JP2010188883A - Vehicular steering control device and vehicular steering control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology of vehicular steering control improving vehicle stability. <P>SOLUTION: By setting a target steering angle<SB>α</SB><SP>*</SP>computed according to a steering angel θ of a steering wheel 1 as a target value, a steering wheel 8 is steering-controlled through a steering actuator 5. When lateral acceleration YG is not less than a predetermined threshold value, as compared with a case that the lateral acceleration is less than the predetermined threshold value, changes of the target steering angle to changes of the steering angle are limited. When a road friction coefficient is small, the predetermined threshold value is made to be smaller as compared with a case that the road surface friction coefficient is large. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steering control technique for controlling the turning angle of a steered wheel in accordance with the traveling state of a vehicle.

As a conventional steering control device, for example, there is a technique described in Patent Document 1. In this technique, it is determined whether or not a normal state in which a lateral force (a force in the direction of the rotation axis input to the wheel, hereinafter also referred to as a lateral force) increases as the wheel slip angle increases. If it determines with not being in the said normal state, the steering angle of a wheel will be returned to a neutral state. Whether or not the vehicle is in a normal state is determined based on whether or not it is within a predetermined slip angle set in advance as a slip angle at which the lateral force of the wheel is maximized.
This prevents the vehicle from becoming a large oversteer state or understeer state.

JP-A-10-324260

In the above-mentioned conventional patent, a predetermined slip angle as a reference is set and controlled. For this reason, even if the wheel slip angle is within the predetermined slip angle, the vehicle may be in an oversteer state. In particular, compared to a high friction coefficient road, it takes time to return to a stable state on a low friction coefficient road even if the steering angle is returned after the vehicle is oversteered. Thus, there is a possibility that sufficient stability cannot be ensured simply by controlling the control amount of the rudder angle to the predetermined slip angle set in advance.
The present invention pays attention to the above points, and an object of the present invention is a vehicle steering control technique that improves vehicle stability.

  In order to solve the above-described problems, the present invention performs vehicle steering control in which steered wheels are steered via a steering actuator using a target steered angle calculated according to the steering angle of a steering wheel as a target value. . When the lateral acceleration is equal to or greater than the predetermined threshold, the change in the target turning angle with respect to the change in the steering angle is suppressed as compared with the case where the lateral acceleration is less than the predetermined threshold. Further, the predetermined threshold is made smaller when the road surface friction coefficient is small than when the road surface friction coefficient is large.

  According to the present invention, when the road surface friction coefficient is small, an increase in the turning angle with respect to an increase in the steering angle is suppressed from a state where the turning angle is small compared to a case where the road surface friction coefficient is large. As a result, when the road surface friction coefficient is small, it is possible to suppress the vehicle from being oversteered compared to when the road surface friction coefficient is large.

1 is a schematic diagram showing a system configuration of a vehicle according to an embodiment based on the present invention. It is a figure which shows the structure of the steering angle control controller which concerns on embodiment based on this invention. It is a figure for demonstrating the structure of the target production | generation part which concerns on embodiment based on this invention. It is a figure explaining the process of the road surface micro estimation part which concerns on embodiment based on this invention. It is a figure explaining the road surface μ estimation method according to an embodiment based on the present invention. It is a figure explaining the process of the target value correction | amendment part which concerns on embodiment based on this invention. It is a figure which shows the relationship between a road surface micro estimation value and a target yaw rate reduction correction value. It is a figure which shows the relationship between the steering angle which concerns on embodiment based on this invention, and target yaw rate. It is a figure which shows the relationship between the road surface μ estimated value and the target yaw rate gain correction value. It is a figure explaining the restriction | limiting of a target yaw rate. It is a figure explaining the operation | movement which concerns on embodiment based on this invention.

Next, embodiments of the present invention will be described with reference to the drawings.
(Constitution)
FIG. 1 shows a configuration diagram of a vehicle according to this embodiment.
First, the configuration will be described with reference to FIG.
A steering input shaft 2 is connected to a steering wheel 1 operated by a driver. A steering output shaft 3 is connected to the steering input shaft 2 via a mechanical backup device (not shown). The mechanical backup device is in a state where torque transmission between the steering input shaft 2 and the steering output shaft 3 is cut off in a normal state. Further, the mechanical backup device connects the steering input shaft 2 and the steering output shaft 3 based on a command from the steering angle control controller 4 so that torque can be transmitted. The mechanical backup device may be omitted, and the steering input shaft 2 and the steering output shaft 3 may be always connected.

  A front wheel steering actuator 5 (also referred to as a front wheel steering motor) is connected to the steering output shaft 3. The front wheel steering actuator 5 rotates and displaces the steering output shaft 3 based on a command from the front wheel steering controller 6. The front wheel steering actuator 5 is provided with a steering actuator rotation angle sensor (not shown). The steering actuator angle sensor detects the rotational angle position of the front wheel steering actuator 5 and outputs the detection signal to the front wheel steering controller 6 as a feedback control amount.

  The steering output shaft 3 is connected to the rack shaft 7 via a rack and pinion mechanism. That is, the pinion gear connected to the steering output shaft 3 meshes with the rack gear of the rack shaft 7. The rack shaft is arranged with the shaft directed in the vehicle width direction. The rack shaft 7 is displaced in the axial direction toward the vehicle width direction by rotationally displacing the steering output shaft 3. The left and right ends of the rack shaft are connected to the knuckles of the front wheels 8 via left and right tie rods, respectively. Thereby, each front wheel 8 can be steered by driving the front wheel steering actuator 5.

In addition, a steering angle sensor 10, a vehicle speed sensor 11, and a lateral G sensor 12 are provided.
The steering angle sensor 10 detects the steering angle θ (rotation angle) of the steering wheel 1 and outputs the detected steering angle θ signal to the steering angle control controller 4. The steering angle sensor 10 is provided on the steering input shaft 2 or the steering wheel 1. The steering angle sensor 10 is composed of, for example, a pulse encoder and detects the steering angle θ operated by the driver.
The vehicle speed sensor 11 detects the vehicle body speed V of the vehicle and outputs the detected vehicle body speed signal to the steering angle controller 4.
The lateral G sensor 12 detects the lateral G (lateral acceleration) of the vehicle and outputs the detected lateral G signal to the steering angle control controller 4.

The steering angle controller 4 calculates a target front wheel turning angle α ^* based on the steering angle θ of the steering wheel 1 detected by the steering angle sensor 10. Specifically, the target front wheel turning angle α ^* with respect to the steering angle θ of the steering wheel 1 is calculated by referring to a map in which the target front wheel turning angle α ^* corresponding to the steering angle θ is stored in advance. The target front wheel turning angle α ^* may be calculated by multiplying the steering angle θ of the steering wheel 1 by a predetermined ratio. A ratio (gear ratio) between the steering angle θ of the steering wheel 1 and the target front wheel turning angle α ^* (gear ratio) may be changed based on the vehicle speed, the vehicle state, or the like.

The front wheel steering controller 6 controls the front wheels 8 that are steered wheels via the front wheel steering actuator 5 so that the actual steering angle of the front wheels matches the target front wheel steering angle α ^* calculated by the steering angle control controller 4. Steering is controlled by feedback. That is, the front wheel steering controller 6 determines the target drive current I _ref based on the deviation between the target front wheel steering angle α ^* calculated by the steering angle controller 4 and the rotation angle θ _ACT of the front wheel steering actuator 5 to be controlled. And the current supplied to the steering actuator 5 based on the calculated target drive current I _ref is controlled by the PWM method or the like, so that the drive current I _ACT corresponding to the target drive current I _ref is supplied to the steering actuator 5. Supply. Thus, it is controlled so that the rotational angle theta _ACT of the front wheel steering actuator 5 follows the target front wheel steering angle alpha ^*. Here, the front wheel turning actuator 5 rotates and displaces the steering output shaft 3, and the front wheel 8 is steered by the rotational displacement of the front wheel turning actuator 5, so the target front wheel turning angle α ^* as a control amount and The rotation angle θ _ACT of the front wheel steering actuator 5 is uniquely determined based on the gear ratio between the pinion gear and the rack gear, and thus can be regarded as equivalent.

As shown in FIG. 2, the rudder angle controller 4 includes a target value generation unit 41, a road surface μ estimation unit 42, a target value correction unit 43, and a target output value generation unit 44.
As shown in FIG. 3, the target value generation unit 41 includes a vehicle model calculation unit 41a and a target value calculation unit 41b.
The vehicle model calculation unit 41a calculates a vehicle parameter related to the yaw rate that specifies the traveling state of the vehicle according to the current steering angle θ and the vehicle body speed V in accordance with the vehicle model to be applied. The state quantities that are vehicle parameters to be calculated are gφ (V), ζφ (V), ωφ (V), and Tφ (V). These state quantities can be calculated from the steering angle θ and the vehicle body speed V according to the following equation (6).
Here, vehicle parameters using the vehicle model will be described.
When a two-wheel model is adopted as a vehicle model, the yaw rate φ ′ and the lateral speed Vy of the vehicle can be expressed by the following equations. That is, the yaw rate and the lateral speed corresponding to the steering angle θ and the vehicle body speed V can be estimated from the state equation of the following expression (1).

here,
φ ′: Yaw rate s: Differential operator θ: Driver steering angle (equivalent to front wheel turning angle)
δ: Rear wheel turning angle Vx, V: Vehicle speed Vy: Lateral speed Iz: Vehicle inertia moment M: Vehicle weight Lf: Front axle to center of gravity distance Lr: Center of gravity center to rear axle distance N: Gear ratio Kf: Front wheel cornering power Kr: rear wheel cornering power Cf: front wheel cornering force Cr: rear wheel cornering force

である。
次に、上記状態方程式から、前輪操舵に対するヨーレイト、及び横速度の伝達関数を求めると、下記（３）式及び（４）式となる。

It is.
Next, when the transfer function of the yaw rate and the lateral velocity for the front wheel steering is obtained from the above state equation, the following equations (3) and (4) are obtained.

  From the above equation (3), the transfer function of yaw rate can be expressed by the following equation.

  From the above equation (4), the transfer function of the lateral velocity can be expressed by the following equation.

From the above, the running state of the vehicle according to the steering angle θ and the vehicle body speed V can be specified by the yaw rate.
And the vehicle parameter (vehicle parameter) related to the yaw rate is as described above,
gφ (V), ζφ (V), ωφ (V), and Tφ (V).
here,
gφ (V) is a state quantity indicating the basic state of the vehicle characteristic mode.
ζφ (V) is a state quantity related to the attenuation term.
ωφ (V) is a state quantity related to the natural frequency of the spring system.
Tφ (V) is a state quantity related to the steady deviation.

Next, the target value calculation unit 41b will be described.
The target value calculation unit 41b calculates the target yaw rate φ ′ ^* of the vehicle from the steering angle θ, the vehicle body speed V, and the vehicle parameters calculated by the vehicle model calculation unit 41a.
First, a target characteristic state quantity corresponding to the set vehicle characteristic mode is calculated.
That is, the vehicle parameter of the vehicle model is multiplied by a gain corresponding to the vehicle characteristic mode as shown in the following equation. Thus, the vehicle parameter calculated by the vehicle model calculation unit 41a-1 is converted into a target characteristic state quantity corresponding to the vehicle characteristic mode.

Target characteristic state ^{quantity, gφ * (V), ζφ} * (V), a ^{ωφ * (V), Tφ *} (V).
gφ ^* (V) = gφ (V) xyrate_gain_map1
ζφ ^* (V) = ζφ (V) xyrate_zeta_map1
ωφ ^* (V) = ωφ (V) xyrate_omegn_map1
Tφ ^* (V) = Tφ (V) xyrate_zeta_map1
However, yrate_gain_map1, yrate_omegn_map1, yrate_zeta_map1, yrate_zero_mapa1, are preset tuning parameters that vary depending on the vehicle speed. When a plurality of vehicle characteristic modes such as a normal mode and a sports mode can be selected, the yrate_gain_map1 has a different map for each vehicle characteristic mode. Then, yrate_gain_map1 corresponding to the selected vehicle characteristic mode is used.

Next, the target value calculation unit 41b applies the target characteristic state quantity corresponding to the vehicle characteristic mode in place of the vehicle parameter calculated by the vehicle model calculation unit 41a, so that the vehicle body speed V and the steering are calculated based on the following equations. The target yaw rate φ ′ ^* is obtained from the angle θ and the target characteristic state quantity.
Here, the target yaw rate φ ′ ^* is expressed by the following equation (9).

Next, the road surface μ estimation unit 42 will be described.
The road surface μ estimation unit 42 estimates the road surface μ estimated value Myu from the target yaw rate φ ′ ^* calculated by the target value generation unit 41, the actual vehicle speed V, and the lateral acceleration YG of the vehicle. In this embodiment, the road surface friction coefficient is estimated from the lateral acceleration value.
The processing of the road surface μ estimation unit 42 will be described with reference to FIG.
First, in step S100, the target lateral acceleration YG_com is calculated according to the following equation. This target lateral acceleration YG_com is a value corresponding to the steering angle θ and the actual vehicle speed V.
YG_com = φ ' ^*・ Vx

  Next, in step S110, it is determined whether or not the difference between the absolute value of the target lateral acceleration YG_com and the detected absolute value of the lateral acceleration YG is equal to or greater than a predetermined value ΔYG1. When the difference between the absolute value of the target lateral acceleration YG_com and the detected absolute value of the lateral acceleration YG is greater than a predetermined value, it can be estimated that the road surface has a low friction coefficient. FIG. 5 shows the relationship between the target lateral acceleration YG_com and the detected lateral acceleration YG. Further, the lateral acceleration YG is detected by the lateral G sensor 12. If the above condition is satisfied, the process proceeds to step S120. On the other hand, if the above condition is not satisfied, the process proceeds to step S130.

In step S120, “1” is set to the road surface friction coefficient determination flag flg_myu. Thereafter, the process proceeds to step S160.
In step S130, it is determined whether or not the difference between the absolute value of the target lateral acceleration YG_com and the detected absolute value of the lateral acceleration YG is equal to or smaller than a predetermined value ΔYG2. If the above condition is satisfied, the process proceeds to step S150. On the other hand, if the above condition is not satisfied, the process proceeds to step S140.

In step S140, it is determined whether or not the absolute value of the target lateral acceleration YG_com is equal to or less than a predetermined value YG0. If the above condition is satisfied, the process proceeds to step S150. On the other hand, if the above condition is not satisfied, the previous value is maintained and the process proceeds to step S160.
In step S150, “0” is set to the road surface friction coefficient determination flag flg_myu. That is, after the road surface friction coefficient determination flag flg_myu is cleared to zero, the process proceeds to step S160.

In step S160, it is determined whether the road surface friction coefficient confirmation flag flg_myu is “1”. If the condition is satisfied, the process proceeds to step S170. On the other hand, if the condition is not satisfied, the process proceeds to step S180.
In step S170, the detected lateral acceleration YG is set as the road surface μ estimated value Myu. Then exit.
Myu = YG

In step S180, it is estimated that the road surface has a high friction coefficient that is equal to or greater than a predetermined value, and a default value 10 is set as the road surface μ estimated value Myu. Then exit.
Myu = 10
Here, the estimation of the road surface friction coefficient is not limited to the above processing. The estimated value of the road surface μ is obtained from the difference between the rotational speed of the driving wheel and the rotational speed of the driven wheel. In other words, the road surface reaction force (road surface friction force) may be detected from the difference in slip rate with respect to the road surface between the wheel generating the driving force and the driven wheel, and the road surface friction coefficient may be estimated from the detected value. Further, the road surface friction coefficient may be estimated from the lateral force of the wheel.

Next, the target value correcting unit 43 calculates a target yaw rate correction value φ ₂ ′ ^* from the original target yaw rate φ ′ ^* calculated by the target position generating unit and the road surface μ estimated value Myu.
As shown in FIG. 6, the target value correction unit 43 includes a target yaw rate maximum value limit calculation unit 43a, a target yaw rate reduction correction calculation unit 43b, and a target yaw rate increase gain correction calculation unit 43c.
The target yaw rate maximum value limit calculation unit 43a limits the target yaw rate φ ′ ^* with the yaw rate that maximizes the lateral force. That is, the target yaw rate maximum value φ ₀ ′ ^* corresponding to the turning equivalent that maximizes the lateral force is obtained.

The target yaw rate maximum value limit calculation unit 43a according to the present embodiment obtains the target yaw rate maximum value φ ₀ ′ ^{* at} which the lateral force is maximum by the following equation.
^{_{φ 0 '* = φ' *}} × (Myu / YG_com)
By limiting the target yaw rate with the yaw rate φ ₀ ′ ^* at which the lateral force is maximum, the target yaw rate can be limited even if the steering angle by the driver's operation increases. As a result, the actual yaw rate and yaw angle can be prevented from becoming excessive, and the vehicle can be prevented from falling into a spin state.

Here, the above expression limits the maximum value of the target yaw rate using the value of the lateral acceleration as the estimated road surface friction coefficient.
Instead, the wheel slip angle β may be calculated by the following equation using the lateral acceleration YG, the yaw rate φ ′, and the vehicle body speed V, and the limit value of the lateral force may be obtained from the slip angle β. The method for obtaining the lateral force limit value is not limited to the above method.
β = ∫ ((YG / V) −γ) dt

Next, the processing of the target yaw rate reduction correction calculation unit 43b will be described.
The target yaw rate reduction correction calculation unit 43b corrects the target yaw rate φ ′ ^* to a value smaller than the yaw rate φ ₀ ′ ^{* at} which the lateral force becomes maximum.
First, the target yaw rate reduction correction calculation unit 43b calculates a target yaw rate reduction correction value Δφ _T ′ ^* according to the estimated road surface μ estimated value based on the “road surface μ estimated value−target yaw rate reduction correction value map” shown in FIG. calculate.

As shown in FIG. 7, the target yaw rate reduction correction value Δφ _T ′ ^* is obtained when the estimated road surface μ is small (when the estimated road friction coefficient is small) or when the estimated road surface μ is large (the estimated road friction coefficient is It becomes a large value compared with the case of large. That is, the reduction correction value Δφ _T ′ ^* is set to be larger as the road surface has a lower friction coefficient. As a result, instability can be prevented and stability can be improved.

Next, target yaw rate reduction correction calculation unit 43b is calculated by the target yaw rate maximum value limiting calculation unit 43a, the target yaw rate and reduced 'from ^* the target yaw rate reduction correction value [Delta] [phi _T' largest target yaw rate phi ₀ ^* by phi ₁ ′ ^* Is calculated.
φ ₁ ′ ^* = φ ₀ ′ ^* −Δφ _T ′ ^*
As shown in FIG. 8, since the target yaw rate is smaller than the maximum target yaw rate φ ₀ ′ ^* as the road surface has a low friction coefficient, the vehicle is oversteered as the road surface has a lower friction coefficient according to the estimated road surface friction coefficient. Since it takes time to return to the stable state even when the steering angle is returned after becoming, the lower the friction coefficient, the closer to the limit (before the lateral force becomes larger), the target yaw rate is controlled.

Next, the process of the target yaw rate increase gain correction calculation unit 43c will be described.
In the target yaw rate increase gain correction calculation unit 43c, first, based on the “road surface μ estimated value to target yaw rate gain correction value map” shown in FIG. 9, a gain correction value GΔφ _B ′ ^* which is an increase gradient of the target yaw rate as a reference ^. Is calculated.
As shown in FIG. 9, the gain correction value GΔφ _B ′ ^* is obtained when the estimated road surface μ is small (when the estimated road friction coefficient is small) or when the estimated road μ is large (the estimated road friction coefficient is large). The value is smaller than that of the case. That is, the gain correction value GΔφ _B ′ ^* , which is an incremental inclination as the road surface with a low friction coefficient, is set to a smaller value.

Next, the target yaw rate correction calculation is performed from the current corrected target yaw rate φ ₁ ′ ^* , the target yaw rate φ ′ ^* before correction, and the target yaw rate gain correction value GΔφ _B ′ ^* by the following calculation.
φ ₂ ′ ^* = φ ₁ ′ ^* + (φ ′ ^* −φ ₁ ′ ^* ) × GΔφ _B ′ ^*
As shown in FIGS. 8 and 10, the target yaw rate increase gain correction is calculated according to the difference between the current corrected target yaw rate φ ₁ ′ ^* and the target yaw rate φ ′ ^* before correction. Thus, as shown in FIG. 11, the corrected target yaw rate (≈front wheel tire turning angle) gradually increases according to the steering angle, and the controllability of the vehicle is improved.

Next, the process of the target output value generation unit 44 will be described.
The target output value generation unit 44 determines the target front wheel steering angle α ^* from the target yaw rate correction value (corrected target yaw rate) φ ₂ ′ ^{* of} the vehicle based on the following equation.
_{^{φ 2 "* = a 11 ×}} φ 2 '* + a 12 × Vy + b f1 × α *
(10)
^{α * = (1 / bf1)} × (φ 2 "* -a 11 × φ 2 '* -a 12 × Vy)
(11)
Then, the target output value generation unit 44 outputs the target front wheel steering angle α ^* to the front wheel steering controller 6.

(Operation)
In order to detect the actual running state of the vehicle as much as possible, the yaw rate that represents the running state of the vehicle is calculated from the steering angle θ and the vehicle body speed V using the vehicle model, and the target yaw rate φ ′ corresponding to the vehicle characteristic mode is calculated. ^* Ask for. Then, it calculates a target front wheel steering angle alpha ^* for realizing the desired yaw rate phi ^'*. That is, the target front wheel turning angle α ^* of the front wheels is calculated according to the steering angle θ.

At this time, the lateral acceleration corresponding to the target yaw rate φ ₁ ′ ^* smaller than the maximum value of the target yaw rate φ ₀ ′ ^{* at} which the lateral force is maximum is set as a predetermined threshold value, and the target yaw rate φ ₁ corresponding to the predetermined threshold value as the reference is set. as compared with the case of less than ^'* less than the target yaw rate phi _2' ^*, if the target yaw rate phi ₂ ^'* or more to suppress the increment of the target yaw rate with respect to the increase in the steering angle theta. As a result, when the acceleration is equal to or higher than the predetermined acceleration corresponding to the target yaw rate φ ₂ ′ ^* , the change in the target turning angle with respect to the change in the steering angle is suppressed. This means that if the lateral acceleration is determined to be greater than or equal to a predetermined threshold value that is smaller than the value that maximizes the lateral force of the wheel, the target turning angle relative to the change in the steering angle is compared with the case where the lateral acceleration is less than the predetermined threshold value. The change will be suppressed. The lateral force is a value correlated with the lateral acceleration. Therefore, if it is determined that the lateral force is equal to or greater than a predetermined threshold value that is smaller than the value at which the lateral force of the wheel is maximum, the change in the target turning angle relative to the change in the steering angle is compared to the case where the lateral force is less than the predetermined threshold value. It is synonymous with suppressing.

That is, as the steering angle before the target yaw rate φ ₀ ′ ^{* at which} the lateral force is maximum is changed to the steering angle corresponding to the target yaw rate φ ₀ ′ ^{* at} which the lateral force is maximum, the increase in the target yaw rate is suppressed. Thus, the closer to the turning angle at which the lateral force becomes maximum, the more gradually the change in the target turning angle with respect to the change in the steering angle is suppressed.
That is, a difference occurs between the target yaw rate φ ₁ ′ ^* reduced and the target yaw rate φ ₀ ′ ^{* at} which the lateral force is maximized by the target yaw rate corresponding to the predetermined threshold corrected by the target yaw rate reduction correction calculation unit 43 b. During this time, the gain for increasing the target yaw rate in accordance with the steering angle is also changed by the road surface μ estimated value Myu. That is, the lower the road surface friction coefficient, the smaller the increase rate. As a result, the change rate of the yaw rate with respect to the change in the steering angle is reduced, that is, the target turning angle of the front wheels with respect to the change in the steering angle is reduced. As a result, the controllability can be improved without impairing the stability on the low friction coefficient road surface that is likely to be unstable.

  Here, the slip angle at which the lateral force is maximized is referred to as a slip angle reference value. The characteristics of the front wheel turning angle with respect to the steering angle change greatly before and after exceeding the slip angle reference value. In other words, the front wheel turning angle with respect to the steering angle before exceeding the slip angle reference value is a certain magnification, but after exceeding the slip angle reference value, the front wheel cannot be cut any more even if the steering angle is entered, and the change in magnification changes. large. On the other hand, in this embodiment, the change of the target turning angle with respect to the change of the steering angle is suppressed as the slip angle reference value is approached from the position before the slip angle reference value. For this reason, it is possible to suppress a change before and after the slip angle reference value. This makes it possible to improve the controllability of the vehicle even on a low coefficient of friction road where the vehicle behavior tends to be unstable.

  Here, the front wheel steering actuator 5 constitutes a steering actuator. The vehicle speed sensor 11 constitutes a vehicle speed acquisition means. The steering angle sensor 10 constitutes a steering angle detection means. The front wheel steering controller 6 constitutes a steering controller. The steering angle controller 4 constitutes a target steering angle calculation means. The target value correcting unit 43 constitutes a steering angle change limiting unit. The target yaw rate reduction correction calculation unit 43b performs processing of a predetermined threshold value. The target yaw rate increase gain correction calculation unit 43c determines the restriction on the change in the target turning angle with respect to the change in the steering angle. The lateral G sensor 12 constitutes lateral acceleration detection means. The road surface μ estimation unit 42 constitutes a friction coefficient detection unit.

(Effect of this embodiment)
(1) The target turning angle calculation means calculates a target turning angle according to the steering angle. The steering controller controls the steering wheel by controlling the steering actuator with the target steering angle as a target value. When the lateral acceleration is equal to or greater than a predetermined threshold value that is smaller than the lateral acceleration at which the lateral force of the wheel is maximum, the steering angle change restricting means Suppress changes in the turning angle. The predetermined threshold is made smaller when it is determined that the road friction coefficient is small than when it is determined that the road friction coefficient is large.

Compared to a high friction coefficient road surface such as a dry road, it takes time to return to a stable state on a low friction coefficient road surface such as a snowy road even when the steering angle is returned after the vehicle is oversteered. In view of this, as the road surface has a lower friction coefficient, the change in the target turning angle with respect to the change in the steering angle from the near position to the vicinity of the limit is limited. As a result, it is possible to prevent instability even on a road surface with a low friction coefficient, and improve stability.
Accordingly, for example, when the vehicle is traveling on a low friction coefficient road surface such as a snowy road or a frozen road, it is possible to suitably suppress oversteer or understeer of the vehicle.

(2) The suppression of the change in the target turning angle with respect to the change in the steering angle is increased as the steering angle increases.
When steering of the steered wheels is stopped at the slip angle that is the limit of the lateral force, the ratio of the steered wheel to the steered angle changes, especially on a low friction coefficient road surface where the vehicle tends to become unstable. Vehicle controllability deteriorates.
On the other hand, on the low friction coefficient road surface, by gradually reducing the control amount of the front wheel rudder angle from before the limit of the lateral force and leaving the driver's control area, in the vicinity of the slip angle that becomes the limit of the lateral force in actual driving But the driver can make fine adjustments. As a result, stability is increased.
Further, the controllability can be improved without impairing the stability by changing the rate of increase of the steering angle of the steered wheels in accordance with the road surface friction coefficient, for example, by reducing the slope as the friction coefficient is lower.

(3) The target turning angle calculation means calculates a target yaw rate based on the vehicle body speed and the steering angle, and calculates a target turning angle based on the target yaw rate. The steering angle change limiting means limits the target yaw rate, thereby suppressing a change in the target turning angle with respect to a change in the steering angle.
By calculating the target turning angle based on the target yaw rate, it is possible to control the turning angle with respect to the turning angle at which the lateral acceleration of the wheel is maximized without directly using the slip angle.

DESCRIPTION OF SYMBOLS 1 Steering wheel 4 Steering angle controller 41 Target value production | generation part 41a Vehicle model calculating part 41b Target value calculating part 42 Road surface micro estimation part 43 Target value correction | amendment part 43a Target yaw rate maximum value restriction | limiting calculation part 43b Target yaw rate reduction correction | amendment calculating part 43c Target Yaw rate increase gain correction calculation unit 5 Front wheel steering actuator 8 Front wheel (steering wheel)
10 Steering angle sensor 11 Vehicle speed sensor 12 Lateral G sensor GΔφ ′ _B gain correction value Myu Road surface μ estimated value V, Vx Vehicle speed Vy Lateral velocity YG Lateral acceleration YG_com Target lateral acceleration α ^* Target front wheel steering angle Δφ _T ′ Target yaw rate reduction correction Value θ Steering angle φ ′ ^* Target yaw rate φ ₀ ′ ^* Target yaw rate maximum value φ ₂ ′ ^* Target yaw rate correction value

Claims (4)

A steering actuator for steering the steered wheels, steering angle detection means for detecting the steering angle of the steering wheel, target turning angle calculation means for calculating a target turning angle according to the steering angle, and target turning A steering controller that controls the steered wheels by controlling the steering actuator with the angle as a target value, a lateral acceleration detection means that detects the lateral acceleration of the vehicle, and a friction coefficient detection that detects the road surface friction coefficient of the traveling road surface Means, and
If the target turning angle calculation means determines that the lateral acceleration is greater than or equal to a predetermined threshold value that is smaller than the value at which the lateral force of the wheel is maximum, the target turning angle calculation means is more sensitive to changes in the steering angle than when the lateral acceleration is less than the predetermined threshold value. A steering angle change limiting means for suppressing changes in the target turning angle,
The vehicle steering control apparatus according to claim 1, wherein the predetermined threshold value is made smaller when it is determined that the road surface friction coefficient is small than when it is determined that the road surface friction coefficient is large.   2. The vehicle steering control device according to claim 1, wherein the amount of suppression of the change in the target turning angle with respect to the change in the steering angle increases as the steering angle increases. A vehicle speed acquisition means for acquiring the vehicle speed is provided,
The target turning angle calculation means calculates a target yaw rate based on the vehicle body speed and the steering angle, calculates a target turning angle based on the target yaw rate,
3. The vehicle steering control according to claim 1, wherein the steering angle change limiting unit suppresses a change in the target turning angle with respect to a change in the steering angle by limiting the target yaw rate. 4. apparatus. In a vehicle steering control method for steering-controlling steered wheels via a steering actuator using a target turning angle calculated according to a steering angle of a steering wheel as a target value,
When the lateral acceleration is greater than or equal to a predetermined threshold, the change in the target turning angle relative to the change in the steering angle is suppressed compared to the case where the lateral acceleration is less than the predetermined threshold. The vehicle steering control method is characterized in that it is made smaller than when the road surface friction coefficient is large.

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