WO2025069169A1 - Steering system control device and control method - Google Patents

Abstract

This steer-by-wire type steering system control device executes: first control in which a turning angle is controlled on the basis of a steering angle of a steering wheel manipulated by a driver and in which reaction torque to the manipulation of the steering wheel is imparted to the steering wheel in accordance with self-aligning torque received by a wheel 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 derived from prescribed driving assistance control and steering assist torque corresponding to the angle difference between the target steering angle and the steering angle or the turning angle is imparted to the steering wheel; and switching control in which, during execution of the second control, the second control is released and switched to the first control as the angle difference increases. If the angle difference increases in a state in which the steering angle or the turning angle is greater than the target steering angle, control is switched from the second control to the first control in a state in which the difference between the steering assist torque and the reaction torque before and after the switching from the second control to the first control is no greater than a prescribed torque difference threshold value.

Description

Steering system control device and control method

This disclosure relates to a control device and control method for a steering system.

In recent years, automatic steering control has been put into practical use in driving assistance controls such as lane keeping control and automatic driving controls that allow vehicles to drive autonomously, in which the road shape and obstacles around the vehicle are recognized to set a target driving trajectory, and the wheels are automatically steered so that the vehicle drives along the target driving trajectory. In such driving assistance controls and automatic driving controls, there is known technology that is configured to accept manual steering operations by the driver even during automatic steering control, so that the driver can correct the driving trajectory, for example to avoid sudden obstacles.

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 the driving assistance target steering angle is calculated as the final target steering angle until the magnitude of the steering torque accordingly exceeds an intervention threshold, 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.

Patent Document 2 also 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 the self-aligning torque acting on the steered wheels when the vehicle is turning.

JP 2022-111970 A JP 2016-107750 A

Here, in the steering system described in Patent Document 1, until the final target steering angle switches from the driving assistance target steering angle to the driver target steering angle, a steering assist torque that causes the wheel steering angle to follow the driving assistance target steering angle is applied to the steering wheel. On the other hand, after the final target steering angle switches to the driver 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.

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. For this reason, when the torque applied switches from steering assist torque to reaction torque, the reaction force received by the driver changes suddenly, and there is a risk that the vehicle's behavior will become unstable.

This disclosure has been made in consideration of the above problems, and the purpose of this disclosure is to provide a steering system control device and control method that can suppress the change in the reaction force that the driver receives through the steering wheel when the torque applied to the steering wheel switches from steering assist torque to reaction torque.

In order to solve the above problem, according to one aspect of the present disclosure, in a control device for a steer-by-wire type steering system, 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 control device controls the steering angle of the wheels based on the steering angle of the steering wheel. The control device controls the steering angle based on the steering angle of the steering wheel operated by the driver, and performs a first control for applying a reaction torque to the steering wheel in response to a self-aligning torque that the wheels receive from the road surface, and a second control for applying a reaction torque to the steering wheel in response to a target steering angle based on a target steering angle by a predetermined driving assistance control. A control device is provided that executes a second control that controls the steering angle and the turning angle based on the target steering angle 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 turning angle, and a switching control that releases 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 in a state in which the steering angle or the turning angle is larger than the target steering angle, the control device switches from the second control to the first control in a state in which the difference between the steering assist torque and the reaction torque before and after switching from the second control to the first control is equal to or less than a predetermined torque difference threshold value for suppressing a preset torque fluctuation.

Furthermore, in solving the above problem, according to another aspect of the present disclosure, in 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 from each other and the steering angle of the wheels is controlled based on the steering angle of the steering wheel, the control device controls the steering angle based on the steering angle of the steering wheel operated by the driver, and performs a first control for applying a reaction torque to the steering wheel against the operation of the steering wheel in accordance with a self-aligning torque that the wheels receive from the road surface, and a second control for applying a reaction torque to the steering wheel in accordance with a target steering angle based on a target steering angle by a predetermined driving assistance control. A control method for a steering system is provided that executes a second control that controls the steering angle and the turning angle 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 turning angle, and a switching control that releases 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 turning angle is larger than the target steering angle, the second control is switched to the first control in a state in which the difference between the steering assist torque and the reaction torque before and after switching from the second control to the first control is equal to or less than a predetermined torque difference threshold value for suppressing a preset torque fluctuation.

As described above, according to the present disclosure, it is possible to suppress the change in the reaction force received by the driver through the steering wheel when the torque applied to the steering wheel switches from steering assist torque to reaction torque.

1 is a schematic diagram illustrating a configuration example of a vehicle equipped with a steering system according to an embodiment of the present disclosure. FIG. 2 is an explanatory diagram showing a configuration example of a steering system according to the embodiment; FIG. 2 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. 4 is an explanatory diagram showing an example of an assist torque map (assist torque information) used in the embodiment. FIG. 11 is an explanatory diagram showing the relationship between the angle difference and the steering assist torque in a reference example. FIG. 11 is an explanatory diagram showing a steering assist torque based on a target steering angle in a reference example, superimposed on an aligning torque map. 4 is an explanatory diagram showing a steering assist torque based on a target steering angle according to the embodiment superimposed on an aligning torque map; FIG. 5 is a flowchart showing a processing operation of a control device of the steering system according to the embodiment. 5 is a flowchart showing a processing operation of a first control by the control device of the steering system according to the embodiment; 5 is a flowchart showing a processing operation of a second control by the control device of the steering system according to the embodiment; 5 is a flowchart showing a processing operation of switching control by the control device of the steering system according to the embodiment; 10 is an explanatory diagram showing an example in which the coefficient of friction of a road surface is small according to the embodiment; FIG.

Below, a preferred embodiment of the present disclosure will be described in detail with reference to the attached drawings. Note that in this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals to avoid redundant description.

<1. Overall configuration of the 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.

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 driving torque output from a driving force source 13 that generates the vehicle's driving torque 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 diesel engine as the driving force source 13, an electric vehicle equipped with a driving motor as the driving force source 13, or a hybrid electric vehicle equipped with both an internal combustion engine and a driving motor as the driving force source 13.

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 corresponding to each wheel. Furthermore, if vehicle 1 is an electric vehicle or hybrid electric vehicle, vehicle 1 is equipped with a secondary battery that stores power supplied to the drive motor, and a generator such as a motor or fuel cell that generates power to charge the battery.

The vehicle 1 is equipped with 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 the 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 devices (ECU: Electronic Control Unit). 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 the target driving torque for automatic driving.

The electric steering system 30 includes an electric motor and a gear mechanism (not shown), and is controlled by a steering control device 60 to adjust the steering angle of the left and right front wheels 3LF, 3RF. 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. During driving assistance control or automatic driving control, the steering control device 60 also controls the steering angle of the front wheels 3LF, 3RF based on a target steering angle that is set according to the target driving trajectory.

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

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, and speeds of obstacles such as other vehicles and pedestrians, road shapes, and travel lane boundaries, and outputs the detection result information to the driving assistance device 17. The surrounding environment sensor 15 includes one or more sensors selected from the group consisting of a camera, LiDAR, radar sensor, and ultrasonic sensor. In FIG. 1, the vehicle 1 includes a pair of left and right stereo cameras 15L, 15R that capture images of the area ahead of the vehicle 1 as the surrounding environment sensor 15.

The driving assistance device 17 uses information on obstacles and other detection results detected by the surrounding environment sensor 15 to execute driving assistance control or automatic driving control of the vehicle 1. 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 the driving lane. Examples of such assistance functions include lane keeping control or automatic driving control. For example, the driving assistance device 17 sets a target driving trajectory for the vehicle 1 to drive, 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.

The vehicle 1 also has 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 load applied to each of the left and right front wheels 3LF, 3RF. For example, the tire force sensors 9L, 9R may be six-component force detectors that detect the load in each of the three axial directions of the vehicle 1, namely the longitudinal direction (x-axis), the width direction (y-axis), and the height direction (z-axis), and the moment generated around each of the three axes.

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

FIG. 2 is a schematic diagram showing an example of the configuration of an 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 the driver and a wheel steering device 40 that steers the wheels are provided mechanically separated, and the steering angle of the front wheels 3LF, 3RF is controlled based on the detected steering angle (rotation angle) of the steering wheel 31.

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 input 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.

The steering device 33 is configured with a steering wheel 31, a steering shaft 32 connected to the steering wheel 31, and a reaction motor 34 coaxially assembled at the axial center 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 through the steering shaft 32 and the steering wheel 31. The drive of the reaction motor 34 is controlled by the 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.

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

The rack shaft 51 is supported by the housing 49 so that it can move freely in the left-right direction of the vehicle 1. A rack gear 54 is provided on the rack shaft 51. A pinion gear 55 meshes with the rack gear 54. A pinion shaft 56 is connected to the pinion gear 55. A steering motor 57 is coaxially mounted at the axial center position of the pinion shaft 56.

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 the 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 be steered.

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 turning motor 57, directly connecting the steering shaft 32 and the pinion shaft 56 and enabling the driver to turn the front wheels 3LF, 3RF.

The steering control device 60 receives sensor signals output from the steering angle sensor 35, steering torque sensor 36, turning angle sensor 58, and tire force sensors 9L, 9R. 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 input torque 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.

Instead of the tire force sensors 9L, 9R, for example, a torque sensor may be provided coaxially on the pinion shaft 56 to detect the self-aligning torque.

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

<3. Steering control device>
The steering control device 60 functions as a device that executes processing to control the electric steering system 30 by executing a computer program by a processor such as one or more CPUs (Central Processing Units). The computer program is a computer program for causing the processor to execute operations to be performed by the steering control device 60, which will be described later. 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.

Recording media for recording computer programs may 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); magnetic optical media such as floptical disks; memory elements such as RAMs (Random Access Memory) and ROMs (Read Only Memory); flash memories such as USB (Universal Serial Bus) memories and SSDs (Solid State Drives); and other media capable of storing programs.

FIG. 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 the whole of the processing unit 61 may be configured with an updatable component such as firmware, or may be a program module executed by a command from a CPU or the like. The steering control device 60 may be configured as a single device, or may be configured by connecting multiple devices so that they can communicate with each other.

The storage unit 63 is composed of one or more storage elements (memories), such as RAM or ROM, communicatively 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.

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 motor 34, and turning motor 57 via a dedicated line or communication means such as CAN (Controller Area Network) or LIN (Local Inter Net).

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 the execution of a computer program by one or more processors. Note that a portion 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.

Below, the function of each part of the processing unit 61 will be briefly explained, and the specific processing operations of the processing unit 61 will be explained in detail later.

(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 against the operation of the steering wheel 31 in accordance with the self-aligning torque that the left and right front wheels 3LF, 3RF receive from the road surface.

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

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. Furthermore, the reaction torque is a restoring force that acts to return the steering wheel 31 to a neutral position (a position where the vehicle 1 is in a straight-ahead state) in response to the self-aligning torque acting on the front wheels 3LF, 3RF.

The self-aligning torque and the reaction torque can be converted into each other using a predetermined coefficient that has been set in advance, 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 instruction value for the reaction motor 34 that can output the set reaction torque, and controls the drive of the reaction motor 34.

Figure 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.

As shown in Figure 4, the larger the steering angle θs is in 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.

(Second control unit)
The second control unit 73 executes a second control that controls the steering angle and the turning angle based on the target steering angle by the driving assistance control of the driving assistance device 17, and applies a steering assist torque to the steering wheel 31 according to the angle 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 mutually convertible, and the target steering angle may be either the target value of the steering angle or the target value of the turning angle, but in this embodiment, an example will be described in which the target steering angle is the target value of the steering angle (hereinafter referred to as the "target steering angle"). Therefore, in the following description, the angle difference between the target steering angle and the steering angle indicates a value obtained by subtracting the absolute value of the target steering angle from the absolute value of the steering angle.

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

The second control unit 73 also 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 the value of the steering assist torque for the angular difference is preset. The second control unit 73 sets a drive instruction value for the reaction motor 34 capable of outputting the set steering assist torque, and controls the drive of the reaction motor 34.

Figure 5 shows an assist torque map in which the 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 the 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. Also, the steering assist torque F_as indicates a force pushing in the direction of reducing the steering angle θs as a positive value and a force pushing in the direction of increasing the steering angle θs as a negative value.

In this embodiment, when the steering angle θs is larger than the target steering angle θs_tgt, the greater the angle difference Δθ between the steering angle θs and the target steering angle θs_tgt, the greater 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. 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 assistance control to the first control based on the driver's steering.

In addition, 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 the steering assist torque F_as is set to zero at the switching threshold value -Δθ_thr at which the steering control is switched from the second control to the first control.

How to set the assist torque map will be explained in detail later.

(Switching control unit)
The switching control unit 75 executes switching control to release the second control and switch the steering control to the first control as the angle 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 vehicle 1 is being automatically steered by the driving assistance function so as to drive 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.

In this embodiment, when the angle difference Δθ between the target steering angle θs_tgt and the actual steering angle θs_act increases when the actual steering angle θs_act is greater 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 is possible 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.

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 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, for example, or a value of 30% or less of the reaction torque F_co, and more preferably set to 0.

In addition, in this embodiment, when the angle difference Δθ between the target steering angle θs_tgt and the actual steering angle θs_act increases when 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 when 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, so that the torque fluctuation when the steering control is switched from the second control to the first control becomes large, which may cause the driver to feel uncomfortable and the behavior of the vehicle 1 to become unstable. 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 a predetermined reference value F_thr or less to suppress the torque fluctuation when the steering control is switched from the second control to the first control.

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 the 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 the aligning torque map.

As shown in FIG. 6, in the reference example, the steering assist torque F_as is set so that its absolute value gradually increases as the angle difference Δθ increases, 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.

As shown in FIG. 7, in the reference example, when the steering angle θs is greater than the target steering angle θs_tgt and the angle 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 occurs, 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 greatly before and after the switching of the steering control from the second control to the first control. For this reason, the steering angle θs of the steering wheel 31 changes abruptly, which may cause the behavior of the vehicle 1 to become unstable.

In addition, in the reference example, when the steering angle θs is smaller than the target steering angle θs_tgt and the angle difference Δθ between the steering angle θs and the target steering angle θs_tgt exceeds the steering angle θs_l at which the switching threshold Δθ_thr occurs, the steering control is switched from the second control to the first control. At this 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. For this reason, the steering angle θs of the steering wheel 31 changes suddenly, which may cause the driver to feel uncomfortable and the behavior of the vehicle 1 to become unstable.

FIG. 8 is an explanatory diagram of this embodiment that shows 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.

As shown in FIG. 8, in this embodiment, when the steering angle θs is greater than the target steering angle θs_tgt, the switchover 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 switchover of steering control from the second control to the first control.

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.

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 the 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 between the steering angle θs and the target steering angle θs_tgt. At this time, as the angular difference Δθ increases, the absolute value of the steering assist torque F_as for making the steering angle θs 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. In the example shown in FIG. 8, the steering assist torque F_as is set to zero at the steering angle θs_l at which the steering control is switched from the second control to the first control. Therefore, an assist force is applied to make the steering angle θs follow the target steering angle θs_tgt, while reducing the discomfort felt by the driver due to the torque direction being reversed before and after switching the steering control from the second control to the first control.

In addition, in Figures 7 and 8, the steering angle θs when the rotation direction of the steering wheel 31 is a right turning direction is shown as a positive value, the torque pushing the steering wheel 31 in the direction returning it 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 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 a left turning direction is shown as a positive value, the torque pushing the steering wheel 31 in the direction returning it 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.

3. Processing Operation of Steering Control Device
So far, an example of the 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. In the following description, the right turning direction and the left turning direction are each understood to be positive values with respect to the neutral position (steering angle θs=0) of the steering wheel 31 as the center. Therefore, the generated torque or applied torque is also understood to be all positive values.

FIG. 9 is a flowchart showing 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 a signal indicating that the driving assistance function is operating is received from the driving assistance device 17. The processing unit 61 may determine whether or not the driving assistance function is operating based on an on/off signal of an operating switch of 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.

If the processing unit 61 does not determine that the driving assistance function is operating (S13/No), the first control unit 71 executes the first control (step S15).

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

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

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). Next, the first control unit 71 multiplies the self-aligning torque F_al by a predetermined coefficient to calculate the reaction torque F_co (step S39).

Then, 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).

In this manner, 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 as a result of the driver's operation. In addition, the first control unit 71 imparts 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.

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

FIG. 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 support 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).

Then, 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.

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 motor 34 to apply the steering assist torque F_as to the steering wheel 31 (step S59).

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

In this manner, 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 to make the actual steering angle θs follow 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 according to the target steering angle θs_tgt set by the driving support device 17, but when the driver intervenes in the operation of the steering wheel 31, the vehicle 1 is steered to reflect the driver's intention.

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

FIG. 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 or not 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 support device 17 (step S71).

If the switching control unit 75 does not determine that the actual steering angle θs_act is greater than the target steering angle θs_tgt (S71/No), it 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). If the switching control unit 75 does not determine that the angle difference Δθ exceeds the predetermined switching threshold Δθ_tgt (S81/No), it ends the switching control as is and continues the second control.

On the other hand, if 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 driving trajectory in a direction to turn the steering wheel 31 back relative to the target steering angle θs_tgt, so the switching control unit 75 overrides the steering control based on the driving assistance function to steering control based on the driver's steering operation.

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 angle difference Δθ matches the switching threshold Δθ_tgt, the steering assist torque F_as is set to zero or a very small value that is less than a predetermined reference value. 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.

If, in step S71 above, the switching control unit 75 determines that the actual steering angle θs_act is greater than the target steering angle θs_tgt (S71/Yes), 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 above (step S73). Next, the switching control unit 75 multiplies the self-aligning torque F_al by a predetermined coefficient to calculate the reaction torque F_co, as in step S39 above (step S75).

Then, 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 that has been set in advance (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.

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 less than the predetermined torque difference threshold ΔF_thr (S75/No), it ends the switching control and continues the second control.

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 less 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 driving 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 steering control based on the driver's steering operation.

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.

Returning to FIG. 9, while the first control or the second control is continuing, 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 or not 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.

<4. Effects>
As described above, in the case where 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 larger than the target steering angle θs_tgt, the control device of the steering system according to the present embodiment switches from the second control to the first control in a state where 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 set in advance. Therefore, a sudden change in the torque applied to the steering wheel 31 due to switching 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. Therefore, it is possible to suppress the behavior of the vehicle 1 from becoming unstable due to switching the steering control.

The control device of the steering system according to this embodiment may also switch 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 zero. In this case, the fluctuation in the torque applied to the steering wheel 31 due to switching the steering control from the second control to the first control becomes extremely small, and the change in the steering angle of the steering wheel 31 can be further suppressed.

5. Modifications
Although the control device for the steering system according to the present embodiment has been described above, various modifications of the above embodiment are possible. Modifications of the control device for the steering system according to the present embodiment will be described below.

In the above embodiment, the steering assist torque F_as applied to the steering wheel 31 while the second control is being executed is 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. However, the steering assist torque F_as may be changed depending on the frictional state of the road surface.

The self-aligning torque F_al that the front wheels 3LF, 3RF receive from the road surface changes depending on the friction coefficient of the road surface. For this reason, if the steering assist torque F_as is not changed according to the friction coefficient of the road surface, in a situation where the steering control is switched from the second control to the first control when 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 differ significantly depending on the friction coefficient of the road surface, even if the target steering angle θs_tgt is the same.

FIG. 13 is an explanatory diagram showing an example in which the friction coefficient of the road surface is smaller than the example shown in FIG. 8. When the friction coefficient of the road surface is relatively small, the reaction torque F_co set according to the self-aligning torque is smaller than the reaction torque F_co' when the friction coefficient of the road surface is relatively large. For this reason, if the setting conditions of 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 a predetermined torque difference threshold ΔF_thr is 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.

For this reason, the second control unit 73 may correct the steering assist torque F_as generated when at least the actual steering angle θs_act is greater than the target steering angle θs_tgt so that the smaller the friction coefficient of the road surface, the smaller the value. This allows 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 greater than the target steering angle θs_tgt.

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 according to the friction coefficient of the road surface. Alternatively, multiple 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 and set the steering assist torque F_as.

　A preferred embodiment of the present disclosure has been described in detail above with reference to the attached drawings, but the present disclosure is not limited to such examples. It is clear that a person with ordinary knowledge in the technical field to which the present disclosure pertains can conceive of various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present disclosure.

For example, in the above embodiment, an example was described in which the target steering angle was the target value of the steering angle, but the target steering angle may also be the 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.

The technology disclosed herein 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-transitory tangible recording medium on which the computer program is recorded.

1: Vehicles 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 turning device 57: Turning motor 58: Turning 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 (5)

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 separately from each other and the steering angle of the wheels is controlled based on the steering angle of the steering wheel,
The control device includes:
a first control for 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;
a second control for controlling the steering angle and the turning angle based on a target steering angle by a predetermined driving assist control, and applying a steering assist torque to the steering wheel according to an angle difference between the target steering angle and the steering angle or the turning angle;
a switching control for canceling the second control and switching to the first control as the angle difference increases during the execution of the second control;
A control device for a steering system, which switches from the second control to the first control in a state where a difference between the steering assist torque and the reaction torque before and after switching from the second control to the first control is equal to or less than a predetermined torque difference threshold for suppressing a preset torque fluctuation when the angle difference increases when the steering angle or the turning angle is larger than the target steering angle. The control device includes:
In the second control, the steering assist torque is controlled based on assist torque information in which the steering assist torque according to the angle difference is set;
correcting the steering assist torque based on a state of a road surface on which the vehicle is traveling;
The steering system control device according to claim 1 . The control device includes:
In the second control, the steering assist torque is controlled based on assist torque information in which the steering assist torque according to the angle difference is set;
The steering system control device according to claim 1 , wherein the assist torque information to be referred to is changed based on a state of a road surface on which the vehicle is traveling. The control device includes:
The steering system control device according to claim 1 , wherein the second control is switched to the first control when a difference between the steering assist torque and the reaction torque before and after switching from the second control to the first control is zero. 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 wheels are provided separately from each other, and a steering angle of the wheels is controlled based on a steering angle of the steering wheel, comprising:
The control device includes:
a first control for 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;
a second control for controlling the steering angle and the turning angle based on a target steering angle by a predetermined driving assist control, and applying a steering assist torque to the steering wheel according to an angle difference between the target steering angle and the steering angle or the turning angle;
a switching control for canceling the second control and switching to the first control as the angle difference increases during the execution of the second control;
A control method for a steering system, in which, when the angle difference increases when the steering angle or the turning angle is larger than the target steering angle, the second control is switched to the first control in a state where a difference between the steering assist torque and the reaction torque before and after switching from the second control to the first control is equal to or less than a predetermined torque difference threshold for suppressing a preset torque fluctuation.

Images