Abstrict A mass rate of flow meter with a first magnet and sensing coil
for detecting the passage of an unrestrained rotor past the sensing
coil. The position of a restrained turbine reactor is sensed by
locating an axially extending magnetic flux linkage near the periphery
of the turbine with an extension that is disposed in an axially
overlapping relationship with a magnet that extends from the periphery
of the rotor. A flux collecting ring on the periphery of the turbine
is coupled to the magnetic flux linkage to provide a low reluctance
magnetic circuit that couples flux from the first magnet to the
sensing coil.
Claims What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. In a mass rate of flow meter including a housing with inlet
and outlet ports for defining a flow passage therebetween, swirl
generating means in the housing at the input port for imparting
angular velocity to a fluid flowing through said housing, a rotor
means in the housing having an input end spaced from the swirl generating
means and an exit end for conveying fluid from the swirl generator
therethrough and for rotating in an unrestrained manner, the rotor
means including first magnetic means at an input end thereof, sensing
coil means disposed on the housing for sensing the passage of the
first magnetic means past the first sensing coil thereby to produce
a first signal, and restrained turbine means having an input end
spaced from the exit end of the rotor means and an output end disposed
adjacent to the output port of the housing, the improvement of means
for sensing the deflection of the restrained turbine, said sensing
means including:
A. second magnetic means disposed near the periphery of the rotor
means at the exit end thereof in a predetermined relationship to
the first magnetic means for generating magnetic flux,
B. magnetic flux linkage means disposed on the turbine means at
a predetermined, circumferential position and having an end portion
extending toward the rotor means,
C. flux collecting means disposed about the periphery of the turbine
and magnetically coupled to said magnetic flux linkage means, and
D. second sensing coil means disposed on the housing and being
axially coextensive with at least a portion of said magnetic flux
linkage means and said flux collecting means whereby the passage
of said second magnetic means by said magnetic flux linkage means
produces a flux change that induces a signal in said second sensing
coil means and wherein the flux collecting means reduces the reluctance
between said magnetic flux linkage means and said second sensing
coil.
2. A mass rate of flow meter as recited in claim 1 wherein the
turbine means has an outer annulus having a longitudinal slot formed
in an outer surface thereof and said linkage means includes a blade
of a permeable material disposed in said slot, said blade including
a main portion that terminates at the input end of the turbine means,
a first end portion spaced from the rotor means and a second end
portion that abuts said flux collecting means.
3. A mass rate of flow meter as recited in claim 1 wherein the
turbine means includes a hub means, a plurality of circumferentially
spaced, radially extending, longitudinal vanes that extend from
the hub means and shroud means about the periphery of the vanes,
at least one of the vanes including a radially extending tab portion
at the outlet end of the turbine means and wherein said flux linkage
means includes a longitudinal vane of permeable material supported
between the hub means and the shroud means and said flux collecting
means includes a ring of permeable material on the periphery of
the vanes and between the shroud means and the tab portion.
Description BACKGROUND OF THE INVENTION
This invention relates to mass rate of flow meters of the angular
momentum type having a swirl generator for imparting swirl to the
measured fluid stream and a torque balance reaction generator for
removing the imparted swirl. More particularly, this invention relates
to such a meter having an improved readout system for indicating
the mass rate of flow.
This invention is particularly adapted for use in a mass rate of
flow meter which utilizes a spring-restrained turbine as the torque
balance reaction generator. One such mass rate of flow meter is
depicted in U.S. Pat. No. 4056976 issued Nov. 8 1977 and titled
Mass Rate of Flow Meter, which patent is assigned to the same assignee
as the present invention. This meter includes a housing that defines
a fluid passage that extends along a longitudinal axis through the
housing and that has an input port and an output port located on
the axis. A swirl generator is located adjacent the input port to
impart a substantially constant angular velocity to an entering
fluid stream. As the fluid leaves the swirl generator, it passes
through an axially displaced, unrestrained rotor that rotates about
the axis. The angular velocity of the rotor accurately represents
the angular velocity of the fluid stream as it leaves the rotor
and passes through an axially spaced, spring-restrained turbine.
The angular momentum of the fluid stream angularly displaces the
turbine about the axis and against the bias of its restraining spring.
Under steady state conditions, this deflection of the turbine is
proportional to the mass rate of flow.
In a spring-restrained flowmeter, the rotor carries two circumferentially
and longitudinally displaced bar magnets. The first magnet is disposed
on the input end of the rotor and is circumferentially poled. A
first sensing coil assembly in a transverse plane through the first
magnet is radially spaced from the magnet and isolated from the
fluid flow. Each time the first magnet passes the first sensing
coil, it induces a "start" voltage pulse in the coil that
indicates the passage of a predetermined point on the rotor past
a predetermined point on the housing.
The second magnet is at the exit end of the rotor and diametrically
opposed to the first magnet. An axially disposed, longitudinally
extending bar of a highly permeable material, such as soft iron,
mounts on the periphery of the turbine. The axial spacing between
the rotor and the turbine interposes an axial air gap between the
bar and the second magnet when they align. A second sensing coil
assembly, that is isolated from the fuel flow, is coaxial with and
longitudinally coextensive with the second magnet and the bar. Each
time the second magnet passes the bar, the flux that the bar couples
to the second sensing coil assembly changes and induces a "stop"
voltage pulse in the second sensing coil. As described in the foregoing
U.S. Pat. No. 4056976 timing circuits convert the start and stop
pulses from the first and second sensing coil assemblies into an
indication of the mass rate of flow through the meter.
The longitudinally extending permeable bar on the periphery of
the turbine forms part of a magnetic path for the flux from the
second magnet that is to link the second coil. The other parts of
the magnetic path include magnetic shields around the second sensing
coil. There is a substantial air gap in this magnetic path so that
the overall reluctance of the magnetic path is relatively high.
In fact, only a small percentage of the total flux from the second
magnet is coupled through this magnetic path. The remaining flux
leaks from the permeable bar to an air path that does not link the
coil. With only a small percentage of the available flux being linked
to the sending coil, the amplitude of the stop pulses is only a
small percentage of the potential maximum amplitude. The resulting
low signal-to-noise ratio can become troublesome, especially at
low rotor speeds when the rate of flux change is reduced thereby
further reducing the potential maximum amplitude. Under some conditions,
therefore, the stop pulses can reach undetectible levels.
SUMMARY
Therefore, it is an object of this invention to provide an improved
mass rate of flow meter with a reliable readout system.
Another object of this invention is to provide an improved mass
rate of flow meter with a reliable readout system that includes
a more efficient magnetic path.
In accordance with this invention the above and further objects
are achieved in a mass rate of flow meter of the angular momentum
type that utilizes a spring-restrained turbine as a torque balance
reaction generator by placing a permeable, annular flux collector
on the turbine. This collector contacts other elements in a longitudinal
magnetic linkage on the turbine and thereby couples the flux from
the magnetic linkage to an annular sensing coil on the housing around
the entire circumference of the flux collector. The resulting magnetic
circuit has a low reluctance, so the amplitude of pulses induced
in the coil is increased.
This invention is pointed out with particularity in the appended
claims. The above and further objects and advantages of this invention
may be better understood by referring to the following description
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal view, in cross section, of a mass rate
of flow meter embodying this invention;
FIG. 2 is a enlarged cross-sectional view of a portion of the mass
rate of flow meter shown in FIG. 1;
FIG. 3 is an end view of a turbine that is shown in FIG. 1; and
FIG. 4 is a sectional view taken along lines 4--4 in FIG. 3.
DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
FIG. 1 illustrates an exemplary flowmeter that incorporates this
invention. It comprises a housing 10 having an inlet port 11 and
an outlet port 12 at the ends of the housing 10 which, with other
elements of the flowmeter, defines a generally annular passage for
a fluid, such as aircraft fuel. The passage is generally disposed
along a longitudinal axis 13. A first sensing coil assembly 14 generates
first timing, or start, pulses and is affixed to the housing 10.
The assembly 14 has a longitudinal axis that is perpendicular to
the axis 13 and is secured in a shield 15.
A second sensing coil assembly 16 generates second, or stop, timing
pulses and is also affixed to the housing 10. The assembly 16 has
a longitudinal axis that is coincident with the axis 13 and includes
a sensing coil 17 that is disposed at a flange 20 at the outlet
port 12. Conductors from both the first sensing coil assembly 14
and the second sensing coil assembly 16 terminate at a connector
assembly (not shown). Both the coil assemblies 14 and 16 are isolated
from the flow of a fluid through the housing 10.
A first inner, or turbine, assembly is radially positioned on the
housing 10 by a housing end flange 20 and an end assembly 22 and
is axially positioned by a retaining ring 21. The end assembly 22
also supports a spring mechanism 23. At the inlet port 11 a second
inner, or rotor, assembly includes a flow straightener 24 that comprises
a plurality of longitudinally extending, circumferentially spaced
vanes 25. The flow straightener 24 is positioned in a tapered bore
26 and is mounted to one end of a shaft 30A. An aligned shaft 30B
is supported by the end assembly 22 and lies on the longitudinal
axis 13.
A forward strut element in the rotor assembly comprises a stationary
annulus 31 and a plurality of struts 32 that extend inwardly from
the annulus 31 and that support a swirl generator 33. The annulus
31 radially positions the rotor assembly and coacts with a retaining
ring 31A to axially position the rotor assembly on the housing 10.
The swirl generator 33 supports the shaft 30A. A flanged ring 34
is carried on the outer surface of the vanes 25 and supports one
end of a variable diameter conduit 35 that includes a plurality
of spring fingers that encircle the swirl generator 33. The conduit
35 acts as a flow reponsive valve. A second ring 36 clamps the conduit
35 and the ring 34 to the vanes 25. This ring 36 also coacts with
the housing 10 to radially position the shaft 30A.
A rotor 37 and a turbine 40 are journaled on shafts 30A and 30B
respectively in an axially spaced relationship. Thrust bearings
41 and 42 support and position the rotor 37 on the shaft 30A; thrust
bearings 43 and 44 the turbine 40 on the shaft 30B. A flat band,
helical spring (not shown) in the spring mechanism 23 is clamped
between the turbine 40 and the shaft 30B to restrain rotation of
the turbine 40 about the shaft 30B.
An outer annulus 45 on the rotor 37 supports a group of permanent
bar magnets 46 in the periphery of the rotor 37. These magnets are
disposed to produce a north-south magnetic axis along a chord near
the periphery of the rotor 37. Each time the magnets 46 rotate past
the sensing coil assembly 14 a start pulse is induced in the coil
assembly 14 that indicates the passage of a predetermined point
on the rotor 37 (i.e., the location of the magnets 46) past a predetermined
point on the housing 10 (i.e., the location of the coil assembly
14).
Referring to FIGS. 1 and 2 another group of permanent magnets
47 also mounts to the outer annulus 45 of the rotor 37. More specifically,
the annulus 45 has an annular extension 50 that extends toward and
overlaps a portion of the turbine, specifically the ends of turbine
blades 51 on the turbine. Longitudinal grooves 52 are cut in the
outer surface of the extension 50 to carry longitudinally extending,
closely spaced, radially poled magnets 47. These magnets 47 also
produce a field with a north-south magnetic axis lying along a chord
near the periphery of the rotor 37.
Now referring to FIGS. 1 through 4 in addition to the turbine
blades 51 the turbine 40 carries an exciter blade 53 of a permeable
material and a diametrically opposed, nonpermeable, balancing blade
53A. An outer band, or shroud, 54 fits over the turbine blade 51
the exciter blade 53 and the balancing blade. The band 54 engages
a flux collecting ring 55 of a permeable material between the band
54 and a radial extension 56 on the turbine 40. The ring 55 bears
against a tab 57 from the exciter blade 53 and a similar tab 58
from the balancing blade.
Each time the magnets 47 pass the exciter blade 53 flux linkages
are coupled to the coil 17 through the exciter blade 53 and the
flux collection ring 55 and induce an electrical stop pulse in the
sensing coil 17 that indicates the passage of another predetermined
point on the rotor 37 (i.e., the location of the magnet 47) past
a predetermined point on the turbine (i.e., the position of the
exciter blade 53). The time between the start and stop pulses is
representative of flow rate.
Referring primarily to FIG. 2 the extension 50 supports the magnets
47 so they rotate in a first circular path. An extension 61 from
a main body 62 of the exciter blade axially overlaps the extension
50 and the magnets 47. However, the extension 61 moves in a second
circular path that is coaxial with the path of the extension 50
so the axially overlapped portions do not interfere during operation.
The air gap between the extension 50 below the magnets 47 and the
extension 61 of the exciter blade 53 and another air gap 63 between
the exciter blade 53 and the ring 54 on one end and the housing
10 in the area of the coil 17 on the other end both comprise radially
extending air gaps. There is no significant axial air gap. Thus,
the inherent stability in the length of the radial air gaps is achieved
so that the length of air gaps is less sensitive to fluctuation
as a result of mechanical vibrations and thermal expansion. The
magnets 47 although coextensive with the coil 17 do not induce
any significant voltage in the coil winding 17 as the rotation of
the magnets 47 does not independently produce flux changes in the
annular coil 17.
Moreover, the permeable flux collector ring 55 shown in FIGS. 1
and 4 further improves the quality of the stop pulses from the coil
17. Specifically, the magnetic circuit associated with the magnets
47 includes the exciter blade 53 the flux collector ring 55 and
magnetic shields that are associated with the coil 17. The presence
of the flux collector ring 55 in close association with the coil
17 about the entire periphery of the turbine 40 greatly reduces
the reluctance of this flux path. As a result a greater percentage
of the total flux from the magnets 47 couples the coil 17 so the
quality of the stop pulses is improved. Specifically the voltage
of the stop pulses, even at low rotor speeds, is reliably detectible.
The flowmeter shown in the FIGURES achieves the objects and advantages
of this invention because the magnetic flux collector operates to
reduce the reluctance of the magnetic circuit that couples flux
to the sensing coil 17. While one specific embodiment of this invention
has been disclosed, it will be apparent to those of ordinary skill
in the art that other embodiments of flowmeters can utilize this
invention. For example, other flux collector ring arrangements could
mounted on the turbine and still provide the improved reluctance
characteristics. Therefore, it is the object of the appended claims
to cover all such variations and modifications as come within the
true spirit and scope of this invention. |