A conductivity sensor

Abstract

The present invention relates to a conductivity sensor, which includes a digital drive circuit, a first electrode, a second electrode, a temperature acquisition circuit, and an operation controller. By setting the digital drive circuit to generate an oscillation signal according to the trigger signal of the operation controller, the oscillation signal is introduced into the conductive liquid in the conductivity cell through the first electrode. At the same time, a temperature acquisition circuit is set to collect the temperature signal of the conductive liquid in the conductivity cell. The operation controller obtains the oscillation signal after passing through the conductivity cell through the second electrode, and the operation controller performs operations on the oscillation signal and the oscillation signal after passing through the conductivity cell to obtain the initial conductivity of the conductive liquid, and performs temperature compensation on the initial conductivity according to the temperature signal to obtain the conductivity of the conductive liquid. The present invention measures the conductivity of the conductive liquid by generating an oscillation signal through the digital drive circuit, and at the same time combines temperature compensation to obtain the final conductivity result, making the measurement result more accurate.

Description

Technical Field

[0001] The present invention belongs to the field of sensors, and particularly relates to a conductivity sensor. Background Art

[0002] Conductivity is the reciprocal of resistivity and reflects the conductivity of an object. Since the conductivity of water is related to the ionic strength in water, in water quality monitoring, conductivity is often used to reflect the salt content in the water to be measured.

[0003] However, the existing conductivity sensors have low measurement accuracy and inaccurate measurement results. Summary of the Invention

[0004] The present invention provides a conductivity sensor to solve the technical problem of inaccurate conductivity measurement in the prior art.

[0005] A conductivity sensor for measuring the conductivity of a conductance liquid includes a digital drive circuit, a first electrode, a second electrode, a temperature acquisition circuit, and an operation controller;

[0006] The digital drive circuit is used to generate an oscillation signal according to the trigger signal of the operation controller, and the oscillation signal is introduced into the conductance liquid through the first electrode;

[0007] The temperature acquisition circuit is used to acquire the temperature signal of the conductance liquid in the conductance cell;

[0008] The operation controller obtains the oscillation signal after passing through the conductance liquid through the second electrode. The operation controller is used to perform operations on the oscillation signal and the oscillation signal after passing through the conductance liquid to obtain the initial conductivity of the conductance liquid, and perform temperature compensation on the initial conductivity according to the temperature signal to obtain the conductivity of the conductance liquid.

[0009] In an embodiment of the present invention, the sensor further includes an analog-to-digital converter, which is used to convert the oscillation signal into a first digital signal, the oscillation signal after passing through the conductance liquid into a second digital signal, and the temperature signal into a third digital signal;

[0010] The operation controller performs operations on the first digital signal and the second digital signal to obtain the initial conductivity of the conductance liquid, and the operation controller performs temperature compensation on the initial conductivity according to the third digital signal to obtain the conductivity of the conductance liquid.

[0011] In an embodiment of the present invention, the digital drive circuit includes a signal generator, a clock circuit, and an inverting proportional amplifier circuit; the digital drive circuit includes a signal generator, a clock circuit, a first operational amplifier, and a second operational amplifier;

[0012] The output end of the clock circuit is connected to the clock signal end of the signal generator. The data end of the signal generator is connected to the arithmetic controller, and the signal generator obtains the trigger signal through the data end. The signal output end of the signal generator is connected to the input end of the inverting proportional amplifier circuit, and the output end of the inverting proportional amplifier circuit is connected to the first electrode.

[0013] In an embodiment of the present invention, the inverting proportional amplifier circuit includes a first operational amplifier and a second operational amplifier;

[0014] The signal output end of the signal generator is connected to the inverting input end of the first operational amplifier, and the non-inverting input end of the first operational amplifier is grounded through a first resistor in series. The output end of the first operational amplifier is connected to the inverting input end of the second operational amplifier through a second resistor. The non-inverting input end of the second operational amplifier is grounded through a third resistor in series, and the output end of the second operational amplifier is connected to the first electrode;

[0015] A first negative feedback circuit is provided between the inverting input end and the output end of the first operational amplifier. The first negative feedback circuit includes an adjustment end for adjusting the feedback coefficient, and the adjustment end of the first negative feedback circuit is connected to the arithmetic controller. A second negative feedback circuit is provided between the inverting input end and the output end of the second operational amplifier.

[0016] In an embodiment of the present invention, the first negative feedback circuit includes a fourth resistor, a fifth resistor, a sixth resistor, a first capacitor, and an analog switch. The analog switch includes a first source terminal, a first drain terminal, a first control terminal, a second source terminal, a second drain terminal, and a second control terminal. The first control terminal is used to control the on-off between the first source terminal and the first drain terminal, and the second control terminal is used to control the on-off between the second source terminal and the second drain terminal;

[0017] One end of the fourth resistor and one end of the first capacitor are connected to the inverting input end of the first operational amplifier, and the other end of the fourth resistor and the other end of the first capacitor are connected to the output end of the first operational amplifier;

[0018] One end of the fifth resistor is connected to the inverting input end of the first operational amplifier, the other end of the fifth resistor is connected to the first source terminal, and the first drain terminal is connected to the output end of the first operational amplifier. The first control terminal is connected to the arithmetic controller;

[0019] One end of the sixth resistor is connected to the inverting input terminal of the first operational amplifier, the other end of the sixth resistor is connected to the second source terminal, and the second drain terminal is connected to the output terminal of the first operational amplifier; the second control terminal is connected to the operation controller.

[0020] In an embodiment of the present invention, the second negative feedback circuit includes a seventh resistor and a second capacitor;

[0021] One end of the seventh resistor and one end of the second capacitor are connected to the inverting input terminal of the second operational amplifier, and the other end of the seventh resistor and the other end of the second capacitor are connected to the output terminal of the second operational amplifier.

[0022] In an embodiment of the present invention, the analog-to-digital converter includes a first input terminal, a second input terminal, a third input terminal, and a data output terminal;

[0023] The first input terminal of the analog-to-digital converter is connected to the output terminal of the digital drive circuit, the second input terminal of the analog-to-digital converter is connected to the second electrode, the third input terminal of the analog-to-digital converter is connected to the output terminal of the temperature acquisition circuit, and the data output terminal of the analog-to-digital converter is connected to the operation controller.

[0024] In an embodiment of the present invention, the mathematical expression of the initial conductivity k0 is:

[0025]

[0026] where θ is the electrode constant of the first electrode and the second electrode, and R is the equivalent impedance of the conductive solution in the conductivity cell;

[0027] where, R i is the input resistance of the inverting proportional amplification circuit, V i is the maximum amplitude of the oscillation signal, V o is the maximum amplitude of the oscillation signal after passing through the conductivity cell.

[0028] In an embodiment of the present invention, the operation controller performs a fast Fourier transform on the oscillation signal and the oscillation signal after passing through the conductivity cell to obtain the maximum amplitude V i of the oscillation signal, and the maximum amplitude V o of the oscillation signal after passing through the conductive solution.

[0029] In an embodiment of the present invention, the mathematical expression of the conductivity k is:

[0030]

[0031] where t is the value of the temperature signal, is the compensation coefficient.

[0032] The present invention provides a conductivity sensor, which has the following beneficial effects: by setting a digital drive circuit to generate an oscillation signal according to the trigger signal of the operation controller, the oscillation signal is introduced into the conductive liquid in the conductivity cell through the first electrode; at the same time, a temperature acquisition circuit is set to acquire the temperature signal of the conductive liquid in the conductivity cell; the operation controller obtains the oscillation signal after passing through the conductivity cell through the second electrode, and the operation controller performs operations on the oscillation signal and the oscillation signal after passing through the conductivity cell to obtain the initial conductivity of the conductive liquid, and performs temperature compensation on the initial conductivity according to the temperature signal to obtain the conductivity of the conductive liquid. The present invention measures the conductivity of the conductive liquid by generating an oscillation signal through a digital drive circuit, and at the same time combines temperature compensation to obtain the final conductivity result, making the measurement result more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a schematic structural diagram of an exemplary conductivity sensor in an embodiment of the present invention;

[0034] Figure 2 is a circuit diagram of an exemplary digital drive circuit in an embodiment of the present invention;

[0035] Figure 3 is a circuit diagram of an exemplary analog-to-digital converter in an embodiment of the present invention;

[0036] Figure 4 is a circuit diagram of an exemplary operation controller in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0037] The following illustrates the embodiments of the present invention through specific specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0038] It should be noted that the diagrams provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner. Therefore, only the components related to the present invention are shown in the diagrams, rather than being drawn according to the number, shape, and size of the components in actual implementation. The types, quantities, and proportions of the components in actual implementation can be arbitrarily changed, and the component layout type may also be more complex.

[0039] In the following description, numerous details are explored to provide a more thorough explanation of the embodiments of the present invention. However, it will be apparent to those skilled in the art that the embodiments of the present invention can be implemented without these specific details.

[0040] First of all, it should be noted that according to electrochemical knowledge, if a DC voltage is applied between the conductance electrodes, during the electrolysis process of the electrolyte solution, oxidation and reduction reactions will occur at the anode and cathode respectively. During this process, electrolysis products will be generated. At the same time, the solution and the electrodes form an electric potential opposite to the polarity of the applied electromotive force, which will reduce the current flowing between the electrodes. Therefore, the equivalent resistance of the aqueous solution will increase. This phenomenon is called the chemical polarization effect of the solution. During the electrolysis process, the ion concentration in the solution near the electrodes will quickly decrease. Since the rate of ion generation in the solution is smaller than the rate of ion loss due to electron exchange, it will lead to an imbalance between the electrode and the contact surface of the solution. This kind of polarization effect is called concentration polarization. Therefore, if a DC signal is selected as the driving signal for measuring the electrolyte solution, the polarization is severe when a DC voltage is applied across the electrodes, seriously affecting the measurement accuracy. Therefore, DC driving cannot be used for conductivity measurement, but AC driving (square wave or sine wave driving) should be used. The commonly used electrode measurement method generally measures the solution resistance between two parallel plates, and then converts the measured resistance value into the conductivity value of the solution. Since the ions in the solution will move between the plates of the measurement electrodes under the action of the voltage, the movement of the ions will generate a polarization potential opposite to the external applied electric potential field, which will hinder the migration of ions in the electric field, equivalent to adding an externally applied resistance with an increasing resistance value to the equivalent resistance of the aqueous solution. The error caused by the polarization effect in the case of AC can be expressed by the formula δ = E 2 / ωR 2It is expressed as follows. In this formula, δ is the measurement deviation, E is the polarization back electromotive force, ω is the driving signal frequency, and R is the resistance of the measured solution. It can be seen from the formula that since the concentration of the measured solution cannot be changed, a certain solution is a fixed value at a certain temperature. The polarization back electromotive force is proportional to the current density, so the polarization back electromotive force can be reduced by reducing the amplitude of the driving signal voltage. A relatively effective way to reduce the error is to change the magnitude of the driving frequency and at the same time reduce the current density flowing through the surface of the measurement electrode. However, if the driving signal frequency is increased excessively, the capacitance effect in the electrode will be enhanced, which will in turn cause a large measurement error value. Therefore, the driving signal frequency needs to be fixed within a suitable range. Usually, the driving signal frequency is selected to be 1000 - 2000 Hz, and when the frequency is high, it is 3000 - 4000 Hz. At the same time, considering that once the measured conductivity increases, its resistance R will become smaller, and the current density in the solution will increase, which will also lead to the enhancement of the polarization effect. This has also become the main reason for the increase in measurement error. However, this situation has a relatively small impact on the measurement error of aqueous solutions with low conductivity. If this error needs to be eliminated, only by reducing the voltage amplitude of the driving signal can it be achieved.

[0041] The measurement principle of conductivity is to place two parallel plates (or cylindrical electrodes) with a fixed distance L between them into the measured solution, and apply a certain potential (i.e., the driving signal; in order to avoid solution electrolysis and polarization effect, it is usually a sine wave or square wave voltage with a frequency of 1 - 3 kHz) at both ends of the plates, and then measure the current flowing between the plates. According to Ohm's law, conductivity is determined by voltage and current. By measuring the current between the plates, the conductivity of the water body to be measured can be measured. Since temperature has a great influence on the conductivity of water, conductivity meters measure the temperature value in the water at the same time to perform temperature compensation on the conductivity.

[0042] As Figure 1 shown, a conductivity sensor provided in the present invention is used to measure the conductivity of the conductance liquid in the conductance cell, and includes a digital driving circuit, a first electrode, a second electrode, a temperature acquisition circuit, and an operation controller;

[0043] The digital driving circuit is used to generate an oscillation signal according to the trigger signal of the operation controller, and the oscillation signal is introduced into the conductance liquid in the conductance cell through the first electrode;

[0044] The temperature acquisition circuit is used to acquire the temperature signal of the conductance liquid in the conductance cell;

[0045] The operation controller obtains the oscillation signal after passing through the conductance cell through the second electrode. The operation controller is used to perform operations on the oscillation signal and the oscillation signal after passing through the conductance cell to obtain the initial conductivity of the conductance liquid, and perform temperature compensation on the initial conductivity according to the temperature signal to obtain the conductivity of the conductance liquid.

[0046] In this embodiment, the conductivity cell is a container for holding the conductive liquid. When measuring the conductivity of the conductive liquid, the conductive liquid is added to the conductivity cell. In addition, the first electrode and the second electrode are integrated in the conductivity cell.

[0047] The operation controller performs a fast Fourier transform (FFT) on the oscillation signal and the oscillation signal after passing through the conductivity cell, so as to obtain the amplitude of the oscillation signal and the amplitude of the oscillation signal after passing through the conductivity cell. Then, by performing operations on the amplitude of the oscillation signal and the amplitude of the oscillation signal after passing through the conductivity cell, the initial conductivity is obtained.

[0048] In this embodiment, the Fourier transform function is called by the operation controller. The operation controller uses a single-chip microcomputer with an ARM architecture, model number STM32F103RET6, which is a 32-bit flash microcontroller chip based on the processor core, with high maturity and strong processing capabilities. The external crystal oscillator is selected as 8 MHz, and serial ports, SPI and other functions are used for internal resource configuration. To improve reliability, protection circuits are added to some I / O ports. At the same time, an external reset chip is added to enable the MCU to recover from an abnormal state and prevent the instrument from crashing, greatly enhancing the product reliability. The historical data is stored in an external EEPROM. The chip uses an SPI bus interface and has built-in write protection functions such as a switch data protection circuit, a hardware write protection pin WP, and a write enable instruction protection, which can effectively protect the data from being accidentally written.

[0049] The fast Fourier transform (FFT) is a method for analyzing signals. It can analyze the components of a signal and also synthesize a signal using these components. Many waveforms can be used as components of a signal, such as sine waves, square waves, sawtooth waves, etc. Here, the square wave measurement signal is associated with the conductivity measurement value through the fast Fourier transform (FFT) algorithm, so as to achieve the purpose of effective measurement.

[0050] In this article, the operation controller's built-in FFT function is used to process the signal. The function arm_rfft_fast_f32 is called to perform a fast Fourier transform on an N-point real sequence.

[0051] The function is defined as follows

[0052] void arm_rfft_fast_f32(arm_rfft_fast_instance_f32*S,float32_t*p,

[0053] float32_t*pOut,uint8_t ifftFlag)

[0054] Parameter definition:

[0055] [in]*S points to an arm_rfft_fast_instance_f32 structure.

[0056] [in]*p points to the input buffer.

[0057] [in]*pOut points to the output buffer.

[0058] [in]ifftFlag RFFT if flag is 0,RIFFT if flag is 1

[0059] The following is the specific code for running the function arm_rfft_fast_f32 to calculate the amplitude-frequency response.

[0060] float arm_rfft_fast_f32_app(float*adc_buf)

[0061] {

[0062] uint8_t ifftFlag = 0; / / 0 for forward transform

[0063] arm_rfft_fast_instance_f32 s; / / Initialize the parameters in structure s

[0064] arm_rfft_fast_init_f32(&s, FFTSIZE); / / Fast Fourier Transform

[0065] arm_rfft_fast_f32(&s, adc_buf, fft_out_array, ifftFlag); / / Solve for the modulus

[0066] arm_cmplx_mag_f32(fft_out_array, fft_out_modulus_array, FFTSIZE);

[0067] }

[0068] In an embodiment of the present invention, the sensor further includes an analog-to-digital converter, which is used to convert the oscillation signal into a first digital signal, the oscillation signal after passing through the conductivity cell into a second digital signal, and the temperature signal into a third digital signal;

[0069] The arithmetic controller performs arithmetic operations on the first digital signal and the second digital signal to obtain the initial conductivity of the conductive liquid. The arithmetic controller performs temperature compensation on the initial conductivity according to the third digital signal to obtain the conductivity of the conductive liquid.

[0070] In this embodiment, the oscillation signal and the oscillation signal after passing through the conductivity cell need to be converted into digital signals first, and then further fast Fourier transform, arithmetic operations, etc. are performed by the arithmetic controller. The temperature signal also needs to be converted into a digital signal first, and then the arithmetic controller performs temperature compensation on the initial conductivity based on the third digital signal.

[0071] As Figure 2 shown, in an embodiment of the present invention, the digital drive circuit includes a signal generator U3, a clock circuit U4, and an inverting proportional amplifier circuit; the digital drive circuit includes a signal generator U3, a clock circuit U4, a first operational amplifier U2A, and a second operational amplifier U2B;

[0072] The output terminal OUT of the clock circuit U4 is connected to the clock signal terminal MCLK of the signal generator U3. The data terminal (SPI1) of the signal generator U3 is connected to the arithmetic controller. The signal generator U3 obtains a trigger signal through the data terminal (SPI1); the signal output terminal VOUT of the signal generator U3 is connected to the input terminal of the inverting proportional amplifier circuit, and the output terminal of the inverting proportional amplifier circuit is connected to the first electrode.

[0073] In this embodiment, the oscillation signal is generated by the signal generator U3. The signal generator U3 is a DDS (Direct Digital Frequency Synthesis) chip, and the model is AD9833. The power supply pin VDD of the signal generator U3 is connected to an external +3.3V digital power supply to supply power to the signal generator U3. The used circuit is an active crystal oscillator C04305-8.000-EXT, and the active crystal oscillator provides a fundamental frequency signal of 8 MHz for the signal generator U3. The digital drive circuit is connected to the arithmetic controller through the SPI1 interface. The lines of the SPI interface include SPI1_NSS, SPI1_SCK, and SPI1_MOSI. The signal generator U3 controls the oscillation signal and its frequency with the arithmetic controller through the SPI1 interface, so as to achieve the purpose of automatically adjusting the frequency of the drive signal. Note that the adjustable frequency output by the AD9833 here can only be a fraction of the 8 MHz fundamental frequency. Subsequently, the output oscillation signal can achieve the purpose of automatically adjusting the amplitude of the drive signal through the cooperation of the analog switch and the operational amplifier.

[0074] In an embodiment of the present invention, the inverting proportional amplifier circuit includes a first operational amplifier U2A and a second operational amplifier U2B;

[0075] The signal output terminal VOUT of the signal generator U3 is connected to the inverting input terminal of the first operational amplifier U2A, and the non-inverting input terminal of the first operational amplifier U2A is grounded through the series-connected first resistor R9; the output terminal VOUT of the first operational amplifier U2A is connected to the inverting input terminal of the second operational amplifier U2B through the second resistor R5, and the non-inverting input terminal of the second operational amplifier U2B is grounded through the series-connected third resistor R11, and the output terminal of the second operational amplifier U2B is connected to the first electrode;

[0076] A first negative feedback circuit is provided between the inverting input terminal and the output terminal of the first operational amplifier U2A. The first negative feedback circuit includes an adjustment terminal for adjusting the feedback coefficient, and the adjustment terminal of the first negative feedback circuit is connected to the operation controller; a second negative feedback circuit is provided between the inverting input terminal and the output terminal of the second operational amplifier U2B.

[0077] In this embodiment, the signal generated by the signal generator U3 is amplified by two-stage operational amplifiers to generate an oscillation signal; wherein both the first operational amplifier U2A and the second operational amplifier U2B form amplifier circuits. The amplification ratio of the first operational amplifier U2A is determined by the feedback coefficient of the first negative feedback circuit, and the amplification ratio of the second operational amplifier U2B is determined by the feedback coefficient of the second negative feedback circuit. Among them, the feedback coefficient of the first negative feedback circuit is adjustable, and the purpose of adjusting the amplitude of the drive signal can be achieved.

[0078] In an embodiment of the present invention, the first negative feedback circuit includes a fourth resistor R1, a fifth resistor R2, a sixth resistor R3, a first capacitor C3, and an analog switch U1; the model of the analog switch is ADG624. The analog switch U1 includes a first source terminal S1, a first drain terminal D1, a first control terminal IN1, a second source terminal S2, a second drain terminal D2, and a second control terminal IN2; the first control terminal IN1 is used to control the on / off between the first source terminal S1 and the first drain terminal D1, and the second control terminal IN2 is used to control the on / off between the second source terminal S2 and the second drain terminal D2;

[0079] One end of the fourth resistor R1 and one end of the first capacitor C3 are connected to the inverting input terminal of the first operational amplifier U2A, and the other end of the fourth resistor R1 and the other end of the first capacitor C3 are connected to the output terminal of the first operational amplifier U2A;

[0080] One end of the fifth resistor R2 is connected to the inverting input terminal of the first operational amplifier U2A, the other end of the fifth resistor R2 is connected to the first source terminal S1, and the first drain terminal D1 is connected to the output terminal of the first operational amplifier U2A; the first control terminal IN1 is connected to the operation controller;

[0081] One end of the sixth resistor R3 is connected to the inverting input terminal of the first operational amplifier U2A, the other end of the sixth resistor R3 is connected to the second source terminal S2, the second drain terminal D2 is connected to the output terminal of the first operational amplifier U2A; the second control terminal IN2 is connected to the operation controller.

[0082] In this embodiment, the first negative feedback circuit is an RC negative feedback circuit composed of the fourth resistor R1, the fifth resistor R2, the sixth resistor R3, and the first capacitor C3. The operation controller controls the number of resistors incorporated into the RC negative feedback circuit through the analog switch U1, thereby changing the feedback coefficient of the RC negative feedback circuit, and further adjusting the amplification ratio of the inverting proportional amplification circuit.

[0083] In an embodiment of the present invention, the second negative feedback circuit includes a seventh resistor R4 and a second capacitor C5;

[0084] One end of the seventh resistor R4 and one end of the second capacitor C5 are connected to the inverting input terminal of the second operational amplifier U2B, and the other end of the seventh resistor R4 and the other end of the second capacitor C5 are connected to the output terminal of the second operational amplifier U2B.

[0085] In this embodiment, the feedback coefficient of the second negative feedback circuit is a fixed value, and the second operational amplifier amplifies the signal at a fixed amplification ratio.

[0086] As Figure 3 shown, in an embodiment of the present invention, the model of the analog-to-digital converter U5 is ADS1256. The analog-to-digital converter U5 includes a first input terminal AIN0, a second input terminal AIN2, a third input terminal (AIN6, AIN7), and a data output terminal SPI3;

[0087] The first input terminal AIN0 of the analog-to-digital converter is connected to the output terminal VOUT of the digital drive circuit, the second input terminal AIN2 of the analog-to-digital converter is connected to the second electrode VIN, the third input terminal (AIN6, AIN7) of the analog-to-digital converter is connected to the output terminal of the temperature acquisition circuit, the data output terminal SPI3 of the analog-to-digital converter is connected to the operation controller, and the data output terminal of the analog-to-digital converter is an SPI interface, including SPI3_SCK, SPI3_MOSI, SPI3_MISO, AD_RDY, SPI3_N55.

[0088] In an embodiment of the present invention, the mathematical expression of the initial conductivity k0 is:

[0089]

[0090] where θ is the electrode constant of the first electrode and the second electrode, and R is the equivalent impedance of the conductive solution in the conductivity cell; the electrode constant is a fixed attribute of the first electrode and the second electrode, which is determined by the cross-sectional area and length of the electrode. L is the length of the electrode, and A is the cross-sectional area of the electrode;

[0091] Among them, R i is the input resistance of the inverting proportional amplifier circuit, V i is the maximum amplitude of the oscillation signal, V o is the maximum amplitude of the oscillation signal after passing through the conductivity cell. The operation controller performs a fast Fourier transform on the oscillation signal and the oscillation signal after passing through the conductivity cell to obtain the maximum amplitude V i of the oscillation signal, and the maximum amplitude V o of the oscillation signal after passing through the conductivity cell.

[0092] In this embodiment, since the inverting proportional amplifier circuit has a two-stage amplification structure, the input resistance R i is the equivalent input resistance of the inverting proportional amplifier circuit, rather than a specific resistor device in the digital drive circuit. The equivalent input resistance can be obtained by connecting a fixed-value resistor to the output end of the digital drive circuit, then acquiring the input voltage value and the output voltage value through a fast Fourier transform, and inversely calculating the equivalent input resistance based on the resistance value of the fixed-value resistor, the input voltage value, and the output voltage value.

[0093] In an embodiment of the present invention, the mathematical expression of the conductivity k is:

[0094]

[0095] Among them, t is the value of the temperature signal, is the compensation coefficient, k is the conductivity compensated to 25 °C, and k0 is the conductivity at the current ambient temperature.

[0096] Figure 4 is the circuit diagram of the operation controller in this embodiment.

[0097] To sum up, the present invention provides a conductivity sensor. By setting a digital drive circuit to generate an oscillation signal according to the trigger signal of the operation controller, the oscillation signal is introduced into the conductive liquid in the conductivity cell through the first electrode; at the same time, a temperature acquisition circuit is set to collect the temperature signal of the conductive liquid in the conductivity cell; the operation controller obtains the oscillation signal after passing through the conductivity cell through the second electrode, and the operation controller performs operations on the oscillation signal and the oscillation signal after passing through the conductivity cell to obtain the initial conductivity of the conductive liquid and perform temperature compensation on the initial conductivity according to the temperature signal to obtain the conductivity of the conductive liquid. The present invention measures the conductivity of the conductive liquid by generating an oscillation signal through the digital drive circuit, and at the same time combines temperature compensation to obtain the final conductivity result, making the measurement result more accurate.

[0098] In the above embodiments, although the present invention has been described in conjunction with specific embodiments of the present invention, many alternatives, modifications, and variations of these embodiments will be apparent to those of ordinary skill in the art based on the foregoing description. Embodiments of the present invention are intended to cover all such alternatives, modifications, and variations that fall within the broad scope of the appended claims.

[0099] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those of ordinary skill in the art within the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A conductivity sensor, characterized in that, For measuring the conductivity of a conductive liquid, comprising a digital drive circuit, a first electrode, a second electrode, a temperature acquisition circuit, and an operation controller; The digital drive circuit is used to generate an oscillation signal according to the trigger signal of the operation controller, and the oscillation signal is introduced into the conductive liquid through the first electrode; The temperature acquisition circuit is used to acquire the temperature signal of the conductive liquid in the conductivity cell; The operation controller obtains the oscillation signal after passing through the conductive liquid through the second electrode. The operation controller is used to perform operations on the oscillation signal and the oscillation signal after passing through the conductive liquid to obtain the initial conductivity of the conductive liquid, and perform temperature compensation on the initial conductivity according to the temperature signal to obtain the conductivity of the conductive liquid; The digital drive circuit includes a signal generator, a clock circuit, and an inverting proportional amplification circuit; the digital drive circuit includes a signal generator, a clock circuit, a first operational amplifier, and a second operational amplifier; The output end of the clock circuit is connected to the clock signal end of the signal generator, the data end of the signal generator is connected to the operation controller, and the signal generator obtains the trigger signal through the data end; the signal output end of the signal generator is connected to the input end of the inverting proportional amplification circuit, and the output end of the inverting proportional amplification circuit is connected to the first electrode; The inverting proportional amplification circuit includes a first operational amplifier and a second operational amplifier; The signal output end of the signal generator is connected to the inverting input end of the first operational amplifier, and the non-inverting input end of the first operational amplifier is grounded through a first resistor; the output end of the first operational amplifier is connected to the inverting input end of the second operational amplifier through a second resistor, the non-inverting input end of the second operational amplifier is grounded through a third resistor, and the output end of the second operational amplifier is connected to the first electrode; A first negative feedback circuit is provided between the inverting input end and the output end of the first operational amplifier. The first negative feedback circuit includes an adjustment end for adjusting the feedback coefficient, and the adjustment end of the first negative feedback circuit is connected to the operation controller; a second negative feedback circuit is provided between the inverting input end and the output end of the second operational amplifier.

2. The conductivity sensor according to claim 1, characterized in that, The sensor further includes an analog-to-digital converter, which is used to convert the oscillation signal into a first digital signal, the oscillation signal after passing through the conductive liquid into a second digital signal, and the temperature signal into a third digital signal; The operation controller performs operations on the first digital signal and the second digital signal to obtain the initial conductivity of the conductive liquid, and the operation controller performs temperature compensation on the initial conductivity according to the third digital signal to obtain the conductivity of the conductive liquid.

3. The conductivity sensor according to claim 1, characterized in that, The first negative feedback circuit includes a fourth resistor, a fifth resistor, a sixth resistor, a first capacitor, and an analog switch; the analog switch includes a first source terminal, a first drain terminal, a first control terminal, a second source terminal, a second drain terminal, and a second control terminal; The first control terminal is used to control the on / off between the first source terminal and the first drain terminal, and the second control terminal is used to control the on / off between the second source terminal and the second drain terminal; One end of the fourth resistor, one end of the first capacitor are connected to the inverting input terminal of the first operational amplifier, and the other end of the fourth resistor, the other end of the first capacitor are connected to the output terminal of the first operational amplifier; One end of the fifth resistor is connected to the inverting input terminal of the first operational amplifier, the other end of the fifth resistor is connected to the first source terminal, and the first drain terminal is connected to the output terminal of the first operational amplifier; the first control terminal is connected to the operation controller; One end of the sixth resistor is connected to the inverting input terminal of the first operational amplifier, the other end of the sixth resistor is connected to the second source terminal, and the second drain terminal is connected to the output terminal of the first operational amplifier; the second control terminal is connected to the operation controller.

4. The conductivity sensor according to claim 1, characterized in that, The second negative feedback circuit includes a seventh resistor and a second capacitor; One end of the seventh resistor, one end of the second capacitor are connected to the inverting input terminal of the second operational amplifier, and the other end of the seventh resistor, the other end of the second capacitor are connected to the output terminal of the second operational amplifier.

5. An electrical conductivity sensor according to claim 2, characterized in that, The analog-to-digital converter includes a first input terminal, a second input terminal, a third input terminal and a data output terminal; The first input terminal of the analog-to-digital converter is connected to the output terminal of the digital drive circuit, the second input terminal of the analog-to-digital converter is connected to the second electrode, the third input terminal of the analog-to-digital converter is connected to the output terminal of the temperature acquisition circuit, and the data output terminal of the analog-to-digital converter is connected to the operation controller.

6. The conductivity sensor according to claim 1, characterized in that, The mathematical expression of the initial conductivity k0 is: where θ is the electrode constant of the first electrode and the second electrode, and R is the equivalent impedance of the conductive solution; wherein, R i is the input resistance of the inverting proportional amplification circuit, V i is the maximum amplitude of the oscillation signal, V o is the maximum amplitude of the oscillation signal after passing through the conductance cell.

7. An electrical conductivity sensor according to claim 6, characterized in that, The operation controller performs a fast Fourier transform on the oscillation signal and the oscillation signal after passing through the conductive liquid to obtain the maximum amplitude V of the oscillation signal i and the maximum amplitude V of the oscillation signal after passing through the conductivity cell o .

8. An electrical conductivity sensor according to claim 6, characterized in that, The mathematical expression of the conductivity k is: where t is the value of the temperature signal, is the compensation coefficient.

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