JP2000241383A - Detecting device for concentration of gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect the concentration of a gas always with good accuracy irrespective of the individual difference of a sensor or irrespective of a change with the passage of time of the sensor. SOLUTION: An A/F sensor 30 is arranged and installed in the exhaust pipe 13 of an engine 10. The A/F sensor 30 is provided with a sensor element part 30a which comprises a solid electrolyte 31 and which comprises an electrode 33 and an electrode 34 which are installed respectively on the opposite faces of the solid electrolyte 31. When a voltage is applied across the electrode 33 and the electrode 34, a current signal which corresponds to the concentration of oxygen in an exhaust gas is output due to the voltage. A microcomputer 20 for air-fuel ratio detection detects an element DC resistance Ri and an AC impedance ZAC. In addition, while a DC resistance conversion value ZDC which is found by converting the AC impedance ZAC is used, a current correction value which corresponds to the difference (Ri-ZDC) between the element DC resistance Ri and the DC resistance conversion value ZDC is calculated. The output of the sensor is corrected by using the correction value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to a gas concentration detecting apparatus using a gas concentration sensor for detecting a concentration of a specific component in a gas to be detected, such as an oxygen concentration in an exhaust gas of a vehicle engine. The present invention relates to a gas concentration detection device for suitably obtaining a current signal output from the gas concentration sensor.

[0002]

2. Description of the Related Art As a gas concentration sensor for detecting the concentration of a specific component in a gas to be detected using a solid electrolyte such as zirconia, for example, a limiting current type air-fuel ratio sensor (A / A) for detecting an oxygen concentration in an exhaust gas. F sensor). This A / F sensor includes a sensor element section having a solid electrolyte and a pair of electrodes. When a voltage is applied to the sensor element section, the voltage is applied to the sensor element section according to the oxygen concentration (air-fuel ratio) in the exhaust gas. Output current signal. The sensor is provided with a heater for maintaining the active state of the sensor element.

FIG. 2 shows a VI characteristic of a limiting current type sensor. In the characteristic of FIG. 2, the limit current (sensor output Ip) is that a straight line parallel to the V axis (horizontal axis) flows through the sensor element.
Corresponds to a limit current detection area for specifying the sensor output I.
An increase or decrease in p corresponds to an increase or decrease in the air-fuel ratio (that is, a degree of lean-rich). That is, the sensor output Ip increases as the air-fuel ratio becomes leaner, and decreases as the air-fuel ratio becomes richer.

In this VI characteristic, a voltage range smaller than a linear portion parallel to the V axis is a resistance dominant region, and the slope of the primary linear portion in the resistance dominant region is specified by element DC resistance Ri. . Element DC resistance Ri
Reflects the active state of the sensor element portion, and is detected from the applied voltage V1 and the sensor output I1 when the voltage V1 is applied in, for example, the resistance dominant region (primary linear portion of the VI characteristic). (Ri = V1 / I1).

In recent years, there has been proposed a method of detecting the AC impedance ZAC of an element by instantaneous change of an applied voltage by utilizing the AC characteristics of a limiting current sensor. In this case, the AC impedance ZAC of the element is obtained from the change amount of the applied voltage and the corresponding current change amount (ZAC =
Voltage change / current change). Note that the element DC resistance Ri defined on the VI coordinate and the AC impedance ZAC of the element have a specific relationship, and in general, “Ri> ZA
It is known that there is a relationship of "C".

Since the AC impedance ZAC detected as described above can be detected in a short time, there is an advantage that the time during which the air-fuel ratio cannot be detected during the impedance detection period is shortened, and wide use can be expected. Therefore, usually, the AC impedance ZAC is detected, and then the AC impedance ZAC is converted into a DC resistance converted value ZDC using a predetermined conversion map. Then, based on the converted value ZDC, an applied voltage control, a heater energization control, and the like are appropriately performed.

More specifically, when controlling the applied voltage of the sensor,
The applied voltage is controlled in accordance with the DC resistance conversion value ZDC so that the applied voltage of the sensor always covers the limit current detection area on the VI characteristic. That is, the AC impedance ZAC is converted into the DC resistance converted value ZDC using the relationship of FIG. 19A, and the target applied voltage of the sensor is set according to the DC resistance converted value ZDC. At this time, as the DC resistance conversion value ZDC is larger, the target applied voltage is set to a larger voltage value on the lean side, and is set to a smaller value on the rich side. In controlling the energization of the heater, the element temperature (temperature of the sensor element section) is obtained from the AC impedance ZAC using the relationship shown in FIG. 19B, and the heater energization is controlled so that the element temperature becomes a predetermined activation temperature. I do.

In short, instead of directly measuring the element DC resistance Ri, the AC impedance ZAC is detected,
Then, the impedance ZAC is converted to a DC resistance conversion value ZD.
C to control the applied voltage, or convert the impedance ZAC to the element temperature to control the heater energization.

[0009]

However, as described above, the AC impedance ZA is used in place of the element DC resistance Ri.
When C is used, the following problems occur due to variations between sensors and changes over time. That is, if there is a variation in individual sensors (variation between individuals), the correspondence between the AC impedance ZAC of the element and the element DC resistance Ri,
20A and 20B, the correspondence between the element impedance ZAC and the element temperature varies for each sensor with respect to the values of the map set in advance as shown in FIGS. In the figure, a solid line sensor A indicates a median product, and a broken line sensor B and a two-dot chain line sensor C indicate sensors having variations.

[0010] When a change with time occurs, the correspondence between the element impedance ZAC and the element DC resistance Ri and the correspondence between the element impedance ZAC and the element temperature are broken, and the relations are shown in FIGS. As shown in b), the element shifts before and after the deterioration. In the figure, a solid line indicates an initial product before deterioration, and a broken line indicates a deteriorated product. For example, when the resistance value of the electrode changes due to thermal deterioration, the correspondence between the AC impedance ZAC and the element DC resistance Ri breaks down, and FIGS.
Invite the state.

When the characteristics of each sensor fluctuate as shown in FIGS. 20 and 21, the applied voltage control is not properly performed, and an accurate sensor output (air-fuel ratio output) cannot be obtained. Similarly, the heater energization control is not properly performed, so that an accurate sensor output (air-fuel ratio output) cannot be obtained.

Such a situation is particularly problematic in realizing high-precision air-fuel ratio feedback control. Originally, a region where the air-fuel ratio is detected with high accuracy, that is, for example, a lean control region (air control region) (Fuel ratio = about 18 to 50), the detection accuracy of the air-fuel ratio is deteriorated, and the control accuracy of the air-fuel ratio is reduced, which may cause a problem such as deterioration of emission.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problem, and has as its object to provide a gas which can always accurately detect a gas concentration irrespective of individual differences between sensors and changes with time. An object of the present invention is to provide a concentration detecting device.

[0014]

A gas concentration detecting device according to the present invention comprises an element section having a solid electrolyte and electrodes provided on opposing surfaces of the solid electrolyte, and a voltage is applied between the electrodes. Then, it is assumed that a gas concentration sensor that outputs a current signal corresponding to the concentration of the specific component in the detected gas is used accordingly. In addition, the sensor is provided with a heater for heating the element portion to a predetermined activation temperature by generating heat by applying a power supply voltage, and the gas concentration detecting device controls the amount of current supplied to the heater.

According to the first aspect of the present invention, a DC resistance detecting means for detecting an element DC resistance in the element section, and a current signal output from a gas concentration sensor is corrected in accordance with the detected element DC resistance. A sensor output correcting unit.

As described above, the element DC resistance Ri is equal to V-
Although it is defined by the I characteristic and originally reflects the active state of the sensor element portion, in the case of a conventional device in which this element DC resistance Ri is substituted by a DC resistance conversion value ZDC obtained by converting the AC impedance ZAC into a map, The accuracy of detecting the gas concentration is reduced due to the variation between the sensors and the change over time. For example, when the resistance value of the electrode changes due to thermal deterioration, the detection accuracy decreases. On the other hand, according to the above configuration, even if thermal deterioration of the electrode occurs, the sensor output correction by the element DC resistance is performed as a reflection thereof, so that a decrease in gas concentration detection accuracy can be avoided. as a result,
The gas concentration can always be detected with high accuracy irrespective of the individual difference of the sensor or the change with time.

According to a second aspect of the present invention, in the first aspect of the present invention, the AC impedance of the element portion is detected, and the detected AC impedance is converted into a DC resistance conversion value. The sensor output correction unit calculates a correction value according to a difference between the element DC resistance and the DC resistance conversion value, and corrects a current signal output from the gas concentration sensor using the calculated correction value. Means to perform.

According to this configuration, since the correction is performed based on the difference between the element DC resistance and the DC resistance conversion value, it is possible to accurately grasp variations between sensors and changes with the passage of time. In the case where the sensor characteristics change as shown in FIG.

According to a third aspect of the present invention, in the second aspect of the present invention, the sensor output correcting means calculates the correction value according to the gas concentration at that time. That is,
If the gas concentration (air-fuel ratio) in the detected gas is different, the difference between the actual sensor output and the sensor output originally desired fluctuates,
The necessary correction value may be naturally different. On the other hand, in the above configuration, the cause of the correction deviation is eliminated, and more accurate sensor output correction can be performed.

According to the present invention, the DC resistance detecting means for detecting the DC resistance of the element in the element section and the voltage applied to the gas concentration sensor are corrected in accordance with the detected DC resistance of the element. And an applied voltage correcting unit.

According to the structure of the fourth aspect, similarly to the first aspect, even if thermal degradation of the electrode occurs, the applied voltage correction by the element DC resistance is performed as a reflection of the degradation. A decrease in detection accuracy can be avoided. as a result,
The gas concentration can always be detected with high accuracy irrespective of the individual difference of the sensor or the change with time.

According to a fifth aspect of the present invention, in the fourth aspect of the present invention, the AC impedance of the element portion is detected, and the detected AC impedance is converted into a DC resistance converted value. The applied voltage correction unit calculates a correction value according to a difference between the element DC resistance and the DC resistance conversion value, and corrects an applied voltage to the gas concentration sensor using the calculated correction value. And

According to this configuration, since the correction is performed based on the difference between the element DC resistance and the DC resistance conversion value, it is possible to accurately grasp the variation between the individual sensors and the change with the passage of time. In the case where the sensor characteristics change as shown in FIG.

According to a sixth aspect of the present invention, in the fifth aspect of the present invention, the applied voltage correction means calculates the correction value according to the gas concentration at each time. That is, if the gas concentration (air-fuel ratio) in the detected gas is different, the difference between the actual sensor applied voltage and the originally desired sensor applied voltage fluctuates, and the required correction value may naturally differ.
On the other hand, in the above configuration, the cause of the correction deviation is eliminated,
More accurate sensor output correction becomes possible.

In the invention according to claim 7, the impedance detecting means for detecting the AC impedance of the element part, the DC resistance detecting means for detecting the element DC resistance of the element part, and And a power supply amount corrector that corrects the power supply amount of the heater determined based on the detected element DC resistance.

According to the structure of claim 7, the above-mentioned claim 1,
As in the case of No. 4, even if thermal deterioration of the electrodes occurs, the amount of current supplied to the heater is corrected by the element DC resistance as a reflection of the deterioration, so that the gas concentration sensor can always be maintained in a desired active state. As a result, the gas concentration can always be detected with high accuracy regardless of the individual difference of the sensor or the change with time.

According to an eighth aspect of the present invention, in the seventh aspect, the detected AC impedance is converted into a DC resistance converted value. The energization amount correction unit calculates a correction value according to a difference between the element DC resistance and the DC resistance conversion value, and corrects the set energization amount of the heater using the calculated correction value. And

According to this configuration, since the correction is performed based on the difference between the element DC resistance and the DC resistance conversion value, it is possible to accurately grasp the variation between the individual sensors and the change with the passage of time. In the case where the sensor characteristics change as shown in FIG.

In the above-described claim 7 or 8, the amount of current supplied to the heater is corrected to an increased side based on the element DC resistance, for example, to compensate for insufficient element activation. Need to decide. Therefore, near the upper limit of heating of the sensor element portion, the accuracy of gas concentration detection is ensured by other corrections instead of increasing the heater power supply amount further.

That is, as set forth in claim 9, a sensor output correcting means for correcting a current signal output from the gas concentration sensor according to the detected element DC resistance is further provided. Alternatively, the apparatus further includes an applied voltage correction unit that corrects an applied voltage to the gas concentration sensor according to the detected element DC resistance.

According to the eleventh aspect, in the second aspect,
In the invention according to any one of the fifth and eighth aspects, the correction value is increased as the difference between the element DC resistance and the DC resistance conversion value increases. That is, as the difference between the element DC resistance and the DC resistance conversion value increases, the deviation width of the current signal from the gas concentration sensor, the sensor applied voltage, or the heater power supply amount increases. In this case, by setting the correction value as described above, the accuracy of the correction calculation is improved.

According to the twelfth aspect of the present invention,
In the invention according to any one of the fifth and eighth aspects, when the difference between the element DC resistance and the DC resistance conversion value exceeds a predetermined allowable value, it is determined that the gas concentration sensor is abnormal. For example, if the difference between the element DC resistance and the DC resistance conversion value becomes extremely large due to thermal deterioration of the element electrode or the like, it is not possible to avoid a decrease in the gas concentration detection accuracy even if the various corrections described above are performed. Therefore, in such a case, it is determined that the sensor is abnormal.

[0033] The DC resistance detecting means may be configured as in the following thirteenth and fourteenth aspects. That is, in claim 13, a voltage is applied in the resistance dominant region on the VI coordinate of the gas concentration sensor, and the element DC resistance is calculated from the applied voltage at that time and the sensor output. In the fourteenth aspect, a circuit for applying a voltage to the gas concentration sensor is momentarily interrupted, and the element DC resistance is detected from a ratio between a voltage change and a current change before and after the momentary interruption. In any of these cases, the element DC resistance can be accurately detected.

As described above, the detection of the element DC resistance requires a longer time than the detection of the AC impedance. Only when the condition is satisfied, the detection of the element DC resistance may be permitted. This minimizes the effect of interrupting the gas concentration detection when detecting the element DC resistance.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) A first embodiment of the present invention embodied in an air-fuel ratio control system for a vehicle engine will be described below with reference to the drawings. In the present control system, the fuel injection amount to the engine is controlled to a desired air-fuel ratio based on the detection result of the air-fuel ratio sensor provided in the engine exhaust pipe. In the following description, a procedure for detecting an air-fuel ratio (A / F) using an air-fuel ratio sensor and a procedure for controlling heater energization provided in the sensor will be described in detail.

FIG. 1 is a configuration diagram showing an outline of the air-fuel ratio control system in the present embodiment. In FIG. 1, an engine 10 is configured as a multi-cylinder four-cycle internal combustion engine. An intake pipe 11 is provided with an injector 12 for injecting and supplying fuel to each cylinder of the engine 10. The exhaust pipe 13 is provided with an A / F sensor 30 composed of a limiting current type air-fuel ratio sensor. The A / F sensor 30 detects the oxygen concentration in the exhaust gas (or the concentration of carbon monoxide or the like in the unburned gas). ), And outputs a wide-area and linear air-fuel ratio signal in proportion to.

The A / F sensor 30 includes a solid electrolyte 31 having a cup-shaped cross section and a diffusion resistance layer 32 laminated on the outside thereof, and a pair of electrodes 33 are provided inside and outside the solid electrolyte 31 (atmosphere side and exhaust side). , 34 are provided. In the present embodiment, the sensor elements 30a are formed by these members 31 to 34.
Is configured. Inside the solid electrolyte 31, a heater 35 for heating the sensor element 30a is provided. Here, the solid electrolyte layer 31 is made of ZrO2, HfO2, ThO.
2, Bi2 O3, etc. to CaO, MgO, Y2 O3, Yb2
The diffusion resistance layer 32 is made of a heat-resistant inorganic material such as alumina, magnesia, silica, spinel, and mullite. The electrodes 33 and 34 are both made of a noble metal having a high catalytic activity such as platinum, and the surfaces thereof are subjected to porous chemical plating or the like. The heater 35 generates heat by power supply from a battery power supply.

In the A / F sensor 30 having the above-described configuration, the sensor element 30a generates a limit current corresponding to the oxygen concentration in a lean region from the stoichiometric air-fuel ratio point. In this case, the sensor element section 30a (solid electrolyte 31) can detect the oxygen concentration with linear characteristics, but a high temperature of about 600 ° C. or more is required to activate the sensor element section 30a. Further, since the active temperature range of the sensor element portion 30a is narrow, the active state cannot be maintained by heating only the exhaust gas of the engine. Therefore, in the present embodiment, the sensor element section 30a is maintained in the active temperature range by controlling the heating of the heater 35. In the region richer than the stoichiometric air-fuel ratio, the concentration of unburned gas such as carbon monoxide (CO) changes almost linearly with respect to the air-fuel ratio, and the sensor element 30a responds to the concentration of CO or the like. Generates a limiting current.

FIG. 2 shows the VI characteristics of the A / F sensor 30. In FIG. 2, the element limit current (sensor output Ip)
The sensor output Ip increases as the air-fuel ratio becomes leaner, and the sensor output Ip decreases as the air-fuel ratio becomes richer. The voltage range smaller than the linear portion (limit current detection range) parallel to the V axis is the resistance dominating region, and the inclination of the primary linear portion in the resistance dominating region is the element DC resistance of the sensor element unit 30a (solid electrolyte 31). Specified by Ri. The slope of the linear portion in the resistance dominant region changes with a change in the element temperature. For example, when the element temperature decreases, the element DC resistance Ri increases and the slope decreases. The AC impedance ZAC is defined as shown in the figure with respect to the element DC resistance Ri (Ri> Z
AC).

On the VI coordinate of FIG. 2, an applied voltage line LX1 is provided in parallel with the primary linear portion of the resistance dominant region, and along the applied voltage line LX1, the applied voltage line LX1 corresponds to the air-fuel ratio at each time. The applied voltage is set. Therefore, if the air-fuel ratio shifts to the lean side and the element limit current (sensor output Ip) flowing to the A / F sensor 30 increases, the applied voltage is changed to a larger value accordingly.

On the other hand, in FIG. 1, the ECU 15 includes an engine control microcomputer 16 for optimally controlling the fuel injection amount by the injector 12. The engine control microcomputer 16 fetches various types of engine operation information from a sensor group (not shown), and detects an engine operation state such as an engine speed, an intake pressure, a water temperature, and a throttle opening from the sensor detection results.

The air-fuel ratio detecting microcomputer 20 is communicably connected to the engine controlling microcomputer 16. The air-fuel ratio detecting microcomputer 20 operates the bias control circuit 40 according to a predetermined control program, and measures a current value flowing through the A / F sensor 30 in accordance with the application of the voltage.
Then, the air-fuel ratio is detected from the measured current value, and the detection result is output to the engine control microcomputer 16. The microcomputer 20 operates the heater control circuit 25 so that the A / F sensor 30 is maintained in the active state, and energizes the heater 35 as necessary.

Here, a bias command signal Vr for applying a voltage to the A / F sensor 30 is input from the air-fuel ratio detecting microcomputer 20 to the D / A converter 21, and the D / A converter 21 converts the analog signal into an analog signal. After being converted into a signal Vb, the signal is input to an LPF (low-pass filter) 22. In addition, LPF22
The output voltage Vc from which the high-frequency component of the analog signal Vb has been removed is input to the bias control circuit 40, and the bias control circuit 40 applies a voltage to the A / F sensor 30. In this case, when detecting the air-fuel ratio (sensor output Ip),
The sensor applied voltage is controlled for each element depending on whether the element DC resistance Ri or the AC impedance ZAC is detected.

The output (sensor output Ip) of the A / F sensor 30 corresponding to the air-fuel ratio at that time is supplied to the bias control circuit 4.
0 is detected by the current detection circuit 50, and the detected value is A
It is input to the air-fuel ratio detection microcomputer 20 via the / D converter 23. The heater control circuit 25 performs duty control on the amount of current supplied to the heater 35 in accordance with the element resistance or the element temperature of the A / F sensor 30 to control the heating of the heater 35.

It should be noted that the air-fuel ratio F / B control by the engine control microcomputer 16 is not the gist of the present invention, and the details of the control are well known. The engine control microcomputer 16 takes in the detection result (voltage signal) of the air-fuel ratio by the A / F sensor 30 and other detection results of various sensors, and based on the detection results, applies a control algorithm such as modern control or PI control. Accordingly, the air-fuel ratio F / B control is performed. That is, the injector 1 is connected to each cylinder of the engine 10 such that the air-fuel ratio at that time matches the target air-fuel ratio.
2 controls the amount of fuel injected and supplied.

Next, the operation of the air-fuel ratio control system configured as described above will be described focusing on the operation of the air-fuel ratio detecting microcomputer 20. In the present embodiment, the air-fuel ratio detection microcomputer 20 follows a main routine (not shown)
Each process such as (a) detection of the AC impedance ZAC, (b) detection of the element DC resistance Ri, (c) detection of the air-fuel ratio (sensor output Ip), and (d) energization control of the heater is sequentially performed. Among the above processes, according to the process (a), A
Impedance Z representing the active state of the / F sensor 30
AC is detected, and according to the process (b), the A / F sensor 3
Element DC resistance Ri indicating the variation between 0 individuals and the state of deterioration
Is detected. According to (c) and (d), the detection of the air-fuel ratio and the heater control are performed while reflecting the detection results of (a) and (b). Hereinafter, each of the above processes will be described in detail with reference to the flowcharts of FIGS.

First, a procedure for detecting the AC impedance ZAC using the sweep method will be described with reference to FIG. FIG.
Is performed, for example, every 128 ms (milliseconds).
However, the execution cycle of the ZAC detection may be changed in accordance with the engine operation state. For example, during normal operation of the engine 10, the change in the air-fuel ratio is relatively small, usually 2 s (second).
Alternatively, the time may be variably set to 128 ms when the air-fuel ratio changes suddenly, such as when the engine 10 is started or during transient operation.

Referring to FIG. 3, in step 101, the bias command signal Vr is manipulated to change the applied voltage Vp (the voltage for detecting the air-fuel ratio) up to that point on a one-time basis to the positive side. The application time of the impedance detection voltage is A /
Several tens to 100 μm in consideration of the frequency characteristics of the F sensor 30
s. Thereafter, in step 102, the voltage change amount Va at that time and the sensor output change amount Ia detected by the current detection circuit 50 are read. In step 103, the AC impedance ZAC is calculated from Va and Ia (ZAC = Va / Ia).

According to the above processing, the LPF 2 shown in FIG.
A voltage having a predetermined time constant is applied to the A / F sensor 30 one by one via the bias control circuit 40. As a result, as shown in FIG. 7, a peak current Ia (current change amount) is detected after a lapse of t time from the application of the voltage, and an AC impedance ZAC is detected from the voltage change amount Va and the peak current Ia at that time. (ZAC = V
a / Ia). In such a case, by applying a one-shot voltage to the A / F sensor 30 via the LPF 22, the occurrence of an excessive peak current is suppressed, and the AC impedance ZA
The detection accuracy of C is improved.

The AC impedance ZAC obtained as described above has a relationship shown in FIG. 19B with respect to the element temperature. That is, as the element temperature is lower, the AC impedance ZAC is dramatically increased.

Next, the procedure for detecting the element DC resistance Ri will be described with reference to the flowchart of FIG. The process of FIG. 4 is executed at a period of, for example, 4 ms. In FIG. 4, first, at step 201, it is determined whether or not a condition for detecting the element DC resistance Ri is satisfied. That is, the detection of the element DC resistance Ri takes a longer time than the detection of the AC impedance ZAC, and during the detection period, the air-fuel ratio (sensor output Ip)
Detection is interrupted. Therefore, the element DC resistance R is set only when the condition that the air-fuel ratio detection can be interrupted is satisfied.
Execute the detection of i. Specifically, when at least one of the following conditions is satisfied: fuel cut is being executed, a signal indicating that the air-fuel ratio F / B control is stopped is transmitted from the engine control microcomputer 16, The detection condition of the element DC resistance Ri is satisfied.

When the result of step 201 is NO, the present process is temporarily terminated. When the result of step 201 is YES, step 202
Proceed to. In step 202, the setting map of the applied voltage is switched from the map for detecting the air-fuel ratio to the map for detecting DC resistance. Actually, as shown in FIG. 8, the applied voltage map for air-fuel ratio detection is given according to the applied voltage line LX1 parallel to the linear portion of the resistance dominating region, whereas An applied voltage map for DC resistance detection is provided according to the applied voltage line LX2 having a negative slope that intersects.

In the following step 203, the target applied voltage Vtg corresponding to the sensor output Ip at that time is set and output using the applied voltage line LX2 of FIG. At this time, considering the influence of the element capacitance component on the current output, for example, 5 mV /
The applied voltage is gradually changed at a change speed of 8 ms or less.
Thus, in the case of FIG. 8, the actual applied voltage (actual applied voltage Vre) is gradually changed to the left side (negative side) from the point A in the figure.

Next, at step 204, the value of the current flowing through the solid electrolyte 31 after the applied voltage is changed, that is, the sensor output Ip detected by the current detection circuit 50 is read. In the subsequent step 205, the applied voltage line LX
2 and the current actual applied voltage Vre
Is determined whether or not. If Vtg ≠ Vre, the process returns to step 203, and the processes of steps 203 to 205 are repeatedly executed. That is, the target applied voltage Vtg is reset every 4 ms, and the sensor output Ip at that time is detected.

When Vtg = Vre, the routine proceeds to step 206, where the element DC resistance Ri is calculated from the latest applied voltage (voltage value when Vtg = Vre) and the sensor output Ip. That is, the element DC resistance Ri is calculated from the calculation result of Ri = Vtg / Ip. After the detection of the Ri value, the present process is temporarily terminated.

According to the processes of steps 203 to 206, the sensor applied voltage is changed from the point A on the applied voltage line LX1 to the point B on the applied voltage line LX2 in FIG. From the sensor output and the element DC resistance Ri
Is calculated. For example, when the applied voltage is changed from 0.45 V to 0.24 V at a voltage change speed of 5 mV / 8 ms, the element DC resistance Ri is detected 336 ms after the start of changing the applied voltage.

In the above configuration, the voltage change speed is limited when the applied voltage is changed. However, an alternative configuration is also possible.
For example, when the applied voltage is changed, the target applied voltage Vtg is set and output at the intersection (point B in FIG. 8) of the applied voltage line LX2 and the linear portion of the resistance dominant region, and then the current output of the element capacitance component Irrespective of the influence of the current, the current output stabilizes, and the element DC resistance Ri is calculated using the sensor output when the current output is stabilized.

Next, the procedure for detecting the air-fuel ratio will be described with reference to FIG. The process of FIG. 5 is executed, for example, at a period of 4 ms.
In FIG. 5, in step 301, the current value flowing through the solid electrolyte 31 due to the application of the voltage, that is, the current detection circuit 5
The sensor output Ip detected by 0 is read.

In step 302, the AC impedance ZAC calculated in FIG. 3 is read, and in step 303, the AC impedance ZAC is converted into a DC resistance conversion value using a preset conversion map of FIG. Convert to ZDC. Further, in step 304,
The element DC resistance Ri calculated in FIG. 4 is read.

Thereafter, at step 305, the difference (Ri-ZDC) between the element DC resistance Ri and the DC resistance converted value ZDC is calculated.
Is smaller than a predetermined value K1. This predetermined value K1 is for determining element breakage, damage to the electrodes, and the like. If Ri-ZDC <K1, step 306 recognizes that an abnormality has occurred, and ends this processing as it is. In this case, processing such as turning on a warning light, interrupting the air-fuel ratio F / B control, and storing diagnostic information in a memory is performed.

If Ri-ZDC ≧ K1, the process proceeds to step 307. In step 307, a current correction value ΔI is calculated according to the difference between the element DC resistance Ri and the DC resistance conversion value ZDC (Ri−ZDC) and the air-fuel ratio (sensor output Ip) at that time according to the relationship in FIG. .

In FIG. 9A, a plurality of characteristic lines are set according to whether the air-fuel ratio is lean or rich, that is, whether the sensor output Ip is positive or negative. = 18,23 as A / F rich characteristic examples
/ F = 12. In the figure, K1 is the determination value used in step 305, and K2 and K3 are determination values for determining whether correction is necessary (where K1 <K2 <K3). Then, the current correction value ΔI is set according to the (Ri−ZDC) value and the air-fuel ratio.

Here, Ri-ZDC <K2 is satisfied by:
This is the case where the element DC resistance Ri decreases with respect to the DC resistance conversion value ZDC. For example, it is conceivable that the sensor element temperature is high and the voltage applied to the sensor element section 30a falls outside the limit current detection range. In this case, if the air-fuel ratio is lean (sensor output Ip> 0), a negative current correction value ΔI is set, and if the air-fuel ratio is rich (sensor output Ip <0),
A positive current correction value ΔI is set. More specifically, when the relationship between the AC impedance ZAC and the element DC resistance Ri in FIG. 20A is broken like “sensor C”, a situation of “Ri−ZDC <K2” occurs, and the correction is performed as described above. The value ΔI is set.

The condition of Ri-ZDC = K2 to K3 (normal range) is due to the element DC resistance Ri and the DC resistance conversion value Z.
This is the case where DC substantially matches. In this case, for example, A
Since it is considered that there is almost no variation between the individual / F sensors 30 and the sensor is not in a deteriorated state, the current correction value ΔI = 0 is set regardless of the air-fuel ratio.

Further, the condition of Ri-ZDC ≧ K3 is as follows.
This is the case where the element DC resistance Ri increases with respect to the DC resistance conversion value ZDC. For example, it is conceivable that the sensor element temperature is low and the voltage applied to the sensor element section 30a falls outside the limit current detection range. In this case, if the air-fuel ratio is lean (sensor output Ip> 0), a positive current correction value ΔI is set, and if the air-fuel ratio is rich (sensor output Ip <0),
A negative current correction value ΔI is set. More specifically, the relationship between the AC impedance ZAC and the element DC resistance Ri in FIG. 20A is broken like “sensor B”, or in FIG.
If the relationship between AC and element DC resistance Ri breaks down, such as “after deterioration”, a situation of “Ri−ZDC ≧ K3” occurs, and the correction value ΔI is set as described above.

The deviation of the sensor output Ip increases as the (Ri-ZDC) value increases. for that reason,
When viewed at the same air-fuel ratio, the more the (Ri-ZDC) value is away from the normal range (K2 to K3), the larger the current correction value ΔI is set to the positive side or the negative side. Incidentally, when the air-fuel ratio is stoichiometric, irrespective of the (Ri-ZDC) value, ΔI =
It becomes 0.

After setting the current correction value ΔI, step 308
Then, the detected sensor output Ip is corrected by the current correction value ΔI. That is, the corrected sensor output Ipf is obtained as Ipf = Ip + ΔI. Thereafter, the corrected sensor output Ipf is output to the engine control microcomputer 16. For example, as shown in FIG. 9B, when A / F = 18 and Ri−ZDC = Kx, “ΔIx” is set as the current correction value ΔI, and the sensor output Ip is corrected by the ΔIx. .

However, the data output to the engine control microcomputer 16 as the air-fuel ratio detection result may be the sensor output itself (corrected sensor output Ipf) or may be converted to an A / F value using a predetermined conversion map. Data may be used.

In step 309, an applied voltage corresponding to the current air-fuel ratio (sensor output Ip before correction) is set and output for the next air-fuel ratio detection. At this time, using the applied voltage line LX1 of FIG.
An applied voltage corresponding to p is set. After that, this processing ends.

FIG. 10 shows how the sensor output Ip is corrected.
This will be described with reference to FIG. In FIG. 10, the VI characteristic during normal activation (before deterioration) is indicated by a solid line, and the VI characteristic during element deterioration is indicated by a two-dot chain line. Specific values in a normal state are ZAC = 20Ω, ZDC = 30Ω, and Ri = 30Ω, and it is assumed that the current Ip1 flows with the application of the voltage Vp. On the other hand, at the time of deterioration, ZAC = 20Ω, ZDC =
It is assumed that the current changes to 30Ω and Ri = 150Ω, and the current Ip2 flows with the application of the voltage Vp.

In this case, in the latter state, (Ri−
The current correction value ΔI is set according to the ZDC) value, and the corrected sensor output Ipf is calculated as Ipf = Ip2 + ΔI. The corrected sensor output Ipf matches the sensor output Ip1 before deterioration.

Next, a control procedure of heater energization will be described with reference to FIG. In FIG. 6, first, at step 401,
The AC impedance ZAC calculated in FIG. 3 is read, and in the subsequent step 402, the preset AC impedance ZAC is set in FIG.
Using the conversion map of (a), the AC impedance ZAC
Is converted to a DC resistance conversion value ZDC.

Thereafter, in step 403, it is determined whether the DC resistance converted value ZDC is equal to or less than a predetermined judgment value (about 200Ω in the present embodiment) for judging the semi-active state of the solid electrolyte 31 (sensor element portion). It is determined whether or not. For example, when the element temperature is low, such as when the engine 10 is started at a low temperature, ZDC> 200Ω, and the routine proceeds to step 404, where “100% energization control” of the heater 35 is performed, and then this processing ends. The 100% energization control is a control for maintaining the duty ratio control signal to the heater 35 at 100%, and when the DC resistance conversion value ZDC becomes 200Ω or less, step 40
It is continuously performed until 3 becomes YES.

If the element temperature rises due to the heater heat and step 403 becomes YES, the routine proceeds to step 405, where the DC resistance converted value ZDC is a predetermined judgment value for starting element impedance F / B control (this embodiment). Then, 40Ω
Degree) or not. The determination value in step 405 is set as a value of about “+ 10Ω” with respect to the target value ZDCtg (30Ω in the present embodiment) of the DC resistance conversion value.

Before completion of sensor activation and ZDC> 4
If the value is 0Ω, the process proceeds to step 406 to control the energization of the heater 35 by “power control”, and thereafter, the present process ends. At this time, the larger the DC resistance conversion value ZDC, the larger the power command value is determined, and the control duty ratio for energizing the heater is calculated according to the power command value.

When the activation of the sensor has been completed,
If 5 is YES, the process proceeds to step 407. In step 407, the element DC resistance Ri calculated in FIG. 4 is read, and in step 408, the element DC resistance Ri is calculated.
It is determined whether or not the difference (Ri-ZDC) between DC and DC resistance converted value ZDC is less than predetermined value K1. This predetermined value K1 may be the same as the K1 value in step 305 in FIG. If Ri-ZDC <K1, step 409 recognizes that an abnormality has occurred, and ends this processing as it is. In this case, processing such as turning on a warning light, interrupting the air-fuel ratio F / B control, and storing diagnostic information in a memory is performed.

If Ri-ZDC ≧ K1, the routine proceeds to step 410. In step 410, a correction value ΔZDCtg for the target value ZDCtg of the DC resistance conversion value is calculated according to the difference (Ri−ZDC) between the element DC resistance Ri and the DC resistance conversion value ZDC in accordance with the relationship of FIG.

In FIG. 11, K1, K2, and K3 have the same values as in FIG. That is, K2 and K3 are determination values for determining whether correction is necessary. Where Ri-Z
If DC <K2, a negative correction value ΔZDCtg is set, and if Ri−ZDC = K2 to K3, the correction value ΔZDC
When tg = 0 is set and Ri-ZDC ≧ K3, a positive correction value ΔZDCtg is set.

For example, in FIGS. 20 (a) and 20 (b), when the characteristic of each sensor is broken like “sensor C”,
A situation of “Ri−ZDC <K2” occurs, and the negative correction value ΔZ
DCtg is set. 20 (a) and 20 (b).
In the case where the characteristic of each sensor collapses as in “sensor B”, or when the deterioration progresses as indicated by the broken lines in FIGS. 21A and 21B, a situation of “Ri−ZDC ≧ K3” occurs and a positive The correction value ΔZDCtg is set.

Thereafter, in step 411, the DC resistance converted target value ZDCtg is corrected by the correction value ΔZDCtg. That is, the corrected target value ZDCtgf is obtained as ZDCtgf = ZDCtg + ΔZDCtg.

In step 412, a duty ratio DUTY for energizing the heater is calculated when the element impedance F / B control is performed. In the present embodiment, a PID control procedure is used as an example. That is, according to the following equations (1) to (3), the proportional term GP and the integral term G
I, a differential term GD is calculated.

GP = KP · (ZDC−ZDCtgf) (1) GI = GIi−1 + KI · (ZDC−ZDCtgf) (2) GD = KD · (ZDC−ZDCi−1) (3) In the equation, “KP” represents a proportional constant, “KI” represents an integral constant, “KD” represents a differential constant, and a subscript “i−1”
Represents the value of the previous processing.

The duty ratio DUTY is calculated by adding the proportional term GP, the integral term GI, and the differential term GD (D
UTY = GP + GI + GD). Then, the heater 35 is energized by the corrected target value ZDCtgf, and thereafter, the present process ends. It should be noted that such heater control is based on the above PID.
The control is not limited to the control, and the PI control or the P control may be performed.

When controlling the energization of the heater with the corrected target value ZDCtgf as described above, there is a concern that the sensor element may be overheated. Therefore, a maximum guard is applied to the element temperature, and when the element temperature reaches the guard value, further correction on the duty increasing side is stopped. In this case, even if the correction of the target value ZDCtg is stopped, the above-described sensor output correction is continuously performed, so that the influence of the individual variation or the like is eliminated.

In this embodiment, "impedance detecting means" is constituted by the processing of FIG. 3, and "DC resistance detecting means" is constituted by the processing of FIG. Also, “sensor output correction means” is performed by steps 307 and 308 in FIG.
Are constituted, and the "energization amount correction means" is constituted by steps 410 to 412 in FIG.

According to the present embodiment described in detail above, the following effects can be obtained. (A) The element DC resistance Ri of the sensor element section 30a is detected, and the sensor output I is determined according to the detected element DC resistance Ri.
p was corrected. Actually, the element DC resistance Ri
And a DC resistance conversion value ZDC, a current correction value ΔI corresponding to the difference (Ri−ZDC) was calculated, and the sensor output Ip was corrected using the current correction value ΔI. According to this configuration, even if thermal degradation of the sensor electrode or the like occurs, the sensor output correction by the element DC resistance Ri is performed as a reflection, and therefore, unlike the conventional device without the correction, the detection accuracy of the air-fuel ratio is different. Reduction can be avoided. As a result, it is possible to always accurately detect the air-fuel ratio regardless of the individual difference of the sensors or the change with time.

(B) Referring to the map for setting the current correction value ΔI, as shown in FIG. 9, the correction value ΔI is calculated according to the magnitude of the (Ri-ZDC) value and the air-fuel ratio at each time. Is done. Thereby, the accuracy of the correction calculation is improved, and more accurate sensor output correction can be performed.

(C) At the time of controlling the energization of the heater, a target value ZDCtg for energizing the heater according to the element DC resistance Ri.
Was corrected. Actually, the correction value ΔZDCtg was calculated according to the (Ri−ZDC) value. According to this configuration, even if thermal deterioration of the electrodes occurs, the heater power supply amount is corrected by the element DC resistance Ri as a reflection, so that the A / F sensor 30 is always maintained in a desired active state. it can. As a result, it is possible to always accurately detect the air-fuel ratio regardless of the individual difference of the sensors or the change with time.

According to the sensor output correction and the heater power supply amount correction, it is possible to perform an appropriate correction when the sensor characteristics change as shown in FIGS. 20 and 21 due to variations among sensors and changes over time. it can.

(D) In the case of the present embodiment in which the sensor output correction based on the element DC resistance Ri and the heater power supply amount correction are performed together, the variation between the sensors and the lapse of time are basically determined by correcting the target value ZDCtg. The effects of change are eliminated,
Further, the deficiency of the correction is compensated by the correction of the sensor output Ip, and as a result, an accurate air-fuel ratio detection value is obtained. In this case, by using both corrections together, the reliability of the air-fuel ratio detection is further improved, and the excessive rise of the temperature of the sensor element portion when the heater is energized is suppressed.

(E) When the difference between the element DC resistance Ri and the DC resistance conversion value ZDC exceeds a predetermined allowable value (Ri-ZDC
<K1), it is determined that the A / F sensor 30 is abnormal. Thereby, it is possible to appropriately cope with occurrence of an abnormality in the sensor such as thermal deterioration of the element electrode.

(F) When detecting the element DC resistance Ri, A
A voltage is applied in the resistance dominant region on the VI coordinate of the / F sensor 30, and the element DC resistance Ri is calculated from the applied voltage and the sensor output at that time. In this case, the element DC resistance Ri can be accurately detected.

(G) The detection of the element DC resistance Ri is permitted only when a predetermined condition for executing the detection of the air-fuel ratio is interrupted. That is, when detecting the element DC resistance Ri as described above, the AC impedance ZAC
Although it takes a longer time than the detection of the above, the effect of interrupting the air-fuel ratio detection at the time of detecting the element DC resistance Ri can be minimized by giving the condition of the execution permission.

(H) As described above, since the detection accuracy of the air-fuel ratio is improved, highly accurate air-fuel ratio F / B control can be realized, and the exhaust emission can be kept good.

Next, second and third embodiments of the present invention will be described. However, in the configurations of the following embodiments, the same components as those of the above-described first embodiment are denoted by the same reference numerals in the drawings, and the description is simplified. The following description focuses on differences from the first embodiment.

(Second Embodiment) In this embodiment,
The difference from the first embodiment is that the method of detecting the element DC resistance Ri is changed. That is, the circuit for applying a voltage to the A / F sensor 30 is momentarily interrupted, and the element DC resistance Ri is detected from the ratio between the voltage change and the current change before and after the momentary interruption. As a change of the device, as shown in FIG.
A switch 60 is provided between the LPF 22 in the ECU 15 and the bias control circuit 40, and the opening and closing of the switch 60 is operated by the air-fuel ratio detecting microcomputer 20.

FIG. 13 is a flowchart showing a procedure for detecting the element DC resistance Ri in the present embodiment. This processing is executed in place of the processing of FIG. In FIG. 13, first, at step 501, it is determined whether or not a condition for detecting the element DC resistance Ri is satisfied. When the result of step 501 is NO, the present process is temporarily terminated, and step 5 is executed.
When 01 is YES, the process proceeds to step 502. That is, the detection of the element DC resistance Ri is permitted only when a condition that may interrupt the air-fuel ratio detection is satisfied, for example, at the time of fuel cut, and the influence of the interruption of the air-fuel ratio detection is minimized. It shall be.

In step 502, the current applied voltage Vp of the A / F sensor 30 and the sensor output Ip are read.
That is, V in the air-fuel ratio detection state (switch closed state)
The p value and the Ip value are read.

Thereafter, at step 503, the switch 60
Is released, and at step 504, the electromotive force Eo of the A / F sensor 30 is detected. In step 505, the switch 60 is connected again. In this case, the shorter the time from the opening of the switch 60 to the reconnection, the better.

Thereafter, in step 506, the element DC resistance Ri is calculated. That is, the element DC resistance Ri is calculated from the calculation result of Ri = (Vp−Eo) / Ip. After the detection of the Ri value, the present process is temporarily terminated. In the present embodiment, the “DC resistance detecting means” is configured by the processing in FIG.

According to the above processing, as shown in FIG. 14, a part of the circuit is momentarily cut off when the switch 60 is opened, so that the voltage is not applied to the sensor element portion, and the current and the voltage are reduced. Decay. Then, the electromotive force Eo is detected at a predetermined timing in consideration of the responsiveness of the circuit. At this time, the detection of the electromotive force Eo according to the oxygen partial pressure difference between the inside and outside of the solid electrolyte 31 and the sensor output Ip
Becomes “0 mA”, the voltage change and the current change are obtained, and the element DC resistance Ri is calculated from the change amounts.

As described above, according to the second embodiment, the first
As in the third embodiment, an excellent effect is obtained such that the air-fuel ratio can always be detected with high accuracy irrespective of individual differences between sensors or changes with time. Further, the above-described element direct resistance Ri
According to the detection method, the element DC resistance Ri can be detected with high accuracy.

(Third Embodiment) The third embodiment differs from the first embodiment in that the applied voltage control in the air-fuel ratio detection process is changed.

In the above embodiment, the sensor output Ip is corrected according to the variation of the element DC resistance Ri. However, in this embodiment, the A / F is adjusted according to the variation of the element DC resistance Ri. The voltage applied to the sensor 30 is corrected.

FIG. 15 is a flowchart showing the procedure for detecting the air-fuel ratio in the present embodiment. This process is executed in place of the process shown in FIG. In FIG. 15, first, in steps 601 to 606, similarly to steps 301 to 306 in FIG. 5, at step 601, the sensor output Ip is read, at step 602, the AC impedance ZAC is read, at step 603, The AC impedance ZAC is converted into a DC resistance converted value ZDC. In step 604, the element DC resistance Ri is read. In step 605, the difference (Ri-ZDC) between the element DC resistance Ri and the DC resistance converted value ZDC is determined. It is determined whether or not the value is less than the value K1. If Ri-ZDC <K1, it is recognized in step 606 that an abnormality has occurred.

As a process different from that of FIG.
When i-ZDC ≧ K1, the process proceeds to step 607, and FIG.
According to the relationship, the element DC resistance Ri and the DC resistance conversion value ZD
The voltage correction value ΔV is calculated according to the difference from C (Ri−ZDC) and the air-fuel ratio (sensor output Ip) at that time.

In FIG. 16 (a), a plurality of characteristic lines are set according to whether the air-fuel ratio is lean or rich, that is, whether the sensor output Ip is positive or negative. = 18,23, A / F = 12 is shown as an example of the air-fuel ratio rich characteristic. In the figure, K1 is the above-mentioned step 605.
, And K2 and K3 are determination values for determining whether correction is necessary (K1 <K2 <K3). Then, the voltage correction value ΔV is set according to the (Ri−ZDC) value and the air-fuel ratio.

Here, Ri-ZDC <K2 is satisfied by:
This is the case where the element DC resistance Ri decreases with respect to the DC resistance conversion value ZDC. For example, it is conceivable that the sensor element temperature is high and the voltage applied to the sensor element section 30a falls outside the limit current detection range. In this case, if the air-fuel ratio is lean (sensor output Ip> 0), a negative voltage correction value ΔV is set, and if the air-fuel ratio is rich (sensor output Ip <0),
A positive voltage correction value ΔV is set. More specifically, when the relationship between the AC impedance ZAC and the element DC resistance Ri in FIG. 20A is broken like “sensor C”, a situation of “Ri−ZDC <K2” occurs, and the correction is performed as described above. The value ΔV is set.

Further, Ri-ZDC = K2 to K3 (normal range) is caused by the element DC resistance Ri and the DC resistance conversion value Z.
This is the case where DC substantially matches. In this case, for example, A
Since it is considered that there is almost no variation between the individual / F sensors 30 and it is not in a deteriorated state, the voltage correction value ΔV = 0 is set regardless of the air-fuel ratio.

Further, the condition of Ri-ZDC ≧ K3 is that
This is the case where the element DC resistance Ri increases with respect to the DC resistance conversion value ZDC. For example, it is conceivable that the sensor element temperature is low and the voltage applied to the sensor element section 30a falls outside the limit current detection range. In this case, if the air-fuel ratio is lean (sensor output Ip> 0), a positive voltage correction value ΔV is set, and if the air-fuel ratio is rich (sensor output Ip <0),
A negative voltage correction value ΔV is set. More specifically, the relationship between the AC impedance ZAC and the element DC resistance Ri in FIG. 20A is broken like “sensor B”, or in FIG.
If the relationship between AC and element DC resistance Ri breaks down, such as "after deterioration", a situation of "Ri-ZDC≥K3" occurs, and the correction value ΔV is set as described above.

Note that as the (Ri-ZDC) value increases, the amount of deviation of the applied voltage Vp increases. Therefore, when viewed at the same air-fuel ratio, as the (Ri-ZDC) value moves away from the normal range (K2 to K3), the voltage correction value ΔV is set to be larger on the positive side or the negative side. Incidentally, when the air-fuel ratio is stoichiometric, ΔV = always regardless of the (Ri−ZDC) value.
It becomes 0.

After setting the voltage correction value ΔV, step 608 is executed.
Then, the applied voltage Vp mapped by the applied voltage line LX1 in FIG. 2 in accordance with the sensor output Ip (the detection value in step 601) is corrected by the voltage correction value ΔV. That is, the applied voltage Vp after the correction as Vpf = Vp + ΔV
Get f. After that, the corrected applied voltage Vpf is output to the bias control circuit 40. For example, FIG.
As shown in (b), A / F = 18 and Ri
When −ZDC = Kx, the voltage correction value ΔV is “ΔVx”
Is set, and the applied voltage Vp is corrected by the ΔVx.

In step 609, after changing to the corrected applied voltage Vpf, the limit current flowing between the electrodes is
It re-detects it as the sensor output Ip and outputs the detection result to the engine control microcomputer 16. After that, this processing ends. In this embodiment, “applied voltage correction means” is constituted by steps 607 and 608 in FIG.

The manner in which the applied voltage Vp is corrected will be described with reference to FIG. In FIG. 17, at normal activation (before degradation)
Are shown by a solid line, and the VI characteristics at the time of device deterioration are shown by a two-dot chain line. The specific value at normal time is
ZAC = 20Ω, ZDC = 30Ω, Ri = 30Ω, and the applied voltage for detecting the sensor output Ip is “Vp
1 ". On the other hand, at the time of deterioration, ZAC = 2
It is assumed that 0Ω, ZDC = 30Ω, and Ri = 150Ω.

In this case, in the latter state, (Ri−
ZDC) value, a voltage correction value ΔV is set, and Vpf
= Vp1 + ΔV, and the corrected applied voltage Vpf is calculated. By applying the applied voltage Vpf after this correction,
The same sensor output Ip as before deterioration is obtained.

In this embodiment, similarly to the first embodiment, the heater energization control (the process of FIG. 6) is performed, and the target value ZDCtg for energizing the heater is set according to the element DC resistance Ri. Although they are corrected, their contents are duplicated, so that their illustration and description are omitted here. Further, the device according to the present embodiment can also be realized in combination with the second embodiment.

As described above, according to the third embodiment, the following effects can be obtained. (A) The element DC resistance Ri of the sensor element section 30a is detected, and the sensor applied voltage Vp is corrected according to the detected element DC resistance Ri. Actually, a voltage correction value ΔV corresponding to the difference (Ri−ZDC) between the element DC resistance Ri and the DC resistance conversion value ZDC was calculated, and the sensor applied voltage Vp was corrected using the voltage correction value ΔV. According to this configuration,
Even if the sensor electrode is thermally degraded or the like, the applied voltage is corrected by the element DC resistance Ri as a reflection of the reflection. Therefore, unlike the conventional device without the correction, a decrease in the air-fuel ratio detection accuracy can be avoided. As a result, it is possible to always accurately detect the air-fuel ratio regardless of the individual difference of the sensors or the change with time.

According to the applied voltage correction, an appropriate correction can be performed in the case where the sensor characteristics change as shown in FIGS. 20 and 21 due to variations among sensors and changes with time.

(B) Referring to the map for setting the voltage correction value ΔV, as shown in FIG. 16, the correction value ΔV is set according to the magnitude of the (Ri-ZDC) value and the air-fuel ratio at each time.
Is calculated. This improves the accuracy of the correction calculation,
More accurate sensor output correction becomes possible.

(C) In the case of the present embodiment in which the correction of the applied voltage based on the element DC resistance Ri and the correction of the amount of current supplied to the heater are performed together, the variation between the sensors and the lapse of time are basically determined by correcting the target value ZDCtg. The effect of the change is eliminated, and the lack of the correction is compensated for by the correction of the applied voltage Vp.
As a result, an accurate air-fuel ratio detection value is obtained. In this case, by using both corrections together, the reliability of the air-fuel ratio detection is further improved, and the excessive rise of the temperature of the sensor element portion when the heater is energized is suppressed.

(D) When the difference between the element DC resistance Ri and the DC resistance conversion value ZDC exceeds a predetermined allowable value (Ri-ZDC
<K1), it is determined that the A / F sensor 30 is abnormal. Thereby, it is possible to appropriately cope with occurrence of an abnormality in the sensor such as thermal deterioration of the element electrode.

The embodiments of the present invention can be embodied in the following forms other than the above. (Alternative Embodiment 1) In the first embodiment, when correcting the sensor output Ip, a difference “Ri−ZDC” between the element DC resistance Ri and the DC resistance conversion value ZDC is obtained, and according to the obtained value, Although the current correction value ΔI has been calculated, this is changed. For example, a ratio “Ri / ZDC” between the element DC resistance Ri and the DC resistance conversion value ZDC may be obtained, and the current correction value ΔI may be calculated according to the obtained value. Similarly, in the third embodiment, the element DC resistance Ri and the DC resistance conversion value Z
The ratio “Ri / ZDC” to DC may be obtained, and the voltage correction value ΔV may be calculated according to the obtained value.

(Alternative Embodiment 2) In the first embodiment, the difference “Ri−ZDC” between the element DC resistance Ri and the DC resistance conversion value ZDC and “0” are set in accordance with the air-fuel ratio at each time. Is calculated (see FIG. 9), and the corrected sensor output Ipf is calculated by an arithmetic expression, Ipf = Ip + ΔI, but this is changed. For example, using the relationship of FIG.
The current correction value ΔIa based on “1” is calculated according to “C” (or Ri / ZDC). At this time, Ri-ZDC
If <K2, ΔIa <1. If Ri-ZDC> K3, ΔIa> 1. Also, the current correction value ΔIa increases as “Ri-ZDC” moves away from “0”.
Then, the corrected sensor output Ipf is calculated by an arithmetic expression: Ipf = Ip · ΔIa. In this case, even if the air-fuel ratio changes, the same correction value ΔIa can be used, and it is not necessary to use the air-fuel ratio as a parameter for calculating the ΔIa value. The above is similarly applicable to the correction of the sensor applied voltage as in the third embodiment. That is,
The voltage correction value ΔVa is calculated according to “Ri−ZDC” (or Ri / ZDC), and the corrected applied voltage Vpf is calculated according to the ΔVa value (Vpf = Vp · ΔVa). Even with the above configuration, the air-fuel ratio can always be detected with high accuracy regardless of the individual difference of the sensor or the change with time as described above.

(Alternative Embodiment 3) In the first embodiment, the target value ZDCtg for energizing the heater is corrected in accordance with the element DC resistance Ri in the energizing control of the heater. May be. For example, the duty ratio DU for energizing the heater according to the DC resistance conversion value ZDC
TY is calculated, and (R
Calculate the duty correction value ΔD (according to i-ZDC). Then, the duty ratio DUTYf after correction is obtained by setting DUTYf = DUTY + ΔD,
The heater is energized by UTYf. Even in this configuration,
The air-fuel ratio can always be detected with high accuracy irrespective of the individual difference of the sensor or the change with time.

(Alternative Embodiment 4) As in the first embodiment, when the sensor output correction based on the element DC resistance Ri and the heater energization amount correction are performed together, according to the element temperature at that time. Both corrections may be selectively performed. For example, when the element temperature is relatively low, the heater power supply amount is corrected, and when the element temperature is relatively high, the sensor output is corrected. Alternatively, as in the third embodiment, the element DC resistance Ri
When the correction of the sensor application voltage and the correction of the amount of electricity supplied to the heater based on the above are performed together, both corrections may be selectively performed according to the element temperature at that time. For example, when the element temperature is relatively low, correction of the heater power supply is performed, and when the element temperature is relatively high, correction of the sensor application voltage is performed. In this case, an excessive temperature rise of the sensor element can be suppressed.

(Alternative Embodiment 5) In the above-described first embodiment, the sensor output correction based on the element DC resistance Ri and the heater energization amount correction are performed together. Good. Alternatively, in the third embodiment, the correction of the sensor applied voltage based on the element DC resistance Ri and the correction of the amount of current supplied to the heater are performed in combination. However, only the correction of the sensor applied voltage may be performed.

(Alternative Embodiment 6) In each of the above embodiments, when comparing the element DC resistance Ri and the DC resistance conversion value ZDC, it is considered that the sensor is abnormal when Ri−ZDC <K1 (FIGS. 5 and 6). , 15), and a new determination value K4 is provided (however, K4> K3), and when Ri−ZDC> K4, the sensor may be regarded as abnormal. In this case, processing such as turning on a warning light, interrupting the air-fuel ratio F / B control, and storing diagnostic information in a memory is performed.

(Alternative Embodiment 7) When detecting the element DC resistance Ri, a negative applied voltage on the VI characteristic is applied and a negative limit current flowing therewith is measured, and the element DC resistance is calculated from these values. Ri may be detected. Applying a negative applied voltage on the VI characteristics means applying a voltage in the resistance dominant region.

(Alternative Embodiment 8) In each of the above embodiments, an example of application to a cup-type A / F sensor has been disclosed, but application to other sensors is also possible. For example, a solid electrolyte, an electrode, a diffusion resistance layer, and a heater are each formed into a plate shape, and these members are laminated to be applied to a so-called laminated A / F sensor. It may be applied to a so-called composite gas sensor that detects the concentration of a gas component. The composite gas sensor generally includes a diffusion resistance portion for introducing a gas to be detected, and a first cell for flowing a current according to the oxygen concentration while discharging excess oxygen in the gas to be detected in the diffusion resistance portion with voltage application. (Pump cell), and a second cell (sensor cell) for flowing a current corresponding to the concentration of the specific component from the gas component after the excess oxygen is discharged in accordance with the voltage application. In the second cell, for example, NOx, C
O, HC, etc. are detected. Even when applied to each of these sensors, as described above, regardless of individual differences and changes over time of the sensors,
The gas concentration can always be accurately detected.

[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.

FIG. 2 is a diagram showing VI characteristics of an A / F sensor.

FIG. 3 is a flowchart showing a procedure for detecting an AC impedance.

FIG. 4 is a flowchart showing a procedure for detecting an element DC resistance.

FIG. 5 is a flowchart showing a procedure for detecting an air-fuel ratio.

FIG. 6 is a flowchart showing a control procedure of heater energization.

FIG. 7 is a waveform chart showing transition of a sensor voltage and a sensor current at the time of impedance detection.

FIG. 8 is a diagram for explaining a set voltage at the time of detecting a device DC resistance.

FIG. 9 is a relationship diagram for setting a current correction value ΔI.

FIG. 10 is a diagram showing how a sensor output is corrected by a current correction value ΔI.

FIG. 11 is a relationship diagram for setting a correction value ΔZDCtg.

FIG. 12 is a configuration diagram illustrating an outline of an air-fuel ratio control system according to a second embodiment.

FIG. 13 is a flowchart illustrating a procedure for detecting a DC resistance of an element in the second embodiment.

FIG. 14 is a diagram showing changes in voltage and current when a circuit is momentarily interrupted.

FIG. 15 is a flowchart showing a procedure for detecting an air-fuel ratio in the third embodiment.

FIG. 16 is a relationship diagram for setting a voltage correction value ΔV.

FIG. 17 is a diagram illustrating a state in which an applied voltage is corrected by a voltage correction value ΔV.

FIG. 18 is a relationship diagram for setting a current correction value ΔIa in another embodiment.

FIG. 19 is a diagram showing a relationship among element DC resistance, AC impedance, and element temperature.

FIG. 20 is a diagram showing a relationship among element DC resistance, AC impedance, and element temperature.

FIG. 21 is a diagram showing a relationship among element DC resistance, AC impedance, and element temperature.

[Explanation of symbols]

Reference numeral 20: an air-fuel ratio detecting microcomputer constituting DC resistance detecting means, sensor output correcting means, applied voltage correcting means, impedance detecting means, and energizing amount correcting means, 30: A / F sensor as a gas concentration sensor (limit current type air Fuel ratio sensor),
30a: Sensor element part, 31: Solid electrolyte, 33, 34
... electrodes, 35 ... heaters.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tomio Kawase 1-1-1, Showa-cho, Kariya-shi, Aichi Prefecture Inside DENSO Corporation (72) Inventor Satoshi Haneda 1-1-1, Showa-cho, Kariya-shi, Aichi Co., Ltd. Inside DENSO

Claims (15)

[Claims] An element portion having a solid electrolyte and electrodes provided on opposing surfaces of the solid electrolyte, respectively, wherein when a voltage is applied between the electrodes, a specific component of a gas to be detected is accordingly generated. In a gas concentration detection device using a gas concentration sensor that outputs a current signal according to a concentration, a DC resistance detection unit that detects an element DC resistance in the element unit, and a current signal output from the gas concentration sensor is detected. And a sensor output correcting means for correcting in accordance with the element DC resistance. 2. A gas concentration detecting device for detecting an AC impedance of said element portion and converting the detected AC impedance into a DC resistance conversion value, wherein said sensor output correction means comprises: said element DC resistance and said DC resistance conversion value. Means for calculating a correction value according to the difference from the value,
2. The gas concentration detection device according to claim 1, further comprising: means for correcting a current signal output from the gas concentration sensor using the calculated correction value. 3. The gas concentration detecting device according to claim 2, wherein said sensor output correcting means calculates said correction value according to a gas concentration at each time. 4. An element section having a solid electrolyte and electrodes respectively provided on opposing surfaces of the solid electrolyte, and when a voltage is applied between the electrodes, a specific component in a gas to be detected is accordingly generated. In a gas concentration detection device using a gas concentration sensor that outputs a current signal corresponding to a concentration, a DC resistance detection unit that detects an element DC resistance in the element unit, and a voltage applied to the gas concentration sensor, A gas concentration detection device, comprising: an applied voltage correction unit that corrects according to a DC resistance. 5. A gas concentration detecting device for detecting an AC impedance of said element section and converting the detected AC impedance into a DC resistance conversion value, wherein said applied voltage correction means comprises: said element DC resistance and said DC resistance conversion value. 5. The gas concentration detection device according to claim 4, further comprising: means for calculating a correction value corresponding to a difference between the value and a means for correcting a voltage applied to the gas concentration sensor using the calculated correction value. 6. The gas concentration detecting device according to claim 5, wherein said applied voltage correcting means calculates said correction value according to a gas concentration at each time. 7. An element section having a solid electrolyte and electrodes respectively provided on opposing surfaces of the solid electrolyte, and an element for heating the element section to a predetermined activation temperature by generating heat by supplying power supply voltage. A gas concentration sensor that includes a heater and outputs a current signal corresponding to the concentration of a specific component in the gas to be detected when a voltage is applied between the electrodes, and controls a power supply amount of the heater. In the detection device, impedance detecting means for detecting the AC impedance of the element portion, DC resistance detecting means for detecting the DC resistance of the element portion, and the amount of power to the heater determined based on the detected AC impedance, A gas concentration detecting device, comprising: an energization amount correction unit that corrects according to the detected element DC resistance. 8. A gas concentration detecting device for converting the detected AC impedance into a DC resistance conversion value, wherein the current supply amount correction means calculates a correction value corresponding to a difference between the element DC resistance and the DC resistance conversion value. 8. The gas concentration detecting device according to claim 7, further comprising: means for calculating, and means for correcting the set amount of power to the heater using the calculated correction value. 9. The gas concentration detecting device according to claim 7, further comprising a sensor output correcting means for correcting a current signal output from the gas concentration sensor according to the detected element DC resistance. Detection device. 10. The gas concentration detecting device according to claim 7, further comprising an applied voltage correcting means for correcting an applied voltage to the gas concentration sensor according to the detected element DC resistance. . 11. The correction value is increased as the difference between the element DC resistance and the DC resistance conversion value increases.
The gas concentration detecting device according to any one of the above. 12. The gas concentration detection device according to claim 2, wherein when the difference between the element DC resistance and the DC resistance conversion value exceeds a predetermined allowable value, it is determined that the gas concentration sensor is abnormal. . 13. The DC resistance detecting means applies a voltage in a resistance dominant region on a VI coordinate of the gas concentration sensor, and calculates an element DC resistance from the applied voltage at that time and a sensor output. 13. The gas concentration detecting device according to any one of claims 12 to 12. 14. The DC resistance detecting means instantaneously interrupts a circuit for applying a voltage to the gas concentration sensor, and detects an element DC resistance from a ratio between a voltage change and a current change before and after the instantaneous interruption. Item 13. The gas concentration detection device according to any one of Items 1 to 12. 15. The device according to claim 1, wherein the DC resistance detecting means permits the detection of the DC resistance of the element only when a predetermined execution condition that allows the gas concentration detection to be interrupted is satisfied. Gas concentration detector.