Motor vehicle steering system

Abstract: A motor vehicle steering system is disclosed which drives a steering mechanism (2, 3) of a motor vehicle (50) on the basis of operation of operation means (1). The system includes: standard value computing means (25) for computing a standard value (γ*; Gγ*) of a behavioral variable (γ; Gγ) indicative of behavior of the motor vehicle (50) in accordance with the operation of the operation means (1); vehicle behavior detecting means (15, 16) for detecting an actual value (γ; Gγ) of the behavioral variable indicative of actual behavior of the motor vehicle (50); traveling state determining means (27) for determining a traveling state of the motor vehicle (50); and a steering controlling section (20, 22, 26, 28) for driving the steering mechanism (2, 3) according to an operation angle (δh) of the operation means (1). On the basis of the result of the determination of the traveling state determining means (27), the steering controlling section (20, 22, 26, 28) modifies a control state of the steering mechanism (2,3) so as to control a change in the behavioral variable (γ; Gγ) in a phase advanced manner, a phase delayed manner or a neutrally balanced manner.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a motor vehicle steering system, which is capable of modifying a relationship between the steering of steerable vehicle wheels and the operation of operation means such as a steering wheel. Description of Related Art A steering system for a motor vehicle has been proposed, in which a steering wheel is mechanically disengaged from a steering mechanism for steering steerable vehicle wheels, and the direction and amount of operation of the steering wheel are detected and a driving force from an actuator such as an electric motor is applied to the steering mechanism on the basis of the result of the detection (see, for example, Japanese Unexamined Patent Publication No.09-142330 (1997)). This arrangement obviates the need for mechanical coupling between the steering mechanism and the steering wheel, so that upward thrust of the steering wheel can be prevented at collision of the motor vehicle. Further, this arrangement allows for structural simplification and weight reduction of the steering mechanism, and more flexible placement of the steering wheel. Moreover, this arrangement makes it possible to employ other operation means such as a steering lever and a steering pedal instead of the steering wheel. In the motor vehicle steering system having the aforesaid construction, a relationship between the operation of the steering wheel and the operation of the steering mechanism can flexibly be modified through an electrical control. Therefore, the maneuverability of the motor vehicle is expected to be drastically improved. The dynamic characteristics of the motor vehicle responsive to the steering operation (over-steering, under-steering, neutral-steering and the like), for example, are determined by various factors (parameters such as weight, wheel base, gravity center and the like) of the motor vehicle. The dynamic characteristics can be optimized by properly setting the relationship between the operation of the steering wheel and the operation of the steering mechanism through an electrical control. The dynamic characteristics of the motor vehicle are represented by dynamic parameters such as responsiveness to the steering operation, rigidity and compliance. However, these parameters are contradictory to each other in some cases due to the nature of the control operation, so that it is difficult to satisfy all the parameters. For example, a higher responsiveness is required in an urgent operation for avoidance of a risk, while a higher rigidity is required in straight traveling for elimination of stagger. It is impossible to satisfy these two requirements. Further, when turning, the steering control is required to be compliant to what a driver wants to do (operation of the steering wheel). Therefore, an emphasis is posed on the compliance, but the rigidity is sacrificed to achieve a higher compliance. A rear wheel steering system comprising the features mentioned in the preamble of claim 1 is disclosed in JP09249145A. SUMMARY OF THE INVENTION It is an object of the present invention to provide a steering system for a motor vehicle which improves the dynamic characteristics of the motor vehicle by properly performing a steering control according to the traveling state of the motor vehicle. In accordance with the present invention, there is provided a motor vehicle steering system for driving a steering mechanism of a motor vehicle on the basis of operation of operation means, the motor vehicle steering system comprising: standard value computing means for computing a standard value of a behavioral variable indicative of behavior of the motor vehicle in accordance with the operation of the operation means; vehicle behavior detecting means for detecting an actual value of the behavioral variable indicative of actual behavior of the motor vehicle; traveling state determining means for determining a traveling state of the motor vehicle; a steering controlling section for driving the steering mechanism according to an operation angle of the operation means; characterized by a traveling state adapting means for causing the steering controlling section to modify a control state of the steering mechanism so as to control a change in the behavioral variable in a phase advanced manner, a phase delayed manner or a neutrally balanced manner on the basis of the result of the determination of the traveling state determining means. The term "phase advanced manner" hereinmeans such a manner that the actual value of the behavioral variable changes more rapidly than the standard value of the behavioral variable, and the term "phase delayed manner" herein means such a manner that the actual value of the behavioral variable changes more slowly than the standard value of the behavioral variable. The term "neutrally balanced manner" herein means a manner which is neither the phase advanced manner nor the phase delayed manner. It is preferred that the operation means is mechanically disengaged from the steering mechanism, and the steering mechanism is electrically controlled according to the operation of the operation means. In accordance with the invention, the steering mechanism is variably controlled so that the change in the vehicle behavioral variable is controlled in the phase advanced manner, the phase delayed manner or the neutrally balanced manner according to the traveling state of the motor vehicle. Thus, the steering control is properly performed according to the traveling state, whereby the dynamic characteristics of the motor vehicle are improved. The traveling state determining means may be capable of distinguishing, for example, between an urgent state where the operation means is operated for urgent steering, a straight traveling state where the motor vehicle is traveling straight, and a turning state where the motor vehicle is turned. In this case, the traveling state adapting means preferably sets the control state of the steering controlling section so as to control the change in the vehicle behavioral variable in the phase advanced manner, when the traveling state determining means determines that the vehicle traveling state is the urgent state. When the traveling state determining means determines that the vehicle traveling state is the straight traveling state, the traveling state adapting means preferably sets the control state of the steering controlling section so as to control the change in the vehicle behavioral variable in the phase delayed manner. Further, when the traveling state determining means determines that the vehicle traveling state is the turning state, the traveling state adapting means preferably sets the control state of the steering controlling section so as to control the change in the vehicle behavioral variable in the neutrally balanced manner. With this arrangement, the responsiveness is improved in the urgent steering by controlling the change in the vehicle behavioral variable in the phase advanced manner. The rigidity is improved in the straight traveling by controlling the change in the vehicle behavioral variable in the phase delayed manner. The compliance is improved in the turning of the motor vehicle by controlling the change in the vehicle behavioral variable in the neutrally balanced manner. Thus, the parameters of the dynamic characteristics of the motor vehicle which are liable to be contradictory to each other can separately be improved according to the traveling state of the motor vehicle, so that the parameters can each be set at an optimum level for drastic improvement of the overall dynamic characteristics of the motor vehicle. The behavioral variable of the motor vehicle may include a lateral acceleration. Further, the behavioral variable of the motor vehicle may include a yaw rate. The lateral acceleration and/or the yaw rate of the motor vehicle are controlled in the phase advanced manner, thephase delayedmanner or the neutrallybalanced manner according to the traveling state of the motor vehicle, whereby the attitude control of the motor vehicle can properly be performed for improvement of the control of the traveling direction of the motor vehicle. The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiment with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a conceptual diagram illustrating the basic construction of a steering system for a motor vehicle in accordance with one embodiment of the present invention; and Fig. 2 is a block diagram for explaining a steering control operation. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a conceptual diagram illustrating the basic construction of a steering system for a motor vehicle in accordance with one embodiment of the present invention. The motor vehicle steering system performs a steering operation by converting a motion of a steering actuator 2 driven in accordance with the rotational operation of a steering wheel (operation means) 1 into a turning motion of right and left front wheels (steerable wheels) 4 by a steering gear 3 without mechanical coupling between the steering wheel 1 and the steering gear 3. In this case, the steering actuator 2, the steering gear 3 and the like constitute a steering mechanism. The steering actuator 2 is comprised of a known electric motor such as a brushless motor. The steering gear 3 has a motion converting mechanism (e.g., a ball screw mechanism) for converting a rotational motion of an output shaft of the steering actuator 2 into a linear axial motion of a steering rod 7 (in a vehicle widthwise direction). The motion of the steering rod 7 is transmitted to knuckle arms 9 via tie rods 8 to rotate the knuckle arms 9. Thus, the wheels 4 supported by the knuckle arms 9 are turned. The steering wheel 1 is coupled to a rotary shaft 10 supported rotatably with respect to a vehicle body. The rotary shaft 10 is provided with a reaction force actuator 19 for applying a steering reaction force to the steering wheel 1. More specifically, the reaction force actuator 19 may be comprised of an electric motor such as a brushless motor having an output shaft unitary to the rotary shaft 10. A resilient member 30 such as a coil spring is provided between the rotary shaft 10 and the vehicle body at an end of the rotary shaft 10 opposite from the steering wheel 1. The resilient member 30 causes the steering wheel 1 to return to a straight steering position by its resilient force when the reaction force actuator 19 applies no torque to the steering wheel 1. An angle sensor 11 for detecting an operation angle δh which is equivalent to the rotation angle of the rotary shaft 10 is provided for detection of an input operation value of the steering wheel 1. The rotary shaft 10 is provided with a torque sensor 12 for detecting an operating torque T applied to the steering wheel 1. A steering angle sensor 13 for detecting a steering angle δ of the wheels 4 is provided as an output value sensor for detecting an output value of the steering actuator 2. The steering angle sensor 13 may be comprised of a potentiometer which detects an operation amount of the steering rod 7 operated by the steering actuator 2. The angle sensor 11, the torque sensor 12 and the steering angle sensor 13 are connected to a steering system controller (steering controlling section) 20 including a computer. The controller 20 is further connected to a lateral acceleration sensor 15 for detecting a lateral acceleration Gy of the motor vehicle, a yaw rate sensor 16 for detecting a yaw rate γ of the motor vehicle and a speed sensor 14 for detecting a vehicle speed V. Besides the sensors for the detection of the operation angle δh and the vehicle speed V, a sensor for detecting a wheel speed which also serves as a variable correlated with the lateral acceleration Gy and the yaw rate γ, for example, may be connected to the controller 20. The controller 20 controls the steering actuator 2 and the reaction force actuator 19 via driver circuits 22, 23. Fig. 2 is a block diagram for explaining a control operation to be performed on the steering actuator 2 by the controller 20. The controller 20 implements the operations of the respective functional sections shown in Fig. 2 through program-based processes to be performed by the computer. More specifically, the controller 20 has a target value computing section 25 for computing on the basis of a vehicle dynamic behavior model a target lateral acceleration Gy* and a target yaw rate γ* which respectively correspond to a standard lateral acceleration and a standard yaw rate for an operation angle δh, a steering angle converting section 26 for converting the operation angle δh into a steering angle C(s)·δh suitable for the control, a traveling state determining section 27 for determining which of an urgent state, a straight traveling state and a turning state is the present traveling state of the motor vehicle, and a target steering angle computing section 28 for computing a target steering angle δ* for the control of the steering actuator 2. The target steering angle computing section 28 determines the target steering angle δ* suitable for the control steering angle C(s)·δh on the basis of the target lateral acceleration Gy* and the target yaw rate γ* computed by the target value computing section 25, the actual lateral acceleration Gy of the motor vehicle detected by the lateral acceleration sensor 15, the actual yaw rate γ of the motor vehicle detected by the yaw rate sensor 16, and the vehicle speed V detected by the speed sensor 14. The steering actuator 2 is feedback-controlled on the basis of the target steering angle δ*, whereby the motor vehicle 50 (more specifically the wheels 4) is properly steered for the control of the traveling direction of the motor vehicle 50. Depending on the result of the determination of the traveling state determining section 27, the steering angle converting section 26 operates so that the lateral acceleration Gy and the yaw rate γ are brought into a phase advanced relation, a phase delayed relation or a neutrally balanced relation with respect to the target values Gy* and γ*. More specifically, a transfer function C(s) is properly modified on the basis of the result of the determination of the traveling state determining section 27 to achieve an attitude control optimum to the traveling state. The transfer function C(s) is represented, for example, by the following expression (1): C(s)= βT·s+1 αT·s+1 wherein T is a time constant, s is a Laplacian, and α and β are variable coefficients. To bring the lateral acceleration Gy and the yaw rate γ into the phase advanced relation with respect to the target values Gy* and γ*, the coefficients α and β are set to satisfy the following expression (2): 0<α<1, β=1 To bring the lateral acceleration Gy and the yaw rate γ into the phase delayed relation with respect to the target values Gy* and γ*, the coefficients α and β are set to satisfy the following expression (3): α=1, 0<β<1 To bring the lateral acceleration Gy and the yaw rate γ into the neutrally balanced relation free from a phase advance and a phase delay with respect to the target values Gy* and γ*, the coefficients α and β are set to satisfy the following expression (4): α=1, β=1 If the traveling state determining section 27 determines that the vehicle traveling state is the urgent state where the steering wheel 1 is operated for urgent steering, the steering angle converting section 26 performs the conversion of the operation angle δh by employing the transfer function C(s) of the expression (1) with the coefficients α, β set as defined by the expression (2) to determine the control steering angle C(s)·δh. Thus, the responsiveness of the control of the lateral acceleration Gy and the yaw rate γ (attitude control) is improved, whereby the urgent steering is achieved according to the operation of the steering wheel 1. If the traveling state determining section 27 determines that the traveling state of the motor vehicle 50 is the straight traveling state, the steering angle converting section 26 performs the conversion of the operation angle δh by employing the transfer function C(s) of the expression (1) with the coefficients α, β set as defined by the expression (3) to determine the control steering angle C(s)·δh. Thus, the responsiveness of the control of the lateral acceleration Gy and the yaw rate γ is reduced but, instead, the rigidity is increased. Therefore, a sufficient rigidity can be provided for elimination of stagger in the straight traveling. If the traveling state determining section 27 determines that the vehicle traveling state is the turning state, the steering angle converting section 26 performs the conversion of the operation angle δh by employing the transfer function C(s) of the expression (1) with the coefficients α, β set as defined by the expression (4) to determine the control steering angle C(s)·δh. Thus, the lateral acceleration Gy and the yaw rate γ satisfactorily follow the operation of the steering wheel 1, so that the compliance is improved. Therefore, the control of the traveling direction of the motor vehicle is improved at the turning, so that the traveling direction of the motor vehicle can be changed as desired by a driver. The traveling state determining section 27 determines the vehicle traveling state, for example, on the basis of the operation angle δh or the steering angle δ, the operating torque T and the change in the behavior of the motor vehicle (lateral acceleration Gy and yaw rate γ). A more specific example of the determination of the traveling state is disclosed, for example, in Japanese Unexamined Patent Publication No. 9-301213 (1997). A process disclosed in this patent publication can be utilized for the operation of the traveling state determining section 27. In accordance with this embodiment, it is determined which of the urgent state, the straight traveling state and the turning state is the present vehicle traveling state, and the steering control (the control of the steering actuator 2) is performed to bring the lateral acceleration Gy and the yaw rate γ into the phase advanced relation, the phase delayed relation or the neutrally balanced relation depending on the result of the determination. Thus, the dynamic characteristics of the motor vehicle can be optimized for the present traveling state, so that the overall vehicle dynamic characteristics can be improved. Therefore, all the parameters including the responsiveness, the rigidity and the compliance can advantageously be satisfied. The present invention may be embodied in any of various ways other than the embodiment described above. For example, the transfer function C(s) represented by the expression (1) is merely one example, and a transfer function C(s) as represented by the following expression (5) may be employed: C(s)= T 1 ·s+1 T 2 s 2 +T 3 s+1 wherein T 1 , T 2 , T 3 are variable time constants. In this case, the steering control can be performed to bring the lateral acceleration Gy and the yaw rate γ into the phase advanced relation, the phase delayed relation or the neutrally balanced relation by properly setting the time constants T 1 , T 2 , T 3 . More specifically, the phase advanced relation can be achieved by setting the time constants T 1 at a higher level, while the phase delayed relation can be achieved by setting the time constants T 3 at a higher level. Although the embodiment described above is directed to a case where two wheels of the four-wheeled motor vehicle are steerable, the present invention is applicable to a system having four steerable wheels. In this case, when it is determined that the vehicle traveling state is the turning state, the steering control is preferably performed separately on the font wheels and the rear wheels so that the lateral acceleration Gy and the yaw rate γ are controlled to be in the phase advanced relation and in the neutrally balanced relation, respectively, at an initial stage of the turning, and then each controlled to be brought into the neutrally balanced relation. Thus, the motor vehicle has a reduced tendency toward spinning with an improved responsiveness at the initial stage of the turning, so that the control of the traveling direction of the motor vehicle can further be improved. Further, the embodiment described above is directed to a case where the steering wheel 1 is used as the operation means, but other operation means such as a lever and a pedal may be employed. While the present invention has been described in detail by way of the embodiment thereof, it should be understood that the foregoing disclosure is merely illustrative of the technical principles of the present invention but not limitative of the same. The scope of the present invention is to be limited only by the appended claims.

Claims: A motor vehicle steering system for a driving steering mechanism (2, 3) of a motor vehicle (50) on the basis of operation of operation means (1), the motor vehicle steering system comprising: standard value computing means (25) for computing a standard value (γ*; Gγ*) of a behavioral variable (γ; Gγ) indicative of behavior of the motor vehicle (50) in accordance with the operation of the operation means (1); vehicle behavior detecting means (15, 16) for detecting an actual value (γ; Gγ) of the behavioral variable indicative of actual behavior of the motor vehicle (50); traveling state determining means (27) for determining a traveling state of the motor vehicle (50); and a steering controlling section (20, 22, 26, 28) for driving the steering mechanism (2, 3) according to an operation angle (δh) of the operation means (1); characterized by a traveling state adapting means (26) for causing the steering controlling section (20, 22, 26, 28) to modify a control state of the steering mechanism (2, 3) so as to control a change in the behavioral variable (γ; Gγ) in a phase advanced manner, a phase delayed manner or a neutrally balanced manner on the basis of the result of the determination of the traveling state determining means (27). The system of claim 1, characterized in that the traveling state determining means (27) is capable of distinguishing between an urgent state where the operation means (1) is operated for urgent steering, a straight traveling state where the motor vehicle (50) is traveling straight, and a turning state where the motor vehicle (50) is turned. The system of claim 2, characterized in that , when the traveling state determining means (27) determines that the vehicle traveling state is the urgent state, the traveling state adapting means (26) sets the control state of the steering controlling section (20, 22, 26, 28) so as to control the change in the vehicle behavioral variable in the phase advanced manner. The system of claim 2 or 3, characterized in that , when the traveling state determining means (27) determines that the vehicle traveling state is the straight traveling state, the traveling state adapting means (26) sets the control state of the steering controlling section (20, 22, 26, 28) so as to control the change in the vehicle behavioral variable in the phase delayed manner. The system of any of claims 2 to 4, characterized in that , when the traveling state determining means (27) determines that the vehicle traveling state is the turning state, the traveling state adapting means (26) sets the control state of the steering controlling section (20, 22, 26, 28) so as to control the change in the vehicle behavioral variable in the neutrally balanced manner. The system of any of claims 1 to 5, characterized in that the operation means (1) is mechanically disengaged from the steering mechanism (2, 3), and the steering mechanism (2, 3) is electrically controlled according to the operation of the operation means (1). The system of any of claims 1 to 6, characterized in that the behavioral variable (γ; Gγ) of the motor vehicle (50) includes at least one of a lateral acceleration (Gγ) and a yaw rate (γ) of the motor vehicle. The system of any of claims 1 to 7, characterized by operation angle detecting means (11) for detecting the operation angle (δh) of the operation means (1), wherein the traveling state adapting means (26) includes steering angle converting means (26) for determining a control steering angle (C(s)·δh) through conversion of the operation angle (δh) detected by the operation angle detecting means (11) by employing a transfer function (C(s)), and modifying the transfer function (C(s)) according to the result of the determination of the traveling state determining means (27). The system of claim 8, characterized in that the transfer function C(s) is represented by the following expression A: C ( s ) = β T · s +1 α T · s +1 wherein T is a time constant, s is a Laplacian, and α and β are variable coefficients, wherein the steering angle converting means (26) variably sets the coefficients α and/or β according to the result of the determination of the traveling state determining means (27). The system of claim 8, characterized in that the transfer function C(s) is represented by the following expression B: C ( s )= T 1 · s + 1 T 2 s 2 + T 3 s + 1 wherein s is a Laplacian, and T 1 , T 2 , T 3 are variable time constants, wherein the steering angle converting means (26) variably sets the time constants T 1 and/or T 2 and/or T 3 according to the result of the determination of the traveling state determining means (27).

Inventors: NAKANO SHIROU, NISHIZAKI KATSUTOSHI, SEGAWA MASAYA, TAKAMATSU TAKANOBU, NAKANO, SHIROU, NISHIZAKI, KATSUTOSHI, SEGAWA, MASAYA, TAKAMATSU, TAKANOBU
Applicants: KOYO SEIKO CO, KOYO SEIKO CO., LTD.
Application year: 2000
Main classification: B62D6
IPC codes: B62D101, B62D6, B62D137, B62D111