JP2003201894A - Air-fuel ratio control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform air-fuel ratio control with good accuracy in an air-fuel ratio control device of an internal combustion engine that performs air-fuel ratio feedback control, while a control gain is calculated by self-tuning. <P>SOLUTION: This air-fuel ratio control device includes a plant model identifying part 223 that identifies a plant model indicating a plant from a fuel injection valve 5 to an air-fuel ratio sensor 13, a stability evaluating part 224 that determines whether an estimated parameter is within the predetermined stability area, a control gain calculating part 225 that calculates a control gain of the feedback control of the fuel injection quantity based on an output of the stability evaluating part 224, and an S/M controlling part 221 that calculates the feedback control quantity using the calculated control gain. The stability evaluating part 224 sets an area, in which another area in which theoretical control is stabilized, has been scaled down at a predetermined rate, as the stabilized area. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for an internal combustion engine, and more particularly to an air-fuel ratio control device for an internal combustion engine that performs air-fuel ratio feedback control while calculating a control gain by self-tuning.

[0002]

2. Description of the Related Art Conventionally, in an internal combustion engine, it is general to feedback control an air-fuel ratio to a target value for the purpose of exhaust gas purification, fuel efficiency improvement, and the like. As a technique for performing such air-fuel ratio feedback control with high accuracy, the applicant of the present application has disclosed that in the previous application (Japanese Patent Application No. 2001-79272), an air-fuel ratio control of an internal combustion engine for calculating a feedback control amount of a fuel injection amount by sliding mode control. It has been proposed that the device is configured to calculate the control gain of the sliding mode control by self-tuning control while performing dead time compensation control by the Smith method.

In such an air-fuel ratio control device, the feedback control amount is calculated as follows. First, the plant between the fuel injection means and the air-fuel ratio detection means is represented by a transfer function as a plant model, and each parameter of this plant model is identified by sequentially estimating it based on the fuel injection amount and the actual air-fuel ratio.

Next, using the parameters of the estimated plant model, the entire system including the plant, the feedback control amount calculation unit (that is, the sliding mode control unit) and the dead time compensation control unit is expressed by one transfer function, The control gain of the sliding mode control is calculated so that the pole matches the desired pole in terms of responsiveness, overshoot, settling time, and the like.

Then, the feedback control amount of the fuel injection amount is calculated by the sliding mode control using the calculated control gain. As a result, the air-fuel ratio control that accurately responds to changes in plant characteristics is executed.

[0006]

By the way, in the calculation of the control gain, in order to prevent the divergence of the control and guarantee the convergence, the stability of whether or not the estimated parameters of the plant model satisfy the stability condition. It is usual to make an evaluation. That is, when the pole of the identified plant model (transfer function) is Zp, | Zp | <1 is a condition for stability, and the estimated parameter is within this theoretical stability region. Stability is evaluated by whether or not there is, and when it is not in the stable region,
The control gain is not calculated using the estimated parameters.

However, since there are many unstable factors in actual control, the control may diverge even when the estimated parameter is within the theoretical stable region. Here, it is possible to prevent the divergence of control by setting a narrow stable region in consideration of the most unstable state of the plant, but in this case, the self-tuning control is used to calculate the control gain. There is a problem that feedback control cannot be effectively implemented.

The present invention has been made in view of the above problems, and when the control gain is calculated by the self-tuning control to perform the air-fuel ratio feedback control, the divergence of the control is surely prevented and the accuracy of the characteristic change is improved. An object of the present invention is to provide an air-fuel ratio control device for an internal combustion engine, which can execute well-suited control.

[0009]

Therefore, an air-fuel ratio control system for an internal combustion engine according to the present invention detects an actual air-fuel ratio by means of an air-fuel ratio detecting means and feedback-controls the air-fuel ratio to a target air-fuel ratio. A fuel ratio control device, an identification means for estimating parameters of a plant model representing a plant between the fuel injection means and the air-fuel ratio detection means based on the fuel injection amount and the detected actual air-fuel ratio, and the identification means It is determined whether the estimated parameter of the estimated plant model is within the set stable region, and when the stable parameter is within the stable region, the estimated parameter is output as it is, while when it is not within the stable region, the preset parameter is set in advance. A determination unit that outputs a predetermined value, and a control gain that calculates a control gain for calculating a feedback control amount of the fuel injection amount based on the output of the determination unit. And a control amount calculation unit that calculates the feedback control amount by using the control gain calculated by the control gain calculation unit, and the determination unit is theoretically a region in which the control is in a stable state. It is characterized in that the area reduced by a predetermined ratio is set as the stable area.

According to a second aspect of the present invention, the judging means comprises:
When the estimated parameter is not within the set stable region, the initial value set according to the engine operating state is output. The invention according to claim 3 is characterized in that the determination means sets the predetermined ratio according to an operating state of the engine.

According to a fourth aspect of the present invention, the judging means comprises:
The smaller the engine intake air amount, the smaller the stable region is set. The invention according to claim 5 is characterized in that the determination means sets the stable region to be smaller as the engine cooling water temperature is lower. The invention according to claim 6 is characterized in that the identifying means estimates the parameters of the plant model by using a recursive least squares method.

The invention according to claim 7 is characterized in that the control amount calculating means comprises dead time compensating means for eliminating the influence of dead time included in the plant by using the plant model. The invention according to claim 8 is characterized in that the control amount calculation means calculates the feedback control amount by sliding mode control.

[0013]

According to the invention of claim 1, the parameters of the plant model representing the plant from the fuel injection means to the air-fuel ratio detection means are estimated, and the stability of the control is evaluated using the estimated parameters. To do. As a result of the evaluation, if the estimated parameter is within the set stable region, the estimated parameter is output as it is, and the control gain is calculated based on the estimated parameter. On the other hand, if the estimated parameter is not within the set stable region, a preset predetermined value is output and the control gain is calculated based on the preset value. Then, the feedback control amount of the fuel injection means is calculated using the control gain calculated in this way. Here, as the stable region to be set when evaluating the stability of the control, a region in which the control is theoretically in a stable state is reduced by a predetermined ratio is set, so that the control gain is prevented from becoming excessive. The divergence of control can be reliably prevented.

According to the second aspect of the present invention, when the estimated parameter is not within the set stable region and the control may diverge, the control is performed using the parameter initial value preset according to the operating state. Since the gain is calculated, it is possible to prevent divergence of control and calculate a stable feedback control amount. According to the invention of claim 3, the ratio of theoretically reducing the region in which the control is in a stable state is set according to the operating state of the engine. Therefore, the estimation error of the parameter caused by the dead time of the plant or the fluctuation of the plant state is set. A stable area corresponding to (identification error) can be set. As a result, in an operating state in which the estimation (identification) error is large, the stable region is further narrowed to reliably prevent control divergence, and in an operating state in which the estimation (identification) error is small, control divergence is prevented. While maximizing the responsiveness limit.

According to the fourth aspect of the present invention, the smaller the intake air amount and the longer the dead time (transportation delay) of the plant, the smaller the stability region used for the stability evaluation of the control. , The divergence of control caused by the increase of the parameter estimation error (identification error) can be reliably prevented. According to the invention of claim 5, when the temperature is low, the state of the plant is unstable and the parameter estimation error (identification error) becomes large. Therefore, the lower the cooling water temperature, the more stable the region used for the stability evaluation of the control becomes. Set smaller. As a result, it is possible to reliably prevent the divergence of control caused by an increase in the parameter estimation error (identification error).

According to the invention of claim 6, by using the recursive least squares method (RLS method), the plant state prediction (identification of the plant model) can be performed accurately and easily. According to the invention of claim 7, the feedback control amount can be accurately calculated while compensating for the influence of the dead time element included in the plant. As a result, it is possible to prevent the control from rising slowly and prevent overshoot, thereby ensuring good control.

According to the invention of claim 8, since the feedback control amount of the fuel injection amount is calculated by the sliding mode control, it is possible to perform the air-fuel ratio feedback control excellent in robustness while suppressing the influence of disturbance. it can.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a system diagram of an engine showing an embodiment of the present invention. As shown in FIG. 1, the intake passage 2 of the engine 1 is provided with an air flow meter 3 for detecting the intake air amount Qa and a throttle valve 4 for controlling the intake air amount Qa.

Further, the fuel injection valve 5 provided in the intake passage 2 is opened and driven by an injection signal from a control unit (C / U) 6 having a built-in microcomputer to inject fuel. A spark plug 8 for performing spark ignition in the combustion chamber 7 is provided in each cylinder, and the air-fuel mixture sucked through the intake valve 9 is ignited by spark ignition.

The combustion exhaust gas is discharged to the exhaust passage 11 via the exhaust valve 10 and discharged to the atmosphere via the exhaust purification device 12. A wide-range air-fuel ratio sensor 13 that linearly detects the air-fuel ratio according to the oxygen concentration in the exhaust is provided in the exhaust passage 11 on the upstream side of the exhaust purification device 12.

Further, a crank angle sensor 14 that outputs a crank angle signal for each predetermined crank angle of the engine 1 and a water temperature sensor 15 that detects the cooling water temperature Tw in the cooling jacket of the engine 1 are provided. The control unit (C / U) 6 controls the fuel injection valve 5 as follows.

First, the engine speed Ne detected based on the intake air amount Qa and the signal from the crank angle sensor 14
To stoichiometric (λ = 1) equivalent basic fuel injection amount Tp = K
× Qa × Ne (K is a constant) is calculated. Then, depending on the operating state, it is determined whether the air-fuel ratio is to be feedback-controlled or open-loop controlled, and when the feedback control is performed, the basic fuel injection amount Tp, the target air-fuel ratio λt and the detection signal of the air-fuel ratio sensor 13 are detected. The final fuel injection amount Ti = Tp × (1 / λt) × α is calculated using the air-fuel ratio feedback correction coefficient α calculated based on In the case of open loop control, the air-fuel ratio feedback correction coefficient α is set to 1 (α = 1), and the final fuel injection amount T
i = Tp × (1 / λt) is calculated.

Here, the fuel injection control in this embodiment will be described. As shown in FIG. 2, the fuel injection control unit in the present embodiment includes an output determination unit 21 that determines the output to the fuel injection valve 5 and an air-fuel ratio feedback control unit 22 shown by a broken line in the figure. Has been done. The output determination unit 21 determines whether to output the feedback control amount calculated by the air-fuel ratio feedback control unit 22 to the fuel injection valve 5 according to the operating state.

When the feedback control amount is not output, a clamp value (α = 1 during open loop control and α = 0 during fuel cut) is output as the control amount. The air-fuel ratio feedback control unit 22 is, as shown in the figure, a sliding mode control unit (S / M control unit).
221, a dead time compensator 222, a plant model identification unit 223, a stability evaluation unit 224, a control gain calculation unit 225, and a dead time calculation unit 226 are included.

The S / M control unit 221 controls the target air-fuel ratio λ
On the basis of the deviation between t and the actual air-fuel ratio λt, the air-fuel ratio feedback correction coefficient α is calculated by sliding mode (S / M) control, and the calculated value is sent to the plant (that is, between the fuel injection valve 5 and the air-fuel ratio sensor 13). The control amount u (t), that is, the feedback control amount of the fuel injection valve 5 is calculated by the following equation (1).

[0026]

[Equation 1]

However, e (t) is sent to the S / M control unit 221.
Input, K_PIs the linear term linear gain, K_DIs a linear term differential gay
N, S_PIs the switching function linear gain, S_DIs the switching function differential gay
N, K _IIs the adaptive law gain, K_NIs the nonlinear gain, and σ (t) is
Switching function, σ (t) = S_pe (t) + S_De (t).
In addition, each of the control gains described above is a control gain calculation unit described later.
225 is calculated.

The dead time compensator 222 is a local feedback type dead time compensator for executing dead time compensation control by the Smith method, and is a dead time (that is, a phase delay of the detected air-fuel ratio) included in the plant. To compensate for the effects of. Specifically, as shown in FIG. 3, the dead time compensator 222 is a plant model 3 that does not include dead time.
1, the plant model 32 including the dead time, and the subtraction unit 3
The output calculated by the plant model 31 that does not include the dead time element (air-fuel ratio)
A deviation e2 between the prediction and the actual output (actual air-fuel ratio) prediction calculated by the plant model 32 including the dead time is calculated,
This is output to the input side of the S / M control unit 21.

Then, the output e2 of the dead time compensator 22 is subtracted from the deviation e1 between the target air-fuel ratio λt and the actual air-fuel ratio λr to calculate e3, and the e3 is calculated by the S / M control section 221.
I am trying to type in. The plant model used in the dead time compensation control is identified by the plant model identification unit 223 described later, and the dead time is calculated by the dead time calculation unit 226 described later.

The plant model identification unit 223 identifies a plant model representing the plant by a transfer function on-line based on the fuel injection amount (input) and the actual air-fuel ratio (output). Specifically, the recursive least squares method (RLS
Method) is used to sequentially estimate the parameters of the plant model (details will be described later). The stability evaluation unit 224
It is determined whether or not the parameter (estimated parameter) estimated by the plant model identification unit 223 is within the set stable region. This is performed in order to prevent divergence of control and guarantee convergence, and when the estimated parameter is within the set stable region, it is output as it is, while when it is not within the stable region, it is output in advance. The set predetermined value is output (details will be described later).

The control gain calculation section 225 uses the S /
The control gain of the M control unit 221 is set to the stability evaluation unit 22.
It is calculated based on the output from 4 (estimated parameter or predetermined value). Specifically, the self-tuning control by the pole placement method is used, and the entire system (that is, the plant (between the fuel injection valve 5 and the air-fuel ratio sensor 13) +
The S / M control unit 221 + dead time compensator 222) is represented by a closed loop transfer function, and the control gain of the S / M control unit 221 is set so that its pole coincides with a desired pole in terms of responsiveness, overshoot amount, settling time, and the like. Calculate (details will be described later).

The dead time calculating section 226 calculates the dead time k of the plant (hereinafter referred to as the normal dead time). This calculates the time (delay time) until the fuel injected from the fuel injection valve 5 burns in the combustion chamber 7 and the air-fuel ratio of the combustion exhaust gas is detected by the air-fuel ratio sensor 13. In the present embodiment, as shown in FIG. 4, the relationship between the intake air amount Qa and the dead time k is tabulated in advance, and the dead time k is calculated by searching the table based on the detected intake air amount Qa. To calculate.

Here, the plant model identification unit 223
Identification of the plant model by the stability evaluation unit 224
The stability evaluation of the control by and the calculation of the control gain by the control gain calculation unit 225 will be described in detail. First, a plant model G that represents a plant by a transfer function
Set _P (z ^-1 ) and estimate the parameters of the plant model. Then, the stability of the control is evaluated by the estimated parameters. On the other hand, the transfer function G _C (z ⁻¹ ) of the S / M control unit 221 and the transfer function G _L (z of the dead time compensator 22.
^-1 ) and obtain the transfer function including the plant model as 1
Closed loop transfer function W of the entire system
(Z ^-1 ) is calculated, and the control gain is calculated so that the pole becomes the set pole.

(A) Setting of plant model The plant between the fuel injection valve 5 and the air-fuel ratio sensor 13 is
Dead time k calculated by the dead time calculation unit 225 (≧
Using 1), for example, it is represented by a quadratic ARX model A (z ⁻¹ ) as in the following equations (2) and (3). A (z ⁻¹ ) y (t) = z ^−k b ₀ u (t) + ε (t) (2) A (z ⁻¹ ) = 1 + a ₁ z ⁻¹ + a ₂ z ⁻² (3) However, , Y (t) is the plant output (that is, the actual air-fuel ratio), u (t) is the plant input value (that is, the fuel injection amount), and ε (t) is the random noise.

Then, the transfer function of the plant model G _P (z
^-1 ) can be expressed as the following equation (4). G _P (z ⁻¹ ) = z ^−k b ₀ / A (z ⁻¹ ) ... (4) The estimated parameter vector θ (t) and the data vector ψ (t−k) are the following (5), ( It can be expressed as in equation 6).

Θ (t) = [a ₁ (t), a ₂ (t), b ₀ (t)] ^T (5) ψ (t-k) = [-y (t-1), -y (t−2), u (t−k)] ^T (6) (B) Identification of plant model (parameter estimation) The plant model set is identified by the plant model identification unit 223. Specifically, the characteristics of the plant are
Since it changes depending on the plant characteristics such as the operating state and the degree of deterioration of the plant itself, the parameter a shown in equation (5)
_A plant model is identified (that is, online identification is performed) by sequentially estimating ₁ (t), a ₂ (t), and b ₀ (t).

In the present embodiment, the least squares method (RLS method) is used for the parameter estimation, and the parameter that minimizes the square of the error between the actual value and the estimated value is sequentially calculated. The specific arithmetic expression is the same as the general weighted recursive least squares method (RLS method). For time update expressions: t = 1, 2, ..., N, the following expressions (7) to
This is done by calculating (9).

[0038]

[Equation 2]

The forgetting factors λ ₁ and λ ₂ are the forgetting factors λ ₁ = λ ₂ = 1 when there is no forgetting element, and λ ₁ = 0.98 and λ ₂ = when the forgetting element is included. It was set to 1. Further, in the present embodiment, the initial value θ0 of each parameter is set in advance according to the operating state,
The time until convergence is shortened (parameter initial values a ₁ (0) = A1, a ₂ (0) = A2, b ₀ (0) = B0).

(C) Evaluation of Stability of Estimated Parameter The parameter estimated by the plant model identification unit 223 is stored in the stability evaluation unit 2 in order to prevent control divergence.
It is determined whether or not it is within the stable region set in 24.
The stable region is set as follows. In this embodiment, as described above, the plant model is identified so that the output error is minimized. In this way, system identification that minimizes the output error is equivalent to adaptive digital filtering (ADF) using an IIR (Infinite Impulse Response) filter as a digital filter. Therefore, the stability condition of the second-order IIR filter is set to the plant model. It is applied to the plant model identified by the identification unit 223.

First, assuming that the pole of the transfer function G _P (z ^-1 ) of the plant model shown in equation (4) is Zp, the following equation (10) is a condition for stability. │Zp│ <1 (10) Then, the pole of the transfer function G _P (z ^-1 ) of the plant model is
From the characteristic polynomial, it is given by the root of the quadratic equation shown in the following equation (11).

F (z) = Z ² + a ₁ Z ¹ + a ₂ = 0 (11) Here, when Expression (11) has a real root, a ₁ ≧ 4a ₂ and at this time, two roots are , -1 to -1. Therefore, the condition is as follows (equation (12)). f (-1) = 1-a 1 + a 2> 0 f (1) = 1 + a 1 + a 2> 0 Therefore, 1> │a1│-a2 ... ( 12) On the other hand, if the expression (11) has a complex roots Is a ₁ <4a ₂ , and the absolute value of the root is given by the following expression (13).

[0043]

[Equation 3]

Therefore, the condition for the absolute value of the root to become smaller than 1 is given by the following equation (14). 1> a ₂ > 0 (14) Therefore, from the above equations (12) and (14), the parameter a _{1 of the} plant model that can ensure the stability of the control,
It is necessary that a _{2 be} in the range indicated by the broken line in FIG. This is a region where control theoretically becomes stable (hereinafter,
The first stable region).

However, in the actual machine, the control may diverge due to the influence of the identification error even if the estimated parameter value is within the first stable region. Therefore, in the present embodiment, a region in which the first stable region is narrowed by a margin amount (reduced by a predetermined ratio) is set as a second stable region, and stability evaluation is performed using the second stable region. I'm going to do it.

However, since the magnitude of the identification error is affected by the operating state of the engine (more specifically, when the dead time of the plant is long or when the state of the plant is unstable), the above-mentioned second When setting the stable region, the ratio of reducing (narrowing) the first stable region is changed according to the operating state of the engine. For example, in the medium to high speed region, the margin time may be small because it is considered that the dead time of the plant is short and the identification error is small.
As shown by the solid line in FIG. 5, a stable region having a size of 90% with respect to the first stable region (stable region A in the figure)
To set. This is to minimize the rate of shrinking the first stable region when setting the second stable region,
The response limit is made as large as possible.

On the other hand, in the low speed region including the idling time, the dead time of the plant becomes long and the identification error is considered to become large, so that the margin margin becomes large, and as shown by the one-dot chain line in FIG. A stable area (stable area B in the figure) having a size of 64% with respect to the stable area 1 is set. This is to make the stable region narrower in order to surely prevent the divergence of the control.

As described above, the second stable region used for stability evaluation is set according to the operating state, and the plant model identifying unit 223 is used by using each stable region (ie, stable region A or B). The stability of the estimated parameters is evaluated. Then, the plant model identification unit 223
If the estimated parameters a ₁ (t) and a ₂ (t) input from the above are within the second stable region (stable region A or B) set according to the operating state, the estimated parameter a ₁ (t) , A ₂ (t) are directly output to the control gain calculator 225 and the dead time compensator 222, and if they are not within the stable region, an initial value a ₁ (0) = A 1, a preset according to the operating state is output.
₂ (0) = A2 is output to the control gain calculation unit 225.

The above control is shown in the flowchart of FIG. In FIG. 6, in step 1 (denoted as S1 in the figure; the same applies hereinafter), the plant model identification unit 223 estimates the parameters of the plant model. In step 2, the intake air amount Qa and the cooling water temperature Tw are read. Step 3
Then, the read intake air amount Qa is compared with a preset predetermined value Q0. If Qa ≧ Q0, step 4
If Qa <Q0, the process proceeds to step 6 to set the stable region B.

In step 4, the cooling water temperature T read
The w is compared with a predetermined temperature T0 set in advance. T
If w ≧ T0, the process proceeds to step 5 to set the stable region A, and if Tw <T0, the process proceeds to step 6 to set the stable region B. Then, in step 7, the parameters a ₁ (t), a ₂ estimated in step 1 are used by using the stable region (A or B) set in step 5 or step 6.
It is determined whether (t) is within the stable region. If it is within the set stable region, the process proceeds to step 8, and the estimated parameters a ₁ (t) and a ₂ (t) are output to the output gain calculation unit 225 as they are. On the other hand, if it is not within the set stable region, the process proceeds to step 9, and the output gain calculation unit 225 sets the initial values A1 and A2 set in advance according to the operating state.
Output to.

As described above, by setting the second stable region used for stability evaluation in accordance with the operating state, it is possible to surely prevent the divergence of the control and set the responsiveness limit to a larger value. In the above description, the intake air amount Qa and the cooling water temperature Tw are used as the values representing the operating state, but any operating state in which the dead time of the plant becomes long may be detected, for example, an engine. The rotation speed Ne or the vehicle speed Vsp may be used.

Further, as the second stable region used for the stability evaluation, the regions of 90% and 64% of the first stable region in which the control theoretically becomes stable are used, but the present invention is not limited to this. Instead of this, for example, the second stable region may be 95%, 80%, etc. of the first operating region, and the reduction ratio may be continuously changed according to the operating condition. You may comprise.

Further, for example, as shown in FIGS. 7A and 7B, a limit value La ₂ is set for the parameter a ₂ with respect to the first stable region, and a reference stable region (solid line in the figure) is provided. Is set) and the stable region of this reference is reduced according to the operating state to set the stable region (stable region C, shown by a dashed line in the figure) to be used for the stability evaluation.
A limit value Lb ₀ may also be set for the parameter b ₀ .

In this case, the estimated parameter a
_{When 1} (t) and a ₂ (t) are not within the stable region C and the estimated parameter b ₀ (t) exceeds the limit value Lb, the control gain calculation unit 225 includes the parameter initial value. (A1, A2, B0) will be output. (D) Calculation of discrete-time transfer function of S / M control unit 221 The S / M control unit 221 is converted into a transfer function as follows.

Y (t) is the plant output value (actual air-fuel ratio λ
r) and ω (t) are set as target values (target air-fuel ratio λt), and e (t)
= Ω (t) -y (t), the plant input (that is, the output from the S / M control unit 221) in one sample u
The difference Δu (t) of (t) is given by the following equation (15).

[0056]

[Equation 4]

Here, e (t) = ω (t) -y (t), e (t)
Since −e (t−1) = Δe (t), the following expression (16) is obtained from the expression (15).

[0058]

[Equation 5]

However, K (z ^-1 ) is expressed by the following equation (17), which is expanded as the equation (18) and calculated based on each control gain.

[0060]

[Equation 6]

Therefore, from the equation (17), the plant input u
(t) is expressed by the following equation (19).

[0062]

[Equation 7]

If the non-linear term is not included, the discrete-time transfer function G _C (z ^-1 ) of the S / M control unit 221 can be expressed by the following equation (20). . G _C (z ⁻¹ ) = K (z ⁻¹ ) / (1-z ⁻¹ ) ... (20) (E) As described above with respect to the calculation of the discrete time transfer function of the dead time compensator 222, the dead time is Since the compensator 222 uses the Smith method for compensating the influence of the dead time element while performing the output prediction after the dead time, the discrete time transfer function G _L (z ⁻¹ ) of the dead time compensator 222 is represented by the following equation ( 21) can be calculated.

G _L (z ⁻¹ ) = z ⁻¹ b ₀ / A (z ⁻¹ ) − z ^−k b ₀ / A (z ⁻¹ ) = (z ⁻¹ −z ^−k ) b ₀ / A (z ⁻¹ ) ... (21) Note that z ⁻¹ b ₀ / A (z ⁻¹ ) is the output prediction when there is no dead time expressed using the plant model, and z ^−k
b ₀ / A (z ⁻¹ ) is an actual output prediction including the dead time similarly expressed using the plant model.

FIG. 8 shows a block diagram using the transfer functions (plant model, S / M control unit 21, dead time compensator) calculated as described above. Next, the closed-loop transfer function conversion of the entire system will be described. Note that the nonlinear term of the S / M control unit 221 is not included as described above. (F) Calculation of Closed Loop Transfer Function W (z ⁻¹ ) of Entire System First, the S / M control unit 221 and the dead time compensator 222 are described.
Is taken out, and one transfer function from the target (target air-fuel ratio λt) to the output (feedback control amount) is calculated. In FIG. 5, the S / M control unit 221
Transfer function G of local loop including dead time compensator 22
_CL (z ^-1 ) can be calculated from the equations (20) and (21) as in the following equation (22).

[0066]

[Equation 8]

Therefore, the closed loop transfer function W of the entire system including the plant and the local loop shown in the equation (22).
(z ^-1 ) can be calculated by the following equation (23).

[0068]

[Equation 9]

FIG. 9 is a block diagram showing the entire air-fuel ratio feedback control in this embodiment based on the above calculation results. (G) Calculation of the control gain of the S / M control unit 221 by the pole placement method The characteristic polynomial of the closed loop transfer function W (z ⁻¹ ) is expressed by Equation (2)
From (3), (1-z ^-1 ) A (z ^-1 ) + z ^-1 b ₀ K (z ^-1 ), which is given by the following equation (24).

[0070]

[Equation 10]

At this time, the control gain of the S / M control section 221 is set as follows by setting T (z ⁻¹ ) which is a desirable pole in terms of responsiveness, overshoot, settling time, etc. To calculate. From equation (24), the following equation (25) is obtained.

[0072]

[Equation 11]

From the equation (18), K (z ^-1 ) = (K _P
＋ K _I・ S _P ＋ K _I・ S _D + K _D )-(K _P ＋ K _I・ S _D ＋ 2K _D )
Since z ⁻¹ + K _D z ⁻² , the switching function linear gain S _P and the switching function differential gain S _D are set to 1, and the linear term linear gain K _P , the adaptive law gain K _I , and the linear term differential gain K _{D are set.} If is a variable parameter, it can be expressed by the following equation (26).

[0074]

[Equation 12]

Then, the following equations (27) to (29) are obtained.

[0076]

[Equation 13]

Therefore, equations (27) to (29) are converted into K _P ,
Each gain can be calculated by solving for K _I and K _D , and representing a ₁ , a ₂ , and b ₀ by the value output from the stability evaluation unit 224 (that is, the estimated parameter or the parameter initial value). . The characteristic polynomial T
As (z ⁻¹ ) = 1 + t ₁ z ⁻¹ + t ₂ z ⁻² , for example, a quadratic continuous-time system when damping ζ = 0.7 and natural angular frequency ω = 30, G (s) It is conceivable to use the denominator of the transfer function when == ω ² / (s ² + 2ζω · s + ω ² ) is discretized at the sample time T _i .

Then, using the control gain calculated in this way, the S / M control section 221 calculates the control amount for the plant. As described above, the entire system is represented by one transfer function using the plant model in which the parameters are sequentially estimated, and the plant is arranged so that its poles coincide with desirable poles in terms of response, overshoot, settling time, etc. Since the control gain of the S / M control unit 221 that calculates the feedback control amount is calculated, a good control gain corresponding to the change in the characteristics of the plant can be calculated, and thus accurate air-fuel ratio feedback control can be executed.

Further, as a feature of this embodiment, the stability evaluation section 224 sets different stable regions according to the operating state, so that the divergence of the control is surely prevented and the stability is secured. At the same time, the responsiveness limit can be maximized.

[Brief description of drawings]

FIG. 1 is a system diagram of an internal combustion engine showing an embodiment of the present invention.

FIG. 2 is a block diagram showing air-fuel ratio control according to the present invention.

FIG. 3 is a block diagram showing the dead time compensation control.

FIG. 4 is a diagram showing a table for similarly calculating dead time.

FIG. 5 is a diagram similarly showing an example of a parameter stable region.

FIG. 6 is a flow chart showing a parameter stability evaluation.

FIG. 7 is a diagram showing another example of the parameter stable region.

FIG. 8 is a block diagram showing a transfer function of an S / M control unit 221 and a dead time compensator 222 according to the present invention.

FIG. 9 is a block diagram showing the entire air-fuel ratio feedback control by sliding mode control using self-tuning control according to the present invention.

[Explanation of symbols]

1 engine 2 Intake passage 3 Air flow meter 4 Throttle valve 5 Fuel injection valve 6 Control unit (C / U) 8 spark plugs 11 exhaust passage 13 A / F sensor 14 Crank angle sensor 15 Water temperature sensor

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G05B 13/02 G05B 13/02 D 13/04 13/04 F term (reference) 3G084 BA09 BA13 DA25 EA11 EB08 EB11 EB13 EB16 EC04 FA07 FA20 FA29 FA33 3G301 JA11 LA01 MA01 NA08 NA09 NB02 NC02 ND01 ND05 ND06 ND15 NE16 PA01Z PB03Z PD04Z PE01Z PE08Z PF01Z 5H004 GA10 GA14 GB12 HA13 HB04 KA61 KA74 KB38C54 K10 K45 KC35 KC39 KC35 KC35 KC39 KC35 KC35 KC35 KC39

Claims (8)

[Claims] 1. An actual air-fuel ratio is detected by an air-fuel ratio detecting means,
An air-fuel ratio control device for an internal combustion engine that feedback-controls an air-fuel ratio to a target air-fuel ratio, and represents a plant between the fuel injection means and the air-fuel ratio detection means based on the fuel injection amount and the detected actual air-fuel ratio. Identification means for estimating the parameters of the plant model, and whether the estimated parameters estimated by the identification means are within the set stable region, and when they are within the stable region, the estimated parameters are output as they are. , A determination means for outputting a predetermined value set in advance when not within the stable region, and a control gain calculation for calculating a control gain for calculating the feedback control amount of the fuel injection amount based on the output of the determination means Means, and control amount calculation means for calculating the feedback control amount using the control gain calculated by the control gain calculation means,
The air-fuel ratio control device for an internal combustion engine, wherein the determining means sets a region obtained by theoretically reducing a region in which the control is in a stable state at a predetermined ratio as the stable region. 2. The determining means outputs an initial value set according to an engine operating state when the estimated parameter is not within a set stable region.
An air-fuel ratio control device for an internal combustion engine as described. 3. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the determining means sets the predetermined ratio according to an operating state of the engine. 4. The air-fuel ratio control device for an internal combustion engine according to claim 3, wherein the determining means sets the stability region to be smaller as the engine intake air amount is smaller. 5. The air-fuel ratio control device for an internal combustion engine according to claim 3, wherein the determining means sets the stable region to be smaller as the engine cooling water temperature is lower. 6. The air-fuel ratio of an internal combustion engine according to claim 1, wherein the identifying means estimates the parameters of the plant model by using a recursive least squares method. Control device. 7. The control amount calculation means comprises dead time compensation means for eliminating the influence of dead time included in the plant by using the plant model.
7. The air-fuel ratio control device for an internal combustion engine according to claim 6. 8. The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the control amount calculation means calculates the feedback control amount by sliding mode control. .