Steering control device for vehicle and steering control method for vehicle

The present invention relates to a steering control device for a vehicle and a steering control method for a vehicle.

This application claims a priority on the basis of Japanese Patent Application No. 2013-226182, filed on Oct. 31, 2013. The contents of the above application are incorporated herein by reference in the designated countries in which the incorporation by reference is accepted.

As a prior art, there is known a steering control device for a vehicle, which controls steering of wheels. In such a steering control device for a vehicle, there is also known a technique to increase steering force to be provided to a steering mechanism, when the vehicle is in a stationary steering state in which the steering is performed while the vehicle is at a standstill, compared with when the vehicle is not in the stationary steering state (see Patent Document 1, for example).

Patent Document

[Patent Document 1] JP 6-239258 A

Problems to be Solved by Invention

In the prior art, however, when the vehicle is in the stationary steering state and the steering mechanism is accordingly provided with an increased steering force, if the vehicle transitions from the stationary steering state to a non-stationary steering state, the increased steering force provided to the steering mechanism will unduly increase a speed of turning the wheels, which may give an uncomfortable feeling to the driver.

Problems to be solved by the present invention include providing a steering control device for a vehicle which can appropriately execute the steering control of wheels.

Means for Solving Problems

In the present invention, the above problems are solved by decreasing steering force to be provided to a steering mechanism when the turning of wheels is detected as a result of providing the steering force with the steering mechanism when the vehicle has been determined to be in the stationary steering state.

Effect of Invention

According to the present invention, when the turning of wheels is detected, the steering force to be provided to the steering mechanism is suppressed. Therefore, when the vehicle transitions from the stationary steering state to a non-stationary steering state, increase in the turning speed of wheels can be suppressed. This can mitigate an uncomfortable feeling given to the driver due to the turning speed of wheels.

FIG. 1 is a schematic diagram illustrating a steering control device according to an embodiment of the present invention.

FIG. 2(A) is a graph illustrating an example of a current command value calculated by using a first control transfer function at the time of non-stationary steering and an example of a current command value calculated by using a second control transfer function at the time of stationary steering, and FIG. 2(B) is a graph illustrating an example of a turning speed of wheels at the time of stationary steering of the scenes illustrated in FIG. 2(A) .

FIG. 3 is a graph for explaining a method of setting a target current command value on the basis of a friction coefficient t.

FIG. 4 is a flowchart illustrating a steering control process according to the embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the present invention will be described by exemplifying a steering control device which is equipped in a vehicle and which can automatically control the steering operation thereby to guide the vehicle to a target position without the driver's steering operation.

FIG. 1 is a schematic diagram which illustrates a steering control device 100 according to the present embodiment. As illustrated in FIG. 1 , the steering control device 100 comprises a steering device 10 , a steering assist control device 20 , an automatic steering control device 30 , an automatic steering control switch 31 , a display 32 , and an electric motor 40 .

The steering device 10 has an electric power steering function and steers wheels (e.g. right and left front wheels) 15 in accordance with the steering angle of a steering wheel 11 when the steering wheel 11 is operated by a driver.

Specifically in the steering device 10 , a steering system between the steering wheel 11 and the wheels 15 is mechanically connected. The steering system is mainly constituted of a steering shaft 12 , a rack gear and pinion gear mechanism 13 , and tie rods 14 . The steering wheel 11 is attached to the upper end of the steering shaft 12 . The rack gear and pinion gear mechanism 13 is connected with the lower end of the steering shaft 12 . In addition, a pinion gear 13 a is attached to the lower end of a pinion shaft which is connected to the steering shaft 12 . The pinion gear 13 a meshes with a rack gear 13 b which is provided to extend in the vehicle width direction. When a rotating operation for the steering wheel (steering shaft 12 ) is performed, the rack gear and pinion gear mechanism 13 converts the rotational motion of the steering wheel 11 (steering shaft 12 ) into linear motion (translational motion) of the rack gear 13 b . Both ends of the rack gear 13 b are connected to respective knuckle arms (not shown) which are provided at the wheels 15 via the tie rods 14 . The linear motion (translational motion) of the rack gear 13 b thus allows steering of the wheels 15 .

A torque sensor 16 is provided to detect a steering torque which corresponds to a steering input force to the steering device 10 . The steering torque detected by the torque sensor 16 is output to the steering assist control device 20 and the automatic steering control device 30 .

A steering angle sensor 17 is provided on or near the steering shaft 12 to detect a rotation angle of the steering shaft 12 as a steering angle. The steering angle detected by the steering angle sensor 17 corresponds to a turning angle of the wheels 15 . The steering angle (turning angle of the wheels 15 ) detected by the steering angle sensor 17 is output to the steering assist control device 20 and the automatic steering control device 30 .

A vehicle speed sensor 18 is provided to detect a rotating state of the wheels 15 . The vehicle speed sensor 18 thereby outputs vehicle speed pulses in accordance with the rotating state of the wheels 15 . For example, the vehicle speed sensor 18 detects the rotation of a gear attached to the center of a wheel by using a magnetic sensor (not shown) and can output the vehicle speed pulses. The vehicle speed pulses are output, as speed information of the vehicle, from the vehicle speed sensor 18 to the steering assist control device 20 and the automatic steering control device 30 .

The electric motor 40 converts the electric power supplied from an onboard battery (not shown) into a torque. The value of a current to be supplied to the electric motor 40 is determined by the steering assist control device 20 or the automatic steering control device 30 . The onboard battery supplies the electric power to the electric motor 40 at the current value determined by the steering assist control device 20 or the automatic steering control device 30 .

The electric motor 40 is supplied with the electric power from the onboard battery. The electric motor 40 generates a torque in accordance with the value of a current supplied from the onboard battery and transmits the generated torque to a decelerator 19 . The torque transmitted to the decelerator 19 is converted into a rotating torque for the steering shaft 12 . This allows the steering device 10 to be supplied with steering power in accordance with the current value.

The steering assist control device 20 is a device for assisting the steering operation by the driver via the steering device 10 . The steering assist control device 20 comprises a microcomputer which is mainly constituted of a CPU, a ROM, a RAM, and an I/O interface. The steering assist control device 20 controls driving of the electric motor 40 in accordance with a control program stored in the ROM. The steering assist control device 20 thereby performs steering assist control to assist the steering force of the driver.

Specifically, the steering assist control device 20 has steering assist characteristics in which the relationship among a steering torque, a vehicle speed and a current command value is predetermined. The steering assist control device 20 calculates a current command value, which is a target value of a current to be supplied to the electric motor 40 on the basis of steering assist characteristics.

After calculating the current command value, the steering assist control device 20 controls the onboard battery (not shown) to supply the electric motor 40 with electric power corresponding to the calculated current command value. In the above described manner, the steering assist control device 20 calculates the target value of a current to be supplied to the electric motor 40 as the current command value, on the basis of the steering torque and the vehicle speed. The onboard battery thereby supplies the electric power to the electric motor 40 at the current of the current command value calculated by the steering assist control device 20 . Thus, the steering assist control device 20 can allow the steering device 10 to be supplied with the steering power for assisting the steering operation of the driver.

In the present embodiment, the steering assist characteristics may be preliminarily stored in the ROM of the steering assist control device 20 in a form of a control map or an arithmetic expression. The steering assist characteristics are configured such that, as the steering torque is lager, the absolute value of the current command value is lager, and as the vehicle speed is higher, the absolute value of the current command value is smaller.

Next, the automatic steering control device 30 will be described. The automatic steering control device 30 comprises a microcomputer which is constituted of a CPU, a ROM, a RAM, and an I/O interface. The automatic steering control device 30 controls driving of the electric motor 40 in accordance with a control program stored in the ROM. The automatic steering control device 30 thereby executes automatic steering control to automatically perform the steering operation of the steering device 10 .

Specifically, when the driver turns on the automatic steering control switch 31 , an image captured by a camera is displayed on the screen of a display 32 which is disposed on the instrument panel. In this situation, the driver can set a target parking position by referring to the image displayed on the screen of the display 32 and operating a touch panel provided with the display 32 . After the target parking position is set by the driver, the automatic steering control device 30 calculates the positional relationship between the present position of the vehicle and the target parking position. The automatic steering control device 30 further calculates a target traveling route required for the vehicle to travel to the target parking position. The automatic steering control device 30 also calculates a target steering angle at each position on the target traveling route on the basis of the target traveling route and the present position of the vehicle. The automatic steering control device 30 further calculates the value of a current required for the electric motor 40 to be supplied to achieve that target steering angle, as the current command value. The current command value calculated by the automatic steering control device 30 is then output to the steering assist control device 20 . A method of calculating the current command value will be described later.

When the automatic steering control switch 31 is turned on, the automatic steering control device 30 outputs a start signal for the automatic steering control to the steering assist control device 20 . This changes the steering assist control mode of the steering assist control device 20 to an automatic steering control mode. The electric motor 40 is supplied with the electric power at a current of the current command value output from the automatic steering control device 30 , under the control by the steering assist control device 20 . Consequently, the electric power at the current command value calculated by the automatic steering control device 30 is converted into a steering torque in the electric motor 40 . The steering torque is transmitted as steering force to the steering device 10 .

Next, a method of calculating the current command value by the automatic steering control device 30 will be described. In the present embodiment, the automatic steering control device 30 stores a control transfer function in a ROM. Parameters of the control transfer function are, for example, the present steering angle of the vehicle (turning angle of the wheels 15 ), target steering angle, steering torque, current command value, and the like. The automatic steering control device 30 can calculate the current command value by inputting the present steering angle of the vehicle (turning angle of the wheels 15 ), target steering angle, steering torque, and the like to the control transfer function stored in the ROM.

In the present embodiment, the automatic steering control device 30 calculates the current command value by using different control transfer functions between in a non-stationary steering state in which the steering is performed while the vehicle is moving and in a stationary steering state in which the steering is performed while the vehicle is at a standstill. Specifically, the automatic steering control device 30 preliminarily stores in the ROM a first control transfer function that is a control transfer function for non-stationary steering and a second control transfer function that is a control transfer function for stationary steering. The automatic steering control device 30 determines whether the vehicle is in the stationary steering state or in the non-stationary steering state, for example, on the basis of the speed information and steering angle of the vehicle. When the vehicle is determined to be in the non-stationary steering state, the automatic steering control device 30 uses the first control transfer function for non-stationary steering to calculate the current command value. When the vehicle is determined to be in the stationary steering state, the automatic steering control device 30 uses the second control transfer function for stationary steering to calculate the current command value.

FIG. 2(A) is a graph illustrating an example of the current command value calculated by using the first control transfer function when the vehicle is in the non-stationary steering state and an example of the current command value calculated by using the second control transfer function when the vehicle is in the stationary steering state. FIG. 2(B) is a graph illustrating an example of a turning speed of the wheels 15 (angular velocity of turning angle) when the vehicle is in the stationary steering state in the scenes illustrated in FIG. 2(A) .

When the vehicle is in the stationary steering state, as illustrated in FIG. 2(A) , the automatic steering control device 30 gradually increases the current command value on the basis of the second control transfer function so that the current command value comes to a first target current value I 1 (details will be described later), from a time t 0 at which the automatic steering control is started to a time t 1 at which turning of the wheels 15 (change in turning angle of the wheels 15 ) is detected. In a similar manner, when the vehicle is in the non-stationary steering state, the automatic steering control device 30 gradually increases the current command value on the basis of the first control transfer function so that the current command value comes to a third target current value I 3 (details will be described later), from a time t 0 at which the automatic steering control is started to a time t 1 at which turning of the wheels 15 is detected. The following description will refer to a first steering control that is control from when the automatic steering control is started to when turning of the wheels 15 is detected.

As illustrated in FIG. 2(A) , in the first steering control which continues until the turning of the wheels 15 is detected, the automatic steering control device 30 sets the first target current value I 1 of the second control transfer function at a higher value than the third target current value I 3 of the first control transfer function. According to this setting, a current command value calculated by using the second control transfer function when the vehicle is in the stationary steering state is to be output as a larger value than the current command value calculated by using the first control transfer function when the vehicle is in the non-stationary steering state. As a result, when the vehicle is in the stationary steering state, the electric motor 40 is supplied with a current of a larger value than that when the vehicle is not in the stationary steering state. Therefore, the steering force for the wheels 15 provided to the steering device 10 can be increased during the stationary steering state in which the frictional force between the wheels 15 and the road surface is larger than that when the vehicle is in the non-stationary steering state. This allows the turning angle of the wheels 15 to vary at a desired rate, as illustrated in FIG. 2(B) , even in the stationary steering state in which the frictional force between the wheels 15 and the road surface is large, as in the case in which the vehicle is in the non-stationary steering state.

As illustrated in FIG. 2(A) , after the turning of the wheels 15 (change in turning angle of the wheels 15 ) is detected, the automatic steering control device 30 sets the target current command value of the second control transfer function to be the same value as a second target current value I 2 that is a target current command value of the first control transfer function. The following description will refer to a second steering control that is control after the turning of the wheels 15 (change in turning angle of the wheels 15 ) is detected.

According to this setting, as illustrated in FIG. 2(A) , when the vehicle is in the stationary steering state, the automatic steering control device 30 gradually decreases the current command value on the basis of the second control transfer function so that the current command value comes to the second target current value I 2 , from the time t 1 at which the turning of the wheels 15 (change in turning angle of the wheels 15 ) is detected. In a similar manner, when the vehicle is in the non-stationary steering state, the automatic steering control device 30 gradually decreases the current command value on the basis of the first control transfer function so that the current command value comes to the second target current value I 2 , from the time t 1 at which the turning of the wheels 15 (change in turning angle of the wheels 15 ) is detected. The value of the second target current value I 2 is not particularly limited provided that the value of the second target current value I 2 is smaller than the first target current value I 1 and the third target current value I 3 . For example, the value of the second target current value I 2 is determined to be a value that can achieve the target steering angle on the basis of the steering torque, present steering angle of the vehicle (turning angle of the wheels 15 ), target steering angle, and the like.

Thus, in the second steering control after the turning of the wheels 15 (change in turning angle of the wheels 15 ) is detected, the target current command value of the second control transfer function is set at the same value as the second target current value I 2 which is the target current command value of the first control transfer function. According to this setting, after the vehicle transitions from the stationary steering state to the non-stationary steering state, the current command value calculated by using the second control transfer function can be the current command value calculated by using the first control transfer function. This can result in suppression of the turning speed of the wheels 15 compared with the case in which the target current command value of the second control transfer function remains at a larger value than the second target current value I 2 after the vehicle transitions from the stationary steering state to the non-stationary steering state. It is therefore possible to mitigate an uncomfortable feeling given to the driver due to the turning speed of the wheels 15 .

As the above, when the vehicle is in the stationary steering state, as illustrated in FIG. 2(A) , the automatic steering control device 30 gradually increases the current command value on the basis of the second control transfer function so that the current command value comes to the first target current value I 1 , from the time t 0 at which the automatic steering control is started. The electric motor 40 is supplied with electric power from the onboard battery at a current in accordance with the current command value, and provides the steering force to the steering device 10 in accordance with the current command value. As a result, as illustrated in FIG. 2(B) , the steering force of the steering device 10 becomes larger than the frictional force between the wheels 15 and the road surface, and the turning of the wheels 15 is started. After the turning of the wheels 15 is started, the automatic steering control device 30 sets the second target current value I 2 as the target current command value of the second control transfer function. According to this setting, as illustrated in FIG. 2(A) , the automatic steering control device 30 gradually decreases the current command value on the basis of the second control transfer function so that the current command value comes to the second target current value I 2 . This can result in suppression of increase in the turning speed of the wheels 15 after the turning of the wheels 15 is detected, and it is thus possible to mitigate an uncomfortable feeling given to the driver due to the turning speed of the wheels 15 .

Moreover, in the present embodiment, the automatic steering control device 30 repeatedly determines whether the turning speed of the wheels 15 is not less than a predetermined speed Vu on the basis of the steering angle (turning angle of the wheels 15 ) acquired from the steering angle sensor 17 . When the turning speed of the wheels 15 becomes not less than the predetermined speed Vu, for example, at a time t 2 as illustrated in FIG. 2(B) , the automatic steering control device 30 resets the current command value to zero or decreases the current command value thereby to suppress the turning speed of the wheels 15 so that the turning speed of the wheels 15 becomes not more than the predetermined speed Vu. By suppressing the turning speed of the wheels 15 in such a manner, it is possible to mitigate an uncomfortable feeling given to the driver due to the turning speed of the wheels 15 , for example, when the driver intervenes into the steering operation.

Furthermore, in the present embodiment, after starting the first steering control, the automatic steering control device 30 calculates a change rate of the turning angle of the wheels 15 at the current command value. Further, the automatic steering control device 30 estimates a friction coefficient μ between the wheels 15 and the road surface on the basis of the calculated change rate. The automatic steering control device 30 then changes the first target current value I 1 of the second control transfer function on the basis of the estimated friction coefficient μ.

FIG. 3(A) is a graph illustrating an example of the current command value calculated by using the second control transfer function in the scene illustrated in FIG. 2(A) , and FIG. 3(B) is a graph illustrating an example of the turning speed of the wheels 15 in the scene illustrated in FIG. 2(B) . As illustrated in FIGS. 3(A) and 3(B) , the automatic steering control device 30 calculates the change rate of the turning angle of the wheels 15 at a current command value Iμ on the basis of the current command value Iμ and a turning speed Vμ of the wheels 15 which are obtained at a time t 3 when a certain period of time has passed from starting the automatic steering control (in an early stage of the automatic steering control). The automatic steering control device 30 estimates the friction coefficient μ between the wheels 15 and the road surface so that the friction coefficient μ becomes larger as the calculated change rate becomes smaller.

The automatic steering control device 30 then changes the first target current value I 1 of the second control transfer function such that the first target current value I 1 of the second control transfer function is larger as the estimated friction coefficient is larger. For example, in the example illustrated in FIG. 3(A) , the estimated friction coefficient μ is large, and therefore the automatic steering control device 30 changes the first target current value I 1 of the second control transfer function to I 1 ′ larger than I 1 in accordance with the estimated friction coefficient μ. According to this operation, the automatic steering control device 30 increases the current command value so that the current command value comes to the changed first target current value I 1 ′ in the first steering control until the turning of the wheels 15 is detected. As a result, the electric motor 40 is supplied with electric power at a current in accordance with the friction coefficient μ between the wheels 15 and the road surface. Then the steering device 10 is provided with steering force in accordance with the friction coefficient μ between the wheels 15 and the road surface. Therefore, the automatic steering control can be appropriately performed regardless of the friction coefficient μ between the wheels 15 and the road surface.

Although not illustrated, also when the vehicle is in the non-stationary steering state and the current command value is calculated by using the first control transfer function, the automatic steering control device 30 may calculate, in a similar manner to the above, the change rate of the turning angle of the wheels 15 at the current command value in the early stage of the automatic steering control. The automatic steering control device 30 then may estimate the friction coefficient μ between the wheels 15 and the road surface on the basis of the calculated change rate and changes the third target current value I 3 of the first control transfer function on the basis of the estimated friction coefficient μ. According to this operation, also when the vehicle is in the non-stationary steering state and the current command value is calculated by using the first control transfer function, the current command value can be appropriately calculated in accordance with the friction coefficient μ between the wheels 15 and the road surface.

Next, with reference to FIG. 4 , a steering control process or sequence according to the present embodiment will be described. FIG. 4 is a flowchart for explaining the steering control process according to the present embodiment.

First, in step S 101 , the automatic steering control device 30 determines whether the automatic steering control is started. For example, the automatic steering control device 30 can determine that the automatic steering control is started when the automatic steering control switch 31 is turned on. If the automatic steering control is determined to be started, the process proceeds to step S 102 , while if the automatic steering control is determined not to be started, the process waits in step S 101 .

In step S 102 , the automatic steering control device 30 starts acquisition of the vehicle information of the vehicle. For example, in the present embodiment, the automatic steering control device 30 acquires a steering torque, which is input to the steering device 10 , from the torque sensor 16 , a steering angle of the steering device 10 (turning angle of the wheels 15 ) from the steering angle sensor 17 , and a vehicle speed from the vehicle speed sensor 18 . The automatic steering control device 30 repeatedly acquires the vehicle information at regular intervals not only in step S 102 but also in the subsequent steps.

In step S 103 , the automatic steering control device 30 determines whether the vehicle is in the stationary steering state in which the steering is performed while the vehicle remains at a standstill. For example, the automatic steering control device 30 can determine that the vehicle is in the stationary steering state when the vehicle is at a standstill and a steering torque, which is a predetermined value or more, is input, on the basis of the vehicle speed of the vehicle and the steering torque which are acquired in step S 102 .

If the vehicle is determined to be in the non-stationary steering state in step S 103 , the process proceeds to step S 104 . In step 104 , the automatic steering control device 30 selects the first control transfer function for non-stationary steering as the control transfer function for calculating the current command value.

On the other hand, if the vehicle is determined to be in the stationary steering state in step S 103 , the process proceeds to step S 105 . In step S 105 , the automatic steering control device 30 selects the second control transfer function for stationary steering as the control transfer function for calculating the current command value.

Then in steps S 106 to S 110 , the automatic steering control device 30 executes the first steering control as illustrated in FIG. 2(A) . Specifically, first in step S 106 , the automatic steering control device 30 starts calculation of the current command value based on the first control transfer function selected in step S 104 or the second control transfer function selected in step S 105 . That is, if the vehicle is in the stationary steering state, calculation of the current command value based on the second control transfer function is started, while if the vehicle is in the non-stationary steering state, calculation of the current command value based on the first control transfer function is started.

In step S 107 , the automatic steering control device 30 estimates the friction coefficient μ between the wheels 15 and the road surface. Specifically, the automatic steering control device 30 calculates the change rate of the turning speed at the current command value Iμ on the basis of the current command value Iμ and the turning speed Vμ of the wheels 15 in the early stage of the first steering control, as illustrated in FIGS. 3(A) and 3(B) . The automatic steering control device 30 then estimates the friction coefficient μ between the wheels 15 and the road surface on the basis of the calculated change rate.

Then in step S 108 , the automatic steering control device 30 changes the first target current value I 1 of the second control transfer function or the third target current value I 3 of the first control transfer function on the basis of the friction coefficient g estimated in step S 107 . Specifically, the automatic steering control device 30 changes the first target current value I 1 or the third target current value I 3 to a larger value as the estimated friction coefficient μ is larger, as illustrated in FIG. 3(A) .

In step S 109 , the automatic steering control device 30 starts calculation of the current command value by using the first target current value I 1 or the third target current value I 3 changed in step S 108 . Then in step S 110 , the automatic steering control device 30 determines whether the turning of the wheels 15 is detected. For example, the automatic steering control device 30 repeatedly acquires the steering angle of the steering device 10 (turning angle of the wheels 15 ), and can determine that the turning of the wheels 15 is detected when the turning angle of the wheels 15 varies. If the turning of the wheels 15 is detected, the process proceeds to step S 111 . While if the turning of the wheels 15 is not detected, the process returns to step S 109 and the calculation of the current command value in the first steering control is repeated.

In step S 111 , the automatic steering control device 30 starts execution of the second steering control which is control after the turning of the wheels 15 is detected. Specifically, as illustrated in FIG. 2(A) , the automatic steering control device 30 changes the first target current value I 1 to the second target current value I 2 smaller than the first target current value I 1 when the current command value is calculated by using the second control transfer function. Similarly, the automatic steering control device 30 changes the third target current value I 3 to the second target current value I 2 smaller than the third target current value I 3 when the current command value is calculated by using the first control transfer function. The automatic steering control device 30 then gradually decreases the current command value on the basis of the first control transfer function or the second control transfer function so that the current command value comes to the second target current value I 2 .

In step S 112 , the automatic steering control device 30 determines whether the turning speed of the wheels 15 is not less than the predetermined speed Vu. If the turning speed of the wheels 15 is determined not to be less than the predetermined speed Vu, the process proceeds to step S 113 . In step 113 , the automatic steering control device 30 suppresses the current command value. This can suppress the turning speed of the wheels 15 so that the turning speed is less than the predetermined speed Vu. On the other hand, if the turning speed of the wheels 15 is determined to be less than the predetermined speed Vu, the process proceeds to step S 114 .

In step S 114 , the automatic steering control device 30 determines whether the automatic steering control has been completed. For example, the automatic steering control device 30 can determine that the automatic steering control has been completed when the vehicle has traveled to a target parking position, when the driver intervenes into the steering operation while the automatic steering control is being executed, or when the automatic steering control switch 31 is turned off. If the automatic steering control is determined to have not been completed, the process returns to step S 112 . In step 112 , the automatic steering of the wheels 15 is performed by the second steering control. While if the automatic steering control is determined to have been completed, the steering control process shown in FIG. 4 is terminated.

As described above, in the present embodiment, in the first steering control until the turning of the wheels 15 is detected, the current command value is calculated as a larger value and the steering device 10 is provided with larger steering force when the vehicle is in the stationary steering state than steering force when the vehicle is in the non-stationary steering state. It is thereby possible to appropriately turn the wheels 15 even in the stationary steering state in which the frictional force between the wheels 15 and the road surface is larger than that in the non-stationary steering state. Moreover, in the present embodiment, when the vehicle is determined to be in the stationary steering state, the electric motor 40 is supplied with a current of a larger current command value than that in the non-stationary steering state. As the result, the wheels 15 start to turn. After the wheels 15 start to turn (that is, in the second steering control after the turning of the wheels 15 is detected), the current command value of the current supplied to the electric motor 40 is suppressed to reduce the steering force provided to the steering device 10 . This can mitigate an uncomfortable feeling given to the driver due to unduly increase of the turning speed of the wheels 15 when the vehicle transitions from the stationary steering state to the non-stationary steering state. More specifically, when the vehicle transitions from the stationary steering state to the non-stationary steering state, if the electric motor 40 is supplied with electric power at a current of the current command value of the stationary steering state even in the non-stationary steering state, the turning speed of the wheels 15 may be unduly increased to give an uncomfortable feeling to the driver, for example, when the driver tries to intervene into the steering operation. In contrast, according to the present embodiment, when the vehicle transitions from the stationary steering state to the non-stationary steering state, the current command value is suppressed to decrease the current to be supplied to the electric motor 40 , and the turning speed of the wheels 15 can thereby be suppressed. This can mitigate an uncomfortable feeling given to the driver due to the turning speed of the wheels 15 .

Moreover, in the present embodiment, when the current command value is calculated by using the second control transfer function in the second steering control, the target current command value in the second steering control is set at the same value as the second target current value I 2 which is set in case of calculating the current command value by using the first control transfer function. According to this setting, even after the turning of the wheels 15 is detected, the turning speed of the wheels 15 can be appropriately suppressed in response to the traveling state of the vehicle as merely by calculating the current command value on the basis of the second control transfer function.

Furthermore, in the present embodiment, the first control transfer function and the second control transfer function are preliminarily stored, and the steering control is performed by using the first control transfer function when the vehicle is in the non-stationary steering state, while the steering control is performed by using the second control transfer function when the vehicle is in the stationary steering state. According to this operation, an appropriate current command value can be relatively easily calculated merely by inputting necessary parameters to the control transfer function which is selected in accordance with whether the vehicle is stationary steering state.

In the present embodiment, the change rate of the turning angle of the wheels 15 at the current command value in the early stage of the automatic steering control is calculated, and a friction coefficient μ between the wheels 15 and the road surface is estimated on the basis of the calculated change rate. Therefore, the current command value can be calculated in accordance with the friction coefficient μ between the wheels 15 and the road surface, as illustrated in FIG. 3(A) , by changing the first target current value I 1 of the second control transfer function or the third target current value I 3 of the first control transfer function on the basis of the friction coefficient μ. The steering device 10 can thus be provided with the steering force in accordance with the friction coefficient μ between the wheels 15 and the road surface. As a result, the automatic steering control can be appropriately performed regardless of the value of the friction coefficient μ between the wheels 15 and the road surface.

In addition, in the present embodiment, it is repeatedly determined whether the turning speed of the wheels 15 is not less than the predetermined speed Vu, and when the turning speed of the wheels 15 becomes not less than the predetermined speed Vu, the current command value is suppressed. This operation can mitigate an uncomfortable feeling given to the driver due to the turning speed of the wheels 15 , for example, when the driver intervenes into the steering operation.

Embodiments heretofore explained are described to facilitate understanding of the present invention and are not described to limit the present invention. Therefore, it is intended that the elements disclosed in the above embodiments include all design changes and equivalents to fall within the technical scope of the present invention.

For example, in the above-described embodiments, the present invention has been described by exemplifying a configuration comprising: determining whether the vehicle is in the stationary steering state and calculating the current command value to be supplied to the electric motor 40 by using the determination result in the scene where the automatic steering control device 30 executes the automatic steering control, but the present invention is not limited to this configuration. For example, another embodiment may be configured such as determining whether the vehicle is in the stationary steering state and calculating the current command value to be supplied to the electric motor 40 on the basis of the determination result in the scene where the steering assist control device 20 assists the steering operation by the driver. In this case, as the first steering control, it is possible to control steering force from when the steering torque is input to when the turning of the wheels 15 is detected.

In the above-described embodiments, the present invention has been described by exemplifying a configuration in which the friction coefficient μ between the wheels 15 and the road surface is estimated and the target current command value is changed on the basis of the estimated friction coefficient μ, but the present invention is not limited to this configuration. For example, another embodiment may be configured such that the current command value is corrected on the basis of the estimated friction coefficient μ. In a possible example of such a configuration, when the vehicle is in the stationary steering state, the current command value calculated by using the second control transfer function is corrected on the basis of the estimated friction coefficient μ, and the electric motor 40 is supplied with electric power at a current in accordance with the corrected current command value.

In the above-described embodiments, the steering device 10 corresponds to the steering mechanism of the present invention, the electric motor 40 corresponds to the drive unit of the present invention, and the automatic steering control device 30 corresponds to the detection unit, determination unit and control unit of the present invention.

100 . . . Steering control device

10 . . . Steering device

16 . . . Torque sensor

17 . . . Steering angle sensor

18 . . . Vehicle speed sensor

20 . . . Steering assist control device

30 . . . Automatic steering control device

40 . . . Electric motor