JP2004219372A - Electromagnetic flowmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transducer, having an input circuit which selects an optimum feedback factor according to the cable length and fluid electrical conductivity, has little signal attenuation, and will not oscillate by providing the feedback factor of a cable capacitance generated at an outer conductor to an amplifier to be variable, when converting a signal from an electrode, using a shielded cable having the outer conductor. <P>SOLUTION: The electromagnetic flowmeter generates a magnetic field by an exciting coil, provided outside a measuring tube and measures a flow rate of a fluid flowing through the measuring tube, by detecting an electromotive force developed between detection electrodes, provided on the measuring tube by the transducer. The transducer comprises the amplifier and a shielding device. The amplifier supplies the signal from the detection electrodes, provided on the measuring tube to the core wire of the cable, having the outer conductor and amplifies the signal of the detection electrodes supplied to the core. The shielding device connects the intermediate point of two resistors, connected to between an output terminal and a common of the amplifier to the outer conductor of the shield cable and feeds it back forwardly to the amplifier. The resistance value of the resistors, constituting the shield device, is set to be variable. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an electromagnetic flowmeter, and more particularly, to an improvement in an input circuit of a converter that amplifies, using a shielded cable, an electromotive force signal obtained by a detection electrode arranged in a measurement tube.
[0002]
[Prior art]
As shown in FIG. 4, the electromagnetic flow meter according to the related art generates a magnetic field by excitation coils 112A and 112B provided outside a measuring tube 111, and is generated between detection electrodes 113A and 113B provided in the measuring tube 111. The converter 114 is configured to measure the flow rate of the fluid flowing through the measuring tube 111 by detecting the electromotive force with the converter 114. The measuring tube includes excitation coils 112A and 112B and detection electrodes 113A and 113B. 111, a shielded cable 117 having an outer conductor 116 having a detection electrode 113B connected to a core 115, a converter 114 for amplifying a signal from the core 115 of the shielded cable 117, and a signal obtained by the converter 114. The flow rate calculation circuit section 118 for calculating and measuring the flow rate is roughly configured.
[0003]
The converter 114 supplies a signal from the detection electrode 113B provided on the measuring tube 111 to the core 115 of the shielded cable 117 having the external conductor 116, and converts the signal Vin of the detection electrode 113B supplied to the core 115 into the signal Vin. An amplifier U to be amplified and a shield device 119 which connects an intermediate point between two resistors R1 and R2 connected between the output terminal of the amplifier U and the common to the outer conductor 116 of the shielded cable 117 and positively feeds back to the amplifier U. , And is provided.
[0004]
Here, in order to apply a magnetic field to the measuring tube 111 at right angles, measure the electromotive force Es between a pair of detection electrodes 113A and 113B provided on the inner surface of the measuring tube 111, and obtain the flow rate in the tube from the measured value, It can be obtained by the following equation (1).
[0005]
Q = π · D · Es / (4 kB) Equation (1)
Q: Volume flow rate (m ³ / S)
π; pi D; inner diameter of measuring tube (m)
Es; electromotive force k; constant B; magnetic flux density
Thus, the flow rate can be measured by the electromotive force Es generated in the measurement tube 111.
To measure this flow rate, a shielded cable 117 is used in the converter 114 as a cable connecting the detection electrode 113B of the measurement tube 111 and the converter 114. Here, if the external conductor 116 of the shielded cable 117 is connected to the circuit common as shown in FIG. 5, the partial resistance ratio of the liquid contact resistance Rs, which is the signal source resistance due to the fluid resistance, to the cable capacitance Cc is calculated. Since the signal (Vin) of the electromotive force Es is input to the converter 114, the cable capacity Cc increases as the cable length of the shielded cable 117 increases, and the signal Vin input to the converter 114 is expressed by the following equation (2). ).
[0007]
Vin = Es / [(ωCc) · (Rs + 1 / ωCc)] Equation (2)
Vin: Input signal to the amplifier constituting the converter Es: Electromotive force Cc: Cable capacity of shielded cable Rs: Wetted resistance
In order to prevent this problem, a buffer output signal is usually applied to the outer conductor 116 of the shielded cable 117. As a result, the potential difference between both ends of the cable capacitance Cc disappears, so that the signal of the electromotive force Es becomes equal to the signal Vin input to the converter 114 as shown in the following equation (2).
[0009]
Es = Vin Equation (3)
Es: electromotive force Vin: input signal to the amplifier constituting the converter
However, since feedback is applied to the non-inverting input terminal of the amplifier U via the cable capacitance Cc, there is a problem in that if positive feedback is applied with the gain being 1, the amplifier U will oscillate.
Therefore, as shown in FIG. 4, by applying a voltage corresponding to the resistances of the resistors R1 and R2, oscillation is prevented and signal attenuation due to the cable length is prevented. This method is generally called a drive shield.
[0011]
[Patent Document 1]
JP-A-8-278180 (page 3 FIG. 1)
[0012]
[Problems to be solved by the invention]
However, in the circuit described in the related art, the resistors R1 and R2 are required for generating a divided voltage for one converter, and the constants of the resistance values are constant. Occurs.
[0013]
{Circle around (1)} When the feedback rate is reduced, the fluid conductivity is reduced, and when the liquid contact resistance Rs is increased, the signal attenuation increases.
(2) Increasing the feedback ratio solves the problem (1). However, when the cable length is long and the liquid contact resistance Rs is small, the impedance connected to the output terminal of the amplifier U becomes almost the cable capacitance Cc. There is a problem that oscillation is likely to occur due to the characteristic load.
[0014]
Therefore, there is a problem to be solved in realizing an input circuit of a converter using a shielded cable for solving the above two problems (1) and (2).
[0015]
[Means for Solving the Problems]
In order to solve the above problems, an electromagnetic flowmeter according to the present invention has the following configuration.
[0016]
(1) An electromagnetic flowmeter generates a magnetic field by an excitation coil provided outside a measuring tube, and detects a electromotive force generated between detection electrodes provided on the measuring tube by a converter, so that a fluid flowing in the measuring tube. In the electromagnetic flowmeter configured to measure the flow rate, the converter supplies a signal from a detection electrode provided in the measurement tube to a core wire of a cable having an outer conductor, and is supplied to the core wire. An amplifier for amplifying the signal of the detection electrode, and a shield device for connecting the midpoint between two resistors connected between the output terminal of the amplifier and the common to the outer conductor of the cable and positively feeding back the amplifier. The resistance value of the resistor constituting the shield device is variable.
(2) The shield device is characterized in that when the cable length is long and the fluid conductivity is large and the liquid contact resistance is small, the feedback value is increased by reducing the resistance value of the resistor. The electromagnetic flow meter according to (1).
(3) The shield device is characterized in that when the cable length is long and the fluid conductivity is small and the liquid contact resistance is large, the feedback value is reduced by increasing the resistance value of the resistor. The electromagnetic flowmeter according to (1).
(4) The electromagnetic flow meter according to (2) or (3), further comprising a function of measuring a fluid conductivity by supplying a constant current between the detection electrodes.
(5) The function of measuring the fluid conductivity is to measure the liquid contact resistance between the detection electrodes by using two detection electrodes,
σ = 2 / (Rs · d)
here,
The electromagnetic flowmeter according to (4), characterized in that it is determined based on a relational expression of σ: fluid conductivity Rs; liquid contact resistance d; electrode diameter.
(6) The function of measuring the fluid conductivity is to measure each liquid contact resistance between the detection electrode and the earth electrode with three electrodes of two detection electrodes and the ground electrode, respectively.
σa = 2 / (Ras · d)
σb = 2 / (Rbs · d)
here,
σa, σb; fluid conductivity Ras, Rbs between each detection electrode and the ground electrode; liquid contact resistance d between each detection electrode and the ground electrode; The electromagnetic flowmeter according to (4).
[0017]
As described above, in a converter having a shield device in an amplifier that amplifies an electromotive force obtained by a detection electrode provided in a measurement tube using a shielded cable, the resistance value of a resistor constituting the shield device is made variable. By appropriately changing the resistance value, it is possible to select an optimal feedback rate according to the cable length and fluid conductivity of the shielded cable, and to provide an input circuit with less signal attenuation and no oscillation. Converter can be realized.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, an embodiment of an electromagnetic flowmeter according to the present invention will be described below with reference to the drawings.
[0019]
As shown in FIG. 1, the electromagnetic flowmeter according to the first embodiment of the present invention generates a magnetic field by exciting coils 12 </ b> A and 12 </ b> B provided outside measurement tube 11, and detects detection electrode 13 </ b> A provided on measurement tube 11. , 13B is an electromagnetic flowmeter configured to measure the flow rate of the fluid flowing in the measuring tube 11 by detecting the electromotive force Es generated between the measuring tube 11 and the electromotive force Es. A shield cable 16 having an excitation coil 12A, 12B, a pair of detection electrodes 13A, 13B provided at opposing positions, and an external conductor 15 in which one of the pair of detection electrodes 13A, 13B is connected to a core wire 14. A converter 17 for amplifying the signal Vin from the electrode obtained via the shielded cable 16, and a flow calculation circuit 18 for calculating the flow rate from the signal amplified by the converter 17. Ranaru.
The flow rate is measured by calculation based on the equation (1) described in the related art, and is calculated by calculation based on the electromotive force Es generated in the measurement tube.
[0020]
The converter 17 supplies signals from the detection electrodes 13A and 13B provided in the measurement tube 11 to the core wire 14 of the shielded cable 16 having the external conductor 15, and the signal of the detection electrode 13B supplied to the core wire 14. An amplifier U that inputs and amplifies Vin through a resistor R, and an intermediate point between two resistors VR and R1 connected between an output terminal of the amplifier U and a common is connected to an external conductor 15 to connect the amplifier U to the amplifier U. And a shield device 19 that positively feeds back. The resistance value of one of the two resistors VR and R1 is a variable resistor VR that can be varied.
Although not shown, the variable resistor VR can be configured to be controlled by the flow rate calculation circuit unit 18.
That is, in the case where the length of the shielded cable 16 is long and the fluid conductivity is large and the liquid contact resistance is small, the shield device 19 reduces the value of the resistor VR to increase the feedback rate. If the length of the shielded cable 19 is long and the fluid conductivity is small and the liquid contact resistance Rs is large, the value of the resistor VR is increased to reduce the feedback rate.
[0021]
The flow rate calculation circuit section 18 calculates the flow rate by a signal obtained by amplifying the signal Vin of the electromotive force Es generated by the liquid contact resistance Rs obtained by the converter, and includes a function of calculating the fluid conductivity. It has. The fluid conductivity can be obtained from the relationship between the liquid contact resistance Rs between the detection electrodes 13A and 13B disposed on the measurement tube 11 and the electrode diameter. Specific equations for obtaining the fluid conductivity are Equation (4) described in the second embodiment and Equation (5) described in the third embodiment, but are not limited thereto. The fluid conductivity measured in advance may be stored in a memory or the like and used as appropriate.
[0022]
In the electromagnetic flow meter having such a configuration, by varying the resistor R or the variable resistor VR for generating the divided voltage, and in the embodiment, the variable resistor VR, the cable length of the shielded cable 16 and the fluid conductivity can be reduced. The feedback ratio is changed according to the value.
In particular,
{Circle around (1)} When the cable length is long and the fluid conductivity is large and the liquid contact resistance Rs is small, the feedback rate is reduced to, for example, 95% to 98% or less.
(2) If the cable length is long and the fluid conductivity is small and the liquid contact resistance Rs is large, the feedback rate is increased to, for example, 99% or more.
[0023]
Further, in the converter 17 having the function of measuring the fluid conductivity, the feedback rate can be changed according to the measured value of the fluid conductivity. That is, since the length of the cable is determined in advance, the length of the cable is stored in the flow rate calculation circuit unit 18. Further, the feedback ratio (the ratio between the resistor R and the variable resistor VR) can be automatically changed by the control signal from the flow rate calculation circuit section 18.
[0024]
Next, an electromagnetic flow meter according to a second embodiment of the present invention will be described below with reference to FIG. Note that the same components as those of the electromagnetic flow meter of the first embodiment will be described with the same reference numerals.
[0025]
As shown in FIG. 2, the electromagnetic flow meter according to the second embodiment of the present invention has a function of calculating fluid conductivity, and is provided with excitation coils 12A and 12B provided outside the measurement tube 11. An electromagnetic flow configured to generate a magnetic field and measure the flow rate of the fluid flowing through the measurement tube 11 by detecting the electromotive force Es generated between the detection electrodes 13A and 13B provided on the measurement tube 11 with the converter 17A. The configuration is such that the excitation coils 12A and 12B provided on the measuring tube 11, a pair of detection electrodes 13A and 13B provided at opposing positions, and one detection electrode 13B of the pair of detection electrodes 13A and 13B are provided. The shield cable 16 connected to the core wire 14, a converter 17A for amplifying the signal Vin from the detection electrodes 13A and 13B obtained through the shield cable 16, and a converter 17A. A flow rate calculation circuit unit 18 for calculating a flow rate from the obtained signal, switches S1 and S2 provided on each of the detection electrodes 13A and 13B, and a constant current source 20 for applying a constant current by turning on the switches S1 and S2; And a switch drive circuit section 21 for controlling ON / OFF of S2.
[0026]
The converter 17A supplies the signal Vin from the detection electrodes 13A and 13B provided in the measurement tube 11 to the core wire 14 of the shielded cable 16 having the outer conductor 15, and the detection electrode 13B supplied to the core wire 14. U which inputs the signal Vin through the resistor R and amplifies the signal Vin, and connects the intermediate point between two resistors VR and R1 connected between the output terminal of the amplifier U and the common to the external conductor 15. The configuration includes a shield device 19A that positively feeds back to the amplifier U. The resistance value of one of the two resistors VR and R1 is a variable resistor VR that can be varied. This variable resistor VR is configured to be controlled by the flow rate calculation circuit section 18.
[0027]
That is, when the length of the shielded cable 16 is long and the fluid conductivity is large and the liquid contact resistance Rs is small, the shield device 19A controls to reduce the value of the resistor VR to increase the feedback rate. When the length of the shielded cable 16 is long and the fluid conductivity is small and the liquid contact resistance Rs is large, control is performed to increase the value of the resistor VR and reduce the feedback rate.
[0028]
The flow rate calculation circuit section 18 calculates a flow rate using a signal obtained by amplifying the signal Vin of the electromotive force Es generated by the liquid contact resistance Rs obtained by the converter 17A. A function is provided for calculating the fluid conductivity when the switches S1 and S2 are controlled to be on. The fluid conductivity can be obtained from the relationship between the liquid contact resistance Rs between the detection electrodes 13A and 13B disposed on the measurement tube 11 and the electrode diameter.
That is, the function of measuring the fluid conductivity is to obtain the fluid conductivity by the following equation (4) by measuring the liquid contact resistance Rs by applying a constant current to the two detection electrodes 13A and 13B. be able to.
[0029]
Here, first, in order to obtain the liquid contact resistance Rs, first, the switch S1 is turned on by the switch drive circuit unit 21, and the switch S2 is turned on. Then, the current from the constant current source 20 flows to the common line via the switch S1 → the detection electrode 13A → the liquid contact resistance Rs → the detection electrode 13B → the switch S2. The flow rate calculation circuit section 18 may measure the voltage generated between the detection electrodes 13A and 13B at this time. The liquid contact resistance Rs is obtained from the voltage generated between the detection electrodes 13A and 13B and the current (known) of the constant current source 20, and the fluid conductivity can be obtained by the following equation (4).
[0030]
σ = 2 / (Rs · d) Equation (4)
σ; fluid conductivity Rs; liquid contact resistance d; electrode diameter
In this manner, by determining the fluid conductivity and making the resistor R or the variable resistor VR for generating the divided voltage, and the variable resistor VR in the embodiment, variable, the cable length of the shielded cable 16 and the fluid conductivity are determined. The feedback ratio can be changed according to the value.
In particular,
{Circle around (1)} When the cable length is long and the fluid conductivity is large and the liquid contact resistance Rs is small, the feedback rate is reduced to, for example, 95% to 98% or less.
(2) If the cable length is long and the fluid conductivity is small and the liquid contact resistance Rs is large, the feedback rate is increased to, for example, 99% or more.
[0032]
Next, an electromagnetic flow meter according to a third embodiment of the present invention will be described below with reference to FIG.
[0033]
As shown in FIG. 3, the electromagnetic flow meter according to the third embodiment of the present invention has a function of calculating fluid conductivity, which is provided with excitation coils 12A and 12B provided outside the measuring tube 11. Generates a magnetic field, and measures the electromotive force generated between each of the detection electrodes 13A and 13B provided on the measurement tube 11 and the ground electrode 22 with the converter 17A to measure the fluid conductivity of the fluid flowing through the measurement tube 11. The electromagnetic flowmeter is configured to include excitation coils 12A and 12B provided on the measurement tube 11, a pair of detection electrodes 13A and 13B provided at opposing positions, and a pair of detection electrodes 13A and 13B. And a shield cable 16 in which one of the pair of detection electrodes 13A and 13B is connected to a core wire 14; A converter 17A for amplifying the signal Vin from the electrode obtained through the cable 16, a flow rate calculation circuit section 18 for calculating a flow rate from the signal amplified by the converter 17A, and a switch S1 for each of the detection electrodes 13A and 13B. , S2, a switch S3 provided on the ground electrode 22, a constant current source 20 for applying a constant current when the switches S1 and S3 or S2 and S3 are turned on, and a switch drive for controlling on / off of the switches S1, S2 and S3 And a circuit section 21.
[0034]
The converter 17A supplies a signal from the detection electrode 13B provided on the measuring tube 11 to the core wire 14 of the shielded cable 16 having the outer conductor 15, and the signal Vin of the detection electrode 13B supplied to the core wire 14 U through a resistor R to amplify the amplifier U, and an intermediate point between two resistors VR and R1 connected between the output terminal of the amplifier U and the common to the external conductor 15 to connect the amplifier U to the amplifier U. The shield device 19A for positive feedback is provided. The resistance value of one of the two resistors VR and R1 is a variable resistor VR that can be varied.
This variable resistor VR is configured to be controlled by the flow rate calculation circuit section 18.
[0035]
That is, when the length of the shielded cable 16 is long and the fluid conductivity is large and the liquid contact resistances Ras and Rbs are small, the shield device 19A controls the value of the resistor VR to be small to increase the feedback rate. I do. When the length of the shielded cable 16 is long and the fluid conductivity is small and the liquid contact resistances Ras and Rbs are large, the value of the resistor VR is increased to reduce the feedback rate.
[0036]
The flow rate calculation circuit section 18 calculates a flow rate based on a signal obtained by amplifying the electromotive force signal Vin generated by the liquid contact resistances Ras and Rbs obtained by the converter 17A, and includes a switch drive circuit section 21. Has the function of calculating the fluid conductivity when the switches S1, S3 or S2, S3 are controlled to be ON. The fluid conductivity can be obtained from the relationship between the liquid contact resistances Ras and Rbs between the detection electrodes 13A and 13B disposed on the measurement tube 11 and the ground electrode 22, and the electrode diameter.
That is, the function of measuring the fluid conductivity is to measure the liquid contact resistances Ras and Rbs by flowing a constant current between the two detection electrodes 13A and 13B and the ground electrode 22, thereby obtaining the following equation (5). ) And (6) can provide fluid conductivity.
[0037]
Here, first, in order to obtain the liquid contact resistance Ras of the detection electrode 13A, first, the switch S1 is turned on by the switch drive circuit unit 21, and the switch S3 is turned on. Then, the current from the constant current source 20 flows to the common line via the switch S1 → the detection electrode 13A → the liquid contact resistance Ras → the earth electrode 22 → the switch S3.
In addition, in order to obtain the liquid contact resistance Rbs of the detection electrode 13B, first, the switch S2 is turned on by the switch drive circuit unit 21, and the switch S3 is turned on. Then, the current from the constant current source 20 flows to the common line via the switch S2 → the detection electrode 13B → the liquid contact resistance Rbs → the earth electrode 22 → the switch S3.
[0038]
The flow rate calculation circuit section 18 may measure the voltage generated between the detection electrodes 13A and 13B and the ground electrode 22 at this time. The liquid contact resistances Ras and Rbs are obtained from the voltage generated between the detection electrodes 13A and 13B and the ground electrode 22 and the current (known) of the constant current source 20, and the fluid conductivity is expressed by the following equations (5) and (6). The relationship can be obtained.
[0039]
σa = 2 / (Ras · d) Equation (5)
σb = 2 / (Rbs · d) (6)
σa, σb; fluid conductivity Ras, Rbs between each detection electrode and earth electrode; liquid contact resistance d between each detection electrode and earth electrode; electrode diameter
In this way, the value of the cable length and the value of the conductivity of the shielded cable 16 can be obtained by varying the resistor R or the variable resistor VR for generating the divided voltage and the variable resistor VR in the embodiment. , It is possible to change the feedback rate.
In particular,
{Circle around (1)} When the cable length is long and the fluid conductivity is large and the liquid contact resistances Ras and Rbs are small, the feedback rate is reduced to, for example, 95% to 98% or less.
(2) If the cable length is long and the fluid conductivity is small and the liquid contact resistances Ras and Rbs are large, the feedback rate is increased to, for example, 99% or more.
[0041]
【The invention's effect】
As described above, the converter according to the present invention realizes a converter having an input circuit that has less signal attenuation and does not oscillate by selecting an optimal feedback rate according to the cable length and the fluid conductivity. There is an effect that can be.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory view showing an electromagnetic flow meter according to a first embodiment of the present invention.
FIG. 2 is a schematic explanatory view showing an electromagnetic flow meter according to a second embodiment of the present invention.
FIG. 3 is a schematic explanatory view showing an electromagnetic flow meter according to a third embodiment of the present invention.
FIG. 4 is a schematic explanatory view showing an electromagnetic flow meter according to the related art.
FIG. 5 is a schematic equivalent circuit of an electromagnetic flow meter according to the related art.
[Explanation of symbols]
11 Measuring tube 12A Exciting coil 12B Exciting coil 13A Detecting electrode 13B Detecting electrode 14 Core wire 15 Outer conductor 16 Shield cable 17 Converter 17A Converter 18 Flow rate operation circuit section 19 Shield device 20 Constant current source 21 Switch drive circuit section

Claims (6)

A magnetic field is generated by an excitation coil provided outside the measuring tube, and an electromotive force generated between detection electrodes provided in the measuring tube is detected by a converter to measure a flow rate of a fluid flowing in the measuring tube. In the obtained electromagnetic flow meter, the converter is:
An amplifier that supplies a signal from a detection electrode provided in the measurement tube to a core wire of a cable having an outer conductor, and amplifies a signal of the detection electrode supplied to the core wire.
A shield device that connects an intermediate point of two resistors connected between an output terminal of the amplifier and a common to an outer conductor of the cable and positively feeds back to the amplifier,
The resistance value of the resistor constituting the shield device is variable. 2. The shield device according to claim 1, wherein when the length of the cable is long and the fluid conductivity is large and the liquid contact resistance is small, the resistance value of the resistor is reduced to increase the feedback rate. An electromagnetic flowmeter according to item 1. 2. The shield device according to claim 1, wherein when the cable length is long and the fluid conductivity is small and the liquid contact resistance is large, the feedback value is reduced by increasing the resistance value of the resistor. An electromagnetic flowmeter according to item 1. The electromagnetic flowmeter according to claim 2, further comprising a function of measuring a fluid conductivity by supplying a constant current between the detection electrodes. The function of measuring the fluid conductivity measures the liquid contact resistance between the detection electrodes by two detection electrodes,
σ = 2 / (Rs · d)
here,
The electromagnetic flowmeter according to claim 4, wherein the flow rate is determined based on a relational expression of σ; fluid conductivity Rs; liquid contact resistance d; The function of measuring the fluid conductivity is to measure each liquid contact resistance between the detection electrode and the ground electrode by three electrodes of two detection electrodes and the ground electrode,
σa = 2 / (Ras · d)
σb = 2 / (Rbs · d)
here,
σa, σb; fluid conductivity Ras, Rbs between each detection electrode and the ground electrode; liquid contact resistance d between each detection electrode and the ground electrode; The electromagnetic flow meter according to claim 4, wherein

Images