Vehicle Steering System

Abstract

To provide a four-wheel turning system in which excellent line trace behavior is realized.SOLUTION: A vehicle steering system includes: an operation member that is operated by a driver; a front-wheel steering device to which the operation member is connected to realize turning of a front wheel by an amount of turning corresponding to an amount of operation of the operation member and that has a drive source 42 for applying a turning force to the front wheel; a rear-wheel steering device 50 that turns a rear wheel independently of the front wheel; and a controller 18 that controls the front-wheel turning device and the rear-wheel turning device. In the vehicle steering system, the controller is configured to execute a front-wheel turning force application control 102 for applying a turning force including as a component a driving assist force for causing a vehicle to travel along a reference travel line 126, to the front wheel and is configured to determine, in the control, the driving assist forces 130, 132, based on a target amount 128 of operation that is an amount of operation of the operation member for the vehicle to travel along the reference travel line and an amount 104 of turning of the rear wheel.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] Among vehicle steering systems, for example, there is a system such as that shown in the following Patent Document. Specifically, in addition to a front-wheel steering device to which a steering operation member (hereinafter sometimes simply referred to as an "operation member") is connected and which has a drive source, there is also a system (hereinafter sometimes referred to as a "four-wheel steering system" or "4WS system") that includes a rear-wheel steering device that steers the rear wheels independently of the front wheels. In the four-wheel steering system described in the following Patent Document, the drive source of the front-wheel steering device applies an operation assist torque to the front wheels as a steering torque that assists the operation torque applied by the driver to the operation member. Taking into consideration a deterioration in the operation feeling felt by the driver, this operation assist torque is determined based on the steering angle of the rear wheels. Incidentally, operation torque, operation assist torque, steering torque, and steering angle are types of operation force, operation assist force, steering force, and steering amount, respectively. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5140662 Summary of the Invention [Problem to be solved by the invention]

[0004] With the recent development of autonomous vehicle driving technology, active steering of the front wheels using a force generated by a drive source of a front-wheel steering device is being considered. For example, applying a steering force to the front wheels that causes the vehicle to travel along a standard driving line, or simply put, a driving assistance force, is being considered. By optimizing this driving assistance force, the practicality of a four-wheel steering system can be improved. The present invention was made in light of such circumstances, and an object of the present invention is to provide a highly practical four-wheel steering system. [Means for solving the problem]

[0005] In order to solve the above problems, the vehicle steering system of the present invention comprises: an operating member operated by the driver; a front wheel steering device having a drive source to which the operation member is connected, for steering the front wheels by an amount corresponding to the amount of operation of the operation member, and for applying a steering force to the front wheels; A rear wheel steering device that steers the rear wheels independently of the front wheels; a controller for controlling the front wheel steering device and the rear wheel steering device; A vehicle steering system comprising: The controller: executes a front wheel steering force application control for applying to the front wheels a steering force including a driving assist force as a component for driving the vehicle along the reference driving line; In the front wheel steering force application control, the driving assist force is (a) a target operation amount which is an operation amount of the operation member for causing the vehicle to travel along a standard travel line; (b) The actual steering amount of the rear wheels, or the steering amount of the rear wheels estimated from the target operation amount. The method is configured to determine the value based on the following: [Effects of the Invention]

[0006] Control for driving a vehicle along a standard driving line is what is known as line tracing control, lane keeping control, etc. (hereinafter, sometimes referred to as "line tracing control, etc."). The driving assist force is a steering force applied to the front wheels by the line tracing control, etc. The vehicle steering system of the present invention (hereinafter, sometimes referred to as "this steering system" or "this system") is a so-called four-wheel steering system, and according to this system, the steering force applied to the front wheels is determined based on the amount of steering of the rear wheels as well, so that, for example, good line tracing behavior can be achieved in a four-wheel steering system. Aspects of the Invention

[0007] The "operating member" in this system is typically a steering wheel, but may also be, for example, a joystick. When the operating member is a steering wheel, the "operating amount" of the operating member can be, for example, the operating angle (steering angle) from the neutral position. The "steering amount" of the wheels is generally the toe angle of the wheels (the turning angle from the neutral position), but it may also be, for example, the operating amount of a member that operates in association with the steering of the wheels, such as a rack bar, or, if the steering device has a driving source such as an electric motor for steering the wheels, the operating amount of the driving source, such as the rotation amount of the electric motor.

[0008] The "front wheel steering device" in this system can be a steering device that constitutes a so-called power steering system. The drive source that applies the steering force can be an electric motor, a hydraulic cylinder, or the like. The steering force applied to the front wheels can also generally be called steering torque, as it is considered to be a force that turns the wheels. The steering force can act as a force that moves the steering rod (rack bar) connecting the left and right front wheels left and right, or it can act as a rotational force on the steering shaft to which the steering wheel is attached.

[0009] The "rear wheel steering device" in this system is not mechanically connected to the front wheel steering device, and is a device that steers the rear wheels independently of the front wheels. If the vehicle is a four-wheel vehicle, the rear wheel steering device may steer the left and right rear wheels together, or may steer them independently of each other. In the latter case, the two rear wheel steering devices can be considered to function as a single rear wheel steering device. The rear wheel steering device may have a drive source that steers the rear wheels using the drive source.

[0010] The "controller" in this system is primarily a computer consisting of a CPU, ROM, RAM, etc., and can include drivers for the drive sources of the front wheel steering device and rear wheel steering device. The controller may be a single unit, or multiple controllers that individually control the front wheel steering device and rear wheel steering device and function as an integrated unit.

[0011] The "front wheel steering force application control" is a control for applying a steering force to the front wheels. In this system, the front wheel steering force application control applies a steering force to the front wheels that includes a "driving assist force" as a component. The driving assist force is a steering force component for driving the vehicle along a standard driving line. The "standard driving line" can be, for example, a virtual line in the center of the dividing lines on both the left and right sides of the road surface. From the perspective of not leaving the vehicle's lane, a driving line with a certain width may also be used. The standard driving line may be set, for example, based on image data obtained by a camera monitoring the front of the vehicle. The standard driving line may be set by a controller of this system, or may be transmitted as data from, for example, a controller of an autonomous driving system. The "target operation amount" may be determined based on the standard driving line.

[0012] If the force applied by the driver to the operating member is defined as the operating force, in this system, the steering force in the front wheel steering force application control may include, as a component, not only the driving assist force but also an operation assist force that assists the operating force. Incidentally, if the operating member is a steering wheel, the operating force can be considered to be the operating torque, and the operation assist force can be considered to be the operation assist torque. The operation assist force may be a force corresponding to the operating force, similar to the assist force in a general power steering system. Specifically, the operation assist force may be set so that the greater the operating force, the greater the operation assist force. By applying a steering force that includes both the driving assist force and the operation assist force as components to the front wheels, an appropriate steering force will be applied to the front wheels even if the driver applies an operating force to the operating member while the driving assist force is being applied. The operating force can be acquired by providing an operating force sensor.

[0013] On the other hand, in this system, the rear wheel steering control, which is the control of the rear wheel steering device, can be performed by, for example, determining the steering amount of the rear wheels as a target steering amount based on the operation amount of an operating member, and controlling the rear wheel steering device so that the actual steering amount of the rear wheels becomes the target steering amount.

[0014] In front wheel steering force application control, when the driving assist force is determined based on a target operation amount and the steering amount of the rear wheels, the steering amount of the rear wheels may be the actual steering amount or may be a steering amount estimated from the target operation amount. The actual steering amount (hereinafter sometimes referred to as the "actual steering amount") may be a value obtained from a steering amount sensor or the operating amount of a drive source that steers the rear wheels. The steering amount estimated from the target operation amount (hereinafter sometimes referred to as the "estimated steering amount") can be obtained by estimating it according to the process for determining the target steering amount of the rear wheels described above. The actual steering amount may have a certain delay compared to the estimated steering amount, and if the driver applies a relatively large operating force to operate the operating member while the driving assist force is being applied, the delay will be relatively large. From the perspective of the above-mentioned good line tracing behavior, it is desirable to use the estimated steering amount of the rear wheels.

[0015] The process for determining the driving assist force is not particularly limited, but may be determined based on, for example, the yaw rate of the vehicle estimated based on a linear two-wheel model or the lateral acceleration occurring in the vehicle. This process will be described in detail later.

[0016] The driving assist force may be determined to include a feedforward component and a feedback component. In this case, the feedforward component may be determined based on a target operation amount and the rear wheel steering amount, and the feedback component may be determined based on the deviation of the actual operation amount from the target operation amount. By determining a driving assist force that includes a feedback component, more appropriate front wheel steering force application control can be executed. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a diagram illustrating a hardware configuration of a vehicle steering system according to an embodiment of the present invention; [Figure 2] FIG. 2 is a block diagram showing the functions of a controller of the vehicle steering system according to the embodiment. [Figure 3] FIG. 1(a) is a diagram showing a state in which the rear wheels are steered in the opposite phase to the front wheels, FIG. 1(b) is a diagram showing a state in which the rear wheels are steered in the same phase as the front wheels, and FIG. 1(c) is a graph showing coefficients used to determine the amount of steering of the rear wheels. [Figure 4] (a) is a diagram showing the concept of the standard driving line, (b) is a diagram showing a linear two-wheel model for front and rear wheel steering, and (c) is a graph showing the coefficients used to determine the driving assistance force. [Figure 5] 10 is a graph for explaining the significance of determining a driving assist force in consideration of the amount of steering of the rear wheels. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, a vehicle steering system according to an embodiment of the present invention will be described in detail with reference to the drawings. In addition to the embodiment described below, the present invention can be embodied in various forms including those described in the above section "Modes of the Invention" and various modifications and improvements based on the knowledge of those skilled in the art. [Example]

[0019] [A] Hardware configuration of vehicle steering system As shown in Fig. 1, the vehicle steering system of the embodiment is mounted on a vehicle having two front wheels 10f (left and right) and two rear wheels 10r (left and right), and is configured to include a steering wheel 12 as an operating member operated by a driver, a front-wheel steering actuator 14 for steering the two front wheels 10f together, two rear-wheel steering actuators 16 for steering the two rear wheels 10r independently of each other and independently of the two front wheels 10f, and a steering electronic control unit (hereinafter sometimes referred to as a "steering ECU") 18 as a controller for controlling the front-wheel steering actuator 14 and the rear-wheel steering actuator 16. Incidentally, the front-wheel steering actuator 14 can be considered a front-wheel steering device, and the two rear-wheel steering actuators 16 can each be considered a rear-wheel steering device, or the two can be considered to constitute a single rear-wheel steering device. The front wheels 10f and rear wheels 10r may be collectively referred to as wheels 10 when there is no need to distinguish between them.

[0020] The steering wheel 12 is rotatably held by a steering column 20 that is fixed to an instrument panel reinforcement. More specifically, the steering wheel 12 is attached to the upper end of a steering shaft 22 that is rotatably held within the steering column 20.

[0021] The front wheel steering actuator 14 is configured to include a housing 24 fixed to the vehicle body, and a steering rod 26 held within the housing 24 so as to be non-rotatable but movable laterally. The left and right ends of the steering rod 26 are connected via link rods 28 to steering knuckles 30 that rotatably hold the left and right front wheels 10f, respectively. A rack 32 is formed on the steering rod 26, and a pinion shaft 34 is rotatably held in the housing 24 in meshing engagement with the rack 32. The upper end of the pinion shaft 34 is connected to the lower end of the steering shaft 22 via an intermediate shaft 36. With this configuration, when the driver operates the steering wheel 12, the front wheels 10f are steered by an amount that corresponds to the operating angle (which is a type of operating amount) of the steering wheel 12.

[0022] Furthermore, in the front wheel steering actuator 14, a thread groove 38 is formed in the steering rod 26, and a nut 40 that holds a bearing ball is threadedly engaged with the thread groove 38 in a state where it is rotatable but immovable in the axial direction. In other words, the steering rod 26 and the nut 40 form a ball screw mechanism. Meanwhile, a front wheel steering motor 42, which is an electric motor, is also provided in the housing 24 as a drive source for the front wheel steering actuator 14. A reducer 44 is connected to the front wheel steering motor 42, and the front wheel steering motor 42 rotates a pulley 46 via the reducer 44. A timing belt 48 is wound around the pulley 46 and the nut 40, and rotation of the front wheel steering motor 42 rotates the nut 40, which in turn moves the steering rod 26 left and right. That is, the front wheel steering motor torque generated by the front wheel steering motor 42 applies to the front wheels 10f a torque for steering the front wheels 10f, that is, a front wheel steering torque which is a type of steering force.

[0023] Each rear wheel steering actuator 16 includes a rear wheel steering motor 50, which is an electric motor serving as a drive source, a reducer 52 connected to the rear wheel steering motor 50, and a steering lever 54 connected to the output shaft of the reducer 52, with the steering lever 54 being connected via a link rod 56 to a steering knuckle 58 that rotatably holds the rear wheel 10r. By rotating the rear wheel steering motor 50, the rear wheel 10f is steered independently of the front wheel 10f.

[0024] The steering ECU 18 is primarily composed of a computer including a CPU, ROM, RAM, etc., and is also configured to include drive circuits (drivers) for the front wheel steering motor 42 and the rear wheel steering motor 50. Each of the front wheel steering motor 42 and the rear wheel steering motor 50 is a brushless DC motor, and each drive circuit is an inverter.

[0025] The vehicle is also provided with an operation angle sensor 70 for detecting the operation angle θ of the steering wheel 12, which is the amount of operation of the operation member; an operation torque sensor 72 for detecting the operation torque (a type of operation force) To, which is the torque applied to the steering wheel 12 by the driver; a front wheel motor rotation angle sensor 74 for detecting the motor rotation angle (rotation phase) φf of the front wheel steering motor 42; and a rear wheel motor rotation angle sensor 76 for detecting the motor rotation angle (rotation phase) φr of each rear wheel steering motor 50. Signals from these sensors 70, 72, 74, and 76 are input to the steering ECU 18 via a CAN (car area network or controllable area network) 90 of the vehicle. The vehicle is also provided with a wheel speed sensor 78 for detecting the wheel speed vw, which is the rotational speed of each wheel 10, and this wheel speed sensor 78 is also connected to the CAN 90. The signal from this wheel speed sensor 78 is sent to a brake electronic control unit (not shown), which estimates the vehicle speed v, which is the traveling speed of the vehicle, based on the signal. A signal related to the estimated vehicle speed v is received by the steering ECU 18 via the CAN 90. Furthermore, the vehicle has a monitoring camera 92 that monitors the front, and image data acquired by the monitoring camera 92 is also input to the steering ECU 18 via the CAN 90.

[0026] [B] Control performed by the vehicle steering system This vehicle steering system performs rear-wheel steering control for steering the rear wheels, and front-wheel steering torque application control (a type of front-wheel steering force application control) for applying a steering torque (a type of steering force) to the front wheels. These controls will be explained below. In detail, these controls are controls of currents supplied to the rear-wheel steering motor and the front-wheel steering motor, which are the drive sources of the rear-wheel steering actuator 16 and the front-wheel steering actuator 14, respectively.

[0027] (a) Functional configuration of the controller related to control The rear wheel steering control and front wheel steering torque application control are executed by a steering ECU 18, which serves as a controller. With regard to these controls, the steering ECU 18 can be considered to have a functional configuration as shown in the block diagram of Fig. 2. Each block in the diagram is a functional part realized by processing executed by the steering ECU 18, that is, a functional unit, and as shown in the diagram, the steering ECU 18 has two rear wheel steering control units 100 corresponding to the rear wheel steering control of each of the two rear wheels 10r, and a front wheel steering torque application control unit 102 corresponding to the front wheel steering torque application control.

[0028] Each rear wheel steering control unit 100 has an actual steering angle determination unit 104 that determines the actual rear wheel steering angle δr (a type of steering amount of the rear wheels) based on the motor rotation angle φr of the rear wheel steering motor 50; a steering angle coefficient determination unit 106 that determines a rear wheel steering angle coefficient Cr that is a coefficient for determining the target rear wheel steering angle δrt, which is the target rear wheel steering angle δr; a target steering angle determination unit 108 that determines the target rear wheel steering angle δrt based on the rear wheel steering angle coefficient Cr and the operation angle θ of the steering wheel 12; a steering torque determination unit 110 that determines a rear wheel steering torque Tr (a type of rear wheel steering force) based on the target rear wheel steering angle δrt and the actual rear wheel steering angle δr; and a steering current supply unit 112 that supplies a rear wheel steering current Ir to the rear wheel steering motor 50 based on the rear wheel steering torque Tr.

[0029] The front wheel steering torque application control unit 102 has an operation assist torque determination unit 114 that determines the magnitude of the operation assist torque Ta, which is the steering torque to be applied to the front wheels 10f to assist the driver in operating the steering wheel 12, according to the operation torque To applied to the steering wheel 12 by the driver, and a driving assist torque determination unit 116 that determines the driving assist torque (a type of driving assist force) Ts, which is the steering torque to be applied to the front wheels 10f to drive the vehicle along the standard driving line.

[0030] The operation assist torque Ta and the driving assist torque Ts are both components of the front wheel turning torque Tf applied to the front wheels 10f, and in order to determine the front wheel turning torque Tf based on the operation assist torque Ta and the driving assist torque Ts, the front wheel turning torque application control unit 102 has multipliers 118, 120 that multiply the operation assist torque Ta and the driving assist torque Ts by an operation assist torque weighting coefficient Wa and a driving assist torque weighting coefficient Ws, respectively, and an adder 122 that adds the operation assist torque Ta and the driving assist torque Ts multiplied by the coefficients Wa and Ws. The front wheel turning torque application control unit 102 also has a turning current supply unit 124 that supplies a front wheel turning current If to the front wheel turning motor 40 based on the front wheel turning torque Tf determined by the addition.

[0031] For the purpose of the driving assist torque Ts, the driving assist torque determination unit 116 has a standard driving line identification unit 126 that identifies a standard driving line based on image data Cam acquired by the monitoring camera 92, and a target operating angle determination unit 128 that determines a target operating angle (a type of target operating amount) θt, which is the operating angle θ of the steering wheel 12 for the vehicle to travel along the standard driving line, based on the identified standard driving line. In addition, the driving assist torque Ts includes a driving feedforward component Tsff and a feedback component Tsfb, and the driving assist torque determination unit 116 has an FF component determination unit 130 and an FB component determination unit 132 that determine these components Tsff and Tsfb, respectively, multipliers 134 and 136 that multiply the feedforward component Tsff and the feedback component Tsfb by a feedforward component weighting coefficient Wsff and a feedback component weighting coefficient Wsfb, respectively, and an adder 138 that adds the feedforward component Tsff and the feedback component Tsfb multiplied by these coefficients Wsff and Wsfb.

[0032] (b) Explanation of rear wheel steering control and front wheel steering torque application control Below, the rear wheel steering control and the front wheel steering torque application control will be explained while explaining the processing in each of the above-mentioned functional units.

[0033] i) Rear wheel steering control In rear-wheel steering control for each of the left and right rear wheels 10r, actual rear-wheel steering angle δr is determined by actual steering angle determination section 104. There is a predetermined gear relationship between actual rear-wheel steering angle δr, i.e., the turning angle of rear wheels 10r when the neutral position (the angular position of the wheels when the vehicle travels straight) is set to 0, and rear-wheel motor rotation angle φr, which is the motor rotation angle of rear-wheel steering motor 50, determined by the structure of speed reducer 52, etc., so actual steering angle determination section 104 determines actual rear-wheel steering angle δr based on that gear ratio and rear-wheel motor rotation angle φr detected by rear-wheel motor rotation angle sensor 76.

[0034] In this steering system, the rear wheels 10r are steered in accordance with the steering angle θ of the steering wheel 12. When the vehicle speed v is low, the rear wheels 10r are steered in the opposite direction to the front wheels 10f, i.e., in the opposite phase, as shown in FIG. 3(a). When the vehicle speed v increases to a certain level, the rear wheels 10r are steered in the same direction as the front wheels 10f, i.e., in the same phase, as shown in FIG. 3(b). In FIGS. 3(a) and 3(b), the dashed lines indicate the vehicle's traveling path when the rear wheels 10r are not steered, and the solid lines indicate the vehicle's traveling path when the rear wheels 10r are steered, although these are extreme examples. As can be seen from these figures, steering the rear wheels 10r in the opposite phase to the front wheels 10f enables turns with a small turning radius, improving the vehicle's maneuverability. Steering the rear wheels 10r in the same phase as the front wheels 10f stabilizes the vehicle's traveling behavior, for example, when changing lanes.

[0035] In this steering system, a rear wheel steering angle coefficient Cr is adopted, and a target rear wheel steering angle δrt is determined in a target steering angle determination unit 108 in accordance with the following equation, based on the operation angle (actual operation angle) θ of the steering wheel 12 detected by the operation angle sensor 70. δrt=Cr·θ (1) Steering ECU 18 stores map data shown in the form of a graph in Figure 3(c), and steering angle coefficient determination unit 106 determines rear wheel steering angle coefficient Cr in accordance with the map data based on the vehicle speed v. Note that rear wheel steering coefficient Cr is set to a positive value when rear wheels 10r are steered in the same phase as front wheels 10f, and is set to a negative value when rear wheels 10r are steered in the opposite phase to front wheels 10f.

[0036] The steering torque determination unit 110 determines the rear wheel steering torque Tr according to a PID feedback control technique based on the rear wheel steering angle deviation Δδr (=δrt-δr), which is the deviation of the actual rear wheel steering angle δr from the target rear wheel steering angle δrt. Specifically, the determination is made according to the following equation: Tr=Grp·Δδr+Gri·∫(Δδr)dt+Grd·d(Δδr) / dt ···(2) Incidentally, Grp, Gri, and Grd are the proportional gain, integral gain, and differential gain, respectively.

[0037] Steering current supply unit 112 is a functional unit whose central component is an inverter. Since rear wheel steering torque Tr applied to rear wheels 10r is roughly proportional to rear wheel steering current Ir, which is the current supplied to rear wheel steering motor 50, steering current supply unit 112 is configured to supply rear wheel steering current Ir of a magnitude corresponding to the rear wheel steering torque Tr determined above to rear wheel steering motor 50.

[0038] ii) Front wheel steering torque control As explained above, the front wheel steering torque Tf includes the operation assist torque Ta and the driving assist torque Ts as components, and the operation assist torque Ta is determined by the operation assist torque determination unit 114, and the driving assist torque Ts is determined by the driving assist torque determination unit 116.

[0039] The operation assist torque determination unit 114 determines the operation assist torque Ta so that an assist torque similar to that of a general power steering device is applied. Specifically, the operation assist torque determination unit 114 determines the operation assist torque Ta based on the operation torque To detected by the operation torque sensor 72, in a direction that reduces the operation torque To, so that the greater the operation torque To, the greater the torque that is applied to the front wheels 10f. Specifically, the operation assist torque Ta is determined according to the following formula: Ta=Cta·To ···(3) Incidentally, Cta is the operation assist torque conversion coefficient.

[0040] As described above, the driving assist torque Ts is a torque for driving the vehicle along a standard driving line, and in the driving assist torque determination unit 116, the standard driving line is identified by the standard driving line identification unit 126. Specifically, the monitoring camera 92 acquires an image of the front of the host vehicle as shown in FIG. 4(a), and image data Cam of the image is transmitted to the standard driving line identification unit 126. Based on the image data Cam, the standard driving line identification unit 126 identifies the standard driving line Ls. This identification method is a common method, so to briefly explain it here, the standard driving line identification unit 126 identifies the left and right dividing lines Le that define the driving lane in which the host vehicle is traveling (hereinafter sometimes referred to as the "host vehicle lane"), and identifies the line midway between these dividing lines Le, in other words, the line that divides the host vehicle lane into left and right equal parts, as the standard driving line Ls.

[0041] In the driving assist torque determiner 116, the target operation angle determiner 128 determines the target operation angle θt based on the specified reference driving line Ls and the actual rear wheel steering angle δr. Incidentally, this determination method may be any known method, and a detailed description thereof will be omitted here.

[0042] The driving assist torque Ts includes a feedforward component (hereinafter sometimes referred to as an "FF component") Tsff and a feedback component (hereinafter sometimes referred to as an "FB component") Tsfb. The FF component determination unit 130 determines the FF component Tsff according to a linear two-wheel model shown in FIG. 4(b). The method for determining the FF component Tsff according to this model will be described in detail below.

[0043] In short, the FF component determination section 130 determines the FF component Tsff according to the following equation. Tsff=Cts Gv Ay (4) Here, Cts is a driving assistance torque conversion coefficient, Gv is a vehicle speed gain, and Ay is a lateral acceleration occurring in the vehicle. Note that the driving assistance torque conversion coefficient Cts is a constant, and the vehicle speed gain Gv is set to a smaller value as the vehicle speed v increases, as shown in the graph in FIG. 4(c), taking into consideration the running stability of the vehicle and the like.

[0044] According to the linear two-wheel model, the yaw rate γ of the vehicle can be expressed by the following equation, where δf is the front wheel steering angle, δr is the rear wheel steering angle, L is the wheelbase, and Kh is the stability factor. γ=v·δf / ((1+Kh·v 2 )·L)-v·δr / ((1+Kh·v 2 )·L) ···(5)

[0045] The lateral acceleration Ay can be expressed by the following equation: Ay=v γ (6) The steering gear ratio Ns (=δf / θ), which is the ratio of the front wheel steering angle δf to the steering angle θ, and the operator Z = (1 + Kh v 2 )·L and adopting the target operation angle θt as the operation angle θ, the FF component Tsff can be calculated by the following equation. Tsff=Cts Gv (v 2 (θt / Ns) / Zv 2 ·δr / Z) ···(7) The steering gear ratio Ns is a constant determined by the structure of the front wheel steering actuator 14, and the wheelbase L is also a constant determined by the structure of the vehicle.

[0046] According to the above formula (7), the FF component Tsff is determined to be a larger value as the target operation angle θt increases. Also, when the rear wheels 10r are steered in the same phase as the front wheels 10f, the FF component Tsff is determined to be a smaller value as the rear wheel steering angle δr increases, and conversely, when the rear wheels 10r are steered in the opposite phase to the front wheels 10f, the FF component Tsff is determined to be a larger value as the rear wheel steering angle δr increases.

[0047] The FF component determination unit 130 determines the FF component Tsff in accordance with the above equation (7) based on the vehicle speed v obtained by the steering ECU 18, the target operating angle θt determined by the target operating angle determination unit 128, and the actual rear wheel steering angle δr identified by the actual steering angle identification unit 104.

[0048] FF component determination section 130 may determine FF component Tsff without using the actual rear wheel steering angle δr. In other words, it is also possible to determine FF component Tsff using rear wheel steering angle δr estimated from target operation angle θt based on rear wheel steering angle coefficient Cr determined by steering angle coefficient determination section 106, instead of actual rear wheel steering angle δr. Specifically, FF component Tsff may be determined according to the following equation. Tsff=Cts Gv (v 2 (θt / Ns) / Zv 2 ·θt·Cr / Z) =Cts Gv v 2 (θt / Ns) (1-Ns Cr) / Z ···(8)

[0049] Furthermore, the FF component determination section 130 may determine the FF component Tsff according to the following equation, instead of the above equation (8). Tsff=Cts Gv v 2 ·(θt / Ns)·(1-Ns·(δr / θt)) / Z ···(9)

[0050] On the other hand, the FB component determination unit 132 determines the FB component Tsfb according to a PID feedback control technique, based on the operation angle deviation Δθ (=θt−θ), which is the deviation of the actual operation angle θ of the steering wheel 12 detected by the operation angle sensor 70, from the target operation angle θt determined by the target operation angle determination unit 128. Specifically, the FB component Tsfb is determined according to the following equation. Tsfb=Gtsp·Δθ+Gtsi·∫(Δθ)dt+Gtsd·(Δδθ / dt) ···(10) Incidentally, Gtsp, Gtsi, and Gtsd are the proportional gain, integral gain, and differential gain, respectively.

[0051] The determined FF component Tsff is multiplied by a feedforward component weighting coefficient Wsff by a multiplier 134, and the determined FB component Tsfb is multiplied by a feedback component weighting coefficient Wsfb by a multiplier 136. Then, an adder 138 adds the FF component Tsff and the FB component Tsfb multiplied by the coefficients Wsff and Wsfb together to determine the driving assist torque Ts. That is, the driving assist torque Ts is determined by the following equation. Ts=Wsff·Tsff+Wsfb·Tsfb ···(11)

[0052] The operation assist torque Ta determined by the operation assist torque determination unit 114 is multiplied by an operation assist torque weighting coefficient Wa by a multiplier 118, and the driving assist torque Ts determined by the driving assist torque determination unit 116 is multiplied by a driving assist torque weighting coefficient Ws by a multiplier 120. Then, an adder 122 adds together the operation assist torque Ta and the driving assist torque Ts multiplied by the coefficients Wa and Ws, and determines the rear wheel steering torque Tf.

[0053] Note that, because the operation assist torque Ta and the driving assist torque Ts are components of the front wheel steering torque Tf that have different purposes, the operation assist torque weighting coefficient Wa and the driving assist torque weighting coefficient Ws are gradually set so as to arbitrate between the operation assist torque Ta and the driving assist torque Ts depending on the situation. Although a detailed explanation will be omitted, specifically, for example, if the driver performs a relatively large operation of the steering wheel 12 while the vehicle is traveling along the standard driving line with the driving assist torque Ts, the driving assist torque weighting coefficient Ws is made relatively small. Furthermore, during such arbitration, the operation assist torque weighting coefficient Wa and the driving assist torque weighting coefficient Ws are gradually changed so as to prevent a sudden change in the front wheel steering torque Tf applied to the front wheels 10f.

[0054] The steering current supply unit 124 is a functional unit whose central component is an inverter. The front wheel turning torque Tf applied to the front wheels 10f is roughly proportional to the front wheel turning current If, which is the current supplied to the front wheel turning motor 42, and so the steering current supply unit 124 is configured to supply the front wheel turning current If, based on the determined front wheel turning torque Tf, to the front wheel turning motor 42, with a magnitude corresponding to that torque Tf.

[0055] iii) Significance of the feedforward component in front wheel steering torque application control In this steering system, as described above, the FF component Tsff of the driving assist torque Ts is determined by any one of equations (7), (8), and (9). That is, regardless of which equation is used, the FF component Tsff is determined not only based on the target operation angle θt determined based on the standard driving line Ls, but also based on the rear wheel steering angle δr.

[0056] Here, for example, consider driving assistance in the present steering system when the rear wheels 10r are steered in the same phase as the front wheels 10f. FIG. 5 shows how the actual operating angle θ changes with time t in response to changes in the target operating angle θt due to the driving assistance torque Ts. When the target operating angle θt changes as shown by the dashed line, if the FF component Tsff is determined without considering the rear wheel steering angle δr, the actual operating angle θ changes as shown by the dashed line. In other words, the FF component Tsff becomes excessive, and the actual operating angle θ overshoots the target operating angle θt. In contrast, when the rear wheel steering angle δr is taken into consideration, that is, when the FF component Tsff is determined based on the rear wheel steering angle δr as described above, the actual operating angle θ changes to generally coincide with the target operating angle θt, as shown by the solid line. In short, by determining the driving assist torque Ts taking into account the rear wheel steering angle δr as in this steering system, a vehicle equipped with this steering system can achieve good line tracing behavior.

[0057] According to the above formula (7), the FF component Tsff is determined based on the actual rear wheel steering angle δr, making it possible to simply determine the FF component Tsff without using the rear wheel steering angle coefficient Cr. Meanwhile, because there is a delay in the steering operation of the rear wheels 10r, this effect is reflected in the determined FF component Tsff. In contrast, according to the formula (8), the FF component Tsff is determined based on the rear wheel steering angle δr estimated from the target steering angle θt, rather than the actual rear wheel steering angle δr, so the actual steering angle θ more closely matches the target steering angle θt. Note that when formula (9) is used, the same results as when formula (7) are obtained; however, when the vehicle is traveling nearly straight, that is, when the target steering angle θt is quite small, the FF component Tsff tends to be excessive. It is also desirable to take measures when the target steering angle θt is completely zero when determining the FF component Tsff. [Explanation of symbols]

[0058] 10f: Front wheels 10r: Rear wheels 12: Steering wheel (steering operation member or operation member) 14: Front wheel steering actuator (front wheel steering device) 16: Rear wheel steering actuator (rear wheel steering device) 18: Steering electronic control unit (steering ECU) (controller) 42: Front wheel steering motor 50: Rear wheel steering motor 70: Operation angle sensor 72: Operation torque sensor 74: Front wheel motor rotation angle sensor 76: Rear wheel motor rotation angle sensor 78: Wheel speed sensor 92: Surveillance camera 100: Rear wheel steering control unit 102: Front wheel steering torque application control unit 104: Actual steering angle identification unit 106: Steering angle coefficient determination unit 108: Target steering angle determination unit 110: Steering torque determination unit 112: Steering current supply unit 114: Operation assist torque determination unit 116: Driving assist torque determination unit 118: Multiplier 120: Multiplier 122: Adder 124: Steering current supply unit 126: Reference driving line specification unit 128: Target operation angle determination unit 130: FF component determination unit 132: FB component determination unit 134: Multiplier 136: Multiplier 138: Adder

Claims

1. an operating member operated by the driver; a front wheel steering device having a drive source to which the operation member is connected, for realizing steering of the front wheels by an amount corresponding to the amount of operation of the operation member, and for applying a steering force to the front wheels; A rear wheel steering device that steers the rear wheels independently of the front wheels; a controller for controlling the front wheel steering device and the rear wheel steering device; A vehicle steering system comprising: The controller: executes a front wheel steering force application control for applying to the front wheels a steering force including a driving assist force as a component for driving the vehicle along the reference driving line; In the front wheel steering force application control, the driving assist force is determined based on (a) a target operation amount, which is an operation amount of the operation member for causing the vehicle to travel along a standard driving line, and (b) an actual steering amount of the rear wheels or an estimated steering amount of the rear wheels from the target operation amount.

2. 2. The vehicle steering system according to claim 1, wherein the front wheel steering force imparting control is a control for imparting to the front wheels a steering force that includes as its components the driving assist force and an operation assist force of a magnitude corresponding to the operating force applied by the driver to an operating member.

3. 2. The vehicle steering system according to claim 1, wherein the controller is configured to execute rear wheel steering control by determining a steering amount of the rear wheels based on an operation amount of the operating member, and controlling the rear wheel steering device so as to steer the rear wheels to the determined steering amount.

4. 2. The vehicle steering system according to claim 1, wherein the controller is configured to determine the driving assist force based on a target operation amount and an actual steering amount of the rear wheels in the front wheel steering force application control.

5. 2. The vehicle steering system according to claim 1, wherein the controller is configured to determine the driving assist force in the front wheel steering force application control based on a target operation amount and a rear wheel steering amount estimated from the target operation amount.

6. 2. The vehicle steering system according to claim 1, wherein the controller is configured to determine a driving assist force based on a yaw rate of the vehicle estimated based on a linear two-wheel model or a lateral acceleration occurring in the vehicle in the front wheel steering force application control.

7. 2. The vehicle steering system according to claim 1, wherein the controller is configured to determine, in the front wheel steering force application control, a driving assist force including a feedforward component and a feedback component, the feedforward component based on a target operation amount and a steering amount of the rear wheels, and to determine the feedback component based on a deviation of an actual operation amount from a target operation amount.

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