Note: Descriptions are shown in the official language in which they were submitted.
MFP/M-8181 336784
1 CURRENT TO PRESSURE TRANSDUCER
2 EMPLOYING MAGNETIC FLUID
s
7 FIELD OF THE I~VENTION
8 This invention relates to the regulation of air
9 pressure in response to electrical signals and in particular
to a transducer for converting an electrical current to a
11 corresponding pressure in a system that uses compressed air.
12
13 BACKGROUND OF THE INVENTION
14 Description of the Prior Art
Compressed air is used in many systems for controlling
16 machinery because compressed air is immune to electrical
17 interference and is safe in explosive environments.
18 Compressed air is generally used, for example, to control
19 valves and other mechanical devices in industrial systems.
When using compressed air in a system, sensors are generally
21 provided that generate small electrical currents, in the
22 range of 4 to 20 milliamperes, for example. These currents
23 are used to establish a corresponding pressure of the
24 compressed air and to provide a sufficient volume of
pressurized air for accomplishing the desired mechanical
26 task. In some systems, the conversion from electrical
27 current to a corresponding pressure is accomplished by use
28 of a current-to-pressure transducer that is capable of
29 regulating the pressure of a small volume of air, wherein
the volume of air is amplified by using standard pneumatic
31 amplifiers. In the conventional current-to-pressure
32 transducer, a nozzle is supplied that directs compressed air
33 to the atmosphere at a rate determined by the proximity of a
34 flapper valve to a nozzle orifice. The flapper valve is
generally mounted on a rotating suspension and is rotated by
36 magnetic forces that are generated by an electromagnet. The
37 flapper is rotated toward the nozzle so that the air that
38 escapes to the atmosphere is reduced. Such prior art
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MFP/M-818
devices are formed as delicate mechanical assemblies that
require several adjustments during fabrication and are
relatively expensive to produce.
It is highly desirable to employ a simple current-to-
pressure transducer that lends itself to facile production
at low cost without the need for individual mechanical
adjustments.
A nozzle is a converging or converging-diverging tube attached
to the outlet of a pipe, hose or pressure chamber. The
purpose of the nozzle is to convert the pressure existing in a
fluid into velocity efficiently. A nozzle allows a pressure
to be carried in a pipe or hose adjacent to the nozzle.
Presently known nozzles used for controlling air flow
generally terminate with an outer diameter slightly larger
than the inner diameter. Typically, the outer diameter of a
nozzle would be 0.035 inch and the inner diameter would be
0.02~ inch, by way of example. The current-to-pressure
transducers that use such type nozzles usually incorporate a
flapper, which is a pivotable paddle-shaped part, or a
diaphragm to vary the flow of air through the nozzle. In
either case, it is necessary that a good seal be provided at
the end of the transducer from which there is the high flow of
air or fluid. In order to achieve the required good seal, the
flapper or diaphragm must be precisely aligned in a plane that
is perpendicular to the axis of the nozzle. If the alignment
is not proper, the flapper or diaphragm will first strike an
edge of the nozzle end and will not advance further towards
making an effective complete seal. It is relatively difficult
to provide the desired orthogonal alignment of the flapper or
diaphragm in a planar orientation relative to the nozzle axis.
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SUMMARY OF THE INVENTION
An object of this invention is to provide a current-
to-pressure transducer that regulates the air pressure within an
air supply line. Henceforth, the word "pressure" will be used
to mean the pressure relative to the environment.
Another object of this invention is to provide a
transducer that is relatively efficient and has uniform
characteristics so that individual adjustments need not be made.
Another object is to provide a current-to-pressure
transducer that can be mass produced at low cost.
Another object is to provide a current-to-pressure
transducer that is insensitive to shock and vibration.
In accordance with this invention, there is provided
a current-to-pressure transducer comprising a baseplate or
housing made of magnetic material; at least one chamber formed
in said baseplate, said chamber having at least one open end; at
least one flexible membrane or diaphragm for sealing said open
end; a volume of magnetic fluid contained within said chamber;
air supply means disposed closely adjacent to said flexible
membrane or diaphragm for supplying air; electromagnetic means
comprising a magnetic circuit and a coil including permanent
magnet means for receiving an input current to provide a
magnetic field closely adjacent to said flexible membrane or
diaphragm for displacing said flexible membrane or diaphragm
relative to said air supply means so that the pressure of said
air in said air supply means is regulated in accordance with
said input current, wherein the displacement of said flexible
membrane or diaphragm is accomplished with the volume of said
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magnetic fluid conserved and maintained constant and wherein
said displacement is not opposed by ambient pressure.
In accordance with another aspect of this invention,
there is provided a current-to-pressure transducer comprising
means for supplying a flow of air; first and second chambers and
a means connecting said chambers, each of said chambers having
an open end; magnetic fluid disposed within the said chambers
and said connection means; first and second flexible diaphragms
respectively positioned against said open ends for containing
said fluid within said chambers; and electromagnetic means
energized by an input electrical current for coacting with said
magnetic fluid to deform a selected one of said diaphragms
thereby varying the air pressure in said supply means.
The transducer can regulate the air pressure within an
air supply line so that the pressure differential between this
line and ambient air pressure varies substantially linearly with
an applied electrical current. In one
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MFP/M-818 1 3 3 6 7 8 4
embodiment pressure sensing means and electronic feedback
are used to achieve the desired linearity between the
pressure within the line supplying air to the nozzle and the
electrical input current.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in greater detail with
reference to the drawing in which:
Figure 1 is a side cross-sectional view of a current-
to-pressure transducer, made in accordance with this
invention;
Figure 2 is a side cross-sectional view of an
alternative implementation of the current-to-pressure
transducer of this invention;
Figure 3 is a cross-sectional view, partly broken away,
of another implementation of the invention;
Figure 4 is an enlarged isometric view, partly broken
away, showing the relationship of a pole piece 11 to the
nozzle 10 and the nozzle air supply line 24, as used in the
transducer of this invention; and
Figure 5 is a representative curve plotting pressure
against current without electronic feedback to aid in the
explanation of the operation of the current-to-pressure
transducer.
Figure 6 is a side view of the nozzle and pole piece, made ln
accordance with this invention;
Figure 7 is an end view at the slotted end of the nozzle and
pole piece structure, such as illustrated in Fig. 6 ;
Figure 8A is an enlarged cross-sectional view, partly broken
away, taken across lines A-A' of Fig.7 ;
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Figure 8B is an enlarged cross-sectional view, partly broken
away, taken across lines B-B' of Fig.7 ;
Figure 9 is an exploded view of an assembly drawing
illustrating the housing which encloses the nozzle and pole
piece structure; and
Figurel0 is an isometric view illustrating an assembled
housing which encloses the nozzle and pole piece structure of
this invention.
Similar numerals refer to similar elements throughout the
drawing.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to Fig. 1, the current-to-pressure
transducer of this invention receives compressed air from a
pneumatic amplifier 32. The current-to-pressure transducer
includes an air supply line 24 having a nozzle 10 at one end
and is coupled at the other end to the pneumatic
amplifier 32.
A baseplate 19 made of magnetically soft material is
joined to a magnetic member 20 by a screw 37 or other
suitable means. A magnetic member 21 is attached to the
member 20 by a screw 38 or other attachment means. A
cylindrical magnetic member 22 is positioned in contact with
and partly within an aperture of the magnetic member 21. As
MFP/M-818 1 3 3 6 7 8 4
1 illustrated in Fig. 4, a pole piece 11 is provided at the
2 upper end of the tubular member 22.
3 A coil 23 is wound around a portion of the magnetic
4 member 22. Baseplate 19, magnetic members 20, 21 and 22,
the gap between pole piece 11 and the magnetic fluid 30 in
6 chamber 14 and the magnetic fluid 30 in chamber 14 form a
7 magnetic circuit. When current is applied to the coil 23,
8 the magnitude of the magnetic flux at the pole piece 11 is
9 varied in accordance with the magnitude of the current
10 signal.
11 The baseplate 19 is formed with two chambers 14 and 16
12 that are connected by a capillary tube 15. In keeping with
13 this invention, a magnetic fluid 30 which may be a colloidal
14 suspension of magnetic particles in a nonmagnetic carrier,
such as Ferrofluid (a trademark of Ferrofluidics
16 Corporation, Nashua, New Hampshire) is provided to
17 chambers 14 and 16 and the capillary tube 15. The magnetic
18 fluid 30 may also be any composite, noncolloidal material
19 that is not capable of supporting shear forces and that
exhibits a magnetic susceptibility. A plug 18 is provided
21 to enable filling the capillary tube 15 and chambers 14 and
22 16 with magnetic fluid 30.
23 In accordance with this invention, flexible membranes
24 or diaphragms 13 and 17 are located respectively at the
lower open ends of the chambers 14 and 16 to seal the ends
26 of the chambers and to contain the magnetic fluid 30 within
27 the chambers. The flexible membranes 13 and 17 are retained
28 by a nonmagnetic retainer element or ring 25 which abuts the
29 diaphragms. The element 25 is fastened at its exposed
surface to the baseplate 19 by screws or other suitable
31 means-
32 In operation, the pneumatic amplifier 32 provides
33 compressed air through the air supply line 24 to the
34 nozzle 10. The air passes through the space between the
diaphragm 13 and the surface of the nozzle 10. Escape
36 holes 12 or other suitable means are provided in the upper
37 portion of the magnetic member 22, as shown in Fig. 4 to
38 prevent undesirable pressure buildup between the pole
MFP/M-818 1 3 3 6 7 8 4
1 piece 11 and diaphragm 13. An air pressure sensor 26 senses
2 the pressure of the compressed air that is passing through
3 the air supply line 24 and generates a signal representative
4 of the pressure value. The signal is provided to an
electronic feedback circuit 27, which also receives the
6 input current through lead 34. The input current and the
7 signal representative of the pressure value are compared in
8 the circuit 27 and a current representative of this
9 comparison is provided to the coil 23. The electronic
feedback circuit 27 adjusts the actual current to the
11 coil 23 so that the pressure in the air line 24 is
12 substantially linear with the input current. The input
13 current during operation maintains the coil in an excited
14 state and as a result the pole piece 11 distributes magnetic
flux in the area adjacent to the diaphragm 13. The
16 magnitude of the magnetic flux emanating from the pole
17 piece 11 varies with variations in the current supplied to
18 the coil 23. The magnetic fluid 30 in chamber 14 is
19 attracted towards the pole piece 11 and the diaphragm 13 is
deformed to an extent directly related to the magnitude of
21 the current which is applied to the coil 23. The
22 diaphragm 13 deforms and moves partially towards the pole
23 piece 11 so that the space between the diaphragm 13 and the
24 nozzle 10 decreases. As a result, the pressure of air
within the air line 24 supplying air to the nozzle 10 is
26 increased. During the deformation of the diaphragm 13
27 resulting from the magnetic fluid 30 being moved towards the
28 pole piece 11, the volume of magnetic fluid that is
29 displaced in chamber 14 associated with the displacement of
diaphragm 13 is provided from chamber 16 to chamber 14. The
31 diaphragm 17 moves inwardly to the chamber 16 in an equal
32 and opposite direction to diaphragm 13.
33 With reference to Fig. 2, the air supply line 24
34 including the nozzle 10 is located under the chamber 16.
The coil 23 and the associated magnetic members 20, 21, 31,
36 baseplate 19 and pole piece 11 remain in association with
37 the chamber 14 for coaction with the diaphragm 13. An
38 increase in current to the coil 23 causes the diaphragm 13
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MFP/M-818
1 to deform toward the pole piece 11 and the volume of
2 magnetic fluid that is displaced from chamber 16 to chamber
3 14 causes the diaphragm 17 to move away from the nozzle
4 10. Consequently, the pressure of the air in line 24 is
decreased. In this embodiment of Fig. 2, the air line 24 is
6 made of nonmagnetic material such as aluminum, or
7 alternatively is magnetically isolated from the magnetic
8 circuit which includes the coil 23 and the magnetic
9 member 22, inter alia.
A feature of this invention is the insensitivity to
11 gravitational or acceleration forces. Because the magnetic
12 fluid 30 is relatively incompressible, the diaphragms 13 and
13 17 move equally in opposite directions. In those
14 embodiments in which the diaphragms are coplanar, the
transducer is insensitive to forces that are applied
16 perpendicularly to the plane of the diaphragms. The
17 transducer is relatively insensitive to forces that are
18 applied perpendicularly to the plane of the drawing and a
19 line through the centers of the chambers, irrespective of
whether the diaphragms are coplanar. Also the viscous
21 damping that is associated with the transport of the
22 fluid 30 through the capillary 15 causes the transducer to
23 be insensitive to shock in any direction. The damping is
24 enhanced as the viscosity of the magnetic fluid 30 is
increased and the conductance of the capillary 15 is
26 decreased. Damping also can be used to limit the high
27 frequency response of the transducer.
28 As illustrated in Fig. 3 in another implementation of
29 the invention, a further increase in sensitivity is achieved
by affixing an element 28 made of a solid, magnetically soft
31 material, such as iron, to the center of the diaphragm 13.
32 The element 28 is disposed within the magnetic fluid in
33 chamber 14 and has a higher saturation magnetization than
34 the magnetic fluid. The element 28 also can provide
stiffness to the central portion of the diaphragm. To
36 preserve shock insensitivity, an element 29 is located in
37 the chamber 16 and is affixed to the diaphragm 17. The
38 element 29 may be substantially identical to the element 28,
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or may be of a nonmagnetic material with suitable size and shape
to achieve the desired insensitivity.
Fig. 5 is a curve representing the changes in pressure
(psi) as a function of current (milliamps). In an actual
implementation of the invention, a 450 Gauss, 400 cp Ferrofluid
was used. The element 28 is a steel slug of 3/8-inch diameter
and 3/16-inch long cemented to the diaphragm 13 with RTV
silicone sealant. The air supply pressure is 18 psi. It should
be understood that these parameters, materials and dimensions
are exemplary and the invention is not limited thereby.
In an alternative approach the transducer comprises a
single chamber and a single flexible diaphragm. In such case an
air space is provided above the level of the magnetic fluid to
allow displacement of the diaphragm.
It should be understood that chambers formed within a
baseplate or housing are equivalent to chambers formed by
concave diaphragms sealed against a flat baseplate.
In another approach, nozzle 10 is made of magnetic
material and functions also as pole piece 11. In this approach,
the outer coaxial member 11 and holes 12 shown in Figure 4 are
eliminated.
The novel current-to-pressure transducer disclosed
herein employs a magnetic fluid to coact with flexible
diaphragms disposed in close juxtaposition to a nozzle of an air
line. The transducer lends itself to mass production and low
cost, is efficient in operation, and does not require individual
adjustments.
In accordance with this invention, a nozzle and pole
piece structure is formed from a rod 40 made of a magnetic
material, such as Carpenter High Permeability "49" Alloy, for
example. The rod 40 has a diameter of about 3/16 inch in this
particular embodiment. As illustrated in Fig. 6, the magnetic
rod 40 is formed with functional pole pieces 41 at a slotted end
of the rod 40. Slots 43 allow the escape of excess air and are
relatively easy to machine, with saw blades or slot cutters, as
compared to the formation of individual bleed
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1 holes 12 and the associated deep circumferential groove
2 between elements 10 and 11 to which the holes connect, shown
3 in Fig. A-. The pole pieces 4~ provide magnetic flux for
4 coaction with electric current flowing through the electrical
coil (not shown) of the electromagnetic circuit. Application
6 of electric current to the coil causes the magnetic fluid 30
7 to move which, in turn, causes the deformation of the flexible
8 diaphragm as explained heretofore thereby controlling air flow
g through the nozzle .
11 The rod ~0 has a threaded part 42 for engagement with a
12 threaded cap 44 of a housing assembly, shown in Fig. ~ The
13 rod 40 also has a hexagonal part 46 formed at the end ddjacent
14 to the threaded part 42 to allow the rod to be turned so that
the height of the nozzle relative to the diaphragm can be
16 adjusted for proper operation, and locked with nut 68.
17
18 Figure 4 shows an end view of the nozzle and pole piece
19 structure 40 viewed from the slotted end. The rod 40 is
formed with one or more of the longitudinal slots 43, which
21 extend inwardly to at least the outer diameter of a relatively
22 shallow groove 46 formed within the end of the rod 40. The
23 slots 43 serve the same purpose as the holes 12 depicted in
24 Fig. 4 to allow the escape of excess air, but are easier to
machine and fabricate than the transverse holes. Groove 46
26 may be eliminated if the slots 43 extend inward to the
27 proximity of the constricted passage 49.
28
29 As depicted in Figs. 8A and 8~B, an open channel 48 is formed
within the interior of the nozzle tube 46 to allow the passage
31 and escape of air. The nozzle tube 46 may be tapered at the
32 end portion that faces the diaphragm so that a constricted
33 passage 49 is formed at the end of the nozzle channel 48. The
34 constricted portion 49 of the channel 48 (Figs. 8A and 8B)
reduces the volume of air that escapes from the nozzle at a
36 given pressure. The amount of air flow from the channel
37 portion 49 is regulated by the position of the diaphragm 13,
38 which is con~ro~le~ by the action of the magnetic fluid in
39 response to the electric current supply to the coil of the
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1 electromagnetic circuit.
3 The exploded view of ~ig.9 shows the main housing 50 for the
4 integral nozzle and pole piece structure which is made of soft
iron. Diaphragms 52 and 54 are spaced by a soft iron spacer
6 56 formed with magnetic fluid chambers 58 and 60. O-ring
7 seals 62 and 64 are provided with the chambers. A threaded
8 aluminum retainer 66 is located adjacent to the diaphragm 54
g for connection to the spacer 56. A lock nut 67 is located
against the retainer 66 and four cap screws 70 tie the spacer
11 56 and retainer 66 with the diaphragms 52 and 54 to the main
12 housing 50. A second nozzle ~not shown) may be threaded into
13 retainer 66 to coact with diaphragm 54, as described in the
14 referenced copending application.
16 At the other end of the housing 50, the threaded element 44,
17 which is made as a soft iron cap with internal threads for
18 engaging the nozzle, is joined with a lock nut 68 by means of
19 four Allen socket cap screws 72 to the main housing 50.
21 Fig.lO depicts the assembled unit which has a notch 74 in the
22 housing 50 to allow connection of electrical circuitry to the
23 electrical coil of the electromagnetic circuit and to permit
24 escape of excess air.
26 By virtue of the integral structure of a nozzle and pole piece
27 which are machined from a single magnetic rod, the end of the
28 nozzle tube 46 and the end of the pole piece 41 are
29 substantially coplanar. When the electromagnetic force is
applied to the top surface of the diaphragm 13 by the magnetic
31 fluid 30, the lower surface of the diaphragm conforms to the
32 shape of the pole piece 41. Since the alignment of the ends
33 of the pole piece 41 and nozzle tube 46 are in substantial
34 planar alignment, the diaphragm will provide a complete seal
at the face of the nozzle. Wlth the present design, the
36 torque applied to the flapper valve acts through a point
37 further from the noæzle than the point of first contact
38 between the flapper valve an~ nozzle 10, and if the flapper
39 valve does no~ contact the nozzle squarely, further torque
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1 will only distort the flapper valve and worsen the incomplete
~ seal. With the nozzle and pole piece structure design as
3 disclosed herein, any canting of the diaphragm 13 is self-
4 corrected because the force on the diaphragm acts between a
point of first contact of the diaphragm 13 with the end of the
6 larger diameter pole piece 41 and the coplanar end of the
7 nozzie tube. The integral nozzle and pole piece structure
8 also is easier to fabricate with the slots 44 formed at the
9 end of the rod structure to allow the desired air escape
instead of with transverse holes as used in prior nozzle
11 assemblies. Such transverse holes either require a difficult
12 process to machine a deep groove between the nozzle 10 and
13 pole piece 11, or require fabricating the nozzle 10 and pole
14 piece 11 separately, in which case it would be difficult to
assemble these parts to achieve the desired coplanarity.
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