JPH09324681A - Air-fuel ratio control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely secure the required ability of catalytic converter devices to purify exhaust gas by calculating an amount of correction for correcting the air-fuel ratio so that the concentration of exhaust gas characteristic components downstream of the catalytic converter devices attains a predetermined value, and controlling the amount of fuel supply in accordance with the amount of correction and the concentration detected by an exhaust gas sensor. SOLUTION: Two three-way catalytic converter devices 4, 5 are installed in a main exhaust pipe 3, and a wide-area air-fuel ratio sensors 6 and an O2 sensor 7 are provided upstream and downstream of the catalytic converter device 4, respectively. A calculating part 13 in a control unit 8 calculates a first correction factor for correcting the basic amount of fuel injection while taking into account the rate of exhaust-gas recirculation, cooling water temperature, and the like. A calculating part 14 calculates a second correction factor while taking into account a desired air-fuel ratio at the wide-area air-fuel ratio sensor 6 and the efficiency of intake air filling, and the amount of fuel injection is corrected in accordance with the correction factors. A calculating part 16 calculates the desired air-fuel ratio by correcting, using adaptive sliding mode control and based on the output of the O2 sensor 7, a reference air-fuel ratio set by a setting part 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for controlling the air-fuel ratio of an internal combustion engine.

[0002]

2. Description of the Related Art For example, in a system for purifying exhaust gas of an internal combustion engine, such as an automobile, by a catalytic device,
From the viewpoint of environmental protection, it is desired to control the air-fuel ratio of the exhaust gas of the internal combustion engine to an appropriate air-fuel ratio such that the exhaust gas purification capacity of the catalyst device becomes good.

The following apparatus has conventionally been proposed by the applicant of the present application as a means for performing such air-fuel ratio control (see, for example, Japanese Patent Application Laid-Open No. 5-321721).

That is, in this air-fuel ratio control device, an exhaust gas sensor (air-fuel ratio sensor) for detecting the air-fuel ratio of the exhaust gas is provided on the upstream side of the catalyst device in the exhaust system of the internal combustion engine, and at the downstream of the catalyst device. On the side, an exhaust gas sensor (oxygen concentration sensor) for detecting the concentration of a specific component of the exhaust gas that has passed through the catalyst device, for example, the oxygen concentration (this corresponds to the air-fuel ratio of the exhaust gas after passing the catalyst device), The reference air-fuel ratio on the upstream side of the catalyst device is set according to the intake pressure and the rotational speed of the internal combustion engine. Then, the reference air-fuel ratio is corrected using PID control so that the oxygen concentration (air-fuel ratio) detected by the exhaust gas sensor on the downstream side of the catalyst device becomes an appropriate value, and the target air-fuel ratio on the upstream side of the catalyst device is corrected. Calculate the fuel ratio. Further, the fuel supply amount to the internal combustion engine is feedback-controlled using PID control or adaptive control so that the air-fuel ratio detected by the exhaust gas sensor on the upstream side of the catalyst device converges to the desired target air-fuel ratio. As a result, the air-fuel ratio of the exhaust gas of the internal combustion engine on the upstream side of the catalyst device is kept within an appropriate range (window) where good purification performance of the catalyst device can be obtained, and the exhaust gas purification capability of the catalyst device is improved. Increase.

By the way, in order to ensure a good purification performance of exhaust gas by such an air-fuel ratio control of the internal combustion engine without depending on aged deterioration of the catalyst device, oxygen of exhaust gas on the downstream side of the catalyst device is required. The inventors of the present application have found through various studies that it is necessary to set the concentration of a specific component such as the concentration to a predetermined appropriate value with high accuracy.

However, in the above-mentioned conventional air-fuel ratio control device, in order to match the oxygen concentration detected by the exhaust gas sensor on the downstream side of the catalyst device with an appropriate value, the reference on the upstream side of the catalyst device is used by using PID control. Since the target air-fuel ratio is determined by correcting the air-fuel ratio, the exhaust gas on the downstream side of the catalyst device is affected by the disturbance acting on the exhaust gas sensor and the like, and the dead time existing in the exhaust system including the catalyst device. It has been difficult to set the oxygen concentration (air-fuel ratio) to an appropriate value with high accuracy.

As a control method for matching the concentration of the specific component detected by the exhaust gas sensor on the downstream side of the catalyst device to an appropriate value, for example, an exhaust system including the catalyst device to be controlled is modeled and A method of constructing an optimal regulator for modern control based on the model is also conceivable. However, in the control method using such an optimum regulator,
When the catalyst device is brand new or the internal combustion engine is in steady operation, the exhaust gas on the downstream side of the catalyst device can be set in a state where the error between the actual control target and the model (model error) can be made sufficiently small. Although it is possible to settle the concentration of the specific component of the catalyst to an appropriate value with relatively high accuracy, if the model error becomes large due to aging deterioration of the catalyst device or changes in the operating state of the internal combustion engine, the model error will be optimized as it is. Since the control output of the regulator is affected, the control performance is deteriorated, and it becomes difficult to match the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device with an appropriate value with high accuracy.

[0008]

SUMMARY OF THE INVENTION In view of the above background, the present invention takes into consideration the concentration of a specific component of exhaust gas on the downstream side of a catalyst device provided in an exhaust system of an internal combustion engine, a disturbance, and a change in an operating state of the internal combustion engine. An air-fuel ratio control device for an internal combustion engine, which can settle to a predetermined appropriate value with high accuracy without depending on aging of the catalyst device, etc., and can reliably secure a required exhaust gas purification capacity of the catalyst device. The purpose is to provide.

In particular, the exhaust gas purifying ability of the catalyst device can be stably maximized, and the exhaust performance of the internal combustion engine via the catalyst device can be made extremely high. The purpose is to provide.

[0010]

In order to achieve such an object, an air-fuel ratio control system for an internal combustion engine of the present invention is provided with an exhaust gas purifying catalyst device provided in an exhaust system of the internal combustion engine, and the catalytic device. A first exhaust gas sensor provided in the exhaust system to detect the air-fuel ratio of the exhaust gas of the internal combustion engine on the upstream side of the exhaust gas of the internal combustion engine passing through the catalytic device on the downstream side of the catalytic device; An air-fuel ratio control device for an internal combustion engine, comprising: a second exhaust gas sensor provided in the exhaust system to detect the concentration of a specific component; and controlling the air-fuel ratio of the internal combustion engine based on the outputs of both exhaust gas sensors. Second
Based on the output of the exhaust gas sensor, by using the adaptive sliding mode control a correction amount for correcting the air-fuel ratio of the internal combustion engine so that the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device becomes a predetermined appropriate value. Based on the required adaptive sliding mode control means, the obtained correction amount, and the output of the first exhaust gas sensor, the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device is converged to the predetermined appropriate value. And a feedback control means for controlling the amount of fuel supplied to the internal combustion engine.

According to the present invention, based on the output of the second exhaust gas sensor on the downstream side of the catalyst device, the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device becomes a predetermined appropriate value. A correction amount for correcting the air-fuel ratio of the engine is obtained by using adaptive sliding mode control, and based on the obtained correction amount and the output of the first exhaust gas sensor, a specific component of exhaust gas on the downstream side of the catalyst device. Feedback control of the amount of fuel supplied to the internal combustion engine is performed so that the concentration of is converged to the predetermined appropriate value.

Thus, by obtaining the correction amount of the air-fuel ratio of the internal combustion engine by using the adaptive sliding mode control, the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device is set to a predetermined appropriate value with high accuracy. Can be made. That is, the adaptive sliding mode control has extremely high stability with respect to the disturbance acting on the controlled object, the model error generated when modeling the controlled object, and the like, as compared with the optimal regulator and the like. Further, due to its extremely high stability, it also serves to absorb an error in control by the feedback control means. Therefore, the correction amount of the air-fuel ratio of the internal combustion engine is obtained by using the adaptive sliding mode control as described above,
Based on the calculated correction amount and the output of the first exhaust gas sensor, the fuel supply amount to the internal combustion engine is adjusted so that the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device converges to the predetermined appropriate value. By performing the feedback control, it becomes possible to control the air-fuel ratio of the internal combustion engine to an optimum air-fuel ratio in order to match the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device with a predetermined appropriate value. Therefore, it becomes possible to set the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device (exhaust gas after passing through the catalyst device) to a predetermined appropriate value with high accuracy.

Thus, according to the present invention, the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device provided in the exhaust system of the internal combustion engine is subject to disturbances, changes in the operating state of the internal combustion engine, and aging of the catalyst device. It is possible to settle to a predetermined appropriate value with high accuracy regardless of deterioration and the like, and it is possible to reliably ensure the required exhaust gas purification capacity of the catalyst device.

In the present invention, preferably, the predetermined appropriate value of the concentration of the specific component is set to a value that maximizes the purification capacity of the catalyst device.

As a result, the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device is set to a value that maximizes the purification capacity of the catalyst device with high precision, so that the exhaust gas of the catalyst device is purified. The capacity can be stably maximized, and the exhaust performance of the internal combustion engine via the catalyst device can be made extremely high.

The second exhaust gas sensor is, for example, an oxygen concentration sensor. In this case, the concentration of the specific component detected by the second exhaust gas sensor becomes the oxygen concentration of the exhaust gas that has passed through the catalyst device, and the oxygen concentration corresponds to the air-fuel ratio of the exhaust gas.

Further, in the present invention, the adaptive sliding mode control means includes an exhaust system including an exhaust system including the purifying device between the exhaust gas sensors, and an exhaust gas space detected in advance by the first and second exhaust gas sensors. The fuel-fuel ratio and the concentration of the specific component are input and output, respectively, and modeled as a model including a delay element of a secondary delay or more, and the air-fuel ratio of the internal combustion engine is corrected so that the output in the model becomes the predetermined appropriate value. A correction amount for the above is obtained by adaptive sliding mode control.

In this way, by modeling the exhaust system from the upstream side to the downstream side of the purification device as a model including a delay element of a secondary delay or more, in performing the adaptive sliding mode control, the exhaust system The actual dynamic characteristics can be represented by a proper model, and the output in the model, that is, the concentration of a specific component of the exhaust gas on the downstream side of the purifier is adapted to match the predetermined proper value in the sliding mode. By obtaining a correction amount for correcting the air-fuel ratio of the internal combustion engine by control, in order to match the air-fuel ratio of the internal combustion engine with the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device to a predetermined appropriate value. It is possible to control to an optimum air-fuel ratio.

In this case, the model is modeled by a spring-mass system including a spring and a damper as delay elements, and having the length of the spring as an amount indicating the concentration of the specific component on the downstream side of the catalyst device. Preferably. By modeling the model for performing the adaptive sliding mode control by the spring-mass system, the model can be made relatively simple.

In the modeling of the exhaust system including the catalyst device as described above, a model error due to a change in the operating state of the internal combustion engine, deterioration of the catalyst device over time, or the like can be absorbed by the adaptive sliding mode control.

[0021] The adaptive sliding mode control means is, concretely, at least the concentration of the specific component on the downstream side of the catalyst device detected by the second exhaust gas sensor and the degree of change of the concentration of the exhaust system. A plurality of state quantities and a predetermined linear function having the plurality of state quantities as variables are defined, and each state quantity is represented by the linear function according to a reaching law and an adaptive law of adaptive sliding mode control. Non-linear input calculation means for obtaining a correction amount of the air-fuel ratio of the internal combustion engine so as to converge on a plane, and the internal combustion engine for confining each state quantity on the hyperplane while converging to an equilibrium point on the hyperplane And an equivalent control input calculating means for calculating a correction amount of the air-fuel ratio of the internal combustion engine, and a correction amount for correcting the air-fuel ratio of the internal combustion engine based on the sum of the correction amounts calculated by both calculating means. Seek.

In the structure of such adaptive sliding mode control means, the exhaust system including at least the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device detected by the second exhaust gas sensor and the degree of change of the concentration. By converging a plurality of state quantities on the hyperplane, those state quantities are affected by disturbance, model error, etc. by the correction amount of the air-fuel ratio of the internal combustion engine obtained by the equivalent control input computing means. Without fail, it surely converges to the equilibrium point on the hyperplane (the point where the concentration of the specific component on the downstream side of the catalyst device matches a predetermined appropriate value). The correction amount obtained by the non-linear input calculation means acts so that the state amount including the concentration of the specific component on the downstream side of the catalyst device converges on the hyperplane. By obtaining according to the reaching law and the adaptive law of the adaptive sliding mode control, the state quantity can be converged on the hyperplane while eliminating the influence of disturbance, model error, and the like.

Therefore, the sum of the correction amounts calculated by the equivalent control input calculation means and the non-linear input calculation means is calculated as the correction amount for correcting the air-fuel ratio of the internal combustion engine, so that the specific component on the downstream side of the catalyst device is obtained. The settling control of the concentration to a predetermined appropriate value can be performed extremely stably against disturbance, model error, and the like.

Further, in the present invention, the stability of the adaptive sliding mode control means for calculating the correction amount for correcting the air-fuel ratio of the internal combustion engine is determined based on the value of the linear function having the state amount as a variable. Stability determining means and a correction amount calculation limiting means for limiting the calculation of the correction amount according to the determination result.

That is, in the adaptive sliding mode control, basically, the state quantity is converged on the hyperplane (on the hyperplane, the value of the linear function is “0”), and further, It is possible to converge to the equilibrium point on the hyperplane, but in the state where the state quantity is not on the hyperplane and the value of the linear function is not "0", the state quantity depends on the kind of disturbance. It is possible that the convergence of on the hyperplane becomes unstable. Therefore, by determining the stability of the adaptive sliding mode control means based on the value of the linear function and limiting the calculation of the correction amount according to the determination result, it is possible to eliminate the above situation. Becomes

In this case, in particular, when the stability determining means determines that the calculation of the correction amount by the adaptive sliding mode control means is unstable, the correction amount is maintained at a predetermined value, The instability of the adaptive sliding mode control for setting the concentration of the specific component on the downstream side of the catalyst device to the predetermined appropriate value can be properly eliminated.

The stability of the adaptive sliding mode control is determined by, for example, the magnitude of the value of the linear function, the magnitude of the changing speed of the value of the linear function, and the value of the linear function and the changing speed thereof. This can be performed by comparing at least one of the products with a predetermined value, and if they are larger than the predetermined value, it can be determined that the control is unstable.

Further, in the present invention, the feedback control means is a recurrence type controller for obtaining the correction amount of the fuel injection amount to the internal combustion engine based on the output of the first exhaust gas sensor and the correction amount. The fuel supply amount to the internal combustion engine is corrected and controlled by the correction amount obtained by the controller.

The controller of the recurrence type is composed of an adaptive controller and an optimum regulator. By using such a controller, the concentration of a specific component on the downstream side of the catalyst device can be adjusted to a predetermined proper value. By correcting and controlling the fuel supply amount to the internal combustion engine so that it converges to a value, the internal combustion engine can be highly accurately tracked against dynamic changes such as changes in the operating state of the internal combustion engine and changes in characteristics over time. The air-fuel ratio can be controlled so that the concentration of the specific component on the downstream side of the catalyst device matches a predetermined appropriate value.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described with reference to FIGS.

Referring to FIG. 1, reference numeral 1 denotes, for example, a four-cylinder engine (internal combustion engine) for performing air-fuel ratio control in this embodiment.
The exhaust pipe 2 for each cylinder of the engine 1 is gathered into a single main exhaust pipe 3 in the vicinity of the engine 1, and two three-way catalytic converters 4, 5 are connected to the main exhaust pipe 3 from the upstream side. It is installed in order. The downstream catalyst device 5 may be omitted.

The air-fuel ratio control system of the present embodiment for controlling the air-fuel ratio of this engine system is a first exhaust gas sensor provided at the upstream side of the catalyst system 4 at the assembly location of the exhaust pipes 2 for each cylinder of the engine 1. Wide-range air-fuel ratio sensor 6 as
The O ₂ sensor (oxygen concentration sensor) 7 as the second exhaust gas sensor provided in the main exhaust pipe 3 on the downstream side of the catalyst device 4 (upstream side of the catalyst device 5) and the outputs of these sensors 6 and 7 The control unit 8 is configured to perform the control described later based on the above. In addition to the wide range air-fuel ratio sensor 6 and the O ₂ sensor 7, the control unit 8 is provided with detection signals from various sensors such as a rotation speed sensor, an intake pressure sensor, and a cooling water temperature sensor, which are not shown. ing.

The wide-range air-fuel ratio sensor 6 is composed of an O ₂ sensor, and the exhaust pipe 2 upstream of the catalyst device 4 is provided.
A signal having a level corresponding to the oxygen concentration (which corresponds to the air-fuel ratio of the air-fuel mixture supplied to the engine 1) indicating the air-fuel ratio of the exhaust gas at the confluence position of is output. In this case, the output signal of the wide-range air-fuel ratio sensor 6 is removed by the linearizer 10 after the high-frequency noise is removed through the filter 9 provided in the control unit 8, over a wide range of the oxygen concentration (air-fuel ratio) of the exhaust gas, It is converted into a signal of a level proportional to that. Hereinafter, in the present embodiment, the wide-range air-fuel ratio sensor 6 obtained by linearizing the output signal in this manner is referred to as an LAF sensor 6.

Further, the O ₂ sensor 7 on the downstream side of the catalyst device 4
Outputs a signal of a level corresponding to the oxygen concentration of the exhaust gas that has passed through the catalyst device 4 (the air-fuel ratio of the exhaust gas after having passed through the catalyst device 4). In this case, the output signal of the O ₂ sensor 7 is, as shown in FIG. 2, in a state where the air-fuel ratio (exhaust gas air-fuel ratio) supplied to the engine 1 is in the vicinity of a predetermined appropriate value. The exhaust gas undergoes a highly sensitive change that is substantially proportional to the oxygen concentration after passing through the catalyst device 4. The output signal of the O ₂ sensor 7 is filtered to remove high-frequency noise by a filter 11 provided in the control unit 8.

The control unit 8 is constructed by using a microcomputer. The main functional components of the control unit 8 are a basic fuel injection amount calculation section 12 for obtaining a basic fuel injection amount Tim for the engine 1 and an engine 1 Exhaust gas recirculation rate (ratio of exhaust gas contained in intake air of engine 1),
A first correction coefficient KTOTAL for correcting the basic fuel injection amount Tim is calculated in consideration of the purge amount of fuel supplied to the engine 1 when the canister (not shown) of the engine 1 is purged, the cooling water temperature of the engine 1, the intake air temperature, and the like. A second correction coefficient calculation unit 13 and a second correction coefficient calculation unit 13 for correcting the basic fuel injection amount Tim from the target air-fuel ratio at the LAF sensor 6 in consideration of the intake air filling efficiency of the engine 1 corresponding to the target air-fuel ratio. The second correction coefficient calculation unit 14 for obtaining the correction coefficient KCMDM, the reference air-fuel ratio setting unit 15 for setting the reference air-fuel ratio KBS (reference air-fuel ratio at the LAF sensor 6) of the engine 1 and the reference air-fuel ratio KBS are set to O. _{2 The} LAF is corrected based on the output of the sensor 7.
A target air-fuel ratio calculation unit 16 that obtains a target air-fuel ratio KCMD at the location of the sensor 6, and a fuel injection amount (fuel supply amount) of the engine 1 so that the air-fuel ratio at the location of the LAF sensor 6 converges to this target air-fuel ratio KCMD. The feedback control unit 17 performs feedback control based on the output of the LAF sensor 6.

In this case, the basic fuel injection amount calculation unit 12
From the rotational speed of the engine 1 and the intake pressure, a reference fuel injection amount defined by them is obtained using a preset map, and the reference fuel injection amount is determined by the effective opening area of a throttle valve (not shown) of the engine 1. The basic fuel injection amount Tim is calculated by making a correction according to.

The specific calculation method of the basic fuel injection amount Tim, the first correction coefficient KTOTAL, and the second correction coefficient KCMDM is disclosed by the applicant of the present application in Japanese Patent Laid-Open No. 5-79374. Therefore, detailed description is omitted here. Further, the first correction coefficient KTOTAL and the second correction coefficient KCMDM
Of the basic fuel injection amount Tim by the first correction coefficient KTOT
This is performed by multiplying the basic fuel injection amount Tim by AL and the second correction coefficient KCMDM, thereby obtaining the required fuel injection amount Tcyl.

Further, the reference air-fuel ratio setting unit 15 obtains the reference air-fuel ratio KBS of the engine 1 defined by the rotational speed and the intake pressure of the engine 1 by using a preset map.

The target air-fuel ratio calculation unit 16 includes the LAF sensor 6
Oxygen from the point O ₂ state quantity of the main exhaust pipe 3 of the catalyst device 4 an exhaust system including the subjected portion of the sensor 7 (the portion denoted by reference symbol A in FIG. 1) (details in part of the O ₂ sensor 7 Based on the state predictor 18 that estimates the value of the concentration and the degree of change in the amount or rate of change) in consideration of the dead time in the exhaust system A, and the state amount estimated by the state predictor 18. An adaptive sliding mode control unit 19 (correction amount calculation means) for obtaining a correction amount of the reference air-fuel ratio KBS by using the adaptive sliding mode control,
The target air-fuel ratio KCMD is calculated by correcting the reference air-fuel ratio KBS with the calculated correction amount (adding the correction amount to the reference air-fuel ratio KBS). The state prediction unit 18 and the adaptive sliding mode control unit 19 of the target air-fuel ratio calculation unit 16
Details of will be described later.

In this embodiment, the feedback control section 17 feedback-controls the overall fuel injection amount to each cylinder of the engine 1 so that the detected air-fuel ratio of the LAF sensor 6 converges to the target air-fuel ratio. Feedback control unit 20 and a local feedback control unit 21 that feedback-controls the fuel injection amount for each cylinder of the engine 1.
It is composed of

Here, the global feedback control unit 20
Is for obtaining a feedback correction coefficient KFB for correcting the required fuel injection amount Tcyl so that the detected air-fuel ratio of the LAF sensor 6 converges to the target air-fuel ratio. in this case,
The global feedback control unit 20 includes a PID control unit 22 that obtains a feedback correction coefficient KFB using well-known PID control so as to eliminate the deviation between the detected air-fuel ratio of the LAF sensor 6 and the target air-fuel ratio, and the LAF sensor. Adaptive control that is a recurrence type controller that adaptively obtains the feedback correction coefficient KFB from the detected air-fuel ratio of No. 6 and the target air-fuel ratio in consideration of dynamic changes such as changes in the operating condition and characteristics of the engine 1. And a part 23 (referred to as STR in the drawing) independently of each other. Then, the global feedback control unit 20 appropriately switches the feedback correction coefficient KFB, which is separately obtained by the PID control unit 22 and the adaptive control unit 23, by the switching unit 24, and one of the feedback correction coefficient KFB is set to the above-mentioned value. This is corrected by multiplying the required fuel injection amount Tcyl. Hereinafter, the feedback correction coefficient KFB by the PID control unit 22 will be referred to as KLAF, and the feedback correction coefficient KFB by the adaptive control unit 23 will be described.
Is referred to as KSTR. Such a global feedback control unit 2
Details of 0 will be described later.

The output of the LAF sensor 6 is input to the PID control unit 22 and the adaptive control unit 23 via filters 24 and 25 having frequency bands matched to their control characteristics.

On the other hand, the local feedback control unit 21
Is the actual air-fuel ratio # nA / F (n = 1,2,3,4) for each cylinder from the detected air-fuel ratio of the LAF sensor 6 (air-fuel ratio of the collecting portion of the exhaust pipe 2 for each cylinder of the engine 1) And the actual air-fuel ratio of each cylinder estimated by the observer 26
A plurality (a number of cylinders) of PID control units 27 that respectively obtain a feedback correction coefficient #nKLAF of the fuel injection amount for each cylinder using PID control so as to eliminate the variation in the air-fuel ratio for each cylinder from # nA / F. And.

Here, the observer 26, to briefly explain it, estimates the actual air-fuel ratio # nA / F for each cylinder as follows. That is, from the engine 1 to the LAF
The system up to the sensor 6 is set to the actual air-fuel ratio #n for each cylinder.
The A / F is used as input, and the LAF sensor 6
It is considered as a system that outputs the air-fuel ratio detected by the air-fuel ratio, and this is a time-dependent contribution of the air-fuel ratio of each cylinder to the detection response delay (for example, first-order delay) of the LAF sensor 6 and the air-fuel ratio of the exhaust pipe 2 collecting portion. Model in consideration of the degree. Then, based on the model, the actual air-fuel ratio # nA / F for each cylinder is estimated backward from the air-fuel ratio detected by the LAF sensor 6.

Since such an observer 26 is disclosed in detail by the applicant of the present application in, for example, Japanese Patent Laid-Open No. 7-83094, detailed description thereof will be omitted here.

Further, each PID control unit 27 of the local feedback control unit 21 determines the detected air-fuel ratio of the LAF sensor 6 for all cylinders of the feedback correction coefficient #nKLAF obtained by each PID control unit 27 at the previous cycle time. Is set as a target value of the air-fuel ratio of each cylinder so that the deviation between the target value and the actual air-fuel ratio # nA / F of each cylinder obtained by the observer 26 is eliminated. , The feedback correction coefficient #nKLAF for each cylinder in this cycle time is calculated. Then, the local feedback control unit 21 multiplies a value obtained by multiplying and correcting the required fuel injection amount Tcyl by the feedback correction coefficient KFB of the global feedback control unit 20 by the feedback correction coefficient #nKLAF for each cylinder. Thus, the output fuel injection amount #nTout (n = 1,2,3,4) of each cylinder is obtained.

The output fuel injection amount #nTout of each cylinder obtained in this way is corrected for each cylinder by the adhesion correction unit 28 provided in the control unit 8 in consideration of adhesion on the wall surface of the intake pipe. The output fuel injection amount #n which is given to the fuel injection device (not shown) of the engine 1 and whose adhesion is corrected
At Tout, fuel injection into each cylinder of the engine 1 is performed. The above-mentioned adhesion correction is disclosed in detail in, for example, JP-A-8-21273 by the applicant of the present invention, and a detailed description thereof will be omitted here.

Next, the state predicting section 18 and the adaptive sliding mode control section 19 of the target air-fuel ratio calculating section 16 will be described in detail.

In the present embodiment, the target air-fuel ratio calculation unit 16
The reference air-fuel ratio KBS is set so that the oxygen concentration of the exhaust gas at the location of the O ₂ sensor 7 on the downstream side of the catalyst device 4 is set to a predetermined appropriate value that maximizes the exhaust gas purification capacity of the catalyst device 4. The target air-fuel ratio KCMD at the location of the LAF sensor 6 on the upstream side of the catalyst device 4 is corrected and the catalyst device 4 of the main exhaust pipe 3 from the location of the LAF sensor 6 to the location of the O ₂ sensor 7 is corrected. The exhaust system A (see FIG. 1) that includes it is the control target (plant). Then, the correction amount for correcting the reference air-fuel ratio KBS as described above is set to the state prediction unit 18 and the adaptive sliding mode control unit 19
Thus, the adaptive sliding mode control is used in consideration of the dead time existing in the exhaust system A which is the control target. In the following description, the air-fuel ratio at the portion of the LAF sensor 6 is referred to as A / F before CAT, and the oxygen concentration at the portion of the O ₂ sensor 7 is referred to as A / F after CAT.

In order to apply the adaptive sliding mode control considering the dead time to the exhaust system A to be controlled (hereinafter referred to as the target exhaust system A) as described above, in the present embodiment, first, the target exhaust system A is controlled. Was modeled by a spring-mass system (second-order lag system) including dead time as shown in FIG.

In FIG. 3, in this spring-mass system, a mass body 29 (assuming that the mass M is "1") is supported by a spring 30 having a spring constant K and an attenuator 31 having a damping coefficient C. Assume that the vibration force applied to 29 corresponds to A / F before CAT, and the displacement amount x of the mass body 29 due to the vibration force.
_It is assumed that ₁ corresponds to A / F after CAT. Also, CA
The pre-T A / F includes an air-fuel ratio component u (herein referred to simply as input u) that can be controlled by the feedback control unit 17 and an uncontrollable air-fuel ratio component L such as noise (herein referred to as disturbance L). It is assumed that it is the sum of (and added). The exhaust system A is connected to the input u and the disturbance L.
It is assumed that the dead time d is included, and the input u (t-d) and the disturbance L (t-d) before the dead time d are input as the excitation force of this spring-mass system.

In such a spring-mass system model, assuming that the value of A / F after CAT corresponding to the displacement amount of the mass body 29 is x ₁ and the rate of change thereof is x ₂ , the equation of state of the model is the above spring constant. It is expressed by the following equation (1) using K, the damping coefficient C, and the like.

[0053]

[Equation 1]

When the state equation (1) is shown in a block diagram, a plant model of the target exhaust system A is obtained as shown in FIG. In the figure, “s” is a Laplace operator.

The state prediction unit 18 and the adaptive sliding mode control unit 19 of this embodiment are constructed based on such a plant model of the target exhaust system A, and their details will be described below.

First, the state predicting section 18 will be described later in detail.
Adaptive sliding mode control unit 19
Waste of target exhaust system A during riding mode control
The LAF sensor 6 is for compensating the time d.
A / F and O before CAT detected by_TwoDetected by sensor 7
A / F after CAT issued and A before CAT until now
O after the dead time d of the exhaust system A corresponding to / F_TwoSensor 7
Estimate the state quantity of A / F after CAT detected by
Of. Here, in the present embodiment, the state quantity is O
_TwoA / F value after CAT detected by sensor 7 (actual
Is O_TwoSensor output level) and A / F after CAT
Change rate or change rate (actually O _TwoSensor output
Level change amount or change speed).

The state predicting section 18 is constructed to perform the following processing in order to make such an estimation.

That is, the state predicting unit 18 omits the dead time term (the portion represented by "e- ^ds " in FIG. 4) from the plant model in FIG. 4 and ^{replaces the} constants C, K and b with each other. The above estimation is performed using the model of the delay element (plant model) shown in FIG. 5, which is replaced by the predetermined set values C _M , K _M , and b _M. In this case, in the delay element model of FIG. 5, the state equation corresponding to the above equation (1) is expressed by the following equation (2).

[0059]

[Equation 2]

Here, in FIG. 6 and equation (2),
x _1M and x _2M are CAT in the model of the delay element in FIG.
It is the value of the rear A / F and its change amount or change speed (state amount). The set values C _M , K _M , and b _M are set based on experiments and the like.

Then, the state prediction unit 18 uses this equation (2)
Using the pre-CAT A / F actually detected by the LAF sensor 6 as the input U (t) of the state equation (2)
To obtain the above state quantities x _1M and x _2M . Furthermore, the _calculated state quantities x _1M and x _2M and the CAT of the current time t
From the state quantities x ₁ and x _{2 of the} rear A / F by the following equation (3),
Estimated value x ₁ hat (= estimated value of A / F after CAT) of the post-CAT A / F after dead time d from the current time t, x ₂ hat (= change amount of A / F after CAT or Estimate the rate of change).

[0062]

(Equation 3)

[0063] Here, in the formula (3), "e ^At" is a matrix exponential function obtained when solving the state equations (2), "d _M" is the dead time d of the object exhaust system A set value (Identification value). In this case, the dead time d _M is set to be equal to or larger than the actual dead time d (d _M ≧ d). In the first term of equation (3), the state quantity actually obtained by the output of the O ₂ sensor 7 x _1, x ₂ (C
A / F value after AT and its change amount or change speed)
Is used.

In the above equation (3), the first term on the right side is the input U (time t−) which is input to the target exhaust system A from the current time t to the time t + d after the dead time d of the target exhaust system A. d
CA detected by the O ₂ sensor 7 after the dead time d when the A / F before CAT from 0 to t is “0”
This is a calculation term for estimating the state quantity of the post-T A / F.

The second and third terms on the right side of the equation (3) are input U (time t-d) input to the target exhaust system A from the current time t to time t + d after the dead time d. O ₂ after dead time d by A / F before CAT from time to time t
This is an arithmetic term for estimating the amount of change in the post-CAT A / F state quantity detected by the sensor 7.

The state predicting section 18 for performing such estimation calculation
Is configured as shown in FIG. 6 in a block diagram. That is, when the configuration is roughly divided, the state prediction unit 18 includes an estimation unit 32 that performs the estimation calculation of the first term on the right side of the equation (3), and an operation that solves the state equation (2) and the equation (3). The estimation unit 33 performs estimation calculation of the second and third terms on the right side.

Then, in order to perform the above-described estimation calculation, the estimation unit 32 causes the state quantity actually obtained from the output of the O ₂ sensor 7 (the post-CAT A / F value x ₁ and its change amount or change). The velocity x ₂ ) is given. In this case, the state quantity obtained by the output of the O ₂ sensor 7 is calculated by the element 3 if necessary.
Filtering and scaling are performed by 4 and the result is provided to the estimation unit 32. In FIG. 6, for convenience of explanation,
Although it has been described that the O ₂ sensor 7 gives both the value x ₁ of the post-CAT A / F and the change amount or the change speed x ₂ thereof, in reality, the change amount of the post-CAT A / F is shown. Alternatively, the rate of change x ₂ is obtained by calculation in the control unit 8.

The pre-CAT A / F actually obtained from the output of the LAF sensor 6 is given to the estimation unit 33 as the input U (= u + L) in order to perform the estimation calculation. In this case, the A / F before CAT obtained from the output of the LAF sensor 6 is provided to the estimation unit 33 after being filtered or scaled by the element 35 as necessary.

Then, the state predicting unit 18 uses the estimating units 3
2 and 33, the values obtained by adding are added, and the resulting values are added to the A / F after CAT detected by the O ₂ sensor 7 after the dead time d.
It is output to the adaptive sliding mode control unit 19 as the estimated values x ₁ hat and x ₂ hat of the state quantity. in this case,
The values obtained by the estimation units 32 and 33 are added after being filtered and scaled by the elements 36 and 37 as necessary, and the addition result (post-CAT A / F after dead time d) is added. Estimated value x of state
₍₁ hat, x ₂ hat) is also output to the adaptive sliding mode control unit 19 after being filtered or scaled by the element 38 as necessary. Hereinafter, the estimated values x ₁ hat and x ₂ hat will be referred to as estimated state quantities x ₁ hat and x ₂ hat.

Next, the adaptive sliding mode controller 1
9 will be described in detail.

First, general sliding mode control will be briefly described with reference to FIG.

The sliding mode control is a variable structure type feedback control method. In this control method, when the state quantities of the controlled object are two, x ₁ and x ₂ , for example, these state quantities x ₁ and x A hyperplane H represented by σ = 0 is designed in advance using a linear function σ = s ₁ x ₁ + s ₂ x ₂ (s ₁ and s ₂ are coefficients) having ₂ as a variable. This hyperplane H is often called a switching line (S) when the phase space is a quadratic system, and the linear function σ is called a switching function. When the order of the phase space increases, the switching line becomes a switching surface, and further becomes a hyperplane which cannot be geometrically illustrated. The hyperplane may also be called a slip surface.

Then, for example, as shown by the point P in FIG.
When the state quantities x ₁ and x ₂ are σ ≠ 0, the so-called
According to the reaching rule, the state quantities x ₁ and x ₂ are converged at high speed on the hyperplane H (σ = 0) by the high gain control (mode 1), and the state quantities x ₁ and x ₂ are changed by the so-called equivalent control input. While constraining on the hyperplane H, it converges to the equilibrium point (convergence point, point of x ₁ = x ₂ = 0) on the hyperplane H (mode 2).

In such a sliding mode control
Therefore, the state quantity x₁, X_TwoEven converge on the hyperplane H
If so, the equivalent control input will be affected by disturbances, etc.
State quantity x₁, X_TwoOn the hyperplane H
With the property that it can be converged to the equilibrium point of
I have. Therefore, in the above mode 1, the state quantity x₁, X _Two
It is important how to stably converge on the hyperplane H.
It becomes an issue. In this case, if there is an influence of disturbance, etc.
Is the state quantity x₁, X_TwoIs a hyperplane H
It is difficult to converge on top stably. Because of this,
For example, on October 20, 1994 by Corona
"Sliding Mode Control-Nonlinear R
Design theory of bust control- ", page 134-
As seen on page 135, in addition to the reaching law,
To converge the state quantity on the hyperplane while eliminating the influence
Hands called adaptive sliding mode control using adaptive law
A law has been proposed.

The adaptive sliding mode control section 19 of the present embodiment uses the adaptive sliding mode control as described above to calculate the reference target space from the estimated state quantities x ₁ hat and x ₂ hat of the post-CAT A / F. It calculates the correction amount of the fuel ratio, and is constructed as follows.

First, the adaptive sliding mode controller 1
The construction of the hyperplane required for the adaptive sliding mode control of 9 and the equivalent control input will be described.

In this embodiment, the adaptive sliding mode control unit 19 obtains the correction amount of the reference air-fuel ratio KBS so that the post-CAT A / F is set to a predetermined proper value. Estimated state quantity of F x ₁ hat,
The target value (that is, the value to be converged) of the x ₂ hat (the estimated value of the value after the dead time d of the A / F after CAT and the estimated value of the amount of change or the changing speed thereof) is set to "appropriate value" and " 0 ".

Therefore, the proper value of A / F after CAT is set to q
, The hyperplane for performing the adaptive sliding mode control is represented by the linear function of the following equation (4).

[0079]

(Equation 4)

On the other hand, the estimated state quantity x₁Hat, x_TwoHa
If the exhaust system is used, the dead time d of the target exhaust system A
Since it is compensated by the measuring unit 18, the target exhaust system in this case
The plant model of A is in the form shown in FIG.
Quantity x_1M, X_2MEstimated state quantity x ₁Hat, x_TwoPlace in a hat
It is expressed as a replacement.

Therefore, the state equation of the plant model is expressed by the following equation (5).

[0082]

(Equation 5)

Here, in the state equation (5), the linear transformation represented by the following equation (6) is performed based on the equation (4),

[0084]

(Equation 6)

Further, when the disturbance L is set to "0", the following equation (7) is obtained.

[0086]

(Equation 7)

Here, when the sliding mode control is performed using the hyperplane represented by the equation (4), the estimated state quantities x ₁ hat and x ₂ hat are constrained on the hyperplane as described above, In the mode 2 in which the light is converged on the equilibrium point on the plane, the condition of the following expression (8) must be satisfied.

[0088]

(Equation 8)

Therefore, the equivalent control input u _eq (= u) required in the mode 2 from the equation (7) is represented by the following equation (9).

[0090]

[Equation 9]

Next, when the estimated state quantities x ₁ hat and x ₂ hat are constrained on the hyperplane by this equivalent control input u _eq , σ = 0, so from the lower equation of equation (7), Equation (10) is obtained.

[0092]

(Equation 10)

Here, for simplification, s ₁ = k, s ₂ =
1 (k = s ₁ / s ₂ ), and the target value q of the estimated state quantity x ₁ hat (appropriate value of A / F after CAT) is time t <0.
In consideration of the fact that the step function input is “0” (constant value) and “q” (constant value) at time t ≧ 0, the following formula (11) is obtained by Laplace transform of the formula (10). can get.

[0094]

[Equation 11]

In the equation (11), X ₁ hat is the Laplace transform of the estimated state quantity x ₁ hat, and s is the Laplace operator.

Therefore, when the equation (11) is subjected to the inverse Laplace transform, the estimated state quantity x ₁ hat is expressed by the following equation (1) on the time axis.
It is represented by 2).

[0097]

(Equation 12)

Therefore, in the equation (12), k> 0 (s
_{If 1} > 0, s ₂ = 1), the estimated state quantity x ₁ hat converges to the target value q at t → ∞. Note that this means that the characteristic root −k (the pole of the control system) of the equation (11) is arranged in the stable region (the region where the real part of the pole is negative) on the complex plane, as shown in FIG. Means that.

Therefore, the hyperplane used in this embodiment is set by the following equation (13).

[0100]

(Equation 13)

It should be noted that the specific value of k in the equation (13) is basically determined based on various experiments and simulations so that the estimated state quantities x ₁ hat and x ₂ hat can quickly converge on the hyperplane. Set. In addition, the value of k is appropriately changed in the present embodiment, which will be described later. Further, in the present embodiment, the adaptive sliding mode control unit 19 is constructed as a servo type controller, so the target value q of the estimated state quantity x ₁ hat is set as q ≠ 0. However, when constructed as a regulator type controller, , And the target value of the estimated state quantity x ₁ hat can be set to “0” and constructed in the same manner as this embodiment.

Next, the reaching law of the adaptive sliding mode control in the adaptive sliding mode control unit 19 of this embodiment is constructed as follows using the hyperplane σ = 0 constructed as described above.

The reaching law of the sliding mode control is a control law for causing the linear function σ to converge on the hyperplane (σ = 0), and various reaching rules are known. And in this embodiment, among those reaching rules,
The acceleration rate law is adopted which has the shortest convergence time to the hyperplane.

According to this acceleration rate rule, the dynamic characteristic of σ (rate of change of the value of σ) is controlled so as to be expressed by the following equation (14).

[0105]

[Equation 14]

In the equation (14), J and α are preset positive constants, and in particular 0 <α <1. Sgn (σ) is the sign function of σ, and when σ <0, sgn (σ)
When (σ) = -1, σ = 0, sgn (σ) = 0, and when σ> 0, sgn (σ) = 1.

Here, when the above equation (13) is time-differentiated, and the state equation (5) is used with the disturbance L = 0, the following equation (15) is obtained.

[0108]

(Equation 15)

Therefore, from the equations (14) and (15),
The input u _sl (= u) to the target exhaust system A is given by the following equation (16).

[0110]

(Equation 16)

Target exhaust system represented by this equation (16)
Input u to A_slHowever, in the present embodiment, the disturbance L = 0
When CAT, the A / F after CAT is settled to an appropriate value q.
Input to be given to the target exhaust system A (A / F before CAT)
It is. Here, the first and second terms of the above equation (16) are
When σ = 0, that is, the estimated state quantity x₁Hat, x_TwoHa
When the focus converges on the hyperplane,
Equivalent control input u _eqMatches That is,

[0112]

[Equation 17]

It should be noted that this means that s ₁ /
If s ₂ = k is set and q is represented by x ₁ hat and x ₂ hat by using the equation (13) and is substituted into the equation (9), it becomes clear.

The third term of the equation (16) is the disturbance L = 0.
In this case, the estimated state quantity x ₁
The control input for converging the hat and x ₂ hat on the hyperplane is shown. Hereinafter, the control input based on this reaching law will be referred to as the reaching control input u _rch . That is,

[0115]

(Equation 18)

Next, the adaptive law of the adaptive sliding mode control in the adaptive sliding mode controller 19 of this embodiment is constructed as follows.

As described above, the hyperplane in this embodiment
Or equivalent control input u_eq, Arrival control input u_rchBuilding outside
Although it was performed based on the premise that the turbulence L = 0, the target exhaust system A
Actually, there are various disturbances, and the above hyperplane etc. are constructed.
The plant model used to build the actual target exhaust
There is a model error for system A. In this case, the estimated letter
Condition x₁Hat, x_TwoIf the hat converges on the hyperplane,
The equivalent control input u _eqTherefore, the estimated state quantity x₁Ha
X_TwoHats are subject to disturbances and model errors.
Instead, it converges to the equilibrium point on the hyperplane.
At the stage where it does not converge on the surface,
Reaching control input u_rchThen, the estimated state quantity x₁Hat, x
_TwoThe hat cannot be converged on the hyperplane.

The adaptive law used in the adaptive sliding mode control section 19 of the present embodiment is for eliminating such inconvenience.

In this embodiment, the adaptive sliding mode is used.
When constructing the adaptive law in the control unit 19, the disturbance L
Time and estimated state quantity x₁Hat, x_TwoDepending on the hat
Given that it is invariant, use the following equation (19)
Integral term u of the linear function σ _adpThe adaptive law term (hereinafter, u
_adpIs referred to as an adaptive control input)
To the right side of the target exhaust system A_slWhen
And decided to ask.

[0120]

[Equation 19]

Therefore, the input u _sl to the target exhaust system A using the adaptive law is obtained by the following equation (20).

[0122]

(Equation 20)

This equation (20) is the simplest form of adaptive sliding mode control, and it is possible to use a further developed adaptive law.

Adaptive Sliding Mode Control of this Embodiment
The control unit 19 calculates the target exhaust gas by performing the calculation of Expression (20).
Input u to system A_slAsk for. In this case, in this embodiment
Is the estimated state quantity x₁Set the hat to a proper value q (x
₁Hat = q, x_TwoHat = 0),
The quasi-air-fuel ratio KBS is corrected, and thereby indirectly after CAT.
Since the A / F is settled to a proper value q,
The riding mode control unit 19 obtains the expression (20).
Input u_slIs output as the correction amount for the reference air-fuel ratio KBS.
I do. Hereinafter, the input u obtained by the equation (20)_slBased on
Quasi air-fuel ratio correction amount u _slCalled.

The adaptive sliding mode control unit 19 constructed as described above is configured as shown in a block diagram of FIG. That is, the adaptive sliding mode control unit 19 has, as its main components, an equivalent control input calculation unit 39 for _{obtaining the} equivalent control input u _eq , the reaching control input u _rch, and the adaptive control input u _ad.
a non-linear input calculation unit 40 for calculating the sum u _nl (= u _rch + u _adp , hereinafter referred to as a non-linear input) of _p , and these calculation units 39, 40 are provided with the estimated state calculated by the state prediction unit 18. Quantity x ₁ hat, x ₂ hat is the element 3
8 is provided.

Then, the adaptive sliding mode control unit 19 basically adds the equivalent control input u _eq obtained by the arithmetic units 39 and 40 and the non-linear input u _nl to the reference air-fuel ratio. Correction amount u _sl (= u _eq + u _nl )
Is output, which is scaled and filtered by the element 41 as necessary, and then stored in a memory (not shown). In this case, the reference air-fuel ratio correction amount u _sl is calculated at a cycle time of a predetermined cycle (constant cycle) determined in advance.

Further, this adaptive sliding mode control
The target air-fuel ratio including the section 19 and the state prediction section 18
The calculator 16 corrects the reference air-fuel ratio stored in the memory.
Quantity u _slIs added to the reference air-fuel ratio KBS,
The target air-fuel ratio KCMD is obtained by correcting the ratio KBS. This place
If the target air-fuel ratio calculation means 16 calculates the target air-fuel ratio KCMD,
Output is the reference by the adaptive sliding mode control unit 19.
Air-fuel ratio correction amount u_slIs asynchronous with the calculation of
It is performed in synchronization with the rank angle cycle (so-called TDC).
However, this will be described later.

As shown in FIG. 9, the adaptive sliding mode control unit 19 of this embodiment has the arithmetic unit 3
In addition to 9, 40, a stability determination unit 42 that further determines the stability of the adaptive sliding mode control, and a correction limiting unit 4 that limits the correction of the reference air-fuel ratio KBS according to the determination result.
3 (correction amount calculation limiting means).

Here, the stability determination unit 42 determines the stability as shown in the flowchart of FIG. 16 every time the reference air-fuel ratio correction amount u _sl is calculated. That is,
First, the stability determination unit 42 first calculates the linear function σ expressed by the equation (13) (this is the nonlinear input calculation unit 40 shown in FIG. 9).
The temporal change rate σ dot (obtained by) is obtained (time differential value of σ) (STEP 16-1). Then, whether the absolute value of the linear function σ is larger than a predetermined value σ ₁ (| σ |> σ ₁ ) or the absolute value of the σ dot is larger than a predetermined value σ ₂ (> 0). greater than or (| σ dot |> σ ₂₎ to determine (STEP16-
2). With this judgment, | σ |> σ ₁ or | σdot |>
If σ ₂ (YES in STEP16-2), the stability determination unit 42 determines that the adaptive sliding mode control is unstable (STEP16-3), and ends the present stability determination. In this case, the estimated state quantity x ₁ hat and x ₂ hat are hyperplane σ when the state is judged to be unstable.
Distant from = 0 or a hyperplane σ = 0
This is a state in which a large temporal change is generated in the direction away from.

When the condition of STEP16-2 is not satisfied (NO in STEP16-2), the stability discriminating unit 42 next determines the product of the value of σ and the value of σdot σ · σdot (this is It is determined whether the Lyapunov function σ ² (corresponding to a time differential function of σ 2/2) is larger than a predetermined value a (≧ 0) (σ · σ dot> a) (STEP 16- 4). In this determination, if σ · σ dot> a (YES in STEP16-4), the stability determination unit 42 determines that the adaptive sliding mode control is unstable (STEP16-3), and The stability determination ends. When the condition of STEP16-4 is not satisfied, the stability determination unit 42 determines that the adaptive sliding mode control is stable (S
TEP16-5) The present stability determination is completed. In this case, the state determined to be unstable is such that the estimated state quantities x ₁ hat and x ₂ hat are displaced from the hyperplane (σ = 0) on the side where σ ² increases. It is in a state.

In this embodiment, STEP16-2,
Although the stability is determined based on the two conditions of STEP16-4, the stability may be determined based on only one of the conditions, or may be determined based on only one condition in STEP16-2.

By the stability determination of the adaptive sliding mode control by the stability determination unit 42, in a situation where the estimated state quantities x ₁ hat and x ₂ hat may not converge to the hyperplane σ = 0, the control is performed. It will be judged to be unstable.

When the stability determining unit 42 determines that the adaptive sliding mode control is unstable, the correction limiting unit 43 calculates the adaptive sliding mode control unit 19 at the current cycle time. The output of the reference air-fuel ratio correction amount u _sl is blocked, and the output of the adaptive sliding mode control unit 19 is held at the reference air-fuel ratio correction amount u _sl calculated in the previous cycle time. Reference air-fuel ratio KB based on fuel ratio correction amount u _sl
Limit the correction of S.

When the stability determining unit 42 determines that the control is stable, the correction limiting unit 43 outputs the reference air-fuel ratio correction amount u _sl calculated in the current cycle time as it is. Excuse me.

In this embodiment, adaptive sliding is used.
When the mode control is unstable, the reference air-fuel ratio correction amount u
_slReference air-fuel ratio correction calculated in the previous cycle time
Quantity u _slHold to limit the correction of the reference air-fuel ratio KBS
However, the adaptive sliding mode control is unstable.
Reference air-fuel ratio correction amount u_slIs forcibly set to “0”
(Do not correct the standard air-fuel ratio KBS)
May be limited.

By the way, in the adaptive sliding mode control used in the present embodiment, the estimated state quantities x ₁ hat and x ₂ hat converge on the hyperplane σ = 0 of equation (13) or its vicinity (σ≈0). At the stage, the stability of the convergence of the estimated state quantities x ₁ hat and x ₂ hat to the target values “q” and “0” (the equilibrium point of the hyperplane) is as follows: In other words, the coefficient k (>
The larger the value of 0), the higher the value. This is shown in FIG.
It is equivalent to that the stability of the system increases as the pole -k of the control system shown in (2) increases in the negative direction of the real axis. Further, as is clear from the equation (12), the larger the value of the coefficient k, the more the convergence time of the estimated state quantities x ₁ hat and x ₂ hat to the target values “q” and “0” on the hyperplane. Also becomes shorter. Therefore, from this point of view, it is preferable to set the coefficient k as large as possible.

However, if the value of the coefficient k in the equation (13) is too large, the estimated state quantities x ₁ hat and x ₂ hat do not converge to the hyperplane σ = 0 at the same equation (13).
As is clear from the above, the value of the linear function σ also becomes large, and therefore the nonlinear input u _nl (= u _{rc h} for converging the estimated state quantities x ₁ hat and x ₂ hat on the hyperplane.
+ U _adp ) also becomes large (equations (18), (19)
reference). When the non-linear input u _nl becomes excessive, the estimated state quantities x ₁ hat and x ₂ hat generate a vibrational response to the hyperplane, and the convergence time on the hyperplane becomes long. As a result, the convergence stability and quick response decrease. Therefore, from this viewpoint, it is not preferable that the value of the coefficient k is too large.

Therefore, the adaptive sliding mode control section 19 of the present embodiment can change the value of the coefficient k in the equation (13) as shown in FIG. 9 in addition to the above-mentioned configuration. A hyperplane variable control unit 44 (hyperplane setting means) for varying the hyperplane of the adaptive sliding mode control is provided.

In this case, the hyperplane variable control unit 44 of this embodiment variably controls the hyperplane of the adaptive sliding mode control as follows.

That is, in the present embodiment, the hyperplane variable control unit 44 uses the current value of the coefficient k to calculate the value of the linear function σ from the estimated state quantities x ₁ hat and x ₂ hat by the equation (13). ), The absolute value of the calculated σ value | σ |
The value of the parameter f defined in advance according to the following equation (21) is obtained according to the magnitude of

[0141]

(Equation 21)

Here, in the above equation (21), σ _limit
Is whether or not the linear function σ corresponding to the current estimated state quantities x ₁ hat and x ₂ hat is substantially in agreement with the hyperplane σ = 0, that is, the estimated state quantities x ₁ hat, x ₂
It is a predetermined threshold value that is determined in advance to determine whether or not the hat is substantially converged on the hyperplane σ = 0.

Then, the hyperplane variable control unit 44 integrates the value of the parameter f determined in this way for each cycle time of the adaptive sliding mode control, and the following equation (2)
As shown in 2), the integrated value sum (f) is calculated,

[0144]

(Equation 22)

From the obtained integrated value sum (f), the value of the coefficient k this time is determined by the following equation (23).

[0146]

(Equation 23)

In the above equation (23), k ₀ is the initial value (> 0) of the coefficient k defining the hyperplane, and γ is a predetermined gain coefficient for adjusting the changing speed of the value of the coefficient k. is there. The initial value k ₀ is set so that the estimated state quantities x ₁ hat and x ₂ hat converge on the hyperplane σ = 0 in the shortest time.

The hyperplane variable control unit 44 transfers the value of the coefficient k thus obtained by the equation (23) to the equivalent control input operation unit 39, the nonlinear input operation unit 40 and the stability determination unit 42. Is given as the value of the coefficient k for performing the above-mentioned calculation and discrimination.

In this embodiment, in order to prevent the value of the coefficient k from becoming a negative value or becoming smaller than the initial value k _{0, the} integrated value sum obtained by the equation (22) is summed.
When (f) is sum (f) <0, equation (23)
The correction coefficient k is obtained by forcibly setting the value of sum (f) at 0 to “0” (in this case, k = k ₀ ). Also,
If the value of the coefficient k becomes excessively large, the decrease of the value of the coefficient k will be delayed when it should be decreased. Therefore, in order to avoid this, the integrated value sum (f ) Is greater than a predetermined value α, the sum in equation (23)
The correction coefficient k is calculated by forcibly setting the value of (f) to “α” (in this case, k = k ₀ + α = the upper limit value of the coefficient k). The coefficient k obtained in this way is the estimated state quantity x. _{At the} stage where the ₁ hat and the x ₂ hat do not converge to the hyperplane σ = 0, the value of the parameter f fluctuates in the vicinity of the initial value k _0. Therefore, the initial value k ₀ is set as described above. The estimated state quantities x ₁ hat and x ₂ hat by
It can be converged to = 0 in almost the shortest time. At the stage where the estimated state quantities x ₁ hat and x ₂ hat have almost converged to the hyperplane σ = 0, the value of the parameter f is fixed to “1” almost constantly, so that the value of the coefficient k gradually increases. Increase to. Therefore, the estimated state quantity x ₁ hat, x
_At the stage where the ₂ hat has almost converged to the hyperplane σ = 0, the inclination of the hyperplane used for the adaptive sliding mode control of the present embodiment is gradually increased, as shown in FIG. Convergence of the estimated state quantities x ₁ hat and x ₂ hat to the target values “q” and “0” (the equilibrium point of the hyperplane) is enhanced, and at the same time, the convergence is performed in a short time (rapid response). Will increase). It should be noted that increasing the value of the coefficient k in this manner means that the pole -k of the control system is moved to the stable region side in the negative direction of the real axis Re on the complex plane of FIG. Is equivalent to that.

The method of setting the value of the coefficient k for making the hyperplane variable is not limited to the above-mentioned mode.
For example, as the value of the parameter f used in Expression (21),
Combinations other than "1" and "-1"("2" and "-"
1 ”,“ 1 ”and“ −2 ”) may be used. By using such a combination, it is possible to determine whether the value of the linear function σ substantially matches the hyperplane σ = 0. The rate of change can be made different, and the value of the parameter f can be given as a function of the value of the linear function σ to change the hyperplane according to the value of the linear function σ. The value of γ in (23) is made different according to the changing direction (whether it is an increasing tendency or decreasing tendency) of the value obtained by the equation (22), or the value of γ is changed according to the value of the linear function σ. It is also possible to change the hyperplane in this way,
The optimum one can be selected according to the control target, and the specific method of obtaining the coefficient k may be determined through experiments or the like in consideration of the stability and quick response of the control.

The contents described above are the details of the adaptive sliding mode control used in this embodiment.

Next, the adaptive control unit 23 included in the global feedback control unit 20 shown in FIG. 1 will be described in detail.

Referring to FIG. 1, the global feedback control unit 20 determines the air-fuel ratio (A / F before CAT) at the LAF sensor 6 as described above by the target air-fuel ratio calculation unit 16 as described above. Feedback control is performed so as to converge to the desired target air-fuel ratio KCMD. At this time, if such feedback control is performed only by well-known PID control, changes in the operating state of the engine 1 and aging may occur. It is difficult to secure stable controllability against dynamic behavior changes such as characteristic changes.

Therefore, in the global feedback control unit 20 of this embodiment, the adaptive control unit 23 capable of compensating the dynamic behavior change as described above is provided together with the PID control unit 22 for performing the well-known PID control. The feedback correction coefficient KFB, which is separately provided by the control units 22 and 23, is switched to perform feedback control.

In this case, the adaptive control section 23 controls the I.D.
D. Using the parameter adjustment rule proposed by Landau et al., As shown in FIG. 11, the parameter adjustment unit 45 that sets a plurality of adaptive parameters, and the feedback correction coefficient KSTR is calculated using the set adaptive parameters. It is composed of a correction coefficient calculation unit 46.

Here, the parameter adjusting unit 45 will be described. In the adjusting rule of Landau et al., The polynomial of the denominator numerator of the transfer function B (Z ^-1 ) / A (Z ^-1 ) of the controlled object of the discrete system is generally When the following equations (24) and (25) are set, the adaptive parameter θ set by the parameter adjusting unit 45 is set.
Hat (j) (j indicates the number of control cycles. The same applies hereinafter) is a vector (transposed vector) as shown in equation (26).
It is represented by In addition, the input ζ to the parameter adjusting unit 45
(j) is expressed as in Expression (27). In this case, in the present embodiment, the engine 1 which is the control target of the feedback control unit 20 is the primary system and the dead time d for three control cycles.
_p (time for 3 combustion cycles of engine 1)
Considering a plant having
With n = 1 and d _p = 3, five adaptive parameters to be set were s ₀ , r ₁ , r ₂ , r ₃ and b ₀ (see FIG. 11). Note that u _s and y _s in the upper and middle stages of the equation (27) generally represent the control input (operation amount) to the control target and the output (control amount) of the control target, respectively. However, in the present embodiment, the control input is the feedback correction coefficient KSTR, and the output of the control target (engine 1) is the A before CAT which is actually detected by the LAF sensor 6.
/ F (hereinafter referred to as KACT), ζ (j) represents the input to the parameter adjusting unit 45 by the lower expression of the expression (27) (see FIG. 11).

[0157]

(Equation 24)

[0158]

(Equation 25)

[0159]

(Equation 26)

[0160]

[Equation 27]

Here, the adaptive parameter θ hat shown in the above equation (26) is a scalar quantity element b ₀ hat ^-1 (j) that determines the gain of the adaptive control unit 23, and control represented by the manipulated variable. The element B _R hat (Z ⁻¹ , j) and the control element S (Z ⁻¹ , j) expressed by using the control amount are respectively expressed by the following equations (28) to (30) ( (See the block diagram of the correction coefficient calculation unit 46 in FIG. 11).

[0162]

[Equation 28]

[0163]

(Equation 29)

[0164]

[Equation 30]

The parameter adjustment unit 45 sets each coefficient of these scalar amount elements and control elements and gives it to the correction coefficient calculation unit 46 as the adaptive parameter θ hat shown in Expression 26. Of the feedback correction coefficient KSTR and the control amount A / F before CAT (= KA
CT) is used to calculate the adaptive parameter θ hat so that the A / F before CAT matches the target air-fuel ratio.

In this case, specifically, the adaptive parameter θ
The hat is calculated by the following equation (31).

[0167]

[Equation 31]

In the equation (31), Γ (j) is a gain matrix (m that determines the setting speed of the adaptive parameter θ hat.
+ N + d _p ), e asterisk (j) indicates the estimation error of the adaptive parameter θ hat, and is expressed by equation (3).
It is expressed by a recurrence formula such as 2) and (33).

[0169]

(Equation 32)

[0170]

[Expression 33]

Here, "D (Z ^-1 )" in the equation (33)
Is an asymptotically stable polynomial for adjusting the convergence, and D (Z ⁻¹ ) = 1 in this embodiment.

Note that λ in equation (33)₁(j), λ_TwoHow to choose (j)
Gives various concrete algorithms. example
If λ₁(j) = 1, λ_TwoIf (j) = λ (0 <λ <2),
Gradual gain algorithm (least squares when λ = 1
Method), λ₁(j) ＝ λ₁(0 <λ₁<1), λ_Two(j) ＝ λ
_Two(0 <λ_Two<Λ), the variable gain algorithm
(Λ _Two= 1, weighted least squares method, λ₁(j) /
λ_Two(j) = η and λ_ThreeIs expressed as equation (34)
Λ₁(j) ＝ λ_ThreeThen the fixed trace algorithm
Becomes Here, “tr Γ (0)” in equation (34) is the line
The trace function of column Γ (0), the sum of the diagonal elements of matrix Γ (0)
(Scalar amount). Also, λ₁(j) = 1, λ _Two(j) = 0
When, it becomes a fixed gain algorithm. in this case,
As is clear from equation (32), Γ (j) = Γ (j-1)
Therefore, Γ (j) has a fixed value. Fuel injection of engine 1
For time-varying plants such as injection or air-fuel ratio, the gain is gradually reduced.
Algorithm, variable gain algorithm, fixed gain algorithm
Both gorism and fixed tracing algorithms
Is suitable.

[0173]

(Equation 34)

Set in the parameter adjusting unit 45 as described above
Adaptive parameter θ hat (s ₀, R₁, R_Two, R
_Three, B₀) And the target air-fuel ratio calculation unit 16 described above.
Using the target air-fuel ratio KCMDM calculated according to
The calculation unit 46 calculates the feed by the recurrence formula of the following formula (35).
Calculate the back correction coefficient KSTR. Correction coefficient calculation unit of FIG.
46 is a block diagram showing the operation of the equation (35).
It is.

[0175]

(Equation 35)

Here, "d '" in the equation (35) is the dead time until the LAF sensor 6 detects the A / F before CAT corresponding to the target air-fuel ratio KCMD, and this dead time. In the present embodiment, d ′ is the crank angle cycle (so-called T
Time for 12 cycles (= 4
d _p ).

The adaptive control unit 23 constructed in this way is
As is clear from the above, the controller is of the recurrence type in consideration of the dynamic behavior change of the engine 1 to be controlled, in other words, to compensate the dynamic behavior change of the engine 1. The controller described in the recurrence form.
Then, more specifically, it can be defined as a controller having an adaptive parameter adjusting mechanism of a recurrence type.

Incidentally, this type of recurrence type controller may be constructed by using a so-called optimum regulator.
In this case, the parameter adjusting mechanism is not generally provided.

The above is the details of the adaptive control unit 23 employed in this embodiment.

Incidentally, the PID control unit 22 provided in the global feedback control unit 20 together with the adaptive control unit 23, as well as the general PID control, performs the A / F (KACT) before CAT detected by the LAF sensor 6. , The proportional term (P term), the integral term (I term) and the derivative term (D term) are calculated from the deviation from the target air-fuel ratio KCMD, and the sum of these terms is calculated as the feedback correction coefficient KLAF. In this case, in the present embodiment, the fuel injection amount is multiplied by the feedback correction coefficient KLAF to correct the fuel injection amount.
When the deviation between (KACT) and its target air-fuel ratio KCMD is "0", the feedback correction coefficient KLAF is set to "1", so the initial value of the integral term (I term) is set to "1". Further, the gains of the proportional term, the integral term, and the derivative term are calculated by the engine 1
It is determined from the number of revolutions and the intake pressure using a predetermined map.

Further, the switching unit 24 of the global feedback control unit 20 operates when the cooling water temperature of the engine 1 is low,
When the combustion of the engine 1 is likely to become unstable, such as during high-speed operation, when the intake pressure is low, or when there is a large change in the target air-fuel ratio KCMD, or immediately after the start of air-fuel ratio feedback control. When the detected air-fuel ratio KACT of the corresponding LAF sensor 6 is lacking in reliability due to the response delay of the LAF sensor 6 or the like, or the operating state of the engine 1 is extremely stable as when the engine 1 is idle, When the high gain control by the adaptive control unit 23 is not required, the feedback correction coefficient KLAF calculated by the PID control unit 22 is output as the feedback correction coefficient KFB for correcting the fuel injection amount, and in other cases than the above In this state, the feedback correction coefficient KSTR calculated by the adaptive control unit 23 is set to the feedback correction coefficient for correcting the fuel injection amount. And outputs it as KFB. This is because the adaptive control unit 23 rapidly adjusts the A / F before CAT detected by the LAF sensor 6 by the high gain control to the target air-fuel ratio.
Since it functions to converge to KCMD, feedback correction of the adaptive control unit 23 is performed in the case where the combustion of the engine 1 becomes unstable as described above or the detected air-fuel ratio KACT of the LAF sensor 6 is lacking in reliability. This is because if the coefficient KSTR is used, control of the air-fuel ratio may become unstable on the contrary.

Since the operation of the switching unit 24 as described above is disclosed in detail in Japanese Patent Application No. 7-227303, the detailed description thereof will be omitted here.

Next, the overall operation of the air-fuel ratio control system of this embodiment will be described.

Referring to the flowcharts of FIGS. 1 and 13, first, the output fuel injection amount #nT for each cylinder of the engine 1
The calculation of out (n = 1,2,3,4) will be described. The control unit 8 calculates the output fuel injection amount #nTout for each cylinder at a cycle time synchronized with the crank angle cycle of the engine 1 as follows. Perform calculation processing.

First, after the outputs of various sensors including the LAF sensor 6 and the O ₂ sensor 7 are read (STEP
13-1), the basic fuel injection amount calculation unit 12 corrects the fuel injection amount corresponding to the rotation speed and intake pressure of the engine 1 according to the effective opening area of the throttle valve as described above. Is required (STEP 13-
2). Further, the first correction coefficient calculation unit 13 calculates the first correction coefficient KTOTAL according to the cooling water temperature of the engine 1, the purge amount of the canister, etc. (STEP 13-3), and the reference air-fuel ratio setting unit 15 calculates the first correction coefficient KTOTAL. The reference air-fuel ratio KBS is set according to the rotation speed of 1 and the intake pressure (S
TEP 13-4).

Next, in the target air-fuel ratio calculation section 16,
Reference air-fuel ratio correction quantity u _sl calculated by the adaptive sliding mode controller 19 is read from the memory (not shown) (STEP13-5), the reference air-fuel ratio correction quantity u _sl
Is added to the reference air-fuel ratio KBS set in STEP 13-4 and the reference air-fuel ratio KBS is corrected to obtain the target air-fuel ratio KBS.
KCMD is required (STEP 13-6).

In the local feedback control section 21, the PI is determined based on the actual air-fuel ratio # nA / F for each cylinder estimated from the output of the LAF sensor 6 by the observer 26.
The D control unit 27 calculates the feedback correction coefficient #nKLAF so as to eliminate the variation for each cylinder (STE
P13-7), and the global feedback control unit 2
The feedback correction coefficient KFB is calculated from 0 (STEP 13-8).

In this case, the feedback correction coefficient KFB is calculated by using the outputs of the various sensors read in STEP 13-1 and the target air-fuel ratio obtained in STEP 13-6.
This is performed using KCMD as shown in the flowchart of FIG. That is, the adaptive control unit 23 and the PID control unit 2
2, the feedback correction coefficients KSTR and KLAF are obtained so that the A / F before CAT detected by the LAF sensor 6 is converged to the target air-fuel ratio KCMD (ST).
EP14-1, 14-2). Then, as described above, in the switching unit 24, it is determined whether or not the operating range in which the adaptive control should be performed depends on whether the combustion of the engine 1 or the detected air-fuel ratio of the LAF sensor 6 is likely to be unstable. Is determined (STEP 14-3), and in the operation region where the adaptive control should be performed, the feedback correction coefficient KSTR obtained by the adaptive control unit 23 is obtained as the feedback correction coefficient KFB for correcting the fuel injection amount of the engine 1. (STEP 14-4), the feedback correction coefficient KLAF calculated by the PID control unit 22 is calculated as the feedback correction coefficient KFB in the operation region where the PID control should be performed (STEP 14-5).

In this case, the feedback correction coefficient KF
When B is switched from the feedback correction coefficient KLAF to the feedback correction coefficient KSTR, in order to avoid the sudden change of the correction coefficient KFB, the adaptive control unit 23 limits the correction coefficient KFB (= KSTR) to this cycle time only. The correction coefficient KSTR is calculated so that is held in the previous correction coefficient KFB (= KLAF). Similarly, when switching the correction coefficient KFB from the correction coefficient KSTR to the correction coefficient KLAF, the PID control unit 22
The correction coefficient KLAF calculated by himself in the previous cycle time is
The correction coefficient KLAF of this time is calculated assuming that the correction coefficient KFB (= KSTR) of the previous time was used.

Returning to FIG. 13, after the calculation of the feedback correction coefficient KFB described above, the second correction coefficient KCMDM corresponding to the target air-fuel ratio KCMD obtained in STEP 13-6 is further calculated.
Is calculated by the second correction coefficient calculation unit 14 (STEP
13-9).

Then, the control unit 8 adds the first correction coefficient KTOTA to the basic fuel injection amount Tim obtained as described above.
L, second correction coefficient KCMDM, feedback correction coefficient KFB
, And the feedback correction coefficient #nKLAF for each cylinder to obtain the output fuel injection amount #nTout for each cylinder (STEP 13-10). Then, the output fuel injection amount #nTout for each cylinder is determined by the adhesion correction unit 28.
After correction in consideration of the adhesion of the wall surface of the intake pipe of the engine 1 (STEP 13-11), the fuel is output to a fuel injection device (not shown) of the engine 1 (STEP 13-12).

In the engine 1, fuel is injected into each cylinder according to the output fuel injection amount #nTout of each cylinder.

The output fuel injection amount #n for each cylinder as described above
The calculation of Tout and the fuel injection to the engine 1 in accordance therewith are sequentially performed at a cycle time in synchronization with the crank angle cycle of the engine 1, whereby the A / F before CAT detected by the LAF sensor 6 is calculated as the target air-fuel ratio. The operating state of the engine 1 is controlled so that it converges to the target air-fuel ratio KCMD calculated by the unit 16. In this case, in particular, when the feedback correction coefficient KSTR obtained by the adaptive control unit 23 is used as the feedback correction coefficient KFB, it is highly stable against behavior changes such as changes in the operating state of the engine 1 and characteristic changes. As a result, the A / F before CAT is rapidly controlled to converge to the target air-fuel ratio KCMD.

On the other hand, the reference air-fuel ratio u _sl read in STEP 13-5 is obtained as shown in the flowchart of FIG. 15 for each cycle time of a predetermined cycle (constant cycle).

That is, referring to FIGS. 6, 9 and 15, after the outputs of the LAF sensor 6 and the O ₂ sensor 7 are read (STEP 15-1), the state prediction unit 18
The estimated state quantities x ₁ hat and x ₂ hat of the post-CAT A / F after the dead time d of the target exhaust system A (the estimated value of the post-CAT A / F value, and its variation amount or variation rate) are It is calculated according to the equations (2) and (3) (STEP 1
5-2).

Next, in the adaptive sliding mode controller 19, after the hyperplane variable controller 44 sets the value of the coefficient k as described above (STEP 15-3),
The equivalent control input computing unit 39 calculates the equivalent control input u _eq according to the equation (17) (STEP 15-4).
Further, after the value of the linear function σ is calculated by the nonlinear input calculation unit 19 according to the equation (13) (STEP15-
5), the adaptive control input u _{ad p} (adaptive law term) is expressed by the equation (1)
It is calculated according to 9) (STEP 15-6).

Further, in the non-linear input calculation section 40,
Absolute value of the linear function σ is a minute predetermined value ε
(STEP15-7), and at this time, | σ |>
If ε, the arrival control input u _rch (reaching law term) is calculated according to the equation (18) (STEP 15-8). Further, if | σ | ≦ ε, that is, in the state where the estimated state quantities x ₁ hat and x ₂ hat converge on a substantially hyperplane, the reaching control input u _{rc h} is forcibly set to “0”. Yes (ST
EP15-9).

Next, the reference air-fuel ratio correction amount u _sl is the equivalent control input u _eq and the arrival control input u obtained as described above.
It is calculated from _rch and the adaptive control input u _{adp according} to the above equation (20) (STEP 15-10).

Next, the stability judging section 42 judges the stability of the adaptive sliding mode control according to the flow chart of FIG. 16 (STEP 15-1).
1) If the result of this determination is "stable" (STEP15
(YES in -12), the reference air-fuel ratio correction amount u _sl obtained in STEP15-10 is output via the correction limiting section 43 (STEP15-13). If the determination result is "unstable" (NO in STEP15-12), the reference air-fuel ratio correction amount u _sl obtained in the previous cycle time is determined.
Is the reference air-fuel ratio correction amount u _{sl of} this time (STEP15-
14), which is output in STEP15-13.
The reference air-fuel ratio correction amount u _sl output in STEP15-13 is held in a memory (not shown), which is stored in the memory shown in FIG.
The target air-fuel ratio KC is read out in STEP 13-5 of No. 3 (this reading method will be described later).
Used for MD calculation.

In this way, the reference air-fuel ratio correction amount u obtained by the adaptive sliding mode control unit 19
_sl is CA detected by the O ₂ sensor 7 as described above.
Since it is required to make the post-T A / F converge to a predetermined proper value q, the target air-fuel ratio KCMD obtained by correcting the reference air-fuel ratio KBS by the reference air-fuel ratio correction amount u _{sl is set} to CA
By performing feedback control on the pre-T A / F by the feedback control unit 17, the post-CAT A / F is indirectly controlled to the proper value q via the feedback control by the feedback control unit 17.

In this case, the sliding mode control performed by the adaptive sliding mode control section 19 is performed by the state amount of the post-CAT A / F (the value of the post-CAT A / F and its change amount) to be settled to a predetermined appropriate value q. Alternatively, as long as the change rate converges on the hyperplane, the equivalent control input u _eq does not affect the disturbance or the model error of the controlled object, and the state quantity reaches the equilibrium point (convergence point) on the hyperplane. It has the characteristic that it can be converged to (point) stably. Therefore, as long as the post-CAT A / F state quantity converges on the hyperplane, the post-CAT A / F does not depend on the change in the operating state of the engine 1 or the deterioration of the catalyst device 4 over time.
Can be set to an appropriate value q.

Then, when the state quantity of the A / F after CAT is converged on the hyperplane, the present embodiment adopts the adaptive sliding mode control in which the influence of disturbance or the like is taken into consideration by using the adaptive law. Therefore, at the stage where the state quantity of the A / F after CAT does not converge on the hyperplane, the influence of disturbance or model error can be minimized and the state quantity can be stably converged on the hyperplane.

In this case, the target exhaust system A to be controlled by the sliding mode control generally has a relatively long dead time d, and this dead time d may cause instability of control. However, in the present embodiment, when obtaining the reference air-fuel ratio correction amount u _sl using the adaptive sliding mode control, the state quantity of the post-CAT A / F detected by the O ₂ sensor 7 in real time is not used as it is, but Since the estimated state quantities x ₁ hat and x ₂ hat obtained by compensating the dead time d by the state prediction unit 18 are used, the estimated state quantities x ₁ hat and x ₂ hat are converged extremely stably on the hyperplane. Can be made. Then, if the estimated state quantities x ₁ hat and x ₂ hat converge on the hyperplane, the estimation error of those estimated state quantities x ₁ hat and x ₂ hat is also absorbed due to the inherent characteristics of the sliding mode control. Will be done.

Therefore, according to the air-fuel ratio control system of the present embodiment, the post-CAT A / F can be controlled without depending on the change in the operating state of the engine 1, the deterioration of the catalyst device 4, the disturbance, the model error, etc.
It is possible to settle to an appropriate value q with extremely high accuracy, and thereby the air-fuel ratio of the engine 1 can be controlled to an air-fuel ratio that maximizes the exhaust gas purification capacity of the catalyst device 4, Optimal emission characteristics can be secured.

In this embodiment, the hyperplane variable control is used.
The control unit 44 calculates the estimated state quantity x of the A / F after CAT.₁Ha
X, x_TwoDepending on how the hat converges to the hyperplane,
By changing the coefficient k that defines the hyperplane, the hyperplane can be changed.
Therefore, the estimated state quantity x ₁Hat, x_TwoSuper hat
It is said that convergence on a plane can be stably performed in a short time.
Sometimes the estimated state quantity x₁Hat, x_TwoThe hat fits in the hyperplane
Estimated state quantity x₁Hat, x_TwoHat
Equilibrium point on the hyperplane, ie x₁Hat = q, x_TwoHa
Stable convergence in a short time to the convergence point where
Can be. Therefore, after CAT, the A / F is set to a short convergence time.
(High responsiveness) and high stability to quickly reach appropriate value q
Can be settled.

By the way, in the present embodiment, the output fuel injection amount #nTout calculated by the feedback control unit 17 (including calculation of each correction coefficient and calculation of the target air-fuel ratio).
Since it has to be performed in synchronization with the rotation of the engine 1, it is performed with a cycle time in synchronization with the crank angle cycle as described above. Therefore, the output fuel injection amount #nTout is calculated at irregular time intervals, not at regular time intervals as shown in the upper part of FIG.

On the other hand, the adaptive sliding mode controller 1
The calculation of the reference air-fuel ratio correction amount u _sl by 9 is performed at a cycle time of a predetermined cycle CT as shown in the lower part of FIG. 12, and the calculated reference air-fuel ratio correction amount u _sl is held in a memory (not shown). It Then, the reference air-fuel ratio correction amount u _sl stored in the memory is updated every time the reference air-fuel ratio correction amount u _sl is newly obtained. Therefore, the timing of calculating the reference air-fuel ratio correction amount u _sl and holding it is determined by the output fuel injection amount #nT.
It is asynchronous with the calculation of out. In this case, in the present embodiment, the cycle CT for calculating the reference air-fuel ratio correction amount u _sl is longer than the normal crank angle cycle.

[0208] Thus, in the present embodiment, since the calculation of the reference air-fuel ratio correction quantity u _sl, it is to perform with the calculated asynchronously output fuel injection quantity #nTout, the reference air-fuel ratio correction quantity u _sl
The process of calculating the target air-fuel ratio KCMD by using the above and further calculating the output fuel injection amount #nTout is performed as follows.

That is, as shown in FIG. 12, when calculating the target air-fuel ratio KCMD and further calculating the output fuel injection amount #nTout, it is calculated by the adaptive sliding mode control unit 19 latest before that. The reference air-fuel ratio correction amount u _sl held in the memory is used. However, in this case, if the calculation timing of the output fuel injection amount #nTout and the calculation timing of the reference air-fuel ratio correction amount u _sl happen to coincide with each other, the reference air-fuel ratio correction amount u _sl already stored in the memory.
The output fuel injection amount #nTout is calculated using
The newly obtained reference air-fuel ratio correction amount u _sl is held in the memory.

As described above, the reference air-fuel ratio correction amount u _sl is calculated in the cycle time independent from the calculation of the output fuel injection amount #nTout, whereby the adaptive sliding mode control unit 19 and the feedback control are performed. Part 17
It is possible to perform arithmetic processing on and with a cycle time suitable for each control characteristic and control target. In particular, the adaptive sliding mode control unit 19 calculates the reference air-fuel ratio correction amount u _sl with a cycle time of a relatively long cycle CT corresponding to the relatively long dead time d and the response delay time existing in the target exhaust system A. By doing so, if the cycle time is constant, d _M in equation (3) may be constant,
The calculation load is reduced and the reference air-fuel ratio correction amount u
_It is possible to calculate _sl with high accuracy while eliminating the calculation error. As a result, the accuracy of settling the A / F after CAT to the proper value q can be improved.

Next, a simulation of control by the air-fuel ratio control system of this embodiment will be described.

In the air-fuel ratio control system of the present embodiment, the inventors of the present invention have shown that the A / A ratio before CAT as shown in FIG.
A simulation was performed on the convergence of A / F after CAT when a disturbance L was applied to F. The result is shown in FIG.
(B). For comparison with this, the same simulation was performed when the reference air-fuel ratio correction amount was determined using the conventional PID control. The result is shown in FIG.

As shown in FIG. 17B, according to the present embodiment, the A / F before CAT settles to the proper value q extremely accurately regardless of the disturbance L, and converges to the proper value q. It takes only a short time to do.

On the other hand, when the conventional PID control is used, the A / F after CAT fluctuates with respect to the proper value q, and it cannot be accurately converged to the proper value q. It was

Therefore, in the air-fuel ratio control system of the present embodiment, the adaptive sliding mode control is used to calculate the reference air-fuel ratio correction amount, so that the post-CAT A / F can be performed with extremely high accuracy regardless of disturbance or the like. It can be seen that the proper value q can be set.

Next, another embodiment of the air-fuel ratio control device of the present invention will be described with reference to FIG. Since the air-fuel ratio control device of the present embodiment is obtained by modifying only a part of the air-fuel ratio control device shown in FIG.
The same reference numerals as those in FIG. 1 are attached and detailed description is omitted.

Referring to FIG. 18, the air-fuel ratio control system of the present embodiment differs from that of FIG. 1 only in the configuration of global feedback control unit 20. Global feedback control unit 20 Is the same as in FIG.
An ID control unit 22, an adaptive control unit 23, and a switching unit 24 are provided, while the LAF sensor 6 to the filters 24 and 25 are provided.
A / F (= KACT) before CAT obtained through the above is divided by the target air-fuel ratio KCMD calculated as described above by the target air-fuel ratio calculation unit 16 (before ACAT / CAT).
The division units 47 and 48 for obtaining the ratio KACT / KCMD between F and the target air-fuel ratio KCMD, and the target value setting unit 49 for setting the target value (= 1) of the ratio KACT / KCMD are provided. In this case, when calculating the ratio KACT / KCMD by the division units 47 and 48, the A / F (= KA before CAT obtained from the LAF sensor 6 is obtained.
CT) and the target air-fuel ratio KCMD calculated by the target air-fuel ratio calculation unit 16, there is a dead time d'shown in the equation (35). KC
MD is provided via a dead time adjusting unit 50 that adjusts the dead time d '.

Then, the ratios KACT / KCMD obtained by the division units 47 and 48 are given to the PID control unit 22 and the adaptive control unit 23, and the target value (=
1) is given from the target value setting unit 49 to the PID control unit 22 and the adaptive control unit 23, and the PID control unit 22 and the adaptive control unit 23 respectively set the given ratio KACT / KCMD to the target value (= 1). In order to agree with each other, the feedback correction coefficients KLAF and KSTR are obtained as in the case of FIG.
In this case, the adaptive control unit 23 uses the “KC in the above equation (35).
MD (j-d ') ”and“ KACT (j) ”are set to“ 1 ”and“ KACT /
The feedback correction coefficient KSTR is obtained by the recurrence formula replaced with KCMD ”.

The other structure is exactly the same as that of FIG.

[0220] Such a global feedback control unit 2
In the air-fuel ratio control apparatus of this embodiment having 0, the target air-fuel ratio corrected by using the adaptive sliding mode control
A / F before CAT detected by KCMD and LAF sensor 6
The feedback correction coefficient KFB (= KLAF or KSTR) is a global feedback so that the ratio KACT / KCMD to and the target air-fuel ratio KCMD and the A / F before CAT match. Since it is obtained by the control unit 20, it is of course possible to obtain the same operational effect as that of FIG. Further, since the target value when the global feedback control unit 20 obtains the feedback correction coefficient KFB is fixed at “1”, the target air-fuel ratio KCMD (which varies from moment to moment) is changed as shown in FIG. The stability of control by the global feedback control unit 20 is improved as compared with the case where the target value is set. Particularly, in the adaptive control unit 23 of the global feedback control unit 20, the stability of the adaptive control unit 23 is improved because the change of the adaptive parameter θ as described above becomes small by fixing the target value. Greatly improved.

In this embodiment, the target air-fuel ratios KCMD and L
Ratio KA with A / F before CAT detected by AF sensor 6
Although the CT / KCMD is made to converge to the target value “1”, the CAT detected by the target air-fuel ratio KCMD and the LAF sensor 6
The deviation from the previous A / F may be obtained and the deviation may be eliminated (the target value of the deviation is set to "0"). Furthermore, it is also possible to directly correct the detection value of the A / F before CAT by the output u _sl of the sliding mode control unit 19 and control it so as to match it with the target value obtained separately.

In each of the embodiments described above, the first
A wide-range air-fuel ratio sensor (LAF sensor) 6 is used as the exhaust gas sensor, but the first exhaust gas sensor may be a normal O ₂ sensor or the like as long as it can detect the air-fuel ratio of the exhaust gas.
Other types of sensors may be used.

Further, in each of the above embodiments, the oxygen concentration sensor (O ₂ sensor) 7 is used as the second exhaust gas sensor, but the second exhaust gas sensor uses the concentration of the specific component of the exhaust gas downstream of the catalyst device to be controlled. Other sensors may be used as long as they can detect the. That is, for example, a CO sensor is controlled when controlling carbon monoxide (CO) in the exhaust gas downstream of the catalyst device, a NO _x sensor is controlled when controlling nitrogen oxides (NO _x ), and a hydrocarbon (HC) is controlled. In some cases, an HC sensor is used. When a three-way catalyst device is used, even if the concentration of any of the above gas components is detected, it is possible to control so as to maximize the purification performance of the catalyst device. Further, when the reduction catalyst device or the oxidation catalyst device is used, the purification performance can be improved by directly detecting the gas component to be purified.

[Brief description of drawings]

FIG. 1 is an overall system configuration diagram of an embodiment of an air-fuel ratio control device for an internal combustion engine of the present invention.

FIG. 2 is an output characteristic diagram of an O ₂ sensor used in the air-fuel ratio control device of FIG.

FIG. 3 is an explanatory diagram for explaining a model of a control target in the air-fuel ratio control device of FIG. 1.

4 is a block diagram of the model of FIG.

5 is a block diagram of a model used for an estimation unit in a state prediction unit of the air-fuel ratio control device of FIG.

FIG. 6 is a block diagram of a state prediction unit in the air-fuel ratio control device of FIG.

FIG. 7 is an explanatory diagram for explaining sliding mode control.

8 is an explanatory diagram showing a pole arrangement of a control system in the air-fuel ratio control device of FIG.

9 is a block diagram of an adaptive sliding mode control unit in the air-fuel ratio control device of FIG.

10 is an explanatory diagram of a hyperplane used by the adaptive sliding mode control unit of FIG.

11 is a block diagram of an adaptive control unit in the air-fuel ratio control device of FIG.

12 is an explanatory diagram for explaining a calculation timing of an output fuel injection amount and a reference air-fuel ratio correction amount in the air-fuel ratio control device of FIG.

13 is a flowchart for explaining the operation of the air-fuel ratio control device in FIG.

14 is a flowchart for explaining the operation of the air-fuel ratio control device in FIG.

15 is a flowchart for explaining the operation of the air-fuel ratio control device in FIG.

16 is a flowchart for explaining the operation of the air-fuel ratio control device in FIG.

17 is an explanatory diagram showing simulation results of the air-fuel ratio control device of FIG. 1 and a conventional device.

FIG. 18 is an overall system configuration diagram of another embodiment of an air-fuel ratio control device for an internal combustion engine of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine (internal combustion engine), 4 ... Catalyst device, 6 ... LAF
Sensor (first exhaust gas sensor), 7 ... O ₂ sensor (second
Exhaust gas sensor), 17 ... Feedback control unit, 19
... adaptive sliding mode control unit, 23 ... adaptive control unit (controller in recurrence form), 39 ... equivalent control input calculation unit,
40 ... Non-linear input operation unit, 42 ... Stability determination unit, 43 ...
Correction limiter (correction amount calculation limiter).

Claims (10)

[Claims] 1. A catalyst device for exhaust gas purification provided in an exhaust system of an internal combustion engine, and a catalyst device provided in the exhaust system for detecting an air-fuel ratio of exhaust gas of the internal combustion engine upstream of the catalyst device. A first exhaust gas sensor, and a second exhaust gas sensor provided in the exhaust system to detect the concentration of a specific component of the exhaust gas of the internal combustion engine that has passed through the catalyst device on the downstream side of the catalyst device. In an air-fuel ratio control device for an internal combustion engine, which controls the air-fuel ratio of the internal combustion engine based on the output of an exhaust gas sensor, based on the output of the second exhaust gas sensor,
Adaptive sliding mode control means for obtaining a correction amount for correcting the air-fuel ratio of the internal combustion engine using adaptive sliding mode control so that the concentration of a specific component of exhaust gas on the downstream side of the catalyst device becomes a predetermined appropriate value; , The fuel to the internal combustion engine so that the concentration of the specific component of the exhaust gas on the downstream side of the catalyst device converges to the predetermined appropriate value based on the obtained correction amount and the output of the first exhaust gas sensor. An air-fuel ratio control device for an internal combustion engine, comprising: feedback control means for controlling a supply amount. 2. The air-fuel ratio control system for an internal combustion engine according to claim 1, wherein the predetermined proper value of the concentration of the specific component is set to a value that maximizes the purification capacity of the catalyst device. . 3. The air-fuel ratio control system for an internal combustion engine according to claim 1, wherein the second exhaust gas sensor is an oxygen concentration sensor. 4. The adaptive sliding mode control means includes an exhaust system including the purifying device between the exhaust gas sensors, the exhaust gas air-fuel ratio detected by the first and second exhaust gas sensors in advance, and the identification value. A correction amount for correcting the air-fuel ratio of the internal combustion engine so that the output in the model becomes the predetermined appropriate value, which is modeled as a model including a second-order lag or more, each of which has the concentration of a component as an input and an output. The air-fuel ratio control device for an internal combustion engine according to any one of claims 1 to 3, wherein is determined by adaptive sliding mode control. 5. The model is modeled by a spring-mass system including a spring and a damper as delay elements, and having a length of the spring as an amount indicating the concentration of the specific component on the downstream side of the catalyst device. The air-fuel ratio control device for an internal combustion engine according to claim 4, wherein. 6. The adaptive sliding mode control means includes a plurality of states of the exhaust system including at least the concentration of the specific component on the downstream side of the catalyst device detected by the second exhaust gas sensor and the degree of change in the concentration. And a predetermined linear function having the plurality of state quantities as variables are defined, and each state quantity is converged on the hyperplane represented by the linear function according to the reaching law and adaptive law of the adaptive sliding mode control. Non-linear input calculation means for obtaining the correction amount of the air-fuel ratio of the internal combustion engine, and the air-fuel ratio of the internal combustion engine so as to converge each state quantity on the hyperplane to the equilibrium point on the hyperplane. And an equivalent control input calculating means for calculating a correction amount of the internal combustion engine, and calculating a correction amount for correcting the air-fuel ratio of the internal combustion engine based on the sum of the correction amounts calculated by both calculating means Air-fuel ratio control system for an internal combustion engine according to any of claims 1 to 5, characterized. 7. A stability determination means for determining the stability of the adaptive sliding mode control means for calculating a correction amount for correcting the air-fuel ratio of the internal combustion engine based on the value of the linear function, and a determination result thereof. The air-fuel ratio control apparatus for an internal combustion engine according to claim 6, further comprising: a correction amount calculation limiting unit that limits the calculation of the correction amount according to the above. 8. The correction amount calculation limiting means sets the correction amount to a predetermined value when the stability determining means determines that the calculation of the correction amount by the adaptive sliding mode control means is unstable. The air-fuel ratio control device for an internal combustion engine according to claim 7, which is maintained. 9. The stability determining means is at least one of a magnitude of a value of the linear function, a magnitude of a changing speed of the value of the linear function, and a product of a value of the linear function and a changing speed thereof. 9. The air-fuel ratio control device for an internal combustion engine according to claim 7, wherein the stability of the calculation of the correction amount is determined by comparing one with a predetermined value. 10. The feedback control means comprises a controller of a recurrence type for obtaining a correction amount of a fuel injection amount to the internal combustion engine based on an output of the first exhaust gas sensor and the correction amount, The air-fuel ratio control device for an internal combustion engine according to any one of claims 1 to 9, wherein the fuel supply amount to the internal combustion engine is corrected and controlled by the correction amount obtained by the controller.