Vehicular steering system

Abstract

A vehicular steering system can suppress, even upon a change in a neutral position of a steering wheel, a right and left difference in the behavior of a vehicle by keeping neutral positions of steerable road wheels from the start of control. The system includes a steering mechanism that steers the steerable road wheels of the vehicle through the steering wheel and an auxiliary steering angle superposition mechanism. A steered angle of the steerable road wheels is detected as an absolute angle. The auxiliary steering angle superposition mechanism is controlled by a target auxiliary steering angle. An activation angle setting section sets, at the start of control, the initial values of a steering wheel angle and an auxiliary steering angle as an activation steering wheel angle and an activation auxiliary steering angle by the steered angle and a transmission characteristic between the steering wheel angle and the steered angle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a vehicular steering system according to a first embodiment of the present invention.

FIG. 2 is an explanatory view illustrating a transmission characteristic for calculating a target steered angle with respect to a steering wheel angle in case where a variable gear ratio mechanism is constructed by using the vehicular steering system of FIG. 1.

FIG. 3 is an explanatory view illustrating the relation between the steering wheel angle and a target auxiliary steering angle when the variable gear ratio mechanism is constructed by using the vehicular steering system of FIG. 1.

FIG. 4 is an explanatory view illustrating the relation between a transmission characteristic between the steering wheel angle and a steered angle in the vehicular steering system of FIG. 1 and the mechanical property of an auxiliary steering angle superposition mechanism.

FIG. 5 is an explanatory view illustrating the relation between the steering wheel angle and an auxiliary steering angle in the vehicular steering system of FIG. 1.

FIG. 6 is an explanatory view illustrating the relation between the steering wheel angle and the steered angle when a deviation occurs between an activation steering wheel angle and an actual activation steering wheel angle in the vehicular steering system of FIG. 1.

FIG. 7 is a timing chart showing the changes over time of a steering wheel angle correction amount and an auxiliary steering angle correction amount in the vehicular steering system of FIG. 1.

FIG. 8 is an explanatory view illustrating the change of the transmission characteristic between the actual steering wheel angle and the steered angle when the steering wheel angle correction amount and the auxiliary steering angle correction amount are caused to change in the vehicular steering system of FIG. 1.

FIG. 9 is a block diagram showing a vehicular steering system according to a second embodiment of the present invention.

FIG. 10 is a timing chart showing the changes over time of a steering wheel angle correction amount and an auxiliary steering angle correction amount in the vehicular steering system of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, preferred embodiments of the present invention will be described in detail while referring to the accompanying drawings. Throughout respective figures, the same or corresponding members or parts are identified by the same reference numerals and characters.

Embodiment 1

Referring to the drawings and first to FIG. 1, there is shown a block diagram of a vehicular steering system according to a first embodiment of the present invention.

In FIG. 1, the vehicular steering system is provided with a steering wheel 1 installed on a vehicle, an auxiliary steering angle superposition mechanism 2 that includes two planetary gear mechanisms 21, 22 and is electrically controlled, a pair of steerable road wheels 5a, 5b of a vehicle, a steering mechanism 3 that steers the steerable road wheels 5a, 5b of the vehicle through the steering wheel 1 and the auxiliary steering angle superposition mechanism 2, and a pair of knuckle arms 4a, 4b that connect between the steering mechanism 3 and the steerable road wheels 5a, 5b, respectively.

In addition, the vehicular steering system is further provided with a steering wheel angle detection section 6, a travel state detection section 7, a transmission characteristic setting section 8, a target auxiliary steering angle calculation section 9, an auxiliary steering angle detection section 10, a driving control section 11, a steered angle detection section 12, and an activation angle setting section 13, which will be described below in detail. The steering wheel angle detection section 6 includes a steering wheel angle detection part 14 and a steering wheel angle correction part 15. Also, the auxiliary steering angle detection section 10 includes an auxiliary steering angle detection part 16 and an auxiliary steering angle correction part 17. Here, note that the transmission characteristic setting section 8, the target auxiliary steering angle calculation section 9, the driving control section 11, the activation angle setting section 13, the steering wheel angle correction part 15, and the auxiliary steering angle correction part 17 are constituted by a microprocessor (not shown) that has a storage section with programs stored therein and a CPU.

The auxiliary steering angle superposition mechanism 2 is composed of a first planetary gear mechanism 21 that is connected with the steering wheel 1, a second planetary gear mechanism 22 that is arranged between the first planetary gear mechanism 21 and the steering mechanism 3, and a drive part 23 that is driven and controlled by the driving control section 11. The first planetary gear mechanism 21 has a sun gear 201 that is connected with the steering wheel 1, planetary gears 202a, 202b that are rotatably supported on a carrier 203 connected with one end of a shaft 210, and a ring gear 204 that is supported on the shaft 210 for rotation therearound and is provided with a worm wheel 205.

The second planetary gear mechanism 22 has a sun gear 206 that is connected with the steering mechanism 3, planetary gears 207a, 207b that are rotatably supported on a carrier 208 connected with the other end of the shaft 210 (the one end thereof being connected with the carrier 203), and a ring gear 209 that is fixedly secured to a case (not shown) of the auxiliary steering angle superposition mechanism 2. The drive part 23 has a worm gear 211 that is in mesh with the worm wheel 205 to rotate the ring gear 204, and an electric motor 212 for rotating the worm gears 211.

The steering mechanism 3 is of a rack-and-pinion type, and is composed of a pinion gear 301 that is connected with the sun gear 206, and a rack gear 302 that is in mesh with the pinion gear 301. The rotation of the pinion gear 301 is converted into a linear motion of the rack gear 302, and the linear motion of the rack gear 302 is converted into a steered angle θ_P of the steerable road wheels 5a, 5b through the knuckle arms 4a, 4b.

The steering wheel angle detection section 6 detects the steering angle of the steering wheel 1 as a steering wheel angle θ_H. The travel state detection section 7 detects the traveling condition of the vehicle (e.g., the vehicle speed or the steering speed of the steering wheel). The transmission characteristic setting section 8 sets a transmission characteristic f(θ_H) between the steering wheel angle θ_H and the steered angle θ_P of the steerable road wheels 5a, 5b in accordance with the traveling condition of the vehicle. The transmission characteristic setting section 8 according to this embodiment stores, as a map, the transmission characteristic including a plurality of nonlinear characteristics selected in accordance with the vehicle speed.

The target auxiliary steering angle calculation section 9 calculates a target auxiliary steering angle θ_SREF to be superposed by the auxiliary steering angle superposition mechanism 2 based on the steering wheel angle θ_H detected by the steering wheel angle detection section 6 and the transmission characteristic f(θ_H) set by the transmission characteristic setting section 8. The auxiliary steering angle detection section 10 detects an auxiliary steering angle θ_S to be superposed by the auxiliary steering angle superposition mechanism 2.

The driving control section 11 controls the driving of the electric motor 212 in the auxiliary steering angle superposition mechanism 2 in such a manner that the auxiliary steering angle θ_S coincides with the target auxiliary steering angle θ_SREF. The steered angle detection section 12 detects the steered angle θ_P of the steerable road wheels 5a, 5b as an absolute angle. At this time, the rotation angle of the pinion gear 301 corresponds, one to one, to the steered angle θ_P of the steerable road wheels 5a, 5b, so the steered angle detection section 12 according to this embodiment detects the rotation angle of the pinion gear 301 as an absolute angle, and detects this rotation angle as the steered angle θ_P of the steerable road wheels 5a, 5b.

Upon actuation of the vehicular steering system, i.e., at the start of control of the auxiliary steering angle superposition mechanism 2, the activation angle setting section 13 sets the initial value of the steering wheel angle θ_H detected by the steering wheel angle detection section 6 as an activation steering wheel angle θ_H0 based on the steered angle θ_P at the time of activation (activation steered angle θ_P0) detected by the steered angle detection section 12 and the transmission characteristic f(θ_H) set by the transmission characteristic setting section 8. Also, the activation angle setting section 13 sets the initial value of the auxiliary steering angle θ_S detected by the auxiliary steering angle detection section 10 as an activation auxiliary steering angle θ_S0 based on the activation steering wheel angle θ_H0 and the transmission characteristic f(θ_H).

Here, note that the vehicle is stopped upon activation of the apparatus, and hence a transmission characteristic corresponding to a vehicle speed of “0” is set in the transmission characteristic setting section 8. The activation steering wheel angle θ_H0 set by the activation angle setting section 13 is input to the steering wheel angle detection part 14, and the activation auxiliary steering angle θ_S0 is input to the auxiliary steering angle correction part 17.

The steering wheel angle detection part 14 detects the steering angle of the steering wheel 1 with respect to the activation steering wheel angle θ_H0 as a steering wheel angle detection value θ_HS in the form of a relative angle. The steering wheel angle correction part 15 adds the steering wheel angle correction amount Δθ_H to the steering wheel angle detection value θ_HS detected by the steering wheel angle detection part 14, and outputs a steering wheel angle θ_H thus obtained. The steering wheel angle correction part 15 sets the initial value of the steering wheel angle correction amount Δθ_H (activation steering wheel angle correction amount Δθ_H0) to “0” at the start of control of the auxiliary steering angle superposition mechanism 2, and sets, after the start of the control, the steering wheel angle correction amount Δθ_H so that the steering wheel angle θ_H, the auxiliary steering angle θ_S, and the steered angle θ_P can satisfy the mechanical property of the auxiliary steering angle superposition mechanism 2.

The auxiliary steering angle detection part 16 detects a rotation angle θ_M of the worm gear 211 as an absolute angle, and divides this rotation angle θ_M by a speed ratio G_S from the worm gear 211 to the pinion gear 301 thereby to obtain an auxiliary steering angle detection value θ_SR. The auxiliary steering angle correction part 17 adds the auxiliary steering angle correction amount Δθ_S to the auxiliary steering angle detection value θ_SR detected by the auxiliary steering angle detection part 16, and outputs the auxiliary steering angle θ_S thus obtained. The auxiliary steering angle correction part 17 sets an initial value of the auxiliary steering angle correction amount Δθ_S (activation auxiliary steering angle correction amount Δθ_S0) based on the activation auxiliary steering angle θ_S0 and the auxiliary steering angle detection value θ_SR, and gradually increases or gradually decreases the auxiliary steering angle correction amount Δθ_S to “0”.

Hereinbelow, reference will be made to the operation of the vehicular steering system according to the first embodiment of the present invention while referring to FIGS. 2 through 8 together with FIG. 1. First of all, reference be made to the case where the steering wheel 1 is rotated with the worm gear 211 being fixed in the auxiliary steering angle superposition mechanism 2.

When the worm gear 211 is not caused to rotate, the ring gear 204 of the first planetary gear mechanism 21 is fixed. Under such a condition, when the driver operates the steering wheel 1, the torque of rotation thereof generated upon steering is transmitted to the sun gear 201 of the first planetary gear mechanism 21. The rotation of the sun gear 201 is transmitted to the planetary gears 202a, 202b, but at this time, the ring gear 204 is fixed, so the rotation of the sun gear 201 is converted into the orbital motion of the carrier 203 that supports the planetary gears 202a, 202b.

Accordingly, the first planetary gear mechanism 21, serving to rotate the shaft 210 for transmission of rotation to the second planetary gear mechanism 22, functions as a speed reducer. Subsequently, the rotation of the shaft 210 is transmitted to the carrier 208 of the second planetary gear mechanism 22. The planetary gears 207a, 207b are driven to revolve around the sun gear 206 in accordance with the rotation of the carrier 208.

Here, in the second planetary gear mechanism 22, the ring gear 209 is fixed, so the revolutions of the planetary gears 207a, 207b cause the sun gear 206 to rotate whereby the pinion gear 301 in the steering mechanism 3 is driven to rotate. At this time, the second planetary gear mechanism 22 operates as a speed increasing gear, as viewed from the shaft 210, and the rotation of the steering wheel 1 is mechanically transmitted to the pinion gear 301 in the steering mechanism 3.

Note that the transmission ratio at this time becomes a value which is obtained by multiplication of the speed reduction ratio (the reciprocal of the speed increasing ratio) of the first planetary gear mechanism 21 and the speed reduction ratio of the second planetary gear mechanism 22, and if the constructions of both the planetary gear mechanisms are the same with respcet to each other, the transmission ratio as a whole becomes “1”. That is, in the construction of the auxiliary steering angle superposition mechanism 2 as shown in FIG. 1, it is found that if the rotation of the worm gear 211 is stopped, the steering system becomes an ordinary steering system in which the steering wheel angle θ_H and the steered angle θ_P correspond one-to-one to each other.

Now, reference will be made to the case where the electric motor 212 is driven to rotate the worm gear 211 with the steering wheel 1 being fixed in the auxiliary steering angle superposition mechanism 2.

When the worm gear 211 is driven to rotate, the ring gear 204 is caused to rotate through the worm wheel 205. At this time, the rotation of the ring gear 204 is transmitted to the planetary gears 202a, 202b, but the sun gear 201 is fixed by the steering wheel 1, so the rotation of the ring gear 204 is transmitted to the shaft 210 through the carrier 203 as the revolutions of the planetary gears 202a, 202b. As the shaft 210 rotates, the steering mechanism 3 is driven to steer the steerable road wheels 5a, 5b through the second planetary gear mechanism 22, as stated above.

Next, reference will be made to the case where the electric motor 212 is driven to rotate the worm gear 211 while operating the steering wheel 1 in the auxiliary steering angle superposition mechanism 2. Here, the steerable road wheels 5a, 5b are steered by means of the steering wheel 1 and the auxiliary steering angle superposition mechanism 2. Thus, the mechanical property of the auxiliary steering angle superposition mechanism 2 is represented by the following expression (1) based on the steering wheel angle θh of the steering wheel 1, the rotation angle θm of the worm gear 211, the steered angle θp of the steerable road wheels 5a, 5b, and the speed ratio G_S from the worm gear 211 to the pinion gear 301.

θp=θh+θm/G_S   (1)

In addition, the auxiliary steering angle θs is a value that is obtained by dividing the rotation angle θm of the worm gear 211 by the speed ratio G_S from the worm gear 211 to the pinion gear 301, so the expression (1) above is transformed into the following expression (2), as stated above. Here, note that the steering wheel angle θh, the rotation angle θm, the steered angle θp, and the auxiliary steering angle θs indicate individual angles in the interior of the entire mechanism from the steering wheel 1 to the steering mechanism 3 through the auxiliary steering angle superposition mechanism 2.

θp=θh+θs   (2)

Next, reference will be made to a variable gear ratio mechanism that changes the transmission characteristic f(θ_H) between the steering wheel angle θ_H and the steered angle θ_P of the steerable road wheels 5a, 5b in accordance with the traveling condition of the vehicle while referring to FIGS. 2 and 3.

FIG. 2 is an explanatory view illustrating the transmission characteristic f(θ_H) for calculating a target steered angle θ_PREF for the steering wheel angle θ_H in case where a variable gear ratio mechanism is constructed by using the vehicular steering system of FIG. 1. FIG. 3 is an explanatory view illustrating the relation between the steering wheel angle θ_H and the target auxiliary steering angle θ_SREF when the variable gear ratio mechanism is constructed by using the vehicular steering system of FIG. 1.

In FIG. 2, the transmission characteristic f(θ_H) is set by the transmission characteristic setting section 8 in accordance with the traveling condition of the vehicle (the vehicle speed in this embodiment). In addition, in FIG. 3, the relation between the steering wheel angle θ_H and the target auxiliary steering angle θ_SREF is obtained from the relation between the steering wheel angle θ_H and the target steered angle θ_PREF shown in FIG. 2.

As described above, the steering wheel angle θ_H output from the steering wheel angle correction part 15 and the transmission characteristic f(θ_H) set by the transmission characteristic setting section 8 in accordance with the traveling condition of the vehicle are input to the target auxiliary steering angle calculation section 9. The target auxiliary steering angle calculation section 9 calculates the target steered angle θ_PREF based on the steering wheel angle θ_H and the transmission characteristic f(θ_H) according to the transmission characteristic shown in FIG. 2. The target steered angle θ_PREF is represented by the following expression (3).

θ_PREF=f(θ_H)   (3)

In addition, the target auxiliary steering angle calculation section 9 calculates the target auxiliary steering angle θ_SREF based on the steering wheel angle θ_H and the target steered angle θ_PREF. Here, note that from the above expression (2), the relation of the following expression (4) holds among the steering wheel angle θ_H, the target steered angle θ_PREF and the target auxiliary steering angle θ_SREF.

θ_PREF=θ_H+θ_SREF   (4)

That is, the target auxiliary steering angle θ_SREF calculated by the target auxiliary steering angle calculation section 9 is represented by the following expression (5) which is transformed from the above expression (4).

θ_SREF=θ_PREF−θ_H   (5)

Subsequently, the driving control section 11 drives the electric motor 212 in the auxiliary steering angle superposition mechanism 2 in such a manner that the auxiliary steering angle θ_S detected by the auxiliary steering angle detection section 10 coincides with the target auxiliary steering angle θ_SREF calculated by the target auxiliary steering angle calculation section 9. The steering wheel angle θ_H and the target steered angle θ_PREF are controlled by controlling the driving of the auxiliary steering angle superposition mechanism 2 by means of the driving control section 11, as shown in FIG. 2.

Here, reference will be made to the case where the steering wheel 1 is steered with the vehicular steering system being stopped while referring to FIG. 4 and FIG. 5.

FIG. 4 is an explanatory view illustrating the relation between the transmission characteristic f(θ_H) between the steering wheel angle θ_H and the steered angle θ_P in the vehicular steering system of FIG. 1 and the mechanical property of the auxiliary steering angle superposition mechanism 2. Also, FIG. 5 is an explanatory view illustrating the relation between the steering wheel angle θ_H and the auxiliary steering angle θ_S in the vehicular steering system of FIG. 1.

Here, note that the auxiliary steering angle θ_S shown in FIG. 5 is a value that is obtained from the above expression (2) based on the steering wheel angle θ_H and the steered angle θ_P shown in FIG. 4.

In FIG. 4 and FIG. 5, a point A indicates a point at which the operation of the vehicular steering system is stopped. Also, this point A always exists on a line of the transmission characteristic f(θ_H) set by the transmission characteristic setting section 8. At this time, when the steering wheel 1 is steered, the vehicular steering system is in a stopped state, so the steering wheel angle θ_H and the steered angle θ_P change according to not the transmission characteristic f(θ_H) but the mechanical property of the auxiliary steering angle superposition mechanism 2. That is, as shown in FIG. 4, the steered angle θ_P deviates from the line of the transmission characteristic f(θ_H) to move to a point B on a line that represents the mechanical property of the auxiliary steering angle superposition mechanism 2. On the other hand, the vehicular steering system is in the stopped state, and hence the auxiliary steering angle θ_S does not change, so it does not move from the point A in FIG. 5.

Subsequently, when the vehicular steering system is activated, the steered angle detection section 12 detects the activation steered angle θ_P0 at the point B in FIG. 4, as an absolute angle. When the activation steered angle θ_P0 is detected, the activation angle setting section 13 estimates the activation steering wheel angle θ_H0 based on the activation steered angle θ_P0 and an inverse characteristic of the transmission characteristic f(θ_H). That is, representing the inverse characteristic of the transmission characteristic f(θ_H) by f⁻¹(θ_P), the activation steering wheel angle θ_H0 is represented by the following expression (6).

θ_H0=f⁻¹(θ_P0)   (6)

This activation steering wheel angle θ_H0 is a value that is calculated based on the transmission characteristic f(θ_H), as shown in FIG. 4, and is different from the actual value of the activation steering wheel angle θ_H0 (actual activation steering wheel angle θ_HR0). In addition, the activation angle setting section 13 estimates the activation auxiliary steering angle θ_S0 from the above expression (2) based on the estimated activation steering wheel angle θ_H0 and the transmission characteristic f(θ_H), as shown in FIG. 5. That is, the activation auxiliary steering angle θ_S0 is represented by the following expression (7).

θ_S0=f(θ_H0)−θ_H0   (7)

Accordingly, as shown in FIG. 4 and FIG. 5, the activation angle setting section 13 estimates the activation steering wheel angle θ_H0 and the activation auxiliary steering angle θ_S0 based on the activation steered angle θ_P0 and the transmission characteristic f(θ_H), assuming that the steering wheel angle θ_H and the auxiliary steering angle θ_S exist at a point C on the line of the transmission characteristic f(θ_H).

Here, the initial value of the auxiliary steering angle detection value θ_SR (activation auxiliary steering angle detection value θ_SR0) detected by the auxiliary steering angle detection part 16 is an absolute angle, and is different from the activation auxiliary steering angle θ_S0. The auxiliary steering angle correction part 17 adds the auxiliary steering angle correction amount Δθ_S to the auxiliary steering angle detection value θ_SR so as to output the auxiliary steering angle θ_S, as previously stated, and the activation auxiliary steering angle correction amount Δθ_S0 to be added is represented by the following expression (8) based on the activation auxiliary steering angle θ_S0 and the activation auxiliary steering angle detection value θ_SR0.

Δθhd S0=θ_S0−θ_SR0   (8)

Accordingly, the auxiliary steering angle θ_S output from the auxiliary steering angle correction part 17 is represented by the following expression (9).

θ_S=θ_SR0+Δθ_S0=θ_S0   (9)

That is, the auxiliary steering angle θ_S output from the auxiliary steering angle correction part 17 becomes equal to the activation auxiliary steering angle θ_S0 estimated by the activation angle setting section 13.

On the other hand, the steering wheel angle detection part 14 detects a relative angle, but can not detect an absolute angle upon activation. Accordingly, the steering wheel angle detection part 14 according to this embodiment detects the steering angle of the steering wheel 1 with respect to the activation steering wheel angle θ_H0 as the steering wheel angle detection value θ_HS in the form of a relative angle with the activation steering wheel angle θ_H0 estimated by the activation angle setting section 13 being made a reference.

FIG. 6 is an explanatory view that illustrates the relation between the steering wheel angle θ_H and the steered angle θ_P when a deviation occurs between the activation steering wheel angle θ_H0 and the actual activation steering wheel angle θ_HR0 in the vehicular steering system of FIG. 1.

In FIG. 6, a zero point of the steering wheel angle detection value θ_HS is different from a zero point of the actual value of the steering wheel angle (actual steering wheel angle θ_HR), and a difference therebetween is equal to a value of “θ_HR0−θ_H0”. The steering wheel angle correction part 15 adds the steering wheel angle correction amount Δθ_H to the steering wheel angle detection value θ_HS, and outputs a steering wheel angle θ_H thus obtained, as stated above. Note that the activation steering wheel angle correction amount Δθ_H0 is “0”, so the steering wheel angle θ_H output from the steering wheel angle correction part 15 becomes equal to the activation steering wheel angle θ_H0 estimated by the activation angle setting section 13.

Here, reference will be made to the relation among the activation steering wheel angle θ_H0 and the activation auxiliary steering angle θ_S0 estimated by the activation angle setting section 13, the activation steered angle θ_P0 detected by the steered angle detection section 12, the activation auxiliary steering angle detection value θ_SR0 detected by the auxiliary steering angle detection part 16, and the actual activation steering wheel angle θ_HR0. From the above expression (2), the relation of the following expression (10) holds among the activation steering wheel angle θ_H0, the activation auxiliary steering angle θ_S0, and the activation steered angle θ_P0.

θ_H0+θ_S0=θ_P0   (10)

On the other hand, from the above expression (2), the relation of the following expression (11) holds among the activation auxiliary steering angle detection value θ_SR0, the actual activation steering wheel angle θ_HR0, and the activation steered angle θ_P0.

θ_HR0+θ_SR0=θ_P0   (11)

Accordingly, the relation of the following expression (12) holds from the above expressions (10) and (11).

θ_HR0−θ_H0=θ_S0−θ_SR=Δθ_S0   (12)

Therefore, the auxiliary steering angle θ_S, which is output from the auxiliary steering angle correction part 17 after the vehicular steering system is activated to start the control operation of the auxiliary steering angle superposition mechanism 2, is represented by the following expression (13) by replacing the activation auxiliary steering angle detection value θ_SR0 with the auxiliary steering angle detection value θ_SR in the above expression (9).

θ_S=θ_SR+Δθ_S0   (13)

In addition, after the start of the control, the actual steering wheel angle θ_HR is represented by the following expression (14) by replacing the actual steering wheel angle upon starting θ_HR0 with the actual steering wheel angle θ_HR, and by replacing the activation steering wheel angle θ_H0 with the steering wheel angle detection value θ_HS in the above expression (12).

θ_HR=θ_HS+ΔθS0   (14)

Accordingly, in order to coincide the steering wheel angle θ_H output from the steering wheel angle correction part 15 with the actual steering wheel angle θ_HR, the steering wheel angle correction amount Δθ_H need only be set to the activation auxiliary steering angle correction amount Δθ_S0.

Next, reference will be made to the operation of the vehicular steering system immediately after the vehicular steering system is activated to set the activation steering wheel angle θ_H0 and the activation auxiliary steering angle θ_S0.

The target auxiliary steering angle calculation section 9 calculates the target auxiliary steering angle θ_SREF to be superposed by the auxiliary steering angle superposition mechanism 2 based on the steering wheel angle θ_H output from the steering wheel angle correction section 15 and the transmission characteristic f(θ_H) set by the transmission characteristic setting section 8, as stated above.

Here, the steering wheel angle correction part 15 outputs the activation steering wheel angle θ_H0, so the target auxiliary steering angle θ_SREF for the activation steering wheel angle θ_H0 is represented by the following expression (15) based on the relation between the steering wheel angle θ_H and the auxiliary steering angle θ_S shown in FIG. 5.

θ_SREF=θ_S0   (15)

In addition, the auxiliary steering angle correction part 17 outputs the activation auxiliary steering angle θ_S0 , as previously stated, so the auxiliary steering angle θ_S and the target auxiliary steering angle θ_SREF coincide with each other, and the driving control section 11 does not drive the electric motor 212 in the auxiliary steering angle superposition mechanism 2. Accordingly, the steering wheel 1 never rotates on its own axis immediately after the activation of the vehicular steering system, and hence no uncomfortable feeling is given to the driver.

Further, under such a situation, when the steering wheel 1 is steered to its neutral position, i.e., when the steering wheel angle θ_H output from the steering wheel angle correction part 15 is made to be “0”, the target auxiliary steering angle θ_SREF becomes “0” based on the relation between the steering wheel angle θ_H and the auxiliary steering angle θ_S shown in FIG. 5.

The driving control section 11 drives the electric motor 212 in the auxiliary steering angle superposition mechanism 2 in such a manner that the auxiliary steering angle θ_S output from the auxiliary steering angle correction part 17 becomes “0”. At this time, in the above expression (14), the steering wheel angle detection value θ_HS becomes “0”, and hence the following expression (16) is obtained.

θ_HR=0+Δθ_S0   (16)

Also, in the above expression (13), the auxiliary steering angle θ_S becomes “0”, and the following expression (17) is obtained.

θ_SR=0−Δθ_S0   (17)

Accordingly, from the above expressions (2), (16) and (17), the relation of the following expression (18) holds among the steered angle θ_P , the actual steering wheel angle θ_HR, and the auxiliary steering angle detection value θ_SR.

θ_P=θ_HR+θ_SR=0   (18)

From the above expression (18), the steered angle θ_P at this time becomes “0”, and the steerable road wheels 5a, 5b are controlled to their neutral positions. That is, the transmission characteristic f(θ_H) between the steering wheel angle θ_H output from the steering wheel angle correction part 15 and the steered angle θ_P of the steerable road wheels 5a, 5b becomes as shown in FIG. 2. On the other hand, between the actual steering wheel angle θ_HR and the steering wheel angle θ_H output from the steering wheel angle correction part 15, there occurs a deviation, as shown in the above expression (14), so control is executed on the actual steering wheel angle θ_HR with the neutral position thereof being deviated or shifted, as shown in FIG. 6. However, the neutral position of the steered angle θ_P is correctly controlled, as shown in the above expression (18), so a right and left difference does not occur in the vehicle behavior to the relative steering of the steering wheel 1.

Now, reference will be made to the operation of the vehicular steering system that serves to make the neutral position of the steering wheel 1 and the neutral positions of the steerable road wheels 5a, 5b coincide with each other.

In order to make the neutral positions of the steering wheel 1 and the steerable road wheels 5a, 5b coincide with each other, it is necessary to make the steering wheel angle θ_H output from the steering wheel angle correction part 15 and the actual steering wheel angle θ_HR coincide with each other, and at the same time to make the auxiliary steering angle θ_S output from the auxiliary steering angle correction part 17 and the auxiliary steering angle detection value θ_SR coincide with each other. That is, as shown in the above expression (14), it is only necessary to set the steering wheel angle correction amount Δθ_H added by the steering wheel angle correction part 15 to the activation auxiliary steering angle correction amount Δθ_S0 and to set the auxiliary steering angle correction amount Δθ_S added by the auxiliary steering angle correction part 17 to “0”.

However, when the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S are rapidly changed, the electric motor 212 in the auxiliary steering angle superposition mechanism 2 is rapidly driven to operate, as a consequence of which when the steering wheel 1 rotates on its own axis or when the driver holds the steering wheel 1, the steerable road wheels 5a, 5b may be rapidly steered, thus giving rise to an uncomfortable feeling to the driver. In order to suppress such an uncomfortable feeling, it is only necessary to change the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S in synchronization with each other.

Hereinafter, reference will be made to the operation of the vehicular steering system that changes the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S in synchronization with each other.

First of all, the steering wheel angle detection value θ_HS output from the steering wheel angle detection part 14 is represented as the following expression (19) by transformation of the above expression (14).

θ_HS=θ_HR−Δθ_S0   (19)

Subsequently, the steering wheel angle θ_H output from the steering wheel angle correction part 15 is represented as the following expression (20) by adding the steering wheel angle correction amount Δθ_H to the steering wheel angle detection value θ_HS detected by the above expression (19).

θ_H=θ_HS+Δθ_H=θ_HR−Δθ_S0+Δθ_H   (20)

On the other hand, the auxiliary steering angle θ_S output from the auxiliary steering angle correction part 17 is represented as the following expression (21) by adding the auxiliary steering angle correction amount Δθ_S to the auxiliary steering angle detection value θ_SR detected by the auxiliary steering angle detection part 16.

θ_S=θ_SR+Δθ_S   (21)

Here, in order to control the neutral positions of the steerable road wheels 5a, 5b in an accurate manner, it is necessary that the steered angle θ_P should be “0” when the steering wheel angle θ_H and the auxiliary steering angle θ_S are “0”, respectively.

Accordingly, the following expression (22) is obtained by assigning “0” to the steering wheel angle θ_H in the above expression (20).

θ_HR=Δθ_S0−Δθ_H   (22)

Also, by assigning “0” to the auxiliary steering angle θ_S in the above expression (21), the following expression (23) is obtained.

θ_SR=−Δθ_S   (23)

Further, the following expression (24) is obtained by assigning the above expressions (22) and (23) to the above expression (18).

(Δθ_S0−Δθ_H)+(−Δθ_S)=θ_P=0

∴Δθ_H=Δθ_S0−Δθ_S   (24)

Accordingly, by changing the steering wheel angle correction amount Δθ_H to be added by the steering wheel angle correction part 15 and the auxiliary steering angle correction amount Δθ_S to be added by the auxiliary steering angle correction part 17 while keeping the relation shown in the above expression (24), it is possible to correctly control the neutral positions of the steerable road wheels 5a, 5b without regard to the correction amounts. Therefore, no right and left difference is generated in the vehicle behavior to the relative steering of the steering wheel 1 even during the time when the correction amounts are being changed.

Here, note that when the steering wheel angle θ_H output from the steering wheel angle correction part 15 and the auxiliary steering angle θ_S output from the auxiliary steering angle correction part 17 are added by using the above expression (20) and the above expression (21), the following expression (25) is obtained.

θ_H+θ_S=(θ_HR−Δθ_S0+Δθ_H)+(θ_SR+Δθ_S)   (25)

In addition, when the above expression (25) is organized by using the above expression (24), the following expression (26) is obtained.

θ_H+θ_S=θ_HR+θ_SR=θ_P   (26)

From the above expression (26), it is found that the steering wheel angle θ_H output from the steering wheel angle correction part 15, the auxiliary steering angle θ_S output from the auxiliary steering angle correction part 17, and the steered angle θ_P satisfy the mechanical property of the auxiliary steering angle superposition mechanism 2 shown in the above expression (2).

FIG. 7 is a timing chart that shows the changes over time of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S in the vehicular steering system of FIG. 1.

In FIG. 7, the changes of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S are started at time point t1. The auxiliary steering angle correction amount Δθ_S is gradually increased from the activation auxiliary steering angle correction amount Δθ_S0 toward “0”, whereas the steering wheel angle correction amount Δθ_H is changed while keeping the relation shown in the above expression (24). The changes of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S are terminated at time point t2. Also, in FIG. 7, the ratio of the changes of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S is set so as not to give an uncomfortable feeling to the driver.

At time point t2, the steering wheel angle correction amount Δθ_H becomes equal to the activation auxiliary steering angle correction amount Δθ_S0 (Δθ_H=Δθ_S0), and the auxiliary steering angle correction amount Δθ_S becomes “0”. At this time, by assigning “Δθ_H=Δθ_So” to the above expression (20), the steering wheel angle θ_H output from the steering wheel angle correction part 15 is represented by the following expression (27).

θ_H=θ_HR   (27)

In addition, by assigning “Δθ_S=0” to the above expression (21), the auxiliary steering angle θ_S output from the auxiliary steering angle correction part 17 is represented by the following expression (28).

θ_S=θ_SR   (28)

That is, from the above expression (27), the steering wheel angle θ_H coincides with the actual steering wheel angle θ_HR, and the auxiliary steering angle θ_S coincides with the auxiliary steering angle detection value θ_SR. Thus, the neutral position of the steering wheel 1 and the neutral positions of the steerable road wheels 5a, 5b coincide with each other.

FIG. 8 is an explanatory view that illustrates the change of the transmission characteristic f(θ_H) between the actual steering wheel angle θ_HR and the steered angle θ_P when the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S are caused to change in the vehicular steering system of FIG. 1.

In FIG. 8, at time point t1 at which the changes of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S are started, the neutral position of the steering wheel 1 and the neutral positions of the steerable road wheels 5a, 5b are different from each other, but at time point t2 at which the changes of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S are terminated, the neutral position of the steering wheel 1 and the neutral positions of the steerable road wheels 5a, 5b coincide with each other.

In addition, it is found from FIG. 8 that even during the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S are being changed, the neutral positions of the steerable road wheels 5a, 5b do not change, thus generating no right and left difference in the vehicle behavior to the relative steering of the steering wheel 1.

According to the vehicular steering system of the first embodiment of the present invention, at the start of control of the auxiliary steering angle superposition mechanism 2, the activation angle setting section 13 sets the initial value of the steering wheel angle θ_H as the activation steering wheel angle θ_H0 based on the steered angle θ_P of the steerable road wheels 5a, 5b detected as an absolute angle and the transmission characteristic f(θ_H) set in accordance with the traveling condition of the vehicle, and also sets the initial value of the auxiliary steering angle θ_S as the activation auxiliary steering angle θ_S0. Accordingly, even when the steering wheel 1 is steered to change the neutral position of the steering wheel 1 with the system being stopped, a right and left difference in the vehicle behavior can be suppressed by keeping the neutral positions of the steerable road wheels 5a, 5b from the start of control of the auxiliary steering angle superposition mechanism 2.

In addition, the auxiliary steering angle correction part 17 sets the activation auxiliary steering angle correction amount Δθ_S0 based on the activation auxiliary steering angle Δ_S0 and the auxiliary steering angle detection value θ_SR, and gradually increases or gradually decreases the auxiliary steering angle correction amount Δθ_S to “0”. The steering wheel angle correction part 15 sets the activation steering wheel angle correction amount Δθ_H0 to “0” at the start of control of the auxiliary steering angle superposition mechanism 2, and sets, after the start of the control, the steering wheel angle correction amount Δθ_H so that the steering wheel angle θ_H, the auxiliary steering angle θ_S, and the steered angle θ_P can satisfy the mechanical property of the auxiliary steering angle superposition mechanism 2. Accordingly, the neutral position of the steering wheel 1 and the neutral positions of the steerable road wheels 5a, 5b can be made to coincide with each other without generating a right and left difference in the vehicle behavior to the relative steering of the steering wheel 1.

Embodiment 2

FIG. 9 is a block diagram that shows a vehicular steering system according to a second embodiment of the present invention.

In FIG. 9, the vehicular steering system is provided with a steering wheel angle detection section 6A, an auxiliary steering angle detection section 10A and an activation angle setting section 13A in place of the steering wheel angle detection section 6, the auxiliary steering angle detection section 10 and the activation angle setting section 13 shown in FIG. 1. The steering wheel angle detection section 6A includes a steering wheel angle detection part 14A and a steering wheel angle correction part 15A. Also, the auxiliary steering angle detection section 10A includes an auxiliary steering angle detection part 16A and an auxiliary steering angle correction part 17A.

The steering wheel angle detection section 6A detects the steering angle of the steering wheel 1 as a steering wheel angle θ_H. The auxiliary steering angle detection section 10A detects an auxiliary steering angle θ_S to be superposed by the auxiliary steering angle superposition mechanism 2. The activation angle setting section 13A sets an activation steering wheel angle θ_H0 and an activation auxiliary steering angle θ_S0 based on an activation steered angle θ_P0 and a transmission characteristic f(θ_H), similar to the above-mentioned first embodiment. Here, note that the activation steering wheel angle θ_H0 set by the activation angle setting section 13A is input to the steering wheel angle detection part 14A, and the activation auxiliary steering angle θ_S0 is input to the auxiliary steering angle detection part 16A.

The steering wheel angle detection part 14A detects the steering angle of the steering wheel 1 with respect to the activation steering wheel angle θ_H0 as a steering wheel angle detection value θ_HREL, and also detects, as a predetermined steering wheel angle θ_HREF, that the steering wheel 1 becomes a predetermined steering angle which is arbitrarily set. Here, note that this predetermined steering angle is mechanically detected, for example, by a groove, a concave, a convex, or the like, formed in a rotation shaft of the steering wheel 1. The steering wheel angle correction part 15A adds the steering wheel angle correction amount Δθ_H to the steering wheel angle detection value θ_HREL detected by the steering wheel angle detection part 14A, and outputs a steering wheel angle θ_H thus obtained.

The steering wheel angle correction part 15A sets the initial value of the steering wheel angle correction amount Δθ_H (activation steering wheel angle correction amount Δθ_H0) to “0” at the start of control of the auxiliary steering angle superposition mechanism 2, also sets, upon detection of the predetermined steering wheel angle θ_HREF, a target correction angle for the steering wheel angle θ_H based on the predetermined steering wheel angle θ_HREF and the steering wheel angle detection value θ_HREL, and gradually increases or gradually decreases the steering angle correction amount Δθ_H to the target correction angle.

The auxiliary steering angle detection part 16A detects a rotation angle θ_M of the worm gear 211 as a relative angle with respect to the activation auxiliary steering angle θ_S0, and divides this rotation angle θ_M by a speed ratio G_S from the worm gear 211 to the pinion gear 301 thereby to obtain an auxiliary steering angle detection value θ_SREL. The auxiliary steering angle correction part 17A adds an auxiliary steering angle correction amount Δθ_S to the auxiliary steering angle detection value θ_SREL detected by the auxiliary steering angle detection part 16A, and outputs an auxiliary steering angle θ_S thus obtained.

The auxiliary steering angle correction part 17A sets the initial value of the auxiliary steering angle correction amount Δθ_S (activation auxiliary steering angle correction amount Δθ_S0) to “0” at the start of control of the auxiliary steering angle superposition mechanism 2, and sets, after the start of the control, the auxiliary steering wheel angle correction amount Δθ_S so that the steering wheel angle θ_H, the auxiliary steering angle θ_S and the steered angle θ_P can satisfy the mechanical property of the auxiliary steering angle superposition mechanism 2.

The construction of this second embodiment other than the above is similar to that of the first embodiment, and a description thereof is omitted.

Hereinbelow, reference will be made to the operation of the vehicular steering system according to the second embodiment of the present invention while referring to FIG. 10 together with FIGS. 2 through 9. Note that an explanation is omitted about an operation of this second embodiment similar to that of the first embodiment.

When the vehicular steering system is activated, the steered angle detection section 12 detects the activation steered angle θ_P0 as an absolute angle. Upon detection of the activation steered angle θ_P0, the activation angle setting section 13A estimates the activation steering wheel angle θ_H0 and the activation auxiliary steered angle θ_S0 based on the activation steered angle θ_P0 and an inverse characteristic of the transmission characteristic f(θ_H).

The activation steering wheel angle θ_H0 estimated by the activation angle setting section 13A is set as the initial value of the steering wheel angle detection part 14A, and the steering wheel angle detection part 14A detects the steering angle of the steering wheel 1 with respect to the activation steering wheel angle θ_H0 as a steering wheel angle detection value θ_HREL in the form of a relative angle. Here, note that the activation steering wheel angle correction amount Δθ_H0 is “0”, so the initial value of the steering wheel angle θ_H output from the steering wheel angle correction part 15A becomes equal to the activation steering wheel angle θ_H0 estimated by the activation angle setting section 13A.

In addition, the activation auxiliary steering angle θ_S0 estimated by the activation angle setting section 13A is set as the initial value of the auxiliary steering angle detection part 16A. The auxiliary steering angle detection part 16A detects a relative angle to the activation auxiliary steering angle θ_S0 as the auxiliary steering angle detection value θ_SREL. Here, the activation auxiliary steering angle correction amount Δθ_{S0 l is “}0”, so the initial value of the auxiliary steering angle θ_S output from the auxiliary steering angle correction part 17A becomes equal to the activation auxiliary steering angle θ_S0 estimated by the activation angle setting section 13A.

Next, reference will be made to the operation of the vehicular steering system immediately after the vehicular steering system is activated to set the activation steering wheel angle θ_H0 and the activation auxiliary steering angle θ_S0.

The target auxiliary steering angle calculation section 9 calculates the target auxiliary steering angle θ_SREF to be superposed by the auxiliary steering angle superposition mechanism 2 based on the steering wheel angle θ_H output from the steering wheel angle correction section 15A and the transmission characteristic f(θ_H) set by the transmission characteristic setting section 8, as stated before.

Here, the steering wheel angle correction part 15A outputs the activation steering wheel angle θH₀, so the target auxiliary steering angle θ_SREF for the activation steering wheel angle θ_H0 becomes the activation auxiliary steering angle θ_S0 based on the relation between the steering wheel angle θ_H and the auxiliary steering angle θ_S shown in FIG. 5. Accordingly, the auxiliary steering angle θ_S and the target auxiliary steering angle θ_SREF coincide with each other, so the driving control section 11 does not drive the electric motor 212 in the auxiliary steering angle superposition mechanism 2. Thus, the steering wheel 1 never rotates on its own axis immediately after the activation of the vehicular steering system, and hence no uncomfortable feeling is given to the driver.

In addition, from the above expression (2), the relation of the following expression (29) holds among the steering wheel angle detection value θ_HREL detected by the steering wheel angle detection part 14A, the auxiliary steering angle detection value θ_SREL detected by the auxiliary steering angle detection part 16A, and the steered angle θ_P detected by the steered angle detection section 12.

θ_P=θ_HREL+θ_SREL   (29)

Subsequently, when the driver steers the steering wheel 1 to a predetermined steering angle that is arbitrarily set, the steering wheel angle detection part 14A detects the predetermined steering wheel angle θ_HREF

As such a predetermined angle, a single angle may be set, or a plurality of angles may instead be set. At this time, as amount of deviation Δθ_HTAG of the neutral position of the steering wheel 1 (target correction angle) is represented by the following expression (30) based on the steering wheel angle detection value θ_HREL detected by the steering wheel angle detection part 14A and the predetermined steering wheel angle θ_HREF.

Δθ_HTAG=θ_HREF−θ_HREL   (30)

That is, the actual steering wheel angle θ_HR is represented by the following expression (31).

θ_HR=θ_HREL+Δθ_HTAG   (31)

Accordingly, in order to make the neutral positions of the steering wheel 1 and the steerable road wheels 5a, 5b coincide with each other, it is necessary to change the steering wheel angle correction amount Δθ_H from “0” up to the amount of deviation Δθ_HTAG of the neutral position of the steering wheel 1. Here, note that from the above expressions (20) and (21), the steering wheel angle θ_H output from the steering wheel angle correction part 15A and the auxiliary steering angle θ_S output from the auxiliary steering angle correction part 17A are represented by the following expressions (32) and (33), respectively.

θ_H=θ_HREL+Δθ_H   (32)

θ_S=θ_SREL+Δθ_S   (33)

Also, it is necessary for the steering wheel angle θ_H output from the steering wheel angle correction part 15A, the auxiliary steering angle θ_S output from the auxiliary steering angle correction part 17A, and the steered angle θ_P to satisfy the relation of the above-mentioned expression (2), so the following expression (34) holds from the above-mentioned expressions (2), (29), (32) and (33).

Δθ_S=−Δθ_H   (34)

Accordingly, by changing the steering wheel angle correction amount Δθ_H to be added by the steering wheel angle correction part 15A from “0” up to the amount of deviation Δθ_HTAG of the neutral position, and at the same time by changing the auxiliary steering angle correction amount Δθ_S while keeping the relation shown in the above expression (34), it is possible to make the neutral position of the steering wheel 1 and the neutral positions of the steerable road wheels 5a, 5b coincide with each other, as in the above-mentioned first embodiment.

FIG. 10 is a timing chart that shows the changes over time of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S in the vehicular steering system of FIG. 9.

In FIG. 10, at time point t3, the changes of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S are started. The steering wheel angle correction amount Δθ_H is gradually increased from “0” up to the amount of deviation Δθ_HTAG of the neutral position, whereas the auxiliary steering angle correction amount Δθ_S is changed while keeping the relation shown in the above expression (34). The changes of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S are terminated at time point t4. Also, in FIG. 10, the ratio of the changes of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S is set so as not to give an uncomfortable feeling to the driver.

According to the vehicular steering system of the second embodiment of the present invention, the steering wheel angle correction part 15A sets the activation steering wheel angle correction amount Δθ_H0 to “0” at the start of control of the auxiliary steering angle superposition mechanism 2, also sets, upon detection of the predetermined steering wheel angle θ_HREF, the target correction angle for the steering wheel angle θ_H based on the predetermined steering wheel angle θ_HREF and the steering wheel angle detection value θ_HREL, and gradually increases or gradually decreases the steering angle correction amount Δθ_H to the target correction angle. The auxiliary steering angle correction part 17A sets the activation auxiliary steering angle correction amount Δθ_S0 to “0” at the start of control of the auxiliary steering angle superposition mechanism 2, and sets, after the start of the control, the auxiliary steering wheel angle correction amount Δθ_S so that the steering wheel angle θ_H, the auxiliary steering angle θ_S and the steered angle θ_P can satisfy the mechanical property of the auxiliary steering angle superposition mechanism 2. Accordingly, the neutral position of the steering wheel 1 and the neutral positions of the steerable road wheels 5a, 5b can be made to coincide with each other without generating a right and left difference in the vehicle behavior to the relative steering of the steering wheel 1, as in the case of the above-mentioned first embodiment.

In the above-mentioned first and second embodiments, the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S are uniformly changed, as shown in FIG. 7 and FIG. 10, respectively, but the present invention is not limited to this. The ratio of the changes of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S may be changed depending on the direction of steering, the speed of steering, etc., of the steering wheel 1. In this case, too, the operational effects similar to those of the above-mentioned first and second embodiments can be achieved.

In addition, by providing information by a lamp or voice, or by giving vibration to the steering wheel 1, etc., at the time when the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S are being changed, the driver may be informed of the changes of the steering wheel angle correction amount Δθ_H and the auxiliary steering angle correction amount Δθ_S. In this case, it is possible to suppress the occurrence of an uncomfortable feeling to the driver.

While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims.

Claims

1. A vehicular steering system having a steering mechanism which includes a steering wheel installed on a vehicle and an electrically controlled auxiliary steering angle superposition mechanism for steering steerable road wheels of the vehicle, the system comprising: a steering wheel angle detection section that detects a steering angle of the steering wheel as a steering wheel angle;an auxiliary steering angle detection section that detects an auxiliary steering angle to be superposed by the auxiliary steering angle superposition mechanism;a steered angle detection section that detects a steered angle of the steerable road wheels as an absolute angle;a travel state detection section that detects the traveling state of the vehicle;a transmission characteristic setting section that sets a transmission characteristic between the steering wheel angle and the steered angle in accordance with the traveling state of the vehicle;a target auxiliary steering angle calculation section that calculates a target auxiliary steering angle to be superpose by the auxiliary steering angle superposition mechanism based on the steering wheel angle and the transmission characteristic;a driving control section that controls driving of the auxiliary steering angle superposition mechanism so as to make the auxiliary steering angle coincide with the target auxiliary steering angle; andan activation angle setting section that sets an initial value of the steering wheel angle detected by the steering wheel angle detection section as an activation steering wheel angle based on the steered angle and the transmission characteristic at the start of control of the auxiliary steering angle superposition mechanism, and also sets an initial value of the auxiliary steering angle detected by the auxiliary steering angle detection section as an activation auxiliary steering angle.

2. The vehicular steering system as set forth in claim 1, wherein the steering wheel angle detection section includes a steering wheel angle detection part that detects the steering angle of the steering wheel as a steering wheel angle detection value and a steering wheel angle correction part that adds a steering wheel angle correction amount to the steering wheel angle detection value thereby to output the steering wheel angle;the auxiliary steering angle detection section includes an auxiliary steering angle detection part that detects an angle to be superposed by the auxiliary steering angle superposition mechanism as an auxiliary steering angle detection value in the form of an absolute angle, and an auxiliary steering angle correction part that adds an auxiliary steering angle correction amount to the auxiliary steering angle detection value thereby to output the auxiliary steering angle;the auxiliary steering angle correction part sets an initial value of the auxiliary steering angle correction amount based on the activation auxiliary steering angle and the auxiliary steering angle detection value, and gradually increases or gradually decreases the auxiliary steering angle correction amount to zero; andthe steering wheel angle correction part sets the initial value of the steering wheel angle correction amount to zero at the start of control of the auxiliary steering angle superposition mechanism, and also sets, after the start of the control, the steering wheel angle correction amount so that the steering wheel angle, the auxiliary steering angle, and the steered angle satisfy a mechanical property of the auxiliary steering angle superposition mechanism.

3. The vehicular steering system as set forth in claim 1, wherein the steering wheel angle detection section includes a steering wheel angle detection part that detects the steering angle of the steering wheel as a steering wheel angle detection value and also detects, as a predetermined steering wheel angle, that the steering angle of the steering wheel has become a predetermined angle, and a steering wheel angle correction part that adds a steering wheel angle correction amount to the steering wheel angle detection value thereby to output the steering wheel angle;the auxiliary steering angle detection section includes an auxiliary steering angle detection part that detects an angle to be superposed by the auxiliary steering angle superposition mechanism as an auxiliary steering angle detection value, and an auxiliary steering angle correction part that adds an auxiliary steering angle correction amount to the auxiliary steering angle detection value thereby to output the auxiliary steering angle;the steering wheel angle correction part sets the initial value of the steering wheel angle correction amount to zero at the start of control of the auxiliary steering angle superposition mechanism, also sets, upon detection of the predetermined steering wheel angle, a target correction angle for the steering wheel angle based on the predetermined steering wheel angle and the steering wheel angle detection value, and gradually increases or gradually decreases the steering angle correction amount to the target correction angle; andthe auxiliary steering angle correction part sets the initial value of the auxiliary steering angle correction amount to zero at the start of control of the auxiliary steering angle superposition mechanism, and also sets, after the start of the control, the auxiliary steering angle correction amount so that the steering wheel angle, the auxiliary steering angle, and the steered angle satisfy a mechanical property of the auxiliary steering angle superposition mechanism.

4. The vehicular steering system as set forth in claim 1, wherein the transmission characteristic includes a nonlinear characteristic.