JP3500775B2 - Oxygen sensor deterioration judgment device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a deterioration determining device applied to a limiting current type oxygen sensor.

[0002]

2. Description of the Related Art In recent years, for example, in an automobile engine, a limiting current type oxygen sensor that linearly detects the air-fuel ratio according to the oxygen concentration in exhaust gas has been adopted. This limiting current type oxygen sensor outputs a substantially constant limiting current corresponding to the oxygen concentration when a voltage is applied, and when used in the air-fuel ratio control system of the automobile engine, the sensor output at that time (limit The air-fuel ratio is calculated according to the current value).

In the above oxygen sensor, there is a demand for a technique for accurately detecting deterioration over time, and as a conventional technique of this type, for example, Japanese Patent Laid-Open No. 4-233447, "Exhaust Concentration Sensor Deterioration Detection Method". Is disclosed.
In this publication, the internal resistance of the oxygen sensor is calculated from the output current of the oxygen sensor when a voltage is applied to the oxygen sensor, and when the internal resistance value increases, it is determined that the oxygen sensor has deteriorated.

[0004]

However, the above-mentioned conventional publications have the following problems. That is, in the case of the oxygen sensor described above, in order to detect the limiting current with high accuracy, the sensor body (solid electrolyte layer or the like) is set at a predetermined activation temperature (for example,
It is necessary to keep the temperature at 650 ° C.), and a heater may be built in the oxygen sensor to control the energization of the heater. In this case, even if the oxygen sensor is deteriorated, the energization amount of the heater is increased, and as a result, the internal resistance is maintained at a substantially constant value. Therefore, there is a problem in that it is impossible to do so even in the originally deteriorated state.

The present invention has been made in view of the above problems, and an object thereof is a deterioration determination device for an oxygen sensor, which is capable of accurately determining the deterioration of the oxygen sensor by a novel method. To provide.

[0006]

In order to achieve the above object, the invention described in claim 1 is applied to an oxygen sensor which outputs a substantially constant limiting current corresponding to the oxygen concentration when a voltage is applied. A determination device, wherein the voltage applied to the oxygen sensor is changed from a positive voltage to a negative voltage or a negative voltage to a positive voltage.
Voltage switching means for switching to the voltage , current change detecting means for detecting a current change due to the voltage switching between immediately after the voltage switching by the voltage switching means and until the current by the oxygen sensor converges, and the current change detection The gist of the present invention is to include deterioration determining means for determining that the oxygen sensor is deteriorated based on a change in current detected by the means.

According to a second aspect of the invention, in the first aspect of the invention, an internal resistance detecting means for detecting the internal resistance of the oxygen sensor, a heater attached to the oxygen sensor, and the internal resistance detection are provided. A heater control unit that feedback-controls energization to the heater in order to eliminate a deviation between the sensor internal resistance detected by the means and a predetermined target value, and the deterioration determination unit is detected by the current change detection unit. It is determined that the sensor is deteriorated when the current change immediately after the voltage switching exceeds the deterioration determination range corresponding to the voltage change by the voltage switching means.

According to a third aspect of the present invention, in the first aspect of the present invention, there is provided convergent current detecting means for detecting the current value when the sensor current converges after the voltage is switched by the voltage switching means, The deterioration determination means,
It is determined that the sensor is deteriorated when the current change detected by the current change detection means immediately after the voltage switching exceeds the deterioration determination range corresponding to the converged current detected by the converged current detection means.

According to a fourth aspect of the present invention, in the third aspect of the present invention, the deterioration determination means is such that the current change becomes larger as the absolute value of the converged current increases. To set.

According to a fifth aspect of the present invention, in the first aspect of the present invention, an internal resistance detecting means for detecting the internal resistance of the oxygen sensor is provided, and the deterioration determining means is detected by the current change detecting means. It is determined that the sensor is deteriorated when the current change immediately after the voltage change exceeds the deterioration determination range corresponding to the internal resistance of the sensor detected by the internal resistance detection means.

According to a sixth aspect of the present invention, in the fifth aspect of the present invention, the deterioration determining means sets the deterioration determining area in such a direction that the current change becomes smaller as the internal resistance of the sensor increases. To do.

[0012]

According to the invention described in claim 1, the voltage switching means changes the voltage applied to the oxygen sensor from a positive voltage to a negative voltage.
Or switch from a negative voltage to a positive voltage . The current change detection means detects a current change due to the voltage switching immediately after the voltage switching by the voltage switching means and before the current converged by the oxygen sensor. The deterioration detecting means determines that the oxygen sensor is deteriorated based on the current change detected by the current change detecting means.

In short, in the limiting current type oxygen sensor, for example, when the applied voltage is switched from the positive voltage to the negative voltage, a steep current in the negative direction (hereinafter referred to as a peak current) is generated, On the contrary, it is known that a steep current (peak current) is generated in the positive direction when the applied voltage is switched from the negative voltage to the positive voltage. In this case, since the magnitude of the peak current corresponds to the deterioration state of the oxygen sensor, the deterioration of the oxygen sensor is accurately detected by the above configuration.

According to the second aspect of the invention, the internal resistance detecting means detects the internal resistance of the oxygen sensor. The heater control means feedback-controls energization to the heater so as to eliminate the deviation between the sensor internal resistance detected by the internal resistance detection means and a predetermined target value. The deterioration determination means determines that the sensor is deteriorated when the current change immediately after the voltage switching detected by the current change detection means exceeds the deterioration determination range corresponding to the voltage change by the voltage switching means.

That is, the limiting current type oxygen sensor can be represented by, for example, an equivalent circuit of FIG. However, "R
“B” is the internal resistance of the electrolyte, “Rd” is the resistance of the interface between the electrolyte and the electrode, and “Cd” is the capacitance of the interface. In this case, if the porous electrode of the oxygen sensor is clogged due to the sensor deterioration, the resistance Rd in FIG. 12A increases. However, if the internal resistance of the oxygen sensor is feedback-controlled to the target value, the resistance Rb will eventually decrease. Therefore, the change in the peak current becomes large in the current path (path in the figure) immediately after switching the applied voltage. As a result, the change in the peak current exceeds the deterioration determination range, and the deterioration is determined.

At this time, for example, the applied voltage is a positive voltage Vp.
When the voltage is switched from the negative voltage Vn to the negative voltage Vn, the peak current value Io is obtained by the following equation. Io = Ip- (Vp-Vn) / Rb Therefore, in order to determine this peak current value Io, it is desirable to perform it based on the deterioration determination range corresponding to the voltage change amount by the voltage switching means. Note that FIG. 12B shows a change in current due to voltage switching, a solid line of the sensor current shows a waveform in a normal state (before deterioration), and a broken line of the sensor current shows a waveform after deterioration.

According to the third aspect of the present invention, the convergent current detecting means detects the current value when the sensor current converges after the voltage switching by the voltage switching means. The deterioration determining means indicates that the sensor is deteriorated when the current change immediately after the voltage switching detected by the current change detecting means exceeds the deterioration determining range corresponding to the converged current detected by the converged current detecting means. To judge.

That is, when the oxygen sensor deteriorates, the internal resistance of the sensor (total resistance value including Rb and Rd in FIG. 12 described above) tends to increase. Therefore, the convergence current (absolute value) after voltage switching. Becomes smaller. Therefore, the ratio between the change in the peak current and the convergence current (absolute value thereof) becomes larger as the deterioration progresses, and that effect is judged by the deterioration judging means. In particular, by setting the deterioration determination area as described in claim 4, more accurate deterioration determination can be realized.

According to the invention of claim 5, the internal resistance detecting means detects the internal resistance of the oxygen sensor. The deterioration determining means determines that the sensor is deteriorated when the current change immediately after the voltage change detected by the current change detecting means exceeds the deterioration determining range corresponding to the sensor internal resistance detected by the internal resistance detecting means. To judge.

That is, when the oxygen sensor deteriorates, the internal resistance of the sensor tends to increase. Therefore, the ratio of the change in the peak current to the internal resistance of the sensor becomes larger as the deterioration progresses, and the deterioration judging means judges that fact. In particular, by setting the deterioration determination area as described in claim 6, more accurate deterioration determination can be realized.

In the structure of claim 2, the heater energization is feedback-controlled so that the internal resistance of the sensor matches the target value in order to judge the deterioration of the sensor only by the change of the peak current when the voltage is switched. In the configurations of claims 3 to 6, since the deterioration determination is performed by relative comparison with the converged voltage or the sensor internal resistance, the feedback control is not required. Therefore, it is possible to embody a device that simply performs energization of the heater by open control (of course,
The configurations of claims 3 to 6 can be embodied in a feedback control device).

[0022]

【Example】

(First Embodiment) A first embodiment in which the present invention is embodied in an air-fuel ratio detecting device for an internal combustion engine for an automobile will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an outline of the air-fuel ratio detecting device in this embodiment. In FIG. 1, an electronic control unit (hereinafter referred to as ECU) 1 is a CPU (central processing unit)
The microcomputer 2 mainly includes a microcomputer 2a, a ROM (read only memory) 2b, and a RAM (random access memory) 2c. The microcomputer 2 inputs a current measurement value of a limiting current type oxygen sensor 5 which will be described later, obtains and outputs an air-fuel ratio according to a predetermined calculation program. An engine control ECU 21 is connected to the microcomputer 2, and the air-fuel ratio obtained by the microcomputer is the engine control EC.
It is output to U21. The engine control ECU 21 performs air-fuel ratio feedback control based on the air-fuel ratio, other internal combustion engine information, and vehicle operation information.

On the other hand, the microcomputer 2 judges the deterioration of the oxygen sensor 5 and outputs the deterioration judgment signal to the engine control ECU 21. ECU2 for engine control
1 displays the warning light 22 in accordance with the deterioration determination signal and displays it.
A driver or the like is notified of the deterioration of the oxygen sensor 5.

The oxygen sensor 5 is provided in an exhaust pipe of an internal combustion engine (not shown) and has a detection element section 6 and a heater 7. The detection element unit 6 generates a limiting current corresponding to the oxygen concentration in the lean air-fuel ratio region or the carbon monoxide (CO) concentration in the air-fuel ratio rich region, and the heater 7 activates the detection element unit 6 at an activation temperature (for example, about 650 ° C or higher). In this case, the heater energization control circuit 3 provided in the microcomputer 2 controls the energization current to the heater 7, so that the temperature of the detection element unit 6 is maintained in the active temperature range. Specifically, the heater energization control circuit 3 has a transistor 3a as a switching element, and one end of the heater 7 is connected to the collector terminal of the transistor 3a. The transistor 3a is turned on / off in response to an energization signal from the microcomputer 2, and duty-controls energization of the heater 7. A battery power source + B is connected to the other end of the heater 7.

A voltage applying section 8 and a current measuring section 12 are connected between the microcomputer 2 and the detecting element section 6. Then, the applied voltage for detecting the limiting current controlled by the microcomputer 2 is applied to the detection element unit 6 via the D / A converter 9, the operational amplifier 10, and the resistor 11 of the voltage application unit 8. Further, the measured value of the limit current generated in the detection element unit 6 is input to the microcomputer 2 via the resistor 11 of the current measurement unit 12, the operational amplifier 13, and the A / D converter 14.

FIG. 2 is a sectional view schematically showing the structure of the oxygen sensor 5. In the detection element portion 6, the exhaust gas side electrode layer 18 is fixed to the outer surface of the solid electrolyte layer 16 formed in a cup-shaped cross section, and the atmosphere side electrode layer 19 is fixed to the inner surface. A diffusion resistance layer 17 is formed outside the exhaust gas side electrode layer 18 by a plasma spraying method or the like. The solid electrolyte layer 16 is made of ZrO ₂ , HfO ₂ , T
CaO, MgO, Y ₂ O ₃ , Y for hO ₂ , Bi ₂ O _3, etc.
The diffusion resistance layer 17 is composed of an oxygen ion conductive oxide sintered body in which b ₂ O ₃ or the like is dissolved as a stabilizer, and the diffusion resistance layer 17 is made of alumina,
It consists of heat-resistant inorganic substances such as magnesia, silica stone, spinel, and mullite. Both the exhaust gas side electrode layer 18 and the atmosphere side electrode layer 19 are made of a precious metal having a high catalytic activity such as platinum and are formed on both surfaces of the solid electrolyte layer 16 as porous chemical plating. The area and thickness of the exhaust gas side electrode layer 18 are 10 to 100 mm ² and 0.5 to 2.0.
On the other hand, the area and thickness of the atmosphere-side electrode layer 19 are 10 mm ² or more and 0.5 to 2.0 μm.

The heater 7 is housed in the atmosphere-side electrode layer 19, and the heat generation energy of the heater 7 causes the atmosphere-side electrode layer 19,
The solid electrode material layer 16, the exhaust gas side electrode layer 18, and the diffusion resistance layer 17 are heated. The heater 7 has sufficient heat generation capacity to activate the detection element unit 6.

In the oxygen sensor 5 having the above structure, the detection element section 6 generates a concentration electromotive force at the stoichiometric air-fuel ratio point and a limiting current corresponding to the oxygen concentration in the lean region from the stoichiometric air-fuel ratio point. In this case, the limiting current corresponding to the oxygen concentration depends on the area of the exhaust gas side electrode layer 18 and the diffusion resistance layer 17.
Thickness, porosity and average pore size. Also,
In a region richer than the stoichiometric air-fuel ratio, the concentration of carbon monoxide (CO), which is unburned gas, changes almost linearly with respect to the air-fuel ratio, and the detection element portion 6 of the oxygen sensor 5 responds to the CO concentration. Generates a limiting current.

Here, the voltage-current characteristics of the oxygen sensor 5 will be described with reference to FIG. That is, as shown in FIG. 3, the characteristic line L1 is a portion where the change in the current flowing through the solid electrolyte layer 16 of the detection element unit 6 is small even if the voltage applied to the solid electrolyte layer 16 changes (voltage in the figure). It has a so-called "limit current generation region" (a straight line portion parallel to the axis) (this region is also referred to as an overvoltage governing region). Then, the limiting current is specified in the limiting current generation region of this straight line portion. The limiting current value of the oxygen sensor 5 is proportional to the air-fuel ratio, increases as the air-fuel ratio becomes leaner, and decreases as the air-fuel ratio becomes richer.

In this voltage-current characteristic, the voltage region smaller than the limit current generation region is the resistance governing region, and the slope of the characteristic line L1 in the resistance governing region is the solid electrolyte layer in the detection element section 6. Specified by 16 internal resistances. Here, since the internal resistance of the solid electrolyte layer 16 changes with a change in temperature, when the temperature of the detection element unit 6 decreases, the inclination increases due to an increase in resistance. In this case, when the temperature decreases, the voltage-current characteristic is specified by the characteristic line L2 shown by the broken line in FIG. The limiting current indicated by the characteristic line L2 substantially matches the limiting current indicated by the characteristic line L1.

The present embodiment corresponds to the invention described in claim 1 or 2, and the CPU 2a in the microcomputer 2 causes the voltage switching means, the current change detecting means, the deterioration determining means, the internal resistance detection. Means and heater control means are configured.

Next, the operation of the air-fuel ratio detecting device constructed as described above will be described. The flowchart of FIG. 4 shows a main routine executed by the CPU 2a in the microcomputer 2, and the CPU 2a repeatedly executes this routine at intervals of several ms.

Now, when the routine of FIG. 4 starts,
First, in step 110, the CPU 2a executes an air-fuel ratio detection routine (A / F detection routine). Normally, only this air-fuel ratio detection routine is repeatedly executed at intervals of several ms, and if any one of steps 120, 140, 160 described later is satisfied, the internal resistance detection routine of step 130 (Zdc detection routine). , Step 15
The heater control routine of 0 or the sensor deterioration determination routine of step 170 is executed.

The contents of each step will be described in detail. Note that FIG.
Is the air-fuel ratio detection routine of step 110, FIG. 6 is the internal resistance detection routine of step 130, FIG. 7 is the heater control routine of step 150, and FIG. 8 is step 170.
The respective sensor deterioration determination routines are shown.

Now, in the air-fuel ratio detection routine of FIG. 5, the CPU 2a firstly executes the oxygen sensor 5 in step 111.
The voltage Vp (positive voltage) is applied to the detection element unit 6 of. The value of the voltage Vp is set to a value that can detect the entire range of the air-fuel ratio (limit current value Ip) to be detected as shown in FIG. For example, if the internal resistance Zdc is 30Ω and it is desired to detect the air-fuel ratio = 12 to 18, Vp = 0.3 to 0.5 [volt].
Good.

After that, the CPU 2a, when the voltage Vp is applied in step 112, the value of the current flowing through the detection element section 6,
That is, the limiting current value Ip is detected. Furthermore, the CPU 2a
In step 113, the limit current value Ip at that time is calculated by using the limit current value-air-fuel ratio map shown in FIG.
F). Further, the CPU 2a uses the following step 1
The air-fuel ratio obtained as described above in 14 is used as an engine control EC
After outputting to U21, this routine ends.

On the other hand, after detecting the air-fuel ratio, in step 120 of FIG. 4, the CPU 2a determines whether or not to detect the internal resistance Zdc of the oxygen sensor 5. At this time, the detection condition of the internal resistance Zdc should follow the temperature change of the exhaust gas. Specifically, when the engine speed, the intake pipe pressure, the intake air amount, the exhaust gas amount, etc. suddenly change, the internal resistance Zdc is changed.
It is determined that the detection is required. Note that this determination may be performed every single time, and the internal resistance Zdc may be detected periodically (for example, every one second). In this case, the engine control ECU
It is not necessary to input internal combustion engine information into the microcomputer 2 via 21.

If step 120 is positively determined, the CPU 2a proceeds to step 130 to execute the internal resistance detection routine of FIG. That is, in FIG. 6, the CPU 2 a applies the voltage Vn (negative voltage) to the detection element unit 6 of the oxygen sensor 5 in step 131. The value of the voltage Vn is a voltage in a resistance-dominated region that does not apply to the limit current generation region as shown in FIG. 3, and specifically, Vn = −
It may be 0.3 to -1 [volt].

Thereafter, the CPU 2a waits for the time t1 in step 132. That is, when the voltage applied to the oxygen sensor 5 is switched from the positive voltage Vp to the negative voltage Vn, a sharp current change (peak current) occurs immediately after the voltage switching as shown in FIG. It converges to a value In (hereinafter, “In” is referred to as a convergence current value). Therefore, after the voltage switching, the CPU 2a waits for a time t1 (several tens of ms to several hundred ms) required for the current to completely converge, and after the time t1 has elapsed, the CPU 2a detects the convergence current value In in step 133, and Value In is RAM2
Store in c.

Further, the CPU 2a returns the voltage applied to the detection element section 6 from the negative voltage Vn to the original positive voltage Vp in step 134, and waits for the time t2 in step 135. This time t2 is also a waiting time (several 10 ms to several 100 ms) for completely converging the peak current at the time of voltage switching as shown in FIG.

Thereafter, in step 136, the CPU 2a uses the negative voltage Vn in step 131 and the converged current value In (negative current value) in step 133 to determine the detection element unit 6 at that time.
Internal resistance Zdc (= Vn / I) of (solid electrolyte layer 16)
n) is calculated. In FIG. 6, as described above, the time t
After waiting for 1, t2, the current value (In, or I in FIG.
By detecting p), a highly accurate detection result can be obtained.

The processing of FIG. 6 will be described more specifically with reference to the time chart of FIG. That is, in FIG.
The voltage is switched from "Vp" to "Vn" at the timing of 1, and the converged current value In is detected at the timing of T2 when the time t1 (several tens ms to several hundred ms) has elapsed, and the voltage returns to the original voltage Vp. Will be returned. Further time t2
The internal resistance Zdc is calculated at the timing of T3 when (several 10 ms to several 100 ms) has elapsed.

Further, step 1 of the main routine shown in FIG.
At 40, the CPU 2a determines whether or not to control the energization of the heater 7. At this time, the execution condition of the heater control is satisfied when the engine speed, the intake pipe pressure, the intake air amount, the exhaust gas amount, etc. suddenly change, as in step 120. Whether or not the heater control is necessary may be determined depending on whether a predetermined time has passed since the previous heater control. In this case, the heater control is simply performed periodically (for example, every one second).

If step 140 is positively determined, the CPU 2a proceeds to step 150 to execute the heater control routine shown in FIG. That is, in this embodiment, the heater control is performed by the duty ratio control by PWM (pulse width modulation). At this time, the control duty Duty of the heater 7 is calculated by the following equations (1), (2) and (3).

GP = KP · (Zdc−ZdcT) (1) GI = GIi−1 + KI · (Zdc−ZdcT) (2) Duty = GP + GI (3) However, in the above equation , "GP" is a proportional term, "GI" is an integral term, "KP" is a proportional constant, "KI" is an integral constant, "Z".
"dcT" represents the target resistance value.

That is, in FIG. 7, the CPU 2a calculates the proportional term GP according to the deviation of the internal resistance Zdc by using the above equation (1) in step 151, and the subsequent step 152
Then, the integral term GI corresponding to the deviation of the internal resistance Zdc is calculated using the above equation (2). Then, the CPU 2a calculates the control duty Duty of the heater 7 by using the above equation (3) in step 153, and then ends this routine.
The heater 7 is energized by the heater energization control circuit 3 of FIG. 1 based on the control duty Duty. In this embodiment, so-called PI control is performed,
It is also possible to change to perform only PID control or I control.

Further, step 1 of the main routine shown in FIG.
At 60, the CPU 2a determines whether or not to make a deterioration determination of the oxygen sensor 5. At this time, as the execution condition of the deterioration determination, the determination may be made based on the driving information such as the traveling distance of the vehicle, or the necessity of the deterioration determination may be determined depending on whether a predetermined time has elapsed since the previous deterioration determination. Good.

If step 160 is positively determined, the CPU 2a proceeds to step 170 to execute the sensor deterioration determination routine of FIG. That is, in FIG. 8, the CPU 2 a determines in step 171 the oxygen sensor 5
The negative voltage Vn is applied to the detection element section 6 (the applied voltage is switched from the positive voltage Vp to the negative voltage Vn). The value of the voltage Vn is a voltage in a resistance-dominated region that does not fall within the limit current generation region as shown in FIG. 3, and may be the same as the voltage Vn in step 131 of FIG.

Thereafter, the CPU 2a waits for the time t3 in step 172. In this case, the time t3 is shorter than the time t1 (convergence time of the peak current at the time of voltage switching) in step 132 of FIG. 6 (t3 << t1), and specifically, it may be 0 to several tens ms. . Then, the CPU 2a waits for a time t3 (0 to several 10 ms), and then at step 173, the current value at that time (hereinafter,
The peak current value Io) is detected and the current value Io is stored in the RAM 2c.

Further, the CPU 2a applies the original positive voltage Vp to the detection element portion 6 in step 174, and stands by for a time t4 in step 175. This time t4
Is a waiting time (several 10 ms to several 100 ms) for converging the peak current when the voltage is switched,
It is the same time as the time t2 in FIG. 6 described above (t4 = t
2).

After that, the CPU 2a determines in step 176 whether the peak current value Io detected in step 173 is smaller than a predetermined deterioration determination value Ios. Here, the deterioration determination value Ios differs depending on the deterioration determination standard for each system, but in the present embodiment, it is set to a value which is smaller by several mA than the peak current value Io in the normal time (when the sensor is normal). In this case, if Io ≧ Ios, the CPU 2a
Judges that the sensor is normal and makes a negative decision in step 176, and ends this routine. On the other hand, Io <I
If os, the CPU 2a considers that the sensor has deteriorated and proceeds to step 177. The CPU 2a proceeds to step 17
After outputting the deterioration determination signal to the engine control ECU 21 in step 7, this routine ends. In such a case, the engine control ECU 21 causes the warning light 22 to be lit and displayed, and
Suspend the air-fuel ratio feedback control.

The process of FIG. 8 will be described more specifically with reference to the time chart of FIG. In the waveform of the sensor current, the solid line shows before deterioration (normal time), and the broken line shows after deterioration. That is, in FIG. 11, the applied voltage is switched from the positive voltage Vp to the negative voltage Vn at the timing of T11. In addition, T when time t3 (0 to several 10 ms) has elapsed
At the timing of 12, the peak current value Io is detected and the voltage is returned to the positive voltage Vp.

Further, time t4 (several 10 ms to several 100 m)
At the timing of T13 when s) has elapsed, the deterioration determination is performed based on the peak current value Io. At this time, if Io ≧ Ios as shown by the solid line, a normality determination is made (step 176 of FIG. 8 is NO). On the other hand, if Io <Ios as shown by the broken line, the deterioration determination is made (YES in step 176 of FIG. 8).

Here, the reason why the amount of change in the peak current value Io at the time of voltage switching increases with the deterioration of the oxygen sensor 5 will be described. FIG. 12 shows an equivalent circuit in the detection element section 6 of the oxygen sensor 6. In FIG. 12, “R
“B” corresponds to the internal resistance in the resistance control region, and “Rd” is the solid electrolyte layer 16 and the electrodes (exhaust gas side electrode layer 18, atmosphere side electrode layer 1) in the limiting current generation region (overvoltage control region).
9) Corresponds to the resistance existing at the interface with. "Cd" is also the capacitance of the interface. However, Zdc = Rb + Rd
Is. In this case, immediately after the applied voltage is switched, the current passes through the path from the resistance Rb to the resistance Rd (path in the figure), so the peak current value Io is determined by the resistance Rb (the following equation (4)). .

Io = Ip− (Vp−Vn) / Rb (4) When the oxygen sensor 5 deteriorates, the inside of the sensor exhibits the following phenomenon. That is, the electrodes of the oxygen sensor 5 (exhaust gas side electrode layer 18 and atmosphere side electrode layer 19) are formed of platinum in a porous form, but are agglomerated due to deterioration, and oxygen is prevented from passing through the solid electrolyte layer 16. Disturbed. In this case, the resistance Rd (interface resistance) increases. And
When the heater control is performed in order to keep the internal resistance Zdc constant in the above-described deteriorated state, the heater 7 is controlled to have a higher temperature than before the deterioration, and the resistance Rb needs to be reduced. Therefore, after deterioration, the internal resistance Zdc (= Rb + Rd) is higher than that before deterioration.
The resistance Rb becomes small even if the same. Therefore, the peak current value Io increases in the negative direction.

According to the first embodiment described in detail above, the deterioration of the oxygen sensor 5 can be determined easily and accurately by monitoring the change in the peak current value Io during the voltage switching. Also, in recent air-fuel ratio control systems,
Since it is desired to reduce the emission as much as possible, it has been proposed to employ the heater energization control (feedback control of FIG. 7) for maintaining the internal resistance Zdc at a constant value as described above. It can be suitably used for a system using the above heater control.

Further, the applied voltage is changed from “Vp” → “Vn” →
When switching to "Vp", a waiting time (usually several 10 ms to several 100 m) for waiting for convergence at each voltage switching.
s) is required, but only a short time (0 to several tens ms) is required to detect the peak current value Io as the deterioration determination element in the deterioration determination of the present embodiment. Therefore, the time required for the deterioration determination can be shortened.

(Second Embodiment) Next, a second embodiment embodying the invention described in claims 3 and 4 will be described with a focus on differences from the first embodiment. In the first embodiment, the heater energization is feedback-controlled to match the internal resistance Zdc of the oxygen sensor 5 with the target value. However, in the second embodiment, the heater energization is changed to open control. There is. The flowchart of FIG. 13 shows the main routine in the second embodiment, which corresponds to the flowchart of FIG. 4 of the first embodiment. The flowchart of FIG. 14 shows the sensor deterioration determination routine in the second embodiment, which corresponds to the flowchart of FIG. 8 of the first embodiment.

In the main routine of FIG. 13, compared with FIG. 4, the processing relating to the internal resistance detection (step 12 in FIG. 4) is performed.
0, 130) and the processing relating to the heater control (steps 140 and 150 in FIG. 4) are omitted, and regarding the heater control, an open control with a constant control duty is performed by a control circuit (not shown).

Now, when the main routine of FIG. 13 is started, the CPU 2a first executes an air-fuel ratio detection routine (A / F detection routine) in step 210. This air-fuel ratio detection routine may be exactly the same as step 110 (FIG. 5) of FIG. 4, and therefore its explanation is omitted here.

Thereafter, the CPU 2a determines whether or not the deterioration of the oxygen sensor 5 is detected in step 220, and if the deterioration is determined, the process proceeds to step 230. Below, FIG.
The sensor deterioration determination routine will be described using.

In FIG. 14, steps 231-233.
Is the same processing as steps 171 to 173 of FIG.
The PU 2a applies the negative voltage Vn in step 231,
In step 132, the process waits for a predetermined time t3 (0 to several tens of ms), and in step 233, the peak current value Io is detected and the detected value is stored in the RAM 2c. Then, after detecting the peak current value Io, the CPU 2a proceeds to step 234.
After waiting for time t5 (several 10 ms to several 100 ms) at step 235, the converged current value In is detected and the RAM is detected.
Store in 2c. Here, the time t5 is the time required for the current to completely converge after the voltage is switched. After that, the CPU 2a applies the original voltage Vp to the detection element unit 6 in step 236.

After that, the CPU 2a uses the Io-In map shown in FIG. 16 in step 237 to judge the deterioration of the oxygen sensor 5 based on the latest current value Io, In (detected value in steps 233 and 235) in the RAM 2c. To do. That is, the map of FIG. 16 is preset in the ROM 2b and is divided into a deterioration area (area A in the drawing) and a normal area (area B in the drawing) with the deterioration determination line L3 as a boundary. . The deterioration determination line L3 is set to draw a rising line as the converged current value In increases toward the negative side. In this case, if the point connecting the converged current value In and the peak current value Io at that time is in the region B, the CPU 2a regards the sensor as normal and makes a negative determination in step 237.

If the point connecting the converged current value In and the peak current value Io at that time is in the area A, the CPU 2a regards it as deteriorated and makes an affirmative decision in step 237.
Then, the CPU 2a proceeds to step 238 and outputs the deterioration determination signal to the engine control ECU 21. After the processing of steps 237 and 238, the CPU 2a executes step 2
At 39, the system waits for time t6 (several tens of ms to several hundreds of ms) so that the next air-fuel ratio detection can be carried out accurately, and then this routine ends.

The process of FIG. 14 will be described more specifically with reference to the time chart of FIG. That is, in FIG.
At the timing of T21, the applied voltage is switched from the positive voltage Vp to the negative voltage Vn, and time t3 (0 to several tens ms)
The peak current value Io is detected at the timing of T22 when is elapsed.

Further, time t5 (several 10 ms to several 100 m)
At the timing of T23 when s) has elapsed, the applied voltage is returned to the original voltage Vp. At this time, the peak current value I
Deterioration determination is performed based on o and the converged current value In. After that, at the timing of T24 when the time t6 has elapsed, FIG.
Routine ends.

Here, in the map of FIG.
The deterioration determination line L3 is set as follows. That is, a line in which the initial characteristics of the oxygen sensor 5 (shown by the broken line in the figure) are measured in advance and the peak current value Io is increased to the negative side by several% is used as the deterioration determination line L3. The rate of increase is changed depending on the degree of determination, and when it is desired to determine the deterioration in the relatively early stage, it may be close to the initial characteristic.
Further, since the relationship between the current value Io and In has some correlation with the limiting current value Ip and the like, when the deterioration determination is performed more strictly, the limiting current value Ip and the like are shown in the two-dimensional map of FIG. Elements may be added to form a three-dimensional or more-dimensional map.

According to the second embodiment detailed above, it is determined that the sensor is deteriorated when the amount of change in the peak current value Io exceeds the deterioration determination range corresponding to the converged current value In. I did it. That is, in the case where the heater energization is open-controlled, when the oxygen sensor 5 deteriorates, the internal resistance Zdc of the sensor 5 tends to increase, so the convergent current (absolute value) after voltage switching becomes small. In this case, the change in peak current is equal to or more than that before deterioration.
Therefore, the ratio between the amount of change in the peak current and (the absolute value of) the converged current increases as the deterioration progresses. As a result, it is possible to easily determine that the oxygen sensor 5 has deteriorated.

Although the heater control is open control in the second embodiment, it may be feedback control as in the first embodiment. In this case, the converged current value In does not change because the internal resistance Zdc does not change, but the peak current value Io changes according to the deterioration state due to the change in the resistance Rb (see FIG. 12). Therefore, it is possible to determine the deterioration of the oxygen sensor 5 using the relationship shown in FIG.

(Third Embodiment) A third embodiment of the present invention as defined in claims 5 and 6 will be described below.
The difference from the second embodiment will be mainly described. In addition, this third
In the embodiment, the deterioration determination of the oxygen sensor 5 is performed according to the relationship between the peak current value Io and the internal resistance Zdc during the voltage switching.

That is, the internal resistance Zdc of the oxygen sensor 5 and the convergent current value In have a relationship of "Zdc = Vn / In". Therefore, instead of the deterioration determination using the convergent current value In of FIG. 14, the deterioration determination using the internal resistance Zdc is performed. The flowchart of FIG. 17 shows the main routine in the third embodiment, which corresponds to the flowchart of FIG. 4 of the first embodiment. The flowchart of FIG. 18 shows the sensor deterioration determination routine in the third embodiment, which corresponds to the flowchart of FIG. 8 of the first embodiment.

In the main routine of FIG. 17, compared with FIG. 4, the processing relating to heater control (step 14 in FIG. 4) is performed.
0, 150) are omitted, and regarding the heater control, open control with a constant control duty is performed by a control circuit (not shown). However, the heater control may be performed by PI control as in the first embodiment, or the oxygen sensor 5 may be used.
The element temperature feedback control may be performed to keep the element temperature at the target temperature.

Now, when the main routine of FIG. 17 is started, the CPU 2a first executes an air-fuel ratio detection routine in step 310. The CPU 2a also executes step 32.
At 0, it is determined whether or not the internal resistance Zdc is detected, and if it is detected, an internal resistance detection routine is executed at step 330. The processing of steps 310 to 330 is
Since the steps 110 (FIG. 5), 120, and 130 (FIG. 6) in FIG. 4 are exactly the same, the description thereof is omitted here.

Thereafter, the CPU 2a determines whether or not the deterioration of the oxygen sensor 5 is detected in step 340. If the deterioration is determined, the process proceeds to step 350. Below, FIG.
The sensor deterioration determination routine will be described using.

Incidentally, in FIG. 18, steps 351 to
Step 355 is the same process as steps 171-175 in FIG. That is, the CPU 2a determines in step 351 that the negative voltage V
n is applied, and in step 352, a predetermined time t3 (0 to several 1
Wait for 0 ms). In step 353, the peak current value Io is detected and the detected value is stored in the RAM 2c.
Remember. In step 354, the voltage is switched from the negative voltage Vn to the positive voltage Vp, and in step 355, the process waits for a predetermined time t5 (several tens to several hundreds ms). After that, the CPU 2a proceeds to step 356, where Io shown in FIG.
-Using the Zdc map, the latest current value I in the RAM 2c
o, internal resistance Zdc (Io detected in step 353,
Deterioration determination of the oxygen sensor 5 is performed based on Zdc) detected in step 330 of FIG. That is, the map of FIG. 20 is preset in the ROM 2b and is divided into a deterioration area (area C in the drawing) and a normal area (area D in the drawing) with the deterioration determination line L4 as a boundary. . Deterioration determination line L4
Is set to draw an increasing line as the internal resistance Zdc decreases. In this case, the internal resistance Zdc at that time
If the point connecting the peak current value Io and the peak current value Io is in the area D, CP
U2a regards the sensor as normal and makes a negative determination in step 356.

If the point connecting the internal resistance Zdc and the peak current value Io at that time is in the region C, the CPU 2a determines that the oxygen sensor 5 is deteriorated and makes a positive determination in step 356. Then, the CPU 2a causes the step 357.
And outputs a deterioration determination signal to the engine control ECU 21. After the processing of steps 356 and 357, the CPU 2a
Ends this routine.

The processing of FIG. 18 will be described more specifically with reference to the time chart of FIG. That is, in FIG.
The voltage is switched from the positive voltage Vp to the negative voltage Vn at the timing of T31, the peak current value Io is detected at the timing of T32 when the time t3 (0 to several 10 ms) has elapsed, and the applied voltage is the positive voltage Vp. Returned to. After that, the deterioration determination is performed based on the peak current value Io and the internal resistance Zdc at the timing of T33 when the time t4 has elapsed.

Here, in the map of FIG.
The deterioration determination line L4 is set as follows. That is, as in the case of FIG. 16, the initial characteristics (indicated by a broken line in the figure) of the oxygen sensor 5 are measured in advance, and then the peak current value I
The line in which o is increased to the negative side by several% is the deterioration determination line L4.
I am trying. The rate of increase is changed depending on the degree of determination, and when it is desired to determine the deterioration in the relatively early stage, it may be close to the initial characteristic.

As described above, according to the third embodiment, when the change amount of the peak current value Io exceeds the deterioration determination range corresponding to the internal resistance Zdc, it is determined that the sensor is deteriorated. . In this case, it is possible to accurately determine the deterioration of the oxygen sensor 5 as in the first and second embodiments.

The present invention can be embodied in the following modes other than the above embodiment. (1) In the above embodiment, the voltage applied to the oxygen sensor 5 in order to detect the internal resistance Zdc is once switched from Vp to Vn, and the current value In is detected by the voltage Vn. Alternatively, it may be obtained by AC impedance. In this case, an alternating voltage is applied to the oxygen sensor 5, and the internal resistance Zdc is measured from the voltage amplitude and the resulting current amplitude. In addition, Japanese Patent Publication No. 7-1883
It may be measured as in JP-A-7.

(2) In the above embodiment, the deterioration of the oxygen sensor 5 was judged based on the change in the peak current generated when the voltage applied to the oxygen sensor 5 was switched from the positive voltage Vp to the negative voltage Vn. A change in the peak current that occurs when the negative voltage Vn is switched to the positive voltage Vp (for example, T in FIG. 10).
The deterioration determination of the sensor 5 may be performed based on the change of the peak current that occurs at the timing 2). Also in this case, the same effect as that of the above embodiment can be obtained.

(3) In the above embodiment, the voltage Vp is set to a fixed value in accordance with the air-fuel ratio range to be detected when detecting the air-fuel ratio, but this may be set variably. That is, FIG.
The applied voltage setting line L5 shown in is used to set the voltage Vp on the setting line L5 according to the limiting current value Ip (air-fuel ratio) at that time (Vp = Z · Ip + Ve). However, the setting line L5
The slope Z of is substantially equal to the internal resistance of the element, and the intercept Ve with respect to the V-axis corresponds to the approximate midpoint of the limiting current region at the ideal air-fuel ratio (Ip = 0 mA).

(4) The temperature of the detection element portion 6 of the oxygen sensor 5 (element temperature) is detected, and the element temperature and internal resistance Zdc are detected.
The internal resistance Zdc may be measured by the correspondence relationship with.

[0085]

According to the invention described in claim 1, a current change (change in peak current value) until the current converges to a predetermined value is detected immediately after switching the applied voltage of the oxygen sensor, and the detection thereof is performed. By performing the deterioration determination based on the result, the excellent effect that the deterioration determination of the oxygen sensor can be accurately performed by a new method is exhibited.

According to the second aspect of the present invention, the device for feedback-controlling the energization of the heater so that the internal resistance of the oxygen sensor matches the target value can be maintained even when the internal resistance is maintained at a constant value. It is possible to accurately determine deterioration of the sensor.

According to the third and fourth aspects of the invention, the deterioration of the oxygen sensor can be accurately determined by using the relationship between the peak current value immediately after the voltage switching and the converged current value after the convergence. it can.

According to the fifth and sixth aspects of the invention, the deterioration of the oxygen sensor can be accurately determined by using the relationship between the peak current value immediately after the voltage switching and the internal resistance of the sensor.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an electrical configuration of an air-fuel ratio detection device in an embodiment.

FIG. 2 is a sectional view showing the configuration of an oxygen sensor.

FIG. 3 is a diagram showing a voltage-current characteristic of an oxygen sensor.

FIG. 4 is a flowchart showing a main routine.

FIG. 5 is a flowchart showing an air-fuel ratio detection routine.

FIG. 6 is a flowchart showing an internal resistance detection routine.

FIG. 7 is a flowchart showing a heater control routine.

FIG. 8 is a flowchart showing a sensor deterioration determination routine.

FIG. 9 is a limit current value-air-fuel ratio map.

FIG. 10 is a time chart for more specifically explaining the operation of the internal resistance detection routine of FIG.

FIG. 11 is a time chart for more specifically explaining the operation of the sensor deterioration determination routine of FIG.

FIG. 12 is a diagram showing an equivalent circuit of an oxygen sensor and waveforms of an applied voltage and a sensor current.

FIG. 13 is a flowchart showing a main routine in the second embodiment.

FIG. 14 is a flowchart showing a sensor deterioration determination routine in the second embodiment.

FIG. 15 is a time chart for more specifically explaining the operation of the sensor deterioration determination routine of FIG.

FIG. 16 is a diagram showing a deterioration region based on a relationship between a peak current value and a converged current value.

FIG. 17 is a flowchart showing a main routine in the third embodiment.

FIG. 18 is a flowchart showing a sensor deterioration determination routine in the third embodiment.

FIG. 19 is a time chart for more specifically explaining the operation of the sensor deterioration determination routine of FIG.

FIG. 20 is a diagram showing a deterioration region based on a relationship between a peak current value and an internal resistance.

FIG. 21 is a diagram in which an applied voltage setting line is added to the voltage-current characteristics of the oxygen sensor.

[Description of Reference Signs] 2a ... Voltage switching means, current change detecting means, deterioration determining means, internal resistance detecting means, heater control means, CPU as convergent current detecting means, 5 ... Oxygen sensor, 7 ... Heater.

Continuation of the front page (56) Reference JP-A-8-327586 (JP, A) JP-A-61-241647 (JP, A) JP-A-62-198751 (JP, A) JP-A-60-218058 (JP , A) JP-A-7-55761 (JP, A) JP-A-1-262460 (JP, A) JP-A-60-192849 (JP, A) JP-A-4-233447 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/26 G01N 27/41 G01N 27/409

Claims (6)

(57) [Claims] 1. A deterioration determination device applied to an oxygen sensor that outputs a substantially constant limiting current corresponding to oxygen concentration when a voltage is applied, wherein the applied voltage of the oxygen sensor is a positive voltage to a negative voltage. Shi
A voltage switching means for switching from a negative voltage to a positive voltage , and a current change detection for detecting a current change accompanying the voltage switching immediately after the voltage switching by the voltage switching means until the current converged by the oxygen sensor. A deterioration determination device for an oxygen sensor, comprising: means and a deterioration determination means for determining that the oxygen sensor is deteriorated based on a current change detected by the current change detection means. 2. An internal resistance detecting means for detecting an internal resistance of the oxygen sensor, a heater attached to the oxygen sensor, a deviation between a sensor internal resistance detected by the internal resistance detecting means and a predetermined target value. Heater control means for feedback control of energization to the heater in order to eliminate the change, the deterioration determination means, the current change immediately after the voltage switching detected by the current change detection means, the voltage change by the voltage switching means The deterioration determination device for an oxygen sensor according to claim 1, wherein it is determined that the sensor is deteriorated when a corresponding deterioration determination range is exceeded. 3. A converged current detecting means for detecting a current value when the sensor current converges after the voltage is switched by the voltage switching means, and the deterioration determining means includes the voltage detected by the current change detecting means. The deterioration determination of the oxygen sensor according to claim 1, wherein it is determined that the sensor is deteriorated when a change in current immediately after switching exceeds a deterioration determination range corresponding to the convergence current detected by the convergence current detection unit. apparatus. 4. The deterioration determination device for an oxygen sensor according to claim 3, wherein the deterioration determination means sets the deterioration determination region in a direction in which the current change increases as the absolute value of the converged current increases. 5. An internal resistance detecting means for detecting an internal resistance of the oxygen sensor is provided, wherein the deterioration determining means detects, by the internal resistance detecting means, a current change immediately after voltage switching detected by the current change detecting means. The deterioration determination device for an oxygen sensor according to claim 1, wherein when the deterioration determination range corresponding to the detected internal resistance of the sensor is exceeded, it is determined that the sensor is deteriorated. 6. The deterioration determination device for an oxygen sensor according to claim 5, wherein the deterioration determination means sets the deterioration determination region in a direction in which the change in the current decreases as the internal resistance of the sensor increases.