JP2002048761A - Heater control device of gas concentration sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suitably detect deterioration of a detection element, and to thereby protect the detection element. SOLUTION: An A/F sensor 30 has the detection element 61 for outputting a limiting current roughly proportional to the oxygen concentration in engine emission gas and a heater 64 for heating the detection element 61. A CPU 22 in a microcomputer 21 executes feedback control of energization to the heater 64 in the range of the energization quantity prescribed beforehand so that impedance of the detection element 61 agrees with a command. When the feedback control of heater energization is executed, the CPU 22 determines whether the heater energization quantity reaches the upper limit of the prescribed range or its periphery or not, and if the heater energization quantity reaches the upper limit of the prescribed range or its periphery and the state is continued as long as a prescribed time or longer, the CPU 22 determines that the detection element 61 is deteriorated. When the detection element 61 is determined to be deteriorated, the CPU 22 corrects the command of the impedance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heater control device for a gas concentration sensor, and more particularly to an air-fuel ratio sensor for detecting an oxygen concentration in exhaust gas of an internal combustion engine. The present invention relates to a technique for suitably controlling energization of a heater.

[0002]

2. Description of the Related Art As a gas concentration sensor of this type, for example, a limiting current type air-fuel ratio sensor for detecting oxygen concentration in exhaust gas of an engine is known. Techniques such as 278279 and JP-A-10-300716 are disclosed.

[0003] In addition, the air-fuel ratio sensor is generally known to have a cup-shaped structure having a cup-shaped cross section and a laminated structure having a plate-shaped detecting element, a heater member, and the like laminated. In recent years, those having a laminated structure which is suitable for miniaturization and cost reduction and has excellent temperature rising characteristics of a detection element are being used frequently. An air-fuel ratio sensor having this laminated structure is disclosed in, for example, Japanese Patent Application Laid-Open No. 11-344466.
Since the detecting element and the heater are arranged close to each other and the difference between the element temperature and the heater temperature is relatively small, the heater power is controlled by the internal resistance (impedance) of the detecting element instead of the heater power control based on the detected value of the heater resistance. Control is performed. That is, the heater power supply amount is feedback-controlled so that the impedance of the detection element becomes a predetermined target value. In the air-fuel ratio sensor having the stacked structure, the heat transfer effect from the heater to the detection element is improved, so that the amount of heat generated by the heater can be suppressed, that is, the current value can be reduced.

[0004]

However, the detection element may be deteriorated due to long-time heating by a heater or poisoning of gasoline components, and the impedance characteristics may be changed. In this case, the element temperature rises due to a change in the impedance characteristic, and there is a risk that further thermal degradation will be promoted by continuing the heater energization in this state.
Further, if high-temperature exhaust gas is discharged from the internal combustion engine in this state, the element temperature may further increase.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide a gas concentration capable of suitably detecting deterioration of a detection element and thus protecting the detection element. The object is to provide a heater control device for the sensor.

[0006]

In the gas concentration sensor, the heater energization amount is feedback-controlled within a predetermined energization amount range so that the element resistance (impedance of the detection element) matches a target value (heater control means). ). When the detection element is activated by the heater energization control, the detection element outputs a limit current substantially proportional to the concentration of a specific component in the exhaust gas of the internal combustion engine. In particular, according to the first aspect of the present invention, when the heater energization feedback control is performed, if the heater energization amount is at or near the upper limit value of the specified range, the deterioration of the detection element is determined from the state (deterioration). Determination means).

In short, when the detection element deteriorates, the element resistance increases, and the deviation from the target value increases. If feedback control for eliminating the deviation is continued,
The heater energization amount increases and reaches the upper limit of the specified range of the feedback control. Therefore, if the heater power supply is at or near the upper limit value of the specified range, deterioration of the detection element can be suitably detected. In this case, if the deterioration of the detection element can be correctly detected, further progress of the deterioration of the detection element is suppressed by limiting the energization of the heater thereafter. Therefore, protection of the detection element can be achieved.

In particular, a gas concentration sensor having a so-called laminated structure in which a heater is laminated on a detection element having a solid electrolyte (Claim 11) is excellent in temperature characteristics, but is liable to cause problems such as element cracking. According to the present invention, the above problem is solved by suitably performing the deterioration determination as described above. Therefore, effects such as element protection can be more effectively obtained.

It is desirable that the deterioration determining means is embodied in the following second or third aspect. That is, in the invention according to claim 2, it is determined whether or not the heater energization amount has reached the upper limit value of the specified range or its vicinity,
If the heater power supply reaches the upper limit of the specified range or its vicinity and the state continues for a predetermined time or more, it is determined that the detection element has deteriorated. According to the third aspect of the present invention, if the average value of the heater power supply amount during the idling operation is at or near the upper limit value of the specified range, it is determined that the detection element is deteriorated. In any of these cases, it can be correctly detected that the detection element has deteriorated.

In the invention according to the fourth aspect, when it is determined that the detection element is deteriorated, the target value of the element resistance is corrected. Heater energization control can be performed. In this case, the temperature of the detection element can be controlled as desired even when the element is deteriorated. Thereby, an excessive rise in temperature of the detection element is suppressed, and the detection element can be protected.

In such a case, as described in claim 5,
The correction means may gradually change the target value so as to eliminate the deviation of the element resistance. At this time, it is preferable to correct the deviation of the element resistance while smoothing it or to correct the deviation with a relatively small predetermined value.

Further, the correcting means may correct the target value of the element resistance only on the increasing side. That is, when the gas concentration sensor (detection element) is exposed to the exhaust gas of the internal combustion engine, if the temperature of the exhaust gas is high,
There is a concern that the deviation of the element resistance temporarily decreases due to the heat received from the exhaust gas, and the target value of the element resistance once corrected to the increasing side is erroneously re-corrected to the decreasing side due to the element deterioration. However, by limiting the correction of the target value to the increase side, the disadvantage that the target value of the element resistance is erroneously corrected is prevented.

Further, in place of the above-mentioned claim 6, the target value of the element resistance is corrected only when the exhaust gas from the internal combustion engine is in a relatively low state. Is also good. For example, when the internal combustion engine is idling, the temperature of the exhaust gas is low, and it is considered that the temperature of the exhaust gas has little effect. Therefore, erroneous correction can be prevented by performing the correction only during such idling operation.

According to the present invention, each time the correction is performed by the correction means, the corrected target value of the element resistance is stored in the temporary storage memory, and is stored in the temporary storage memory at a longer time interval. The target value of the element resistance thus set is stored in the backup memory. In the present invention, even if the target value of the element resistance is erroneously corrected, it is temporarily stored in the temporary storage memory (normal RAM) and is not immediately stored in the backup memory (standby RAM). Therefore, there is no inconvenience that the target value of the element resistance is erroneously learned.

According to the ninth aspect of the present invention, at the beginning of heater energization at the start of the internal combustion engine, a target value stored in the backup memory is read out and used for feedback control of heater energization. Thus, it is possible to perform suitable heater energization control using the target value learned last time from the start of the internal combustion engine.

Further, when the present invention is applied to an air-fuel ratio control system for an internal combustion engine, the heater of the gas concentration sensor is preferably energized as described above, so that the gas concentration sensor ( The appropriate activation state of the air-fuel ratio sensor is maintained, and the air-fuel ratio can be correctly detected. Therefore, the air-fuel ratio control can be performed with high accuracy.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) An embodiment in which the present invention is embodied in an air-fuel ratio control system for a vehicle-mounted engine will be described below with reference to the drawings. The present control system controls the fuel injection amount to the engine at a desired air-fuel ratio based on the detection result by an air-fuel ratio sensor (A / F sensor). In the present embodiment, in particular, the A / F sensor The details of the heater energization control for satisfactorily activating the power supply will be described below.

FIG. 1 is an overall configuration diagram showing an outline of an air-fuel ratio control system according to the present embodiment. In FIG. 1, a limiting current type air-fuel ratio sensor (hereinafter, referred to as an A / F sensor) 30 is attached to an exhaust pipe 12 extending from an engine body 11 of an engine 10, and is provided from an electronic control unit (hereinafter, referred to as an ECU) 20. With the application of the commanded voltage, a linear air-fuel ratio detection signal (sensor current signal) proportional to the oxygen concentration in the exhaust gas is output.

The ECU 20 includes a microcomputer (hereinafter, referred to as a microcomputer) 21 which is a center of various controls. The microcomputer 21 includes a CPU 22 for executing various arithmetic programs, and a ROM 23 for storing various programs and control data in advance. And an NRAM (normal RAM) 24 for temporarily storing operation data, and a data backup SRAM (standby RAM) 25 for retaining the stored contents even when the power is turned off. The microcomputer 21 controls the applied voltage to the A / F sensor 30 and controls the heater energization of the A / F sensor 30 in addition to controlling the fuel injection and ignition of the engine 10.

Briefly describing the control regarding the A / F sensor 30, the microcomputer 21 sends a bias command signal Vr for applying a voltage to the A / F sensor 30 to the D / A converter 26 and an LPF (low-pass filter). 27 to the bias control circuit 40. At this time, after the bias command signal Vr is converted into the analog signal Vb by the D / A converter 26 and the high frequency component of the analog signal Vb is removed by the LPF 27, the output voltage Vc is changed to the bias control circuit 40.
Is input to

The current detection circuit 5 in the bias control circuit 40
0 detects the value of the current flowing with the application of the voltage to the A / F sensor 30. The analog signal of the current value detected by the current detection circuit 50 is input to the microcomputer 21 via the A / D converter 28. The microcomputer 21 takes in the sensor current at a predetermined time period (for example, every several milliseconds) and converts the current value into an A / F. Also, the A / F sensor 30
When the impedance is detected, a single-shot voltage having a predetermined time constant is applied to the A / F sensor 30 by a rectangular bias command signal Vr output from the microcomputer 21.

The microcomputer 21 further includes a heater control circuit 2
9 to output a heater control signal. Thus, A
The energization of the heater 64 provided in the / F sensor 30 is duty-controlled.

The A / F sensor 30 has a stacked sensor element section (cell) 60, and its structure is shown in FIG.
This will be described with reference to FIG. Here, FIG. 2 is a cross-sectional view illustrating the overall configuration of the A / F sensor 30, and FIG. 3 is a cross-sectional view of the sensor element unit 60 configuring the A / F sensor 30.

As shown in FIG. 2, the A / F sensor 30
Is a cylindrical metal housing 31 screwed to the exhaust pipe wall.
The element cover 32 is attached to the lower opening of the housing 31. In the element cover 32,
The distal end (lower end) of the long plate-shaped sensor element section 60 is provided. The element cover 32 has a bottomed double structure, and has a plurality of exhaust ports 32a for taking exhaust gas into the inside of the cover. The sensor element section 60 extends upward in the drawing so as to penetrate the insulating member 33 provided in the housing 31, and a pair of lead sections 34 is connected to the upper end thereof.

A main body cover 35 is provided on the upper end of the housing 31.
Is being swaged. Further, a dust cover 36 is attached above the main body cover 35, and the upper part of the sensor is protected by the double structure of the main body cover 35 and the dust cover 36. Each cover 35, 36 is provided with a plurality of air ports 35a, 36a for taking air into the covers.

Next, the structure of the sensor element section 60 will be described with reference to FIG. The sensor element section 60 is roughly composed of a detection element 61 made of a solid electrolyte, a gas diffusion resistance layer 62, an air introduction duct 63, and a heater 64, and is formed by laminating these members. A protective layer 65 is provided around each member.

A rectangular plate-like detection element (solid electrolyte) 61
Is a sheet made of partially stabilized zirconia. On its upper surface (gas diffusion resistance layer 62 side), a porous measurement electrode 66 made of platinum or the like is formed, and on its lower surface (atmosphere introduction duct 63 side), A porous atmosphere-side electrode 67 made of platinum or the like is formed. Lead portions 66a, 67a are connected to the measurement electrode 66 and the atmosphere-side electrode 67,
The other end is connected to the ECU 20.

The gas diffusion resistance layer 62 has a gas permeable layer 62a made of a porous sheet for introducing exhaust gas to the measurement electrode 66, and a gas shielding layer 62b made of a dense layer for suppressing the transmission of exhaust gas. . Each of the gas permeable layer 62a and the gas shielding layer 62b is formed by molding a ceramic such as alumina, spinel, or zirconia by a sheet molding method or the like. It has become something. In this case, since the surface of the gas permeable layer 62a is covered with the gas shielding layer 62b, the exhaust gas around the sensor element portion enters from the side of the gas permeable layer 62a (the left-right direction in the drawing) and reaches the measurement electrode 66. .

The air introduction duct 63 is made of a highly heat-conductive ceramic such as alumina, and an air chamber 68 is formed by the duct 63. The air introduction duct 63 plays a role of introducing air into the atmosphere-side electrode 67 in the atmosphere chamber 68. Incidentally, the atmosphere chamber 68 is provided with the cover 3 shown in FIG.
The air ports 5a and 36a communicate with the air ports 35a and 36a.

A heater 64 is provided on the lower surface of the air introduction duct 63.
Is attached. The heater 64 includes a heating element 64a that generates heat when energized by a vehicle-mounted battery, and an insulating sheet 64b that covers the heating element 64a. Leads 64c are connected to both ends of the heating element 64a. However, in addition to the configuration shown in FIG. 3, a configuration in which the heating element 64a is embedded in the detection element 61 or the heating element 64a is embedded in the gas diffusion resistance layer 62 is also possible.

The A / F sensor 30 having the above configuration has a voltage-current characteristic shown in FIG. That is, the sensor element unit 60
The (detection element 61) is capable of detecting the oxygen concentration with a linear characteristic, and generates a limit current according to the oxygen concentration. The increase / decrease of the limit current (sensor current) corresponds to the increase / decrease of the A / F value (that is, the degree of lean / rich),
The limit current increases as the A / F value becomes leaner, and A / F
The limiting current decreases as the F value becomes richer. Here, there is a correlation between the impedance (element resistance) of the detection element 61 and the temperature (element temperature) of the element 61, and the impedance tends to decrease as the element temperature increases.
In this case, the element temperature is maintained at the target temperature (for example, 750 ° C.) by performing the F / B control of the heater energization so that the impedance of the detection element 61 becomes the target value (for example, 30Ω).

Next, an outline of the heater energization control will be described. Here, FIGS. 6 to 8 are flowcharts showing a control program executed by the CPU 22 in the microcomputer 21. FIG. 6 is executed as an initial routine when the engine is started. FIG. 7 shows a heater control amount calculation routine, which is executed, for example, every 131 milliseconds. FIG. 8 shows a target impedance update process, which is executed, for example, every one second.

When the process of FIG. 6 is started with the start of the engine, in step 101, the target impedance Ztg (learned value up to the previous time) already stored in the SRAM 25 is read, and the Ztg is stored in the NRAM 24. Thereafter, the heater 64 using the Ztg value of the NRAM 24
, And the heater control reflecting the learning result up to the previous operation is performed.

On the other hand, in FIG. 7, roughly, the full energization control, the first heater F / B control, and the second heater F / B control are sequentially performed. , The full energization control is performed, and in the full energization control, the heater control amount (Duty) is controlled at 100%.
Further, the first heater F / B control is performed following the full energization control. In the first heater F / B control, the heater control amount (Duty) is set to 0 to 0 according to the deviation of the impedance.
It is controlled within a specified range of 80%. Further, the second heater F / B control is performed subsequent to the first heater F / B control. In the second heater F / B control, the heater control amount (Duty) is 0 to 0 according to the impedance deviation. 60%
Is controlled within the specified range.

Specifically, in FIG. 10 showing the impedance characteristics when the sensor is new, the actual impedance Zre
When the target impedance Ztg is “30Ω” corresponding to the element temperature of 750 ° C., the impedance deviation ΔZ (= Zre−Ztg)
Is larger than a predetermined value K1 (for example, 20Ω), the full energization control is performed. Also, the impedance deviation Δ
Z is within a range of predetermined values K2 to K1 (for example, 10 to 20).
Ω), the first heater F / B control is performed, and Δ
If Z is smaller than a predetermined value K2 (for example, 10Ω),
The second heater F / B control is performed.

Further, when the second heater F / B control is performed, if the heater control amount (Duty) is maintained at the upper limit (60%) of the F / B control range, the detection element 61 deteriorates. Is determined. In other words, when the detection element 61 deteriorates, the deviation of the impedance from the target value increases. When the F / B control for eliminating the deviation is continued, the duty increases and reaches the upper limit of the F / B control range. Thus, the deterioration of the detection element 61 is detected.

Here, the real impedance Zre is
The voltage is detected by a well-known sweeping method. Specifically, as shown in FIG. 5, the voltage applied to the A / F sensor 30 is temporarily changed in the positive direction and the negative direction. Then, the real impedance Zre is calculated from either the positive or negative voltage change amount ΔV and the current change amount ΔI at the time of the voltage change (Zre = ΔV / ΔI). Hereinafter, the detailed contents of the heater energization will be described with reference to FIG.

First, in step 201 of FIG. 7, the deterioration of the detecting element 61 is determined. Here, the deterioration determination is performed when the heater control amount (D
uty) remains at the upper limit value (60%) for a predetermined time or more. In practice, it is assumed that 300 seconds have elapsed since the engine was started, and the value of a counter Cmax described later is set to a predetermined time. If it is (for example, 20 seconds) or more, it is determined that the detection element 61 has deteriorated.

If there is no element deterioration, step 2
Go to 04. If the element has deteriorated, the process proceeds to step 202, where the target impedance Ztg is corrected. That is, in step 202, the target impedance Ztg, which is the NRAM value, is corrected according to the impedance deviation ΔZ. However, the target impedance Ztg (NRAM
Value) is limited to the increase side only.
The reason will be described later.

In correcting the target impedance Ztg, it is desirable to perform correction using a technique such as smoothing calculation. As an example, the previous value Ztg of the target impedance is calculated by a calculation such as Ztg = Ztg (i-1) + ΔZ / 4.
(I-1) is corrected, and the current value is calculated. Note that the ΔZ value in the previous process may be used for the correction. Of course, with a relatively small constant update amount α, the target impedance Ztg
(Ztg = Ztg (i-1) +
α). After the correction of the target impedance Ztg, in step 203, the counter Cmax is cleared.

Thereafter, in step 204, the deviation ΔΔ from the actual impedance Zre and the target impedance Ztg is calculated.
Z is calculated (ΔZ = Zre−Ztg). Next, in step 205, it is determined whether or not a heater control permission condition is satisfied. The permission conditions include that the engine speed has increased to a predetermined value (for example, 200 rpm) or more after the engine has started, that the battery voltage has not dropped, and that there is no abnormality in other sensors involved in heater control. , Etc., and if these conditions hold, the heater control is permitted. If the heater control permission condition is not satisfied, the routine proceeds to step 206, where the heater control amount (Duty) is set to 0%.

When the condition for permitting the heater control is satisfied, the routine proceeds to step 207, where it is determined whether or not the heater is fully energized. As the conditions for performing the full energization of the heater, the elapsed time after the start of the full energization is within a predetermined time (for example, 10 seconds), and the impedance deviation ΔZ (= Zre−Z)
tg) is equal to or more than the predetermined value K1, and since the actual impedance Zre is a very large value at the time of starting the engine at a low temperature or the like, step 207 becomes YES and the heater is fully energized. That is, step 20
Proceeding to 8, the heater control amount (Duty) is set to 100%.

If step 207 is NO, the process proceeds to step 209, where it is determined whether or not the impedance deviation ΔZ is larger than a predetermined value K2. Then, if step 209 is YES, the process proceeds to step 210 to execute the first heater F / B control. At this time, the duty is set in the range of 0 to 80% as described above. However, in practice, immediately after the start of the engine, since the impedance deviation ΔZ is still large, a duty of about 80% is set.

If NO in step 209, the process advances to step 211 to execute the second heater F / B control. At this time, Dut in the range of 0 to 60% as described above.
y is set. In the case of the present embodiment, the heater control amount (Duty) is set to 0 according to the impedance deviation ΔZ.
%, 20%, 40%, and 60% (however, 80% is added to this in the first heater F / B control). Specifically, referring to FIG. 10, when the element temperature is higher than the target value,
If ΔZ <−K4, Duty = 0% and ΔZ = −
If K3 to -K4, Duty = 20%. If the element temperature is near the target value, that is, | ΔZ | ≦ K
If it is 3, Duty = 40 so that this temperature can be maintained.
%. When the element temperature is lower than the target value, if ΔZ = K3 to K2, Duty = 60%.

Thereafter, at step 212, the current D
It is determined whether or not uty is at the upper limit (60% or near). If YES, the process proceeds to step 213, where the counter Cmax is incremented. If NO in step 212, the flow advances to step 214 to clear the counter Cmax (steps 208 and 210).
After the treatment of the same). According to steps 212 and 213, if the heater control amount (Duty) is continued at the upper limit value in the case where the second heater F / B control is performed, the continuation time is measured. When the A / F sensor 30 has deteriorated, the state in which step 212 becomes YES continues, so the counter Cmax is counted up.

Finally, at step 215, in order to prevent a sudden change in the heater control amount, the currently set heater control amount (D
uty). For example, Duty = (3 × Duty (i−1) + Current Duty)
The duty is set by an operation such as / 4.

In this embodiment, steps 209 to 209 are performed.
211 corresponds to “heater control means” of the present invention, step 201 corresponds to “deterioration determination means”, and step 202
Corresponds to a “correction unit”.

On the other hand, in the target impedance updating process shown in FIG. 8, the SRAM 25 is updated at intervals of, for example, 30 minutes. That is, when the process of FIG. 8 starts in a one-second cycle, first, in step 301, it is determined whether or not a counter Cimp for measuring the update interval of the SRAM value has reached a value corresponding to 30 minutes. For example, the process proceeds to step 302, where the counter Cimp is incremented.

If step 301 is YES, the process proceeds to step 303, where SR of target impedance Ztg is calculated.
Update the AM value. That is, in the process of FIG.
The target impedance Ztg is corrected and its value is set to NRA
If it is temporarily stored in M24, the NRAM value (corrected Ztg) is written to SRAM25. However, at this time, it is desirable to perform the smoothing process on the SRAM value up to that time. For example, the SRAM value = (7 × SRAM value (i
-1) + NRAM value) / 8
A RAM value (Ztg) is obtained and written to the SRAM 25. Then, in step 304, the counter C
Clear imp.

In short, N of the target impedance Ztg
Since the RAM value is updated each time it is changed by the correction, the latest value is immediately reflected in the heater control. On the other hand, since the SRAM value of the target impedance Ztg is updated slowly at relatively long time intervals, erroneous learning and the like can be suppressed.

Here, the reason why the correction of the target impedance Ztg at the time of element deterioration is performed only on the increasing side will be described. Comparing the impedance characteristics of the A / F sensor 30 when it is new and when it is deteriorated, as shown in FIG. 11, the impedance at the time of deterioration generally increases, and it is assumed that the heater energization is controlled with the same target impedance Ztg (for example, 30Ω). Also, when the sensor deteriorates, the element temperature increases. In this case, by raising the target impedance Ztg, an increase in the element temperature is suppressed. In addition, the exhaust temperature of the engine changes according to the operating state of the engine.Comparing the case where the exhaust temperature is low as in idling operation and the case where the exhaust temperature is high as in high-speed running, the Changes. That is, as shown in FIG.
Even if heater energization is controlled at ty, the higher the exhaust temperature, the higher the element temperature. Therefore, if the exhaust gas temperature is high, the actual impedance becomes small, and the impedance deviation ΔZ at the time of sensor deterioration becomes small. Conversely, if the exhaust gas temperature is low, the actual impedance increases, and the impedance deviation ΔZ at the time of sensor deterioration increases.

In this case, the target impedance Ztg is uniformly determined by the impedance deviation ΔZ regardless of the exhaust gas temperature.
Is corrected, the amount of correction of the target impedance Ztg differs depending on the difference in the exhaust temperature even if the deterioration state is the same. Inconvenience such that the temperature is erroneously re-corrected on the decreasing side when the temperature rises). Figure 1
According to No. 3, it can be seen that when the exhaust temperature is low at idle and when the exhaust temperature is high, the former is more remarkable in element degradation (the degree of impedance increase due to degradation is greater). .

In this embodiment, the target impedance Z
By limiting the correction of tg only to the increase side, the target impedance Ztg can be corrected substantially only during idling operation with the lowest exhaust temperature during the entire operation period of the engine, that is, only under the condition where the impedance deviation is the largest. Will be done. At this time, the A / F sensor 30
Has been degraded, the inconvenience that the impedance must be corrected in accordance with the degraded state but the proper correction cannot be performed is solved.

When the exhaust gas temperature is high, even if the A / F sensor 30 is deteriorated, the actual impedance Zre substantially matches the target impedance Ztg due to the heat received from the exhaust gas, and the element temperature is high. Nevertheless, it is conceivable that the heater control amount (Duty) decreases. Therefore, the impedance cannot be corrected due to the element deterioration. However, if the impedance is corrected during the idling operation as described above, the element temperature rise can be suppressed even when the exhaust gas temperature is high.

Next, a more specific operation of the heater energization control will be described with reference to a time chart of FIG. FIG. 9 shows how the heater is energized when the engine 10 is started at a low temperature.

When heater control is permitted immediately after the engine is started, full energization control of the heater 64 is started. At this time, the heater control amount (Duty) is controlled at 100%. Thereafter, the real impedance Zre gradually decreases as the element temperature rises, and at time t1, when the impedance deviation ΔZ falls below a predetermined value K1 (or when 10 seconds have elapsed after the start of all energization), Duty = 0 to 80% Is started as the control range. At this time, the heater control amount (Duty) is controlled at approximately 80%, and the heater control is performed with priority given to the element temperature rise.

Thereafter, at time t2, the impedance deviation ΔZ decreases to a predetermined value K2, and Duty = 0 to 60%
Is started as the control range. Alternatively, when 300 seconds have elapsed after the start, the second heater F / B control is started. At this time, if the detection element 61 has deteriorated, the impedance deviation ΔZ remains large, so that the duty is set to the upper limit (60) of the F / B control range.
%), And the state is continued. Therefore, the counter Cmax is counted up.

Thereafter, the Cmax value becomes 20 in that state.
When the value becomes equivalent to seconds, it is determined that the detection element 61 is deteriorated. Then, the target impedance Ztg is corrected to the increasing side according to the impedance deviation ΔZ.
Further, if the impedance deviation ΔZ is large even after the Ztg value is corrected and the duty is stuck to the upper limit (60%), the counter Cmax is re-counted, and when the value again becomes a value equivalent to 20 seconds, Again, the target impedance Ztg is corrected to the increasing side according to the impedance deviation ΔZ. Note that the corrected target impedance Ztg
Is updated each time as an NRAM value.

By repeating the correction of the Ztg value in this way, the impedance deviation ΔZ gradually decreases,
When the state where the upper limit of the heater control amount (Duty) is stuck is eliminated (time t3), thereafter, the heater energization is feedback-controlled in accordance with the deviation ΔZ such that the actual impedance Zre matches the target impedance Ztg.

After the engine is started, the counter Cimp
Is counted up, and when the Cimp value becomes a value equivalent to 30 minutes, the target impedance Ztg in the NRAM 24 is written into the SRAM 25 (time t4). That is, 3
The update of the SARM value is repeated at 0 minute intervals. Then, at the time of the next engine start, the Ztg value updated this time is read from the SRAM 25 to the NRAM 24 and used for heater control from the beginning of the start.

According to the embodiment described above, the following effects can be obtained. When the heater control amount (Duty) reaches the upper limit value of the specified range and the state is continued for a predetermined time or more when the F / B control of the heater energization is performed, it is determined that the detection element 61 is deteriorated. I did it. When the detection element 61 is deteriorated, the target impedance Zt
g was corrected. In this case, the deterioration of the detection element 61 can be correctly detected, and the heater energization control according to the deterioration state (degree of deterioration) can be performed.
Therefore, even when the element is deteriorated, the element temperature can be controlled as desired. Thereby, an excessive rise in temperature of the detection element 61 is suppressed, and further deterioration of the detection element 61 is suppressed.

In particular, the A / F sensor 30 having a laminated structure has excellent temperature characteristics, but is liable to cause a problem such as element cracking. However, by performing the deterioration judgment suitably as described above, the problem such as the element cracking is solved. Is done. Therefore, effects such as element protection can be more effectively obtained.

At the time of the Ztg correction due to the element deterioration, the target impedance Ztg is gradually changed by the smoothing process, so that the erroneous correction of the Ztg value is prevented.
In addition, since the correction is performed according to the impedance deviation ΔZ, when the impedance deviation ΔZ is large,
When data in the SRAM 25 is cleared, such as when the battery is replaced, the convergence is improved.

Further, since the target impedance Ztg is corrected only on the increasing side, erroneous Ztg correction due to the influence of the exhaust gas temperature is prevented. As a result, the control accuracy of heater energization is improved.

Each time the target impedance Ztg is corrected, the corrected Ztg value is stored in the NRAM (temporary storage memory) 24, and the Ztg value of the NRAM 24 is stored at a longer time interval.
Since the tg value is stored in the SRAM (backup memory) 25, even if the Ztg value is erroneously corrected, it is temporarily stored in the NRAM 24 and immediately
It is not stored in the AM 25. Therefore, there is no inconvenience that the Ztg value is erroneously learned.

At the beginning of the heater energization upon engine start, the Ztg value stored in the SRAM 25 is read and used for heater energization control. Therefore, a suitable heater energization control is performed using the previously learned target value from the engine start. Can be implemented.

When the temperature of the detection element 61 is raised from the cold state, the heater 64 is fully energized at the beginning of energization. Thereafter, as the temperature of the detection element 61 increases, the first heater F / B control and the second Heater F / B control is sequentially performed. As a result, the duty F / B control range is changed according to the warm-up state of the detection element 61, and both promotion of warm-up (early activation) of the detection element 61 and protection of the element can be achieved.

In the engine air-fuel ratio control system,
As described above, the energization of the heater of the A / F sensor 30 is preferably performed, so that the appropriate activation state of the A / F sensor 30 is maintained, and the air-fuel ratio can be correctly detected. Therefore, the air-fuel ratio control can be performed with high accuracy.

(Second Embodiment) Next, a second embodiment of the present invention will be described focusing on differences from the above-described first embodiment. In the above embodiment,
When the second heater F / B control is performed, if the heater control amount (Duty) is maintained at the upper limit (60%) of the F / B control range, the detection element 61 is considered to be deteriorated. Although the target impedance Ztg has been corrected in this way, this is changed as follows. FIG. 14 is a flowchart showing the procedure of the Ztg correction, which is performed in place of steps 201 to 203 in the flowchart of FIG. Accordingly, steps 212 to 21 in FIG.
4 is deleted.

In FIG. 14, in step 401, it is determined whether or not the engine is in an idle operation state. If YES, the process proceeds to step 402, where the average heater control amount at that time (idle control average value Dav) Is calculated. In step 403, it is determined whether the calculated idle control amount average value Dav is larger than a predetermined value.
Here, the predetermined value for determining the idle control amount average value Dav is an upper limit Duty in the F / B energization control.
(60%) or a value in the vicinity thereof, and if step 403 is YES, it is regarded as element deterioration and the target impedance Ztg is corrected. That is, in step 404, the target impedance Z
Correct tg (NRAM value). For the Ztg correction, it is desirable to use a method such as a smoothing operation as described above. In step 405, the average idle control amount Dav is cleared. In the present embodiment, step 403 corresponds to “deterioration determination means”, and step 404
Corresponds to a “correction unit”.

As described above, according to the second embodiment, the first
Similarly to the embodiment, the deterioration of the detection element 61 can be correctly detected, and the heater energization control according to the deterioration state (degree of deterioration) can be performed. Therefore, even when the element is deteriorated, the element temperature can be controlled as desired. Thereby, an excessive rise in temperature of the detection element 61 is suppressed, and further deterioration of the detection element 61 is suppressed.

Further, the heater control amount during idle operation (D
(uty) is monitored to determine the deterioration, so that erroneous deterioration determination and Ztg correction due to the influence of the exhaust gas temperature are prevented.
As a result, the control accuracy of heater energization is improved.

The present invention can be embodied in the following forms other than the above. In the first embodiment (the processing in FIG. 7), the correction of the target impedance Ztg (NRAM value) is limited to the increase side, but this is changed as follows. That is, the reason for limiting the Ztg correction to the increasing side is as follows.
This was to eliminate erroneous corrections other than during idle operation (high temperature of exhaust gas). Therefore, instead of limiting the Ztg correction to the increase side, a requirement such as “is the exhaust gas temperature low?” Or “is the idling operation?” Is added as a condition for performing the correction. Then, the Ztg correction is permitted only when the exhaust gas temperature is low or during idling operation (except for a state in which the exhaust gas temperature becomes high immediately after high-speed running). Also in this case, erroneous Ztg correction due to the influence of the exhaust gas temperature is prevented. As a result, the control accuracy of heater energization is improved.

In the first embodiment, if the heater control amount (Duty) is within the upper limit (60%) of the specified range,
Deterioration judgment was performed by operating the counter Cmax,
Even if the heater control amount (Duty) is not near the upper limit (60%) but is near the upper limit (for example, 55 to 60%), the counter Cmax may be operated to perform the deterioration determination. .

In the apparatus of the above embodiment, the presence or absence of element deterioration is determined, and if the element is deteriorated, the target impedance Ztg is corrected, but this is changed. For example, it may be embodied simply as a deterioration determination device that determines whether or not the detection element 61 has deteriorated. Further, when the element is deteriorated, the heater energization may be prohibited, code information indicating the element deterioration may be stored in the memory, or a driver or the like may be warned of the element deterioration.

In the above-described embodiment, the air-fuel ratio control system is embodied using the stacked A / F sensor. However, a so-called cup A / F sensor having a detection element having a cup-shaped cross section may be used. Further, the present invention can be applied to devices other than the air-fuel ratio detecting device using the A / F sensor. That is, NO
The present invention can also be applied to a gas concentration detection device using a gas concentration sensor capable of detecting gas concentration components such as x, HC, and CO. By using the same method as that of the above-described embodiment also in the other gas concentration detection device, it is possible to appropriately detect the deterioration of the detection element and to protect the detection element.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an outline of an air-fuel ratio control system according to an embodiment of the invention.

FIG. 2 is a sectional view showing the structure of the A / F sensor.

FIG. 3 is a sectional view of a sensor element unit.

FIG. 4 is a diagram showing output characteristics of an A / F sensor.

FIG. 5 is a waveform diagram of a voltage and a current at the time of impedance detection.

FIG. 6 is a flowchart showing an initial routine.

FIG. 7 is a flowchart illustrating a heater control amount calculation routine.

FIG. 8 is a flowchart showing a target impedance update process.

FIG. 9 is a time chart showing a state of energization of a heater when the engine is started.

FIG. 10 is a diagram illustrating impedance characteristics of a detection element.

FIG. 11 is a diagram showing impedance characteristics at the time of element deterioration.

FIG. 12 is a diagram showing a difference in element temperature due to a difference in exhaust temperature.

FIG. 13 is a diagram showing a difference in impedance characteristics due to a difference in exhaust temperature.

FIG. 14 is a flowchart showing a procedure for correcting a target impedance in another embodiment.

[Explanation of symbols]

10 engine (internal combustion engine), 12 exhaust pipe, 20 E
CU, 21: microcomputer, 22: CPU, 24: NRAM
(Temporary storage memory), 25 SRAM (backup memory), 30 A / F sensor (gas concentration sensor), 6
1: detection element, 64: heater.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G01N 27/46 331

Claims (11)

[Claims] 1. A gas concentration sensor comprising: a detection element for outputting a limit current substantially proportional to the concentration of a specific component in exhaust gas of an internal combustion engine; and a heater for heating the detection element; A heater control means for performing feedback control of energization of the heater within a range of a predetermined energization amount so as to coincide with the target value; and
A heater control device for a gas concentration sensor, comprising: a deterioration determination unit configured to determine deterioration of a detection element from the state when the heater power supply amount is at or near an upper limit value of a specified range. 2. The deterioration judging means judges whether or not the heater energization amount has reached the upper limit of the specified range or its vicinity, and determines whether the heater energization amount has reached or has reached the upper limit of the specified range or not. The heater control device for a gas concentration sensor according to claim 1, wherein when it is continued for more than a time, it is determined that the detection element is deteriorated. 3. A means for monitoring the amount of heater current during idle operation of the internal combustion engine and calculating an average value within a predetermined period, wherein the deterioration determining means determines that the average value of heater current during idle operation is 2. The heater control device for a gas concentration sensor according to claim 1, wherein it is determined that the detection element is deteriorated if the upper limit value of the specified range is at or near the upper limit. 4. The gas concentration sensor according to claim 1, further comprising correction means for correcting a target value of the element resistance when the deterioration determination means determines that the detection element is deteriorated. Heater control device. 5. The heater control device for a gas concentration sensor according to claim 4, wherein said correction means gradually changes a target value so as to eliminate a deviation of element resistance. 6. The heater control device for a gas concentration sensor according to claim 4, wherein the correction means corrects the target value of the element resistance only on the increasing side. 7. The heater control device according to claim 4, wherein the correction means corrects the target value of the element resistance only when the exhaust gas from the internal combustion engine is in a relatively low state. Sensor heater control device. 8. The heater control apparatus according to claim 4, wherein the corrected target value of the element resistance is stored in a temporary storage memory every time the correction is performed by the correction means, and the time is longer than the corrected target value. A heater control device for a gas concentration sensor, wherein a target value of the element resistance stored in the temporary storage memory is stored in a backup memory at intervals. 9. The heater control device according to claim 8, wherein a target value stored in the backup memory is read out at the beginning of heater energization accompanying the start of the internal combustion engine.
A heater control device for a gas concentration sensor that uses it for feedback control of heater energization. 10. An air-fuel ratio control system for controlling an air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine, wherein the gas concentration sensor outputs a limiting current substantially proportional to an oxygen concentration in exhaust gas of the internal combustion engine. 10. An air-fuel ratio sensor.
A heater control device for a gas concentration sensor according to any one of the above. 11. The heater control device for a gas concentration sensor according to claim 1, wherein said gas concentration sensor has a stacked structure in which a heater is stacked on a detection element having a solid electrolyte.