JP2010237028A - Humidity measurement apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a humidity measurement apparatus for simplifying a constitution, and accurately measuring the humidity due to a minute change in an electrostatic capacitance. <P>SOLUTION: The humidity measurement apparatus includes: a humidity sensing section having a humidity sensing element whose electrostatic capacitance is variable; and an output signal converting section for converting the change in the electrostatic capacitance of the humidity sensing element detected by a signal processing section into an electrical signal indicating the corresponding relative humidity, and outputting it. A stray capacitance of a first conductive cable added to the electrostatic capacitance of the humidity sensing element 14 is equal to a stray capacitance of a second conductive cable added to a reference electrostatic capacitance. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a humidity measuring device that measures the relative humidity of a measurement environment by using a characteristic that a humidity sensing element changes its capacitance value depending on relative humidity.

  A capacitive humidity sensor, which is one of the capacitive sensors, measures the relative humidity of the measurement environment by using the characteristic that the humidity sensing element changes its capacitance value due to a change in relative humidity. In this capacitive humidity sensor, the amount of change in capacitance is converted into a voltage or the like and output as a humidity measurement value. At this time, the change in capacitance is several tens of pF or less (20% relative to 100% relative humidity). ˜30 pF or less). For this reason, when a humidity sensor of a humidity sensor and a circuit unit (a transmission circuit, an amplifier circuit, an output signal conversion circuit, etc.) that processes a detection signal from the humidity sensor are connected by a cable, the capacity of the cable is reduced. It changes greatly under the influence of temperature changes and bending, and the accuracy of humidity measurement values deteriorates.

Therefore, it is desirable that the moisture sensitive element and the circuit unit should be connected in close proximity, but the temperature environment of the site where the humidity is measured exceeds the limit temperature of use of the electronic components constituting the amplifier circuit in the circuit unit. If this is the case, it is no longer possible to connect the moisture sensitive element and the circuit part in close proximity, and the circuit part must be isolated from the site where the moisture sensitive element is installed and connected to each other with a cable.
For example, Patent Documents 1 and 2 disclose conventional techniques for solving the deterioration in humidity measurement accuracy caused by the change in the capacitance of the cable connecting the humidity sensing element and the circuit unit.

In the sensor system of Patent Document 1, the sensor head includes a first electrode and a second electrode arranged to face each other with a predetermined distance between the first electrode, and the measurement device includes a signal terminal, A shield terminal and a bias terminal are provided, and the measurement circuit is configured so that the voltage at the signal terminal is always equal to the voltage at the shield terminal. Also, a triple coaxial cable is used to connect the sensor head and the measuring device, the first electrode is connected to the signal terminal at the center axis of the triple coaxial cable, and the second electrode is biased by the outer sheathed wire of the triple coaxial cable It is connected to the terminal, and the inner coated wire of the triple coaxial cable is connected to the shield terminal.
Thus, since the signal line corresponding to the central axis of the triple coaxial cable is electrically interrupted by the outer covered wire and the inner covered wire which are shielding means having the same potential as the signal line, the capacity of the cable The adverse effect of changes on the accuracy of measured values can be eliminated.

  Further, the capacitance-voltage conversion device disclosed in Patent Document 2 has an inverting input terminal, a non-inverting input terminal, and an output terminal, and an operation in which the output terminal and the inverting input terminal are connected by a feedback resistor. One end is connected to the amplifier and the inverting input terminal, a signal line connecting the capacitive element having capacitance to the other end, and at least part of the signal line is surrounded and connected to the non-inverting input terminal A shield, an AC signal generator that applies an AC signal to the non-inverting input terminal, and an adjustment circuit that adjusts the amplitude and phase of the operational amplifier so that the output signal and the AC signal cancel each other. It has. By configuring in this way, the capacitance of the capacitive element connected to the signal line can be converted into a voltage without being affected by the stray capacitance of the signal line. The corresponding voltage can be obtained with high accuracy.

JP 2002-157671 A Japanese Patent No. 3302377

  Patent Document 1 is premised on the use of a triple coaxial cable having a conductor outer covered wire and an inner covered wire as shield means. Such a triple coaxial cable is not used for a general humidity sensor, and in Patent Document 1, a coaxial cable that is commercially available as a standard product cannot be used, and is not versatile.

  Since Patent Document 2 requires an adjustment circuit that adjusts the amplitude and phase of the operational amplifier so that the output signal and the AC signal cancel each other, the circuit configuration is complicated.

  The present invention has been made to solve the above-described problems, and it is intended to obtain a humidity measuring device capable of measuring with high accuracy even with humidity due to a minute change in capacitance with a simple configuration. Objective.

  A humidity measuring device according to the present invention includes a humidity sensing unit having a humidity sensing element whose capacitance changes according to the humidity in the measurement environment atmosphere, a reference capacitance unit for obtaining a reference capacitance, and a humidity sensing element. A first conductive cable connected to the reference capacitance unit, a second conductive cable connected to the reference capacitance unit, and a capacitance of the moisture sensitive element through the first conductive cable, and a reference capacitance unit A signal processing unit that obtains the reference capacitance of the moisture sensing element and detects a change in the capacitance of the moisture sensing element from the capacitance of the moisture sensing element and the reference capacitance, and the humidity sensing element detected by the signal processing unit In a humidity measuring device comprising an output signal conversion unit that converts the change in electrostatic capacitance into an electrical signal representing the corresponding relative humidity and outputs the electrical signal, the stray capacitance of the first conductive cable and the second conductive cable The stray capacitances are equal to each other.

  According to the present invention, since the stray capacitance of the first conductive cable is equal to the stray capacitance of the second conductive cable, the stray capacitance of the first and second conductive cables is determined as the change in the capacitance of the moisture sensitive element. Is canceled and a true change in capacitance is obtained, so that the corresponding humidity can be accurately measured even if the change in capacitance of the humidity sensing element is very small.

It is a figure which shows roughly the humidity measuring device by Embodiment 1 of this invention. 1 is a block diagram illustrating a configuration of a humidity measuring device according to Embodiment 1. FIG. 2 is a cross-sectional view showing an example of a cable according to Embodiment 1. FIG. FIG. 3 is a diagram illustrating a connection relationship of cables according to the first embodiment. It is a figure which shows the flow of a process by the signal processing part in FIG. 6 is a cross-sectional view showing another example of the cable according to Embodiment 1. FIG.

Embodiment 1 FIG.
FIG. 1 schematically shows a humidity measuring apparatus according to Embodiment 1 of the present invention. In FIG. 1, a humidity measuring device 1 according to Embodiment 1 is a capacitive humidity sensor that measures humidity by a change in capacitance of a moisture sensitive element, and a circuit unit 2 provided on a circuit unit substrate housed in a case 2A. And a moisture sensing part 3 having a moisture sensing element, and a cable 4 for connecting the circuit part 2 and the moisture sensing part 3.

  The circuit unit 2 is a component that performs signal processing on a detection signal from the moisture sensing element of the moisture sensing unit 3, and includes a transmission circuit, an amplifier circuit, a power supply circuit, an external terminal, and the like. The moisture sensing element 14 of the moisture sensing unit 3 is a moisture sensing element having a characteristic that the capacitance value changes due to a change in relative humidity of the measurement environment, and the circuit unit 2 has a capacitance value of the moisture sensing element 14. Is converted into a voltage or the like and output as a humidity measurement value. In the example shown in FIG. 1, the moisture sensing unit 3 is configured by combining a moisture sensing element 14 and a resistance temperature detector (temperature sensor) 22 that measures temperature by a resistance temperature detector three-wire measurement. Humidity-temperature measurement is possible.

  Here, assuming that the site where the humidity measurement is performed may exceed the use limit temperature of the electronic parts constituting the circuit unit 2, as shown in FIG. 1, the moisture sensitive unit 3 is arranged at the measurement site, Place the case 2A containing the circuit unit 2 in a place that is not affected by the temperature environment at the measurement site, and connect the electronic components in the circuit unit 2 and the moisture sensitive element in the moisture sensitive unit 3 via the cable 4. ing.

In addition, the cable 4 is preferably a coaxial cable with little change in capacitance, but in the first embodiment, two coaxial cables of the same type and length are used as described later with reference to FIG. The moisture-sensitive element and the element-side circuit of the circuit unit 2 are connected by the coaxial cable, and the other coaxial cable is not connected to the moisture-sensitive element and is connected to the reference-side circuit in the circuit unit 2 as an open circuit.
At this time, two coaxial cables are used as one composite coaxial cable. As a result, even if the stray capacitance of the coaxial cable connected to the moisture-sensitive element changes due to bending or the like, the stray capacitance of the reference-side coaxial cable also changes in the same way. By dynamically amplifying, it is possible to cancel the influence on the measurement value output due to the change in the cable capacity.

  FIG. 2 is a block diagram showing a configuration of the humidity measuring apparatus according to the first embodiment. 2, the circuit unit 2 includes a signal processing unit 5, a power supply circuit 6, an output signal conversion unit 7, and an external terminal 8. The signal processing unit 5 includes a CV conversion circuit (transmitting circuit) 9 and a differential amplification. The circuit 10 is provided, and the output signal converter 7 includes an output amplifier circuit 11. The power supply circuit 6 supplies the power input through the external terminal 8 to the signal processing unit 5 and the output signal conversion unit 7 to drive them.

  The CV conversion circuit 9 includes a reference side circuit that converts a reference capacitance provided by the reference capacitance unit into a voltage, and an element side circuit that converts the capacitance of the moisture sensitive element 14 into a voltage. A coaxial cable (first conductive cable) 12 is connected to the element side circuit, and the stray capacitance Cc1 of the coaxial cable 12 is added to the electrostatic capacitance Ce of the moisture sensitive element 14. Further, a coaxial cable (second conductive cable) 13 is connected to the reference side circuit, and the stray capacitance Cc2 of the coaxial cable 13 is added to the reference capacitance Cr provided by the reference capacitance portion.

  The differential amplifier circuit 10 receives the output voltage of the element side circuit of the CV conversion circuit 9 and the output voltage of the reference side circuit and differentially amplifies them. The output signal conversion unit 7 obtains the voltage difference corresponding to the change in the capacitance of the humidity sensitive element 14 with respect to the reference capacitance obtained by the differential amplifier circuit 10 as an electric signal representing the relative humidity corresponding thereto. The output amplifier circuit 11 amplifies the output of the differential amplifier circuit 10.

  As the moisture sensing element 14 provided in the moisture sensing unit 3, for example, a polymer capacitive humidity sensing element configured by sandwiching a moisture sensitive polymer between two electrodes is used. In this case, depending on the amount of water vapor in the measurement environment, the amount of moisture adsorbed on the moisture-sensitive polymer changes, and the dielectric constant of the moisture-sensitive polymer changes depending on the amount of moisture adsorbed. This change in dielectric constant is measured as the change in capacitance between the electrodes. This change in capacitance is proportional to the relative humidity of the measurement environment.

  FIG. 3 is a sectional view showing an example of the cable according to the first embodiment. As shown in FIG. 3, the cable 4 is a composite coaxial cable in which coaxial cables 12 and 13 are covered with a sheath 15 and a shield 16. The coaxial cables 12 and 13 are coaxial cables of the same type and the same length provided with an inner insulator 18, a shield 19, and an outer insulator 20 coaxially with a conductor 17 that propagates a signal.

  FIG. 4 is a diagram showing a cable connection relationship according to the first embodiment, and FIG. 5 is a diagram showing a processing flow by the signal processing unit in FIG. As shown in FIG. 4, the signal conductor of the coaxial cable 12 is connected to one electrode of the moisture sensitive element 14, and the other electrode is grounded together with the shield of the coaxial cable 12.

  The moisture sensitive element 14 is connected to the resistor R in the element side circuit of the circuit unit 2, and in the element side circuit, as shown in FIG. 5, a rectangular wave signal (element side) corresponding to the detected humidity of the moisture sensitive element 14. Output waveform) is output. In this element-side transmission waveform, the change time T1 during which the capacitance Ce of the moisture sensitive element 14 changes depends on the product of the detected humidity (capacitance Ce) of the moisture sensitive element 14 and the resistance R.

  On the other hand, also in the reference side circuit of the circuit unit 2, a resistor R is connected to the reference capacitance unit, and a rectangular wave signal (reference side transmission waveform) corresponding to the reference capacitance is output. In this reference-side transmission waveform, the change time T2 when the reference capacitance Cr changes depends on the product of the reference capacitance Cr and the resistance R.

  The CV conversion circuit 9 performs RC integration on the element-side transmission waveform and the reference-side transmission waveform, respectively, so that the voltage value corresponding to the change in the capacitance Ce of the moisture sensitive element 14 is obtained as shown in FIG. The voltage value corresponding to the reference capacitance Cr is output. The differential amplifier circuit 10 amplifies the voltage difference between these output voltage values to obtain the voltage output shown in FIG. This voltage output is a voltage difference corresponding to a change in the capacitance Ce of the moisture sensitive element 14 with respect to the reference capacitance Cr. The output signal converter 7 amplifies the voltage output and converts it into an electrical signal representing the corresponding relative humidity, and outputs it as a humidity measurement value via the external terminal 8.

  As described above, the stray capacitance of the coaxial cable 12 is added to the capacitance Ce of the moisture-sensitive element 14, and the stray capacitance of the coaxial cable 13 is added to the reference capacitance Cr. In the present invention, the coaxial cables 12 and 13 are used as one composite coaxial cable 4, and the stray capacitance of the coaxial cable 12 and the stray capacitance of the coaxial cable 13 are substantially matched (Cc1 = Cc2). Thereby, the above-mentioned change times T1 and T2 become equal, and the influence on the voltage output of the stray capacitance of the coaxial cables 12 and 13 can be canceled by differentially amplifying the output on the element side and the reference side.

The cable 4 is not limited to the configuration shown in FIG.
FIG. 6 is a cross-sectional view showing another example of the cable according to the first embodiment. The cable 4 shown in FIG. 6 is a 5-core composite coaxial cable, and includes three coaxial cables 21 serving as signal lines in addition to the coaxial cables 12 and 13. For example, in the humidity-temperature measuring apparatus shown in FIG. 1, the humidity sensing element 14 and the coaxial cables 12 and 13 are connected, and 3 is used as the signal line of the resistance temperature detector 22 for measuring the temperature by the resistance temperature detector 3-wire measurement. A coaxial cable 21 is used.
In this way, the sensor unit such as the moisture sensing unit 3 and the circuit unit 2 that performs signal processing are connected by a composite coaxial cable, and thus not only for capacitive humidity sensors but also for signals of other sensor elements. Coaxial cables can also be combined into a single composite coaxial cable. In addition to the above, signal lines such as a wind speed sensor may be combined into a composite coaxial cable.

  As described above, according to the first embodiment, the floating capacitance of the coaxial cable 13 added to the reference capacitance Cr and the floating capacitance of the coaxial cable 12 added to the capacitance Ce of the moisture sensitive element 14 are described. Since the capacitance is made equal, the stray capacitance of the coaxial cables 12 and 13 is canceled out as the change in the capacitance of the moisture sensing element 14, and the change in the true capacitance is obtained. Even if the change in the capacitance Ce is very small, the corresponding humidity can be measured with high accuracy.

  Further, according to the first embodiment, the coaxial cables 12 and 13 are coaxial cables of the same type and the same length, and the composite coaxial cable 4 constituted by the coaxial cables 12 and 13 is connected to the humidity sensitive element 14 and the reference. Since the connection with the electrostatic capacity unit is performed, the stray capacitance of the coaxial cable 12 and the stray capacitance of the coaxial cable 13 can be easily made the same. In addition, since the coaxial cables 12 and 13 are combined into one composite coaxial cable 4, the cable can be easily handled and installed at the measurement site. Further, the coaxial cables 12 and 13 are used as the composite coaxial cable 4. Since they are arranged in the same place, conditions such as the temperature at the measurement site can be made the same, and the measurement accuracy is improved.

DESCRIPTION OF SYMBOLS 1 Humidity measuring device 2 Circuit part 2A Case 3 Humidity sensing part 4 Cable 5 Signal processing part 6 Power supply circuit 7 Output signal conversion part 8 External terminal 9 CV conversion circuit 10 Differential amplification circuit 11 Output amplification circuit 12, 13 Coaxial cable (First conductive cable, second conductive cable)
14 Humidity sensing element 15 Sheath 16, 19 Shield 17 Conductor 18, 20 Insulator 21 Coaxial cable 22 Resistance temperature detector

Claims (2)

A moisture sensing part having a moisture sensing element whose capacitance changes according to the humidity in the measurement environment atmosphere;
A reference capacitance unit for obtaining a reference capacitance;
A first conductive cable connected to the moisture sensitive element;
A second conductive cable connected to the reference capacitance unit;
The capacitance of the moisture sensitive element is acquired via the first conductive cable, the reference capacitance of the reference capacitance unit is acquired, and the capacitance of the moisture sensitive element and the reference capacitance are acquired. A signal processing unit for detecting a change in capacitance of the moisture sensitive element from the capacitance;
In a humidity measuring device comprising: an output signal conversion unit that converts a change in capacitance of the humidity sensing element detected by the signal processing unit into an electrical signal representing a corresponding relative humidity, and outputs the electrical signal.
A humidity measuring apparatus, wherein the stray capacitance of the first conductive cable and the stray capacitance of the second conductive cable are equal. The first and second conductive cables are coaxial cables of the same type and length,
The humidity measurement according to claim 1, wherein the humidity sensing element and the reference capacitance unit are connected by a single composite coaxial cable composed of the first and second conductive cables. apparatus.

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