JP3292124B2 - Element impedance detecting device of oxygen concentration sensor and oxygen concentration detecting device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen concentration sensor for detecting, for example, an oxygen concentration in exhaust gas of a vehicle-mounted engine, and more particularly, to detecting an element impedance (AC impedance) of the oxygen concentration sensor. The present invention relates to an element impedance detection device for an oxygen concentration sensor and an oxygen concentration detection device having a function of detecting the element impedance of the oxygen concentration sensor.

[0002]

2. Description of the Related Art In recent years, in air-fuel ratio control of an on-vehicle engine, there has been a demand for improving control accuracy and a demand for lean burn, for example.
2. Description of the Related Art A linear air-fuel ratio sensor (oxygen concentration sensor) for linearly detecting an air-fuel ratio (oxygen concentration in exhaust gas) of an air-fuel mixture taken into an engine over a wide area is provided.

In order to maintain the detection accuracy of such an air-fuel ratio sensor, it is necessary to keep the sensor in an active state. Generally, the air-fuel ratio sensor is controlled by energizing a heater attached to the sensor. The sensor element (sensor element) is heated to maintain the active state.

[0004] Incidentally, in such a heater energization control, a technique of detecting the temperature (element temperature) of a sensor element and performing feedback control so that the element temperature becomes a desired activation temperature (for example, about 700 ° C). Is conventionally disclosed. In this case, in order to detect the temperature of the element at that time, it is conceivable to attach a temperature sensor to the element and derive it from the result of the detection. However, this would increase the size of the sensor and increase the cost.

For this reason, it has been proposed to detect the element impedance by utilizing the fact that the impedance of the sensor element (element impedance) has a predetermined correspondence with the element temperature, and to derive the element temperature from the detected element impedance. Have been. The detection result of the element impedance is also used for determining the degree of deterioration of the sensor, for example.

[0006]

Therefore, the present inventor has proposed:
As an air-fuel ratio detecting device (oxygen concentration detecting device) having a function of detecting the element impedance of the air-fuel ratio sensor, the following configuration and method were considered.

First, as shown in FIG. 10, the air-fuel ratio detecting apparatus 100 includes a microcomputer (hereinafter referred to as a microcomputer) 102 having an A / D converter and a D / A converter built therein. Two terminals DAC1 and DAC2 are output terminals of the D / A converter, and the other two terminals ch11 and ch12 are
This is an input terminal of the D converter.

The device 100 includes a microcomputer 1
An output circuit 106 as a buffer that inputs the voltage Vp output from the terminal DAC1
A low-pass filter 108 composed of a resistor R1 and a capacitor C1 for integrating the output of the output circuit 106, the output of the low-pass filter 108 is input to a non-inverting input terminal (+), and the inverting input terminal (-) is connected via a resistor R2. One terminal of the air-fuel ratio sensor AFS (the positive terminal in this example)
Operational amplifier OP1 connected to the air-fuel ratio sensor AFS
And the voltage Vm output from the terminal DAC2 of the microcomputer 102 are input to the shunt resistor R3 for current detection connected between the above-mentioned plus terminal and the output terminal of the operational amplifier OP1. An output circuit 104 as a buffer for outputting to the other terminal (in this example, a negative terminal) is provided.

Further, in the apparatus 100, an operational amplifier OP2 having a non-inverting input terminal connected to the plus side terminal of the air-fuel ratio sensor AFS, and an inverting input terminal and an output terminal commonly connected, and an operational amplifier OP2. Output of resistor R
4 through the terminal ch of the microcomputer 102 as it is.
11, an operational amplifier OP3 having a non-inverting input terminal connected to the output terminal of the operational amplifier OP1, and an inverting input terminal and an output terminal connected in common, and an output of the operational amplifier OP3 connected to a resistor R5. , And an input circuit 112 for outputting the signal to the terminal ch12 of the microcomputer 102 as it is.

[0010] The air-fuel ratio detecting device 10 constructed as described above.
0, the voltage V output from the low-pass filter 108
The same voltage Vo as the voltage o is applied to the positive terminal of the air-fuel ratio sensor AFS by the operational amplifier OP1. That is, the operational amplifier OP1 changes its own output voltage so that the output voltage Vo of the low-pass filter 108 matches the voltage of the positive terminal of the air-fuel ratio sensor AFS.

When the voltage Vp output from the terminal DAC1 of the microcomputer 102 does not change in a steady state, the voltage Vp is output from the low-pass filter 108 as the voltage Vo. The voltage Vm output from the terminal DAC2 of the microcomputer 102 is applied to the negative terminal of the air-fuel ratio sensor AFS via the output circuit 104.

Therefore, in the steady state, the voltages Vp and V output from the terminals DAC1 and DAC2 of the microcomputer 102, respectively.
The differential pressure of m (Vp-Vm) is applied to both ends of the air-fuel ratio sensor AFS as a voltage for detecting the air-fuel ratio (oxygen concentration in exhaust gas) of the air-fuel mixture. For example, if the voltage Vp is 3.3 V and the voltage Vm is 3.0 V, 0.3 V is applied to the air-fuel ratio sensor AFS. Then, with the application of the voltage, a current flows through the air-fuel ratio sensor AFS according to the oxygen concentration in the exhaust gas at that time.

Then, the same current as the current (sensor current) I flowing to the air-fuel ratio sensor AFS flows to the shunt resistor R3, and the potential difference between both ends of the shunt resistor R3 becomes a value proportional to the sensor current I. Further, the air-fuel ratio sensor AFS
(Ie, the voltage on the air-fuel ratio sensor AFS side of the shunt resistor R3) Vo applied to the plus terminal of the microcomputer 102 is input to the terminal ch11 of the microcomputer 102 by the operational amplifier OP2 and the input circuit 110. , The same voltage Vi as the voltage on the opposite side of the shunt resistor R3 from the air-fuel ratio sensor AFS (that is, the output voltage of the operational amplifier OP1).
Is input to the terminal ch12 of the microcomputer 102 by the operational amplifier OP3 and the input circuit 112.

For this reason, the microcomputer 102 is connected to the terminal ch1
2 and the voltage Vo input to the terminal ch11 are detected by digital conversion by an internal A / D converter, and the difference (Vi-Vo) between the two voltages Vi and Vo is detected.
Is divided by the resistance value of the shunt resistor R3 (hereinafter referred to as the shunt resistance value) to detect the sensor current I.
Further, the air-fuel ratio (oxygen concentration in the exhaust gas) of the air-fuel mixture is obtained from the detected sensor current I.

On the other hand, in the air-fuel ratio detecting device 100, the microcomputer 102 changes the voltage applied to the air-fuel ratio sensor AFS to detect the air-fuel ratio at every predetermined time T1 as shown in FIG. The element impedance of the fuel ratio sensor AFS is detected. In this example, the microcomputer 102 changes the voltage applied to the air-fuel ratio sensor AFS by changing the output voltage Vp of the terminal DAC1, and FIG. The waveform of the voltage Vo input to the terminal ch11 of 102 (that is, the voltage applied to the positive terminal of the air-fuel ratio sensor AFS) Vo is shown. FIG. 11 (b)
Is an enlarged view of a portion surrounded by an ellipse in FIG.

More specifically, the microcomputer 102
According to the following procedures (1) to (7), the air-fuel ratio sensor AF
The element impedance of S is detected. (1) As shown in FIG. 11, first, at time t1, the voltage Vo input to the terminal ch11 is detected. Hereinafter, the voltage Vo detected at the time t1 is referred to as "Vo (t1)".

(2) Further, at time t2 immediately after time t1,
The voltage Vi input to the terminal ch12 is detected. Hereinafter, the voltage Vi detected at the time t2 is referred to as “Vi (t2
) ". (3) Further, at time t3 immediately after time t2, a command is given to the internal D / A converter, so that the terminal DA
The output voltage Vp of C1 is changed to a voltage lower than the normal voltage for detecting the air-fuel ratio by a predetermined voltage ΔVa.

(4) Next, at time t4 when a predetermined time T2 has elapsed from time t3, the voltage Vi input to the terminal ch12 is detected. Hereinafter, at time t4
Is referred to as "Vi (t4)". Also,
The predetermined time T2 is set to a time at which the change ΔI in the sensor current I is expected to peak from time t3.

(5) Further, at time t5 immediately after time t4,
The voltage Vo input to the terminal ch11 is detected. Hereinafter, the voltage Vo detected at the time t5 is referred to as "Vo (t5
) ". If the shunt resistance value is Rs,
The element impedance Z of the air-fuel ratio sensor AFS is calculated by the following equation 1.

[0020]

(Equation 1)

In the above equation (1), ΔVo (t1) −
Vo (t5)} is the variation of the applied voltage △ V, and the other part is the reciprocal (1 / △) of the variation of the sensor current I △ I.
I). Also, ΔVo (t1) −Vo (t
5)｝ can be replaced with a known △ Va.

(6) Thereafter, at time t6 when a predetermined time T3 has elapsed from time t3, the output voltage Vp of the terminal DAC1 is changed to a voltage higher by a predetermined voltage ΔVb than the normal voltage. (7) A predetermined time T from time t6
4 at time t7, the output voltage Vp of the terminal DAC1.
To the normal voltage.

That is, in the air-fuel ratio detecting device 100,
As can be seen from Equation 1, the sensor current “ΔVi (t2) −Vo before changing the voltage applied to the air-fuel ratio sensor AFS.
(t1)｝ / RS ”and the sensor current“ ｛Vi (t4) −Vo when a predetermined time T2 has elapsed since the applied voltage was changed.
(t5)｝ / RS ”, and the element impedance Z is calculated.

The reason why the output voltage Vp of the terminal DAC1 is changed to the positive side from the normal voltage at the time t6 when the detection of the element impedance Z is completed is to speed up the convergence of the sensor current. That is, when the voltage applied to the air-fuel ratio sensor AFS is returned to the normal voltage again, if the voltage is directly switched to the original normal voltage, the electric charge stored in the capacitance component of the element of the air-fuel ratio sensor AFS is obtained. , The sensor current generates a peak current immediately after the return of the voltage, and as a result, the time required to converge to the original current value becomes longer. Therefore, in this example, when the applied voltage is returned to the original normal voltage, a voltage in a direction opposite to the previous voltage change is applied for a short time, and the discharge of the electric charge in the capacitance component of the element is completed in a short time. Thus, the stabilization time of the sensor current is shortened. Therefore, it is more effective if the voltage waveform is set so that the amount of charge moving in the element of the air-fuel ratio sensor AFS becomes substantially the same regardless of whether the amount of charge is positive or negative.

Here, the air-fuel ratio detecting device 10 as described above
At 0, when a D / A converter (hereinafter referred to as a simple D / A converter) 120 having a small circuit configuration and an inexpensive structure as shown in FIG. It has been found that the element impedance of the sensor AFS cannot be accurately detected. FIG. 12 shows a configuration in the case where the number of output terminals of the D / A converter is four (that is, in the case of four channels).

That is, as shown in FIG. 12, a simple D / A converter 120 of this type includes output terminals DAC1 to DAC1.
A register group 12 composed of four target value registers (DAC1 target value register to DAC4 target value register) in which a digital value indicating a voltage (analog output) to be output from the DAC 4 is set by the CPU in the microcomputer 102, respectively.
2 and D that outputs a voltage corresponding to the input digital value
/ A conversion processing unit 124 and four output holding circuits (DAC1 output holding circuits) that hold the output voltages of the D / A conversion processing unit 124 and output the held voltages from their corresponding output terminals DAC1 to DAC4. ＤＡDAC4 output holding circuit) and a digital value set in each target value register of the register group 122 are sequentially switched and input to the D / A conversion processing unit 124 at predetermined time intervals (for example, 12 μs). The second switching in which the output voltages of the first switching unit 128 and the D / A conversion processing unit 124 are sequentially switched and input to each output holding circuit of the output unit 126 at a timing synchronized with the switching timing of the first switching unit 128. Part 12
9 is provided.

The simple D / A converter 120
In the register group 122, the digital value set in the DAC1 target value register corresponding to the output terminal DAC1 in the register group 122 is transmitted to the D / A conversion processing unit 12 by the first switching unit 128.
4, the output voltage of the D / A conversion processing unit 124 is input by the second switching unit 129 to the DAC1 output holding circuit corresponding to the output terminal DAC1 in the output unit 126. Similarly, in the register group 122, the digital value set in the DAC2 target value register corresponding to the output terminal DAC2 is supplied to the D / A by the first switching unit 128.
When input to the conversion processing unit 124, the D / A conversion processing unit 1
24 is output by the second switching unit 129 to the output unit 1
26, the signal is input to a DAC2 output holding circuit corresponding to the output terminal DAC2. And such a switching operation,
The processing is sequentially performed for each of the output terminals DAC1 to DAC4.

Therefore, in such a simple D / A converter 120, the CPU in the microcomputer 102 rewrites the digital value of the desired target value register to arbitrarily change the voltage of the output terminal corresponding to the target value register. Can be changed. However, an unpredictable variation occurs in the response time from the rewriting of the digital value to the change in the voltage of the output terminal corresponding to the rewriting of the digital value. This is because the rewriting timing of the digital value by the CPU and the switching operation of the first switching unit 128 and the second switching unit 129 are asynchronous. For example, when the switching operation of the first switching unit 128 and the second switching unit 129 is performed every 12 μs, the variation of the response time is 0 μs to
It is about 48 μs.

For this reason, the above-described air-fuel ratio detecting device 100
When the simple D / A converter 120 as shown in FIG. 12 is used, the CPU in the microcomputer 102 operates at the time t3.
Therefore, even if the digital value of the target value register corresponding to the terminal DAC1 is rewritten to change the output voltage Vp of the terminal DAC1, the output voltage Vp of the terminal DAC1 is not changed until the output voltage Vp of the terminal DAC1 actually changes as shown in FIG. As shown, an unpredictable delay time Td occurs. In addition, FIG.
FIG. 11B shows a case where there is no delay time Td as in FIG. 11B, which is also shown for comparison with FIG. 13B.

When such a delay time Td occurs, as shown in FIG. 13B, even if the voltage Vi input to the terminal ch12 is detected at time t4 when the above-mentioned predetermined time T2 has elapsed from time t3. , Its detection voltage Vi (t4)
Is the value to be detected originally (that is, the variation of the sensor current I)
Value when I reaches its peak), and as a result,
This makes it impossible to accurately detect the element impedance Z of the air-fuel ratio sensor AFS.

As shown in FIG. 13 (b), the CPU in the microcomputer 102 operates the terminal DAC1 at time t7.
If the delay occurs after the output voltage Vp of the terminal DAC1 returns to the normal voltage until the output voltage Vp of the terminal DAC1 returns to the normal voltage, the stabilization of the sensor current is delayed, and the normal air-fuel ratio detection is affected. There is also a possibility that it will affect.

If the number of channels (the number of output terminals) of the D / A converter is only one, the above-described delay time Td does not occur.
In many cases, the microcomputer 102, which forms the main part of 00, must output a variable voltage to an object other than the air-fuel ratio sensor AFS in order to perform other control such as engine control. D / A converter is required.

On the other hand, a D / A converter having a configuration in which the D / A conversion processing section 124 shown in FIG. 12 is provided independently for each output terminal (a so-called ladder type D / A converter) is used.
A converter), even if there are multiple channels,
The output voltage can be changed without generating the delay time Td as described above.

However, such a ladder type D
When the / A converter is used, the circuit configuration becomes large-scale and the cost is increased. And
The above-described problems are not limited to the case where the D / A converter is built in the microcomputer 102, and the same applies to the case where the D / A converter is provided outside the microcomputer 102.

The present invention has been made in view of such a problem, and has an element impedance detecting device for an oxygen concentration sensor capable of accurately detecting the element impedance of an oxygen concentration sensor despite its simple configuration. It is an object to provide an oxygen concentration detecting device.

[0036]

According to the first aspect of the present invention, there is provided a device for detecting an element impedance of an oxygen concentration sensor, comprising the steps of: The voltage applied by the voltage application means for applying a voltage for detecting the voltage (hereinafter referred to as oxygen concentration detection voltage) is changed by the applied voltage changing means in order to detect the element impedance of the oxygen concentration sensor. Before the applied voltage changing means performs the operation of changing the output voltage of the voltage applying means, the first current detecting means detects a current flowing through the oxygen concentration sensor.

When a predetermined time has passed since the application voltage changing means performed the operation of changing the output voltage of the voltage application means, the second current detection means detects the current flowing through the oxygen concentration sensor. To detect. Further, the impedance calculating means calculates the element impedance of the oxygen concentration sensor based on the difference between the current detected by the first current detecting means and the current detected by the second current detecting means.

That is, similarly to the air-fuel ratio detection device 100 shown in FIG. 10, the sensor current before changing the applied voltage to the oxygen concentration sensor and the sensor current when a predetermined time has elapsed after changing the applied voltage. The element impedance is calculated based on the difference. Here, in particular, in the element impedance detection device according to claim 1, the voltage application unit includes:
An output circuit that outputs a voltage input to the input unit to the oxygen concentration sensor, a constant voltage circuit that outputs a predetermined constant voltage Vc to an input unit of the output circuit via an output resistor, and an output resistor And a voltage switching circuit composed of a switching element and a voltage-dividing resistor connected in series between a signal path extending from the control circuit to the input portion of the output circuit and a set voltage Vs different from the constant voltage Vc.

Therefore, if the switching element of the voltage switching circuit is in the off state, the constant voltage Vc from the constant voltage circuit becomes
A voltage that is output from the output circuit to the oxygen concentration sensor and, when the switching element is in the ON state, a voltage obtained by dividing the differential pressure between the set voltage Vs and the constant voltage Vc by a voltage dividing resistor and an output resistor. Vb is output from the output circuit to the oxygen concentration sensor.

Therefore, the applied voltage changing means changes the output voltage of the voltage applying means (that is, the applied voltage to the oxygen concentration sensor) by turning on or off the switching element of the voltage switching circuit. For example, when the constant voltage Vc from the constant voltage circuit is applied to the oxygen concentration sensor as the oxygen concentration detection voltage, the switching element is turned on from the off state to reduce the voltage applied to the oxygen concentration sensor. It can be changed to the divided voltage Vb. Conversely, when the divided voltage Vb is applied to the oxygen concentration sensor as the oxygen concentration detection voltage, the switching element is turned off from the on state, so that the voltage applied to the oxygen concentration sensor is reduced. Constant voltage Vc
Can be changed to

According to the element impedance detecting device of the first aspect, the applied voltage changing means changes the output voltage of the voltage applying means (ie, the operation of turning on or off the switching element). Is performed, the output voltage of the voltage application means changes immediately, so that the second current detection means reliably detects the sensor current when a predetermined time has elapsed since the change in the voltage applied to the oxygen concentration sensor. can do. Therefore, according to the element impedance detection device, the voltage applied to the oxygen concentration sensor can be changed without delay, and the element impedance of the sensor can be accurately detected despite its simple configuration. Become.

According to a second aspect of the present invention, there is provided the device for detecting the element impedance of the oxygen concentration sensor according to the first aspect, wherein the voltage applying means includes two of the voltage switching circuits. Then, the applied voltage changing means turns on / off the switching element of each voltage switching circuit, thereby changing the output voltage of the voltage applying means to both positive and negative sides.

Therefore, according to the element impedance detecting device of the second aspect, the air-fuel ratio detecting device 1 of FIG.
As described with reference to FIG. 11 for 00, the sensor current after detecting the element impedance of the oxygen concentration sensor can be stabilized early.

By the way, in the element impedance detecting device according to the second aspect, for example, when the set voltage Vs to which the two voltage switching circuits are connected is the same voltage,
The output voltage of the voltage applying means can be switched among three types as shown in the following (A) to (C). Note that, here, the switching element and the voltage-dividing resistor constituting one voltage switching circuit are Sa and Ra, respectively, and the switching element and the voltage-dividing resistor constituting the other voltage switching circuit are respectively represented by: Sb and Rb.

(A) If both the switching elements Sa and Sb are turned off, the constant voltage Vc from the constant voltage circuit is output from the output circuit to the oxygen concentration sensor. (B) If only one of the switching elements Sa and Sb (for example, Sa) is turned on, the set voltage V
A voltage Vb1 obtained by dividing the differential pressure between s and the constant voltage Vc by the voltage dividing resistor Ra and the output resistor is output from the output circuit to the oxygen concentration sensor.

(C) If both the switching elements Sa and Sb are turned on, the differential pressure between the set voltage Vs and the fixed voltage Vc is divided by the combined resistance of the two voltage dividing resistors Ra and Rb and the output resistor. The compressed voltage Vb2 is output from the output circuit to the oxygen concentration sensor. Therefore, in this case, each of the switching elements Sa and Sb is normally driven as in the above (B), and the divided voltage Vb1 is applied to the oxygen concentration sensor as the oxygen concentration detection voltage. When detecting the element impedance of the sensor, the on / off state of each switching element Sa, Sb is changed from (B) → (A) →
By switching in the order of (C) or in the order of (B) → (C) → (A), the output voltage of the voltage applying unit can be changed from the oxygen concentration detection voltage to both positive and negative sides.

However, in this case, one of the two switching elements Sa and Sb must be turned on during a period in which the element impedance of the oxygen concentration sensor is not detected, and power consumption increases. There are also disadvantages. Therefore, in the device impedance detecting device of the oxygen concentration sensor according to the third aspect, in the device according to the second aspect, first, the constant voltage circuit of the voltage applying means includes an oxygen concentration detecting voltage (a voltage in the gas to be detected). A voltage for detecting the oxygen concentration) is output as the constant voltage Vc.

The set voltage Vs to which one of the two voltage switching circuits is connected is set to a voltage higher than the constant voltage Vc, and the other voltage switching circuit is connected. The set voltage Vs is set to a voltage lower than the constant voltage Vc. And furthermore
The applied voltage changing means changes the output voltage of the voltage applying means from the constant voltage Vc to the positive side by turning on a switching element of the voltage switching circuit connected to a voltage higher than the constant voltage Vc. By turning on the switching element of the voltage switching circuit connected to a voltage lower than Vc, the output voltage of the voltage
Change from c to the negative side.

According to the element impedance detecting device of the third aspect, by switching the two switching elements and sequentially turning them on, the output voltage of the voltage applying means is changed to the oxygen concentration detection voltage (constant voltage Vc). Can be changed to both positive and negative sides. Then, during a period in which the element impedance of the oxygen concentration sensor is not detected,
Since it is only necessary to turn off both switching elements,
There is no increase in power consumption.

On the other hand, in the oxygen concentration detecting device according to the fourth aspect, the first voltage applying means applies the first voltage to one terminal of the oxygen concentration sensor, and the second voltage applying means applies the first voltage to the oxygen concentration sensor. A second voltage different from the first voltage is applied to the other terminal. That is, a differential pressure between the first voltage and the second voltage is applied to both ends of the oxygen concentration sensor as an oxygen concentration detection voltage.

The applied voltage changing means changes the output voltage of the first voltage applying means in order to detect the element impedance of the oxygen concentration sensor. Before performing the operation of changing the output voltage, the first current detecting means detects a current flowing through the oxygen concentration sensor. Further, the oxygen concentration calculating means calculates the oxygen concentration in the detected gas based on the current detected by the first current detecting means.

Further, when a predetermined time has passed since the applied voltage changing means performed the operation of changing the output voltage of the first voltage applying means, the second current detecting means is provided with:
The current flowing to the oxygen concentration sensor is detected. Then, the impedance calculating means calculates the element impedance of the oxygen concentration sensor based on the difference between the current detected by the first current detecting means and the current detected by the second current detecting means.

That is, in this oxygen concentration detecting device, the oxygen concentration in the gas to be detected is calculated based on the sensor current at the time of normal application of the differential pressure between the first voltage and the second voltage to the oxygen concentration sensor. Further, similarly to the air-fuel ratio detection device 100 of FIG. 10, the difference between the sensor current before changing the applied voltage to the oxygen concentration sensor and the sensor current when a predetermined time has elapsed after changing the applied voltage. The element impedance is calculated based on.

In this case, in particular, in the oxygen concentration detecting device according to the fourth aspect, the first voltage applying means converts the voltage input to the input section into the oxygen concentration as in the element impedance detecting device according to the first aspect. An output circuit for outputting to the sensor,
A constant voltage circuit that outputs a predetermined constant voltage to an input section of the output circuit via an output resistor; a signal path from the output resistor to the input section of the output circuit; and a set voltage different from the constant voltage. And a voltage switching circuit composed of a switching element and a voltage-dividing resistor connected in series. Then, the applied voltage changing unit changes the output voltage of the first voltage applying unit by turning on or off the switching element of the voltage switching circuit.

Thus, according to the oxygen concentration detecting device of the fourth aspect, similarly to the element impedance detecting device of the first aspect, the operation of the applied voltage changing means changing the output voltage of the first voltage applying means ( That is, if the switching element is turned on or off), the output voltage of the first voltage applying means changes immediately. Therefore, the second current detecting means changes the applied voltage to the oxygen concentration sensor. , It is possible to reliably detect the sensor current when a predetermined time has elapsed. Therefore, according to this oxygen concentration detection device, the element impedance of the oxygen concentration sensor can be accurately detected by changing the applied voltage to the oxygen concentration sensor without delay despite its simple configuration. In other words, heater control for maintaining the oxygen concentration sensor in the active state and determination of the degree of deterioration of the sensor can be accurately performed.

Next, in the oxygen concentration detecting device according to the fifth aspect, in the oxygen concentration detecting device according to the fourth aspect, three voltage generation devices connected in series between a predetermined power supply voltage and a ground potential are sequentially connected. A resistor is provided, and the three voltage generating resistors generate the first voltage and the second voltage by dividing the power supply voltage.

The constant voltage circuit of the first voltage applying means receives the first voltage generated by the voltage generating resistor, and outputs the first voltage as the constant voltage.
The applied voltage changing means changes the output voltage of the first voltage applying means from the first voltage by turning on a switching element of the voltage switching circuit.

Further, the second voltage applying means receives the second voltage generated by the voltage generating resistor and outputs the second voltage to the other terminal of the oxygen concentration sensor.
According to the oxygen concentration detecting device of the fifth aspect, the first voltage output to each terminal of the oxygen concentration sensor and the second voltage
Since the voltage generation source is three resistors (voltage generation resistors) connected in series, for example, even if the ambient temperature of the device changes, the oxygen concentration set in advance for the oxygen concentration sensor The detection voltage can be stably applied, and the oxygen concentration in the gas to be detected can be accurately detected. That is, since the resistance value of each voltage generating resistor changes in the same tendency with respect to the temperature change, the ratio of each resistance value does not largely change, and as a result, the first signal output to each terminal of the oxygen concentration sensor is changed. This is because the voltage and the second voltage do not change.

Next, in the oxygen concentration detecting device according to a sixth aspect, in the oxygen concentration detecting device according to the fifth aspect, the first voltage applying means includes two of the voltage switching circuits. The set voltage to which one of the two voltage switch circuits is connected is set to a voltage higher than the first voltage, and the set voltage to which the other voltage switch circuit is connected is , Is set to a voltage lower than the first voltage. Further, the applied voltage changing means turns on / off the switching element of each voltage switching circuit,
The output voltage of the first voltage applying means is changed from the first voltage to both positive and negative sides.

According to the oxygen concentration detecting device of the sixth aspect, similarly to the device impedance detecting device of the second aspect, the sensor current after detecting the element impedance of the oxygen concentration sensor can be quickly changed. Can be stabilized. Further, similarly to the above-described element impedance detecting device of the third aspect, by switching two switching elements and sequentially turning them on, the output voltage of the first voltage applying means can be changed from the first voltage to both positive and negative sides. ,
During a period in which the element impedance of the oxygen concentration sensor is not detected, both switching elements may be turned off, so that power consumption does not increase.

On the other hand, in the oxygen concentration detecting device according to claim 7, in the oxygen concentration detecting device according to claim 4, the second voltage applying means is configured so that its output voltage can be arbitrarily changed. According to the oxygen concentration detecting device of the seventh aspect, the operating region of the oxygen concentration sensor can be optimized by arbitrarily changing the oxygen concentration detecting voltage applied to the oxygen concentration sensor. Become.

When the oxygen concentration detecting voltage is changed, the second voltage applying means is not required to be as quick as in detecting the element impedance.
2 can be configured as a main part. Next, in the oxygen concentration detecting device according to claim 8, in the oxygen concentration detecting device according to claim 7, the first voltage applying unit includes the two voltage switching circuits, and the applied voltage changing unit includes By turning on / off the switching element of each voltage switching circuit, the output voltage of the first voltage applying means is changed to both positive and negative sides.

According to the oxygen concentration detecting device of the eighth aspect, the sensor current after detecting the element impedance of the oxygen concentration sensor can be stabilized early, as in the element impedance detecting device of the second aspect. Will be able to do that. Further, in the oxygen concentration detecting device according to the ninth aspect, in the oxygen concentration detecting device according to the eighth aspect, first, the constant voltage circuit of the first voltage applying means includes the first voltage applying means.
A voltage is output as the constant voltage.

The set voltage to which one of the two voltage switching circuits is connected is the first voltage switching circuit.
The voltage is set higher than the voltage, and the set voltage to which the other voltage switching circuit is connected is set to a voltage lower than the first voltage. Further, the applied voltage changing means changes the output voltage of the first voltage applying means from the first voltage to the positive side by turning on a switching element of a voltage switching circuit connected to a voltage higher than the first voltage. Further, by turning on the switching element of the voltage switching circuit connected to a voltage lower than the first voltage, the output voltage of the first voltage applying means is changed from the first voltage to the negative side.

According to the oxygen concentration detecting device of the ninth aspect, similar to the device impedance detecting device of the third aspect, the first voltage application means is switched by sequentially switching on two switching elements. Can be changed from the first voltage to both positive and negative sides, and both switching elements may be turned off during a period in which the element impedance of the oxygen concentration sensor is not detected, which may increase power consumption. Absent.

[0066]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An air-fuel ratio detecting device according to an embodiment of the present invention will be described below with reference to the drawings.
The air-fuel ratio detection device of the present embodiment is used for an electronically controlled gasoline injection engine mounted on an automobile. In the air-fuel ratio control system of the engine, the engine is controlled based on the detection result by the air-fuel ratio detection device. Is controlled to a desired air-fuel ratio. In the following description, a configuration and processing for detecting the air-fuel ratio using the air-fuel ratio sensor and detecting the element impedance (AC impedance) of the sensor will be described in detail.

FIG. 1 is a circuit diagram showing the configuration of the air-fuel ratio detecting device 1 according to the first embodiment. In FIG. 1, the same members as those of the air-fuel ratio detection device 100 of FIG. 10 described above are denoted by the same reference numerals. As shown in FIG. 1, a limiting current type air-fuel ratio sensor AFS as an oxygen concentration sensor is connected to the air-fuel ratio detection device 1, and the air-fuel ratio sensor AFS is an exhaust pipe extending from an engine body (not shown). 10 (see FIG. 2). And
A current proportional to the oxygen concentration in the exhaust gas flows through the air-fuel ratio sensor AFS with the application of the voltage.

FIG. 2 is a sectional view showing the structure of the air-fuel ratio sensor AFS. As shown in FIG. 2, the air-fuel ratio sensor AFS protrudes toward the inside of the exhaust pipe 10. The sensor AFS is roughly divided into a cover 21, a sensor body 22, and a heater 23. . The cover 21 has a U-shaped cross section, and a plurality of small holes 21a communicating with the inside and outside of the cover are formed in the peripheral wall.
The sensor body 22 generates a limit current corresponding to the oxygen concentration in the air-twist ratio lean region or the unburned gas (CO, HC, H2, etc.) concentration in the air-fuel ratio rich region.

The structure of the sensor main body 22 will be described in detail. First, in the sensor main body 22, the exhaust gas side electrode layer 26 is fixed to the outer surface of the solid electrolyte layer 24 formed in a cup-shaped cross section, Has an atmosphere-side electrode layer 27 fixed thereto. A diffusion resistance layer 25 is formed outside the exhaust gas side electrode layer 26 by a plasma spraying method or the like.

The solid electrolyte layer 24 is made of ZrO 2, HfO 2
, ThO2, Bi2O3, etc. to CaO, MgO, Y2O3,
The diffusion resistance layer 25 is made of an oxygen ion conductive oxide sintered body in which Yb2O3 or the like is dissolved as a stabilizer.
It consists of heat-resistant inorganic substances such as magnesia, siliceous, spinel, and mullite. Both the exhaust gas side electrode layer 26 and the atmosphere side electrode layer 27 are made of a noble metal having high catalytic activity such as platinum, and the surfaces thereof are subjected to porous chemical plating or the like. The area and thickness of the exhaust gas side electrode layer 26 are as follows:
10 to 100 square millimeters and about 0.5 to 2.0 micrometers. The area and thickness of the atmosphere-side electrode layer 27 are 10 square millimeters or more and 0.5 to 2.0 millimeters.
It is about 2.0 micrometers.

On the other hand, the heater 23 is accommodated in the atmosphere-side electrode layer 27, and the heat generated by the heater 23 causes the sensor body 22 (the atmosphere-side electrode layer 27, the solid electrolyte layer 24, the exhaust gas-side electrode layer 26, and the diffusion resistance). Heat layer 25). still,
The heater 23 has a heat generating capacity sufficient to activate the sensor main body 22.

In the air-fuel ratio sensor AFS having the above configuration, the sensor main body 22 generates a limit current corresponding to the oxygen concentration in a lean region from the stoichiometric air-fuel ratio point. In this case, the limit current corresponding to the oxygen concentration is determined by the area of the exhaust gas side electrode layer 26, the thickness of the diffusion resistance layer 25, the porosity, and the average pore diameter.

The sensor body 22 is capable of detecting the oxygen concentration with a linear characteristic.
A high temperature of about 600 ° C. or more is required to activate the fuel cell 2, and the active temperature range of the sensor main body 22 is narrow. Therefore, the active area cannot be controlled by heating only the exhaust gas of the engine. Therefore, in the present embodiment, the heater 23
By the heating control described above, the sensor body 22 is heated to the activation temperature range. In the region richer than the stoichiometric air-fuel ratio,
The concentration of unburned gas such as carbon monoxide (CO) changes almost linearly with the air-fuel ratio, and the sensor body 22 generates a limiting current according to the concentration of CO and the like.

The voltage-current characteristics of the sensor body 22 will be described with reference to FIG. As shown in FIG. 3, in the air-fuel ratio sensor AFS, a current flowing into the solid electrolyte layer 24 of the sensor body 22 according to the detected air-fuel ratio (A / F) and a voltage applied to the solid electrolyte layer 24 are different. It turns out that it has a linear characteristic. In this case, the straight line parallel to the voltage axis V specifies the limit current of the sensor body 22, and the increase / decrease of this limit current (sensor current) depends on the increase / decrease of the air-fuel ratio (ie,
Lean rich). That is, the limit current increases as the air-fuel ratio becomes leaner, and decreases as the air-fuel ratio becomes richer.

In the voltage-current characteristics of FIG. 3, the voltage range smaller than the linear portion parallel to the voltage axis V is the resistance dominant region, and the slope of the primary linear portion in the resistance dominant region is the sensor main body. It is specified by the internal impedance of the solid electrolyte layer 24 at 22 (this is called element impedance). Since the element impedance changes with a change in temperature, when the temperature of the sensor main body 22 decreases, the slope decreases due to an increase in the element impedance.

Next, the air-fuel ratio detecting device 1 to which the above-described air-fuel ratio sensor AFS is connected, as shown in FIG.
1 includes a microcomputer 2 having a built-in A / D converter.
The input terminal of the A / D converter as in the case of the air-fuel ratio detection device 100 of 0. On the other hand, the other two terminals PB20 and PB21 of the microcomputer 2 are terminals of a normal output port.

Further, in the air-fuel ratio detecting device 1, similarly to the air-fuel ratio detecting device 100 of FIG. 10, the inverting input terminal is connected to the plus side terminal of the air-fuel ratio sensor AFS via the resistor R 2 (that is, the atmospheric air of FIG. 2). Terminal connected to the side electrode layer 27) AF +
Operational amplifier OP1 connected to the air-fuel ratio sensor AFS
A shunt resistor R3 connected between the plus side terminal AF + of the operational amplifier OP1 and the non-inverting input terminal is connected to the plus side terminal AF + of the air-fuel ratio sensor AFS. Are commonly connected, an input circuit 4 that inputs the output of the operational amplifier OP2 via a resistor R4 and outputs the output to a terminal ch11 of the microcomputer 2 as it is, and a non-inverting input terminal is an operational amplifier OP
An operational amplifier OP3 connected to the output terminal of the first operational amplifier 1 and having an inverting input terminal and an output terminal commonly connected to each other;
And an input circuit 6 for inputting the output of the microcomputer 2 via the resistor R5 and outputting the output to the terminal ch12 of the microcomputer 2 as it is.

The input circuits 4 and 6 correspond to the input circuits 110 and 112 in FIG. And
As shown in a dotted line in FIG. 1, the input circuit 4 includes an operational amplifier OP6 as a buffer having an inverting input terminal and an output terminal commonly connected, and an output terminal of the operational amplifier OP6 and a ground potential (GND: 0 V). , A resistor R18 having one end connected to the output terminal of the operational amplifier OP6, and one end connected to an end of the resistor R19 opposite to the operational amplifier OP6. Resistor R20, an anode is connected to a connection point between the resistor R19 and the resistor R20, and a cathode is connected to a power supply voltage VCC (5V in the present embodiment).
1 and a noise removing capacitor C4 connected between the end of the resistor R20 opposite to the resistor R19 and the ground potential. Further, in the input circuit 4, the non-inverting input terminal of the operational amplifier OP6 is connected to the output terminal of the operational amplifier OP2 via the resistor R4, and the connection point between the resistor R20 and the capacitor C4 is connected to the terminal ch11 of the microcomputer 2.
It is connected to the. And the input circuit 6 is also the input circuit 4
The circuit configuration is exactly the same.

Further, the air-fuel ratio detecting device 1 of the first embodiment
In the same manner as in the air-fuel ratio detection device 100 of FIG.
A capacitor C1 is connected between the non-inverting input terminal of the operational amplifier OP1 and the ground potential.
Constitutes a low-pass filter together with resistors R15, R16 and R17 to be described later.

Here, in particular, the air-fuel ratio detecting device 1 of the first embodiment has a power supply voltage VCC (= 5 V) and a ground potential (= 0).
V), three resistors R10, R as three voltage generating resistors for generating a first voltage V1 and a second voltage V2 by dividing the power supply voltage VCC.
11 and R12. In this embodiment,
The resistance values of the resistors R10, R11, R12 are respectively
It is set to 2.21 kΩ, 390 Ω, and 3.9 kΩ. Therefore, the first voltage V1 generated at the connection point between the resistor R10 and the resistor R11 is 3.3V, and the second voltage V2 generated at the connection point between the resistor R11 and the resistor R12 is 3.0.
V.

In the air-fuel ratio detecting device 1, the non-inverting input terminal is connected to the connection point between the resistors R10 and R11, and the inverting input terminal and the output terminal are commonly connected.
An operational amplifier OP5 as a constant voltage circuit that outputs the first voltage V1 (= 3.3 V) from its output terminal, and is connected between an output terminal of the operational amplifier OP5 and a non-inverting input terminal of the operational amplifier OP1. A resistor R15 as an output resistor, an NPN transistor Tr1 as a switching element having an emitter connected to the ground potential (<V1) and a base connected to the terminal PB21 of the microcomputer 2.
And one end is connected to the collector of the transistor Tr1,
The other end is connected to a signal path from the resistor R15 to the non-inverting input terminal of the operational amplifier OP1 as a resistor R16 as a voltage dividing resistor, the emitter is connected to the power supply voltage VCC (> V1), and the base is connected to the microcomputer. PNP transistor Tr2 as a switching element connected to the second terminal PB20, and one end connected to the collector of the transistor Tr2, and the other end connected to a signal path from the resistor R15 to the non-inverting input terminal of the operational amplifier OP1. And a resistor R17 as a voltage dividing resistor.

Further, in the air-fuel ratio detecting device 1, the non-inverting input terminal is connected to the connection point between the resistors R11 and R12, and the output terminal is connected to the negative side of the air-fuel ratio sensor AFS via the resistor R13. The terminal (that is, the terminal connected to the exhaust gas side electrode layer 26 in FIG. 2) AF−, and the inverting input terminal is connected to the above-mentioned minus side terminal AF− of the air-fuel ratio sensor AFS via a resistor R14. Operational amplifier OP
4 is provided.

The air-fuel ratio detecting device 1 has an operational amplifier O
A noise removing capacitor C2 connected between the non-inverting input terminal of P4 and the ground potential, a noise removing capacitor C3 connected between the non-inverting input terminal of the operational amplifier OP5 and the ground potential, A capacitor C5 connected between the signal path from the resistor R13 to the negative terminal AF- of the air-fuel ratio sensor AFS and the ground potential, and a signal path from the shunt resistor R3 to the positive terminal AF + of the air-fuel ratio sensor AFS. Is connected to a diode D2 having an anode connected to the battery voltage VB (normally 12 V).
And a cathode connected to a signal path from the shunt resistor R3 to the positive terminal AF + of the air-fuel ratio sensor AFS,
A diode D3 whose anode is connected to the ground potential. The capacitor C5 and the two diodes D2 and D3 are connected to the air-fuel ratio detection device 1 and the air-fuel ratio sensor A.
It is provided to prevent a high voltage surge or static electricity from entering the inside of the device 1 from a signal line connecting the FS.

The air-fuel ratio detecting device 1 configured as described above
In the above, the output circuit composed of the operational amplifier OP1, the resistor R2, and the shunt resistor R3 converts the voltage Vo input to the non-inverting input terminal of the operational amplifier OP1 as its input part into the positive side of the air-fuel ratio sensor AFS. Output to terminal AF +.

Therefore, the two transistors Tr1, Tr
2 are both in the OFF state, the first voltage V1 (= 3.3 V) output from the operational amplifier OP5 is input to the non-inverting input terminal of the operational amplifier OP1, so that the air-fuel ratio sensor AFS
The first voltage V1 is applied to the plus side terminal AF +.

The two transistors Tr1 and Tr2
If only the transistor Tr1 is in the ON state, the differential pressure between the first voltage V1 and the ground potential (that is, the first voltage V1)
Is divided by a resistor R15 and a resistor R16 (V1
−ΔVa) is input to the non-inverting input terminal of the operational amplifier OP1, so that the positive terminal AF + of the air-fuel ratio sensor AFS
Is applied with the divided voltage (V1−ΔVa) lower than the first voltage V1 by ΔVa.

On the contrary, the two transistors Tr1 and Tr2
If only the transistor Tr2 is in the ON state, the voltage (V1 + △) obtained by dividing the differential pressure (VCC-V1) between the power supply voltage VCC and the first voltage V1 by the resistors R17 and R15.
Vb) is input to the non-inverting input terminal of the operational amplifier OP1, so that the divided voltage (V1 + ΔVb) higher by ΔVb than the first voltage V1 is applied to the plus terminal AF + of the air-fuel ratio sensor AFS. Is done.

In this embodiment, each resistor R15, R
The resistance values of R16 and R17 are 200Ω and 3.09k, respectively.
Ω and 1.5 kΩ. Therefore, the divided voltage (V1− △ Va) becomes 3.1V, and the divided voltage (V1 + △ Vb) becomes 3.5V. That is, both ΔVa and ΔVb are 0.2V.

On the other hand, the operational amplifier OP4 and the resistors R13 and R13 are connected to the negative terminal AF− of the air-fuel ratio sensor AFS.
14, the second voltage V2 (= 3.0 V) input to the non-inverting input terminal of the operational amplifier OP4
Is always applied. Therefore, two transistors Tr
Normally, when both Tr1 and Tr2 are off, the differential pressure between the first voltage V1 and the second voltage V2 generated by the three resistors R10, R11, R12 (V1-V2 = 0.3V).
Is applied to both ends of the air-fuel ratio sensor AFS as a voltage (oxygen concentration detection voltage) for detecting the air-fuel ratio (oxygen concentration in exhaust gas) of the air-fuel mixture. Then, with the application of the voltage, a current flows through the air-fuel ratio sensor AFS according to the oxygen concentration in the exhaust gas at that time.

Also, in the air-fuel ratio detection device 1 of this embodiment, the same current as the current (sensor current) I flowing to the air-fuel ratio sensor AFS is supplied to the shunt resistor R3, as in the air-fuel ratio detection device 100 of FIG. Therefore, the potential difference between both ends of the shunt resistor R3 has a value proportional to the sensor current I. Further, the same voltage Vo as the voltage Vo applied to the plus terminal AF + of the air-fuel ratio sensor AFS (that is, the voltage of the shunt resistor R3 on the air-fuel ratio sensor AFS side) Vo is supplied to the microcomputer 2 by the operational amplifier OP2 and the input circuit 4. Terminal ch
11 and the voltage on the opposite side of the shunt resistor R3 from the air-fuel ratio sensor AFS (ie, the operational amplifier OP1).
(The output voltage of the microcomputer 2) is input to the terminal ch12 of the microcomputer 2 by the operational amplifier OP3 and the input circuit 6.

For this reason, the air-fuel ratio detecting device 1 of this embodiment
Then, the microcomputer 2 periodically executes the detection process shown in FIG. 4 to detect the sensor current I flowing through the air-fuel ratio sensor AFS to obtain the air-fuel ratio (oxygen concentration in the exhaust gas) of the air-fuel mixture. The element impedance of the air-fuel ratio sensor AFS is detected by a procedure similar to the procedure shown in FIG. In the present embodiment, the times corresponding to the times T1, T2, T3, and T4 shown in FIG.
They are set to 128 ms, 135 μs, 200 μs, and 200 μs.

The detection process executed by the microcomputer 2 of the air-fuel ratio detection device 1 will be described below with reference to the flowchart of FIG. Note that the detection processing in FIG. 4 is executed every 4 ms. The terminals PB20 and PB20 of the microcomputer 2
21 has an initial output level of two transistors Tr1 and Tr2.
The terminal PB20 is at a high level (5V) and the terminal PB21 is at a low level (0V) so that both Tr2s are turned off.

As shown in FIG. 4, when the microcomputer 2 starts executing the detection processing, first, a step (hereinafter simply referred to as “S”) is performed.
At 110, the voltage V input to the terminal ch11
o, and the detected voltage Vo is applied to the aforementioned Vo (t1).
To be stored. Next, in S120, the voltage Vi input to the terminal ch12 is detected, and the detected voltage Vi is stored as the above-mentioned Vi (t2).

Then, at S130, the voltage Vi (t2) detected at S120 and the voltage Vo detected at S110 are output.
The current value (limit current value) of the sensor current I is calculated by dividing the difference (Vi (t2) -Vo (t1)) from (t1) by the shunt resistance value (resistance value of the shunt resistor R3) RS. Further, the air-fuel ratio (oxygen concentration in the exhaust gas) of the air-fuel mixture is calculated using the current value and a characteristic map stored in the ROM in the microcomputer 2 in advance.

Next, at S140, the RA
The value of the counter CT set to M is equal to a predetermined time T1 (= 128 ms) which is a detection cycle of the element impedance.
Is determined to be “31” corresponding to
If the value of T is not "31", the value of the counter CT is incremented by 1 in the following S150, and the detection process is terminated.

On the other hand, if it is determined in step S140 that the value of the counter CT is "31", it is determined that the detection timing of the element impedance has arrived, and the flow shifts to step S160, where the voltage Vi ( It is determined whether or not t2) is greater than a predetermined reference voltage. The reference voltage is the output of the operational amplifier OP1 when a current of a predetermined value (for example, a central value) within the dynamic range (current detectable region) of the air-fuel ratio sensor AFS shown in FIG. 3 flows through the shunt resistor R3. The voltage is set to Vi.

If it is determined in step S160 that the voltage Vi (t2) is higher than the reference voltage (that is, if the current sensor current I is higher than the predetermined value within the dynamic range), the process proceeds to step S170. Then, as shown at time t3 in FIG. 5A, the output level of the terminal PB21 is changed from the low level to the high level to turn on the transistor Tr1, and thereafter, the process proceeds to S190. Then, FIG.
As shown at time t3 in (a), the voltage Vo applied to the plus terminal AF + of the air-fuel ratio sensor AFS is a voltage (= 3) lower than the first voltage V1 by ΔVa (= 0.2 V) described above. .1V), and accordingly, the input voltage Vi to the terminal ch12 (the output voltage of the operational amplifier OP1) Vi and the sensor current I also change to the negative side.

If it is determined in step S160 that the voltage Vi (t2) is not higher than the reference voltage (that is, if the current sensor current I is not higher than the predetermined value within the dynamic range), the process proceeds to step S180. Figure 5
As shown at time t3 in (b), the output level of the terminal PB20 is changed from high to low,
r2 is turned on, and then the process proceeds to S190. Then, as shown at time t3 in FIG. 5B, the air-fuel ratio sensor AFS
Changes to a voltage (= 3.5 V) higher than the first voltage V1 by the above-mentioned ΔVb (= 0.2 V), and accordingly, the terminal ch12
The input voltage Vi and the sensor current I also change to the positive side.

In S190, the above S170 or the above S
180, the change in the sensor current I ΔI
Is expected to peak, T2 (= 135 μs)
It is determined whether or not has elapsed, and it waits until the time T2 has elapsed. When it is determined that the time T2 has elapsed, the process proceeds to S200, where the voltage Vi input to the terminal ch12 is detected, and the detected voltage Vi is referred to as the aforementioned Vi.
(t4). Further, in S210, the voltage Vo input to the terminal ch11 is detected, and the detected voltage Vo is stored as Vo (t5).

Then, in the subsequent S220, the above S11
0, Vo (t1), Vi (t2), Vi (t4) and Vo (t5) detected this time in S120, S200 and S210.
Then, the element impedance Z of the air-fuel ratio sensor AFS is calculated from the shunt resistance value RS and the above equation (1).

Note that the element impedance Z calculated in S220 is used at least for heating control of the heater 23 provided in the air-fuel ratio sensor AFS. That is, the microcomputer 2 executes a heating control process (not shown),
The amount of power to the heater 23 necessary to eliminate the difference between the element impedance Z calculated in S220 and the target element impedance which is considered to be sufficient for activation of the air-fuel ratio sensor AFS is determined. And heater 2
The duty control of the current supplied to 3 is performed via a heater drive circuit (not shown).

Next, in the following S230, the microcomputer 2
After performing the processing of S170 or S180,
It is determined whether or not a predetermined time T3 (= 200 μs) has elapsed, and the process waits until the time T3 has elapsed. When it is determined that the time T3 has elapsed, the process proceeds to S240, and it is determined whether or not the voltage Vi (t2) detected in S120 is higher than the reference voltage, just as in S160.

If it is determined in step S240 that the voltage Vi (t2) is higher than the reference voltage, the process proceeds to step S25.
0, as shown at time t6 in FIG.
21 from the high level to the low level to turn off the transistor Tr1,
The output level is changed from the high level to the low level to turn on the transistor Tr2. Thereafter, the flow shifts to S270. Then, as shown at time t6 in FIG. 5A, the voltage Vo applied to the plus side terminal AF + of the air-fuel ratio sensor AFS is higher than the first voltage V1 by ΔVb (= 0.2
V), the input voltage Vi to the terminal ch12 and the sensor current I also change to the positive side.

If it is determined in step S240 that the voltage Vi (t2) is not higher than the reference voltage, the process proceeds to step S260.
Then, as shown at time t6 in FIG. 5 (b), the output level of the terminal PB20 is returned from the low level to the high level to turn off the transistor Tr2, and the terminal PB20 is turned off.
The output level of the transistor 21 is changed from the low level to the high level to turn on the transistor Tr1, and thereafter, the process proceeds to S270. Then, as shown at time t6 in FIG. 5B, the voltage Vo applied to the plus terminal AF + of the air-fuel ratio sensor AFS is higher than the first voltage V1 by ΔVa (= 0.2
V), the input voltage Vi to the terminal ch12 and the sensor current I also change to the negative side.

In S270, the above-mentioned S250 or the above-mentioned S
A predetermined time T4 (= 200 μ
It is determined whether or not s) has elapsed, and the process waits until the time T4 has elapsed. When it is determined that the time T4 has elapsed, the process proceeds to S280, where the output level of the terminal PB21 is set to the initial low level and the transistor Tr1 is turned off, as shown at time t7 in FIGS. 5A and 5B. At the same time, the output level of the terminal PB20 is set to the initial high level to turn off the transistor Tr2. Then, FIG.
As shown at time t7 in (a) and (b), the voltage Vo applied to the plus terminal AF + of the air-fuel ratio sensor AFS returns to the original first voltage V1, and the input voltage Vi to the terminal ch12 and the sensor The current I also returns to the original state.

At last, at S290, the value of the counter CT is reset to "0", and thereafter, the detection processing is terminated. That is, in the detection processing of FIG.
By the processing of 0, S150, and S290, the predetermined time T1 (= 128 m
s), and every time it is determined that the predetermined time T1 has elapsed (S140: YES), the two transistors Tr1 and Tr2 are turned on by the processing of S160 to S180 and S230 to S280 as the applied voltage changing means. / Off, and the voltage Vo applied to the plus side terminal AF + of the air-fuel ratio sensor AFS is changed from the first voltage V1 to both positive and negative sides as shown in FIG. 5A or 5B.

Then, S1 as the first current detecting means
10 and S120, S170 or S18
Before changing the voltage Vo applied to the plus side terminal AF + in the process of 0, the sensor current I flowing through the air-fuel ratio sensor AFS is changed to each voltage Vi (t2), both ends of the shunt resistor R3,
It is detected as Vo (t1), and the predetermined time T2 (after the change of the voltage Vo applied to the plus side terminal AF + in the processing of S170 or S180 by the processing of S190 to S210 as the second current detection means. = 135 μs)
Has elapsed (S190: YES), the air-fuel ratio sensor A
The sensor current I flowing through the FS is detected as voltages Vi (t4) and Vo (t5) across the shunt resistor R3.

Further, by the processing of S220 as the impedance calculating means, S110, S120, S2
00 and Vo (t1) and Vi (t2) detected in S210.
), Vi (t4) and Vo (t5), ie, the sensor current "{Vi (t2) -Vo (t1)} before changing the voltage applied to the air-fuel ratio sensor AFS.
/ RS ”and the sensor current“ {Vi (t4) −Vo (t5)} / when a predetermined time T2 has elapsed since the applied voltage was changed.
The element impedance Z of the air-fuel ratio sensor AFS is calculated on the basis of the difference from "RS".

S130 as oxygen concentration calculating means
From the voltages Vi (t2) and Vo (t1) detected as the sensor current I in the processing of S110 and S120,
The limit current value of the air-fuel ratio sensor AFS is calculated, and the air-fuel ratio (oxygen concentration in exhaust gas) of the air-fuel mixture is calculated based on the current value.

In the first embodiment, a circuit portion including the resistors R10 to R12, R15 to R17, and R2, the shunt resistor R3, the operational amplifiers OP1 and OP5, and the transistors Tr1 and Tr2 in FIG. Corresponds to a first voltage applying means, and includes an operational amplifier OP4 in FIG.
A circuit portion including the resistors R13 and R14 corresponds to a second voltage applying unit. Further, a circuit portion obtained by combining a circuit portion corresponding to the first voltage applying device and a circuit portion corresponding to the second voltage applying device corresponds to the voltage applying device according to the first aspect. And the transistor Tr
1 and a resistor R16, and a circuit portion including a transistor Tr2 and a resistor R17, respectively.
It corresponds to a voltage switching circuit.

According to the air-fuel ratio detection device 1 of the first embodiment described in detail above, the microcomputer 2 includes the transistors Tr1 and T
If the operation of turning on / off r2 is performed, the air-fuel ratio sensor A
Since the voltage applied to the plus side terminal AF + of the FS changes immediately, the sensor current I when a predetermined time T2 has elapsed since the change in the applied voltage can be reliably detected. Therefore, according to the air-fuel ratio detection device 1, the voltage applied to the air-fuel ratio sensor AFS can be changed without delay to accurately detect the element impedance of the air-fuel ratio sensor AFS despite its simple configuration. The heater 23 for maintaining the air-fuel ratio sensor AFS in the active state
And the determination of the degree of deterioration of the air-fuel ratio sensor AFS can be accurately performed.

The air-fuel ratio detecting device 1 of the first embodiment
By turning on / off the two transistors Tr1 and Tr2, the voltage applied to the air-fuel ratio sensor AFS is changed to both positive and negative sides. Therefore, FIG.
As described with reference to FIG. 11 for the air-fuel ratio detection device 100, when the applied voltage is restored to the original voltage, the capacitance component (the grain boundary capacitance at the interface of the particles of the solid electrolyte layer 24) of the air-fuel ratio sensor AFS. Discharge at the electrode interface capacitance) in a short time, and as a result, the sensor current after detecting the element impedance of the air-fuel ratio sensor AFS can be stabilized at an early stage.

Further, in the air-fuel ratio detecting device 1 of the first embodiment, the emitter of the transistor Tr2 is connected to the operational amplifier O
Power supply voltage V higher than first voltage V1 output from P5
In addition to the connection to CC, the emitter of the transistor Tr1 is connected to a ground potential lower than the first voltage V1. Then, by turning on the transistor Tr2, the voltage applied to the plus side terminal AF + of the air-fuel ratio sensor AFS is changed to a positive side from the first voltage V1, and by turning on the transistor Tr1, the air-fuel ratio sensor AFS is turned on. The voltage applied to the plus side terminal AF + is changed to a more negative side than the first voltage V1.

Therefore, according to the air-fuel ratio detection device 1 of the first embodiment, the two transistors Tr1 and Tr2 are switched on and turned on sequentially as shown in FIG. The voltage applied to the terminal AF + can be changed to both positive and negative sides, and during the period in which the element impedance is not detected, both the transistors Tr1 and Tr2 may be turned off, which may increase power consumption. Absent.

This feature will be described in detail. For example, the transistor Tr2 may be changed to the same NPN transistor as the transistor Tr1, and the emitter of the transistor Tr2 may be connected to the ground potential similarly to the transistor Tr1. , Air-fuel ratio sensor AFS
Are applied to the plus side terminal AF + of
As shown in (c), three types can be switched.

(A) If both transistors Tr1 and Tr2 are turned off, the first voltage V1 from the operational amplifier OP5
Is output to the plus terminal AF + of the air-fuel ratio sensor AFS. (B) If only one of the transistors Tr1 and Tr2 (for example, Tr1) is turned on, the differential pressure between the first voltage V1 and the ground potential (that is, the first voltage V1) is applied to the resistor R1.
5 and the resistor R16, the voltage divided by the air-fuel ratio sensor A
Output to the plus side terminal AF + of the FS.

(C) If both transistors Tr1 and Tr2 are turned on, the voltage obtained by dividing the first voltage V1 by the resistor R15 and the combined resistance of the two resistors R16 and R17 becomes
It is output to the plus terminal AF + of the air-fuel ratio sensor AFS. Therefore, in this case, each transistor Tr1, T
r2 is normally driven as in (b) above, and when detecting the element impedance of the air-fuel ratio sensor AFS, the on / off state of each of the transistors Tr1 and Tr2 is
(B) → (a) → (c), or (b) → (c) →
By switching in the order of (a), the voltage applied to the plus terminal AF + of the air-fuel ratio sensor AFS can be changed from the normal voltage to both positive and negative sides. This means that the transistor Tr1 is changed to the same PNP transistor as the transistor Tr2, and the transistor T1
The same applies to the case where the emitter of r1 is connected to the power supply voltage VCC as in the case of the transistor Tr2.

However, when the above change is made, one of the two transistors Tr1 and Tr2 must be turned on during a period in which the element impedance is not detected, which is disadvantageous in that power consumption increases. There are also aspects. On the other hand, according to the air-fuel ratio detection device 1 of the first embodiment, an increase in power consumption can be eliminated.

Further, the air-fuel ratio detecting device 1 of the first embodiment
In order to detect the air-fuel ratio, the air-fuel ratio sensor AF
First voltage V1 (= 3.3 V) output to each terminal of S
And the second voltage V2 (= 3.0 V) are three resistors R10, R11 and R12 connected in series between the power supply voltage VCC and the ground potential.

For this reason, even if the ambient temperature of the air-fuel ratio detecting device 1 changes, it is necessary to stably apply the preset oxygen concentration detecting voltage (= 0.3 V) to the air-fuel ratio sensor AFS. Therefore, the air-fuel ratio can be accurately detected. That is, each of the resistors R10
Since the resistance values of R12 to R12 change in the same tendency with respect to the temperature change, the ratio of each resistance value does not largely change.
First voltage V1 output to each terminal of air-fuel ratio sensor AFS
And the second voltage V2 does not change.

On the other hand, in the air-fuel ratio detecting device 1 of the first embodiment, as shown in FIG. 5, when the sensor current at the time of detecting the air-fuel ratio is larger than a predetermined value within the dynamic range (S160: YES), the voltage applied to the air-fuel ratio sensor AFS is changed to a negative side (that is, a smaller value) (S
170), the element impedance is calculated from the current change accompanying the change in the applied voltage, and conversely, if the sensor current at the time of detecting the air-fuel ratio is not larger than the predetermined value (S16)
0: NO), the voltage applied to the air-fuel ratio sensor AFS is changed to the positive side (that is, the voltage increases) (S180), and the element impedance is calculated from the current change accompanying the change in the applied voltage. .

For this reason, when detecting the element impedance, it is possible to reliably prevent the applied voltage from changing in the direction out of the dynamic range, and to detect the sensor current without departing from the dynamic range. As a result, the element impedance can be accurately detected.

The air-fuel ratio detecting device 1 of the first embodiment has only one air-fuel ratio sensor AFS connected. However, in general, an engine (V-type six-cylinder engine) having two exhaust pipes is generally used. Or a V-type eight-cylinder engine), an air-fuel ratio sensor AFS is attached to each exhaust pipe. Therefore, in the case of an engine having two air-fuel ratio sensors AFS, two sets of circuit parts other than the microcomputer 2 shown in FIG. 1 are provided for each air-fuel ratio sensor AFS. Can be considered.

However, with such a configuration, the circuit scale becomes almost twice as large as that shown in FIG. Then, next, as a second embodiment, two air-fuel ratio sensors A
FIG. 6 shows a preferred configuration example of the air-fuel ratio detection device connected to the FS.
Shown in

FIG. 6 is a circuit diagram showing the configuration of the air-fuel ratio detection device 31 of the second embodiment. In FIG. 6, the same members and voltage signals as those of the air-fuel ratio detection device 1 of FIG. The same reference numerals are given. Also, in FIG. 6, members and voltage signals that have the same role as the air-fuel ratio detection device 1 of FIG. 1 and are added to the air-fuel ratio detection device 1 of FIG. Are added. Therefore, a detailed description of each member and the voltage signal is omitted here.

As shown in FIG. 6, the air-fuel ratio detecting device 31 according to the second embodiment is a circuit other than the microcomputer 2 shown in FIG. Portion is set to 2 corresponding to each of the air-fuel ratio sensors AFS and AFS '.
Has a pair. And the circuit part enclosed by the dotted line is
Circuits for each of the air-fuel ratio sensors AFS and AFS 'are shared. The terminal ch of the microcomputer 2
Reference numerals 21 and 22 are input terminals of the A / D converter, similarly to the terminals ch11 and ch12 described above.

Therefore, according to the air-fuel ratio detecting device 31 of the second embodiment, it is possible to reduce the size and cost of the circuit configuration. Next, an air-fuel ratio detection device according to a third embodiment will be described with reference to FIGS. First, FIG. 7 is a circuit diagram illustrating a configuration of the air-fuel ratio detection device 41 according to the third embodiment. Note that, in FIG. 7, the same members and voltage signals as those of the air-fuel ratio detection device 1 of FIG. 1 are denoted by the same reference numerals, and a detailed description thereof will be omitted.

As shown in FIG. 7, the air-fuel ratio detecting device 41 of the third embodiment is different from the air-fuel ratio detecting device 1 of the first embodiment.
The following four points are different from (FIG. 1). : Inside the microcomputer 2, not only the A / D converter but also a simple D / A converter as shown in FIG. The terminals DAC1 and D of the microcomputer 2
AC2 is an output terminal of the simple D / A converter.

The connection state of the air-fuel ratio sensor AFS is reversed. That is, the negative terminal AF− of the air-fuel ratio sensor AFS is connected to the connection point between the resistor R2 and the shunt resistor R3, and the positive terminal AF + of the air-fuel ratio sensor AFS is connected to the resistor R13 and the resistor R13. It is connected to the connection point with the device R14.

: Three resistors R10, R11, R12
Has been removed. The non-inverting input terminal of the operational amplifier OP4 is connected to the terminal D of the microcomputer 2 via the resistor R21.
The non-inverting input terminal of the operational amplifier OP5 is connected to the terminal DAC2 of the microcomputer 2 via the resistor R22.

The microcomputer 2 has a built-in simple D / A
The converter always outputs 3.0 V from the terminal DAC 2, and variably outputs a voltage higher than 3.0 V (normally 3.3 V) from the terminal DAC 1. In the third embodiment, the fixed voltage of 3.0 V output from the terminal DAC2 corresponds to the first voltage, and the voltage higher than 3.0V output from the terminal DAC1 is the second voltage. Is equivalent to

The microcomputer 2 executes the detection processing shown in FIG. 8 instead of the detection processing of FIG. Then, as shown in FIG. 8, in the detection processing executed in the third embodiment,
The only difference is that the process of S155 is added to the detection process of FIG. That is, in the detection process executed in the third embodiment, after the value of the counter CT is incremented by 1 in S150, the process proceeds to S155. Then, in S155, the optimum applied voltage to the air-fuel ratio sensor AFS is determined from the air-fuel ratio calculated this time in S130,
The output value of the voltage output from the terminal DAC1 is changed according to the determined applied voltage.

That is, the characteristic line L1 in FIG.
In the line that connects the center of the limit current range in each detected air-fuel ratio of the air-fuel ratio sensor AFS), the applied voltage corresponding to the air-fuel ratio calculated this time is set as the optimum applied voltage to be applied from the next time, and the applied voltage is The output voltage of DAC2 (3.
0V) is set as the output value from the terminal DAC1. Then, within a predetermined maximum delay time of the simple D / A converter in the microcomputer 2, the terminal DAC1
Will change to the output value set above.

After performing the processing of S155,
The detection processing ends. In the air-fuel ratio detection device 41 according to the third embodiment, the two transistors Tr
In a normal state in which both the first and Tr2 are off, the voltage (3.0 V) output from the terminal DAC2 of the microcomputer 2 is applied to the minus terminal AF− of the air-fuel ratio sensor AFS, and the terminal DAC1 of the microcomputer 2 The output voltage (> 3.0 V) is applied to the positive terminal A of the air-fuel ratio sensor AFS.
Applied to F +.

Also, with this air-fuel ratio detecting device 41, the microcomputer 2 turns on / off the transistors Tr1 and Tr2.
If the operation of turning off is performed, the applied voltage to the minus side terminal AF− of the air-fuel ratio sensor AFS changes immediately, so that the sensor current I when a predetermined time T2 has elapsed since the change of the applied voltage is surely obtained. By detecting, the element impedance of the air-fuel ratio sensor AFS can be accurately detected.

Particularly, according to the air-fuel ratio detecting device 41 of the third embodiment, as shown in FIG.
The voltage applied to the plus side terminal AF + of the FS is a simple D / A
Since it can be changed by the converter, S155 in FIG.
Processing, the plus side terminal AF of the air-fuel ratio sensor AFS
+, And the voltage applied to both ends of the air-fuel ratio sensor AFS is arbitrarily changed, and the air-fuel ratio sensor A
The operation area of the FS can be optimized.

When the applied voltage for detecting the air-fuel ratio is changed, quick response is not required as in the case where the element impedance is detected. Therefore, the voltage applied to the plus terminal AF + of the air-fuel ratio sensor AFS is not required. Is changed by the simple D / A converter, there is no problem.

On the other hand, in the third embodiment, the portion including the simple D / A converter in the microcomputer 2, the operational amplifier OP4, and the resistors R13, R14, R21 corresponds to the second voltage applying means. I have. Further, in the third embodiment, since the connection state of the air-fuel ratio sensor AFS is opposite to that of the first embodiment, the voltage Vi is determined in S160 described above.
If it is determined that (t2) is larger than the reference voltage, it means that the sensor current is not larger than a predetermined value within the dynamic range. However, in this case, S17
In the process of 0, the transistor Tr1 is turned on, and the voltage applied to the negative terminal AF- of the air-fuel ratio sensor AFS changes to the negative side. As a result, similarly to the first embodiment, the transistor Tr1 is turned on. Is changed to the positive side (that is, to the larger side). Similarly, the aforementioned S
If it is determined at 160 that the voltage Vi (t2) is not higher than the reference voltage, it means that the sensor current is higher than a predetermined value within the dynamic range. However, in this case, the transistor Tr2 is turned on by the processing of S180, and the negative terminal AF of the air-fuel ratio sensor AFS is turned on.
Since the applied voltage to the negative side changes to the positive side, the applied voltage to the air-fuel ratio sensor AFS eventually changes to the negative side (that is, the smaller one), as in the first embodiment.
Therefore, similarly to the air-fuel ratio detection device 1 of the first embodiment, the air-fuel ratio detection device 41 of the third embodiment can detect the sensor current without deviating from the dynamic range when detecting the element impedance. Can be.

As described above, one embodiment of the present invention has been described. However, the present invention is not limited to each of the above embodiments, and can adopt various aspects. For example, in the air-fuel ratio detection devices 1 and 31 of the first and second embodiments, when detecting the element impedance, the air-fuel ratio sensor AF
Although the voltage applied to the positive terminal AF + of S is changed, the voltage applied to the negative terminal AF- may be changed.

Similarly, in the air-fuel ratio detecting device 41 of the third embodiment, when detecting the element impedance, the voltage applied to the minus terminal AF- of the air-fuel ratio sensor AFS is changed. The configuration may be such that the voltage applied to the plus side terminal AF + is changed. In this case, the voltage applied to the minus side terminal AF- of the air-fuel ratio sensor AFS may be changed by a simple D / A converter.

In the air-fuel ratio detecting device 41 of the third embodiment, the fixed voltage (= 3.0 V) output from the terminal DAC2 of the microcomputer 2 is input to the non-inverting input terminal of the operational amplifier OP5. However, the non-inverting input terminal of the operational amplifier OP5 may be configured to input a fixed voltage generated by voltage division by a resistor, for example, instead of the terminal DAC2 of the microcomputer 2.

On the other hand, in the air-fuel ratio detecting devices 1, 31, and 41 of each of the above embodiments, two transistors Tr1 and R16 and two transistors Tr2 and R17 as voltage switching circuits are provided. The voltage applied to the AFS is changed to both positive and negative sides. By providing only one of them and turning on or off the transistor, the voltage applied to the air-fuel ratio sensor is changed only to the positive side or only the negative side. You may make it do.

In each of the above embodiments, the order of connection between the transistor Tr1 and the resistor R16 and the order of connection between the transistor Tr2 and the resistor R17 are shown in FIGS.
However, the present invention is not limited to the one shown in FIG. However, the connection order of the above-described embodiments is advantageous in that the transistors Tr1 and Tr2 can be turned on / off reliably and easily by a signal from the microcomputer 2.

On the other hand, the air-fuel ratio detection devices 1, 31, 41 of the above embodiments use the cup-type air-fuel ratio sensor AFS shown in FIG.
Although the air-fuel ratio is detected from the limit current flowing through the FS, the present invention provides a stacked air-fuel ratio sensor having two solid electrolyte layers forming a so-called pumping cell and a sensing cell as an oxygen concentration sensor. The same can be applied when used.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a configuration of an air-fuel ratio detection device according to a first embodiment.

FIG. 2 is a cross-sectional view illustrating a configuration of an air-fuel ratio sensor.

FIG. 3 is a graph showing voltage-current characteristics of an air-fuel ratio sensor.

FIG. 4 is a flowchart illustrating a detection process executed by a microcomputer provided in the air-fuel ratio detection device according to the first embodiment.

FIG. 5 is a time chart for explaining the operation of the detection processing of FIG. 4;

FIG. 6 is a circuit diagram illustrating a configuration of an air-fuel ratio detection device according to a second embodiment.

FIG. 7 is a circuit diagram illustrating a configuration of an air-fuel ratio detection device according to a third embodiment.

FIG. 8 is a flowchart illustrating a detection process executed by a microcomputer provided in the air-fuel ratio detection device according to the third embodiment.

FIG. 9 is a time chart illustrating the operation of the air-fuel ratio detection device according to the third embodiment.

FIG. 10 is a circuit diagram illustrating a configuration of an air-fuel ratio detection device having a problem to be solved by the present invention.

FIG. 11 is a time chart illustrating a procedure for detecting an element impedance of the air-fuel ratio sensor.

FIG. 12 is a schematic configuration diagram showing a configuration of a small-scale and inexpensive D / A converter (simple D / A converter) having a small circuit configuration.

FIG. 13 is a time chart for explaining a problem to be solved by the present invention.

[Explanation of symbols]

1, 31, 41 Air-fuel ratio detection device 2 Microcomputer (microcomputer) 4, 6 Input circuit 10 Exhaust pipe AFS Air-fuel ratio sensor AF + Plus terminal AF- Minus terminal Tr1, Tr2 Transistor R3 Shunt resistors R10 to R12: resistor for voltage generation R15: resistor for output R16, R17: resistor for voltage division OP1 to OP6: operational amplifier

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-10-300716 (JP, A) JP-A-10-232220 (JP, A) JP-A 10-26599 (JP, A) JP-A 8- 15216 (JP, A) JP-A-7-72113 (JP, A) JP-A-59-230154 (JP, A) JP-A-59-18449 (JP, A) JP-A-57-192856 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01N 27/41 G01N 27/419

Claims (9)

(57) [Claims] 1. An output voltage of a voltage applying means for applying a voltage for detecting the oxygen concentration in a gas to be detected to an oxygen concentration sensor in which a current corresponding to the oxygen concentration in the gas to be detected flows with the application of a voltage. And voltage applied to the oxygen concentration sensor to change the output voltage of the oxygen concentration sensor. The applied voltage changing means changes the output voltage of the oxygen concentration sensor. First current detecting means for detecting a current; and flowing to the oxygen concentration sensor when a predetermined time has elapsed after the applied voltage changing means has performed an operation of changing the output voltage of the voltage applying means. A second current detecting means for detecting a current; and a difference between a current detected by the first current detecting means and a current detected by the second current detecting means. An impedance calculating means for calculating an element impedance of the oxygen concentration sensor, wherein the voltage applying means outputs a voltage input to an input unit to the oxygen concentration sensor. An output circuit, a constant voltage circuit that outputs a predetermined constant voltage to an input section of the output circuit via an output resistor, a signal path from the output resistor to an input section of the output circuit, and A voltage switching circuit including a switching element and a voltage dividing resistor connected in series between a constant voltage and a set voltage different from the constant voltage, wherein the applied voltage changing unit turns on or turns off the switching element of the voltage switching circuit. Being turned off to change the output voltage of the voltage applying means. Child impedance detector. 2. The element impedance detection device for an oxygen concentration sensor according to claim 1, wherein the voltage applying unit includes two of the voltage switching circuits, and the applied voltage changing unit includes the voltage switching circuits. An element impedance detection device for an oxygen concentration sensor, characterized in that the switching element is turned on / off to change the output voltage of the voltage applying means to both positive and negative sides. 3. The element impedance detecting device for an oxygen concentration sensor according to claim 2, wherein the constant voltage circuit of the voltage applying means sets a voltage for detecting an oxygen concentration in the gas to be detected as the constant voltage. And the setting voltage to which one of the two voltage switching circuits is connected is higher than the fixed voltage, and the other voltage switching circuit is connected to the setting voltage. The set voltage is a voltage lower than the fixed voltage.Further, the applied voltage changing means turns on a switching element of a voltage switching circuit connected to a voltage higher than the fixed voltage, thereby turning on the voltage applying means. The voltage is applied by changing an output voltage to a positive side and turning on a switching element of a voltage switching circuit connected to a voltage lower than the constant voltage. A device for detecting an element impedance of the oxygen concentration sensor, wherein the output voltage of the means is changed to a negative side. 4. A first voltage applying means for applying a first voltage to one terminal of an oxygen concentration sensor through which a current according to an oxygen concentration in a gas to be detected flows with application of a voltage, and the other of the oxygen concentration sensor A second voltage applying means for applying a second voltage different from the first voltage to a terminal of the first voltage applying means, and an applied voltage for changing an output voltage of the first voltage applying means to detect an element impedance of the oxygen concentration sensor. Changing means; first current detecting means for detecting a current flowing through the oxygen concentration sensor before the applied voltage changing means performs an operation of changing the output voltage of the first voltage applying means; Based on the current detected by the current detecting means,
Oxygen concentration calculating means for calculating the oxygen concentration in the gas to be detected; and when a predetermined time has elapsed after the applied voltage changing means has performed an operation of changing the output voltage of the first voltage applying means. A second current detecting means for detecting a current flowing through the oxygen concentration sensor, and a difference between a current detected by the first current detecting means and a current detected by the second current detecting means, An oxygen concentration detection device comprising: an impedance calculation unit that calculates an element impedance of the oxygen concentration sensor. The oxygen concentration detection device further comprises: an output circuit that outputs a voltage input to an input unit to the oxygen concentration sensor. A constant voltage circuit that outputs a predetermined constant voltage to an input section of the output circuit via an output resistor; and from the output resistor to an input section of the output circuit. A voltage switching circuit comprising a switching element and a voltage-dividing resistor connected in series between a signal path to reach and a set voltage different from the constant voltage, and the applied voltage changing unit includes the voltage switch. By turning on or off the switching element of the circuit, the first
An oxygen concentration detecting device, characterized in that it is configured to change the output voltage of the voltage applying means. 5. The oxygen concentration detecting device according to claim 4, wherein the first voltage and the second voltage are sequentially connected in series between a predetermined power supply voltage and a ground potential to divide the power supply voltage. The voltage generating means includes three voltage generating resistors for generating two voltages, wherein the constant voltage circuit of the first voltage applying means receives the first voltage generated by the voltage generating resistor and receives the first voltage.
A voltage is output as the constant voltage, and the applied voltage changing unit is configured to change an output voltage of the first voltage applying unit by turning on a switching element of the voltage switching circuit. Further, the second voltage applying means is configured to input the second voltage generated by the voltage generating resistor, and output the second voltage to the other terminal of the oxygen concentration sensor. An oxygen concentration detection device, characterized in that: 6. The oxygen concentration detecting device according to claim 5, wherein the first voltage applying means includes two of the voltage switching circuits, and one of the two voltage switching circuits. The set voltage to which is connected is higher than the first voltage, and the set voltage to which the other voltage switching circuit is connected is lower than the first voltage. The means is configured to change an output voltage of the first voltage applying means to both positive and negative sides by turning on / off a switching element of each of the voltage switching circuits. 7. The oxygen concentration detecting device according to claim 4, wherein the output voltage of the second voltage applying means is arbitrarily changeable. 8. The oxygen concentration detecting device according to claim 7, wherein the first voltage applying means includes two of the voltage switching circuits, and the applied voltage changing means switches the voltage switching circuits. An oxygen concentration detecting device, characterized in that an output voltage of the first voltage applying means is changed to both positive and negative sides by turning on / off an element. 9. The oxygen concentration detecting device according to claim 8, wherein the constant voltage circuit of the first voltage applying means is configured to output the first voltage as the constant voltage, and the two voltages The setting voltage to which one of the switching circuits is connected is higher than the first voltage, and the setting voltage to which the other voltage switching circuit is connected is lower than the first voltage. And the applied voltage changing means changes the output voltage of the first voltage applying means to a positive side by turning on a switching element of a voltage switching circuit connected to a voltage higher than the first voltage. And turning on a switching element of a voltage switching circuit connected to a voltage lower than the first voltage to change the output voltage of the first voltage applying means to a negative side. An oxygen concentration detecting device, characterized in that: