U.S. patent application number 09/754727 was filed with the patent office on 2001-08-23 for burst mode ultrasonic flow sensor.
Invention is credited to Feller, Murray F..
Application Number | 20010015107 09/754727 |
Document ID | / |
Family ID | 23820299 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010015107 |
Kind Code |
A1 |
Feller, Murray F. |
August 23, 2001 |
Burst mode ultrasonic flow sensor
Abstract
An ultrasonic transit time flow sensor employs two transducers
spaced out along a direction of fluid flow. A variable frequency
acoustic signal is simultaneously transmitted by both transducers
in a burst. After an interval corresponding to the expected transit
time between transducers, both transducers are switched to their
respective receiving states and a phase difference between their
received signals is used as a measure of the fluid flow rate. The
invention also provides a feedback arrangement for controlling the
acoustic frequency to maintain stable operation under a variety of
operating conditions.
Inventors: |
Feller, Murray F.;
(Micanopy, FL) |
Correspondence
Address: |
DAVID KIEWIT
5901 THIRD ST SOUTH
ST PETERSBURG
FL
33705
US
|
Family ID: |
23820299 |
Appl. No.: |
09/754727 |
Filed: |
January 4, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09754727 |
Jan 4, 2001 |
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09458315 |
Dec 10, 1999 |
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6178827 |
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Current U.S.
Class: |
73/861.27 |
Current CPC
Class: |
G01F 1/662 20130101;
G01F 1/667 20130101; G01F 5/00 20130101 |
Class at
Publication: |
73/861.27 |
International
Class: |
G01F 001/66 |
Claims
1. A flow sensor for determining a flow rate of a fluid from
measurements of acoustic transmissions through the fluid, the
sensor comprising: first and second transducers adapted to be
located in acoustic contact with the fluid, each of the transducers
having a respective transmitting state and a respective receiving
state, each of the transducers adapted to transmit acoustic energy
when in its respective transmitting state, each transducer adapted
to receive acoustic energy transmitted by the other transducer when
said each transducer is in its respective receiving state; a
carrier signal oscillator circuit adapted to generate an electrical
carrier signal at a controlled carrier frequency; a transmitting
switching means adapted to operate under control of a timing
circuit to switch between a closed state in which the transmitting
switching means simultaneously connects the carrier signal
oscillator to both of the transducers, thereby placing both of the
transducers in their respective transmitting states, and an open
state in which the transmitting switching means disconnects both of
the transducers from the carrier signal oscillator; a receiving
switching means adapted to operate under control of the timing
circuit to switch between a closed state in which it separately
electrically connects each of the transducers to a respective
receiver, and an open state in which it disconnects each of the
transducers from the respective receiver; first and second phase
detectors having respective outputs representative of a phase
difference between two inputs, each of the phase detectors having a
respective first input from a respective one of the receivers and a
second input from the carrier signal oscillator; a frequency
control circuit having an input from at least one of the phase
detectors, the frequency control circuit adapted to control the
carrier signal oscillator to operate at the controlled frequency;
and means for comparing the outputs from the two phase detectors
and for generating a rate output signal responsive to the
comparison.
2. The sensor of claim 1 further comprising a transmitting
amplifier electrically connected between the carrier signal
oscillator and the transmitting switching means.
3. The sensor of claim 1 wherein each of the phase detectors has
its respective output connected to a low pass filter, the low pass
filter having an output to the means for comparing.
4. The sensor of claim 1 wherein the means for comparing the
outputs from the two phase detectors comprises two sample and hold
circuits, each of the sample and hold circuits receiving an input
from a respective one of the phase detectors, each of the sample
and hold circuits having a respective output to a differential
amplifier.
5. The sensor of claim 1 further comprising signal route exchange
switching means adapted to interchange the connections between the
transducers and the signal processing circuitry so that when the
route exchange switching means is in a first of two settings one of
the transducers is connected to a first of the two receivers and
that phase detector receiving an input from said first of the two
receivers provides an output to a first of two connections to the
means for comparing the outputs, and when the route exchange
switching means is in the second setting said one of the
transducers is connected to the second receiver and that phase
detector receiving an input from said second receiver provides an
output to the first connection to the means for comparing the
outputs.
6. The sensor of claim 5 further comprising a frequency divider
adapted to toggle the signal route exchange switching means after a
transit time interval.
7. The sensor of claim 5 wherein the route exchange switching means
comprises a pair of relays.
8. The sensor of claim 1 wherein the transmitting and receiving
switching means comprise separate relays.
9. The sensor of claim 1 wherein the frequency control circuit has
an input from only one of the phase detectors.
10. The sensor of claim 1 wherein the frequency control comprises a
circuit element having a long time constant.
11. The sensor of claim 1 wherein the controlled frequency is a
frequency selected to cause the output of at least one of the phase
detectors to be in the middle of its operating range.
12. The sensor of claim 1 wherein the first and second transducers
directly face each other.
13. The sensor of claim 1 wherein the fluid flows through a pipe
and wherein the first and second transducers are disposed external
to the pipe.
14. The sensor of claim 13 wherein the pipe comprises at least one
portion adapted to reflect the transmitted acoustic energy from one
of the transducers to the other of the transducers.
15. The sensor of claim 1, further comprising a reset detector
adapted to momentarily connect the frequency control to a reference
voltage input and thereby to control the carrier signal oscillator
to operate at a center of its operating range.
16. The sensor of claim 15, wherein the reset detector is adapted
to operate responsive to a low frequency oscillator.
17. The sensor of claim 15, wherein the reset detector is adapted
to operate responsive to an operating condition.
18. A method of measuring a flow rate of a fluid flowing in a
predetermined flow direction, the method comprising the steps of:
a) spacing two transducers out along the flow direction so that
each of the transducers is in acoustic contact with the fluid; b)
simultaneously connecting both of the transducers for a selected
interval to a carrier signal oscillator having an output at a
controlled frequency, thereby causing each of the transducers to
transmit a respective acoustic signal into the fluid; c)
disconnecting, after the conclusion of the selected interval, both
of the transducers from the oscillator and connecting each of the
transducers to a respective receiver, whereupon each receiver
outputs a respective received signal representative of the acoustic
signal generated by the transducer other than the one to which it
is then connected; d) supplying each of the respective received
outputs as respective first inputs to respective phase detectors,
each of the phase detectors having a second input from the carrier
signal oscillator, each of the phase detectors having an output
representative of a respective phase difference between the
oscillator and that receiver from which it is supplied; e)
measuring a difference between the outputs of the two phase
detectors and supplying that difference as an output representative
of the flow rate; and f) supplying an output from at least one of
the phase detectors to a frequency control circuit controlling the
carrier signal oscillator to operate at the controlled
frequency.
19. The method of claim 18 wherein the outputs of the phase
detectors are respective time varying voltage signals that are
filtered by respective low pass filters, sampled and held by
respective sample and hold circuits and supplied as respective
inputs to a differential amplifier having a DC voltage level output
that is representative of the flow rate.
20. The method of claim 18 wherein the two transducers are spaced
apart by a known distance, and wherein the interval during which
the two transducers are connected to the carrier signal oscillator
is selected to be equal to a transit time required for an acoustic
signal from one of the transducers to travel to the other of the
transducers.
21. The method of claim 18 wherein steps b) through d) are
repeated, wherein a pair of signal exchange relays is toggled
between repetitions so that each transducer is connected to a
different one of the two receivers on successive repetitions of
steps b) through d) and wherein the step of measuring the
difference between the outputs of the two phase detectors comprises
averaging the outputs of the phase detectors over a selected number
of repetitions.
22. The method of claim 18 wherein each of the two transducers is
wetted by the fluid.
23. The method of claim 18 wherein at least one of the two
transducers is attached to the outside of a pipe through which the
fluid flows.
24. The method of claim 18 wherein the outputs of the phase
detectors are respective time varying voltage signals and wherein
at least one of the outputs of the phase detectors is supplied as
an input to the frequency control circuit comprising a differential
amplifier having an output acting to control the controlled
frequency of the carrier signal oscillator so that an average
output of the at least one of the phase detectors is maintained at
a center of its operating range.
25. The method of claim 18 wherein the carrier signal oscillator
has a predetermined operating range and wherein the method further
comprises a reset step of forcing the signal oscillator to operate
in a middle of its operating range.
26. The method of claim 25 wherein the reset step occurs
periodically responsive to a low frequency oscillator.
27. The method of claim 25 wherein the reset step is taken
responsive to an operating condition.
28. A transit-time flow sensor for measuring an acoustic
propagation time difference between upstream and downstream
acoustic transmissions in a fluid and for determining the flow rate
of the fluid therefrom, the sensor comprising: first and second
transducers, each of the transducers adapted to be wetted by the
fluid, each of the transducers having a respective transmitting
functional state and a respective receiving functional state, each
of the transducers adapted to transmit acoustic energy at a first
frequency along an axis parallel to a direction of flow of said
fluid when said each transducer is in its respective transmitting
state, each transducer adapted to receive acoustic energy
transmitted by the other transducer when said each transducer is in
its receiving state; means for alternating the transmitting and
receiving states of the transducers so as to always have one of the
two transducers in its respective transmitting state and the other
of the two transducers in its respective receiving state, the means
for alternating adapted to act at a second frequency substantially
less than the first frequency; a carrier signal oscillator circuit
adapted to provide a continuous carrier signal at the first
frequency to the transmitting one of said transducers; and a phase
detector containing a low pass filter for providing an output
signal responsive to a phase difference between electrical signals
corresponding to, and of the same frequency as, the transmitted and
received acoustic energy.
29. The sensor of claim 28 wherein the first and second transducers
directly face each other.
30. The sensor of claim 28 wherein the fluid flows through a pipe
comprising at least one portion adapted to reflect the transmitted
acoustic energy from one of the transducers to the other of the
transducers.
31. The sensor of claim 28, further comprising means to correct a
drift in said phase difference between said transmitted and
received acoustic energy by changing the first frequency.
32. The sensor of claim 28, further comprising means to correct a
drift in said phase difference between said transmitted and
received acoustic energy by a phase shift change of the phase
relationship of said electrical signals corresponding to the
transmitted and received acoustic energy.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 09/458,315, filed Dec. 10, 1999 and presently
pending.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to a simplified method for
measuring the flow rate of a fluid in which the propagation times
of ultrasonic signals transmitted through the fluid can be detected
to determine fluid flow rate. The invention further relates to
improvement of a probe flow sensor configuration and to its
installation, and similar sensing devices.
[0005] 2. Background Information
[0006] Transit-time ultrasonic flow sensors, also known as
"time-of-flight" ultrasonic flow sensors, detect the acoustic
propagation time difference between the upstream and downstream
ultrasonic transmissions in a moving fluid and process this
information to derive a fluid flow rate. Several different sensor
configurations have been used including: 1) direct measurement of
the propagation time of a pulse emitted by a first transducer and
received by a second transducer where the change in time is a
function of fluid flow rate; 2) dual `sing-around` sound
velocimeters, where the difference in "sing-around" frequency
between the velocimeters is a function of the fluid flow rate; 3)
sensors producing continuous waves using two widely different high
frequency carriers but commonly modulated with another much lower
frequency signal where the phase difference of the modulated signal
on the received carriers is a function of the fluid flow rate; and
4) sensors producing bursts of continuous waves using a single
frequency on a pair of transducers, the burst duration being less
than the acoustic propagation time between the transducers, where
the time difference between the received transmissions is a
function of flow rate.
[0007] The transducers of transit-time ultrasonic flow sensors are
most often field mounted, and are commonly individually attached to
the outside of a pipe, thereby offering the advantage of not having
to break into the pipe in order to make the flow measurement.
However, the uncertainty of the pipe wall integrity and the effects
of its surface condition, and the uncertainties of Locating,
attaching and acoustically coupling the transducers to the pipe, as
well as uncertainties of the reflection from the interior of the
pipe when it is used to complete the acoustic path between the
transducers, can often lead to substantial measurement error. Even
when the transducers are in contact with the fluid being measured
(i.e., wetted), their mechanical location may result in
misalignment, being spaced at the wrong distance or set a the wrong
angle, all of which can result in measurement error. As a result,
these sensors are usually equipped with supporting electronics
containing sophisticated diagnostic means for confining proper
installation and operation. Overall, these sensors are relatively
expensive and have a reputation for sometimes producing erroneous
measurements.
[0008] A notable example of prior art in this area is U.S. Pat. No.
4,221,128 to Lawson, who teaches an acoustic flow meter in which
bursts of acoustic energy are periodically and simultaneously
emitted by each of two transducers. The bursts are shorter than a
transit time between the two transducers and a portion of the
acoustic energy from each transducer is received by the other of
the two transducers. Because of the fluid flow, there is a relative
phase shift between the two received signals. This phase shift is
measured by beating each of the received signals against a common
reference signal to produce signals at a lower frequency at which
the phase difference is more easily measured by means of electronic
circuitry that is less expensive, consumes less power, and is more
stable than older prior art equipment. The prior art methods cited
by Lawson directly measured the phase difference at the higher
frequency selected for acoustic transmission.
[0009] Further teaching in this area is provided by the inventor in
U.S. patent application Ser. No. 09/592,313, filed on Jun. 13,
2000. In this application the inventor teaches a transit time
ultrasonic flow sensor having two simultaneously transmitting
transducers. This sensor is configured to compensate for
circuit-related drifts in the flow rate output signal. During
acoustic transmission some of the transmitting signal is also
routed through the receiving circuits to generate a reference
signal that is used to compensate the received signals. The
disclosure of U.S. patent application Ser. No. 09/592,313 is
incorporated herein by reference.
BRIEF SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to provide means
for reducing the complexity and cost of transit-time ultrasonic
flow sensors, to improve their measurement reliability, and to make
their installation easier.
[0011] It is a further object of the present invention to provide a
cost effective means for enabling transit-time flow sensors to
measure a fluid flow rate along two axes.
[0012] The above and other objects may be satisfied with a
transit-time flow sensor configured as a single modular unit as
exemplified in accordance with some preferred embodiments of the
present invention. One of the preferred embodiments includes an
insertion probe with two permanently mounted transducers which
enters a relatively small opening in a pipe carrying a flowing
fluid. A probe-mounted acoustically reflective surface is also
provided to enable the acoustic path to be completed within the
entity of the probe so that it does not depend upon any other
reflective surface for its operation. This sensor is thus more
simple in construction, easier to install correctly, and provides
more reliable operation. The probe may of course be supplied
factory mounted and calibrated in a short section of pipe, and
thereby be considered a "full bore" sensor for installation between
two similar pipe sections.
[0013] In an embodiment of the invention configured as a probe, the
transducers are spaced out along the flow direction so that one is
upstream relative to the other. These transducers are in line with
and at an angle to the fluid flow, and are directly wetted by the
fluid. In this configuration the sensor is isolated from the
attenuation and multipath problems which occur when the transducers
are pipe mounted. The supporting electronics may be simplified in
concept and incorporate cost effective components while still
offering good measurement precision.
[0014] One method of flow rate sensing used with the present
invention differs from the four methods listed in the foregoing
"Discussion of Prior Art", in that it uses a variable frequency
acoustic signal which is continuously transmitted by either one or
the other transducers as they alternate between transmitting and
receiving states. In this arrangement, a relatively low alternation
frequency is the exclusive modulation source and the primary
detection of time difference occurs at the transmitted acoustic
frequency without using an intermediate frequency. This method, in
its basic form, is unstable because of both acoustic path and
electronic related drifts and frequency-related uncertainties.
Special frequency control provisions have been employed to correct
these deficiencies so that high flow sensing sensitivity, good zero
stability and low noise level are obtained.
[0015] In a second method of flow rate sensing used with the of the
present invention, the variable frequency acoustic signal is
simultaneously transmitted by both transducers in a burst. After an
interval corresponding to the expected transit time between
transducers, both transducers are switched to their respective
receiving states and a phase difference between their received
signals is used as a measure of the fluid flow rate.
[0016] The invention provides transmission modes, both continuous
or in bursts, and means of providing operating stability by
controlling the acoustic frequency, that are also applicable to
"full bore" sensors where the transducers are individually mounted
rather than being configured as a probe. Furthermore, when using
the burst mode flow sensing may be performed with the acoustic
energy being conveyed through the pipe wall on which the
transducers are mounted.
[0017] In yet other preferred embodiments, two pairs of transducers
are used within a single sensor housing and are located to
determine fluid flow rate components along two orthogonal axes,
[0018] In further embodiments, an electric current is passed
between surfaces of the sensor in the proximity of the acoustic
path so that, by the process of electrolysis, those surfaces will
be maintained clean.
[0019] Although it is believed that the foregoing recital of
features and advantages may be of use to one who is skilled in the
art and who wishes to learn how to practice the invention, it will
be recognized that the foregoing recital is not intended to list
all of the features and advantages. Moreover, it may be noted that
various embodiments of the invention may provide various
combinations of the hereinbefore recited features and advantages of
the invention, and that less than all of the recited features and
advantages may be provided by some embodiments.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0020] The various advantages of the present invention will become
apparent to one skilled in the art by reading the following
specification and subjoined claims and by referencing the following
drawings in which:
[0021] FIG. 1 is a simplified side cross-sectional view of a
preferred embodiment of the ultrasonic transit-time flow sensor of
the present invention;
[0022] FIG. 1A is a simplified end cross-sectional view of the
sensor depicted in FIG. 1;
[0023] FIG. 2 is a block diagram indicating the functional blocks
of electronic circuitry of a continuous transmitting embodiment of
the invention.
[0024] FIG. 3 is a schematic depiction of an arrangement of two
pairs of transducer elements in accordance with a particular
embodiment of the present invention.
[0025] FIG. 4 is a block diagram of a burst transmitting embodiment
of the invention.
[0026] FIG. 4A is a block diagram showing an alternative
arrangement of the receiver portion of FIG. 4.
[0027] FIG. 5 is a partly schematic view of two transducers
attached to the outside of a pipe.
[0028] FIG. 6 is a partly schematic depiction of fluid flow in a
chamber having two facing detectors disposed at its ends.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Turning now to FIGS. 1 and 1A, one finds a depiction of a
flow sensor 10 in accordance with a preferred embodiment of the
present invention. This sensor is shown as it would be typically
mounted in a pipe. Fluid, the flow direction of which is
represented by arrow 12, enters the pipe 14 and passes between
transducers 16 and 18, which are preferably piezoelectric
transducers, but which may be other sorts of transducers known in
the acoustic flow measurement arts. A reflector 20 is preferably
used as a portion of the acoustic path so as to avoid using
surfaces of unknown and uncontrolled quality, such as an inside of
a pipe wall. Transducers 16 and 18 are preferably each mounted to
reduce internal acoustic reflections and to direct the transfer of
acoustic energy between each other by way of a reflector 20 so that
at least a component of the acoustic energy is along an axis
parallel to the direction 12 of flow. A material 34 such as cork
can be used as an acoustic energy absorbing material surrounding
each transducer and as a barrier between the transducers, as may be
required to provide the acoustic isolation needed for proper
operation. The dominant acoustic energy transferred between the
transducers 16 and 18 is beamed along the lines indicated with the
reference numeral 22. A housing 24, mounting the transducers 16 and
18, joins with a stem 26 upon which is mounted an electronics
enclosure 28. A reflector 20 is supported by posts 36 that are a
part of the probe. The stem 26 may, for example, be mounted through
a fitting 30, facilitating insertion depth control, which engages a
pipe mounted fitting 32.
[0030] During one mode of operation, one of the transducers 16 or
18 transmits while the other receives. Then they alternate
functional states so that the transducer formerly transmitting
receives, and the transducer formerly receiving transmits. In this
mode, acoustic energy is transmitted by one or the other transducer
16 or 18 all of the time and is received by the non-transmitting
one of the transducers 18, 16 all of the time. The difference in
phase shift between the acoustic energy signals of the two
functional states is representative of fluid flow rate. With these
and other refinements discussed subsequently herein, the
acoustically self contained module which sensor 10 exemplifies
exhibits low noise and high stability and measurement precision.
The sensor 10 further enables the electronics to be simplified and
the mechanical assembly to be standardized to cover a wide range of
pipe sizes. Furthermore, the risk of improper operation due to
installation error or pipe related factors is greatly reduced.
[0031] FIG. 2 is a block diagram of representative electronic
circuitry used with an embodiment of the present invention that
employs wetted transducers. In this depiction an oscillator circuit
246 generates a carrier signal at a selected high frequency (e.g.,
4 MHz). This carrier signal is amplified by amplifier 248 and
routed through a relay 260 to a transmitting transducer 16. The
acoustic energy received by the other transducer 18 is converted to
a corresponding electrical signal which is routed through a relay
260 to an amplifier 238, and then to a limiter 240. The output from
the limiter 240 enters a first Schmitt trigger 268 which provides
inverted signal to both a phase detector 244 and a second Schmitt
trigger 270. The output signal from the second Schmitt trigger 270
enters a phase detector 242. Both phase detectors 244 and 242 also
receive a common input from the carrier signal oscillator 246, and
each provides an output signal to the corresponding low pass
filters 250 and 251, whose outputs enter a differential amplifier
252. The alternating signal component of the signal from this
amplifier 252 passes through a differentiating circuit 253 to a
buffer amplifier 255 and then to an inverting buffer amplifier 257.
The output of the inverting buffer amplifier 257 is routed by a
relay 258 to the low pass filters 254 and 280 and then to a
differential amplifier 256. The output from the differential
amplifier 256 enters the switch 249 and is routed through a buffer
amplifier 272, a low pass filter 261, and an amplifier 259 to
provide the rate output signal.
[0032] In this embodiment, an oscillator 278 activates relays 260
and 258. Another oscillator 266 activates two switches 276, 249. An
amplifier 274 detects the voltage difference between the output
from the low pass filter 250 and a reference voltage "A", and
produces an output signal which passes through another low pass
filter 263 to the frequency control 264 in order to vary the
frequency of the carrier signal oscillator 246 as in a phase locked
loop. The output from the low pass filter 263 is also switched to a
reference voltage "B" when the switch 276 is activated by the
oscillator 266. Balanced circuits are indicated to reduce drift and
increase dynamic operating range.
[0033] The carrier signal oscillator 246 produces a continuous
carrier signal at a high frequency, which may be on the order of 4
MHz. This signal is increased in magnitude by an amplifier 248 and
routed through relay 260 to transducer 16 which emits a
corresponding acoustic signal. A portion of that acoustic signal is
received by transducer 18 which converts it into a corresponding
electrical signal that is routed by a relay 260 to an amplifier 238
to increase its magnitude and then to a limiter 240 to produce a
relatively large output signal for the smallest received signal of
interest. This insures that the Schmitt trigger 268 is reliably
switched between its trigger points. The Schmitt trigger 268 drives
the second Schmitt trigger 270 so that phase detectors 244 and 242
provide 180 degree phase opposed received signals while their other
inputs from oscillator 246 are of the same phase. The outputs from
the phase detectors 244 and 242 are therefore phase opposed and
enter low pass filters 250 and 251 to remove the high frequency
components prior to entering a differential amplifier 252. The
differential amplifier 252 extracts their difference voltage, the
magnitude of that voltage being a function of the phase difference
between the oscillator 246 signal and the signal provided by
transducer 218.
[0034] Relays 258 and 260 are activated by oscillator an 278, which
produces a square wave at a relatively low frequency, typically 100
Hz. Relay 260 comprises means for alternating receiving and
transmitting functional states for the two transducers and so that
the difference in the transit times between the upstream and
downstream acoustic paths resulting from relative movement between
the transducers and the fluid flow being sensed produces an
alternating signal at the output of amplifier 252. The DC component
of the output signal of amplifier 252 is removed by differentiating
circuit 253 so that only the magnitude of the alternating component
of that signal enters buffer amplifier 255 and then its inverting
amplifier 257. DC drifts present at the output of amplifier 252 are
thereby eliminated so that they will not affect the output and, in
particular, the zero stability of the rate output signal.
[0035] In this embodiment the phase opposed signals from the
amplifier 255 and the inverter 257 are synchronously switched by
the output relay 258 at the 100 Hz frequency provided by the
oscillator 278. This provides rectification of the signals, which
are then smoothed by low pass filters 254 and 280 prior to entering
differential amplifier 256.
[0036] The output signal from amplifier 256 is thus a DC voltage
responsive to the magnitude of the difference in transit time
between the mode of operation where transducer 16 transmits while
transducer 18 receives, and the mode of operation where transducer
18 transmits while transducer 16 receives. The output from
amplifier 256 enters the normally closed switch 249, which has a
capacitor in parallel with its output for retaining the most recent
output voltage from the amplifier 256 in the event the switch 249
is opened. Amplifier 272 is a high impedance buffer amplifier used
to minimally discharge the capacitor when switch 249 opens and to
provide the signal through the lowpass filter 261 to the output
amplifier 259.
[0037] Transducers exhibit the equivalent of acoustic phase drift
in characteristics due to aging, changes in temperature, mechanical
stress and other factors. All of these affect the precision of the
flow rate measurement. The mechanical mounting of the transducers
and their relationship to the acoustic reflecting surfaces that
they interact with, particularly if those surfaces are of plastic,
may be unstable over a period of time. These instabilities may
arise from moisture absorption, accumulation of surface
contamination and stress relief, among other causes. Mechanical
stress and temperature change can also cause acoustic phase drift.
Because of the desire to produce a small insertion probe, the
spacing between the transducers is relatively small and relatively
small mechanical changes increase the drift problem. The higher
frequencies desired to increase phase detection sensitivity coact
with the small transducer-to-transducer spacing to produce further
instabilities. Add the above drift promoting factors to the phase
drifts produced by the supporting electronics, particularly when
they are of relatively low cost commercial quality and designed
with a clear intent to reduce costs, and the total impact of these
drifts can be catastrophic for a transit-time flow sensing product.
It can now be appreciated that the prior art does not overcome
these difficulties to produce a commercial product competitive with
probes relying on other flow sensing technologies, while the
present invention provides several successful approaches.
[0038] It may at first seem obvious that the phase locked loop
would alone be sufficient to provide frequency control of the
carrier signal oscillator 246 so that the average output from phase
detector 242 is maintained in the center of its operating range.
However, this is not the case. Consider for example, that while the
frequency range of the carrier signal oscillator 246 needs to be
great enough to compensate for the few acoustic wavelengths of
maximum possible phase drift, it should be no greater than that in
order to minimize oscillator phase noise and drift. This would be,
for example, the equivalent of one or two acoustic wavelengths on
either side of its center frequency. However, should phase lock
occur with the carrier signal oscillator 246 operating close to a
limit of its frequency range, and where subsequent phase drift
would require the oscillator 246 to change its frequency in the
direction that it is incapable of accommodating, the resultant will
be sensor performance degradation, or actual failure. The
oscillator 246 might become phase locked close to a frequency limit
when operating power is applied, for example, or because the
acoustic path has been broken by insufficient flowing fluid or
because of absorption of the acoustic energy. This could also occur
because of reflection or scattering of the acoustic beams due to
transient changes in the composition of the flowing fluid, or
because of phase drift inherent in the sensor. In any case, a means
is required to both initialize and maintain the frequency of
oscillator the carrier signal oscillator 246 and the phase detector
242 in the nominal center of their operating ranges.
[0039] The above means is augmented by providing a switch 249 which
is activated by oscillator 266, and which produces a pulse at a
very low frequency--e.g., once every minute. This enables the
frequency control circuits to reset to the center of their
operating range. During the time of this pulse and for a short time
afterwards, as required to enable the circuits to stabilize the
output, the differential output amplifier 259 is maintained at the
value existing just before the pulse occurred to avoid upsetting
the rate output signal. This may, for example involve a total
duration of two seconds.
[0040] The oscillator 266 of FIG. 2 also activates the relay 276 to
connect the reference voltage "B" to the capacitor of long time
constant filter 263 during the oscillator 266 pulse. This forces it
to the voltage which acts on the frequency control circuit 264 to
adjust the frequency of the carrier signal oscillator 246 to the
nominal center of its operating range. At all other times, the
signal representative of the phase difference between the
oscillator 246 and Schmitt trigger 270 signals, as it appears at
the output of low pass filter 250, is used in conjunction with
amplifier 274 and low pass filter 263 as in a conventional phase
locked loop. Reference voltage A is of the magnitude corresponding
to the center of the operating range of the detector 242 so that
the amplifier 274 will vary its output voltage to maintain the
voltage on its positive input essentially equal to that on its
negative input, thereby locking the loop. The low pass filter 263
has a relatively long time constant so that its output does not
contain a significant 100 Hz component. This enables the phase
locked loop to respond only to the average of the phase detector
242 output, while the full alternating component of its signal
enters amplifier 252.
[0041] An alternative method of control is to have a phase shift
block with a similar number of acoustic cycles of phase shift
capacity located between oscillator 246 and the phase detectors 242
and 244, which is similarly controlled by phase detector 242, low
pass filter 250, amplifier 274 and low pass filter 263 to maintain
the phase detector 242 output centered in its operating range. In
this case, the phase shift block is periodically connected by
switch 276 to the reference "B" voltage to force its operation to
the nominal center of its operating range. In this method,
oscillator 246 is not frequency controlled and the frequency
control block 264 is omitted.
[0042] Reset of the phase locked loop occurs periodically as a
function of the frequency of the oscillator 266. Reset can also
occur by alternate means, for example, by detecting when power is
first applied, when the average output from phase detector 242 is
detected to be sufficiently removed from its center value, when the
acoustic signal is reestablished after loss or when other criteria
are met.
[0043] The use of balanced circuits, as in the case of phase
detectors 242 and 244, for example, reduces the upsetting effect of
the reset pulse. This is because the oscillator 246 is never phase
locked exactly to the center of its frequency range and must make
an abrupt frequency change before resuming stable phase lock. In
some applications this disruption may be small enough to be
insignificant so that relay 249 and amplifier 272 may be omitted.
Furthermore, relay 249 and amplifier 272 may be omitted if the
reset pulse occurs infrequently enough for the intended sensor
application, although it produces a significant change in the rate
output signal.
[0044] The reset means and overall high sensitivity signal-to-noise
ratio employed in the present invention, as exemplified in
particular in FIG. 2, make the sensor very tolerant of operating
conditions. Flow probes can now be made economically with small
dimensions which make them easy to install. These probes can have
their transducers set at small angles, such as 75 degrees with
respect to the flow axis, and thereby be relatively small so that
the hole in the pipe through which the probe passes during
installation is likewise relatively small. For example, a probe may
be made with a 0.8" transducer-to-transducer spacing, which allows
it to fit through a 1.0" hole in the wall of a pipe. Furthermore,
the probe will be relatively tolerant of variations in the distance
between the reflector and the transducers so that it can even be
supplied without the reflector and rely instead on the opposite
inside wall of the pipe to act as the reflector and complete the
acoustic path between transducers.
[0045] A relatively small amount of low pass filtering enables the
amplifier 256 output to respond to rapidly changing flow rates so
that characteristics of the fluid system in which the probe is
used, such as pulsations due to pump impellers, can be detected and
provided as an output signal indicative of impeller condition. The
sensor 10 may then additionally serve as a flow system maintenance
monitor.
[0046] The amplifier 256 output may also be used to detect flow
variations due to vortex shedding or fluidic oscillators for
example. If only flow detection of flow variations is required, the
circuit may be simplified by eliminating relay 260, amplifiers 257,
272 and 259, low pass filters 261 and 280, and relay 249. Flow rate
pulsations due to Karman vortices, as may be produced by a bluff
body 38 in the flow stream, can set up a street of vortices that
pass through one or both acoustic beams of the transducers 16 and
18, of sensor 10. In this arrangement the frequency of the detected
pulsations is proportional to flow rate. The bluff body 38 may be
located to one side of or between the acoustic beams 22. However,
if the sensor 10 is rotated in its fitting 30 by ninety degrees,
one of the posts 36 may be used as the bluff body produce the
vortices.
[0047] In the process of detecting the phase difference between the
received and transmitted signals, DC offsets appear at the output
signal of amplifier 252, which may drift over a period of time and
seriously affect the zero flow stability of the sensor. By removing
the DC component of that signal with a differentiating circuit 253,
that source of error is eliminated. In the sensor referred to
earlier, zero stability equivalent to a sensed flow rate of 0.05
inches/second has been observed. This is significant particularly
when it is noted that no high stability or matched filters or other
sophisticated or inherently expensive components were used, the
transducers were not impedance matched for both the receiving and
transmitting functions. Furthermore, conventional and relatively
low cost commercial components were used throughout to reflect the
commercial intent of future products based upon the present
invention. Additionally, only three adjustments, these being output
zero, output span and center frequency, are required for the entire
circuit.
[0048] Exclusive-OR gates operating at the carrier frequency are
preferably used as the phase detectors 242 and 244 to detect the
phase shift between the locally generated signal from oscillator
246 and the received signal from transducer 16. There is no
translation of frequency, and therefore no corresponding phase
noise and drift. The circuit would also function if the phase
detectors were supplied with receive signals having the same phase
while the signals from oscillator 246 were phase opposed. The
carrier frequency used is relatively high, typically 4-10 MHz, so
that a substantial flow related phase shift occurs for a physically
small probe.
[0049] It is noteworthy that the transmitting signals may be
seriously distorted without seriously affecting the overall
operation of the sensor. The output of amplifier 248 is a square
wave, whereby only a small inductive impedance in series with relay
260 is used with the transducer capacitance to provide a semblance
of series resonance or low pass filtering. Furthermore, the circuit
is very tolerant of impedance mismatch of the transducers 16, 18,
between the functions of transmitting and receiving, so that in an
existing practical sensor design there is no provision for such
matching.
[0050] Because the transmitted and received acoustic signals are
always present in this embodiment of the present invention in which
each transducer operates half of the time to transmit and the other
half to receive, phase detection occurs over many carrier cycles.
Hence, signal detection averaging is extensive and the
signal-to-noise ratio is high. Therefore, at the relatively low
alternating frequency of 100 Hz (compared to a 4 Mhz carrier
frequency) and the relatively short acoustic transit time (because
of the short length of the acoustic path between the transducers),
any error introduced because of resultant signal delays in the
acoustic path between the transducers will, on the average, be a
very small factor of the rate output signal and will therefore be
negligible. However, should the transit time become significant,
one or more functional blocks for receiving or detecting the rate
related signals, such as a portion of relay 260 and amplifiers 238
and 268, limiter 240, inverter 270, and phase detectors 242 and
244, would be disabled for the required interval during the
transducers transmitting/receiving alternation.
[0051] When the acoustic signals are reflected between the
transducers by a flat reflector, the received signal is a summation
of transmitted signals, including those having taken more than one
path because of beam spreading. As the selected transmitted
frequency changes to maintain phase lock and center the operating
points of phase detectors 242 and 244, the change in the relative
magnitudes and phase relationships of the components of the
received signal may cause increased noise and drift in the rate
output signal. This problem may be reduced by curving the surface
of the reflector facing the transducers in the direction along the
axis between the transducers with a radius equal to about two times
the distance between the midpoint between the transducers and
reflector. Because this degree of curvature is very small, it
should not affect fluid flow sensing in most applications. The
inside surface of a pipe would not provide the same benefits
because its curvature is orthogonal to that desired.
[0052] In some applications it will be desired to operate the
sensor with low power. Considerable power reduction without serious
loss of flow measurement precision may obtained by enabling all the
sensor's functional blocks operating at the acoustic path frequency
to operate for only a few alternating cycles within a larger period
of operation, whereby low pass filter 263 becomes equipped with a
sample-and-hold circuit to retain its frequency control voltage
during that period. Relay 249 acts in effect as a sample and hold
circuit and would be switched to the amplifier 256 only during the
latter portion of the alternating frequency cycles when the
amplifier 256 output signal has stabilized. Since the functional
blocks operating at the acoustic path frequency consume almost all
of the operating power of the sensor, a large power reduction is
possible. For example, with a larger period of one second and the
acoustic path functional blocks being active for 100 milliseconds,
the power reduction is nearly 90%.
[0053] The flow sensor configuration of probe 10 is also suitable
for use in open channels and in large bodies of water for example,
as it provides for the complete reflective path within itself and
can operate with low power. Furthermore, a second set of
transducers 370, 372 located in an enlarged form of housing 24, and
mounted orthogonally to transducers 16 and 18, which similarly beam
to and receive from reflector 20, will provide a measurement of
flow rate in a direction orthogonal to the first set so that their
rate and directional components enable a resultant flow rate and
angle to be determined by electronic computation. Such a transducer
arrangement is illustrated in FIG. 3 where transducers 16, 18 of
probe 10 establish an acoustic energy beam line 22. The beam line
22 is reflected by reflector 20 to sense the component of fluid
flow which moves horizontally across the page, in the direction of
arrow 12. Transducers 370 and 372, mounted in the same housing,
establish a second acoustic energy beam, represented by lines 374,
to sense the component of flow moving orthogonally to the first
beam. It is possible for both sets of transducers 16, 18 and 370,
322 to operate at the same time with their own supporting
electronics when the acoustic beam angles are narrow if there is
otherwise good acoustic and electrical isolation between them.
However, they may also time share the same electronics or operate
at different frequencies.
[0054] The installation of probe 10 in a pipe for example,
typically requires that it be angled to the flow precisely if the
best flow sensing accuracy is to be achieved. Observing an
indication of a flow rate measurement while adjusting the probe
angle for the maximum indication is useful but often
unsatisfactory, considering that flows are often not constant
enough. A pipe-mounted flow probe configured as in FIG. 3 however,
would do very nicely because the second set of transducers 370 and
372, sensing fluid flow orthogonal to primary flow direction 12
through the pipe, would be aligned to produce an output signal null
when the probe angle is exactly correct. As a null indication, the
magnitude of the fluid flow rate is not important as long as some
flow is occurring so that a steady flow rate is no longer
necessary. A null type of indication is also more sensitive and can
be made to respond faster.
[0055] In some fluid handling systems, the fluid flow is not
reasonably uniform and straight, and may have a variability
depending on flow rate and other factors such as nearby valve
states. The affects of these non-uniformities may also be sensed by
the second set of transducers 370 and 372, with their flow
responsive signals being used for correcting the flow measurements
derived from the other transducers 16 and 18.
[0056] The second set of transducers 370 and 372, if not used to
provide a precision rate measurement, need not be mechanically
located as shown, and do not even have to make use of the reflector
20. Transducers 370 and 372 merely need to lie parallel to and
facing each other with the flow and acoustic energy passing between
them so as to respond to the component of flow which is not
parallel. Since transducers 370, 372 have the axis between them
perpendicular to the fluid flow, they may additionally be used to
directly measure the speed of sound of the fluid whereby the output
signal obtained therefrom is used, for example, to vary the gain of
amplifier 259 in order to compensate for flow rate errors due to
fluid temperature or composition changes. Any suitable supporting
electronics configurations including those listed in the earlier
"Discussion of Prior Art" section may be used to obtain the related
output signals. Compensation means may also be had by measuring the
temperature of the flowing fluid and using the output signal
obtained therefrom to similarly vary the gain of amplifier 259 when
there is close correlation between the fluid temperature and the
speed of sound.
[0057] Although the immediately preceding discussion is primarily
directed toward a probe configuration, the apparatus and method of
the present invention are also applicable to other configurations
using similar digital processing electronics. Such arrangements
could include an in line flow sensor containing transducers which
are well acoustically isolated from each other and from whatever
housing is employed. For example, the depiction of FIG. 5 shows a
fluid flowing through a generally U-shaped pipe 14 having two
transducers 16, 18 in acoustic contact with the fluid and arrayed
along a straight line so that they are directly facing each other.
An alternate embodiment is shown in FIG. 6, where two transducers
16, 18 are shown attached to the outside of a pipe 14 by means of
appropriate adapters 602. Moreover, the reflector 20 described
above with respect to the probe configuration, is not an essential
element. In some cases (e.g., FIG. 5) no reflector is needed, in
others (e.g., FIG. 6) a portion of an existing structure, such as
the pipe, can be used as a reflector. In yet others, more than one
acoustic reflector may be used to convey the acoustic signal
between the transducers.
[0058] Turning now to FIG. 4, one finds a block diagram of an
electronic circuit for a second embodiment of the present
invention. In this circuit, the carrier signal oscillator 246
generates a high frequency carrier signal which is amplified by a
carrier amplifier 248 and routed to phase detectors 342 and 344, as
well as passing through a transmitting switching means, which is
preferably a relay 312, to the transducers 16 and 18 when the relay
312 is briefly closed, thus causing both transducers 16, 18 to
simultaneously emit respective bursts of acoustic energy into the
fluid. After a time interval determined by the
transducer-to-transducer spacing and the speed of sound in the
fluid, each transducer 16, 18 begins to receive a portion of the
acoustic energy earlier emitted by the other 18, 16 transducer. The
transducers 16 and 18 convert the respective received acoustic
energy to electrical signals which are switched by a receiving
switching means, which may be a relay 314, to enter respective
receivers A, 320, and B, 322. The outputs from these receivers
enter respective phase detectors 342 and 344. Each of the phase
detector 342, 344 outputs, which is a time-varying voltage, passes
through a respective low pass filter 324, 326, a respective sample
and hold 328, 330, low pass filters 304, and finally a differential
output amplifier 259 to provide a DC voltage level as the rate
output signal.
[0059] In addition to providing means for generating, receiving and
processing the acoustic signals, the circuit of FIG. 4 is also
configured to provide several ancillary features that aid in
operational stability. The output of one of the sample and holds
328 is applied as an input to an amplifier 274 along with a
reference voltage (labeled Reference A in the drawing). The output
from the amplifier 274, applied through a low pass filter 363, is
supplied as an input to a frequency control circuit 264 and is used
to control the frequency of the carrier signal oscillator 246 so
the phase detector, on average, operates in the middle of its
operating range. In addition to the above recited elements, the
circuit of FIG. 4 comprises a reset detector 313 arranged to
operate a relay 376 or other appropriate switching means to exert
momentary control of the frequency control circuit 264.
[0060] A pulse generator 332 and timing circuits 334 provide the
enabling signals to operate the various circuit blocks depicted in
FIG. 4 to provide signals having a proper duration and in a correct
sequence. As discussed in a subsequent portion of this document,
the control pulses supplied by these control circuits 332, 334 are
preferably configured to provide pulse durations on the order of
tens of microseconds occurring in a fixed sequence at repetition
rate on the order of 100 Hz. Those skilled in the electronic arts
will realize there are many design choices that can be used to
provide for these functional features.
[0061] During typical operation, the carrier signal oscillator 246
generates a high frequency carrier signal at for example, 4 MHz. To
conserve electrical power, the carrier signal oscillator is not
operated constantly. Generally speaking, it needs to be turned on
shortly before an acoustic transmission is to be made so that its
frequency and amplitude stabilize before the transmitting relay 312
is closed. The oscillator 246 can be turned off immediately after
the phase detection process (or, more specifically, the sample and
hold 328 and 330 operation) has been completed. For a probe
application as illustrated in 10 of FIG. 1, where the transducer 16
to transducer 18 spacing is 0.8" and the transducer to reflector 20
spacing is 1.8", the oscillator need only operate for a few hundred
microseconds. The amplifier 248, which provides isolation for the
carrier signal oscillator 246 and which powers the phase detectors
342 and 344 and transducers 16 and 18, is typically operated for a
shorter period, in fact just long enough so that the acoustic
signals can be transmitted and received. For the above recited
dimensions of a probe 10 this requires about one hundred and twenty
six microseconds for an acoustic excitation to make a round trip
between the two transducers and to detect the flow rate signal.
[0062] During the acoustic transmission of sixty three
microseconds, the transmitting relay 312 is energized to connect
transducers 16 and 18 in parallel with the transmitting amplifier
248, thereby minimizing any change in phase between the two
transmitted signals. During the next sixty three microseconds, in
which the acoustic energy is being received by transducers 18 and
16, this relay 312 is de-energized while the receiving relay 314 is
energized to route the transducer signals to the respective
receiver, 322, 320, which then provide those two signals, along
with that from the transmitting amplifier 248, to the phase
detectors 342 and 344. It may be noted that this step of
referencing measured phase shifts to the transmitting oscillator
provides an improved degree of operational stability over prior art
circuits that do not provide this reference and that are therefore
subject to drift errors. Although the depiction of FIGS. 4 and 4A
show separate relays in the interest of clarity of presentation,
one skilled in the circuitry arts will recognize that the
transmitting and receiving switching means can be provided by other
components, such as a single more complex relay that can act as
both the transmitting and the receiving means, solid state
switching elements, and the like.
[0063] The outputs from phase detectors 342 and 344 enter
respective low pass filters 324 and 326 which respond fast enough
to capture the full magnitude of the phase detected pulse during
the receive period while minimizing any high frequency ripple. The
sample and hold circuits 328, 330 are enabled during the latter
portions of the phase detected pulses, for example the last 40
microseconds, to capture signal values from well stabilized
pulses.
[0064] The output signals from the sample and hold circuits 328,
330 pass through a low pass filters 304 to suppress fast signal
changes, and then to an output amplifier 259 which extracts the
difference in voltage between the two sample and hold circuits.
This difference is representative of the fluid flow rate and is the
output signal provided by 259. It will be appreciated that the
output amplifier 259 may also include additional low pass
filtering.
[0065] The output from one of the sample and hold circuits 328 is
also supplied as an input to an amplifier 274 that has a reference
voltage A applied to it as well. Typically, the reference voltage A
corresponds to the center of the operating range of one of the
phase detectors 342. Moreover, the phase relationship of the
amplifier 274 and the frequency control module 264 is such as to
control the frequency of the oscillator 246 so as to maintain the
average output from the phase detector in the center of its
operating range, as is done in a conventional phase locked loop,
thus providing the same functionality as was done by other means in
the circuit of FIG. 2. The low pass filter 363 incorporates a long
time constant circuit so that the frequency change is very slow so
that the controlled frequency does not change responsive to
measured phase shifts. One skilled in the art will realize that
although the depicted circuit uses feedback from only one of the
two sample-and-hold components, other feedback arrangements using
an output from at least one sample-and-hold are possible. For
example, one could invert the output from the second
sample-and-hold, average that inverted value with the value from
the first sample-and-hold and then use the newly formed composite
signal as a feedback input to the amplifier 274.
[0066] In the depicted circuit, a reset relay 376 is activated by
reset detector 313 to momentarily connect the frequency control 264
to a reference voltage, which is typically reference voltage A, to
force to the oscillator 246 to operate in the center of its
operating range. The purpose and functionality of the reset
mechanism is similar to that described with respect to the circuit
of FIG. 2. The reset mechanism may similarly be activated
periodically by a low frequency oscillator or by detection of a
condition which requires reset, such as a power fluctuation or the
frequency control 264 operating point being too close to or at an
operating limit. It is also noted that the range of oscillator
frequency control may be narrowed and the reset mechanism omitted
in the flow sensing systems illustrated in either FIG. 2 or FIG. 4
if the circuit components and operational environment be adequately
stable.
[0067] A portion 4A of the circuitry shown in FIG. 4 can be
replaced by the circuitry of FIG. 4A in order to null out errors
and fluctuations arising from electronic component sources--i.e.,
from circuitry sources, rather than from flow fluctuations or
inhomogeneities. Each receiver/phase detector/low pass filter path
provides its own contribution of drift and offset which is
ultimately canceled by appearing as a common mode signal. The
modification comprises the addition of two signal route exchange
relays 308, 310 operated together to interchange the input and
output connections of the received signal processing circuits. For
example, during a transmission/reception cycle in which the route
exchange relays 308, 310 are in their respective first settings, as
depicted in FIG. 4A, the received signal from transducer 16 is
routed through relays 314 and 308, receiver 320, phase detector
342, low pass filter 324 and relay 310 to sample and hold 328.
During the next transmission/reception cycle, the route exchange
relays 308, 310 switched to their respective second setting and the
same signal processing elements (i.e., receiver 320, phase detector
342 and low pass filter 324) process the signal from transducer 18
and provide that processed signal to the sample and hold 330
associated with that transducer 18. The signal path togging can be
provided by means of a frequency divider 306 which energizes the
route exchange relays 308 and 310. This frequency divider 306 is
triggered by a pulse generator 332 to alternate the signal routing.
In a preferred embodiment, the frequency divider 306 operates at
one half the selected repetition rate so that the signal routing is
toggled after each transit time interval. This alternation in the
routing of the received signals between the parallel receiving
circuits that provide the amplification and phase detection
functions allows one to average out the differential phase error
introduced by those circuits by averaging the outputs of the phase
detectors over a selected number of repetitions of the
transmission/reception cycle.
[0068] In addition to exhibiting a high degree of stability, a flow
sensor incorporating the features of FIGS. 4 or FIG. 4A can be
designed to consume relatively little power. For example, if the
acoustic signal processing circuits are active for 200 microseconds
and their repetition rate is once every 10 milliseconds (100 Hz),
the duty cycle is {fraction (1/50)} or 2%. If those circuits
require, for example, 5 volts at 50 milliamperes when active, their
average consumption is only 1 milliampere. If the repetition rate
were only once every 100 milliseconds, the average consumption
would be only 100 microamperes. This illustration does not of
course take into account the circuits which must be continuously
powered such as the timing 334, sample and hold and amplifier
circuits. However, these circuits can usually be designed to
consume very little power.
[0069] The configurations of FIG. 4 and FIG. 4A offer high
stability with relatively low cost and complexity. The transducers
16 and 18 are powered from the same source amplifier 248 and
thereby exhibit low differential phase error. The receiving signal
processing circuits are arranged to minimize circuit related phase
errors. This is particularly true if one incorporates the blocks of
FIG. 4A.
[0070] Another advantage of the configuration depicted in FIG. 4 is
that because acoustic transmission and reception occur for both
transducers at the same time, rapidly occurring acoustic path
changes introduce minimal error. With sensor configurations which
rely on alternating transducer functions where at one time a first
transducer transmits and a second receives, and at another time the
second transducer transmits while the first receives, acoustic path
changes may introduce significant error when relatively long
intervals occur between their alternating states.
[0071] The embodiment discussed with respect to FIG. 2 of the
drawing has each transducer in either a transmitting or a receiving
mode at all times. In this arrangement there are no "dead" times in
which a receiving portion of the sensor ignores acoustic artifacts
associated with extraneous reflections which may arise from pipe
interfaces if the transducers are external to a pipe through which
fluid flow. Hence, the embodiment discussed with respect to FIG. 2
is generally restricted to use with wetted transducers. The
burst-mode embodiment, however, can easily be configured with such
dead periods and therefore permits the use of one or more
non-wetted transducers installed on the outside of a pipe through
which fluid flows.
[0072] It may be noted that the simultaneous transmission
arrangement depicted in FIG. 4 and 4A, like the alternating
transmission arrangement of FIG. 2, can be extended by the addition
of a second pair of transducers 370, 372 that operate with an
acoustic beam 374 disposed orthogonally to the beam 22 generated by
the first pair 16, 18 of transducers. As noted above with respect
to the alternating transmission arrangement, the second pair 370,
372 of transducers may be operated either with separate circuitry,
or may time-share the same circuitry used for the first pair 16,
18.
[0073] When used in flow environments which encourage the
accumulation of surface coatings, debris or biogrowths,
electrolytic means may be used to clean or maintain clean the
acoustically active surfaces when wetted transducers are employed.
This would consist, in a sea water environment for example, of a
positive potential being applied to the flow sensing or nearby
surfaces which had been platinum plated so as to cause a
corresponding electric current to flow through the water and
generate chlorine gas at those active surfaces. Nearby insulated
electrodes or conductive surfaces with a corresponding negative
potential complete the current path. Low currents of several
milliamperes and less have been found effective in maintaining the
surfaces of a small flow sensor of a few square inches in surface
area clean in such environments.
[0074] Those skilled in the art can now appreciate from the
foregoing description that the broad teachings of the present
invention can be implemented in a variety of forms where a
transit-time ultrasonic flow sensor is configured as a self
contained modular system. Therefore, while this invention has been
described in connection with particular examples thereof, the true
scope of the invention should not be so limited since other
modifications will become apparent to the skilled practitioner upon
a study of the drawings, specifications and claims.
[0075] Although the present invention has been described with
respect to several preferred embodiments, many modifications and
alterations can be made without departing from the invention.
Accordingly, it is intended that all such modifications and
alterations be considered as within the spirit and scope of the
invention as defined in the attached claims.
* * * * *