Liquid sensor and detection method

Abstract

To provide a liquid sensor and a detection method that can detect a parameter having a correlation with the degree of deterioration of liquid.SOLUTION: A liquid sensor comprises a pair of electrodes and a control unit. The pair of electrodes is immersed in liquid for use. The control unit sweeps the frequency of an AC voltage applied between the pair of electrodes. The control unit detects the impedance of the liquid for every frequency, and calculates a second parameter that is related to a value having a correlation with the impedance or a first parameter that is the impedance. The second parameter is the difference between a first degree of change and a second degree of change. The first degree of change and the second degree of change each indicate the amount of change in the first parameter relative to the amount of change in the frequency. The liquid sensor further comprises a storage unit. The storage unit stores relationship information indicating the correspondence between a third parameter having a correlation with the degree of deterioration of liquid and the second parameter. The control unit detects the third parameter on the basis of the second parameter and the relationship information.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a liquid sensor and a detection method. [Background technology]

[0002] Japanese Patent No. 6910037 (Patent Document 1) discloses an oil state determination system, which measures the resistance value of the oil and determines that the oxidation state of the oil has changed when the change in the resistance value of the oil reverses from a decreasing trend to an increasing trend (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6910037 Summary of the Invention [Problem to be solved by the invention]

[0004] The oil state determination system disclosed in the above-mentioned Patent Document 1 determines the change in the oxidation state of oil (an example of a "liquid"), but does not detect a parameter that has a correlation with the degree of deterioration of the oil.

[0005] The present invention has been made to solve such problems, and an object of the present invention is to provide a liquid sensor and a detection method capable of detecting a parameter that has a correlation with the degree of deterioration of the liquid. [Means for solving the problem]

[0006] A liquid sensor according to an aspect of the present invention includes a pair of electrodes and a control unit. The pair of electrodes are used in a state where they are immersed in a liquid. The control unit sweeps the frequency of an AC voltage applied between the pair of electrodes. The control unit detects the impedance of the liquid for each frequency, and calculates a second parameter related to a first parameter, which is a value correlated with the impedance or the impedance. The second parameter is a difference between the first rate of change and the second rate of change. Each of the first rate of change and the second rate of change indicates an amount of change in the first parameter relative to an amount of change in frequency. The liquid sensor further includes a memory unit. The memory unit stores relationship information indicating a correspondence relationship between the second parameter and a third parameter correlated with a degree of deterioration of the liquid. The control unit detects the third parameter based on the second parameter and the relationship information.

[0007] The inventor(s) found that a third parameter having a correlation with the deterioration degree of the liquid has a relatively high correlation with the second parameter. In this liquid sensor, the third parameter is detected based on the relationship information indicating the correspondence between the third parameter and the second parameter and the calculated second parameter. Therefore, according to this liquid sensor, the third parameter can be detected with a relatively high accuracy.

[0008] In the above liquid sensor, the first parameter may be a resistance value of the liquid or a capacitance of the liquid.

[0009] In the above liquid sensor, the third parameter may be a base number of the liquid or an acid number of the liquid.

[0010] The liquid sensor may further include a temperature sensor that detects the temperature of the liquid, and the relationship information may be provided for each temperature of the liquid, and the control unit may detect a third parameter based on the temperature of the liquid, the relationship information corresponding to the temperature of the liquid, and the second parameter.

[0011] The inventor(s) found that the relationship between the third parameter and the second parameter may be affected by the temperature of the liquid. In this liquid sensor, the third parameter is detected based on the temperature of the liquid, relationship information corresponding to the temperature of the liquid, and the second parameter. Therefore, according to this liquid sensor, the third parameter is detected taking into account the temperature of the liquid, so that the third parameter can be detected with higher accuracy.

[0012] In the above liquid sensor, the control unit may switch between using the resistance value and the capacitance as the first parameter depending on the detected value of the third parameter.

[0013] The inventor(s) found that which of the second parameter calculated based on the resistance value and the second parameter calculated based on the capacitance has a higher correlation with the third parameter varies depending on the value of the third parameter. In this liquid sensor, whether the resistance value or the capacitance is used as the first parameter is switched depending on the value of the detected third parameter. Therefore, according to this liquid sensor, the second parameter having a higher correlation with the third parameter is used to detect the third parameter, so that the third parameter can be detected with higher accuracy.

[0014] A detection method according to another aspect of the present invention includes the steps of sweeping the frequency of an AC voltage applied between a pair of electrodes immersed in a liquid, detecting the impedance of the liquid for each frequency, and calculating a second parameter related to a first parameter that is a value correlated with the impedance or the impedance. The second parameter is a difference between the first rate of change and the second rate of change. Each of the first rate of change and the second rate of change indicates an amount of change in the first parameter relative to an amount of change in frequency. The detection method further includes the steps of storing relationship information indicating a correspondence between the second parameter and a third parameter correlated with a deterioration level of the liquid, and detecting the third parameter based on the second parameter and the relationship information.

[0015] In this detection method, the third parameter is detected based on the relationship information indicating the correspondence between the third parameter and the second parameter and the calculated second parameter, and therefore, according to this detection method, the third parameter can be detected with relatively high accuracy. Effect of the Invention

[0016] According to the present invention, it is possible to provide a liquid sensor and a detection method capable of detecting a parameter that has a correlation with the degree of deterioration of a liquid. [Brief description of the drawings]

[0017] [Figure 1] FIG. 2 is a diagram illustrating a configuration of a liquid sensor. [Diagram 2] FIG. 2 is a diagram showing a schematic parallel equivalent circuit of a liquid. [Diagram 3] 1 is a diagram illustrating an example of a relationship between an AC voltage (input signal) applied between a pair of electrodes and a current (measurement signal) generated in a circuit including the pair of electrodes. [Figure 4] FIG. 4 is a diagram illustrating the relationship between impedance, resistance value, capacitance, and phase difference. [Diagram 5] 13 is a flowchart showing an example of a procedure for generating relationship information. [Figure 6] FIG. 10 is a diagram illustrating an example of a graph generated through detection of resistance values. [Figure 7] FIG. 4 is a diagram for explaining a method of calculating first and second change rates. [Figure 8] FIG. 13 is a diagram showing an example of the relationship between the base number (estimated value) of a liquid detected using relationship information and the analytical value of the base number of the liquid. [Figure 9] FIG. 13 is a diagram showing an example of the relationship between the base number (estimated value) of a liquid detected using relationship information and the analytical value of the base number of the liquid, in a case where one degree of change is used as a second parameter. [Figure 10] 4 is a flowchart showing a procedure for detecting the base number of a liquid in the liquid sensor. [Figure 11]FIG. 13 is a diagram showing an example of the relationship between the acid value (estimated value) of a liquid detected using relationship information and the analysis value of the acid value of the liquid, in a case where the acid value of the liquid is used as a third parameter. [Figure 12] 13 is a flowchart showing a procedure for identifying relationship information used to detect a third parameter. [Figure 13] FIG. 11 is a diagram for explaining an image regarding a change of a first parameter. [Figure 14] 10 is a flowchart showing another example of the procedure for detecting the base number of a liquid in the liquid sensor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, an embodiment according to one aspect of the present invention (hereinafter also referred to as "the present embodiment") will be described in detail with reference to the drawings. Note that the same or corresponding parts in the drawings are given the same reference numerals and their description will not be repeated. Also, each drawing is drawn in a schematic manner with objects appropriately omitted or exaggerated for ease of understanding.

[0019] [1. Liquid sensor configuration] FIG. 1 is a diagram showing a schematic configuration of a liquid sensor 10 according to the present embodiment. The liquid sensor 10 is attached in a tank 20, and is configured to detect a parameter (e.g., base number) that correlates with the degree of deterioration of a liquid L1 (e.g., oil). For example, the liquid sensor 10 is attached in an oil tank or a pipe of a vehicle or the like, and detects the base number of the oil. The base number of the oil correlates with the degree of deterioration of the oil. The liquid sensor 10 is used in a state where at least a portion is immersed in the liquid L1.

[0020] As shown in FIG. 1, the liquid sensor 10 includes a detection system 100 and a substrate 110. When the liquid sensor 10 is used, the substrate 110 is immersed in a liquid L1. The substrate 110 has, for example, a substantially rectangular shape in a plan view. The substrate 110 includes a substrate body 111, a pair of electrodes 112, and a temperature sensor 113. The substrate body 111 is a so-called fluororesin substrate. Since a fluororesin substrate has excellent weather resistance and chemical resistance, the substrate 110 including the substrate body 111 can withstand use in harsh environments. Note that the substrate body 111 does not necessarily have to be made of a fluororesin substrate, but is preferably made of a substrate having excellent chemical resistance, for example.

[0021] The pair of electrodes 112 are formed on the substrate body 111. The pair of electrodes 112 are used to detect the impedance of the liquid L1. Each electrode 112 has a comb-like shape. On the substrate 110, the pair of electrodes 112 are arranged such that the teeth of each electrode 112 are positioned alternately. The pair of electrodes 112 are formed, for example, by patterning a conductive layer formed on one surface of the substrate body 111.

[0022] The temperature sensor 113 is mounted on the substrate body 111. The temperature sensor 113 is used to detect the temperature of the liquid L1. The temperature sensor 113 is configured with a temperature detection element such as a resistance temperature detector (RTD), a thermistor, or a thermocouple. Note that the temperature sensor 113 is not necessarily required.

[0023] The detection system 100 includes a memory unit 106, a measurement unit 102, a control unit 104, and a notification unit 108. The memory unit 106 is configured, for example, with an auxiliary storage device such as a flash memory. The memory unit 106 stores, for example, "related information" used in detecting the base number of the liquid L1. The related information will be described in detail later.

[0024] The measuring unit 102 includes, for example, a power source and an ammeter. The measuring unit 102 applies a measurement voltage (AC voltage) between the pair of electrodes 112 and measures a current generated in a circuit including the pair of electrodes 112. The measuring unit 102 also applies a measurement voltage to the temperature sensor 113 and measures a current generated in the circuit including the temperature sensor 113, for example.

[0025] The control unit 104 includes, for example, an arithmetic circuit, a CPU (Central Processing Unit), a RAM (Random Access Memory), and a ROM (Read Only Memory). The control unit 104 detects the impedance between the pair of electrodes 112 based on, for example, the value of a voltage applied between the pair of electrodes 112 and the value of a current generated in a circuit including the pair of electrodes 112. When the pair of electrodes 112 is entirely immersed in the liquid L1, the impedance between the pair of electrodes 112 is regarded as the impedance of the liquid L1.

[0026] Although details will be described later, the control unit 104 uses the electrical resistance value (hereinafter also simply referred to as "resistance value") of the liquid L1 to detect the base number of the liquid L1. The control unit 104 calculates the resistance value of the liquid L1 based on the impedance of the liquid L1. Next, an example of a procedure for calculating the resistance value of the liquid L1 based on the impedance of the liquid L1 will be described.

[0027] Fig. 2 is a schematic diagram showing a parallel equivalent circuit of liquid L1. As shown in Fig. 2, liquid L1 can be considered to be electrically equivalent to a circuit in which a resistor and a capacitor are connected in parallel. In other words, the impedance Z1 of liquid L1 can be separated into a resistance value R1 and a capacitance C1.

[0028] 3 is a diagram showing an example of a relationship between an AC voltage (input signal) applied between a pair of electrodes 112 and a current (measurement signal) generated in a circuit including the pair of electrodes 112. With reference to FIG. 3, the horizontal axis indicates time, and the vertical axis indicates a voltage value or a current value. In this example, the difference between the phase of the input signal and the phase of the measurement signal is θ. The control unit 104 detects the phase difference θ based on the input signal and the measurement signal.

[0029] Fig. 4 is a diagram showing a schematic relationship between impedance Z1, resistance value R1, capacitance C1, and phase difference θ. Referring to Fig. 4, the control unit 104 calculates the resistance value R1 by multiplying the detected impedance Z1 by cos θ. Through this procedure, the control unit 104 calculates the resistance value R1 of the liquid L1 based on the impedance Z1 of the liquid L1. The details will be described later, but the control unit 104 detects the base number of the liquid L1 by using the resistance value R1 of the liquid L1 and related information, etc.

[0030] 1 again, the control unit 104 detects the resistance value of the temperature sensor 113, for example, based on the voltage applied to the temperature sensor 113 and the current generated in a circuit including the temperature sensor 113. The control unit 104 detects the temperature of the liquid L1 based on the resistance value of the temperature sensor 113. The memory unit 106 stores in advance, for example, the relationship between the temperature and the resistance value.

[0031] The notification unit 108 includes, for example, a display. For example, the notification unit 108 displays an image indicating the base value detected by the control unit 104. In this way, the base value of the liquid L1 is notified to the user.

[0032] [2. Detection of base value using relational information] For example, there is a technique for detecting a change in the oxidation state of oil (an example of the liquid L1) and determining the timing of oil replacement. However, depending on such a technique, a parameter (for example, base number) having a correlation with the degree of deterioration of the liquid L1 is not directly detected. On the other hand, if a parameter having a correlation with the degree of deterioration of the liquid L1 is directly detected, for example, it becomes possible to determine the replacement timing of the liquid L1 with higher accuracy.

[0033] The first parameter, the second parameter, and the third parameter are defined as follows. The first parameter is a value having a correlation with the impedance Z1 of the liquid L1 or a parameter corresponding to the impedance Z1 itself (for example, the resistance value R1 of the liquid L1). The second parameter is the difference between the first change degree and the second change degree. The control unit 104 is configured to sweep the frequency of the measurement voltage applied between the pair of electrodes 112. The control unit 104 is configured to detect the resistance value R1 of the liquid L1 when the measurement voltage of each of a plurality of predetermined frequencies (for example, frequencies f1, f2, f3, f4 (f1 < f2 < f3 < f4)) is applied. Each of the first change degree and the second change degree indicates the change amount of the first parameter with respect to the change amount of the frequency (for example, the first change degree is the change amount of the first parameter with respect to the frequency f4 - (minus) frequency f1, and the second change degree is the change amount of the first parameter with respect to the frequency f3 - (minus) frequency f2).). The third parameter is a parameter having a correlation with the degree of deterioration of the liquid L1 (for example, the base number of the liquid L1).

[0034] The present inventor(s) have found that the third parameter having a correlation with the degree of deterioration of the liquid L1 and the second parameter have a relatively high correlation. In the liquid sensor 10 according to the present embodiment, the relationship information indicates the correspondence relationship between the third parameter and the second parameter, and the third parameter is detected based on the calculated second parameter and the relationship information. Therefore, according to the liquid sensor 10, the third parameter can be detected with relatively high accuracy. As described above, the relationship information is generated in advance and stored in the storage unit 106. The generation procedure of the relationship information will be described next.

[0035] 5 is a flowchart showing an example of a procedure for generating relationship information. Each step shown in this flowchart is performed by the control unit 104 or an operator. Note that a plurality of samples (liquids L1) each having a different base number are prepared in advance, and the steps shown in this flowchart are started with any one of the plurality of samples in the tank 20 (FIG. 1). Also, the actual base number of each of the plurality of samples is measured in advance.

[0036] Referring to FIG. 5, the control unit 104 detects the resistance value R1 of the liquid L1 with a measurement voltage of each frequency applied between a pair of electrodes 112 (step S100), and completes the detection of the resistance value at all predetermined frequencies for all samples (step S110).

[0037] FIG. 6 is a diagram showing an example of a graph generated through detection of the resistance value R1 of the liquid L1. This diagram shows the relationship between the frequency of the measurement voltage and the resistance value R1 for a specific sample. With reference to FIG. 6, the horizontal axis indicates the frequency of the measurement voltage, and the vertical axis indicates the detected resistance value R1. Each point P1 indicates the resistance value R1 of a specific sample (liquid L1) when a measurement voltage of a specific frequency is applied between a pair of electrodes 112. By repeating the sweep of the measurement voltage and the detection of the resistance value R1 for each sample, a graph such as that shown in FIG. 6 is generated for each of all the samples.

[0038] FIG. 7 is a diagram for explaining the first and second change rates. Referring to FIG. 7, as described above, each point P1 indicates the resistance value R1 of a specific sample (liquid L1) when a measurement voltage of a specific frequency is applied between a pair of electrodes 112. The change rate DC1 is an example of the first change rate, and the change rate DC2 is an example of the second change rate. Each of the change rates DC1 and DC2 indicates the change rate of the resistance value R1 (first parameter) relative to the change rate of the frequency of the measurement voltage. In this example, the change rate DC1 indicates the change rate of the resistance value R1 relative to the frequency f3-(negative) frequency f2, and the change rate DC2 indicates the change rate of the resistance value R1 relative to the frequency f4-(negative) frequency f1. Each of the change rates DC1 and DC2 can be said to be the slope of a straight line connecting any two points each indicating the correspondence relationship between the frequency of the measurement voltage and the resistance value R1.

[0039] 5 again, the operator plots the difference between the first and second change rates in a specific combination of the first and second change rates for all samples (step S120). This generates a graph showing the relationship between the difference between the first and second change rates and the base number of the liquid L1. The operator completes the plot of the difference between the first and second change rates in a specific combination of the first and second change rates for all samples for all combinations of the first and second change rates (step S130). This generates a graph showing the relationship between the difference between the first and second change rates and the base number of the liquid L1 for each combination of the first and second change rates.

[0040] The worker calculates a correlation coefficient based on each generated graph (each plot result) (step S140). For example, various known methods are used to calculate the correlation coefficient. For example, the correlation coefficient is calculated by using a computer. The worker generates "relationship information" based on the graph with the maximum correlation coefficient (step S150). The relationship information is generated by such a procedure. The relationship information may be, for example, a linear function, a quadratic function, or a cubic or higher degree function. Note that the relationship information indicates a correspondence relationship between the difference between the first and second change rates in the combination of the first and second change rates used to generate the graph with the maximum correlation coefficient and the base value of the liquid L1 (an example of a third parameter). That is, in the relationship information, two frequencies for calculating the first change rate are predetermined, and two frequencies for calculating the second change rate are predetermined.

[0041] Fig. 8 is a diagram showing an example of the relationship between the base number (estimated value) of liquid L1 detected by liquid sensor 10 according to the present embodiment using relationship information and the analytical value of the base number of liquid L1. Referring to Fig. 8, the horizontal axis indicates the analytical value, and the vertical axis indicates the estimated value or error. Each white point P2 indicates an estimated value for liquid L1 having a specific analytical value, and each black point P3 indicates the error between the estimated value and the analytical value. Reference line RL1 indicates the upper limit of error defined by the JIS standard (JIS K 2501:2003 potentiometric titration method (base number-hydrochloric acid method)), and reference line RL2 indicates the lower limit of error defined by the JIS standard.

[0042] As shown in FIG. 8, the error of each estimated value falls within the error range defined by the JIS standard. This also shows that there is a high correlation between the second parameter (the difference between the first rate of change and the second rate of change) and the third parameter (for example, base number). In liquid sensor 10 according to the present embodiment, the relationship information indicates the corresponding relationship between the third parameter and the second parameter, and the third parameter is detected based on the calculated second parameter and the relationship information. Therefore, liquid sensor 10 can detect the third parameter with relatively high accuracy.

[0043] Suppose that the second parameter is not the difference between the first and second change rates, but is a single change rate (the amount of change in the first parameter relative to the amount of change in frequency). In this case, it is assumed that relationship information indicating the correspondence between the third parameter and the second parameter is generated in advance. It is also assumed that the base number of the liquid L1 is estimated using this relationship information.

[0044] Fig. 9 is a diagram showing an example of the relationship between the base number (estimated value) of liquid L1 detected using the relationship information and the analytical value of the base number of liquid L1 when one degree of change is used as the second parameter. As shown in Fig. 9, the errors of some of the estimated values ​​do not fall within the error range defined by the JIS standard. It can be seen that when one degree of change is used as the second parameter, the second parameter and the third parameter do not have a high correlation.

[0045] In the liquid sensor 10 according to the present embodiment, the difference between the first rate of change and the second rate of change is determined as the second parameter, and then the third parameter is detected based on the calculated second parameter and the relationship information. Therefore, the liquid sensor 10 can detect the third parameter with a relatively high degree of accuracy.

[0046] [3. Procedure for detecting base number] 10 is a flowchart showing a procedure for detecting the base number of the liquid L1 in the liquid sensor 10. The process shown in this flowchart is executed by the control unit 104.

[0047] 10, the control unit 104 detects the resistance value R1 (first parameter) of the liquid L1 in a state where a measurement voltage of a predetermined frequency (for example, any of four frequencies predetermined in the relationship information) is applied between the pair of electrodes 112 (step S200). The control unit 104 determines whether or not the detection of the resistance value R1 has been performed in a state where each measurement voltage of a plurality of predetermined frequencies (for example, four) is applied between the pair of electrodes 112 (step S210). If it is determined that the detection of the resistance value R1 has not been completed for at least some frequencies (NO in step S210), the control unit 104 controls the measurement unit 102 to sweep the frequency of the measurement voltage (step S220). Thereafter, the process of step S200 is performed again.

[0048] When it is determined that detection of the resistance value R1 has been completed for all the predetermined frequencies (YES in step S210), the control unit 104 calculates the first and second change rates based on each detection result of the resistance value R1 (step S230). The control unit 104 calculates the difference between the first change rate and the second change rate (second parameter) (step S240). The control unit 104 detects the base number (third parameter) of the liquid L1 by substituting the second parameter into the relationship information (e.g., a function) stored in the storage unit 106 (step S250).

[0049] [4. Features] As described above, the liquid sensor 10 according to the present embodiment includes a pair of electrodes 112 and a control unit 104. The pair of electrodes 112 are used in a state of being immersed in the liquid L1. The control unit 104 sweeps the frequency of the AC voltage applied between the pair of electrodes 112. The control unit 104 detects the impedance of the liquid L1 for each frequency, and calculates a second parameter related to a first parameter (e.g., resistance value R1) which is a value correlated with the impedance or an impedance. The second parameter is a difference between the first rate of change and the second rate of change. Each of the first rate of change and the second rate of change indicates the amount of change in the first parameter relative to the amount of change in frequency. The liquid sensor 10 further includes a storage unit 106. The storage unit 106 stores relationship information indicating the correspondence between the second parameter and a third parameter (e.g., base number) correlated with the deterioration degree of the liquid L1. The control unit 104 detects the third parameter based on the second parameter and the relationship information. According to the liquid sensor 10, since the third parameter and the second parameter have a relatively high correlation, and the third parameter is detected based on the second parameter and relationship information indicating the correspondence between the third parameter and the second parameter, the third parameter can be detected with relatively high accuracy.

[0050] 5. Other embodiments The concept of the above embodiment is not limited to the embodiment described above. Hereinafter, an example of another embodiment to which the concept of the above embodiment can be applied will be described.

[0051] <5-1> In the above embodiment, the first parameter is the resistance value R1 of the liquid L1. However, the first parameter does not necessarily have to be the resistance value R1 of the liquid L1. The first parameter may be, for example, the capacitance C1 of the liquid L1 or the impedance Z1 of the liquid L1. The relationship information may be generated in advance with the capacitance C1 or the impedance Z1 of the liquid L1 as the first parameter, and the third parameter of the liquid L1 may be detected according to the flowchart shown in FIG. 10 with the capacitance C1 or the impedance Z1 of the liquid L1 as the first parameter.

[0052] <5-2> In the above embodiment, the third parameter is the base value of the liquid L1. However, the third parameter does not necessarily have to be the base value of the liquid L1. The third parameter may be, for example, the acid value of the liquid L1. The relationship information may be generated in advance with the acid value of the liquid L1 as the third parameter, and the third parameter of the liquid L1 may be detected according to the flowchart shown in FIG. 10 with the acid value of the liquid L1 as the third parameter.

[0053] FIG. 11 is a diagram showing an example of the relationship between the acid value (estimated value) of the liquid L1 detected using the relationship information and the analytical value of the acid value of the liquid L1 when the acid value of the liquid L1 is the third parameter. With reference to FIG. 11, the horizontal axis indicates the analytical value, and the vertical axis indicates the estimated value or the error. Each white point P4 indicates an estimated value (acid value) for the liquid L1 having a specific analytical value (acid value), and each black point P5 indicates the error between the estimated value and the analytical value. The reference line RL3 indicates the upper limit of the error defined by the JIS standard (JIS K 2501:2003 Potentiometric titration method (acid value)), and the reference line RL4 indicates the lower limit of the error defined by the JIS standard. As shown in FIG. 11, the error of each estimated value falls within the error range defined by the JIS standard.

[0054] <5-3> In the above embodiment, the same relationship information is always used to detect the third parameter. However, the relationship information used to detect the third parameter may be changed according to a predetermined condition. For example, a plurality of pieces of relationship information may be prepared in advance, and each of the plurality of pieces of relationship information may correspond to a different temperature range of the liquid L1. The plurality of pieces of relationship information may be generated in advance, for example, by changing the temperatures of the plurality of samples (liquid L1). The plurality of pieces of relationship information may be stored in the storage unit 106.

[0055] Fig. 12 is a flowchart showing a procedure for identifying relationship information used to detect the third parameter. The process shown in this flowchart is executed by, for example, the control unit 104. Referring to Fig. 12, the control unit 104 detects the temperature of the liquid L1 based on the output of the temperature sensor 113 (step S300). The control unit 104 identifies relationship information corresponding to the detected temperature of the liquid L1 (step S310). Using the identified relationship information, the third parameter of the liquid L1 may be detected according to the flowchart shown in Fig. 10.

[0056] The inventor(s) found that the relationship between the third parameter and the second parameter may be affected by the temperature of the liquid L1. In this liquid sensor 10, the third parameter is detected based on the temperature of the liquid L1, relationship information corresponding to the temperature of the liquid L1, and the second parameter. Therefore, according to this liquid sensor 10, the third parameter is detected taking into account the temperature of the liquid L1, so that the third parameter can be detected with higher accuracy.

[0057] <5-4> In the above embodiment, the resistance value R1 is used as the first parameter regardless of the value of the third parameter. However, the first parameter may be changed depending on the value of the third parameter.

[0058] FIG. 13 is a diagram for explaining an image regarding the change of the first parameter. With reference to FIG. 13, for example, when the base number (third parameter) is 4 or more, the capacitance C1 may be used as the first parameter, and when the base number is less than 4, the resistance value R1 may be used as the first parameter. The present inventor(s) found that which of the second parameter calculated based on the resistance value R1 and the second parameter calculated based on the capacitance C1 has a higher correlation with the third parameter varies depending on the value of the third parameter. More specifically, the present inventor(s) found that the higher the base number, the higher the correlation of the second parameter calculated based on the capacitance C1 with the third parameter, and the lower the base number, the higher the correlation of the second parameter calculated based on the resistance value R1 with the third parameter.

[0059] 14 is a flowchart showing another example of the procedure for detecting the base number of the liquid L1 in the liquid sensor 10. The process shown in this flowchart is executed by the control unit 104.

[0060] 14, the control unit 104 detects the resistance value R1 (first parameter) of the liquid L1 in a state where a measurement voltage of a predetermined frequency (for example, any of four frequencies predetermined in the relationship information) is applied between the pair of electrodes 112 (step S400). The control unit 104 determines whether or not the detection of the resistance value R1 has been performed in a state where each measurement voltage of a plurality of predetermined frequencies (for example, four) is applied between the pair of electrodes 112 (step S405). If it is determined that the detection of the resistance value R1 has not been completed for at least some frequencies (NO in step S405), the control unit 104 controls the measurement unit 102 to sweep the frequency of the measurement voltage (step S410). Thereafter, the process of step S400 is performed again.

[0061] When it is determined that detection of the resistance value R1 has been completed for all the predetermined frequencies (YES in step S405), the control unit 104 calculates the first and second change rates based on the detection results of the resistance value R1 (step S415). The control unit 104 calculates the difference between the first change rate and the second change rate (second parameter) (step S420). The control unit 104 detects the base number (third parameter) of the liquid L1 by substituting the second parameter into the relationship information (e.g., a function) stored in the storage unit 106 (step S425). Note that the relationship information used here is generated in advance based on the frequency characteristics of the resistance value R1.

[0062] The control unit 104 determines whether the detected base number is equal to or greater than a predetermined value (e.g., "4") (step S430). If it is determined that the detected base number is less than the predetermined value (NO in step S430), the process shown in this flowchart ends, and the base number detected in step S425 is regarded as the base number of the liquid L1.

[0063] On the other hand, if it is determined that the detected base number is equal to or greater than the predetermined value (YES in step S430), the control unit 104 detects the capacitance C1 (first parameter) of the liquid L1 in a state where a measurement voltage of a predetermined frequency is applied between the pair of electrodes 112 (step S435). The control unit 104 determines whether or not the detection of the capacitance C1 has been performed in a state where each measurement voltage of a plurality of predetermined frequencies is applied between the pair of electrodes 112 (step S440). If it is determined that the detection of the capacitance C1 has not been completed for at least some frequencies (NO in step S440), the control unit 104 controls the measurement unit 102 to sweep the frequency of the measurement voltage (step S445). After that, the process of step S435 is performed again.

[0064] When it is determined that the detection of the capacitance C1 is completed for all the predetermined frequencies (YES in step S440), the control unit 104 calculates the first and second change rates based on the detection results of the capacitance C1 (step S450). The control unit 104 calculates the difference between the first change rate and the second change rate (second parameter) (step S455). The control unit 104 detects the base number (third parameter) of the liquid L1 by substituting the second parameter into the relationship information (e.g., a function) stored in the storage unit 106 (step S460). Note that the relationship information used here is generated in advance based on the frequency characteristics of the capacitance C1. When the base number is detected in step S460, the process shown in this flowchart ends, and the base number detected in step S460 is considered to be the base number of the liquid L1.

[0065] In this manner, in the liquid sensor 10, whether to use the resistance value R1 or the capacitance C1 as the first parameter is switched depending on the value of the third parameter. Therefore, according to the liquid sensor 10, the second parameter having a higher correlation with the third parameter is used to detect the third parameter, so that the third parameter can be detected with higher accuracy.

[0066] The above describes the embodiments of the present invention by way of example. That is, the detailed description and the accompanying drawings are disclosed for the purpose of illustrative description. Therefore, some components described in the detailed description and the accompanying drawings may be non-essential components for solving the problem. Therefore, just because the non-essential components are described in the detailed description and the accompanying drawings, it should not be immediately determined that the non-essential components are essential.

[0067] Moreover, the above-described embodiments are merely examples of the present invention in every respect. Various improvements and modifications of the above-described embodiments are possible within the scope of the present invention. For example, at least a part of the configuration of any of the embodiments may be combined with at least a part of the configuration of any of the other embodiments. In other words, in implementing the present invention, specific configurations can be appropriately adopted according to the embodiment. [Explanation of symbols]

[0068] 10 liquid sensor, 20 tank, 100 detection system, 102 measurement section, 104 control section, 106 memory section, 108 notification section, 110 substrate, 111 substrate body, 112 electrode, 113 temperature sensor, DC1, DC2 degree of change, L1 liquid, P1-P5 points, RL1-RL4 reference line.

Claims

1. A pair of electrodes that are used in a state of being immersed in a liquid; a control unit that sweeps a frequency of the AC voltage applied between the pair of electrodes, The control unit detects the impedance of the liquid for each of the frequencies, and calculates a value correlated with the impedance or a second parameter related to a first parameter that is the impedance; the second parameter is a difference between the first rate of change and the second rate of change, each of the first change rate and the second change rate indicates an amount of change in the first parameter relative to an amount of change in the frequency; a storage unit configured to store relationship information indicating a correspondence relationship between the second parameter and a third parameter having a correlation with a deterioration level of the liquid; The control unit detects the third parameter based on the second parameter and the relationship information.

2. The liquid sensor according to claim 1 , wherein the first parameter is a resistance value of the liquid or a capacitance of the liquid.

3. The liquid sensor according to claim 1 or 2, wherein the third parameter is a base number of the liquid or an acid number of the liquid.

4. A temperature sensor for detecting a temperature of the liquid is further provided. The relationship information is provided for each temperature of the liquid, The liquid sensor according to claim 1 , wherein the control unit detects the third parameter based on the temperature of the liquid, the relationship information corresponding to the temperature of the liquid, and the second parameter.

5. The liquid sensor according to claim 2 , wherein the control unit switches between using the resistance value or the capacitance as the first parameter depending on the detected value of the third parameter.

6. Sweeping the frequency of an AC voltage applied between a pair of electrodes immersed in a liquid; detecting the impedance of the liquid for each of the frequencies; calculating a second parameter related to a first parameter that is a value correlated with the impedance or the impedance; the second parameter is a difference between the first rate of change and the second rate of change, each of the first change rate and the second change rate indicates an amount of change in the first parameter relative to an amount of change in the frequency; storing relationship information indicating a correspondence relationship between the second parameter and a third parameter having a correlation with the deterioration level of the liquid; and detecting the third parameter based on the second parameter and the relationship information.

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