STEER-BY WIRE STEERING SYSTEM

Abstract

A steer-by-wire steering system for a vehicle, comprising: an operation member operable by a vehicle driver; an urging device configured to generate an urging force to urge the operation member; a steering device configured to steer a wheel; and a controller configured to control the urging device and the steering device. In a normal operation, the controller enables the wheel to be steered in accordance with an operation of the operation member and causes the urging force to function as an operation reaction force against the operation of the operation member. In an automatic steering operation in which the wheel is steered without depending on the operation of the operation member, the controller enables the operation member to be moved in accordance with steering of the wheel by the urging force and causes at least part of the urging force generated in the normal operation not to be generated.

Claims

1. A steer-by-wire steering system for a vehicle, comprising: an operation member operable by a driver of the vehicle; an urging device configured to generate an urging force to urge the operation member; a steering device configured to steer a wheel; and a controller configured to control the urging device and the steering device, wherein, in a normal operation, the controller enables the wheel to be steered in accordance with an operation of the operation member and causes the urging force to function as an operation reaction force against the operation of the operation member, and wherein, in an automatic steering operation in which the wheel is steered without depending on the operation of the operation member, the controller enables the operation member to be moved in accordance with steering of the wheel by the urging force and causes at least part of the urging force generated in the normal operation not to be generated.

2. The steer-by-wire steering system according to claim 1, wherein the automatic steering operation is an operation of the steering system performed in automatic parking of the vehicle.

3. The steer-by-wire steering system according to claim 1, wherein the urging force includes a plurality of components, one of which is a positive-movement component for causing the operation member to positively move in accordance with the steering of the wheel, and wherein the controller causes the positive-movement component to be generated only in the automatic steering operation and causes a non-generating component not to be generated in the automatic steering operation, the non-generating component being at least part of the plurality of components except for the positive-movement component.

4. The steer-by-wire steering system according to claim 3, wherein the plurality of components includes an assist component for assisting the operation of the operation member by the driver, a compensation component for compensating an operation feeling of the operation member given to the driver, and a steering-load-dependent component that is based on a load of the steering device with respect to the steering of the wheel.

5. The steer-by-wire steering system according to claim 3, wherein the controller gradually increases the positive-movement component when the automatic steering operation starts and gradually decreases the positive-movement component when the automatic steering operation ends.

6. The steer-by-wire steering system according to claim 3, wherein the controller gradually decreases the non-generating component when the automatic steering operation starts.

7. The steer-by-wire steering system according to claim 3, wherein, in the automatic steering operation, the controller keeps generating at least part of the non-generating component and adds, to the positive-movement component, a canceling component for canceling the at least part of the non-generating component.

8. The steer-by-wire steering system according to claim 7, wherein the controller immediately stops generating both the canceling component and the at least part of the non-generating component when the automatic steering operation ends.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The objects, features, advantages, and technical and industrial significance of the present disclosure will be better understood by reading the following detailed description of an embodiment, when considered in connection with the accompanying drawings, in which:

[0019] FIG. 1 is a view schematically illustrating a hardware configuration of a steering system according to one embodiment of the present disclosure;

[0020] FIG. 2 is a functional block diagram illustrating a functional configuration of a controller of the steering system according to the embodiment;

[0021] FIG. 3A is a block diagram illustrating a first switching portion included in the functional configuration of the controller;

[0022] FIG. 3B is a block diagram illustrating a second switching portion included in the functional configuration of the controller;

[0023] FIG. 3C is a block diagram illustrating a third switching portion included in the functional configuration of the controller;

[0024] FIG. 4 is a table indicating whether or not to generate various components of an urging force applied to an operation member in each of a normal operation and an automatic steering operation; and

[0025] FIG. 5 illustrates graphs showing a change in the urging force applied to the operation member and changes in some components of the urging force, in a changeover between the normal operation and the automatic steering operation.

DETAILED DESCRIPTION OF THE EMBODIMENT

[0026] Referring to the drawings, there will be described below in detail a steer-by-wire steering system according to one embodiment of the present disclosure. It is to be understood that the present disclosure is not limited to the details of the following embodiment but may be embodied based on the forms described in Various Forms and may be changed and modified based on the knowledge of those skilled in the art.

A. Hardware Configuration of Steering System

[0027] As schematically illustrated in FIG. 1, a steering system according to the present embodiment is of a steer-by-wire type. The steering system includes: a steering wheel 10 (as one example of an operation member) operable by a vehicle driver; an operation portion 14 including a reaction force actuator 12 for applying an operation reaction force to the steering wheel 10; a steering portion 20 including a steering actuator 18 (as one example of a steering device) for steering a wheel 16; and a steering electronic control unit (hereinafter abbreviated as “steering ECU” where appropriate) 22 configured to control the reaction force actuator 12 and the steering actuator 18. The steering ECU 22 is one example of a controller.

[0028] The operation portion 14 will be described. The steering wheel 10 is fixed to a distal end portion of a steering shaft 30. The reaction force actuator 12 includes: a reaction force motor 32, which functions as a force generation source; and a speed reducing mechanism 38 including a worm 34 attached to a motor shaft of the reaction force motor 32 and a worm wheel 36 attached to the steering shaft 30. The reaction force actuator 12 is an urging device configured to generate an urging torque Tq.sub.C that depends on a motor torque of the reaction force motor 32 and to urge, by the urging torque, the steering wheel 10 through the steering shaft 30. (The urging torque is a subordinate concept of an urging force.) The reaction force actuator 12 causes the urging torque Tq.sub.C to function as a reaction force torque Tq.sub.C against the operation of the steering wheel 10, so that the reaction force actuator 12 functions as a reaction-force application device. (The reaction force torque is a subordinate concept of the operation reaction force.) It is noted that the urging torque Tq.sub.C functions mainly as the reaction force torque. Thus, the urging torque Tq.sub.C will be hereinafter referred to as the reaction force torque Tq.sub.C where appropriate.

[0029] The reaction force motor 32 is a three-phase brushless motor. The reaction force motor 32 includes a motor rotational angle sensor 40 for detecting a rotational phase of the motor shaft of the reaction force motor 32, that is, for detecting a rotational angle θ.sub.MC of the reaction force motor 32 (hereinafter referred to as “reaction-force-motor rotational angle” where appropriate). The steering shaft 30 includes upper and lower shaft portions coupled to each other via a torsion bar 42. The operation portion 14 includes an operation torque sensor 44 for detecting a torsional amount of the torsion bar 42 to thereby detect an operation torque Tq.sub.O that the vehicle driver applies to the steering wheel 10. (The operation torque is a subordinate concept of an operation force.) The signal indicative of the reaction-force-motor rotational angle θ.sub.MC detected by the motor rotational angle sensor 40 and the signal indicative of the operation torque Tq.sub.O detected by the operation torque sensor 44 are sent to the steering ECU 22.

[0030] The steering portion 20 will be described. The steering actuator 18 includes a steering rod 50 extending in the right-left direction and a housing 52 holding the steering rod 50 such that the steering rod 50 is movable in the right-left direction. A threaded groove 54 of a ball screw mechanism is formed on the steering rod 50. A nut 56 holding bearing balls and threadedly engaging with the threaded groove 54 is held by the housing 52 so as to be rotatable and immovable in the right-left direction. A steering motor 58, which is a drive source, is attached to the housing 52. A timing belt 62 is looped over a pulley 60 attached to the motor shaft of the steering motor 58 and an outer circumferential portion of the nut 56 functioning as another pulley. Rotation of the motor shaft of the steering motor 58, namely, rotation of the steering motor 58, causes the nut 56 to be rotated to thereby move the steering rod 50 in the right-left direction. The steering rod 50 has right and left ends coupled, via respective link rods (not illustrated), to respective knuckle arms of right and left steering knuckles that rotatably hold the right and left wheels 16. The movement of the steering rod 50 in the right-left direction causes the right and left wheels 16 to be turned, namely, to be steered.

[0031] A rack 64 is formed on the steering rod 50, and a pinion shaft 66 meshing with the rack 64 is rotatably held by the housing 52. The steering actuator 18 of the steer-by-wire steering system according to the present embodiment need not have the rack 64 and the pinion shaft 66. In the present steering system, if the pinion shaft 66 and the steering shaft 30 of the operation portion 14 are coupled, an ordinary power steering system is constructed. That is, the present steering system is constructed by slightly modifying an ordinary power steering system. It is noted that the steering rod 50 with the rack 64 may also be referred to as a rack bar.

[0032] The steering motor 58 is a three-phase brushless motor. The steering motor 58 includes a motor rotational angle sensor 68 for detecting a rotational phase of a motor shaft of the steering motor 58, namely, for detecting a rotational angle θ.sub.MS of the steering motor 58 (hereinafter referred to as “steering-motor rotational angle” where appropriate). The signal indicative of the steering-motor rotational angle θ.sub.MS detected by the motor rotational angle sensor 68 is sent to the steering ECU 22.

[0033] The steering ECU 22 includes a computer constituted by a CPU, a ROM, a RAM, etc., an inverter functioning as a drive circuit for the reaction force motor 32, and an inverter functioning as a drive circuit for the steering motor 58. As later described in detail, when the vehicle automatically parks, the present steering system performs an automatic steering operation in which the wheel 16 is automatically steered without depending on the operation of the steering wheel 10 by the vehicle driver. To perform the automatic steering operation, the steering ECU 22 is connected to an automatic parking controller 70. The steering ECU 22 receives the signal indicative of a running speed v (hereinafter referred to as “vehicle speed v” where appropriate) of the vehicle from a vehicle speed sensor 72 configured to detect the vehicle speed v.

B. Functions of Controller

[0034] The steering ECU 22, which is a controller for the present steering system, has a functional configuration illustrated in a functional block diagram of FIG. 2. The computer executes a predetermined program to effectuate the functional configuration. The functional configuration may be effectuated by a dedicated circuit such as an ASIC. The steering ECU 22 is classified roughly into a reaction force control section 100 and a steering control section 102. To and from the constituent elements illustrated in FIG. 2, there are input and output signals indicative of values of a torque, components of the torque, a steering angle, an operation angle, etc. For avoiding redundancy of the description, the following description simply expresses in such a way that the torque, the components of the torque, the steering angle, the operation angle, etc., are input to and output from the constituent elements.

(a) Reaction Force Control Section

[0035] The reaction force control section 100 is a functional portion that controls the urging torque Tq.sub.C (reaction force torque Tq.sub.C) generated by the reaction force actuator 12, which is the urging device. The reaction force control section 100 includes an assist component determining portion 104 for determining an assist component Tq.sub.C-A, a compensation component determining portion 106 for determining a compensation component Tq.sub.C-C, a positive-movement component determining portion 108 for determining a positive-movement component Tq.sub.C-M, and a steering-load-dependent component determining portion 110 for determining a steering-load-dependent component Tq.sub.C-L. Each of the assist component Tq.sub.C-A, the compensation component Tq.sub.C-C, the positive-movement component Tq.sub.C-M, and he steering-load-dependent component Tq.sub.C-L is a component of the urging torque Tq.sub.C.

[0036] In the control of the present steering system, an operation angle θ.sub.O is utilized as an operation amount of the steering wheel 10. Accordingly, the reaction force control section 100 includes an operation angle converting portion 112 for converting the reaction-force-motor rotational angle θ.sub.MC detected by the motor rotational angle sensor 40 of the reaction force motor 32 to the operation angle θ.sub.O. The operation angle θ.sub.O and a cumulative amount of the reaction-force-motor rotational angle θ.sub.Mc have a relationship to satisfy a speed reduction ratio of the speed reducing mechanism 38. The conversion of the reaction-force-motor rotational angle θ.sub.MC to the operation angle θ.sub.O is carried out based on the speed reduction ratio. Though not described in detail, the present steering system includes a sensor (not illustrated) for detecting the operation angle θ.sub.O from the neutral position of the steering wheel 10 (that is a position of the steering wheel 10 in a straight traveling state of the vehicle). Based on the detection value by the sensor, a calibration of the operation angle θ.sub.O converted by the operation angle converting portion 112 is performed at predetermined timing.

[0037] For performing the automatic steering operation, the reaction force control section 100 includes a target operation angle determining portion 114 for determining, as a target operation angle θ.sub.O*, the operation angle θ.sub.O corresponding to the steering angle θ.sub.S at the present time point in a state in which the operation angle θ.sub.O and the steering angle θ.sub.S indicative of the steering amount of the wheel 16 have a relation to satisfy a specific steering gear ratio γ.sub.0.

[0038] The components of the urging torque Tq.sub.C described above are determined as follows. The assist component Tq.sub.C-A is a component similar to an assist force in what is called power steering. The assist component determining portion 104 determines the assist component Tq.sub.C-A based on the vehicle speed v and the operation torque Tq.sub.O detected by the operation torque sensor 44. In short, the assist component determining portion 104 determines the assist component Tq.sub.C-A to be a greater value with an increase in the operation torque Tq.sub.O. Further, the assist component determining portion 104 determines the assist component Tq.sub.C-A to be a smaller value when the vehicle speed v is high for giving the vehicle driver a heavy operation feeling with respect to the operation of the steering wheel 10 and to be a greater value when the vehicle speed v is low for giving the vehicle driver a light operation feeling with respect to the operation of the steering wheel 10. The operation feeling of the steering wheel 10 felt by the vehicle driver will be hereinafter referred to as “steering operation feeling” or simply referred to as “operation feeling” where appropriate. The direction of the assist component Tq.sub.C-A is the same as the steering operation direction in which the steering wheel 10 is operated.

[0039] The compensation component Tq.sub.C-C includes: a return compensation component for returning or retaining the steering wheel 10 to or at the neutral position; a hysteresis compensation component for imitating a hysteresis characteristic due to mechanical friction in the operation of the steering wheel 10; a damping compensation component for viscously preventing or reducing a micro-vibration generated in the steering wheel 10; and an inertia compensation component for preventing or reducing a catching feeling (response lag) at the start of the operation of the steering wheel 10 and a carried-away feeling (overshoot) at the end of the operation of the steering wheel 10. The compensation component determining portion 106 determines these components and sums up the determined components, so as to determine the compensation component Tq.sub.C-C.

[0040] Specifically, the return compensation component is determined based on the operation torque Tq.sub.O, the vehicle speed v, the operation angle θ.sub.O, and an operation speed θ.sub.O′ obtained by differentiating the operation angle θ.sub.O. In short, where the operation angle θ.sub.O when the steering wheel 10 is located at the neutral position is defined as a neutral angle, the return compensation component is determined to be a greater value with an increase in a difference between the operation angle θ.sub.O and the neutral angle. The hysteresis compensation component is determined based on the operation angle θ.sub.O and the vehicle speed v such that the hysteresis characteristic described above is optimized. The damping compensation component is determined based on the vehicle speed v and the operation speed θ.sub.O′ obtained by differentiating the operation angle θ.sub.O. In short, the damping compensation component is determined to be a greater value with an increase in the operation speed θ.sub.O′. The inertia compensation component is determined based on the vehicle speed v and operation acceleration θ.sub.O″ obtained by differentiating the operation speed θ.sub.O′. In short, the inertia compensation component is determined to be a greater value with an increase in the operation acceleration θ.sub.O″. The direction of the compensation component Tq.sub.C-C obtained by summing up these components may be the same as or opposite to the steering operation direction.

[0041] The steering-load-dependent component Tq.sub.C-L is considered as a main component of the reaction force torque. The steering-load-dependent component Tq.sub.C-L is a component for causing the vehicle driver to feel a steering force necessary for steering the wheel 16. The steering-load-dependent component Tq.sub.C-L may be considered as a component based on a force that acts on the steering rod 50 of the steering actuator 18 in the axial direction of the steering rod 50, i.e., the axial force. The steering-load-dependent component Tq.sub.C-L is a component for causing the vehicle driver to also feel a force that acts on the wheel 16 from the road surface, in addition to the steering force described above. The direction of the steering-load-dependent component Tq.sub.C-L is generally opposite to the steering operation direction.

[0042] Specifically, the steering-load-dependent component Tq.sub.C-L includes: a theoretical component that is based on the operation angle θ.sub.O of the steering wheel 10, the steering angle θ.sub.S of the wheel, and so on; an actual-load dependent component that is based on an actual load of the steering actuator 18; a steering-end-dependent component for causing the vehicle driver to feel steering ends of the wheel 16; and a steering-hysteresis-dependent component that is based on a hysteresis characteristic of mechanical friction of the steering actuator 18. The steering-load-dependent component determining portion 110 determines these components and sums up the components, so as to determine the steering-load-dependent component Tq.sub.C-L.

[0043] Specifically, the theoretical component is a component not taking account of friction between the road surface and the wheel 16. The theoretical component is determined based on a target steering angle θ.sub.S* that is a steering angle θ.sub.S to which the wheel 16 should be steered. In short, the theoretical component is determined in consideration of the self-aligning torque of the wheel 16 so as to be a greater value with an increase in the target steering angle θ.sub.S and with an increase in the vehicle speed v. Here, it is considered that the load of the steering actuator 18 is proportional to a steering current Is, which is a supply current to the steering motor 58. Thus, the actual-load dependent component is determined, based on the steering current Is, so as to be a greater value with an increase in the steering current Is. The steering-end-dependent component is determined, based on the target steering angle θ.sub.S*, so as to steeply rise when the target steering angle θ.sub.S* gets close to each steering end to a certain extent. The steering-hysteresis-dependent component is determined based on the operation angle θ.sub.O and the vehicle speed v such that the hysteresis characteristic is optimized.

[0044] The positive-movement component Tq.sub.C-M is for positively moving the steering wheel 10. In the present steering system, the positive-movement component Tq.sub.C-M is generated in the automatic steering operation when the vehicle performs automatic parking. The positive-movement component determining portion 108 determines, according to the feedback control law, the positive-movement component Tq.sub.C-M based on an operation angle deviation Δθ.sub.O, which is a deviation of the operation angle θ.sub.O at the present time point with respect to the target operation angle θ.sub.O* determined by the target operation angle determining portion 114. Specifically, the positive-movement component Tq.sub.C-M is determined according to a proportional control, namely, the positive-movement component Tq.sub.C-M is determined as a component whose magnitude corresponds to the magnitude of the operation angle deviation Δθ.sub.O. In other words, the positive-movement component Tq.sub.C-M is determined to be a greater value with an increase in the operation angle deviation Δθ.sub.O. The direction of the positive-movement component Tq.sub.C-M is the same as a direction in which the steering wheel 10 is moved. Thus, the positive-movement component Tq.sub.C-M is a co-directional component with respect to the assist component Tar-A and a counter-directional component with respect to the steering-load-dependent component Tq.sub.C-L.

[0045] The assist component Tq.sub.C-A determined by the assist component determining portion 104 is input to the adder 116, and the compensation component Tq.sub.C-C determined by the compensation component determining portion 106 is input to the adder 116 via a first switching portion 118. The positive-movement component Tq.sub.C-M determined by the positive-movement component determining portion 108 is input to a preliminary adder 122 via a second switching portion 120, and the steering-load-dependent component Tq.sub.C-L determined by the steering-load-dependent component determining portion 110 is input to the preliminary adder 122 via a third switching portion 124. The preliminary adder 122 adds up the positive-movement component Tq.sub.C-M and the steering-load-dependent component Tq.sub.C-L input thereto, and a resultant added component is input to the adder 116. The adder 116 adds up the assist component Tq.sub.C-A, the compensation component Tq.sub.C-C, and a sum of the positive-movement component Tq.sub.C-M and the steering-load-dependent component Tq.sub.C-L, and a resultant added component is input to a final adder 126. The steering-load-dependent component Tq.sub.C-L determined by the steering-load-dependent component determining portion 110 is input also to the final adder 126 via the third switching portion 124. The final adder 126 subtracts the steering-load-dependent component Tq.sub.C-L input by the third switching portion 124 from the component input by the adder 116. As a result, the urging torque Tq.sub.C is determined. Each of the first switching portion 118, the second switching portion 120, and the third switching portion 124 is a functional portion for switching whether or not to generate the corresponding component in a changeover between the normal operation and the automatic steering operation.

[0046] The first switching portion 118 has a functional configuration illustrated in FIG. 3A. Specifically, the first switching portion 118 includes an operation mode determiner 128, a gain changeover switch 130, a bidirectional change-amount limiter 132, and a multiplier 134. The operation mode determiner 128 receives a flag value of an automatic steering flag ASF from the automatic parking controller 70 and the operation torque Tq.sub.O detected by the operation torque sensor 44. The automatic steering flag ASF is configured such that its flag value is set to “1” (ASF=“1”) when automatic steering is instructed and set to “0” (ASF=“0”) when automatic steering is not instructed. The operation mode determiner 128 determines that the operation of the steering system is the automatic steering operation when the following two conditions are satisfied: i) ASF=“1”; and ii) the operation torque Tq.sub.O is less than a threshold operation torque Tq.sub.O-TH, namely, the steering wheel 10 is not being operated by the vehicle driver. The operation mode determiner 128 determines that the operation of the steering system is the normal operation when at least any one of the above two conditions is not satisfied.

[0047] Based on the determination made by the operation mode determiner 128, the gain changeover switch 130 outputs “0” in the case of the automatic steering operation and “1” in the case of the normal operation. The bidirectional change-amount limiter 132 prevents an abrupt change of a value of a gain G in a changeover from 1 to 0 and from 0 to 1. Specifically, in a case where the value of the gain G after a lapse of a predetermined cycle time changes from a value before the lapse of the predetermined time by a predetermined value or more, the change of the gain G is made as the predetermined value. The gain G passed through the bidirectional change-amount limiter 132 is input to the multiplier 134. The multiplier 134 also receives the compensation component Tq.sub.C-C determined by the compensation component determining portion 106. The multiplier 134 multiplies the compensation component Tq.sub.C-C by the gain G, and the compensation component Tq.sub.C-C after multiplication is output from the first switching portion 118.

[0048] The second switching portion 120 has a functional configuration illustrated in FIG. 3B. Specifically, the second switching portion 120 includes the operation mode determiner 128, the bidirectional change-amount limiter 132, and the multiplier 134 similar to those of the first switching portion 118. The second switching portion 120 includes a gain changeover switch 136. Unlike the gain changeover switch 130 of the first switching portion 118, the gain changeover switch 136 of the second switching portion 120 outputs “1” in the case of the automatic steering operation and “0” in the case of the normal operation. The positive-movement component Tq.sub.C-M determined by the positive-movement component determining portion 108 undergoes the process by the second switching portion 120 and is output from the second switching portion 120.

[0049] The third switching portion 124 has a functional configuration illustrated in FIG. 3C. Specifically, the third switching portion 124 includes the operation mode determiner 128 and the multiplier 134 similar to those of the first switching portion 118 and the gain changeover switch 136 similar to that of the second switching portion 120. The third switching portion 124 also includes a change-amount limiter, specifically, an increasing-direction change-amount limiter 138. The increasing-direction change-amount limiter 138 prevents an abrupt change of the value of the gain G in a changeover from 0 to 1 but allows an abrupt change of the value of the gain G in a changeover from 1 to 0.

[0050] The third switching portion 124 further includes a resetter 140. The resetter 140 receives the steering-load-dependent component Tq.sub.C-L determined by the steering-load-dependent component determining portion 110 and the flag value of the automatic steering flag ASF. When the flag value of the automatic steering flag ASF is set to 0 (ASF=“0”), namely, when the automatic steering is not instructed, the resetter 140 resets the steering-load-dependent component Tq.sub.C-L to 0 and subsequently gradually increases the steering-load-dependent component Tq.sub.C-L from 0 when the normal operation is started thereafter. The steering-load-dependent component Tq.sub.C-L processed by the resetter 140 is output therefrom not only to the multiplier 134 but also directly to the final adder 126.

[0051] The urging torque Tq.sub.C output from the final adder 126 is input to a reaction-force-current control portion 142. The reaction-force-current control portion 142 includes an inverter that is a drive circuit (driver) for the reaction force motor 32. The reaction-force-current control portion 142 determines a reaction force current I.sub.C to be supplied to the reaction force motor 32 based on the urging torque Tq.sub.C input thereto and supplies the reaction force current I.sub.C form the inverter to the reaction force motor 32.

(b) Steering Control Section

[0052] The steering control section 102 is a functional portion configured to control the steering angle θ.sub.S of the wheel 16 steered by the steering actuator 18, which is the steering device. The steering control section 102 includes a target steering angle determining portion 150, a target steering angle changeover switch 152, a steering torque determining portion 154, and a steering-current control portion 156.

[0053] In the control of the present steering system, the steering angle θ.sub.S is utilized as the steering amount of the wheel 16. Thus, the steering control section 102 includes a steering angle converting portion 158 for converting the steering-motor rotational angle θ.sub.MS detected by the motor rotational angle sensor 68 of the steering motor 58 to the steering angle θ.sub.S. In this respect, though a toe angle of the wheel 16 may be employed as the steering angle θ.sub.S, the rotational angle of the pinion shaft 66 is employed as the steering angle θ.sub.S in the control of the present steering system. The steering angle θ.sub.S and a cumulative amount of the steering-motor rotational angle θ.sub.MS have a relationship to satisfy a predetermined speed reduction ratio, namely, a speed reduction ratio determined based on the speed reducer of the steering motor 58, the lead angle of the ball screw mechanism of the steering actuator 18, the diameter of the pinion shaft 66, etc. Thus, the conversion of the steering-motor rotational angle θ.sub.MS to the steering angle θ.sub.S is performed based on the speed reduction ratio. Though not described in detail, the present steering system includes a sensor (not illustrated) for detecting a rotational angle of the pinion shaft 66 from a rotational position of the pinion shaft 66 in the straight traveling state of the wheel 16. Based on the detection value by the sensor, a calibration of the steering angle θ.sub.S converted by the steering angle converting portion 158 is performed at predetermined timing.

[0054] The target steering angle determining portion 150 determines a target steering angle θ.sub.S*, which is a control target of the steering angle θ.sub.S, based on the operation angle θ.sub.O converted by the operation angle converting portion 112 of the reaction force control section 100. The present steering system is capable of changing a steering gear ratio γ, namely, a ratio of the steering angle θ.sub.S with respect to the operation angle θ.sub.O, depending upon the vehicle speed v. The target steering angle determining portion 150 determines the target steering angle θ.sub.S* based on the operation angle θ.sub.O and the vehicle speed v referring to stored map data. The technique of changing the steering gear ratio γ is known, a detailed description of which is dispensed with.

[0055] The target steering angle θ.sub.S* determined by the target steering angle determining portion 150 is employed in the normal operation whereas the target steering angle θ.sub.S* based on the signal sent from the automatic parking controller 70 is employed in the automatic steering operation described above. The target steering angle changeover switch 152 is for switching the target steering angle θ.sub.S* to be employed. Though not described in detail, the target steering angle changeover switch 152 includes a determiner similar to the operation mode determiner 128 of the first switching portion 118 of the reaction force control section 100. Based on the determination made by the determiner, the target steering angle changeover switch 152 switches the target steering angle θ.sub.S* to be employed.

[0056] The steering torque determining portion 154 is a functional portion for determining a steering torque Tq.sub.S necessary for steering the wheel 16. The steering torque Tq.sub.S may be considered as a torque to be generated by the steering motor 58, for instance. Specifically, the steering torque determining portion 154 determines a steering angle deviation Δθ.sub.S, which is a deviation of the steering angle θ.sub.S with respect to the target steering angle θ.sub.S*, based on the target steering angle θ.sub.S* and an actual steering angle θ.sub.S at the present time point converted by the steering angle converting portion 158. According to a PID feedback control law based on the thus determined steering angle deviation Δθ.sub.S, the steering torque determining portion 154 determines the steering torque Tq.sub.S. The technique according to the feedback control law is known, a detailed description of which is dispensed with.

[0057] The steering-current control portion 156 includes an inverter that is a drive circuit (driver) for the steering motor 58. Based on the steering torque Tq.sub.S determined as described above, the steering-current control portion 156 determines the steering current Is, which is a current to be supplied to the steering motor 58, and supplies the steering current Is to the steering motor 58 from the inverter. The steering ECU 22 includes a current sensor 160 for detecting the steering current Is supplied to the steering motor 58. The steering current Is detected by the current sensor 160 is utilized in determining the steering-load-dependent component Tq.sub.C-L described above.

C. Urging Force in Normal Operation and Automatic Steering Operation and Changeover of Urging Force Between Normal Operation and Automatic Steering Operation

[0058] The urging torque Tq.sub.C is controlled by the steering ECU 22 having the functional configuration described above, namely, the urging torque Tq.sub.C is controlled by the reaction force control section 100 of the steering ECU 22. As described above, the present steering system switches the operation mode between the normal operation and the automatic steering operation performed in automatic parking and switches the urging torque Tq.sub.C accordingly.

[0059] More specifically, the first switching portion 118, the second switching portion 120, the third switching portion 124, the preliminary adder 122, the adder 116, and the final adder 126 switch, between the normal operation and the automatic steering operation, whether or not to generate the assist component Tq.sub.C-A, the compensation component Tq.sub.C-C, the positive-movement component Tq.sub.C-M, and the steering-load-dependent component Tq.sub.C-L, each of which is a component of the urging torque Tq.sub.C, as illustrated in the table of FIG. 4. Specifically, the assist component Tq.sub.C-A is generated in both the normal operation and the automatic steering operation. The compensation component Tq.sub.C-C and the steering-load-dependent component Tq.sub.C-L are generated in the normal operation but are not generated in the automatic steering operation. The positive-movement component Tq.sub.C-M is not generated in the normal operation but is generated in the automatic steering operation.

[0060] It is particularly noted that the steering-load-dependent component Tq.sub.C-L is at least part of the non-generating component not generated in the automatic steering operation and is the counter-directional component with respect to the positive-movement component Tq.sub.C-M. In the automatic steering operation, the steering-load-dependent component Tq.sub.C is not simply configured not to be generated but is canceled by adding the same component as the steering-load-dependent component Tq.sub.C-L input to the final adder 126 in the normal operation, to the adder 116 together with the positive-movement component Tq.sub.C-M via the preliminary adder 122. Consequently, the steering-load-dependent component Tq.sub.C-L is not generated in the automatic steering operation.

[0061] In the normal operation, the urging torque Tq.sub.C suitably functions as the reaction force torque Tq.sub.C against the operation of the steering wheel 10 performed by the vehicle driver owing to the assist component Tq.sub.C-A, the compensation component Tq.sub.C-C, and the steering-load-dependent component Tq.sub.C-L, as apparent from the table of FIG. 4. In the automatic steering operation, the steering wheel 10 is appropriately moved so as to achieve the operation angle θ.sub.O corresponding to the steering angle θ.sub.S of the wheel 16 owing to the urging torque Tq.sub.C including the positive-movement component Tq.sub.C-M. Because the compensation component Tq.sub.C-C and the steering-load-dependent component Tq.sub.C-L are not generated in the automatic steering operation, the appropriate movement of the steering wheel 10 is not impaired. That is, the steering wheel 10 is prevented from being inappropriately moved due to the compensation component Tq.sub.C-C and the steering-load-dependent component Tq.sub.C-L. The assist component Tq.sub.C-A remains in the automatic steering operation. It is, however, noted that the assist component Tq.sub.C-A is a component based on the operation torque Tq.sub.O applied to the steering wheel 10 by the driver. In the automatic steering operation that does not depend on the operation of the steering wheel 10 by the driver, the assist component Tq.sub.C-A hardly influences the operation of the steering wheel 10. In summary, the present steering system is configured to generate the positive-movement component Tq.sub.C-M only in the automatic steering operation and not to generate, in the automatic steering operation, at least part of the plurality of components of the urging torque Tq.sub.C except for the positive-movement component Tq.sub.C-M, i.e., the non-generating component.

[0062] When the normal operation is switched to the automatic steering operation, namely, at the start of the automatic steering operation, the bidirectional change-amount limiters 132 of the first switching portion 118 and the second switching portion 120 gradually increase the positive-movement component Tq.sub.C-M and gradually decrease the compensation component Tq.sub.C-C. Similarly, when the automatic steering operation is switched to the normal operation, namely, at the end of the automatic steering operation, the bidirectional change-amount limiters 132 of the first switching portion 118 and the second switching portion 120 gradually decrease the positive-movement component Tq.sub.C-M and gradually increase the compensation component Tq.sub.C-C. Thus, in the changeover between the normal operation and the automatic steering operation, the bidirectional change-amount limiters 132 prevent or reduce an abrupt movement of the steering wheel 10 that would be otherwise caused due to an abrupt change of the urging torque Tq.sub.C.

[0063] Like the positive-movement component Tq.sub.C-M, the steering-load-dependent component Tq.sub.C-L is gradually increased when the normal operation is switched to the automatic steering operation. Specifically, when the normal operation is switched to the automatic steering operation, namely, at the start of the automatic steering operation, the increasing-direction change-amount limiter 138 of the third switching portion 124 gradually increases the steering-load-dependent component Tq.sub.C-L input to the preliminary adder 122 for the cancellation described above. In contrast, when the automatic steering operation is switched to the normal operation, namely, at the end of the automatic steering operation, the resetter 140 of the third switching portion 124 immediately stops generating both the steering-load-dependent component Tq.sub.C-L input to the preliminary adder 122 for the cancellation and the steering-load-dependent component Tq.sub.C-L input to the final adder 126, in other words, the resetter 140 resets both the components to 0. The graphs of FIG. 5 respectively illustrate a change in the steering-load-dependent component Tq.sub.C-L input to the final adder 126, a change in the steering-load-dependent component Tq.sub.C-L input to the preliminary adder 122, a change in the positive-movement component Tq.sub.C-M, and a change in a total urging torque Tq.sub.C, with a lapse of time t. Here, the total urging torque Tq.sub.C is a sum of the steering-load-dependent component Tq.sub.C-L input to the final adder 126, the steering-load-dependent component Tq.sub.C-L input to the preliminary adder 122, and the positive-movement component Tq.sub.C-M. In this respect, the steering-load-dependent component Tq.sub.C-L input to the final adder 126 is indicated as a negative value for clarifying that the steering-load-dependent component Tq.sub.C-L input to the final adder 126 is the counter-directional component. The value of the positive-movement component Tq.sub.C-M in the automatic steering operation is a value sufficient and necessary for moving the steering wheel 10. The steering-load-dependent component Tq.sub.C-L determined by the steering-load-dependent component determining portion 110 remains to a considerable extent at the end of the automatic steering operation.

[0064] As illustrated in the graph of FIG. 5, in the automatic steering operation, the steering-load-dependent component Tq.sub.C-L is canceled as described above, and the change in the steering-load-dependent component Tq.sub.C-L does not adversely influence the movement of the steering wheel 10 by the positive-movement component Tq.sub.C-M. Even when the wheel 16 is located at the neutral position in automatic parking, for instance, it is highly probable that the steering-load-dependent component Tq.sub.C-L determined by the steering-load-dependent component determining portion 110 somewhat remains due to deformation of the tire, for instance. The remaining component may cause the steering wheel 10 to be unnecessarily or inappropriately moved when the automatic steering operation is switched to the normal operation. In the present steering system, the steering-load-dependent component Tq.sub.C-L is reset to 0 at the end of the automatic steering operation, thus obviating the unnecessary movement of the steering wheel 10 caused by the steering-load-dependent component Tq.sub.C-L otherwise remaining at the end of the automatic steering operation. The steering-load-dependent component Tq.sub.C-L in the urging torque Tq.sub.C is made equal to 0 by the cancelation described above in the automatic steering operation. Thus, even if the steering-load-dependent component Tq.sub.C-L is abruptly reset to 0 at the end of the automatic steering operation, the urging torque Tq.sub.C is not influenced.