_H. Accordingly, the higher the flow rate F of the fluid through the conduit 12, the more the flap 14 pivots away from the rest position. In the position of the flap 14 in Fig. 3, the Hall Effect sensor 36 upstream of the flap 14 will issue a larger signal than the downstream Hall Effect sensor 36. The output signal provided by the Hall Effect sensors 36 is inversely proportional to the square of the distance between the flap magnet 24 and the Hall Effect sensor 36.

5 When the flap 14 is in the position shown in Fig. 4. the upstream Hall Effect sensor issues an even larger signal 38 than that of Fig. 3 and the downstream Hall Effect sensor issues an even lower signal than that of Fig. 3. When the output signals of the Hall Effect sensors are processed by the differential amplifier it produces a signal in Volts (v) which, when plotted against a range of known flow rates (litres per minute ι o of air), produces the characteristic plot of Fig. 5.

As shown in Fig. 5, the apparatus 10 provides extremely desirable output characteristics as, in the lower flow rates, for example below 20 litres per minute, a relatively steep linear relationship exists between flow rate and output signal voltage which provides a higher level of accuracy at this lower range. When the flow rate

15 increases, for example, to above 50 litres per minute, a shallower linear relationship develops which enables the apparatus to monitor the flow rate at up to 150 litres per minute and above.

Figs. 6 to 11 show a second embodiment of an apparatus 50 for indicating the flow rate of a fluid F through a conduit 12. Like reference numerals to those used in

20 describing the first embodiment will be used to denote like features in the second embodiment.

The apparatus 50 includes a housing 51 having two housing halves 52 and 54 which are held together by clamps 56 to define a right-angled section of conduit 12 having open ends 58 and 60.

25 The aluminium flap 14 is mounted adjacent one edge to a vertical shaft 62 which has ends protruding through both of the housing halves 52, 54. A first flap magnet 24a is eccentrically mounted to one end of the shaft 62 external the conduit 14. The flap magnet 24a is repelled by the conduit magnets 26 mounted within circular recesses 55. The repulsion between the magnets 24 and 26 biases the flap 14 to the

30 central rest position shown in phantom in Fig. 6.

The other end of the shaft 62 also includes an external eccentrically mounted second flap magnet 24b. The magnet 24b pivots between the Hall Effect sensors 36 mounted on a printed circuit board 66. The housing half 54 includes boss 68 and recessed lug 70 to locate the housing 51 with respect to the board 66 and thus position

35 the Hall Effect sensors 36 adjacent the second flap magnet 24b.

The Hall Effect sensors 36 each produce a voltage output which is supplied to respective inputs of a differential amplifier whose output is supplied to an analogue to digital (a-d) converter having an a-d count output. Fig. 12 shows a plot of a-d output (counts) over a range of air flow rates (litres/min). The housing halves 52, 54 each also include a dampening magnet 72 disposed to attract each other. A steel yoke 74 closes the magnetic field therebetween. The flap 14 includes a radial fin 76 extending partially around the shaft 62, as best shown in Fig. 10. The fin 76 is positioned between the facing poles of the dampening magnets 72 and, as the flap 14 pivots, the fin 76 moves through the magnets' magnetic field and creates eddy currents. The eddy currents produce their own magnetic field which opposes the movement of the fin 76 and dampens oscillation in the flap 14.

The fin 76 is constructed of a conductive, non ferro-magnetic material such as Aluminium. Fig. 13 shows a third embodiment of a flow indicating apparatus 80, similar to the second embodiment, but in which the position of the flap 14 and corresponding rotation of the shaft 62 is directly measured by a linearly variable differential transformer 82 connected to the shaft 62.

Fig. 14 shows a fourth embodiment of a flow indicating apparatus 90 in which the position of the flap 14 and corresponding rotation of an optically encoded disc 92 mounted on the shaft 62 is sensed by an optical sensor 94. The disc 92 can have a rotary Grey scale thereon.

Fig. 15 shows a fifth embodiment of a flow indicating apparatus 100 in which the position of the flap 14 is measured by sensing the amount of light from a light source 102 that is incident on a photodetector 104. The corner 106 includes a transparent portion 108 to permit the light to enter the conduit 12.

Fig. 16 is a diagrammatical representation of the flap 14 within the conduit 12 of the first embodiment.

Fig. 17 is a diagrammatical representation of a sixth embodiment having an irregular shaped flap 110 provided within a correspondingly shaped conduit 112 to exemplify that the flap 110 and the conduit 112 can be of many different shapes.

Fig. 18 is a diagrammatical representation of a seventh embodiment having a flap 120 which pivots about the horizontal axis 16 near the upper edge 122 of the flap

120 to exemplify that the off-centre pivot axis can be placed at different positions in the flap 120. Further, in this embodiment, gravity alone, or with the assistance of a weight

124, can bias the flap 120 to the rest position.

Figs. 19 and 20 show an eighth embodiment 130 in which a flap magnet 132 is used to attract a conduit magnet 134 to bias the flap 14 to the rest position shown. In a variation, one of the flap magnet or the conduit magnet can be replaced by ferro- magnetic metal.

In the embodiments of Fig. 1 and Fig 6, the flap 14 substantially occludes the cross sectional area of the conduit 12 in the rest position. This is a preferred configuration only and in other embodiments (not shown) the flap 14 only partially occludes the conduit when in the rest position. Similarly, in the embodiment of Fig. 1 the flap 14, in the rest position, is substantially perpendicular to the direction of fluid flow F through the conduit 12. This is also a preferred configuration and in other embodiments (not shown) the flap 14 is angled with respect to the flow direction at the rest position.

5 The apparatus 10, as shown in Fig. 1, is inherently damped due to the drag of the flap 14 as it rotates. Drag between the flap 14 and the fluid tends to resist rotation of the flap 14 and increases with fast rotations thereby providing true damping. The damping effect is more pronounced with denser fluids. Further, if flap size is increased then more drag and thereby more damping is created. ιo The movement of the flap 14 can also be damped by providing friction between the pivot shafts 17 and the pivot bores 20.

The apparatus' described above possess the previously described advantages over the fixed area restriction as they function in the manner of a variable area restriction. Further, the apparatus' are they also advantageous over the flexible flap i s restriction previously described as the pivotable flap cannot take on a permanent deflection or set through constant use thereby increasing the accuracy of the apparatus' when indicating flow rate. Also, the function between the flow rate and the output signals of the Hall Effect sensors can be altered to provide other desired relationships by varying the flap size, shape, pivot axis or the force of the biasing means. At

20 extremely large flow rates, the function tends to an asymptote of substantially constant output voltage. Additionally, by comparing the output signals of the upstream and downstream Hall Effect sensors the apparatus can also provide an indication of flow direction.

Moreover, the apparatus' are relatively inexpensive as they avoid the use of

25 expensive prior art electronic differential pressure transducers.

Also, when the apparatus is used in conjunction with the magnetic or optical position sensing means, then no contact occurs between the fluid and the sensing electronics making the apparatus' particularly advantageous for use with the fluids that are highly corrosive, infectious or flammable. This is also an advantage if cleaning of

30 the apparatus' is important. In particular, the apparatus' can be configured so that the only materials in contact with the fluid are those of the flap and the conduit. If magnets or ferro-magnetic material are fitted to the flap, then they can be fitted inside the flap and fully encased by the material from which the flap is produced.

Embodiments of the apparatus' also provide good signal sensitivity at low

35 flows and thereby avoid the need for expensive amplifiers and other circuitry to boost the signal.

Further, the apparatus' are more thermally stable than devices using differential pressure transducer as such devices commonly use thermal elements which do not compensate ambient temperature variations very accurately. The apparatus' also have a large and accurate measurement range compared to prior art devices. More particularly, the apparams' exhibit high accuracy at low flow rates and slightly lower, but acceptable, accuracy at large flow rates.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art, that the invention can be embodied in many other forms.

Claims

1. An apparatus for indicating the flow rate of a fluid through a conduit, the apparams comprising: a flap mounted in said conduit, said flap being off-centre pivotally mounted to 5 pivot in response to the flow of the fluid through the conduit; biasing means associated with the flap to rotationally urge the flap against the action of the flow; and means to sense the position of the flap and issue a signal indicative of the flap position, wherein the flap position signal is indicative of the flow rate of the fluid ╬╣ o through the conduit.

2. An apparatus as claimed in claim 1 , wherein the biasing means urges said flap to a rest position when no fluid is flowing through the conduit.

3. An apparatus as claimed in claim 2. wherein the flap substantially occludes the conduit in the rest position.

15 4. An apparatus as claimed in claim 2 or 3, wherein the flap in the rest position is substantially perpendicular to the general direction of fluid flow through the conduit.

5. An apparams as claimed in any one of the preceding claims, wherein the flap progressively pivots to progressively increase the flow area of the conduit in

20 response to increasing fluid flow therethrough.

6. An apparams as claimed in any one of claims 2 to 5, wherein the biasing means includes a magnet mounted on the flap which, in the rest position, is positioned between at least two magnets mounted on the conduit remote from the flap magnet.

25 7. An apparatus as claimed in any one of claims 2 to 6, wherein the flap is mounted on a pivotable shaft and the biasing means includes a flap magnet eccentrically mounted on the shaft, the flap magnet, in the rest position, being positioned between at least two magnets mounted on the conduit remote the flap magnet.

30 8. An apparams as claimed in claim 7, wherein the shaft passes through a wall of the conduit and the flap magnet is mounted exterior the conduit.

9. An apparams as claimed in claim 6, 7 or 8, wherein the conduit magnets are symmetrically mounted either side of the flap magnet.

10. An apparatus as claimed in any one of claims 6 to 9, wherein the flap 35 magnet and the conduit magnets are adapted to provide a repelling force therebetween to urge the flap to the rest position.

11. An apparams as claimed in any one of claims 2 to 6, wherein an attracting magnet is placed on the conduit adjacent the flap magnet, when the flap is in the rest position, to attract the flap magnet and urge the flap into the rest position.

12. An apparatus as claimed in any one of claims 2 to 6, wherein a ferro- magnetic metal is placed on the flap and is attracted by the conduit magnet to bias the flap to the rest position.

13. An apparatus as claimed in any one of claims 2 to 6, wherein a ferro- 5 magnetic metal is placed on the conduit and a flap magnet used to attract the flap into the rest position.

14. An apparatus as claimed in any one of claims 2 to 6, wherein a spring a clock spring, has one end attached to the flap and the other end attached to the conduit to urge the flap to the rest position. ╬╣ o

15. An apparatus as claimed in any one of claims 2 to 6, wherein gravity is used to urge the flap to the rest position.

16. An apparatus as claimed in claim 15, wherein a weight is placed on the portion of the flap below the hinge axis.

17. An apparatus as claimed in claim 15, wherein the portion of the flap 15 below the hinge axis is configured to be larger and thereby heavier than that above.

18. An apparatus as claimed in any one of the preceding claims, wherein the flap position sensing means is one or more continuous output Hall Effect sensors mounted on or adjacent the conduit and adapted to issue an output signal in response to their distance from a magnet disposed on the flap.

20 19. An apparatus as claimed in claim 18, wherein the magnet associated with the Hall Effect sensor(s) is also used to provide the biasing force in combination with either conduit magnets or ferro- magnetic material mounted on the conduit.

20. An apparams as claimed in claim 18, wherein the magnet associated with the Hall Effect sensors is disposed remote from any magnets or metal associated

25 with the biasing means.

21. An apparatus as claimed in claim 20, wherein two Hall Effect sensors are each arranged either side of the flap rest position and the difference in their signals used to indicate the flap position.

22. An apparatus as claimed in any one of claims 1 to 17, wherein the flap 30 position sensing means is a linearly variable differential transformer having a first and second part adapted for relative rotation therebetween, wherein the output signal of the transformer being indicative of flap position.

23. An apparams as claimed in claims 1 to 17, wherein one of the first or second parts is non-rotatably mounted to the flap and the other of the first or second

35 parts is non-rotatably mounted to the conduit.

24. An apparatus as claimed in any one of claims 1 to 17, wherein the flap position sensing means includes a light source and a photodetector each arranged either side of the flap, wherein flap rotation alters the amount of light passing the flap and incident on the photodetector which thereby issues a signal indicative of flap position.

25. An apparatus as claimed in any one of claims 1 to 17, wherein an optically encoded disc is attached to the flap having for example a rotary Grey scale and an optical sensor is mounted on or adjacent the conduit and adapted to recognise regions of the scale and thereby indicate the amount of rotation or position of the flap.

26. An apparatus as claimed in claim 25, wherein the optically encoded disc has a Grey scale thereon.

27. An apparatus as claimed in any one of the preceding claims wherein the flap includes a conductive, non ferro-magnetic fin extending radially therefrom, the fin being adapted to pass through a magnetic field to dampen the movement of the flap.

28. An apparatus as claimed in claim 27, wherein the fin is aluminium.