Steering control method and steering control device

Abstract

In this steering control method, a front-wheel slip angle is estimated, and: when the road surface on which a vehicle is travelling is a low-μ road surface and the axial force difference between a feedback axial force and a standard axial force or a feedforward axial force is less than a first prescribed value, a steering reaction force based on the feedforward axial force is applied (S6); when the road surface is a low-μ road surface and the axial force difference is greater than or equal to the first prescribed value, a steering reaction force based on the feedback axial force is applied (S9); when the road surface is not a low-μ road surface and the front-wheel slip angle is less than a third prescribed value, a steering reaction force based on the feedforward axial force is applied, (S12); and when the road surface is not a low-μ road surface and the front-wheel slip angle is greater than or equal to the third prescribed value, a steering reaction force based on the feedback axial force is applied (S13).

Description

[Technical Field]

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

[0002] The steering control device described in Patent Document 1 below reflects the influence of external forces acting on the steered wheels in the steering reaction force by driving a reaction motor based on a control amount of the steering reaction force that is based on the steering angle and a control amount calculated by multiplying the current of the steering motor by a set gain. [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, the control amount of the steering reaction force based on the steering angle may become inappropriate, for example, when the vehicle behavior becomes large. The present invention aims to provide a steering control device that can apply a more appropriate steering reaction force in a steer-by-wire system in which the steering wheel and the steered wheels are mechanically separated. [Means for solving the problem]

[0005] In one aspect of the steering control method of the present invention, when the road surface is a low μ road surface and the axial force difference between the feedback axial force and one of the standard axial force or the feedforward axial force, which is the steering rack axial force according to the actual steering angle of the steered wheels, is less than a first predetermined value, the feedback axial force is not used and a steering reaction force based on the feedforward axial force is applied; when the road surface is a low μ road surface and the axial force difference is equal to or greater than the first predetermined value, a steering reaction force based on the feedback axial force is applied; when the road surface is not a low μ road surface and the front wheel slip angle is less than a third predetermined value, the feedback axial force is not used and a steering reaction force based on the feedforward axial force is applied; and when the road surface is not a low μ road surface and the front wheel slip angle is equal to or greater than the third predetermined value, a steering reaction force based on the front feedback axial force is applied. [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. 10 is an explanatory diagram of axial force switching control for each driving scene. [Figure 5] 4 is a flowchart of an example of a steering control method according to an embodiment. [Figure 6] FIG. 10 is a block diagram of an example of a functional configuration of an axial force mixing unit. [Figure 7] 10(a) to 10(c) are explanatory diagrams of examples of setting the mixing ratio. [Figure 8] FIG. 10 is a block diagram illustrating an example of a functional configuration of an axial force difference-dependent mixture ratio calculation unit. [Figure 9] 10(a) and 10(b) are diagrams illustrating an example of setting a correction gain. [Figure 10] 10(a) and 10(b) are diagrams illustrating an example of setting a switching range limit value. [Figure 11] FIG. 10 is an explanatory diagram of an example of setting a bank correction value. [Figure 12] FIG. 10 is an explanatory diagram of an example of setting an axial force difference dependent mixture ratio. [Figure 13] FIG. 10 is a block diagram illustrating an example of the functional configuration of a front wheel slip angle dependent mixture ratio calculation unit. [Figure 14] FIG. 10 is an explanatory diagram of an example of setting a front wheel slip angle dependent mixture ratio. DETAILED DESCRIPTION OF THE INVENTION

[0008] (First Example) (composition) FIG. 1 is a schematic diagram of an example of a steering control device according to an embodiment. 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. Steering angle sensor 3 detects the steering angle δ of steering wheel 1a. Steering angle sensor 4 detects the actual steering angle θ of front wheels (steered wheels) 2. Vehicle speed sensor 5 detects the vehicle speed V of the host vehicle. Acceleration sensor 6 detects the lateral acceleration Gy acting on the host vehicle. Yaw rate sensor 7 detects the actual yaw rate γa of the host vehicle. 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 current detection unit 8B detects steering current Itm flowing through steering motor 8A. Steering current detection unit 8B outputs a signal indicative of the steering current Itm to steering motor drive unit 8C and controller 11. Based on a target turning current Itt calculated by controller 11, steering motor drive unit 8C controls the turning current Itm of steering motor 8A so that the turning current Itm detected by steering current detection unit 8B matches the target turning current Itt. In this way, steering motor drive unit 8C drives steering motor 8A.

[0009] 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 force motor 9A is connected to the steering shaft 1b via a reducer. The reaction force motor 9A is driven by the reaction force motor drive unit 9C and applies a rotational torque to the steering wheel 1a via the steering shaft 1b. This causes the reaction force motor 9A to generate a steering reaction force. The reaction force current detection unit 9B detects a reaction force current Ism flowing through the reaction force motor 9A. The reaction force current detection unit 9B then 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 drive 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 detection unit 9B coincides with the target reaction force current Ist. In this way, the reaction force motor drive unit 9C drives the reaction force motor 9A.

[0010] The backup clutch 12 is provided between the steering shaft 1b and the pinion shaft 10b. The pinion shaft 10b is connected to the steering rack 10a via a pinion gear 10c, and when the backup clutch 12 is engaged, the steering wheel 1a is mechanically connected to the front wheels 2. When the backup clutch 12 is released, the steering wheel 1a is mechanically disconnected from the front wheels 2.

[0011] Controller 11 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 processor 13 and peripheral components such as storage device 14. The functions of controller 11 are realized, for example, by processor 13 executing a computer program stored in storage device 14. An example functional configuration of controller 11 is shown in FIG. 2. Target steering angle calculation unit 11A calculates target steering angle θt, which is a target value for actual steering angle θ, based on steering angle δ and vehicle speed V. Target steering angle θt may be calculated, for example, by multiplying steering angle δ by a variable gear ratio. Subtractor 11D calculates deviation Δθ by subtracting actual steering angle θ from target steering angle θt. Target steering current calculation unit 11C calculates target turning current Itt based on the deviation Δθ. Target turning current calculation unit 11C outputs target turning current Itt to steering motor drive unit 8C.

[0012] The target steering reaction force calculation unit 11B calculates a target reaction force current Ist based on the steering angle δ, vehicle speed V, actual steering angle θ, actual yaw rate γa, lateral acceleration Gy, and steering current Itm. The target steering reaction force calculation unit 11B outputs the calculated target reaction force current Ist to the reaction force motor drive unit 9C. FIG. 3 is a block diagram of an example of the functional configuration of the target steering reaction force calculation unit. In the following description and drawings, the feedforward axial force may be referred to as the "FF axial force," and the feedback axial force may be referred to as the "FB axial force." The FF axial force calculation unit 20 calculates the FF axial force Fff, 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. For example, the FF axial force Fff may be calculated based on the target steering angle θt calculated based on the steering angle δ and the vehicle speed V, and on the pinion stiffness, pinion viscosity, rack inertia, and rack viscosity of the pinion and rack of the steering mechanism. For example, the FF axial force Fff may be an axial force applied to the steering rack that includes at least a proportional component according to the target steering angle θt and a damping component according to the steering angular velocity.

[0013] The FB axial force calculation unit 21 calculates the FB axial force Ffb based on the steering current Itm, the vehicle speed V, the lateral acceleration Gy, and the actual yaw rate γa. The FB axial force Ffb is calculated by dividing the force transmitted from the road surface to the front wheels 2 by the steering reaction force of the steering wheel 1. a The steering rack axial force is applied to the steering wheel 2 and transmitted to the driver's tactile sense. The FB axial force calculation unit 21 calculates the steering rack axial force that reflects the influence of the tire lateral force acting on the steered wheels 2 based on the turning current Itm. The steering rack axial force calculated based on the turning current is referred to as the "current axial force."

[0014] Furthermore, the FB axial force calculation unit 21 calculates the steering rack axial force that reflects the influence of the tire lateral force acting on the steered wheels 2 based on the lateral acceleration Gy. The steering rack axial force calculated based on the lateral acceleration Gy is referred to as the "lateral G axial force." The FB axial force calculation unit 21 calculates the steering rack axial force that reflects the influence of the tire lateral force acting on the steered wheels 2 based on the actual yaw rate γa. The steering rack axial force calculated based on the actual yaw rate γa is referred to as the "yaw rate axial force." The FB axial force calculation unit 21 calculates the FB axial force Ffb by combining the current axial force, the lateral G axial force, and the yaw rate axial force.

[0015] The axial force mixing unit 23 estimates the front wheel slip angle, which is the slip angle of the front wheels 2, and determines whether the road surface on which the vehicle is traveling is a low μ road surface with a friction coefficient equal to or less than a threshold value. The axial force mixing unit 23 includes a road surface μ estimating unit 23a that determines whether the road surface on which the vehicle is traveling is a low μ road surface. If the road surface is a low μ road surface, the FB axial force Ffb will be small even if the FF axial force Fff is large. The road surface μ estimating unit 23a estimates the FF axial force Fff is equal to or greater than the upper limit Fff2, and FB axial force F fb If the FF axial force Fff is less than the lower limit Ffb1, the road surface is determined to be a low μ road surface. fb If is equal to or greater than the upper limit value Ffb2, it is determined that the road surface is not a low μ road surface.

[0016] When the road surface is a low μ road surface and the axial force difference ΔFs between the FF axial force Fff and the FB axial force Ffb is less than a first predetermined value, the axial force mixing unit 23 calculates the estimated rack axial force Frk based on the FF axial force Fff without using the FB axial force Ffb. Note that the difference between the standard axial force Frm and the FB axial force Ffb may be used as the axial force difference ΔFs. This is because the standard axial force Frm is a steering rack axial force calculated using a vehicle model corresponding to the actual steering angle θ, and in a steady state, the difference between the standard axial force Frm and the FF axial force Fff is very small. When the road surface is a low μ road surface and the axial force difference ΔFs is equal to or greater than a first predetermined value, the axial force mixing unit 23 determines that the vehicle is in an understeer state (hereinafter referred to as the “US state”) and calculates the estimated rack axial force Frk based on the FB axial force Ffb. If the road surface is not a low μ road surface and the front wheel slip angle is less than a third predetermined value, the estimated rack axial force Frk is calculated based on the FF axial force Fff without using the FB axial force Ffb, and if the road surface is not a low μ road surface and the front wheel slip angle is equal to or greater than the third predetermined value, it is determined that the vehicle is in a US state and the estimated rack axial force Frk is calculated based on the FB axial force Ffb.

[0017] The conversion unit 24 converts the estimated rack axial force Frk calculated by the axial force mixing unit 23 into a target steering reaction force. The conversion unit 24 may convert the estimated rack axial force into the target steering reaction force using a conversion map that stores target steering reaction forces corresponding to each value of the estimated rack axial force. The target reaction force current calculation unit 25 calculates a target reaction force current based on the target steering reaction force output from the conversion unit 24. The target reaction force current calculation unit 25 outputs the target reaction force current to the reaction force motor drive unit 9C.

[0018] In this way, the controller 11 changes the determination of whether the vehicle is in the US state depending on whether the road surface is a low μ road surface. The reason for this will be explained with reference to FIG. FIG. 4 is an explanatory diagram of axial force switching control for each driving situation. When the vehicle's tires are in a grip state, a steering reaction force is applied based on the FF axial force Fff. This is because, in a grip state, using the FF axial force makes it possible to apply a steering reaction force that is not affected by external disturbances. On the other hand, in an oversteer state (hereinafter referred to as the "OS state") or a US state, a steering reaction force is applied based on the FB axial force Ffb. This is because using the FB axial force Ffb makes it possible to convey road surface information as a steering reaction force. Furthermore, if the road surface is not a low μ road surface, the system judges whether or not the vehicle is in the US state based on the front tire slip angle. By basing the judgment on the front wheel slip angle, which is information from the tire edge, the delay in detecting the US state can be reduced. On the other hand, on a low μ road surface, the effect of a decrease in road surface μ is less likely to appear on the front wheel slip angle. For this reason, on a low μ road surface, the system judges the US state based on the axial force difference, which directly reflects the US state caused by a decrease in road surface μ. Whether or not the vehicle is in the OS state can be determined based on the yaw rate difference or the vehicle body vinegar The determination is based on the lip angular velocity difference.

[0019] (operation) FIG. 5 is a flowchart of an example of a steering control method according to an embodiment. In step S1, the controller 11 calculates the FF axial force Fff and the FB axial force Ffb. In step S2, the controller 11 estimates the front wheel slip angle. In step S3, the controller 11 estimates whether the road surface is a low μ road surface. If the road surface is a low μ road surface (step S4: Y), the process proceeds to step S5. If the road surface is not a low μ road surface (step S4: N), the process proceeds to step S10. In step S5, the controller 11 determines whether the axial force difference is less than a first predetermined value. If the axial force difference is less than the first predetermined value (step S5: Y), the process proceeds to step S6. If the axial force difference is not less than the first predetermined value (step S5: N), the process proceeds to step S7. In step S6, the controller 11 applies a steering reaction force according to the FF axial force Fff, and in step S7, the controller 11 applies a steering reaction force according to the FB axial force Ffb. The process then ends.

[0020] In step S8, the controller 11 determines whether the front wheel slip angle is less than a third predetermined value. If the front wheel slip angle is less than the third predetermined value (step S8: Y), the process proceeds to step S9. If the front wheel slip angle is not less than the third predetermined value (step S8: N), the process proceeds to step S10. In step S9, the controller 11 applies a steering reaction force according to the FF axial force Fff, and in step S10, applies a steering reaction force according to the FB axial force Ffb. The process then ends.

[0021] (Variation) The axial force mixing unit 23 may set a larger first predetermined value when the bank angle of the road surface is large compared to when it is small. This reduces the effect of estimation errors on the FB axial force Ffb due to the bank. For example, the axial force mixing unit 23 calculates a lateral G offset Gyo, which is the lateral acceleration added to the lateral acceleration Gy output by the acceleration sensor 6 due to the influence of the bank angle or cant of the road surface. For example, the axial force mixing unit 23 may calculate the lateral G offset Gyo = V × γa - Gy based on the vehicle speed V, the actual yaw rate γa, and the lateral acceleration Gy. The axial force mixing unit 23 then calculates a bank correction value Cb, which is a correction value for the first predetermined value and the second predetermined value according to the bank correction value Cb. See FIG. 11. The bank correction value Cb increases from "0" to a value Cb2 as the lateral G offset Gyo increases. For example, when the lateral G offset Gyo is in a range smaller than the value Gy1, the bank correction value Cb is "0." As the lateral G offset Gyo increases from the value Gy1 to the value Gy2, the bank correction value Cb increases from "0" to the value Cb1. As the lateral G offset Gyo increases from the value Gy2 to the value Gy3, the bank correction value Cb increases from the value Cb1 to the value Cb2. The axial force mixing unit 23 corrects the first predetermined value and the second predetermined value by adding the bank correction value Cb. As a result, the first predetermined value becomes larger when the bank angle of the road surface is large.

[0022] (Second Example) In the second embodiment, the steering wheel 1 aWhen the steering speed ω is equal to or greater than a predetermined speed, a steering reaction force based on the FB axial force is applied to the steering wheel 1 (see Fig. 3). a The axial force mixing unit 23 calculates the steering angular velocity ω of the FF axial force Ffb. The axial force mixing unit 23 determines whether the steering velocity ω is equal to or greater than a predetermined velocity. If the steering velocity ω is equal to or greater than the predetermined velocity, the estimated rack axial force Frk is calculated based on the FB axial force Ffb, regardless of whether the road surface is a low μ road surface, whether the axial force difference ΔFs is equal to or greater than a second predetermined value, or whether the front wheel slip angle is equal to or greater than a fourth predetermined value. For example, the axial force mixing unit 23 may calculate a steering angular velocity-dependent mixing ratio R1 according to the steering angular velocity ω, and calculate the estimated rack axial force Frk by mixing the FF axial force Fff and the FB axial force Ffb at the steering angular velocity-dependent mixing ratio R1. The steering angular velocity-dependent mixing ratio R1 is set so that the proportion of the FF axial force Fff is reduced when the steering angular velocity ω is high. See FIG. 7(a). The steering angular velocity-dependent mixing ratio R1 transitions from "1" to "0" as the steering angular velocity ω increases. For example, in a range where the steering angular velocity ω is lower than a lower limit value ω1, the steering angular velocity dependent mixing rate R1 is "1." In a range where the steering angular velocity ω is higher than an upper limit value ω2, the steering angular velocity dependent mixing rate R1 is "0." In the range from the lower limit value ω1 to the upper limit value ω2, the steering angular velocity dependent mixing rate R1 decreases from "1" to "0" as the steering angular velocity ω increases. The lower limit value ω1 and the upper limit value ω2 may be variable according to the vehicle speed V. The axial force mixing unit 23 may calculate the estimated rack axial force Frk=R1×Fff+(1−R1)×Ffb by mixing the FF axial force Fff and the FB axial force Ffb at the steering angular velocity dependent mixing rate R1.

[0023] (Third Example) In the third embodiment, in addition to the controls of the first and second embodiments, a steering reaction force based on the FB axial force is applied when the host vehicle is in the OS state. For example, when the host vehicle is in the OS state, the yaw rate difference Δγ, which is the difference between the reference yaw rate γm, which is the reference value of the yaw rate calculated according to the vehicle model, and the actual yaw rate γa, becomes large. Therefore, when the yaw rate difference Δγ is large, a steering reaction force based on the FB axial force is applied. Also, for example, when the host vehicle is in the OS state, the vehicle body slip angular velocity difference Δα, which is the difference between the reference vehicle body slip angular velocity αm, which is the reference value of the vehicle body slip angular velocity calculated according to the vehicle model, and the actual vehicle body slip angular velocity αa of the host vehicle, becomes large. Therefore, when the vehicle body slip angular velocity difference Δα is large, a steering reaction force based on the FB axial force is applied.

[0024] FIG. 6 is a block diagram of an example of the functional configuration of the axial force mixing unit 23 of the third embodiment. The axial force mixing unit 23 calculates a steering angular velocity-dependent mixing ratio R1 according to the steering angular velocity ω and calculates the first mixed axial force Fmf by mixing the FF axial force Fff and the FB axial force Ffb at the steering angular velocity-dependent mixing ratio R1. The axial force mixing unit 23 also calculates a yaw rate difference-dependent mixing ratio R2 according to the yaw rate difference Δγ, a vehicle body slip angular velocity difference-dependent mixing ratio R3 according to the vehicle body slip angular velocity difference Δα, an axial force difference-dependent mixing ratio R4 according to the axial force difference ΔFs, and a front wheel slip angle-dependent mixing ratio R5 according to the front wheel slip angle. The axial force mixing unit 23 selects one of these mixing ratios R2 to R5 as the final mixing ratio Rf according to the driving situation. The axial force mixing unit 23 calculates the estimated rack axial force Frk by mixing the first mixed axial force Fmf and the FB axial force Ffb at the final mixing ratio Rf.

[0025] The axial force mixing unit 23 includes a steering angular velocity-dependent mixing rate calculation unit 30, mixers 31 and 32, a model calculation unit 33, a yaw rate difference-dependent mixing rate calculation unit 34, a vehicle body slip angular velocity difference-dependent mixing rate calculation unit 35, an axial force difference-dependent mixing rate calculation unit 36, a front wheel slip angle-dependent mixing rate calculation unit 37, and a selector 38. As described above with reference to FIG. 7(a), the steering angular velocity-dependent mixing rate calculation unit 30 calculates a steering angular velocity-dependent mixing rate R1 based on at least the steering angular velocity ω. The mixer 31 calculates a first mixed axial force Fmf=R1×Fff+(1−R1)×Ffb by mixing the FF axial force Fff and the FB axial force Ffb at the steering angular velocity-dependent mixing rate R1. The mixer 32 mixes the reference axial force Frm and the FB axial force Ffb at a steering angular velocity dependent mixing ratio R1 to calculate a second mixed axial force Fmm=R1×Frm+(1−R1)×Ffb.

[0026] The model calculation unit 33 calculates the reference yaw rate γm and the reference vehicle body slip angular velocity αm in accordance with a predetermined linear two-wheel model based on the actual steering angle θ and the vehicle speed V. An example of the linear two-wheel model is shown in the following equations.

[0027]

number

[0028] V x is the longitudinal speed of the vehicle, and V y is the transverse velocity, and θ f is the steering angle of the front wheels, and θ r is the steering angle of the rear wheels, M is the mass of the vehicle, and I z is the moment of inertia, and L f is the length from the center of gravity to the front wheel, and L r is the distance from the center of gravity to the rear wheel, and K f is the cornering power of the front wheels, and K ris the cornering power of the rear wheels. The model calculation unit 33 also calculates a first estimated front wheel slip angle βfm1, which is a front wheel slip angle calculated based on the actual yaw rate γa, and a second estimated front wheel slip angle βfm2, which is a front wheel slip angle calculated based on the standard yaw rate γm. For example, the model calculation unit 33 may calculate the first estimated front wheel slip angle βfm1 and the second estimated front wheel slip angle βfm2 in accordance with the following equations. βfm1=θ / N-(V y +L f ×γa) / V x βfm2=θ / N-(V y +L f ×γm) / V x In the formula, N is the overall gear ratio. Furthermore, the model calculation unit 33 calculates the reference axial force Frm based on the actual steering angle θ. For example, the model calculation unit 33 calculates the front wheel cornering power K f to estimate the tire lateral force of the front wheel 2, and convert the estimated tire lateral force into the reference axial force Frm.

[0029] The yaw rate difference-dependent mixing ratio calculation unit 34 calculates a yaw rate difference-dependent mixing ratio R2 for calculating an estimated rack axial force Frk based on the FB axial force Ffb when the host vehicle is in the OS state. The yaw rate difference-dependent mixing ratio R2 may be calculated based on at least the standard yaw rate γm and the actual yaw rate γa. The yaw rate difference-dependent mixing ratio R2 is set so that the ratio of the first mixed axial force Fmf is reduced (so that the ratio of the FF axial force Fff is reduced) when the yaw rate difference Δγ between the standard yaw rate γm and the actual yaw rate γa is large. See FIG. 7(b). The yaw rate difference-dependent mixing ratio R2 transitions from "1" to "0" as the yaw rate difference Δγ increases. For example, when the yaw rate difference Δγ is smaller than the lower limit value Δγ1, the yaw rate difference-dependent mixing ratio R2 is "1." When the yaw rate difference Δγ is larger than the upper limit value Δγ2, the yaw rate difference-dependent mixing ratio R2 is "0." In the range from the lower limit value Δγ1 to the upper limit value Δγ2, as the yaw rate difference Δγ increases, the yaw rate difference dependent mixture ratio R2 decreases from "1" to "0." The lower limit value Δγ1 and the upper limit value Δγ2 may be variable depending on the vehicle speed V.

[0030] Referring to FIG. 6, the vehicle body slip angular velocity difference-dependent mixing ratio calculation unit 35 calculates a vehicle body slip angular velocity difference-dependent mixing ratio R3 for calculating an estimated rack axial force Frk based on the FB axial force Ffb when the host vehicle is in the OS state. The vehicle body slip angular velocity difference-dependent mixing ratio calculation unit 35 calculates an actual vehicle body slip angular velocity αa = γa - Gy / V based on the vehicle speed V, actual yaw rate γa, and lateral acceleration Gy, calculates a vehicle body slip angular velocity difference Δα between the reference vehicle body slip angular velocity αm and the actual vehicle body slip angular velocity αa, and calculates the vehicle body slip angular velocity difference-dependent mixing ratio R3 based on the vehicle body slip angular velocity difference Δα. The vehicle body slip angular velocity difference-dependent mixing ratio R3 is set so that the proportion of the first mixed axial force Fmf is reduced when the vehicle body slip angular velocity difference Δα is large.

[0031] See FIG. 7(c). The vehicle body slip angular velocity difference-dependent mixture ratio R3 transitions from "1" to "0" as the vehicle body slip angular velocity difference Δα increases. For example, when the vehicle body slip angular velocity difference Δα is smaller than the lower limit value Δα1, the vehicle body slip angular velocity difference-dependent mixture ratio R3 is "1." When the vehicle body slip angular velocity difference Δα is larger than the upper limit value Δα2, the vehicle body slip angular velocity difference-dependent mixture ratio R3 is "0." Within the range from the lower limit value Δα1 to the upper limit value Δα2, the vehicle body slip angular velocity difference-dependent mixture ratio R3 decreases from "1" to "0" as the vehicle body slip angular velocity difference Δα increases. The lower limit value Δα1 and the upper limit value Δα2 may be variable depending on the vehicle speed V. See FIG. 6. The vehicle body slip angular velocity difference-dependent mixture ratio calculation unit 35 calculates the above-mentioned lateral G offset Gyo.

[0032] The axial force difference-dependent mixing ratio calculation unit 36 ​​calculates the axial force difference-dependent mixing ratio R4. See FIG. 8. The axial force difference-dependent mixing ratio calculation unit 36 ​​includes a subtractor 40, a low-pass filter 41, correction gain calculation units 42 and 43, multipliers 44 and 45, a lower limit value calculation unit 46, an upper limit value calculation unit 47, a bank correction value calculation unit 48, adders 49 and 50, and a mixing ratio calculation unit 51. The subtractor 40 calculates the axial force difference ΔFs between the second mixed axial force Fmm and the FB axial force Ffb. Instead of the second mixed axial force Fmm, the axial force difference between the FF axial force Fff and the FB axial force Ffb may be calculated as the axial force difference ΔFs. The axial force difference ΔFs, from which high-frequency components have been removed by the low-pass filter 41, is input to the multiplier 44.

[0033] The correction gain calculation units 42 and 43 calculate a first correction gain G1 and a second correction gain G2 depending on whether the road surface is a low-μ road surface. As a result, when the road surface is a low-μ road surface, an axial force difference-dependent mixture ratio R4 is calculated according to the axial force difference ΔFs. When the road surface is not a low-μ road surface, the axial force difference-dependent mixture ratio R4 is fixed to "1." When the road surface is a low-μ road surface, even if the FF axial force Fff is large, the FB axial force Ffb is small. Therefore, when the FF axial force Fff is equal to or greater than the upper limit Fff2 and the FB axial force Ffb is less than the lower limit Ffb1, the correction gain calculation units 42 and 43 determine that the road surface is a low-μ road surface and set the first correction gain G1 and the second correction gain G2 to "1." See FIG. 9(a). The first correction gain G1 transitions from "1" to "0" as the FB axial force Ffb increases. For example, when the FB axial force Ffb is in a range smaller than the lower limit value Ffb1, the first correction gain G1 is "1," and when it is in a range larger than the upper limit value Ffb2, the first correction gain G1 is "0." In the range from the lower limit value Ffb1 to the upper limit value Ffb2, the first correction gain G1 decreases from "1" to "0" as the FB axial force Ffb increases.

[0034] Conversely, if the FF axial force Fff is equal to or less than the lower limit Fff1 or the FB axial force Ffb is equal to or greater than the upper limit Ffb2, it is determined that the road surface is not a low μ road surface, and the first correction gain G1 and the second correction gain G2 are set to "0." See FIG. 9(b). The second correction gain G2 transitions from "0" to "1" as the standard axial force Frm increases. For example, when the standard axial force Frm is less than the lower limit Fff1, the second correction gain G2 is "0," and when the standard axial force Frm is greater than the upper limit Fff2, the second correction gain G2 is "1." Within the range from the lower limit Fff1 to the upper limit Fff2, the second correction gain G2 increases from "0" to "1" as the standard axial force Frm increases. Multipliers 44 and 45 calculate the corrected axial force difference ΔFsc = G1 × G2 × ΔFs. The mixing ratio calculation unit 51 calculates the axial force difference-dependent mixing ratio R4 based on the axial force difference ΔFsc. As a result, if the road surface is a low μ road surface, the axial force difference ΔFs is input as is to the mixing ratio calculation unit 51. Therefore, the mixing ratio calculation unit 51 calculates the axial force difference-dependent mixing ratio R4 according to the axial force difference ΔFs. On the other hand, if the road surface is not a low μ road surface, the axial force difference ΔFsc=0 is input to the mixing ratio calculation unit 51, and the axial force difference-dependent mixing ratio R4 is fixed to "1" as described below.

[0035] The lower limit value calculation unit 46 calculates a switching range lower limit value LL1, which is the lower limit value of a switching range in which the axial force difference-dependent mixing ratio R4 is switched between "1" and "0" depending on the axial force difference ΔFsc. The upper limit value calculation unit 47 calculates a switching range upper limit value LU1, which is the upper limit value of the switching range. For example, the lower limit value calculation unit 46 and the upper limit value calculation unit 47 may calculate the switching range lower limit value LL1 and the switching range upper limit value LU1 depending on the vehicle speed V. See FIG. 10(a). The switching range lower limit value LL1 may be a constant value L1 at any vehicle speed V, or may be variable depending on the vehicle speed. See FIG. 10(b). The switching range upper limit value LU1 may be a constant value L2 at any vehicle speed V, or may be variable depending on the vehicle speed.

[0036] See Figure 8. The bank correction value calculation unit 48 calculates the bank correction value Cb described above with reference to Figure 11. Adders 49 and 50 add the bank correction value Cb to the switching range lower limit value LL1 and the switching range upper limit value LU1, respectively, to calculate a corrected switching range lower limit value LL2 = LL1 + Cb and a corrected switching range upper limit value LU2 = LU1 + Cb. As described above, the greater the bank angle or cant of the road surface, the greater the value of the lateral G offset Gyo. Therefore, the switching range lower limit value LL2 and the switching range upper limit value LU2 are set to larger values ​​as the bank angle or cant of the road surface increases.

[0037] The mixing ratio calculation unit 51 calculates the axial force difference-dependent mixing ratio R4 based on the axial force difference ΔFsc. The axial force difference-dependent mixing ratio R4 is set so that the proportion of the first mixed axial force Fmf is reduced when the axial force difference ΔFsc is large. Refer to FIG. 12. The axial force difference-dependent mixing ratio R4 transitions from "1" to "0" as the axial force difference ΔFsc increases. For example, when the axial force difference ΔFsc is smaller than the switching range lower limit LL2, the axial force difference-dependent mixing ratio R4 is "1." Therefore, when the road surface is not a low-μ road surface, ΔFsc = 0 is input, and the axial force difference-dependent mixing ratio R4 is fixed at "1." When the axial force difference ΔFsc is larger than the switching range upper limit LU2, the axial force difference-dependent mixing ratio R4 is "0." In the range from the switching range lower limit LL2 to the switching range upper limit LU2, the axial force difference-dependent mixing ratio R4 decreases from "1" to "0" as the axial force difference ΔFsc increases. The switching range lower limit LL2 and the switching range upper limit LU2 are examples of the "first predetermined value" and "second predetermined value" respectively.

[0038] Refer to FIG. 13. The front-wheel slip-angle-dependent mixture ratio calculation unit 37 calculates the front-wheel slip-angle-dependent mixture ratio R5. The front-wheel slip-angle-dependent mixture ratio calculation unit 37 includes a road surface condition estimation unit 60, mixture ratio calculation units 61 and 62, a DRY switch range lower limit value calculation unit 63, a WET switch range lower limit value calculation unit 64, mixers 65 and 68, a DRY switch range upper limit value calculation unit 66, a WET switch range upper limit value calculation unit 67, and a selector 69. The road surface condition estimation unit 60 determines whether the road surface is dry (or wet) based on the axial force difference ΔFs, and calculates the mixture ratio R6 corresponding to the dry (or wet) road surface state. The mixture ratio R6 transitions from "1" to "0" as the axial force difference ΔFs increases. Therefore, when the road surface is dry, the mixture ratio R6 is "1," and decreases to "0" as the road surface becomes wetter. The road surface condition estimation unit 60, like the correction gain calculation units 42 and 43 and the multipliers 44 and 45 in FIG. 8, calculates the FB axial force Ffb and Normative axial force Frm The axial force difference ΔFs may be corrected with a correction gain corresponding to the difference, and the mixing ratio R6 may be calculated according to the corrected axial force difference. Also, similar to the lower limit value calculation unit 46, the upper limit value calculation unit 47, the bank correction value calculation unit 48, and the adders 49 and 50, the lower limit value and the upper limit value of the switching range in which the mixing ratio R6 transitions between "1" and "0" may be set according to the vehicle speed V.

[0039] The mixing ratio calculation unit 61 calculates the first front-wheel slip angle-dependent mixing ratio R5a based on the first estimated front-wheel slip angle βfm1. The first front-wheel slip angle-dependent mixing ratio R5a is set so that the proportion of the first mixed axial force Fmf decreases when the first estimated front-wheel slip angle βfm1 is large. See FIG. 14. The first front-wheel slip angle-dependent mixing ratio R5a transitions from "1" to "0" as the first estimated front-wheel slip angle βfm1 increases. For example, when the first estimated front-wheel slip angle βfm1 is smaller than the switching range lower limit LL5, the first front-wheel slip angle-dependent mixing ratio R5a is "1." When the first estimated front-wheel slip angle βfm1 is larger than the switching range upper limit LU5, the first front-wheel slip angle-dependent mixing ratio R5a is "0." Within the range from the switching range lower limit value LL5 to the switching range upper limit value LU5, as the first estimated front wheel slip angle βfm1 increases, the first front wheel slip angle dependent mixture rate R5a decreases from 1 to 0. The switching range lower limit value LL5 and the switching range upper limit value LU5 are examples of the "third predetermined value" and "fourth predetermined value" respectively. The mixing ratio calculation unit 62 calculates the second front-wheel slip angle-dependent mixing ratio R5b based on the second estimated front-wheel slip angle βfm2. The calculation method for the second front-wheel slip angle-dependent mixing ratio R5b is the same as the calculation method for the first front-wheel slip angle-dependent mixing ratio R5a, except that the second estimated front-wheel slip angle βfm2 is used instead of the first estimated front-wheel slip angle βfm1.

[0040] The DRY switching range lower limit calculation unit 63, the WET switching range lower limit calculation unit 64, and the mixer 65 calculate a switching range lower limit value LL5. For example, the DRY switching range lower limit calculation unit 63 and the WET switching range lower limit calculation unit 64 calculate a switching range lower limit value LL3 for a dry state and a switching range lower limit value LL4 for a wet state, respectively. The switching range lower limits LL3 and LL4 may be constant at any vehicle speed V or may be variable depending on the vehicle speed. The mixer 65 calculates the switching range lower limit value LL5 by mixing the switching range lower limits LL3 and LL4 at a mixing ratio R6. The DRY switching range upper limit calculation unit 66, the WET switching range upper limit calculation unit 67, and the mixer 68 calculate a switching range upper limit value LU5. For example, the DRY switching range upper limit calculation unit 66 and the WET switching range upper limit calculation unit 67 calculate a switching range upper limit value LU3 for a dry state and a switching range upper limit value LU4 for a wet state, respectively. Switching range upper limits LU3 and LU4 may be constant at any vehicle speed V, or may be variable depending on the vehicle speed. A mixer 68 calculates switching range upper limit LU5 by mixing switching range upper limits LU3 and LU4 at a mixing ratio R6. A selector 69 selects and outputs the smaller of the first front wheel slip angle-dependent mixing ratio R5a and the second front wheel slip angle-dependent mixing ratio R5b as the front wheel slip angle-dependent mixing ratio R5.

[0041] See FIG. 6. The selector 38 selects one of these mixing ratios R2 to R5 as the final mixing ratio Rf depending on the driving situation. For example, if the yaw rate difference-dependent mixing ratio R2 is less than "1," it may be determined that the host vehicle is in the OS state, and the yaw rate difference-dependent mixing ratio R2 may be selected. Also, if the vehicle body slip angular velocity difference-dependent mixing ratio R3 is less than "1," it may be determined that the host vehicle is in the OS state, and the vehicle body slip angular velocity difference-dependent mixing ratio R3 may be selected. Also, if the axial force difference-dependent mixing ratio R4 is less than "1," it may be determined that the host vehicle is in the US state and the road surface is a low μ road surface, and the axial force difference-dependent mixing ratio R4 may be selected. Also, if the front wheel slip angle-dependent mixing ratio R5 is less than "1," it may be determined that the host vehicle is in the US state and the road surface is a low μ road surface, and the front wheel slip angle-dependent mixing ratio R5 may be selected. The selector 38 may select the smallest of the mixing ratios R2 to R5 as the final mixing ratio Rf. The selector 38 may calculate a mixed axial force obtained by mixing the first mixed axial force Fmf and the FB axial force Ffb at the final mixing ratio Rf as the estimated rack axial force Frk. This makes it possible to mitigate a sudden change in the steering reaction force caused by the estimated rack axial force Frk suddenly switching between the FF axial force Fff and the FB axial force Ffb.

[0042] (Effects of the embodiment) (1) In the steering control method, when the road surface is a low μ road surface and the axial force difference is less than a first predetermined value, a steering reaction force based on the FF axial force is applied, and when the road surface is a low μ road surface and the axial force difference is equal to or greater than a second predetermined value that is equal to or greater than the first predetermined value, a steering reaction force based on the FB axial force is applied. As a result, when the road surface is a low μ road surface, it is possible to switch between the FF axial force and the FB axial force according to the axial force difference that can detect a μ decrease, so that when the road surface is in the US state, it is possible to quickly switch to a steering reaction force based on the FB axial force, and it is possible to appropriately notify the driver of a decrease in road surface μ. If the road surface is not a low μ road surface and the front wheel slip angle is less than a third predetermined value, a steering reaction force based on the FF axial force is applied, and if the road surface is not a low μ road surface and the front wheel slip angle is equal to or greater than a fourth predetermined value that is equal to or greater than the third predetermined value, a steering reaction force based on the FB axial force is applied. As a result, if the road surface is not a low μ road surface and the US state is in effect, the steering reaction force can be quickly switched to that based on the FB axial force, and the road surface condition can be appropriately communicated to the driver.

[0043] (2) When the steering speed is equal to or greater than a predetermined speed, a steering reaction force based on the FB axial force may be applied regardless of whether the road surface is a low μ road surface or whether the front wheel slip angle is equal to or greater than a fourth predetermined value. This allows the steering reaction force to be switched based on the FB axial force, which can appropriately convey the road surface condition to the driver in an emergency avoidance situation. (3) When the difference between the standard yaw rate and the actual yaw rate is greater than a predetermined value, a steering reaction force based on the front-wheel axial force may be applied regardless of whether the road surface is a low μ road surface or whether the front wheel slip angle is equal to or greater than a fourth predetermined value. Furthermore, if the difference between the reference vehicle body slip angular velocity and the actual vehicle body slip angular velocity is greater than a predetermined value, a steering reaction force based on the FB axial force may be applied regardless of whether the road surface is a low μ road surface or whether the front wheel slip angle is equal to or greater than a fourth predetermined value. This makes it possible to switch to a steering reaction force based on the FB axial force that can appropriately convey vehicle behavior to the driver when the vehicle is in the OS state.

[0044] (4) If the FF axial force Fff is equal to or greater than a fifth predetermined value and the FB axial force is less than a sixth predetermined value, it may be determined that the road surface is a low μ road surface. This makes it possible to determine with a simple configuration whether the road surface is a low μ road surface. (5) A larger first predetermined value is set when the bank angle of the road surface is large than when it is small, thereby reducing the influence of estimation errors on the FB axial force Ffb due to the bank. [Explanation of symbols]

[0045] 1a...steering wheel, 1b...steering shaft, 2...front wheels, 3...steering angle sensor, 4...turning angle sensor, 5...vehicle speed sensor, 6...acceleration sensor, 7...yaw rate sensor, 8...turning control unit, 8A...turning motor, 8B...turning current detection unit, 9...reaction force control unit, 9A...reaction force motor, 9B...reaction force current detection unit, 10a...steering rack, 10b, 10d...pinion shaft, 10c, 10e...pinion gear, 12...backup clutch

Claims

1. A steering control method for a vehicle in which a steering wheel and steered wheels are mechanically separated, the method comprising: calculating a feedforward axial force, which is a steering rack axial force that provides a steering reaction force to be transmitted to the steering rack, when a steering rack connected to the steered wheels is driven by a steering motor in response to steering of the steering wheel; and calculating a feedback axial force, which is a steering rack axial force that is transmitted to the steering rack from a road surface on which the vehicle is traveling via the steered wheels; and applying a steering reaction force to the steering wheel based on at least one of the feedforward axial force and the feedback axial force. Estimating a front wheel slip angle, which is the slip angle of the steered wheels; determining whether the road surface is a low μ road surface having a friction coefficient equal to or less than a threshold value; When the road surface is a low μ road surface and an axial force difference between the feedback axial force and one of a reference axial force, which is a steering rack axial force corresponding to an actual steering angle of a steered wheel or the feedforward axial force, is less than a first predetermined value, the feedback axial force is not used and a steering reaction force based on the feedforward axial force is applied; When the road surface is a low μ road surface and the axial force difference is equal to or greater than the first predetermined value, a steering reaction force based on the feedback axial force is applied, When the road surface is not a low μ road surface and the front wheel slip angle is less than a third predetermined value, the feedback axial force is not used, and a steering reaction force based on the feedforward axial force is applied; When the road surface is not a low μ road surface and the front wheel slip angle is equal to or greater than the third predetermined value, a steering reaction force based on the feedback axial force is applied. A steering control method comprising:

2. When the road surface is a low μ road surface and the axial force difference is equal to or greater than the first predetermined value and less than a second predetermined value equal to or greater than the first predetermined value, a steering reaction force based on a mixed axial force obtained by mixing the feedforward axial force and the feedback axial force is applied, When the road surface is not a low μ road surface and the front wheel slip angle is equal to or greater than the third predetermined value and is equal to or greater than the third predetermined value and less than a fourth predetermined value, a steering reaction force based on a mixed axial force obtained by mixing the feedforward axial force and the feedback axial force is applied. The steering control method according to claim 1 .

3. 3. The steering control method according to claim 2, wherein, when the steering speed is equal to or greater than a predetermined speed, a steering reaction force based on the feedback axial force is applied regardless of whether the road surface is a low μ road surface, whether the axial force difference is equal to or greater than the second predetermined value, and whether the front wheel slip angle is equal to or greater than the fourth predetermined value.

4. an actual yaw rate occurring in the vehicle and a vehicle speed of the vehicle are detected by a sensor; 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 of the vehicle, and a vehicle model; 3. The steering control method according to claim 2, wherein, when a difference between the standard yaw rate and the actual yaw rate is greater than a predetermined value, a steering reaction force based on the feedback axial force is applied regardless of whether the road surface is a low μ road surface, whether the axial force difference is equal to or greater than the second predetermined value, and whether the front wheel slip angle is equal to or greater than the fourth predetermined value.

5. an actual yaw rate and lateral acceleration occurring in the vehicle, and a vehicle speed of the vehicle are detected by sensors; calculating a reference vehicle body slip angular velocity that is a reference value of a vehicle body slip angular velocity generated in the vehicle based on the steering angle of the steered wheels, the vehicle speed of the vehicle, and a vehicle model; calculating an actual vehicle body slip angular velocity based on the actual yaw rate, the lateral acceleration, and the vehicle speed; 3. The steering control method according to claim 2, wherein, when the difference between the reference vehicle body slip angular velocity and the actual vehicle body slip angular velocity is larger than a predetermined value, a steering reaction force based on the feedback axial force is applied regardless of whether the road surface is a low μ road surface, whether the axial force difference is equal to or larger than the second predetermined value, and whether the front wheel slip angle is equal to or larger than the fourth predetermined value.

6. 2. The steering control method according to claim 1, wherein the road surface is determined to be a low μ road surface when the feedforward axial force is equal to or greater than a fifth predetermined value and the feedback axial force is less than a sixth predetermined value.

7. 2. The steering control method according to claim 1, wherein the first predetermined value is set to be larger when the bank angle of the road surface is large than when the bank angle is small.

8. a steering angle sensor for detecting the steering angle of a steering wheel in a vehicle in which the steering wheel and the steered wheels are mechanically separated; a reaction motor that applies a steering reaction force to the steering wheel; a controller that calculates a feedforward axial force, which is a steering rack axial force that provides a steering reaction force to be transmitted to the steering rack when a steering rack connected to the steered wheels is driven by a steering motor in response to steering of the steering wheel, and a feedback axial force, which is a steering rack axial force that is transmitted to the steering rack from a road surface on which the vehicle is traveling via the steered wheels, and causes the reaction motor to generate a steering reaction force to be applied to the steering wheel based on at least one of the feedforward axial force and the feedback axial force; A steering control device comprising: A front wheel slip angle, which is the slip angle of the steered wheels, is estimated, and it is determined whether the road surface is a low μ road surface having a friction coefficient equal to or less than a threshold value. When the road surface is a low μ road surface and an axial force difference between the feedback axial force and one of a reference axial force, which is a steering rack axial force according to an actual steering angle of the steered wheels, or the feedforward axial force, is less than a first predetermined value, the feedback axial force is not used, and a steering reaction force based on the feedforward axial force is generated in the reaction motor. When the road surface is a low μ road surface and the axial force difference is a steering control device that generates a steering reaction force based on the feedback axial force in the reaction motor when the road surface is not a low μ road surface and the front wheel slip angle is less than a third predetermined value, without using the feedback axial force, a steering reaction force based on the feedforward axial force in the reaction motor, and that generates a steering reaction force based on the feedback axial force in the reaction motor when the road surface is not a low μ road surface and the front wheel slip angle is not less than the third predetermined value.

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