Steering control method and steering control device

Abstract

To impart further proper steering reaction force, in a steer-by-wire type steering control device in which a steering wheel and a steering ring are mechanically separated from each other.SOLUTION: In a steering control method, feedforward axial force being rack axial force for imparting steering reaction force corresponding to a steering angle of a steering wheel which is mechanically separated from a steering ring is calculated (S1), feedback axial force being rack axial force for returning force from a road face as steering reaction force, to a driver is calculated (S2), a slip angle of a front wheel is estimated (S3), a mixing rate of the feedforward axial force and the feedback axial force is set according to the estimated slip angle so as to lower a ratio of the feedforward axial force when a change rate of the rack axial force with respect to the estimated slip angle is equal to or lower than a prescribed value (S4), and the steering reaction force is imparted on the basis of mixed axial force which is obtained by mixing the feedforward axial force and the feedback axial force at the mixing rate (S5, S6).SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a steering control method and a steering control device. [Background technology]

[0002] One known steering control device is described in Patent Document 1. This steering control device drives a reaction motor based on a control amount of the steering reaction force, which is based on the steering angle, and a control amount calculated by multiplying the current of the steering motor by a set gain. As a result, in this conventional technology, the influence of an external force acting on the steered wheels is reflected in the steering reaction force. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-108914 Summary of the Invention [Problem to be solved by the invention]

[0004] In the prior art, the reaction motor is driven based on a control amount of the steering reaction force based on the steering angle and a control amount calculated by multiplying the current of the steering motor by a set gain. Therefore, in the prior art, there is a possibility that the steering reaction force may become inappropriate if, for example, the accuracy of the control amount of the steering reaction force based on the steering angle decreases. An object of the present invention is to provide a steering control device of a steer-by-wire system in which the steering wheel and the steered wheels are mechanically separated, so that a more appropriate steering reaction force can be applied. [Means for solving the problem]

[0005] A steering control method according to one aspect of the present invention calculates a feedforward axial force, which is a rack axial force that provides a steering reaction force according to the steering angle of a steering wheel that is mechanically separated from the steered wheels; calculates a feedback axial force, which is a rack axial force that returns a force from the road surface to the driver as a steering reaction force; estimates a slip angle of the front wheels; sets a mixing ratio of the feedforward axial force and the feedback axial force according to the estimated slip angle so as to reduce the proportion of the feedforward axial force when the rate of change of the rack axial force with respect to the estimated slip angle is equal to or less than a predetermined value; and applies a steering reaction force based on a mixed axial force obtained by mixing the feedforward axial force and the feedback axial force at the mixing ratio. [Effects of the Invention]

[0006] According to the present invention, a more appropriate steering reaction force can be applied in a steer-by-wire type steering control device in which the steering wheel and the steered wheels are mechanically separated. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a schematic configuration diagram of an example of a steering control device according to an embodiment; [Figure 2] FIG. 2 is a block diagram of an example of a functional configuration of a controller. [Figure 3] FIG. 4 is a block diagram of an example of a functional configuration of a target steering reaction force calculation unit. [Figure 4] FIG. 4 is a diagram showing an example of an axial force-steering reaction force conversion map. [Figure 5] FIG. 4 is an explanatory diagram of the relationship between the front wheel slip angle and the rack axial force. [Figure 6] FIG. 2 is a block diagram illustrating an example of a functional configuration of a mixture ratio setting unit according to the first embodiment. [Figure 7] FIG. 4 is a diagram showing an example of a mixture ratio coefficient map according to the first embodiment. [Figure 8] 4 is a flowchart of an example of a steering control method according to an embodiment. [Figure 9] FIG. 10 is a block diagram illustrating an example of the functional configuration of a mixture ratio setting unit according to a second embodiment. [Figure 10]FIG. 10 is a diagram illustrating an example of a mixture ratio coefficient map according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the drawings are schematic and may differ from the actual product. Furthermore, the embodiments of the present invention shown below are examples of devices and methods for embodying the technical concept of the present invention, and the technical concept of the present invention does not limit the structure, arrangement, etc. of component parts to those described below. The technical concept of the present invention can be modified in various ways within the technical scope defined by the claims.

[0009] (First embodiment) (composition) 1 is a schematic diagram of an example of a steering control device according to an embodiment. In the following description, a vehicle equipped with the steering control device according to the embodiment will be referred to as a "host vehicle." The steering control device according to the embodiment is a steer-by-wire type steering control device that can mechanically separate a steering wheel 1a from front wheels 2, which are steered wheels. The steering control device of the embodiment includes a steering angle sensor 3, a turning angle sensor 4, a vehicle speed sensor 5, an acceleration sensor 6, a yaw rate sensor 7, a turning control unit 8, a reaction force control unit 9, and a controller 11.

[0010] The steering angle sensor 3 detects the steering angle δ of the steering wheel 1. For example, the steering angle sensor 3 may calculate the steering angle δ based on the amount of rotation of the steering shaft 1b. The steering angle sensor 3 outputs information on the detected steering angle δ to the controller 11. The steering angle sensor 4 detects the steering angle θ of the front wheels 2. For example, the steering angle sensor 4 may detect the steering angle θ based on the rack movement amount of the steering rack or the rotation angle of the pinion shaft. The steering angle sensor 4 outputs information about the detected steering angle θ to the controller 11. The vehicle speed sensor 5 detects the vehicle speed V of the host vehicle. The vehicle speed sensor 5 outputs information on the detected vehicle speed V to the controller 11.

[0011] The acceleration sensor 6 detects the lateral acceleration Gy acting on the vehicle and outputs information on the detected lateral acceleration Gy to the controller 11. The yaw rate sensor 7 detects the yaw rate γa of the host vehicle. In the following description, the yaw rate detected by the yaw rate sensor 7 will be referred to as the “actual yaw rate.” The yaw rate sensor 7 outputs information on the actual yaw rate γa to the controller 11.

[0012] The steering control unit 8 includes a steering motor 8A, a steering current detection unit 8B, and a steering motor drive unit 8C. Steering motor 8A is connected to pinion shaft 10d via a reducer. Steering motor 8A is driven by steering motor drive unit 8C and moves steering rack 10a left and right via pinion shaft 10d and pinion gear 10e. In this way, steering motor 8A steers front wheels 2. Steering motor 8A may be driven, for example, by controlling steering current Itm, which is the current flowing through steering motor 8A.

[0013] The steering current detection unit 8B detects the steering current Itm and outputs a signal indicating the steering current Itm to the steering motor drive unit 8C and the controller 11. Based on the target turning current Itt calculated by controller 11, steering motor drive section 8C controls the turning current Itm of steering motor 8A so that the turning current Itm detected by steering current detection section 8B matches the target turning current Itt. In this way, steering motor drive section 8C drives steering motor 8A. The target turning current Itt is a target value for the current flowing through steering motor 8A.

[0014] The reaction force control unit 9 includes a reaction force motor 9A, a reaction force current detection unit 9B, and a reaction force motor drive unit 9C. The reaction motor 9A is connected to the steering shaft 1b via a reducer. The reaction motor 9A is driven by a reaction motor drive unit 9C and applies a rotational torque to the steering wheel 1a via the steering shaft 1b. This causes the reaction motor 9A to generate a steering reaction force. The reaction motor 9A may be driven, for example, by controlling a reaction current Ism flowing through the reaction motor 9A. The reaction force current detection unit 9B detects the reaction force current Ism and outputs a detection signal indicating the reaction force current Ism to the reaction force motor drive unit 9C and the controller 11. The reaction force motor driving unit 9C controls the reaction force current Ism of the reaction force motor 9A based on the target reaction force current Ist calculated by the controller 11 so that the reaction force current Ism detected by the reaction force current detecting unit 9B coincides with the target reaction force current Ist. In this way, the reaction force motor driving unit 9C drives the reaction force motor 9A. The target reaction force current Ist is a target value for the current flowing through the reaction force motor 9A.

[0015] Backup clutch 12 is provided between steering shaft 1b and pinion shaft 10b. Pinion shaft 10b is connected to steering rack 10a via pinion gear 10c, and when backup clutch 12 is engaged, steering shaft 1b and pinion shaft 10b are connected, thereby mechanically connecting steering wheel 1a and front wheels 2. When backup clutch 12 is released, steering shaft 1b and pinion shaft 10b are disconnected, thereby mechanically disconnecting steering wheel 1a and front wheels 2. Backup clutch 12 is engaged when some abnormality occurs in the system, such as an abnormality in steering motor 8A or reaction motor 9A, or when the vehicle ignition switch is turned off (for example, when parked), and is released under normal circumstances, such as when the vehicle is running or the ignition switch is turned on. In the following description, it is assumed that backup clutch 12 is in a released state, and that steering wheel 1a and front wheels 2 are mechanically separated.

[0016] Controller 11 is an electronic control unit that controls the driving of steering motor 8A by steering control unit 8 and the driving of reaction force motor 9A by reaction force control unit 9. Controller 11 may include a processor 20 and peripheral components such as a storage device 21. Processor 20 may be, for example, a CPU (Central Processing Unit) or an MPU (Micro-Processing Unit). The storage device 21 may include any of a semiconductor storage device, a magnetic storage device, and an optical storage device. The storage device 21 may include memories such as a register, a cache memory, a ROM (Read Only Memory) used as a main storage device, and a RAM (Random Access Memory). The functions of the controller 11 described below are realized by, for example, the processor 20 executing a computer program stored in the storage device 21. The controller 11 may be realized by a functional logic circuit set in a general-purpose semiconductor integrated circuit. For example, the controller 11 may have a programmable logic device (PLD) such as a field-programmable gate array (FPGA).

[0017] 2 shows an example of the functional configuration of controller 11. Controller 11 includes target turning angle calculation section 11A, target steering reaction force calculation section 11B, target turning current calculation section 11C, and subtractor 11D. Target steering angle calculation unit 11A calculates target steering angle θt, which is a target value for steering angle θ, based on steering angle δ detected by steering angle sensor 3 and vehicle speed V detected by vehicle speed sensor 5. Target steering angle θt may be calculated, for example, by multiplying steering angle δ by a variable gear ratio for steering angle δ and steering angle θ. Subtractor 11D calculates deviation Δθ by subtracting the steering angle detected by steering angle sensor 4 from target steering angle θt. Target steering current calculation unit 11C calculates target steering current Itt based on deviation Δθ. Target steering current calculation unit 11C outputs target steering current Itt to steering motor drive unit 8C.

[0018] Target steering reaction force calculation unit 11B calculates target reaction force current Ist based on steering angle δ detected by steering angle sensor 3, vehicle speed V detected by vehicle speed sensor 5, turning angle θ detected by turning angle sensor 4, actual yaw rate γa detected by yaw rate sensor 7, lateral acceleration Gy detected by acceleration sensor 6, and turning current Itm detected by turning current detection unit 8B. Target steering reaction force calculation unit 11B outputs the calculated target reaction force current Ist to reaction force motor drive unit 9C. See Fig. 3. Target steering reaction force calculation unit 11B includes a feedforward axial force calculation unit 30, a feedback axial force calculation unit 31, a mixture ratio setting unit 32, a subtractor 33, multipliers 34 and 35, an adder 36, a conversion unit 37, and a target reaction force current calculation unit 38. In the following description and drawings, the feedforward axial force may be referred to as the "FF axial force" and the feedback axial force as the "FB axial force."

[0019] The FF axial force calculation unit 30 calculates the FF axial force Tff, which is a steering rack axial force that provides a steering reaction force corresponding to the steering angle δ, based on the steering angle δ and the vehicle speed V. The steering rack axial force is the rack axial force applied to the steering rack 10a. The FF axial force calculation unit 30 may calculate the FF axial force Tff, for example, according to the following equation (1). Tff=(Ks+Cs·s) / (Jr·s 2 +(Cr+Cs)·s+Ks)·k·V / (1+A·V 2 )·θ+Ks(Jr·s 2 +Cr·s) / (Jr·s 2 +(Cr+Cs)·s+Ks)·θ … (1) In equation (1), Ks is pinion stiffness, Cs is pinion viscosity, Jr is rack inertia, Cr is rack viscosity, and k and A are preset constants. The FF axial force calculation unit 30 outputs the calculation result to the multiplier 34.

[0020] The FB axial force calculation unit 31 calculates the FB axial force Tfb based on the lateral acceleration Gy, the steering current Itm, the actual yaw rate γa, and the vehicle speed V. The FB axial force Tfb is a steering rack axial force that applies a force from the road surface to the steering wheel 1a as a steering reaction force and returns it to the driver. For example, the FB axial force calculation unit 31 may calculate the FB axial force Tfb as shown below. First, the FB axial force calculation unit 31 calculates the steering rack axial force (hereinafter sometimes referred to as "lateral G axial force") corresponding to the lateral acceleration Gy according to the following equation (2). Lateral G axial force = front wheel load × lateral acceleration Gy × link ratio … (2) In equation (2), the link ratio is a constant that depends on the angle of the link and the suspension.

[0021] Further, the FB axial force calculation unit 31 calculates the steering rack axial force (hereinafter, sometimes referred to as "current axial force") corresponding to the steering current Itm according to the following equation (3). Current axial force = steering current Itm × motor gear ratio × torque constant [Nm / A] / pinion radius [m] × efficiency … (3) In equation (2), the motor gear ratio is the gear ratio of the reduction gear that transmits the motor torque of steering motor 8A, the torque constant is the torque constant of steering motor 8A, and the pinion radius is the pinion radius of pinion gear 10e.

[0022] The FB axial force calculation unit 31 also calculates the steering rack axial force (hereinafter sometimes referred to as "yaw rate axial force") according to the vehicle speed V and the actual yaw rate γa in accordance with the following equation (4). Yaw rate axial force = front wheel load × vehicle speed V × actual yaw rate γa × link ratio … (4) Then, the FB axial force calculation unit 31 calculates the FB axial force Tfb in accordance with the following equation (5) based on the calculated lateral G axial force, current axial force, and yaw rate axial force. Tfb = Lateral G axial force × K1 + Current axial force × K2 + Yaw rate axial force × K3 … (5)

[0023] The mixing ratio setting unit 32 sets a mixing ratio Gf:(1-Gf) for calculating a mixed axial force by mixing the FF axial force Tff and the FB axial force Tfb. In the following description, the coefficient X that determines the mixing ratio X:(1-X) may be referred to as a "mixing ratio coefficient X." The details of the mixture ratio setting unit 32 will be described later. A subtractor 33, multipliers 34 and 35, and an adder 36 mix the FF axial force Tff and the FB axial force Tfb at a mixing ratio Gf:(1-Gf) to calculate a mixed axial force given by the following equation (6). Mixed axial force=Tff×Gf+Tfb×(1-Gf) … (6)

[0024] Conversion unit 37 calculates a target steering reaction force based on the mixed axial force calculated by subtractor 33, multipliers 34 and 35, and adder 36. The target steering reaction force is a target value of the steering reaction force. For example, conversion unit 37 may convert the mixed axial force into the target steering reaction force using an axial force-to-steering reaction force conversion map that defines a target steering reaction force corresponding to the vehicle speed V and the axial force. An example of the axial force-to-steering reaction force conversion map is shown in FIG. 4. The axial force-to-steering reaction force conversion map is set for each vehicle speed V. In the axial force-to-steering reaction force conversion map, the larger the final axial force, the larger the value of the target steering reaction force. See Fig. 3. Target reaction force current calculation unit 38 calculates target reaction force current Ist according to the following equation (7) based on the target steering reaction force calculated by conversion unit 37. Target reaction force current calculation unit 38 outputs the calculation result to reaction force motor drive unit 9C. Target reaction force current Ist = target steering reaction force × gain … (7)

[0025] Next, a description will be given of the mixture ratio setting unit 32. The mixture ratio setting unit 32 sets the mixture ratio Gf:(1-Gf) according to the driving scene of the host vehicle. In everyday driving situations, for example, situations where the vehicle behavior can respond to steering input from the driver, that is, in situations where the force generated in the tires of the front wheels 2, which are steered wheels, is smaller than the friction limit, the mixture ratio setting unit 32 sets the mixture ratio Gf:(1-Gf) with a high proportion of the FF axial force Tff so that a steering reaction force is applied using a stable FF axial force Tff that is not affected by external disturbances. On the other hand, in unusual situations, such as situations where the vehicle behavior cannot keep up with the steering input from the driver, such as when the force generated in the front wheel 2 tires approaches or exceeds the friction limit (i.e., when understeer occurs), the accuracy of the FF axial force Tff decreases. Therefore, in such situations, the mixing ratio Gf:(1-Gf) is set by increasing the proportion of the FB axial force Tfb (i.e., decreasing the proportion of the FF axial force Tff) so that the steering reaction force is applied using the FB axial force Tfb, which makes it easier to understand the friction limit and information from the road surface. This makes it possible to switch the rack axial force used to calculate the target steering reaction force from FF axial force Tff to FB axial force Tfb.

[0026] As a method for switching between the FF axial force Tff and the FB axial force Tfb, for example, when the absolute value of the axial force difference, which is the difference between the FF axial force Tff and the FB axial force Tfb, is equal to or greater than a set value, it is possible to switch the mixture ratio Gf:(1-Gf) to reduce the proportion of the FF axial force Tff and increase the proportion of the FB axial force Tfb. However, with this method, the mixing ratio Gf:(1-Gf) is changed after the axial force difference occurs, so the change may be delayed until the force generated in the front tires approaches or exceeds the friction limit. Also, since the index such as the actual yaw rate γa used to calculate the FB axial force Tfb acts after the influence of the friction limit appears in the vehicle behavior, the axial force difference itself may also be delayed.

[0027] Referring to Figure 5, the problem caused by a delay in switching the mixture ratio Gf:(1-Gf) will be explained. The solid line L1 schematically shows the relationship between the slip angle βf of the front wheels 2 and the rack axial force actually acting on the steering rack 10a. The dashed line L2 schematically shows the relationship between the slip angle βf of the front wheels 2 and the FF axial force Tff. The slip angle βmax indicates the saturated slip angle at which the force acting on the tires of the front wheels 2 reaches the friction limit. In the following explanation, the slip angle of the front wheels 2 may be referred to as the "front wheel slip angle." As the front wheel slip angle βf increases and approaches the saturation slip angle βmax, the rate of increase in the rack axial force decreases. When the front wheel slip angle βf reaches the saturation slip angle βmax, the actual rack axial force saturates and no longer increases even if the front wheel slip angle βf increases. When the front wheel slip angle βf exceeds the saturation slip angle βmax, the actual rack axial force begins to decrease. For this reason, when the front wheel slip angle βf exceeds the saturation slip angle βmax, the difference between the actual rack axial force (solid line) and the FF axial force Tff (dashed line) becomes large.

[0028] Therefore, if the change in the mixing ratio Gf:(1-Gf) is delayed and begins after the front wheel slip angle βf exceeds the saturated slip angle βmax, the mixed axial force will have the characteristics shown by the dashed dotted line L3. If a steering reaction force is generated in response to a mixed axial force such as that indicated by the dashed-dotted line L3, the strong steering reaction force felt until the friction limit is exceeded will suddenly decrease after the friction limit is exceeded, which may cause the driver to feel uncomfortable.

[0029] Therefore, the mixing ratio setting unit 32 sets the mixing ratio Gf:(1-Gf) so that the proportion of the FF axial force Tff is reduced before the force generated in the tires of the front wheels 2 reaches the friction limit. Specifically, the front wheel slip angle βf is estimated, and the mixing ratio Gf:(1-Gf) is set in accordance with the estimated front wheel slip angle βf so that the proportion of the FF axial force Tff is reduced when the rate of change of the rack axial force with respect to the estimated front wheel slip angle βf is equal to or less than a predetermined value. This allows the steering reaction force to be gradually reduced before the force acting on the front tire reaches the friction limit, allowing the driver to predict the friction limit before the force acting on the front tire reaches the friction limit and reflect this in subsequent steering.

[0030] 6 is a block diagram of an example of the functional configuration of the mixing ratio setting unit 32 of the first embodiment. The mixing ratio setting unit 32 includes a first estimated front wheel slip angle calculation unit 40, a second estimated front wheel slip angle calculation unit 41, an estimated rear wheel slip angle calculation unit 42, a front wheel lateral force estimator 43, a rear wheel lateral force estimator 44, a yaw rate lateral velocity estimator 45, a first mixing ratio calculation unit 46, a second mixing ratio calculation unit 47, and a selection unit 48.

[0031] First estimated front wheel slip angle calculation unit 40 calculates a first estimated front wheel slip angle βf1, which is an estimate of front wheel slip angle βf, based on actual yaw rate γa, steering angle θ, vehicle speed V, and estimated lateral velocity Vye, which is an estimate of the lateral velocity of the host vehicle calculated based on a vehicle model by yaw rate lateral velocity estimator 45. As the vehicle model, for example, a linear two-wheel model may be used. For example, the first estimated front wheel slip angle calculation section 40 may calculate the first estimated front wheel slip angle βf1 according to the following equation (8). βf1=θ / N-(Vye+Lf×γa) / V … (8) In equation (8), N is the overall gear ratio, and Lf is the length from the center of gravity of the vehicle to the front wheel axle.

[0032] The second estimated front-wheel slip angle calculation unit 41 calculates a second estimated front-wheel slip angle βf2, which is an estimate of the front-wheel slip angle βf based on the vehicle model, based on the estimated yaw rate γe, which is the yaw rate calculated by the yaw-rate lateral velocity estimation unit 45 based on the vehicle model, the turning angle θ, the vehicle speed V, and the estimated lateral velocity Vye. The estimated yaw rate γe is an example of the "standard yaw rate" described in the claims. For example, the second estimated front wheel slip angle calculation section 41 may calculate the second estimated front wheel slip angle βf2 according to the following equation (9). βf2=θ / N-(Vye+Lf×γe) / V … (9) The estimated rear wheel slip angle calculation unit 42 calculates an estimated rear wheel slip angle βr, which is an estimated value of the slip angle of the rear wheels based on the vehicle model, based on the estimated yaw rate γe, the vehicle speed V, the estimated lateral velocity Vye, and the vehicle model. For example, the estimated rear wheel slip angle calculation section 42 may calculate the estimated rear wheel slip angle βr based on the following equation (10). βr=-(Vye+Lr×γe) / V … (10) In equation (10), Lr is the length from the center of gravity of the vehicle to the rear wheel axle.

[0033] The front wheel lateral force estimator 43 calculates an estimated front wheel lateral force Cf, which is an estimated value of the front wheel lateral force based on the vehicle model, based on the second estimated front wheel slip angle βf2. For example, the front wheel lateral force estimating section 43 may calculate the estimated front wheel lateral force Cf based on the following equation (11). Cf=Kf×βf2 … (11) In equation (10), Kf is the cornering power of the front wheels. The rear wheel lateral force estimating unit 44 calculates an estimated rear wheel lateral force Cr, which is an estimated value of the rear wheel lateral force based on the vehicle model, based on the estimated rear wheel slip angle βr. For example, the front wheel lateral force estimating section 43 may calculate the estimated rear wheel lateral force Cr based on the following equation (12). Cr=Kr×βr … (12) In equation (12), Kr is the cornering power of the rear wheels.

[0034] The yaw rate / lateral velocity estimating unit 45 calculates the estimated yaw rate γe and the estimated lateral velocity Vye based on the estimated front wheel lateral force Cf, the estimated rear wheel lateral force Cr, and the vehicle speed V. For example, the yaw rate / lateral velocity estimating unit 45 may calculate the estimated yaw rate γe and the estimated lateral velocity Vye based on the following equations (13) to (16). Iz×γ'=2×(Lf×Cf-Lr×Cr) … (13) M×Vy'=2×(Cf+Cr)-M×Vx×γe … (14) γe=∫γ'dt … (15) Vye=∫Vy'dt … (16) In equation (13), Iz is the yaw moment of inertia, γ' is the yaw angular acceleration, and in equation (14), Vy' is the lateral acceleration.

[0035] The first mixing ratio calculation unit 46 calculates a first mixing ratio coefficient Gf1 based on a first estimated front wheel slip angle βf1 calculated based on the actual yaw rate γa. The second mixing ratio calculation unit 47 calculates a second mixing ratio coefficient Gf2 based on a second estimated front wheel slip angle βf2 calculated based on the estimated yaw rate γe. For example, the first mixing ratio calculation unit 46 and the second mixing ratio calculation unit 47 may calculate the first and second mixing ratio coefficients Gf1 and Gf2 from the first and second estimated front wheel slip angles βf1 and βf2 using a mixing ratio coefficient map that defines mixing ratio coefficients corresponding to the front wheel slip angles.

[0036] 7 is a diagram showing an example of a mixture ratio coefficient map according to the first embodiment. When the first and second estimated front wheel slip angles βf1 and βf2 are within a range equal to or less than the threshold value β1, the first and second mixture ratio coefficients Gf1 and Gf2 are set to a preset value g1 (e.g., 1.0) regardless of the magnitudes of the first and second estimated front wheel slip angles βf1 and βf2. When the first and second estimated front wheel slip angles βf1 and βf2 are in a range equal to or greater than the threshold value β2, the first and second mixture ratio coefficients Gf1 and Gf2 are set to a value g2 (e.g., 0.0) smaller than the value g1, regardless of the magnitude of the first and second estimated front wheel slip angles βf1 and βf2. When the first and second estimated front wheel slip angles βf1, βf2 are in a range greater than threshold value β1 and smaller than threshold value β2, the first and second mixing ratio coefficients Gf1, Gf2 are decreased as the first and second estimated front wheel slip angles βf1, βf2 increase. For example, the first and second mixing ratio coefficients Gf1, Gf2 may be decreased linearly as the first and second estimated front wheel slip angles βf1, βf2 increase.

[0037] As described with reference to FIG. 5, as the front wheel slip angle βf increases and approaches the saturated slip angle βmax, the rate of increase in the rack axial force relative to the front wheel slip angle βf decreases. Therefore, the front wheel slip angle βf at which the rate of increase in rack axial force relative to the front wheel slip angle βf is equal to or less than a predetermined value is determined in advance by calculation or experiment, and set as the threshold value β1. As a result, the threshold value β1 is set to a value smaller than the saturated slip angle βmax. By setting the threshold value β1 in this manner, the first and second mixture ratio coefficients Gf1 and Gf2 can be set so as to decrease when the rate of increase in rack axial force relative to the front wheel slip angle βf is equal to or less than a predetermined value.

[0038] The selector 48 selects the smaller of the first mixing ratio coefficient Gf1 and the second mixing ratio coefficient Gf2 and outputs it to the subtractor 33 and the multiplier 34 as the mixing ratio coefficient Gf. By using this mixing ratio coefficient Gf to calculate the mixed axial force = Tff × Gf + Tfb × (1 - Gf), the proportion of the FF axial force Tff in the mixed axial force can be reduced when the rate of increase in the rack axial force relative to the front wheel slip angle βf is equal to or less than a predetermined value. As a result, before the force acting on the front tires reaches the friction limit, the rack axial force used to calculate the target steering reaction force can be switched from the FF axial force Tff to the FB axial force Tfb. This allows the steering reaction force to be gradually reduced before the friction limit is reached, allowing the driver to detect in advance that the friction limit is being approached and reflect this in subsequent steering.

[0039] (operation) FIG. 8 is a flowchart of an example of a steering control method according to an embodiment. In step S1, the FF axial force calculation unit 30 calculates the FF axial force Tff. In step S2, the FB axial force calculation unit 31 calculates the FB axial force Tfb. In step S3, the first estimated front wheel slip angle calculation section 40 and the second estimated front wheel slip angle calculation section 41 calculate the first estimated front wheel slip angle βf1 and the second estimated front wheel slip angle βf2, respectively.

[0040] In step S4, the first mixing ratio calculation unit 46, the second mixing ratio calculation unit 47, and the selection unit 48 set the mixing ratio Gf:(1-Gf) based on the first estimated front wheel slip angle βf1 and the second estimated front wheel slip angle βf2. In step S5, the subtractor 33, multipliers 34 and 35, and adder 36 calculate a mixed axial force by mixing the FF axial force Tff and the FB axial force Tfb at a mixing ratio Gf:(1-Gf). The conversion unit 37 calculates a target steering reaction force based on the mixed axial force. In step S6, the target reaction force current calculation unit 38 and the reaction force motor drive unit 9C drive the reaction force motor 9A based on the target steering reaction force, after which the process ends.

[0041] (Second embodiment) The saturated slip angle, at which the force acting on the tires of the front wheels 2 reaches the friction limit, varies depending on the friction coefficient of the road surface. That is, when the friction coefficient is small, the saturated slip angle is smaller than when the friction coefficient is large. For this reason, the mixing ratio setting unit 32 of the second embodiment estimates the friction coefficient of the road surface, and sets the mixing ratio Gf:(1-Gf) so that the proportion of the FF axial force Tff is reduced at smaller front wheel slip angles βf when the friction coefficient is small compared to when the friction coefficient is large.

[0042] Fig. 9 is a block diagram of an example of the functional configuration of the mixing ratio setting unit 32 of the second embodiment. The mixing ratio setting unit 32 of the second embodiment has a configuration similar to that of the mixing ratio setting unit 32 of the first embodiment shown in Fig. 6. Therefore, the same or similar components are denoted by the same reference numerals. The mixture ratio setting unit 32 of the second embodiment includes a road surface friction coefficient estimating unit 49 that estimates the friction coefficient of the road surface on which the vehicle is traveling. For example, road surface friction coefficient estimating unit 49 may estimate the friction coefficient based on the ratio of wheel speed to driving force, the ratio of wheel speed to braking force, the wheel speed difference between driving wheels and driven wheels, etc. Also, for example, it may recognize whether the weather at the location of the host vehicle is sunny, cloudy, rainy, or snowy from an image of the surroundings of the host vehicle captured by a camera or the like, and estimate whether the road surface on which the host vehicle is traveling is in a dry state with a relatively high friction coefficient, or in a wet state with a relatively low friction coefficient.

[0043] The first mixing ratio calculation unit 46 changes the threshold value of the first estimated front wheel slip angle βf1 at which the first mixing ratio coefficient Gf1 is switched, depending on the result of friction coefficient estimation by the road surface friction coefficient estimation unit 49. Similarly, the second mixing ratio calculation unit 47 changes the threshold value of the second estimated front wheel slip angle βf2 at which the second mixing ratio coefficient Gf2 is switched, depending on the result of friction coefficient estimation by the road surface friction coefficient estimation unit 49. Specifically, when the friction coefficient is small, the threshold values of the first and second estimated front wheel slip angles βf1, βf2 for reducing the first and second mixture ratio coefficients Gf1, Gf2 are made smaller than when the friction coefficient is large.

[0044] For example, the first mixing ratio calculation unit 46 and the second mixing ratio calculation unit 47 may have two types of maps: a mixing ratio coefficient map to be used in a dry state where the coefficient of friction is relatively large, and a mixing ratio coefficient map to be used in a wet state where the coefficient of friction is relatively small. The first mixing ratio calculation unit 46 and the second mixing ratio calculation unit 47 may calculate the first and second mixing ratio coefficients Gf1, Gf2 based on the mixing ratio coefficient map for a dry state when the road surface is estimated to be in a dry state, and may calculate the first and second mixing ratio coefficients Gf1, Gf2 based on the mixing ratio coefficient map for a wet state when the road surface is estimated to be in a wet state.

[0045] 10 is a diagram showing an example of a mixing ratio coefficient map according to the second embodiment. The solid line Ld represents the mixing ratio coefficient map for dry conditions, and the dashed-dotted line Lw represents the mixing ratio coefficient map for wet conditions. The slip angles βdmax and βwmax represent the saturated slip angles in dry and wet conditions, respectively. First, the front wheel slip angle βf at which the rate of increase in rack axial force relative to the front wheel slip angle βf is equal to or less than a predetermined value in both dry and wet conditions is determined in advance as appropriate through calculation or experiment, and set as threshold values βd1 and βw1. As a result, the threshold values βd1 and βw1 are set to values smaller than the saturated slip angles βdmax and βwmax, respectively. The threshold value βw1 is also set to a value smaller than the threshold value βd1.

[0046] In the case of the mixture ratio coefficient map for dry conditions (solid line), when the first and second estimated front wheel slip angles βf1, βf2 are within the range of threshold value βd1 or less, the first and second mixture ratio coefficients Gf1, Gf2 are set to the value g1 regardless of the magnitude of the first and second estimated front wheel slip angles βf1, βf2. When the first and second estimated front wheel slip angles βf1, βf2 are in a range equal to or greater than the threshold value βd2, the first and second mixture ratio coefficients Gf1, Gf2 are set to the value g2 regardless of the magnitude of the first and second estimated front wheel slip angles βf1, βf2. When the first and second estimated front wheel slip angles βf1, βf2 are in a range greater than the threshold value βd1 and smaller than the threshold value βd2, the first and second mixture ratio coefficients Gf1, Gf2 are decreased as the first and second estimated front wheel slip angles βf1, βf2 increase.

[0047] In the case of the mixture ratio coefficient map for wet conditions (dashed line), when the first and second estimated front wheel slip angles βf1, βf2 are within the range of threshold value βw1, the first and second mixture ratio coefficients Gf1, Gf2 are set to the value g1 regardless of the magnitude of the first and second estimated front wheel slip angles βf1, βf2. When the first and second estimated front wheel slip angles βf1, βf2 are equal to or greater than the threshold value βw2, the first and second mixing ratio coefficients Gf1, Gf2 are set to the value g2 regardless of the magnitude of the first and second estimated front wheel slip angles βf1, βf2. Note that the threshold value βw2 is set to a value smaller than the threshold value βd2 of the mixing ratio coefficient map for dry conditions. When the first and second estimated front wheel slip angles βf1, βf2 are in a range greater than the threshold value βw1 and smaller than the threshold value βw2, the first and second mixture ratio coefficients Gf1, Gf2 are decreased as the first and second estimated front wheel slip angles βf1, βf2 increase.

[0048] The mixture ratio coefficient map in FIG. 10 shows an example in which two mixture ratio coefficient maps, one for dry conditions and one for wet conditions, are used. However, three or more mixture ratio coefficient maps may be used in accordance with three or more levels of road surface friction coefficient. In addition, the mixture ratio coefficient map may be continuously changed according to the friction coefficient so that the slip angle threshold (e.g., threshold β1 in FIG. 7) at which the first and second mixture ratio coefficients Gf1 and Gf2 start to decrease becomes smaller as the friction coefficient of the road surface becomes smaller.

[0049] (Effects of the embodiment) (1) The controller 11 calculates a feedforward axial force, which is a rack axial force that provides a steering reaction force according to the steering angle of a steering wheel that is mechanically separated from the steered wheels, calculates a feedback axial force, which is a rack axial force that returns a force from the road surface to the driver as a steering reaction force, estimates a slip angle of the front wheels, sets a mixing ratio of the feedforward axial force and the feedback axial force according to the estimated slip angle so as to reduce the proportion of the feedforward axial force when the rate of change of the rack axial force with respect to the estimated slip angle is equal to or less than a predetermined value, and applies a steering reaction force based on a mixed axial force obtained by mixing the feedforward axial force and the feedback axial force at the mixing ratio. This allows a steering reaction force to be applied that gradually decreases before the force acting on the front tires reaches the friction limit when understeer occurs, allowing the driver to predict the friction limit before the force acting on the front tires reaches the friction limit and reflect this in subsequent steering.

[0050] (2) The controller 11 may estimate the friction coefficient of the road surface, and set the mixing ratio so that the proportion of the feedforward axial force is reduced at a smaller slip angle when the friction coefficient is small compared to when the friction coefficient is large. This allows for the application of a steering reaction force that gradually begins to decrease before the force acting on the front tire reaches the friction limit, even if the friction coefficient of the road surface decreases and the front wheel slip angle at which the friction limit is reached becomes smaller.

[0051] (3) An actual yaw rate that is an actual yaw rate occurring in the vehicle may be detected, a first estimated slip angle that is an estimate of the slip angle of the front wheels may be calculated based on the detected actual yaw rate, a first mixing ratio that is a mixing ratio of the feedforward axial force and the feedback axial force may be set based on the first estimated slip angle, a standard yaw rate that is a standard value of the yaw rate occurring in the vehicle may be calculated based on the steering angle of the steered wheels, the vehicle speed, and a vehicle model, a second estimated slip angle that is an estimate of the slip angle of the front wheels may be calculated based on the calculated standard yaw rate, a second mixing ratio that is a mixing ratio of the feedforward axial force and the feedback axial force may be set based on the second estimated slip angle, and the mixed axial force may be calculated by mixing the feedforward axial force and the feedback axial force at either the first mixing ratio or the second mixing ratio, whichever has a smaller proportion of the feedforward axial force. In this way, by making a judgment based on the front wheel slip angle calculated from the actual yaw rate, it is possible to generate a steering reaction force that reflects the tendency toward understeer at an early stage. On the other hand, when oversteer occurs, the front wheel slip angle calculated from the actual yaw rate tends to be smaller than the actual slip angle. By using the front wheel slip angle calculated from the standard yaw rate in combination, it is possible to set an appropriate mixing ratio even when oversteer occurs. [Explanation of symbols]

[0052] 1a...steering wheel, 1b...steering shaft, 2...front wheels, 3...steering angle sensor, 4...steering angle sensor, 5...vehicle speed sensor, 6...acceleration sensor, 7...yaw rate sensor, 8...steering control unit, 8A...steering motor, 8B...steering current detection unit, 8C...steering motor drive unit, 9...reaction force control unit, 9A...reaction force motor, 9B...reaction force current detection unit, 9C...reaction force motor drive unit, 10a...steering rack, 10b, 10d...pinion shaft, 10c, 10e...pinion gear, 11...controller, 11A...target steering angle calculation unit, 11B...target steering reaction force calculation unit, 11C...target steering current calculation unit calculation unit, 12...backup clutch, 20...processor, 21...storage device, 30...feedforward axial force calculation unit, 31...feedback axial force calculation unit, 32...mixing ratio setting unit, 33...subtractor, 34, 35...multiplier, 36...adder, 37...conversion unit, 38...target reaction force current calculation unit, 40...first estimated front wheel slip angle calculation unit, 41...second estimated front wheel slip angle calculation unit, 42...estimated rear wheel slip angle calculation unit, 43...front wheel lateral force estimator, 44...rear wheel lateral force estimator, 45...yaw rate lateral velocity estimator, 46...first mixing ratio calculation unit, 47...second mixing ratio calculation unit, 48...selection unit, 49...road surface friction coefficient estimator

Claims

1. A feedforward axial force is calculated, which is a rack axial force that provides a steering reaction force according to the steering angle of a steering wheel that is mechanically separated from the steered wheels. Calculates the feedback axial force, which is the rack axial force that returns the force from the road surface to the driver as a steering reaction force, Estimate the slip angle of the front wheels, a mixing ratio of the feedforward axial force and the feedback axial force is set in accordance with the estimated slip angle so as to reduce a ratio of the feedforward axial force when a rate of change of the rack axial force with respect to the estimated slip angle is equal to or less than a predetermined value; applying a steering reaction force based on a mixed axial force obtained by mixing the feedforward axial force and the feedback axial force at the mixing ratio; A steering control method comprising:

2. Estimate the friction coefficient of the road surface, 2. The steering control method according to claim 1, wherein the mixing ratio is set so as to reduce the proportion of the feedforward axial force at a smaller slip angle when the friction coefficient is small compared to when the friction coefficient is large.

3. Detecting an actual yaw rate that is an actual yaw rate occurring in the vehicle; calculating a first estimated slip angle, which is an estimated value of the slip angle of the front wheels, based on the detected actual yaw rate; setting a first mixture ratio, which is a mixture ratio of the feedforward axial force and the feedback axial force, based on the first estimated slip angle; calculating a reference yaw rate that is a reference value of a yaw rate generated in the vehicle based on the steering angle of the steered wheels, the vehicle speed, and a vehicle model; calculating a second estimated slip angle, which is an estimated value of the slip angle of the front wheels, based on the calculated standard yaw rate; setting a second mixture ratio, which is a mixture ratio of the feedforward axial force and the feedback axial force, based on the second estimated slip angle; calculating the mixed axial force by mixing the feedforward axial force and the feedback axial force at one of the first mixing ratio and the second mixing ratio, whichever has a smaller ratio of the feedforward axial force; 3. The steering control method according to claim 1 or 2.

4. a steering angle sensor that detects the steering angle of a steering wheel that is mechanically separated from the steered wheels; an actuator that applies a steering reaction force to the steering wheel; a controller that calculates a feedforward axial force which is a rack axial force that provides a steering reaction force in accordance with the steering angle, calculates a feedback axial force which is a rack axial force that returns a force from the road surface to a driver as a steering reaction force, estimates a slip angle of a front wheel, sets a mixing ratio of the feedforward axial force and the feedback axial force in accordance with the estimated slip angle so as to reduce a proportion of the feedforward axial force when a rate of change of the rack axial force with respect to the estimated slip angle is equal to or less than a predetermined value, and generates a steering reaction force by the actuator based on a mixed axial force obtained by mixing the feedforward axial force and the feedback axial force at the mixing ratio.

Images