CN105806900A - Humidity detection circuit - Google Patents

Abstract

A humidity detection circuit belongs to the technical field of humidity sensors. The problem that humidity detection is unstable due to the fact that a humidity sensor changes along with temperature change in a low-temperature environment is solved. The humidity detection circuit is a humidity measurement circuit based on a capacitance charging and discharging and comparison method, so that temperature drift and zero drift are effectively inhibited, and the influence of parasitic capacitance on a measurement result is reduced. It is used for humidity sensor.

Description

A humidity detection circuit

technical field

The present invention belongs to the field of humidity sensor technology. The present invention is a divisional application of the invention patent "Heating Humidity Sensor for Airsonde and Its Preparation Method and a Humidity Detection Circuit" filed on August 20, 2014 with application number 201410411744.2 .

Background technique

The operational level of upper-air meteorological detection is one of the main references to measure the scientific level of a country's atmospheric detection, and humidity detection is an important part of upper-air meteorological detection. Due to the harsh high-altitude environment, drastic changes in humidity, and the lowest ambient temperature reaches -90°C, this requires the humidity sensor to have the advantages of low temperature resistance, fast response, and strong anti-interference ability, and the capacitive humidity sensor has the above characteristics, and the manufacturing cost is relatively low. , has become one of the important research directions of humidity sensors used in radiosondes. However, due to the structural characteristics of the capacitive humidity sensor, when it is in a high-humidity environment, the surface of the humidity sensor is prone to condensation, which increases the measurement error and even causes the sensor to fail. At present, the research focus of relevant scholars at home and abroad tends to optimize the sensor structure and improve the humidity-sensitive materials. Developed countries such as Europe and the United States have always been in the leading position in the field of research in this field. For example, the polymer capacitive humidity sensor developed by the Austrian E+E company solves the problem of low-temperature humidity measurement from the aspect of moisture-sensing materials. Its response time is about 1.5s and the resolution About 1%, with an uncertainty of about 5%, and can work normally at -80°C; the RS92 radiosonde developed by Vaisala, Finland, solves the problem of low-temperature humidity measurement from the sensor structure and working mode, using two pieces with heating function The humidity sensor works alternately, its response time is less than 0.5s, the resolution is about 1%RH, and the uncertainty is about 5%, which is currently recognized as the standard for high-altitude humidity detection.

Therefore, the current humidity sensor throws the problem that the humidity measurement effect is not good in a low temperature environment.

The inductance of the humidity sensor itself and the equivalent inductance of the external leads have a certain influence on the capacitance of the humidity sensor. Moreover, the parasitic capacitance between the humidity sensor and the ground and the parasitic capacitance between the lead wires change with the change of temperature in a low-temperature environment, which makes the humidity detection unstable.

Contents of the invention

The purpose of the present invention is to solve the problem that the humidity sensor changes with the change of temperature in a low temperature environment, making the humidity detection unstable. The present invention provides a humidity detection circuit.

A humidity detection circuit of the present invention, said humidity detection circuit includes a heating type humidity sensor C _m for radiosondes, a standard capacitance C _s , an analog resistance R _P , a parasitic capacitance C _P , a resistance R1, a resistance R2, three calculation Amplifier, 2 SPDT switches and power supply;

One end of the humidity sensor C _m and one end of the standard capacitor C _s are connected to the ground terminal of the power supply at the same time, the other end of the humidity sensor C _m is connected to a static end of the first single-pole double-throw switch, and the other end of the standard capacitor C _s connected with the other static terminal of the first SPDT switch,

The moving end of the first SPDT switch is connected to one end of the analog resistor R _P , and the other end of the analog resistor R _P is connected to one end of the parasitic capacitor C _P , one end of the resistor R1 and the positive input end of the first operational amplifier to supply power simultaneously. The Vcc end of the power supply, the other end of the parasitic capacitor C _P is connected to the ground end of the power supply,

The other end of the resistor R1 is connected to the moving end of the second SPDT switch, one static end of the second SPDT switch is connected to the Vcc end of the power supply, and the other static end of the second SPDT switch is connected to the ground end of the power supply ,

The signal output terminal of the first operational amplifier is connected with the reverse signal input terminal of the first operational amplifier at the same time with one end of the resistor R2, the positive pole of the power supply of the first operational amplifier is connected with the Vcc end of the power supply, the power supply of the first operational amplifier The ground end of the power supply is connected to the ground end of the power supply, and the other end of the resistor R2 is simultaneously connected to the forward signal input end of the second operational amplifier and the forward signal input end of the third operational amplifier, and the reverse signal input end of the second operational amplifier end is connected with the Vcc end of the power supply, and the reverse signal input end of the third operational amplifier is connected with the Vcc end of the power supply,

The positive pole of the power supply of the second operational amplifier is connected to the Vcc terminal of the power supply, and the ground terminal of the power supply of the second operational amplifier is connected to the ground terminal of the power supply.

The beneficial effect of the present invention is that the present invention provides a humidity detection circuit that has good temperature stability, can effectively suppress temperature drift and zero point drift, and reduces the influence of parasitic capacitance on measurement results.

Description of drawings

Fig. 1 is a schematic diagram of the principle of the heating type humidity sensor for radiosonde described in the first embodiment.

Fig. 2 is a schematic structural diagram of the first heater electrode in the first embodiment.

Fig. 3 is a schematic structural diagram of the second heater electrode in the first embodiment.

FIG. 4 is a schematic structural view of a third heater electrode in Embodiment 1. FIG.

FIG. 5 is a schematic structural diagram of a fourth heater electrode in Embodiment 1. FIG.

Fig. 6 is a schematic structural view of the serpentine heater electrode in the first embodiment.

Fig. 7 is the equivalent circuit of the heated humidity sensor for the radiosonde described in the tenth specific embodiment;

FIG. 8 is a schematic diagram of the principle of the humidity detection circuit in the tenth embodiment.

Figure 9 shows the charging and discharging curves of the standard capacitance and the humidity sensor under the same conditions;

Fig. 10 is the humidity detection performance test curve of the heating type humidity sensor for radiosondes of the present invention in +30°C environment;

Fig. 11 is the temperature rise and fall measurement characteristic curve of the heating type humidity sensor for radiosondes of the present invention;

Fig. 12 is a test curve of the time constant of the humidity sensor of the heating type humidity sensor used in the radiosonde of the present invention.

detailed description

Specific embodiment one: this embodiment is described in conjunction with Fig. 1, and the heating type humidity sensor for radiosonde described in this embodiment includes a base 1, a first insulating layer 2, a serpentine heater electrode 8, a second insulating Layer 9, lower electrode 10, moisture sensing layer 11 and porous upper electrode 12; wherein, the upper surface of the substrate 1 is laid with a first insulating layer 2; the upper surface of the first insulating layer 2 is provided with a serpentine heater electrode 8;

The serpentine heater electrode 8 includes a first heater pad 5, a second heater pad 6, a first extraction electrode, a first part of the serpentine electrode, a second part of the serpentine electrode, a third part of the serpentine electrode and the second lead-out electrode;

One end of the first lead-out electrode is connected to the head end of the first part of the serpentine electrode, the end of the first part of the serpentine electrode is connected to the head end of the second part of the serpentine electrode, and the end of the second part of the serpentine electrode is connected to the third part of the serpentine electrode. The first end of the electrode is connected, the end of the third part of the serpentine electrode is connected to one end of the second lead-out electrode, the other end of the first lead-out electrode is connected to the first heater pad 5, and the other end of the second lead-out electrode is connected to the second lead-out electrode. Heater pad 6 connection;

The first part of the serpentine electrode and the third part of the serpentine electrode are mirror-symmetrical on both sides of the second part of the serpentine electrode, and the serpentine arrangement direction of the first part of the serpentine electrode is the same as the serpentine arrangement of the second part of the serpentine electrode directions perpendicular to each other;

The second insulating layer 9 is laid on the serpentine heater electrode 8, and exposes the first heater pad 5 and the second heater pad 6;

The lower electrode 10 is laid on the second insulating layer 9; the moisture-sensing layer 11 is laid on the lower electrode 10; the porous upper electrode 12 is laid on the moisture-sensing layer 11; Groove 13 formed after hollowing out.

Since the saturated water vapor content of humid air is directly proportional to the air temperature. When the air temperature is high, the water vapor that can exist in the air is more, and when the air temperature is low, the water vapor that can exist in the air is less, even if it contains little water vapor, condensation will occur. Therefore, even if the humid air itself is not saturated, when the surface temperature of the humidity sensor is lower than the saturation temperature of the humid air, the water vapor on the surface of the object will condense, resulting in condensation. If the surface temperature of the humidity sensor can be kept within a certain temperature range without affecting the measurement characteristics of the humidity sensor, so that the surface temperature of the humidity sensor is higher than the ambient temperature, then condensation on the surface of the humidity sensor can be avoided.

This embodiment is a flat sandwich capacitive humidity sensor with a serpentine heater electrode. By controlling the heating power of the heater under different ambient temperature conditions, the surface temperature of the humidity sensor is kept constant in the ideal temperature range, thereby effectively solving the problem. Condensation problem of humidity sensor in high-altitude environment.

At the same time, in this embodiment, polyimide is used as the moisture-sensitive layer 11 of the humidity sensor. Polyimide has excellent mechanical properties and dielectric properties between -200°C and +260°C. It has good dimensional stability, excellent high temperature resistance, low temperature resistance, radiation resistance, wear resistance, and has the characteristics of easy modification, diverse processing forms, and synthetic diversity. By heating the humidity sensor, the polyimide moisture-sensitive film works under constant temperature conditions. Since the moisture-sensing properties of polyimide change with temperature, this requires that the surface temperature distribution of the humidity sensor is uniform during the heating process, and the heating area must cover the effective humidity-sensing area, so as to ensure the stability and reliability of the humidity sensor measurement. sex.

In this embodiment, platinum metal is used to prepare serpentine heaters. In order to obtain the best heating effect, four heater structures as shown in Figure 2 to Figure 5 are used, and the four heaters are loaded into the humidity sensor structure. Perform finite element simulation analysis, including heat transfer process analysis of humidity sensor and heat transfer analysis of substrate material.

First, the heat conduction model of the humidity sensor is established. In the high-altitude environment, the heat of the humidity sensor is mainly lost in three forms of heat conduction, heat convection and heat radiation, which are respectively expressed as Q _Cond , Q _Conv , and Q _Rad . T ₁ and T ₂ represent the internal temperature of the sensor and the ambient temperature, respectively. The total heat loss Q of the humidity sensor can be expressed by formula (1).

Q=Q _Cond +Q _Conv +Q _Rad , (1)

In order to simplify the analysis process, the humidity sensor is approximated as a regular cuboid, assuming that the area of the cuboid is S and the height is h, then the heat conduction equation of the humidity sensor can be expressed by formula (2)

${\mathrm{<span\; class="notranslate">\; wxya</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; no</span>}\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}\mathrm{<span\; class="notranslate">\; S</span>}\frac{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Where λ is the thermal conductivity, dt/dx is the temperature gradient vector, and the direction points to the direction of temperature increase. Integrate x from 0 to h in the above formula to get

${\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; no</span>}\mathrm{<span\; class="notranslate">\; d</span>}}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; h</span>}}\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}\mathrm{<span\; class="notranslate">\; S</span>}{<span\; class="notranslate">\; \&Integral;</span>}_{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

The essence of heat convection is the heat transfer caused by the macroscopic motion of fluid particles. The heat convection heat transfer equation can be expressed as

_QConv = μ(T ₁ -T ₂ ), (4)

Where μ is the air convection coefficient, generally 10W/(m ² ·K).

Thermal radiation refers to the heat transfer method in which an object emits heat energy in the form of electromagnetic radiation. This heat transfer method does not depend on any external conditions. According to the Stefan-Boltzmann law (Stefan-BoltzmannLaw), the heat radiation heat transfer equation can be expressed as

Q _Rad = 2Sσε(T ₁ ⁴ -T ₂ ⁴ ), (5)

Where S is the area of the heating area, σ=5.67×10 ^-8 W/(m ² ·K ⁴ ), and is Boltzmann's constant. In order to simplify the analysis process, assume that the sensor is an absolute black body, then ε=1.

Assuming that the ambient temperature is -70°C and the heat generation rate is 1.16×1011W/m ³ , in order to simplify the simulation process, the following two conventions are made: Ignore the change in thermal conductivity of the material due to changes in temperature and humidity; Contact thermal resistance.

From the simulation results, it can be seen that the first type of heater structure is the easiest to implement, and the heat distribution is relatively uniform, but the temperature decreases in steps, and the temperature at the edge of the effective moisture-sensitive surface is different; in order to make the temperature of the effective moisture-sensitive area the same, the A heater structure has been improved. From the simulation results, it can be seen that although the target temperature coverage area has increased, the temperature is discontinuous and the temperature distribution of the effective humidity-sensing surface is uneven; the optimization is carried out on the basis of the first two structures. Improvement, the third heater structure is obtained. From the simulation results, it can be seen that compared with the first two structures, the heat distribution of the third structure is improved, and the target temperature coverage area is further increased, but it still does not cover the entire effective moisture-sensitive surface. ; The fourth structure has the largest target temperature coverage area, and the target temperature covers the effective wet-sensing surface, but the temperature at the center of the sensor is discontinuous.

After optimizing and improving on the basis of the above four structures, this embodiment provides the structure of the serpentine heater electrode as shown in FIG. 6 .

The total volume of the heater is V≈2.9×10 ^-8 m ³ . The heating electrode resistance can be calculated by formula (6), where ρ=1×10 ^-3 Ω·m is the resistivity of the platinum resistor, L=33.95×10 ^-3 mm is the total length of the heating electrode, S=8.5× 10 ^-7 m is the cross-sectional area of the heating electrode.

$\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&rho;</span>}\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; S</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; 10</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; \&times;</span>\mathrm{<span\; class="notranslate">\; 33.95</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; 10</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 3</span>}}}{\mathrm{<span\; class="notranslate">\; 8.5</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; 10</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 7</span>}}}<span\; class="notranslate">\; \&ap;</span>\mathrm{<span\; class="notranslate">\; 40</span>}\mathrm{<span\; class="notranslate">\; \&Omega;</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

The structure of the serpentine heater electrode shown in Figure 6 is simulated. From the simulation results, the maximum temperature of the humidity sensor is about 12.5°C, and the average temperature of the effective humidity sensing surface of the humidity sensor is about 2°C. The temperature distribution of the humidity sensor is uniform, and the target temperature covers the effective humidity sensing surface.

When the silicon substrate is etched with an etching process, the heat generation rate is still 1.16×1011W/m ³ , and the ambient temperature is -70°C. It can be seen from the simulation of the humidity sensor after the substrate is hollowed out that the center of the humidity sensor The maximum temperature is about +30°C, and the average temperature of the humidity sensor’s effective wet-sensing surface is about +20°C. After etching the silicon substrate, under the same heating power and ambient temperature, the temperature rise of the humidity sensor is higher, which improves the heating efficiency. efficiency, reducing power consumption. The heat on the back of the humidity sensor is simulated. From the simulation results, it can be seen that the SiO ₂ prepared on the silicon substrate not only has an insulating effect, but also has a good heat insulation effect. The maximum temperature on the back of the humidity sensor is about -47°C. The structure effectively reduces the heat loss of the sensor.

According to the horizontal and vertical distribution curves of the simulated surface temperature after heating the humidity sensor, the average temperature of the 13.5mm×16.3mm area on the surface of the humidity sensor is about 20°C, which covers the effective humidity sensing area of the humidity sensor. In the high-altitude and low-temperature environment, the temperature range is +35°C to -90°C. By measuring the ambient temperature, the heating power is determined so that the temperature on the effective area of the humidity sensor is maintained at around +20°C. On the one hand, it ensures that the humidity sensor The surface temperature is higher than the ambient temperature to avoid condensation on the humidity sensor; on the other hand, the static test results show that when the surface of the humidity sensor is maintained at around +20°C, the measurement characteristics of the humidity sensor are the best.

When conducting thermal simulation analysis, +10°C to -70°C is selected as the ambient temperature, and the target temperature on the surface of the humidity sensor is +20°C. The relationship between the heating power and the ambient temperature in Table 1 is obtained.

Table 1 The relationship between heating power and ambient temperature

ambient temperature temperature rise Q heating power +10°C +10°C 0.235×10 ¹¹ W/m ³ 6.7815W 0°C +20°C 0.351×10 ¹¹ W/m ³ 10.1289W -10°C +30°C 0.467×10 ¹¹ W/m ³ 13.4764W -20°C +40°C 0.583×10 ¹¹ W/m ³ 16.8239W -30°C +50°C 0.699×10 ¹¹ W/m ³ 20.1713W -40°C +60°C 0.815×10 ¹¹ W/m ³ 23.5188W -50°C +70°C 0.931×10 ¹¹ W/m ³ 26.8663W -60°C +80°C 1.047×10 ¹¹ W/m ³ 30.2138W -70°C +90°C 1.163×10 ¹¹ W/m ³ 33.5612W

Curve fitting was carried out on the above data by the method of least squares, and the fitting equation was shown in formula (7).

y=-0.0116x+0.3510, (7)

The fitting equation such as formula (7) expresses the functional relationship between the ambient temperature y and the heating power x, so that different heating powers are provided to the humidity sensor in different temperature environments, so that the surface temperature of the humidity sensor is maintained at +20°C, so that the humidity The sensor works under ideal temperature conditions.

The dimensions of the serpentine heater electrodes in this embodiment are shown in FIG. 6 .

Specific embodiment two: this embodiment is the preparation method of the heating type humidity sensor described in specific embodiment one for sonde, and described method comprises the following steps:

Step 1: preparing the substrate 1 of the sensor, and cleaning the prepared substrate 1 with deionized water;

Step 2: Oxidize the surface of the substrate 1 prepared in Step 1 to form a layer of dense SiO ₂ as the first insulating layer 2;

Step 3: Obtain the upper surface of the insulating layer in step 2, and prepare the serpentine heater electrode 8 by photolithography and magnetron sputtering;

Step 4: Prepare an Al ₂ O ₃ protective layer on the upper surface of the serpentine heater electrode 8 prepared in Step 3 by radio frequency sputtering as the second insulating layer 9;

Step 5: Prepare the lower electrode 10 on the upper surface of the second insulating layer 9 by photolithography and magnetron sputtering;

Step 6: Hollowing out the upper surface of the lower electrode 10 prepared in step 5 and the lower surface of the substrate 1 in step 1 by etching and hollowing out;

Step 7: Prepare a moisture-sensitive layer 11 on the upper surface of the lower electrode 10 after hollowing out;

Step 8: Prepare a porous upper electrode 12 on the upper surface of the moisture-sensing layer 11 by using an evaporation coating machine.

Specific embodiment three: this embodiment is a further limitation on the preparation method of the heating humidity sensor for radiosondes described in specific embodiment two. In step one, the substrate 1 is a single crystal with a thickness of 400 μm and a crystal orientation of 100 silicon;

In step 2, the thickness of the insulating layer is 500nm-1000nm.

Embodiment 4: This embodiment is a further limitation on the preparation method of the heating humidity sensor for radiosondes described in Embodiment 3. In step 3, the upper surface of the insulating layer is obtained in step 2, and the photolithography process and The method for preparing serpentine heater electrode 8 by the method of magnetron sputtering is:

Using the hollow pattern of the serpentine heater electrode 8 as a mask, evenly coat the photoresist on the upper surface of the insulating layer obtained in step 2, then bake at 80°C-100°C for 20min-40min, and expose for 15s-30s, Transfer to the developing solution for 20s~40s, rinse in deionized water for 20s~30s, and then harden the film at 100℃~120℃ for 30min~40min;

The upper surface of the photoresist is coated by magnetron sputtering, the target material is 99.99% platinum, the target size is Φ60×2.5mm, and the vacuum degree reaches 1×10 ^-5 Pa～2×10 ^{- At 5} Pa, pass argon gas into the sputtering chamber, the flow rate of argon gas is 15ml/min~25ml/min, and the pressure of argon gas is 1.5Pa~2.5Pa; use acetone to dissolve the photoresist, and ultrasonically reach the electrodes of the serpentine heater 8 patterns are clear. Embodiment 5: This embodiment is a further limitation on the preparation method of the heating type humidity sensor for radiosondes described in Embodiment 4. In step 4, the serpentine heating method prepared in step 3 is adopted by radio frequency sputtering The method for preparing the Al ₂ O ₃ protective layer on the upper surface of the device electrode 8 is:

Apply the photoresist evenly on the upper surface of the serpentine heater electrode 8 prepared in step 3, then bake at 80°C to 100°C for 20min to 40min, cover the pattern of the pad on the photoresist After the upper surface is exposed for 15s~30s, it is transferred to the developer solution for 20s~40s, rinsed in deionized water for 20s~30s, and then hardened at 100℃~120℃ for 30min~40min;

When the vacuum degree reaches 1×10 ^-5 Pa～2×10 ^-5 Pa, argon is passed into the sputtering chamber, the flow rate of argon is 15ml/min～25ml/min, and the pressure of argon is 1.5Pa～2.5Pa. The sputtering power is 60W-80W, the time is 120min-180min, and the pressure during coating is controlled below 0.5Pa to obtain the Al ₂ O ₃ protective layer.

Embodiment 6: This embodiment is a further limitation on the preparation method of the heating humidity sensor for sondes described in Embodiment 5. In step 5, the method of photolithography and magnetron sputtering is used in the second step. The method for preparing the lower electrode 10 on the upper surface of the insulating layer 9 is:

Utilize acetone to dissolve the photoresist on the second insulating layer 9, peel off the second insulating layer 9 of the first heater pad 5 and the second heater pad 6 area, adopt the method for magnetron sputtering on the second insulating layer Coating on the upper surface of 9, the target material is 99.99% gold, the size of the target material is Φ60×2.5mm, when the vacuum degree reaches 1×10 ^-5 Pa～2×10 ^-5 Pa, argon gas is passed into the sputtering chamber , the flow rate of argon is 15ml/min～25ml/min, and the pressure of argon is 1×10 ^-5 Pa～2×10 ^-5 Pa;

Apply the photoresist evenly on the upper surface of the second insulating layer 9, cover the mask of the pattern of the lower electrode 10 on the corresponding position of the photoresist surface, after exposing for 15s-30s, transfer to the developing solution to develop for 20s-40s , rinse in deionized water for 20s to 30s, then harden the film at 100°C to 120°C for 30min to 40min, etch the exposed gold film with a saturated solution of iodine and ammonium iodide to obtain the lower electrode 10 .

Embodiment 7: This embodiment is a further limitation of the preparation method of the heating humidity sensor for sondes described in Embodiment 6. In step 6, the lower electrode 10 prepared in step 5 is corroded and hollowed out. The method of hollowing out the upper surface and the lower surface of the substrate 1 described in step 1 is as follows;

Coating photoresist on the upper surface of the lower electrode 10 prepared in step five and the lower surface of the substrate 1 described in step one respectively, and then baking at 80° C. to 100° C. for 20 minutes to 40 minutes, using the upper surface of the lower electrode 10 as the Cover the upper surface of the lower electrode 10 with the mask plate of the plate-making pattern, cover the mask plate with the hollow pattern as the plate-making pattern on the corresponding position of the lower surface of the substrate 1, expose on both sides for 15s to 30s, and put the exposed substrate 1 Develop in the developer solution for 20s~40s, rinse in deionized water for 20s~30s, then harden the film at 100℃~120℃ for 30min~40min, and at a temperature of 70℃~90℃, use a mass fraction of 35%~40 % potassium hydroxide solution to hollow out the lower surface of the substrate 1 to form grooves 13 with a depth of about 350 μm.

Embodiment 8: This embodiment is a further limitation on the preparation method of the heating type humidity sensor for radiosondes described in Embodiment 7. In step 7, a moisture-sensitive sensor is prepared on the upper surface of the lower electrode 10 after the hollowing out process. The layer 11 method is:

Apply the polyimide moisture-sensitive solution on the upper surface of the lower electrode 10, and then keep the temperature at 80°C-100°C for 5min-10min;

Cover the upper surface of the lower electrode 10 with a mask plate whose pattern of the moisture-sensitive film is a plate-making pattern, expose for 15s to 30s, develop in N,N-dimethylacetamide for 20s to 40s, and rinse in deionized water for 20s to 30s , and then harden the film at 100° C. to 120° C. for 30 minutes to 40 minutes to obtain the moisture-sensitive layer 11 , and the thickness of the moisture-sensitive layer 11 is about 1 μm.

Specific embodiment nine: This embodiment is a further limitation of the preparation method of the heating type humidity sensor for radiosondes described in specific embodiment eight. In step eight, an evaporation coating machine is used to prepare a porous layer on the upper surface of the moisture-sensitive layer 11 The method of upper electrode 12 is:

The evaporation raw material is gold foil, and the pattern of the porous upper electrode 12 is used as a mask plate of the plate-making pattern as a template, and the film is formed in an evaporation coating machine. The evaporation current is 110A-120A, and the evaporation time is 5s-10s, and the porous upper electrode 12 is obtained.

With the preparation method of this embodiment, the capacitance of the humidity sensor is about 100pF˜160pF.

Embodiment 10: This embodiment is a humidity detection circuit,

When the high-altitude humidity changes, the capacitance value of the humidity sensor changes accordingly. If the change of the capacitance value of the humidity sensor can be accurately obtained, the change of the high-altitude environment humidity can be obtained. In most cases, the capacitive humidity sensor can be considered as a pure capacitance, but when working under high temperature, low temperature and high humidity conditions, the capacitance loss of the humidity sensor cannot be ignored, and when working at high frequencies, the inductive effect cannot be ignored . Therefore, when considering the loss and inductance effect of the humidity sensor, the humidity sensor cannot be considered as a pure capacitance, and the equivalent circuit is shown in Figure 7. Among them, R _s is the resistance loss between the circuit board leads and the plates, and its value gradually increases with the increase of the operating frequency. R _s is usually very small, even when the operating frequency is above megahertz, so it is only when the operating frequency The influence of R _s on the measurement results needs to be considered only when the temperature is extremely high; L is the inductance of the humidity sensor itself and the equivalent inductance of the external lead. The shorter the inductance, the smaller the inductance; R _P is the parallel loss resistance, including the dielectric loss and leakage loss between the plates. When the humidity sensor works at a low frequency, the loss is large, and gradually decreases with the increase of the operating frequency. The total loss coefficient of the humidity sensor capacitance can be expressed by formula (8).

$\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}\mathrm{<span\; class="notranslate">\; \&omega;</span>}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}\mathrm{<span\; class="notranslate">\; \&omega;</span>}\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; )</span>$

where ω is the circular frequency. It can be seen from Figure 7 that the equivalent circuit of the humidity sensor capacitance has a resonant frequency. When the humidity sensor is at the resonant frequency, it will not work normally. When designing the circuit, it is necessary to avoid the resonant frequency. It can be seen from the formula (8) that the first item As the resonant frequency increases, its value decreases, and the second term increases as the resonant frequency increases, so there is an optimal frequency f _b , which minimizes the total loss coefficient D, which can be minimized by formula (8) value to get f _b , as shown in formula (9).

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}\mathrm{<span\; class="notranslate">\; C</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 9</span>}<span\; class="notranslate">\; )</span>$

Then the effective capacitance C _e of the humidity sensor can be expressed by formula (10).

$\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; j\&omega;C</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; j</span>}\mathrm{<span\; class="notranslate">\; \&omega;</span>}\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; j</span>}\mathrm{<span\; class="notranslate">\; \&omega;</span>}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; )</span>$

The actual variation of the capacitance of the humidity sensor can be expressed by formula (11).

$\frac{{\mathrm{<span\; class="notranslate">\; \&Delta;C</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; C</span>}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; L</span>}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 11</span>}<span\; class="notranslate">\; )</span>$

The above formula shows that the inductance of the humidity sensor itself and the equivalent inductance of the external leads have a certain influence on the capacitance of the humidity sensor. Moreover, the parasitic capacitance between the humidity sensor and the ground and the parasitic capacitance between the lead wire and the humidity sensor change with the change of temperature in a low-temperature environment.

In view of the above problems, combined with the requirements of the high-altitude meteorological detection service for humidity measurement, this embodiment provides a humidity detection circuit based on capacitor charging and discharging and comparison method, as shown in FIG. 8 .

The humidity detection circuit includes a heating type humidity sensor C _m for a radiosonde, a standard capacitance C _s , an analog resistance R _P , a parasitic capacitance C _P , a resistance R1, a resistance R2, three operational amplifiers, two single-pole double-throw switches and Power supply;

One end of the humidity sensor C _m and one end of the standard capacitor C _s are connected to the ground terminal of the power supply at the same time, the other end of the humidity sensor C _m is connected to a static end of the first single-pole double-throw switch, and the other end of the standard capacitor C _s connected with the other static end of the first SPDT switch,

The moving end of the first SPDT switch is connected to one end of the analog resistor R _P , and the other end of the analog resistor R _P is connected to one end of the parasitic capacitor C _P , one end of the resistor R1 and the positive input end of the first operational amplifier to supply power simultaneously. The Vcc end of the power supply, the other end of the parasitic capacitor C _P is connected to the ground end of the power supply,

The other end of the resistor R1 is connected to the moving end of the second SPDT switch, one static end of the second SPDT switch is connected to the Vcc end of the power supply, and the other static end of the second SPDT switch is connected to the ground end of the power supply ,

The signal output terminal of the first operational amplifier is connected with the reverse signal input terminal of the first operational amplifier at the same time with one end of the resistor R2, the positive pole of the power supply of the first operational amplifier is connected with the Vcc end of the power supply, the power supply of the first operational amplifier The ground end of the power supply is connected to the ground end of the power supply, and the other end of the resistor R2 is simultaneously connected to the forward signal input end of the second operational amplifier and the forward signal input end of the third operational amplifier, and the reverse signal input end of the second operational amplifier end is connected with the Vcc end of the power supply, and the reverse signal input end of the third operational amplifier is connected with the Vcc end of the power supply,

The positive pole of the power supply of the second operational amplifier is connected to the Vcc terminal of the power supply, and the ground terminal of the power supply of the second operational amplifier is connected to the ground terminal of the power supply.

In this embodiment, both the second operational amplifier and the third operational amplifier are implemented by integrated circuit NE5532, and the voltage range input by the reverse signal input terminal of the second operational amplifier and the reverse signal input terminal of the third operational amplifier is 1.5v-2.5 v, the first operational amplifier is realized by the integrated circuit CA3140, and the power supply provides voltages of 1.5v, 2.5v and 5v in this embodiment.

R _P is the on-resistance and lead resistance of the analog switch. Since the humidity sensor and the standard capacitor are in the same measurement circuit, and the other end of the humidity sensor and the standard capacitor is connected to the ground, the parasitic capacitance C _P is the parasitic capacitance of the connection between the humidity sensor and the standard capacitor and the parasitic capacitance between the ground.

When the capacitor is not charged, the output of the second operational amplifier and the third operational amplifier (NE5532) are both zero. When the capacitor is charged, the input voltage of the same input terminal of the two operational amplifiers NE5532 gradually increases. When the input voltage reaches 1.5 When V, the output of the second operational amplifier changes from low level to high level, which triggers an external interrupt of the MCU and starts timing; when the input voltage reaches 2.5V, the output of the third operational amplifier changes from low level to high level , trigger the MCU external interrupt again, and the timing ends, so that the charging time of the capacitor can be obtained. Therefore, the charging time T _sc of the standard capacitor and the charging time T _mc of the humidity sensor can be obtained. The capacitance value of the standard capacitor is known. Assuming it is C _s , then the capacitance value of the humidity sensor

The standard capacitance and the humidity sensor are charged under the same conditions, and the capacitance charge and discharge curve is shown in Figure 9.

Among them, T _C is the charging time, T _D is the discharging time, and V _m is the charging saturation voltage. Assume that the time required for the standard capacitor to charge from the voltage of V _s to _Ve is T _sc , the time required for the humidity sensor to charge from the voltage of V _s to _Ve is T _mc , and the capacitance value of the standard capacitor is C _s , So

${\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&times;</span>\frac{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; c</span>}}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 12</span>}<span\; class="notranslate">\; )</span>$

K ₁ is to consider the influence of the analog switch and lead resistance and parasitic capacitance on the capacitance value of the humidity sensor. K ₂ is to consider the influence of the analog switch and lead resistance and parasitic capacitance on the standard capacitance value. Since the humidity sensor and the standard capacitance are in the same loop, K ₁ =K ₂ , then the measured capacitance C _m can be obtained from formula (13).

${\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&times;</span>\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; c</span>}}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 13</span>}<span\; class="notranslate">\; )</span>$

This embodiment has good temperature stability, can effectively suppress temperature drift and zero point drift, and reduces the influence of parasitic capacitance on measurement results. It should be noted that: considering the influence of distributed capacitance and leakage current, the voltage cannot be selected in the range of 0V to full scale during charging; since the capacitance value of the humidity sensor is between 100pF and 160pF, the capacitance value is small, and the charging time constant needs to be adjusted reasonably , the charging time is not too long or too short, and the measurement time is shortened while ensuring high measurement accuracy; the charging current is not too large, so as to avoid breakdown of the humidity sensor; it is necessary to choose a sensor with high precision, good temperature characteristics, and high reliability. The capacitor with high performance and stability is used as the standard capacitor; the operational amplifier in the charging circuit needs to have high input impedance and low leakage current.

To the ground test of the present invention:

Normal temperature characteristic test: The static experiment of humidity measurement was carried out by using the double pressure method humidity generator. The working range of the humidity sensor is 0% RH ~ 100% RH. According to the principle of selection of test points and the size of sample space, 5 test points of 10% RH, 30% RH, 50% RH, 70% RH and 90% RH were selected. The ambient temperature is +30°C, and the data is recorded after each humidity point is stable for 20 minutes, and the relative humidity rises from 10% RH to 90% RH. The experimental results are shown in Figure 10, and the fitting curve expression is shown in (14).

y=2.336x-256.553, (14)

It can be concluded from the experimental results that the coefficient of determination is 0.9963, the humidity measurement results have good linearity, the sensitivity is 2.336%RH/pF, the standard deviation σ=1.917, the capacitance measurement resolution is 0.1pF, and the humidity measurement resolution is about 0.2% RH. In order to obtain the hysteresis and repeatability of humidity measurement, a continuous test of humidity rise and fall was carried out, and the experimental results are shown in Figure 11.

The test temperature is +30°C, the humidity range is 10%RH-90%RH, and 5 test points of 10%RH, 30%RH, 50%RH, 70%RH, and 90%RH are selected. According to the experimental results, the humidity The maximum difference ΔH _M ≈0.7%RH in the rising process and the falling process curve, the full-scale output Y _FS =100%RH, the measurement hysteresis is about 0.7%F·S, and the repeatability is about 1.9%. Hysteresis uncertainty U _H ≈ 0.35%, linear uncertainty U _L = 3%, repeatability uncertainty U _R ≈ 1.9%, then the total uncertainty U ≈ 4%.

Temperature alternating characteristic test: In the case of no heating control of the humidity sensor, static experiments were carried out at +10°C, 0°C, -10°C, -40°C, and -70°C. By controlling the heating power of the humidity sensor, the surface temperature of the sensor is maintained at around +20°C to +30°C under different ambient temperature conditions, so as to ensure that the humidity sensor has good measurement characteristics and avoid condensation on the surface of the humidity sensor. Refer to the data in Table 1 for control. It can be seen from the test results that when the humidity sensor works in a low temperature environment, the surface temperature of the humidity sensor is higher than the ambient temperature by heating the humidity sensor, and its measurement characteristics are significantly improved, avoiding the phenomenon of sensor condensation.

When the humidity sensor works in +10°C, 0°C, -10°C, -40°C, -70°C temperature environments, the sensitivity, average error, hysteresis, and repeatability of the humidity sensor are shown in Table 2 after heating the humidity sensor .

Table 2 Measurement characteristics after heating the humidity sensor at different temperatures

Operating temperature sensitivity average error hysteresis repeatability +10 2.338%RH/pF 1.12%RH 1.16% F·S 1.94%RH 0 2.322%RH/pF 0.74%RH 0.93%F·S 1.93%RH -10 2.327%RH/pF 0.79%RH 1.40% F·S 1.89%RH -40 2.261%RH/pF 1.13%RH 1.96%F·S 1.55%RH -70 2.182%RH/pF 1.40%RH 1.98% F·S 2.60%RH

Time constant test: Put the self-made sealed cabin in the humidity generator, install the humidity sensor in the sealed cabin, lead out the signal of the humidity sensor through the cable, and control the up and down movement of the cylinder connected to the sealed cover in the sealed cabin. The opening or closing of the airtight compartment. Set the temperature of the humidity generator to +30°C, open the airtight hatch, set the initial humidity of the humidity generator H ₀ =30%, and close the airtight cabin after the humidity is stable; set the humidity of the humidity generator to H ₁ =70 %, after the humidity is stable, open the airtight cabin, and record the humidity value every 0.05s at the same time, assuming that the humidity value H _n measured by the humidity sensor at a certain time t _n satisfies the formula (15), then t _n is the time constant of the humidity sensor .

$\frac{{\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}}{{\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 63.2</span>}\mathrm{<span\; class="notranslate">\; \%</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 15</span>}<span\; class="notranslate">\; )</span>$

Figure 12 shows the test results of the humidity sensor time constant. It can be seen from the test results that the time constant of the humidity sensor is about 0.5s, which is the same as the time constant of the foreign advanced RS92 radiosonde humidity sensor, and is better than the time constant of the GTS1 radiosonde humidity sensor currently in use in China.

The invention provides a flat sandwich capacitive humidity sensor structure and a process preparation method with serpentine heater electrodes, and also provides a humidity detection circuit, and conducts a ground characteristic experiment. The experimental results show that the designed humidity sensor has a sensitivity of about 2.3RH%/pF, a hysteresis of about 0.7% F S, a repeatability of about 1.9%, a total measurement uncertainty of about 4%, and a time constant of about 0.5s. By heating the sensor, the humidity sensor can ensure the stability of its measurement characteristics in a low temperature environment, which verifies the scientificity and effectiveness of the method and lays a technical foundation for the next step of engineering application.

Claims (2)

1. A humidity detection circuit, characterized in that, the humidity detection circuit comprises a heating type humidity sensor C _m for a radiosonde, a standard capacitance C _s , an analog resistance R _P , a parasitic capacitance C _P , a resistance R1, a resistance R2, Three operational amplifiers, two SPDT switches and power supplies; One end of the humidity sensor C _m and one end of the standard capacitor C _s are connected to the ground terminal of the power supply at the same time, the other end of the humidity sensor C _m is connected to a static end of the first single-pole double-throw switch, and the other end of the standard capacitor C _s connected with the other static end of the first SPDT switch, The moving end of the first SPDT switch is connected to one end of the analog resistor R _P , and the other end of the analog resistor R _P is connected to one end of the parasitic capacitor C _P , one end of the resistor R1 and the positive input end of the first operational amplifier to supply power simultaneously. The Vcc end of the power supply, the other end of the parasitic capacitor C _P is connected to the ground end of the power supply, The other end of the resistor R1 is connected to the moving end of the second SPDT switch, one static end of the second SPDT switch is connected to the Vcc end of the power supply, and the other static end of the second SPDT switch is connected to the ground end of the power supply , The signal output terminal of the first operational amplifier is connected with the reverse signal input terminal of the first operational amplifier at the same time with one end of the resistor R2, the positive pole of the power supply of the first operational amplifier is connected with the Vcc end of the power supply, the power supply of the first operational amplifier The ground end of the power supply is connected to the ground end of the power supply, and the other end of the resistor R2 is simultaneously connected to the forward signal input end of the second operational amplifier and the forward signal input end of the third operational amplifier, and the reverse signal input end of the second operational amplifier end is connected with the Vcc end of the power supply, and the reverse signal input end of the third operational amplifier is connected with the Vcc end of the power supply, The positive pole of the power supply of the second operational amplifier is connected to the Vcc terminal of the power supply, and the ground terminal of the power supply of the second operational amplifier is connected to the ground terminal of the power supply. 2. A kind of humidity detecting circuit according to claim 1, it is characterized in that, the voltage range of the reverse signal input terminal of the second operational amplifier and the reverse signal input terminal of the third operational amplifier is 1.5v-2.5v .