A mass rate of flow meter with a first magnet and a sensing coil
assembly for detecting the passage of an unrestrained rotor past
the sensing coil assembly. The sensing coil assembly includes a
coil wound on a copper coil form that provides phase shifts to start
signals from the sensing coil assembly at low speeds to compensate
phase shifts that are produced in a second coil assembly that generates
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 means for defining
a passage for a fluid, swirl generator means, unrestrained rotor
means and restrained turbine means axially displaced along a first
longitudinal axis extending (through) along the housing means, first
sensing means including magnetic means at a predetermined point
on the rotor means and magnetic flux sensing means located on said
housing for producing a first pulse when said magnetic means comes
into flux exchange relationship with said magnetic flux sensing
means and second sensing means for detecting the passage of another
predetermined point on the rotor means past a predetermined point
on the turbine means, including a further magnetic means and a further
magnetic flux sensing means for producing a second pulse when the
magnetic means in said second sensing means comes into flux exchange
relationship with said further flux sensing means, the interval
between said first and second pulses being a measure of the mass
rate of fluid flow, the improvement comprising means for compensating
for any phase shift in said second pulse due to eddy currents in
said housing when the magnetic means in said sensing means comes
into flux exchange relationship with said further magnetic flux
sensing means including, means for shifting the phase of said first
pulse by an amount equal to any eddy current induced phase shift
in said second pulse including an electrically shorted turn associated
with the magnetic flux sensing means in said first sensing means.
2. A mass rate of flow meter as recited in claim 1 wherein said
support means includes
i. a hollow core composed of a non-magnetic, electrically conductive
ii. an end plate affixed to each end of said core and composed
of a nonmagnetic, electrically conductive material, said coil being
wrapped about said core between said end plates.
3. A mass rate of flow meter as recited in claim 2 wherein said
nonmagnetic, electrically conductive material is copper.
4. A mass rate of flow meter as recited in claim 2 wherein said
flowmeter additionally comprises magnetic core means composed of
a permeable material disposed in said hollow core.
5. A mass rate of flow meter as recited in claim 1 wherein the
magnetic flux sensing means and the shorted turn associated therewith
in the first sensing means comprises:
A. Non magnetic, electrically conductive support means disposed
on the periphery of the housing with a second longitudinal axis
that is transverse to the first longitudinal axis and that passes
through the magnetic means, and,
B. A coil on said support means about the second longitudinal axis.
6. A mass rate of flow meter comprising:
A. A housing means for defining a passage for fluid along a first
longitudinal axis and including an input port and an exit port,
B. Swirl generator means affixed to said housing having its axis
on said first longitudinal axis and positioned adjacent to said
input port for imparting a constant, angular velocity to fluid entering
C. Unrestrained rotor means axially displaced from said swirl generator
means for rotating in response to the angular velocity of the fluid
passing through said housing, said rotor means including first and
second magnetic means located at first and second predetermined
points on said rotor that are axially and circumferentially displaced
with respect to each other,
D. Restrained turbine means spaced axially from said rotor means
and having a magnetically permeable blade means affixed at a predetermined
position on said turbine means, said blade means being closely coupled
to said second magnetic means when the second predetermined point
on said rotor means passes said blade means,
E. First sensor means mounted on said housing at a predetermined
location for sensing the passage of the first predetermined point
on the rotor means and producing a first pulse; in response thereto,
F. Second sensor means mounted on said housing coaxially with said
turbine and axially coextensive with at least a portion of said
turbine means, said second sensor generating a second pulse each
time the second predetermined point on said rotor means passes the
predetermined position on said turbine means, the interval between
said second and first pulses being a measure of the mass rate of
G. Means for compensating for any phase shift in said second pulse
including means for shifting the phase of said first pulse by an
amount equal to any phase shift in said second pulse.
7. A mass rate of flow meter as recited in claim 1 wherein said
support means includes:
a. a hollow coil form core composed of a non-magnetic, electrically
conductive material, and
ii. an end plate affixed to each end of said core and composed
of a non-magnetic, electrically conductive material, said coil being
wrapped about said core and between said end plates.
8. A mass rate of flow meter as recited in claim 7 wherein said
non-magnetic, electrically conductive material is copper.
9. A mass rate of flow meter as recited in claim 7 wherein said
flowmeter additionally comprises magnetic core means composed of
a permeable material disposed in said hollow core.
10. A mass rate of flow meter as recited in claim 9 wherein said
first sensing means additionally includes permeable cover means
mounted to said housing and surrounding said first sensing means.
11. A mass rate of flow meter as recited in claim 10 wherein said
coil form core additionally includes magnetic core means in magnetic
circuit with said cover means.
12. A mass rate of flow meter as recited in claim 6 wherein said
first sensor has a shorted turn associated therewith which includes;
i. Non-magnetic, electrically conductive support means with a second
longitudinal axis that is transverse to the first longitudinal axis
and that passes through said first magnetic means, and
ii. Coil means coaxial with the second longitudinal axis on said
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 flow meter, 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" 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"
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.
In this type of flowmeter, the unrestrained rotor rotates at different
angular velocities. In some applications, the range is from one
revolution per second up to and exceeding six revolutions per second.
Rotor speeds below four revolutions per second are considered to
be low rotor speeds. During various tests, it has been found that
at low rotor speeds the indicated rate of flow is less than the
actual rate of flow through the flowmeter.
This error, in part, arises because the relative timing of the
start and stop pulses produced in the sensing coils is dependent,
in part, upon rotor speed. This dependence can adversely affect
the flow indicator because the timing circuits respond to negative-going
zero crossings of the start and stop pulses. Specifically, it has
been found that flux changes that produce the stop pulses also induce
eddy currents in the aluminium housing. These eddy currents shift
the phase of the stop pulses without a corresponding shift in the
phase of the start pulses. Consequently, the measured time interval
between the start and stop pulses is dependent upon both fuel flow
and rotor speed. As a result, at low flow rates the flowmeter can
indicate a flow rate that is less than the actual flow rate. cl
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 which provides reliable indications of mass flow
at low speeds of the unrestrained rotor.
In accordance with this invention, the above objects are achieved
by improving a sensor that detects the passage of a predetermined
point on the rotor past a predetermined point on the housing and
from which successive start pulses indicate rotor speed. More specifically,
this indicator includes a magnet on the periphery of the rotor and
a coil that is wrapped about a coil support on the housing. The
coil support is composed of a nonmagnetic, electrically conductive
material that acts as a shorted turn and shifts the phase of the
start pulses to compensate the phase shift in the stop signals produced
at low rotor speeds.
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 detailed
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 an enlarged cross-sectional view of a portion of a sensor
that is shown is FIG. 1;
FIG. 3 is a cross-sectional view of a coil form that is depicted
in FIG. 2;
FIG. 4 is a top view of the coil form of FIG. 3;
FIG. 5 is a sectional view taken along lines 5--5 in FIG. 2; and
FIG. 6 is a graph that depicts the improved operating results achieved
with this invention.
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
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 responsive 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
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.
In addition to the turbine blades 51 the turbine 40 carries an
exciter blade 53 of a permeable material and a diametrically opposed,
non-permeable, balancing blade (not shown). 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 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.
FIG. 2 depicts a portion of the rotor 37 and the housing 10 at
the first sensing coil assembly 14. The magnets 46 on the annulus
45 are positioned adjacent to the sensing coil assembly 14. The
shield 15 is composed of highly magnetically permeable material
and mounts around the coil assembly 14 thereby to magnetically shield
the coil assembly from electromagnetic interference. It is affixed
to the housing 10. There is also mounted to the housing 10 a coil
form assembly 60 including a coil form core 61 and end plates 62
and 63. A sensing coil 64 wraps around the coil form core 61 between
the end plates 62 and 63.
Referring to FIGS. 3 and 4 the coil form core 61 has an oblong
shape, thereby to define an oblong slot 65 vertically through the
coil form core 61. The walls of the coil form core 61 have upper
and lower recesses 66 and 67 that accept and position the end plates
62 and 63. As shown more specifically in FIG. 4 the end plate 62
is generally rectangular with radiused corners 70. As is most readily
seen in FIG. 3 the end plates 62 and 63 thereby nest onto the coil
form core 61.
Referring to FIGS. 2 and 5 the coil assembly 14 additionally includes
a magnetic core 71 that reduces the reluctance of the magnetic current
associated with the magnets 46. This magnetic core 71 is L-shaped.
One leg 71A is disposed in the oblong slot 65 through the hollow
coil form core 61. The free end of the leg 71A abuts the housing
10. The other leg 71B extends between and is in contact with the
lower end plate 63 and the magnetic shield 15.
The coil form core 61 and end plates 62 and 63 are composed of
copper. Thus they constitute a coil form assembly 60 that electrically
is a shorted turn. Referring again to FIG. 2 the coil form assembly
60 is positioned with a longitudinal axis 72 through the coil form
60 lying in a plane that is transverse to the longitudinal axis
13 and that intersects the magnets 46 that are centered longitudinally
on the axis 72.
During operation of the flowmeter, eddy currents induced in the
housing 10 produce the phase shift in the signals from the coil
assembly 16 at low rotor speeds. Without the copper coil form assembly
60 shown in FIG. 2 the dashed Graph A in FIG. 6 results. As can
be seen, the timing error increases as the rotor speed decreases.
This means that the flowmeter indicates a flow rate that is less
than the actual flow rate. Graph B in FIG. 6 depicts the error that
exists when the copper coil form assembly 60 of FIGS. 3 and 4 is
utilized. It can be seen that the addition of a coil form assembly
60 constructed in accordance with this invention significantly reduces
the error rate over a rotor speed range from four revolutions per
second to one revolution per second.
These particular graphs are representative of a test that is produced
with a 1200 turn coil 64. In Graph A, the coil was wound on a conventional
coil form assembly of an insulation material and Graph B depicts
the test results when the coil 16 is wrapped about a 65 mil thick
copper core 61 having 15 mil thick end plates 62 and 63. Experimentation
has shown that the phase shift is more responsive to changes in
the thickness of the copper core 61 than it is to the end plates
62 and 63. Thus additional corrections can be achieved by changing
the thickness of the core.
In summary, there is disclosed a copper coil form assembly that
acts as a shorted turn and introduces correcting relative phase
shifts into the timing signals of a restrained reaction turbine
flowmeter, especially at low rotor speeds. As a result, the flowmeter
produces more accurate indications of flow rate over a wider range
of operating conditions. While a specific embodiment of a particular
sensing coil form assembly has been disclosed, 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.