Abstrict A sonic flow meter for measuring the rate of flow of a fluid within
an enclosed axially elongated hollow conduit is disclosed. First
and second transducers are secured to the exterior surface of the
conduit. The transducers are spaced from each other along the axis
of the conduit such that the first transducer is located downstream
from the second transducer with respect to the direction of flow
of the fluid. Each of the transducers are adapted to transmit to
each other, and receive from each other, sinusoidal sonic signals
which pass through the fluid in the conduit. Sonic signal transmission
means are provided for causing a plurality of sonic signals to be
transmitted in an upstream direction from the first to the second
transducer means and for causing an equal plurality of sonic signals
to be transmitted in a downstream direction from the second to the
first transducer means. Finally, a flow measurement circuit is provided
for measuring the rate of flow of the fluid as a function of the
difference in time it takes to transmit the sonic signals in the
upstream and downstream directions. The flow measurement circuit
determines the time difference by measuring the upstream and downstream
transmission times with reference to a plurality of zero cross-over
points of the transmitted sonic signals.
Claims What is claimed is:
1. A sonic flow meter for measuring the rate of flow of a fluid
within an enclosed axially elongated hollow conduit, comprising:
first and second transducer means secured to the exterior surface
of said conduit; said first and second transducer means being spaced
from each other along the axis of said conduit such that said first
transducer means is located downstream from said second transducer
means with respect to the direction of flow of said fluid; each
of said transducer means being adapted to transmit to each other,
and to receive from each other, sinusoidal sonic signals which pass
through said fluid in said conduit;
sonic signal transmission means for causing said first transducer
means to transmit N sonic signals in an upstream direction from
said first to said second transducer means and for causing said
second transducer means to transmit N sonic signals in a downstream
direction from said second to said first transducer means during
each of a plurality of successive transmission cycles; and
flow measurement circuit means for measuring the rate of flow of
said fluid as a function of the difference in time it takes to transmit
said sonic signals in said upstream and said downstream directions,
said flow measurement circuit means determining said time difference
by measuring said upstream and downstream transmission times with
reference to a single zero cross-over point of each of said received
sonic signals, the particular zero cross-over point referenced during
any given transmission cycle being the same for each sonic signal
received in that transmission cycle, the particular zero cross-over
point referenced during each successive transmission cycle being
different than the zero cross-over point referenced during a prior
transmission cycle.
2. The sonic flow meter of claim 1 wherein:
said flow measurement circuit means determines the difference between
said upstream and downstream transmission times at the end of each
of said transmission cycles; and
said flow measurement circuit means measures said rate of flow
by averaging said difference between said upstream and downstream
transmission times over a plurality of said transmission cycles.
3. The sonic flow meter of claim 2 wherein the location of said
plurality of zero cross-over points is affected by noise associated
with said conduit, the location of some of said zero cross-over
points being affected by said noise to a lesser degree than the
remaining zero cross-over points, and wherein said flow measurement
circuit means determines said difference between said upstream and
downstream transmission times with respect to those zero cross-over
points whose location is affected by said noise to said lesser degree
at a greater frequency than the remaining said zero cross-over points.
4. The sonic flow meter of claim 2 wherein said flow measurement
circuit means comprises:
signal recognition circuit means for generating a recognition signal
upon receipt of a transmitted sonic signal by either of said transducer
means, said signal recognition circuit generating said recognition
signal responsive to the detection of one of said plurality of zero
cross-over points;
zero cross-over logic means for causing said signal recognition
circuit to generate said recognition signal responsive to the detection
of a different one of said plurality of zero cross-over points during
each successive said transmission cycle, said zero cross-over logic
means also for causing said signal recognition circuit to generate
said recognition signal responsive to the detection of the same
zero cross-over point during any given transmission cycle;
flow computer means for generating a flow signal indicative of
the rate of flow of said fluid in said conduit at the end of each
of said transmission cycles whereby each of said flow signals is
associated with a different said transmission cycle, said flow computer
means generating said flow signal as a function of the difference
in time it takes to transmit said sonic signals in said upstream
and said downstream directions as indicated by said recognition
signal in the transmission cycle associated therewith; and
means for computing an average rate of flow of said fluid in said
conduit as a function of a plurality of said flow signals.
5. The sonic flow meter of claim 4 wherein said flow measurement
circuit means further includes display means for displaying said
average flow rate.
6. The sonic flow meter of claim 4 wherein said flow computer
means computes said flow rate in accordance with the following equation:
wherein V.sub.f is a signal indicating the rate of flow of said
fluid through said conduit, K is a proportionality constant, .DELTA.T
is the difference in the upstream and downstream transmission times
and T.sub.L is the time the sonic signal spends in the fluid during
a single transmission between said first and second transducer means.
7. A method for measuring the arrival time of a repetitive sinusoidal
wave form pulse which is accompanied by noise; said method comprising
the steps of: repetitively detecting the occurrence of a plurality
of zero cross-over points of said wave form, averaging the time
of arrival of each said cross-over points within said wave form
to determine the arrival time for each such cross-over without the
effect of the presence of noise.
8. A method for measuring the rate of flow of a fluid within an
enclosed axially elongated hollow conduit, comprising the steps
of:
securing first and second transducer means to the exterior surface
of said conduit such that said first and second transducer means
are spaced from each other along the axis of said conduit, said
first transducer means being located downstream from said second
transducer means with respect to the direction of flow of said fluid;
transmitting N sonic signals in an upstream direction from said
first transducer means to said second transducer means and transmitting
N sonic signals in a downstream direction from said second to said
first transducer means during each of a plurality of transmission
cycles, each of said sonic signals having a plurality of zero cross-over
points; and
measuring the rate of flow of said fluid by determining the difference
in time it takes to transmit said sonic signals in said upstream
and said downstream directions, said step of determining said time
difference including the step of measuring said upstream and downstream
transmission times with reference to a single said zero cross-over
point of each of said received sonic signals, the particular zero
cross-over point referenced during any given transmission cycle
being the same for each sonic signal received in that transmission
cycle, the particular zero cross-over point referenced during each
successive transmission cycle being different than the zero cross-over
point referenced during a prior transmission cycle.
9. The method of claim 8 wherein said step of determining the
difference between said upstream and downstream transmission times
includes the step of making such determination at the end of each
said transmission cycle.
10. The method of claim 9 wherein said step of measuring the rate
of flow of said fluid includes the step of averaging the determinations
of said differences between said upstream and downstream transmission
times over a plurality of said transmission cycles.
11. The method of claim 10 wherein the location of said plurality
of zero cross-over points is affected by noise associated with said
conduit, the location of some of said zero cross-over points being
affected by said noise to a lesser degree than the remaining said
zero cross-over points, and wherein said difference between said
upstream and downstream transmission times is measured with respect
to those zero cross-over points whose location is affected by said
noise to said lesser degree at a greater frequency than the remaining
said zero cross-over points.
12. The method of claim 11 wherein said flow rate is computed
in accordance with the following equation:
wherein V.sub.f is a signal indicating the rate of flow of said
fluid through said conduit, K is a proportionality constant, .DELTA.T
is the difference in the upstream and downstream transmission times
and T.sub.L is the time the sonic signal spends in the fluid during
a single transmission between said first and second transducer means.
13. Apparatus including means responsive to the receipt of a repetitive
sinusoidal pulse signal; said apparatus including means for measuring
the arrival time of repetitive sinusoidal waveform pulses which
are accompanied by noise, means for detecting the zero cross-over
point of each of a plurality of loops in said sinusoidal waveform
pulse, means averaging the time of arrival of each of the corresponding
zero cross-over points in each repetitively received signal, and
means connecting a signal related to the averaged time of arrival
of at least a selected one of said zero cross-over points to said
means responsive to said signal.
Description BACKGROUND OF THE INVENTION
The present invention relates to a liquid flow meter, more particularly
a sonic liquid flow meter. Liquid flow meters of the foregoing type
measure the rate of flow of a liquid within a conduit (e.g., a pipe)
by transmitting a plurality of sonic pulses through the fluid in
both an upstream and downstream direction and by computing the rate
of flow as a function of the difference in time it takes for the
upstream beam to travel through the fluid with respect to the time
it takes for the downstream beam to flow through the fluid.
Typical of such flow meters are U.S. Pat. Nos. 3869915 (hereinafter
the '915 patent) and 3987674 (hereinafter the '674 patent) both
issued to Joseph Baumoel. In accordance with the teachings of these
patents, a sonic beam is transmitted between first and second transducers
located adjacent the outer walls of the conduit. The transducers
are spaced in such a manner that one transducer is located upstream
from the other transducer. In each of these patents, a plurality
of sonic beams are transmitted from the upstream to the downstream
transmitter and then from the downstream to the upstream transmitter.
By measuring the difference in travel time of the upstream and downstream
pulses, an indication of flow rate of the fluid in the conduit is
provided.
The primary advantage of sonic flow meters of the foregoing type
is that they do not require an invasion of the conduit walls. As
such, installation of the sonic flow system does not require a shutdown
of the system being monitored nor does it require the cutting into
the conduits. Additionally, since the sonic flow meter does not
require physical contact between the measuring apparatus and the
liquid whose rate is being measured, there is no possibility that
the sonic apparatus will hinder flow or will be adversely affected
by the chemical nature of the fluid being monitored.
While sonic flow meters such as those described in the foregoing
patents overcome the drawbacks of standard mechanical flow meters,
their accuracy depends on their ability to precisely detect the
time interval between the instant at which the sonic beam is transmitted
from one transducer (e.g., the downstream transducer) until it is
received by the second transducer (e.g., the upstream transducer).
In practice, at a flow rate of 30 feet per second the difference
between the upstream and downstream travel times for a sonic beam
is only about 0.2 percent of the time taken to travel either upstream
or downstream. Since the travel times are extremely small (approximately
20 microseconds per inch in water) the difference in the upstream
and downstream travel time is only a small percentage of the period
of the sonic beam being transmitted (which beam is normally a sinusoidal
pulse with an exponential envelope in form). In order to obtain
the desired accuracy, it is imperative that receipt of the transmitted
sonic beam is detected with reference to the same point of the pulse
during each upstream-downstream pair of transmissions. In the foregoing
patents, this result was obtained by detecting a specific zero crossing
point of each of the sinusoidal sonic pulses transmitted.
BRIEF DESCRIPTION OF THE INVENTION
While the foregoing detection procedure has been found to be generally
satisfactory, it has been determined that flow readings are affected
by systematic noise contained in the fluid container (hereinafter
pipe noise). This pipe noise is produced as a result of the ringing
of the transmitting transducer as well as other miscellaneous factors.
The noise signal is phase coherent with the transmitted sonic beam
and can combine with the received liquid signal in a manner which
causes a random phase shift of the zero crossover points of the
sonic beam if the phase of the pipe noise shifts relative to the
liquid signal. This phenomena can best be understood with reference
to FIGS. 1A-1D.
FIG. 1A illustrates the wave form of the transmitted sonic pulse
as it passes through the liquid in the pipe. As shown therein, this
pulse is exponentially sinusoidal in nature, having a frequency
determined by the transducer parameters and the excitation signals
applied to the transducer. FIG. 1B illustrates a typical wave form
of noise contained in the wall of the pipe through which the liquid
flows and FIG. 1C illustrates the signal detected by the receiving
transducer (for example, the upstream transducer). This signal is
a combination of the transmitted sonic beam illustrated in FIG.
1A and the pipe noise illustrated in FIG. 1B. While the magnitude
A.sub.N of the pipe noise is fairly small with respect to the magnitude
A.sub.L of the transmitted sonic beam, it is sufficiently large
to cause phase shifts in the received signal. This phase shift can
best be seen in FIG. 1D which illustrates the relative phase of
the transmitted sonic beam, the pipe noise and the received signal.
As shown therein, the zero cross-over point of the received signal
is phase shifted with respect to the transmitted sonic beam due
to the pipe noise. Since the frequency and phase of the pipe noise
is phase coherent with the signal, its effect on each zero cross-over
point of the transmitted sonic pulse will also be systematic, causing
a shift of reading as a result of variations in the perceived arrival
time of the signal caused by, for example, small changes in liquid
velocity, pipe properties, or liquid travel time which might result
from changes in temperature or fluid properties. Additionally, since
the frequency of the pipe noise is substantially similar to that
of the transmitted sonic pulse, it is nearly impossible to filter
this noise out of the received signal. As a result, the zero cross-over
points of the received signal will exhibit a systematic phase shift
which can significantly affect the measurement of the upstream and
downstream transmission times.
In order to overcome the foregoing problem, the present invention
measures the upstream and downstream transmission times of the transmitted
sonic beams at each of a plurality of zero cross-over points (in
the example illustrated in FIG. 1A, points A-E). Since the probability
that the phase relationship between the pipe noise and the transmitted
sonic beam will be identical at each of the zero cross-over points
A-E is very small, the average effect of the noise for a large number
of upstream and downstream transmissions will be negligible.
BRIEF DESCRIPTION OF THE DRAWINGS
For the purpose of illustrating the invention, there are shown
in the drawings several embodiments which are presently preferred;
it being understood, however, that this invention is not limited
to the precise arrangements and instrumentality shown.
FIG. 1 is a graph showing a plurality of wave forms which demonstrate
a phase shift problem overcome by the present invention;
FIG. 2 is a schematic diagram of one embodiment of the liquid flow
meter of the present invention;
FIG. 3 is a block diagram of the flow measurement circuit of FIG.
2;
FIG. 4 is a block diagram of the signal transmission circuit of
FIG. 3;
FIG. 5 is a block diagram of the timing and control circuit of
FIG. 3;
FIG. 6 is a timing diagram for the flow measurement circuit of
FIG. 3;
FIG. 7 is a block diagram of the signal recognition circuit of
FIG. 3;
FIG. 8 is a block diagram of the four cross logic circuit of FIG.
3;
FIG. 9 is a block diagram of the cycle counter of FIG. 3;
FIG. 10 is a timing diagram for the timing and control circuit
of FIG. 5;
FIG. 11 is a timing diagram for the four cross logic circuit of
FIG. 8; and
FIG. 12 is a block diagram of the flow computer of FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings wherein like numerals indicate like
elements, there is shown in FIG. 2 a liquid flow meter constructed
in accordance with the principles of the present invention and designated
generally as 10. Liquid flow meter 10 includes a pair of transducers
12 and 14 and a flow measurement circuit 16. Transducers 12 and
14 are coupled to the outer wall of pipe 18 by transducer housings
12', 14', respectively, and are selectively energized by flow measurement
circuit 16 so as to transmit a plurality of sonic beams in the upstream
and downstream directions between transducers 12 and 14. One suitable
transducer structure is disclosed in the '674 patent, the disclosure
of which is incorporated herein by reference. The transducer disclosed
therein establishes a shear mode wave form in the wall of a pipe
18 by selecting the angle .alpha. between the face of the transducer
12 and the surface of the outer wall of pipe 18 such that the tangential
component f.sub.t of the incident wave in the pipe wall is equal
to the velocity v.sub.ps of the pipe shear mode vibration characteristic
for a pipe 18 of a given wall thickness and material. While it is
preferable to utilize a transducer structure which will set up a
shear mode vibration in the pipe wall 18 it should be recognized
that the present invention is not limited to such a structure and
other transducers which do not establish such wave forms may also
be utilized without departing from the spirit or scope of the present
invention. When a tangential mode wave is not induced in pipe 18
flow equations other than those set forth below must be used. By
way of example, those equations set forth in the '915 patent may
be utilized. For the purposes of the following description of the
invention, it will be assumed that the transducer structure of the
'674 patent is utilized.
In addition to selectively enabling transducers 12 and 14 flow
measurement circuit 16 measures the difference in travel time of
the upstream and downstream pulses generated by transducers 12
14 respectively. Flow measurement circuit 16 further computes the
flow rate V.sub.f of the liquid in pipe 18 as a function of this
time differential. In making this computation, flow measurement
circuit 16 relies upon the following relationship between the difference
in travel time .DELTA. T of the upstream and downstream sonic beams
and the velocity of flow V.sub.f of liquid in pipe 18:
wherein T.sub.L is the time that the sonic beam spends in the liquid
during its transmission from the transmitting to the receiving transducer;
and K is a constant determined by the pipe parameters and the number
of transmissions N. The proof for this relationship may be found
in the '674 patent.
Solving equation (1) for the flow velocity V.sub.f, we get:
As will be shown below, flow measurement circuit 16 includes apparatus
for measuring both .DELTA.T and T.sub.L and for dividing these values
to arrive at the flow rate V.sub.f.
Referring now to FIG. 3 there is shown a block diagram of a flow
measurement circuit 16 constructed in accordance with the principles
of the present invention. Flow measurement circuit 16 includes a
signal transmission circuit 22 a signal recognition circuit 24
a cycle counter 26 a flow computer 28 a timing and control circuit
30 a four cross logic circuit 32 and a totalizer and display circuit
34. The function of each of these blocks will be described briefly
at this point and will be described in some detail below.
In the following description, a transmission sequence in which
N sonic pulses are transmitted from upstream transducer 12 to downstream
transducer 14 and then N sonic pulses are transmitted from downstream
transducer 14 to upstream transducer 12 will be referred to as a
"transmission cycle". It should be recognized, however,
that this order may be reversed without departing from the spirit
and scope of the present invention. In either case, one "transmission
cycle" comprises N upstream and N downstream transmissions.
Any desired number N can be utilized.
During each transmission cycle, transducers 12 and 14 each receive
N transmission pulses from signal transmission circuit 22 at a frequency
determined by timing and control circuit 30. Each time a transducer
12 14 receives a transmission signal from circuit 22 it generates
a sonic beam which is transmitted through the liquid in the pipe
18 and is received by the remaining transducer. Receipt of this
signal V.sub.SIG is detected by signal recognition circuit 24 which
identifies receipt of the transmitted sonic beam by detecting a
particular one of the zero cross-over points of the received signal
V.sub.SIG (see FIG. 1A). The particular zero cross-over point which
is detected during any given transmission cycle is determined by
four cross logic circuit 32 which controls the operation of signal
recognition circuit 24 in such a manner that a different zero cross-over
point is utilized for each successive transmission cycle. For example,
four cross logic circuit 32 may cause signal recognition circuit
24 to detect zero cross over point A during a first transmission
cycle, zero cross over point B during a second transmission cycle,
zero cross over point C during a third transmission cycle, and zero
cross over point D during a fourth transmission cycle. Thereafter,
this sequence may be repeated. Generally, any desired sequence of
zero cross over points may be utilized as long as the overall effect
of averaging the .DELTA. T readings for each transmission cycle
is to substantially reduce the effect of noise on the ultimate flow
reading of flow detector circuit 10 over a large number of transmission
cycles.
Each time signal recognition circuit 24 identifies the zero cross
over point selected by logic circuit 32 it generates a recognition
signal T.sub.D which is applied to both cycle counter 26 and timing
and control circuit 30. Cycle counter 26 counts the number of recognition
signals T.sub.D generated by signal recognition circuit 24 and generates
output signals NCT and EOC responsive thereto. An output signal
NCT is generated whenever N recognition signals T.sub.D have been
received by cycle counter 26 and divides each transmission cycle
into an upstream and a downstream portion. This signal, in conjunction
with recognition signal T.sub.D, controls the operation of timing
and control circuit 30. During the upstream portion of each cycle,
timing and control circuit 30 causes signal transmission circuit
22 to apply a transmit signal to downstream transmitter 14 a predetermined
time delay after the generation of recognition signal T.sub.D. During
the downstream portion of each cycle, timing and control circuit
30 causes signal transmission circuit 22 to apply a transmit signal
to upstream transducer 12 a predetermined delay after generation
of the recognition signal T.sub.D. In both cases, each sonic pulse
generated by transducer 12 14 is generated responsive to recognition
of the appropriate zero crossing (as determined by four cross logic
32) by signal recognition circuit 24.
The end of cycle signal EOC generated by cycle counter 26 serves
two functions; it causes the output of four cross logic circuit
32 to change at the end of each successive transmission cycle (thereby
changing the zero cross-over point detected by signal recognition
circuit 24), and it causes flow computer 28 to compute a new flow
rate V.sub.f at the end of each transmission cycle.
Flow computer 28 includes circuitry for generating an output voltage
V.sub.f proportional to the rate of flow of liquid through pipe
18 in accordance with equation (2), supra. A new signal V.sub.f
is generated at the end of each transmission cycle and is applied
to totalizer and display circuit 34. Totalizer and display circuit
34 memorizes each successively generated flow signal V.sub.f and
averages these readings over a large number of transmission cycles.
Totalizer and display circuit 34 converts the average signal into
appropriate flow or mass units and displays the same in an appropriate
digital or analog display.
Having briefly reviewed the operation of flow measurement circuit
16 the structure and operation of each of the individual elements
thereof will now be reviewed in greater detail.
Signal Transmission Circuit
A block diagram of signal transmission circuit 22 is illustrated
in FIG. 4. As shown therein, transmission circuit 22 includes a
pair of multiplexers 36 38 and a pulse pattern generator 40. Pulse
pattern generator 40 receives enabling pulses T.sub.DX from timing
and control circuit 30 responsive to generation of a recognition
signal T.sub.D by signal recognition circuit 24 (compare FIGS. 6A
and 6M). Signal T.sub.DX has a pulse width equal to the desired
time delay between the generation of the recognition signal T.sub.D
and the generation of the next transmission pulse T.sub.X. As best
shown in FIG. 6B, transmission pulse T.sub.X is preferably a plurality
of transmission pulses, the first two of which are in phase with
the sine wave generated by transmitting transducer 12 or 14 and
the latter of which is out of phase with this signal. While three
transmission pulses T.sub.X are preferably generated responsive
to each enabling pulse T.sub.D, these three pulses will be referred
to herein as a single transmission pulse T.sub.X.
The transmission pulse T.sub.X generated by pulse pattern generator
40 is applied to the transmitting transducer 12 or 14 via multiplexer
36 or 38 respectively. If flow measurement circuit 16 is in the
upstream portion of a given transmission cycle, transmission signal
T.sub.X is applied to downstream transducer 14 via multiplexer 38
and transducer 14 acts as the transmitting transducer. In this case,
transducer 12 operates as the receiving transducer and the sonic
signal V.sub.SIG received thereby is applied to the output 42 of
signal transmission circuit 22 via multiplexer 36. When flow measurement
circuit 16 is in the downstream portion of a given transmission
cycle, the transmission pulses T.sub.X are applied to upstream transducer
12 via multiplexer 36 causing transducer 12 to operate as the transmitting
transducer. In this case, downstream transducer 14 operates as the
receiving transducer and the sonic signal V.sub.SIG received thereby
is applied to output 42 of signal transmission circuit 22 via multiplexer
38. Control over multiplexers 36 and 38 is determined by the upstream/downstream
voltage signal V.sub.U /V.sub.D generated by timing and control
circuit 30. By way of example, when this voltage signal is a binary
"1" (representing an upstream portion of a transmission
cycle), multiplexer 38 applies the transmission signal T.sub.X to
transducer 14 and multiplexer 36 applies the sonic signal V.sub.SIG
received by transducer 12 to the output of signal transmission circuit
22. Conversely, when the upstream/downstream signal V.sub.U /V.sub.D
is a binary "0", multiplexer 36 applies the transmit signal
T.sub.X to upstream transducer 12 and applies the sonic signal received
by transducer 14 to the output of signal transmission circuit 22.
Pulse pattern generator 40 also receives an inhibit signal INHIB
which is generated by timing and control circuit 30 (FIG. 5). The
purpose of this signal is to disable (that is to prevent its turning
on) pulse pattern generator 40 during the intervals between transmission
cycles during which flow computer 28 computes the flow rate of the
liquid within the pipe 18 and during the transition time between
upstream and downstream pulsing. The specific function of inhibit
signal INHIB will be discussed below with reference to FIG. 10.
Signal Recognition Circuit
A block diagram of signal recognition circuit 24 is shown in FIG.
7. The operation of signal recognition circuit 24 will be described
with reference to wave forms of FIG. 6.
The sonic signal V.sub.SIG generated by signal transmission circuit
22 is applied to an RF amplifier 44 which normalizes the peak magnitude
of the received signal to a predetermined reference value R.sub.REF
in cooperation with the automatic gain control (AGC) circuit 46.
The resultant wave form KV.sub.SIG is illustrated in FIG. 6F. While
any suitable automatic gain control circuit may be utilized, one
such circuit is disclosed in copending application Ser. No. 6183
filed Jan. 24 1979 in the name of Joseph Baumoel.
The amplified sonic signal KV.sub.SIG is applied to the zero crossing
comparator 48 and to the T.sub.B generator 50. Zero crossing comparator
48 generates a positive logic output ZCO and a negative logic output
ZCO. The wave forms of these signals are illustrated in FIGS. 6J
and 6K, respectively. As shown therein, the outputs of comparator
48 toggle between a binary "1" and a binary "0"
responsive to each zero crossing of the normalized wave form KV.sub.SIG.
The two outputs of comparator 48 are applied to gates 52 54 which
also receive a ZCO ENBL and ZCO ENBL signals from four cross logic
circuit 32 (see FIG. 8). As will be described below, four cross
logic circuit 32 only generates one of these enabling signals during
each transmission cycle. As such, either output ZCO or ZCO is applied
to T.sub.D generator 56 during any transmission cycle. In the diagram
illustrated in FIG. 6 it is assumed that four cross logic circuit
32 is presently generating the ZCO ENBL signal and that the ZCO
output of comparator 48 is applied to T.sub.D generator 56. The
significance of this fact will become apparent below.
As mentioned above, the normalized sonic signal KV.sub.SIG generated
by RF amplifier 44 is also applied to T.sub.B generator 50. T.sub.B
generator 50 compares the normalized signal KV.sub.SIG with reference
voltage V.sub.REF and generates a T.sub.B pulse (see FIG. 6G) whenever
the magnitude of KV.sub.SIG rises above V.sub.REF. T.sub.B signal
generator cooperates with memory 58 to determine the average transmission's
time T.sub.BA for each sonic signal V.sub.SIG transmitted from the
transmitting to the receiving transducer as measured between the
transmission instant defined by the trailing edge of the T.sub.DX
signal and the receipt instant defined by the signal T.sub.B. Since
the exact instant at which the signal T.sub.B is generated will
vary as a function of the magnitude of the normalized signal KV.sub.SIG
(which, in turn, is controlled by the gain of AGC circuit 46), this
time is not very precise. Accordingly, the signal T.sub.B is utilized
to establish a time zone during which the zero crossing selected
by logic circuit 32 should appear. The actual recognition instant
will be determined by the appropriate zero crossover point as described
below.
The gain determined by automatic gain control circuit 46 will not
always be sufficient to normalize the sonic signal to the value
V.sub.REF due to, for example an instantaneous partial blocking
of the sonic beam by air or solids contained in the liquid. As such,
the T.sub.B signal will not be generated responsive to some of the
transmitted sonic signals. For this reason, it is necessary to memorize
the average position of the T.sub.B signal with respect to the transmission
signal T.sub.X and to utilize this memorized signal as the basis
for determining the location of the sonic signal V.sub.SIG and therefore
the appropriate zero crossing. To this end, memory 58 memorizes
the average transmission time of the last several sonic transmissions
as determined by the time delay between the transmission instant
identified by the trailing edge of the signal T.sub.DX and the reception
instant identified by the T.sub.B signal. The T.sub.BA signal generated
by memory 58 is illustrated in FIG. 6L. As shown therein, the T.sub.BA
signal has a leading edge corresponding to the transmission instant
T.sub.X and a trailing edge corresponding to the average signal
reception instant as determined by T.sub.B. This trailing edge defines
the time zone around which zero crossing selected by logic circuit
32 should appear. The signal T.sub.BA includes first and second
portions T.sub.BA1 and T.sub.BA2. The second portion, T.sub.BA2
is of constant duration and is equal in time to one period of the
sonic signal V.sub.SIG. The first portion T.sub.BA1 is adjusted
in length to insure that the entire interval T.sub.BA follows the
actual transmission time as indicated by the signal T.sub.B. The
signal T.sub.BA1 is applied to delay logic block 59 which generates
the pules T.sub.BE and T.sub.BL responsive thereto. As shown in
FIGS. 6H and 6I, signals T.sub.BE and T.sub.BL are equal in length
to a single period of the sonic signal V.sub.SIG and straddle either
side of the end of the signal T.sub.BA.
Delay logic block 59 may be formed utilizing two cascaded one shots,
the first of which receives the signal T.sub.BA1 and generates the
signal T.sub.BE, the second of which receives the signal T.sub.BE
and generates the signal T.sub.BL. The signals T.sub.BE and T.sub.BL
are applied to gates 60 62 respectively, and represent the early
and late zero crossover points with reference to the T.sub.B instant.
Gates 60 62 also receive enabling signals T.sub.BE ENBL and T.sub.BL
ENBL, respectively, from four cross logic circuit 32. Four cross
logic circuit 32 generates only one of these enable signals during
any given transmission cycle in order that only one of the signals
T.sub.BE or T.sub.BL are applied to T.sub.D generator 56 during
any given transmission cycle. In the example illustrated in FIG.
6 it is assumed that the signal T.sub.BE ENBL is generated by four
cross logic circuit 32 and therefore that signal T.sub.BE is applied
to T.sub.D generator 56. In this condition, T.sub.D generator 56
receives the ZCO signal from gate 54 and the T.sub.BE signal from
gate 60. T.sub.D generator 56 generates a recognition pulse T.sub.D
(see FIG. 6M) whenever a binary "1" is applied to its
input 64 and the trailing edge of a positive going pulse is applied
to its input 66. Accordingly, the recognition pulse T.sub.D is generated
at the zero crossover point A shown in FIG. 6F. During the entire
transmission period during which four cross logic circuit 32 generates
the output signals ZCO ENBL and T.sub.BE ENBL, T.sub.D generator
56 receives the signals ZCO and T.sub.BE and generates the recognition
signal T.sub.D when the first zero crossover point A of the sonic
signal V.sub.SIG is detected. During the next succeeding transmission
period, four cross logic circuit 32 will change the enabling inputs
into signal recognition circuit 24 causing T.sub.D generator 56
to generate the recognition signal T.sub.D responsive to a different
zero crossing (e.g., zero crossing B) of the transmitted sonic signal
V.sub.SIG. This phenomena will be discussed in greater detail below.
The signal T.sub.BA ' is generated by T.sub.BA ' generator 70 and
represents location of the first zero crossing point after the normalized
sonic signal KV.sub.SIG increases in magnitude above the value V.sub.REF.
The signal T.sub.BA ' is applied to flow computer 28 and is utilized
to compute the time T.sub.L that the sonic beam spends in the liquid
during a single transmission from the transmitting to the receiving
transducer. The manner in which this computation is made is described
hereinafter, and is described in previously referred-to U.S. Pat.
No. 3869915.
Cycle Counter
Referring to FIG. 9 the recognition signals T.sub.D generated
by signal recognition circuit 24 are applied to cycle counter 26
which includes a programmable N counter 72 and a divide-by-two counter
74. The programmable N counter 72 counts the number of recognition
signals T.sub.D applied to its count input and generates an output
signal NCT whenever it has counted a pre-programmed number N of
the recognition signals.
The output signal NCT generated by N counter 72 is applied to its
reset input in order to reset the N counter 72 to zero at the end
of each upstream or downstream transmission period. Alternatively,
the count in N counter 72 may be reset to the predetermined value
N and the count in counter 72 may be counted down to zero responsive
to the recognition signals T.sub.D in order to generate the output
signal NCT. The output signal NCT is applied to the divide-by-two
counter 74 which generates an end-of-cycle signal EOC responsive
to every other NCT signal. This signal indicates the end of each
transmission cycle.
Timing and Control Circuit
A block diagram of timing and control circuit 30 is illustrated
in FIG. 5. As shown therein, timing and control circuit 30 includes
a transmission cycle control section 76 and a transmission signal
control section 78. Transmission cycle control section 76 receives
the output pulses NCT generated by cycle counter 26 and generates
the output signals which define the upstream and downstream portions
of each of the transmission cycles. Particularly, transmission cycle
control section 76 generates the upstream/downstream control voltage
V.sub.U /V.sub.D, the count-gate signal CNTGT and the inhibit signal
INHIB (see FIG. 10).
Each NCT signal generated by cycle counter 26 is applied to upstream/downstream
flip-flop 80 whose output toggles between a binary "1"
and a binary "0" responsive to the negative edge of each
NCT signal. The upstream/downstream signal V.sub.U /V.sub.D is applied
to multiplexers 36 38 of signal transmission circuit 22 in order
to insure that the transmission pulses are applied to the appropriate
transducer 12 14. The upstream/downstream signal V.sub.U /V.sub.D
is also applied to the up/down input of a CNTGT counter 82 (FIG.
12) located in flow computer 28 (FIG. 3). As will be described in
further detail below, counter 82 determines the time differential
between the upstream and downstream transmission times. Each NCT
signal is also applied to count-gate generator 84 (FIG. 5) which
generates a count-gate signal CNTGT (see FIG. 10C). As shown therein,
the CNTGT signal is preferably the inverse of the NCT signal.
Count gate signal CNTGT is applied to inhibit generator 86 which
generates an inhibit signal INHIB (see FIG. 10D) which is applied
to pulse pattern generator 40 (FIG. 4) to disable pulse pattern
generator 40 at the end of each upstream and downstream portion
of each transmission cycle.
The transmission control section 78 of timing and control circuit
30 controls the transmission instant of each successive sonic signal
V.sub.SIG as a function of the T.sub.D signal generated by signal
recognition circuit 24.
As shown in FIG. 5 transmission signal control section 78 of timing
and control circuit 30 includes a retransmit delay circuit 88 an
auto start circuit 90 and a P.sub.INHIB one shot 92. Retransmit
delay circuit 88 receives the T.sub.D pulse generated by signal
recognition circuit 24 and generates an enabling pulse T.sub.DX
a predetermined time period after receipt of the T.sub.D signal.
The purpose of auto start circuit 90 is to insure that a sonic pulse
is transmitted should no pulse be recognized for some given length
of time.
Enabling signal T.sub.DX is applied to both pulse pattern generator
40 (as described above), and P.sub.INHIB one shot 92. P.sub.INHIB
one shot 92 generates the P.sub.INHIB signal responsive thereto
and applies this signal to AGC circuit 46 and T.sub.B generator
50.
Four Cross Logic Circuit 32
One possible four cross logic circuit 32 is illustrated in FIG.
8. As shown therein, four cross logic circuit 32 includes a ZCO
enable flip-flop 94 and a T.sub.BE -T.sub.BL enable flip-flop 96.
ZCO enable flip-flop 94 receives the end of cycle signal EOC and
toggles its Q and Q outputs responsive to a leading edge of each
end-of-cycle signal EOC. The form of the ZCO ENBL and ZCO ENBL outputs
is illustrated in FIGS. 11D and 11E, respectively. As shown therein,
only one of the two outputs will be at a binary "1" level
during any given transmission period. The Q output of flip-flop
94 is applied to the input of flip-flop 96 whose Q and Q outputs
toggle responsive thereto. The resulting T.sub.BE ENBL and T.sub.BL
ENBL outputs are illustrated in FIGS. 11F and 11G, respectively.
Again, only one of these outputs will be at a binary "1"
level during any given transmission period. Using the particular
sequence of enabling signals illustrated in FIGS. 11D-11G, four
cross logic circuit 32 causes signal recognition circuit 24 to recognize
the zero crossing points A, D, C, and B in successive transmission
cycles. The sequence repeats every fourth transmission cycle.
While four cross logic circuit 32 has been shown utilizing a pair
of flip-flops, it should be recognized that any logic circuit 32
may be utilized as long as it generates output signals which have
the effect of causing signal recognition circuit 24 to recognize
a different zero crossover point during each successive transmission
cycle. The particular order in which the zero crossover points are
recognized may be selected as desired as long as the overall effect
of utilizing the different zero crossover points is to average out
the effect of noise on the received signal.
In the above-described embodiment of the present invention, transmission
time measurements are made with reference to four cross-over points
(i.e., A, B, C, D). Additionally, each cross-over point is utilized
with the same frequency (i.e., one in four). While such a system
will favorably compensate for the effects of noise on the flow meter,
improved results may be obtained by utilizing a greater number of
crossover points (e.g., A. B, C, D and E) and by utilizing those
cross-over points which are least affected by noise at a greater
frequency than the remaining crossover points. Referring to FIG.
6F, it can be seen that cross-over points B, C and D are associated
with the peak magnitude cycle of the received signal V.sub.SIG.
Since this cycle has the greatest magnitude (i.e., V.sub.SIG .apprxeq.V.sub.REF),
it is least affected by pipe noise and the phase shift of the zero
cross-over points B, C and D is less than that of the remaining
cross-over points (e.g., A, E). By utilizing cross-over points B,
C and D at a greater frequency (e.g., twice as often) than the remaining
cross-over points, it is possible to improve the measurement accuracy
of the flow meter.
In accordance with one possible modification of the circuit described
above, T.sub.B generator 50 may be modified to generate a negative
peak signal T.sub.BN corresponding to the negative peak of the received
signal V.sub.SIG in addition to the positive peak signal T.sub.B.
Referring to FIGS. 6F and 6G, the illustrated T.sub.B signal would
be designated the positive peak signal T.sub.BP and a negative peak
signal T.sub.BN would be generated in the area of the crest of the
wave form between points C and D (specifically at the point the
received wave form falls below some negative reference voltage.
Memory 58 would also be modified to memorize the time of occurrence
of T.sub.BN as well as the time of occurrence of T.sub.BP. During
the appropriate transmission cycles, the memorized signal T.sub.BA1
associated with the memorized negative peak signal T.sub.BN would
be applied to delay logic block 59 so as to generate early and late
peak signals T.sub.BEN and T.sub.BLN, respectively. These latter
signals would be identical to the signals T.sub.BE and T.sub.BL,
respectively, illustrated in FIGS. 6H and 6I, but would straddle
either side of the negative peak signal T.sub.BN. As such, these
signals would be shifted over to the negative peak of the received
wave form lying between points C and D. Finally, four cross logic
circuit 32 would be modified to generate a positive peak control
signal POSPK during four successive transmission cycles followed
by a negative peak control signal POSPK during the next four transmission
cycles. The signal would be applied to T.sub.B generator 50 and
to memory 58 and would inform the T.sub.B generator 50 whether to
generate the positive or negative peak signal T.sub.BP or T.sub.BN
during any given transmission cycle. This signal would also inform
the memory 58 whether to utilize the memorized T.sub.BA signal associated
with the positive peak or the negative peak.
Flow Computer
The preferred structure of flow computer 28 is illustrated in FIG.
12. As shown therein, flow computer 28 includes a .DELTA.T computer
98 a T.sub.L computer 102 a 1/T.sub.L generator 104 and a V.sub.f
generator 106. The primary component of .DELTA.T computer 98 is
.DELTA.CNTGT counter 82. .DELTA.CNTGT counter 82 determines the
time differential between the upstream and downstream transmission
times of the sonic signals transmitted during a single transmission
cycle. To this end, the count in counter 82 is increased by a high
speed clock 106 during the upstream transmission portion of each
transmission cycle thereafter counted down by the high speed clock
106 during the downstream portion of each transmission cycle. The
residue count in counter 82 at the end of the complete transmission
cycle is representative of the time differential between the upstream
and downstream transmission times and is utilized to generate an
output pulse whose duration is proportional to that time differential.
High speed clock 106 generates a high frequency pulse train and
applies this pulse train to both multiplexer 108 and divide by P
counter 110. Whenever the end-of-count signal EOC is a binary "0"
multiplexer 108 applies the output of high speed clock 106 to the
count input of counter 82. Whenever the end-of-count signal EOC
is a binary "1" multiplexer 108 applies the output of
counter 110 to the count input of counter 82.
Counter 82 counts up, at a rate determined by clock 106 during
the upstream portion of each transmission cycle and counts down
at this rate during the downstream portion thereof. The exact counting
intervals are defined by the two count gate signals CNTGT applied
to the enable input of counter 82. Since the length of the upstream
CNTGT signal will be greater than the length of the downstream CNTGT
signal (due to the rate of flow of the liquid in pipe 16), the end
of the downstream transmission portion of each transmission cycle
is representative of the difference in the upstream and downstream
transmission times. This residue count is converted into a pulse
.DELTA.T whose width is proportional to the difference in transmission
times by counting counter 82 down to zero at a frequency determined
by the output of divide by P counter 110. Particularly, the end
of cycle signal EOC is applied both to multiplexer 108 and to the
enable input of counter 82 causing the multiplexer 108 to apply
the output of counter 110 to the count input of counter 82 and causing
counter 82 to count down at the rate determined by the output of
counter 110 to zero.
The end of cycle signal EOC is also applied to the set input of
flip-flop 112. The positive edge of the EOC signal causes the Q
output of flip-flop 112 to toggle to a binary "1" level.
Once the count in counter 82 has counted down from its residue level
to zero, it generates a short pulse at its output 114 which is
applied to the reset input of flip-flop 112 and causes the Q output
of flip-flop 112 to toggle to a binary "0" level. As a
result, flip-flop 112 generates an output pulse whose width is proportional
to the difference in the upstream and downstream transmission times
.DELTA.T.
As shown in equation (2), supra, the flow rate V.sub.f is computed
by dividing the difference in upstream and downstream transmission
times .DELTA.T by the time T.sub.L which each sonic beam transmitted
between transducers 12 14 spends in the liquid in pipe 18. In order
to perform this computation, flow computer 28 includes a T.sub.L
generator 102. As set forth in the '915 patent, the time T.sub.L
which the sonic signal spends in the liquid varies with varying
liquid temperatures and densities. Accordingly, some means must
be provided for continually updating the value of the time T.sub.L.
When traveling between transducers 12 and 14 the sonic beam pulse
travels through the transducer housings 12', 14', through the wall
of pipe 18 and through the liquid in the pipe. The total time T.sub.T
is the time between the instant the transmission pulse T.sub.X is
applied to the transmitting transducer and the instant (as determined
by the signal T.sub.BA ') at which the peak of the transmitted pulse
is recognized by the flow measuring circuit. Accordingly, the time
T.sub.T is equal to the time the sonic pulse spends in the liquid
(T.sub.L) plus a fixed time T.sub.F which includes the time the
sonic beam spends in the walls of pipe 18 the time the sonic beam
spends in the transducer housings 12', 14' and certain electronic
delay times between the instant at which the transmit signal T.sub.X
is applied to the transducer and the instant the sonic beam is actually
recognized by the measuring circuit. The foregoing relationship
may be expressed mathematically as follows:
Solving the foregoing equation for the time in the liquid T.sub.L
we get:
T.sub.L computer 102 relies upon the foregoing relationship to
compute the time in the liquid T.sub.L. Particularly, T.sub.L computer
102 measures the total time difference between the generation of
the transmit pulse T.sub.X (as defined by the trailing edge of the
enabling pulse T.sub.DX) and the instant at which the sonic signal
is recognized by the measuring circuit (as defined by the T.sub.BA
' signal) and subtracts the fixed time delay T.sub.F therefrom.
Referring again to FIG. 12 T.sub.L computer 102 includes a fixed
time delay one shot 116 which receives the enabling signal T.sub.DX
and generates a pulse of a fixed duration (equal in length to the
fixed time delay T.sub.F) responsive to the trailing edge thereof.
The T.sub.F pulse generated by one shot 116 is applied to the set
input of T.sub.L generator flip-flop 120 causing the Q output thereof
to toggle to a binary "1" level. The output remains at
this level until the T.sub.BA ' signal is applied to the reset input
of flip-flop 120 at which point the output of flip-flop 120 toggles
to a binary "0" level. As a result, flip-flop 120 generates
an output pulse whose width is proportional to the time in the liquid
T.sub.L. T.sub.L memory 122 may be a capacitor which generates an
output voltage V.sub.T.sbsb.L which is representative of the time
in the liquid T.sub.L.
As noted above, the difference in transmission time .DELTA.T must
be divided by the time in the liquid T.sub.L to arrive at the flow
rate V.sub.f. In order to obtain this relationship, the signal generated
by T.sub.L computer 102 is applied to a 1/T.sub.L generator 104
which generates an output signal whose magnitude is proportional
to 1/T.sub.L. The outputs of .DELTA.T computer 98 and 1/T.sub.L
generator 104 are applied to V.sub.f generator 100 which effectively
multiplies these two signals to arrive at the flow signal V.sub.f.
By way of example, V.sub.f generator 106 may include a capacitor
which is charged by a current generated by 1/T.sub.L generator 104
for a time period determined by the pulse generated by .DELTA.T
computer 98.
Totalizer and Display Circuit
Totalizer and display circuit 34 memorizes each successive generated
flow signal V.sub.f and averages these readings over a large number
of transmission cycles. By way of example, the flow signals V.sub.f
generated by flow computer 28 may be averaged for a total period
of four seconds at the end of which an average flow signal output
is generated. This signal may be converted into any appropriate
flow or mass units and displayed in an appropriate digital or analog
display.
It should be noted that while the process and apparatus of the
invention have been described with reference to sonic measurement
equipment and sonic pulses that the invention has broader application
and could be used with electrical signals and circuits.
The present invention may be embodied in other specific forms without
departing from the spirit or essential attributes thereof and, accordingly,
reference should be made to the appended claims, rather than to
the foregoing specification as indicating the scope of the invention. |