JP2019184544A - Converter for electromagnetic flow meter, electromagnetic flow meter, and method for operating flow rate - Google Patents

Abstract

To provide a converter for an electromagnetic flow meter, an electromagnetic flow meter, and a method for operating a flow rate which can secure a stable interval for an excitation current by simple processing regardless of the type of a detector and the diameter of a measurement tube.SOLUTION: The present invention relates to a converter for an electromagnetic flow meter which determines the flow rate of a fluid on the basis of a signal electromotive force proportional to the rate of a fluid flowing in the measurement tube of a detector having an excitation coil. The converter includes: a current measurement unit for measuring the magnitude of an excitation current as an excitation current value before a signal becomes stable at an excitation switching point; and a current controller for fixing the magnitude of the excitation current at an excitation current value while the signal is stable at the excitation switching point.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a converter for an electromagnetic flow meter that measures the flow rate of a fluid having conductivity, an electromagnetic flow meter, and a flow rate calculation method.

  The electromagnetic flow meter generates a magnetic field in the measurement tube by passing a rectangular wave excitation current through the excitation coil of the detector, detects a signal electromotive force proportional to the flow velocity of the fluid flowing through the measurement tube, and generates a detected signal electromotive force. It is a device for measuring the flow rate of a fluid based on it.

  In the electromagnetic flow meter, an accurate flow rate cannot be obtained unless the signal electromotive force is detected when the magnetic flux density in the measuring tube is stable. Here, the magnetic flux density in the measuring tube is proportional to the exciting current flowing through the exciting coil. The exciting current takes a certain amount of time until the current value is stabilized due to the inductance of the exciting coil. In other words, the excitation current changes according to the number of turns of the excitation coil, the voltage applied to the excitation coil, the application time of the voltage to the excitation coil, and the like. Therefore, in order to accurately determine the flow rate of the fluid, it is necessary to secure a certain stable section when the exciting current is stabilized and to sample the signal electromotive force.

  Therefore, the conventional electromagnetic flow meter employs a technique of sampling the signal electromotive force when a certain waiting time has elapsed after switching the excitation direction (see, for example, Patent Document 1).

JP 2001-235352 A

  However, since the rise time until the excitation current is stabilized differs depending on the type of detector and the diameter of the measurement tube, the waiting time may be insufficient depending on the detector. In this case, the converter samples the signal electromotive force where the magnetic flux density is not stable.

  Therefore, conventionally, in order to secure a certain stable interval, precise verification and adjustment combined with a transducer are performed at the time of detector design. That is, in the conventional electromagnetic flow meter, it is necessary to determine the sampling timing of the excitation frequency, the excitation voltage, and the signal electromotive force for each detector. Therefore, register information on the excitation frequency, excitation voltage, and signal electromotive force sampling timing of each of multiple types of detectors in the converter, or design all detectors to have the same characteristics. There is a need.

  The present invention has been made to solve the above-described problems, and is a converter for an electromagnetic flowmeter that secures a stable section of excitation current by simple processing regardless of the type of detector and the diameter of a measurement tube. An object is to provide an electromagnetic flow meter and a flow rate calculation method.

  The converter of the electromagnetic flowmeter according to the present invention is a converter for obtaining a flow rate of a fluid based on a signal electromotive force proportional to a flow velocity of the fluid flowing through a measurement tube of a detector having an excitation coil, the excitation coil Between the current measurement unit that measures the magnitude of the excitation current as the excitation current value and the signal stabilization time to the excitation switching point before the signal stabilization time at the excitation switching point, which is the timing at which the polarity of the excitation current supplied to the And a current control unit for fixing the magnitude of the exciting current to the exciting current value.

  An electromagnetic flowmeter according to the present invention includes an excitation coil that applies a magnetic field to a fluid flowing in a measurement tube, a detector that detects a signal electromotive force generated according to the flow velocity of the fluid flowing in the measurement tube, and a detector And the above-described converter for obtaining the flow rate of the fluid based on the detected signal electromotive force.

  The flow rate calculation method according to the present invention acquires the magnitude of the excitation current as the excitation current value before the signal stabilization time of the excitation switching point, which is the timing at which the polarity of the excitation current supplied to the excitation coil of the detector is switched, During the signal stabilization time to the excitation switching point, the magnitude of the excitation current is fixed to the excitation current value, and the signal electromotive force proportional to the flow velocity of the fluid flowing through the measuring tube of the detector is changed from the excitation switching point to the signal stabilization time. Sampling is performed in the meantime, and the flow rate of the fluid is obtained using the sampled data.

  According to the present invention, during the signal stabilization time until the excitation switching point, the magnitude of the excitation current is fixed to the excitation current value that is the magnitude of the excitation current before the signal stabilization time at the excitation switching point. As a result, a stable section for the signal stabilization time can be secured for the excitation current and the signal electromotive force, so that a stable section for the excitation current can be secured by simple processing regardless of the type of detector and the diameter of the measuring tube. can do.

It is a block diagram which shows roughly the structure of the electromagnetic flowmeter which concerns on embodiment of this invention. It is a block diagram which shows the functional structure of the control apparatus of FIG. It is explanatory drawing which shows the example of a change of an electric current value when an exciting coil is magnetically saturated comparatively early when an exciting current is sent through the exciting coil of an electromagnetic flowmeter. It is explanatory drawing which shows the example of a change of an electric current value when an excitation coil flows into an excitation coil of an electromagnetic flowmeter, and an excitation coil magnetically saturates comparatively late. It is explanatory drawing which shows the example of a change of an electric current value when the polarity of an exciting current switches before an exciting coil magnetically saturates when an exciting current is sent through the exciting coil of an electromagnetic flowmeter. It is explanatory drawing which shows the example of a change of an electric current value when an exciting current is sent through the exciting coil of an electromagnetic flowmeter, and the stable area is ensured with the converter of FIG. It is explanatory drawing which showed the change of the electric current value at the time of supplying an exciting current to the exciting coil of an electromagnetic flowmeter, and ensuring the stable area with the converter of FIG. 1 compared with the case of FIG. It is explanatory drawing which showed the change of the electric current value when an exciting current is sent through the exciting coil of an electromagnetic flowmeter, and the stable area was ensured with the converter of FIG. 1 compared with the case of FIG. FIG. 2 is an explanatory diagram illustrating the operation of the switch circuit and the sample hold circuit of FIG. 1 in association with changes in excitation current and signal electromotive force. It is explanatory drawing which illustrated the excitation voltage control by the voltage control part of FIG. It is a flowchart which shows the operation | movement regarding stabilization of an excitation current in case the electromagnetic flowmeter of FIG. 1 is a 2-wire type electromagnetic flowmeter. It is a flowchart which shows the operation | movement regarding stabilization of exciting current in case the electromagnetic flowmeter of FIG. 1 is a 4-wire type electromagnetic flowmeter. It is a flowchart which shows the operation | movement of the flow volume calculation by the electromagnetic flowmeter of FIG. It is a flowchart which shows the operation | movement of the voltage control by the electromagnetic flowmeter of FIG.

Embodiment.
FIG. 1 is a block diagram schematically showing the configuration of an electromagnetic flow meter according to an embodiment of the present invention. With reference to FIG. 1, the whole structure of the electromagnetic flowmeter 100 is demonstrated.

  The electromagnetic flow meter 100 is a device that measures the flow rate of a conductive fluid. As shown in FIG. 1, the electromagnetic flow meter 100 includes a detector 10 and a converter 20. The detector 10 has a measuring tube 11, an excitation coil 12, an electrode 13 a, an electrode 13 b, and a ground electrode 14. The measurement tube 11 is formed in a pipe shape and allows fluid to pass therethrough. The electromagnetic flow meter 100 is disposed so that a conductive fluid flows in the measurement tube 11. The exciting coil 12 generates a magnetic field in the measurement tube 11 and applies a magnetic field to the fluid flowing in the measurement 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 measurement 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 measurement tube 11 and the direction of the magnetic field generated by the excitation coil 12. Is arranged. The electrodes 13a and 13b constitute a pair of electrodes for extracting a signal electromotive force E based on an electromotive force generated when a fluid flows in a magnetic field generated by the exciting coil 12. The signal electromotive force E is a voltage generated between the electrode 13a and the electrode 13b and proportional to the flow rate of the fluid. The detector 10 detects the signal electromotive force E using the electrode 13 a and the electrode 13 b and outputs the detected signal electromotive force E to the converter 20.

  The converter 20 obtains the flow rate of the fluid flowing through the measurement tube 11 based on the signal electromotive force E proportional to the flow velocity of the fluid flowing through the measurement tube 11 of the detector 10 having the excitation coil 12. That is, the converter 20 obtains the flow rate of the fluid flowing through the measurement 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 35, an AD converter 40, a control device 50, a storage device 58, an excitation circuit 60, an excitation current generation unit 70, and an excitation voltage generation unit 80. And a current measuring unit 90 and a measuring resistance Ro.

  The excitation circuit 60 generates an excitation current Iex having an excitation frequency F whose polarity changes alternately according to a frequency command signal from the control device 50, and supplies the generated excitation current Iex to the excitation coil 12. The excitation circuit 60 includes an excitation switching circuit 61 and a switch circuit 62.

  The switch circuit 62 includes 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 switching elements such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect 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 connection point N, and the connection point Q between the third switch SW3 and the fourth switch SW4 is connected to the excitation current generator 70. Yes. A connection point R between the first switch SW1 and the third switch SW3 is connected to a terminal Y of the detector 10. A connection point S between the second switch SW2 and the fourth switch SW4 is connected to a terminal X of the detector 10. That is, the exciting coil 12 is connected between the connection point R and the connection point S.

  In accordance with the frequency command signal from the control device 50, the excitation switching circuit 61 turns on the second switch SW2 and the third switch SW3 and turns off the first switch SW1 and the fourth switch SW4, and the first switch The second state in which the SW1 and the fourth switch SW4 are turned on and the second switch SW2 and the third switch SW3 are turned off is repeated at a predetermined cycle. That is, the excitation switching circuit 61 generates a rectangular wave excitation current Iex whose polarity is switched alternately by controlling the on / off operation of each switch of the switch circuit 62. In the present embodiment, a positive exciting current Iex flows through the exciting coil 12 in the first state, and a negative exciting current Iex flows through the exciting coil 12 in the second state.

  The AC amplifier circuit 30 acquires the signal electromotive force E from the detector 10 and amplifies it. The sample hold circuit 35 is disposed on the output side of the AC amplifier circuit 30 and operates in accordance with the SH time control signal from the control device 50. The sample hold circuit 35 is a signal amplified in the AC amplifier circuit 30 for the signal stabilization time Ts from the excitation stabilization point signal stabilization time Ts, which is the timing at which the polarity of the excitation current Iex supplied to the excitation coil 12 is switched. The electromotive force E is sampled.

  Here, the signal stabilization time Ts is set so that the accuracy of the signal electromotive force E sampled during the signal stabilization time Ts as data for calculating the flow velocity can be ensured. The signal stabilization time Ts is set to 7 [ms], for example, and can be changed as appropriate.

  The AD converter 40 is an AD converter having a DC amplification function. The AD converter 40 includes a gain setting unit 41, a direct current amplification unit 42, and a conversion unit 43. The gain setting unit 41 is made of, for example, PGA (Programmable Gain Amplifier), and sets the gain G of the converter 20 in accordance with a gain adjustment signal from the control device 50. The DC amplifier 42 differentially amplifies the signal electromotive force E sampled by the sample hold circuit 35 using the gain G set by the gain setting unit 41. The converter 43 generates an AD count value corresponding to the flow velocity V from the analog signal based on the signal electromotive force E. That is, the conversion unit 43 converts the analog signal electromotive force E amplified by the DC amplification unit 42 into an AD count value that is a digital value.

  The control device 50 controls operations of the sample hold circuit 35, the AD converter 40, the excitation circuit 60, the excitation current generation unit 70, and the excitation voltage generation unit 80. In addition to the operation program of the control device 50, the storage device 58 stores various data used by the control device 50 for arithmetic processing.

  For example, the storage device 58 stores voltage conversion data for converting an AD count value into a voltage value. The voltage conversion data is linear data with the AD count value and the voltage value as variables, or table information in which the AD count value and the voltage value are associated with each other. In the voltage conversion data, the AD count value and the voltage value are associated with each other so that the voltage value increases as the AD count value increases. The storage device 58 stores flow velocity conversion data indicating the relationship between the voltage value and the flow velocity V. Further, the storage device 58 stores a reference excitation current EXI that serves as a reference for obtaining the fluid flow velocity V from the excitation current Iex.

The current measurement unit 90 is a circuit that measures the magnitude of the excitation current Iex as the excitation current value EXI _fix before the signal stabilization time Ts at the excitation switching point, which is the timing at which the polarity of the excitation current Iex is switched. The current measuring unit 90 includes an AD converter 91 that converts an analog signal into a digital signal. The current measuring unit 90 measures the current flowing through the measuring resistor Ro provided between the connection point M and the connection point N before the signal stabilization time Ts at the excitation switching point, and the measured current value is the excitation current value. It outputs to the control apparatus 50 as EXI _fix . For example, a resistance of 1 [Ω] is used as the measurement resistance Ro.

The exciting current generator 70 is a circuit that can continuously change the magnitude of the exciting current Iex. The exciting current generator 70 includes a DA converter 71 that converts a digital signal into an analog signal, and generates an exciting current Iex having a magnitude corresponding to the current command signal from the control device 50. That is, the excitation current generator 70 fixes the magnitude of the excitation current Iex to the excitation current value EXI _fix included in the current command signal from the control device 50. The exciting current generator 70 of the present embodiment can adjust the exciting current Iex between 0 [mA] and 320 [mA].

  The excitation voltage generator 80 is a circuit that can continuously change the excitation voltage Vex supplied to the excitation coil 12. The excitation voltage generator 80 includes a DA converter 81 that converts a digital signal into an analog signal, and generates an excitation voltage Vex having a magnitude corresponding to a voltage command signal from the control device 50. In the present embodiment, the excitation voltage generator 80 can adjust the excitation voltage Vex between 0 [V] and 36 [V].

  FIG. 2 is a block diagram showing a functional configuration of the control device of FIG. A functional configuration of the control device 50 will be described with reference to FIG. The control device 50 includes a switching control unit 51, a current acquisition unit 52, a current control unit 53, a voltage control unit 54, a flow rate calculation unit 55, a sampling processing unit 56, and a gain adjustment unit 57. ing.

  The switching control unit 51 controls the excitation frequency F of the excitation current Iex by outputting a frequency command signal instructing switching of the excitation direction of the excitation current Iex to the excitation switching circuit 61. That is, the switching control unit 51 controls the switch circuit 62 via the excitation switching circuit 61 and performs a switching process of the excitation direction of the excitation current Iex flowing through the excitation coil 12. In the excitation direction switched by the switching control unit 51, the excitation current Iex flows in the order of the terminal X, the excitation coil 12, and the terminal Y, and the excitation current Iex flows in the order of the terminal Y, the excitation coil 12, and the terminal X. There is a direction of the second state. That is, the above-described excitation switching point is a timing at which the switching control unit 51 switches the excitation direction of the excitation current Iex.

The current acquisition unit 52 acquires the excitation current value EXI _fix from the current measurement unit 90 before the signal stabilization time Ts of the excitation switching point, and acquires the acquired excitation current value EXI _fix from the current control unit 53, the voltage control unit 54, And output to the flow rate calculation unit 55.

The current control unit 53 _fixes the magnitude of the excitation current Iex to the excitation current value EXI _fix during the signal stabilization time Ts until the excitation switching point. More specifically, the current control unit 53 generates a current command signal instructing current adjustment based on the excitation current value EXI _fix acquired from the current acquisition unit 52, and outputs the current command signal to the excitation current generation unit 70. As a result, the current control unit 53 sets the excitation current value EXI _fix in the DA converter 71 of the excitation current generation unit 70. That is, the current control unit 53 determines the magnitude of the excitation current Iex generated by the excitation current generation unit 70 before the signal stabilization time Ts at the excitation switching point, and the excitation current value EXI _fix measured by the current measurement unit 90 at that time. Secure to.

  The voltage control unit 54 performs control so that the magnitude of the excitation voltage Vex supplied to the excitation coil 12 becomes a preset switching voltage value Vo at the excitation switching point. The switching voltage value Vo is a positive voltage value within a range that can be generated by the excitation voltage generation unit 80, and is set to be relatively large. That is, the voltage control unit 54 sets the magnitude of the excitation voltage Vex supplied from the excitation voltage generation unit 80 to the excitation coil 12 to the switching voltage value Vo that is a relatively large voltage value at the excitation switching point.

  Here, at the excitation switching point, the switching voltage value Vo is set to, for example, the maximum voltage that can be generated by the excitation voltage generation unit 80 in order to quickly raise the excitation current Iex. However, the switching voltage value Vo is set to a value smaller than the maximum voltage that can be generated by the excitation voltage generator 80 if a signal-to-noise ratio (Signal-to-Noise ratio) greater than a predetermined value in the excitation current Iex can be secured. May be.

In addition, the voltage control unit 54 reduces the magnitude of the excitation voltage Vex to the lower limit voltage value EXV set to be smaller than the switching voltage value Vo before the signal stabilization time Ts at the excitation switching point. The lower limit voltage value EXV is set to a relatively small voltage value within a range in which the magnitude of the excitation current Iex can be maintained at the excitation current value EXI _fix . The voltage control unit 54 obtains the lower limit voltage value EXV using the exciting current value EXI _fix and the internal resistance _RL of the exciting coil.

  More specifically, the voltage control unit 54 obtains the lower limit voltage value EXV by the following equation (1). Here, α is a compensation coefficient set in consideration of heat generation in the excitation voltage generator 80, and is set to a value of 0 or more. The compensation coefficient α is a coefficient for compensating power consumption due to external factors. As an external factor, heat generation in the internal configuration of the converter 20 such as the excitation current generation unit 70 and the excitation voltage generation unit 80 is assumed.

[Equation 1]
EXV [V] = EXI _fix [A] × R _L [Ω] × α (1)

In other words, the voltage control unit 54 multiplies the excitation current value EXI _fix measured by the current measurement unit 90 before the signal stabilization time Ts of the excitation switching point by the internal resistance _RL and the compensation coefficient α to thereby set the lower limit voltage value EXV. Ask for. Then, the voltage control unit 54 generates a voltage command signal instructing the change to the obtained lower limit voltage value EXV and outputs the voltage command signal to the excitation voltage generation unit 80.

By the way, the internal resistance _RL is preferably calculated by applying a specified voltage to both ends of the exciting coil 12, using the measured value of the current flowing at that time, and stored in the storage device 58. Note that the current measuring unit 90 can be used for the calculation of the internal resistance _RL . The compensation coefficient α is set to 1.2, for example, and can be appropriately changed according to the circuit configuration of the converter 20. The lower limit voltage value EXV can be set to a minimum voltage that can be maintained by the exciting current generating unit 70 such that the magnitude of the exciting current Iex is the exciting current value EXI _fix by adjusting the compensation coefficient α.

  In the present embodiment, the voltage controller 54 causes the excitation voltage generator 80 to generate a positive excitation voltage Vex when in the first state, and causes the excitation voltage generator 80 to generate a negative excitation voltage Vex when in the second state. Is generated. That is, the voltage control unit 54 causes the excitation voltage generation unit 80 to generate a positive excitation voltage Vex having a switching voltage value Vo at the excitation switching point at which the first state is switched. The voltage control unit 54 causes the excitation voltage generation unit 80 to generate a negative excitation voltage Vex having a switching voltage value Vo at the excitation switching point at which the second state is switched.

  Further, the voltage control unit 54 sets the excitation voltage Vex to be generated by the excitation voltage generation unit 80 to a positive voltage value having a lower limit voltage value EXV before the signal stabilization time Ts at the excitation switching point in the first state. To lower. The voltage control unit 54 increases the excitation voltage Vex generated by the excitation voltage generation unit 80 to a negative voltage value whose magnitude is the lower limit voltage value EXV before the signal stabilization time Ts at the excitation switching point in the second state. Let

  The flow rate calculation unit 55 obtains the flow rate of the fluid per unit time based on the signal electromotive force E detected by the detector 10 for each measurement period ΔT. In the present embodiment, the flow rate calculation unit 55 obtains an instantaneous flow rate that is a flow rate of fluid per 1 [s]. Here, the measurement cycle ΔT is set to 200 [ms], for example, and can be changed as appropriate.

The flow rate calculation unit 55 is based on the signal electromotive force E, and the relationship between the reference excitation current EXI serving as a reference for obtaining the flow velocity V from the excitation current Iex and the excitation current value EXI _fix measured by the current measurement unit 90. Is used to generate a converted voltage V _con corresponding to the reference excitation current EXI. And the flow volume calculating part 55 calculates _| requires the flow volume of the fluid using the produced _| generated conversion voltage _Vcon .

More specifically, the flow rate calculation unit 55 compares the AD count value output from the AD converter 40 with the voltage-converted data at each measurement period ΔT, so that the signal voltage V that is a voltage value corresponding to the AD count value is obtained. _Find in. The flow rate calculation unit 55, the signal voltage _{V in,} by the following equation (2), obtaining the converted voltage _{V con} which is proportional to the flow velocity V of the fluid. That is, the flow rate calculation unit 55, the signal voltage _{V in,} by multiplying the value obtained by dividing the reference exciting current EXI by the excitation current value _{EXI fix,} determine the conversion voltage _{V con.}

[Equation 2]
V _con [V] = V _in [V] × (EXI / EXI _fix ) (2)

And the flow volume calculating part 55 calculates _| requires the flow velocity V from the conversion voltage _Vcon using the flow velocity conversion data which show the relationship between a voltage value and the flow velocity V. FIG. The flow rate conversion data is data indicating a relationship that, for example, if the flow rate is 1 [m / s], the voltage value becomes 100 [μV]. In the case of this example, if the conversion voltage V _con is 1 [mV], the flow rate calculation unit 55 can obtain 10 [m / s] as the flow velocity from the flow velocity conversion data.

  Further, the flow rate calculation unit 55 obtains an instantaneous flow rate from the obtained flow velocity V according to the following equation (3). The cross-sectional area of the equation (3) is the cross-sectional area of the measuring tube 11 and is obtained from the diameter of the measuring tube 11 as in the following equation (4).

[Equation 3]
Instantaneous flow rate [m ³ / s] = flow velocity V [m / s] × cross-sectional area [m ² ] (3)

[Equation 4]
Cross-sectional area [m ² ] = π × caliber [m] × caliber [m] / 4 (4)

  Information on the cross-sectional area of the measurement tube 11 may be stored in the storage device 58 in advance, or the flow rate calculation unit 55 may calculate it each time according to the equation (4). In any case, the flow rate calculation unit 55 calculates the instantaneous flow rate by multiplying the calculated flow velocity by the cross-sectional area of the measurement tube 11. And the flow volume calculating part 55 outputs the information which shows the calculated | required instantaneous flow volume outside, or displays it on a display part (not shown).

  The sampling processing unit 56 outputs an SH time control signal including a clock used for sampling the signal electromotive force E to the sample hold circuit 35. That is, the sampling processing unit 56 causes the sample hold circuit 35 to sample the signal electromotive force E during the signal stabilization time Ts before the excitation switching point signal stabilization time Ts, that is, during the signal stabilization time Ts until the excitation switching point. .

  The gain adjusting unit 57 adjusts the gain G of the converter 20 according to the change in the flow velocity V calculated by the flow rate calculating unit 55. That is, the gain adjustment unit 57 generates a gain adjustment signal instructing the adjustment of the gain G according to the flow velocity V, and outputs the generated gain adjustment signal to the gain setting unit 41, whereby the gain used by the DC amplification unit 42 is obtained. Adjust G.

  FIG. 3 is an explanatory diagram showing an example of a change in current value when the exciting coil is magnetically saturated relatively quickly when an exciting current is passed through the exciting coil of the electromagnetic flow meter. FIG. 4 is an explanatory diagram showing an example of a change in current value when the exciting coil is magnetically saturated relatively slowly when an exciting current is passed through the exciting coil of the electromagnetic flow meter. FIG. 5 is an explanatory diagram showing a change example of the current value when the polarity of the excitation current is switched before the excitation coil is magnetically saturated when an excitation current is passed through the excitation coil of the electromagnetic flow meter. 3 to 5, for convenience, the same reference numerals are assigned to times used in the same meaning. With reference to FIGS. 3 to 5, the change pattern of the current value when the exciting current Iex is passed through the exciting coil of the conventional electromagnetic flow meter will be described.

In general, when a DC voltage _VL is applied to the exciting coil 12, the current i flowing through the exciting coil 12 changes as shown in the following equation (6) until the exciting coil 12 is magnetically saturated. Here, L is the inductance of the exciting coil 12, and t is the application time of the DC voltage _VL .

[Equation 6]
i = (V _L / L) × t (6)

In the example of FIG. 3, after switching the excitation direction, the excitation current Iex gradually rises, and the excitation coil 12 is magnetically saturated at time t _h1 . Therefore, the signal stabilization time Ts before time t ₁ is the excitation switching point, in other words the time t _01, already excitation current Iex is stable. Similarly, in the example of FIG. 3, after time _{t 1,} the excitation current Iex gradually decreases at time _{t h2,} the excitation coil 12 is magnetically saturated. Therefore, the signal stabilization time Ts before time t ₂ is the excitation switching point, in other words the time t _02, already excitation current Iex is stable. That is, in the case as shown in FIG. 3, it is possible to secure a stable section longer than the signal stabilization time Ts.

Meanwhile, in the example of FIG. 4, the time _{t h1} of the excitation coil 12 is magnetically saturated because is later than the time _{t 01,} the time _{t 01} the excitation current Iex is not stable. Similarly, time _{t h2} of the excitation coil 12 is magnetically saturated because is later than the time _{t 02,} the time _{t 02} the excitation current Iex is not stable. Therefore, in the case as shown in FIG. 4, it is not possible to secure a stable section longer than the signal stabilization time Ts, and it is not possible to sample an appropriate amount of stable signal electromotive force E. Further, in the example of FIG. 5, the excitation coil 12 is not magnetically saturated between the excitation current Iex rising and the excitation switching point. Therefore, even in the case of FIG. 5, a sufficient stable section cannot be secured, and an appropriate amount of stable signal electromotive force E cannot be sampled.

  FIG. 6 is an explanatory diagram showing an example of a change in current value when an excitation current is passed through the excitation coil of the electromagnetic flow meter and a stable section is secured by the converter of FIG. 6 is determined by the excitation voltage Vex and the inductance of the excitation coil 12.

In FIG. 6, the control device 50 acquires “I ₁ ” as the excitation current value EXI _fix from the current measurement unit 90 before the signal stabilization time Ts at time t ₁ that is the excitation switching point, that is, at time t _01. Assumed. In this case, as shown in FIG. 6, the control device 50 fixes the current value of the excitation current Iex generated by the excitation current generator 70 to “I ₁ ”. Similarly, in FIG. 6, the control device 50 acquires “I ₂ ” as the excitation current value EXI _fix from the current measurement unit 90 before the signal stabilization time Ts of time t ₂ that is the excitation switching point, that is, at time t _02. Assuming that In this case, as shown in FIG. 6, the control device 50 fixes the current value of the excitation current Iex generated by the excitation current generator 70 to “−I ₂ ”.

  Thus, even when various types of detectors are applied as the detector 10, the electromagnetic flow meter 100 according to the present embodiment converts the difference in the rising characteristics of the excitation current Iex, which is different for each detector 10, into a converter. 20 can be absorbed. That is, the converter 20 of the present embodiment can ensure a stable section of the excitation current Iex regardless of the type of the detector 10. In other words, the converter 20 is combined with various detectors 10 having different rising characteristics of the excitation current Iex without precise verification and adjustment, and the electromagnetic flow meter 100 in which the stable section of the signal stabilization time Ts is secured. Can be configured.

FIG. 7 is an explanatory diagram showing a change in the current value when an excitation current is passed through the excitation coil of the electromagnetic flow meter and a stable section is secured by the converter of FIG. 1 in comparison with the case of FIG. FIG. 8 is an explanatory diagram showing a change in the current value when an excitation current is passed through the excitation coil of the electromagnetic flow meter and a stable section is secured by the converter of FIG. 1 in comparison with the case of FIG. In FIG. 7, the waveform shown in FIG. 4 is indicated by a broken line, and the waveform in the present embodiment is indicated by a solid line. In FIG. 8, the waveform shown in FIG. 5 is indicated by a broken line, and the waveform in the present embodiment is indicated by a solid line. 7 and 8, it is assumed that the control device 50 has acquired the same excitation current value EXI _fix from the current measurement unit 90 as in FIG. 6.

  Even in the case of FIG. 4 in which a certain stable section could not be secured depending on the conventional configuration, if the converter 20 of the present embodiment is used, the signal stabilization time Ts can be increased as shown in FIG. A stable section can be secured. Similarly, even in the case of FIG. 5 where a certain stable section could not be ensured depending on the conventional configuration, if the converter 20 of the present embodiment is used, as shown in FIG. A stable section of time Ts can be secured. That is, even when the converter 20 cannot sufficiently secure a stable section due to magnetic saturation of the excitation current Iex, the converter 20 can reliably secure a stable section of the signal stabilization time Ts by simple processing.

  FIG. 9 is an explanatory diagram illustrating the operation of the switch circuit and the sample hold circuit of FIG. 1 in association with changes in excitation current and signal electromotive force. In FIG. 9, the first state where the second switch SW2 and the third switch SW3 are on and the second state where the first switch SW1 and the fourth switch SW4 are on are alternately repeated. Since the control device 50 controls the excitation current Iex as described above, a stable section of the excitation current Iex is ensured for the signal stabilization time Ts.

Therefore, the stable section of the signal electromotive force E is also secured for the signal stabilization time Ts. That is, the waveform of the signal electromotive force E includes a section that is not stable due to the influence of differential noise or the like, but as shown in FIG. 9, at least signals from time t ₀ to time t ₇ that are excitation switching points. During the stabilization time Ts, the signal electromotive force E is stabilized. Therefore, the sample hold circuit 35 can sample the highly stable signal electromotive force E amplified by the AC amplifier circuit 30 during the signal stabilization time Ts until the excitation switching point. Since the AD converter 40 differentially amplifies the signal electromotive force E sampled by the sample and hold circuit 35 and can accurately obtain the AD count value, the control device 50 allows the fluid flowing through the measurement tube 11 to flow. The flow rate can be accurately determined.

  FIG. 10 is an explanatory diagram illustrating excitation voltage control by the voltage control unit of FIG. With reference to FIG. 10, the excitation voltage control by the voltage control part 54 is demonstrated concretely.

  When the excitation voltage Vex applied to the excitation coil 12 is increased at the excitation switching point, the excitation current Iex can be increased quickly, so that the SN ratio equal to or higher than a predetermined value can be reached quickly. On the other hand, if the high excitation voltage Vex is maintained even after reaching the stable section, that is, before the signal stabilization time Ts of the excitation switching point, the excitation current generator 70 generates heat and consumes useless power. For this reason, the converter 20 of the present embodiment can adjust the excitation voltage Vex using the excitation voltage generation unit 80.

  The voltage control unit 54 sets the magnitude of the excitation voltage Vex supplied from the excitation voltage generation unit 80 to the excitation coil 12 to the switching voltage value Vo at the timing when the polarity of the excitation current Iex is switched. That is, when the switch circuit 62 is switched to the first state, the voltage control unit 54 sets the excitation voltage Vex to a positive voltage value whose magnitude is the switching voltage value Vo. Further, when the switch circuit 62 is switched to the second state, the voltage control unit 54 sets the excitation voltage Vex to a negative voltage value whose magnitude is the switching voltage value Vo. FIG. 10 illustrates a case where the switching voltage value Vo is set to 36 [V], which is the maximum voltage that can be generated by the excitation voltage generator 80.

Furthermore, the voltage control unit 54 sets the magnitude of the excitation voltage Vex to the lower limit voltage value EXV when the signal stabilization time Ts before the excitation switching point is reached. That is, when the time t ₀₁ in the first state is reached, the voltage control unit 54 multiplies I ₁ as the excitation current value EXI _fix by the internal resistance _RL and the compensation coefficient α to obtain the lower limit voltage value EXV. “V ₁ ” is obtained. Then, as shown in FIG. 10, the voltage control unit 54 reduces the excitation voltage Vex to a positive voltage value whose magnitude is “V ₁ ”. Further, when the time t ₀₂ in the second state is reached, the voltage control unit 54 multiplies I ₂ as the excitation current value EXI _fix by the internal resistance _RL and the compensation coefficient α to obtain the lower limit voltage value EXV. “V ₂ ” is obtained. Then, as illustrated in FIG. 10, the voltage control unit 54 increases the excitation voltage Vex to a negative voltage value having a magnitude of “V ₂ ”.

  The control device 50 can be configured by an arithmetic device such as a CPU (Central Processing Unit) and an operation program that realizes the various functions described above in cooperation with such an arithmetic device. The storage device 58 can be configured by a RAM (Random Access Memory) and a ROM (Read Only Memory), a PROM (Programmable ROM) such as a flash memory, or an HDD (Hard Disk Drive).

  FIG. 11 is a flowchart showing an operation relating to excitation current stabilization when the electromagnetic flow meter of FIG. 1 is a two-wire electromagnetic flow meter. With reference to FIG. 11, the stabilization process of the exciting current Iex in the flow rate calculation method by the converter 20 will be described.

The current acquisition unit 52 waits until the excitation switching point before the signal stabilization time Ts (step S101 / No). The current acquisition unit 52 acquires the excitation current value EXI _fix from the current measurement unit 90 when the signal stabilization time Ts of the excitation switching point is reached (step S101 / Yes). That is, the current acquisition unit 52 acquires the magnitude of the excitation current Iex in the signal stabilization time Ts at the excitation switching point as the excitation current value EXI _fix from the current measurement unit 90. Then, the current acquisition unit 52 outputs the acquired excitation current value EXI _fix to the current control unit 53, the voltage control unit 54, and the flow rate calculation unit 55 (step S102).

Next, the current control unit 53 _fixes the magnitude of the excitation current Iex to the excitation current value EXI _fix during the signal stabilization time Ts until the excitation switching point (step S103). The switching control unit 51 waits until the signal stabilization time Ts elapses before the signal stabilization time Ts before the excitation switching point and the excitation switching point is reached (step S104 / No). Then, when the signal stabilization time Ts has elapsed and the excitation switching point is reached (Yes at Step S104), the switching control unit 51 switches the excitation direction of the excitation current Iex flowing through the excitation coil 12 (Step S105). .

The control device 50 executes the above-described series of processing of steps S101 to S105 for each cycle in which the polarity of the excitation current Iex is switched. That is, when the electromagnetic flowmeter 100 is a two-wire type, it is necessary to change the excitation voltage Vex and the excitation frequency F by outputting a 4-20 mA current indicating the flow rate of the fluid. Therefore, it is necessary to acquire the excitation current value EXI _fix from the current measurement unit 90 every time the excitation stabilization point signal stabilization time Ts is reached.

  FIG. 12 is a flowchart showing an operation relating to excitation current stabilization when the electromagnetic flow meter of FIG. 1 is a four-wire electromagnetic flow meter. With reference to FIG. 12, the stabilization process of the exciting current Iex in the flow rate calculation method by the converter 20 will be described.

The control device 50 executes the processes in steps S101 and S102 in the same manner as in FIG. Moreover, the current acquisition unit 52 stores the excitation current value EXI _fix acquired from the current measurement unit 90 in the storage device 58 (step S201).

Next, the control device 50 executes the processes of steps S103 to S105 as in the case of FIG. The current control unit 53 waits until the excitation switching point signal stabilization time Ts is reached (step S202 / No). When the excitation switching point signal stabilization time Ts comes (step S202 / Yes), the storage device 58 receives the excitation current. The value EXI _fix is read (step S203). Then, the current control unit 53 _fixes the magnitude of the excitation current Iex in the signal stabilization time Ts until the next excitation switching point to the excitation current value EXI _fix read from the storage device 58 (step S204).

And the control apparatus 50 performs a series of processes of said step S104, S105, S202-S204 for every period in which the polarity of the exciting current Iex switches. By the way, when the electromagnetic flowmeter 100 is a four-wire type, the inductance of the exciting coil 12 of the detector 10 is not influenced by the type of fluid flowing in the measuring tube 11 and the change in flow velocity. Therefore, if the excitation frequency F and the excitation voltage Vex are constant, the excitation current value EXI _fix before the signal stabilization time Ts at the excitation switching point is constant. Therefore, it is not necessary to acquire the excitation current value EXI _fix from the current measuring unit 90 every time the excitation stabilization point signal stabilization time Ts is reached. That is, when the electromagnetic flowmeter 100 is a four-wire type, it is only once when the power is turned on, when adjusting in combination with the new detector 10, or when the excitation frequency Fex or the excitation voltage Vex applied at the time of excitation switching is changed. The excitation current value EXI _fix may be acquired. During the measurement, the excitation current Iex may be controlled using the excitation current value EXI _fix acquired first.

  FIG. 13 is a flowchart showing the flow rate calculation operation by the electromagnetic flow meter of FIG. With reference to FIG. 13, the flow rate calculation method by the converter 20 will be described focusing on the processing by the flow rate calculation unit 55.

  First, the sampling processing unit 56 causes the sample and hold circuit 35 to sample the signal electromotive force E during the signal stabilization time Ts to each excitation switching point in the measurement period ΔT. Then, the AD converter 40 differentially amplifies the signal electromotive force E sampled by the sample hold circuit 35 to generate an AD count value, and outputs the AD count value to the flow rate calculation unit 55. Thereby, the flow volume calculating part 55 acquires AD count value (step S301).

Then, the flow rate calculation unit 55 calculates the signal voltage _{V in} the light of the AD count value to the voltage conversion data (step S302). Then, the flow rate calculation unit 55, the above equation (2), by applying an excitation current value _{EXI fix} the signal voltage _{V in} obtaining the converted voltage _{V con.} Here, the flow rate calculation unit 55 uses the excitation current value EXI _fix output from the current acquisition unit 52 in step S102 or the excitation current value EXI _fix stored in the storage device 58 in step S201 as the excitation current value EXI _fix . Used (step S303).

Subsequently, the flow rate calculation unit 55 obtains the flow velocity V using the flow velocity conversion data based on the converted voltage V _con (step S304). And the flow volume calculating part 55 calculates | requires the instantaneous flow volume which is the flow volume of the fluid per 1 [s] from said flow velocity V calculated | required by step S304 by said Formula (3) (step S305). The control apparatus 50 performs a series of processes of said step S301-S305 for every measurement period (DELTA) T.

  FIG. 14 is a flowchart showing the voltage control operation by the electromagnetic flow meter of FIG. With reference to FIG. 14, the flow rate calculation method by the converter 20 will be described focusing on the excitation voltage Vex change processing by the voltage control unit 54.

The voltage control unit 54 sets the magnitude of the excitation voltage Vex supplied to the excitation coil 12 to the switching voltage value Vo at the excitation switching point (step S401). Next, the voltage control unit 54 stores the excitation current value EXI _fix output from the current acquisition unit 52 in step S102 or the storage device 58 in step S201 before the signal stabilization time Ts of the next excitation switching point. An excitation current value EXI _fix is acquired (step S402). Next, the voltage control unit 54 obtains the lower limit voltage value EXV by applying the acquired excitation current value EXI _fix to the above equation (1) (step S403). Then, the voltage control unit 54 outputs a voltage command signal instructing the change to the obtained lower limit voltage value EXV to the excitation voltage generation unit 80, and changes the magnitude of the excitation voltage Vex to the lower limit voltage value EXV (step). S404). The voltage control unit 54 executes the series of processes of steps S401 to S404 for each cycle in which the polarity of the excitation current Iex is switched.

As described above, according to the converter 20 and the electromagnetic flow meter 100 in the present embodiment, during the signal stabilization time Ts to the excitation switching point, the magnitude of the excitation current Iex is changed to the signal stabilization time Ts at the excitation switching point. The previous excitation current value EXI _fix is fixed. Therefore, since it is possible to secure a stable section corresponding to the signal stabilization time Ts for the excitation current Iex and the signal electromotive force E, the excitation current can be reduced by simple processing regardless of the type of the detector 10 and the diameter of the measurement tube. A stable section can be secured. The converter 20 samples the signal electromotive force E proportional to the flow velocity of the fluid flowing through the measurement tube 11 of the detector 10 during the signal stabilization time Ts from the excitation switching point, and uses the sampled data to Find the flow rate. Therefore, the fluid flow rate can be obtained with high accuracy.

  That is, the converter 20 can detect various detectors such as the existing detector 10, the new detector 10, or the detector 10 with unknown characteristics without checking the rising characteristics of the excitation current Iex for each detector 10. 10 can be used in combination. Since a stable section of the signal stabilization time Ts can be secured, stable data acquisition and generation can be realized.

Further, the flow rate calculation unit 55 generates a converted voltage V _con corresponding to the reference excitation current EXI using the relationship between the reference excitation current EXI and the excitation current value EXI _fix based on the signal electromotive force E, The flow rate of the fluid is obtained using the converted voltage _Vcon . Therefore, even if the excitation current value EXI _fix is different from the reference excitation current EXI, the flow velocity and flow rate can be accurately obtained using the converted voltage V _con .

Further, the voltage control unit 54 reduces the magnitude of the excitation voltage Vex, which is the switching voltage value Vo at the excitation switching point, to the lower limit voltage value EXV before the signal stabilization time Ts at the excitation switching point. . That is, when the voltage control unit 54 reaches the stable interval, the voltage control unit 54 increases the magnitude of the excitation voltage Vex up to a relatively small lower limit voltage value EXV that can maintain the excitation current value EXI _fix that is the magnitude of the excitation current Iex at that time. Lower. Therefore, heat generation in the internal circuit of the converter 20 can be suppressed, and wasteful power consumption can be prevented.

In addition, the voltage control unit 54 obtains the lower limit voltage value EXV using the excitation current value EXI _fix and the internal resistance _RL of the excitation coil 12. That is, the voltage control unit 54 obtains the lower limit voltage value EXV by multiplying the excitation current value EXI _fix by the internal resistance _RL and the compensation coefficient α. Therefore, even if an extra voltage is required to maintain the magnitude of the excitation current value EXI _fix with the excitation current Iex due to the power consumed by the heat generated in the internal circuit of the converter 20, etc., the excitation current Iex Can be reliably maintained at the exciting current value EXI _fix .

  Each embodiment mentioned above is a suitable example in a converter of an electromagnetic flow meter, an electromagnetic flow meter, and a flow rate calculation method, and the technical scope of the present invention is not limited to these modes. For example, in the above description, the case where the voltage control unit 54 changes the excitation voltage Vex before the signal stabilization time Ts at the excitation switching point is illustrated, but the present invention is not limited to this. That is, the converter 20 may be configured without providing the excitation voltage generation unit 80 that can continuously change the excitation voltage Vex. However, it is possible to save energy by providing the converter 20 with the excitation voltage generator 80 and reducing the magnitude of the excitation voltage Vex before the signal stabilization time Ts at the excitation switching point.

Further, since the internal resistance _RL of the exciting coil 12 can be measured by the current measuring unit 90, the control device 50 performs failure diagnosis such as disconnection or short using the measured value by the current measuring unit 90. May be. In addition, even if the current measurement unit 90 is not highly accurate, if the excitation current generation unit 70 is configured to generate an accurate excitation current Iex, the accuracy of the excitation current control can be ensured.

  In addition, in the said embodiment, although the case where AD converter 40 was provided with the direct current | flow amplification function was illustrated, it is not limited to this. The converter 20 may include a DC amplifier that functions similarly to the gain setting unit 41 and the DC amplification unit 42 and an AD converter that functions similarly to the conversion unit 43 instead of the AD converter 40. Good.

DESCRIPTION OF SYMBOLS 10 Detector, 11 Measuring tube, 12 Excitation coil, 13a, 13b Electrode, 14 Ground electrode, 20 Converter, 30 AC amplifier circuit, 35 Sample hold circuit, 40 AD converter, 41 Gain setting part, 42 DC amplifier part, 43 conversion unit, 50 control device, 51 switching control unit, 52 current acquisition unit, 53 current control unit, 54 voltage control unit, 55 flow rate calculation unit, 56 sampling processing unit, 57 gain adjustment unit, 58 storage device, 60 excitation circuit , 61 Excitation switching circuit, 62 Switch circuit, 70 Excitation current generation unit, 71, 81 DA converter, 80 Excitation voltage generation unit, 90 Current measurement unit, 91 AD converter, 100 Electromagnetic flow meter, E signal electromotive force, EXI reference excitation current, EXV lower limit voltage value, F excitation frequency, G gain, H leading edge, Iex excitation current, M, N, P~S connection point, _{R L} Part resistance, Ro measuring resistor, SW1 first switch, SW2 second switch, SW3 third switch, SW4 fourth switch, Ts signal stabilization time, V flow velocity, _{V L} DC _{V oltage, V con} converted voltage, Vex excitation V oltage, V _in signal voltage, Vo switching voltage value, X and Y terminals, ΔT measurement period, α compensation coefficient.

Claims (7)

A converter for obtaining a flow rate of the fluid based on a signal electromotive force proportional to a flow velocity of the fluid flowing through a measurement tube of a detector having an excitation coil,
A current measurement unit that measures the magnitude of the excitation current as an excitation current value before the signal stabilization time of the excitation switching point, which is a timing at which the polarity of the excitation current supplied to the excitation coil is switched;
An electromagnetic flowmeter converter comprising: a current control unit that fixes the magnitude of the excitation current to the excitation current value during a signal stabilization time to the excitation switching point.   Based on the signal electromotive force, the reference excitation current is obtained by using a relationship between a reference excitation current that is a reference for obtaining a flow velocity from the excitation current and the excitation current value measured in the current measurement unit. The converter of the electromagnetic flowmeter of Claim 1 which has the flow volume calculating part which produces | generates the conversion voltage corresponding to 1 and calculates | requires the flow volume of the fluid using the produced | generated said conversion voltage. A voltage control unit that controls the excitation voltage supplied to the excitation coil to have a preset switching voltage value at the excitation switching point;
The voltage controller is
3. The electromagnetic flow meter according to claim 1, wherein the excitation voltage is reduced to a lower limit voltage value set smaller than the switching voltage value before the signal stabilization time of the excitation switching point. Converter. The voltage controller is
The converter of the electromagnetic flowmeter according to claim 3, wherein the lower limit voltage value is obtained using the exciting current value and an internal resistance of the exciting coil. The voltage controller is
The electromagnetic flowmeter converter according to claim 4, wherein the lower limit voltage value is obtained by multiplying the excitation current value by the internal resistance and a compensation coefficient for compensating power consumption due to an external factor. . A detector for detecting a signal electromotive force generated in accordance with a flow velocity of the fluid flowing in the measurement tube, the excitation coil applying a magnetic field to the fluid flowing in the measurement tube;
An electromagnetic flowmeter comprising: the converter according to any one of claims 1 to 3, wherein a flow rate of the fluid is obtained based on the signal electromotive force detected by the detector. Before the signal stabilization time of the excitation switching point, which is the timing at which the polarity of the excitation current supplied to the excitation coil of the detector is switched, the magnitude of the excitation current is acquired as the excitation current value.
During the signal stabilization time to the excitation switching point, the magnitude of the excitation current is fixed to the excitation current value,
A flow rate calculation method of sampling a signal electromotive force proportional to the flow velocity of the fluid flowing through the measurement tube of the detector during the signal stabilization time from the excitation switching point, and obtaining the flow rate of the fluid using the sampled data .

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