Abstrict An input unit for receiving a signal from an oscillator corresponds
to an ultrasonic wave received by the oscillator has an input impedance
significantly smaller than an impedance of an inter-electrode capacitance
in the oscillator. This arrangement allows the delay of timing of
the received wave to be influenced by a change in the input impedance
of the input unit than by a change in the impedance of the oscillator.
Since the influence of the change in the impedance of the oscillator
against the delay of timing of the received wave is reduced, a flow
meter can measure a propagation duration of the ultrasonic wave
accurately. More specifically, the flow meter can measure the propagation
duration accurately regardless of a change of the inter-electrode
capacitance of the oscillator by temperature, thus measuring a flow
rate of fluid correctly.
Claims What is claimed is:
1. A flow meter comprising: first and second oscillators provided
at a fluid conduit in which fluid flows, for transmitting and receiving
an ultrasonic wave; a switching unit for switching the transmitting
and receiving of the ultrasonic wave between the first and second
oscillators; an input unit having an input impedance smaller than
respective impedances of the first and second oscillators and arranged
for receiving, from the first and second oscillators, a signal corresponding
to the ultrasonic wave received by the first and second oscillator;
a received-wave detector for detecting a reception of the ultrasonic
wave according to a signal from the input unit; a driver for driving
the first and second oscillators to transmit the ultrasonic wave;
a time counter for counting a propagation duration of the ultrasonic
wave between the first and second oscillators; and a flow calculator
for calculating a flow rate of the fluid from the counted propagation
duration.
2. The flow meter according to claim 1 wherein the input impedance
of the input unit is not higher than 1/2 of each of the impedances
of the first and second oscillators.
3. The flow meter according to claim 1 wherein a time allowance
(.DELTA.t) required for counting of the propagation duration, a
change (.DELTA.C1) of an inter-electrode capacitance of one of the
first and second oscillators, and the input impedance (Z) of the
input unit satisfy the relationship:
.DELTA.t>.DELTA.C1.times.Z.
4. The flow meter according to claim 3 wherein the change (.DELTA.C1)
of the inter-electrode capacitance is in a predetermined temperature
range between a minimum temperature and a maximum temperature.
5. The flow meter according to claim 1 wherein a time allowance
(.DELTA.t) required for counting of the propagation duration, a
change (.DELTA.C2) of a difference between respective inter-electrode
capacitances of the first and second oscillators, and the input
impedance (Z) of the input unit satisfy the relationship:
6. The flow meter according to claim 5 wherein the change (.DELTA.C2)
of the inter-electrode capacitance is in a predetermined temperature
range between a minimum temperature and a maximum temperature.
7. The flow meter according to claim 1 wherein the input unit
includes: a resistor connected in parallel with the first and second
oscillators; and an amplifier for amplifying a voltage at both ends
of the resistor.
8. The flow meter according to claim 1 wherein the input unit
includes an impedance converter.
9. The flow meter according to claim 1 wherein the input unit
is connected in parallel with the first and second oscillators.
10. The flow meter according to claim 1 wherein the first and
second oscillators having properties different from each other.
11. The flow meter according to claim 1 wherein the inter-electrode
capacitances of the first and second oscillators varies according
to a temperature, a elapsing time, and a moisture.
Description FIELD OF THE INVENTION
The present invention relates to a flow meter for measuring a flow
rate of fluid.
BACKGROUND OF THE INVENTION
FIG. 6 illustrates a conventional flow meter. At a conduit 1 in
which fluid flows, a first oscillator 2 and a second oscillator
3 which face each other across the fluid for transmitting and receiving
an ultrasonic wave are mounted. A driver 4 drives the first oscillator
2 to produce and transmit an ultrasonic wave. The ultrasonic wave
is then received by the second oscillator 3 and transferred to a
received-wave detector 7 for detecting the timing of receiving the
wave. A timer 8 counts a time from the start of actuation of the
driver 4 for driving the first oscillator 2 to the detection of
the timing by the received-wave detector 7 and determines a propagation
duration T1 of the ultrasonic wave. Then, a switching unit switches
the direction of the transmitting/receiving of the ultrasonic wave,
and the time counter 8 measures a propagation duration T2 of the
ultrasonic wave in a reverse direction. A flow-rate calculator 9
then calculates the flow rate of the fluid from durations T1 and
T2 in both the directions.
The received-wave detector 7 will be explained in more detail referring
to FIG. 7.
The received-wave detector 7 includes a third-peak detector for
detecting a third peak W3 from the rise of the waveform, and a zero-cross
detector for detecting a zero-cross point of the received wave.
Upon receiving the propagated wave, the third-peak detector detects
a third peak W3 (denoted by "A" in FIG. 7) from the rise
of the waveform, and outputs its detection signal for actuating
the zero-cross detector. The zero-cross detector then detects a
zero-cross point (denoted by B in FIG. 7) succeeding the peak, and
determines the timing of receiving the wave. The timing is then
transferred to the time counter 8. The timing of receiving the propagated
wave determined by the above sequence provides the durations T1
and T2 of the ultrasonic wave. Then, the flow-rate calculator 9
calculating the flow rate Q from the duration T1 and T2 as equation
1
where K is a constant determined according to a cross sectional
area of the conduit, a propagating distance of the ultrasonic wave,
the positional relationship between the oscillators and the conduit,
and their units.
Another conventional flow meter is disclosed in Japanese Patent
Laid-open Publication No. 8-70926. In the meter, a circuit connected
to two oscillators is matched in impedance so as to be equal in
impedance in both a transmission mode and a reception mode and then
connected to a transmitter/receiver circuit. The impedance in the
circuit is low and constant. Another conventional flow meter detects
the amplitude of a received wave at some points in time synchronized
with a reference clock signal, and produces data of a phase against
the reference clock signal according to the relationship between
the detected amplitude and the timing of the reference clock signal.
Then, the propagation duration of the wave can be determined from
a combination of rough timings of the reference clock signal and
the data of the phase having high resolution. This measuring method
may however create an error due to a change of the amplitude of
the received wave converted into the data of the timing. The method
requires that the waveform of the ultrasonic wave propagated from
its upstream to downstream and the waveform of a reverse ultrasonic
wave propagated from the downstream to the upstream are shaped identical
to each other. Thus, respective impedances between the transmission
mode and the reception mode are matched.
As shown in equation 1 a relative accuracy of the durations T1
and T2 i.e., a difference (T1-T2) affects the accuracy of the measurement
of the flow rate more than respective absolute value of the durations
T1 and T2. For increasing the relative accuracy of the durations
T1 and T2 delay times for which the ultrasonic wave is received
by the oscillator and detected as a converted electrical signal
by at the received-wave detector 7 have to be identical between
both the directions of the transmission and reception.
As shown in an equivalent circuit of FIG. 2 a receiving-side oscillator
of the conventional flow meter includes a signal source 11 for converting
the ultrasonic wave into an electric signal, an internal impedance
12 (Zo), and an inter-electrode capacitance 13 (C). Upon receiving
the ultrasonic wave, the oscillator produces an electric signal
from the oscillation of the ultrasonic wave with a delay determined
by the internal impedance 12 and the inter-electrode capacitance
13. More particularly, the delay is proportional to (C.times.Zo),
thus increasing as the internal impedance 12 and the inter-electrode
capacitance 13 are increased.
When an ambient temperature varies, a change in the inter-electrode
capacitance 13 increases significantly, thus changing the delay
of the output signal from the oscillator. This makes measurement
of the duartions T1 and T2 inaccurate. The higher an input impedance
of the received-wave detector 7 the greater a voltage of the output
signal increases. Thus, the output signal from the oscillator is
generally received by a high impedance circuit.
The flow meter disclosed in the publication No. 8-70926 is intricate
in circuitry, and requires the wave forms of the ultrasonic wave
in both the directions similar to each other. Thus, an interference
of waves reflected on an inner wall of the conduit has to be considered.
The conventional flow meter may be designed more difficultly under
the consideration of variance at its mass production.
In the case that the two oscillators have identical properties,
whichever of the oscillators is assigned to a receiving oscillator,
the value (C.times.Zo) of each oscillator is unchanged, and the
delay times are identical to each other. This arrangement does not
create an error in the measured propagation durations, which are
essential for calculating the flow rate. However, in case that the
two oscillators do not have identical properties, the first oscillator
2 and the second oscillator 3 have the value (C.times.Zo) different
from each other. Therefore, the delay time of the output signal
from the receiving oscillator of one of the two oscillators is not
equal to that of the receiving oscillators of other of the two oscillators.
This makes the time counter 8 fail to measure the propagation durations
accurately when the two oscillators are switched in the transmitting/receiving,
and makes the flow-rate calculator 19 determine inaccurate flow
rate.
As described above, the conventional flow meters hardly measure
the flow rate accurately unless the two ultrasonic oscillators have
properties identical to each other. Further, since a change in the
inter-electrode capacitance caused by a temperature change is not
uniform between the oscillators, as apparent from FIG. 3 a pair
of oscillators having their properties substantially identical to
each other has to be prepared. This preparation is a troublesome
bearing process in which variations in the internal impedance and
in the inter-electrode capacitance are checked while the temperature
varies.
SUMMARY OF THE INVENTION
The flow meter provides accurate measurement even if employing
the oscillators having their properties different from each other.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a flow meter according to exemplary
embodiment 1 of the present invention;
FIG. 2 illustrates an equivalent circuit of an oscillator in the
flow meter;
FIG. 3 is a temperature characteristic of an inter-electrode capacitance
of oscillators in the flow meter according to the embodiment;
FIGS. 4A and 4B illustrate an input unit of the flow meter;
FIG. 5 is a block diagram of a flow meter according to exemplary
embodiment 2 of the invention;
FIG. 6 is a block diagram of a conventional flow meter; and
FIG. 7 illustrates a timing of detecting a received wave in the
flow meter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(Exemplary Embodiment 1)
FIG. 1 is a block diagram of a flow meter according to exemplary
embodiment 1 of the present invention. FIG. 2 illustrates an equivalent
circuit of an oscillator in the flow meter of embodiment 1. FIG.
3 is a temperature characteristic of inter-electrode capacitances
of first and second oscillators of the flow meter of embodiment
1.
As shown in FIG. 1 fluid to be measured in its flow rate flows
through a fluid conduit 1. A first oscillator 2 and a second oscillator
3 transmits and receives an ultrasonic wave. The oscillators are
driven by a driver 4. An input unit 5 receives signals output from
the oscillators 2 and 3 in a low impedance. A switching unit 6 switches
the connection of the oscillators 2 and 3 between the driver 4 and
the input unit 5. A received-wave detector 7 detects the timing
of receiving the ultrasonic wave according to a signal output from
the input unit 5. A time counter 8 counts a time interval from a
release of a startup signal for transmitting the ultrasonic wave
to a reception of the signal output from the received-wave detector
7. A flow-rate calculator 9 then calculates a flow rate of the fluid
from a signal output from the time counter 8. A controller 10 sets
the switching unit 6 and then sends the startup signal to the driver
4 and the time counter 8.
An operation of the flow meter of embodiment 1 will be described
in more detail. First, the controller 10 directs the switching unit
6 to set up for measuring a propagation duration of an ultrasonic
wave. In order to measure the propagation duration of the ultrasonic
wave from the first oscillator 2 to the second oscillator 3 the
switching unit 6 connects the driver 4 to the first oscillator 2
and connects the input unit 5 to the second oscillator 3. Then,
the controller 10 sends the startup signal to the driver 4 and the
time counter 8. Upon receiving the startup signal, the driver then
outputs a signal for actuating the first oscillator 2 and the time
counter 8 starts counting the time interval. Upon being driven,
the first oscillator 2 transmits the ultrasonic wave. Upon receiving
the ultrasonic wave, the second oscillator 3 outputs a signal corresponding
to the received ultrasonic wave to the input unit 5 and the received-wave
detector 7 then detects the receiving. Then, the counting of the
time counter 8 is canceled. The time interval counted by the time
counter 8 is transferred to the flow-rate calculator 9. Then, the
controller 10 directs the switching unit 6 to switch the connection
for measuring the propagation duration in a reverse direction of
the ultrasonic wave. More particularly, the switching unit 6 connects
the driver 4 to the second oscillator 3 and connects the input unit
5 to the first oscillator 2. Similarly, the time counter 8 measures
the propagation duration in the reverse direction of the ultrasonic
wave, and sends the measured duration to the flow-rate calculator
9. The flow-rate calculator 9 calculates the flow rate of the fluid
through multiplying a difference between respective inverted values
of the two measured durations by a predetermined constant.
FIG. 2 illustrates an equivalent circuit of each of the oscillators.
The oscillator includes a signal source 11 for converting a received
ultrasonic wave into an electric signal, an internal impedance 12
(Zo), and an inter-electrode capacitance 13 (C). The electric signal
is then delayed by a time determined by the impedance 12 and the
inter-electrode capacitance 13. The greater the impedance 12 and
the inter-electrode capacitance 13 the greater the delay is. The
oscillator generally includes a pair of electrodes, a piezoelectric
device provided between the electrodes, and an oscillating strip
fixedly joined to one of the electrodes. Upon receiving a voltage
between the electrodes, the piezoelectric device of the oscillator
starts oscillating to produce an ultrasonic wave. The ultrasonic
wave is then received by the oscillating strip of the other oscillator
and is converted into a voltage by the piezoelectric device.
As apparent from FIG. 3 the inter-electrode capacitance 13 and
its change according to a temperature may be different between the
oscillators. This fact makes the delay of a signal output from the
oscillator not uniform, thus resulting in s measurement error.
The input unit 5 of embodiment 1 receiving the signal output from
the oscillator has an input impedance lower than the impedance 12
at the inter-electrode capacitance 13 of the oscillator. The input
impedance of the input unit 5 is low, thus reducing influence of
a change and a variation of the inter-electrode capacitance 13 to
the delay time determined by the inter-electrode capacitance 13
and the input unit 5. This arrangement improves the accuracy of
the measured propagation durations of the ultrasonic wave. Even
if the inter-electrode capacitance 13 of the oscillator varies according
to a temperature, elapsing time, moisture, or any other factor,
the flow meter of embodiment 1 can accurately measure the propagation
duration, thus calculates the flow rate accurately.
The input impedance of the input unit 5 may preferably be equal
to 1/2 of the impedance of a circuit including the internal impedance
12 and the inter-electrode capacitance 13 connected in parallel
with the impedance 12. This arrangement allows the input unit 5
to affects the delay time of the signal from the oscillator at the
receiving side more than the inter-electrode capacitance 13 hence
reducing the influence of a change of the inter-electrode capacitance
13 to the timing of receiving the ultrasonic wave. Accordingly,
the propagation duration is measured accurately. Even if the inter-electrode
capacitance 13 of the oscillator varies according to a temperature,
the elapsing time, and the moisture, the flow meter can measures
the propagation duration correctly thus calculating the flow rate
accurately.
As shown in FIG. 2 the oscillator at the receiving side is connected
to the input unit 5 having the input impedance Z. For example, if
the input impedance Z is 1/10 of the impedance Zo of the oscillator,
a total impedance Zt of the impedance Z and the input impedance
connected in parallel is determined by: ##EQU1##
Accordingly, the delay time of the oscillator of the flow meter
of embodiment 1 including the input unit 5 having the low input
impedance is 1/11 of that of the conventional flow meter not including
such the input unit. More particularly, if the inter-electrode capacitance
13 is different between the first oscillator 2 and the second oscillator
3 its adverse effect over the accuracy of the measurement of the
propagating duration can successfully be reduced to 1/11.
The input unit 5 may have an input impedance (Zi1) to satisfy the
relationship:
where .DELTA.t is an allowance of the measurement, and .DELTA.C1
is a change of the inter-electrode capacitance 13 of each of the
first oscillator 2 and the second oscillator 3. A change of the
delay time is substantially .DELTA.C1.times.Zi1. Accordingly, even
if the inter-electrode capacitance 13 of the oscillator varies according
to the temperature, the elapsing time, and the moisture, the flow
meter can measure the propagation duration of the ultrasonic wave
within the allowance of the delay time, thus calculating the flow
rate accurately.
The change .DELTA.C1 will be explained in more detail referring
to FIG. 3. The inter-electrode capacitance of the second oscillator
3 has a temperature characteristic, for example, as shown in FIG.
3. The flow meter is usually used within a temperature range from
a minimum temperature .theta.1 of -25.degree. C. to a maximum temperature
.theta.2 of 60.degree. C. The inter-electrode capacitance varies
according to a temperature between the temperatures .theta.1 and
.theta.2 by a difference expressed by a change .DELTA.C1 in FIG.
3. The input impedance Zi1 of the input unit 5 is then determined
so that the relationship between the change .DELTA.C1 and the allowance
.DELTA.t of the delay time is expressed by the above relationship.
In the calculation of the flow rate, a difference between respective
inverse values of measured propagation durations is multiplied by
the constant. Therefore, a change of the difference of the delay
times affects the accuracy of the measurement more than respective
absolute values of the delay times of the signals output from the
oscillators. For compensation, the input unit 5 may preferably have
an input impedance Zi2 satisfying the relationship:
where .DELTA.t is an allowance of the difference, and .DELTA.C2
is a change of the difference between respective inter-electrode
capacitances of the first oscillator 2 and the second oscillator
3. Accordingly, even if the difference of the inter-electrode capacitance
13 between the oscillators varies according to the temperature,
the elapsing time, and the moisture, the difference of the change
of the delay times stays in a range of about (.DELTA.C2.times.Zi2).
Since .DELTA.C2<.DELTA.C1 and .DELTA.t satisfies the above relationship,
the impedances Zi2 and Zi1 satisfy Zi2>Zi1 thus providing the
input unit 5 with the lower input impedance easily. This arrangement
allows the flow meter of embodiment 1 to measure the propagation
duration of the ultrasonic wave within a necessarily-accurate range
of the delay time, and thus to calculate the flow rate accurately.
The change of the difference .DELTA.C2 will be explained in more
detail referring to FIG. 3. At a temperature .theta.x, the difference
between the inter-electrode capacitances of the first oscillator
2 and the second oscillator 3 is .DELTA.Cx. The difference .DELTA.Cx
varies according to the temperature .theta.x. The difference is
.DELTA.C21 at the maximum temperature .theta.2 and is .DELTA.C11
at the minimum temperature .theta.1. The change .DELTA.C2 is expressed
as:
The input impedance (Zi2) of the input unit 5 is determined to
satisfy the above equations with the allowance .DELTA.t and the
change .DELTA.C2 of the difference between respective inter-electrode
capacitances of the first oscillator 2 and the second oscillator
3.
FIGS. 4A and 4B illustrate details of the input unit 5. The input
unit 5 shown in FIG. 4A includes a resistor 14 having a. low resistance
and an amplifier 15. Since the input signal is received by the resistor
14 having the resistance lower than the impedance of the oscillator,
the input impedance Z is determined mainly by the resistance of
the resistor 14. The voltage at both ends of the resistor 14 is
then amplified to a desired level by the amplifier 15. Accordingly,
the input unit 5 can have optimum input/output characteristics while
having a simple arrangement.
Another input unit 5 shown in FIG. 4B includes a transformer 16
which functions as an impedance converter. Its input impedance is
determined by the ratio between an input side winding 17 and an
output side winding 18 and the resistance of a resistor 19 connected
to the output side winding 18. As the result, the input unit 5 can
have a low input impedance while having a simple arrangement.
(Exemplary Embodiment 2)
FIG. 5 illustrates a flow meter according to exemplary embodiment
2 of the present invention. Like components are denoted by like
numerals as those of embodiment 1 shown in FIG. 1 and will be described
in no more detail.
A repeating unit 20 is provided for producing and transmitting
a repeat signal to a trigger generator 21 in response to a signal
from a comparator 7a in a received-wave detector 7. Upon receiving
a signal output from the trigger generator 21 a delay unit 22 counts
a delay time.
Upon receiving the signal from the trigger generator 21 and a signal
indicating the end of the delay time from the delay unit 22 a driver
4 starts driving the oscillator through the switching unit 6. A
time counter 8 counts a time interval from a release of a startup
signal from a starter 24 to the reception of a signal at an input
unit 5 having a low input impedance which indicates that a ultrasonic
wave is received by a receiving oscillator. The flow-rate calculator
9 then calculates a flow rate of fluid flowing in a fluid conduit
1 from the measured delay tome of the time counter 8 and the delay
time at the delay unit 22.
An operation of the flow meter of embodiment 2 having the above-explained
arrangement will be described in more detail.
When the starter 24 outputs the startup signal, the repeating unit
20 directs the switching unit 6 to set the first oscillator 2 in
a transmission mode and to set the second oscillator 3 in a reception
mode. More specifically, while the first oscillator 2 is connected
to the driver 4 the second oscillator 3 is connected to a resistor
14 having a small resistance in the input unit 5. This permits the
ultrasonic wave to be propagated from an upstream to a downstream
of a flow of the fluid.
Then, the number of repeating times in the repeating unit 20 is
set to an initial value, and the time counter 8 starts to count
a time. The repeating unit 20 actuates the trigger generator 21
and the generator 21 then sends a trigger signal to the delay unit
22 for starting to count a time until a delay time. When receiving
the trigger signal from the trigger generator 21 and a signal indicating
the end of the counting of the delay unit 22 the driver 4 directs
the first oscillator 2 to transmit the ultrasonic wave through the
conduit 1.
The ultrasonic wave is propagated through the conduit 1 and then
reaches the second oscillator 3 after a propagation duration. The
signal corresponding to the received ultrasonic wave is transferred
from the second oscillator 3 to the input unit 5 including a receiving
unit 14 having a small input impedance for receiving the signal.
The input unit 5 then transmits the signal to a received-wave detector
7 for detecting a timing of the signal.
More specifically, the received-wave signal is amplified to a desired
level by the amplifier 15 and then is transferred to a comparator
7a in the received-wave detector 7. The comparator 7a compares the
amplified signal with a reference level. If the received signal
is greater than the reference level, the comparator 7a supplies
a repeat signal to the repeating unit 20.
Then, the repeating unit 20 directs the trigger generator 21 to
make the delay unit 22 start counting time until the delay time.
When the delay unit 22 finishes the counting, the trigger generator
21 directs the first oscillator 2 to transmit the ultrasonic wave
again. The above cycle including transmission and reception of the
ultrasonic wave and cancellation of the transmission and reception
by the delay unit 22 is repeated until the number of the repeating
times in the repeating unit 20 reaches a predetermined number.
After the predetermined number of the cycles are repeated, the
time counter 8 stops the counting, and the flow-rate calculator
9 reads the propagation duration (T) counted by the time counter
8. The propagation duration (T1) of the ultrasonic wave from the
first oscillator 2 to the second oscillator 3 is calculated by:
where N is the number of the repeating times, and Td is the delay
time determined for the delay unit 22. Then, the calculated duration
is stored in the flow-rate calculator 9.
Then, in order to start the measurement of the propagation duration
the starter 14 set the number of the repeating times in the repeating
unit 20 and the time counted by the time counter 8 to the initial
values. The switching unit 6 then connects the first oscillator
2 with the input unit 5 and connects the second oscillator 3 with
the driver 4. At this moment, the ultrasonic wave is propagated
from the downstream to the upstream of the flow. Similar to the
duration (T1), the propagation duration (T2) of the ultrasonic wave
from the second oscillator 3 to the first oscillator 2 is calculated
by:
where N is the number of the repeating times, and Td is the delay
time for the delay unit 22. The calculated duration is stored in
the flow-rate calculator 9.
The propagation duration T2 of the ultrasonic wave from the second
oscillator 3 to the first oscillator 2 i.e., from the downstream
to the upstream of the flow increases as the fluid flows through
the conduit 1. Therefore, the durations T1 and T2 satisfy T1>T2.
The flow-rate calculator 9 calculates a difference between an inverse
value of the duration T1 and an inverse value of the duration T2
and calculates the flow rate in consideration of the cross section
of the conduit 1 and the behavior of the flow.
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