Excitation circuit of electromagnetic flowmeter

Abstract

To suppress heat generation in a component due to a difference in a series resistance value of an excitation coil.SOLUTION: An excitation circuit of an electromagnetic flowmeter includes: an excitation switching circuit 1 that switches a polarity of an excitation current supplied to an excitation coil L1; diodes D1 and D2 with cathodes connected to a voltage input terminal of the excitation switching circuit 1; a DC / DC converter 2 that supplies a low voltage VexL; a constant current circuit 3 with an input terminal connected to an output terminal of the DC / DC converter 2, and an output terminal connected to an anode of the diode D1; and a switch SW5 with a first contact terminal connected to a high voltage VexH, and a second contact terminal connected to an anode of the diode D2, which turns on during a period from an excitation period start point to a rising point of the excitation current within the excitation period, and turns off during a period from the rising point to an excitation period end point. A feedback voltage to the DC / DC converter 2 is set to an anode side voltage of the diode D1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an excitation circuit for an electromagnetic flowmeter. [Background technology]

[0002] An electromagnetic flowmeter is equipped with an excitation coil that generates a magnetic field perpendicular to the flow direction of the fluid flowing through a measuring pipe, and a pair of detection electrodes that are placed inside the measuring pipe and are arranged in a direction perpendicular to the magnetic field generated by the excitation coil.The electromagnetic flowmeter measures the flow rate of the fluid flowing through the measuring pipe by detecting the electromotive force generated between the detection electrodes while alternately switching the polarity of the excitation current passed through the excitation coil.

[0003] Generally, methods for improving the measurement stability of an electromagnetic flowmeter include increasing the excitation current to raise the resulting flow signal level S, or increasing the excitation frequency to reduce the 1 / f noise N contained in the flow signal and improve the S / N ratio (Signal to Noise Ratio).

[0004] The excitation circuit of the electromagnetic flowmeter disclosed in Patent Document 1 is shown in Fig. 5. In Fig. 5, 100 is a constant current circuit, 101 is a control circuit that outputs polarity switching signals EXD1 and EXD2 for the excitation current Iex, 102 is an excitation current rise detection circuit, L1 is an excitation coil, A1 is an operational amplifier, Q1 is a power MOS-FET, D1 and D2 are diodes, R1 and R2 are current detection resistors, and SW1 to SW5 are switches. The constant current circuit 100 is composed of an operational amplifier A1, a power MOS-FET Q1, and a current detection resistor R2.

[0005] 5, in order to speed up the rise of the excitation current Iex when switching the excitation polarity, two power supplies, a high voltage VexH and a low voltage VexL, are prepared in advance, and excitation is performed with the high voltage VexH when the excitation current Iex rises, and with the low voltage VexL during steady state excitation. This switching from the high voltage VexH to ​​the low voltage VexL reduces heat generation in the power MOS-FET Q1 of the constant current circuit 100.

[0006] Furthermore, in the excitation circuit shown in Figure 5, the constant current circuit 100 has been moved to the low-voltage power supply side, and during high-voltage excitation, voltage is supplied directly to the excitation coil L1 without passing through the constant current circuit 100. Therefore, even if the applied voltage during high-voltage excitation is made higher than conventional levels, the heat generation of the power MOS-FET Q1 does not increase. Therefore, high-voltage excitation can speed up the rise of the excitation current Iex, making it possible to increase the excitation frequency.

[0007] In the excitation circuit shown in Figure 5, during low-voltage excitation, switch SW5 is turned off, causing the excitation current Iex to flow through the power MOS-FET Q1 of the constant-current circuit 100. A constant-voltage power supply is generally used to supply the low voltage VexL. The voltage value VexL of this constant-voltage power supply is designed with a margin, taking into account the DC resistance value of the excitation coil L1 and other factors. However, excitation coils L1 come in a variety of diameters depending on the application of the electromagnetic flowmeter. Therefore, the number of turns, wire diameter, and other specifications of the excitation coil L1 must be changed for each diameter, resulting in significant differences in the DC resistance value of the excitation coil L1. Furthermore, the DC resistance value of the excitation coil L1 changes depending on the temperature of the fluid flowing through the excitation coil L1, due to the influence of heat from the fluid.

[0008] In order to supply a predetermined excitation current Iex to the excitation coil L1 with the maximum DC resistance value within the range of DC resistance values ​​that can be assumed for the excitation coil L1 as described above, it is necessary to set the low voltage VexL to a somewhat high value. In this case, when an excitation coil L1 with a low DC resistance value is connected, the constant current circuit 100 consumes excess power, which causes a problem of increased heat generation in the power MOSFET Q1.

[0009] Figure 6(A) shows the voltage distribution of the drain-source voltage Vd-s of the power MOSFET Q1 and the voltage Vcoil of the excitation coil L1 when the DC resistance of the excitation coil L1 is large, while Figure 6(B) shows the voltage distribution when the DC resistance of the excitation coil L1 is small. Note that Figures 6(A) and 6(B) ignore residual voltages such as the forward voltage drop of the diode D1 and the voltage drop due to the current detection resistors R1 and R2. Figure 6(B) shows that when the DC resistance of the excitation coil L1 is small, the voltage Vcoil of the excitation coil L1 also becomes small, and the drain-source voltage Vd-s of the power MOSFET Q1 becomes larger accordingly, resulting in increased heat generation from the power MOSFET Q1. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent No. 6985185 Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention has been made to solve the above-mentioned problems, and has an object to provide an excitation circuit for an electromagnetic flowmeter that can suppress heat generation in components due to differences in the series resistance values ​​of the excitation coils. [Means for solving the problem]

[0012] The excitation circuit of the electromagnetic flowmeter of the present invention comprises: an excitation switching circuit configured to switch the polarity of the excitation current supplied to the excitation coil of the electromagnetic flowmeter between positive and negative polarity for each positive and negative excitation period that is repeated at a constant cycle; first and second backflow prevention diodes having cathodes connected to the voltage input terminal of the excitation switching circuit; a DC / DC converter configured to supply a first voltage; a constant current circuit having an input terminal connected to the output terminal of the DC / DC converter and an output terminal connected to the anode of the first backflow prevention diode; and a switch having a first contact terminal connected to a second voltage higher than the first voltage and a second contact terminal connected to the anode of the second backflow prevention diode, the switch being configured to be on during the period from the start of the excitation period to the rising point of the excitation current and to be off during the period from the rising point to the end of the excitation period, and the feedback voltage to the DC / DC converter is the anode voltage of the first backflow prevention diode.

[0013] In addition, in one configuration example of the excitation circuit of the electromagnetic flowmeter of the present invention, the constant current circuit is characterized in that it is composed of a current detection resistor having one end connected to the output terminal of the DC / DC converter, a transistor having a drain connected to the other end of the current detection resistor and a source connected to the output terminal of the constant current circuit, and an operational amplifier having an output terminal connected to the gate of the transistor, configured to compare the voltage at the other end of the current detection resistor with a reference voltage and control the transistor based on the comparison result obtained. Furthermore, one configuration example of the excitation circuit of the electromagnetic flowmeter of the present invention further includes a rising edge detection circuit configured to detect a rising edge of the excitation current for each excitation period, and the rising edge detection circuit outputs a control signal that turns on the switch during the period from the start of the excitation period to the rising edge of the excitation current, and turns off the switch during the period from the rising edge to the end of the excitation period. [Effects of the Invention]

[0014] According to the present invention, by supplying the first voltage during low-voltage excitation from the DC / DC converter and using the anode voltage of the first reverse current prevention diode as the feedback voltage to the DC / DC converter, the first voltage can be controlled to the minimum necessary for various excitation coil series resistance values, thereby suppressing heat generation in the constant current circuit. Therefore, the present invention can increase the excitation current to improve the S / N ratio of the flow signal, and enable miniaturization of the constant current circuit and miniaturization by eliminating the heat dissipation mechanism. Furthermore, by using the anode voltage of the first reverse current prevention diode as the feedback voltage to the DC / DC converter, feedback operation is possible without being affected by the back electromotive force of the excitation coil or external noise. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a circuit diagram showing the configuration of an excitation circuit of an electromagnetic flowmeter according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing the voltage distribution of the drain-source voltage of the power MOSFET and the voltage of the excitation coil in the excitation circuit according to the embodiment of the present invention. [Figure 3] FIG. 3 shows voltage and current waveforms at various points in a conventional excitation circuit. [Figure 4] FIG. 4 is a diagram showing voltage and current waveforms at various parts of the excitation circuit according to the embodiment of the present invention. [Figure 5] Figure 5 shows the excitation circuit of a conventional electromagnetic flowmeter. [Figure 6] FIG. 6 is a diagram showing the voltage distribution of the drain-source voltage of a power MOSFET and the voltage of the excitation coil in the excitation circuit of a conventional electromagnetic flowmeter. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a circuit diagram showing the configuration of an excitation circuit of an electromagnetic flowmeter according to an embodiment of the present invention. The excitation circuit of the electromagnetic flowmeter includes an excitation switching circuit 1 that switches the polarity of the excitation current supplied to the excitation coil L1 of the electromagnetic flowmeter between positive and negative polarities for each positive / negative excitation period that is repeated at a constant cycle, backflow prevention diodes D1 and D2 whose cathodes are connected to a voltage input terminal (Vout) of the excitation switching circuit 1, a current detection resistor R1 whose one end is connected to a ground-side terminal of the excitation switching circuit 1 and whose other end is connected to ground, a DC / DC converter 2 that supplies a low voltage VexL (first voltage), a constant current circuit 3 whose input terminal is connected to the output terminal of the DC / DC converter 2 and whose output terminal is connected to the anode of the backflow prevention diode D1, and an excitation current I The excitation current rise detection circuit 5 includes a control circuit 4 that outputs polarity switching signals EXD1 and EXD2 for ex; a switch SW5 having a first contact terminal connected to a high voltage VexH (second voltage) higher than the low voltage VexL and a second contact terminal connected to the anode of a backflow prevention diode D2, which is turned on during the excitation period from the start of the excitation period to the rise of the excitation current Iex and turned off during the period from the rise to the end of the excitation period; and an excitation current rise detection circuit 5 that outputs a control signal that turns on the switch SW5 during the period from the start of the excitation period to the rise of the excitation current Iex and turns off the switch SW5 during the period from the rise to the end of the excitation period.

[0017] The excitation switching circuit 1 is composed of a switch SW1 having a control terminal to which a polarity switching signal EXD1 is input, a first contact terminal connected to one end of the excitation coil L1 of the detector, and a second contact terminal connected to the voltage input terminal (Vout) of the excitation switching circuit 1; a switch SW2 having a control terminal to which a polarity switching signal EXD2 complementary to the polarity switching signal EXD1 is input, a first contact terminal connected to one end of the excitation coil L1, and a second contact terminal connected to the ground terminal of the excitation switching circuit 1 (one end of the current detection resistor R1); a switch SW3 having a control terminal to which the polarity switching signal EXD2 is input, a first contact terminal connected to the other end of the excitation coil L1, and a second contact terminal connected to the voltage input terminal of the excitation switching circuit 1; and a switch SW4 having a control terminal to which the polarity switching signal EXD1 is input, a first contact terminal connected to the other end of the excitation coil L1, and a second contact terminal connected to the ground terminal of the excitation switching circuit 1.

[0018] The excitation switching circuit 1 has a function of switching the polarity of the excitation current Iex supplied to the excitation coil L1 between positive and negative polarity for each positive / negative excitation period that is repeated at a constant cycle. Specifically, the switches SW1 and SW4 are switches that switch the excitation current Iex to positive polarity and apply it to the excitation coil L1 by turning on when the polarity switching signal EXD1 is significant (the polarity switching signal EXD2 is insignificant). The switches SW2 and SW3 are switches that switch the excitation current Iex to negative polarity and apply it to the excitation coil L1 by turning on when the polarity switching signal EXD2 is significant (the polarity switching signal EXD1 is insignificant).

[0019] The constant current circuit 3 has a function of making the excitation current Iex supplied from the DC / DC converter 2 to the excitation coil L1 constant. As in the conventional case, the constant current circuit 3 is composed of a current detection resistor R2 having one end connected to the output terminal of the DC / DC converter 2, a power MOS-FET Q1 having a drain connected to the other end of the current detection resistor R2 and a source connected to the output terminal of the constant current circuit 3, and an operational amplifier A1 having an output terminal connected to the gate of the power MOS-FET Q1, which compares the voltage at the other end of the current detection resistor R2 with a reference voltage VREF and controls the power MOS-FET Q1 based on the comparison result.

[0020] The excitation current rise detection circuit 5 receives the terminal voltage of the current detection resistor R1 as an input and has the function of detecting the rising point of the excitation current Iex when it switches from negative to positive polarity and the rising point of the excitation current Iex when it switches from positive to negative polarity. A specific configuration example of the excitation current rise detection circuit 5 is disclosed in Patent Document 1. The excitation current rise detection circuit 5 outputs a control signal to turn on the switch SW5 from the start of the excitation period (the point at which the polarity of the excitation current Iex switches) to the rising point of the excitation current Iex.

[0021] As a result, the switch SW5 is on during the period from the start of the excitation period to the rising point of the excitation current Iex, i.e., the high-voltage excitation period, and is off during the period from the rising point to the end of the excitation period (the point at which the next polarity switches), i.e., the low-voltage excitation period. Therefore, during the high-voltage excitation period, the high voltage VexH is supplied to the excitation switching circuit 1 via the diode D2, and during the low-voltage excitation period when the switch SW5 is off, the low voltage VexL is supplied to the excitation switching circuit 1 via the constant current circuit 3 and diode D1.

[0022] In this embodiment, a DC / DC converter 2 is used to make the low voltage VexL variable rather than a constant voltage as in the conventional case. Also, since the low voltage VexL is made variable by the DC resistance value of the excitation coil L1, a configuration is adopted in which feedback is performed to the DC / DC converter 2. Furthermore, the feedback voltage VFB to the DC / DC converter 2 is the anode voltage of the backflow prevention diode D1 connected to the constant current circuit 3. The DC / DC converter 2 outputs a voltage VexL proportional to the feedback voltage VFB.

[0023] By feeding back the voltage VFB, which varies depending on the DC resistance value of the excitation coil L1, to the DC / DC converter 2, it becomes possible to control the low voltage VexL in accordance with the series resistance value of the excitation coil L1. This makes it possible to control the low voltage VexL to the minimum necessary for each of various series resistance values ​​of the excitation coil L1, thereby suppressing heat generation in the power MOS-FET Q1 of the constant current circuit 3. Therefore, in this embodiment, it is possible to increase the excitation current Iex to improve the S / N ratio of the flow rate signal, and it becomes possible to miniaturize the power MOS-FET and to reduce the size by removing the heat dissipation mechanism.

[0024] 2(A) shows the drain-source voltage Vd-s of the power MOSFET Q1 and the voltage Vcoil of the excitation coil L1 when the DC resistance of the excitation coil L1 is large in this embodiment, while FIG. 2(B) shows the voltage distribution when the DC resistance of the excitation coil L1 is small in this embodiment. Note that FIGS. 2(A) and 2(B) ignore residual voltages such as the forward voltage drop of the diode D1 and the voltage drop across the current detection resistors R1 and R2. According to FIGS. 2(A) and 2(B), when the DC resistance of the excitation coil L1 is small, the voltage Vcoil of the excitation coil L1 decreases. However, in this embodiment, the voltage VexL also decreases, so the drain-source voltage Vd-s of the power MOSFET Q1 does not increase, and heat generation by the power MOSFET Q1 can be suppressed.

[0025] Figure 3 shows the voltage waveforms of the polarity switching signals EXD1 and EXD2, the waveform of the excitation current Iex, the waveform of the cathode voltage Vout of the diodes D1 and D2, the waveform of the voltage Vcoil of the excitation coil L1, and the waveform of the drain-source voltage Vd-s of the power MOSFET Q1 in the conventional excitation circuit of Figure 5. If the DC resistance of the excitation coil L1 is R, then during low-voltage excitation, the voltage Vcoil of the excitation coil L1 is R × Iex. Therefore, when the DC resistance R is small, the voltage Vcoil is small. During low-voltage excitation, the drain-source voltage Vd-s of the power MOSFET Q1 is VexL - Vcoil. Therefore, when the voltage Vcoil is small, the drain-source voltage Vd-s increases, resulting in increased heat generation by the power MOSFET Q1. The shaded portion of the Vd-s waveform in Figure 3 contributes to heat generation.

[0026] FIG. 4 shows the voltage waveforms of the polarity switching signals EXD1 and EXD2, the waveform of the excitation current Iex, the waveform of the cathode voltage Vout of the diodes D1 and D2, the waveform of the voltage Vcoil of the excitation coil L1, and the waveform of the drain-source voltage Vd-s of the power MOSFET Q1 in this embodiment. In this embodiment, when switching to low-voltage excitation, control is performed to reduce the voltage VexL to the minimum necessary value. With this control, if the voltage Vcoil of the excitation coil L1 is small, the voltage VexL is also reduced, thereby making it possible to reduce the drain-source voltage Vd-s of the power MOSFET Q1. This makes it possible to suppress heat generation in the power MOSFET Q1.

[0027] Furthermore, in this embodiment, the feedback voltage VFB to the DC / DC converter 2 is set to the anode voltage of the backflow prevention diode D1 of the constant current circuit 3, thereby enabling feedback operation without being affected by the back electromotive force of the exciting coil L1 or external noise. [Explanation of symbols]

[0028] 1... excitation switching circuit, 2... DC / DC converter, 3... constant current circuit, 4... control circuit, 5... excitation current rise detection circuit, A1... operational amplifier, Q1... power MOS-FET, D1, D2... backflow prevention diodes, R1, R2... current detection resistors, switches SW1 to SW5.

Claims

1. an excitation switching circuit configured to switch the polarity of an excitation current supplied to an excitation coil of the electromagnetic flowmeter between positive and negative polarities for each positive and negative excitation period that is repeated at a constant cycle; first and second backflow prevention diodes, the cathodes of which are connected to the voltage input terminal of the excitation switching circuit; a DC / DC converter configured to provide a first voltage; a constant current circuit having an input terminal connected to the output terminal of the DC / DC converter and an output terminal connected to the anode of the first reverse current prevention diode; a switch having a first contact terminal connected to a second voltage higher than the first voltage and a second contact terminal connected to an anode of the second reverse current prevention diode, the switch being configured to be turned on during a period from a start point of the excitation period to a rising point of the excitation current during the excitation period, and to be turned off during a period from the rising point to an end point of the excitation period; 10. An excitation circuit for an electromagnetic flow meter, wherein a feedback voltage to the DC / DC converter is an anode voltage of the first backflow prevention diode.

2. 2. The excitation circuit of claim 1, The constant current circuit is a current detection resistor having one end connected to an output terminal of the DC / DC converter; a transistor having a drain connected to the other end of the current detection resistor and a source connected to an output terminal of the constant current circuit; an operational amplifier having an output terminal connected to the gate of the transistor, which is configured to compare the voltage at the other end of the current detection resistor with a reference voltage and control the transistor based on the comparison result.

3. 2. The excitation circuit of claim 1, a rising edge detection circuit configured to detect a rising edge of the excitation current for each excitation period; the rising edge detection circuit outputs a control signal that turns on the switch during a period from the start of the excitation period to the rising edge of the excitation current, and turns off the switch during a period from the rising edge to the end of the excitation period.

Images