CN105806900A - Humidity detection circuit - Google Patents

Abstract

The invention relates to a humidity detection circuit and belongs to the technical field of a humidity sensor. The invention aims to solve the problem that the humidity detection is unstable because the humidity sensor changes along with the change of the temperature under low-temperature environment. The humidity detection circuit provided by the invention is arranged as a humidity measuring circuit based on a capacitance charge-discharge and comparison method, so that the temperature drift and the zero drift are effectively restrained and the influence of stray capacitance on measuring result is reduced. The humidity detection circuit is applied to the humidity sensor.

Description

A kind of humidity measuring circuit

Technical field

The invention belongs to moisturt register sensor technical field, the present invention is the divisional application of the patent of invention " sonde heated type humidity sensor and preparation method thereof and a kind of humidity measuring circuit " of application on August 20th, 2014, application number 201410411744.2.

Background technology

Aerological sounding professional skill is one of Primary Reference weighing a national Atmospheric Survey scientific level, and humidity detection is aerological sounding important step.Owing to high altitude environment is severe, humidity change is acutely, ambient temperature is minimum reaches-90 DEG C, this just requires that humidity sensor has the advantages such as low temperature resistant, response is fast, capacity of resisting disturbance is strong, and capacitance type humidity sensor possesses These characteristics, and manufacturing cost is relatively low, become one of important directions of sonde humidity sensor research.But due to capacitance type humidity sensor own structural characteristics, when in high humidity environment, wetness sensor surface easily produces dew condensation phenomenon so that measurement error increases, and even causes sensor failure.Relevant scholar's research emphasis tends to optimize sensor construction, improve the aspects such as humidity-sensitive material both at home and abroad at present.Developed country's researchs in this field such as America and Europe are constantly in status advanced in the world, the High-Polymer Capacitance formula humidity sensor that E+E company of Austria develops solves low temperature moisture measurement problem in wet sensory material, its response time is about 1.5s, resolution about 1%, uncertainty about 5%, it is possible at-80 DEG C of normal operation；The RS92 type sonde of Vaisala Oy's research and development solves low temperature moisture measurement problem from sensor construction and mode of operation, two panels is adopted to have the humidity sensor alternation of heating function, its response time is less than 0.5s, resolution is about 1%RH, uncertainty about 5%, is the standard of the high-altitude humidity detection generally acknowledged at present.

So current humidity sensor is thrown away and be there is the problem of poor of moisture measurement effect at low ambient temperatures.

Humidity sensor capacitance tool is had a certain impact by the inductance of humidity sensor self and the equivalent inductance of outside lead.And parasitic capacitance between humidity sensor and ground and and lead-in wire between parasitic capacitance variation with temperature at low ambient temperatures and change, make Humidity Detection unstable.

Summary of the invention

The invention aims to solve humidity sensor variation with temperature at low ambient temperatures and change, make the problem that Humidity Detection is unstable, the present invention provides a kind of humidity measuring circuit.

A kind of humidity measuring circuit of the present invention, described humidity measuring circuit includes sonde heated type humidity sensor C_m, standard capacitance C_s, artifical resistance R_P, parasitic capacitance C_P, resistance R1, resistance R2, three operational amplifiers, 2 single-pole double-throw switch (SPDT)s and power supply；

Described humidity sensor C_mOne end and standard capacitance C_sOne end meet the ground end of power supply, humidity sensor C simultaneously_mThe other end and quiet end of the first single-pole double-throw switch (SPDT) connect, standard capacitance C_sThe other end and the first single-pole double-throw switch (SPDT) another quiet end connect,

The moved end of the first single-pole double-throw switch (SPDT) and artifical resistance R_POne end connect, artifical resistance R_PThe other end and parasitic capacitance C_POne end, one end of resistance R1 and the positive input of the first operational amplifier be simultaneously connected with the Vcc end of power supply, parasitic capacitance C_PAnother termination power supply ground end,

Another of resistance R1 terminates the moved end of the second single-pole double-throw switch (SPDT), the Vcc end of a quiet termination power supply of the second single-pole double-throw switch (SPDT), the ground end of another quiet termination power supply of the second single-pole double-throw switch (SPDT),

The signal output part of the first operational amplifier is connected with the non-inverting signal input thereof of the first operational amplifier with one end of resistance R2 simultaneously simultaneously, the power supply positive pole of the first operational amplifier is connected with the Vcc end of power supply, the power supply ground end of the first operational amplifier is connected with the ground end of power supply, the other end of resistance R2 is connected with the forward signal input of the second operational amplifier and the forward signal input of the 3rd operational amplifier simultaneously, the non-inverting signal input thereof of the second operational amplifier is connected with the Vcc end of power supply, the non-inverting signal input thereof of the 3rd operational amplifier is connected with the Vcc end of power supply,

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

The beneficial effects of the present invention is, the present invention provides a kind of humidity measuring circuit, has good temperature stability, it is possible to effective suppression temperature drift and null offset, reduces the parasitic capacitance impact on measurement result.

Accompanying drawing explanation

Fig. 1 is the principle schematic of the sonde heated type humidity sensor described in detailed description of the invention one.

Fig. 2 is the structural representation of the first heater electrode in detailed description of the invention one.

Fig. 3 is the structural representation of the second heater electrode in detailed description of the invention one.

Fig. 4 is the structural representation of the third heater electrode in detailed description of the invention one.

Fig. 5 is the structural representation of the 4th kind of heater electrode in detailed description of the invention one.

Fig. 6 is the structural representation of snakelike heater electrode described in detailed description of the invention one.

Fig. 7 is the equivalent circuit of sonde heated type humidity sensor described in detailed description of the invention ten；

Fig. 8 is the principle schematic of humidity measuring circuit in detailed description of the invention ten.

Standard capacitance and humidity sensor are charged respectively by Fig. 9 in identical situation, capacitor charge and discharge curve；.

Figure 10 is sonde heated type humidity sensor Humidity Detection performance test curve under+30 DEG C of environment of the present invention；

Figure 11 is the sonde heated type humidity sensor humidity rise and fall measurement characteristics curve of the present invention；

Figure 12 is the sonde heated type humidity sensor humidity sensor time constant test curve of the present invention.

Detailed description of the invention

Detailed description of the invention one: present embodiment is described in conjunction with Fig. 1, sonde described in present embodiment heated type humidity sensor, it includes by electrode 12 in substrate the 1, first insulating barrier 2, snakelike heater electrode the 8, second insulating barrier 9, bottom electrode 10, humidity sensing layer 11 and porous；Wherein, the upper surface of substrate 1 lays the first insulating barrier 2；Upper surface at the first insulating barrier 2 is provided with snakelike heater electrode 8；

Described snakelike heater electrode 8 includes primary heater pad 5, secondary heater pad the 6, first extraction electrode, the snakelike electrode of Part I, the snakelike electrode of Part II, the snakelike electrode of Part III and the second extraction electrode；

One end of first extraction electrode is connected with the head end of the snakelike electrode of Part I, the end of the snakelike electrode of Part I is connected with the head end of the snakelike electrode of Part II, the end of the snakelike electrode of Part II is connected with the head end of the snakelike electrode of Part III, the end of the snakelike electrode of Part III and one end of the second extraction electrode connect, the other end of the first extraction electrode is connected with primary heater pad 5, and the other end of the second extraction electrode is connected with secondary heater pad 6；

The snakelike electrode of Part I and the snakelike electrode of Part III are specular in the both sides of the snakelike electrode of Part II, and the snakelike arragement direction of the snakelike electrode of Part I is orthogonal with the snakelike arragement direction of the snakelike electrode of Part II；

Described second insulating barrier 9 is laid on snakelike heater electrode 8, and exposes primary heater pad 5 and secondary heater pad 6；

Described bottom electrode 10 is laid on the second insulating barrier 9；Described humidity sensing layer 11 is laid on bottom electrode 10；In described porous, electrode 12 is laid on humidity sensing layer 11；The lower surface of described substrate 1 is provided with the groove 13 formed after hollow out.

Owing to the saturated steam content of humid air is directly proportional to air themperature.When air themperature is higher, the steam that can exist in air is many, and when air themperature is relatively low, the steam that can exist in air is few, even if the steam contained seldom also can produce condensation.So, even if humid air self is not up to saturation, when wetness sensor surface temperature is lower than the saturation temperature of humid air, the steam of body surface also can condense, and produces condensation.If when not affecting humidity sensor measurement characteristic, by wetness sensor surface temperature constant in some temperature range, wetness sensor surface temperature can be made higher than ambient temperature, then be just avoided that wetness sensor surface condenses.

Present embodiment is a kind of sandwich capacitance type humidity sensor of flat board with snakelike heater electrode, by controlling the heating power of heater under varying environment temperature conditions, make wetness sensor surface temperature constant in desirable temperature range, thus efficiently solving the problem of condensation under humidity sensor high altitude environment.

Simultaneously, in present embodiment, adopt polyimides as the humidity sensing layer 11 of humidity sensor, polyimides has excellent mechanical property and dielectric properties between-200 DEG C～+260 DEG C, there is good dimensional stability in this temperature range, there is outstanding high temperature resistant, low temperature resistant, radiation hardness, abrasion resistance properties, and have easily modified, process the feature such as Morphological Diversity, synthesis multiformity.By humidity sensor is heated so that when polyimides humidity-sensitive film is operated in stationary temperature.Changing due to polyimides humidity sensing characteristic variation with temperature, this just requires that humidity sensor its surface temperature distribution in heating process is uniform, and heating surface (area) (HS need to cover effective feeling wet zone, thus ensureing stability and the reliability of humidity sensor measurement.

Present embodiment adopts platinum to prepare snakelike heater, in order to obtain the heats of optimum, have employed four kinds of heater structures as shown in Figures 2 to 5, and four kinds of heaters are loaded in humidity sensor structure, carry out Finite Element Simulation Analysis, including the analysis of humidity sensor heat transfer process, backing material analysis of Heat Transfer.

Having initially set up humidity sensor heat conduction model, in high altitude environment, humidity sensor heat mainly scatters and disappears with conduction of heat, thermal convection current and three kinds of forms of heat radiation, is expressed as Q_Cond、Q_Conv、Q_Rad。T₁、T₂Represent sensor internal temperature and ambient temperature respectively.The total heat leakage Q of humidity sensor can be represented by formula (1).

Q=Q_Cond+Q_Conv+Q_Rad, (1)

Process for simplifying the analysis, is approximately a regular cuboid by humidity sensor, it is assumed that cuboid area is S, is highly h, then the humidity sensor equation of heat conduction can be represented by formula (2)

${\mathrm{dQ}}_{Cond}=-\mathrm{\λ}S\frac{dt}{dx},---\left(2\right)$

Wherein λ is heat conductivity, and dt/dx is thermograde vector, and the direction that temperature raises is pointed in direction.The integration that x in above formula is 0～h can obtain

$Q_{Cond}{\∫}_0^{h}dx=-\mathrm{\λ}S{\∫}_{T_1}^{T_2}dx=\frac{\mathrm{\λ}S(T_1-T_2)}{h},---\left(3\right)$

The essence of thermal convection current is the heat transmission caused due to the macroscopic motion of fluid particle.Thermal convection current heat transfer equation is represented by

Q_Conv=μ (T₁-T₂), (4)

Wherein μ is cross-ventilation coefficient, generally takes 10W/ (m²·K)。

Heat radiation refers to that the heat transfer type that heat energy is outwards distributed by object in the form of electromagnetic radiation, this heat transfer type are independent of extraneous any condition.According to Stefan-Boltzmann law (Stefan-BoltzmannLaw), heat radiation heat transfer equation is represented by

Q_Rad=2S σ ε (T₁ ⁴-T₂ ⁴), (5)

Wherein S is thermal treatment zone area, σ=5.67 × 10^-8W/(m²·K⁴), for Boltzmann constant, for simplifying the analysis process, it is assumed that sensor is absolute black body, then ε=1.

Assumptions' environment temperature is-70 DEG C, heat generation rate 1.16 × 1011W/m³, in order to simplify simulation process, make following two agreement: ignore the change of the heat conductivity that material causes because of temperature, humidity change；Ignore the thermal contact resistance of each interlayer of sensor.

From simulation result it can be seen that the first heater structure technique realizes the simplest, heat distribution is relatively uniform, but the stepped decline of temperature, effective feeling wetted surface marginal portion temperature differs；So that effective feeling wetted surface temperature is identical, having improved on the first heater structure basis, from simulation result it is recognized that while target temperature overlay area increases to some extent, but temperature is discontinuous, and effective feeling wetted surface temperature distributing disproportionation is even；First two architecture basics carries out Optimal improvements, has obtained the third heater structure, from simulation result, compared with first two structure, the third structure heat distribution makes moderate progress, and target temperature overlay area increases further, but does not cover whole effective feeling wetted surface yet；4th kind of structural object temperature overlay area is maximum, and target temperature covers effective feeling wetted surface, but center sensor place temperature is discontinuous.

Above four kinds of architecture basics after being optimized and improve, present embodiments provide for the structure of snakelike heater electrode as shown in Figure 6.

Heater cumulative volume V ≈ 2.9 × 10^-8m³.Add hot electrode resistance can calculate by through type (6), wherein ρ=1 × 10^-3Ω m, for the resistivity of platinum resistance, L=33.95 × 10^-3Mm, for adding the total length of thermode, S=8.5 × 10^-7M, for adding the cross-sectional area of thermode.

$R=\frac{\mathrm{\ρ}L}{S}=\frac{1\×{10}^{-3}\×33.95\×{10}^{-3}}{8.5\×{10}^{-7}}\≈40\mathrm{\Ω},---\left(6\right)$

The structure of the snakelike heater electrode shown in Fig. 6 is emulated, from simulation result, humidity sensor maximum temperature is about 12.5 DEG C, humidity sensor effective feeling wetted surface mean temperature is about 2 DEG C, experiment proves, the uniformity of temperature profile of the humidity sensor of present embodiment, target temperature covers effective feeling wetted surface.

When adopting etching process to carry out silicon substrate etching substrate, heat generation rate remains as 1.16 × 1011W/m³Ambient temperature is-70 DEG C, humidity sensor sensor after substrate hollow out is carried out emulation can be seen that, humidity sensor center maximum temperature is about+30 DEG C, and humidity sensor effective feeling wetted surface mean temperature is about+20 DEG C, after carrying out silicon substrate etching, under identical heating power and ambient temperature, humidity sensor temperature rise is higher, improves the efficiency of heating, reduces power consumption.Humidity sensor back side heat being emulated, can be seen that from simulation result, SiO prepares in side on a silicon substrate₂Not only having the effect of insulation, and have good effect of heat insulation, humidity sensor back side maximum temperature is about-47 DEG C, and this structure effectively reduces the heat loss of sensor.

According to humidity sensor heated post-simulation surface temperature laterally, genesis analysis curve, show that the mean temperature in wetness sensor surface 13.5mm × 16.3mm region is about 20 DEG C, this region overlay humidity sensor effective feeling wetted surface.Under high-altitude low temperature environment, range of temperature is+35 DEG C～-90 DEG C, by the measurement to ambient temperature, determine heating power, the temperature on humidity sensor effective area is made to maintain about+20 DEG C, on the one hand, it is ensured that wetness sensor surface temperature is higher than ambient temperature, it is to avoid humidity sensor condenses；On the other hand, static experimental result shows, when wetness sensor surface maintains about+20 DEG C, the measurement characteristics of humidity sensor is best.

When carrying out hot simulation analysis, choosing+10 DEG C～-70 DEG C for ambient temperature, wetness sensor surface target temperature is+20 DEG C, obtains the relation of heating power and ambient temperature in table 1.

Table 1 heating power and ambient temperature relation table

Ambient temperature	Temperature rise	Q	Heating power
				+10℃	+10℃	0.235×10 11W/m3	6.7815W
0℃	+20℃	0.351×10 11W/m3	10.1289W
				-10℃	+30℃	0.467×10 11W/m3	13.4764W
-20℃	+40℃	0.583×10 11W/m3	16.8239W
				-30℃	+50℃	0.699×10 11W/m3	20.1713W
-40℃	+60℃	0.815×10 11W/m3	23.5188W
				-50℃	+70℃	0.931×10 11W/m3	26.8663W
-60℃	+80℃	1.047×10 11W/m3	30.2138W
				-70℃	+90℃	1.163×10 11W/m3	33.5612W

Adopt method of least square that data above is carried out curve fitting, shown in fit equation such as formula (7).

Y=-0.0116x+0.3510, (7)

Fit equation such as formula (7) represents the functional relationship of ambient temperature y and heating power x, make to provide different heating powers to humidity sensor under different temperature environments, wetness sensor surface temperature is made to maintain+20 DEG C, so that humidity sensor is operated under desirable temperature conditions.

The size of the snakelike heater electrode in present embodiment is as shown in Figure 6.

Detailed description of the invention two: present embodiment is the preparation method of the sonde heated type humidity sensor described in detailed description of the invention one, and described method comprises the steps:

Step one: prepare the substrate 1 of sensor, and adopt deionized water to clean the substrate 1 of preparation；

Step 2: the surface oxidation of the substrate 1 step one prepared, generates SiO one layer fine and close₂, as the first insulating barrier 2；

Step 3: obtain insulating barrier upper surface in step 2, adopts the method for photoetching process and magnetron sputtering to prepare snakelike heater electrode 8；

Step 4: the method for employing radio-frequency sputtering prepares Al at the upper surface of snakelike heater electrode 8 prepared by step 3₂O₃Protective layer, as the second insulating barrier 9；

Step 5: adopt the method for photoetching process and magnetron sputtering to prepare bottom electrode 10 at the upper surface of the second insulating barrier 9；

Step 6: described in the upper surface of the bottom electrode 10 that step 5 is prepared by the method for employing corrosion hollowed-out and step one, the lower surface of substrate 1 carries out hollow out process respectively；

Step 7: the upper surface of the bottom electrode 10 after hollow out processes prepares humidity sensing layer 11；

Step 8: adopt evaporation coating machine to prepare electrode 12 in porous at the upper surface of humidity sensing layer 11.

Detailed description of the invention three: present embodiment is the further restriction of the preparation method to the sonde heated type humidity sensor described in detailed description of the invention two, and in step one, described substrate 1 is the monocrystal silicon in thickness 400 μm, crystal orientation 100；

In step 2, the thickness of described insulating barrier is 500nm～1000nm.

Detailed description of the invention four: present embodiment is the further restriction of the preparation method to the sonde heated type humidity sensor described in detailed description of the invention three, in step 3, obtaining insulating barrier upper surface in step 2, the method for employing photoetching process and magnetron sputtering is prepared the method for snakelike heater electrode 8 and is:

With the pierced pattern of snakelike heater electrode 8 for mask plate, photoresist is evenly coated in and obtains insulating barrier upper surface through step 2, then at 80 DEG C～100 DEG C, dry 20min～40min, after exposure 15s～30s, it is transferred in developer solution development 20s～40s, rinse 20s～30s in deionized water, then post bake 30min～40min at 100 DEG C～120 DEG C；

Adopting the method upper surface plated film at described photoresist of magnetron sputtering, target is the platinum of 99.99%, and target size is Φ 60 × 2.5mm, reaches 1 × 10 in vacuum^-5Pa～2 × 10^-5During Pa, toward the logical argon of sputtering chamber, the flow of argon is 15ml/min～25ml/min, and ar pressure is 1.5Pa～2.5Pa；Utilize acetone solution photoresist ultrasonic to snakelike heater electrode 8 clear patterns.Detailed description of the invention five: present embodiment is the further restriction of the preparation method to the sonde heated type humidity sensor described in detailed description of the invention four, in step 4, the method for employing radio-frequency sputtering prepares Al at the upper surface of snakelike heater electrode 8 prepared by step 3₂O₃The method of protective layer is:

Photoresist is evenly coated in the upper surface of snakelike heater electrode 8 prepared by step 3, then at 80 DEG C～100 DEG C, dry 20min～40min, the upper surface at photoresist will be covered for mask plate with the pattern of pad, after exposure 15s～30s, it is transferred in developer solution development 20s～40s, rinse 20s～30s in deionized water, then at 100 DEG C～120 DEG C post bake 30min～40min；

When vacuum reaches 1 × 10^-5Pa～2 × 10^-5During Pa, toward the logical argon of sputtering chamber, the flow of argon is 15ml/min～25ml/min, and ar pressure is 1.5Pa～2.5Pa, and sputtering power is 60W～80W, and the time is 120min～180min, and during plated film, pressure controls at below 0.5Pa, it is thus achieved that Al₂O₃Protective layer.

Detailed description of the invention six: present embodiment is the further restriction of the preparation method to the sonde heated type humidity sensor described in detailed description of the invention five, in step 5, the method that the method for photoetching process and magnetron sputtering prepares bottom electrode 10 at the upper surface of the second insulating barrier 9 is adopted to be:

Utilize the photoresist on acetone solution the second insulating barrier 9, peel off primary heater pad 5 and secondary heater pad 6 region the second insulating barrier 9, the method adopting magnetron sputtering carries out plated film at the upper surface of the second insulating barrier 9, target is the gold of 99.99%, the size Φ 60 × 2.5mm of target, reaches 1 × 10 in vacuum^-5Pa～2 × 10^-5During Pa, toward the logical argon of sputtering chamber, the flow of argon is 15ml/min～25ml/min, and ar pressure is 1 × 10^-5Pa～2 × 10^-5Pa；

Photoresist is evenly coated in the upper surface of the second insulating barrier 9, the mask plate of bottom electrode 10 pattern is covered on the relevant position on photoresist surface, after exposure 15s～30s, it is transferred in developer solution development 20s～40s, rinse 20s～30s in deionized water, then, post bake 30min～40min at 100 DEG C～120 DEG C, adopt iodine and iodate ammonia saturated solution to erode the golden film exposed, obtain bottom electrode 10.

Detailed description of the invention seven: present embodiment is the further restriction of the preparation method to the sonde heated type humidity sensor described in detailed description of the invention six, in step 6, the lower surface of substrate 1 described in the upper surface of the bottom electrode 10 that step 5 is prepared by the method for employing corrosion hollowed-out and step one carries out the method for hollow out process respectively and is；

nullDescribed in the upper surface of bottom electrode 10 prepared by step 5 and step one, the lower surface of substrate 1 coats photoresist respectively，Then at 80 DEG C～100 DEG C, dry 20min～40min，The mask that upper surface is plate-making figure with described bottom electrode 10 is covered on the upper surface of bottom electrode 10，To cover on the relevant position of the lower surface of substrate 1 with the mask that hollow out figure is plate-making figure，Double-sided exposure 15s～30s，Development 20s～40s in developer solution is put in the substrate 1 exposed，Rinse 20s～30s in deionized water，Post bake 30min～40min at 100 DEG C～120 DEG C again，At the temperature of 70 DEG C～90 DEG C，The lower surface of substrate 1 is carried out hollow out by the potassium hydroxide solution adopting mass fraction to be 35%～40%，Form groove 13，The degree of depth of groove 13 is about 350 μm.

Detailed description of the invention eight: present embodiment is the further restriction of the preparation method to the sonde heated type humidity sensor described in detailed description of the invention seven, in step 7, the upper surface of the bottom electrode 10 after hollow out processes is prepared the method for humidity sensing layer 11 and is:

Wet for polyimides sense solution is coated in the upper surface of bottom electrode 10, then constant temperature 5min～10min at 80 DEG C～100 DEG C；

The mask plate that humidity-sensitive film pattern is plate-making figure is covered the upper surface at bottom electrode 10, exposure 15s～30s, then at N, develop in N-dimethyl acetylamide 20s～40s, rinse 20s～30s in deionized water, then post bake 30min～40min at 100 DEG C～120 DEG C, namely obtains humidity sensing layer 11, and humidity sensing layer 11 thickness is about 1 μm.

Detailed description of the invention nine: present embodiment is the further restriction of the preparation method to the sonde heated type humidity sensor described in detailed description of the invention eight, in step 8, adopt evaporation coating machine to prepare the method for electrode 12 in porous at the upper surface of humidity sensing layer 11 to be:

Evaporation raw material is native gold, and the mask plate adopting the pattern of electrode 12 in porous to be plate-making figure is template, film forming in evaporation coating machine, evaporation current 110A～120A, and namely evaporation time 5s～10s obtains electrode 12 in porous.

The preparation method adopting present embodiment, humidity sensor capacitance is about 100pF～160pF.

Detailed description of the invention ten: present embodiment is a kind of humidity measuring circuit,

When high-altitude humidity changes, humidity sensor capacitance changes therewith, if it is possible to be accurately obtained the change of humidity sensor capacitance, it becomes possible to obtain the change of high altitude environment humidity.In most of the cases, capacitance type humidity sensor be considered one purely capacitive, but when working under high temperature, low temperature, super-humid conditions, humidity sensor capacity loss can not be ignored, and when being operated in high frequency, inductive effect can not be ignored.Accordingly, it is considered to when the loss of humidity sensor and inductive effect, humidity sensor be not construed as one purely capacitive, equivalent circuit is as shown in Figure 7.Wherein R_sFor the resistance loss between circuit board leads and pole plate, its value progressively becomes big along with increasing of operating frequency, R_sGenerally only small, though operating frequency megahertz more than time also only small, so only when operating frequency is high just need consider R_sImpact on measurement result；L is the equivalent inductance of the inductance of humidity sensor self and outside lead, and the inductance of self is relevant with its version, and outside lead equivalent inductance is then relevant with wire length, goes between more short, and inductance is more little；R_PFor parallel loss resistance, including the dielectric loss between pole plate and leakage loss, when humidity sensor is operated in low frequency, its loss is relatively big, along with increasing of operating frequency tapers into.Humidity sensor electric capacity total loss coefficient can be represented by formula (8).

$D=\frac{1}{R_p\mathrm{\ω}C}+R_s\mathrm{\ω}C,---\left(8\right)$

Wherein ω is circular frequency.There is resonant frequency in the equivalent circuit of humidity sensor electric capacity as can be known from Fig. 7, when humidity sensor is in resonant frequency by cisco unity malfunction, resonant frequency need to be avoided during design circuit, from in formula (8), Section 1 reduces along with its value of rising of resonant frequency, Section 2 increases along with its value of rising of resonant frequency, so there is optimal frequency f_bSo that total loss coefficient D is minimum, it is possible to obtain f by formula (8) is minimized_b, as shown in formula (9).

$f_b=\frac{1}{2\mathrm{\π}C}\sqrt{\frac{1}{R_p{R}_s}},---\left(9\right)$

So humidity sensor effective capacitance C_eCan be represented by formula (10).

$\frac{1}{{\mathrm{j\ωC}}_e}=j\mathrm{\ω}L+\frac{1}{j\mathrm{\ω}C},---\left(10\right)$

Humidity sensor electric capacity actual change amount can be represented by formula (11).

$\frac{{\mathrm{\ΔC}}_e}{C_e}=\frac{\mathrm{\Δ}C/C}{1-{\mathrm{\ω}}^{2}LC},---\left(11\right)$

Above formula shows, humidity sensor capacitance tool is had a certain impact by the inductance of humidity sensor self and the equivalent inductance of outside lead.And parasitic capacitance between humidity sensor and ground and and lead-in wire between parasitic capacitance variation with temperature at low ambient temperatures and change.

For the problems referred to above, in conjunction with the requirement to moisture measurement of the aerological sounding business, present embodiments provide for a kind of humidity measuring circuit based on capacitor charge and discharge and relative method, as shown in Figure 8.

Described humidity measuring circuit includes sonde heated type humidity sensor C_m, standard capacitance C_s, artifical resistance R_P, parasitic capacitance C_P, resistance R1, resistance R2, three operational amplifiers, 2 single-pole double-throw switch (SPDT)s and power supply；

Described humidity sensor C_mOne end and standard capacitance C_sOne end meet the ground end of power supply, humidity sensor C simultaneously_mThe other end and quiet end of the first single-pole double-throw switch (SPDT) connect, standard capacitance C_sThe other end and the first single-pole double-throw switch (SPDT) another quiet end connect,

The moved end of the first single-pole double-throw switch (SPDT) and artifical resistance R_POne end connect, artifical resistance R_PThe other end and parasitic capacitance C_POne end, one end of resistance R1 and the positive input of the first operational amplifier be simultaneously connected with the Vcc end of power supply, parasitic capacitance C_PAnother termination power supply ground end,

Another of resistance R1 terminates the moved end of the second single-pole double-throw switch (SPDT), the Vcc end of a quiet termination power supply of the second single-pole double-throw switch (SPDT), the ground end of another quiet termination power supply of the second single-pole double-throw switch (SPDT),

The signal output part of the first operational amplifier is connected with the non-inverting signal input thereof of the first operational amplifier with one end of resistance R2 simultaneously simultaneously, the power supply positive pole of the first operational amplifier is connected with the Vcc end of power supply, the power supply ground end of the first operational amplifier is connected with the ground end of power supply, the other end of resistance R2 is connected with the forward signal input of the second operational amplifier and the forward signal input of the 3rd operational amplifier simultaneously, the non-inverting signal input thereof of the second operational amplifier is connected with the Vcc end of power supply, the non-inverting signal input thereof of the 3rd operational amplifier is connected with the Vcc end of power supply,

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

In present embodiment, the second operational amplifier and the 3rd operational amplifier all adopt integrated circuit NE5532 to realize, the voltage range of the non-inverting signal input thereof of the second operational amplifier and the non-inverting signal input thereof input of the 3rd operational amplifier is 1.5v-2.5v, first operational amplifier adopts integrated circuit CA3140 to realize, and in present embodiment, power supply provides 1.5v, 2.5v and 5v voltage.

R_PFor analog switch conducting resistance and lead resistance.Owing to humidity sensor and standard capacitance are in same measurement loop, and humidity sensor and the standard capacitance other end are connected to ground, parasitic capacitance C_PFor humidity sensor and standard capacitance wiring parasitic capacitance and and ground between parasitic capacitance.

When electric capacity not being charged, second operational amplifier and the output of the 3rd operational amplifier (NE5532) are all zero, when charging to electric capacity, the input voltage of input in the same direction of two operational amplifier NE5532 gradually rises, when input voltage reaches 1.5V, the output of the second operational amplifier is become high level from low level, triggers MCU external interrupt, starts timing；When input voltage reaches 2.5V, the output of the 3rd operational amplifier is become high level from low level, again triggers MCU external interrupt, and timing terminates, and thus obtains the charging interval of electric capacity.So the charging interval T of standard capacitance can be obtained_sc, and the charging interval T of humidity sensor_mc, the capacitance of standard capacitance is it is known that be assumed to be C_s, then the capacitance of humidity sensor

Standard capacitance and humidity sensor are charged respectively in identical situation, and capacitor charge and discharge curve is as shown in Figure 9.

Wherein T_CFor charging interval, T_DFor discharge time, V_mFor charging saturation voltage.Tentative standard electric capacity is V from voltage_s, charge to V_eThe required time is T_sc, humidity sensor is V from voltage_s, charge to V_eThe required time is T_mc, the capacitance of standard capacitance is C_s, then

$K_1{C}_m=T_{mc}\×\frac{K_2{C}_s}{T_{sc}},---\left(12\right)$

K₁For considering analog switch and lead resistance, the parasitic capacitance impact on humidity sensor capacitance, K₂For considering analog switch and lead resistance, the parasitic capacitance impact on standard capacitor-value, owing to humidity sensor and standard capacitance are at the same circuit, K₁=K₂, then measured capacitance C_mCan be obtained by formula (13).

$C_m=T_{mc}\×\frac{C_s}{T_{sc}},---\left(13\right)$

Present embodiment has good temperature stability, it is possible to effective suppression temperature drift and null offset, reduces the parasitic capacitance impact on measurement result.It is to be noted that consider the impact of distribution capacity and leakage current, in charging process, voltage not can select that 0V～full scale is interval；Owing to humidity sensor capacitance is between 100pF～160pF, capacitance is less, needs Reasonable adjustment charge constant, and the charging interval is not easily long to be not easy to too short, shortens the measurement time while ensureing higher certainty of measurement；Charging current is not easily excessive, it is to avoid puncture humidity sensor；The electric capacity with high accuracy, good temperature characterisitic, high reliability and stability need to be selected as standard capacitance；Operational amplifier in charging circuit need to have higher input impedance, relatively low leakage current.

Ground experiment to the present invention:

Room temperature characteristic test: adopt two-pressure method to carry out moisture measurement static experiment.Humidity sensor working range is 0%RH～100%RH, according to selecting test point principle and sample space size selection principle, have chosen 10%RH, 30%RH, 50%RH, 70%RH, 90%RH5 test point, ambient temperature is+30 DEG C, each humidity point carries out data record after stablizing 20min, and relative humidity rises to 90%RH from 10%RH.As shown in Figure 10, matched curve expression formula is as shown in (14) for experimental result.

Y=2.336x-256.553, (14)

Can drawing from experimental result, it is determined that coefficient is 0.9963, and humidity measurement results has the good linearity, sensitivity is 2.336%RH/pF, standard deviation sigma=1.917, and capacitance measurement resolving power is 0.1pF, and moisture measurement resolving power is about 0.2%RH.In order to obtain sluggishness and the repeatability of moisture measurement, having carried out humidity and risen and decline follow-on test, experimental result is as shown in figure 11.

Test temperature is+30 DEG C, and range of humidity variation is 10%RH-90%RH, have chosen 10%RH, 30%RH, 50%RH, 70%RH, 90%RH5 test point, according to experimental result it can be seen that humidity is at the maximum difference Δ H of uphill process Yu decline conditional curve_M≈ 0.7%RH, Full-span output Y_FS=100%RH, measures sluggishness and is about 0.7%F S, and repeatability is about 1.9%.Sluggish uncertainty U_H≈ 0.35%, linear uncertainty U_L=3%, repeatability uncertainty U_R≈ 1.9%, then overall uncertainty U ≈ 4%.

Temperature alternating characteristic test: when not being heated humidity sensor controlling, respectively at+10 DEG C, 0 DEG C ,-10 DEG C ,-40 DEG C, carry out static experiment under-70 DEG C of temperature conditions.By controlling the heating power of humidity sensor, sensor sheet surface temperature is made all to maintain+20 DEG C～about+30 DEG C under varying environment temperature conditions, thus ensureing that humidity sensor has good measurement characteristics, it is to avoid wetness sensor surface condenses, and heating power is controlled with reference to table 1 data.From test result it can be seen that when humidity sensor works at low ambient temperatures, by humidity sensor is heated so that wetness sensor surface temperature is higher than ambient temperature, and its measurement characteristics obtains obvious improvement, it is to avoid the phenomenon of sensor condensation.

When humidity sensor is at+10 DEG C, 0 DEG C ,-10 DEG C ,-40 DEG C, when working under-70 DEG C of temperature environments, by its sensitivity after humidity sensor is heated, mean error, sluggishness, repeatability as shown in table 2.

Measurement characteristics after heating humidity sensor under table 2 different temperatures

Operating temperature	Sensitivity	Mean error	Sluggish	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 is tested: be placed in humidity generator by homemade sealed compartment, humidity sensor is arranged in sealed compartment, by cable, the signal of humidity sensor is drawn, by controlling the up and down motion of the cylinder being connected in sealed compartment with sealing lid, it is possible to control opening or closing of sealed compartment.Set the temperature of humidity generator as+30 DEG C, open plug hatch, set humidity generator initial humidity H₀=30%, after moisture stable, close sealed compartment；Set the humidity H of humidity generator₁=70%, after moisture stable, open sealed compartment, every 0.05s records a humidity value simultaneously, it is assumed that certain moment t_nThe humidity value H that humidity sensor records_nMeet formula (15), then t_nIt it is the time constant of humidity sensor.

$\frac{H_1-H_n}{H_1-H_0}=63.2\%,---\left(15\right)$

Figure 12 is humidity sensor time constant test result.It can be seen that humidity sensor time constant is about 0.5s from test result, identical with external advanced RS92 type sonde humidity sensor time constant, it is better than the domestic GTS1 type sonde humidity sensor time constant being currently in use at present.

The invention provides a kind of sandwich capacitance type humidity sensor structure of flat board with snakelike heater electrode and its preparation process, additionally provide humidity measuring circuit, carried out ground characteristics experiment.Test result indicate that, the humidity sensor sensitivity of design is about 2.3RH%/pF, sluggishness is about 0.7%F S, repeatability is about 1.9%, measures overall uncertainty and is about 4%, and time constant is about 0.5s, by sensor is heated, humidity sensor be ensure that at low ambient temperatures, and its measurement characteristics is stable, demonstrates science and the effectiveness of the method, establishes technical foundation for the application of next step through engineering approaches.

Claims (2)

1. a humidity measuring circuit, it is characterised in that described humidity measuring circuit includes sonde heated type humidity sensor C_m, standard capacitance C_s, artifical resistance R_P, parasitic capacitance C_P, resistance R1, resistance R2, three operational amplifiers, 2 single-pole double-throw switch (SPDT)s and power supply；

Described humidity sensor C_mOne end and standard capacitance C_sOne end meet the ground end of power supply, humidity sensor C simultaneously_mThe other end and quiet end of the first single-pole double-throw switch (SPDT) connect, standard capacitance C_sThe other end and the first single-pole double-throw switch (SPDT) another quiet end connect,

The moved end of the first single-pole double-throw switch (SPDT) and artifical resistance R_POne end connect, artifical resistance R_PThe other end and parasitic capacitance C_POne end, one end of resistance R1 and the positive input of the first operational amplifier be simultaneously connected with the Vcc end of power supply, parasitic capacitance C_PAnother termination power supply ground end,

Another of resistance R1 terminates the moved end of the second single-pole double-throw switch (SPDT), the Vcc end of a quiet termination power supply of the second single-pole double-throw switch (SPDT), the ground end of another quiet termination power supply of the second single-pole double-throw switch (SPDT),

The signal output part of the first operational amplifier is connected with the non-inverting signal input thereof of the first operational amplifier with one end of resistance R2 simultaneously simultaneously, the power supply positive pole of the first operational amplifier is connected with the Vcc end of power supply, the power supply ground end of the first operational amplifier is connected with the ground end of power supply, the other end of resistance R2 is connected with the forward signal input of the second operational amplifier and the forward signal input of the 3rd operational amplifier simultaneously, the non-inverting signal input thereof of the second operational amplifier is connected with the Vcc end of power supply, the non-inverting signal input thereof of the 3rd operational amplifier is connected with the Vcc end of power supply,

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

2. a kind of humidity measuring circuit according to claim 1, it is characterised in that the voltage range of the non-inverting signal input thereof of the second operational amplifier and the non-inverting signal input thereof input of the 3rd operational amplifier is 1.5v-2.5v.