JP4671481B2 - Ultrasonic flow meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an ultrasonic flowmeter that measures the propagation time of ultrasonic waves in a fluid both upstream and downstream (forward direction) and downstream to upstream (reverse direction), calculates a flow velocity, and further obtains and integrates the flow rate. .
[0002]
[Prior art]
As an example of the measurement principle, as shown in FIG. 5, one transmitter / receiver 1 of a set of ultrasonic transducers arranged at the upstream and downstream of the flow tube 3 at a distance L in the fluid is sent and received from the other. The forward propagation time T ₁ to the correlator 2 is as follows: C is the sound velocity of the ultrasonic wave in the static fluid, and V is the flow velocity of the fluid.
T ₁ = L / (C + V)
It becomes.
[0003]
The backward propagation time T ₂ from the transducer 2 to the transducer 1 is
T ₂ = L / (C−V)
It becomes.
[0004]
From this propagation time T ₁ and T ₂ , the flow velocity V is
V = (L / 2) {(1 / T ₁ ) − (1 / T ₂ )}
Was asking.
[0005]
Further, in order to increase the resolution of the propagation time measurement, it is not necessary to simply measure the times T ₁ and T ₂ from one transmission to the reception, but to perform the next transmission at the same time as receiving a plurality of times (n times). ) By repeating, each of the n propagation times T ₁ and T ₂ is continued, and the time nT ₁ or nT ₂ from the first (first) transmission to the last (nth) reception is measured. Yes.
[0006]
A flow meter used in such a measuring method will be described with reference to a block diagram shown in FIG. 6 (hereinafter referred to as the first prior art).
[0007]
The transducers 1 and 2 are each composed of an ultrasonic transducer and can be used for both transmission and reception.
[0008]
Both transducers transmit and receive ultrasonic waves in the fluid from upstream to downstream and from downstream to upstream. The reception wave detection unit 4 outputs a reception wave detection signal when a reception-side transducer is connected and a reception wave is detected. When receiving the first transmission command signal from the control unit 6, the transmitter / receiver driving unit 5 first drives the transmitting-side transmitter / receiver, and thereafter drives whenever it receives the received wave detection signal from the received wave detection unit 4. However, when the nth received wave detection signal is received from the first counter 7, the driving is stopped until a new first transmission command signal is received thereafter.
[0009]
The first counter 7 counts the reception wave detection signal from the reception wave detector 4 and outputs the nth reception wave detection signal. The counter 7 is reset by a first transmission command signal from the control unit 6.
[0010]
The second counter 8 measures the accumulated propagation time from the first transmission command signal to the nth received wave detection signal. The control unit 6 reads the time (count value). In this example, the count value is cleared to zero by the first transmission command signal, and counting of the reference clock from the built-in reference clock generator is started.
[0011]
The control unit 6 inverts the transmission / reception switching signal at regular intervals to switch the roles of the two transducers 1 and 2.
[0012]
After each switching, the first transmission command signal is output after a time when noise or the like due to the switching is stopped each time. When the n-th received wave detection signal is input, the measurement (count value) of the counter 8 is read, and the flow rate flow rate between them is calculated using the measurement value in the reverse direction performed immediately before.
[0013]
In such a measurement method, there is a method of detecting a zero-cross point of a specific wave as a reception detection method for specifying a time when an ultrasonic wave reaches a transmitter / receiver on the receiving side, that is, an arrival point.
[0014]
This detection method will be described with reference to FIG.
[0015]
FIG. 7 shows a transmission drive signal and a reception wave indicating the timing of transmission. The actual received wave is very small and is first amplified. The received wave in the figure shows the waveform after amplification.
[0016]
a is the arrival point, and the amplitude gradually increases. After that, it becomes maximum amplitude and gradually decreases.
[0017]
However, the arrival point a is hidden behind noise and cannot be detected. Therefore, the following method is performed.
[0018]
A threshold V _TH as a reference voltage level that is sufficiently larger than noise is determined, and a zero cross point that passes through the zero level after the wave that first reaches this level, for example, the third wave in FIG. This is a method of detecting c and setting it as reception detection.
[0019]
The threshold value V _TH is always set so as to detect the zero cross point of a certain specific wave (for example, the third wave), and the actual arrival time T is the time τ from point a to point c. It is obtained in advance by storing and subtracting the time τ from the value corresponding to the measured time T + τ. The arrival time T corresponds to the propagation times T ₁ and T ₂ .
[0020]
As in the first prior art, the reception wave detection signal is received by the reception wave detection unit 4 and at the same time the transmitter on the transmission side is driven again by the transmitter drive unit 5 to perform the next transmission. There are the following disadvantages.
[0021]
The ultrasonic wave transmitted from the transmitter / receiver is reflected by the receiver / transmitter and returned to the transmitter / receiver, and further reflected by the transmitter / receiver. There are waves that reach the handset. This wave becomes noise. Such noise is hereinafter referred to as 1.5 round-trip noise.
[0022]
That is, as shown in FIG. 8, when the received wave A of the ultrasonic wave transmitted by the first drive by the first transmission command signal (not shown) is detected, the second drive is performed simultaneously with this detection and the received wave is received. B is detected, and the third drive is performed simultaneously with the detection of the received wave to detect the received wave C. When the received wave C is detected by the third drive, 1.5 reciprocations are performed by the first drive. The noise D reaches the receiving-side transducer, and the 1.5 round-trip noise D overlaps the received wave C. Although not shown in the figure, the received wave by the fourth drive overlaps with 1.5 round-trip noise by the second drive, and the same can be said for all the received waves hereinafter.
[0023]
Such 1.5 round-trip noise deforms the received wave, resulting in the result that the zero cross point of the specific wave is deviated from the normal time point, and adversely affects the arrival time measurement result.
[0024]
In particular, since the flow is stable near the flow rate of zero, the 1.5 round-trip noise has the same timing with respect to the received wave. Therefore, even if it repeats n times, the said bad influence will remain, without being averaged.
[0025]
Therefore, there is a drawback that the flow rate measurement accuracy is poor.
[0026]
In addition, since the distance between elements is short in a small flow meter, 1.5 round-trip noise is large and reverberation tends to remain.
[0027]
Therefore, the applicant of the present application has proposed an ultrasonic flowmeter that solves the above-mentioned drawbacks in Japanese Patent Application No. 9-173667 (Japanese Patent Laid-Open No. 11-23333) (hereinafter referred to as “second prior art”).
[0028]
This second prior art is provided with at least one pair of ultrasonic transducers acting on both the transmission side and the reception side, and transmits and receives ultrasonic waves in the fluid flow from upstream to downstream and from downstream to upstream, It is an ultrasonic flowmeter that determines the flow velocity and flow rate from the arrival time in each direction, and transmits the transmitter / receiver on the transmission side, and the received wave detector that receives the signal of the transmitter / receiver on the reception side receives the received wave. When detected, the transmitter / receiver on the transmission side is transmitted again, and this is repeated a certain number of times (n times), and the time from the first transmission to the reception of the certain number of times (n times) is measured. In the case where the arrival time is obtained from the result, a fixed time is set between the detection of the reception wave by the reception wave detection unit and the transmission side transmitter / receiver being transmitted again. Until a certain number of times Was ultrasonic flowmeter that was to determine the arrival time by correcting the time is long minute.
[0029]
[Problems to be solved by the invention]
However, since the ultrasonic wave transmitted from one transducer is reflected not only at the transducer but also at other locations, the time for generating 1.5 round-trip noise is considerably long. Therefore, in order to prevent 1.5 round-trip noise from overlapping with the original received wave as in the second prior art, the certain time (delay time) must be increased. Then, the accuracy of the fixed time (delay time) itself becomes a problem. Naturally, if a certain period of time is set between reception and the next transmission (transmission), the total arrival time increases accordingly. Therefore, a certain amount of time (delay time) is subtracted from the total arrival time and used in subsequent calculations. Since it is difficult to measure, a value obtained by measuring and storing a predetermined time (delay time) in advance by another means is used in the calculation. Therefore, the difference between the actual fixed time (delay time) and the stored value is a measurement error. In particular, if the fixed time (delay time) is large, the error is a large proportion of the true arrival time, and the obtained flow rate has a problem that includes a larger error.
[0030]
Further, if the predetermined time (delay time) is increased, the measurement time becomes longer, and the current consumption increases accordingly.
[0031]
Such problems of measurement error and increase in current consumption are caused by a sufficiently large delay time so that 1.5 round-trip noise does not overlap with the original received wave.
[0032]
Therefore, the present invention does not prevent 1.5 round-trip noise from overlapping with the original received wave, but aims at minimizing the effect on measurement error even if it overlaps. An object is to provide a sonic flow meter.
[0033]
[Means for Solving the Problems]
In order to achieve the above object, the invention of claim 1 is provided with at least one pair of ultrasonic transducers acting on both the transmitting side and the receiving side, and in the fluid flow from upstream to downstream and from downstream to upstream. An ultrasonic flowmeter that transmits and receives ultrasonic waves and obtains the flow rate and flow rate from the arrival time in each direction,
First, the transmitter / receiver on the transmission side is transmitted, and when the received wave detector that receives the signal of the transmitter / receiver on the reception side detects the zero cross point of the specific wave of the received wave, the transmitter / receiver on the transmission side is transmitted again. In this way, this is repeated a certain number of times (n times), the time from the first transmission to the zero-cross point of the specific wave of the received wave at the certain number of times (n times) is measured, and the arrival time is obtained from the result. It was something like
By thereby originating the transducer of the receiving wave detection unit again sender after detecting the zero-crossing point of a particular wave of a received wave, and transmitting performed without setting time, the resonance frequency of the ultrasonic transducer It is possible to select a transmission to be performed after half the period of time, and to select one of the two transmissions in succession twice and to select the other transmission twice in succession Are repeated alternately ,
An ultrasonic flowmeter that corrects the amount of time from the first transmission to the zero cross point of the specific wave of the received wave at a certain number of times due to the transmission performed at a predetermined time to obtain the arrival time. .
[0034]
The period of the ultrasonic wave is the period of the natural frequency (resonance frequency) of the ultrasonic transducer, and is also the period of the received wave.
[0035]
[Action]
The case where noise, for example, 1.5 round-trip noise overlaps with the received wave described in FIG. 7, will be described with reference to FIGS. In both figures, the vicinity of the zero cross point c of the third wave of the received wave shown in FIG. Note that the magnification of the time axis (horizontal axis) is larger than that of the vertical axis, so that the slope of the received wave A is gentler than in FIG. Further, the received wave is a sine wave whose amplitude gradually changes, but in FIGS. 1 and 2, only the vicinity of the zero cross is shown in an enlarged manner, and therefore the received wave A is a straight line with a lower right.
[0036]
In FIG. 1, the broken line indicated by symbol B is a received wave in which plus noise + e overlaps with the original received wave A, and when the plus noise overlaps in this way, the zero cross point moves from point c to point d, and the arrival time Shifts in the direction of increasing and an error occurs. FIG. 2 shows a received wave in which a negative noise -e is superimposed on the original received wave A by a symbol B ′. In this way, when the negative noise overlaps, the zero cross point moves from the point c to the point d ′ and shifts in a direction in which the arrival time is reduced, resulting in an error.
[0037]
In both figures, for simplicity of explanation, the noise is shown as a positive constant voltage + e or a negative constant voltage -e, but the actual 1.5 round-trip noise can swing in a substantially sinusoidal shape centered on the zero level. It is a waveform.
[0038]
As described with reference to FIGS. 1 and 2, an error occurs in the measured value of the zero cross point due to the influence of noise. In both figures, the noise voltage is described as positive or negative. However, in the case of 1.5 reciprocating noise, the period is the same as the period of the original received wave, that is, the period of ultrasonic waves. The error on the measured value of the zero cross point varies depending on the phase of the.
[0039]
Next, the operation of the present invention will be described with reference to FIG. In the figure, t is an ultrasonic half-cycle. Considering each reception, the transmission that is the 1.5 round-trip noise source that is affected is the previous three transmissions if the previous transmission is the previous one. Therefore, when the arrival time of the ultrasonic wave is T, the time from the transmission that is the target of 1.5 round-trip noise of each reception is
In case of reception 3 3T
In case of reception 4 3T + t
In case of reception 5 3T + 2t
In case of reception 6 3T + t
In case of reception 7 3T
In case of reception 8 3T + t
In case of reception 9 3T + 2t
In case of reception 10 3T + t
...
It becomes. That is, the time from the target transmission is shifted by a half period t of the ultrasonic wave every time. The 1.5 round-trip noise waveform is almost the same because the conditions are the same every time. Therefore, the difference in half cycle means a phase difference of 180 °, and the influence on the zero cross point is reversed every time and the magnitude is almost the same, so they cancel each other out. It will not be affected by.
[0040]
Moreover, in the present invention, the phase of the adjacent times is zero and one period with respect to the + half cycle, that is, a phase difference of −180 ° and + 180 °, which is 1.5 reciprocations as compared with the case of a simple 180 ° difference. The influence of noise can be effectively canceled out.
[0041]
DETAILED DESCRIPTION OF THE INVENTION
Next, a preferred embodiment of the present invention will be described with reference to the example of FIG.
[0042]
The transducers 1 and 2 are ultrasonic transducers that can be used for both transmission and reception. Both transducers transmit and receive ultrasonic waves in the fluid from upstream to downstream and from downstream to upstream.
[0043]
The reception wave detection unit 4 is connected to a transmitter / receiver on the reception side as in the prior art, and outputs a reception wave detection signal when a specific wave of the reception wave, for example, a zero cross point of the third wave is detected. When receiving the first transmission command signal from the control unit 6, the transmitter driving unit 5 first drives the transmitting-side transmitter / receiver, and thereafter drives every time a received wave detection signal is received from the received wave detection unit 4. To do.
[0044]
However, the received wave detection signal is input to the transmitter drive unit 5 via the reception point transmission unit 9, and the reception wave detection signal is transmitted without delay or is delayed by time t corresponding to a half cycle. Can be selected by the control input.
[0045]
Further, when the nth received wave detection signal is received from the counter 7, the driving is stopped thereafter until the first transmission command signal is newly received. The counter 7 counts the received wave detection signal from the received wave detection unit 4 and outputs the nth received wave detection signal when the nth received wave detection signal is received. The counter 7 is reset by a first transmission command signal from the control unit 6.
[0046]
The counter 8 measures the time from the first transmission command signal to the nth received wave detection signal. The control unit 6 reads the time (count value). In this example, the count value is cleared to zero by the first transmission command signal, and the counting of the reference clock from the built-in reference clock generator is started.
[0047]
The control unit 6 switches the roles of the two transducers 1 and 2 by inverting the transmission / reception switching signal at regular intervals.
[0048]
After each switching, the first transmission command signal is output after a time when noise or the like due to the switching is stopped each time. When the nth received wave detection signal is input, the measurement value (count value) of the counter 8 is read, and the flow rate flow rate between them is calculated using the measurement value in the reverse direction performed immediately before.
[0049]
The increment of the measured value of the counter 8 due to placing a certain time t in the transmission from 2 out of 4 transmissions is measured in advance, stored in the control unit 6, and subtracted when calculating the flow velocity. It is corrected.
[0050]
The reception point transmission unit 9 is a circuit of FIG. 4B that inputs the reception wave detection signal from the reception wave detection unit 4 and the output of the counter 10 to the control input, and is a time constant circuit composed of a capacitor C and a resistor R. The delay time t of the half cycle is determined by the time constant CR.
[0051]
The counter 10 counts the rising edge of the received wave detection signal, and its Q2 output is a signal that repeats high and low every two pulses of the received wave detection signal. In this embodiment, when the Q2 output is high, that is, when the control input of the receiving point transmission unit 9 is high, a signal delayed by a half cycle t of the ultrasonic wave is transmitted to the transmitter driving unit 5. Yes. The transmitter driver 5 drives the transmitter-side transmitter / receiver at the falling edge of the signal from the reception point transmitter 9.
[0052]
In this way, the transmission of ultrasonic waves is repeated in the order of delay, none, half cycle t, and half cycle t.
[0053]
【The invention's effect】
The ultrasonic flowmeter of the present invention is configured as described above, and the delay time from the detection of the zero cross point of the specific wave of the received wave to the next transmission is compared with the arrival time, which is a half cycle of the ultrasonic wave. Therefore, the delay time itself does not adversely affect the measurement accuracy. And the error resulting from 1.5 round-trip noise can be reduced effectively.
[0054]
In addition, since the delay time (t) is short, the time required for the measurement is shortened, the flow rate can be measured with low power consumption, and it contributes to the practical use of an ultrasonic flow meter using a battery as a power source.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating the operation of the present invention.
FIG. 2 is a diagram illustrating the operation of the present invention.
FIG. 3 is a diagram illustrating the operation of the present invention.
4A is a block diagram, FIG. 4B is an electric circuit diagram of a main part, and FIG. 4C is a timing diagram according to an embodiment of the present invention.
FIG. 5 is a diagram illustrating the principle of an ultrasonic flow meter.
FIG. 6 is a block diagram of a conventional ultrasonic flowmeter.
FIG. 7 is a diagram for explaining a zero cross point of a received wave.
FIG. 8 is a diagram showing the relationship between a received wave and 1.5 round-trip noise according to the prior art.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1, 2 Ultrasonic transmitter / receiver 4 Received wave detection part 5 Transmitter drive part 6 Control part 7, 8, 10 Counter 9 Reception point transmission part t Delay time T Arrival time c, d, d 'Zero cross point

Claims (1)

Provide at least one pair of ultrasonic transducers acting on both the transmitting and receiving sides, and transmit and receive ultrasonic waves from upstream to downstream and from downstream to upstream in the fluid flow. Furthermore, an ultrasonic flowmeter for obtaining a flow rate,
First, the transmitter / receiver on the transmission side is transmitted, and when the received wave detector that receives the signal of the transmitter / receiver on the reception side detects the zero cross point of the specific wave of the received wave, the transmitter / receiver on the transmission side is transmitted again. In this way, this is repeated a certain number of times (n times), the time from the first transmission to the zero-cross point of the specific wave of the received wave at the certain number of times (n times) is measured, and the arrival time is obtained from the result. It was something like
By thereby originating the transducer of the receiving wave detection unit again sender after detecting the zero-crossing point of a particular wave of a received wave, and transmitting performed without setting time, the resonance frequency of the ultrasonic transducer It is possible to select a transmission to be performed after half the period of time, and to select one of the two transmissions in succession twice and to select the other transmission twice in succession Are repeated alternately ,
The ultrasonic flowmeter which correct | amends the part which the time from the said first transmission to the zero cross point of the specific wave of the received wave of the fixed number of times by the transmission performed after the said time is calculated | required, and calculates | requires the said arrival time.

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