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 travels is a low-[mu] road surface and the axial force difference between a feedback axial force and either a standard axial force or a feed-forward axial force is less than a first prescribed value, a steering reaction force based on the feed-forward axial force is applied (S6), and the steering reaction force is applied (S6). When the road surface is a low [mu] road surface and the axial force difference is equal to or greater than a first predetermined value, a steering reaction force based on the feedback axial force is applied (S9), and when the road surface is not a low [mu] road surface and the front wheel slip angle is less than a third predetermined value, a steering reaction force based on the feed-forward axial force is applied (S12). When the road surface is not a low [mu] road surface and the front wheel slip angle is equal to or greater than a third predetermined value, a steering reaction force based on the feedback axial force is applied (S13).

Description

Technical Field

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

[0002] The steering control device described in Patent Document 1 below drives a reaction motor based on a control amount of a steering reaction force based on a steering angle and a control amount calculated by multiplying a current of a steering motor by a set gain, thereby reflecting the influence of an external force acting on a steering wheel in the steering reaction force.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2000-108914

[0006] Problems to be Solved by the Invention

[0007] In the prior art, the reaction force motor is driven based on the control amount of the steering reaction force based on the steering angle and the control amount calculated by multiplying the current of the steering motor by a set gain. Therefore, in the prior art, for example, when the vehicle behavior becomes larger, the control amount of the steering reaction force based on the steering angle may become inappropriate. The purpose of the present invention is to apply a more appropriate steering reaction force in a steering control device of a steer-by-wire type in which a steering wheel and a steering wheel are mechanically separated. Summary of the invention

[0008] In a steering control method of one embodiment of the present invention, when the road surface is a low μ road surface and the axial force difference between the steering rack axial force corresponding to the actual steering angle of the steering wheel, i.e., the standard axial force or the feedforward axial force, and the feedback axial force is less than a first specified value, a steering reaction force based on the feedforward axial force is applied without using the feedback axial force; when the road surface is a low μ road surface and the axial force difference is greater than a first specified 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 specified value, a steering reaction force based on the feedforward axial force is applied without using the feedback axial force; when the road surface is not a low μ road surface and the front wheel slip angle is greater than a third specified value, a steering reaction force based on the feedback axial force is applied.

[0009] Effects of the Invention

[0010] According to the present invention, a more appropriate steering reaction force can be applied in a steer-by-wire steering control device in which a steering wheel and a steering wheel are mechanically separated. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a schematic configuration diagram of an example of a steering control device according to an embodiment.

[0012] Figure 2 This is a block diagram of an example of the functional configuration of a controller.

[0013] Figure 3 This is a block diagram of an example of the functional configuration of a target steering reaction force calculation unit.

[0014] Figure 4 It is an illustration of the axial force switching control for each driving scenario.

[0015] Figure 5 This is a flowchart of an example of the steering control method according to the embodiment.

[0016] Figure 6 This is a block diagram of an example of the functional structure of the axial force mixing section.

[0017] Figure 7 (a) to (c) are explanatory diagrams of examples of setting the mixing ratio.

[0018] Figure 8 This is a block diagram of an example of the functional structure of the axial force difference-dependent mixing ratio calculation unit.

[0019] Fig. 9 (a) and Fig. 9 (b) is an explanatory diagram of a setting example of the correction gain.

[0020] Fig.10 (a) and Fig.10 (b) is an explanatory diagram of a setting example of the switching range limit value.

[0021] Fig.11 This is a diagram for explaining an example of setting the tilt correction value.

[0022] Fig.12 This is a diagram for explaining an example of setting the axial force difference depending on the mixing ratio.

[0023] Fig.13 This is a block diagram of an example of the functional configuration of a front wheel slip angle dependent mixture ratio calculation unit.

[0024] Fig.14 It is an explanatory diagram of an example of setting the front wheel slip angle dependent mixture ratio. DETAILED DESCRIPTION

[0025] (First embodiment)

[0026] (constitute)

[0027] Figure 11 is a schematic structural diagram of an example of a steering control device of an embodiment. The steering control device of the embodiment is a steering control device of a steer-by-wire type that can mechanically separate the steering wheel 1a and the front wheel 2 as a steering wheel. The steering angle sensor 3 detects the steering angle δ of the steering wheel 1a. The steering angle sensor 4 detects the actual steering angle θ of the front wheel (steering wheel) 2. The vehicle speed sensor 5 detects the vehicle speed V of the host vehicle. The acceleration sensor 6 detects the lateral acceleration Gy acting on the host vehicle. The yaw rate sensor 7 detects the actual yaw rate (yaw rate: yaw rate) γa of the host vehicle. The steering control unit 8 includes: a steering motor 8A, a steering current detection unit 8B, and a steering motor drive unit 8C. The steering motor 8A is connected to the pinion shaft 10d via a speed reducer. The steering motor 8A is driven by the steering motor drive unit 8C to move the steering rack 10a to the left and right via the pinion shaft 10d and the pinion 10e. As a result, the steering motor 8A steers the front wheel 2. The steering current detection unit 8B detects the steering current Itm flowing through the steering motor 8A. The steering current detection unit 8B outputs a signal indicating the steering current Itm to the steering motor drive unit 8C and the controller 11. The steering motor drive unit 8C controls the steering current Itm of the steering motor 8A based on the target steering current Itt calculated by the controller 11 so that the steering current Itm detected by the steering current detection unit 8B coincides with the target steering current Itt. Thus, the steering motor drive unit 8C drives the steering motor 8A.

[0028] 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 speed 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. As a result, the reaction force motor 9A generates a steering reaction force. The reaction force current detection unit 9B detects the reaction force current Ism flowing through the reaction force motor 9A. Then, the reaction force current detection unit 9B outputs a detection signal representing 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 is consistent with the target reaction force current Ist. As a result, the reaction force motor drive unit 9C drives the reaction force motor 9A.

[0029] 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 the pinion 10c. When the backup clutch 12 is in the engaged state, the steering wheel 1a is mechanically connected to the front wheels 2. When the backup clutch 12 is in the released state, the steering wheel 1a is mechanically disconnected from the front wheels 2.

[0030] The controller 11 controls the driving of the steering motor 8A by the steering control unit 8 and the driving of the reaction motor 9A by the reaction force control unit 9. The controller 11 may include peripheral components such as a processor 13 and a storage device 14. For example, the functions of the controller 11 are realized by the processor 13 executing a computer program stored in the storage device 14. Figure 2 : An example of the functional structure of the controller 11 is shown. The target steering angle calculation unit 11A calculates a target steering angle θt which is a target value of the actual steering angle θ based on the steering angle δ and the vehicle speed V. For example, the target steering angle θt can be calculated by multiplying the steering angle δ by the variable gear ratio. The subtractor 11D calculates a deviation value Δθ obtained by subtracting the actual steering angle θ from the target steering angle θt. The target steering current calculation unit 11C calculates a target steering current Itt based on the deviation value Δθ. The target steering current calculation unit 11C outputs the target steering current Itt to the steering motor drive unit 8C.

[0031] The target steering reaction force calculation unit 11B calculates the target reaction force current Ist based on the steering angle δ, the vehicle speed V, the actual steering angle θ, the actual yaw rate γa, the lateral acceleration Gy, and the 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. Figure 3 It is a block diagram of a functional structure example of a target steering reaction force calculation unit. In addition, in the following description and drawings, the feedforward axial force is sometimes expressed as "FF axial force" and the feedback axial force is sometimes expressed as "FB axial force". The FF axial force calculation unit 20 calculates the steering rack axial force that imparts the steering reaction force corresponding to the steering angle δ, that is, the FF axial force Fff, based on the steering angle δ and the vehicle speed V. The steering rack axial force refers to the rack axial force applied to the steering rack 10a. For example, the FF axial force Fff can be calculated based on the target steering angle θt calculated based on the steering angle δ and the vehicle speed V, the pinion rigidity of the pinion and rack of the steering mechanism, the pinion viscosity, the rack inertia, and the rack viscosity. For example, the FF axial force Fff can be an axial force applied to the steering rack that includes at least a proportional component corresponding to the target steering angle θt and a damping component corresponding to the steering angular velocity.

[0032] 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 a steering rack axial force that applies the force transmitted from the road surface to the front wheel 2 as a steering reaction force to the steering wheel 1a and transmits it 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 steering wheel 2 based on the steering current Itm. The steering rack axial force calculated based on the steering current is expressed as "current axial force".

[0033] In addition, the FB axial force calculation unit 21 calculates the steering rack axial force reflecting the influence of the tire lateral force acting on the steering wheel 2 based on the lateral acceleration Gy. The steering rack axial force calculated based on the lateral acceleration Gy is expressed as "lateral G axial force". The FB axial force calculation unit 21 calculates the steering rack axial force reflecting the influence of the tire lateral force acting on the steering wheel 2 based on the actual yaw rate γa. The steering rack axial force calculated based on the actual yaw rate γa is expressed as "yaw rate axial force". The FB axial force calculation unit 21 mixes the current axial force, the lateral G axial force and the yaw rate axial force to calculate the FB axial force Ffb.

[0034] The axial force mixing unit 23 estimates the slip angle of the front wheel 2, i.e., the front wheel slip angle, and determines whether the road surface on which the vehicle is traveling is a low μ road surface having a friction coefficient below a threshold value. The axial force mixing unit 23 has a road surface μ estimation unit 23a for determining whether the road surface on which the vehicle is traveling is a low μ road surface. In the case where the road surface is a low μ road surface, even if the FF axial force Fff is large, the FB axial force Ffb becomes smaller. The road surface μ estimation unit 23a determines that the road surface is a low μ road surface when the FF axial force Fff is greater than the upper limit value Fff2 and the FB axial force Ffb is less than the lower limit value Ffb1. In the case where the FF axial force Fff is less than the lower limit value Fff1, or the FB axial force Ffb is greater than the upper limit value Ffb2, it is determined that the road surface is not a low μ road surface.

[0035] 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 specified 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. In addition, as the axial force difference ΔFs, the difference between the standard axial force Frm and the FB axial force Ffb can also be used. This is because the standard axial force Frm is a steering rack axial force calculated according to the vehicle model based on the actual steering angle θ, and the difference with the FF axial force Fff in a stable state is very small. In addition, when the road surface is a low μ road surface and the axial force difference ΔFs is greater than a first specified value, the axial force mixing unit 23 determines that the vehicle is in an understeering state (hereinafter referred to as "US state"), and calculates the estimated rack axial force Frk based on the FB axial force Ffb. When the road surface is not a low μ road surface and the front wheel slip angle is less than the third specified value, the FB axial force Ffb is not used and the estimated rack axial force Frk is calculated based on the FF axial force Fff. When the road surface is not a low μ road surface and the front wheel slip angle is greater than the third specified value, it is judged that the vehicle is in the US state and the estimated rack axial force Frk is calculated based on the FB axial force Ffb.

[0036] 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 also convert the estimated rack axial force into the target steering reaction force using a conversion map that stores the target steering reaction forces corresponding to the respective values ​​of the estimated rack axial force. The target reaction force current calculation unit 25 calculates the 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.

[0037] As described above, the controller 11 changes the determination of whether the host vehicle is in the US state according to whether the road surface is a low μ road surface. Figure 4 State the reason.

[0038] Figure 4 It is an illustration of the axial force switching control for each driving scenario. When the tires of this vehicle are in a gripping state, a steering reaction force is applied based on the FF axial force Fff. This is because, in the gripping state, by using the FF axial force, a steering reaction force that is not affected by interference can be applied. On the other hand, in an oversteering state (hereinafter referred to as "OS state") or a US state, a steering reaction force is applied based on the FB axial force Ffb. This is because by using the FB axial force Ffb, road surface information can be transmitted as a steering reaction force.

[0039] Furthermore, when the road surface is not a low μ road surface, whether it is in the US state is determined based on the front wheel tire slip angle. By making a judgment based on the front wheel slip angle as information on the tire end, the delay in the detection timing of the US state can be reduced. On the other hand, on a low μ road surface, the influence of the reduction of the road surface μ is not easy to appear in the front wheel slip angle. Therefore, on a low μ road surface. The US state is determined based on the axial force difference that directly reflects the US state caused by the reduction of the road surface μ. In addition, whether the vehicle is in the OS state is determined based on the yaw rate difference or the body slip angular velocity difference.

[0040] (action)

[0041] Figure 5Flowchart of an example of a steering control method of 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. In the case where the road surface is a low μ road surface (step S4: Yes), the processing enters step S5. In the case where the road surface is not a low μ road surface (step S4: No), the processing enters step S10. In step S5, the controller 11 determines whether the axial force difference is less than a first prescribed value. In the case where the axial force difference is less than the first prescribed value (step S5: Yes), the processing enters step S6. In the case where the axial force difference is not less than the first prescribed value (step S5: No), the processing enters step S7. In step S6, the controller 11 applies a steering reaction force corresponding to the FF axial force Fff, and in step S7, applies a steering reaction force corresponding to the FB axial force Ffb. The processing then ends.

[0042] In step S8, the controller 11 determines whether the front wheel slip angle is less than the third specified value. If the front wheel slip angle is less than the third specified value (step S8: Yes), the process proceeds to step S9. If the front wheel slip angle is not less than the third specified value (step S8: No), the process proceeds to step S10. In step S9, the controller 11 applies a steering reaction force corresponding to the FF axial force Fff, and in step S10, applies a steering reaction force corresponding to the FB axial force Ffb. The process then ends.

[0043] (Variation Example)

[0044] The axial force mixing unit 23 may also set a larger first prescribed value when the inclination angle of the road surface is larger than that of the smaller one. Thus, the influence of the estimated error of the FB axial force Ffb caused by the inclination can be reduced. For example, the axial force mixing unit 23 calculates the lateral acceleration that is added to the lateral acceleration Gy output by the acceleration sensor 6 due to the influence of the inclination angle or slope of the road surface, that is, the lateral G offset Gyo. 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 calculates the correction value of the first prescribed value and the second prescribed value corresponding to the inclination correction value Cb, that is, the inclination correction value Cb. Refer to Fig.11. The tilt correction value Cb increases from "0" to value Cb2 as the lateral G offset Gyo increases. For example, in the range where the lateral G offset Gyo is less than value Gy1, the tilt correction value Cb is "0", and as the lateral G offset Gyo increases from value Gy1 to value Gy2, the tilt correction value Cb increases from "0" to value Cb1, and as the lateral G offset Gyo increases from value Gy2 to value Gy3, the tilt correction value Cb increases from value Cb1 to value Cb2. The axial force mixing section 23 corrects the first prescribed value and the second prescribed value by adding the tilt correction value Cb. As a result, when the inclination angle of the road surface is large, the first prescribed value becomes larger.

[0045] (Second embodiment)

[0046] In the second embodiment, when the steering speed ω of the steering wheel 1a is equal to or higher than a predetermined speed, a steering reaction force based on the FB axial force is applied. Figure 3 . The differentiator 22 calculates the steering angular velocity ω of the steering wheel 1a by differentiating the steering angle δ. The axial force mixing unit 23 determines whether the steering angular velocity ω is above the specified speed. When the steering angular velocity ω is above the specified speed, 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 above the second specified value, and whether the front wheel slip angle is above the fourth specified value. For example, the axial force mixing unit 23 can calculate the estimated rack axial force Frk by calculating the steering angular velocity dependent mixing ratio R1 corresponding to the steering angular velocity ω, and mixing the FF axial force Fff and the FB axial force Ffb with the steering angular velocity dependent mixing ratio R1. The steering angular velocity dependent mixing ratio R1 is set to reduce the proportion of the FF axial force Fff when the steering angular velocity ω is high. Refer to Figure 7 (a). As the steering angular velocity dependent mixing ratio R1 and the steering angular velocity ω become higher, the steering angular velocity dependent mixing ratio R1 changes from "1" to "0". For example, in the range where the steering angular velocity ω is lower than the lower limit ω1, the steering angular velocity dependent mixing ratio R1 is "1". In the range where the steering angular velocity ω is higher than the upper limit ω2, the steering angular velocity dependent mixing ratio R1 is "0". In the range from the lower limit ω1 to the upper limit ω2, as the steering angular velocity ω becomes higher, the steering angular velocity dependent mixing ratio R1 decreases from "1" to "0". The lower limit ω1 and the upper limit ω2 can also be made variable corresponding to the vehicle speed V. The axial force mixing unit 23 can 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 with the steering angular velocity dependent mixing ratio R1.

[0047] (Third Embodiment)

[0048] In the third embodiment, in addition to the control of the first and second embodiments, when the vehicle is in the OS state, a steering reaction force based on the FB axial force is applied. For example, when the vehicle is in the OS state, the yaw rate difference Δγ, which is the difference between the standard yaw rate γm, which is the standard value of the yaw rate calculated by the vehicle model, and the actual yaw rate γa, becomes larger. Therefore, when the yaw rate difference Δγ is large, a steering reaction force based on the FB axial force is applied. In addition, for example, when the vehicle is in the OS state, the body slip angular velocity difference Δα, which is the difference between the standard body slip angular velocity αm, which is the standard value of the body slip angular velocity calculated by the vehicle model, and the actual body slip angular velocity αa of the vehicle, becomes larger. Therefore, when the body slip angular velocity difference Δα is large, a steering reaction force based on the FB axial force is applied.

[0049] Figure 6 This is a block diagram of a functional structure example of the axial force mixing unit 23 of the third embodiment. The axial force mixing unit 23 calculates the steering angular velocity dependent mixing ratio R1 corresponding 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 with the steering angular velocity dependent mixing ratio R1. In addition, the axial force mixing unit 23 calculates the yaw rate difference dependent mixing ratio R2 corresponding to the yaw rate difference Δγ, the body slip angular velocity difference dependent mixing ratio R3 corresponding to the body slip angular velocity difference Δα, the axial force difference dependent mixing ratio R4 corresponding to the axial force difference ΔFs, and the front wheel slip angle dependent mixing ratio R5 corresponding to the front wheel slip angle. The axial force mixing unit 23 selects any one of these mixing ratios R2 to R5 as the final mixing ratio Rf according to the driving scene. 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 with the final mixing ratio Rf.

[0050] The axial force mixing unit 23 includes a steering angular velocity dependent mixing ratio calculation unit 30, mixers 31 and 32, a model calculation unit 33, a yaw rate difference dependent mixing ratio calculation unit 34, a vehicle body slip angular velocity difference dependent mixing ratio calculation unit 35, an axial force difference dependent mixing ratio calculation unit 36, a front wheel slip angle dependent mixing ratio calculation unit 37, and a selector 38. Figure 7 (a) As described above, the steering angular velocity dependent mixing ratio calculation unit 30 calculates the steering angular velocity dependent mixing ratio R1 based on at least the steering angular velocity ω. The mixer 31 calculates the 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 ratio R1. The mixer 32 calculates the second mixed axial force Fmm=R1×Frm+(1-R1)×Ffb by mixing the standard axial force Frm and the FB axial force Ffb at the steering angular velocity dependent mixing ratio R1.

[0051] The model calculation unit 33 calculates the standard yaw rate γm and the standard vehicle body slip angular velocity αm according to a predetermined linear two-degree-of-freedom model based on the actual steering angle θ and the vehicle speed V. An example of the linear two-degree-of-freedom model is shown in the following equation.

[0052] [Formula 1]

[0053]

[0054] V x is the front-rear speed of the vehicle, V y is the lateral velocity, θ f is the steering angle of the front wheel, θ r is the steering angle of the rear wheels, M is the mass of the vehicle, I z is the moment of inertia, L f is the length from the center of gravity to the front wheel, L r is the length from the center of gravity to the rear wheel, K f is the angular stiffness of the front wheel (tire deflection stiffness), K r is the angular stiffness of the rear wheel. In addition, the model calculation unit 33 calculates the first estimated front wheel slip angle βfm1 which is the front wheel slip angle calculated based on the actual yaw rate γa, and the second estimated front wheel slip angle βfm2 which is the front wheel slip angle calculated based on the standard yaw rate γm. For example, the model calculation unit 33 can calculate the first estimated front wheel slip angle βfm1 and the second estimated front wheel slip angle βfm2 according to the following formula.

[0055] βfm1=θ / N-(V y +L f ×γa) / V x

[0056] βfm2=θ / N-(V y +L f ×γm) / V x

[0057] Where N is the total speed ratio. Furthermore, the model calculation unit 33 calculates the standard axial force Frm based on the actual steering angle θ. For example, the model calculation unit 33 may multiply the first estimated front wheel slip angle βfm1 or the second estimated front wheel slip angle βfm2 by the front wheel angular stiffness K f The tire lateral force of the front wheel 2 is estimated, and the estimated tire lateral force is converted into a standard axial force Frm.

[0058] The yaw rate difference dependent mixing ratio calculation unit 34 calculates the yaw rate difference dependent mixing ratio R2 for calculating the estimated rack axial force Frk based on the FB axial force Ffb when the vehicle is in the OS state. The yaw rate difference dependent mixing ratio R2 can 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 to reduce the proportion of the first mixed axial force Fmf (reduce the proportion of the FF axial force Fff) when the yaw rate difference Δγ between the standard yaw rate γm and the actual yaw rate γa is large. Refer to Figure 7 (b). The yaw rate difference dependent mixing ratio R2 changes from "1" to "0" as the yaw rate difference Δγ increases. For example, in a range where the yaw rate difference Δγ is less than a lower limit Δγ1, the yaw rate difference dependent mixing ratio R2 is "1". In a range where the yaw rate difference Δγ is greater than an upper limit Δγ2, the yaw rate difference dependent mixing ratio R2 is "0". In the range from the lower limit Δγ1 to the upper limit Δγ2, the yaw rate difference dependent mixing ratio R2 decreases from "1" to "0" as the yaw rate difference Δγ increases. The lower limit Δγ1 and the upper limit Δγ2 may also be variable according to the vehicle speed V.

[0059] Reference Figure 6 . The body slip angular velocity difference-dependent mixing ratio calculation unit 35 calculates the body slip angular velocity difference-dependent mixing ratio R3 for calculating the estimated rack axial force Frk based on the FB axial force Ffb when the vehicle is in the OS state. The body slip angular velocity difference-dependent mixing ratio calculation unit 35 calculates the actual body slip angular velocity αa=γa-Gy / V based on the vehicle speed V, the actual yaw rate γa and the lateral acceleration Gy, calculates the body slip angular velocity difference Δα between the standard body slip angular velocity αm and the actual body slip angular velocity αa, and calculates the body slip angular velocity difference-dependent mixing ratio R3 based on the body slip angular velocity difference Δα. The body slip angular velocity difference-dependent mixing ratio R3 is set to reduce the ratio of the first mixed axial force Fmf when the body slip angular velocity difference Δα is large.

[0060] Reference Figure 7 (c). The vehicle body slip angular velocity difference dependent mixing ratio R3 changes from "1" to "0" as the vehicle body slip angular velocity difference Δα becomes larger. For example, in the range where the vehicle body slip angular velocity difference Δα is less than the lower limit Δα1, the vehicle body slip angular velocity difference dependent mixing ratio R3 is "1". In the range where the vehicle body slip angular velocity difference Δα is greater than the upper limit Δα2, the vehicle body slip angular velocity difference dependent mixing ratio R3 is "0". In the range from the lower limit Δα1 to the upper limit Δα2, the vehicle body slip angular velocity difference dependent mixing ratio R3 decreases from "1" to "0" as the vehicle body slip angular velocity difference Δα becomes larger. The lower limit Δα1 and the upper limit Δα2 may also be made variable according to the vehicle speed V. Refer to Figure 6The vehicle body slip angular velocity difference-dependent mixture ratio calculation unit 35 calculates the lateral G offset amount Gyo.

[0061] The axial force difference-dependent mixing ratio calculation unit 36 ​​calculates the axial force difference-dependent mixing ratio R4. Figure 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 calculation unit 46, an upper limit calculation unit 47, an inclination 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 can also be calculated as the axial force difference ΔFs. The axial force difference ΔFs from which the high-frequency components are removed by the low-pass filter 41 is input to the multiplier 44.

[0062] The correction gain calculation units 42 and 43 calculate the first correction gain G1 and the second correction gain G2 corresponding to whether the road surface is a low μ road surface. Thus, when the road surface is a low μ road surface, the axial force difference-dependent mixing ratio R4 corresponding to the axial force difference ΔFs is calculated, and when the road surface is not a low μ road surface, the axial force difference-dependent mixing 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 becomes smaller. Therefore, when the FF axial force Fff is above the upper limit value Fff2 and the FB axial force Ffb is less than the lower limit value 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". Refer to Fig. 9 (a). The first correction gain G1 changes from "1" to "0" as the FB axial force Ffb increases. For example, in the range where the FB axial force Ffb is less than the lower limit value Ffb1, the first correction gain G1 is "1", and in the range where the FB axial force Ffb is greater 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.

[0063] On the contrary, when the FF axial force Fff is less than the lower limit value Fff1 or the FB axial force Ffb is greater than the upper limit value 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". Fig. 9(b). The second correction gain G2 changes from "0" to "1" as the standard axial force Frm increases. For example, in the range where the standard axial force Frm is less than the lower limit value Fff1, the second correction gain G2 is "0", and in the range where the standard axial force Frm is greater than the upper limit value Fff2, the second correction gain G2 is "1". In the range from the lower limit value Fff1 to the upper limit value 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. Therefore, in the case where the road surface is a low μ road surface, the axial force difference ΔFs is directly input into the mixing ratio calculation unit 51. Therefore, the mixing ratio calculation unit 51 calculates the axial force difference-dependent mixing ratio R4 corresponding to the axial force difference ΔFs. On the other hand, when the road surface is not a low μ road surface, the axial force difference value ΔFsc=0 is input to the mixture ratio calculation unit 51, and the axial force difference value is fixed to “1” depending on the mixture ratio R4 as described later.

[0064] The lower limit value calculation unit 46 calculates the switching range lower limit value LL1, which is the lower limit value of the switching range of the axial force difference dependent mixture ratio R4 between "1" and "0" according to the axial force difference value ΔFsc. The upper limit value calculation unit 47 calculates the switching range upper limit value LU1 as 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 can calculate the switching range lower limit value LL1 and the switching range upper limit value LU1 according to the vehicle speed V. 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 according to the vehicle speed. 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 according to the vehicle speed.

[0065] Reference Figure 8 The tilt correction value calculation unit 48 calculates the reference Fig.11 The tilt correction value Cb is added to the switching range lower limit LL1 and the switching range upper limit LU1 by the adders 49 and 50, respectively, to calculate the corrected switching range lower limit LL2=LL1+Cb and the corrected switching range upper limit LU2=LU1+Cb. As described above, the greater the inclination angle or the slope of the road surface, the greater the value of the lateral G offset Gyo. Therefore, the greater the inclination angle or the slope of the road surface, the greater the switching range lower limit LL2 and the switching range upper limit LU2 are set to.

[0066] 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 to reduce the ratio of the first mixed axial force Fmf when the axial force difference ΔFsc is large.

[0067] Reference Fig.12 . The axial force difference-dependent mixing ratio R4 changes from "1" to "0" as the axial force difference ΔFsc becomes larger. For example, in the range where the axial force difference ΔFsc is less than the switching range lower limit LL2, the axial force difference-dependent mixing ratio R4 is "1". Therefore, in the case where the road surface is not a low μ road surface, since ΔFsc=0 is input, the axial force difference-dependent mixing ratio R4 is fixed to "1". In the range where the axial force difference ΔFsc is greater 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, as the axial force difference ΔFsc becomes larger, the axial force difference-dependent mixing ratio R4 decreases from "1" to "0". The switching range lower limit LL2 and the switching range upper limit LU2 are examples of the "first specified value" and "second specified value" respectively recorded in the scope of the claim.

[0068] Reference 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 state estimation unit 60, a mixture ratio calculation unit 61 and 62, a dry (DRY) switching range lower limit value calculation unit 63, a wet (WET) switching range lower limit value calculation unit 64, mixers 65 and 68, a dry switching range upper limit value calculation unit 66, a wet switching range upper limit value calculation unit 67, and a selector 69. The road surface state estimation unit 60 determines the dry state (or wet state) of the road surface based on the axial force difference ΔFs, and calculates the mixture ratio R6 corresponding to the dry state (or wet state) of the road surface. The mixture ratio R6 changes 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 wet. The road surface state estimation unit 60 and Figure 8 Similarly to the correction gain calculation units 42 and 43 and the multipliers 44 and 45, the axial force difference ΔFs can be corrected using the correction gain corresponding to the FB axial force Ffb and the standard axial force Frm, and the mixture ratio R6 can be calculated based on the corrected axial force difference. In addition, similar to the lower limit calculation unit 46, the upper limit calculation unit 47, the tilt correction value calculation unit 48, the adders 49 and 50, the lower limit and upper limit of the switching range for switching the mixture ratio R6 between "1" and "0" can be set according to the vehicle speed V.

[0069] The mixture ratio calculation unit 61 calculates the first front wheel slip angle dependent mixture ratio R5a based on the first estimated front wheel slip angle βfm1. The first front wheel slip angle dependent mixture ratio R5a is set to reduce the ratio of the first mixture axial force Fmf when the first estimated front wheel slip angle βfm1 is large. Fig.14 . The first front wheel slip angle dependent mixture ratio R5a changes from "1" to "0" as the first estimated front wheel slip angle βfm1 increases. For example, in a range where the first estimated front wheel slip angle βfm1 is less than the switching range lower limit LL5, the first front wheel slip angle dependent mixture ratio R5a is "1". In a range where the first estimated front wheel slip angle βfm1 is greater than the switching range upper limit LU5, the first front wheel slip angle dependent mixture ratio R5a is "0". In a range from the switching range lower limit LL5 to the switching range upper limit LU5, as the first estimated front wheel slip angle βfm1 increases, the first front wheel slip angle dependent mixture ratio R5a decreases from "1" to "0". The switching range lower limit LL5 and the switching range upper limit LU5 are examples of the "third prescribed value" and the "fourth prescribed value" respectively described in the scope of the claim.

[0070] The mixture ratio calculation unit 62 calculates the second front wheel slip angle dependent mixture ratio R5b based on the second estimated front wheel slip angle βfm2. The calculation method of the second front wheel slip angle dependent mixture ratio R5b is the same as the calculation method of the first front wheel slip angle dependent mixture ratio R5a, except that the second estimated front wheel slip angle βfm2 is used instead of the first estimated front wheel slip angle βfm1.

[0071] The dry switching range lower limit value calculation unit 63, the wet switching range lower limit value calculation unit 64 and the mixer 65 calculate the switching range lower limit value LL5. For example, the dry switching range lower limit value calculation unit 63 and the wet switching range lower limit value calculation unit 64 calculate the switching range lower limit value LL3 for the dry state and the switching range lower limit value LL4 for the wet state, respectively. The switching range lower limit values ​​LL3 and LL4 may be constant values ​​at any vehicle speed V, or may be variable according to the vehicle speed. The mixer 65 calculates the switching range lower limit value LL5 by mixing the switching range lower limit values ​​LL3 and LL4 at a mixing ratio R6. The dry switching range upper limit value calculation unit 66, the wet switching range upper limit value calculation unit 67 and the mixer 68 calculate the switching range upper limit value LU5. For example, the dry switching range upper limit value calculation unit 66 and the wet switching range upper limit value calculation unit 67 calculate the switching range upper limit value LU3 for the dry state and the switching range upper limit value LU4 for the wet state, respectively. The switching range upper limit values ​​LU3 and LU4 may be constant at any vehicle speed V, or may be variable according to the vehicle speed. The mixer 68 calculates the switching range upper limit value LU5 by mixing the switching range upper limit values ​​LU3 and LU4 at a mixing ratio R6. The selector 69 selects the smaller one 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 and outputs it.

[0072] Reference Figure 6 . The selector 38 selects any one of these mixture ratios R2 to R5 as the final mixture ratio Rf according to the driving scenario. For example, when the yaw rate difference dependent mixture ratio R2 is less than "1", it can be judged that the vehicle is in the OS state and the yaw rate difference dependent mixture ratio R2 is selected. In addition, when the body slip angular velocity difference dependent mixture ratio R3 is less than "1", it can be judged that the vehicle is in the OS state and the body slip angular velocity difference dependent mixture ratio R3 is selected. In addition, when the axial force difference dependent mixture ratio R4 is less than "1", it can be judged that the vehicle is in the US state and the road surface is a low μ road surface, and the axial force difference dependent mixture ratio R4 is selected. In addition, when the front wheel slip angle dependent mixture ratio R5 is less than "1", it can be judged that the vehicle is in the US state and the road surface is a low μ road surface, and the front wheel slip angle dependent mixture ratio R5 is selected.

[0073] In addition, the selector 38 may select the smallest one of the mixture ratios R2 to R5 as the final mixture ratio Rf. The selector 38 may calculate the mixed axial force obtained by mixing the first mixed axial force Fmf and the FB axial force Ffb at the final mixture ratio Rf as the estimated rack axial force Frk. Thus, the sudden change in the steering reaction force caused by the sudden switching of the estimated rack axial force Frk between the FF axial force Fff and the FB axial force Ffb can be alleviated.

[0074] (Effects of Embodiments)

[0075] (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 greater than a second predetermined value that is greater than the first predetermined value, a steering reaction force based on the FB axial force is applied. Thus, when the road surface is a low μ road surface, the FF axial force and the FB axial force can be switched according to the axial force difference that can detect a decrease in μ, so that when the state is US, the steering reaction force based on the FB axial force can be quickly switched, and the decrease in the road surface μ can be appropriately transmitted to the driver.

[0076] When the road surface is not a low μ road surface and the front wheel slip angle is less than the third predetermined value, the steering reaction force based on the FF axial force is applied, and when the road surface is not a low μ road surface and the front wheel slip angle is greater than the third predetermined value and greater than a fourth predetermined value, the steering reaction force based on the FB axial force is applied. Thus, when the road surface is not a low μ road surface, in the case of the US state, the steering reaction force based on the FB axial force can be quickly switched, and the road surface state can be appropriately transmitted to the driver.

[0077] (2) When the steering speed is greater than or equal to 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 the front wheel slip angle is greater than or equal to a fourth predetermined value. Thus, in an emergency avoidance scenario, the steering reaction force may be switched based on the FB axial force that can appropriately transmit the road surface condition to the driver.

[0078] (3) When the difference between the standard yaw rate and the actual yaw rate is greater than a predetermined value, the 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 the front wheel slip angle is greater than a fourth predetermined value.

[0079] Furthermore, when the difference between the standard 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 and whether the front wheel slip angle is greater than a fourth predetermined value. Thus, it is possible to switch to a steering reaction force based on the FB axial force that can appropriately transmit the vehicle behavior to the driver in the case of the OS state.

[0080] (4) When the FF axial force Fff is equal to or greater than the fifth predetermined value and the FB axial force is less than the sixth predetermined value, the road surface may be determined to be a low μ road surface. In this way, whether the road surface is a low μ road surface can be determined with a simple structure.

[0081] (5) When the inclination angle of the road surface is larger than that of the smaller road surface, a larger first predetermined value is set. This can reduce the influence of the estimation error of the FB axial force Ffb caused by the inclination.

[0082] Explanation of symbols

[0083] 1a: Steering wheel, 1b: Steering shaft, 2: Front wheel, 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, 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, 12: Backup clutch.

Claims

1. A steering control method, in a vehicle in which a steering wheel is mechanically separated from a steering wheel, wherein a feedforward axial force, which is a steering rack axial force imparted to the steering rack as a steering reaction force when a steering rack connected to the steering wheel is driven by a steering motor in accordance with steering of the steering wheel, and a feedback axial force, which is a steering rack axial force transmitted from a road surface on which the vehicle is traveling via the steering wheel to the steering rack, are calculated, and a steering reaction force is applied to the steering wheel based on at least one of the feedforward axial force and the feedback axial force, wherein: The slip angle of the steering wheel, i.e., the front wheel slip angle, is estimated. determining whether the road surface is a low μ road surface having a friction coefficient below a threshold value, When the road surface is a low μ road surface and the axial force difference between the steering rack axial force corresponding to the actual steering angle of the steering wheel, that is, the standard axial force or the feedforward axial force and the feedback axial force is less than a first specified value, a steering reaction force based on the feedforward axial force is applied without using the feedback axial force, When the road surface is a low μ road surface and the axial force difference is greater than the first predetermined value, applying a steering reaction force based on the feedback axial force, When the road surface is not a low μ road surface and the front wheel slip angle is smaller than a third predetermined value, applying a steering reaction force based on the feedforward axial force without using the feedback axial force, 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.

2. The steering control method according to claim 1, characterized in that: When the road surface is a low μ road surface and the axial force difference is greater than the first predetermined value and less than a second predetermined value greater than the first predetermined value, a steering reaction force based on a mixed axial force of 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 greater than or equal to the third predetermined value and less than a fourth predetermined value greater than or equal to the third predetermined value, a steering reaction force based on a mixed axial force of the feedforward axial force and the feedback axial force is applied.

3. The steering control method according to claim 2, characterized in that: When the steering speed is greater than a prescribed 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 greater than the second prescribed value, and whether the front wheel slip angle is greater than the fourth prescribed value.

4. The steering control method according to claim 2, characterized in that: The actual yaw rate and the vehicle speed of the vehicle are detected by using a sensor. A standard yaw rate, that is, a standard value of the yaw rate generated in the vehicle is calculated based on the steering angle of the steering wheel, the vehicle speed and the vehicle model, When the difference between the standard yaw rate and the actual yaw rate is greater than the specified 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 greater than the second specified value, and whether the front wheel slip angle is greater than the fourth specified value.

5. The steering control method according to claim 2, characterized in that: The actual yaw rate, lateral acceleration and vehicle speed of the vehicle are detected by using sensors. A standard body slip angular velocity, that is, a standard body slip angular velocity generated in the vehicle is calculated based on the steering angle of the steering wheel, the vehicle speed and the vehicle model; Calculating an actual vehicle body slip angular velocity based on the actual yaw rate, the lateral acceleration and the vehicle speed, When the difference between the standard vehicle body slip angular velocity and the actual vehicle body slip angular velocity is greater than a specified 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 greater than the second specified value, and whether the front wheel slip angle is greater than the fourth specified value.

6. The steering control method according to claim 1, characterized in that: When the feedforward axial force is equal to or greater than a fifth predetermined value and the feedback axial force is smaller than a sixth predetermined value, it is determined that the road surface is a low μ road surface.

7. The steering control method according to claim 1, characterized in that: When the inclination angle of the road surface is large, the first predetermined value is set to be larger than when the inclination angle of the road surface is small.

8. A steering control device comprising: A steering angle sensor for detecting a steering angle of a steering wheel in a vehicle in which the steering wheel is mechanically separated from the steering wheel; a reaction force motor which applies a steering reaction force to the steering wheel; a controller that calculates a steering rack axial force, i.e., a feedforward axial force, which is a steering rack axial force imparted to the steering rack as a steering reaction force transmitted to the steering rack when the steering motor drives the steering rack connected to the steering wheel according to the steering of the steering wheel, and a steering rack axial force, i.e., a feedback axial force, which is a steering rack axial force transmitted from a road surface on which the vehicle is traveling via the steering wheel to the steering rack, and causes the reaction force motor to generate a steering reaction force applied to the steering wheel based on at least one of the feedforward axial force and the feedback axial force, The controller is characterized in that it performs the following processing: The method further comprises estimating a slip angle of the steering wheel, i.e., a front wheel slip angle, determining whether the road surface is a low μ road surface having a friction coefficient of less than a threshold value, and causing the reaction force motor to generate a steering reaction force based on the feedforward axial force without using the feedback axial force when the road surface is a low μ road surface and an axial force difference between one of a steering rack axial force, i.e., a standard axial force or the feedforward axial force, corresponding to an actual steering angle of the steering wheel is less than a first predetermined value, causing the reaction force motor to generate a steering reaction force based on the feedforward axial force without using the feedback axial force, and causing the reaction force motor to generate a steering reaction force based on the feedback axial force when the road surface is a low μ road surface and the axial force difference is greater than the first predetermined value, causing the reaction force motor to generate a steering reaction force based on the feedback axial force, and causing the reaction force motor to generate a steering reaction force based on the feedforward axial force without using the feedback axial force when the road surface is not a low μ road surface and the front wheel slip angle is less than a third predetermined value, and causing the reaction force motor to generate a steering reaction force based on the feedback axial force when the road surface is not a low μ road surface and the front wheel slip angle is greater than the third predetermined value.

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