Control device and control method for steering system

Abstract

This control device for a steer-by-wire type steering system executes: first control in which the turning angle is controlled on the basis of the steering angle of a steering wheel operated by a driver, and a reaction torque for the operation of the steering wheel is applied to the steering wheel in accordance with a self-aligning torque that a wheel receives from the road surface; second control in which the steering angle and the turning angle are controlled on the basis of a target steering angle according to predetermined driving assist control, and a steering assist torque according to an angle difference between the target steering angle and the steering angle or the turning angle is applied to the steering wheel; and switching control in which the second control is cancelled and switched to the first control as the angle difference increases during the execution of the second control. When the angle difference increases in a state in which the steering angle or the turning angle is smaller than the target steering angle, the control device switches the second control to the first control in a state in which the absolute value of the steering assist torque is less than or equal to a predetermined reference value which is set in advance to reduce torque fluctuations.

Description

Steering system control device and control method

[0001] The present disclosure relates to a control device and a control method for a steering system.

[0002] In recent years, in driving assistance controls such as lane keeping control and automatic driving controls that allow a vehicle to travel autonomously, automatic steering controls have been put into practical use, which recognize the road shape and obstacles around the vehicle, set a target driving trajectory, and automatically steer the wheels so that the vehicle travels along the target driving trajectory.In such driving assistance controls and automatic driving controls, there is known a technique configured to accept manual steering operation by the driver even during automatic steering control, so that the driver can correct the driving trajectory, for example, to avoid an unexpected obstacle.

[0003] For example, Patent Document 1 discloses a steering system in which, while lane keeping assistance is being performed to keep the vehicle in the center of the driving lane, the driver's intentional steering operation causes the driver's target steering angle to deviate from the driving assistance target steering angle, and until the magnitude of the steering torque accordingly exceeds an intervention threshold, the driving assistance target steering angle is calculated as the final target steering angle, and once the magnitude of the steering torque exceeds a switching threshold, the driver's target steering angle is calculated as the final target steering angle as is.

[0004] Furthermore, Patent Document 2 discloses a technology in a steer-by-wire steering system that generates a steering reaction torque that urges the steering wheel toward a neutral position in response to a self-aligning torque acting on the steered wheels when the vehicle is turning.

[0005] JP 2022-111970 A JP 2016-107750 A

[0006] In the steering system described in Patent Document 1, a steering assist torque that causes the wheel steering angle to follow the driving assistance target steering angle is applied to the steering wheel until the final target steering angle is switched from the driving assistance target steering angle to the driver's target steering angle. On the other hand, after the final target steering angle is switched to the driver's target steering angle, it is desirable to apply a reaction torque corresponding to the self-aligning torque to the steering wheel, as described in Patent Document 2.

[0007] However, the assist torque curve, which indicates the steering assist torque applied to make the steering angle (steering angle) follow the target steering angle (target steering angle), is different from the reaction torque curve, which indicates the reaction torque corresponding to the self-aligning torque. In particular, when the steering angle (steering angle) is smaller than the target steering angle (target steering angle), the steering assist torque acts in a direction to turn the steering wheel further, while the reaction torque corresponding to the self-aligning torque acts in a direction to return the steering wheel to the neutral position. Therefore, when the applied torque switches from the steering assist torque to the reaction torque, the reaction force received by the driver is in the opposite direction, which may cause the driver to feel uncomfortable.

[0008] The present disclosure has been made in consideration of the above-mentioned problems, and an object of the present disclosure is to provide a control device and a control method for a steering system that can reduce the discomfort experienced by the driver when the torque applied to the steering wheel switches from steering assist torque to reaction torque.

[0009] In order to solve the above-described problems, according to an aspect of the present disclosure, there is provided a control device for a steer-by-wire steering system in which a steering wheel operated by a driver of a vehicle and a wheel steering device for steering wheels are provided mechanically separated from each other and the steering angle of the wheels is controlled based on a steering angle of the steering wheel, the control device controlling the steering angle based on the steering angle of the steering wheel operated by the driver, and performing a first control of applying a reaction torque to the steering wheel in response to a self-aligning torque received by the wheels from a road surface, and a second control of applying a reaction torque to the steering wheel in response to a self-aligning torque received by the wheels from a road surface, and executing a second control for controlling the steering angle and the turning angle based on a target steering angle controlled by a first control, and applying a steering assist torque to the steering wheel according to an angular difference between the target steering angle and the steering angle or the turning angle, and a switching control for canceling the second control and switching to the first control as the angular difference increases while the second control is being executed, and when the angular difference increases while the steering angle or the turning angle is smaller than the target steering angle, the control switches from the second control to the first control when the absolute value of the steering assist torque is equal to or less than a predetermined reference value set in advance for suppressing torque fluctuations.

[0010] Further, in solving the above-described problems, according to another aspect of the present disclosure, there is provided a control method for a steer-by-wire steering system in which a steering wheel operated by a driver of a vehicle and a wheel steering device for steering wheels are provided mechanically separated from each other, and a steering angle of the wheels is controlled based on a steering angle of the steering wheel, the control device performing a first control of controlling the steering angle based on the steering angle of the steering wheel operated by the driver, and applying a reaction torque to the steering wheel in response to a self-aligning torque that the wheels receive from a road surface, and a predetermined driving support. A control method for a steering system is provided, which executes a second control that controls the steering angle and the steering angle based on a target steering angle by an assist control, and applies a steering assist torque to the steering wheel according to the angular difference between the target steering angle and the steering angle or the steering angle, and a switching control that cancels the second control and switches to the first control as the angular difference increases while the second control is being executed, and when the angular difference increases while the steering angle or the steering angle is smaller than the target steering angle, the absolute value of the steering assist torque is switched from the second control to the first control when it is equal to or less than a predetermined reference value for suppressing torque fluctuations that has been set in advance.

[0011] As described above, according to the present disclosure, it is possible to reduce the discomfort experienced by the driver when the torque applied to the steering wheel switches from the steering assist torque to the reaction torque.

[0012] FIG. 1 is a schematic diagram showing an example of the configuration of a vehicle equipped with a steering system according to an embodiment of the present disclosure. FIG. 2 is an explanatory diagram showing an example of the configuration of the steering system according to the embodiment. FIG. 3 is a block diagram showing an example of the configuration of a control device of the steering system according to the embodiment. FIG. 4 is an explanatory diagram showing an example of an aligning torque map (aligning torque information) used in the embodiment. FIG. 5 is an explanatory diagram showing an example of an assist torque map (assist torque information) used in the embodiment. FIG. 6 is an explanatory diagram showing the relationship between an angle difference and steering assist torque according to a reference example. FIG. 7 is an explanatory diagram showing the steering assist torque based on a target steering angle according to the reference example superimposed on an aligning torque map. FIG. 8 is an explanatory diagram showing the steering assist torque based on a target steering angle according to the embodiment superimposed on an aligning torque map. FIG. 9 is a flowchart showing the processing operation of a control device of the steering system according to the embodiment. FIG. 10 is a flowchart showing the processing operation of first control by the control device of the steering system according to the embodiment. FIG. 11 is a flowchart showing the processing operation of second control by the control device of the steering system according to the embodiment. FIG. 12 is a flowchart showing the processing operation of switching control by the control device of the steering system according to the embodiment. FIG. 13 is an explanatory diagram showing an example of an assist torque map (assist torque information) according to a first modified example. FIG. 14 is an explanatory diagram showing an example of an assist torque map (assist torque information) according to a second modified example. Fig. 10 is an explanatory diagram showing an example of an assist torque map (assist torque information) according to a third modified example. Fig. 11 is an explanatory diagram showing another example of an assist torque map (assist torque information) according to the third modified example. Fig. 12 is an explanatory diagram showing an example of an assist torque map (assist torque information) according to a fourth modified example. Fig. 13 is an explanatory diagram showing an example of a case where the friction coefficient of the road surface is small according to a sixth modified example.

[0013] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant description will be omitted.

[0014] 1. Overall Configuration of Vehicle First, an example of the overall configuration of a vehicle equipped with a control device for a steering system according to an embodiment of the present disclosure (hereinafter also referred to as a "steering control device") will be described.

[0015] Fig. 1 is a schematic diagram showing an example configuration of a vehicle 1. The vehicle 1 shown in Fig. 1 is configured as a front-wheel drive four-wheel vehicle in which drive torque output from a drive power source 13 that generates drive torque for the vehicle is transmitted to left and right front wheels 3LF, 3RF. The vehicle 1 may be a vehicle equipped with an internal combustion engine such as a gasoline engine or a diesel engine as the drive power source 13, an electric vehicle equipped with a drive motor as the drive power source 13, or a hybrid electric vehicle equipped with both an internal combustion engine and a drive motor as the drive power source 13.

[0016] The combination of drive wheels and the drive method are not limited. For example, vehicle 1 may be a rear-wheel drive vehicle, a four-wheel drive vehicle, or an electric vehicle equipped with a drive motor for each wheel. Furthermore, if vehicle 1 is an electric vehicle or a hybrid electric vehicle, vehicle 1 is equipped with a secondary battery that stores power supplied to the drive motor, and a motor or a generator such as a fuel cell that generates power to charge the battery.

[0017] The vehicle 1 includes a driving force source 13, an electric steering system 30, and brake devices 7LF, 7RF, 7LR, and 7RR (hereinafter collectively referred to as "brake devices 7" unless a distinction is required) as devices used to control the driving of the vehicle 1. The driving force source 13 outputs a driving torque that is transmitted to a front-wheel drive shaft 5 via a front-wheel differential mechanism (not shown). The driving of the driving force source 13 is controlled by a vehicle control device 11 that includes one or more electronic control units (ECUs). During manual driving, the vehicle control device 11 controls the driving of the driving force source 3 based on the accelerator opening operated by the driver. Furthermore, during execution of driving assistance control or automatic driving control, the vehicle control device 11 controls the driving of the driving force source 3 based on a target driving torque for automatic driving.

[0018] The electric steering system 30 includes an electric motor and a gear mechanism (not shown), and adjusts the steering angle of the left and right front wheels 3LF, 3RF by being controlled by a steering control device 60. During manual driving, the steering control device 60 controls the steering angle of the front wheels 3LF, 3RF based on the steering angle of the steering wheel 31 by the driver. Furthermore, during execution of driving assistance control or automatic driving control, the steering control device 60 controls the steering angle of the front wheels 3LF, 3RF based on a target steering angle set according to a target driving trajectory.

[0019] The brake devices 7LF, 7RF, 7LR, and 7RR apply braking forces to the front, rear, left, and right wheels 3LF, 3RF, 3LR, and 3RR, respectively. The brake devices 7 are configured, for example, as hydraulic friction brake devices, and generate predetermined braking forces by adjusting the hydraulic pressure supplied to each brake device 7. If the vehicle 1 is an electric vehicle or a hybrid electric vehicle, the brake devices 7 are used in combination with regenerative braking using a drive motor. The hydraulic pressure supplied to the brake devices 7 is controlled by a hydraulic pressure control unit (not shown).

[0020] The vehicle 1 also includes a surrounding environment sensor 15 and a driving assistance device 17. The surrounding environment sensor 15 is one or more sensors that detect information about the surrounding environment of the vehicle 1. The surrounding environment sensor 15 detects the positions, distances, speeds, etc. of obstacles such as other vehicles and pedestrians, road shapes, and boundary lines of driving lanes, and outputs information about the detection results to the driving assistance device 17. The surrounding environment sensor 15 includes, for example, one or more sensors selected from a camera, a LiDAR, a radar sensor, and an ultrasonic sensor. In FIG. 1 , the vehicle 1 includes, as the surrounding environment sensor 15, a pair of left and right stereo cameras 15L, 15R that capture images ahead of the vehicle 1.

[0021] The driving assistance device 17 executes driving assistance control or automatic driving control of the vehicle 1 using information on obstacles and other detection results detected by the surrounding environment sensor 15. In this embodiment, the driving assistance device 17 has at least an assistance function for automatically driving the vehicle 1 at a predetermined position in a driving lane. Examples of such assistance functions include lane keeping control and automatic driving control. For example, the driving assistance device 17 sets a target driving trajectory for the vehicle 1, sets target values ​​for the acceleration / deceleration of the vehicle 1 and the steering angle of the steering wheel 31 so that the vehicle 1 drives along the target driving trajectory, and outputs information on the target values ​​to the vehicle control device 11 and the steering control device 60.

[0022] The vehicle 1 also includes tire force sensors 9L, 9R corresponding to the left and right front wheels 3LF, 3RF. The tire force sensors 9L, 9R are provided at the connection between the front wheel drive shaft 5 and the front wheels 3LF, 3RF, respectively. The tire force sensors 9L, 9R are load sensors that detect the loads applied to the left and right front wheels 3LF, 3RF, respectively. For example, the tire force sensors 9L, 9R may be six-component force detectors that detect loads in each of the three axial directions of the vehicle 1, namely the longitudinal direction (x-axis), the transverse direction (y-axis), and the height direction (z-axis), as well as moments generated around each of the three axes.

[0023] 2. Steering System Next, an example of the overall configuration of the electric steering system 30 according to this embodiment will be described.

[0024] 2 is a schematic diagram showing an example of the configuration of the electric steering system 30. The electric steering system 30 is constructed as a steer-by-wire type steering system in which a steering wheel 31 operated by a driver and a wheel steering device 40 that steers the wheels are provided mechanically separated from each other, and the steering angle of the front wheels 3LF, 3RF is controlled based on a detected value of the steering angle (rotation angle) of the steering wheel 31.

[0025] The electric steering system 30 has a steering device 33 and a wheel steering device 40 that steers the front wheels (steerable wheels) 3RFLF, 3RF. The steering device 33 receives steering inputs from a steering wheel 31 operated by the driver. The electric steering system 30 is a steer-by-wire type steering system in which the steering device 33 and the wheel steering device 40 are mechanically separated (linkless) and controlled in conjunction with each other by a steering control device 60.

[0026] The steering device 33 is configured to include a steering wheel 31, a steering shaft 32 connected to the steering wheel 31, and a reaction motor 34 coaxially incorporated at an axially intermediate position of the steering shaft 32. The reaction motor 34 is a means for applying a reaction force (rotational resistance) to the driver operating the steering wheel 31 via the steering shaft 32 and the steering wheel 31. The drive of the reaction motor 34 is controlled by a steering control device 60, and the output torque of the reaction motor 34 is transmitted to the steering wheel 31 via the steering shaft 32.

[0027] The wheel steering device 40 is configured to include a rack shaft 51 extending in the left-right direction of the vehicle body, and knuckle arms 53L, 53R connected via tie rods 52L, 52R to both ends of the rack shaft 51. The left and right front wheels 3LF, 3RF are connected to the knuckle arms 53L, 53R, respectively.

[0028] Rack shaft 51 is supported by housing 49 so as to be movable in the left-right direction of vehicle 1. Rack gear 54 is provided on rack shaft 51. Pinion gear 55 meshes with rack gear 54. Pinion gear 55 is connected to pinion shaft 56. A steering motor 57 is coaxially mounted on pinion shaft 56 at an intermediate position in the axial direction.

[0029] The steering motor 57 is an electric motor that steers the left and right front wheels 3LF, 3RF, and the drive of the steering motor 57 is controlled by a steering control device 60. The output torque of the steering motor 57 is transmitted to the rack shaft 51 via the pinion shaft 56, the pinion gear 55, and the rack gear 54. This causes the left and right front wheels 3LF, 3RF to turn.

[0030] A clutch mechanism 37 is provided between the steering shaft 32 and the pinion shaft 56. The clutch mechanism 37 is engaged by the steering control device 60 in the event of a failure of the reaction motor 34 or the steering motor 57, thereby directly connecting the steering shaft 32 and the pinion shaft 56 and enabling the driver to steer the front wheels 3LF, 3RF.

[0031] Sensor signals output from the steering angle sensor 35, steering torque sensor 36, turning angle sensor 58, and tire force sensors 9L, 9R are input to the steering control device 60. The steering angle sensor 35 detects the steering angle (rotation angle) of the steering wheel 31. The steering torque sensor 36 detects the steering torque, which is the torque input to the steering wheel 31 by the driver. The steering torque corresponds to the reaction torque that the driver receives from the steering wheel 31. The turning angle sensor 58 detects the turning angle (wheel angle) of the left and right front wheels 3LF, 3RF. The tire force sensors 9L, 9R detect the self-aligning torque applied to the left and right front wheels 3LF, 3RF from the road surface.

[0032] In place of the tire force sensors 9L, 9R, for example, a torque sensor that is provided coaxially with the pinion shaft 56 and detects the self-aligning torque may be provided.

[0033] Based on the inputs from these sensors, steering control device 60 controls the drive of reaction force motor 34 and steering motor 57. In other words, steering control device 60 controls linkless steering device 33 and wheel steering device 40 in conjunction with each other (steer-by-wire control) according to a pre-stored program.

[0034] 3. Steering Control Device The steering control device 60 functions as a device that executes processing to control the electric steering system 30 by having one or more processors, such as CPUs (Central Processing Units), execute a computer program. The computer program is a computer program that causes the processor to execute operations, described below, that should be executed by the steering control device 60. The computer program executed by the processor may be recorded on a recording medium that functions as a storage unit (memory) provided in the steering control device 60, or may be recorded on a recording medium built into the steering control device 60 or any recording medium that can be externally attached to the steering control device 60.

[0035] Examples of recording media for recording computer programs include magnetic media such as hard disks, floppy disks, and magnetic tapes; optical recording media such as CD-ROMs (Compact Disc-Read Only Memory), DVDs (Digital Versatile Discs), and Blu-ray (registered trademark); magneto-optical media such as floptical disks; memory elements such as RAMs (Random Access Memory) and ROMs (Read Only Memory); and flash memories such as USB (Universal Serial Bus) memories and SSDs (Solid State Drives), as well as other media capable of storing programs.

[0036] 3 is a block diagram showing an example of the configuration of the steering control device 60. The steering control device 60 includes a processing unit 61 and a storage unit 63. The processing unit 61 is configured to include one or more processors. A part or all of the processing unit 61 may be configured as an updatable unit such as firmware, or may be a program module executed by commands from a CPU or the like. The steering control device 60 may be configured as a stand-alone device, or may be configured as a plurality of devices connected to each other so that they can communicate with each other.

[0037] The storage unit 63 is configured with one or more storage elements (memories), such as RAM or ROM, communicably connected to the processing unit 61. However, the number and type of storage units 63 are not particularly limited. The storage unit 63 stores computer programs executed by the processing unit 61, various parameters used in arithmetic processing, detection data, calculation results, and other data. A part of the storage unit 63 functions as a work area for the processing unit 61.

[0038] The steering control device 60 is communicatively connected to the driving assistance device 17, tire force sensors 9L, 9R, steering angle sensor 35, steering torque sensor 36, turning angle sensor 58, tire force sensors 9L, 9R, reaction force motor 34, and turning motor 57 via a dedicated line or via communication means such as CAN (Controller Area Network) or LIN (Local Internet).

[0039] The processing unit 61 includes a first control unit 71, a second control unit 73, and a switching control unit 75. The first control unit 71, the second control unit 73, and the switching control unit 75 are functions realized by execution of a computer program by one or more processors. Note that part of the first control unit 71, the second control unit 73, and the switching control unit 75 may be configured by hardware such as an analog circuit.

[0040] The function of each part of the processing unit 61 will be briefly explained below, and the specific processing operation of the processing unit 61 will be explained in detail later.

[0041] (First control unit) The first control unit 71 controls the steering angle based on the steering angle of the steering wheel 31 operated by the driver, and also performs a first control that applies a reaction torque to the steering wheel 31 in response to the self-aligning torque that the left and right front wheels 3LF, 3RF receive from the road surface.

[0042] When the driver manually steers vehicle 1, first control unit 71 detects the steering angle of steering wheel 31 based on the sensor signal of steering angle sensor 35, and sets target steering angles for left and right front wheels 3LF, 3RF based on the actual steering angle. Hereinafter, the steering angle detected based on the sensor signal of steering angle sensor 35 will be referred to as the actual steering angle. Steering angle and steering angle can be converted into each other using a predetermined coefficient, and first control unit 71 sets the target steering angle based on the actual steering angle using the coefficient. In addition, first control unit 71 converts the target steering angle into a rotation angle of steering motor 57 and controls the rotation angle of steering motor 57.

[0043] Furthermore, while executing the first control, the first control unit 71 detects the self-aligning torque that the front wheels 3LF, 3RF receive from the road surface based on the sensor signals of the tire force sensors 9L, 9R, and sets the reaction torque to be applied to the steering wheel 31 based on the self-aligning torque. The self-aligning torque is a restoring force that acts to return the orientation of the front wheels 3LF, 3RF to the fore-and-aft direction of the vehicle 1 while the vehicle 1 is turning. The reaction torque is a restoring force that acts in response to the self-aligning torque acting on the front wheels 3LF, 3RF to return the steering wheel 31 to the neutral position (a position where the vehicle 1 is traveling straight).

[0044] The self-aligning torque and the reaction torque can be converted into each other using a predetermined coefficient, and the first control unit 71 sets the reaction torque based on the self-aligning torque using the coefficient. The first control unit 71 also sets a drive command value for the reaction motor 34 that can output the set reaction torque, and controls the drive of the reaction motor 34.

[0045] 4 shows an aligning torque map (aligning torque information) that indicates the reaction torque F_co according to the steering angle θs of the steering wheel 31. The steering angle θs is shown with the neutral position θ0 as the center, with the right turning direction θs_R as a positive value and the left turning direction θs_L as a negative value. Furthermore, the reaction torque F_co is shown with the force pushing the steering wheel 31 in the left turning direction as a positive value and the force pushing the steering wheel 31 in the right turning direction as a negative value.

[0046] 4, the larger the steering angle θs in each of the right turning direction θs_R and the left turning direction θs_L, the larger the absolute value of the reaction torque F_co that acts to return the steering angle θs to the neutral position θ0 is set to. The relationship between the steering angle θs and the reaction torque F_co shown in the aligning torque map can also be seen as the relationship between the steering angle of the front wheels 3LF, 3RF and the self-aligning torque that the front wheels 3LF, 3RF receive from the road surface.

[0047] (Second control unit) The second control unit 73 controls the steering angle and the turning angle based on the target steering angle determined by the driving assist control of the driving assist device 17, and also executes second control to apply a steering assist torque to the steering wheel 31 according to the angular difference between the target steering angle and the steering angle or the turning angle. As described above, the steering angle and the turning angle are convertible to each other, and the target steering angle may be either a target value of the steering angle or a target value of the turning angle. However, in this embodiment, an example will be described in which the target steering angle is a target value of the steering angle (hereinafter referred to as the "target steering angle"). Therefore, in the following description, the angular difference between the target steering angle and the steering angle refers to the value obtained by subtracting the absolute value of the target steering angle from the absolute value of the steering angle.

[0048] Second control unit 73 controls the steering angle of front wheels 3LF, 3RF based on a target steering angle set by an assistance function that causes vehicle 1 to automatically travel in a predetermined position in the driving lane, such as lane keeping control or automatic driving control by driving assistance device 17. Second control unit 73 sets a target steering angle based on the target steering angle using a predetermined coefficient. Second control unit 73 also converts the target steering angle into a rotation angle of steering motor 57 and controls the rotation angle of steering motor 57.

[0049] Furthermore, the second control unit 73 controls the steering angle of the steering wheel 31 based on the target steering angle set by the driving assistance device 17. At this time, the second control unit 73 sets a steering assist torque according to the angular difference between the target steering angle and the actual steering angle. The steering assist torque is an assist force that acts to make the actual steering angle follow the target steering angle. For example, the second control unit 73 sets the steering assist torque using an assist torque map (assist torque information) in which a value of the steering assist torque relative to the angular difference is set in advance. The second control unit 73 sets a drive command value for the reaction force motor 34 that can output the set steering assist torque, and controls the drive of the reaction force motor 34.

[0050] 5 shows an assist torque map in which a steering assist torque F_as to be generated according to the angular difference Δθ between the target steering angle θs_tgt and the steering angle θs is set. As described above, the angular difference Δθ indicates a difference value obtained by subtracting the absolute value of the target steering angle θs_tgt from the absolute value of the steering angle θs, and is shown with the angular difference Δθ=0 as the center, with a positive value indicating that the steering angle θs is larger than the target steering angle θs_tgt and a negative value indicating that the steering angle θs is smaller than the target steering angle θs_tgt. Furthermore, the steering assist torque F_as indicates a force in a direction to reduce the steering angle θs as a positive value and a force in a direction to increase the steering angle θs as a negative value.

[0051] In this embodiment, when the steering angle θs is larger than the target steering angle θs_tgt, the absolute value of the steering assist torque F_as that acts to make the steering angle θs follow the target steering angle θs_tgt is set to a larger value as the angle difference Δθ between the steering angle θs and the target steering angle θs_tgt increases. Note that the illustrated switching threshold Δθ_thr is a threshold at which the steering control is switched (overridden) from the second control based on the driving assist control to the first control based on the driver's steering.

[0052] Furthermore, when the steering angle θs is smaller than the target steering angle θs_tgt, the absolute value of the steering assist torque F_as gradually increases as the absolute value of the angle difference Δθ between the steering angle θs and the target steering angle θs_tgt increases, but at a switching threshold value −Δθ_thr at which the steering control is switched from the second control to the first control, the absolute value of the steering assist torque F_as is set to a value equal to or less than a predetermined reference value |F_as_α| for suppressing torque fluctuations that has been set in advance. The predetermined reference value |F_as_α| is an arbitrary value that is zero or close to zero, and may be any value that can reduce the force that pushes the steering wheel 31 in the direction to turn it further.

[0053] In the illustrated example, the steering assist torque F_as at the switching threshold value -Δθ_thr is set to zero, which is smaller than the predetermined reference value |F_as_α|. Furthermore, the steering assist torque F_as has one peak in the region where the angular difference Δθ is between zero and the switching threshold value -Δθ_thr. In other words, as the angular difference Δθ increases, the steering assist torque F_as gradually increases and then decreases to zero.

[0054] The method for setting the assist torque map will be explained in detail later.

[0055] (Switching control unit) The switching control unit 75 executes switching control to cancel the second control and switch the steering control to the first control as the angular difference between the target steering angle and the steering angle increases during execution of the second control. In other words, the switching control unit 75 detects that the driver is intentionally attempting to correct the driving trajectory while the driving assistance function is automatically steering the vehicle 1 so that it travels along the target driving trajectory, and switches (overrides) to the first control that controls the steering angle based on the steering operation by the driver.

[0056] In this embodiment, when the angle difference Δθ between the target steering angle θs_tgt and the actual steering angle θs_act increases while the actual steering angle θs_act is larger than the target steering angle θs_tgt, the switching control unit 75 switches the steering control from the second control to the first control when the difference ΔF between the steering assist torque F_as and the reaction torque F_co before and after switching the steering control from the second control to the first control is equal to or less than a predetermined torque difference threshold ΔF_thr for suppressing a preset torque fluctuation. When the steering wheel 31 is being turned further, the direction of the force of the steering assist torque F_as by the second control and the direction of the force of the reaction torque F_co by the first control are the same, so switching control can be performed on the condition that the difference ΔF between the steering assist torque F_as and the reaction torque F_co is equal to or less than the predetermined torque difference threshold ΔF_thr.

[0057] The predetermined torque difference threshold ΔF_thr is set in advance to an appropriate value that can suppress a large fluctuation in the torque applied to the steering wheel 31 and a sudden change in the steering angle θs when the torque applied to the steering wheel 31 is switched from the steering assist torque F_as to the reaction torque F_co as the steering control is switched from the second control to the first control. The predetermined torque difference threshold ΔF_thr is preferably set to a value of 0.5 N·m or less, or a value of 30% or less of the reaction torque F_co, for example, and more preferably to 0.

[0058] Furthermore, in this embodiment, when the angular difference Δθ between the target steering angle θs_tgt and the actual steering angle θs_act increases while the actual steering angle θs_tgt is smaller than the target steering angle θs_tgt, the steering control is switched from the second control to the first control while the absolute value of the steering assist torque is equal to or less than a predetermined reference value. When the steering wheel 31 is being turned back, the direction of the force of the steering assist torque F_as by the second control and the direction of the force of the reaction torque F_co by the first control are opposite to each other. This increases torque fluctuation when the steering control is switched from the second control to the first control, which may cause the driver to feel uncomfortable and may cause unstable behavior of the vehicle 1. For this reason, the steering assist torque F_as when the steering control is switched from the second control to the first control while the steering wheel 31 is being turned back is set to equal to or less than a predetermined reference value F_thr, thereby suppressing torque fluctuation when the steering control is switched from the second control to the first control.

[0059] 6 and 7 are diagrams for explaining a reference example in which the steering assist torque F_as is set to gradually increase as the angular difference Δθ between the target steering angle θs_tgt and the steering angle θs increases in the second control, and the steering control is switched from the second control to the first control when the angular difference Δθ exceeds a switching threshold Δθ_thr. Fig. 6 is an explanatory diagram showing the relationship between the angular difference Δθ and the steering assist torque F_as, and corresponds to the assist torque map according to this embodiment shown in Fig. 5. Fig. 7 is an explanatory diagram of a reference example in which the steering assist torque F_as based on the target steering angle θs_tgt when the vehicle 1 is turning right is superimposed on an aligning torque map.

[0060] As shown in Figure 6, in the reference example, regardless of the magnitude relationship between the steering angle θs and the target steering angle θs_tgt, and regardless of the value of the target steering angle θs_tgt, the steering assist torque F_as is set so that its absolute value gradually increases as the angle difference Δθ increases.

[0061] 7 , in the reference example, when the steering angle θs is greater than the target steering angle θs_tgt and the angular difference Δθ between the steering angle θs and the target steering angle θs_tgt exceeds the steering angle θs_h at which the switching threshold Δθ_thr is reached, the steering control is switched from the second control to the first control. At this time, if the steering assist torque F_as greatly exceeds the reaction torque F_co at the steering angle θs_h, the torque applied to the steering wheel 31 changes significantly before and after the switching of the steering control from the second control to the first control. As a result, the steering angle θs of the steering wheel 31 changes abruptly, which may cause the behavior of the vehicle 1 to become unstable.

[0062] In the reference example, when the steering angle θs is smaller than the target steering angle θs_tgt and the angular difference Δθ between the steering angle θs and the target steering angle θs_tgt exceeds a steering angle θs_l at which the switching threshold Δθ_thr is reached, the steering control is switched from the second control to the first control. At the steering angle θs_l, the steering assist torque F_as and the reaction torque F_co are torques in opposite directions, and the torque applied to the steering wheel 31 changes suddenly before and after the switching of the steering control from the second control to the first control. As a result, the steering angle θs of the steering wheel 31 changes suddenly, which may cause the driver to feel uncomfortable and may cause the behavior of the vehicle 1 to become unstable.

[0063] FIG. 8 is an explanatory diagram of this embodiment showing the steering assist torque F_as based on the target steering angle θs_tgt when the vehicle 1 is turning right, superimposed on the aligning torque map.

[0064] As shown in Figure 8, in this embodiment, when the steering angle θs is larger than the target steering angle θs_tgt, the switch of steering control from the second control to the first control is determined not based on the angle difference Δθ between the steering angle θs and the target steering angle θs_tgt, but based on the difference between the steering assist torque F_as and the reaction torque F_co before and after the switch of steering control from the second control to the first control.

[0065] Specifically, the switching control unit 75 releases the second control and switches the steering control to the first control when the difference ΔF between the steering assist torque F_as and the reaction torque F_co before and after switching the steering control from the second control to the first control becomes equal to or less than a predetermined torque difference threshold ΔF_thr for suppressing a preset torque fluctuation. In the example shown in FIG. 8 , the second control is released and the steering control is switched to the first control when the difference ΔF between the steering assist torque F_as and the reaction torque F_co before and after switching the steering control from the second control to the first control becomes zero (steering angle θs = θs_c). Therefore, it is possible to suppress a sudden change in the steering angle of the steering wheel 31 when the steering control is switched from the second control to the first control.

[0066] In addition, in this embodiment, when the steering angle θs is smaller than the target steering angle θs_tgt, the steering control is switched from the second control to the first control when the steering angle θs reaches a steering angle θs_l at which the angular difference Δθ between the steering angle θs and the target steering angle θs_tgt becomes a predetermined switching threshold Δθ_thr, based on the angular difference Δθ. At this time, as the angular difference Δθ increases, the absolute value of the steering assist torque F_as for causing the steering angle θs to follow the target steering angle θs_tgt gradually increases, but at the steering angle θs_l at which the steering control is switched from the second control to the first control, the steering assist torque F_as is set to a predetermined reference value or less.

[0067] In the example shown in FIG. 8 , at the steering angle θs_l at which the steering control is switched from the second control to the first control, the steering assist torque F_as is set to zero, which is equal to or less than a predetermined reference value |F_as_α|. Therefore, while applying an assist force that causes the steering angle θs to follow the target steering angle θs_tgt, it is possible to reduce the discomfort felt by the driver due to the torque direction being reversed before and after the steering control is switched from the second control to the first control. Furthermore, the steering assist torque F_as has one peak in the region where the angle difference Δθ is between zero and the switching threshold Δθ_thr. Therefore, before the angle difference Δθ reaches the switching threshold Δθ_thr, the steering assist torque F_as increases and then decreases, allowing the driver to recognize through the steering wheel 31 that the timing at which the steering control is switched from the second control to the first control is approaching. This reduces the discomfort felt by the driver due to torque fluctuations when the steering control is switched from the second control to the first control.

[0068] 7 and 8, the steering angle θs when the rotation direction of the steering wheel 31 is in the right turning direction is shown as a positive value, the torque pushing the steering wheel 31 in the direction to return to the neutral position is shown as a positive value, and the torque pushing the steering wheel 31 further in the right turning direction is shown as a negative value. The explanation based on these Figures 7 and 8 can also be understood in the same way if the steering angle θs when the rotation direction of the steering wheel 31 is in the left turning direction is shown as a positive value, the torque pushing the steering wheel 31 in the direction to return to the neutral position is shown as a positive value, and the torque pushing the steering wheel 31 further in the left turning direction is shown as a negative value.

[0069] 3. Processing Operation of Steering Control Device Up to this point, an example configuration of the steering control device 60 according to this embodiment has been described. Next, the processing operation of the steering control device 60 will be specifically described. Note that in the following description, the right turning direction and the left turning direction are understood to be positive values ​​with respect to the neutral position (steering angle θs=0) of the steering wheel 31. Therefore, the generated torque or applied torque is also understood to be a positive value.

[0070] 9 shows a flowchart of the processing operation of the steering control device 60. First, when the steering system is started (step S11), the processing unit 61 determines whether or not the driving assistance function is operating (step S13). For example, the processing unit 61 determines that the driving assistance function is operating when it receives a signal indicating that the driving assistance function is operating from the driving assistance device 17. The processing unit 61 may also determine whether or not the driving assistance function is operating based on an on / off signal of an activation switch for the driving assistance function. As described above, the driving assistance function is an assistance function that automatically drives the vehicle 1 in a predetermined position in the driving lane, and may be, for example, lane keeping control or automatic driving control.

[0071] When the processing unit 61 does not determine that the driving assistance function is in operation (S13 / No), the first control unit 71 executes the first control (step S15).

[0072] 10 is a flowchart showing the processing operation of the first control by first control unit 71. First, first control unit 71 detects the actual steering angle θs_act of steering wheel 31 based on the sensor signal of steering angle sensor 35 (step S31). Next, first control unit 71 sets the target steering angle θw_tgt based on the actual steering angle θs_act (step S33). For example, first control unit 71 calculates the target steering angle θw_tgt by multiplying the actual steering angle θs_act by a predetermined coefficient set in advance.

[0073] Next, first control unit 71 drives steering motor 57 to control the steering of front wheels 3LF, 3RF (step S35). For example, first control unit 71 refers to a map that defines the relationship between the steering angle and the drive command value for steering motor 57, obtains the drive command value that corresponds to target steering angle θw_tgt, and drives steering motor 57.

[0074] Next, the first control unit 71 detects the self-aligning torque F_al that the left and right front wheels 3LF, 3RF receive from the road surface based on the sensor signals of the tire force sensors 9L, 9R (step S37), and then multiplies the self-aligning torque F_al by a predetermined coefficient to calculate the reaction torque F_co (step S39).

[0075] Next, the first control unit 71 drives the reaction motor 34 to apply a reaction torque F_co corresponding to the self-aligning torque F_al to the steering wheel 31 (step S41).

[0076] In this way, the first control unit 71 controls the steering angle θw of the left and right front wheels 3LF, 3RF based on the actual steering angle θs_act of the steering wheel 31 operated by the driver. The first control unit 71 also applies to the steering wheel 31 a reaction torque F_co that corresponds to the self-aligning torque F_al that the left and right front wheels 3LF, 3RF receive from the road surface, causing the driver to feel a reaction force that corresponds to the self-aligning torque F_al.

[0077] Returning to Figure 9, on the other hand, in the above step S13, if the processing unit 61 determines that the driving assistance function is operating (S13 / Yes), the second control unit 73 performs steering control using the second control (step S17).

[0078] 11 is a flowchart showing the processing operation of the second control by the second control unit 73. First, the second control unit 73 acquires information on the target steering angle θs_tgt transmitted from the driving assistance device 17 (step S51). Next, the second control unit 73 detects the actual steering angle θs_act of the steering wheel 31 based on the sensor signal of the steering angle sensor 35 (step S53).

[0079] Next, the second control unit 73 calculates the angle difference Δθ by subtracting the target steering angle θs_tgt from the actual steering angle θs_act (step S55). Here, it is determined whether the actual steering angle θs_act is turned too far or too far from the target steering angle θs_tgt.

[0080] Next, the second control unit 73 sets the steering assist torque F_as based on the calculated angular difference Δθ (step S57). For example, the second control unit 73 sets the steering assist torque F_as according to the angular difference Δθ based on a preset assist torque map (see FIG. 5). Next, the second control unit 73 drives the reaction force motor 34 to apply the steering assist torque F_as to the steering wheel 31 (step S59).

[0081] Next, second control unit 73 sets target steering angle θw_tgt based on actual steering angle θs_act (step S61), similar to step S33 described above. Next, second control unit 73 drives steering motor 57 to control the steering of front wheels 3LF, 3RF (step S63), similar to step S35 described above.

[0082] As described above, the second control unit 73 sets the steering assist torque F_as based on the angle difference Δθ between the target steering angle θs_tgt set by the driving support device 17 and the actual steering angle θs, and applies the steering assist torque F_as to the steering wheel 31 so that the actual steering angle θs follows the target steering angle θs_tgt. Then, the second control unit 73 controls the steering angle θw of the left and right front wheels 3LF, 3RF based on the actual steering angle θs_act. Therefore, in the second control, the steering of the vehicle 1 is basically controlled in accordance with the target steering angle θs_tgt set by the driving support device 17, but when the driver intervenes in operating the steering wheel 31, the vehicle 1 is steered in a manner reflecting the driver's intention.

[0083] Returning to FIG. 9, in the execution state of the second control, the switching control unit 75 executes the switching control (step S19).

[0084] 12 is a flowchart showing the processing operation of the switching control by the switching control unit 75. First, the switching control unit 75 determines whether the actual steering angle θs_act detected based on the sensor signal of the steering angle sensor 35 is larger than the target steering angle θs_tgt set by the driving assistance device 17 (step S71).

[0085] When the switching control unit 75 does not determine that the actual steering angle θs_act is greater than the target steering angle θs_tgt (No in S71), the switching control unit 75 determines whether the angle difference Δθ obtained by subtracting the actual steering angle θs_act from the target steering angle θs_tgt exceeds a predetermined switching threshold Δθ_tgt (step S81). When the switching control unit 75 does not determine that the angle difference Δθ exceeds the predetermined switching threshold Δθ_tgt (No in S81), the switching control ends as is and the second control continues.

[0086] On the other hand, when the switching control unit 75 determines that the angle difference Δθ exceeds the predetermined switching threshold Δθ_tgt (S81 / Yes), it cancels the second control and switches the steering control to the first control (step S79). In other words, it is determined that the driver is intentionally attempting to correct the traveling trajectory in a direction to turn the steering wheel 31 back with respect to the target steering angle θs_tgt, so the switching control unit 75 overrides the steering control based on the driving assistance function to the steering control based on the steering operation of the driver.

[0087] At this time, as shown in Fig. 8, when the actual steering angle θs_act is smaller than the target steering angle θs_tgt and the angular difference Δθ matches the switching threshold Δθ_tgt, the steering assist torque F_as is set to zero, which is less than or equal to a predetermined reference value |F_as_α|. This makes it possible to reduce the discomfort felt by the driver due to the torque applied to the steering wheel 31 being in the opposite direction before and after the steering control is switched from the second control to the first control.

[0088] If the switching control unit 75 determines in step S71 that the actual steering angle θs_act is greater than the target steering angle θs_tgt (Yes in step S71), it detects the self-aligning torque F_al that the left and right front wheels 3LF, 3RF receive from the road surface based on the sensor signals of the tire force sensors 9L, 9R, as in step S37 (step S73). Next, the switching control unit 75 calculates the reaction torque F_co by multiplying the self-aligning torque F_al by a predetermined coefficient, as in step S39 (step S75).

[0089] Next, the switching control unit 75 determines whether the difference ΔF between the calculated reaction torque F_co and the steering assist torque F_as applied to the steering wheel 31 is equal to or less than a predetermined torque difference threshold ΔF_thr (step S75). For example, the switching control unit 75 determines whether the difference ΔF between the reaction torque F_co and the steering assist torque F_as is zero.

[0090] If the switching control unit 75 does not determine that the difference ΔF between the reaction torque F_co and the steering assist torque F_as is less than or equal to the predetermined torque difference threshold ΔF_thr (S75 / No), it terminates the switching control and continues the second control.

[0091] On the other hand, if the switching control unit 75 does not determine that the difference ΔF between the reaction torque F_co and the steering assist torque F_as is equal to or smaller than the predetermined torque difference threshold ΔF_thr (S75 / No), it cancels the second control and switches the steering control to the first control (step S79). In other words, it is determined that the driver is intentionally attempting to correct the traveling trajectory in a direction to turn the steering wheel 31 more with respect to the target steering angle θs_tgt, so the switching control unit 75 overrides the steering control based on the driving support function to the steering control based on the steering operation of the driver.

[0092] At this time, as shown in Fig. 8, the steering control is switched from the second control to the first control in a state where the difference between the steering assist torque F_as and the reaction torque F_co applied to the steering wheel 31 before and after the steering control is switched from the second control to the first control is zero or extremely small. Therefore, a sudden change in the torque applied to the steering wheel 31 due to the switching of the steering control from the second control to the first control is suppressed, and a sudden change in the steering angle of the steering wheel 31 can be suppressed.

[0093] 9 , while the first control or the second control is continued, or after the steering control is switched from the second control to the first control during the second control, the processing unit 61 determines whether the activation of the steering system 10 has been stopped (step S21). If the processing unit 61 does not determine that the activation of the steering system 10 has been stopped (S21 / No), the processing unit 61 returns to step S13 and executes the processing of each step described above. On the other hand, if the processing unit 61 determines that the activation of the steering system 10 has been stopped (S21 / Yes), the processing unit 61 ends the steering control.

[0094] 4. Effects As described above, when the angle difference Δθ between the target steering angle θs_tgt and the actual steering angle θs_act increases in a state where the actual steering angle θs_act is smaller than the target steering angle θs_tgt, the control device for the steering system according to this embodiment switches from the second control to the first control in a state where the steering assist torque F_as at the time of switching from the second control to the first control is small and is equal to or smaller than a predetermined reference value |F_as_α|. This suppresses abrupt fluctuations in torque caused by the steering assist torque F_as and the reaction torque F_co being in opposite directions, thereby reducing the sense of discomfort experienced by the driver.

[0095] Furthermore, in the control device for the steering system according to this embodiment, the steering assist torque F_as is set to zero when the steering control is switched from the second control to the first control. In this case, the fluctuation in the torque applied to the steering wheel 31 due to the switching of the steering control from the second control to the first control becomes extremely small, and the sense of discomfort experienced by the driver can be further alleviated.

[0096] Furthermore, in the control device for the steering system according to this embodiment, the steering assist torque F_as has a peak in the region where the angle difference Δθ is between zero and the switching threshold value Δθ_thr. This makes it possible to make the driver aware that the timing for switching the steering control from the second control to the first control is approaching, and further reduces the discomfort felt by the driver due to torque fluctuations when the steering control is switched from the second control to the first control.

[0097] 5. Modifications The steering system control device according to this embodiment has been described so far, but the above embodiment can be modified in various ways. Modifications of the steering system control device according to this embodiment will now be described.

[0098] (First Modification) When the angle difference Δθ increases in a state where the actual steering angle θact is smaller than the target steering angle θs_tgt, the setting of the steering assist torque F_as that can suppress torque fluctuations that occur when the steering control is switched from the second control to the first control is not limited to the example in the above embodiment. For example, the absolute value of the steering assist torque F_as that is generated in a state where the actual steering angle θs is smaller than the target steering angle θs_tgt may be set to be equal to or less than a predetermined reference value |F_as_α|.

[0099] FIG. 13 shows an example of an assist torque map according to a first modification. In the assist torque map shown in FIG. 13, the absolute value of the steering assist torque F_as gradually increases in the initial stage when the actual steering angle θs and the target steering angle θs_thr start to increase from zero when the actual steering angle θs is smaller than the target steering angle θs_tgt. However, thereafter, until the angle difference Δθ reaches the switching threshold Δθ_thr, the absolute value of the steering assist torque F_as is set to a constant value that is equal to or less than a predetermined reference value |F_as_α| and exceeds zero. As a result, an assist force is generated to make the actual steering angle θs follow the target steering angle θs_tgt until the steering control is switched from the second control to the first control, while torque fluctuations are reduced when the steering control is switched from the second control to the first control, thereby reducing the sense of discomfort experienced by the driver.

[0100] (Second Modification) The absolute value of the steering assist torque F_as generated when the actual steering angle θs is smaller than the target steering angle θs_tgt may be set to be equal to or less than a predetermined reference value |F_as_α| and to gradually increase as the angle difference Δθ increases.

[0101] FIG. 14 shows an example of an assist torque map according to a second modification. In the assist torque map shown in FIG. 14, the absolute value of the steering assist torque F_as gradually increases as the actual steering angle θs and the target steering angle θs_thr increase when the actual steering angle θs is smaller than the target steering angle θs_tgt. The absolute value of the steering assist torque F_as when the angle difference Δθ becomes equal to the switching threshold value Δθ_thr is set to be equal to or less than a predetermined reference value |F_as_α|. This allows an assist force for making the actual steering angle θs follow the target steering angle θs_tgt to be generated in accordance with the angle difference Δθ until the timing at which the steering control is switched from the second control to the first control, while reducing torque fluctuations when the steering control is switched from the second control to the first control, thereby reducing the sense of discomfort experienced by the driver.

[0102] (Third variant) In the assist torque map (assist torque information), the steering assist torque F_as is zero when the angular difference Δθ is zero, the absolute value of the steering assist torque F_as when the angular difference Δθ is the switching threshold value Δθthr is less than or equal to a predetermined reference value |F_as_α|, and the absolute value of the steering assist torque F_as in at least a portion of the region between the angular difference Δθ of zero and the switching threshold value Δθ_thr may exceed the predetermined reference value |F_as_α|.

[0103] FIG. 15 shows an example of an assist torque map according to a third modification. In the assist torque map shown in FIG. 15, the absolute value of the steering assist torque F_as in a portion of the region between the angular difference Δθ of zero and the switching threshold Δθ_thr is set to exceed a predetermined reference value |F_as_α|. That is, the peak value of the steering assist torque F_as is set to exceed the predetermined reference value |F_as_α|. Therefore, the steering assist torque F_as increases and then decreases before the angular difference Δθ reaches the switching threshold Δθ_thr, increasing the degree of decrease. This increases the reliability of making the driver aware that the timing for switching the steering control from the second control to the first control is approaching. This reduces the discomfort felt by the driver due to torque fluctuations when the steering control is switched from the second control to the first control.

[0104] In the third modified example, the first angle difference |Δθ_A| at which the absolute value of the steering assist torque F_as reaches its maximum value (peak value) may be a value that exceeds 50% (|Δθ_thr×0.5|) of the switching threshold value |Δθ_thr|. This allows the driver to recognize the timing of switching the steering control from the second control to the first control through the steering wheel 31 when the timing of switching the steering control from the second control to the first control approaches.

[0105] 16, the first angle difference |Δθ_A| at which the absolute value of the steering assist torque F_as reaches its maximum value (peak value) may be a value less than 50% (|Δθ_thr×0.5|) of the switching threshold value |Δθ_thr|. This causes the steering assist torque F_as to increase and then gradually decrease, approaching the timing at which the steering control is switched from the second control to the first control, allowing the driver to recognize through the steering wheel 31 that the timing at which the steering control is switched is approaching.

[0106] (Fourth variant) In the assist torque map (assist torque information), in the region where the actual steering angle θs is smaller than the target steering angle θs_tgt and the angle difference Δθ is between zero and the switching threshold value Δθ_thr, a steering assist torque F_as acting in the direction of returning the actual steering angle θs to the target steering angle θs_tgt may be applied, and then a steering assist torque F_as acting in the direction of returning the actual steering angle θs to the neutral position may be applied.

[0107] FIG. 17 shows an example of an assist torque map according to a fourth modification. In the assist torque map shown in FIG. 17 , in the region where the angular difference Δθ is between zero and the switching threshold Δθ_thr, and in the region up to the second angular difference Δθ_B, a steering assist torque F_as is set in a first direction that returns the actual steering angle θs to the target steering angle θs_tgt. Furthermore, in the region where the angular difference Δθ is between the second angular difference Δθ_B and the switching threshold Δθ_thr, a steering assist torque F_as is set in a second direction opposite to the first direction that returns the actual steering angle θs to the neutral position. This further increases the reliability with which the driver is made aware that the timing for switching the steering control from the second control to the first control is approaching. This reduces the sense of discomfort experienced by the driver due to torque fluctuations when the steering control is switched from the second control to the first control.

[0108] (Fifth Modification) In the above embodiment or any of the first to fifth modifications, the second control unit 73 may, at any position on the way that the angular difference Δθ approaches the switching threshold value Δθ_tgt, amplify the steering assist torque F_as to apply a slight vibration to the steering wheel 31. This increases the reliability with which the driver is made aware through the steering wheel 31 that the timing for switching the steering control is approaching, and can reduce the discomfort felt by the driver due to torque fluctuations when the steering control is switched.

[0109] (Sixth Variant) In the above embodiment, the steering assist torque F_as applied to the steering wheel 31 while the second control is being executed was set based on the angle difference Δθ obtained by subtracting the target steering angle θs_tgt from the actual steering angle θs_act regardless of the value of the target steering angle θs_tgt, but the steering assist torque F_as may also be changed depending on the frictional state of the road surface.

[0110] The self-aligning torque F_al that the front wheels 3LF, 3RF receive from the road surface varies depending on the friction coefficient of the road surface. Therefore, if the steering assist torque F_as is not changed in accordance with the friction coefficient of the road surface, in a situation where the steering control is switched from the second control to the first control while the actual steering angle θs_act is larger than the target steering angle θs_tgt, the steering angle θs at which the steering control is switched may vary significantly depending on the friction coefficient of the road surface, even if the target steering angle θs_tgt is the same.

[0111] Fig. 18 is an explanatory diagram showing an example in which the coefficient of friction of the road surface is smaller than the example shown in Fig. 8. When the coefficient of friction of the road surface is relatively small, the reaction torque F_co set in accordance with the self-aligning torque is smaller than the reaction torque F_co' when the coefficient of friction of the road surface is relatively large. For this reason, if the setting conditions for the steering assist torque F_as are the same, the steering angle θs_c at which the difference ΔF between the steering assist torque F_as and the reaction torque F_co is equal to or smaller than the predetermined torque difference threshold ΔF_thr will be smaller than the steering angle θs_c' at which the difference ΔF between the steering assist torque F_as and the reaction torque F_co' is equal to or smaller than the predetermined torque difference threshold ΔF_thr.

[0112] Therefore, the second control unit 73 may correct the steering assist torque F_as, which is generated at least when the actual steering angle θs_act is larger than the target steering angle θs_tgt, so that the smaller the friction coefficient of the road surface, the smaller the value of the steering assist torque F_as. This makes it possible for the second control unit 73 to prevent a large difference in the timing at which the steering control is switched from the second control to the first control at least when the actual steering angle θs_act is larger than the target steering angle θs_tgt.

[0113] For example, the second control unit 73 corrects the steering assist torque F_as calculated based on an assist torque map in which a reference steering assist torque F_as is set, with a correction coefficient corresponding to the friction coefficient of the road surface. Alternatively, a plurality of assist torque maps set according to the friction coefficient of the road surface may be stored in advance in the storage unit 63, and the second control unit 73 may change the assist torque map to be used according to the friction coefficient of the road surface to set the steering assist torque F_as.

[0114] Although the preferred embodiments of the present disclosure have been described in detail above with reference to the accompanying drawings, the present disclosure is not limited to such examples. It is clear that a person skilled in the art to which the present disclosure pertains can conceive of various modifications or alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present disclosure.

[0115] For example, in the above embodiment, an example was described in which the target steering angle was a target value of the steering angle, but the target steering angle may also be a target value of the turning angle. In this case, as already mentioned, since the steering angle and the turning angle can be converted into each other using a predetermined coefficient, the same effect can be obtained by implementing the angle difference between the actual steering angle and the target steering angle in the above embodiment as the angle difference between the actual turning angle and the target turning angle.

[0116] In addition, the technology of the present disclosure can also be realized as a vehicle equipped with the steering control device described in the above embodiment, a steering control method using the steering control device, a computer program that causes a computer to function as the above steering control device, and a non-temporary tangible recording medium on which the computer program is recorded.

[0117] 1: Vehicle 3LF / 3RF: Front wheels 9L: Tire force sensor 9R: Tire force sensor 10: Steering system 17: Driving assistance device 30: Electric steering system 31: Steering wheel 33: Steering device 34: Reaction motor 35: Steering angle sensor 36: Steering torque sensor 40: Wheel steering device 57: Steering motor 58: Steering angle sensor 60: Steering control device 61: Processing unit 63: Memory unit 71: First control unit 73: Second control unit 75: Switching control unit

Claims

1. A control device for a steer-by-wire type steering system in which a steering wheel operated by a driver of a vehicle and a wheel steering device for steering the wheels are provided mechanically separated from each other and the steering angle of the wheels is controlled based on the steering angle of the steering wheel, the control device executes: a first control for controlling the steering angle based on the steering angle of the steering wheel operated by the driver, and applying to the steering wheel a reaction torque against the operation of the steering wheel in accordance with a self-aligning torque that the wheels receive from the road surface; a second control for controlling the steering angle and the steering angle based on a target steering angle by a predetermined driving assistance control, and applying to the steering wheel a steering assist torque in accordance with an angular difference between the target steering angle and the steering angle or the steering angle; and a switching control for releasing the second control and switching to the first control as the angular difference increases while the second control is being executed, A control device for a steering system, which switches from the second control to the first control when the absolute value of the steering assist torque is equal to or less than a predetermined reference value for suppressing a preset torque fluctuation when the angle difference increases while the steering angle or the turning angle is smaller than the target steering angle.

2. A control device for a steering system as described in claim 1, wherein in the second control, the absolute value of the steering assist torque generated when the steering angle or the turning angle is smaller than the target steering angle is set to be equal to or less than the predetermined reference value.

3. A control device for a steering system as described in claim 2, wherein in the second control, the absolute value of the steering assist torque generated when the steering angle or the turning angle is smaller than the target steering angle is set to a constant value that is less than the predetermined reference value and exceeds zero.

4. A control device for a steering system as described in claim 2, wherein in the second control, the absolute value of the steering assist torque generated when the steering angle or the turning angle is smaller than the target steering angle is set to be equal to or less than the predetermined reference value and to gradually increase as the angle difference increases.

5. The control device of a steering system as described in claim 1, wherein, in the second control, the control device sets the steering assist torque based on assist torque information in which the steering assist torque is set according to the angular difference, and in the switching control, when the steering angle or the turning angle is smaller than the target steering angle and the angular difference exceeds a predetermined switching threshold, the second control is released and switched to the first control, and in the assist torque information, the steering assist torque is zero when the angular difference is zero, and the absolute value of the steering assist torque when the angular difference is the predetermined switching threshold is less than or equal to the predetermined reference value.

6. A control device for a steering system as described in claim 5, wherein the steering angle or the turning angle in the assist torque information is smaller than the target steering angle, and the absolute value of the steering assist torque in at least a portion of the region between zero and the predetermined switching threshold value where the angle difference is greater than the predetermined reference value.

7. A control device for a steering system as described in claim 5, wherein the assist torque information is such that the absolute value of the steering assist torque becomes maximum at a predetermined first angle difference where the steering angle or the turning angle is smaller than the target steering angle and the angle difference is included in a region between zero and the predetermined switching threshold, and the first angle difference is set to a value exceeding 50% of the predetermined switching threshold.

8. A control device for a steering system as described in claim 5, wherein, in the assist torque information, when the steering angle or the turning angle is smaller than the target steering angle and the angle difference is in a region between zero and a predetermined second angle difference among the region between zero and the predetermined switching threshold, the steering assist torque is set in a first direction that returns the steering angle to a steering angle corresponding to the target steering angle, and when the angle difference is in a region between the predetermined second angle difference and the predetermined switching threshold, the steering assist torque is set in a second direction opposite to the first direction.

9. A control method for a steer-by-wire type steering system in which a steering wheel operated by a driver of a vehicle and a wheel steering device for steering the wheels are provided mechanically separated and the steering angle of the wheels is controlled based on the steering angle of the steering wheel, wherein the control device executes a first control for controlling the steering angle based on the steering angle of the steering wheel operated by the driver and applying to the steering wheel a reaction torque against the operation of the steering wheel in accordance with a self-aligning torque that the wheels receive from a road surface, a second control for controlling the steering angle and the steering angle based on a target steering angle by a predetermined driving assistance control and applying to the steering wheel a steering assist torque in accordance with an angular difference between the target steering angle and the steering angle or the steering angle, and a switching control for releasing the second control and switching to the first control as the angular difference increases during the execution of the second control, A control method for a steering system, in which, when the angle difference increases while the steering angle or the turning angle is smaller than the target steering angle, the second control is switched to the first control while the absolute value of the steering assist torque is equal to or smaller than a predetermined reference value for suppressing a preset torque fluctuation.

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