JP2010266324A - Moisture quantity measuring sensor, moisture quantity measuring device, and water supply quantity control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive piercing-type moisture quantity measuring sensor capable of measuring a moisture quantity in a plant or the like directly and stably at site. <P>SOLUTION: This moisture quantity measuring sensor includes: a needle-shaped probe body 20 pierceable into, for example, a measuring object (plant 6); a water-insoluble polymer water-sensitive material (PES 24) formed on the outside surface thereof, and used for absorption and separation of moisture, whose impedance is changed corresponding to the moisture quantity; and an electrode 26 for detecting an impedance of the polymer water-sensitive material. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a water content measurement sensor, a water content measurement device, and a water supply amount control device, and more particularly, a low-cost water content that can be directly measured for water content in plants, suitable for use in irrigation cultivation of plants. The present invention relates to a measurement sensor, a moisture content measurement device using the moisture content measurement sensor, and a water supply amount control device using the moisture content measurement device.

  In recent years, as a technique for cultivating high-quality crops such as tomatoes, mangoes, melons, grapes, mandarin oranges, and strawberries with high sugar content, irrigation cultivation that finely regulates the amount of irrigation (the amount of water given to the crops) has attracted attention. . In this irrigation cultivation, when a plant is placed in a water-deficient state and stress due to lack of water (referred to as water stress) is applied, the plant accumulates sugar and increases the sugar content. However, plants are withered when water stress is applied too much, so how to control the amount of watering to the plants is a major issue.

  The plant stem is filled with conduits, phloem tubes, or cells, and the moisture content in the plant is constantly changing due to various external factors such as soil moisture and weather. In irrigated cultivation, the amount of water in the plant and the flow of water greatly affect the yield and quality of the crop. Therefore, in order to cultivate a high-quality crop by irrigation cultivation, it is necessary to constantly monitor the water state in the plant and control the irrigation amount.

  Until now, in the institutional cultivation, a method for indirectly estimating the water state from changes in environmental factors related to the water state in the plant such as the water potential of the soil and the amount of solar radiation has been taken. Especially for measuring the water potential of soil, using the relative dielectric constant of water being very large, the TDR sensor that calculates the amount of water between the propagation speed of electromagnetic waves between two points, the thermal conductivity of the soil A heat probe sensor for measuring, a capacitance sensor for measuring the capacitance of soil (see Patent Document 1), an optical sensor using microwaves and short-wavelength infrared light (see Non-Patent Document 1), etc. Research has been done. However, the water status in plants is affected by many environmental factors other than soil water potential and solar radiation, so accuracy is not sufficient for high-quality crop cultivation that requires advanced water management. It was.

  Therefore, methods for knowing the moisture state of a plant from information on the plant itself have been studied. The pressure chamber method and the cyclometer method can measure the water potential, which is an accurate indicator of the moisture state, but it is a destructive method for collecting a part of a plant and it takes time to measure it. It is a problem. Moreover, since the method of estimating the water state in the plant from the change in stem diameter (see Patent Document 2) and the amount of transpiration is non-destructive, continuous monitoring is possible. The relationship between the amount and the moisture state is unclear, and the accuracy is not sufficient. In recent years, research on optical sensors such as near-infrared spectroscopy, infrared imaging, and thermal imaging for measuring the water content of plant leaves has been active. These optical sensors are capable of non-destructive and long-time monitoring, but they are expensive and have a large apparatus, and there is a problem in that it is impossible to measure crops on the spot in actual farming. is there.

  For this reason, there is no effective sensor that is actually put into practical use, and the current situation is that the irrigation amount depends on the experience of the producer and the intuition of many years.

  On the other hand, Patent Document 3 describes an insertion-type botanical drug treatment means, but it does not measure water content.

  As described in Non-Patent Document 2, the inventor manufactured the glass by stretching a micro glass tube having an outer diameter of 2.0 mm and an inner diameter of 1.7 mm as described in Non-Patent Document 2. A silver film 12 serving as one electrode is formed on the inner surface of the probe body 10 having a tip outer shape of 200 μm, while a resistance value is set on the outer side of the silver wire 14 serving as the other electrode according to the amount of water absorbed. Polyvinyl alcohol (PVA), which is a polymer water-sensitive material that changes, is formed into a film, and this is inserted into the probe body 10 so that the silver wire 14 is at the center. A moisture sensor was developed. This sensor can insert a probe directly into a plant and directly measure the amount of water in the plant with a water-sensitive material in the probe. In the figure, 18 is a lead wire.

JP 2001-21517 A JP-A-7-289082 JP-T 63-501262

SANYO TECHNICAL REVIEW, Vol.35, No.2, 2003, pp 40-47 T.A. Iwashita, K.M. Ariga and N.M. Miki, “Development of a Plant Water Content Sensor Using a Micro Probe” PROCEEDINGS OF THE 24TH SENSOR SYMPOSIUM, 2007, pp 457-460

  However, as a result of experiments by the inventors, it has been found that the sensor proposed in Non-Patent Document 2 has the following problems. (1) PVA has very strong hydrophilicity and is soluble in warm water. Therefore, if a large amount of water is absorbed, the PVA itself swells or dissolves and flows out, so that the measured value is not constant and lacks stability, and when repeatedly used, the value changes and good reproducibility is obtained. Therefore, it is not suitable for a sensor for measuring the amount of water in a plant. (2) Since it is necessary to pass the silver wire 14 through the center of the probe body 10, it is difficult to make it. (3) Since the PVA 16 is disposed inside the probe body 10, the moisture content cannot be measured unless moisture enters the probe body 10, but moisture enters the probe body 10 well from the probe tip. Absent.

  The present invention has been made to solve the above-mentioned conventional problems, and provides a low-cost and practical water content measurement sensor capable of directly and stably measuring the water content of plants and the like. The issue is to provide.

  In the present invention, a polymer electric resistance sensor using a polymer whose resistance value changes according to the amount of moisture to be absorbed is used. Polymers that are water-sensitive materials cause ionic conduction by moisture (humidity). In the polymer that has absorbed moisture (humidity), as shown in FIG. 2, there are fixed ions that cannot move freely, and mobile ions (hydrogen ions) that are free and easily move by ionization. Most conduction is caused by this mobile ion, and the mobile ion concentration increases as the amount of moisture absorbed increases. Conversely, when the amount of moisture absorption decreases, the mobile ion concentration decreases. By applying a voltage to this polymer, the change in the concentration of mobile ions, that is, the amount of moisture absorbed can be regarded as a change in resistance. In a polymer actually used as a sensor, moisture (moisture) is reversibly absorbed and desorbed. Here, the reason for using a polymer material as the water-sensitive material is that processing is easy even in a minute region and it is relatively inexpensive.

  Next, the principle of the penetration type moisture measurement sensor will be described.

  As shown in FIG. 3, when the sensor 8 is inserted into the plant 6, the water-sensitive material in the probe portion absorbs moisture in the plant (for example, the stem) 6. Moisture absorbed from the tip of the probe (the right end in FIG. 3) is transmitted to the water sensitive material at the end (the left end in FIG. 3). Since the water-sensitive material at the end is in contact with air, moisture is released into the atmosphere. When the amount of water absorbed by the water-sensitive material from the plant 6 is equal to the amount of water released from the water-sensitive material into the atmosphere, the apparent water movement stops. If the temperature and humidity in the atmosphere are constant and the amount of moisture released into the atmosphere is constant, the amount of moisture contained in the water-sensitive material reflects the amount of moisture in the plant 6. Since the impedance of the water-sensitive material changes depending on the amount of moisture absorbed, the moisture content in the plant 6 can be measured by measuring the impedance.

  Although the polymer water-sensitive material usually has a high moisture absorption capacity, it is not suitable for measuring the moisture content of a solid having a large moisture content, even though it is suitable for measuring humidity. However, by always releasing moisture into the atmosphere, it becomes possible to measure the moisture content of the solid using the polymer water-sensitive material.

  Moreover, the plant which is the main target in the present invention is mainly cultivated in a greenhouse, and it can be assumed that the temperature is controlled to some extent and is not affected by weather such as rain.

  The present invention has been made on the basis of the above knowledge, and is formed on the outer surface of the probe body and its outer surface, so that it does not dissolve in water and the impedance changes according to the amount of water. The above-mentioned problem is solved by a water content measuring sensor comprising a polymer water-sensitive material and an electrode for detecting the impedance of the polymer water-sensitive material.

  Here, the probe body can be shaped like a needle and can be inserted into a measurement object.

  The polymer water-sensitive material can be a porous polymer (for example, polyethersulfone).

  The present invention also provides the moisture content measurement sensor, an impedance detection circuit for detecting an impedance between the electrodes by applying a voltage between the electrodes of the moisture content measurement sensor, and the detected impedance as a moisture content. And a circuit for converting the amount into a water content.

  The present invention also provides a water supply amount control device that controls the supply amount of water to the measurement object based on the output of the water content measurement device.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to measure the moisture content, such as a plant, directly and stably on the spot with a low-cost sensor. Therefore, for example, by controlling the amount of water supplied by plant irrigation cultivation, it becomes possible to cultivate, for example, high-sugar crops.

(A) perspective view and (b) cross-sectional view showing the configuration of a conventional insertion-type moisture measurement sensor described in Non-Patent Document 2. Diagram showing the principle of the present invention The figure which shows the principle of the penetration type moisture content measuring sensor which concerns on this invention The (a) perspective view and (b) cross-sectional view which show the structure of embodiment of the moisture content measuring sensor which concerns on this invention The figure which shows the sensor manufacture process in this embodiment The figure which also shows the PES film formation process The schematic diagram which shows the structure which performed the evaluation experiment of the said embodiment The figure which shows the calibration curve of the relationship between the moisture content and impedance in the said embodiment. Figure showing the effect of humidity The figure which shows the state which inserted the moisture content measuring sensor of the said embodiment in the bag. Figure showing the measurement results of moisture content change The block diagram which shows the structure of the moisture content measuring apparatus using the said embodiment. The figure which shows the example which applied the said embodiment to the irrigation apparatus

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  As shown in FIG. 4, the embodiment of the present invention applied to the insertion-type in-plant moisture content sensor is formed on a probe body 20 having a needle-like tip produced by stretching a micro glass tube and an outer surface thereof. Polyether sulfone (PES) 24, which is a polymer water-sensitive material that does not dissolve in water and changes impedance depending on moisture, and a pair of electrodes 26 for applying a voltage to the film of the PES 24, It is composed of In the figure, reference numeral 28 denotes a lead wire connected to the electrode 26.

  Since the moisture content measurement sensor of the present embodiment (hereinafter also simply referred to as a sensor) is directly inserted into a plant, the insertion portion needs to be fine and strong enough to minimize the destruction of the plant. Therefore, in this embodiment, a microprobe using a micro glass tube is used as the probe body 20. The microprobe is manufactured by stretching a micro glass tube with a machine called a puller. Since glass has a very small heat conduction and is close to a non-conductive heat body, the range of deformation due to the heat of the heater is small, and the glass can be stretched short and thin. The puller used in this embodiment uses the gravity of a weight as a force in the tensile direction. By changing the amount of heat and weight applied by the heater, the shape of the micro glass tube can be controlled. In this embodiment, the microprobe was manufactured using a micro glass tube having an outer diameter of 2.0 mm and an inner diameter of 1.7 mm. At that time, it was designed to be as thin and short as possible in order to obtain strength and ease of sticking. The tip of the probe has an outer diameter of 200 μm in consideration of strength and ease of sticking.

  The PES 24 does not dissolve in water, is chemically stable, and has excellent biocompatibility. Instead of the PES 24, other sulfone-based polymer water-sensitive materials or polymer water-sensitive materials that do not dissolve in water and whose impedance changes according to the amount of water can also be used.

  FIG. 5 shows a sensor manufacturing process. First, for example, a glass tube having an outer diameter of 2.0 mm and an inner diameter of 1.7 mm is stretched while being heated, and, for example, is cut at a position where the outer diameter is 200 μm, thereby achieving both strength and ease of sticking as shown in FIG. Thus, the probe body 20 having a sharp tip is manufactured.

  Next, as shown in FIG. 5B showing a BB cross section of FIG. 5A, a PTFE tape 42 having a width of 1.5 mm, for example, is formed on the side surface of the probe body 20 for patterning and separating the electrodes. After pasting, as shown in FIG. 5C, first, a titanium 44 film is formed as a contact metal for improving the adhesion between the electrode metal (gold in this embodiment) and the glass, and the material of the electrode 26 is formed thereon. A gold film is formed. In this way, by using a contact metal that strongly adsorbs both gold and glass, gold having a high adsorption strength can be formed on the glass. The electrode metal is not limited to gold, and the contact metal can be omitted if there is no problem with the adhesion to the substrate.

  Thereafter, as shown in FIG. 5 (d), the PTFE tape 42 is peeled off to obtain the state shown in FIG. 5 (e). Further, the PES 24 is formed by the wet inversion method shown in FIG. A sensor having a cross section as shown in FIG.

  The Wet Inversion method shown in FIG. 6 is prepared by dissolving the probe body 20 in the state of FIG. 5 (e) by dissolving PES and a solubilizing agent polyvinyl pyrrolidone (PVP) in a solvent N-methylpyrrolidone (NMP). After immersing in the PES solution 30 as shown in FIG. 6A, the PES 24 is formed by immersing in pure water 32 as shown in FIG. 6B. The film thickness is determined by the PES concentration. In this embodiment, the film thickness is adjusted by changing the component ratio of the PES solution 30. Specifically, four types of solutions having different component ratios were used, and the film thickness of PES formed with each solution was measured, and the films were evaluated. The component ratios of the solutions and the results are shown in Table 1.

  As a result, the ratio of PES and PVP in the solution increased and the film became thicker as the solution viscosity increased. Further, when the formed film was evaluated, the solution 1 was not uniformly formed to the tip because of its low viscosity, but the solutions 2 to 4 could form a good film uniformly. did it.

  The influence of the film thickness appears when the sensor is inserted into the plant, and the thicker the film, the easier it is to peel off during the insertion. Therefore, in this embodiment, the solution 2 (PES 10.0%, PVP 10.0%, NMP 80%) that can form the thinnest and good film is determined as the optimum solution.

  In addition, when the surface of the PES film was observed using a scanning electron microscope (SEM), there was no difference in the performance as a water-sensitive material although there was a difference in the pore size due to the difference in the component ratio of the solution. It was.

  The material of the probe body 20 is not limited to a glass tube, and may be a solid glass rod. In addition, as long as it is insulative and easy to stab, it may be plastic or a metal with an insulating coating. Furthermore, a structure may be used in which a metal is used as one electrode with the probe body 20 and a polymer water sensitive agent and the other electrode are coated on the outer surface thereof. The cross-sectional shape is not limited to a circle.

  When the sensor is inserted into a plant and the PES absorbs moisture, the impedance between the electrodes changes. By detecting this, the amount of water can be measured.

  Sensor characteristics were evaluated using the experimental apparatus shown in FIG. This sensor is originally intended to measure the amount of water in the plant, but in order to evaluate the characteristics of the sensor, a constant amount of water was maintained as a measurement target in a glove box where the humidity was kept constant. The tip of the sensor 8 was brought into contact with the absorbent cotton 58, and the impedance at that time was measured.

  Specifically, the signal of the AC power generated by the function generator 52 is amplified by the piezo driver 54 to act as an AC power supply device. From here, a constant frequency and an alternating voltage were applied between the electrodes 26 of the sensor 8 in which the tip was inserted into the absorbent cotton 58 with adjusted moisture, and the current value at that time was measured by the multimeter 56. Furthermore, the impedance was calculated from the measured current value. By combining the function generator 52 and the piezo driver 54, it is possible to apply a frequency of 1 to 1 MHz and a voltage of 0.1 to 200V. A multimeter 56 that can measure up to 0.1 μA was used. Here, an AC voltage of 5 V was applied at 1 kHz because the application of DC voltage causes PES to undergo electrolysis and corrode.

  First, when the time required for the sensor 8 to absorb and release moisture is measured, the response time is about 10 minutes at the maximum, which is sufficiently practical compared to the time scale of several hours in irrigation of plants. Was confirmed.

  A calibration curve created from the moisture content of the absorbent cotton 58 and the measured impedance is shown in FIG. As the moisture content of the absorbent cotton increased, the impedance decreased exponentially, and it was confirmed that there was a good correlation between the two. The impedance change width at that time is as large as 2 to 3 digits. In addition, the measurable range is from 17% to 62% of the moisture content, and it is possible to measure in a very high moisture region as compared with a humidity sensor or the like in which a polymer water-sensitive material has been used so far. From this result, the effectiveness of the manufactured sensor could be confirmed.

  As described above, this sensor always measures the amount of moisture in the plant by continuously releasing moisture from the PES into the atmosphere. Therefore, the amount of moisture released into the atmosphere greatly affects the measured value. For example, even if plants with the same amount of moisture are measured, the impedance increases if the amount of moisture released into the atmosphere is large, and conversely, the impedance decreases if the amount of moisture released into the atmosphere is small. The amount of moisture released depends on humidity, temperature, and convection, but irrigated cultivation is intended to be used in a temperature-controlled greenhouse. It is thought that we receive. Specifically, when the humidity increases, the amount of water released decreases and the impedance decreases. On the contrary, it is expected that when the humidity is lowered, the amount of water released is increased and the impedance is increased.

  Therefore, the humidity in the glove box was changed to 30%, 40%, and 50% with the apparatus shown in FIG. At that time, the moisture content of the absorbent cotton 58 was changed to 29 to 62%, and an experiment was conducted. As a result, as expected, as shown in FIG. 9, it was confirmed that the impedance increased as the humidity decreased. Comparing the values of 30% and 40% humidity, there is a difference of 10 to 15% in the amount of water even at the same impedance. In particular, the lower the humidity, the more pronounced the effect. Further, the range of change was smaller in the region where the amount of moisture was high. Therefore, it is desirable to perform humidity correction when using this sensor. Conversely, it is also possible to measure the atmospheric humidity using this sensor.

  Furthermore, the moisture state of the cocoon was monitored using this sensor. As shown in FIG. 10, the sensor 8 produced at the beginning was inserted into the stalk stem 6 and fixed. After the insertion, it was left as it was for several hours until the measured value became stable. After several hours, the measurement was started. Measurements were taken every 2 hours (4 hours after irrigation every hour). The plants were watered twice at the timings (12 hours and 27 hours after the start) when the soil was thought to be sufficiently dry. The measurement results are shown in FIG.

  From FIG. 11, it can be seen that the impedance is reduced immediately after irrigation. This is thought to be because the amount of water in the plant increased due to irrigation. There is a difference in the change width of the impedance between the two irrigations, which is thought to be affected by the difference in the time zone during which irrigation was performed. Second, it can be seen that the impedance gradually increases during the daytime, and hardly changes during the nighttime. It is exposed to sunlight during the day and consumes water for photosynthesis, but does not consume water because it does not perform photosynthesis at night. Therefore, it is considered that such a result was obtained.

  Based on these verifications, it is considered that the characteristics of changes in water content in plants, such as an increase in water content in plants due to irrigation and a decrease in water content due to photosynthesis, were successfully captured as changes in impedance. As a result, the feasibility of the insertion type moisture measurement sensor could be verified.

  An example of a water content measuring apparatus 60 using the water content measuring sensor 8 according to the present invention is shown in FIG. In the figure, 60A is an AC power source corresponding to the combination of the function generator 52 and the piezo driver 54 shown in FIG. 7, 60B is a current detection circuit corresponding to the multimeter 56, and 60C is a voltage applied by the AC power source 60A. An impedance calculation circuit for calculating an impedance from V and the current I detected by the current detection circuit 60B, 60D is a moisture amount conversion circuit for obtaining a moisture amount from the calculated impedance, for example, using a calibration curve shown in FIG. This is a display device for displaying the measured water content. As the display device 62, in addition to an analog or digital meter that directly displays the amount of water, for example, a display that displays water in a state that requires watering according to the amount of water, a speaker that informs by sound, or the like is used. be able to.

  The water content measurement sensor according to the present invention not only measures the water content of plants, but also constitutes an irrigation device that controls the amount of water supplied to the measurement object by its output as illustrated in FIG. Is possible. In the figure, 60 is a water content measuring device having the same configuration as FIG. 12, 64 is an irrigation amount control device for feedback control of the water supply amount according to the measured water content, and 66 is supplied from a pipe 68. This is a valve for controlling the flow rate of water.

  The configuration of the sensor of the present invention is not limited to the insertion type, and the shape, size, and material of the probe body and electrodes are not limited to the embodiment. The measurement object is not limited to plants, and for example, moisture in food or moisture (humidity) in air can be measured. Furthermore, the application target is not limited to the irrigation apparatus.

6 ... Plant (stem)
8 ... sensor 20 ... probe body 24 ... PES (membrane)
DESCRIPTION OF SYMBOLS 26 ... Electrode 28 ... Lead wire 52 ... Function generator 54 ... Piezo driver 56 ... Multimeter 60 ... Water content measuring device 60A ... AC power supply 60B ... Current detection circuit 60C ... Impedance calculation circuit 60D ... Water content conversion circuit 62 ... Display device 64 ... irrigation amount control device 66 ... valve 68 ... piping

Claims (5)

A probe body;
A polymer water-sensitive material that is filmed on its outer surface, does not dissolve in water, and changes impedance according to the amount of water, in order to cause moisture absorption and desorption.
An electrode for detecting the impedance of the polymer water-sensitive material;
A water content measuring sensor comprising:   The moisture content measurement sensor according to claim 1, wherein the probe body is needle-shaped and can be inserted into a measurement target.   The moisture measuring sensor according to claim 1 or 2, wherein the polymer water-sensitive material is a porous polymer. The water content measurement sensor according to any one of claims 1 to 3,
An impedance detection circuit for detecting an impedance between the electrodes by applying a voltage between the electrodes of the water content measurement sensor;
A circuit that converts the detected impedance into a moisture content;
A water content measuring device comprising:   A water supply amount control device, wherein the water supply amount to the measurement object is controlled by the output of the water content measurement device according to claim 4.