Wireless passive film type humidity sensor

Abstract

The invention discloses a wireless passive thin film type humidity sensor which comprises an LC resonance circuit, a first isolation thin film, a humidity sensitive thin film, a second isolation thin film, a magnetic thin film and a substrate thin film. The LC resonance circuit is composed of a metal coil, a substrate and an interdigital capacitor, and the metal coil and the interdigital capacitor are placed on the two sides of the substrate respectively and connected in series through through holes formed in the substrate to form the LC resonance circuit; the first isolation film is arranged between the LC resonance circuit and the humidity-sensitive film, the second isolation film is arranged between the humidity-sensitive film and the magnetic film, and the substrate film is arranged on the other side of the magnetic film. When the humidity of the external environment changes, the humidity-sensitive film expands or shrinks due to the change of the water content, so that the distance between the LC resonance circuit and the magnetic film is changed, the inductance is changed along with the change, the resonance frequency of the LC resonance circuit is changed, and wireless passive detection of the humidity is realized by measuring the resonance frequency.

Description

Technical Field

[0001] The invention relates to the technical field of humidity detection, in particular to a wireless passive thin-film humidity sensor. Background Art

[0002] Humidity is closely related to human production and life, so people have started to measure humidity very early. Humidity sensors are developing from traditional discrete humidity sensors to intelligent integrated humidity sensors, and also from wired sensors to wireless forms. Traditional humidity sensors meet most practical application needs, but problems such as large size still restrict their application in portable devices and microelectronic products. Semiconductor integrated humidity sensors have advantages that traditional discrete humidity sensors do not have, such as small size, fast response and low power consumption. At the same time, with the advancement of semiconductor processing technology, the humidity measurement principles of many traditional discrete hygrometers can be transplanted to semiconductor integrated humidity sensors. In addition, with the emergence of MEMS technology, MEMS humidity sensors such as high-depth ratio structure and polysilicon microbridge structure have been developed.

[0003] Various types of humidity sensors have been emerging in the market, including capacitive humidity sensors, resistive humidity sensors, semiconductor and optical humidity sensors, etc. However, most of the above sensors are active wired sensors and require external power supplies. These power supplies are mainly lithium batteries or button batteries. The use of batteries means that the size of such sensors is large, which is not conducive to the integration of design circuits, the working time is limited, and the battery needs to be replaced uninterruptedly, and the problem of chemical pollution of discarded batteries will also occur. The above problems limit the development of existing humidity sensors. In addition, for some harsh special scenarios, such as high temperature, closed or rotating equipment for humidity measurement, wired sensors will no longer be applicable, and most of them adopt wireless working mode. Wireless active humidity sensors generally use radio frequency transceiver circuits for data transmission, and their transceiver circuits rely on batteries for power supply. In contrast, wireless passive humidity sensors generally use the inductive near-field coupling principle to transmit data. The sensor structure is simple and does not require battery power supply, which is particularly suitable for applications in harsh environments. Therefore, in order to broaden the application scope of humidity sensors, it is necessary to develop wireless passive thin-film humidity sensors with high sensitivity, long life, low power consumption and small size. Summary of the invention

[0004] In response to the above needs, the present invention provides a wireless passive thin-film humidity sensor, which responds to changes in ambient humidity through an LC resonant circuit based on the law of volume changes of the hygroscopic material when the humidity changes. The sensor with this structure can achieve high-sensitivity, passive wireless detection and low-power humidity detection, solve the problems in materials, reliability, repeatability and production cost, and thus provide support and guarantee for the industrial application of passive wireless humidity sensors.

[0005] A wireless passive thin-film humidity sensor of the present invention comprises an LC resonant circuit, a first isolation film, a humidity-sensitive film, a second isolation film, a magnetic film and a substrate film. The LC resonant circuit is composed of a metal coil, a substrate and an interdigital capacitor, wherein the metal coil and the interdigital capacitor are respectively placed on both sides of the substrate and connected in series through a through hole provided on the substrate to form an LC resonant circuit; the first isolation film is provided between the LC resonant circuit and the humidity-sensitive film; and the second isolation film is provided between the humidity-sensitive film and the magnetic film.

[0006] The LC resonant circuit is a circuit composed of an inductor and a capacitor connected in series. The capacitor stores energy through the electric field, and the inductor stores energy through the magnetic field. Under the action of the inductor and the capacitor, the energy in the LC resonant circuit will be in a resonant state, that is, in each cycle, the energy is exchanged back and forth between the inductor and the capacitor. At the resonant frequency, the energy exchange between the inductor and the capacitor reaches the maximum, the impedance of the circuit is the minimum, and the phase difference between the current and the voltage is zero. The LC resonant circuit has the advantages of simple circuit structure, easy design and implementation, high quality factor (Q value), low energy loss, and low cost.

[0007] Preferably, the inductor in the LC resonant circuit adopts a planar spiral structure inductor structure made of a metal coil. Planar spiral inductor is a spiral inductor wound on a plane, which is mainly used in the fields of radio frequency integrated circuits, microwave equipment and high-frequency power supplies. Its structure is more compact than traditional wire-wound inductors, easy to integrate and manufacture, and suitable for applications with limited size and weight; suitable for high-frequency applications because it can reduce the influence of parasitic capacitance and parasitic resistance; has good thermal stability; can flexibly adjust performance parameters through the optimization of its parameters such as number of turns, line width, line spacing, etc. and can be used in conjunction with other devices.

[0008] The use of an LC resonant circuit based on a planar spiral coil inductor is the key to enabling the sensor of the present invention to work in a wireless passive manner. First, the planar spiral inductor made of a metal coil can achieve energy exchange with an external alternating electromagnetic field through the principle of electromagnetic induction, so that energy can be introduced into the LC resonant circuit when the sensor is working, without the need for the LC resonant circuit to have its own power supply, thereby achieving the effect of passive sensing. Secondly, in the process of realizing wireless energy transmission through electromagnetic induction, the external electromagnetic field can load a specific signal, thereby realizing wireless signal transmission between the LC resonant circuit and the outside world.

[0009] Preferably, the capacitor in the LC resonant circuit is in the form of a finger capacitor. The finger capacitor has a compact structure, can be designed in a miniaturized manner, and is convenient for connection with the planar spiral inductor; and the performance of the capacitor can be optimized by adjusting the number, width, spacing and other parameters of the finger.

[0010] Preferably, the metal coil and the interdigital capacitor are made of copper, because copper has a high electrical conductivity, which can effectively reduce the impedance of the LC resonant circuit, improve the quality factor, and improve the sensitivity of the sensor.

[0011] Preferably, in order to enhance the corrosion resistance of the sensor, a protective layer is coated on the surface of the metal coil and the interdigital capacitor.

[0012] Preferably, the copper wire thickness of the metal coil and the interdigital capacitor is 0.08-0.25 mm. When the copper wire thickness is too small, the resistance value is too large, the quality factor Q is reduced, and the sensitivity of the sensor is reduced; when the thickness is too large, the manufacturing difficulty increases and the overall structure is affected, which is not conducive to the installation of the sensor itself and other devices.

[0013] Preferably, the substrate in the LC resonant circuit is made of polyimide film. Polyimide is a commonly used circuit substrate material for flexible circuit boards. Compared with hard substrates used for PCB circuit boards, polyimide is light in weight, has stable physical and chemical properties, and is not easily affected by the external environment.

[0014] Preferably, the LC resonant circuit is arranged in the center of the humidity sensitive film, and the area of ​​the LC resonant circuit is less than 25% of the area of ​​the humidity sensitive film. Such a structural design is adopted to ensure that the humidity sensitive film has sufficient contact area with the external environment, so as to realize effective diffusion and transportation of water molecules between the environment and the humidity sensitive film, thereby achieving rapid response to changes in environmental humidity, improving the performance of the humidity sensor, and expanding its application range.

[0015] Preferably, the humidity sensitive film adopts polymer hydrogel. Polymer hydrogel has extremely high water absorption, and its structure is usually three-dimensional network-like, composed of polymer chains connected by crosslinking points. In the dry state, this network structure is relatively loose. After absorbing water, water molecules will enter the network structure of the polymer, interact with the functional groups between the polymer chain and the crosslinking point, and the water molecules will fill the gaps in the polymer network, resulting in the volume expansion of the polymer network. Therefore, the polymer hydrogel will undergo significant volume expansion after moisture absorption, and this volume expansion can be used to measure changes in humidity levels. In addition, the water molecules in the polymer hydrogel are kept in the polymer network, rather than simply adsorbed on the surface, so that the expansion of the polymer hydrogel is reversible, that is, when the humidity decreases, it can release water molecules and return to its original state. Therefore, the polymer hydrogel can quickly absorb or release water, and can usually respond quickly to humidity changes. As a humidity sensitive film, the polymer hydrogel can obtain advantages such as high sensitivity, rapid response, controllability and lightweight in wireless passive humidity sensors.

[0016] The swelling ratio is generally used to characterize the hygroscopic expansion properties of polymer hydrogels. Swelling is the phenomenon that the volume of a polymer expands in a solvent. Cross-linked polymers swell due to the absorption of a certain amount of solvent. When the swelling equilibrium is reached, the ratio of its volume to the volume before swelling is called the swelling ratio. Specifically, the swelling ratio q is calculated as follows:

[0017]

[0018] In the formula,

[0019] V a It is the volume of the solvent absorbed when the sample is in swelling equilibrium, unit: mL;

[0020] W0 is the mass of dry gel, unit: g;

[0021] W is the mass of the sample at swelling equilibrium, unit: g;

[0022] ρ a is the density of the solvent, unit: g / cm 3 .

[0023] When selecting polymer hydrogels, materials with higher swelling ratios need to be considered.

[0024] Furthermore, the polymer hydrogel is an anisotropic hydrogel, which has the characteristics of absorbing moisture and expanding between the layers of the humidity sensor, but remaining stable within the film plane.

[0025] Preferably, the polymer hydrogel is one of anisotropic nanocellulose hydrogel, anisotropic polyvinyl alcohol hydrogel, anisotropic MXene conductive hydrogel, anisotropic poly N-isopropylacrylamide (PNIPAM) fiber hydrogel, and anisotropic acrylamide (AM) / 2-acrylamide-2-methylpropanesulfonic acid (AMPS) / Fe3O4-R hydrogel.

[0026] Furthermore, the polymer hydrogel has a thickness of 1 to 3 mm.

[0027] Preferably, a first isolation film is provided between the LC resonant circuit and the humidity sensitive film. The function of the first isolation film is:

[0028] (1) Isolating the LC resonant circuit and the humidity-sensitive film prevents water molecules in the hygroscopic film from diffusing into the LC resonant circuit, thereby causing the loss of water molecules in the humidity-sensitive film and causing measurement errors; at the same time, it also prevents excessive water molecules from contacting the LC resonant circuit, thereby ensuring the stability and life of the LC resonant circuit.

[0029] (2) Because the volume of the humidity-sensitive film changes after absorbing moisture and expanding, the isolation film can be set to prevent the humidity-sensitive film from being embedded in the gap of the metal line of the LC resonant circuit after absorbing moisture and expanding, thereby hindering the volume shrinkage of the humidity-sensitive film during the dehydration process, thereby ensuring the repeatability and stability of the humidity sensor measurement.

[0030] (3) When the humidity sensitive film is deformed, it will generate uneven swelling force. The first isolation film can increase the effective area of ​​the swelling force, so that it is evenly dispersed on the force-bearing surface in the LC resonant circuit, so that the LC resonant circuit can achieve overall vertical translation without bending deformation. If the LC resonant circuit generates bending deformation, the inductance value of the planar spiral inductor will change, thereby causing measurement errors.

[0031] Therefore, by providing the first isolation film between the LC resonant circuit and the humidity sensitive film, not only the structural consistency and stability of the LC resonant circuit are ensured, but also the life of the sensor is improved.

[0032] Preferably, the first isolation film is made of polyimide film. This is because polyimide film has a relatively high molecular weight, which makes the polyimide film have excellent chemical stability, and also has excellent mechanical strength and rigidity, which can meet the performance requirements of the first isolation film.

[0033] Furthermore, the area of ​​the first isolation film is the same as the area of ​​the LC resonant circuit.

[0034] Preferably, the magnetic film is one of a ferrite film and a nickel-iron alloy film. Ferrite film and nickel-iron alloy film have relatively high magnetic permeability, which enables them to effectively absorb and concentrate magnetic fields. In addition, the magnetic film material exhibits good corrosion resistance in a humid environment, which enables the sensor to continue to work stably in a humid or harsh environment.

[0035] Preferably, the relative magnetic permeability of the magnetic film is in the range of 10 to 1000. When the humidity-sensitive film is affected by humidity changes, the straight-line distance between the planar spiral inductor and the magnetic film will change. Due to the higher magnetic permeability of the magnetic film, the magnetic flux of the planar spiral inductor will change significantly, resulting in a change in inductance, causing a change in the resonant frequency of the LC resonant circuit. In addition, the relative magnetic permeability range is large, which can increase the stability of the sensor. Even in an environment with large humidity changes, the response of the sensor can remain stable, and unstable measurement results will not occur due to humidity fluctuations. In addition, the relative magnetic permeability range can enable the sensor to generate a larger response signal, thereby reducing the power consumption required for the sensor to read data, thereby reducing overall energy consumption.

[0036] Preferably, a second isolation film is provided between the humidity sensitive film and the magnetic film to prevent the humidity sensitive film from corroding the magnetic film after sensing humidity, and to evenly disperse the swelling force generated when the humidity sensitive film is deformed on the force-bearing surface of the magnetic film, thereby preventing stress concentration from causing deformation of the overall structure of the sensor, thereby affecting the consistency and stability of the sensor measurement.

[0037] Preferably, the area of ​​the second isolation film is the same as the area of ​​the humidity sensitive film.

[0038] Preferably, the second isolation film is made of polyethylene terephthalate (PET). PET material has a high molecular weight, orderly molecular arrangement and crystallinity, so that PET has high strength and toughness.

[0039] Preferably, a substrate film is provided on the other side of the magnetic film for carrying the entire humidity sensor and isolating it from the surface of the placed object.

[0040] The working principle of the scheme of the present invention is: when the humidity of the external environment changes, the anisotropic polymer hydrogel of the humidity-sensitive film expands or contracts due to water absorption or dehydration, and the anisotropic polymer hydrogel has the characteristics of absorbing moisture and expanding between the humidity sensor layers, but maintaining stability within the film plane, thereby ensuring that when the environmental humidity changes, the humidity-sensitive film can ensure the structural stability of the sensor layers. On this basis, the thickness of the humidity-sensitive film changes, thereby changing the distance between the LC resonant circuit and the magnetic film. The relationship between the inductance value of the metal coil and the separation distance between the ferrite film and the LC resonant circuit can be expressed as:

[0041]

[0042] Where, L is the inductance of the metal coil, μ r is the relative magnetic permeability of the magnetic film, d is the separation distance between the magnetic film and the LC resonant circuit, l e is the effective length of the metal coil.

[0043] The distance between the LC resonant circuit and the ferrite film is thereby converted into a change in inductance, which is further converted by the sensor into a detectable change in the resonant frequency of the LC resonant circuit, thereby calculating the magnitude of the humidity change.

[0044] Due to the high relative magnetic permeability of the magnetic film, the magnetic induction intensity is large, and the inductance changes more significantly, so that the resonant frequency of the LC resonant circuit changes more significantly. In addition, the magnetic film can avoid the interference of conductive materials and overcome the problem of poor robustness.

[0045] The relationship between the quality factor of the LC resonant circuit and the capacitance and inductance is:

[0046]

[0047] Among them, Q is the quality factor, W is the power frequency when the circuit resonates, L is the inductance, R is the resistance, and C is the capacitance. When the external humidity increases, the inductance increases, and the effective quality factor Q of the sensor also increases, which overcomes the problem of effective quality factor Q loss in most existing flexible wireless passive humidity sensors.

[0048] Beneficial effect: The humidity sensor uses a hygroscopic material with good moisture sensing properties as the humidity sensitive layer. The volume of the hygroscopic material changes when the humidity changes, causing the distance between the LC resonant circuit and the magnetic film to change, causing the inductance to change, and bringing about a change in the resonant frequency of the LC resonant circuit, thereby realizing wireless passive detection of humidity by measuring the resonant frequency.

[0049] The wireless passive thin film humidity sensor of the present invention provides a method for realizing wireless passive detection based on a humidity sensitive layer and an LC resonant circuit, and has greatly improved sensitivity and anti-interference ability. At the same time, the device also has the advantages of low power consumption, long life, easy integration, low measurement error, small size, process compatibility, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is the structure diagram of wireless passive thin film humidity sensor;

[0051] Figure 2 It is a cross-sectional view of the LC resonant circuit layer;

[0052] Figure 3 This is the inductance sensitivity test diagram of the wireless passive thin film humidity sensor;

[0053] Figure 4 This is the resonant frequency sensitivity test diagram of the wireless passive thin film humidity sensor;

[0054] Figure 5 This is a test chart of synchronization and stability of wireless passive thin film humidity sensor;

[0055] Figure 6 This is the response / recovery time test graph of the wireless passive thin film humidity sensor;

[0056] Figure 7 It is the hysteresis loop and hysteresis difference diagram of the wireless passive thin film humidity sensor;

[0057] Figure 8 This is the temperature test diagram of the wireless passive thin film humidity sensor;

[0058] Fig. 9 This is a repeatability test chart of a wireless passive thin-film humidity sensor.

[0059] Description of reference numerals:

[0060] 1 is an LC resonant circuit, 2 is a first isolation film, 3 is a moisture-sensitive film, 4 is a second isolation film, 5 is a magnetic film, 6 is a substrate film, 11 is a metal coil, 12 is a substrate, and 13 is an interdigital capacitor. DETAILED DESCRIPTION

[0061] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings.

[0062] A wireless passive thin film humidity sensor of the present invention comprises an LC resonant circuit 1, a first isolation film 2, a humidity sensitive film 3, a second isolation film 4, a magnetic film 5 and a substrate film 6. The LC resonant circuit 1 is composed of a metal coil 11, a substrate 12 and an interdigital capacitor 13, wherein the metal coil 11 and the interdigital capacitor 13 are respectively placed on both sides of the substrate 12 as an inductor and a capacitor, and are connected in series through a through hole provided on the substrate 12 to form the LC resonant circuit 1; the first isolation film 2 is provided between the LC resonant circuit 1 and the humidity sensitive film 3; and the second isolation film 4 is provided between the humidity sensitive film 3 and the magnetic film 5.

[0063] The metal coil 11 in the LC resonant circuit 1 adopts a planar spiral structure inductor structure. In order to improve the sensitivity of the sensor and enhance the accuracy and precision of parameter measurement. Preferably, the spiral inductor made of the metal coil 11 is square, forming a square planar spiral inductor. The square planar spiral inductor is arranged by arranging the wires on a plane in a square spiral shape. Due to the structure of the square spiral, the magnetic field is more evenly distributed inside the wire. When inductors with the same inductance value are made at the same time, the area of ​​the square planar spiral inductor made is smaller than that of the circular planar spiral inductor and the weak coupling inductor. Therefore, when the magnetic film 5 causes a change in magnetic flux, the change in the inductance value of the square planar spiral inductor is more obvious than that of the circular planar spiral inductor and the weak coupling inductor.

[0064] Specifically, when the humidity sensor of the present invention is applied to the design of a smart home humidity monitoring system, practical factors such as sensing frequency, communication standard, antenna size, circuit design and application environment are considered. In smart home applications, a smaller inductance value is usually selected, such as a range of 50 to 200 mH.

[0065] In order to realize the miniaturization of the sensor and improve the accuracy of the sensor parameter measurement, the capacitor in the LC resonant circuit 1 adopts the form of forked electrodes. The forked capacitor 13 composed of the forked electrodes is realized by introducing a forked structure between two metal electrodes. Compared with the capacitor of the parallel plate structure, the capacitor of the forked structure has a smaller volume, lower inductive coupling, lower crosstalk and mutual inductance effect due to its special structure, which can improve the stability of the circuit and reduce the mutual influence between signals. At the same time, the sensitivity of the forked capacitor 13 to external electromagnetic interference is relatively low, so that the forked capacitor 13 can provide more reliable performance in a high noise environment. Reduce the volume of the sensor and improve the accuracy of the sensor parameter measurement.

[0066] The metal coil 11 and the interdigital capacitor 13 are made of copper. In order to enhance the corrosion resistance of the sensor, SL-362 environmentally friendly copper alloy passivation liquid is coated on the surface of the metal coil 11 and the interdigital capacitor 13. SL-362 environmentally friendly copper alloy passivation liquid can quickly form a dense, anti-oxidation passivation film on the surface of copper, and the copper substrate can provide good adhesion for the formed passivation film, forming a good combination and extending the service life of the sensor.

[0067] The copper wire thickness of the metal coil 11 and the interdigital capacitor 13 is 0.15 mm. When the copper wire thickness is too small, the resistance value is too large, the quality factor Q is reduced, and the sensitivity of the sensor is reduced; when the thickness is too large, the manufacturing difficulty increases and the overall structure is affected, which is not conducive to the installation of the sensor itself and other devices.

[0068] The substrate 12 in the LC resonant circuit 1 is made of polyimide film. Polyimide is a commonly used circuit substrate material for flexible circuit boards. Compared with hard substrates used for PCB circuit boards, polyimide is light in weight, has stable physical and chemical properties, and is not easily affected by the external environment.

[0069] The LC resonant circuit 1 is arranged in the center of the humidity sensitive film 3, and the area of ​​the LC resonant circuit 1 is 20% of the area of ​​the humidity sensitive film 3. Such a structural design is adopted to ensure that the humidity sensitive film 3 has sufficient contact area with the external environment, so as to realize the effective diffusion and transportation of water molecules between the environment and the humidity sensitive film 3, thereby achieving a rapid response to changes in environmental humidity, improving the performance of the humidity sensor, and expanding its application range.

[0070] The humidity sensitive film 3 adopts anisotropic polymer hydrogel, specifically, anisotropic hydrogel prepared by wood and polyacrylamide through natural cellulose skeleton orientation method. Its tensile strength parallel to the hydrogel orientation direction and perpendicular to the hydrogel orientation direction are 18MPa and 0.8MPa respectively, and the anisotropy index is 22.88. At the same time, there are many hydrophilic groups in its hydrogel network. The hydrophilic groups can bond with water molecules in the form of hydrogen bonds, thereby achieving the absorption of water molecules.

[0071] The thickness of the humidity sensitive film 3 is 2 mm.

[0072] A first isolation film 2 is provided between the LC resonant circuit 1 and the humidity sensitive film 3. The first isolation film 2 is made of a polyimide film, because the polyimide film has a relatively high molecular weight, which makes the polyimide film have excellent chemical stability, and also has excellent mechanical strength and rigidity, which can meet the performance requirements of the first isolation film 2.

[0073] The area of ​​the first isolation film 2 is the same as the area of ​​the LC resonance circuit 1 .

[0074] The magnetic film 5 is made of nickel zinc ferrite (NiZn), the chemical formula of which is Ni a Zn 1-a Fe2O4. The film is formed by physical vapor deposition, chemical vapor deposition or sputtering. The nickel-zinc ferrite film has high magnetic permeability and magnetic field strength. Therefore, when the distance between the LC resonant circuit 1 and the magnetic film 5 changes, the magnetic flux of the metal coil 11 in the LC resonant circuit 1 changes accordingly, causing a change in inductance. At the same time, nickel-zinc ferrite has good magnetic stability and thermal stability, can maintain its magnetic properties under high temperature conditions, and work for a long time without significant performance degradation or damage. And the nickel-zinc ferrite film has good antioxidant properties and can resist oxidation and corrosion. It helps to maintain its performance and stability, so that the sensor has a longer life and reliability.

[0075] The relative magnetic permeability of the magnetic film 5 is 100.

[0076] A second isolation film 4 is provided between the humidity sensitive film 3 and the magnetic film 5. This is to prevent the humidity sensitive film 3 from corroding the magnetic film 5 after sensing humidity, and at the same time, the swelling force generated when the humidity sensitive film 3 is deformed can be evenly dispersed on the force-bearing surface of the magnetic film 5, to prevent stress concentration from causing deformation of the overall structure of the sensor, thereby affecting the consistency and stability of the sensor measurement.

[0077] The material used for the second isolation film is polyethylene terephthalate (PET). PET material has a high molecular weight, orderly molecular arrangement and crystallinity, so that PET has high strength and toughness.

[0078] The area of ​​the second isolation film 4 is the same as that of the humidity sensitive film 3 , ensuring that the second isolation film 4 can isolate and protect the entire area of ​​the magnetic film 5 .

[0079] A substrate film 6 is provided on the other side of the magnetic film 5 for carrying the entire humidity sensor and isolating it from the surface of the placed object.

[0080] The humidity sensor in the present invention uses a humidity-sensitive material with good humidity-sensing properties as the humidity-sensitive film 3. When the humidity changes, the volume of the humidity-sensitive film 3 changes, causing the distance between the LC resonant circuit 1 and the magnetic film 5 to change, causing the inductance to change, resulting in a change in the resonant frequency of the LC resonant circuit 1, and then realizing wireless passive detection of humidity by measuring the resonant frequency. The wireless passive humidity film sensor in the present invention provides a method for realizing wireless passive detection based on a humidity-sensitive unit and an LC resonant circuit 1, and has greatly improved sensitivity and anti-interference ability. At the same time, the device also has the advantages of low power consumption, long life, easy integration, low measurement error, small size, and process compatibility.

[0081] In order to further confirm the performance of the humidity sensor provided by the patent of this invention, relevant tests are performed on it.

[0082] (1) Inductance sensitivity test:

[0083] like Figure 3 The figure shows the relative inductance change of the humidity sensor based on the anisotropic hydrogel prepared by the humidity sensitive film 3 of wood and polyacrylamide through the natural cellulose skeleton orientation method between the relative humidity of 20%-80%. It can be seen that when the humidity sensitive film 3 absorbs moisture and expands, the magnetic film 5 will be away from the coil magnetic field, and the sensitivity of the inductance is 7.67nH / %RH.

[0084] (2) Resonance frequency sensitivity test:

[0085] like Figure 4 The figure shows the change of the resonant frequency of the LC resonant circuit 1 between 20% and 80% relative humidity. It can be seen from the figure that the resonant frequency increases from 26.65MHz to 29.78MHz when the relative humidity increases from 20% to 80%. The sensitivity of the resonant frequency is 52.17KHz / %RH.

[0086] (3) Synchronicity and stability test:

[0087] In order to determine the synchronization and stability of the sensor during the dynamic humidity change process, a dynamic cycle experiment was carried out on the sensor. The inductance change curve in the dynamic cycle experiment is shown in Figure 5 shown.

[0088] In the experiment, a humidifier was used to humidify for 10 seconds and then dehumidify for 10 seconds, and the relative humidity in the controlled environment was cycled between 40% and 60%. Figure 5 The highest and lowest values ​​of the inductance in the four cycles in (a) remain consistent, indicating that the volume change of the humidity sensor remains consistent with the change of the ambient humidity. Figure 5 (b) Yes Figure 5 (a) is a separate analysis of the fourth cycle. In the figure, 60-70s indicates that during the humidification process, the volume almost reaches stability around 63s, and during the dehumidification process of 70-80s, the volume almost reaches stability around 73s. The change from low humidity to medium humidity is faster, and the change from medium humidity to high humidity is slower, which is consistent with the curve in this section. It can be seen that the humidity sensor can maintain good synchronization with the ambient humidity during dynamic humidity changes, and maintain good stability during continuous humidity changes.

[0089] (4) Response / recovery time test

[0090] In order to determine the dynamic response / recovery time of the humidity sensor, a dynamic test was performed on it, such as Figure 6 As shown in the figure, the sensor is repeatedly inserted into a saturated MgCl2 (magnesium chloride) solution (relative humidity of about 32%) and a K2CO3 (potassium carbonate) solution (relative humidity of about 43%), and the dynamic response curve of the sensor is measured. It can be clearly seen that after each switch between high and low humidity environments, the response curve of the sensor remains basically consistent, indicating that the humidity sensor has good repeatability and stability. And from the dynamic response curve, the response time and recovery time of the sensor when switching back and forth between high and low humidity environments can be obtained. Calculation shows that the response time of the sensor is about 3.1s, and the recovery time is about 4.3s.

[0091] (5) Hysteresis loop and hysteresis difference

[0092] In order to obtain the hysteresis loop of the humidity sensor, the sensor is placed in humidity environments from low humidity to high humidity, and then placed in humidity environments from high humidity to low humidity. The inductance value of the humidity sensor is recorded each time it is placed, and the result is plotted as follows: Figure 7 The hysteresis loop is shown. Calculated from the test results, we know that the hysteresis of the sensor is small, and the maximum hysteresis is about 3%RH, which means that after the humidity-sensitive film 3 made of polymer hydrogel fully absorbs water molecules in a high humidity environment, the water molecules can be relatively quickly and completely separated from the humidity-sensitive film when the ambient humidity decreases.

[0093] (6) Temperature test:

[0094] The humidity sensor was tested at a relative humidity of 40% and a temperature range of -45°C to 45°C. The relative inductance changes are shown in Figure 8 As shown in the figure, the relative inductance change of the humidity sensor at -45℃~45℃ is almost 0, so it can be considered that the humidity sensor is almost unaffected by temperature (-45℃~45℃). When it is subsequently used in daily life or industry, external temperature fluctuations have almost no effect on the sensor performance.

[0095] (7) Repeatability test:

[0096] The humidifier was tested alternately at medium humidity (50% RH) and high humidity (80% RH) twice a day for 5 days. The inductance response is as follows: Fig. 9 During the 10 tests, the inductance output of the sensor maintained good repeatability.

[0097] The above is a specific description of the preferred implementation of the present invention, but the present invention is not limited to the above-mentioned implementation mode. Technical personnel familiar with the field can also make various equivalent deformations or substitutions without violating the spirit of the present invention. These equivalent deformations or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A wireless passive thin film humidity sensor, characterized in that: It includes an LC resonant circuit, a first isolation film, a humidity-sensitive film, a second isolation film, a magnetic film and a substrate film; The LC resonant circuit is composed of a metal coil, a substrate and an interdigital capacitor, wherein the metal coil and the interdigital capacitor are respectively placed on both sides of the substrate and connected in series through a through hole provided on the substrate to form an LC resonant circuit; The first isolation film is arranged between the LC resonant circuit and the humidity sensitive film; The second isolation film is arranged between the humidity sensitive film and the magnetic film; The substrate film is arranged on the other side of the magnetic film; The LC resonant circuit is arranged at the center of the humidity-sensitive film, and the area of ​​the LC resonant circuit is less than 25% of the area of ​​the humidity-sensitive film; the area of ​​the first isolation film is the same as the area of ​​the LC resonant circuit.

2. The wireless passive thin film humidity sensor according to claim 1, characterized in that: The metal coil is a planar spiral inductor structure.

3. The wireless passive thin film humidity sensor according to claim 1, characterized in that: The metal coil and the interdigital capacitor are made of copper, and a protective film is coated on their surfaces.

4. The wireless passive thin film humidity sensor according to claim 1, characterized in that: The material of the first isolation film is polyimide.

5. The wireless passive thin film humidity sensor according to claim 1, characterized in that: The material used for the humidity sensitive film is anisotropic polymer hydrogel.

6. The wireless passive thin film humidity sensor according to claim 1, characterized in that: The magnetic film includes one of a ferrite film and a nickel-iron alloy film.

7. The wireless passive thin film humidity sensor according to claim 1, characterized in that: The relative magnetic permeability of the magnetic film ranges from 10 to 1000.

8. A method for using the wireless passive thin film humidity sensor according to claim 1, characterized in that: When the humidity of the external environment changes, the humidity-sensitive film expands or contracts due to changes in water content, thereby changing the distance between the LC resonant circuit and the magnetic film, causing inductance changes, resulting in changes in the resonant frequency of the LC resonant circuit, and then realizing wireless passive detection of humidity by measuring the resonant frequency.

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