Abstrict A target-type flow meter uses a target in a flowing fluid and selectively
changes the orientation of the target with respect to the direction
of flow of the fluid between two or mote orientations, where the
target provides a different flow impedance in each of the orientations.
This change in flow impedance gives rise to a corresponding difference
in drag forces exerted on the target by the flowing fluid. Those
forces, or displacements associated with them, are measured to determine
the rate of fluid flow. In some cases the target may be a vane attached
to a shaft rotated by a motor. In others, the target may be a vane
structure attached to a post in a flexible fashion so that it can
be oscillated transverse to the flow direction by fixed electromagnets
acting on a permanent magnet portion of the vane structure.
Claims What is claimed is:
1. Apparatus for measuring a rate at which a fluid flows, the apparatus
comprising: a target arranged so that the fluid impinges on the
target when the fluid is flowing, the target having a flow impedance
that varies with its angular orientation with respect to a direction
of the flow; and an electromechanical transducer for both varying
the orientation of the target and for supplying an output to a signal
processor for sensing a variation in drag force associated with
the corresponding variation of the flow impedance of the target.
2. The apparatus of claim 1 wherein the target comprises a rigid
vane extending outwardly from a shaft and the transducer comprises
an electric motor for rotating the shaft.
3. The apparatus of claim 1 wherein the target comprises a vane
attached to a post and the transducer comprises at least one electric
winding fixedly connected to the post and cooperating with a permanent
magnet fixedly attached to the vane.
4. Apparatus for measuring a rate at which a fluid flows, the apparatus
comprising: a target arranged so that the fluid impinges on the
target when the fluid is flowing, the target moving by an amount
that depends on the rate of fluid flow and on an external force
applied to the target; and an electromechanical transducer for supplying
the external force to the target and for supplying an output to
a signal processor for sensing the amount that the target moves.
5. The apparatus of claim 4 wherein the target comprises a rigid
vane extending outwardly from a shaft and the transducer comprises
an electric motor for rotating the shaft.
6. The apparatus of claim 4 wherein the target comprises a vane
attached to a post and the transducer comprises at least one electric
winding fixedly connected to the post and cooperating with a permanent
magnet fixedly attached to the vane.
7. A method of measuring a rate at which a fluid flows, the method
comprising the steps of: disposing a vane extending radially outward
from one of a shaft or a non-rotating post in the fluid; supplying,
by means of an electromechanical transducer, an external force to
change the orientation of the vane; providing, as a representation
of the rate at which the fluid flows, an electrical signal responsive
to a flow-induced drag force on the vane, the flow-induced drag
force varying with the orientation.
8. The method of claim 7 wherein the vane is attached to a non-rotating
post in a compliant fashion and in which the vane is oscillated
by electromagnetic means comprising at least one winding fixedly
connected to the post cooperating with a ferromagnetic portion of
the vane.
9. The method of claim 7 wherein the vane is rigidly attached to
a shaft rotated by an electric motor and wherein the electrical
signal responsive to the flow-induced drag is representative of
an electric current supplied to the motor.
10. The method of claim 7 wherein the vane is rigidly attached
to a shaft rotated by an electric motor and wherein the electrical
signal responsive to the flow-induced drag is representative of
an electric voltage supplied to the motor.
11. The method of claim 7 wherein the step of providing the output
representative of the rate at which the fluid flows comprises the
substeps of acquiring an output signal from the electromechanical
transducer; separating, by means of a signal processor, a time varying
component from the output signal from the transducer; determining
a magnitude of the time varying component; and supplying the magnitude
as the electrical signal responsive to a flow-induced drag force
on the vane.
12. Apparatus for measuring a rate at which fluid flows along a
flow direction past a measurement location at which a support is
disposed, the apparatus comprising: a vane extending from the support
along the flow direction, the vane attached to the post in a compliant
fashion to permit angular oscillation of the vane, the vane comprising
a ferromagnetic portion; at least one electric winding fixedly attached
to the support adjacent the ferromagnetic portion of the vane, the
winding having an axis transverse to the flow direction; a power
supply for supplying an electric current to the winding in order
to move the vane along the axis of the winding and to thereby change
an angle between the vane and the flow direction by an amount responsive
to the flow rate of the fluid; and a signal processor for determining
the flow rate of the fluid from a known functional relation between
the magnitude of the change of the angle, the electric current and
the fluid flow rate.
13. The apparatus of claim 12 wherein the ferromagnetic portion
of the vane comprises a permanent magnet.
14. The apparatus of claim 12 wherein the vane comprises a flexible
sheet of material directly attached to the post.
15. The apparatus of claim 12 wherein the vane is attached to the
post by means comprising an axle and a bearing.
Description FIELD OF THE INVENTION
This invention relates to apparatus for determining the rate of
flow of a fluid by sensing the force exerted by the fluid on a moveable
element in the flow stream.
BACKGROUND INFORMATION
Target meters are commercially available to measure the flow rates
of fluids. They are generally relatively simple, low cost devices
consisting basically of a target, such as a disc, suspended in the
flow stream by a rod that is attached to a force transducer. The
force or torque arising from flowing fluid impacting the disc is
sensed by the transducer, which provides an electrical output signal
responsive to the flow rate of the fluid. These meters provide typical
accuracies specified as several percent of span over a 10 to 1 flow
range. The relatively poor accuracy of these meters at the lower
flow rates limits their applications, and it is an object of this
invention to improve their accuracy.
BRIEF SUMMARY OF THE INVENTION
The above and other objects are attained by target meters in accordance
with various preferred embodiments of the present invention. Preferred
embodiments of the present invention use a target in a flowing fluid
where the orientation of the target to the direction of flow of
the fluid can be selectively changed between two or more orientations,
and where the target provides a different flow impedance in each
of the orientations. This change in flow impedance gives rise to
a corresponding difference in drag forces exerted on the target
by the flowing fluid. Those forces, or the displacement associated
with them are measured to determine the rate of fluid flow.
In some embodiments, the target orientation is changed by continuous
rotation and electrical signals responsive to the variable drag
on that target are processed to provide a measurement of fluid flow
rate. In one preferred embodiment, the target is in the form of
a rigid vane, extending radially outwards from a shaft which is
attached to a transducer and rotated in the fluid flow stream. The
drag forces imposed on the vane by the stream cyclically vary from
a maximum to a minimum and are converted by the transducer into
corresponding electrical signals. The magnitude of only the variational
component of the electrical signal is used to determine flow rate.
The transducer signals, which may form part of its output signal
as may be necessitated by its power requirements, either AC or DC,
or friction related loads, for example, are not used. By this means,
the flow sensing error and in particular transducer zero drift relating
to the conversion of target drag to a flow rate signal at low flow
rates, is reduced thereby enabling the meter to be effectively used
at lower flow rates.
In another preferred embodiment the angular orientation of a vane
in the fluid flow stream is cyclically oscillated in alternate directions
whereby the corresponding variation in drag forces are used by the
transducer to provide flow rate responsive signals as in the first
embodiment.
Although it is believed that the foregoing recital of features
and advantages may be of use to one who is skilled in the art and
wishes to learn how to practice the invention, it will be recognized
that the foregoing is not intended to list all of the features and
advantages. Moreover, it may be noted that various embodiments of
the invention may provide various combinations of the hereinbefore
recited features and advantages of the invention, and that less
than all of the recited features and advantages or the invention,
my be provided by some embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic presentation of the principle components
of a flow meter in accordance the present invention.
FIG. 2 is a side cross sectional view of meter of FIG. 1 showing
greater component detail pertaining to a preferred embodiment of
the present invention.
FIG. 3 is a side view of the vane and transducer assembly of a
preferred embodiment of the present invention.
FIG. 4 is a bottom view section of the configuration of FIG. 3
along lines 4--4.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 schematically depicts some the components of a flow meter
10 comprising a transducer 12 a shaft 14 and a target or vane
18 that the transducer can move angularly as indicated by the arrow
28. A fluid 20 is in contact with and flows past the target 18 in
a flow direction indicated by arrows 22. The transducer 12 is powered
by an appropriate power supply 24 and provides a signal representative
of its mechanical loading to a signal processor 16. The power supply
24 and the signal processor 16 may be integral to or remotely located
from meter 10 and incorporate functions and techniques well established
in the flow measurement art.
During operation of some embodiments of the invention, the transducer
12 provides a rotary motion through the shaft 14 to the vane 18.
As the vane 18 rotates, its angular orientation with respect to
the direction of fluid flow 22 changes. For half of a complete rotation
a fluid flow acts to aid the rotation of the vane 18. For the other
half of the rotation the fluid flow acts to impede the vane's motion.
In this embodiment the transducer 12 provides both the rotary motion
to the vane 18 and an electrical signal representing the torque
load it is accommodating. When the fluid 20 flows, the transducer
12 supplies an electrical signal varying in magnitude according
to the variable loading experienced by vane 18 to the signal processor
16. In the signal processor 16 this varying component of the signal,
being an AC signal, is extracted from any non-varying portion of
the transducer signal with a high pass filter (not shown) and is
then magnitude detected to provide a signal responsive to the fluid
flow rate. This signal is then further processed for linearization,
temperature and other factors as required for the application.
On the other hand, when the fluid 20 is stationary its loading
on the vane 18 is constant and therefore the transducer 12 output
signal will not vary with angle during the course of a complete
rotation. Transducer-related signal drifts affected by factors such
as friction, temperature and time tend to change relatively slowly
with respect to the rotational rate of a vane 18 and will be eliminated
by the high pass filter in the signal processor 16. As a result,
the output signal from the signal processor 16 at zero flow rate
is more stable than that of prior art target meters which utilize
transducer signals from an essentially fixed target. With well defined
and stable zero and span calibration points, the meter 10 is therefore
useful over a broader operating range of flow rates and may be specified
for providing flow measurement accuracies as a percentage of flow
rate rather than of span, as is typical of the prior art meters.
The transducer 12 is conveniently configured as a single device
providing both the rotary motion and sensing the drag effects of
the fluid load on the vane 18. Alternately, the transducer 12 may
have separate components for each function. Furthermore, a complete
three hundred sixty degree rotation of the vane 18 is not necessary
for it to experience a variable loading responsive to fluid flow.
For example, the vane 18 could be oscillated by some acute angle--e.g.,
thirty degrees--about a central position in which it was in alignment
with the flow direction 22 in order to sense fluid loading effects.
A relatively large vane could also be used in an arrangement in
which its rotational angle with respect to the fluid flow direction
22 is controlled to provide a relatively constant flow induced drag.
For example, a vane 18 could be angled at thirty degrees with respect
to the flow direction 22 at low fluid flow rates and at a lesser
angle of, say, five degrees at a high fluid flow rate in order to
produce the same drag as was experienced at the low flow rate. This
has the effect of increasing the measurement range of the meter
and improving its linearity. The oscillating vane 18 may, in any
case, be driven in both directions from the central position.
FIG. 2 depicts one embodiment of the flow meter 10. In this case
the output from a motor 30 drives a gearbox 32 to rotate a drive
magnet 34 housed within a protective barrier 36 used to seal the
drive components off from wetted portions of the apparatus. On the
other side of barrier 36 a slave magnet 38 is attached by means
of a shaft 14 supported by bearings 40 to a vane 18. A suitable
housing 48 encloses the meter components and mechanically couples
the apparatus to a pipe or other flow passage 46 in which fluid
20 flows. Although the examples described herein are conveniently
arranged to address the flow of fluid in a stationary pipe to which
a sensing apparatus is fixedly attached, those skilled in the art
will recognize that the same discussion applies to a variety of
measurements of flow relative to a measurement location at which
a flow target support is disposed. Such measurements embrace, but
are not limited to, measurement of flow in open channels; measurement
of water currents flowing past an inertially fixed structure, such
as a dock; and flow of water past the hull of moving ship, in which
case the flow measured is that of the ship relative to the water.
In the embodiment depicted in FIG. 2 a preferred motor 30 has
operating power requirements that vary over a wide range in response
to load changes. An example of such a motor is the type 1616E018ST
manufactured by MicroMo Electronics, Inc. When used with a 10 volt
power supply this motor requires only 4 milliamperes when unloaded
but over 40 milliamperes fully loaded. The motor 30 drives a gear
train 32 which increases its output torque and reduces its rotational
speed. This causes parasitic motor generated signals, such as commutation
ripple, to be much higher in frequency than the rotational speed
of the vane 18 and thus allows them to be easily filtered out. A
MicroMo gear train type 16AK with an 11.8:1 ratio is for example,
attached to the above motor rotates the drive magnet 34 which is
magnetically coupled through protective barrier 36 to the slave
magnet 38. The slave magnet 38 is coupled by the shaft 14 to rotate
the vane 18. In this embodiment the current supplied to the motor
is representative of the shaft load and therefore of the flow-induced
drag. Those skilled in the arts will recognized that one could alternately
select a motor operated from a constant current source that produced
a voltage variation responsive to load changes. Constant current
operation is also advantageously used if the vane 18 is made to
oscillate so that the magnitude of its deflection is responsive
to loading effect of the fluid flow.
The rotational rate of vane 18 is typically slow and may range
from a fraction of a turn per second to several turns per second.
The off-center-of-gravity load represented by the vane 18 may be
balanced by a compensating weight either on the shaft 14 or on the
slave magnet 38. A small amount of mechanical vibration is normally
present in any flow meter application so that the slave magnet 38
will tend to, on the average, be only slightly affected by mechanical
friction and will smoothly follow the movement of the drive magnet
34 thus resulting in a precise transfer of torque between the gear
train 32 and vane 18 at both high and low fluid flow rates. Because
the variable torque loading on the motor 30 is transformed by the
motor into a flow rate responsive signal, the variation in angular
displacement between the drive and slave magnets 34 and 38 which
occurs at different angular locations of the vane 18 does not seriously
affect the precision of the flow rate measurement. As a result of
the above, the meter 10 may be rugged and incorporate relatively
large bearings 40 for long life and the ability to withstand the
impacts which may be present, for example, during startup conditions
when a liquid line is not full, or a steam line when solids or slugs
of condensate are circulated.
The motor 30 will tend to have a higher shaft rotational rate when
torque due to the flowing fluid is aiding the rotation and a lower
shaft rotational rate when the flow is opposing the rotation. The
resulting speed change, which reduces the dynamic range of the motor
and may add further nonlinearities to the meter response, may be
minimized by operating the motor 30 at a regulated constant speed.
However, if the motor 30 speed is allowed to vary with vane loading
the time differential between the rotational half cycle in which
the vane torque aids the rotation and the rotational half cycle
when that torque is in opposition can also be used to produce a
signal responsive to flow rate.
Although the transducer 12 is depicted in FIGS. 1 and 2 as being
connected to the vane 18 by means of a shaft 14 the transducer
may also be located close to the vane 18 and may even directly engage
it in some embodiments of the invention. An example of such an arrangement
is illustrated in the vane and transducer assembly of FIGS. 3 and
4 which comprise a preferred embodiment of the present invention.
FIG. 3 is a side view of such an assembly while FIG. 4 shows a central
cross section taken as shown by the double-headed arrow 4--4. In
this embodiment both a streamlined wetted transducer enclosure 72
and a vane 18 are extend radially outward from a non-rotating post
52. The vane 18 in this embodiment, is attached to the post 52
in a compliant fashion in order to permit angular oscillations of
the vane as indicated by the double headed arrow 74 in FIG. 4. The
compliant attachment can take many forms, including the depicted
combination of ell-shaped axle members 54 and bearings 56 by springs,
or by directly attaching a vane made of a flexible sheet of material
to the shaft 52. Moreover, the post 52 may readily be configured
as a bluff body so as to efficiently produce a street of Karman
vortices that would affect the vane 18.
In the preferred embodiment depicted in FIGS. 3 and 4 the transducer
12 comprises a magnet 62 attached to the vane 18 that cooperates
with two electromagnets 64 66 supported by the wetted enclosure
72 and energized to produce magnetic flux of opposing polarities
in the space between them. These electromagnets may lie on the same
axis, as depicted, or may be tilted away from that axis so as to
be approximately tangent to the arc along which the permanent magnet
62 moves when the vane oscillates.
When the electromagnets 64 and 66 are energized to provide magnetic
fields with the polarities indicated in FIG. 4 the permanent magnet
62 is attracted to move up further into the magnet 64 at the same
time it is being pushed out from magnet 66 below. When the electrical
current through electromagnets 64 and 66 is reversed, the polarities
of their magnetic fields also reverse and the magnetic forces act
on the permanent magnet 62 to move it in the opposite direction.
The vane 18 being fixedly attached to the permanent magnet 62
will oscillate as indicated by the double headed arrow 74. These
movements provide the angular displacement of vane 18 required for
implementing the present invention. Those skilled in the art will
recognize that other electromagnetic transducers could be used in
place of the preferred embodiment depicted in the drawing. Alternate
versions include, but are not limited to, one comprising a ferromagnetic
portion of the vane, which may comprise a permanent magnet or other
piece of metal attached thereto, arranged to cooperate with one
or more windings fixedly attached to the enclosure in order to oscillate
the vane.
If the permanent magnet 62 is of the alnico, samarium-cobalt or
neodymium-iron-boron types, its presence within the magnetic fields
of the electromagnets 64 and 66 substantially increases their electrical
losses at high frequencies, for example several hundred kilohertz.
Ferrite magnets, depending on their composition, may have a similar
characteristic or may reduce the high frequency losses. The electrical
signals used to produce the magnetic fields that operate to oscillate
the vane are of much lower frequency, so that the magnetic generating
and loss sensing signals can be easily separated by filtering. These
losses are responsive to the penetration depth of the permanent
magnet 62 into the electromagnets 64 and 66. By detecting these
losses, a means is provided for determining the mechanical displacement
of permanent magnet 62 and therefore the angular displacement of
the vane 18. This is useful in a meter operational mode in which
the forces producing the vane's angular deflection are constant,
and the vane's deflection angle is responsive to the fluid induced
drag forces used for determining fluid flow rate, or where a specific
vane angular displacement is maintained and the forces required
to provide that displacement are used to determine fluid flow rate.
A regulated constant current through the electromagnets is a convenient
way of providing precision control of the forces needed to produce
the angular displacement and, just as in the case with the rotated
vane, a transducer having a directly driven vane may be operated
in several modes.
Alternately, the apparatus depicted in FIGS. 3 and 4 may be used
in sensing arrangements in which fluid flow variations, rather than
electromagnetic forces, drive the vane 18. These flow variations
may be due, for example, to fluidic oscillators or vortex shedding,
as may be the case if the post 52 is configured as a bluff body.
Electrical signals responsive to angular movements of the vane 18
are then generated by motion of the permanent magnet 62 within the
electromagnets 64 and 66 and can be to be electrically processed
to become output signals representative of fluid flow rate. High
frequency signal processing may also be used to determine the angular
translation of the vane 18 if desired. |