Abstrict A flow meter measures a flow rate of fluid flowing through a flow
passage based on a duration of propagation of an ultrasonic wave
along the flow passage. A signal of the received ultrasonic wave
is then compared with a reference voltage to detect an arrival of
the ultrasonic wave. A voltage setting unit determines the reference
voltage to an appropriate level according to a signal output from
a propagation measuring unit which measures a duration of propagation
of the ultrasonic wave along the flow passage from the start of
transmission of the ultrasonic wave to the output of a signal from
a judging unit as the reference voltage being changed. The reference
voltage is determined readily and accurately, the flow meter measures
the flow rate with the appropriate reference voltage.
Claims What is claimed is:
1. A flow meter for measuring a flow rate of fluid flowing through
a flow passage, said flow meter comprising: first and second oscillators
provided across said flow passage for transmitting and receiving
an ultrasonic wave; a transmitter for driving said first and second
oscillators; a switching unit for switching transmission of said
ultrasonic wave between said first and second oscillators; an amplifier
for amplifying a signal received from said first and second oscillators;
a flow-rate calculator for calculating said flow rate based on a
duration of propagation of said ultrasonic wave between said first
and second oscillators; a reference comparator for comparing a voltage
of said amplified signal with a reference voltage to output a signal
which indicates a time point where a relationship between said voltage
of said amplified signal and said reference voltage changes; a judging
unit for detecting an arrival time of said ultrasonic wave at said
first and second oscillators based on said signal output from said
reference comparator and said signal output from said amplifier
to output a signal which indicates said arrival time; and a reference
setting unit including a propagation measuring unit for measuring
a duration of propagation of said ultrasonic wave along said flow
passage from a start of transmission of said ultrasonic wave to
said arrival time detected by said judging unit, and a voltage setting
unit for determining said reference voltage based on said duration
of propagation and for said reference voltage to said reference
comparator.
2. The flow meter according to claim 1 wherein said reference
setting unit further includes a time difference measuring unit for
measuring a time difference between said signal output from of said
reference comparator and said signal output from said judging unit,
and wherein said voltage setting unit determines said reference
voltage based on a signal output from said time difference measuring
unit.
3. The flow meter according to claim 2 wherein said reference
setting unit determines said reference voltage based on a plurality
of inflection points where said time difference measured by said
time difference measuring unit significantly varies according to
a change of said reference voltage.
4. The flow meter according to claim 3 wherein said reference
setting unit determines said reference voltage between two inflection
points of said plurality of inflection points most distanced from
each other.
5. The flow meter according to claim 4 wherein said reference
setting unit determines said reference voltage at a middle between
said two inflection points.
6. The flow meter according to claim 3 wherein said reference
setting unit changes said reference voltage when said time difference
measured by said time difference measuring unit is out of a predetermined
range.
7. The flow meter according to claim 3 wherein said reference
setting unit determines said reference voltage when said time difference
measured by said time difference measuring unit according to a change
of said reference voltage from a minimum to a maximum of a range
of said reference voltage contain both a maximum and a minimum of
a predetermined range of said time difference.
8. The flow meter according to claim 3 wherein said reference
setting unit changes said reference voltage when said time difference
measured by said time difference measuring unit according to a change
of said reference voltage from a minimum to a maximum of said reference
voltage is out of a predetermined rage a predetermined number of
times.
9. The flow meter according to claim 3 wherein, when a difference
between time differences measured by said time difference measuring
unit at transmission of said ultrasonic wave in two opposite directions
becomes larger than a predetermined level, said reference voltage
is determined for each of said directions.
10. A flow meter for measuring a flow rate of fluid flowing through
a flow passage, said flow meter comprising: first and second oscillators
provided across said flow passage for transmitting and receiving
an ultrasonic wave; a transmitter for driving said first and second
oscillators; a switching unit for switching transmission of said
ultrasonic wave between said first and second oscillators; an amplifier
for amplifying a signal received by said first and second oscillators;
a flow-rate calculator for calculating said flow rate based on a
duration of propagation of said ultrasonic wave between said first
and second oscillators; a reference comparator for comparing a voltage
of said amplified signal with a reference voltage to output a signal
which indicates a time point where a relationship between said voltage
of said amplified signal and said reference voltage changes; a judging
unit for detecting an arrival time of said ultrasonic wave at said
first and second oscillators based on said signal output from said
reference comparator and said signal output from said amplifier
to output a signal which indicates said arrival time; and a reference
setting unit for determining said reference voltage based on said
signal output from said reference comparator according to a change
of said reference voltage.
11. A flow meter for measuring a flow rate of fluid flowing through
a flow passage, said flow meter comprising: first and second oscillators
provided across said flow passage for transmitting and receiving
an ultrasonic wave; a transmitter for driving said first and second
oscillators; a switching unit for switching transmission of said
ultrasonic wave between said first and second oscillators; an amplifier
for amplifying a signal received by said first and second oscillators;
a flow-rate calculator for calculating said flow rate based on a
duration of propagation of said ultrasonic wave between said first
and second oscillators; a reference comparator for comparing a voltage
of said amplified signal with a reference voltage to output s signal
which indicates a time point where a relationship between said voltage
of said amplified signal and said reference voltage changes; a judging
unit for detecting an arrival time of said ultrasonic wave at said
first and second oscillators based on said signal output from said
reference comparator and said signal output from said amplifier
to output a signal which indicates said arrival time; a time difference
measuring unit for measuring a time difference between said signal
output from said reference comparing unit and said signal output
from said judging unit; and a reference setting unit for determining
said reference voltage based on said time difference measured by
said time difference measuring unit as said reference voltage is
gradually decreased from a level near a peak of said amplified signal.
Description FIELD OF THE INVENTION
[0001] The present invention relates to a flow meter for measuring
a flow rate of fluid, such as a gas, with ultrasonic waves.
BACKGROUND OF THE INVENTION
[0002] FIG. 20 illustrates a conventional flow meter. A first ultrasonic
oscillator 32 and a second ultrasonic oscillator 33 provided across
a flow passage 31 in which fluid, such as gas, flows transmit and
receive ultrasonic wave. A switch unit 34 switches transmitting
and receiving operations of the ultrasonic oscillators. A transmitter
35 drives one of the first and second ultrasonic oscillators 32
and 33 to transmit an ultrasonic wave. An amplifier 34 amplifies
an ultrasonic wave received by the other oscillator through the
switching unit 34 to a predetermined amplitude. A reference comparator
37 compares a voltage of the signal amplified by the amplifier 36
with the amplitude of a reference voltage. A judging unit 38 outputs
a signal D upon detecting the first zero-crossing point Ta of the
detection signal in time after the comparator 37 detects that the
received signal is larger than the reference voltage, as shown in
FIG. 21. A repeating unit 39 counts the number of the signals D
received from the judging unit 38 and simultaneously transfers the
signals D to a controller 42. A time counter 40 measuring a duration
of time before the repeating unit 39 counts the signals up to a
predetermined number. A flow-rate calculator 41 calculates a flow-rate
of the fluid from an output signal of the time counter 40. The controller
42 controls the transmitter 35 according to signals from the flow-rate
calculator 41 and the repeating unit 39.
[0003] An operation of the conventional flow meter will be described
in more detail. First, the controller 42 drives the transmitter
35 and the switching unit 34 to start transmitting an ultrasonic
wave from the first ultrasonic wave oscillator 32. The ultrasonic
wave is then propagated through fluid and is received by the second
ultrasonic wave oscillator 33 and is then amplified by the amplifier
36. The reference comparator 37 outputs a signal C falling when
the output signal of the amplifier 36 become larger than the reference
voltage. The judging unit 38 outputs a signal D falling at the first
zero-crossing point Ta in time after the output signal of the amplifier
36 becomes larger than the reference voltage. The repeating unit
39 transfers the signal D from the judging unit 38 to the controller
42. This operation is repeated N times predetermined, and the duration
of the repeating is measured by the time counter 40. Then, the switching
unit 34 switches the transmission of the ultrasonic signal from
the first ultrasonic wave oscillator 32 to the second ultrasonic
wave oscillator 33 and then, the same procedures are executed.
The flow-rate calculator 41 receives, from the time counter 40
the duration in a forward direction from an upstream to a downstream
of the fluid and the duration in a reverse direction from the downstream
to the upstream of the fluid, and calculates a flow rate Q by:
Q=K.multidot.S.multidot.v=K.multidot.S.multidot.L/2.multidot.(n/t1-n/t2).m-
ultidot.cos .phi. (Equation 1)
[0004] where L is an effective distance in a flowing direction
of the fluid between the ultrasonic wave oscillators 32 and 33
t1 is the duration of the signal D transmitted N times in the forward
direction, t2 is the duration of the signal D transmitted N times
in the reverse direction, v is a measured velocity of the fluid,
S is a cross section of the flow passage, .phi. is an angle formed
by a line extending between the ultrasonic wave oscillators 32
33and the flowing direction, and K is a constant determined according
to the flow rate.
[0005] The amplifier 36 has its gain adjusted such that the signal
received by the ultrasonic wave oscillator is output at a constant
amplitude, and peak values range in a predetermined range. More
specifically, while the repeating unit 39 counts the number of the
signals D up to the predetermined number, the number of times that
the peak of the received signal is smaller than the lower limit
of a predetermined-voltage range, as shown by dotted line AL in
FIG. 22 is counted, and the number of times that the peak of the
received signal is larger than the upper limit of the range, as
shown by dotted line AH in FIG. 22 is counted. According to the
counted numbers, the gain for the subsequence measuring of flow
rate is determined. For example, if the number of times that the
peak of the received signal is smaller than the lower limit is greater
than the number of times that the peak of the received signal is
larger than the upper limit, the gain is increased so that the peaks
of the received signal denoted by real line A in FIG. 22 stay between
the upper limit and the lower limit of the range.
[0006] The reference voltage of the reference comparator 37 to
be compared with the voltage of the signal from the amplifier 36
is determined for the judging unit 38 to detect the position of
zero crossing. More particularly, as shown in FIG. 21 the reference
voltage is determined to be a substantial middle between the second
peak and the third peak of the received signal in time, thus enabling
the judging unit 38 to detect the first zero-crossing point Ta in
time after the third peak P3. This assures a margin against the
case that the second peak P2 increases or the third peak P3 declines
according to a change in the flow rate or a temperature, thus allowing
the judging unit 38 to detect the zero-crossing point Ta.
[0007] In the conventional flow meter having the above arrangement,
the reference voltage to be compared with the voltage of the received
signal amplified to a desired voltage is determined by resistance-voltage
division implemented by a fixed resistor and a variable resistor.
The division however requires the variable resistor to be manually
adjusted for determining the reference voltage while monitoring
the voltage, thus taking a considerable length of time and possibly
creating an error in the adjusting. Further, the reference voltage
remains unchanged during the measurement of the flow rate, and the
gain of the amplifier for amplifying the received signal is fixed.
Therefore, the voltage of the received ultrasonic wave varies during
the measurement, and the relationship between the voltage and the
reference voltage may accordingly change. For example, when the
received signal significantly declines in its amplitude, the third
peak P3 used for detecting the arrival of the wave may not reach
the reference voltage. Accordingly, the reference comparator may
output the signal C at the timing of the fourth peak P4 thus causing
the judging unit to detect the first zero-crossing point after the
fourth peak P4. As the result, the time counter measures the duration
incorrectly, thus permitting the flow-rate calculator to calculate
a incorrect flow rate.
SUMMARY OF THE INVENTION
[0008] A flow meter measures a flow rate of fluid flowing through
a flow passage. The flow meter includes first and second oscillators
provided across the flow passage for transmitting and receiving
an ultrasonic wave, a transmitter for driving the first and second
oscillators, a switching unit for switching transmission of the
ultrasonic wave between the first and second oscillators, an amplifier
for amplifying a signal received from the first and second oscillators,
a flow-rate calculator for calculating the flow rate based on a
duration of propagation of the ultrasonic wave between the first
and second oscillators, a reference comparator for comparing a voltage
of the amplified signal with a reference voltage to output a signal
which indicates a time point where a relationship between the voltage
of the amplified signal and the reference voltage changes, a judging
unit for detecting an arrival time of the ultrasonic wave at the
first and second oscillators based on the signal output from the
reference comparator and the signal output from the amplifier to
output a signal which indicates the arrival time, and a reference
setting unit. The reference setting unit includes a propagation
measuring unit for measuring a duration of propagation of the ultrasonic
wave along the flow passage from a start of transmission of the
ultrasonic wave to the arrival time detected by the judging unit,
and a voltage setting unit for determining the reference voltage
based on the duration of propagation and for the reference voltage
to the reference comparator.
[0009] In the flow meter, the reference voltage is automatically
set to a level assuring a margin against a change in the amplitude
of the received signal, and the reference voltage is determined
readily and accurately according to a change in the amplitude of
the received signal during the measurement of the flow rate. This
allows the judging unit to precisely detect a desired point or the
first zero-crossing point in time after a desired point, e.g. the
third peak P3 of the received signal, thus enabling the flow meter
to measure the flow rate accurately against a change in the amplitude
of the received signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a block diagram of a flow meter according to exemplary
embodiment 1 of the present invention.
[0011] FIG. 2 is an explanatory view for illustrating an operation
of the flow meter of embodiment 1.
[0012] FIG. 3 is a block diagram of a flow meter according to exemplary
embodiment 2 of the invention.
[0013] FIG. 4 is an explanatory view for illustrating an operation
of the flow meter of embodiment 2.
[0014] FIG. 5 is a characteristic diagram for showing an operation
of the flow meter of embodiment 2.
[0015] FIG. 6 is a block diagram of a flow meter according to exemplary
embodiment 3 of the invention.
[0016] FIG. 7 is an explanatory view for illustrating an operation
of the flow meter of embodiment 3.
[0017] FIG. 8 is a flow chart of an operation of the flow meter
of embodiment 3.
[0018] FIG. 9 is a block diagram of a flow meter according to exemplary
embodiment 4 of the invention.
[0019] FIG. 10 is an explanatory view for illustrating an operation
of the flow meter of embodiment 4.
[0020] FIG. 11 is a flow chart of an operation of the flow meter
of embodiment 4.
[0021] FIG. 12 is a flow chart of an operation of the flow meter
of embodiment 4.
[0022] FIGS. 13A and 13B are explanatory diagrams for illustrating
an operation of the flow meter of embodiment 4.
[0023] FIG. 14 is a flow chart of an operation of another flow
meter of embodiment 4.
[0024] FIG. 15 is a block diagram of a flow meter according to
exemplary embodiment 5 of the invention.
[0025] FIG. 16 is a flow chart of an operation of the flow meter
of embodiment 5.
[0026] FIG. 17 is a block diagram of a flow meter according to
exemplary embodiment 6 of the invention.
[0027] FIG. 18 is an explanatory view for illustrating an operation
of the flow meter of embodiment 6.
[0028] FIG. 19 is an explanatory view for illustrating an operation
of the flow meter of embodiment 6.
[0029] FIG. 20 is a block diagram of a conventional flow meter.
[0030] FIG. 21 is an explanatory view for illustrating an operation
of the conventional flow meter.
[0031] FIG. 22 is an explanatory view for illustrating an operation
of the conventional flow meter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] (Exemplary Embodiment 1)
[0033] FIG. 1 is a block diagram of a flow meter according to exemplary
embodiment 1 of the present invention, and FIG. 2 illustrates an
operation of the flow meter. A first ultrasonic wave oscillator
2 and a second ultrasonic wave oscillator 3 for transmitting and
receiving ultrasonic wave are provided across a flow passage 1 where
fluid flows at an angle .phi. to the direction of the flow of the
fluid. A transmitter 5 drives the first and second ultrasonic wave
oscillators 2 3 to transmit an ultrasonic wave. A switching unit
4 switches the transmitting and receiving operation between the
first ultrasonic wave oscillator 2 and the second ultrasonic wave
oscillator 3. An amplifier 6 has its gain to output a signal received
from the ultrasonic wave oscillator at a constant amplitude. A reference
setting unit 13 determines a reference voltage according to a signal
which is output from the amplifier 6. The reference voltage is then
fed to a reference comparator 7 for comparing the voltage with the
signal output from the amplifier 6. A judging unit 8 detects the
arrival of the ultrasonic wave based on a signal output from the
reference comparator 7 and the signal output from the amplifier
6. A repeating unit 9 counts a predetermined number of signals output
from the judging unit 8 and supplies the signals output from the
judging unit to a controller 12. A time counter 10 measures a duration
of time while the repeating unit 9 counts the predetermined number
of the signals output from the judging unit 8. A flow-rate calculator
11 calculates a flow rate of the fluid based on the duration measured
by the time counter 10 in consideration with the cross section of
the flow passage and conditions of the flow. The controller 12 controls
the transmitter 5 and the amplifier 6 in responsive to signals output
from the repeating unit 9 and the flow-rate calculator 11. A propagation-time
measuring unit 13b measures a propagating duration of the ultrasonic
wave based on a start signal from the controller 12 for starting
transmitting the ultrasonic wave and the signal output from the
judging unit 8. A voltage setting unit 13a determines the reference
voltage based on a signal output from the propagation-time measuring
unit 13b, and outputs the reference voltage to the reference comparator
7. The propagation-time measuring unit 13b and the voltage setting
unit 13a constructs a reference setting unit 13.
[0034] An operation of the flow meter of embodiment 1 for determining
the reference voltage level will be explained.
[0035] FIG. 2 illustrates the operation of the flow meter of embodiment
1. Upon starting the measuring of the flow rate, the controller
12 drives the transmitter 5 to transmit an ultrasonic wave from
the first ultrasonic wave oscillator 2. The ultrasonic wave transmitted
from the first ultrasonic wave oscillator 2 is propagated in the
flow of the fluid and received by the second ultrasonic wave oscillator
3. The amplifier 6 amplifies a signal output according to the ultrasonic
wave received by the second ultrasonic wave oscillator 3 by a gain
so as to outputs a signal having a constant amplitude to the reference
comparator 7 and the judging unit 8.
[0036] Upon receiving the start signal from the controller 12 and
the signal from the judging unit 8 the propagation-time measuring
unit 13b starts measuring the duration of the ultrasonic wave propagated
through the flow passage.
[0037] Then while the propagation-time measuring unit 13b measures
the duration, the voltage setting unit 13a decreases the reference
voltage to a voltage Vb for permitting the judging unit 8 to detect
the first zero-crossing point T1 after the second peak P2 of the
amplified signal. This allows the reference comparator 7 and the
judging unit 8 to output signals B1 and B2 respectively. Accordingly,
the duration measured by the propagation time measuring unit 13b
becomes shorter by one period of the ultrasonic wave, e.g. 2 .mu.s
for a ultrasonic wave of 500 kHz than the duration until the judging
unit 8 detects the first zero-crossing point after the third peak
P3 of the amplified signal.
[0038] If the duration measured by the propagation time measuring
unit 13b becomes shorter by one period of the ultrasonic wave, the
voltage setting unit 13a increases the reference voltage to a voltage
Vc for permitting the first zero-crossing point T2 in time after
the fourth peak P4 of the amplified signal to be detected. This
allows the reference comparator 7 and the judging unit 8 to output
signals C1 and C2 respectively. Accordingly, the duration of the
propagation measured by the propagation-time measuring unit 13b
becomes longer by one period of the ultrasonic wave than the duration
until the judging unit 8 detects the first zero-crossing point in
time after the third peak P3.
[0039] By this manner, the reference voltage varies from near the
second peak P2 to near the third peak P3 as shown in FIG. 2. The
reference voltage is then set to an intermediate level between the
two peaks by the voltage setting unit 13a and is supplied to the
reference comparator 7. The reference comparator 7 compares the
signal output from the amplifier 6 with the reference voltage, and
outputs, to the judging unit 8 a signal indicating the time point
when that the signal from the amplifier 6 exceeds the reference
voltage. The judging unit 8 judges the first zero-crossing point
Ta, when the signal output from the amplifier 6 shifts from positive
to negative after the time point as an arrival time of the ultrasonic
wave, and outputs the signal indicating the point to the repeating
unit 9.
[0040] As described, in the flow meter of embodiment 1 the range
of the reference voltage is determined based on the propagation
duration changing according to the reference voltage changed by
the reference setting unit 13. The range of the reference voltage
enables the judging unit 6 to detect a specified time in the signal
output from the amplifier 6 for example, the first zero-crossing
point after the third peak P3. The reference voltage is then set
to an intermediate level in the range. As the result, the flow meter
stably detects a specified time in the signal. The reference voltage
of the flow meter is determined readily and accurately at periodic
intervals or at every time of the measurement of the flow rate,
thus being maintained in optimum.
[0041] (Exemplary Embodiment 2)
[0042] FIG. 3 is a block diagram of a flow meter according to exemplary
embodiment 2 of the present invention, and FIGS. 4 and 5 illustrate
operations of the flow meter. A time difference measuring unit 13c
measures a time difference between a signal output from a reference
comparator 7 and a signal output from a judging unit 8. The time
difference measuring unit 13c and a voltage setting unit 13a constructs
a reference setting unit 131. The other elements are identical to
those of embodiment 1 and are explained in no more detail.
[0043] An operation of the another flow meter will be explained.
Upon starting of measurement of a flow rate, the controller 12 drives
the transmitter 5 to transmit an ultrasonic wave from the first
ultrasonic oscillator 2. The ultrasonic wave transmitted from the
first ultrasonic wave oscillator 2 is propagated through the flow
passage 1 and is received by the second ultrasonic wave oscillator
3. The received signal is amplified by the amplifier 6 at a gain
controlled by the controller 12 such that the amplifier 6 outputs
the amplified signal having a constant amplitude to the reference
comparator 7. The reference comparator 7 comparing the amplifier
6 with a reference voltage Vc, outputs a signal C1 to the judging
unit 8 and the time difference measuring unit 13c at the time point
when the signal output from the amplifier 6 exceeds the reference
voltage Vc.
[0044] The judging unit 8 judges the first zero-crossing point
Ta in time after the time point as shown in FIG. 4 as an arrival
time of the ultrasonic wave, and then outputs a signal C2 to the
time difference measuring unit 13c. At the first zero-crossing point
Ta, the signal output from the amplifier 6 has its sign change.
The time difference measuring unit 13c measures a time difference
td between the signal C1 output from the reference comparator 7
and the signal C2 output from the judging unit 8 as shown in FIG.
4. Based on the time difference td measured by the time difference
measuring unit 13c, the voltage setting unit 13a increases and decreases
the reference voltage Vc. Accordingly, the time difference td changes
according to the reference voltage Vc, as shown in FIG. 5.
[0045] When the reference voltage Vc is a voltage V2 which is substantially
equal to the second peak P2 for enabling the judging unit 8 to detect
the first zero-crossing point Tb after the second peak P2 of the
received signal, the time difference td0 is 1/4 of the period of
the received signal, i.e., 500 ns. Then, the reference voltage Vc
is increased from the voltage V2 to a voltage for enabling the first
zero-crossing point Ta after the third peak P3 of the signal to
be detected, and the time difference td becomes local-maximum, as
shown in FIG. 5. Then, as the reference voltage Vc is further increased,
the time difference td decreases. The time difference td becomes
the time difference td0 when the reference voltage is a voltage
V3 substantially equal to the third peak P3 of the signal.
[0046] As described, the voltage setting unit 13a sets the reference
voltage to a voltage making the time difference td be an intermediate
between the maximum time difference and the minimum time difference
(1/4 of the period). This setting permits the time difference td
measured by the time difference measuring unit 13c to be set to
an intermediate between the maximum and minimum time differences
according to the change of the reference voltage. Accordingly, the
reference voltage is maintained so that a specified zero-crossing
point Ta in the received signal is detected stably. Since the reference
voltage in the flow meter is set readily and accurately at periodic
intervals or at every time at the measurement of the flow rate,
the reference voltage is maintained in optimum.
[0047] (Exemplary Embodiment 3)
[0048] FIG. 6 is a block diagram of a flow meter according to exemplary
embodiment 3 of the present invention. FIG. 7 illustrates an operation
of the flow meter. FIG. 8 is a flowchart of the operation. A first
ultrasonic wave oscillator 2 and a second ultrasonic wave oscillator
3 for transmitting and receiving ultrasonic waves are provided at
an angle .phi. to a direction of the flow of fluid across a flow
passage 1. A transmitter 5 drives the first and second ultrasonic
wave oscillators 2 and 3 to transmit an ultrasonic wave. A switching
unit 4 switches the transmitting and receiving of the waves between
the first ultrasonic wave oscillator 2 and the second ultrasonic
wave oscillator 3. An amplifier 6 amplifies a signal received from
the ultrasonic wave oscillator at a receiving side with a gain controlled
by a controller 12. A reference comparator 7 compares the signal
amplified and output by the amplifier 6 with a reference voltage.
A judging unit 8 determines an arrival time of the ultrasonic wave
according to a signal output from the reference comparator 7 and
the signal output from the amplifier 6. A repeating unit 9 counts
a predetermined number of signals output from the judging unit 8
and outputs a signal to the controller 12. A time counter 10 measures
a duration of time for the repeating unit 9 to count the predetermined
number of signals output from the judging unit 8. A flow-rate calculator
11 calculates a flow rate of the fluid according to the duration
measured by the time counter 10 in consideration with the cross
section of the flow passage and conditions of the flow. The controller
12 controls the transmitter 6 and the amplifier 6 based on signals
output from the repeating unit 9 and the flow-rate calculator 11.
A reference setting unit 14 including a digital-to-analog (D/A)
converter or an electronic variable resistor determines the reference
voltage used in the reference comparator 7. The reference setting
unit 14 determines the reference voltage based on a signal which
the reference comparator 7 outputs while the reference voltage is
changed.
[0049] An operation of the flow meter of embodiment 3 will be explained
referring to FIG. 8. The controller 12 when being turned on, starts
an initial setting procedure for determining the reference voltage
by adjusting a gain of the amplifier 6. After the gain is adjusted,
the received signal is amplified, e.g., so that the fifth peak P5
of the signal stays in a desired range as shown in FIG. 7. When
the signal is exceeds the upper limit of the range, the amplifier
6 supplies a peak detection signal PD to the controller 12 and the
reference setting unit 14. The gain of the amplifier 6 is modified
to a first gain for maintaining the signal received by the ultrasonic
wave oscillator at the receiving side in constant (Step 1). Then,
the received signal is amplified with a second gain which is larger
than the first gain of the amplifier 6 while a change of a peak,
e.g. the second peak P2 just before a peak, the third peak P3
used for determining the arrival time of the received signal is
considered (Step 2). The resultant amplified signal is shown in
FIG. 7.
[0050] In FIG. 7 the signal amplified at the first gain is denoted
by the dotted line, while the signal amplified at the second gain
is denoted by the real line. The reference setting unit 14 increases
the reference voltage Vref by one control unit (e.g., 5 mV) from
the minimum level of the voltage for comparison with the signal
A amplified at the second gain, as shown in FIG. 7 (Step 3). According
to embodiment 3 the minimum level of the reference voltage Vref
is larger than the first peak P1 but smaller than the second peak
P2 of the signal A. The controller 12 sets the predetermined number
used in the repeating unit 9 to one and drives the transmitter 5
to transmit an ultrasonic wave from the first ultrasonic wave oscillator
2 (Step 4). The ultrasonic wave transmitted from the first ultrasonic
wave oscillator 2 is propagated through the flow passage 1 and received
by the second ultrasonic wave oscillator 3. The signal output from
oscillator 3 is amplified by the amplifier 6 and is then transferred
to the reference comparator 7. The reference comparator 7 compares
the signal output from the amplifier 6 i.e., the signal A, with
the reference voltage Vref. If the signal A is larger than the voltage
Vref, the comparator 7 outputs signals C2 C3 C4 and C5 to both
the reference setting unit 14 and the judging unit 8 as shown in
FIG. 7 (Step 5). The reference setting unit 14 counts the number
of the signals C2 to C5 output from the reference comparator 7 (Step
6). Steps 5 and 6 are repeated until the amplifier 6 outputs the
peak detection signal (Step 7). When receiving the peak detection
signal from the amplifier 6 the reference setting unit 14 examines
whether the number of the signals from the reference comparator
7 is three or not (Step 8). When the number is not three, the reference
voltage Vref is increased by the one control unit (Step 3), and
then, steps 4 to 8 are repeated. When the number of the signals
output from the reference comparator 7 is three, the gain of the
amplifier 6 is set back to the first gain (Step 9).
[0051] That is, if the reference voltage Vref is not larger than
the second peak P2 of the received signal, the reference comparator
7 outputs the four signals C2 to C5 before the peak detection signal
is received from the amplifier 6. Then, when the reference voltage
Vref exceeds the second peak P2 the reference comparator 7 outputs
three of the signals C3 to C5 and does not output the signal C2.
The reference setting unit 14 stops increasing the reference voltage
Vref just when the number of the signals outputs from the reference
comparator 14 shifts from four to three. When the gain of the amplifier
6 is set back to the first gain, the reference voltage Vref of the
reference setting unit 14 becomes slightly larger than the second
peak P2 of the received signal. A difference between the reference
voltage Vref and the second peak P2 is predetermined so that the
second peak P2 varies during the measurement of the flow rate. This
prevents the second peak P2 from exceeding the reference voltage
Vref even if the received signal varies in its amplitude during
the measurement of the flow rate. Therefore, the reference voltage
Vref can have a large margin against the third peak P3 thus allowing
the judging unit 8 to judge the arrival of the received signal of
the ultrasonic wave regardless of any change in the amplitude of
the received signal.
[0052] As described, according to this embodiment, the reference
voltage to be determined by the reference setting unit 14 according
to the signal output from the reference comparator 7. More specifically,
the reference voltage is set to an optimum level in consideration
of the maximum varying amplitude of the peak just before the peak
to be detected for detecting the arrival of the received signal.
Accordingly, the peak just before the peak to be detected for detecting
the arrival of the received signal does not exceed the reference
voltage, hence allowing the reference voltage to have the large
margin against the peak of the wave used for detecting the arrival
of the received signal. As the result, the flow meter of embodiment
3 can be improved in the detecting of the signal against its amplitude
change, thus measuring the flow rate of the fluid accurately. According
to embodiment 3 the amplifier 6 amplifies the received signal at
the gain higher rate than the gain set before the reference voltage
is determined. However, the reference voltage can thus be set to
the voltage Vref which is larger than the peak (e.g. the second
peak P2) of the received signal just before the peak (e.g. the third
peak P3) to be detected for detecting the arrival of the received
signal in consideration that the second peak P2 may vary during
the measurement of the flow rate. Moreover, since the number of
the signals output from the reference comparator 7 during the change
of the reference voltage indicates the voltages of the peaks of
the received signal, the controller 12 can detect the relationship
between the received signal and the reference voltage according
to the number of the output signals of the reference comparator
7. The controller 12 may set the reference voltage Vref to an arbitrary
intermediate level between the third peak P3 used for detecting
the arrival of the received signal and the second peak P2 just before
the third peak P3.
[0053] (Exemplary Embodiment 4)
[0054] FIG. 9 is a block diagram of a flow meter according to exemplary
embodiment 4 of the present invention. FIGS. 10 and 13 illustrate
operations of the flow meter. FIGS. 11 and 12 are flowcharts of
the operations.
[0055] In FIG. 9 a time difference measuring unit 15 measures
a time difference between a signal output from the reference comparator
7 and a signal output from the judging unit 8. Differently from
embodiment 3 a reference setting unit 14 determines a reference
voltage in a reference comparator 7 according to the signal output
from the time difference measuring unit 15. The other components
are identical to those of embodiment 3 shown in FIG. 6 and are explained
in no more detail.
[0056] An operation of the flow meter having above arrangement
will be described referring to FIG. 11. The controller 12 when
being turned on, changes a gain of an amplifier 6 and sets the reference
voltage. According to embodiment 4 the gain of the amplifier 6
is adjusted similarly to embodiment 3 and an operation of the adjusting
of the gain is explained in no more detail.
[0057] First, the controller 12 changes the gain such that a signal
received by the ultrasonic wave oscillator at a receiving side is
amplified and output at a desired amplitude (Step 101). Then, the
reference setting unit 14 sets the reference voltage to a minimum
level of a adjustable range of the voltage (Step 102). The controller
12 sets the number of signals used in the repeating unit 9 to one
and drives the transmitter 5 to transmit an ultrasonic wave from
the first ultrasonic wave oscillator 2 (Step 103).
[0058] The ultrasonic wave output from the first ultrasonic wave
oscillator 2 is propagated through the flow passage 1 and received
by the second ultrasonic wave oscillator 3. A signal output from
the oscillator 3 is amplified by the amplifier 6 and then is transferred
to the reference comparator 7.
[0059] FIG. 10 shows the resultant amplified signal. The reference
comparator 7 compares the signal output from the amplifier 6 (a
signal A) with the reference voltage. At a time point Tc when the
signal A is larger than the reference voltage, the comparator 7
outputs a signal C to both the time difference measuring unit 15
and the judging unit 8 (Step 104). Upon receiving the signal C,
the time difference measuring unit 15 starts measuring a time (Step
105). The judging unit 8 judges that the first zero-crossing point
Ta in time after the time point Tc of the signal A is detected as
an arrival time of the ultrasonic wave, thus outputting a signal
D to both the repeating unit 9 and the time difference measuring
unit 15 (Step 106). When receiving the signal D, the time difference
measuring unit 15 stops measuring the time and outputs a measured
time difference Td to the reference setting unit 14 (Step 107).
The reference setting unit 14 increases the reference voltage by
one control unit (e.g., 2 mV) in the range (Step 108). Since the
number determined for the repeating unit 9 is one, the controller
12 receives, from the repeating unit 9 a signal indicating that
one repeating operation has been completed, and again drives the
transmitter 5 to transmit an ultrasonic wave from the first ultrasonic
wave oscillator 2. The procedure from Step 103 to Step 108 is repeated
until the reference setting unit 14 sets the reference voltage to
the maximum level in the range.
[0060] While the reference setting unit 14 sets the reference voltage
between the minimum and the maximum, the time difference measuring
unit 15 measures plural inflection points at which time differences
vary significantly from just before the points. For example, the
reference setting unit 14 may determine the reference voltage at
the middle between two inflection points distanced most of plural
inflection points between which the time difference changes largely,
for example, between which the time difference exceeds 1.3 times
greater than an initial of the time difference.
[0061] An operation at step 110 will be explained in more detail
referring to FIGS. 13A and 13B. FIG. 13B illustrates a profile of
the time difference measured by the time difference measuring unit
15 when the reference voltage of the reference setting unit 14 varies
from the minimum to the maximum. The time difference is an interval
between the signal output from the reference comparator 7 and the
signal output from the judging unit 8 as shown in FIG. 10. The
time difference measuring unit 15 measures time differences Tp1
Tp2 Tp3 Tp4 . . . corresponding to the first peak P1 the second
peak P2 the third peak P3 the fourth peak P4 . . . of the received
signal, respectively, with the reference voltage set to near the
peaks. Each of the time differences Tp1 Tp2 Tp3 and Tp4 is substantially
equal to 1/4 of the period of the ultrasonic wave (e.g., 500 ns
at the ultrasonic wave of 500 kHz). As the reference voltage exceeds
each peak, the time difference measured by the time difference measuring
unit 15 abruptly increases, thus being exhibited as inflection points
Tp1 Tp2 Tp3 and Tp4 as shown in FIG. 13. For example, when the
reference voltage having been smaller than the second peak P2 exceeds
the second peak P2 the inflection point Tp2 is measured. This means
that the reference voltage at the inflection point of the time difference
is close to the peak. Accordingly, the difference of the reference
voltage between any two adjacent inflection points represents a
difference of the voltage between the two adjacent peaks.
[0062] As shown in FIG. 13 the difference between the points Tp1
and Tp2 represents a difference of the voltage between the first
peak P1 and the second peak P2 and the difference of the voltage
between the points Tp2 and Tp3 represents a difference of the voltage
between the second peak P2 and the third peak P3. More specifically,
the reference voltage is changed from the minimum to the maximum,
the inflection points in the time difference accordingly appear.
The reference voltage is determined between the second peak P2 and
the third peak P3 as shown in FIG. 13 thus being determined at
the largest voltage difference of the received signal. Thus, the
reference voltage Vref is determined at an intermediate level between
the second peak P2 and the third peak P3 as shown in FIG. 13. This
allows the reference voltage to have a large margin against the
voltages at the peaks P2 and P3. Accordingly, the judging unit 8
detects the arrival of the ultrasonic wave stability regardless
of any change in the amplitude of the received signal.
[0063] The reference voltage of the reference setting unit 14 has
been set in the above manner, and the controller 12 sets the repeating
unit 9 to have a desired number of repeating times (e.g., 256) and
starts the measurement of the flow rate.
[0064] An operation of the reference setting unit 14 after starting
the measurement of the flow rate will be described referring to
FIG. 12. First, the controller 12 drives the transmitter 5 to transmit
an ultrasonic wave from the first ultrasonic wave oscillator 2 (Step
12). The ultrasonic wave (signal) is received by the second ultrasonic
wave oscillator 3 is amplified by the amplifier 6 and is transferred
to both the reference comparator 7 and the judging unit 8. The reference
comparator 7 compares the received signal with the reference voltage
(Step 13). When the received signal exceeds the reference voltage,
the time difference measuring unit 13 starts measuring a time difference
(Step 14) and continues the measuring until the judging unit 8 detects
the first zero-crossing point in time after the start (Steps 15
and 16).
[0065] When the judging unit 8 detects the first zero-crossing
point (i.e. the arrival of the ultrasonic wave), the controller
12 drives the transmitter 5 to repeat transmitting an ultrasonic
wave from the first ultrasonic wave oscillator 2. The procedure
from Step 12 to Step 16 is repeated by the predetermined number
of times (Step 11). A range of the time difference is determined
between the inflection points Tp2 and Tp3 (from 500 ns to 800 ns)
at the initial setting of the reference voltage, as shown in FIG.
13. More particularly, the time difference may range from 20% to
80% of the range, i.e., from 560 ns (=(800 ns-500 ns).times.0.2+500)
to 740 ns (=(800 ns-500 ns).times.0.8+500). After the procedure
has been repeated the predetermined number of times, the reference
setting unit 14 examines whether or not the time difference measured
by the time difference measuring unit 15 is out of the range during
the procedure being repeated and determines whether the reference
voltage is set again or not according to a distribution of the measured
time difference. In other words, the reference setting unit 14 examines
whether or not the measured time differences is smaller than the
lowermost (560 ns) of the range (Step 17). When the differences
are smaller than the lowermost, the reference voltage is set again
in the same manner as of the initial setting (Step 19). When the
differences are not small, the reference setting unit 14 examines
whether or not the measured time differences are larger than the
uppermost (740 ns) of the range (Step 18). When the differences
are larger, the reference voltage is set again by the same manner
as of the initial setting (Step 19). When the measured time differences
are smaller than the range or larger than the range, the initial
setting of the controller 12 is repeated. If the measured time differences
are within the range, the measurement of the flow rate is finished,
and the time measuring unit 10 measures the duration from the start
to the end of the repeating measured. Then, the transmitting of
an ultrasonic wave is switched by the switching unit 4 from the
first ultrasonic wave oscillator 2 to the second ultrasonic wave
oscillator 3 and the flow rate is the measured by the above described
steps. The duration of propagating the ultrasonic waves is measured
from the upstream to the downstream and from the downstream to the
upstream. Finally, the flow-rate calculator 11 calculates the flow
rate of the fluid from a difference between the measured durations
in consideration with the cross section of the flow passage and
conditions of the flow.
[0066] Another operation of the reference setting unit 14 will
be described. FIG. 14 is a flowchart showing the operation of the
reference setting unit 14. The procedure from Step 11 to Step 16
is identical to that from Step 11 to Step 16 shown in FIG. 12.
[0067] After the transmission of an ultrasonic wave has been repeated
a desired number of times predetermined by the repeating unit 9
the reference setting unit 14 examines whether or not a time difference
measured by the time difference measuring unit 15 during the repeating
is out of the range, that is, larger than the uppermost or smaller
than the lowermost of a range which is defined by multiplying the
difference between the two adjacent inflection points at the initial
setting of the reference voltage (Step 21) by a certain rate (e.g.
20%). When the time difference is out of the range, the reference
setting unit 14 sets the reference voltage again by the same manner
as of the initial setting (Step 22).
[0068] If no time difference is out of the range, the reference
setting unit 14 further examines whether or not the measured time
difference is larger than the uppermost (740 ns) of the range (Step
23). When the time difference is larger, the reference setting unit
examines whether the number of times the larger time differences
are measured is greater than a predetermined number (e.g., ten)
(Step 24). If the number of times is greater, the reference setting
unit 14 increases the reference voltage by one control unit (2 mV)
per repeated time, i.e., two control units (Steps 25 and 26). If
the number of times is not greater than the specific number, the
reference setting unit 14 increases the reference voltage by one
control unit (2 mV) (Step 26).
[0069] The reference setting unit 14 then examines whether or not
the measured time difference is smaller than the lowermost (560
ns) of the range (Step 27). If the difference is smaller, the reference
setting unit examines whether the number of times the measured time
difference smaller than the lowermost is greater than a predetermined
number (e.g., ten) (Step 28). If the number of times is greater,
the reference setting unit 14 decreases the reference voltage by
one control unit (2 mV) per repeated time, i.e., two control units
(Steps 29 and 30). If the number of times is not greater than the
predetermined number, the reference setting unit 14 decreases the
reference voltage by one control unit (2 mV) (Step 30). According
to a result of comparison with the uppermost of the range (Step
23) and the lowermost of the range (Step 27), the measured time
differences are all in the range, the operation terminates without
modifying the reference voltage.
[0070] As described, the reference voltage once set can be modified
depending on the number of times that the time difference measured
by the time difference measuring unit 15 during the measurement
of the flow rate is out of the predetermined range of the time difference.
[0071] According to Embodiment 4 the reference voltage is first
set at the middle between two adjacent inflection points having
the largest distance of the inflection points, in which the time
difference measured by the time difference measuring unit 15 significantly
varies. The reference voltage may be set not to the middle between
the time differences but to a lower level if the received signal
has its amplitude biased to a specific (increasing or decreasing)
aspect. For example, when the amplitude of the received signal tends
to decreasingly varied, the reference voltage has a large margin
against a change (the decreasing) of the amplitude of the received
signal. More particularly, the reference voltage may be set to 1/3
of the range between two maximum distanced inflection points. The
reference voltage can be set to a desired level corresponding to
a property of a change of the amplitude of the received signal.
[0072] As described above, the reference voltage is initially set
to a level such that the arrival of the ultrasonic wave is detected
most steadily regardless of any change in the amplitude of the received
signal indicating the receiving of the ultrasonic wave. Then, when
the time difference measured by the time difference measuring unit
15 during the measurement of the flow rate becomes out of the range
of the time difference which has been determined at the initial
setting of the reference voltage, the reference voltage can be modified
to an optimum level. As the reference voltage is automatically modified
to a desired level, the voltage can remain optimum throughout the
measurement of the flow rate by the flow meter.
[0073] (Embodiment 5)
[0074] FIG. 15 is a block diagram of a flow meter according to
exemplary embodiment 5 of the present invention. FIG. 16 is a flowchart
of showing an operation of the flow meter. A reference setting unit
14 determines a reference voltage according to a signal output from
the controller 12 which indicates the direction of an ultrasonic
wave transmitted from a first ultrasonic wave oscillator or a second
ultrasonic wave oscillator. The other components are identical to
those of embodiment 4 and will be explained in no more detail.
[0075] An operation of the reference setting unit 14 in the flow
meter different from that of embodiment 4 will now be described.
The controller 12 when being turned on, performs an initial setting
for determining a gain of an amplifier 6 and the reference voltage.
The reference voltage is set, by the same manner as of Embodiment
4 to the middle between two inflection points of time difference
being distanced most while inflection points are exhibited while
the time differences measured by a time difference measuring unit
15 significantly varies during a change of the reference voltage
from the minimum to the maximum. The controller 12 operates in the
same manner as of embodiment 2 for repeating transmitting an ultrasonic
wave a desired number of times predetermined for the repeating unit
9 to measure a flow rate of fluid in one direction (Step 51). After
the measurement, the controller 12 calculates an average of the
time differences measured by the time difference measuring unit
15 (Step 52). The controller 12 then drives the switching unit 4
to switch the transmission of an ultrasonic wave between the first
ultrasonic wave oscillator 2 and the second ultrasonic wave oscillator
3 (Step 53), repeats the measurement of the flow rate in a reverse
direction (Step 54), and calculates again an average of the time
differences measured by the time difference measuring unit 15 (Step
55). The reference setting unit 14 compares the two averages of
the time differences for the ultrasonic wave transmission of both
the first ultrasonic wave oscillator 2 and the second ultrasonic
wave oscillator 3 (Step 56). When a difference between the two averages
of the time differences is greater than a predetermined level, for
example, one average is 600 ns, and the other is 670 ns creating
a 10% difference, the reference setting unit 14 determines the reference
voltage for each of the directions of the ultrasonic wave transmission
(Step 57), hence allowing the flow rate to be measured based on
respective reference voltages.
[0076] According to embodiment 5 the detection signal indicating
the reception of an ultrasonic wave is different in the perception
sensitivity between two opposite directional flows of the fluid
from the upstream to the downstream and from the downstream to the
upstream, thus changing the relationship between the amplitude of
the received signal and the reference voltage. The reference voltage
can be set to an optimum level for each directions so that the time
difference measured by the time difference measuring unit varies
depending on the direction of transmission of the ultrasonic wave
along the flow. The flow meter of embodiment 5 allows the reference
voltage to be at its optimum level even when the received signal
is different in the amplitude between the two opposite directions
of transmission of the ultrasonic wave.
[0077] (Embodiment 6)
[0078] FIG. 17 is a block diagram of a flow meter according to
exemplary embodiment 6 of the present invention. FIGS. 18 and 19
illustrate an operation of the flow meter particularly for determining
a reference voltage. As shown in FIG. 17 a reference setting unit
7a includes an electronic variable resistor for setting the reference
voltage to a desired level or modifying the reference voltage. A
comparator 7b compares the reference voltage determined by the reference
setting unit 7a with the amplitude of a signal received by an ultrasonic
wave oscillator at a receiving side. A time difference measuring
unit 7c measures a time difference between respective signals output
from the comparing unit 7b and the judging unit 8. Differently from
other embodiments, the reference comparator 7 of embodiment 6 includes
the reference setting unit 7a, the comparator 7b, and the time difference
measuring unit 7c. A signal width detector 16 calculates an interval
of time between two points in the signal output from the amplifier
6 after the reference comparator 7 outputs a signal. The other components
are identical to those of Embodiment 3 and will be explained in
no more detail.
[0079] An operation of the flow meter having the above arrangement
will be described. The reference comparator 7 determines the reference
voltage through the following steps. A controller 12 drives the
reference comparator 7 to start an initial setting of the reference
voltage upon being turned on and to change the reference voltage
when the flow-rate calculator 11 produces an error output or when
the judging unit 8 fails to detect the arrival of the ultrasonic
wave. When receiving a command from the controller 12 the reference
comparator 7 is set to a mode for determining the reference voltage,
and the reference setting unit 7a sets the reference voltage near
a peak of the received signal amplified by the amplifier 6. More
specifically, as shown in FIG. 18 if the fourth peak P4 is a peak
as a whole of the received signal, the reference setting unit 7a
set the reference voltage to a voltage Vref at the fourth peak P4.
The reference setting unit 7a gradually decreases the reference
voltage from the voltage Vref, and the time difference measuring
unit 7c accordingly measures a time difference between the signal
output from the comparing unit 7b and the signal output from the
judging unit 8.
[0080] Assuming that the reference voltage Vref is denoted by a
dotted line in FIG. 18 the judging unit 8 outputs a signal E which
falls down at the first zero-crossing point Ta in time after the
amplitude of the signal output from the amplifier 6 reaches the
voltage Vref. Then, the time difference measuring unit 7c measures
a time difference td between the signal E output from the judging
unit 8 and the signal output from the comparing unit 7b.
[0081] FIG. 19 illustrates a profile of the time difference td
measured by the time difference measuring unit 7c when the reference
voltage is decreased. The difference td is 1/4 of the period of
the ultrasonic wave at peaks (the second peak P2 the third peak
P3 . . . ) in the received signal. The time difference between the
peaks becomes larger according to a decrease of the reference voltage.
[0082] The time difference td and the reference voltage are correlated
with each other. The reference voltage is a voltage for each peak
at which the time difference td is 1/4 of the period of the ultrasonic
wave to be received. Then, the reference setting unit 7a stores
an intermediate voltage Vr2 between the second peak and the third
peak and an intermediate voltage Vr1 between the first peak and
the second peak, and then the mode for determining the reference
voltage of the reference comparator 7 terminates.
[0083] If the third peak P3 in the received signal is used for
detecting the arrival of the received, the controller 12 sets the
reference voltage to the voltage Vr2 between the second peak P2
and the third peak P3 and starts measuring the flow rate. According
to embodiment 6 the reference voltage is determined as being gradually
decreased from its maximum at the peak after the gain of the amplifier
9 is adjusted. Alternatively, the reference voltage may, with equal
success, be set to a desired level between any two adjacent peaks
determined according to the time difference measured by the time
difference measuring unit 7c, e.g., the second peak P2 and the third
peak P3 as being gradually increased from the first peak P1 where
the amplitude of the received signal is lowest.
[0084] According to embodiment 6 the reference voltage set to
the intermediate voltage between the two peaks in the received signal,
and the reference voltage may be determined similarly to embodiment
2 to a level between two specific peaks such that the level provides
a large margin against a change of the amplitude of the received
signal which is biased in (increasing or decreasing) aspect. More
particularly, the reference voltage may be set to 1/3 of the amplitude
between the two peaks adjacent to each other, as described in embodiment
2.
[0085] As set forth above, in the flow meter according to embodiment
6 the reference setting unit 7a determines and stores the reference
voltage based on the time difference between the signal output from
the comparing unit 7b and the signal output from the judging unit
8 measured by the time difference measuring unit 7c. The reference
voltage is determined according to the actual signal of the ultrasonic
wave, and is set to the optimum level, and therefore the flow meter
of embodiment 6 calculates the flow rate of the fluid accurately.
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