JP2000016268A - Method for controlling wheel speed of vehicle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control wheel speed to an optimum value irrespective of the friction coefficient of a road surface. SOLUTION: An error err between desired slip velocity Vslt and actual slip velocity Vsl and its integrated value (z) are computed (S20) and an equivalent control input ueq is computed on the basis of the error err of the slip velocity and the actual slip velocity Vsl (S30) while a switching function σ is computed on the basis of the integrated value (z) (S40). Whether or not the sign of the switching function σ has changed within a time T1 is determined (S50), and if the result is negative, a tire generated force (f) (kb) is computed on the basis of slip rate kb to correct parameters θand β (S60), whereas if the result is affirmative, the parameters θ and β are not corrected (S70). Further, a nonlinear input unl is computed (S80) and a control input (u) is computed as the sum of the equivalent control input ueq and the nonlinear input unl (S90) and braking torque tb is controlled to equal the control input (u) (S100).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling a wheel speed of a vehicle, and more particularly, to a vehicle wheel speed control for applying a sliding mode control so that a wheel slip speed becomes a given target slip speed. According to the method.

[0002]

2. Description of the Related Art As a control law of a wheel speed in consideration of the non-linearity of the movement of a tire or a vehicle, a control to which a sliding mode control is applied is already known.
Japanese Patent No. 267737 discloses an ABS control to which a sliding mode control is applied, in which a switching braking torque calculated from a switching function and an equivalent braking torque calculated from a wheel load, a vehicle body acceleration, a vehicle speed, and a wheel speed. A wheel speed control method is described in which a target braking torque of a wheel is calculated as the sum of the above, and a braking force is controlled such that the braking torque of the wheel becomes the target braking torque.

According to this wheel speed control method, the switching braking torque is calculated from the switching function, and the equivalent braking torque is calculated from the wheel load, the vehicle body acceleration, the vehicle speed, and the wheel speed. The target braking torque of the wheel is calculated as the sum of the above, and the braking force is controlled so that the braking torque of the wheel becomes the target braking torque, so that the braking distance of the vehicle is reduced and the behavior of the vehicle during braking is stabilized. The vehicle can be stopped smoothly.

[0004]

However, in the above-described conventional wheel speed control method to which the sliding mode control is applied, a desired friction coefficient is high when the road surface has a high friction coefficient and the tire can generate a high force. In order to obtain braking characteristics, it is necessary to increase the gain of the controller.However, if the gain of the controller is set to a high value, the control input becomes excessive when the friction coefficient of the road surface is low and the force that can generate the tire is low. Therefore, there is a problem that a desired braking force cannot be obtained, the rotational motion of the wheels becomes oscillatory, and sufficient wheel speed control performance cannot be obtained.

The present invention has been made in view of the above-mentioned problems in the conventional wheel speed control method to which the sliding mode control is applied. The switching function is composed of the term of the integral value of the deviation from the target slip velocity and the term of the actual slip velocity, and the control input is composed as the sum of the equivalent control input term and the nonlinear input term. In other words, the wheel speed is optimally controlled regardless of the friction coefficient of the road surface.

[0006]

According to the present invention, there is provided a vehicle for which a sliding mode control is applied so that the slip speed of a wheel becomes a given target slip speed. In the wheel speed control method, the switching function is composed of the term of the integral value of the deviation between the actual slip speed of the wheel and the target slip speed and the term of the actual slip speed, and the control input is nonlinear with the term of the equivalent control input. A first parameter for calculating the force generated by the tire based on the tire model, and a second parameter representing a non-linear element other than the first parameter. The first and second parameters are modified in accordance with the value of the switching function, thereby achieving the vehicle wheel speed control method.

According to the first aspect of the present invention, the switching function includes the term of the integral value of the deviation between the actual slip speed of the wheel and the target slip speed and the term of the actual slip speed, and the control input is an equivalent control input. And a non-linear input term, wherein the non-linear input term represents a first parameter for calculating the force generated by the tire based on the tire model and a non-linear element other than the first parameter. It is configured based on the second parameter, and the first and second parameters are modified according to the value of the switching function, so that the possibility of excessive control input regardless of the road surface friction coefficient is reduced, Further, the error between the tire model and the actual vehicle is reduced.

According to the present invention, in order to effectively attain the above-mentioned main object, in the configuration of the first aspect, when the value of the switching function is oscillating across zero, the second It is configured to stop correcting the first and second parameters or to reduce the magnitude of the first and second parameters (configuration of claim 2).

In general, when the first and second parameters are excessively corrected, the rotational state of the wheel becomes rather oscillating. However, according to the configuration of the second aspect, the value of the switching function becomes zero. Since the correction of the first and second parameters is stopped or the magnitudes of the first and second parameters are reduced when vibrating while sandwiching, due to excessive correction of the parameters, This prevents the rotation state of the wheels from becoming vibrating.

[0010]

[Principle of the present invention] (1) Modeling The controlled object is modeled as follows. That is, the dynamic characteristics of the actuator system of the braking device are considered to be negligible, and the range from the wheel cylinder pressure to the wheel speed is considered as a single-injection model including two equations of motion relating to one wheel rotation and parallel movement on a spring as follows.

Ma Vxd = −Fx (1) It ωtd = Fx Rt−Tb (2)

In the above equations (1) and (2), Ma
Is the vehicle weight, Vxd is the differential value of the front-rear speed Vx of the vehicle body, Fx is the force that the tire receives from the road surface, It
Is the moment of inertia of the wheel, ωtd is the angular velocity of the wheel ω
t is the derivative of R, Rt is the effective radius of the tire,
b is the braking torque.

The force Fx applied to the tire from the road surface is generally obtained by the following equation (3), using Cx as the braking stiffness.

(Equation 2)

Here, for the sake of simplicity, it is assumed that Cxv varies according to the vehicle speed, as shown in the following equations (4) and (5).

Fx = Cxv (Vx-.omega.tRt) (4) Cxv = Cx / Vx (5)

If (Vx-ωt Rt) in the equation (4) is set as the slip velocity Vsl, the differential value Vsld of the slip velocity is expressed by the following equation (6) from the equations (1) to (5).

(Equation 4)

(2) Application to Servo A control system is designed using the theory of sliding mode control with respect to equation (6).

Here, it is assumed that Cxv is represented by a nominal value Cxv0 and a variation ΔCxv as shown in the following equation (7).

Cxv = Cxv0 + ΔCxv (7)

In order to simplify the notation, equation (6) is rewritten as equation (8) below. The non-linear part elements to be controlled are collected in h.

[0018]

Vsld = AVsl + Bu + Bh (8) where:

(Equation 7)

In order to construct a servo system, an integral value z of an error between the target slip speed Vslt and the actual slip speed Vsl is set.
Is introduced, and zd is a differential value of z, and the control target is placed as in the following equations (9) and (10).

Zd = Vslt-Vsl (9) Vsld = AVsl + Bu + Bh (10)

For equations (9) and (10), a sliding mode controller is designed.

(3) Design of Switching Surface First, a switching function σ is set as in the following equation.

(Equation 9)

When the sliding mode is generated, the following equation (12) is established.

Σ = s1 z + s2 Vsl = 0 (12)

Therefore, if s1 = -1 / Ts and s2 = 1, the following equation (13) is established, and the output Vs1 of the system follows the target value Vslt with a first-order delay of the time constant Ts. In this case, the differential value σd of σ is expressed by the following equation (14).

[0024]

[Equation 11]

(Equation 12)

(4) Controller Design The control input u is converted to an equivalent control input ueq as shown in the following equation (15).
And the non-linear input unl.

## EQU13 ## u = ueq + unl (15)

The non-linear input unl is an input for setting σ = 0, and since unl is 0 when the sliding mode is generated, the above equation (15) is expressed as the following equation (16).

U = ueq (16)

The equivalent control input ueq is obtained from the equations (14) and (16) as follows, assuming that h = 0.

(Equation 15)

Next, consider a non-linear input unl for generating a sliding mode. The present invention is to design a controller with a gain as low as possible in response to a change in the system such as a change in road surface conditions. The following adaptive mechanism is used to minimize the unl gain while maintaining stability. Think about suppressing it. First, the non-linear element h of the system is divided into a part (hp) that can be parameterized and a part (Δh) that cannot be parameterized as in the following equation (18).

H = hp + Δh (18)

As shown in the following equation (19), hp is formulated by a state quantity X (for example, Vslt, Vsl, Vx, ωt∈X) inside and outside the system, a function f at time t, and a parameter θ.

Hp = f (X, t) θ (19)

Further, it is assumed that Δh has an upper bound value function ρ as in the following equation (20) and can be defined by a parameter β.

(18) {Δh} ≦ ρ (X, t, β) (20)

The above assumptions do not make this system lose generality. Unl is given as in the following equation (21).

[Equation 19]

Θp and βp are adaptation parameters, and Γ1
, Γ2 correspond to the adaptive gain that determines the adaptive speed.

(5) Guarantee of safety It is shown that the sliding mode (σ → 0) is achieved by the control input represented by the equations (15), (17) and (21). The following equation (2) is used as a candidate for the Lyapunov function.
Consider V represented by 3).

(Equation 20)

The differential value Vd of V is represented by the following equation (24).

Vd = σB ⁻¹ σd + (θp−θ) Γ1 ⁻¹ θpd + (βp−β) Γ2 ⁻¹ βpd (24)

In the equation (24), the equations (14), (15), (1)
By substituting 7) and (21), Vd is given by the following equation (25).
It is represented as

(Equation 22)

Further, from h = hp + Δh, equation (1) is added to equation 25.
9), (20), and (22), the following equation (2) is obtained.
6) holds, and V is negatively determined with respect to σ, so σ becomes 0. Therefore, the output follows Vslt as described above in the section of “(3) Design of switching surface”.

(Equation 23)

(6) Modification of Adaptive Algorithm In the logic shown in the above equation (22), adaptive parameters are constantly updated when σ is other than 0. In particular, βp simply increases. If the response of the actuator is sufficiently fast relative to the behavior of the system, the state of the system is constrained to be very close to σ = 0, so that the adaptive parameters converge to a constant value, but the actuator is delayed, When the control input is saturated, the state of the system vibrates fluctuating with σ = 0, so that the adaptive parameters become vibratory or diverge. For the above reason, when σ becomes oscillating near 0, it is determined that the gain of the controller is sufficiently large, and the correction of the adaptive parameter is stopped or the absolute value of the adaptive parameter is reduced. Further, when the vibration continues, the absolute value of the adaptive parameter is reduced. That is, the adaptive logic represented by the equation (22) is changed to the following equations (27) and (28). Here, t0 is the time when σ finally becomes 0, tp is the time for stopping the adaptation logic, and Tp is the time constant for decreasing the adaptation parameter.

(Equation 24)

(7) Modification of sgn Function In order to reduce the chattering of the system due to the discontinuity of the above equation (21), the sgn function in the equation (21) is converted into a continuous equation represented by the following equation (30). Saturation function (sgnp (x))
To fix. The sgn function and the sgnp function are shown in the following equations (29) and (30), respectively.

(Equation 25)

[0039]

According to a preferred aspect of the present invention, in the above-mentioned structure, the target slip speed is determined by controlling the vehicle behavior by controlling the wheel braking force. The calculation is performed based on the target slip ratio or the target wheel speed (preferred mode 1).

According to another preferred aspect of the present invention, in the configuration of the second aspect, when the magnitudes of the first and second parameters are reduced, the magnitudes of the first and second parameters are reduced. (Preferred mode 2).

[0041]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a schematic diagram showing one embodiment of a vehicle braking force control apparatus to which a wheel speed control method according to the present invention is applied.

In FIG. 1, 10FL, 10FR, 10RL,
10RR indicates left and right front wheels and left and right rear wheels of the vehicle 12, respectively. Each wheel has a braking device 14FL, 14FR, 1
4RL and 14RR are provided. Each brake device 14FL-1
The 4RR controls the braking force of the corresponding wheel by controlling the braking hydraulic pressure by the hydraulic control circuit 16, and the hydraulic control circuit 16 is controlled by the electronic control device 18.

The electronic control unit 18 includes a ground speed sensor 20.
Signal indicating the ground vehicle speed Vx, the wheel speed sensor 22FL
Wheels 10FL, 10FR, 10RL, 10 from 22RR
A signal indicating the wheel speeds Vwfl, Vwfr, Vwrl, Vwrr of the RR, and the braking device 1 based on the hydraulic pressure sensors 24FL to 24RR, respectively.
4FL, 14FR, 14RL, 14RR braking oil pressure Pfl, Pfr,
Signals indicating Prl and Prr are input.

The electronic control unit 18 calculates a target wheel speed necessary for controlling the behavior of the vehicle, which is well known in the art, and calculates a target slip speed obtained from the target wheel speed according to the flowchart shown in FIG. The wheel speed is controlled by applying the sliding mode control to the speed.

In particular, the switching function σ is the actual slip speed V of the wheel.
The control input u is composed of the term of the integral value of the deviation between sl and the target slip velocity Vslt and the term of the actual slip velocity. The control input u is composed of the sum of the term of the equivalent control input ueq and the term of the nonlinear input unl. Is configured based on the first parameter θ corresponding to the product of the road surface friction coefficient and the wheel load and the second parameter β of the nonlinear element, and the first and second parameters are set to the value of the switching function. Will be modified accordingly. However, when the value of the switching function oscillates across 0, the first and second parameters are not corrected, and the magnitudes of these parameters are reduced as necessary.

The electronic control unit 18 includes, for example, a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), and an input / output port device, which are connected by a bidirectional common bus. It may be a microcomputer of a general configuration connected to each other.

Next, the operation of the illustrated embodiment will be described with reference to FIG. The control according to the flowchart shown in FIG. 2 is started when an ignition switch (not shown) is closed, and is repeatedly executed at predetermined time intervals for each wheel.

First, at step 10, a signal or the like indicating the ground vehicle speed Vx detected by the ground vehicle speed sensor 20 is read. At step 20, a behavior control routine not shown in the figure is used to calculate. Target wheel speed Vwti and actual wheel speed Vwi (i = fl, fr, rl, or rr)
And the target slip speed Vsl of the wheel based on the ground vehicle speed Vx
t and the actual slip speed Vsl are calculated, the error err between the target slip speed Vslt and the actual slip speed Vsl is calculated according to the following equation 31, and the integral value z of the error err is further calculated.

Err = Vslt−Vsl (31)

In step 30, based on the slip speed error err and the actual slip speed Vsl, the following equation 32 is used.
The equivalent control input ueq is calculated according to

Ueq = (It / Rt) {(1 / Ts) err + KeqVsl} (32)

In step 40, the actual slip speed V
The switching function σ is calculated according to the following equation 33 based on the integrated value z of the error of the slip and the error of the slip speed.

Σ = Vsl−z / Ts (33)

In step 50, it is determined whether or not the sign of the switching function σ has changed within the time T1, using T1 as the adaptive operation permission determination time (positive constant). Slip rate kb in step 60
Based on (= Vsl / Vx), the tire force f (kb) is calculated from the map corresponding to the graph shown in FIG. 3, and the parameters θ and β are calculated by the following equations (34) and (3), respectively.
5), the corrected θ and β are stored as the previous values θold and βold for the next cycle, and when the affirmative determination is made, the parameters θ and β are corrected in step 70. Without their values being set to the previous values θold and βold for the next cycle. If the affirmative determination in step 50 is performed subsequently, the magnitudes of the parameters θ and β are reduced.

(29) θ = θold + f (kb) Γ1σ (34) β = βold + Γ2 | σ | (35)

In the illustrated embodiment, the amount of calculation is reduced by using the simple tire model shown in FIG. 3 as a nonlinear component that can be formalized. For example, a model closer to the actual tire characteristics, such as a brush model or a magic formula, may be used. In this case, the amount of calculation increases, but the control performance improves.

In step 80, based on the switching function σ, sgn is obtained from the map corresponding to the graph shown in FIG.
(Σ) is calculated, and the nonlinear input unl is calculated according to the following equation (36) using Knl1 and Knl2 as positive constants.

[Mathematical formula-see original document] unl =-([beta] + Knl1) sgn ([sigma])-Knl2 [sigma] -f (kb) [theta] (36)

In step 90, the control input u is calculated as the sum (ueq + unl) of the equivalent control input ueq calculated in step 30 and the non-linear input unl calculated in step 80. In this case, the hydraulic circuit 16 is designed so that the braking torque Tb estimated from the braking oil pressure Pi (i = fl, fr, rl, rr) becomes equal to the control input u.
Is controlled to control the braking force of the corresponding wheel, and thereafter, the process returns to step 10.

As can be seen from the above description, in the illustrated embodiment, in step 20, the error err between the target slip speed Vslt and the actual slip speed Vsl and the integral z thereof are calculated, and step 30 is executed. In step 40, the equivalent control input ueq is calculated based on the slip speed error err and the actual slip speed Vsl. In step 40, the switching function σ is calculated based on the integral value z.

Then, in step 50, it is determined whether or not the sign of the switching function σ has changed within the time T1, that is, whether or not the value of the switching function oscillates across 0 is determined. When the determination is made, the tire generation force f (kb) is calculated based on the slip ratio kb in step 60, and the parameters θ and β are corrected. When the determination is affirmative, the parameters θ and β are corrected. However, the magnitude of the parameter is reduced if necessary.

Further, in step 90, the equivalent control input u
The control input u is calculated as the sum of eq and the non-linear input unl, and the hydraulic circuit 16 is controlled in step 100 so that the braking torque Tb estimated from the braking oil pressure Pi becomes equal to the control input u. The braking force of the wheels is controlled.

Thus, according to the illustrated embodiment, θ and β are corrected according to the value of the actual wheel rotation state σ for the desired wheel rotation state (σ = 0). In particular, the first parameter θ corresponds to (− (K / B) × maximum braking force), and θ
f (κb) gives the tire force in a feedforward manner. Further, σ includes an error integral (when σ <0, the slip ratio is shallower than the target value) as an element, and when σ <0, such as when the road surface friction coefficient is high, the estimation of the tire force is considered to be small. The value of θ is reduced (the absolute value is increased). Conversely, when σ> 0, such as when the friction coefficient of the road surface is high, it is considered that an input greater than the force that can be generated on the actual road surface is given, and the value of θ is increased (absolute Value will be smaller).

The second parameter β represents a non-linear element that cannot be represented by θf (kb). Even if the assumed tire characteristics (FIG. 3) deviate from the actual tire characteristics, the state of the wheel rotation is determined. When σ 保 つ 0, β is considered to be too small, and the value of β is increased.

When the actual state of the wheel rotation is oscillating with (σ = 0) therebetween, an affirmative determination is made in step 50, so that the values of the parameters θ and β are not corrected, and , The magnitude of the parameter is reduced. Therefore, when σ = 0 cannot be maintained continuously due to the delay of the hydraulic circuit, the braking device, the sensor, etc., θ or β
Is corrected more than necessary to prevent the wheel rotation state from becoming vibratory.

When this effect is insufficient, θ, β
Is considered to be excessively large. For example, when the affirmative determination is continuously performed for a predetermined number of times or more in step 50, for example, the magnitudes of θ and β according to the above equations (27) and (28) May be gradually reduced, in which case the vibration of the wheel rotation is reliably prevented.

Therefore, according to the illustrated embodiment, the adaptive effect of the control gain can reduce the possibility that the control input becomes excessive irrespective of the change in the friction coefficient of the road surface and reduce the vibration of the wheel rotation. In addition, it is possible to correct a difference between the tire characteristics of the model and the actual tire characteristics, and thereby it is possible to optimally control the wheel speed.

Although the present invention has been described in detail with reference to specific embodiments, the present invention is not limited to the above-described embodiments, and various other embodiments may be included within the scope of the present invention. It will be clear to those skilled in the art that is possible.

For example, in the above-described embodiment, the target wheel speed Vwti is calculated as the wheel speed necessary for the behavior control by controlling the braking force of the wheel. May be calculated.

In the above-described embodiment, the longitudinal speed of the vehicle body is determined by the ground vehicle speed Vx detected by the ground vehicle speed sensor.
However, the longitudinal speed of the vehicle body may be calculated based on the wheel speed of a wheel whose braking force is not controlled, for example, in behavior control by controlling the braking force of the wheel.

Further, in the above embodiment, sgn
(Σ) is calculated from a map corresponding to the graph shown in FIG. 4 based on the switching function σ.
May be calculated from the map corresponding to the graph shown in FIG.

[0068]

As is apparent from the above description, according to the first aspect of the present invention, since the first and second parameters are modified according to the value of the switching function, the coefficient of friction of the road surface is improved. Regardless of this, the risk of excessive control input can be reduced, and the error between the tire model and the actual vehicle can be reduced, whereby the wheel speed can be optimally controlled.

According to the second aspect of the present invention, when the value of the switching function oscillates across 0, the correction of the first and second parameters is stopped or the magnitude of the first and second parameters is reduced. Is reduced, so that it is possible to reliably prevent the rotational state of the wheels from becoming vibrating due to excessive parameter correction.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of a vehicle braking force control device to which a wheel speed control method according to the present invention is applied.

FIG. 2 is a flowchart illustrating a wheel speed control routine.

FIG. 3 is a graph showing a relationship between a slip ratio kb and a braking force f (kb).

FIG. 4 is a graph showing an example of a saturation function.

FIG. 5 is a graph showing another example of the saturation function.

[Explanation of symbols]

 14FL-14RR ... Brake device 16 ... Hydraulic control circuit 18 ... Electronic control device 20 ... Ground vehicle speed sensor 22FL-22RR ... Wheel speed sensor 24FL-24RR ... Hydraulic sensor

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yoshikazu Hattori 41-41, Chuchu, Yokomichi, Nagakute-cho, Aichi-gun, Aichi F-term in Toyota Central R & D Laboratories Co., Ltd. 3D046 BB28 HH23 HH26 HH29 HH36 HH52 JJ06 KK08

Claims (2)

[Claims] In a vehicle wheel speed control method for applying a sliding mode control so that a wheel slip speed becomes a given target slip speed, an integral value of a deviation between an actual slip speed of the wheel and a target slip speed is provided. And the actual slip speed term, constitute a switching function, the control input is constituted as the sum of the equivalent control input term and the non-linear input term, and the non-linear input term is generated by the tire based on the tire model. The first and second parameters representing the non-linear element other than the first parameter to calculate the force to be configured based on the non-linear element, the first and second parameters according to the value of the switching function A wheel speed control method for a vehicle, wherein the method is modified. 2. When the value of the switching function is oscillating across 0, stopping the correction of the first and second parameters or reducing the magnitude of the first and second parameters. The vehicle wheel speed control method according to claim 1, wherein: