In a liquid level sensor according to the present invention, a first detecting electrode being always in a liquid to be measured, a second detecting electrode which measures a level of the liquid to be measured, and a third detecting electrode being always out of the liquid to be measured are arranged, an operation of charging capacitor 45 for a time being in proportion to a ratio of a length of a part of the second detecting electrode in the liquid to be measured to a total length of the second detecting electrode and an operation of discharging the electric charge of the capacitor for a time being in proportion to a ratio of a length of a part of the second detecting electrode being out of the liquid to be measured to the total length of the second detecting electrode is repeated. With this configuration, the liquid level can be precisely detected without arranging a complex operational device even though a dielectric constant or a temperature of the liquid to be measured changes.

TECHNICAL FIELD

The present invention relates to a liquid level sensor which detects a liquid level of a liquid stored in a vessel and, in particular, to a liquid level sensor which detects a liquid level of an engine oil or a fuel for an automobile, a construction machine, or the like.

BACKGROUND ART

As a liquid level sensor which detects a liquid level of an engine oil or a fuel in an automobile, a construction machine, or the like, a liquid level sensor as shown inFIG. 33or34is known (see Patent Document 1).

FIG. 33is a front view of a detecting unit of a conventional liquid level sensor. InFIG. 33, first comb-shaped detecting electrode unit2is arranged on a lower end portion of rectangular substrate1which vertically extends. Second comb-shaped detecting electrode unit3is arranged on a central portion of substrate1.

First detecting electrode unit2is configured by a plurality of linear electrodes4vertically arranged at predetermined intervals. Linear electrodes4are alternately connected to extraction lines5and6which vertically extend along both side edges of substrate1.

Second detecting electrode unit3is configured by a plurality of linear electrodes7which are arranged to extend from an upper end portion to a lower end portion at predetermined intervals from side to side. Linear electrodes7have upper ends alternately connected to extraction lines8and9.

In liquid level measurement, the detecting unit is dipped in a liquid to be measured. More specifically, first detecting electrode unit2is arranged to be always dipped in the liquid to be measured. On the other hand, second detecting electrode unit3crosses to the liquid level to be measured, and a part dipped in the liquid increases or decreases in size with rising and falling of the liquid level.

FIG. 34is a detecting circuit diagram of a conventional liquid level sensor. InFIG. 34, a detecting circuit is configured by oscillating circuit10and processing circuit17. Oscillating circuit10has inverters11,12, and13and resistor14. First and second detecting electrode units2and3which configure the detecting unit inFIG. 33are connected between inverters12and13through analog switches15and16.

Processing circuit17having a microcomputer closes analog switch15first calculates a dielectric constant of the liquid to be measured from an oscillation frequency determined by resistor14and a capacitance of first detecting electrode unit2to store the dielectric constant. Processing circuit17closes other analog switch16in place of analog switch15. A liquid level is calculated based on an oscillation frequency determined by resistor14and a capacitance of second detecting electrode unit3and the dielectric constant of the liquid to be measured.

As information of prior art document related to the invention of the application, for example, Patent Document 1 is known.

However, in the conventional liquid level sensor, a dielectric constant of a liquid to be measured is calculated from an oscillation frequency of oscillating circuit10determined by a capacitance between the electrodes of first detecting electrode unit2always dipped in the liquid to be measured and the dielectric constant is stored. Thereafter, the level of the liquid to be measured is calculated from the oscillation frequency of oscillating circuit10determined by a capacitance between the electrodes of second detecting electrode unit3crossing to the liquid level to be measured and having a part which is dipped in the liquid and increases or decreases in size with rising and falling of the liquid level, and the dielectric constant of the liquid to be measured. Therefore, an operational device to calculate the liquid level becomes complex and has a large scale.

PRIOR ART DOCUMENT

DISCLOSURE OF THE INVENTION

The present invention provides a liquid level sensor which can accurately detect a liquid level without a complex operational device even though a dielectric constant or a temperature of a liquid to be measured changes.

A liquid level sensor according to the present invention includes a detecting unit having a first detecting electrode always being in a liquid to be measured, a second detecting electrode which measures a liquid level of the liquid to be measured, and a third detecting electrode always being out of the liquid to be measured; and a circuit which repeats an operation of charging for a time which is in proportion to a ratio of a length of a part of the second detecting electrode dipped in the liquid to be measured to a total length of the second detecting electrode and an operation of discharging the charged electric charge for a time which is in proportion to a ratio of a length of a part of the second detecting electrode being out of the liquid to be measured to the total length of the second detecting electrode. With this configuration, a voltage being in proportion to a liquid level can be always outputted without arranging a complex operational device even though a dielectric constant, a temperature, or the like of the liquid to be measured changes. For this reason, a highly sensitive liquid level sensor can be easily provided.

PREFERRED EMBODIMENTS FOR CARRYING OUT OF THE INVENTION

A liquid level sensor according to Embodiment 1 of the present invention will be described below with reference to the accompanying drawings.

FIG. 1is a front view of a detecting unit in the liquid level sensor according to Embodiment 1 of the present invention. InFIG. 1, a pair of first comb-shaped detecting electrodes22made of carbon are arranged on a lower end portion of vertically extending rectangular detecting unit21made of a polyimide film or the like. A pair of second comb-shaped detecting electrodes23made of carbon are arranged at the center of detecting unit21. Furthermore, similarly, a pair of third comb-like detecting electrodes24made of carbon are arranged on an upper end portion of detecting unit21. First, second, and third detecting electrodes22,23, and24are connected to terminals26,27,28, and29with vertically extending extraction lines25.

FIG. 2is a detecting circuit diagram of the liquid level sensor according to Embodiment 1 of the present invention. InFIG. 2, terminal26of detecting unit21is connected to first potential30configured by a GND potential. Terminals27,28, and29of detecting unit21are connected to second potential34configured by a 5-V power supply potential through resistors31,32, and33, respectively. In this manner, first detecting electrodes22, second detecting electrodes23, and third detecting electrodes24are connected to resistors31,32, and33, respectively. At this time, in a state in which first, second, and third detecting electrodes22,23, and24are out of a liquid to be measured, a time constant determined by an inter-electrode capacitance of first detecting electrodes22and resistor31, a time constant determined by an inter-electrode capacitance of second detecting electrodes23and resistor32, and a time constant determined by an inter-electrode capacitance of third detecting electrodes24and resistor33are set to be substantially equal to one another.

A first midpoint potential between resistor31and first detecting electrode22is compared with a threshold value given by resistors36and37in first comparing unit35configured by a comparator. Similarly, a second midpoint potential between resistor32and second detecting electrode23and a third midpoint potential between resistor33and third detecting electrode24is compared with a threshold value given by resistors36and37in second comparing unit38and third comparing unit39which are configured by comparators, respectively.

Output signals from first, second, and third comparing units35,38, and39are inputted to logic circuit40configured by a logic element or a flip-flop. At a subsequent stage of logic circuit40, first analog switch41and second analog switch42which are open-and-close-controlled by an output signal from logic circuit40are arranged. One end of fourth resistor43is connected to a midpoint between first analog switch41and second analog switch42, and the other end is connected to output terminal44. Furthermore, one end of capacitor45is connected to first potential30, and the other end is connected between fourth resistor43and output terminal44.

In this manner, an electronic circuit of the liquid level sensor according to Embodiment 1 of the present invention is configured.

A circuit operation of the liquid level sensor according to Embodiment 1 of the present invention will be described below with reference toFIGS. 3A to 3H.

FIGS. 3A to 3Hshow voltage waveforms of each unit of the liquid level sensor according to Embodiment 1 of the present invention. Detecting unit21of the liquid level sensor shown inFIG. 1is dipped in a liquid to be measured such as an engine oil in an oil pan (not shown). At this time, first detecting electrodes22are always dipped in the liquid to be measured, and third detecting electrodes24are arranged out of the liquid to be measured. Second detecting electrodes23cross to a level of the liquid to be measured, and a part dipped in the liquid increases or decreases in size with rising and falling of the liquid level.

In an initial state (t0) before power supply is turned on, electric charges are not present between first, second, and third detecting electrodes22,23, and24. Therefore, all the first midpoint potential between resistor31and first detecting electrode22, the second midpoint potential between resistor32and second detecting electrode23, and the third midpoint potential between resistor33and third detecting electrode24are equal to first potential30(V1).

InFIG. 3A, when the power supply is turned on, the third midpoint potential between resistor33and third detecting electrode24exponentially increases from first potential30(V1) to second potential34(V2) at the time constant determined by resistor33and the inter-electrode capacitance of third detecting electrodes24.

InFIG. 3B, the second midpoint potential between resistor32and second detecting electrode23exponentially increases from first potential30(V1) to second potential34(V2) at the time constant determined by resistor32and the inter-electrode capacitance of second detecting electrodes23. At this time, since the part of second detecting electrode23is in the liquid to be measured, the time constant determined by resistor32and the electrostatic capacitance of second detecting electrodes23is larger than the time constant determined by resistor33and third detecting electrode24.

InFIG. 3C, the first midpoint potential between resistor31and first detecting electrode22exponentially increases from first potential30(V1) to second potential34(V2) at the time constant determined by resistor31and the inter-electrode capacitance of first detecting electrodes22. At this time, since first detecting electrodes22are always dipped in the liquid to be measured, the time constant determined by resistor31and the electrostatic capacitance of first detecting electrodes22is larger than the time constant determined by resistor32and second detecting electrodes23.

InFIG. 3D, when the third midpoint potential between resistor33and third detecting electrode24reaches threshold voltage Vthdetermined by resistors36and37, an output from third comparing unit39configured by a comparator shifts from high to low (t1).

InFIG. 3E, when the second midpoint potential between resistor32and second detecting electrode23reaches threshold voltage Vthdetermined by resistors36and37, an output from second comparing unit38configured by a comparator shifts from high to low (t2).

InFIG. 3F, when the first midpoint potential between resistor31and first detecting electrode22reaches threshold voltage Vthdetermined by resistors36and37, an output from first comparing unit35configured by a comparator shifts from high to low. At the same time, electric charges accumulated in first, second, and third detecting electrodes22,23and24are discharged to first potential30(V1) through element46having an open-collector configuration. Therefore, all the first midpoint potential between resistor31and first detecting electrode22, the second midpoint potential between resistor32and second detecting electrode23, and the third midpoint potential between resistor33and third detecting electrode24return to first potential30(V1). Furthermore, each of outputs from first, second, and third comparing units35,38, and39shifts from low to high (t3) as shown inFIGS. 3F,3E, and3D.

Thereafter, the third midpoint potential between resistor33and third detecting electrode24, the second midpoint potential between resistor32and second detecting electrode23, and the first midpoint potential between resistor31and first detecting electrode22exponentially increase again from first potential30(V1) to second potential34(V2), as shown inFIGS. 3A,3B, and3C, at the time constants determined by resistor33and the inter-electrode capacitance of third detecting electrodes24, resistor32and inter-electrode capacitance of second detecting electrodes23, and resistor31and inter-electrode capacitance of first detecting electrodes22.

When the third midpoint potential between resistor33and third detecting electrode24reaches threshold voltage Vthdetermined by resistors36and37, an output from third comparing unit39configured by a comparator shifts from high to low (t4) as shown inFIG. 3D.

In the same manner as described above, when the second midpoint potential between resistor32and second detecting electrode23reaches threshold voltage Vthdetermined by resistors36and37, an output from second comparing unit38configured by a comparator shifts from high to low (t5) as shown inFIG. 3E.

When the first midpoint potential between resistor31and first detecting electrode22reaches threshold voltage Vthdetermined by resistors36and37, an output from first comparing unit35configured by a comparator shifts from high to low as show inFIG. 3F. At the same time, electric charges accumulated in first, second, and third detecting electrodes22,23, and24are discharged to first potential30through element46having an open-collector configuration. Therefore, all the first midpoint potential between resistor31and first detecting electrode22, the second midpoint potential between resistor32and second detecting electrode23, and the third midpoint potential between resistor33and third detecting electrode24return to first potential30(V1). Furthermore, each of outputs from first, second, and third comparing units35,38, and39shifts from low to high (t6) as shown inFIGS. 3F,3D, and3E. The same operations as described above will be repeated subsequently.

Output signals from first, second, and third comparing units35,38, and39are inputted to logic circuit40configured by a logic element or a flip-flop. A signal shown inFIG. 3Gis outputted to first analog switch41. A signal shown inFIG. 3His outputted to second analog switch42.

In this case, when a signal inputted to first analog switch41is at high, first analog switch41is “opened”. When the signal is at low, first analog switch41is “closed”. When a signal inputted to second analog switch42is at high, second analog switch42is “closed”. When the signal is at low, second analog switch42is “opened”.

In this manner, for times t1to t2and times t4to t5, first analog switch41is “closed”, and second analog switch42is “opened”. For this reason, capacitor45is charged from second potential34through fourth resistor43. For times t2to t3and times t5to t6, first analog switch41is “opened”, and second analog switch42is “closed”. For this reason, electric charges accumulated in capacitor45are discharged to first potential30(V1) through fourth resistor43.

For times t0to t1and times t3to t4, both first analog switch41and second analog switch42are “opened”, electric charges accumulated in capacitor45are stored. In this manner, first analog switch41and second analog switch42are alternately opened and closed for a period determined by a length of a part of second detecting electrode23dipped in the liquid to be measured and a length of a part being out of the liquid to be measured to discharge capacitor45so as to make it possible to output a liquid level of the liquid to be measured to output terminal44as an analog voltage.

The above circuit operations will be further described by using numerical expressions. In order to briefly describe the circuit operations, electrostatic capacitances per unit length obtained when first, second, third detecting electrodes22,23, and24are out of a liquid to be measured are represented by C0, resistances of resistors31and33are represented by r1, a resistance of resistor32is represented by r2, lengths of first and third detecting electrodes22and24are represented by A, a length of second detecting electrode23is represented by B, a relative dielectric constant of the liquid to be measured is represented by ∈r, a length of second detecting electrode23dipped in the liquid to be measured is represented by αB, and a length of second detecting electrode23being out of the liquid to be measured is represented by (1−α)B. Note that α denotes an arbitrary positive number in the range of 0 to 1. First potential30(V1) is represented by 0 [V], and second potential34(V2) is represented by Vdd[V]. At this time, the time constant determined by first detecting electrode22and resistor31is expressed by Equation 1.
[Numerical Expression 1]
C0Ar1∈25rEquation 1

A time constant determined by second detecting electrode23and resistor32is expressed by Equation 2.
[Numerical Expression 2]
C0Br2[α∈r+(1−α)]  Equation 2

A time constant determined by third detecting electrode24and resistor33is expressed by Equation 3.
[Numerical Expression 3]
C0Ar1Equation 3

As described above, in a state in which all first, second, and third detecting electrodes22,23, and24are out of a liquid to be measured, the time constant determined by an inter-electrode capacitance of first detecting electrodes22and resistor31, the time constant determined by an inter-electrode capacitance of second detecting electrodes23and resistor32, and the time constant determined by an inter-electrode capacitance of third detecting electrodes24and resistor33are set to be substantially equal to one another. Therefore,
C0Ar1=C0Br2
is given. This value is newly defined as D.

When the power supply is turned on, third midpoint potential Vn3between resistor33and third detecting electrode24is expressed by Equation 4.

According to this equation, t1inFIGS. 3A to 3His expressed by Equation 5.

Second midpoint potential Vn2between resistor32and second detecting electrode23is expressed by Equation 6.

According to Equation 6, t2inFIGS. 3A to 3His expressed by Equation 7.

In this manner, time Tcfor which capacitor45is charged is expressed by Equation 8.

Time Tdfor which capacitor45is discharged is expressed by Equation 9.

According to Equation 8 and Equation 9 described above, it is understood that charging time Tcis in proportion to length αB of a part of second detecting electrode23dipped in a liquid to be measured and that discharging time Tdis determined by a length of a part of second detecting electrode23being out of the liquid to be measured. Furthermore, it is understood that, even though dielectric constant of the liquid to be measured is changed by a change in temperature, deterioration and denaturation of the liquid to be measured, and the like, or even though C0, i.e., D is changed by a change in vapor pressure of the liquid to be measured with a change in temperature or the like, although Tcand Tdchange, a ratio of Tcto Tddoes not change.

Output voltage V0generated at output terminal44is expressed by a numerical expression. In the circuit shown inFIG. 2, as described inFIGS. 3A to 3H, for the periods of t0to t1, t3to t4, and the like, both first and second analog switches41and42are “opened”. Therefore, an electric charge accumulated in capacitor45does not change, and output voltage V0does not also change.

In the following description, these periods are ignored, and a change in output voltage when charging time Tcand discharging time Tdare repeated will be considered. An output voltage obtained immediately after the power supply is turned on is set to 0 [V]. When a resistance of fourth resistor43is represented by R and when a capacitance of capacitor45is represented by C, output voltage V1cobtained after the first charge is expressed by Equation 10.

Output voltage V1dobtained after the first discharge is expressed by Equation 11.

Output voltage V2cobtained after the second charge is expressed by Equation 12.

Output voltage V2dobtained after the second discharge is expressed by Equation 13.

Output voltage V3cobtained after the third charge and output voltage V3dobtained after discharge are expressed by Equation 14 and Equation 15, respectively.

By the same manner as described above, output voltages obtained after charge and discharge can be calculated.FIGS. 4 to 7are obtained by calculating output voltage V0when resistance R of fourth resistor43and capacitance C of capacitor45are set to 500 kΩ and 100 pF, respectively.

FIG. 4is a characteristic graph showing a change of output voltage V0with time when a ratio of a length of a part of the second detecting electrode dipped in the liquid to be measured to a length of a part being out of the liquid to be measured is 1:4 in the liquid level sensor according to Embodiment 1 of the present invention.FIG. 4shows a change of output voltage V0with time when charging time Tcand discharging time Tdare set to 1 μsec and 4 μsec, respectively, more specifically, a ratio of Tcto Td, i.e., a ratio of a length of a part of second detecting electrodes23dipped in the liquid to be measured to a length of a part being out of the liquid to be measured is 1:4. It is understood that output voltage V0made by superposing a ripple having an amplitude of about ±0.04 [V] on a DC component of 1 [V] is obtained after about 500 μsec have elapsed. This ripple can be removed by using an appropriate low-pass filter.

FIG. 5is a characteristic graph showing a change of output voltage V0with time when a ratio of a length of a part of the second detecting electrode dipped in a liquid to be measured to a length of a part being out of the liquid to be measured is 1:1 in the liquid level sensor according to Embodiment 1 of the present invention.FIG. 5shows a change of output voltage V0with time when charging time Tcand discharging time Tdare set to 2.5 μsec and 2.5 μsec, respectively, more specifically, a ratio of Tcto Td, i.e., a ratio of a length of a part of second detecting electrode23dipped in the liquid to be measured to a length of a part being out of the liquid to be measured is 1:1. It is understood that output voltage V0made by superposing a ripple having an amplitude of about ±0.06 [V] on a DC component of 2.5 [V] is obtained after about 500 μsec have elapsed.

FIG. 6is a characteristic graph showing a change of output voltage with time when a ratio of a length of a part of the second detecting electrode dipped in a liquid to be measured to a length of a part being out of the liquid to be measured is 4:1 in the liquid level sensor according to Embodiment 1 of the present invention.FIG. 6shows a change of output voltage V0with time when charging time Tcand discharging time Tdare set to 4 μsec and 1 μsec, respectively, more specifically, a ratio of Tcto Td, i.e., a ratio of a length of a part of second detecting electrode23dipped in the liquid to be measured to a length of a part being out of the liquid to be measured is 4:1. It is understood that output voltage V0made by superposing a ripple having an amplitude of about ±0.04 [V] on a DC component of 4 [V] is obtained after about 500 μsec have elapsed. In this manner, output voltage V0which is in proportion to the ratio of the length of the part of second detecting electrode23dipped in the liquid to be measured to the length of the part dipped in the liquid to be measured is generated at output terminal44.

FIG. 7is a characteristic graph showing a change of output voltage V0with time when a ratio of a length of a part of the second detecting electrode dipped in a liquid to be measured to a length of a part being out of the liquid to be measured is 1:4 in the liquid level sensor according to Embodiment 1 of the present invention and when a charging time and a discharging time are elongated by 20%.FIG. 7shows a change of output voltage V0with time when charging time Tcand discharging time Tdare set to 4.8 μsec and 1.2 μsec, respectively, i.e., Tcand Tdare each longer than those inFIG. 6by 20%. InFIG. 7, it is understood that output voltage V0made by superposing a ripple having an amplitude of about ±0.05 [V] on a DC component of 4 [V] is obtained after about 500 μsec have elapsed. More specifically, it is understood that, even though dielectric constant ∈rof the liquid to be measured is changed by a change in temperature, deterioration and denaturation of the liquid to be measured, and the like, or even though C0, i.e., D is changed by a change in vapor pressure of the liquid to be measured with a change in temperature or the like to change Tcand Td, if the ratio of Tcto Td, i.e., the ratio of the length of the part of second detecting electrode23dipped in the liquid to be measured to the length of the part being out of the liquid to be measured is constant, a DC component of output voltage V0does not change.

As described above, the liquid level sensor according to Embodiment 1 of the present invention can always automatically output a voltage which is in proportion to a liquid level of the liquid to be measured without arranging a complex operational device in the liquid level sensor even though the dielectric constant or temperature of the liquid to be measured changes. In this manner, a highly sensitive liquid level sensor can be easily provided.

A liquid level sensor according to Embodiment 2 of the present invention will be described below with reference to the accompanying drawings.

FIG. 8is a front view of a detecting unit of the liquid level sensor according to Embodiment 2 of the present invention. InFIG. 8, a pair of first comb-shaped detecting electrodes222made of carbon are arranged on a lower end portion of vertically extending rectangular detecting unit221made of a polyimide film or the like. A pair of second comb-shaped detecting electrodes223made of carbon are arranged at the center of detecting unit221. Furthermore, similarly, a pair of third comb-shaped detecting electrodes224made of carbon are arranged on an upper end portion of detecting unit221. First, second, and third detecting electrodes222,223, and224are connected to terminals229,230,231, and232by common extraction line225and extraction lines226,227, and228which vertically extend. First cancel electrode233arranged in the same direction as that of extraction line226of first detecting electrode222is arranged to extend from the upper end portion of detecting unit221to the upper end portion of first detecting electrode222. The distance between first cancel electrode233and extraction line226of first detecting electrode222is made substantially equal to that between common extraction line225and extraction line226of the first detecting electrode. Furthermore, first cancel electrode233is connected to terminal234.

FIG. 9is a detecting circuit diagram of the liquid level sensor according to Embodiment 2 of the present invention. InFIG. 9, a pulse from pulse generating circuit235is inputted to terminal229of detecting unit221. Terminal230of detecting unit221is connected to one end of first resistor236. A signal obtained by inverting a pulse is inputted to terminal234of detecting unit221. In the detecting circuit, a signal branched from an input of a NOR gate in the final stage of pulse generating circuit235is inputted to terminal234of detecting unit221. The other end of first resistor236is connected to an output side of first differential amplifier237. A first node potential between terminal230and first resistor236is inputted to a negative terminal of first differential amplifier237. A first threshold value determined by resistors238,239,240, and241is inputted to a positive terminal. Similarly, terminals231and232of detecting unit221are connected to one ends of second and third resistors242and243, respectively. Further, other ends of second and third resistors242and243are connected to output sides of second and third differential amplifiers244and245, respectively. A second node potential between terminal231and second resistor242and a third node potential between terminal232and third resistor243are inputted to negative terminals of second and third differential amplifiers244and245. A first threshold value determined by resistors238,239,240, and241is inputted to positive terminals of second and third differential amplifiers244and245. In Embodiment 2 of the present invention, the first threshold value is set to ½ of a power supply voltage. Furthermore, a diode and a resistor are connected in series between an input and an output of each of the differential amplifiers.

With this configuration, first detecting electrode222, second detecting electrode223, and third detecting electrode224are connected to first resistor236, second resistor242, third resistor243, first differential amplifier237, second differential amplifier244, and third differential amplifier245, respectively. At this time, in a state in which all first, second, and third detecting electrodes222,223, and224are out of the liquid to be measured, a time constant determined by an inter-electrode capacitance of first detecting electrodes222and first resistor236, a time constant determined by an inter-electrode capacitance of second detecting electrodes223and second resistor242, and a time constant determined by an inter-electrode capacitance of third detecting electrodes224and third resistor243are set to be substantially equal to one another.

An output potential from first differential amplifier237is compared with a second threshold value determined by resistors238,239,240, and241in first comparing unit246. Similarly, an output potential from second differential amplifier244and an output potential from third differential amplifier245are compared with a second threshold value determined by resistors238,239,240, and241in second comparing unit247and third comparing unit248which are each configured by comparators. In Embodiment 2 of the present invention, the second threshold value is set to ¼ of the power supply voltage.

Output signals from first, second, and third comparing units246,247, and248are inputted to logic circuit249configured by a logic element and a flip-flop. First analog switch250and second analog switch251which are open/close-controlled based on the output signal from logic circuit249are arranged on the subsequent stage of logic circuit249between first potential252and second potential253. Fourth resistor254has one end connected to a midpoint between first analog switch250and second analog switch251. The other end of fourth resistor254is connected to output terminal255. Capacitor256has one end connected to first potential252and the other end connected between fourth resistor254and output terminal255.

A circuit operation of the liquid level sensor according to Embodiment 2 of the present invention will be described below.FIGS. 10A to 10Iare voltage waveform charts of each unit in the liquid level sensor according to Embodiment 2 of the present invention. Detecting unit221of the liquid level sensor shown inFIG. 8is dipped in a liquid to be measured such as an engine oil in an oil pan (not shown). At this time, first detecting electrodes222are always dipped in the liquid to be measured, and third detecting electrodes224are always arranged out of the liquid to be measured. Second detecting electrodes223cross to the level of the liquid to be measured, and a part dipped in the liquid to be measured increases or decreases in size with rising and falling of the liquid level.

In an initial state (t0) before power supply is turned on, electric charges are not present between first, second, and third detecting electrodes222,223, and224. Therefore, all a first node potential between resistor236and first detecting electrode222, a second node potential between resistor242and second detecting electrode223, and a third node potential between resistor243and third detecting electrode224are equal to first potential52(V1).

InFIG. 10A, when the power supply is turned on (t0), a pulse from pulse generating circuit235is inputted to terminal229of detecting unit221.

InFIG. 10B, due to the pulse, an output potential from third differential amplifier245exponentially increases from first potential252(V1) to second potential253(V2) at the time constant determined by third resistor243and an inter-electrode capacitance of third detecting electrodes224.

InFIG. 10C, an output potential from second differential amplifier244exponentially increases from first potential252(V1) to second potential253(V2) at the time constant determined by second resistor242and an inter-electrode capacitance of second detecting electrodes223. At this time, since a part of second detecting electrode223is in the liquid to be measured, the time constant determined by second resistor242and an electrostatic capacitance of second detecting electrodes223is larger than the time constant determined by third resistor243and third detecting electrode224.

InFIG. 10D, an output potential from first differential amplifier237exponentially increases from first potential252(V1) to second potential253(V2) at the time constant determined by first resistor236and an inter-electrode capacitance of first detecting electrodes222. At this time, first detecting electrodes222are always dipped in the liquid to be measured. Therefore, the time constant determined by first resistor236and an electrostatic capacitance of first detecting electrodes222is larger than the time constant determined by second resistor242and second detecting electrode223.

In this case, between terminals229and230of detecting unit221, an electric charge accumulated in an electrostatic capacitance by first detecting electrode222and an electric charge accumulated in an electrostatic capacitance by common extraction line225and extraction line226of first detecting electrode222are present. Since an electrostatic capacitance obtained by common extraction line225and extraction line226of first detecting electrode222changes depending on a level of the liquid to be measured, the electrostatic capacitance of first detecting electrodes222is apparently measured as if the electrostatic capacitance increases. As a result, measurement of the level of the liquid to be measured has an error.

In Embodiment 2 of the present invention, a distance between first cancel electrode233and extraction line226of first detecting electrode222is made substantially equal to a distance between common extraction line225and extraction line226of first detecting electrode222to make the electrostatic capacitances substantially equal to each other. A signal obtained by inverting an output signal from pulse generating circuit235is inputted to terminal234of detecting unit221.

With this configuration, an electric charge the amount of which is substantially equal to an electric charge accumulated between extraction line226of first detecting electrode222and common extraction line225and the sign of which is opposite to the electric charge is accumulated between extraction line226of first detecting electrode222and first cancel electrode233. Therefore, electric charges accumulated between extraction line226of first detecting electrode222and common extraction line225are canceled. This eliminates that an electrostatic capacitance of the liquid to be measured that is detected by first detecting electrodes222is influenced by the level of the liquid to be measured.

InFIG. 10E, when the third node potential between third resistor243and third detecting electrode224reaches first threshold voltage Vthdetermined by resistors238,239,240, and241, an output from third comparing unit248configured by a comparator shifts from high to low (t1).

InFIG. 10F, when the second node potential between second resistor242and second detecting electrode223reaches first threshold voltage Vthdetermined by resistors238,239,240, and241, an output from second comparing unit247configured by a comparator shifts from high to low (t2).

InFIG. 10G, when the first node potential between first resistor236and first detecting electrode222reaches first threshold voltage Vthdetermined by resistors238,239,240, and241, an output from first comparing unit246configured by a comparator shifts from high to low. At the same time, pulse generation from pulse generating circuit235is stopped. Therefore, output voltages from first differential amplifier237, second differential amplifier244, and third differential amplifier245increase to second potential (V1) (t3).

Thereafter, diodes connected between the inputs and the outputs of first differential amplifier237, second differential amplifier244, and third differential amplifier245are turned on. At this time, output voltages from the differential amplifiers rapidly decrease. When the output voltages reach first threshold potentials given to the positive inputs of the differential amplifiers, outputs from the differential amplifiers return to first potential252(V1). At the same time, each of outputs from first, second, and third comparing units246,247, and248shifts from low to high as shown inFIGS. 10G,10F, and10E. As shown inFIG. 10A, a pulse is generated from pulse generating circuit235and inputted to terminal229of detecting unit221(t4).

Furthermore, thereafter, output potentials from first differential amplifier237, second differential amplifier244, and third differential amplifier245increase again from first potential252(V1) to second potential253(V2). More specifically, as shown inFIGS. 10D,10C, and10B, the output potentials exponentially increase at time constants determined between third resistor243the inter-electrode capacitance of third detecting electrodes224, between second resistor242and the inter-electrode capacitance of second detecting electrodes223, and between first resistor236and the inter-electrode capacitance of first detecting electrodes222. Subsequently, the same operations as those for a period of t0to t4are repeated.

InFIGS. 10H and 10I, output signals from first, second, and third comparing units246,247, and248are inputted to logic circuit249configured by a logic element and a flip-flop. A signal shown inFIG. 10His outputted to first analog switch250. A signal shown inFIG. 10Iis outputted to second analog switch251.

When a signal inputted to first analog switch250is at high, second analog switch251is “closed”. When a signal inputted to first analog switch250is at low, second analog switch251is “opened”.

When a signal inputted to second analog switch251is at high, second analog switch251is “closed”. When a signal inputted to second analog switch251is at low, second analog switch251is “opened”.

In this manner, for times t1to t2and times t5to t6, first analog switch250is “closed”, and second analog switch251is “opened”. Therefore, capacitor256is charged from second potential253through fourth resistor254. For times t2to t3and times t6to t7, first analog switch250is “opened”, and second analog switch251is “closed”. Therefore, an electric charge accumulated in capacitor256is discharged to first potential252(V1) through fourth resistor253.

For times t0to t1, times t3to t5, and the like, both first analog switch250and second analog switch251are “opened”. Therefore an electric charge accumulated in capacitor256is stored.

In this manner, first analog switch250and second analog switch251are alternately opened and closed for a time determined by a length of a part of second detecting electrode223dipped in the liquid to be measured and a length of a part being out of the liquid to be measured to charge and discharge capacitor256, so that a level of the liquid to be measured can be outputted to output terminal255as an analog voltage.

The above circuit operations will be further described by using numerical expressions. An electrostatic capacitance per unit length obtained when first, second, third detecting electrodes222,223, and224are out of a liquid to be measured is represented by C0, resistances of first resistor236and third resistor243are represented by r1, a resistance of second resistor242is represented by r2, lengths of first detecting electrode222and third detecting electrode224are represented by A, a length of second detecting electrode223is represented by B, and a relative dielectric constant of the liquid to be measured is represented by ∈r. A length of second detecting electrode223dipped in the liquid to be measured is represented by αB, and a length of second detecting electrode223being out of the liquid to be measured is represented by (1−α)B.

Note that α is an arbitrary positive number in the range of 0 to 1. First potential252(V1) is represented by 0 [V], and second potential253(V2) is represented by Vdd[V]. At this time, the time constant determined by first detecting electrode222and first resistor236is expressed by Equation 16.
[Numerical Expression 16]
C0Ar1∈r25Equation 16

A time constant determined by second detecting electrode223and resistor242is expressed by Equation 17.
[Numerical Expression 17]
C0Br2[α∈r+(1−α)]  Equation 17

A time constant determined by third detecting electrode224and resistor243is expressed by Equation 18.
[Numerical Expression 18]
C0Ar1Equation 18

In this case, as described above, in a state in which all first, second, and third detecting electrodes222,223, and224are out of the liquid to be measured, the time constant determined by an inter-electrode capacitance of first detecting electrodes222and resistor236, the time constant determined by an inter-electrode capacitance of second detecting electrodes223and resistor242, and the time constant determined by an inter-electrode capacitance of third detecting electrodes224and resistor243are set to be substantially equal to one another. Therefore,
C0Ar1=C0Br2
is given. This value is newly defined as D.

When the power supply is turned on, third node potential Vn3between resistor243and third detecting electrode224is expressed by Equation 19.

According to this equation, t1is expressed by Equation 20.

Similarly, second node potential Vn2between resistor242and second detecting electrode223is expressed by Equation 21.

According to the equation, t2is expressed by Equation 22.

In this manner, time Tcfor which capacitor256is charged is expressed by Equation 23.

By the same manner as described above, time Tdfor which capacitor256is discharged is expressed by Equation 24.

According to Equation 23 and Equation 24 described above, it is found that charging time Tcis in proportion to length αB of a part of second detecting electrode223dipped in a liquid to be measured and that discharging time Tdis determined by a length of a part of second detecting electrode223being out of the liquid to be measured. Furthermore, it is understood that, even though dielectric constant ∈rof the liquid to be measured is changed by a change in temperature, deterioration and denaturation of the liquid to be measured, and the like, or even though C0, i.e., D is changed by a change in vapor pressure of the liquid to be measured with a change in temperature or the like, although Tcand Tdchange, a ratio of Tcto Tddoes not change.

Output voltage V0generated at output terminal255is expressed by a numerical expression. In the circuit shown inFIG. 9, as described inFIGS. 10A to 10I, for the periods of t0to t1, t3to t6, and the like, both first and second analog switches250and251are “opened”. Therefore, an electric charge accumulated in capacitor256does not change, and output voltage V0does not also change.

Therefore, in the following description, these periods are ignored, and a change in output voltage when charging time Tcand discharging time Tdare repeated will be considered. An output voltage obtained immediately after the power supply is turned on is set to 0 [V]. When a resistance of fourth resistor254is represented by R and when a capacitance of capacitor256is represented by C, output voltage V1cobtained after the first charge is expressed by Equation 25.

Output voltage V1dobtained after the first discharge is expressed by Equation 26.

Output voltage V2cobtained after the second charge is expressed by Equation 27.

Output voltage V2dobtained after the second discharge is expressed by Equation 28.

Output voltage V3cobtained after the third charge and output voltage V3dobtained after discharge are expressed by Equation 29 and Equation 30, respectively.

By the same manner as described above, output voltages obtained after charge and discharge can be calculated.FIGS. 11 to 14are obtained by calculating output voltage V0when resistance R of fourth resistor254and capacitance C of capacitor256are set to 500 kΩ and 100 pF, respectively.

FIG. 11is a characteristic graph showing a change of output voltage V0with time when a ratio of a length of a part of the second detecting electrode dipped in the liquid to be measured to a length of a part being out of the liquid to be measured is 1:4 in the liquid level sensor according to Embodiment 2 of the present invention.FIG. 11shows a change of output voltage V0with time when charging time Tcand discharging time Tdare set to 1 μsec and 4 μsec, respectively, more specifically, a ratio of Tcto Td, i.e., a ratio of a length of a part of second detecting electrode223dipped in the liquid to be measured to a length of a part being out of the liquid to be measured is 1:4. It is understood that output voltage V0made by superposing a ripple having an amplitude of about ±0.04 [V] on a DC component of 1 [V] is obtained after about 500 μsec have elapsed. This ripple can be removed by using an appropriate low-pass filter.

FIG. 12is a characteristic graph showing a change of output voltage V0with time when a ratio of a length of a part of the second detecting electrode dipped in a liquid to be measured to a length of a part being out of the liquid to be measured is 1:1 in the liquid level sensor according to Embodiment 2 of the present invention.FIG. 12shows a change of output voltage V0with time when charging time Tcand discharging time Tdare set to 2.5 μsec and 2.5 μsec, respectively, more specifically, a ratio of Tcto Td, i.e., a ratio of a length of a part of second detecting electrode223dipped in the liquid to be measured to a length of a part being out of the liquid to be measured is 1:1. Output voltage V0made by superposing a ripple having an amplitude of about ±0.06 [V] on a DC component of 2.5 [V] is obtained after about 500 μsec have elapsed.

FIG. 13is a characteristic graph showing a change of output voltage V0with time when a ratio of a length of a part of the second detecting electrode dipped in a liquid to be measured to a length of a part being out of the liquid to be measured is 4:1 in the liquid level sensor according to Embodiment 2 of the present invention.FIG. 13shows a change of output voltage V0with time when charging time Tcand discharging time Tdare set to 4 μsec and 1 sec, respectively, more specifically, a ratio of Tcto Td, i.e., a ratio of a length of a part of second detecting electrode223dipped in the liquid to be measured to a length of a part being out of the liquid to be measured is 4:1. Output voltage V0made by superposing a ripple having an amplitude of about ±0.04 [V] on a DC component of 4 [V] is obtained after about 500 μsec have elapsed.

In this manner, output voltage V0which is in proportion to the ratio of the length of the part of second detecting electrode223dipped in the liquid to be measured to the length of the part being out of the liquid to be measured is generated at output terminal255.

FIG. 14is a characteristic graph showing a change of output voltage V0with time when a ratio of a length of a part of the second detecting electrode dipped in a liquid to be measured to a length of a part being out of the liquid to be measured is 1:4 in the liquid level sensor according to Embodiment 2 of the present invention and when a charging time and a discharging time are elongated by 20%.FIG. 14shows a change of output voltage V0with time when charging time Tcand discharging time Tdare set to 4.8 μsec and 1.2 μsec, respectively, i.e., Tcand Tdare each longer than those inFIG. 13by 20%. Output voltage V0made by superposing a ripple having an amplitude of about ±0.05 [V] on a DC component of 4 [V] is obtained after about 500 μsec have elapsed.

More specifically, even though dielectric constant ∈rof the liquid to be measured is changed by a change in temperature, deterioration and denaturation of the liquid to be measured, and the like, or even though C0, i.e., D is changed by a change in vapor pressure of the liquid to be measured with a change in temperature or the like to change Tcand Td, if the ratio of Tcto Td, i.e., the ratio of the length of the part of second detecting electrode223is dipped in the liquid to be measured to the length of the part being out of the liquid to be measured is constant, a DC component of output voltage V0does not change.

A liquid level sensor according to Embodiment 3 of the present invention will be described below.

FIG. 15is a front view of a detecting unit in the liquid level sensor according to Embodiment 3 of the present invention. InFIG. 15, a pair of first comb-shaped detecting electrodes322made of carbon are arranged on a lower portion of vertically extending rectangular detecting unit321made of a polyimide film or the like. Above first detecting electrodes322, a pair of second comb-shaped detecting electrodes323made of carbon are arranged. Above second detecting electrodes323, similarly a pair of third comb-shaped detecting electrodes324made of carbon are similarly arranged. Further, a pair of fourth comb-shaped detecting electrodes325made of carbon are similarly arranged on a lower end portion of detecting unit321. First, second, third, and fourth detecting electrodes322,323,324, and325are connected to terminals326,327,328,329, and330with vertically extending extraction lines.

FIG. 16is a sectional view of fourth detecting electrodes325taken along line16-16inFIG. 15. InFIG. 16, fourth detecting electrode325is configured by covering an entire area of opposite electrodes331with metal layer333through insulator332.

With this configuration, since electric flux lines generated between opposite electrodes331do not pass through an liquid to be measured, an electrostatic capacitance between the electrodes331measured by first detecting electrode322is not influenced by a level of the liquid to be measured and a dielectric constant held by the liquid to be measured.

FIG. 17is a detecting circuit diagram of the liquid level sensor according to Embodiment 3 of the present invention. InFIG. 17, terminal326of detecting unit321is connected to, for example, first potential334configured by a GND potential. Terminals327,328,329, and330of detecting unit321are connected to, for example, second potential339configured by a 5-V power supply potential through first resistor335, second resistor336, third resistor337, and fourth resistor338, respectively. In this manner, first detecting electrode322, second detecting electrode323, third detecting electrode324, and fourth detecting electrode325are connected to first resistor335, second resistor336, third resistor337, and fourth resistor338, respectively.

At this time, in a state in which all first, second, and third detecting electrodes322,323, and324are out of the liquid to be measured, a time constant determined by an inter-electrode capacitance of first detecting electrodes322and first resistor335, a time constant determined by an inter-electrode capacitance of second detecting electrodes323and second resistor336, and a time constant determined by an inter-electrode capacitance of third detecting electrodes324and third resistor337are set to be substantially equal to one another. A time constant determined by an inter-electrode capacitance of fourth detecting electrodes325and fourth resistor338is set to be smaller than the time constant determined by the inter-electrode capacitance of first detecting electrodes322and first resistor335in a state in which first detecting electrodes322are dipped in the liquid to be measured and to be larger than the time constant determined by the inter-electrode capacitance of third detecting electrodes324and third resistor337in a state in which third detecting electrodes324are out of the liquid to be measured.

A first midpoint potential between first resistor335and first detecting electrode322is compared with a threshold value determined by resistors341and342in first comparing unit340configured by a comparator. Similarly, a second midpoint potential between second resistor336and second detecting electrode323, a third midpoint potential between third resistor337and third detecting electrode324, and a fourth midpoint potential between fourth resistor338and fourth detecting electrode325are compared with a threshold value determined by resistors341and342in second comparing unit343, third comparing unit344, and fourth comparing unit345which are configured by comparators, respectively.

Output signals from first, second, third comparing units340,343, and344are inputted to first logic circuit346configured by a logic element. Furthermore, output signals from first, third, and fourth comparing units340,344, and345are inputted to second logic circuit347configured by a logic element and a D flip-flop. Output signals from first logic circuit346and second logic circuit347are inputted to third logic circuit348configured by a logic element.

First analog switch349and second analog switch350which are open/close-controlled by an output signal from third logic circuit348are arranged on the subsequent stage of third logic circuit348. Fifth resistor351has one end connected to a midpoint between first analog switch349and second analog switch350, and the other end connected to output terminal352. Capacitor353has one end connected to first potential334, and the other end connected between fifth resistor351and output terminal352.

A circuit operation of a liquid level sensor according to an embodiment of the present invention will be described below.

FIGS. 18A to 18Jare voltage waveform charts of each unit of the liquid level sensor according to Embodiment 3 of the present invention. Detecting unit321of the liquid level sensor shown inFIG. 15is dipped in a liquid to be measured such as an engine oil in an oil pan (not shown). At this time, first detecting electrodes322and fourth detecting electrodes325are always dipped in the liquid to be measured, third detecting electrodes324are always arranged out of the liquid to be measured, second detecting electrodes323cross to a level of the liquid to be measured, and a part of second detecting electrodes323dipped in the liquid increases or decreases with rising or falling of the liquid level.

In an initial state (t0) before the power supply is turned on, electric charges are not present between the electrode pairs of first, second, third, and fourth detecting electrodes322,323,324, and325. Therefore, all the first midpoint potential between first resistor335and first detecting electrode322, the second midpoint potential between second resistor336and second detecting electrode323, the third midpoint potential between third resistor337and third detecting electrode324, and the fourth midpoint potential between fourth resistor338and fourth detecting electrode325are equal to first potential334.

When the power supply is turned on, the third midpoint potential between third resistor337and third detecting electrode324exponentially increases from first potential334to second potential339at the time constant determined by third resistor337and the inter-electrode capacitance of third detecting electrodes324. The second midpoint potential between second resistor336and second detecting electrode323exponentially increases from first potential334to second potential339at the time constant determined by second resistor336and an inter-electrode capacitance of second detecting electrodes323. At this time, since the part of second detecting electrode323is in the liquid to be measured, the time constant determined by the second resistor336and the electrostatic capacitance of second detecting electrodes323is larger than the time constant determined by third resistor337and third detecting electrode324. The first midpoint potential between first resistor335and first detecting electrodes322exponentially increases from first potential334to second potential339at the time constant determined by first resistor335and an inter-electrode capacitance of first detecting electrodes322. At this time, first detecting electrode322is always dipped in the liquid to be measured. Therefore, the time constant determined by first resistor335and the electrostatic capacitance of first detecting electrodes322is larger than the time constant determined by second resistor336and second detecting electrode323. Furthermore, the fourth midpoint potential between fourth resistor338and fourth detecting electrode325exponentially increases from first potential334to second potential339at the time constant determined by fourth resistor338and the inter-electrode capacitance of fourth detecting electrodes325. At this time, the time constant determined by the inter-electrode capacitance of fourth detecting electrodes325and fourth resistor338is set to be smaller than the time constant determined by the inter-electrode capacitance of first detecting electrodes322and first resistor335in the state in which first detecting electrodes322are dipped in the liquid to be measured and to be larger than the time constant determined by the inter-electrode capacitance of third detecting electrodes324and third resistor337in the state in which third detecting electrodes324are out of the liquid to be measured.

InFIG. 18A, the third midpoint potential between third resistor337and third detecting electrode324reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from third comparing unit344configured by a comparator shifts from high to low (t1).

InFIG. 18B, the second midpoint potential between second resistor336and second detecting electrode323reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from second comparing unit343configured by a comparator shifts from high to low (t3).

InFIG. 18D, the fourth midpoint potential between fourth resistor338and fourth detecting electrode325reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from fourth comparing unit345configured by a comparator shifts from high to low (t2).

InFIG. 18C, the first midpoint potential between first resistor335and first detecting electrode322reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from first comparing unit340configured by a comparator shifts from high to low (t4). Since electric charges accumulated in first, second, third, and fourth detecting electrodes322,323,324, and325are discharged to first potential334through element354having an open-collector configuration, all the first midpoint potential between first resistor335and first detecting electrode322, the second midpoint potential between second resistor336and second detecting electrode323, the third midpoint potential between third resistor337and third detecting electrode324, and the fourth midpoint potential between fourth resistor338and fourth detecting electrode325return to first potential334. At the same time, each of outputs from first, second, third, and fourth comparing units340,343,344, and345shifts from low to high (t5).

Thereafter, the third midpoint potential between third resistor337and third detecting electrode324, the second midpoint potential between second resistor336and second detecting electrode323, the first midpoint potential between first resistor335and first detecting electrode322, and the fourth midpoint potential between fourth resistor338and fourth detecting electrode325exponentially increase again from first potential334to second potential339at the time constants determined by third resistor337and the inter-electrode capacitance of third detecting electrodes324, second resistor336and the inter-electrode capacitance of second detecting electrodes323, first resistor335and the inter-electrode capacitance of first detecting electrodes322, and fourth resistor338and the inter-electrode capacitance of fourth detecting electrodes325.

Subsequently, an output from third comparing unit344shifts from high to low (t6), an output from second comparing unit343shifts from high to low (t8), an output from fourth comparing unit345shifts from high to low (t7), and an output from first comparing unit340shifts from high to low (t9), and the same operations as described above are repeated.

InFIGS. 18E and 18F, output signals from first, second, and third comparing units340,343, and344are inputted to first logic circuit346configured by a logic element. A signal shown inFIG. 18Eis outputted to an output of first NOR element355in first logic circuit346. A signal shown inFIG. 18Fis outputted to an output of second NOR element356.

InFIGS. 18G and 18H, output signals from first, third, and fourth comparing units340,344, and345are inputted to second logic circuit347configured by a logic element and a D flip-flop. An always-high signal is outputted to a Q negative output of first D flip-flop357in second logic circuit347as shown inFIG. 18G. An always-high signal is outputted to a Q output of second flip-flop358as shown inFIG. 18H.

InFIGS. 18I and 18J, an output signal from first logic circuit346and an output signal from second logic circuit347are inputted to third logic circuit348. A signal shown inFIG. 18Iis outputted to first analog switch349. A signal shown inFIG. 18Jis outputted to second analog switch350.

In this case, when a signal inputted to first analog switch349is at high, first analog switch349is “closed”. When the signal inputted to first analog switch349is at low, first analog switch349is “opened”. When a signal inputted to second analog switch350is at high, second analog switch350is “closed”. When the signal inputted to second analog switch350is at low, second analog switch350is “opened”.

In this manner, for times t1to t3and times t6to t8, i.e., for a period from when an output from third comparing unit344shifts to low to when an output from second comparing unit343shifts to low, first analog switch349is “closed”, and second analog switch350is “opened”. Therefore, capacitor353is charged from second potential339through fifth resistor351. For times t3to t4and times t8to t9, i.e., in a period from when an output from second comparing unit343shifts to low to when an output from first comparing unit340shifts to low, first analog switch349is “opened”, and second analog switch350is “closed”. Therefore, an electric charge accumulated in capacitor353is discharged to first potential334through fifth resistor351.

For the other times, both first analog switch349and second analog switch350are “opened”. Therefore, an electric charge accumulated in capacitor353is stored.

In this manner, for first analog switch349and second analog switch350are alternately opened and closed for a time determined by a length of a part of second detecting electrode323dipped in a liquid to be measured and a length of a part being out of the liquid to be measured to charge and discharge capacitor353, so that a level of the liquid to be measured can be outputted to output terminal352as an analog voltage.

FIGS. 19A to 19Jare voltage waveform charts of each unit of the liquid level sensor when a liquid level of a liquid to be measured exceeds an upper end of second detecting electrode323and rises to a center of third detecting electrode324in the liquid level sensor according to Embodiment 3 of the present invention.

In an initial state (t0) before power supply is turned on, electric charges are not present between the electrode pairs of first, second, third, and fourth detecting electrodes322,323,324, and325. Therefore, all the first midpoint potential between first resistor335and first detecting electrode322, the second midpoint potential between second resistor336and second detecting electrode323, the third midpoint potential between third resistor337and third detecting electrode324, and a fourth midpoint potential between fourth resistor338and fourth detecting electrode325are equal to first potential334.

When the power supply is turned on, the third midpoint potential between third resistor337and third detecting electrode324exponentially increases from first potential334to second potential339at the time constant determined by third resistor337and the inter-electrode capacitance of third detecting electrodes324. The second midpoint potential between second resistor336and second detecting electrode323exponentially increases from first potential334to second potential339at the time constant determined by second resistor336and the inter-electrode capacitance of second detecting electrodes323. The first midpoint potential between first resistor335and first detecting electrode322exponentially increases from first potential334to second potential339at the time constant determined by first resistor335and the inter-electrode capacitance of first detecting electrodes322.

At this time, although only the part of third detecting electrode324is in the liquid to be measured, second detecting electrode323and first detecting electrode322are dipped in the liquid to be measured. Therefore, the time constant determined by third resistor337and the electrostatic capacitance of third detecting electrodes324is smaller than the time constant determined by second resistor336and second detecting electrode323and the time constant determined by first resistor335and first detecting electrode322.

InFIG. 19A, the third midpoint potential between third resistor337and third detecting electrode324reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from third comparing unit344configured by a comparator shifts from high to low (t2).

InFIG. 19B, the second midpoint potential between second resistor336and second detecting electrode323reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from second comparing unit343configured by a comparator shifts from high to low (t3).

InFIG. 19D, the fourth midpoint potential between fourth resistor338and fourth detecting electrode325reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from fourth comparing unit345configured by a comparator shifts from high to low (t1).

InFIG. 19C, the first midpoint potential between first resistor335and first detecting electrode322reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from first comparing unit340configured by a comparator shifts from high to low (t3).

Electric charges accumulated in first, second, third, and fourth detecting electrodes322,323,324, and325are discharged to first potential334through element354having an open-collector configuration. For this reason, all the first midpoint potential between first resistor335and first detecting electrode322, the second midpoint potential between second resistor336and second detecting electrode323, the third midpoint potential between third resistor337and third detecting electrode324, and the fourth midpoint potential between fourth resistor338and fourth detecting electrode325return to first potential334. At the same time, each of outputs from first, second, third, and fourth comparing units340,343,344, and345shifts from low to high (t4) as shown inFIGS. 19C,19B,19A, and19D.

Subsequently, the same operations as described above are repeated (t5to t9).

InFIGS. 19E and 19F, output signals from first, second, and third comparing units340,343, and344are inputted to first logic circuit346configured by a logic element. A signal shown inFIG. 19Eis outputted to an output of first NOR element355in first logic circuit346. A signal shown inFIG. 19Fis outputted to an output of second NOR element356. Output signals from first, third, and fourth comparing units340,344, and345are inputted to second logic circuit347configured by a logic element and a D flip-flop. Furthermore, outputs from first logic circuit346and second logic circuit347are inputted to third logic circuit348configured by a logic element.

At this time, when time t2at which the output from third comparing unit344shifts from high to low is before time t1at which the output from fourth comparing unit345shifts from high to low, i.e., when the length of the part of third detecting electrodes324clipped in the liquid to be measured is short, a Q negative output from first D flip-flop357in second logic circuit347and a Q output from second flip-flop358are always at high as inFIGS. 18G and 18H.

In this manner, a signal inFIG. 19Eis directly given to first analog switch349, and a signal inFIG. 19Fis directly given to second analog switch350. At this time, for only times t2to t3and times t6to t7, first analog switch349is “closed”. For this reason, capacitor353is charged from second potential339through fifth resistor351.

Since second analog switch350is always “open”, an electric charge charged in capacitor353is not discharged to capacitor353. Therefore, an output potential from output terminal352is equal to second potential339.

As shown inFIGS. 19A and 19G, a case in which time t2at which the output from third comparing unit344shifts from high to low is after time t1at which the output from fourth comparing unit345shifts from high to low, i.e., the length of the part of third detecting electrode324dipped in the liquid to be measured becomes long will now be considered. In this case, an electrostatic capacitance measured by third detecting electrodes324increases. Therefore, a time constant determined by the electrostatic capacitance measured by third detecting electrodes324and third resistor337is larger than a time constant determined by an electrostatic capacitance measured by fourth detecting electrodes325and fourth resistor338. In this case, as shown inFIGS. 19G and 19H, although a Q output of second flip-flop358in second logic circuit347is kept high, a Q negative output from first D flip-flop357shifts from high to low at t2.

A signal given from third logic circuit348to first analog switch349goes to high after t2as shown inFIG. 19I. Therefore, first analog switch349is “closed”, capacitor353is charged from second potential339through fifth resistor351. A signal given from third logic circuit348to second analog switch350is kept low as shown inFIG. 19J. Therefore, second analog switch350is always “opened”. In this manner, since an electric charge charged in capacitor353is not discharged, an output potential from output terminal352is equal to second potential339.

FIGS. 20A to 20Jare voltage waveform charts of each unit of the liquid level sensor when a liquid level of a liquid to be measured exceeds a lower end of second detecting electrode323and falls to a center of first detecting electrode322in the liquid level sensor according to Embodiment 3 of the present invention.

In an initial state (t0) before power supply is turned on, electric charges are not present between the electrode pairs of first, second, third, and fourth detecting electrodes322,323,324, and325. Therefore, all the first midpoint potential between first resistor335and first detecting electrode322, the second midpoint potential between second resistor336and second detecting electrode323, the third midpoint potential between third resistor337and third detecting electrode324, and a fourth midpoint potential between fourth resistor338and fourth detecting electrode325are equal to first potential334.

When the power supply is turned on, the third midpoint potential between third resistor337and third detecting electrode324exponentially increases from first potential334to second potential339at the time constant determined by third resistor337and the inter-electrode capacitance of third detecting electrodes324. The second midpoint potential between second resistor336and second detecting electrode323exponentially increases from first potential334to second potential339at the time constant determined by second resistor336and the inter-electrode capacitance of second detecting electrodes323. The first midpoint potential between first resistor335and first detecting electrode322exponentially increases from first potential334to second potential339at the time constant determined by first resistor335and the inter-electrode capacitance of first detecting electrodes322.

At this time, although only the part of first detecting electrodes322is in the liquid to be measured, second detecting electrodes323and third detecting electrodes324are out of the liquid to be measured. Therefore, the time constant determined by first resistor335and the electrostatic capacitance of first detecting electrodes322is larger than the time constant determined by second resistor336and second detecting electrode323and the time constant determined by third resistor337and third detecting electrode324.

InFIG. 20A, the third midpoint potential between third resistor337and third detecting electrode324reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from third comparing unit344configured by a comparator shifts from high to low (t1).

InFIG. 20B, the second midpoint potential between second resistor336and second detecting electrode323reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from second comparing unit343configured by a comparator shifts from high to low (t1).

InFIG. 20D, the fourth midpoint potential between fourth resistor338and fourth detecting electrode325reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from fourth comparing unit345configured by a comparator shifts from high to low (t3).

InFIG. 20C, the first midpoint potential between first resistor335and first detecting electrode322reaches threshold voltage Vthdetermined by resistors341and342. At this time, an output from first comparing unit340configured by a comparator shifts from high to low (t2).

Electric charges accumulated in first, second, third, and fourth detecting electrodes322,323,324, and325are discharged to first potential334through element354having an open-collector configuration. For this reason, all the first midpoint potential between first resistor335and first detecting electrode322, the second midpoint potential between second resistor336and second detecting electrode323, the third midpoint potential between third resistor337and third detecting electrode324, and the fourth midpoint potential between fourth resistor338and fourth detecting electrode325return to first potential334. At the same time, each of outputs from first, second, third, and fourth comparing units340,343,344, and345shifts from low to high (t4) as shown inFIGS. 20A,20B,20C, and20D.

Subsequently, the same operations as described above are repeated (t4to t8).

Output signals from first, second, and third comparing units340,343, and344are inputted to first logic circuit346configured by a logic element. A signal shown inFIG. 20Eis outputted to an output of first NOR element355in first logic circuit346. A signal shown inFIG. 20Fis outputted to an output of second NOR element356.

Output signals from first, third, and fourth comparing units340,344, and345are inputted to second logic circuit347configured by a logic element and a D flip-flop. Furthermore, outputs from first logic circuit346and second logic circuit347are inputted to third logic circuit348configured by a logic element.

At this time, when time t2at which the output from first comparing unit340shifts from high to low is before time t3at which the output from fourth comparing unit345shifts from high to low, i.e., when the length of the part of third detecting electrode324dipped in the liquid to be measured is long, a Q negative output from first D flip-flop357in second logic circuit347and a Q output from second flip-flop358are always at high as inFIGS. 18G and 18H.

In this manner, a signal inFIG. 20Eis directly given to first analog switch349, and a signal inFIG. 20Fis directly given to second analog switch350. For this reason, for only times t1to t2and times t5to t6, second analog switch350is “closed”, and fifth resistor351and capacitor353are connected to first potential334. Since first analog switch349is always “opened”, capacitor353is not charged with an electric charge. Therefore, a potential of output terminal352is equal to first potential334.

As shown inFIGS. 20C and 20D, a case in which time t2at which the output from first comparing unit340shifts from high to low is before time t3at which the output from fourth comparing unit345shifts from high to low, i.e., the level of the liquid to be measured falls to shorten the length of the part of first detecting electrodes322dipped in the liquid to be measured will now be considered. In this case, an electrostatic capacitance measured by first detecting electrodes322decreases. Therefore, a time constant determined by the electrostatic capacitance measured by first detecting electrodes322and first resistor335is smaller than a time constant determined by an electrostatic capacitance measured by fourth detecting electrodes325and fourth resistor338. In this case, as shown inFIGS. 20G and 20H, although a Q negative output of first D flip-flop357in second logic circuit347is kept high, a Q output from second flip-flop358shifts from high to low at t2. For this reason, a signal given from third logic circuit348to second analog switch350goes to high after t2as shown inFIG. 20J. Therefore, second analog switch350is “closed”, fifth resistor351and capacitor353are connected to first potential334.

Since a signal given from third logic circuit348to first analog switch349is kept low as shown inFIG. 20I, first analog switch349is always “opened”. In this manner, since capacitor353is not charged, an output potential from output terminal352is equal to first potential334.

As is apparent from the above description, the liquid level sensor according to Embodiment 3 of the present invention can always output a voltage being in proportion to a level of a liquid to be measured without a complex operational device when the level of the liquid to be measured crosses to second detecting electrodes323. Even though the level of the liquid to be measured crosses to first detecting electrodes322and third detecting electrodes324everywhere, a fixed voltage can be always outputted. Therefore, a highly sensitive liquid level sensor can be easily provided.

The liquid level sensor according to Embodiment 3 of the present invention in which fourth detecting electrodes325are arranged under first detecting electrodes322has been described. However, the present invention is not limited to the embodiment. Even though fourth detecting electrodes325are arranged at an arbitrary position such as a side or a backside of first detecting electrodes322, the same operational advantage as that in Embodiment 3 of the present invention can be obtained.

A liquid level sensor according to Embodiment 4 of the present invention will be described below.

FIG. 21is a front view of a detecting unit of the liquid level sensor according to Embodiment 4 of the present invention. InFIG. 21, a pair of first comb-shaped detecting electrodes422made of carbon are arranged on a lower end portion of vertically extending rectangular detecting unit421made of a polyimide film or the like. Above first detecting electrodes422, a pair of second comb-shaped detecting electrodes423made of carbon are arranged. A pair of third comb-shaped detecting electrodes424are similarly arranged above second detecting electrodes423, and a pair of fourth comb-shaped detecting electrodes425made of carbon are similarly arranged on an upper end portion of detecting unit421. First, second, third, and fourth detecting electrodes422,423,424, and425are connected to terminals426,427,428,429, and430by vertically extending extraction lines.

FIG. 22is a sectional view of first detecting electrodes422taken along line22-22inFIG. 21. InFIG. 22, first detecting electrode422is configured by covering an entire area of opposite electrodes455with metal layer457through insulator456. With this configuration, since electric flux lines generated between opposite electrodes455do not pass through an liquid to be measured, an electrostatic capacitance between electrodes455measured by first detecting electrode422is not influenced by a dielectric constant held by the liquid to be measured.

As insulator456, a liquid to be measured, a solid material which impregnates a liquid to be measured, or a material having substantially the same dielectric-constant-temperature characteristic as that of a liquid to be measured is desirably selected. In this manner, an influence of a change in temperature in liquid quality measurement can be removed.

In an operation of the liquid level sensor according to Embodiment 4 of the present invention, since first detecting electrodes422and second detecting electrodes423are always dipped in a liquid to be measured such as an oil having a high heat conductivity, temperatures of first detecting electrodes422and second detecting electrodes423are substantially equal to each other.

FIG. 23is a detecting circuit diagram of the liquid level sensor according to Embodiment 4 of the present invention. InFIG. 23, terminal426of detecting unit421is connected to, for example, first potential431configured by a GND potential. Terminals427,428,429, and430of detecting unit421are connected to second potential452configured by a 5-V power supply potential through first resistor432, second resistor433, third resistor434, and fourth resistor435, respectively. In this manner, first detecting electrode422, second detecting electrode423, third detecting electrode424, and fourth detecting electrode425are connected to first resistor432, second resistor433, third resistor434, and fourth resistor435, respectively.

At this time, in a state in which all second, third, and fourth detecting electrodes423,424, and425are out of the liquid to be measured, a time constant determined by an inter-electrode capacitance of second detecting electrodes423and second resistor433, a time constant determined by an inter-electrode capacitance of third detecting electrodes424and third resistor434, and a time constant determined by an inter-electrode capacitance of fourth detecting electrodes425and fourth resistor435are set to be substantially equal to one another. A time constant determined by first detecting electrode422and first resistor432is set to be smaller than the time constant determined by the inter-electrode capacitance of second detecting electrodes423and second resistor433in a state of being dipped in the liquid to be measured.

A first midpoint potential between first resistor432and first detecting electrode422is compared with a threshold value determined by resistors453and454in first comparing unit436configured by a comparator. In the same manner as described above, a second midpoint potential between second resistor433and second detecting electrode423, a third midpoint potential between third resistor434and third detecting electrode424, and a fourth midpoint potential between fourth resistor435and fourth detecting electrode425are compared with a threshold value determined by resistors453and454in second comparing unit437, third comparing unit438, and fourth comparing unit439which are configured by comparators, respectively.

Output signals from first and second comparing units436and437are inputted to first logic circuit440configured by a logic element. Furthermore, output signals from second, third, and fourth comparing units437,438, and439are inputted to second logic circuit441configured by a logic element.

First analog switch442and second analog switch443which are open/close-controlled by an output signal from first logic circuit440are arranged on a subsequent stage of first logic circuit440. Resistor444has one end connected to a midpoint between first analog switch442and second analog switch443and the other end connected to first output terminal445. Capacitor446has one end connected to first potential431and the other end connected between fifth resistor444and first output terminal445.

In the same manner as described above, third analog switch447and fourth analog switch448which are open/close-controlled by an output signal from second logic circuit441are arranged on a subsequent stage of second logic circuit441. Sixth resistor449has one end connected to a midpoint between third analog switch447and fourth analog switch448, and the other end connected to second output terminal450. Capacitor451has one end connected to first potential431and the other end connected between sixth resistor449and second output terminal450.

A circuit operation of the liquid level sensor according to Embodiment 4 of the present invention will be described below.

FIGS. 24A to 24Lare voltage waveform charts of each unit of the liquid level sensor according to Embodiment 4 of the present invention. Detecting unit421shown inFIG. 21is dipped in a liquid to be measured such as an engine oil in an oil pan (not shown). At this time, first detecting electrodes422and second detecting electrodes423are always dipped in the liquid to be measured, and fourth detecting electrodes425are always arranged out of the liquid to be measured. Third detecting electrodes424cross to a level of the liquid to be measured, and a part dipped in the liquid increases or decreases with rising and falling of the liquid level.

In an initial state (t0) before the power supply is turned on, since electric charges are not present between the electrode pairs of first, second, third, and fourth detecting electrodes422,423,424, and425, all the first midpoint potential between first resistor432and first detecting electrode422, the second midpoint potential between second resistor433and second detecting electrode423, the third midpoint potential between third resistor434and third detecting electrode424, and a fourth midpoint potential between fourth resistor435and fourth detecting electrode425are equal to first potential431(V1).

When the power supply is turned on, the fourth midpoint potential between fourth resistor435and fourth detecting electrode425exponentially increases, as shown inFIG. 24A, from first potential431(V1) to second potential452(V2) at a time constant determined by fourth resistor435and an inter-electrode capacitance of fourth detecting electrodes425. The third midpoint potential between third resistor434and third detecting electrode424exponentially increases, as shown inFIG. 24B, from first potential431(V1) to second potential452(V2) at a time constant determined by third resistor434and an inter-electrode capacitance of third detecting electrodes424. Since a part of third detecting electrodes424is in the liquid to be measured, a time constant determined by third resistor434and an electrostatic capacitance of third detecting electrodes424is larger than a time constant determined by fourth resistor435and fourth detecting electrode425. The second midpoint potential between second resistor433and second detecting electrode423, as shown inFIG. 24C, exponentially increases from first potential431(V1) to second potential452(V2) at the time constant determined by second resistor433and the inter-electrode capacitance of second detecting electrodes423. At this time, since second detecting electrodes423are always dipped in the liquid to be measured, a time constant determined by second resistor433and an inter-electrode capacitance of second detecting electrodes423is larger than a time constant determined by third resistor434and third detecting electrode424. Furthermore, the first midpoint potential between first resistor432and first detecting electrode422, as shown inFIG. 24D, exponentially increases from first potential431(V1) to second potential452(V2) at a time constant determined by first resistor432and an inter-electrode capacitance of first detecting electrodes422. At this time, as described above, the time constant determined by first detecting electrode422and first resistor432is set to be smaller than a time constant determined by an inter-electrode capacitance of second detecting electrodes423and second resistor433in a state of being dipped in the liquid to be measured.

Thereafter, when the fourth midpoint potential between fourth resistor435and fourth detecting electrode425reaches threshold voltage Vthdetermined by resistors453and454, an output from fourth comparing unit439configured by a comparator shifts from high to low (t1) as shown inFIG. 24E. In the same manner as described above, the third midpoint potential between third resistor434and third detecting electrode424reaches threshold voltage Vthdetermined by resistors453and454, an output from third comparing unit438configured by a comparator shifts from high to low (t2) as shown inFIG. 24F. When the first midpoint potential between first resistor432and first detecting electrode422reaches threshold voltage Vthdetermined by resistors453and454, an output from first comparing unit436configured by a comparator shifts from high to low (t3) as shown inFIG. 24H. Furthermore, when the second midpoint potential between second resistor433and second detecting electrode423reaches threshold voltage Vthdetermined by resistors453and454, an output from second comparing unit437configured by a comparator shifts from high to low (t4) as shown in FIG.24G. Since electric charges accumulated in first, second, third, and fourth detecting electrodes422,423,424, and425are discharged to first potential431(V1) through element451having an open-collector configuration, all the first midpoint potential between first resistor432and first detecting electrode422, the second midpoint potential between second resistor433and second detecting electrode423, the third midpoint potential between third resistor434and third detecting electrode424, and the fourth midpoint potential between fourth resistor435and fourth detecting electrode425return to first potential431(V1). Each of outputs from first, second, third, and fourth comparing units436,437,438, and439shifts from low to high (t5) as shown inFIGS. 24H,24G,24F, and24E.

Thereafter, the fourth midpoint potential between fourth resistor435and fourth detecting electrode425, the third midpoint potential between third resistor434and third detecting electrode424, the second midpoint potential between second resistor433and second detecting electrode423, and the first midpoint potential between first resistor432and first detecting electrode422exponentially increase again from resistor431(V1) to second potential452(V2), as shown inFIGS. 24A,24B,24C, and24D, at the time constants determined by fourth resistor435and the inter-electrode capacitance of fourth detecting electrodes425, third resistor434and the inter-electrode capacitance of third detecting electrodes424, second resistor433and the inter-electrode capacitance of second detecting electrodes423, and first resistor432and the inter-electrode capacitance of first detecting electrodes422, respectively. Subsequently, an output from fourth comparing unit439shifts from high to low (t6), an output from third comparing unit438shifts from high to low (t7), an output from second comparing unit437shifts from high to low (t8), and an output from first comparing unit436shifts from high to low (t9), and the same operations as described above are repeated.

Output signals from first and second comparing units436and437are inputted to logic circuit440configured by a logic element, a signal shown inFIG. 24Kis outputted to first analog switch442, and a signal shown inFIG. 24Jis outputted to second analog switch443.

In the same manner as described above, output signals from second, third, and fourth comparing units437,438, and439are inputted to logic circuit441configured by a logic element, a signal shown inFIG. 24Iis outputted to third analog switch447, and a signal shown inFIG. 24Jis outputted to fourth analog switch448.

In this case, when a signal inputted to each analog switch is at high, an analog switch is “closed”. When the signal is at low, the analog switch is “opened”. In this manner, for times t3to t4and times t8to t9, first analog switch442is “closed”, and second analog switch443is “opened”. For this reason, capacitor446is charged from second potential452through fifth resistor444. For times t0to t3and times t5to t8, since first analog switch442is “opened”, and second analog switch443is “closed”. For this reason, an electric charge accumulated in capacitor446is discharged to first potential431(V1) through fifth resistor444.

In this manner, an operation of charging capacitor446for a time being in proportion to a difference between an electrostatic capacitance measured by first detecting electrodes422and an electrostatic capacitance measured by second detecting electrodes423and an operation of discharging an electric charge charged in capacitor446for a time being in proportion to the electrostatic capacitance measured by first detecting electrodes422is repeated to make it possible to output a voltage being in proportion to liquid quality of the liquid to be measured to first output terminal445.

In the same manner as described above, for times t1to t2and times t6to t7, third analog switch447is “closed”, and fourth analog switch448is “opened”. For this reason, capacitor451is charged from second potential452through sixth resistor449. For times t2to t4and times t7to t9, third analog switch447is “opened”, and fourth analog switch448is “closed”. For this reason, the electric charge accumulated in capacitor451is discharged to first potential431(V1) through sixth resistor449. In this manner, for times determined by a length of part of third detecting electrodes424dipped in the liquid to be measured and a length of a part being out of the liquid to be measured, third analog switch447and fourth analog switch448are alternately opened and closed to charge and discharge capacitor451. For this reason, a level of the liquid to be measured can be outputted to second output terminal450as an analog voltage.

A circuit operation of a liquid quality measuring unit will be further described by using numerical expressions. An electrostatic capacitance of first detecting electrodes422is represented by C1, an electrostatic capacitance obtained when second detecting electrodes423are out of a liquid to be measured is represented by C2, resistances of first resistor432and second resistor433are represented by r0, and a relative dielectric constant of the liquid to be measured is represented by ∈L. First potential431(V1) is set to 0 [V], and second potential452(V2) is represented by Vdd[V]. When first detecting electrodes422and second detecting electrodes423are dipped in the liquid to be measured, the electrostatic capacitance of first detecting electrodes422is not influenced by a dielectric constant held by the liquid to be measured. For this reason, a time constant determined by first detecting electrode422and first resistor432is expressed by Equation 31.
[Numerical Expression 31]
C1r0Equation 31

Since the electrostatic capacitance of second detecting electrodes423is influenced by the dielectric constant held by the liquid to be measured, the time constant determined by second detecting electrode423and second resistor433is expressed by Numerical Expression 32.
[Numerical Expression 32]
∈LC2r0Equation 32

When power supply is turned on, second midpoint potential Vn2between second detecting electrode423and second resistor433is expressed by an equation shown in Equation 33.

In the same manner as described above, first midpoint potential Vn1between first detecting electrode422and first resistor432is expressed by Equation 34.

According to this, time Tcfor which capacitor446is charged is expressed by Equation 35.

Similarly, time Tdfor which capacitor446is discharged is expressed by Equation 36.

According to Equation 35 and Equation 36, it is understood that charging time Tcis in proportion to a difference between an electrostatic capacitance of second detecting electrodes423and an electrostatic capacitance of first detecting electrodes422(∈LC2−C1) and that discharging time Tdis determined by the electrostatic capacitance of first detecting electrodes422.

Output voltage V0generated at first output terminal445is expressed by a numerical expression. In the circuit shown inFIG. 23, both first and second analog switches442and443are “opened” in a period of t4to t5or the like. For this reason, an electric charge accumulated in capacitor446does not change, and output voltage V0does not also change. Therefore, in the following description, these periods are ignored, a change in output voltage when charging time Tcand discharging time Tdare repeated will be considered. An output voltage obtained immediately after the power supply is turned on is set to 0 [V]. When a resistance of fifth resistor444and a capacitance of capacitor446are represented by R and C, respectively, output voltage V1cafter the first charge is expressed by Equation 37.

Output voltage V1dobtained after the first discharge is expressed by Equation 38.

Output voltage V2cobtained after the second charge is expressed by Equation 39.

Output voltage V2dobtained after the second discharge is expressed by Equation 40.

Furthermore, output voltage V3cobtained after the third charge and output voltage V3dobtained after discharge are expressed by Equation 41 and Equation 42.

In the same manner as described above, the output voltages obtained after the charge and the discharge can be calculated.

FIGS. 25 to 27are obtained by calculating output voltage V0obtained when resistance R of fifth resistor444and capacitance C of capacitor446are set to 500 kΩ and 100 pF, respectively.

FIG. 25is a characteristic graph showing a change of output voltage V0with time when a ratio of an electrostatic capacitance of a first detecting electrodes to an electrostatic capacitance of a second detecting electrodes in the liquid level sensor according to Embodiment 4 of the present invention is 4:5. More specifically,FIG. 25shows a change of output voltage V0with time when charging time Tcand discharging time Tdare set to 1 μsec and 4 μsec, more specifically, a ratio of Tcand Td, i.e., a ratio of electrostatic capacitance C1of first detecting electrodes422dipped in the liquid to be measured to electrostatic capacitance ∈LC2of second detecting electrodes423is 4:5. InFIG. 25, after 500 μsec have elapsed, output voltage V0made by superposing a ripple having an amplitude of ±0.04 [V] on a DC component of 1 [V] is obtained. The ripple can be removed by using an appropriate low-pass filter.

FIG. 26is a characteristic graph showing a change of output voltage V0with time when an electrostatic capacitance of second detecting electrodes in the liquid level sensor according to Embodiment 4 of the present invention increases by 5%. More specifically,FIG. 26shows a change of output voltage V0with time obtained when only charging time Tcincreases to 1.25 μsec when dielectric constant ∈Lof the liquid to be measured increases by 5% due to deterioration or the like in the state shown inFIG. 25. InFIG. 26, after about 500 μsec have elapsed, output voltage V0made by superposing a ripple having an amplitude of about ±0.05 [V] on a DC component of 1.24 [V] is obtained.

FIG. 27is a characteristic graph showing a change of output voltage V0with time when both the electrostatic capacitance of the first detecting electrodes and the electrostatic capacitance of the second detecting electrodes in the liquid level sensor according to Embodiment 4 of the present invention increase by 10%. More specifically,FIG. 27shows a change of output voltage V0with time when discharging time Tdand charging time Tcincrease to 1.1 μsec and 4.4 μsec, respectively when both the electrostatic capacitance of first detecting electrodes422and the electrostatic capacitance of second detecting electrodes423increase by 10% in the state shown inFIG. 25. InFIG. 27, after about 500 μsec have elapsed as inFIG. 25, output voltage V0made by superposing a ripple having an amplitude of about ±0.04 [V] is obtained.

In this manner, change rates of the electrostatic capacitance of first detecting electrodes422and the electrostatic capacitance of second detecting electrodes423are made equal to each other to make it possible to measure liquid quality of the liquid to be measured regardless of a temperature.

In the same manner as described above, for times determined by a length of part of third detecting electrode424dipped in the liquid to be measured and a length of a part being out of the liquid to be measured, third analog switch447and fourth analog switch448are alternately opened and closed to charge and discharge capacitor451. In this manner, a level of the liquid to be measured can be outputted to second output terminal450as an analog voltage.

As is apparent from the above description, the liquid level sensor according to Embodiment 4 of the present invention can always output a voltage being in proportion to liquid quality without arranging a complex operational device. Therefore, a highly sensitive liquid level sensor can be easily provided.

A liquid level sensor according to Embodiment 5 of the present invention will be described below.

FIG. 28is a front view of a detecting unit in the liquid level sensor according to Embodiment 5 of the present invention. InFIG. 28, a pair of first comb-shaped detecting electrodes522made of carbon or the like are arranged on a lower end portion of vertically extending rectangular detecting unit521made of a polyimide film or the like. Above first detecting electrodes522, a pair of second comb-shaped detecting electrodes523made of carbon or the like are arranged. A pair of third comb-shaped detecting electrodes524are similarly arranged above second detecting electrodes523, and a pair of fourth comb-shaped detecting electrodes525made of carbon or the like are similarly arranged above third detecting electrodes524. First and second detecting electrodes522and523are connected to terminals529,530, and531by common extraction line526and extraction lines527and528. First cancel electrode532is connected to terminal533and arranged along extraction line527of first detecting electrode522. First cancel electrode532is arranged throughout the upper end portion of first detecting electrode522. Third and fourth detecting electrodes524and525are connected to terminals537,538, and539by common extraction line534and extraction lines535and536. Second cancel electrode540is connected to terminal541and arranged between extraction line528of second detecting electrode523and extraction line535of third detecting electrode524. Second cancel electrode540is arranged throughout an upper end portion of second detecting electrode523and a lower end portion of third detecting electrode524. Third cancel electrode542is connected to terminal543and arranged along extraction line536of fourth detecting electrode525. Third cancel electrode542is arranged throughout a lower end portion of fourth detecting electrode525.

FIG. 29is a sectional view of second detecting electrode523taken along line29-29inFIG. 28. InFIG. 29, second detecting electrode523is configured by covering an entire area of opposite electrodes544with metal layer546through insulator545. With this configuration, since electric flux lines generated between opposite electrodes544do not pass through an liquid to be measured, an electrostatic capacitance between opposite electrodes544measured by second detecting electrode523is not influenced by a dielectric constant held by the liquid to be measured.

As insulator545, a liquid to be measured, a solid material which impregnates a liquid to be measured, or a material having substantially the same dielectric-constant-temperature characteristic as that of a liquid to be measured is desirably selected. In this manner, an influence of a change in temperature in liquid quality measurement can be removed.

FIG. 30is a detecting circuit diagram of the liquid level sensor according to Embodiment 5 of the present invention. InFIG. 30, pulses from pulse generating circuit551are inputted to terminals529and537of detecting unit521, and signals obtained by inverting the pulses are inputted to terminals533,541, and543of detecting unit521through level adjusters552,553, and554. In this detecting circuit, signals branched from an input of a NOR gate on the final stage of pulse generating circuit551are inputted to terminals533,541, and543of detecting unit521.

When the pulses from pulse generating circuit551are inputted to terminals529and537of detecting unit521, an electric charge accumulated in an electrostatic capacitance by first detecting electrodes522and an electric charge accumulated in an electrostatic capacitance between common extraction line526and extraction line527of first detecting electrode522are present between terminals529and530of detecting unit521. An electric charge accumulated in an electrostatic capacitance by second detecting electrodes523and an electric charge accumulated in an electrostatic capacitance between common extraction line526and extraction line528of second detecting electrode523are present between terminals529and531of detecting unit521.

In the same manner as described above, an electric charge accumulated in an electrostatic capacitance by third detecting electrodes524and an electric charge accumulated in an electrostatic capacitance between common extraction line534and extraction line535of third detecting electrode524are present between terminals537and538of detecting unit521. An electric charge accumulated in an electrostatic capacitance by fourth detecting electrodes525and an electric charge accumulated in an electrostatic capacitance between common extraction line534and extraction line536of fourth detecting electrode525are present between terminals537and539of detecting unit521.

First cancel electrode532is arranged along extraction line527of first detecting electrode522. Second cancel electrode540is arranged between extraction line528of second detecting electrode523and extraction line535of third detecting electrode524.

Third cancel electrode542is arranged along extraction line536of fourth detecting electrode525. Signals obtained by inverting pulses from pulse generating circuit551are inputted to first cancel electrode532, second cancel electrode540, and third cancel electrode542through level adjusters552,553, and554. In this manner, electric charges accumulated between extraction line527of first detecting electrode522and common extraction line526of first detecting electrode522and second detecting electrode523are canceled. Electric charges accumulated between extraction line528of second detecting electrode523and common extraction line526of first detecting electrode522and second detecting electrode523are canceled. Furthermore, electric charges accumulated between extraction line535of third detecting electrode524and common extraction line534of third detecting electrode524and fourth detecting electrode525are canceled. Electric charges accumulated between extraction line536of fourth detecting electrode525and common extraction line534of third detecting electrode524and fourth detecting electrode525are canceled.

A node voltage between terminal530of detecting unit521and one end of first resistor556is inputted to one terminal of first differential amplifier555. In the same manner as described above, a node voltage between terminal531of detecting unit521and one end of second resistor518, a node voltage between terminal538of detecting unit521and one end of third resistor559, and a node voltage between terminal539of detecting unit521and fourth resistor561are inputted to negative terminals of second differential amplifier557, third differential amplifier558, and fourth differential amplifier560, respectively, and a threshold value (not shown) is inputted to positive terminals. In Embodiment 5 of the present invention, the threshold value is set to ½ of the power supply voltage. The other ends of first, second, third, and fourth resistors556,518,559, and561are connected to output sides of first, second, third, and fourth differential amplifiers555,557,558, and560, respectively. Furthermore, a diode and a resistor are connected in series with each other between the input and the output of each of the differential amplifiers.

In this manner, first detecting electrode522, second detecting electrode523, third detecting electrode524, and fourth detecting electrode525are connected to first resistor556and first differential amplifier555, second resistor518and second differential amplifier557, third resistor559and third differential amplifier558, and fourth resistor561and fourth differential amplifier560, respectively. At this time, in a state in which all first, third and fourth detecting electrodes522,524, and525are out of the liquid to be measured, a time constant determined by an inter-electrode capacitance of first detecting electrodes522and first resistor556, a time constant determined by an inter-electrode capacitance of third detecting electrodes524and third resistor559, and a time constant determined by an inter-electrode capacitance of fourth detecting electrodes525and fourth resistor561are set to be substantially equal to one another.

In a state in which both first detecting electrodes522and second detecting electrodes523are clipped in the liquid to be measured, a time constant determined by second detecting electrode523and second resistor518is set to be smaller than a time constant determined by the inter-electrode capacitance of first detecting electrodes522and first resistor556.

An output potential of first differential amplifier555is compared with a threshold value from a threshold value generating unit (not shown) in first comparing unit562. In the same manner as described above, an output potential from second differential amplifier557, an output potential from third differential amplifier558, and an output potential from fourth differential amplifier560are compared with a threshold value from a threshold value generating unit (not shown) in second comparing unit563, third comparing unit564, and fourth comparing unit565which are configured by comparators, respectively. In Embodiment 5 of the present invention, the threshold value is set to ¼ of the power supply voltage.

Output signals from first, third, and fourth comparing units562,564, and565are inputted to logic circuit566configured by a logic element and a flip-flop. On the subsequent stage of logic circuit566, first analog switch567and second analog switch568which are open/close-controlled by an output signal from logic circuit566are arranged between first potential569and second potential570. Fifth resistor571has one end connected to a midpoint between first analog switch567and second analog switch568, and the other end connected to output terminal572. Capacitor573has one end connected to first potential569, and the other end connected between fifth resistor571and output terminal572.

Output signals from first and second comparing units562and563are inputted to logic circuit574configured by a logic element and a flip-flop. On the subsequent stage of logic circuit574, first analog switch567and second analog switch576which are open/close-controlled by an output signal from logic circuit574are arranged between first potential569and second potential570. Sixth resistor577has one end connected to a midpoint between first analog switch575and second analog switch576, and the other end connected to output terminal578. Capacitor579has one end connected to first potential569, and the other end connected between sixth resistor577and output terminal578.

Connections between terminals of detecting unit521and pulse generating circuits551, resistors556,558,559, and561, and the like are made in minimum dimensions not to generate a stray capacitance.

A circuit operation of the liquid level sensor according to Embodiment 5 of the present invention will be described below.

FIGS. 31A to 31Mare voltage waveform charts of each part in the liquid level sensor according to Embodiment 5 of the present invention. Detecting unit521of the liquid level sensor shown inFIG. 28is dipped in a liquid level sensor such as an engine oil in an oil pan (not shown). At this time, first detecting electrodes522and second detecting electrodes523are always dipped in the liquid to be measured, and fourth detecting electrodes525are always arranged out of the liquid to be measured. Third detecting electrodes524cross to the level of the liquid to be measured, and a part dipped in the liquid increases or decreases with rising or falling of the liquid level.

In the liquid level sensor according to Embodiment 5 of the present invention, in an initial state (t0) before power supply is turned on, since electric charges are not present between first, second, third, and fourth detecting electrodes522,523,524, and525, all node potential between first resistor556and first detecting electrode522, a node potential between second resistor518and second detecting electrode523, a node potential between third resistor559and third detecting electrode524, and a node potential between fourth resistor561and fourth detecting electrode525are set to first potential569(V1).

When the power supply is turned on (t0), as shown inFIG. 31A, pulses from pulse generating circuit551are inputted to terminals529and537of detecting unit521. The pulses are differentiated by a differentiating circuit configured by fourth resistor561, an inter-electrode capacitor of fourth detecting electrodes525, and fourth differential amplifier560, and output potential from fourth differential amplifier560exponentially increases, as shown inFIG. 31B, from first potential569(V1) to second potential570(V2) at the time constant determined by fourth resistor561and the inter-electrode capacitance of fourth detecting electrodes525. An output potential from a differentiating circuit configured by third resistor559, an inter-electrode capacitor of third detecting electrodes524, and third differential amplifier558exponentially increases, as shown inFIG. 31C, from first potential569(V1) to second potential570(V2) at the time constant determined by third resistor559and the inter-electrode capacitor of third detecting electrodes524. At this time, since a part of third detecting electrodes524is in the liquid to be measured, the time constant determined by third resistor559and the electrostatic capacitance of third detecting electrodes524is larger than the time constant determined by fourth resistor561and fourth detecting electrode525. In the same manner as described above, an output potential from a differentiating circuit configured by first resistor556, an inter-electrode capacitor of first detecting electrodes522, and first differential amplifier555exponentially increases, as shown inFIG. 31D, from first potential569(V1) to second potential570(V2) at the time constant determined by first resistor556and the inter-electrode capacitance of first detecting electrodes522. At this time, since first detecting electrodes522are always dipped in the liquid to be measured, the time constant determined by first resistor556and an electrostatic capacitance of first detecting electrodes522is larger than the time constant determined by third resistor559and third detecting electrode524. Furthermore, an output potential from a differentiating circuit configured by second resistor518, an inter-electrode capacitor of second detecting electrodes523, and second differential amplifier557exponentially increases, as shown inFIG. 31E, from first potential569(V1) to second potential570(V2) at the time constant determined by second resistor558and the inter-electrode capacitance of second detecting electrodes523. At this time, in a state in which both first detecting electrodes522and second detecting electrodes523are dipped in the liquid to be measured, the time constant determined by second detecting electrode523and second resistor518is smaller than the time constant determined by the inter-electrode capacitance of first detecting electrodes522and first resistor556.

When an output potential from fourth differential amplifier560reaches threshold voltage Vthdetermined by a threshold value generating unit (not shown), an output from fourth comparing unit565configured by a comparator shifts from high to low (t1) as shown inFIG. 31F. In the same manner as described above, when an output potential from third differential amplifier558reaches threshold voltage Vthdetermined by a threshold value generating unit (not shown), an output from third comparing unit564configured by a comparator shifts from high to low (t2) as shown inFIG. 31G. When an output potential from second differential amplifier557reaches threshold voltage Vthdetermined by a threshold value generating unit (not shown), an output from second comparing unit563configured by a comparator shifts from high to low (t3) as shown inFIG. 31I. Furthermore, when an output potential from first differential amplifier555reaches threshold voltage Vthdetermined by a threshold value generating unit (not shown), an output from first comparing unit562configured by a comparator shifts from high to low as shown inFIG. 31H, pulse generation from pulse generating circuit551is stopped. For this reason, output voltages from first differential amplifier555, second differential amplifier557, third differential amplifier558, and fourth differential amplifier560increase to second potential (V1) (t4).

Thereafter, since diodes connected between inputs and outputs of first differential amplifier555, second differential amplifier557, third differential amplifier558, and fourth differential amplifier560are turned on, output voltages from the differential amplifiers sharply decrease to reach threshold potentials given to positive inputs of the differential amplifiers, the outputs from the differential amplifiers return to first potential569(V1). At the same time, each of outputs from first, second, third, and fourth comparing units562,563,564, and565shifts from low to high as shown inFIGS. 31I,31H,31G, and31F, and pulses are generated from pulse generating circuit551and inputted to terminals530and538of detecting unit521as shown inFIG. 31A(t5).

Thereafter, output potentials from first differential amplifier555, second differential amplifier557, third differential amplifier558, and fourth differential amplifier560exponentially increase again from first potential569(t1) to second potential570(t2), as shown inFIGS. 31D,31E,31C, and31B, at time constants determined by first resistor556and the inter-electrode capacitance of first detecting electrodes522, second resistor518and the inter-electrode capacitance of second detecting electrodes523, third resistor559and the inter-electrode capacitance of third detecting electrodes524, and fourth resistor561and the inter-electrode capacitance of fourth detecting electrodes525. Subsequently, the same operation as that in a zone of t0to t5is repeated.

Output signals from first, third, and fourth comparing units562,564, and565are inputted to logic circuit566configured by a logic element and a flip-flop, a pulse signal having a pulse width from a falling edge of a pulse shown inFIG. 31Fto a falling edge of a pulse shown inFIG. 31Gis outputted to first analog switch567as shown inFIG. 31J, and a pulse signal having a pulse width from the falling edge of the pulse shown inFIG. 31Gto a falling edge of a pulse shown inFIG. 31His outputted to second analog switch568as shown inFIG. 31K.

In this case, when a signal inputted to first analog switch567is at high, first analog switch567is “closed”. When the signal is at low, first analog switch567is “opened”. When a signal inputted to second analog switch568is at high, second analog switch568is “closed”. When the signal is at low, second analog switch568is “opened”. In this manner, for times t1to t2and times t6to t7, first analog switch567is “closed”, and second analog switch568is “opened”. For this reason, capacitor573is charged from second potential570through fifth resistor571. For times t2to t4and times t7to t9, first analog switch567is “opened”, and second analog switch568is “closed”. For this reason, an electric charge accumulated in capacitor573is discharged to first potential569(V1) through fifth resistor571.

For times t0to t1and times t4to t5or the like, since both first analog switch567and second analog switch568are “opened”, an electric charge accumulated in capacitor573is stored. In this manner, first analog switch567and second analog switch568are alternately opened and closed for times determined by a length of a part of third detecting electrodes524dipped in the liquid to be measured and a length of a part being out of the liquid to be measured to charge and discharge capacitor573, so that a liquid level of the liquid to be measured can be outputted to output terminal572as an analog voltage.

FIG. 32is a characteristic graph showing a change of output voltage V0with time when a ratio of a length of a part of the second detecting electrode dipped in a liquid to be measured to a length of a part being out of the liquid to be measured is 1:4 in the liquid level sensor according to Embodiment 5 of the present invention. More specifically, the characteristic graph is obtained by simulating analog voltage V0outputted across both ends of capacitor573when resistance R of fifth resistor571and a capacitance C of capacitor573are set to 500 kΩ and 100 pF, respectively, when charging time Tcand discharging time Tdare set to 1 μsec and 4 μsec, respectively, and when a ratio of Tcto Tdand a ratio of a length of a part of third detecting electrodes524dipped in the liquid to be measured to a length of a part being out of the liquid to be measured are 1:4.

As is apparent fromFIG. 32, after about 500 μsec have elapsed, output voltage V0made by superposing a ripple having an amplitude of about ±0.04 [V] on a DC component of 1 [V] is obtained. Since the ripple is removed by a low-pass filter, a DC voltage expressing a level of the liquid to be measured is outputted to output terminal572after a predetermined period of time has elapsed since the power supply is turned on.

Output signals from first and second comparing units562and563are inputted to logic circuit574configured by a logic element and a flip-flop, a pulse signal having a pulse width from a falling edge of a pulse shown inFIG. 31Ito a falling edge of a pulse shown inFIG. 31His outputted to first analog switch575as shown inFIG. 31L, and a pulse signal having a pulse width from the falling edge of the pulse shown inFIG. 31Hto a falling edge of a pulse shown inFIG. 31Iis outputted to second analog switch576as shown inFIG. 31M.

In this case, when a signal inputted to each analog switch is at high, the analog switch is “closed”. When the signal is at low, the analog switch is “opened”. In this manner, for times t3to t4and times t8to t9, first analog switch575is “closed”, and second analog switch576is “opened”. Therefore, capacitor579is charged from second potential570through sixth resistor577. For times t0to t3and times t5to t8, first analog switch575is “opened”, and second analog switch576is “closed”. For this reason, an electric charge accumulated in capacitor579is discharged to first potential569(V1) through sixth resistor577.

In this manner, an operation of charging capacitor579for a time being in proportion to a difference between an electrostatic capacitance measured by first detecting electrodes522always dipped in the liquid to be measured and influenced by a dielectric constant held by the liquid to be measured and an electrostatic capacitance measured by second detecting electrodes523which are not influenced by the dielectric constant held by the liquid to be measured and an operation of discharging an electric charge charged in capacitor579for a time being in proportion to an electrostatic capacitance measured by first detecting electrodes522are repeated to make it possible to output a voltage being in proportion to liquid quality of the liquid to be measured to first output terminal578.

A role of a cancel electrode arranged in the liquid level sensor according to Embodiment 5 of the present invention will be further described.

In the detecting unit of the liquid level sensor according to Embodiment 5 of the present invention shown inFIG. 28, when first cancel electrode532is not arranged, a position (liquid level) where third detecting electrodes524cross to the level of the liquid to be measured changes, and a position where extraction line528on the level of the liquid to be measured and extraction line527of first detecting electrode522cross each other also changes. For this reason, an electrostatic capacitance generated between extraction line528and extraction line527of first detecting electrode522changes. In this manner, the falling edge of the pulse shown inFIG. 31Hchanges, and an output voltage value expressing a liquid level has an error.

When second cancel electrode540is not arranged, a position (liquid level) where third detecting electrodes524cross to the level of the liquid to be measured changes, and a position where common extraction line526on the level of the liquid to be measured and extraction line528of second detecting electrode523cross each other also changes. For this reason, an electrostatic capacitance generated between common extraction line526and extraction line528of second detecting electrode523changes. In this manner, the falling edge of the pulse shown inFIG. 31Jchanges, and a reference to measure liquid quality changes. For this reason, and an output voltage value expressing the liquid quality has an error. At the same time, a position (liquid level) where third detecting electrodes524cross to the level of the liquid to be measured changes. Since a position where common extraction line534on the level of the liquid to be measured and extraction line535of third detecting electrode524cross each other also changes, an electrostatic capacitance generated between common extraction line534and extraction line535of third detecting electrode524changes. In this manner, the falling edge of the pulse shown inFIG. 31Jand the falling edge of the pulse shown inFIG. 31Gchange, and an output voltage value expressing the liquid level has an error.

In the same manner as described above, when third cancel electrode542is not arranged, a position (liquid level) where third detecting electrodes524cross to the level of the liquid to be measured changes. Since a position where common extraction line534on the level of the liquid to be measured and extraction line536of fourth detecting electrode525cross each other also changes, an electrostatic capacitance generated between common extraction line534and extraction line536of fourth detecting electrode525changes. In this manner, the falling edge of the pulse shown inFIG. 31Fchanges, and an output voltage value expressing the liquid level has an error.

In contrast to this, in the liquid level sensor according to Embodiment 5 of the present invention, first cancel electrode532is arranged along extraction line527of first detecting electrode522, second cancel electrode540is arranged between extraction line528of second detecting electrode523and extraction line535of third detecting electrode524, and third cancel electrode542is arranged along extraction line536of fourth detecting electrode525. Signals obtained by inverting pulses from pulse generating circuit551are inputted through level adjusters552,553, and554to cancel electric charges accumulated between extraction lines and a common extraction line of the detecting electrodes. In this manner, electrostatic capacitances measured between terminals529and530, between terminals529and531, between terminals537and538, terminals537and539are only electrostatic capacitances measured by comb-shaped detecting electrodes522,523,524, and525.

For this reason, an electrostatic capacitance of the liquid to be measured detected by the detecting electrodes is not influenced by a level of the liquid to be measured. As a result, an output voltage value which accurately expresses a liquid level or liquid quality can be obtained.

As is apparent from the above description, since the liquid level sensor according to Embodiment 5 of the present invention can output a voltage which is accurately proportional to a liquid level or liquid quality without arranging a complex operational device, a highly sensitive sensor can be easily provided.

INDUSTRIAL APPLICABILITY

The liquid level sensor according to the present invention can always automatically output a voltage which is in proportion to a liquid level without arranging a complex operational device even though a dielectric constant of a liquid to be measured or a temperature changes. In this manner, the present invention can advantageously easily provide a highly sensitive sensor and is useful as a liquid level sensor which detects a liquid level of an engine oil or a fuel for an automobile, a construction machine, and the like.