Abstrict A micromachined, monolithic silicon flow meter includes a vane
28 from which projects a hinge. The hinge is provided by torsion
bars 24. The hinge supports the vane 28 for rotation about the torsion
bars 24. A deflection sensor, consisting of a torsion sensor 42
incorporated into at least one of the torsion bars 24 senses deflection
of the vane 28 responsive to fluid impinging thereupon. The frame
22 the torsion bars 24 the vane 28 and the torsion sensor 42 are
all monolithically fabricated in a semi-conductor single-crystal
silicon layer of a substrate.
Claims What is claimed is:
1. A micromachined vane flow meter adapted for use in sensing a
rate at which a fluid flows through the flow meter, the flow meter
comprising:
a support;
two opposing torsion bars that are aligned along a common axis,
and which project from said support;
a vane that is joined to and supported from said support by said
torsion bars that dispose said vane in a rest position while no
external force is applied to said vane, said vane being supported
by the torsion bars in the rest position for rotation about the
common axis of the torsion bars by a force applied to said vane
thereby imposing a shear stress on said torsion bars; and
deflection sensing means for sensing deflection of said vane from
the rest position by fluid impinging upon and flowing around said
vane; said support, torsion bars, vane and deflection sensing means
all being monolithically fabricated in a semiconductor single-crystal
silicon layer of a substrate.
2. The flow meter of claim 1 wherein said deflection sensing means
is disposed on at least one of said torsion bars for generating
a torsion signal that indicates deflection of said vane from the
rest position.
3. The flow meter of claim 2 wherein said deflection sensing means
comprises:
at least four electrical ohmic contacts with a pair of said ohmic
contacts being disposed along a line that is substantially parallel
to the axis of the torsion bar; and
means for applying an electric current across a first pair of said
ohmic contacts while the torsion signal is sensed from a second
pair of said ohmic contacts that are oriented perpendicularly to
a line joining the first pair of said ohmic contacts.
4. The flow meter of claim 3 wherein alternating current ("AC")
is applied across the pair of ohmic contacts whereby the torsion
signal becomes a modulation envelope of the AC.
5. The flow meter of claim 1 wherein said substrate is a silicon
material which has both a [100] crystallographic direction and a
[110 ] crystallographic direction, and said torsion bars are oriented
along the [110 ] crystallographic direction for an n-type silicon
layer.
6. The flow meter of claim 1 wherein said semiconductor substrate
is a silicon material which has both a [100] crystallographic direction
and a [100] crystallographic direction, and said torsion bars are
oriented in the [100] crystallographic direction for a p-type silicon
layer.
7. The flow meter of claim 1 wherein said single crystal-silicon
layer is in a Simox wafer.
8. The flow meter of claim 1 wherein said single crystal-silicon
layer is in a silicon-on-insulator wafer.
9. The flow meter of claim 1 wherein rounded corners join said
torsion bars to said support.
10. The flow meter of claim 1 wherein rounded corners join said
torsion bars to said vane.
11. The flow meter of claim 1 wherein said torsion bars have a
surface layer of silicon carbide or silicon nitride formed thereon.
12. The flow meter of claim 1 wherein said support is frame-shaped
and encircles said vane.
13. The flow meter of claim 1 wherein the vane is substantially
thinner than the support.
14. The flow meter of claim 1 wherein mass around the center of
said vane is mostly etched away.
15. The flow meter of claim 1 wherein mass around the center of
said vane is completely etched away whereby said vane has a frame-shape.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of fluid flow measurement and,
more particularly, to miniaturized flow meters capable of measuring
flow rates of tens to hundreds of microliters/minute (".mu.l/min").
2. Description of the Prior Art
In many fields, the measurement of small fluid flows is extremely
important. Particularly in the fields of biological, medical sciences,
analytical instrumentation, and drug delivery, the measurement of
extremely small quantifies of fluids for dosimetry or titration,
either of liquids or for gases, is a common problem. Frequently
the quantity of fluid to be measured is almost in the nanoliter
range. Often it is desirable, especially to avoid contamination
in medical tests, that the flow meter be disposable and hence of
low cost. Absolute accurate measurements are not always necessary,
but precise and repeatable measurements are. For many of these applications,
traditional flow meters, scaled down to smaller geometries, are
infeasible. For example, flow meters that employ a pressure measurement
principle become very difficult to implement.
An article by Gass, V., van der Schoot, B. H., and de Rooij, N.
F. entitled "Nanofluid Handling by Micro-Flow-Sensor Based
on Drag Force Measurements," published in the Proceedings of
the IEEE Micro-Electronics Mechanical Systems conference held in
Fort Lauderdale, Fla. during 1993 IEEE Catalog no. 93CH3265-6 ("the
Gass article"), discloses a micro-flow-sensor for liquids that
operates over a flow range extending from 5 microliters/minute (".mu.l/min")
to 500 .mu.l/min. The micro-flow-sensor described in the Gass article
employs a cantilevered beam that carries piezo-resistors arranged
to form a Wheatstone bridge. Under laminar flow conditions, with
a low Reynolds number, the force due to the pressure difference
in the liquid on opposite sides of the cantilevered beam is negligible.
However, liquid flowing past a free end of the cantilevered beam,
and that flow's associated drag on the end of the beam due to viscous
shear in the liquid, bends the beam and thereby stresses the piezo-resistors
making up the Wheatstone bridge.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a flow meter for
measuring minute fluid flows.
Another object of the present invention is to provide an inexpensive
flow meter for measuring minute fluid flows.
Yet another object of the present invention is to provide a flow
meter for measuring minute fluid flows that is easy to manufacture.
Yet another object of the present invention is to provide a flow
meter for measuring minute fluid flows that is economical to manufacture.
Yet another object of the present invention is to provide a flow
meter for measuring minute fluid flows which is simple.
Yet another object of the present invention is to provide a flow
meter for measuring minute fluid flows which is reliable.
The present invention is a micromachined, monolithic silicon vane-type
flow meter for measuring very small flows. The flow meter includes
an outer frame from which inwardly projects a hinge. A vane, which
the hinge joins to and supports from the frame, is disposed in a
rest position with respect to the frame if no external force is
applied to the vane. The hinge permits deflection of the vane from
the rest position by a torque applied to the vane such as fluid
flowing through the flow meter. The flow meter also includes a deflection
sensing means for sensing deflection of the vane from the rest position.
The frame, hinge, vane and deflection sensing means are all monolithically
fabricated in a semiconductor single-crystal silicon layer of a
substrate.
In a preferred embodiment of the flow meter, the hinge is formed
by opposing torsion bars that are aligned along a common axis for
supporting the vane within the frame. The torsion bars support the
vane within the frame for rotation about the common axis of the
torsion bars. In this embodiment, the deflection sensing means is
integrated into at least one of the torsion bars for generating
a torsion signal that indicates deflection of the vane from the
rest position. The sensor integrated into the torsion bar is inexpensive
and is very sensitive. An alternative embodiment of the flow meter
employs a flexure hinge, whose bending is sensed by a piezo-resistive
bridge. In either embodiment, fluid impacting upon the vane, or
dynamic or flow pressure on the vane, either urges the vane to rotate
about the torsion bars' common axis, or bends the flexure hinge.
These and other features, objects and advantages will be understood
or apparent to those of ordinary skill in the art from the following
detailed description of the preferred embodiment as illustrated
in the various drawing figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a plan view illustrating a preferred embodiment of the
micromachined, vane-type flow meter using a torsion bar hinge to
support the vane within a frame, and having a sensor integrated
into the torsion bar;
FIG. 1b is a cross-sectional elevational view illustrating the
preferred embodiment of the flow meter using a torsion bar hinge
and sensor integrated into the torsion bar taken along the line
1b--1b in FIG. 1a;
FIG. 2a is a plan view illustrating an alternative embodiment of
the micromachined flow meter that is provided with a symmetrically
shaped inlet and outlet;
FIG. 2b is a cross-sectional elevational view illustrating the
alternative embodiment of the flow meter taken along the line 2b--2b
in FIG. 2a;
FIG. 3 is a plan view illustrating a four-terminal torsion sensor
that is integrated into the torsion bar taken along the line 3--3
in FIG. 1a;
FIG. 4a is a plan view illustrating an alternative embodiment of
the torsion bar hinge and vane of the flow meter which provides
for a better match to etching geometries;
FIG. 4b is a cross-sectional elevational view illustrating the
alternative embodiment of the torsion bar hinge and vane of the
flow meter taken along the line 4b--4b in FIG. 4a;
FIG. 5a is plan view illustrating an embodiment of the flow meter
in which a central flexure hinge, that incorporates a piezo-resistive
bridge arrangement, supports and senses deflection of the vane;
FIG. 5b is a cross-sectional elevational view illustrating the
embodiment of the flow meter having a central flexure hinge that
incorporates the piezo-resistive bridge taken along the line 5b--5b
of FIG. 5a;
FIG. 5c is a cross-sectional elevational view illustrating the
embodiment of the flow meter having a central flexure hinge that
incorporates the piezo-resistive bridge taken along the line 5c--5c
of FIG. 5a;
FIG. 6a is a plan view illustrating an alternative embodiment of
the flow meter having a spaced-apart pair of flexure hinges that
incorporates a piezo-resistive bridge arrangement for measuring
bending into at least one of the flexure hinges;
FIG. 6b is a cross-sectional elevational view illustrating the
embodiment of the flow meter having a spaced-apart pair of flexure
hinges that incorporates a piezo-resistive bridge arrangement taken
along the line 6b--6b of FIG. 6a;
FIG. 6c is a cross-sectional elevational view illustrating the
embodiment of the flow meter having a spaced-apart pair of flexure
hinges that incorporates a piezo-resistive bridge arrangement taken
along the line 6c--6c of FIG. 6a;
FIG. 7 is a cross-sectional view of a large diameter tube that
has a flow meter supported at the center of the tube thereby adapting
the flow meter for measuring a flow rate for larger volume lows
using a force-feedback technique;
FIG. 8 is a plan view depicting an alternative embodiment of the
central flexure hinge flow meter depicted in FIG. 5a that is adapted
for directly measuring flow rate using the force-feedback technique
employed for the larger volume flow meter depicted in FIG. 7;
FIG. 9 is a cross-sectional view of a "vortex shedding"
flowmeter that may incorporate any of the flow-meters depicted respectively
in FIGS. 1a, 1b; 4a, 4b; 5a, 5b, 5c; 6a, 6b and 6c;
FIG. 10a is a plan view of an alternative embodiment vane-type
flow meter in which the vane surrounds a central mounting pillar;
FIG. 10b is a cross-sectional view of the alternative embodiment
flow meter in which the vane surrounds a central mounting pillar
taken along the line 10b--10b in FIG. 10a; and
FIG. 11 is a cross-sectional view of the alternative embodiment
flow meter in which the vane surrounds a central mounting pillar
adapted for sensing the flow rate of a larger volume flow.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIGS. 1a and 1b illustrate a preferred embodiment of the micromachined
flow meter referred to by the general reference character 20. The
flow meter 20 includes an outer frame 22 from which inwardly projects
a pair of opposing torsion bars 24. The torsion bars 24 are aligned
along a common axis 26 for supporting a vane 28 in a rest position
illustrated in FIG. 1b. A gap 32 separates the vane 28 from the
encircling frame 22 except where the torsion bars 24 join the vane
28 to the frame 22 and the vane 28 is shaped asymmetrically with
respect to the common axis 26 of the torsion bars 24. However, in
principle, torsion bars 24 need not be located at one edge of the
vane 28 as depicted in FIG. 1a, but may be more symmetrically located.
In general, the gap 32 between the vane 28 and the frame 22 is made
as small as possible to avoid leakage and loss of flow meter sensitivity.
The gap 32 as narrow as a few microns, may be easily fabricated
using micromachining. The torsion bars 24 functioning as hinges,
support the vane 28 within the frame 22 for rotation with respect
to the frame 22 about the common axis 26 of the torsion bars 24.
The frame 22 torsion bars 24 and vane 28 are all monolithically
fabricated from a semiconductor single-crystal silicon layer of
a substrate wafer. When using a common [100] oriented silicon wafer,
the torsion bars 24 are preferably oriented along the [100] crystallographic
direction, or the [110] crystallo-graphic direction. The physical
properties of the torsion bars 24 in relationship to the vane 28
are those described in U.S. patent application Ser. No. 08/139397
filed Oct. 18 1993 entitled "Micromachined Torsional Scanner."
This ensures that the vane 28 rotates about the common axis 26 upon
application of a torque. The flow meter 20 is generally fabricated
using SOI (Silicon on Insulator) type silicon wafers, such as Simox
or bonded wafers as described in the patent application identified
above. Such a substrate provides a natural etch stop in the process
of fabricating the flow meter 20 a good definition for the vane
28 and stress-free, single crystal torsion bars 24 which are essential
to the functioning of the flow meter 20. Reactive ion etching or
wet etching can be used to define the vane 28. The torsion bars
24 and a central portion 34 of the vane 28 are made from the top
silicon on the SOI wafer, similar to scanner fabrication described
in the patent application identified above. The vane 28 may be formed
from the full thickness of the substrate, or the bulk of the silicon
may be removed from the backside of the substrate by aniso-tropic
etching leaving a pyramidally-shaped, box-shaped reinforcing frame
36 encircling the perimeter of the central portion 34 of the vane
28.
At least one of the torsion bars 24 carries a torsion sensor 42
a four-terminal version of which is illustrated in greater detail
in FIG. 3. The torsion sensor 42 is of the type described both in
U.S. patent application Ser. No. 08/139397 identified above, and
in U.S. Pat. No. 5488862 entitled "Monolithic Silicon Rate-Gyro
With Integrated Sensors" which issued on Feb. 6 1996 both
of which were filed by the inventors of the present application.
Both the patent application and issued patent are hereby incorporated
herein by reference.
As described in the patent application and patent identified above,
the orientation of the torsion bars 24 is along the [100] direction
if the active material of the sensor is p-type silicon, or in the
[110] direction if the active torsion sensor material is n-type.
The torsion sensor 42 is located in diffused or implanted region
44 within the torsion bar 24. The torsion sensor 42 preferably has
two pairs of metallic sensor leads 46a and 46b which make ohmic
contact with the implanted region 44 and which are elsewhere insulated
from the torsion bar 24 and from the frame 22. The sensor leads
46a and 46b terminate on the frame 22 in individual torsion sensor
pads, not illustrated in any of the FIGs. The torsion sensor pads
permit bonding or soldering the sensor leads 46a and 46b to external
leads, also not illustrated in any of the FIGs. An electric current
is applied to the torsion sensor 42 through a pair of leads 46a,
and a sensor output voltage is measured between a pair of leads
46b. The implanted region 44 restrains the electric current to the
top surface of the torsion bar 24 so the electric current is used
optimally for sensing twisting of the torsion bar 24. The sensor
output voltage appearing across leads 46b is proportional to the
current flowing between leads 46a, and the rotational-position of
the vane 28 about the common axis 26 of the torsion bars 24 with
respect to the frame 22. The torsion sensor 42 can resolve very
small rotations, on the order of a microradian, and can be thermally
compensated as known in the art.
The torsion bars 24 may be hardened by conversion of a surface
layer thereof into silicon carbide or silicon nitride. The torsion
sensor 42 may be also overcoated with dielectrics such as oxides
or nitrides, or polymers, to isolate the torsion sensor 42 electrically
from the fluid if so desired. If the torsion sensor 42 is partially
overlapped by the underlaying silicon support, then the oxide is
preferentially etched out sideways from underneath the torsion sensor
42 to permit the torsion bars 24 to freely rotate. For use with
liquids, the flow meter 20 may be coated with a wetting agent to
avoid entrapping gases. The flow meter 20 may also be operated in
a vertical orientation, with the hinge at the bottom. Such an orientation
for the flow meter 20 helps remove trapped gas bubbles if any are
present.
Referring again to FIGS. 1a and 1b, the flow meter 20 is bonded
or otherwise glued or attached to a support base 52 possibly also
made out of silicon or glass. The support base 52 includes a cavity
54 large enough to accommodate the vane 28 when deflected by pressure
applied to the vane 28 by fluid flowing through the flow meter 20.
One or more ports 56 only one of which is illustrated in FIG. 1b,
vent fluid from the flow meter 20. The sensor leads 46a and 46b
for the torsion sensor 42 may pass between the frame 22 and the
juxtaposed support base 52. In principle, the support base 52 may
be made from any material suitable for attachment to the frame 22
such as a plastic material. Thus the support base 52 need not be
made from silicon.
Operation of the flow meter 20 is, in principle, identical to a
macroscopic vane flow meter. With no fluid flow, the vane 28 is
undeflected, and the output voltage from the torsion sensor 42 is
zero. When a flow of fluid, indicated by arrows 58 in FIG. 1b, passes
through the flow meter 20 the vane 28 deflects downward into the
cavity 54 thereby rotating around the common axis 26 of the torsion
bars 24. As a result, the torsion sensor 42 produces an output voltage
on the leads 46b that is related to how fast the fluid is flowing
through the flow meter 20. The output signal produced by the torsion
sensor 42 is not necessarily linearly related to the flow rate,
but the relationship is unique. Because the torsion sensor 42 in
the torsion bar 24 is extremely sensitive, very good resolution
and repeatability can be obtained. The output signal from the torsion
sensor 42 on the leads 46b is bipolar. Thus, the output signal of
the torsion sensor 42 reverses if the vane 28 deflects away from
the cavity 54 rather than into the cavity 54. An alternating current
("AC") may be applied across the leads 46a which causes
the electrical signal produced by the torsion sensor 42 on the leads
46b to become a modulation envelope of the applied AC thereby removing
any direct current ("DC") offset. If AC is applied to
the torsion sensor 42 the modulated AC signal present on the leads
46b reverses phase if the vane 28 deflects away from the cavity
54 rather than toward the cavity 54. Regardless of whether a direct
current ("DC") or AC current is applied to the torsion
sensor 42 the relationships between fluid flow rate and the output
signal produced by the torsion sensor 42 if the vane 28 is deflected
in opposite directions are not necessarily identical.
FIGS. 2a and 2b respectively depict plan and cross-sectional elevational
views of a slightly different embodiment of the flow meter 20. In
the embodiment depicted in FIGS. 2a and 2b, both an inlet 64 and
outlet 66 of the flow meter 20 are shaped symmetrically. Those elements
depicted in FIGS. 2a and 2b that are common to the flow meter 20
depicted in FIGS. 1a and 1b carry the same reference numeral distinguished
by a prime (" ' ") designation. The embodiment depicted
in FIGS. 2a and 2b is readily fabricated from silicon by anisotropic
etching. Again, the frame 22' may be attached to the support base
52' by standard silicon bonding techniques, or they may simply be
glued together.
An alternative configuration for the vane 28 and torsion bars 24
is illustrated for the alternative embodiment of the flow meter
20 depicted in FIGS. 4a and 4b. Those elements depicted in FIGS.
4a and 4b that are common to the flow meter 20 depicted in FIGS.
1a and 1b carry the same reference numeral distinguished by a double
prime (" " ") designation. The torsion bars 24"
depicted in FIGS. 4a and 4b are re-entrant in that both of the torsion
bars 24" extend for some distance along one edge of the vane
28". This shape for the vane 28" and the torsion bars
24" including the torsion sensor 42" now fits within a
simple rectangular area in the frame 22". This configuration
for the vane 28" and the torsion bars 24" fits better
to the anisotropic etching geometries which may be used for the
support base 52. As before, for best and most sensitive operation,
the gap 32" between the frame 22" and vane 28" is
made as narrow as practicable. The vane 28" may or may not
include the pyramidally shaped reinforcing frame 36" encircling
the perimeter of the central portion 34" of the vane 28",
or the vane 28" may equal the full thickness of the wafer substrate,
not illustrated in any of the FIGs.
FIGS. 5a, 5b and 5c depict another alternative embodiment of the
flow meter 20. Those elements depicted in FIGS. 5a, 5b and 5c that
are common to the flow meter 20 depicted in FIGS. 1a and 1b carry
the same reference numeral distinguished by a triple prime ("
'" ") designation. In the embodiment depicted in FIGS.
5a, 5b and 5c, the vane 28'" is attached by a flexure hinge
72 to the frame 22'". The flexure hinge 72 simply bends, rather
than twisting as the torsion bars 24. The stresses in the flexure
hinge 72 caused by bending are sensed in the flow meter 20'"
depicted in FIGS. 5a, 5b and 5c by a four terminal, piezo-resistor
Wheatstone bridge as is known in the art. Accordingly, the flow
meter 20'" includes at least one piezo-resistor 74a located
on the flexure hinge 72 and three compensating piezo-resistors
74b, 74c and 74d located elsewhere on the flow meter 20'".
The piezo-resistors 74a-74d are interconnected on the frame 22'"
to form a Wheatstone bridge. Stretching or compression of the piezo-resistor
74a due to bending of the flexure hinge 72 unbalances the Wheatstone
bridge. Note that the flow meter 20'" may or may not include
the reinforcing frame 36'".
FIGS. 6a, 6b and 6c depict an alternative embodiment of the flow
meter 20'" illustrated in FIGS. 5a, 5b and 5c. Those elements
depicted in FIGS. 6a, 6b and 6c that are common to the flow meter
20'" depicted in FIGS. 5a, 5b and 5c carry the same reference
numeral distinguished by a quadruple prime (" ""
") designation. The flow meter 20"" depicted in FIGS.
6a, 6b and 6c replaces the central flexure hinge 72 of the embodiment
depicted in FIGS. 5a, 5b and 5c with two symmetrically located flexure
hinges 72"" that attach the vane 28"" to the
frame 22"". Again, at least one of the flexure hinges
72"" carries the piezo-resistor 74a"" while
the flow meter 20"" carries the piezo-resistors 74b"",
74c"" and 74d"". Both flexure hinges 72""
may include the piezo-resistor 74a"", and the signals
the pair of piezo-resistors 74a"" may be combined to cancel
parasitic effects. Regardless of whether the flow meter 20""
includes one or two piezo-resistors 74a"", the piezo-resistor
74a"" produces an electrical signal responsive to stress
in the flexure hinge 72"" created by bending of the flexure
hinge 72"". Similar to the embodiment of the flow meter
20'" depicted in FIGS. 5a, 5b and 5c, the piezo-resistors 74a""-74d""
are interconnected to form a Wheatstone Bridge, and the vane 28""
may include or omit the reinforcing frame 36.
FIG. 7 depicts an adaptation of the flow meter 20 for measuring
larger volume flows in which the flow meter 20 is supported at the
center of a large diameter external tube 82. Most of the fluid flowing
through external tube 82 bypasses the flow meter 20 but pressure
across the flow meter 20 applies a deflecting force to the vane
28. For the application of the flow meter 20 depicted in FIG. 7
the vane 28 is preferably maintained in its undeflected rest position,
for example by supplying an electric current through a coil 84 formed
on the vane 28 that lies in a magnetic field established by a permanent
magnet 86 mounted on the frame 22. The output signal from the torsion
sensor 42 depicted in FIG. 7 (or the Weatstone bridge formed by
the piezo-resistors 74a-74d for the flexure hinge 72) is employed
in a feedback circuit located externally to the flow meter 20 to
establish a force-feedback operating mode for the flow meter 20.
The force-feedback operating mode maintains the vane 28 in its undeflected
rest position. Operating in such a force-feedback mode, the electric
current passing through the coil 84 measures the rate at which fluid
flows past the flow meter 20.
Omitting the external tube 82 FIG. 8 depicts an alternative embodiment
of the flow meter 20'" depicted in FIG. 5a, 5b and 5c that
is adapted for using the force-feedback principle in measuring minute
flow rates. Those elements depicted in FIG. 8 that are common to
the flow meter 20'" depicted in FIGS. 5a, 5b and 5c and to
the flow meter 20 depicted in FIG. 7 carry the same reference numeral
distinguished by a quintuple prime (" '"" ")
designation. In the alternative embodiment of the flow meter 20'""
depicted in FIG. 7 the gap 32'"" between vane 28'""
and frame 22'"" is deliberately enlarged to provide a
passage for fluid flowing through the flow meter 20'"".
Similar to the flow meter 20 depicted in FIG. 7 the vane 28'""
is maintained in its undeflected rest position against the static
pressure of flowing fluid by forces resulting from a magnet field
established by the permanent magnet 86'"" and electric
current flowing through the coil 84'"". The signal from
the piezo-resistors 74a-74d is supplied to an external feed-back
circuit which supplies the electric current to the coil 84'""
which maintains the vane 28'"" in its undeflected rest
position. The flow meter 20'"" illustrated in FIG. 8 uses
the flexure hinge 72'"", but can, of course, also be fabricated
with the torsion bars 24. If the forces applied to the vane 28'""
by fluid flowing past the flow meter 20'"" through the
gap 32'"" are substantial, then a permanent magnet may
be mounted on the vane 28'"", and a magnetic field established
by an electromagnet external to the flow meter 20'"" may
be used to maintain the vane 28'"" in its undeflected
rest position. The size of the vane 28'"" can be adjusted
with respect to the gap 32'"" to match the forces applied
to the vane 28'"" by fluid flowing through the flow meter
20'"" with the forces applied to the vane 28'""
by the permanent magnet and the electromagnet.
Thus far two different classes of micromachined vane-type flow
meters have been described, those which employ torsion bars 24 for
supporting the moveable vane 28 or those which employ the flexure
hinge 72 for supporting the moveable vane 28. In connection with
both classes of flow meters, the moving vane 28 has been described
as being oriented to obstruct the fluid flowing through the flow
meter 20. Impeding at least part of the fluid flow in this way applies
forces to the vane 28 which causes it either to twist the torsion
bars 24 or to bend the flexure hinge 72 thereby producing an electrical
signal either in the torsion sensor 42 or in the piezo-resistors
74a-74d. Depending upon a particular application for the flow meter
20 it becomes necessary to select a particular size for the vane
28 and for the gap 32 which tailors the flow meter 20 so the torsion
sensor 42 or the piezo-resistors 74a-74d produce a signal which
accurately measures flow rate over the anticipated range of minimum
and maximum flow rates. Also, in some applications, movement of
the vane 28 responsive to fluid flowing through the flow meter 20
affects the rate of fluid flow slightly, resulting in a non-linear
flow rate vs. pressure relationship across the flow meter 20.
In addition to an orientation in which the vane 28 obstructs fluid
flow through the flow meter 20 another orientation for the flow
meter 20 exists in which the vane 28 is oriented substantially parallel
to the flowing fluid. With the vane 28 oriented substantially parallel
to the flowing fluid, the flow meter 20 may operate as a so-called
a "vortex shedding" flow meter. FIG. 9 is a cross-sectional
diagram depicting such a "vortex shedding" flow meter
in which a tube 92 establishes a channel 94 in which a vortex bluff
96 is located upstream from the flow meter 20. In such a configuration,
the flow meter 20 measures fluid flow rate by detecting vortices
shed by fluid as it flows past the vortex bluff 96 at a Reynolds
number high enough to form a von Karman vortex street downstream
from the vortex bluff 96. In such a flow meter, the vortex shedding
rate is directly proportional to the fluid flow rate. In practice,
this means that a body immersed in the flow stream receives a vibrational
input at a frequency directly proportional to fluid flow rate. The
most sensitive orientation for the vane 28 places the common axis
26 in a plane that is parallel to the flowing fluid so that the
motion of the vane 28 is perpendicular to the flowing fluid. Oriented
in this way, the signal produced by the torsion sensor 42 or by
the piezo-resistors 74a-74d will have a frequency proportional to
the fluid flow rate. In such an orientation, the vane 28 moves in
response to vortices in the flowing fluid, but does not appreciably
affect the flow of the fluid. An advantage of this orientation for
the flow meter 20 is that it permits measuring flow rates in circumstances
in which it would be impractical to allow the flow meter 20 including
the vane 28 to impede the flow of fluid past the flow meter 20
or in which for reasons of linearity it is undesirable to allow
the vane 28 to change orientation depending upon the fluid flow
rate.
To avoid a large change in the magnitude of the output signal from
the torsion sensor 42 or the piezo-resistors 74a-74d as a function
of frequency, a flow meter 20 adapted for use as a "vortex
shedding" flow meter will preferably have a resonant frequency
of oscillation about the common axis 26 that is much higher than
the vortex frequencies anticipated during normal operation. At frequencies
above the resonant frequency of rotation of the vane 28 about the
reinforcing frame 36 the output signal from the torsion sensor
42 or from the piezo-resistors 74a-74d decreases swiftly with increasing
frequency; while below that resonant frequency the amplitude of
the output signal remains substantially constant. However, operating
the flow meter 20 in the "vortex shedding" manner reduces
sensitivity of the flow rate measurement to the output signal produced
by the torsion sensor 42 or from the piezo-resistors 74a-74d. Since
the frequency of vortex shedding is proportional to the fluid flow
rate, the frequency of the output signal from the torsion sensor
42 or from the piezo-resistors 74a-74d represents the true fluid
flow rate past the flow meter 20.
This is independence of fluid flow rate from the amplitude of the
output signal produced by the torsion sensor 42 or by the piezo-resistors
74a-74d is one of the fundamental advantages of the "vortex
shedding" sensing orientation for the flow meter 20. The amplitude
of the output signal from the torsion sensor 42 or from the piezo-resistors
74a-74d may vary significantly without adversely affecting resolution
of the frequency of the output signal, i.e. the fluid flow rate.
This fact permits great flexibility in the adapting the flow meter
20 to various applications. For instance, the resistance of piezo-resistors
74a-74d may vary with temperature independent of bending of the
flexure hinge 72. In many applications, it is necessary to compensate
the piezo-resistors 74a-74d for temperature changes or to hold the
temperature constant if the amplitude of the output signal measures
the fluid flow rate. In the "vortex shedding" configuration,
the amplitude of the output signal is less important, since the
frequency of the output signal may be measured independently of
the output signal's amplitude. Consequently, in the "vortex
shedding" configuration temperature variations do not significantly
affect the accuracy of the flow rate measurement. Reducing or eliminating
any need for temperature compensation or temperature regulation
can makes a substantial difference in the cost of the system used
to measure the output signal from the piezo-resistors 74a-74d.
FIGS. 10a and 10b depict a less preferred alternative embodiment
of the flow meter 20 in which a central pillar 102 supports a surrounding
vane. Those elements depicted in FIGS. 10a and 10b that are common
to the flow meter 20 depicted in FIGS. 1a and 1b carry the same
reference numeral distinguished by a sextuple prime (" """
") designation. In the alternative embodiment depicted in FIG.
2 torsion bars 24""" extending outward from the
central pillar 102 support the vane 28""", which
surrounds the central pillar 102 for rotation about the common
axis 26""". As illustrated in FIG., fluid flowing
through the support base 52""", indicated by the
arrows 58""", impinges upon the vane 28""".
While FIGS. 10a and 10b depict the alternative embodiment flow meter
20""" as employing the torsion bars 24"""
to support the vane 28""", it is readily apparent
that the vane 28""" could also be supported by either
of the flexure hinges 72 with their piezo-resistors 74a-74d depicted
respectively in FIGS. 5a, 5b and 5c; or 6a, 6b and 6c.
FIG. 11 depicts the less preferred alternative embodiment of the
flow meter 20 in which the central pillar 102 supports a surrounding
vane adapted for sensing the flow rate of a larger volume flow.
Those elements depicted in FIG. 11 that are common to the flow meter
20""" depicted in FIGS. 10a and 10b carry the same
reference numeral distinguished by a septuple prime (" '"""
") designation. In the embodiment depicted in FIG. 11 the
support base 52'""" supports the flow meter 20'"""
within a larger diameter tube 104 that permits much of the flow
to pass around the vane 28'""".
The flow meter 20 is not easily clogged. The area of the vane 28
can be easily varied between a few tens of square microns to a square
cm or more. For a vane 28 having such a large area, the vane might
be the thickness of the wafer as described in the patent application
and patent identified above. The thickness of the torsion bars 24
is established by the thickness of the top SOI layer, and the width
and length of the torsion bars 24 may range from a few microns to
several hundred microns, depending upon flow rate to be sensed.
The flow meter 20 may have a thickness "t," indicated
by a double headed arrow 106 in FIG. 5b, ranges between 50 and 500
microns, but more commonly is between 100 microns and 500 microns.
The thickness of the flow meter 20 may be as little as 1.0 mm, even
less if so desired. The thickness of the vane 28 may equal that
of the torsion bars 24 (i.e. the upper silicon layer of the SOI
substrate), the full wafer thickness, or may alternatively be formed
as the reinforcing frame 36 which provides stiffness, yet lowers
the mass of the vane 28.
Although the present invention has been described in terms of the
presently preferred embodiment, it is to be understood that such
disclosure is purely illustrative and is not to be interpreted as
limiting. Consequently, without departing from the spirit and scope
of the invention, various alterations, modifications, and/or alternative
applications of the invention will, no doubt, be suggested to those
skilled in the art after having read the preceding disclosure. Accordingly,
it is intended that the following claims be interpreted as encompassing
all alterations, modifications, or alternative applications as fall
within the true spirit and scope of the invention. |