An electrode conductivity sensor conductivity measuring circuit and method

Abstract

The present application relates to the field of solution measurement, in particular to an electrode conductivity sensor conductivity measurement circuit and method, first, the AC excitation generation module generates an amplitude and frequency adjustable AC sine excitation signal under the control of the single-chip microcomputer; then the AC sine excitation signal passes through the precision resistor R0 and the conductivity cell of the electrode sensor to the system ground in turn; then the differential amplification circuit differentially amplifies the voltage between the precision resistor R0 and the electrode sheet of the conductivity cell; the amplified voltage signal is collected by the ADC, and then the voltage data is transmitted to the single-chip microcomputer; finally, the single-chip microcomputer can obtain the conductivity of the solution after the collected voltage data is operated. The present application directly collects the peak and valley voltage values of the AC signal, subtracts to obtain the amplitude of the AC signal, and does not need a rectification filter circuit to detect the amplitude of the AC signal, which can quickly respond to the change of the solution conductivity, eliminates the influence of common-mode interference factors, and simplifies the circuit design.

Description

Technical Field

[0001] The present invention relates to the field of solution measurement, and in particular to a conductivity measurement circuit and method of an electrode-type conductivity sensor. Background Art

[0002] Conductivity is a key physicochemical property of electrolyte solutions, an electrical quantity that measures their ability to conduct electricity. Liquid conductivity measurement plays a crucial role in environmental monitoring, chemical engineering, microelectronics, biopharmaceuticals, food processing, electroanalytical chemistry, clinical medicine, oceanography, hydrology, light industry, and metallurgy.

[0003] Currently, there are two main methods for measuring the conductivity of solutions: inductive and electrode. The electrode method is widely used due to its high sensitivity, fast response, strong anti-interference capabilities, and wide measurement range. Depending on the number of electrodes, electrode conductivity sensors come in various types, including two-electrode, three-electrode, four-electrode, and seven-electrode.

[0004] Different types of electrode-type conductivity sensors have different performance and costs, but the principles of measuring solution conductivity are basically the same. Taking a two-electrode conductivity sensor as an example, the sensor has a conductivity cell with two electrodes. The electrolyte liquid between the two electrodes can be regarded as a conductor. According to the calculation formula of the conductor resistance: Where R is the conductor resistance, ρ is the resistivity, l is the conductor length, and S is the conductor cross-sectional area. The conductivity formula can be obtained based on the fact that conductivity C is the inverse of resistivity ρ: The ratio of the length to the cross-section of a fixed conductivity cell is a fixed value, which is set as the conductivity cell constant K. The calculation formula for conductivity can be changed to: The conductivity cell can be placed in an electrolyte solution with a known fixed conductivity. The conductor resistance of the conductivity cell can be measured by a digital multimeter, and the conductivity cell constant K of the conductivity cell can be calculated. Once K is known, the sensor can be connected to the conductivity measurement circuit and finally output the conductivity value.

[0005] Current conductivity measurement methods primarily utilize a rectifier-filter circuit to convert the AC signal amplitude into a DC signal for sampling. Due to the presence of the filter circuit, this method results in a delayed response to conductivity changes, thus limiting rapid conductivity measurement. For example, Chinese patents with publication numbers CN102809697A, CN106291119B, and CN109188099B all employ rectifier-filtering solutions in their circuit designs. Summary of the Invention

[0006] To solve the above problems, the present invention provides a fast-response measurement circuit and conductivity measurement method suitable for electrode-type conductivity sensors. The circuit directly collects the peak and trough voltage values ​​of the AC signal, subtracts them and obtains the AC signal amplitude. No rectification and filtering circuit is required to detect the AC signal amplitude, and the circuit can quickly respond to changes in solution conductivity while eliminating the influence of common-mode interference factors.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] A conductivity measurement circuit for an electrode-type conductivity sensor includes a single-chip microcomputer, an AC excitation generation module, an ADC, a differential amplifier circuit, a precision resistor R0, and an electrode-type sensor. The AC excitation generation module is used to generate an AC sinusoidal excitation signal with adjustable amplitude and frequency under the control of the single-chip microcomputer. The AC sinusoidal excitation signal sequentially passes through the precision resistor R0 and a conductivity cell of the electrode-type sensor to a system ground. The differential amplifier circuit is used to differentially amplify the voltage between the precision resistor R0 and the conductivity cell electrodes. The amplified voltage signal is collected by the ADC and the voltage data is transmitted to the single-chip microcomputer. The single-chip microcomputer calculates the collected voltage data to obtain the conductivity of the solution.

[0009] Furthermore, the original waveform generating device of the AC excitation generating module adopts a DAC chip.

[0010] Furthermore, the ADC, under the control of the single chip microcomputer, samples at the peak of the AC signal, and the amplitude of the AC signal can be obtained by subtracting the voltage collected at the peak from the voltage collected at the trough.

[0011] The present invention also provides a conductivity measurement method of an electrode-type conductivity sensor, which uses the above-mentioned measurement circuit to implement measurement and includes the following steps:

[0012] S1, AC excitation generation module generates accurate AC sinusoidal excitation signal Ui with adjustable amplitude and frequency under the control of single chip microcomputer;

[0013] S2, AC excitation signal Ui passes through the precision resistor R0 and the conductivity cell equivalent resistor R1 to the system ground, where the voltage across the precision resistor R0 is The voltage across the equivalent resistance of the conductivity cell Using the ratio of U0 to U1, we can get: So the equivalent resistance of the conductivity cell is: Then by It can be seen that the conductivity of the electrolyte liquid in the conductivity cell at this time is:

[0014] S3, using the differential amplifier circuit to amplify the voltage across the precision resistor R0 and the voltage between the conductivity cell electrodes, and then sending the amplified signal to the ADC;

[0015] S4. Under the control of the single-chip microcomputer, the ADC samples the peak of the AC signal, subtracts the voltage collected at the peak from the voltage collected at the trough to obtain the amplitude of the AC signal, and transmits it to the single-chip microcomputer;

[0016] S5. The single chip microcomputer calculates the collected voltage data to obtain the conductivity of the solution.

[0017] Compared with the prior art, the beneficial effects of the present invention are: the present invention directly collects the peak and trough voltage values ​​of the AC signal, subtracts them and then obtains the AC signal amplitude, without the need for a rectifier and filter circuit to detect the AC signal amplitude, can quickly respond to changes in solution conductivity, and at the same time eliminates the influence of common-mode interference factors and simplifies circuit design. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a principle block diagram of the solution conductivity measurement circuit of the present invention.

[0019] Figure 2 Schematic diagram of the simplified measurement principle of the electrode-type conductivity sensor.

[0020] Figure 3 Schematic diagram of the waveform of the AC excitation signal and ADC sampling pulse. DETAILED DESCRIPTION

[0021] The present invention will be described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those skilled in the art, several variations and improvements can be made without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0022] like Figure 1 As shown, a conductivity measurement circuit of an electrode-type conductivity sensor of the present invention includes a single-chip microcomputer, an AC excitation generation module, an ADC, a differential amplifier circuit, a precision resistor R0, and an electrode-type sensor. The AC excitation generation module generates an accurate AC sinusoidal excitation signal with adjustable amplitude and frequency under the control of the single-chip microcomputer. The original waveform generating device here is implemented using a DAC chip. The advantage of using a DAC chip is that the amplitude and frequency can be software-adjusted through the control of the single-chip microcomputer, avoiding the influence of the accuracy of external resistors and capacitors on the AC excitation signal. At the same time, it can cooperate with the single-chip microcomputer to accurately control the sampling position of the ADC.

[0023] The AC sinusoidal excitation signal generated by the AC excitation generator first passes through a precision resistor R0 and then through the conductivity cell of the electrode-type conductivity sensor. The precision resistor R0 and the conductivity cell form a series structure. As analyzed in the background technology, the electrolyte liquid between the two electrodes in the conductivity cell can be regarded as a conductor, so a simplified schematic diagram of the measurement principle of the electrode-type conductivity sensor can be obtained, as shown in the figure below. Figure 2 As shown in the figure, the AC excitation signal Ui passes through the precision resistor R0 and the conductivity cell equivalent resistor R1 to the system ground, where the voltage across the precision resistor R0 is: The voltage across the equivalent resistance of the conductivity cell is: Using the ratio of U0 to U1, we can get: So the equivalent resistance of the conductivity cell is: Then by It can be seen that the conductivity of the electrolyte liquid in the conductivity cell at this time is: The method of measuring the equivalent resistance of the conductivity cell using a precision resistor divider can effectively eliminate the interference of factors such as power supply voltage fluctuation, temperature, or components on the amplitude of the excitation signal, which ultimately affects the calculation of the equivalent resistance of the conductivity cell. The principle is as follows: Assuming that factors such as power supply voltage fluctuation, temperature, or components cause interference ΔU on the amplitude of the excitation signal, the AC excitation signal Ui becomes: Ui′=Ui+ΔU. At this time, Equation (4) will become: Since the power supply disturbance exists in both the numerator and denominator of the fraction, they can cancel each other out, and equation (5) still holds. Therefore, the disturbance ΔU will not affect the measurement of the equivalent resistance of the conductivity cell.

[0024] A differential amplifier circuit is then used to amplify the voltage across the precision resistor R0 and the voltage between the conductivity cell electrodes. The amplified signal is then sent to an ADC (analog-to-digital converter). It should be noted that while the present invention uses a conductivity sensor with a two-electrode conductivity cell as an example to illustrate its principles, the circuit and method of the present invention are actually applicable to electrode-type conductivity sensors with any number of electrodes.

[0025] Under the control of the microcontroller, the ADC accurately samples the peak of the AC signal. The voltage collected at the peak is subtracted from the voltage collected at the trough to obtain the amplitude of the AC signal. By sampling the voltage in each excitation signal cycle, a conductivity value can be obtained in each excitation signal cycle, so the conductivity response speed is faster. Theoretically, the maximum output rate is equal to the excitation signal frequency. The schematic diagram of the AC excitation signal and the ADC sampling pulse is shown in the figure below. Figure 3Compared with the method of obtaining the AC signal amplitude through rectification and filtering, the method of obtaining the AC signal amplitude through subtracting the voltage collected at the peak and trough can eliminate the influence of temperature and common mode interference such as amplifier output bias voltage on the output AC signal amplitude, thereby obtaining more accurate measurement results. The principle is: let the voltage at the peak of the AC signal in one cycle be V H , the voltage at the valley is V L , then the amplitude of the AC signal is: If the AC signal is offset by V due to temperature and common mode interference such as amplifier output bias voltage, offset , at this time the voltage at the peak becomes: V′ H =V H +V offset , the voltage at the valley becomes V′ L =V L +V offaet , the amplitude of the AC signal becomes: The amplitude is consistent with that before interference, indicating that this method can eliminate the influence of common-mode interference factors.

[0026] Finally, the ADC converts the collected voltage into a digital signal and transmits it to the microcontroller. The microcontroller processes the data to obtain the conductivity of the solution.

[0027] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various variations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A conductivity measurement method of a seven-electrode conductivity sensor, characterized in that: The conductivity measurement circuit of the seven-electrode conductivity sensor includes a DDS digital frequency synthesizer, a seven-electrode conductivity sensor, an amplifier circuit, an analog switch, a precision full-wave rectifier circuit, a low-pass filter circuit, an ADC sampling module, and a main control module. The DDS digital frequency synthesizer outputs a sine wave excitation signal under the control of the main control module to excite the seven-electrode conductivity sensor, collects two feedback signals of the seven-electrode conductivity sensor and the voltage signal of the excitation signal on the precision sampling resistor, amplifies the three voltage signals by the amplifier circuit, uses the analog switch for patrol switching, transmits the transmission signal to the precision full-wave rectifier circuit and the low-pass filter circuit to convert the feedback signal into a DC voltage signal, and collects it through the ADC sampling module. The main control module calculates the ratio of the DC voltage value of the two signals to the DC voltage value on the precision resistor to solve the conductivity value; The seven-electrode conductivity sensor is composed of seven platinum electrode rings and a quartz conductivity cell; The middle four electrodes of the seven-electrode conductivity sensor serve as excitation electrodes, and the sinusoidal excitation signal generated by the DDS is input thereto. Electrodes 2 and 3, and electrodes 5 and 6 form two symmetrical pairs of voltage electrodes. The feedback signals on the two pairs of electrodes are converted into DC voltage signals through a precision full-wave rectifier circuit and low-pass filtering. Electrodes 1 and 7 are ground electrodes to eliminate polarization effects and electromagnetic interference. The seven-electrode conductivity sensor conductivity measurement method uses a seven-electrode conductivity sensor conductivity measurement circuit to achieve measurement, and the specific steps are as follows: S1, DDS digital frequency synthesizer outputs a sine wave excitation signal to excite the seven-electrode conductivity sensor under the control of the main control module; S2, collecting the sinusoidal signals of electrodes 2 and 3 and electrodes 5 and 6 of the seven-electrode conductivity sensor and the voltage signal of the excitation signal on the precision sampling resistor, and using an analog switch for patrol switching; S3, transmitting the transmission signal to the precision full-wave rectifier circuit and low-pass filter circuit to convert the feedback signal into a DC voltage signal; S4, collecting the DC voltage signal through the ADC sampling module, and converting the collected voltage into a digital signal and transmitting it to the main control module; S5. The main control module calculates the conductivity value by calculating the ratio of the DC voltage value of the two signals to the DC voltage value on the precision resistor.

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