A feedback controllable excitation magnetic field generator for an inductive conductivity sensor

Abstract

The invention discloses a feedback controllable excitation magnetic field generator for an inductive conductivity sensor, and relates to the technical field of electromagnetic sensors. The magnetic field generator mainly comprises a C-shaped magnetic ring (1), an excitation coil (2), a zero point and amplitude adjustment circuit (3), a forward operational amplifier circuit (4), a power amplifier circuit (5), a Hall sensor (6), a differential amplifier circuit (7), and a constant current source circuit (8). Each link is connected to form a closed-loop negative feedback magnetic field control system. Through the control of the closed-loop system, the magnetic field generated by the excitation coil (2) in the C-shaped magnetic ring (1) follows the change of the input waveform, thereby generating a circular ring-shaped controlled excitation magnetic field consistent with the input waveform, thereby solving the problem of excitation magnetic field time domain waveform distortion caused by the hysteresis and magnetic saturation effect of the magnetic ring and the excitation magnetic field size change with temperature caused by the temperature coefficient of the magnetic ring.

Description

Technical Field

[0001] The invention patent relates to the technical field of electromagnetic sensors. More specifically, the invention patent relates to a feedback-type controllable excitation magnetic field generator for an inductive conductivity sensor. Background Art

[0002] Compared with electrode conductivity sensors, inductive conductivity sensors have the advantages of corrosion resistance, no polarization effect and capacitance effect, and are therefore widely used in the measurement of liquid conductivity in industry. Inductive conductivity sensors consist of an excitation coil and an induction coil. An excitation signal is input on the excitation coil side. Based on the principle of electromagnetic induction, an AC voltage signal related to the conductivity of the measured medium can be induced on the induction coil side through the conduction and coupling of electromagnetic energy by the measured medium, thereby achieving the measurement of the conductivity of the liquid medium.

[0003] Usually, the excitation coil part and the induction coil part of the induction conductivity sensor are both magnetic core coils wound with an O-shaped magnetic ring (a radially air-gap-free annular magnetic core), and the two parts of the magnetic ring are installed in parallel and coaxially. In order to measure the conductivity by electromagnetic induction, it is necessary to apply electromagnetic energy that changes with time on the excitation coil side. The excitation method used by the existing induction conductivity sensors is to apply a sinusoidal voltage signal or a sinusoidal current signal at both ends of the excitation coil, which can generate a changing magnetic field in the excitation magnetic ring, and then generate an AC voltage signal in the induction coil. For example, CN102124321B discloses an inductive conductivity sensor, which uses two coplanar and coaxially mounted annular coils to measure conductivity; CN103412009B discloses a device and method for measuring fluid conductivity, wherein the transmitting coil and the receiving coil are two concentrically arranged annular coils, and the transmitting coil is provided with alternating current by a power supply; CN106199203A discloses an inductive conductivity sensor and a production method thereof, wherein the transmitter coil and the receiver coil, which are designed as rotationally symmetrical annular coils ("spiral coils"), are coaxially arranged one after another, and the transmitter coil is excited or flows through by an input signal, i.e., an alternating voltage; CN108445298A discloses an electric field coupled inductive conductivity sensor and a characteristic compensator thereof, wherein the excitation coil and the induction coil of the sensor are respectively tightly wound on a ferrite magnetic ring to form an annular magnetic core coil, the two annular magnetic core coils are coaxially mounted, and an alternating voltage signal is applied to both ends of the excitation coil.

[0004] However, existing inductive conductivity sensors all use O-shaped magnetic ring excitation coils and directly use a sinusoidal AC voltage source or current source as excitation, which is equivalent to the magnetic field in the magnetic ring being in an open-loop and uncontrollable state. Due to the hysteresis and magnetic saturation effects of the magnetic ring, under the action of the sinusoidal excitation voltage or current, the annular magnetic field source in the magnetic ring often undergoes time domain waveform distortion and becomes a non-sinusoidal wave, thereby causing the voltage waveform ultimately output by the induction coil to have greater distortion, generating high-order harmonics, and affecting the linearity and accuracy of the measurement. In addition, due to the temperature coefficient of the excitation magnetic ring, under the action of a constant excitation voltage or current, the magnitude of the magnetic field generated in the magnetic ring will vary greatly with changes in temperature, and the stability of the excitation magnetic field is poor, thereby causing the sensor measurement to have a large temperature coefficient.

[0005] In order to overcome the above problems, the process of the excitation coil generating an excitation magnetic field in the magnetic ring needs to be closed-loop controlled. To perform closed-loop control, the structural design of the magnetic ring, the feedback and control process of the magnetic field are relatively complex, which makes it very difficult to implement. To this end, the patent of this invention provides an inductive conductivity sensor feedback controllable excitation magnetic field generator.

[0006] Invention patent content

[0007] The technical problem to be solved by the patent of this invention is to directly use sinusoidal AC voltage or current excitation, that is, when the excitation magnetic field is uncontrollable, the time domain waveform distortion of the excitation magnetic field caused by the hysteresis and magnetic saturation effect of the magnetic ring and the change of the excitation magnetic field size with temperature due to the temperature coefficient of the magnetic ring are solved, thereby improving the measurement performance of the sensor.

[0008] In order to solve the technical problem, the present invention provides a feedback controllable excitation magnetic field generator for an inductive conductivity sensor, which is mainly composed of a C-shaped magnetic ring, an excitation coil, a zero point and amplitude adjustment circuit, a forward operational amplifier circuit, a power amplifier circuit, a Hall sensor, a differential amplifier circuit, and a constant current source circuit. The excitation coil is wound on the C-shaped magnetic ring, the waveform input end is connected to the input end of the zero point and amplitude adjustment circuit, the output end of the zero point and amplitude adjustment circuit is connected to the positive phase input end of the forward operational amplifier circuit, the output end of the forward operational amplifier circuit is connected to the input end of the power amplifier circuit, and the output end of the power amplifier circuit and the system reference ground are connected. The ends are respectively connected to the two ends of the excitation coil, the Hall sensor is installed in the air gap of the C-shaped magnetic ring, the current output end of the constant current source circuit is connected to the excitation current input end of the Hall sensor, and the voltage output end of the Hall sensor is connected to the differential input end of the differential amplifier circuit, the output end of the differential amplifier circuit is connected to the inverting input end of the forward operational amplifier circuit, and each link is connected to form a closed-loop negative feedback magnetic field control system. Through the control action of the closed-loop system, the magnetic field generated by the excitation coil in the C-shaped magnetic ring follows the change of the input waveform, thereby generating a circular ring-shaped controlled excitation magnetic field consistent with the input waveform, which can provide a controllable excitation magnetic field source for the inductive conductivity sensor.

[0009] The C-shaped magnetic ring is an open-gap magnetic ring made of soft magnetic material with a relative magnetic permeability greater than 1000. The magnetic ring has a 2mm air gap in the radial direction. The excitation coil is evenly wound on the C-shaped magnetic ring in the form of a spiral ring.

[0010] The zero point and amplitude adjustment circuit is composed of an inverting amplifier and an inverting adder in cascade connection, and the inverting amplifier and the inverting adder are both composed of operational amplifiers, the first stage is an inverting amplifier, and the gain is set to 1, the second stage is an inverting adder, the output end of the inverting amplifier is connected to the first input end of the inverting adder, a first potentiometer is connected between a positive power supply and a negative power supply as a zero point adjustment potentiometer, a center tap of the zero point adjustment potentiometer is connected to the second input end of the inverting adder, and a feedback channel of the inverting adder is connected to the second potentiometer as an amplitude adjustment potentiometer.

[0011] The forward operational amplifier circuit adopts an open-loop gain greater than 1×10 6 The integrated operational amplifier makes the entire closed-loop negative feedback magnetic field control system a closed-loop deep negative feedback magnetic field control system.

[0012] The power amplifier circuit is a Class AB complementary symmetrical power amplifier circuit mainly composed of complementary NPN and PNP power transistors, and a diode connected in series in the same direction to overcome crossover distortion is connected between the bases of the two transistors. The input signal of the power amplifier circuit is connected from the middle point of the series connection of the two diodes, and its output signal is connected from the middle point of the series connection of the emitters of the two complementary symmetrical transistors.

[0013] The Hall sensor is a linear Hall sensor in TO-92 package, and the sheet-shaped Hall sensor is installed in parallel in the center of the air gap of the C-shaped magnetic ring.

[0014] The differential amplifier circuit is composed of an instrument amplifier with a differential input impedance greater than 1MΩ. The two voltage output terminals of the Hall sensor are respectively connected to the two differential input terminals of the instrument amplifier. A magnetic field feedback coefficient adjustment potentiometer and a magnetic field feedback zero-point residual voltage elimination potentiometer are connected in the circuit.

[0015] The constant current source circuit is composed of a current series negative feedback circuit mainly composed of a voltage reference chip, an operational amplifier, a current amplifying transistor, and a feedback resistor. The constant voltage output by the voltage reference chip is input to the positive input terminal of the operational amplifier, the output terminal of the operational amplifier is connected to the base of the current amplifying transistor, the collector of the current amplifying transistor is connected to the positive power supply, the emitter of the current amplifying transistor is connected to the excitation current inflow terminal of the Hall sensor, the excitation current outflow terminal of the Hall sensor is connected to the inverting input terminal of the operational amplifier, and the feedback resistor is connected between the inverting input terminal of the operational amplifier and the negative power supply terminal.

[0016] The present invention patent includes at least the following beneficial effects:

[0017] (1) The magnetic field generated by the excitation magnetic ring is controlled by a closed-loop negative feedback control method. The excitation magnetic field is in a controllable state, and the magnetic field waveform can completely follow the changes of the input waveform. The closed-loop regulation effect prevents the magnetic field time domain waveform from being distorted due to the hysteresis and magnetic saturation effects of the magnetic ring, ensuring that the excitation magnetic field waveform is a complete sine wave and avoiding the generation of high-order harmonics.

[0018] (2) The closed-loop negative feedback control of the magnetic field has the function of resisting various interferences, especially it can overcome the influence of temperature on the size of the excitation magnetic field, so that the amplitude of the excitation magnetic field waveform does not change accordingly with the temperature change, thereby improving the stability of the excitation magnetic field.

[0019] (3) It provides a technical idea and technical means to improve the performance of the sensor as a whole, which is beneficial to improve the measurement linearity and accuracy and reduce the temperature coefficient.

[0020] Other advantages, objectives and features of the present invention will be reflected in part through the following description, and in part will be understood by those skilled in the art through research and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1It is a schematic diagram of the composition principle of the feedback controllable excitation magnetic field generator of the inductive conductivity sensor described in the present invention. The elliptical dotted line in the magnetic ring in the figure is the magnetic flux line of the controllable excitation magnetic field generated by the generator;

[0022] Figure 2 This is a specific implementation circuit diagram of the feedback-type controllable excitation magnetic field generator for the inductive conductivity sensor described in the patent of this invention. DETAILED DESCRIPTION

[0023] The present invention is further described in detail below in conjunction with the accompanying drawings so that those skilled in the art can implement it with reference to the description. It should be understood that the terms such as "having", "including" and "comprising" used herein do not exclude the existence or addition of one or more other elements or their combinations.

[0024] In order to solve the technical problem, the present invention provides an inductive conductivity sensor feedback controllable excitation magnetic field generator, such as Figure 1 As shown, the magnetic field generator is mainly composed of a C-shaped magnetic ring 1, an excitation coil 2, a zero point and amplitude adjustment circuit 3, a forward operational amplifier circuit 4, a power amplifier circuit 5, a Hall sensor 6, a differential amplifier circuit 7, and a constant current source circuit 8. Among them, the excitation coil 2 is wound on the C-shaped magnetic ring 1, the waveform input end is connected to the input end of the zero point and amplitude adjustment circuit 3, the output end of the zero point and amplitude adjustment circuit 3 is connected to the non-phase input end of the forward operational amplifier circuit 4, the output end of the forward operational amplifier circuit 4 is connected to the input end of the power amplifier circuit 5, the output end of the power amplifier circuit 5 and the system reference ground end are respectively connected to the two ends of the excitation coil 2, and the Hall sensor The device 6 is installed in the air gap of the C-shaped magnetic ring 1, the current output end of the constant current source circuit 8 is connected to the excitation current input end of the Hall sensor 6, and the voltage output end of the Hall sensor 6 is connected to the differential input end of the differential amplifier circuit 7, and the output end of the differential amplifier circuit 7 is connected to the inverting input end of the forward operational amplifier circuit 4. All links are connected to form a closed-loop negative feedback magnetic field control system. Through the control action of the closed-loop system, the magnetic field generated by the excitation coil 2 in the C-shaped magnetic ring 1 follows the change of the input waveform, thereby generating a circular ring-shaped controlled excitation magnetic field consistent with the input waveform, which can provide a controllable excitation magnetic field source for the inductive conductivity sensor.

[0025] like Figure 1 As shown, the C-shaped magnetic ring 1 is an open air gap magnetic ring made of laminated silicon steel sheets with a relative magnetic permeability greater than 1000, and the magnetic ring has a 2mm air gap in the radial direction. The excitation coil 2 is evenly wound on the C-shaped magnetic ring 1 in the form of a spiral ring.

[0026] like Figure 2As shown, the zero point and amplitude adjustment circuit 3 is composed of an inverting amplifier and an inverting adder cascaded, and the inverting amplifier and the inverting adder are both composed of an operational amplifier LF353. The first stage is an inverting amplifier, and its input resistance R1 = 10kΩ, the feedback resistance R2 = 10kΩ, so the gain is set to 1, the second stage is an inverting adder, the output end of the inverting amplifier is connected to the first input end of the inverting adder, and the positive power supply V CC and negative power supply V SS A potentiometer R3=1kΩ is connected between the two input terminals as a zero adjustment potentiometer, and the center tap of the zero adjustment potentiometer R3 is connected to the second input terminal of the inverting adder. The input resistances of the two input terminals of the inverting adder are R4=10kΩ and R5=10kΩ respectively. The feedback channel of the inverting adder is connected to a potentiometer R6=50kΩ as an amplitude adjustment potentiometer. Therefore, the gain of the inverting adder can be adjusted between 0 and 5, which is convenient for adjusting the amplitude of the input waveform.

[0027] like Figure 2 As shown, the forward operational amplifier circuit 4 adopts an open-loop gain greater than 1×10 6 The integrated operational amplifier OP27 is composed of an integrated operational amplifier. Since the open-loop gain of the amplifier used is extremely large, it is far greater than the feedback coefficient of the magnetic field feedback channel, making the entire closed-loop negative feedback magnetic field control system a closed-loop deep negative feedback magnetic field control system.

[0028] like Figure 2 As shown, the power amplifier circuit 5 is a class A and B complementary symmetrical power amplifier circuit mainly composed of a complementary NPN power transistor TIP31 and a PNP power transistor TIP32, and diodes D1 and D2 are connected in series in the same direction between the bases of the two power transistors Q1 and Q2 to overcome crossover distortion, and the input signal of the power amplifier circuit 5 is connected from the middle point of the series connection of the two diodes, and R7=2kΩ, R8=2kΩ current limiting resistors are connected in the input loop, and the output signal of the power amplifier circuit 5 is connected from the middle point of the series connection of the emitters of the two complementary symmetrical transistors.

[0029] like Figure 1 As shown, the Hall sensor 6 adopts a linear Hall sensor HG-302C in TO-92 package. The sheet-shaped Hall sensor 6 is installed in parallel in the center of the air gap of the C-shaped magnetic ring 1. The circular excitation magnetic field B generated by the excitation current in the C-shaped magnetic ring 1 vertically passes through the upper and lower surfaces of the Hall sensor 6, and a constant current I is passed through the excitation current input end of the Hall sensor 6. H , then the output voltage U of the Hall sensor 6 H It can be expressed as

[0030] U H =U H+ -U H- =kIH B (1)

[0031] Where, k is the Hall coefficient of the Hall sensor 6, U H+ , U H- are the potentials of the positive phase output terminal and the negative phase output terminal of the Hall sensor 6, I H is the constant current passed into the excitation current input terminal of the Hall sensor 6 , and B is the magnetic induction intensity of the annular excitation magnetic field generated in the C-shaped magnetic ring 1 .

[0032] like Figure 2 As shown, the differential amplifier circuit 7 is composed of an instrument amplifier AD627 with a differential input impedance greater than 1MΩ. The two voltage output terminals of the Hall sensor 6 are respectively connected to the two differential input terminals of the instrument amplifier, and a gain adjustment potentiometer R9 is connected between the 1st and 8th pins of the AD627. The output bias terminal of the AD627, that is, the 5th pin, is connected to the potentiometer R 10 = 10kΩ center tap output voltage U z , then the output voltage U of the differential amplifier circuit 7 F It can be expressed as

[0033] U F =U H ×(5+200 / R9)+U z (2)

[0034] Where U F It is the magnetic field feedback voltage signal of the closed-loop magnetic field control system. R9 is the resistance value of the gain adjustment potentiometer, and its value range is 0~50kΩ, which is used to adjust the feedback coefficient of the magnetic field feedback channel. 10 Center tap output voltage U z To offset the zero-point residual voltage of the Hall sensor 6, R9 is used as a magnetic field feedback coefficient adjustment potentiometer. 10 As a magnetic field feedback zero point residual voltage elimination potentiometer.

[0035] like Figure 2 As shown, the constant current source circuit 8 mainly comprises a voltage reference chip TL431, an operational amplifier OP07, a current amplifier transistor 9014, a feedback resistor R 14 =250Ω is composed of a current series negative feedback circuit, and at the positive power supply terminal V CC There is R in series between the output terminal of TL431 11 =2kΩ voltage divider resistor, TL431 has a 2.5V reference voltage source inside, and the output of TL431 and the negative power supply terminal V SS There is R in series 12 =10kΩ, R 13 = 10kΩ two resistors, R 12, R 13 The middle point of the series connection is connected to the feedback input terminal of TL431. The feedback factor of TL431 is set to 0.5. Then the output of TL431 is relative to the negative power supply terminal V SS The 5V constant voltage output by the voltage reference chip TL431 is input to the non-inverting input terminal of the operational amplifier OP07. The output terminal of the operational amplifier OP07 is connected to the base of the current amplifier transistor 9014. The collector of the current amplifier transistor 9014 is connected to the positive power supply V CC The emitter of the current amplifier transistor 9014 is connected to the excitation current inflow end of the Hall sensor 6, and the excitation current outflow end of the Hall sensor 6 is connected to the inverting input end of the operational amplifier OP07. The feedback resistor R 14 Connect to the inverting input terminal of the operational amplifier OP07 and the negative power supply terminal V SS Therefore, according to the steady-state relationship between the input voltage and the output current of the current series negative feedback, the excitation current input to the Hall sensor 6 is I H =5V / 250Ω=20mA.

[0036] In addition, the embodiment adopts a positive and negative symmetrical dual power supply V CC With V SS Power the system, V CC The value range is 5~12V, V SS The value range is -5 to -12V.

[0037] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and the embodiments. It can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily realized. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to the specific details and the illustrations shown and described here.

Claims

1. A feedback controllable excitation magnetic field generator for an inductive conductivity sensor, characterized in that: The magnetic field generator mainly comprises a C-shaped magnetic ring (1), an excitation coil (2), a zero point and amplitude adjustment circuit (3), a forward operational amplifier circuit (4), a power amplifier circuit (5), a Hall sensor (6), a differential amplifier circuit (7), and a constant current source circuit (8), wherein the excitation coil (2) is wound on the C-shaped magnetic ring (1), the waveform input end is connected to the input end of the zero point and amplitude adjustment circuit (3), the output end of the zero point and amplitude adjustment circuit (3) is connected to the positive phase input end of the forward operational amplifier circuit (4), the output end of the forward operational amplifier circuit (4) is connected to the input end of the power amplifier circuit (5), and the output end of the power amplifier circuit (5) and the system reference ground end are respectively connected to both ends of the excitation coil (2), the Hall sensor (6) is installed in the air gap of the C-shaped magnetic ring (1), the current output end of the constant current source circuit (8) is connected to the excitation current input end of the Hall sensor (6), and the voltage output end of the Hall sensor (6) is connected to the differential input end of the differential amplifier circuit (7), and the output end of the differential amplifier circuit (7) is connected to the inverting input end of the forward operational amplifier circuit (4), and each link is connected to form a closed-loop negative feedback magnetic field control system. Through the control action of the closed-loop system, the magnetic field generated by the excitation coil (2) in the C-shaped magnetic ring (1) follows the change of the input waveform, thereby generating a circular ring-shaped controlled excitation magnetic field consistent with the input waveform; The C-shaped magnetic ring (1) is an open-gap magnetic ring made of a soft magnetic material with a relative magnetic permeability greater than 1000, and the magnetic ring has a 2 mm air gap in the radial direction. The excitation coil (2) is evenly wound on the C-shaped magnetic ring (1) in the form of a spiral ring. The zero point and amplitude adjustment circuit (3) is composed of an inverting amplifier and an inverting adder connected in cascade, and the inverting amplifier and the inverting adder are both composed of operational amplifiers, the first stage is an inverting amplifier, and the gain is set to 1, the second stage is an inverting adder, the output end of the inverting amplifier is connected to the first input end of the inverting adder, a first potentiometer is connected between the positive power supply and the negative power supply as a zero point adjustment potentiometer, the center tap of the zero point adjustment potentiometer is connected to the second input end of the inverting adder, and the feedback channel of the inverting adder is connected to the second potentiometer as an amplitude adjustment potentiometer.

2. The feedback controllable excitation magnetic field generator for an inductive conductivity sensor according to claim 1, characterized in that: The forward operational amplifier circuit (4) adopts an open-loop amplification factor greater than 1×10 6 The integrated operational amplifier makes the entire closed-loop negative feedback magnetic field control system a closed-loop deep negative feedback magnetic field control system.

3. The feedback controllable excitation magnetic field generator for an inductive conductivity sensor according to claim 1, characterized in that: The power amplifier circuit (5) is a class AB complementary symmetrical power amplifier circuit mainly composed of complementary NPN and PNP power transistors, and a diode connected in series in the same direction to overcome crossover distortion is connected between the bases of the two transistors, and the input signal of the power amplifier circuit (5) is connected from the middle point of the series connection of the two diodes, and its output signal is connected from the middle point of the series connection of the emitters of the two complementary symmetrical transistors.

4. The feedback controllable excitation magnetic field generator for an inductive conductivity sensor according to claim 1, characterized in that: The Hall sensor (6) is a linear Hall sensor in a TO-92 package. The sheet-shaped Hall sensor (6) is installed in parallel at the center of the air gap of the C-shaped magnetic ring (1).

5. The feedback controllable excitation magnetic field generator for an inductive conductivity sensor according to claim 1, characterized in that: The differential amplifier circuit (7) is composed of an instrument amplifier with a differential input impedance greater than 1 MΩ. The two voltage output terminals of the Hall sensor (6) are respectively connected to the two differential input terminals of the instrument amplifier. A magnetic field feedback coefficient adjustment potentiometer and a magnetic field feedback zero-point residual voltage elimination potentiometer are connected in the circuit.

6. The feedback controllable excitation magnetic field generator for an inductive conductivity sensor according to claim 1, characterized in that: The constant current source circuit (8) is composed of a current series negative feedback circuit mainly composed of a voltage reference chip, an operational amplifier, a current amplifying transistor, and a feedback resistor. The constant voltage output by the voltage reference chip is input to the positive input terminal of the operational amplifier, the output terminal of the operational amplifier is connected to the base of the current amplifying transistor, the collector of the current amplifying transistor is connected to the positive power supply, the emitter of the current amplifying transistor is connected to the excitation current inflow terminal of the Hall sensor (6), the excitation current outflow terminal of the Hall sensor (6) is connected to the inverting input terminal of the operational amplifier, and the feedback resistor is connected between the inverting input terminal of the operational amplifier and the negative power supply terminal.

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