Steering control method and steering control device

Abstract

This steering control method comprising: calculating a target steering angle of a steering wheel (S1); driving a steering motor on the basis of a difference between the target steering angle and an actual steering angle of the steering wheel (S2); estimating first estimated current axial force which is steering rack axial force, by multiplying a steering current, which is the current that drives the steering motor, by a first coefficient (S3); estimating second estimated current axial force which is steering rack axial force, by multiplying the steering current by a second coefficient larger than the first coefficient (S4); calculating an estimated steering rack axial force on the basis of the first estimated current axial force, when the steering state of the steering wheel is a largely-turned steering state (S6); calculating an estimated steering rack axial force on the basis of the second estimated current axial force when the steering state is a turn-back steering state (S7); calculating a target reaction force current on the basis of the estimated steering rack axial force (S8); and driving a reaction force motor that applies a steering reaction force to the steering wheel on the basis of the estimated target reaction force current (S9).

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] However, the relationship between the magnitude of the current of the steering motor and the strength of the steering rack axial force changes depending on whether the steering device is in a turning-to-return state or a returning-to-turn state. Therefore, if the gain used when estimating the steering rack axial force from the current of the steering motor is fixed, an inappropriate steering reaction force may be generated, causing the driver to feel uncomfortable. An object of the present invention is to improve the accuracy of estimating the steering rack axial force from the current of the steering motor. [Means for solving the problem]

[0005] In one aspect of the steering control method of the present invention, a target steering angle, which is a target value for the steering angle of the steered wheels, is calculated based on the steering angle of a steering wheel that is mechanically separated from the steered wheels; a steering motor that turns the steered wheels is driven based on the difference between the target steering angle and the actual steering angle of the steered wheels; a first current estimated axial force, which is the steering rack axial force, is estimated by multiplying a steering current, which is a current that drives the steering motor, by a first coefficient; and a second current estimated axial force, which is the steering rack axial force, is estimated by multiplying the steering current by a second coefficient that is greater than the first coefficient; when the steering state of the steering wheel is a turn-back steering state, the estimated steering rack axial force is calculated based on the first current estimated axial force, and when the steering state is a return steering state, the estimated steering rack axial force is calculated based on the second current estimated axial force; a target reaction force current, which is a target value for a current that drives a reaction force motor that applies a steering reaction force to the steering wheel, is calculated based on the estimated estimated steering rack axial force; and the reaction force motor is driven based on the estimated target reaction force current. [Effects of the Invention]

[0006] According to the present invention, it is possible to improve the accuracy of estimating the steering rack axial force from the current of the steering motor. The objects and advantages of the invention will be realized and attained by means of the elements and combinations set forth in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. [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 a block diagram of an example of the functional configuration of an FB axial force calculation unit. [Figure 5]FIG. 4 is a characteristic diagram showing the relationship between the current axial force calculated by multiplying the steering current by a fixed torque constant and the actual steering rack axial force. [Figure 6] FIG. 10 is a characteristic diagram showing the relationship between a positive current axial force and an actual steering rack axial force. [Figure 7] FIG. 10 is a characteristic diagram showing the relationship between the inverse efficiency current axial force and the actual steering rack axial force. [Figure 8] FIG. 10 is an explanatory diagram of an example of a mixture ratio of axial forces according to a steering angular velocity. [Figure 9] FIG. 4 is a characteristic diagram showing the relationship between the feedback axial force and the actual steering rack axial force. [Figure 10] 4 is a flowchart of an example of a steering control method according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] (composition) 1 is a schematic diagram of an example of a steering control device according to an embodiment. In the following description, a vehicle equipped with the steering control device according to the embodiment will be referred to as a "host vehicle." The steering control device according to the embodiment is a steer-by-wire type steering control device that can mechanically separate a steering wheel 1a from front wheels 2, which are steered wheels. The steering control device of the embodiment includes a steering angle sensor 3, a turning angle sensor 4, a vehicle speed sensor 5, an acceleration sensor 6, a turning control unit 8, a reaction force control unit 9, and a controller 11.

[0009] Steering angle sensor 3 detects steering angle δ of steering wheel 1a. Steering angle sensor 3 outputs information about the detected steering angle δ to controller 11. Turning angle sensor 4 detects steering angle θ of front wheels (steered wheels) 2. Turning angle sensor 4 outputs information about the detected steering angle θ to controller 11. Vehicle speed sensor 5 detects vehicle speed V of the host vehicle. Vehicle speed sensor 5 outputs information about the detected vehicle speed V to controller 11. Acceleration sensor 6 detects lateral acceleration Gy acting on the host vehicle. The acceleration sensor outputs information about the detected lateral acceleration Gy to controller 11.

[0010] The steering control unit 8 includes a steering motor 8A, a steering current detection unit 8B, and a steering motor drive unit 8C. Steering motor 8A is connected to pinion shaft 10d via a reducer. Steering motor 8A is driven by steering motor drive unit 8C and moves steering rack 10a left and right via pinion shaft 10d and pinion gear 10e. In this way, steering motor 8A steers front wheels 2. Steering motor 8A may be driven, for example, by controlling steering current Itm, which is the current flowing through steering motor 8A.

[0011] The steering current detection unit 8B detects the steering current Itm and outputs a signal indicating the steering current Itm to the steering motor drive unit 8C and the controller 11. Based on the target turning current Itt calculated by controller 11, steering motor drive section 8C controls the turning current Itm of steering motor 8A so that the turning current Itm detected by steering current detection section 8B matches the target turning current Itt. In this way, steering motor drive section 8C drives steering motor 8A. The target turning current Itt is a target value for the current flowing through steering motor 8A.

[0012] The reaction force control unit 9 includes a reaction force motor 9A, a reaction force current detection unit 9B, and a reaction force motor drive unit 9C. The reaction motor 9A is connected to the steering shaft 1b via a reducer. The reaction motor 9A is driven by a reaction motor drive unit 9C and applies a rotational torque to the steering wheel 1a via the steering shaft 1b. This causes the reaction motor 9A to generate a steering reaction force. The reaction motor 9A may be driven, for example, by controlling a reaction current Ism flowing through the reaction motor 9A. The reaction force current detection unit 9B detects the reaction force current Ism and outputs a detection signal indicating the reaction force current Ism to the reaction force motor drive unit 9C and the controller 11. The reaction force motor driving unit 9C controls the reaction force current Ism of the reaction force motor 9A based on the target reaction force current Ist calculated by the controller 11 so that the reaction force current Ism detected by the reaction force current detecting unit 9B coincides with the target reaction force current Ist. In this way, the reaction force motor driving unit 9C drives the reaction force motor 9A. The target reaction force current Ist is a target value for the current flowing through the reaction force motor 9A.

[0013] Backup clutch 12 is provided between steering shaft 1b and pinion shaft 10b. Pinion shaft 10b is connected to steering rack 10a via pinion gear 10c, and when backup clutch 12 is in an engaged state, steering shaft 1b and pinion shaft 10b are connected, thereby mechanically connecting steering wheel 1a and front wheels 2. When backup clutch 12 is in a disengaged state, steering shaft 1b and pinion shaft 10b are disconnected, thereby mechanically disconnecting steering wheel 1a and front wheels 2. In the following description, it is assumed that backup clutch 12 is in a disengaged state and steering wheel 1a and front wheels 2 are mechanically disconnected.

[0014] Controller 11 is an electronic control unit that controls the driving of steering motor 8A by steering control unit 8 and the driving of reaction force motor 9A by reaction force control unit 9. Controller 11 may include a processor 20 and peripheral components such as a memory device 21. Processor 20 may be, for example, a CPU (Central Processing Unit) or an MPU (Micro-Processing Unit). Memory device 21 may include any of a semiconductor memory device, a magnetic memory device, and an optical memory device. Memory device 21 may include memories such as a register, a cache memory, and a ROM (Read Only Memory) and RAM (Random Access Memory) used as a main memory device. The functions of controller 11 described below are realized, for example, by processor 20 executing a computer program stored in memory device 21.

[0015] 2 shows an example of the functional configuration of controller 11. Controller 11 includes target turning angle calculation section 11A, target steering reaction force calculation section 11B, target turning current calculation section 11C, subtractor 11D, and differentiator 11E. Target steering angle calculation unit 11A calculates target steering angle θt, which is a target value for steering angle θ, based on steering angle δ detected by steering angle sensor 3 and vehicle speed V detected by vehicle speed sensor 5. Target steering angle θt may be calculated, for example, by multiplying steering angle δ by a variable gear ratio for steering angle δ and steering angle θ. Subtractor 11D calculates deviation Δθ by subtracting steering angle θ detected by steering angle sensor 4 from target steering angle θt. Target steering current calculation unit 11C calculates target steering current Itt based on deviation Δθ. Target steering current calculation unit 11C outputs target steering current Itt to steering motor drive unit 8C. Differentiator 11E calculates target turning angular velocity ωt by differentiating target turning angle θt. Target turning angular velocity ωt is an example of the "turning velocity" recited in the claims.

[0016] Target steering reaction force calculation unit 11B calculates target reaction force current Ist based on steering angle δ detected by steering angle sensor 3, vehicle speed V detected by vehicle speed sensor 5, lateral acceleration Gy detected by acceleration sensor 6, steering current Itm detected by steering current detection unit 8B, and target turning angular velocity ωt. Target steering reaction force calculation unit 11B outputs the calculated target reaction force current Ist to reaction force motor drive unit 9C. See Fig. 3. Target steering reaction force calculation unit 11B includes a feedforward axial force calculation unit 30, a feedback axial force calculation unit 31, a mixture ratio setting unit 32, a subtractor 33, multipliers 34 and 35, an adder 36, a conversion unit 37, and a target reaction force current calculation unit 38. In the following description and drawings, the feedforward axial force may be referred to as "FF axial force" and the feedback axial force as "FB axial force."

[0017] The FF axial force calculation unit 30 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 or the target steering angle θt (i.e., the steering command value) 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 calculation unit 30 may calculate the FF axial force Fff based on the target steering angle θt calculated based on the steering angle δ and the vehicle speed V, and 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 corresponding to the target steering angle θt and a damping component corresponding to the steering angular velocity. The FF axial force calculation unit 30 outputs the calculated FF axial force Fff to the multiplier 34.

[0018] The FB axial force calculation unit 31 calculates the FB axial force Ffb based on the lateral acceleration Gy, the turning current Itm, the target turning angular velocity ωt, and the vehicle speed V. The FB axial force Ffb is a steering rack axial force that applies a force from the road surface to the steering wheel 1a as a steering reaction force and returns it to the driver. The configuration and function of the FB axial force calculation unit 31 will be described later. The mixing ratio setting unit 32 sets a mixing ratio Gf:(1-Gf) for calculating a mixed axial force by mixing the FF axial force Fff and the FB axial force Ffb. For example, the mixing ratio setting unit 32 may set the mixing ratio Gf:(1-Gf) according to the axial force difference between the FF axial force Fff and the FB axial force Ffb. A subtractor 33, multipliers 34 and 35, and an adder 36 mix the FF axial force Fff and the FB axial force Ffb at a mixing ratio Gf:(1-Gf) to calculate a mixed axial force given by the following equation (1). Mixed axial force=Fff×Gf+Ffb×(1-Gf) … (1)

[0019] The conversion unit 37 calculates a target steering reaction force based on the mixed axial force calculated by the subtractor 33, the multipliers 34 and 35, and the adder 36. The target steering reaction force is a target value of the steering reaction force. For example, the conversion unit 37 may convert the mixed axial force into the target steering reaction force using an axial force-steering reaction force conversion map that defines a target steering reaction force corresponding to the vehicle speed V and the axial force. The target reaction force current calculation unit 38 calculates the target reaction force current Ist according to the following equation (2) based on the target steering reaction force calculated by the conversion unit 37. The target reaction force current calculation unit 38 outputs the calculation result to the reaction force motor drive unit 9C. Target reaction force current Ist = target steering reaction force × gain … (2)

[0020] 4 is a block diagram of an example of the functional configuration of the FB axial force calculation unit 31. The FB axial force calculation unit 31 includes a first current axial force conversion unit 40, a first correction unit 41, a second current axial force conversion unit 42, a second correction unit 43, a lateral G axial force conversion unit 44, a first mixed axial force calculation unit 45, a second mixed axial force calculation unit 46, and an axial force switching unit 47. The first current axial force conversion unit 40 converts the turning current Itm into a positive efficiency current axial force Fcf0=Itm×Nf by multiplying the turning current Itm by the positive efficiency torque constant Nf. When the steering state of the steering wheel 1a is in a turning-to-add state, the FB axial force Ffb is estimated based on the positive efficiency current axial force Fcf0. The positive efficiency torque constant Nf and the positive efficiency current axial force Fcf0 are examples of a "first coefficient" and a "first current estimated axial force" respectively described in the claims.

[0021] The first correction unit 41 corrects the positive efficiency current axial force Fcf0 by removing a friction component caused by the steering of the steered wheels 2 from the positive efficiency current axial force Fcf0, and outputs the corrected positive efficiency current axial force Fcf. For example, if the sign of the positive efficiency current axial force Fcf0 before correction is positive, a predetermined friction component (constant) is subtracted from the positive efficiency current axial force Fcf0, and if the sign of the positive efficiency current axial force Fcf0 is negative, the friction component is added to the positive efficiency current axial force Fcf0 to calculate the corrected positive efficiency current axial force Fcf. The corrected positive efficiency current axial force Fcf is an example of a "second axial force component based on the first current estimated axial force" as defined in the claims. The positive efficiency torque constant Nf and the friction component are set so that the positive efficiency current axial force Fcf and the steering rack axial force actually applied to the steering rack 10a are approximately equal during further steering.

[0022] 5 is a characteristic diagram showing the relationship between the current axial force calculated by multiplying the steering current by a fixed torque constant and the actual steering rack axial force. In the further steering state, the self-aligning torque acts in the opposite direction to the rotation direction of steering motor 8A, and in the return steering state, the self-aligning torque acts in the same direction as the rotation direction of steering motor 8A. For this reason, the ratio (gradient) of the current axial force to the actual steering rack axial force is smaller in the return steering state than in the further steering state. Furthermore, the friction component always acts in the opposite direction to the rotation direction of steering motor 8A as steered wheels 2 are steered (i.e., while steering motor 8A is rotating). For this reason, when the absolute value of the steering angle decreases and passes the neutral position, a discrepancy occurs between the timing when the actual steering rack axial force becomes "0" and the timing when the current axial force becomes "0". As a result, the relationship between the current axial force and the actual steering rack axial force has a hysteresis characteristic as shown in Figure 5, and the larger the steering rack axial force, the greater the deviation between the current axial force in the further steering state and the current axial force in the return steering state. Also, in the characteristic diagram of Figure 5, the intersection with the vertical axis is shifted from "0".

[0023] 6 is a characteristic diagram showing the relationship between the positive efficiency current axial force Fcf and the actual steering rack axial force. By setting the positive efficiency torque constant Nf so that the slope of the positive efficiency current axial force Fcf0 relative to the actual steering rack axial force (i.e., the ratio (positive efficiency current axial force Fcf0 / actual steering rack axial force)) is 1 in the case of further steering, and removing the friction component with the first correction unit 41, it is possible to make the positive efficiency current axial force Fcf and the actual steering rack axial force roughly coincide with each other in the case of further steering.

[0024] See Fig. 4. Second current axial force conversion unit 42 converts the turning current Itm into an inverse efficiency current axial force Fcr0 = Itm × Nr by multiplying the turning current Itm by the inverse efficiency torque constant Nr. When the steering state of the steering wheel 1a is in the returning state, the FB axial force Ffb is estimated based on the inverse efficiency current axial force Fcr0. The inverse efficiency torque constant Nr and the inverse efficiency current axial force Fcr0 are examples of a "second coefficient" and a "second current estimated axial force" respectively described in the claims. As described above, in the return-to-steer steering state, the self-aligning torque acts in the same direction as the rotation direction of steering motor 8A, so the steering current Itm is smaller than in the further-steer steering state. Therefore, if the inverse efficiency torque constant Nr used to estimate the FB axial force Ffb in the return-to-steer steering state is set to the same magnitude as the normal efficiency torque constant Nf used to estimate the FB axial force Ffb in the further-steer steering state, the inverse efficiency current axial force Fcr0 will be too small. For this reason, the inverse efficiency torque constant Nr is set to a value larger than the normal efficiency torque constant Nf.

[0025] The second correction unit 43 corrects the inverse efficiency current axial force Fcr0 by removing a friction component associated with the steering of the steered wheels 2 from the inverse efficiency current axial force Fcr0, and outputs the corrected inverse efficiency current axial force Fcr. For example, if the sign of the inverse efficiency current axial force Fcr0 before correction is positive, a predetermined friction component (constant) is added to the inverse efficiency current axial force Fcr0, and if the sign of the inverse efficiency current axial force Fcr0 is negative, the friction component is subtracted from the inverse efficiency current axial force Fcr0 to calculate the corrected inverse efficiency current axial force Fcr. The corrected inverse efficiency current axial force Fcr is an example of a "second axial force component based on a second current estimated axial force" as defined in the claims. The inverse efficiency torque constant Nr and the friction component are set so that the inverse efficiency current axial force Fcr and the steering rack axial force actually applied to the steering rack 10a in the return steering state approximately match.

[0026] 7 is a characteristic diagram showing the relationship between the inverse efficiency current axial force Fcr and the actual steering rack axial force. By setting the inverse efficiency torque constant Nr so that the slope of the inverse efficiency current axial force Fcr0 relative to the actual steering rack axial force (i.e., the ratio (inverse efficiency current axial force Fcr0 / actual steering rack axial force)) is 1 in the return-to-steer state and removing the friction component with the second correction unit 43, it is possible to make the inverse efficiency current axial force Fcr and the actual steering rack axial force roughly coincide with each other in the return-to-steer state.

[0027] Referring to Fig. 4, the lateral G axial force converter 44 calculates the lateral G axial force Ft, which is the steering rack axial force corresponding to the lateral acceleration acting on the vehicle. For example, the lateral G axial force converter 44 may calculate the lateral G axial force Ft by multiplying the lateral acceleration Gy by a gain, which is a coefficient corresponding to the vehicle speed V. The first mixed axial force calculation unit 45 calculates a first mixed axial force Fm1 by mixing, in a predetermined ratio, the uncorrected positive current axial force Fcf0 calculated by the first current axial force conversion unit 40 and the lateral G axial force Ft calculated by the lateral G axial force conversion unit 44. The first mixed axial force Fm1 is an example of a "first axial force component" as defined in the claims.

[0028] The second mixed axial force calculation unit 46 calculates the second mixed axial force Fm2 by mixing the inverse efficiency current axial force Fcr and the first mixed axial force Fm1. See the range of "return-back steering state" in Figure 8. When target turning angular velocity ωt is equal to or greater than "0" and less than first turning angular velocity ω1, second mixed axial force calculation unit 46 outputs first mixed axial force Fm1 as second mixed axial force Fm2. When target turning angular velocity ωt is equal to or greater than second turning angular velocity ω2 that is greater than first turning angular velocity ω1, second mixed axial force calculation unit 46 outputs inverse efficiency current axial force Fcr as second mixed axial force Fm2. When target turning angular velocity ωt is equal to or greater than first turning angular velocity ω1 and less than second turning angular velocity ω2, second mixed axial force calculation unit 46 calculates second mixed axial force Fm2 by mixing first mixed axial force Fm1 and inverse efficiency current axial force Fcr at a mixing ratio according to target turning angular velocity ωt. The higher the target turning angular velocity ωt, second mixed axial force calculation unit 46 increases the mixing ratio of inverse efficiency current axial force Fcr.

[0029] See Fig. 4. Axial force switching unit 47 determines whether the steering state of steering wheel 1a is an increase steering state or a return steering state. For example, axial force switching unit 47 may determine that the steering state is an increase steering state when the sign of steering current Itm and the sign of target turning angular velocity ωt are the same, and may determine that the steering state is a return steering state when the sign of steering current Itm and the sign of target turning angular velocity ωt are different. It may also be determined that the steering state is an increase steering state when the sign of positive efficiency current axial force Fcf0 instead of steering current Itm and the sign of target turning angular velocity ωt are the same.

[0030] The axial force switching unit 47 switches the axial force to be output as the FB axial force Ftb between the positive efficiency current axial force Fcf and the second mixed axial force Fm2 based on the steering state and the target turning angular velocity ωt. For example, in the case of a return-to-steady steering state, axial force switching unit 47 outputs second mixed axial force Fm2 as FB axial force Ftb. Therefore, as shown in Fig. 8, when the steering state is a return-to-steady steering state and the target turning angular velocity ωt is equal to or greater than the second turning angular velocity ω2, the inverse efficiency current axial force Fcr is output as FB axial force Ftb. Also, when the steering state is a return-to-steady steering state and the target turning angular velocity ωt is equal to or greater than the first turning angular velocity ω1 and less than the second turning angular velocity ω2, an axial force obtained by mixing the first mixed axial force Fm1 and the inverse efficiency current axial force Fcr at a mixing ratio according to the target turning angular velocity ωt is output as FB axial force Ftb. Also, when the steering state is a return-to-steady steering state and the target turning angular velocity ωt is equal to or greater than 0 and less than the first turning angular velocity ω1, the first mixed axial force Fm1 is output as FB axial force Ftb.

[0031] On the other hand, when the steering state is an additional steering state and the target turning angular velocity ωt is equal to or greater than the third turning angular velocity ω3, axial force switching unit 47 outputs the positive efficiency current axial force Fcf as the FB axial force Ftb. When the steering state is an additional steering state and the target turning angular velocity ωt is equal to or greater than 0 and less than the third turning angular velocity ω3, an axial force obtained by mixing the second mixed axial force Fm2 and the positive efficiency current axial force Fcf at a mixing ratio according to the target turning angular velocity ωt is output as the FB axial force Ftb. The higher the target turning angular velocity ωt, the higher the mixing ratio of the positive efficiency current axial force Fcf becomes.

[0032] The value of third turning angular velocity ω3 may be set to the same value as first turning angular velocity ω1, for example. As described above, when target turning angular velocity ωt is equal to or greater than "0" and less than first turning angular velocity ω1, second mixed axial force-calculating section 46 outputs first mixed axial force Fm1 as second mixed axial force Fm2. Therefore, when the steering state is an additional steering state and the target turning angular velocity ωt is equal to or greater than "0" and less than the third turning angular velocity ω3, the axial force switching unit 47 outputs an axial force obtained by mixing the first mixed axial force Fm1 and the positive efficiency current axial force Fcf at a mixing ratio according to the target turning angular velocity ωt as the FB axial force Ftb.

[0033] The value of third turning angular velocity ω3 may be set to a value different from, for example, first turning angular velocity ω1. In this case, for example, a third mixed axial force calculation section (not shown) that calculates a third mixed axial force that is a mixture of first mixed axial force Fm1 and positive efficiency current axial force Fcf may be added to FB axial force calculation section 31. The third mixed axial force calculation unit outputs the positive efficiency current axial force Fcf as the third mixed axial force when the target turning angular velocity ωt is equal to or greater than the third turning angular velocity ω3. When the target turning angular velocity ωt is equal to or greater than "0" and less than the third turning angular velocity ω3, the third mixed axial force calculation unit outputs an axial force obtained by mixing the first mixed axial force Fm1 and the positive efficiency current axial force Fcf at a mixing ratio according to the target turning angular velocity ωt. The third mixed axial force calculation unit increases the mixing ratio of the positive efficiency current axial force Fcf the higher the target turning angular velocity ωt. The axial force switching unit 47 outputs the third mixed axial force as the FB axial force Ftb in the case of further steering, and outputs the second mixed axial force as the FB axial force Ftb in the case of return steering.

[0034] By generating the FB axial force Ffb as described above, the mixture ratio of the axial forces contained in the FB axial force Ffb becomes as shown in FIG. When the steering state of the steering wheel 1a is the further steering state and the target turning angular velocity ωt is equal to or greater than the third turning angular velocity ω3, the positive efficiency current axial force Fcf is output as the FB axial force Ffb. Furthermore, when the steering state of the steering wheel 1a is the return steering state and the target turning angular velocity ωt is equal to or greater than the second turning angular velocity ω2, the inverse efficiency current axial force Fcr is output as the FB axial force Ffb.

[0035] On the other hand, when the steering state is a turn-back steering state and the target turning angular velocity ωt is equal to or greater than 0 and less than the third turning angular velocity ω3, an axial force obtained by mixing the first mixed axial force Fm1 and the positive efficiency current axial force Fcf at a mixing ratio according to the target turning angular velocity ωt is output as the FB axial force Ftb. When the steering state is a return steering state and the target turning angular velocity ωt is equal to or greater than 0 and less than the first turning angular velocity ω1, the first mixed axial force Fm1 is output as the FB axial force Ffb. This reduces the estimation error of the FB axial force Ffb when the steering speed is low (for example, when the steering wheel is held steady). In other words, if the estimated steering rack axial force is always calculated based on the positive effective current axial force Fcf when the steering speed is low in order to prevent chattering of the current axial force when the steering speed is low, the calculation result may be too small. By incorporating the lateral G axial force Ft according to the lateral acceleration, the error can be reduced and the estimation accuracy can be improved.

[0036] Furthermore, when the steering state is a return steering state and the target turning angular velocity ωt is equal to or greater than the first turning angular velocity ω1 and less than the second turning angular velocity ω2, an axial force obtained by mixing the first mixed axial force Fm1 and the inverse efficiency current axial force Fcr at a mixing ratio according to the target turning angular velocity ωt is output as the FB axial force Ftb. This allows the FB axial force Ffb to be switched gradually until the target steering angular velocity ωt reaches the second steering angular velocity ω2, so that the estimated result of the FB axial force Ffb does not change suddenly when the steering state switches between the further steering state and the return steering state.

[0037] 9 is a characteristic diagram showing the relationship between the FB axial force Ffb and the actual steering rack axial force. It can be seen that the difference between the estimated FB axial force Ffb in the turn-to-turn steering state and the estimated FB axial force Ffb in the return steering state is reduced, and the friction component is removed, resulting in a good agreement between the FB axial force Ffb and the actual steering rack axial force. In particular, the deviation between the estimated result of the FB axial force Ffb in the turning-back steering state and the estimated result of the FB axial force Ffb in the turning-back steering state in the region where the steering rack axial force is large becomes smaller, and the bulge in the hysteresis characteristic can be reduced. Furthermore, by limiting the target steering angular velocity ωt, which includes the lateral G axial force Ft in the FB axial force Ffb, to a low steering angular velocity range, it is possible to suppress a decrease in the estimation accuracy of the FB axial force Ffb due to a phase lag of the lateral G axial force Ft with respect to the positive efficiency current axial force Fcf and the negative efficiency current axial force Fcr.

[0038] (operation) FIG. 10 is a flowchart of an example of a steering control method according to an embodiment. In step S1, target turning angle calculation section 11A calculates target turning angle θt, which is a target value of turning angle θ. In step S2, steering motor drive section 8C drives steering motor 8A based on the difference between target steering angle θt and the actual steering angle of steered wheels 2. In step S3, the first current axial force conversion unit 40 and the first correction unit 41 calculate the positive efficiency current axial force Fcf.

[0039] In step S4, the second current axial force conversion unit 42 and the second correction unit 43 calculate the inverse efficiency current axial force Fcr. In step S5, the axial force switching unit 47 determines whether the steering state of the steering wheel 1a is in an additional steering state or a returning state. If the steering state is in an additional steering state, the process proceeds to step S6. If the steering state is in a returning state, the process proceeds to step S7. In step S6, second mixed axial force calculation unit 46 and axial force switching unit 47 calculate the positive efficiency current axial force Fcf as the FB axial force Ftb. However, if the target turning angular velocity ωt is equal to or greater than "0" and less than the third turning angular velocity ω3, the axial force calculated by mixing the first mixed axial force Fm1 and the positive efficiency current axial force Fcf at a mixture ratio according to the target turning angular velocity ωt is calculated as the FB axial force Ftb. Thereafter, the processing proceeds to step S8.

[0040] In step S7, second mixed axial force calculating unit 46 and axial force switching unit 47 calculate the inverse efficiency current axial force Fcr as the FB axial force Ftb. However, if the target turning angular velocity ωt is equal to or greater than "0" and less than the first turning angular velocity ω1, the first mixed axial force Fm1 is calculated as the FB axial force Ffb. If the target turning angular velocity ωt is equal to or greater than the first turning angular velocity ω1 and less than the second turning angular velocity ω2, the axial force obtained by mixing the first mixed axial force Fm1 and the inverse efficiency current axial force Fcr at a mixing ratio according to the target turning angular velocity ωt is calculated as the FB axial force Ftb. Thereafter, the processing proceeds to step S8.

[0041] In step S8, the conversion unit 37 of the target steering reaction force calculation unit 11B (FIG. 3) calculates the target steering reaction force based on a mixed axial force obtained by mixing the FF axial force Fff and the FB axial force Ffb. The target reaction force current calculation unit 38 calculates the target reaction force current Ist based on the target steering reaction force. In step S9, the reaction force motor drive unit 9C drives the reaction force motor 9A based on the target reaction force current Ist. The process then ends.

[0042] (Variation) (1) In the above embodiment, when the steering state is an additional steering state and the target turning angular velocity ωt is equal to or greater than 0 and less than the third turning angular velocity ω3, the FB axial force calculation unit 31 outputs, as the FB axial force Ftb, an axial force obtained by mixing the first mixed axial force Fm1 and the positive efficiency current axial force Fcf at a mixing ratio according to the target turning angular velocity ωt. Alternatively, when the steering state is an additional steering state and the target turning angular velocity ωt is equal to or greater than 0 and less than the third turning angular velocity ω3, the first mixed axial force Fm1 may be output as the FB axial force Ftb.

[0043] (2) In the above embodiment, when the steering state is a return-to-steer steering state and the target turning angular velocity ωt is equal to or greater than the first turning angular velocity ω1 and less than the second turning angular velocity ω2, the FB axial force calculation unit 31 outputs, as the FB axial force Ftb, an axial force obtained by mixing the first mixed axial force Fm1 and the inverse efficiency current axial force Fcr at a mixing ratio according to the target turning angular velocity ωt. Alternatively, when the steering state is a return-to-steer steering state and the target turning angular velocity ωt is equal to or greater than the first turning angular velocity ω1 and less than the second turning angular velocity ω2, the inverse efficiency current axial force Fcr may be output as the FB axial force Ftb.

[0044] (Effects of the embodiment) (1) In a steering control method, a target steering angle, which is a target value for the steering angle of the steered wheels, is calculated based on the steering angle of a steering wheel that is mechanically separated from the steered wheels; a steering motor that turns the steered wheels is driven based on the difference between the target steering angle and the actual steering angle of the steered wheels; a first current estimated axial force, which is the steering rack axial force, is estimated by multiplying a steering current, which is a current that drives the steering motor, by a first coefficient; and a second current estimated axial force, which is the steering rack axial force, is estimated by multiplying the steering current by a second coefficient that is greater than the first coefficient; when the steering state of the steering wheel is a turn-back steering state, an estimated steering rack axial force is calculated based on the first current estimated axial force, and when the steering state is a return steering state, an estimated steering rack axial force is calculated based on the second current estimated axial force; a target reaction force current, which is a target value for a current that drives a reaction force motor that applies a steering reaction force to the steering wheel, is calculated based on the estimated steering rack axial force; and the reaction force motor is driven based on the estimated target reaction force current. This allows the current axial force to be estimated by multiplying the coefficient according to the characteristics of the steering state when the steering is in a turn-in steering state and when the steering is in a return steering state, thereby improving the estimation accuracy of the steering rack axial force in either steering state.

[0045] (2) When the steering state is a return state and the steering speed is less than a predetermined steering speed, the estimated steering rack axial force may be calculated based on the axial force obtained by adding the steering rack axial force corresponding to the lateral acceleration acting on the vehicle and the first current estimated axial force. For example, when the steering state is a turn-in steering state and the steering speed is less than a predetermined steering speed, the estimated steering rack axial force may be calculated based on an axial force that is a mixture of the steering rack axial force corresponding to the lateral acceleration acting on the vehicle, a first axial force component obtained by adding the first current estimated axial force, and a second axial force component based on the first current estimated axial force, and the mixture ratio of the second axial force component may be made higher when the steering speed is high than when the steering speed is low. If the estimated steering rack axial force is always calculated based on the first current estimated axial force when the steering speed is low in order to prevent chattering of the current axial force when the steering speed is low (for example, when the steering wheel is held steady), the calculation result may be too small. By incorporating the steering rack axial force according to the lateral acceleration, the error can be reduced and the estimation accuracy can be improved.

[0046] (3) When the steering state is a return steering state and the steering speed is equal to or greater than a predetermined first steering speed and less than a predetermined second steering speed, the estimated steering rack axial force may be calculated based on an axial force that is a mixture of a first axial force component obtained by adding a steering rack axial force corresponding to the lateral acceleration acting on the vehicle to the first current estimated axial force, and a second axial force component based on the second current estimated axial force, and the mixture ratio of the second axial force component may be made higher when the steering speed is high than when the steering speed is low. This allows the estimated steering rack axial force to be switched gradually until the steering speed reaches the second steering speed, so that the estimated steering rack axial force does not change suddenly when the steering state switches between the further steering state and the return steering state.

[0047] All examples and conditional terms described herein are intended for educational purposes to aid the reader in understanding the present invention and the concepts provided by the inventor for the advancement of technology, and should be construed without limitation to the specifically described examples and conditions above, and the configuration of examples herein for illustrating the advantages and disadvantages of the present invention. Although the embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions, and alterations can be made thereto without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0048] 1a...steering wheel, 1b...steering shaft, 2...front wheels, 3...steering angle sensor, 4...steering angle sensor, 5...vehicle speed sensor, 6...acceleration sensor, 8...steering control unit, 8A...steering motor, 8B...steering current detection unit, 8C...steering motor drive unit, 9...reaction force control unit, 9A...reaction force motor, 9B...reaction force current detection unit, 9C...reaction force motor drive unit, 10a...steering rack, 10b, 10d...pinion shaft, 10c, 10e...pinion gear, 11...controller, 12...backup clutch, 20...processor, 21...storage device

Claims

1. A target steering angle is calculated based on the steering angle of a steering wheel mechanically separated from the steered wheels, and driving a steering motor that steers the steered wheels based on a difference between the target steering angle and an actual steering angle of the steered wheels; a first current estimated axial force, which is a steering rack axial force, is estimated by multiplying a steering current, which is a current that drives the steering motor, by a first coefficient; multiplying the steering current by a second coefficient greater than the first coefficient to estimate a second current estimated axial force, which is a steering rack axial force; calculating an estimated steering rack axial force based on the first current estimated axial force when the steering state of the steering wheel is a turn-back steering state, and calculating the estimated steering rack axial force based on the second current estimated axial force when the steering state is a turn-back steering state; calculating a target reaction force current, which is a target value of a current for driving a reaction force motor that applies a steering reaction force to the steering wheel, based on the estimated steering rack axial force; driving the reaction motor based on the estimated target reaction current; A steering control method comprising:

2. 2. The steering control method according to claim 1, wherein, when the steering state is a return steering state and the steering speed is less than a predetermined steering speed, the estimated steering rack axial force is calculated based on an axial force obtained by adding the first current estimated axial force to a steering rack axial force corresponding to a lateral acceleration acting on the vehicle.

3. when the steering state is an additional steering state and the steering speed is less than a predetermined steering speed, the estimated steering rack axial force is calculated based on an axial force that is a mixture of a steering rack axial force corresponding to a lateral acceleration acting on the vehicle, a first axial force component obtained by adding the steering rack axial force corresponding to the lateral acceleration acting on the vehicle and the first current estimated axial force, and a second axial force component based on the first current estimated axial force; When the steering speed is high, the mixing ratio of the second axial force component is made higher than when the steering speed is low.

2. The steering control method according to claim 1.

4. when the steering state is a return steering state and the steering speed is equal to or greater than a predetermined first steering speed and less than a predetermined second steering speed, the estimated steering rack axial force is calculated based on an axial force that is a mixture of a first axial force component obtained by adding a steering rack axial force corresponding to a lateral acceleration acting on the vehicle to the first current estimated axial force, and a second axial force component based on the second current estimated axial force; When the steering speed is high, the mixing ratio of the second axial force component is made higher than when the steering speed is low.

2. The steering control method according to claim 1.

5. a steering angle sensor that detects the steering angle of a steering wheel that is mechanically separated from the steered wheels; a steering motor for steering the steered wheels; a reaction motor that applies a steering reaction force to the steering wheel; a controller that calculates a target steering angle, which is a target value for the steering angle of the steered wheels, based on the steering angle, drives the steering motor based on a difference between the target steering angle and the actual steering angle of the steered wheels, estimates a first current estimated axial force, which is the steering rack axial force, by multiplying a steering current, which is a current that drives the steering motor, by a first coefficient, estimates a second current estimated axial force, which is the steering rack axial force, by multiplying the steering current by a second coefficient that is greater than the first coefficient, calculates an estimated steering rack axial force based on the first current estimated axial force when the steering state of the steering wheel is a turn-back steering state, and calculates the estimated steering rack axial force based on the second current estimated axial force when the steering state is a turn-back steering state, calculates a target reaction force current, which is a target value for a current that drives a reaction force motor that imparts a steering reaction force to the steering wheel, based on the estimated estimated steering rack axial force, and drives the reaction force motor based on the estimated target reaction force current; A steering control device characterized by the above.

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