JP2010210307A - Liquid level sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid level sensor capable of reducing cost of a detection circuit by reducing a chip area by summarizing analog circuit parts, and improving liquid level measurement accuracy. <P>SOLUTION: This liquid level sensor includes: a first pulse generation means 38 for generating alternately a first pulse having a pulse width in proportion to a capacitance of a first detection electrode 22 always put in liquid to be measured, and a third pulse having a pulse width in proportion to a capacitance of a third detection electrode 24 always put out of the liquid to be measured; and a second pulse generation means 41 for generating a second pulse having a pulse width in proportion to a capacitance of a second detection electrode 23 for measuring the level of the liquid to be measured. In the sensor, operation is repeated, for charging as long as a time from fall of the third pulse until fall of the second pulse, and for discharging as long as a time from fall of the second pulse until fall of the first pulse. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a liquid level sensor that detects a liquid level of a liquid stored in a container, and more particularly to a liquid level sensor that detects a liquid level of engine oil or fuel of an automobile, a construction machine, or the like.

  As a liquid level sensor for detecting the liquid level of engine oil or fuel for automobiles, construction machines, etc., those shown in FIGS. 11 and 12 are known (see Patent Document 1). FIG. 11 is a front view of a detection unit of a conventional liquid level sensor. In FIG. 11, reference numeral 1 denotes a rectangular substrate extending vertically, and the lower end portion of the substrate 1 has a first comb-shaped shape. The detection electrode 2 is provided with a comb-shaped second detection electrode 3 at a substantially central portion. The first detection electrode 2 is composed of a plurality of linear electrodes 4 arranged at predetermined intervals in the vertical direction, and these are alternately arranged on lead lines 5 and 6 extending vertically along both side edges of the substrate 1. It is connected to the. The second detection electrode 3 includes a plurality of linear electrodes 7 arranged so as to extend from the upper end portion to the lower end portion at predetermined intervals on the left and right sides, and the upper ends of these linear electrodes 7 are leads. The lines 8 and 9 are alternately connected.

  At the time of liquid level measurement, the detection unit is immersed in the liquid to be measured. At this time, the first detection electrode 2 is always arranged to be immersed in the liquid to be measured. On the other hand, the second detection electrode 3 intersects the liquid surface to be measured, and the portion immersed in the liquid increases and decreases as the liquid level rises and falls.

  FIG. 12 is a detection circuit diagram using the above-described detection unit, and includes an oscillation circuit 10 and a processing circuit 11. The oscillation circuit 10 includes inverters 12, 13, and 14 and a resistor 15, and the first and second detection electrodes 2 and 2 in the detection unit are connected between the inverters 13 and 14 via analog switches 16 and 17, respectively. 3 is connected. The processing circuit 11 having a microcomputer first closes the analog switch 16 and calculates the dielectric constant of the liquid to be measured from the oscillation frequency determined by the resistor 15 and the capacitance of the first detection electrode 2. Remember. Subsequently, the processing circuit 11 closes the other analog switch 17 in place of the one analog switch 16, and the oscillation frequency determined by the resistor 15 and the capacitance of the second detection electrode 3 and the liquid to be measured are measured. The liquid level is calculated from the dielectric constant, and the accurate liquid level can be known even when the dielectric constant of the liquid to be measured fluctuates.

  However, in the above-described conventional liquid level sensor, the liquid quality of the liquid to be measured is determined from the measured value obtained from the first detection electrode 2 that is always immersed in the liquid to be measured and the standard value stored in advance. While calculating, the part which cross | intersects the to-be-measured liquid surface and is immersed in a liquid is obtained from the measured value obtained from the 2nd detection electrode 3 which increases / decreases with the raising / lowering of a liquid level, and the said 1st detection electrode 2. Since the liquid level of the liquid to be measured is calculated from the measured value, there is a problem that the calculation device becomes complicated and large.

  In order to solve the above problems, the present inventors have developed a liquid level sensor as shown in FIGS. FIG. 13 is a front view of the detection unit of the liquid level sensor. In FIG. 13, 201 is a detection unit made of a rectangular polyimide film or the like extending vertically, and a comb is placed at the lower end of the detection unit 201. A pair of first detection electrodes 202 made of tooth-shaped carbon is provided. In addition, a pair of second detection electrodes 203 made of comb-shaped carbon is provided in the approximate center of the detection unit 201, and a pair of second detection electrodes 203 made of comb-shaped carbon is also formed at the upper end of the detection unit 201. 3 detection electrodes 204 are provided. The first, second, and third detection electrodes 202, 203, and 204 are connected to terminals 206, 207, 208, and 209 by lead wires 205 extending vertically.

  FIG. 14 shows a detection circuit diagram of the liquid level sensor. In FIG. 14, the terminal 206 of the detection unit 201 is connected to the first potential 210. The terminals 207, 208, and 209 of the detection unit 201 are connected to the second potential 214 via resistors 211, 212, and 213, respectively. As a result, the first detection electrode 202, the second detection electrode 203, and the third detection electrode 204 are connected to the resistors 211, 212, and 213, respectively. At this time, a time constant determined by the interelectrode capacitance of the first detection electrode 202 and the resistance 211 in a state where all of the first, second, and third detection electrodes 202, 203, and 204 are outside the liquid to be measured, The time constant determined by the interelectrode capacitance of the detection electrode 203 and the resistor 212 is made substantially equal to the time constant determined by the interelectrode capacitance of the third detection electrode 204 and the resistance 213.

  Further, the midpoint potential of the resistor 211 and the first detection electrode 202 is compared with the threshold value generating means comprising resistors 216 and 217 in the first pulse generating means 215 comprising a comparator. Similarly, the midpoint potential of the resistor 212 and the second detection electrode 203 and the midpoint potential of the resistor 213 and the third detection electrode 204 are respectively second pulse generation means 218 comprising a comparator. The third pulse generation means 219 compares the threshold generation means including resistors 216 and 217. The output signals of the first, second, and third pulse generating means 215, 218, and 219 are input to a logic circuit 220 made up of logic elements. A first analog switch 221 and a second analog switch 222 that are controlled to open and close by an output signal of the logic circuit 220 are provided at the subsequent stage of the logic circuit 220. Reference numeral 223 denotes a resistor having one end connected to the midpoint between the first analog switch 221 and the second analog switch 222 and the other end connected to the output terminal 224. A capacitor 225 has one end connected to the first potential 210 and the other end connected between the resistor 223 and the output terminal 224.

  The circuit operation of the liquid level sensor will be described with reference to FIG. FIGS. 15 (a), (b), (c), (d), (e), (f), (g), and (h) show voltage waveforms of respective parts of the liquid level sensor. The detection part of the liquid level sensor shown in FIG. 13 is immersed in a liquid to be measured such as engine oil in an oil pan (not shown). At this time, the first detection electrode 202 is always immersed in the liquid to be measured, and the third detection electrode 204 is always disposed outside the liquid to be measured. The second detection electrode 203 intersects with the liquid surface to be measured, and the portion immersed in the liquid is increased or decreased as the liquid level is raised or lowered.

Since the liquid level sensor has no electric charge between the electrode pairs of the first, second, and third detection electrodes 202, 203, 204 in the initial state (t ₀ ) before the power is turned on, The midpoint potential of the first detection electrode 202, the midpoint potential of the resistor 212 and the second detection electrode 203, and the midpoint potential of the resistor 213 and the third detection electrode 204 are all the first potential 210 (V ₁ )

When the power is turned on, the midpoint potential of the resistor 213 and the third detection electrode 204 is changed from the _first potential 210 (V ₁ ) to the second potential 214 as shown in FIG. Toward (V ₂ ), it rises exponentially with a time constant determined by the resistance 213 and the interelectrode capacitance of the third detection electrode 204. Further, the midpoint potential between the resistor 212 and the second detecting electrode 203, toward the first potential 210 (V ₁₎ from the second potential 214 (V ₂₎ as shown in FIG. 15 (b) , And rises exponentially with a time constant determined by the resistance 212 and the interelectrode capacitance of the second detection electrode 203. At this time, since a part of the second detection electrode 203 is in the liquid to be measured, the time constant determined by the resistance 212 and the capacitance of the second detection electrode 203 is the resistance 213 and the third detection. It becomes larger than the time constant determined by the electrode 204. Further, the midpoint potential of the resistor 211 and the first detection electrode 202 is directed from the _first potential 210 (V ₁ ) to the second potential 214 (V ₂ ) as shown in FIG. , And rises exponentially with time constants determined by the resistor 211 and the interelectrode capacitance of the first detection electrode 202, respectively. At this time, since the first detection electrode 202 is always immersed in the liquid to be measured, the time constant determined by the resistance 211 and the capacitance of the first detection electrode 202 is the resistance 212 and the second resistance. It becomes larger than the time constant determined by the detection electrode 203.

When the midpoint potential of the resistor 213 and the third detection electrode 204 reaches the threshold voltage V _th determined by the resistors 216 and 217, the output of the third pulse generating means 219 comprising a comparator is shown in FIG. As shown, the transition from high to low (t ₁ ) generates a third pulse 227 having a pulse width proportional to the capacitance measured by the third detection electrode 204. Similarly, when the midpoint potential of the resistor 212 and the second detection electrode 203 reaches the threshold voltage _Vth determined by the resistors 216 and 217, the output of the second pulse generating means 218 comprising a comparator is shown in FIG. As shown in FIG. 15E, the transition is made from high to low (t ₂ ), and a second pulse 228 having a pulse width proportional to the capacitance measured by the second detection electrode 203 is generated. When the midpoint potential between the resistor 211 and the first detection electrode 202 reaches the threshold voltage _Vth determined by the resistors 216 and 217, the output of the first pulse generating means 215 comprising a comparator is shown in FIG. f) transition from high to low (t ₃ ) to generate a first pulse 229 having a pulse width proportional to the capacitance measured by the first detection electrode 202, and Since the charges accumulated in the second and third detection electrodes 202, 203, and 204 are discharged to the _first potential 210 (V ₁ ) through the element 226 having an open collector configuration, the resistor 211 and the second The midpoint potential of one detection electrode 202, the midpoint potential of the resistor 212 and the second detection electrode 203, and the midpoint potential of the resistor 213 and the third detection electrode 204 are all the first potential 210. to (V ₁₎ Ri, and the first, second, transition from the third output of the pulse generating means 215,218,219 of low, as shown in FIGS 15 (f) (e) ( d) to high (t ₄₎ .

Thereafter, the midpoint potential of the resistor 213 and the third detection electrode 204, the midpoint potential of the resistor 212 and the second detection electrode 203, and the midpoint potential of the resistor 211 and the first detection electrode 202 are toward the first potential 210 (V ₁₎ from the second potential 214 (V ₂₎ again, respectively Figure 15 (c) (b) and the resistor 213 as shown in (a) third detection electrode 204 The time constant determined by the interelectrode capacitance, the interelectrode capacitance of the resistor 212 and the second detection electrode 203, and the interelectrode capacitance of the resistor 211 and the first detection electrode 202, and increases exponentially as before. The operation (t ₅ , t ₆ , t ₇ ) is repeated.

  The output signals of the first, second, and third pulse generating means 215, 218, and 219 are input to a logic circuit 220 made up of logic elements, and the first analog switch 221 has an output signal as shown in FIG. In addition, from the fall of the third pulse 227 proportional to the capacitance measured by the third detection electrode 204, the second pulse 228 proportional to the capacitance measured by the second detection electrode 203 is detected. A fourth pulse 230 having a pulse width up to the falling edge is output. Further, as shown in FIG. 15H, the second analog switch 222 performs the first detection from the falling edge of the second pulse 228 proportional to the capacitance measured by the second detection electrode 203. A fifth pulse 231 having a pulse width up to the fall of the first pulse 229 proportional to the capacitance measured by the electrode 202 is output.

In this case, the first analog switch 221 is “closed” when the signal input to the first analog switch 221 is high, and “open” when the signal is low. The second analog switch 222 is “closed” when the signal input to the second analog switch 222 is high, and “open” when the signal is low. As a result, the outputs of the second pulse generating means 218 change to low after the time t _{1 to} t ₂ and t _{5 to} t ₆ in FIG. 15, ie, the output of the third pulse generating means 219 changes to low. In the period until the first analog switch 221 is “closed” and the second analog switch 222 is “open”, the capacitor 225 is charged from the second potential 214 through the fourth resistor 223.

And also, the output of the first pulse generating means 215 changes to the low output of the time t ₂ ~t ₃ and t ₆ ~t _7, that is, the second pulse generating means 218 of FIG. 15 transitions low Until the first analog switch 221 is “open” and the second analog switch 222 is “closed”, the charge accumulated in the capacitor 225 is discharged to the first potential 210 through the resistor 223. .

  Further, at other times, the first analog switch 221 and the second analog switch 222 are both “open”, so that the charge accumulated in the capacitor 225 is stored. In this way, the first analog switch 221 and the second analog switch 222 are set for a time determined by the length of the portion where the second detection electrode 203 is immersed in the liquid to be measured and the length of the portion outside the measurement target. Are alternately opened and closed to charge and discharge the capacitor 225, whereby the liquid level of the liquid to be measured can be output as an analog voltage to the output terminal 224.

  According to the configuration of the liquid level sensor described above, when the liquid level of the liquid to be measured is between the second detection electrodes 203, the liquid level can be detected with high accuracy without providing a complicated arithmetic device. It is.

As prior art document information relating to the invention of this application, for example, Patent Document 1 is known.
JP-A-63-79016

  However, if the detection circuit of the liquid level sensor having the configuration shown in FIG. 14 is to be integrated on a one-chip IC, the need for three pulse generating means for handling analog signals in parallel is a major obstacle. The reason for this is that the logic circuit that handles digital signals can be configured efficiently with low current consumption, so it can be configured with a small number of transistors, whereas the analog circuit has high current consumption, and has a certain level of accuracy and resistance. In order to maintain noise characteristics, a large number of transistors, resistors, etc. occupying a large area are required. As a result, in the conventional liquid level sensor including three pulse generating means, the chip area becomes large, and the cost of the IC increases. In a typical example, an element constituting the analog circuit portion requires about 20 times the area of an element constituting the logic circuit portion. Second, since there are variations in switching voltage, rising characteristics, etc. between a plurality of analog circuit units, the accuracy of liquid level measurement decreases as the number of analog circuits used increases.

  The present invention solves the above-described problems. By consolidating analog circuit portions, the chip area can be reduced, the cost of the detection circuit can be reduced, and the liquid level measurement accuracy can be improved. An object of the present invention is to provide a liquid level sensor that can be realized.

  In order to achieve the above object, the present invention has the following configuration.

  According to the first aspect of the present invention, the first detection electrode always in the liquid to be measured, the second detection electrode for measuring the liquid level of the liquid to be measured, and the third detection electrode always outside the liquid to be measured. A detection unit having a detection electrode; and a first pulse generation unit and a second pulse generation unit that generate a pulse having a pulse width proportional to the capacitance measured by each detection electrode. From the fall of the third pulse having a pulse width proportional to the capacitance measured by the third detection electrode generated by one of the pulse generation means and the second pulse generation means. Charging for the time until the fall of the second pulse having a pulse width proportional to the capacitance measured by the second detection electrode generated by the other pulse generating means, and generating the one pulse The second detection electrode generated by the means A pulse width proportional to the capacitance measured by the first detection electrode generated by the other pulse generation means from the trailing edge of the second pulse having a pulse width proportional to the determined capacitance. The liquid level of the liquid to be measured is measured by repeating the operation of discharging only for the time until the falling edge of the first pulse having the above. Since the circuit section can be integrated into two, the cost of the detection circuit can be reduced, and the liquid level measurement accuracy can be improved, thereby enabling an inexpensive and highly sensitive liquid level sensor. It has the effect that it can be provided.

  According to a second aspect of the present invention, the first detection electrode, the second detection electrode, and the third detection electrode are provided in order from the lower side to the upper side. Separately from the second and third detection electrodes, a detection unit provided with a fourth detection electrode for measuring the capacitance between the electrodes without being affected by the dielectric constant of the liquid to be measured; First pulse generating means and second pulse generating means for generating a pulse having a pulse width proportional to the capacitance measured by the detection electrode, the first pulse generating means and the second pulse generating means From the falling edge of the third pulse having a pulse width proportional to the capacitance measured by the third detection electrode generated by one of the pulse generating means, the other pulse generating means Static electricity measured by the second detection electrode A pulse that is charged for the time until the fall of the second pulse having a pulse width proportional to the amount, and that is proportional to the capacitance measured by the second detection electrode generated by the one pulse generating means From the fall of the second pulse having a width to the fall of the first pulse having a pulse width proportional to the capacitance measured by the first detection electrode generated by the other pulse generating means. At the falling edge of the fourth pulse having a pulse width proportional to the capacitance measured by the fourth detection electrode generated by the one pulse generating means. If the third pulse having a pulse width proportional to the capacitance measured by the third detection electrode generated by the pulse generating means is at a high level, the discharge is stopped and only charging is performed; and Said one The first pulse generated by the other pulse generating means at the time of falling of the fourth pulse having a pulse width proportional to the capacitance measured by the fourth detection electrode generated by the pulse generating means. When the first pulse having a pulse width proportional to the capacitance measured by the detection electrode is at a low level, the liquid to be measured is repeated by stopping the charge and performing only the discharge. The liquid level is measured, and according to this configuration, the analog circuit portion, which has conventionally been required to be four, is consolidated into two, so that the cost of the detection circuit can be reduced. In addition, it is possible to improve the liquid level measurement accuracy, thereby providing an inexpensive and highly sensitive liquid level sensor, and when the liquid level of the liquid to be measured intersects at any position of each detection electrode But always fixed This has the effect of being able to output an output voltage.

  As described above, the liquid level sensor of the present invention has the first detection electrode always in the liquid to be measured and the second detection electrode for measuring the liquid level of the liquid to be measured, and the third sensor which is always outside the liquid to be measured. A detection unit having a detection electrode; and a first pulse generation unit and a second pulse generation unit that generate a pulse having a pulse width proportional to the capacitance measured by each detection electrode. From the fall of the third pulse having a pulse width proportional to the capacitance measured by the third detection electrode generated by one of the pulse generation means and the second pulse generation means. Charging for the time until the fall of the second pulse having a pulse width proportional to the capacitance measured by the second detection electrode generated by the other pulse generating means, and generating the one pulse Said second detection electrode generated by means A pulse width proportional to the capacitance measured by the first detection electrode generated by the other pulse generation means from the falling edge of the second pulse having a pulse width proportional to the measured capacitance. Since the liquid level of the liquid to be measured is measured by repeating the operation of discharging only for the time until the first pulse falls, the number of analog circuit parts to be used can be reduced. Thereby, the outstanding effect that an inexpensive and highly accurate liquid level sensor can be provided is produced.

  Hereinafter, a liquid level sensor according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a front view of a detection unit of a liquid level sensor according to an embodiment of the present invention. In FIG. 1, reference numeral 21 denotes a detection unit made of a rectangular polyimide film or the like extending vertically. A pair of first detection electrodes 22 made of comb-shaped carbon is provided at the lower end of 21. In addition, a pair of second detection electrodes 23 made of comb-shaped carbon is provided in the approximate center of the detection unit 21, and a pair of second detection electrodes 23 made of comb-shaped carbon is also formed at the upper end of the detection unit 21. 3 detection electrodes 24 are provided. The first, second, and third detection electrodes 22, 23, and 24 are connected to terminals 26, 27, 28, and 29 by lead wires 25 extending vertically.

  FIG. 2 shows a detection circuit diagram of the liquid level sensor. In FIG. 2, the terminal 26 of the detection unit 21 is connected to a first potential 30. The terminal 27 of the detection unit 21 is connected to the second potential 33 through the resistor 31 and the first switch 32. Similarly, the terminal 28 of the detection unit 21 is connected to the second potential 33 via the resistor 34, and the terminal 29 of the detection unit 21 is connected to the second potential 33 via the resistor 35 and the second switch 36. As a result, the first detection electrode 22, the second detection electrode 23, and the third detection electrode 24 are connected to the resistors 31, 34, and 35, respectively. At this time, a time constant determined by the interelectrode capacitance of the first detection electrode 22 and the resistance 31 in a state where all of the first, second, and third detection electrodes 22, 23, and 24 are outside the liquid to be measured, The time constant determined by the interelectrode capacitance of the detection electrode 23 and the resistor 34 and the time constant determined by the interelectrode capacitance of the third detection electrode 24 and the resistance 35 are made substantially equal.

  Further, the midpoint potential of the resistor 31 and the first detection electrode 22 is input to the first pulse generation means 38 formed of a comparator via the third switch 37, and the threshold voltage formed of the resistors 39 and 40 is detected. To be compared. In the same manner, the midpoint potential of the resistor 34 and the second detection electrode 23 is input to the second pulse generating means 41 formed of a comparator, and the resistor 35 and the third detection electrode 24 are used. The midpoint potential is input to the first pulse generating means 38 comprising a comparator via the fourth switch 42 and compared with the threshold voltage comprising the resistors 39 and 40.

  The first switch 32 and the third switch 37 are controlled to open and close by a signal (SEL_Q) from the switching signal generation circuit 43. Similarly, the second switch 36 and the fourth switch 42 are controlled to open and close by a signal (SEL_A) from the switching signal generation circuit 43. In this case, each switch is “closed” when the signal applied to each switch is high, while each switch is “open” when the signal applied to each switch is low.

  44 is a logic circuit composed of logic elements and flip-flops. The logic circuit 44 receives the output signals of the first and second pulse generating means 38 and 41 and the signal from the switching signal generating circuit 43. . Further, a first analog switch 45 and a second analog switch 46 that are controlled to open and close by an output signal of the logic circuit 44 are arranged between the first potential 30 and the second potential 33 at the subsequent stage of the logic circuit 44. Is provided. Reference numeral 47 denotes a resistor having one end connected to the midpoint of the first analog switch 45 and the second analog switch 46 and the other end connected to the output terminal 48. A capacitor 49 has one end connected to the first potential 30 and the other end connected between the resistor 47 and the output terminal 48.

  Next, the circuit operation of the liquid level sensor according to the embodiment of the present invention will be described with reference to FIG.

  3 (a), (b), (c), (d), (e), (f), (g), (h), and (i) show voltage waveforms at various parts of the liquid level sensor, which are shown in FIG. The detection unit 21 of the liquid level sensor is immersed in a liquid to be measured such as engine oil in an oil pan (not shown). At this time, the first detection electrode 22 is always immersed in the liquid to be measured, and the third detection electrode 24 is always disposed outside the liquid to be measured. The second detection electrode 23 intersects the liquid surface to be measured, and the portion immersed in the liquid increases or decreases as the liquid level rises and falls.

In the liquid level sensor according to the embodiment of the present invention, electric charge exists between the electrode pairs of the first, second, and third detection electrodes 22, 23, 24 in the initial state (t ₀ ) before the power is turned on. Therefore, the midpoint potential of the resistor 31 and the first detection electrode 22, the midpoint potential of the resistor 34 and the second detection electrode 23, and the midpoint potential of the resistor 35 and the third detection electrode 24 are all. At first potential 30.

  When the power is turned on, the SEL_A signal is output from the switching signal generation circuit 43 as shown in FIG. 3A, and the second switch 36 and the fourth switch 42 are “closed”. The midpoint potential of the resistor 35 and the third detection electrode 24 is changed from the first potential 30 toward the second potential 33 as shown in FIG. It rises exponentially with a time constant determined by the interelectrode capacitance of the electrode 24. Further, the midpoint potential between the resistor 34 and the second detection electrode 23 is changed from the first potential 30 toward the second potential 33 as shown in FIG. It rises exponentially with a time constant determined by the interelectrode capacitance of the detection electrode 23.

  At this time, since the second detection electrode 23 intersects the liquid surface to be measured and the third detection electrode 24 is always outside the liquid to be measured, the resistance 34 and the capacitance of the second detection electrode 23 The time constant determined is larger than the time constant determined by the resistor 35 and the third detection electrode 24.

When the midpoint potential of the resistor 35 and the third detection electrode 24 reaches the threshold voltage _Vth determined by the resistors 39 and 40, the output of the first pulse generating means 38 comprising a comparator is shown in FIG. As shown, a transition from high to low (t ₁ ) generates a third pulse 51 proportional to the capacitance measured at the third detection electrode 24. Similarly, the midpoint potential between the resistor 34 and the second detection electrode 23 reaches the threshold voltage V _th which is determined by the resistor 39 and 40, the output of the second pulse generating means 41 comprising a comparator Figure As shown in 3 (f), the transition from high to low (t ₂ ) occurs, and a second pulse 52 proportional to the capacitance measured by the second detection electrode 23 is generated.

When the SEL_A signal falls (t ₃ ), the signal from the logic circuit 44 drives the element 50 having an open collector configuration, whereby charges accumulated in the second detection electrode 23 and the third detection electrode 24 are accumulated. Is discharged to the first potential 30, the midpoint potential of the resistor 34 and the second detection electrode 23 and the midpoint potential of the resistor 35 and the third detection electrode 24 are respectively shown in FIG. Return to the first potential 30 as shown in c).

Then, Figure 3 SEL_Q signal from the switching signal generating circuit 43 as shown in (b) is outputted (t _4), since the first switch 32 and third switch 37 becomes "closed", the resistor 31 As shown in FIG. 3E, the midpoint potential between the resistor 31 and the first detection electrode 22 moves from the first potential 30 toward the second potential 33 as shown in FIG. It rises exponentially with a time constant determined by the interspace. Further, the midpoint potential of the resistor 34 and the second detection electrode 23 is changed again from the first potential 30 toward the second potential 33 as shown in FIG. It rises exponentially with a time constant determined by the interelectrode capacitance of the detection electrode 23.

  At this time, since the first detection electrode 22 is always immersed in the liquid to be measured, the time constant determined by the resistor 31 and the first detection electrode 22 is determined by the resistance 34 and the second detection electrode 23. The time constant is determined by the capacitance between the electrodes.

When the midpoint potential of the resistor 34 and the second detection electrode 23 reaches the threshold voltage _Vth determined by the resistors 39 and 40, the output of the second pulse generating means 41 comprising a comparator is shown in FIG. As shown, the transition from high to low (t ₅ ) generates a second pulse 53 proportional to the capacitance measured by the second detection electrode 23. Similarly, when the midpoint potential of the resistor 31 and the first detection electrode 22 reaches the threshold voltage _Vth determined by the resistors 39 and 40, the output of the first pulse generating means 38 comprising a comparator is shown in FIG. As shown in 3 (g), the transition from high to low (t ₆ ) occurs, and a first pulse 54 proportional to the capacitance measured by the first detection electrode 22 is generated.

When the SEL_Q signal falls (t ₇ ), the signal from the logic circuit 44 drives the element 50 having an open collector configuration, whereby charges accumulated in the second detection electrode 23 and the first detection electrode 22 are accumulated. Is discharged to the first potential 30, the midpoint potential of the resistor 34 and the second detection electrode 23 and the midpoint potential of the resistor 31 and the first detection electrode 22 are respectively shown in FIG. Return to the first potential 30 as shown in e).

Next, the SEL_A signal is output again from the switching signal generation circuit 43 (t ₈ ), and the second switch 36 and the fourth switch 42 are “closed”, so that the resistor 35, the third detection electrode 24, The middle point potential is a time constant determined by the resistance 35 and the interelectrode capacitance of the third detection electrode 24 from the first potential 30 toward the second potential 33 as shown in FIG. It rises exponentially. Further, the midpoint potential between the resistor 34 and the second detection electrode 23 is changed from the first potential 30 toward the second potential 33 as shown in FIG. It rises exponentially with a time constant determined by the interelectrode capacitance of the detection electrode 23. Then, thereafter, it repeats the same operation as the above t ₀ ~t ₇ interval.

  In this way, the first pulse generator 38 outputs a pulse train as shown in FIG. 3G, and the second pulse generator 41 outputs a pulse train as shown in FIG. Will be.

  The output signals of the first and second pulse generators 38 and 41 are input to a logic circuit 44 composed of logic elements and flip-flops, and the first analog switch 45 receives the first signal as shown in FIG. From the fall of the third pulse 51 proportional to the capacitance measured by the third detection electrode 24 to the fall of the second pulse 52 proportional to the capacitance measured by the second detection electrode 23 A fourth pulse 55 having a pulse width of is output. Further, as shown in FIG. 3I, the second analog switch 46 performs the first detection from the trailing edge of the second pulse 53 proportional to the capacitance measured by the second detection electrode 23. A fifth pulse 56 having a pulse width up to the fall of the first pulse 54 proportional to the capacitance measured at the electrode 22 is output.

Here, the first analog switch 45 is “closed” when the signal input to the first analog switch 45 is high, and “open” when the signal is low. The second analog switch 46 is “closed” when the signal input to the second analog switch 46 is high, and “open” when the signal is low. Thus, during the time t _{1 to} t ₂ in FIG. 3, that is, during the period when the fourth pulse 55 is high, the first analog switch 45 is “closed” and the second analog switch 46 is “open”. The capacitor 49 is charged from the second potential 33 through the fourth resistor 47.

Then, during the time t _{5 to} t ₆ in FIG. 3, that is, during the period when the fifth pulse 56 is high, the first analog switch 45 is “open” and the second analog switch 46 is “closed”. The charge accumulated in 49 is discharged to the first potential 30 through the fourth resistor 47.

  At other times, since the first analog switch 45 and the second analog switch 46 are both “open”, the charge accumulated in the capacitor 49 is stored. In this way, the first analog switch 45 and the second analog switch 46 are set for a time determined by the length of the portion where the second detection electrode 23 is immersed in the liquid to be measured and the length of the portion outside the measurement target. Are alternately opened and closed to charge and discharge the capacitor 49, whereby the liquid level of the liquid to be measured can be output to the output terminal 48 as an analog voltage.

FIG. 4 shows the calculation of the output voltage V ₀ appearing at the output terminal 48 when the first potential is 0 [V], the second potential is 5 [V], the resistance 47 is 500 kΩ, and the capacitance of the capacitor 49 is 100 pF. It is. In FIG. 4, assuming that charging and discharging are repeated continuously, the charging time (T _c ), that is, the pulse width of the fourth pulse 55 is set to 3 μs, and the discharging time (T _d ), that is, the pulse of the fifth pulse 56 is used. When the width is 2 μs, that is, the ratio between T _c and T _d , and therefore the ratio between the length of the portion of the second detection electrode 23 immersed in the liquid to be measured and the length of the portion outside the liquid to be measured is The time change of the output voltage V _{0 in} the case of 3: 2 is shown. In FIG. 4, it can be seen that an output voltage V ₀ in which a ripple having an amplitude of approximately ± 0.06 [V] is superimposed on a DC component of 3 [V] is obtained after approximately 500 μsec. Since this ripple can be removed by using an appropriate low-pass filter, the liquid level of the liquid to be measured can be output as an analog voltage.

  As is clear from the above description, the detection circuit of the liquid level sensor according to the embodiment of the present invention has three to two analog circuit portions that require an extremely large element area compared to the digital circuit portion. In addition, the liquid level measurement accuracy can be improved by reducing the number of analog circuit units that have variations in switching voltage, rise characteristics, etc. The position sensor can be provided.

  FIG. 5 is a front view of a detection unit of a liquid level sensor according to another embodiment of the present invention. In FIG. 5, reference numeral 61 denotes a detection unit made of a rectangular polyimide film or the like extending vertically. A pair of first detection electrodes 62 made of comb-shaped carbon is provided below the portion 61. A pair of second detection electrodes 63 made of comb-shaped carbon is provided on the first detection electrode 62, and the comb-shaped carbon is also formed on the second detection electrode 63. A pair of third detection electrodes 64 are provided, and a pair of fourth detection electrodes 65 made of comb-shaped carbon is also provided at the lower end of the detection unit 61. The first, second, third, and fourth detection electrodes 62, 63, 64, 65 are connected to terminals 66, 67, 68, 69, 70 by lead wires extending vertically.

  FIG. 6 is a cross-sectional view taken along line AA of the fourth detection electrode 65 in FIG. 5. As shown in FIG. 6, the fourth detection electrode 65 insulates the entire opposing electrode 71 from each other. It is configured to be covered with a metal layer 73 with an object 72 interposed therebetween. With such a configuration, the lines of electric force generated between the opposing electrodes 71 do not pass through the liquid to be measured, and therefore the capacitance between the electrodes 71 measured by the first detection electrode 62 is measured. It is not affected by the liquid level of the liquid or the dielectric constant of the liquid to be measured.

  FIG. 7 shows a detection circuit diagram of a liquid level sensor according to another embodiment of the present invention. In FIG. 7, a terminal 66 of the detection unit 61 is connected to a first potential 74. The terminal 67 of the detection unit 61 is connected to the second potential 77 via the first resistor 75 and the first switch 76. Similarly, the terminal 68 of the detection unit 61 is connected via a second resistor 78 and a second switch 79, and the terminal 69 of the detection unit 61 is connected via a third resistor 80 and a third switch 81. Further, the terminal 70 of the detection unit 61 is connected to the second potential 77 through the fourth resistor 82 and the fourth switch 83, respectively. As a result, the first detection electrode 62, the second detection electrode 63, the third detection electrode 64, and the fourth detection electrode 65 are connected to the first to fourth resistors 75, 78, 80, and 82, respectively. .

  At this time, a time constant determined by the interelectrode capacitance of the first detection electrode 62 and the first resistor 75 in a state where the first, second, and third detection electrodes 62, 63, 64 are all outside the liquid to be measured. And the time constant determined by the interelectrode capacitance of the second detection electrode 63 and the second resistor 78 and the time constant determined by the interelectrode capacitance of the third detection electrode 64 and the third resistor 80 are substantially the same. It is intended to be equal to. The time constant determined by the interelectrode capacitance of the fourth detection electrode 65 and the fourth resistor 82 is the electrode of the first detection electrode 62 in a state where the first detection electrode 62 is immersed in the liquid to be measured. The inter-electrode capacitance of the third detection electrode 64 and the third resistance 80 are smaller than the time constant determined by the inter-electrode capacitance and the first resistance 75, and the third detection electrode 64 is outside the liquid to be measured. Is set to be larger than the time constant determined by.

  The midpoint potential of the first resistor 75 and the first detection electrode 62 and the midpoint potential of the third resistor 80 and the third detection electrode 64 are respectively the fifth switch 84. The first pulse generation means 85 comprising a comparator via the sixth switch 88 is compared with the threshold generation means comprising resistors 86 and 87. Similarly, the midpoint potential of the second resistor 78 and the second detection electrode 63 and the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 are respectively The second pulse generation means 91 comprising a comparator through the seventh switch 89 and the eighth switch 90 is compared with the threshold generation means comprising the resistors 86 and 87.

  The first switch 76 and the fifth switch 84 are based on the signal (SEL_Q) from the switching signal generating circuit 92, and the second switch 79 and the seventh switch 89 are based on the signal (SEL_L) from the switching signal generating circuit 92. Are controlled to open and close. Similarly, the third switch 81 and the sixth switch 88 are switched by the signal (SEL_A) from the switching signal generating circuit 92, and the fourth switch 83 and the eighth switch 90 are switched by the switching signal generating circuit 92. Open / close control is performed by a signal (SEL_R) from In this case, each switch is “closed” when the signal applied to each switch is high, while each switch is “open” when the signal applied to each switch is low.

  93 is a logic circuit composed of logic elements and flip-flops. The logic circuit 93 receives the output signals of the first and second pulse generating means 85 and 91 and the signal from the switching signal generating circuit 92. . Further, a first analog switch 94 and a second analog switch 95 that are controlled to open and close by an output signal of the logic circuit 93 are arranged between the first potential 74 and the second potential 77 at the subsequent stage of the logic circuit 93. Is provided. Reference numeral 96 denotes a resistor having one end connected to the midpoint of the first analog switch 94 and the second analog switch 95 and the other end connected to the output terminal 97. Reference numeral 98 denotes a capacitor having one end connected to the first potential 74 and the other end connected between the resistor 96 and the output terminal 97.

  Next, the circuit operation of the liquid level sensor according to another embodiment of the present invention will be described with reference to FIG.

  8 (a), (b), (c), (d), (e), (f), (g), (h), (i), (j), (k), and (l) show voltage waveforms at various parts of the liquid level sensor. Therefore, the detection unit 61 of the liquid level sensor shown in FIG. 5 is immersed in a liquid to be measured such as engine oil in an oil pan (not shown). At this time, the first detection electrode 62 and the fourth detection electrode 65 are always immersed in the liquid to be measured, the third detection electrode 64 is always disposed outside the liquid to be measured, and the second detection electrode 63 is The part that intersects the liquid surface to be measured and is immersed in the liquid is assumed to increase or decrease as the liquid level rises and falls.

Liquid level sensor according to another embodiment of the present invention, in an initial state before power-on (t _0), the first, second, third, fourth detection electrodes 62, 63, 64, 65 Since there is no electric charge between the electrode pair, the midpoint potential between the first resistor 75 and the first detection electrode 62, the midpoint potential between the second resistor 78 and the second detection electrode 63, and the third resistance The midpoint potential of 80 and the third detection electrode 64 and the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 are all at the first potential 74.

  When the power is turned on, as shown in FIGS. 8A and 8B, the switching signal generation circuit 92 outputs the SEL_A signal and the SEL_L signal, and the second switch 79, the third switch 81, Since the sixth switch 88 and the seventh switch 89 are “closed”, the midpoint potential of the third resistor 80 and the third detection electrode 64 is the first as shown in FIG. The potential rises exponentially from the potential 74 toward the second potential 77 with a time constant determined by the third resistor 80 and the interelectrode capacitance of the third detection electrode 64. Further, the midpoint potential between the second resistor 78 and the second detection electrode 63 is changed from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the resistance 78 and the interelectrode capacitance of the second detection electrode 63.

  At this time, since the second detection electrode 63 intersects the liquid surface to be measured and the third detection electrode 64 is always outside the liquid to be measured, the electrostatic resistance between the second resistor 78 and the second detection electrode 63 is obtained. The time constant determined by the capacitance is larger than the time constant determined by the third resistor 80 and the third detection electrode 64.

When the midpoint potential of the third resistor 80 and the third detection electrode 64 reaches the threshold voltage _Vth determined by the resistors 86 and 87, the output of the first pulse generating means 85 comprising a comparator is shown in FIG. As shown in j), a transition from high to low (t ₁ ) occurs, and a third pulse 99 proportional to the capacitance measured by the third detection electrode 64 is generated. Similarly, when the midpoint potential of the second resistor 78 and the second detection electrode 63 reaches the threshold voltage V _th determined by the resistors 86 and 87, the second pulse generating means 91 comprising a comparator As shown in FIG. 8I, the output transitions from high to low (t ₂ ), and a second pulse 100 proportional to the capacitance measured by the second detection electrode 63 is generated.

When the SEL_A signal falls (t ₃ ), the signal from the logic circuit 93 drives the element 101 having an open collector configuration, whereby charges accumulated in the second detection electrode 63 and the third detection electrode 64 are accumulated. Is discharged to the first potential 74, so that the midpoint potential of the second resistor 78 and the second detection electrode 63 and the midpoint potential of the third resistor 80 and the third detection electrode 64 are: Each returns to the first potential 74 as shown in FIGS.

Then, Figure 8 SEL_Q signal from the switching signal generating circuit 92 as shown in (c) is outputted (t _4), since the first switch 76 and fifth switch 84 is "closed", the first The midpoint potential between the first resistor 75 and the first detection electrode 62 is from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the interelectrode capacitance of the detection electrode 62. Further, the midpoint potential of the second resistor 78 and the second detection electrode 63 is changed from the first potential 74 toward the second potential 77 again as shown in FIG. Increases exponentially with a time constant determined by the resistance 78 and the interelectrode capacitance of the second detection electrode 63.

  At this time, since the first detection electrode 62 is always immersed in the liquid to be measured, the time constant determined by the first resistance 75 and the first detection electrode 62 is the second resistance 78 and the second resistance. The time constant is determined by the interelectrode capacitance of the two detection electrodes 63.

When the midpoint potential of the second resistor 78 and the second detection electrode 63 reaches the threshold voltage V _th determined by the resistors 86 and 87, the output of the second pulse generating means 91 comprising a comparator is shown in FIG. As shown in i), a transition from high to low (t ₅ ) occurs, and a second pulse 102 proportional to the capacitance measured by the second detection electrode 63 is generated. Similarly, when the midpoint potential of the third resistor 80 and the first detection electrode 62 reaches the threshold voltage V _th determined by the resistors 86 and 87, the first pulse generating means 85 comprising a comparator As shown in FIG. 8J, the output transitions from high to low (t ₆ ), and the first pulse 103 proportional to the capacitance measured by the first detection electrode 62 is generated.

When the SEL_Q signal falls (t ₇ ), the signal from the logic circuit 93 drives the element 101 having an open collector configuration, whereby charges accumulated in the second detection electrode 63 and the first detection electrode 62 are accumulated. Is discharged to the first potential 74, the midpoint potential of the second resistor 78 and the second detection electrode 63 and the midpoint potential of the first resistor 75 and the first detection electrode 62 are: Each returns to the first potential 74 as shown in FIGS.

Next, the SEL_A signal and the SEL_R signal are output again from the switching signal generation circuit 92 (t ₈ ), and the third switch 81, the sixth switch 88, the fourth switch 83, and the eighth switch 90 are “closed”. It becomes. Thereby, the midpoint potential of the resistor 80 and the third detection electrode 64 is changed from the first potential 74 toward the second potential 77 as shown in FIG. 8E. And an exponential function with a time constant determined by the interelectrode capacitance of the third detection electrode 64. Further, the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 is changed from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the resistance 82 and the interelectrode capacitance of the fourth detection electrode 65.

  At this time, as described above, the time constant determined by the interelectrode capacitance of the fourth detection electrode 65 and the fourth resistor 82 is the third detection in a state where the third detection electrode 64 is outside the liquid to be measured. It is set to be larger than the time constant determined by the interelectrode capacitance of the electrode 64 and the third resistor 80.

When the midpoint potential of the third resistor 80 and the third detection electrode 64 reaches the threshold voltage _Vth determined by the resistors 86 and 87, the output of the first pulse generating means 85 comprising a comparator is shown in FIG. transitions from high to low as shown in j) (t _9), the third pulse 104 that is proportional to the capacitance measured by the third sensing electrode 64 is generated. Similarly, when the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 reaches the threshold voltage V _th determined by the resistors 86 and 87, the second pulse generating means 91 comprising a comparator The output transitions from high to low (t ₁₀ ) as shown in FIG. 8 (i), and a fourth pulse 105 proportional to the capacitance measured by the fourth detection electrode 65 is generated.

When the SEL_A signal falls (t ₁₁ ), the signal from the logic circuit 93 drives the element 101 having an open collector configuration, whereby charges accumulated in the third detection electrode 64 and the fourth detection electrode 65 are accumulated. Is discharged to the first potential 74, the midpoint potential of the third resistor 80 and the third detection electrode 64 and the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 are: Each returns to the first potential 74 as shown in FIGS.

Then, the switching SEL_Q signal from the signal generating circuit 92 is outputted (t ₁₂₎ as shown in FIG. 8 (c), since the first switch 76 and fifth switch 84 is "closed", the first The midpoint potential between the first resistor 75 and the first detection electrode 62 is from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the interelectrode capacitance of the detection electrode 62. Further, the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 is changed from the first potential 74 toward the second potential 77 again as shown in FIG. Increases exponentially with a time constant determined by the resistance 82 and the interelectrode capacitance of the fourth detection electrode 65.

  At this time, as described above, the time constant determined by the interelectrode capacitance of the fourth detection electrode 65 and the fourth resistor 82 is the first time when the first detection electrode 62 is immersed in the liquid to be measured. The time constant determined by the interelectrode capacitance of the first detection electrode 62 and the first resistor 75 is set to be smaller.

When the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 reaches the threshold voltage _Vth determined by the resistors 86 and 87, the output of the second pulse generating means 91 comprising a comparator is shown in FIG. As shown in i), a transition from high to low (t ₁₃ ) occurs, and a fourth pulse 106 proportional to the capacitance measured by the fourth detection electrode 65 is generated. Similarly, when the midpoint potential of the first resistor 75 and the first detection electrode 62 reaches the threshold voltage V _th determined by the resistors 86 and 87, the first pulse generating means 85 comprising a comparator The output transitions from high to low as shown in FIG. 8J (t ₁₄ ), and a first pulse 107 proportional to the capacitance measured by the first detection electrode 62 is generated.

When the SEL_Q signal falls (t ₁₅ ), the signal from the logic circuit 93 drives the element 101 having an open collector configuration, whereby charges accumulated in the first detection electrode 62 and the fourth detection electrode 65 are accumulated. Is discharged to the first potential 74, the midpoint potential of the first resistor 75 and the first detection electrode 62 and the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 are: Each returns to the first potential 74 as shown in FIGS.

Next, the SEL_A signal and the SEL_L signal are output again from the switching signal generation circuit 92 (t ₁₆ ), and the second switch 79, the third switch 81, the sixth switch 88, and the seventh switch 89 are “closed”. Therefore, the midpoint potential of the third resistor 80 and the third detection electrode 64 is changed from the first potential 74 toward the second potential 77 as shown in FIG. 3 rises exponentially with a time constant determined by the resistance of the third resistor 80 and the interelectrode capacitance of the third detection electrode 64. The midpoint potential of the second resistor 78 and the second detection electrode 63 is the same as that of the resistor 78 from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the interelectrode capacitance of the second detection electrode 63. Thereafter, the same operation as in the section from t ₀ to t ₁₅ is repeated.

  In this way, the first pulse generator 85 outputs a pulse train as shown in FIG. 8 (i), and the second pulse generator 91 outputs a pulse train as shown in FIG. 8 (j). Will be.

  The output signals of the first and second pulse generating means 85 and 91 are inputted to a logic circuit 93 made up of logic elements and flip-flops, and the first analog switch 94 has a first signal as shown in FIG. From the fall of the third pulse 99 proportional to the capacitance measured by the third detection electrode 64 to the fall of the second pulse 100 proportional to the capacitance measured by the second detection electrode 63 A fifth pulse 108 having a pulse width of is output. Further, as shown in FIG. 3L, the second analog switch 95 detects the first detection from the falling edge of the second pulse 102 proportional to the capacitance measured by the second detection electrode 63. A sixth pulse 109 having a pulse width up to the fall of the first pulse 103 proportional to the capacitance measured by the electrode 62 is output.

Here, the first analog switch 94 is “closed” when the signal input to the first analog switch 94 is high, and “open” when the signal is low. The second analog switch 95 is “closed” when the signal input to the second analog switch 95 is high, and “open” when the signal is low. As a result, the first analog switch 94 is “closed” and the second analog switch 95 is “open” during the period of time t _{1 to} t ₂ of FIG. The capacitor 98 is charged from the second potential 77 through the fifth resistor 96.

Then, during the time t _{5 to} t ₆ in FIG. 8, that is, during the period when the sixth pulse 109 is high, the first analog switch 94 is “open” and the second analog switch 95 is “closed”. The electric charge accumulated in 98 is discharged to the first potential 74 through the fifth resistor 96.

  The logic circuit 93 inputs a high signal to the first analog switch 94 when the third pulse 104 is at the high level when the fourth pulse 105 falls, and the fourth pulse 106 When the first pulse 107 is at a low level at the time of falling, it has a function of inputting a high signal to the second analog switch 95.

  However, when the first detection electrode 62 and the fourth detection electrode 65 are always immersed in the liquid to be measured, and the third detection electrode 64 is always disposed outside the liquid to be measured, FIG. ) As shown in (j), the third pulse 104 is at the low level when the fourth pulse 105 falls, and the first pulse 107 is at the high level when the fourth pulse 106 falls. Therefore, a high signal is not input to the first analog switch 94 and the second analog switch 95.

Therefore, in the time t _{1 to} t ₂ in FIG. 8, that is, the period in which the fifth pulse 108 is high, and in the time t _{5 to} t _{6 in} FIG. 8, that is, in the time other than the period in which the sixth pulse 109 is high, Since both the first analog switch 94 and the second analog switch 95 are “open”, the electric charge accumulated in the capacitor 98 is stored. In this manner, the first analog switch 94 and the second analog switch 95 are set for a time determined by the length of the portion where the second detection electrode 63 is immersed in the liquid to be measured and the length of the portion outside the measurement target. Are alternately opened and closed to charge and discharge the capacitor 98, whereby the liquid level of the liquid to be measured can be output as an analog voltage to the output terminal 97.

  Next, in the liquid level sensor according to another embodiment of the present invention, the operation when the liquid level of the measurement liquid rises and the third detection electrode 64 is immersed in the liquid to be measured will be described.

  9 (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) are voltages of each part of the liquid level sensor in the above case. The waveform is shown.

In the liquid level sensor according to another embodiment of the present invention, the first, second, third, and fourth detection electrodes 62, 63, 64, 65 are in the initial state (t ₀ ) before the power is turned on. Since there is no electric charge between the electrode pair, the midpoint potential between the first resistor 75 and the first detection electrode 62, the midpoint potential between the second resistor 78 and the second detection electrode 63, and the third resistance The midpoint potential of 80 and the third detection electrode 64 and the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 are all at the first potential 74.

  When the power is turned on, as shown in FIGS. 9A and 9B, the switching signal generation circuit 92 outputs the SEL_A signal and the SEL_L signal, and the second switch 79, the third switch 81, Since the sixth switch 88 and the seventh switch 89 are “closed”, the midpoint potential of the third resistor 80 and the third detection electrode 64 is the first as shown in FIG. The potential rises exponentially from the potential 74 toward the second potential 77 with a time constant determined by the third resistor 80 and the interelectrode capacitance of the third detection electrode 64. Further, the midpoint potential of the second resistor 78 and the second detection electrode 63 is changed from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the resistance 78 and the interelectrode capacitance of the second detection electrode 63.

  At this time, since the first detection electrode 62, the second detection electrode 63, and the third detection electrode 64 are all immersed in the liquid to be measured, the static resistance between the first resistor 75 and the first detection electrode 62 is obtained. A time constant determined by the capacitance, a time constant determined by the capacitance of the second resistor 78 and the second detection electrode 63, and a time determined by the third resistor 80 and the third detection electrode 64. It is equal to a constant.

When the midpoint potential of the third resistor 80 and the third detection electrode 64 reaches the threshold voltage _Vth determined by the resistors 86 and 87, the output of the first pulse generating means 85 comprising a comparator is shown in FIG. j) transition from high to low (t ₁ ), and a third pulse 110 proportional to the capacitance measured by the third detection electrode 64 is generated. Similarly, when the midpoint potential of the second resistor 78 and the second detection electrode 63 reaches the threshold voltage V _th determined by the resistors 86 and 87, the second pulse generating means 91 comprising a comparator The output changes from high to low as shown in FIG. 9 (i), and the second pulse 111 proportional to the capacitance measured by the second detection electrode 63 is generated.

When the SEL_A signal falls (t ₂ ), the signal from the logic circuit 93 drives the element 101 having an open collector configuration, whereby charges accumulated in the second detection electrode 63 and the third detection electrode 64 are accumulated. Is discharged to the first potential 74, so that the midpoint potential of the second resistor 78 and the second detection electrode 63 and the midpoint potential of the third resistor 80 and the third detection electrode 64 are: Each returns to the first potential 74 as shown in FIGS.

Next, as shown in FIG. 9C, the SEL_Q signal is output from the switching signal generation circuit 92 (t ₃ ), and the first switch 76 and the fifth switch 84 are “closed”. The midpoint potential between the first resistor 75 and the first detection electrode 62 is from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the interelectrode capacitance of the detection electrode 62. Further, the midpoint potential of the second resistor 78 and the second detection electrode 63 is changed from the first potential 74 toward the second potential 77 again as shown in FIG. Increases exponentially with a time constant determined by the resistance 78 and the interelectrode capacitance of the second detection electrode 63.

  At this time, as described above, since the first detection electrode 62, the second detection electrode 63, and the third detection electrode 64 are all immersed in the liquid to be measured, the first resistor 75 and the first detection electrode are used. A time constant determined by the capacitance of the electrode 62, a time constant determined by the capacitance of the second resistor 78 and the second detection electrode 63, the third resistor 80 and the third detection electrode 64. Is equal to the time constant determined by.

When the midpoint potential of the second resistor 78 and the second detection electrode 63 reaches the threshold voltage _Vth determined by the resistors 86 and 87, the output of the second pulse generating means 91 comprising a comparator is shown in FIG. As shown in i), a transition from high to low (t ₄ ) occurs, and a second pulse 112 proportional to the capacitance measured by the second detection electrode 63 is generated. Similarly, when the midpoint potential of the first resistor 75 and the first detection electrode 62 reaches the threshold voltage V _th determined by the resistors 86 and 87, the first pulse generating means 85 comprising a comparator The output transitions from high to low as shown in FIG. 9 (j), and a first pulse 113 proportional to the capacitance measured by the first detection electrode 62 is generated.

When the SEL_Q signal falls (t ₅ ), the signal from the logic circuit 93 drives the element 101 having an open collector configuration, whereby charges accumulated in the second detection electrode 63 and the first detection electrode 62 are accumulated. Is discharged to the first potential 74, the midpoint potential of the second resistor 78 and the second detection electrode 63 and the midpoint potential of the first resistor 75 and the first detection electrode 62 are: Each returns to the first potential 74 as shown in FIGS.

Then, FIG. 9 (a) and again SEL_A signal and SEL_R signal from the switching signal generating circuit 92 as shown in (d) is outputted (t _6), the third switch 81, the sixth switch 88, the fourth The switch 83 and the eighth switch 90 are “closed”. As a result, the midpoint potential of the third resistor 80 and the third detection electrode 64 is changed from the first potential 74 toward the second potential 77 as shown in FIG. Increases exponentially with a time constant determined by the resistance 80 and the interelectrode capacitance of the third detection electrode 64. Further, the midpoint potential between the fourth resistor 82 and the fourth detection electrode 65 is changed from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the resistance 82 and the interelectrode capacitance of the fourth detection electrode 65.

  At this time, since the third detection electrode 64 is immersed in the liquid to be measured as described above, the time constant determined by the third resistor 80 and the interelectrode capacitance of the third detection electrode 64 is the fourth constant. And a time constant determined by the inter-electrode capacitance of the fourth detection electrode 65.

When the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 reaches the threshold voltage V _th determined by the resistors 86 and 87, the output of the second pulse generating means 91 comprising a comparator is shown in FIG. transitions from high to low as shown in i) (t _7), the fourth pulse 114 which is proportional to the capacitance measured by the fourth detection electrode 65 is generated. Similarly, when the midpoint potential of the third resistor 80 and the third detection electrode 64 reaches the threshold voltage V _th determined by the resistors 86 and 87, the first pulse generating means 85 comprising a comparator As shown in FIG. 9J, the output transitions from high to low (t ₈ ), and a third pulse 115 proportional to the capacitance measured by the third detection electrode 64 is generated.

When the SEL_A signal falls (t ₉ ), the signal from the logic circuit 93 drives the element 101 having an open collector configuration, whereby charges accumulated in the third detection electrode 64 and the fourth detection electrode 65 are accumulated. Is discharged to the first potential 74, the midpoint potential of the third resistor 80 and the third detection electrode 64 and the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 are: Each returns to the first potential 74 as shown in FIGS.

Next, as shown in FIG. 9C, the SEL_Q signal is output from the switching signal generation circuit 92 (t ₁₀ ), and the first switch 76 and the fifth switch 84 are “closed”. The midpoint potential between the first resistor 75 and the first detection electrode 62 is from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the interelectrode capacitance of the detection electrode 62. Further, the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 is changed from the first potential 74 toward the second potential 77 again as shown in FIG. And rises exponentially with a time constant determined by the interelectrode capacitance of the fourth detection electrode 65.

  At this time, since the first detection electrode 62 is immersed in the liquid to be measured as described above, the time constant determined by the first resistance 75 and the interelectrode capacitance of the first detection electrode 62 is the fourth constant. And a time constant determined by the inter-electrode capacitance of the fourth detection electrode 65.

When the midpoint potential between the fourth resistor 82 and fourth detection electrode 65 reaches the threshold voltage V _th which is determined by the resistor 86 and 87, the output of the second pulse generating means 91 comprising a comparator 9 ( As shown in i), a transition from high to low (t ₁₁ ) occurs, and a fourth pulse 116 proportional to the capacitance measured by the fourth detection electrode 65 is generated. Similarly, when the midpoint potential of the first resistor 75 and the first detection electrode 62 reaches the threshold voltage V _th determined by the resistors 86 and 87, the first pulse generating means 85 comprising a comparator As shown in FIG. 9J, the output transitions from high to low (t ₁₂ ), and a first pulse 117 proportional to the capacitance measured by the first detection electrode 62 is generated.

When the SEL_Q signal falls (t ₁₃ ), the signal from the logic circuit 93 drives the element 101 having an open collector configuration, whereby charges accumulated in the first detection electrode 62 and the fourth detection electrode 65 are accumulated. Is discharged to the first potential 74, the midpoint potential of the first resistor 75 and the first detection electrode 62 and the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 are: Each returns to the first potential 74 as shown in FIGS.

Then, a re SEL_A signal and SEL_L signal from the switching signal generating circuit 92 is outputted (t _14), the second switch 79, third switch 81, the switch 88 of the sixth switch 89 of the seventh "closed" Therefore, the midpoint potential of the third resistor 80 and the third detection electrode 64 is changed from the first potential 74 toward the second potential 77 as shown in FIG. 3 rises exponentially with a time constant determined by the resistance of the third resistor 80 and the interelectrode capacitance of the third detection electrode 64. Further, the midpoint potential of the second resistor 78 and the second detection electrode 63 is changed from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the resistance 78 and the interelectrode capacitance of the second detection electrode 63. Thereafter, the same operation as in the section from t ₀ to t ₁₃ is repeated.

  In this way, the first pulse generating means 85 outputs a pulse train as shown in FIG. 9 (i), and the second pulse generating means 91 outputs a pulse train as shown in FIG. 9 (j). Will be.

The output signals of the first and second pulse generating means 85 and 91 are input to a logic circuit 93 composed of logic elements and flip-flops, but the liquid level of the measurement liquid rises and the third detection electrode 64 becomes the liquid to be measured. When immersed in the second detection electrode 63, the second pulse 111 falls in proportion to the capacitance measured by the second detection electrode 63 and the third capacitance proportional to the capacitance measured by the third detection electrode 64. The falling edge of the second pulse 112 coincides with the falling edge of the pulse 110 and is proportional to the capacitance measured at the second detecting electrode 63 and the capacitance measured at the first detecting electrode 62. since the fall of the first pulse 113 proportional also matches, FIG. 9 (k) (l) in the interval t ₀ ~t ₆ as shown output to the first analog switch 94 and the second analog switch 95 Signal is always low, 1, the second of the analog switch 94 and 95 is always "open". Further, as shown in FIGS. 9 (i) and 9 (j), since the first pulse 117 is at the high level when the fourth pulse 116 falls, a high signal is input to the second analog switch 95. Instead, as shown in FIG. 9L, the second analog switch 95 is always “open” during the period after time t ₆ . Then, if the if the input signal is always low time t ₆ to the first analog switch 94 in the subsequent period, the output terminal 97 to become an electrical floating state, no output voltage normally determined is obtained It will be.

However, the third pulse 115 at the fall time of the fourth pulse 114 due to the high level, as shown in FIG. 9 (k), the time t ₇ after periods of high to the first analog switch 94 A signal is input.

Accordingly, in the period after time t ₇ in FIG. 9 (k), the first analog switch 94 is “closed” and the second analog switch 95 is “open”. The capacitor 98 is charged through the resistor 96, and the potential of the output terminal 97 becomes equal to the second potential 77.

  In this way, the determined output voltage can be output even when the liquid level of the measurement liquid rises and the third detection electrode 64 is immersed in the liquid to be measured.

  Next, in the liquid level sensor according to another embodiment of the present invention, the operation when the liquid level of the measurement liquid is lowered and the first detection electrode 62 is exposed to the outside of the liquid to be measured will be described.

  10 (a), (b), (c), (d), (e), (f), (g), (h), (i), (j), (k), and (l) are the voltages of each part of the liquid level sensor in the above case. The waveform is shown.

In the liquid level sensor according to another embodiment of the present invention, the first, second, third, and fourth detection electrodes 62, 63, 64, 65 are in the initial state (t ₀ ) before the power is turned on. Since there is no electric charge between the electrode pair, the midpoint potential between the first resistor 75 and the first detection electrode 62, the midpoint potential between the second resistor 78 and the second detection electrode 63, and the third resistance The midpoint potential of 80 and the third detection electrode 64 and the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 are all at the first potential 74.

  When the power is turned on, as shown in FIGS. 10A and 10B, the switching signal generation circuit 92 outputs the SEL_A signal and the SEL_L signal, and the second switch 79, the third switch 81, Since the sixth switch 88 and the seventh switch 89 are “closed”, the midpoint potential of the third resistor 80 and the third detection electrode 64 is the first potential as shown in FIG. From 74 to the second potential 77, it rises exponentially with a time constant determined by the third resistor 80 and the interelectrode capacitance of the third detection electrode 64. The midpoint potential of the resistor 78 and the second detection electrode 63 is the same as that of the second resistor 78 from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the interelectrode capacitance of the second detection electrode 63.

  At this time, since the first detection electrode 62, the second detection electrode 63, and the third detection electrode 64 are all exposed to the liquid to be measured, the first resistor 75 and the first detection electrode 62 It is determined by a time constant determined by the capacitance, a time constant determined by the capacitance of the second resistor 78 and the second detection electrode 63, and the third resistor 80 and the third detection electrode 64. It is equal to the time constant.

When the midpoint potential of the third resistor 80 and the third detection electrode 64 reaches the threshold voltage _Vth determined by the resistors 86 and 87, the output of the first pulse generating means 85 comprising a comparator is as shown in FIG. j) transition from high to low (t ₁ ) and a third pulse 120 proportional to the capacitance measured at the third sensing electrode 64 is generated. Similarly, when the midpoint potential of the second resistor 78 and the second detection electrode 63 reaches the threshold voltage V _th determined by the resistors 86 and 87, the second pulse generating means 91 comprising a comparator The output changes from high to low as shown in FIG. 10 (i), and a second pulse 121 proportional to the capacitance measured by the second detection electrode 63 is generated.

When the SEL_A signal falls (t ₂ ), the signal from the logic circuit 93 drives the element 101 having an open collector configuration, whereby charges accumulated in the second detection electrode 63 and the third detection electrode 64 are accumulated. Is discharged to the first potential 74, so that the midpoint potential of the second resistor 78 and the second detection electrode 63 and the midpoint potential of the third resistor 80 and the third detection electrode 64 are: Each returns to the first potential 74 as shown in FIGS.

Next, as shown in FIG. 10C, the SEL_Q signal is output from the switching signal generation circuit 92 (t ₃ ), and the first switch 76 and the fifth switch 84 are “closed”. The midpoint potential between the resistor 75 and the first detection electrode 62 is from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by 62 interelectrode capacitance. Further, the midpoint potential of the second resistor 78 and the second detection electrode 63 is changed from the first potential 74 toward the second potential 77 again as shown in FIG. Increases exponentially with a time constant determined by the resistance 78 and the interelectrode capacitance of the second detection electrode 63.

  At this time, as described above, since the first detection electrode 62, the second detection electrode 63, and the third detection electrode 64 are all exposed outside the liquid to be measured, the first resistor 75 and the first detection electrode A time constant determined by the capacitance of the detection electrode 62, a time constant determined by the capacitance of the second resistor 78 and the second detection electrode 63, the third resistor 80, and the third detection electrode The time constant determined by 64 is equal.

When the midpoint potential of the second resistor 78 and the second detection electrode 63 reaches the threshold voltage _Vth determined by the resistors 86 and 87, the output of the second pulse generating means 91 comprising a comparator is shown in FIG. As shown in i), a transition from high to low (t ₄ ) occurs, and a second pulse 122 proportional to the capacitance measured by the second detection electrode 63 is generated. Similarly, when the midpoint potential of the first resistor 75 and the first detection electrode 62 reaches the threshold voltage V _th determined by the resistors 86 and 87, the first pulse generating means 85 comprising a comparator As shown in FIG. 10J, the output transitions from high to low, and a first pulse 123 proportional to the capacitance measured by the first detection electrode 62 is generated.

When the SEL_Q signal falls (t ₅ ), the signal from the logic circuit 93 drives the element 101 having an open collector configuration, whereby charges accumulated in the second detection electrode 63 and the first detection electrode 62 are accumulated. Is discharged to the first potential 74, the midpoint potential of the second resistor 78 and the second detection electrode 63 and the midpoint potential of the first resistor 75 and the first detection electrode 62 are: Each returns to the first potential 74 as shown in FIGS.

Next, as shown in FIGS. 10A and 10D, the SEL_A signal and the SEL_R signal are output again from the switching signal generation circuit 92 (t ₆ ), and the third switch 81, the sixth switch 88, the fourth switch, The switch 83 and the eighth switch 90 are “closed”. Thereby, the midpoint potential of the third resistor 80 and the third detection electrode 64 is changed from the first potential 74 toward the second potential 77 as shown in FIG. Increases exponentially with a time constant determined by the resistance 80 and the interelectrode capacitance of the third detection electrode 64. Further, the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 is changed from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the resistance 82 and the interelectrode capacitance of the fourth detection electrode 65.

  At this time, since the third detection electrode 64 is exposed to the outside of the liquid to be measured as described above, the time constant determined by the third resistor 80 and the interelectrode capacitance of the third detection electrode 64 is the first constant. 4 and the time constant determined by the interelectrode capacitance of the fourth detection electrode 65.

When the midpoint potential of the third resistor 80 and the third detection electrode 64 reaches the threshold voltage _Vth determined by the resistors 86 and 87, the output of the first pulse generating means 85 comprising a comparator is as shown in FIG. j) transition from high to low (t ₇ ), and a third pulse 124 proportional to the capacitance measured at the third detection electrode 64 is generated. Similarly, when the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 reaches the threshold voltage V _th determined by the resistors 86 and 87, the second pulse generating means 91 comprising a comparator As shown in FIG. 10I, the output transitions from high to low (t ₈ ), and a fourth pulse 125 proportional to the capacitance measured by the fourth detection electrode 65 is generated.

When the SEL_A signal falls (t ₉ ), the signal from the logic circuit 93 drives the element 101 having an open collector configuration, whereby charges accumulated in the third detection electrode 64 and the fourth detection electrode 65 are accumulated. Is discharged to the first potential 74, the midpoint potential of the third resistor 80 and the third detection electrode 64 and the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 are: Each returns to the first potential 74 as shown in FIGS.

Next, as shown in FIG. 10C, the SEL_Q signal is output from the switching signal generation circuit 92 (t ₁₀ ), and the first switch 76 and the fifth switch 84 are “closed”. The midpoint potential between the first resistor 75 and the first detection electrode 62 is from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the interelectrode capacitance of the detection electrode 62. Further, the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 is changed from the first potential 74 toward the second potential 77 again as shown in FIG. Increases exponentially with a time constant determined by the resistance 82 and the interelectrode capacitance of the fourth detection electrode 65.

  At this time, as described above, since the first detection electrode 62 is exposed to the outside of the liquid to be measured, the time constant determined by the first resistor 75 and the interelectrode capacitance of the first detection electrode 62 is the first constant. 4 and the time constant determined by the interelectrode capacitance of the fourth detection electrode 65.

When the midpoint potential of the first resistor 75 and the first detection electrode 62 reaches the threshold voltage _Vth determined by the resistors 86 and 87, the output of the first pulse generating means 85 comprising a comparator is as shown in FIG. j) transition from high to low (t ₁₁ ), and a first pulse 126 proportional to the capacitance measured by the first detection electrode 62 is generated. Similarly, when the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 reaches the threshold voltage V _th determined by the resistors 86 and 87, the second pulse generating means 91 comprising a comparator As shown in FIG. 10I, the output transitions from high to low (t ₁₂ ), and a fourth pulse 127 proportional to the capacitance measured by the fourth detection electrode 65 is generated.

When SEL_Q signal falls (t _13), by a signal from the logic circuit 93 drives the element 101 is an open collector configuration, the first detection electrode 62 fourth charges accumulated in the detection electrodes 65 Is discharged to the first potential 74, the midpoint potential of the first resistor 75 and the first detection electrode 62 and the midpoint potential of the fourth resistor 82 and the fourth detection electrode 65 are: Each returns to the first potential 74 as shown in FIGS.

Next, the SEL_A signal and the SEL_L signal are output again from the switching signal generation circuit 92 (t ₁₄ ), and the second switch 79, the third switch 81, the sixth switch 88, and the seventh switch 89 are “closed”. Therefore, the midpoint potential of the third resistor 80 and the third detection electrode 64 is changed from the first potential 74 toward the second potential 77 as shown in FIG. 3 rises exponentially with a time constant determined by the resistance of the third resistor 80 and the interelectrode capacitance of the third detection electrode 64. Further, the midpoint potential of the second resistor 78 and the second detection electrode 63 is changed from the first potential 74 toward the second potential 77 as shown in FIG. It rises exponentially with a time constant determined by the resistance 78 and the interelectrode capacitance of the second detection electrode 63. Thereafter, the same operation as in the section from t ₀ to t ₁₃ is repeated.

  In this way, the first pulse generation means 85 outputs a pulse train as shown in FIG. 10 (j), and the second pulse generation means 91 outputs a pulse train as shown in FIG. 10 (i). Will be.

The output signals of the first and second pulse generating means 85 and 91 are input to a logic circuit 93 made up of logic elements and flip-flops. When exposed to the outside, the fall of the second pulse 121 proportional to the capacitance measured by the second detection electrode 63 and the third capacitance proportional to the capacitance measured by the third detection electrode 64. The falling edge of the second pulse 122 coincides with the falling edge of the pulse 120 and is proportional to the capacitance measured at the second detecting electrode 63 and the capacitance measured at the first detecting electrode 62. Since the falling edge of the proportional first pulse 123 coincides with the first analog switch 94 and the second analog switch 95 during the period from t ₀ to t ₆ as shown in FIGS. The output signal is always low. , The first and second analog switches 94 and 95 is always "open". Further, as shown in FIGS. 10 (i) and 10 (j), since the third pulse 124 is at the low level at the time of falling of the fourth pulse 125, a high signal is input to the first analog switch 94. Sarezu, as shown in FIG. 9 (k), the first analog switch 94 is also a period of time t ₆ after is always "open". If the input signal to the first analog switch 94 is always low during the period after the time t ₆ , the output terminal 97 is in an electric floating state, so that a normal and determined output voltage cannot be obtained. It will be.

However, the first pulse 126 at the falling time of the fourth pulse 127 is due to the low level, as shown in FIG. 9 (l), the time t ₁₂ and subsequent periods of high to the second analog switch 94 A signal is input.

Thus, the time t ₁₂ after the period of FIG. 10 (l), since the first analog switch 94 is "open" in the second analog switch 95 is "closed", the potential of the output terminal 97 is first Is equal to the potential 74.

  In this way, the determined output voltage can be output even when the liquid level of the measurement liquid is lowered and the first detection electrode 62 is exposed to the outside of the liquid to be measured.

  As is apparent from the above description, the detection circuit of the liquid level sensor according to another embodiment of the present invention is composed of two analog circuit portions, which conventionally required four, so that the detection circuit has a low level. It is possible to realize cost reduction and improve the liquid level measurement accuracy, thereby providing an inexpensive and highly sensitive liquid level sensor, and the liquid level of the liquid to be measured is set to each detection electrode. Even when crossing at any of the positions, it is possible to always output a fixed output voltage.

  In one embodiment and other embodiments of the present invention, in FIG. 9, the third detection electrode 64 is immersed in the liquid to be measured, and in FIG. 10, the first detection electrode is outside the liquid to be measured. The time constant determined by the fourth resistor 82 and the fourth detection electrode 65 is equal to the first resistance when the first detection electrode 62 is immersed in the liquid to be measured. The time constant determined by 75 and the first detection electrode 62 and the time constant determined by the third resistor 80 and the third detection electrode 64 when the third detection electrode 64 is outside the liquid to be measured. By setting the intermediate value, the output terminal is set to the first potential 74 when the liquid surface to be measured crosses a position approximately equal to or lower than that of the first detection electrode 62, and approximately equal to or higher than that of the third detection electrode. So that the output terminal reaches the second potential 77 when the liquid surface to be measured crosses the position of It is also as it can be.

  The liquid level sensor according to the present invention can reduce the chip area and reduce the cost of the detection circuit by consolidating the analog circuit section, and can improve the liquid level measurement accuracy. Therefore, it is possible to provide an inexpensive and highly accurate liquid level sensor, which is particularly useful as a liquid level sensor for detecting the level of engine oil or fuel in automobiles, construction machinery, etc. is there.

The front view of the detection part of the liquid level sensor in one embodiment of this invention Detection circuit diagram of the same level sensor (A)-(i) Waveform diagram for explaining circuit operation of the same liquid level sensor The time change of the output voltage V ₀ when the ratio of the length of the portion of the second detection electrode immersed in the liquid to be measured and the length of the portion outside the liquid to be measured is 3: 2 in the same level sensor. Characteristic diagram The front view of the detection part of the liquid level sensor in other embodiment of this invention Sectional drawing of the 4th detection electrode in the same liquid level sensor Detection circuit diagram of the same level sensor (A)-(l) Waveform diagram for explaining the circuit operation of the liquid level sensor (A)-(l) Waveform diagram for demonstrating the circuit operation | movement of the same level sensor when the 3rd detection electrode is immersed in to-be-measured liquid. (A)-(l) Waveform diagram for explaining circuit operation of the liquid level sensor when the first detection electrode is exposed from the liquid to be measured Front view of conventional liquid level sensor detector Detection circuit diagram of the same level sensor Front view of the detection part of another conventional liquid level sensor Detection circuit diagram of the same level sensor (A)-(h) Waveform diagram for explaining circuit operation of the liquid level sensor

21 Detection Unit 22 First Detection Electrode 23 Second Detection Electrode 24 Third Detection Electrode 25 Lead Wire 38 First Pulse Generation Unit 41 Second Pulse Generation Unit 51 Third Pulse 52, 53 Second Pulse 54 1st pulse 62 1st detection electrode 63 2nd detection electrode 64 3rd detection electrode 65 4th detection electrode 85 1st pulse generation means 91 2nd pulse generation means 99,104 3rd pulse 100, 102 Second pulse 103, 109 First pulse 105, 106 Fourth pulse

Claims (2)

A first detection electrode that is always in the liquid to be measured; a second detection electrode that measures the liquid level of the liquid to be measured; a detection unit that has a third detection electrode that is always outside the liquid to be measured; The first pulse generating means and the second pulse generating means for generating a pulse having a pulse width proportional to the capacitance measured in step 1 and any one of the first pulse generating means and the second pulse generating means The third pulse generated by the other pulse generating means from the fall of the third pulse having a pulse width proportional to the capacitance measured by the third detection electrode generated by one of the pulse generating means. The second detection electrode is charged by the time until the fall of the second pulse having a pulse width proportional to the capacitance measured by the two detection electrodes, and is generated by the one pulse generation means. Pal proportional to the measured capacitance From the fall of the second pulse having a width to the fall of the first pulse having a pulse width proportional to the capacitance measured by the first detection electrode generated by the other pulse generating means. A liquid level sensor which measures the liquid level of the liquid to be measured by repeating the operation of discharging only for a predetermined time. The first detection electrode, the second detection electrode, and the third detection electrode are provided in order from the lower side to the upper side, and are provided separately from the first, second, and third detection electrodes. And a detection unit provided with a fourth detection electrode for measuring the capacitance between the electrodes without being affected by the dielectric constant of the liquid to be measured, and the capacitance measured by each of the detection electrodes First pulse generation means and second pulse generation means for generating a pulse having a pulse width are provided, and generated by one of the first pulse generation means and the second pulse generation means. The electrostatic capacitance measured by the second detection electrode generated by the other pulse generation means from the falling edge of the third pulse having a pulse width proportional to the electrostatic capacitance measured by the third detection electrode. Second with pulse width proportional to capacitance From the trailing edge of the second pulse, which is charged for the time until the trailing edge of the pulse and has a pulse width proportional to the capacitance measured by the second detection electrode generated by the one pulse generating means. , Discharging for the time until the fall of the first pulse having a pulse width proportional to the capacitance measured by the first detection electrode generated by the other pulse generating means, and further, the one pulse The third pulse generated by the other pulse generating means at the falling edge of the fourth pulse having a pulse width proportional to the capacitance measured by the fourth detecting electrode generated by the generating means. When the third pulse having a pulse width proportional to the capacitance measured by the detection electrode is at a high level, the discharge is stopped and only the charging is performed, and the one generated by the one pulse generating means is used. First The capacitance measured by the first detection electrode generated by the other pulse generating means at the time of falling of the fourth pulse having a pulse width proportional to the capacitance measured by the detection electrode A liquid in which the liquid level of the liquid to be measured is measured by repeating the operation of stopping the charge and performing only the discharge when the first pulse having a pulse width proportional to is at the low level. Position sensor.

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