Vehicle steering system

Abstract

To improve practicability of a vehicular steering system steering right and left wheels independently.SOLUTION: A steering system, having a pair of wheel steering devices for steering right and left wheels independently of each other, executes the following operations: i) While maintaining a ratio of steering quantities ψO* to ψI* of right and left wheels at a setting ratio, ordinary control of steering the right and left wheels is performed according to a steering request δ and ii) While maintaining a steering quantity ψI* of one of the right and left wheels at a steering quantity of a steering limit ψIlim in place of the ordinary control if one of the right and left wheels reaches the steering limit, one wheel steering limit control of increasing only a steering quantity ψO* of the other one of the right and left wheels is performed according to the steering request. Even if one of the right and left wheels reaches the steering limit, it is possible to decrease the turning radius of a vehicle equipped with the steering system by steering the other wheel further.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to a steering system for a vehicle that independently steers left and right wheels.

Background Art

[0002] Recently, for example, a steering system for a vehicle as described in the following patent documents, specifically, a steering system that independently steers left and right wheels (hereinafter, may be referred to as a "left and right independent steering type steering system") has been studied.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The left and right independent steering type steering system has not yet had enough time since the development started, and at present, it is hardly put into practical use. That is, there is much room for improvement in the left and right independent steering type steering system, and by making some improvements, it is possible to improve the practicality of the system. The present invention has been made in view of such circumstances, and an object thereof is to provide a highly practical left and right independent steering type steering system.

Means for Solving the Problems

[0005] To solve the above problems, the steering system for a vehicle of the present invention includes a steering operation member operated by a driver, a pair of wheel steering devices that independently steer the left and right wheels respectively, a controller that controls the pair of wheel steering devices A steer-by-wire type vehicle steering system including a steering operation member and a pair of wheel steering devices that are not mechanically connected, wherein the controller executes normal control to steer the left and right wheels in accordance with the operation amount of the steering member as a steering request while maintaining the ratio of the steering amounts of the left and right wheels at a set ratio. When one of the left and right wheels reaches the steering limit, instead of the normal control, while maintaining the steering amount of that one wheel at the steering amount that becomes the steering limit, only the steering amount of the other of the left and right wheels is increased in accordance with the operation amount of the steering operation member, and one-wheel steering limit control is executed. Operation

Advantages of the Invention

[0006] According to the present invention, even if one of the left and right wheels reaches the steering limit, by further steering the other wheel, it is possible to reduce the turning radius of the vehicle equipped with the steering system. Aspects of the Invention

[0007] The steering amounts of the left and right wheels in the vehicle steering system of the present invention are displacement amounts from the positions of the wheels when the vehicle is moving straight ahead, and generally, can be considered as the steering angles of the left and right wheels, in other words, the toe angles. If the ratio of the steering amounts of the left and right wheels is called the "steering ratio", the steering ratio can be considered as, for example, the ratio of the steering amount of the wheel located on the outer side of the turn (hereinafter sometimes referred to as the "outer turning wheel") of the left and right wheels to the steering amount of the wheel located on the inner side of the turn (hereinafter sometimes referred to as the "inner turning wheel") of the left and right wheels. The steering ratio may be fixed, or may vary in accordance with, for example, the traveling speed of the vehicle (hereinafter sometimes referred to as the "vehicle speed") and the like.

[0008] ​Regarding the steering ratio, to explain further, there is a steering ratio called the so-called Ackermann ratio. As will be explained later, in the state where the steering ratio becomes the Ackermann ratio, that is, in the steering state according to the Ackermann geometry, the slip of the tires of the left and right wheels during turning can be suppressed, and the turning resistance of the vehicle can be suppressed. On the other hand, in the state where the steering amounts of the left and right wheels are equal, that is, in the steering state according to the parallel geometry, the turning performance of the vehicle is improved, and a sharp (sporty) turn is realized. Here, if the concept of "Ackermann ratio" is adopted, the steering state according to the Ackermann geometry can be called the steering state with an Ackermann ratio of 100%, and the steering state according to the parallel geometry can be called the steering state with an Ackermann ratio of 0%.

[0009] From the above, for example, in the above normal control, for example, it is desirable to set the above setting ratio to the Ackermann ratio at least when the vehicle speed is below the set speed. The set speed in this case may be set, for example, to a speed at which the wheels may be steered to the steering end, specifically, 10 to 30 km / h. Incidentally, it is also possible to set the steering ratio to the Ackermann ratio regardless of the vehicle speed. On the other hand, in view of the turning stability of the vehicle, when the vehicle speed is quite high (for example, 80 to 120 km / h or more), it is desirable to set the steering ratio to 1, that is, to make the steering amounts of the left and right wheels equal. In other words, it is desirable to realize the steering state according to the parallel geometry. When changing the steering ratio, it is desirable to change it gradually rather than suddenly.

[0010] In normal control, the left and right wheels are steered according to the steering request. That is, the target steering amount, which is the steering amount to be steered at that time, may be determined based on the steering request. Specifically, when the vehicle is steered by the driver operating a steering operation member such as a steering wheel, the operation amount of the steering operation member may be used as the steering request and determined based on that operation amount. On the other hand, when the vehicle is in autonomous driving, for example, a command from an autonomous driving controller (a command corresponding to the operation amount of the above-mentioned steering operation member, a command regarding the target slip angle of the vehicle, etc.) may be used as the steering request and determined based on that command.

[0011] The above-mentioned "steering limit" of the wheel can also be called the so-called steering end. The steering limit may be defined, for example, by providing a mechanical stopper so that the wheel itself does not interfere with the body or the like, and the steering knuckle or the like abuts against the stopper, or may be defined by the control of the steering device.

[0012] When steering the wheel within a range where it does not interfere with the body or the like, the steering limit often differs between the case where the wheel becomes the inner turning wheel and the case where it becomes the outer turning wheel. In the vehicle steering system of the present invention, by utilizing this difference in the steering limit, when one of the left and right wheels reaches the steering limit, the above-described one-wheel steering limit control, that is, the control of further steering only the other wheel that has not reached the steering limit is executed. By this one-wheel steering limit control, the turning radius of the vehicle, in other words, the minimum turning radius becomes small.

[0013] In the one-wheel steering limit control, if the wheel that has reached the steering limit is defined as the "steering limit wheel" and the wheel whose steering angle can be further increased is defined as the "steerable wheel", in the vehicle steering system of the present invention, in order to make the turning radius smaller, in the one-wheel steering limit control, the increase gradient of the steerable wheel is made larger than the increase gradient of the steerable wheel immediately before the steering limit wheel exceeds the steering limit. Specifically, it is desirable to increase the steering angle of the steerable wheel so that the increase gradient of the steerable wheel is 2 times or more the increase gradient of the steerable wheel immediately before the steering limit wheel exceeds the steering limit.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0015] Hereinafter, as embodiments for carrying out the present invention, a vehicle steering system of an embodiment and its modified examples will be described in detail with reference to the drawings. It should be noted that the present invention can be implemented in various forms obtained by making various changes and improvements based on the knowledge of those skilled in the art, in addition to the vehicle steering system of the following embodiments and the modified examples, and the forms described in the section of

Aspects of the Invention

Embodiment

[0016] [A] Overall Configuration of a Vehicle Equipped with a Vehicle Steering System As schematically shown in FIG. 1, the steering system of the embodiment 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 driving wheels and steering wheels. 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, and when it is not necessary to distinguish between the front wheels 10F and the rear wheels 10R, they may simply be collectively referred to as wheels 10.

[0017] This steering system is a so-called steer-by-wire type steering system, and includes a pair of wheel steering devices 12 respectively provided for two front wheels 10F to steer the two front wheels 10F independently of each other, an operation device 14 for receiving the operation of the driver, a pair of steering electronic control units (hereinafter may be abbreviated as "steering ECU") 16 for controlling the pair of wheel steering devices 12 respectively, and an operation electronic control unit (hereinafter may be abbreviated as "operation ECU") 18 for controlling the operation device 14 and overall controlling the steering ECU 16. The configuration and control of this steering system will be described in detail later, but it can be considered that the controller of the steering system is constituted by the two steering ECUs 16 and the operation ECU 18.

[0018] In addition, this vehicle is equipped with a vehicle drive system having a pair of wheel drive units 20 respectively provided for the two front wheels 10F and rotationally 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 may be abbreviated as "drive ECU") 26 for controlling 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 configuration and general control is performed, the description of the configuration and control of the vehicle drive system will be omitted.

[0019] Furthermore, the vehicle is provided with a hydraulic braking system. The braking system includes a brake pedal 30 as a brake operating member operated by the driver, a master cylinder 32 connected to the brake pedal 30, a hydraulic fluid supply device 34 having a hydraulic fluid source such as a pump for pressurizing the hydraulic fluid, four brake devices 36 provided on each of the four wheels 10 for braking each wheel by the pressure of the hydraulic fluid from the hydraulic fluid supply device 34, and a brake electronic control unit (hereinafter sometimes referred to as "brake ECU") 38 for controlling the operation of the hydraulic fluid supply device 34. The braking system is a so-called brake-by-wire type system, and the brake ECU 38 controls the braking force applied to the vehicle by controlling the pressure of the hydraulic fluid supplied from the hydraulic fluid supply device 34 to the brake 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. Since the braking system has a general configuration and general control is performed, the description of the configuration and control of the braking system is omitted.

[0020] The vehicle is provided with a CAN (car area network or controllable area network) 44, 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, 38 communicate with each other via the CAN 44 and execute the control that each should perform. Incidentally, each of these ECUs 16, 18, 26, 38 includes a computer having a CPU, a ROM, a RAM, etc., and a driver (drive circuit) for driving components (for example, an electric motor, a valve, a pump, etc.) based on the instructions of the computer. For each of the rear wheels 10R, a wheel speed sensor 48 for detecting its respective wheel rotation speed (hereinafter sometimes referred to as "wheel speed") v W is provided, and these wheel speed sensors 48 are also connected to the CAN 44.

[0021] [B] Hardware Configuration of Vehicle Steering System Each of the pair of wheel steering devices 12 of the vehicle steering system according to the present embodiment is incorporated in a wheel arrangement module 50. In the wheel arrangement module 50, one of the pair of wheel drive units 20 of the above-described vehicle drive system and one of the four brake devices 36 of the brake system are also incorporated. The wheel arrangement module (hereinafter, may be simply abbreviated as "module") 50 is a module for arranging the wheel 10b with the tire 10a mounted thereon on the vehicle body, as shown in FIG. 2. Although the wheel 10b itself can be considered as a wheel, in the present embodiment, for convenience, the wheel 10b with the tire 10a mounted thereon will be referred to as the wheel 10.

[0022] Describing the wheel steering device 12 of the present steering system while explaining the configuration of the module 50, the above-described wheel drive unit 20 disposed in the present module 50 includes a housing 20a, an electric motor as a drive source built in the housing 20a, and a speed reducer (both not shown in the figure) for reducing the rotation of the electric motor, and an axle hub (invisible in the figure) to which the wheel 10b is attached. The wheel drive unit 20 is disposed inside the rim of the wheel 10b and is what is called an in-wheel motor unit. Since the wheel drive unit 20 has a well-known structure, the description of its structure will be omitted.

[0023] The present module 50 is configured to include a MacPherson type suspension device (also called "MacPherson strut type"). In this suspension device, the housing 20a of the wheel drive unit 20 functions as a carrier for rotatably holding the wheel. More specifically, the housing 20a functions as a steering knuckle in the wheel steering device 12 and is allowed to move up and down with respect to the vehicle body. Therefore, the suspension device includes a lower arm 52 which is a suspension arm, the housing 20a of the wheel drive unit 20, a shock absorber 54, and a suspension spring 56.

[0024] Since the suspension device itself has a general structure, to briefly explain, the lower arm 52 has a shape called a so-called L-arm, and its base end portion is divided into two parts in the vehicle longitudinal direction. At its base end portion, it is supported by a side member of the vehicle body (not shown) via a first bush 58 and a second bush 60 so as to be rotatable around an arm rotation axis LL. The housing 20a of the wheel drive unit 20 is rotatably connected to the tip end portion of the lower arm 52 via an arm connecting ball joint 62 which is a first joint (hereinafter sometimes referred to as "the first joint 62") at its lower portion.

[0025] The shock absorber 54 has its lower end portion fixedly supported by the housing 20a of the wheel drive unit 20, and its upper end portion is supported by the upper part of the tire housing of the vehicle body via an upper support 64. The upper end portion of the suspension spring 56 is also supported by the upper part of the tire housing of the vehicle body via the upper support 64, and the lower end portion of the suspension spring 56 is supported by a lower support 54a provided in a flange shape on the shock absorber 54. That is, the suspension spring 56 and the shock absorber 54 are arranged in parallel with each other between the lower arm 52 and the vehicle body.

[0026] As described above, this module 50 has a brake device 36. The brake device 36 includes a disk rotor 66 which is attached to an axle hub together with the wheel 10b and rotates with the wheel 10, and a brake caliper 68 which is held by the housing 20a of the wheel drive unit 20 so as to straddle the disk rotor 66, and is a disk brake device. Although detailed description is omitted, this brake caliper 68 has a brake pad as a friction member and a hydraulic cylinder, and the brake device 36 is configured to generate a braking force for stopping the rotation of the wheel 10 by pressing the brake pad against the disk rotor 66 depending on the pressure of the hydraulic fluid supplied from the hydraulic fluid supply device 34 to the hydraulic cylinder.

[0027] The wheel steering device 12 is a single-wheel independent steering device for independently steering only one of a pair of left and right front wheels 10F with respect to the other, and generally includes a housing 20a of a wheel drive unit 20 that functions as a steering knuckle as described above (hereinafter, when handled as a component of the wheel steering device 12, it may be referred to as the "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.

[0028] The steering actuator 70 includes a steering motor 70a that is an electric motor as a drive source, a speed reducer 70b that reduces the rotation of the steering motor 70a, and an actuator arm 70c that is rotated by the rotation of the steering motor 70a via the speed reducer 70b and functions as a pitman arm. The base end portion of the tie rod 72 is connected to the actuator arm 70c via a rod base end portion connecting ball joint 74 that is a second joint (hereinafter, may be referred to as the "second joint 74"), and the tip end portion of the tie rod 72 is connected to a knuckle arm 20b of the steering knuckle 20a via a rod tip end portion ball joint 76 that is a third joint (hereinafter, may be referred to as the "third joint 76").

[0029] In the wheel steering device 12, the line connecting the center of the upper support 64 and the center of the first joint 62 serves as the kingpin axis KP. By operating the steering motor 70a, as indicated by the thick arrow in the figure, the actuator arm 70c of the steering actuator 70 rotates about the actuator axis AL. This rotation is transmitted by the tie rod 72, and the steering knuckle 20a is rotated about the kingpin axis KP. That is, as indicated by the thick arrow in the figure, the front wheel 10F is steered. Due to such a structure, in the wheel steering device 12, an operation conversion mechanism 78 is provided that includes the actuator arm 70c, the tie rod 72, the knuckle arm 20b, etc., and converts the rotational operation of the steering motor 70a into the steering operation of the front wheel 10F.

[0030] The wheel steering device 12 has the steering actuator 70 disposed on the lower arm 52. Therefore, the work of assembling the module 50 to the vehicle body can be easily performed. To put it simply, by attaching the base end portion of the lower arm 52 to the side member of the vehicle body and attaching the upper support 64 to the upper part of the tire housing of the vehicle body, the suspension device, the brake device, and the wheel steering device can be mounted on the vehicle. That is, the present module 50 is an excellent module in terms of mountability to the vehicle.

[0031] Here, if the rear wheel 10R is briefly described, each of the rear wheels 10R is held by the vehicle body via a trailing arm type suspension device. Although a brake device 36 is provided for each rear wheel 10F, since each rear wheel 10R is neither a drive wheel nor a steering wheel, the wheel drive unit 20 and the wheel steering device 12 are not provided.

[0032] The operation device 14 has a general structure in a steer-by-wire type steering system. Briefly described, as shown in FIG. 1, a steering wheel 80 as a steering operation member steered by a driver, a steering sensor 82 for detecting a steering operation angle, which is the rotation angle of the steering wheel 80, as an operation amount from the straight-ahead state position of the steering operation member, and a reaction force applying device 84 for applying an operation reaction force to the steering wheel 80 are included. The reaction force applying device 84 includes a reaction force motor 84a which is an electric motor as a power source, and a speed reducer 84b for transmitting the force of the reaction force motor 84a to the steering wheel 80.

[0033] [C] Steering control of a vehicle steering system In this steering system, as the steering control of the front wheels 10F, normal control and one-wheel steering limit time control are executed. Briefly speaking, the normal control is a control for steering the left and right front wheels 10F according to a steering request while maintaining the ratio of the steering amounts of the left and right front wheels 10F at a set ratio. The one-wheel steering limit time control is a control for increasing only the steering amount of the other of the left and right front wheels 10F according to a steering request while maintaining the steering amount of one of the left and right front wheels 10F at the steering amount that reaches the steering limit when one of the left and right front wheels 10F reaches the steering limit. Incidentally, in this steering system, the steering angle ψ of the left and right front wheels 10F is adopted as the steering amount. Hereinafter, the normal control and the one-wheel steering limit time control will be described respectively, and then the control flow for executing these controls will be described. In this steering system, the control of the operation reaction force in the operation device 14 is also executed, but since the control may be the same as the control in a general steer-by-wire type steering system, the description in this specification is omitted.

[0034] i) Normal control In this steering system, the operation ECU 18 sets the target steering angle ψ which is the target of the steering amount of each front wheel 10F * as the target steering angle ψ of the left front wheel 10FL L *and the target steering angle ψ of the right front wheel R * are determined, and based on these target steering angles ψ L * , ψ R * a pair of steering ECUs 16 control the corresponding wheel steering devices 12 respectively to steer the left front wheel 10FL and the right front wheel 10FR to the respective steering angles ψ L , ψ R such that the target steering angle ψ L * , ψ R * is achieved.

[0035] Specifically, the operation ECU 18 determines the target vehicle body slip angle β that the vehicle body is to achieve based on the steering request, that is, the steering operation angle δ acquired by the steering sensor 82. Incidentally, when the vehicle is performing autonomous driving, information about the target vehicle body slip angle β S which is the target vehicle body slip angle β S * is sent from an autonomous driving system (not shown) as a steering request via the CAN 44. The operation ECU 18 determines which of the left and right front wheels 10FL and 10FR will be the outer turning wheel (the wheel on the side far from the turning center, hereinafter sometimes referred to as "outer turning wheel 10FO") and which will be the inner turning wheel (the wheel on the side close to the turning center, hereinafter sometimes referred to as "inner turning wheel 10FI") based on the target vehicle body slip angle β S * . S * The operation ECU 18 determines the target steering angles ψ

[0036] for the respective steering angles ψ L , ψ R of the left and right front wheels 10FL and 10FR. In this steering system, the steering angles ψ L * , ψ R * as the ratio of the steering amounts of the left and right wheels (steering amount ratio) L , ψ RTo change the ratio (hereinafter sometimes referred to as the "steering angle ratio") R according to the vehicle running speed v, which is the vehicle speed, the operation ECU 18 determines the target steering angle ψ L * , ψ R * based on the steering angle ratio R (a kind of "set ratio") preset based on the vehicle speed v.

[0037] Here, the steering angle ratio R will be described in detail with reference to FIG. 3. If the steering angle of the outer turning wheel 10FO is ψ O , and the steering angle of the inner turning wheel 10FI is ψ I , then the steering angle ratio R can be defined, for example, as R = ψ O / ψ I .

[0038] FIG. 3(a) schematically shows a steering state according to so-called parallel geometry. In this steering state, generally speaking, the steering angle ψ O of the outer turning wheel 10FO and the steering angle ψ I of the inner turning wheel 10FI are equal to each other, and the steering angle ratio R is "1". The direction of the inner turning wheel 10FI is perpendicular to the line connecting the turning center TC and the center C I of the ground contact surface of the inner turning wheel 10FI, but the direction of the outer turning wheel 10FO is not perpendicular to the line connecting the turning center TC and the center C O of the ground contact surface of the outer turning wheel 10FO. In this steering system, for the sake of convenience, the steering angle ψ I of the inner turning wheel 10FI and the vehicle body slip angle β S are handled as being equal.

[0039] On the other hand, FIG. 3(b) schematically shows a steering state according to so-called Ackermann geometry. Generally speaking, the direction of the inner turning wheel 10FI is perpendicular to the line connecting the turning center TC and the center C I of the ground contact surface of the inner turning wheel 10FI, and the direction of the outer turning wheel 10FO is such that the center C Ois perpendicular to the line connecting them. Therefore, the steering angle ψ of the outer turning ring 10FO O is smaller than the steering angle ψ of the inner turning ring 10FI I , and the steering angle ratio R is the Ackermann steering angle ratio R which is a specific value in this steering state A .

[0040] The Ackermann ratio A can be considered to be 0% in the steering state according to the parallel geometry and 100% in the steering state according to the Ackermann geometry, respectively. Incidentally, the relationship between the Ackermann ratio A and the steering angle ratio R can be expressed by the following formula. A = (1 - R) / (1 - R A ) × 100%

[0041] When the Ackermann ratio A is high, the slip of the tire 10a during vehicle turning can be suppressed, and the wear of the tire 10a and the skid sound (squeal) generated by the tire 10a can be suppressed. On the other hand, when the Ackermann ratio A is low, the turning performance of the vehicle is improved. Briefly speaking, the running of the vehicle becomes sharp (sporty). In view of these, in this steering system, the operation ECU18 changes the steering angle ratio R so as to change the Ackermann ratio A according to the vehicle speed v.

[0042] Figure 3(c) is a graph showing the relationship between the vehicle speed v and the steering angle ratio R. As shown in this graph, the operation ECU18 increases the steering angle ratio R as the vehicle speed v increases, and conversely, decreases the steering angle ratio R as the vehicle speed v decreases. Specifically, the steering angle ratio R is the Ackermann steering angle ratio R when the vehicle speed v is equal to or lower than the lower limit vehicle speed v L (for example, 20 km / h), and is 1 when the vehicle speed v is equal to or higher than the upper limit vehicle speed v A (for example, 100 km / h). Between the lower limit vehicle speed v U and the upper limit vehicle speed v L , as the vehicle speed v increases, the Ackermann steering angle ratio R U is determined to approach 1 A .

[0043] The operation ECU 18, based on the target vehicle body slip angle β described above, S * determines the target steering angle ψ of the turning inner wheel 10FI, * which is the target inner wheel steering angle ψ, I * according to the following formula: ψ I * = β S * and determines the target inner wheel steering angle ψ such that the target inner wheel steering angle ψ I * and the vehicle speed v are used to identify the steering angle ratio R while referring to the map data as shown in Fig. 3(c), and then, based on the identified steering angle ratio R and the target inner wheel steering angle ψ of the turning inner wheel 10FI I * determines the target steering angle ψ of the turning outer wheel 10FO, O * which is the target outer wheel steering angle ψ, O * according to the following formula: ψ O * = ψ I * × R Incidentally, the operation ECU 18 identifies the vehicle speed v based on each wheel speed v of the front wheels 10F that depends on the rotational speed of the drive motor of the wheel drive unit 20 W and each wheel speed v of the rear wheels 10R that depends on the detection of the wheel speed sensor 48. W

[0044] When the left front wheel 10FL is the turning outer wheel 10FO, the operation ECU 18 sets the target steering angle ψ of the left front wheel 10FL, * which is the left wheel target steering angle ψ, L * to ψ O * and sets the target steering angle ψ of the right front wheel 10FR, * which is the right wheel target steering angle ψ, R * to ψ I * respectively. When the right front wheel 10FR is the turning outer wheel 10FO, the left wheel target steering angle ψL * , ψ I * , the right wheel target steering angle ψ R * , ψ O * The operation ECU 18 determines the target steering angle ψ L * , ψ R * The information on the above is transmitted via the CAN 44 to each of the two steering ECUs 16 corresponding to the left and right front wheels 10F.

[0045] Each steering ECU 16 controls the wheel steering device 12 corresponding to itself to detect the target steering angle ψ of the front wheel 10F corresponding to itself. * More specifically, since wheel turning device 12 does not have a turning angle sensor for directly detecting turning angle ψ of front wheels 10F, in this steering system, the turning ECU 16 utilizes the specific relationship between the turning angle ψ of front wheels 10F and the rotation angle (hereinafter sometimes referred to as "motor rotation angle") θ of turning motor 70a to control the turning force generated by turning actuator 70 based on motor rotation angle θ of turning motor 70a. Since the turning force generated by turning actuator 70 is equivalent to turning torque Tq, which is the torque generated by turning motor 70a, specifically, turning ECU 16 controls the turning force generated by turning actuator 70 based on target turning torque Tq, which is the turning torque Tq that should be generated by turning motor 70a. * is determined based on the motor rotation angle θ of the steering motor 70a. Incidentally, the motor rotation angle θ can be considered as the displacement angle of the motor shaft from the state when the vehicle is traveling straight, and is accumulated over 360°.

[0046] Target steering torque Tq * Specifically, the steering ECU 16 determines the target steering angle ψ for each front wheel 10F. * Based on this, a target motor rotation angle θ *Determine. 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 grasps the actual motor rotation angle θ, which is the current motor rotation angle θ based on the detection of this motor rotation angle sensor with reference to the reference motor rotation angle. The steering ECU 16 obtains the motor rotation angle deviation Δθ, which is the deviation of the actual motor rotation angle θ with respect to the target motor rotation angle θ * and determines the target steering torque Tq according to the following formula based on this motor rotation angle deviation Δθ (= θ * - θ). * That is to say. Tq * =G P ·Δθ + G D ·(dΔθ / dt) + G I ·∫Δθdt Note that the above formula is a formula according to the feedback control law 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.

[0047] There is a specific relationship between the steering torque Tq and the supply current I to the steering motor 70a. In other words, since the steering torque Tq depends on the force exerted by the steering motor 70a, the steering torque Tq and the supply current I are generally in a proportional relationship. Accordingly, the steering ECU 16 determines the target supply current I * which is the target of the supply current I to the steering motor 70a based on the determined target steering torque Tq * and supplies the target supply current I * to the steering motor 70a.

[0048] ii) On the other hand, when the steering limit of one wheel is reached, control Generally, when the steering angle of the wheel 10 increases, for example, when the wheel 10 interferes with the body, further steering becomes impossible. Considering this, a steering limit is set for the steering of the wheel 10. In this steering system, the target steering angles ψ * , specifically, the target outer wheel steering angle ψ O * of the outer wheel 10FO during turning and the target inner wheel steering angle ψ I * of the inner wheel 10FI during turning are respectively made not to exceed the outer wheel limit steering angle ψ Olim and the inner wheel limit steering angle ψ Ilim .

[0049] In this steering system, as shown in Fig. 4(a), the outer wheel limit steering angle ψ Olim and the inner wheel limit steering angle ψ Ilim are set at approximately the same angle in view of interference with the body B. However, when the front wheel 10F is steered to the steering limit, generally, it is the case when the vehicle speed v is below the lower limit vehicle speed v L . In that case, as shown in Fig. 4(b), the steering angle ratio R is the Ackermann steering angle ratio R A . When the steering requirement increases, the target inner wheel steering angle ψ I * of the inner wheel 10FI during turning reaches the inner wheel limit steering angle ψ Ilim first. Briefly speaking, the inner wheel 10FI during turning reaches the steering end first. In this steering system, by the one-wheel steering limit time control, after the inner wheel 10FI during turning reaches the steering end, further steering of the inner wheel 10FI is prohibited. On the other hand, since there is still a margin M for steering in the outer wheel 10FO during turning, the outer wheel 10FO during turning is further steered using that margin M. In other words, in this steering system, by the one-wheel steering limit time control, when one of the left and right wheels reaches the steering limit, the steering amount of that one is maintained at the steering amount that is the steering limit, and only the steering amount of the other of the left and right wheels is increased according to the steering requirement.

[0050] The steering angle ratio R is the Ackermann steering angle ratio R AWhen it is in such a state, the steering requirement, that is, the target outer wheel steering angle ψ of the turning outer wheel 10FO with respect to the increase in the steering operation angle δ O * , and the target inner wheel steering angle ψ of the turning inner wheel 10FI I * The increase is shown graphically in FIG. 5. As can be understood from this graph, when the steering operation angle δ reaches the one-wheel interference operation angle δ int , the target inner wheel steering angle ψ of the turning inner wheel 10FI I * becomes the inner wheel limit steering angle ψ Ilim , and even if the steering operation angle δ becomes larger, the target inner wheel steering angle ψ I * is maintained at the inner wheel limit steering angle ψ Ilim . On the other hand, when the steering operation angle δ reaches the one-wheel interference operation angle δ int , the target outer wheel steering angle ψ of the turning outer wheel 10FO O * becomes the outer wheel steering angle ψ at the inner wheel limit time point Ogrd , but as the steering operation angle δ becomes larger, the target outer wheel steering angle ψ O * is allowed to become even larger until it reaches the outer wheel limit steering angle ψ Olim . Regarding the increase gradient of the target outer wheel steering angle ψ O * with respect to the steering operation angle δ, when the target outer wheel steering angle ψ O * exceeds the outer wheel steering angle ψ at the inner wheel limit time point Ogrd , the increase gradient is set to be more than twice (twice in this steering system) the increase gradient before exceeding the outer wheel steering angle ψ at the inner wheel limit time point Ogrd . In other words, in this steering system, when one of the left and right front wheels 10F reaches the steering limit, due to the one-wheel steering limit time control, the increase gradient of the other of the left and right front wheels 10F is made larger than the increase gradient immediately before one of the left and right front wheels 10F exceeds the steering limit, so that the steering amount of the other of the left and right front wheels 10F is increased.

[0051] In this steering system, even after one of the left and right front wheels 10F reaches the steering limit by the one-wheel steering limit control as described above, the vehicle can turn with a smaller turning radius.

[0052] iii) Control Flow The control of the present steering system described above is performed by the computer of the operation ECU 18 repeatedly executing the steering integrated control program whose flowchart is shown in FIG. 6, and the computer of each steering ECU 16 repeatedly executing the wheel steering program whose flowchart is shown in FIG. 7 at a short time pitch (for example, several msec to several tens of msec). Hereinafter, the control flow of the steering system will be briefly described by explaining the processing according to the flowcharts of these programs.

[0053] In the processing according to the steering integrated control program, first, in Step 1 (hereinafter abbreviated as "S1". The same applies to other steps), it is determined whether or not the vehicle is in automatic driving. If it is not in automatic driving, in S2, the steering operation angle δ is acquired by detecting with the steering sensor 82, and in S3, based on the steering operation angle δ, the target vehicle body slip angle β S * is determined. If it is in automatic driving, in S4, based on the information from the automatic driving system, the target vehicle body slip angle β S * is acquired.

[0054] Subsequently, in S5, based on the target vehicle body slip angle β S * , specifically, based on its sign, it is determined which of the left and right front wheels 10FL, 10FR is the turning outer wheel 10FO and which is the turning inner wheel 10FI. Then, in S6, the target vehicle body slip angle β S * is determined as the target inner wheel steering angle ψ I * .

[0055] In the next S7, the determination process of the steering angle ratio R is executed. In this process, based on the wheel speed v of each wheel 10, the current vehicle speed v is specified, and based on the vehicle speed v and the determined target inner wheel steering angle ψ W , the steering angle ratio R is determined by referring to the map data as shown in Fig. 3(c). Subsequently, in S8, based on the steering angle ratio R, the target outer wheel steering angle ψ I * is determined. O *

[0056] Subsequently, in S9, the one-wheel steering limit time process is performed. This process is performed by executing the one-wheel steering limit time process subroutine whose flowchart is shown in Fig. 8. In the process according to this subroutine, first, in S21, it is determined whether the target inner wheel steering angle ψ I * has reached the above-mentioned inner wheel limit steering angle ψ Ilim . If the target inner wheel steering angle ψ I * has not reached the inner wheel limit steering angle ψ Ilim , the process according to the subroutine ends. Briefly speaking, the one-wheel steering limit time control is not executed.

[0057] If the target inner wheel steering angle ψ I * has reached the inner wheel limit steering angle ψ Ilim , then in S22, the target inner wheel steering angle ψ I * is determined to be the inner wheel limit steering angle ψ Ilim . Subsequently, in S23, the outer wheel steering angle ψ at the time when the turning inner wheel 10FI reaches the steering limit, that is, the outer wheel steering angle ψ at the inner wheel limit time ψ Ogrd is determined by multiplying the inner wheel limit steering angle ψ Ilim by the steering angle ratio R. Based on this outer wheel steering angle ψ at the inner wheel limit time ψ Ogrd , in S24, the target outer wheel steering angle ψ O * is corrected. Specifically, the increasing gradient of the target outer wheel steering angle ψ O * is adjusted according to the target inner wheel steering angle ψI * is the inner ring limit steering angle ψ Ilim It is corrected according to the following formula so that it becomes a gradient twice that of the increasing gradient immediately before reaching ψ O * = ψ Ogrd + 2×(ψ O * - ψ Ogrd )

[0058] In the subsequent S25, it is determined whether the corrected target outer ring steering angle ψ O * has reached the outer ring limit steering angle ψ Olim If it has reached, in S26, the target outer ring steering angle ψ O * is determined to be the outer ring limit steering angle ψ Olim If the target outer ring steering angle ψ O * has not reached the outer ring limit steering angle ψ Olim the corrected target outer ring steering angle ψ O * is adopted as it is.

[0059] After the one-wheel steering limit time process, in S10, it is determined whether the left front wheel 10FL is the turning outer wheel. If the left front wheel 10FL is the turning outer wheel, in S11, the left wheel target steering angle ψ L * is set to the target outer ring steering angle ψ O * and the right wheel target steering angle ψ R * is set to the target inner ring steering angle ψ I * If the left front wheel 10FL is not the turning outer wheel, in S12, the left wheel target steering angle ψ L * is set to the target inner ring steering angle ψ I * and the right wheel target steering angle ψ R * is set to the target outer ring steering angle ψ O * Then, in S13, the left wheel target steering angle ψ L *, Right wheel target steering angle ψ R * Information about this is transmitted to the steering ECUs 16 corresponding to the left front wheel 10FL and the right front wheel 10FR, respectively.

[0060] In the process according to the wheel steering program executed by each steering ECU 16, at S41, information about the target steering angle ψ of the corresponding front wheel 10F * is received from the operation ECU 18, and at S42, based on the target steering angle ψ * the target motor rotation angle θ of the steering motor 70a is determined. At the subsequent S43, the actual motor rotation angle θ, which is the actual rotation angle of the steering motor 70a, is acquired, and at S44, the motor rotation angle deviation Δθ, which is the deviation of the actual motor rotation angle θ with respect to the target motor rotation angle θ * is determined. At the next S45, based on the motor rotation angle deviation Δθ, the target steering torque Tq * is determined according to the above formula, and at S46, the target supply current I, which is the current to be supplied to the steering motor 70a based on the target steering torque Tq * is determined. Then, at S47, based on the target supply current I * the steering motor 70a is supplied with current. * *

[0061] [D] Modification example In the vehicle steering system of the above embodiment, as the steering requirement increases, the turning inner wheel 10FI interferes with the body B first and reaches the steering limit. However, depending on the vehicle structure, the turning outer wheel 10FO may reach the steering limit first. Hereinafter, a vehicle steering system adopted when the turning outer wheel 10FO reaches the steering limit first will be described as a modified example of the vehicle steering system.

[0062] In the steering system of the modification example, when the steering angle ratio R is the Ackermann steering angle ratio R A the target outer wheel steering angle ψ of the turning outer wheel 10FO with respect to an increase in the steering requirement, that is, the steering operation angle δ O ​​* , the target inner wheel steering angle ψ of the turning inner wheel 10FI I * The increase of is shown graphically in FIG. 9. As can be understood from this graph, when the steering operation angle δ becomes the one-wheel interference operation angle δ int , the target inner wheel steering angle ψ of the turning outer wheel 10FO O * becomes the outer wheel limit steering angle ψ Olim , and even when the steering operation angle δ becomes larger than that, the target outer wheel steering angle ψ O * is maintained at the outer wheel limit steering angle ψ Olim . On the other hand, when the steering operation angle δ becomes the one-wheel interference operation angle δ int , the target inner wheel steering angle ψ of the turning inner wheel 10FI I * becomes the inner wheel steering angle ψ at the outer wheel limit time point Igrd , but as the steering operation angle δ becomes larger than that, the target inner wheel steering angle ψ I * is allowed to become even larger until it reaches the inner wheel limit steering angle ψ Ilim . Regarding the increase gradient of the target inner wheel steering angle ψ I * with respect to the steering operation angle δ, in the steering system of the modification, when the target inner wheel steering angle ψ I * exceeds the inner wheel steering angle ψ at the outer wheel limit time point Igrd , the increase gradient is 1.5 times the increase gradient before exceeding the inner wheel steering angle ψ at the outer wheel limit time point Igrd . In other words, also in the steering system of the modification, when one of the left and right front wheels 10F reaches the steering limit, by the one-wheel steering limit time control, the increase gradient of the other of the left and right front wheels 10F becomes larger than the increase gradient immediately before one of the left and right front wheels 10F exceeds the steering limit, so that the steering amount of the other of the left and right front wheels 10F is increased.

[0063] The control flow of the steering system for a vehicle in the modified example differs from that of the steering system for a vehicle in the embodiment only in the above-described one-wheel steering limit time processing. This processing is performed by executing the one-wheel steering limit time processing subroutine whose flowchart is shown in FIG. 10. In the processing according to this subroutine, first, in S51, it is determined whether the target outer wheel steering angle ψ O * has reached the above-described outer wheel limit steering angle ψ Olim . If the target outer wheel steering angle ψ O * has not reached the outer wheel limit steering angle ψ Ilim , the processing according to the subroutine ends. Briefly speaking, the one-wheel steering limit time control is not executed.

[0064] If the target outer wheel steering angle ψ O * has reached the outer wheel limit steering angle ψ Olim , then in S52, the target outer wheel steering angle ψ O * is determined to be the outer wheel limit steering angle ψ Olim . Subsequently, in S53, the inner wheel steering angle ψ at the outer wheel limit time ψ Igrd , which is the steering angle of the inner wheel 10FI during turning when the outer wheel 10FO during turning reaches the steering limit, is determined by dividing the outer wheel limit steering angle ψ Olim by the steering angle ratio R. Based on this inner wheel steering angle ψ at the outer wheel limit time ψ Igrd , in S54, the target inner wheel steering angle ψ I * is corrected. Specifically, the increase gradient of the target inner wheel steering angle ψ I * is corrected according to the following formula so that it becomes 1.5 times the increase gradient immediately before the target outer wheel steering angle ψ O * reaches the outer wheel limit steering angle ψ Olim . ψ I * =ψ Igrd +1.5×(ψ I * -ψ Igrd )

[0065] In subsequent S55, it is determined whether the corrected target inner-wheel steering angle ψ I * has reached the inner-wheel limit steering angle ψ Ilim . If it has reached, in S56, the target inner-wheel steering angle ψ I * is determined to be the inner-wheel limit steering angle ψ Ilim . If the target inner-wheel steering angle ψ I * has not reached the inner-wheel limit steering angle ψ Ilim , the corrected target inner-wheel steering angle ψ I * is adopted as it is.

Explanation of Signs

[0066] 10: Wheel 10F: Front wheel 10FL: Left front wheel 10FR: Right front wheel 10FO: Outer turning wheel 10FI: Inner turning wheel 12: Wheel steering device 14: Operating device 16: Steering electronic control unit [Controller] 18: Operating electronic control unit [Controller] 20: Wheel drive unit 20a: Steering knuckle 44: CAN 48: Wheel speed sensor 50: Wheel arrangement module 52: Lower arm 70: Steering actuator 70a: Steering motor 72: Tie rod 80: Steering wheel [Steering operation member] 82: Steering sensor TC: Turning center C O : Center C of the ground contact surface of the outer turning wheel I : Center B of the ground contact surface of the inner turning wheel Body M: Margin δ: Steering operation angle δ int : One-wheel interference operation angle β S : Vehicle body slip angle β S * : Target vehicle body slip angle ψ: Steering angle [Steering amount] of the wheel ψ L : Steering angle ψ of the left wheel R : Steering angle ψ of the right wheel O : Steering angle ψ of the outer turning wheel I : Steering angle ψ of the inner turning wheel Olim : Outer-wheel limit steering angle ψ Ilim : Inner-wheel limit steering angle ψ Ogrd : Outer-wheel steering angle ψ at the inner-wheel limit time Igrd : Inner-wheel steering angle ψ at the outer-wheel limit time *: Target steering angle ψ L * : Left wheel target steering angle ψ R * : Right wheel target steering angle ψ O * : Target outer wheel steering angle ψ I * : Target inner wheel steering angle R: Steering angle ratio [Ratio of steering amounts] R A : Ackermann steering angle ratio A: Ackermann ratio v: Vehicle running speed (vehicle speed) v L : Lower limit vehicle speed v U : Upper limit vehicle speed v W : Wheel rotation speed (wheel speed)

Claims

1. A steering operation member operated by a driver, A pair of wheel steering devices that steer the left and right wheels independently of each other, A controller that controls the pair of wheel steering devices A steer-by-wire type vehicle steering system comprising the above, wherein the steering operation member and the pair of wheel steering devices are not mechanically connected, The controller, While maintaining the ratio of the steering angles of the left and right wheels at a set ratio, it executes normal control to steer the left and right wheels according to the operation amount of the steering operation member as a steering request. When one of the left and right wheels reaches the steering limit, instead of the normal control, while maintaining the steering amount of that one wheel at the steering amount that becomes the steering limit, only the steering amount of the other of the left and right wheels is increased according to the operation amount of the steering operation member, and a vehicle steering system configured to execute one-wheel steering limit time control.

2. The controller, In the one-wheel steering limit time control, the steering system for a vehicle according to claim 1, wherein the increase gradient of the other of the left and right wheels is configured to increase the steering amount of the other of the left and right wheels so as to be greater than the increase gradient immediately before the one of the left and right wheels exceeds the steering limit.

3. The vehicle steering system according to claim 2, wherein the increase gradient of the other of the left and right wheels in the one-wheel steering limit time control is set to be at least twice the increase gradient immediately before the one of the left and right wheels exceeds the steering limit.

4. The vehicle steering system according to any one of claims 1 to 3, wherein the set ratio in the normal control is an Ackermann ratio, at least when the traveling speed of the vehicle is equal to or lower than the set speed.

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