Steering control method and steering control device

Abstract

A steering control method (S5) in which, for a steering mechanism in which a steering wheel and a steered wheel are mechanically separate, a steering counterforce is provided to the steering wheel on the basis of a rack axial force including at least a feedback axial force, which is a steering rack axial force that is transmitted to the steering rack via the steered wheel from a road surface on which a vehicle is traveling, wherein low-pass filter processing is performed (S3) on the feedback axial force, a limited feedback axial force is determined by limiting the post-low-pass-filtered feedback axial force in accordance with the pre-low-pass-filtered feedback axial force, and a rack axial force including the limited feedback axial force is set (S4).

Description

Steering control method and steering control device

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

[0002] Japanese Patent Application Laid-Open No. 2006-124494 (JP-A-2006-124494) describes a technology that achieves a good steering feel by detecting the torque component transmitted from the road surface to the steering shaft with a torque detection means and feeding back a reaction force corresponding to the torque component to the steering wheel. It also describes removing high-frequency noise components of the torque detected by the torque detection means with a low-pass filter.

[0003] Japanese Patent Application Laid-Open No. 2003-137124

[0004] However, when high-frequency noise components of the reaction torque applied to the steering wheel are removed using a low-pass filter, there is a risk that the steering feel may be deteriorated due to a delay in the low-frequency components. An object of the present invention is to reduce the deterioration of the steering feel due to a delay in the steering reaction torque while suppressing the high-frequency components of the steering reaction torque applied to the steering wheel.

[0005] According to one aspect of the present invention, there is provided a steering control method for applying a steering reaction force to the steering wheel of a vehicle in a steering mechanism in which the steering wheel and steered wheels are mechanically separated, based on a rack axial force including at least a feedback axial force, which is a steering rack axial force transmitted from a road surface on which the vehicle is traveling to the steering rack via the steered wheels. In the steering control method, a low-pass filter is applied to the feedback axial force, and a limited feedback axial force is obtained by limiting the feedback axial force after the low-pass filter processing in accordance with the feedback axial force before the low-pass filter processing, and a rack axial force including the limited feedback axial force is set.

[0006] According to the present invention, it is possible to suppress high frequency components of the steering reaction force applied to the steering wheel while reducing the deterioration of steering feeling due to the delay of the steering reaction force. The objects and advantages of the present invention are realized and achieved by using the elements and combinations thereof set forth in the claims. It should be understood that both the foregoing general description and the following detailed description are merely exemplary and explanatory and are not intended to limit the invention as defined by the claims.

[0007] 1 is a schematic configuration diagram of an example of a steering control device of an embodiment. FIG. 2 is a block diagram of an example of the functional configuration of a controller. FIG. 3 is a block diagram of an example of the functional configuration of a target steering reaction force calculation unit of a first embodiment. FIG. 4 is a block diagram of an example of the functional configuration of a noise suppression unit of an embodiment. FIG. 5 is a block diagram of an example of the functional configuration of a restriction amount calculation unit of an embodiment. (a) and (b) are diagrams showing characteristics of a first restriction amount and a second restriction amount with respect to axial force, respectively. FIG. 6 is a block diagram of an example of the functional configuration of a filter processing unit of an embodiment. FIG. 7 is an example of a time chart of an FB axial force before low-pass filter processing, an FB axial force after low-pass filter processing, and an FB axial force after limitation. FIG. 8 is a characteristic diagram of an example of an axial force-steering reaction force conversion map. FIG. 9 is a flowchart of an example of a steering control method of an embodiment. FIG. 10 is a block diagram of an example of the functional configuration of a target steering reaction force calculation unit of a second embodiment. FIG. 11 is a block diagram of an example of the functional configuration of a target steering reaction force calculation unit of a third embodiment.

[0008] (First embodiment) (Configuration) Fig. 1 is a schematic configuration diagram of an example of a steering control device of an embodiment. In the following description, a vehicle equipped with a steering control device of an embodiment will be referred to as "host vehicle." The steering control device of the embodiment is a steer-by-wire type steering control device that can mechanically separate a steering wheel 1a from front wheels 2 that 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] Steering control unit 8 includes steering motor 8A, steering current detection unit 8B, and 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] Steering current detection unit 8B detects steering current Itm. Steering current detection unit 8B outputs a signal indicating the steering current Itm to steering motor drive unit 8C and controller 11. Steering motor drive unit 8C controls the steering current Itm of steering motor 8A based on the target steering current Itt calculated by controller 11 so that the steering current Itm detected by steering current detection unit 8B matches the target steering current Itt. In this way, steering motor drive unit 8C drives steering motor 8A. The target steering current Itt is a target value of 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 force motor 9A is connected to the steering shaft 1b via a reducer. The reaction force motor 9A is driven by the reaction force motor drive unit 9C and applies a rotational torque to the steering wheel 1a via the steering shaft 1b. As a result, the reaction force motor 9A generates a steering reaction force. The reaction force motor 9A may be driven, for example, by controlling a reaction force current Ism flowing through the reaction force motor 9A. The reaction force current detection unit 9B detects the reaction force current Ism. The reaction force current detection unit 9B then outputs a detection signal indicating the reaction force current Ism to the reaction force motor drive unit 9C and the controller 11. The reaction force motor drive unit 9C controls the reaction force current Ism of the reaction force motor 9A based on the target reaction force current Ist calculated by the controller 11 so that the reaction force current Ism detected by the reaction force current detection unit 9B coincides with the target reaction force current Ist. As a result, the reaction force motor drive unit 9C drives the reaction force motor 9A. The target reaction force current Ist is a target value of the current flowing through the reaction force motor 9A.

[0013] The backup clutch 12 is provided between the steering shaft 1b and the pinion shaft 10b. The pinion shaft 10b is connected to the steering rack 10a via a pinion gear 10c, and when the backup clutch 12 is engaged, the steering shaft 1b and the pinion shaft 10b are connected, thereby mechanically connecting the steering wheel 1a and the front wheels 2. When the backup clutch 12 is disengaged, the steering shaft 1b and the pinion shaft 10b are disconnected, thereby mechanically disconnecting the steering wheel 1a and the front wheels 2. In the following description, it is assumed that the backup clutch 12 is in a disengaged state and the steering wheel 1a and the 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 storage device 21. Processor 20 may be, for example, a CPU (Central Processing Unit) or an MPU (Micro-Processing Unit). Storage device 21 may include any of a semiconductor storage device, a magnetic storage device, and an optical storage device. Storage device 21 may include memories such as a register, a cache memory, a ROM (Read Only Memory), and a RAM (Random Access Memory). The functions of controller 11 described below are realized, for example, by processor 20 executing a computer program stored in storage device 21.

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

[0016] Target steering reaction force calculation unit 11B calculates a target reaction force current Ist based on the steering angle δ detected by steering angle sensor 3, the vehicle speed V detected by vehicle speed sensor 5, the lateral acceleration Gy detected by acceleration sensor 6, the steering current Itm detected by steering current detection unit 8B, and the target steering 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 noise suppression unit 32, a mixing unit 33, a conversion unit 34, and a target reaction force current calculation unit 35. In the following description and drawings, the feedforward axial force may be referred to as the "FF axial force," and the feedback axial force may be referred to as the "FB axial force."

[0017] The FF axial force calculation unit 30 calculates an FF axial force Fff, which is a steering rack axial force that applies 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 a 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.

[0018] The FB axial force calculation unit 31 calculates an 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 FB axial force calculation unit 31 calculates the steering rack axial force that reflects the influence of the tire lateral force acting on the steered wheels 2 based on the turning current Itm. The steering rack axial force calculated based on the turning current is referred to as the "current axial force." The FB axial force calculation unit 31 also calculates the steering rack axial force that reflects the influence of the tire lateral force acting on the steered wheels 2 based on the lateral acceleration Gy. The steering rack axial force calculated based on the lateral acceleration Gy is referred to as the "lateral G axial force." The FB axial force calculation unit 31 calculates the steering rack axial force that reflects the influence of the tire lateral force acting on the steered wheels 2 based on the actual yaw rate γa. The steering rack axial force calculated based on the actual yaw rate γa is referred to as the “yaw rate axial force.” The FB axial force calculation unit 31 calculates the FB axial force Ffb by combining the current axial force, the lateral G axial force, and the yaw rate axial force.

[0019] The noise suppression unit 32 performs low-pass filtering on the FB axial force Ffb. This allows the noise suppression unit 32 to suppress high-frequency noise contained in the FB axial force Ffb. The noise suppression unit 32 calculates the limited FB axial force Ffb1 by limiting the value of the FB axial force Ffb after the low-pass filtering process in accordance with the value of the FB axial force Ffb before the low-pass filtering process.

[0020] 4 is a block diagram of an example of the functional configuration of noise suppression unit 32. Noise suppression unit 32 includes a restriction amount calculation unit 40 that calculates a restriction amount R that limits low-pass filtering of FB axial force Ffb, and a filter processing unit 50 that performs low-pass filtering on FB axial force Ffb in accordance with a restriction based on the restriction amount R. Restriction amount calculation unit 40 calculates the restriction amount R based on the FB axial force Ffb before low-pass filtering by filter processing unit 50 and the target turning angular velocity ωt. Restriction amount R is an example of a "predetermined value" in the claims.

[0021] 5 is a block diagram of an example of the functional configuration of restriction amount calculation unit 40 according to the embodiment. Restriction amount calculation unit 40 includes a steering state determination unit 41, a first restriction amount calculation unit 42, a second restriction amount calculation unit 43, and a restriction amount switching unit 44. Steering state determination unit 41 determines whether the steering state of steering wheel 1a (i.e., the steering state of the steering mechanism) is further steering or return steering, based on the sign (positive or negative) of FB axial force Ffb and the sign (positive or negative) of target turning angular velocity ωt. Target turning angular velocity ωt is an example of the "steering speed" described in the claims.

[0022] It should be noted that "additional steering" refers to an operation of rotating the steering wheel 1a in a direction that increases the steering angle δ, and "return-back steering" refers to an operation of rotating the steering wheel 1a in a direction that decreases the steering angle δ. For example, steering state determination unit 41 determines that the steering state is additional steering when the sign of FB axial force Ffb and the sign of target turning angular velocity ωt are the same, and determines that the steering state is return-back steering when the sign of FB axial force Ffb and the sign of target turning angular velocity ωt are different.

[0023] In this embodiment, the sign of the axial force that steers the steered wheels 2 to the left (i.e., the axial force that increases the steering angle to the left) is defined as positive, and the sign of the axial force that steers the steered wheels 2 to the right (i.e., the axial force that increases the steering angle to the right) is defined as negative. Also, the sign of the target turning angular velocity ωt that increases the steering angle to the right is defined as positive, and the sign of the target turning angular velocity ωt that increases the steering angle to the left is defined as negative. However, the present invention is not limited to this, and the definitions of the signs of the axial force and target turning angular velocity ωt may be opposite to those in this embodiment. Steering state determination unit 41 outputs the steering state determination result to restriction amount switching unit 44.

[0024] The first restriction amount calculation unit 42 calculates a first restriction amount R1 to be used as the restriction amount R in the case of further steering, and the second restriction amount calculation unit 43 calculates a second restriction amount R2 to be used as the restriction amount R in the case of return steering. The first restriction amount calculation unit 42 and the second restriction amount calculation unit 43 calculate the first restriction amount R1 and the second restriction amount R2, respectively, based on the FB axial force Ffb before low-pass filtering by the filtering unit 50. Figures 6(a) and 6(b) are diagrams showing the characteristics of the first restriction amount R1 and the second restriction amount R2 with respect to the FB axial force Ffb.

[0025] When the FB axial force Ffb is large, the first regulating amount R1 and the second regulating amount R2 are smaller than when the FB axial force Ffb is small. For example, the larger the FB axial force Ffb, the smaller the first regulating amount R1 and the second regulating amount R2. For example, when the FB axial force Ffb is 0, the first regulating amount R1 and the second regulating amount R2 may have values ​​Ra and Rb (>0) greater than 0. The values ​​Ra and Rb may be equal to or different from each other. As the FB axial force Ffb increases, the first regulating amount R1 and the second regulating amount R2 may decrease, and when the FB axial force Ffb reaches F1 and F2, the first regulating amount R1 and the second regulating amount R2 may reach 0, respectively. Furthermore, the absolute value of the ratio of the decrease in the first restriction amount R1 to the increase in the FB axial force Ffb (|Decrease in the first restriction amount R1 / Increase in the FB axial force Ffb|) is greater than the absolute value of the ratio of the decrease in the second restriction amount R2 to the increase in the FB axial force Ffb (|Decrease in the second restriction amount R2 / FB axial force Ffb|). For example, the axial force F1 at which the first restriction amount R1 reaches 0 may be smaller than the axial force F2 at which the second restriction amount R2 reaches 0. When the steering state determination unit 41 determines that the steering state is turn-in steering, the restriction amount switching unit 44 selects and outputs the first restriction amount R1 as the restriction amount R, and when the steering state determination unit 41 determines that the steering state is return steering, the restriction amount switching unit 44 selects and outputs the second restriction amount R2 as the restriction amount R.

[0026] 7 is a block diagram of an example of the functional configuration of the filter processing unit 50 of the embodiment. The filter processing unit 50 includes a low-pass filter (LPF) 51 and a limiting unit 52. The low-pass filter 51 includes coefficient multipliers 53, 54, and 55, delay units 56 and 57, and a subtractor 58, and performs low-pass filtering on the FB axial force Ffb and outputs a low-pass filtered FB axial force FfbL. The limiting unit 52 limits the low-pass filtered FB axial force FfbL in accordance with the regulating amount R and the FB axial force Ffb before low-pass filtering, and outputs the axial force obtained as a limited FB axial force Ffb1.

[0027] For example, the limiting unit 52 limits the FB axial force FfbL after low-pass filtering so that the difference between the FB axial force Ffb before low-pass filtering and the FB axial force FfbL after low-pass filtering is within a restriction amount R corresponding to the FB axial force Ffb before low-pass filtering. For example, if the difference (FfbL-Ffb) obtained by subtracting the FB axial force Ffb before low-pass filtering from the FB axial force FfbL after low-pass filtering is greater than the restriction amount R, the limiting unit 52 sets the sum of the FB axial force Ffb before low-pass filtering and the restriction amount R as the restricted FB axial force Ffb1=Ffb+R.

[0028] Furthermore, for example, when the difference (FfbL-Ffb) is smaller than (-1) x the restricted amount R, the difference obtained by subtracting the restricted amount R from the FB axial force Ffb before low-pass filtering is set as the restricted FB axial force Ffb1=Ffb-R. In other cases (i.e., when the absolute value of the difference (FfbL-Ffb) is equal to or smaller than the restricted amount R), the FB axial force FfbL after low-pass filtering is set as the restricted FB axial force Ffb1=FfbL.

[0029] 8 is an example of a time chart of the FB axial force Ffb before low-pass filtering, the FB axial force FfbL after low-pass filtering, and the FB axial force Ffb1 after limitation. The dashed line indicates the FB axial force Ffb before low-pass filtering, the dashed-dotted line indicates the FB axial force FfbL after low-pass filtering, and the solid line indicates the FB axial force Ffb1 after limitation. Although high-frequency noise components have been removed from the FB axial force FfbL after low-pass filtering (dashed line), the FB axial force FfbL after low-pass filtering (dashed line) lags behind the FB axial force Ffb before low-pass filtering (dashed line).

[0030] In contrast, when focusing on the FB axial force Ffb1 (solid line) after limitation, in a region Ra where the FB axial force is relatively low, the high-frequency noise components of the FB axial force Ffb1 (solid line) after limitation are reduced, and the delay is reduced by restricting the difference from the FB axial force Ffb (dashed line) before low-pass filtering to be within the restriction amount R. Furthermore, in a region Rb where the FB axial force is relatively high, a smaller restriction amount R is set, thereby further reducing the difference (delay) from the FB axial force Ffb (dashed line) before low-pass filtering. For example, by setting the restriction amount R to "0", it is possible to make the FB axial force Ffb1 match the FB axial force Ffb (dashed line) before low-pass filtering (no delay).

[0031] The mixing unit 33 mixes the FF axial force Fff and the limited FB axial force Ffb1 at a mixing ratio Gf:(1-Gf) to calculate a mixed axial force Fmx=Fff×Gf+Ffb1×(1-Gf). The mixing unit 33 may set the mixing ratio Gf:(1-Gf) depending on the axial force difference between the FF axial force Fff and the FB axial force Ffb, the steering speed, the deviation of the actual yaw rate from the target yaw rate, and the deviation of the actual vehicle body slip angular velocity from the target vehicle body slip angular velocity.

[0032] The conversion unit 34 calculates the target steering reaction force based on the mixed axial force Fmx. The target steering reaction force is a target value of the steering reaction force. For example, the conversion unit 34 may convert the mixed axial force into the target steering reaction force using an axial force-to-steering reaction force conversion map that defines the target steering reaction force corresponding to the vehicle speed V and the axial force. FIG. 9 is a characteristic diagram of an example of the axial force-to-steering reaction force conversion map. As shown in FIG. 9, in the neutral axial force region where the magnitude of the axial force is relatively small, the characteristics of the steering reaction force are set so that the ratio of the increase in the steering reaction force to the increase in the axial force ((increase in steering reaction force) / (increase in axial force)) is larger than in the region where the magnitude of the axial force is relatively large. This is because the driver is required to feel a sufficient response at the beginning of steering in the neutral axial force region, and therefore the sensitivity of the steering reaction force to the increase in the axial force is set high.

[0033] 3 , the target reaction force current calculation unit 35 calculates the target reaction force current Ist in accordance with the following equation (1) based on the target steering reaction force calculated by the conversion unit 34: Target reaction force current Ist = Target steering reaction force × Gain (1) The target reaction force current calculation unit 35 outputs the calculated target reaction force current Ist to the reaction force motor drive unit 9C. In this way, the noise suppression unit 32 limits the value of the FB axial force Ffb after low-pass filtering in accordance with the value of the FB axial force Ffb before low-pass filtering. This makes it possible to increase the amount of regulation of the low-pass filtering on the FB axial force Ffb in, for example, the neutral axial force region where the value of the FB axial force Ffb is small, while decreasing the amount of regulation of the low-pass filtering in the region where the value of the FB axial force Ffb is large.

[0034] In the neutral axial force region where the value of the FB axial force Ffb is small, the sign of the torque generated by the reaction motor 9A reverses between positive and negative, and therefore the reaction vibration, in which the torque sign repeatedly switches between positive and negative, is easily transmitted to the steering wheel as acceleration. Furthermore, as shown in the axial force-steering reaction force conversion map of FIG. 9 , in the neutral axial force region, the ratio of the increase in steering reaction force to the increase in axial force ((increase in steering reaction force) / (increase in axial force)) is larger, and therefore noise contained in the axial force is easily reflected in the steering reaction force. Relaxing the limitations of the low-pass filter processing in the neutral axial force region can sufficiently remove noise that causes reaction vibration, improving the steering feel. On the other hand, when the value of the FB axial force Ffb is large, such as during steering, strongly restricting the effect of the low-pass filter processing can suppress delay in low-frequency components and maintain the response felt by the driver.

[0035] (Operation) Fig. 10 is a flowchart of an example of a steering control method of an embodiment. In step S1, the FF axial force calculation unit 30 calculates the FF axial force Fff. In step S2, the FB axial force calculation unit 31 calculates the FB axial force Ffb. In step S3, the noise suppression unit 32 performs low-pass filter processing (LPF processing) on ​​any one of the FF axial force Fff, the FB axial force Ffb, or the mixed axial force Fmx. In the example of the first embodiment, the low-pass filter processing is performed on the FB axial force Ffb. In the flowchart of Fig. 10, the axial force that is subjected to low-pass filter processing by the noise suppression unit 32, out of the FF axial force Fff, the FB axial force Ffb, or the mixed axial force Fmx, is denoted as the "target axial force".

[0036] In step S4, the noise suppression unit 32 limits the target axial force after low-pass filtering according to the target axial force before low-pass filtering, and calculates the limited target axial force. In the first embodiment, the FB axial force Ffb after low-pass filtering is limited according to the FB axial force Ffb before low-pass filtering, and calculates the limited FB axial force Ffb1. The target steering reaction force calculation unit 11B sets an estimated axial force including the limited target axial force. In the first embodiment, the mixer 33 mixes the FF axial force Fff and the limited FB axial force Ffb1 to calculate a mixed axial force Fmx as an estimated axial force. The estimated axial force is an example of the "rack axial force" described in the claims. In step S5, the conversion unit 34, the target reaction force current calculation unit 35, and the reaction force motor drive unit 9C apply a steering reaction force based on the estimated axial force. In the first embodiment, the steering reaction force is applied based on the mixed axial force Fmx.

[0037] Second Embodiment FIG. 11 is a block diagram of an example of the functional configuration of a target steering reaction force calculation unit 11B according to a second embodiment. While the noise suppression unit 32 in the target steering reaction force calculation unit 11B of the first embodiment performs low-pass filtering only on the FB axial force Ffb before mixing by the mixer 33, the noise suppression unit 32 of the second embodiment performs low-pass filtering on the FF axial force Fff before mixing by the mixer 33. This allows the noise suppression unit 32 to suppress high-frequency noise contained in the FF axial force Fff. Because the FF axial force Fff is calculated based on the steering angle δ, the detection noise of the steering angle sensor 3 can be suppressed. The noise suppression unit 32 calculates a limited FF axial force Fff1 by limiting the value of the FF axial force Fff after low-pass filtering according to the value of the FF axial force Fff before low-pass filtering. The configuration of the noise suppression unit 32 is similar to that of the noise suppression unit 32 of the first embodiment, for example.

[0038] The mixer 33 mixes the limited FF axial force Fff1 and FB axial force Ffb at a mixing ratio Gf:(1-Gf) to calculate a mixed axial force Fmx = Fff1 x Gf + Ffb x (1-Gf). The mixed axial force Fmx in the second embodiment is an example of the "rack axial force" described in the claims. Note that, instead of the noise suppressor 32 in the target steering reaction force calculator 11B in the first embodiment performing low-pass filtering on only the FB axial force Ffb before mixing by the mixer 33, the noise suppressor 32 in the second embodiment may perform low-pass filtering on each of the FB axial force Ffb and the FF axial force Fff before mixing by the mixer 33, and then mix them in the mixer 33.

[0039] (Third Embodiment) Figure 12 is a block diagram of an example of the functional configuration of a target steering reaction force calculation unit 11B of the third embodiment. A mixer 33 in the target steering reaction force calculation unit 11B of the third embodiment mixes the FF axial force Fff and the FB axial force Ffb at a mixing ratio Gf:(1-Gf) to calculate a mixed axial force Fmx = Fff x Gf + Ffb x (1-Gf). While the noise suppression unit 32 of the first embodiment performs low-pass filtering on only the FB axial force Ffb before mixing, the noise suppression unit 32 of the third embodiment performs low-pass filtering on the mixed axial force Fmx after mixing the FF axial force Fff and the FB axial force Ffb. This allows the noise suppression unit 32 to suppress high-frequency noise contained in the FB axial force Ffb and also suppress high-frequency noise contained in the FF axial force Fff.

[0040] The noise suppression unit 32 calculates a limited mixed axial force mx1 by limiting the value of the mixed axial force mx after low-pass filtering in accordance with the value of the mixed axial force mx before low-pass filtering. The configuration of the noise suppression unit 32 is similar to the configuration of the noise suppression unit 32 in the first embodiment, for example. The conversion unit 34 calculates a target steering reaction force based on the limited mixed axial force Fmx1. The limited mixed axial force Fmx1 in the third embodiment is an example of the "rack axial force" described in the claims.

[0041] Effects of the Embodiments (1) The steering control device controls a steering mechanism in which the steering wheel and steered wheels of a vehicle are mechanically separated. The steering control device applies a steering reaction force to the steering wheel based on a rack axial force that includes at least a feedback axial force, which is a steering rack axial force transmitted from the road surface on which the vehicle is traveling to the steering rack via the steered wheels. The controller 11 performs low-pass filtering on the feedback axial force and a mixed axial force that combines the feedforward axial force and the feedback axial force, and limits the feedback axial force after low-pass filtering in accordance with the feedback before low-pass filtering to obtain a limited feedback axial force, and sets the rack axial force including the limited feedback axial force. This suppresses high-frequency components of the steering reaction force applied to the steering wheel while reducing deterioration in steering feel due to a delay in the steering reaction force.

[0042] (2) The controller 11 may limit the feedback axial force after low-pass filtering so that the difference between the feedback axial force before low-pass filtering and the feedback axial force after low-pass filtering is within a predetermined value according to the feedback axial force before low-pass filtering. This makes it possible to convey necessary information about the vehicle behavior and road surface to the driver according to the feedback axial force.

[0043] (3) The controller 11 may decrease the predetermined value as the feedback axial force before low-pass filtering increases. This suppresses high-frequency vibration components when the feedback axial force is relatively small, and provides the driver with appropriate information regarding vehicle behavior and road surface when the feedback axial force is relatively large. (4) The controller 11 may increase the ratio of the change in the predetermined value to the change in the feedback axial force before low-pass filtering in the case of further steering of the steering mechanism compared to the case of return steering of the steering mechanism. This allows the driver to receive appropriate information regarding vehicle behavior and road surface, for example, in further steering (e.g., steering for emergency avoidance, etc.).

[0044] (5) The controller 11 may determine whether the steering mechanism is performing further steering or return steering based on the steering speed of the steered wheels. This allows for appropriate determination of further steering or return steering. (6) The controller 11 may reduce the predetermined value as the mixed axial force before low-pass filtering increases. This allows appropriate information regarding the vehicle behavior and the road surface to be accurately transmitted at an appropriate time based on the mixed axial force. Furthermore, the controller 11 may reduce the predetermined value as the feedback axial force before low-pass filtering increases. This allows appropriate information regarding the vehicle behavior and the road surface to be accurately transmitted at an appropriate time based on the feedback axial force. (7) The controller 11 may apply a steering reaction force to the steering wheel based on a rack axial force including a feedforward axial force, which is the steering rack axial force generated in response to driving by the steering motor, and a feedback axial force, which is the steering rack axial force transmitted from the road surface on which the vehicle is traveling to the steering rack via the steered wheels, perform low-pass filtering on the rack axial force, and limit the rack axial force after low-pass filtering in accordance with the rack axial force before low-pass filtering to obtain a limited rack axial force. This makes it possible to suppress the high frequency components of the steering reaction force applied to the steering wheel, while reducing the deterioration of steering feeling due to the delay in the steering reaction force.

[0045] 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.

[0046] 1a...Steering wheel, 1b...Steering shaft, 2...Front wheels (steered wheels), 3...Steering angle sensor, 4...Turning angle sensor, 5...Vehicle speed sensor, 6...Acceleration sensor, 8...Turning control unit, 8A...Turning motor, 8B...Turning current detection unit, 8C...Turning motor drive unit, 9...Reaction force control unit, 9A...Reaction force motor, 9B...Reaction force current detection unit, 9C...Reaction force motor drive unit, 10a...Steering rack, 10b, 10d...Pinion shaft, 10c, 10e...Pinion gear, 11...Controller, 11A...Target turning angle calculation unit, 11B...Target steering reaction force calculation unit, 11C...Target turning Steering current calculation unit, 11D, 58... subtractor, 11E... differentiator, 12... backup clutch, 20... processor, 21... storage device, 30... feedforward axial force calculation unit, 31... feedback axial force calculation unit, 32... noise suppression unit, 33... mixing unit, 34... conversion unit, 35... target reaction force current calculation unit, 40... restriction amount calculation unit, 41... steering state determination unit, 42... first restriction amount calculation unit, 43... second restriction amount calculation unit, 44... restriction amount switching unit, 50... filter processing unit, 51... low pass filter (LPF), 52... limiting unit, 53, 54, 55... coefficient multiplier, 56, 57... delay device

Claims

1. A steering control method for a vehicle in a steering mechanism in which the steering wheel and steered wheels are mechanically separated, which applies a steering reaction force to the steering wheel based on a rack axial force that includes at least a feedback axial force, which is a steering rack axial force transmitted from the road surface on which the vehicle is traveling to the steering rack via the steered wheels, the steering control method comprising: performing low-pass filtering on the feedback axial force; determining the limited feedback axial force by limiting the feedback axial force after low-pass filtering in accordance with the feedback axial force before low-pass filtering; and setting the rack axial force that includes the limited feedback axial force.

2. A steering control method according to claim 1, characterized in that the feedback axial force after the low-pass filter processing is limited so that the difference between the feedback axial force before the low-pass filter processing and the feedback axial force after the low-pass filter processing is within a predetermined value according to the feedback axial force before the low-pass filter processing.

3. A steering control method according to claim 2, wherein the predetermined value is made smaller as the feedback axial force before the low-pass filter processing increases.

4. A steering control method as described in claim 2 or 3, characterized in that the ratio of the change in the predetermined value to the change in the feedback axial force before the low-pass filter processing is larger in the case of further steering of the steering mechanism than in the case of return steering of the steering mechanism.

5. A steering control method according to claim 4, wherein a determination is made as to whether the steering mechanism is turning further or turning back based on the steering speed of the steered wheels.

6. A steering control method according to any one of claims 2 to 5, characterized in that the greater the mixed axial force before the low-pass filter processing, the smaller the predetermined value.

7. A steering control method according to any one of claims 2 to 5, characterized in that the larger the feedback axial force before the low-pass filter processing is, the smaller the predetermined value is.

8. A steering control method according to claim 1, characterized in that a steering reaction force is applied to the steering wheel based on a rack axial force including a feedforward axial force, which is a steering rack axial force generated in response to driving by the steering motor, and a feedback axial force, which is a steering rack axial force transmitted to the steering rack from the road surface on which the vehicle is traveling via the steered wheels, and the rack axial force after low-pass filtering is subjected to low-pass filtering, and the rack axial force after low-pass filtering is limited in accordance with the rack axial force before low-pass filtering, thereby determining the rack axial force after limiting.

9. A steering control device in a steering mechanism in which the steering wheel and steered wheels of a vehicle are mechanically separated, which applies a steering reaction force to the steering wheel based on a rack axial force that includes at least a feedback axial force, which is a steering rack axial force transmitted to the steering rack from the road surface on which the vehicle is traveling via the steered wheels, characterized in that it comprises a controller that executes the following processes: a process of performing low-pass filtering on the feedback axial force; and a process of determining the limited feedback axial force by limiting the feedback axial force after low-pass filtering in accordance with the feedback axial force before low-pass filtering, and setting the rack axial force that includes the limited feedback axial force.

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