U.S. patent application number 09/985063 was filed with the patent office on 2002-05-16 for motor vehicle steering system.
Invention is credited to Hayama, Ryouhei, Nakano, Shiro, Nishizaki, Katsutoshi, Segawa, Masaya.
Application Number | 20020059021 09/985063 |
Document ID | / |
Family ID | 18820866 |
Filed Date | 2002-05-16 |
United States Patent
Application |
20020059021 |
Kind Code |
A1 |
Nishizaki, Katsutoshi ; et
al. |
May 16, 2002 |
Motor vehicle steering system
Abstract
A motor vehicle steering system is provided which includes: an
operation member for steering a motor vehicle; a steering mechanism
for turning steerable wheels of the motor vehicle; a first actuator
and a second actuator each operative to apply a driving force to
the steering mechanism; and a clutch mechanism operative to
mechanically engage and disengage the operation member to or from
the steering mechanism. When the first and second actuators operate
normally, the clutch mechanism is brought into the disengaged
state, and a driving force is applied to the steering mechanism
from the first and second actuators and, when one of the first and
second actuators malfunctions, the clutch mechanism is brought into
the engaged state, and the steering assist force is generated by
the other normal actuator in accordance with the operation of the
operation member.
Inventors: |
Nishizaki, Katsutoshi;
(Nabari-shi, JP) ; Nakano, Shiro; (Osaka, JP)
; Segawa, Masaya; (Tenri-shi, JP) ; Hayama,
Ryouhei; (Nabari-shi, JP) |
Correspondence
Address: |
RABIN & CHAMPAGNE, PC
1101 14TH STREET, NW
SUITE 500
WASHINGTON
DC
20005
US
|
Family ID: |
18820866 |
Appl. No.: |
09/985063 |
Filed: |
November 1, 2001 |
Current U.S.
Class: |
701/41 ;
180/443 |
Current CPC
Class: |
B62D 5/006 20130101;
B62D 5/003 20130101 |
Class at
Publication: |
701/41 ;
180/443 |
International
Class: |
B62D 005/04 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 14, 2000 |
JP |
2000-347081 |
Claims
What is claimed is:
1. A motor vehicle steering system comprising: an operation member
for steering a motor vehicle; a steering mechanism for turning
steerable wheels of the motor vehicle; a first actuator and a
second actuator each operative to apply a driving force to the
steering mechanism; a clutch mechanism operative to mechanically
engage and disengage the operation member with respect to the
steering mechanism; and a steering control section which, when the
first and second actuators operate normally, brings the clutch
mechanism into a disengaged state and causes the first and second
actuators to generate the driving force for driving the steering
mechanism and, when one of the first and second actuators
malfunctions, brings the clutch mechanism into an engaged state and
causes the other normal actuator to generate a steering assist
force according to operation of the operation member.
2. A motor vehicle steering system as set forth in claim 1, wherein
the first actuator is a primary steering actuator coupled to the
steering mechanism; and wherein the second actuator is a secondary
steering actuator coupled to the steering mechanism.
3. A motor vehicle steering system as set forth in claim 2, further
comprising a counter-force actuator for applying an operation
reaction force to the operation member, wherein, when one of the
primary steering actuator and the secondary steering actuator
malfunctions, the steering control section causes the counter-force
actuator to generate an additional steering assist force according
to the operation of the operation member.
4. A motor vehicle steering system as set forth in claim 2, further
comprising a counter-force actuator for applying an operation
reaction force to the operation member, wherein, when the
counter-force actuator malfunctions, the steering control section
brings the clutch mechanism into the engaged state to cause the
primary steering actuator and the secondary steering actuator to
generate the steering assist force according to the operation of
the operation member.
5. A motor vehicle steering system as set forth in claim 1, wherein
the first actuator is a steering actuator coupled to the steering
mechanism; wherein the second actuator is a counter-force actuator
for applying an operation reaction force to the operation member;
and wherein, when the steering actuator operates normally, the
steering control section brings the clutch mechanism into a
disengaged state and causes the steering actuator to generate the
driving force for driving the steering mechanism and, when the
steering actuator malfunctions, brings the clutch mechanism into an
engaged state and causes the counter-force actuator to generate a
steering assist force according to operation of the operation
member.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a motor vehicle steering
system which is adapted to turn steerable vehicle wheels on the
basis of the operation of an operation member such as a steering
wheel.
[0003] 2. Description of Related Art
[0004] A steering system for a motor vehicle (so-called
steer-by-wire system) has been proposed, which has no mechanical
coupling between a steering wheel and a steering mechanism for
turning steerable vehicle wheels, and is adapted to detect the
direction and degree of the operation of the steering wheel and
apply a driving force from a steering actuator such as an electric
motor to the steering mechanism on the basis of the result of the
detection.
[0005] This arrangement makes it possible to flexibly change the
ratio (gear ratio) of the degree of the rotation of the steering
wheel to the degree of the turning of the steerable vehicle wheels
depending on the traveling state of the vehicle for improvement of
the maneuverability of the motor vehicle. The aforesaid arrangement
advantageously prevents upward thrust of the steering wheel in the
event of the collision of the motor vehicle, and allows for more
flexible placement of the steering wheel.
[0006] Since the steer-by-wire system has no mechanical coupling
between the steering wheel and the steering mechanism, it is
important to give a fail-safe consideration to a steering drive
system. To this end, it is proposed to provide a clutch between the
steering wheel and the steering mechanism for engagement and
disengagement therebetween, whereby a mechanical coupling is
established between the steering wheel and the steering mechanism
with the clutch brought into engagement in the event of malfunction
of the system.
[0007] With the clutch interposed between the steering wheel and
the steering mechanism, however, there is a possibility that the
turning of the steerable vehicle wheels by the steering actuator
undesirably causes passive rotation of the steering wheel when the
drive control of the steering actuator is continued after the
engagement of the clutch. Therefore, the driving of the steering
actuator is stopped even if any component other than the steering
actuator malfunctions. As a result, the steering wheel feels very
heavy after the engagement of the clutch, so that a greater load is
exerted on the clutch.
SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to provide a motor
vehicle steering system which is capable of properly performing a
steering operation even if a malfunction occurs.
[0009] The motor vehicle steering system according to the present
invention comprises: an operation member for steering a motor
vehicle; a steering mechanism for turning steerable wheels of the
motor vehicle; a first actuator and a second actuator each
operative to apply a driving force to the steering mechanism; and a
steering control section which, when the first and second actuators
operate normally, causes at least one of the first and second
actuators to apply the driving force to the steering mechanism and,
when one of the first and second actuators malfunctions, causes the
other normal actuator to apply the driving force to the steering
mechanism.
[0010] In accordance with a first aspect of the present invention,
more specifically, there is provided a motor vehicle steering
system, which comprises: an operation member for steering a motor
vehicle; a steering mechanism for turning steerable wheels of the
motor vehicle; a primary steering actuator and a secondary steering
actuator coupled to the steering mechanism; a clutch mechanism
operative to mechanically engage and disengage the operation member
with respect to the steering mechanism; and a steering control
section which, when the primary steering actuator and the secondary
steering actuator operate normally, brings the clutch mechanism
into a disengaged state to cause the primary steering actuator and
the secondary steering actuator to generate a driving force for
driving the steering mechanism and, when one of the primary
steering actuator and the secondary steering actuator malfunctions,
brings the clutch mechanism into an engaged state to cause the
other normal steering actuator to generate a steering assist force
according to operation of the operation member.
[0011] According to this arrangement, when the primary steering
actuator and the secondary steering actuator operate normally, the
driving force is applied to the steering mechanism from both of the
primary steering actuator and the secondary steering actuator. Even
if the primary steering actuator and the secondary steering
actuator each have a relatively small maximum output, the driving
force can sufficiently be applied to the steering mechanism to
properly perform a steering operation.
[0012] If one of the primary steering actuator and the secondary
steering actuator malfunctions, the clutch mechanism is brought
into the engaged state, and the other normal steering actuator is
controlled as a source of the steering assist force. Thus, the
normal steering actuator applies the steering assist force to the
steering mechanism according to the operation of the operation
member. Therefore, the operation member can easily be operated even
after the engagement of the clutch, allowing for a practical
fail-safe steering operation.
[0013] The motor vehicle steering system may further comprise a
counter-force actuator for applying an operation reaction force to
the operation member. In this case, the steering control section is
preferably adapted to cause the counter-force actuator to generate
an additional steering assist force according to the operation of
the operation member when one of the primary steering actuator and
the secondary steering actuator malfunctions.
[0014] With this arrangement, the counter-force actuator is also
controlled as the source of the steering assist force if the
primary steering actuator or the secondary steering actuator
malfunctions. Thus, a further greater steering assist force can be
applied to the steering mechanism, so that the operation member can
more easily be operated after the engagement of the clutch
mechanism.
[0015] Where the motor vehicle steering system further comprises
the counter-force actuator for applying the operation reaction
force to the operation member, the steering control section is
adapted to bring the clutch mechanism into the engaged state to
cause the primary steering actuator and the secondary steering
actuator to generate the steering assist force according to the
operation of the operation member when the counter-force actuator
malfunctions.
[0016] With this arrangement, the clutch mechanism is brought into
the engaged state, and the primary steering actuator and the
secondary steering actuator are controlled as the source of the
steering assist force if the counter-force actuator malfunctions.
Therefore, the operation member (e.g., steering wheel) can easily
be operated, allowing for a practical fail-safe steering
operation.
[0017] In accordance with a second aspect of the present invention,
there is provided a motor vehicle steering system, which comprises:
an operation member for steering a motor vehicle; a steering
mechanism for turning steerable wheels of the motor vehicle; a
primary steering actuator and a secondary steering actuator coupled
to the steering mechanism and having substantially the same maximum
output; and steering control means which, when the primary steering
actuator and the secondary steering actuator operate normally,
causes the primary steering actuator and the secondary steering
actuator to generate a driving force for driving the steering
mechanism and, when one of the primary steering actuator and the
secondary steering actuator malfunctions, causes the other normal
steering actuator to generate the driving force for driving the
steering mechanism.
[0018] According to the present invention, when the primary
steering actuator and the secondary steering actuator operate
normally, the driving force is applied to the steering mechanism
from both of the primary steering actuator and the secondary
steering actuator. Therefore, the maximum outputs of the primary
steering actuator and the secondary steering actuator can each be
set at a smaller level.
[0019] The primary steering actuator and the secondary steering
actuator preferably each have a maximum output which is 60% to 70%
of an output sufficient to drive the steering mechanism. In this
case, the driving force can sufficiently be applied to the steering
mechanism to properly perform the steering operation, when the
primary steering actuator and the secondary steering actuator
operate normally. Even if only one of the primary steering actuator
and the secondary steering actuator operates normally, a driving
force sufficient to maintain the direction of the steerable vehicle
wheels can be applied to the steering mechanism.
[0020] In accordance with a third aspect of the present invention,
there is provided a motor vehicle steering system, which comprises:
an operation member for steering a motor vehicle; a steering
mechanism for turning steerable wheels of the motor vehicle; a
steering actuator coupled to the steering mechanism; a clutch
mechanism operative to mechanically engage and disengage the
operation member with respect to the steering mechanism; a
counter-force actuator for applying an operation reaction force to
the operation member; and a steering control section which, when
the steering actuator operates normally, brings the clutch
mechanism into a disengaged state to cause the steering actuator to
generate a driving force for driving the steering mechanism and,
when the steering actuator malfunctions, brings the clutch
mechanism into an engaged state to cause the counter-force actuator
to generate a steering assist force according to operation of the
operation member.
[0021] According to the present invention, the clutch mechanism is
brought into the engaged state and the counter-force actuator is
controlled as a source of the steering assist force, if the
steering actuator malfunctions. Thus, the counter-force actuator
applies the steering assist force to the steering mechanism
according to the operation of the operation member. Therefore, the
operation member can easily be operated after the engagement of the
clutch mechanism, allowing for a practical fail-safe steering
operation.
[0022] The foregoing and other objects, features and effects of the
present invention will become more apparent from the following
description of the preferred embodiments with reference to the
attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a conceptual diagram for explaining the basic
construction of a motor vehicle steering system according to one
embodiment of the present invention;
[0024] FIG. 2 is a flow chart for explaining a process to be
performed by a steering controlling section provided in the motor
vehicle steering system shown in FIG. 1;
[0025] FIG. 3 is a conceptual diagram for explaining the basic
construction of a motor vehicle steering system according to
another embodiment of the present invention; and
[0026] FIG. 4 is a flow chart for explaining a process to be
performed by a steering controlling section provided in the motor
vehicle steering system shown in FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] FIG. 1 is a conceptual diagram for explaining the basic
construction of a steering system for a motor vehicle in accordance
with one embodiment of the present invention. The motor vehicle
steering system includes a steering mechanism 10 for causing a pair
of steerable vehicle wheels (typically, front wheels) W, W to
perform a steering operation, a steering wheel 30 to be brought
into or out of mechanical engagement with the steering mechanism 10
via a clutch 20, and a counter-force actuator 40 for applying a
reaction force to the steering wheel 30.
[0028] The motor vehicle steering system has a dual steering drive
system including a primary steering drive system 50M and a
secondary steering drive system 50S for driving the steering
mechanism 10. The primary steering drive system 50M includes a
primary steering actuator 51M, and a primary rotation angle sensor
52M for detecting a rotation angle of the primary steering actuator
51M. The secondary steering drive system 50S includes a secondary
steering actuator 51S, and a secondary rotation angle sensor 52S
for detecting a rotation angle of the secondary steering actuator
51S. The primary steering actuator 51M and the secondary steering
actuator 51S each comprise an electric motor, for example, and have
substantially the same maximum output which is equal to 60% to 70%
of an output sufficient for the primary steering actuator 51M or
the secondary steering actuator 51S alone to drive the steering
mechanism 10.
[0029] The steering mechanism 10 includes a steering shaft 11
extending transversely of a body of the motor vehicle, and knuckle
arms 13, 13 respectively coupled to opposite ends of the steering
shaft 11 via tie rods 12, 12 for supporting the steerable vehicle
wheels W, W. The steering shaft 11 is supported in a housing 14 in
an axially slidable manner, and the primary steering actuator 51M
is incorporated in the midst of the steering shaft 11 in a coaxial
relation.
[0030] A rack gear 11a is provided as apart of the steering shaft
11, and meshed with a pinion gear 22 provided at a distal end of a
shaft 21 coupled to one side of the clutch 20. The shaft 21 is
coupled to the secondary steering actuator 51S, so that a driving
force generated by the secondary steering actuator 51S is inputted
to the shaft 21.
[0031] With this arrangement, when the primary steering actuator
51M is driven, a rotational motion of the primary steering actuator
51M is converted into a sliding motion of the steering shaft 11 by
a motion converting mechanism comprising a ball thread and the
like, so that the steerable vehicle wheels W, W are turned by the
sliding motion of the steering shaft 11. When the secondary
steering actuator 51S is driven, a rotational motion of the shaft
21 caused by the secondary steering actuator 51S is converted into
the sliding motion of the steering shaft 11 by the pinion gear 22
and the rack gear 11a, so that the steerable vehicle wheels W, W
are turned by the sliding motion of the steering shaft 11.
[0032] A shaft 23 coupled to the other side of the clutch 20 is
coupled to the steering wheel 30. If the clutch 20 is in an engaged
state, therefore, a steering torque applied to the steering wheel
30 is mechanically transmitted to the steering shaft 11 via the
shaft 23, the clutch 20, the shaft 21, the pinion gear 22 and the
rack gear 11a.
[0033] The counter-force actuator 40 comprises an electric motor
(e.g., three-phase brushless motor) which has a rotation shaft
defined by the shaft 23 connecting the clutch 20 to the steering
wheel 30 and a casing fixed to an appropriate portion of the
vehicle body. The counter-force actuator 40 is provided with a
torque sensor 41 for detecting a steering torque inputted from the
steering wheel 30, and an operation angle sensor 42 for detecting
an operation angle of the steering wheel 30.
[0034] The clutch 20, the counter-force actuator 40, the primary
steering actuator 51M and the secondary steering actuator 51S are
controlled by a steering controlling section 60 which comprises a
microprocessor and the like. More specifically, detection signals
of the steering angle sensor 42, the primary rotation angle sensor
52M and the secondary rotation angle sensor 52S are inputted to the
steering controlling section 60. A steering position sensor 15 for
detecting an axial position of the steering shaft 11 is provided in
association with the steering shaft 11, and a detection signal of
the steering position sensor 15 is also inputted to the steering
controlling section 60. Further, a detection signal of a vehicle
speed sensor 70 for detecting a vehicle speed is inputted to the
steering controlling section 60. The vehicle speed sensor 70 may
comprise, for example, a vehicle wheel speed sensor for detecting a
rotation speed of each vehicle wheel. The steering controlling
section 60 applies control signals to driver circuits 24, 43, 53M,
53S for driving the clutch 20, the counter-force actuator 40, the
primary steering actuator 51M and the secondary steering actuator
51S, respectively, on the basis of the signals inputted from the
respective sensors.
[0035] FIG. 2 is a flow chart for explaining a process to be
performed by the steering controlling section 60. The steering
controlling section 60 constantly checks if the primary steering
actuator 51M, the secondary steering actuator 51S and the
counter-force actuator 40 each operate normally, i.e., if any of
the primary steering actuator 51M, the secondary steering actuator
51S and the counter-force actuator 40 malfunctions (Step S1,
S2).
[0036] If neither the primary steering actuator 51M nor the
secondary steering actuator 51S malfunctions (NO in Step S1) , and
if the counter-force actuator 40 does not malfunction (NO in Step
S2), the steering controlling section 60 controls the primary
steering actuator 51M and the secondary steering actuator 51S for
driving thereof (Step S3). In this embodiment, the steering
controlling section 60 has a VGR (variable gear ratio) function,
and is adapted to set the ratio (gear ratio) of the degree of the
rotation of the steering wheel 30 to the degree of the turning of
the steerable vehicle wheels W, W according to the traveling state
of the motor vehicle. The steering controlling section 60 further
sets voltage command values for the primary steering actuator 51M
and the secondary steering actuator 51S on the basis of the gear
ratio thus set and the degree of the operation of the steering
wheel 30, and apply control signals corresponding to the respective
voltage command values to the driver circuits 53M, 53S. Thus,
torques are respectively outputted from the primary steering
actuator 51M and the secondary steering actuator 51S for sliding
the steering shaft 11 in a direction corresponding to the direction
of the operation of the steering wheel 30, whereby the steering
operation is properly performed according to the traveling state of
the motor vehicle and the operation of the steering wheel 30.
[0037] If one of the primary steering actuator 51M and the
secondary steering actuator 51S malfunctions during the drive
control thereof, the steering controlling section 60 deactuates the
VGR function (Step S4), and controls the other normal steering
actuator to adjust the position of the steerable vehicle wheels W,
W so that the rotation angle of the steering wheel 30 and the
steering angle of the steerable vehicle wheels W, W are brought
into a proper relationship (i.e., a relationship where the neutral
point of the steering wheel 30 coincides with the neutral point of
the steerable vehicle wheels W, W). After the positional adjustment
of the steerable vehicle wheels W, W, the engagement of the clutch
20 is established (Step S5), and the driving of the counter-force
actuator 40 is stopped (Step S6).
[0038] The engagement of the clutch 20 establishes the mechanical
coupling between the steering mechanism 10 and the steering wheel
30 and, thereafter, the motor vehicle steering system functions as
a power steering system employing the normal steering actuator (the
primary steering actuator 51M or the secondary steering actuator
51S) and the counter-force actuator 40 as a source of a steering
assist force. That is, after the engagement of the clutch 20, the
steering controlling section 60 generates a steering actuator PWM
(pulse width modulation) control signal and a counter-force
actuator PWM control signal according to the operation angle of the
steering wheel 30 (steering wheel angle) , and inputs these PWM
control signals to the driver circuits of the normal steering
actuator and the counter-force actuator 40 so as to cause the
normal steering actuator and the counter-force actuator 40 to
generate the steering assist force (Steps S7, S8). Thus, the
steering assist force is properly generated by the normal steering
actuator and the counter-force actuator 40, whereby a steering
assist operation can properly be performed according to the
operation of the steering wheel 30.
[0039] If the counter-force actuator 40 malfunctions, the steering
controlling section 60 first stops the driving of the counter-force
actuator 40 (Step S9), and then deactuates the VGR function (Step
S10). The steering controlling section 60 controls the normal
steering actuator to adjust the position of the steerable vehicle
wheels W, W so that the rotation angle of the steering wheel 30 and
the steering angle of the steerable vehicle wheels W, W are brought
into the proper relationship, and then establishes the engagement
of the clutch 20 (Step S11).
[0040] After the engagement of the clutch 20, the steering
mechanism 10 and the steering wheel 30 are mechanically coupled to
each other, so that the motor vehicle steering system functions as
a power steering system employing the primary steering actuator 51M
and the secondary steering actuator 51S as the source of the
steering assist force. That is, after the engagement of the clutch
20, the steering controlling section 60 generates a primary
steering actuator PWM control signal and a secondary steering
actuator PWM control signal according to the operation angle of the
steering wheel 30 (steering wheel angle), and inputs these PWM
control signals to the driver circuits 53M and 53S of the primary
steering actuator 51M and the secondary steering actuator 51S so as
to cause the primary steering actuator 51M and the secondary
steering actuator 51S to generate the steering assist force (Step
S12). Thus, the steering assist force is properly generated by the
primary steering actuator 51M and the secondary steering actuator
51S, whereby the steering assist operation can properly be
performed according to the operation of the steering wheel 30.
[0041] In accordance with this embodiment, as described above, the
steering mechanism 10 is driven by both of the primary steering
system 50M and the secondary steering system 50S when the
counter-force actuator 40, the primary steering actuator 51M and
the secondary steering actuator 51S operate normally. Even if the
primary steering actuator 51M and the secondary steering actuator
51S each have a relatively small maximum output, therefore, the
driving force can sufficiently be applied to the steering mechanism
10, so that the steering operation can properly be performed.
[0042] If one of the primary steering actuator 51M and the
secondary steering actuator 51S malfunctions, the engagement of the
clutch 20 is established, and the other normal steering actuator is
PWM-controlled as the source of the steering assist force. Thus,
the steering assist force is applied to the steering mechanism 10
from the normal steering actuator according to the operation of the
steering wheel 30. Therefore, the steering wheel 30 can easily be
operated even after the engagement of the clutch 20, allowing for a
practical fail-safe steering operation.
[0043] In this embodiment, the counter-force actuator 40 is also
PWM-controlled as the source of the steering assist force when the
primary steering actuator 51M or the secondary steering actuator
51S malfunctions. This makes it possible to apply a further greater
steering assist force to the steering mechanism 10, so that the
steering wheel 30 can more easily be operated after the engagement
of the clutch 20.
[0044] If the counter-force actuator 40 malfunctions, the
engagement of the clutch 20 is established, and the primary
steering actuator 51M and the secondary steering actuator 51S are
PWM-controlled as the source of the steering assist force.
Therefore, the steering wheel 30 can easily be operated, allowing
for a practical fail-safe steering operation.
[0045] FIG. 3 is a conceptual diagram for explaining the basic
construction of a steering system for a motor vehicle in accordance
with another embodiment of the present invention. The motor vehicle
steering system according to this embodiment is different from the
motor vehicle steering system shown in FIG. 1 in that the steering
drive system for driving the steering mechanism 10 is not a dual
system. In FIG. 3, components corresponding to those shown in FIG.
1 are denoted by the same reference characters as in FIG. 1.
[0046] The motor vehicle steering system includes a steering
mechanism 10 for causing a pair of steerable vehicle wheels
(typically, front wheels) W, W to perform a steering operation, a
steering wheel 30 to be brought into or out of mechanical
engagement with the steering mechanism 10 via a clutch 20, and a
counter-force actuator 40 for applying a reaction force to the
steering wheel 30.
[0047] The steering mechanism 10 includes a steering shaft 11
extending transversely of a body of the motor vehicle, and knuckle
arms 13, 13 respectively coupled to opposite ends of the steering
shaft 11 via tie rods 12, 12 to support the steerable vehicle
wheels W, W. The steering shaft 11 is supported in a housing 14 in
an axially slidable manner, and a steering actuator 51 is
incorporated in the midst of the steering shaft 11 in a coaxial
relation. The steering actuator 51 comprises, for example, an
electric motor having an output sufficient to drive the steering
mechanism 10. With this arrangement, when the steering actuator 51
is driven, a rotational motion of the steering actuator 51 is
converted into a sliding motion of the steering shaft 11 by a
motion converting mechanism comprising a ball thread and the like,
so that the steerable vehicle wheels W, W are turned by the sliding
motion of the steering shaft 11.
[0048] A rack gear 11a is provided as a part of the steering shaft
11, and meshed with a pinion gear 22 provided at a distal end of a
shaft 21 coupled to one side of the clutch 20. A shaft 23 coupled
to the other side of the clutch 20 is coupled to the steering wheel
30. If the clutch 20 is in an engaged state, therefore, a steering
torque applied to the steering wheel 30 is mechanically transmitted
to the steering shaft 11 via the shaft 23, the clutch 20, the shaft
21, the pinion gear 22 and the rack gear 11a.
[0049] The counter-force actuator 40 comprises an electric motor
(e.g., three-phase brushless motor) which has a rotation shaft
defined by the shaft 23 connecting the clutch 20 to the steering
wheel 30 and a casing fixed to an appropriate portion of the
vehicle body. The counter-force actuator 40 is provided with a
torque sensor 41 for detecting a steering torque inputted from the
steering wheel 30, and a steering angle sensor 42 for detecting an
operation angle of the steering wheel 30.
[0050] The clutch 20, the counter-force actuator 40 and the
steering actuator 51 are controlled by a steering controlling
section 60 which comprises a microprocessor and the like. More
specifically, detection signals of the steering angle sensor 42 and
a rotation angle sensor 52 are inputted to the steering controlling
section 60. A steering position sensor 15 for detecting an axial
position of the steering shaft 11 is provided in association with
the steering shaft 11, and a detection signal of the steering
position sensor 15 is also inputted to the steering controlling
section 60. Further, a detection signal of a vehicle speed sensor
70 for detecting a vehicle speed is inputted to the steering
controlling section 60. The vehicle speed sensor 70 may comprise,
for example, a vehicle wheel speed sensor for detecting a rotation
speed of each vehicle wheel. The steering controlling section 60
applies control signals to driver circuits 24, 43, 53 for driving
the clutch 20, the counter-force actuator 40 and the steering
actuator 51, respectively, on the basis of the signals inputted
from the respective sensors.
[0051] FIG. 4 is a flow chart for explaining a process to be
performed by the steering controlling section 60. The steering
controlling section 60 constantly checks if the steering actuator
51 operates normally (Step T1),
[0052] If the steering actuator 51 does not malfunction, the
steering controlling section 60, for example, sets the gear ratio
of the degree of the rotation of the steering wheel 30 to the
degree of the turning of the steerable vehicle wheels W, W
according to the traveling state of the motor vehicle (VGR
function). The steering controlling section 60 further sets a
voltage command value for the steering actuator 51 on the basis of
the gear ratio thus set and the degree of the operation of the
steering wheel 30, and applies a control signal corresponding to
the voltage command value to the driver circuit 53 (Step T2). Thus,
a torque is outputted from the steering actuator 51 for sliding the
steering shaft 11 in a direction corresponding to the direction of
the operation of the steering wheel 30, whereby the steering
operation is properly performed according to the traveling state of
the motor vehicle and the operation of the steering wheel 30.
[0053] If the steering actuator 51 malfunctions during the drive
control thereof, the steering controlling section 60 deactuates the
VGR function (Step T4), and establishes the engagement of the
clutch 20 (Step T3).
[0054] The engagement of the clutch 20 establishes the mechanical
coupling between the steering mechanism 10 and the steering wheel
30 and, thereafter, the motor vehicle steering system functions as
a power steering system employing the counter-force actuator 40 as
a source of a steering assist force. That is, after the engagement
of the clutch 20, the steering controlling section 60 generates a
PWM control signal according to the operation angle of the steering
wheel 30 (steering wheel angle), and inputs the PWM control signal
to the driver circuit 43 of the counter-force actuator 40 so as to
cause the counter-force actuator 40 to generate the steering assist
force (Step T5). Thus, the steering assist force is properly
generated by the counter-force actuator 40, whereby a steering
assist operation can properly be performed according to the
operation of the steering wheel 30.
[0055] In accordance with this embodiment, the engagement of the
clutch 20 is established and the counter-force actuator 40 is
PWM-controlled as the source of the steering assist force, if the
steering actuator 51 malfunctions. Thus, the steering assist force
is applied to the steering mechanism 10 from the counter-force
actuator 40 according to the operation of the steering wheel 30.
Therefore, the steering wheel 30 can easily be operated even after
the engagement of the clutch 20, allowing for a practical fail-safe
steering operation.
[0056] While the two specific embodiments of the present invention
have thus been described, the invention may be embodied in any
other ways. In accordance with the invention, the primary steering
actuator 51M and the secondary steering actuator 51S are designed
so as to have substantially the same maximum output and to be both
controlled for driving thereof when the primary steering actuator
51M and the secondary steering actuator 51S operate normally. This
arrangement is applied not only to the motor vehicle steering
system according to the first embodiment but also to a motor
vehicle steering system in which a steering drive system for
driving the steering mechanism 10 is a dual drive system and no
clutch 20 is provided. In this case, the motor vehicle steering
system may be constructed so that, when the primary steering
actuator 51M and the secondary steering actuator 51S operate
normally, the driving force for driving the steering mechanism 10
is generated by both of the primary steering actuator 51M and the
secondary steering actuator 51S and, when one of the primary
steering actuator 51M and the secondary steering actuator 51S
malfunctions, the driving force for driving the steering mechanism
10 is generated by the other normal steering actuator alone.
[0057] Although the steering wheel 30 is employed as the operation
member in the embodiments described above, a lever and the like may
be employed as the operation member.
[0058] While the present invention has been described in detail by
way of the embodiments thereof, it should be understood that the
foregoing disclosure is merely illustrative of the technical
principles of the present invention but not limitative of the same.
The spirit and scope of the present invention are to be limited
only by the appended claims.
[0059] This application corresponds to Japanese Patent Application
No. 2000-347081 filed to the Japanese Patent Office on Nov. 14,
2000, the disclosure thereof being incorporated herein by
reference.
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