Steering system

Abstract

The present invention provides a steering system with independent left and right steering, which has excellent steering handling stability. In a steer-by-wire type steering system including a pair of wheel steering devices respectively provided for left and right wheels, and a controller for controlling the pair of wheel steering devices, the controller is configured to perform steering stabilization control (S11) for suppressing changes in the steering angle ψ in a condition of being steered by an external force (S8: Yes, S9: Yes, S), which is a condition where the left and right wheels are steered or easily steered by forces acting on the respective wheels from the outside. Specifically, when the braking force (F B ) or the driving force (F D ) is relatively large in a vehicle straight-ahead traveling state, when determining the torque (Tq S ) to be generated by the steering motors of the respective wheel steering devices based on the motor rotation angle deviation (Δθ) and according to the PID feedback control rule, the derivative term gain (G D ) is increased.

Description

Technical Field

[0001] The present invention relates to a steering system mounted on a vehicle. Background Art

[0002] A force acts on a wheel from the outside (e.g., "road surface"). In a steering system mounted on a vehicle, it is desirable to take measures against this force (hereinafter, there are cases where it is called "external acting force"). For example, in the technology described in the following patent documents, when emergency braking is applied, in order to assist the braking force, the steering amount (which can be considered synonymous with "caster angle") of the steered wheel is changed.

[0003] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2019-171905

[0004] In the case where a relatively large external acting force acts on the steered wheel, in the technology described in the above patent document, the steering amount (which can also be considered as "steering angle") of the wheel is positively changed. On the other hand, it is also necessary to consider that the steering amount of the wheel changes independently of the driver's intention due to such external input. In particular, in a system different from the steering system described in the above patent document, specifically, a system in which the left and right wheels are not mechanically connected and are independently steered by left and right actuators (hereinafter, there are cases where it is called "left-right independent steering type steering system"), the influence caused by the external acting force is large. Moreover, suppressing the change in the steering amount of the wheel caused by the external acting force is related to the stabilization of the steering operation. The present invention has been completed in view of such actual situations, and its problem is to provide a steering system with excellent steering operation stability. Summary of the Invention

[0005] In order to solve the above problems, the steering system of the present invention is a steer-by-wire type steering system, which includes: a pair of wheel steering devices respectively provided on the left and right wheels, each including an actuator to individually steer the left and right wheels; and a controller that controls the pair of wheel steering devices so that the left and right wheels are respectively steered to a steering amount corresponding to a request. The controller is configured to: in a situation where the left and right wheels are respectively steered or easily steered by a force acting on each wheel from the outside, that is, a situation of being steered according to an external force, perform steering stabilization control for suppressing the change in the steering amount.

[0006] In the steering system of the present invention, in the above situation of being steered according to an external force, the above steering stabilization control is performed, so the change in the steering amount of the wheel is suppressed. As a result, according to the present invention, a steering system with excellent steering operation stability is realized.

[0007] The "condition of steering according to an external force" in the present invention is, for example, a condition where a braking force or a driving force equal to or greater than a set magnitude is being applied or has been applied to the vehicle. As will be described in detail later, when an external force acts on a wheel and the wheel is a steered wheel, a moment about the kingpin axis is generated with respect to the wheel (hereinafter, there are cases where it is referred to as a "steering moment"). In emergency braking or the like, this steering moment is quite large.

[0008] The steering moments caused by the braking force and the driving force are in opposite directions in the left and right wheels. In a general steering system, that is, in a steering system where the left and right wheels are mechanically connected by a connecting member such as a rack and are steered together, the steering moments generated with respect to the left and right wheels due to the braking force and the driving force are eliminated by the connecting member, in other words, they are balanced with each other, so it can be considered that there is no influence. Since the present invention is applied to a left-right independent steering type steering system, its application significance is great.

[0009] In addition, as will be described in detail later, for example, when a crosswind or the like acts on the vehicle body or when the vehicle is traveling on a wavy road surface, the above-mentioned steering moment is also generated. Therefore, these cases can also be regarded as the condition of steering according to an external force.

[0010] When the vehicle is turning, the wheels are steered, and of course, an external force acts on the wheels. Therefore, it is preferably considered that the condition of steering according to an external force for performing steering stabilization control is a condition other than normal vehicle turning, etc., that is, a condition not desired by the driver. In addition, in view of the fact that an external force is generated when the vehicle is normally turning, the influence of steering the wheels by the external force becomes particularly problematic when the vehicle is moving straight ahead. Therefore, it is preferable to perform the steering stabilization control on the condition that the vehicle is moving straight ahead.

[0011] The steering system of the present invention is a steer-by-wire type steering system. For example, the controller controls the left and right wheel steering devices so that the respective steering positions of the left and right wheels (for example, concepts that can be considered the same as the "steering angle with respect to the intermediate position" and the "steering amount with respect to the intermediate position") become steering positions corresponding to the request. In such a steering system, the "steering stabilization control" for suppressing changes in the steering amount can, for example, become a control that does not easily change the steering position of the wheels from the position corresponding to the steering amount corresponding to the request.

[0012] If described in more detail, the controller may perform the following control as normal steering control, that is, determine a target steering amount as the steering amount to be the target according to a request, based on a steering amount deviation which is the deviation of the actual steering amount with respect to the target steering amount, and according to the method of PID feedback control, determine a steering force as the force that the actuator should generate. As steering stabilization control, it is sufficient to perform control to increase the differential term gain in the method of the above PID feedback control. By increasing the differential term gain, the steering position of the wheel is less likely to change from the position corresponding to the steering amount (target steering amount) corresponding to the request. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 FIG. is a schematic diagram showing the overall structure of a vehicle equipped with a steering system according to an embodiment.

[0014] Figure 2 FIG. is a perspective view showing a wheel arrangement module including a wheel steering device constituting a steering system according to an embodiment.

[0015] Figure 3 is for Figure 2 FIG. is a schematic diagram for explaining the suspension geometry of the wheel arrangement module shown.

[0016] Figure 4 FIG. is an arithmetic expression showing the torque applied to the knuckle by an external force.

[0017] Figure 5 FIG. is a flowchart of a steering control program executed by the steering system according to an embodiment.

[0018] REFERENCE SIGNS LIST

[0019] 10... wheel; 12... wheel steering device; 14... operating device; 16... steering electronic control unit (steering ECU) [controller]; 18... operation electronic control unit (operation ECU) [controller]; 20... wheel drive unit; 22... accelerator pedal; 26... vehicle drive electronic control unit (drive ECU); 30... brake pedal; 32... master cylinder; 34... working fluid supply device; 36... braking device; 38... brake electronic control unit (brake ECU); 44... CAN; 50... wheel arrangement module; 52... lower arm; 54... shock absorber; 56... suspension spring; 66... disc rotor; 68... brake caliper; 70... steering actuator; 70a... steering motor; 72... tie rod; 78... motion conversion mechanism; 80... steering wheel [steering operation member]; 82... steering sensor; 84... reaction force imparting device; 84a... reaction force motor; 90... spring / absorber Assy. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] Hereinafter, as a mode for carrying out the present invention, a steering system as an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In addition, the present invention can be implemented in various modes in which various changes and improvements have been made based on the knowledge of those skilled in the art, starting with the mode described in the above [Mode of the Invention] section, in addition to the following embodiments.

[0021]

Embodiment

[0022] [A] Overall Structure of Vehicle Equipped with Steering System

[0023] As Figure 1 schematically shown, the steering system is mounted on a vehicle having left and right front wheels 10FL, 10FR and left and right rear wheels 10RL, 10RR. The left and right front wheels 10FL, 10FR are drive wheels and also steering wheels. In addition, when it is not necessary to distinguish between the left and right front wheels 10FL, 10FR, they are collectively referred to as front wheels 10F. When it is not necessary to distinguish between the left and right rear wheels 10RL, 10RR, they are collectively referred to as rear wheels 10R. When it is not necessary to distinguish between the front wheels 10F and the rear wheels 10R, they are simply collectively referred to as wheels 10.

[0024] This steering system is a so-called steer-by-wire type steering system and is a left and right independent steering type steering system that steers the two front wheels 10F independently of each other. This steering system is configured to include a pair of wheel steering devices 12 respectively provided for these front wheels 10F, an operation device 14 for receiving the operation of the driver, a pair of steering electronic control units (hereinafter, sometimes abbreviated as "steering ECU", and shown as "S-ECU" in the drawings) 16 for respectively controlling the pair of wheel steering devices 12, and an operation electronic control unit (hereinafter, sometimes abbreviated as "operation ECU", and shown as "O-ECU" in the drawings) 18 for controlling the operation device 14 and comprehensively controlling the steering ECU 16. The structure and control of this steering system will be described in detail later, but it can be considered that the controller of this steering system is composed of two steering ECUs 16 and an operation ECU 18.

[0025] In addition, this vehicle is equipped with a vehicle drive system having a pair of wheel drive units 20. The pair of wheel drive units 20 are respectively provided on two front wheels 10F and are rotated by being driven by electric motors. The vehicle drive system includes an accelerator pedal 22 as an accelerator operation member operated by the driver, an accelerator operation amount sensor 24 for detecting the operation amount of the accelerator pedal 22, and a vehicle drive electronic control unit (hereinafter, sometimes simply referred to as "drive ECU", and represented as "A-ECU" in the drawings) 26 that controls the operation of the pair of wheel drive units 20 based on the accelerator operation amount detected by the accelerator operation amount sensor 24. Since the vehicle drive system has a general structure and performs general control, the description of the structure and control of the vehicle drive system is omitted.

[0026] Furthermore, a hydraulic braking system is provided in this vehicle. The braking system includes a brake pedal 30 as a brake operation member operated by the driver, a master cylinder 32 connected to the brake pedal 30, a working fluid supply device 34 having a hydraulic source composed of a pump or the like for pressurizing the working fluid, four braking devices 36 respectively provided on the four wheels and for braking the four wheels by the pressure of the working fluid from the working fluid supply device 34, and a brake electronic control unit (hereinafter, sometimes referred to as "brake ECU", and represented as "B-ECU" in the drawings) 38 that controls the operation of the working fluid supply device 34. The braking system is a so-called by-wire braking system. The brake ECU 38 controls the pressure of the working fluid supplied from the working fluid supply device 34 to the braking devices 36 of each wheel 10 based on the brake operation amount, which is the operation amount of the brake pedal 30 detected by the brake operation amount sensor 40, thereby controlling the braking force applied to the vehicle. Since the braking system has a general structure and performs general control, the description of the structure and control of the braking system is omitted.

[0027] A CAN (Car Area Network or Controllable Area Network) 44 is provided in the vehicle, and two steering ECUs 16, an operation ECU 18, a drive ECU 26, and a brake ECU 38 are connected to the CAN 44. These ECUs 16, 18, 26, and 38 communicate with each other via the CAN 44 and perform the respective controls that they should carry out. Moreover, these ECUs 16, 18, 26, and 38 are each configured to include a computer having a CPU, a ROM, a RAM, etc., and a driver (drive circuit) for driving structural elements (e.g., an electric motor, a valve, a pump, etc.) based on the instructions of the computer. In addition, in the vehicle, a longitudinal acceleration sensor 46 for detecting the longitudinal acceleration, which is the acceleration in the longitudinal direction generated in the vehicle, is provided. Additionally, wheel speed sensors 48 for detecting their respective wheel rotation speeds (hereinafter, there are cases where it is referred to as "wheel speed") v W are provided for the rear wheels 10R respectively. These longitudinal acceleration sensors 46 and wheel speed sensors 48 are also connected to the CAN 44.

[0028] [B] Hardware Structure of the Steering System

[0029] A pair of wheel steering devices 12 constituting the steering system of this embodiment are respectively installed in the wheel mounting module 50. In the wheel mounting module 50, one of the pair of wheel drive units 20 of the above-described vehicle drive system and one of the four braking devices 36 of the braking system are also installed. As Figure 2 shown, the wheel mounting module (hereinafter, there are cases where it is abbreviated as "module") 50 is a module for mounting the wheel 10b with the tire 10a on the vehicle body. The wheel 10b itself can be regarded as a wheel, but in this embodiment, for convenience, the wheel 10b with the tire 10a installed is referred to as the wheel 10.

[0030] If the structure of the module 50 is described and the wheel steering device 12 of this steering system is described, the above-described wheel drive unit 20 disposed in this module 50 has a housing 20a, an electric motor as a drive source built in the housing 20a, and a speed reducer for reducing the rotation of the electric motor (both are omitted from the illustration), and a hub for mounting the wheel 10b (hidden and not visible in the figure). The wheel drive unit 20 is disposed inside the rim of the wheel 10b and is a drive unit called an in-wheel motor unit. The wheel drive unit 20 is a drive unit with a widely known structure, so the description of its structure is omitted.

[0031] This module 50 is configured to include a MacPherson strut suspension device (also known as a "MacPherson strut"). In this suspension device, the housing 20a of the wheel drive unit 20 serves as a bracket for holding the wheel in a rotatable manner. Further, the housing 20a functions as a knuckle in the wheel steering device 12, allowing vertical movement relative to the vehicle body. Therefore, the suspension device is configured to include a lower arm 52 serving as a suspension arm, the housing 20a of the wheel drive unit 20, a shock absorber 54, and a suspension spring 56.

[0032] The suspension device itself is a device with a general structure. Therefore, if simply explained, the lower arm 52 is an arm in the shape of a so-called L-arm. The base end portion is divided into two parts in the vehicle front-rear direction. At its base end portion, it is pivotally supported on the longitudinal beam of the vehicle body (not shown) via a first bushing 58 and a second bushing 60 so as to be rotatable about an arm rotation axis LL. The housing 20a of the wheel drive unit 20 is rotatably connected to the front end portion of the lower arm 52 via a ball joint 62 for arm connection serving as a first joint (hereinafter, there is a case where it is referred to as the "first joint 62").

[0033] Regarding the shock absorber 54, the lower end portion is fixedly supported on the housing 20a of the wheel drive unit 20, and the upper end portion is supported on the upper part of the tire housing of the vehicle body via an upper support member 64. The upper end portion of the suspension spring 56 is also supported on the upper part of the tire housing of the vehicle body via the upper support member 64, and the lower end portion of the suspension spring 56 is supported by a lower support member 54a formed in a flange shape on the shock absorber 54. That is, the suspension spring 56 and the shock absorber 54 are arranged side by side between the lower arm 52 and the vehicle body.

[0034] As described above, this module 50 has a braking device 36. This braking device 36 is a disc braking device, which is configured to include a disc rotor 66 mounted on the axle hub together with the wheel 10b to rotate with the wheel 10, and a brake caliper 68 held on the housing 20a of the wheel drive unit 20 so as to straddle the disc rotor 66. Although detailed description is omitted, this brake caliper 68 is configured to have a brake pad serving as a friction member and a hydraulic cylinder block. The braking device 36 presses the brake pad against the disc rotor 66 based on the pressure of the working fluid supplied from the working fluid supply device 34 to the hydraulic cylinder, thereby generating a braking force for stopping the rotation of the wheel 10.

[0035] The wheel steering device 12 is an independent steering device for steering only one of a pair of left and right wheels 10 independently of the other, and is generally configured to include a housing 20a of a wheel drive unit 20 that functions as a steering knuckle as described above (hereinafter, when disposed as a structural element of the wheel steering device 12, there is a case where it is referred to as "steering knuckle 20a"), a steering actuator 70 disposed on the lower arm 52 at a position near the base end of the lower arm 52, and a tie rod 72 that connects the steering actuator 70 and the steering knuckle 20a.

[0036] The steering actuator 70 is configured to include an electric motor, i.e., a steering motor 70a, as a drive source, a speed reducer 70b that reduces the rotation of the steering motor 70a, and an actuator arm 70c that rotates by the rotation of the steering motor 70a decelerated via the speed reducer 70b and functions as a steering pitman arm. The base end of the tie rod 72 is connected to the actuator arm 70c via a rod base end connecting ball joint 74 as a second joint (hereinafter, there is a case where it is referred to as "second joint 74"), and the front end of the tie rod 72 is connected to a steering arm 20b provided on the steering knuckle 20a via a rod front end ball joint 76 as a third joint (hereinafter, there is a case where it is referred to as "third joint 76").

[0037] In this wheel steering device 12, the line connecting the center of the upper support member 64 and the center of the first joint 62 becomes the kingpin axis KP. By operating the steering motor 70a, as indicated by the thick arrow in the drawing, the actuator arm 70c of the steering actuator 70 rotates about the actuator axis AL. This rotation is transmitted through the tie rod 72, causing the steering knuckle 20a to rotate about the kingpin axis KP. That is, as indicated by the thick arrow in the drawing, the wheel 10 is steered. According to such a configuration, in this wheel steering device 12, there is an operation conversion mechanism 78 that includes the actuator arm 70c, the tie rod 72, the steering arm 20b, etc., and converts the rotational motion of the steering motor 70a into the steering motion of the wheel 10.

[0038] The steering actuator 70 of the wheel steering device 12 is disposed on the lower arm 52. Therefore, the assembly operation of the module 50 to the vehicle body can be easily performed. To put it simply, by simply installing the base end of the lower arm 52 on the longitudinal beam of the vehicle body and installing the upper support member 64 on the upper part of the tire housing of the vehicle body, the suspension device, the braking device, and the wheel steering device can be mounted on the vehicle. That is, this module 50 is a module with excellent mountability to the vehicle.

[0039] The operation device 14 has a general configuration in a steer-by-wire type steering system. Briefly described, as Figure 1As shown in the figure, it is configured to include a steering wheel 80 as a steering operation component operated by a driver's steering operation, a steering sensor 82 for detecting the rotation angle of the steering wheel 80, that is, the steering operation angle, as the operation amount of the steering operation component from the straight-ahead state position (neutral position), and a reaction force applying device 84 for applying an operation reaction force to the steering wheel 80. The reaction force applying device 84 is configured to include an electric motor as a force source, that is, a reaction force motor 84a, and a speed reducer 84b for transmitting the force of the reaction force motor 84a to the steering wheel 80.

[0040] [C] Control of Steering System

[0041] i) Basic Steering Control

[0042] Briefly speaking, the steering control in this steering system is to control the two front wheels 10F to turn to the steering angles ψ corresponding to the steering requests respectively, which is executed by each steering ECU 16. The steering angle ψ is synonymous with the so-called camber angle, and can be considered as the phase displacement amount starting from the reference steering angle, that is, the steering amount, when the position that should be located in the straight-ahead state of the vehicle, that is, the straight-ahead state position (neutral position), is used as the reference steering angle. Hereinafter, the steering control executed by each steering ECU 16, specifically, the basic steering control as the basic steering control, will be described. In addition, since the two steering ECUs 16 execute the same steering control, hereinafter, the processing of the steering control in one steering ECU 16 will be described.

[0043] In addition, as the degree of the steering request (the degree of the steering operation), the operation ECU 18 obtains the operation angle of the steering wheel 80 detected based on the detection of the steering sensor 82, that is, the steering operation angle δ (hereinafter, there are cases where it is simply referred to as "operation angle δ"). The operation ECU 18 sends the information of the detected operation angle δ to each steering ECU 16 responsible for controlling each wheel steering device 12 via the CAN 44. It can be considered that the operation angle δ is the position change amount starting from the reference operation position, that is, the steering operation amount, when the straight-ahead state position (neutral position) as the position for the vehicle to move straight ahead is used as the reference operation position.

[0044] The steering ECU 16 determines a target steering angle ψ* of the wheel 10 to be achieved, which is the steering angle ψ, based on the received information about the operation angle δ and according to the set steering gear ratio. Each wheel steering device 12 does not have a steering angle sensor for directly detecting the steering angle ψ of the wheel 10. Therefore, in this steering system, taking advantage of the specific relationship between the steering angle ψ of the wheel 10 and the operating position of the steering motor 70a, the steering ECU 16 performs steering control based on the operating position of the steering motor 70a, i.e., the motor rotation angle θ. Thus, the steering ECU 16 determines a target motor rotation angle θ* of the motor rotation angle θ to be achieved based on the target steering angle ψ*. Moreover, it can be considered that the motor rotation angle θ is a displacement angle measured from the reference motor rotation angle, which is the reference operating position when the vehicle is moving straight forward.

[0045] The steering force generated by the steering actuator 70 and the steering torque Tq, which is the torque generated by the steering motor 70a S are in a specific relationship. Specifically, they are generally in a proportional relationship. Therefore, the steering ECU 16 determines the steering torque Tq that the steering motor 70a should generate based on the actual motor rotation angle θ, which is the actual motor rotation angle, and the target motor rotation angle θ*. S In addition, the steering motor 70a is a brushless DC motor and has a motor rotation angle sensor (such as a Hall IC, resolver, etc.) for phase switching in the current supply to itself. The steering ECU 16 detects the actual motor rotation angle θ based on the detection of this motor rotation angle sensor.

[0046] If the determination of the steering torque Tq S is explained in detail, the steering ECU 16 calculates the motor rotation angle deviation Δθ, which is the deviation of the motor rotation angle θ from the target motor rotation angle θ*, and based on this motor rotation angle deviation Δθ (= θ* - θ), determines the steering torque Tq according to the following formula S .

[0047] Tq S = G P ·Δθ + G D ·(dΔθ / dt) + G I ·∫Δθdt

[0048] The above formula is based on the PID feedback control rule based on the motor rotation angle deviation Δθ. The first term, the second term, and the third term are the proportional term, the differential term, and the integral term respectively. G P , G D , G I are the proportional gain, the differential gain, and the integral gain respectively.

[0049] The steering torque Tq Sis in a specific relationship with the steering current I that is the supply current to the steering motor 70a. Specifically, the steering torque Tq S depends on the force exerted by the steering motor 70a. Therefore, the steering torque Tq S is generally in a proportional relationship with the steering current I S . In this case, based on the determined steering torque Tq S , the steering ECU 16 determines the steering current I that is the supply current to the steering motor 70a S , and supplies this steering current I S to the steering motor 70a S .

[0050] ii) Reaction force control

[0051] Reaction force control is control for applying an operating reaction force, which is a reaction force relative to the steering operation by the driver, to the steering wheel 80, and is executed by the operation ECU 18. Reaction force control is general control, and the operating reaction force can be determined by various methods. In this steering system, the operation ECU 18 determines the reaction force torque Tq that the reaction force motor 84a of the reaction force applying device 84 should generate according to a typical method, namely the following formula C (which is a type of operating reaction force).

[0052] Tq C = Gδ·δ + G v ·v

[0053] Gδ and G v are weighting gains with respect to the operating angle δ and the vehicle traveling speed (hereinafter, sometimes referred to as "vehicle speed") v, respectively. Briefly, it is set such that the larger the steering operation amount and the higher the vehicle speed v, the larger the reaction force torque Tq applied C . Moreover, the operation ECU 18 determines the vehicle speed v based on the wheel speed v detected via the wheel speed sensor 48 W .

[0054] Based on the reaction force torque Tq determined as described above C , the operation ECU 18 determines the reaction force current I that is the supply current to the reaction force motor 84a C , and supplies this determined reaction force current I C to the reaction force motor 84a. In addition, the operation ECU 18 has a drive circuit (driver) for the reaction force motor 84a, and supplies the current I to the reaction force motor 84a via this drive circuit C .

[0055] The above basic steering control is for the control during driving when the driver operates the steering wheel 80. However, when the vehicle is an autonomous driving-capable vehicle and is actually in autonomous driving, the target steering angle ψ* is not determined based on the operation angle δ transmitted from the operation ECU 18. For example, the steering control can be performed based on the information of the target steering angle ψ* sent from the autonomous driving controller.

[0056] iii) Influence generated by external forces

[0057] The module 50 is configured to include a general strut-type suspension device, and the suspension geometry of the module 50 will be described while referring to the Figure 3 schematic diagram. Moreover, the rotation axis of the wheel 10 is defined as the wheel axis WL, and the line passing through the ground contact center SC, which is the center of the ground contact surface of the wheel 10, and perpendicular to the ground contact surface is defined as the vertical line VL. As viewed from the Figure 3 (a) in the direction of the wheel axis, which is an extension of the wheel axis WL, and from the Figure 3 (b) in the front-rear direction, it can be seen that the vertical line VL intersects the wheel axis WL at a right angle, that is, the wheel axis WL is parallel to the road surface. In addition, in the following description, the direction of the wheel axis, which is an extension of the wheel axis WL, is sometimes referred to as the Y direction, the up-down direction is referred to as the Z direction, and the direction perpendicular to the wheel axis direction and horizontal is referred to as the X direction (refer to Figure 3 ). Moreover, when the wheel 10 is not steered, the X direction coincides with the front-rear direction of the vehicle.

[0058] In Figure 3 , the shock absorber 54 and the suspension spring 56 are integrated and represented as the spring / absorber Assy90. As described above, the line connecting the center of the first joint 62 and the center of the upper support member 64 (the center of the support of the spring / absorber Assy90 to the vehicle body) is the kingpin axis KP. When the kingpin axis KP and the vertical line VL are projected onto a plane perpendicular to the wheel axis WL (hereinafter, there are cases where it is called the "XZ plane"), the angle formed by the kingpin axis KP and the vertical line VL in this XZ plane is the caster angle Φ S , and when the kingpin axis KP and the vertical line VL are projected onto a plane parallel to the wheel axis WL and perpendicular to the road surface (hereinafter, there are cases where it is called the "YZ plane"), the angle formed by these kingpin axis KP and the vertical line VL in this YZ plane is the kingpin angle Φ K .

[0059] Moreover, as Figure 3As shown in (b), the horizontal distance between the intersection of the wheel axis WL and the vertical line VL in the YZ plane and the kingpin axis KP is the kingpin offset δ1, and the distance between the intersection of the kingpin axis KP and the ground surface and the center SC of the ground surface (distance in the wheel axis direction) is the kingpin offset δ2. In addition, as shown in (a) of Figure 3 , the distance between the center SC of the ground surface in the XZ plane and the intersection of the kingpin axis KP and the ground surface is the trail δ3, and the horizontal distance between the wheel axis WL and the kingpin axis KP is the caster offset δ4. Moreover, although not shown in Figure 3 , in reality, the ground surface has a certain area, and the kingpin axis KP passes through the ground surface.

[0060] The force in the vehicle longitudinal direction (longitudinal force) F X , the force in the vehicle width direction (lateral force) F Y , and the force in the vertical direction (vertical force) F Z act on the wheel 10 as external acting forces from the outside, that is, from the road surface. Strictly speaking, it is assumed to act at the center SC of the ground surface. By these longitudinal forces F X , lateral forces F Y , and vertical forces F Z , the wheel 10 is steered. In other words, the knuckle 20a is rotated around the kingpin axis KP. If the torques acting on the knuckle 20a by these longitudinal forces F X , lateral forces F Y , and vertical forces F Z are respectively defined as the torque M x based on the longitudinal force, the torque M y based on the lateral force, and the torque M z based on the vertical force, then these torques M x based on the longitudinal force, the torque M y based on the lateral force, and the torque M z are expressed as in the formula of Figure 4 . Moreover, A, B, and C in these formulas are coefficients determined by the caster angle Φ S and the kingpin angle Φ K .

[0061] For example, when the vertical force F Z acts due to road surface undulations or the like, if the wheel 10 is steered by this action, the actual steering angle ψ in the steering control described above changes, and current I S is supplied to the steering motor 70a in such a way that the steering angle ψ becomes the target steering angle ψ*. Similarly, in the case where the longitudinal force F X acts by applying driving force, braking force, etc. to the vehicle, or in the case where the lateral force F YWhen it comes into effect, the wheel 10 will also turn due to the above-mentioned effect. To eliminate this turning, current I is supplied to the steering motor 70a S .

[0062] In a steering system with a two-wheel steering device that mechanically connects the left and right steering knuckles through a steering rod or the like, the steering directions of the wheels 10 caused by the longitudinal force F X are opposite to each other on the left and right. Therefore, the torques M x of the left and right wheels 10 based on the longitudinal force cancel each other out. Therefore, from the viewpoints of energy consumption and steering operation stability, it does not pose a problem. Moreover, the steering directions of the wheels 10 caused by the vertical force F Z are not necessarily in opposite directions, but in a steering system with a two-wheel steering device, the torques M z based on the vertical force are canceled to a certain extent. That is, since this steering system is a left-right independent steering type system, the energy consumption and the reduction of steering operation stability caused by the steering of the wheels 10 caused by the longitudinal force F X and the vertical force F Z become problems, and this problem is relatively large. In addition, the action of the lateral force F Y generates torques M y in the same direction on the left and right wheels 10 based on the lateral force, so even in a two-wheel steering device, they cannot be canceled. However, when the wheels 10 are steered by the steering actuator 70 and the vehicle turns, of course, torques M y based on the lateral force are generated, so there is no need to deliberately consider the influence caused by external forces

[0063] iv) Handling of changes in the steering angle caused by external forces

[0064] In this steering system, the change in the steering angle ψ of the wheel 10 caused by the above-mentioned longitudinal force F X as an external force is regarded as a factor that hinders the stability of the steering operation, and steering stabilization control for suppressing its change is performed. Specifically, in view of the fact that it is easy to identify the hindrance of the external force to the stability of the steering operation when the vehicle is moving straight ahead, when the actual steering angle ψ determined based on the actual motor rotation angle θ is below the threshold steering angle ψ TH which is set to a level that can be regarded as the vehicle moving straight ahead, the steering ECU 16 will apply a braking force F B to the entire vehicle that is equal to or greater than the set critical braking force F BTH , or a driving force F D applied to the vehicle that is equal to or greater than the set critical driving force F DTHThe above situation is recognized as a situation of external force steering in which the wheel 10 is steered by an external force or is likely to be steered, and steering stabilization control is performed in this situation of external force steering. Moreover, the braking force F is obtained based on the information transmitted from the brake ECU 38 and the drive ECU 26 via the CAN 44 respectively B , the driving force F D .

[0065] In the steering stabilization control, the steering ECU 16 increases the above-mentioned steering torque Tq in order to suppress the change of the steering angle ψ S the value of the differential term gain G in the determination formula of D . The differential term in the above formula is related to the change of the motor rotation angle θ with respect to the target motor rotation angle θ*, that is, the change of the steering angle ψ with respect to the target steering angle ψ*. By increasing the differential term gain G D , the effect of suppressing its change becomes higher. That is, the responsiveness to the change of the steering angle ψ is improved. In other words, the steering stabilization control is a control that does not easily change the steering position of the wheel 10 from the position corresponding to the requested steering amount. Specifically, the steering ECU 16 will set the differential term gain G that is G D1 in the basic steering control D to G in the steering stabilization control D2 (>G D1 ).

[0066] v) Flow of the steering control process

[0067] The computer of each steering ECU 16 repeatedly executes the steering control program represented by the flowchart in Figure 5 at a short time interval (for example, several msec to several tens of msec) to perform steering control including the above-mentioned steering stabilization control. Hereinafter, along with this flowchart, the process flow in the steering control will be briefly described

[0068] In the process according to the steering control program, first, in step 1 (hereinafter, briefly denoted as "S1". The same applies to other steps), the operation angle δ of the steering wheel 80 detected by the operation ECU 18 is obtained via the CAN 44. In S2, the target steering angle ψ* of the wheel 10 is determined based on this operation angle δ. In the next S3, the target motor rotation angle θ* is determined based on this target steering angle ψ*. In S4, the actual motor rotation angle θ is detected. Next, in S5, the actual steering angle ψ is determined based on the detected actual motor rotation angle θ. In S6, the motor rotation angle deviation Δθ is determined

[0069] In S7, it is determined whether the actual steering angle ψ is the threshold steering angle ψ THNext, that is, it is determined whether the vehicle is moving straight ahead. When the vehicle is moving straight ahead, the determinations in S8 and S9 are performed, that is, it is determined whether any one of the braking force F B and the driving force F D applied to the vehicle is equal to or greater than the set critical braking force F BTH and the critical driving force F DTH .

[0070] When the vehicle is not moving straight ahead, or even when it is moving straight ahead, the braking force F BTH equal to or greater than the critical braking force F B and the driving force F DTH equal to or greater than the critical driving force F D are not applied to the vehicle. In S10, the above-mentioned differential term gain G D is set to G D1 . When the vehicle is moving straight ahead and a braking force F BTH equal to or greater than the critical braking force F B or a driving force F DTH equal to or greater than the critical driving force F D is applied, in S11, the differential term gain G D is set to G D1 which is greater than G D2 .

[0071] After setting the differential term gain G D , in S12, the steering torque Tq S is determined according to the above formula based on the PID feedback control rule. In S13, the steering current I S is determined based on the determined steering torque Tq S . Moreover, in S14, a current is supplied to the steering motor 70a based on the determined steering current I S , and one execution of this program ends.

Claims

1. A steering system is a steer-by-wire type steering system, which includes: A pair of wheel steering devices, which are respectively arranged on the left and right wheels, and each has an actuator to individually steer the left and right wheels; and A controller that controls the pair of wheel steering devices so that the left and right wheels are respectively steered to a steering amount corresponding to a request. Wherein, The controller is configured to: In a situation where the left and right wheels are respectively steered or easily steered by a force acting on each wheel from the outside, that is, a situation of being steered according to an external force, perform steering stabilization control to suppress changes in the steering amount. Taking that the vehicle is moving straight forward as a condition, when a braking force or a driving force greater than a set magnitude is applied to the vehicle, it is determined that the vehicle is in the situation of being steered according to an external force. As the steering stabilization control, the controller performs control that does not easily change the steering position of the wheel from the position corresponding to the steering amount corresponding to the request. The controller is configured to: Perform the following control, that is, determine a target steering amount as a target steering amount to be achieved according to a request, based on a steering amount deviation that is a deviation of the actual steering amount from the target steering amount, and according to the method of PID feedback control, determine a steering force as the force that the actuator should generate. As the steering stabilization control, perform control to increase the differential term gain in the method of the PID feedback control.

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