JP6077843B2 - Electromagnetic flow meter - Google Patents

Description

  The present invention relates to a flow rate integrated value calculation technique for an electromagnetic flow meter that measures the flow rate of a fluid having conductivity in various process systems.

  In general, in an electromagnetic flow meter that measures the flow rate of a fluid having electrical conductivity, an excitation current whose polarity is alternately switched to an excitation coil arranged so that the direction of magnetic field generation is perpendicular to the flow direction of the fluid flowing in the measurement tube And detects an electromotive force generated between a pair of electrodes arranged in the measurement tube perpendicular to the magnetic field generated from the excitation coil, and based on the electromotive force generated between the electrodes, the flow rate of the fluid flowing in the measurement tube Is measuring.

  Such an electromagnetic flow meter has a function of calculating a flow integrated value obtained by integrating a flow rate measurement value as one of standard functions. In general, the flow rate integrated value can be obtained by integrating the sampling period with the flow rate measurement value (instantaneous flow rate) obtained by sampling the electromotive force at a constant cycle (for example, Patent Document 1) reference).

Japanese Patent Laid-Open No. 5-126608

However, in such a conventional technique, the flow rate integrated value is obtained by simply integrating the sampling period consisting of a fixed value to the flow rate measured value (instantaneous flow rate). There was a problem that an error occurred.
In the electromagnetic flow meter, in order to improve the S / N ratio with respect to the power frequency noise, there is a case where a power frequency synchronization method for performing flow rate measurement in synchronization with the power frequency is used. In this method, a power supply synchronization signal synchronized with a power supply frequency is taken into a control circuit, and an electromotive force is sampled in synchronization with the control circuit.

At this time, the sampling cycle of the electromotive force varies in synchronization with the power supply frequency, but the control circuit uses a fixed value set in advance as the sampling cycle used to calculate the flow rate integrated value.
On the other hand, the power supply frequency includes a deviation of about 0.2 to 0.3 Hz with respect to the frequency of 50 Hz / 60 Hz. For this reason, when the power supply frequency fluctuates, an error occurs between the electromotive force sampling period and the sampling period used to calculate the flow rate integrated value, resulting in an error in the flow rate integrated value.

For example, since the reference period is 1/50 = 20 ms at a power supply frequency of 50 Hz, when the sampling period is set to 10 times of the reference period, the sampling period is 200 ms.
On the other hand, when the power supply frequency is shifted by 0.2 Hz and fluctuates to 50.2 Hz, the reference period is 1 / 50.2 = 19.92 ms, so the sampling period is 199.2 ms.
Therefore, when the actual power supply frequency is 50.2 Hz, when the fixed value of 200 ms is used as the sampling period when calculating the flow rate integrated value, the flow rate integrated value is (200-199.2) / 200 = An error of 0.4% will occur.

  An object of the present invention is to solve such a problem, and an object of the present invention is to provide a flow rate integrated value calculation technique capable of calculating a flow rate integrated value with high accuracy even when a power supply frequency varies.

In order to achieve such an object, the electromagnetic flow meter according to the present invention detects an electromotive force generated in a fluid in response to excitation of the fluid flowing in the measurement tube by an electrode, and amplifies the electromotive force by alternating current. An AC flow signal obtained by sampling at a sampling period synchronized with an external AC power source, and calculating and outputting a flow rate measurement value of the fluid based on the obtained DC flow rate signal. and synchronizing signal extracting circuit for extracting a power-supply sync signal synchronized with the period of the external AC power from the AC power supply, and timing the sampling period for each said sampling period based on the power-supply sync signal, to the time measurement result, the sampling period And a control circuit for calculating a flow integrated value by sequentially integrating values obtained by multiplying the measured flow rate values.

  According to the present invention, a value that coincides with the sampling period for calculating the flow rate measurement value (instantaneous flow rate) is used as the sampling period for calculating the integrated flow rate value. Therefore, even if the power supply frequency fluctuates, the fluctuation is reflected in the sampling cycle for calculating the flow integrated value, so that the flow integrated value can be calculated with high accuracy.

It is a block diagram which shows the structure of the electromagnetic flowmeter of this invention. It is a signal waveform diagram which shows a power supply synchronizing signal. It is a flowchart which shows the flow volume integrated value calculation process of the electromagnetic flowmeter of this invention.

Next, embodiments of the present invention will be described with reference to the drawings.
[Configuration of the embodiment]
First, with reference to FIG. 1, the electromagnetic flowmeter 1 concerning this Embodiment is demonstrated. FIG. 1 is a block diagram showing the configuration of the electromagnetic flow meter of the present invention.

  The electromagnetic flow meter 1 is composed of a detector 1A and a converter 1B as a whole, and has a function of measuring the flow rate of a fluid having conductivity in various process systems. In the present embodiment, the electromagnetic flow meter 1 operates with an external AC power supply AC, and is connected to a host device (not shown) such as a controller via a 4-wire communication line W. A case of a 4-wire electromagnetic flow meter that performs transmission will be described as an example. It should be noted that the present embodiment can be similarly applied to a two-wire electromagnetic flow meter as long as it operates with an external AC power source AC.

The detector 1A is provided with a measuring tube P, electrodes TA and TB, and an exciting coil L as main components.
The measuring tube P is made of a nonmagnetic metal cylinder such as stainless as a whole, and constitutes a flow path through which a fluid to be measured flows.
The exciting coil L is composed of a pair of coils arranged to face the outside of the measuring tube P, and is orthogonal to the flow direction of the fluid flowing through the flow path in accordance with the AC exciting current supplied from the converter 1B. It has a function of generating a magnetic field in the direction.

The electrodes TA and TB are composed of a pair of electrodes arranged in contact with the fluid flowing in the flow path on the inner wall of the measuring tube P, facing the direction perpendicular to the direction of the magnetic field generated by the exciting coil L, It has a function of detecting an electromotive force E0 generated in the fluid in response to excitation of the fluid by a magnetic field and outputting it to the converter 1B.
Note that the potential of the measuring tube P is connected to the ground potential of the converter 1B via the ground terminal TG.

  The converter 1B includes, as main circuit units, an excitation circuit 10, an AC amplifier circuit 11, a sample hold circuit (SH) 12, an A / D converter 13, a control circuit 14, a communication circuit 15, a power supply circuit 17, and a display. A vessel 16 is provided.

The excitation circuit 10 has a function of supplying an AC excitation current having a predetermined excitation frequency to the detector 1A.
The AC amplifier circuit 11 has a function of inputting the electromotive force E0 obtained by the detector 1A through the capacitive element C, amplifying the electromotive force E0, and outputting it as an AC flow rate signal E1.

Based on the sampling control signals SH1 and SH2 from the control circuit 14, the sample and hold circuit 12 samples and holds a voltage in a specific period in which the waveform is stable among the positive side waveform and the negative side waveform of the AC flow rate signal E1. And has a function of outputting as a DC flow rate signal V1.
The A / D converter 13 has a function of A / D converting the voltage value of the DC flow rate signal V <b> 1 and outputting it to the control circuit 14.

  The control circuit 14 includes a CPU and its peripheral circuits as a whole, and has a function of executing various processes by reading a program from a memory (not shown) and executing it. The main processes executed by the control circuit 14 include an excitation control process, a sampling control process, a flow rate measurement value calculation process, a communication control process, and a flow rate integrated value calculation process.

  The excitation control process is a process for driving the excitation circuit 10 based on the excitation frequency based on the power supply synchronization signal T from the synchronization signal extraction circuit 18. The sampling control processing is processing for generating sampling control signals SH1 and SH2 based on the excitation frequency based on the power supply synchronization signal T from the synchronization signal extraction circuit 18 and outputting the sampling control signals SH1 and SH2 to the sample hold circuits 12 and 22. The flow rate measurement value calculation process is a function that calculates a flow rate measurement value based on the voltage value of the DC flow rate signal V1. The communication control process is a process for notifying the calculated flow rate measurement value from the communication circuit 15 to the host device via the communication line W.

  The flow rate integrated value calculation processing is performed by sequentially integrating the product of the value measured by the sampling period determined based on the power supply synchronization signal T from the synchronization signal extraction circuit 18 and the flow rate measurement value (instantaneous flow rate). This is a process for calculating the integrated value.

The communication circuit 15 has a function of notifying information such as a flow rate measurement value and a flow rate integrated value by communicating with a host device via the communication line W.
The display 16 includes a display element such as an LCD or an LED, and has a function of displaying various information such as a measured current value and a sky detection result in accordance with an instruction from the control circuit 14.

The power supply circuit 17 has a function of distributing the power generated from the external AC power supply AC to each circuit unit in the converter 1B.
The synchronization signal extraction circuit 18 has a function of extracting the power supply synchronization signal T from the external AC power supply AC of the power supply circuit 17 and outputting it to the control circuit 14.

  FIG. 2 is a signal waveform diagram showing the power supply synchronization signal. Here, a power supply synchronization signal T having a reference period Ts of the external AC power supply AC is shown. When the external AC power supply AC is 50 Hz, the reference period Ts is 20 ms. As a method for generating the power supply synchronization signal T, for example, a known technique such as generation by dividing the external AC power supply AC and detecting polarity inversion by a comparator may be used.

[Operation of this embodiment]
Next, the operation of the electromagnetic flow meter 1 according to the present embodiment will be described with reference to FIG. FIG. 3 is a flowchart showing a flow rate integrated value calculation process of the electromagnetic flow meter of the present invention.

The excitation circuit 10 supplies an AC excitation current having a predetermined excitation frequency to the excitation coil L of the detector 1A based on the excitation control process of the control circuit 14.
The exciting coil L generates a magnetic field in a direction orthogonal to the flow direction of the fluid flowing through the flow path in accordance with the AC exciting current. As a result, an electromotive force E0 having a magnitude corresponding to the flow rate is generated in the fluid flowing through the measurement pipe P.
In a normal state where the fluid is in contact with the electrodes TA and TB, the electrodes TA and TB detect the electromotive force E0 generated in the fluid and output it to the converter 1B.

The AC amplifying circuit 11 of the converter 1B AC amplifies the electromotive force E0 from the detector 1A input via the capacitive coupling by the capacitive element C, and outputs it as an AC flow signal E1.
The sample hold circuit 12 is based on the sampling control signals SH1 and SH2 output by the sampling control processing of the control circuit 14, and is a specific period in which the waveform is stable among the positive side waveform and the negative side waveform of the AC flow rate signal E1. The A / D converter 13 A / D converts the voltage value of the DC flow rate signal V1 and outputs it to the control circuit 14.

Thereafter, the control circuit 14 executes the flow rate integrated value calculation process of FIG. 3 as the flow rate measurement value calculation process. Hereinafter, the flow of the flow rate integrated value calculation process will be described with reference to FIG.
First, the control circuit 14 determines the voltage value of the DC flow rate signal V1 acquired from the A / D converter 13 by the flow rate integrated value calculation process at each sampling period Ts based on the power supply synchronization signal T from the synchronization signal extraction circuit 18. Based on the above, a flow rate measurement value (instantaneous flow rate) Fs is calculated (step 100).

Next, the control circuit 14 acquires the flow rate integrated value Fe immediately before being stored in the internal memory (not shown) of the CPU (step 101), and the flow rate measurement value (instantaneous) calculated by the flow rate measurement value calculation process. The flow rate Fs is multiplied by the sampling period Ts and added to the flow rate integrated value Fe, thereby calculating a new flow rate integrated value Fe (step 102).
Subsequently, the control circuit 14 stores the new integrated flow rate value Fe in the internal memory (step 103), and ends the series of integrated flow rate calculation processing. This flow rate integrated value Fe is read from the internal memory by the control circuit 14 in response to the arrival of an integration cycle such as 1 minute or 1 hour, and is notified from the communication circuit 15 to the host device via the communication line W. .

  In the flow rate integrated value calculation process, the control circuit 14 measures the sampling period Ts based on the power supply synchronization signal T from the synchronization signal extraction circuit 18 (step 104). For example, when it corresponds to 10 times of the sampling period Ts and the reference period Tc of the power supply synchronization signal T, the period of 10 times for the reference period Tc of the power supply synchronization signal T is measured by the timer function of the CPU. At this time, the period from the start to the end of the sampling period Ts may be measured continuously, or the time measured for each reference period Tc constituting the sampling period Ts may be totaled.

[Effect of the first embodiment]
Thus, in the present embodiment, the synchronization signal extraction circuit 18 extracts the power supply synchronization signal T synchronized with the cycle of the external AC power supply AC from the external AC power supply AC, and the control circuit 14 converts the power supply synchronization signal T into the power supply synchronization signal T. The integrated sampling period Ts is timed, and the flow rate integrated value Fe is calculated by sequentially integrating a value obtained by multiplying the measured result Ts by the flow rate measurement value (instantaneous flow rate) Fs.

  As a result, a value that matches the sampling cycle Ts for calculating the flow rate measurement value (instantaneous flow rate) Fs is used as the sampling cycle Ts for calculating the flow rate integrated value Fe. Therefore, even if the power supply frequency fluctuates, the fluctuation is reflected in the sampling period Ts for calculating the flow integrated value Fe, so that the flow integrated value Fe can be calculated with high accuracy.

In the present embodiment, the control circuit 14 measures the sampling period Ts every sampling period Ts, and multiplies the measured result Ts by the flow rate measurement value (instantaneous flow rate) Fs obtained in the sampling period Ts. The flow rate integrated value Fe may be calculated by sequentially integrating the obtained values.
Thereby, when calculating the flow rate integrated value Fe, the time measurement result Ts and the flow rate measurement value (instantaneous flow rate) Fs obtained in the same period can be used, and even if the power supply frequency fluctuates in a short period, it is extremely high. It becomes possible to calculate the flow rate integrated value Fe with accuracy.

  Note that the sampling period Ts is measured at a period longer than the sampling period Ts, periodically stored in the CPU internal memory, and the sampling period read from the internal memory when calculating the flow rate integrated value Fe. The time measurement result of Ts may be used, the processing load on the control circuit 14 can be reduced, and the power consumption can be reduced.

[Extended embodiment]
The present invention has been described above with reference to the embodiments, but the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

  DESCRIPTION OF SYMBOLS 1 ... Electromagnetic flowmeter, 1A ... Detector, 1B ... Converter, 10 ... Excitation circuit, 11 ... AC amplification circuit, 12 ... Sample hold circuit, 13 ... A / D converter, 14 ... Control circuit, 15 ... Communication circuit , 16 ... Display, 17 ... Power supply circuit, 18 ... Synchronization signal extraction circuit, P ... Measuring tube, TA, TB ... Electrode, L ... Excitation coil, E0 ... Electromotive force, E1 ... AC flow rate signal, V1 ... DC flow rate signal SH1, SH2 ... Sampling control signal, AC ... External AC power supply, T ... Power supply synchronization signal, Ts ... Sampling cycle, Fs ... Flow rate measurement value, Fe ... Flow rate integrated value.

Claims (1)

An electromotive force generated in the fluid in response to excitation of the fluid flowing in the measuring tube is detected by an electrode, and an AC flow signal obtained by AC amplification of this electromotive force is sampled at a sampling period synchronized with an external AC power source. An electromagnetic flow meter that calculates and outputs a flow rate measurement value of the fluid based on the obtained DC flow rate signal,
A synchronization signal extraction circuit for extracting a power supply synchronization signal synchronized with the cycle of the external AC power supply from the external AC power supply;
For each sampling period based on the power supply synchronization signal, the sampling period is timed, and by integrating the value obtained by multiplying the time measurement result by the flow rate measurement value measured in the sampling period , the flow rate integrated value is obtained. An electromagnetic flow meter comprising: a control circuit for calculating.

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