JP2838650B2 - Electromagnetic flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flow meter for measuring the flow rate of a fluid having conductivity in various kinds of process systems, and more particularly, to a method for measuring a current flowing through a cable connected to a DC power supply via an external load. The present invention relates to an electromagnetic flowmeter that outputs a measured value by adjusting a current range.

[0002]

2. Description of the Related Art Conventionally, in this type of electromagnetic flowmeter, an exciting current is supplied at a predetermined frequency to an exciting coil arranged so that the direction of generation of a magnetic field is perpendicular to the direction of flow of a fluid flowing in a measuring tube. Detecting a signal electromotive force (a signal proportional to the flow rate) obtained between electrodes arranged in the measurement tube at right angles to the magnetic field generated by the exciting coil, and performing arithmetic processing in the CPU based on the detected signal electromotive force. The measured value is 0
Calculated as 100% value, converted through integration circuit consisting of a resistor and a capacitor the reference signal V ₁ of the duty ratio corresponding to the measurement values thus determined to the DC voltage V _2, the DC voltage V
_2. Controlling the current I _out (cable current) flowing through a cable connected to a DC power supply (24 VDC) via an external load by controlling the transistor in its active area according to ₂ in a current range of 4 to 20 mA. And

FIG. 3 is a circuit diagram showing a main part of a conventional electromagnetic flow meter. In the figure, 100 is an electromagnetic flow meter,
It is connected to a DC power supply (DC 24 V) 200 via an external load R ₀ . L1 and L2 are cables connecting the electromagnetic flow meter 100 and the DC power supply 200. In the electromagnetic flow meter 100, a measured value is obtained as a value of 0 to 100% by arithmetic processing in a CPU (not shown), and a duty signal I _DUTY corresponding to the obtained measured value is sent from the CPU to the current output circuit 1. Will be sent. In the current output circuit 1, the switch SW 1 is turned on / off by the duty signal I _DUTY , that is, the reference voltage V _REF generated by the reference voltage generator 2 is turned on / off, and the output of the buffer 3 corresponds to the measured value. pulse signal having a duty ratio (a reference signal) V ₁ is obtained. This reference signal V ₁ is converted into a DC voltage V ₂ through an integrating circuit 4 including a capacitor C ₁ and a resistor R _1, and supplied to a CCS circuit 5. CCS circuit 5
_{Are the} operational amplifier OP, the transistor Tr1, and the resistors R _F1 and R _F1 .
And a _F2 and switch SW2, usually if, that does not communicate, the switch SW2 is turned off. Thus, CCS circuit 5, usually, corresponding to the DC voltage V ₂ is input, and a resistor R _F1 and R _F2 as a feedback resistor, and controls the transistor T _r1 in the active region,
The cable current I _out is adjusted in a current range of 4 to 20 mA.

On the other hand, at the time of communication, the electromagnetic flow meter 1 is used.
At 00, the duty signal I _DUTY corresponds to the measured value of 0%. As a result, the DC voltage V ₂ is set to a value corresponding to the measured value of 0%, and the cable current I _out becomes 4 mA. Here, a communication signal T _XD is _supplied to the switch SW2 as digital data to be communicated, and the switch SW2 is turned on when the communication signal T _XD is at “H” level, and is turned off when the communication signal T _XD is at “L” level. That is, when the communication signal T _{XD is at} the “H” level, both ends of the resistor R _F2 are short-circuited to make the resistor R _F1 a feedback resistor in the CCS circuit 5, and when the communication signal T _{XD is at} the “L” level, the resistance R _F2 The short-circuit state at both ends is released, and the resistors R _F1 and R _F2 are used as feedback resistors in the CCS circuit 5. Thus, when the communication signal T _{XD is at} the “H” level, the cable current I _out is 20
mA, and at the time of the “L” level, the cable current I _out is 4 mA. That is, if the DC voltage V ₂ corresponding to the measured value of 0% is 1 V, the series resistance value of the resistors R _F1 and R _F2 is 250Ω, so that 1 V / 250Ω = 4 m
As A, the cable current I _out at the time of the “L” level of the communication signal T _XD is obtained. By setting the value of the resistor R _F1 to 50Ω, 1V / 50Ω = 20 mA at the time of the “H” level of the communication signal T _XD . The cable current I _out is obtained. Due to the change in the cable current I _out , the cables L1 and L
Handy terminal connected between the two, DC power supply 20
0 and a remote device with an external load R ₀ . FIG. 4 shows the signal waveform of each part for reference.

[0005]

[SUMMARY OF THE INVENTION However, according to such a conventional electromagnetic flow meter, because it is to perform communication by switching the feedback resistance at the CCS circuit 5 switch SW2, transistor T _r1
Feedback resistance R _F1 + R in consideration of the on resistance of
_A large value (for example, 250 Ω) is required as _F2 , which makes it impossible to increase the resistance value of the external load R ₀ , the communication waveform rises and falls steeply, and crosstalk occurs with other lines. Was likely to occur.

The present invention has been made to solve such a problem, and an object of the present invention is to make a rising and falling of a communication waveform gentle and to cause a crosstalk with another line. It is an object of the present invention to provide an electromagnetic flowmeter capable of suppressing the occurrence of a flow.

[0007]

To SUMMARY OF THE INVENTION To achieve the above object, (the invention according to claim 1) a first invention, after having the time of communication, the duty ratio of the reference signal V ₁ is 100%, In addition, the time constant in the integrating circuit is shortened, and when the digital data to be communicated changes from the first level to the second level, the constant in the integrating circuit is switched to a predetermined constant to change the DC voltage V ₂ to the _{second level} . 1 to a second voltage value to change the current flowing through the cable from the maximum value to the minimum value of the current range, while changing the digital data to be communicated from the second level to the first level. when, by switching the constant of the integration circuit to the original constant switching to the first voltage value DC voltage V ₂ from the second voltage value, to vary the current flowing through the cable from the minimum value of the current range to a maximum value It is like that. Further, the second invention (the invention according to claim 2), when communicating, the duty ratio of the reference signal V ₁ 10
0%, the capacity of the capacitor in the integration circuit is made small, and when the digital data to be communicated changes from the first level to the second level, the resistance value in the integration circuit is set to a predetermined resistance value. To change the DC voltage V ₂ from the first voltage value to the second voltage value to change the current flowing through the cable from the maximum value to the minimum value of the current range, and to change the second value of the digital data to be communicated. when the change from the level to the first level, switched DC voltage V ₂ from the second voltage value by switching the resistance value of the integration circuit to the original resistance value to the first voltage value, the current flowing in the cable The current range is changed from the minimum value to the maximum value.

[0008]

Therefore, according to the present invention, it is possible to make the feedback resistance in the CCS circuit the same during non-communication and during communication, and to set its value small.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on embodiments. FIG. 1 is a circuit diagram showing a main part of an electromagnetic flow meter according to an embodiment of the present invention. The same reference numerals as those in FIG. 3 denote the same or equivalent components, and a description thereof will be omitted. In this embodiment, in the integration circuit 4 'of the current output circuit 1', the resistance R
It is assumed that a resistor R ₂ between ₁ and capacitor C _1. Further, the switch SW2 across resistor R ₁ provided, and the switch SW3 provided in the connecting line between the capacitor C ₁ and resistor R _2. The capacitor C having a small capacity in parallel with the series circuit of the capacitor C ₁ and the switch SW3
₂ and a resistor R in parallel with the capacitor C ₂ .
₃ is connected. Further, as a feedback resistance of the CCS circuit 5 ′, a resistor R (for example, 50Ω) having a value smaller than the series resistance of the resistor R _F1 and the resistor R _F2 shown in FIG.
I'm using _F.

Next, the characteristic operation of the electromagnetic flowmeter will be described with the function of each part. [At the time of non-communication] A measured value is obtained as a value of 0 to 100% by arithmetic processing in a CPU (not shown), and a duty signal I _DUTY corresponding to the obtained measured value is transmitted from the CPU to the current output circuit 1 '. Come. In the current output circuit 1 ',
The switch SW1 is turned on by the duty signal I _DUTY .
The control is turned off, that is, the reference voltage V _REF generated by the reference voltage generator 2 is turned on / off, and a pulse signal (reference signal) V ₁ having a duty ratio corresponding to the measured value is obtained as an output of the buffer 3.

Here, during non-communication, the switch SW2 is turned on because the communication signal T _XD maintains the “L” level, and the switch SW3 is turned on because the COM signal is at the “H” level. Is done. As a result, the reference signal V ₁ is converted as a divided voltage of the resistors R ₂ and R ₃ into a DC voltage V ₂ smoothed by the capacitors C ₁ and C ₂ and supplied to the CCS circuit 5 ′. In this case, since the capacitor C ₁ is provided as a large-capacity, the time constant of the integrating circuit 4 'it is long. CCS circuit 5 ', in response to the DC voltage V ₂ is input, the resistance R _F as a feedback resistor, and controls the transistor T _r1 in the active region,
The cable current I _out is adjusted in a current range of 4 to 20 mA.

[During communication] During communication, the duty signal I _DUTY
Corresponds to the 100% measured value. As a result, the switch SW1 maintains the ON state, and the reference signal V1
Is 100% (V1 = V _REF ).
For this reason, the DC voltage V ₂ is set to a value corresponding to the measured value of 100%, and the cable current I _out is 20 mA. Also,
In this case, the COM signal is set to the “L” level, and the switch SW3 is turned off. Thus, the capacitor C ₁ of a large capacity is disconnected, only the capacitor C ₂ of a small capacity is intended to be used, the time constant of the integrating circuit 4 'is shortened. Here, the communication signal T _XD is given to the switch SW2 as digital data to be communicated, and the communication signal T _XD is set to “H”.
The switch SW2 is turned off at the time of the level, and the switch SW2 is turned on at the time of the "L" level. That is, the communication signal T
_XD at "L" level of the short-circuited at both ends of the resistor R _1, when "H" level of the communication signal T _XD opens the short-circuit state between both ends of the resistor R _1. Thus, when the communication signal T _{XD is at} the “H” level, V ₂ = V _REF × ｛R ₃ / (R ₁ + R ₂ +
R ₃ )｝ and V when the communication signal T _{XD is at} the “L” level.
₂ = V _REF × {R ₃ / (R ₂ + R ₃ )}.

In this embodiment, R _F = 50
And _{Ω, R F × 20mA = V} REF × {R 3 / (R 2 + R 3)} ··· (1) R F × 4mA = V REF × {R 3 / (R 1 + R 2 + R 3)} · The values of the resistors R ₁ , R ₂ , and R ₃ are determined so as to satisfy (2). Thus, according to the present embodiment, when the communication signal T _{XD is at} the “L” level, the cable current I _out is 20 mA, and when the communication signal T _XD is at the “H” level, the cable current I _out is 4 mA. That is, V _REF × {R ₃ / (R ₂ + R ₃ )}
= 1.0 V, V _REF × {R ₃ / (R ₁ + R ₂ + R ₃ )}
= 0.2V, the communication signal T _XD
Of the "L" level when the cable current I _out of 1.0V / 50Ω = 20mA, the communication signal T _XD "H" at the time the level of 0.2V / 50Ω = 4mA cable current I _out
It is said. Due to the change in the cable current _Iout , communication can be performed between a handy terminal connected between the cables L1 and L2 and a remote device including the DC power supply 200 and the external load _R0 . FIG. 2 shows a signal waveform of each part for reference.

Here, it should be noted that the feedback resistance R _F in the CCS circuit 5 ′ is the same between non-communication and communication, and its value is set to 50 Ω smaller than the conventional 250 Ω. Is to be. As a result, according to the present embodiment, the value of the external load R ₀ can be increased, the rising and falling of the communication waveform can be made smooth, and the occurrence of crosstalk with other lines can be suppressed. Will be able to Moreover, when communicating, since the time constant of the high-capacity capacitor C ₁ is disconnected and the integrating circuit 4 'of the short, it is possible to obtain a rapid response waveforms of the rising and falling as the DC voltage V _2, the communication signal Communication according to the level change of T _XD can be performed reliably. If here unless disconnected capacitors C _1, is long time constant of the integrating circuit 4 ',
A signal waveform corresponding to the level change of the communication signal T _XD cannot be obtained as the DC voltage V ₂ . During non-communication,
Integrating circuit 4 the capacitor C ₁ of a large capacity is used '
And the ripple of the cable current I _out of 4 to 20 mA corresponding to the measured value can be reduced.

[0015]

As is apparent from the above description, according to the present invention, it is possible to make the feedback resistance in the CCS circuit the same during non-communication and during communication, and to set the value to be small. Increase the value of the external load to make the rising and falling of the communication waveform gentle,
The occurrence of crosstalk with other lines can be suppressed. Further, according to the present invention, since the time constant of the integration circuit is shortened during communication, it is possible to obtain a fast rising and falling response waveform as the DC voltage V ₂ , and to change the level of digital data to be communicated. It is possible to reliably perform communication according to

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a main part of an electromagnetic flow meter according to an embodiment of the present invention.

FIG. 2 is a diagram showing, as a reference, signal waveforms of various parts of the electromagnetic flow meter during non-communication and during communication.

FIG. 3 is a circuit diagram showing a main part of a conventional electromagnetic flow meter.

FIG. 4 is a diagram showing, as a reference, a signal waveform of each part of the electromagnetic flow meter during non-communication and during communication.

[Explanation of symbols]

 1 ': current output circuit, 2: reference voltage generator, 3: buffer
, 4 '... integration circuit, 5' ... CCS circuit, SW1-SW
3 ... Switch, R₁~ R_Three... resistance, C₁, C_Two ... Conde
Sensor, R_F... feedback resistance, OP ... operational amplifier,
T_r1... transistor, R₀... external load, 100 ... electromagnetic current
Meter, 200 ... DC power supply.

Claims (2)

(57) [Claims] An exciting current is supplied at a predetermined frequency to an exciting coil arranged so that the direction of generation of a magnetic field is perpendicular to the direction of flow of a fluid flowing in a measurement tube, and the exciting current is orthogonal to the generated magnetic field of the exciting coil. A signal electromotive force obtained between the electrodes arranged in the measurement tube is detected, a measured value is calculated as a value of 0 to 100% based on the signal electromotive force, and a reference signal V having a duty ratio corresponding to the obtained measured value is obtained. ₁ was converted to a DC voltage V ₂ through the integration circuit, by controlling the switching element in its active region in response to the DC voltage V _2, given the current flowing in the cable connected to the DC power supply through an external load the electromagnetic flowmeter for adjustment of the current range, when communicating, after the duty ratio of the reference signal V ₁ is 100%, also after having short time constant in the integration circuit, de to communicate When changing from a first level Ijitarudeta to the second level, switching and the DC voltage V ₂ by switching the constant in the integration circuit to a predetermined constant from the first voltage value to a second voltage value Changing the current flowing through the cable from the maximum value of the current range to the minimum value,
When changing from the second level of the digital data to be communicated to the first level, the first voltage value the DC voltage V ₂ from the second voltage value by switching the constant in the integration circuit to the original constant And a communication unit for changing a current flowing through the cable from a minimum value to a maximum value of the current range. 2. An exciting current is supplied at a predetermined frequency to an exciting coil arranged so that its magnetic field generation direction is perpendicular to the flow direction of a fluid flowing through the measuring tube, and the excitation current is orthogonal to the magnetic field generated by the excitation coil. A signal electromotive force obtained between the electrodes arranged in the measurement tube is detected, a measured value is calculated as a value of 0 to 100% based on the signal electromotive force, and a reference signal V having a duty ratio corresponding to the obtained measured value is obtained. ₁ is converted to a DC voltage V ₂ through an integrating circuit including a resistor and a capacitor,
By controlling the switching element in its active region in accordance with the DC voltage V ₂ , an electromagnetic flowmeter that adjusts a current flowing through a cable connected to a DC power supply via an external load within a predetermined current range is used. , Ue duty ratio of the reference signal V ₁ is 100%, also after having a capacitor in the integrator circuit and a small capacity, when the change from the first level of the digital data to be communicated to the second level, The DC voltage V ₂ is switched from a first voltage value to a second voltage value by switching a resistance value of the integration circuit to a predetermined resistance value, and a current flowing through the cable is reduced from a maximum value of the current range to a minimum value. When the digital data to be communicated changes from the second level to the first level, the resistance value in the integration circuit is switched to the original resistance value to change the DC voltage V. _2. An electromagnetic flowmeter comprising: communication means for switching ₂ from a second voltage value to a first voltage value and changing a current flowing through the cable from a minimum value to a maximum value of the current range.