A drag-type flow meter is disclosed of the type having a drag plate
disposed in the fluid flow stream, the velocity of which is to be
measured. The drag plate is mounted on a cantilevered arm which
is pivitally mounted externally of the fluid flow passage, with
a tensioned wire counteracting the drag force tending to pivot the
arm. The tensioned wire is disposed in a magentic field and an electrical
current is passed through the wire to cause vibration of the wire,
the frequency of which varies linearly with the drag force exerted
on the drag plate due to the corresponding change in wire tension.
Varying frequency of the current provides a digital electrical signal
which varies linearly with fluid flow velocity. Embodiments are
described enclosing the wire mounting components in a vacuum housing
in order to minimize the air damping of the wire vibration, and
a mounting arrangement minimizing the effects of temperature variation
on the output signal.
The embodiments of the invention in which an exclusive property
or privilege is claimed are defined as follows:
1. A fluid flow meter for generating a linearly varying output
signal having a linear variation with respect to the flow of the
fluid, said meter comprising:
a conduit having a passage through which a fluid flows;
drag body means having an arm extending through the wall of said
conduit and pivotally connected thereto, a drag body connected at
one end of said arm and positioned transverse to the direction of
the flow of the fluid, said arm pivoting in response to the flow
of the fluid against said drag body;
a vibrating wire sensor having a tension wire operatively connected
to the other end of said arm and responsive to the pivotal motion
of said arm for varying both the longitudinal tension in said wire
and the frequency of vibration of said wire;
said wire and said drag body means being fabricated from materials
having matching coefficients of expansion; and
electrical signal generating means responsive to the frequency
of vibration of said wire sensor and operable for generating an
electrical signal varying linearly with respect to the flow of fluid
against said drag body.
2. The fluid flow meter according to claim 1 further including
means enclosing said vibrating wire sensor in a vacuum for minimizing
the damping effects of the atmospheric air pressure on said vibration
of said wire and means for sealing said arm in said enclosing means
throughout the range of pivotable motion of said arm.
This invention concerns fluid flow meters of the type used to measure
various flow parameters as fluid flow velocity and mass rate of
It has long been known to measure fluid flow velocity in a flow
stream by disposing a drag body or target in the flow stream and
measuring the drag force exerted on the body by the fluid flow by
various arrangements. The drag force varies with the velocity of
flow and thus provides a measure of the average fluid flow velocity
in the stream.
This approach, while simple, has the disadvantage of yielding an
analog output signal which varies nonlinearly with fluid flow. That
is, the drag force varies with the square of the velocity of fluid
flow. Also, the analog form of the output signals is relatively
inconvenient for handling in digital signal processing circuitry.
It has also been known in the prior art to utilize flow meters
which have included means for causing oscillation or turbulence
of the fluid in a conduit, the frequency of which is proportional
to the fluid velocity. The oscillations are measured by various
arrangements, the frequency of which are related back to the flow
velocity. This, of course, produces a relatively large pressure
loss due to the requirement for substantial turbulent flow about
A typical such arrangement is disclosed in Jannsen et al U.S. Pat.
It is accordingly the object of the present invention to provide
a simple drag force measuring flow meter in which the output signal
is in digital form and in which there is established a linear relationship
between the output signal and the flow rate.
SUMMARY OF THE INVENTION
This and other objects of the present invention, which will become
apparent upon a reading of the following specification and claims,
are achieved by an arrangement wherein the drag body is mounted
on a pivoted arm, in turn drivingly connected to a tensioned, electrically
conductive wire, the tension of which is varied to a degree by the
drag force. The tensioned wire is disposed in a magnetic field and
an electrical current caused to flow through the wire to produce
a vibration of the wire by the force generated by the magnetic field,
which varies with the degree of drag exerted on the wire force by
the varying of the tension of the wire.
The relationship between the tensioning of the wire and the effect
on the frequency of vibration is such that the output signal, i.e.,
the varying frequency of vibration of the conductor wire, is related
linearly to the fluid flow rate over the drag body. The output signal
being in the form of a frequency signal is easily digitized to thus
achieve the above-recited object of the invention.
The wire is preferably disposed within a housing which is sealed
so as to enable evacuation thereof and to thus minimize the effects
of air dampening on the vibration of the wire. Various mounting
arrangements for the wire are disclosed in which the change in tensioning
of the wire due to temperature induced vibrations in the effective
length of the mounting arm is minimized.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic representation of the drag flow meter
according to the present invention.
FIG. 2 is a transverse sectional view of an actual embodiment of
a transducer of the flow meter according to the present invention
shown installed in a flow conduit.
FIG. 3 is a top view of the flow meter arrangement shown in FIG.
FIG. 4 is an endwise view of the flow meter depicted in FIGS. 2
FIG. 5 is a transverse sectional view of an alternate embodiment
of the transducer of the drag flow meter according to the present
FIG. 6 is an endwise view of the transducer shown in FIG. 5.
FIG. 7 is a top view of the flow meter shown in FIGS. 5 and 6.
In the following detailed description, certain specific terminology
will be employed for the sake of clarity and a particular embodiment
described in accordance with the requirements of 35 USC 112 but
it is to be understood that the same is not intended to be limiting
and should not be construed inasmuch as the invention is capable
of taking many forms and variations within the scope of the appended
As developed above, the concept according to the present invention
utilizes a tensioned wire arrangement in conjunction with the drag
body or target, which tensioned wire is caused to vibrate at a frequency
which varies with the drag force.
The relationship between the frequency of vibration and the wire
tension varies according to the square root of the tension, and
by suitable connection of the drag target with the wire, a linearized
output signal is obtained in relation to the fluid flow velocity.
This occurs since this latter relationship offsets the drag force
increase as a function of the square of fluid velocity.
Such an arrangement is depicted diagrammatically in FIG. 1 mounted
to a flow passage 10 through which the fluid flow rate is to be
determined, and includes a drag body 12 disposed so as to be impinged
by the fluid flow and a resultant drag force generated. The drag
body 12 is mounted on an arm 14 mounted for pivotal movement about
a pivot axis 16. The effective length of the arm section to which
the drag body 12 is attached is defined as length L2 while the
shorter arm section is defined at length L1. The arm L1 is connected
to a tensioned conductive wire 18 which is fixed to a pedestal 20
such that as the drag force increases and decreases, the tension
of the tensioned wire 18 varies accordingly.
The tensioned wire 18 is disposed within a magnetic field established
as by permanent magnets 22 and 24 which have poles positioned opposite
each other as indicated. The presence of the magnetic field induces
a transverse force to be generated acting on the tensioned wire
18 in a manner causing the tensioned wire to be vibrated by the
passage of an electrical current therethrough. The magnetic field
also generates an output signal consisting of the varying electrical
voltage which is induced as the tensioned wire 18 vibrates within
the magnetic field.
An output signal and current generating circuit is advantageously
provided by a conventional feedback control loop circuit, indicated
in block diagram form in FIG. 1. Since suitable such circuits are
very well known in the art, the details of the same are not included
here, but typically include an amplifier connected so as to maintain
a current in the tensioned wire 18 by feedback of the induced current
to the amplifier, and at the same time generating an amplified output
The linear relationship between the variation in frequencies of
the output signal, drag force and fluid flow velocity may be better
understood by the following analysis:
The frequency of vibration of a stretched wire is given by the
n=mode (123 . . . )
F.sub.1 =wire tension
A.sub.w =wire area
d=density of wire
The drag force on the target is given by the relationship:
C.sub.D =drag coefficient (constant for disc)
V=average velocity of flow
Note that the drag force is related to the tension by the expression:
##EQU2## The equation (1) becomes: ##EQU3##
which shows that frequency is linear with velocity. The important
point to recognize is that the frequency is related to the square
root of the tension (Eq. 1). Then, the tension is proportion to
the square of the velocity (Eq. 2). The result is Eq. 4 where the
frequency is related to the first power of the velocity.
The tensioned wire 18 has a force exerted thereon by the passage
of a current therethrough in the presence of the magnetic field
which will tend to vibrate at the natural frequency which is a function
of the wire tension. FIGS. 2 3 and 4 depict an actual implementation
of the concept of a flow meter depicted diagrammatically in FIG.
The tensioned wire 18 is preferably vibrated in a vacuum since
the presence of the surrounding atmosphere tends to produce an undesirable
damping of the wire vibration which will disturb the calibration
of the meter. Accordingly, the flow meter 26 depicted in FIG. 2
is provided with a housing 28 and mating cover 30 which is secured
over the housing 28 with a seal 32 provided such that a vacuum may
be established within the interior of the housing 28 through an
evacuation port 34 closed off after evacuation by a sealing screw
The housing 28 is provided with a pedestal 38 which is secured
by means of threaded fasteners 40 to the fluid flow conduit 42.
A drag flow body 44 is disposed within the interior of the fluid
flow conduit 42 and is mounted on a force arm 46 which extends through
an opening 48 formed in the fluid flow conduit 42.
In order to maintain the vacuum with the interior 50 the housing
cover assembly includes a sealing bellows 52 preferably of thin
flexible metal which is sealed to the exterior of the force arm
46 and to the interior of the bore 54 formed in the housing 28.
The force arm 46 is pivotally mounted by a pair of bearings 56
carried by the force arm 46 and engaged by tapered pivot pins 58
carried by the openings formed in the housing 28 and retained by
means of threaded lugs 60.
The force arm 46 is overhung about the pivot point and has a bifurcated
section 62 which carries an end fitting 64 secured to a length
of electrically conductive wire. A high tensile strength material
such as tungsten is preferred since the wire 66 is pretensioned
to relatively high stress levels.
At the opposite end of wire 66 is an end fitting 70 which is received
within an insulated bushing 72 of an electrically insulating material
such as boron nitride. A tensioning adjustment nut 74 threadedly
engages the end fitting 70 and presets the tension level of the
A support block 76 is also received within the interior bore of
the housing 28 having a projecting boss portion 77 upon which is
mounted a ferromagnetic yoke 78 which is keyed over the boss portion
77. The boss portion 77 supports the insulated bushing 72. The ferrmagnetic
yoke 78 serves to provide a mounting for a pair of opposite pole
oriented magnets 80 and 82 which are disposed at the ends with the
wire 66 passing between the ends. Each of the magnets 80 and 82
is supported on an arm opposite the ferromagnetic yoke 78 and threaded
fasteners 84 and 86 allow adjustment of the intermediate gap.
Thus, the wire 66 is disposed in a magnetic field such that an
electrical current will generate a force acting on the wire tending
to establish a transversely directed vibration of the wire 66 the
frequency of which is a function of the tension of the wire, which
in turn corresponds to the force acting on the drag flow body 44
by the fluid flow.
As developed above, the drag force being a function of the square
of the velocity of the fluid flow, and the vibration frequency being
a function of the square root of the wire tension, there is essentially
a linear relationship between the fluid flow velocity and the frequency
of vibration of the wire 66.
In order to minimize the effects of temperature shifts, the housing
28 and force arm 46 are preferably constructed of metal exhibiting
coefficients of expansion closely matched to that of the wire, such
In order to make the necessary electrical connection to the feedback
control loop circuitry, feed through connectors 88 are employed
which are in turn adapted to be connected to short electrical leads
(not shown) extending from either end of the wire 66.
The drag flow meter device according to the present concept is
applicable to pipe flow monitoring applications and may also be
applied to ducted flow meter applications such as for oceanographic
and limnological studies, or for a vane oriented wind velocity sensor.
In this case, as indicated in the embodiment depicted in FIGS.
5 through 7 a duct 90 is provided to which is mounted a flow meter
92 according to the present invention.
In similar fashion, a drag disc plate 94 is provided secured to
a force arm 96. Various sizes of drag discs may be provided to afford
many different flow rate ranges and differing fluid densities.
A two-part enclosure defined by an endcap 98 and cover 100 are
provided which enables a vacuum to be maintained within the interior
for the purposes described above. In this case, a soft foam annular
sealing disc 104 is provided in order to maintain the vacuum throughout
the slight excursion of force arm 96.
The force arm 96 is pivotally supported at a point intermediate
its length between a pair of clevises 106 with an upper bifurcated
end section 108 supporting a wire fitting 110 to which is secured
a length of tungsten wire 112.
Support body 114 is secured to the endcap 98 by a pair of cap screws
116 and serves to mount the opposite end of the support body 114
which holds an insulating bushing 118. Insulating bushing 118 receives
the opposite end of the wire 112 with an end fitting 120 secured
to the opposite end, being threadably engaged with a tension adjusting
nut 122. Insulating bushing 118 is locked in position by means of
a set screw 124.
The support body 114 also serves to mount a pair of opposed cylindrical
permanent magnets 126 and 128 having oppositely directed polarities
which are juxtaposed with a gap therebetween so as to establish
a magnetic field through which the wire 112 passes.
The mounting of the magnets 126 and 128 is by means of threaded
elements 130 and 132 which control the position of the magnets in
respective bores 134 and 136 formed in plug bushings 138 and 140
An access plug 142 is provided to enable the tension adjusting
nut 122 to be reached through the end of the cover 100.
Electrical leads 144 and 146 are secured to either end of the wire
112 and are passed out through a sealing grommet 148.
Accordingly, it can be appreciated that the object of the present
invention has been achieved by the disclosed drag flow meters, which
produce an electrical output signal which is easily digitized by
the use of counters and is thus compatible with digital signal processing
circuits. The offsetting relationship between the variation in frequency
with tension and the increasing drag force with fluid flow velocity
produces an essentially linear relationship between fluid flow and
the output signal constituted by the varying frequency electrical
The arrangement itself is relatively simple and rugged and, accordingly,
is compatible with many fluid flow measuring applications.
While the fluid flow meter described provides a measurement of
average flow velocity, the same may also be employed for the measurement
of mass rate of flow, inasmuch as the drag force provides a measurement
of the fluid density as well as the average velocity of flow.