CN210464785U - Bipolar capacitance type vacuum gauge and corresponding measuring circuit thereof - Google Patents

Abstract

The utility model relates to the field of vacuum gauge measurement, in particular to a bipolar capacitance type vacuum gauge and a corresponding measuring circuit thereof, which comprises a shell, a diaphragm, a fixed substrate and a fixed electrode; the diaphragm is fixedly arranged in the shell and divides the shell into two parts, one side is a vacuum cavity, and the other side is a detection cavity; the fixed substrate is fixedly arranged in the vacuum cavity, the fixed electrode is arranged on the fixed substrate, when the diaphragm deforms under stress, the capacitance on the fixed electrode changes, and the pressure in the detection cavity can be obtained by detecting the change of the capacitance; the fixed electrode comprises an annular electrode and a circular electrode, the circular electrode is positioned in the annular electrode, and a gap is formed between the circular electrode and the annular electrode, so that the fixed electrode is divided into the annular electrode and the circular electrode, and the precision of the vacuum gauge can be effectively improved; the measuring circuit detects and processes capacitance changes on the annular electrode and the circular electrode and outputs a sine wave, and the pressure can be obtained by measuring the sine wave.

Description

Bipolar capacitance type vacuum gauge and corresponding measuring circuit thereof

Technical Field

The utility model relates to a vacuum gauge measurement field especially relates to a bipolar capacitance formula vacuum gauge and measurement circuit that corresponds thereof.

Background

The metal film vacuum gauge is a vacuum instrument which is recognized as a working secondary standard of low vacuum measurement (0.01-100 Pa), and is also a vacuum degree measuring instrument with legal metrological calibration verification rules in China (calibration reference rule: Q/WHJ46-1998 standard type capacitance film vacuum gauge calibration rule). The metal capacitance film vacuum gauge is a vacuum gauge for absolute pressure and total pressure measurement, and is made according to the principle that the elastic detection diaphragm generates strain under the action of pressure difference to cause capacitance change. The measurement of the vacuum gauge is a direct measurement type full-pressure vacuum gauge which directly reflects the change value of vacuum pressure and is only related to pressure and gas components.

The difficulty of vacuum measurement is small range and high accuracy. Under the existing process conditions, the single-capacitor metal film vacuum gauge can achieve the full scale of 10Torr and the precision of 0.25% of the reading value, but the process is difficult to realize under the existing process of the single-capacitor metal film vacuum gauge when the range is continuously reduced, for example, the range is reduced to 0.1Torr and the precision value of the single-capacitor metal film vacuum gauge is still 0.25% of the reading value. Therefore, under the existing material and processing conditions, the problem of how to improve the precision needs to be solved.

SUMMERY OF THE UTILITY MODEL

Technical problem to be solved

The utility model provides a bipolar capacitance formula vacuum gauge and measurement circuit that corresponds thereof aims at solving prior art, the not high problem of vacuum gauge precision.

(II) technical scheme

In order to solve the above problems, the present invention provides a bipolar capacitance type vacuum gauge including a case, a diaphragm, a fixed substrate and a fixed electrode;

the diaphragm is fixedly arranged in the shell and divides the shell into two parts, one side is a vacuum cavity, and the other side is a detection cavity;

the fixed substrate is fixedly arranged in the vacuum cavity, and the fixed electrode is arranged on the fixed substrate;

the fixed electrode comprises an annular electrode and a circular electrode, the circular electrode is positioned in the annular electrode, and a gap is formed between the circular electrode and the annular electrode.

Preferably, the detection cavity is provided with a detection hole for detecting pressure.

Preferably, the circular electrode and the annular electrode are disposed on a side of the fixed substrate facing the diaphragm, and an insulating layer is disposed at a space between the circular electrode and the annular electrode.

Preferably, when the diaphragm deforms under stress, the distance variation amount of the diaphragm relative to the annular electrode is Δ d_{Outer cover}The amount of change in distance from the circular electrode is Δ d_{Inner part}；

The area ratio of the circular electrode to the annular electrode is equal to Δ d_{Inner part}And said Δ d_{Outer cover}The ratio of.

Preferably, the area ratio of the circular electrode to the annular electrode is 1: 1-1.5: 1.

preferably, the diaphragm is a circular diaphragm, and the ratio of the outer diameter of the ring electrode to the diameter of the diaphragm is 0.7: 1-1: 1.

preferably, the membrane is parallel to the fixed substrate when not under force.

Preferably, a measuring circuit for measuring the pressure in the detection chamber in a bipolar capacitance type vacuum gauge, the measuring circuit includes a common-substrate bridge type wave detection circuit and an oscillation circuit;

the common-substrate bridge detection circuit is connected with the circular electrode and the annular electrode, and outputs induction voltage according to the capacitance variation on the circular electrode and the annular electrode;

the common-substrate bridge detection circuit is connected with the oscillation circuit, the oscillation circuit outputs sine waves corresponding to the induction voltage according to the induction voltage, the amplitude of the sine waves corresponds to the magnitude of the induction voltage, and the frequency of the sine waves corresponds to the frequency of the induction voltage.

Preferably, the common-substrate bridge-type detector circuit comprises a first diode, a second diode, a third diode and a fourth diode, and the first diode, the second diode, the third diode and the fourth diode are sequentially connected end to form a closed loop;

the common-substrate bridge detection circuit comprises a first input end and a second input end, wherein the first input end is positioned at the joint of the first diode and the second diode, and the second input end is positioned at the joint of the third diode and the fourth diode;

the common-substrate bridge type detection circuit further comprises a first output end and a second output end, the first output end is located at the joint of the second diode and the third diode, and the second output end is located at the joint of the fourth diode and the first diode.

Preferably, the first input end is connected to the ring electrode through a wire, the second input end is connected to the circular electrode, and the first output end and the second output end are connected to the oscillation circuit.

(III) advantageous effects

The utility model discloses an increase fixed electrode to set up fixed electrode into circular electrode and ring electrode, when the diaphragm takes place deformation, can arouse the change of zero-bit electric capacity and ring electrode polar plate zero-bit electric capacity on the circular electrode polar plate, join in marriage the outside measuring circuit who corresponds with it, the deformation that can make the change volume of electric capacity and measure the diaphragm is close to linear relation, compare with single electrode capacitance type film vacuum gauge, its precision improves nearly twice, can accurately improve the precision of small-scale vacuum gauge.

Drawings

FIG. 1 is a schematic diagram of a bipolar capacitance type vacuum gauge;

FIG. 2 is a schematic diagram of a bipolar capacitance type vacuum gauge when a diaphragm is bent under a force;

FIG. 3 is a top view of a circular electrode and a ring electrode in a bipolar capacitance vacuum gauge;

FIG. 4 is a schematic diagram of a measurement circuit of a bipolar capacitance gauge;

fig. 5 is a specific circuit diagram of a measuring circuit of the bipolar capacitance type vacuum gauge.

[ description of reference ]

1: a housing; 11: a vacuum chamber; 12: a detection chamber; 13: a detection hole; 2: a membrane; 3: fixing the substrate; 31: an insulating layer; 4: a ring electrode; 5: a circular electrode; 6: a common-substrate bridge detection circuit; 7: an oscillation circuit; 71: an oscillation control circuit; 72: a bridge type oscillation circuit; 73: an RC filter circuit; m1: a first diode; m2: a second diode; m3: a third diode; m4: and a fourth diode.

Detailed Description

For a better understanding of the present invention, reference will now be made in detail to the present invention, examples of which are illustrated in the accompanying drawings.

It should be noted that all the directional indicators (such as upper, lower, left, right, front and rear … …) in the embodiment of the present invention are only used to explain the relative position relationship between the components, the motion situation, etc. in a specific posture (as shown in the drawings), and if the specific posture is changed, the directional indicator is changed accordingly.

In addition, descriptions in the present application as to "first", "second", and the like are for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicit to the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, "a plurality" means at least two, e.g., two, three, etc., unless specifically limited otherwise.

In the present application, unless expressly stated or limited otherwise, the terms "connected" and "fixed" are to be construed broadly, e.g., "fixed" may be fixedly connected or detachably connected, or integrally formed; can be mechanically or electrically connected; they may be directly connected or indirectly connected through intervening media, or they may be connected internally or in any other suitable relationship, unless expressly stated otherwise. The specific meaning of the above terms in the present invention can be understood according to specific situations by those skilled in the art.

The utility model provides a bipolar capacitance type vacuum gauge (as shown in figures 1to 3), which comprises a shell 1, a diaphragm 2, a fixed substrate 3 and a fixed electrode;

the diaphragm 2 is fixedly installed in the housing 1, in a preferred embodiment, the diaphragm 2 is installed in the housing 1 by a welding installation method, the diaphragm 2 divides the housing 1 into two parts, one side is a vacuum chamber 11, the other side is a detection chamber 12, and the vacuum chamber 11 and the detection chamber 12 are two independent chambers, it should be noted that the diaphragm 2 can also adopt other installation methods as long as the housing 1 can be divided into two independent chambers.

The fixed substrate 3 is fixedly arranged in the vacuum cavity 11, and a fixed electrode is arranged on the fixed substrate 3; fixed electrode and diaphragm 2 have constituteed two polar plates of a variable capacitance, and diaphragm 2 produces elastic deformation after receiving the pressure in detecting chamber 12 for the distance between diaphragm 2 and the fixed electrode changes, and variable capacitance changes the back in the distance between two polar plates, and the electric capacity on the fixed electrode also can change, can calculate the size that detects chamber 12 internal pressure through the change volume of measuring the electric capacity on the fixed electrode. The pressure measuring mode is only related to the pressure and is not related to the gas components, and the measuring precision is high.

The fixed electrode comprises a ring electrode 4 and a circular electrode 5, the circular electrode 5 is located in the ring electrode 4, and a space is formed between the circular electrode 5 and the ring electrode 4, in a preferred scheme, as shown in fig. 2 and 3, the circular electrode 5 and the ring electrode 4 are arranged on one side of the fixed substrate facing the diaphragm 2, an insulating layer 31 is arranged at the space between the circular electrode 5 and the ring electrode 4, and the insulating layer 31 between the circular electrode 5 and the ring electrode 4 can be a ceramic insulating layer, which has a good insulating effect and avoids mutual transfer of electrons between the circular electrode 5 and the ring electrode 4.

As shown in fig. 2, when the measured pressure P is measured₁>P_bDue to the pressure difference (P) in the pressure gauge₁-P_b) The deformation of the diaphragm 2 causes a capacitance (C) on the circular electrode_{Inner part}) Andcircular ring polar plate zero capacitance (C)_{Outer cover}) Is changed. The capacitance formula C is K epsilon S/d:

wherein:

P₁: the measured pressure;

P_b: high vacuum system (P)_b<10^-3Pa), i.e. the reference pressure;

C_{inner part}: when the diaphragm 2 is not stressed, the capacitance on the circular electrode 5 is increased;

C_{outer cover}: when the diaphragm 2 is not stressed, the capacitance on the annular electrode 4 is increased;

epsilon: a dielectric permittivity;

s: the effective area between the two polar plates of the flat capacitor;

k: an electrostatic force constant;

d: the distance between the two polar plates.

Then, for the capacitance on the circular electrode 5, the distance change Δ d under the measured pressure is_{Inner part}＝d₀-d₁₁The effective area of the circular electrode 5 relative to the membrane 2 is S_{Round (T-shaped)}；

For the capacitance on the ring electrode 4, its distance variation Δ d under the measured pressure_{Outer cover}＝d₀-d₂₁The effective area of the circular electrode 5 relative to the membrane 2 is S_Ring；

d₀: the diaphragm 2 is away from the annular electrode 4 and the circular electrode 5 when deformed;

d₁₁: when the diaphragm 2 is deformed under stress, a gap is formed between the diaphragm 2 and the circular electrode 5;

d₂₁: when the diaphragm 2 is deformed by stress, a gap is formed between the diaphragm 2 and the annular electrode 4.

When the displacement of the diaphragm 2 changes: the displacement of the membrane 2 relative to the circular electrode 5 is deltad_{Inner part}While the gap d between the membrane 2 and the circular electrode 5₁₁＝d₀-Δd_{Inner part}(ii) a The displacement of the membrane 2 relative to the ring electrode 4 is deltad_{Outer cover}While the gap d between the membrane 2 and the circular electrode 4₂₁＝d₀-Δd_{Outer cover}(ii) a The corresponding capacitance of the circular electrode is

The corresponding capacitance of the ring plate at this moment is

Then:

the relative change amounts of the capacitance values on the circular electrode 5 and the ring electrode 4 are respectively:

when Δ d_{Inner part}/d₀<<1, Δ C_{Inner part}/C_{Inner part}≈Δd_{Inner part}/d₀；

Equivalent to delta d_{Outer cover}/d₀<<1, Δ C_{Outer cover}/C_{Outer cover}≈Δd_{Outer cover}/d₀；

Then Δ C_{Inner part}/C_{Inner part}And Δ d_{Inner part}/d₀、ΔC_{Inner part}/C_{Inner part}And Δ d_{Inner part}/d₀Approximately linear.

The utility model provides a pair of bipolar capacitance-type vacuum gauge's precision:

and the accuracy K of the single-electrode film vacuum gauge_Sheet＝Δd/d₀。

The utility model provides a precision K of bipolar electrode capacitance type vacuum gauge_{Double is}Precision K of single electrode film vacuum gauge_SheetCompared with the prior art, the precision is improved by nearly two times.

Taking into account Δ C/C₀The linear term and the cubic term in (b) to obtain a relative linearity error r which is approximately:

when in use

Time of flight

Taking into account Δ C/C₀Linear and quadratic terms in (b), then:

the relative non-linearity error of a bipolar capacitance vacuum gauge:

relative non-linear error of prior art single-pole capacitance type vacuum gauge

r_Sheet＞r_{Double is}Therefore, the utility model discloses bipolar capacitance type vacuum gauge's measurement is more accurate.

A detection hole 13 for detecting pressure is formed in the detection cavity 12, the detection hole 13 is communicated with the detection cavity 12, the detection hole 13 is set to be in an open state, and the diaphragm 2 can detect the pressure in the detection cavity 12; the detection hole 13 is communicated with the outside and the detection cavity 12, so that the detection operation is convenient, and the structure is simple.

When the diaphragm 2 is deformed under stress, the distance variation relative to the annular electrode 4 is delta d_{Outer cover}The distance change amount with respect to the circular electrode 5 is Δ d_{Inner part}When the materials of the diaphragms 2 are different and the installation modes of the diaphragms 2 are different, the diaphragms 2 are stressedThe deformation varies with respect to the distance between the circular electrode 5 and the annular electrode 4, but the area S of the circular electrode 5 is determined_{Round (T-shaped)}Area S of the ring electrode 4_RingThe ratio is set equal to Δ d_{Inner part}And said Δ d_{Outer cover}Compared with the prior art, the influence of the null shift on the measuring circuit can be effectively reduced.

In a preferred embodiment, the area S of the circular electrode 5 is defined_{Round (T-shaped)}Area S of the ring electrode 4_RingThe ratio of 1: 1-1.5: 1, the influence of the null shift on a measuring circuit can be effectively reduced. This is because during the forced flexing of the membrane 2

The method comprises the following steps:

when the diaphragm 2 is installed in the housing 1, the specific installation process may be welding, riveting, etc., so that stress concentration is generated at the installation position of the diaphragm 2 and the housing 1, deformation generated by stress is not uniform, and thus an edge effect is generated, and the area S of the circular electrode 5 is reduced_{Round (T-shaped)}Area S of the ring electrode 4_RingRatio of (A) to (B) being equal to

The structure solves the problem of the consistency of the ratio of the internal capacitance variation and the external capacitance variation, eliminates the edge effect and solves the problem of null shift stability.

Meanwhile, the diaphragm 2 is a circular diaphragm, and the ratio of the outer diameter of the ring electrode 4 to the diameter of the diaphragm 2 is 0.7: 1-1: 1, the external diameter with ring electrode 4, diaphragm 2 are parallel to each other with fixed baseplate 3 when not atress, make the utility model discloses bipolar capacitance type vacuum gauge's measuring range is bigger change.

The membrane 2 is preferably made of a nickel-based alloy thin film, so that the membrane 2 has better corrosion resistance and can be applied to complex environments.

As shown in FIG. 4, a measuring circuit for measuring the pressure in a detection chamber 12 in a bipolar capacitance type vacuum gauge includes a common-substrate bridge type detection circuit 6 and an oscillation circuit 7.

The common-base bridge type detection circuit 6 is connected to the circular electrode 5 and the ring electrode 4, and the common-base bridge type detection circuit 6 outputs an induced voltage according to the capacitance variation on the circular electrode 5 and the ring electrode 4.

The common-substrate bridge type detection circuit 6 is connected with the oscillation circuit 7, the oscillation circuit 7 outputs sine waves corresponding to the induced voltage according to the induced voltage, the amplitude of the sine waves corresponds to the magnitude of the induced voltage, and the frequency of the sine waves corresponds to the frequency of the induced voltage.

The common-substrate bridge type detection circuit 6 comprises a first diode M1, a second diode M2, a third diode M3 and a fourth diode M4, wherein the first diode M1, the second diode M2, the third diode M3 and the fourth diode M4 are sequentially connected end to form a closed loop;

in a preferred embodiment, the four diodes in common-substrate bridge detector circuit 6 are implemented as an integrated circuit having six diodes mounted on a common monolithic substrate, five of which are used independently, and the sixth diode shares a common terminal with the substrate (referred to as common ground in this circuit).

The common-substrate bridge detector circuit 6 comprises a first input terminal Vin1 and a second input terminal Vin2, the first input terminal Vin1 is located at the junction of a first diode M1 and a second diode M2, and the second input terminal Vin2 is located at the junction of a third diode M3 and a fourth diode M4;

the common-substrate bridge detector circuit 6 further includes a first output terminal Vout1 and a second output terminal Vout2, the first output terminal Vout1 is located at the junction of the second diode M2 and the third diode M3, and the second output terminal Vout2 is located at the junction of the fourth diode M4 and the first diode M1.

The first input terminal Vin1 is connected to the ring electrode 4, the second input terminal Vin2 is connected to the circular electrode 5, and the first output terminal Vout1 and the second output terminal Vout2 are connected to the oscillation circuit 7.

For external measurement circuits, circuit emphasis of the present design and prior artAs with the medium single-stage capacitive vacuum gauge, the voltage signal needs to be proportional to Δ C; however, the voltage signal of the bipolar capacitance type vacuum gauge of the present invention is proportional to (Δ C)_{Inner part}+ΔC_{Outer cover}). To achieve the design goal, a relative change in capacitance value needs to be obtained. The utility model discloses a total base bridge detection circuit 6 acquires the capacitance variation (delta C) on circular polar plate 5 and the ring polar plate 4 specially for bipolar electric formula vacuum gauge_{Inner part}+ΔC_{Outer cover}) Signal and utility model's circuit.

The oscillation circuit 7 includes an oscillation control circuit 71 and a bridge oscillation circuit 72, as shown in fig. 4, the first output terminal Vout1 and the second output terminal Vout2 are combined into a total output terminal Vout3, the induced voltage flows to the bridge oscillation circuit 72 and the oscillation control circuit 71, respectively, and the bridge oscillation circuit 72 and the oscillation control circuit 71 generate two waves with the same frequency according to the induced voltage.

The oscillation control circuit 71 rectifies, filters, amplifies, and oscillates the induced voltage to generate an alternating voltage, the alternating voltage generated by the oscillation control circuit 71 is a sine wave having a certain amplitude and frequency, and the alternating voltage generated by the oscillation control circuit 71 flows to the bridge oscillation circuit 72. The bridge oscillator circuit 72 outputs a specific sinusoidal alternating voltage at point P, the amplitude of which corresponds to the amplitude of the alternating voltage generated by the oscillation control circuit 71, in accordance with the frequency of the induced voltage and the alternating voltage generated by the oscillation control circuit 71. The pressure in the vacuum gauge can be obtained by measuring the sine alternating voltage.

As shown in fig. 5, in a specific measuring circuit of the present invention, the oscillation control circuit 71 includes an operational amplifier LM308, a field effect device J232, and a plurality of resistors and capacitors; the bridge oscillation circuit 72 comprises an operational amplifier NE5534 and a plurality of resistors and capacitors, wherein the operational amplifier NE5534 and the RC are connected in series and in parallel to form an RC bridge oscillator; the total output terminal Vout3 on the common-substrate bridge detector circuit 6 is connected to the inverting input terminal of the operational amplifier LM308, and the operational amplifier LM308 amplifies the induced voltage input from the inverting input terminal of the operational amplifier LM 308. The total output end Vout3 of the common-substrate bridge-type detector circuit 6 is further connected to the forward input end of the operational amplifier NE5534 through a frequency-selective network composed of a resistor and a capacitor, the output end Vout4 of the operational amplifier LM308 is connected to the reverse input end of the operational amplifier NE5534 through a field effect device J232, the output end Vout5 of the operational amplifier NE5534 is connected to an RC filter circuit 73, and the output point P in the RC filter circuit 73 is the output end of the oscillator circuit 7. The voltage measured at point P is:

U＝U_P*{(ΔC_{inner part}/C₃)+(ΔC_{Outer cover}/C)₂}；

Get C₂＝C₃If U is equal to U_P*ΔC/C₂；

Wherein U is_PIs the input sine wave amplitude; Δ C ═ Δ C_{Inner part}+ΔC_{Outer cover}；

As shown in FIG. 5, C can be accurately determined by adjusting the value of C1 in the present circuit_{Inner part}And C_{Outer cover}And (4) the initial value.

Table 1 shows the comparison between the bipolar capacitance type vacuum gauge manufactured according to the design of the present invention and the single-electrode capacitance type vacuum gauge in the prior art when measuring data. Under the same environment, two vacuum gauges with the same range measure the measurement data when the actual values are zero, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and full of the range and the zero drift condition of the vacuum gauge at the normal temperature of 27 ℃ after zero setting; and calculating the precision grades of the two vacuum gauges.

TABLE 1

The percentages in Table 1 refer to the percentage of the full scale, with 0Pa to 1333.2Pa being the scale (scale of 0-10Torr), 0Pa representing the zero position, and 1333.2Pa representing the full scale, i.e., the full position.

As can be seen from the table 1, the utility model provides a bipolar capacitance type vacuum gauge compares with the single electrode capacitance type vacuum gauge among the prior art, and the precision has obtained great improvement, and receives the influence of null shift less.

It should be understood that the above description of the embodiments of the present invention is only for illustrating the technical lines and features of the present invention, and the purpose of the present invention is to enable those skilled in the art to understand the contents of the present invention and to implement the present invention accordingly, but the present invention is not limited to the above specific embodiments. All changes and modifications that come within the scope of the claims are to be embraced within their scope.

Claims (10)

1. A bipolar capacitance type vacuum gauge is characterized by comprising a shell, a diaphragm, a fixed substrate and a fixed electrode;

the diaphragm is fixedly arranged in the shell and divides the shell into two parts, one side is a vacuum cavity, and the other side is a detection cavity;

the fixed substrate is fixedly arranged in the vacuum cavity, and the fixed electrode is arranged on the fixed substrate;

the fixed electrode comprises an annular electrode and a circular electrode, the circular electrode is positioned in the annular electrode, and a gap is formed between the circular electrode and the annular electrode.

2. The bipolar capacitance vacuum gauge of claim 1, wherein the sensing chamber is provided with a sensing hole for sensing pressure.

3. The bipolar capacitance vacuum gauge of claim 1, wherein the circular electrode and the ring electrode are disposed on a side of the stationary substrate facing the membrane, and an insulating layer is disposed at a space between the circular electrode and the ring electrode.

4. The bipolar capacitance vacuum gauge of claim 1, wherein the diaphragm, when deformed by a force, changes distance Δ d from the ring electrode_{Outer cover}The amount of change in distance from the circular electrode is Δ d_{Inner part}；

The area ratio of the circular electrode to the annular electrode is equal to Δ d_{Inner part}And said Δ d_{Outer cover}The ratio of.

5. The bipolar capacitance vacuum gauge of claim 4, wherein the ratio of the area of the circular electrode to the area of the ring electrode is 1: 1-1.5: 1.

6. the bipolar capacitance vacuum gauge of any of claims 1-5, wherein the diaphragm is a circular diaphragm and the ratio of the outer diameter of the ring electrode to the diameter of the diaphragm is 0.7: 1-1: 1.

7. the bipolar capacitance vacuum gauge of any of claims 1-5, wherein the membrane is parallel to the stationary substrate when unstressed.

8. A measuring circuit for measuring the pressure in the detection chamber in the bipolar capacitance type vacuum gauge according to any one of claims 1to 7, wherein the measuring circuit comprises a common-substrate bridge type wave detecting circuit and an oscillating circuit;

the common-substrate bridge detection circuit is connected with the circular electrode and the annular electrode, and outputs induction voltage according to the capacitance variation on the circular electrode and the annular electrode;

the common-substrate bridge detection circuit is connected with the oscillation circuit, the oscillation circuit outputs sine waves corresponding to the induction voltage according to the induction voltage, the amplitude of the sine waves corresponds to the magnitude of the induction voltage, and the frequency of the sine waves corresponds to the frequency of the induction voltage.

9. The measurement circuit of claim 8, wherein the common-substrate bridge detector circuit comprises a first diode, a second diode, a third diode, and a fourth diode, and wherein the first diode, the second diode, the third diode, and the fourth diode are sequentially connected end-to-end to form a closed loop;

the common-substrate bridge detection circuit comprises a first input end and a second input end, wherein the first input end is positioned at the joint of the first diode and the second diode, and the second input end is positioned at the joint of the third diode and the fourth diode;

the common-substrate bridge type detection circuit further comprises a first output end and a second output end, the first output end is located at the joint of the second diode and the third diode, and the second output end is located at the joint of the fourth diode and the first diode.

10. The measurement circuit of claim 9, wherein the first input is connected to the ring electrode by a wire, the second input is connected to the circular electrode, and the first output and the second output are connected to the oscillation circuit.

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