Flow meter

Details Flow meter Flow meter

2003-04-09

2004-08-10

Patel, Harshad (Department: 2855)

Measuring and testing

Volume or rate of flow

By measuring vibrations or acoustic energy

C073S861270, C702S051000

Reexamination Certificate

active

06772643

ABSTRACT:

FIELD OF THE INVENTION
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
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
.
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·S·v=K·S·L/
2·(
n/t
1
−n/t
2)·cos &phgr;  (Equation 1)
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, &phgr; is an angle formed by a line extending between the ultrasonic wave oscillators
32
,
33
and the flowing direction, and K is a constant determined according to the flow rate.
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.
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 P
3
. This assures a margin against the case that the second peak P
2
increases or the third peak P
3
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.
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 P
3
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 P
4
, thus causing the judging unit to detect the first zero-crossing point after the fourth peak P
4
. 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
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 re

Affiliated with

Also associated with

Rating

Flow meter does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Flow meter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Flow meter will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3342925