JP6801414B2 - Moisture detector - Google Patents

Description

The present invention relates to a water content detector.

The following patent documents disclose a moisture detection device and method (MOISTURE DETECTION APPARATUS AND METHOD) for detecting the moisture content of a detection object as a capacitance. That is, in this moisture detection device and method, a clock signal (reference signal) having a predetermined frequency is input to a detection electrode in contact with the detection object via a resistor, and the static state of the detection object changes depending on the amount of moisture. A detection signal whose rise is slow due to the influence of electric capacity is acquired from the detection electrode, and a DC voltage (output voltage) indicating the amount of water is generated by detecting the phase difference between this detection signal and the reference signal. It is a thing.

U.S. Pat. No. 6,904,789

By the way, the background technique has a problem that the output voltage is liable to fluctuate due to the influence of the ambient temperature. That is, in the above background technique, there is a problem that the temperature characteristics of the output voltage are not always sufficient, that is, the degree of agreement of the output voltages is not sufficient when the same amount of water is detected at different ambient temperatures.

The present invention has been made in view of the above circumstances, and an object of the present invention is to improve the temperature characteristics of the output voltage as compared with the conventional case.

In order to achieve the above object, in the present invention, as a first solution for a water content detector, a detection electrode and a signal supply means for supplying a detection input signal to the detection electrode via a predetermined resistor. The phase detection means that detects the phase difference between the detection signal obtained from the detection electrode and the detection input signal and outputs the DC voltage indicating the phase difference as the detection voltage, and the detection input signal. A means of including a reference voltage generating means for generating a DC voltage as an internal reference voltage and an amplification means for differentially amplifying the detected voltage and the internal reference voltage is adopted.

In the present invention, as a second solution for the water content detector, in the first solution, the signal supply means includes a drive circuit that outputs the detection input signal to the resistor.
A means is adopted in which a delay circuit for delaying the detection input signal by a time corresponding to the delay time in the drive circuit is further provided in front of the phase detection means.

In the present invention, as a third solution relating to the water content detector, in the first or second solution, the signal supply means includes a reference signal generator that generates the detection input signal and the reference. A means is adopted that includes a first drive circuit that amplifies the current of the detection input signal input from the signal generator and outputs it to the resistor.

In the present invention, as a fourth solution relating to the water content detector, in the third solution, the reference voltage generating means current-amplifies the detection input signal input from the reference signal generator. A means is adopted that includes a second drive circuit and a voltage conversion circuit that converts the detection input signal input from the second drive circuit into a DC voltage.

In the present invention, as a fifth solution relating to the water content detector, in the fourth solution, the means that the voltage conversion circuit is a low-pass filter is adopted.

In the present invention, as a sixth solution relating to the water content detector, in any of the third to fifth solutions, the phase detection means is the detection input input from the reference signal generator. A means is adopted that includes a phase detector that compares the phase of the signal with the detection signal, and a second low-pass filter that extracts a DC component from the output of the phase detector and outputs it as the detection voltage.

According to the present invention, since the reference voltage generating means generates a DC voltage corresponding to the detection input signal as the internal reference voltage, the temperature characteristic of the internal reference voltage shows the same tendency as the temperature characteristic of the detection input signal. It becomes. That is, both the internal reference voltage and the detection voltage differentially amplified by the amplification means have the same tendency in temperature characteristics. Therefore, according to the present invention, the temperature characteristics of the output voltage output from the amplification means can be improved as compared with the conventional case.

It is a block diagram which shows the functional structure of the water content detector which concerns on one Embodiment of this invention. It is a schematic diagram which shows the appearance structure of the water content detector which concerns on one Embodiment of this invention. It is a waveform diagram (a) which shows the operation of the water content detector which concerns on one Embodiment of this invention, and the graph (b) which shows the output characteristic.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
The water content detector according to the present embodiment is a device that detects the water content of various detection objects, and as shown in FIG. 1, a reference signal generator 1, a drive circuit 2 (first drive circuit), and a resistor. Instrument 3, pair of detection electrodes 4a, 4b, delay circuit 5, phase detector 6, low-pass filter 7 (second low-pass filter), buffer amplifier 8, differential amplifier 9, drive circuit 10 (second drive circuit) , Low-pass filter 11 (voltage conversion circuit), buffer amplifier 12, buffer amplifier 13, 5V power supply circuit 14, 3.3V power supply circuit 15. Of these components, the components other than the pair of detection electrodes 4a and 4b constitute the main circuit A.

Here, among these components, the reference signal generator 1, the drive circuit 2 (first drive circuit), and the resistor 3 constitute the signal supply means in the present invention. Further, the delay circuit 5, the phase detector 6 and the low-pass filter 7 (second low-pass filter) constitute the phase detection means in the present invention. Further, the drive circuit 10 (second drive circuit), the low-pass filter 11 and the buffer amplifier 12 constitute the reference voltage generating means in the present invention. Further, the buffer amplifier 8, the differential amplifier 9, and the buffer amplifier 12 constitute the amplification means in the present invention.

The reference signal generator 1 is, for example, an oscillator that generates a square wave signal (clock signal) having a predetermined frequency and a predetermined duty ratio. The frequency is, for example, 50 MHz, but an optimum value is appropriately set according to the detection target. The duty ratio is, for example, 50%, but the optimum value is appropriately set according to the object to be detected. Such a reference signal generator 1 outputs the square wave signal to the drive circuit 2, the delay circuit 5, and the drive circuit 10. The drive circuit 2 is a kind of current amplification circuit, and the square wave signal is current-amplified and output to the resistor 3. Since the reference signal generator 1 is operated by the 3.3V power supply supplied from the 3.3V power supply circuit 15, the peak value (amplitude) of the square wave signal (clock signal) is 3.3V.

The resistor 3 is a two-terminal electronic element having a predetermined resistance value, and is provided between the drive circuit 2 and one of the detection electrodes 4a. That is, one end of the resistor 3 is connected to the output end of the drive circuit 2, and the other end is connected to one of the detection electrodes 4a. Such a resistor 3 together with a pair of detection electrodes 4a and 4b constitutes a load on the drive circuit 2. Further, the resistor 3 is a component that controls the detection dynamic range of the water content detector, and the resistance value is optimally set so that the detection dynamic range is maximized.

As shown in FIG. 2, the pair of detection electrodes 4a and 4b are conductive members that come into contact with a detection object such as soil, and are arranged so as to face each other at a predetermined distance. Here, the object to be detected is a dielectric having a capacitance having a size corresponding to the amount of water. That is, the pair of detection electrodes 4a and 4b together with the detection target form a capacitor (electronic element) having the above capacitance.

Further, since such a pair of detection electrodes 4a and 4b function as the above-mentioned capacitor in a circuit, the CR circuit J in the electronic circuit together with the above-mentioned resistor 3, that is, the capacitance C of the capacitor and the resistance of the resistor 3 It constitutes a passive circuit having a time constant τ based on the value R. That is, the voltage waveform of the contact P between one of the detection electrodes 4a and the resistor 3 in the CR circuit J is integrated by the square wave signal input from the drive circuit 2 as the detection input signal based on the time constant τ. It will be the one that was done. Such a CR circuit J outputs a signal obtained by integrating the detection input signal (square wave signal) from the contact P, that is, one of the detection electrodes 4a, to the phase detector 6 as a detection signal.

The delay circuit 5 is provided in front of the phase detector 6 and delays the square wave signal input from the reference signal generator 1 by a time (delay time τ) corresponding to the delay time of the square wave signal in the drive circuit 2. It is a circuit and outputs a square wave signal delayed by the delay time τ to the phase detector 6. The delay circuit 5 has exactly the same circuit configuration as, for example, the drive circuit 2.

The phase detector 6 is a phase comparator that compares the phase difference between the square wave signal input from the delay circuit 5 and the detection signal input from the CR circuit J. As is well known, there are various types of circuits in a phase comparator, and the phase detector 6 is a digital comparator that performs phase comparison processing on a detection signal and a square wave signal as digital signals. Such a phase detector 6 outputs a signal (phase difference signal) indicating the phase difference between the detection signal and the square wave signal to the low-pass filter 7.

The low-pass filter 7 is a passive filter circuit having a predetermined cutoff frequency, and a DC voltage (detection voltage) indicating a phase difference between a detection signal and a square wave signal is obtained by extracting a DC component from a phase difference signal. Output to. The cutoff frequency is, for example, 16 Hz. The buffer amplifier 8 is a kind of impedance converter, and the detected voltage is reduced in impedance and output to the differential amplifier 9.

The differential amplifier 9 is a differential amplifier that differentially amplifies the detection voltage input from the buffer amplifier 8 and the internal reference voltage input from the buffer amplifier 12 described later with a predetermined amplification degree. Since the detected voltage is a relatively low voltage value, for example, on the order of several hundred mV, it is necessary to amplify it in order to secure a sufficient S / N ratio in order to output it to the outside. The differential amplifier 9 differentially amplifies such a detected voltage using an internal reference voltage, and outputs a signal (output voltage) obtained by the differential amplification to the buffer amplifier 13.

On the other hand, the drive circuit 10 is a kind of current amplification circuit, and the square wave signal (clock signal) input from the reference signal generator 1 is current-amplified and output to the low-pass filter 11. The low-pass filter 11 is a passive filter circuit having a predetermined cutoff frequency, converts a square wave signal into an internal reference voltage which is a DC voltage, and outputs the square wave signal to the buffer amplifier 12. The cutoff frequency of the low-pass filter 11 is, for example, 16 Hz. Since the peak value of the square wave signal is set to 3.3V, the internal reference voltage is the average voltage (DC voltage) of the square wave signal, that is, 1,65V.

The buffer amplifier 12 is a kind of impedance converter, and outputs the internal reference voltage to the differential amplifier 9 by lowering the impedance. The buffer amplifier 13 is a kind of current amplifier circuit, which amplifies the output voltage input from the differential amplifier 9 and outputs it to the outside. The 5V power supply circuit 14 converts an external power supply (for example, a 12V power supply) supplied from the outside into a 5V power supply and supplies the voltage to the buffer amplifiers 8, 12, 13 and the differential amplifier 9 and the 3.3V power supply circuit 15. The 3.3V power supply circuit 15 converts the 5V power supply input from the 5V power supply circuit 14 into a 3,3V power supply and supplies it to the reference signal generator 1, the drive circuits 2, 10 and the phase detector 5.

A water content detector having such an electrical configuration has a mechanical configuration as shown in FIG. That is, this water content detector includes a printed circuit board B, a connection cable D, and a resin mold E. As shown in the drawing, the printed circuit board B has a shape having two comb tooth portions b1 and b2 facing each other in parallel, and one of the comb tooth portions b1 and b2 described above is on the surface of one of the comb tooth portions b1. The detection electrode 4a of the above is formed as a part of the wiring pattern, and the other detection electrode 4b is formed on the surface of the other detection electrode 4b. Further, the above-mentioned main circuit A is formed on the printed circuit board B in a region other than the comb teeth portions b1 and b2, that is, in the body portion b3.

The connection cable D is an electric wire for transmitting the above-mentioned external power supply and output voltage, and is composed of a plurality of core wires and a sheath (covering member) covering the core wires. Such a connection cable D is connected to the printed circuit board B by soldering or the like. This water content detector is used by a person grasping the body portion b3 and inserting the pair of comb tooth portions b1 and b2 into a detection object such as soil. The resin mold E is an insulating resin material coated on the body portion b3 in order to electrically and mechanically protect the body portion b3 on which the main circuit A is formed.

Next, the operation of the water content detector configured in this way will be described in detail with reference to FIG.

In this water content detector, the pair of comb tooth portions b1 and b2 are inserted into a detection object such as soil, so that the pair of detection electrodes 4a and 4b formed on the pair of comb tooth portions b1 and b2 are formed. Contact the detection target such as soil. As a result, the pair of detection electrodes 4a and 4b function as capacitors having a capacitance C corresponding to the amount of water in the object to be detected. Then, the resistor 3 having such a pair of detection electrodes 4a and 4b and the resistance value R forms a CR circuit J having a time constant τ defined by the capacitance C and the resistance value R.

In this water content detector, the square wave signal oscillated by the reference signal generator 1 is applied to the CR circuit J via the drive circuit 2. That is, in the CR circuit J, the square wave signal is input to one of the detection electrodes 4a as a detection input signal via the resistor 3. Then, as shown in FIG. 3A, the CR circuit J outputs a detection signal whose rise of the square wave signal is delayed according to its own time constant τ (that is, capacitance C) to the phase detector 5. To do. The delay time of the detection signal with respect to the square wave signal, that is, the phase difference θ between the square wave signal and the detection signal indicates the water content of the detection object.

In the phase detector 6, a phase difference signal corresponding to the phase difference θ between the detection signal input from the CR circuit J (that is, one of the detection electrodes 4a) and the square wave signal input from the delay circuit 5 is generated. Will be done. This phase difference signal is a pulse signal whose duty ratio changes according to the phase difference θ. Such a phase difference signal is input to the buffer amplifier 8 as a detection voltage (DC voltage) indicating the amount of water in the detection target by removing the high frequency component by the low-pass filter 7. Then, this detection voltage is lowered in impedance by the buffer amplifier 8 and input to the − input end (inverting input end) of the differential amplifier 9.

In order to normally amplify the detection voltage with this differential amplifier 9, it is necessary to input an internal reference voltage that defines the operating point of differential amplification to the + input end (non-inverting input end) of the differential amplifier 9. However, in this water content detector, the internal reference voltage is generated by processing the square wave signal input from the reference signal generator 1 with the drive circuit 10 and the low pass filter 7.

That is, the internal reference voltage is the average DC voltage (1) of the peak value (3.3 V) of the square wave signal by removing the high frequency component by the low-pass filter 7 after the square wave signal is current-amplified by the drive circuit 10. It is generated as .65V). Then, the internal reference voltage generated in this way is reduced in impedance by the buffer amplifier 12 after voltage adjustment, and is input to the + input end (non-inverting input end) of the differential amplifier 9.

Here, the detection voltage and the internal reference voltage input to the differential amplifier 9 are both DC voltages generated based on the square wave signal output by the reference signal generator 1, and therefore have the same tendency as the temperature characteristics. .. That is, the detected voltage and the internal reference voltage show the same tendency with respect to the change in ambient temperature. For example, if the detected voltage has a temperature characteristic that rises as the ambient temperature rises, the internal reference voltage also has a temperature characteristic that rises as the ambient temperature rises.

Therefore, according to this water content detector, the temperature characteristic of the detected voltage and the temperature characteristic of the internal reference voltage show the same tendency, so that the temperature characteristic is improved in the amplification process of the detected voltage by the differential amplifier 9 as compared with the conventional case. Can be done.

Then, the detection voltage is voltage-amplified by the differential amplifier 9 using the internal reference voltage, further current-amplified by the buffer amplifier 13, and output to the outside as the output voltage of the water content detector. As shown in FIG. 3B, this output voltage has a characteristic of linearly changing with respect to a change in the water content (volume water content) of the object to be detected. Such a detection voltage is input to the processing device in the subsequent stage by the connection cable D described above, and is converted into a water content (volume water content).

Further, since the delay circuit 5 is provided in this water content detector, it is possible to eliminate the error factor of the phase comparison in the phase detector 6. That is, when the delay circuit 5 is not provided, the square wave signal input to the phase detector 6 and the detection signal have a time relationship that is deviated by the delay time originally possessed by the drive circuit 2, but the delay circuit 5 When is provided, the time-related deviation is eliminated, so that the square wave signal and the detection signal can be phase-compared more accurately. Therefore, according to such a water content detector, it is possible to detect the water content more accurately.

The present invention is not limited to the above embodiment, and for example, the following modifications can be considered.
(1) In the above embodiment, the detection electrodes 4a and 4b are formed as a wiring pattern on the surfaces of the comb tooth portions b1 and b2 formed on the printed circuit board B, but this fixes the distance between the pair of detection electrodes 4a and 4b. It is for stabilization (stabilization). That is, since the capacitance C, which changes depending on the water content of the object to be detected, also changes depending on the distance between the pair of detection electrodes 4a and 4b, in order to detect the water content more accurately, the pair of detections It is important that the distance between the electrodes 4a and 4b is fixed.

However, the present invention is not limited to integrally providing the pair of detection electrodes 4a and 4b on the printed circuit board B. For example, a pair of conductive members provided separately from the printed circuit board B and electrically connected to the printed circuit board B using an electric wire or the like may be the detection electrodes 4a and 4b. However, in this case, it is necessary to fix the distance between the pair of conductive members with an insulating support member.

(2) In the above embodiment, the reference signal generator 1, the drive circuit 2, the phase detector 5, and the drive circuit 10 are driven by the 3.3 V power supply generated by the 3.3 V power supply circuit 15, and the buffer amplifier 8 and the differential are differential. The amplifier 9, the buffer amplifier 12, and the buffer amplifier 13 are driven by a 5V power supply generated by the 5V power supply circuit 14, but the present invention is not limited thereto. If it is not necessary to consider the reduction of power consumption, all the circuits may be driven by a 5V power supply. Further, if the dynamic range of the output voltage output to the outside may be sacrificed to some extent, all the circuits may be driven by a 3.3 V power supply.

(3) In the above embodiment, the reference signal generator 1 generates a square wave signal, but the present invention is not limited to this. For example, a trapezoidal wave signal may be generated instead of the square wave signal.

1 Reference signal generator 2 Drive circuit (1st drive circuit)
3 Resistor 4a, 4b Detection electrode 5 Delay circuit 6 Phase detector 7 Low-pass filter (second low-pass filter)
8 Buffer amplifier 9 Differential amplifier 10 Drive circuit (second drive circuit)
11 Low-pass filter (voltage conversion circuit)
12 Buffer amplifier 13 Buffer amplifier 14 5V power supply circuit 15 3.3V power supply circuit A Main circuit B Printed circuit board b1, b2 Comb tooth part b3 Body part D Connection cable E Resin mold

Claims (5)

With the detection electrode
A signal supply means for supplying a detection input signal to the detection electrode via a predetermined resistor, and
A phase detecting means for detecting the phase difference between the detection signal obtained from the detection electrode and the detection input signal as a detection voltage , and
A reference voltage generating means that generates a DC voltage corresponding to the detection input signal as an internal reference voltage,
It is provided with an amplification means for differentially amplifying the detection voltage and the internal reference voltage .
The signal supply means includes a drive circuit that outputs the detection input signal to the resistor.
A water content detector further provided with a delay circuit for delaying the detection input signal by a time corresponding to a delay time in the drive circuit in front of the phase detection means . The signal supply means includes a reference signal generator that generates the detection input signal, and a first drive circuit that amplifies the detection input signal input from the reference signal generator and outputs the current to the resistor. water content detector according to claim 1, characterized in that it comprises and. The reference voltage generating means has a second drive circuit that amplifies the detection input signal input from the reference signal generator and a DC voltage of the detection input signal input from the second drive circuit. The water content detector according to claim 2 , further comprising a voltage conversion circuit for converting to . The water content detector according to claim 3, wherein the voltage conversion circuit is a low-pass filter . The phase detection means extracts a DC component from a phase detector that compares the phase of the detection input signal input from the reference signal generator with the detection signal, and the output of the phase detector, and extracts the detection voltage. The water content detector according to any one of claims 2 to 4, further comprising a second low-pass filter that outputs as .

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