JP5158067B2 - Liquid concentration measurement device - Google Patents

Description

  The present invention relates to a liquid concentration measuring device for measuring alcohol concentration and the like.

  Low-pollution alcohol-mixed gasoline has attracted attention as a fuel for internal combustion engines. In order to control the alcohol-mixed gasoline so as to have an optimum air-fuel ratio, it is important to measure the alcohol content in the mixed gasoline, that is, the alcohol concentration.

  In order to accurately measure such alcohol concentration, it is desirable to use a physical constant having a relatively high change ratio. Therefore, conventionally, a method for detecting a change in relative permittivity has been disclosed. For example, since the relative permittivity is obtained from a change in capacitance, a liquid concentration meter that measures capacitance by arranging a pair of electrodes facing each other has been proposed (for example, see Patent Document 1). The liquid concentration meter disclosed herein repeatedly charges and discharges the concentration sensor via a changeover switch that is switched at a constant period by a control circuit, and obtains an output voltage proportional to the concentration of the fluid to be measured.

JP-A-6-3313

  However, when the capacitance is obtained by arranging a pair of electrodes facing each other, if there are many impurities, that is, if gasoline is inferior, resistance between the electrodes (hereinafter referred to as “leak resistance”) becomes relatively small. There is a problem. In other words, gasoline that does not contain impurities is in an insulated state (leak resistance is infinite) and the conductivity between the electrodes is almost “0”. However, when the impurities increase, the conductivity becomes relatively large. turn into.

  Therefore, in order to accurately measure the alcohol concentration, a measuring device that eliminates the influence of leakage resistance is required. In this regard, the liquid concentration meter described in Patent Document 1 is affected by leakage resistance, and it may be difficult to accurately measure the alcohol concentration.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a liquid concentration measuring apparatus capable of eliminating the influence of leakage resistance generated between detection electrodes when measuring the liquid concentration. Is to provide.

  According to the first aspect of the present invention, charging and discharging of the detection electrode are switched by the switch means. The switch means is switched at a predetermined cycle by the operation signal output means. A voltage for charging the detection electrode is generated in accordance with the operation of the switch means. Thereby, a detection electrode repeats charging / discharging with a predetermined period. At this time, the measurement value output means outputs a voltage corresponding to the capacitance of the detection electrode as a measurement value.

  In particular, in the present invention, an operation signal having a first duty ratio for switching the switch means at the first duty ratio and an operation signal having a second duty ratio for switching the switch means at the second duty ratio are provided. Output it. When the measurement value output means outputs the first measurement value based on the first duty ratio and the second measurement value based on the second duty ratio, the calculation means multiplies the first measurement value by the second duty ratio. The difference from the second measurement value multiplied by the first duty ratio is calculated.

  When the leak resistance is Rp, a constant term including (1 / Rp) appears in the calculation formula of the voltage as the measurement value. Therefore, when the leak resistance Rp is reduced, the influence becomes relatively large, and the measured value varies.

  On the other hand, in the present invention, the first measurement value based on the first duty ratio and the second measurement value based on the second duty ratio are output. Assuming that the first duty ratio is d1 and the second duty ratio is d2, the first measurement value includes the term d1 (1 / Rp), and the second measurement value includes the term d2 (1 / Rp). included. Therefore, in the present invention, a difference between a value obtained by multiplying the first measured value by the second duty ratio (d2) and a value obtained by multiplying the second measured value by the first duty ratio (d1) is taken. Thereby, the constant term including (1 / Rp) can be erased, and the influence of the leakage resistance can be eliminated.

  According to a second aspect of the present invention, the reference voltage generating means generates a reference voltage. With respect to this reference voltage, the first measurement value and the second measurement value are AC coupled by the AC coupling means. AC coupling itself is also called AC coupling, and is a technique well known to those skilled in the art. Further, the first measurement value and the second measurement value AC-coupled to the reference voltage are amplified by the amplifying means. In this way, the fluctuation of the measured value becomes centered on the reference voltage, which contributes to the amplification of the measured value in the measurable range.

  Incidentally, the AC coupling means is generally configured using a capacitor and a resistor. At this time, when the charge amount of the capacitor is adjusted, the variation in the measured value becomes centered on the reference voltage. However, depending on the resistance value of the resistor constituting the AC coupling means, the time required for charging becomes long, and it takes several seconds to several tens of seconds until the fluctuation of the measured value becomes centered on the reference voltage. It may take such a case.

  In view of this, it is preferable to provide charge amount adjustment time switching means capable of switching the time required for adjusting the charge amount of the capacitor constituting the AC coupling means. Specifically, it is conceivable to arrange a switching element and a resistor having a small resistance value in parallel with the resistor of the AC coupling means. The switching element is turned on / off by, for example, a microcomputer. In this sense, the “charge amount adjustment time switching means” may include a control unit such as a microcomputer. Here, when the switching element is turned on, the capacitor can be charged (charge amount adjustment) in a short time via a resistor having a small resistance value. As a result, the fluctuation of the measured value immediately becomes centered on the reference voltage. Actually, although depending on the selection of the resistor, the measured value can be centered on the reference voltage in a period of 100 ms or less (for example, a period of 10 ms or 20 ms).

  At this time, as shown in claim 4, the time required for adjusting the charge amount of the capacitor is periodically switched. As described above, the present invention is characterized by taking a difference based on the first measurement value and the second measurement value. Here, one cycle is from the time when the operation signal having the first duty ratio is output to the time when the operation signal having the second duty ratio is output and the operation signal having the first duty ratio is output again. In this case, for example, it is conceivable to take a difference by using an average of the first and second measurement values obtained during a period of 4 cycles. Then, “periodically” can be considered to be every 4 cycles here. Alternatively, it may be performed every cycle. Alternatively, it may be performed at every switching timing of the first and second operation signals (every 0.5 cycle). In this way, the concentration can be appropriately measured even if the electrical conductivity in the fuel changes midway due to the shape of the tank or the like.

  There is also a desire to know the alcohol concentration in the fuel as early as possible when starting the engine. This is to appropriately control the air-fuel ratio at the time of engine start. Therefore, as shown in claim 5, when the ignition switch before starting the engine is turned on, the time required for adjusting the charge amount of the capacitor is exemplified. In this way, measurement can be started at an early stage after the engine is started (for example, within 10 ms and 20 ms from the start).

  By the way, there is a possibility that the measured value may exceed the measurable range until the variation of the measured value becomes centered on the reference voltage. Accordingly, as shown in claim 6, the operation signal output means has a duty ratio that is an intermediate duty ratio between the first duty ratio and the second duty ratio in accordance with the adjustment period switched by the charge amount adjustment time switching means. You may make it output the operation signal of 3 duty ratios. In the first duty ratio and the second duty ratio, measured values appear up and down with a predetermined voltage as a reference. Therefore, if the third duty ratio, which is an intermediate duty ratio, is used, the variation of the measured value is set to almost “0”. Can do. In this way, when the measured value exceeds the measurable range, a fail-safe abnormality signal is not output.

  According to a seventh aspect of the present invention, the measurement value output means has smoothing means for smoothing the first measurement value and the second measurement value. In this way, since the output voltage is smoothed, the subsequent processing of the measured values becomes relatively simple.

  By the way, as described above, the influence of the leakage resistance can be eliminated by taking the difference based on the first measurement value and the second measurement value. However, if each measurement value itself has the influence of the leakage resistance, It gets bigger. This is because a constant term including (1 / Rp) appears in the calculation formula of the voltage as the measurement value. Therefore, there is a concern that when the influence of the leak resistance becomes large, the measurement range is exceeded and measurement becomes impossible.

  In this respect, according to the eighth aspect, since the coupling capacitor is connected to the positive terminal of the detection electrode, the leakage current flowing through the detection electrode can be further reduced. Thereby, the influence of the leak resistance appearing in each measured value can be further reduced. As a result, exceeding the measurable range is suppressed, and there is a high possibility that a situation in which measurement is impossible can be avoided. Further, by connecting a coupling capacitor, charges are alternately accumulated in both electrodes of the detection electrode, which contributes to suppression of electrolytic corrosion (electrolysis of the detection electrode).

  According to the ninth aspect, since the negative terminal of the detection electrode is directly grounded, it is advantageous in that it is less affected by static electricity. For example, it is possible to suppress the switches constituting the switch means from being damaged by static electricity. For example, it can suppress that a switch malfunctions by electromagnetic waves. In addition, this makes the configuration of the switch means relatively simple.

  In the tenth aspect, the measurement value output means is configured as a crawling circuit. The “crawl type” is one of the switching methods in the switched capacitor circuit. In this way, the first measured value and the second measured value can be made relatively small, although the configuration of the switch means is slightly complicated. As a result, the measurable range can be made relatively large.

  According to the eleventh aspect, the operation signal having the first duty ratio and the operation signal having the second duty ratio are switched and output every predetermined period. For example, the operation signal is switched in anticipation of the convergence period of the output voltage or by detecting the convergence of the output voltage. This is advantageous in that the circuit configuration is simplified as compared with a circuit operating at two duty ratios in parallel.

  In the twelfth aspect, an operation signal output means is realized as a function of the microcomputer. This is advantageous in that the circuit configuration is simplified compared to the configuration including the oscillation circuit.

  In claim 13, the conductivity of the sensing electrode is measured by the conductivity measuring means based on at least one of the first measured value and the second measured value, and measured by the conductivity and a preset switch-on resistance. The result is corrected. In this way, measurement accuracy can be improved even when the leakage resistance is extremely small.

  In the fourteenth aspect, the switch-on resistance measuring unit measures the switch-on resistance generated when the switch unit is operated. Then, the measurement result is corrected based on the measured switch-on resistance. In this way, even if there are variations in leakage resistance and temperature characteristics, the measurement accuracy can be further improved.

  In the fifteenth aspect, the conductivity of the detection electrode is measured by the conductivity measuring means based on at least one of the first measurement value and the second measurement value. Further, when the measured conductivity exceeds a predetermined threshold by the abnormal signal output means, an abnormal signal is output. In this way, fail safe can be realized.

It is a circuit diagram which shows the structure of the alcohol concentration sensor of 1st Embodiment. It is a circuit diagram for demonstrating the basic operation | movement of an alcohol concentration sensor. It is explanatory drawing which shows the electric current which generate | occur | produces in a detection electrode and leak resistance. It is explanatory drawing which shows the difference based on an output voltage. It is a circuit diagram which shows the basic composition at the time of providing a coupling capacitor. It is a circuit diagram which shows the electric current which generate | occur | produces in the detection electrode and leak resistance at the time of providing a coupling capacitor. It is explanatory drawing which shows the difference based on the output voltage at the time of providing a coupling capacitor. It is a circuit diagram which shows the structure of the alcohol concentration sensor of 2nd Embodiment. It is a circuit diagram which shows the structure of the alcohol concentration sensor of 3rd Embodiment. It is a circuit diagram which shows the structure of the alcohol concentration sensor of 4th Embodiment. It is a circuit diagram which shows the basic composition in case switch-on resistance is considered. It is a circuit diagram which shows the structure of the alcohol concentration sensor of 5th Embodiment. It is explanatory drawing which shows typically transition of the center voltage used as the center of the fluctuation | variation of a measured value.

Hereinafter, a plurality of embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 is a circuit diagram showing a circuit configuration of an alcohol concentration sensor 1 of the present embodiment.
The alcohol concentration sensor 1 of this embodiment is a sensor that measures the concentration of ethanol, and is specifically used by being mounted on a vehicle. The alcohol concentration sensor 1 uses the battery 10 shown at the left end in FIG. 1 as the power supply voltage Vcc (5 V in this embodiment), and outputs the measurement result to the terminal 11 shown at the right end in FIG. The power supply voltage Vcc is stably supplied by a constant voltage IC (three-terminal regulator) 12 at the upper left in FIG.

The alcohol concentration sensor 1 includes a first oscillation unit 20, a second oscillation unit 25, a detection unit 40, a reference voltage generation unit 50, an AC coupling unit 60, an amplification unit 70, and a microcomputer 80. .
The first oscillating unit 20 includes a Schmitt trigger 21 provided with hysteresis in the determination operation, a resistor 22 connected in parallel to the Schmitt trigger 21, and a capacitor connected between the input side of the Schmitt trigger 21 and the ground potential. 23. With this configuration, the first oscillating unit 20 outputs a pulse wave (operation clock) having a duty ratio d1. Similarly, the second oscillation unit 25 includes a Schmitt trigger 26, a resistor 27, and a capacitor 28. With this configuration, the second oscillation unit 25 outputs a pulse wave (operation clock) having a duty ratio d2. In this embodiment, the Schmitt triggers 21 and 26 are used to configure the circuit, but other configurations may be employed as long as the same output can be obtained.

  Here, output terminals from the first oscillating unit 20 and the second oscillating unit 25 are connected to duty changeover switches 31 and 32, respectively. The two duty changeover switches 31 and 32 are controlled by the microcomputer 80 so as to be alternately turned on. That is, when these duty changeover switches 31 and 32 are controlled by the microcomputer 80, a circuit to be described later operates with a pulse wave having a duty ratio d1 or a duty ratio d2.

  The pulse wave from the first oscillating unit 20 and the second oscillating unit 25 switches the two switches sw1 and the switch sw2. Here, an inverter 33 is connected between one switch sw <b> 1 and the duty changeover switches 31 and 32. With this configuration, when the duty changeover switch 31 is on, the switches sw1 and sw2 are alternately turned on and off by the pulse wave having the duty ratio d1 output by the first oscillation unit 20. Similarly, when the duty changeover switch 32 is on, the switches sw1 and sw2 are alternately turned on / off alternately by the pulse wave having the duty ratio d2 output by the second oscillation unit 25.

  The detection unit 40 includes a detection electrode 41. This detection electrode 41 is installed in the fuel path of the vehicle. The detection electrode 41 constitutes what is called a capacitor by being arranged facing. In this embodiment, the ethanol concentration is measured by measuring the capacitance of the detection electrode 41. At this time, a leak resistance Rp exists as a factor that hinders the measurement. That is, the resistance Rp shown in the detection unit 40 varies depending on the contamination of impurities. This leakage resistance Rp can be considered as being connected in parallel with the detection electrode 41. One of the features of this embodiment is that the ethanol concentration can be measured without being affected by the leak resistance Rp.

  The positive terminal of the detection electrode 41 is connected to the inverting input terminal of the operational amplifier (operational amplifier) 43 through the coupling capacitor 42 and the switch sw1 in order. Further, a capacitor 44 and a gain resistor Rg are connected in parallel between the output terminal and the inverting input terminal of the operational amplifier 43. Furthermore, resistors 45 and 46 are sequentially connected between the power supply voltage Vcc and the ground potential, and a connection point between the resistors 45 and 46 is connected to a non-inverting input terminal of the operational amplifier 43. The positive terminal of the detection electrode 41 is grounded via the switch sw2, and the negative terminal is directly grounded.

  The output terminal of the operational amplifier 43 is connected to the non-inverting input terminal of the operational amplifier 48 via the resistor 47. The non-inverting input terminal is grounded via a capacitor 49. With this configuration, the output voltage of the operational amplifier 43 is input to the non-inverting input terminal of the operational amplifier 48 as the smoothed output voltage Va. The output terminal and the inverting input terminal of the operational amplifier 48 are commonly connected. The output of the operational amplifier 48 is an input to the AC coupling unit 60.

The reference voltage generation unit 50 generates a reference voltage Vr, and includes resistors 51, 52, and 53 and an operational amplifier 54.
The resistors 51 and 52 are sequentially connected between a power supply voltage Vcc (5V in this embodiment) and a ground potential (= 0V), and by dividing the power supply voltage Vcc, the resistors 51 and 52 are 2.5V in this embodiment. ) Is generated. A non-inverting input terminal of an operational amplifier 54 is connected to a connection point between the resistors 51 and 52. The inverting input terminal and the output terminal of the operational amplifier 54 are commonly connected, and the output terminal is grounded via the resistor 53. With this configuration, the operational amplifier 54 functions as a buffer that outputs the reference voltage Vr.

  In the AC coupling unit 60, an AC coupling is configured by a coupling capacitor 61 and a resistor 62. The output terminal of the operational amplifier 48 is connected to the non-inverting input terminal of the operational amplifier 71 via the capacitor 61. The connection point between the capacitor 61 and the non-inverting input terminal of the operational amplifier 71 is connected to the output terminal of the operational amplifier 54 via the resistor 62.

  The amplification unit 70 includes an operational amplifier 71 and resistors 72 and 73. The output terminal of the operational amplifier 71 is connected to the inverting input terminal via the resistor 72. Further, the output terminal of the operational amplifier 54 is connected to the inverting input terminal via the resistor 73. With this configuration, the amplification unit 70 amplifies the voltage Vb with reference to the reference voltage Vr, and outputs the voltage Vc to the microcomputer 80.

  The microcomputer 80 has a power supply voltage Vcc connected to its terminal VCC. The output voltage Vc from the amplifier 70 is input to the terminal A / D2 of the microcomputer 80. The microcomputer 80 performs AD conversion on the output voltage Vc, performs a predetermined difference calculation, and outputs the result to the terminal 11. This terminal 11 is connected to an ECU (not shown). The output terminal of the operational amplifier 48 is connected to the terminal AD1 of the microcomputer 80 so that an abnormality in the output voltage can be detected. For example, the circuit is operated with one duty ratio (for example, duty ratio d1), and warning processing is performed when a voltage exceeding the measurable range is detected. In this sense, the microcomputer 80 constitutes “conductivity measuring means” and “abnormal signal output means”.

Next, the operation of the basic part of the alcohol concentration sensor 1 will be described.
As described above, the switches sw1 and sw2 are alternately turned on and off by the pulse waves (operation clocks) from the first oscillation unit 20 and the second oscillation unit 25 (see FIG. 1). Here, based on FIG. 2, the flow of current when a pulse wave having a duty ratio d is input and the switches sw1 and sw2 are turned on / off will be described. The change in current will be described with reference to FIG. FIG. 2 shows a part of the circuit of FIG.

  When the pulse wave is at the low level, as shown in FIG. 2A, one switch sw1 is turned on and the other switch sw2 is turned off. In this case, the operational amplifier 43 operates so that the potentials of the non-inverting input terminal and the inverting input terminal are the same. As a result, a current is generated in the gain resistor Rg by the power supply voltage E. Here, the current generated in the detection electrode 41 is indicated as i1, and the current generated in the leak resistance Rp is indicated as i2.

  At this time, as indicated by periods T1 and T3 in FIG. 3, the current i1 generated in the detection electrode 41 rises first and becomes “0” when the detection electrode 41 is charged. On the other hand, the current i2 generated in the leakage resistance Rp connected in parallel with the detection electrode 41 has a constant value. Strictly speaking, the currents i1 and i2 do not rise simultaneously (although the rise of the current i2 is delayed), the current i2 is described as a constant value for convenience.

  When the pulse wave is at a high level, as shown in FIG. 2B, one switch sw1 is turned off and the other switch sw2 is turned on. In this case, since the positive side of the detection electrode 41 is grounded, the charged detection electrode 41 is discharged. Therefore, a current i1 in the direction opposite to that when the pulse wave is at the low level is generated at the detection electrode 41.

  At this time, as indicated by periods T2 and T4 in FIG. 3, the current i1 flowing through the detection electrode 41 rises in the opposite direction and becomes “0” when the discharge of the detection electrode 41 is completed. On the other hand, the current i2 flowing through the leak resistance Rp connected in parallel with the detection electrode 41 is “0”.

Next, the output voltage of the operational amplifier 43 when the switches sw1 and sw2 are switched with the pulse wave having the duty ratio d will be described.
First, from FIG. 3, the average of the current i2 is as shown by the following expression 1.

また、検知電極に溜まる電荷は、検知電極４１の静電容量をＣｐとすると、電源電圧Ｅであるため、次の式２で示すごとくとなる。

Further, since the charge accumulated in the detection electrode is the power supply voltage E when the capacitance of the detection electrode 41 is Cp, the charge is as shown in the following Expression 2.

電流ｉ１の平均は、電荷の時間微分であるため、式２を用いて、次の式３で示すごとくとなる。ここでは、周期Ｔ０（＝１／ｆ）とした（図３参照）。

Since the average of the current i1 is a time derivative of the electric charge, the following equation 3 is obtained using equation 2. Here, the cycle is T0 (= 1 / f) (see FIG. 3).

したがって、出力電圧Ｖは、式１、式３を用いて、次の式４で示すごとくとなる。

Therefore, the output voltage V is expressed by the following expression 4 using the expressions 1 and 3.

この式４によれば、リーク抵抗Ｒｐが無限大に近い場合、出力電圧Ｖにばらつきは生じない。つまり、精度よくエタノール濃度が測定できることになる。しかしながら、リーク抵抗Ｒｐが小さくなった場合、すなわち不純物が多く含まれているような場合には、測定誤差が大きくなってしまう。

According to Equation 4, when the leakage resistance Rp is close to infinity, the output voltage V does not vary. That is, the ethanol concentration can be measured with high accuracy. However, when the leak resistance Rp is small, that is, when a large amount of impurities is contained, the measurement error becomes large.

  Therefore, in the present embodiment, as shown in FIG. 1, the first oscillating unit 20 and the second oscillating unit 25 are provided, and the switches sw1 and sw2 are switched by pulse waves having two different duty ratios d1 and d2. The difference between the output voltages V (d1) and V (d2) of the operational amplifier 43 is taken. Specifically, the difference between the output voltage V (d1) multiplied by the duty ratio d2 and the output voltage V (d2) multiplied by the duty ratio d1 is taken. This calculation is performed by the microcomputer 80. That is, as shown in the following formula 5.

このようにすれば、リーク抵抗Ｒｐの影響を受けず、検知電極４１の静電容量Ｃｐを出力電圧Ｖに基づく差分として測定することができる。

In this way, the electrostatic capacitance Cp of the detection electrode 41 can be measured as a difference based on the output voltage V without being affected by the leakage resistance Rp.

FIG. 4 is an explanatory diagram showing a difference based on the output voltage Va.
The output voltage V from the operational amplifier 43 is smoothed by the resistor 47 and the capacitor 49 shown in FIGS. In FIG. 4, the switches sw1 and sw2 are first switched with a pulse wave having a duty ratio d2, but it has almost converged by time t1. Further, the switches sw1 and sw2 are switched with the pulse wave having the duty ratio d1 from the time t1, but they are almost converged by the time t2. Therefore, the switching timing of the duty ratios d1 and d2 may be controlled by the microcomputer 80 based on such a change in the voltage Va.

  Incidentally, the voltage Va takes a large value when the leak resistance Rp is small, as can be seen from the above equation 4. FIG. 4 shows a comparison between the case where the leak resistance Rp is infinite and the case where it is 1 k ohm. That is, the difference between the output voltages d2 · V (d1) and d1 · V (d2) is not affected by the leakage resistance Rp, but the output voltage Va itself is increased by the influence of the leakage resistance Rp. is there. In extreme cases, the output voltage Va may exceed the measurable range.

  Therefore, in this embodiment, as shown in FIG. 5, the positive terminal of the detection electrode 41 is AC-coupled by the coupling capacitor 42. In this case, the basic operation is the same as that described in FIG. At this time, as shown in FIG. 6, the average of the current i2 is d (1-d) · E / Rp, which is smaller than when the coupling capacitor 42 is not inserted. Specifically, the area indicated by symbol A and the area indicated by symbol B are the same. As a result, as can be seen from Equation 4, the influence of the leakage resistance Rp can be d (1-d). That is, as shown in FIG. 7, when the coupling capacitor 42 is not inserted, the output voltage is as shown by a two-dot chain line. However, by providing the coupling capacitor 42, even when the leak resistance Rp is the same 1 k ohm. The output voltage can be reduced. Moreover, since it can suppress that an electric charge accumulates only in one electrode of the detection electrode 41 by doing in this way, the electrolytic corrosion of the detection electrode 41 can be suppressed.

  The output voltage Va as described above becomes the output voltage Vb based on the reference voltage Vr (2.5 V in this embodiment) by the operational amplifier 48, the reference voltage generation unit 50, and the AC coupling unit 60 (see FIG. 1). ). Further, the output voltage Vb is amplified by the amplifying unit 70 to become the output voltage Vc. Thereby, a measurement result can be taken out suitably.

  In this embodiment, the switches sw1 and sw2 constitute “switch means”, and the first oscillation unit 20, the second oscillation unit 25, the duty changeover switches 31 and 32, and the microcomputer 80 constitute “operation signal output means”. The operational amplifier 43, the gain resistor Rg, the capacitor 44, the resistors 45, 46, and 47, and the capacitor 49 constitute “measurement value output means”, and the resistor 47 and the capacitor 49 constitute “smoothing means”. Further, the reference voltage generation unit 50 constitutes “reference voltage generation means”, the AC coupling unit 60 constitutes “AC coupling means”, the amplification unit 70 constitutes “amplification means”, and the microcomputer 80 constitutes “calculation means”. Is configured.

As described above in detail, according to the alcohol concentration sensor 1 of the present embodiment, the ethanol concentration can be accurately measured without being affected by the leakage resistance Rp.
Moreover, since the output voltage Va can be reduced by providing the coupling capacitor 42, the measurable range can be made relatively large. In addition, since electric charges are alternately accumulated on both electrodes of the detection electrode 41, the electrolytic corrosion of the detection electrode 41 can be suppressed.
Furthermore, since the minus terminal of the detection electrode 41 is directly grounded, a circuit can be configured by the two switches sw1 and sw2, and the circuit configuration becomes relatively simple. In addition, the switches sw1 and sw2 can be prevented from being damaged by static electricity, and malfunctions of the switches sw1 and sw2 due to electromagnetic waves can be suppressed.

(Second Embodiment)
The alcohol concentration sensor 2 of the present embodiment is different from the above embodiment in the connection of the detection electrode 41 of the detection unit 40. Therefore, only the difference from the above embodiment will be described, and the description of the same portion as the above embodiment will be omitted. In addition, the same reference numerals are assigned to the same components as those in the above embodiment.

  As shown in FIG. 8, in the alcohol concentration sensor 2, resistors 91 and 92 are sequentially connected between the power supply voltage Vcc and the ground potential. A connection point between the resistors 91 and 92 is connected to a non-inverting input terminal of the operational amplifier 90. Further, the output terminal and the inverting input terminal of the operational amplifier 90 are connected in common. Furthermore, the output terminal of the operational amplifier 90 is connected to the inverting input terminal of the operational amplifier 43 through the four switches sw3, sw4, sw2, and sw1 in order. Here, the connection point between the switches sw2 and sw4 is grounded and connected to the non-inverting input terminal of the operational amplifier 43. When the switches sw1 and sw4 are ON as shown in FIG. 8, the switches sw2 and sw3 are OFF, and conversely When sw1 and sw4 are OFF, the switches sw2 and sw3 are ON.

  By adopting such a configuration (crawl type), four switches sw1, sw2, sw3, and sw4 are required, but the measurable range can be further increased.

(Third embodiment)
The alcohol concentration sensor 3 of the present embodiment is different from the above embodiment in the configuration of the first oscillation unit 20 and the second oscillation unit 25. Here, only the difference from the above embodiment will be described, and the description of the same portion as the above embodiment will be omitted. In addition, the same reference numerals are assigned to the same components as those in the above embodiment.

  As shown in FIG. 9, the alcohol concentration sensor 3 of the present embodiment is configured to output from the microcomputer 80 both a pulse wave having a duty ratio d1 and a pulse wave having a duty ratio d2. In this sense, the microcomputer 80 realizes a function as “operation signal output means”. In this way, the configuration of the first oscillating unit 20 and the second oscillating unit 25 and the duty changeover switches 31 and 32 are not required, so that the configuration of the alcohol concentration sensor 3 is relatively simple. Moreover, since the number of parts is reduced, it contributes to cost reduction.

(Fourth embodiment)
The alcohol concentration sensor 4 of this embodiment is capable of measuring the switch-on resistances of the switches sw1 and sw2 with respect to the alcohol concentration sensor 3 of the above embodiment. Here, only the difference from the above embodiment will be described, and the description of the same portion as the above embodiment will be omitted. In addition, the same reference numerals are assigned to the same components as those in the above embodiment.

  As shown in the lower right part of FIG. 10, a switch sw3 controlled by the microcomputer 80 is further provided, and a resistor 93 and a switch sw3 are sequentially connected between the power supply voltage Vcc and the ground potential. The connection point between the resistor 93 and the switch sw3 is connected to the terminal AD3 of the microcomputer 80. The switch sw3 is preferably included in the same package as the other two switches sw1 and sw2, for example. In this case, the microcomputer 80 measures the switch-on resistance generated by turning on the switch sw3 based on the input voltage from the terminal AD3. This is regarded as the switch-on resistance of the other switches sw1 and sw2, and the output voltage is corrected. In this sense, the microcomputer 80 constitutes “switch-on resistance measuring means”.

Here, a method for correcting the influence of the switch-on resistance by measuring the switch-on resistance will be described.
As shown in FIG. 11, the switch-on resistance Rsw1 of one switch sw1 can be considered to be connected in series to the switch sw1. Similarly, it can be considered that the switch-on resistance Rsw2 of the other switch sw2 is connected in series to the switch sw2. The description will be continued below assuming that the switch-on resistance Rsw1 (= Rsw2).

  The output voltage V is as shown by the following Expression 6.

そして、デューティ比ｄ１のときの出力電圧Ｖ（ｄ１）とデューティ比ｄ２のときの出力電圧Ｖ（ｄ２）とに基づく差分は、式６を用いて、次の式７で示すごとくである。

The difference based on the output voltage V (d1) when the duty ratio is d1 and the output voltage V (d2) when the duty ratio is d2 is as shown in the following Expression 7 using Expression 6.

この式７から検知電極４１の静電容量Ｃｐを求め、次に出力電圧Ｖ（ｄ１）（あるいは出力電圧Ｖ（ｄ２））からＲｐを求めることができる。したがって、補正係数は、

Ｒｐ／（Ｒｐ＋Ｒｓｗ１） ・・・式８

として求めることができる。

From this equation 7, the capacitance Cp of the detection electrode 41 can be obtained, and then Rp can be obtained from the output voltage V (d1) (or output voltage V (d2)). Therefore, the correction factor is

Rp / (Rp + Rsw1) Formula 8

Can be obtained as

  Since this embodiment can correct the influence of the switch-on resistances Rsw1 and Rsw2, it is effective for an extremely small leak resistance Rp.

(Fifth embodiment)
The alcohol concentration sensor 5 of the present embodiment is obtained by adding a circuit for adjusting the charge amount of the capacitor in a short time to the AC coupling unit 60 with respect to the alcohol concentration sensor 1 of the above embodiment. Here, only the difference from the above embodiment will be described, and the description of the same portion as the above embodiment will be omitted. In addition, the same reference numerals are assigned to the same components as those in the above embodiment.

  As in the above embodiment, the output voltage Va becomes the output voltage Vb based on the reference voltage Vr (2.5 V in this embodiment) by the operational amplifier 48, the reference voltage generation unit 50, and the AC coupling unit 60 (FIG. 12). reference). Further, the output voltage Vb is amplified by the amplifying unit 70 to become the output voltage Vc. That is, AC amplification is performed, and the measurement result can be suitably extracted.

  However, in order to obtain the output voltage Vb based on the reference voltage Vr, the charge amount of the capacitor 61 constituting the AC coupling unit 60 must be adjusted by the reference voltage Vr. However, it usually takes several seconds to several tens of seconds before the electrode is charged via the resistor 62.

  Therefore, in this embodiment, the resistor 63 and the switching element 64 are arranged in parallel with the resistor 62 that constitutes the AC coupling unit 60. Here, the resistor 63 having a resistance value sufficiently smaller than that of the resistor 62 is used. The switching element 64 is exemplified by an FET or the like, and is controlled by the microcomputer 80.

  Under such a configuration, in this embodiment, the switching element 64 is turned on by the microcomputer 80 periodically, for example, in accordance with the measurement timing. The switching element 64 is turned on for a period necessary for charging the capacitor 61 described above, for example, a period of 100 ms or less (10 ms, 20 ms, etc.). During this time, the capacitor 61 is charged by the reference voltage Vr, so that the output Vb from the AC coupling unit 60 immediately becomes centered on the reference voltage Vr (during 100 ms at the latest).

Here, in order to facilitate the understanding of the above configuration, a description using the drawings will be added.
FIG. 13 is an explanatory diagram schematically showing the transition of the center voltage of the measured value Vb when the duty ratio d1 and the duty ratio d2 are alternately output. The “center voltage” refers to a voltage that is the center of fluctuation of the measured value. The same applies to the following.
As shown in FIG. 13, the voltage Ve is initially the center voltage due to the influence of the leak resistance Rp, and finally the reference voltage Vr is the center voltage.

  At this time, since the electrode is normally charged through the resistor 62, the center voltage gradually approaches the reference voltage Vr as indicated by the symbol J, and the time (symbol from several seconds to several tens of seconds). (Time indicated by T0) is required.

  On the other hand, in this embodiment, the resistor 63 and the switching element 64 are arranged in parallel with the resistor 62 constituting the AC coupling unit 60 (see FIG. 12). Since the resistor 63 having a resistance value sufficiently smaller than that of the resistor 62 is used, as shown by the symbol K, the center voltage becomes the reference voltage Vr within 100 ms (out of 10 ms and 20 ms). As indicated by the symbol T1.

  Thereby, even if the influence of the leak resistance Rp is large, the measured value Vb centered on the reference voltage Vr can be obtained immediately.

  The resistor 63, the switching element 64, and the microcomputer 80 in this embodiment form “charge amount adjustment time switching means”.

  In this embodiment, for example, the switching element 64 is periodically turned on in accordance with the measurement timing. However, for example, the switching element 64 may be turned on when an ignition switch before starting the engine is turned on. In this way, even if the influence of the leak resistance Rp is large, measurement can be started before the engine is started.

  Further, the measured value Vb may exceed the measurable range until the variation of the measured value Vb becomes centered on the reference voltage Vr. Therefore, during the period when the switching element 64 is turned on (period T1 in FIG. 13), a duty ratio d3 intermediate between the duty ratio d1 and the duty ratio d2 may be output. At the duty ratio d1 and the duty ratio d2, the measurement value Vb appears up and down with respect to the center voltage. Therefore, if the intermediate duty ratio d3 is used, the fluctuation of the measurement value Vb can be made almost “0”. In this way, the measured value Vb does not exceed the measurable range during the charge amount adjustment.

Incidentally, the adjustment of the charge amount of the capacitor 61 by the resistor 63, the switching element 64, and the microcomputer 80 can be adopted in any of the above forms.
(Other embodiments)
Although the said form has been demonstrated as a sensor which measures ethanol concentration, alcohol concentration, such as methanol concentration, can be measured by the same method.
As mentioned above, this invention is not limited to the said form at all, In the range which does not deviate from the meaning of invention, it can implement with a various form.

  1-4: alcohol concentration sensor, 10: battery, 11: terminal, 20: first oscillation unit, 25: second oscillation unit, 31, 32: duty switch, 40: detection unit, 41: detection electrode, 42: Coupling capacitor, 43: operational amplifier, 44: capacitor, 45: resistor, 47: resistor, 49: capacitor, 50: reference voltage generation unit, 60: AC coupling unit, 61: coupling capacitor, 62: resistor, 63: resistor, 64: switching element, 70: amplifier, 80: microcomputer, 90: operational amplifier, 91 to 93: resistor, Rg: gain resistor, Rp: leak resistor, Rsw1, Rsw2: switch-on resistor, sw1 sw4: Switch

Claims (15)

A sensing electrode in which the electrodes are arranged opposite to each other;
Switch means for switching between charging and discharging of the detection electrode;
An operation signal output means for outputting an operation signal for operating the switch means in order to perform switching of the switch means at a predetermined period;
A measurement value output means capable of generating a voltage for charging the detection electrode according to the operation of the switch means and outputting a voltage according to the capacitance of the detection electrode as a measurement value;
A calculation means for performing a calculation based on the measurement value output by the measurement value output means;
A liquid concentration measuring apparatus comprising:
The operation signal output means has a first duty ratio operation signal for switching the switch means at a first duty ratio and a second duty ratio operation signal for switching the switch means at a second duty ratio. Output
The measurement value output means outputs a first measurement value based on a first duty ratio and a second measurement value based on a second duty,
The calculation means outputs a difference between a value obtained by multiplying the first measured value by the second duty ratio and a value obtained by multiplying the second measured value by the first duty ratio as a measurement result. Liquid concentration measuring device. The concentration measuring apparatus for liquid according to claim 1,
A reference voltage generating means for generating a reference voltage;
AC coupling means for AC coupling the first measurement value and the second measurement value to the reference voltage generated by the reference voltage generation means;
Amplifying means for amplifying the first measurement value and the second measurement value AC-coupled to a reference voltage by the AC coupling means;
A liquid concentration measuring apparatus comprising: The concentration measuring apparatus for liquid according to claim 2,
A liquid concentration measuring device comprising charge amount adjustment time switching means capable of switching a time required for adjusting a charge amount of a capacitor constituting the AC coupling means. The liquid concentration measuring apparatus according to claim 3,
The concentration measuring apparatus for liquid, wherein the charge amount adjustment time switching means periodically switches a time required for adjusting the charge amount of the capacitor. In the liquid concentration measuring device according to claim 3 or 4,
Assuming that it is installed and used in the engine,
The liquid concentration measuring device, wherein the charge amount adjusting time switching means switches a time required for adjusting the charge amount of the capacitor when the ignition switch before starting the engine is turned on. In the concentration measuring apparatus for liquids as described in any one of Claims 3-5,
The operation signal output means operates in accordance with an adjustment period switched by the charge amount adjustment time switching means, and operates with a third duty ratio that is an intermediate duty ratio between the operation signal with the first duty ratio and the second duty ratio. A concentration measuring apparatus for liquid, characterized by outputting a signal. In the concentration measuring apparatus for liquids as described in any one of Claims 1-6,
The liquid measurement apparatus according to claim 1, wherein the measurement value output means includes a smoothing means for smoothing the first measurement value and the second measurement value. In the concentration measuring apparatus for liquids as described in any one of Claims 1-7,
The concentration measuring apparatus for liquid, wherein the detection electrode is configured by connecting a coupling capacitor to a positive terminal thereof. In the liquid concentration measuring apparatus according to any one of claims 1 to 8,
The concentration measuring apparatus for liquid, wherein the negative electrode terminal of the detection electrode is directly grounded. In the liquid concentration measuring apparatus according to any one of claims 1 to 9,
The liquid concentration measurement apparatus, wherein the measurement value output means is configured as a crawl type circuit with respect to the detection electrode. In the liquid concentration measuring apparatus according to any one of claims 1 to 10,
The liquid concentration measuring device according to claim 1, wherein the operation signal output means switches and outputs the operation signal having the first duty ratio and the operation signal having the second duty ratio every predetermined period. In the liquid concentration measuring apparatus according to any one of claims 1 to 11,
The liquid concentration measuring apparatus, wherein the operation signal output means is realized as a function of a microcomputer. In the concentration measuring apparatus for liquids as described in any one of Claims 1-12,
Conductivity measuring means for measuring the conductivity of the detection electrode based on at least one of the first measurement value and the second measurement value;
A measurement result which is a difference based on the first measurement value and the second measurement value, based on the conductivity measured by the conductivity measuring means and a switch-on resistance generated when the switch means is set in advance. A liquid concentration measuring device characterized by correcting the above. The liquid concentration measuring device according to claim 13,
A switch-on resistance measuring means for measuring the switch-on resistance;
The liquid concentration measuring apparatus, wherein the measurement result is corrected based on the switch-on resistance measured by the switch-on resistance measuring means. In the concentration measuring apparatus for liquids as described in any one of Claims 1-14,
Conductivity measuring means for measuring the conductivity of the detection electrode based on at least one of the first measurement value and the second measurement value;
An apparatus for measuring liquid concentration, comprising: an abnormal signal output means for outputting an abnormal signal when the conductivity measured by the conductivity measuring means exceeds a predetermined threshold value.

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