JP2003315295A - Moisture sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a moisture sensor capable of being reduced in size and cost and having an excellent noise resistance. <P>SOLUTION: A sensor section 1 has a pair of detection electrodes insulated from each other, and an output from an oscillation circuit 4 is applied to cause a current corresponding to an impedance across the electrodes to flow between the electrodes. An I/V conversion circuit 5 converts the current flowing between the electrodes into a voltage value. A mixer circuit 6 mixes an output from the circuit 5 and an output from the circuit 4, and outputs a sum frequency and a differential frequency components of a frequency of the output from the circuit 5 and a frequency of the output from the circuit 4. A filter circuit 7 detects a signal of a specified reference frequency from the output from the circuit 6, and outputs it to an integral amplifier circuit 8. An FM modulation circuit 11 modulates oscillation frequency of an oscillation circuit 3 in such a manner that a differential frequency between the frequency of the output of the circuit 5 and the frequency of the output of the circuit 4 changes within a specified range of frequencies including the reference frequency. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a water amount sensor for detecting the amount of water contained in an object to be detected.

[0002]

2. Description of the Related Art As a water content sensor of this type, there has been conventionally provided a sensor for detecting the water content contained in the food waste thrown into the processing tank of the food waste processing apparatus. FIG. 9 shows the positional relationship between the detection object and the water content sensor. A window hole 21 is formed through the metal processing tank 20 into which the detection object 22 such as garbage is put, and the resin is passed through the window hole 21. The electrode case 16 of the molded product is exposed.
A pair of detection electrodes 2 and 2 insulated from each other is provided on the back surface side of the. This water content sensor is a capacitance type sensor, and utilizes the property that water is a substance that polarizes (dielectric), and determines the water content contained in the detection target 22 existing between the detection electrodes 2 and 2. It is detected from the impedance change between the detection electrodes 2 and 2.

FIG. 8 shows a block diagram of a conventional water content sensor. This water content sensor includes a pair of detection electrodes 2 and 2 which are insulated from each other, and the detection electrodes 2 and 2 are arranged according to the water content.
Between the sensor unit 1 in which the impedance changes between the crystal unit X1 and the crystal unit X2, and the crystal unit X1 oscillates at a predetermined frequency f1 and supplies an oscillation output to the sensor unit 1.
An oscillating circuit 4 that oscillates at a predetermined frequency f2 (≠ f1), an I / V converting circuit 5 that converts a current flowing between the detection electrodes 2 and 2 of the sensor unit 1 into a voltage value, and an I / V From the output of the mixer circuit 6, which mixes the output of the conversion circuit 5 and the output of the oscillation circuit 4, and outputs the signal of the frequency (f1 ± f2) of the sum and difference of the frequencies f1 and f2 of both, /
A filter circuit 7 for detecting a signal of a frequency component of a difference (f1-f2) between the frequency f1 of the output of the V conversion circuit 5 and the frequency f2 of the output of the oscillation circuit 4, and the output of the filter circuit 7 is integrated and then amplified. By doing so, it is composed of an integral amplification circuit 8 that generates an output of a voltage value according to the impedance value between the detection electrodes 2 and 2. Incidentally, C4, R1 and R2 in the figure are external capacitors and resistors for adjusting the integration time constant and the amplification factor of the integration amplifier circuit 8.

In this moisture content sensor, the oscillation output of the oscillation circuit 4 is applied to the sensor section 1. When the impedance between the detection electrodes 2 and 2 changes according to the moisture content, the impedance between the detection electrodes 2 and 2 changes. Since a current according to the current flows between the detection electrodes 2 and 2, a change in the current value due to a change in impedance at the oscillation frequency f1 of the oscillation circuit 3 is I / V.
The conversion circuit 5 converts it into a voltage change and extracts the water content as a voltage value.

In the capacitance type moisture content sensor, the impedance between the detection electrodes 2 and 2 fluctuates due to various fluctuation factors. Therefore, the oscillation frequency f1 of the oscillation circuit 3 is changed to the impedance between the detection electrodes 2 and 2. It is set to a frequency in the tens of MHz band that enables stable and accurate detection. Therefore, in order to facilitate the signal processing in the circuit in the subsequent stage, the output of the sensor unit 1 is mixed with the oscillation output of the oscillation circuit 4 and further detected by the filter circuit 7, whereby the frequency of the input to the integration amplification circuit 8 is detected. Is lowered.

[0006]

In the water content sensor having the above structure, in order to improve noise resistance, the mixer circuit 6 is used.
It is necessary to narrow the pass frequency band of the filter circuit 7 provided at the next stage, and it is necessary to use expensive parts such as the ceramic filter 7a in the mixer circuit 6, which causes a cost increase. .

Further, since the pass frequency band of the filter circuit 7 is a narrow band, it is necessary to stably and highly accurately control the difference between the oscillation frequencies of the two oscillation circuits 3 and 4. Therefore, the crystal oscillator X1, Since expensive parts such as X2 have to be used, the cost has been increased.

Further, when the circuit of the water content sensor is made into an IC, parts such as the crystal resonators X1 and X2 and the ceramic filter 7a remain as external parts, so that there are sufficient advantages such as miniaturization and cost reduction by the IC. There was also a problem that I could not get it.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a water content sensor which can be downsized and reduced in cost and has good noise resistance. is there.

[0010]

In order to achieve the above-mentioned object, in the invention of claim 1, a sensor unit is provided with a plurality of detection electrodes insulated from each other, and the impedance between the detection electrodes changes according to the amount of water. A first oscillating circuit section that applies an oscillating output to the sensor section; a second oscillating circuit section that oscillates at a frequency different from that of the first oscillating circuit section; and an output of the sensor section and the second oscillating circuit section. A mixing circuit section that mixes the output, a detection circuit section that detects a frequency component of a predetermined reference frequency from the output of the mixing circuit section, and an output that corresponds to the impedance between the detection electrodes is generated from the output of the detection circuit section. An output circuit section is provided, and the intermediate frequency of the difference between the output frequency from the sensor section and the oscillation frequency of the second oscillation circuit section, which is substantially equal to the oscillation frequency of the first oscillation circuit section, is constant including the reference frequency. Changes in the frequency range of So that the, characterized in that a frequency modulation unit for modulating either one of the oscillation frequency of the first or second oscillator circuit portion.

According to a second aspect of the present invention, the sensor section is provided with a plurality of detection electrodes insulated from each other, and the impedance between the detection electrodes changes according to the amount of water, and the first oscillation for applying an oscillation output to the sensor section. A circuit section; a second oscillation circuit section that oscillates at a frequency different from that of the first oscillation circuit section; a mixing circuit section that mixes the output of the sensor section and the output of the second oscillation circuit section; The first oscillation circuit section includes a detection circuit section that detects a frequency component of a predetermined reference frequency from the output, and an output circuit section that generates an output corresponding to the impedance between the detection electrodes from the output of the detection circuit section. Any one of the first and second oscillation circuit units is set so that the intermediate frequency of the difference between the output frequency from the sensor unit and the oscillation frequency of the second oscillation circuit unit, which is approximately equal to the oscillation frequency, becomes the reference frequency. Change the oscillation frequency of either It is characterized in that a frequency varying unit for controlling the frequency is provided.

A third aspect of the present invention is characterized in that, in the second aspect of the invention, the frequency varying unit changes the oscillation frequency based on the intermediate frequency.

According to a fourth aspect of the present invention, in the second aspect of the invention, the frequency varying section changes one oscillation frequency based on the oscillation frequency of the other oscillation circuit section.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (Embodiment 1) FIG. 1 is a block diagram of a water content sensor of this embodiment. Since the basic structure is the same as that of the water content sensor shown in FIG. 8 described in the related art, the same components are designated by the same reference numerals and the description thereof will be omitted.

This water content sensor comprises a pair of detection electrodes 2 and 2 which are insulated from each other, and an oscillation frequency is formed by a sensor unit 1 in which the impedance between the detection electrodes 2 and 2 changes according to the water content and a capacitor C1. The f1 is determined, and the oscillation circuit 3 as the first oscillation circuit section that supplies the oscillation output to the sensor section 1 and the FM modulation circuit as the frequency modulation section that modulates the oscillation frequency of the oscillation circuit 3 within a predetermined frequency range 11
An oscillation circuit 4 as a second oscillation circuit unit such as a VCO that oscillates at a frequency according to an input voltage, and an I / V conversion circuit 5 that converts a current flowing through the sensor unit 1 into a voltage value.
And the output of the I / V conversion circuit 5 and the output of the oscillation circuit 4 are mixed, and the sum of the frequencies f1 and f2 of both and the difference frequency (f
Mixer circuit 6 as a mixing circuit unit that outputs a signal of 1 ± f2), and the difference (f1) between the frequency f1 of the output of the I / V conversion circuit 5 and the frequency f2 of the output of the oscillation circuit 4 from the output of the mixer circuit 6. The voltage corresponding to the impedance value between the detection electrodes 2 and 2 by integrating the output of the filter circuit 7 which is a detection circuit unit for detecting the signal of the frequency component of -f2) and the output of the filter circuit 7 and then amplifying it. An integral amplifier circuit 8 as an output circuit unit for generating a value output, a comparator circuit 9 for comparing the output of the mixer circuit 6 input via the filter circuit 7 with a predetermined reference voltage, and an output of the comparator circuit 9. The integrating circuit 10 outputs a voltage obtained by integration to the oscillating circuit 4. C2 and C3 in the figure are capacitors for adjusting the oscillation frequency of the oscillation circuit 4 and capacitors for adjusting the integration time constant of the integrating circuit 10, respectively. Also,
The configuration of the sensor unit 1 is the same as the configuration of FIG. 9 described in the related art, and thus the description thereof will be omitted.

Here, the oscillation frequencies of the oscillation circuits 3 and 4 are set to f1 and f2, respectively, and the center frequency of the pass frequency band of the filter circuit 7 composed of a bandpass filter is set to the oscillation frequencies of the oscillation circuits 3 and 4. When the reference frequency Δf (= f1−f2), which is the difference between the values, is set, the oscillation frequencies of one or both of the oscillation circuits 3 and 4 are set to the set values f1 and f2 due to variations in parts and changes in the surrounding environment. , The intermediate frequency, which is the difference between the oscillation frequencies of the two oscillation circuits 3 and 4, deviates from the center frequency (reference frequency Δf) of the pass frequency band of the filter circuit 7. here,
The filter circuit 7 is a highly accurate one using an external ceramic filter 7a, and its pass band is narrow. Therefore, if the frequency of the output of the mixer circuit 6 deviates from the center frequency (reference frequency Δf), the filter circuit 7 The output of 7 becomes very small, and the value according to the amount of water cannot be obtained,
There is a problem that the wrong value is output.

On the other hand, in the present embodiment, the FM modulation circuit 11 causes the oscillation frequency of the oscillation circuit 3 so that the difference between the oscillation frequencies of the oscillation circuits 3 and 4 changes in a constant frequency range including the reference frequency Δf. Even if the oscillation frequencies of the oscillation circuits 3 and 4 fluctuate due to variations in parts and changes in the surrounding environment, by modulating the oscillation frequency f1 ′ of the oscillation circuit 3 after fluctuation, the mixer circuit 6 A signal of the center frequency (reference frequency Δf) of the filter circuit 7 is also output, and the integral amplification circuit 8 smoothes the output of the filter circuit 7 and further amplifies it to obtain a value according to the water content. You can

For example, when the oscillation frequency of the oscillation circuit 3 changes from f1 to f1 '(= f1 + df1) and the oscillation frequency of the oscillation circuit 4 changes from f2 to f2' (= f2 + df2), the mixer circuit 6 after the change. Output frequency is f1 '
± f2 ′ = (f1 + df1) ± (f2 + df2), which means that the center frequency Δf (f1-f2) of the filter circuit 7 deviates by (df1-df2), but the oscillation frequency of the oscillation circuit 3 becomes (± fm). Since the modulation is performed in the range, the frequency of the output of the mixer circuit 6 also changes in the range of (± fm). Therefore, the frequency modulation width f
By setting m to an appropriate value according to the usage conditions,
(F1'-f2 ')-fm <Δf <(f1'-f2')
It can be set to + fm, and the signal of the center frequency Δf of the filter circuit 7 can be output from the mixer circuit 6.

As described above, in this embodiment, the oscillation frequency of the oscillation circuit 3 is modulated so that the frequency of the output of the mixer circuit 6 matches the center frequency of the filter circuit 7 to obtain a value corresponding to the amount of water. Therefore, compared with the conventional moisture content sensor, the FM modulation circuit 11 is newly required, but since it is not necessary to control the oscillation frequencies of the oscillation circuits 3 and 4 with high precision, high-precision and expensive crystal vibration is required. The children X1 and X2 are unnecessary, and as a result, the oscillator circuits 3 and 4 can be constructed at low cost. Also, when this circuit is integrated into an IC, a capacitor C1 for adjustment is used.
~ You can also remove C3 and eliminate external parts,
Compared with the conventional water content sensor, the advantages of smaller size and lower cost can be obtained.

Further, in the moisture sensor of this embodiment, the circuit including the oscillation circuit 4, the mixer circuit 6, the comparison circuit 9 and the integration circuit 10 causes the oscillation circuits 3 and 4 even if the oscillation frequency of the oscillation circuit 3 varies. The oscillating frequency of the oscillating circuit 4 is controlled so that the difference in the oscillating frequency becomes the predetermined reference frequency f0. 2 shows the relation of the output voltage with respect to the input frequency of the comparison circuit 9, and FIG. 3 shows the relation of the output frequency with respect to the input voltage of the oscillation circuit 4, respectively.
Then, as the frequency of the input signal (output of the filter circuit 7) increases, the output voltage thereof increases. In addition, the oscillation frequency of the oscillation circuit 4 increases as the input voltage increases.

For example, when the oscillation frequency of the oscillator circuit 3 drops from a preset frequency f1 to f1 '(= f1-df1), the output frequency of the mixer circuit 6 is f1' ± f.
2 = (f1-df1) ± f2. Here, in the comparison circuit 9, the mixer circuit 6 input via the filter circuit 7 is input.
Is compared with the reference voltage, the oscillation frequency of the oscillation circuit 3 decreases from f1 to f1 ', and the output frequency of the mixer circuit 6 decreases from (f1-f2) to (f1'-f2). Then, the output of the comparison circuit 9 drops from V0 to V0 '. Then, the output of the integration circuit 10 decreases from V1 to V1 'in accordance with the change in the output of the comparison circuit 9, and accordingly the oscillation frequency of the oscillation circuit 4 decreases from f2 to f2' (see FIG. 4). . In this way, when the oscillation frequency of the oscillation circuit 3 decreases, the oscillation frequency of the oscillation circuit 4 also decreases accordingly, so that the difference between the oscillation frequencies of the oscillation circuits 3 and 4 becomes approximately the same value. The oscillation frequency of can be tracked. In contrast to the above, when the oscillation frequency of the oscillation circuit 3 becomes higher than the set value f1, the difference between the oscillation frequencies of the oscillation circuits 3 and 4 becomes large, so the comparison circuit 9
Since the output of the integration circuit 10 and the output of the integration circuit 10 increase, and the oscillation frequency of the oscillation circuit 4 increases accordingly, the oscillation frequency of the oscillation circuit 3 is adjusted so that the difference between the oscillation frequencies of the oscillation circuits 3 and 4 becomes approximately the same value. Can be followed.

(Embodiment 2) FIG. 5 shows a block diagram of a water content sensor of the present embodiment. Since the basic configuration is the same as that of the first embodiment, the same components are designated by the same reference numerals and the description thereof will be omitted.

This water content sensor comprises a pair of detection electrodes 2 and 2 which are insulated from each other, and an oscillation frequency is formed by a sensor unit 1 in which the impedance between the detection electrodes 2 and 2 changes according to the water content and a capacitor C1. f1 is determined, the oscillation circuit 3 as the first oscillation circuit unit that supplies the oscillation output to the sensor unit 1, and the VCO that oscillates at a frequency according to the input voltage
Such as the second oscillating circuit section, an I / V converting circuit 5 for converting a current flowing through the sensor section 1 into a voltage value, an output of the I / V converting circuit 5, and the oscillating circuit 4. And a mixer circuit 6 as a mixing circuit unit that outputs a signal of a frequency (f1 ± f2) that is the sum and difference of both frequencies f1 and f2, and I / V conversion from the output of the mixer circuit 6. A low-pass filter (hereinafter abbreviated as LPF) 12 as a detection circuit unit that detects and outputs a signal of a frequency component of a difference (f1-f2) between the output frequency f1 of the circuit 5 and the output frequency f2 of the oscillation circuit 4. The output of the mixer circuit 6 and the integration amplifier circuit 8 as an output circuit section that generates an output of a voltage value according to the impedance value between the detection electrodes 2 and 2 by integrating and then amplifying the output of the LPF 12. LPF1
And a comparison circuit 9 for comparing the output obtained by conversion by the F / V conversion unit 12a in 2 and the level of a predetermined reference voltage.
And an integrating circuit 10 that outputs a voltage obtained by integrating the output of the above to the oscillating circuit 4. C2, C3, C5 in the figure
Are capacitors for adjusting the oscillation frequency of the oscillator circuit 4, capacitors for adjusting the integration time constant of the integrating circuit 10, and capacitors for setting the cutoff frequency of the LPF 12. Further, the configuration of the sensor unit 1 is similar to the configuration of FIG. 9 described in the related art, and thus the description thereof will be omitted.

FIG. 6 shows the relationship of the output voltage with respect to the input voltage of the comparison circuit 9, and FIG. 3 shows the relationship of the output frequency with respect to the input voltage of the oscillation circuit 4. In the comparison circuit 9, the F / V conversion section 12a is used. As the input voltage increases, the output voltage increases, and in the oscillation circuit 4, the oscillation frequency increases as the input voltage (output of the integrating circuit 10) increases. Note that Va in FIG. 6 indicates the output of the F / V conversion unit 12a when the intermediate frequency is (f1-f2).

Here, the intermediate frequency, which is the difference between the oscillation frequencies of the oscillation circuit 3 and the oscillation circuit 4, depending on whether the oscillation frequency of the oscillation circuit 3 is increased or the oscillation frequency of the oscillation circuit 4 is decreased due to variations in parts or changes in the surrounding environment. Becomes larger than the reference frequency Δf (that is, the input voltage to the comparison circuit 9 increases), the output voltage of the integration circuit 10 increases, and the oscillation frequency of the oscillation circuit 4 increases accordingly. The difference (intermediate frequency) can be reduced. On the contrary to the above, if the oscillation frequency of the oscillation circuit 3 becomes lower or the oscillation frequency of the oscillation circuit 4 becomes higher, the intermediate frequency, which is the difference between the oscillation frequencies of the two, becomes larger than the reference frequency Δf ( That is, when the input voltage to the comparison circuit 9 decreases), the output voltage of the integration circuit 10 decreases, and the oscillation frequency of the oscillation circuit 4 decreases accordingly. Therefore, the difference (intermediate frequency) between the two oscillation frequencies can be increased. . Here, the comparison circuit 9, the integration circuit 10, and the F / V conversion unit 12a constitute a frequency variable unit that changes the oscillation frequency of the oscillation circuit 4.

As described above, in the present embodiment, based on the frequency of the output of the mixer circuit 6, the oscillation is performed so that the intermediate frequency, which is the difference between the oscillation frequencies of the oscillation circuits 3 and 4, matches the predetermined reference frequency Δf. By changing the oscillation frequency of the circuit 4, the frequency of the output of the mixer circuit 6 is made lower than the cutoff frequency of the LPF 12 to obtain a value corresponding to the amount of water. Since it is not necessary to control the oscillation frequencies of the circuits 3 and 4 with high precision, the highly accurate and expensive crystal oscillators X1 and X2 are unnecessary, and as a result, the oscillation circuits 3 and 4 can be configured at low cost.

Further, in the first embodiment, since the filter circuit 7 composed of the bandpass filter is used, it is necessary to set the center frequency with high accuracy, and as a result, expensive parts such as a ceramic filter are required. Since the LPF 12 is used in the present embodiment, it is not necessary to set the cutoff frequency with high accuracy, and therefore expensive parts such as a ceramic filter are not required,
Cost reduction can be achieved. In addition, this circuit
When converting to C, the capacitors C1 to C5 for adjustment can be eliminated and external parts can be eliminated, and the merit of downsizing and cost reduction can be obtained as compared with the conventional moisture sensor.

In this embodiment, the oscillation frequency of the oscillation circuit 4 is changed based on the output frequency of the mixer circuit 6 so that the difference between the frequencies of the oscillation circuits 3 and 4 matches a predetermined reference frequency Δf. However, instead of changing the oscillation frequency of the oscillation circuit 4, the oscillation frequency of the oscillation circuit 3 may be changed based on the frequency of the output of the mixer circuit 6, and the same effect as described above is obtained. be able to.

(Embodiment 3) FIG. 7 shows a block diagram of a water content sensor of the present embodiment. Since the basic configuration is the same as that of the second embodiment, the same components are designated by the same reference numerals and the description thereof will be omitted.

This water content sensor comprises a pair of detection electrodes 2 and 2 which are insulated from each other, and a sensor section 1 in which the impedance between the detection electrodes 2 and 2 changes according to the water content and a capacitor C1 provide an oscillation frequency. f1 is determined, the oscillation circuit 3 as the first oscillation circuit unit that supplies the oscillation output to the sensor unit 1, and the VCO that oscillates at a frequency according to the input voltage
Such as the second oscillating circuit section, an I / V converting circuit 5 for converting a current flowing through the sensor section 1 into a voltage value, an output of the I / V converting circuit 5, and the oscillating circuit 4. And a mixer circuit 6 as a mixing circuit unit that outputs a signal of a frequency (f1 ± f2) that is the sum and difference of both frequencies f1 and f2, and I / V conversion from the output of the mixer circuit 6. A low-pass filter (hereinafter abbreviated as LPF) 13 as a detection circuit unit that detects and outputs a signal of a frequency component of a difference (f1-f2) between the output frequency f1 of the circuit 5 and the output frequency f2 of the oscillation circuit 4. And the output of the oscillation circuit 3 and the integration amplifier circuit 8 as an output circuit section that generates an output of a voltage value according to the impedance value between the detection electrodes 2 and 2 by integrating and then amplifying the output of the LPF 13. And oscillator circuit 4
Mixer circuit 1 which mixes the outputs of the two and outputs signals of the sum and difference frequencies (f1 ± f2) of both frequencies f1 and f2.
4, an F / V conversion circuit 15 that converts the frequency of the output of the mixer circuit 14 into a voltage value, a comparison circuit 9 that compares the output of the F / V conversion circuit 15 and the reference voltage, and a comparison circuit 9.
And an integrating circuit 10 that outputs a voltage obtained by integrating the output of the above to the oscillating circuit 4. C2, C3, C5 in the figure
Are capacitors for adjusting the oscillation frequency of the oscillation circuit 4, capacitors for adjusting the integration time constant of the integrating circuit 10, and capacitors for setting the cutoff frequency of the LPF 13. Further, the configuration of the sensor unit 1 is similar to the configuration of FIG. 9 described in the related art, and thus the description thereof will be omitted.

As described in the second embodiment, FIG. 6 shows the relationship between the input voltage of the comparator circuit 9 and the output voltage, and FIG. 3 shows the relationship between the input voltage of the oscillator circuit 4 and the output frequency. In 9, the output voltage increases as the input voltage from the F / V conversion circuit 15 increases, and in the oscillation circuit 4, the oscillation frequency increases as the input voltage (output of the integrating circuit 10) increases. There is.

Here, the intermediate frequency, which is the difference between the oscillation frequencies of the oscillation circuit 3 and the oscillation circuit 4, depending on whether the oscillation frequency of the oscillation circuit 3 is high or the oscillation frequency of the oscillation circuit 4 is low due to variations in parts or changes in the surrounding environment. Becomes larger than a predetermined reference frequency Δf (that is, the frequency of the output of the mixer circuit 14 becomes higher), the output of the F / V conversion circuit 15 (the input of the comparison circuit 9) increases, and the output of the comparison circuit 9 becomes Increased,
In response to this, the output of the integration circuit 10 increases, and the oscillation circuit 4
Since the oscillating frequency becomes high, the difference between the oscillating frequencies can be reduced. Contrary to the above, depending on whether the oscillation frequency of the oscillation circuit 3 is low or the oscillation frequency of the oscillation circuit 4 is high, the intermediate frequency, which is the difference between the oscillation frequencies of the two, is smaller than the predetermined reference frequency Δf. Then (that is, when the frequency of the output of the mixer circuit 14 becomes low), the output of the F / V conversion circuit 15 (input of the comparison circuit 9) becomes small,
The output of the comparison circuit 9 decreases, and accordingly the integration circuit 10
Output decreases and the oscillation frequency of the oscillation circuit 4 decreases, so that the difference between the oscillation frequencies of the two can be increased.

As described above, in the present embodiment, the oscillation circuit 3
The output frequency of the mixer circuit 6 is changed by changing the oscillation frequency of the oscillation circuit 4 so that the intermediate frequency which is the difference between the oscillation frequencies of the oscillation circuits 3 and 4 becomes a predetermined reference frequency Δf. Since the frequency is set lower than the cutoff frequency of the LPF 13 to obtain a value corresponding to the water content, it is not necessary to control the oscillation frequencies of the oscillation circuits 3 and 4 with high accuracy as compared with the conventional water content sensor. , The high-precision and expensive crystal units X1 and X2 are no longer required, and as a result, the oscillation circuit 3,
4 can be constructed at low cost. In the second embodiment, the LPF1
Based on the output of 2, the intermediate frequency is a predetermined reference frequency Δf
The oscillation frequency of the oscillation circuit 4 is shifted from a certain frequency so that the oscillation frequency of the oscillation circuit 3 follows.
Since the output of the LPF 12 changes according to the amount of water to be detected, even if the frequency before tracking is the same when the output is large, there is a possibility that an error may occur in the frequency after tracking. In the embodiment, the oscillation frequency of the oscillation frequency 4 is changed based on the oscillation frequency of the oscillation circuit 3,
Since the magnitude of the output of the oscillation circuit 3 is constant regardless of the amount of moisture to be detected, it is possible to reduce the error in tracking the oscillation frequency.

Further, in the first embodiment, since the filter circuit 7 consisting of a bandpass filter is used, it is necessary to set the center frequency with high accuracy, and as a result, expensive parts such as a ceramic filter are required. In the present embodiment, since the LPF 13 is used, it is not necessary to set the cutoff frequency with high accuracy, and therefore expensive parts such as a ceramic filter are not required,
Cost reduction can be achieved. In addition, this circuit
When converting to C, the capacitors C1 to C5 for adjustment can be eliminated and external parts can be eliminated, and the merit of downsizing and cost reduction can be obtained as compared with the conventional moisture sensor.

In this embodiment, the two oscillator circuits 3,
Based on the oscillation frequency of one of the oscillation circuits 3, the oscillation frequency of the other oscillation circuit 4 is changed so that the intermediate frequency, which is the difference between the frequencies of the oscillation circuits 3 and 4, becomes a predetermined reference frequency Δf. However, the oscillation frequency of the one oscillation circuit 3 may be changed based on the oscillation frequency of the other oscillation circuit 4 so that the intermediate frequency becomes the predetermined reference frequency Δf. The effect of can be obtained.

[0036]

As described above, according to the first aspect of the present invention, the sensor unit is provided with a plurality of detection electrodes insulated from each other, and the impedance between the detection electrodes changes according to the amount of water. A first oscillating circuit section that applies a voltage, a second oscillating circuit section that oscillates at a frequency different from that of the first oscillating circuit section, and a mixing circuit that mixes the output of the sensor section and the output of the second oscillating circuit section. Section, a detection circuit section that detects a frequency component of a predetermined reference frequency from the output of the mixing circuit section, and an output circuit section that generates an output according to the impedance between the detection electrodes from the output of the detection circuit section, The frequency of the output from the sensor unit that is substantially equal to the oscillation frequency of the first oscillator circuit unit;
The intermediate frequency of the difference from the oscillation frequency of the second oscillation circuit section is
As it changes in a certain frequency range including the reference frequency,
A frequency modulation section for modulating the oscillation frequency of either the first or second oscillation circuit section is provided, and the frequency modulation section oscillates one of the first and second oscillation circuit sections. By modulating the frequency, the intermediate frequency of the difference between the output frequency from the sensor unit and the oscillation frequency of the second oscillation circuit unit, which is approximately equal to the oscillation frequency of the first oscillation circuit unit, is kept constant including the reference frequency. Since it is changed in the frequency range of, even if the oscillation frequency of the oscillation circuit unit changes due to variations in parts and changes in the surrounding environment, it is possible to output a signal of a frequency component equal to the reference frequency from the mixing circuit unit, Since the detection circuit unit can detect a value corresponding to the amount of water, there is an effect that noise resistance is improved. Moreover, since it is not necessary to set the oscillation frequencies of the first and second oscillation circuit units with high accuracy, it is not necessary to use a highly accurate and expensive crystal oscillator unlike the conventional moisture sensor.
If the first and second oscillation circuit units can be constructed at low cost, and if an external component such as a crystal oscillator is used when making the moisture sensor into an IC, the merit of miniaturization cannot be sufficiently obtained.
There is also an effect that further miniaturization can be achieved by eliminating external parts such as a crystal oscillator.

According to a second aspect of the invention, there is provided a sensor section comprising a plurality of detection electrodes insulated from each other, the impedance between the detection electrodes changing in accordance with the amount of water, and a first oscillation for applying an oscillation output to the sensor section. A circuit section; a second oscillation circuit section that oscillates at a frequency different from that of the first oscillation circuit section; a mixing circuit section that mixes the output of the sensor section and the output of the second oscillation circuit section; The first oscillation circuit section includes a detection circuit section that detects a frequency component of a predetermined reference frequency from the output, and an output circuit section that generates an output corresponding to the impedance between the detection electrodes from the output of the detection circuit section. Any one of the first and second oscillation circuit units is set so that the intermediate frequency of the difference between the output frequency from the sensor unit and the oscillation frequency of the second oscillation circuit unit, which is approximately equal to the oscillation frequency, becomes the reference frequency. Change the oscillation frequency of either And a frequency varying unit for modulating the oscillation frequency of either one of the first and second oscillation circuit units.
Since the intermediate frequency of the difference between the output frequency from the sensor unit and the oscillation frequency of the second oscillator circuit unit, which is approximately equal to the oscillation frequency of the oscillator circuit unit, is matched with the reference frequency, there are variations in parts and the surrounding environment. Even if the oscillation frequency of the oscillating circuit changes due to the change of, the frequency of the output of the mixing circuit can be matched with the reference frequency, and the value corresponding to the moisture content can be detected by the detection circuit. Has the effect of improving. Moreover, since it is not necessary to set the oscillation frequencies of the first and second oscillation circuit units with high accuracy, it is not necessary to use a highly accurate and expensive crystal oscillator unlike the conventional water content sensor. If the first and second oscillation circuit units can be constructed at low cost, and if an external component such as a crystal oscillator is used when integrating the moisture content sensor into an IC, the merit of downsizing cannot be sufficiently obtained. There is also an effect that further miniaturization can be achieved by eliminating external parts such as a vibrator.

According to a third aspect of the present invention, in the second aspect of the invention, the frequency varying section changes the oscillation frequency based on the intermediate frequency, and has the same effect as the second aspect of the invention.

According to a fourth aspect of the present invention, in the second aspect of the invention, the frequency variable section changes one oscillation frequency based on the oscillation frequency of the other oscillation circuit section, and is based on the intermediate frequency. If the oscillation frequency is changed by changing the oscillation frequency, the magnitude of the intermediate frequency signal changes depending on the amount of water to be detected.Therefore, an error is likely to occur when the signal is large and when it is small, but the output of the oscillation circuit section is detected. Since it is constant irrespective of the amount of water to be used, there is an effect that an error that occurs can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram of a water content sensor according to a first embodiment.

FIG. 2 is an explanatory diagram of input / output characteristics of a comparison circuit used in the above.

FIG. 3 is an explanatory diagram of input / output characteristics of a second oscillator circuit used in the above.

FIG. 4 is an explanatory diagram illustrating operations of a comparison circuit and a second oscillation circuit used in the above.

FIG. 5 is a block diagram of a water content sensor according to a second embodiment.

FIG. 6 is an explanatory diagram of input / output characteristics of a comparison circuit used in the above.

FIG. 7 is a block diagram of a water content sensor according to a third embodiment.

FIG. 8 is a block diagram of a conventional moisture sensor.

FIG. 9 is an explanatory diagram illustrating a positional relationship between the water content sensor and a detection target of the above.

[Explanation of symbols]

1 sensor 3 oscillator circuits 4 oscillator circuit 5 I / V conversion circuit 6 mixer circuit 7 Filter circuit 8 Integral amplifier circuit 11 FM modulation circuit

Continued front page    F-term (reference) 2G060 AC01 AE16 AF06 HA02 HC07                       HC10 HC12                 5J050 AA49 AA50 BB22 EE34 EE35                       EE40 FF29

Claims (4)

[Claims] 1. A sensor section having a plurality of detection electrodes insulated from each other, wherein impedance between the detection electrodes changes according to the amount of water, and a first oscillation circuit section for applying an oscillation output to the sensor section. A second oscillating circuit section that oscillates at a frequency different from that of the first oscillating circuit section, a mixing circuit section that mixes the output of the sensor section and the output of the second oscillating circuit section, and the mixing circuit section A detection circuit section that detects a frequency component of a predetermined reference frequency from the output of the output circuit, and an output circuit section that generates an output according to the impedance between the detection electrodes from the output of the detection circuit section, The intermediate frequency of the difference between the frequency of the output from the sensor unit that is substantially equal to the oscillation frequency of the oscillation circuit unit and the oscillation frequency of the second oscillation circuit unit changes within a constant frequency range including the reference frequency. In front Note 1st or 2nd
2. A water content sensor provided with a frequency modulation section for modulating the oscillation frequency of any one of the oscillation circuit sections. 2. A sensor section comprising a plurality of detection electrodes insulated from each other, the impedance between the detection electrodes changing according to the amount of water, and a first oscillation circuit section for applying an oscillation output to the sensor section. A second oscillation circuit section that oscillates at a frequency different from that of the first oscillation circuit section; a mixing circuit section that mixes an output of the sensor section and an output of the second oscillation circuit section; and the mixing circuit section A detection circuit section that detects a frequency component of a predetermined reference frequency from the output of the output circuit, and an output circuit section that generates an output according to the impedance between the detection electrodes from the output of the detection circuit section, The first or second frequency is set so that the intermediate frequency of the difference between the frequency of the output from the sensor section that is substantially equal to the oscillation frequency of the oscillation circuit section and the oscillation frequency of the second oscillation circuit section becomes the reference frequency. Within the oscillator circuit Any moisture content sensor, characterized in that one provided with a frequency controller for varying the oscillation frequency. 3. The water content sensor according to claim 2, wherein the frequency variable unit changes the oscillation frequency based on the intermediate frequency. 4. The water content sensor according to claim 2, wherein the frequency variable unit changes one oscillation frequency based on the oscillation frequency of the other oscillation circuit unit.