JP2019007813A - Electromagnetic flowmeter and method for correcting electromagnetic flowmeter - Google Patents

Abstract

To obtain an electromagnetic flowmeter, etc., that easily corrects a gain with high accuracy in correspondence to a setting change.SOLUTION: The electromagnetic flowmeter comprises: an excitation circuit 210 for supplying an AC excitation current; a detector 100 for outputting a detection signal that corresponds to the flow rate of a liquid; a reference voltage generation circuit 280 for outputting a pseudo detection signal based on a reference voltage by the excitation current; an AC amplification circuit 220 for amplifying the detection signal; a sample-and-hold circuit 230 for sampling the detection signal amplified by the AC amplification circuit 220 and outputting a DC signal; a DC amplification circuit 240 for amplifying the DC signal; an AD converter 250 for converting the amplified DC signal into a digital signal that includes the corresponding AD count value; and a control computation unit 260 for processing the AD count value. When settings are changed, the control computation unit 260 corrects the gains of the AC amplification circuit 220 and DC amplification circuit 240 on the basis of the AD count values obtained by the pseudo detection signal before and after the change.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electromagnetic flow meter for measuring a flow rate of a fluid having conductivity in various process systems, and a correction method for the electromagnetic flow meter.

  An electromagnetic flow meter is a device that includes, for example, a detector and a transducer, and can measure a fluid flow velocity, a flow rate, and the like. The electromagnetic flow meter includes a detector and a converter. The detector generates a magnetic field in the measurement tube through which the fluid passes, and outputs a signal based on the electromotive force generated by the passage of the fluid. The converter performs a process of converting into a flow rate value or the like based on the signal output from the detector.

  FIG. 4 is a diagram for explaining calibration of a conventional electromagnetic flow meter. In the electromagnetic flow meter as described above, the calibrator 300 is used when performing calibration related to the input / output of the converter (see, for example, Patent Document 1). The calibrator 300 is a device that can artificially generate a voltage waveform signal corresponding to the reference flow rate in accordance with the excitation current from the detector.

JP, 2006-206080, A

  Here, in an electromagnetic flow meter, for example, the optimum excitation frequency and the time during which the signal is stabilized differ depending on the type of detector, the diameter of the pipe through which the fluid flows, and the like. For this reason, there is a demand for changing the excitation frequency when the magnetic field is generated in the detector, the sample hold time when sampling the signal in the converter, and the like in accordance with the measurement situation. Here, when the excitation frequency is changed, the gain of the amplifier circuit and the sample hold time of the converter change. When the sample hold time is changed, the charge / discharge time of the capacitor in the converter changes, and the voltage of the signal input to the AD converter in the converter changes. For this reason, it is necessary to adjust the zero point and the gain again in the converter.

  The adjusted values for the zero point and gain are set at the time of shipment from the factory, and it is difficult to change the adjusted values at the site. For example, if the calibrator 300 of FIG. 4 described above is used, the zero point and gain can be readjusted. However, it takes time to carry the electromagnetic flowmeter into the factory for readjustment. In addition, even when the calibrator 300 is brought to the site and readjustment is performed, it is necessary to change the wiring and connect to the converter for adjustment, and after adjustment, the detector and the converter must be connected again. It takes time and effort. From the above, it was difficult to realize readjustment of the electromagnetic flowmeter.

  The present invention has been made in order to solve such a problem. When the setting is changed, an electromagnetic flowmeter and an electromagnetic flowmeter capable of easily performing high-accuracy correction for the changed gain are provided. An object is to provide a correction method.

  In order to achieve such an object, an electromagnetic flow meter according to the present invention is based on an excitation circuit that supplies an alternating excitation current, a magnetic field based on the excitation current, and an electromotive force that corresponds to the flow rate of the fluid. A detector that outputs a detection signal, a reference voltage generation circuit that outputs a pseudo detection signal based on a reference voltage by an excitation current, an AC amplification circuit that amplifies the detection signal, and a detection signal that is amplified by the AC amplification circuit is sampled A sample-and-hold circuit that processes and outputs a DC signal, a DC amplifier circuit that amplifies the DC signal, and a DC signal amplified by the DC amplifier circuit is converted into a digital signal including an AD count value corresponding to the amplified DC signal And a control arithmetic unit that processes the AD count value. When the setting is changed, the control arithmetic unit performs pseudo detection before and after the change. Based on the AD count value obtained by the signal, and performs gain correction of the AC amplifier and the DC amplifier circuit.

  According to the present invention, the gain in the amplifier circuit is corrected based on the AD count value obtained from the pseudo detection signal output from the reference voltage generation circuit. Therefore, it is possible to easily perform highly accurate gain correction.

It is a block diagram which shows the structure of the electromagnetic flowmeter which concerns on Embodiment 1 of this invention. It is a figure explaining the procedure which concerns on the correction | amendment of the gain G which concerns on Embodiment 1 of this invention. It is a figure which shows the relationship between the duty X and gain G which concern on the sample hold time concerning Embodiment 3 of this invention. It is a figure explaining the calibration of the conventional electromagnetic flowmeter.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a configuration of an electromagnetic flow meter according to Embodiment 1 of the present invention. Hereinafter, embodiments of the present invention will be described with reference to the drawings. The electromagnetic flow meter according to the first embodiment includes a detector 100 and a converter 200.

  The detector 100 has an excitation coil 110 and a measurement tube 120. The exciting coil 110 generates a magnetic field based on the alternating exciting current supplied from the converter 200. The measurement tube 120 is a tube through which the fluid to be measured passes. The exciting coil 110 is disposed so as to be included in the generated magnetic field. Moreover, the measuring tube 120 has an electrode for detecting an electromotive force. According to Faraday's law, an electromotive force is induced in a direction orthogonal to the flowing direction of the conductive fluid and the direction of the magnetic field. The measuring tube 120 detects an electromotive force generated when the fluid flows in the magnetic field. The electromotive force corresponds to the flow rate of the fluid. Then, the fluid flow rate can be derived from the fluid flow velocity and the cross-sectional area of the measurement tube 120. The measurement tube 120 outputs a detection signal corresponding to the flow rate based on the detected electromotive force. The detection signal output from the detector 100 is an AC signal.

  For example, the converter 200 supplies an excitation current to the excitation coil 110 of the detector 100 when performing measurement. And the process of converting the detection signal sent from the detector 100 into a flow value is performed. The converter 200 includes an excitation circuit 210, an AC amplifier circuit 220, a sample hold circuit 230, a DC amplifier circuit 240, an AD (Analog-to-Digital) converter 250, a control arithmetic unit 260, a display unit 270, and a reference voltage generation circuit 280. And a selection circuit 290.

  The excitation circuit 210 generates an excitation current and supplies it to the detector 100. The excitation current is an alternating current based on the excitation frequency. The AC amplifier circuit 220 amplifies the detection signal sent from the detector 100.

  The sample hold circuit 230 is installed on the output side of the AC amplifier circuit 220. By turning on the switch S1 at the timing of the positive side of the exciting current, the signal sampling process is performed on the positive side, and the voltage in the detection signal when the exciting current is positive is held. Further, by turning on the switch S2 at the timing of the negative side of the exciting current, the signal sampling process is performed on the negative side, and the voltage in the detection signal when the exciting current is negative is held. By measuring the voltage of these differences, a detection signal corresponding to the flow rate (flow velocity) is output.

  The DC amplifier circuit 240 amplifies the differential voltage of the detection signal output from the sample hold circuit 230. Although not particularly limited here, the DC amplifier circuit 240 has a programmable gain amplifier, and can change the voltage of the DC signal input to the AD converter 250.

  The AD converter 250 converts the signal obtained by amplifying the differential voltage by the DC amplifier circuit 240 into a signal (digital signal) including an AD count value expressed as a numerical value indicated by the detection signal. For example, the control calculator 260 performs a process of calculating a flow rate value based on the AD count value. Then, a display signal for displaying a flow rate value or the like is sent to the display 270. In addition, the control arithmetic unit 260 of the first embodiment adjusts the zero point and adjusts the gain of the entire amplifier circuit (the AC amplifier circuit 220 and the DC amplifier circuit 240) when, for example, the setting of the electromagnetic flow meter is changed. Processing for performing correction can be performed. The gain correction will be described later. The display 270 performs display based on the display signal sent from the control arithmetic unit 260.

  The reference voltage generation circuit 280 is a circuit that generates a reference voltage inside the converter 200 and outputs a pseudo detection signal based on the excitation current from the excitation circuit 210. The reference voltage generation circuit 280 of the first embodiment, for example, applies two voltages of the same magnitude in opposite directions so that the reference voltage can be stably generated over time and the change due to temperature is suppressed. Has a power supply. As for the magnitude of the voltage applied by the DC power supply, the accuracy is improved if a voltage as high as possible is selected within a range in which the AC amplifier circuit 220, the DC amplifier circuit 240, and the AD converter 250 are not saturated. Therefore, here, for example, a voltage equivalent to a relatively large voltage is applied among the electromotive forces detected by the detector 100, such as ± 1.0 mV.

  For example, the selection circuit 290 performs signal connection between the detector 100 and the AC amplifier circuit 220 or signal connection between the reference voltage generation circuit 280 and the AC amplifier circuit 220 based on an instruction from the control arithmetic unit 260. The circuit to be selected. Therefore, either the detection signal from the detector 100 or the pseudo detection signal from the reference voltage generation circuit 280 is sent to the AC amplification circuit 220.

  Next, the operation of each circuit and the like when measuring the fluid flow rate and the like will be described. The excitation circuit 210 supplies an excitation current to the detector 100. The detector 100 generates a magnetic field based on the excitation current. Fluid in the tube passes through the magnetic field. At this time, an electromotive force based on the flow velocity (flow rate) of the fluid is generated. The detector 100 detects an electromotive force and outputs a detection signal. The detection signal is input to the converter 200. Here, the detection signal input from the detector 100 to the converter 200 is a minute signal on the order of μV. Therefore, in the converter 200, the AC amplification circuit 220 amplifies the input detection signal into a signal having a voltage level sufficient for the conversion of the sample and hold circuit 230. The sample hold circuit 230 converts an AC detection signal into a DC detection signal and outputs it. The DC amplifier circuit 240 amplifies the voltage to a sufficient level for the AD converter 250 to convert the flow rate value. The AD converter 250 converts the detection signal amplified by the DC amplification circuit 240 into a digital signal including an AD count value. Based on the AD count value included in the digital signal from the AD converter 250, the control calculator 260 converts the flow rate value, the flow velocity value, and other data into the display device 270. .

  Next, a method for correcting the zero point and the gain G of the entire amplifying circuit in the electromagnetic flow meter when the settings such as the excitation frequency and the sample hold time are changed in the first embodiment will be described. When the excitation frequency, the sample hold time, etc. are changed, it is necessary to adjust the zero point and gain G inside the converter 200. The zero point is a point adjusted by subtracting a bias such as an offset current that flows even when the fluid flow rate is zero. Here, after the installation, it is possible to create a state where the actual flow rate of the fluid is zero. For this reason, the zero point can be readjusted after installation. Therefore, the gain G corresponding to the setting change may be determined. Here, the gain G can be expressed by the following equation (1). Here, the AD count value is a value that the AD converter 250 represents a direct-current signal as a numerical value and is included in the digital signal.

  G = AD count value at specified flow rate-AD count value at zero point (1)

  For example, the calibrator 300 shown in FIG. 4 described above can accurately generate a voltage signal corresponding to a prescribed flow rate, prescribed flow rate, and the like. Here, the ratio between the gain G before the setting change and the gain G after the setting change is the ratio between the AD count value obtained at the same voltage and the two AD count values obtained at the zero point before and after the change. Will be the same. And the voltage at this time does not need to be a signal voltage corresponding to a prescribed flow rate or flow velocity.

  Therefore, in the first embodiment, the gain G is corrected based on the pseudo detection signal based on the reference voltage related to the generation of the reference voltage generation circuit 280 before and after the setting change. Accordingly, the AD count value for the accurate signal voltage corresponding to the prescribed flow rate (flow velocity) using the calibrator 300 may be only the value acquired at the time of factory shipment (initial setting). The gain G adjusted at the time of factory shipment reflects the AD count value using the calibrator 300 as shown in FIG. This eliminates the need to use a calibrator 300 as shown in FIG.

Then, as expressed by the following equation (2), the gain G related to the correction after the setting change can be derived. Here, the parameters in the equation (2) are defined as follows. Here, G0 is a gain G set at the time of factory shipment.
(A) Adjustment value when operated at excitation frequency or sample hold time before setting change (factory shipment) AD count value corresponding to zero point generated inside converter 200 ... Zref
AD count value corresponding to a reference voltage generated inside converter 200... Gref
(B) Adjustment value when operated at excitation frequency or sample hold time after setting change AD count value corresponding to zero point generated inside converter 200 ... Zset
AD count value corresponding to a reference voltage generated inside converter 200... Gset
(C) Adjustment value set by the calibrator at the time of factory shipment AD count value corresponding to the zero point generated by the calibrator ... Zship
AD count value corresponding to the reference voltage generated by the calibrator ... Gship

G = (Gship−Zship) × (Gset−Zset)
/ (Gref-Zref)
= G0 * (Gset-Zset) / (Gref-Zref) (2)

  FIG. 2 is a diagram for explaining a procedure related to the correction of the gain G according to the first embodiment of the present invention. Here, the description will be made assuming that the control arithmetic unit 260 performs processing. The control arithmetic unit 260 acquires the AD count value Zref and the AD count value Gref when operated at the excitation frequency or sample hold time before the setting change (step ST1). Next, an AD count value Zset and an AD count value Gset when operating at the excitation frequency or sample hold time after the setting change are acquired (step ST2). Then, based on the above-described equation (2), a new gain G for correction is calculated and set (step ST3).

  From the above, the electromagnetic flow meter of the first embodiment has the reference voltage generation circuit 280, so that the AD count value corresponding to the zero point and the reference obtained when the excitation frequency, the sample hold time, and the like are changed. The calibrator 300 as shown in FIG. 4 even when the excitation frequency, the sample hold time, etc. are changed in the field based on the AD count value corresponding to the reference voltage obtained by the voltage applied through the voltage generation circuit 280. The gain G can be easily corrected without using.

Embodiment 2. FIG.
Although not particularly shown in Embodiment 1 described above, in the electromagnetic flow meter of FIG. 1, the DC amplifier circuit 240 has a programmable gain amplifier. For example, conventionally, when a gain in a programmable gain amplifier is switched and a DC signal is sent to the AD converter 250, a zero or the like for each of a plurality of gains related to the switching is previously obtained at the factory or the like using the calibrator 300 of FIG. Adjustments are made using points or the like, and adjustment values are stored as data. In the field, when the gain is switched, the adjustment value is changed using the adjustment value data stored in advance.

  Here, based on the reference voltage from the reference voltage generation circuit 280, the gain can be corrected by the pseudo detection signal. Therefore, for example, in a factory, an adjustment value for one gain related to switching is stored as data by a calibrator 300 as shown in FIG. And about the gain concerning other switching, the control arithmetic unit 260 performs the process demonstrated in Embodiment 1, and calculates by correction | amendment. Thereby, a plurality of gain correction values can be automatically set. Therefore, for example, it is not necessary to store a plurality of gains, and the amount of data to be stored can be reduced.

Embodiment 3 FIG.
FIG. 3 is a diagram showing a relationship between the duty X and the gain G related to the sample hold time according to the third embodiment of the present invention. The configuration of the electromagnetic flow meter used in the third embodiment is basically the same as that of the electromagnetic flow meter described in the first embodiment.

  As described above, the gain G can be expressed by equation (1). Further, the peak voltage due to charging / discharging of the capacitor C1 and the capacitor C2 included in the sample hold circuit 230 is held and becomes a voltage of a DC signal. When the excitation frequency and the sample hold time are changed, as described above, the charge / discharge time of the capacitor C1 and the capacitor C2 in the sample hold circuit 230 of the converter 200 is changed, and the voltage of the DC signal input to the AD converter 250 is changed. Changes. The voltage of the DC signal is proportional to the duty X obtained by dividing the sample hold time by the excitation cycle (the reciprocal of the excitation frequency).

  On the other hand, when the excitation frequency changes, the gain of the amplifier circuit changes. If the influence is changed within the range of the excitation frequency where the influence is small, the relationship between the gain G and the duty X can be approximated by a linear function as G = aX + b as shown in FIG. In FIG. 3, dotted thin lines are lines connecting main actual measurement values. A thick straight line represents a straight line by a linear function in the range of duty X and gain G to which gain correction by approximation can be applied. A thick dotted line indicates a portion that is not applicable.

  Therefore, in the adjustment at the time of factory shipment, the values of the slope (change ratio) a and the intercept (constant) b are stored as adjustment values in a storage device (not shown) included in the control calculator 260. When at least one of the excitation frequency and the sample hold time is changed, the control calculator 260 calculates the duty X, and corrects it with the gain G obtained by inputting the calculated duty X into a linear function.

  As described above, according to the electromagnetic flow meter of the third embodiment, the gain G can be easily corrected based on the changed setting by approximating the relationship between the gain G and the duty X with a linear function. it can.

Embodiment 4 FIG.
In the first to third embodiments described above, the control arithmetic unit 260 has been described as performing the process of correcting the gain G. However, the present invention is not limited to this. For example, a person may correct the gain G based on the obtained AD count value.

  The present invention can be applied not only to an electromagnetic flow meter but also to a calorimeter that measures the amount of heat based on a flow rate related to the measurement of the electromagnetic flow meter, for example.

  DESCRIPTION OF SYMBOLS 100 ... Detector, 110 ... Excitation coil, 120 ... Measuring tube, 200 ... Converter, 210 ... Excitation circuit, 220 ... AC amplification circuit, 230 ... Sample hold circuit, 240 ... DC amplification circuit, 250 ... AD converter, 260 ... control arithmetic unit, 270 ... display, 280 ... reference voltage generation circuit, 290 ... selection circuit, 300 ... calibrator.

Claims (5)

An excitation circuit for supplying an alternating excitation current;
A detector that generates a magnetic field based on the excitation current and outputs a detection signal based on an electromotive force corresponding to the flow rate of the fluid;
A reference voltage generation circuit that outputs the pseudo detection signal based on a reference voltage by the excitation current;
An AC amplifier circuit for amplifying the detection signal;
A sample-and-hold circuit that samples the detection signal amplified by the AC amplifier circuit and outputs a DC signal;
A DC amplifier circuit for amplifying the DC signal;
An AD converter that converts the DC signal amplified by the DC amplifier circuit into a digital signal including an AD count value corresponding to the amplified DC signal;
A control arithmetic unit for processing the AD count value,
The control arithmetic unit is:
When the setting is changed, an electromagnetic flow rate for correcting the gain in the AC amplifier circuit and the DC amplifier circuit based on the AD count value obtained from the pseudo detection signal before and after the change. Total. The preset gain is G0, the AD count value obtained by the pseudo detection signal before the change is Gref, the AD count value related to the zero point before the change is Zref, and the pseudo value after the change is When the AD count value obtained by the detection signal is set as Gset and the AD count value related to the changed zero point is set as Zset,
The control arithmetic unit is:
G = G0 × (Gset−Zset) / (Gref−Zref)
The electromagnetic flow meter according to claim 1, wherein the gain G is calculated to correct the gain.   When the setting of at least one of the excitation frequency in the excitation current and the sample hold time for holding the signal sampled in the sample hold circuit is changed, or when the gain in the programmable gain amplifier needs to be switched, The electromagnetic flow meter according to claim 1 or 2, wherein gain correction is performed.   4. The circuit according to claim 1, further comprising: a selection circuit that selects a signal connection between the detector and the AC amplifier circuit or a signal connection between the reference voltage generation circuit and the AC amplifier circuit. 5. The electromagnetic flow meter as described in the paragraph. An excitation circuit for supplying an alternating excitation current;
A detector that generates a magnetic field based on the excitation current and outputs a detection signal based on an electromotive force corresponding to the flow rate of the fluid;
A reference voltage generation circuit that outputs the pseudo detection signal based on a reference voltage by the excitation current;
An AC amplifier circuit for amplifying the detection signal;
A sample-and-hold circuit that samples the detection signal amplified by the AC amplifier circuit and outputs a DC signal;
A DC amplifier circuit for amplifying the DC signal;
A method for correcting an electromagnetic flowmeter comprising: an AD converter that converts the DC signal amplified by the DC amplifier circuit into a digital signal including an AD count value corresponding to the amplified DC signal,
When changing settings in the circuit,
Obtaining the AD count value based on the pseudo detection signal before the change and the AD count value related to the zero point before the change;
Obtaining the AD count value based on the pseudo detection signal after the change and the AD count value related to the zero point after the change;
And correcting the gain in the AC amplifier circuit and the DC amplifier circuit based on the acquired AD count value.

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