Patent Publication Number: US-11035893-B2

Title: Sensor device

Description:
TECHNICAL FIELD 
     The present invention relates to a sensor device, and in more detail, to a sensor device that measures the resistance value of a medium such as liquid to determine a property state of the medium. 
     BACKGROUND ART 
     In general, when the properties of a liquid are changed, such as composition and characteristics, the dielectric constant and resistivity of the liquid correspondingly change in many cases. However, what influence is exerted on the dielectric constant or the resistivity is different depending on the type of a target liquid or a property of interest. For example, after a long-term use, engine oil for vehicles is deteriorated due to the interfusion of foreign substance such as soot to reduce resistivity, and therefore it is conceivable that the deterioration can be determined by measuring the resistance of the engine oil. 
     For example, in the past, an oil deterioration detector adapted to electrically detect the deterioration of oil has been known (Patent Literature 1). This oil deterioration detector detects the deterioration of the oil by arranging two electrodes in an engine oil flow path and measuring the conductivity and dielectric constant of the oil. 
     When measuring the electrical characteristics of a liquid with an electrode pair immersed in the liquid, there has been a problem that the electrode pair is deteriorated due to corrosion. Specifically, when measuring the resistance of the liquid using an electrode pair made of copper, there has been a problem that the electrode pair in direct contact with the liquid is corroded to make it difficult to ensure the durability of the detector. 
     Also, in order to prevent the corrosion of the electrode pair, it is conceivable to form the electrode pair using an electrically conductive material having corrosion resistance, such as carbon nanotubes or diamond-like carbon. However, there has been a problem that manufacturing cost is significantly increased. Further, it is also conceivable to prevent the corrosion by gold-plating the electrode pair. However, there has still been a problem of increased manufacturing cost. 
     CITATION LIST 
     Patent Literatures 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2009-02693 
     SUMMARY OF INVENTION 
     Technical Problem 
     The present invention has been made in consideration of the above situations and intends to provide a sensor device that measures the resistance value of a medium such as a liquid to determine the properties of the medium. Also, the present invention intends to improve the durability of the sensor device. Further, the present invention intends to suppress the manufacturing cost of the sensor device and provide the sensor device at low cost. 
     In particular, the present invention intends to provide a sensor device having both improved durability and improved property determination accuracy. Further, the present invention intends to provide the sensor device that achieves the improvement of durability and the improvement of property determination accuracy without significantly increasing manufacturing cost. 
     Solution to Problem 
     A sensor device according to a first aspect of the present invention includes: aa first electrode pair that is formed on a board and covered by an insulating protective film; a resistance measurement part that supplies AC current to the first electrode pair to measure the resistance value of a medium around the first electrode pair; a storage part that holds the parasitic capacitance of the first electrode pair; and a property determination part that, on the basis of the resistance value and the parasitic capacitance, determines a property of the medium. 
     By covering the first electrode pair for measuring the resistance value of the medium with the insulating protective film, the corrosion of the first electrode pair can be prevented to improve the durability of the device without significantly increasing manufacturing cost. Also, by supplying the AC current to the first electrode pair covered by the insulating protective film to measure the resistance value of the medium around the first electrode pair, the property of the medium can be determined without impairing the durability. Further, by determining the property of the medium on a basis of the resistance value and parasitic capacitance of the first electrode pair, the property determination accuracy of the medium can be improved. Accordingly, employing the above configuration makes it possible to achieve both the improvement of the durability and the improvement of the property determination accuracy while suppressing manufacturing cost. 
     A sensor device according to a second aspect of the present invention is, in addition to the above configuration, configured to further include a measurement error correction part that, on the basis of the parasitic capacitance, corrects a measurement error of the resistance value, in which the property determination part determines the property of the medium on the basis of the resistance value after the correction. 
     Employing such a configuration makes it possible to, when measuring the resistance value of the medium around the first electrode pair using the AC current, suppress the influence of the parasitic capacitance of the first electrode pair to accurately obtain the resistance value of the medium. Accordingly, the property determination accuracy of the medium can be improved. 
     A sensor device according to a third aspect of the present invention is, in addition to the above configuration, configured such that the resistance measurement part measures the impedance of the first electrode pair at the time of supplying AC current having a frequency of 5 kHz or less. 
     Employing the above configuration makes it possible to suppress the influence of the capacitance of the medium around the first electrode pair on the impedance of the first electrode pair. For this reason, by measuring the impedance of the first electrode pair at the time of supplying the AC current, the resistance value of the medium around the first electrode pair can be accurately obtained. 
     A sensor device according to a fourth aspect of the present invention is, in addition to the above configuration, configured such that the parasitic capacitance has a value measured as the impedance of the first electrode pair at the time of supplying the AC current to the first electrode pair that is not close to the medium. 
     Employing such a configuration makes it possible to correct the measured value of the resistance using the parasitic capacitance of the first electrode pair, which is related to the resistance value measurement and accurate. For this reason, the influence of the parasitic capacitance of the first electrode pair on the measured value of the resistance can be effectively suppressed, and thereby the resistance value of the medium around the first electrode pair can be further accurately obtained. 
     A sensor device according to a fifth aspect of the present invention in addition to the above configuration, configured to further include a medium container tank in which the board is arranged; a capacitance measurement part that supplies AC current having a first frequency to said first electrode pair to measure capacitance of said first electrode pair; a medium amount detection part that, on a basis of the capacitance measured by said capacitance measurement part, detects a medium amount in said container tank; and a medium amount compensation part that, on a basis of said medium amount, corrects said resistance value; in which said first electrode pair extends in a direction intersecting with a horizontal direction; said resistance measurement part supplies AC current having a second frequency different from the first frequency to said first electrode pair to measure a resistance value of a medium around the first electrode pair; and said property determination part determines a property of said medium on a basis of said resistance value after the correction and said parasitic capacitance. 
     Employing such a configuration makes it possible, by using the same electrode pair, to measure the capacitance of the electrode pair to detect the medium amount as well as to measure the resistance value of the medium around the electrode pair to determine the property of the medium. Accordingly, the property of the medium can also be determined using the board for detecting the medium amount. 
     A sensor device according to a sixth aspect of the present invention in addition to the above configuration, configured to further include a medium container tank in which the board is arranged; a second electrode pair that is formed on the board, arranged above the first electrode pair, and extends in a direction intersecting with a horizontal direction; a capacitance measurement part that measures capacitance of said second electrode pair; and a medium amount detection part that, on a basis of the capacitance measured by said capacitance measurement part, detects a medium amount in said container tank; in which said protective film covers said first electrode pair and said second electrode pair; said resistance measurement part supplies AC current to said first electrode pair to measure a resistance value of a medium around said first electrode pair; and said storage part holds parasitic capacitance of said first electrode pair. 
     Employing such a configuration makes it possible to form on the same board the first electrode pair for determining the property of the medium and the second electrode pair for detecting the medium amount, and cover the first electrode pair and the second electrode pair with the same insulating protective film. For this reason, the durability of the first and second electrode pairs can be improved without increasing manufacturing cost. 
     A sensor device according to a seventh aspect of the present invention is, in addition to the above configuration, configured to further include a measurement error correction part that, on the basis of the parasitic capacitance, corrects a measurement error of the resistance value, in which the property determination part determines the property of the medium on the basis of the resistance value after the correction. 
     Employing such a configuration makes it possible to, when measuring the resistance value of the medium around the first electrode pair using the AC current, suppress the influence of the parasitic capacitance of the first electrode pair to accurately obtain the resistance value of the medium. Accordingly, the property determination accuracy of the medium can be improved. 
     A sensor device according to an eighth aspect of the present invention is, in addition to the above configuration, configured to include a temperature measurement part that measures the temperature of the board, and a temperature compensation part that, on the basis of the temperature, corrects the resistance value, in which the property determination part determines the property of the medium on the basis of the resistance value after the correction. 
     Employing such a configuration enables the influence of the temperature characteristics of the resistance of the medium to be suppressed by measuring the temperature of the medium and correcting the resistance value of the medium and the property determination accuracy of the medium to be improved. 
     A sensor device according to a ninth aspect of the present invention is, in addition to the above configuration, configured such that the temperature measurement part obtains the temperature of the board by measuring the resistance value of an electrode formed on the board. 
     Employing such a configuration makes it possible to measure the temperature only with the electrode on the board without using a temperature measuring element such as a thermistor. For this reason, manufacturing cost can be suppressed. 
     A sensor device according to a tenth aspect of the present invention is, in addition to the above configuration, configured such that the electrode is covered by the protective film 
     Employing such a configuration makes it possible to prevent the corrosion of the electrode for temperature measurement to improve the durability of the device without significantly increasing manufacturing cost. 
     A sensor device according to an eleventh aspect of the present invention is, in addition to the above configuration, configured such that the medium is liquid, and the property determination part determines the deterioration state of the liquid. 
     A sensor device according to a twelfth aspect of the present invention is, in addition to the above configuration, configured such that the liquid is oil, and the property determination part determines the deterioration state of the oil. 
     Advantageous Effects of Invention 
     According to the present invention, a sensor device that measures the resistance value of a medium such as a liquid and determines the properties of the medium can be provided. Also, the durability of the sensor device can be improved. Further, the manufacturing cost of the sensor device can be suppressed to make it possible to provide it at low cost. 
     In particular, a sensor device that achieves both the improvement of durability and the improvement of property determination accuracy can be provided. Further, the sensor device having improved durability and improved property determination accuracy can be provided without significantly increasing manufacturing cost. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an example of an oil tank  10  in which an oil sensor device  1  according to a first embodiment of the present invention is attached. 
         FIG. 2  is a development perspective view illustrating the structure of a sensor board  2  in  FIG. 1 . 
         FIG. 3  is a cross-sectional view when cutting the sensor board  2  along an A-A section line in  FIG. 2 . 
         FIG. 4  is a diagram illustrating an example of the relationship between liquid level height and the capacitance of an electrode pair  21 . 
         FIG. 5  is a diagram illustrating an example of the relationship among the deterioration state, temperature, and resistance value of oil  4 . 
         FIG. 6  is a diagram illustrating an equivalent circuit between opposite elements immersed in the oil  4 . 
         FIG. 7  is a diagram illustrating an equivalent circuit between opposite elements not immersed in the oil  4 . 
         FIG. 8  is a block diagram illustrating a detailed configuration of a control circuit  3  in  FIG. 1 . 
         FIG. 9  is a diagram illustrating a configuration example of a sensor board  2  according to a second embodiment of the present invention. 
         FIG. 10  is a development perspective view illustrating the structure of the sensor board  2  in  FIG. 9 . 
         FIG. 11  is a cross-sectional view when cutting the sensor board  2  along a B-B section line in  FIG. 10 . 
         FIG. 12  is a block diagram illustrating a configuration example of a control circuit  3  according to the second embodiment of the present invention. 
         FIG. 13  is a diagram illustrating another configuration example of the sensor board  2  according to the second embodiment of the present invention. 
         FIG. 14  is a diagram illustrating a configuration example of a sensor board  2  according to a third embodiment of the present invention. 
         FIG. 15  is a block diagram illustrating a configuration example of a control circuit  3  according to the third embodiment of the present invention. 
         FIG. 16  is a diagram illustrating an example of the relationship between liquid level height and the resistance value of an electrode pair  21 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In each of the following embodiments, as an example of a sensor device that detects the amount and property of a medium contained in a tank, an oil sensor device that detects the liquid amount and deterioration state of oil contained in an oil tank will be described. Note that the oil sensor device described as each of the embodiments is exemplified, and the present invention is not limited only to a specific configuration described in each of the embodiments. 
     First Embodiment 
       FIG. 1  is a diagram illustrating an example of an oil tank  10  in which an oil sensor device  1  according to a first embodiment of the preset invention is attached. The oil sensor device  1  is a device adapted to detect the liquid amount and deterioration state of oil  4  in the oil tank  10  and configured to include a sensor board  2  and a control circuit  3 . 
     (A1) Oil Tank  10   
     The oil tank  10  is a container for containing the oil  4 , and for example, an oil tank connected to a circulation path of engine oil for vehicles. The oil tank  10  includes an inflow port  12  and an outflow port  13 , and the oil  4  supplied from the outflow port  13  to the oil circulation path returns into the oil tank  10  through the inflow path  12  after passing through an engine and an oil filter. The oil tank  10  may be an independent container for storing the oil  4 . Also, as long as the liquid amount and deterioration state of the oil  4  is stable enough to be measurable at the time of measurement, the oil tank  10  may be a container from which and into which part of the oil  4  as a measurement target flows. 
     (A2) Tank Cover  11   
     The tank cover  11  is a cover for closing the upper opening of the oil tank  10 , and attached with the oil sensor device  1 . The lower surface side of the tank cover  11  is attached with the sensor board  2 , and in a casing on the upper surface side of the tank cover  11 , the control circuit  3  is contained. 
     (A3) Sensor Board  2   
     The sensor board  2  is a board arranged in the oil tank  10 , and one principal surface thereof is provided with an electrode pair  21 , an electrode pair  22 , an electrode pair  23 , and a thermistor  23   s , whereas the other principal surface is provided with a ground plate  25 . The liquid amount and deterioration state of the oil  4  can be detected by immersing the sensor board  2  in the oil  4 . The sensor board  2  has an elongated rectangular shape, and is arranged with the longer direction thereof intersecting with the horizontal direction. The illustrated sensor board  2  extends vertically downward with the upper end thereof fixed to the tank cover  11 , and the lower end thereof is arranged near the bottom part of the oil tank  10 . 
     (A4) Control Circuit  3   
     The control circuit  3  is an electric circuit for controlling the sensor board  2 , and arranged outside the oil tank  10 . The control circuit  3  is configured to include an unillustrated power supply circuit, a DC converter, a microprocessor, and the like, and connected with the electrode pairs  21  to  23  and the ground plate  25  on the sensor board  2 . 
       FIGS. 2 and 3  are views illustrating a configuration example of the sensor board  2  in  FIG. 1 .  FIG. 2  is a development perspective view illustrating the structure of the sensor board  2 .  FIG. 3  is a cross-sectional view when cutting the sensor board  2  along an A-A section line in  FIG. 2 . The configuration of the sensor board  2  will be described here in more detail with reference to  FIGS. 1 to 3 . The sensor board  2  is configured to include a base material  200 , a protective film  201 , the electrode pairs  21  to  23 , the thermistor  23   s , the ground plate  25 , and a protective film  203 . 
     (B1) Base Material  200   
     The base material  200  is made of a dielectric material processed in an elongated rectangular flat-plate shape and having corrosion resistance, such as a fluorine resin. On one principal surface (hereinafter referred to as a first surface) of the base material  200 , the electrode pairs  21  to  23  and the thermistor  23   s  are provided. The electrode pairs  21  to  23  are formed as a patterned electrically conductive metal layer. For example, the electrode pairs  21  to  23  having desired patterns are formed by etching copper foil put on the base material  200 , and then the thermistor  23   s  is soldered. On the other principal surface (hereinafter referred to as a second surface) of the base material  200 , the ground plate  25  is provided. The ground plate  25  is formed as an electrically conductive metal layer covering the entire surface of the base material  200 . For example, the ground plate  25  is made of copper foil put on the base material  200 . 
     (B2) Protective Film  201   
     The protective film  201  is a film formed on the first surface of the base material  200 , and made of a dielectric material having corrosion resistance, such as a fluorine resin. The protective film  201  is formed on the entire first surface of the base material  200 , and covers the electrode pairs  21  to  23  and the thermistor  23   s . For this reason, the electrode pairs  21  to  23  and the thermistor  23   s  are sealed by the base material  200  and the protective film  201  and do not contact with the oil  4  or air, and therefore the electrode pairs  21  to  23  and the thermistor  23   s  can be prevent from being deteriorated and damaged by corrosion or the like. Also, by covering all the parts on the sensor board  2  with the same protective film  201 , manufacturing cost can be suppressed to obtain the sensor device  1  at low cost. 
     (B3) Electrode Pair  21   
     The electrode pair  21  is an electrode pair used to detect the liquid level height of the oil  4 , and consists of two electrodes  21   a  and  21   b . The electrodes  21   a  and  21   b  respectively include two or more opposite elements  21   n  and  21   m.    
     The electrodes  21   a  and  21   b  both have an elongated shape having a substantially uniform width, and extend mutually parallel along the longer direction of the base material  200 , and one ends thereof are connected to the control circuit  3 . That is, the electrodes  21   a  and  21   b  both have a shape extending in a direction intersecting with the horizontal direction, for example, in the vertical direction, and the upper ends thereof are connected to the control circuit  3 . 
     The opposite elements  21   n  and  21   m  all have an elongated shape having a substantially uniform width, extend in the shorter direction of the base material  200 , and are closely arranged mutually parallel. The electrode  21   a  is connected with the two or more opposite elements  21   n , and the electrode  21   b  is connected with the two or more opposite elements  21   m . Between the electrodes  21   a  and  21   b , three or more opposite elements  21   n  and  21   m  are alternately arrayed along the longer direction of the base material  200 , and pairs of adjacently arranged opposite elements  21   n  and  21   m  respectively form electrode pairs. 
     When the liquid amount of the oil  4  changes, the liquid level of the oil  4  moves in the vertical direction, and the number of the opposite elements  21   n  and  21   m  immersed in the oil  4  is changed depending on the liquid level height of the oil  4 . The capacitance between adjacent opposite elements  21   n  and  21   m  is significantly changed depending on whether the opposite elements  21   n  and  21   m  are present in air or in the oil. For this reason, by measuring the capacitance of the electrode pair  21 , the liquid level height of the oil  4  can be measured. 
     In the case of the electrode pair  21  in the diagram, the two or more opposite elements  21   n  and  21   m  closely arranged in the vertical direction form an opposite element group  21   g , and two or more opposite element groups  21   g  are arrayed in the vertical direction at intervals wider than the gap between adjacent opposite elements  21   n  and  21   m . Employing such a configuration makes it possible to discretely detect the liquid level height. In addition, if all the opposite elements  21   n  and  21   m  are closely arranged and the electrode pair  21  is configured to have only one opposite element group  21   g , the liquid level height can be continuously detected. 
     (B4) Electrode Pair  22   
     The electrode pair  22  is an electrode pair used to determine the deterioration state of the oil  4 , and consists of two electrodes  22   a  and  22   b . The electrodes  22   a  and  22   b  respectively include one or more opposite elements  22   n  and  22   m.    
     The electrodes  22   a  and  22   b  both have an elongated shape having a substantially uniform width, and extend mutually parallel along the longer direction of the base material  200 , and one ends thereof are connected to the control circuit  3 . That is, the electrodes  22   a  and  22   b  both have a shape extending in a direction intersecting with the horizontal direction, for example, in the vertical direction, and the upper ends thereof are connected to the control circuit  3 . 
     The opposite elements  22   n  and  22   m  all have an elongated shape having a substantially uniform width, extend in the shorter direction of the base material  200 , and are closely arranged mutually parallel. The electrode  22   a  is connected with the one or more opposite elements  22   n , and the electrode  22   b  is connected with the one or more opposite elements  22   m . When three or more opposite elements  22   n  and  22   m  are arranged, the opposite elements  22   n  and  22   m  are alternately arrayed between the electrodes  22   a  and  22   b , and adjacent opposite elements  22   n  and  22   m  respectively form an electrode pair. Also, the opposite elements  22   n  and  22   m  are arranged downward of the opposite elements  21   n  and  21   m , and desirably arranged below the thermistor  23   s . In addition, the opposite elements  22   n  and  22   m  may be arranged above the thermistor  23   s.    
     When the oil  4  is deteriorated after a long-term use, the resistivity of it reduces. For this reason, by measuring the resistance value of the electrode pair  22  with the opposite elements  22   n  and  22   m  immersed in the oil  4 , the deterioration state of the oil  4  can be determined. However, since the electrode pair  22  is covered with the protective film  201 , DC resistance cannot be measured. For this reason, the electrode pair  22  has to be supplied with AC current to measure the resistance of the oil  4 . 
     (B5) Electrode Pair  23   
     The electrode pair  23  consists of two electrodes  23   a  and  23   b . The electrodes  23   a  and  23   b  are respectively connected to the terminals of the thermistor  23   s , and therefore by measuring the resistance of the electrode pair  23 , the temperature of the sensor board  2  can be measured. 
     The electrodes  23   a  and  23   b  both have an elongated shape having a substantially uniform width, and extend mutually parallel along the longer direction of the base material  200 , and the upper ends thereof are connected to the control circuit  3 . The thermistor  23   s  is a well-known temperature detecting element whose resistance value has a relatively large temperature characteristic, and for example, a surface mounting element is attached on the base material  200 . The temperature of the sensor board  2  is substantially equal to the temperature of the oil  4  around the sensor board  2 , and therefore by measuring the resistance of the electrode pair  23 , the temperature of the oil  4  can be acquired. When the thermistor  23   s  is immersed in the oil  4 , the temperature of the oil  4  can be more accurately measured, and therefore the thermistor  23   s  is arranged below the opposite elements  21   n  and  21   m.    
     (B6) Ground Plate  25   
     The ground plate  25  is a ground layer formed on the entire second surface of the base material  200 , and connected to the ground of the control circuit  3 . Arranging the ground plate  25  in parallel with the electrode pairs  21  to  23  across the base material  200  makes it possible to stabilize the potentials of the electrode pairs  21  to  23  and improve anti-noise characteristics. 
     (B7) Protective Film  203   
     The protective film  203  is a film formed on the second surface of the base material  200 , and made of a dielectric material having corrosion resistance, such as a fluorine resin. The protective film  203  is formed on the entire second surface of the base material  200 , and covers the ground plate  25 . For this reason, the ground plate  25  is sealed by the base material  200  and the protective film  203  and does not contact with the oil  4  or air, and therefore the ground plate  25  can be prevent from being deteriorated and damaged by corrosion or the like. In addition, the ground plate  25  and the protective film  203  are provided as needed, and can also be omitted. 
     (C1) Method for Detecting Liquid Amount of Oil 
       FIG. 4  is a diagram illustrating an example of the relationship between the liquid level height of the oil  4  and the capacitance of the electrode pair  21 . This diagram is one represented with the liquid level height of the oil  4  in the oil tank  10  taken as the horizontal axis and the capacitance of the electrode pair  21  taken as the vertical axis. A characteristic  40  in the diagram exhibits an upward-sloping straight line. That is, it turns out that the capacitance of the electrode pair  21  increases corresponding to the liquid level height of the oil  4 , and both have a substantially linear relationship. In addition, when the electrode pair  21  is configured to discretely measure the liquid level height of the oil  4 , the measured capacitance of the electrode pair  21  exhibits a characteristic obtained by changing the straight line in  FIG. 4  to the stepwise shape. 
     Since the electrode pair  21  extends in the vertical direction and the many opposite elements  21   n  and  21   m  are also arrayed in the vertical direction, the rise of the liquid level height increases the number of opposite elements  21   n  and  21   m  immersed in the oil  4 . For this reason, the rise of the liquid level height also monotonously increases the capacitance of the electrode pair  21 , thus obtaining the characteristic as illustrated in the diagram. Accordingly, as long as the capacitance of the electrode pair  21  can be measured, the liquid level height can be obtained, and the liquid amount of the oil  4  in the oil tank  10  can be detected. 
     (C2) Method for Detecting Deterioration State of Oil 
       FIG. 5  is a diagram illustrating an example of the relationship among the deterioration state, temperature, and resistance value of the oil  4 . This diagram is one represented with the temperature of the oil  4  taken as the horizontal axis and the resistance value of the oil  4  taken as the vertical axis. The resistance value of the oil  4  is a value measured using the electrode pair  22 , and shows the resistance value of the oil  4  around the opposite elements  22   n  and  22   m . Also, a characteristic  41  in the diagram is a characteristic of the oil  4  that is unused and not deteriorated, whereas a characteristic  42  in the diagram is a characteristic of the oil  4  that is used and deteriorated. 
     When comparing the characteristics  41  and  42 , it turns out that the resistance value of the oil  4  reduces in association with the deterioration of the oil  4 . For example, in the case of engine oil, a long-term use causes the interfusion of impurities such as carbon and metal powder. For this reason, as long as the resistance value of the oil  4  can be measured, the deterioration state of the oil  4  can be obtained. 
     Also, when referring to the characteristics  41  and  42 , it turns out that the resistance value of the oil  4  is significantly changed depending on the temperature. For this reason, it turns out that in order to determine the deterioration state on the basis of the resistance of the oil  4 , it is necessary to make the determination after correcting the measured value of the resistance to suppress the influence of the temperature. 
     (D1) Impedance Measurement Principle 
       FIGS. 6 and 7  are explanatory diagrams on the principle of measuring the impedance between adjacent opposite elements  2   n  and  2   m .  FIG. 6  illustrates an equivalent circuit between the opposite elements  2   n  and  2   m  that are immersed in the oil  4 , whereas  FIG. 7  illustrates an equivalent circuit between the opposite elements  2   n  and  2   m  that are not immersed in the oil  4 . In addition, the opposite elements  2   n  and  2   m  respectively refer to adjacent opposite elements  21   n  and  21   m  or adjacent opposite elements  22   n  and  22   m , and in  FIGS. 6 and 7 , the opposite elements  2   n  and  2   m  are used as generic names for both cases. 
     As illustrated in  FIG. 6 , when the adjacent opposite elements  2   n  and  2   m  are immersed in the oil  4 , an equivalent circuit of the oil  4  around the opposite elements  2   n  and  2   m  is a parallel circuit of capacitance C 1  and resistance R 1 . Both ends of the parallel circuit are respectively connected to the opposite elements  2   n  and  2   m  via the capacitance C 2  of the protective film  201 . Also, between the opposite elements  2   n  and  2   m , parasitic capacitance Cs 1  through the protective film  201 , parasitic capacitance Cs 2  through the base material  200 , and parasitic capacitance Cs 3  through the base material  200  and the ground plate  25  are present. Given that the sum of these parasitic capacitances Cs 1  to Cs 3  is parasitic capacitance Cs, the impedance Z between the opposite elements  2   n  and  2   m  can be expressed by Expression (1) below. 
                   Z   =       (         R   ⁢           ⁢   1       1   +     j   ⁢           ⁢   ω   ⁢           ⁢   C   ⁢           ⁢     1   ·   R     ⁢           ⁢   1         +     2     j   ⁢           ⁢   ω   ⁢           ⁢   C   ⁢           ⁢   2         )     //     1     j   ⁢           ⁢   ω   ⁢           ⁢   Cs                 (   1   )               
In addition, the symbol // represents a parallel connection operator.
 
     When the frequency f(=ω/2π) has a sufficiently large value, Expression (1) above can be approximated as Expression (2) below. Expression (2) below does not include the resistance R 1  of the oil  4 . That is, 
     by supplying AC current having a sufficiently high frequency f 1  to the opposite elements  2   n  and  2   m  and obtaining the impedance Z as the ratio between the effective values of voltage and current at the time, the capacitance between the opposite elements  2   n  and  2   m  can be measured. Also, the capacitance C 2  and the parasitic capacitance Cs have values not varied by the liquid level height, and therefore by measuring the capacitance between the opposite elements  2   n  and  2   m , the liquid level height can be obtained. 
     
       
         
           
             
               
                 
                   
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                   ) 
                 
               
             
           
         
       
     
     On the other hand, when the frequency f has a sufficiently small value, Expression (1) above can be approximated as Expression (3) below. Expression (3) below includes the resistance R 1  of the oil  4 . That is, by supplying AC current having a sufficiently low frequency f 2  to the opposite elements  2   n  and  2   m  and obtaining the impedance Z as the ratio between the effective values of voltage and current at the time, the resistance R 1  between the opposite elements  2   n  and  2   m  can be obtained. Note that the resulting value includes an error due to the capacitance C 2  and the parasitic capacitance Cs. 
                     Z   ≈     (       R   ⁢           ⁢   1     +     2     j   ⁢           ⁢   ω   ⁢           ⁢   C   ⁢           ⁢   2         )       //     1     j   ⁢           ⁢   ω   ⁢           ⁢   C   ⁢           ⁢   s               (   3   )               
(D2) Resistance Value Error Compensation
 
     As illustrated in  FIG. 7 , when the adjacent opposite elements  2   n  and  2   m  are not immersed in the oil  4 , an equivalent circuit of surrounding air is capacitance C 3 . For this reason, the equivalent circuit between the opposite elements  2   n  and  2   m  is a circuit obtained by connecting the parasitic capacitance Cs in parallel to a circuit where the capacitance C 2  of the protective film  201  is connected in series to both ends of the capacitance C 3 . Accordingly, the impedance Z between the opposite elements  2   n  and  2   m  can be expressed by the following expression. 
     
       
         
           
             
               
                 
                   Z 
                   = 
                   
                     
                       ( 
                       
                         
                           1 
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ω 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                         + 
                         
                           2 
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ω 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                       
                       ) 
                     
                     // 
                     
                       1 
                       
                         j 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Cs 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Note that the capacitance C 3  of the surrounding air has a sufficiently small value as compared with the parasitic capacitance Cs of the opposite elements  2   n  and  2   m . For this reason, Expression (4) above can be approximated as Expression (5) below. 
     
       
         
           
             
               
                 
                   Z 
                   ≈ 
                   
                     1 
                     
                       j 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ω 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Cs 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     That is, by measuring the impedance Z of the opposite elements  2   n  and  2   m  not immersed in the oil  4 , the parasitic capacitance Cs that is the sum of the parasitic capacitances Cs 1  to Cs 3  can be obtained. Accordingly, by preliminarily measuring the impedance Z of the opposite elements  2   n  and  2   m  not immersed in the oil  4 , a correction of eliminating the influence of the parasitic capacitance Cs (error compensation) can be made when measuring the resistance R 1  between the opposite elements  2   n  and  2   m , and therefore the resistance R 1  of the oil  4  can be accurately measured. 
       FIG. 8  is a block diagram illustrating a detailed configuration of the control circuit  3  in  FIG. 1 . The control circuit  3  detects the liquid amount of the oil  4  using the electrode pair  21 , as well as determines the deterioration state of the oil  4  using the electrode pairs  22  and  23 . The control circuit  3  is configured to include a capacitance measurement part  301 , a resistance measurement part  302 , a temperature measurement part  303 , a parasitic capacitance storage part  304 , a liquid amount detection part  310 , a measurement error correction part  311 , a temperature compensation part  312 , and a deterioration determination part  313 . 
     (E1) Capacitance Measurement Part  301   
     The capacitance measurement part  301  is means adapted to measure the capacitance of the electrode pair  21  in order to detect the liquid amount of the oil  4 . The capacitance of the electrode pair  21  can be measured as the impedance of the electrode pair  21  at the time of supplying AC current having a predetermined frequency f 1 . 
     The impedance of the electrode pair  21  can be obtained, for example, as the ratio between the effective values of voltage and current at the time of applying AC voltage to the electrode pair  21 . At this time, by selecting a sufficiently high frequency f 1 , the capacitance of the electrode pair  21  can be measured as the impedance in which the influence of the resistance R 1  of the oil  4  is suppressed as given in Expression (2). For such capacitance measurement, a frequency f 1  of 10 kHz or more can be used. For example, a frequency of 50 kHz to 1 MHz, more desirably a frequency of 100 kHz to 500 kHz is used. 
     (E2) Resistance Measurement Part  302   
     The resistance measurement part  302  is means adapted to measure the resistance value of the electrode pair  22  in order to determine the deterioration state of the oil  4 . The resistance value of the electrode pair  22  can be measured as the impedance of the electrode pair  22  at the time of supplying AC current having a predetermine frequency f 2 . 
     The impedance of the electrode pair  22  can be obtained, for example, as the ratio between the effective values of voltage and current at the time of applying AC voltage to the electrode pair  22 . At this time, by selecting a sufficiently low frequency f 2 , the resistance value R 1  of the oil  4  around the electrode pair  22  can be measured as the impedance in which the influence of the capacitance C 1  of the oil  4  is suppressed as given in Expression (3). For such resistance value measurement, a frequency f 2  of 10 kHz or less can be used. For example, a frequency of 50 Hz to 5 kHz, more desirably a frequency of 100 Hz to 1 kHz is used. 
     (E3) Temperature Measurement Part  303   
     The temperature measurement part  303  measures the temperature of the sensor board  2  using the thermistor  23   s  in order to compensate for the temperature characteristic of the resistance value of the oil  4 . Since the thermistor  23   s  is connected with the electrode pair  23 , the temperature measurement part  303  detects the temperature of the oil  4  by measuring the resistance of the electrode pair  23 . 
     (E4) Parasitic Capacitance Storage Part  304   
     The parasitic capacitance storage part  304  is storage means adapted to hold the parasitic capacitance Cs of the electrode pair  22 . The parasitic capacitance storage part  304  holds the parasitic capacitance Cs of the electrode pair  22 , which influences the measurement of the resistance value of the electrode pair  22 . 
     For example, when the parasitic capacitance Cs of the electrode pair  22  is known, the parasitic capacitance storage part  304  holds a preliminarily given value. On the other hand, when the parasitic capacitance Cs of the electrode pair  22  is unknown, the parasitic capacitance storage part  304  holds the parasitic capacitance Cs of the electrode pair  22 , which was preliminarily measured. The parasitic capacitance Cs of the electrode pair  22  varies depending on a sensor board  2 , and therefore it is desirable to preliminarily measure the parasitic capacitance Cs for each sensor board  2  and store the measured parasitic capacitance Cs in the parasitic capacitance storage part  304 . 
     The parasitic capacitance Cs of the electrode pair  22  can be measured as impedance at the time of supplying AC current having a predetermined frequency to the electrode pair  22  not immersed in the oil  4 . The impedance of the electrode pair  22  can be obtained, for example, as the ratio between the effective values of voltage and current at the time of applying AC voltage to the electrode pair  22 , as in the case of measuring the resistance value R 1 . A frequency at the time of measuring the parasitic capacitance Cs is desirably the same as the frequency f 2  at the time of measuring the resistance value R 1 , but a different frequency can also be used. 
     (E5) Liquid Amount Detection Part  310   
     The liquid amount detection part  310  detects the liquid amount of the oil  4  in the oil tank  10  on the basis of the capacitance measured by the capacitance measurement part  301 . As long as the relationship between the capacitance of the electrode pair  21  and the liquid level height of the oil  4  illustrated in  FIG. 4  is preliminarily given, the liquid level height can be obtained from the measured capacitance, and the liquid level height is outputted as liquid amount information. 
     (E6) Measurement Error Correction Part  311   
     The measurement error correction part  311  corrects the measurement error of the resistance measurement part  302  using the parasitic capacitance Cs of the electrode pair  22  held by the parasitic capacitance storage part  304 , and suppresses the influence of the parasitic capacitance Cs of the electrode pair  22  included in a corresponding measured value. 
     As given in Expression (3), in the measured value of the resistance measurement part  302 , the influence of the capacitance C 1  of the oil  4  connected in parallel to the resistance value R 1  of the oil  4  is suppressed by using a low frequency f 2 , but the influence of the capacitance C 2  of the protective film  201  and the parasitic capacitance Cs of the electrode pair  22  is exerted. For this reason, by correcting the measured value of the resistance measurement part  302  using the parasitic capacitance Cs of the electrode pair  22  held in the parasitic capacitance storage part  304 , the influence of the parasitic capacitance Cs of the electrode pair  22  can be suppressed and the resistance value R 1  of the oil  4  can be more accurately obtained. 
     (E7) Temperature Compensation Part  312   
     The temperature compensation part  312  corrects the measured value of the resistance measurement part  302  to suppress the influence of the temperature characteristics of the oil  4  on the basis of the measured value of the temperature measurement part  303 . That is, the temperature compensation part  312  compensates for a change in the resistance value R 1  of the oil  4  due to the temperature. This temperature compensation is desirably made for a value after the correction by the measurement error correction part  311 . 
     (E8) Deterioration Determination Part  313   
     The deterioration determination part  313  determines the deterioration state of the oil  4  on the basis of the parasitic capacitance Cs and the resistance value R 1  after the temperature correction. For example, the deterioration determination part  313  determines the deterioration state by comparing the corrected resistance value R 1  with a threshold value, and outputs the determined deterioration state as deterioration information. The deterioration information may be binary information indicating the presence or absence of the deterioration or ternary or more information indicating the degree of the deterioration. 
     Second Embodiment 
     In the above-described embodiment, the sensor board  2  provided with the thermistor  23   s  is described. On the other hand, in the present embodiment, an example of the sensor board  2  provided with the wiring electrode  24  for temperature measurement will be described. 
       FIGS. 9 to 11  are diagrams illustrating a configuration example of the sensor board  2  according to the second embodiment of the present invention.  FIG. 9( a )  illustrates one principal surface (first surface) of the sensor board  2 , and  FIG. 9( b )  illustrates the other principal surface (second surface). Also,  FIG. 10  is a development perspective view illustrating the structure of the sensor board  2  in  FIG. 9 .  FIG. 11  is a cross-sectional view when cutting the sensor board  2  along a B-B section line in  FIG. 10 . 
     When comparing the sensor board  2  in  FIGS. 9 to 11  with the sensor board  2  in  FIGS. 1 to 3  (first embodiment), the former is different from the latter in that the first surface is not provided with the electrode pair  23  and the thermistor  23   s , but the second surface is provided with the wiring electrode  24 . Also, the former is different from the latter in that a protective film  202  is formed on the second surface. In addition, the same components as those of the sensor board  2  in  FIGS. 1 to 3  are marked with the same symbols and redundant description will be omitted. 
     On the first surface of the sensor board  2 , the electrode pairs  21  and  22  are formed, and the protective film  201  covering the entire first surface is formed. On the second surface of the sensor board  2 , the wiring electrode  24  is formed and the protective film  202  covering the entire second surface is formed. 
     (F1) Wiring Electrode  24   
     The wiring electrode  24  is formed as a patterned electrically conductive metal layer. For example, the wiring electrode  24  having desired pattern is formed by etching copper foil put on the second surface of the base material  200 . When the ground plate  25  and the protective film  203  are formed on the second surface of the base material  200 , the wiring electrode  24  is formed on the protective film  203 . The wiring electrode  24  is means adapted to detect temperature by measuring resistance. That is, the temperature measurement is performed using the temperature characteristics of the resistance. For this reason, it is desirable to use a material having large temperature characteristics. Also, it is preferable to have a narrower and longer shape. 
     The wiring electrode  24  illustrated in  FIGS. 9 to 10  have a substantially uniform line width and have a shape arranged using substantially the entire second surface of the base material  200 . Specifically, the wiring electrode  24  has a pattern that extends from one end side to the other end side along the longer direction of the sensor board  2  while meandering in a reciprocating manner along the shorter direction of the sensor board  2 , and after reaching the vicinity of the other end, returns toward the one end side along the longer direction of the sensor board  2 . By forming such a wiring electrode  24  for temperature measurement on the second surface of the base material  200 , the need for the thermistor  23   s  can be eliminated to suppress manufacturing cost. 
     (F2) Protective Film  202   
     The protective film  202  is a film formed on the second surface of the base material  200 , and made of a dielectric material having corrosion resistance, such as a fluorine resin. The protective film  202  is formed on the entire second surface of the base material  200  and covers the wiring electrode  24 . For this reason, the wiring electrode  24  is sealed by the base material  200  and the protective film  202 , and does not contact with the oil  4  or air, and therefore the wiring electrode  24  can be prevented from being deteriorated and damaged by corrosion or the like. In addition, both surfaces of the base material  200  are covered with the protective films  201  and  202  of the same type, and thereby manufacturing cost can be suppressed. 
       FIG. 12  is a block diagram illustrating a configuration example of a control circuit  3  according to the second embodiment of the present invention. When comparing the control circuit  3  in  FIG. 12  with the control circuit  3  in  FIG. 8  (first embodiment), the former is different from the latter in that a temperature control part  303  is connected with the wiring electrode  24 . In addition, the same components as those of the control circuit  3  in  FIG. 8  are marked with the same symbols and redundant description will be omitted. 
     (F3) Temperature Measurement Part  303   
     The temperature measurement part  303  detects the temperature of the oil  4  by measuring the resistance of the wiring electrode  24 . 
     (F4) Another Pattern Example 
       FIG. 13  is a diagram illustrating another configuration example of the sensor board  2  according to the second embodiment of the present invention.  FIG. 13( a )  illustrates the first surface of the sensor board  2 , and  FIG. 13( b )  illustrates the second surface of the sensor board  2 . 
     When comparing the sensor board  2  in  FIG. 13  with the sensor board  2  in  FIG. 9  (second embodiment), only the pattern of the wiring electrode  24  is different. In addition, the rest of the configuration is the same, and therefore redundant description will be omitted. 
     The wiring electrode  24  illustrated in  FIG. 13  has a substantially uniform line width, and has a shape arranged using the substantially entire second surface of the base material  200 . Specifically, the wiring electrode  24  has a pattern that extends from one end side to the other end side along the shorter direction of the sensor board  2  while meandering in a reciprocating manner along the longer direction of the sensor board  2 . By forming such a wiring electrode  24  for temperature measurement on the second surface of the base material  200 , the need for the thermistor  23   s  can be eliminated to suppress manufacturing cost. 
     Third Embodiment 
     In the above-described second embodiment, the liquid sensor device that, with the sensor board  2  provided with the two electrode pairs  21  and  22 , measures the capacitance of the electrode pair  21  to detect the liquid amount, and measures the resistance value of the electrode pair  22  to determine the deterioration is described. On the other hand, in the present embodiment, an example of a liquid sensor device that, with a sensor board  2  provided with one electrode pair  21 , measures the capacitance and resistance value of the electrode pair  21  to detect a liquid amount and determine deterioration will be described. 
       FIG. 14  is a diagram illustrating a configuration example of the sensor board  2  according to the third embodiment of the present invention.  FIG. 14( a )  illustrates one principal surface (first surface) of the sensor board  2 , and  FIG. 14( b )  illustrates the other principal surface (second surface). 
     When comparing the sensor board  2  in  FIG. 14  with the sensor board  2  in  FIG. 9  (second embodiment), the former is different from the latter in that the first surface is not provided with the electrode pair  22 . In addition, the same components as those of the senor board  2  in  FIG. 9  are marked with the same symbols, and redundant description will be omitted. The electrode pair  21  is one whose capacitance is measured to detect the liquid amount and whose resistance value is measured to determine the deterioration state. For this reason, the need for the electrode pair  22  can be eliminated to further suppress manufacturing cost. Also, the electrode pair  21  can be arranged extending to the vicinity of the bottom surface of the oil tank  10 , and therefore a detectable liquid amount range can be expanded. 
       FIG. 15  is a block diagram illustrating a configuration example of a control circuit  3  according to the third embodiment of the present invention. When comparing the control circuit  3  in  FIG. 15  with the control circuit  3  in  FIG. 12  (first embodiment), the former is different from the latter in that a resistance measurement part  302  is connected with the electrode pair  21  and a liquid amount compensation part  314  is added. In addition, the same components as those of the control circuit  3  in  FIG. 12  are marked with the same symbols, and redundant description will be omitted. 
     (G1) Resistance Measurement Part  302   
     The resistance measurement part  302  is means adapted to measure the resistance value of the electrode pair  21  in order to determine the deterioration state of oil  4 . The resistance value of the electrode pair  21  can be measured as the impedance of the electrode pair  21  at the time of supplying AC current having a predetermined frequency f 2 . A method for measuring the impedance is the same as in the case of the electrode pair  22 . 
     (G2) Liquid Amount Compensation Part  314   
     The liquid amount compensation part  314  corrects the measured value of the resistance measurement part  302  on the basis of the liquid amount from the liquid amount detection part  310 , and suppresses the influence of the liquid amount on the measured value of the resistance measurement part  302 . The electrode pair  21  for detecting the liquid amount has a shape extending in the vertical direction, and when a liquid level rises, the number of opposite elements  21   n  and  21   m  immersed in the oil  4  is increased. For this reason, even when the resistivity of the oil  4  is constant, the resistance value of the electrode pair  21  is changed depending on the liquid amount. 
       FIG. 16  is a diagram illustrating an example of the relationship between the liquid level height of the oil  4  and the resistance value of the electrode pair  21 . This diagram is one represented with the liquid level height of the oil  4  in the oil tank  10  taken as the horizontal axis and the resistance value R 1  of the electrode pair  21  taken as the vertical axis. A characteristic  43  in the diagram exhibits a downward-sloping hyperbola. That is, it turns out that the resistance value of the electrode pair  21  is decreased depending on the liquid level height of the oil  4 , and both have a substantially inversely proportional relationship. 
     As described above, when the oil  4  is deteriorated after a long-term use, the resistivity of it reduces. For this reason, if the number of opposite elements  21   n  and  21   m  immersed in the oil  4  is constant, the deterioration state of the oil  4  can be determined on the basis of the resistance value of the electrode pair  21 . However, the resistance value of the electrode pair  21  is changed depending on the liquid amount even when the resistivity of the oil is constant. Therefore, in order to determine the deterioration state of the oil  4 , it is necessary to correct the resistance value on the basis of the liquid amount and correct the influence of the liquid amount. The liquid amount compensation part  314  corrects the resistance value of the electrode pair  21  measured by the resistance measurement part  302  on the basis of the liquid amount detected by the liquid amount detection part  310 , and obtains the resistance value from which the influence of the liquid amount is removed. The resistance value obtained in this manner is inputted to the measurement error correction part  311 . 
     In the above-described embodiments, the examples of the oil sensor device  1  targeting the oil  4  for detection are described, however the application target of the present invention is not limited only to such a device. That is, the present invention is applicable to a liquid sensor device targeting any liquid for detection. For example, the present invention is also applicable to a device targeting a fuel such as gasoline or light oil for detection. Further, the present invention is also applicable to a sensor device that targets a medium having fluidity other than liquid, for example, a deformable medium such as powder, grain, or gel for measurement, and measures the capacitance of the medium to measure a medium amount, as well as measures the resistance of the medium to determine the properties of the medium. Still further, the present invention can also target, for measurement, a medium whose dielectric constant or resistivity is changed depending on some condition regardless of the amount or properties of the medium, and sense an arbitrary state of the medium by measuring a change in the capacitance or resistance of the medium. 
     Also, in the above-described embodiments, the examples of the device that determines the deterioration state of a detection target are described. However, the present invention is not limited only to such cases. The present invention is applicable to a device that determines an arbitrary property of a detection target. For example, the present invention is also applicable to a device that determines changes in the composition and characteristics of a detection target. 
     REFERENCE SIGNS LIST 
     
         
           1  oil sensor device 
           2  sensor board 
           2   n ,  2   m  opposite element 
           200  base material 
           201 - 203  protective film 
           21  electrode pair 
           21   a ,  21   b  electrode 
           21   n ,  21   m  opposite element 
           22  electrode pair 
           22   a ,  22   b  electrode 
           22   n ,  22   m  opposite element 
           23  electrode pair 
           23   a ,  23   b  electrode 
           23   s  thermistor 
           24  wiring electrode 
           25  ground plate 
           3  control circuit 
           301  capacitance measurement part 
           302  resistance measurement part 
           303  temperature measurement part 
           304  parasitic capacitance storage part 
           310  liquid amount detection part 
           311  measurement error correction part 
           312  temperature compensation part 
           313  deterioration determination part 
           314  liquid amount compensation part 
           4  oil 
         C 1  capacitance of oil 
         C 2  capacitance of protective film 
         C 3  capacitance of air 
         Cs, Cs 1 -Cs 3  parasitic capacitance 
         R 1  resistance of oil