Moisture Sensor

Abstract

This soil moisture sensor determines the amount of moisture in soil. The soil moisture sensor comprises: a thermoelectric element that is disposed in the soil and is capable of changing the temperature of the soil; a first temperature information acquisition unit for acquiring first temperature information indicating the change over time in the temperature of the soil resulting from the operation of the thermoelectric element; a second temperature information acquisition unit for acquiring second temperature information indicating the change over time in the temperature of the soil resulting from the operation of the thermoelectric element; and a computation device for obtaining moisture information on the basis of the temperature information. The computation device comprises an inclination information acquisition unit for using the first temperature information and second temperature information to determine an amount of temperature variation per unit of time and a conversion unit for using conversion information indicating the relationship between the amount of temperature variation and the amount of moisture to convert the amount of temperature variation into an amount of moisture.

Description

[Technical Field]

[0001] The present invention relates to a moisture sensor that obtains information about the moisture contained in a measurement object. [Background technology]

[0002] In the agricultural field, the decline in the number of agricultural workers and the aging of agricultural workers are seen as problems. Precision agriculture is being considered as a technology for cultivating high-quality crops without relying on the experience of agricultural workers. In precision agriculture, the condition of the soil in which crops are grown is measured using various sensors. By using the results of measuring the soil condition, it is possible to obtain the optimum amount of moisture and soil components for crops.

[0003] Non-Patent Document 1 discloses a technology related to a soil moisture sensor. The soil moisture sensor of Non-Patent Document 1 places a measurement probe in the soil. The probe of Non-Patent Document 1 includes a case made of a porous material and a transistor housed in the case. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Pedro Carvalhaes Dias et.al., "Proposal of a Novel Heat Dissipation Soil Moisture Sensor," IEEE Recent Researches in Circuits, Systems and Signal Processing, July 2011, pp. 124-127. Summary of the Invention [Problem to be solved by the invention]

[0005] Indicators of soil condition include, for example, ion concentration, moisture content, pH, and temperature. For example, TDR and capacitance methods are being considered as technologies for obtaining moisture content. TDR uses the reflection of electromagnetic waves. Capacitance methods use electrical impedance and dielectric constant. TDR and capacitance methods obtain moisture content using dielectric constant, which is based on the combined capacitance of the object being measured. Dielectric constant is affected by the ion concentration in water. Therefore, even if the moisture content is the same, the measured value may vary due to the influence of ion concentration.

[0006] An object of the present invention is to provide a moisture sensor that can obtain stable measurement results. [Means for solving the problem]

[0007] One aspect of the present invention is a moisture sensor that obtains moisture information related to moisture contained in a measurement object. The moisture sensor includes a temperature controller that is disposed on the measurement object and is capable of changing the temperature of the measurement object, a temperature information acquisition unit that acquires temperature information indicating changes in the temperature of the measurement object over time due to operation of the temperature controller, and a calculation unit that obtains the moisture information based on the temperature information. The calculation unit includes a slope information acquisition unit that uses the temperature information to obtain the amount of temperature change per unit time, and a conversion unit that uses conversion information indicating the relationship between the amount of temperature change and the moisture information to convert the amount of temperature change into moisture information.

[0008] A moisture sensor obtains the amount of temperature change when an operation is performed to change the temperature of an object being measured. The amount of temperature change is related to information about the moisture contained in the object being measured. The amount of temperature change is based on the specific heat of the object being measured. The specific heat of the object being measured is a physical quantity. As a physical quantity, specific heat is less susceptible to external disturbances. Therefore, by using the amount of temperature change caused by specific heat to obtain information about moisture, stable measurement results can be obtained.

[0009] The temperature control unit of one embodiment of the moisture sensor may be a thermoelectric element that converts electrical energy into thermal energy. The thermoelectric element can switch between heating and cooling by switching the voltage between positive and negative. This characteristic allows the temperature of the object to be measured to be maintained within a predetermined range.

[0010] The thermoelectric element of one embodiment of the moisture sensor may include a first heat input / output surface and a second heat input / output surface that are in contact with the object to be measured and provide heat to the object to be measured and remove heat from the object to be measured. The temperature information acquisition unit may include a first temperature measurement unit attached to the first heat input / output surface and a second temperature measurement unit attached to the second heat input / output surface. This configuration makes it possible to obtain temperature change amounts based on two different modes. As a result, more stable measurement results can be obtained.

[0011] In one embodiment of the moisture sensor, the thermoelectric element may be capable of providing heat to the object to be measured from one of the first heat input / output surface and the second heat input / output surface, and removing heat from the object to be measured from the other of the first heat input / output surface and the second heat input / output surface. The gradient information acquisition unit may acquire a first temperature change amount using first temperature information obtained from the first temperature measurement unit, and may acquire a second temperature change amount using second temperature information obtained from the second temperature measurement unit. The conversion unit may acquire moisture information using the conversion information, the first temperature change amount, and the second temperature change amount. This configuration can improve the accuracy of the measurement results.

[0012] In one aspect of the moisture sensor, the gradient information acquiring unit may obtain the amount of temperature change by using the difference between the first temperature and the second temperature during a transient period, among the changes over time in the temperature of the measurement object.

[0013] In one aspect of the moisture sensor, the temperature information acquisition unit may be disposed so as to be in contact with the object to be measured.

[0014] Another aspect of the present invention is a moisture sensor that obtains moisture information related to the moisture contained in a measurement object. The moisture sensor of another aspect includes a thermoelectric element that is placed on the measurement object and is capable of changing the temperature of the measurement object, a power supply unit that supplies current to the thermoelectric element and is capable of switching the direction of the current supplied to the thermoelectric element, a temperature information acquisition unit that obtains the temperature of the measurement object, and a moisture content acquisition unit that uses the temperature obtained by the temperature information acquisition unit to control the direction of the current supplied to the thermoelectric element by the power supply unit and obtain moisture information.

[0015] In another aspect of the moisture sensor, the moisture amount acquisition section may have a switching control section that controls the direction of the current that the power supply section applies to the thermoelectric element using the temperature of the object to be measured.

[0016] In another form of moisture sensor, the switching control unit may perform a switching operation to switch the direction of the current that the power supply unit applies to the thermoelectric element when the temperature of the object to be measured reaches at least one predetermined threshold value.

[0017] In another form of moisture sensor, the switching control unit may perform the switching operation N times (N is an integer equal to or greater than 1). The moisture amount acquisition unit may obtain moisture information using the elapsed time required for the N switching operations.

[0018] In another aspect of the moisture sensor, the switching control unit may use one predetermined threshold value to control the direction of the current that the power supply unit supplies to the thermoelectric element.

[0019] In another aspect of the moisture sensor, the switching control unit may use a plurality of predetermined threshold values different from one another to control the direction of the current that the power supply unit supplies to the thermoelectric element.

[0020] In another form of moisture sensor, the absolute value of the threshold used when cooling the measurement object may be smaller than the absolute value of the threshold used when heating the measurement object.

[0021] In another form of moisture sensor, the thermoelectric element may include a first thermal input / output surface and a second thermal input / output surface that are in contact with the object to be measured and provide heat to the object to be measured and remove heat from the object to be measured. The temperature information acquisition unit may include a first temperature measurement unit attached to the first thermal input / output surface. The multiple thresholds may include one or more upper thresholds that are higher than the temperature of the object to be measured before the start of measurement and one or more lower thresholds that are lower than the temperature of the object to be measured before the start of measurement. The switching control unit may switch the direction of the current when the temperature acquired by the first temperature measurement unit reaches the upper threshold or the lower threshold.

[0022] In another form of moisture sensor, the thermoelectric element may include a first thermal input / output surface and a second thermal input / output surface that are in contact with the object to be measured and provide heat to the object to be measured and remove heat from the object to be measured. The temperature information acquisition unit may include a first temperature measurement unit attached to the first thermal input / output surface and a second temperature measurement unit attached to the second thermal input / output surface. The multiple thresholds may include one or more upper thresholds that are higher than the temperature of the object to be measured before measurement begins. The switching control unit may switch the direction of the current when the temperature acquired by the first temperature measurement unit reaches the upper threshold or when the temperature acquired by the second temperature measurement unit reaches the upper threshold.

[0023] In another aspect of the moisture sensor, the temperature information acquiring unit may acquire the temperature of the object to be measured before measurement starts as a reference temperature, and the switching control unit may stop the supply of current from the power supply unit to the thermoelectric element when it is determined that the temperature acquired by the first temperature measuring unit has reached the reference temperature while the power supply unit is supplying current to the thermoelectric element.

[0024] In another form of moisture sensor, the thermoelectric element may include a first heat input / output surface and a second heat input / output surface that are in contact with the object to be measured and provide heat to the object to be measured and remove heat from the object to be measured. The temperature information acquisition unit may include a first temperature measurement unit attached to the first heat input / output surface and a second temperature measurement unit attached to the second heat input / output surface opposite to the first heat input / output surface. The multiple thresholds may include one or more lower thresholds that are lower than the temperature of the object to be measured before measurement begins. The switching control unit may switch the direction of the current when the temperature acquired by the first temperature measurement unit reaches the lower threshold or when the temperature acquired by the second temperature measurement unit reaches the lower threshold.

[0025] In another form of moisture sensor, the switching control unit may stop the supply of current from the power supply unit to the thermoelectric element when it is determined that the temperature acquired by the first temperature measuring unit and the temperature acquired by the second temperature measuring unit match.

[0026] In another aspect of the moisture sensor, the moisture amount acquisition section may have a switching control section that controls the direction of the current that the power supply section applies to the thermoelectric element every time a predetermined switching time elapses.

[0027] In another form of moisture sensor, the moisture amount acquisition section may obtain moisture information by utilizing at least one extreme value of the temperature of the measurement object.

[0028] In another form of moisture sensor, the thermoelectric element may include a first thermal input / output surface and a second thermal input / output surface that are in contact with the object to be measured and provide heat to the object to be measured and remove heat from the object to be measured. The temperature information acquisition unit may include a first temperature measurement unit attached to the first thermal input / output surface and a second temperature measurement unit attached to the second thermal input / output surface. The switching control unit may stop the supply of current from the power supply unit to the thermoelectric element when it is determined that the temperature acquired by the first temperature measurement unit matches the temperature acquired by the second temperature measurement unit. [Effects of the Invention]

[0029] According to the present invention, a moisture sensor capable of obtaining stable measurement results is provided. [Brief explanation of the drawings]

[0030] [Figure 1] FIG. 1 is a diagram schematically showing the configuration of a moisture sensor according to the first embodiment. [Figure 2] FIG. 2 is a diagram for explaining the principle of obtaining the moisture content. [Figure 3] FIG. 3 is a diagram illustrating an example of the physical configuration of the arithmetic unit. [Figure 4] FIG. 4 is a diagram illustrating an example of the electrical configuration of the temperature acquisition unit included in the moisture sensor of FIG. [Figure 5] FIG. 5 is a graph showing an example of the relationship between the degree of moisture saturation and the amount of temperature change. [Figure 6] FIG. 6 is a diagram for explaining the measurement principle employed by the moisture sensor of the second embodiment. [Figure 7] FIG. 7 is a diagram schematically showing the configuration of a moisture sensor according to the second embodiment. [Figure 8] FIG. 8 is a diagram showing in detail the configuration of the moisture sensor shown in FIG. [Figure 9] Fig. 9(a) is a diagram illustrating a first threshold setting method, Fig. 9(b) is a diagram illustrating a second threshold setting method, and Fig. 9(c) is a diagram illustrating a third threshold setting method. [Figure 10] FIG. 10 is a flow diagram showing the operation of the moisture sensor shown in FIG. [Figure 11] FIG. 11 shows the history of the first temperature and the second temperature when the threshold value is fixed. [Figure 12] FIG. 12 is a diagram showing in detail the configuration of the moisture sensor of the third embodiment. [Figure 13] FIG. 13 shows the history of the first temperature and the second temperature when the threshold value is variable. [Figure 14] FIG. 14 is a flow chart showing the operation of the moisture sensor shown in FIG. [Figure 15] 15(a) is a graph showing the results of Experimental Example 1. FIG. 15(b) is a graph showing the results of Experimental Example 2. [Figure 16] 16(a) is a graph showing the results of Experimental Example 3. FIG. 16(b) is a graph showing the results of Experimental Example 4. [Figure 17] FIG. 17 is a diagram showing in detail the configuration of the moisture sensor of the fourth embodiment. [Figure 18] FIG. 18 shows the history of the first temperature and the second temperature obtained by switching between heating and cooling at a predetermined time. [Figure 19] FIG. 19 is a flow chart showing the operation of the moisture sensor shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0031] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicated explanations will be omitted.

[0032] As shown in FIG. 1, the moisture sensor obtains the amount of moisture contained in an object to be measured. In the first embodiment, soil 100 is exemplified as the object to be measured. In the following description, the moisture sensor will be referred to as a soil moisture sensor 1. The soil moisture sensor 1 has a sensor unit 10 and a control unit 20. The sensor unit 10 is embedded in the soil 100. The control unit 20 is placed outside the soil 100. The sensor unit 10 is electrically connected to the control unit 20 by a cable or the like.

[0033] The sensor unit 10 has a thermoelectric element 11 (temperature control unit), a first temperature measurement unit 12, and a second temperature measurement unit 13. An example of the thermoelectric element 11 is a Peltier element. The thermoelectric element 11 is electrically connected to the control unit 20 via a cable. The thermoelectric element 11 receives a predetermined voltage from the control unit 20. When a voltage is applied to the thermoelectric element 11, the temperature of one surface increases. When a voltage is applied to the thermoelectric element 11, the temperature of the other surface decreases. These two surfaces, which operate differently from each other, are referred to as a first heat input / output surface 11a and a second heat input / output surface 11b.

[0034] The thermoelectric element 11 has a first heat input / output surface 11a and a second heat input / output surface 11b. When the thermoelectric element 11 is flat, the first heat input / output surface 11a is the main surface. The second heat input / output surface 11b is the back surface. The portion of the soil 100 that contacts the first heat input / output surface 11a is different from another portion of the soil 100 that contacts the second heat input / output surface 11b.

[0035] The thermoelectric element 11 can switch between heating and cooling depending on the polarity of the input voltage. When the thermoelectric element 11 heats and / or cools the soil 100 for measurement, the temperature of the soil 100 changes from its initial state. Therefore, a voltage of the opposite polarity to that used during measurement is applied to the thermoelectric element 11. The heated portion of the soil 100 is cooled. Another portion of the cooled soil 100 is heated. As a result, the temperature can be quickly restored to its initial state. It is possible to suppress the impact of the temperature of the soil 100 on crops.

[0036] In other words, the thermoelectric element 11 can alternate between a heating section and a cooling section. To achieve this function of alternately switching between a heating section and a cooling section, a positive current and a negative current are applied alternately to the thermoelectric element 11 for each measurement. As a result, a large increase in the temperature of the measurement area can be suppressed. Furthermore, a large decrease in the temperature of the measurement area can also be suppressed. Furthermore, there is no need for the probe to cool down after measurement. As a result, continuous measurement is possible. Furthermore, the moisture content can be measured without being affected by the ion concentration in the water.

[0037] The first temperature measuring unit 12 is fixed to the first heat input / output surface 11a. The first temperature measuring unit 12 measures the temperature of the first heat input / output surface 11a. Therefore, the first temperature measuring unit 12 does not measure the temperature of the soil 100 alone. The first temperature measuring unit 12 measures the temperature of a system including the thermoelectric element 11 and the soil 100 in contact with the thermoelectric element 11. The temperature measured by the first temperature measuring unit 12 is affected by the thermal characteristics of the thermoelectric element 11, such as the heat capacity and thermal conductivity, and the thermal characteristics of the soil 100. For example, a factor affected by the thermal characteristics of the thermoelectric element 11 is the time lag between the start of application of electricity and the start of a temperature rise. The soil 100 is in contact with the first heat input / output surface 11a. Furthermore, the first temperature measuring unit 12 is also provided on the first heat input / output surface 11a. This configuration makes it possible to eliminate the influence of the thermal characteristics of the thermoelectric element 11.

[0038] The first temperature measuring unit 12 is electrically connected to the control unit 20 via a cable. The first temperature measuring unit 12 outputs a signal related to temperature to the control unit 20. The second temperature measuring unit 13 is fixed to the second heat input / output surface 11b. The second temperature measuring unit 13 obtains the temperature of the second heat input / output surface 11b. The second temperature measuring unit 13 is electrically connected to the control unit 20 via a cable. The second temperature measuring unit 13 outputs a signal related to temperature to the control unit 20.

[0039] The control unit 20 operates the sensor unit 10. The control unit 20 obtains the moisture content of the soil 100 by processing the signal transmitted from the sensor unit 10. The control unit 20 has a calculation device 30 (calculation section) and a power supply device 50 (power supply section).

[0040] The computing device 30 outputs a control signal to the power supply device 50. The power supply device 50 includes a DC power supply 50a (see FIG. 4). The power supply device 50 outputs a voltage according to the control signal to the thermoelectric element 11. As a result, the thermoelectric element 11 operates. The power supply device 50 outputs a DC voltage to the thermoelectric element 11. The power supply device 50 can adjust the voltage level according to the control signal. The power supply device 50 can set the positive and negative polarities of the voltage according to the control signal. By reversing the positive and negative polarities of the voltage, for example, it is possible to switch a first heat input / output surface that was a heating surface to a cooling surface.

[0041] The calculation device 30 receives temperature-related signals from the first temperature measurement unit 12 and the second temperature measurement unit 13. The temperature-related signals may be, for example, voltage signals. The calculation device 30 obtains the moisture content of the soil 100 using the voltage signals.

[0042] The process of obtaining the moisture content performed by the arithmetic unit 30 will be described below.

[0043] FIG. 2 shows temperature histories obtained when the thermoelectric element 11 is operated to provide heat to the soil 100 from the first heat input / output surface 11a and remove heat from the soil 100 from the second heat input / output surface 11b. Graphs G2a and G2b are temperature histories obtained from the first temperature measurement unit 12. Graphs G2c and G2d are temperature histories obtained from the second temperature measurement unit 13. The moisture state of the soil 100 shown in graph G2a is different from the moisture state of the soil 100 shown in graph G2b. Specifically, the moisture content of the soil 100 shown in graph G2a is less than the moisture content of the soil 100 shown in graph G2b. As can be seen from comparing graphs G2a and G2b, the temperature in the steady-state period S2 differs when the moisture state differs. The temperature difference that appears in the steady-state period S2 is caused by the difference in thermal conductivity of the soil 100. The thermal conductivity of the soil 100 is due to the difference in moisture content. Therefore, the moisture content can be obtained by utilizing the temperature difference that appears during the steady period S2.

[0044] The inventors discovered that differences in the thermal properties of the soil 100 due to differences in moisture content result in differences in thermal conductivity and specific heat. Specifically, when the porosity of the soil 100 is constant, an increase in the water content increases the specific heat. As a result, the thermal conductivity also increases. In terms of thermal properties, differences in specific heat are reflected in the amount of temperature change per unit time. In the following description, "amount of temperature change" is defined as the amount of temperature change per unit time. When the thermoelectric element 11 starts to provide heat, the temperature of the soil 100 in contact with the first heat input / output surface 11a rises. The amount of temperature change per unit time shown in graph G2a is clearly different from the amount of temperature change per unit time shown in graph G2b. The inventors discovered that the moisture content can be obtained by utilizing the amount of temperature change per unit time.

[0045] Figure 5 shows the experimental results of measuring temperature change using soil with a predetermined moisture saturation level. The horizontal axis of the graph represents moisture saturation. The experiment used soils with moisture saturation levels of 0%, 10%, 30%, and 50%. The vertical axis of the graph represents temperature change. Plot G5a in Figure 5 represents the temperature history of the cooling side. Plot G5b represents the temperature history of the heating side. As shown in plots G5a and G5b, a predetermined relationship can be observed between moisture saturation and temperature change. Specifically, as moisture saturation increased, the temperature change decreased. It was also found that there was no significant difference in the relationship between moisture saturation and temperature change, regardless of whether the soil was cooled or heated. The temperature change per unit time when the temperature of the soil 100 increased was the same as the temperature change per unit time when the temperature of the soil 100 decreased. Therefore, the temperature change when the temperature increased can also be used to obtain the moisture content. The amount of moisture can also be obtained by utilizing the amount of temperature change when the temperature is decreased.

[0046] When calculating the moisture content, the temperature change per unit time that occurs in the transient period S1 has several advantages over the temperature change per unit time that occurs in the steady period S2. As mentioned above, the difference in the temperature change per unit time is based on the difference in specific heat. The specific heat is less affected by factors such as the ion concentration of the soil 100. Since the specific heat is less affected by noise, it is advantageous from the perspective of improving measurement accuracy.

[0047] When the thermoelectric element 11 starts operating, a transient period S1 occurs. After the transient period S1, a steady period S2 occurs. The temperature change per unit time is obtained during the transient period S1. The temperature during the steady period S2 is obtained during the steady period S2. The temperature change per unit time obtained during the transient period S1 can be obtained at an earlier timing than the temperature during the steady period S2. It is difficult to clearly determine whether the steady period S2 has been reached. It is easier to determine whether the transition period S1 has begun than to determine whether the steady period S2 has been reached.

[0048] The soil 100 in which crops are grown is managed to have conditions suitable for growing crops. The conditions suitable for growing crops include moisture content, ion concentration, and temperature. It is desirable that the temperature of the soil 100 be within a predetermined range. Under these conditions, the temperature immediately after the start of operation of the thermoelectric element 11 is considered to be the desired temperature. In this case, the temperature during the transient period S1 deviates less from the desired temperature than the temperature during the steady period S2. Therefore, the impact of measuring the moisture content on the crops can be reduced.

[0049] FIG. 3 is a block diagram showing the hardware configuration of the arithmetic device 30. The arithmetic device 30 is a computer. The arithmetic device 30, which is a computer, physically includes one or more processors 30a, a storage device 30b such as a random access memory (RAM) and a read-only memory (ROM), an input device 30c such as a keyboard, a display device 30d such as a display, and a communication device 30e which is a communication interface for transmitting and receiving data. The arithmetic device 30 loads a predetermined computer program into hardware such as the processor 30a, thereby causing each piece of hardware to operate under the control of the processor 30a. The arithmetic device 30 reads data from the storage device 30b. The arithmetic device 30 writes data to the storage device 30b. These operations realize the various functions of the arithmetic device 30 shown in FIG. 1. The arithmetic device 30 does not necessarily have to include all of these components.

[0050] 1 again, the calculation device 30 includes a voltage control unit 31, a first data logger 32, a second data logger 33, a memory unit 34, a slope information acquisition unit 35, an equilibrium temperature acquisition unit 36, and a conversion unit 37.

[0051] The voltage control unit 31 outputs a control signal to the power supply device 50. The control signal includes information such as the voltage level and whether the voltage is positive or negative.

[0052] As shown in FIG. 4, the first data logger 32 measures the voltage generated in the first temperature measurement unit 12. The first data logger 32 converts the voltage into temperature information. The first data logger 32 outputs the temperature information to the storage unit 34. The first data logger 32 includes a first voltmeter 32a, a first DC power supply 32b, and a first temperature conversion unit 32c. The first voltmeter 32a measures the potential difference (voltage) generated in the first temperature measurement unit 12. The first voltmeter 32a outputs the voltage to the first temperature conversion unit 32c. The first temperature conversion unit 32c converts the voltage input from the first voltmeter 32a into temperature. The relationship between voltage and temperature may be information acquired in advance. The relationship between voltage and temperature may be stored in the storage unit 34. The information indicating the relationship between voltage and temperature may be a mathematical formula. The information indicating the relationship between voltage and temperature may be a conversion table.

[0053] The second data logger 33 has the same configuration as the first data logger 32. The second data logger 33 also has a second voltmeter 33a, a second DC power supply 33b, and a second temperature conversion unit 33c. The operation of the second voltmeter 33a, etc. is similar to that of the first voltmeter 32a, etc. Therefore, detailed description of the second voltmeter 33a, the second DC power supply 33b, and the second temperature conversion unit 33c will be omitted.

[0054] The first temperature measuring unit 12 and the first data logger 32 constitute a first temperature information acquiring unit M1, and the second temperature measuring unit 13 and the second data logger 33 constitute a second temperature information acquiring unit M2.

[0055] The first temperature measurement unit 12 includes a first diode 12a. When a constant voltage is input to the diode, a predetermined potential difference (voltage) occurs between the input and output of the diode. The voltage occurring between the input and output of the diode changes depending on the temperature. For example, a relationship can be obtained by prior testing, such as when the voltage increases, the temperature decreases. Therefore, it is possible to obtain the temperature using the voltage. By measuring the voltage, the temperature of the diode can be known.

[0056] The input of the first diode 12a is connected to the positive electrode of the first DC power supply 32b via a first resistor 32d. The first resistor 32d is disposed between the first diode 12a and the first DC power supply 32b. The output of the first diode 12a is connected to the negative electrode of the first DC power supply 32b. The first voltmeter 32a is connected to the input and output of the first diode 12a. The first voltmeter 32a outputs information relating to the voltage to a first temperature converter 32c. The converter 37 converts the voltage into temperature. The converter 37 outputs the temperature information.

[0057] The second temperature measurement unit 13 also includes a second diode 13a, similar to the first temperature measurement unit 12. The connection configuration and operation of the second diode 13a are the same as those of the first temperature information acquisition unit M1. Therefore, a detailed description of the second temperature measurement unit 13 will be omitted.

[0058] The memory unit 34 corresponds to the storage device 30b shown in Fig. 3. The memory unit 34 receives the first temperature information D1 from the first data logger 32. The memory unit 34 receives the second temperature information D2 from the second data logger 33. The memory unit 34 is referenced by the slope information acquisition unit 35. The memory unit 34 outputs the first temperature information D1 and the second temperature information D2 to the slope information acquisition unit 35. The memory unit 34 is referenced by the conversion unit 37. The memory unit 34 outputs conversion information D3 to the memory unit 34.

[0059] The memory unit 34 stores at least first temperature information D1, second temperature information D2, and conversion information D3. The first temperature information D1 indicates the change in the first temperature over time (for example, graph G2a in FIG. 2). The first temperature information D1 is based on the output of the first temperature information acquisition unit M1. The second temperature information D2 indicates the change in the second temperature over time (for example, graph G2c in FIG. 2). The second temperature information D2 is based on the output of the second temperature information acquisition unit M2. The conversion information D3 is information for converting the slope of the temperature change amount into moisture content. The conversion information D3 may be a mathematical formula with the slope of the temperature change amount as an input variable and the moisture content as a dependent variable. The conversion information D3 may also be a conversion table that lists moisture content corresponding to the slope of the temperature change amount.

[0060] The slope information acquisition unit 35 acquires the first slope information using the first temperature information D1. The specific processing for acquiring the first slope information is not particularly limited. For example, as shown in FIG. 2, a transient period S1 and a steady period S2 are distinguished in the first temperature information D1 (graph G2a). Next, in the transient period S1, a predetermined time span d1 (t2-t1) and a temperature difference d2 (T2a-T1a) occurring during the time span d1 are acquired. The time span d1 may be appropriately set based on any conditions. For example, the start point for acquiring the slope may be set based on a time difference based on the thermal characteristics of the thermoelectric element 11, with the start of current application as the reference point. The start point for acquiring the slope is, for example, temperature T1a in FIG. 2. The start point for acquiring the slope may be set to the timing when a temperature change is detected. The processing is not limited to acquiring the temperature difference d2 based on the time span d1. The time span d1 may also be acquired based on the temperature difference d2. The time duration d1 and the temperature difference d2 are used to obtain first slope information. The slope information obtaining unit 35 performs the same process on the second temperature information D2. As a result, the slope information obtaining unit 35 obtains the first slope information and the second slope information. The slope information obtaining unit 35 may output the first slope information and the second slope information to the storage unit 34. The slope information obtaining unit 35 may output the first slope information and the second slope information to the conversion unit 37.

[0061] The information output by the tilt information acquisition unit 35 is not limited to the first tilt information and the second tilt information. For example, the tilt information acquisition unit 35 may output the difference between the first tilt information and the second tilt information as the overall tilt information.

[0062] The equilibrium temperature acquisition unit 36 obtains first equilibrium temperature information using the first temperature information D1. There are no particular limitations on the specific processing content for obtaining the first equilibrium temperature information. The equilibrium temperature acquisition unit 36 performs similar processing on the second temperature information D2 to obtain second equilibrium temperature information. The equilibrium temperature acquisition unit 36 may be provided as needed. The calculation device 30 may also omit the equilibrium temperature acquisition unit 36.

[0063] The conversion unit 37 converts the first slope information into first moisture information. The conversion unit 37 converts the second slope information into second moisture information. The conversion unit 37 receives the first slope information from the slope information acquisition unit 35 or the memory unit 34. The conversion unit 37 receives conversion information D3 from the memory unit 34. The conversion unit 37 converts the first slope information into first moisture information using the conversion information D3. The conversion unit 37 performs similar processing on the second slope information. The conversion unit 37 obtains total moisture information using the first moisture information and the second moisture information. There are no particular limitations on the process of obtaining the total moisture information from the first moisture information and the second moisture information. For example, the average value of the first moisture information and the second moisture information may be used as the total moisture information.

[0064] The following describes the effects of the soil moisture sensor 1, while illustrating the background of the soil moisture sensor 1 of the first embodiment and the problems with the conventional technology.

[0065] In the fields of agriculture and disaster prevention, it is important to measure the amount of water contained in soil 100 (soil moisture content). One technology for measuring soil moisture content is a sensor that uses the TDR method, which utilizes the reflection of electromagnetic waves. Another technology for measuring soil moisture content is a sensor that uses the capacitance method, which detects differences in relative dielectric constant by measuring electrical impedance. Both of these sensors measure the relative dielectric constant calculated from the combined capacitance of a substance. However, the relative dielectric constant is affected by the ion concentration in the water. Differences in ion concentration result in noise when measuring moisture content.

[0066] In consideration of the above circumstances, the soil moisture sensor 1 includes a thermoelectric element 11 disposed in the soil 100 and capable of changing the temperature of the soil 100, a first temperature information acquisition unit M1 that acquires first temperature information D1 indicating a change in temperature of the soil 100 over time due to the operation of the thermoelectric element 11, a second temperature information acquisition unit M2 that acquires second temperature information D2 indicating a change in temperature of the soil 100 over time due to the operation of the thermoelectric element 11, and a calculation device 30 that acquires moisture information based on the temperature information. The calculation device 30 includes a slope information acquisition unit 35 that acquires the amount of temperature change per unit time using the first temperature information D1 and the second temperature information D2, and a conversion unit 37 that converts the amount of temperature change into moisture content using conversion information D3 indicating the relationship between the amount of temperature change and moisture content.

[0067] When the soil moisture sensor 1 performs an operation that changes the temperature of the soil 100, it obtains the amount of temperature change as information. The amount of temperature change is related to information about the moisture contained in the soil 100. The amount of temperature change is based on the specific heat of the soil 100. The specific heat of the soil 100 is a physical quantity. Therefore, the amount of temperature change is not easily affected by external disturbances. Therefore, by obtaining information about moisture using the amount of temperature change caused by the specific heat, stable measurement results can be obtained.

[0068] The soil moisture sensor 1 can obtain the necessary measurement information in a short time. As a result, it is possible to measure using only the specific heat and thermal conductivity without generating Joule heat. With this configuration, it is possible to measure moisture information without increasing the average temperature of the heating and cooling parts.

[0069] Sensors that employ the TDR method, which utilizes the reflection of electromagnetic waves, require a signal with a high sampling frequency during measurement. To generate this signal, TDR sensors require complex and large systems. On the other hand, the soil moisture sensor 1 of the first embodiment does not require the high sampling frequency required by TDR devices. As a result, the system can be simplified. Furthermore, the soil moisture sensor 1 can also be miniaturized. Therefore, the soil moisture sensor 1 is suitable for precision agriculture, which requires a compact sensor capable of measuring a wide range and multiple points.

[0070] Although the embodiments of the present invention have been described above, the soil moisture sensor 1 of the present invention is not limited to the above embodiments.

[0071] The soil moisture sensor 1 of the first embodiment is not limited to use in precision agriculture where nutrients are highly concentrated. For example, it can be used in soil with high levels of contamination and for calibrating moisture measurement sensors using impedance. Furthermore, the object to be measured is not limited to the soil 100. For example, it can also be applied to the field of skin regenerative medicine. As an example, it can be applied to measuring moisture content in high ion concentrations, such as in physiological saline.

[0072] The soil moisture sensor 1 that utilizes the thermal characteristics of the object to be measured may be applied to a multimodal sensor that measures multiple soil property values such as the temperature, ion concentration, and pH value of the soil 100 in real time.

[0073] The soil moisture sensor 1 may obtain the moisture content using only the temperature change per unit time, or may obtain the moisture content by combining the temperature change per unit time with the steady-state temperature.

[0074] In the soil moisture sensor 1 of the first embodiment, the first temperature measuring unit 12 is directly attached to the first heat input / output surface 11a. However, the first temperature measuring unit 12 does not necessarily have to be in direct contact with the first heat input / output surface 11a. For example, the first temperature measuring unit 12 may be attached to the first heat input / output surface 11a via a member such as thermally conductive grease or adhesive. A member may be disposed between the first temperature measuring unit 12 and the first heat input / output surface 11a. The soil 100, which is the measurement target, may be present between the first temperature measuring unit 12 and the first heat input / output surface 11a. When the sensor unit 10 is removed from the soil 100, a gap may exist between the first temperature measuring unit 12 and the first heat input / output surface 11a. As long as the gap between the first temperature measuring unit 12 and the first heat input / output surface 11a is a few centimeters (e.g., 4 centimeters) or less, the same effect as that of the soil moisture sensor 1 of the first embodiment described above can be obtained. The same applies to the relationship between the second temperature measuring unit 13 and the second heat input / output surface 11b.

[0075] Second Embodiment A soil moisture sensor of the second embodiment will now be described. The soil moisture sensor 1 described above utilizes the fact that the amount of moisture contained in the soil 100 corresponds to specific heat to measure the amount of moisture. Differences in specific heat corresponding to the amount of moisture can be evaluated using various methods. The soil moisture sensor 1 of the first embodiment described above evaluated using the degree of temperature rise (amount of temperature change per unit time) when heat was applied to the soil 100. Then, the soil moisture sensor 1 of the first embodiment converted the amount of temperature change per unit time into the amount of moisture.

[0076] Like the soil moisture sensor 1 of the first embodiment, the soil moisture sensor 1A of the second embodiment also measures the amount of moisture contained in the soil 100 by utilizing the fact that the amount of moisture corresponds to the specific heat. On the other hand, the soil moisture sensor 1A evaluates the difference in the specific heat corresponding to the amount of moisture using a method different from that of the soil moisture sensor 1 of the first embodiment. The soil moisture sensor 1A will be described in detail below.

[0077] First, the principle of measuring moisture content employed by the soil moisture sensor 1A will be described. The graph in Figure 6 shows the temperature change over time of the soil 100. The horizontal axis of Figure 6 represents the elapsed time from the start of heating by the thermoelectric element 11. The vertical axis of Figure 6 represents the temperature change over time of the soil 100. Graph G6a shows the temperature change over time of soil 100 with a low moisture content. Graph G6b shows the temperature change over time of soil 100 with a high moisture content.

[0078] For example, let us look at graph G6a. As shown in graph G6a, the soil moisture sensor 1A repeatedly heats and cools the soil 100 multiple times. The soil moisture sensor 1A switches the operation of the thermoelectric element 11 from heating to cooling when the amount of temperature change in the soil 100 reaches a predetermined upper threshold φ4H. The soil moisture sensor 1A also switches the operation of the thermoelectric element 11 from cooling to heating when the amount of temperature change in the soil 100 reaches a predetermined lower threshold φ4L.

[0079] The amount of moisture contained in the soil 100 affects the specific heat of the soil 100. Specifically, when the moisture content is low, the specific heat is low. Therefore, soil 100 with a low moisture content has a large amount of temperature change. For example, when heated, the time t1 required to reach the upper threshold value φ4H is also short. On the other hand, when the moisture content is high, the specific heat is high. Therefore, soil 100 with a high moisture content has a small amount of temperature change. When heated, the time t2 required to reach the upper threshold value φ4H is longer than the time t1.

[0080] The difference in the time required to reach the threshold value, which varies depending on the moisture content, is accumulated each time the heating and cooling operations are repeated. As a result, the times t3 and t4 required to perform a predetermined number of switching operations differ depending on the moisture content of the soil 100. For example, the time t4 required to perform the predetermined number of operations when the soil 100 has a high moisture content (graph G6b) is longer than the time t3 required to perform the predetermined number of operations when the soil 100 has a low moisture content (graph G6a). Therefore, the soil moisture sensor 1A evaluates the difference in specific heat corresponding to the moisture content using the times t3 and t4 required to perform the predetermined number of switching operations.

[0081] 7, the soil moisture sensor 1A has a sensor unit 10 and a control unit 20A. The sensor unit 10 is the same as the sensor unit 10 provided in the soil moisture sensor 1 of the first embodiment. Therefore, a detailed description of the sensor unit 10 will be omitted.

[0082] The control unit 20A has an arithmetic device 30A and a power supply device 50A. The arithmetic device 30A receives a first voltage φ1 output by the first temperature measuring unit 12. The arithmetic device 30A receives a second voltage θ1 output by the second temperature measuring unit 13. The arithmetic device 30A uses the first voltage φ1 and the second voltage θ1 to output a moisture content φ9.

[0083] The power supply device 50A supplies either a first current π1 or a second current π2 to the thermoelectric element 11. The direction of the first current π1 is opposite to the direction of the second current π2. The power supply device 50A switches the direction of the current by switching the polarity of the voltage connected to the thermoelectric element 11. For example, the power supply device 50A includes a switch 51, a switch 52, and a DC power supply 50s. The power supply device 50A controls the switches 51 and 52 to switch the polarity of the DC power supply 53 connected to the thermoelectric element 11. The switch 51 switches between terminals 53a and 53b connected in response to control signals φ7a and φ8c. The switch 51 maintains connection to terminal 53a or terminal 53b until it receives the next control signal φ7a or φ8c. The switch 52 switches between terminals 54a and 54b connected in response to a control signal φ7b. The switch 52 maintains connection to the terminal 54a or 54b until it receives the next control signal φ7b. The control signals φ7a, φ7b, and φ8c are provided by the arithmetic unit 30A. The circuit configuration of the power supply device 50A shown in FIG. 7 is an example. Any circuit configuration may be adopted for the power supply device 50A as long as it is possible to switch the direction of the current provided to the thermoelectric element 11.

[0084] As shown in FIG. 8, the arithmetic device 30A has a first data logger 32A, a second data logger 33A, a storage unit 34A, and a moisture content acquisition unit 40A.

[0085] The first data logger 32A receives a first voltage φ1. The first data logger 32A converts the first voltage φ1 into a first temperature change amount φ2. The first data logger 32A outputs the first temperature change amount φ2. The second data logger 33A receives a second voltage θ1. The second data logger 33A converts the second voltage θ1 into a second temperature change amount θ2. The second data logger 33A outputs the second temperature change amount θ2.

[0086] In the following description, the "first temperature change amount φ2" and the "second temperature change amount θ2" are defined as the difference from the reference temperature TS, which is the temperature of the soil 100 immediately before the start of measurement.

[0087] The storage unit 34A receives the first temperature change amount φ2. The storage unit 34A stores the first temperature change amount φ2 in association with time. The storage unit 34A receives the second temperature change amount θ2. The storage unit 34A stores the second temperature change amount θ2 in association with time.

[0088] The storage unit 34A has several values for the process of calculating the moisture amount φ9. The storage unit 34A holds conversion information φ3 for converting the evaluation value into the moisture amount φ9 as a value for the process of calculating the moisture amount φ9.

[0089] The memory unit 34A has several values for controlling the sensor unit 10. For example, the memory unit 34A stores a plurality of thresholds φ4 as values for controlling the sensor unit 10. The memory unit 34A stores a specified number of times φ5 as a value for controlling the sensor unit 10. The specified number of times φ5 is the number of switching operations performed in one moisture content measurement operation. The specified number of times φ5 may be defined as one switching from heating to cooling. Similarly, the specified number of times φ5 may be defined as one switching from cooling to heating. Furthermore, the specified number of times φ5 may be defined as one switching from heating to cooling and one switching from cooling to heating together.

[0090] The moisture content acquiring unit 40A receives the first temperature change amount φ2 from the memory unit 34A. The moisture content acquiring unit 40A uses the first temperature change amount φ2 to generate control signals φ7a, φ7b, and φ8c for the power supply device 50A. The moisture content acquiring unit 40A uses the first temperature change amount φ2 to generate a moisture content φ9.

[0091] The moisture content acquisition unit 40A includes a switching control unit 41A and a calculation processing unit 42A. The switching control unit 41A receives a first temperature change amount φ2 and a threshold value φ4 from the memory unit 34A. The switching control unit 41A generates control signals φ7a, φ7b, and φ8c for the power supply device 50A using the first temperature change amount φ2 and the threshold value φ4. The calculation processing unit 42A receives an elapsed time φ8b from the switching control unit 41A. Furthermore, the calculation processing unit 42A receives conversion information φ3 from the memory unit 34A. The calculation processing unit 42A converts the elapsed time φ8b into a moisture content φ9 using the conversion information φ3.

[0092] The switching control unit 41A includes a switching determination unit 41a, a switching instruction unit 41b, and a switching counting unit 41c.

[0093] The switching determination unit 41a receives the first temperature change amount φ2 from the memory unit 34A. Furthermore, the switching determination unit 41a receives the threshold value φ4 from the memory unit 34A. The switching determination unit 41a determines whether the first temperature change amount φ2 has reached the threshold value φ4. The switching determination unit 41a outputs a determination result φ6 according to the determination result. The determination result φ6 is either information indicating that the first temperature change amount φ2 has reached the threshold value φ4, or information indicating that the first temperature change amount φ2 has not reached the threshold value φ4.

[0094] Here, the threshold value φ4 will be described in detail. The threshold value φ4 is a condition for switching between heating and cooling of the thermoelectric element 11. There are several methods for setting the threshold value φ4. In this embodiment, three setting methods are exemplified.

[0095] <First setting method> In the first setting method, only the first temperature change amount φ2 is used for control of switching. In the first setting method, the second temperature change amount θ2 is not used for control of switching. Graph G9a in FIG. 9(a) shows the first temperature change amount φ2. In the first setting method, two thresholds are used. In the first setting method, an upper threshold φ4H and a lower threshold φ4L are used.

[0096] The upper threshold value φ4H is, for example, higher than the reference temperature TS of the soil 100 immediately before the start of measurement. As an example, the upper threshold value φ4H is 4 degrees higher than the reference temperature TS of the soil 100 immediately before the start of measurement. When the soil 100 is heated by the thermoelectric element 11 and the first temperature change amount φ2 reaches the upper threshold value φ4H (see symbol P1H), the operation of the thermoelectric element 11 is switched from heating to cooling.

[0097] The lower threshold value φ4L is lower than the reference temperature TS of the soil 100 immediately before the start of measurement. As an example, the lower threshold value φ4L is 4 degrees lower than the reference temperature TS of the soil 100 immediately before the start of measurement. When the soil 100 is cooled by the thermoelectric element 11 and the first temperature change amount φ2 reaches the lower threshold value φ4L (see symbol P1L), the operation of the thermoelectric element 11 is switched from cooling to heating.

[0098] When the first setting method is adopted, the temperature of the soil 100 in contact with the first heat input / output surface 11a can be reliably controlled. Specifically, the first temperature change φ2 of the soil 100 in contact with the first heat input / output surface 11a does not exceed the upper threshold value φ4H. Similarly, the first temperature change φ2 of the soil 100 in contact with the first heat input / output surface 11a does not exceed the lower threshold value φ4L.

[0099] <Second setting method> In the second setting method, the first temperature change amount φ2 and the second temperature change amount θ2 are used for control of switching. Graph G9b in FIG. 9(b) shows the first temperature change amount φ2. Graph G9b in FIG. 9(b) shows the second temperature change amount θ2. In the second setting method, one upper threshold value φ4H is used.

[0100] When the soil 100 is heated by the first heat input / output surface 11a of the thermoelectric element 11 and the first temperature change amount φ2 reaches the upper threshold value φ4H, the operation of the thermoelectric element 11 is switched (see symbol P1H). The switching of operation causes the second heat input / output surface 11b of the thermoelectric element 11 to start heating the soil 100. In other words, the second temperature change amount θ2 measured by the second temperature measuring unit 13 provided on the second heat input / output surface 11b starts to increase. Then, when the second temperature change amount θ2 reaches the upper threshold value φ4H, the operation of the thermoelectric element 11 is switched (see symbol P2H).

[0101] When the second setting method is adopted, the first temperature change φ2 of the soil 100 in contact with the first heat input / output surface 11a does not exceed the upper threshold value φ4H. Similarly, the second temperature change θ2 of the soil 100 in contact with the second heat input / output surface 11b does not exceed the upper threshold value φ4H.

[0102] <Third setting method> In the second setting method, the first temperature change amount φ2 and the second temperature change amount θ2 are compared with the upper threshold value φ4H. As shown in FIG. 9(c), in the third setting method, the first temperature change amount φ2 and the second temperature change amount θ2 are compared with the lower threshold value φ4L. That is, when the first temperature change amount φ2 reaches the lower threshold value φ4L (see symbol P1L), the operation of the thermoelectric element 11 is switched. Similarly, when the second temperature change amount θ2 reaches the lower threshold value φ4L (see symbol P2L), the operation of the thermoelectric element 11 is switched.

[0103] When the third setting method is adopted, the first temperature change φ2 of the soil 100 in contact with the first heat input / output surface 11a does not exceed the lower threshold value φ4L. Similarly, the temperature θ2 of the soil 100 in contact with the second heat input / output surface 11b does not exceed the lower threshold value φ4L.

[0104] Referring again to Figure 8, the switching instruction unit 41b receives the determination result φ6 from the switching determination unit 41a. When the switching instruction unit 41b receives the determination result φ6 indicating that the threshold value φ4 has been reached, it outputs control signals φ7a and φ7b to the power supply device 50A for switching the direction of the current. When the switching instruction unit 41b receives the determination result φ6 indicating that the threshold value φ4 has not been reached, it is not necessary for it to output any signal in order to maintain the direction of the current.

[0105] The switching counter 41c receives the determination result φ6 from the switching determination unit 41a. When the switching counter 41c receives the determination result φ6 indicating that the threshold value φ4 has been reached, it updates the count value φ8a. When the switching instruction unit 41b receives the determination result φ6 indicating that the threshold value φ4 has not been reached, it does not update the count value φ8a.

[0106] Furthermore, the switching counter 41c determines whether the switching count value φ8a has reached the specified number of times φ5. When the switching count value φ8a has reached the specified number of times φ5, the switching counter 41c outputs the elapsed time φ8b until the specified number of times φ5 is reached. The elapsed time φ8b is an evaluation value that can evaluate the difference in specific heat corresponding to the moisture content. Therefore, the elapsed time φ8b can be converted into the moisture content φ9. Furthermore, when the switching count value φ8a has reached the specified number of times φ5, the switching counter 41c outputs a signal φ8c to stop the output of the power supply device 50A.

[0107] The calculation processing unit 42A has a conversion unit 42a. The conversion unit 42a receives the elapsed time φ8b from the switching counting unit 41c. Furthermore, the conversion unit 42a receives conversion information φ3 from the memory unit 34. The conversion unit 42a converts the elapsed time φ8b into a moisture content φ9 using the conversion information φ3. The conversion unit 42a outputs the moisture content φ9.

[0108] <Operation flow of the soil moisture sensor of the second embodiment> Next, the operation of the soil moisture sensor 1 A will be described. Figure 10 is a flow chart showing the operation of the soil moisture sensor 1 A. The operation shown in Figure 10 is performed when the first setting method described above is adopted.

[0109] First, the threshold value φ4 is set (S11). For example, in S11, an upper threshold value φ4H and a lower threshold value φ4L are set. Next, the first temperature change amount φ2 is obtained (S12).

[0110] Next, it is determined whether the first temperature change amount φ2 has reached the upper threshold value φ4H (S13). If it is determined as a result of executing S13 that the first temperature change amount φ2 has reached the upper threshold value φ4H (S13: YES), the direction of the current is switched (S14). By switching the direction of the current, the operation of the first heat input / output surface 11a switches from heating to cooling. After executing S14, the count value of the switching operation is updated (S15). If it is determined as a result of executing S13 that the first temperature change amount φ2 has not reached the upper threshold value φ4H (S13: NO), the direction of the current is not switched. Then, after a predetermined time has elapsed, S12 is executed again.

[0111] After executing S15, it is determined whether the count value φ8a has reached the specified number of times φ5 (S16). As a result of executing S16, if it is determined that the switching count value φ8a has reached the specified number of times φ5 (S16: YES), the measurement operation is terminated and an operation to calculate the moisture amount φ9 is performed (S23, S24, S25). S23, S24, and S25 will be described later. As a result of executing S21, if it is determined that the switching count value φ8a has not reached the specified number of times φ5 (S16: NO), the first temperature change amount φ2 is acquired (S17).

[0112] Next, it is determined whether the first temperature change amount φ2 has reached the lower threshold value φ4L (S18). If it is determined as a result of executing S18 that the first temperature change amount φ2 has reached the lower threshold value φ4L (S18: YES), the direction of the current is switched (S19). By switching the direction of the current, the operation of the first heat input / output surface 11a switches from cooling to heating. After executing S19, the count value of the switching operation is updated (S21). If it is determined as a result of executing S18 that the first temperature change amount φ2 has not reached the lower threshold value φ4L (S18: NO), the direction of the current is not switched. Then, after a predetermined time has elapsed, S17 is executed again.

[0113] After executing S21, it is determined whether the switching count value φ8a has reached the specified number of times φ5 (S22). If it is determined that the switching count value φ8a has reached the specified number of times φ5 as a result of executing S22 (S22: YES), the measurement operation is terminated and an operation to calculate the moisture amount φ9 is performed (S23, S24, S25). If it is determined that the switching count value φ8a has not reached the specified number of times φ5 as a result of executing S22 (S22: NO), after a predetermined time has elapsed, the steps are executed again in order from S12.

[0114] The operation of calculating the moisture content φ9 upon completion of the measurement operation will be described below. First, the supply of current is stopped (S23). Next, the elapsed time φ8b is obtained (S24). S24 is executed by the switching counting unit 41c. Then, the elapsed time φ8b is converted into the moisture content φ9 (S25).

[0115] <Effects of the Second Embodiment> Like the soil moisture sensor 1, the soil moisture sensor 1A can provide stable measurement results.

[0116] The soil moisture sensor 1A obtains the moisture content φ9 using the elapsed time φ8b obtained as a result of multiple switching operations. By using the results of multiple switching operations, the influence of variations in the time required for each heating or cooling operation can be suppressed.

[0117] In the second embodiment, whether to end the measurement is determined based on whether the switching count value φ8a has reached the specified number of times φ5. When determining whether to end the measurement, further conditions may be added. For example, the switching control unit 41A continues heating or cooling when it is determined that the switching count value φ8a has reached the specified number of times φ5. Then, the switching control unit 41A may stop the supply of current to the thermoelectric element 11 when it is determined that the first temperature change amount φ2 has satisfied a predetermined condition.

[0118] Such an operation may be performed by the termination determination unit 41d shown in FIG. 8. The termination determination unit 41d is an additional element that may be provided as needed. The termination determination unit 41d receives the first temperature change amount φ2. When a condition is met, the termination determination unit 41d outputs a signal φ10 to the power supply device 50A to stop outputting current.

[0119] When it is determined that the first temperature change amount φ2 is zero, the termination determination unit 41d outputs a signal φ10 to the power supply device 50A to stop the supply of current to the thermoelectric element 11. The case where it can be determined that the first temperature change amount φ2 is zero does not necessarily mean that the first temperature change amount φ2 is strictly zero. For example, the termination determination unit 41d sets a predetermined tolerance range based on zero. Then, when it is determined that the first temperature change amount φ2 is within the tolerance range, the termination determination unit 41d may determine that the first temperature change amount φ2 is zero.

[0120] <Third embodiment> For example, the soil moisture sensor 1A may be used in precision agriculture. When used in precision agriculture, the soil 100 near the soil moisture sensor 1A may contain crop roots. The temperature of the soil 100 affects the quality of the crops. Therefore, when using the soil moisture sensor 1A in precision agriculture, it is desirable to reduce the difference between the temperature of the soil 100 before measurement and the temperature of the soil 100 after measurement.

[0121] One factor that causes a change in the temperature of the soil 100 before and after measurement is Joule heat from the thermoelectric element 11. FIG. 11 shows the first temperature change amount φ2 (graph G11a) and the second temperature change amount θ2 (graph G11b) when switching from heating to cooling when the first temperature change amount φ2 reaches the upper threshold value φ4H, and when switching from cooling to heating when the first temperature change amount φ2 reaches the lower threshold value φ4L. Graphs G11a and G11b are actual measured values. In FIG. 11, graph G11c is the difference between the first temperature change amount φ2 and the second temperature change amount θ2. This difference indicates the temperature caused by Joule heat.

[0122] Focus on the first temperature change amount φ2 (graph G11a). Because the first temperature change amount φ2 is subject to control, it does not significantly deviate from the upper threshold value φ4H and the lower threshold value φ4L. On the other hand, the second temperature change amount θ2 (graph G11b) gradually increases in its extreme value during heating (see P11a and P11b in FIG. 11) with each repetition of heating and cooling. As a result, the second temperature change amount θ2 may become higher than the upper threshold value φ4H.

[0123] The phenomenon in which the extreme values P11a and P11b of the second temperature change amount θ2 gradually increase can be explained by the Joule heat generated by the thermoelectric element 11.

[0124] For example, assume that heat is emitted from the first heat input / output surface 11a and absorbed from the second heat input / output surface 11b. When a current is applied to the thermoelectric element 11, heat is emitted from the first heat input / output surface 11a due to the Peltier effect. Furthermore, when a current is applied to the thermoelectric element 11, Joule heat is generated due to the current. The Joule heat is emitted from the first heat input / output surface 11a. In other words, the first heat input / output surface 11a emits heat due to the Peltier effect and Joule heat.

[0125] On the other hand, when a current is applied to the thermoelectric element 11, heat due to the Peltier effect is absorbed from the second heat input / output surface 11b. However, Joule heat is also emitted from the second heat input / output surface 11b. In other words, the difference between the heat absorbed due to the Peltier effect and the Joule heat is the actual heat absorbed by the second heat input / output surface 11b.

[0126] As in the above, assume that heat is emitted from the first heat input / output surface 11a and absorbed from the second heat input / output surface 11b. Because the first heat input / output surface 11a emits Joule heat in addition to heat due to the Peltier effect, it reaches the upper threshold φ4H in a short time. When the first temperature change amount φ2 reaches the upper threshold φ4H, the second temperature change amount θ2 does not reach the lower threshold φ4L. This is because the second heat input / output surface 11b is absorbing heat due to the Peltier effect while emitting Joule heat.

[0127] Now, let's assume that the situation has switched to one in which heat is absorbed from the first heat input / output surface 11a and released from the second heat input / output surface 11b. The first heat input / output surface 11a absorbs heat based on the Peltier effect while emitting Joule heat. Therefore, it takes a considerable amount of time for the first temperature change φ2 to reach the lower threshold φ4L. Meanwhile, the second heat input / output surface 11b not only releases heat based on the Peltier effect, but also releases Joule heat. Therefore, the amount of released heat increases, and the temperature of the soil 100 on the second heat input / output surface 11b side is likely to rise.

[0128] Therefore, the second temperature change amount θ2, which is not subject to control, increases in extreme value each time heating and cooling are repeated.

[0129] The soil moisture sensor 1B, which will be described next, operates so that the extreme value of the second temperature change θ2, which is not subject to control, does not exceed the upper threshold φ4H. As a result, the soil moisture sensor 1B can reduce the difference between the temperature of the soil 100 before and after measurement.

[0130] In the soil moisture sensor 1A of the second embodiment, the upper threshold value φ4H is a constant value. In other words, the upper threshold value φ4H is a single value. The same is true for the lower threshold value φ4L. In contrast, the soil moisture sensor 1B employs multiple upper threshold values φ4Ha, φ4Hb, and φ4Hc that are different from one another. Furthermore, the soil moisture sensor 1B employs multiple lower threshold values φ4La and φ4Lb that are different from one another.

[0131] As shown in FIG. 12, the soil moisture sensor 1B has a sensor unit 10 and a control unit 20B. The sensor unit 10 is the same as the sensor unit 10 provided in the soil moisture sensor 1 of the first embodiment. Therefore, a detailed description of the sensor unit 10 will be omitted. The control unit 20B has a calculation device 30B and a power supply device 50B. The power supply device 50B is the same as the power supply device 50A provided in the soil moisture sensor 1A. Therefore, a detailed description of the power supply device 50B will be omitted.

[0132] The calculation device 30B has a first data logger 32B, a second data logger 33B, a memory unit 34B, and a moisture content acquisition unit 40B. The first data logger 32B and the second data logger 33B are the same as the first data logger 32A and the second data logger 33A provided in the soil moisture sensor 1A. Therefore, detailed description of the first data logger 32B and the second data logger 33B will be omitted.

[0133] The storage unit 34B stores the first temperature change amount φ2, the second temperature change amount θ2, the conversion information φ3, and the specified number of times φ5. The storage unit 34B also stores a threshold value φ4B. The moisture content acquisition unit 40B has a switching control unit 41B and a calculation processing unit 42B.

[0134] As shown in FIG. 13, the threshold value φ4B includes multiple upper threshold values φ4Ha, φ4Hb, and φ4Hc. In this embodiment, three upper threshold values are illustrated as an example, but the number of upper threshold values may be set appropriately depending on the specified number of switching operations φ5. Each of the upper threshold values φ4Ha, φ4Hb, and φ4Hc is also provided with information indicating that it will be used for the nth heating operation (n is an integer greater than or equal to 1 and less than or equal to N). For example, the upper threshold value φ4Ha is used for the first heating operation. The upper threshold value φ4Hb is used for the second heating operation. The upper threshold value φ4Hc is used for the third heating operation.

[0135] The threshold value φ4B further includes a plurality of lower threshold values φ4La and φ4Lb. In this embodiment, three lower threshold values are exemplified, but the number of lower threshold values may be set appropriately depending on the specified number of switching operations φ5. Each of the lower threshold values φ4La and φ4Lb is also provided with information indicating that it will be used for the nth cooling operation (n is an integer greater than or equal to 1 and less than or equal to N). For example, the lower threshold value φ4La is used for the first cooling operation. The lower threshold value φ4Lb is used for the second cooling operation.

[0136] Heating and cooling are performed alternately. Therefore, the five thresholds are used in the following order: upper threshold φ4Ha, lower threshold φ4La, upper threshold φ4Hb, lower threshold φ4Lb, and upper threshold φ4Hc.

[0137] The provision that the thresholds are different can be rephrased as the absolute values of the thresholds being different. The above five thresholds are set so that their absolute values become smaller and smaller.

[0138] More specifically, the absolute value of the lower threshold φ4La used after the upper threshold φ4Ha is smaller than the absolute value of the upper threshold φ4Ha. In other words, when heating and cooling are repeated, the absolute value of the threshold used during cooling is smaller than the absolute value of the threshold used during heating performed immediately before the cooling. As an example, the upper threshold φ4Ha may be +4 degrees, and the lower threshold φ4La may be -3 degrees.

[0139] In contrast, the absolute value of the upper threshold φ4Hb used after the lower threshold φ4La may be the same as the absolute value of the lower threshold φ4La. In other words, when heating and cooling are repeated, the absolute value of the threshold used during heating may be set to be the same as the absolute value of the threshold used for cooling performed immediately before that heating. As an example, the lower threshold φ4La may be -3 degrees, and the upper threshold φ4Hb may be +3 degrees.

[0140] In summary, the thresholds may be set as follows: Upper threshold φ4Ha: +4 degrees. Lower threshold φ4La: -3 degrees. Upper threshold φ4Hb: +3 degrees. Lower threshold φ4Lb: -2 degrees. Upper threshold φ4Hc: +2 degrees.

[0141] The switching determination unit 41a determines whether to switch between heating and cooling using five thresholds and the count value φ8a. The count value φ8a may be used to control which of the five thresholds is used for the first temperature change amount φ2.

[0142] The operations of the switching instruction unit 41b and the switching counting unit 41c are the same as those in the second embodiment, and therefore detailed descriptions thereof will be omitted. Furthermore, the operations of the calculation processing unit 42 are also the same as those in the second embodiment, and therefore detailed descriptions thereof will be omitted.

[0143] <Operation flow of the soil moisture sensor of the third embodiment> 14 shows the operational flow of the soil moisture sensor 1B of the third embodiment. The difference from the operational flow of the second embodiment is that after updating the count value φ8a (S15, S21), an operation of changing the threshold value (S15a, S21a) is added. The other steps are the same as those of the operational flow of the second embodiment, so detailed explanations will be omitted.

[0144] <Effects of the Third Embodiment> The soil moisture sensor 1B described above can obtain stable measurement results, similar to the soil moisture sensor 1.

[0145] The soil moisture sensor 1B reduces the absolute value of the threshold value each time heating and cooling are repeated, so that the second temperature change θ2 of the second heat input / output surface 11b, which is not the control target, does not exceed the highest upper threshold value φ4Ha.

[0146] Furthermore, the soil moisture sensor 1B reduces the absolute value of the threshold value with each cycle of heating and cooling. This operation gradually shortens the time it takes to reach the threshold value with each cycle of heating and cooling. As a result, the elapsed time φ8b required to reach the specified number of cycles φ5 can be shortened. Joule heat is proportional to the time that current flows. Therefore, the total amount of Joule heat generated by the thermoelectric element 11 during measurement operation can be reduced. Therefore, due to the above two actions, the soil moisture sensor 1B can preferably preserve the temperature state of the soil 100, which is the measurement target.

[0147] In the third embodiment, as in the second embodiment, whether to end the measurement is determined based on whether the switching count value φ8a has reached the specified number of times φ5. When determining whether to end the measurement, further conditions may be added. For example, the switching control unit 41B may continue heating or cooling when it is determined that the switching count value φ8a has reached the specified number of times φ5. Then, the switching control unit 41B may stop the supply of current to the thermoelectric element 11 when it is determined that the first temperature change amount φ2 and the second temperature change amount θ2 satisfy a predetermined condition.

[0148] Such an operation may be performed by the termination determination unit 41d shown in FIG. 12. The termination determination unit 41d receives the first temperature change amount φ2 and the second temperature change amount θ2. For example, when the termination determination unit 41d determines that the first temperature change amount φ2 and the second temperature change amount θ2 match, the termination determination unit 41d outputs a signal φ10 to the power supply device 50B to stop the supply of current to the thermoelectric element 11. The case where it can be determined that the first temperature change amount φ2 and the second temperature change amount θ2 match does not necessarily mean that the first temperature change amount φ2 and the second temperature change amount θ2 exactly match. For example, the termination determination unit 41d obtains the difference between the first temperature change amount φ2 and the second temperature change amount θ2. The case where the first temperature change amount φ2 and the second temperature change amount θ2 exactly match means that the difference is zero. When the difference is zero, it may be determined that the first temperature change amount φ2 and the second temperature change amount θ2 match. Furthermore, the termination determination unit 41d sets a predetermined tolerance range based on zero, and may determine that the first temperature change φ2 and the second temperature change θ2 match when it determines that the difference between the first temperature change φ2 and the second temperature change θ2 is within the tolerance range.

[0149] <Experimental Example 1 of the Soil Moisture Sensor of the Third Embodiment> An experiment was conducted to confirm the effectiveness of the soil moisture sensor 1B. In this experiment, it was confirmed that the soil moisture sensor 1B can control the temperature change amount that is not the object of control so that it does not deviate from the threshold value. The experimental conditions are as follows. Absolute value of voltage applied to thermoelectric element: 1.3V. Upper threshold φ4Ha: +4 degrees. Lower threshold φ4La: -3 degrees. Upper threshold φ4Hb: +3 degrees. Lower threshold φ4Lb: -2 degrees. Upper threshold φ4Hc: +2 degrees. Measurement target: simulated soil with 0% moisture content.

[0150] FIG. 15(a) shows the temperature history of the first temperature change amount φ2 and the second temperature change amount θ2. The horizontal axis shows the measurement time. The vertical axis shows the temperature change amount from the reference temperature TS. Graph G15a shows the first temperature change amount φ2. Graph G15b shows the second temperature change amount θ2. Graphs G15a and G15b are actual measured values. Graph G15c shows the temperature caused by Joule heat. The temperature caused by Joule heat is the difference between the first temperature change amount φ2 and the second temperature change amount θ2. Therefore, graph G15c is a calculated value.

[0151] The first temperature change amount φ2 (graph G15a) is the object of control. Therefore, the first temperature change amount φ2 (graph G15a) never exceeds the upper threshold value φ4Ha. The second temperature change amount θ2 (graph G15b) is not the object of control. However, by changing the threshold value, the second temperature change amount θ2 also never exceeds the upper threshold value φ4Ha. Therefore, it was found that the soil moisture sensor 1B can control the temperature change amount that is not the object of control so that it does not deviate from the threshold value.

[0152] <Experimental Example 2 of the Soil Moisture Sensor of the Third Embodiment> Another experiment (Experimental Example 2) was conducted to confirm the effect of the soil moisture sensor 1B. In this experiment, it was confirmed that the soil moisture sensor 1B can suppress the temperature rise caused by Joule heat at the end of measurement. The experimental conditions are as follows: Absolute value of voltage applied to thermoelectric element: 1.3V. Upper threshold φ4Ha: +4 degrees. Lower threshold φ4La: -3 degrees. Upper threshold φ4Hb: +3 degrees. Lower threshold φ4Lb: -2 degrees. Upper threshold φ4Hc: +2 degrees. Measurement object (A): Simulated soil with 0% moisture content. Measurement object (B): Simulated soil with a moisture content of 10%. Measurement object (C): Simulated soil with a moisture content of 30%. Measurement object (D): Simulated soil with a moisture content of 50%.

[0153] Furthermore, as a comparative example, an experiment was also carried out under the following conditions. Absolute value of voltage applied to thermoelectric element: 3V. Upper threshold φ4H: +4 degrees. Lower threshold φ4L: -4 degrees. Measurement object (A): Simulated soil with 0% moisture content. Measurement object (B): Simulated soil with a moisture content of 10%. Measurement object (C): Simulated soil with a moisture content of 30%. Measurement object (D): Simulated soil with a moisture content of 50%.

[0154] The horizontal axis of the graph in Fig. 15(b) represents the amount of moisture. The vertical axis of the graph in Fig. 15(b) represents the temperature TJ increased due to Joule heat. Graph G15d shows the results of Experimental Example 2. Graph G15e shows the results of the comparative example.

[0155] Referring to graph G15e, which is a comparative example, it was found that the temperature TJ was higher than +1 degree Celsius regardless of the moisture content. In contrast, referring to graph G15d, which is experimental example 2, it was found that the temperature TJ was lower than +1 degree Celsius regardless of the moisture content. Therefore, it was found that the soil moisture sensor 1B can suppress the rise in temperature caused by Joule heat at the end of measurement.

[0156] <Experimental Example 3 of the Soil Moisture Sensor of the Third Embodiment> Another experiment (Experimental Example 3) was conducted to confirm the effect of the soil moisture sensor 1B. In this experiment, it was confirmed that the soil moisture sensor 1C can improve the resolution for measuring the moisture content. The experimental conditions were the same as those of Experimental Example 2.

[0157] The horizontal axis in Fig. 16(a) represents the moisture content, and the vertical axis in Fig. 16(b) represents the time required to reach the threshold value, which is in other words the elapsed time φ8b.

[0158] Referring to graph G16b, which is a comparative example, it was found that the difference between the threshold arrival time when the moisture content was 0% and the threshold arrival time when the moisture content was 50% was approximately 0.1 minutes (6 seconds). In contrast, referring to graph G16a, which is experimental example 3, it was found that the difference between the threshold arrival time when the moisture content was 0% and the threshold arrival time when the moisture content was 50% was approximately 0.4 minutes (24 seconds). This large difference means that there is a large difference in the threshold arrival time depending on the moisture content. In other words, even a slight difference in moisture content can be perceived as a large change in the threshold arrival time. Therefore, it was found that the soil moisture sensor 1B can improve the resolution for measuring moisture content.

[0159] <Experimental Example 4 of the Soil Moisture Sensor of the Third Embodiment> Another experiment (Experimental Example 4) was conducted to confirm the effect of the soil moisture sensor 1B. In this experiment, it was confirmed that the resolution of the moisture content can be improved by increasing the number of times the sensor is heated and cooled. The experimental conditions are as follows: Absolute value of voltage applied to thermoelectric element: 1.3V. Upper threshold φ4Ha: +4 degrees. Lower threshold φ4La: -3 degrees. Upper threshold φ4Hb: +3 degrees. Lower threshold φ4Lb: -2 degrees. Upper threshold φ4Hc: +2 degrees. Measurement object (A): Simulated soil with 0% moisture content. Measurement object (D): Simulated soil with a moisture content of 50%.

[0160] The horizontal axis of Fig. 16(b) represents measurement time. The vertical axis of Fig. 16(b) represents temperature change. Graph G16c shows the history of temperature change in simulated soil with a moisture content of 0%. Graph G16d shows the history of temperature change in simulated soil with a moisture content of 50%.

[0161] First, in the first heating, the difference in time (Δt1) until the upper threshold φ4Ha was reached was found to be 0.12 minutes. Then, in the third heating, the difference in time (Δt2) until the upper threshold φ4Ha was reached was found to be 0.42 minutes. In other words, it was found that by increasing the number of times the heating and cooling were switched in one measurement operation, the difference in time until the specified number of times could be increased. Therefore, it was found that the resolution of the moisture content could also be improved by increasing the number of times the heating and cooling were switched.

[0162] <Fourth embodiment> The soil moisture sensor 1A of the second embodiment uses temperature as a switching condition for switching between heating and cooling. The soil moisture sensor 1A obtains the moisture content by utilizing the time required to perform a specified number of switching operations between heating and cooling. The soil moisture sensor 1C of the fourth embodiment uses time as a switching condition. The soil moisture sensor 1C obtains the moisture content by utilizing the extreme values of temperature that occur due to heating and cooling.

[0163] As shown in FIG. 17, the soil moisture sensor 1C has a sensor unit 10 and a control unit 20C. The sensor unit 10 is the same as the sensor unit 10 provided in the soil moisture sensor 1 of the first embodiment. Therefore, a detailed description of the sensor unit 10 will be omitted. The control unit 20C has a calculation device 30C and a power supply device 50C. The power supply device 50C is the same as the power supply device 50A provided in the soil moisture sensor 1A. Therefore, a detailed description of the power supply device 50C will be omitted.

[0164] The calculation device 30C has a first data logger 32C, a second data logger 33C, a memory unit 34C, and a moisture content acquisition unit 40C. The first data logger 32C and the second data logger 33C are the same as the first data logger 32A and the second data logger 33A provided in the soil moisture sensor 1A. Therefore, detailed description of the first data logger 32C and the second data logger 33C will be omitted.

[0165] The storage unit 34C stores the first temperature change amount φ2, the second temperature change amount θ2, conversion information φ3C, and the specified number of times φ5. The storage unit 34B also stores a switching time φ4C. The switching time φ4C includes a heating maintenance time φ4Ca and a cooling maintenance time φ4Cb. In the following description, the heating maintenance time φ4Ca is the heating time on the second heat input / output surface 11b. Similarly, the cooling maintenance time φ4Cb is the cooling time on the second heat input / output surface 11b. Note that when the second heat input / output surface 11b is in heating mode, the first heat input / output surface 11a is in cooling mode. Therefore, the heating maintenance time φ4Ca is the cooling maintenance time from the perspective of the first heat input / output surface 11a. Similarly, the cooling maintenance time φ4Cb is the heating maintenance time from the perspective of the first heat input / output surface 11a.

[0166] The soil moisture sensor 1C converts the temperature change amount into the moisture content. The conversion information φ3C includes information for converting the temperature change amount into the moisture content. The conversion information φ3C may be, for example, a function with the temperature change amount as the independent variable and the moisture content as the dependent variable.

[0167] For example, during a period in which heating and cooling are repeated, the soil moisture sensor 1C switches to cooling when the heating maintenance time φ4Ca has elapsed after starting heating of the second heat input / output surface 11b.The soil moisture sensor 1C switches to heating when the cooling maintenance time φ4Cb has elapsed after starting cooling of the second heat input / output surface 11b.

[0168] The moisture content acquisition unit 40C has a switching control unit 41C and a calculation processing unit 42C. The switching control unit 41C uses the heating maintenance time φ4Ca and the cooling maintenance time φ4Cb to obtain control signals φ7a and φ7b to be provided to the power supply device 50C. The calculation processing unit 42C uses the first temperature change amount φ2 or the second temperature change amount θ2 to obtain the moisture content φ9.

[0169] The switching determination unit 41a of the switching control unit 41C receives the heating maintenance time φ4Ca and the cooling maintenance time φ4Cb. The switching determination unit 41a determines whether the elapsed time has reached the heating maintenance time φ4Ca at a predetermined timing during the execution of the heating operation of the second heat input / output surface 11b. Similarly, the switching determination unit 41a determines whether the elapsed time has reached the cooling maintenance time φ4Cb at a predetermined timing during the execution of the cooling operation of the second heat input / output surface 11b. The switching determination unit 41a outputs a determination result φ6 when the elapsed time has reached the heating maintenance time φ4Ca or when the elapsed time has reached the cooling maintenance time φ4Cb.

[0170] The switching instruction unit 41b and the switching counting unit 41c are the same as those in the second embodiment, and therefore detailed description thereof will be omitted. Note that the soil moisture sensor 1C does not use the elapsed time φ8b to obtain the moisture content. Therefore, the switching counting unit 41c of the soil moisture sensor 1C does not need to output the elapsed time φ8b.

[0171] The calculation processing unit 42C has a preprocessing unit 42s and a conversion unit 42a. The preprocessing unit 42s receives the first temperature change amount φ2. The preprocessing unit 42s obtains a preprocessed value φ2s using the first temperature change amount φ2. The preprocessing unit 42s outputs the preprocessed value φ2s to the conversion unit 42a. The conversion unit 42a receives the preprocessed value φ2s and conversion information φ3C. The conversion unit 42a converts the preprocessed value φ2s into a moisture content φ9 using the conversion information φ3C.

[0172] Here, the pretreatment value φ2s will be explained. The pretreatment value φ2s is the amount of temperature change. FIG. 18 shows the change in the amount of temperature change over time when switching between heating and cooling every time a certain amount of time has passed. The horizontal axis of FIG. 18 represents time. The vertical axis of FIG. 18 represents the amount of temperature change. Graph G18a shows the first amount of temperature change φ2. Graph G18b shows the second amount of temperature change θ2.

[0173] Graphs G18a and G18b show extreme values P18a, P18b, and P18c on the high-temperature side. The extreme values P18a and P18c on graph G18a represent the temperature of the first heat input / output surface 11a. The extreme value P18b on graph G18b represents the temperature of the second heat input / output surface 11b. For example, the highest extreme value P18c of the extreme values P18a, P18b, and P18c may be used as the pretreatment value φ2s. Alternatively, the total value of the extreme values P18a, P18b, and P18c may be used as the pretreatment value φ2s. Alternatively, the average value of the extreme values P18a, P18b, and P18c may be used as the pretreatment value φ2s.

[0174] The pretreatment value φ2s may be a value based on the extreme values P18d, P18e, and P18f on the low-temperature side that exist in the graphs G18a and G18b. The extreme value P18e in the graph G18a is the temperature of the first heat input / output surface 11a. The extreme values P18d and P18f in the graph G18b are the temperatures of the second heat input / output surface 11b. For example, the lowest extreme value P18d among the extreme values P18d, P18e, and P18f may be used as the pretreatment value φ2s. Alternatively, the total value of the extreme values P18d, P18e, and P18f may be used as the pretreatment value φ2s. Furthermore, the average value of the extreme values P18d, P18e, and P18f may be used as the pretreatment value φ2s.

[0175] Furthermore, the pretreatment value φ2s may be a value obtained as the difference between the first temperature change amount φ2 and the second temperature change amount θ2. In Fig. 18, the difference is shown as graph G18c.

[0176] <Operation flow of the soil moisture sensor of the fourth embodiment> 19 shows the operation flow of the soil moisture sensor 1C of the fourth embodiment. First, the heating maintenance time φ4Ca is set (S51). Next, the cooling maintenance time φ4Cb is set (S52). Next, the supply of current is started (S53). When the supply of current starts, the elapsed heating time is also counted.

[0177] Next, it is determined whether the elapsed heating time has reached the heating maintenance time φ4Ca (S54). If it is determined that the elapsed heating time has not reached the heating maintenance time φ4Ca (S54: NO), S54 is executed again after a predetermined time has elapsed. If it is determined that the elapsed heating time has reached the heating maintenance time φ4Ca (S54: YES), the direction of the current is switched (S55). When the direction of the current is switched, counting of the elapsed cooling time is started as a new elapsed time. Next, the switching count value φ8a is updated (S56). Next, it is determined whether the number of switchings has reached the specified number of times φ5 (S57). If it is determined that the number of switchings has reached the specified number of times φ5 (S57: YES), the process proceeds to the process of obtaining the moisture content (S63, S64, S65).

[0178] If it is determined that the number of switching times has not reached the specified number φ5 (S57: NO), it is determined whether the elapsed cooling time has reached the cooling maintenance time φ4Cb (S54). If it is determined that the elapsed cooling time has not reached the cooling maintenance time φ4Cb (S58: NO), S58 is executed again after a predetermined time has elapsed. If it is determined that the elapsed cooling time has reached the cooling maintenance time φ4Cb (S58: YES), the direction of the current is switched (S59). When the direction of the current is switched, counting the elapsed heating time begins as a new elapsed time.

[0179] Next, the switching count value φ8a is updated (S61). Next, it is determined whether the number of switchings has reached the specified number of times φ5 (S62). If it is determined that the number of switchings has reached the specified number of times φ5 (S62: YES), the process proceeds to the process of obtaining the moisture content (S63, S64, S65). If it is determined that the number of switchings has not reached the specified number of times φ5 (S62: NO), it is again determined whether the elapsed heating time has reached the heating maintenance time φ4Ca (S54).

[0180] In the process of obtaining the moisture content φ9, first, the supply of current is stopped (S63). Next, a pre-processing value φ2s is obtained. Then, the pre-processing value φ2s is converted into the moisture content φ9 (S65).

[0181] In the fourth embodiment, the determination of whether to terminate measurement may be made using the same method as in the third embodiment. That is, the termination determination unit 41d may terminate measurement on the condition that the first temperature change amount φ2 and the second temperature change amount θ2 match, as shown by point P18g in FIG. 18. As described in the third embodiment, the match between the first temperature change amount φ2 and the second temperature change amount θ2 does not require a strict match. When it is determined that the difference between the first temperature change amount φ2 and the second temperature change amount θ2 is within the allowable range, it may be determined that the first temperature change amount φ2 and the second temperature change amount θ2 match.

[0182] <Effects of the Fourth Embodiment> Like the soil moisture sensor 1, the soil moisture sensor 1C described above can obtain stable measurement results by obtaining information about moisture using the amount of temperature change caused by specific heat.

[0183] The soil moisture sensor 1C switches between heating and cooling every time a predetermined time elapses. The first temperature change φ2 and second temperature change θ2 (see FIG. 18) obtained by this operation can be corrected to remove the temperature effects caused by Joule heat. For example, the extreme values P18a, P18b, and P18c included in graph G18a in FIG. 18 increase in a roughly linear fashion over time. This increase is expected to be due to the effects of Joule heat. Therefore, an approximation line connecting the extreme values P18a, P18b, and P18c is set. Then, a function representing the approximation line is obtained. By correcting the first temperature change φ2 using this function, temperature history information can be obtained with the effects of Joule heat removed. [Explanation of symbols]

[0184] 1, 1A, 1B, 1C... soil moisture sensor, 10... sensor unit, 11... thermoelectric element (temperature control unit), 11a... first heat input / output surface, 11b... second heat input / output surface, 12... first temperature measurement unit, 13... second temperature measurement unit, 20, 20A, 20B, 20C... control unit, 30, 30A, 30B, 30C... calculation device (calculation unit), 31... voltage control unit, 32... first data logger, 33... second data logger, 34... storage Memory unit, 35...slope information acquisition unit, 36...equilibrium temperature acquisition unit, 37...conversion unit, 40A, 40B, 40C...moisture content acquisition unit, 41A, 41B, 41C...switching control unit, 41a...switching determination unit, 41b...switching instruction unit, 41c...switching counting unit, 42A, 42B, 42C...calculation processing unit, 42a...conversion unit, 42s...preprocessing unit, 50...power supply (power supply unit), M1...first temperature information acquisition unit, M2...second temperature information acquisition unit, 100...soil.

Claims

1. A moisture sensor that obtains moisture information regarding the moisture contained in a measurement object, a temperature control unit disposed on the measurement object and capable of changing the temperature of the measurement object; a temperature information acquiring unit that acquires temperature information indicating a change in temperature of the measurement object over time due to the operation of the temperature control unit; a calculation unit that obtains the moisture information based on the temperature information, The calculation unit a gradient information acquisition unit that acquires a temperature change amount per unit time using the temperature information; a conversion unit that converts the temperature change amount into the moisture information using conversion information that indicates the relationship between the temperature change amount and the moisture information, the temperature control unit is a thermoelectric element that converts electrical energy into thermal energy, the thermoelectric element includes a first thermal input / output surface and a second thermal input / output surface that are in contact with the measurement object and provide heat to the measurement object and remove heat from the measurement object, and is capable of providing heat to the measurement object from one of the first thermal input / output surface and the second thermal input / output surface and removing heat from the measurement object from the other of the first thermal input / output surface and the second thermal input / output surface, the temperature information acquisition unit includes a first temperature measurement unit attached to the first heat input / output surface and a second temperature measurement unit attached to the second heat input / output surface; the gradient information acquisition unit acquires a first temperature change amount by using first temperature information acquired from the first temperature measurement unit, and acquires a second temperature change amount by using second temperature information acquired from the second temperature measurement unit; The conversion unit obtains the moisture information by using the conversion information, the first temperature change amount, and the second temperature change amount.

2. (delete)

3. (delete)

4. (delete)

5. The moisture sensor according to claim 1 , wherein the gradient information acquisition unit obtains the temperature change amount by using a difference between a first temperature and a second temperature during a transient period of the temperature change over time of the measurement object.

6. The moisture sensor according to claim 5 , wherein the temperature information acquisition unit is disposed so as to be in contact with the object to be measured.

7. A moisture sensor that obtains moisture information regarding the moisture contained in a measurement object, a thermoelectric element disposed on the measurement object and capable of changing the temperature of the measurement object; a power supply unit that supplies a current to the thermoelectric element and is capable of switching the direction of the current supplied to the thermoelectric element; a temperature information acquisition unit for acquiring the temperature of the measurement object; a moisture content acquisition unit that uses the temperature acquired by the temperature information acquisition unit to control the direction of the current that the power supply unit applies to the thermoelectric element and acquires the moisture information, the moisture content acquisition unit has a switching control unit that controls the direction of the current that the power supply unit applies to the thermoelectric element by using the temperature of the measurement object; The switching control unit performs a switching operation N times (N is an integer equal to or greater than 1), The moisture amount acquisition unit obtains the moisture information by utilizing the elapsed time required for the N number of switching operations.

8. (delete)

9. The moisture sensor according to claim 7, wherein the switching control unit performs a switching operation to switch the direction of the current supplied from the power supply unit to the thermoelectric element when the temperature of the object to be measured reaches at least one predetermined threshold value.

10. (delete)

11. The moisture sensor according to claim 9 , wherein the switching control unit controls the direction of the current supplied from the power supply unit to the thermoelectric element by using one predetermined threshold value.

12. The moisture sensor according to claim 9 , wherein the switching control unit controls the direction of the current supplied from the power supply unit to the thermoelectric element by using a plurality of predetermined threshold values that are different from one another.

13. 10. The moisture sensor according to claim 9, wherein an absolute value of the threshold value used when cooling the measurement object is smaller than an absolute value of the threshold value used when heating the measurement object.

14. the thermoelectric element includes a first thermal input / output surface and a second thermal input / output surface that are in contact with the measurement object and provide heat to the measurement object and remove heat from the measurement object, the temperature information acquisition unit includes a first temperature measurement unit attached to the first heat input / output surface, the plurality of threshold values include one or more upper threshold values that are higher than the temperature of the measurement object before the start of measurement, and one or more lower threshold values that are lower than the temperature of the measurement object before the start of measurement, The moisture sensor according to any one of claims 9, 11 to 13, wherein the switching control unit switches the direction of the current when the temperature acquired by the first temperature measuring unit reaches the upper threshold or the lower threshold.

15. the thermoelectric element includes a first thermal input / output surface and a second thermal input / output surface that are in contact with the measurement object and provide heat to the measurement object and remove heat from the measurement object, the temperature information acquisition unit includes a first temperature measurement unit attached to the first heat input / output surface and a second temperature measurement unit attached to the second heat input / output surface; the plurality of threshold values include one or more upper threshold values that are higher than the temperature of the measurement object before the start of measurement, A moisture sensor as described in any one of claims 9, 11 to 13, wherein the switching control unit switches the direction of the current when the temperature acquired by the first temperature measuring unit reaches the upper threshold value or when the temperature acquired by the second temperature measuring unit reaches the upper threshold value.

16. the temperature information acquisition unit acquires the temperature of the measurement object before measurement starts as a reference temperature; The moisture sensor of claim 15, wherein the switching control unit stops the supply of current from the power supply unit to the thermoelectric element when it determines that the temperature acquired by the first temperature measuring unit has reached the reference temperature while the power supply unit is supplying current to the thermoelectric element.

17. the thermoelectric element includes a first thermal input / output surface and a second thermal input / output surface that are in contact with the measurement object and provide heat to the measurement object and remove heat from the measurement object, the temperature information acquisition unit includes a first temperature measurement unit attached to the first heat input / output surface and a second temperature measurement unit attached to a second heat input / output surface opposite to the first heat input / output surface, the plurality of threshold values include one or more lower threshold values that are lower than the temperature of the measurement object before the start of measurement, A moisture sensor as described in any one of claims 9, 11 to 13, wherein the switching control unit switches the direction of the current when the temperature acquired by the first temperature measuring unit reaches the lower threshold value or when the temperature acquired by the second temperature measuring unit reaches the lower threshold value.

18. The moisture sensor of claim 17, wherein the switching control unit stops the supply of current from the power supply unit to the thermoelectric element when it is determined that the temperature acquired by the first temperature measuring unit and the temperature acquired by the second temperature measuring unit match.

19. (delete)

20. A moisture sensor that obtains moisture information regarding the moisture contained in a measurement object, a thermoelectric element disposed on the measurement object and capable of changing the temperature of the measurement object; a power supply unit that supplies a current to the thermoelectric element and is capable of switching the direction of the current supplied to the thermoelectric element; a temperature information acquisition unit for acquiring the temperature of the measurement object; a moisture content acquisition unit that uses the temperature acquired by the temperature information acquisition unit to control the direction of the current that the power supply unit applies to the thermoelectric element and acquires the moisture information, the moisture content acquisition unit has a switching control unit that controls the direction of the current that the power supply unit applies to the thermoelectric element every time a predetermined switching time elapses; The moisture amount acquisition unit obtains the moisture information by utilizing at least one extreme value of the temperature of the object to be measured.

21. the thermoelectric element includes a first thermal input / output surface and a second thermal input / output surface that are in contact with the measurement object and provide heat to the measurement object and remove heat from the measurement object, the temperature information acquisition unit includes a first temperature measurement unit attached to the first heat input / output surface and a second temperature measurement unit attached to the second heat input / output surface; The moisture sensor of claim 20, wherein the switching control unit stops the supply of current from the power supply unit to the thermoelectric element when it is determined that the temperature acquired by the first temperature measuring unit and the temperature acquired by the second temperature measuring unit match.

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