JP2017106771A - Electromagnetic flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electromagnetic flowmeter capable of reducing measurement error even when the length of an excitation cable connecting an excitation circuit and an exciting coil is long.SOLUTION: The electromagnetic flowmeter includes: an excitation circuit that outputs excitation current; and an exciting coil, which are connected to each other via excitation cable. The excitation circuit corrects an excitation signal, which is detected based on the excitation current, based on characteristics including cable length of the excitation cable, characteristics of the exciting coil and the frequency of the excitation current.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electromagnetic flow meter, and more particularly, to an electromagnetic flow meter that can reduce measurement errors even when the length of an excitation cable connecting an excitation circuit and an excitation coil is long.

  Electromagnetic flowmeters that measure the flow rate of conductive fluid using electromagnetic induction are widely used in industrial applications and the like because they are robust and highly accurate. The electromagnetic flow meter measures the generated electromotive force using a pair of electrodes by flowing a conductive fluid to be measured in a measurement tube to which a magnetic field is applied in an orthogonal direction. Since this electromotive force is proportional to the flow velocity of the fluid to be measured, the volume flow rate of the fluid to be measured can be obtained based on the measured electromotive force.

  FIG. 5 is a diagram showing a configuration of a conventional electromagnetic flow meter 500. As shown in the figure, the electromagnetic flow meter 500 includes a detector 510 and a converter 520, which are electrically connected by a signal cable 530 and an excitation cable 540.

  The detector 510 includes a measurement tube 511 through which a fluid to be measured flows, and an excitation coil 515, and a pair of measurement electrodes 513 and a ground electrode 514 are attached to the measurement tube 511.

  The converter 520 includes an excitation circuit 521, a differential amplifier circuit 522, an A / D circuit 523, an arithmetic circuit 524, and an output circuit 525. The differential amplifier circuit 522 and the measurement electrode 513 are connected by a signal cable 530, and the excitation circuit 521 and the excitation coil 515 are connected by an excitation cable 540.

  In accordance with the excitation control signal from the arithmetic circuit 524, the excitation circuit 521 outputs an excitation current. This exciting current flows to the exciting coil 515 via the exciting cable 540 and gives a magnetic field in the measuring tube 511.

  The electromotive force generated in the fluid to be measured is detected by the pair of measurement electrodes 513, is input to the differential amplifier circuit 522 via the signal cable 530, and is output as a flow rate signal. The A / D circuit 523 digitally converts the excitation signal having a voltage corresponding to the excitation current output from the excitation circuit 521 and the flow rate signal, and outputs the digital signal to the arithmetic circuit. The arithmetic circuit 524 calculates the flow rate of the fluid to be measured based on these signals, specifically based on the ratio between the flow rate signal and the excitation signal, and outputs it to the outside via the output circuit 525.

  FIG. 6 shows a configuration example of the excitation circuit 521 connected to the excitation coil 515 via the excitation cable 540. In this example, as shown in FIG. 7, an example of outputting an excitation current that alternately repeats a positive excitation interval and a negative excitation interval is shown. However, there may be an excitation current including a non-excitation interval or an excitation current combining two long and short frequencies.

  As shown in this figure, the excitation circuit 521 includes a current supply unit 526, four switches Q1, Q2, Q3, Q4, switch control units SW1, SW2, SW3, SW4 corresponding to the respective switches, and excitation current detection. A resistor Rd is provided.

  The switches Q1 and Q3 are arranged in series and connected to the current supply unit 526, the switches Q2 and Q4 are arranged in series and connected to the current supply unit 526, the connection point between the switches Q1 and Q3, and the switch Q2 An excitation cable 540 is connected to the connection point with Q4. At this time, one end of the excitation cable 540 is connected across the excitation current detection resistor Rd. The voltage generated at the excitation current detection resistor Rd is output as the excitation signal Vd.

  The exciting coil 515 connected to the exciting cable 540 includes an inductance component L and a resistance component R. Here, the current flowing through the exciting current detection resistor Rd is Ir, and the current flowing through the exciting coil 515 is Iex.

  FIG. 8 is a control example for the switches Q1, Q2, Q3, and Q4 of the switch control units SW1, SW2, SW3, and SW4. As shown in FIG. 9, in the positive excitation interval, the switch control unit SW1 turns on / off the switch Q1 corresponding to the increase / decrease of the voltage generated in the excitation current detection resistor Rd with the switches Q2 and Q3 off and the switch Q4 on. Thus, PWM control is performed so that the current Ir becomes constant. In the negative excitation interval, the switch Ir is turned on and off by the switch control unit SW2 while the switches Q1 and Q4 are turned off and the switch Q3 is turned on. PWM control is performed so that

JP 2004-325100 A JP 2002-168667 A

  The excitation cable 540 can be considered as a distributed constant circuit including a resistance component, an inductance component, and a capacitance component. When the cable length of the excitation cable 540 is not so long, these components can be ignored. Therefore, the current Ir flowing through the excitation current detection resistor Rd (detection excitation current Ir) and the current Iex flowing through the excitation coil 515 (actual excitation current) Iez) can be considered the same.

  However, when the cable length of the excitation cable 540 is long, the current Ic flowing through the capacitor Ci cannot be ignored as shown in FIG. That is, the actual excitation current Iez is a value obtained by subtracting the current Ic from the detected excitation current Ir. Since a large amount of current Ic flows when the current is reversed, the detected excitation current Ir and the actual excitation current Iex have a relationship as shown in FIG.

  Thus, the detected excitation current Ir detected as the excitation signal Vd by the excitation current detection resistor Rd has a difference from the actual excitation current Iex that actually flows through the excitation coil 515. As described above, since the flow rate of the fluid to be measured is calculated based on the ratio between the flow rate signal and the excitation signal, this difference becomes a measurement error of the electromagnetic flow meter.

  Therefore, an object of the present invention is to reduce measurement errors even when the length of an excitation cable connecting an excitation circuit and an excitation coil is long in an electromagnetic flow meter.

In order to solve the above-described problem, an electromagnetic flow meter according to a first aspect of the present invention is an electromagnetic flow meter in which an excitation circuit that outputs an excitation current and an excitation coil are connected by an excitation cable. The excitation signal detected based on the excitation current is corrected based on a characteristic including a cable length of the excitation cable, a characteristic of the excitation coil, and a frequency of the excitation current.
Here, the characteristics of the excitation cable may include a capacitance per unit length of the excitation cable.
The characteristics of the exciting coil may include an inductance component.
In order to solve the above-described problem, an electromagnetic flow meter according to a second aspect of the present invention is an electromagnetic flow meter in which an excitation circuit that outputs an excitation current and an excitation coil are connected by an excitation cable. The excitation signal detected based on the excitation current is corrected using a relational expression between the current flowing through the excitation coil and the excitation signal prepared in advance.
In any aspect, the excitation circuit may include a detection resistor through which the excitation current flows, and the excitation signal may be a voltage generated by the detection resistor.
A measurement tube attached to the measurement electrode; and the flow rate of the fluid to be measured is calculated based on the corrected excitation signal and the flow rate signal detected by the measurement electrode. it can.

  According to the present invention, in the electromagnetic flow meter, even when the cable length of the excitation cable connecting the excitation circuit and the excitation coil is long, the measurement error can be reduced.

It is a figure which shows the structure of the electromagnetic flowmeter of this embodiment. It is a model figure in case the cable length of an excitation cable is long. It is a flowchart explaining operation | movement of the electromagnetic flowmeter of this embodiment. It is a figure which shows another example of the electromagnetic flowmeter of this embodiment. It is a figure which shows the structure of the conventional electromagnetic flowmeter. It is a figure explaining an excitation circuit. It is a figure which shows a positive excitation area and a negative excitation area. It is a chart explaining operation | movement of a switch control part. It is a wave form diagram explaining operation | movement of a switch control part. It is a figure explaining the influence when the cable length of an excitation cable is long. It is a figure which shows the relationship between the detection exciting current Ir and the actual exciting current Iex.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration of an electromagnetic flow meter 100 of the present embodiment. As shown in the figure, the electromagnetic flow meter 100 includes a detector 110 and a converter 120, which are electrically connected by a signal cable 130 and an excitation cable 140.

  The detector 110 includes a measurement tube 111 through which a fluid to be measured flows, and an excitation coil 115, and a pair of measurement electrodes 113 and a ground electrode 114 are attached to the measurement tube 111.

  The converter 120 includes an excitation circuit 121, a differential amplifier circuit 122, an A / D circuit 123, an arithmetic circuit 124, and an output circuit 125. The differential amplifier circuit 122 and the measurement electrode 113 are connected by a signal cable 130, and the excitation circuit 121 and the excitation coil 115 are connected by an excitation cable 140.

  In accordance with the excitation control signal from the arithmetic circuit 124, the excitation circuit 121 outputs an excitation current. This exciting current flows to the exciting coil 115 via the exciting cable 140 and gives a magnetic field in the measuring tube 111.

  The electromotive force generated in the fluid to be measured is detected by the pair of measurement electrodes 113, is input to the differential amplifier circuit 122 via the signal cable 130, and is output as a flow rate signal. The A / D circuit 123 digitally converts the excitation signal corresponding to the excitation current output from the excitation circuit 121 and the flow rate signal, and outputs the digital signal to the arithmetic circuit. The arithmetic circuit 124 calculates the flow rate of the fluid to be measured based on the ratio between the flow rate signal and the excitation signal, and outputs it to the outside via the output circuit 125.

  The configuration of the excitation circuit 121 is the same as the conventional one. That is, the exciting current detection resistor Rd is provided, and the detected exciting current Ir is detected. Specifically, it is detected as a voltage generated in the excitation current detection resistor Rd and input to the arithmetic circuit 124 as the excitation signal Vd.

  The arithmetic circuit 124 includes an excitation signal correction unit 126. The excitation signal correction unit 126 corrects the excitation signal Vd based on the detected excitation current Ir to the excitation signal Vex based on the actual excitation current Iex. The arithmetic circuit 124 calculates the flow rate using the excitation signal Vex corrected by the excitation signal correction unit 126. That is, the flow rate of the fluid to be measured is calculated based on the ratio between the flow rate signal and the corrected excitation signal. Thereby, even when the length of the excitation cable 140 is long, the flow rate calculation with a small error can be performed.

  FIG. 2 is a simplified model when the cable length of the excitation cable 140 is long. The detection excitation current Ir flowing through the excitation current detection resistor Rd flows into the excitation cable 140, and a part thereof becomes the current Ic flowing through the capacitance component Ic. Then, the remaining current becomes the actual excitation current Iex to excite the excitation coil 115 and give a magnetic field in the measurement tube 111.

  Here, the impedance of the capacitive component Ci included in the excitation cable 140 is Zc. The exciting coil 115 has an inductance component L (H) and a resistance component R (Ω), and its impedance is Z. Note that the resistance component Ri and the inductance component Li of the excitation cable 140 are not a problem in this embodiment.

When the angular frequency of the excitation current is ω (rad / s), the capacitance per unit length of the excitation cable 140 is Cco (F / m), and the cable length of the excitation cable 140 is A (m), the capacitance component Ci The impedance Zc is [Equation 1].
The angular frequency ω can be calculated by [Equation 2] when the frequency of the excitation current is Fc (Hz).
As [Equation 1] indicates, as the cable length A of the excitation cable 140 increases, the cable capacity increases and the current Ic flowing through the cable capacity Ci increases. As a result, the actual exciting current Iex flowing through the exciting coil 115 is reduced, and the electromotive force actually generated becomes smaller than the electromotive force that should be originally obtained.

The impedance Z of the exciting coil 115 is [Equation 3].
From the above, the relationship between the current Ic flowing through the cable capacitance Ci and the detected excitation current Ir can be expressed by [Equation 4].
If Ir = 100 (mA), R = 100 (Ω), L = 0.2 (H), Cco = 0.1 (nF / m), and Fc = 50 (Hz), the cable length A is If 200 (m), Ic = 0.1022 (mA), and if the cable length A is 2000 (m), Ic = 1.014 (mA). That is, as the cable length A of the excitation cable 140 increases, the result is that the current Ic flowing through the cable capacitance Ci increases.

The excitation signal Vd, which is a voltage generated by the excitation current detection resistor Rd, also includes a voltage generated by the current Ic. Therefore, the excitation signal Vex, which is a voltage generated by the actual excitation current Iex, can be obtained according to [Equation 5] with a correction coefficient k.
The excitation signal correction unit 126 calculates an excitation signal Vex obtained by correcting the excitation signal Vd according to [Equation 5]. For this reason, the excitation signal correction unit 126 receives in advance registration of the cable length A (m) of the excitation cable 140 and the capacitance Cco (F / m) per unit length. Further, based on the specifications of the detector 110, the inductance component L (H) and the resistance component R (Ω) of the exciting coil 115 are recorded. If the resistance component R (Ω) is sufficiently small, it may be ignored. The frequency of the excitation current can be acquired from the arithmetic circuit 124.

  Next, the flow measurement operation of the electromagnetic flow meter 100 of this embodiment will be described with reference to the flowchart of FIG.

  When the flow rate measurement is started, the arithmetic circuit 124 acquires a digitally converted flow rate signal (S101). Further, the excitation signal Vd that has been digitally converted is acquired (S102).

  With respect to the acquired excitation signal Vd, the excitation signal correction unit 126 refers to correction parameters such as the cable length A (m) of the excitation cable 140 and the capacitance Cco (F / m) per unit length (S103). Then, the correction according to [Equation 5] is performed, and the corrected excitation signal Vex is calculated (S104).

  Then, the arithmetic circuit 124 calculates the flow rate of the fluid to be measured based on the ratio between the flow rate signal and the corrected excitation signal Vex (S105), and outputs it to the outside via the output circuit 125 (S106). The electromagnetic flow meter 100 repeats the above processing until the measurement is completed (S107).

  When the excitation cable 140 is short, no correction calculation is required, and the correction function of the excitation signal correction unit 126 may be switched on and off.

  In the above example, the correction method using the simple model is used. However, as shown in FIG. 4, the actual excitation current Iex flowing in the vicinity of the excitation coil 115 is measured using the ammeter 127 prior to the actual measurement. Then, based on the relationship between the actual excitation current Iex and the excitation signal Vd, a conversion equation for converting the excitation signal Vd into the excitation signal Vex (= Iex × Rd) may be created. It is desirable to create a conversion formula for each excitation current frequency. In the case where a plurality of excitation cables 140 are used after replacement, the excitation cables 140 are created for each excitation cable 140.

  In this case, at the time of measurement, the excitation signal Vex can be obtained by correcting the excitation signal Vd based on a conversion formula created in advance.

DESCRIPTION OF SYMBOLS 100 ... Electromagnetic flow meter, 110 ... Detector, 111 ... Measuring tube, 113 ... Measuring electrode, 114 ... Ground electrode, 115 ... Excitation coil, 120 ... Converter, 121 ... Excitation circuit, 122 ... Differential amplification circuit, 123 ... A / D circuit, 124 ... arithmetic circuit, 125 ... output circuit, 126 ... excitation signal correction unit, 127 ... ammeter, 130 ... signal cable, 140 ... excitation cable

Claims (6)

An electromagnetic flowmeter in which an excitation circuit that outputs an excitation current and an excitation coil are connected by an excitation cable,
An excitation signal detected based on the excitation current in the excitation circuit is corrected based on a characteristic including a cable length of the excitation cable, a characteristic of the excitation coil, and a frequency of the excitation current. Electromagnetic flow meter.   The electromagnetic flowmeter according to claim 1, wherein the characteristic of the excitation cable includes a capacitance per unit length of the excitation cable.   The electromagnetic flowmeter according to claim 1, wherein the characteristics of the exciting coil include an inductance component. An electromagnetic flowmeter in which an excitation circuit that outputs an excitation current and an excitation coil are connected by an excitation cable,
An electromagnetic flowmeter, wherein an excitation signal detected based on the excitation current in the excitation circuit is corrected using a relational expression between a current flowing through the excitation coil and the excitation signal, which is created in advance. The excitation circuit includes a detection resistor through which the excitation current flows,
The electromagnetic flow meter according to claim 1, wherein the excitation signal is a voltage generated in the detection resistor. A measuring tube to which a measuring electrode is attached and a fluid to be measured flows;
6. The electromagnetic flow meter according to claim 1, wherein the flow rate of the fluid to be measured is calculated based on the corrected excitation signal and a flow rate signal detected by the measurement electrode. .