JP4807017B2 - Vehicle steering control device - Google Patents

Description

  The present invention relates to a vehicle steering control device.

  2. Description of the Related Art Conventionally, a vehicle steering control device that can independently control the steering angle of a front wheel and a rear wheel is known. For example, Patent Document 1 describes a technique for performing rear wheel steering control based on at least one of a yaw rate, a vehicle body slip angle, and a differential value of the vehicle body slip angle. Patent Document 2 describes a technique for compensating for fluctuations in steering torque generated by four-wheel steering with the assist force of power steering.

JP-A-5-208681 Japanese Patent Laid-Open No. 3-220067

  However, in the techniques described in Patent Document 1 and Patent Document 2 described above, the relationship between the steering torque and the lateral acceleration changes during the steering operation, and the driver may feel uncomfortable. For this reason, there is a possibility that the driver performs an erroneous steering.

  The present invention has been made in order to solve such problems. The object of the present invention is to provide a steering assist in consideration of the relationship between the steering torque and the lateral acceleration, thereby providing a steering fee during turning. An object of the present invention is to provide a vehicle steering control device capable of improving a ring.

In one aspect of the present invention, a vehicle steering control device includes a steering control unit that independently steers a front wheel and a rear wheel of a vehicle, and steering assist control that performs steering assist based on an assist characteristic that a steering torque and an assist torque satisfy. And the steering assist control means includes assist characteristic correction means for correcting the assist characteristic so that the relational characteristic between the steering torque and the lateral acceleration is kept constant during the steering operation. The steering control means includes a steering angle calculation means for calculating a front wheel steering angle to be set for the front wheel and a rear wheel steering angle to be set for the rear wheel, and the steering assist control means A lateral force load distribution ratio indicating a ratio between the total lateral force generated in the vehicle and the lateral force generated in the front wheel based on the front wheel steering angle and the rear wheel steering angle calculated by the steering angle calculating means. A change for calculating a change rate corresponding to a ratio of the lateral force load distribution ratio calculated by the lateral force load distribution ratio calculation unit with respect to the lateral force load distribution ratio during steady turning The assist characteristic correcting unit corrects the assist characteristic based on the change rate calculated by the change rate calculating unit.

The above-described vehicle steering control device is suitably used for independently steering-controlling the front wheels and the rear wheels of the vehicle. Specifically, the steering control means performs control for independently steering the front wheels and the rear wheels, and the steering assist control means performs steering assist control based on an assist characteristic that the steering torque and the assist torque satisfy. Specifically, the steering assist control means includes assist characteristic correction means for correcting the assist characteristic so that the relational characteristic between the steering torque and the lateral acceleration is kept constant during the steering operation (during the steering operation). Therefore, according to the above-described vehicle steering control device, the relational characteristic between the steering torque and the lateral acceleration can be maintained constant regardless of the turning state, and thus the steering feeling during turning can be improved. Can do.
Specifically, the steering control means includes a steering angle calculation means for calculating a front wheel steering angle to be set for the front wheel and a rear wheel steering angle to be set for the rear wheel, and the steering assist control means includes Lateral force distribution indicating the ratio between the total lateral force generated in the vehicle and the lateral force generated in the front wheel based on the front wheel steering angle and the rear wheel steering angle calculated by the steering angle calculating means A lateral force load distribution ratio calculating means for calculating a ratio, and a change ratio corresponding to the ratio of the lateral force load distribution ratio calculated by the lateral force load distribution ratio calculating means with respect to the lateral force load distribution ratio during steady turning is calculated. A change rate calculating unit that corrects the assist characteristic based on the change rate calculated by the change rate calculating unit. This makes it possible to effectively keep the relationship between the steering torque and the lateral acceleration constant.

  In a preferred embodiment, the steering angle calculation means can calculate the front wheel steering angle and the rear wheel steering angle so as to obtain a target yaw rate and a target vehicle body slip angle.

  Preferred embodiments of the present invention will be described below with reference to the drawings.

[Vehicle configuration]
First, an overall configuration of a vehicle to which a steering control device according to an embodiment of the present invention is applied will be described with reference to FIG.

  FIG. 1 is a schematic diagram showing a schematic configuration of a vehicle. FIG. 1 is a view of the vehicle observed from above, with the left showing the front of the vehicle and the right showing the rear of the vehicle.

  The vehicle mainly includes an engine 1, a front wheel 2f, a rear wheel 2r, a front wheel steering shaft 3f, a rear wheel steering shaft 3r, a steering wheel (steering wheel) 4, a steering wheel angle sensor 5, and a vehicle speed sensor. 6, a front wheel actuator 7 f, a rear wheel actuator 7 r, an EPS (Electric Power Steering) actuator 8, and a system controller 10.

  The engine 1 is an internal combustion engine that generates power by exploding an air-fuel mixture in a combustion chamber. The power generated by the engine 1 is transmitted to at least one of the front wheels 2f and the rear wheels 2r via a torque converter, a transmission, a drive shaft, and the like (not shown).

  The steering angle of the front wheel 2f is controlled by the front wheel actuator 7f via the front wheel steering shaft 3f. The steering angle of the rear wheel 2r is controlled by the rear wheel actuator 7r via the rear wheel steering shaft 3r. That is, the steering angles of the front wheels 2f and the rear wheels 2r are independently controlled (that is, individually steered).

  The steering wheel 4 is operated so that the driver turns the vehicle, and the steering force by the driver is transmitted to the front wheel actuator 7f via the steering shaft 4a. An angle (handle angle, steering angle) MA by which the driver rotates the handle 4 is detected by the handle angle sensor 5. The handle angle MA detected by the handle angle sensor 5 is output to the system controller 10 as a detection signal S1. The vehicle speed sensor 6 detects a vehicle speed (vehicle speed) V and outputs the detected vehicle speed V to the system controller 10 as a detection signal S2. Thus, the steering wheel angle sensor 5 functions as steering angle detection means, and the vehicle speed sensor 6 functions as vehicle speed detection means.

  The front wheel actuator 7f and the rear wheel actuator 7r acquire the steering angle determined by the system controller 10 as control signals S3f and S3r. The front wheel actuator 7f and the rear wheel actuator 7r respectively steer the front wheel 2f and the rear wheel 2r with the acquired steering angles δf and δr via the front wheel steering shaft 3f and the rear wheel steering shaft 3r.

  The EPS actuator 8 is a device capable of changing (for example, amplifying) a steering torque corresponding to a driver's steering operation. Specifically, the EPS actuator 8 is configured by a motor or the like, and is controlled by a control signal S4 supplied from the system controller 10. The EPS actuator 8 generates a torque corresponding to the control signal S4 (hereinafter, this torque is referred to as “assist torque”), and transmits this assist torque to the steering shaft 4a. Thus, the EPS actuator 8 functions as an electric power steering and assists the steering.

  The system controller 10 is configured by a so-called ECU (Electric Control Unit) or the like, and includes a CPU, a ROM, a RAM, an A / D converter, an input / output interface, and the like. The system controller 10 controls the EPS actuator 8 as well as the front wheel actuator 7 f and the rear wheel actuator 7 r based on the handle angle acquired from the handle angle sensor 5 and the vehicle speed acquired from the vehicle speed sensor 6. That is, the system controller 10 performs steering control and steering assist control.

[System controller configuration]
Here, a specific configuration of the system controller 10 described above will be described with reference to FIG.

  FIG. 2 is a block diagram illustrating a schematic configuration of the system controller 10. The system controller 10 includes a steering controller 10a and an EPS controller 10b. The steering controller 10a is a controller for performing steering control, and the EPS controller 10b is a controller for performing steering assist control. The steering controller 10a functions as a steering control unit, and the EPS controller 10b functions as a steering assist control unit.

  The steering controller 10a includes a target yaw rate calculation unit 11, a target vehicle body slip angle calculation unit 12, a steering angle calculation unit 13, a front wheel steering angle command value transmission unit 14f, a rear wheel steering angle command value transmission unit 14r, Is provided.

  The steering wheel angle sensor 5 supplies a detection signal S1 corresponding to the detected steering wheel angle MA to the target yaw rate calculation unit 11 and the target vehicle body slip angle calculation unit 12. Further, the vehicle speed sensor 6 supplies a detection signal S2 corresponding to the detected vehicle speed V to the target yaw rate calculation unit 11 and the target vehicle body slip angle calculation unit 12.

  The target yaw rate calculation unit 11 calculates a target yaw rate based on the acquired steering wheel angle MA and vehicle speed V. Then, the target yaw rate calculation unit 11 supplies the calculated target yaw rate as a signal S11 to the steering angle calculation unit 13. Further, the target vehicle body slip angle calculation unit 12 calculates the target vehicle body slip angle based on the acquired steering wheel angle MA and the vehicle speed V. Then, the target vehicle body slip angle calculation unit 12 supplies the calculated target vehicle body slip angle to the steering angle calculation unit 13 as a signal S12.

  The steering angle calculation unit 13 sets the steering angle to be set for the front wheel 2f (hereinafter referred to as “front wheel steering angle”) and the rear wheel 2r based on the acquired target yaw rate and target vehicle body slip angle. A steering angle to be set (hereinafter referred to as “rear wheel steering angle”) is calculated. The steering angle calculation unit 13 supplies the calculated front wheel steering angle as a signal S13f to the front wheel steering angle command value transmission unit 14f, and uses the calculated rear wheel steering angle as a signal S13r as a rear wheel steering angle command value transmission unit 14r. To supply. In addition, the steering angle calculation unit 13 supplies the calculated front wheel steering angle and rear wheel steering angle to the lateral force load distribution ratio calculation unit 15 as a signal S13a. Thus, the target yaw rate calculation unit 11, the target vehicle body slip angle calculation unit 12, and the steering angle calculation unit 13 function as a steering angle calculation unit.

  The front wheel steering angle command value transmission unit 14f supplies a command value S3f corresponding to the acquired front wheel steering angle to the front wheel actuator 7f. Further, the rear wheel steering angle command value transmission unit 14r supplies a command value S3r corresponding to the acquired rear wheel steering angle to the rear wheel actuator 7r. Then, the front wheel actuator 7f steers the front wheel 2f based on the acquired front wheel steering angle, and the rear wheel actuator 7r steers the rear wheel 2r based on the acquired rear wheel steering angle.

  On the other hand, the EPS controller 10 b includes a lateral force load distribution ratio calculation unit 15, a change rate calculation unit 16, an assist characteristic correction unit 17, and an assist torque command value transmission unit 18.

  The lateral force load distribution ratio calculation unit 15 is based on the front wheel steering angle and the rear wheel steering angle acquired from the steering angle calculation unit 13 and the total lateral force generated in the vehicle and the lateral force generated in the front wheels 2f (hereinafter referred to as the following). , Which is also referred to as “front wheel lateral force”) (side force distribution ratio). That is, the total lateral force obtained by adding the front wheel lateral force and the lateral force generated on the rear wheel 2r (hereinafter also referred to as “rear wheel lateral force”) is distributed to the front wheel 2f. Calculate the ratio of the lateral force. Then, the lateral force burden distribution ratio calculation unit 15 supplies the calculated lateral force burden distribution ratio to the change ratio calculation unit 16 as a signal S15. As described above, the lateral force load distribution ratio calculation unit 15 functions as a side force load distribution ratio calculation unit.

  The change rate calculation unit 16 calculates the ratio (change rate) of the lateral force load distribution ratio supplied from the side force load distribution ratio calculation unit 15 to the side force load distribution ratio during steady turning. Then, the change rate calculation unit 16 supplies the calculated change rate to the assist characteristic correction unit 17 as a signal S16. Thus, the change rate calculation unit 16 functions as a change rate calculation unit.

  The assist characteristic correction unit 17 corrects the assist characteristic based on the acquired change ratio. This assist characteristic refers to the relationship between assist torque and steering torque. Specifically, the assist characteristic correction unit 17 corrects an assist characteristic (hereinafter referred to as “basic assist characteristic”) set during steady turning based on the change rate. Further, the assist characteristic correction unit 17 obtains the assist torque according to the corrected assist characteristic. Then, the assist characteristic correction unit 17 supplies the corrected assist torque to the assist torque command value transmission unit 18 as a signal S17. As described above, the assist characteristic correction unit 17 functions as an assist characteristic correction unit.

  The assist torque command value transmission unit 18 supplies a command value S4 corresponding to the acquired assist torque to the EPS actuator 8. Then, the EPS actuator 8 is driven so that the acquired assist torque is output.

[Control method]
Next, the control method according to the present embodiment will be specifically described.

  In the present embodiment, steering control is performed for setting the steering angle independently for the front wheels 2f and the rear wheels 2r, and steering assist control for assisting the steering torque is performed. Specifically, in the steering control, the target yaw rate and the target vehicle body slip angle are calculated based on the vehicle state (vehicle speed, steering wheel angle, etc.), and the front wheel steering is performed so that the calculated target yaw rate and target vehicle body slip angle are obtained. The angle and rear wheel steering angle are controlled.

  Here, the steering assist control according to the present embodiment will be described in detail. In the present embodiment, the steering assist is performed so that the relational characteristic between the steering torque and the lateral acceleration is kept constant during the turning transition period. Specifically, the basic assist characteristic is corrected so that the relational characteristic between the steering torque and the lateral acceleration is kept constant, and the steering assist is performed using the assist torque determined based on the corrected assist characteristic. . As a result, the relational characteristic between the steering torque and the lateral acceleration is kept constant regardless of the turning state, so that the driver's steering feeling is improved.

  FIG. 3 is a diagram for explaining the basic concept of the steering assist control according to the present embodiment. FIG. 3 shows lateral acceleration in the downward direction, front wheel lateral force in the right direction, rack axial force in the upward direction, and steering torque in the left direction. The lower right graph in FIG. 3 shows the relationship between lateral acceleration and front wheel lateral force, the upper right graph in FIG. 3 shows the relationship between front wheel lateral force and rack axial force, and the upper left graph in FIG. The relationship between rack axial force and steering torque is shown, and the lower left graph in FIG. 3 shows the relationship between steering torque and lateral acceleration. The rack axial force generally corresponds to the assist torque.

  First, the case where the vehicle is making a steady turn will be described. Here, a case is considered in which the lateral acceleration and the front wheel lateral force indicated by the symbol A1 in the lower right graph of FIG. A line segment 61 indicates the relationship between the lateral acceleration and the front wheel lateral force that are established during steady turning, and the point indicated by reference numeral A1 is located on the line segment 61. Next, referring to the upper right graph in FIG. 3, the front wheel lateral force indicated by reference numeral A1 corresponds to the rack axial force indicated by reference numeral A2. A line segment 62 indicates the relationship between the front wheel lateral force and the rack axial force, and the point indicated by reference numeral A <b> 2 is located on the line segment 62. Such a relationship between the front wheel lateral force and the rack axial force is determined irrespective of the turning state. That is, the rack axial force is basically uniquely determined by the front wheel lateral force.

  Next, referring to the upper left graph of FIG. 3, the rack axial force indicated by reference numeral A2 corresponds to the steering torque indicated by reference numeral A3. A line segment 63 indicates the relationship between the rack axial force and the steering torque that are established during steady turning, and the point indicated by reference numeral A3 is located on the line segment 63. The relationship between the rack axial force and the steering torque indicated by a line segment 63 indicates a relationship that the assist torque and the steering torque satisfy during steady turning, and corresponds to a preset basic assist characteristic. Next, referring to the lower left graph of FIG. 3, the steering torque indicated by the symbol A3 and the lateral acceleration indicated by the symbol A1 are located at a point indicated by the symbol A4. The point indicated by the symbol A4 is located on the line segment 64. A line segment 64 indicates a relational characteristic between the steering torque and the lateral acceleration, which is established during steady turning.

  Here, a case where the vehicle is in a turning transition period will be described. Here, a case is considered where the lateral acceleration and the front wheel lateral force indicated by the symbol B1 in the lower right graph of FIG. 3 are acting on the vehicle. The lateral acceleration indicated by reference sign B1 is the same as the lateral acceleration indicated by reference sign A1, but the front wheel lateral force indicated by reference sign B1 is smaller than the front wheel lateral force indicated by reference sign A1. Normally, when steering control is performed independently for the front wheel 2f and the rear wheel 2r, even if the lateral acceleration is the same during the steady turning and the turning transition period, such a difference in the front wheel lateral force is obtained. Occurs. Next, referring to the upper right graph of FIG. 3, the rack axial force corresponding to the front wheel lateral force indicated by reference numeral B1 is based on the line segment 62 indicating the relationship between the front wheel lateral force and the rack axial force described above. The rack axial force is indicated by the symbol B2.

  Next, referring to the graph in the upper left of FIG. 3, the rack axial force indicated by reference numeral B2 becomes the steering torque indicated by reference numeral B3 based on the line segment 63 indicating the basic assist characteristics. The steering torque indicated by the symbol B3 is smaller than the steering torque indicated by the symbol A3 described above. Next, referring to the lower left graph of FIG. 3, the steering torque indicated by reference sign B3 and the lateral acceleration indicated by reference sign B1 are located at points indicated by reference sign B4. The point indicated by the symbol B4 is not located on the line segment 64 described above.

  From the above, it can be seen that the relationship characteristic between the steering torque and the lateral acceleration, which is established during steady turning, is not established during the turning transition period. That is, it is understood that the steering torque changes according to the turning state even with the same lateral acceleration. As a result, the relationship between the steering torque felt by the driver and the lateral acceleration changes, and the driver may feel uncomfortable. For example, in the example of FIG. 3, the driver feels that the actual steering torque is smaller than the steering torque expected from experience based on the lateral acceleration experienced. Therefore, the driver may feel uneasy when turning, or may make a mistake in the steering operation.

  The above-mentioned problems are that the front wheel lateral force changes, that is, the lateral force load distribution ratio changes, and the same basic assist characteristics are used during steady turning and turning transients. It is caused by that. Therefore, in this embodiment, in order to prevent the occurrence of such a problem, the basic assist is based on the change rate of the lateral force distribution ratio so that the relational characteristic between the steering torque and the lateral acceleration is kept constant. Steering assist is performed by correcting the characteristics.

  Here, the procedure for correcting the basic assist characteristics will be described with reference to FIG. 3 again. In the example of FIG. 3, the lateral acceleration during steady turning and the lateral acceleration during turning transient are the same (see reference A1 and reference B1). Therefore, in the present embodiment, the basic assist characteristics are corrected so that the steering torque in the turning transition period is the same as the steering torque obtained during the steady turning (see symbol A3). Specifically, the basic assist characteristic indicated by the line segment 63 is corrected to the assist characteristic indicated by the line segment 73. As a result, during the turning transition period, the steering torque is changed from the point indicated by the symbol B3 to the point indicated by the symbol C. In this case, the amount of the changed steering torque (steering torque corresponding to the distance between the point indicated by the symbol B3 and the point indicated by the symbol C) is supplemented by the driving force by the EPS actuator 8. By correcting the assist characteristics as described above, the steering torque indicated by the reference symbol C and the lateral acceleration indicated by the reference symbol B1 are on the line segment 64 indicating the relational characteristic between the steering torque and the lateral acceleration that are established during steady turning. Come to be located. That is, the relational characteristic between the steering torque and the lateral acceleration becomes the same between the steady turning and the turning transition period.

  As described above, according to the steering assist control according to the present embodiment, the relational characteristic between the steering torque and the lateral acceleration can be kept constant regardless of the turning state, and thus the steering feeling during turning can be reduced. Can be improved. As a result, the driver can perform steering with peace of mind.

[Method of calculation]
Next, calculations performed in the above-described steering control and steering assist control will be described.

  First, calculations performed in the steering control will be described with reference to FIGS. 4 and 5. Here, the calculation method of the target yaw rate and the target vehicle body slip angle will be mainly described.

  FIG. 4 is a schematic diagram (two-wheel model) showing the state of the vehicle during a turn. In the example of FIG. 4, the various dimensions of the vehicle are the wheel base L, the longitudinal distance Lf from the center of gravity G to the front wheel 2f, and the longitudinal distance Lr from the center of gravity G to the rear wheel 2r. The vehicle is turning at a vehicle speed V (speed at the center of gravity G).

  Here, a method for calculating the target yaw rate γ will be described. The target yaw rate γ is calculated by the target yaw rate calculator 11 in the steering controller 10a based on the steering wheel angle MA and the vehicle speed V. Specifically, the target yaw rate calculation unit 11 calculates the target yaw rate γ using a map indicating the relationship between the steering wheel angle MA and the vehicle speed V and the target yaw rate γ.

  FIG. 5A shows an example of a map used for calculating the target yaw rate γ. FIG. 5A shows the vehicle speed V on the horizontal axis and the yaw rate gain Gγ on the vertical axis. The yaw rate gain Gγ is expressed by the following equation (1).

Gγ = γ / MA Formula (1)
The target yaw rate calculation unit 11 determines the yaw rate gain Gγ corresponding to the vehicle speed V using the map illustrated in FIG. 5A, and calculates the target yaw rate γ using the steering angle MA from the determined yaw rate gain Gγ. . Specifically, the target yaw rate γ is obtained by using Expression (2) obtained by modifying Expression (1).

γ = Gγ × MA Formula (2)
Next, a method for calculating the target vehicle body slip angle β will be described. The target vehicle body slip angle β is calculated based on the steering wheel angle MA and the vehicle speed V by the target vehicle body slip angle calculation unit 12 in the steering controller 10a. Specifically, the target vehicle body slip angle calculation unit 12 calculates the target vehicle body slip angle β using a map showing the relationship between the steering wheel angle MA and the vehicle speed V and the target vehicle body slip angle β.

  FIG. 5B shows an example of a map used for calculating the target vehicle body slip angle β. FIG. 5B shows the vehicle speed V on the horizontal axis and the vehicle body slip angle gain Gβ on the vertical axis. The vehicle body slip angle gain Gβ is expressed by the following equation (3).

Gβ = β / MA Formula (3)
The target vehicle body slip angle calculation unit 12 determines a vehicle body slip angle gain Gβ corresponding to the vehicle speed V using the map illustrated in FIG. 5B, and uses the steering wheel angle MA from the determined vehicle body slip angle gain Gβ. A target vehicle body slip angle β is calculated. Specifically, the target vehicle body slip angle β is obtained by using Expression (4) obtained by modifying Expression (3).

β = Gβ × MA Formula (4)
The steering angle calculation unit 13 uses the target yaw rate γ and the target vehicle body slip angle β obtained as described above, and the front wheel steering angle to be set for the front wheels 2f and the rear wheel to be set for the rear wheels 2r. Calculate the steering angle. Specifically, the steering angle calculation unit 13 calculates the front wheel steering angle and the rear wheel steering angle by using formulas (8) and (9) described later.

  Next, calculation performed in the steering assist control will be described. In this case, as shown in FIG. 4, the front wheel lateral force Yf and the rear wheel lateral force Yr act on the vehicle, the angle of the front wheel 2f is the front wheel steering angle δf, and the angle of the rear wheel 2r is the rear wheel steering. The angle is δr.

  First, the lateral force load distribution ratio Wc of the front wheels 2f during steady turning will be described. Expression (5) is an expression indicating the lateral force distribution ratio Wc of the front wheels 2f during steady turning.

Wc = Lr / (Lf + Lr)
= Lr / L Formula (5)
From equation (5), it can be seen that the lateral force burden distribution ratio Wc can be expressed using the wheel base L and the longitudinal distance Lr from the center of gravity G to the rear wheel 2r. That is, the lateral force burden distribution ratio Wc is a predetermined constant.

  Next, the lateral force burden distribution ratio Wt of the front wheels 2f in the turning transition period will be described. The following equation (6) shows the front wheel lateral force Yf in the turning transition period, and equation (7) shows the rear wheel lateral force Yr in the turning transition period.

2 × Yf = 2 × Kf × (δf−β−γ × Lf / V) Equation (6)
2 × Yr = 2 × Kr × (δr−β + γ × Lr / V) Equation (7)
In Equation (6) and Equation (7), “Kf” represents the cornering power per front wheel 2f, and “kr” represents the cornering power per rear wheel 2r. The front wheel lateral force Yf and the rear wheel lateral force Yr indicate the lateral force per wheel. Therefore, the lateral force acting on the front two wheels is “2 × Yf”, and the lateral force acting on the rear two wheels is “2 × Yr”.

  Here, according to the equation of motion in the vehicle, the front wheel steering angle δf is expressed by equation (8), and the rear wheel steering angle δr is expressed by equation (9). Note that “I” in the equations (8) and (9) indicates the vehicle yawing moment of inertia.

  By using the above formulas (6) to (9), the lateral force load distribution ratio Wt of the front wheel 2f in the turning transition period can be expressed by the formula (10).

  The upper equation of equation (10) indicates the total lateral force acting on the front wheel 2f and the rear wheel 2r in the denominator, and the front wheel lateral force in the numerator. The transformation from the upper expression to the lower expression in Expression (10) is performed by substituting Expression (8) into “δf” in Expression (6), and “δr” in Expression (7). The equation obtained by substituting equation (9) into is substituted by “Yf” and “Yr” in the upper equation of equation (10).

  As described above, it is possible to calculate the change rate Z of the lateral force distribution ratio Wt of the front wheels 2f in the turning transition period with respect to the lateral force distribution ratio Wc in the steady state of turning. Specifically, Expression (11) indicating the change ratio Z is obtained by using Expression (5) and Expression (10).

  At the time of steady turning, “dβ / dt” and “dγ / dt” in equation (11) are “0”, so the change ratio Z is “1”. On the other hand, in the turning transition period, when the lateral force load distribution ratio Wt becomes larger than the lateral force load distribution ratio Wc, the change ratio Z becomes larger than “1”. This case corresponds to a case where the ratio of the lateral force distributed to the front wheels 2f with respect to the total lateral force is larger than that during steady turning. On the contrary, when the lateral force burden distribution ratio Wt becomes smaller than the lateral force burden distribution ratio Wc, the change ratio Z becomes smaller than “1”. This case corresponds to a case where the ratio of the lateral force distributed to the front wheels 2f with respect to the total lateral force is smaller than that during steady turning.

  Using such a change ratio Z, an assist torque obtained by correcting the basic assist torque can be obtained. Specifically, the corrected assist torque can be obtained using the following equation (12).

Ts = Ta × Z Formula (12)
In Expression (12), “Ta” indicates the basic assist torque used in the basic assist characteristics, and “Ts” indicates the corrected assist torque. According to Expression (12), the assist torque is corrected according to the change rate Z. Specifically, when the change ratio Z is larger than 1, the corrected assist torque Ts is made larger than the basic assist torque Ta. That is, the amount of assisting the steering torque is increased. On the other hand, when the change ratio Z is smaller than 1, the corrected assist torque Ts is made smaller than the basic assist torque Ta. That is, the amount of assisting the steering torque is reduced.

  In the above example, the assist torque Ts obtained by correcting the basic assist torque Ta used in the basic assist characteristic is obtained instead of obtaining the assist characteristic obtained by correcting the basic assist characteristic. This is because in the steering assist control, the corrected assist torque Ts is directly used, and the assist characteristic obtained by correcting the basic assist characteristic is not directly used.

  FIG. 6 is a diagram illustrating a specific example of the relationship between the steering torque and the assist torque when the assist characteristics are changed. FIG. 6 shows the steering torque on the horizontal axis and the assist torque on the vertical axis. A curve 80 shows the relationship between the steering torque and the assist torque when the change ratio Z is “1”, that is, when the vehicle is in steady turning. A curve 81 shows an example of the relationship between the steering torque and the assist torque when the change ratio Z is larger than “1”. From this, it can be seen that the assist torque used for the same steering torque is larger than that during steady turning. On the other hand, the curve 82 shows an example of the relationship between the steering torque and the assist torque when the change ratio Z is smaller than “1”. From this, it can be seen that the assist torque used for the same steering torque is smaller than that during steady turning.

  By correcting the assist torque according to such a calculation method and performing the steering assist control using the corrected assist torque, it becomes possible to effectively keep the relational characteristic between the steering torque and the lateral acceleration constant. . Therefore, the steering feeling at the time of turning can be improved reliably.

1 is a schematic diagram illustrating a schematic configuration of a vehicle to which a steering control device according to an embodiment of the present invention is applied. It is a block diagram which shows schematic structure of a system controller. It is a figure for demonstrating the basic concept of the steering assist control which concerns on this embodiment. It is a figure which shows the two-wheel model of the vehicle at the time of turning. It is a figure for demonstrating a target yaw rate and a target vehicle body slip angle. It is a figure which shows the specific example of the relationship between steering torque and assist torque when changing an assist characteristic.

Explanation of symbols

1 Engine 2f Front Wheel 2r Rear Wheel 4 Handle 7f Front Wheel Actuator 7r Rear Wheel Actuator 8 EPS Actuator 10 System Controller 10a Steering Controller 10b EPS Controller 13 Steering Angle Calculation Unit 17 Assist Characteristic Correction Unit

Claims (2)

Steering control means for independently steering the front and rear wheels of the vehicle;
Steering assist control means for assisting steering based on an assist characteristic that the steering torque and the assist torque satisfy,
The steering assist control means includes assist characteristic correction means for correcting the assist characteristic so that a relational characteristic between the steering torque and lateral acceleration is kept constant during a steering operation .
The steering control means has a steering angle calculation means for calculating a front wheel steering angle to be set for the front wheel and a rear wheel steering angle to be set for the rear wheel,
The steering assist control means includes
A lateral force load distribution ratio indicating a ratio between the total lateral force generated in the vehicle and the lateral force generated in the front wheel based on the front wheel steering angle and the rear wheel steering angle calculated by the steering angle calculating means. A lateral force burden distribution ratio calculating means for calculating
Change rate calculating means for calculating a change rate corresponding to the ratio of the lateral force burden distribution ratio calculated by the lateral force load distribution ratio calculating means with respect to the lateral force load distribution ratio during steady turning;
The vehicle steering control device , wherein the assist characteristic correcting unit corrects the assist characteristic based on the change rate calculated by the change rate calculating unit . 2. The vehicle steering control device according to claim 1 , wherein the steering angle calculation means calculates the front wheel steering angle and the rear wheel steering angle so that a target yaw rate and a target vehicle body slip angle are obtained.

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