Abstrict An acoustic fluid flow meter has a timer for determining downstream
and upstream transit times values of acoustic pulses transmitted
between acoustic signal transceivers, operable alternately as transmitters
and receivers, respectively, with and against the direction of fluid
flow. Additionally an error signal indicator is supplied with the
determined transit time values and emits an error signal indicative
of a false flow reading if their sum and/or difference, particularly
the sum of derived values, differs from a corresponding control
value by a predetermined amount.
Claims I claim as my invention:
1. An acoustic fluid flow meter comprising;
an acoustic transmitter and receiver arrangement disposed relative
to a flowing medium, said acoustic transmitter and receiver arrangement
including a first element acting as a first transmitter and a first
receiver operable alternately, said first transmitter emitting downstream
acoustic pulses into said flowing medium, propagating downstream
relative to said flowing medium, and said first receiver receiving
said downstream acoustic pulses after propagation through said flowing
medium, and second element acting as a second transmitter and a
second receiver operable alternately, said second transmitter emitting
upstream acoustic pulses into said flowing medium, propagating upstream
relative to said flowing medium, and said second receiver receiving
said upstream acoustic pulses after propagation through said flowing
medium, said acoustic transmitter and receiver arrangement emitting
respective signals into said flowing medium, the respective signals
identifying when said downstream acoustic pulses are emitted, when
said downstream acoustic pulses are received, when said upstream
acoustic pulses are emitted and when said upstream acoustic pulses
are received;
a timer connected to said acoustic transmitter and receiver arrangement,
said timer measuring a downstream transit time of said downstream
acoustic pulses and an upstream transit time of said upstream acoustic
pulses from the respective signals; and
an error signal indicator in said timer, supplied with said downstream
transit time and said upstream transit time, said error signal indicator
deriving a derived value from said downstream transit time and said
upstream transit time and emitting an error signal indicating
a false flow reading when said derived value differs from a control
value by a predetermined amount.
2. An acoustic fluid flow meter as claimed in claim 1 wherein said
first element is a first acoustic transceiver alternatingly operable
as said first transmitter of said downstream acoustic pulses and
as said first receiver of said upstream acoustic pulses, and said
second element is a second acoustic transceiver, alternatingly operable
as the second receiver of said downstream acoustic pulses, when
said first transceiver is operated as said first transmitter, and
as the second transmitter of said upstream acoustic pulses, when
said first transceiver is operated as the first receiver.
3. A fluid flow meter as claimed in claim 2 wherein said timer
includes means for processing said control value from a downstream
transit time and an upstream transit time obtained during a laminar
flow of said flowing medium.
4. A fluid flow meter as claimed in claim 2 wherein said timer
includes means for processing said control value from a downstream
transit time and an upstream transit time obtained when said flowing
medium has a substantially zero flow rate.
5. A fluid flow meter as claimed in claim 1 wherein said error
signal indicator forms a sum of said upstream transit time and said
downstream transit time as said derived value.
6. A fluid flow meter as claimed in claim 1 wherein said error
signal indicator forms a difference between said downstream transit
time and said upstream transit time as said derived value.
7. A fluid flow meter as claimed in claim 6 wherein said error
signal indicator emits said error signal when said difference exceeds
the control value.
8. A fluid flow meter as claimed in claim 1 further comprising
a valve disposed relative to said acoustic transmitter and receiver
arrangement for controlling a flow of said flowing medium past said
acoustic transmitter and receiver arrangement.
9. A fluid flow meter as claimed in claim 1 wherein said upstream
transit time and said downstream transit time are currently-obtained
values, and wherein said timer includes a processor, said processor
including means for determining previous upstream and downstream
transit times determined by the timer, preceding said current values,
means for storing said previous upstream and downstream transit
times, and means for periodically updating said control value dependent
on said previous upstream and downstream transit times.
10. A fluid flow system comprising:
a conduit through which a fluid flows;
an acoustic transmitter and receiver arrangement connected to said
conduit and acoustically coupled to said fluid in said conduit,
said acoustic transmitter and receiver arrangement including a first
element acting as a first transmitter and a first receiver operable
alternately, said first transmitter emitting downstream acoustic
pulses into said fluid, propagating downstream relative to said
fluid, and said first receiver receiving said downstream acoustic
pulses after propagation through said fluid, and a second element
acting as a second transmitter and a second receiver operable alternately,
said second transmitter emitting upstream acoustic pulses into said
fluid, propagating upstream relative to said fluid, and said second
receiver receiving said upstream acoustic pulses after propagation
through said fluid, said acoustic transmitter and receiver arrangement
emitting respective signals into said fluid, said respective signals
identifying when said downstream acoustic pulses are emitted, when
said downstream acoustic pulses are received, when said upstream
acoustic pulses are emitted and when said upstream acoustic pulses
are received;
a timer connected to said acoustic transmitter and receiver arrangement
said timer measuring a downstream transit time of said downstream
acoustic pulses and an upstream transit time of said upstream acoustic
pulses from said respective signals; and
an error signal indicator in said timer, supplied with said downstream
transit time and said upstream transit time, said error signal indicator
deriving a derived value from said downstream transit time and said
upstream transit time and emitting an error signal indicating a
false flow reading when said derived value differs from a control
value by a predetermined amount.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acoustic flow meter and in
particular to an ultrasound flow meter useable in the monitoring
of gas flow in breathing aid devices such as ventilators or respirators.
2. Description of the Prior Art
Flow meters in which the time of flight of an acoustic (usually
ultrasonic) pulse is used to determine the velocity (and hence the
flow rate) of the fluid through which the pulse was transmitted
are well known in the art. Devices such as those described in PCT
Application WO 94/28790 and U.S. Pat. No. 5247826 improve on
this basic methodology by arranging for the transit times of ultrasonic
pulses to be measured both upstream (T.sub.u) and downstream (T.sub.d)
of the fluid flow. These transit times are then supplied to a microprocessor
which is set to calculate the fluid flow rate using standard algorithms.
A thermometer is also included in both devices to measure the ambient
temperature of the fluid. Since the velocity of sound in a medium
changes with its temperature a more accurate transit time can be
derived with a knowledge of the ambient temperature.
In PCT Application WO 94/28790 a pair of cells, each having a piezoelectric
transmitter and receiver, are placed so that an ultrasonic pulse
can travel between the cells at an angle to the direction of fluid
flow. By having the transmitter in each cell transmit an ultrasonic
pulse for reception by the receiver the other cell, both T.sub.u
and T.sub.d can be measured. The device described in U.S. Pat. No.
5247826 achieves the same result by arranging for a pair of ultrasonic
transceivers, which are spaced apart in an elongate coiled tube
through which gas can flow, to alternately operate as transmitters
and receivers.
A piezoelectric crystal does not emit a single pulse when energized
with a single electrical pulse. Rather the crystal is caused to
oscillate at a characteristic resonant frequency to emit a "packet"
that comprises a number of pulses. The envelope of the transmitter
signals decays rapidly with time, usually producing a train of six
or so cycles. Thus small errors in the determination of the flow
rate may result if the determination is made using different pulses
from within the packet.
A problem may therefore arise when conventional devices are used
in situations where it is critical to maintain flow rates within
fine tolerances, for example in medical applications such as monitoring
breathing gas flow rates in ventilators and respirators. In these
applications flow meters must be capable of accurately and reliably
detecting small changes in gas flow rates. Conventional devices,
however, may record small changes which on the face of it look correct
but which do not actually result from flow rate changes but rather
from registering the arrival time of the wrong acoustic pulse from
within a particular packet.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an acoustic
flow meter in which such erroneous flow rates can be automatically
identified.
This object is achieved by a flow meter wherein an error signal
indicator receives the measured transit times T.sub.u and T.sub.d
and compares a derived value (that is, their sum or their difference)
with corresponding control values and, if their difference exceeds
a corresponding predetermined threshold value, emits an error signal.
This error signal may be subsequently used, for example, to vary
operating parameters of the flow meter to avoid such errors, to
instruct circuitry to ignore the reading, or to provide a detectable
warning to an operator so that corrective measures can be manually
implemented.
Especially when the flow meter is to measure continuous flows,
the error signal indicator can include circuitry to form the sum
of the transit times T.sub.u and T.sub.d (which sum, because the
fluid flow rate has equal but opposite effects on the transit times,
should be constant for any flow rate), to compare it to a control
value dependent on the expected sum, which may be a measured value
or a calculated value (calculated for example by using the well
known equation for the speed of sound in an ideal gas provided that
the temperature and the composition of the gas is known), to emit
an error signal should the difference between the formed value and
the control value exceed a predetermined threshold, which may be
0. In this way the flow meter can be continuously monitored for
erroneous signals.
The circuitry may, for example, be configured to measure the difference
between the formed sum and a control value consisting of a previously
formed sum and may additionally be adapted to replace the control
value with the formed sum if the difference does not exceed the
predetermined threshold, to thereby update the control value.
Preferably, the error signal indicator forms the control value
from transit times measured during substantially laminar fluid flow
conditions, for example at zero or low flow rates, thus allowing
the flow meter to be made self-calibrating. Preferably, the self-calibration
is performed periodically throughout the operation of the flow meter
to provide self-compensation for changes in the velocity of the
ultrasound caused by changes in ambient conditions, such as temperature,
or in the condition of the timer. This has the further advantage
that the construction of the flow meter may be simplified since
additional components, such as a thermometer, that are employed
to monitor the ambient conditions need not be included in the flow
meter.
Additionally or alternatively the error signal indicator may include
circuitry to form the difference of the transit times T.sub.u and
T.sub.d, to compare it to a control value dependent on the expected
or a measured difference and to emit an error signal should the
difference between the control value and the formed value exceed
a predetermined threshold.
As will be appreciated by those skilled in the art, formed difference
values, unlike formed sum values, will be dependent on and change
as the flow rate changes. However, such error monitoring may be
used at times when there is a known or zero flow or by arranging
the circuitry to utilize a previously formed difference value as
a control value, and by replacing the control value with new formed
values so that the time between determining the control and formed
values is less than the measurable changes in the flow rate.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a known flow meter.
FIG. 2 illustrates a commonly used mode of operating the known
flow meter of FIG. 1.
FIG. 3 illustrates a flow meter according to the present invention.
FIG. 4 shows a logic flow diagram of an exemplary mode of operating
the flow meter according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
To better understand the present invention a known flow meter will
first be described. Considering FIG. 1 a flow meter 1 has a tube
2 through which a fluid can flow, for example in the direction of
the arrows. Branches 34 are formed with respective fluid tight
ends 56 in the tube 2 and respective piezoelectric transceivers
78 are placed in the branches 34 so that acoustic pulses emitted
from one transceiver 78 can cross the path of the fluid flow at
an angle to be received at the other transceiver 87. An electrical
pulse generator 9 and timer 10 which can be formed by a suitably
programmed microprocessor 11 operating with a known internal clock
frequency, are connected to each other and to each of the piezoelectric
transceivers 78 such that each transceiver 78 can be operated
in turn as a transmitter and as a receiver. The connections between
the pulse generator 9 the timer 10 and each of the transceivers
78 are switchable such that when the pulse generator 9 is switched
to supply electrical pulses to the transceiver 7 the timer 10 is
switched to receive electrical signals only from the transceiver
8 and vice versa.
Further description of the operation of the flow meter 1 will be,
for simplicity, made for a state where the transceiver 7 is set
to act as a transmitter and the transceiver 8 is set to act as a
receiver.
The pulse generator 9 supplies a single electrical pulse 14 shown
generally by the inset of FIG. 2 to the piezoelectric transmitter
7 and provides a signal to the microprocessor 11 of the timer 10
to begin counting pulses from its internal clock. This electrical
pulse 14 causes the transmitter 7 to emit a wave train 15 which
has zero amplitude crossings at P.sub.1-5 as shown in FIG. 2. This
wave train 15 passes through the fluid in tube 2 at an angle to
the direction of fluid flow, and is received at the piezoelectric
receiver 8 where it causes an electrical wave train 15 to be generated
with analogous voltage amplitude variations. This electrical signal
is supplied as an input to the timer 10.
The timer 10 also includes a zero crossing detector 12 well known
in the art, to detect voltage zero crossings P.sub.1-5 and is configured
to always determine the same voltage zero crossing (for example
P.sub.3) associated with the electrical pulse 14. On detecting this
single zero crossing, a signal is transmitted to the processor 11
which interrupts the clock pulse count. From a knowledge of the
internal clock frequency of the processor 11 and the number of counted
clock pulses between the generation and receipt of the ultrasonic
signal, the processor 11 can be programmed to calculate the transit
time of the ultrasonic pulses. When fluid flows through the tube
2 in the direction shown in FIG. 1 then with the transceiver 7
acting as a transmitter and the transceiver 8 acting as a receiver,
the downstream transit time T.sub.d will be determined by the timer
10.
When the roles of the transceivers 78 are reversed then the slower,
upstream transit time T.sub.u can be similarly determined. The processor
11 of the timer 10 is further programmed to determine the fluid
flow rate from T.sub.u and T.sub.d and by using an equation well
known to those knowledgeable in the physics of sound propagation
as exemplified in U.S. Pat. No. 5247826 and which may be expressed
as:
wherein V is the bulk flow rate and K is a constant dependent on,
inter alia, physical dimensions of the flow meter 1 and which may
be calculated or determined experimentally without undue effort.
Clearly, each electrical pulse 14 provides a received signal 15
having a number of voltage zero crossing points P.sub.1-5. While
it is intended to use only a single crossing point, for example
P.sub.3 for each electrical pulse any of the zero crossing points
P.sub.1-5 could be registered, which would lead to errors in the
determination of the transit time values T.sub.u -T.sub.d and hence
errors in the calculated flow rate can result.
In an attempt to remove this problem it is known to include in
the flow meter 1 a discriminator 13 for example at the input stage
of the timer 10 to prevent registering any crossing point but,
for example, P.sub.3. Referring to FIG. 2 the zero crossing detector
12 and the discriminator 13 co-operate generally by looking for
a zero crossing in which the signal 15 goes from negative to positive
(or positive to negative) but only after the signal 15 has fallen
below (above) a preset threshold voltage V. In this way all but
the crossing P.sub.3 can be rejected. Measurement errors, however,
may still occur. For example if the threshold voltage is set too
low (V') it may be possible to register one of several zero crossing
points P.sub.2.sub.3. Even if the voltage is set to a correct level,
such as V" variations 15' in amplitude of the signal 15 for
example caused by noise or time dependent changes in the operational
characteristics of the transceivers 78 could still mean that it
is possible to detect one of several zero crossing points P.sub.2-3.
A flow meter 1 according to the present invention is shown in FIG.
3 and, to the extent it is similar to that of the known flow meter
of FIG. 1 is shown with common components having the same numbering
as FIG. 1. Upstream and downstream transit times, T.sub.u and T.sub.d
respectively are determined as previously described and are entered
into the microprocessor 11 of the timer 10. Differing from the previously
described known flow meter, an error signal indicator 16 is included
within the timer 10 as part of the programmed microprocessor 11
which operates according to the logic flow chart shown in FIG. 4.
In use the error signal indicator 16 makes the determination of
a false transit time reading based on the summation of T.sub.u and
T.sub.d, namely T.sub.sum. The microprocessor 11 holds in memory
a control value, T.sub.c, which is a summed transit time value formed
at zero (or low flow producing laminar flow) flow. This can be obtained
at start up before fluid flows through the meter 1 or may be obtained
when a flow control valve 17 (here shown as part of the meter 1
but which could be in the fluid system outside the flow meter 1)
is closed to prevent fluid flowing in the meter 1 or may be obtained
during periods when it is known that no fluid will be flowing through
the meter 1. This last option may be preferred, for example when
the meter 1 is used in breathing assist systems, such as in the
expiration or inspiration sides of known ventilator systems, where
by the very nature of the breathing process there will be periods
when no fluid flows in one or other of the sides. Additionally,
the control value T.sub.c may be updated from new summations made
at the zero flow condition or from measured values of T.sub.sum
obtained when the error signal indicator 16 indicates that accurate
transit time values were collected.
The flow meter 1 operates in a manner previously described in connection
with FIG. 1 to obtain instantaneous values of T.sub.u and T.sub.d.
The error signal indicator 16 forms a value of T.sub.sum using these
instantaneous values and compares it with the control value T.sub.c
to determine the absolute value of T.sub.c -T.sub.sum. If this absolute
value is greater than a preset threshold value T.sub.t, which may
be set at zero, then an error signal is generated by the error signal
indicator 16 of the processor 11. This signal may then be used in
ways apparent to those skilled in the art, for example to inhibit
a flow rate reading from being made, or for varying operating parameters
of the device to which the flow meter is connected to adjust the
flow rate, or for initiating correction algorithms within the flow
meter to generate an estimated flow value, perhaps based on trends
in the flow rate from previously calculate "good" flow
rate measurements. If no error signal is generated, the processor
11 then calculates the flow rate using equation (1). A detectable
representation of this flow rate can then made, for example as an
output on a computer screen or a dial.
It will be appreciated by those skilled in the art that a flow
meter according to the present invention as described above with
the aid of FIG. 4 could easily be modified to make error determinations
based on the difference T.sub.diff between T.sub.u and T.sub.d.
In this case T.sub.c would be, for example, T.sub.d -T.sub.u and
the error signal generator 16 would operate to determine whether
.vertline.T.sub.c --T.sub.diff .vertline. exceeds a set amount.
Although modifications and changes may be suggested by those skilled
in the art, it is the intention of the inventor to embody within
the patent warranted hereon all changes and modifications as reasonably
and properly come within the scope of his contribution to the art. |