Patent Description:
A steering system has been recently proposed in which a steering device including a drive source enables a wheel to be steered in accordance with an operation of an operation member, such as a steering wheel, without depending on an operation force applied to the operation member by a driver of a vehicle. That is, what is called steer-by-wire steering system has been proposed. The steer-by-wire steering system includes a reaction-force application device to apply, to the operation member, an operation reaction force that is a reaction force against the operation of the operation member, namely, the steer-by-wire steering system includes an urging device configured to generate an urging force for urging the operation member. As disclosed in <CIT>, for instance, the urging device generates the urging force composed of some components to apply the operation reaction force and changes a ratio of the components depending on situations. A steering system having the features of the preamble of claim <NUM> is known from <CIT>.

In a case where a vehicle is automatically parked, the steering system performs an automatic steering operation, for instance. In the automatic steering operation, a wheel is automatically steered without the operation member being operated. In the steer-by-wire steering system, the wheel to be steered and the operation member are not mechanically coupled. It is thus possible to utilize the urging force of the urging device to enable the operation member to be moved in accordance with steering of the wheel in the automatic steering operation. The urging device described above is, however, designed to generate the urging force in an attempt to apply the operation reaction force. It is thus expected that an appropriate movement of the operation member based on the steering of the wheel is not ensured if the urging force composed of some components indicated above is generated. In other words, there is a possibility that the operation member unnaturally moves. Such an unnatural movement of the operation member undesirably lowers the utility of the steer-by-wire steering system. Accordingly, an aspect of the present disclosure is directed to a steer-by-wire steering system with high utility, and is defined by the subject-matter of claim <NUM>. Advantageous embodiments are laid out in the dependent claims.

In one aspect of the present disclosure, a steer-by-wire steering system for a vehicle includes: 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. 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.

The steer-by-wire steering system according to the present disclosure does not generate, in the automatic steering operation, at least part of the urging force generated by the urging device and functioning as the operation reaction force in the normal operation. Thus, the operation member moves appropriately in accordance with the steering of the wheel.

While situations in which the automatic steering operation of the steering system according to the present disclosure is performed are not limited, the automatic steering operation of the present steering system is preferably applicable to automatic parking in which the wheel is automatically steered without depending on the operation of the operation member by the vehicle driver. For instance, it is proposed to transport the vehicle by automated driving in facilities such as production plants, namely, automated-driving transportation is proposed. The automatic steering operation of the present steering system is preferably applicable to the automated-driving transportation.

As described above, the urging force may be composed of various components. For instance, the urging force generated in the normal operation may include 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.

Specifically, the assist component is a component similar to an assist force in what is called power steering. For instance, the assist component may be a component that increases with an increase in the operation force applied by the driver to the operation member.

The compensation component may include a return compensation component, a hysteresis compensation component, a damping compensation component, and an inertia compensation component, for instance. The return compensation component is a component for returning or retaining the operation member to or at an operational position thereof in a straight traveling sate of the vehicle (hereinafter referred to as "neutral position" where appropriate). The hysteresis compensation component is a component for imitating a hysteresis characteristic due to mechanical friction in the operation of the operation member. The damping compensation component is a component for viscously preventing or reducing a micro-vibration generated in the operation member. The inertia compensation component is a component for preventing or reducing a catching feeling (response lag) at the start of the operation of the operation member and a carried-away feeling (overshoot) at the end of the operation of the operation member.

The steering-load-dependent component is considered as a main component of the operation reaction force. The steering-load-dependent component is a component for causing the vehicle driver to feel a steering force necessary for steering the wheel. The steering-load-dependent component is considered as a component based on an axial force that acts on a steering rod (which may also be referred to as a rack bar) coupling right and left wheels in ordinary steering systems. The steering-load-dependent component is a concept widely including not only the steering force described above but also a force that acts on the wheel from the road surface. The steering-load-dependent component acts in a direction generally opposite to a direction in which the assist component acts. That is, the assist component acts in the same direction as the operation direction of the operation member, and the steering-load-dependent component acts in the direction opposite to the operation direction of the operation member.

The steering-load-dependent component may include a theoretical component, an actual-load dependent component, a steering-end-dependent component, and a steering-hysteresis-dependent component, for instance. The theoretical component is a component based on an operation amount of the operation member or a steering amount of the wheel. The actual-load dependent component indicates an actual load obtained based on a supply current to an electric motor in a case where the steering device includes the electric motor as a drive source. The steering-end-dependent component is a component for causing the vehicle driver to feel steering ends. The steering-hysteresis-dependent component is a component based on a hysteresis characteristic of the steering device.

The components described above can be generated in the normal operation. In contrast, a positive-movement component can be generated in the automatic steering operation. The positive-movement component is a component for causing the operation member to positively move in accordance with the steering of the wheel, so as to enable the operation member to move in accordance with the steering of the wheel. It is desirable that the positive-movement component function as a main urging force in the automatic steering operation. In view of a possibility that the assist component, the compensation component, the steering-load-dependent component, etc., which constitute the operation reaction force, may impair an appropriate movement of the operation member in the automatic steering operation, it is desirable not to generate at least part of those components in the automatic steering operation, namely, it is desirable not to generate at least part of all components except for the positive-movement component in the automatic steering operation. The at least part of the components not to be generated in the automatic steering operation will be hereinafter referred to as a non-generating component.

The positive-movement component is determined as follows, for instance. The operation amount of the operation member corresponding to the steering amount of the wheel is determined as a target operation amount based on the steering amount of the wheel. Based on a deviation of an actual operation amount with respect to the target operation amount, the positive-movement component is determined according to the feedback control law.

In a case where the positive-movement component is generated only in the automatic steering operation, it is expected that the operation member abruptly moves in a changeover between generation of the positive-movement component and non-generation of the positive-movement component. It is thus desirable to gradually increase the positive-movement component when the automatic steering operation starts and to gradually decrease the positive-movement component in returning to the normal operation, namely, when the automatic steering operation ends. Meanwhile, if the non-generating component is abruptly changed at the start of the automatic steering operation, the operation member is expected to abruptly move due to the abrupt change of the non-generating component. In view of this, it is desirable to gradually decrease the non-generating component when the automatic steering operation starts.

One example of the configuration in which the steering system does not generate at least part of the non-generating component in the automatic steering operation is a configuration in which, in the automatic steering operation, a component for cancelling the at least part of the non-generating component is added to the positive-movement component while the at least part of the non-generating component is kept generated. (The component to be added to the positive-movement component for the cancellation will be hereinafter referred to as "cancelling component" where appropriate. ) More specifically, the non-generating component is classified into: a co-directional component that acts in the same direction as a direction in which the positive-movement component acts with respect to the movement of the operation member, namely, with respect to the direction in which the operation member moves; and a counter-directional component that acts in a direction opposite to the direction in which the positive-movement component acts with respect to the movement of the operation member. For causing the counter-directional component not to be generated in the automatic steering operation, a component having the same magnitude as the counter-directional component is added to the positive-movement component to thereby cancel the counter-directional component.

In the configuration in which the cancelling component is added, when the automatic steering operation ends, the controller immediately stops generating, namely, abruptly decreases, both the cancelling component and the at least part of the non-generating component, thereby making it possible to sufficiently reduce or obviate an inappropriate movement of the operation member at the end of the automatic steering operation caused by the at least part of the non-generating component otherwise remaining at the end of the automatic steering operation. In a case where the at least part of the non-generating component to be cancelled is the counter-directional component described above, the controller immediately stops generating, namely, abruptly decreases, both the counter-directional component and the above-indicated component having the same magnitude as the counter-directional component at the end of the automatic steering operation, thereby making it possible to sufficiently reduce or obviate an inappropriate movement of the operation member at the end of the automatic steering operation caused by the counter-directional component otherwise remaining at the end of the automatic steering operation. The steering-load-dependent component described above is the counter-directional component and will probably remain even after the automatic steering operation ends. Thus, to immediately stop generating the steering-load-dependent component offers a great merit.

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:.

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.

As schematically illustrated in <FIG>, a steering system according to the present embodiment is of a steer-by-wire type. The steering system includes: a steering wheel <NUM> (as one example of an operation member) operable by a vehicle driver; an operation portion <NUM> including a reaction force actuator <NUM> for applying an operation reaction force to the steering wheel <NUM>; a steering portion <NUM> including a steering actuator <NUM> (as one example of a steering device) for steering a wheel <NUM>; and a steering electronic control unit (hereinafter abbreviated as "steering ECU" where appropriate) <NUM> configured to control the reaction force actuator <NUM> and the steering actuator <NUM>. The steering ECU <NUM> is one example of a controller.

The operation portion <NUM> will be described. The steering wheel <NUM> is fixed to a distal end portion of a steering shaft <NUM>. The reaction force actuator <NUM> includes: a reaction force motor <NUM>, which functions as a force generation source; and a speed reducing mechanism <NUM> including a worm <NUM> attached to a motor shaft of the reaction force motor <NUM> and a worm wheel <NUM> attached to the steering shaft <NUM>. The reaction force actuator <NUM> is an urging device configured to generate an urging torque TqC that depends on a motor torque of the reaction force motor <NUM> and to urge, by the urging torque, the steering wheel <NUM> through the steering shaft <NUM>. (The urging torque is a subordinate concept of an urging force. ) The reaction force actuator <NUM> causes the urging torque TqC to function as a reaction force torque TqC against the operation of the steering wheel <NUM>, so that the reaction force actuator <NUM> 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 TqC functions mainly as the reaction force torque. Thus, the urging torque TqC will be hereinafter referred to as the reaction force torque TqC where appropriate.

The reaction force motor <NUM> is a three-phase brushless motor. The reaction force motor <NUM> includes a motor rotational angle sensor <NUM> for detecting a rotational phase of the motor shaft of the reaction force motor <NUM>, that is, for detecting a rotational angle θMC of the reaction force motor <NUM> (hereinafter referred to as "reaction-force-motor rotational angle" where appropriate). The steering shaft <NUM> includes upper and lower shaft portions coupled to each other via a torsion bar <NUM>. The operation portion <NUM> includes an operation torque sensor <NUM> for detecting a torsional amount of the torsion bar <NUM> to thereby detect an operation torque TqO that the vehicle driver applies to the steering wheel <NUM>. (The operation torque is a subordinate concept of an operation force. ) The signal indicative of the reaction-force-motor rotational angle θMC detected by the motor rotational angle sensor <NUM> and the signal indicative of the operation torque TqO detected by the operation torque sensor <NUM> are sent to the steering ECU <NUM>.

The steering portion <NUM> will be described. The steering actuator <NUM> includes a steering rod <NUM> extending in the right-left direction and a housing <NUM> holding the steering rod <NUM> such that the steering rod <NUM> is movable in the right-left direction. A threaded groove <NUM> of a ball screw mechanism is formed on the steering rod <NUM>. A nut <NUM> holding bearing balls and threadedly engaging with the threaded groove <NUM> is held by the housing <NUM> so as to be rotatable and immovable in the right-left direction. A steering motor <NUM>, which is a drive source, is attached to the housing <NUM>. A timing belt <NUM> is looped over a pulley <NUM> attached to the motor shaft of the steering motor <NUM> and an outer circumferential portion of the nut <NUM> functioning as another pulley. Rotation of the motor shaft of the steering motor <NUM>, namely, rotation of the steering motor <NUM>, causes the nut <NUM> to be rotated to thereby move the steering rod <NUM> in the right-left direction. The steering rod <NUM> 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 <NUM>. The movement of the steering rod <NUM> in the right-left direction causes the right and left wheels <NUM> to be turned, namely, to be steered.

A rack <NUM> is formed on the steering rod <NUM>, and a pinion shaft <NUM> meshing with the rack <NUM> is rotatably held by the housing <NUM>. The steering actuator <NUM> of the steer-by-wire steering system according to the present embodiment need not have the rack <NUM> and the pinion shaft <NUM>. In the present steering system, if the pinion shaft <NUM> and the steering shaft <NUM> of the operation portion <NUM> 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 <NUM> with the rack <NUM> may also be referred to as a rack bar.

The steering motor <NUM> is a three-phase brushless motor. The steering motor <NUM> includes a motor rotational angle sensor <NUM> for detecting a rotational phase of a motor shaft of the steering motor <NUM>, namely, for detecting a rotational angle θMS of the steering motor <NUM> (hereinafter referred to as "steering-motor rotational angle" where appropriate). The signal indicative of the steering-motor rotational angle θMS detected by the motor rotational angle sensor <NUM> is sent to the steering ECU <NUM>.

The steering ECU <NUM> includes a computer constituted by a CPU, a ROM, a RAM, etc., an inverter functioning as a drive circuit for the reaction force motor <NUM>, and an inverter functioning as a drive circuit for the steering motor <NUM>. As later described in detail, when the vehicle automatically parks, the present steering system performs an automatic steering operation in which the wheel <NUM> is automatically steered without depending on the operation of the steering wheel <NUM> by the vehicle driver. To perform the automatic steering operation, the steering ECU <NUM> is connected to an automatic parking controller <NUM>. The steering ECU <NUM> 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 <NUM> configured to detect the vehicle speed v.

The steering ECU <NUM>, which is a controller for the present steering system, has a functional configuration illustrated in a functional block diagram of <FIG>. 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 <NUM> is classified roughly into a reaction force control section <NUM> and a steering control section <NUM>. To and from the constituent elements illustrated in <FIG>, 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.

The reaction force control section <NUM> is a functional portion that controls the urging torque TqC (reaction force torque TqC) generated by the reaction force actuator <NUM>, which is the urging device. The reaction force control section <NUM> includes an assist component determining portion <NUM> for determining an assist component TqC-A, a compensation component determining portion <NUM> for determining a compensation component TqC-C, a positive-movement component determining portion <NUM> for determining a positive-movement component TqC-M, and a steering-load-dependent component determining portion <NUM> for determining a steering-load-dependent component TqC-L. Each of the assist component TqC-A, the compensation component TqC-C, the positive-movement component TqC-M, and he steering-load-dependent component TqC-L is a component of the urging torque TqC.

In the control of the present steering system, an operation angle θO is utilized as an operation amount of the steering wheel <NUM>. Accordingly, the reaction force control section <NUM> includes an operation angle converting portion <NUM> for converting the reaction-force-motor rotational angle θMC detected by the motor rotational angle sensor <NUM> of the reaction force motor <NUM> to the operation angle θO. The operation angle θO and a cumulative amount of the reaction-force-motor rotational angle θMC have a relationship to satisfy a speed reduction ratio of the speed reducing mechanism <NUM>. The conversion of the reaction-force-motor rotational angle θMC to the operation angle θO 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 θO from the neutral position of the steering wheel <NUM> (that is a position of the steering wheel <NUM> in a straight traveling state of the vehicle). Based on the detection value by the sensor, a calibration of the operation angle θO converted by the operation angle converting portion <NUM> is performed at predetermined timing.

For performing the automatic steering operation, the reaction force control section <NUM> includes a target operation angle determining portion <NUM> for determining, as a target operation angle θO*, the operation angle θO corresponding to the steering angle θS at the present time point in a state in which the operation angle θO and the steering angle θS indicative of the steering amount of the wheel <NUM> have a relation to satisfy a specific steering gear ratio γ<NUM>.

The components of the urging torque TqC described above are determined as follows. The assist component TqC-A is a component similar to an assist force in what is called power steering. The assist component determining portion <NUM> determines the assist component TqC-A based on the vehicle speed v and the operation torque TqO detected by the operation torque sensor <NUM>. In short, the assist component determining portion <NUM> determines the assist component TqC-A to be a greater value with an increase in the operation torque TqO. Further, the assist component determining portion <NUM> determines the assist component TqC-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 <NUM> 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 <NUM>. The operation feeling of the steering wheel <NUM> 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 TqC-A is the same as the steering operation direction in which the steering wheel <NUM> is operated.

The compensation component TqC-C includes: a return compensation component for returning or retaining the steering wheel <NUM> 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 <NUM>; a damping compensation component for viscously preventing or reducing a micro-vibration generated in the steering wheel <NUM>; and an inertia compensation component for preventing or reducing a catching feeling (response lag) at the start of the operation of the steering wheel <NUM> and a carried-away feeling (overshoot) at the end of the operation of the steering wheel <NUM>. The compensation component determining portion <NUM> determines these components and sums up the determined components, so as to determine the compensation component TqC-C.

Specifically, the return compensation component is determined based on the operation torque TqO, the vehicle speed v, the operation angle θO, and an operation speed θO' obtained by differentiating the operation angle θO. In short, where the operation angle θO when the steering wheel <NUM> 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 θO and the neutral angle. The hysteresis compensation component is determined based on the operation angle θO 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 θO' obtained by differentiating the operation angle θO. In short, the damping compensation component is determined to be a greater value with an increase in the operation speed θO'. The inertia compensation component is determined based on the vehicle speed v and operation acceleration θO" obtained by differentiating the operation speed θO'. In short, the inertia compensation component is determined to be a greater value with an increase in the operation acceleration θO". The direction of the compensation component TqC-C obtained by summing up these components may be the same as or opposite to the steering operation direction.

The steering-load-dependent component TqC-L is considered as a main component of the reaction force torque. The steering-load-dependent component TqC-L is a component for causing the vehicle driver to feel a steering force necessary for steering the wheel <NUM>. The steering-load-dependent component TqC-L may be considered as a component based on a force that acts on the steering rod <NUM> of the steering actuator <NUM> in the axial direction of the steering rod <NUM>, i.e., the axial force. The steering-load-dependent component TqC-L is a component for causing the vehicle driver to also feel a force that acts on the wheel <NUM> from the road surface, in addition to the steering force described above. The direction of the steering-load-dependent component TqC-L is generally opposite to the steering operation direction.

Specifically, the steering-load-dependent component TqC-L includes: a theoretical component that is based on the operation angle θO of the steering wheel <NUM>, the steering angle θS of the wheel, and so on; an actual-load dependent component that is based on an actual load of the steering actuator <NUM>; a steering-end-dependent component for causing the vehicle driver to feel steering ends of the wheel <NUM>; and a steering-hysteresis-dependent component that is based on a hysteresis characteristic of mechanical friction of the steering actuator <NUM>. The steering-load-dependent component determining portion <NUM> determines these components and sums up the components, so as to determine the steering-load-dependent component TqC-L.

Specifically, the theoretical component is a component not taking account of friction between the road surface and the wheel <NUM>. The theoretical component is determined based on a target steering angle θS* that is a steering angle θS to which the wheel <NUM> should be steered. In short, the theoretical component is determined in consideration of the self-aligning torque of the wheel <NUM> so as to be a greater value with an increase in the target steering angle θS and with an increase in the vehicle speed v. Here, it is considered that the load of the steering actuator <NUM> is proportional to a steering current IS, which is a supply current to the steering motor <NUM>. 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 θS*, so as to steeply rise when the target steering angle θS* gets close to each steering end to a certain extent. The steering-hysteresis-dependent component is determined based on the operation angle θO and the vehicle speed v such that the hysteresis characteristic is optimized.

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

The assist component TqC-A determined by the assist component determining portion <NUM> is input to the adder <NUM>, and the compensation component TqC-C determined by the compensation component determining portion <NUM> is input to the adder <NUM> via a first switching portion <NUM>. The positive-movement component TqC-M determined by the positive-movement component determining portion <NUM> is input to a preliminary adder <NUM> via a second switching portion <NUM>, and the steering-load-dependent component TqC-L determined by the steering-load-dependent component determining portion <NUM> is input to the preliminary adder <NUM> via a third switching portion <NUM>. The preliminary adder <NUM> adds up the positive-movement component TqC-M and the steering-load-dependent component TqC-L input thereto, and a resultant added component is input to the adder <NUM>. The adder <NUM> adds up the assist component TqC-A, the compensation component TqC-C, and a sum of the positive-movement component TqC-M and the steering-load-dependent component TqC-L, and a resultant added component is input to a final adder <NUM>. The steering-load-dependent component TqC-L determined by the steering-load-dependent component determining portion <NUM> is input also to the final adder <NUM> via the third switching portion <NUM>. The final adder <NUM> subtracts the steering-load-dependent component TqC-L input by the third switching portion <NUM> from the component input by the adder <NUM>. As a result, the urging torque TqC is determined. Each of the first switching portion <NUM>, the second switching portion <NUM>, and the third switching portion <NUM> 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.

The first switching portion <NUM> has a functional configuration illustrated in <FIG>. Specifically, the first switching portion <NUM> includes an operation mode determiner <NUM>, a gain changeover switch <NUM>, a bidirectional change-amount limiter <NUM>, and a multiplier <NUM>. The operation mode determiner <NUM> receives a flag value of an automatic steering flag ASF from the automatic parking controller <NUM> and the operation torque TqO detected by the operation torque sensor <NUM>. The automatic steering flag ASF is configured such that its flag value is set to "<NUM>" (ASF="<NUM>") when automatic steering is instructed and set to "<NUM>" (ASF="<NUM>") when automatic steering is not instructed. The operation mode determiner <NUM> determines that the operation of the steering system is the automatic steering operation when the following two conditions are satisfied: i) ASF="<NUM>"; and ii) the operation torque TqO is less than a threshold operation torque TqO-TH, namely, the steering wheel <NUM> is not being operated by the vehicle driver. The operation mode determiner <NUM> 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.

Based on the determination made by the operation mode determiner <NUM>, the gain changeover switch <NUM> outputs "<NUM>" in the case of the automatic steering operation and "<NUM>" in the case of the normal operation. The bidirectional change-amount limiter <NUM> prevents an abrupt change of a value of a gain G in a changeover from <NUM> to <NUM> and from <NUM> to <NUM>. 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 <NUM> is input to the multiplier <NUM>. The multiplier <NUM> also receives the compensation component TqC-C determined by the compensation component determining portion <NUM>. The multiplier <NUM> multiplies the compensation component TqC-C by the gain G, and the compensation component TqC-C after multiplication is output from the first switching portion <NUM>.

The second switching portion <NUM> has a functional configuration illustrated in <FIG>. Specifically, the second switching portion <NUM> includes the operation mode determiner <NUM>, the bidirectional change-amount limiter <NUM>, and the multiplier <NUM> similar to those of the first switching portion <NUM>. The second switching portion <NUM> includes a gain changeover switch <NUM>. Unlike the gain changeover switch <NUM> of the first switching portion <NUM>, the gain changeover switch <NUM> of the second switching portion <NUM> outputs "<NUM>" in the case of the automatic steering operation and "<NUM>" in the case of the normal operation. The positive-movement component TqC-M determined by the positive-movement component determining portion <NUM> undergoes the process by the second switching portion <NUM> and is output from the second switching portion <NUM>.

The third switching portion <NUM> has a functional configuration illustrated in <FIG>. Specifically, the third switching portion <NUM> includes the operation mode determiner <NUM> and the multiplier <NUM> similar to those of the first switching portion <NUM> and the gain changeover switch <NUM> similar to that of the second switching portion <NUM>. The third switching portion <NUM> also includes a change-amount limiter, specifically, an increasing-direction change-amount limiter <NUM>. The increasing-direction change-amount limiter <NUM> prevents an abrupt change of the value of the gain G in a changeover from <NUM> to <NUM> but allows an abrupt change of the value of the gain G in a changeover from <NUM> to <NUM>.

The third switching portion <NUM> further includes a resetter <NUM>. The resetter <NUM> receives the steering-load-dependent component TqC-L determined by the steering-load-dependent component determining portion <NUM> and the flag value of the automatic steering flag ASF. When the flag value of the automatic steering flag ASF is set to <NUM> (ASF="<NUM>"), namely, when the automatic steering is not instructed, the resetter <NUM> resets the steering-load-dependent component TqC-L to <NUM> and subsequently gradually increases the steering-load-dependent component TqC-L from <NUM> when the normal operation is started thereafter. The steering-load-dependent component TqC-L processed by the resetter <NUM> is output therefrom not only to the multiplier <NUM> but also directly to the final adder <NUM>.

The urging torque TqC output from the final adder <NUM> is input to a reaction-force-current control portion <NUM>. The reaction-force-current control portion <NUM> includes an inverter that is a drive circuit (driver) for the reaction force motor <NUM>. The reaction-force-current control portion <NUM> determines a reaction force current IC to be supplied to the reaction force motor <NUM> based on the urging torque TqC input thereto and supplies the reaction force current IC form the inverter to the reaction force motor <NUM>.

The steering control section <NUM> is a functional portion configured to control the steering angle θS of the wheel <NUM> steered by the steering actuator <NUM>, which is the steering device. The steering control section <NUM> includes a target steering angle determining portion <NUM>, a target steering angle changeover switch <NUM>, a steering torque determining portion <NUM>, and a steering-current control portion <NUM>.

In the control of the present steering system, the steering angle θS is utilized as the steering amount of the wheel <NUM>. Thus, the steering control section <NUM> includes a steering angle converting portion <NUM> for converting the steering-motor rotational angle θMS detected by the motor rotational angle sensor <NUM> of the steering motor <NUM> to the steering angle θS. In this respect, though a toe angle of the wheel <NUM> may be employed as the steering angle θS, the rotational angle of the pinion shaft <NUM> is employed as the steering angle θS in the control of the present steering system. The steering angle θS and a cumulative amount of the steering-motor rotational angle θMS 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 <NUM>, the lead angle of the ball screw mechanism of the steering actuator <NUM>, the diameter of the pinion shaft <NUM>, etc. Thus, the conversion of the steering-motor rotational angle θMS to the steering angle θS 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 <NUM> from a rotational position of the pinion shaft <NUM> in the straight traveling state of the wheel <NUM>. Based on the detection value by the sensor, a calibration of the steering angle θS converted by the steering angle converting portion <NUM> is performed at predetermined timing.

The target steering angle determining portion <NUM> determines a target steering angle θS*, which is a control target of the steering angle θS, based on the operation angle θO converted by the operation angle converting portion <NUM> of the reaction force control section <NUM>. The present steering system is capable of changing a steering gear ratio γ, namely, a ratio of the steering angle θS with respect to the operation angle θO, depending upon the vehicle speed v. The target steering angle determining portion <NUM> determines the target steering angle θS* based on the operation angle θO 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.

The target steering angle θS* determined by the target steering angle determining portion <NUM> is employed in the normal operation whereas the target steering angle θS* based on the signal sent from the automatic parking controller <NUM> is employed in the automatic steering operation described above. The target steering angle changeover switch <NUM> is for switching the target steering angle θS* to be employed. Though not described in detail, the target steering angle changeover switch <NUM> includes a determiner similar to the operation mode determiner <NUM> of the first switching portion <NUM> of the reaction force control section <NUM>. Based on the determination made by the determiner, the target steering angle changeover switch <NUM> switches the target steering angle θS* to be employed.

The steering torque determining portion <NUM> is a functional portion for determining a steering torque Tqs necessary for steering the wheel <NUM>. The steering torque Tqs may be considered as a torque to be generated by the steering motor <NUM>, for instance. Specifically, the steering torque determining portion <NUM> determines a steering angle deviation ΔθS, which is a deviation of the steering angle θS with respect to the target steering angle θS*, based on the target steering angle θS* and an actual steering angle θS at the present time point converted by the steering angle converting portion <NUM>. According to a PID feedback control law based on the thus determined steering angle deviation ΔθS, the steering torque determining portion <NUM> determines the steering torque TqS. The technique according to the feedback control law is known, a detailed description of which is dispensed with.

The steering-current control portion <NUM> includes an inverter that is a drive circuit (driver) for the steering motor <NUM>. Based on the steering torque TqS determined as described above, the steering-current control portion <NUM> determines the steering current IS, which is a current to be supplied to the steering motor <NUM>, and supplies the steering current IS to the steering motor <NUM> from the inverter. The steering ECU <NUM> includes a current sensor <NUM> for detecting the steering current IS supplied to the steering motor <NUM>. The steering current IS detected by the current sensor <NUM> is utilized in determining the steering-load-dependent component TqC-L described above.

The urging torque TqC is controlled by the steering ECU <NUM> having the functional configuration described above, namely, the urging torque TqC is controlled by the reaction force control section <NUM> of the steering ECU <NUM>. 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 TqC accordingly.

More specifically, the first switching portion <NUM>, the second switching portion <NUM>, the third switching portion <NUM>, the preliminary adder <NUM>, the adder <NUM>, and the final adder <NUM> switch, between the normal operation and the automatic steering operation, whether or not to generate the assist component TqC-A, the compensation component TqC-C, the positive-movement component TqC-M, and the steering-load-dependent component TqC-L, each of which is a component of the urging torque TqC, as illustrated in the table of <FIG>. Specifically, the assist component TqC-A is generated in both the normal operation and the automatic steering operation. The compensation component TqC-C and the steering-load-dependent component TqC-L are generated in the normal operation but are not generated in the automatic steering operation. The positive-movement component TqC-M is not generated in the normal operation but is generated in the automatic steering operation.

It is particularly noted that the steering-load-dependent component TqC-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 TqC-M. In the automatic steering operation, the steering-load-dependent component TqC-L is not simply configured not to be generated but is canceled by adding the same component as the steering-load-dependent component TqC-L input to the final adder <NUM> in the normal operation, to the adder <NUM> together with the positive-movement component TqC-M via the preliminary adder <NUM>. Consequently, the steering-load-dependent component TqC-L is not generated in the automatic steering operation.

In the normal operation, the urging torque TqC suitably functions as the reaction force torque TqC against the operation of the steering wheel <NUM> performed by the vehicle driver owing to the assist component TqC-A, the compensation component TqC-C, and the steering-load-dependent component TqC-L, as apparent from the table of <FIG>. In the automatic steering operation, the steering wheel <NUM> is appropriately moved so as to achieve the operation angle θO corresponding to the steering angle θS of the wheel <NUM> owing to the urging torque TqC including the positive-movement component TqC-M. Because the compensation component TqC-C and the steering-load-dependent component TqC-L are not generated in the automatic steering operation, the appropriate movement of the steering wheel <NUM> is not impaired. That is, the steering wheel <NUM> is prevented from being inappropriately moved due to the compensation component TqC-C and the steering-load-dependent component TqC-L. The assist component TqC-A remains in the automatic steering operation. It is, however, noted that the assist component TqC-A is a component based on the operation torque TqO applied to the steering wheel <NUM> by the driver. In the automatic steering operation that does not depend on the operation of the steering wheel <NUM> by the driver, the assist component TqC-A hardly influences the operation of the steering wheel <NUM>. In summary, the present steering system is configured to generate the positive-movement component TqC-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 TqC except for the positive-movement component TqC-M, i.e., the non-generating component.

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 <NUM> of the first switching portion <NUM> and the second switching portion <NUM> gradually increase the positive-movement component TqC-M and gradually decrease the compensation component TqC-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 <NUM> of the first switching portion <NUM> and the second switching portion <NUM> gradually decrease the positive-movement component TqC-M and gradually increase the compensation component TqC-C. Thus, in the changeover between the normal operation and the automatic steering operation, the bidirectional change-amount limiters <NUM> prevent or reduce an abrupt movement of the steering wheel <NUM> that would be otherwise caused due to an abrupt change of the urging torque TqC.

Like the positive-movement component TqC-M, the steering-load-dependent component TqC-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 <NUM> of the third switching portion <NUM> gradually increases the steering-load-dependent component TqC-L input to the preliminary adder <NUM> 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 <NUM> of the third switching portion <NUM> immediately stops generating both the steering-load-dependent component TqC-L input to the preliminary adder <NUM> for the cancellation and the steering-load-dependent component TqC-L input to the final adder <NUM>, in other words, the resetter <NUM> resets both the components to <NUM>. The graphs of <FIG> respectively illustrate a change in the steering-load-dependent component TqC-L input to the final adder <NUM>, a change in the steering-load-dependent component TqC-L input to the preliminary adder <NUM>, a change in the positive-movement component TqC-M, and a change in a total urging torque TqC, with a lapse of time t. Here, the total urging torque TqC is a sum of the steering-load-dependent component TqC-L input to the final adder <NUM>, the steering-load-dependent component TqC-L input to the preliminary adder <NUM>, and the positive-movement component TqC-M. In this respect, the steering-load-dependent component TqC-L input to the final adder <NUM> is indicated as a negative value for clarifying that the steering-load-dependent component TqC-L input to the final adder <NUM> is the counter-directional component. The value of the positive-movement component TqC-M in the automatic steering operation is a value sufficient and necessary for moving the steering wheel <NUM>. The steering-load-dependent component TqC-L determined by the steering-load-dependent component determining portion <NUM> remains to a considerable extent at the end of the automatic steering operation.

Claim 1:
A steer-by-wire steering system for a vehicle, comprising: an operation member (<NUM>) operable by a driver of the vehicle; an urging device (<NUM>) configured to generate an urging force to urge the operation member (<NUM>); a steering device (<NUM>) configured to steer a wheel (<NUM>); and a controller (<NUM>) configured to control the urging device (<NUM>) and the steering device (<NUM>),
wherein, in a normal operation, the controller (<NUM>) enables the wheel (<NUM>) to be steered in accordance with an operation of the operation member (<NUM>) and causes the urging force to function as an operation reaction force against the operation of the operation member (<NUM>), and
wherein, in an automatic steering operation in which the wheel (<NUM>) is steered without depending on the operation of the operation member (<NUM>), the controller (<NUM>) enables the operation member (<NUM>) to be moved in accordance with steering of the wheel (<NUM>) by the urging force and causes at least part of the urging force generated in the normal operation not to be generated, characterized in that
said at least part of the urging force comprises a steering-load-dependent component as a component that is based on an axial force that acts on a steering rod or rack bar and that includes a force that acts on the wheel (<NUM>) from the road surface, the steering-load-dependent component acting in the direction opposite to the operation direction of the operation member (<NUM>).