JPH08105774A - Electromagnetic flowmeter - Google Patents

Abstract

PURPOSE: To provide an electromagnetic flowmeter, which can use a transistor, whose rated power is small, as the transistor of a constant current circuit and can suppress the temperature rising in the inside of a converter even if the flowmeter is set in the different environment or state. CONSTITUTION: A load adjusting part 21 is provided between an exciting part 12 and a coil 2. When a cable connecting a detector 1 and a converter 10 is relatively long, the resistance becomes large. Therefore, the variable resistance is set at the small value at the time of provision or at the time of maintenance and adjustment. When the cable 5 is relatively short conversely, the resistance becomes small. Therefore, the variable resistance is set at the large value. Thus, the electric power, which is dissipated in the load viewed from the exciting part 12, is adjusted to the constant value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flowmeter, and more particularly to a connecting cable connecting a detector for detecting a signal electromotive force from a fluid to be measured and a converter for calculating and outputting a flow rate according to the detected signal electromotive force. The present invention relates to a separate type electromagnetic flow meter which is installed at a remote place via the.

[0002]

2. Description of the Related Art Conventionally, a separation type electromagnetic flowmeter has a structure as shown in FIG. In the figure, 1 is a detector for generating a predetermined magnetic field and detecting a signal electromotive force from a fluid, 10 is a signal electromotive force from the detector 1 for signal processing to calculate a flow rate, and a predetermined process control signal is obtained. The converters 5 for converting and outputting are cables for connecting the converters 10 and the detectors 1, which are installed at distant locations, respectively. In the detector 1, 2 is a coil for generating a predetermined magnetic field,
Reference numeral 3 denotes a tube through which a fluid to be measured flows, and reference numerals 4a and 4b denote detection electrodes arranged so as to face the inner surface of the tube 3 and come into contact with the fluid.

In the converter 10, 11 is a power supply section for generating a predetermined power supply voltage from a loop current supplied from a process control device (not shown), and 12 is an exciting section for outputting an exciting current of a predetermined frequency to the coil 2. , 13 are detection electrodes 4a,
4b is a signal processing unit for amplifying the signal electromotive force and then sampling and outputting it as digital information. Reference numeral 14 is a calculation / control for calculating the flow rate from the output of the signal processing unit 13 and controlling each unit in the converter 10. Part, 15 is calculation
The output interface unit outputs a predetermined process control signal based on the flow rate information from the control unit 14.

The exciting section 12 excites the coil 2 by outputting an exciting signal of a constant current having alternating polarities via the cable 5 based on a control signal from the arithmetic / control section 14. As a result, a magnetic field orthogonal to the fluid flowing in the pipe 3 from the coil 2 is generated, and a signal electromotive force proportional to the flow velocity of the fluid is generated between the detection electrodes 4a and 4b provided on the inner wall surface of the pipe 3. This is input to the signal processing unit 13 and converted into digital information, the calculation / control unit 14 calculates the flow rate, and the process control signal corresponding to this is output from the output interface unit 15.

FIG. 7 is a circuit diagram of a portion related to an exciting circuit in the electromagnetic flow meter. In FIG. 7, SW1 to SW are shown.
A switch 4 is controlled based on a control signal from the arithmetic / control unit 14, and in particular, SW1 and SW4 are paired and controlled in the same phase, while SW2 and S4 are controlled.
W3 is paired and controlled in the opposite phase to SW1 and SW4. Further, Q is a transistor, and Rf is a feedback resistor, both of which are connected in series to the coil 2, and also have an operational amplifier AMP, a constant voltage diode ZD, and a resistor R.
A constant current circuit is constituted by 1 and R2, whereby an exciting current having a predetermined current value, which is a rectangular wave current whose polarity alternates at a predetermined frequency of the control signal, is output to the coil 2. Will be things.

Here, when the detector 1 and the converter 10 are installed near each other, the resistance RC of the cable 5 is relatively small, but the detector 1 and the converter 10 are installed several hundred meters apart. In that case, the resistance RC of the cable 5 connecting the detector 1 and the converter 10 becomes a value that cannot be ignored as compared with the resistance RL of the coil 2. For example, for a cable with a diameter of 2 mm, 2
It becomes 5Ω / Km, and this is 300m one way (600 round trips)
m) When used, RC = 15Ω, several tens of Ω
The coil resistance is about the same as the coil resistance RL. The resistance RL of the coil 2 and the resistance RC of the cable 5 also fluctuate depending on the environmental temperature in which the electromagnetic flow meter is installed. For example, the resistance value at 100 ° C for copper is about 1.4 times the resistance value at 0 ° C. Becomes

Therefore, in the conventional electromagnetic flowmeter, when the load consisting of the resistance RL of the coil 2 and the resistance RC of the cable 5 is maximum, the collector-emitter voltage VCE of the transistor Q is minimum and the load is reduced. The constant current circuit is configured so that the voltage VCE becomes maximum when the voltage VCE becomes minimum, and load fluctuation due to the coil 2 and the cable 5 is compensated by the voltage VCE between the collector and emitter of the transistor Q. That is, assuming that the voltage drop in the feedback resistor RF is VF (= IRF: constant), the power supply voltage VEX is VEX = I (RC + RL) + VCE + VF , The power consumption corresponding to the difference is compensated by the collector-emitter voltage VCE of the transistor Q, that is, the power consumption of the transistor Q.

[0008]

Therefore, in such a conventional electromagnetic flowmeter, since the load variation due to the coil 2 and the cable 5 is configured to be compensated by the transistor Q, it is consumed by the transistor Q. It is necessary to use one with a large maximum rated power (collector loss) according to the power, and to configure a means for heat dissipation, and it is also necessary to design each part in consideration of the temperature rise inside the converter. Therefore, there is a problem that the product becomes expensive and the design work becomes complicated. The present invention is intended to solve such a problem, and it is possible to use a transistor with a small maximum rated power as a transistor used in a constant current circuit, and even if it is installed in a different environment or situation, a converter is used. It is an object of the present invention to provide an electromagnetic flow meter capable of suppressing an internal temperature rise.

[0009]

In order to achieve such an object, an electromagnetic flowmeter according to the present invention includes a detector having a coil for generating a magnetic field for obtaining a signal electromotive force according to the flow rate, and a coil. In an electromagnetic flowmeter consisting of a converter having an exciting part for supplying a constant current as an exciting current to the, and a wire connecting the coil of the detector and the exciting part of the converter, the electromagnetic flowmeter is provided between the coil and the exciting part. The load adjusting unit adjusts the electric power consumed by the load on the coil side as seen from the exciting unit.

Further, the load adjusting unit is provided inside the detector or inside the converter. Further, the load adjusting unit is composed of a variable resistor connected in series with the wiring. Further, the load adjusting unit is composed of variable voltage drop means for causing a voltage drop for compensating a load caused by the wiring to be generated in series in the wiring in response to an instruction operation based on the length of the displayed wiring. .

[0011]

Therefore, the load adjusting section provided between the coil of the detector and the exciting section of the converter adjusts the electric power consumed by the load on the coil side as seen from the exciting section. In addition, the variable resistor connected in series to the wiring adjusts the power consumed by the coil-side load as seen from the exciting unit.

[0012]

Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an electromagnetic flowmeter which is an embodiment of the present invention, in which the same or equivalent parts as those in the above description (see FIG. 4) are designated by the same reference numerals. In FIG. 1, reference numeral 21 is a load adjusting unit provided between the exciting unit 12 and the coil 2 for arbitrarily adjusting the power consumed by the load viewed from the exciting unit 12, and in this case, the detector. Even if the load differs depending on the length of the cable 5 that connects the detector 1 and the converter 10 and the like, the power consumed by the load is adjusted and set to an arbitrary value.

FIG. 2 is a circuit diagram showing a portion relating to the excitation circuit of the electromagnetic flow meter shown in FIG. 1, and the same or equivalent portions as those in the above description (see FIG. 6) are designated by the same reference numerals.
In this case, the load adjusting unit 21 provided in the detector 1 is realized by a variable resistor as a variable voltage drop means for generating an arbitrary voltage drop, and the resistance value RV is adjusted according to the load, and the converter The load viewed from the excitation unit 12 of 10 is adjusted to a predetermined value, and the power consumed by the load becomes constant.
When SW1 and SW4 are controlled to be conductive, the exciting current I supplied to the coil 2 is equal to the power supply voltage VEX-SW1.
-Cable 5-Variable resistor-Coil 2-Cable 5-S
W4-transistor Q-feedback resistor-ground voltage GND.

Here, the power supply voltage VEX becomes VEX = I (RC + RL + RV) + VCE + VF, and the resistance RL of the coil 2 and the resistance R of the cable 5 are obtained.
If C varies depending on the environment and situation at the time of installation, increase or decrease the variable resistance RV to reduce the power consumption P due to the load.
Can be set to a predetermined value, and as the predetermined value, the power consumption of the transistor Q is a power value for generating an emitter-collector voltage VCE sufficient to absorb a relatively small load change after installation. By setting the load power consumption P such that the power consumption in the transistor Q can be minimized.

That is, assuming that an ideal emitter-collector voltage which has a sufficient emitter-collector voltage to absorb a relatively small load fluctuation after installation and has a small power consumption in the transistor Q is VCE ', the above equation is obtained. VEX = I. (RC + RL + RV) + VCE '+ VF. Here, the resistance when the cable 5 is short is RL '
When (RL '<RL), the load between SW1 and SW4, that is, the load R (=
RC + RL + RV) is reduced by RL -RL '.

Conventionally, the difference power I · (RL −R
L ') is consumed by the transistor Q, but in the present invention, the load R is kept constant by increasing the variable resistance of the load adjusting unit 21 from RV to RL-RL', and an ideal VCE 'Is suppressed and the transistor Q
It keeps power consumption to a minimum. Therefore, when the detector 1 and the converter 10 are installed at distant places and the cable 5 connecting them is relatively long, the resistance R C becomes large. Therefore, the load R is made constant by reducing the variable resistance R V. On the contrary, when the detector 1 and the converter 10 are installed close to each other and the cable 5 is relatively short, the resistance R C becomes small, so that the load R becomes constant by increasing the variable resistance R V. To be kept.

As described above, the load adjusting unit 21 is provided between the exciting unit 12 and the coil 2 so that the load R viewed from the exciting unit 12 is adjusted to be constant during installation or maintenance adjustment. It is possible to use a transistor having a small maximum rated power as the transistor Q, and it is possible to suppress an increase in temperature inside the converter even if the transistor Q is installed in different environments or situations. Further, since the load adjusting unit 21 uses the variable voltage drop means capable of arbitrarily setting the voltage to be dropped, it is possible to reliably adjust the power even with respect to the exciting current supplied as the constant current.

In the above description, the load adjusting unit 2
Although the case where 1 is provided inside the detector 1 has been described, it may be provided inside the converter 10, or the detector 1 may be provided inside.
And the converter 10 may be provided. FIG. 3 is a block diagram when the load adjusting unit 21 is provided inside the converter 10. The load adjusting unit 21 is provided between the exciting unit 12 and the cable 5, and the load viewed from the exciting unit 12 is shown. Is held constant, and it is possible to obtain the same actions and effects as those described above.

Further, the case where the load adjusting section 21 is constituted by the variable resistance RV has been described, but the present invention is not limited to this, and a plurality of resistive circuit elements having different fixed values, for example, fixed resistors, diodes or constant voltage. A diode or the like may be provided to switch and connect as necessary. FIG. 4 is a circuit diagram showing the configuration of the load adjusting unit 21, in which fixed resistors are switched and connected, and a fixed resistor or a constant voltage diode having a value corresponding to the change of the load should be selected. This makes it possible to obtain the same action and effect as those described above.

Further, in the above description, the load adjusting unit 2
1. At each operating position such as the knob of the variable resistor RV that composes 1, or the changeover switch that switches the fixed resistor, etc.
The length of the cable or the length and diameter of the cable which can be compensated for by the voltage drop occurring at each operating position may be displayed. FIG. 5 shows the load adjusting unit 21.
It is an explanatory view showing the operation panel of FIG.
Is an adjusting knob, and around the adjusting knob 31, the length of the cable that can be compensated for by the voltage drop generated at the operating position is displayed by one way.

Therefore, at the time of load adjusting work, the load adjusting section 2 is selected by selecting the operation position according to the length of one way of the cable 5 connecting the detector 1 and the converter 10.
In 1, the load generated in the cable 5 (reciprocating portion) of that length is compensated, and a voltage drop occurs such that the load seen from the excitation unit 12 becomes constant. In this way, by selecting the position where the cable length or the cable length and diameter are displayed, the optimum load corresponding to the cable is selected, and the load adjustment work becomes easy. At the same time, it becomes possible to suppress adjustment errors.

[0022]

As described above, according to the present invention, the load adjusting section is provided between the coil of the detector and the exciting section of the converter so that the load seen from the exciting section is set at the time of installation or maintenance adjustment. Since the power consumption is adjusted to be constant, it is possible to use transistors with a small maximum rated power as the transistors that make up the constant current circuit of the excitation part, and install them in different environments and situations. It is also possible to suppress the temperature rise inside the converter.

Further, since the load adjusting section is composed of the variable resistor connected in series to the wiring, it is possible to continuously and easily adjust it to an arbitrary value. Also,
Since the load adjusting unit is configured by the variable voltage drop unit that generates a voltage drop for compensating the load caused by the wiring in series in accordance with the instruction operation based on the displayed length of the wiring, By selecting the operation position where the length is displayed, the optimum load corresponding to the wiring is selected, which facilitates the load adjustment work and suppresses the adjustment error.

[Brief description of drawings]

FIG. 1 is a block diagram of an electromagnetic flow meter according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a portion related to an exciting circuit of the electromagnetic flow meter of the present invention.

FIG. 3 is a block diagram of an electromagnetic flow meter according to another embodiment of the present invention.

FIG. 4 is a circuit diagram showing a configuration of a load adjusting unit.

FIG. 5 is an explanatory diagram showing an operation panel of a load adjusting unit.

FIG. 6 is a block diagram of a conventional electromagnetic flow meter.

FIG. 7 is a circuit diagram of a portion related to an excitation circuit of a conventional electromagnetic flow meter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Detector, 2 ... Coil, 3 ... Tube, 4a, 4b ... Detection electrode, 5 ... Cable, 10 ... Converter, 11 ... Power supply part, 12
... Excitation section, 13 ... Signal processing section, 14 ... Calculation / control section, 1
5 ... Output interface unit, 21 ... Load adjusting unit.

Claims (4)

[Claims] 1. A detector having a coil for generating a magnetic field for obtaining a signal electromotive force according to a flow rate, a converter having an exciting section for supplying a constant current as an exciting current to the coil, and a detector of the detector. In an electromagnetic flowmeter comprising a coil and a wiring connecting the exciting part of the converter, the electromagnetic flowmeter is provided between the coil and the exciting part, and is consumed by a load on the coil side viewed from the exciting part. An electromagnetic flowmeter comprising a load adjusting unit for adjusting electric power. 2. The electromagnetic flowmeter according to claim 1, wherein the load adjustment unit is provided inside the detector or inside the converter. 3. The electromagnetic flowmeter according to claim 1, wherein the load adjusting unit includes a variable resistor connected in series to the wiring. 4. The electromagnetic flowmeter according to claim 1, wherein the load adjusting unit responds to an instruction operation based on the length of the displayed wiring.
An electromagnetic flow meter, comprising variable voltage drop means for generating a voltage drop for compensating a load generated by the wiring in series with the wiring.