Abstrict An electroacoustical flow meter for measuring flow rates in open
and closed channels includes two sets of transducers, one set being
positioned upstream relative to the other set. Flow rates are determined
by computing the difference between upstream and downstream transit
times of acoustical signals transmitted along various paths through
the channel. A receiver in the flow meter monitors the acoustical
signals received after each transmission. When an acoustical signal
is received which has been attenuated due to disturbances such as
obstructions along the signal path or interfering reflected signals,
the receiver transmits a flag signal. This flag signal is utilized
to minimize flow rate reading errors which normally result from
such disturbances.
Claims What I claim as new and desire to secure by Letters Patent of the
United States is:
1. Apparatus for determining the flow rate of a fluid medium through
a channel of the type comprising first and second transducer means
for transmitting and receiving acoustical signals along a path through
the fluid medium, the acoustical signals oscillating in magnitude
with a series of positive-going and negative-going peaks, said first
transducer means being positioned upstream relative to said second
transducer means, means for measuring the difference between upstream
and downstream transit times of the acoustical signals transmitted
between said first and second transducer means, said measuring means
being responsive to a selected one of the peaks of the acoustical
signals received by said first and second transducer means, respectively,
and means responsive to said measuring means for deriving an output
signal indicative of the flow rate of the fluid medium through the
channel, said apparatus being characterized by
A. means for determining whether the selected peak in the acoustical
signals received by said first and second transducer means respectively,
has a magnitude at least equal to a predetermined normal level,
and
B. means for generating a flag signal indicative of the presence
and absence of said predetermined normal magnitude in the selected
peak.
2. Apparatus as recited in claim 1 in which said determining means
determines whether the first negative going peak in the received
signal has a magnitude at least equal to said predetermined normal
level.
3. Apparatus as recited in claim 1 in which each of said first
and second transducer means convert the received acoustical signal
to an electrical signal having a waveshape corresponding to that
of the received acoustical signal, in which said testing means includes
a first threshold detector for receiving the electrical signal and
for transmitting a first signal when the received electrical signal
attains a first threshold level and a second threshold detector
for receiving the electrical signal and for transmitting a second
signal when the received electrical signal attains a second threshold
level, said second threshold level being greater in magnitude than
said first threshold level, and in which said flag signal generating
means is responsive to the first and second signals and generates
said flag signal at a first level when the second signal is received
within a predetermined time interval of the first signal and at
a second level when the second signal is not received within said
predetermined time interval of the first signal.
4. Apparatus as recited in claim 3 in which said testing means
further includes pulse generating means responsive to the first
signal for generating an output pulse of duration equal to said
predetermined time interval, and in which said flag signal generating
means includes gating means responsive to the second signal and
the output pulse from said pulse generating means for transmitting
an output pulse when the second signal is received in coincidence
with the output pulse from said pulse generating means, and means
including a set and reset condition and responsive to said gating
means for transmitting said flag signal at the first level when
set by the output pulse from said gating means and at second level
when not set by the output pulsed from said gating means.
5. Apparatus as recited in claim 4 in which said testing means
further includes resettable means coupled between said first threshold
detector and said pulse generating means and responsive to the activation
of said pulse generating means by the first signal for disabling
said pulse generating means.
6. Apparatus as recited in claim 3 in which said second threshold
level is in the range of about 5 to 10 times greater in magnitude
than said first threshold level.
7. Apparatus as recited in claim 3 in which the received acoustical
signal oscillates in magnitude with a periodicity T, and in which
said predetermined time interval is approximately equal to 1/2T.
8. Apparatus as recited in claim 3 in which said first and second
threshold detectors are identical, and in which testing means further
includes means for attenuating the electrical signal applied to
said second threshold detector relative to the electrical signal
applied to said first threshold detector so that said second threshold
detector responds to a greater magnitude of the electrical signal
than said first threshold detector.
9. Apparatus for determining the flow rate of a fluid medium through
a channel comprising:
first and second transducer means for transmitting and receiving
acoustical signals along a path through the fluid medium, said first
transducer means being positioned upstream relative to said second
transducer means, the acoustical signals oscillating in magnitude
with a series of positive going and negative going peaks;
transmitter means coupled to said first and second transducer means,
respectively, for transmitting an activating signal which activates
one of said transducer means to transmit an acoustical signal through
the fluid medium;
receiver means coupled to said first and second transducer means,
respectively, for detecting the receipt by one of said transducer
means of an acoustical signal transmitted by the other of said transducer
means and for transmitting a timing signal in response thereto,
the timing signal being generated in response to a selected one
of the peaks of the acoustical signal received by said receiver
means attaining a timing signal threshold; pg,28
monitoring means coupled to said receiver means for determining
whether the selected peak of the acoustical signal received by said
receiver means has a predetermined normal magnitude greater than
the timing signal threshold and for generating a flag signal indicative
of the presence and absence of said predetermined normal magnitude
in said selected peak.
10. Apparatus as recited in claim 9 in which said selected peak
of the received acoustical signal is the first negative going peak
therein.
11. Apparatus as recited in claim 10 in which said receiver means
has a threshold level and transmits the timing signal when the first
negative-going peak in the received acoustical signal builds up
in amplitude to said threshold level, and in which said predetermined
normal amplitude of the first negative-going peak in the received
acoustical signal is in the range of 5 to 10 times greater than
the threshold level of said receiver means.
12. Apparatus for determining the flow of a fluid medium through
a channel, said apparatus comprising:
first and second electro-acoustical transducer means for transmitting
and receiving signals along a path through the fluid medium, one
of said transducer means being positioned upstream relative to the
other of said transducer means;
means for transmitting an activating signal which activates one
of said transducer means to transmit an acoustical signal through
the fluid medium, the acoustical signal oscillating with alternate
positive and negative half cycles;
means responsive to the receipt of the acoustical signal by the
other of said transducer means for transmitting a timing signal
responsive to a selected half cycle of the received acoustical signal
attaining a predetermined threshold magnitude;
means responsive to the activating signal and to the timing signal
for measuring the transit of acoustical signals transmitted upstream
and downstream relative to the fluid medium between said first and
second transducer means;
means responsive to said timing means for deriving an output indicative
of the flow rate of the fluid medium; and
signal quality monitoring means including means for generating
a flag signal indicative of an acoustical signal received under
normal propagation conditions through the fluid medium, said generating
means generating the flag signal responsive to the selected half
cycle of the received acoustical signal attaining a predetermined
normal magnitude in excess of said predetermined threshold magnitude,
said monitoring means further including means for preventing the
generation of the flag signal responsive to other half cycles of
the received acoustical signal attaining said predetermined normal
magnitude.
13. Apparatus as recited in claim 12 in which the acoustical signal
half cycles are defined by first and second zero axis crossings,
the second zero axis crossing being spaced by a time interval of
T/2 from the first zero axis crossing, in which said detecting means
includes a first threshold detector for generating the timing signal
responsive to the selected half cycle of the received acoustical
signal attaining said predetermined threshold magnitude, in which
said flag signal generating means comprises a second threshold detector
for generating the flag signal responsive to the selected half cycle
of received acoustical signal attaining said predetermined normal
magnitude within a time interval of T/2 after generation of the
timing signal, and in which said preventing means includes means
for disabling said flag signal generating means upon the expiration
of the T/2 time interval.
14. Apparatus as recited in claim 13 in which said predetermined
threshold magnitude is such that said first threshold detector generates
the timing signal responsive to the selected half cycle of the received
acoustical signal completing the first zero axis crossing, said
predetermined threshold magnitude being displaced sufficiently from
zero to avoid the generation of the timing signal as a result of
noise in the received acoustical signal and in which said predetermined
normal magnitude is between 5 and 10 times greater than said predetermined
threshold magnitude.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to fluid flow measurement, and, more particularly,
to an electroacoustical flow meter with signal quality monitoring
capabilities.
2. Description of the Prior Art
There are numerous techniques for measuring fluid flow rates in
open and closed channels. Some of these techniques are embodied
in systems of the electroacoustical type which include one or more
acoustical transducer pairs. Each pair includes an upstream and
a downstream transducer spaced a fixed distance apart. The flow
rate is determined by computing the difference between the upstream
and downstream transit times of acoustical signals transmitted through
the fluid between the transducers. Prior systems of this type are
shown, for example, in U.S. Pat. Nos. 3237453 and 3564912.
More particularly, in these systems, an acoustical signal is transmitted
through the fluid by a first transducer and is received by a second
transducer spaced a fixed distance D away from the first. If the
acoustical signal is transmitted from an upstream transducer to
a downstream transducer, the acoustical propagation is aided by
the flow of the fluid and the transit time is reduced. The reverse
is true if the acoustical signal is propagated against the flow
direction. The difference in the effective speed of sound along
and against the direction of flow is proportional to the velocity
of the flowing medium relative to the axis between the transducers.
Thus, by accurately measuring the difference between the upstream
and downstream transit times, T.sub.u and T.sub.d, respectively,
the flow velocity can be determined. The mathematical relationship
for determining the flow velocity V relative to the transducer axis
is easily derived and may be expressed as follows:
where .DELTA. T= T.sub.u - T.sub.d, C is the speed of sound in
the fluid and D is the distance between transducers.
Systems of this type must employ a relatively accurate timer for
measuring acoustical signal transit times through the fluid, particularly
for channels with low flow velocities. Additionally, the timer must
be started and stopped at precisely the right time. The same electrical
impulse that is used to activate the transmitting transducer may
also be used to start the transit time timer in the system. Thus,
it is fairly simple to synchronize the start of the timer with the
start of the acoustical signal transmission through the fluid. Problems
are encountered, however, in stopping the timer exactly at the time
that the acoustical signal is received by the receiving transducer.
Typically, a system of this type utilizes a threshold receiver,
connected to the receiving transducer, which emits a timing signal
when the output of the transducer reaches a readily sensed point
in its waveform, usually the first zero-axis crossing. Assuming,
for example, that the transducer output undergoes a positive excursion
prior to the first axis crossing, the crossing is detected by sensing
a negative polarity in the signal. In practice this is done with
a threshold detector whose threshold is set at a sufficiently negative
level to avoid false triggering because of noise. Because of the
steep slope of the waveform in this region, timing errors resulting
from noise-like effects on the threshold level are negligible.
The timing signal is used to stop the timer. As long as the acoustical
signals received after each pass through the fluid have substantially
the same magnitude, the timing signal provides an accurate indication
of the end of the transit time period for each pass. However, in
many channels, the flow conditions often change from one pass to
another causing the magnitude of the acoustical signals received
also to change.
Specifically, inhomogeneities in the fluid properties, such as
temperature and salinity, or the presence of obstructions, such
as air bubbles, silt and debris, can attenuate and distort a propagating
acoustical signal. In open channels, random reflections from the
upper and lower surfaces of the channel often combine with a direct
path signal to produce a received signal, the magnitude of which
is dependent on the phase relationship between the reflected and
direct path signals. If the attenuation is severe enough, so that
no portion of a propagated signal has an excursion greater than
the threshold of the zero-crossing detector, these signals go completely
undetected. Also, the signal attenuation may be such that the first
zero crossing does not have an excursion greater than the threshold
of the zero-crossing detector, but the second zero crossing does,
causing the detector to respond to the wrong half cycle of the signal.
These factors give rise to instabilities in the receiver timing
signals, and thereby result in inaccurate and unreliable flow rate
readings.
It is therefore, an object of the present invention to provide
a flow metering system of improved accuracy.
Another object of the invention is to provide an electroacoustical
flow meter with signal quality monitoring capabilities.
Still another object of the invention is to provide an electroacoustical
flow meter of the type described which minimizes flow reading errors
due to fluid disturbances.
Yet still another object of the invention is to provide an electroacoustical
flow meter of the type described which minimizes flow reading errors
due to reflected acoustical signals.
SUMMARY
In accordance with the present invention, an electroacoustical
flow meter includes a receiver which monitors the quality of each
acoustical signal received from the fluid whose flow rate is being
measured. Generally, the receiver tests each received signal for
the presence of a predetermined normal characteristic, and generates
a flag signal indicative of the presence or absence of this characteristic.
More specifically, and in accordance with an illustrative embodiment
of the invention, the receiver determines whether a selected portion
of a received acoustical signal, e.g., the first negative-going
peak, has a predetermined normal magnitude greater than a timing
signal threshold of the receiver. As each transmitted acoustical
signal is received, the receiver not only transmits a timing signal
which is used in determining the transit time of received signal,
but also transmits a flag signal which specifies whether the received
signal is normal or "bad". In this way, the receiver discriminates
between signals which are received under normal propagation conditions,
and bad signals which have been unduly attenuated or distorted,
in one manner or another, during transmission through the fluid.
The flag signal is then used to reduce flow-rate reading errors
that normally result from the bad signals. For example, the flag
signal may be coupled to a processor which activates a bad signal
alarm and restarts or otherwise alters the flow measurement process
each time a bad signal is received.
Preferably, the receiver comprises first and second threshold detectors
which are coupled to each receiving transducer in the system. An
input attenuator section coupled between the threshold detectors
and the receiving transducers causes the second threshold detector
to respond to a larger magnitude of the received signal than the
first. The first, lesser magnitude, threshold detector generates
the timing signal which stops a timer. The second, higher amplitude
threshold detector has a threshold corresponding essentially to
the peak level of the first negative-going peak of a normal signal.
An output logic section compares the outputs from the first and
second threshold detectors to determine if the first negative-going
peak in the received signal has at least the predetermined normal
level. Based upon this comparison, the logic section generates the
flag signal as a logic signal, the level of which specifies whether
the received signal is normal or bad.
The invention is pointed out with particularity in the appended
claims. The foregoing and other features and advantages of the invention
will be better understood from the following detailed description
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a flow metering system embodying the
principles of the invention;
FIG. 2 which comprises FIGS. 2A, 2B and 2C, illustrates waveshapes
of signals received in a system of the type shown in FIG. 1; and
FIG. 3 illustrates further details of a signal quality monitoring
receiver of FIG. 1.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT
In FIG. 1 I have illustrated an open channel 10 whose flow rate
is to be measured using the system of the present invention. This
channel 10 may, for example, be a relatively large river or stream,
which conducts water to a reservoir serving a pumping station, a
power plant or other facility. Illustratively, the system monitors
the flow rate on a continuous basis so that operators can open and
close various control gates to regulate the water flow to the facility.
It should be understood, however, that this system has equal application
for measuring flow rates through closed channels and pipes.
The system includes two sets of electroacoustical transducers distributed
across the channel 10 being monitored. One transducer set is positioned
upstream from the other set. The transducer sets may include any
number of transducers. In FIG. 1 for example, the upstream set
is shown with three transducers 12U, 14U and 16U, and the downstream
set is shown with three transducers 12D, 14D and 16D. In practice,
however, particularly for relatively large channels more than three
transducers in each set would typically be used. Arrow 18 indicates
the direction of the fluid flow in the channel 10.
Path selection switches 20 and 22 connect any one of the upstream
transducers 12U, 14U, and 16U and the corresponding one of the downstream
transducers 12D, 14D and 16D in a circuit with a receiver 24 and
a transmitter 26. When the transmitter 26 is activated, it sends
an electrical impulse to the transducer to which it is connected
by the switch 20 causing that transducer to transmit an acoustical
signal through the fluid. This same electrical impulse is fed via
a line 27 to a timing mechanism comprising a transit-time counter
32 and a clock 36. The transmitter-generated impulse on the line
27 enables the counter 32 to start counting clock pulses from the
clock 36.
The system illustratively operates on opposing transducer pairs
only. For example, if one of the upstream transducers 12U, 14U or
16U transmits the acoustical signal, it is received by its opposing
downstream transducer 12D, 14D or 16D, respectively. Alternatively,
if one of the downstream transducers 12D, 14D or 16D transmits the
acoustical signal, it is received by its opposing upstream transducer
12U, 14U or 16U, respectively. The receiving transducer converts
the received acoustical signal to a corresponding electrical signal
which is passed through the switch 22 to the receiver 24. When the
received signal builds up to the timing signal threshold of the
receiver 24 the receiver 24 transmits a timing signal over a line
29. The timing signal stops the counter 32.
A controller 30 controls the path selection switches 20 and 22.
Each of the switches 20 and 22 has six possible positions. The controller
30 sequences the switches 20 and 22 incrementally through each of
these positions so that transit time data is collected for each
opposing transducer pair and each transducer in each pair serves
both as a transmitting transducer and a receiving transducer. In
this way, both the upstream and downstream transit times are collected
for each path.
For example, as viewed in FIG. 1 the controller 30 initially connects
the switch 20 to its leftmost position and initially connects the
switch 22 to its rightmost position. The upstream transducer 12U
is thus serving as the transmitting transducer, while the downstream
transducer 12D is serving as the receiving transducer. An electrical
impulse is sent by the transmitter 26 to the transducer 12U. The
time required for an acoustical signal to pass downstream from the
transducer 12U to the transducer 12D is measured by the counter
32. After the counter 32 is stopped, the number contained therein,
corresponding to the downstream transit time for that particular
pair, is transferred to a processor 34 and the counter 32 is reset.
The controller 30 then increments the switch 20 one position to
the right in FIG. 1 and increments the switch 22 one position to
the left in FIG. 1. This causes the transducer 14U to serve as the
transmitting transducer, and the transducer 14D to serve as the
receiving transducer. The timing process is repeated. The number
in the counter 32 in this case corresponding to the downstream
transit time for the transducer pair 14U- 14D, is also transferred
to the processor 34.
The controller 30 continues to increment the switch 20 to the right
and the switch 22 to the left, and the timing process is repeated
after each increment of the switches. It is noted that after the
third increment of the switches 20 and 22 the downstream transducers
in the system are serving as transmitting transducers and the upstream
transducers are serving as receiving transducers. The numbers transferred
to the processor 34 in these cases correspond to the upstream transit
times for each pair. After the measurement sequence is completed,
the switches 20 and 22 may be returned to their initial positions
by the controller 30 in preparation for the next measurement sequence.
Alternatively, the direction in which the switches 20 and 22 are
incremented may be reversed during the next measurement sequence.
All fixed constants associated with the different acoustical paths
between each transducer pair are stored in the processor 34. Thus,
for each transducer pair, numbers are stored corresponding to the
path length, D, between the transducers, the angle .theta., that
the acoustical path makes with the fluid flow direction, and the
speed of sound, C, through the fluid. With the propagation time
data supplied by the counter 32 the processor 34 can thus calculate
the fluid flow velocity as measured for each path according to the
mathematical relationship set forth previously. The processor 34
then averages the velocities from all the paths to obtain an accurate
flow rate measurement for the channel 10. Various weighting factors
may also be stored in the processor 34 for use in the averaging
function. The processor 34 also controls a display device 38 to
provide a visual reading of the channel flow rate.
With the exception of the receiver 24 each of the elements shown
in FIG. 1 may be of conventional construction. In accordance with
this invention, the receiver 24 not only performs the conventional
timing function, but also monitors the quality of the electrical
signal received from each receiving transducer after each acoustical
signal is passed through the fluid. The receiver 24 sets certain
predetermined conditions on the magnitude of the received signal.
If a received signal does not meet these conditions, the receiver
24 transmits a "bad" signal flag over a line 40. This
flag is then used to reduce the flow rate reading errors that typically
arise when a bad signal is received.
Before proceeding with a more detailed description of the receiver
24 of my invention, I make reference to FIGS. 2A- 2C of the drawings.
These figures illustrate typical signal waveshapes in the output
of a receiving transducer in an electroacoustical system of the
type shown in FIG. 1. As indicated, the signals oscillate with a
series of negative-going and positive-going peaks which initially
build up in magnitude and then decay to zero. This general waveshape
results from the resonant character of the transmitting and receiving
transducers.
Each signal has a periodicity, T, which is also determined by the
transducer characteristics. Consequently, the period between successive
zero crossings (i.e., 1/2T), is relatively stable from one pass
through the fluid to the next. Also relatively stable from pass
to pass are the relative magnitudes of the first two peaks of the
same polarity in each of the signals. That is, the ratio of the
magnitudes of the first and second positive-going peaks and the
ratio of the magnitudes of the first and second negative going peaks
are substantially constant in each received signal regardless of
variations in the fluid properties. Typically, these magnitude ratios
are about 3 to 1. The absolute magnitudes of the peaks in the signals,
however, vary widely with fluid conditions.
These factors are illustrated in FIGS. 2A-2C of the drawings. FIG.
2A represents a signal received under normal acoustical propagation
conditions. FIGS. 2B and 2C represent signals which have been attenuated
to different degrees, for example, due to obstructions passing by
in the fluid or to interferring reflected signals. In each of the
signals shown in FIGS. 2A-2C, the zero crossings occur at essentially
the same time and the magnitude ratios of the first two peaks of
the same polarity are essentially equal. The magnitudes of corresponding
peaks in the signals of FIGS. 2A, 2B and 2C, however, differ depending
upon the degree of attenuation experienced by the acoustical signal
during propagation through the fluid.
The dashed line labeled A in FIGS. 2A-2C represents a typical receiver
timing signal threshold. Because the time period between successive
zero crossings in the received signals are relatively constant,
but the magnitudes of the peaks in the signals vary, it is desirable
to use a portion of the signal as near as practical to a selected
zero crossing in generating the timing signal. It is thus common
to choose the timing signal threshold of the receiver such that
the timing signal is transmitted shortly after the received signal
first goes negative. The dashed line labeled B in each of FIGS.
2A-2C corresponds essentially to the magnitude of the first negative-going
peak in a signal, such as that shown in FIG. 2A, which is received
under normal propagation conditions.
Selecting the threshold of a receiver at a low magnitude level,
such as the level A indicated in FIGS. 2A-2C, reduces, but does
not eliminate, problems resulting from deviations in signal magnitudes
from normal levels. Specifically, the first negative-going peak
in the signal of FIG. 2B builds up in magnitude more slowly than
the first negative-going peak in the normal signal of FIG. 2A. Consequently,
when a signal such as that shown in FIG. 2B is received, rather
than the normal signal shown in FIG. 2A, the transmission of the
timing signal is delayed by a short but significant period of time.
With a signal of the type shown in FIG. 2C, the timing signal is
generated in response to the second negative-going peak and not
the first, since the first negative-going peak has a magnitude less
than the level A. Thus, the timing signal is generated in response
to the wrong half cycle of the signal and is delayed by an even
greater length of time when the received signals are of the type
illustrated in FIG. 2C. In either case, the delay results in an
inaccurate flow-rate reading.
My signal quality monitoring receiver 24 shown in block form in
FIG. 1 is illustrated in further detail in FIG. 3 of the drawings.
The receiver 24 determines whether the selected portion of the received
signal (e.g., the first negative-going peak) which causes the transmission
of the timing signal also has a normal magnitude such as that indicated
by the level B in FIGS. 2A-2C. The receiver 24 thus discriminates
between normal signals of the type shown in FIG. 2A and signals
such as those shown in FIGS. 2B and 2C which have been attentuated,
in one manner or another, during propagation through the fluid.
Specifically, the receiver detects if and when the received signal
first attains the levels A and B, respectively. If the received
signal attains the level B within one half cycle (i.e. 1/2T) of
the time it attains the level A, then the receiver 24 recognizes
that the signal is normal. Otherwise, the receiver 24 recognizes
that the signal is bad and transmits the bad signal flag.
As indicated in FIG. 3 the electrical signal from a receiving
transducer in the system of FIG. 1 is fed through an attenuator
section comprising potentiometers 50 and 52 to a pair of threshold
detectors 54 and 56. Each of the threshold detectors 54 and 56 includes
conventional components for amplifying and shaping the incoming
signal and for generating a logic level output pulse when the voltage
of the input signal attains a predetermined threshold level. Preferably,
the threshold detectors 54 and 56 have the same gains and inherent
threshold levels. The potentiometer 50 provides a combined sensitivity
adjustment for the detectors 54 and 56. The potentiometer 52 further
attenuates the signal fed to the detector 56 relative to the signal
fed to the detector 54. Thus, the detector 56 responds to a larger
magnitude than does the detector 54. The received signal is tested
by comparing the outputs from the detectors 54 and 56.
Specifically, the threshold detector 54 is selected to have a low
magnitude negative threshold level such as that indicated by the
dashed line A in FIGS. 2A-2C. Thus, the detector 54 transmits an
output pulse shortly after the received signal first goes negative.
The leading edge of this pulse from the detector 54 is the timing
signal and is used to stop the counter 32.
The potentiometer 52 is adjusted so that the detector 56 develops
an output pulse when the incoming signal attains the level indicated
by the line B in FIGS. 2A-2C. As noted, the level B is close in
value to the magnitude of the first negative-going peak in a normal
signal. The specific level B used, of course, varies depending upon
the characteristics of the particular components in the system and
of the particular channel whose flow rate is being measured. Typically,
the level B used is determined on the basis of a number of sample
passes through the fluid prior to taking flow rate measurements.
As seen in FIG. 3 the output of the detector 54 is also coupled
to an output logic section including an AND gate 60 and a monostable
multivibrator 62. Gate 60 is enabled by the reset state of a flip-flop
64. When the detector 54 transmits its output pulse, the leading
edge of this pulse triggers the multivibrator 62. The multivibrator
62 emits a pulse, the duration of which is selected to be approximately
equal to one-half the acoustical signal period T (FIGS. 2A-2C).
The multivibrator pulse determines the time interval during which
the outputs of the detectors 54 and 56 are compared.
The pulse from the multivibrator 62 enables another AND gate 68
for a time interval equal to 1/2T. If the first negative-going peak
of the incoming signal has a magnitude at least equal to the level
B, the detector 56 transmits an output pulse during this time interval.
The output pulse from the detector 56 is thus passed by the gate
68 and sets a flip-flop 70. The corresponding output of the flip-flop
70 is a signal quality flag which asserts that the incoming signal
is normal. This output is coupled by the flag line 40 of FIG. 1
to the processor 34. When the processor 34 receives the transit-time
data from the counter 32 of FIG. 1 it senses the level of the flag
line 40. If the flag line 40 indicates that the incoming is normal,
the measurement process is allowed to continue in the normal sequence.
If, however, the incoming signal is of the type shown in FIG. 2B,
the first negative-going peak thereof does not have sufficient magnitude
to activate the detector 56. The multivibrator 62 of FIG. 3 is triggered
by the output pulse from the detector 54 and the gate 68 is enabled,
but no pulse is passed by the gate 68 since the detector 56 is not
activated within 1/2T of the activation of the detector 54. The
flip-flop 70 is thus not set. The flag line 40 from the output of
the flip-flop 70 thus remains at a level which represents that the
incoming signal is bad. Upon receiving the transit time data from
the counter 32 of FIG. 1 the processor 34 senses the level of the
flag line 40 and recognizes that the received signal is bad. The
processor 34 in this event activates the alarm 42 of FIG. 1 and
initiates the appropriate corrective operation. For example, the
processor 34 resets the controller 30 via the line 44 in FIG. 1
so that the measurement sequence for the particular transducer pair
being used is repeated.
It is noted that the second negative-going peak in the signal of
FIG. 2B has sufficient magnitude to activate both threshold detectors
54 and 56. The receiver 24 of FIG. 3 however, prevents this portion
of the incoming signal from being erroneously interpreted as a normal
signal. Specifically, the output of the multivibrator 62 is also
coupled to set the flip-flop 64. Thus, when the detector 54 transmits
an output pulse responsive to the first negative-going peak of the
signal, the resulting output pulse from the multivibrator 62 sets
the flip-flop 64.
The output of the flip-flop 64 in turn, disables the input gate
60. If the detector 54 transmits another output pulse responsive
to the second negative-going peak in the signal, the multivibrator
62 is not triggered thereby. The gate 68 remains disabled and the
flip-flop 70 remains in its reset state. The flag line 40 thus remains
at a level representing that the incoming signal is bad. The gate
60 is again enabled only after the flip-flop 64 is reset by the
controller 30 of FIG. 1 under the control of the processor 34 or
at the start of the next measurement sequence.
If the received signal is of the type shown in FIG. 2C, no portion
of the signal has sufficient magnitude to activate the detector
56. As a result, the flip-flop 70 is not set, and the flag line
40 remains at a level representing that the received signal is bad.
Optimum discrimination between normal signals and bad signals is
obtained when the ratio B/A of the thresholds of the detectors 54
and 56 is in the range of about 5 to 10. This is due to the fact
that the magnitude of the second negative going peak in a received
signal is about 3 times the magnitude of the first negative going
peak, as noted earlier. Thus, for example, if the detector 54 responds
to a signal level of about -1 millivolt, the detector 56 preferably
responds to a level of about -5 to -10 millivolts. The ratio B/A
can be adjusted to optimize signal discrimination in each case simply
by adjusting the potentiometer 52.
In summary, therefore, I have described an electroacoustical flow
metering system which incorporates a receiver with signal quality
monitoring capabilities. The receiver tests each received acoustical
signal for the presence of a predetermined, normal characteristic.
This characteristic is illustratively determined on the basis of
the magnitude of the first negative going peak in a signal received
under normal propagation conditions. The presence of signal-altering
disturbances in the fluid whose flow rate is being measured is thus
detected. The effect such disturbances have on the accuracy of system
flow readings is thus minimized.
The above-described system is illustrative of only one possible
embodiment of the invention. Various modifications and additions
to the specific system described may be devised by those skilled
in the art without departing from the scope of the invention. For
example, the specific orientation and number of transducers used
may vary from that shown in FIG. 1 and would depend on the accuracy
of the measurements required and the specific characteristics of
the channel whose flow rate is being measured. Additionally, although
the receiver 24 of FIG. 3 has been described as responsive to the
first negative-going peak in each received signal, the receiver
24 can be readily modified to respond to other portions of the signals
including a selected positive going peak. Also, the specific programmed
response of the processor 34 to a bad signal flag may vary in complexity,
depending upon the particular accuracy requirements in each case.
It is, therefore, the object of the appended claims to cover all
such variations and modifications as come within the true spirit
and scope of this invention.
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