JPH0317087B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a humidity sensor.

Its purpose is to create a humidity sensor that independently combines a humidity sensing part and a detection part. It can detect changes in humidity directly as an electrical signal, has excellent responsiveness, and can quickly measure humidity. It is an object of the present invention to provide a new and useful humidity sensor that is not easily affected by atmospheric gases, has great durability, is suitable for mass production, and can be produced at low cost.

Hair-type sensing means and wet-bulb sensing means, which have been used in hygrometers for a long time, have the disadvantage of slow response to changes in humidity. Humidity sensors that extract changes in electrical conductivity and capacitance as electrical signals have been developed, and hygrometers, dew point meters, etc. that utilize these sensors have become commercially available.

However, since this conventional humidity sensor uses the electrical properties of the humidity sensor itself, the electrical signal may be distorted due to the influence of atmospheric gases, especially gases that ionize when humid, such as carbon dioxide, sulfur dioxide, and nitrogen dioxide. The disadvantage is that it tends to cause
They are easily affected by dust, have a short lifespan, and have unsatisfactory responsiveness.Furthermore, those that rely on changes in capacitance require the extraction electrode to be formed on a moisture-sensitive film. They had various problems, including manufacturing difficulties.

With the above points in mind, the inventors of the present invention focused on the fact that resistance-type strain gauges cause changes in resistance value during bending deformation, and as a result of intensive research, they combined a moisture-sensitive membrane and this strain gauge to detect humidity. He invented a humidity sensor with a structure with independent parts.

The present invention provides a strain gauge in which a strain gauge resistance pattern is formed on an insulating substrate that has low hygroscopicity and elasticity, and is bonded to the surface of the pattern and the exposed insulating substrate by a plasma polymerization reaction. A moisture-sensitive film made of a thin organic film is provided, and the amount of warpage caused by the difference in expansion rate during moisture absorption and desorption between the moisture-sensitive film and the insulating substrate is detected as a change in the resistance value of the strain gauge. The gist of the present invention is a humidity sensor characterized by the following.

The present invention will be explained with reference to the drawings.

FIG. 1 is an enlarged front view showing an example of a strain gauge used in the sensor of the present invention.

As shown in this figure, strain gauge a used in the present invention
In this example, a gauge resistor 2 is formed as a pattern on one side of an insulating substrate 1.

This insulating substrate 1 needs to be made of a material with low hygroscopicity and elasticity; for example, a plastic film, an extremely thin glass plate, a ceramic plate, or a metal plate whose surface has been subjected to insulation treatment can be used. be.

Further, the gauge resistor 2 formed as a pattern on one side of the insulating substrate 1 may be made of various known metals, alloys, etc. whose electrical resistance value changes when bent, such as a copper-nickel alloy. can be used as the preferred material.

Note that 3 in FIG. 1 is a connection lead line.

FIG. 2 is an enlarged side sectional view of one embodiment of the sensor of the present invention.

As shown in this figure, the sensor of the present invention uses, for example, two strain gauges a and insulating substrates 1 are bonded together, and gauge resistors 2 and 2' are installed on both sides, and the gauge is protected on the surface. A film 4 is provided, and a moisture-sensitive film 5, which is an organic thin film, is further deposited on only one side of the gauge resistor 2 using a thin film forming means using a plasma polymerization reaction.

In other words, in the embodiment shown in this figure, two strain gauges a are combined and have gauge resistors 2 and 2' on both sides, and a moisture-sensitive film 5 is formed on the entire surface of only one of them by plasma polymerization reaction. be.

The moisture-sensitive film 5 formed by plasma polymerization is an organic thin film that exhibits the characteristic of absorbing moisture, expanding, dehumidifying, shortening, and returning to its original state. For example, it is made of allylamine polymer, acrylamide polymer, acrylonitrile polymer Organic thin films obtained by plasma polymerization of nitrogen-containing monomers such as , etc. have shown good results, and in experiments conducted by the present inventors, in particular, those obtained by plasma polymerization of allylamine have been found to be the most excellent moisture-sensitive films that can be used.

The thickness of this moisture-sensitive film 5 is extremely thin, for example, in the case of the plasma-polymerized thin film of allylamine mentioned above, the thickness is 1000~
It only needs to be 5000 Å thick, and because it is such a very thin film, it absorbs and releases moisture quickly, making it a humidity sensor with excellent responsiveness.

FIG. 3 is an enlarged side sectional view showing how the embodiment shown in FIG. 2 absorbs moisture and warps.

As shown in this figure, even if the moisture sensitive film 5 absorbs moisture and expands greatly, the expansion is slight because the insulating substrate 1 hardly absorbs moisture, and due to this difference in expansion rate, the gauge resistor 2' side of this sensor is on the inside. In other words, the moisture sensitive membrane 5 side is bent outward.

As a result, the electrical resistance value of the inner gauge resistor 2' decreases, and the electrical resistance value of the outer gauge resistor 2 increases. The increase or decrease in resistance value naturally changes depending on the degree of warpage of the sensor, and the degree of warpage is determined by the degree of moisture absorption of the moisture-sensitive film 5.

In other words, the expansion and contraction of the moisture-sensitive membrane 5 due to moisture absorption and desorption causes a warpage change in the strain gauge a, and this can be directly detected as an electrical signal in the form of a change in the gauge resistance value.

As explained above, the present invention is a humidity sensor that combines a strain gauge and a moisture-sensitive membrane, and the moisture-sensitive membrane that expands and contracts depending on humidity and the detection section that extracts this as a change in resistance value are integrated independently of each other. This is a completely new technical idea that converts humidity directly into an electrical signal regardless of the electrical properties of the moisture-sensitive membrane, so the influence of atmospheric gases and dirt is extremely small due to its structure, and the reliability of detected values is high. It also has excellent durability.

Furthermore, in the present invention, the strain gauge that senses the amount of warpage is located on the lower surface of the moisture-sensitive membrane, meaning that the sensing element is not present on the outer surface of the moisture-sensitive membrane, and the entire surface of the moisture-sensitive membrane is exposed to the outside. Therefore, the sensitivity of the moisture-sensitive membrane is high, and there is nothing that interferes with moisture absorption and desorption, which has an excellent effect on its responsiveness.

In addition, the sensor of the present invention has fewer manufacturing troubles because the strain gauge part and the humidity sensing part do not affect each other in terms of characteristics, and the plasma reaction device of thin film technology can be used as is to form the humidity sensitive film. The process is the same as the conventional technology, and the reproducibility is also good, and the film thickness can be set arbitrarily.
It has excellent mass productivity and enables the production of inexpensive humidity sensors.

As mentioned above, the sensor of the present invention has an extremely thin moisture-sensitive film, so that the moisture absorption and desorption rate is fast, and humidity detection with excellent responsiveness can be achieved.

As described above, the sensor of the present invention exhibits various effects and usefulness.

Example Two strain gauges a were prepared using a Mylar film with a thickness of 50μ as the insulating substrate 1, forming a gauge pattern of copper-nickel alloy as the gauge resistor 2, and bonding a protective film 4 to the surface of the gauge pattern.
These two insulating substrates 1 were bonded together with an adhesive to create a strain gauge with a double-sided structure.

Next, on one side of this double-sided strain gauge, allylamine was polymerized and deposited using a plasma polymerization thin film forming apparatus to form a moisture sensitive film 5 having a thickness of 3000 Å, thereby producing a humidity sensor of the present invention as shown in FIG.

A detection circuit as shown in FIG. 4 was created by combining two of these humidity sensors.

In other words, the detection circuit consists of 4 gauge resistors (120Ω)
The gauge output is 1.
It is four times the size of a gauge, and it is also possible to compensate for drift due to temperature changes.

Note that R ₁ and R ₂ in the figure indicate the gauge resistance within one humidity sensor, and R ₃ and R ₄ indicate the gauge resistance within the other humidity sensor. That is, when moisture is absorbed, R ₁ and R ₃ show an increase in resistance, and R ₂ and R ₄ show a decrease in resistance, and when moisture is removed, the opposite is shown.

Further, P in FIG. 4 represents an amplifier and a recorder, where the sensor output is detected.

When we investigated the relationship between relative humidity (RH) and sensor output (μV) using this detection circuit, we found that
The results shown in the figure were obtained.

Note that humidity fixed points of various salt-saturated aqueous solutions were used as constant humidity atmosphere for humidity measurement.

As shown in Figure 5, from 0%RH to 98%RH.
The changes up to this point were curves with a slightly steeper slope on the low humidity side and a smaller slope on the high humidity side, but it was clear that humidity changes could be interpreted as changes in sensor output.

Also, the response speed during this measurement is 10 to 20 when moisture is absorbed.
It was concluded that the response time was very fast (about 40 to 60 seconds when dehumidifying), which was slightly slower than when absorbing moisture, but it still responded within 1 minute, so it had a much faster response time than conventional products. Note that this response speed can be further reduced by using an insulating substrate that is less hygroscopic.

Furthermore, when the above-mentioned sensor was examined for the influence of atmospheric gas (mainly the concentration of carbon dioxide gas) at the same humidity, the value obtained as the sensor output hardly changed.

[Brief explanation of drawings]

FIG. 1 is an enlarged front view showing an example of a strain gauge used in the sensor of the present invention. FIG. 2 is an enlarged side sectional view of one embodiment of the sensor of the present invention. FIG. 3 is an enlarged side sectional view showing how the embodiment shown in FIG. 2 absorbs moisture and warps. 1... Insulating substrate, 2... Gauge resistor, 3... Connection lead wire, a... Strain gauge consisting of 1, 2, and 3, 4... Gauge protection film, 5... Moisture sensitive film, Fig. 4 1 is a detection circuit diagram in an embodiment of the present invention. FIG. 5 is a graph showing the relationship between the relative humidity (RH) determined by the detection circuit of FIG. 4 and the sensor output (μV).

Claims (1)

[Claims] 1. A strain gauge in which a strain gauge resistance pattern is formed on an elastic insulating substrate with low hygroscopicity. A moisture-sensitive film made of an organic thin film is provided, and the amount of warpage caused by the difference in expansion rate during moisture absorption and desorption between the moisture-sensitive film and the insulating substrate is expressed as a change in the resistance value of the strain gauge. A humidity sensor that detects humidity. 2. The humidity sensor according to claim 1, wherein the moisture sensitive organic thin film is a plasma polymerized thin film of allylamine.