JP5494564B2 - Exhaust gas sensor signal processing device - Google Patents

Description

  The present invention relates to a signal processing device for an exhaust gas sensor.

  Conventionally, for example, as disclosed in Japanese Patent Application Laid-Open No. 2002-303601, in a configuration in which an output signal of an exhaust gas sensor is filtered (filtered) by a low-pass filter (LPF), a cutoff frequency (cut-off frequency) of the low-pass filter There is known a configuration for appropriately changing (adjusting). The low-pass filter is a low-pass type frequency filter that selectively passes a signal having a frequency equal to or lower than a predetermined cutoff frequency. In the technique according to the above publication, the cutoff frequency of the low-pass filter is changed according to the temperature rise state of the A / F sensor and the operating state of the internal combustion engine. Specifically, the cutoff frequency of the low-pass filter is changed to A / F. It is set high during the temperature rise of the F sensor, and is set low after the temperature rise of the A / F sensor is completed. By doing so, stabilization of control is achieved while realizing early activation required for the A / F sensor.

JP 2002-303601 A JP 2000-292411 A JP-A-11-6813 JP 2006-112232 A JP 2006-97521 A JP 2004-143967 A JP-A-5-187297

  When an exhaust gas sensor output signal that has passed through a frequency filter (actually, a low-pass filter) is used for air-fuel ratio control, information necessary for air-fuel ratio control can be read with sufficient accuracy from the signal that has passed through this frequency filter. Is desired. On the other hand, if the cutoff frequency of the filter is not an appropriate value, it becomes difficult to accurately detect the exhaust gas atmosphere from the signal after passing through the filter, and the accuracy of the air-fuel ratio control is lowered. In the prior art, no consideration is given to the influence of such deterioration in accuracy of air-fuel ratio control due to the filter cutoff frequency. Due to such a lack of study, there is a risk that the sensitivity of air-fuel ratio detection for performing air-fuel ratio control will greatly deviate from an appropriate state as the cut-off frequency is changed as necessary. As a result, the accuracy of the air-fuel ratio control may be adversely affected.

  The present invention has been made to solve the above-described problems, and suppresses deterioration in the accuracy of air-fuel ratio control when air-fuel ratio control is performed using an exhaust gas sensor output signal after passing through a filter. An object of the present invention is to provide a signal processing device for an exhaust gas sensor that can be used.

In order to achieve the above object, a first invention is a signal processing device for an exhaust gas sensor,
An exhaust gas sensor that is provided in an exhaust passage of the internal combustion engine and changes an output according to an oxygen concentration in the exhaust gas;
A low-pass filter that has a cutoff frequency variable and performs filtering so as to pass a signal having a frequency equal to or lower than the cutoff frequency with respect to the output signal of the exhaust gas sensor;
Means for transmitting a signal that has passed through the low-pass filter as information to be used for controlling an air-fuel ratio of the internal combustion engine to a control device for the internal combustion engine;
A high-pass filter that performs filtering so as to pass a signal having a frequency equal to or higher than a cutoff frequency with respect to an output signal of the exhaust gas sensor;
A frequency adjusting means for adjusting a cutoff frequency of the low-pass filter based on a signal that has passed through the high-pass filter;
It is characterized by providing.

The second invention is the first invention, wherein
The frequency adjusting unit is configured to control the low-pass signal according to the number or magnitude of signals having a frequency equal to or higher than a cutoff frequency of the high-pass filter included in the signal that has passed through the high-pass filter. And a frequency shift means for shifting the cutoff frequency of the filter of the type to a relatively low frequency side.

The third invention is the first or second invention, wherein
The exhaust gas sensor includes a downstream air-fuel ratio sensor provided downstream of a catalyst disposed in the internal combustion engine,
Specific operating condition frequency that shifts the cutoff frequency of the low-pass filter to the high frequency side when the internal combustion engine is operated under at least one operating condition before completion of fuel cut, starting operation, and catalyst warm-up It includes a shift means.

According to a fourth invention, in any one of the first to third inventions,
A catalyst is provided in the exhaust passage of the internal combustion engine,
The exhaust gas sensor
An upstream exhaust gas sensor of a limiting current type provided upstream of the catalyst;
A downstream exhaust gas sensor of a limiting current type provided downstream of the catalyst;
Including
The low-pass filter includes a first low-pass filter that filters a signal having a frequency equal to or lower than a cutoff frequency with respect to an output signal of the upstream exhaust gas sensor, and the downstream exhaust gas. A second low-pass filter that performs filtering so as to pass a signal having a frequency equal to or lower than the cutoff frequency with respect to the output signal of the gas sensor,
The high-pass filter includes a first high-pass filter that filters a signal having a frequency equal to or higher than a cutoff frequency with respect to an output signal of the upstream exhaust gas sensor, and the downstream exhaust gas. A second high-pass filter that performs filtering so as to pass a signal having a frequency equal to or higher than the cutoff frequency with respect to the output signal of the gas sensor,
The frequency adjusting means includes
First frequency adjusting means for adjusting a cutoff frequency of the first low-pass filter based on a signal that has passed through the first high-pass filter;
Second frequency adjusting means for adjusting a cutoff frequency of the second low-pass filter based on a signal that has passed through the second high-pass filter;
It is characterized by including.

  According to the first aspect of the invention, the cutoff frequency of the low-pass filter can be adjusted based on a signal having a predetermined frequency or higher included in the exhaust gas sensor output signal. As a result, the cutoff frequency of the low-pass filter is adjusted so that the air-fuel ratio detection sensitivity is appropriate when air-fuel ratio control is performed using the exhaust gas sensor output signal after passing through the low-pass filter. It becomes possible to do. As a result, it is possible to adjust the air-fuel ratio detection sensitivity and suppress deterioration in accuracy of the air-fuel ratio control.

  According to the second aspect of the present invention, the cutoff frequency can be adjusted based on the number or magnitude of signals having a predetermined frequency or higher included in the exhaust gas sensor output signal. Thereby, it can suppress that the precision of the air-fuel ratio control deteriorates because the sensitivity of detection by the exhaust gas sensor becomes excessively good.

  According to the third aspect of the present invention, it is possible to cope with a specific operating condition in which the output response in the transition may be smoothed (smoothed in the time axis direction) beyond the level required for control. In addition, the cutoff frequency of the low-pass filter can be adjusted.

  According to the fourth aspect of the invention, limiting current type exhaust gas sensors are arranged upstream and downstream of the catalyst, which is promising in the future from the viewpoint of coping with future emission regulations, OBD regulations, reduction of catalyst precious metals, etc. In the air-fuel ratio control system, the cutoff frequency of the low-pass filter can be adjusted so that the sensitivity of air-fuel ratio detection is appropriate. By adjusting the cut-off frequency, it is possible to adjust the air-fuel ratio detection sensitivity and suppress deterioration in accuracy of the air-fuel ratio control.

It is a figure which shows the structure of the signal processing apparatus of the exhaust gas sensor concerning embodiment of this invention with a part of structure of the internal combustion engine to which this is applied. It is a block diagram for demonstrating the structure and operation | movement of the signal processing apparatus of the exhaust gas sensor concerning embodiment of this invention. It is a figure which shows the modification of the structure of the signal processing apparatus of the exhaust gas sensor concerning embodiment of this invention with a part of structure of the internal combustion engine to which this is applied.

Embodiment.
FIG. 1 is a diagram showing the configuration of an exhaust gas sensor signal processing device according to an embodiment of the present invention, together with a part of the configuration of an internal combustion engine to which the exhaust gas sensor is applied. The internal combustion engine to which the control device of the present embodiment is applied is an internal combustion engine for automobiles, and more specifically, a premixed combustion type 4-stroke 1-cycle reciprocating engine. The signal processing apparatus of the present embodiment is realized as one function of an ECU (Electronic Control Unit) that comprehensively controls the operation of such an internal combustion engine.

(Configuration of internal combustion engine and system)
Hereinafter, a specific configuration of the internal combustion engine 10 will be described. However, illustration of each component is omitted. The internal combustion engine 10 has a piston inside, and this piston is connected to a crankshaft via a crank mechanism. A crank angle sensor is provided in the vicinity of the crankshaft. The crank angle sensor is configured to detect a rotation angle (hereinafter referred to as “crank angle”) CA of the crankshaft. A cylinder head is assembled to the upper part of the cylinder block, and the space from the upper surface of the piston to the cylinder head forms a combustion chamber. The cylinder head is provided with a spark plug that ignites the air-fuel mixture in the combustion chamber.

The cylinder head of the internal combustion engine 10 includes an intake port that communicates with the combustion chamber, and an intake valve is provided at a connection portion between the intake port and the combustion chamber. An intake passage is connected to the intake port, and an injector that injects fuel in the vicinity of the intake port is provided in the intake passage. A direct injection injector that injects fuel directly into the combustion chamber may be provided.
A throttle valve is provided upstream of the injector. The throttle valve is an electronically controlled valve that is driven by a throttle motor. The throttle valve is driven based on the accelerator opening AA detected by the accelerator opening sensor. A throttle opening sensor for detecting the throttle opening is provided in the vicinity of the throttle valve. A hot-wire air flow meter is provided upstream of the throttle valve. The air flow meter is configured to detect an intake air amount Ga. An air cleaner is provided upstream of the air flow meter.

The cylinder head of the internal combustion engine 10 includes an exhaust port communicating with the combustion chamber. An exhaust valve is provided at the connection between the exhaust port and the combustion chamber. An exhaust passage 20 is connected to the exhaust port. The exhaust passage 20 is provided with an S / C catalyst 24 (hereinafter referred to as “catalyst”) for purifying exhaust gas. The catalyst 24 is assumed to be a low noble metal catalyst with a reduced noble metal content. A limit current type air-fuel ratio sensor 22 is provided at a position upstream of the catalyst 24 in the exhaust passage 20. A limit current type air-fuel ratio sensor 26 is also provided at a position downstream of the catalyst 24 in the exhaust passage 20. These air-fuel ratio sensors 22 and 26 are both connected to the ECU 50.
As the future emission regulations are strengthened, OBD regulations are strengthened, and catalytic precious metals are reduced, an air-fuel ratio control system with high controllability and robustness is required. In response to this requirement, a system in which air-fuel ratio sensors (for example, limit current type air-fuel ratio sensors) are arranged before and after the three-way catalyst (S / C) is promising in the future. The present embodiment applies the output processing technique of the air-fuel ratio sensor according to the present invention in such an air-fuel ratio control system in which air-fuel ratio sensors are arranged before and after the catalyst.

(Configuration of air-fuel ratio sensor)
The air-fuel ratio sensors 22 and 26 have sensor element portions (not shown). The sensor element portions of the air-fuel ratio sensors 22 and 26 have a solid electrolyte layer as a detection element. This solid electrolyte layer is made of partially stabilized zirconia and has oxygen ion conductivity. A measurement electrode is provided on one surface of the solid electrolyte layer. In addition, an atmosphere-side electrode (also referred to as “reference gas-side electrode”) is provided on the other surface of the solid electrolyte layer. Both the measurement electrode and the atmosphere side electrode are made of platinum or the like, and are connected to an ECU 50 (described later) via leads. A porous diffusion resistance layer is formed on one surface of the solid electrolyte layer. The porous diffusion resistance layer has a gas permeable layer for covering the measurement electrode, for introducing exhaust gas into the measurement electrode, and a gas blocking layer for suppressing permeation of the exhaust gas. These gas permeable layers and gas barrier layers are made of ceramics such as alumina and zirconia, and have different average pore diameters and porosity.
An air introduction duct is formed on the other surface of the solid electrolyte layer. The air introduction duct has an air chamber (also referred to as “reference gas chamber”) at the top. The atmosphere side electrode is disposed in the atmosphere chamber. The air introduction duct is made of high thermal conductivity ceramics such as alumina. A heater is provided on the lower surface of the air introduction duct. The heater includes a plurality of heating elements that generate heat when energized, and an insulating layer b that covers the heating elements. The heating element is connected to the ECU via a lead.
The sensor element unit can detect the oxygen concentration with a linear characteristic, and can output a limit current corresponding to the detected oxygen concentration to the ECU 50. This air-fuel ratio sensor output (limit current) has a correlation with the air-fuel ratio of the exhaust gas. Specifically, the limit current increases as the air-fuel ratio of the exhaust gas becomes leaner, and the limit current decreases as the air-fuel ratio of the exhaust gas becomes richer.

(Configuration of control device and signal processing device)
As shown in FIG. 1, the present embodiment includes an ECU (Electronic Control Unit) 50 as a control device. An ignition plug, an injector, a throttle motor, and the like are connected to the output side of the ECU 50. Connected to the input side of the ECU 50 are a coolant temperature sensor, a crank angle sensor, an accelerator opening sensor, a throttle opening sensor, an air flow meter, an air-fuel ratio sensor, and the like. The ECU 50 calculates the engine speed NE based on the output of the crank angle sensor. Further, the ECU calculates the engine load KL based on the accelerator opening AA detected by the accelerator opening sensor. The ECU determines the fuel injection amount based on the engine speed NE, the engine load KL, and the like.

The ECU 50 performs air-fuel ratio control for adjusting the fuel injection amount so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio. More specifically, as air-fuel ratio control, one comprising main feedback control based on the output signal of the catalyst upstream sensor and sub feedback control based on the output signal of the catalyst downstream sensor is known. The main feedback control is a process for making the output signal of the catalyst upstream sensor coincide with the target air-fuel ratio, and the sub feedback control is a process for compensating for a steady error included in the output signal of the catalyst upstream sensor. In the configuration of the internal combustion engine in the embodiment, the air-fuel ratio sensor 22 corresponds to the catalyst upstream sensor, and the air-fuel ratio sensor 26 corresponds to the catalyst downstream sensor.
As described above, in this embodiment, a system is used in which limit current type air-fuel ratio sensors 22 and 26 are respectively arranged before and after the three-way catalyst (S / C). According to such a system, the exhaust gas concentration (that is, oxygen concentration, air-fuel ratio) downstream of the catalyst can be detected linearly, and appropriate monitoring of the internal state of the catalyst 24 can be performed.

  FIG. 2 is a block diagram for explaining the configuration and operation of the signal processing apparatus for the exhaust gas sensor according to the embodiment of the present invention. In FIG. 2, the air-fuel ratio sensor is indicated as “A / F sensor”. The exhaust gas sensor signal processing apparatus according to this embodiment includes low-pass filters (LPF) 30 and 40 and high-pass filters (HPF) 32 and 42. The low-pass filters 30 and 40 are assumed to use filter circuits that can change their cutoff frequencies (cut-off frequencies). Since various techniques are already known for the filter circuit that makes the cut-off frequency variable, detailed description is omitted here. Specific configurations of these filters 30, 32, 40, and 42 according to the embodiment can be appropriately selected and adopted from various types of frequency filter circuits such as a digital filter, an analog filter, a passive filter, and an active filter. . In the signal processing device for the exhaust gas sensor according to the embodiment, the functions of these filters are provided in the ECU 50, and processing for changing the cutoff frequency is also executed in the ECU 50. However, the configuration is not limited to the configuration built in the ECU 50, and other external processing units may include a filter circuit. Further, these frequency filters may be incorporated in a part of the output signal processing circuit attached to the exhaust gas sensor instead of the main arithmetic processing unit and its peripheral devices for controlling the internal combustion engine. Between each of these filters (the low-pass filters 30 and 40 and the high-pass filters 32 and 42) and the ECU 50 or other arithmetic processing unit, there is an interface for transmitting signals between these components. .

  As shown in FIG. 2, the low-pass filter 30 performs filtering on the output signal capture processing S100 of the air-fuel ratio sensor (air-fuel ratio sensor 22) upstream of the catalyst, and the signal after passing through the low-pass filter 30 is the air-fuel ratio. Used for control (processing S104). That is, a signal that has passed through the low-pass filter 30 is input to the ECU 50 and used for air-fuel ratio control (more specifically, fuel injection amount control) of the internal combustion engine 10. Further, in the output signal capture processing S100 of the air-fuel ratio sensor upstream of the catalyst (air-fuel ratio sensor 22), filtering by the high-pass filter 32 is also performed, and the signal after passing through the high-pass filter 32 is input to the ECU 50 and is empty. This is used for determining the failure of the fuel ratio sensor 22. On the other hand, as shown in FIG. 2, the output signal S120 of the air-fuel ratio sensor (air-fuel ratio sensor 26) downstream of the catalyst is filtered by the low-pass filter 40, and the signal after passing through the low-pass filter 40 is input to the ECU 50. And used for air-fuel ratio control (step S124). Further, the output signal S120 of the air-fuel ratio sensor (air-fuel ratio sensor 26) downstream of the catalyst is also filtered by the high-pass filter 42, and the signal after passing through the high-pass filter 42 is input to the ECU 50 to be output from the air-fuel ratio sensor 26. Used for failure determination.

  In the present embodiment, the low-pass filter 30 is a filter that has a cutoff frequency (cutoff frequency) set to 100 Hz and selectively passes a signal having a frequency of 100 Hz or less. The high-pass filter 32 is a filter that selectively sets a cutoff frequency (cutoff frequency) to 1 kHz and allows a signal having a frequency of 1 kHz or more to pass therethrough. On the other hand, the low-pass filter 40 is a filter that has a cutoff frequency (cutoff frequency) set to 10 Hz and selectively passes a signal having a frequency of 10 Hz or less. The high-pass filter 42 is a filter that selectively sets a cutoff frequency (cutoff frequency) to 1 kHz and allows a signal having a frequency of 1 kHz or more to pass therethrough.

(Output signal processing common to upstream and downstream air-fuel ratio sensors)
When the air-fuel ratio sensors 22 and 26 are arranged upstream and downstream of the catalyst 24 and their output signals are used, it is preferable to perform output signal processing in consideration of the following points. That is, as a matter common to the air-fuel ratio sensors arranged upstream and downstream of the catalyst, it is preferable to deal with electrical signal noise peculiar to the current detection type air-fuel ratio sensor. This electrical signal noise is not only an electrical noise due to external factors, but it is also possible to monitor the state of changes in exhaust gas sensed by the air-fuel ratio sensor by taking into account the reaction time constant of the air-fuel ratio sensor. It is preferable to perform an appropriate filtering process. Specifically, for example, the responsiveness of a general air-fuel ratio sensor using zirconia is about 100 ms at the fastest, and it is preferable to perform a filtering process taking this reaction rate into consideration. In consideration of this point, in the exhaust gas sensor signal processing device according to the embodiment, the cutoff frequency of the low-pass filter 30 is set to 100 Hz. As a result, it is possible to perform a filtering process that allows a signal of 100 Hz or less to pass through the output signal of the air-fuel ratio sensor 22 disposed upstream of the catalyst 24.

  Furthermore, when an air-fuel ratio sensor is used downstream of the catalyst, it is preferable to set the filter characteristics in consideration of the reaction speed in the catalyst in addition to the gas reaction time in the air-fuel ratio sensor. That is, if the air-fuel ratio sensor output is used with high responsiveness without taking into consideration the reaction speed in the catalyst, in the air-fuel ratio control, not only the reflection of the actual air-fuel ratio change in the catalyst, but more than that, A slight change in the air-fuel ratio or a sensitive air-fuel ratio sensor output fluctuation is reflected. If this happens, fluctuations in the value (detected air-fuel ratio) to be used as the basis of air-fuel ratio control become more sensitive than necessary, which is undesirable in terms of emission characteristics, particularly in the case of a low noble metal catalyst system. Will cause an immediate deterioration of emissions. From such a viewpoint, it is preferable to consider both the gas reaction speed in the catalyst and the air-fuel ratio sensor reaction time, and the responsiveness of the air-fuel ratio sensor when both are taken into consideration is on the order of several hundred ms. Considering this point, in the exhaust gas sensor signal processing apparatus according to the embodiment, the cutoff frequency of the low-pass filter 40 is set to 10 Hz. As a result, it is possible to perform a filtering process that allows a signal of 10 Hz or less to pass through the output signal of the air-fuel ratio sensor 26 disposed downstream of the catalyst 24.

(Adjustment of cutoff frequency of low-pass filter)
Furthermore, in the exhaust gas sensor signal processing apparatus according to the embodiment, as described below, the high-pass filter 32 included in the signals that have passed through the high-pass filters 32 and 42 (that is, the signals that have passed through the high-pass filters 32 and 42). The variable cutoff frequency of the low-pass filters 30 and 40 is adjusted based on the amount of the signal having a frequency equal to or higher than the cutoff frequency of 42.

  In an air-fuel ratio control system using a low precious metal catalyst, the responsiveness of the air-fuel ratio sensor significantly affects the emission. In such a system, if the air-fuel ratio sensor output exhibits output characteristics (behavior) with an excessively high response, accurate detection of the exhaust gas atmosphere and the atmosphere in the catalyst is hindered. In other words, when a crack occurs in the diffusion layer of the limiting current air-fuel ratio sensor, the fluctuation in output increases due to an instantaneous change in gas sensitivity due to the crack, or electrical noise increases due to poor contact. There is a case. Such an increase in output fluctuation and noise increase hinders accurate detection of the exhaust gas atmosphere and the catalyst atmosphere. In this regard, according to the low-pass filters 30 and 40, signals exceeding the cutoff frequency among the output signals of the air-fuel ratio sensors 22 and 26 can be blocked (filtered), that is, unnecessary signals in a frequency range higher than the cutoff frequency. Can be removed. However, on the other hand, when the exhaust gas sensor output signal passed through the low-pass filter is used for air-fuel ratio control, if the cutoff frequency of the low-pass filter is not an appropriate value, the exhaust gas atmosphere can be detected accurately. It becomes difficult. In particular, when the cutoff frequency of the low-pass filter is changed as necessary for some purpose, the sensitivity of air-fuel ratio detection for performing air-fuel ratio control may greatly deviate from an appropriate state. As a result, the accuracy of the air-fuel ratio control may be adversely affected.

  Therefore, in the exhaust gas sensor signal processing apparatus according to the embodiment, the high-pass filters 32 and 42 for detecting the high-frequency noise component which is a problem in the air-fuel ratio control are provided in the output processing unit for processing the sensor output signal. did. As described above, the high-pass filters 32 and 42 are filters that selectively pass a signal having a cutoff frequency (cutoff frequency) of 1 kHz and a frequency of 1 kHz or higher. In the exhaust gas sensor signal processing apparatus according to the embodiment, the cut-off frequency of the low-pass filters 30 and 40 increases as the amount of the signal that passes through the high-pass filters 32 and 42 increases as in the processing S108 and S128 shown in FIG. Was shifted to the low frequency side. The “amount of the signal that has passed through the high-pass filters 32, 42” here means that “the air-fuel ratio sensor failure determination processing (processing S 110, S 130) should be a basis for determining that the air-fuel ratio sensor has failed. This means how many, large, or strong signals appear in the signal after passing through the high-pass filters 32 and 42 (for example, the output level of the signal, the frequency, etc.). Hereinafter, the “amount of signal that has passed through the high-pass filters 32 and 42” described here is also referred to as “passed signal amount” for convenience of explanation. A state in which the amount of passing signal is large is a state in which the degree to which a signal having a frequency higher than the cutoff frequency of the high-pass filters 32 and 42 is sensed is increased, and the output sensitivity of the high-pass filters 32 and 42 is increased. It can be said. In other words, the signal processing device for the exhaust gas sensor according to the embodiment shifts the cutoff frequency of the low-pass filters 30 and 40 toward the low frequency side as the output sensitivity of the high-pass filters 32 and 42 increases. In addition, the shift to the low frequency side according to the increase in output sensitivity of the high pass filters 32 and 42 is caused by the number or magnitude of signals having a frequency equal to or higher than the cutoff frequency included in the signals that have passed through the high pass filters 32 and 42. Specifically, for example, when the amount of passing signal is relatively large, the cutoff frequency of the low-pass filter is shifted to the lower frequency side, or the amount of passing signal is greater than or equal to a predetermined value. In addition, the cutoff frequency of the low-pass filter may be shifted by a predetermined width to the low frequency side.

  Thereby, it is possible to suppress the air-fuel ratio detection using the air-fuel ratio sensors 22 and 26 from being performed with better sensitivity than necessary, and to suppress deterioration in accuracy of the air-fuel ratio control and increase in emissions.

(Deterioration judgment of air-fuel ratio sensor)
In the internal combustion engine according to the embodiment, when the amount of signal recognized after passing through the high-pass filters 32 and 42 exceeds a predetermined value, the ECU 50 as the control device (or the high-pass filters 32 and 42). A process of determining that the air-fuel ratio sensor has deteriorated (failed) when the output sensitivity exceeds a predetermined value is executed (processes S110 and S130 in FIG. 2). Such a determination process is realized by executing a program stored in advance in the ECU 50.
If the air-fuel ratio sensors 22 and 26 are in a normal state, it is difficult to assume that the output signal of each air-fuel ratio sensor includes a signal having a frequency that is high enough to pass through the high-pass filters 32 and 42. On the other hand, when an increase in the high-pass filter passing signal is recognized (that is, when sensitivity in a frequency region higher than the cutoff frequency is increased), the exhaust gas or the atmosphere in the catalyst is caused by a diffusion layer crack or the like. It can be determined that deterioration or failure at a level that cannot be accurately detected has occurred in the air-fuel ratio sensor. According to the execution of the above determination process, such an abnormality can be detected. As a result, it is possible to quickly determine the failure of each of the air-fuel ratio sensors 22 and 24, and to detect and suppress emission deterioration in the air-fuel ratio control system, particularly the low noble metal catalyst system.

(Adjustment of cutoff frequency of low-pass filter)
It should be noted that under certain operating conditions, there is a possibility that the output response in the transition will be smoothed (smoothed in the time axis direction) to a level that is required for control. The specific operating condition specifically refers to a state before completion of catalyst warm-up at the time of fuel cut or engine start. Under these operating conditions, an air-fuel ratio sensor (air-fuel ratio sensor 26 downstream of the catalyst). It is preferable to set the pass frequency range of the low-pass filter 40 that filters the output signal of 100) to 100 Hz or less. Therefore, in the exhaust gas sensor signal processing device according to the embodiment, the low-pass filter 40 is also a filter circuit whose cutoff frequency is variable. When the operating conditions of the internal combustion engine 10 correspond to the above operating conditions, the low-pass filter 40 is shut off. Set the frequency to 100 Hz.

As described above, according to the signal processing apparatus of the exhaust gas sensor according to the embodiment, based on a signal having a frequency equal to or higher than the cutoff frequency (1 kHz) included in the signals that have passed through the high-pass filters 32 and 42 (particularly, In the embodiment, the cutoff frequency of each of the low-pass filters 30 and 40 can be adjusted (based on the amount of passing signal described above) (processing S108 and S128). As a result, the low-pass filters 30 and 40 are each based on the amount of a signal having a frequency equal to or higher than a predetermined frequency included in the output signals of the air-fuel ratio sensors 22 and 26 (higher than the cutoff frequency of the high-pass filters 32 and 42, specifically 1 kHz or higher). The cutoff frequency can be adjusted. According to such adjustment of the cut-off frequency, the low-pass filter 30 and the low-pass filter 30 and 40 are used so that the air-fuel ratio detection sensitivity when the air-fuel ratio control is performed using the air-fuel ratio sensor output signal is appropriate. It becomes possible to adjust the cutoff frequency (cut-off frequency) of the filters 30 and 40. As a result, it is possible to adjust the air-fuel ratio detection sensitivity and suppress deterioration in accuracy of the air-fuel ratio control.
In particular, according to the exhaust gas sensor signal processing device according to the embodiment, the cutoff frequency of each of the low-pass filters 30 and 40 is set to a relatively low frequency side according to the number or magnitude of signals having a frequency equal to or higher than the cutoff frequency. Can be shifted. As a result, it is possible to prevent the accuracy of air-fuel ratio control from deteriorating due to excessively high detection sensitivity by the air-fuel ratio sensors 22 and 26.

(Modification)
FIG. 3 is a view showing a modified example of the configuration of the signal processing apparatus for the exhaust gas sensor according to the embodiment of the present invention, together with a part of the configuration of the internal combustion engine to which the exhaust gas sensor is applied. In the configuration of FIG. 3, a tandem catalyst 34 is provided in the exhaust passage 20 instead of the catalyst 24 in FIG. 1. In place of the air-fuel ratio sensor 26 arranged at the downstream position of the catalyst 24, an air-fuel ratio sensor 36 is arranged in the center of the tandem catalyst 34 in FIG. In such a system, the filter processing (FIG. 2) performed by the signal processing apparatus for the exhaust gas sensor according to the above-described embodiment may be similarly applied.

As described in the section “Adjustment of cut-off frequency of low-pass filter” in the above-described embodiment, in the signal processing device for the exhaust gas sensor according to the embodiment, the output sensitivity of the high-pass filters 32 and 42 increases. The cut-off frequency of the low-pass filters 30 and 40 was shifted to the low frequency side. However, the present invention is not limited to this. Not only the increase in output sensitivity of the high-pass filters 32 and 42 but also the increase in the ratio of the output signal that has passed through the high-pass filters 32 and 42 to the output signal that has passed through the low-pass filters 30 and 40 (sensitivity ratio). You may shift to the low frequency side.
Further, as described in the column of “determination determination of air-fuel ratio sensor” in the above embodiment, in the exhaust gas sensor signal processing device according to the embodiment, the ECU 50 is configured to pass through the high-pass filters 32, 42. When the amount of the recognized signal exceeds a predetermined value (or when the output sensitivity of the high-pass filters 32 and 42 exceeds the predetermined value), it is determined that the air-fuel ratio sensor has deteriorated (failed). However, the present invention is not limited to this. When the “sensitivity ratio” equal to or higher than a predetermined value with respect to the low-pass filter output is reached, processing for determining that the air-fuel ratio sensor has deteriorated may be executed (steps S110 and S130 in FIG. 2).
Here, the “sensitivity ratio” specifically refers to the ratio of the high-pass filter passing signal to the low-pass filter passing signal. If the air-fuel ratio sensors 22 and 26 are in a normal state, it is usually difficult to think that the output signal of each air-fuel ratio sensor includes a signal having a frequency that is high enough to pass through the high-pass filters 32 and 42. The value of the sensitivity ratio should also be a specific and constant value corresponding to this. Therefore, when the sensitivity ratio is larger than normal (specifically, a predetermined value or more), the deterioration of the air-fuel ratio is such that the exhaust gas or the atmosphere in the catalyst cannot be accurately detected due to diffusion layer cracks or the like. It can be judged that it has occurred in the sensor. Such a determination based on the sensitivity ratio can quickly determine the failure of each of the air-fuel ratio sensors 22 and 24, and detect and suppress the deterioration of emissions in the air-fuel ratio control system, particularly a low noble metal catalyst system. May be.

  In the above embodiment, the low-pass filter 30 has a cutoff frequency (cut-off frequency) set to 100 Hz, the low-pass filter 40 has a cutoff frequency set to 10 Hz, and the high-pass filters 32 and 42 have a cutoff frequency (cut-off frequency). Off frequency) is set to 1 kHz. However, the cutoff frequency of each filter in the present invention is not limited to the specific numerical values in the above-described embodiment. Considering the fact that the response (time constant) of an air-fuel ratio sensor using general zirconia is about 100 ms at the earliest as described above, The low-pass filter applied to the upstream air-fuel ratio sensor output is in the vicinity of 100 Hz (for example, 99 Hz, 98 Hz,... 90 Hz or lower frequency, for example, 101 Hz, 102 Hz,... 110 Hz or higher frequency. A filter circuit having a predetermined cut-off frequency may be used. Further, as described above, considering that both the gas reaction speed in the catalyst and the air-fuel ratio sensor reaction time are taken into account, the response (time constant) of the air-fuel ratio sensor is on the order of several hundreds of ms. The low-pass filter applied to the exhaust gas sensor output is a filter circuit having a predetermined cutoff frequency in the vicinity of 10 Hz (for example, a frequency of 9 Hz, 8 Hz, or lower, or a frequency of, for example, 11 Hz, 12 Hz, or higher). Use it. Note that a filter circuit with a variable cutoff frequency may be used so that each cutoff frequency can be adjusted in consideration of variations in each product, environmental changes, and the like, and the high-pass filter 32 and the high-pass filter 42 are not necessarily the same. The cut-off frequency may not be used, and the cut-off frequency of each filter may be different from the viewpoint of optimization or the like.

In the above-described embodiment, the low-pass filters 30 and 40 and the high-pass filters 32 and 42 are used as the frequency filters. For example, a band-pass filter (BPF) that passes a predetermined low-frequency band is used as the low-pass filter. It may be used as an alternative to 30, 40. For example, a bandpass filter that passes a predetermined high frequency band may be used as an alternative to the highpass filters 32 and 42. In this way, a configuration in which a low-pass filter or a high-pass filter is replaced with a band-pass filter is also applicable to a “low-pass filter that performs filtering so as to pass a signal having a frequency lower than the cutoff frequency” or “a frequency higher than the cutoff frequency. It is included in the technical scope of the present invention as a “high-pass filter that performs filtering so as to pass a signal”. For this reason, the structure using such a band pass filter is also included in the technical scope of the present invention.
In the above-described embodiment, the output signal processing technology of the exhaust gas sensor according to the embodiment of the present invention is applied to the air-fuel ratio control system in which the air-fuel ratio sensor is disposed before and after the catalyst. However, the present invention is not limited to this. Of the signal processing apparatus of the exhaust gas sensor according to the embodiment, the air-fuel ratio control system having one air-fuel ratio sensor also depends on the positional relationship (upstream or downstream) between the air-fuel ratio sensor and the catalyst. The signal processing contents applied to the air-fuel ratio sensor 22 or the air-fuel ratio sensor 26 may be similarly applied.

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 20 Exhaust passage 22 Air fuel ratio sensor 24 Catalyst 26 Air fuel ratio sensor 30 Low pass filter 32 High pass filter 34 Tandem catalyst 36 Air fuel ratio sensor 40 Low pass filter 42 High pass filter

Claims (4)

An exhaust gas sensor that is provided in an exhaust passage of the internal combustion engine and changes an output according to an oxygen concentration in the exhaust gas;
A low-pass filter that has a cutoff frequency variable and performs filtering so as to pass a signal having a frequency equal to or lower than the cutoff frequency with respect to the output signal of the exhaust gas sensor;
Means for transmitting a signal that has passed through the low-pass filter as information to be used for controlling an air-fuel ratio of the internal combustion engine to a control device for the internal combustion engine;
A high-pass filter that performs filtering so as to pass a signal having a frequency equal to or higher than a cutoff frequency with respect to an output signal of the exhaust gas sensor;
A frequency adjusting means for adjusting a cutoff frequency of the low-pass filter based on a signal that has passed through the high-pass filter;
An exhaust gas sensor signal processing apparatus comprising:   The frequency adjusting unit is configured to control the low-pass signal according to the number or magnitude of signals having a frequency equal to or higher than a cutoff frequency of the high-pass filter included in the signal that has passed through the high-pass filter. 2. The signal processing apparatus for an exhaust gas sensor according to claim 1, further comprising frequency shift means for shifting the cutoff frequency of the filter of the type to a relatively low frequency side. The exhaust gas sensor includes a downstream air-fuel ratio sensor provided downstream of a catalyst disposed in the internal combustion engine,
Specific operating condition frequency that shifts the cutoff frequency of the low-pass filter to the high frequency side when the internal combustion engine is operated under at least one operating condition before completion of fuel cut, starting operation, and catalyst warm-up 3. The exhaust gas sensor signal processing apparatus according to claim 1, further comprising a shift unit. A catalyst is provided in the exhaust passage of the internal combustion engine,
The exhaust gas sensor
An upstream exhaust gas sensor of a limiting current type provided upstream of the catalyst;
A downstream exhaust gas sensor of a limiting current type provided downstream of the catalyst;
Including
The low-pass filter includes a first low-pass filter that filters a signal having a frequency equal to or lower than a cutoff frequency with respect to an output signal of the upstream exhaust gas sensor, and the downstream exhaust gas. A second low-pass filter that performs filtering so as to pass a signal having a frequency equal to or lower than the cutoff frequency with respect to the output signal of the gas sensor,
The high-pass filter includes a first high-pass filter that filters a signal having a frequency equal to or higher than a cutoff frequency with respect to an output signal of the upstream exhaust gas sensor, and the downstream exhaust gas. A second high-pass filter that performs filtering so as to pass a signal having a frequency equal to or higher than the cutoff frequency with respect to the output signal of the gas sensor,
The frequency adjusting means includes
First frequency adjusting means for adjusting a cutoff frequency of the first low-pass filter based on a signal that has passed through the first high-pass filter;
Second frequency adjusting means for adjusting a cutoff frequency of the second low-pass filter based on a signal that has passed through the second high-pass filter;
4. The exhaust gas sensor signal processing device according to claim 1, comprising:

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