JPH0726861B2 - Flow rate measuring device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a flow rate measuring device applied to, for example, a pipeline for fuel or air supplied to a heating furnace.

(Prior Art) Conventionally, in this type of flow rate measurement, as shown in FIG. 7, an orifice provided in the fluid pipe 11 to be measured is used. A fixed throttle mechanism 12 such as a venturi and a differential pressure sensor 13 such as a differential pressure gauge or a differential pressure converter for measuring the pressure difference between the fixed throttle mechanism 12 front and rear.
And calculate the flow rate based on the signal from the differential pressure sensor 13,
It is usual to use a single device in combination with the arithmetic controller 14 for output.

(Problem to be Solved by the Invention) As is well known, the differential pressure of the fixed throttle mechanism 12 is proportional to the square of the flow rate, and sharply decreases as the flow rate decreases. As shown in FIG. 8 (when the zero drift of the differential pressure sensor is ± 1%), there is a problem that the flow rate rapidly increases as the flow rate decreases.

On the other hand, FIG. 9 shows a flow rate measuring device for avoiding an error in a low flow rate region.
The variable throttle mechanism 22 provided in the, the differential pressure sensor 23 for detecting the differential pressure before and after the variable throttle mechanism 22, the opening sensor 24 attached to the variable throttle mechanism 22, the variable throttle drive mechanism 25, from the above sensors And the arithmetic controller 26 which calculates and outputs the flow rate based on the signal of (1). Incidentally, in the example shown in FIG.
The variable aperture mechanism 22 is controlled via the variable aperture drive mechanism 25 based on the calculated flow rate.

The flow rate measurement error in this device is mainly due to the error relating to the differential pressure sensor 23 and the error relating to the opening degree of the variable throttle mechanism 22, and is shown in FIG. 10 (zero drift of the differential pressure sensor is ± 1%, opening degree). Zero drift with respect to ±
In the high flow rate region, as in the case of 0.5%), there is a problem that the flow rate measurement error by the variable throttle mechanism 22 becomes larger than the flow rate measurement error by the flow rate measurement method by the fixed throttle mechanism 12.

Generally, the error related to the differential pressure sensor and the error related to the opening degree of the variable throttle mechanism constantly fluctuate, and the deterioration of the fuel consumption rate and the deterioration of the product quality due to the decrease of the flow rate measurement control accuracy due to the error related to these sensors. In order to avoid a loss such as a decrease, it is necessary to frequently perform error correction for each sensor, that is, so-called calibration. Conventionally, these calibrations require the operator to stop the operation of the device and use a standard device or the like, and an increase in the frequency of calibration causes a loss of production opportunities due to the suspension of the operation, calibration work labor, etc. It will increase.

Therefore, in the past, in many cases, they have been operating at an economic compromise between them.

The present invention has been made to solve the above-mentioned conventional problems, and automatically calibrates at a high frequency during operation, that is, automatically corrects an error relating to each sensor, thereby making it easy to obtain a general purpose An object of the present invention is to provide a simple flow rate measuring device with high accuracy in the entire flow rate region while using a pressure sensor.

(Means for Solving the Problems) In order to solve the above problems, the present invention relates to a fixed throttle mechanism and a variable throttle mechanism provided in a fluid pipe line to be measured,
A variable throttle drive mechanism and an opening sensor attached to the variable throttle mechanism, a first differential pressure sensor and a second differential pressure sensor for detecting a differential pressure before and after each of the fixed throttle mechanism and the variable throttle mechanism, and the fixed sensor. Flow rate measurement value based on a signal from the first differential pressure sensor of the fixed throttle system including the throttle mechanism and the first differential pressure sensor, and opening of the variable throttle system including the variable throttle mechanism, the opening sensor, and the second differential pressure sensor. Of the flow rate measurement values based on the signals from the velocity sensor and the second differential pressure sensor, and the error relating to each of the above sensors based on the "general expression of error spread" for both calculation equations. Approximately equal to the difference between the approximate expressions of the flow rate measurement error due to To the error By calculating the approximate value of the error, the measured value of each sensor is corrected by the approximate value of the error, and the flow rate measurement value is calculated from the corrected value and output from the arithmetic controller. Formed.

(Embodiment) Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows a flow rate measuring device according to the present invention, in which a fixed throttle mechanism 2 is provided in a fluid pipeline 1 to be measured, a relationship of a flow rate coefficient with respect to an opening is known, and an opening sensor 3 and a variable throttle are provided. The variable throttle mechanism 5 to which the drive mechanism 4 is attached is provided, and the fixed throttle mechanism 2 and the first and second differential pressure sensors 6 and 7 for detecting the differential pressure before and after each of the variable throttle mechanism 5 are provided.

Further, an arithmetic controller 8 for inputting signals from the first and second differential pressure sensors 6 and 7 and a signal from the opening sensor 3 is provided. This arithmetic controller 8 is formed by using a microprocessor or the like, and is stored in advance in the form of a mathematical expression of the flow coefficient of the fixed throttle mechanism 2 and the flow coefficient with respect to the opening of the variable throttle mechanism 5, or in the form of a table. A fixed throttle system including a fixed throttle mechanism 2 and a first differential pressure sensor 6 and a variable throttle mechanism including a variable throttle mechanism 5, an opening sensor 3 and a second differential pressure sensor 7 based on the relations shown above and the above sensor signals. The difference between the calculation formulas for calculating both flow rate measured values by the system and the difference between the approximate formulas for the flow rate measurement error due to the error related to each sensor based on the "general formula of the spread of error" for both calculation formulas are equal. The approximate value is calculated by forming a simultaneous equation by substituting the actual measurement value of each sensor at the required number of flows at an appropriate interval into the approximate error equation and solving the error relating to each sensor. Further, the arithmetic controller 8 issues an alarm signal when these approximate values become larger than a predetermined value, and / or corrects the actual measurement value of the sensor by the approximate value, and the corrected value The flow rate measurement value is calculated every moment and output.

It should be noted that the arithmetic controller 8 is used for the flow rate or the variable throttle mechanism 5.
It also has a control function for setting the differential pressure or the opening to a predetermined value, and sends an opening control signal to the variable diaphragm drive mechanism 4 to operate the opening of the variable diaphragm mechanism 5.

Therefore, first, a method of calculating the approximate value of the error for each sensor will be described in detail.

Assuming that the flow coefficient of the fixed throttle mechanism 2 is A and the measured value of the differential pressure thereof, that is, the differential pressure value including the error related to the first differential pressure sensor 6 and the differential pressure is Pa, the measured flow rate value Qa is ) Is represented by the formula.

The flow coefficient of the variable throttle mechanism 5 is a function f (s) of the opening, an actual measurement value of the opening, that is, an opening value including an error related to the opening sensor 3 and the opening s, an actual measurement value of the differential pressure thereof, That is, the second
Letting Pb be a differential pressure value including an error related to the differential pressure sensor 7 and differential pressure measurement, the flow rate measurement value Qb is expressed by the following equation (2).

These two flow rate measurement values are due to the error related to each sensor,
Except for coincidence, it generally makes a difference.

Here, an approximate expression based on the well-known “general expression of error propagation” is obtained for the difference between the right side of the expression (1) and the right side of the expression (2), and the following expression (3) is obtained.

Since the expression (3) is an approximate expression of a difference obtained by subtracting the right side of the expression (1) from the right side of the expression (2), the following expression (4), which is assumed to be equal to each other, is calculated by using the following expression (4). Is a basic formula for calculating an approximate value of, and is tentatively called an error approximate formula.

Since the function f (s) of the opening of the flow coefficient of the variable throttle mechanism 5 and the flow coefficient A of the fixed throttle mechanism 2 in this equation (4) are known, the proper interval in the equation (4), for example, the maximum flow rate A system of simultaneous linear equations is created by formulating a required number for every 5%, that is, a required number obtained by substituting the actual measurement value of each sensor at the same number of flow rates (three in this embodiment) as the number of sensors, which is the number of unknowns. And error for each sensor (δS, δPb, δPa)
Is used as an unknown value, and an approximate value of the error for each sensor is obtained by a known method, for example, the Gaussian elimination method.

The arithmetic and control unit 8 issues an alarm signal when these approximate values become larger than a predetermined value, and / or corrects the actual measurement value by the sensor based on the approximate value, and measures the flow rate by the corrected value. Calculate the value.

Next, the logic for selecting the actually measured value by each sensor for taking in and storing the actually measured value by each sensor and calculating the approximate value of the error relating to them will be described in detail with reference to FIG.

First, a plurality of set flow rate values are determined in advance over the entire required flow rate region at appropriate intervals (# 2-1), and the flow rate measurement values obtained by the fixed throttle system or the variable throttle system are read (# 2-2). Every time this coincides with this set flow rate value, the process proceeds to the next step (# 2-3), and the measured value of each sensor is taken into the storage area corresponding to each set flow rate value (# 2-
4) The stored data is updated (# 2-5).

Every time the measured value is taken in by each sensor and the stored data is updated, including the updated one,
The actual measured values of the latest required number of sensors with different flow rate setting values are selected (# 2-6), and the approximate value of the error for each sensor is calculated by a simultaneous linear equation based on the error approximate expression that substitutes these values. (# 2-7), this calculation ends, and the actual measurement value by each sensor is corrected based on this approximate value.

According to the calculation method described in detail so far, since each sensor is calibrated in advance, the error related to each sensor is small, for example, ± 1% or less, and the required flow rate region is limited to the high flow rate region. In the range of 60 to 80% of the maximum flow rate, it is practically sufficient because the approximate value of the error for each sensor can be calculated with sufficient accuracy, for example, ± 0.03% or less. As the flow area increases,
The approximation error of the approximate value of the error for each sensor due to the approximation of the error approximation formula increases.

A calculation method for sufficiently reducing this approximation error will be described in detail with reference to FIG.

The measured values by a required number of each set of sensors having different set flow rate values are read (# 3-1), and an approximate value of the error for each sensor is calculated (# 3-2). It is judged whether or not the difference from the previous approximate value (0 at the beginning) is smaller than a predetermined value (# 3-3). If not, the measured value of each sensor is corrected by the integrated value of the approximate values. (# 3-5, # 3-6), the same approximate value as above is calculated for the corrected actual measurement value of each sensor (# 3-
2). The calculation of the approximate value, the integration thereof, and the repetition of the correction of the measured value are repeated until the difference between the calculated approximate value and / or its approximate value and the previous approximate value becomes smaller than a predetermined value. That is, at the end of the above iteration,
Iterative convergence calculation is performed until the approximation error of the approximate value of the error for each sensor due to the approximation of the error approximation formula becomes a sufficiently small value, and the integrated value of the approximate value of the convergence result is the value of each sensor before correction. Since the approximation error of the error concerning each sensor included in the actual measurement value is a sufficiently small approximation value, the actual measurement value is corrected by this integrated value (# 3-4).

Furthermore, when the error related to each sensor becomes large, for example, when most of the calibration immediately after manufacturing is simplified by applying automatic calibration, or when the required flow rate region becomes wide, for example, in combustion control with a large turndown ratio, When used, the approximation error of the approximation value of the error relating to each sensor due to the approximation of the error approximation formula becomes excessive and may diverge or not converge in the above-described iterative calculation.

A calculation method for preventing this divergence will be described in detail next with reference to FIG.

In the iterative convergence calculation as described above, the actual measurement values by a required number of sets of sensors having different set flow rate values are read (# 4-1), and the approximate value of the error is calculated by the simultaneous linear equations based on the error approximation formula. Calculated (# 4-2), and the calculated error approximation value for each sensor has a coefficient of 1 or less, for example, 0.
Multiply by 5 (# 4-3), and / or if these approximate values are greater than a predetermined value, for example, 10%, the value is limited to a predetermined value (# 4-7), and the integrated value (# 4 -8) and iterative convergence calculation of repeating the correction (# 4-9) of the measured value by the integrated value, and when the approximation error of the error is sufficiently small (# 4-5), the measured value is calculated by the integrated value. Is corrected (# 4-6).

According to the calculation method described in detail so far, there is no problem if the actual measurement value by each sensor and the fluctuation of the error related to each sensor are more gradual than the frequency of correction, but these fluctuations are frequent (short cycle). In this case, the flow rate measurement value suddenly changes due to the fluctuation of the correction value every time.In particular, when the flow rate measurement value is used for control, the control accuracy becomes unstable due to instability of the control and the life of the moving part is shortened. It often causes problems.

A calculation method for avoiding the sudden change in the flow rate measurement value will be described in detail with reference to FIG.

The fluid to be measured and controlled often has steady short-term fluctuations such as pulsations due to pumps and blowers, pressure fluctuations due to fluctuations in the amount used in the system, etc. In many cases, the actual measurement value of each differential pressure sensor varies greatly.

The first method of suppressing the variation of the correction value due to this type of variation
Is a moving average of the measured values of each sensor (filtering of high-frequency components, also known as exponential smoothing) to reduce the actual measured value of each sensor before calculating the approximate value of the error of each sensor. By keeping the periodic fluctuation small enough (# 5-1), the averaged sensor values are read (# 5-2), and the approximate value and the correction value of the error are calculated based on this (# 5-). 3).

Since the fluctuation of the error relating to each sensor is the fluctuation of the actual measurement value by each sensor, it can be considerably suppressed by smoothing the fluctuation of the actual measurement value as described above, but it is not always perfect.

If the problem due to the fluctuation of the correction value remains, the moving average of the calculated correction value as the second method of suppressing the fluctuation of the correction value is also used (# 5-4), and the measured value by each sensor is used. Is corrected (# 5-5).

When the fluctuation of the actual measurement value is slight as described above, the moving average of the correction value may be sufficient.

According to the calculation method described in detail up to this point, the error related to each sensor is mainly a so-called zero drift in which a constant value is added (or reduced) over the entire measurement range, and a sufficient error is obtained when other errors can be ignored. Correction can be performed, but if so-called proportional error that increases in proportion to the measured value and instrumental error peculiar to each sensor (when not corrected in advance) cannot be ignored, it is necessary to obtain the corrected value corresponding to the measured value. is there.

Regarding the calculation method to obtain the correction value corresponding to this measured value,
Next, a detailed description will be given according to FIG.

In order to calculate the approximate value of the error for each sensor, it is necessary to have the number of sensors, that is, the actual measurement value group of each sensor in the flow rate group of the same number as the unknown number, and the error for each sensor by the simultaneous equations based on the error approximation formula. Since the calculation of the approximate value of is based on the assumption that those errors are the same in the actual measurement values of the group, if they differ, it is considered that those errors are the same. The error is included in the calculated approximate value, and the actual measurement value corresponding to the approximate value becomes uncertain.

Therefore, assuming that the calculated approximate value corresponds to the median value of the measured value group for the time being, a formula or data table of the correction values over the entire required measurement region was sequentially created, and the correction was performed by this. Correction of the measured value by each sensor is performed by sequentially repeating the calculation of the approximate value by the measured value group (# 6-1) and the correction of the formula of the correction value by the approximate value (# 6-2). (# 6-3) is intended to gradually reduce the assumed error and establish the corresponding measured value.

(Effects of the Invention) As is apparent from the above description, according to the present invention, the fixed throttle mechanism and the variable throttle mechanism provided in the fluid pipe to be measured, and the variable throttle drive mechanism attached to the variable throttle mechanism. And an opening sensor, a first differential pressure sensor and a second differential pressure sensor for detecting a differential pressure before and after each of the fixed throttle mechanism and the variable throttle mechanism, and a fixed throttle mechanism including the fixed differential mechanism and the first differential pressure sensor. Flow rate measurement value based on the signal from the first differential pressure sensor of the throttle system, and the signal from the second throttle sensor of the variable throttle system including the variable throttle mechanism, the opening sensor, and the second differential pressure sensor. The difference between the calculation formulas for calculating each of the flow rate measurement values based on the above, and the difference in the approximate formula of the flow rate measurement error due to the error related to each sensor based on the "general formula of the spread of error" for both calculation formulas. Assumed equal An approximate value of the error is calculated by forming a simultaneous equation in which the measured values of the respective sensors at the required number of flows at appropriate intervals are substituted into the error approximate expression and solving the error regarding the respective sensors. The measurement value of each sensor is corrected based on the approximate value of the error, and the flow rate measurement value is calculated based on the corrected value, and the arithmetic controller is provided.

That is, the error of each sensor is automatically corrected during operation by combining and utilizing the flow rate measuring means with the fixed throttle mechanism and the variable throttle mechanism, and a highly accurate flow rate is obtained using an easily available general-purpose sensor. It is possible to obtain the measuring device and to significantly reduce the calibration work.

[Brief description of drawings]

FIG. 1 is a system diagram of a flow rate measuring device according to the present invention, and FIGS.
FIG. 6 is a flow chart showing the procedure of reading the actual measurement value of each sensor, calculating the approximate value of the error of each sensor, correcting, etc. FIG. 7 is a system diagram of a flow rate measuring device using a conventional fixed throttle mechanism, 8 is a diagram showing the relationship between flow rate measurement error and flow rate due to zero drift of the differential pressure sensor in FIG. 7, FIG. 9 is a system diagram of a flow rate measuring device using a variable throttle mechanism, and FIG.
The figure is a diagram showing the relationship between the flow rate measurement error and the flow rate due to the zero drift of the differential pressure sensor and the zero drift of the opening sensor in FIG. 1 ... Fluid line, 2 ... Fixed throttle mechanism, 3 ... Opening sensor, 5
... Variable throttle mechanism, 6, 7 ... First and second differential pressure sensor, 8 ... Arithmetic controller.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Akira Nakayasu 2-4-7 Kyomachibori, Nishi-ku, Osaka City, Osaka Prefecture Chugai Prox Co., Ltd. (72) Hideaki Narahara 2-4-4 Kyomachibori, Nishi-ku, Osaka City, Osaka Prefecture No. 7 inside Chugai Prox Co., Ltd.

Claims (1)

[Claims] 1. A fixed throttle mechanism and a variable throttle mechanism provided in a fluid pipe to be measured, a variable throttle drive mechanism and an opening sensor attached to the variable throttle mechanism, and the fixed throttle mechanism and the variable throttle mechanism. Flow rate measurement based on signals from the first differential pressure sensor and the second differential pressure sensor that detect the differential pressure before and after each, and the fixed throttle mechanism and the first differential pressure sensor of the fixed throttle system including the first differential pressure sensor. Of the flow rate measurement value based on the signal from the variable throttle mechanism including the variable throttle mechanism, the opening sensor, and the second differential pressure sensor, and the signal from the second differential pressure sensor. In the error approximation formula, which is assumed to be equal to the difference, the difference in the approximate formula of the flow rate measurement error due to the error related to each sensor based on the "general formula of the spread of error" for both arithmetic expressions At the flow rate of The approximate value of the error is calculated by forming a simultaneous equation in which the measured value of each sensor is substituted and solving the error related to each sensor, and the measured value of each sensor is corrected by the approximate value of this error. A flow rate measuring device comprising: an arithmetic controller that calculates and outputs a flow rate measurement value based on the corrected value.