JP7221633B2 - Excitation circuit and electromagnetic flowmeter - Google Patents

Description

The present invention relates to an electromagnetic flowmeter that measures the flow rate of fluid in various process systems, and an excitation circuit that supplies an excitation current to an excitation coil of the electromagnetic flowmeter.

In general, an electromagnetic flowmeter consists of an excitation coil that generates a magnetic field in a direction perpendicular to the flow direction of a fluid flowing in a measuring pipe, and an excitation coil that is placed in the measuring pipe and generates a magnetic field in a direction perpendicular to the flow direction of the fluid and the direction of the magnetic field . a detector having a pair of sensing electrodes positioned opposite each other; Such an electromagnetic flowmeter measures the flow rate of the fluid flowing through the measuring tube based on the electromotive force generated between the detection electrodes when the fluid flows through the magnetic field generated by the excitation coil.

In such an electromagnetic flowmeter, in order to eliminate the influence of polarization in the detection electrodes, a voltage is applied to the excitation coil while switching the polarity at a predetermined excitation frequency (hereinafter referred to as the excitation coil for excitation). The applied voltage is called "excitation voltage"). As a result, an alternating magnetic field is generated by supplying a current (hereinafter, the current flowing through the exciting coil is referred to as an "exciting current") to the exciting coil, and when the exciting current has a steady value, that is, a constant intensity The electromotive force (hereinafter sometimes referred to as "flow rate signal") generated between the detection electrodes is measured during the period when a strong magnetic field is generated.

However, when an alternating voltage consisting of a rectangular wave is applied to the exciting coil, the rise/fall of the exciting current is dull due to the influence of the self-inductance of the exciting coil. As a result, if the excitation frequency is increased, the period during which a magnetic field of constant strength is generated will be shortened, making it difficult to sample the flow rate signal stably, causing an increase in measurement error in the flow rate value. .

Therefore, for example, in the electromagnetic flowmeter disclosed in Patent Document 1, an excitation circuit that supplies an excitation current to an excitation coil is a DC voltage source having two different output voltage values, a high output voltage value and a low output voltage value. A high output voltage value is selected as the excitation voltage during the transient period of the excitation current after the polarity of the excitation voltage is switched, and a low output voltage value is selected during the steady period when the excitation current is at a steady value. . According to Patent Document 1, by selecting a high output voltage value in the transient period of the excitation current, the excitation current is brought to a steady value more quickly, and by selecting a low output voltage value in the steady period, It is described that power consumption can be suppressed.

However, in the electromagnetic flowmeter disclosed in Patent Document 1, since the two voltage values are fixed values, during the transient period of the excitation current, the excitation voltage is a high output voltage regardless of the change in the magnitude of the excitation current. voltage is applied. Therefore, as the excitation current increases, the electric power wasted in the constant current circuit increases, and heat generation in the excitation circuit becomes a problem.

On the other hand, the electromagnetic flowmeter disclosed in Patent Document 2 includes a variable voltage DC/DC converter, a constant voltage power supply connected in series with the excitation coil, and a constant voltage power supply connected in series with the excitation coil. A constant current circuit for prescribing the current flowing through the coil to a predetermined value, a residual voltage detection circuit for detecting the residual voltage of this constant current circuit (voltage drop in the constant current circuit), and outputting a predetermined reference voltage and a comparison circuit for comparing the residual voltage with a predetermined reference voltage. The electromagnetic flowmeter disclosed in Patent Document 2 controls the output voltage of the constant voltage source according to the comparison result between the residual voltage of the constant current circuit and a predetermined reference voltage.

In the electromagnetic flowmeter disclosed in Patent Document 2, the constant voltage power supply compares the residual voltage detected by the residual voltage detection circuit with a predetermined reference voltage, and adjusts the output voltage so that the comparison result is minimized. Control. In addition, the variable voltage DC/DC converter of the constant voltage power supply applies a high voltage to the excitation coil so as to facilitate current flow when the power supply of the excitation coil is turned on, and after the current starts to flow, the voltage is lowered to a certain level. Apply a constant voltage.

In addition, according to the electromagnetic flowmeter disclosed in Patent Document 2, an excitation circuit with extremely low power loss can be configured by using a DC/DC converter to generate the minimum required output voltage. It is described that by controlling the output voltage of the constant voltage source according to the residual voltage of the current circuit, it is possible to supply the optimum voltage even if exciting coils with different characteristics are connected.

JP-A-53-020956 Japanese Patent No. 5065620

By the way, when connecting the excitation circuit of the electromagnetic flowmeter described in Patent Document 2 to an excitation coil with different characteristics, for example, by replacing it with a detector with a different diameter, the characteristics of the new excitation coil, that is, the DC It is necessary to change the reference voltage to be compared with the residual voltage of the constant current circuit according to the resistance and inductance.

However, in general, the reference voltage is configured from another power supply using a regulator circuit or the like. In the excitation circuit described in Patent Document 2, in order to change the reference voltage to be compared with the residual voltage of the constant current circuit according to the characteristics of the new excitation coil, the physical circuit constants must be changed. In addition, in order to make the reference voltage variable, it is necessary to consider the configuration of the reference element and the generation of the signal that changes the output voltage, resulting in an extremely complicated circuit configuration.

It is an object of the present invention to provide an excitation circuit and an electromagnetic flowmeter in which the excitation voltage can be changed according to the characteristics of the excitation coil and the magnitude of the excitation current without comparing the residual voltage of the constant current circuit with the reference voltage. aim.

In order to solve the above-described problems, an exciting circuit according to the present invention provides an exciting coil that receives an exciting current and generates a magnetic field perpendicular to the flow direction of a fluid flowing inside a measuring tube at a predetermined exciting frequency. an excitation circuit configured to apply an excitation voltage while switching polarity, the variable voltage power supply circuit configured to output an excitation voltage whose magnitude can be changed according to a voltage control signal; a switch circuit configured to apply an excitation voltage output from a variable voltage power supply circuit to the excitation coil by switching the polarity at a predetermined excitation frequency; and an excitation current supplied to the excitation coil through the switch circuit. a current control signal generating circuit for generating a current control signal based on the magnitude thereof; and a control element for controlling the magnitude of the exciting current based on the current control signal, so as to apply a predetermined constant current to the exciting coil. and a voltage control circuit that generates the voltage control signal from the current control signal generated by the current control signal generation circuit and inputs the voltage control signal to the variable voltage power supply circuit. and

Moreover, in the excitation circuit according to the present invention, the voltage control circuit may include a signal detection circuit that detects the current control signal.

Moreover, in the excitation circuit according to the present invention, the voltage control circuit may include a time constant circuit that gradually changes the voltage control signal based on the current control signal.

Further, in the excitation circuit according to the present invention, the voltage control circuit delays a change in the current control signal and reflects it in the voltage control signal each time the polarity of the excitation voltage is switched, so that the excitation current reaches a predetermined value. A delay circuit configured to reduce the excitation voltage after a predetermined delay time has elapsed after the current becomes constant may be provided.

Further, in the excitation circuit according to the present invention, the variable voltage power supply circuit may include a variable voltage DC/DC converter.

Further, in the excitation circuit according to the present invention, the excitation current control circuit includes a resistor whose one end and the other end are connected to the output terminal and the ground terminal of the control element, respectively, and detects the magnitude of the excitation current. and an amplifier for outputting the current control signal to be input to the control terminal of the control element based on the magnitude of the excitation current detected by the resistor, the control element being connected to the switch circuit and the An input terminal to which an excitation current is input, an output terminal to output the excitation current, and the control terminal to which the current control signal is input. A three-terminal control element may be included for controlling the magnitude of the excitation current flowing between the input terminal and the output terminal accordingly.

Further, in order to solve the above-described problems, the electromagnetic flowmeter according to the present invention includes a measuring tube for flowing a fluid, and an electromagnetic flowmeter which is arranged near the measuring tube, receives an excitation current, and generates a current inside the measuring tube. and a pair of detection electrodes facing each other in a direction perpendicular to the flow direction of the fluid flowing inside the measuring tube and the direction of the magnetic field. and an excitation voltage is applied to the excitation coil while switching its polarity at a predetermined excitation frequency to supply an excitation current to the excitation coil, and the flow direction of the fluid flowing inside the measurement tube is an excitation circuit configured to generate an orthogonal alternating magnetic field; a power supply circuit for supplying DC power to the excitation circuit; a control circuit that calculates the flow rate of the fluid flowing through the measurement tube based on the detection signal detected from the detection electrode while switching the excitation frequency, wherein the excitation circuit is the excitation circuit described above. characterized by being

According to the present invention, the current control signal is generated based on the magnitude of the exciting current supplied to the exciting coil, and the voltage control signal is generated from the current control signal and input to the variable voltage power supply circuit. The excitation voltage can be changed according to the characteristics of the excitation coil and the magnitude of the excitation current without comparing the voltage with the reference voltage.

FIG. 1 is a block diagram showing the configuration of an electromagnetic flowmeter according to an embodiment of the invention. FIG. 2 is a block diagram showing the configuration of an excitation circuit according to the embodiment of the invention. FIG. 3 is a signal waveform diagram showing the operation of the excitation circuit according to the embodiment of the invention.

[Embodiment]
Preferred embodiments of the present invention will now be described in detail with reference to FIGS. 1 to 3. FIG.
First, an excitation circuit 12 according to an embodiment of the present invention and an electromagnetic flowmeter 1 using the same will be described with reference to FIGS. 1 and 2. FIG.

[Electromagnetic Flowmeter]
The electromagnetic flowmeter 1 according to the present embodiment includes a power supply circuit 10, a detector 11, an excitation circuit 12, a control circuit 13, and a setting/manipulator 14, as shown in FIG.

Here, the detector 11 includes a measuring tube Pex for flowing the fluid to be measured, and a flow direction of the fluid flowing inside the measuring tube Pex upon receiving the supply of the exciting current Iex. and a pair of detection electrodes facing each other in a direction perpendicular to the flow direction of the fluid flowing inside the measuring tube Pex and the direction of the magnetic field generated by the excitation coil Lex. E1, E2.

Under the control of the control circuit 13, the excitation circuit 12 applies an excitation voltage V _Eout to the excitation coil Lex of the detector 11 while switching the excitation polarity at a predetermined excitation frequency to excite the excitation coil Lex. It is configured to supply a current Iex and generate an alternating magnetic field perpendicular to the flow direction of the fluid flowing inside the measuring tube Pex. Details of the configuration and operation of the excitation circuit 12 will be described later.

The control circuit 13 controls the excitation circuit 12 to switch the polarity of the excitation voltage V _Eout applied to the excitation coil Lex at a predetermined excitation frequency. It is configured to measure the flow rate of fluid flowing through the tube Pex.

More specifically, the control circuit 13 outputs polarity switching signals EXD1 and EXD2, which are binary signals complementary to each other, to the excitation circuit 12 to periodically change the polarity of the excitation voltage V _Eout applied to the excitation coil Lex. while the excitation current is at a steady value, i.e., during a period in which a magnetic field of constant strength is generated, the flow rate signal generated between the detection electrodes E1 and E2 is amplified, and the amplified flow rate is The signal is sampled and its value is held, and the flow rate of the fluid flowing through the measuring pipe Pex is measured from the sampled and held value and the cross-sectional area of the measuring pipe Pex. Although not shown, such a control circuit 13 can be composed of an amplifier, a central processing circuit (CPU), a signal processing circuit, an I/F circuit, or the like.

In this embodiment, the control circuit 13 is configured to output the calculated flow rate to the setting/manipulator 14, which will be described later. A flow rate signal or flow rate may be output to a higher-level device that does not operate.

On the other hand, the power supply circuit 10 receives the supply of the AC power ACin to generate DC power, and supplies the DC power to the excitation circuit 12 and the control circuit 13 . Such a power supply circuit 10 can be composed of, for example, rectifier circuits 10A, 10D, 10E, a switching control circuit 10B, and a transformer 10C.

Here, the rectifier circuit 10A rectifies the AC power ACin, converts it into a DC power, and outputs it to the switching control circuit 10B. The switching control circuit 10B switches the DC voltage of the DC power supply at a high frequency and supplies it to the primary winding of the transformer 10C.
The rectifier circuit 10</b>D rectifies the high-frequency signal output from the secondary winding of the transformer 10</b>C, generates an operating voltage for DC analog signal processing and a ground voltage, and supplies them to the control circuit 13 .
Also, the rectifier circuit 10E rectifies the high-frequency signal output from the secondary winding of the transformer 10C, generates a DC excitation high voltage (for example, 80 V), and supplies it to the excitation circuit 12. FIG.

The setting/operating device 14 detects setting inputs and operation inputs of the operator and outputs them to the control circuit 13 . In addition, the setting/operating device 14 displays information indicating the detected flow rate or the like on an LED or LCD according to a signal from the control circuit 13 .

[Excitation circuit]
Next, the excitation circuit 12 will be described with reference to FIG.
Under the control of the control circuit 13, the excitation circuit 12 applies an excitation voltage V _Eout to the excitation coil Lex of the detector 11 while switching the excitation polarity at a predetermined excitation frequency to alternate the excitation coil Lex. A circuit that supplies an excitation current Iex to generate a magnetic field. As shown in FIG. 2, such an excitation circuit 12 includes a variable voltage power supply circuit 12A, an excitation direction switching circuit 12B, an excitation current control circuit 12C, and a voltage control circuit 12D. The magnitude of the excitation voltage V _Eout applied to the excitation coil is changed according to the magnitude Iex' (the magnitude Iex' of the excitation current Iex detected in the excitation current control circuit 12C).

The variable voltage power supply circuit 12A is basically a circuit that receives supply of DC power from the rectifier circuit 10E and outputs an excitation voltage V _Eout . The magnitude of the excitation voltage V _Eout can be changed according to the voltage control signal _SET . Such a variable voltage power supply circuit 12A can include, for example, a variable voltage DC/DC converter. The excitation voltage V _Eout whose voltage value is changed by the variable voltage power supply circuit 12A is input to the excitation direction switching circuit 12B.

The excitation direction switching circuit 12B is a circuit that acts as a switch circuit in the present invention, and applies the excitation voltage V _Eout output by the variable voltage power supply circuit 12A to the excitation coil Lex by switching its polarity at a predetermined excitation frequency. . By switching the polarity of the excitation voltage applied to the excitation coil Lex by the excitation direction switching circuit 12B at a predetermined excitation frequency, the polarity of the excitation current Iex supplied to the excitation coil Lex is switched between positive and negative.

Specifically, the excitation direction switching circuit 12B can be composed of transistors Tr1, Tr2, Tr3, and Tr4 that are turned on/off based on the polarity switching signals EXD1 and EXD2 from the control circuit 13. FIG. That is, as shown in FIG. 2 , the excitation direction switching circuit 12B includes two pairs of transistors Tr1 and Tr2 and transistors Tr3 and Tr4 which are connected in series and which are connected in parallel to the node N0 and the node N3. It can be constructed from a transistor circuit.
As the transistors Tr1 to Tr4, known three-terminal control elements such as FETs and bipolar transistors can be used.

An exciting coil Lex of the detector 11 is connected to a connection point between the transistors Tr1 and Tr2 and a connection point between the transistors Tr3 and Tr4 via nodes N1 and N2, respectively.

One (EXD1) of the polarity switching signals EXD1 and EXD2, which are binary signals complementary to each other and output from the control circuit 13, is input to the gate terminal of the transistor Tr1 and the gate terminal of the transistor Tr4. , the other (EXD2) of the polarity switching signals EXD1 and EXD2 is input to the gate terminal of the transistor Tr2 and the gate terminal of the transistor Tr3. That is, the transistor Tr1 is connected between the node N0 and the node N1, and turns on and off based on the polarity switching signal EXD1. The transistor Tr2 is connected between the node N1 and a node N3, which is the output terminal of the excitation direction switching circuit 12B, and is turned on and off based on the polarity switching signal EXD2. The transistor Tr3 is connected between the node N0 and the node N2, and turns on and off based on the polarity switching signal EXD2. The transistor Tr4 is connected between the node N2 and the node N3, and turns on and off based on the polarity switching signal EXD1.

Therefore, by turning on/off the transistors Tr1, Tr2, Tr3 and Tr4 in accordance with the polarity switching signals EXD1 and EXD2, the voltage supplied from the variable voltage power supply circuit 12A to the node N0 which is the input terminal of the excitation direction switching circuit 12B. The excitation voltage V _Eout is applied to the excitation coil Lex of the detector 11 connected between the nodes N1 and N2 while switching its polarity. As a result, the excitation current flowing into the node N0 of the excitation direction switching circuit 12B is output from the node N3.

The excitation current control circuit 12C is a control circuit that is connected to the node N3 of the excitation direction switching circuit 12B and configured to apply a predetermined constant current to the excitation coil Lex. As shown in FIG. 2 , such an excitation current control circuit 12C includes a transistor Tr5 whose source terminal is connected to the node N3 of the excitation direction switching circuit 12B, and one end and the other end of which are respectively connected to the drain terminal of the transistor Tr5 and the ground. and the magnitude Iex' of the exciting current Iex detected by the resistor _RE connected to the terminal and detecting the magnitude Iex' of the exciting current _Iex , that is, based on the drain voltage of the transistor Tr5. , and an operational amplifier (amplifier) A that outputs a current control signal V _Aout to be input to the gate terminal of the transistor Tr5.

Here, the non-inverting input terminal (+) of the operational amplifier A is connected to the reference voltage _Vref , and the inverting input terminal (-) is connected to the drain terminal of the transistor Tr5. A desired voltage may be set as the reference voltage V _ref from a power supply (not shown) or the like. The current control signal V _Aout output by the operational amplifier A is a voltage corresponding to the difference between the voltage (drain voltage of the transistor Tr5) corresponding to the magnitude Iex' of the exciting current Iex detected by the resistor _RE and the reference voltage _Vref . is a signal. Therefore, the current detection resistor R _E and the operational amplifier A constitute a feedback circuit for keeping the magnitude Iex' of the exciting current Iex constant, and act as a "current control signal generating circuit" in the present invention. Further, the transistor Tr5 acts as a control element for controlling the magnitude Iex' of the exciting current Iex based on the current control signal V _Aout .

In the present embodiment, an example of a voltage control circuit using a well-known three-terminal control element FET as the transistor Tr5 and using a voltage signal as a current control signal has been described. A conventional current control circuit may be used.

In this embodiment, the current control signal V _Aout output from the operational amplifier A is input to the control terminal of the transistor Tr5 and also to the voltage control circuit 12D.

The voltage control circuit 12D controls the magnitude of the excitation voltage V _Eout supplied from the variable voltage power supply circuit 12A based on the current control signal V _Aout generated by the current detection resistor R _E and the operational amplifier A in the excitation current control circuit 12C. This is a circuit that generates a voltage control signal S _ET to control the voltage and inputs it to the variable voltage power supply circuit 12A.
In this embodiment, the voltage control circuit 12D includes a signal detection circuit 12E, a delay circuit 12F, and a time constant circuit 12G, as shown in FIG .

Here, the signal detection circuit 12E is a circuit that detects the current control signal V _Aout output from the operational amplifier A and outputs a detection signal V _det corresponding to the detected current control signal V _Aout . The signal detection circuit 12E can be configured by a well-known operational circuit such as an operational amplifier or a comparator. For example, the signal detection circuit 12E outputs a detection signal V _det corresponding to the magnitude of the current control signal V _Aout and performs threshold processing on the current control signal V _Aout to detect the value of the current control signal V _Aout . is below a set threshold voltage _Vth , it may be detected that the magnitude Iex' of the exciting current Iex has reached a predetermined constant current value. A detection signal V _det output from the signal detection circuit 12E is input to the delay circuit 12F.

The delay circuit 12F is a circuit that delays the detection signal V _det output by the signal detection circuit 12E, ie, the detection result of the current control signal V _Aout by the signal detection circuit 12E, by a predetermined delay time T2 and outputs the result. Such a delay circuit 12F can be configured using, for example, a CR circuit including a capacitive element and a resistor. In this case, the delay time T2 can be set to a desired delay time by adjusting the values of the capacitance and resistance, and also by setting a threshold value.

The delay circuit 12F is reset each time the polarity of the excitation voltage V _Eout is switched, and outputs a signal V _del obtained by delaying the detection signal V _det corresponding to the current control signal V _Aout by a predetermined delay time T2. This signal V _del is input to the subsequent time constant circuit 12G.
Such a delay circuit 12F achieves the object of the present invention to change the excitation voltage according to the characteristics of the excitation coil and the magnitude of the excitation current without comparing the residual voltage of the constant current circuit and the reference voltage. Although it is not a necessary component, by providing the delay circuit 12F as in the present embodiment, the change in the current control signal V _Aout is delayed and reflected in the voltage control signal _SET , which will be described later. As shown in FIG. 3, the excitation voltage V _Eout can be lowered after a predetermined delay time T2 has passed since the magnitude Iex' of the excitation current Iex reaches a predetermined constant current value. It becomes possible to realize stable operation.

The time constant circuit 12G is a circuit that gradually changes the voltage control signal _SET based on the current control signal _VAout . The time constant circuit 12G can be configured by, for example, an integrating circuit having a capacitive element and a resistor. More specifically, the time constant circuit 12G generates a voltage control signal _SET according to the signal _Vdel input from the delay circuit 12F and outputs _it to the variable voltage power supply circuit 12A.
By providing such a time constant circuit 12G, it is possible to gently change the excitation voltage V _Eout output by the variable voltage power supply circuit 12A.
In particular, by configuring the capacitive element to discharge after the magnitude Iex' of the excitation current Iex reaches a predetermined constant current value, the magnitude Iex' of the excitation current Iex reaches a predetermined constant current value. until the excitation voltage V _Eout is gradually increased according to a predetermined time constant, and after the magnitude Iex' of the excitation current Iex reaches a predetermined constant current value (when the delay circuit 12F is provided, the excitation current The excitation voltage V _Eout can be gradually lowered after the delay time T2 from when the magnitude Iex of Iex reaches a predetermined constant current value.

[Excitation circuit operation]
Next, operation of the excitation circuit 12 according to the present embodiment will be described with reference to signal waveforms shown in FIG. In FIG. 3, the polarity switching signals EXD1 and EXD2 output from the control circuit 13, the excitation current Iex and its magnitude Iex', the current control signal V _Aout , and the detection signal V _det are delayed by a delay time T2. The signal V _del , the voltage control signal _SET and the excitation voltage V _Eout are shown along a common time axis.

In FIG. 3, a period during which the polarity switching signal EXD2 is at H level and the polarity switching signal EXD1 is at L level is defined as a "negative excitation period TN", and the polarity switching signal EXD1 is at H level and the polarity switching signal EXD2 is at L level. The period during which the voltage is at the level is called a "positive excitation period TP".
During the negative excitation period TN, the transistors Tr1 and Tr4 are turned off and the transistors Tr2 and Tr3 are turned on, so that the excitation current Iex flows from the node N2 to the node N1. On the other hand, during the positive excitation period TP, the transistors Tr1 and Tr4 are turned on and the transistors Tr2 and Tr3 are turned off, so the excitation current Iex flows from the node N1 to the node N2.

At time T0 in each of the negative excitation period TN and the positive excitation period TP, when the polarity of the excitation voltage V _Eout switches, the direction of the excitation current Iex also switches (FIG. 3, _Iex ). The magnitude Iex' of the exciting current Iex gradually increases to a preset current value due to the influence of the inductance of the exciting coil Lex and the action of the time constant circuit 12G (FIG. 3, T0 of Iex and Iex' ~T1).

At this time as well, the operational amplifier A of the excitation current control circuit 12C constantly compares the input signal V _RE and the reference voltage V _ref , and the magnitude Iex' of the excitation current Iex flowing through the resistor _RE becomes a predetermined constant current value. , the transistor Tr5 is controlled according to the comparison result by outputting the current control signal V _Aout .

The current control signal _VAout , which is the output of the operational amplifier A, maintains the H level voltage until the magnitude Iex' of the exciting current Iex reaches a predetermined constant current value. When the constant current value is reached, the current control signal V _Aout output from the operational amplifier A drops to the set voltage of L level (FIG. 3, V _Aout , T1: T0 to T1, T1': T1 to T0). The H level period T1 and the L level period T1' of the current control signal V _Aout vary depending on the inductance of the exciting coil Lex.

The signal detection circuit 12E of the voltage control circuit 12D detects the current control signal V _Aout and outputs the detection signal V _det to the delay circuit 12F. The delay circuit 12F delays the detection signal V _det by the delay time T2. Output _Vdel .

The reason for adding the delay to the current control signal V _Aout will now be explained. A current continues to flow through the exciting coil Lex until the exciting current Iex becomes a constant current. When the magnitude Iex' of the excitation current Iex reaches a predetermined constant current value, the excitation current Iex flowing through the excitation coil Lex changes due to the repulsion of the energy of the current that continues to flow through the excitation coil Lex. , a situation occurs in which the current applied to the exciting coil Lex is not stable.

If the excitation voltage V _Eout is switched during such an unstable period of the excitation current Iex, the excitation current Iex becomes even more unstable. Therefore, in the present embodiment, the delay circuit 12F is provided to provide a delay of the delay time T2 from the current control signal V _Aout to switch the excitation voltage V _Eout .

The signal V _del output from the delay circuit 12F is delayed from the current control signal V _Aout and the detection signal V _det by the delay time T2. At the timing when the polarity is switched from the period TN to the positive excitation period TP and at the timing when the positive excitation period TP is switched to the negative excitation period TN, the delay processing of the delay circuit 12F is reset, and the signal V _del becomes the current control signal V _Aout and It becomes H level corresponding to the detection signal V _det (FIG. 3, V _del , T0).

After that, the time constant circuit 12G generates the voltage control signal _SET based on the signal _Vdel inputted from the delay circuit 12F, more specifically, time-integrates the signal _Vdel . The voltage control signal S _ET has a smaller slope when the magnitude Iex' of the exciting current Iex reaches the constant current value earlier. In the time constant circuit 12G, the integration effect is reset when the polarity is switched from the negative excitation period TN to the positive excitation period TP.

The variable voltage power supply circuit 12A outputs an excitation voltage V _Eout according to the voltage control signal _SET generated by the time constant circuit 12G. The peak of the excitation voltage V _Eout coincides with the peak of the voltage control signal _SET and the edge of the delayed signal V _del (FIG. 3, V _Eout , T3).

In this manner, the voltage control signal _SET for the variable voltage power supply circuit 12A is generated from the current control signal V _Aout output from the operational amplifier A, and the variable voltage is maintained until the voltage of the current control signal V _Aout drops to the set voltage. The voltage of the power supply circuit 12A is operated to be a high voltage. That is, until the excitation current Iex flowing through the excitation coil Lex becomes a constant current, the voltage of the variable voltage power supply circuit 12A is increased, and the excitation voltage V _Eout applied to the excitation coil Lex and the voltage applied to the excitation current control circuit 12C be operated to raise the

The excitation voltage V _Eout generated by the variable voltage power supply circuit 12A applied to the excitation direction switching circuit 12B is gradually increased with a time constant. Therefore, since the voltage between the input terminal and the output terminal of the transistor Tr5 of the exciting current control circuit 12C is not increased more than necessary, loss in the exciting current control circuit 12C can be suppressed.

As described above, according to the excitation circuit 12 of the present embodiment, the current control signal V _Aout output from the operational amplifier A of the excitation current control circuit 12C is detected, and the detected current control signal V _Aout Based on this, the voltage control signal _SET is generated. Since the excitation voltage V _Eout generated by the variable voltage power supply circuit 12A is variably controlled by the voltage control signal S _ET , the characteristics of the excitation coil and the excitation current can be controlled without comparing the residual voltage of the constant current circuit and the reference voltage. The excitation voltage can be varied according to the magnitude of .

In addition, the excitation circuit 12 according to the present embodiment gradually increases the excitation voltage V _Eout as the magnitude Iex′ of the excitation current Iex increases, so power consumption during the transition period can be suppressed. Furthermore, when a constant excitation current flows through the excitation coil Lex, the power consumption during the steady period can be further reduced by gradually lowering the excitation voltage V _Eout .

In addition, in the excitation circuit 12 according to _the present embodiment, the excitation current Iex reaches a constant current in a shorter time, particularly when an excitation coil Lex having a relatively small inductance is used. It is possible to control the excitation voltage V _Eout generated by the variable voltage power supply circuit 12A without increasing it more than necessary.

In addition, since the excitation circuit 12 according to the present embodiment variably controls the excitation voltage V _Eout , the drain voltage of the control element Tr5 of the excitation current control circuit 12C, which is a constant current circuit, can also be lowered. suppressed.

Further, the excitation circuit 12 according to the present embodiment has a more simplified configuration, so that the electromagnetic flowmeter 1 including the excitation circuit 12 can be made more compact.

Although the embodiments of the excitation circuit and the electromagnetic flowmeter of the present invention have been described above, the present invention is not limited to the described embodiments, and can be envisioned by those skilled in the art within the scope of the invention described in the claims. Various modifications are possible.

REFERENCE SIGNS LIST 1 electromagnetic flowmeter 10 power supply circuit 10A, 19D, 10E rectifier circuit 10B switching control circuit 10C transformer 11 detector 12 excitation circuit 12A variable voltage power supply circuit 12B excitation Direction switching circuit 12C Excitation current control circuit 12D Voltage control circuit 12E Signal detection circuit 12F Delay circuit 12G Time constant circuit 13 Control circuit 14 Setting/operator A Operational amplifier Pex... Measuring tube, E1, E2... Detection electrode, Lex... Excitation coil, Tr1, Tr2, Tr3, Tr4, Tr5... Transistor, _RE ... Resistance, N0, N1, N2, N3... Node, EXD1, EXD2... Polarity switching signals, Iex...excitation current, Iex'...magnitude of excitation current, _VEout ...excitation voltage, _VRE ...input signal, _VAout ...current control signal, _Vref ...reference voltage, _Vdet ...detection signal, _Vdel ... Signal, _SET ... voltage control signal, V _th ... threshold voltage, Tex... constant period, TP... positive excitation period, TN... negative excitation period.

Claims (6)

An excitation circuit configured to apply an excitation voltage while switching the polarity at a predetermined excitation frequency to an excitation coil that receives an excitation current and generates a magnetic field orthogonal to the flow direction of the fluid flowing inside the measuring tube. There is
a variable voltage power supply circuit configured to output an excitation voltage whose magnitude can be changed according to a voltage control signal;
a switch circuit configured to switch the polarity of the excitation voltage output from the variable voltage power supply circuit at a predetermined excitation frequency and apply the excitation voltage to the excitation coil;
a current control signal generation circuit for generating a current control signal based on the magnitude of the excitation current supplied to the excitation coil through the switch circuit; and a control element for controlling the magnitude of the excitation current based on the current control signal. and an excitation current control circuit configured to apply a predetermined constant current to the excitation coil;
a voltage control circuit that generates the voltage control signal from the current control signal generated by the current control signal generation circuit and inputs the voltage control signal to the variable voltage power supply circuit ;
The voltage control circuit is
A time constant circuit that gradually changes the voltage control signal based on the current control signal is provided.
An excitation circuit characterized by: In the excitation circuit according to claim 1,
The voltage control circuit is
An excitation circuit comprising a signal detection circuit that detects the current control signal. In the excitation circuit according to claim 1 or 2 ,
The voltage control circuit is
Each time the polarity of the excitation voltage is switched, the change in the current control signal is delayed and reflected in the voltage control signal, and after a predetermined delay time has elapsed after the excitation current becomes a predetermined constant current, the An excitation circuit comprising a delay circuit configured to reduce an excitation voltage. In the excitation circuit according to any one of claims 1 to 3 ,
The excitation circuit, wherein the variable voltage power supply circuit includes a variable voltage DC/DC converter. In the excitation circuit according to any one of claims 1 to 4 ,
The excitation current control circuit is
a resistor having one end and the other end connected to the output terminal and the ground terminal of the control element, respectively, and detecting the magnitude of the excitation current;
an amplifier that outputs the current control signal to be input to the control terminal of the control element based on the magnitude of the excitation current detected by the resistor,
The control element is
An input terminal connected to the switch circuit to which the excitation current is input, an output terminal to output the excitation current, and the control terminal to which the current control signal is input, and are input to the control element. and a three-terminal control element for controlling the magnitude of the excitation current flowing between the input terminal and the output terminal according to the magnitude of the current control signal. a measuring tube for causing a fluid to flow; an exciting coil disposed near the measuring tube and receiving an excitation current to generate a magnetic field perpendicular to the flow direction of the fluid flowing inside the measuring tube; a detector having a pair of detection electrodes arranged opposite to each other in a direction orthogonal to the flow direction of the fluid flowing inside and the direction of the magnetic field;
An excitation voltage is applied to the excitation coil while switching its polarity at a predetermined excitation frequency, an excitation current is supplied to the excitation coil, and an alternating magnetic field perpendicular to the flow direction of the fluid flowing inside the measurement tube is generated. an excitation circuit configured to generate
a power supply circuit that supplies DC power to the excitation circuit;
While controlling the excitation circuit to switch the polarity of the excitation voltage applied to the excitation coil at the predetermined excitation frequency, the flow rate of the fluid flowing through the measurement tube is calculated based on the detection signal detected by the detection electrode. a control circuit for
with
An electromagnetic flowmeter, wherein the excitation circuit is the excitation circuit according to any one of claims 1 to 5 .

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