High-precision impedance detection circuit capable of overcoming the influence of distributed parameters and line resistance

Abstract

The present invention relates to a high-precision impedance detection circuit that can overcome the influence of distributed parameters and line resistance, which is composed of an impedance sensor, an integrated operational amplifier, and a current detection resistor; during detection, a known excitation voltage is applied to the measured medium through the sensor, and then the response current of the measured medium is detected; the excitation is connected to the non-inverting input terminal of the first operational amplifier, and its output terminal is connected to the excitation electrode of one plate of the sensor through the shielding layer of the shielded wire. The sensing electrode of the same plate is fed back to the inverting input terminal of the first operational amplifier through the inner core of the shielded wire; the sensing electrode of the other plate of the sensor is connected to the inverting input terminal of the third operational amplifier through the inner core of the triple coaxial cable, and the excitation electrode is connected to the current detection resistor through the inner shielding layer of the triple coaxial cable and then connected to the output terminal of the third operational amplifier, and its non-inverting input terminal is grounded; on the instrument side, the inner shielding layer of the triple coaxial cable is connected to its outer shielding layer after being driven by the voltage follower of the second operational amplifier; the circuit impedance detection of the present invention has high precision and strong anti-interference ability.

Description

Technical Field

[0001] The present invention relates to the technical field of detection, and relates to a high-precision impedance detection circuit that can overcome the influence of distributed parameters and line resistance. Background Art

[0002] Impedance is an electrical parameter of a material object. There are specific correlation relationships between many physical and chemical properties of a material system and its impedance. People often obtain various performance information by detecting its impedance value or impedance characteristics. Therefore, impedance detection is widely used in many fields, and modern impedance spectroscopy technology is also based on the impedance detection of the object to be measured.

[0003] The detection of the impedance of a material object is usually realized by means of a flat impedance sensor. The main structure of this sensor is two relatively flat plate electrodes with conductive surfaces, and the medium to be measured is filled in the middle. This sensor has the characteristics of simple structure, fast dynamic response, strong environmental adaptability, high working reliability, and average effect. During impedance detection, the instrument circuit outputs a known excitation voltage u (constant or time-varying) and acts on both sides of the medium to be measured through the flat plate electrodes of the sensor, and then the corresponding instrument output response current i is detected as the current flowing through the medium to be measured, or a known excitation current i (constant or time-varying) is output and introduced into the medium to be measured through the flat plate electrodes, and then the response voltage u at the output end of the instrument is detected as the voltage on both sides of the medium to be measured. Finally, the impedance obtained by dividing the voltage u by the current i is used as the impedance of the medium to be measured system. This impedance detection method has a fast data acquisition speed, a simple circuit structure, low cost, and high circuit stability.

[0004] However, the most commonly used impedance detection method mentioned above is also affected by many adverse factors, resulting in low detection accuracy. First, there are distributed parameters such as distributed resistance and distributed capacitance between the lines of the detection circuit, between the sensor leads and adjacent components such as external lines and instrument enclosures. These distributed parameters will shunt the excitation current or response current output by the instrument, resulting in the current detected by the instrument circuit being greater than the true current flowing through the measured medium between the sensor plates. Second, the line resistance composed of the lead resistance of the connecting wire from the instrument output terminal to the sensor plates, the contact resistance at the line joints, the film resistance of various films such as the oxide film / reaction film / polarization film on the sensor plate surface, etc., is in series with the impedance of the measured medium between the sensor plates, and will also produce a series voltage division effect, resulting in the actual voltage across the measured medium being less than the voltage at the output terminal of the instrument circuit. The influence of the above two factors will result in the voltage u and the corresponding current i at the output terminal of the instrument not being the true voltage across the measured medium and the true current flowing through the measured medium. Therefore, there is an obvious error in using the voltage u at the output terminal of the instrument divided by the corresponding current i to represent the impedance of the measured medium; moreover, because the shunting effect of the distributed parameters and the voltage division effect of the line resistance are variable and unmeasurable, this error cannot be corrected.

[0005] The impedance detection circuit involved in this patent is precisely to weaken or overcome the influence of the aforementioned distributed parameters and line resistance, so as to improve the impedance detection accuracy, thereby meeting the high-precision detection requirements for impedance in many fields. Summary of the Invention

[0006] The present invention provides an impedance detection circuit, by providing a circuit structure, to overcome the influence of distributed parameters and line resistance on impedance detection, improve the impedance detection accuracy, make it applicable to more fields requiring high-precision measurement, and expand its scope of use and application scenarios.

[0007] The high-precision impedance detection circuit provided by the present invention that can overcome the influence of distributed parameters and line resistance is as follows:

[0008] In a first aspect, it includes a first operational amplifier that provides a specified first excitation voltage to the non-inverting input terminal of the first operational amplifier; it includes a shielding device and an impedance measurement sensor connected to the shielding device. The inverting input terminal and the output terminal of the first operational amplifier are connected to the impedance measurement sensor through the shielding device; the impedance measurement sensor includes a first plate and a second plate, and a measured medium is provided between the first plate and the second plate; excitation electrodes and sensing electrodes are provided on both the first plate and the second plate.

[0009] The shielding device includes a first shielded wire, and the first shielded wire includes a shielding layer and an inner core.

[0010] The output terminal of the first operational amplifier is connected to the excitation electrode of the first plate of the impedance measurement sensor through the shielding layer of the first shielded wire, and the sensing electrode of the first plate is connected to the inverting input terminal of the first operational amplifier through the inner core of the first shielded wire.

[0011] Based on the first aspect, a first implementation manner of other aspects is provided. It further includes a triaxial cable. One end of the triaxial cable is connected to the impedance measurement sensor, and the other end is connected to a current detection resistor, a third operational amplifier, and a voltage follower. The other end of the current detection resistor is connected to the output terminal of the third operational amplifier. The non-inverting input terminal of the third operational amplifier is grounded. The voltage across the current detection resistor is the output signal of the detection circuit.

[0012] The triaxial cable includes an outer shielding layer, an inner shielding layer, and an inner core.

[0013] The excitation electrode of the second plate of the impedance measurement sensor is connected to the inner shielding layer at one end of the triaxial cable, and the sensing electrode of the second plate of the impedance measurement sensor is connected to the inner core at the same end of the triaxial cable; the inner core at the other end of the triaxial cable is connected to the inverting input terminal of the third operational amplifier. The inner shielding layer at the same end of the triaxial cable is connected to the output terminal of the third operational amplifier after being connected to the current detection resistor on one hand, and is connected to the outer shielding layer of the triaxial cable after being connected to the voltage follower on the other hand.

[0014] The voltage follower is formed by connecting the inverting input terminal and the output terminal of the second operational amplifier. The non-inverting input terminal of the second operational amplifier is connected to the inner shielding layer of the triaxial cable, and the output terminal is connected to the outer shielding layer of the triaxial cable.

[0015] The first excitation voltage can be a constant voltage or a time-varying voltage; the response current flowing through the measured medium can be obtained by dividing the voltage of the output signal by the current detection resistor, and the impedance of the measured medium can be obtained by dividing the first excitation voltage by the response current.

[0016] Based on the first aspect, a second implementation manner of other aspects is provided, which further includes an impedance reference sensor having the same structure as the impedance measurement sensor, a second operational amplifier, and a third operational amplifier. The shielding device further includes a second shield wire, a third shield wire, and a fourth shield wire having the same structure as the first shield wire. A specified second excitation voltage is provided to the non-inverting input terminal of the second operational amplifier. The inverting input terminal of the second operational amplifier is connected to the sensing electrode of the first plate of the impedance reference sensor through the inner core of the second shield wire, and the output terminal of the second operational amplifier is connected to the excitation electrode of the first plate of the impedance reference sensor through the shielding layer of the second shield wire. The excitation electrode and the sensing electrode of the second plate of the impedance measurement sensor are connected to the inner core of one end of the third shield wire, and the excitation electrode and the sensing electrode of the second plate of the impedance reference sensor are connected to the inner core of one end of the fourth shield wire. At the other ends of the third shield wire and the fourth shield wire, the shielding layers of the two are interconnected and grounded, and at the same time, the inner cores of the two are interconnected and then connected to the inverting input terminal of the third operational amplifier. A current detection resistor is connected in parallel between the inverting input terminal and the output terminal of the third operational amplifier, and the non-inverting input terminal is grounded.

[0017] Based on the first aspect, a third implementation manner of other aspects is provided, which further includes an impedance reference sensor having the same structure as the impedance measurement sensor, a second operational amplifier, and a third operational amplifier. The shielding device further includes a second shield wire and a third shield wire having the same structure as the first shield wire. A specified second excitation voltage is provided to the non-inverting input terminal of the second operational amplifier. The inverting input terminal of the second operational amplifier is connected to the sensing electrode of the first plate of the impedance reference sensor through the inner core of the second shield wire, and the output terminal of the second operational amplifier is connected to the excitation electrode of the first plate of the impedance reference sensor through the shielding layer of the second shield wire. The sensing electrodes and the excitation electrodes of the second plates of the impedance measurement sensor and the impedance reference sensor are interconnected and connected to the inner core of one end of the third shield wire. The inner core of the other end of the third shield wire is connected to the inverting input terminal of the third operational amplifier. The shielding layer of the same end of the third shield wire and the non-inverting input terminal of the third operational amplifier are both connected to ground, and a current detection resistor is connected between the inverting input terminal and the output terminal of the third operational amplifier.

[0018] Based on the second implementation or the third implementation in other aspects, a reference medium is provided between the first electrode plate and the second electrode plate of the impedance reference sensor. The second excitation voltage and the first excitation voltage are equal in amplitude but opposite in polarity direction. The first excitation voltage and the second excitation voltage can be a constant voltage or a time-varying voltage. The voltage across the current detection resistor is the output signal of the detection circuit. The difference current between the response current flowing through the measured medium and the response current flowing through the reference medium can be obtained by dividing the voltage of the output signal by the current detection resistor. The difference in admittance between the measured medium and the reference medium can be obtained by dividing the difference current by the first excitation voltage. Then, by using the known admittance value or impedance value of the reference medium, the admittance or impedance of the measured medium can be obtained.

[0019] Advantages of the above invention:

[0020] The high-precision impedance detection circuit provided by the present invention can overcome the influence of distributed parameters and line resistance. A known excitation voltage is applied to the object to be measured, and then the response current flowing through the object to be measured is detected, so as to obtain the impedance value of the object to be measured. The main advantages of the invention are that it significantly reduces or even overcomes the influence of the distributed parameters of the detection circuit and the line resistance on the impedance detection accuracy, can significantly improve the impedance detection accuracy, and at the same time has the advantages of fast data acquisition speed, simple circuit structure, low cost, and high circuit stability, and can be applied to more fields that require high-precision measurement. Brief Description of the Drawings

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0022] Figure 1 Schematic diagram of the circuit structure of the first embodiment of the high-precision impedance detection circuit provided by the invention that can overcome the influence of distributed parameters and line resistance;

[0023] Figure 2 Schematic diagram of the circuit structure of the second embodiment of the high-precision impedance detection circuit provided by the invention that can overcome the influence of distributed parameters and line resistance;

[0024] Figure 3 Schematic diagram of the circuit structure of the third embodiment of the high-precision impedance detection circuit provided by the invention that can overcome the influence of distributed parameters and line resistance;

[0025] Figure 4 Structural diagram of the impedance sensor of the high-precision impedance detection circuit provided by the present invention that can overcome the influence of distributed parameters and line resistance;

[0026] Figure 5 This is a schematic diagram of the three coaxial lines of a high-precision impedance detection circuit provided by the present invention that can overcome the influence of distributed parameters and line resistance.

[0027] Illustration:

[0028] 110 - First shield wire; 120 - Impedance measurement sensor; 130 - Three coaxial lines; 140 - Instrument circuit;

[0029] 210 - Second shield wire; 220 - Impedance reference sensor; 230 - Third shield wire; 240 - Fourth shield wire; 250 - Measured medium; 260 - Reference medium;

[0030] 310 - First electrode plate; 320 - Second electrode plate; 330 - Excitation electrode of the first electrode plate; 340 - Excitation electrode of the second electrode plate; 350 - Sensing electrode of the first electrode plate; 360 - Sensing electrode of the second electrode plate; 370 - Measured medium or reference medium;

[0031] 410 - Outer shield layer; 420 - Inner shield layer; 430 - Inner core. Detailed implementation manners

[0032] It should be understood that the exemplary embodiments described herein should be considered only as descriptive and not for the purpose of limitation. The description of the features or aspects in each exemplary embodiment should generally be considered applicable to similar features or aspects in other exemplary embodiments.

[0033] The above description is provided with reference to the accompanying drawings to assist in a comprehensive understanding of the various embodiments of the present invention defined by the claims. It includes various specific details to assist in this understanding, but these details should be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and improvements can be made to the various embodiments described herein without departing from the scope of the present invention defined by the appended claims. In addition, descriptions of well-known functions and configurations may be omitted for clarity and conciseness.

[0034] The terms and words used in the following description and claims are not limited to the written meanings, but are used only by the inventor to allow a clear and consistent understanding of the present invention. Accordingly, it will be apparent to those skilled in the art that the following description of the various embodiments of the present invention is provided only for the purpose of explanation and not for the purpose of limiting the present invention defined by the appended claims.

[0035] Throughout the description and claims of this application, the words "comprising" and "including" and variations of those words, such as "comprises" and "includes", mean "including but not limited to", and do not exclude other components, integers or steps.

[0036] Features, integers or characteristics described in connection with a particular aspect, embodiment or example of the present invention will be understood to be applicable to any other aspect, embodiment or example described herein, unless incompatible therewith.

[0037] It should be understood that the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. In the present invention, the expression "or" includes any and all combinations of the words listed together. For example, "A or B" can include A or B, or can include both A and B.

[0038] Although terms such as "first" and "second" may be used to describe various elements of the present invention, they are not intended to limit the corresponding elements. For example, the above terms are not intended to limit the order or importance of the corresponding elements. The above terms can be used to distinguish one component from another.

[0039] The terminology used in this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the technical field to which this application belongs. It should also be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and this specification, and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0040] The following specifically describes a high-precision impedance detection circuit that can overcome the influence of distributed parameters and line resistance provided for the embodiments of the present invention:

[0041] Embodiment 1

[0042] Refer to Figure 1 , the high-precision impedance detection circuit provided in this embodiment that can overcome the influence of distributed parameters and line resistance includes a first operational amplifier A1, a second operational amplifier A2, and a third operational amplifier A3. Among them, a specified first excitation voltage, i.e., excitation voltage u, is provided to the non-inverting input terminal of the first operational amplifier A1 i。It also includes a shielding device, which is the first shield wire 110 in this embodiment. The output end of the first operational amplifier A1 is connected to the first shield wire 110, and the first shield wire 110 is connected to the impedance measurement sensor 120. In this embodiment, the impedance measurement sensor 120 is connected to the inverting input end of the first operational amplifier via the first shield wire 110.

[0043] In this embodiment, referring to Figure 4 , the impedance measurement sensor 120 includes a first electrode plate 310 and a second electrode plate 320. A measured medium is disposed between the first electrode plate and the second electrode plate, and the impedance of the measured medium between the electrode plates is Z X , where excitation electrode regions 330 / 340 and sensing electrode regions 350 / 360 are both provided on the first electrode plate 310 and the second electrode plate 320. Among them, the surfaces of the excitation electrode regions and the sensing electrode regions are conductive and electrically separated from each other and independent of each other. The excitation electrode region almost covers the entire electrode plate, while the sensing electrode region is composed of thin lines or small dot regions that are uniformly distributed inside the excitation electrode region and are interconnected. The area of the excitation electrode region is much larger than the area of the sensing electrode region. The excitation electrode region forms the excitation electrode, and the sensing electrode region forms the sensing electrode. The excitation electrodes 330 / 340 are used to input an excitation voltage and a corresponding response current to the measured medium between the first electrode plate 310 and the second electrode plate 320, and the sensing electrodes 350 / 360 are used to sense the potential difference between the two medium surfaces at the contact interface between the medium and the two electrode plates. This potential difference is also the true excitation voltage borne by the measured medium.

[0044] In this embodiment, the first shield wire 110 includes a shielding layer and a core.

[0045] In this embodiment, based on this, the output end of the first operational amplifier A1 is connected to the excitation electrode of the first electrode plate 310 of the impedance measurement sensor 120 via the shielding layer of the first shield wire 110, and the sensing electrode of the same electrode plate of the impedance measurement sensor 120, that is, the first electrode plate 310, is connected to the inverting input end of the first operational amplifier A1 via the core of the first shield wire 110.

[0046] In this embodiment, the detection circuit further includes a triple coaxial line 130.

[0047] Referring to Figure 5, the triple coaxial cable 130 includes an outer shielding layer 410, an inner shielding layer 420, and an inner core 430. The excitation electrode of the second electrode plate 320 of the impedance measurement sensor 120 is connected to the inner shielding layer of the triple coaxial cable 130, and the sensing electrode of the second electrode plate 320 is connected to the inner core of the triple coaxial cable 130. At the other end of the triple coaxial cable 130, the inner core of the triple coaxial cable is connected to the inverting input terminal of the third operational amplifier A3. On the one hand, the inner shielding layer of the triple coaxial cable is connected to the output terminal of the third operational amplifier A3 after connecting the current detection resistor Rs, and on the other hand, it is connected to the outer shielding layer of the triple coaxial cable 130 after passing through a voltage follower; the non-inverting input terminal of the third operational amplifier A3 is grounded.

[0048] In this embodiment, the voltage follower is formed by connecting the inverting input terminal and the output terminal of the second operational amplifier A2, where the non-inverting input terminal of the second operational amplifier A2 is connected to the inner shielding layer of the triple coaxial cable 130, and the output terminal is connected to the outer shielding layer of the triple coaxial cable 130.

[0049] In this embodiment, the voltage u across the current detection resistor Rs o is the output signal of the detection circuit, which is used to reflect the current flowing through the current detection resistor Rs, that is, the response current i flowing through the measured medium between the two electrode plates of the impedance measurement sensor 120. This current is i = u o / Rs, and the impedance of the measured medium is Z x = u i / i = u i ·Rs / u o .

[0050] In this embodiment, the current detection resistor Rs is usually selected as a precision resistor, and its value is determined by the range of the response current flowing through the impedance measurement sensor 120 and the allowable range of the output voltage u o that can be accepted by the input side of the subsequent detection circuit. Usually, it is taken as the maximum allowable value of the output voltage u o divided by the maximum value of the response current i.

[0051] In this embodiment, the first operational amplifier A1, the second operational amplifier A2, and the third operational amplifier A3 are respectively precision operational amplifiers with high input impedance, low bias current, and low offset voltage.

[0052] The impedance detection circuit provided in this embodiment can effectively overcome the influence of the series voltage division effect of the line resistance on the detection accuracy of the impedance of the measured medium. The principle lies in:

[0053] First, the output of the first operational amplifier A1 is connected to the inverting input terminal of the first operational amplifier A1 through the shielding layer of the first shielded wire 110, the excitation electrode of the first plate 310 of the impedance measurement sensor 120, the measured medium, the sensing electrode of the first plate 310 of the impedance measurement sensor 120, and the inner core of the first shielded wire 110 to form negative feedback. The feedback regulation of the output voltage or current by the internal circuit of the first operational amplifier A1 ensures that the potential at its inverting input terminal is equal to the excitation voltage u at its non-inverting input terminal i ; the bypass shunt caused by the distributed parameters between the first shielded wire 110 and the external lines and components is provided by the output terminal of the first operational amplifier A1 through the shielding layer of the first shielded wire 110. Therefore, the inner core of the first shielded wire 110 is not affected by the external distributed parameter bypass shunt. At the same time, due to the extremely high input impedance of the input terminal of the first operational amplifier A1, the current flowing through the path from its inverting input terminal through the inner core of the first shielded wire 110 and the sensing electrode of the first plate 310 of the impedance measurement sensor 120 to the measured medium is zero, and the current on the surface of the measured medium from the sensing electrode to the excitation electrode in the first plate 310 is also zero. This ensures that the potential on the surface of the measured medium in contact with the first plate 310 of the impedance measurement sensor 120 is equal to the potential at the inverting input terminal of the first operational amplifier A1, and is also equal to the excitation voltage u at the non-inverting input terminal i .

[0054] Second, the output of the third operational amplifier A3 is connected to its inverting input terminal through the current detection resistor Rs, the inner shielding layer of the triple coaxial cable 130, the excitation electrode of the second plate 320 of the impedance measurement sensor 120, the measured medium, the sensing electrode of the second plate 320 of the impedance measurement sensor 120, and the inner core of the triple coaxial cable 130 to form negative feedback. The feedback regulation of the output voltage or current by the internal circuit of the third operational amplifier A3 ensures that the potential at its inverting input terminal is equal to the potential at its non-inverting input terminal, and is equal to 0 because the non-inverting input terminal is grounded; the inner core of the triple coaxial cable 130 is not affected by the external distributed parameter bypass shunt due to the double protection of the inner shielding layer and the outer shielding layer. At the same time, due to the extremely high input impedance of the input terminal of the third operational amplifier A3, the current flowing through the path from its inverting input terminal through the inner core of the triple coaxial cable 130 and the sensing electrode of the second plate 320 of the impedance measurement sensor 120 to the measured medium is zero, and the current on the surface of the measured medium from the sensing electrode to the excitation electrode in the second plate 320 is also zero. This ensures that the potential on the surface of the measured medium in contact with the second plate 320 of the impedance measurement sensor 120 is equal to the potential at the inverting input terminal of the third operational amplifier A3, and is also equal to the potential 0 at its non-inverting input terminal

[0055] The above two aspects jointly ensure that the voltage across the two sides of the measured medium between the two plates of the impedance measurement sensor 120 is exactly equal to the excitation voltage u i, thus eliminating the influence of the series voltage division of the line resistance between the instrument excitation output end and the surface of the measured medium in the traditional measurement method, and overcoming the problem that the output voltage of the instrument is not equal to the actual voltage borne on both sides of the measured medium.

[0056] The impedance detection circuit provided in this embodiment can also effectively overcome the influence of the bypass shunt effect of distributed parameters on the impedance detection accuracy of the measured medium. The principle is as follows:

[0057] The response current flowing from the excitation electrode of the first electrode plate 310 of the impedance measurement sensor 120 through the measured medium to the excitation electrode of the second electrode plate 320 continues to flow into the output end of the third operational amplifier A3 after passing through the inner shielding layer of the triple coaxial line 130 and the current detection resistor Rs; since the inner shielding layer of the triple coaxial line 130 is connected to the outer shielding layer after passing through the voltage follower driver composed of the second operational amplifier A2, the inner shielding layer and the outer shielding layer are at the same potential, and there will be no leakage current between the inner shielding layer and the outer shielding layer, which ensures that the current flowing through the inner shielding layer to the current detection resistor Rs is exactly equal to the response current flowing through the measured medium. Therefore, the current measured by the voltage u o across the current detection resistor Rs is exactly equal to the response current flowing through the measured medium. At the same time, the bypass shunt generated by distributed parameters such as distributed resistance and distributed capacitance between the triple coaxial line 130 and external circuits and components such as the chassis is provided by the output end of the second operational amplifier A2 through the outer shielding layer of the triple coaxial line 130, and will not affect the response current flowing through the inner shielding layer, thereby overcoming and avoiding the influence of the bypass shunt effect of distributed parameters on the response current.

[0058] In addition, since an integrated operation is used to form the detection circuit, this solution has the characteristics of high input impedance and low output impedance.

[0059] Embodiment 2

[0060] Refer to Figure 2 , the impedance detection circuit provided in this embodiment has the same points as that in Embodiment 1 in that it is provided with the first operational amplifier A1, the first shielded wire 110 and the impedance measurement sensor 120 which are the same as the corresponding components in Embodiment 1, and the mutual connection relationship of these three components and the specified excitation voltage u iBoth it and its connection mode with the first operational amplifier A1 are the same as those in Embodiment 1; meanwhile, the impedance detection circuit provided in Embodiment 2 also includes a second operational amplifier A2, a third operational amplifier A3, and a current detection resistor Rs whose connection relationships are different from those of the corresponding components in Embodiment 1; moreover, the impedance detection circuit provided in Embodiment 2 further includes an impedance reference sensor 220 having the same structure as the impedance measurement sensor 120, and a second shield wire 210, a third shield wire 230, and a fourth shield wire 240 having the same structure as the first shield wire 110. A reference medium is provided between the two plates of the impedance reference sensor 220.

[0061] In this embodiment, the description is mainly focused on the circuit structure different from that in Embodiment 1. In this embodiment, a specified second excitation voltage -u i with the same amplitude but opposite polarity as the first excitation voltage u i is provided to the non-inverting input terminal of the second operational amplifier A2. The inverting input terminal of the second operational amplifier A2 is connected to the sensing electrode of the first plate 310 of the impedance reference sensor 220 through the inner core of the second shield wire 210, and the output terminal of the second operational amplifier A2 is connected to the excitation electrode of the first plate 310 of the impedance reference sensor 220 through the shield layer of the second shield wire 210; the excitation electrode and the sensing electrode of the second plate 320 of the impedance measurement sensor 120 are connected to the inner core of one end of the third shield wire 230, and the excitation electrode and the sensing electrode of the second plate 320 of the impedance reference sensor 220 are connected to the inner core of one end of the fourth shield wire 240. At the other ends of the third shield wire 230 and the fourth shield wire 240, the shield layers of the two are interconnected and grounded, and at the same time, the inner cores of the two are interconnected and then connected to the inverting input terminal of the third operational amplifier A3. A current detection resistor Rs is connected across the inverting input terminal and the output terminal of the third operational amplifier A3, and the non-inverting input terminal is grounded.

[0062] In this embodiment, the detection circuit includes two symmetric channels: a measurement channel and a reference channel. The measurement channel is composed of the first operational amplifier A1, the first shield wire 110, the impedance measurement sensor 120, the third shield wire 230, the current detection resistor Rs, and the third operational amplifier A3; the reference channel is composed of the second operational amplifier A2, the second shield wire 210, the impedance reference sensor 220, the fourth shield wire 240, the current detection resistor Rs, and the third operational amplifier A3. Since the connection relationships among the second excitation voltage -u i of the reference channel with the second operational amplifier A2, the second shield wire 210, and the first plate 310 of the impedance reference sensor 220 are the same as the connection relationships among the first excitation voltage +u iThe connection relationships among the first operational amplifier A1, the first shield wire 110, and the first electrode plate 310 of the impedance measurement sensor 120 are symmetric and identical. Therefore, according to the principle analyzed in Embodiment 1, in this Embodiment 2, it is ensured that the inner cores of the first shield wire 110 and the second shield wire 210 are not affected by the bypass shunt of external distributed parameters. At the same time, it is also ensured that the potential of the surface of the measured medium in contact with the first electrode plate 310 of the impedance measurement sensor 120 is equal to the first excitation voltage +u i , and the potential of the surface of the reference medium in contact with the first electrode plate 310 of the impedance reference sensor 220 is equal to the second excitation voltage -u i . This eliminates the series voltage division effect of the line resistance between the output end of the first excitation voltage +u i and the surface of the measured medium on the first electrode plate side of the measurement sensor, and also eliminates the series voltage division effect of the line resistance between the output end of the second excitation voltage -u i and the surface of the reference medium on the first electrode plate side of the reference sensor, overcoming the problem that the potential of the instrument excitation output end, namely +u i and -u i , is not equal to the potential of the corresponding medium surface (i.e., the surface of the measured medium in contact with the first electrode plate of the measurement sensor and the surface of the reference medium in contact with the first electrode plate of the reference sensor).

[0063] At the same time, in this embodiment, since the non-inverting input terminal of the third operational amplifier A3 is grounded, and due to the negative feedback effect from its output terminal through the current detection resistor Rs to its inverting input terminal, the potential of its inverting input terminal and the inner cores of the third shield wire 230 and the fourth shield wire 240 connected thereto are all equal to the zero potential of its non-inverting input terminal; and since the shielding layers of the third shield wire 230 and the fourth shield wire 240 are interconnected and also connected to the ground, for the third shield wire 230 and the fourth shield wire 240, the potential of the inner core is equal to that of the shielding layer, and both are equal to the ground potential, that is, zero potential. Therefore, there will be no leakage current between the inner core and the shielding layer, and the bypass shunt generated by the distributed parameters between the shielding wire and the external components is provided by the ground of the instrument, that is, the common reference pole of the instrument power supply, through the shielding layers of the third shield wire 230 and the fourth shield wire 240. This ensures that the current flowing through the inner core of the third shield wire 230 to the current detection resistor Rs and then into the output terminal of the third operational amplifier A3 is exactly equal to the response current i of the measured medium flowing through the impedance measurement sensor 120 X , and also ensures that the current flowing from the output terminal of the third operational amplifier A3 through the current detection resistor Rs and the inner core of the fourth shield wire 240 to the reference sensor 220 is also exactly equal to the response current i flowing through the reference medium R , that is, it ensures that the net current flowing through the current detection resistor Rs to the output terminal of the third operational amplifier A3 is exactly equal to the difference current between the response current of the measured medium and the response current of the reference medium (i X -iR ), namely, the voltage u across the current detecting resistor Rs o can accurately detect the differential current (i X - i R ), without being affected by the shunt of the distributed parameters.

[0064] In this embodiment, if the line impedance and its voltage division of the two sections, namely, the section from the excitation electrode and the sensing electrode of the second plate 320 of the measurement sensor 120 through the inner core of the third shield wire 230 to the current detecting resistor Rs, and the section from the current detecting resistor Rs through the inner core of the fourth shield wire 240 to the excitation electrode and the reference electrode of the second plate 320 of the reference sensor 220, are not considered, the excitation voltages borne by the measured medium and the reference medium are respectively (+u i - 0 = u i ) and (0 - (-u i ) = u i ). Dividing the excitation voltage by the impedance Z X of the measured medium and the impedance Z R of the reference medium respectively, the response currents flowing through the measured medium and the reference medium can be obtained as i X = u i / Z X and i R = u i / Z R respectively. Therefore, the output voltage across the current detecting resistor is: u o = Rs · (i X - i R ) = Rs · (u i / Z X - u i / Z R ). From this, the impedance of the measured medium can be obtained as:

[0065] The detection circuit provided in this embodiment can effectively overcome the influence of distributed parameters and significantly reduce the influence of line resistance compared with the traditional impedance detection circuit. Moreover, the differential detection structure composed of the measurement channel and the reference channel can significantly improve the impedance detection range of the measured medium, and at the same time significantly improve the sensitivity of impedance detection. It has the advantages of high sensitivity, wide measurement range, good stability, and strong anti-interference ability, and is suitable for impedance measurement occasions with high precision, large range, and high sensitivity.

[0066] Embodiment 3

[0067] Refer to Figure 3, this embodiment provides a detection circuit structure diagram based on Embodiment 2 for the case where the impedance measurement sensor 120 and the impedance reference sensor 220 are arranged in a very close proximity. Different from the detection circuit provided in Embodiment 2, the electrical connection structure between the impedance measurement sensor 120 and the impedance reference sensor 220 and the third operational amplifier A3 is different.

[0068] Specifically: In this embodiment, the sensing electrodes and the excitation electrodes of the second electrode plates 320 of the impedance measurement sensor 120 and the impedance reference sensor 220 are interconnected and connected to one end of the inner core of the third shielded wire 230. The other end of the inner core of the third shielded wire 230 is connected to the inverting input terminal of the third operational amplifier A3. The shield layer of the third shielded wire 230 is grounded to the non-inverting input terminal of the third operational amplifier A3. The fourth shielded wire 240 in Embodiment 2 is removed and not used in this embodiment. The remaining circuit structure, connection relationship, application position of the excitation signal, output signal of the circuit, etc. are exactly the same as those in Embodiment 2. The characteristics, effects, impedance detection working process, and calculation and solution of the impedance of the measured medium of the circuit provided in this embodiment are also the same as those in Embodiment 2, so they will not be elaborated here.

[0069] The above various embodiments are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, various modifications and changes can be made to the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A high-precision impedance detection circuit that can overcome the influence of distributed parameters and line resistance, characterized in that Comprising a first operational amplifier, and providing a specified first excitation voltage to the non-inverting input terminal of the first operational amplifier; Comprising a shielding device, and an impedance measurement sensor connected to the shielding device, the inverting input terminal and the output terminal of the first operational amplifier are connected to the impedance measurement sensor after passing through the shielding device; the impedance measurement sensor includes a first electrode plate and a second electrode plate, and a measured medium is provided between the first electrode plate and the second electrode plate; excitation electrodes and sensing electrodes are provided on both the first electrode plate and the second electrode plate; the shielding device includes a first shielded wire, and the first shielded wire includes a shielding layer and an inner core; the output terminal of the first operational amplifier is connected to the excitation electrode of the first electrode plate of the impedance measurement sensor through the shielding layer of the first shielded wire, and the sensing electrode of the first electrode plate is connected to the inverting input terminal of the first operational amplifier through the inner core of the first shielded wire; further comprising a triple coaxial cable, one end of the triple coaxial cable is connected to the impedance measurement sensor, and the other end is connected with a galvanometer resistance, a third operational amplifier and a voltage follower, the other end of the galvanometer resistance is connected to the output terminal of the third operational amplifier, the non-inverting input terminal of the third operational amplifier is grounded, and the voltage across the two ends of the galvanometer resistance is the output signal of the detection circuit; the excitation electrode of the second electrode plate of the impedance measurement sensor is connected to the inner shielding layer at one end of the triple coaxial cable, and the sensing electrode of the second electrode plate of the impedance measurement sensor is connected to the inner core at the same end of the triple coaxial cable; the inner core at the other end of the triple coaxial cable is connected to the inverting input terminal of the third operational amplifier, and the inner shielding layer at the same end of the triple coaxial cable is connected to the output terminal of the third operational amplifier after connecting the galvanometer resistance on the one hand, and is connected to the outer shielding layer of the triple coaxial cable after connecting the voltage follower on the other hand.

2. The high-precision impedance detection circuit capable of overcoming the influence of distributed parameters and line resistance according to claim 1, characterized in that, The voltage follower is formed by connecting the inverting input terminal and the output terminal of a second operational amplifier, the non-inverting input terminal of the second operational amplifier is connected to the inner shielding layer of the triple coaxial cable, and the output terminal is connected to the outer shielding layer of the triple coaxial cable.

3. The high-precision impedance detection circuit capable of overcoming the influence of distributed parameters and line resistance according to claim 1, wherein Dividing the voltage of the output signal by the galvanometer resistance can obtain the response current flowing through the measured medium, and dividing the first excitation voltage by the response current can obtain the impedance of the measured medium.

4. The high-precision impedance detection circuit capable of overcoming the influence of distributed parameters and line resistance according to claim 3, wherein The first excitation voltage is a constant voltage.

5. The high-precision impedance detection circuit capable of overcoming the influence of distributed parameters and line resistance according to claim 3, wherein The first excitation voltage is a time-varying voltage.

6. The high-precision impedance detection circuit capable of overcoming the influence of distributed parameters and line resistance according to claim 1 is further characterized in that, It further includes an impedance reference sensor having the same structure as the impedance measurement sensor, a second operational amplifier, and a third operational amplifier. The shielding device further includes a second shield wire, a third shield wire, and a fourth shield wire having the same structure as the first shield wire. A specified second excitation voltage is provided to the non-inverting input terminal of the second operational amplifier. The inverting input terminal of the second operational amplifier is connected to the sensing electrode of the first plate of the impedance reference sensor through the inner core of the second shield wire, and the output terminal of the second operational amplifier is connected to the excitation electrode of the first plate of the impedance reference sensor through the shielding layer of the second shield wire. The excitation electrode and the sensing electrode of the second plate of the impedance measurement sensor are connected to the inner core of one end of the third shield wire, and the excitation electrode and the sensing electrode of the second plate of the impedance reference sensor are connected to the inner core of one end of the fourth shield wire. At the other ends of the third shield wire and the fourth shield wire, the shielding layers of the two are interconnected and grounded, and at the same time, the inner cores of the two are interconnected and connected to the inverting input terminal of the third operational amplifier. A current detection resistor is connected in parallel between the inverting input terminal and the output terminal of the third operational amplifier, and the non-inverting input terminal is grounded.

7. The high-precision impedance detection circuit capable of overcoming the influence of distributed parameters and line resistance according to claim 1, further characterized in that, It further includes an impedance reference sensor having the same structure as the impedance measurement sensor, a second operational amplifier, and a third operational amplifier. The shielding device further includes a second shield wire and a third shield wire having the same structure as the first shield wire. A specified second excitation voltage is provided to the non-inverting input terminal of the second operational amplifier. The inverting input terminal of the second operational amplifier is connected to the sensing electrode of the first plate of the impedance reference sensor through the inner core of the second shield wire, and the output terminal of the second operational amplifier is connected to the excitation electrode of the first plate of the impedance reference sensor through the shielding layer of the second shield wire. The sensing electrodes and the excitation electrodes of the second plates of the impedance measurement sensor and the impedance reference sensor are interconnected and connected to the inner core of one end of the third shield wire. The inner core of the other end of the third shield wire is connected to the inverting input terminal of the third operational amplifier. The shielding layer of the same end of the third shield wire and the non-inverting input terminal of the third operational amplifier are both connected to ground, and a current detection resistor is connected between the inverting input terminal and the output terminal of the third operational amplifier.

8. The high-precision impedance detection circuit capable of overcoming the influence of distributed parameters and line resistance according to any one of claims 6-7, characterized in that A reference medium is provided between the first plate and the second plate of the impedance reference sensor. The second excitation voltage has the same amplitude as the first excitation voltage but opposite in polarity direction. The voltage across the current detection resistor is the output signal of the detection circuit. By dividing the voltage of the output signal by the current detection resistor, the differential current between the response current flowing through the measured medium and the response current flowing through the reference medium can be obtained. By dividing the differential current by the first excitation voltage, the difference in admittance between the measured medium and the reference medium can be obtained. Then, using the known admittance value or impedance value of the reference medium, the admittance or impedance of the measured medium can be obtained.

9. The high-precision impedance detection circuit capable of overcoming the influence of distributed parameters and line resistance according to claim 8, wherein The first excitation voltage is either a constant voltage or a time-varying voltage.

Images