In a flow meter of the traveling wave type having a flexible undulating
membrane, velocity detecting means is utilized to determine the
velocity of propagation of the traveling wave associated with the
undulating flexible membrane between at least two known locations
on the flexible membrane. The velocity of propagation is proportional
to the volumetric flow rate of fluid through the flow meter.
1. A positive displacement flow meter comprising:
means for conducting fluid;
flexure means for undulating in response to fluid flow within said
conducting means; and
means for detecting the velocity of propagation of a traveling
wave associated with said undulating flexure means.
2. An apparatus according to claim 1 wherein said velocity detecting
means includes means for generating output signals at a plurality
of known locations on said flexure means.
3. An apparatus according to claim 2 wherein said means for generating
output signals includes at least two metallized polymer piezoelectric
strips affixed to said flexure means and having output leads coupled
4. An apparatus according to claim 3 wherein said polymer piezoelectric
strips are affixed to means for holding said flexure means.
5. An apparatus according to claim 2 wherein said velocity detecting
means includes strain gauges.
6. An apparatus according to claim 1 wherein said flexure means
includes a flexible membrane of piezoelectric polymer material having
known metallized locations thereon and having output leads coupled
to said metallized locations.
7. An apparatus according to claim 3 4 5 or 6 wherein said velocity
detecting means includes means for determining time .DELTA.t.sub.ij
for traveling waves to propagate a distance .DELTA.X.sub.ij between
known locations, and means for determining velocity V.sub.ij =.DELTA.X.sub.ij
8. An apparatus according to claim 7 which further includes means
for providing flow rates which are proportional to the detected
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to devices for measuring flow rates, and
more specifically to a traveling wave flow meter having an undulating
flexible membrane, wherein the velocity of propagation of a traveling
wave associated with the flexible membrane is a function of volumetric
2. Description of the Prior Art
A traveling wave flow meter having an undulating flexible membrane
whose frequency is a function of volumetric flow is described in
U.S. Pat. No. 4358954 entitled "Traveling Wave Flow Meter",
issued Nov. 16 1982 to Joannes de Jong. The flow meter includes
a strip of flexible material having a length X which is forced to
occupy a length of channel L, wherein (L-X). As a result, the flexible
membrane buckles and assumes a wavelike shape inside the channel.
When fluid is forced to flow through the channel, the flexible membrane
undulates in a traveling wave type of displacement and the frequency
of the traveling wave is a function of the volumetric flow rate
of the fluid through the channel.
The response time of the above described traveling wave flow meter
is relatively slow, since the response time is dictated by the frequency
of the undulating flexible membrane. In interfacing the traveling
wave flow meter with electronic processing devices, the relatively
slow response time may prove to be particularly disadvantageous.
Accordingly, there is a need for a traveling wave flow meter having
a relatively fast response time.
SUMMARY OF THE INVENTION
A traveling wave flow meter having a relatively fast response time
includes a flexible membrane within a channel and means for detecting
the velocity of propagation of a traveling wave associated with
the undulating flexible membrane. The velocity of propagation is
detected by choosing at least two known locations on the flexible
membrane and measuring the distance therebetween as well as measuring
the travel time or phase difference of a traveling wave between
the known locations. Preferably, either strain gauges or piezoelectric
polymer strips are disposed at known locations on the flexible membrane,
and as the flexible membrane undulates output signals are generated.
Since the distances between the strain gauges or piezoelectric polymer
strips are known, the velocity of the traveling wave may be determined
by timing and processing the generated output signals. In an alternate
embodiment of the present invention, the flexible membrane may be
fabricated from a piezoelectric material which is metallized in
known locations. Leads coupled to the metallized locations provide
the necessary output signals as the flexible piezoelectric membrane
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of the apparatus of the present invention.
FIG. 2 is a sectional front view of the apparatus of the present
FIG. 3 is an enlarged plan view of a portion of the apparatus of
FIG. 4 is an alternate embodiment of the apparatus of FIG. 3.
FIGS. 5a and 5b depict a flexible membrane included in the apparatus
of the present invention.
FIG. 6 is a block diagram of a propagation velocity and volumetric
flow rate computer.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIGS. 1 and 2 plan and sectional front views
of a flow meter 10 are provided. The flow meter 10 includes a housing
11 and a channel 12 which is formed therein. Preferably, the housing
11 is made of aluminum bar stock or other suitable material and
is sealed with a cover 21 which may be made of plexiglass or other
suitable material. The cover 21 is secured by screws 20 which are
threaded into the housing 11. The generally rectangular channel
12 is approximately 8" in length and includes an inlet 13 and
an outlet 14 through which a continuous flow of fluid may be conducted.
Disposed within the channel 12 is a flexible membrane 15 of a predetermined
length X which is forced to occupy a predetermined length L of a
portion of channel 12. The flexible membrane 15 is preferably fabricated
from mylar or other suitable polymer, and in an alternate embodiment
of the present invention it is fabricated from a piezoelectric polymer
such as polyvinylidene fluoride PVF.sub.2. Typically, the flexible
membrane 15 has dimensions of approximately 0.005 inch thick, 13/16
inch wide, and 3 inches long. Moreover, the flexible membrane 15
is accurately dimensioned in width such that there is adequate clearance
for free movement of the flexible membrane 15 inside the channel
without excessive leakage flow. Forcing the flexible membrane 15
to assume length L in a portion of channel 12 results in a buckling
of the flexible membrane 15.
The ends of the flexible membrane 15 are illustrated as being held
by membrane holders 16 17 which are slotted to allow the passage
of fluid along the flexible membrane 15. A lead screw 18 may be
threaded through the housing 11 and coupled to the membrane holder
17 such that the distance L between the membrane holders 16 17
may be precisely adjusted. A set screw 22 may be threaded through
the housing 11 to secure the membrane holder 16 within the channel
12. In the preferred embodiment at least two strips of piezoelectric
polymer 24 are affixed to the flexible membrane 15 at known locations.
It should be noted that in further embodiments of the present invention,
strain gauges of the type having a resistive element whose output
is proportional to the amount of deformation may be substituted
for the strips of piezoelectric polymer 24. In still further embodiments
of the invention the flexible membrane 15 itself may be fabricated
from a piezoelectric polymer instead of affixing the strips of piezoelectric
Referring now to FIG. 3 an enlarged plan view depicts a strip
of piezoelectric polymer 24 affixed to the flexible membrane 15.
The piezoelectric polymer strip 24 is shown as being affixed to
the flexible membrane 15 near the membrane holder 17. Preferably,
the piezoelectric strip is fabricated from a piece of PVF.sub.2
approximately 0.001" thick which is metallized on both sides.
PVF.sub.2 having a sputtered metal coating is a commercially procurable
product. A wire lead 30 is attached to the metallized piezoelectric
strip 24 by a bead of conductive epoxy 32. A hole 33 approximately
1/16" in diameter is drilled into the flexible membrane 15
which is epoxied to the membrane holder 17 by layers of epoxy 35.
The wire lead 30 is then fed through the hole 33 and nonconductive
epoxy is used to affix the metallized piezoelectric strip 24 to
the flexible membrane 15. A second wire lead 31 is attached to the
metallized piezoelectric strip 24 by a bead of conductive epoxy
34. The wire leads 30 31 are epoxied to the membrane holder 17
and attached to pins 19 19' disposed in the plexiglass cover 21.
Referring now to FIG. 4 an alternate technique for affixing the
piezoelectric polymer strip 24 to the flexible membrane 15 is illustrated.
The membrane holder 17 is fabricated from a nonconductive material
or the interior portion is covered with an insulating material.
The wire leads 30 31 are epoxied to both sides of the metallized
piezoelectric polymer strip 24. The flexible membrane 15 and the
metallized piezoelectric polymer strip are epoxied together as well
as the membrane holder 17 by layers of epoxy 35. Wire leads 3031
are also epoxied to the membrane holder 17.
Referring now to FIGS. 5a, and 5b, plan and front views illustrate
the flexible membrane 15 in a flattened and extended disposition.
Illustrating the flexible membrane 15 in this disposition is useful
in explaining an alternate embodiment of the present invention and
useful in explaining the relationship of known points on the flexible
membrane 15. An alternate embodiment of the present invention may
be realized by fabricating the entire flexible membrane 15 from
the piezoelectric polymer PVF.sub.2. In known locations on the flexible
membrane 15 for example, S.sub.1 S.sub.2 . . . S.sub.j, the piezoelectric
polymer is metallized. It should be noted that when the flexible
membrane 15 is undulating in a traveling wave type of displacement
inside channel 12 locations S.sub.1 S.sub.2 . . . S.sub.j on
the flexible membrane 15 are displaced in a direction substantially
traverse to the X axis which is parallel to the direction of fluid
flow. Accordingly, when the flexible membrane is undulating the
distance between any two known locations S.sub.i and S.sub.j may
be designated as .DELTA.X.sub.ij, a substantially constant known
distance. Electrical output signals may be generated by attaching
wire leads to each of the metallized locations S.sub.1 S.sub.2
S.sub.3 S.sub.4 S.sub.5 on the piezoelectric flexible membrane
15 in FIG. 5.
Referring back to FIG. 1 it can be appreciated that in the preferred
embodiment of the present invention there are two piezoelectric
polymer strips 24 separated by a substantially constant distance
.DELTA.X. In operation, when the flexible membrane undulates, the
piezoelectric polymer strips 24 are subject to alternating tension
and compression forces which generate a pair of output signals at
the output terminals or pins 19 19'. The output signals have a
generally sinusoidal waveform. The peak to peak time difference
.DELTA.t.sub.ij between the two generally sinusoidal waveforms or
the phase difference therebetween are proportional to the time required
for the traveling wave to propagate the distance .DELTA.X.sub.ij,
i.e., the known distance between piezoelectric polymer strips 24.
Referring to FIG. 6 since the distance .DELTA.X.sub.ij is known
and since the time difference .DELTA.t.sub.ij may be measured with
electronic time difference detector 25 the velocity of propagation
between points S.sub.i and S.sub.j may be computed by divider 26
using the equation V.sub.ij =.DELTA.X.sub.ij .DELTA./T.sub.ij. The
traveling wave flow meter is a positive displacement type of flow
meter, and the average velocity of fluid flow is equal to the velocity
of propagation of the traveling wave, i.e., V.sub.ave =V.sub.ij.
It is well known that volumetric flow rate Q can be computed from
the equation V.sub.ave =Q/A, where A is the cross sectional area
of the channel 12. Thus, the volumetric flow through the channel
12 may be computed by cross sectional area multiplier 27 from the
equation Q=A.DELTA.X.sub.ij /.DELTA.t.sub.ij.
Accordingly, it should be appreciated that the response time of
the traveling wave flow meter can be made considerably faster by
using the velocity of propagation rather than by using the frequency
of the undulating flexible membrane to compute the volumetric flow
as disclosed in copending application Ser. No. 210088. It should
be further appreciated that by increasing the number of known locations
on the flexible membrane at which an output signal is generated
the response time of the traveling wave flow meter may be made even
faster. For example, if the number of known locations is doubled
from two to four, then the response time can be cut in half.
While the invention has been described in its preferred embodiments,
it is to be understood that the words which have been used are words
of description rather than limitation and that changes may be made
within the purview of the appended claims without departing from
the true scope and spirit of the invention in its broader aspects.