阅读说明：本技术 车辆用转向系统 (Steering system for vehicle ) 是由 中田大辅 于 2021-04-13 设计创作，主要内容包括：本发明提供一种车辆用转向系统,是实用性高的左右独立转向型转向系统。所述车辆用转向系统具备：一对车轮转向装置,使左右车轮分别独立地转向；及控制器,控制所述一对车轮转向装置,其中,将该控制器构成为根据转向要求(β-(S))来决定左右车轮各自的标准转向量(ψ-(L)、ψ-(R)),并且能够执行使左右车轮各自的转向量相对于左右车轮各自的标准转向量向彼此相反方向偏移(Δψ-(L)、Δψ-(R))的反相偏移转向,并且基于在该反相偏移转向中对左右车轮分别施加的转向力,来推定该车辆行驶的路面的摩擦系数。能够在维持稳定的车辆的行驶的状态下,推定该车辆行驶的路面的摩擦系数。(The invention provides a vehicle steering system, which is a left-right independent steering type steering system with high practicability. The vehicle steering system includes: a pair of wheel steering devices for independently steering left and right wheels; and a controller for controlling the pair of wheel steering devices, wherein the controller is configured to respond to a steering request (beta) S ) To determine the respective standard steering amounts (psi) of the left and right wheels L 、ψ R ) And shifting (Δ ψ) the respective steering amounts of the left and right wheels in directions opposite to each other with respect to the respective standard steering amounts of the left and right wheels can be performed L 、Δψ R ) And based on the rotation applied to the left and right wheels respectively in the reverse offset steeringThe friction coefficient of the road surface on which the vehicle runs is estimated from the directional force. The coefficient of friction of the road surface on which the vehicle is running can be estimated while maintaining stable running of the vehicle.)

1. A vehicle steering system mounted on a vehicle, comprising: a pair of wheel steering devices for independently steering left and right wheels; and a controller that controls the pair of wheel turning devices, wherein,

the controller is configured to determine a standard steering amount for each of the left and right wheels in accordance with a steering request, execute reverse offset steering for offsetting the steering amount of each of the left and right wheels in opposite directions from the standard steering amount of each of the left and right wheels, and estimate a friction coefficient of a road surface on which the vehicle travels based on steering forces applied to the left and right wheels during the reverse offset steering.

2. The vehicular steering system according to claim 1, wherein,

the controller is configured to execute the reverse offset steering while maintaining the vehicle body slip angle at the standard vehicle body slip angle when the vehicle body slip angle in the case where the right and left wheels are steered by the standard steering amount is defined as the standard vehicle body slip angle.

3. The vehicular steering system according to claim 1 or 2, wherein,

the controller is capable of performing the reverse offset steering when the vehicle is running straight as follows: the respective steering amounts of the left and right wheels are made equal to each other in offset amount with respect to the standard steering amount.

4. The vehicular steering system according to any one of claims 1 to 3, wherein,

the controller is capable of performing the reverse phase offset steering as follows when the vehicle turns: the amount of deviation of the steering amount of the outer-side of turning of the left and right wheels from the standard steering amount is made smaller than the amount of deviation of the steering amount of the inner-side of turning of the left and right wheels from the standard steering amount.

5. The vehicular steering system according to any one of claims 1 to 4,

the controller is capable of performing the reverse offset steering as follows: the offset amount of the steering amount of each of the left and right wheels with respect to the standard steering amount is periodically varied.

6. The vehicular steering system according to claim 5, wherein,

the controller is configured to estimate a friction coefficient of a road surface on which the vehicle travels, based on steering forces applied to the left and right wheels, respectively, when the offset amount reaches a peak value.

7. The vehicular steering system according to any one of claims 1 to 6,

the controller is configured to estimate a degree of magnitude of a friction coefficient of a road surface based on a degree of steering force applied to each of the left and right wheels during the reverse-phase offset steering.

8. The vehicular steering system according to any one of claims 1 to 7, wherein,

the controller is configured to recognize that a vehicle is traveling on a low-friction-coefficient road, and estimate a friction coefficient of a road surface on which the vehicle is traveling.

9. The vehicular steering system according to claim 8, wherein,

the controller is configured to determine that the vehicle is traveling on a low-friction-coefficient road when a steering force of at least one of the left and right wheels in reverse-phase offset steering is smaller than a set steering force.

10. The vehicular steering system according to claim 8 or 9, wherein,

the controller is configured to execute a recognition process for causing a driver to recognize that the vehicle is traveling on a low-friction-coefficient road.

11. The vehicular steering system according to claim 10, wherein,

the vehicle steering system includes:

a steering operation member operated by a driver; and

a reaction force applying device that applies an operation reaction force to the steering operation member under the control of the controller,

the controller is configured to reduce the operation reaction force when the vehicle is traveling on the low-friction-coefficient road as compared to when the vehicle is not traveling on the low-friction-coefficient road as the identification process.

Technical Field

The present invention relates to a steering system mounted on a vehicle and configured to steer left and right wheels independently.

Background

In the field of a vehicle steering system (hereinafter, may be simply referred to as a "steering system"), for example, as described in the following patent documents, there is a technology for estimating the degree of understeer during turning of a vehicle based on an axial force (a force for steering wheels, corresponding to a "steering force").

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2019-64568

Disclosure of Invention

Problems to be solved by the invention

The degree of understeer during turning of the vehicle depends on the friction coefficient of the road surface on which the vehicle is traveling (hereinafter, sometimes referred to as "road surface μ"), and therefore, it is advantageous to be able to estimate the road surface μ if the stability of turning of the vehicle is taken into consideration. On the other hand, in the development of a steering system capable of independently steering the left and right wheels (hereinafter, sometimes referred to as "independent left and right steering system"), it is advantageous to estimate the road surface μ in the independent left and right steering system, and the use of a unique estimation method for the independent left and right steering system can improve the practicability of the independent left and right steering system. The present invention has been made in view of such circumstances, and an object thereof is to provide a left-right independent steering type steering system having high practicability.

Means for solving the problems

In order to solve the above problem, a vehicle steering system according to the present invention includes: a pair of wheel steering devices for independently steering left and right wheels; and a controller for controlling the pair of wheel steering devices, characterized in that,

the controller is configured to determine a standard steering amount for each of the left and right wheels in accordance with a steering request, execute reverse offset steering for offsetting the steering amount of each of the left and right wheels in opposite directions from the standard steering amount of each of the left and right wheels, and estimate a friction coefficient of a road surface on which the vehicle travels based on steering forces applied to the left and right wheels during the reverse offset steering.

Effects of the invention

Generally, when the road surface μ is small, the steering force, which is a force for steering the wheels, is small. Therefore, the road surface μ can be easily estimated based on the steering force. In the case of the left-right independent steering type steering system, the left and right wheels can be steered in opposite directions, that is, in opposite phases, so in the present steering system, the above-described opposite-phase offset steering is performed. According to the reverse offset steering, for example, although some load is applied to the wheel steering apparatus, the left and right wheels can be steered to some extent in a state where the vehicle body slip angle generated by the steering of the wheels is not greatly deviated from the vehicle body slip angle according to the steering request. Therefore, according to the steering system of the present invention, the road surface μ of the road on which the vehicle travels can be estimated while maintaining stable travel of the vehicle.

[ MEANS FOR INVENTING ] A method for producing a semiconductor device

The "estimation of the friction coefficient of the road surface" in the present invention includes not only the numerical value of the estimated road surface μ but also the degree to which the degree of the estimated road surface μ is. Specifically, for example, the degree of the magnitude of the friction coefficient of the road surface may be estimated from the degree of the steering force applied to each of the left and right wheels in the reverse-phase offset steering. In this case, the degree of the size of road surface μmay be estimated discretely, i.e., stepwise. As the estimation of the road surface μ, for example, it is assumed that the vehicle is traveling on a low friction coefficient road (hereinafter, sometimes referred to as a "low μ road") which is a road surface on which the road surface μ is relatively small. Specifically, it may be determined that the vehicle is traveling on the low- μ road in a case where the steering force of at least one of the left and right wheels in the reverse-phase offset steering is smaller than the set steering force. Further, it is also conceivable that the vehicle is traveling on a road surface, i.e., a cross-road surface, where the difference in road surface μ between the portions where the left and right wheels pass is large.

The "steering request" may be grasped based on, for example, the amount of operation of a steering operation member such as a steering wheel by the driver. In the case of autonomous driving, it is sufficient to grasp the driving state based on an instruction from the autonomous driving system. A case where the vehicle is intended to be driven straight, such as a case where the steering member is not operated, may be considered as one of the steering requests. The "steering amount of the wheel" may be regarded as a steering angle of the wheel with respect to the wheel in the straight traveling state. The "standard steering amount" may be that the left and right wheels are identical to each other, or may be different from each other in the left and right wheels, for example, in accordance with ackermann geometry or the like. The "steering force" is a force applied to the wheel by the wheel steering device in order to steer the wheel or in order to maintain the steering amount of the wheel, but when the wheel steering device is configured to steer the wheel by a force exerted by the electric motor, it can be considered as the magnitude of the torque of the electric motor, and further, as the magnitude of the current supplied to the electric motor.

In the case where the vehicle body slip angle in the case where the right and left wheels are steered by the standard steering amount is defined as the standard vehicle body slip angle, the steering system of the invention preferably executes the reverse offset steering while maintaining the vehicle body slip angle at the standard vehicle body slip angle. For example, the following reverse offset steering may also be performed when the vehicle is traveling straight: the respective steering amounts of the left and right wheels are made equal to each other in offset amount with respect to the standard steering amount. In addition, the following reverse phase offset steering may be performed when the vehicle turns: the amount of deviation of the steering amount of the outer-side of turning of the left and right wheels from the standard steering amount is made smaller than the amount of deviation of the steering amount of the inner-side of turning of the left and right wheels from the standard steering amount. Since the ground contact load of the turning outer wheel is larger than that of the turning inner wheel, according to this aspect, the vehicle body slip angle in the reverse offset steering during turning of the vehicle can be made close to or maintained at the standard vehicle body slip angle.

The reverse offset steering may be performed such that the amount of offset of the steering amount of each of the left and right wheels with respect to the standard steering amount periodically varies. According to such reverse offset steering, since the time period in which the offset amount with respect to the left and right wheels is increased can be made relatively short, the factor that hinders the stability of the traveling of the vehicle can be reduced. In addition, in the case where the offset amount is periodically varied, if the friction coefficient of the road surface on which the vehicle is running is estimated based on the steering forces applied to the left and right wheels when the offset amount reaches the peak, the estimation is accurate.

In the steering system of the present invention, it is also possible to execute recognition processing for causing the driver to recognize that the vehicle is traveling on a low- μ road. Specifically, for example, the driver may be recognized by a display on the display of the instrument panel or a warning sound from a speaker. In addition, in the case where the vehicle steering system includes a steering operation member operated by the driver and a reaction force applying device that applies an operation reaction force to the steering operation member, it is also possible to execute, as the recognition processing, processing for reducing the operation reaction force when the vehicle is traveling on a low friction coefficient road as compared with when the vehicle is not traveling on a low friction coefficient road. According to this processing, the driver can be effectively made to feel the low μ road running.

Drawings

Fig. 1 is a schematic diagram showing an overall configuration of a vehicle mounted with a vehicle steering system according to an embodiment.

Fig. 2 is a perspective view showing a wheel arrangement module in which a wheel steering device constituting a vehicle steering system according to the embodiment is incorporated.

Fig. 3 is a graph and a schematic diagram for explaining the reverse phase offset steering performed in the vehicular steering system of the embodiment when the vehicle is running straight.

Fig. 4 is a graph and a schematic diagram for explaining the reverse phase offset steering performed in the steering system for a vehicle of the embodiment when the vehicle turns.

Fig. 5 is a flowchart of a steering overall control routine executed in the vehicle steering system of the embodiment.

Fig. 6 is a flowchart of a wheel turning routine executed in the vehicular steering system of the embodiment.

Fig. 7 is a flowchart of an operation reaction force control routine executed in the vehicular steering system of the embodiment.

Fig. 8 is a flowchart of the reverse phase offset steering processing subroutine and the standard torque storage processing subroutine executed in the steering overall control program.

Fig. 9 is a flowchart of a road surface μ estimation processing subroutine executed in the steering integrated control program.

Detailed Description

Hereinafter, a vehicle steering system, which is an embodiment of the present invention, will be described in detail with reference to the accompanying drawings. The present invention can be implemented in various ways, including the way described in the above-mentioned [ invention mode ], in addition to the following examples, by carrying out various modifications and improvements based on the knowledge of those skilled in the art.

[ examples ] A method for producing a compound

[A] Vehicle integrated structure mounted with vehicle steering system

As schematically shown in fig. 1, the steering system of the embodiment is mounted on a vehicle having front left and right wheels 10FL, 10FR and rear left and right wheels 10RL, 10 RR. The right and left front wheels 10FL, 10FR are driving wheels and steered wheels. When it is not necessary to distinguish between the right and left front wheels 10FL and 10FR, these wheels are collectively referred to as the front wheels 10F, when it is not necessary to distinguish between the right and left rear wheels 10RL and 10RR, these wheels are collectively referred to as the rear wheels 10R, and when it is not necessary to distinguish between the front wheels 10F and the rear wheels 10R, these wheels may be collectively referred to as the wheels 10 alone.

The steering system is a so-called steer-by-wire type steering system, and includes: a pair of wheel steering devices 12 provided for the front wheels 10F to steer the two front wheels 10F independently of each other; an operation device 14 for receiving an operation by a driver; a pair of steering electronic control units (hereinafter, sometimes simply referred to as "steering ECUs") 16 for controlling the pair of wheel steering devices 12, respectively; and an operating electronic control unit (hereinafter, sometimes simply referred to as "operating ECU") 18 for controlling the operating device 14 and collectively controlling the steering ECU 16. The configuration and control of the steering system will be described in detail later, but it is considered that the two steering ECUs 16 and the operation ECU18 constitute a controller of the steering system.

The vehicle is equipped with a vehicle drive system including a pair of wheel drive units 20 provided to the two front wheels 10F, respectively, and configured to rotationally drive the front wheels by electric motors. A vehicle drive system is provided with: an accelerator pedal 22 as an acceleration operation member operated by the driver; an accelerator operation amount sensor 24 for detecting an operation amount of the accelerator pedal 22; and a vehicle drive electronic control unit (hereinafter, may be simply referred to as "drive ECU") 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 configuration and performs general control, the description of the configuration and control of the vehicle drive system will be omitted.

The vehicle is provided with a hydraulic brake system. The brake system is provided with: a brake pedal 30 as a brake operating member operated by a driver; a master cylinder 32 connected to the brake pedal 30; a working fluid supply device 34 that has a hydraulic pressure source constituted by a pump or the like and pressurizes a working fluid; four brake devices 36 provided to the four wheels, respectively, for braking the respective wheels by the pressure of the working fluid from the working fluid supply device 34; and a brake electronic control unit (hereinafter, may be referred to as "brake ECU") 38 that controls the operation of the working fluid supply device 34. The brake system is a so-called brake-by-wire type system, and the brake ECU38 controls the pressure of the hydraulic fluid supplied from the hydraulic fluid supply device 34 to the brake devices 36 of the respective wheels 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, to control the braking force applied to the vehicle. Since the brake system has a general configuration and performs general control, the description of the configuration and control of the brake system is omitted.

A CAN (car area network or controllable area network) 44 is provided in the vehicle, and two steering ECUs 16, an operation ECU18, a drive ECU26, and a brake ECU38 are connected to the CAN 44. These ECUs 16, 18, 26, and 38 execute control to be performed, while communicating with each other via CAN 44. Incidentally, each of these ECUs 16, 18, 26, 38 is configured to include a computer having a CPU, ROM, RAM, and the like, and a driver (drive circuit) for driving constituent elements (e.g., an electric motor, a valve, a pump, and the like) based on instructions of the computer. The vehicle is provided with a yaw rate sensor 46 for detecting the yaw rate of the vehicle, and each rear wheel 10R is provided with a wheel speed sensor 48 for detecting the wheel rotation speed (hereinafter, sometimes referred to as "wheel speed") of each rear wheel 10R. The yaw rate sensor 46 and the wheel speed sensor 48 are also connected to the CAN 44.

[B] Hardware structure of steering system for vehicle

The pair of wheel turning devices 12 of the vehicle steering system of the present embodiment are respectively assembled to the wheel arrangement module 50. One of the pair of wheel drive units 20 of the vehicle drive system and one of the four brake devices 36 of the brake system are also assembled to the wheel-arrangement module 50. As shown in fig. 2, a wheel arrangement module (hereinafter, simply referred to as "module") 50 is a module for arranging a wheel 10b, to which a tire 10a is attached, on a vehicle body. The wheel 10b itself may be regarded as a wheel, but in the present embodiment, the wheel 10b with the tire 10a mounted thereon is referred to as a wheel 10 for convenience.

When the description is given of the wheel steering device 12 of the steering system of the present invention while describing the structure of the module 50, the wheel driving unit 20 provided in the module 50 includes: a housing 20 a; an electric motor as a drive source and a speed reducer (both not shown) for reducing the rotation of the electric motor, which are incorporated in the housing 20 a; and an axle hub (hidden from view) to which the wheel 10b is mounted. The wheel drive unit 20 is disposed inside the rim of the wheel 10b, and is referred to as a so-called in-wheel motor unit. Since the wheel drive unit 20 has a well-known configuration, a description of the configuration thereof is omitted.

The module 50 is configured to include a mcpherson type suspension device (also referred to as a "mcpherson strut type"). In this suspension device, the housing 20a of the wheel drive unit 20 functions as a bracket that rotatably holds the wheel, and further, the housing 20a functions as a knuckle in the wheel steering device 12 and allows vertical movement with respect to the vehicle body. Therefore, the suspension device is configured to include the lower arm 52 as a suspension arm, the housing 20a of the wheel drive unit 20, the shock absorber 54, and the suspension spring 56.

Since the suspension device itself has a general structure, for simplicity of explanation, the lower arm 52 has a shape called an L-arm, and has a base end portion divided into two parts in the vehicle longitudinal direction, and the lower arm 52 is supported at the base end portion via a first bush 58 and a second bush 60 to a side member (not shown) of the vehicle body so as to be rotatable about an arm rotation axis LL. The housing 20a of the wheel drive unit 20 is rotatably coupled to the distal end portion of the lower arm 52 at the lower portion thereof via an arm coupling ball joint 62 (hereinafter, sometimes referred to as "first joint 62") as a first joint.

The lower end of the damper 54 is fixedly supported by the casing 20a of the wheel drive unit 20, and the upper end of the damper 54 is supported by the upper portion of the tire casing of the vehicle body via the upper support 64. The upper end portion of the suspension spring 56 is also supported on the upper portion of the tire casing of the vehicle body via an 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 damper 54 are disposed in parallel with each other between the lower arm 52 and the vehicle body.

As described above, the present module 50 includes the brake device 36, and the brake device 36 is a disc brake device including the disc rotor 66 and the caliper 68, the disc rotor 66 being attached to the axle hub together with the wheel 10b and rotating together with the wheel 10, and the caliper 68 being held on the housing 20a of the wheel drive unit 20 across the disc rotor 66. Although not described in detail, the brake caliper 68 includes 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 disc rotor 66 depending on the pressure of the hydraulic fluid supplied from the hydraulic fluid supply device 34 to the hydraulic cylinder.

The wheel steering device 12 is a single-wheel independent steering device for steering only one of the pair of left and right wheels 10 independently of the other wheel, and is roughly configured to include: a housing 20a of the wheel drive unit 20 that functions as a knuckle as described above (hereinafter, may be referred to as a "knuckle 20 a" when handled as a component of the wheel steering apparatus 12); a steering actuator 70 disposed on the lower arm 52 at a position close to the base end portion of the lower arm 52; and a tie rod 72 connecting the steering actuator 70 and the knuckle 20 a.

The steering actuator 70 includes: a steering motor 70a as an electric motor serving as a driving source; a speed reducer 70b that reduces the rotation of the steering motor 70 a; and an actuator arm 70c that rotates by rotation of the steering motor 70a via the reduction gear 70b and functions as a steering rocker arm. The base end portion of the tie rod 72 is coupled to the actuator arm 70c via a rod base end portion coupling ball joint 74 (hereinafter, sometimes referred to as "second joint 74") which is a second joint, and the tip end portion of the tie rod 72 is coupled to the knuckle arm 20b included in the knuckle 20a via a rod tip end portion ball joint 76 (hereinafter, sometimes referred to as "third joint 76") which is a third joint.

In the vehicle wheel steering device 12, a line connecting the center of the upper support 64 and the center of the first joint 62 is a kingpin axis KP. By operating the steering motor 70a, the actuator arm 70c of the steering actuator 70 is rotated about the actuator axis AL as indicated by the thick arrow in the figure. This rotation is transmitted by the tie rod 72, causing the knuckle 20a to rotate about the kingpin axis KP. That is, the wheels 10 are steered as indicated by thick arrows in the drawing. According to such a configuration, the vehicle wheel steering device 12 is provided with a motion conversion mechanism 78, and the motion conversion mechanism 78 includes the actuator arm 70c, the tie rod 72, the knuckle arm 20b, and the like, and converts the rotational motion of the steering motor 70a into the steering motion of the wheels 10.

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. In short, the suspension device, the brake device, and the wheel steering device can be mounted on the vehicle by simply 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 portion of the tire casing of the vehicle body. That is, the present module 50 is excellent in mountability to a vehicle.

The operation device 14 has a general structure of a steer-by-wire type steering system, and as shown in fig. 1, the operation device includes: a steering wheel 80 as a steering member operated by the driver; a steering sensor 82 for detecting a steering angle, which is a rotation angle of the steering wheel 80, as an operation amount of the steering member from the straight-ahead position; and a reaction force applying device 84 that applies an operation reaction force to the steering wheel 80. The reaction force applying device 84 includes: an electric motor as a force source, i.e., a reaction force motor 84 a; and a speed reducer 84b for transmitting the force of the reaction force motor 84a to the steering wheel 80.

[C] Control of a steering system for a vehicle

i) Basic control

In the present steering system, the operation ECU18 determines the target steering angle ψ of the front left wheel 10FL_LTarget steering angle ψ of front right wheel and front left wheel_RTargets for steering angle ψ, which is the steering amount of each front wheel 10F, based on these target steering angles ψ_L*、ψ_RThe pair of steering ECUs 16 control the wheel steering devices 12 corresponding to the steering ECUs, respectively, to steer the front left wheel 10FL and the front right wheel 10FR at the steering angles ψ, respectively_L、ψ_RBecomes a target steering angle psi_L*、ψ_RThe pattern is reversed.

Specifically, the operation ECU18 determines the vehicle body slip angle β to be achieved in the vehicle body based on the steering operation angle δ obtained from the steering sensor 82, which is the steering request_SI.e. target body slip angle beta_S*. Incidentally, in the case where the own vehicle is automatically driving, information about the target vehicle body slip angle β is sent from an automatic driving system (not shown) via CAN44_SThe information of the star serves as a steering requirement. In addition, even when the steering wheel 80 is not operated or when a straight-ahead command is issued from the automated driving system, it is assumed that there is a steering request to the effect that the vehicle should go straight, and the ECU18 is operated to set the target vehicle body slip angle β_SDecided as "0".

The operation ECU18 determines the steering angle ψ of each of the right and left front wheels 10FL and 10FR_L、ψ_RTarget steering angle psi of the target_L*、ψ_R*. The ratio of the steering amounts of the left and right wheels (steering amount ratio) can be arbitrarily set, but in the present steering system, the target steering angle ψ of the left and right front wheels 10FL, 10FR is set in parallel geometry for the sake of simplicity of explanation_L*、ψ_RIs set to itThis equality (strictly speaking, the target steering angle ψ)_L*、ψ_RPositive and negative signs of each other). Incidentally, the target steering angle ψ determined in this basic control is sometimes to be set_L*、ψ_RIn the relation with the inverted offset steer described later, the standard steer angle ψ as a standard steer amount_L*、ψ_R*. If the front wheels 10FL, 10FR are to be steered by the standard steering angle ψ_L*、ψ_RSlip angle of vehicle body in the case of_SThe target vehicle body slip angle β determined as described above can be set as the standard vehicle body slip angle_SConsider the standard body slip angle. The operating ECU18 will determine the target steering angle ψ_L*、ψ_RThe information of these points is sent to the two steering ECUs 16 corresponding to the left and right front wheels 10F, respectively, via the CAN 44.

Each steering ECU16 controls the corresponding wheel steering device 12 so that the steering angle ψ of the corresponding front wheel 10F becomes the transmitted target steering angle ψ. Specifically, since the wheel steering device 12 does not include a steering angle sensor for directly detecting the steering angle ψ of the wheels 10, the steering ECU16 controls the steering force generated by the steering actuator 70 based on the motor rotation angle θ of the steering motor 70a in the present steering system, using the fact that there is a specific relationship between the steering angle ψ of the wheels 10 and the rotation angle θ of the steering motor 70a (hereinafter, sometimes referred to as "motor rotation angle") in the present steering system. Since the steering force generated by the steering actuator 70 is equivalent to the steering torque Tq that is the torque generated by the steering motor 70a, specifically, the steering ECU16 determines the target steering torque Tq that is the steering torque Tq to be generated by the steering motor 70a based on the motor rotation angle θ of the steering motor 70 a. The motor rotation angle θ can be considered as a displacement angle of the motor shaft from a state when the vehicle travels straight, and is accumulated over 360 °.

Specifically describing the determination of the target steering torque Tq, the steering ECU16 determines the target motor rotation angle θ, which is the target of the motor rotation angle θ, for each wheel 10 based on the target steering angle ψ. The steering motor 70a is a brushless DC motor, and includes a motor rotation angle sensor (e.g., a hall IC, a resolver, etc.) for switching the phase during current supply to the steering motor itself. The steering ECU16 recognizes the actual motor rotation angle θ, which is the motor rotation angle θ at the current time point with reference to the reference motor rotation angle, based on the detection of the motor rotation angle sensor. The steering ECU16 obtains a motor rotation angle deviation Δ θ, which is a deviation of the actual motor rotation angle θ from the target motor rotation angle θ, and determines the target steering torque Tq according to the following equation based on the motor rotation angle deviation Δ θ (═ θ - θ).

Tq*＝G_P·Δθ+G_D·(dΔθ/dt)+G_I·∫Δθdt

The expression is based on a feedback control rule based on the motor rotation angle deviation Δ θ, and the first term, the second term, and the third term are a proportional term, a differential term, and an integral term, respectively, G_P、G_D、G_IRespectively, proportional gain, differential gain, integral gain.

Steering torque Tq and supply current I to steering motor 70a_SHave a specific relationship. 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_SRoughly proportional. Accordingly, the steering ECU16 determines the supply current I to be supplied to the steering motor 70a based on the determined target steering torque Tq_STarget of (I) target supply current I_SAnd supplying current I based on the target_SSupplies current to steering motor 70 a.

The operation ECU18 also executes control for applying an operation reaction force, that is, operation reaction force control, to the steering wheel 80 as a reaction force against the steering operation. The operation reaction force control is a general control, and the operation reaction force can be determined by various methods. In the steering system of the present embodiment, the operation ECU18 converts the reaction torque Tq to be generated by the reaction motor 84a of the reaction force applying device 84 in accordance with a typical method, that is, in accordance with the following equation_CI.e., target reaction torque Tq_CIs determined as the target operating reaction force.

Tq_C*＝Gδ·δ+G_v·v+G_T·(Tq_L+Tq_R)/2

Gδ、G_v、G_TThe steering angle δ, the vehicle speed v, and the actual steering torque Tq determined by each steering ECU16 (the steering torque Tq of the front left wheel 10F may be referred to as a left wheel steering torque Tq_LThe steering torque Tq of the right front wheel 10F is referred to as a right wheel steering torque Tq_R) In short, the weighting gain is such that the larger the steering angle δ, the higher the vehicle speed v, and the larger the steering torque Tq, the larger the reaction torque Tq is applied_C. Further, the operation ECU18 is based on the respective wheel speeds v of the front wheels 10F depending on the rotation speed of the electric motor of the wheel drive unit 20_WAnd the respective wheel speeds v of the rear wheels 10R depending on the detection of the wheel speed sensors 48_WTo determine the vehicle speed v.

The operation ECU18 determines the target reaction torque Tq based on the above_CTo determine the supply current I to be supplied to the reaction force motor 84a_CTarget of (I) target supply current I_CAnd supplying current I based on the target_CCurrent is supplied to the reaction force motor 84 a.

ii) estimation of the coefficient of friction of the road surface

In the steering system of the present embodiment, the ECU18, which is the controller, estimates the coefficient of friction of the road surface on which the vehicle is traveling. In detail, the operation ECU18 makes the steering angle ψ of the right and left front wheels 10F during running of the vehicle_L、ψ_ROffset and based on the steering torque Tq in the state after the offset_L、Tq_RThe case of running on a low- μ road with a low road surface μ is assumed.

In a state where the vehicle speed v is low, if the road surface μ is large, a large steering torque Tq is required to steer the wheel 10, and conversely, if the road surface μ is small, the wheel 10 can be steered by a small steering torque Tq. Further, since the self-aligning torque acts on the wheels 10, when the wheels 10 are steered at a certain steering angle ψ or when the wheels 10 are maintained at the certain steering angle ψ, the wheels 10 must be steered or maintained against the self-aligning torque, and the steering torque Tq required is larger as the vehicle speed v is higher and the steering torque Tq required is larger as the steering angle ψ is larger. This torque also increases when the road surface μ is large, and decreases when the road surface μ is small.

In the steering system of the present embodiment, the steering angle ψ of the right and left front wheels 10F is made to take advantage of the above_L、ψ_RThe displacement estimates the road surface μ. However, if the steering angle ψ of the right and left front wheels 10F is made during running of the vehicle_L、ψ_RIf the vehicle body is shifted in the same direction, the shift will be caused by the slip angle beta of the vehicle body_SThe variation in the speed variation(s) deteriorates the driving stability (concept including turning stability) of the vehicle. Therefore, in the steering system of the present embodiment, steering angle ψ of the left and right front wheels 10F is executed_L、ψ_RThe reverse offset turns to offset in opposite directions to each other.

Reverse phase offset steering by applying the previously described standard steering angle psi_L*、ψ_RShift is performed. When the offset amount is set to the offset amount Δ ψ, the ECU18 is operated to periodically vary the offset amount Δ ψ in order to shorten the time for increasing the offset amount Δ ψ. The offset amount Δ ψ can be expressed as a function of the passage of time t as a parameter, that is, a function of the following equation.

Δψ＝f(t)

The ECU18 is operated to set the target steering angle ψ of the left and right front wheels 10F_L*、ψ_RFrom the nominal steering angle psi_L*、ψ_ROffset by an offset amount Δ ψ. Specifically, the offset amount of the left front wheel 10F is set to Δ ψ_LThe offset of the right front wheel is set to Δ ψ_RThen, the target steering angle ψ of the right and left front wheels 10F is made_L*、ψ_RVaries according to the following formula.

ψ_L*＝ψ_L*+Δψ_L

ψ_R*＝ψ_R*-Δψ_R

Specifically, for example, when the vehicle is traveling straight, the offset amount Δ ψ of the left front wheel 10F_LAnd the offset amount [ delta ] ψ of the right front wheel 10F_RAre all Δ ψ, are equal to each other, and target steering angles ψ of the left and right front wheels 10F_L*、ψ_RAs shown in fig. 3(a), the steering angle ψ of the front wheel 10F is changed as shown in fig. 3(b)_L、ψ_RPeriodically varying. Therefore, even if the reverse offset steering is performed, the vehicle body slip angle β_SWill also be maintained at the target body slip angle beta_S*。

On the other hand, when the vehicle turns in the forward left direction, the vehicle body shared load of the turning outer wheel is larger than the vehicle body shared load of the turning inner wheel due to the centrifugal force, and therefore the offset amount Δ ψ with respect to the front wheel 10F serving as the turning outer wheel is smaller than the offset amount Δ ψ of the front wheel 10F serving as the turning inner wheel. Specifically, when the vehicle is turning in the right-left direction, the offset amount Δ ψ for the right and left front wheels 10F is determined in accordance with the following equation_L、Δψ_R。

Δψ_L＝Δψ×τ_I

Δψ_R＝Δψ×τ_O

On the other hand, when the vehicle is turning right, the offset amount Δ ψ of the right and left front wheels 10F is determined in accordance with the following equation_L、Δψ_R。

Δψ_L＝Δψ×τ_O

Δψ_R＝Δψ×τ_I

In addition, tau_OIs a weighting factor, τ, for the outer wheel of a turn_IIs a weighting coefficient for the inside wheel of a turn. Weighting coefficient tau_O、τ_IIs composed of

τ_O<τ_I≤1，

Based on the actual yaw rate γ detected by the yaw rate sensor 46, the vehicle body slip angle β is determined_SMaintained at the target slip angle beta_S*。

Therefore, for example, when the vehicle is turning in the right and left direction, the target steering angle ψ of the right and left front wheels 10F_L*、ψ_RAs shown in fig. 4(a), the steering angle ψ of the front wheel 10F is changed as shown in fig. 4(b)_L、ψ_RPeriodically varying.

In the steering system of the present embodiment, the road surface μ is estimated in view of the steering of the wheels 10 against the self-aligning torque, and the ECU18 is operated with the vehicle speed v as the set vehicle speed v₀(e.g., 30 km/h)) In the above, the reverse offset steering is performed on the premise that the vehicle body behavior is appropriate. Further, the operation ECU18 converges the actual yaw rate γ detected by the yaw rate sensor 46 on the target vehicle body slip angle β based on the vehicle speed v and the target vehicle body slip angle_SSet a margin ± Δ γ to the determined target yaw rate γ_MWhen the obtained allowable yaw rate is within the range, it is determined that the vehicle body behavior is appropriate.

The ECU18 is operated to perform the reverse phase offset steering in units of a plurality of cycles. Each steering ECU16 detects the actual supply current I actually supplied to the steering motor 70a_SAnd based on the detected actual supply current I_SThe actual steering torque Tq, which is the steering torque actually exerted, is determined. Then, each steering ECU16 transmits information relating to the actual steering torque Tq. The operation ECU18 receives this information to acquire the actual steering torque Tq, and stores the time point t at which the offset amount Δ ψ reaches a peak in each cycle in the fluctuation of the offset amount Δ ψ in a plurality of cycles_PThe actual steering torque Tq. Specifically, the actual steering torque Tq depends on the vehicle speed v and the vehicle slip angle β_STherefore, in order to be used as a parameter for estimating road surface μ, operation ECU18 is operated based on vehicle speed v and target vehicle body slip angle β_SThe actual steering torque Tq is normalized to obtain a reference torque Tq of the right and left front wheels 10F_LS、Tq_RSThe operating ECU18 stores these standard torques Tq_LS、Tq_RS. In addition, the standard torque Tq_LS、Tq_RSCan be regarded as the slip angle beta with the vehicle speed v and the vehicle body_SAn unrelated unified torque.

The ECU18 is operated when the set time t corresponding to a plurality of cycles has elapsed₀(hereinafter, it may be referred to as "unit offset time t₀") of one unit reverse offset steering, based on the stored standard torque Tq of the left and right front wheels 10F_LS、Tq_RSThe road surface mu is estimated. If explained in detail, the operation ECU18 determines the stored standard torques Tq for the left and right front wheels 10F, respectively_LS、Tq_RSWhether or not the torque is lower than the threshold value, and the standard torque Tq lower than the threshold value exists in any one of the left and right front wheels 10F_LS、Tq_RSIn the state ofUnder the circumstances, it is determined that the vehicle is traveling on a low μ road (low friction coefficient road). More specifically, two threshold torques, that is, a first threshold torque Tq, are set as the threshold values₁And the first threshold torque Tq₁Small second threshold torque Tq₂. Operating the ECU18 when there is more than a first threshold torque Tq₁Small reference torque Tq_LS、Tq_RSAnd there is no torque below the second threshold torque Tq₂Standard torque Tq of_LS、Tq_RSIn the case where the road on which the vehicle is traveling is determined to be a first low- μ road having a road surface μ smaller than a normal road surface μ, the presence of the torque below the second threshold torque Tq is determined to be lower₂Standard torque Tq of_LS、Tq_RSIn the case of (2), the road on which the vehicle is traveling is determined to be a second low- μ road having a road surface μ smaller than that of the first low- μ road. Therefore, the first threshold torque Tq₁Second threshold torque Tq₂It can be considered as a set steering force for recognizing that the vehicle is traveling on a low- μ road.

In other words, in the steering system of the present embodiment, the operation ECU18 is configured to assume that the vehicle is traveling on the low μ road when the steering force of at least one of the left and right wheels in the reverse-phase offset steering is smaller than the set steering force, and further configured to estimate the degree of the magnitude of the road surface μ in stages in accordance with the degree of the steering force applied to the wheels 10F.

When the vehicle is determined to be traveling on a low- μ road, the operation ECU18 issues a warning to that effect via the annunciator 86 (see fig. 1) provided in the instrument panel of the vehicle. This warning is one of recognition processes for making the driver recognize that the vehicle is traveling on a low- μ road. The annunciator 86 has a display and a speaker, and performs an alarm by using an image displayed on the display and a sound from the speaker.

As another identification process, the operation ECU18 reduces the operation reaction force when the vehicle is traveling on the low μ road in the operation reaction force control described above, as compared to when the vehicle is not traveling on the low μ road. Specifically, when the vehicle is traveling on a low- μ road, the target reaction torque Tq is set_CDecreasing the reducing torque Δ Tq_C. Specifically, the torque Δ Tq is reduced_CSet with a first reducing torque DeltaTq_C1And for further reducing the reaction torque Tq_CSecond reduction torque Δ Tq_C2(＞ΔTq_C1) These two methods are to make the target reaction torque Tq when at least one of the front left and right wheels 10F is passing the first low μ road and neither of the front left and right wheels 10F is passing the second low μ road_CReducing the first reducing torque Δ Tq_C1When at least one of the right and left front wheels 10F is passing through the second low μ road, the target reaction torque Tq is set_CReducing the second reducing torque Δ Tq_C2。

By the recognition processing as described above, the driver can recognize that the vehicle is passing through the low μ road before the behavior of the vehicle becomes unstable due to the low μ road, in other words, without the behavior of the vehicle becoming unstable due to the low μ road. Further, by reducing the operation reaction force in two stages, the degree of the low μ road, that is, the degree of the road surface μ can be recognized. Further, when the steering torque Tq of only one of the right and left front wheels 10F is small, it can be recognized that the vehicle is traveling on a so-called crossroad, and therefore, the vehicle can be warned of this fact, that is, the fact that the vehicle is traveling on a crossroad.

iii) control flow

The control of the steering system described above is performed by the computer operating the ECU18 repeatedly executing the steering overall control routine shown in the flowchart in fig. 5 at short time intervals (for example, several msec to several tens of msec), the computer of each steering ECU16 repeatedly executing the wheel steering routine shown in the flowchart in fig. 6 at short time intervals (for example, several msec to several tens of msec), and the computer operating the ECU18 repeatedly executing the operation reaction force control routine shown in the flowchart in fig. 7 at short time intervals (for example, several msec to several tens of msec). Hereinafter, the flow of control of the steering system will be briefly described by describing the processing of the flowcharts according to these programs.

In the processing according to the steering summary control program, first, in step 1 (hereinafter, simply referred to as "step 1")"S1", and the same applies to other steps), it is determined whether or not the vehicle is under automatic driving. If the vehicle is not under automatic driving, the steering angle δ is acquired by detection of the steering sensor 82 in S2, and the target vehicle body slip angle β is determined based on the steering angle δ in S3_S*. If the vehicle is under automatic driving, at S4, the target vehicle body slip angle beta is obtained based on information from the automatic driving system_S*。

In the next S5, the vehicle speed v is determined, and in S6, the vehicle speed v is determined based on the target vehicle body slip angle β_STo determine the target steering angle ψ of the left and right front wheels 10F_L*、ψ_R*. In next S7, based on the vehicle speed v and the target vehicle body slip angle β_SA target yaw rate γ is determined, and in S8, an actual yaw rate γ is acquired based on the detection by the yaw rate sensor 46.

At S9 and S10, it is determined whether or not the vehicle speed v is the set vehicle speed v₀Whether or not the actual yaw rate γ is within the above-described allowable yaw rate range. The allowable yaw rate range is a range suitable for the traveling of the vehicle, and is a range in which a margin ± Δ γ is provided with respect to a target yaw rate γ_MBut a set range. When the vehicle speed v is the set vehicle speed v₀If the actual yaw rate γ is within the above-described allowable yaw rate range, the reverse phase offset steering process is executed in S11. When the vehicle speed v is less than the set vehicle speed v₀Or when the actual yaw rate γ is out of the allowable yaw rate range, the time counter t described later is reset in S12, and the standard torque Tq stored in the memory of the computer described later is cleared in S13_LS、Tq_RS。

The reverse phase offset diversion process of S11 is performed by executing a reverse phase offset diversion process subroutine of which the flowchart is shown in fig. 8. In this processing, first, in S21, the time counter t is incremented by a time Δ t corresponding to the execution interval of the program. The time counter t is a counter for detecting completion of one unit of reverse offset steering, and the initial value, which is the value at the start of one unit of reverse offset steering, is "0". In the next S22, it is determined whether the time counter t has reached the unit offset time t₀I.e., whether one unit of reverse offset steering is completed. Reaching a unit offset time t at a time counter t₀In the case of (1), in S23, road surface μ estimation processing described later is executed. When it is determined that the time counter t has not reached the unit offset time t₀In the case of (3), in S24, the offset Δ ψ to be realized at the current time point is determined based on the function f (t) described above.

After the offset amount Δ ψ is determined, at S25, the vehicle body slip angle β is determined based on the target vehicle body slip angle β_STo determine whether the vehicle is in a straight run. When the vehicle is traveling straight, the offset amount of the left front wheel 10F, i.e., the left wheel offset amount Δ ψ is set at S26_LAnd the right front wheel offset amount delta phi of the right front wheel 10F_RBoth are determined as the determined offset Δ ψ. When the vehicle is not traveling straight, that is, when the vehicle is turning, in S27, the weighting coefficient τ described above is determined as described above based on the actual yaw rate γ detected by the yaw rate sensor 46_O、τ_IAt next S28, based on the target vehicle body slip angle β_STo determine whether the vehicle is turning left. In the case where the vehicle is making a left turn, the left wheel offset amount Δ ψ is set in S29_LThe determined offset amount [ Delta ] phi is multiplied by a weighting coefficient [ tau ] for the turning inner wheel_IThe obtained value, right wheel offset Δ ψ_RThe determined offset amount [ Delta ] phi is multiplied by a weighting coefficient [ tau ] for the outer wheel of the curve_OThe resulting value. On the other hand, when the vehicle is not turning left, that is, when the vehicle is turning right, the left wheel offset amount Δ ψ is set at S30_LThe determined offset amount [ Delta ] phi is multiplied by a weighting coefficient [ tau ] for the outer wheel of the curve_OThe obtained value, right wheel offset Δ ψ_RThe determined offset amount [ Delta ] phi is multiplied by a weighting coefficient [ tau ] for the turning inner wheel_IThe resulting value. The offset amount Deltaphi of the left and right front wheels 10F is determined_L、Δψ_RThereafter, in S31, the left wheel is offset by Δ ψ_LTarget steering angle ψ added to the previously determined left front wheel 10F_LAnd, from the previously determined rightTarget steering angle ψ of front wheels 10F_RSubtracting the right wheel offset Δ ψ_R. The target steering angle ψ of the right and left front wheels 10F for performing the reverse offset steering is thus determined_L*、ψ_RAfter that, the processing of this subroutine is ended.

After the reverse phase offset steer processing or after the reverse phase offset steer processing is skipped, in S14, the target steer angle ψ is decided_L*、ψ_RThe information of the left and right is sent to each steering ECU 16. Next, in S15, it is determined whether or not the time counter t has reached the peak time point t described above_P. At the time point t when the time counter t reaches the peak value_PThen, the standard torque storing process of S16 is executed, and the time counter t does not reach the peak time point t_PIf so, the standard torque storing process of S16 is skipped, and the process of the present steering collective control routine is ended once.

The standard torque storing process of S16 is performed by executing a standard torque storing process subroutine of which the flowchart is shown in fig. 8. In the processing according to this subroutine, first, in S41, based on the information transmitted from each steering ECU16, the left wheel steering torque Tq, which is the actual steering torque Tq of each of the left and right front wheels 10F, is acquired_LRight wheel steering torque Tq_R. In the following S42, the vehicle speed v and the target vehicle body slip angle β are based on_SSteering left wheel torque Tq_LRight wheel steering torque Tq_RThe right and left front wheels 10F are normalized to determine a right wheel reference torque Tq which is a reference torque of the right and left front wheels 10F_LSRight wheel standard torque Tq_RSIn the next S43, these left wheel standard torques Tq are set_LSRight wheel standard torque Tq_RSStored in the memory of the computer.

As previously explained, in the course of executing the reverse phase offset steering process, the time counter t reaches the unit offset time t in S22₀In the case of (2), the road surface μ estimation process is executed in S23. The road surface μ estimation process is performed by executing a road surface μ estimation process subroutine of which a flowchart is shown in fig. 9.

In the processing according to the road surface μ estimation processing subroutine,first, in S51, the stored standard torque Tq for the left wheel is determined_LSIs present or not is present than the above-mentioned first threshold torque Tq₁Small left wheel standard torque. In the absence of less than the first threshold torque Tq₁In the case of the left wheel standard torque of (4), in S52, the left wheel road surface μ sign F μ_LIs set to "0". That is, it is considered that the road surface through which the left front wheel 10F is passing is not the low μ road. In the presence of a first threshold torque Tq₁In the case of a small left wheel reference torque, at S53, it is determined that the stored left wheel reference torque Tq is equal to or greater than the stored reference torque Tq_LSIs present or not is present than the above-mentioned second threshold torque Tq₂Small left wheel standard torque. In the absence of less than the second threshold torque Tq₂In the case of the left wheel standard torque of (4), in S54, the left wheel road surface μ sign F μ_LIs set to "1". That is, the road surface through which the left front wheel 10F is passing is regarded as the above-described first low μ road. In the presence of less than a second threshold torque Tq₂In the case of the left wheel standard torque of (4), in S55, the left wheel road surface μ sign F μ_LIs set to "2". That is, the road surface through which the left front wheel 10F is passing is regarded as the second low μ road described above.

In the next S56, the stored right wheel standard torque Tq is determined_RSIs present or not is present than the above-mentioned first threshold torque Tq₁Small right wheel standard torque. In the absence of a torque greater than the first threshold torque Tq₁In the case of a small right wheel standard torque, in S57, the right wheel road surface μ sign F μ_RIs set to "0". That is, the road surface through which the right front wheel 10F is assumed to be passing is not a low μ road. In the presence of a first threshold torque Tq₁In the case of a small right wheel reference torque, at S58, it is determined that the stored right wheel reference torque Tq is equal to_RSIs present or not is present than the above-mentioned second threshold torque Tq₂Small right wheel standard torque. In the absence of less than the second threshold torque Tq₂In the case of the right wheel standard torque of (2), in S59, the right wheel road surface μ sign F μ_RIs set to "1". That is, the road surface on which the right front wheel 10F is passing is regarded as the first low μ road described above. In the presence of less than a second threshold torque Tq₂Right wheel standard torque conditionIn S60, the road surface μmark F μ for the right wheel_RIs set to "2". That is, the road surface on which the right front wheel 10F is passing is regarded as the second low μ road described above.

In the following S61 and S62, the left wheel road surface μ sign F μ is judged_LWhether the road surface is 1 or 2 or not and the road surface mu mark F mu of the right wheel_RWhether it is "1" or "2". Mu mark F mu on left wheel road surface_LRoad surface mu mark F mu of right wheel_RIf either one of the values is "1" or "2", at S63, a warning is issued from the annunciator 86 as described above. Mu mark F mu on left wheel road surface_LRoad surface mu mark F mu of right wheel_RIf both are "0", the warning from the annunciator 86 is prohibited or stopped at S64.

At next S65, the stored standard torque Tq of the left wheel is cleared from the memory_LSRight wheel standard torque Tq_RS. Then, in S66, time counter t is reset to "0", and the process according to the present road surface μ estimation process subroutine ends.

In the processing of the wheel steering program executed by the computer of each steering ECU16, first, information about the target steering angle ψ of the corresponding front wheel 10F is received from the operation ECU18 in S71, and the target motor rotation angle θ of the steering motor 70a is determined based on the target steering angle ψ in S72. Next, at S73, an actual motor rotation angle θ, which is an actual rotation angle of the steering motor 70a, is acquired, and at S74, a motor rotation angle deviation Δ θ, which is a deviation of the actual motor rotation angle θ from the target motor rotation angle θ, is determined. In next S75, a target steering torque Tq is determined according to the above equation based on the motor rotation angle deviation Δ θ, and in S76, a target supply current I, which is a current to be supplied to the steering motor 70a, is determined based on the target steering torque Tq_S*. Then, in S77, the current I is supplied based on the target_SSupplies current to steering motor 70 a.

In the next S78, the current I actually supplied to the steering motor 70a is detected_SIn S79, based on the detected supply current I_STo determine the actual steering forceI.e., the actual steering torque Tq, which is the torque actually generated by the steering motor 70 a. Information about this actual steering torque Tq is sent to operation ECU18 in S80.

In the processing of the operation reaction force control program executed in parallel with the steering collective control program according to the computer of the operation ECU18, first, the steering operation angle δ is acquired, the vehicle speed v is determined, and the actual steering torque Tq of each of the left and right front wheels 10F is received in S81 to S83_L、Tq_R. In next S84, the steering angle δ, the vehicle speed v, and the actual steering torque Tq are used as the basis_L、Tq_RThe target reaction torque Tq is determined according to the above equation_C*。

Next, when the vehicle is traveling on a low μ road, the central part of the above-described recognition processing for allowing the driver to recognize the situation is performed. First, in S85, the left wheel road surface μ sign F μ is determined_LRoad surface mu mark F mu of right wheel_RWhether any of them is "1" or "2". Mu mark F mu on left wheel road surface_LRoad surface mu mark F mu of right wheel_RIn the case of both "0", the reaction torque Tq is reduced in S86 so that the recognition processing is not substantially performed_CReduced torque Δ Tq_CThe decision is "0". Mu mark F mu on left wheel road surface_LRoad surface mu mark F mu of right wheel_RWhen either one of them is "1" or "2", the left wheel road surface μ sign F μ is determined in S87_LRoad surface mu mark F mu of right wheel_RIs "2". Mu mark F mu on left wheel road surface_LRoad surface mu mark F mu of right wheel_RIf none of the values is "2", at S88, the torque Δ Tq is decreased_CDetermined as the first reduction torque DeltaTq_C1On the left wheel road surface mu mark F mu_LRoad surface mu mark F mu of right wheel_RWhen any one of the values is "2", at S89, the torque Δ Tq is decreased_CIs determined to be set to a value more than the first reduction torque DeltaTq_C1Large second reducing torque Δ Tq_C2。

Then, at S90, the target reaction torque Tq determined previously is used_CSubtract as aboveThe reduction torque Δ Tq determined as described above_CTo redetermine the target reaction torque Tq_C*. Next, at S91, the target reaction torque Tq is used_CDetermines a target supply current I to be supplied to the reaction force motor 84a_CIn S92, the current I is supplied based on the target_CCurrent is supplied to the reaction force motor 84 a.

Description of the reference symbols

10: a wheel; 12: a wheel steering device; 14: an operating device; 16: steering electronic control unit [ controller ]; 18: operating the electronic control unit [ controller ]; 20: a wheel drive unit; 20 a: a housing (knuckle); 44: CAN; 46: a yaw rate sensor; 48: a wheel speed sensor; 50: a wheel arrangement module; 70: a steering actuator; 70 a: a steering motor; 72: a tie rod; 78: a motion conversion mechanism; 80: steering wheel [ steering operation member ]; 82: a steering sensor; 84: a reaction force applying device; 84 a: a reaction force motor; 84 b: a speed reducer; 86: a notification device.