Abstrict A Coriolis mass flow meter of the type including a continuous flexible
flow tube fixedly mounted at its ends and adapted for receiving
fluid flow. A driver oscillates the flow tube at a fixed frequency.
A drive brace is attached at one end to the flow tube mount and
at the opposite end to the flow tube substantially at the position
of the applied oscillation of the driver. The brace limits the displacement
of the flow tube transverse to the direction of the applied oscillation
without substantially restricting the motion of the flow tube due
to the Coriolis reaction of the fluid to the driver oscillation.
The mounting of the flow tube being made by the engagement of gripping
portions having bushings therein. The mount limiting transfer of
forces acting on the inlet and outlet ends of the flow tube. Resonant
pins are provided at each end of the flow tube adjacent the fixed
mounting to resiliently clamp the flow tube and fix the resonant
frequency thereof throughout its useful life. The Coriolis reaction
of the fluid being measured by sensors so as to determine the mass
flow rate of the fluid.
Claims I claim:
1. A Coriolis mass flow meter comprising: a continuous flow tube
adapted to receive and exhaust fluid flow; a base, the flow tube
fixedly mounted at its ends to the base; a driver adapted to oscillate
the flow tube at a resonant frequency thereof; sensors positioned
on opposite sides of the flow tube equidistantly from the driver;
and brace means attached at one end to the base and at the opposite
end to the flow tube, the brace means attached to the flow tube
at substantially the same position as the driver and adapted to
limit lateral displacement of the flow tube transverse to the direction
of the applied oscillation without substantially restricting the
Coriolis reaction of the fluid on the flow tube in response to the
applied oscillation of the driver.
2. A Coriolis mass flow meter as claimed in claim 1 wherein the
brace means is attached to the flow tube at the center of the flow
tube length between the fixed ends.
3. A Coriolis mass flow meter as claimed in claim 1 wherein the
driver is adapted to oscillate the flow tube at a resonant frequency
of the flow tube higher than the fundamental resonant frequency
thereof, the oscillation applied by the driver creating local maximum
amplitudes of vibration along the flow tube length, the sensors
positioned adjacent the local maximum amplitudes of vibration.
4. A Coriolis mass flow meter as claimed in claim 3 further comprising
means on the flow tube for varying the mass of the flow tube between
opposite ends and for producing a vibrational pattern along the
length of the flow tube created by the applied oscillation of the
driver that is coincident and substantially in resonance with the
Coriolis reaction of the fluid in response to the oscillation of
the flow tube.
5. A Coriolis mass flow as claimed in claim 3 wherein the frequency
of oscillation applied by the driver is substantially at the second
resonant frequency of the flow tube.
6. A Coriolis mass flow meter as claimed in claim 1 further comprising
means positioned at each end of the flow tube adjacent the fixed
mounting thereof adapted to resiliently clamp the flow tube adjacent
the ends thereof to define the resonant frequency of the unsupported
length.
7. A Coriolis mass flow meter comprising: a continuous flexible
flow tube adapted to receive and exhaust fluid flow; a base, the
flow tube fixedly mounted to the base; a driver adapted to oscillate
the flow tube transverse to the direction of flow therein; sensors
positioned on opposite sides of the flow tube and adapted to output
a signal proportional to the motion of the flow tube in response
to the driver oscillation; and brace means attached at one end to
the base and at the opposite end to the flow tube, the brace means
limiting lateral displacement of the flow tube transverse to the
direction of the applied oscillation without substantially restricting
the Coriolis reaction of the fluid on the flow tube in response
to the applied oscillation of the driver, the brace means having
a first portion and a second portion, the first and second portions
generally forming an A-frame with the apex of the frame attached
to the flow tube and the opposite ends of the first and second portions
attached to the base.
8. A Coriolis mass flow meter comprising: a continuous flexible
flow tube adapted to receive and exhaust fluid flow; a base, the
flow tube fixedly mounted at its ends to the base; the base having
mounting means including a first portion and a second portion, the
first and second portions having mating surfaces including gripping
portions therein, the gripping portions adapted to engage the inlet
and outlet ends of the flow tube over a substantially large surface
area as compared to the cross-sectional area of the flow tube; a
driver adapted to oscillate the flow tube transverse to the direction
of flow therein; sensors positioned on opposite sides of the flow
tube equidistantly from the driver; and brace means attached at
one end to the base and at the opposite end to the flow tube, the
brace means adapted to limit lateral displacement of the flow tube
transverse to the direction of the applied oscillation without substantially
restricting the Coriolis reaction of the fluid on the flow tube
in response to the applied oscillation by the driver.
9. A Coriolis mass flow meter as claimed in claim 8 wherein the
mounting means further comprises bushing means provided within the
gripping portions, the outside dimensions of the bushing means engaged
by the first and second portions and the inside dimension engaging
the outside surface of the flow tube.
10. A Coriolis mass flow meter as claimed in claimed 9 wherein
the mounting means has a substantially large mass as compared to
the mass of the flow tube and the flow tube and the flow tube is
formed such that it is stabilized about its center of gravity and
the center of gravity of the mounting means.
11. A Coriolis mass flow meter comprising: a flow tube having an
inlet end and an outlet end adapted to receive and exhaust fluid
flow; a driver adapted to oscillate the flow tube at a resonant
frequency thereof; sensors means adapted to generate signals proportional
to the motion of the flow tube; mounting means having a first portion
and a second portion, the first and second portions having mating
surfaces including gripping means adapted for engaging the inlet
and outlet ends of the flow tube, the portion of the flow tube between
the engagement of the gripping means defining a sensing portion;
bushing means adapted to be engaged on its outside dimension by
the gripping means, the inside dimension of the bushing means adapted
to engage the outside surfaces of the inlet and outlet ends of the
flow tube, the bushing means adapted to limit the transfer of forces
acting on the inlet and outlet ends of the flow tube to the sensing
portion of the flow tube.
12. A Coriolis mass flow meter according to claim 11 further comprising
brace means attached at one end to the mounting means and at the
opposite end to the flow tube, the brace means adapted to limit
lateral displacement of the flow tube transverse to the direction
of the applied oscillation of a driver without substantially restricting
the motion due to the Coriolis reaction of the fluid in response
to the applied oscillation.
13. A Coriolis mass flow meter as claimed in claim 12 wherein the
driver is adapted to oscillate the sensing portion at a resonant
frequency of the flow tube greater than its fundamental resonant
frequency so as to create a vibrational wave pattern having at least
two local maximum amplitudes of vibration therein.
14. A Coriolis mass flow meter as claimed in claim 13 wherein the
frequency of the applied oscillation of the drive is substantially
the mode 2 frequency of the flow tube.
15. A Coriolis mass flow meter as claimed in claim 14 further comprising
resonant pin means positioned at each end of the sensing portion
of the flow tube adjacent the mounting means, the pin means adapted
to resiliently clamp the flow tube so as to fix the resonant frequency
of the sensing portion, the gripping force of the pin means being
within the elastic deformation limits of the pin.
16. A Coriolis mass flow meter comprising: a continuous flow tube
fixedly mounted at both ends, the flow tube adapted to receive fluid
flow; a driver adapted to oscillate the flow tube at resonant frequency
thereof; sensor means positioned on opposite sides of the flow tube
equidistantly from the driver, the driver adapted to oscillate the
flow tube at the second resonant frequency of the flow tube, the
driver oscillation producing a vibrational pattern within the length
of the flow tube having at least two local maximum of amplitudes
of vibration and wherein the Coriolis reaction of the flow tube
is substantially in resonance with the vibrational pattern; and
means positioned at each end of the flow tube adjacent the fixed
mounting thereof adapted to resiliently clamp the flow tube to fix
the resonant frequency thereof.
17. A Coriolis mass flow meter as claimed in claim 16 further comprising
brace means attached at one end to the fixed mounting of the flow
tube and the opposite end to the flow tube substantially at the
point of the application of the oscillation frequency by the driver,
the brace means having a first portion and a second portion, the
first and second portions generally forming an A-frame with the
apex of the frame attached to the flow tube and the opposite ends
of the first and second portions attached at separate locations
on the fixed mounting.
18. A Coriolis mass flow meter comprising: a continuous flexible
flow tube having an inlet and outlet portion adapted to receive
and exhaust fluid flow, respectively; a driver adapted to oscillate
the flow tube; sensors positioned on opposite sides of the flow
tube equidistantly from the driver; and means positioned at each
end of the flow tube adjacent the inlet and outlet portions of the
flow tube for resiliently clamping the flow tube and to define the
resonant frequency thereof, and means for fixedly mounting the flow
tube, the mounting means comprising a first portion and a second
portion, the first and second portions having mating surfaces including
gripping means adapted for engaging the inlet and outlet portions
of the flow tube, and bushing means adapted to be engaged on its
outside dimensions by the gripping portions and the inside dimensions
adapted to engage the outside surfaces of the inlet and outlet portions
of the flow tube, the bushing means further adapted to limit transfer
of forces acting on the inlet and outlet portions of the flow tube
to the portion of the flow tube between the resilient clamp means.
19. A Coriolis mass flow meter comprising: a continuous flow tube
adapted to receive fluid flow from a defined fluid stream, the flow
tube fixedly mounted at its inlet and outlet ends and defining an
unsupported length therebetween; vibration means adapted to oscillate
the flow tube substantially at a resonant frequency thereof, the
vibration means including flexible brace means fixed at one end
to a base and at the opposite end to the flow tube, the brace means
adapted to limit the amplitude of vibration of the flow tube at
the position of attachment thereto, the vibration means further
adapted to create local maximum amplitudes of vibration along the
unsupported flow tube length on opposite sides of the brace means,
the local maximums being substantially coincident with local maximum
amplitudes of rotation of the flow within the flow tube; and sensor
means adapted to generate signals proportional to the motion of
the flow tube, the sensor means positioned at substantially the
local maximum amplitudes of vibration created by the oscillation
of the vibration means.
20. A Coriolis mass flow meter as claimed in claim 19 wherein the
vibration means further comprises resonant weights attached to the
unsupported length of the flow tube between the fixed end and the
brace means, the resonant weights having a mass sufficient to create
an amplification of the oscillation of the flow tube so as to produce
the amplitude maximums as a result of the oscillation of the flow
tube.
21. A Coriolis mass flow meter comprising: a continuous flexible
flow tube including an inlet end and an outlet end adapted for communicating
with a defined fluid stream and for receiving a fluid flow; mounting
means having a first portion and a second portion, the first and
second portions having mating surfaces including gripping means
adapted for engaging the inlet and outlet ends of the flow tube
over a substantially large surface area as compared to the cross-sectional
area of the flow tube; bushing means provided within the gripping
means, the outside dimension of the bushing means being engaged
by the first and second portions, the inside dimension of the bushing
means engaging the outside surfaces of the flow tube, the bushings
adapted to limit transfer of forces acting on the inlet and outlet
ends of the flow tube to the portion of the flow tube between the
gripping portions; means on opposite sides of the flow tube adjacent
the gripping portions adapted to resiliently clamp the flow tube
to fix the resonant frequency of the unsupported flow tube length;
driver means adapted to oscillate the unsupported length of the
flow tube substantially at the second resonant frequency of the
flow tube, the driver positioned at the center of the unsupported
length of the flow tube; sensor means positioned on opposite sides
of the flow tube equidistantly from the driver means, the sensor
means having a predetermined mass supported on the flow tube; and
flexible bracing means attached at one end to the mounting means
and at the opposite end to the flow tube at the same position as
the driver, the bracing means adapted to substantially limit displacement
of the flow tube transverse to the direction of applied oscillation
of the driver without substantially restricting the relative motion
of the opposite sides of the flow tube due to the Coriolis reaction
of the fluid in response to the oscillation, the bracing means further
adapted to limit the amplitude of oscillation of the flow tube at
its attachment thereto, the driver means, mass of the sensor means
and bracing means reacting with one another so as to produce a vibrational
wave pattern along the length of the flow tube having two local
maximum amplitudes of vibration on opposite sides of the driver
means and producing a Coriolis reaction by the flow within the flow
tube having two local maximum amplitudes of rotation substantially
coincident with the vibrational amplitude maximums, whereby the
Coriolis reaction of the flow means and the vibrational pattern
along the flow tube length are substantially in resonance.
22. An apparatus for measuring the mass flow rate of a fluid comprising:
a continuous, flexible flow tube having a fixedly mounted input
and outlet end for receiving an exhausting fluid flow; oscillation
means for vibrating the flow tube perpendicular to the flow therein
substantially at the second resonant frequency of the flow tube
to induce a vibrating wave pattern that varies in amplitude along
the length of the flow tube, the vibrating wave pattern producing
two local maximum amplitudes of vibration moving in the same direction
transverse to the direction of flow and having two local maximum
amplitudes of rotation substantially coincident with the maximum
amplitudes of vibration; sensor means located on or adjacent the
flow tube for generating signals representative of the motion of
the flow tube due to the vibrational wave pattern and the Coriolis
reaction pattern of the fluid in response to the vibration by the
oscillation means; and means for determining the mass flow rate
through the flow tube from the sensor signals.
23. An apparatus as claimed in claim 22 wherein the oscillation
means further comprises brace means attached to the flow tube at
a position between the relative maximum amplitudes of vibration.
24. An apparatus as claimed in claim 23 wherein the oscillation
means further comprises an electromagnetic driver, the brace means
attached to the flow tube at substantially the same position as
the driver.
25. A Coriolis mass flow meter comprising: a continuous flow tube
adapted to receive an exhaust fluid flow, the flow tube fixedly
mounted at opposite ends and substantially encircling its overall
center of gravity; a base, the flow tube supported on the base at
its ends; sensor means for emitting signals proportional to the
motion of the flow tube; means for determining the mass flow through
the flow tube from the sensor means signals; means for oscillating
the flow tube transverse to the direction of flow and for creating
two local maximum amplitudes of vibration; and means associated
with the oscillating means for limiting the amplitude of displacement
of the flow tube between amplitude maximums, the oscillating means
attached to the flow tube at substantially the same position at
the limiting means.
26. A Coriolis mass flow meter as claimed in claim 25 wherein the
limiting means comprises a flexible brace means attached at one
end to the base and at its opposite end to the flow tube.
27. A Coriolis mass flow meter as claimed in claim 26 wherein the
flexible brace means has two side portions attached to one another
to form an apex, the apex attached to the flow tube, and the opposite
ends of the side members attached to the base.
28. A Coriolis mass flow meter as claimed in claim 26 wherein the
oscillating means comprises resonant weights attached to the flow
tube on opposite sides of the limiting means, the resonant weights
having a mass sufficient to create an amplification of the oscillation
of the flow tube on opposite sides of the limiting means and to
produce the amplitude maximums as a result of the oscillation.
29. A Coriolis mass flow meter as claimed in claim 28 wherein the
oscillating means is an electro-magnetic driver adapted to oscillate
the flow tube substantially at a resonant frequency thereof.
30. A Coriolis mass flow meter as claimed in claim 29 wherein the
resonant frequency of oscillation by the driver is substantially
at a resonant frequency of the flow tube greater than the fundamental
resonance.
31. A Coriolis mass flow meter as claimed in claim 29 wherein the
frequency of oscillation by the driver is substantially the second
resonant frequency of the flow tube.
Description BRIEF SUMMARY OF THE INVENTION
The present invention relates to an apparatus for measuring the
mass flow rate and/or density flow rate of a fluid or slurry. Particularly
the invention relates to a flow meter which operates on the Coriolis
principle. The present invention has found application to the accurate
measurement of the mass flow of a fluid having a low flow rate,
such as in the range of 0.01 to 1 pounds per minute.
BACKGROUND OF THE INVENTION
Coriolis type mass flow meters operate on the principle that fluent
material passing through a conduit tubing, when exposed to a deflection
or oscillation transverse to the direction of flow, will react with
a measurable force (the Coriolis force) on the walls of the tubing.
The Coriolis reaction is generated by the material moving in an
instantaneously changing curvilinear path, and acts with a force
directly proportional to the momentum of the material in the tubing.
The Coriolis reaction tends to apply a force in the opposite direction
of the motion of the deflection of the flow tube on the inlet side
thereof and with an equal and oppositely directed force on the outlet
side thereof. The measurement of this reaction is used to determine
the mass flow rate passing through the flow tube.
The flow tube portion of the Coriolis mass flow meter of the present
invention is preferably formed in accordance with commonly assigned,
co-pending application Ser. No. 912893 filed Sep. 26 1986. A
Coriolis flow meter of this type includes a stabilized flow tube
having a centralized center of gravity position substantially proximal
the center of gravity of its mounting. This centralized mounting
provides a stable oscillating structure which is less susceptible
to sensor signal contamination, due to external noise or vibrational
influences unrelated to the applied oscillation. A stable flow tube
form permits more flexible tubing to be utilized, increasing sensitivity
without increasing the detrimental effects on the Coriolis reaction
measurement due to external mechanical noise and the like from the
flow meter environment. Additionally, the present invention contemplates
an oscillation applied to the flow tube at a frequency that is higher
than the natural fundamental resonance of the flow tube. This applied
oscillation produces a vibrational wave pattern along the flow tube
length.
Increasing flexibility of the flow tube within a Coriolis mass
flow meter increases the sensitivity of the meter to the Coriolis
reaction, since the tubing will more readily react in response to
the Coriolis force of the fluid. However, the detrimental effects
of outside mechanical and vibrational noise on the Coriolis reaction
measurement, is also typically increased by a more flexible flow
tube. Further, the lateral stability (i.e. transverse to the direction
of the applied oscillation) of the flow tube may also be detrimentally
effected by increasing flexibility, introducing errors in the sensor
signals as well as effecting the frequency of oscillation applied
to the flow tube by the driver.
U.S. Pat. No. 4711132 issued Dec. 8 1987 in FIGS. 11 and 12
shows stiffening portions on the flow tube which are intended to
resist what is termed "wobble" and "roll" created
by lengthening the projection of the flow tube from its mounting
to increase flexibility. FIG. 11 shows stiffening plates which are
attached to the sides of the projected flow tube to resist curvilinear
bending, called "wobble", of these sides during application
of the driver oscillation. FIG. 12 shows struts fixedly attached
at both ends to two positions on the flow tube on the same side
of the applied oscillation. These struts are intended to prevent
displacement of the flow tube laterally, called "roll",
with respect to the plane thereof. These struts and stiffening plates
also limit the ability of the flow tube to flex in response to the
Coriolis reaction of the fluid. These structures, therefore, are
considered inapplicable to flow meters for measuring low flow rates,
since sensitivity is an important feature within this flow range.
SUMMARY OF THE INVENTION
The present invention relates to a Coriolis mass flow meter comprising
a continuous flow tube having an inlet and an outlet adapted to
communicate with a define fluid stream or pipeline. The flow tube
is oscillated by a driver transverse to the direction of flow therein.
Sensors for measuring the translation, velocity or acceleration
of the flow tube are typically positioned on opposite sides of the
flow tube equidistantly from the driver. In the preferred embodiment
of the present invention, the frequency of oscillation by the driver
is substantially at a resonant frequency of the flow tube that is
higher than the natural fundamental resonance thereof.
One portion of the present invention comprises a flexible brace
attached at one end to a fixed base and at the opposite end to the
flow tube, preferably, at the position of the applied oscillation
of the driver. The brace is formed so as to limit the ability of
the flow tube to displace laterally, i.e. transverse to the direction
of the applied oscillation, without substantially restricting the
motion of the flow tube due to the Coriolis reaction of the fluid.
Further, the amplitude of oscillation is preferably limited solely
by the damping factor of the flow tube and brace system. The brace
stabilizes the frequency of oscillation by more precisely defining
the frequency band width applied by the driver. The brace is tuned
so that its resonance is closely related to that of the flow tube.
Another portion of the present invention comprises mounting the
flow tube at its inlet and outlet ends by means of mating gripping
portions within a mounting block. The gripping portions are adapted
to engage the flow tube so as to limit the transfer of torque and
other forces generated along the defined fluid stream or pipeline
to the sensing portion of the flow tube. The gripping portions preferably
include bushings made of a material that is substantially softer,
i.e. more resilient, than the mounting block so as to maximize the
grip. Additionally, the thermal coefficient of expansion of the
bushings is preferably closer to that of the flow tube material
than the mounting blocks.
A third feature contemplated by the present invention comprises
resonant pins adapted to resiliently fix the resonant frequency
of the unsupported length of the flow tube over its useful life.
The resonant frequency of the flow tube operating at a harmonic
frequency above the fundamental resonance must remain constant over
the life of the flow meter so as to hold its calibration. The resonant
pins fix the length of the tubing by engaging the flow tube with
a resilient force within the elastic limits of the material that
is clamping the flow tube or by a resilient spring force, thereby
substantially eliminating the need for welding the tubing.
An additional feature of the invention includes resonant control
weights positioned about the driver and sensors to tune the vibrational
wave pattern of the unsupported tube segment to the pattern of the
Coriolis reaction of the fluid on the flow tube. This tuning maximizes
the signal to noise ratio of the oscillation pattern of the flow
tube and significantly increases the measurability of the Coriolis
reaction.
The features of the present invention are preferably contemplated
for use within a single tube flow meter having sufficient flexibility
to accurately determine the Coriolis reaction of a fluid flowing
at a rate within the range of 0.01 to 1 pounds per minute. Such
low flow rates require substantial measurement accuracy due to small
reaction force acting on the flow tube. The features of invention
may be used either alone or in combination as desired and are contemplated
to be applicable to various flow meter structures beyond those presently
described. Further features of the invention, including its applicability
to meters for determining relatively greater flow rates, will become
apparent by describing a preferred embodiments thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
For the purpose of illustrating the invention, there is shown in
the drawings a form which is presently preferred; it being understood,
however, that this invention is not limited to the precise arrangements
and instrumentalities shown.
FIG. 1 shows a perspective view of a Coriolis mass flow meter as
contemplated by the present invention.
FIG. 2 shows a top plan view of the flow meter shown in FIG. 1.
FIG. 3 shows a cross-sectional view thereof.
FIG. 4 shows a second cross-sectional view thereof.
FIG. 5 shows a cross-sectional view of a portion of the invention.
FIG. 6 shows a second cross-sectional view of the portion of the
invention shown in FIG. 5.
FIG. 7 shows a cross-sectional view of an alternate embodiment
of that portion of the invention shown in FIGS. 5 and 6.
FIG. 8 shows a second cross-sectional view of the portion of the
invention shown in FIG. 7.
FIG. 9 shows another portion of the present invention.
FIG. 10 shows a partial view of a further portion of the invention.
FIG. 11 shows a partial view of an alternate embodiment of the
portion of the invention shown in FIG. 10.
FIG. 12 shows a variation of the embodiment of the portion of the
invention shown in FIG. 11.
FIGS. 13A, 13B, 13C and 13D show various vibrational wave patterns
of a flow tube oscillated at various resonant frequencies.
FIGS. 14A, 14B, 14C and 14D are illustrations of the vibrational
wave patterns show in FIGS. 13A-D, respectively, including a representation
of the Coriolis reaction produced therefrom.
FIG. 15 is a graph (not to scale) of the phase shift of opposite
sides of the flow tube of the present invention as a function of
the frequency of the applied oscillation.
FIG. 16 shows a cross-sectional view of another alternate embodiment
of the portion of the invention shown in FIGS. 5-8.
FIG. 17 shows a second cross-sectional view of the portion of the
invention shown in FIG. 16.
DETAILED DESCRIPTION OF THE INVENTION
In the drawings where like numerals indicate like elements there
is shown a Coriolis mass flow meter 10 including a continuous, flexible
flow tube 12. The flow tube 12 is preferably formed in the manner
described in commonly assigned, co-pending application Ser. No.
912893 filed Sep. 26 1986. This application is herein incorporated
by reference. However, various other flow tube forms are applicable
and contemplated for use with the features of the invention.
As illustrated in FIGS. 1 and 2 the Coriolis mass flow meter 10
generally includes the flow tube 12 an electromagnetic drive 14
and sensors 16 18. The driver, as illustrated, is positioned at
the center of the flow tube length. The sensors 16 18 are positioned
on opposite sides of the driver 14 equidistantly therefrom. It
should be noted, however, that the position of the driver and the
position of the sensors may vary as desired depending on the shape
of the flow tube, its frequency of oscillation, and its pattern
of vibration.
The means for mounting the flow tube 12 effects the accuracy of
the flow meter 10 in determining the mass flow. As illustrated in
FIGS. 2 3 and 4 the flow tube 12 is supported by mount 22. The
flow tube 12 includes an inlet 24 and an outlet 26. The inlet 24
and outlet 26 are formed as flanges which are adapted to be connected
to a pipeline or defined fluid stream. Extending from the inlet
24 to mount 22 is an inlet connecting section 28. There is also
a corresponding outlet connecting section 30 formed substantially
parallel to the inlet connecting section 28 and extends from the
outlet 26 to mount 22. The inlet and outlet connecting sections
28 30 are engaged by mount 22 so as to fix the position of the
flow tube 12.
Mount 22 includes a upper portion 32 and a lower portion 34. The
upper portion 32 is secured to the lower portion by means of bolts
36. Bolts 36 also secure mount 22 to a plate 38 which forms the
base of the flow meter 10. Base plate 38 may be secured to any convenient
surface when the flow meter 10 is positioned in its operating environment.
Upper portion 32 and lower portion 34 form mating surfaces 40 passing
substantially through the center of the mount 22. Mount 22 also
includes gripping portions 42 44 in the form of circular shape
channels. Inlet and outlet connecting sections 28 30 extend through
the gripping portions 42 44 and are engaged thereby to fix the
position of the flow tube 12.
Provided within each gripping portion 42 44 is a bushing 46. The
outside diameter of the bushings 46 conforms to the inside diameter
of the gripping portions 42 44. The inside diameter of the bushings
46 conforms to the outside diameter of the inlet and outlet connecting
sections 28 30 of the flow tube 12. The bushings 46 may comprise
mating upper and lower portions forming a substantially circular
channel therethrough. It is contemplated for assembly and manufacturing
purposes that the outside diameter of the bushings 46 will remain
constant while the inside diameter may be varied over a range of
flow tube diameters. The outside diameter of the bushings 46 is
fixed so as to be firmly engaged by the gripping portions 42 44.
The engagement force is preferably constant at all anticipated operating
temperatures for the flow meter 10. This engagement restricts transfer
of forces generated at the inlet 24 or outlet 26 of the flow meter
10 such that these forces will not be transmitted through the connecting
sections 28 30 within mount 22 to the unsupported length of the
flow tube 12.
Bushings 46 are preferably made of bronze or other soft material.
Mount 22 is contemplated to be made of steel or the like. Thus,
the bushings 46 are softer than portions 32 and 34 of the mount
22 to maximize the engagement force on the flow tube 12. In addition,
bushings 46 preferably have a thermal coefficient of expansion closer
to the flow tube 12 than that of the mount 22. Moreover, it is also
contemplated that the engagement of the gripping portions will be
provided over relatively long length as compared to the inside diameter
of the bushings 46.
The sensing portion of the flow meter 10 includes the unsupported
length of the flow tube 12 between the mounted ends 28 30. This
unsupported length, sometimes called the sensing portion, is that
portion which may deflect in response to the applied oscillation
of the driver 14. Thus, the inlet 48 to the sensing portion of the
flow tube 12 is defined at the exit of the inlet connecting section
28 from bushing 46 within gripping portion 42. The outlet 50 of
the sensing portion is defined by the passage of the flow tube 12
from bushing 46 within gripping portion 44.
The resonance of the sensing portion or unsupported length of the
flow tube 12 particularly when oscillated in a higher than fundamental
mode, must remain constant throughout the useful life of the flow
meter 10 so as to hold calibration. This resonance is fixed by the
resonant pins 52 54 positioned adjacent the inlet 48 and the outlet
50. The structure of the resonant pins 52 54 is particularly shown
in FIGS. 5-8. It is contemplated that both pins 52 and 54 will be
substantially identical. The following description relates specifically
to pin 52 with pin 54 having the same structure.
As particularly illustrated in FIGS. 5 and 6 resonant pin 52 includes
a base 56 which is fixed to base plate 38 by bolts 58. Base 56 projects
from the base plate 38 and terminates in a U-shaped end forming
an opening 60. At the bottom of the opening 60 is positioned a support
tube 62 preferably having a rounded upper surface. The support tube
62 is preferably made of a resilient or soft material, i.e. a material
having a large range of elastic deformation such as aluminum. The
flow tube 12 is position within the opening 60 and rests on the
support tube 62. The flow tube 12 is fixed into position by means
of block 64. Block 64 is mounted on the base 56 by means of bolts
66 which extend into the projections of the U-shaped end. Shims
68 may be provided between the block 64 and base 56 so as to fix
the clamping force on the flow tube 12 between surface 70 of block
64 and the support tube 62.
The clamping force created by the surface 70 via bolts 66 preferably
causes a slight crushing or deformation of the support tube 62 within
the elastic limit of its material. Other known fixing techniques,
such as welding, normally tend to change slightly with the age.
The clamping force by pin 52 on the relatively stiff flow tube 12
as compared to the soft material of the support tube 62 is within
the elastic deformation limits of the support tube 62 so as to assure
flow tube 12 stability over its useful life.
An alternate embodiment of a resonant pin 52' is shown in FIGS.
7 and 8. This alternate embodiment includes base 56 connected to
base plate 38 via bolts 58 similar to that formed in the embodiment
shown in FIGS. 5 and 6. Within the opening 60 is positioned two
clamping jaws 62' which are resiliently biased against the flow
tube 12 by spring 71. The spring 71 may be any type, as desired,
including a bellville or a coil type. Block 64 forms the top of
the pin 52' and is rigidly attached to the base 56 via bolts 66
without requiring shims. Spring 71 provides sufficient clamping
force to fix the flow tube 12 over the design temperature range
of the flow meter 10. However, since the spring 71 provides a resilient
bias against the flow tube 12 jaws 64' may be made of steel or
other relatively rigid material. A groove is provided within jaws
64' to engage the flow tube 12 and maintain lateral as well as vertical
stability of the tubing at this point.
Another alternate embodiment of a structure for fixing the ends
of the flow tube 12 is shown in FIGS. 16 and 17. The fixing means
in this embodiment is in the form of a resonant clamp 150. Clamp
150 includes a base 152 connected to base plate 38 via bolts 154.
The base includes two upstanding projections 156 forming a gap 158.
A flange 160 is attached to the flow tube 12 at the desired position
by brazing or the like. Flange 160 is fixed within the gap 158 by
engaging slot 162 on the inside surfaces of the projections 156.
A clamping block 164 is attached to the top of projections 156 by
screws 166. Base 152 and block 164 may also include slots for receiving
the edge of the flange 160. A spring 168 is provided to fix the
flange 160 within slot 162 to account for tolerance variations of
the clamp structures. However, if the tolerances are closely machined,
spring 168 need not be included. The resonant clamp 150 maintains
the lateral and vertical stability of the ends of the flow tube
12 for fixing the resonant frequency of the unsupported flow tube
length.
The flow tube 12 of the present invention, in that such is intended
to handle small flow rates, is relatively small in diameter. In
the embodiment shown here, the length of the unsupported portion
of the flow tube 12 between pins 52 and 54 as compared to the
cross-sectional area of the flow tube 12 creates flexibility. This
flexibility increases the ability of the tubing to react in response
to the Coriolis reaction of the fluid. However, a flexible flow
tube 12 may tend to be unstable in the X-Y plane (FIG. 1) causing
a transverse motion, i.e. a motion perpendicular to the direction
of the applied oscillation by driver 14. This instability can be
excited by fluctuation of the flow rate, by the influence of mechanical
or other noise from the flow meter environment, or by the operation
of the driver 14. This transverse movement within the X-Y plane
affects the resultant vibrational frequency as seen by the flow
tube 12 when oscillated by the driver 14. This resultant frequency
may vary considerably due to lateral movement of the flow tube.
Lateral displacement causes misalignment of the magnet within the
coil 112 of the driver 14.
As illustrated in FIGS. 2 10 and 11 sensor 14 includes an attachment
structure 85 that is fixed to the flow tube 12 by screw 142. Screw
142 is fixed in length so that its end does not crush the tubing.
A drive brace 72 is also attached to the flow tube 12 at the position
of the driver 14. Brace 72 is contemplated to be relatively thin
and flexible and have a large dimension within the X-Y or lateral
plane as compared to its other dimensions. Brace 72 is attached
at its opposite end to the mount 22 at support block 74. Attachment
to the mount 22 is made by means of a pin or small screw 76. The
brace 72 includes a stepped portion directly adjacent its attachment
to support block 74. The width of the brace 72 in this embodiment
is approximately 0.1 inch, whereas the stepped or notched portion
has a dimension of approximately 0.05 inch. The thickness of the
material is contemplated to be approximately 0.01 inch.
The drive brace 72 is contemplated to be tuned to the resonant
frequency of the unsupported flow tube segment. The brace 72 preferably
has a resonant frequency close to that of the preferred operational
frequency of oscillation of the flow tube 12 so as to offer little
or no resistance to the applied oscillation of the driver 14 in
the Z direction. However, the relative dimensions of the brace 72
are such that the damping factor within the X-Y or lateral plane
is higher as compared to that in the Z direction. This dimensional
relationship stabilizes the flow tube 12 in the X-Y plane and has
the effect of substantially narrowing the band width of the resultant
oscillation frequency applied by driver 14. Thus, the brace 72 assists
in maintaining the oscillation of the driver 14 at the desired frequency.
Furthermore, the relative flexibility of the brace 72 in the Z direction
will not substantially effect the measurable Coriolis reaction on
the flow tube 12.
FIG. 11 shows an alternate embodiment of a bracing structure 72'.
In this embodiment, two brace arms 78 80 form an A-frame with respect
to the mount 22. The apex of the A-frame is attached to the flow
tube 12 at driver 14. The opposite ends of the brace arms 78 80
are attached to portion 32 of mount 22 at openings 82 on the base
of brace 72'. The opening provided within the center of the A-frame
of brace 72' includes radiused corners. The brace 72' should be
relatively stiff with respect to the unsupported segment of the
flow tube 12 having a thickness in the range of 0.006 to 0.010
inch. Further, the brace arms 78 80 are tapered away from mount
22. The brace 72' is contemplated to be made of a stainless steel,
such as a spring stock material SST304 which has been tempered
to one half hard. The material may be formed by a photoresistive
etching process from a shim stock. If formed in this manner, it
is preferred that the grains of the shim stock due to the forging
and rolling processes be aligned generally in the direction of the
apex of the A-frame. This alignment will provide both arms 78 80
of brace 72' with substantially the same flexibility.
The brace structures 72 and 72', as shown in FIGS. 10 and 11 are
attached to the structure of the driver 14 by means of plate 140.
The end of the brace 72 or 72' is folded upwardly and is engaged
by plate 140. (See FIG. 4.) Plate 140 is secured to the driver 14
by screw 144 which projects through plate 140 and the end of the
brace 72 or 72'. As illustrated, screw 144 is engaged within resonant
control weight 104. A pin type structure may also be used as a substitute
for the screw 144. Other means of securing the end of the brace
72 or 72' to the driver 14 are also contemplated.
FIG. 12 shows an alternate embodiment of a means for securing the
bracing structure 72 or 72' to the flow tube 12 as well as for forming
the driver 14. The securing means 114 includes a base portion 116
having a substantially "L" shape, and a clamp 122. The
projection of the base portion 116 includes an opening 118 for insertion
of a tab 120 on clamp 122. Tab 120 is inserted into slot 118. The
opposite end of clamp 122 is fixed to the base 116 by screw 124.
The base portion 116 and clamp 122 form a grove which conforms to
the flow tube 12 diameter. The driver magnet 110 is mounted on the
lower end of the base 116 so as to project substantially vertically
downward when the securing means 114 is mounted on the flow tube
12.
A slot 126 is provided on the lower surface of the base 116 for
receipt of the brace 72 or 72'. An enlargement of the attachment
end of brace 72 or the apex of brace 72' will provide a large surface
area for securing the brace within slot 126. Tee brace 72 or 72'
may be secured within slot 126 by a high temperature epoxy. It is
contemplated that this joint will experience a significant shear
stress during operation due to the internal pressure within the
flow tube 12. This internal pressure, because of the shape of the
flow tube, will tend to force the straight crossing portion (at
the driver) outwardly in the Y direction (as shown in FIGS. 1 and
2). A pin 128 may be inserted therein for further securing the brace
72 or 72' to the clamping means 114. The brace 72 or 72' may also
be braised or otherwise fixed within the slot 126.
The engagement of the clamping means 114 on the flow tube 12 in
this manner is contemplated to be capable of maintaining the magnet
110 in a fixed position over the life of the flow meter 10 without
substantially effecting the drive frequency. Variations in the A-frame
structure as well as other bracing structures may be contemplated
for use along with the present invention. These variations will
likely be dependent upon the size of the tubing and the overall
frequency of oscillation during operation.
FIG. 9 illustrates a sensor structure for use with the present
invention. Sensors 16 and 18 positioned on opposite sides of the
flow meter 10 are contemplated to have the same or similar structure.
The sensors 16 18 are limited only in that, preferably, a relatively
small mass should be mounted on the flow tube 12. Sensor 16 includes
an attachment structure 84 and a U-shaped bracket 86 which supports
magnets 88 adjacent opposite sides of a coil 90. Magnets 88 may
be mounted on the bracket 86 within openings 130 at the projected
ends. The positioning of magnets 88 within the opening 130 may be
further supported by a high temperature epoxy. Other suitable means
is contemplated such as by directly mounting the magnets 88 on the
inside surface of the bracket 86 by means of an epoxy. Other means
for mounting, as well as other sensor structures, should be apparent
to those skilled in the art.
Attachment means 84 fixes the position of the sensor 16 on the
flow tube 12 by insertion in a notch 132. A screw 134 projects downwardly
through the mounting means 84 to engage the flow tube within the
notch 132. The sensor 16 may also be supported on the flow tube
12 by including an opening which substantially conforms to the outside
diameter of the flow tube. The attachment structure 84 can then
be slid onto the ends of the flow tube 12 to the proper position.
The sensor 16 may then be secured onto the flow tube 12 at that
position by means of screw 134 by braising or the like. Movement
of the magnets 88 induce a current within the coil 90 sending a
sensor signal. The coil 90 may be fixed to a support 92 attached
to the base plate 38 via bolts 94.
The sensing portion of the flow meter 10 includes the unsupported
length of the flow tube 12 the sensors 16 18 the driver 14 and
the drive brace 72 or 72'. The resonant frequency of the sensing
portion of the flow meter 10 for a given flow tube 12 can be varied
by controlling the mass suspended on the flow tube 12 and the stiffness
of the brace 72 or 72'. The resultant resonant frequency of this
flow meter sensing portion is a complex combination of these elements.
The attachment structure 85 of the sensors 16 18 includes resonance
control weights 100 and 102. Weight 104 is also included on the
flow tube 12 about the driver 14. Control of the mass of the sensors
16 18 and the driver 14 by these weights 100 102 104 optimize
the signal to noise ratio of the sensing portion of the flow tube
12. Further, by controlling the mass of the sensors 16 18 the
overall oscillation pattern of the sensing portion of the flow meter
10 may be tuned to the pattern of the Coriolis reaction of the fluid.
This tuning results in an increase in the amplitude of the Coriolis
reaction and, thus, increases the measurability of that reaction.
Tuning is accomplished by the addition or deletion to these weights
100 102 104. Weight 100 as shown in FIG. 9 is mounted on the
attachment structure 85 of the driver 14 and is adapted to control
the mass of the vibrating element of the flow meter and set the
oscillating frequency thereof. Weights 100 102 are positioned about
the sensors 16 18 respectively, to tune the segment of the unsupported
flow tube length adjacent thereto. Once the desired position of
the sensors 16 18 and the other aspects of the mass flow meter
operation have been determined, the sensor and driver structure
may be fixed. Therefore, further adjustment is not necessarily required
upon tuning the relative structures of the flow meter.
A number of embodiments are contemplated for the present invention.
Specifically, the size and bore of the flow tube meter may vary
dependent upon the desires of the user. The following examples are
for purposes of representing the relative adjustment of the flow
meter structures to tune the operation of the flow meter. Each embodiment
contemplates an unsupported flow tube length of approximately 9.85
inches (as measured along the centerline of the tubing). The flow
tube material is further contemplated to be made of a stainless
steel material (SST 316L). A first embodiment includes tubing having
an outside diameter of 0.062 inch and a wall thickness of 0.008
inch. The sensors 16 and 18 in this embodiment each have a total
mass of 1 gram (.+-.0.1 grams). This embodiment further includes
the A-frame-type bracing structure 72' as described above. The thickness
of the brace arms 78 80 is approximately 0.008 inch. This structure
provides a second resonance at approximately 114 Hz. A second embodiment
includes tubing having an outside diameter of 0.093 inch with a
0.008 inch wall thickness. The sensors have a mass of approximately
1.75 grams. The brace is approximately 0.006 inch thick. This structure
provides a second resonance at approximately 126 Hz. A third embodiment
has a tube diameter of 0.042 inch with a 0.008 wall thickness. The
mass of the sensors is approximately 0.4 gram. The thickness of
the brace is approximately 0.010 inch. The second resonant frequency
of this structure is approximately 113 Hz.
In FIGS. 13A-D there is shown various vibrational oscillation patterns
of the flow tube 12 shown in FIGS. 1 and 2. FIGS. 14A-D makes reference
to the oscillation patterns shown in FIGS. 13A-D, respectively.
FIG. 14 is a representation of the linear deflection along the length
of the flow tube due to the oscillation of the driver 14. This FIG.
14 further represents the Coriolis reaction gradient corresponding
to the vibrational pattern shown. It should be noted that the Coriolis
reaction as well as the amplitude of the oscillation is constantly
changing. FIGS. 14 A-D show peak deflection and reaction during
one half cycle of oscillation. Thus, the proportional relationship
of these reactions to the deflection is represented. In FIGS. 13A,
13B, and 13C at that particular time the brace 72 or 72' has not
been included as part of the sensing portion.
In FIG. 13A and FIG. 14A there is shown a flow tube oscillated
at the fundamental resonant frequency of the tubing between resonant
pins 52 54. In FIG. 13A the maximum deflection of the flow tube
12 on opposite sides of the neutral plan are labeled A.sub.1 and
A.sub.2. In FIG. 14A only the deflection in the A.sub.1 direction
is shown. As can be seen in FIG. 14A, the Coriolis reaction is in
the form of a gradient and is dependent upon the relative rotation
of the flow. With the flow moving from left to right across the
page, the Coriolis reaction at the inlet end of the flow tube 12
is in a direction opposite of the deflection by the driver 14. This
reaction reaches a maximum substantially at a position between the
inlet and the maximum deflection or center position of the flow
tube 12. This maximum reaction is a maximum rotation or change in
direction of the flow due to the deflection of the flow tube 12
by the driver. A similar reaction is found on the opposite side
of the center of the flow tube 12. This opposite side reaction is
generally in the same direction as the vibrational deflection A.sub.1.
This type reaction pattern at the fundamental resonance or mode
1 is generally referred to as lagging the deflection of the flow
table by the driver on the inlet side and leading on the outlet
side. By positioning sensors on opposite sides of the maximum deflection,
the effect of the Coriolis reaction on opposite sides of the driver
is determined. This reaction is proportional to the mass flow.
FIGS. 13B and 14B are generally representative of a flow tube oscillated
in the second mode or at the second resonant vibrational frequency.
The vibrational pattern represented by B.sub.1 and B.sub.2 includes
a node N at the center of the flow tube length. A node is generally
defined as a local minimum amplitude of vibrational about which
the flow tube length appears to flex. A node is comparable to a
free floating ball and socket joint with the flow tube forming flexible
extensions from both sides of the socket. Advantages obtained by
a nodal type vibrational pattern are generally described in commonly
assigned co-pending application Ser. No. 912893 filed Sep. 26
1986. FIG. 14B also represents the reaction gradient of the fluid
in response to this vibrational pattern. A maximum reaction is created
substantially at the node N. This maximum is the result of the fluid
experiencing a maximum change in angular direction at that position.
The fluid is being rotated at the greatest rate at node N. The translational
motion of the remaining portions of the flow tube diminishes the
relative rotation of these portions as compared to the node N. The
vibrational patterns B.sub.1 also includes two local maximum amplitudes
of vibration on opposite sides of the node N.
FIGS. 13C and 14C generally show a flow tube oscillated in the
third mode or at the third resonant vibrational frequency. The vibrational
pattern C.sub.1 and C.sub.2 includes two nodes N located symmetrically
on opposite sides of the flow tube. The two nodes N isolate the
oscillation of the flow tube from its fixed ends at the resonant
pins 52 and 54. The nodes N are relatively free floating points
in space which are not substantially effected by vibrational influences
and external noise. As can be seen, the Coriolis reaction pattern
includes two local maximums due to the rotation of the flow at the
nodes N. Preferably, the sensing of the Coriolis reaction in this
embodiment is made adjacent the local maximum amplitudes of vibration.
These vibrational maximums are located on opposite sides of the
flow tube, between a fixed end and a node.
In each of the vibrational and reaction patterns shown in FIGS.
13A-C and FIGS. 14A-C, respectively, the mode of the oscillation
of the flow tube is out of phase with respect to the "mode"
of the Coriolis reaction. As shown in FIG. 14A, the mode 1 vibrational
pattern A.sub.1 includes what can be labeled as a "mode 2"
reaction pattern. In FIG. 14B, the mode 2 vibrational pattern B.sub.1
includes a "mode 3" reaction pattern. In FIG. 14C, the
mode 3 vibrational pattern C.sub.1 includes a "mode 4"
reaction pattern. Additional modes of vibration would include similar
reaction pattern relationships.
The present invention contemplates the tuning of the vibrational
pattern of the flow tube to that of the Coriolis reaction pattern.
This tuning of the flow tube 12 is a function of the resonant weights
100 102 and 104 and the brace 72 or 72'. In the preferred embodiment
of the present invention, the brace 72 or 72' is a spring preferably
having a spring coefficient that is stiffer or greater than that
of the unsupported length of the flow tube. The brace in this respect
serves to limit the amplitude of deflection of the flow tube at
its central position. The weight of the sensors 16 and 18 on opposite
sides of the driver, when set at the desired mass, accentuate the
deflection of the flow tube between the driver and the fixed ends.
Thus, the flow tube on opposite sides of the driver translates a
greater distance from the neutral position than at the driver which
is limited by the brace.
As shown in FIGS. 13D and 14D, the vibrational D.sub.1 pattern
D.sub.1 includes two local maximum amplitudes of vibration. These
local maximums are positioned on opposite sides of the center position
of the flow tube 12 and are moving in the same direction. The flow
tube at these local maximums are also moving in the same direction
as the driver. The pattern of the Coriolis reaction substantially
coincides with this vibrational pattern such that the reaction is
in resonance with the oscillation.
As can be seen in FIG. 15 the frequency of the applied oscillation
of the driver also substantially effects the vibrational and reaction
patterns shown in FIGS. 13D and 14D. The curve shown in FIG. 15
is a plot of the relative phase difference between sensor 16 and
sensor 18 as a function of the driver frequency. Point 1 on the
graph generally represents the phase difference at a drive frequency
which is substantially the fundamental resonant frequency of the
sensing portion of the flow meter. The phase difference created
by the Coriolis reaction of the fluid for example 1 above is in
the range of 30 microseconds. (This value is determined by the phase
of sensor 18 on the outlet end of the flow tube less the phase of
the signal generated by sensor 16 on the inlet side of the flow
tube.) Point 2 on the graph illustrates the phase difference produced
by the present invention when oscillated at a mode 2 equivalent
frequency for the flow tube. This mode 2 frequency generally produces
a reaction in the range of -3000 microseconds. (The negative value
for this phase difference is due to the equation, whereby the signal
from sensor 16 is subtracted from the signal from sensor 18.) Point
3 on the graph generally represents the phase difference, 8 microseconds,
at a mode 3 drive frequency.
The substantial increase in the measurable phase difference is
a result of the tuning of the flow meter to create the vibrational
pattern illustrated in FIGS. 13D and 14D. The tuning of the flow
meter includes either adjustment of the mass of the sensors and
driver for a given flow tube or adjustment of the stiffness of the
brace. The resonant weights 100 102 on the sensors control the
"resonant" frequency of the portion of the flow tube between
the fixed inlet end and the position of the driver and the second
portion of the flow tube between the driver and the fixed outlet
end. These adjustments are made to tune the resonant frequencies
of these flow tube portions to be approximately half of that of
the flow tube. The amplitude of oscillation of the flow tube at
the driver is essentially fixed by the operation of the driver and
the spring force of the entire flow tube and the brace. However,
in the embodiments discussed above, by driving the flow tube at
the mode 2 frequency, the second resonance of the sensing portion
of the flow meter substantially corresponds to the mode frequency
of each half of the flow tube. Thus, the flow tube becomes compliant
to the resonance of the flow tube portions greatly amplifying the
vibration of the flow tube on opposite sides of the driver. The
resultant vibrational pattern also falls into resonance with the
reaction pattern of the Coriolis response. This resonance permits
the Coriolis reaction to act on the flow tube without conflicting
with the vibrational pattern of the flow tube. Thus, limiting the
ability of the Coriolis reaction to deflect the flow tube.
If the sensing portion of the flow meter, including brace 72 or
72', is oscillated at around the fundamental resonance of the flow
tube, the Coriolis reaction is not tuned to the vibrational pattern.
This is due to the fact that the mode 1 resonance of the two half
portions of the flow meter length is not reached. The reaction pattern
of the Coriolis response would be substantially the same as that
illustrated in FIGS. 13A and 14A with the vibrational pattern dominating.
Further, if the flow tube is oscillated at around its mode 3 frequency,
the vibrational pattern illustrated in FIGS. 13C and 14C would substantially
be created. Again, the opposite portions of the flow tube are out
of resonance with the drive frequency and the vibrational pattern
dominates over the Coriolis reaction pattern.
As can be seen, the vibrational pattern D.sub.1 shown in FIG. 14D
is similar to the vibrational pattern C.sub.1 shown in FIG. 14C.
Each pattern includes two local maximum amplitudes of vibration
moving in the same direction on opposite sides of the center position
of the flow tube Further, the Coriolis reaction patterns are for
each example includes a "mode 4" type reaction pattern.
However, the tuning of the reaction pattern to the vibrational pattern
of the flow tube in FIG. 14D substantially increases the measurability
of the Coriolis reaction as compared to that shown in FIG. 14C.
This is due to the fact that the reaction maximums in FIG. 14C are
dominated by the vibrational pattern C.sub.1. In FIG. 14D the Coriolis
reactions are substantially coincident with the vibrational maximums.
This "resonance" of the vibrational and reaction pattern
substantially increases the measurability of the Coriolis reaction.
This tuning of the reaction and vibrational pattern substantially
contributes to the determination of the mass flow rate for the low
flow rates, as contemplated by the present invention.
Points 4 and 5 on the graph shown in FIG. 15 generally illustrate
the further tuning of the flow meter by means of the resonant weights
100 102 and 104 on the sensors 16 and 18 and driver 14 respectively,
and the stiffness of the brace 72 or 72'. If the flow tube 12 were
permitted to oscillate exactly at its mode 2 resonant frequency,
the vibrational pattern is unstable. Minute changes in the frequency
of oscillation will greatly effect the phase shift for reasons other
than the Coriolis reaction. This is represented by the large peak
reaction at the mode 2 frequency, point 2. By adjusting the mass
of the resonant weights 100 102 and 104 the flow meter may be
tuned slightly out of resonance to provide a more stable oscillation.
The decrease in the phase difference from point 2 to points 4 or
5 is not considered significant in view of the relatively large
value therefore and the resultant stability of the phase of the
sensor signals produced.
Tuning of the flow meter may also be accomplished by adjusting
the relative stiffness of the spring once the mass of the resonant
control weights has been determined. For example, if the driver
frequency is operating below the flow tube resonance, the dimensions
of the brace may be increased to stiffen the spring force as compared
to the prior operating structure. Likewise, if the driver frequency
is operating above the flow tube resonance, the spring force of
the brace may be relaxed, by decreasing the dimensions or changing
the materials. This change in the overall vibrational frequency
of the sensing portion of the flow meter does not substantially
effect the resonance of the opposite sides of the flow tube. This
is due to the fact that the resonance of the local maximum amplitudes
of vibration are not dependent upon the spring force of the brace.
Rather, this resonance is dependent upon the length of the tubing
between the fixed ends and the center position and the mass of the
sensors. It should be noted that the vibrational and reaction pattern
shown in FIGS. 13D and 14D may be produced by other structured and
such are contemplated. For example, in relatively heavier tubing
capable of handling greater flow rates, two drivers may be utilized
to produce the desired reaction pattern. These drivers would be
positioned on opposite sides of the center of the flow tube length.
However, the brace structure would also be included at the center
position. The brace structure in this embodiment is contemplated
to provide a substantial restriction to the ability of the flow
tube to oscillate at the center position. The drivers would create
local maximum amplitudes of vibration similar to those illustrated
in FIGS. 13D and 14D. The brace would act as a restriction to the
amplitude of vibration. Other embodiments of a flow meter in accordance
with the teachings of the present invention may become apparent
to those skilled in the art after reviewing the above disclosure.
The electronics of the flow meter 10 including wires communicating
with the driver 14 or the sensors 16 18 may be attached to an
electrical connector 96 supported on cover 98 which surrounds the
flow meter 10. Processing of the signals from the sensors 16 18
to determine the mass flow rate and to control of the driver frequency
can be made in any manner as desired or as known in the art. One
contemplated method of determining the mass flow rate from the signals
generated by the sensors is described in U.S. Pat. No. 4782711.
The flow meter 10 of the present invention is intended to handle
small flow rates between 0.01 to 1 pounds per minute. The background
mechanical noise is isolated in part from sensing portion of the
flow tube 12 by means of mounting block 22. Various flow tube materials,
such as stainless steel, Hastelloy (registered trademark), monnel,
titanium, etc., can be used as part of the present invention without
the need to develop corresponding welding and or brazing techniques
for fixing the ends of the tubing. However, other methods of fixing
the ends of the flow tube 12 may be utilized. The gripping portions
42 44 of the mount 22 may be adapted to handle various diameters
and/or configurations of the flow tube by modifying the dimensions
of the bushings 46. Thus, a range of applications can be fit into
a standard mounting block 22 making manufacturing more cost effective.
Additionally, a faulty, plugged or eroded flow tube may be economically
repaired and/or replaced, even within the field, without requiring
replacement of the entire flow meter.
The present invention may be embodied in other specific forms without
departing from the spirit or essential attributes thereof and, accordingly,
reference should be made to the appended claims, rather than to
the foregoing specifications, as indicating the scope of the invention.
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