WO2009154115A1

WO2009154115A1 - Two-wire electromagnetic flow meter

Info

Publication number: WO2009154115A1
Application number: PCT/JP2009/060607
Authority: WO
Inventors: 敬祐名越
Original assignee: 株式会社キーエンス
Priority date: 2008-06-20
Filing date: 2009-06-10
Publication date: 2009-12-23
Also published as: JPWO2009154115A1; JP5635907B2

Abstract

A capacitive electromagnetic flow meter is realized by the two-wire method. Provision is made for: a power supply unit (14) which is provided with a primary side input capable of connecting two transmission lines DL, and a secondary side output insulated from the primary side input, and can convert the DC power supplied from the transmission lines DL to the specified power and output the power from the secondary side output; and a calculation means which calculates the flow of the target fluid for detection flowing through the flow path of a measuring tube based on the voltage signal detected by a detection circuit (34) and can output an excitation control signal for controlling the drive state of an excitation circuit (24) to the excitation circuit (24), and a power supply control signal for controlling the drive state of the power supply unit (14) to the power supply unit (14). The excitation circuit (24) and the detection circuit (34) are connected to the secondary side output of the power supply unit (14). The calculation means controls the current flow from the transmission lines DL supplied to the primary side input of the power supply unit (14) in response to the calculated flow rate of the target fluid for detection.

Description

2-wire electromagnetic flow meter

The present invention relates to a two-wire electromagnetic flow meter that detects a flow rate of a fluid to be detected by a capacitance method.

The electromagnetic flow meter generates an electromotive force proportional to the flow velocity of the detected fluid by flowing the detected fluid in the magnetic field, and calculates the flow rate based on the electromotive force detected by the electrode. Such an electromagnetic flow meter is roughly classified into two types, a wetted electrode type and a non-wetted electrode type. A wetted electrode type electromagnetic flow meter is also called a wetted type or an electrode type electromagnetic flow meter, and the electrode is in direct contact with the fluid to be detected and directly detects the electromotive force generated in the fluid to be detected. On the other hand, non-wetted electrode type electromagnetic flowmeters are also called (electrostatic) capacity type electromagnetic flowmeters, and the electrodes do not directly contact the fluid to be detected, and the electromotive force generated in the fluid to be detected is It detects through the electrostatic capacitance between electrodes.

In a liquid contact electrode type electromagnetic flow meter, the electrode is always in contact with the liquid of the fluid to be detected, so that the electrode needs to be a liquid that does not corrode. In addition, there are problems such as an insulating deposit or an oil film contained in the liquid adhering to the electrode to increase resistance, rusting of the electrode, and the need for a waterproof structure in the electrode arrangement portion. In contrast, a capacitive electromagnetic flow meter generates an electromotive force that is proportional to the product of the strength of the applied magnetic field and the fluid flow velocity when a conductive fluid to be detected flows perpendicular to the magnetic field. Is attached to the outer surface of the measuring tube through which the fluid to be detected flows, and is detected based on the principle that the flow rate can be measured by detecting the capacitance with a pair of electrodes that are not in contact with the fluid to be detected. FIG. 52 shows an example of a block diagram of a capacitive electromagnetic flow meter. In this capacitive electromagnetic flow meter 900, an electrode 930 is attached to the outside of a measurement tube 910 through which a fluid to be detected flows, and a capacitor is formed with the pipe thickness of the measurement tube 910. A pair of exciting coils 922 are arranged in a direction orthogonal to the electric field detected by the electrodes 930, and an alternating magnetic field is generated by the exciting circuit 924. In this configuration, since the electrode 930 does not come into contact with the fluid to be detected, it is difficult to be affected by the insulating deposit, and it is not necessary to consider the deterioration of the electrode or the waterproof structure, and maintenance is easy. There is an advantage that even a low-conductivity fluid can be used to eliminate the generation factor. On the other hand, the internal circuit has a high impedance and is also susceptible to noise.

On the other hand, as a connection method of such an electromagnetic flow meter, a four-wire system in which a power line and a signal line are separated and a two-wire system in which a signal is integrated into the power line are known. A two-wire electromagnetic flow meter is suitable for laying a long distance because of the small number of wires. However, in the case of the 2-wire type, the amount of current supplied to the transmission line is limited to the range of 4 mA to 20 mA, and the flow rate information is transmitted by the amount of current. Therefore, when the flow rate is low, the supply current is also reduced. There is a problem that a sufficient amount of current cannot be supplied to the excitation circuit for exciting the excitation coil. In order to solve this problem, the two-wire electromagnetic flowmeters of

Patent Documents

1 and 2 have been proposed.
Japanese Patent Publication No. 7-9375 JP 2002-340638 A

FIG. 53 shows the two-wire electromagnetic flow meter disclosed in Patent Document 1. The two-wire electromagnetic flow meter shown in this figure includes a wetted electrode 572 in a measurement tube 571, and a current output circuit 575 for adjusting the current flowing through the excitation circuit 573, the transmission line 574, and the excitation coil to be constant. , And an analog circuit such as a signal processing circuit 576 such as a CPU. In this two-wire electromagnetic flow meter, the excitation circuit 573 and the signal processing circuit 576 for calculating the flow rate are connected in series, and the transmission line 574 is connected to the excitation circuit 573 side that consumes the most current. Specifically, in order to effectively use the current supplied from the transmission line 574, the excitation circuit 573 is first energized, and then the current output from the excitation circuit 573 is used for other circuits connected in series. To do.

However, in this method, since the common potentials of the excitation circuit 573 and the signal processing circuit 576 are different, an insulation circuit for insulating the signal between them is necessary, which causes a problem that the structure is complicated and the cost is increased. . That is, in a two-wire electromagnetic flow meter, the common potential of the external power supply is often different from piping such as a measurement tube. In general, since the signal amplifier circuit uses the common potential of the pipe and the current output circuit uses the external power source as the common potential, the signal amplifier circuit always requires an insulation circuit. In the circuit example of FIG. 53, an insulating power source is provided in the signal processing circuit 576.

In addition, since the output current is directly supplied to the excitation circuit, it is difficult to flow an excitation current higher than the output current, and thus it is necessary to prepare an additional power supply circuit. In particular, when the capacity type electromagnetic flow meter is a two-wire type, the voltage signal is weaker than the liquid contact type because of the structure in which the electrode is not in direct contact with the fluid to be detected. For this reason, in order to accurately detect the flow rate, it is necessary to increase the amount of current in the excitation circuit according to the ampere-turn law. However, in the 2-wire system, the amount of current that can be energized is limited as described above. I had a problem that I could not do. Furthermore, since the power supply voltage needs to be equal to or higher than the voltage required for the excitation circuit and the voltage required for the signal processing circuit, the power supply voltage must be increased to some extent. On the other hand, even when a high power supply voltage can be supplied, a voltage higher than (voltage required for the excitation circuit) + (voltage required for the signal processing circuit) depending on the circuit operating state is wasted and voltage loss There was also a problem of growing. Furthermore, since the current that is not used for excitation becomes heat of the transistor in the constant voltage circuit, it is necessary to discard the useless current as heat.

On the other hand, a two-wire electromagnetic flow meter disclosed in Patent Document 2 is shown in FIG. This two-wire electromagnetic flow meter also has a liquid contact type electrode 582 in the measuring tube 581, an excitation circuit 583, a current output circuit 585 for adjusting the amount of current applied to the transmission line 584 to a range of 4 mA-20 mA, A signal processing circuit 586 is provided. In this two-wire electromagnetic flow meter, an excitation circuit 583, a signal processing circuit 586, and a current output circuit 585 are connected in parallel. However, even in this method, an insulating circuit 588 for insulating the analog amplifier circuit 587 and the signal processing circuit 586 is required. In addition, the insulated signal is vulnerable to noise, and there is a problem that the flow rate measurement cannot be performed at all if the data of the flow rate signal is corrupted. In particular, a non-wetted type electromagnetic flow meter has a higher impedance than the wetted type, and thus is relatively vulnerable to noise and is susceptible to such data corruption. Further, even in this method, since the output current is supplied to the excitation circuit as it is, it is difficult to flow an excitation current higher than the output current.

Furthermore, in either method, a method of increasing the excitation current by increasing / decreasing the voltage of the excitation circuit is also conceivable. However, in this case, an extra power supply circuit for step-up / step-down is required, and there is a problem that the circuit becomes complicated. For example, in the circuit example of the two-wire electromagnetic flow meter shown in FIG. 55, a DC / DC conversion circuit 599 for driving the current output circuit 595 is disposed before the excitation circuit 593 to once step down / boost, and further the excitation circuit The DC / DC conversion circuit 560 for the signal processing circuit 596 is disposed at the subsequent stage of the signal 593 to further perform step-down / boost, and two extra power supply circuits are required.

It is also conceivable to provide a required current by providing a rest period and charging the capacitor during that period and discharging it intermittently. However, when such intermittent excitation is performed, the power consumption in the excitation circuit changes dynamically, resulting in large fluctuations in the output current, and a large capacity capacitor is required to avoid this. . For these reasons, it has been conventionally difficult to realize a capacitive electromagnetic flow meter with a two-wire system.

The present invention has been made to solve such conventional problems. An object of the present invention is to provide a two-wire electromagnetic flow meter capable of realizing a capacitive electromagnetic flow meter with a two-wire type.

Means for Solving the Problems and Effects of the Invention

In order to achieve the above object, according to the two-wire electromagnetic flow meter according to the first invention, the flow rate of the fluid to be detected is detected, and signal transmission and power supply of the detected flow rate are shared by two transmissions. In a two-wire electromagnetic flow meter performed by a wire, at least a pair of excitation coils disposed so as to be orthogonal to the flow path of the measurement pipe, a measurement pipe that constitutes a flow path through which a fluid to be detected passes, and the excitation coil An at least pair of electrodes arranged so as to be orthogonal to the flow path of the measuring tube, and a voltage signal detected by the electrodes Detection circuit, a primary side input capable of connecting two transmission lines, and a secondary side output insulated from the primary side input, and the DC power supplied from the transmission line is predetermined. Power that can be converted into power from the secondary side and output from the secondary output And calculating the flow rate of the fluid to be detected that passes through the flow path of the measurement tube based on the voltage signal detected by the detection circuit, and an excitation control signal for controlling the drive state of the excitation circuit. The excitation circuit and a calculation means capable of outputting a power control signal for controlling the driving state of the power supply unit to the power supply unit, respectively, and a secondary side output of the power supply unit to the excitation circuit, A detection circuit is connected, and the calculation means can be configured to control the amount of current from the transmission line supplied to the primary side input of the power supply unit according to the calculated flow rate of the fluid to be detected. As a result, the primary side input current value is changed according to the flow rate of the fluid to be detected, while the excitation circuit is driven by the secondary side output of the power supply unit insulated from the primary side output. The current can be adjusted separately from the primary-side input current value, and the voltage signal detected by increasing the drive current without increasing the power supply voltage can be increased to achieve highly accurate detection.

Further, according to the two-wire electromagnetic flow meter according to the second aspect of the present invention, the common potential between the primary side input of the power supply unit and the calculation unit is further separated from the calculation unit to the power supply unit. A control signal insulation circuit for sending a power control signal is provided, and the computing means is arranged at the secondary output of the power supply unit and can be connected to the output side of the detection circuit. As a result, the analog voltage signal detected by the detection circuit can be input to the arithmetic means without being insulated, and the insulation circuit therebetween can be eliminated. In addition, since the calculation means and the excitation circuit are arranged on the same secondary side output, the excitation control signal from the calculation means to the excitation circuit can also be output without passing through the insulation circuit, and the insulation circuit between them is unnecessary. As a result, it is sufficient to arrange the insulating circuit only between the arithmetic means and the power supply unit, so that the number of necessary power supply units can be reduced and the circuit configuration can be simplified.

Furthermore, according to the two-wire electromagnetic flow meter according to the third aspect of the invention, the power supply unit can control a primary side input current value for energizing the transmission line based on a power supply control signal from the calculation means. And a switching circuit that is controlled by the current output circuit and that can convert the DC power of the primary side input current supplied from the transmission line into predetermined power and output it to the secondary side. As a result, the secondary side output current insulated from the primary side by the switching circuit can be arbitrarily set, and the excitation current of the excitation coil is within the range of 4 mA to 20 mA as in the conventional two-wire electromagnetic flowmeter. Since a higher excitation current can be supplied without being restricted, a high voltage signal can be obtained, and accurate flow rate detection can be realized.

Furthermore, according to the two-wire electromagnetic flow meter according to the fourth aspect of the invention, the switching circuit further serves as a secondary side output for exciting circuit output for generating driving power for driving the exciting circuit, and the calculation. And an output for calculating means for generating drive power for driving the means. As a result, appropriate driving power can be individually supplied to the excitation circuit and the calculation means, and at the same time, insulation between them can be achieved.

Furthermore, according to the two-wire electromagnetic flow meter according to the fifth invention, the detection circuit can be constituted by an analog signal amplification circuit that amplifies a voltage signal detected by the electrode.

Furthermore, according to the two-wire electromagnetic flow meter according to the sixth aspect of the invention, the output voltage value for exciting the excitation coil is adjusted and supplied in parallel with the excitation circuit and in series with the excitation coil. A possible constant voltage power supply and a charge charge monitor circuit for instructing a voltage required to supply an excitation current to the excitation circuit by the constant voltage power supply can be provided.

1 is an external perspective view showing a two-wire electromagnetic flow meter according to Embodiment 1 of the present invention. FIG. 2 is a front view of the two-wire electromagnetic flow meter of FIG. 1. FIG. 2 is a side view of the two-wire electromagnetic flow meter of FIG. 1. FIG. 2 is a block diagram of the two-wire electromagnetic flow meter of FIG. 1. It is a circuit diagram which shows the state which connected the 2-wire type electromagnetic flowmeter to the external power supply. It is a circuit diagram which shows the circuit example of a current output circuit. It is a circuit diagram which shows the circuit example of the switching circuit of a power supply part. It is a circuit diagram which shows the circuit example of an excitation circuit. It is a graph which shows an example of the waveform pattern of the coil current which flows into an exciting coil, and an exciting current. 5 is a block diagram showing a two-wire electromagnetic flow meter according to Embodiment 2. FIG. It is a block diagram which shows the example which comprised the power supply part with the insulation type switching power supply. It is a block diagram which shows the example which comprised the power supply part with the non-insulated type power supply. It is a wave form diagram which shows operation | movement when making an output current small. It is a wave form diagram which shows operation | movement when enlarging output current. It is a wave form diagram which shows operation | movement when a power supply voltage rises. It is a wave form diagram which shows operation | movement when a power supply voltage falls. It is a block diagram which shows the 2-wire type electromagnetic flow meter which added the surplus current adjustment circuit. It is a graph which shows the relationship between a flow volume and output current. It is a block diagram which shows the 2-wire type electromagnetic flow meter using the electric power variable excitation circuit. It is a circuit diagram which shows the example of an excitation circuit. It is a graph which shows the pattern of the continuous excitation which excites pn alternately. It is a graph which shows the pattern of the intermittent excitation which excites pn alternately. It is a graph which shows the pattern of the intermittent excitation which changes alternately the polarity of the 1st excitation restarted after a rest period. It is a graph which shows the waveform pattern at the time of performing intermittent excitation using the excitation circuit of FIG. It is a circuit diagram which shows the specific structural example of an excitation circuit. It is a wave form diagram which shows a mode that a rest period is controlled by a charge charge monitor circuit in the case of intermittent excitation. It is a wave form diagram which shows the operation example when output current increases. It is a circuit diagram which shows the charge means which produces | generates the high voltage _VH and the low voltage _VL . It is a wave form diagram which shows the operation | movement of continuous excitation. It is a block diagram of an electromagnetic flowmeter provided with an amplifier mode switching function. It is a block diagram of an output current adjustment circuit provided with an amplifier mode switching function. It is a graph which shows the pattern of the intermittent excitation which set the reference. It is a circuit diagram which shows an electromagnetic flowmeter provided with an offset compensation circuit. It is a wave form diagram which shows the excitation operation | movement by a reference excitation function. It is a table | surface which shows the excitation state of FIG. It is a circuit diagram which shows an analog reset circuit. It is a wave form diagram which shows the operation | movement waveform at the time of performing continuous excitation with the analog reset circuit of FIG. It is a wave form diagram which shows the input and output waveform of P-> N-> pause-> P-> N intermittent excitation which does not perform reference excitation. It is a wave form diagram which shows P-> N-> P-> pause-> N-> P-> N intermittent excitation. It is a wave form diagram which shows the input and output waveform of P-> N-> P-> pause-> N-> P-> N intermittent excitation. It is a block diagram which shows the 2-wire type electromagnetic flow meter system which connected the converter to the 2-wire type electromagnetic flow meter. It is a block diagram which shows the other 2-wire type electromagnetic flowmeter system which connected the converter to the 2-wire type electromagnetic flowmeter. It is a circuit diagram which shows the 2-wire type electromagnetic flowmeter provided with the amplifier mode switching function. It is a circuit diagram which shows an excitation circuit provided with an excitation control function. It is the schematic which shows the outline | summary of a damping process. It is a block diagram which shows an electromagnetic flowmeter provided with a damping function. It is a graph which shows an example of a damping process. It is a graph which shows the other example of a damping process. It is an image figure which performs the setting of damping. It is a graph which shows the example of the setting of a span, and the change of the analog output with respect to the flow velocity. It is an image figure which performs the setting of a span. It is a block diagram which shows a capacity | capacitance type electromagnetic flowmeter. It is a circuit diagram which shows the conventional 2-wire type electromagnetic flowmeter. It is a circuit diagram which shows the other conventional 2-wire type electromagnetic flowmeter. It is a circuit diagram which shows the other conventional 2-wire type electromagnetic flowmeter.

100, 200, 300, 400, 700 ...

electromagnetic flowmeter

500, 600 ... converter 10 ... measuring tube 12 ... control signal insulation circuit; 12B ... excitation signal insulation circuit; 12C ... indication signal insulation circuit 14 ...

power supply unit

16, 16B ... current output circuit 162 ... operational amplifier; 164 ... adder; 166 ... output current adjustment circuit; 167 ... operational amplifier 168 ... transistor 17 ... DC / DC conversion circuit 18 ... switching

circuits

181 and 181B ... switching elements; 182 and 182B ... Oscillation circuit 184 ... transistor control circuit; 185 ... excess current adjustment circuit 19 ... switching control circuit 20 ... transformer 22 ... excitation coil; 24 ... excitation circuit 25 ... constant voltage power supply 26 ... charge charge monitor circuit 28 ... excitation polarity switching circuit 29 ... exciting constant current circuit 30 ... electrode; 31 ... adder circuit; 33 ... amplifier mode switching 34 ... Detection circuit; 34C ... Signal amplification circuit 341 ... Buffer circuit; 342 ... Differential amplifier; 343 ... Differential amplifier 344 ... Offset compensation circuit; 38 ... A / D converter 40 ... Calculation means; 41 ... Addition / subtraction instruction circuit; 42 ... Memory unit; 43 ... Reference voltage switching circuit 44 ... Damping unit; 45 ... Auto mode switching unit 50 ... Display unit; 51 ... Display unit; 52 ... Display screen; 54 ... Operation panel 60 ... Output unit; 80 ... setting unit 110 ... main body; 111 ... flow path port 180 ... residual voltage detection circuit; 192 ... power supply switch 501 ... step-up power supply; 502 ... insulation transformer; 503 ... insulation type switching control circuit 504 ... current detection resistor; Voltage subtraction circuit; 506, pulse transformer 507, output circuit 560, conversion circuit, 571, measuring tube, 572, electrode, 573, excitation Circuit: 574 Transmission line 575 Current output circuit 576 Signal processing circuit 581 Measuring tube 582 Electrode 583 Excitation circuit 584 Transmission line 585 Current output circuit 586 Signal processing circuit 587 Analog Amplifier circuit 588 Insulating circuit 593 Excitation circuit 595 Current output circuit 596 Signal processing circuit 599 Conversion circuit 601 Boost switching power supply 602 Conversion circuit 604 Current detection resistor 605 Voltage subtraction circuit 608 ... Converter; 609 ... Calculation unit; 610 ... Conversion circuit 611 ... Integrated pulse alarm output circuit 900 ... Capacitive electromagnetic flow meter; 910 ... Measurement tube; 922 ... Excitation coil; 924 ... Excitation circuit 930 ... Electrode DL ... Transmission line; PL ... Power line; OL ... Output line L ... Coil _Ro ... Output current detection resistor; _RE ... Excitation current detection resistor; _Ra , _Rb , _Rc ... Resistor C _C ... charge capacitor; C _C2 ... second charge capacitor C _S1 ... smoothing capacitor; C ₀₁ , C ₀₂ ... hold capacitors Tr, Tr ₅ , Tr ₆ ... transistors; Tr ₁ to Tr ₄ ... bridge transistors A ₂ , A ₆ , A ₇ ... operational amplifier A _E1 ... error amplifier LPF ... low pass filter DC ... external DC power supply SW _B1 , SW _B2 , SW _B3 , SW _B4 ... bridge switch; SW _O1 , SW _O2 , SW ₇ ... switch SW _Vref ... reference Voltage selector switches V _ref1 , V _ref3 , V _ref4 ... Reference voltage; V _refc ... Charge voltage instruction voltage V _refo ... Inverted input of differential amplifier; V _RE ... Excitation current detection voltage V _L ... Low voltage; V _H ... High voltage V _O ... Output voltage; V _IO ... Output current instruction voltage; V _p ... Output current instruction voltage V _c ... Charge voltage; V ₂ ... Charge voltage instruction voltage; V _S ... Output of differential amplifier; _OS ... differential amplifier output V _AI3 ... gate voltage of the transistor; the drain voltage of V _T1 ... voltage V _Tr ... transistor Tr ₅ is input to the inverting input of the primary voltage V _n ... error amplifier transformer I _E ... Excitation current I _L ... Coil current I _O ... Output current I _add ... Additional current T _off ... Off time T _on ... Charging time CHG_COMP ... Charge completion signal AN_RES_T1, AN_RES_T2 ... Reset signal S _RV ... Residual voltage detection timing signal S _VH ... Power supply Switching signal S _ET ... Excitation timing signal S _refs ... Reference voltage switching signal S _VO ... Output current instruction signal S _AP ... Amplifier mode instruction signal

Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the embodiment described below exemplifies a two-wire electromagnetic flow meter for embodying the technical idea of the present invention, and the present invention specifies the two-wire electromagnetic flow meter as follows. do not do. Further, the present specification by no means specifies the members shown in the claims as the members of the embodiments. In particular, the dimensions, materials, shapes, relative arrangements, and the like of the component parts described in the embodiments are not intended to limit the scope of the present invention unless otherwise specified, but are merely described. It is just an example. In addition, the size, positional relationship, and the like of members illustrated in each drawing may be exaggerated for clarity of explanation. Furthermore, in the following description, the same name and reference numeral indicate the same or the same members, and detailed description will be omitted as appropriate. Furthermore, each element constituting the present invention may be configured such that a plurality of elements are constituted by the same member and the plurality of elements are shared by one member, and conversely, the function of one member is constituted by a plurality of members. It can also be realized by sharing.
Example 1

1 to 5 show a two-wire electromagnetic flow meter 100 according to Embodiment 1 of the present invention. In these drawings, FIG. 1 is an external perspective view of a two-wire electromagnetic flow meter, FIG. 2 is a front view of the two-wire electromagnetic flow meter of FIG. 1, FIG. 3 is a side view, FIG. 5 shows circuit diagrams in a state where the two-wire electromagnetic flow meter 100 is connected to an external power source. This electromagnetic flow meter is a non-wetted capacity type.
(appearance)

The two-wire electromagnetic flow meter shown in FIGS. 1 to 3 includes a main body 110 that constitutes a two-wire electromagnetic flow meter main body and a display unit 50. This two-wire electromagnetic flow meter allows the fluid to be detected to pass through the flow passage port 111 opened at both end faces of the main body 110, detects the flow rate, outputs it through the transmission line DL, and displays it as necessary. Display on the unit 50. The main body 110 may be made of metal or made of PPS resin or the like. The main body 110 is provided with a flange-like flange portion on a cylindrical end surface, and opens a screw hole for screwing with a pipe such as a pipe and a screw. In order to ensure the mechanical strength when piping by screwing, the flange portion is preferably formed integrally with the main body 110 and metal. Further, the flow path port 111 opened in the flange portion forms a flow path with the measurement tube 10 built in the main body 110. The diameter of the flow path is set to substantially the same diameter from one end to the other end of the flow path port 111 to reduce loss when the detected fluid flows in one direction in the flow path. Further, the display unit 50 is fixed to the upper surface of the main body 110 so as to be orthogonal as shown in FIG. By fixing the display surface of the display unit in a posture orthogonal to the cylindrical main body, the flow rate can be easily seen from the side surface through which the fluid to be detected flows through the pipe-shaped measuring tube 10.
(Display unit 50)

The display unit 50 is a member for displaying information such as the flow rate of the fluid to be detected, and includes a display screen 52 as the display unit 51 as shown in FIG. In the example of FIG. 2, a 7-segment display is used as a numerical display area for displaying numerical values on the display screen 52, and the flow rate and the like are displayed numerically. The 7-segment display displays numerical values such as instantaneous flow rate and integrated flow rate for the detected flow rate. The display screen 52 shown in this figure includes two stages of 7-segment displays, and can display the integrated flow rate and the set value simultaneously. However, a 7-segment display may be provided for only one screen so that the display of the integrated flow rate, instantaneous flow rate, set value, and the like can be switched. Furthermore, instead of the segment-type display screen 52 using LEDs or the like, a display screen using liquid crystal or organic EL can be used. In this way, the display screen 52 can display not only numerical values such as flow rate but also figures and images such as arrows or alternatively, and can easily display the detected flow rate information to the user. Can be displayed.

The display unit 50 includes an operation panel 54 as a setting unit 80 for performing various settings. The operation panel 54 includes keys and buttons for performing various settings. In the example of FIG. 2 and the like, a 4-digit 7-segment display is arranged in two stages on the display screen 52, an operation panel 54 is provided below, and buttons are arranged. The setting unit 80 functions as a reset setting unit for setting an initial value of the integrated flow rate or a predetermined reset value.

In this example, the display circuit that displays the display screen 52 incorporates a calculation means 40 that is connected to the detection circuit 34 and the excitation circuit 24 and controls them. However, it goes without saying that the calculation means can be constituted by individual members and incorporated in the main body 110. It is also possible to integrate a calculation means, a detection circuit, an excitation circuit, and the like.

Further, the display unit 50 can include an input unit. The input unit is an interface for inputting an input signal from an external device such as a temperature sensor, a reset signal for resetting the integrated value, and various setting information. As the input unit, a communication unit capable of data communication, an I / O terminal, a memory card, or the like can be used. In the example of FIG. 1, the display unit 50 is separated from the main body 110. However, the display unit 50 may be integrated with the main body 110. Further, the display unit does not need to be fixed to the main body case 110, and may be a separation type in which the display unit is arranged at another position.

This two-wire electromagnetic flow meter can output an analog current corresponding to the measured instantaneous flow rate, integrated flow rate, etc., with the current value of the transmission line DL as an analog output. In this example, when the instantaneous flow rate changes in the range of 0 to the rated value, an analog current is output in the range of 4 mA to 20 mA. For this purpose, an analog current output circuit 16 is provided as an output unit. Analog currents have better noise immunity than voltage signals, and can be used for data recording and analysis by outputting them externally.
(Block Diagram)

Next, the configuration of the two-wire electromagnetic flow meter will be described based on the block diagram of FIG. As shown in this figure, a two-wire electromagnetic flow meter 100 is wound around a measurement tube 10 that passes a detection fluid and a pole piece, and applies a magnetic field to the detection fluid from the outside of the measurement tube 10. An excitation coil 22, an excitation circuit 24 for generating an alternating magnetic field by the excitation coil 22, and an electrode 30 for detecting an electromotive force generated by the detected fluid passing through the magnetic field generated by the excitation coil 22. A detection circuit 34 for detecting an electromotive force via the electrode 30, an excitation circuit 24 and a detection circuit 34, and a calculation means 40 for calculating a flow rate from the detected signal, and a calculation means 40. And a display unit 51 for displaying the calculated flow rate. The calculation means 40 functions as a flow rate calculation unit capable of calculating the integrated flow rate based on the flow rate of the fluid to be detected detected by the main body case 110 constituting the flow rate detection means. Further, the calculation means 40 includes a memory unit 42 for adding or integrating the instantaneous flow rate calculated with the flow rate calculation value and holding the integrated flow rate. The integrated flow rate is updated by sequentially adding the newly measured instantaneous flow rate to the integrated flow rate, and the updated integrated value is held in the memory unit 42 as needed. Further, as necessary, an output unit 60 for outputting an output signal, an input unit 70 for inputting various input signals such as an external reset signal, a setting unit 80 for performing various settings, and the like are provided. Also good. The calculation means 40, the display unit 51, the output unit 60, the input unit 70, the setting unit 80, and the like are configured as a unit that is a separate member from the main body case 110 as the display unit 50.
(Measurement tube 10)

The measuring tube 10 is an insulating lining that allows a fluid to be detected to pass through the inside of the tube. The measuring tube 10 is required to have excellent chemical resistance as a pipe through which a fluid to be detected passes and electrical characteristics for constituting a capacitor. From the viewpoint of mechanical characteristics, the measuring tube 10 is a strength matrix that bears the force of tension or compression based on the expansion and contraction of the pipe due to pressure and temperature changes of the fluid to be detected, and the required inner diameter, thickness, and length to withstand it. A rigid structural member having On the other hand, from the viewpoint of electrical characteristics, the measuring tube 10 is desirably a dielectric material as a nonmagnetic insulating member. In particular, the electrode 30 is made of a material having a high dielectric constant in order to enhance the capacitive coupling between the electrode 30 attached around the measuring tube 10 and the fluid to be detected and to improve the S / N ratio. Such a material can be made of ceramics, plastic, or PPS resin mixed with ceramics. In particular, the latter is relatively strong and can secure molding accuracy and high dielectric properties. PPS resin is excellent in oil resistance and chemical resistance. A lining is applied to the inner surface of the measuring tube 10 as necessary.

The fluid to be detected is water or a non-corrosive liquid, and a liquid having a predetermined conductivity. Unlike wetted electromagnetic flowmeters, non-wetted electromagnetic flowmeters measure even liquids that corrode electrodes that could not be used in the past because the electrode 30 is not in direct contact with the fluid to be detected. it can. Moreover, it can respond to various to-be-detected fluids by selecting the material of the measuring tube 10. FIG. In particular, the material of the measuring tube 10 can be selected according to the dielectric constant required in accordance with the measurement accuracy and the resistance to the fluid to be detected. In particular, the two-wire electromagnetic flowmeter according to the present embodiment uses the measuring tube 10 as a separate member from the main body 110, so that only the measuring tube 10 is changed and other components are shared, so that two-wires with various specifications are used. A type electromagnetic flow meter can be configured, which is advantageous in terms of product preparation.

The measurement tube 10 is a separate member from the main body case 110. Thereby, the material suitable for a capacitor | condenser can be selected for the member which comprises the measurement pipe | tube 10. FIG. On the other hand, a resin that can be easily molded into a complicated shape can be used for the main body 110. Thus, by making the measuring tube 10 a separate member from the main body 110, it can be configured by a member suitable for each. In particular, since the high dielectric material constituting the measuring tube is generally expensive, only necessary portions are made of expensive members, and the other members can be made of less expensive materials to reduce the overall cost. Further, the measuring tube 10 made of an appropriate material can be selected according to the accuracy required for the two-wire electromagnetic flow meter. Further, it can be replaced with a measuring tube having a different diameter. Thus, a measuring tube made of an appropriate material can be selected according to the detection purpose and application of the two-wire electromagnetic flow meter, the required specifications and cost. Further, by making it possible to set a plurality of measuring tubes on one 2-wire electromagnetic flow meter, the members of various types of 2-wire electromagnetic flow meters can be shared and provided at low cost.
(Electrode 30)

The electrode 30 is disposed around the measuring tube 10. As the electrode 30, an insulating tape such as polyimide coated with a copper foil can be used. The electrode 30 is a planar conductor that is curved along the outer periphery of the cylindrical measurement tube 10, and the pair of electrodes 30 are disposed so as to face each other with the measurement tube 10 interposed therebetween. Thus, by electrostatic coupling between the pair of electrodes 30 and the fluid to be detected, the electromotive force generated in the fluid can be taken out from the measuring tube 10 and the flow rate can be detected. Each electrode 30 is affixed to the outer periphery of the measuring tube 10 without a gap. Tape or adhesive can be used for pasting. The electrode 30 is preferably made of a flexible member and can be fixed to the outer surface of the measuring tube 10 without a gap.

In order to prevent conduction between a pair of electrodes due to corrosion or condensation of the electrode which is a planar conductor, the conductor is preferably covered with an insulating layer. Although a pair of electrodes is used in this example, two or more sets of electrodes can be used. When a plurality of sets of electrodes are used, the positions of the electrodes are adjusted so that the electric field detected by each electrode is orthogonal to the magnetic field.

The operation principle of the two-wire electromagnetic flow meter 100 will be described with reference to FIG. The measuring tube 10 that guides the fluid to be detected is generated by a pair of exciting coils 22 disposed on the left and right sides of the measuring tube 10 and is disposed so as to be orthogonal to the substantially parallel magnetic field guided to the pole piece. The pair of electrodes 30 arranged to face the upper and lower surfaces of the measuring tube 10 are arranged so as to detect the electromotive force generated in the direction perpendicular to the magnetic field generated by the exciting coil 22 and the direction of passage of the fluid to be detected. Has been. In this configuration, when the fluid to be detected flows in the measuring tube 10, that is, when the conductive fluid moves in a direction orthogonal to the magnetic field, the moving speed (flow velocity) of the fluid to be detected is increased according to Faraday's law of electromagnetic induction. Proportional electromotive force is generated. At this time, the electromotive force is proportional to the product of the magnetic flux density, the flow velocity, and the measurement tube diameter according to Faraday's law. The electrode 30 faces the fluid to be detected through the tube wall of the measuring tube 10 made of a derivative and is capacitively coupled, and functions to electrically extract an electromotive force generated in the fluid. The extracted electromotive force is transmitted to the calculation means 40, converted into a flow rate signal, displayed on the display unit 51, or output as an electrical signal.

In this two-wire electromagnetic flow meter, a pair of excitation coils 22 are arranged apart from each other, and the excitation coil 24 is energized and excited by an excitation circuit 24 to generate a magnetic field between the excitation coils. As a result, when a liquid having conductivity as a fluid to be detected is caused to flow through the measurement tube 10, an electromotive force is generated in a direction orthogonal to the direction of movement of the liquid. In the example of FIG. 4, two excitation coils 22 are used and are provided on the left and right sides of the measurement tube 10. However, the excitation coil may be one.

The output unit 60 can function as an integrated value output unit or an instantaneous value output unit. Note that the output unit may omit any of the control output, analog output, and timeout output, or may include another output terminal. Furthermore, you may provide the output indicator lamp which shows the output state of each output terminal.

Next, the details of the operation of the two-wire electromagnetic flow meter will be described with reference to FIG. A two-wire electromagnetic flow meter 100 shown in this figure detects a measurement tube 10, an electrode 30, an excitation coil 22, an excitation circuit 24 for exciting the excitation coil 22, and an analog signal detected by the electrode 30. And an A / D converter 38 for A / D converting the analog signal detected by the detection circuit 34, an arithmetic means 40, a control signal insulation circuit 12, and a power supply unit 14. Since members having the same names as those described above can be basically used, detailed description thereof is omitted. Further, only one excitation coil 22 is shown for the sake of simplicity, but it goes without saying that one pair or more may be used as described above.

A charge capacitor _CC is connected in parallel to the secondary side output of the transformer 20 of the power supply unit 14 as a charging means for charging the electric charge from the power supply unit 14. Further the power supply unit 14 based on the power control signal from the operation means 40, primary-side input current for energizing the transmission line DL (the output current I _O) capable of controlling the current output circuit 16, the current output circuit 16 And a switching circuit 18 capable of converting the DC power of the primary side input current supplied from the transmission line DL into a predetermined power and outputting it to the secondary side. As a result, the secondary side output current _IO isolated from the primary side by the switching circuit 18 can be arbitrarily set, and the excitation current _IE of the excitation coil 22 can be set as the output current as in a conventional two-wire electromagnetic flow meter. Since a higher excitation current _IE can be supplied without being restricted by I _O or less, a high voltage signal can be obtained, and accurate flow rate detection can be realized.
(Power supply unit 14)

The power supply unit 14 is connected to two transmission lines DL for the external power supply. The power supply unit 14 includes a primary side input capable of connecting two transmission lines DL, and a secondary side output insulated from the primary side input. The power supply unit 14 converts the DC power supplied from the transmission line DL into predetermined power and outputs it from the secondary output. An excitation circuit 24, a detection circuit 34, and the like are connected to the secondary side output of the power supply unit 14. That is, the excitation circuit 24 and the detection circuit 34 are insulated from the current output circuit 16 arranged on the primary side input side by the power supply unit 14, and the power supply unit 14 can also be used as an insulation circuit.

As shown in FIG. 5, the two transmission lines DL are composed of a pair of a HIGH-side first power supply line and a LOW-side first common line. By using a two-wire system, it is possible to dispose an electromagnetic flow meter at a long distance of several kilometers.

On the secondary side of the power supply unit 14, a measurement tube 10 that allows the fluid to be detected to pass through, an excitation coil 22 that is wound around the pole piece and applies a magnetic field to the fluid to be detected from the outside of the measurement tube 10, and excitation An excitation circuit 24 for generating an alternating magnetic field in the coil 22, an electrode 30 for detecting an electromotive force generated by the detected fluid passing through the magnetic field generated in the excitation coil 22, and the electrode 30 A detection circuit 34 for detecting an electromotive force, an A / D converter 38 for converting an analog signal obtained by the detection circuit 34 into a digital signal, and a digital signal of the A / D converter 38, Arithmetic means 40 for sending a power supply control signal to the power supply unit 14 is arranged so that the primary side input current value corresponding to the flow rate is obtained from the detected signal.

The power supply unit 14 is an insulating switching power supply that DC / DC converts power supplied from an external power supply via the transmission line DL, and includes an output current detection resistor _Ro , a current output circuit 16, and a switching circuit 18. Is done. The current output circuit 16 controls the primary side input current value for energizing the transmission line DL based on the power supply control signal from the calculation means 40. Further, the switching circuit 18 includes a switching control circuit 19 and a transformer 20, and is controlled by the current output circuit 16, and a DC voltage obtained from the primary side input current supplied from the transmission line DL is converted into a predetermined voltage by DC / DC. The converter circuit 17 performs DC / DC conversion, and outputs it to the secondary side of the transformer 20. As a result, drive power is supplied to the excitation circuit 24, the arithmetic means 40, the detection circuit 34, etc. connected to the secondary side. In particular, the power supply unit 14 shown in FIG. 5 is provided with an excitation circuit output connected to the excitation circuit 24 and a calculation means output connected to the calculation means 40 as secondary outputs of the transformer 20. As a result, the number of turns for the primary winding can be adjusted and adjusted individually to the power required for driving the excitation circuit 24 and the calculation means 40. However, the secondary side output may be shared, and the excitation circuit 24 and the arithmetic unit 40 may be connected and driven in parallel on the circuit. In this case, since the excitation circuit 24 and the computing means 40 are connected to the same second power supply line, a design such that both members can be driven at the same potential, or a voltage is divided by a resistor can be used.

In addition, the DC / DC conversion circuit 17 can be used to generate the minimum required output voltage, and the excitation circuit 24 with very little power loss can be configured. In particular, the DC / DC conversion circuit 17 is a step-down converter that performs a switching operation, and there is little power loss when performing voltage conversion, and an advantage that power loss can be extremely reduced is obtained.

Unlike the conventional two-wire electromagnetic flow meter, this configuration is different from the configuration in which the calculation means 40 such as a CPU, the excitation circuit 24, and the detection circuit 34 such as an analog signal amplification circuit are arranged on the primary side. Arrangement on the side, that is, the exciting coil 22 side insulated from the input current has various advantages. Specifically, the electromotive force detected by the analog signal amplifier circuit can be directly input to the CPU as a flow rate analog signal without insulation. Therefore, the insulation circuit between them, which has been conventionally required, becomes unnecessary, and the problem of signal corruption caused by insulation can be avoided.

In addition, the number of power supply circuits can be reduced by using the power supply unit 14 in which an insulated power supply for an analog signal detection circuit and a power supply for an excitation circuit, which are conventionally required, are integrated. In other words, it is possible to simplify the configuration by reducing the number of power supply circuits by sharing the essential insulated power supply for the analog signal amplifier circuit also for the excitation circuit.

Furthermore, no insulation is required between the excitation circuit 24 and the CPU, and an excitation control signal for controlling the drive of the excitation circuit 24 can be directly output from the CPU. Therefore, in this configuration, the control circuit isolation circuit 12 that separates the common potential (ground) between the CPU and the current output circuit 16 of the power supply unit 14 is sufficient. Thus, the circuit configuration can be further simplified in that the number of insulating circuits can be reduced.

Furthermore, since only the switching circuit 18 and the current output circuit 16 exist on the primary side of the transformer 20 in the configuration of FIG. 5, it is possible to deal with a wide range of power supply voltages with respect to the external power supply on the input side. There are also benefits. For example, even when the voltage of the external power supply is low, the secondary voltage can be boosted by increasing the duty ratio of the switching circuit 18 or increasing the number of turns of the transformer 20. Therefore, even if it is difficult to operate in the past, such as when the voltage required for driving the excitation circuit 24 is higher than the external power supply voltage, the operation is boosted by the isolated switching power supply according to the present embodiment. Can be made.

In addition, there is an advantage that the circuit configuration is simplified when excitation is performed by the intermittent excitation method. That is, if the excitation current is increased with a conventional two-wire electromagnetic flow meter having an excitation circuit on the primary side, it is conceivable to perform intermittent excitation instead of continuous excitation. In this case, since the fluctuation of the primary side input current value according to the flow rate is large, the fluctuation of the consumption current in the excitation circuit is also large, and a large-capacity capacitor is required to keep the output current constant, There is a problem that the load on the current output circuit becomes large. In the configuration of FIG. 5 with respect to this, because fluctuations in the supply current to the primary side is not larger circuit, or even greater fluctuations in the output current I _O internal power consumption is varied, the output current I _O It is not necessary to provide a large-capacitance capacitor to suppress fluctuations. Therefore using a charge capacitor C _C a large capacity on the secondary side smoothing part, by monitoring the charge amount of charge, perform excitation at a timing charge necessary for excitation stored, lean efficient intermittent Excitation is possible (details will be described later). In this way, the circuit configuration in the case of the intermittent excitation system circuit for increasing the excitation current can be greatly simplified, and there is an advantage that there is no waste of current and the efficiency can be increased.
(Current output circuit 16)

As shown in FIG. 5, the current output circuit 16 is connected in parallel with the switching circuit 18 between the two transmission lines DL, that is, between the HIGH-side first power supply line and the LOW-side first common line. ing. A circuit example of such a current output circuit 16 is shown in FIG. In the current output circuit 16 shown in this figure, a low-pass filter LPF, an operational amplifier 162, an adder 164, and an output current adjustment circuit 166 are connected in series. The output current adjustment circuit 166 includes an operational amplifier 167, a transistor 168, and a resistor. The + input terminal of the operational amplifier 167 is grounded, and the output is connected to the base side of the transistor 168. The emitter of the transistor 168 is grounded to the first common line, and is connected to the output current detection resistor _Ro through the first common line. The output current detection resistor R _o converts the output current I _O returned to the external power source through the first common line into an output current detection voltage V _o (negative voltage). Therefore, the relationship of V _o = -I _{_{_O}} R _o is established.

The operation of the current output circuit 16 will be described with reference to FIG. First, an arithmetic means (not shown in FIG. 6) sends an output current instruction signal as a power supply control signal to the current output circuit 16 via the control signal insulation circuit 12 in the PWM method. The current output circuit 16 converts the PWM signal into an output current instruction voltage V _p using a low-pass filter LPF and an operational amplifier 162. Further, the adder 164 and the output current adjustment circuit 166 adjust the output current I _O to the instructed current value. Here, the adder 164 and the output current adjustment circuit 166 operate so that V _p / R ₁ + V _o / R ₂ = 0. In the above equation, since V _o = −I _o R _o , I _o = −V _o / R _o = {R ₂ / (R _o R ₁ )} × V _p .
(Switching circuit 18)

The switching circuit 18 of the power supply unit 14 in FIG. 5 includes a switching control circuit 19 and a transformer 20. A circuit example of the switching control circuit 19 is shown in FIG. The switching control circuit 19 shown in this figure includes a smoothing capacitor C _S1 connected in parallel to the primary side of the transformer 20, and a switching connected to the first common line side between the smoothing capacitor C _S1 and the transformer 20. An element 181 and an oscillation circuit 182 connected to the switching element 181 are provided. A power MOSFET or the like can be used for the switching element 181. The oscillation circuit 182 includes an operational amplifier, a resistor, and a capacitor. The switching circuit 18 generates a driving pulse by the oscillation circuit 182, and inputs the driving pulse to the gate of the transistor Tr to perform ON / OFF switching, thereby driving the transformer 20 and adjusting the output on the secondary side. Thus, the power supply unit 14 that drives the switching element 181 with the switching control circuit 19 to control the power ON / OFF and stabilize the output can be configured to be small and light.
(Measurement tube 10)

The measuring tube 10 constitutes a flow path through which the fluid to be detected passes. A pair of exciting coils 22 and electrodes 30 are arranged so as to be orthogonal to the flow path of the measuring tube 10. The excitation coil 22 and the electrode 30 are fixed to each other so that the pair of the excitation coil 22 and the electrode 30 face each other so that the measurement tube 10 is sandwiched therebetween. The electrode 30 is fixed to the outside of the measuring tube 10 and is isolated from the fluid to be detected and does not come into contact. By arranging the exciting coil 22, the electrode 30, and the measuring tube 10 so as to be orthogonal to each other, an electromotive force is generated as described above, and the flow rate can be detected. The detection circuit 34 detects the voltage signal of the electromotive force detected by the electrode 30.
(Calculation means 40)

The computing means 40 computes the flow rate of the fluid to be detected that passes through the flow path of the measuring tube 10 based on the voltage signal detected by the detection circuit 34. The computing means 40 outputs an excitation control signal for controlling the drive state of the excitation circuit 24 to the excitation circuit 24, and further outputs a power supply control signal for controlling the drive state of the power supply unit 14 to the power supply unit 14. Based on these signals, the calculation means 40 controls the amount of current from the transmission line DL supplied to the primary side input of the power supply unit 14 according to the calculated flow rate of the fluid to be detected. The computing means 40 is composed of a CPU or the like.
(Control signal insulation circuit 12)

The control signal insulation circuit 12 sends a power control signal from the computing means 40 to the power supply unit 14 while separating the common potential between the primary side input and the computing means 40. On the other hand, since the calculation means 40 is disposed at the secondary output of the power supply unit 14, it can be connected to the output side of the detection circuit 34. As a result, the excitation circuit 24 and the calculation means 40 can be arranged on the secondary side output and insulated from the primary side input with the transmission line DL. An insulation circuit can be eliminated. Further, the power supply unit 14 can be controlled from the calculation unit 40 with high reliability in a state where the calculation unit 40 and the power supply unit 14 are insulated from the common potential. Furthermore, since the number of parts to be insulated can be reduced, the number of necessary insulation circuits can also be reduced.
(Detection circuit 34)

The detection circuit 34 shown in FIG. 5 includes an analog signal amplification circuit that amplifies the voltage signal detected by the electrode 30. The analog signal amplifier circuit includes a buffer circuit 341 connected to each electrode 30, a differential amplifier 342 that inputs an output of the buffer circuit 341, and an amplifier 343 that inputs an output of the differential amplifier 342. The buffer circuit 341 constitutes a preamplifier that amplifies the signal detected by the electrode 30. In a two-wire electromagnetic flow meter, since the capacitive coupling between the electrode 30 and the fluid to be detected is generally as small as several tens of pF, the impedance of the resistance when providing a filter for passing an electrical signal is extremely high. For this reason, the buffer circuit 341 is connected to each electrode 30 to reduce the impedance. The electric signals detected by the electrodes 30 are respectively input to the inputs of the differential amplifier 342 via the buffer circuit 341, and after the difference is amplified by the amplifier 343, the arithmetic means 40 via the A / D converter 38. Is output. The analog signal amplifier circuit may be provided with a periodic reset circuit as necessary as a circuit for resetting a voltage detected by receiving a reset signal periodically sent from the computing means 40.
(Excitation circuit 24)

In order to generate the alternating magnetic field by exciting the exciting coil 22, an exciting circuit 24 is used. Power supply unit 14, as a constant voltage power supply for driving the excitation circuit 24, a low voltage V _L and the high voltage V _H. The excitation circuit 24 connects a low voltage V _L and a high voltage V _{H of} a constant voltage power source and an excitation constant current circuit 29 (described later) to the excitation coil 22 via an excitation polarity switching circuit. A constant current is generated by the excitation constant current circuit 29 by a constant DC voltage supplied from the low voltage V _L and the high voltage V _H , and switching is performed by an excitation polarity switching circuit having a switch connected in a bridge shape to generate an alternating current. An alternating current is passed through the coil 22.

The excitation circuit 24 is connected to a second power supply line of the secondary side output of the power supply unit 14. FIG. 8 shows an example of the excitation circuit 24. Excitation circuit 24 shown in this figure, the transistors Tr1 ~ 4 which are connected like a bridge, and the exciting coil 22 connected to the center of the bridge, the coil current I _L to be energized exciting coil 22 to a constant value The excitation constant current circuit 29 is provided. Here, the coil current I _L is a current that is passed through the excitation coil 22 and is an alternating current, and the excitation current I _E is a current that is passed from the excitation coil 22 through the bridge circuit to the excitation constant current circuit 29. It is a direct current. That excitation current I _E is equal to the absolute value of the coil current I _L. In FIG. 8, the exciting coil 22 is disposed in the center of the bridge for the sake of explanation. However, in an actual apparatus, the exciting coil 22 is disposed in the vicinity of the measuring tube 10 as shown in FIG.
(Excitation constant current circuit 29)

Exciting the constant current circuit 29 includes transistors Tr _5, an operational amplifier A _2, and the reference voltage V _ref1, constituted by the excitation current detecting resistor R _E. The inverting input (−) of the operational amplifier A ₂ is connected to the exciting current detection resistor R _E , the non-inverting input (+) is connected to the reference voltage V _ref1, and the output side is input to the gate of the transistor Tr ₅ . . In this excitation constant current circuit 29, the excitation current I _E is detected by the excitation current detection resistor R _E , converted into the excitation current detection voltage V _RE and input to the inverting input (−) of the operational amplifier A ₂ . . The operational amplifier A ₂ controls the transistor Tr ₅ so that the excitation current detection voltage V _RE becomes equal to the reference voltage V _ref1 . That the difference between the exciting current detection voltage V _RE and the reference voltage V _ref1 is input to the gate of the transistor Tr _5, by adjusting the output of the transistor Tr _5, feedback control is performed for adjusting the excitation current I _E. The gates of the bridge transistors Tr ₁ to Tr ₄ are connected to the calculation means 40 (not shown in FIG. 8), and each bridge transistor is used as an excitation control signal sent from the calculation means 40 by an excitation timing signal _SET. Tr ₁ to Tr ₄ are driven ON / OFF. The calculating means 40 controls the timing of causing the coil current _IL to flow through the exciting coil 22 by the bridge transistors Tr ₁ to Tr ₄ by the excitation timing signal S _ET . The operational amplifier A ₂ controls the transistor Tr ₅ so that the excitation current detection voltage V _RE becomes equal to the reference voltage V _ref1 , thereby making the excitation current _IE constant.

FIG. 9 shows an example of a waveform pattern of the coil current I _L and the excitation current I _E flowing through the excitation coil 22. In FIG. 9, (a) is a bridge transistor Tr ₁ , Tr ₄ , (b) is a bridge transistor Tr ₂ , Tr ₃ , (c) is a coil current I _L , and (d) is a waveform pattern of an excitation current _IE. ing. As shown in this figure, the direction of the coil current I _L flowing through the exciting coil 22 is changed by alternately turning on / off the bridge transistors Tr ₁ , Tr ₄ and Tr ₂ , Tr ₃ of FIG. An alternating magnetic field can be generated by changing in the directions of A and B shown in FIG. Thereby, the AC coil current _IL can be obtained from the DC excitation current _IE .

In general, even when a voltage is applied to the exciting coil, a coil current does not flow immediately, and a certain amount of time is required for this to reach a constant current range. Therefore, as shown in FIG. 9, it occurs gentle current change when switching the direction of the coil current I _L. In order to improve this part, a high voltage V _H is applied to the exciting coil 22 when the coil current I _L is applied to the exciting coil 22 and switched to the low voltage V _L after the constant current is reached. It is preferable to switch and control the second power supply line shown in FIG. 8 (details will be described later).
(Example 2)

In the above FIG. 5, the example in which the excitation circuit 24 and the computing means 40 are connected in parallel to the secondary side output of the power supply unit 14 has been described. However, the present invention is not limited to this configuration, and the excitation circuit 24 and other circuits arranged on the secondary side can be connected in series. An example of such a circuit is shown in FIG. 10 as a two-wire electromagnetic flow meter 200 according to the second embodiment. The two-wire electromagnetic flow meter 200 shown in this figure can use the same configuration as that in FIG. 5 on the primary side. Moreover, since the member similar to FIG. 5 can be utilized also for the member of a secondary side, these detailed description is abbreviate | omitted. The two-wire electromagnetic flow meter 200 of FIG. 10 connects the excitation circuit 24 to the excitation circuit power supply line which is the second power supply line of the secondary side output of the power supply unit 14. Further, the calculation means 40 and the A / D converter 38 are connected in series with the excitation circuit 24 and then connected to the circuit common line of the secondary output. The arithmetic means 40 and the A / D converter 38 are connected in parallel, and are connected to the excitation circuit common line via the excitation circuit 24. Therefore, the excitation circuit common line becomes a circuit power supply line other than the excitation circuit, such as the arithmetic means 40 and the A / D converter 38.

However, in this configuration, the circuit common line other than the excitation circuit such as the arithmetic unit 40 is different from the excitation circuit common line. For this reason, since the level of the excitation control signal sent from the arithmetic means 40 to the excitation circuit 24, that is, the excitation timing signal _SET is different, it is necessary to insulate them. Therefore, an excitation signal insulation circuit 12B is provided on the excitation timing signal line between the excitation circuit 24 and the calculation means 40.
(Power supply unit 14)

Next, a configuration example of the power supply unit 14 will be described with reference to FIG. The two-wire electromagnetic flow meter shown in this figure shows a simplified configuration of the secondary output of the power supply unit 14. Here, the DC / DC conversion circuit 17 and the current output circuit 16 are integrated as an insulating switching power supply, and the secondary output and the output current _IO are adjusted. The power supply unit 14 shown in this figure includes a switching element 181, an oscillation circuit 182 that controls ON / OFF of the switching element 181, an output current detection resistor _Ro , a transformer 20, and a smoothing capacitor C _S1 . The power supply unit 14 performs a switching operation using the switching element 181 and smoothes it with the smoothing capacitor C _S1 . As the switching element 181, a power transistor Tr such as a MOSFET can be used. The oscillation circuit 182 can change the switching pattern of the switching element 181 by adjusting the oscillation frequency and the duty. The output current I _O is adjusted by changing the switching pattern of the switching element 181 by the oscillation circuit 182.

The power supply unit 14 does not have a configuration in which the excess current that cannot be consumed in the circuit as in the prior art is applied to the transistor and consumed as heat so as to match the book. Instead, the oscillation circuit 182 adjusts the duty of the transistor Tr so that the instructed output current I _O is obtained, and the power sent to the secondary side of the switching power source is made variable, so that the output current I _O is reduced. It is adjusted. As a result, it is resistant to fluctuations in the power supply voltage and output current _IO , and can be maintained at a high efficiency with a wide power supply voltage and output current. In particular, any power supply voltage or output current is DC / DC converted so that power is sent to the secondary side with a switching power supply as much as possible, so no extra power is generated and consumed as heat. There is an advantage that is not necessary. In addition, even when the power supply voltage rises, the advantage that it can be effectively used in the internal circuit without wastefully consuming the increased power is an excellent feature that cannot be obtained by other circuit formats. In the case of a two-wire electromagnetic flow meter, the excitation power circuit consumes the largest amount of power, so a circuit for intermittent excitation with a constant excitation current (described later) or a circuit for dynamically changing the excitation current. Even in the configuration, highly efficient operation can be realized.

In the example of FIG. 11, the insulated switching power supply using a transformer has been described. However, the present invention is not limited to this, and for example, a non-insulated power supply that does not use a transformer can be used. FIG. 12 shows such a two-wire electromagnetic flow meter as a modification. The power supply unit of the two-wire electromagnetic flow meter shown in this figure includes a smoothing capacitor C _S1 , a switching element 181B, a coil L, a charge capacitor C _C, and an oscillation circuit 182B. The circuit operation of this two-wire electromagnetic flow meter is almost the same as that of the above-described isolated switching power supply, and by operating the oscillation circuit 182B so that the output current I _O becomes a value indicated by the output current instruction signal, Adjust the output current _IO .

Next, how the switching speed of the transistor Tr is changed using the oscillation circuit 182 will be described based on the waveform patterns of FIGS. In these figures, FIG. 13 shows an operation when the output current I _O is reduced, FIG. 14 shows an operation when the output current I _O is increased, FIG. 15 shows an operation when the power supply voltage is increased, and FIG. 16 shows a decrease. The operation of each case is shown. In each figure, V _AI3 indicates the gate voltage of the transistor Tr shown in FIG. 11, and V _{T1 also} indicates the primary voltage of the transformer 20.

First, the case of reducing the output current I _O, the lower the output current command voltage V _IO as the output coulometric instruction signal outputted from the operation means 40. If a constant charging time T _on, since the duty is relatively smaller only downtime V _AI3 is long, the switching duty of the transistor Tr becomes smaller, the output current I _O is reduced.

On the other hand, when increasing the output current I _O is to increase the output current indicating voltage V _IO. Then, as shown in FIG. 14, since the duty of V _AI3 increases, the switching duty of the transistor Tr increases, and the output current I _O increases.

In this way, the output current I _O is changed by changing the duty ratio for switching the primary side of the transformer 20. The method of changing the output current is not limited to this configuration, and the same change can be realized by changing the switching frequency, for example.

The switching power supply also functions as a stabilized power supply that maintains a stable output even when the power supply voltage of the external power supply fluctuates. For example, as shown in FIG. 15, when the power supply voltage increases, the primary side voltage V _{T1 of} the transformer 20 also increases and the amplitude value increases. In this case, the power that can be transmitted by switching the transformer 20 once increases. In other words, the current consumption for switching the transformer 20 once increases. Therefore, the oscillation circuit 182 works so as to lengthen the off time _Toff for switching the transformer 20, so that the current can be relatively reduced and the output current _IO can be maintained at the indicated value.

On the other hand, when the power supply voltage decreases, the power that can be transmitted by switching the transformer 20 once decreases. In other words, the current consumed by switching the transformer 20 once is reduced. Therefore, by working the oscillation circuit 182 so as to shorten the off time T _off of the switching transformer 20 as shown in FIG. 16, relatively on time long, namely by increasing the current value, the output current I _O can be maintained at the indicated value.

In this way, by adopting a method of changing the switching duty and frequency so as to maintain the output current _IO at a desired indication value, the switching power supply can respond to the change even when the power supply voltage changes. Thus, the output current _IO can be kept constant, and the output can be stabilized and the reliability can be improved. As a result, the switching power supply can perform the maximum switching with respect to the power supply voltage that is currently supplied and the output current _IO that is currently output, and the power can be efficiently transferred to the output side of the switching power supply at all times.

As described above, since the oscillation circuit 182 performs switching so that the output current value is determined according to the detected flow rate, unnecessary current consumption is eliminated, and the efficiency is improved with the instructed output current value. Switching can be performed well. For this reason, compared with the conventional system, there is no part which converts electric power into heat and is consumed, and it is excellent in efficiency. In particular, this feature is beneficial because the two-wire electromagnetic flowmeter originally has less power available. In addition, a strong advantage is obtained against fluctuations in the power supply voltage, and even if the power supply voltage fluctuates, the switching duty can be adjusted so that the indicated current value is obtained at the power supply voltage.
(Surplus current adjustment circuit 185)

Even in a switching power supply with variable duty and frequency, the power consumption that can be varied is limited. That is, when the power consumption that can be transmitted by the switching power supply reaches the upper limit, the remaining power cannot be consumed. In view of this, an example of a two-wire electromagnetic flow meter to which a surplus current adjusting circuit for consuming surplus current is added so as to cope with such a case is shown in FIG. 17 as a modified example. This two-wire electromagnetic flow meter includes a DC / DC conversion circuit 17 as an insulating switching power supply, an oscillation circuit 182 including a duty adjustment circuit, a surplus current adjustment circuit 185 including a transistor Tr ₆ and a resistor, a transistor Tr _{6 includes} a transistor control circuit 184 for controlling ₆ , an output current detection resistor _Ro, and a control signal insulation circuit 12. The two-wire electromagnetic flow meter shown in this figure uses a current output circuit 16B similar to the conventional one in combination with the switching power supply described above. That is, when the power that can be transmitted by a switching power supply has reached the upper limit, the excess current regulating circuit 185 for consumption by energizing the surplus current to the transistor Tr _6, is provided on the primary side of the transformer 20 . The surplus current adjustment circuit 185 can also be arranged on the output side of the switching power supply. FIG. 18 is a graph showing the relationship between the flow rate and the output current _IO . As shown in this figure, when the output current I _O is changed in the range of 4 mA to 20 mA, the current is transmitted by the switching power supply in the range up to the upper limit that can be transmitted by the switching power supply, and the current is consumed inside the circuit. On the other hand, the surplus current adjusting circuit 185 consumes the surplus current in the range equal to or higher than the switching power transmission upper limit indicated by cross hatching in FIG. That is, the transistor Tr ₆ is turned ON, and the surplus is consumed by the resistor and the transistor Tr ₆ .
Example 3 Excitation circuit that performs intermittent excitation

Furthermore, an excitation circuit with variable power can be used on the output side of the switching power supply. A two-wire electromagnetic flow meter using such an excitation circuit is shown in FIG. A two-wire electromagnetic flow meter 300 shown in this figure includes a power supply unit 14 including a DC / DC conversion circuit 17, an excitation circuit 24 connected to the secondary side of the power supply unit 14, and a charge voltage supplied to the excitation circuit 24. a charge charge monitoring circuit 26 which monitors the V _c, calculation means for outputting the excitation timing signal S _ET instructing excitation start of excitation circuit 24 receives the charging completion signal from the charge charge monitoring circuit 26 to the excitation circuit 24 40, a constant voltage power supply 25 for supplying power to the arithmetic means 40 and performing signal amplification, a measuring tube 10, an electrode 30, a detection circuit 34, and an A / D converter 38. Of the members, the members having the same names as those described above are basically the same as those in the above-described embodiment, and detailed description thereof will be omitted.

The excitation circuit 24 maintains the coil current I _L for exciting the excitation coil 22 substantially constant. If the coil current is changed as in a conventional electromagnetic flow meter, the linearity of the signal may deteriorate. However, by keeping the coil current constant at an appropriate value without changing the coil current, the linearity is improved and the flow rate is highly accurate. Detection can be realized. Further, when the output current value is made constant, setting of the value is important. In other words, if the current value is set low, it is necessary to design the excitation circuit so that the excitation coil can be excited with the low current value. However, if the output current value is high in this design, the current that is not consumed by the excitation circuit increases. It is wasted due to heat and the like. Therefore, in this embodiment, the current value is not set to the lowest current value but is set to a higher current value. In this case, since excitation cannot be performed in the flow range where the current value is low, intermittent excitation is performed in this range to temporarily stop the excitation. The specific current value is set to a current value at a flow rate of 50% (12 mA when the output current _{IO is in} the range of 4-20 mA). Thereby, continuous excitation can be performed in a current range of 50% or more, and highly accurate flow rate detection can be stably performed. In the current range of 50% or less, by performing as intermittent excitation described above, it maintains the coil current I _L at a constant value. Further, the rising characteristics are improved by using the charging of the rising voltage (high voltage V _H ) of the exciting coil 22 described later without throwing away the surplus current even in the region of 50% or more.

During intermittent excitation, the power exceeding the power required for circuit drive is stored during the excitation pause period, and when the excitable power is obtained, the pause period is switched to the excitation period. Start. In addition, after the start of excitation, when electric power is consumed and the continuation of excitation becomes impossible, the excitation period is again switched to the rest period, and switching from discharging to charging is performed. Specifically, charging of the power to the charge capacitor is started at the timing when the excitation current is turned off. In the example of FIG. 19, a constant voltage power supply 25 and a charge charge monitor circuit 26 are connected in parallel with the excitation circuit 24. The constant voltage power supply 25 performs DC / DC conversion on the electric power received from the power supply unit 14, and adjusts and supplies it to a voltage value for driving the computing means 40. On the other hand charge the charge monitor circuit 26 monitors the voltage necessary to supply the coil current I _L to the excitation circuit 24. Time Specifically, the when the charge voltage instruction signal from the excitation circuit 24 to the charge charge monitoring circuit 26 during the intermittent excitation is instructed, the start of the charge to the charge capacitor C _C, a predetermined voltage is charged To supply the charge voltage V _c to the excitation circuit 24.

Next, FIG. 20 shows a two-wire electromagnetic flow meter that constitutes such an excitation circuit. The two-wire electromagnetic flow meter shown in this figure shows an excitation circuit 24 and its peripheral circuit in the secondary side portion of the power supply unit 14. The two-wire electromagnetic flow meter includes a switching control circuit 19, a transformer 20, a charge capacitor C _C that generates a voltage for exciting the excitation coil 22 on the secondary side of the power supply circuit, and a charge capacitor C as a charging means. a charge charge monitoring circuit 26 for monitoring the charge charge stored and _C, the calculating means 40 for delivering the excitation timing signal S _ET to indicate the timing of the excitation on the basis of the monitoring result of the charge charge monitor circuit 26, the excitation circuit 24 as, an exciting coil 22, an exciting polarity switching circuit 28 for switching the polarity of the excitation of the exciting coil 22, an exciting constant current circuit 29 to keep the coil current I _L flowing through the exciting coil 22 constant, the residual voltage detection circuit 180 Is provided.

In the circuit of FIG. 20, an excitation circuit 24 using a charge charge monitor circuit 26 is provided on the secondary side of the power supply unit 14 which is a switching power supply, and intermittent excitation is performed. Charge charge monitor circuit 26 monitors the charge is charged from the power supply unit 14 during the excitation dead time to charge the capacitor C _C, when the prescribed power is able to charge, and sends a charging completion signal CHG_COMP to computing means 40 . Computing means 40, upon receiving the charge completion signal CHG_COMP, each bridge switches SW _B1 ~ SW _B4 constituting a bridge-like excitation polarity switching circuit 28 sends an excitation timing signal S _ET, starts excitation.

On the other hand, the arithmetic means 40 sends a residual voltage detection timing signal S _RV to the residual voltage detection circuit 180 and maintains the residual voltage of the exciting coil 22 constant. The exciting constant current circuit 29, the residual voltage is caused by the voltage drop of the transistor Tr ₅ and the exciting current detection resistor R _E. In particular the voltage drop of the transistor Tr ₅ causes fever becomes the power loss. This residual voltage is detected by the residual voltage detection circuit 180 and controlled so that the residual voltage becomes small.

In this way, the residual voltage detection circuit 180 sends a charge voltage instruction signal for instructing the charge voltage V _c to the charge charge monitor circuit 26 so that the current consumption in the excitation constant current circuit 29 is reduced. The charge charge monitor circuit 26 monitors the amount of charge stored in the charge capacitor C _C with the charge voltage V _c . In addition, the charge voltage V _{c for} determining that excitation can be started is set so that the residual voltage of the excitation coil 22 becomes constant. As a result, the charge voltage V _c corresponding to the excitation coil 22 is set and the power is adjusted to an appropriate power regardless of the size and characteristics of the excitation coil, so that wasteful current consumption can be reduced to a minimum. Here, the charge voltage V _c is the same as the low voltage V _L described above.
(Intermittent excitation)

The excitation polarity switching circuit 28 of FIG. 20 includes four bridge switches SW _B1-B4 connected in a bridge shape with the central excitation coil 22 interposed therebetween. A pair of bridge switches SW _B1 and SW _B4 and SW _B2 and SW _B3 positioned obliquely with respect to the exciting coil 22 are simultaneously turned ON / OFF, and these sets are alternately turned ON / OFF, thereby exciting the pair. the coil 22 is energized the coil current I _L alternately in forward and reverse directions. We note in order to distinguish the current direction of the coil current I _L, called the coil current I _L P side excitation the excitation of the forward (rightward in FIG. 8), the reverse excitation of the N side exciting. Here, the pattern of continuous excitation and intermittent excitation will be described with reference to FIGS. FIG. 21 shows a pattern of continuous excitation, in which P-side excitation and N-side excitation are alternately repeated. On the other hand, FIG. 22 repeats the operation of performing the P-side excitation and the N-side excitation again after the P-side excitation and the N-side excitation are continuously excited as a set, and then a pause period is provided. In addition, intermittent excitation is not limited to this pattern. For example, as shown in FIG. 23, after an odd number of excitations such as P-side excitation, N-side excitation, P-side excitation, N-side excitation, and P-side excitation are continued, And starts from N-side excitation when restarting. As a result, the polarity of the excitation that is initially performed at every excitation can be alternately switched, and excitation can be performed in a well-balanced manner.

Further, an example of operation when the exciting coil 22 is intermittently excited in the circuit of FIG. 20 is shown in the waveform pattern of FIG. 24A shows the basic period of excitation, FIG. 24B shows the voltage across the excitation coil 22, FIG. 24C shows the coil current I _L , and FIG. 24D shows the excitation current detection voltage. V _RE , FIG. _24E shows the power supply switching signal S _VH output from the computing means 40, FIG. _24F shows the low voltage V _L , and FIG. _24G shows the charge completion signal output from the charge charge monitor circuit 26. CHG_COMP and FIG. 24H show the waveforms of the residual voltage detection timing signal S _RV , respectively. FIG. 24A shows the timing for alternately performing P-side excitation and N-side excitation in intermittent excitation with a pause period. Of the four bridge switches SW _{B1 -B4} of the excitation polarity switching circuit 28 provided with the excitation coil 22 shown in FIG. 20, SW _B1 and SW _B4 and SW _B2 and SW _B3 are alternately paired, and the lower right part of FIG. It is to oN / OFF by the excitation timing signal S _ET from the calculating means 40 shown, energized alternately an alternating current to the exciting coil 22 generates an alternating magnetic field. FIG. 24 illustrates an example in which the excitation with the excitation period of one (odd number) is repeated, in which the P-side excitation is performed once and then the N-side excitation is performed once with a pause period in between.

Excitation is started by instructing the excitation timing signal _SET from the calculation means 40 to the bridge switches SW _B1 to SW _B4 . When a voltage is applied to the exciting coil 22, it is preferable to apply a high voltage so that the coil current _IL can easily flow. Therefore, as shown in FIG. 24 (e), the output (power supply switching signal S _VH ) of the calculation means 40 is set to HIGH, the power supply changeover switch 192 is turned on, and the high voltage V of the exciting coil power supply as shown in FIG. 24 (b). _{Apply H.} If then the coil current I _L to the excitation coil 22 begins to flow, the exciting current detection voltage V _RE of the excitation constant current circuit 29 as shown in FIG. 24 (d) is gradually increased. When the excitation current detection voltage V _RE exceeds a predetermined threshold value, the power source switching signal S _VH is turned off and the power source switching switch 192 is turned off as shown in FIG. Thus, the voltage is switched to the low voltage V _L and applied to the exciting coil 22. In this state, the coil current I _L becomes a substantially constant value as shown in FIG. Note that the broken line shown in FIG. 24C indicates that the coil current I _L increases when the low voltage V _L is applied from the beginning. As indicated by the broken line, the excitation coil 22 has poor rise characteristics. Therefore, by increasing the applied voltage at the time of voltage application, it becomes possible to obtain a stable excitation in a short time with a sharp rise.

On the other hand, for the low voltage V _L , as shown in FIG. 24 (f), the residual voltage detection circuit 180 is set so that the excitation coil residual voltage becomes a predetermined value when the excitation coil starts up and shifts to a normal constant current. Has been adjusted. Further, when the charge completion signal CHG_COMP charge charge monitor circuit 26 of FIG. 24 (g) is inputted to the arithmetic unit 40, is output excitation timing signal S _ET from the arithmetic unit 40 to the excitation polarity switching circuit 28, FIG. 24 ( Excitation is started as shown in a).

As described above, in the third embodiment, the pause period during intermittent excitation is not constant, and the circuit performs the excitation by determining that the pause period is as short as possible. As a result, the most efficient excitation can be performed within the supplied power. Further, since the flow rate calculation and the like are not required during the suspension period, the CPU constituting the calculation means 40 also reduces power consumption and performs only a minimum operation. As a method of reducing the power consumption of the CPU, a method of lowering the operating frequency such as clock division and having a sub clock separately from the main clock can be used.

A specific example constituting the circuit of FIG. 20 is shown in FIG. Each bridge switches SW _B1-B4 of the excitation polarity switching circuit 28 is constituted by bridge transistors Tr ₁ ~ Tr _4, the ON / OFF control by the excitation timing signal S _ET from the calculating means 40, not shown.
(Charge Charge Monitor Circuit 26)

FIG. 26 shows a waveform pattern for controlling the pause period by the charge charge monitor circuit 26 during intermittent excitation. 26A shows the excitation timing, FIG. _26B shows the charge voltage V _c , and FIG. 26C shows the waveform of the charge completion signal CHG_COMP. In the excitation period, the flows excitation current I _E, Vc due to the discharge of the charge capacitor C _C is reduced. The excitation is ended quiescent period, the exciting current I _E is stopped, the charge to the charge capacitor C _C is started. Then Vc rises, and reaches the charge voltage command voltage V _2, is output from the operation unit 40 shown in FIG. 20 the excitation timing signal S _ET bridge switches SW _B1-B4, again exciting current I _E is the excitation coil 22 is energized. Power stored in the charge capacitor C _C in this way, is consumed by the excitation coil 22 to the exciting period, it is charged from the power supply unit 14 to pause.

The excitation circuit 24, without the pause period is constant, dynamically changes according to the charge amount of the charge voltage command voltage V ₂ and a charge capacitor C _C. That is, since the rapidly energized upon reaching the charge voltage command voltage V ₂ which is instructed is started, the charging of the charge capacitor C _C is completed earlier, is shortened rest period by that amount. The shorter the pause period, the longer the period of excitation, which can contribute to improved flow rate detection accuracy and stable operation.

In this circuit, if is sufficient supply amount of charge to the charge capacitor C _C, it can maintain a continuous excitation without shifting the intermittent excitation. FIG. 27 shows an operation example in the case where the output current I _O increases and the continuous excitation can be maintained. Similarly to FIG. 26, (a) shows the excitation timing, (b) shows the charge voltage V _c , and (c) shows the waveform of the charge completion signal CHG_COMP. As shown in FIG. 27, since the power is charged to the charge capacitor C _C is, if exceeding the power consumed by the excitation circuit 24, no less than the charge voltage command voltage V ₂ which Vc is instructed, the exciting current I _E Can be continuously excited to maintain the current. For this reason, by adding an output current in an amplifier mode, which will be described later, continuous excitation can be maintained without performing intermittent excitation, and highly accurate flow rate detection can be performed.

Thus, the charge charge monitor circuit 26 determines the timing of intermittent excitation. Although it is conceivable that the intermittent excitation cycle is defined by the output current value, in this method, it is necessary to consider an offset or the like, and the current is wasted. In this embodiment the contrary, to monitor the amount of charge charge charge monitoring circuit 26 is stored in the charge capacitor C _C, because it configured to discharge when it becomes chargeable voltage, the charge amount of the actual It is possible to operate in accordance with high efficiency driving without wasting current. In this method, since the actual voltage value is monitored, the actual charge amount can always be monitored and the excitation possible timing can be determined even if the characteristics of the excitation coil etc. are changed. The advantage is also obtained.

In the charge charge monitor circuit 26 of the present embodiment, only one excitation voltage is applied to the excitation coil, but a plurality of excitation voltages may be provided as shown in FIGS. In particular, the excitation coil has a slow rise at the time of voltage application, and therefore it is preferable to apply a voltage higher than the normal voltage when a current is applied. FIG. 28 shows an example of charging means for generating a voltage V _H higher than the normal low voltage V _L as the voltage at the time of starting up the coil. This charging means includes a plurality of coils, rectifier diodes, and charge capacitors _CC having different turns on the secondary side of the power supply unit 14 so as to supply a high voltage V _H and a low voltage V _L. In this circuit, a second charge capacitor C _C2 is further stacked on the charge voltage V _c to obtain a high voltage V _H. In this configuration, since the high voltage V _H is generated based on the charge voltage V _C , V _H also fluctuates up and down together with the up and down fluctuation of Vc.

Next, details of the circuit of FIG. 25 will be described with reference to the waveform pattern of FIG. Two types of power sources for the exciting coil 22 shown in FIG. 25 are provided, that is, a charge voltage V _c that is a normal voltage (low voltage V _L ) and a high voltage V _H for starting up the exciting coil. Normally, excitation is performed with the charge voltage V _c , but when a voltage is applied, the rise of the excitation current _IE becomes faster when a high voltage is applied. Therefore, these are switched by the power switch 192 for use. The bridge transistors Tr ₁ to Tr ₄ constituting the bridge switches SW _{B1 to B4} of the excitation polarity switching circuit 28 are a combination of Tr ₁ · Tr ₄ and Tr ₂ · Tr ₃ , and apply both positive and negative excitation voltages to the excitation coil 22. As a result, an alternating current flows through the exciting coil 22. FIG. 29 shows a waveform pattern during continuous excitation operation, where (a) is the ON / OFF timing of the bridge transistors Tr ₁ and Tr ₄ , (b) is the ON / OFF timing of the bridge transistors Tr ₂ and Tr ₃ , c) Excitation voltage applied to the excitation coil 22, (d) Excitation current I _E , (e) Excitation current detection voltage V _RE , (f) Power supply switching signal S _VH , (h) Residual voltage detection The waveforms of the timing signal S _RV are shown. In (c), V _C and V _H are slightly inclined because of discharge of the charge capacitor.
(Excitation constant current circuit 29)

The excitation constant current circuit 29 includes an operational amplifier A ₂ , a transistor Tr ₅ , an excitation current detection resistor R _E , and a reference voltage V _ref1 . The non-inverting input of the operational amplifier A ₂ and the reference voltage V _ref1, inverting input and between the exciting current detection resistor R _E and the transistor Tr _5, are connected, and the output side is connected to the transistor Tr _5. When the voltage at the inverting input of the operational amplifier A ₂ (between the exciting current detection resistor R _E and the transistor Tr ₅₎ and exciting current detection voltage V _RE, the operational amplifier A ₂ becomes V _ref1 = exciting current detection voltage V _RE driving the transistor Tr ₅ as. As a result, control is performed so that the excitation current I _E = V _RE (≈V _ref1 ) / R ₂ .

The excitation constant current circuit 29 controls the transistor Tr ₅ so that the excitation current detection voltage V _RE approaches the reference voltage V _ref1 . Specifically, when the excitation current detection voltage V _RE is lower than a predetermined reference value, the excitation coil 22 is driven by the high voltage V _H by turning on the transistor Tr ₆ constituting the power supply switch 192. Conversely, when it is high, the transistor Tr ₆ is turned off and the exciting coil 22 is driven by the charge voltage V _c .

Also conversely, it outputs a charge completion signal CHG_COMP output from the charge charge monitoring circuit 26 to the arithmetic unit 40, instead of the configuration for generating the excitation timing signal S _ET in computing means 40, without using the operation means A dedicated circuit can also be provided. However, in this case, it is not possible to enjoy the advantage of reducing power consumption by changing the operating frequency of the calculation means during the suspension period.
(Example 4 amplifier mode)

In addition to the normal mode in which the output current _IO is operated at 4-20 mA, the electromagnetic flow meter can also have an amplifier mode switching function for switching to an amplifier mode in which the output current is increased in order to improve measurement accuracy. . In the amplifier mode, for example, by operating at 24-40 mA, which is a 4-20 mA consumption current plus an additional current (base current) of 20 mA, the excitation current _IE is increased and the electromotive force is increased, resulting in more stable performance. It will be obtained. As a result, it is possible to reduce or eliminate the pause period of intermittent excitation and realize stable measurement with continuous excitation.

An electromagnetic flow meter having such an amplifier mode switching function is shown in FIG. The electromagnetic flow meter 400 shown in this figure has substantially the same configuration as the electromagnetic flow meter 300 in FIG. 19 except that the calculation means 40 functions as a mode switching means, and detailed description of the same members as those in FIG. 19 is omitted. To do. Calculation means 40 outputs the normal mode, the amplifier mode instruction signal S _AP instructing switching of adding a predetermined additional current to the output current to the amplifier mode to output, to the power unit 14 and the excitation circuit 24. Power supply unit 14 and the excitation circuit 24 receives the amplifier mode instruction signal S _AP, the process proceeds to the operation of the amplifier mode with add additional current. Further, in the amplifier mode, the output current I _O is increased, so that an external device that receives the output current also needs a mechanism for detecting the flow rate corresponding to the increase in current. For example, the converter corresponding to amplifier mode is connected and it converts into the flow volume which considered the increase in electric current appropriately. The amplifier mode switching means may also be configured to automatically detect that it is connected to an amplifier mode compatible external device such as a converter and switch to the amplifier mode.

Next, FIG. 31 shows an example of an output current adjusting circuit of an electromagnetic flow meter having an amplifier mode switching function. The electromagnetic flow meter shown in this figure includes a DC / DC conversion circuit 17 that is an insulating switching power supply, and an oscillation circuit 182 that includes a duty adjustment circuit as a switching control circuit 19 that performs switching control of the DC / DC conversion circuit 17; Low-pass filter LPF, calculation means 40, control signal insulation circuit 12 that insulates the power supply control signal from calculation means 40 and sends it to the current output circuit 16, and amplifier mode from calculation means 40 that includes amplifier mode switching means 33 an instruction signal S amp mode changeover switch for switching the ON / OFF at _AP, and the instruction signal insulation circuit 12C for insulating the amplifier mode instruction signal S _AP, as an additional programmable current output circuit an additional current without lowering the resolution, additional the output current I _o and Anpumo so as to be able to effectively take advantage of the current consumption and the amplifier mode An adding circuit 31 for adding the reference voltage V _a at the time. The adder circuit 31 includes an operational amplifier A ₂ and resistors R _a , R _b , and R _c .

The calculation means 40 serves also as an amplifier mode switching means 33 switches the reference voltage at the amplifier mode instruction signal S _AP. Specifically, the two controls are switched so as to stabilize the operation of the electromagnetic flowmeter by raising the excitation current _IE and the excitation voltage by changing the reference voltage. That is, (A) the threshold value of timing for switching the excitation voltage of the excitation coil 22 from V _H to V _L is switched. (B) to switch the range of the excitation current I _E, to increase the partial excitation current I _E plus additional current.

The electromagnetic flow meter, the amplifier mode instruction signal S _AP from the amplifier mode switching means 33 switches the ON / OFF of the amplifier mode switch. Here, when the amplifier mode selector switch is OFF, the normal mode is selected, and when it is ON, the amplifier mode is selected. When simply adding an additional current to the output current in the amplifier mode, for example, if a current of 20 mA is added, the current range may be expanded from 4-20 mA to 4-40 mA, resulting in a relative decrease in resolution. There is. Therefore, the output current adjusting circuit in the amplifier mode by sending the amp mode instruction signal S _AP to the arithmetic unit 40 is the adding circuit 31, by shifting the current range 24-40MA, remains current consumption maintaining the resolution It is configured to allow base up.

Based on the circuit diagram of FIG. 31, an operation example in which the output current adjusting circuit adds the additional current in the amplifier mode will be described. The voltage input to the duty adjustment circuit is V _p , and is a voltage suitable for PWM control of the duty adjustment circuit through the low-pass filter LPF. On the other hand, the voltage V _n of the duty adjustment circuit is V _n = R _b / R _a × I _o _{Ro since the} amplifier mode selector switch is OFF in the normal mode. On the other hand, in the amplifier mode, the amplifier mode changeover switch is turned on and the adder circuit 31 operates, resulting in V _n = R _b / R _a × I _o _Ro −R _b / R _c × V _a . When the duty adjusting circuit operates such that V _p = V _n, the output current I _O in the normal mode, the _{_{_{I o = R a / R o}}} × V p / R b amplifier mode, I _o = R _a / R _o × (V _p / R _b + V _a / R _c ). That is, in addition to the output current I _O in the normal mode, the amplifier mode changeover switch is turned ON / OFF in the amplifier mode to add the current R _a / R _o × V _a / R _c as the additional current I _add. It can be energized. With this output current adjustment circuit, the span can be set arbitrarily in normal mode and amplifier mode, so it can be switched to 4-20mA or 24-40mA current output without reducing the resolution of PWM control in the duty adjustment circuit. And can.

As described above, when more stable operation is required, the amplifier mode is set for operation. Since the obtained signal has an additional current of 20 mA, it is necessary to subtract 20 mA on the signal receiving side. Alternatively, a dedicated converter corresponding to the amplifier mode operation is used. In such a dedicated converter, the obtained signal is received by a resistor of, for example, 250Ω, and converted to a voltage signal of 6-10V. That is, by subtracting the voltage of 5 V corresponding to the additional current of 20 mA, it can be converted into a voltage signal of 1-5 V. This 1-5V voltage signal is a commonly used voltage signal and can be easily processed.

By using the amplifier mode, there is an advantage that the excitation current can be secured to a certain level regardless of the output current. In the normal mode, for example, even when the flow rate is small and the output current _IO is operating at 4 mA, the 30 mA excitation current _IE can be intermittently supplied. In other words, the excitation current can be set to any value, but if intermittent excitation is performed, the number of excitations in a fixed time is smaller than that in continuous excitation, so the amplifier mode is used more frequently and requires high accuracy. Set the excitation current so that continuous excitation is possible with the output current value. In the normal mode, for example, when the output current I _O of 12 mA or more is frequently used and high accuracy is required, it is desirable to determine the excitation current _IE so that continuous excitation is possible with the output current I _O of 12 mA.
(Reference excitation function)

In intermittent excitation, the offset compensation circuit is used and offset compensation is performed by an analog circuit in order to remove the low frequency offset component without increasing the number of A / D conversions as much as possible. At this time, as shown in FIG. 32, a reference setting function for adding half-period excitation for setting the reference may be provided. Thereby, it is possible to realize offset compensation that removes the variation of the zero point. Conventional offset compensation circuits cannot be used as they are because accuracy is deteriorated in intermittent excitation. On the other hand, in the present embodiment, odd-numbered half-period excitation is performed, and sampling is not performed by the first one-time excitation, and only the offset compensation circuit is driven, thereby enabling offset compensation without reducing accuracy. Yes.

Next, an example of an electromagnetic flow meter provided with such an offset compensation circuit is shown in FIG. The electromagnetic flow meter shown in this figure includes an exciting coil 22, an exciting circuit 24, a measuring tube 10, an electrode 30, a signal amplifying circuit 34C, an A / D converter 38, and a computing means 40. About the member of the same name as the member mentioned above, substantially the same composition can be used, and detailed explanation is omitted. The signal amplification circuit 34C includes an offset compensation circuit 344 between the differential amplifier 342 and the differential amplifier 343 of the detection circuit. The offset compensation circuit 344 includes switches SW _O1 and SW _O2 , resistors R _O1 and R _O2 , and hold capacitors C _O1 and C _O2 . The reason why the offset compensation circuit 344 has two charging / discharging paths in this way is to correspond to P-side excitation and N-side excitation, respectively. The output V _S of the differential amplifier 342 is connected to the inverting input of the differential amplifier 343. On the other hand, the switch SW _O1 of the offset compensation circuit 344 is connected to the output V _S of the differential amplifier 342, and the switch SW _O2 is connected to the non-inverting input of the differential amplifier 343. That is, the offset compensation circuit 344 receives the output V _S of the differential amplifier 342, charges one of the hold capacitors C _O1 and C _O2 , and outputs it to discharge the non-inverting input of the differential amplifier 343. .

The calculation means 40 outputs the reference voltage switching signal S _refs to the offset compensation circuit 344, and switches the switches SW _O1 and SW _O2 . As shown in FIG. 33, the switches SW _O1 and SW _O2 are turned ON / OFF so that one is connected to the hold capacitor C _O1 and the other is connected to the hold capacitor C _O2 . As a result, a sample-and-hold circuit is configured in which the output of the differential amplifier 342 is charged and held in one of the hold capacitors C _O1 and C _O2 , and the comparison result with the output V _S of the previous stage is output from the differential amplifier 343. Is done.

In the example of FIG. 32, sampling is always performed in the order of P-side excitation → N-side excitation. Preferably, in the reference excitation, the polarity is alternately changed every excitation period. In the present embodiment, as shown in FIG. 34, when excitation is performed in the order of P-side excitation → N-side excitation, in the next excitation resumed after the pause period, N-side excitation → P-side excitation, Set the polarity of the excitation to be started to be reversed. Thereby, the P-side excitation and the N-side excitation can be handled equally. In order to perform such excitation with alternating polarity, half-period excitation is generated an odd number of times in the excitation period. An example of this waveform pattern is shown in FIG. 34A shows the timing of the excitation pattern, FIG. 34B shows the reference voltage switching signal S _refs output from the calculation means 40, and FIG. 34C shows the output V _S of the differential amplifier 342 (ie, the offset compensation circuit 344). (D) is the inverting input V _refo of the differential amplifier 343 (ie, the output of the offset compensation circuit 344), (e) is the output V _OS of the differential amplifier 343, and (f) is the calculation means 40. The timing for sampling the detection signal is shown. In this example, it is assumed that there is an offset portion linearly inclined as shown in FIG. 34C in the output V _S of the differential amplifier 342 which is an analog circuit. In each excitation period, the first half-cycle excitation is the reference excitation, and no signal sampling is performed as shown in FIG. Further, in reference excitation, the output V _refo of the offset compensation circuit 344 is offset because of sample hold. Further, as the output V _refo of the offset compensation circuit 344, the last voltage before a half cycle is output even in the _idle period. This situation is shown in the table of FIG. If the excitation periods of PNP → NPN → PNP... Are expressed as 1, 2, 3,..., 2n,. Here, the voltage Vs in the k-th state is expressed as Vs (k).

When n is an odd number, if capacitor C ₀₁ is charged,
VC ₀₁ (2n) = VC ₀₁ (2n-1) = V _s (2n-1)
VC ₀₂ (2n + 1) = VC ₀₂ (2n) = V _s (2n)
V _ref0 (2n) = VC ₀₁ (2n) = V _s (2n−1)
V _ref0 (2n + 1) = VC ₀₂ (2n + 1) = V _s (2n)
It becomes.

Thus, V _ref0 (N) = V _s (N−1) regardless of whether N is an even number or an odd number. Therefore,
V _os (N) = Gain × (V _s (N) −V _ref0 (N)) = Gain × (V _s (N) −V _s (N−1))
It becomes. Here, when the electromotive force is Es (N) and the offset component is En,
V _s (N) = Es (N) + En
Even in this case, V _oS (N) = Gain × ((Es (N) + En) − (Es (N−1) + En)) = Es (N) −Es (N−1)
Thus, differential amplification can be performed regardless of the offset component.

In the example of FIG. 34, a half-cycle excitation is added to the pair of signals on the P-side excitation and the N-side excitation first as a reference excitation, so that a total of three half-cycle excitations are performed within the excitation period. . Further, since a half cycle may be added to the even number of times of excitation such as P → N → P → N, the total may be 5 or 7 times. When 2n + 1 half-period excitation is performed, signal sampling is actually performed 2n times excluding the first reference excitation, and thus signal sampling is not performed at a rate of 1 / (2n + 1). Therefore, it is desirable to take n as large as possible from the viewpoint of efficiency. Therefore, it is preferable to perform sampling of the maximum number possible in a charge amount accumulated in the charge capacitor C _C. Thereby, even if an offset occurs on either the P side or the N side, it can be effectively eliminated.

Here, offset compensation, that is, an analog reset operation will be described with reference to FIGS. FIG. 36 is a circuit diagram of an analog reset circuit, and FIG. 37 shows operation waveforms when continuous excitation is performed in this circuit. 37A shows the input waveform of the analog detection signal, FIG. 37B shows the waveform of the reset signal AN_RES_T1, and FIG. 37B shows the waveform of the reset signal AN_RES_T2. The circuit in FIG. 36 has a sample hold circuit dedicated to P side excitation and N side excitation, respectively, as in FIG. 33. Each sample hold circuit includes hold capacitors C ₀₁ and C ₀₂ and is operated via a switch. Connected to the non-inverting input of amplifier 343. The switch can be switched ON / OFF by a reset signal. In FIG. 36, each analog switch shows a low state, and the P side is connected to the non-inverting input side of the operational amplifier 343. The hold capacitors C ₀₁ and C ₀₂ can be charged even during signal sampling. When the P-side waveform is amplified, the N-side excitation value before the half cycle is used as a reference, and when the N-side waveform is amplified, the P-side excitation before the half cycle is used. With the value of as a reference. As a result, as shown in FIG. 37, one hold capacitor is charged during each A / D sampling period, and at the same time, the voltage of the other hold capacitor is maintained, thereby enabling a reference operation. (Modification)

The above method is inferior in efficiency because the exciting current is applied even during the period when the signal is not sampled, and wasteful power that does not contribute to measurement is consumed. Therefore, it is possible to drive only the offset compensation circuit without using excitation while using the same circuit as in FIG. FIG. 38 shows an example of such a driving method. As shown in FIG. 38A, when intermittent excitation of P → N → pause → P → N is performed, signal sampling is performed using the circuit of FIG. An output as shown in FIG. 38 (b) is obtained. With this method, the number of excitations and the number of signal samplings are the same, so that unnecessary excitation current is eliminated and efficient excitation can be performed with low consumption. FIG. 39 shows a waveform pattern in the case where odd-numbered half-period excitations such as P → N → P → pause → N → P → N are continued by this method as in FIG. Also in FIG. 39, (a) shows the timing of the excitation pattern, (b) shows the reference voltage switching signal S _refs output by the computing means 40, (c) shows the output V _S of the differential amplifier 342, and (d). Is the inverting input V _refo of the differential amplifier 343, (e) is the output V _OS of the differential amplifier 343, and (f) is the timing at which the computing means 40 samples the detection signal. Also in this example, it is assumed that the output V _S of the differential amplifier 342 which is an analog circuit has a linearly inclined offset as shown in FIG. 39C.

In this method, as shown in FIG. 39 (e) and FIG. 38 (b), the signal of the first one out of the odd number of half-period excitations is halved. Since the offset compensation circuit 344 has a gain, the gain is lowered because there is no signal before the half cycle in the first excitation after the pause period. Therefore, this can be dealt with by performing an operation with gain only for the first signal, or by performing an operation with the first signal set as P and N. For example, when an odd number of half-period excitations such as P → N → P → pause → N → P → N are continued as shown in FIG. 40 (a), each excitation period is shown in FIG. 40 (b). The low gains A and B appearing at the beginning of the signal are combined into one signal. This makes it possible to compensate for a gain that is halved, and it can be handled in the same way as an excitation signal for a normal half cycle.

The offset compensation method is not limited to the above method, and for example, offset compensation can also be realized by a method in which the output voltage of the amplifier 343 is fed back and used as an input reference.
(converter)

Next, FIG. 41 shows an example of a two-wire electromagnetic flow meter system in which a converter is connected to such a two-wire electromagnetic flow meter. Here, an insulation type converter that insulates the two-wire electromagnetic flow meter and the external DC power supply in a circuit manner will be described. The converter 500 shown in this figure includes a step-up power supply 501, an insulation transformer 502, an insulation type switching control circuit 503, a current detection resistor 504, a voltage subtraction circuit 505, an insulation type pulse transformer 506, and an output circuit 507. Is provided. The converter 500 converts the power received from the external DC power source DC through the power line PL by a switching method, and supplies the converted power to the two-wire electromagnetic flow meter 400 through the transmission line DL, while the two-wire electromagnetic flow rate. The output current I _O corresponding to the flow rate detected by the total 400 is converted into an output voltage V _O and output from the output circuit as a 1-5 V voltage signal via the output line OL.

FIG. 42 shows an example of another two-wire electromagnetic flow meter system. This converter 600 is a high-functional type provided with an integrated pulse alarm output that outputs an integrated pulse of flow rate in addition to a normal voltage output. The converter 600 includes a step-up switching power supply 601, a current detection resistor 604, a voltage subtraction circuit 605, an A / D converter 608, a calculation unit 609, a DC / DC conversion circuit 602, and an F / V conversion circuit 610. And an integrated pulse alarm output circuit 611. As in FIG. 41, the converter 600 subtracts the additional voltage from the output current I _O corresponding to the flow detected by the two-wire electromagnetic flow meter 400 by the voltage subtracting circuit 605, and then the A / D converter 608. The A / D converted signal is sent to the calculation unit 609. The calculation unit 609 calculates the flow rate based on the detected digital signal, and outputs the digital signal whose frequency is changed according to the flow rate to the F / V conversion circuit 610 and the integrated pulse alarm output circuit 611. The F / V conversion circuit 610 performs frequency / voltage conversion and outputs a voltage signal of 1-5V corresponding to the flow rate. On the other hand, the integrated pulse alarm output circuit 611 outputs the integrated value of the flow rate signal as a transistor output. Thereby, an alarm output is obtained based on the integrated value of the flow signal.
(Amplifier mode switching function)

These two-wire electromagnetic flow meters can be switched to an amplifier mode in which an additional current is added in order to improve measurement accuracy in addition to the normal mode in which the output current I _O is operated at 4-20 mA. Switching between the normal mode and the amplifier mode is performed by the amplifier mode switching means 33. Amp mode switching unit 33 outputs the amp mode instruction signal S _AP, instructs the switching of the normal mode and the amp mode. In the example of FIGS. 41 and 42, an additional current of 20 mA is added to the output current in the amplifier mode. As a result, since the output current _IO can be operated at 24-40 mA in the amplifier mode, the excitation coil 22 can be continuously excited even when the flow rate is small, and stable flow rate measurement can be realized.

Also, the converter switches the output signal according to the presence / absence of the amplifier mode or ON / OFF. In the example of FIG. 41, when the converter 500 detects the amplifier mode with the current detection resistor, the output voltage is subtracted by an additional voltage (for example, 5V) by the voltage subtracting circuit and insulated via the pulse transformer, and then the output circuit 1-5V, a voltage signal of 1-5V corresponding to the flow rate is output in both the normal mode and the amplifier mode.

An example of a two-wire electromagnetic flow meter having such an amplifier mode switching function is shown in FIG. The two-wire electromagnetic flow meter shown in this figure is connected to an external DC power source DC by a pair of transmission lines DL, and a current output circuit 16 is provided between the transmission lines DL. The current output circuit 16 includes an operational amplifier A ₆ and a transistor. The current output circuit 16 is connected to the addition / subtraction instruction circuit 41. The addition / subtraction instruction circuit 41 includes an operational amplifier A ₇ and a resistor. The switch SW ₇ is connected to the inverting input of the operational amplifier A ₇ via a resistor. Switch SW ₇ is switched to ON / OFF the amplifier mode instruction signal S _AP output from the amplifier mode switching means 33 of the internal circuit. The inverting input of the operational amplifier A ₇ is connected to the internal circuit via a further resistor. As a result, the output current instruction signal _SVO output from the internal circuit is input to the inverting side input side via the resistor.

The internal circuit of FIG. 43 is designed to consume 4-20 mA in accordance with the flow rate range in the normal mode. On the other hand, in the amplifier mode, an additional current is added. In this case, for example, if 20 mA is simply added as an additional current, an instruction circuit that consumes 4-40 mA is required for the flow rate range, and the resolution is lowered. In contrast, in the example of FIG. 43, the amplifier mode switch SW ₇ by an amplifier mode instructing signal S _AP is ON or OFF, to change the reference voltage of the subtraction instruction circuit 41. As a result, the operation of the current output circuit 16 is switched to operate so that the output current I _O changes in a range of 24-40 mA instead of 4-40 mA. In this way, the current offset can be added without reducing the resolution of the output current instruction signal _{SVO by} the addition / subtraction instruction circuit 41 in the amplifier mode.

With such a configuration, it is possible to operate by switching between the normal mode and the amplifier mode. In particular, by raising the excitation current _IE and the excitation voltage, a stable operation as an electromagnetic flow meter is realized. When the user requests stable operation by measuring the flow rate, the amplifier mode is set. However, an additional current of 20 mA is added to the signal obtained in this case. Therefore, it is necessary to perform 20 mA subtraction on the signal receiving side. Alternatively, a dedicated converter (or distributor) that supports amplifier mode operation is used. As an example of the latter, in FIG. 41 described above, the converter 500 receives the obtained signal with a current detection resistor (for example, a resistor of 250 Ω) to obtain a voltage signal of 6-10V. Here, by subtracting the additional voltage 5V corresponding to the additional current 20 mA, it can be easily converted into a signal of 1-5V. Since the 1-5V voltage signal is a widely used voltage signal, it is easy to process.
(Excitation circuit with excitation control function)

By increasing the current that can be used in the amplifier mode, the exciting coil 22 can be continuously excited. For this reason, a circuit for increasing the excitation current and the excitation voltage is required so that the consumption current added in the amplifier mode can be effectively utilized. Next, FIG. 44 shows an example of an excitation circuit having an excitation control function capable of controlling the excitation current and voltage corresponding to such an amplifier mode. The electromagnetic flow meter, a charge charge monitoring circuit 26, a residual voltage detection circuit 180, an exciting polarity switching circuit 28, the excitation and constant current circuit 29, the amplifier mode switching means amplifier mode (not shown) instructing signal S _AP receiving, and a reference voltage switching circuit 43 for switching these residual voltage detection circuit 180, the excitation polarity switching circuit 28, a reference voltage exciting the constant current circuit 29 is referenced from V _ref3 to V _ref4.

The excitation circuit 24 switches between three controls in order to realize a stable operation of the electromagnetic flow meter. That is, (1) the excitation current _IE is switched. (2) The timing for switching the excitation voltage of the excitation coil 22 is switched. Here, the threshold value of the timing for switching the excitation voltage from V _H to V _L is switched by the excitation constant current circuit 29. (3) The voltage supplied to the exciting coil 22 is increased. In other words, the excitation current _IE is changed. When the excitation current _IE is changed, the residual voltage needs to be changed. As a result of changing the residual voltage, the charge voltage instruction signal V _ref from the residual voltage detection circuit 180 to the charge charge monitor circuit 26 changes. And control charge charge monitoring circuit 26, changes the charge voltage command voltage V _2. As a result, the coil voltage V _L increases. With such a configuration, the efficiency can be further improved by adjusting the excitation timing in addition to the switching operation to the amplifier mode.
(Damping function)

Also, this electromagnetic flow meter has a damping function to stabilize the signal. When the flow velocity detected by the electromagnetic flow meter decreases, the value fluctuation increases and the measurement stability decreases. Therefore, the stability is improved by increasing the damping amount at a low flow rate at which the noise component increases. Thus, damping is weighting of averaging in the moving averaging process or the like, and FIG. 45 shows an outline of the damping process (averaging process). As shown in FIG. 45 (a), the output value after the damping process for the input value is output as shown in FIG. 45 (c) when the input changes stepwise as shown in FIG. 45 (b). Is defined as the time to reach 63%. As shown in FIG. 45 (c), as the damping is increased, the stability of the value is improved, but the response time to the input change is deteriorated. On the contrary, if the damping is small, the response time to the input change is fast, but the stability of the value is low, and it is easy to touch the variation of the value.

FIG. 46 shows an example of an electromagnetic flow meter having such a damping function. In addition to the above-described configuration, the electromagnetic flow meter 700 shown in this figure further includes a damping means 44 and an auto mode switching means 45, and is further connected to a setting unit 80 for performing these settings. Here, the damping means 44 is a member for performing a damping calculation based on the set damping amount. The auto mode switching means 45 is a means for switching between manual mode and auto mode. The setting unit 80 is a member that sets a damping amount, an auto mode switching timing, and the like, and includes a console, a display monitor, a communication port, and the like as necessary. About other members, it is the same as that of the Example mentioned above, and detailed description is abbreviate | omitted. In the example of FIG. 46, the damping unit 44 and the auto mode switching unit 45 are integrated with a calculation unit 40 that performs flow rate calculation. However, it goes without saying that the damping means 44 and / or the auto mode switching means 45 can be provided separately from the calculation means.

In this electromagnetic flow meter, the damping unit 44 performs a damping process and the auto mode switching unit 45 can switch between a manual mode and an auto mode. Here, the manual mode is a mode in which the damping unit 44 maintains the damping amount for performing the damping calculation at a predetermined value, and one auto mode is a mode in which the damping unit 44 changes the damping amount according to the flow rate. It is a mode to make it. As a result, the electromagnetic flow meter can perform stable measurement by switching the damping amount from a fixed value to a variable value and increasing the damping amount by the automatic mode switching means 45 switching from the manual mode to the automatic mode according to the predetermined conditions. The result can be obtained. Hereinafter, this process will be described in detail.
(Damping calculation)

47 and 48 show an example of the damping process. In these examples, since the detection signal is stable in the region where the flow velocity is large, the damping amount is constant, and the damping amount is increased in the region where the flow rate is small, thereby stabilizing the signal. In the example of FIG. 47, the damping amount is greatly changed linearly, and in the example of FIG. 48, the damping amount is changed exponentially. These changes are set according to the required measurement accuracy and responsiveness, changes in signal detection stability, and the like.

Note that the damping amount is typically a damping time constant indicating the time for performing the averaging process, and is variable in the range of 0.5 s to 30 s, for example.

Further, the reference for making the damping amount variable, that is, the timing for switching from the manual mode to the auto mode is determined by the auto mode switching means 45. The reference for switching is an area where the damping process needs to be increased, and the flow rate, output current, etc. can be used. For example, when the calculated flow velocity is in the low flow velocity range (0 m / s to 0.5 m / s, etc.), the signal value is small, so stabilization is necessary. Even in the range where the analog output of the output current _IO is 4 to 12 mA, the excitation current is small, so the detection signal is also small, and similarly stabilization by damping processing is necessary. In addition, when the coil current of the exciting coil is switched from continuous current to intermittent excitation with a pause period, the stability of the signal similarly deteriorates, so switching to intermittent excitation can be used as a reference. Further, the auto mode and the manual mode may be switched at an arbitrary timing designated by the user.

In such a low speed region, the mode can be shifted to the auto mode, and the stability can be improved by increasing the damping amount as the flow rate decreases. Conversely, by suppressing the damping amount as the flow rate increases, it is possible to reduce the averaging process and increase the responsiveness to changes in the flow rate, thereby making it possible to maintain both measurement accuracy and improve stability.

Fig. 49 shows an image for setting the damping. Here, an example of a setting screen displayed on the monitor of the setting unit 80 is shown, and the user sets each item from the console. From this screen, the damping time constant in the manual mode can be switched between 0.5 s, 1.0 s, and 1.5 s as the setting of the damping amount that the damping means 44 performs the damping process. It is not limited to the selection of such a fixed value, but the user may be able to specify an arbitrary value.

Also, the operation of the auto mode switching means 45 is selected from specified conditions (

level

1, 2, 3, etc.). Each level is defined by, for example, a flow velocity value, an excitation current value, or the like. It is also possible to select the operation in the manual mode in which the auto mode is turned off, that is, the damping amount is maintained at a constant value without being limited to the flow velocity.

In addition, when switching to the auto mode with the output current I _O , it is possible to define the span specified by the user. The span is a flow velocity or a flow rate when outputting 20 mA that is the maximum value of the analog output of the output current _IO . As described above, if the user assigns the relationship between the flow rate and the output current value, the user can also assign the auto mode switching threshold. FIG. 50 shows an example of such a span setting and a change in analog output with respect to the flow velocity. Here, the analog output linearly changes from 4 mA to 20 mA from the flow velocity 0 toward the span flow velocity value. Such a span can be set in units of a flow rate (m / s), a flow rate (L / min), a mass flow rate (kg / s), or the like. An example of setting such a span is shown in FIG. This example also shows a setting screen image of the setting unit 80 as in FIG. Here, a span is selected as a setting item, and the user can select a desired value from choices (for example, a flow rate of 500 L / min, a flow rate of 2 m / s, and a mass flow rate of 100 kg / h).

The two-wire electromagnetic flow meter of the present invention can be suitably applied as a capacitive electromagnetic flow meter that detects the flow rate of a conductive liquid in a non-wetted state.

Claims

In a two-wire electromagnetic flowmeter that detects the flow rate of the fluid to be detected and performs signal transmission and power supply of the detected flow rate using two common transmission lines.
A measuring tube constituting a flow path through which the fluid to be detected passes,
At least a pair of exciting coils arranged to be orthogonal to the flow path of the measuring tube;
An excitation circuit for exciting the excitation coil;
A straight line connecting the pair of excitation coils, and at least a pair of electrodes arranged to be orthogonal to the flow path of the measurement tube;
A detection circuit capable of detecting a voltage signal detected by the electrode;
A primary side input capable of connecting two transmission lines and a secondary side output insulated from the primary side input are converted into a secondary power by converting DC power supplied from the transmission line into predetermined power. Power supply that can output from the side output;
Based on the voltage signal detected by the detection circuit, the flow rate of the fluid to be detected that passes through the flow path of the measurement tube is calculated, and an excitation control signal for controlling the driving state of the excitation circuit is calculated in the excitation circuit. And a calculation means capable of outputting a power control signal for controlling a driving state of the power supply unit to the power supply unit, respectively.
With
The excitation circuit and the detection circuit are connected to the secondary side output of the power supply unit,
The two-wire system characterized in that the calculation means is configured to control the amount of current from the transmission line supplied to the primary side input of the power supply unit according to the calculated flow rate of the fluid to be detected. Electromagnetic flow meter.
The two-wire electromagnetic flow meter according to claim 1, further comprising:
A control signal insulation circuit for sending a power supply control signal from the calculation means to the power supply section while separating a common potential between the primary side input of the power supply section and the calculation means;
The two-wire electromagnetic flowmeter, wherein the arithmetic means is disposed at a secondary output of the power supply unit and connected to an output side of the detection circuit.
The two-wire electromagnetic flow meter according to claim 2,
The power supply unit is
A current output circuit capable of controlling a primary-side input current value for energizing the transmission line based on a power supply control signal from the arithmetic means;
A switching circuit controlled by the current output circuit and capable of converting the DC power of the primary side input current supplied from the transmission line into a predetermined power and outputting it to the secondary side;
A two-wire electromagnetic flow meter comprising:
The two-wire electromagnetic flow meter according to claim 3,
The switching circuit is further used as a secondary output,
An excitation circuit output for generating drive power for driving the excitation circuit;
A two-wire electromagnetic flow meter comprising: an output for calculating means for generating driving power for driving the calculating means.
The two-wire electromagnetic flow meter according to claim 1,
The two-wire electromagnetic flowmeter, wherein the detection circuit is an analog signal amplification circuit that amplifies a voltage signal detected by the electrode.
The two-wire electromagnetic flow meter according to claim 1, further comprising:
In parallel with the excitation circuit,
A constant voltage power source connected in series with the exciting coil and capable of adjusting and supplying an output voltage value for exciting the exciting coil;
A charge charge monitor circuit for instructing a voltage required to supply an excitation current to the excitation circuit by the constant voltage power supply;
A two-wire electromagnetic flow meter comprising: