JPH06222031A - Measuring equipment employing capacitive sensor having nonnegligible resistive component - Google Patents

Abstract

PURPOSE:To allow highly accurate measurement of the quantity of specific component in a specimen using a capacitive sensor having nonnegligible resistive component. CONSTITUTION:A pulse generating circuit is constituted of a CMOS type Schmidt inverter 11, a pulse frequency determining element, i.e., a fixed resistor R0, connected across the inverter 11, and another pulse frequency determining element, i.e., a capacitive sensor SC having a resistive component Rh, connected between the input side of the Schmidt inverter 11 and the ground through a fixed series capacitor C0. Variation of the quantity of specific component in a specimen is detected based on the variation of impedance of the capacitive sensor SC and then the variation is converted through a pulse generating circuit into a pulse signal having frequency relevant to the quantity of specific component. The pulse output is then processed to detect a frequency and the quantity of specific component corresponding to the detected frequency is calculated.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention generally relates to analytes such as temperature sensors, humidity sensors, gas sensors (oxygen sensors, hydrogen sensors, hydrocarbon sensors, carbon monoxide sensors, nitrogen oxide sensors, etc.). TECHNICAL FIELD The present invention relates to a measuring device that uses a sensor that detects the amount of a specific component contained therein, and particularly relates to a measuring device that measures the amount of a specific component in a subject such as a gas phase or a liquid phase using a capacitive sensor in which a resistance component cannot be ignored. .

[0002]

2. Description of the Related Art For example, the amount of a specific component in a sample such as a gas phase or a liquid phase (a specific example is the amount of water in the air, a sealed gas in a closed container such as a transformer filled with insulating oil). Various measuring devices for measuring the amount of water in the oil inside or in the insulating oil, the amount of water in the seal gas in various oil tanks, the amount of water in oil, etc. are semiconductors whose impedance changes according to the change in the amount of the specific component. Type sensor, electrolyte type sensor, or capacitance type sensor is used.

The applicant of the present invention has previously proposed a measuring device capable of measuring the amount of a specific component in a subject with high accuracy by using a capacitive sensor. An example of the detection circuit of this measuring device is shown in FIG.

The detection circuit is, for example, a C-MOS type Schmitt inverter 11, a fixed resistor R0 which is a pulse frequency determining element connected between the input and output of the inverter 11 (inserted in a feedback circuit), and the inverter 11 A pulse generation circuit that generates a pulse signal having a frequency related to the amount of a specific component in the subject is constituted by another capacitive frequency sensor SC which is a pulse frequency determination element connected between the input side and the ground. The capacitance component Ch of the capacitance type sensor SC, which is the determining element, changes in accordance with the change of the specific component amount, so that the output frequency F of this pulse generating circuit
o is correspondingly changed, the frequency output, that is, a square wave pulse signal is sent to, for example, a microcomputer (not shown), the pulse signals input in the microcomputer are counted, and the output frequency Fo of the pulse generation circuit is determined. It is detected by arithmetic processing, and a specific component amount corresponding to the frequency is calculated and measured. This frequency Fo is
If the resistance value of the fixed resistor R0 is R0 and k is a constant, then Fo = 1 / k · Ch · R0.

The operation of the pulse generating circuit will be briefly described. The Schmitt inverter 11 converts the voltage Vc accumulated in the capacitance component Ch of the capacitive sensor SC into a pulse signal. The Schmitt inverter 11 has two threshold voltages, a high level threshold voltage V _TH and a low level threshold voltage V _TL , and the input voltage is higher than the high level threshold voltage V _TH . When it is low, a high level output voltage V _H is generated, and when the input voltage reaches a high level threshold voltage V _TH , the output voltage switches from the high level V _H to the low level V _L , and the input voltage is until drops to a low level threshold voltage V _TL holds the output voltage V _L of the low level, the output voltage when the input voltage drops to a low level threshold voltage V _TL is high from the low level V _L It operates so as to switch to the level V _H. Therefore, when no charge is stored in the capacitance component Ch of the capacitive sensor SC, the output voltage is at a high level V _H , and the charging voltage Vc of the capacitance component Ch of the capacitive sensor SC is at a high level threshold voltage. When it becomes equal to V _TH , the output voltage of the Schmitt inverter 11 switches from the high level to the low level V _L. Further, when the charging voltage Vc of the capacitance component Ch of the capacitive sensor SC drops to the low level threshold voltage V _TL of the Schmitt inverter 11 due to discharging, the output voltage of the Schmitt inverter 11 changes from the low level V _L to the high level V _L. Switch to _H. Therefore, the Schmitt inverter 11 outputs a pulse voltage having a cycle corresponding to charging and discharging of the capacitive component Ch of the capacitive sensor SC, and thus the amount of the specific component in the subject is converted into the frequency signal Fo.

[0006]

By the way, the capacitive sensor has a resistance component Rh, and when the resistance component Rh can be ignored, the amount of the specific component in the subject can be accurately measured with the above circuit configuration. However, when the resistance component Rh cannot be ignored, the circuit configuration is substantially the same as that in which the resistance component Rh is connected in parallel with the capacitance component Ch as shown in FIG.

Thus, the resistance component R of the capacitive sensor SC is
When h is connected in parallel with the capacitance component Ch, the input voltage Vi (= Vc) of the Schmitt inverter 11 is fixed resistor R0.
And the division of the resistance component Rh of the capacitive sensor SC results in Vi = Vdd · Rh / (R0 + Rh), and when the resistance component Rh is large, Vi is always smaller than the high-level threshold voltage V _{TH of the} Schmitt inverter 11. In this case, there is a serious defect that an output signal cannot be obtained from the pulse generation circuit (detection circuit).

On the other hand, if the resistance component Rh is small, the oscillation frequency becomes too high, which causes problems such as exceeding the frequency range used by the sensor. There is also a problem that the voltage applied to the sensor contains a DC component.

Therefore, an object of the present invention is to provide a measuring device capable of measuring the amount of a specific component in a subject with high accuracy even if a capacitive sensor in which a resistance component cannot be ignored is used.

[0010]

The above object is achieved by a measuring device according to the present invention which uses a capacitive sensor whose resistance component cannot be ignored. In summary, the present invention detects a change in the amount of a specific component in a subject by a change in the impedance of a capacitive sensor, and a measuring device using the capacitive sensor configured to measure the amount of the specific component, It has a high level threshold voltage and a low level threshold voltage, and when the input voltage is lower than the high level threshold voltage, it generates a high level output voltage, and the input voltage is a high level threshold voltage. When the output voltage reaches the low level, the output voltage switches from the high level to the low level, and the low level output voltage is held until the input voltage drops to the low level threshold voltage. Output voltage is switched from low level to high level when the voltage drops to, and a voltage-pulse conversion means capable of converting the input voltage into a pulse signal, a fixed resistor connected between the input and output of the conversion means, and a fixed capacitance. In series The detection circuit comprises a capacitive sensor connected to the input side of the conversion means via a resistance component that cannot be ignored, and a change in the amount of a specific component in the subject is detected by a change in the impedance of the capacitive sensor. A capacitive type in which a resistance component cannot be ignored, which is characterized by detecting and converting to a pulse signal having a frequency related to the specific component amount, detecting the frequency of the pulse signal, and measuring the specific component amount corresponding to the frequency. It is a measuring device using a sensor.

[0011]

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a circuit configuration diagram showing a detection circuit of an embodiment of a measuring apparatus using a capacitive sensor in which a resistance component according to the present invention cannot be ignored. For example, a C-MOS type Schmitt inverter 11 and this inverter are shown. Another pulse frequency determining element connected to the input side of the Schmitt inverter via a fixed resistor R0 which is a pulse frequency determining element connected between the input and output of 11 and a fixed capacitance C0 in series on the ground side. And a capacitive sensor SC having a resistance component Rh which is a pulse generation circuit for generating a pulse signal having a frequency relating to a specific component amount in the subject. Since it is easy to perform the arithmetic processing of the pulse signal from the pulse generating circuit to detect the frequency and calculate the specific component amount corresponding to the detected frequency by a microcomputer or the like, the description thereof will be omitted here. .

In the detection circuit having the above structure, Vdd = 2
V, current I = 0.03 mA, fixed resistor R0 is 10
A resistance of 0 KΩ and a capacitance of 10 nF are connected as the fixed capacitance C0, and the resistance component Rh of the capacitance type sensor SC is 1 K.
When the output frequency Fo of the pulse generation circuit with respect to the change of the capacitance component Ch was measured using the capacitance type sensors of Ω, 10 KΩ, 50 KΩ, 100 KΩ, 500 KΩ, and ∞, the characteristics shown in FIG. 2 were obtained. . Further, when only the fixed capacitance C0 was changed to 22 nF (other values are the same as above), the characteristics shown in FIG. 3 were obtained. Furthermore, Vd
When d = 5V, a fixed resistance R0 of 10 KΩ and a fixed capacitance C0 of 22 nF are connected, and the resistance component Rh of the capacitive sensor SC is 1 KΩ, 2 KΩ, 5 K.
When the output frequency Fo of the pulse generation circuit with respect to the change of the capacitance component Ch was actually measured using the capacitance type sensors of Ω, 10 KΩ, 100 KΩ, and ∞, the characteristics shown in FIG. 4 were obtained.

The characteristics obtained from the characteristics of FIGS. 2 to 4 are that, under the condition of Rh <R0 / 10, the output frequency Fo from the pulse generating circuit hardly changes even if the capacitance component Ch of the sensor SC changes. That's what it means. This gives a very important point in the design of the detection circuit.

Next, the reason why the characteristics shown in FIGS. 2 to 4 are obtained by the detection circuit of this embodiment will be described with reference to the equivalent circuit at the time of charging in FIG.

When the resistance component Rh of the capacitance type sensor SC is large, the capacitance component Ch and the fixed capacitance C0 are charged by the current I2 flowing through the capacitance component Ch side, and the input voltage Vi of the Schmitt inverter 11 rises. On the other hand, when the resistance component Rh is small, the input voltage Vi of the Schmitt inverter 11 rises because the fixed capacitance C0 is charged by the current I1 flowing on the resistance component Rh side. In this case, even if the capacitance component Ch changes, the time when the input voltage Vi reaches the high level threshold voltage V _TH does not change, and the output frequency of the pulse generation circuit also remains constant. Therefore, the resistance component Rh of the sensor SC has the above-described conditions, and the resistance component of the sensor SC is
It must be greater than R0 / 10.

Therefore, for example, when the amount of water in the subject is measured using a capacitance type sensor for detecting water, the resistance component Rh of the sensor SC changes with respect to the amount of water as shown in FIG. 6, for example. Then, assuming that the lower limit value of the resistance component Rh is, for example, 5 KΩ, the fixed resistance R0 becomes
R0> 50KΩ must be satisfied.

However, the fixed resistance R0 cannot be determined only by this condition. Because the resistance component R
Since h has frequency dependence, the lower limit of the resistance component Rh may be 5 KΩ or less only under the condition of R0> 50 KΩ. That is, the upper limit of the frequency determined by _{Fo max = 1 / k · Ch} min · Rh min, Rh min is determined by Fo _max.

As is apparent from the above description, in this embodiment, since the fixed capacitance is connected in series to the capacitive sensor whose resistance component cannot be ignored, the frequency output from the detection circuit (pulse generation circuit) is caused by the resistance component. The defect that the resistance component cannot be obtained does not occur. Therefore, the amount of various specific components in the sample such as the gas phase and the liquid phase can be measured with high accuracy by the capacitive sensor in which the resistance component cannot be ignored.

From the characteristics shown in FIGS. 2 to 4, it can be seen that if the condition R0 <Rh <R0 / 10 is set, the output frequency Fo changes remarkably due to the change in the resistance component Rh of the capacitive sensor SC. By using this non-linear region, not only the capacitance component Ch of the capacitive sensor SC but also its resistance component R
The amount of the specific component in the subject can also be detected by the change of h.

The above embodiment is merely an example of the present invention, and it goes without saying that the circuit configuration shown in the above embodiment, the elements used, and the like can be arbitrarily changed as necessary. Of course, an inverter other than the C-MOS type Schmitt inverter or other voltage-pulse conversion means may be used, and an element other than the microcomputer may be used. Furthermore, the pulse generation circuit may generate a pulse other than a square wave.

[0022]

As described above, according to the present invention,
Since the defect that the frequency output cannot be obtained from the detection circuit (pulse generation circuit) can be eliminated by the resistance component of the capacitive sensor, the capacitive sensor in which the resistance component cannot be ignored can be used to detect the gas phase, liquid phase, etc. There is a remarkable effect that the amounts of various specific components can be measured with high accuracy. Further, there is an effect that the amount of the specific component in the subject can be detected with high accuracy not only by the capacitance component of the capacitance type sensor but also by the change of the resistance component.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing a detection circuit of an embodiment of a measuring apparatus using a capacitive sensor in which a resistance component cannot be ignored according to the present invention.

FIG. 2 is a characteristic diagram showing a relationship between a capacitance component of a capacitive sensor and an output frequency in the detection circuit of FIG.

3 is a characteristic diagram showing a relationship between a capacitance component of a capacitive sensor and an output frequency in the detection circuit of FIG.

4 is a characteristic diagram showing a relationship between a capacitance component of a capacitive sensor and an output frequency in the detection circuit of FIG.

5 is an equivalent circuit diagram of the detection circuit of FIG. 1 during charging.

FIG. 6 is a characteristic diagram showing changes in the resistance component of the sensor with respect to the water content when the water content in the subject is measured using the capacitive sensor.

FIG. 7 is a circuit configuration diagram showing a detection circuit of an embodiment of a measuring apparatus using a conventional capacitive sensor.

[Explanation of symbols]

 11 C-MOS type Schmidt inverter SC capacitive sensor Ch capacitive component of capacitive sensor SC Rh resistive component of capacitive sensor SC R0 fixed resistance C0 fixed capacitance

Claims (2)

[Claims] 1. A measuring device using a capacitive sensor, which detects a change in the amount of a specific component in a subject by a change in the impedance of the capacitive sensor and measures the amount of the specific component. It has a threshold voltage and a low level threshold voltage, it generates a high level output voltage when the input voltage is lower than the high level threshold voltage, and the input voltage reaches the high level threshold voltage. The output voltage then switches from the high level to the low level, and the low level output voltage is held until the input voltage drops to the low level threshold voltage, and the input voltage drops to the low level threshold voltage. Sometimes the output voltage switches from low level to high level, and the voltage-pulse conversion means capable of converting the input voltage into a pulse signal,
The detection circuit comprises a fixed resistor connected between the input and output of the conversion means, and a capacitive sensor connected to the input side of the conversion means through a fixed capacitance in series and a resistance component of which cannot be ignored. , Detecting a change in the amount of a specific component in the subject by a change in the impedance of the capacitive sensor, converting it into a pulse signal having a frequency related to the amount of the specific component, detecting the frequency of the pulse signal, and responding to the frequency A measuring device using a capacitive sensor in which a resistance component cannot be ignored, which is characterized in that the amount of the specific component is measured. 2. When the resistance value of the fixed resistance of the detection circuit is R0 and the resistance value of the resistance component of the capacitive sensor in which the resistance component cannot be ignored is Rh, the following expression Rh ≧ R0 / 10 is satisfied. 2. The measuring device according to claim 1, wherein a capacitive type sensor having a resistance component is used for the detection circuit.