JP4759829B2 - Flow measuring device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a flow rate measuring device that measures the flow rate of a liquid or gas.
[0002]
[Prior art]
In this type of conventional flow rate measuring device, various proposals have been made to obtain an accurate flow rate even when pressure fluctuations occur. For example, as described in Japanese Patent Application Laid-Open No. 11-44563. There was a thing. The operation of this type of flow rate measuring device will be described with reference to FIG.
[0003]
In FIG. 4, a first vibrator 2 and a second vibrator 3 as flow rate detection means are attached to the fluid flow path 1 so as to face each other in the flow direction, and the flow rate measurement means 16 includes these two vibrators. Ultrasonic waves are transmitted and received between them, and the flow rate value is obtained using the time required for ultrasonic propagation at that time. In such a configuration, when a pressure fluctuation occurs in the flow path 1, the flow rate changes due to the influence of the pressure fluctuation, and an accurate flow rate value cannot be obtained. Therefore, the pressure detection means 10 is attached, and the AC component of the output signal of the pressure detection means 10 is input to the pulsation measurement means 11, and the zero cross passage point of the signal level is detected by the comparison means 12 and synchronized therewith. Thus, the start / stop of the flow rate measuring means is controlled. With this configuration, it was possible to control the measurement time in accordance with the pressure cycle and obtain an accurate average flow rate during pulsation.
[0004]
[Problems to be solved by the invention]
However, the conventional flow rate measuring device has the following problems. That is, the pressure signal waveform is not necessarily a simple waveform such as a sine wave, and in many cases is a complex waveform in which a plurality of frequency components are synthesized. Even if the waveform is relatively close to a sine wave, taking a waveform for each period may include some variation in the period and level. For this reason, even if measurement is performed in synchronization with the zero cross, the flow rate in the pressure fluctuation cycle is not necessarily averaged, so that an accurate average flow rate cannot be obtained, and there is a problem in terms of measurement accuracy.
[0005]
SUMMARY OF THE INVENTION The present invention solves the above-described conventional problems, and an object thereof is to provide a flow rate measurement device capable of optimizing the flow rate measurement timing and accurately measuring the flow rate by accurately grasping the pressure fluctuation period. .
[0006]
[Means for Solving the Problems]
In order to solve the above-mentioned conventional problems, the flow rate measuring device of the present invention can be set with a flow rate detecting means for detecting a fluid flow rate, a pulsation detecting means for detecting a fluid pressure fluctuation, and an output of the pulsation detecting means. A comparison means for comparing with a comparison reference value, a period detection means for measuring a pulsation period from the output of the comparison means, a setting change means for changing a comparison reference value of the comparison means, and a period measurement means. Measurement control means for controlling the operation time of the flow rate detection means according to the cycle.
[0007]
In this way, by accurately determining the comparison reference value, it is possible to capture the exact pressure fluctuation period and optimize the flow measurement timing, thereby realizing highly accurate flow measurement that is not affected by pressure fluctuation. .
[0008]
DETAILED DESCRIPTION OF THE INVENTION
According to the first aspect of the present invention , the flow rate detecting means for detecting the flow rate of the fluid, the pulsation detecting means for detecting the fluid pressure fluctuation, and the comparison for comparing the output of the pulsation detecting means with a settable comparison reference value. A setting change to increase the comparison reference value of the comparison means if the pulsation period measured by the period detection means is smaller than a predetermined value. means and a measurement control means for causing operation of said flow rate detection means at a period determined by the period detecting means, a flow rate measuring device provided with a. As a result, by appropriately setting the comparison reference value, it is possible to capture the accurate pressure fluctuation period and optimize the flow rate measurement timing, thereby realizing highly accurate flow rate measurement that is not affected by the pressure fluctuation. Further, even if the average cycle is short and the waveform has a steep change gradient, the cycle can be detected by capturing a point where the pressure fluctuation is gentle, so that variations in the detection cycle can be reduced.
[0009]
According to the second aspect of the present invention, in particular, the setting change unit according to the first aspect is configured so that the comparison reference value of the comparison unit is made small if the output change of the comparison unit is not more than a predetermined time. Since the reference value is not increased excessively, the comparison reference value can be optimized.
[0010]
According to the third aspect of the present invention, in particular, the time for pressure fluctuation is obtained by adopting a configuration in which the start / stop signal of the measurement control unit according to the first or second aspect is output in synchronization with the output change point of the comparison unit. Since it is possible to measure with a small delay, it is possible to improve the followability to pressure fluctuation.
[0011]
【Example】
Embodiments of the present invention will be described below with reference to the drawings.
[0012]
Example 1
FIG. 1 is a block diagram showing a flow rate measuring apparatus according to Embodiment 1 of the present invention. In FIG. 1, a first vibrator 2 that transmits ultrasonic waves and a second vibrator 3 that receives ultrasonic waves are disposed in the flow direction in the middle of a fluid flow path 1. 4 is a transmission circuit to the first vibrator 2, 5 is a reception circuit for signal processing of ultrasonic waves received by the second vibrator 3, and 6 is a switch for switching transmission / reception between the first vibrator 2 and the second vibrator 3. Means 7 is a repeating means that repeats transmission from the first vibrator 2 and reception by the second vibrator 3 after detecting ultrasonic waves by the receiving circuit 5, and 8 is a plurality of times of superposition performed by the repeating means 7. Time measuring means 9 for measuring the time required for sound wave propagation, 9 is a flow rate calculating means for obtaining a flow rate from the measured value of the time measuring means 8. Further, 10 is a pressure detecting means for detecting the pressure in the flow path 1, 11 is a pulsation detecting means for taking out an AC component of the signal output of the pressure detecting means 11 through a capacitor, and 12 is an AC signal taken out by the pulsation detecting means. The comparison means 13 compares the magnitude relationship with the set voltage, converts the magnitude relationship into a binary signal and outputs the binary signal, and 13 measures the interval of the output change point from the small value to the large value of the binary signal of the comparison means 12 to determine the pulsation cycle. Detecting period detecting means, 14 is a measurement control means for controlling the flow rate measurement time according to the output result of the frequency detecting means 13, and 15 is a setting changing means for changing the comparison reference value of the comparing means 12.
[0013]
Next, the operation and action will be described. When the velocity of sound in the static fluid is c and the velocity of the fluid flow is v, the propagation velocity of the ultrasonic wave in the forward direction is (c + v) and the propagation velocity in the reverse direction is (cv). The distance between the transducers 2 and 3 is L, the angle between the ultrasonic propagation axis and the central axis of the flow path is θ, the propagation time of the ultrasonic wave transmitted in the forward direction of the flow is t ₁ , and the reverse of the flow When the time for propagation of the ultrasonic wave transmitted in the direction t _2,
t ₁ = L / (c + v cos θ) (1)
t ₂ = L / (c−v cos θ) (2)
It becomes. When the flow velocity v is obtained from the equations (1) and (2),
v = (L / 2 cos θ) · (1 / t ₁ −1 / t ₂ ) (3)
If L and θ are known, t ₁ and t ₂ are measured to obtain the flow velocity v. Here, if the channel cross-sectional area is S and the correction coefficient is K, the flow rate Q is Q = K · S · v (4)
It becomes. As is clear from the equations (3) and (4), the flow rate Q is obtained by obtaining the propagation time. On the other hand, the flow velocity v is not constant when a pressure fluctuation occurs in the flow path 1, but when a periodic fluctuation occurs, the propagation time between the pressure fluctuation n cycles is measured, If the average value is obtained, the influence of fluctuation is canceled and an accurate value can be obtained.
[0014]
Next, the measurement control procedure will be described with reference to FIG. The comparison unit 12 performs a comparison process between the output signal of the pulsation detecting unit 11 and the comparison reference value 0V. The comparison means 12 outputs a binary signal of H if the AC signal of the pulsation detection means 11 is higher than the comparison reference value, and L if it is lower. The measurement control unit 14 outputs a repeated measurement start signal to the repetition unit 7 in synchronization with the rising waveform of the comparison unit 12, that is, the change point t ₁ a where the output signal changes from L to H. At this time, the switching means 6 has a system for measuring the propagation time of transmitting the ultrasonic wave in the forward direction by connecting the first vibrator 2 to the transmission circuit 4 and the second vibrator 3 to the reception circuit 5 in advance. Has been taken. Each time transmission / reception is started upon receipt of a start signal output from the measurement control means 14 and one transmission / reception ends, the repetition means 7 counts the number of transmission / reception and instructs the transmission circuit 4 to transmit ultrasonic waves. In parallel with the repetitive measurement by the repetitive means 7, the time measuring means 8 measures the time required for transmission / reception. Then, the measurement control means 14 again outputs a measurement stop signal at a point t _1b where the output of the comparison means 12 changes from L to H. When receiving the stop signal from the measurement control means 14, the repeat means 7 stops new transmission / reception. Here, based on the propagation time measured by the time measuring means 8 and the number of repetitions of transmission / reception performed by the repetition means 7, the flow rate calculation means 9 determines the forward propagation time t1. Thereafter, the switching means 6 first oscillator 2, take the system for measuring the propagation time obtained by transmitting an ultrasonic wave in the opposite direction of flow by changing connect the reception circuit 5 to the second oscillator 3. At the points t _2a and t _{2b when} the signal of the comparison unit 12 switches from L to H, the measurement control unit 14 outputs a start signal and a stop signal, respectively, and the reverse measurement is performed in the same procedure as the forward direction. . Based on the forward and reverse propagation times obtained as described above, the flow rate calculation means 9 obtains the flow rate Q using the equations (3) and (4).
[0015]
Here, the comparison reference value in the comparison means 12 is set to 0 V, but this value is not fixed. By setting the appropriate value according to the cause, period, and absolute level of the pulsation waveform generated by the setting change means 15, the accuracy of period detection is improved, and as a result, the accuracy of flow rate measurement is improved.
[0016]
As described above, in this embodiment, the setting change means appropriately determines the comparison reference value, so that the accurate pressure fluctuation cycle can be captured and the timing of flow measurement can be optimized, so that it is not affected by the pressure fluctuation. Highly accurate flow measurement can be realized.
[0017]
In particular, by increasing the comparison reference value of the comparison unit 12 as the average cycle of the cycle detection unit 13 becomes smaller, even if the average cycle is short and the waveform has a steep change gradient, the pressure fluctuation is gentle. Since the period can be detected by capturing a point, variation in the detection period can be reduced.
[0018]
(Example 2)
FIG. 3 is a timing chart for explaining the operation of the second embodiment of the present invention. If the pulsation waveform is a sine wave as shown in FIG. 2, zero crossing occurs twice in one cycle, but if the waveform is as shown in FIG. 3, zero crossing occurs four times during one cycle. Hereinafter, such a phenomenon is referred to as a middle cracking phenomenon. When the initial value of the comparison reference value of the comparison means 12 is set to 0, the output of the comparison means 12 changes as shown in the figure, and the detection value of the period detection means 13 changes alternately with T _a and T _b . In this case, in the period T _a, the pressure average is positive, since the swing in the negative side in a period of T _b, it is impossible to obtain the true value of the flow rate average value even when averaged over any given period. The setting change means 15 performs a comparison process with a predetermined value T _s (T _b > T _s > T _a ) each time the period detection means 15 obtains a period, and if a value smaller than T _s is detected, the setting change means 15 it is determined that the waveform changes the comparison reference value of the comparator 12 to V _1. As a result, the output of the comparison means 12 changes as in the latter half of FIG. 3, and a constant period _Tc is always obtained. As a result, the influence of pulsation is canceled and an accurate flow rate value can be obtained.
[0019]
As described above, in this embodiment, if the measurement period of the period detection unit is smaller than a predetermined value, the comparison reference value of the comparison unit is increased, so that fluctuations generated in a short time are not accepted. The accurate fluctuation period can be obtained by avoiding the influence of the crack of the wave meter.
[0020]
As another method, the determination unit 16 may change the comparison reference value of the comparison unit 12 based on the variation in the period. In this case, even if the pressure cycle is substantially constant, the comparison reference value can be optimized in response to a case where the variation of the waveform cycle becomes large, so that an accurate pressure cycle can be obtained. The method for obtaining the variation may be a method for obtaining the standard deviation within a certain period, or a method for obtaining the difference between the maximum value and the minimum value.
[0021]
Furthermore, in this embodiment, if the comparison reference value is changed too much, the output of the comparison means 12 may remain unchanged at L. In this case, when the determination unit 16 cannot detect the rising signal of the comparison unit 12 for a predetermined time or longer, the comparison reference value may be changed small. As a result, the comparison reference value is not increased excessively, so that the comparison reference value can be optimized.
[0022]
In each of the embodiments, the configuration is described in which the rising edge of the output of the comparison means 12 is synchronized. However, the same effect can be obtained even with a falling signal. In either case of rising and falling, by measuring the flow rate in synchronization with the output change of the comparison means 12, it is possible to measure with a small time delay with respect to the pressure fluctuation, so that the followability to the pressure fluctuation can be improved. .
[0023]
Further, as another method, there is a method of performing measurement processing asynchronously with the comparison signal by performing arithmetic processing for converting the time obtained by the period detection means 13 into the number of repetitions of the repetition means 13. In this case, since there is no risk that the operation of the flow rate measuring means is hindered by the noise of the pulsation detecting means or the comparing means, it is possible to avoid the influence of unnecessary signals during flow rate measurement. High measurement is possible.
[0024]
Furthermore, although each example demonstrated what used the ultrasonic transducer | vibrator, even if it is other than that, for example, a heat ray type flow sensor, an equivalent effect is acquired. Needless to say, the same effect can be obtained not only with a gas flow rate measuring device but also with a liquid flow rate measuring device.
[0025]
【The invention's effect】
As described above, according to the inventions described in claims 1 to 3 of the present invention, it is possible to capture an accurate fluctuation cycle by appropriately setting a comparison reference value, and it is possible to obtain a high accuracy that is not affected by pressure fluctuations. Can be measured.
[Brief description of the drawings]
FIG. 1 is a block diagram of a flow rate measuring apparatus according to a first embodiment of the present invention. FIG. 2 is a timing chart illustrating the operation of the apparatus. FIG. 3 is a timing illustrating the operation of the flow rate measuring apparatus according to a second embodiment of the present invention. Chart [Figure 4] Block diagram of a conventional flow measurement device [Explanation of symbols]
2 First vibrator (flow rate detection means)
3 Second vibrator (flow rate detection means)
11 Pulsation detection means 12 Comparison means 13 Period detection means 14 Measurement control means 15 Setting change means

Claims (3)

Flow rate detection means for detecting the flow rate of the fluid;
Pulsation detecting means for detecting fluid pressure fluctuations;
Comparing means for comparing the output of the pulsation detecting means with a settable comparison reference value;
Period detection means for measuring a pulsation period from the output of the comparison means;
If the pulsation cycle measured by the cycle detection unit is smaller than a predetermined value , a setting change unit that increases the comparison reference value of the comparison unit;
Flow rate measuring apparatus provided with a operation makes measuring control the flow rate detecting means at a period determined by the period detecting means. The setting change unit, the flow rate measuring device according to claim 1 and a small comparison reference value of the comparison means unless the change in the output of the comparison means a predetermined time or more. The measurement start stop signal of the control means, the flow rate measuring apparatus according to claim 1 or 2 which is output in synchronism with the output change point of the comparison means.

Images