JPS6242015A - Temperature correcting method for ultrasonic flow meter - Google Patents

Abstract

PURPOSE:To obtain the correct propagating time even when the vibrator resonance frequency is changed together with temperature by setting the measuring point of the third propagating time to the maximum vicinity of the receiving signal in the system to deduct the time for several waves from the rise of the flow meter to next maximum zero cross point and measure the true propagating time. CONSTITUTION:A receiving signal (d) obtained from the ultrasonic flow meter is changed to the signal having the first gate off width (a) by the first gate 29, an obtained receiving signal (e) is compared with the first threshold (b) and the first time measuring completing signal (f) is obtained. Thus, when the first propagating time (g) is measured and the first gate receiving signal comes to be the maximum, the second time measuring completing signal (h) is sent, and thus, the second propagating time (i) is measured, the second gate off width (j) is set and the second gate receiving signal (k) is obtained. Therefore, the third time measuring completing signal (m) is obtained from the first zero cross point of the signal (k), the third propagating time (m) is measured, the time from the zero cross point to the next zero point equivalent to serveral periods is measured, divided by the number of the included wave and a period signal (o) is obtained.

Description

[Detailed Description of the Invention] [Field to which the invention pertains] The present invention relates to a temperature correction method for an ultrasonic flowmeter, and in particular, it involves disposing ultrasonic transducers upstream and downstream of a flow for mutual transmission and reception. The present invention relates to a temperature correction method for a non-insertion type transmission type ultrasonic flowmeter that measures the flow rate by taking advantage of the difference in propagation time from transmission to reception depending on the flow velocity of the fluid.

[Prior art and its problems]

This type of non-insertion type transmission type ultrasonic flow meter is shown in FIG.

Ultrasonic transducers (2a) and (2b) are arranged in a fluid-guiding pipe (1) with their positions shifted from each other in the direction of movement of the fluid, and the ultrasonic waves emitted from the ultrasonic transducer (2a) are The time required for the ultrasonic waves emitted from the ultrasonic transducer (2b) to reach the ultrasonic transducer (2b)
The flow rate of the fluid in the pipe (1) is determined from the velocity of the fluid in the pipe (1) based on the time difference between the time at which it reaches point a). As an ultrasonic transducer used in this manner, one having a structure shown in FIG. 2 is known. Reference numeral (3) in the figure is an ultrasonic transducer. This ultrasonic transducer (3)
For example, a piezoelectric element made of zircon-lead titanate-based ceramics is molded onto a disk, and silver electrodes are deposited on both sides of the piezoelectric element. It is for transmitting the ultrasonic wave to the pipe (1), and is made up of ultrasonic propagation members such as epoxy resin, acrylic resin, and metal. In addition, the bottom surface (5) of the wedge (4) is closely attached to the pipe (1) with an intervening material (6) for acoustic matching when the ultrasonic transducer is used. The ultrasonic transducer mounting surface of this wedge (4) (the force is applied to the bottom surface (
5), so that the ultrasonic waves generated from the ultrasonic transducer (3) are radiated obliquely to the bottom surface (5). When an alternating current of a predetermined frequency is applied to the ultrasonic vibrator (3) through the lead wires (81, (9)), the ultrasonic vibrator (3) vibrates at the same frequency and generates an ultrasonic pulse signal P. This ultrasonic pulse signal P passes through the wedge (4), enters the fluid to be measured (00) via the intervening material (6) and the pipe (1), and is reflected on the opposing surface of the pipe (1). Then, the ultrasonic transducer (
3) and is converted into a measurement signal by an electric circuit connected to lead wires (8) and (9). In this manner, the ultrasonic transducer shown in FIG. 5 transmits and receives ultrasonic waves.

In a non-insertion type transmission type ultrasonic flowmeter having such a configuration, if the temperature of the ultrasonic vibrator changes due to a change in fluid temperature or a change in ambient temperature, the following drawbacks occur. The resonant frequency of an ultrasonic transducer has temperature dependence. Figure 6 shows temperature characteristic data of the resonant frequency for each transducer provided by the transducer manufacturer. As shown in Figure 6, if the vibration frequency of the vibrator is IMHz at 20°C and the temperature characteristic is 300ppm/l, then at I°C it is 1.015MHz.
At 1too°C, it becomes 1.030MHz〇On the other hand, the resonant frequency of the vibrator is determined by the product of the frequency constant determined by the material of the vibrator and the thickness of the vibrator〇Generally, the thickness of the vibrator is set to 1 μm for a given dimension. It is difficult to match with precision and is also a cause of high costs. Figure 7 shows variations in the resonant frequencies of the delivered ultrasonic transducers.

However, no matter how precisely this is finished to the specified dimensions, it cannot be used as a single vibrator; in the case of a flowmeter, the vibrator is glued to a wedge, so the resonant frequency in this case is no longer the same as when using a single vibrator. This is different from the resonant frequency. Table 1 shows the difference in resonant frequency between a single vibrator and a wedge.

Table 1 From the above, two or more ultrasonic vibrators used in an ultrasonic flowmeter always have a frequency difference of several percent of the resonant frequency. On the other hand, when an ultrasonic flowmeter is applied to a small-diameter pipe, it depends on the thickness, inner diameter, and fluid to be measured of the pipe, but for example, if the pipe is 3.2 m thick, the inner diameter is 25 mm, and the fluid to be measured is water, one of the The propagation time of the sound wave from when it is transmitted from the ultrasonic transducer to when it is received by the ultrasonic transducer is approximately 5 μ in the V method and 12.5 μ in the Z method, and the fluid flow velocity is 1 m/s. The propagation time from the transmission of the ultrasonic transducer (2a) to the reception of the ultrasonic transducer (2b) and the propagation time from the transmission of the ultrasonic transducer (2b) to the reception of the ultrasonic transducer (2a) when The propagation time difference with V method is about 25nsl! c.
, in the case of the Z method, it is approximately 12.51 seconds. Since the measurement accuracy of a commercially available ultrasonic flow meter at a flow rate of 1 m/s is 1%, it is necessary to measure this propagation time difference with at least 0.1 nsec. Measurement of propagation time is performed as follows. The moment the ultrasonic transducer (2a) in the transmitting state is transmitted, it starts time measurement and the ultrasonic transducer (2a)
When the received wave in b) reaches the desired darkness value, the time measurement ends.0 In the case of an ultrasonic flowmeter, the ultrasonic transducer (2b) in the receiving state is used to detect waves other than the sound waves propagating in the fluid. , for example, also receives noise sound waves such as wrap-around waves propagating around the pipes. Therefore, this dark value is due to noise,
It is set higher than the noise sound wave level to avoid accidentally measuring the propagation time. In other words, the received wave at the point exceeding the dark value is not the sound wave that propagated at the moment the ultrasonic transducer (2a) was transmitted, but the sound wave that was transmitted several waves later. 0 In other words, the measured propagation time (the time from the moment of transmission until the received wave exceeds the dark value) is the true propagation time (from the moment of transmission,
The ultrasonic transducer (2b) receives the sound wave at the moment it is transmitted.
It consists of the sum of the time it takes for the signal to be received by the receiver) and the time corresponding to the period of several waves of the sound wave. The measured propagation time from upstream to downstream, downstream to upstream, even if the flow velocity and true propagation time are the same,
This results in a difference in time corresponding to a period difference of several waves of sound waves due to the difference in the resonance frequency of the two vibrators, and it is indicated as if there is a flow even though the flow velocity is zero. When the temperature changes, the resonant frequency difference of the vibrator changes depending on the temperature, and the time difference corresponding to the period difference of several waves of sound waves changes, giving an instruction as if the flow velocity had changed. Figure 8 shows how the propagation time difference occurs due to the resonance frequency difference of the ultrasonic transducer when the flow velocity is zero.0 The upstream transmission wave αυ is sent from the upstream transducer and propagated through the piping by the downstream transducer. The sound wave consisting of the wrap-around wave (19) and the underwater propagation wave (a) in the pipe that has propagated through the pipe and the fluid is received, and the downstream received wave 03
get. When this downstream received wave 03) exceeds a predetermined threshold value (15), a downstream received wave presence signal QO is output. The time from this upstream side transmission to the downstream side received signal is the sum of the propagation time of the light from the transmission to point A and the time from the 3-cycle child in the figure to the dark value αω. A similar waveform is obtained when transmitting from the downstream transducer to the upstream transducer. Since the time from transmission to point A is the same on both the upstream and downstream sides, we will examine the time from point A to the dark value (15). The downstream transducer receives the sound wave with the transmission frequency fl from the upstream transducer, and the upstream transducer receives the sound wave with the transmission frequency f2 from the downstream transducer, so the threshold value ( 15
) (from point A to point B, from point A to 0 point) is naturally 0, i.e., (1/ f+ i, /rz) * (3 period fertility to darkness value) time difference is 0. Therefore, Even though the flow velocity is zero, a propagation time difference (from point B to 0 point) occurs, and the flow velocity is displayed as if it were not zero. If the temperature of the fluid changes, the resonance frequency difference of the upstream vibrator also changes. Therefore, the indicated value also changes, giving the false impression that the flow velocity has changed.On the other hand, if this dark value (19) is set to a low level, the effect of the change in the resonant frequency of the vibrator is small, but the point A The received wave on the 0th floor near the point is a wrap-around wave (a superimposed wave on the underwater propagation wave side in the pipe that has propagated through the pipe and the fluid), S
The poor /N ratio causes problems such as poor accuracy in propagation time measurement, leading to a dilemma. In this way, conventional ultrasonic flowmeters have a narrow applicable temperature range of about 0 to 40 degrees Celsius.
Outside this applicable temperature range, there is a problem in that the flow rate indication value is offset and sufficient measurement accuracy cannot be obtained. An object of the present invention is to provide a temperature correction method for a non-insertion type transmission type ultrasonic flowmeter, which is capable of reducing the offset of the flow rate indication value caused by the noise and measuring the flow rate with high accuracy.

[Key points of the invention]

In order to achieve the above object, the present invention measures the time (1'') from transmission to the zero cross point after the received wave reaches its maximum, and sets the time to be slightly larger than the level of the wraparound wave of the received wave. Compare the darkness value with the received wave level] -1 Calculate the time E k corresponding to the moment of transmission in the received wave, and calculate the time from the time (E) corresponding to the moment of transmission in the received wave to the zero loss point (by DJ Calculate the wave number of the included sound wave, find the product of the separately measured period near the maximum of the received wave and the wave number of this sound wave, and calculate the product of the period and the wave number of the sound wave from the time p) until the zero cross point. (F') and calculate the time fG) corresponding to the moment of transmission of the received wave.
The measurement is carried out from downstream to upstream respectively, and the true propagation time is determined by correcting the influence of the difference in the resonant frequency of the vibrator included in the measured propagation time, and the flow rate is determined with high accuracy.

[Embodiments of the invention]

An embodiment of the temperature correction method for an ultrasonic flowmeter according to the present invention will be described below with reference to FIGS. 1 to 3.

In FIG. 1, reference numeral 21 indicates a measurement condition input section, into which the pipe diameter, pipe thickness, and installation position of an ultrasonic transducer and the like are input. A first gate off width signal is input from the measurement condition input section 21 to the first gate off width setting section 22, and the first gate off width signal is input to the first gate section 4. Also, from the measurement condition input section 21, the first
The signal sent to the threshold setting section 5 is input to the first combiner 30 as a signal indicating the first threshold value. The above-mentioned first gate-off width a is determined by calculating the time (estimated propagation time) from when the underwater propagation wave in the pipe is received by the receiver 24 after it is transmitted from the transmitter n, and from this estimated propagation time, it is 5 to 8 μs. It is set by subtracting the degree. On the other hand, the first threshold value is
It is set to a value slightly larger than the value of the wraparound wave, which is related to the pipe diameter, pipe thickness, placement position of the ultrasonic transducer, etc. input in the eleventh initial value setting section 5. Also, when transmitting sound waves, the transmission timing circuit is used, and the transmission pulse C is transmitted from the transmission circuit n.
is input to , and transmit +5 is excited. This sound wave is received by the receiver 24, and the received signal d is input to the receiving amplifier path, amplified, and then input to the first gate section 4.

The first gate unit 4 that receives the received signal d selects a signal with the first gate off width a, obtains the received signal from the vicinity where the wrap-around wave has slightly occurred, and obtains the first gated received signal e'a'. Input to I. The first ”:'7 parator addition compares the first gated received signal e and the first a value, and
When the gated reception signal e exceeds the first fA value S, a first time measurement end signal f is sent out and input to the first time measurement circuit 31. The first time measurement circuit 31 measures the first propagation time g from the transmission pulse C from the transmission timing circuit 26 and the first time measurement end signal f, and inputs the result to the calculation unit 40 .

On the other hand, the maximum value of the first gated reception signal e is detected by the maximum value detection circuit 32, and this detection circuit 32 detects the second time measurement end signal when the first gated reception signal e reaches the maximum value. The signal is sent out and input to the second time measurement circuit 33.

The second time measurement circuit 33 measures the second propagation time i from the transmission pulse C from the transmission timing circuit 26 and the second time measurement end signal, and inputs it to each second gate-off width setting section. The second gate off width J output from the second gate off width setting part is input to the second gate part A, the first gated reception signal e is selected, and the second gated reception signal k is outputted, and this output signal is It is input to the second comparator 37, where it is compared with the second value t of the second #l value setting section Akara, and the second
When the gated reception signal k exceeds the second threshold, a third time measurement end signal m is sent and input to the third time measurement circuit. This third time measurement circuit calculates a third propagation time n from the transmission pulse C and the third time measurement end signal m, and inputs it to the calculation section 40.

Furthermore, the period measuring circuit 39 measures the periods of several waves after the received signal d reaches its maximum value, and sends out a periodic signal O, which is input to the calculation section 40. In this calculation section 40, the first propagation time Calculate the true propagation time p from g, second propagation time 1 and period 0 In other words, to calculate the true propagation time, first propagation time is X1 second propagation time is Y1 period is Z, true propagation time is W Then, 0 W=Y-n(Z-2), which is calculated by the following formula. However, if Y-X ) / (Z/2) = n, n=n(Y-X)/(Z/2) =n+c if n
=n+10 is an integer Figure 2 is a flowchart showing the operation of propagation time correction according to the present invention, and Figure 3 is a flowchart showing the operation of propagation time correction according to the change in the resonance frequency of the ultrasonic transducer when the flow velocity is zero. The received signal (fd) is gated by the first gate off width (at) to obtain the first gated received signal (e), and the received signal (fd) is gated by the first gate off width (at) to obtain the first gated received signal (e).
, and obtain the first time measurement end signal). This measures the first propagation time (g). When the first gated reception signal reaches the maximum, the second time measurement end signal (h) is sent, the second propagation time (i) is measured from this, the second gate off width (j) is set, and the second time measurement end signal (h) is measured. Di-gated received signal (k)
get. A third time measurement end signal h-+) is obtained from the first zero cross point of the second gated received signal (k), and the third propagation time (n) is measured. The time from this zero cross point to the next zero cross point corresponding to several cycles is measured, divided by the number of waves included in it, and the periodic signal (o)
get.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, the time from transmission to the next maximum zero cross point of the received signal by the receiver is measured, and the period of several waves after the maximum of the received signal is measured, In the method of measuring the true propagation time by subtracting the time corresponding to the wave number from the rising edge of the received signal to the next maximum zero crossing point from the time measurement value, (11. The point at which the third propagation time is measured is Since it is set near the maximum of the received signal, the influence of wrap-around waves on time measurement is reduced, and the accuracy of propagation time measurement is improved. (2) From the third propagation time, from the rise of the received signal to the next maximum zero cross point. By subtracting the time by the number of waves to find the propagation time, even if the resonant frequency of the vibrator changes with temperature, it is possible to always find the correct propagation time with temperature correction.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a propagation time correction circuit using an ultrasonic flowmeter according to the present invention, FIG. 2 is a flow chart showing a propagation time correction operation according to the present invention, and FIG. Fig. 4 is a schematic diagram showing the arrangement of a non-insertion type transmission type ultrasonic flowmeter, and Fig. 5 is a time chart showing the correction of propagation time due to changes in the resonance frequency of the ultrasonic transducer. Fig. 6 is a diagram showing the temperature characteristics of the resonant frequency of the ultrasonic transducer; Fig. 7 is a diagram showing the variation of the ultrasonic transducer; Fig. 8 The figure is a diagram showing how the propagation time difference occurs due to the resonance frequency difference of the ultrasonic transducers when the flow velocity is zero. 1... Arrangement s 2a, 2b... Ultrasonic transducer,
3. Ultrasonic transducer, 4. Wedge, 10. Fluid.
11...Upstream side transmission wave, 12...Downstream side transmission wave, 1
3... Downstream side received wave, 14... Upstream side received wave, 1
5... Threshold value, 18... Propagation time difference, 19... Surrounding wave, addition... Underwater propagation wave, 21... Measurement condition input section, n... First gate off width setting section, field... ... Transmitter, 24... Receiver, 5... First threshold value setting section, 26.
...Transmission timing circuit, four...first gate section, 31
...First time measurement circuit, 33...Second time measurement circuit, 34...Second gate off width setting section, A...Second gate section, 36...Second darkness value setting section, Feather...Third time measurement circuit, 39...Period measurement circuit, 40...Calculation section

Claims (1)

[Claims] 1. A pair of ultrasonic transducers are disposed facing each other on the outside of the pipe so that sound waves propagate obliquely to the flow, from the upstream side to the downstream side, and from the downstream side to the upstream side. In an ultrasonic flowmeter that transmits and receives sound waves alternately, measures the propagation time of the sound waves propagating in both directions, and calculates the flow rate by determining the flow velocity from the difference in this propagation time, the wave from the transmission to the reception is the maximum. Measure the time until the zero cross point after the signal has passed, compare the received wave level with a threshold set slightly larger than the level of the wraparound wave of the received wave, and find the time corresponding to the instant of transmission in the received wave. Calculate the wave number of the sound wave included in the time from this time to the zero cross point, find the product of the separately measured period near the maximum of the received wave and the wave number of this sound wave, and calculate the period from the time to the zero cross point. A temperature correction method for an ultrasonic flowmeter, characterized in that the product of the wave number of the received wave and the wave number of the sound wave is subtracted, the time corresponding to the moment of transmission of the received wave is calculated, and the true propagation time is obtained. 2. In the temperature correction method for an ultrasonic flowmeter as set forth in claim 1, the measurement point of one cycle of the received wave is set near the maximum of the received wave, and the propagation time measurement interval is set from the transmission to the maximum of the received wave. A temperature correction method characterized in that the temperature is corrected up to the zero cross point after the temperature change. 3. In the temperature correction method for an ultrasonic flowmeter according to claim 1, the received wave reaches its maximum when the wave number included in the propagation time reaches a level slightly larger than the wraparound wave. A temperature correction method characterized by using a wave number included up to a zero cross point after