Non-contact electrical impedance sensor based on negative resistance and inductance synergistic compensation

Abstract

This invention discloses a non-contact impedance sensor based on the synergistic compensation of negative resistance and inductance, belonging to the field of non-contact conductivity detection. The sensor mainly includes: an excitation electrode, a detection electrode, an inductor, and a negative resistance module. The inductive reactance of the inductor is used to offset the capacitive reactance of the coupling capacitance formed between the excitation electrode and the detection electrode through the wall of the insulating pipe. The negative resistance module generates an equivalent negative resistance. The value of the equivalent negative resistance generated by the negative resistance module is configured to be equal to the value of the equivalent series internal resistance of the inductor, thereby offsetting the equivalent series internal resistance. This invention more thoroughly solves the background problems in non-contact impedance conductivity sensors, significantly improving the sensor's sensitivity and measurement range, enabling its application in the detection of important two-phase flow systems such as water electrolysis and boiling heat transfer.

Description

Technical Field

[0001] This invention belongs to the field of non-contact conductivity detection and relates to a non-contact impedance sensor based on the synergistic compensation of negative resistance and inductance. Background Technology

[0002] Conductive single-phase and multiphase flows are widely used in many industrial fields such as boiling heat transfer, electrolytic hydrogen production, and oil and gas transportation. Utilizing fluid conductivity to invert multiphase flow parameters (flow pattern, phase content) is a promising detection method. In conductivity detection technology, non-contact conductivity detection is a suitable technology for industrial applications. Its sensor electrodes are installed on the outer wall of the pipe being tested, without direct contact with the fluid inside the pipe. This avoids problems such as flow interference and electrode corrosion associated with traditional contact conductivity sensors, effectively extending the electrode's lifespan.

[0003] Compared to traditional contact-based conductivity detection solutions, this method completely avoids common problems such as electrode polarization and electrochemical corrosion. It effectively extends the lifespan of the sensing electrode without disrupting the normal flow of the fluid, and also offers practical advantages such as easy assembly and low application cost. However, because the sensing electrode and the measured solution are separated by a pipe wall, the coupling capacitance of the pipe wall acts as the background capacitance of the entire equivalent measurement circuit, significantly reducing measurement accuracy and sensitivity. In particular, since the pipe wall coupling capacitance is in series with the resistance of the measured solution, it primarily interferes with the measurement of low-resistance fluids. Many useful multiphase flow systems in industrial settings are precisely low-resistance; for example, in water electrolysis, high-concentration potassium hydroxide solutions are often used, which have high conductivity. In boiling heat exchange, due to the high temperature of the solution, its conductivity is also high, and the resistance of the solution is low. Therefore, eliminating the adverse effects of coupling capacitance on measurement is the first problem to be solved when applying non-contact conductivity sensors to many industrial scenarios.

[0004] Considering that the impedance of the coupling capacitor decreases with increasing frequency, increasing the measurement frequency is a feasible approach. However, increasing the frequency leads to a greater adverse effect of stray and parasitic capacitances on the measurement, thus limiting the potential for improvement and limiting the overall measurement performance. Some researchers employ a series resonance method to counteract the background capacitive reactance of the coupling capacitor. This involves connecting an inductor in series in the measurement path and adjusting the sensor's excitation frequency to the resonant frequency, using the inductor's inductive reactance to offset the capacitive reactance. However, this method has a significant drawback. Due to the parasitic capacitance between the inductor coils, the inductor exhibits a large equivalent internal resistance (determined by the inductor's quality factor) at the resonant frequency. Since the coupling capacitance of non-contact sensors is relatively small (often less than 10 pF), a larger inductance and a higher excitation frequency are often required. In this case, the inductor's equivalent internal resistance significantly exceeds the measured resistance of the solution. Although the background capacitive reactance of the coupling capacitor is mitigated by the inductor's inductive reactance, the inductor's equivalent internal resistance becomes a new background problem, interfering with the measurement and failing to fundamentally solve the background impedance issue.

[0005] It is worth noting that existing technologies already employ virtual inductors to replace actual inductors. For example, CN106199205A proposes a capacitively coupled non-contact conductivity measurement device and method based on a single-sided virtual inductor. This scheme utilizes the single-sided virtual inductor to generate inductive reactance, thereby offsetting the capacitive reactance of the coupling capacitor formed between the excitation electrode and the detection electrode through the tube wall, thus making the measurement circuit exhibit purely resistive behavior in the resonant state. However, the core module of this scheme is the virtual inductor, which can essentially be considered a negative capacitor. It can only offset various capacitive background impedances in the measurement circuit, and has no effect on offsetting resistive background impedances in the measurement circuit (such as electrode contact resistance, equivalent series internal resistance of inductance, etc.).

[0006] Furthermore, while the virtual inductor module disclosed in CN106199205A includes two parameters—equivalent inductance L and equivalent resistance Req—Req, representing the equivalent internal resistance of the virtual inductor, is positive. This not only fails to eliminate the background resistance in the measurement circuit but also becomes a new background resistance itself, interfering with the detection. This solution minimizes the equivalent internal resistance of the virtual inductor by selecting a large-value resistor, thus approximating its influence. However, this approximation cannot fundamentally eliminate the background resistance. Therefore, how to simultaneously eliminate both background capacitive reactance and background resistance in the measurement circuit remains a technical problem that needs to be solved in this field. Summary of the Invention

[0007] This invention addresses the interference of background capacitive reactance on measurements in non-contact conductivity detection technology, as well as the problem that existing inductor-based methods for eliminating coupling capacitance inevitably introduce new background resistance. It proposes a non-contact impedance sensor and measurement method based on the synergistic compensation of negative resistance and inductance. This invention employs a combination of inductor and negative resistance modules to completely eliminate the adverse effects of coupling capacitance on measurements, thereby improving detection accuracy and sensitivity.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] This invention provides a non-contact impedance sensor based on the synergistic compensation of negative resistance and inductance, which includes: an excitation electrode, a detection electrode, an inductor, and a negative resistance module;

[0010] The excitation electrode and the detection electrode are spaced apart on the outer wall of the insulating pipe containing the fluid to be measured; the excitation electrode, the coupling capacitor formed by the insulating pipe wall, the equivalent resistance of the fluid to be measured, the detection electrode, and the inductor and negative resistance module connected in series therebetween constitute the measurement circuit.

[0011] The inductor is connected in series in the measurement circuit of the non-contact impedance sensor; the inductive reactance of the inductor is used to cancel the capacitive reactance of the coupling capacitance formed between the excitation electrode and the detection electrode through the wall of the insulating pipe;

[0012] The negative resistance module is connected in series to the measurement circuit to generate an equivalent negative resistance; the resistance value of the equivalent negative resistance generated by the negative resistance module is configured to be equal to the resistance value of the equivalent series internal resistance of the inductor, so as to cancel the equivalent series internal resistance.

[0013] Furthermore, the excitation electrode is used to connect to an external AC excitation source, and the detection electrode is connected to an external current-to-voltage module via a negative resistor module; the excitation frequency of the measurement circuit is configured to satisfy the resonance condition, so that the sum of the inductive reactance of the inductor and the total capacitive reactance of the coupling capacitor is zero.

[0014] Furthermore, the negative resistance module includes operational amplifier A1, resistor R1, resistor R2, and resistor R3;

[0015] The resistor R1 is connected between the inverting input terminal and the output terminal of the operational amplifier A1;

[0016] The first end of the resistor R2 is connected to the non-inverting input terminal of the operational amplifier A1, and the second end of the resistor R2 is connected to the output terminal of the operational amplifier A1.

[0017] The first end of resistor R3 is connected to the non-inverting input of operational amplifier A1, and the second end of resistor R3 is connected to the input of an external current-to-voltage module as the output of the negative resistance module; the input of the external current-to-voltage module is a virtual ground.

[0018] For the negative resistance module to work properly, the second terminal (output terminal) of resistor R3 needs to be grounded. In this invention, the input terminal of the current-to-voltage module is a virtual ground terminal. Connecting the output terminal of R3 to the input terminal of the current-to-voltage module allows the negative resistance module to work properly.

[0019] Furthermore, the equivalent negative resistance value of the negative resistance module R1, R2, and R3 are the resistance values ​​of the corresponding resistors.

[0020] Furthermore, the total resistivity Z of the measurement circuit x In the resonant state, it is represented as: Z x =R L +R x +R eq , where R L R is the equivalent series internal resistance of the inductor. x R is the resistance of the fluid being measured. eq The equivalent negative resistance generated by the negative resistance module; the value of the equivalent negative resistance is configured to be equal to the value of the equivalent series internal resistance of the inductor, i.e., R. eq =-R L The total resistivity Z x =R x .

[0021] The present invention also provides a method for measuring fluid conductivity using the aforementioned non-contact impedance sensor, comprising:

[0022] An AC excitation voltage is applied to the measurement circuit through an AC excitation source;

[0023] Adjust the frequency of the AC excitation voltage so that the inductive reactance of the inductor and the capacitive reactance of the coupling capacitor cancel each other out;

[0024] Adjust the parameters of the negative resistance module so that the equivalent negative resistance generated by the negative resistance module is equal to the equivalent series internal resistance of the inductor.

[0025] The alternating current signal in the measurement circuit is detected, and the conductivity of the fluid under test is inverted based on the alternating current signal.

[0026] Furthermore, adjusting the frequency of the AC excitation voltage includes: monitoring the voltage and current phase of the measurement circuit using an oscilloscope; when the voltage and current phases are consistent, confirming that the inductive reactance of the inductor and the capacitive reactance of the coupling capacitor cancel each other out.

[0027] Measurement range and sensitivity are crucial performance parameters for non-contact conductivity sensors (CID). The capacitive reactance of the pipe wall coupling capacitance is the primary reason why CID sensors lag far behind contact conductivity sensors in these two parameters (especially in measurements of high-conductivity solutions). Traditional CID sensors reduce the capacitive reactance by increasing the frequency of the AC detection current. However, since the pipe wall capacitance is typically small, a very high excitation frequency is often required to reduce the capacitive reactance to a level that does not significantly affect the measurement (the coupling capacitance is in series with the resistance of the fluid being measured; the larger the capacitive reactance, the greater the impact). At higher frequencies, various stray and parasitic capacitances in the sensor (these capacitances are in parallel with the resistance of the fluid being measured; the smaller the capacitive reactance, the greater the impact) become new background capacitances, weakening the sensor's measurement range and sensitivity. Therefore, finding a method to eliminate the pipe wall coupling capacitance at a lower frequency is essential to fundamentally improve the measurement range and sensitivity of CID sensors.

[0028] Existing methods mainly include capacitive reactance cancellation, phase-sensitive demodulation, and series resonance. However, capacitive reactance cancellation places the main measurement path containing capacitance in the operational amplifier feedback path and requires an additional operational amplifier with a capacitor as the feedback path, which easily leads to operational amplifier instability and unusability. Phase-sensitive demodulation requires two excitation signals, increasing cost, and necessitates the use of multipliers and other multi-stage modules, causing errors to propagate step by step and increasing measurement errors. Although series resonance eliminates coupling capacitance using inductors, the presence of parasitic inductor capacitance means that at the sensor's operating frequency, the inductor has a significant equivalent internal resistance, replacing the coupling capacitance and becoming a new background resistance that limits the sensor's measurement range and sensitivity.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] This invention further introduces a negative resistance module based on the series resonance method. Its output terminal is connected to the virtual ground input terminal of the current-to-voltage module, allowing it to be integrated into the detection circuit of a non-contact sensor. This achieves the elimination of capacitive reactance by inductance through pipe wall coupling, while the negative resistance eliminates the equivalent internal resistance of the inductance. This more thoroughly solves the background problems in non-contact impedance sensors, significantly improving the sensor's sensitivity and measurement range, enabling its application in the detection of important two-phase flow systems such as water electrolysis and boiling heat transfer.

[0031] Furthermore, this invention differs fundamentally from existing schemes using virtual inductors (such as CN106199205A) in its technical approach. The core module of existing virtual inductor schemes is the virtual inductor itself, which functions equivalently to a negative capacitor. It can only cancel the capacitive reactance of the coupling capacitor in the measurement circuit and has no effect on resistive background impedance. The equivalent internal resistance Req of its virtual inductor is positive, thus becoming a new background resistance itself. This resistance can only be approximated by selecting a large resistance value (and due to limitations such as operational amplifier stability, the value is not flexible; in practice, only a relatively large Req resistance value can be chosen). This invention, however, uses a combination of a real inductor and a negative resistance module. The equivalent negative resistance Req generated by the negative resistance module is negative and configured to be equal to the equivalent series internal resistance of the inductor, thereby completely canceling the equivalent internal resistance of the inductor. It can further cancel various background resistances such as electrode contact resistance and wire resistance, making the total resistivity Zx = Rx in the resonant state. The inductor used in this invention can be either a real inductor or a virtual inductor as used in existing solutions. When a virtual inductor is used, the negative resistance module can further offset its residual equivalent internal resistance, resulting in more thorough compensation. This invention can significantly improve the sensitivity and measurement range of the sensor, enabling its application in the detection of important two-phase flow systems such as water electrolysis and boiling heat transfer. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the non-contact impedance sensor based on the synergistic compensation of negative resistance and inductance of the present invention.

[0033] Figure 2 This is the equivalent circuit diagram of the non-contact impedance sensor based on the synergistic compensation of negative resistance and inductance of the present invention;

[0034] Figure 3 This is the voltage waveform across the resistor under test during the test of the negative resistance module in Example 1;

[0035] Figure 4 This refers to the definitions of bubbly flow and slug flow involved in Example 3. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0037] To better illustrate the non-contact impedance sensor based on the synergistic compensation of negative resistance and inductance proposed in this invention, the principle of the sensor is explained in detail below, based on its schematic diagram. For a section of insulated pipe, such as... Figure 1Two electrodes are installed at or around the bottom of the flow channel (depending on the channel shape), one as an excitation electrode and the other as a detection electrode. During detection, an AC excitation voltage of a certain frequency is applied to the excitation electrode, thus forming an AC path between the excitation electrode, the pipe wall, the fluid being measured, the pipe wall, and the detection electrode. An AC current is received on the detection electrode. The amplitude of this AC current is controlled by the conductivity of the fluid being measured, and the conductivity of the fluid being measured can be inferred from the AC current. Figure 2 This is the equivalent circuit diagram of the entire sensor, where R x C1 and C2 are the resistance of the fluid, and the coupling capacitance formed by the coupling between the excitation electrode and the detection electrode through the pipe wall and the fluid inside the pipe. It can be clearly seen that C1 and C2 are background capacitances, and their impedance will have an adverse effect on the measurement and must be eliminated.

[0038] Existing technologies generally reduce the capacitive reactance of C1 and C2 by increasing the excitation frequency to suppress their influence. However, because increasing the excitation frequency leads to a greater impact from various parasitic and stray capacitances, the frequency cannot be increased indefinitely, thus failing to truly solve the problem. In addition, some existing technologies introduce an inductor L and use the inductive reactance of the inductor in series to offset the capacitive reactance. However, due to the low capacitance of the coupling capacitor, the inductor often needs to operate at a high frequency. In this case, due to the presence of internal parasitic capacitances, the inductor's equivalent internal resistance R increases. L It can become very large (depending on the inductor's quality factor), becoming a new background impedance that interferes with the measurement.

[0039] To address this issue, the non-contact impedance sensor based on the synergistic compensation of negative resistance and inductance proposed in this invention further includes a negative resistance module. The excitation electrode, the coupling capacitance formed by the insulating pipe wall, the equivalent resistance of the fluid being measured, the detection electrode, and the inductor and negative resistance module connected in series therethrough constitute a measurement circuit. The inductor is connected in series in the measurement circuit of the non-contact impedance sensor; the inductive reactance of the inductor is used to cancel the capacitive reactance of the coupling capacitance formed by the excitation electrode and the detection electrode through the insulating pipe wall; the negative resistance module is connected in series in the measurement circuit to generate an equivalent negative resistance; the resistance value of the equivalent negative resistance generated by the negative resistance module is configured to be equal to the resistance value of the equivalent series internal resistance of the inductor, so as to cancel the equivalent series internal resistance.

[0040] like Figure 2 As shown, the negative resistance module includes operational amplifier A1, resistor R1, resistor R2 and resistor R3;

[0041] The resistor R1 is connected between the inverting input terminal and the output terminal of the operational amplifier A1;

[0042] The first end of the resistor R2 is connected to the non-inverting input terminal of the operational amplifier A1, and the second end of the resistor R2 is connected to the output terminal of the operational amplifier A1.

[0043] The first end of the resistor R3 is connected to the non-inverting input of the operational amplifier A1, and the second end of the resistor R3 is connected to the inverting input of the operational amplifier A2 (the input of the current-to-voltage module) of the external current-to-voltage module as the output of the negative resistor module; the inverting input of the operational amplifier A2 is a virtual ground.

[0044] like Figure 2 As shown, the current-to-voltage module includes an operational amplifier A2, resistors R4 and R5; resistor R4 is connected between the inverting input and output of operational amplifier A2; one end of resistor R5 is connected to the non-inverting input of operational amplifier A2, and the other end is grounded. The output of operational amplifier A2 is the output of the current-to-voltage module, and the output of the current-to-voltage module is connected to the RMS detection module.

[0045] The RMS detection module is used to output a DC signal that is proportional to the conductivity of the fluid being measured.

[0046] Based on the aforementioned non-contact impedance sensor, this invention proposes a fluid conductivity measurement method based on the synergistic compensation of inductance and negative resistance to completely solve the background impedance problem. First, the inductive reactance of the introduced inductor is used to offset the capacitive reactance of the background capacitance. Then, the equivalent negative resistance R formed by the negative resistance module is used... eq (The negative resistance value depends on R1, R2, and R3 in the negative resistance module) cancels the internal resistance RL of the inductor.

[0047]

[0048] Note: The potential on the right side of R3 is 0 (connected to the inverting input of operational amplifier A2, in a "virtual ground" state).

[0049] The impedance Z of the entire measurement path x for:

[0050]

[0051] in It is the excitation angular frequency.

[0052] Control frequency satisfies

[0053]

[0054] The total resistivity Z of the measurement circuit x In the resonant state, it is represented as: Z x =R L +Rx +R eq

[0055] Simultaneously adjust the values ​​of R1, R2, and R3 so that

[0056]

[0057] At this point, the inductive reactance cancels out the capacitive reactance of the coupling capacitor, and the negative resistance R eq With R L Internal resistance cancels out, and the impedance of the entire measurement path is equal. The sensor's output is only related to R. x It is unaffected by the background coupling capacitor reactance and the equivalent internal resistance of the inductor.

[0058] Example 1:

[0059] To verify that the negative resistance module of the sensor of this invention can function properly after being connected to the current-to-voltage module, the following test was performed. A 10 kΩ resistor was selected as the test resistor, with one end connected to the signal generator and the other end connected to the input terminal of the negative resistance module via an inductor. Simultaneously, the output terminal of the negative resistance module was connected to the input terminal of the current-to-voltage module (the input terminal was grounded). With this connection method, the current flowing through the test resistor (the current of the entire circuit) can be obtained by measuring the voltage across the test resistor, and then the equivalent resistance of the entire circuit and the equivalent value of the negative resistance can be obtained according to Ohm's law. Figure 3 As shown, the yellow waveform represents the input voltage waveform of the resistor under test, and the blue waveform represents the output voltage waveform. The voltages across the test resistor have a 180-degree phase difference (reverse). This proves that the current flowing through the test resistor is significantly greater than the ratio of the AC excitation to the test resistor (this value is the current value without the negative resistor to cancel the resistance), indicating that the negative resistor module is working normally. With the resistance value of the negative resistor module fixed, the value of the test resistor was changed. The experimental results are shown in Tables 1 and 2. It can be clearly seen that the current-to-voltage output decreases monotonically as the resistance value of the test resistor increases. The output corresponds one-to-one with the resistance value of the resistor under test, which is in line with the experimental expectations, indicating that the negative resistor module is working normally. Comparing the outputs of the traditional CID and the CID sensor using the technical route of this invention, it can be seen that when the resistance value of the test resistor increases from 1.7 kΩ to 170 kΩ, the sensor output using the traditional CID sensor only decreases from 0.314V to 0.248V, a decrease of less than 25%. This shows that the sensor sensitivity is low due to the influence of background capacitance. The sensor output using the CID sensor of this invention dropped from 8.43V to 1.010V, a decrease of more than 85%, indicating that the sensor using the CID sensor of this invention has higher sensitivity.

[0060] Table 1. Simulation Experiment of Traditional CID Sensor

[0061]

[0062] Table 2. CID sensor simulation experiments recorded using the technology of this invention

[0063]

[0064] Example 2:

[0065] After completing the simulation experiment, the CID sensor of this invention was used to measure the conductivity of fluids in an actual pipeline. The pipeline had an inner diameter of 3 mm, an outer diameter of 4 mm, and a length of 10 cm. The excitation electrode and detection electrode of the CID sensor were installed in the middle of the pipeline. During the test, a potassium chloride conductivity solution with a conductivity ranging from 100 uS / cm to 1000 uS / cm was passed into the pipeline (the higher the conductivity, the lower the resistance), and the sensor output was recorded. The experimental results are shown in Tables 3 and 4. It can be clearly seen that the traditional CID sensor does not change significantly after the conductivity of the measured solution reaches 60 uS / cm, indicating low sensor sensitivity. However, the CID sensor using the method of this invention still has significant sensitivity when the conductivity exceeds 100 uS / cm.

[0066] Table 3. Conductivity Testing Experiments for Traditional CID Sensors

[0067]

[0068] Table 4. Conductivity Test Experiments of CID Sensor Using the Technical Route of the Invention

[0069]

[0070] Example 3:

[0071] The sensor of this invention is used for two-phase flow pattern identification in pipelines. The sensor is constructed by attaching copper tape to a circular pipe (hydraulic diameter of the flow channel is 2 mm), and a T-type mixer is used to simultaneously introduce air and electrolytes with different conductivity (50 uS / cm and 200 uS / cm) into the flow channel, forming a gas-liquid two-phase flow. The aqueous phase flow rate is 10-100 ml / min, the gas phase flow rate is 0-30 ml / min, and the flow patterns are single-phase flow, bubbly flow, and slug flow. During the experiment, a high-speed camera is used to calibrate the flow patterns. Flows with bubble sizes smaller than the flow channel size are considered bubbly flow, and those with bubble sizes larger than the flow channel size are considered slug flow (see...). Figure 4(The area enclosed by the yellow dashed box contains air bubbles). During manifold identification, a windowing method was used to collect the sensor's output signal within 0.1 seconds, and the mean and variance were calculated. The mean and variance were then combined with a neural network for manifold identification. 200 data points were obtained for each manifold, with 100 sets used for training and 100 sets for testing. The results using / not using this method under different electrolyte conductivity conditions are shown in Tables 5 to 8.

[0072] Table 5. Flow pattern recognition accuracy of traditional CID sensors (electrolyte conductivity 50 uS / cm)

[0073]

[0074] Table 6. Flow pattern recognition accuracy of traditional CID sensors (electrolyte conductivity 200 uS / cm)

[0075]

[0076] Table 7. Flow pattern recognition accuracy using the CID sensor of this invention (electrolyte conductivity 50 uS / cm)

[0077]

[0078] Table 8. Flow pattern recognition accuracy using the CID sensor of the present invention (electrolyte conductivity 200 uS / cm)

[0079]

[0080] It is evident that traditional CID sensors achieve good flow pattern identification performance (accuracy of over 90% for each pattern) when the solution conductivity is low. However, as the solution conductivity increases, the sensor's sensitivity and signal-to-noise ratio decrease significantly due to the influence of background capacitance, and the identification accuracy of bubbly and slug flows drops significantly. This is particularly true for bubbly flows with small bubble volumes, where the resulting change in two-phase flow resistance is minimal, leading to a significant decrease in identification accuracy. In contrast, the CID sensor using this method maintains high flow pattern identification accuracy even at high conductivity. Considering the high conductivity of solutions in boiling heat transfer and electrolytic water processes, the CID sensor using this method has greater application potential.

[0081] It should be further noted that in the embodiments provided by this invention, the conductivity of the measured solution is in the range of 100 μS / cm to 1000 μS / cm. In scenarios such as electrolytic hydrogen production and electrochemical reactors, high-conductivity electrolytes are typically used to reduce the resistance of the anode and cathode, and wide flow channel structures are used to increase the reaction area, resulting in a further reduction in the equivalent resistance of the measured fluid. In such low-resistance systems, even small resistances such as electrode contact resistance, wire resistance, and the equivalent series internal resistance of the inductor can affect the measurement. If only the existing virtual inductor scheme is used to eliminate the capacitive reactance of the coupling capacitor, although the equivalent internal resistance of the virtual inductor can be made smaller by selecting a large-value resistor, it is still a positive value and constitutes the background resistance itself. This invention introduces a negative resistance module, which, on the basis of the inductor canceling the capacitive reactance of the coupling capacitor, further cancels the equivalent series internal resistance of the inductor and various contact resistances, so that the total resistivity Zx = Rx of the measurement circuit in the resonant state is only related to the resistance of the measured fluid. Therefore, the sensor of the present invention still has sufficient sensitivity and measurement range in low resistance systems, and can be applied to the detection of high conductivity two-phase flow systems such as water electrolysis and boiling heat exchange.

[0082] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A non-contact impedance sensor based on the synergistic compensation of negative resistance and inductance, characterized in that, include: Excitation electrode, detection electrode, inductor and negative resistor module; The excitation electrode and the detection electrode are spaced apart on the outer wall of the insulating pipe containing the fluid to be measured; the excitation electrode, the coupling capacitor formed by the insulating pipe wall, the equivalent resistance of the fluid to be measured, the detection electrode, and the inductor and negative resistance module connected in series therebetween constitute the measurement circuit. The inductor is connected in series in the measurement circuit of the non-contact impedance sensor; the inductive reactance of the inductor is used to cancel the capacitive reactance of the coupling capacitance formed between the excitation electrode and the detection electrode through the wall of the insulating pipe; The negative resistance module is connected in series to the measurement circuit to generate an equivalent negative resistance; the resistance value of the equivalent negative resistance generated by the negative resistance module is configured to be equal to the resistance value of the equivalent series internal resistance of the inductor, so as to cancel the equivalent series internal resistance.

2. The non-contact impedance sensor according to claim 1, characterized in that, The excitation electrode is used to connect to an external AC excitation source, and the detection electrode is connected to an external current-to-voltage module via a negative resistor module; the excitation frequency of the measurement circuit is configured to satisfy the resonance condition, so that the sum of the inductive reactance of the inductor and the total capacitive reactance of the coupling capacitor is zero.

3. The non-contact impedance sensor according to claim 1, characterized in that, The negative resistance module includes operational amplifier A1, resistor R1, resistor R2 and resistor R3; The resistor R1 is connected between the inverting input terminal and the output terminal of the operational amplifier A1; The first end of the resistor R2 is connected to the non-inverting input terminal of the operational amplifier A1, and the second end of the resistor R2 is connected to the output terminal of the operational amplifier A1. The first end of resistor R3 is connected to the non-inverting input of operational amplifier A1, and the second end of resistor R3 is connected to the input of an external current-to-voltage module as the output of the negative resistance module; the input of the external current-to-voltage module is a virtual ground.

4. The non-contact impedance sensor according to claim 3, characterized in that, The equivalent negative resistance value of the negative resistance module , R1, R2, and R3 are the resistance values ​​of the corresponding resistors.

5. The non-contact impedance sensor according to claim 3, characterized in that, The total resistivity Z of the measurement circuit x In the resonant state, it is represented as: Z x =R L +R x +R eq , where R L R is the equivalent series resistance of the inductor. x R is the resistance of the fluid being measured. eq The equivalent negative resistance generated by the negative resistance module; the value of the equivalent negative resistance is configured to be equal to the value of the equivalent series internal resistance of the inductor, i.e., R. eq =-R L The total resistivity Z x =R x .

6. The non-contact impedance sensor according to claim 3, characterized in that, The inverting input terminal of the operational amplifier A1 is connected to the inductor as the input terminal of the negative resistor module.

7. The non-contact impedance sensor according to claim 3, characterized in that, The output terminal of the current-to-voltage module is connected to the input terminal of the RMS detection module, which is used to output a DC signal that is proportional to the conductivity of the fluid being measured.

8. The non-contact impedance sensor according to claim 1, characterized in that, The excitation electrode and the detection electrode are copper tapes adhered to the outer wall of the insulating pipe.

9. A method for measuring fluid conductivity using a non-contact impedance sensor as described in any one of claims 1 to 8, characterized in that, include: An AC excitation voltage is applied to the measurement circuit through an AC excitation source; Adjust the frequency of the AC excitation voltage so that the inductive reactance of the inductor and the capacitive reactance of the coupling capacitor cancel each other out; Adjust the parameters of the negative resistance module so that the equivalent negative resistance generated by the negative resistance module is equal to the equivalent series internal resistance of the inductor. The alternating current signal in the measurement circuit is detected, and the conductivity of the fluid under test is inverted based on the alternating current signal.

10. The method according to claim 9, characterized in that, The adjustment of the frequency of the AC excitation voltage includes: monitoring the phase of the voltage and current of the measurement circuit using an oscilloscope; when the voltage and current are in phase, confirming that the inductive reactance of the inductor and the capacitive reactance of the coupling capacitor cancel each other out.

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