JP6937672B2 - 2-wire electromagnetic flowmeter - Google Patents

Description

The present invention relates to a two-wire electromagnetic flow meter that measures the flow rate of a conductive fluid.

The two-wire electromagnetic flowmeter applies a magnetic field perpendicular to the flow direction of the fluid flowing in the measuring tube, and detects an electromotive force proportional to the flow velocity of the fluid. Then, the two-wire electromagnetic flowmeter adjusts the current output indicating the flow rate of the fluid based on the detected electromotive force. The current output is a current signal in the range of 4 mA to 20 mA output to an external device. Since the 2-wire electromagnetic flowmeter operates using the power supplied from the current output of 4-20 mA, the power consumption can be suppressed, and the power supply and the output by the current are performed by a pair of signal lines. Therefore, it is widely used because it can save wiring.

Here, in the two-wire electromagnetic flowmeter, the component that consumes the most power is an excitation circuit that creates a magnetic field by passing a current through the coil. According to Faraday's law of electromagnetic induction, the signal electromotive force, which indicates a voltage proportional to the flow rate generated between the electrodes, is proportional to the magnetic flux density. Since the magnetic flux density is proportional to the exciting current, the exciting current is proportional to the signal electromotive force.

The current value of the current output of the 2-wire electromagnetic flowmeter changes depending on the flow rate obtained from the signal electromotive force. Therefore, when the flow rate of the fluid in the measuring tube becomes low, the current value of the current output becomes as small as about 4 mA, and the electric power and the exciting current supplied to each circuit become insufficient. Therefore, the two-wire electromagnetic flowmeter needs to change the exciting current according to the change in the current output.

Therefore, in the conventional two-wire electromagnetic flowmeter, a threshold value for the current output is set, and the exciting current is changed stepwise to adjust so that the electric power is not insufficient (see, for example, Patent Document 1). However, when the exciting current is changed stepwise, a phenomenon such as a whiskers showing a sudden change occurs in the measured value in the transient section of the switching of the exciting current due to the predetermined span correction. Therefore, in some 2-wire electromagnetic flowmeters, the flow rate calculation is interrupted in 1 second immediately after the switching of the exciting current, and the previous value hold process using the flow rate immediately before the switching of the exciting current is performed.

Japanese Unexamined Patent Publication No. 2004-61451

However, unlike the conventional two-wire electromagnetic flowmeter, when the previous value hold process is performed, there is a problem that the accuracy of the current output for one second during which the flow rate calculation is interrupted is lowered. Further, if the magnitude relationship between the current output and the threshold value is reversed in 1 second of the previous value hold process, the flow rate immediately before the initial switching is maintained for another 1 second. That is, if the reversal of the magnitude relationship between the current output and the threshold value is repeated within 1 second of the previous value hold process, the current output sticks and the accuracy of the current output is further lowered.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a two-wire electromagnetic flow meter that realizes an improvement in the accuracy of a current output indicating the flow rate of a fluid flowing in a measuring tube.

The two-wire electromagnetic flowmeter according to the present invention has an exciting coil that applies a magnetic field to the fluid flowing in the measuring tube, and has a detector that detects a signal electromotive force generated according to the flow velocity of the fluid flowing in the measuring tube. A converter that adjusts the current output that indicates the flow rate of the fluid flowing through the measuring tube based on the signal electromotive force detected by the detector is provided, and the converter applies an exciting current of a predetermined exciting frequency to the exciting coil. The excitation circuit to be supplied, the AD converter that generates an AD count value indicating the flow velocity from the analog signal based on the signal electromotive force, the current control circuit that adjusts the current output, and the change in the AD count value generated by the AD converter. A control device that continuously changes the exciting current supplied by the exciting circuit to the exciting coil according to the current output according to the current output.

According to the present invention, since the exciting current is continuously changed according to the change of the AD count value by following the current output, the occurrence of whiskers can be prevented and the previous value holding process becomes unnecessary. Therefore, it is possible to improve the accuracy of the current output indicating the flow rate of the fluid flowing in the measuring tube.

It is a block diagram which shows schematic structure of the electromagnetic flowmeter which concerns on Embodiment 1 of this invention. It is explanatory drawing which shows typically the flow of the electric current when the 2nd switch and the 3rd switch are turned on in the electromagnetic flowmeter of FIG. It is explanatory drawing which shows typically the flow of the electric current when the 1st switch and the 4th switch are turned on in the electromagnetic flowmeter of FIG. It is a block diagram which shows the functional structure of the control device of FIG. It is a graph which shows the change of the AD count value and the flow rate ratio when the flow velocity is lowered with time at a constant rate in the electromagnetic flow meter of FIG. FIG. 5 is a graph showing changes in the AD count value and the flow rate ratio when the flow velocity is decreased with time at a constant rate and the gain is switched several times in the electromagnetic flow meter of FIG. 1. It is a graph which shows the change of the AD count value and the flow rate ratio when the exciting current and the flow velocity are lowered with time at a constant rate, and the gain is switched several times in the electromagnetic flow meter of FIG. It is a comparative example of the simulation of FIG. 7, and is a graph which shows the change of the AD count value and the flow rate ratio when the current correction is not performed. It is explanatory drawing which illustrated the calibrator used for the simulation of FIGS. It is a block diagram which shows schematic structure of the electromagnetic flowmeter which concerns on Embodiment 2 of this invention. It is a block diagram which shows the functional structure of the control device of FIG. FIG. 5 is a graph showing changes in the AD count value and the flow rate ratio when the flow velocity is decreased with time at a constant rate in the electromagnetic flow meter of FIG. 10. FIG. 5 is a graph showing changes in the AD count value and the flow rate ratio when the exciting current and the flow velocity are reduced with time at a constant rate and the gain is switched several times in the electromagnetic flow meter of FIG. 10. It is a block diagram which shows schematic structure of the electromagnetic flowmeter which concerns on Embodiment 3 of this invention. It is a table which shows the ratio of the signal electromotive force at a certain exciting current to the signal electromotive force at another exciting current for each of the electromagnetic flowmeters provided with detectors having different diameters of measuring tubes. It is a block diagram which shows an example of the functional structure of the control device of FIG. FIG. 6 is a block diagram showing another example of the functional configuration of the control device of FIG. It is a graph which shows the state which the exciting current was switched stepwise in the conventional 2-wire type flowmeter. It is a graph which shows the change of the AD count value when the exciting current is changed as shown in FIG. 18 in the conventional two-wire flowmeter. It is a graph which shows the change of the AD count value when the span correction is switched in the initial time zone of FIG. It is a graph which shows the change of the AD count value when the span correction is switched in the middle time zone of FIG. It is a graph which shows the change of the AD count value when the span correction is switched in the late time zone of FIG.

Embodiment 1.
FIG. 1 is a block diagram schematically showing a configuration of an electromagnetic flowmeter according to a first embodiment of the present invention. FIG. 2 is an explanatory diagram schematically showing the current flow when the second switch and the third switch are turned on in the electromagnetic flowmeter of FIG. FIG. 3 is an explanatory diagram schematically showing the current flow when the first switch and the fourth switch are turned on in the electromagnetic flowmeter of FIG. The overall configuration of the electromagnetic flowmeter 100 will be described with reference to FIGS. 1 to 3.

The electromagnetic flow meter 100 is a two-wire electromagnetic flow meter that measures the flow rate of a conductive fluid. A voltage is supplied to the electromagnetic flowmeter 100 from the external power supply 500 via a pair of signal lines, the signal line L1 and the signal line L2. As the external power supply 500, for example, a DC power supply of 24 V DC can be used. A resistor of, for example, 250Ω is connected to the signal line L2 as an external load RL. Therefore, the voltage supplied to the electromagnetic flowmeter 100 is obtained by subtracting the voltage drop in the external load RL from the external voltage Vs by the external power supply 500.

That is, the electromagnetic flowmeter 100 controls the current flowing through the signal line L1 and the signal line L2, which are a pair of signal lines connected to the external power supply 500, and thereby indicates a current output indicating the flow rate of the fluid flowing in the measuring tube 11. Adjust Iout. Then, the current output Iout flows through the external load RL, and the external device measures from the voltage across the external load RL (1V to 5V when the resistance value of the external load RL is 250Ω). The flow rate of the fluid flowing in the pipe 11 can be known.

As shown in FIG. 1, the electromagnetic flowmeter 100 includes a detector 10 and a converter 20. The detector 10 includes a measuring tube 11, an exciting coil 12, an electrode 13a, an electrode 13b, and a ground electrode 14. The measuring tube 11 is formed in a pipe shape and allows a fluid to pass through. The electromagnetic flowmeter 100 is arranged so that a conductive fluid flows through the measuring tube 11. The exciting coil 12 generates a magnetic field in the measuring tube 11 and applies the magnetic field to the fluid flowing in the measuring tube 11. The exciting coil 12 is arranged so that the direction in which the magnetic field is generated is perpendicular to the flow direction of the fluid flowing in the measuring tube 11.

The electrodes 13a and 13b are arranged in the measuring tube 11 so that the straight line connecting the electrodes 13a and 13b is orthogonal to the flow direction of the fluid flowing in the measuring tube 11 and the direction of the magnetic field generated by the exciting coil 12. Is located in. The electrodes 13a and 13b form a pair of electrodes for extracting the signal electromotive force E. The signal electromotive force E is a voltage proportional to the flow rate of the fluid generated between the electrodes 13a and 13b. That is, the detector 10 detects the signal electromotive force E based on the electromotive force generated by the fluid flowing in the magnetic field generated by the exciting coil 12 by the electrodes 13a and 13b, and detects the detected signal electromotive force E. Output to the converter 20.

Here, the signal electromotive force E, the magnetic flux density B [N / A · m], the diameter (inner diameter) D [m] of the measuring tube 11, and the flow velocity of the fluid flowing in the measuring tube 11 (fluid velocity in the average axial direction). ) V [m / s] has the relationship of the following equation (1) using the constant k.

[Number 1]
E = kBDV ・ ・ ・ (1)

The converter 20 obtains the flow rate of the fluid in the measuring tube 11 based on the signal electromotive force E detected by the detector 10. The converter 20 includes an AC amplifier circuit 30, a sample hold circuit 40, an AD converter 50, a control device 60, a storage device 70, an excitation circuit 80, and a current control circuit 90.

The exciting circuit 80 generates an exciting current Iex having an exciting frequency F whose polarity changes alternately in response to a frequency command signal from the control device 60, and supplies the generated exciting current Iex to the exciting coil 12. The excitation circuit 80 includes an excitation switching circuit 81 and a switch circuit 82.

The switch circuit 82 has a first switch SW1, a second switch SW2, a third switch SW3, and a fourth switch SW4. The first switch SW1, the second switch SW2, the third switch SW3, and the fourth switch SW4 are configured by a switching element such as a MOSFET (Metal-Oxide-Semiconductor Field-Effective Transistor) or an IGBT (Insulated Gate Bipolar Transistor). be able to.

The connection point P between the first switch SW1 and the second switch SW2 is connected to the positive side of the external power supply 500. The connection point Q between the third switch SW3 and the fourth switch SW4 is connected to the current control circuit 90. The connection point R between the first switch SW1 and the third switch SW3 is connected to the terminal Y of the detector 10. The connection point S between the second switch SW2 and the fourth switch SW4 is connected to the terminal X of the detector 10. That is, the exciting coil 12 is connected between the connection point R and the connection point S.

The excitation switching circuit 81 is in an XY state in which the second switch SW2 and the third switch SW3 are turned on as shown in FIG. 2 according to the frequency command signal from the control device 60, and the first switch SW1 and the fourth as shown in FIG. The YX state in which the switch SW4 is turned on is repeated at a predetermined cycle. That is, the excitation switching circuit 81 generates a rectangular wave-shaped exciting current Iex in which the polarities are alternately switched by controlling the on / off operation of each switch of the switch circuit 82. In the XY state, the first switch SW1 and the fourth switch SW4 are off, and in the YX state, the second switch SW2 and the third switch SW3 are off. In the XY state, the exciting current Iex flows from the terminal X to the terminal Y, and in the YX state, the exciting current Iex flows from the terminal Y to the terminal X.

Here, in the electromagnetic flowmeter 100, the two switches constituting the switch circuit 82, the exciting coil 12, and the current control circuit 90 are connected in series between the signal line L1 and the signal line L2. That is, in the state of FIG. 2, the external power supply 500, the second switch SW2, the exciting coil 12, the third switch SW3, the current control circuit 90, and the external load RL form a closed loop circuit. Further, in the state of FIG. 3, the external power supply 500, the first switch SW1, the exciting coil 12, the fourth switch SW4, the current control circuit 90, and the external load RL form a closed loop circuit. Therefore, in the electromagnetic flow meter 100, the magnitude of the current output Iout and the magnitude of the exciting current Iex are equal to each other.

The current control circuit 90 adjusts the current output Iout in the range of 4 mA to 20 mA according to the current control signal from the control device 60. More specifically, the current control circuit 90 is configured so that the resistance value is variable, and adjusts the current output Iout by changing the resistance value according to the current control signal from the control device 60. In the electromagnetic flow meter 100, since the current output Iout and the exciting current Iex are equal, the current control circuit 90 also adjusts the exciting current Iex according to the current control signal from the control device 60.

The AC amplifier circuit 30 acquires the signal electromotive force E from the detector 10 and amplifies it. The sample hold circuit 40 is arranged on the output side of the AC amplifier circuit 30, and operates in response to the SH time control signal from the control device 60. The sample hold circuit 40 holds the signal electromotive force E amplified by the AC amplifier circuit 30 at a value immediately before the polarity of the exciting current Iex is switched, and holds the sample.

The AD converter 50 is an AD converter having a DC amplification function. The AD converter 50 includes a gain setting unit 51, a DC amplification unit 52, and a conversion unit 53. The gain setting unit 51 is composed of, for example, a PGA (Programable Gain Amplifier), and sets the gain G of the converter 20 according to the gain adjustment signal from the control device 60. The DC amplification unit 52 amplifies the signal electromotive force E sample-held by the sample-hold circuit 40 by using the gain G set by the gain setting unit 51. The conversion unit 53 generates an AD count value indicating the flow velocity V from the analog signal based on the signal electromotive force E. That is, the conversion unit 53 converts the analog signal electromotive force E amplified by the DC amplification unit 52 into an AD count value which is a digital value.

The storage device 70 stores various data related to the equations (2) to (5) described later, an operation program of the control device 60, and the like. For example, the storage device 70, a zero reference value D _0, and span the reference value D _S, it is stored. Zero reference value D ₀ and span the reference value D _S, respectively, and the reference excitation current I ₀ is the reference of the excitation current Iex, and the reference excitation frequency F ₀ is a measure of the excitation frequency F, which is a reference of the gain G reference It is obtained by using the gain G _0.

That is, the zero reference value D ₀ is the AD count value at the zero point when the reference excitation current I ₀ , the reference excitation frequency F ₀ , and the reference gain G _0. The zero point is a point adjusted by subtracting a bias such as an offset current that flows even when the flow rate of the fluid is zero. More specifically, the zero reference value D ₀ has the excitation current Iex as the reference excitation current I ₀ , the excitation frequency F as the reference excitation frequency F ₀ , the gain G as the reference gain G _0, and the contact A and the contact in FIG. It is an AD count value obtained in a state where B and contact C are short-circuited.

Span reference value _{D S} is the reference excitation current _{I 0,} the reference excitation frequency _{F 0,} and when the reference gain _{G 0,} is AD count value based on the maximum flow rate. More specifically, the span reference value D _S is a value obtained by subtracting the zero reference value D ₀ from the maximum adjustment count value D _MAX. The maximum adjustment count value D _MAX is the maximum value of the detectable flow velocity when the exciting current Iex is set to the reference exciting current I ₀ , the exciting frequency F is set to the reference exciting frequency F ₀ , and the gain G is set to the reference gain G _0. It is an AD count value based on the electromotive force corresponding to (100% value).

For example, the reference excitation current I ₀ is set to 20 [mA], the reference excitation frequency F ₀ is set to 22.5 [Hz], and the reference gain G ₀ is set to 1. Zero reference value D ₀ and span the reference value D _S is set like before shipment of the manufacturing plant.

FIG. 4 is a block diagram showing a functional configuration of the control device of FIG. The control device 60 reads the AD count value from the AD converter 50 for each measurement cycle, and continuously changes the exciting current Iex according to the change in the read AD count value by following the current output Iout. The measurement cycle is set to, for example, 200 [ms] and can be changed as appropriate. As shown in FIG. 4, the control device 60 includes a zero adjustment value calculation unit 61, a span adjustment value calculation unit 62, a flow velocity calculation unit 63, a current control unit 64, an excitation control unit 65, and a gain adjustment unit 66. And an SH time adjusting unit 67.

The zero adjustment value calculation unit 61 obtains _{the zero adjustment count value C ZERO} at an arbitrary excitation current Iex and gain G by correcting the _{zero reference value D 0 using the gain G.} That is, the zero adjustment value calculation unit 61 obtains the zero adjustment count value C _{ZERO by multiplying the zero reference value D 0} by the gain G based on the following equation (2).

[Number 2]
C _ZERO = D ₀ x G ... (2)

Span adjustment value calculation unit 62, by correcting the span reference value D _S by using the ratio and the gain G to the reference excitation current I ₀ of the excitation current Iex, span adjustment in the case of any excitation current Iex and gain G Find the count value C _SPAN . That is, the span adjustment value calculation unit 62, based on the following equation (3), the span reference value _{D S,} the span adjustment count value _{C SPAN} by multiplying the rate and gain G to the reference excitation current _{I 0} of the excitation current Iex Ask.

[Number 3]
_{C SPAN = D S × (Iex} / I 0) × G ··· (3)

The flow velocity calculation unit 63 uses the AD count value, the zero adjustment count value C _ZERO , and the span adjustment count value C _SPAN for each measurement cycle generated by the AD converter 50, and uses the following equation (4) to perform the flow velocity according to the following equation (4). Calculate V. Here, the maximum flow velocity (100% flow velocity) is the maximum value of the detectable flow velocity V, and is determined according to the variation in the circuit characteristics of the electromagnetic flow meter 100, etc., and is stored in the storage device 70 in advance in calibration or the like. It will be remembered.

[Number 4]
Flow velocity V [m / s] = Maximum flow velocity [m / s] × (AD count value-C _ZERO ) / C _SPAN ... (4)

That is, the flow velocity calculation unit 63 _{divides the value obtained by multiplying the maximum value of the flow velocity V by the value obtained by subtracting the zero adjustment count value C ZERO} from the current AD count value by the span adjustment count value C _SPAN , and divides the flow velocity. Find V. The flow velocity calculation unit 63 uses the flow velocity V obtained by the equation (4) and the diameter D of the measuring tube 11 to obtain the instantaneous flow rate, which is the flow rate per 1 [s], by the following equation (5). May have.

[Number 5]
Instantaneous flow rate [m ³ / s] = flow velocity V [m / s] x π x caliber [m] x caliber [m] / 4 ... (5)

The current control unit 64 adjusts the resistance value of the current control circuit 90 according to the change of the flow velocity V determined according to the change of the AD count value, the exciting current Iex, and the gain G, and continuously changes the current output Iout. .. That is, the current control unit 64 adjusts the resistance value of the current control circuit 90 so that the current output Iout increases as the flow velocity V increases and the current output Iout decreases as the flow velocity V decreases, and the current output Iout is adjusted. Change continuously. More specifically, the current control unit 64 generates a current control signal according to the flow velocity V calculated by the flow velocity calculation unit 63, and outputs the current control signal to the current control circuit 90, whereby the current control circuit 90 Adjust the resistance value. As a result, the current output Iout and the exciting current Iex continuously change according to the change in the flow velocity V.

The excitation control unit 65 controls the excitation frequency F of the excitation current Iex. That is, the excitation control unit 65 adjusts the excitation frequency F of the excitation current Iex by generating a frequency command signal indicating the excitation frequency F of the excitation current Iex and outputting the frequency command signal to the excitation circuit 80.

The excitation control unit 65 controls the excitation frequency F according to a change in the current output Iout, that is, a change in the flow velocity V calculated by the flow velocity calculation unit 63. That is, the excitation control unit 65 lowers the excitation frequency F when the current output Iout is relatively small, and raises the excitation frequency F when the current output Iout is relatively large.

The gain adjusting unit 66 adjusts the gain G of the converter 20 according to a change in the current output Iout, that is, a change in the flow velocity V calculated by the flow velocity calculation unit 63. That is, the gain adjusting unit 66 adjusts the gain G used by the DC amplification unit 52 by outputting a gain adjusting signal instructing the adjustment of the gain G according to the change in the current output Iout to the gain setting unit 51. The SH time adjustment unit 67 outputs an SH time control signal including a clock used for sample holding of the signal electromotive force E to the sample hold circuit 40.

The control device 60 can be configured by an arithmetic unit such as a CPU (Central Processing Unit) or a DSP (Digital Signal Processor), and an operation program that realizes the above-mentioned various functions in cooperation with such an arithmetic unit. The storage device 70 can be configured by a RAM (Random Access Memory) and a ROM (Read Only Memory), a PROM (Programmable ROM) such as a flash memory, an HDD (Hard Disk Drive), or the like.

FIG. 5 is a graph showing changes in the AD count value and the flow rate ratio when the flow velocity is decreased with time at a constant rate in the electromagnetic flow meter of FIG. 1. FIG. 6 is a graph showing changes in the AD count value and the flow rate ratio when the flow velocity is decreased with time at a constant rate and the gain is switched several times in the electromagnetic flow meter of FIG. 1. FIG. 7 is a graph showing changes in the AD count value and the flow rate ratio when the exciting current and the flow velocity are decreased at a constant rate over time in the electromagnetic flow meter of FIG. 1 and the gain is switched several times. be. FIG. 8 is a comparative example of the simulation of FIG. 7, and is a graph showing changes in the AD count value and the flow rate ratio when the current correction is not performed. FIG. 9 is an explanatory diagram illustrating the calibrator used in the simulation of FIGS. 5 to 8. In FIGS. 5 to 8, the horizontal axis represents time [s], and the vertical axis represents the AD count value and the flow rate ratio [%].

Here, the calibrator 300 illustrated in FIG. 9 is generally used in an electromagnetic flowmeter when calibrating the input / output of a converter. The calibrator 300 has a function of generating a signal having a voltage waveform corresponding to a reference flow rate in a pseudo manner according to the exciting current Iex from the converter 20.

The simulations of FIGS. 5 to 8 confirm whether or not the flow velocity V can be accurately obtained when the gain G, the exciting frequency F, and the exciting current are continuously changed in conjunction with the flow rate of the fluid. This is the result. In the simulations of FIGS. 5 to 8, the calibrator 300 is used to generate a signal voltage synchronized with the excitation frequency F and proportional to the current output Iout. The calibrator 300 is set to reduce the signal voltage by 0.1 [%] at 1-second intervals from the value corresponding to the flow rate ratio of 100 [%]. The excitation frequency F is set to be continuously lowered from 22.5 [Hz] to 3.125 [Hz] with respect to the flow rate ratio of 100 [%] to 0 [%]. The exciting current Iex and the current output Iout are the same.

In the simulations of FIGS. 6 to 8, the upper limit of the AD count value is set to "200,000" and the lower limit of the AD count value is set to "50,000". The converter 20 is set so that the gain G is switched in four steps of 1 time, 4 times, 16 times, and 64 times. That is, the converter 20 is set so that the initial value of the gain G is set to 1 and the gain G is switched when the AD count value drops to the lower limit value. The converter 20 calculates the flow rate ratio [%] from the measured AD count value by a calculation based on the above equations (2) to (5) and records a trend of 1000 seconds in a sampling cycle of 200 [ms]. It is set to do. Here, the flow rate ratio [%] is a value obtained by converting the instantaneous flow rate into a ratio, where the maximum value of the instantaneous flow rate obtained by the equation (5) is 100 [%].

According to the electromagnetic flow meter 100 of the first embodiment, the flow rate ratio [%] is a signal not only when the gain G is not changed as shown in FIG. 5 but also when the gain G is changed as shown in FIG. It can be seen that the voltage changes continuously following the change in voltage. By the way, in FIG. 6, the ideal straight line of the flow rate ratio [%] is shown by a broken line. Here, comparing the ideal straight line with the straight line representing the measured value of the flow rate ratio [%], although there are slight differences in the slope and the low flow rate region, according to the electromagnetic flow meter 100, the exciting current as in the conventional case. The measured value can be calculated more accurately than the configuration in which the Iex is switched stepwise.

Further, as in the comparative example of FIG. 8, _{unless the current correction corresponding to “Iex / I 0} ” of the equation (3) is performed, the flow rate ratio [%] does not change linearly, and an accurate measured value can be obtained. Cannot calculate. In this respect, the electromagnetic flowmeter 100 calculates the flow velocity V using the _{span adjustment count value CSPAN} obtained by the equation (3) including the current correction. Therefore, as shown in FIG. 7, the flow rate ratio [%] can be linearly changed by following the change in the signal voltage.

As described above, the electromagnetic flow meter 100 of the first embodiment continuously changes the exciting current Iex according to the change of the AD count value by following the current output Iout, thereby preventing the occurrence of whiskers. This can be done, and the previous value hold process becomes unnecessary. Therefore, it is possible to improve the accuracy of the current output Iout, which indicates the flow rate of the fluid flowing in the measuring tube.

Further, the electromagnetic flow meter 100 is configured so that the magnitude of the current output Iout and the magnitude of the exciting current Iex are equal to each other. Therefore, since it is not necessary to control the current output Iout and the exciting current Iex individually, it is necessary to separately provide a constant current circuit for controlling the current output Iout and a constant current circuit for controlling the exciting current Iex. Therefore, the exciting voltage circuit as in Patent Document 1 is not required. That is, as shown in FIG. 1, the electromagnetic flowmeter 100 needs to be provided with only one current control circuit 90 as a constant current circuit for current control, so that the configuration can be simplified and the cost can be reduced. , It is possible to simplify the arithmetic processing.

Furthermore, the electromagnetic flow meter 100, as a reference value for determining the flow velocity V, it is sufficient to configure only two of the zero reference value D ₀ and span the reference value D _S. Therefore, as compared with the conventional configuration in which the exciting current Iex is changed stepwise, the number of adjustment points at the time of shipment can be reduced, so that the time required for adjustment can be reduced. Since the electromagnetic flowmeter 100 incorporates the current correction, the calculation accuracy of the flow velocity V is improved, so that the accuracy of the current output Iout can be improved.

In addition, in the conventional configuration in which the exciting current is changed stepwise, when the calibrator 300 performs zero-span adjustment or pseudo input, if the calibrator 300 generates a signal voltage corresponding to the stepwise switching of the exciting current. Since the switching determination of the calibrator 300 and the switching determination of the converter are not synchronized, a contradiction may occur in the signal voltage at the switching point. On the other hand, when the electromagnetic flow meter 100 that continuously changes the exciting current Iex is used, even if the calibrator 300 does not have the function of accurately measuring the alternating current exciting current Iex, the current of 4-20 mA which is a direct current is 4-20 mA. From the output Iout, the calibrator 300 can output a signal voltage corresponding to the exciting current Iex and the exciting frequency F required by the converter 20.

By the way, in the first embodiment, the reference excitation frequency F ₀ is set to 22.5 [Hz], but the present invention is not limited to this, and the reference excitation frequency F ₀ starts from 3.125 [Hz]. It can be set within a range of up to 22.5 [Hz] and changed as appropriate. Similarly, the reference exciting current I ₀ and the reference gain G ₀ can be changed as appropriate.

Embodiment 2.
FIG. 10 is a block diagram schematically showing the configuration of the electromagnetic flowmeter according to the second embodiment of the present invention. Since the overall configuration of the electromagnetic flowmeter 100A of the second embodiment is the same as that of the first embodiment, the same reference numerals are used for equivalent constituent members, and the description thereof will be omitted.

The storage device 70 of the second embodiment includes various data related to the above-mentioned equations (4) and (5) and the following equations (6) to (9), an operation program of the control device 60, and the like. Is remembered. For example, the storage device 70, a first zero reference value _{D 01,} and the second zero reference value _{D 02,} the first span reference value _{D S1,} a second span reference value _{D S2,} are stored.

The first zero reference value D ₀₁ is an AD count value at the zero point when the reference excitation current I ₀ , the first reference excitation frequency _FL , and the reference gain G _0. The second zero reference value D ₀₂ is an AD count value at the zero point when the reference excitation current I ₀ , the second reference excitation frequency _HF , and the reference gain G _0. The first span reference value _DS1 is an AD count value based on the electromotive force corresponding to the maximum value of the detectable flow velocity when the reference excitation current I ₀ , the first reference excitation frequency _FL , and the reference gain G _0. be. The second span reference value _DS2 is an AD count value based on the electromotive force corresponding to the maximum value of the detectable flow velocity when the reference excitation current I ₀ , the second reference excitation frequency F _H , and the reference gain G _0. be. Here, the second reference excitation frequency F _H is set to be higher than the first reference excitation frequency F _L.

For example, the reference exciting current I ₀ is set to 20 mA and the reference gain G ₀ is set to 1. The first reference excitation frequency _{F L} is set to the minimum value of the excitation frequency F 3.125 [Hz], the second reference excitation frequency _{F H} is the maximum value of the excitation frequency F 22.5 [ It is set to [Hz]. The first zero reference value D ₀₁ , the second zero reference value D ₀₂ , the first span reference value D _S1 , and the second span reference value D S ₂ are set before shipment at the manufacturing factory.

FIG. 11 is a block diagram showing a functional configuration of the control device of FIG. As shown in FIG. 11, the control device 60A includes a zero adjustment value calculation unit 61A, a span adjustment value calculation unit 62A, a flow velocity calculation unit 63, a current control unit 64, an excitation control unit 65, and a gain adjustment unit 66. And an SH time adjusting unit 67.

The zero adjustment value calculation unit 61A uses the first zero reference value D ₀₁ and the second zero reference value D _{02 according} to the following equation (6) to obtain an arbitrary exciting current Iex, an exciting frequency F, and a gain G. The zero adjustment count value C _ZERO at the time of is obtained.

[Number 6]
_{C ZERO = [(D 02 -D} 01) × (F-F L) / (F H -F L) + D 01] × G ··· (6)

That is, zero adjustment value calculation unit 61A includes a first zero reference value _{D 01} and the first reference excitation frequency _{F L,} by linear interpolation based on the second zero reference value _{D 02} and the second reference excitation frequency _{F H,} the current The zero reference count value corresponding to the excitation frequency F of is obtained. Then, the zero adjustment value calculation unit 61A obtains the zero adjustment count value C _ZERO by multiplying the obtained zero reference count value by the gain G of the converter 20.

Span adjustment value calculation unit 62A includes a first span reference value _{D S1,} by using the second span reference value _{D S2,} by the following equation (7), any excitation current Iex, excitation frequency F, and the gain G The span adjustment count value _CSPAN at the time of is obtained.

[Number 7]
_{C SPAN = [(D S2 -D} S1) × (F-F L) / (F H -F L) + D S1] × (Iex / I 0) × G ··· (7)

That is, the span adjustment value calculation unit 62A includes a first span reference value _{D S1} and the first reference excitation frequency _{F L,} by linear interpolation based on the second span reference value _{D S2} and second reference excitation frequency _{F H,} the current The span reference count value corresponding to the excitation frequency F of is obtained. Then, the span adjustment value calculation unit 62A obtains the span adjustment count value C _{SPAN by multiplying the obtained span reference count value by the gain G.}

Here, the reference exciting current _{I 0} is set to 20 mA, the first reference excitation frequency _{F L} is set to 3.125 [Hz], the second reference excitation frequency _{F H} is set to 22.5 [Hz] In this case, the equation (6) becomes the following equation (8), and the equation (7) becomes the following equation (9).

[Number 8]
C _ZERO = [(D _02- D ₀₁ ) x (F-3.125) /19.375 + D ₀₁ ] x G ... (8)

[Number 9]
_{C SPAN = [(D S2 -D} S1) × (F-3.125) / (19.375) + D S1] × (Iex / I 0) × G ··· (9)

The flow velocity calculation unit 63 of the second embodiment includes the current AD count value, the zero adjustment count value C _{ZERO obtained by the zero adjustment value calculation unit 61A, and the span adjustment count value C SPAN} obtained by the span adjustment value calculation unit 62A. Is used to calculate the flow velocity V from the equation (4). Other configurations of the control device 60A are the same as those of the control device 60 of the first embodiment.

FIG. 12 is a graph showing changes in the AD count value and the flow rate ratio when the flow velocity is reduced with time at a constant rate in the electromagnetic flow meter of FIG. FIG. 13 is a graph showing changes in the AD count value and the flow rate ratio when the exciting current and the flow velocity are reduced with time at a constant rate and the gain is switched several times in the electromagnetic flow meter of FIG. be. The simulations of FIGS. 12 and 13 are performed under the same conditions as the simulations of FIGS. 5 to 8.

The electromagnetic flowmeter 100A of the second embodiment incorporates excitation frequency correction, which is a linear complement between two points, in the calculation of the _{zero adjustment count value C ZERO} and the span adjustment count value C _SPAN. Therefore, as shown in FIG. 12, the slope of the straight line representing the measured value of the flow rate ratio [%] is closer to the ideal straight line than in the case of FIG. More specifically, it can be seen that the slope of the straight line representing the measured value of the flow rate ratio [%] is closer to the slope of the ideal straight line in the electromagnetic flow meter 100A than in the electromagnetic flow meter 100. Therefore, in FIG. 12, it is difficult to visually recognize the ideal straight line except in the low flow rate region. Further, in FIG. 12, it can be seen that the accuracy of the measured value is improved as compared with the case of FIG. 6 even in the low flow rate region.

As described above, the electromagnetic flow meter 100A of the second embodiment can also improve the accuracy of the current output Iout indicating the flow rate of the fluid flowing in the measuring tube, as in the electromagnetic flow meter 100 of the first embodiment. can. Further, since the electromagnetic flow meter 100A is also configured so that the magnitude of the current output Iout and the magnitude of the exciting current Iex are equal to each other, the configuration can be simplified, the cost can be reduced, and the arithmetic processing can be simplified. Can be planned.

In addition, the electromagnetic flowmeter 100A has a first zero reference value D ₀₁ , a second zero reference value D ₀₂ , a first span reference value D _S1 , and a second span reference value D as reference values for obtaining the flow velocity V. _{Only four with S2} need to be set. Therefore, as compared with the conventional configuration in which the exciting current Iex is changed stepwise, the number of adjustment points at the time of shipment can be reduced, so that the time required for adjustment can be reduced. Since the electromagnetic flowmeter 100A incorporates the excitation frequency correction together with the current correction, the calculation accuracy of the flow velocity V is further improved, so that the accuracy of the current output Iout can be improved. Further, since the electromagnetic flow meter 100A continuously changes the exciting current Iex, the exciting current Iex required by the converter 20 is input by inputting the current output Iout of 4-20 mA, which is a direct current, to the calibrator 300. And the signal voltage corresponding to the excitation frequency F can be output.

Incidentally, in the second embodiment, the first reference excitation frequency _{F L} is set to 3.125 [Hz], but the second reference excitation frequency _{F H} is an example that is set to 22.5 [Hz] , Not limited to this. The first reference excitation frequency _{F L} and the second reference excitation frequency _{F H,} unless the condition that _"F L _{<F H",} in the range from 3.125 [Hz] to 22.5 [Hz], as appropriate You can change the settings. Similarly, the reference exciting current I ₀ and the reference gain G ₀ can be changed as appropriate.

Embodiment 3.
FIG. 14 is a block diagram schematically showing the configuration of the electromagnetic flowmeter according to the third embodiment of the present invention. FIG. 15 is a table showing the ratio of the signal electromotive force at one exciting current to the signal electromotive force at another exciting current for each of the electromagnetic flowmeters provided with detectors having different diameters of measuring tubes. Since the overall configuration of the electromagnetic flowmeter 100B of the third embodiment is the same as that of the first and second embodiments, the same reference numerals are used for the equivalent constituent members, and the description thereof will be omitted.

The calculation of the flow velocity V described in the first and second embodiments is correct on the assumption that the exciting current Iex and the magnetic flux density B (signal electromotive force E) are in a proportional relationship. However, in reality, the magnetic flux density B is curved due to the characteristics of the coil of the detector 10. This point will be described with reference to FIG.

In FIG. 15, the diameter D of the measuring tube 11 is 5 [mm], 10 [mm], 15 [mm], 25 [mm], 40 [mm], 50 [mm], 65 [mm], 80 [mm]. ], 100 [mm], and 200 [mm], and various data relating to each of the electromagnetic flowmeters 100 are shown. The measurement process with the electromagnetic flow meter 100 having the diameters D of the measuring tube 11 of 5 [mm], 100 [mm], and 200 [mm] is performed by setting the excitation frequency F to 17.5 [Hz]. Measurement with an electromagnetic flowmeter 100 whose diameters of the measuring tube 11 are 10 [mm], 15 [mm], 25 [mm], 40 [mm], 50 [mm], 65 [mm], and 80 [mm]. The processing is performed with the excitation frequency F set to 22.5 [Hz].

In the column of "Signal electromotive force ratio by exciting current" in FIG. 15, the signal electromotive force E when the exciting current Iex is 11.9 [mA] is set to 1. Then, in the column of 7.0 [mA], the ratio of the signal electromotive force E when the exciting current Iex is 7.0 [mA] to the signal electromotive force E when the exciting current Iex is 11.9 [mA]. (Ratio value) is described. Similarly, in the column of 4.9 [mA], the signal electromotive force E when the exciting current Iex is 4.9 [mA] with respect to the signal electromotive force E when the exciting current Iex is 11.9 [mA]. The ratio (ratio value) is listed.

From FIG. 15, it can be seen that the smaller the exciting current Iex, the higher the signal electromotive force E. It is presumed that this is because when the exciting current Iex is relatively large, the energy for creating the magnetic field is consumed by heat or the like, and the magnetic field decreases. Therefore, in the third embodiment, the relationship between the exciting current Iex and the magnetic flux density B (signal electromotive force E) is corrected by using the correction coefficient Co (Iex) for correcting the flow velocity V.

FIG. 16 is a block diagram showing an example of the functional configuration of the control device of FIG. As shown in FIG. 16, as shown in FIG. 13, the control device 60B includes a zero adjustment value calculation unit 61, a span adjustment value calculation unit 62, a current control unit 64, an excitation control unit 65, and a gain adjustment unit. It has 66 and an SH time adjusting unit 67. Further, the control device 60B has a correction coefficient calculation unit 68B and a flow velocity calculation unit 63B.

The flow velocity calculation unit 63B uses the following equation (10) instead of the above equation (4). That is, the flow velocity calculation unit 63B multiplies the maximum value of the flow velocity V by the value obtained by subtracting the _{zero adjustment count value C ZERO} from the current AD count value, and corrects the value by dividing by the _{span adjustment count value C SPAN.} Multiply by the coefficient Co (Iex) to obtain the flow velocity V.

[Number 10]
Flow velocity V [m / s] = Maximum flow velocity [m / s] x (AD count value-C _ZERO ) / C _SPAN x Co (Iex) ... (10)

The correction coefficient calculation unit 68B obtains the correction coefficient Co (Iex) of the equation (10) based on the detector constant set for each detector 10 and the change in the exciting current Iex. That is, since the signal electromotive force E changes depending on the magnitude of the exciting current Iex, the correction coefficient calculation unit 68B changes the correction coefficient Co (Iex) according to the characteristics of the exciting current Iex and the detector 10. It is configured. Here, the detector constant is a constant different for each detector 10 of the electromagnetic flow meter 100. The detector constant is obtained at the time of actual flow calibration in which a fluid is actually flowed through the detector 10 so that the processing accuracy of parts, variations in assembly, and the like do not affect the accuracy. In the electromagnetic flow meter 100, the flow rate can be accurately measured by setting the detector constant in the converter 20.

In the third embodiment, the correction coefficient calculation unit 68B determines the correction coefficient Co (Iex) by actual flow calibration at three points. That is, the storage device 70 of the third embodiment stores the first detector constant EX1, the second detector constant EX2, and the third detector constant EX3. First detector constant EX1 is a detector constant when the excitation current Iex real flow calibration factor reference current _{I 1,} a reference correction coefficient Co (Iex). Second detector constant EX2 is excitation current Iex real flow calibration is detector constant when the judging reference current I _2. Third detector constant EX3 is excitation current Iex real flow calibration is detector constant when low flow reference current I _3. There is a relationship of _{"I 1} > I ₂ > I ₃ " between the coefficient reference current I ₁ and the determination reference current I ₂ and the low flow rate reference current I _3. That is, the low flow rate reference current I ₃ is smaller than the judgment reference current I ₂ , and the coefficient reference current I ₁ is larger than the judgment reference current I _2.

Here, the detector constant corresponds to the signal electromotive force E when the exciting current Iex is constant at a flow velocity of 1 [m / s]. Therefore, as can be seen from FIG. 15, there is a relationship of "EX1 <EX2 <EX3" between the first detector constant EX1, the second detector constant EX2, and the third detector constant EX3. That is, there is a relationship between the exciting current Iex and the detector constant that the detector constant decreases as the exciting current Iex increases and the detector constant increases as the exciting current Iex decreases. The first detector constant EX1, the second detector constant EX2, and the third detector constant EX3 are described on the name plate of the detector 10, respectively. By setting the respective values of the first detector constant EX1, the second detector constant EX2, and the third detector constant EX3 in the converter 20, various combinations of the detector 10 and the converter 20 can be accommodated. The calculation of the flow velocity V can be performed accurately.

More specifically, the correction coefficient calculation unit 68B determines whether or not _{the exciting current Iex is less than the determination reference current I 2. If the exciting current Iex is less than the determination reference current I 2} , the correction coefficient calculation unit 68B obtains a low flow rate constant which is a value linearly complemented between the second detector constant EX2 and the third detector constant EX3. The ratio of the low flow constant to the first detector constant EX1 is obtained as the correction coefficient Co (Iex). The low flow rate constant is a value obtained by the calculation in the curly braces of the following equation (11).

[Number 11]
Co (Iex) = [(Iex -I 3) × (EX2-EX3) / (I 2 -I 3) + EX3] / EX1 ··· (11)

That is, the correction coefficient calculation unit 68B linearly complements the second detector constant EX2 and the determination reference current I ₂ and the third detector constant EX3 and the low flow rate reference current I _{3 based on the equation (11).} To obtain a low flow rate constant corresponding to the current exciting current Iex. Then, the correction coefficient calculation unit 68B obtains the correction coefficient Co (Iex) by dividing the low flow rate constant by the first detector constant EX1 based on the equation (11).

Further, if the exciting current Iex is the determination reference current I ₂ or more, the correction coefficient calculation unit 68B linearly interpolates between the first detector constant EX1 and the second detector constant EX2, and obtains a stable flow rate constant. Ask. Then, the correction coefficient calculation unit 68B obtains the ratio of the obtained stable flow rate constant to the first detector constant EX1 as the correction coefficient Co (Iex). The stable flow rate constant is a value obtained by the calculation in the curly braces of the following equation (12).

[Number 12]
Co (Iex) = [(Iex-I ₂ ) x (EX1-EX2) / (I _1- I ₂ ) + EX2] / EX1 ... (12)

That is, the correction coefficient calculation unit 68B, based on the equation (12), a first detector constant EX1 and the coefficient reference current _{I 1,} the linear interpolation based on the second detector constants EX2 and Criteria current _{I 2} , Find the stable flow rate constant corresponding to the current exciting current Iex. Then, the correction coefficient calculation unit 68B obtains the correction coefficient Co (Iex) by dividing the stable flow rate constant by the first detector constant EX1 based on the equation (12).

For example, the coefficient reference current I ₁ is set to 20.0 [mA], the determination reference current I ₂ is set to 12.0 [mA], and the low flow rate reference current I ₃ is set to 8.0 [mA]. In this case, the equation (11) becomes the following equation (13), and the equation (12) becomes the following equation (14).

[Number 13]
Co (Iex) = [(Iex-8) x (EX2-EX3) / 4 + EX3] / EX1 ... (13)

[Number 14]
Co (Iex) = [(Iex-12) x (EX1-EX2) / 8 + EX2] / EX1 ... (14)

In this case, the correction coefficient calculation unit 68B determines whether or not the exciting current Iex is less than 12.0 [mA]. Then, if the exciting current Iex is less than 12.0 [mA], the correction coefficient calculation unit 68B obtains the correction coefficient Co (Iex) by the equation (8). On the other hand, if the exciting current Iex is 12.0 [mA] or more, the correction coefficient calculation unit 68B obtains the correction coefficient Co (Iex) by the equation (9).

The first detector constant EX1, the second detector constant EX2, and the third detector constant EX3 are not limited to the above settings, respectively. That is, the combination of the coefficient reference current I ₁ and the judgment reference current I ₂ and the low flow rate reference current I ₃ is limited to the combination of 20.0 [mA], 12.0 [mA], and 8.0 [mA]. Not done. The coefficient reference current I ₁ , the determination reference current I _2, and the low flow rate reference current I ₃ can be set to arbitrary current values as long as the relationship of "I ₁ > I ₂ > I _{3" is satisfied.}

FIG. 17 is a block diagram showing another example of the functional configuration of the control device of FIG. As shown in FIG. 17, the control device 60B replaces the zero adjustment value calculation unit 61 and the span adjustment value calculation unit 62 with the zero adjustment value calculation unit 61A and the span adjustment value calculation unit 62A in the second embodiment. You may have. In this case, since the electromagnetic flowmeter 100B can execute not only the current correction but also the excitation frequency correction, the calculation accuracy of the flow velocity V can be further improved, and the accuracy of the current output Iout can be further improved.

As described above, the electromagnetic flow meter 100B of the third embodiment can also improve the accuracy of the current output Iout indicating the flow rate of the fluid flowing in the measuring tube, as in the electromagnetic flow meter 100 of the first embodiment. can. Further, since the electromagnetic flow meter 100B is also configured so that the magnitude of the current output Iout and the magnitude of the exciting current Iex are equal to each other, the configuration can be simplified, the cost can be reduced, and the arithmetic processing can be simplified. Can be planned.

By the way, conventionally, since it is difficult to perform actual flow calibration at a low flow rate such that the current output Iout is 4.9 mA (flow rate ratio 5.6%), the detector constant is set to a constant value (constant value). Default value). In this respect, since the electromagnetic flow meter 100B continuously changes the exciting current Iex, a complementary calculation can be performed using the result of actual flow calibration with the _{determination reference current I 2} and the low flow rate reference current I _3. Therefore, the correction suitable for the actual excitation characteristic of the detector 10 can be performed, and the accuracy of the current output Iout can be further improved. Other effects are the same as in the first and second embodiments.

In the third embodiment, a case where Co (Iex) is obtained by using three detector constants of the first detector constant EX1, the second detector constant EX2, and the third detector constant EX3 has been illustrated. However, it is not limited to this. The electromagnetic flow meter 100B may determine Co (Iex) using four or more detector constants corresponding to different exciting currents Iex. For example, when the electromagnetic flowmeter 100B obtains Co (Iex) using four detector constants, three electromagnetic flowmeters 100B are configured in the same manner as in the equations (13) and (14) according to the exciting current Iex. It is good to use the calculation formula properly. That is, when the electromagnetic flowmeter 100B obtains Co (Iex) using n (n is a natural number of 3 or more) detector constants, the equations (13) and (14) are expressed according to the exciting current Iex. It is advisable to properly use n-1 arithmetic expressions configured in the same manner.

Here, the effect obtained by the electromagnetic flow meter 100 will be described in detail with reference to FIGS. 18 to 22. FIG. 18 is a graph showing how the exciting current is switched stepwise in the conventional two-wire flowmeter. FIG. 19 is a graph showing a change in the AD count value when the exciting current is changed as shown in FIG. 18 in a conventional two-wire flowmeter. FIG. 20 is a graph showing a change in the AD count value when the span correction is switched in the initial time zone of FIG. FIG. 21 is a graph showing a change in the AD count value when the span correction is switched in the middle time zone of FIG. FIG. 22 is a graph showing a change in the AD count value when the span correction is switched in the latter time zone of FIG.

When the exciting current is switched stepwise as in the conventional 2-wire electromagnetic flowmeter, the signal electromotive force E changes according to the change in the exciting current, so the fluid flow rate does not change in the transitional section of the switching. However, the AD count value output from the AD converter changes. That is, in the 2-wire electromagnetic flowmeter, when the exciting current is increased, the magnetic field applied to the fluid in the measuring tube becomes large and the signal electromotive force E rises, so that the AD count is performed regardless of the change in the flow rate of the fluid. The value increases. Similarly, in a 2-wire electromagnetic flowmeter, when the exciting current is reduced, the magnetic field applied to the fluid in the measuring tube becomes smaller and the signal electromotive force E decreases. The count value becomes smaller.

Therefore, conventionally, two AD count values for adjustment are set for each exciting current set stepwise, and the change of the AD count value due to the switching of the exciting current is changed by span correction using these. It is suppressing. As the two AD count values for adjustment, a zero adjustment AD value which is an AD count value when the flow rate is 0% and a maximum adjustment AD value which is an AD count value when the flow rate is 100% are set. The span correction is, for example, a correction for calculating a normalized value which is a normalized value from a zero adjusted AD value and a maximum adjusted AD value. That is, in the conventional two-wire electromagnetic flowmeter, the span correction at a certain exciting current is performed by the following equation (15) to obtain the normalized value. Then, the conventional two-wire electromagnetic flowmeter calculates the flow velocity by multiplying the normalized value by the maximum flow velocity, and obtains the current output Iout based on the calculated flow velocity.

[Number 15]
Normalized value = (AD count value-zero adjustment AD value) / (maximum adjustment AD value-zero adjustment AD value) ... (15)

Here, it is assumed that the conventional two-wire electromagnetic flowmeter switches the exciting current in four stages of 4.9 mA, 7.0 mA, 11.9 mA, and 14 mA. In this example, as shown in FIG. 18, when the exciting current is switched from 7.0 mA to 11.9 mA at _{time t 0, the signal electromotive force E increases as the exciting current increases.} Therefore, as shown in FIG. 19, in the transitional section of switching, the AD count value output from the AD converter gradually increases even if the flow rate of the fluid does not change.

The conventional 2-wire electromagnetic flowmeter uses a common value for each exciting current as the zero adjustment AD value and the maximum adjustment AD value in the equation (15) except for the exciting current switching point, but the exciting current switching point. Then, the zero-adjusted AD value and the maximum-adjusted AD value of the equation (15) are suddenly changed. Therefore, various whiskers are generated in the current output Iout according to the switching point of the exciting current. For example, if the timing of changing from the span correction of the exciting current of 7.0 mA to the span correction of the exciting current of 11.9 mA is different as shown in FIG. 19, the span correction is applied to the current output Iout as shown in FIGS. 20 to 22. Different whiskers occur depending on the change timing of. This whiskers are particularly generated because the maximum adjusted AD value of the exciting current 11.9 mA is larger than the maximum adjusted AD value of the exciting current 7.0 mA.

That is, when you change the span correction at time t ₁ of the initial time period (A) during the transient of FIG. 19, as shown in FIG. 20, whiskers is generated in the minus side. Also, if you change the span correction at time t ₂ of the medium-term time zone (B) during the transient of FIG. 19, as shown in FIG. 21, whiskers occurs in both the negative and the positive side. If you change the span correction at time t ₃ the late hours of the transition period of Figure 19 (C), as shown in FIG. 21, whiskers are generated on the plus side. Then, due to such a whiskers, the current output changes across the threshold value, so that a hunting phenomenon occurs in which switching of the exciting current occurs frequently. Further, as described above, even if the previous value hold process is adopted, there is a problem that the accuracy of the current output is further lowered.

In this respect, since the electromagnetic flowmeter in each of the above embodiments continuously changes the exciting current Iex, it is possible to prevent the occurrence of whiskers, holds, and steps when the exciting current is switched, as in the conventional case. That is, as shown in FIGS. 5 to 7, 12, and 13, even if the gain G, the exciting frequency F, and the exciting current Iex are dynamically changed in conjunction with the flow rate of the fluid, each of the above implementations is performed. In the electromagnetic flowmeter in the form of, it is converted to a normal flow rate value. That is, the flow velocity V and the instantaneous flow rate do not take discontinuous values.

In addition, since the conventional 2-wire electromagnetic flowmeter performs span correction, it is necessary to obtain and store two adjustment AD values for each exciting current set in stages at the time of adjustment such as at the time of factory shipment. be. For example, in the case of a two-wire electromagnetic flowmeter that switches the exciting current in four stages of 4.9 mA, 7.0 mA, 11.9 mA, and 14 mA as in the above example, eight adjusted AD values are obtained and stored. It is necessary to keep it, and the adjustment process takes time. Further, if a configuration for changing the excitation frequency is added, the number of adjustment points increases, and the time spent for the adjustment process increases. In addition, as the flow rate becomes smaller, the AD count value is less likely to change. Therefore, if a setting for increasing the gain of the amplifier is added at a low flow rate, the number of adjustment points increases and the adjustment processing time further increases. For example, if two reference values for the excitation frequency are set and two reference values for the gain are set, 32 adjustment points are required in the configuration in which the excitation current is switched in four stages. Further, if the two adjusted AD values for each exciting current set stepwise are out of order, a step is generated in the current output Iout when the exciting current is switched, even if there is no transient period.

In this regard, in the electromagnetic flowmeter 100 of the first embodiment and the electromagnetic flowmeter 100B of the third embodiment provided with the control device 60B of FIG. 16, only two reference values for obtaining the flow velocity V should be set. Just do it. Further, in the electromagnetic flowmeter 100A of the second embodiment and the electromagnetic flowmeter 100B of the third embodiment provided with the control device 60B of FIG. 17, only four reference values for obtaining the flow velocity V can be set. good. Therefore, according to the electromagnetic flowmeter in each of the above embodiments, the number of adjustment points at the time of shipment can be reduced as compared with the conventional configuration in which the exciting current Iex is changed stepwise, so that the time required for adjustment can be reduced. be able to.

Each of the above-described embodiments is a suitable specific example in a two-wire electromagnetic flowmeter, and the technical scope of the present invention is not limited to these embodiments. For example, in each of the above embodiments, the case where the AD converter 50 has a DC amplification function has been illustrated, but the present invention is not limited to this. Even if the converter 20 has, instead of the AD converter 50, a DC amplifier that functions in the same manner as the gain setting unit 51 and the DC amplification unit 52, and an AD converter that functions in the same manner as the conversion unit 53. good.

Further, in the above description, the case where the switch circuit 82 repeats the XY state and the YX state at a predetermined cycle has been illustrated, but the present invention is not limited to this. For example, a fully off state in which the first switch SW1, the second switch SW2, the third switch SW3, and the fourth switch SW4 are all turned off may be provided between the XY state and the YX state. For example, the excitation control unit 65 generates a frequency command signal that repeats three states in a predetermined pattern such as "XY state-> all off state-> YX state-> all off state-> XY state" to excite. It may be output to the switching circuit 81. In this way, since the section in which the exciting current Iex does not flow can be provided, the current value can be lowered, and the power consumption can be suppressed. Then, by adjusting the set time in the all-off state, the flow velocity V is calculated accurately even when the flow rate of the fluid flowing in the measuring tube 11 decreases and the current output Iout becomes 4.0 [mA] or less. can do.

That is, the electromagnetic flowmeter of Patent Document 1 sets the minimum exciting current Iex to 4.8 [mA], which is higher than the minimum value of the output current of 4-20 mA, so that the current output is 4.0 [mA] or less. There is a limitation that it cannot be done. In this regard, if the all-off state is applied to the electromagnetic flowmeters of the above embodiments, the power consumption can be dynamically reduced at the time of burnout (when a serious failure occurs) or at a low flow rate. Therefore, the lower limit value of the current output Iout can be lowered.

10 Detector, 11 Measuring tube, 12 Exciting coil, 13a, 13b electrode, 14 Ground electrode, 20 Converter, 30 AC amplification circuit, 40 Sample hold circuit, 50 AD converter, 51 Gain setting unit, 52 DC amplification unit, 53 Conversion unit, 60, 60A, 60B control device, 61, 61A Zero adjustment value calculation unit, 62, 62A Span adjustment value calculation unit, 63, 63B Flow velocity calculation unit, 64 Current control unit, 65 Excitation control unit, 66 Gain adjustment Unit, 67 SH time adjustment unit, 68B correction coefficient calculation unit, 70 storage device, 80 excitation circuit, 81 excitation switching circuit, 82 switch circuit, 90 current control circuit, 100, 100A, 100B electromagnetic flowmeter, 300 calibrator, 500 external Power supply, AC contacts, C _SPAN span adjustment count value, C _ZERO zero adjustment count value, Co (Iex) correction coefficient, D caliber, D ₀ zero reference value, D ₀₁ 1st zero reference value, D ₀₂ 2nd zero reference _{value, D MAX} maximum adjustment count, _{D S} span reference _{value, D S1} first span reference _{value, D S2} second span reference value, E signal EMF, EX1 first detector constant, EX2 second detector constants , EX3 third detector constant, F the excitation frequency, _{F 0} reference excitation frequency, _{F H} second reference excitation frequency, _{F L} first reference excitation frequency, G gain, _{G 0} reference gain, I exciting current, _{I 0} reference excitation Current, I ₁ coefficient reference current, I ₂ judgment reference current, I ₃ low flow reference current, Iex excitation current, Iout current output, L1, L2 signal line, PS connection point, RL external load, SW1 first switch, SW2 2nd switch, SW3 3rd switch, SW4 4th switch, V flow velocity.

Claims (7)

A detector having an exciting coil that applies a magnetic field to the fluid flowing in the measuring tube and detecting a signal electromotive force generated according to the flow velocity of the fluid flowing in the measuring tube.
A converter that adjusts the current output indicating the flow rate of the fluid flowing in the measuring tube based on the signal electromotive force detected by the detector is provided.
The converter
An exciting circuit that supplies an exciting current with a predetermined exciting frequency to the exciting coil,
An AD converter that generates an AD count value indicating the flow velocity from an analog signal based on the signal electromotive force, and
A current control circuit that adjusts the current output and
It has a control device that continuously changes the exciting current supplied by the exciting circuit to the exciting coil according to the change of the AD count value generated in the AD converter according to the current output. , 2-wire electromagnetic flowmeter. The two-wire electromagnetic flowmeter according to claim 1, wherein the magnitude of the current output and the magnitude of the exciting current are configured to be equal to each other. Stores the zero reference value, which is the AD count value at the zero point, and the span reference value, which is the AD count value based on the maximum flow velocity, at the reference excitation current, the reference excitation frequency, and the reference gain. Has a storage device
The control device is
A zero adjustment value calculation unit that obtains a zero adjustment count value by multiplying the zero reference value by the gain of the converter.
A span adjustment value calculation unit for obtaining a span adjustment count value by multiplying the span reference value by the ratio of the exciting current to the reference exciting current and the gain.
A flow velocity calculation unit that calculates the flow velocity using the AD count value, the zero adjustment count value, and the span adjustment count value generated by the AD converter.
The invention according to claim 1 or 2, further comprising a current control unit that adjusts the resistance value of the current control circuit according to the change in the flow velocity calculated by the flow velocity calculation unit and continuously changes the current output. 2-wire electromagnetic flowmeter. The first zero reference value, which is the AD count value at the zero point at the reference excitation current, the first reference excitation frequency, and the reference gain, and the first span reference, which is the AD count value based on the maximum flow velocity. The maximum value, the reference exciting current, the second reference excitation frequency larger than the first reference excitation frequency, and the second zero reference value which is the AD count value at the zero point at the reference gain. It has a storage device for storing the second span reference value which is the AD count value based on the flow velocity.
The control device is
The zero reference count value corresponding to the excitation frequency is obtained by linear complementation based on the first zero reference value and the first reference excitation frequency, and the second zero reference value and the second reference excitation frequency, and the zero reference is obtained. A zero adjustment value calculation unit that obtains a zero adjustment count value by multiplying the count value by the gain of the converter.
The span reference count value corresponding to the excitation frequency is obtained by linear complementation based on the first span reference value and the first reference excitation frequency, and the second span reference value and the second reference excitation frequency, and the span reference is obtained. A span adjustment value calculation unit that obtains the span adjustment count value by multiplying the count value by the gain.
A flow velocity calculation unit that calculates the flow velocity using the AD count value, the zero adjustment count value, and the span adjustment count value generated by the AD converter.
The invention according to claim 1 or 2, further comprising a current control unit that adjusts the resistance value of the current control circuit according to the flow velocity calculated by the flow velocity calculation unit to continuously change the current output. 2-wire electromagnetic flowmeter. The control device is
It has a coefficient calculation unit for obtaining a correction coefficient for correcting the flow velocity based on a detector constant set in the detector and a change in the exciting current.
The flow velocity calculation unit
The two-wire electromagnetic flowmeter according to claim 3 or 4, wherein the flow velocity is obtained by further using the correction coefficient obtained by the coefficient calculation unit. The flow velocity calculation unit
The maximum value of the flow velocity is multiplied by the value obtained by subtracting the zero adjustment count value from the AD count value, and the value divided by the span adjustment count value is multiplied by the correction coefficient to obtain the flow velocity. The two-wire electromagnetic flowmeter according to claim 5. The storage device is
In the actual flow calibration, the first detector constant when the exciting current is the coefficient reference current, in the actual flow calibration, the second detector constant when the exciting current is the judgment reference current, and in the actual flow calibration, the exciting current is It stores the third detector constant at the time of low flow rate reference current,
The low flow rate reference current is smaller than the determination reference current, and the coefficient reference current is larger than the determination reference current.
The coefficient calculation unit
When the exciting current is less than the determination reference current,
A low flow rate constant corresponding to the exciting current is obtained by linear complementation based on the second detector constant and the determination reference current, and the third detector constant and the low flow rate reference current, and the low flow rate constant is used as the first. 1 Divide by the detector constant to obtain the correction coefficient.
When the exciting current is equal to or greater than the determination reference current,
The stable flow rate constant corresponding to the exciting current is obtained by linear complementation based on the first detector constant and the coefficient reference current, and the second detector constant and the determination reference current, and the stable flow rate constant is used as the first. The two-wire electromagnetic flowmeter according to claim 5 or 6, wherein the correction coefficient is obtained by dividing by a detector constant.

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