U.S. patent application number 09/778713 was filed with the patent office on 2001-08-30 for electric power steering system.
Invention is credited to Hayama, Ryouhei, Kada, Tomoyasu, Nakano, Shiro.
Application Number | 20010017229 09/778713 |
Document ID | / |
Family ID | 18575101 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017229 |
Kind Code |
A1 |
Kada, Tomoyasu ; et
al. |
August 30, 2001 |
Electric power steering system
Abstract
An electric power steering system, which controls an electric
motor on the basis of a target electric current determined in
accordance with a steering torque and applies a driving force from
the electric motor to a steering mechanism. The system includes: a
steering torque sensor; a target current setting circuit for
determining the target electric current in accordance with the
steering torque on the basis of an assist characteristic indicative
of a relationship between the steering torque and the target
electric current; a motor controlling circuit for controlling the
electric motor on the basis of the target electric current; a
traveling state judging circuit for judging whether or not a motor
vehicle is in a slalom traveling state; and an assist
characteristic modifying circuit for offsetting the assist
characteristic from an initial characteristic level when the motor
vehicle is in the slalom traveling state.
Inventors: |
Kada, Tomoyasu; (Osaka,
JP) ; Nakano, Shiro; (Osaka, JP) ; Hayama,
Ryouhei; (Nabari-shi, JP) |
Correspondence
Address: |
RABIN & CHAMPAGNE, PC
1101 14TH STREET, NW
SUITE 500
WASHINGTON
DC
20005
US
|
Family ID: |
18575101 |
Appl. No.: |
09/778713 |
Filed: |
February 8, 2001 |
Current U.S.
Class: |
180/446 |
Current CPC
Class: |
B62D 6/007 20130101;
B62D 5/0463 20130101 |
Class at
Publication: |
180/446 |
International
Class: |
B62D 005/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 29, 2000 |
JP |
2000-53766 |
Claims
What is claimed is:
1. An electric power steering system which is adapted to control an
electric motor on the basis of a target electric current determined
in accordance with a steering torque applied to an operation member
and apply a driving force from the electric motor to a steering
mechanism for steering assist, the electric power steering system
comprising: steering torque sensor for detecting the steering
torque; target current setting circuit for determining the target
electric current in accordance with the steering torque detected by
the steering torque sensor on the basis of an assist characteristic
indicative of a relationship between the steering torque and the
target electric current; motor controlling circuit for controlling
the electric motor for driving thereof on the basis of the target
electric current determined by the target current setting circuit;
traveling state judging circuit for judging whether or not a motor
vehicle equipped with the electric power steering system is in a
slalom traveling state; and assist characteristic modifying circuit
for offsetting the assist characteristic from an initial
characteristic level when the traveling state judging circuit
judges that the motor vehicle is in the slalom traveling state.
2. An electric power steering system as set forth in claim 1,
wherein the traveling state judging circuit judges that the motor
vehicle is in the slalom traveling state if a frequency at which
the steering torque detected by the steering torque sensor
increases to not smaller than a first predetermined torque level
reaches a predetermined reference frequency level.
3. An electric power steering system as set forth in claim 1,
wherein the traveling state judging circuit includes a circuit
which judges that the motor vehicle is brought out of the slalom
traveling state when the steering torque detected by the steering
torque sensor is continuously kept at not greater than a second
predetermined torque level for not shorter than a first time
period.
4. An electric power steering system as set forth in claim 1,
wherein the assist characteristic modifying circuit includes
forward stroke steering judging circuit for judging whether or not
a forward stroke steering operation is performed, and offsets the
assist characteristic from the initial characteristic level in
response to the judgment that the forward stroke steering operation
is performed.
5. An electric power steering system as set forth in claim 4,
wherein the forward stroke steering judging circuit includes a
circuit which judges that the forward stroke steering operation is
performed when the steering torque detected by the steering torque
sensor is continuously kept at not smaller than a third
predetermined torque level for not shorter than a second time
period.
6. An electric power steering system as set forth in claim 1,
wherein the assist characteristic modifying circuit sequentially
modifies the assist characteristic in accordance with a change in
the steering torque detected by the steering torque sensor.
7. An electric power steering system as set forth in claim 1,
wherein the assist characteristic modifying circuit includes return
stroke steering judging circuit for judging whether or not a return
stroke steering operation is performed, and returns the assist
characteristic to the initial characteristic level when it is
judged that the return stroke steering operation is performed.
8. An electric power steering system as set forth in claim 7,
wherein the return stroke steering judging circuit includes a
circuit which judges that the return stroke steering operation is
performed when a time-based differential value of the steering
torque detected by the steering torque sensor decreases to smaller
than a predetermined value.
9. An electric power steering system as set forth in claim 7,
wherein the return stroke steering judging circuit includes a
circuit which judges that the return stroke steering operation is
performed when the steering torque detected by the steering torque
sensor is continuously kept reduced for longer than a third time
period.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an electric power steering
system which assists a steering operation by applying a driving
force from an electric motor to a steering mechanism, the electric
motor being controlled for driving thereof in accordance with a
steering torque.
[0003] 2. Description of Related Art
[0004] An electric power steering system which assists a steering
operation by transmitting a driving force from an electric motor to
a steering mechanism is conventionally mounted in a motor vehicle.
The electric motor is feedback-controlled on the basis of a target
electric current determined in accordance with a steering torque
applied to a steering wheel, whereby a steering assist force
according to the steering torque is applied to the steering
mechanism.
[0005] When a motor vehicle travels along a mountain road having
continuous meanders, for example, the motor vehicle experiences a
so-called slalom traveling state. During the slalom traveling, the
steering angle and the steering torque tend to be greater than
during ordinary traveling. This increases a burden on a driver even
with the steering assist, so that an improvement is required for
providing a more satisfactory steering feeling.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide an
electric power steering system which alleviates the burden on the
driver during the slalom traveling for the improvement of the
steering feeling.
[0007] The present invention is directed to an electric power
steering system which is adapted to control an electric motor on
the basis of a target electric current determined in accordance
with a steering torque applied to an operation member and apply a
driving force from the electric motor to a steering mechanism for
steering assist. The system comprises: a steering torque sensor for
detecting the steering torque; a target current setting circuit for
determining the target electric current in accordance with the
steering torque detected by the steering torque sensor on the basis
of an assist characteristic indicative of a relationship between
the steering torque and the target electric current; a motor
controlling circuit for controlling the electric motor for driving
thereof on the basis of the target electric current determined by
the target current setting circuit; a traveling state judging
circuit for judging whether or not a motor vehicle equipped with
the electric power steering system is in a slalom traveling state;
and an assist characteristic modifying circuit for offsetting the
assist characteristic from an initial characteristic level when the
traveling state judging circuit judges that the motor vehicle is in
the slalom traveling state.
[0008] In accordance with the present invention, the assist
characteristic which is employed as a basis for the determination
of the target electric current according to the steering torque is
modified to be offset from the initial characteristic level when
the motor vehicle is in the slalom traveling state. During the
slalom traveling, a steering assist force is generated, which is
different from that to be generated when the motor vehicle is in
any other traveling state.
[0009] That is, when the motor vehicle is in the slalom traveling
state, the assist characteristic is offset from the initial
characteristic level to cause the electric motor to generate a
greater driving force than with the initial characteristic level,
whereby a greater steering assist force is applied to the steering
mechanism than in an ordinary traveling state. Thus, a slalom
steering operation can advantageously be assisted, in which a
steering direction is frequently changed and a steering angle is
changed by greater degrees. Therefore, the burden on the driver can
be alleviated for the improvement of the steering feeling.
[0010] The traveling state judging circuit maybe adapted to judge
that the motor vehicle is in the slalom traveling state if a
frequency at which the steering torque detected by the steering
torque sensor increases to not smaller than a first predetermined
torque level (e.g., 3 Nm) reaches a predetermined reference
frequency level. Thus, the slalom traveling state can assuredly be
detected when a greater steering torque level is detected at a
higher frequency.
[0011] The traveling state judging circuit may include a circuit
which is adapted to judge that the motor vehicle is brought out of
the slalom traveling state when the steering torque detected by the
steering torque sensor is continuously kept at not greater than a
second predetermined torque level (which is preferably smaller than
the first predetermined torque level, e.g., 2 Nm) for not shorter
than a first time period (e.g., 30 seconds).
[0012] Thus, the fact that the motor vehicle is brought out of the
slalom traveling state can be detected on the basis of the detected
steering torque level.
[0013] The assist characteristic modifying circuit preferably
includes a forward stroke steering judging circuit for judging
whether or not a forward stroke steering operation is performed,
and is adapted to offset the assist characteristic from the initial
characteristic level in response to the judgment that the forward
stroke steering operation is performed.
[0014] With this arrangement, a greater steering assist force can
be applied to the steering mechanism when the forward stroke
steering operation is performed during the slalom traveling.
Therefore, the burden on the driver can effectively be
alleviated.
[0015] The forward stroke steering judging circuit preferably
includes a circuit which is adapted to judge that the forward
stroke steering operation is performed when the steering torque
detected by the steering torque sensor is continuously kept at not
smaller than a third predetermined torque level (which is
preferably smaller than the first predetermined torque level and
may be substantially equal to the second predetermined torque
level, e.g., 2 Nm) for not shorter than a second time period (which
is preferably shorter than the first time period, e.g., 0.1
second).
[0016] With this arrangement, the forward stroke steering operation
of the operation member during the slalom traveling is detected on
the basis of the detected steering torque level, and the assist
characteristic is modified so as to generate a greater steering
assist force in this state.
[0017] The assist characteristic modifying circuit is preferably
adapted to sequentially modify the assist characteristic in
accordance with a change in the steering torque detected by the
steering torque sensor.
[0018] With this arrangement, the assist characteristic is
sequentially modified in accordance with the change in the steering
torque during the slalom traveling. Therefore, the steering assist
force can properly be generated at any time during the slalom
traveling.
[0019] The assist characteristic modifying circuit preferably
includes a return stroke steering judging circuit for judging
whether or not are turn stroke steering operation is performed, and
is adapted to return the assist characteristic to the initial
characteristic level when it is judged that the return stroke
steering operation is performed.
[0020] With this arrangement, the steering assist force is reduced
to increase the steering torque during the return stroke steering
operation, so that a proper return steering feeling can be provided
for the improvement of the steering feeling.
[0021] The return stroke steering judging circuit preferably
includes a circuit which is adapted to judge that the return stroke
steering operation is performed when a time-based differential
value of the steering torque detected by the steering torque sensor
decreases to smaller than a predetermined value (e.g., -10
Nm/second).
[0022] With this arrangement, a state where the operation member is
relatively quickly returned to a neutral position for the return
stroke steering operation during the slalom traveling is detected
on the basis of the time-based differential value of the steering
torque, and the assist characteristic is returned to the initial
characteristic level in this state.
[0023] The return stroke steering judging circuit may include a
circuit which is adapted to judge that the return stroke steering
operation is performed when the steering torque detected by the
steering torque sensor is continuously kept reduced for longer than
a third time period (which is preferably shorter than the first
time period and substantially equal to the second time period,
e.g., 0.1 second).
[0024] With this arrangement, a state where the operation member is
relatively slowly returned to the neutral position for the return
stroke steering operation during the slalom traveling is detected
on the basis of the detected steering torque value, and the assist
characteristic is returned to the initial characteristic level in
this state.
[0025] 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
[0026] FIG. 1 is a block diagram illustrating the electrical
construction of an electric power steering system according to one
embodiment of the present invention;
[0027] FIG. 2 is a diagram illustrating an exemplary assist
characteristic;
[0028] FIG. 3 is a flow chart for explaining an exemplary process
for modifying the assist characteristic;
[0029] FIG. 4 is a flow chart for explaining the exemplary process
for modifying the assist characteristic; and
[0030] FIG. 5 is a flow chart for explaining the exemplary process
for modifying the assist characteristic.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] FIG. 1 is a block diagram illustrating the electrical
construction of an electric power steering system according to one
embodiment of the present invention. A steering torque applied to a
steering wheel 1 as an operation member is transmitted to a
steering mechanism 3 via a steering shaft 2. A driving force
generated by an electric motor 20 is transmitted as a steering
assist force to the steering mechanism 3.
[0032] The steering shaft 2 is divided into an input shaft 2A
coupled to the side of the steering wheel 1 and an output shaft 2B
coupled to the side of the steering mechanism 3. The input shaft 2A
and the output shaft 2B are coupled to each other by a torsion bar
4. The torsion bar 4 experiences a torsion in accordance with a
steering torque T, and the direction and magnitude of the torsion
are detected by a torque sensor 5. An output signal of the torque
sensor 5 is inputted to a controller (ECU) 10.
[0033] The controller 10 applies a driving electric current to the
electric motor 20 in accordance with the steering torque T detected
by the torque sensor 5 to control the electric motor 20 for driving
thereof so that a steering assist force is applied to the steering
mechanism 3 in accordance with the steering torque T. The
controller 10 receives, in addition to the output signal of the
torque sensor 5, an output signal of a steering angle sensor 6 for
detecting a steering angle 8 as the rotation angle of the steering
wheel 1 and an output signal of a vehicle speed sensor 7 for
detecting the speed V of a motor vehicle equipped with the
inventive electric power steering system.
[0034] The controller 10 causes a built-in microprocessor to
perform program-based processes so as to effect the function of a
target current setting section 11 for determining a target electric
current according to the steering torque T, the function of a motor
controlling section 12 for feedback-controlling the electric motor
20 on the basis of the target electric current determined by the
target current setting section 11, the function of an assist
characteristic modifying section 13 for modifying an assist
characteristic indicative of a characteristic relationship of the
steering torque versus the target electric current in the target
current setting section 11, and the function of a traveling state
judging section 14 for judging whether or not the motor vehicle is
in a slalom traveling state.
[0035] The traveling judging section 14 judges, on the basis of the
vehicle speed V detected by the vehicle speed sensor 7 and the
steering torque T detected by the torque sensor 5, whether or not
the motor vehicle is in the slalom traveling state, and applies a
signal indicative of the judgment result to the assist
characteristic modifying section 13.
[0036] FIG. 2 is a diagram illustrating an exemplary assist
characteristic which is to be employed when the target current
setting section 11 determines the target electric current. An
initial characteristic level is indicated by a curve L0 and, when
the motor vehicle is in a traveling state other than the slalom
traveling state, the target current setting section 11 determines
the target electric current for the steering torque T on the basis
of the curve L0. The steering torque T detected by the torque
sensor 5 has a plus sign if its direction is for the right turn of
the steering wheel 1 (hereinafter referred to as "right rotation
torque") and has a minus sign if its direction is for the left turn
of the steering wheel 1 (hereinafter referred to as "left rotation
torque"). The target electric current has a positive value when a
torque is to be applied to the steering mechanism 3 for right turn
thereof, and has a negative value when a torque is to be applied to
the steering mechanism 3 for left turn thereof.
[0037] A predetermined range of the steering torque T around zero
is defined as a so-called dead zone NS, and the target electric
current is set at zero irrespective of the value of the steering
torque T within the dead zone NS. For convenience of explanation, a
positive portion of the curve L0 away from the dead zone NS is
hereinafter referred to as a positive initial assist characteristic
curve L0p, and a negative portion of the curve L0 away from the
dead zone NS is herein after referred to as a negative initial
assist characteristic curve L0n. Similarly, a positive portion of
the assist characteristic curve away from the dead zone NS is
referred to as a positive assist characteristic curve, and a
negative portion of the assist characteristic curve away from the
dead zone NS is referred to as a negative assist characteristic
curve.
[0038] In this embodiment, when the slalom traveling state is
detected with a forward stroke steering operation being performed
to turn the steering wheel 1 away from a neutral position, the
positive assist characteristic curve or the negative assist
characteristic curve is offset from the initial assist
characteristic curve L0p or L0n by the operation of the assist
characteristic modifying section 13. More specifically, when the
forward stroke steering operation is detected with a right rotation
torque being applied to the steering wheel 1 during the slalom
traveling, the positive assist characteristic curve is gradually
offset from the positive initial assist characteristic curve L0p in
such a direction as to increase the target electric current.
Similarly, when the forward stroke steering operation is detected
with a left rotation torque being applied to the steering wheel 1
during the slalom traveling, the negative assist characteristic
curve is gradually offset from the negative initial assist
characteristic curve L0n in such a direction as to reduce the
target electric current (or to increase the absolute value of the
target electric current).
[0039] Even during the slalom traveling, the target electric
current is determined on the basis of the initial assist
characteristic curve L0p or L0n if a return stroke steering
operation is performed to return the steering wheel 1 to the
neutral position.
[0040] That is, when the forward stroke steering operation is
performed during the slalom traveling, a steering assist force
greater than that to be applied when the motor vehicle is in any
other traveling state is applied to the steering mechanism 3 by the
electric motor 20. Thus, a burden on a driver who holds the
steering wheel 1 is alleviated.
[0041] In this embodiment, the assist characteristic curve is
further modified in accordance with a change in the steering torque
T when the forward stroke steering operation is performed during
the slalom traveling. More specifically, the positive assist
characteristic curve is modified within a zone defined between the
positive initial characteristic curve L0p and a positive upper
limit assist characteristic curve L1p so as to increase or reduce
the target electric current. Similarly, the negative assist
characteristic curve is further modified within a zone defined
between the negative initial characteristic curve L0n and a
negative lower limit assist characteristic curve L1n so as to
increase or reduce the target electric current.
[0042] When the forward stroke steering operation is performed
during the slalom traveling, the assist characteristic is modified
in accordance with the change in the steering torque T applied to
the steering wheel 1, so that the steering assist force is properly
applied to the steering mechanism 3. Thus, a satisfactory steering
feeling can be provided.
[0043] FIGS. 3, 4 and 5 are flow charts for explaining a specific
example of a process to be performed by the traveling state judging
section 14 and the assist characteristic modifying section 13. In
this example, the zones defined between the positive initial assist
characteristic curve L0p and the positive upper limit assist
characteristic curve L1p and between the negative initial assist
characteristic curve L0n and the negative lower limit assist
characteristic curve L1n are each divided into 64 levels, and an
assist characteristic curve corresponding to one of these levels is
selected on the basis of an offset value OS
(0.ltoreq.OS.ltoreq.64). The positive and negative initial assist
characteristic curves L0p, L0n each correspond to an offset value
of OS=0, while the positive upper limit assist characteristic curve
L1p and the negative lower limit assist characteristic curve L1n
each correspond to an offset value of OS=64.
[0044] More specifically, an offset target electric current is
obtained by addition (T>0) or subtraction (T<0) of a value
corresponding to the offset value OS (e.g., a value proportional to
the offset value OS) to/from the target electric current (normal
value) determined on the basis of the steering torque T in
accordance with the positive or negative initial assist
characteristic curve L0p or L0n. As a result, a target electric
current level according to the offset assist characteristic curve
is determined.
[0045] The process shown in FIGS. 3, 4 and 5 is repeatedly
performed in a predetermined control cycle during the operation of
the electric power steering system (during which an ignition key
switch is on).
[0046] More specifically, it is first judged whether or not a
slalom control mode flag indicative of the slalom traveling state
is off (Step S1). If the motor vehicle is not in the slalom
traveling state, the slalom control mode flag is off. In this case,
it is judged whether or not the vehicle speed V detected by the
vehicle speed sensor 7 is within a range, for example, between 10
km and 80 km per hour (Step S2). If the vehicle speed V is out of
this range, there is a high possibility that the motor vehicle is
not in the slalom traveling state. In this case, a judgment value C
indicative of whether or not the motor vehicle is in the slalom
traveling state is decremented by "1" (Step S5). However, the
judgment value C has a lower limit of zero for the
decrementation.
[0047] If it is judged in Step S2 that the vehicle speed V is
within the range between 10 km and 80 km per hour, the steering
torque T detected by the torque sensor 5 is checked. If a condition
that the absolute value of the steering torque T (hereinafter
expressed as "steering torque .vertline.T.vertline.") is not
smaller than 3 Nm (first predetermined torque level) is satisfied
(YES in Step S3), the judgment value C is incremented by "5" (Step
S4).
[0048] After the judgment value C is updated in Step S4 or S5, it
is judged whether or not the judgment value C reaches a reference
judgment value Cref (Step S6). If it is judged that the judgment
value C reaches the reference judgment value Cref, the slalom
control mode flag is turned on (Step S7). If the judgment value C
does not reach the reference judgment value Cref, a process
sequence from Step S1 is repeated.
[0049] The judgment value C indicates a frequency at which the
steering torque .vertline.T.vertline. increases to not smaller than
3 Nm. During the slalom traveling, the forward stroke steering
operation in which the steering wheel 1 is turned away from the
neutral position and the return stroke steering operation in which
the steering wheel 1 is returned to the neutral position are
alternately repeated. Therefore, the steering torque
.vertline.T.vertline. is not necessarily kept at greater levels,
but alternately increases to a greater level and decreases to a
smaller level. Since the judgment value C is incremented by "5"
when the steering torque .vertline.T.vertline. is not smaller than
3 Nm and the judgment value C is decremented by "1" when the
steering torque .vertline.T.vertline. is smaller than 3 Nm, the
slalom traveling state of the motor vehicle can promptly and
assuredly be detected.
[0050] When the slalom control mode flag is turned on (Step S7),
the process goes to Step S8 in FIG. 4 to judge whether or not the
slalom control implementation flag is on. Immediately after the
slalom control mode flag is turned on, the slalom control
implementation flag is off and, therefore, the process goes to Step
S9.
[0051] In Step S9, it is judged whether or not the forward stroke
steering operation is performed. That is, it is judged whether a
state where the steering torque .vertline.T.vertline. detected by
the torque sensor 5 is, for example, not smaller than 2 Nm (third
predetermined torque level is maintained, for example, for 0.1
second (second time period). If the judgment is positive, it is
judged that the forward stroke steering operation is performed, and
the slalom control implementation flag is turned on (Step S10). On
the contrary, if the state where the steering torque
.vertline.T.vertline. is not smaller than 2 Nm is not maintained
for 0.1 second or longer (NO in Step S9), a process shown in FIG. 5
is performed to judge whether or not the motor vehicle is brought
out of the slalom traveling state.
[0052] That is, it is judged whether a period during which the
steering torque .vertline.T.vertline. is kept, for example, at not
greater than 2 Nm (second predetermined torque level) reaches 30
seconds (first time period) (Step S11). If this judgment is
positive, it is judged that the motor vehicle is brought out of the
slalom traveling state, and the slalom control mode flag is turned
off (Step S12) If the judgment in Step S11 is negative, it is
judged that the motor vehicle is kept in the slalom traveling
state, and the slalom control mode flag is kept on.
[0053] If the slalom control mode flag is on and the slalom control
implementation flag is on, it is judged whether or not the steering
torque .vertline.T.vertline. is increased as compared with a
steering torque level observed in the preceding control cycle (Step
S13). If it is judged that the steering torque
.vertline.T.vertline. is increased, it is determined whether the
offset value OS is smaller than "64" which is the upper limit
thereof (Step S14). If the offset value is smaller than the upper
limit "64", the offset value OS is incremented by "1" (Step S15).
Then, the assist characteristic is modified on the basis of the
offset value OS thus determined (Step S16).
[0054] On the other hand, if it is judged in Step S13 that the
steering torque .vertline.T.vertline. is not increased as compared
with the steering torque level observed in the preceding control
cycle, it is judged whether a time-based differential value T' of
the steering torque .vertline.T.vertline. is smaller than -10
Nm/second, for example (Step S17). If the time-based differential
value T' of the steering torque .vertline.T.vertline. is smaller
than -10 Nm/second, it is judged that the return stroke steering
operation is performed to relatively quickly return the steering
wheel 1 to the neutral position, and the offset value OS is set at
zero which is the initial value (Step S21). After the slalom
control implementation flag is turned off (Step S22), the target
electric current of the electric motor 20 is determined on the
basis of the assist characteristic corresponding to the offset
value of OS=0, i.e., the ordinary assist characteristic (initial
assist characteristic curve L0p or L0n) (Step S16). Therefore, the
steering assist force is applied to the steering mechanism 3 in
accordance with the ordinary assist characteristic when the return
stroke steering operation is performed during the slalom traveling
state.
[0055] When the return stroke steering operation is performed to
slowly return the steering wheel 1 to the neutral position, the
judgment in Step S17 maybe negative. In such a case, it is judged
whether the steering torque .vertline.T.vertline. is kept reduced,
for example, for not shorter than 0.1 second (Step S18). If this
judgment is positive, it is judged that the return stroke steering
operation is relatively slowly performed, so that a process
sequence of Steps S21 and S22 is performed to generate the steering
assist force in accordance with the initial assist characteristic
curve L0p or L0n.
[0056] On the other hand, if the judgments in Steps S17 and S18 are
negative so that it is judged that the return stroke steering
operation is not performed but the steering torque
.vertline.T.vertline. is reduced, the offset value OS is
decremented by "1" (Step S20) on condition that the offset value OS
is greater than zero (Step S19). The assist characteristic is
determined on the basis of the offset value OS thus decremented,
and the target electric current of the electric motor 20 is
determined in accordance with the assist characteristic thus
modified. This situation may occur, for example, when the motor
vehicle travels along a composite curve having different curvature
radii during the slalom traveling, i.e., along a curve having a
smaller curvature radius at the beginning thereof and a greater
curvature radius at the middle thereof.
[0057] Only when the forward stroke steering operation is performed
during the slalom traveling, the assist characteristic is thus
properly modified within the zone between the positive initial
assist characteristic curve L0p and the upper limit assist
characteristic curve L1p or between the negative initial assist
characteristic curve L0n and the lower limit assist characteristic
curve L1n. When the forward stroke steering operation is performed
during the slalom traveling, a greater steering assist force is
provided as compared with a case where the motor vehicle is in the
ordinary traveling state. Therefore, a steering burden on a driver
who repeatedly performs the forward stroke steering operation is
alleviated. Even during the slalom traveling, the offset value OS
is set at zero when the return stroke steering operation is
performed, and the ordinary assist characteristic is employed.
Therefore, a returning resistance can be provided to the steering
wheel 1 to provide a satisfactory steering feeling.
[0058] When the forward stroke steering operation is performed, the
assist characteristic can be set in accordance with the steering
torque .vertline.T.vertline. by increasing or reducing the offset
value OS in accordance with the change in the steering torque
.vertline.T.vertline.. Therefore, the steering assist force can be
applied to the steering mechanism 3 without any excess or
deficiency. Thus, the steering feeling can be improved during the
slalom traveling.
[0059] More specifically, the positive assist characteristic is
modified so that the target electric current is increased in
accordance with an increase in the offset value OS and reduced in
accordance with a reduction in the offset value OS. Similarly, the
negative assist characteristic is modified so that the target
electric current is reduced (or the absolute value thereof is
increased) in accordance with an increase in the offset value OS
and the target electric current is increased (or the absolute value
thereof is reduced) in accordance with a reduction in the offset
value OS.
[0060] While one embodiment of the present invention has thus been
described, the invention can be embodied in any other ways.
Although the frequency of the occurrence of the increase in the
steering torque .vertline.T.vertline. is detected on the basis of
the judgment value C which is incremented by "5" when the steering
torque .vertline.T.vertline. increases to not smaller than 3 Nm and
decremented by "1" when the steering torque .vertline.T.vertline.
decreases to smaller than 3 Nm in the embodiment described above,
the judgment on the slalom traveling state of the motor vehicle may
be based on a condition that the number of times of occurrence of
the state where the steering torque increases to not smaller than 3
Nm exceeds a predetermined reference value within a predetermined
time period.
[0061] When the forward stroke steering operation is performed
during the slalom traveling, the assist characteristic is modified
in accordance with the change in the steering torque
.vertline.T.vertline. in the embodiment described above, but the
offset value may be set at a constant level so that a steering
assist force which is generally greater than that determined on the
basis of the ordinary assist characteristic can be applied to the
steering mechanism 3.
[0062] Although the judgment on the forward stroke steering
operation is based on the condition that the steering torque
.vertline.T.vertline. is continuously kept at not smaller than 2 Nm
for not shorter than 0.1 second in the embodiment described above,
the detection of the forward stroke steering operation may be
achieved, for example, on the basis of the steering angle .theta.
detected by the steering angle sensor 6 or a time-related change in
the steering angle. Similarly, the detection of the return stroke
steering operation may be achieved on the basis of the steering
angle .theta. detected by the steering angle sensor 6 or the
time-related change in the steering angle.
[0063] While the present invention has been described in detail by
way of the embodiment 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.
[0064] This application corresponds to Japanese Patent Application
No. 2000-053766 filed to the Japanese Patent Office on Feb. 29,
2000, the disclosure thereof being incorporated herein by
reference.
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