JP2002206956A - Electromagnetic flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electromagnetic flowmeter of the constant-current control type capable of detecting a short circuit in an excitation coil by detecting an exciting current flowing in the coil. SOLUTION: A voltage limiter 50 for limiting the voltage applied to the excitation coil 3 of a detector, so that it will not drop below a certain value, is interposed between the excitation coil 3 and the excitation circuit of a converter, and a detection resistance 60 for detecting the exciting current is connected to a path of connection between the excitation coil 3 and the excitation circuit. A voltage derived from the exciting current detected by the resistance 60 is supplied to an operational amplifier 71 to which a reference voltage Es is imparted. In this arrangement, if the exciting current 3 is short-circuited, the voltage applied to the coil 3 is limited by the voltage limiter 50 and will not drop below a threshold voltage, so the exciting current is increased and the voltage at each end of the resistance 60 is also increased. The operational amplifier 71 compares the voltage Ed detected by the resistance 60 to the reference voltage Es; if the voltage Ed is greater than the reference voltage Es, the operational amplifier 71 outputs a short circuit detection signal.

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 an electromagnetic flowmeter having a short-circuit detecting function for detecting a short-circuit of an exciting coil.

[0002]

2. Description of the Related Art An electromagnetic flowmeter of this type takes a fluid to be measured flowing in a measuring tube as a conductor, generates a voltage by applying a magnetic field to the flowing conductor, and detects the voltage with an electrode to measure the flow rate. The configuration is generally composed of a detector and a converter as shown in FIG. The detector comprises: a measuring tube 1 having a pair of electrodes 2a and 2b provided on an inner wall thereof for flowing a fluid to be measured; an exciting coil 3 for applying a magnetic field in a direction orthogonal to the axial direction of the measuring tube 1 and the direction of electrode arrangement; Consisting of

On the other hand, the converter comprises preamplifiers 4a and 4b for amplifying an electromotive force (voltage) signal corresponding to a flow rate derived between a pair of electrodes 2a and 2b, a differential amplifier 5, and an exciting coil of a detector. 3
An excitation circuit 6 composed of a constant current source 6a for supplying an excitation current to the motor and a changeover switch 6b for switching the direction of the excitation current. The output signal of the differential amplifier 5 is arithmetically processed to calculate a fluid flow rate. The arithmetic processing / control unit 7 that controls the changeover switch 6b of the excitation circuit 6 so as to provide a positive / negative or zero period between positive and negative, and generates a low-frequency rectangular wave exciting current in the exciting coil 3. Become.

[0004] When exciting the exciting coil in this type of electromagnetic flowmeter, the voltage applied to the exciting coil is controlled so that the current (exciting current) flowing through the exciting coil becomes a predetermined value regardless of the impedance of the exciting coil. There are a constant voltage control method and a constant current control method with a constant current source that sets the exciting current to a constant current. However, the constant voltage control method cannot perform high-speed flow measurement and has a complicated circuit configuration. From that
Normally, a constant current control method capable of high-speed measurement and having a relatively simple circuit configuration is used. For this purpose, as shown in FIG. 3, the exciting circuit 6 is a constant current source 6a, and a changeover switch 6b connected to the constant current source 6a is switched so that a low-frequency rectangular wave constant exciting current flows through the exciting coil 3. Have been.

In such a configuration, when the fluid to be measured flows into the measuring tube 1, an electromotive force (voltage) signal corresponding to the flow rate drawn between the pair of electrodes 2a and 2b is generated, and this voltage is The impedance is converted by the preamplifiers 4a and 4b and the differential amplifier 6
Is differentially amplified. The arithmetic processing / control unit 7 includes an excitation circuit 6
In accordance with the alternate switching timing of the changeover switch 6b, the output of the differential amplifier 5 is taken in at the time when the excitation of the positive electrode and the excitation of the negative electrode are stabilized, and the fluid flow rate is calculated. I do.

As shown in FIG. 4, the excitation circuit of the constant current control system basically includes a power supply section 10, a constant current circuit 20 as a constant current control section, and an excitation switching section 30, and the power supply section 10 , A rectifier 12 that forms a rectifying and smoothing circuit that converts an AC voltage to a DC voltage, and a capacitor 13.The constant current circuit 20 includes an operational amplifier 21, a reference voltage source 22, and a current control element. For example, it comprises a transistor 23 and a detection resistor 24.

Excitation switching unit 30 for switching the direction of the excitation current
Are switching elements (hereinafter referred to as switches) S1 and S4
Is a bridge circuit in which switches S2 and S3 are connected in series, and these series circuits are connected in parallel, between a connection point between switches S1 and S4 and a connection point between switches S2 and S3. Is connected to the excitation coil 3 of the detector. The connection point between the switches S1 and S2 of the excitation switching unit 30 is connected to the power supply unit.
The connection point between the cathode of the rectifier 12 and the capacitor 13 and the connection point between the switches S3 and S4 are connected to the transistor 23 in which the emitter of the constant current circuit 20 is grounded via the resistor 24.
Connected to the collector. In addition, transistor 23
Is connected to an inverting input terminal of an operational amplifier 21 having a non-inverting input terminal connected to a reference voltage source 22, and an output terminal of the operational amplifier 21 is connected to a base of a transistor 23.

Therefore, in the excitation circuit shown in FIG. 4, the operational amplifier 21 of the constant current circuit 20 includes an electromotive force (voltage) proportional to the excitation current of the resistor 24 connected in series with the excitation coil 3 and the reference voltage source 22. The reference voltage is compared with the reference voltage, the base potential of the transistor 23 is controlled based on the difference, and the exciting current flowing through the exciting coil 3 is controlled to a constant current value set by the reference voltage of the reference voltage source 22.

The switches S1, S2, S of the excitation switching unit 30
3 and S4 are switched by a timing signal from the arithmetic processing / control unit 7 (see FIG. 3).
For example, a constant excitation current in the form of a rectangular wave that repeats positive, negative, zero, positive, zero, and negative as one cycle flows. The detector is attached to the pipe, the converter is attached to the console, and the detector and the converter are connected by an excitation cable and a signal cable.

[0010]

In this type of electromagnetic flowmeter, the exciting coil of the detector and the exciting cable connecting the exciting coil to the exciting circuit of the converter are short-circuited (hereinafter, both are collectively referred to as the exciting coil). In this case, the constant current circuit is broken or the power supply fuse is disconnected, which hinders the measurement of the fluid flow rate.

In a constant voltage control type electromagnetic flow meter,
If the exciting coil is short-circuited, the applied voltage of the exciting coil changes due to the change in impedance of the exciting coil. Therefore, the short-circuit of the exciting coil can be detected by detecting the voltage.

However, in the constant current control type electromagnetic flow meter, the constant current circuit controls the excitation current to a constant current value regardless of the change in the impedance of the excitation coil. , For example, a short circuit of an exciting coil cannot be detected by a voltage drop caused by an exciting current of a resistor connected in series to an exciting coil system from a detector to a converter.

In view of the above, the present invention provides an electromagnetic flowmeter that controls an exciting current supplied to an exciting coil by a constant current control method, whereby a short circuit of the exciting coil can be detected by measuring the exciting current due to a change in impedance of the exciting coil. An object of the present invention is to provide an electromagnetic flowmeter having a short-circuit detection function.

[0014]

In order to solve the above problems, an electromagnetic flow meter according to the present invention comprises an exciting coil between a constant current circuit for controlling a constant current on a converter side and an exciting coil on a detector side. The excitation current is detected by interposing a voltage limiter that limits the applied voltage of the excitation coil so as not to fall below a certain value, and a short circuit of the excitation coil is detected based on the detected value.

Specifically, a constant current circuit for controlling a constant current, a voltage limiter for limiting an applied voltage of an exciting coil interposed between the circuit and the exciting coil so as not to fall below a certain value, A detection resistor for detecting a voltage proportional to an excitation current connected in series with the excitation coil in a connection path between the limiter and the excitation coil, and a detection unit for detecting a short circuit of the excitation coil based on the detected voltage. It is characterized by being. Further, it is preferable that the detection means includes a comparison circuit that compares the detected voltage with the reference voltage and detects a short circuit of the exciting coil based on the comparison result. The term short-circuiting the exciting coil in claim 1 includes not only short-circuiting of the exciting coil itself on the detector side but also short-circuiting of the exciting cable connecting the exciting coil and the exciting circuit on the converter side.

According to the present invention, a voltage limiter is provided between the exciting coil and the constant current circuit to limit the applied voltage of the exciting coil so that the applied voltage does not drop below a predetermined value. The constant current circuit controls the excitation current to a constant current value set by the reference voltage.However, the applied voltage of the excitation coil does not fall below the fixed value determined by the action of the voltage limiter, and is fixed at the predetermined voltage value. You.

As a result, the exciting current changes in accordance with the impedance of the exciting coil and the exciting cable connecting the exciting coil and the constant current circuit. More specifically, the exciting current increases as the impedance decreases. Therefore, by measuring the exciting current, a short circuit of the exciting coil and the exciting cable connecting the exciting coil and the exciting circuit can be detected.

At this time, the exciting current is generated by connecting a detection resistor in series with the exciting coil to generate an electromotive force (drop voltage) due to the exciting current.
If the short circuit of the exciting coil is measured by comparing the measured voltage and the reference voltage, the configuration of the short circuit detecting means can be simplified. Note that the threshold voltage value of the voltage limiter that limits the applied voltage of the exciting coil so that it does not drop below a certain value is determined by the rated exciting current of the electromagnetic flowmeter. Further, the comparison reference voltage as a threshold value voltage for determining that a “short circuit” occurs when the detection voltage exceeds a certain value is experimentally obtained.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an electromagnetic flow meter according to the present invention will be described with reference to the drawings. FIG. 1 shows one embodiment of a constant current control type excitation circuit having a short-circuit detecting function which is a main part of the present invention. In the block diagram showing the schematic configuration of the example, components having the same functions as those in FIG. 4 are denoted by the same reference numerals, and description thereof will be omitted.

In FIG. 1, reference numeral 40 denotes an exciting coil short-circuit detecting circuit provided according to the present invention, which includes a voltage limiter 50, a detecting resistor 60 for detecting an exciting current, a differential amplifier 61, and a detecting circuit 70.
It is composed of The voltage limiter 50 limits the voltage applied to the exciting coil so that it does not drop below a certain value. The voltage limiter 50 is connected between the exciting circuit and the exciting coil 3, and the detection resistor 60 is connected between the exciting coil 3 and the exciting circuit. The connection path is connected in series with the exciting coil 3, and both ends thereof are connected to the non-inverting input terminal and the inverting input terminal of the differential amplifier 61. Also,
The detection circuit 70 includes an operational amplifier 71 and a reference voltage source 72.A reference voltage source 72 is connected to a non-inverting input terminal of the operational amplifier 71, and an output signal of the differential amplifier 61 is connected to an inverting terminal of the rectifier 73.
Connected through. The non-inverting input terminal and the inverting input terminal of the differential amplifier 61 are

Next, the operation of the short circuit detecting circuit 40 having the above configuration will be described. In a normal state where the exciting coil or the exciting cable connecting the exciting coil and the exciting circuit of the converter is not short-circuited, the voltage across the detection resistor 60 is constant, and the voltage limiter 50 shown in the characteristic diagram of FIG. Since the constant value is equal to or higher than the threshold voltage, the voltage limiter 50 does not have the voltage limiting function.
The operational amplifier 21 of the constant current circuit 20 generates an electromotive force (voltage) proportional to the exciting current of the resistor 24 connected in series with the exciting coil 3.
Is compared with the reference voltage of the reference voltage source 22, and the base potential of the transistor 23 is controlled based on the difference.
Is controlled to a constant current value set by the reference voltage.

When the exciting coil is short-circuited, the voltage limiter 50
As a result, the current is increased because the voltage is limited to the threshold voltage shown in the characteristic diagram of FIG. 2 and does not fall below the threshold voltage. As a result, the voltage across the detection resistor 60 connected in series with the exciting coil 3 increases. The voltage detected by the detection resistor 60 is supplied to a differential amplifier 61, and its output is a rectangular wave AC voltage, so that the rectifier 73 converts the voltage to a DC voltage, and the detected DC voltage Ed
Is compared with a reference voltage Es, which is a threshold voltage of the reference voltage source 72, by an operational amplifier 71 as a comparator of the detection circuit 70. When the detected voltage Ed exceeds the reference voltage Es (Ed> Es),
71 outputs an output signal, and this output signal is supplied to the monitor 80. The monitor 80 displays an indication that the exciting coil is short-circuited,
No warning is given. Thereby, the operator can recognize that the exciting coil is short-circuited.

In the embodiment, since the short circuit is detected by comparing with the DC reference voltage, it functions as a comparator of the detection circuit constituting the short circuit detection circuit by rectifying the voltage by the exciting current detected by the detection resistor. Is applied to the non-inverting terminal of the operational amplifier, but if the reference voltage applied to the non-inverting terminal of the operational amplifier is a rectangular wave reference voltage synchronized with the rectangular wave exciting current, the detection voltage is converted to a DC voltage. It is not necessary. Even when the detection voltage is converted to a DC voltage, the rectified voltage may be smoothed and applied to the inverting terminal of the comparison circuit. However, since the impedance (resistance) of the exciting coil differs greatly between the normal state and the short-circuited state, the short-circuit detection circuit does not malfunction even if the pulsating detection voltage is simply rectified as in the embodiment. Can be

The constant current circuit is not limited to the circuit of the embodiment, but may be another constant current circuit such as a field effect transistor as a current control element. Further, in the embodiment, the monitor is operated by the output of the short-circuit detection circuit to display the fact that the excitation coil is short-circuited.
It is also possible to stop the operation of the excitation circuit. Also,
In the embodiment, the output of the operational amplifier as the comparator is monitored to detect the short-circuit of the exciting coil. However, the rectified output of the preceding differential amplifier may be monitored to detect the short-circuit of the exciting coil. It is possible.

[0025]

According to the electromagnetic flowmeter of the present invention, by measuring the exciting current flowing through the exciting coil, the exciting coil on the detector side is short-circuited and the exciting coil is connected to the exciting circuit on the converter side. A short circuit in the connection path can be detected, and the configuration of the short circuit detecting means therefor is simple.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of an excitation circuit according to an embodiment of a main part of the present invention.

FIG. 2 is a characteristic diagram of the voltage limiter in FIG.

FIG. 3 is a block diagram illustrating a schematic configuration of an electromagnetic flowmeter.

FIG. 4 is a block diagram showing a configuration of an excitation circuit of a conventional electromagnetic flowmeter.

[Explanation of symbols]

 3: Excitation coil 10: Power supply unit 11 ... Commercial power supply 12 ... Rectifier 13 ... Capacitor 20: Constant current circuit (constant current control unit) 21 ... Operational amplifier 22 ... Reference voltage source 23 ... Transistor (current control element) 24 ... Resistance 30 : Excitation switching section S1 to S4 ... Switch (switching element) 40: Short circuit detection circuit 50 ... Voltage limiter 60 ... Detection resistor 61 ... Differential amplifier 70 ... Detection circuit 71 @ Operational amplifier 72 @ Reference voltage source 73 @ Rectifier 80: Monitor

Claims (3)

[Claims] An electromagnetic flow meter for controlling an exciting current flowing through an exciting coil with a constant current, wherein a voltage applied to the exciting coil between a constant current circuit for controlling the constant current and the exciting coil does not drop below a certain value. An electromagnetic flowmeter characterized by detecting an exciting current through a voltage limiter for limiting an exciting current and detecting a short circuit of an exciting coil based on the detected value. 2. A constant current circuit for controlling a current supplied to an exciting coil with a constant current, and a voltage for limiting an applied voltage of the exciting coil interposed between the circuit and the exciting coil so as not to drop below a certain value. A limiter, a detection resistor that detects a voltage proportional to an excitation current connected in series with the excitation coil in a connection path between the voltage limiter and the excitation coil, and a detection unit that detects a short circuit of the excitation coil based on the detected voltage. An electromagnetic flowmeter comprising: 3. The electromagnetic flow meter according to claim 2, wherein:
An electromagnetic flowmeter, wherein the detection means comprises a comparison circuit that compares the detected voltage with a reference voltage and detects a short circuit of the exciting coil.