JP2001317400A - Activity judging device for air-fuel ratio sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To determine activity by detecting impedance of a sensor element of an air-fuel ratio sensor and prevent worsening of judgement precision due to the fluctuation of a supply voltage. SOLUTION: An AC voltage is applied to a Nernst cell part of the air-fuel ratio sensor provided with the Nernst cell part and a pump cell part to measure impedance of the Nernst cell part based on a current value flowing in the Nernst cell part due to the application of the AC voltage. An activity condition of the air-fuel ratio sensor is judged based on the measured impedance. Besides, the application of the AC voltage and the measurement of impedance are inhibitted, to say concretely, (1) during cranking, (2) when battery voltage is reduced, (3) when battery voltage is fluctuated, and (4) during driving by revolutions at a high speed under predetermined driving conditions causing the fluctuation of the supply voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an activity judging device for an air-fuel ratio sensor mounted on an exhaust system of an internal combustion engine and used for air-fuel ratio control.

[0002]

2. Description of the Related Art As an air-fuel ratio control device for an internal combustion engine, an air-fuel ratio sensor detects an actual air-fuel ratio based on the oxygen concentration in exhaust gas and the like, and supplies a fuel to the engine such that the actual air-fuel ratio becomes a target air-fuel ratio. Is known that performs feedback control on the control.

In order to perform the above-described air-fuel ratio feedback control, a precondition is that the air-fuel ratio sensor is activated. Further, when diagnosing the response deterioration of the air-fuel ratio sensor based on the sensor output during the air-fuel ratio feedback control, it is a precondition that the air-fuel ratio sensor is activated.

[0004] The air-fuel ratio sensor is activated when its element temperature reaches a predetermined activation temperature. Therefore, conventionally, the air-fuel ratio sensor is energized based on the energization time from the start of the heater for heating the sensor element provided in the air-fuel ratio sensor. The element temperature is estimated to determine the activation state of the air-fuel ratio sensor.
As disclosed in Japanese Patent No. 3481, the element temperature is estimated from the catalyst temperature and the like detected by the catalyst temperature sensor, and the activation state of the air-fuel ratio sensor is determined.

On the other hand, Japanese Patent Application Laid-Open No. Sho 61-12556 discloses that the impedance of a sensor element is controlled in order to control the amount of electric power to a heater for heating a sensor element provided in an air-fuel ratio sensor based on the element temperature. It is disclosed that the element temperature is detected by using the impedance of the sensor element because it depends on the element temperature.

Specifically, a high-frequency AC voltage is applied to the sensor element of the air-fuel ratio sensor, and the impedance of the sensor element is measured from the current value (amplitude) flowing through the sensor element by the application of the AC voltage. The element temperature is detected more.

Therefore, it is naturally conceivable to determine the activation state of the air-fuel ratio sensor based on the measured impedance.

[0008]

However, the above-mentioned conventional apparatus for determining the activity of the air-fuel ratio sensor has the following problems.

When estimating the element temperature of the air-fuel ratio sensor from the energization time to the heater and other temperatures, variations in individual sensors due to element shape, heater capacity, deterioration, etc., variations in the heater control circuit including voltage fluctuations, It is difficult to accurately estimate the element temperature due to variations in environmental conditions including heat removal from the exhaust pipe due to outside air temperature, rainfall, and the like, and there is a risk of erroneous activity determination.

When measuring the impedance of the sensor element, although the element temperature can be detected more accurately than the above estimation, during the cranking of the internal combustion engine, when the battery voltage drops or fluctuates in a low temperature state, and when the rotation speed increases, When the power supply voltage fluctuates greatly, such as during operation, the measured value of impedance fluctuates due to fluctuations in the power supply voltage, and the detection accuracy of the element temperature decreases.

The present invention has been made in view of such a conventional problem and makes it possible to detect the impedance of the sensor element of the air-fuel ratio sensor to determine the activation, while preventing deterioration of the determination accuracy due to fluctuations in the power supply voltage. An object of the present invention is to provide an activity determination device for an air-fuel ratio sensor.

[0012]

Therefore, in the invention according to claim 1, as shown in FIG. 1, an AC voltage applying means for applying an AC voltage to the sensor element of the air-fuel ratio sensor, While providing an impedance measuring means for measuring the impedance of the sensor element from a current value flowing through the sensor element and an active state determining means for determining an active state of the air-fuel ratio sensor based on the measured impedance, a power supply voltage fluctuates. Under a predetermined operating condition, a measurement prohibiting unit for prohibiting the application of the AC voltage and the measurement of the impedance is provided.

[0013] In the invention according to claim 2, the air-fuel ratio sensor includes a Nernst cell unit for generating a voltage corresponding to a lean-rich air-fuel ratio, and a lean-rich air-fuel ratio detected by the Nernst cell unit. A pump cell section in which a predetermined voltage is applied in a corresponding direction and a current value continuously changes in accordance with an air-fuel ratio, wherein the AC voltage applying means applies an AC voltage to the Nernst cell section. Wherein the impedance measuring means measures the impedance of the Nernst cell unit from a current value flowing through the Nernst cell unit by applying an AC voltage.

[0014] The invention according to claim 3 is characterized in that the predetermined operating condition is at least during cranking of the internal combustion engine. The invention according to claim 4 is characterized in that the predetermined operating condition is at least when the battery voltage is equal to or lower than a predetermined value.

According to a fifth aspect of the present invention, the predetermined operating condition is at least when a fluctuation amount of the battery voltage is equal to or more than a predetermined value. The invention according to claim 6 is characterized in that the predetermined operating condition is at least a high-speed operation of the internal combustion engine.

In the invention according to claim 7, the active state determination means compares the measured impedance with a predetermined value, and determines that the active state is established when the impedance becomes equal to or less than the predetermined value. Features.

According to an eighth aspect of the present invention, the impedance measuring means has means for smoothing the measured impedance.

[0018]

According to the first aspect of the present invention, an AC voltage is applied to the sensor element of the air-fuel ratio sensor, and the impedance of the sensor element is measured from a current value flowing through the sensor element by the application of the AC voltage. It is possible to know the element temperature without being affected by various variations, and to accurately determine the activation state of the air-fuel ratio sensor. By prohibiting the measurement of the impedance, it is possible to prevent the erroneous determination from being caused by the fluctuation of the impedance measurement value due to the fluctuation of the power supply voltage.

According to the second aspect of the present invention, in the air-fuel ratio sensor including the Nernst cell section and the pump cell section, an AC voltage is applied to the Nernst cell section to measure the impedance of the Nernst cell section. The impedance can be detected without affecting the air-fuel ratio detection in the section.

According to the third aspect of the invention, during cranking of the internal combustion engine, the fluctuation of the impedance measurement value due to the fluctuation of the power supply voltage occurs. At this time, the application of the AC voltage and the measurement of the impedance are prohibited. Thus, erroneous determination can be reliably prevented.

According to the fourth aspect of the invention, when the battery voltage drops, the impedance measurement value fluctuates. At this time, the application of the AC voltage and the measurement of the impedance are prohibited, so that an erroneous determination is made. It can be reliably prevented.

According to the fifth aspect of the invention, when the battery voltage fluctuates, the impedance measurement value fluctuates. At this time, the application of the AC voltage and the measurement of the impedance are prohibited, so that an erroneous determination is made. It can be reliably prevented.

According to the sixth aspect of the present invention, when the internal combustion engine operates at a high speed, the impedance measurement value fluctuates due to the power supply voltage fluctuation. At this time, the application of the AC voltage and the measurement of the impedance are prohibited. By doing so, erroneous determination can be reliably prevented.

According to the seventh aspect of the present invention, when the impedance of the sensor element becomes equal to or less than the predetermined value, it is determined that the sensor element is in the active state.

According to the eighth aspect of the present invention, the measured impedance is smoothed and used for activity determination, thereby improving the accuracy of the determination.

[0026]

Embodiments of the present invention will be described below. FIG. 2 shows a system configuration of an air-fuel ratio feedback control device for an internal combustion engine according to an embodiment of the present invention.

An internal combustion engine (hereinafter referred to as an engine) 1 includes:
A fuel injection valve 3 is provided for each cylinder so as to face the intake passage 2 or the combustion chamber. The fuel injection of each fuel injection valve 3 is controlled by the control unit 4.

The control unit 4 calculates a stoichiometric value based on, for example, an intake air amount Qa detected based on a signal from the air flow meter 5 and an engine speed Ne detected based on a signal from the crank angle sensor 6. λ =
1) Calculate a corresponding basic fuel injection amount Tp = K × Qa / Ne (K is a constant), and calculate the air-fuel ratio based on a signal from an air-fuel ratio sensor 8 disposed in the exhaust passage 7 in addition to the target air-fuel ratio tλ. A final fuel injection amount Ti = Tp × (1 / tλ) × α is calculated by correcting with a feedback correction coefficient α, and a fuel injection pulse having a pulse width corresponding to this Ti is synchronized with the engine rotation. Output to each fuel injection valve 3.

Here, the air-fuel ratio sensor 8 is disposed in the exhaust passage 7 and outputs a signal corresponding to the oxygen concentration in the exhaust gas. The control unit 4 is based on the signal from the air-fuel ratio sensor 8. The air-fuel ratio λ is detected by detecting the air-fuel ratio λ of the air-fuel mixture supplied to the engine 1 and increasing or decreasing the air-fuel ratio feedback correction coefficient α by proportional integral control or the like so that the air-fuel ratio λ becomes the target air-fuel ratio tλ. Feedback control is performed to the target air-fuel ratio tλ.

As the air-fuel ratio sensor 8, a so-called wide-range air-fuel ratio sensor capable of linearly detecting the air-fuel ratio is used. FIG. 3 shows a sensor element structure of the wide-range air-fuel ratio sensor 8, which will be described.

The main body 20 of the sensor element is formed in a porous layer of a solid electrolyte material such as zirconia having oxygen ion conductivity, and is placed in an exhaust passage. Inside the main body 20, a heater 21, an atmosphere chamber 22, and a gas diffusion chamber 23 are provided from below in the figure.

The heater 21 can heat the sensor element by energizing it. The atmosphere chamber 22 is formed outside the exhaust passage so as to communicate with the atmosphere, which is a reference gas.

The gas diffusion chamber 23 is formed so as to communicate with exhaust gas through a protective layer 25 such as γ-alumina through an exhaust gas introduction hole 24 formed from the upper surface side of the main body 20 in the drawing. Here, an electrode 26A provided on the upper wall of the atmosphere chamber 22;
The Nernst cell part 26 is constituted by the electrode 26B provided on the lower wall of the gas diffusion chamber 23.

The pump cell unit 27 is composed of an electrode 27A provided on the upper wall of the gas diffusion chamber 23 and an electrode 27B provided on the upper wall of the main body 20 and covered with a protective layer 28. The Nernst cell section 26 generates a voltage according to the oxygen partial pressure ratio between the Nernst cell section electrodes 26A and 26B which is affected by the oxygen ion concentration (oxygen partial pressure) in the gas diffusion chamber 23.

Accordingly, the Nernst cell electrodes 26A, 2
By detecting the voltage generated by the oxygen partial pressure ratio between 6B, it is possible to detect whether the air-fuel ratio is lean or rich with respect to stoichiometry (λ = 1).

When a predetermined voltage is applied to the pump cell section 27, the oxygen ions in the gas diffusion chamber 23 move by this, and a current flows between the pump cell section electrodes 27A and 27B. .

Then, the pump cell electrodes 27A, 27B
When a predetermined voltage is applied between the electrodes, the current value (limit current value) Ip flowing between the electrodes is affected by the oxygen ion concentration in the gas diffusion chamber 23. Therefore, if the current value Ip is detected,
The air-fuel ratio of the exhaust can be detected.

That is, as shown in FIG. 4 (A), the voltage-current characteristic of the pump cell section 27 changes according to the air-fuel ratio λ, and the exhaust air becomes less than the current value Ip when a predetermined voltage Vp is applied. The fuel ratio λ can be detected.

In the Nernst cell section 27, the lean
By inverting the voltage application direction to the pump cell unit 27 based on the rich output, the air flows through the pump cell unit 27 in the air-fuel ratio region in both the lean region and the rich region as shown in FIG. Based on the current value Ip, it is possible to detect a wide range of the air-fuel ratio λ.

According to the present invention, a high-frequency AC voltage is applied to the sensor element (particularly, the Nernst cell section 26) to measure the impedance of the sensor element having the above-described structure and characteristics. Make a decision.

FIG. 5 shows a control circuit for the sensor elements (Nernst cell section and pump cell section) of the air-fuel ratio sensor and a heater for heating the sensor element. The Nernst cell section 26 includes a microcomputer 3 for detecting impedance.
Under the control of 0, a high-frequency AC voltage (frequency f = 3 KHz, amplitude 1.75 V) is applied from the AC power supply 31, whereby the current value Is flowing through the Nernst cell unit 26 is changed to a current detection resistor 32 and a detection amplifier. 33 for voltage conversion.

The signal from the detection amplifier 33 is input to an impedance detection circuit 34 composed of, for example, a high-pass filter and an integrator to extract only the AC component and detect the impedance Ri from the amplitude. Thereby, the impedance Ri of the Nernst cell unit 26 can be measured.

The signal from the detection amplifier 33 is input to a low-pass filter 35 to extract only a DC component and detect a voltage generated in the Nernst cell unit 26 in accordance with the oxygen concentration. As a result, the oxygen concentration is
Rich can be detected.

A predetermined voltage Vp is applied to the pump cell unit 27 from the DC power supply 36 under the control of the microcomputer 30, and the application direction is in accordance with the lean / rich oxygen concentration detected by the Nernst cell unit 26. The current Ip flowing through the pump cell unit 27 is converted into a voltage by the current detection resistor 37 and the detection amplifier 38 and detected.
Thereby, the air-fuel ratio λ can be detected.

A battery voltage VB is applied to the heater 21 from a battery.
9 are provided. Therefore, the duty of the switching element 39 is controlled by the microcomputer 30 to control the amount of power to the heater 21.

FIG. 6 is a flowchart of the impedance measurement, heater control, and activation determination executed by the microcomputer 30 at predetermined time intervals. In step 1 (referred to as S1 in the figure, the same applies hereinafter), various operating conditions are read.

In step 2, it is determined whether a predetermined impedance (Ri) measurement permission condition is satisfied. Here, the impedance measurement permission condition is a condition in which none of the following prohibition conditions (1) to (4) is satisfied.

(1) During cranking, specifically, when the start switch is ON and the engine speed Ne is equal to or less than a predetermined value (at less than a complete explosion, for example, 100 rpm). (2) When the battery voltage VB is (3) When the amount of change | ΔVB | of the battery voltage VB is equal to or more than a predetermined value (4) During high-speed operation, specifically, the engine speed Ne
Is 4400 rpm or more.

If the impedance measurement permission condition is satisfied, steps 3 to 6 are executed. In step 3, the AC power supply 31 is turned on, and the Nernst cell unit 26 is turned on.
High frequency AC voltage (frequency f = 3 KHz, amplitude 1.75)
V). This portion corresponds to an AC voltage applying unit.

In step 4, based on the current value (amplitude) flowing through the Nernst cell section 26 due to the application of the AC voltage,
The impedance Ri of the Nernst cell unit 26 is measured via the impedance detection circuit 34 and the like. Here, the impedance Ri is measured to be larger at a lower temperature and smaller at a higher temperature. This part corresponds to impedance measuring means.

In step 5, the measured impedance Ri is smoothed. This part corresponds to the smoothing processing means. Specifically, the smoothing process is performed by the following method (a) or (b).

(A) For example, when the impedance Ri is measured every 20 ms, the latest measured Ri is calculated as Ri (n
ew), assuming that Ri measured 200 ms before is Ri (old), an average value AVGRi is calculated by AVGRi = [Ri (new) + Ri (old)] / 2 based on these two Ri data. Then, a smoothing process is performed.

(B) For example, when the impedance Ri is measured every 20 ms, based on the latest data of ten Ris, for example, AVGRi = ΣRi / 10 (where ΔRi is the total value of the ten Ris) , An average value AVGRI to calculate a smoothing process.

At step 6, the flag F = 1 is set to indicate that the impedance measurement permission condition is satisfied and Ri measurement is being performed, and the routine proceeds to step 9. On the other hand, when the impedance measurement permission condition is not satisfied, that is, when any of the above-described prohibition conditions (1) to (4) is satisfied, the process proceeds to step 7 and the AC power supply 31 is turned off. , Without performing impedance measurement,
In step 8, a flag F is set to indicate that the impedance measurement permission condition is not satisfied and Ri measurement is not performed.
= 0 and proceed to step 9. This part corresponds to the measurement prohibiting means.

In step 9, it is determined whether a predetermined heater control permission condition is satisfied. Here, the heater control permission condition is, for example, when the battery voltage is equal to or higher than a predetermined value while the engine is rotating, and the air-fuel ratio sensor and its heater are diagnosed as having no failure.

If the heater control permission condition is satisfied, the routine proceeds to step 10, where the value of the flag F is determined. When F = 1, the impedance measurement permission condition is satisfied, Ri measurement is performed, and the heater 2 based on Ri is used.
Since the feedback control of step 1 is possible, step 11,
12, the heater duty HDUTY is calculated and controlled.

In step 11, the real impedance AV
The heater duty HD is set so that GRi becomes a target impedance TGRi corresponding to a target temperature (for example, 800 ° C.).
PID of feedback correction coefficient KRi for UTY
Calculated by control. Specifically, the following [1] to [5]
Calculate according to the following procedure.

[1] Based on the actual impedance AVGRi and the target impedance TGRi, P
Calculate the minutes. P component = GP × (AVGRi−TGRi) / AVGRi Here, GP is a predetermined P component gain.

[2] Based on the actual impedance AVGRi and the target impedance TGRi, I
Calculate the minutes. I minute = previous I minute + GI × (AVGRi−TGRi) /
AVGRi Here, GI is a predetermined I gain.

[3] Based on the actual impedance AVGRi and the previous actual impedance AVGRiold, the D component is calculated from the following equation. D component = GD × (AVGRi−AVGRiold) Here, GD is a predetermined D component gain.

[4] The P, I, and D components are added to calculate the error amount according to the following equation. Error amount = P minute + I minute + D minute The calculated error amount is appropriately subjected to a limiter process.

[5] Based on the error amount, a feedback correction coefficient KRi is calculated with reference to a predetermined table. Here, when the error amount = 0, the feedback correction coefficient KRi = 1, and as the error amount increases to the plus side, the feedback correction coefficient KRi becomes smaller than 1, and as the error amount increases to the minus side, the feedback correction coefficient KRi increases. The coefficient KRi is set to be larger than 1.

In step 12, the heater duty HDUTY is corrected by the feedback correction coefficient KRi.
Is calculated. Here, the heater duty HDUTY is
It is calculated by the following equation using the feedback correction coefficient KRi.

HDUTY = 100 (%) × KVB × KA
FCV × KRi KVB is a battery voltage correction coefficient, and KAFCUT is a fuel cut correction coefficient.

On the other hand, when F = 0, the impedance measurement permission condition is not satisfied, the Ri measurement is not performed, and the feedback control of the heater 21 based on the Ri cannot be performed. The duty HDUTY is calculated and controlled.

In step 13, the feedback correction coefficient KRi is set to 1 (no correction). In step 14,
In the equation used in step 12, assuming that the feedback correction coefficient KRi = 1 and the heater duty HDUTY =
100 (%) × KVB × KAFCUT is calculated and controlled, and this routine ends.

On the other hand, if the heater control permission condition is not satisfied, the routine proceeds from step 9 to step 15, sets the heater duty HDUTY = 0, and ends this routine. Therefore, in this case, the heater 21 is not energized.

Next, the activity determination will be described. When the heater control permission condition is satisfied, and the impedance measurement permission condition is satisfied and the heater duty HDUTY is feedback-controlled based on the Ri measurement,
Then, the process proceeds to step S16.

In step 16, the actual impedance AV
GRi is set to a predetermined value (for example, 80 to 100Ω,
(Equivalent to 50 ° C.). When AVGRi> predetermined value, this routine is terminated as it is. When AVGRi ≦ predetermined value, the routine proceeds to step 17, the activation determination flag is set to 1, and this routine is terminated.

When AVGRi ≦ predetermined value, the element temperature of the air-fuel ratio sensor has reached a predetermined activation temperature and the air-fuel ratio sensor has been activated, so that the activation determination flag is set to 1 to indicate this. You do it. Therefore, this part corresponds to the activity determining means.

FIG. 7 shows the impedance change after the start. The heater impedance and the exhaust temperature rise after the start cause the element temperature of the air-fuel ratio sensor to rise with time, so that the actual impedance AVGRi gradually decreases. Then, when the value becomes equal to or less than the predetermined value, the activity is determined.

By the activation determination (setting of the activation determination flag), the air-fuel ratio feedback control based on the signal from the air-fuel ratio sensor is started, and the failure diagnosis of the air-fuel ratio sensor is also possible.

As described above, according to the present invention, an AC voltage is applied to the sensor element (Nernst cell section 26) of the air-fuel ratio sensor, and the current value (amplitude) flowing through the sensor element due to the application of the AC voltage is applied to the sensor. By measuring the impedance of the element, without being affected by various variations,
While it is possible to know the element temperature and accurately determine the activation state of the air-fuel ratio sensor, on the other hand, under predetermined operating conditions that cause fluctuations in power supply voltage, specifically, (1) during cranking,
(2) When the battery voltage drops, (3) when the battery voltage fluctuates, and (4) during high-speed operation, etc., by prohibiting the application of the AC voltage and the measurement of the impedance, an erroneous measurement due to the fluctuation of the impedance measurement value occurs. The determination can be prevented from occurring.

[Brief description of the drawings]

FIG. 1 is a functional block diagram showing a configuration of the present invention.

FIG. 2 is a system diagram of an air-fuel ratio feedback control device for an internal combustion engine showing an embodiment of the present invention.

FIG. 3 is a diagram showing a sensor element structure of an air-fuel ratio sensor.

FIG. 4 is a characteristic diagram of a sensor element of an air-fuel ratio sensor.

FIG. 5 is a control circuit diagram for a sensor element and a heater of the air-fuel ratio sensor.

FIG. 6 is a flowchart of impedance measurement, heater control, and activation determination.

FIG. 7 is a diagram showing a change in impedance after starting.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine 3 Fuel injection valve 4 Control unit 8 Air-fuel ratio sensor 20 Sensor element main body 21 Heater 22 Atmosphere chamber 23 Gas diffusion chamber 24 Exhaust introduction hole 25 Protective layer 26 Nernst cell part 27 Pump cell part 28 Protective layer 30 Microcomputer 31 AC power supply 34 Impedance Detection circuit 36 DC power supply 39 Switching element

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G01N 27/46 327P F-term (Reference) 3G084 BA09 CA09 DA27 EA11 EB11 EB25 FA29 FA36 FA38 3G301 JA08 KA25 NA02 NA08 ND01 ND41 NE00 NE17 NE19 PD04B PD04Z PE03Z PF16Z PG01Z

Claims (8)

[Claims] An apparatus for determining the activity of an air-fuel ratio sensor mounted on an exhaust system of an internal combustion engine, comprising: an AC voltage applying means for applying an AC voltage to a sensor element of the air-fuel ratio sensor; While providing an impedance measuring means for measuring the impedance of the sensor element from a current value flowing through the sensor element and an active state determining means for determining the active state of the air-fuel ratio sensor based on the measured impedance. An activity determination device for an air-fuel ratio sensor, comprising: a measurement prohibiting unit that prohibits the application of the AC voltage and the measurement of the impedance under operating conditions. 2. An air-fuel ratio sensor comprising: a Nernst cell for generating a voltage corresponding to a lean / rich air-fuel ratio; and a predetermined direction in a direction corresponding to a lean / rich air-fuel ratio detected by the Nernst cell. A pump cell section to which a voltage is applied and a current value continuously changes in accordance with an air-fuel ratio. The AC voltage applying means applies an AC voltage to the Nernst cell section, and the impedance measurement 2. The activity judging device for an air-fuel ratio sensor according to claim 1, wherein the means measures the impedance of the Nernst cell unit from a current value flowing through the Nernst cell unit by applying an AC voltage. 3. The apparatus according to claim 1, wherein the predetermined operation condition is at least during cranking of the internal combustion engine. 4. The apparatus according to claim 1, wherein the predetermined operating condition is at least when a battery voltage is equal to or lower than a predetermined value. 5. The apparatus according to claim 1, wherein the predetermined operating condition is at least when a variation amount of a battery voltage is equal to or more than a predetermined value. 6. The air-fuel ratio sensor activity judging device according to claim 1, wherein the predetermined operating condition is at least a high-speed operation of the internal combustion engine. 7. The apparatus according to claim 1, wherein said active state determining means compares the measured impedance with a predetermined value and determines that the state is an active state when the impedance becomes equal to or less than a predetermined value. The activity determination device for an air-fuel ratio sensor according to any one of claims 6 to 6. 8. The air-fuel ratio sensor according to claim 1, wherein said impedance measuring means has means for smoothing the measured impedance. Activity determination device.