JP2002013958A - Ultrasonic flow meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve reliability by optimizing a trigger level for the reception wave-form of an ultrasonic flow meter. SOLUTION: This ultrasonic flow meter is provided with ultrasonic sensors 1A and 1B, a transmitting/receiving circuit section 31 transmitting and receiving ultrasonic waves, a comparing section 32, a propagation delay time measuring circuit section 33, a CPU 34, and a D-A converter 35. The comparison level to the ultrasonic signal received by the transmitting/receiving circuit section 31 is changed via the CPU 34 and the D-A converter 35, a comparison level/ propagation delay time characteristic is obtained by the CPU 34 via the propagation delay time measuring circuit section 33, and the measurement precision is improved by optimizing the comparison level from this characteristic.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic flowmeter for measuring the flow velocity and flow rate of a fluid using ultrasonic waves, and more particularly to an ultrasonic flowmeter capable of optimizing a trigger level of an ultrasonic signal. It is.

[0002]

2. Description of the Related Art FIG. 6 shows a conventional example of this kind. In other words, the ultrasonic flowmeter is largely composed of the ultrasonic sensors 1A and 1B,
It comprises a pipe 2 and a converter 3. The pipe 2 has a pair of ultrasonic sensors 1A and 1B disposed so as to sandwich the center line s and to face the center line s at a predetermined angle θ and mutually transmit and receive ultrasonic signals. . The pair of ultrasonic sensors 1A and 1B include an ultrasonic sensor 1
A converter 3 for converting signals from A and 1B into flow velocity and flow rate is connected.

In such a configuration, the ultrasonic flow meter transmits an ultrasonic signal from one ultrasonic sensor (for example, 1A) and receives the transmitted ultrasonic signal from the other ultrasonic sensor (1B). The flow rate of the fluid FL in the pipeline is measured by alternately switching the transmission and reception of the ultrasonic signal and measuring the difference between the propagation time of the ultrasonic signal to the upstream side and the propagation time of the ultrasonic signal to the downstream side. The flow rate can be measured by multiplying the flow velocity by the sectional area of the pipe 2. The relational expression for obtaining the flow rate is as follows. Both "propagation" and "propagation" are used, but have the same meaning.

T1 = L / (C + Vcosθ) (1) t2 = L / (C−Vcosθ) (2) From the equations (1) and (2), V = L (1 / t1-1 / t2) / 2 cos θ (3) Q = (π / 4) × D ² × V × K (4) The meanings of the above symbols are as follows. t1: Propagation time of ultrasonic waves transmitted from ultrasonic sensors 1A to 1B t2: Propagation time of ultrasonic waves transmitted from ultrasonic sensors 1B to 1A C: Sound velocity of fluid L: Ultrasonic sensor 1
Distance between A and 1B V: Measured flow velocity K: Flow velocity distribution coefficient D: Inner diameter of pipe 2 Q: Flow rate θ: Angle between pipe axis and sensor installation axis

[0005] An ultrasonic signal from one ultrasonic sensor is
The signal is amplified by the reception circuit in the transmission / reception circuit unit 31 in FIG. 6, is compared with the reference value VR by the comparison unit 32, is binarized, and the arrival time of the first pulse is regarded as the propagation time of the ultrasonic signal. Propagation time measurement circuit 33
Measure the propagation time. Higher-order arithmetic circuit (CPU) 34
Calculates the flow velocity or flow rate from the propagation time by performing calculations as in the above equations (3) and (4). FIG.
Shows the timing and waveform of the comparison of the ultrasonic signal. That is, the ultrasonic signal 7B is compared with the reference potential 7A (see the comparator output 7C), and the pulse which is first compared, for example, the time until the second wave of the ultrasonic signal 7B is determined as the propagation time 7D of the ultrasonic signal. Is shown. The propagation time of the ultrasonic signal is not limited to the second wave, and may be the propagation time of another wave.

[0006]

However, in the system as shown in FIG. 6, the ultrasonic flowmeter is installed due to a change in the installation conditions (temperature, pressure, etc.), a repair exchange of the ultrasonic sensor itself, and an aging. When the waveform itself of the sound signal changes and the comparator output 8C does not appear at the first peak of the ultrasonic signal 8B, that is, the second wave as shown in FIG. 8, the propagation time 8D becomes Δt as shown in FIG. This may be longer than in the case of FIG. 7, and this may cause a large error in the flow rate measurement.

[0007] The flow velocity V in consideration of the above ΔT
V = L {1 / t1-1 / (t2 + ΔT)} / 2cos θ (5) When the ultrasonic signal frequency is 100 KHz and the pipe diameter is 50 A (50 mm), the ΔT is about 20 m / s in terms of flow velocity, which is an extremely large error of 66% for a full-scale flow rate of 30 m / s. Therefore, the error due to ΔT cannot be ignored.

[0008] As described above, when the initially assumed ultrasonic signal waveform is different due to a change in installation conditions, repair / replacement of the ultrasonic sensor itself, aging, etc., the comparison position of the ultrasonic signal is shifted, and as a result, the measurement is performed. There is a problem that a large error occurs in the flow rate. As a countermeasure, a method of adjusting the reference potential (trigger level) of the comparator with respect to the ultrasonic signal for each installation condition of the flow meter can be considered. There is also a problem that adjustment must be performed, and it is difficult to realize. Therefore, an object of the present invention is to provide a high-accuracy ultrasonic flowmeter that, even when an ultrasonic signal other than the assumed one is received, causes the trigger level to follow the changed ultrasonic signal so that the optimum level is obtained. Is to do.

[0009]

In order to solve such a problem, according to the first aspect of the present invention, a pair of ultrasonic waves which are opposed to each other are arranged along a flow of a fluid or inclined at a predetermined angle in the flow direction. A sensor, a transmitting and receiving unit that alternately performs a transmitting and receiving operation of transmitting an ultrasonic signal from one ultrasonic sensor and receiving the other, and a comparing unit that compares the ultrasonic signal with a predetermined reference potential; An ultrasonic flowmeter comprising a time measuring unit for measuring a propagation time from transmission to reception of an ultrasonic signal based on an output, and a calculating unit for calculating a flow velocity or a flow rate of a fluid based on the propagation time obtained from the time measuring unit. In the reference unit, the reference potential is changed from the center of the ultrasonic signal waveform to the maximum amplitude to determine the reference potential vs. propagation time characteristic of the ultrasonic signal, and the ultrasonic signal waveform to be detected from this characteristic. Seeking suitable trigger level, and obtains the propagation time of the ultrasonic signal on the basis of this optimal trigger level.

[0010]

FIG. 1 is a block diagram showing an embodiment of the present invention. As is clear from the figure, the D / A (digital / analog) conversion unit 3 shown in FIG.
The characteristic feature is that 5 is added. Hereinafter, the differences will be mainly described. An ultrasonic signal received by one of the ultrasonic sensors, for example, 1B, is amplified by a reception circuit in the transmission / reception circuit unit 31, and is compared by the D / A conversion unit 35 in the comparison unit 32.
Is compared with the reference value TRG given through
The arrival time of the pulse that has been converted into the value and first compared is treated as the propagation time of the ultrasonic signal, and the propagation time is measured by the propagation time measurement circuit unit 33 in the subsequent stage.

Next, in order to capture the waveform pattern of the ultrasonic reception signal, the reference value (trigger level) TRG from the D / A converter 35 is set at the center of the ultrasonic reception waveform (the center value of the waveform having both positive and negative polarities). ) To the maximum value, and
Is measured by the propagation time measuring circuit 33 as time data. Thereby, it is possible to obtain a pattern indicating the relationship between the trigger level and the propagation time, and by using this, even if the ultrasonic reception signal changes, it is possible to determine the optimum trigger level corresponding thereto, Trigger errors can be eliminated. The control of the D / A converter 35 for changing the trigger level as described above and the processing of collecting or tabulating the ultrasonic signal propagation time data for each trigger level are performed by a higher-order arithmetic circuit (CPU). 3
4 can be easily performed.

FIG. 2 shows an example of the relationship between the ultrasonic signal waveform and the trigger level versus the propagation time. Here, for the ultrasonic signal 2D, when the trigger level TRG is within 2A from a position slightly above the center of the waveform, the trigger level is compared with the first wave of the ultrasonic signal 2D, and the amplitude of the received waveform is The propagation time changes from T1 to T2 according to the change. If the trigger level slightly exceeds the maximum value of the first wave (the trigger level is
If it is slightly raised from the position of A), the propagation time is measured as T3. Similarly, the position of the trigger level is from 2A to 2B
In the case of, the propagation time is measured from T3 to T4, and
When the trigger level is from 2B to 2C, the propagation time is from T5 to T6.

In this way, when the trigger level is changed from the center of the ultrasonic signal to the maximum value, it can be grasped as a propagation time change pattern with respect to the trigger level of the ultrasonic signal. FIG. 3 shows a change pattern with respect to the ultrasonic signal of FIG.
Shown in Here, the first wave, the second wave, and the third wave of the ultrasonic signal
The wave appears stepwise in FIG. 3 and is between the propagation times T2 and T3, T
4 and T5, there is a time difference of about the cycle of the ultrasonic signal, between T1 and T2, between T3 and T4, and between T5 and T5.
Since it is sufficiently larger than the number between the six, the distinction of the wave number of the ultrasonic signal can be easily determined by the upper-level arithmetic circuit unit (CPU) 34, and the change pattern as shown in FIG. 3 can be easily tabulated. When the second wave is the target waveform in FIG. 3, for example, the most stable position of the trigger level is
(2A + 2B) / 2, which is just half of the peak value of the first wave and the peak value of the second wave, which is the optimum value of the trigger level.

FIG. 4 shows an example of the relationship between the trigger level and the propagation time when the ultrasonic signal waveform changes, and FIG. 5 shows the change pattern at that time. FIG. 4 shows an example in which the amplitude of the second wave of the ultrasonic signal 4D is lower than that in FIG. 2, but even in such a case, the propagation time T4 for the trigger level 4B and the propagation time for the trigger level 4A Only a change in the time (T4−T3) between T3 and T3 results in a change pattern as shown in FIG. Thus, even when the ultrasonic signal waveform changes, it is possible to obtain the optimum trigger level for the ultrasonic signal waveform in the same manner as in the above case.

[0015]

According to the present invention, the trigger level corresponding to the change of the received waveform is obtained by changing the trigger level for the ultrasonic waveform and obtaining the change pattern of the propagation time of the ultrasonic signal corresponding to the change. Can be determined. This makes it possible to optimize the trigger level even when the ultrasonic signal waveform is constantly changed due to changes in various conditions, and has the advantage that highly accurate measurement without trigger errors can be performed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a relationship between a certain ultrasonic signal waveform, a trigger level, and a propagation time.

FIG. 3 is an explanatory diagram showing an example of a change pattern with respect to the waveform of FIG. 2;

FIG. 4 is a diagram illustrating the relationship between another ultrasonic signal waveform and a trigger level versus propagation time.

FIG. 5 is an explanatory diagram showing an example of a change pattern with respect to the waveform of FIG. 4;

FIG. 6 is a configuration diagram showing a conventional example.

FIG. 7 is a waveform diagram for explaining an operation of detecting an ultrasonic signal in FIG. 6;

FIG. 8 is a waveform diagram for explaining a detection operation when the ultrasonic signal waveform changes in FIG.

[Explanation of symbols]

1A, 1B ... ultrasonic sensor, 2 ... piping, 3 ... converter, 3
DESCRIPTION OF SYMBOLS 1 ... Transmission / reception circuit part, 32 ... Comparison part, 33 ... Propagation time measurement circuit part, 34 ... Upper-order arithmetic circuit part (CPU), 35 ... D /
A (digital / analog) converter, FL: fluid.

Claims (1)

[Claims] 1. A pair of ultrasonic sensors arranged to face each other along a flow of a fluid or inclined at a predetermined angle in a flow direction, and an ultrasonic signal is transmitted from one ultrasonic sensor and received by the other. A transmission / reception unit for alternately performing transmission / reception operations, a comparison unit for comparing an ultrasonic signal with a predetermined reference potential, and a time measurement unit for measuring a propagation time from transmission to reception of the ultrasonic signal based on an output from the comparison unit And an arithmetic unit configured to calculate the flow velocity or flow rate of the fluid based on the propagation time obtained from the time measuring unit, wherein the reference potential in the comparing unit is from the center of the ultrasonic signal waveform to the maximum amplitude. By changing the reference potential versus propagation time characteristic of the ultrasonic signal, the optimum trigger level of the ultrasonic signal waveform to be detected is determined from this characteristic, and the ultrasonic signal is determined based on the optimum trigger level. Ultrasonic flow meter and obtains the propagation time.