JPH0687048B2 - Operating circuit of organic polymer humidity sensor - Google Patents

Description

TECHNICAL FIELD The present invention relates to an operating circuit of a humidity sensor necessary for controlling humidity of an atmosphere, and particularly to an operation suitable for a humidity sensor using an organic polymer material as a moisture sensitive film. Regarding the circuit.

[Background of the Invention]

As the humidity control has been considered as an important element as a part of the air conditioning technology, the applicable range of the humidity sensor has been widened, and great efforts have been made to improve the performance of the humidity sensor.

From such a point of view, one of the humidity sensors that has been attracting attention in recent years is a humidity sensor having a moisture sensitive film made of an organic polymer material.

An example of such an organic polymer humidity sensor is shown in FIGS. 1 and 2.

In these figures, FIG. 1 is a plan view and FIG. 2 is a cross-sectional view taken along the line AA ′, where 11 is a substrate, 12 and 13 are electrodes, 14 is a moisture sensitive film, and 15 and 16 are terminals. Is. In addition, 1 represents the entire humidity sensor.

The substrate 11 is made of a dielectric material such as aluminum oxide and has a rectangular shape, and a pair of electrodes 12 and 13 made of a plurality of narrow strips formed of a thin film of gold or the like on one surface of the substrate 11 are mutually predetermined. Are arranged in a fitted state with a gap therebetween.
And, on the surface of the substrate 11 between these electrodes 12, 13,
A layer of an organic polymer material, whose conductivity and inductivity change according to the ambient humidity, is provided by coating, etc.
Is formed.

Examples of the organic polymer material used for the moisture sensitive film 14 include sulfonated polystyrene and a copolymer of cation monomer (eg, methacryl / oxy / trimethyl / ammonium chloride) and vinyl monomer (eg, methyl / methacrylate). Is used. As an example of the size of the humidity sensor 1, the substrate 11 is a plate material having a thickness of 1 mm and is 15 mm square, the thickness of the electrodes 12 and 13 is about 10 μm, and the gap between the electrodes is about 0.2 mm. , The thickness of the moisture sensitive film 14 is 10μ
It is about m.

An example of the characteristics of the humidity sensor 1 is shown in FIG. 3, where A is the electric resistance characteristic between the terminals 15 and 16 with respect to humidity, and B is the capacitance characteristic. As is clear from this characteristic diagram, in this humidity sensor, the electric resistance and the electrostatic capacitance show an exponential change with respect to the relative humidity, and the humidity can be detected. Incidentally, in this FIG. 3 is an equivalent circuit of the humidity sensor shown in FIGS. 1 and 2 is regarded as a parallel circuit of the electrical resistance and capacitance, the results measured at the original frequency of 1 kH _z This is an example.

Now, in order to detect the humidity using such a humidity sensor, it is necessary to measure the electric resistance and the electrostatic capacitance as described above and to detect the humidity according to the measured value. An operating circuit for that is required.

Then, as an operation circuit conventionally used in such a case, a predetermined voltage is applied to the humidity sensor as in JP-A-55-40935, and the current flowing through the humidity sensor is directly measured by this. A method in which humidity is calculated or a humidity sensor is used as one resistance element of a resistance voltage divider to measure the output voltage of the resistance voltage divider to calculate the humidity is mainly used.

However, when the organic polymer type humidity sensor as described above is used by such a conventionally used operation circuit, the electrode is deteriorated in a relatively short time, and the humidity detection characteristics change significantly over time. There was a drawback that it would end up.

[Object of the Invention]

The object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, and even when the humidity detection device is configured by being applied to an organic polymer humidity sensor, there is no fear of electrode deterioration of the humidity sensor, and a sufficiently stable detection is performed. An object is to provide an operating circuit of an organic polymer type humidity sensor that can be used while maintaining its characteristics.

[Outline of Invention]

To achieve this object, the present invention provides:
The feature is that the voltage applied between the terminals of the humidity sensor is always kept below a predetermined value so that this voltage does not exceed the ionization voltage of the electrode.

Example of Invention

Hereinafter, the operating circuit of the organic polymer humidity sensor according to the present invention will be described with reference to the illustrated embodiment.

FIG. 4 shows an embodiment of the present invention, 1 is an organic polymer humidity sensor, 2 is an AC voltage source, 3 is a detection circuit, 4 is a coupling capacitor, and 5 is an output terminal.

The organic polymer type humidity sensor 1 uses an organic polymer material as the moisture sensitive film as described with reference to FIGS. 1 and 2, and has a characteristic as shown in FIG.

The AC voltage source 2 includes an operational amplifier 21, a capacitor 22, a rectangular wave oscillator composed of resistors 23 to 25, and a buffer amplifier composed of an operational amplifier 28, and an organic polymer type humidity sensor 1 (hereinafter, simply referred to as a sensor) via a capacitor 4. Referred to as) to supply an AC voltage. Here, the operational amplifier (hereinafter,
21), which is called an operational amplifier, constitutes an astable vibrator.
CR determined by the product of capacitance value of capacitor 22 and resistance value of resistor 23
It oscillates at a frequency according to the time constant, and the amplitude of the output rectangular wave signal at this time is determined by the DC power supply voltages V _CC and V _EE . Therefore, the DC voltage V _CC , V stabilized by the stabilized power supply, etc.
_{If EE} is supplied to the operational amplifier 21, a rectangular wave with a constant amplitude can be obtained from the output. Therefore, the output V _{o of the} operational amplifier 21 is divided into a predetermined value V by the resistors 26 and 27, and is taken out by the operational amplifier 28 to which 100% of negative feedback is applied to operate as an AC voltage source having a predetermined constant output. Become. The detection circuit 3 includes an operational amplifier 31 having a capacitor 32 and diodes 33 to 35, rectifies the AC current flowing from the AC voltage source 2 to the sensor 1 via the capacitor 4, and further the voltage-current non-linearity of the diode 33. Logarithmically converted by
The exponential function characteristic of the sensor 1 as shown in FIG. 3 is converted into a linear characteristic and the detection output is taken out to the output terminal 5.

The capacitor 4 serves to prevent or prevent the direct current component contained in the output of the alternating current voltage source 2 from being added to the sensor 1, thereby preventing the sensor 1 from being deteriorated. In addition,
At this time, the impedance of the capacitor 4 must be set to a value sufficiently smaller than the impedance of the sensor 1 so that it can be practically ignored.

Next, the operation of this embodiment will be described.

Since the amplification degree of the operational amplifier 31 is usually extremely high, the potential of its − input is maintained at almost 0 voltage, and the impedance of the capacitor 4 is almost negligible as compared with the impedance of the sensor 1. Is applied with the AC output voltage V of the AC voltage source 2 as it is, and as a result, a current determined by the impedance of the sensor 1 and the AC voltage V flows into the operational amplifier 31, and the DC voltage obtained by logarithmically converting the current is output to the output terminal 5. Appears.

Therefore, when the impedance of the sensor 1 changes as shown in FIG. 3 according to the ambient humidity, the operational amplifier is correspondingly changed.
The current flowing into the minus input of 31 also changes, and a DC voltage that changes according to humidity is obtained at the output terminal 5. Moreover, at this time, since the operational amplifier 31 performs logarithmic conversion, the DC voltage that appears at the output terminal 5 is obtained. Has a value almost proportional to the change in humidity, and the change in humidity can be detected as a DC voltage that changes in proportion thereto.

On the other hand, at this time, the AC voltage applied between the terminals 15 and 16 of the sensor 1 is the DC voltage V supplied to the operational amplifier 21.
The maximum value is determined by the voltage division ratio of _CC , V _EE and resistors 26, 27.

Therefore, by setting this AC voltage V to a predetermined value,
In this embodiment, it is possible to completely prevent the electrode of the sensor 1 from being deteriorated, and it is possible to prevent the sensor characteristic from changing with time. The detailed description will be given later.

Next, FIG. 5 shows another embodiment of the present invention, which is different from the embodiment of FIG. 4 in that the content of the AC voltage source 2 is only a rectangular wave oscillator composed of an operational amplifier 21. 1
A voltage divider for making the AC voltage V to be applied to the predetermined value is constituted by the resistors 6 and 7, and further, the detection circuit 3
The inside is a voltage comparator composed of an operational amplifier 31 and resistors 37 and 38, and accordingly, a rectifying diode 35 and a smoothing capacitor 32 are connected to the input side of the operational amplifier 31, and the others are. This is the same as in the case of FIG.

In the embodiment shown in FIG. 5, the output of the AC voltage source 2 is the output voltage V _o of the operational amplifier 21 constituting the rectangular wave oscillator, which is connected to one terminal of the sensor 1 via the capacitor 4.
It is applied directly to 15.

On the other hand, the other terminal 16 of the sensor 1 is grounded by the resistor 7, and the resistor 6 is connected between these terminals 15 and 16.

Therefore, ignoring the impedance of the capacitor 4 and ignoring the capacitance of the sensor 1, the AC voltage V applied to the sensor 1 is given by the following equation.

Where R _{x is} the resistance value of the sensor 1 R _{p is} the resistance value of the resistor 6 R _{s is} the resistance value of the resistor 7 As a result, the AC voltage V _x appearing across the resistor 7 becomes the resistance value of the sensor 1. It changes all the time.

Therefore, in the embodiment of FIG. 5, the AC voltage V _{x of the} resistor 7 is
Is rectified by a diode 33 and smoothed by a capacitor 32 to obtain a DC voltage E _x proportional to the AC voltage V _x.
Supply to the + input of.

On the other hand, this operational amplifier 31 has a voltage E _s obtained by dividing it from the DC power supply V _CC by resistors 3 and 38, and is applied to its − input, so that it operates as a voltage comparator using the voltage E _s as a reference voltage. I'm running.

Therefore, in the embodiment of FIG. 5, the voltage E _x rises as the ambient humidity rises, and when it becomes larger than the reference voltage E _s , the output terminal 5 becomes high level and the voltage E _x becomes the reference voltage. When it is smaller than E _{s, the} output terminal 5 operates so as to maintain a low level, and a change in ambient humidity can be detected as a switching between two level signals depending on the vertical relationship with respect to a certain set humidity.

Also in this embodiment, the applied voltage V to the sensor 1 given by the above equation (1) is set so as to be kept at a predetermined value even when the resistance value R _x of the sensor 1 shows the maximum value. As a result, the characteristics of the sensor 1 can be prevented from changing over time, and a highly reliable sensor can be obtained.

Here, the reason why the ionization of the electrode of the sensor is prevented by the present invention and the change with time of the characteristic thereof can be eliminated will be described.

Returning to FIG. 3, the characteristic indicated by broken lines A ′ and B ′ in the figure is that the sensor 1 described in FIGS. 1 and 2 is energized with an AC voltage having a frequency of 50 Hz and a peak value of 2 V for about 500 hours. At this time, the state is changed from the characteristics A ′ and B shown by the solid lines. The material of the moisture sensitive film 14 is the above-mentioned methacrylic
It is a copolymer of oxy-trimethyl-ammonium chloride and methyl-methacrylate, and the counter ion contained in the copolymer is chlorine.

In this case, the amount of change from the initial characteristics is about 3% in terms of relative humidity (RH), which is a considerable problem in practical use.

Further, in this case, not only the characteristics changed but also discoloration appeared at the end of the electrode (in this case, the gold electrode).

From this, the following can be understood. That is, during the half cycle of the alternating current applied to the sensor, the gold electrode is ionized and becomes a chlorine compound, which makes it difficult for chlorine ions to work, and the reaction as shown in the following formula (2) is an alternating current. It also occurs when the power is turned on.

_{^{A u + 4Cl - = AuCl -}} 4 + 3e ........................ (2) that is, the resulting change with time in characteristics of the sensor can also be used by applying an AC voltage, the ionization of the electrodes is generated This is because the mobile ions in the moisture-sensitive film are gradually restrained by this, and in order to prevent characteristic changes, it is sufficient to prevent ionization of the electrodes.

By the way, this ionization can be reduced by increasing the frequency of the applied AC voltage and shortening the half-cycle energization period, but this is not an essential solution.

Therefore, the voltage value of the AC voltage applied to the above sensor is
When the change with time was changed for each 0.5V and the characteristics were examined with time, the results shown in FIG. 6 were obtained. This 6th
As is clear from the figure, the characteristic changes only when the peak value of the applied voltage exceeds 1.0V, and it is understood that the characteristic does not change at a voltage lower than this.

On the other hand, there is a standard electrode potential of the electrode reaction that has a threshold value in the vicinity of 1.0 V, and it is known that this is determined by the electrode material and the ion species. As an example thereof ^{_{A u + I - = AuI -}} + e ...... 0.53VA u + 4Br = AuBr - 4 + 3e ...... 0.87VA u + 4Cl - = AuCl - 4 + 3e ...... 1.00VA u = A u 3+ + 3e ...... 1.50 It is V.

Therefore, in response to this, a copolymer having counter ions of fluorine, chlorine, bromine, iodine, and hydroxyl group was prepared, and gold was used when a DC voltage was applied for a short time to a sensor using these as a moisture sensitive film. The ionization of the electrode was examined by the occurrence of its discoloration, and an example of the result is shown in FIG. In FIG. 7, a circle indicates that no change occurred at the end of the gold electrode, and a cross indicates that discoloration occurred.

Comparing the results of FIG. 7 with the standard electrode potentials described above, these show a good agreement except that discoloration in the presence of chlorine ions occurs at 0.9 V. It turns out that the result is theoretically correct.

Even in the coexistence of chlorine ions in FIG. 7, the degree of color change is small at the voltage at which the color change starts to occur, and the degree of color change increases as the voltage increases. Therefore, the effect of discoloration at a voltage of 0.9V to 1.0V at which discoloration begins to occur is practically well tolerated.

Therefore, it can be understood from the results of FIG. 7 that, except for the case of the hydroxyl group, among the remaining four kinds of halogen elements, the elements having the largest ionization tendency, namely, fluorine, chlorine, bromine, and iodine are threshold values in this order. It can be seen that the voltage is high, and in the case of fluorine, the discoloration at the edge of the gold electrode is most unlikely to occur, and in any case, there is a threshold voltage, and use below this voltage For example, it can be seen that it is possible to prevent the sensor from changing its characteristics. When a hydroxyl group is used as a counter ion, ionization does not occur up to 1.2 V. Therefore, a sensor using a copolymer having a hydroxyl group as a counter ion may cause characteristic changes even when the applied voltage is considerably increased. It is also understood that stable operation can be obtained.

By the way, as already described, according to the embodiments of FIGS. 4 and 5, the AC voltage V applied to the sensor 1 is always limited to the predetermined value.

Therefore, if the voltage V is set to be a predetermined value equal to or lower than the ionization voltage depending on the type of counter ions contained in the moisture sensitive film 14 of the sensor 1 and the materials of the electrodes 12 and 13, There is no possibility that the characteristics of the sensor 1 will change over time, and the humidity detection operation can always be performed reliably and reliably.

In each of the above embodiments, a rectangular wave oscillator is used as the AC voltage source 2, but this is one embodiment, and it is stable even if any other AC voltage source is used. It goes without saying that the present invention can be referred to as an embodiment of the present invention as long as it can provide a predetermined AC output.

Further, the sensor 1 is not limited to the examples shown in FIGS. 1 and 2, but any humidity sensor may be used as long as it uses an organic polymer material as a moisture sensitive film, and for example, the electrodes 12, 13 Needless to say, the present invention may be carried out by using a conductive compound such as ruthenium oxide, zirconia boride, silicon carbide, or boron nitride instead of gold. Although a sensor using silver as the electrodes 12 and 13 is also known, the reaction formulas in the case of silver electrodes are the following formulas (3) and (4).

^{Ag = Ag + + e ...... (} 3) AgCl - = AgCl + e ...... (4) and these threshold voltages are both in comparison with the case of gold lower, for example, (3) In the formula about 0.8 V, ( In equation 4)
The voltage is 0.2V, so there is not much merits when silver is used as the electrode.

〔The invention's effect〕

As described above, according to the present invention, even if the humidity sensor using the organic polymer material as the moisture sensitive film is used, it is possible to eliminate the possibility that the detection characteristic is deteriorated. Except for this, the humidity of the organic polymer humidity sensor can be fully utilized, and a humidity detecting device that can perform stable and accurate detection operation over a long period of time can be obtained.

Further, according to the present invention, even if the frequency of the AC voltage applied to the organic polymer type humidity sensor is lowered, it is possible to sufficiently prevent the deterioration of the sensor, so that the sensor can be operated as it is at the commercial AC frequency and the cost can be reduced. Can provide a humidity detection device.

[Brief description of drawings]

FIG. 1 is a plan view showing an example of an organic polymer humidity sensor,
2 is a cross-sectional view taken along the line AA ′ in FIG. 1. FIG. 3 is a characteristic diagram showing an example of the characteristics of the organic polymer humidity sensor, and FIG. 4 is the operating circuit of the organic polymer humidity sensor according to the present invention. FIG. 5 is a circuit diagram showing one embodiment, FIG. 5 is a circuit diagram showing another embodiment, FIG. 6 is a characteristic diagram for explaining the operation, and FIG. 7 is an explanatory diagram of ionization. 1 ... Organic polymer type sensor, 2 ... AC voltage source, 3 ...
Detection circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroko Kawakami 3-1-1, Saiwaicho, Hitachi-shi, Ibaraki Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Shigeki Tsuchiya 3-chome, Hitachi-shi, Ibaraki No. 1-1 Hitachi Research Laboratory, Hitachi, Ltd. (72) Inventor Noriyuki Kaneshiro 3-1-1, Saiwaicho, Hitachi City, Ibaraki Hitachi Co., Ltd. Hitachi Research Laboratory (72) Inventor, Shuichi Ohara Hitachi, Ibaraki Prefecture 3-1, 1-1 Sachimachi, Hitachi Research Laboratory, Hitachi, Ltd. (72) Inventor Mitsuyoshi Kashiwagi 1380-1, Nishihara, Katsuta-shi, Ibaraki, Nishihara, Hitachi Chemical Co., Ltd. (56) References Sho 55-40935 (JP, A)

Claims (1)

[Claims] 1. A humidity sensor comprising a moisture sensitive film made of an organic polymer material containing counter ions between a pair of electrodes, and detecting ambient humidity by a change in impedance between the pair of electrodes. An AC constant voltage source whose voltage output terminal is connected to the electrode of the capacitor via a capacitor is provided, and the upper limit of the AC voltage appearing between the pair of electrodes during the humidity detection operation is determined by the output voltage of this AC constant voltage source. An operating circuit of an organic polymer humidity sensor, characterized in that it is configured to be set to be equal to or lower than an ionization voltage of an electrode.