U.S. patent application number 09/760991 was filed with the patent office on 2001-08-02 for steering device for vehicle.
This patent application is currently assigned to Koyo Seiko Co., Ltd.. Invention is credited to Nakano, Shiro, Nishizaki, Katsutoshi, Segawa, Masaya, Takamatsu, Takanobu.
Application Number | 20010011201 09/760991 |
Document ID | / |
Family ID | 18548963 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010011201 |
Kind Code |
A1 |
Nishizaki, Katsutoshi ; et
al. |
August 2, 2001 |
Steering device for vehicle
Abstract
In a steering device for vehicle, the movement of a steering
actuator, which is driven in accordance with the rotating operation
of an operating member, is transmitted to the vehicle wheels such
that the steering angle changes without mechanically coupling the
operating member to the wheels. A target yaw rate, which accords
with a load torque, that is sum of the control torque generated by
an operating actuator and driver operating torque, and an operating
angle of the operating member resulting from the functioning of
this load torque, are computed. The steering actuator is controlled
such that the vehicle yaw rate follows the target yaw rate. The
operating actuator is controlled such that the operating angle
follows a target operating angle of the operating member
corresponding to a behavior index value comprising at least yaw
rate of the vehicle.
Inventors: |
Nishizaki, Katsutoshi;
(Nabari-shi, JP) ; Nakano, Shiro; (Osaka, JP)
; Takamatsu, Takanobu; (Osaka, JP) ; Segawa,
Masaya; (Tenri-shi, JP) |
Correspondence
Address: |
Jordan and Hamburg
122 East 42nd Street
New York
NY
10168
US
|
Assignee: |
Koyo Seiko Co., Ltd.
|
Family ID: |
18548963 |
Appl. No.: |
09/760991 |
Filed: |
January 16, 2001 |
Current U.S.
Class: |
701/41 ;
180/443 |
Current CPC
Class: |
B62D 5/006 20130101;
B62D 6/008 20130101; B62D 6/003 20130101 |
Class at
Publication: |
701/41 ;
180/443 |
International
Class: |
B62D 006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2000 |
JP |
2000-22690 |
Claims
What is claimed is:
1. A steering device for vehicle, comprising: an operating member
operated by being rotated; a steering actuator driven in accordance
with operation of said operating member; means for transmitting the
movement of said steering actuator to wheels of the vehicle such
that steering angle changes in accordance with said movement
without mechanically coupling said operating member to the wheels;
an operating actuator for generating control torque, which acts on
said operating member; means for determining load torque, which is
sum of said control torque and operating torque exerted on said
operating member by a driver; a sensor for determining operating
angle of the operating member which is operated by the action of
said load torque; means for computing target behavior index value
of the vehicle, comprising at least target yaw rate corresponding
to said determined load torque and operating angle, based on a
stored relationship between said load torque, operating angle and
target behavior index value; a sensor for determining a value,
comprising at least yaw rate of the vehicle, as a behavior index
value corresponding to change of behavior of the vehicle; means for
controlling said steering actuator such that said determined
behavior index value follows said target behavior index value;
means for computing target operating angle of said operating member
corresponding to said determined behavior index value, based on a
stored relationship between said behavior index value and target
operating angle; and means for controlling said operating actuator
such that said determined operating angle follows said computed
target operating angle.
2. The steering device for vehicle according to claim 1, wherein a
steering gear is employed as means for transmitting the movement of
said steering actuator to the wheels such that the steering angle
changes in accordance with said movement.
3. The steering device for vehicle according to claim 1, wherein
lateral acceleration and vehicle velocity are determined in
addition to the yaw rate as said behavior index value, said target
operating angle has a component corresponding to a value arrived at
by dividing said lateral acceleration by vehicle velocity, and a
component corresponding to the value of the yaw rate, and the ratio
of the component corresponding to the value of the yaw rate in said
target operating angle changes in accordance with the vehicle
velocity.
4. The steering device for vehicle according to claim 3, wherein
the setting value of the vehicle velocity, at the time when the
component corresponding to a value arrived at by dividing said
lateral acceleration by vehicle velocity is equal to the component
corresponding to the value of the yaw rate in said target operating
angle, can be changed.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a steering device for
vehicle, which utilizes a so-called steer by electric wire
system.
DESCRIPTION OF THE RELATED ART
[0002] In a vehicle steering device that employs a steer by
electric wire system, the movement of a steering actuator, which
corresponds to the operation of an operating member modeled on a
steering wheel, is transmitted to the wheels of the vehicle in such
a manner that the steering angle changes without this operating
member being coupled mechanically to the wheels. In a vehicle that
employs a steer by electric wire system such as this, a proposal
has been made for computing a target yaw rate corresponding to the
amount of operation of the operating member, and controlling the
steering actuator such that the target yaw rate coincides with the
actual yaw rate so as to stabilize the behavior of the vehicle.
[0003] FIG. 13 shows an example of a control block diagram of a
vehicle steering device employing a conventional steer by electric
wire system.
[0004] In the control block diagram, K1 is the gain of a target yaw
rate .gamma.* relative to the operating angle .delta.h of an
operating member 101, and a steering device computes a target yaw
rate .gamma.* from the stored relationship of
.gamma.*=K1.multidot..delta.h, and an operating angle .delta.h
detected by a sensor. K2 is the gain of a target steering angle
.delta.* relative to the deviation between the target yaw rate
.gamma.* and the actual yaw rate .gamma. of a vehicle 100, and a
steering device computes a target steering angle .delta.* from the
stored relationship of .delta.*=K2.multidot.(.gamma.*-.gamma.), the
computed target yaw rate .gamma.*, and a yaw rate .gamma. detected
by a sensor. The gain K2 is regarded as a function of velocity V,
and is set such that it decreases in line with an increase in
velocity V in order to ensure stability at high speeds. Ga is the
transfer function of the target drive current Ia* of the steering
actuator 102 relative to the deviation between a target steering
angle .delta.* and the actual steering angle .delta. of the
vehicle, and the steering device computes a target drive current
Ia* from the stored relationship of Ia*=Ga.multidot.(.delta.*-.de-
lta.), the computed target steering angle .delta.*, and a steering
angle .delta. detected by a sensor. The transfer function Ga is
set, for example, such that proportional integral (PI) control is
performed. K3 is the gain of a target operating torque Th* relative
to the operating angle .delta.h of operating member 101, and the
steering device computes a target operating torque Th* from the
stored relationship of Th*=K3.multidot..delta.h and an operating
angle .delta.h detected by a sensor. Gb is the transfer function of
the target drive current Ib* of the operating actuator 103 relative
to the deviation between the target operating torque Th* and the
actual operating torque Th, and the steering device computes a
target drive current Ib* from the stored relationship
Ib*=Gb.multidot.(Th*-Th), the computed target operating torque Th*
and an operating torque Th detected by a sensor. The transfer
function Gb is set, for example, such that proportional integral
(PI) control is performed.
[0005] In the above-mentioned conventional constitution, because
the actual yaw rate .gamma. of a vehicle does not increase when the
coefficient of friction between the surface of a road and the tires
is reduced by surface icing, or when tire lateral force reaches its
limit, a saturated state results in which the yaw rate .gamma. does
not attain the target yaw rate .gamma.* when operating torque Th
increases, and there is a possibility of the steering angle .delta.
diverging, and of vehicle behavior becoming unstable.
[0006] That is, FIG. 14 (1) and FIG. 14 (2) is one example of
simulation results in a steering device constituting the
above-mentioned conventional steer by electric wire system, showing
changes over time in the yaw rate .gamma., target yaw rate .gamma.*
and steering angle .delta. relative to a step input of 2.7
N.multidot.m operating torque Th at times t1 to t2 (0.5 to 5
seconds), in a vehicle travelling at a velocity of 60 km/hour,
wherein the coefficient of friction between the vehicle and the
surface of the road is regarded as 1 up until t3 (2.5 seconds), and
is regarded as 0.1 thereafter. The fact that the deviation between
the yaw rate .gamma. and the target yaw rate .gamma.* increases,
and the steering angle .delta. diverges in accordance with the drop
in the coefficient of friction is shown.
[0007] Further, FIG. 15 (1) and FIG. 15 (2) depict Bode diagrams
showing an example of yaw rate .gamma. frequency response
simulation relative to operating torque input in a steering device
constituting the above-mentioned conventional steer by electric
wire system, wherein a vehicle is travelling at a velocity of 20
km/hour. Further, FIG. 15 (3) and FIG. 15 (4) depict Bode diagrams
showing an example of yaw rate .gamma. frequency response
simulation relative to operating torque input in a conventional
steering device in which a steering wheel is mechanically coupled
to the vehicle wheels, wherein a vehicle is travelling at a
velocity of 20 km/hour. FIG. 15 (1) through FIG. 15 (4) indicate
that, at low travelling velocity, yaw rate responsiveness relative
to operating torque input decreases more in a vehicle steering
device employing a conventional steer by electric wire system than
in a steering device in which a steering wheel is mechanically
coupled to the vehicle wheels.
[0008] An object of the present invention is to provide a vehicle
steering device capable of solving the above-mentioned problem.
SUMMARY OF THE INVENTION
[0009] A steering device for vehicle of the present invention
comprises an operating member operated by being rotated; a steering
actuator driven in accordance with the operation of the operating
member; means for transmitting the movement of the steering
actuator to wheels of the vehicle such that the steering angle
changes in accordance with the movement without mechanically
coupling the operating member to the wheels; an operating actuator
for generating control torque, which acts on the operating member;
means for determining a load torque, which is sum of the control
torque and the operating torque exerted on the operating member by
a driver; means for determining the operating angle of the
operating member which is operated by the action of the load
torque; means for computing a target behavior index value of the
vehicle, comprising at least a target yaw rate corresponding to the
determined load torque and operating angle based on a stored
relationship between the load torque, operating angle, and target
behavior index value; means for determining a value, comprising at
least the yaw rate of the vehicle, as a behavior index value
corresponding to change of behavior of the vehicle; means for
controlling the steering actuator such that the determined behavior
index value follows the target behavior index value; means for
computing a target operating angle of the operating member
corresponding to the determined behavior index value, based on a
stored relationship between the behavior index value and the target
operating angle; and means for controlling the operating actuator
such that the determined operating angle follows the computed
target operating angle.
[0010] According to the constitution of the present invention, the
operating angle is generated by the operation of the operating
member in accordance with the load torque, which is sum of the
control torque outputted by the operating actuator and the
operating torque inputted by the driver. This control torque
functions so as to do away with the deviation between the operating
angle and the target operating angle. Accordingly, in a case in
which the operating angle has not attained the target operating
angle, the control torque serves as an auxiliary force for the
operation of the operating member, and in a case in which the
operating angle has exceeded the target operating angle, the
control torque serves as a reactive force against the operation of
the operating member.
[0011] The steering actuator is controlled such that the behavior
index value follows the target behavior index value corresponding
to the operating angle and load torque. The vehicle behavior index
value comprising the yaw rate changes in accordance with the
control of the steering actuator. The target operating angle
corresponds to the behavior index value comprising the yaw rate,
and the operating angle corresponds to the target behavior index
value.
[0012] Accordingly, in a case in which the behavior index value has
not attained the target behavior index value, since the operating
angle exceeds the target operating angle, the above reactive force
against the operation of the operating member functions. In
accordance therewith, in a case in which the yaw rate does not
increase due to a drop in the coefficient of friction between the
surface of the road and the tires, or tire lateral force having
reached its limit even when the operating torque is increased, the
reactive force against the operation of the operating member can be
made to function. Even if the driver increases operating torque at
this time, the operating torque increase can be offset by the
increase of this reactive force, and the load torque acting on the
operating member can be maintained approximately constant,
preventing an increase of the target behavior index value
corresponding to the load torque and operating angle. That is,
because the operating angle and load torque, and in turn, the
target behavior index value can be held in check by this reactive
force, the divergence of the steering angle can be prevented, and
vehicle behavior can be stabilized. Further, in a case in which a
delay occurs in the behavior index value following the target
behavior index value due to a delay in the response of the steering
actuator relative to an operation input, because the above reactive
force functions, it is possible to alleviate the wrong feeling
resulting from the delayed response of this steering actuator, thus
enabling improved steering feel.
[0013] It is desirable that lateral acceleration and velocity are
determined in addition to yaw rate as the above-mentioned behavior
index value, that the target operating angle has a component
corresponding to a value arrived at by dividing the lateral
acceleration by vehicle velocity, and a component corresponding to
the value of the yaw rate, and that the ratio of the component
corresponding to the value of the yaw rate in the target operating
angle changes in accordance with the vehicle velocity. Furthermore,
it is desirable that this ratio increase in accordance with an
increase in vehicle velocity.
[0014] In accordance therewith, it is possible to control the
steering device with accommodating to vehicle behavior
characteristics, such that the yaw rate becomes smaller at low
velocity by making the affect of lateral acceleration greater at
low vehicle velocity, and making the affect of the yaw rate greater
pursuant to an increase in vehicle velocity, in response to the
target operating angle corresponding to the behavior index
value.
[0015] Furthermore, it is possible to control the steering device
with accommodating even closer to vehicle behavior characteristics
by making it possible to change setting value of the vehicle
velocity, at the time when the component corresponding to a value
arrived at by dividing the lateral acceleration by vehicle velocity
is equal to the component corresponding to the value of the yaw
rate in the target operating angle.
[0016] According to the present invention, in a vehicle that
employs a steer by electric wire system, it is possible to provide
a steering device, which prevents vehicle behavior from becoming
unstable and steering feel from deteriorating, by controlling the
torque acting on the operating member in accordance with vehicle
behavior and setting a target behavior index value in accordance
with the torque acting on the operating member.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a block diagram illustrating a steering device of
the embodiment of the present invention;
[0018] FIG. 2 is a control block diagram of the steering device of
the embodiment of the present invention;
[0019] FIG. 3 is a diagram showing the state of a vehicle in a
steady circular turning state;
[0020] FIG. 4 (1) is a diagram showing the oversteer state of
vehicle that is slipping sideways, and FIG. 4 (2) is a diagram
showing the understeer state of a vehicle that is slipping
sideways;
[0021] FIG. 5 in a flowchart showing the control procedures of the
steering device of the embodiment of the present invention;
[0022] FIG. 6 (1) and FIG. 6 (2) are Bode diagrams showing yaw rate
response to operating torque input at a vehicle velocity of 20
km/hour in the steering device of the embodiment of the present
invention, and FIG. 6 (3) is a Nyquist diagram;
[0023] FIG. 7 (1) and FIG. 7 (2) are Bode diagrams showing yaw rate
response to operating torque input at a vehicle velocity of 60
km/hour in the steering device of the embodiment of the present
invention, and FIG. 7 (3) is a Nyquist diagram;
[0024] FIG. 8 (1) and FIG. 8 (2) are Bode diagrams showing yaw rate
response to operating torque input at a vehicle velocity of 100
km/hour in the steering device of the embodiment of the present
invention, and FIG. 8 (3) is a Nyquist diagram thereof;
[0025] FIG. 9 (1) and FIG. 9 (2) are Bode diagrams showing yaw rate
.gamma. response to a target yaw rate .gamma.* at a vehicle
velocity of 60 km/hour in a simulation model of a comparative
example, and FIG. 9 (3) is a Nyquist diagram thereof;
[0026] FIG. 10 (1) and FIG. 10 (2) are Bode diagrams showing yaw
rate y response to a target yaw rate .gamma.* at a vehicle velocity
of 100 km/hour in the simulation model of a comparative example,
and FIG. 10 (3) is a Nyquist diagram thereof;
[0027] FIG. 11 is a control block diagram of a simulation model of
the comparative example;
[0028] FIG. 12 (1) is a diagram showing changes over time in the
yaw rate and the target yaw rate relative to the step input of
operating torque in the steering device of the embodiment of the
present invention, and FIG. 12 (2) is a diagram showing changes
over time ill the steering angle and operating member operating
angle relative to the step input of operating torque in the
steering device of the embodiment of the present invention;
[0029] FIG. 13 is a control block diagram of a conventional
steering device;
[0030] FIG. 14 (1) is a diagram showing changes over time in the
yaw rate and the target yaw rate relative to the step input of
operating torque in the conventional steering device, and FIG. 14
(2) is a diagram showing changes over time in the steering angle
relative to the step input of operating torque in the conventional
steering device; and
[0031] FIG. 15 (1) and FIG. 15 (2) are Bode diagrams showing the
response of the yaw rate relative to operating torque input at a
vehicle velocity of 20 km/hour in the steering device constituting
a conventional steer by electric wire system, and FIG. 15 (3) and
FIG. 15 (4) are Bode diagrams showing the response of the yaw rate
relative to operating torque input at a vehicle velocity of 20
km/hour in a steering device, in which a steering wheel is
mechanically coupled to the wheels.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0032] The vehicle steering device shown in FIG. 1 comprises an
operating member 1 modeled on a steering wheel; a steering actuator
2 driven in accordance with the rotating operation of the operating
member 1; and a steering gear 3 for transmitting the movement of
the steering actuator 2 to the front left and right wheels 4 such
that the steering angle changes in accordance with the movement
without mechanically coupling the operating member 1 to the wheels
4.
[0033] The steering actuator 2 can be constituted from an electric
motor, such as, for example, the well-known brushless motor. The
steering gear 3 has a motion conversion mechanism for converting
the rotary motion of the output shaft of this steering actuator 2
to the linear motion of a steering rod 7. The movement of this
steering rod 7 is transmitted to the wheels 4 by way of tie rods 8
and knuckle arms 9, and the toe angle of the wheels 4 change. As
for the steering gear 3, a well-known steering gear can be
utilized, and as long as the motion of the steering actuator 2 can
be transmitted to the wheels 4 such that the steering angle
changes, the constitution thereof is not limited. Furthermore, in a
state wherein the steering actuator 2 is not being driven, the
wheel alignment is set such that the wheels 4 can return to the
straight-forward steering position by self-aligning torque.
[0034] The operating member 1 is coupled to a rotating shaft 10,
which is supported rotatably by the vehicle body. An operating
actuator 19 for generating control torque that acts on this
operating member 1 is provided. The operating actuator 19 can be
constituted from an electric motor such as a brushless motor having
an output shaft integrated together with the rotating shaft 10.
[0035] There is provided an elastic member 30 for providing elastic
force in the direction in which the operating member 1 is made to
return to ;a straight-forward steering position. This elastic
member 30 can be constituted, for example, from a spring that
provides the elastic force to the rotating shaft 10. When the
above-mentioned operating actuator 19 is not furnishing torque to
the rotating shaft 10, the operating member 1 can return to the
straight-forward steering position in accordance with the elastic
force.
[0036] There is provided an angle sensor 11 for detecting the
rotation angle of the rotating shaft 10 as the operating angle of
the operating member 1. The operating member 1 is operated by the
action of load torque, which is sum of the control torque and
operating torque exerted on the operating member 1 by the
driver.
[0037] There is provided a torque sensor 12 for detecting torque
transferred by the rotating shaft 10 as the operating torque, which
is exerted on the operating member 1 by the driver.
[0038] A steering angle sensor 13 for detecting steering angle of
the vehicle is constituted by a potentiometer, which detects the
amount of movement of the steering rod 7 corresponding to the
steering angle.
[0039] There is provided a velocity sensor 14 for detecting vehicle
velocity as a vehicle behavior index value corresponding to change
in the behavior of the vehicle.
[0040] There is provided a lateral acceleration sensor 15 for
detecting lateral acceleration as a behavior index value. 5 There
is provided a yaw rate sensor 16 for detecting yaw rate as a
behavior index value.
[0041] There is provided an electric current sensor 19a for
detecting load current of the operating actuator 19 as a value
corresponding to the control torque Tm, which the operating
actuator 19 generates.
[0042] The angle sensor 11, torque sensor 12, steering angle sensor
13, velocity sensor 14, lateral acceleration sensor 15, yaw rate
sensor 16, and electric current sensor 19a are connected to a
controller 20 constituted by a computer. The controller 20 controls
the steering 15 actuator 2 and the operating actuator 19 by way of
driving circuits 22, 23.
[0043] FIG. 2 shows a control block diagram of the above-mentioned
constitution, and the symbols in the figure are as follows.
[0044] .delta.h: Operating angle of the operating member 1
[0045] .delta.h*: Target operating angle of the operating member
1
[0046] .delta.: Steering angle detected by the steering angle
sensor 13
[0047] .delta.*: Target steering angle
[0048] Th: Driver operating torque detected by the torque sensor
12
[0049] 25 Tm: Control torque generated by the operating actuator
19
[0050] T: Load torque of the operating member 1
[0051] .gamma.*: Target yaw rate
[0052] .gamma.: Vehicle yaw rate detected by the yaw rate sensor
16
[0053] V: Vehicle velocity detected by the velocity sensor 14
[0054] Gy: Lateral acceleration of the vehicle detected by the
lateral acceleration sensor 15
[0055] Is*: Target driving current of the steering actuator 2
[0056] It*: Target driving current of the operating actuator 19
[0057] The controller 20 computes the load torque T, which is sum
of the operating torque Th and the control torque Tm. The operating
torque Th is detected by the above-mentioned torque sensor 12, and
the control torque Tm is computed based on the load current
detected by the above-mentioned electric current sensor 19a.
[0058] The controller 20 stores a predetermined relationship
between the load torque T, operating angle .delta.h and target yaw
rate .gamma.* and based on this stored relationship, computes the
target yaw rate .gamma.* corresponding to the above-mentioned
computed load torque T and detected operating angle .delta.h. In
this embodiment, the predetermined relationship is stored as the
following expression having K.gamma. and Kt as coefficients of
proportionality.
.gamma.*=K.gamma..multidot.(Kt.multidot.T+.delta.h)
[0059] The coefficients of proportionality K.gamma., Kt are set so
as to enable optimum control.
[0060] The controller 20 stores a predetermined relationship
between the target yaw rate .gamma.*, yaw rate .gamma., and target
steering angle .delta.*, and based on this stored relationship,
computes the target steering angle .delta.* corresponding to the
above-mentioned computed target yaw rate .gamma.* and detected yaw
rate .gamma.. In this embodiment, the predetermined relationship is
a transfer function G1 of the target steering angle .delta.*
relative to the deviation (.gamma.*-.gamma.) between the target yaw
rate .gamma.* and detected yaw rate .gamma., and is expressed as
G1=(Ka+Kb/s) such that PI control is performed, with having Ka as
proportional gain, Kb as integral gain, and s as the Laplace
operator. That is, the following expression is stored in the
controller 20.
.delta.*=G1.multidot.(.gamma.*-.gamma.)
[0061] Each gain Ka, Kb is set so as to enable optimum control. In
this embodiment, the gains Ka, Kb are regarded as functions of
vehicle velocity, and are set so as to decrease pursuant to an
increase in vehicle velocity V in order to ensure stability at high
speeds.
[0062] The controller 20 stores a predetermined relationship
between the target steering angle .delta.*, steering angle .delta.,
and target driving current Is* of the steering actuator 2, and
based on this stored relationship, computes the target driving
current Is* corresponding to the above-mentioned computed target
steering angle .delta.* and detected steering angle .delta.. The
steering angle .delta. changes by the movement of the steering
actuator 2 being driven in accordance with the target driving
current Is*. In this embodiment, the predetermined relationship is
a transfer function G2 of the target driving current Is* relative
to the deviation (.delta.*-.delta.) between the target steering
angle .delta.* and detected steering angle .delta., and is
expressed as G2=(Kd+Ke/s) such that PI control is performed, with
having Kd as proportional gain, Ke as integral gain, and s as the
Laplace operator. That is, the following expression is stored in
the controller 20.
Is*=G2.multidot.(.delta.*-.delta.)
[0063] Each gain Kd, Ke is set so as to enable optimum control.
[0064] The controller 20 stores a predetermined relationship
between the target operating angle .delta.h* and the yaw rate
.gamma., lateral acceleration Gy, and vehicle velocity V, which are
the behavior index value of the vehicle, and based on this stored
relationship, computes the target operating angle .delta.h*
corresponding to the detected yaw rate .gamma., detected lateral
acceleration Gy and detected vehicle velocity V. In this
embodiment, the predetermined relationship is stored as the
following expression, in which K.delta. is a proportional constant
and Vo is a crossover velocity.
.delta.h*=K.delta..multidot.(Gy/V+Vo.multidot..gamma.)
[0065] That is, the target operating angle .delta.h* has an
component K.delta..multidot.Gy/V corresponding to a value arrived
at by dividing the lateral acceleration Gy by vehicle velocity V,
and a component K.delta..multidot.Vo.multidot..gamma. corresponding
to the value of the yaw rate .gamma.. The ratio of the component
corresponding to the yaw rate .gamma.in the target operating angle
.delta.h* increases in accordance with increase in the vehicle
velocity V. The coefficient of proportionality K.delta. is set so
as to enable optimum control.
[0066] The crossover velocity Vo represents the yaw rate dependence
factor in the control of the steering device. That is, in FIG. 3,
for vehicle 100 turning at velocity V in the direction indicated by
arrow 40, the relationship between the lateral acceleration Gy
acting in the direction indicated by arrow 41 and the yaw rate
.gamma. acting in the direction indicated by arrow 42 is
approximated as .gamma.=Gy/V, when the vehicle 100 is assumed to be
in a steady turning state. Further, for a vehicle 100 which is
slipping sideways in an oversteer state as shown in FIG. 4 (1), or
for a vehicle 100 which is slipping sideways in an understeer state
as shown in FIG. 4 (2), the angle formed by the center line of the
body of the vehicle depicted as a dotted line paralleling the
longitudinal direction of this vehicle 100, and the direction
indicated by a broken line in which the vehicle 100 would advance
if it were not sliding sideways, is regarded as vehicle sideslip
angle .beta.. The sideslip angle .beta. is approximately determined
by a time integral value of (Gy/V-.gamma.), that is, by
.beta.=.intg.(Gy/V-.gamma.) dt. Accordingly, if the vehicle
velocity at which the sign of sideslip angle .beta. of the vehicle
in a steady turning state changes from positive to negative is
represented as V.sub.NS, the sign of the sideslip angular velocity
represented by d.beta./dt (=Gy/V-.gamma.) also change at V.sub.NS
up until the steady turning state is reached. That is, in a
transient state, when vehicle velocity is lower than V.sub.NS, Gy/V
is larger than .gamma., while when vehicle speed is higher than
V.sub.NS, Gy/V is smaller than .gamma.. Consequently, if Vo=1 in
the expression of the target operating angle .delta.h*, the control
based on the target operating angle .delta.h* depends more on
lateral acceleration than on yaw rate at a vehicle velocity slower
than V.sub.NS, and depends more on yaw rate than on lateral
acceleration at a vehicle velocity faster than V.sub.NS. For
example, if Vo=1, the vehicle velocity at which the control based
on the target operating angle .delta.h* changes from lateral
acceleration dependent state to yaw rate dependent state when
vehicle velocity is increasing, that is, the vehicle velocity at
the time when the component K.delta..multidot.Gy/V corresponding to
a value arrived at by dividing the lateral acceleration Gy by
vehicle velocity V is equal to the component
K.delta..multidot.Vo.multidot..gamma. corresponding to the value of
the yaw rate .gamma. in the target operating angle .delta.h*
(herein-below referred to as the yaw rate dependent velocity Vc),
becomes equal to V.sub.NS. Further, if Vo>1, then
Vc>V.sub.NS, and if Vo<1, then Vc<V.sub.NS. In this
embodiment, the setting value of the yaw rate dependent velocity Vc
is such that it is capable of being changed, by changing the
crossover velocity Vo so as to enable optimum control.
[0067] The controller 20 stores a predetermined relationship
between the target operating angle .delta.h*, operating angle
.delta.h, and target driving current It* of the operating actuator
19, and based on this stored relationship, computes the target
driving current It* corresponding to the above-mentioned computed
target operating angle .delta.h* and detected operating angle
.delta.h. The control torque Tm is generated by the operating
actuator 19 being driven in accordance with the target driving
current It*. In this embodiment, the predetermined relationship is
the transfer function G3 of the target driving current It* relative
to the deviation (.delta.h*-.delta.h) between the target operating
angle .delta.h* and detected operating angle .delta.h, and is
expressed as G3=(Kg+Kh/s) such that PI control is performed, with
having Kg as the proportional gain, Kh as the integral gain, and s
as the Laplace operator. In accordance therewith, the following
expression is realized, and the expression is stored in the
controller 20.
It*=G3.multidot.(.delta.h*-.delta.h)
[0068] Each gain Kg, Kh is set so as to enable optimum control.
Each gain Kg, Kh can be regarded as a function of vehicle velocity
and increased pursuant to an increase in vehicle velocity V, to
enhance stability by increasing the operating torque Th required to
operate the operating member 1 at high speeds.
[0069] The control procedure by the above-mentioned controller 20
is explained by referring to the flowchart of FIG. 5. First, data
detected by each sensor is read in (Step 1). Next, a load torque T,
which is sum of the determined operating torque Th and control
torque Tm, is computed (Step 2). The target yaw rate .gamma.*
corresponding to the computed load torque T and determined
operating angle .delta.h is computed from the stored relationship
.gamma.*=K.gamma..multidot.(Kt.multidot.T+.delta.h) (Step 3). The
target steering angle .delta.* corresponding to the deviation
between the computed target yaw rate .gamma.* and determined yaw
rate .gamma. is computed from the stored relationship
.delta.*=G1.multidot.(.gamma.*-.gamma.) (Step 4). The target
driving current Is* of the steering actuator 2 corresponding to the
deviation between the computed target steering angle .delta.* and
determined steering angle .delta. is computed from the stored
relationship Is*=G2.multidot.(.delta.*-.delta.) (Step 5). The
steering actuator 2 is driven in accordance with the target driving
current Is* such that the steering angle .delta. corresponds to the
target steering angle .delta.*. In accordance therewith, the
steering actuator 2 is controlled such that the determined yaw rate
.gamma. follows the target yaw rate .gamma.* (Step 6). Next, the
target operating angle .delta.h* corresponding to the determined
yaw rate .gamma., lateral acceleration Gy and vehicle velocity V is
computed from the stored relationship
.delta.h*=K.delta..multidot.(G- y/V+Vo.multidot..gamma.) (Step 7).
The target driving current It* of the operating actuator 19
corresponding to the deviation between the computed target
operating angle .delta.h* and determined operating angle .delta.h
is computed from the stored relationship
It*=G3.multidot.(.delta.h*-.delt- a.h) (Step 8). The control torque
Tm is generated by driving the operating actuator 19 in accordance
with the target driving current It*. In accordance therewith, the
operating actuator 19 is controlled such that the determined
operating angle .delta.h follows the computed target operating
angle .delta.h* (Step 9). Next, a determination is made as to
whether or not to terminate control in accordance with, for
example, whether or not the vehicle ignition switch is ON (Step
10), and in a case in which processing is not terminated,
processing returns to Step 1.
[0070] Examples of settings K.gamma., K.delta. in the
above-mentioned constitution is explained. In a steady turning
state, wherein the target yaw rate .gamma.* and actual detected yaw
rate .gamma. are identical, and the target operating angle
.delta.h* and actual detected operating angle .delta.h are
identical, the following expressions (1), (2) are realized from the
above-mentioned expressions .gamma.*=K.gamma..multidot.(Kt.multi-
dot.T+.delta.h) and
.delta.h*=K.delta..multidot.(Gy/V+Vo.multidot..gamma.)- .
.gamma.=K.gamma..multidot.(Kt.multidot.T+.delta.h) (1)
.delta.h=K.delta..multidot.(Gy/V+Vo.multidot..gamma.) (2)
[0071] Further, in the steady turning state, by treating the spring
constant of the elastic member 30 for returning the operating
member 1 to a straight-forward steering position as Ks. Because the
relationships are approximately T=Ks.multidot..delta.h,
Gy/V=.gamma., if these expressions are substituted for the
above-mentioned expressions (1), (2), the following expressions
(3), (4) are realized.
.gamma.=K.gamma..multidot.(Kt.multidot.Ks+1).multidot..delta.h
(3)
.delta.h=K.delta..multidot.(Vo+1).multidot..gamma. (4)
[0072] In accordance therewith, if it is supposed, for example,
that Ks=3.183 N.multidot.m/rad, Kt=0.5, Vo=1.0, and
.gamma./.delta.h=0.2222, from expressions (3), (4), the settings
K.gamma.=0.0857, K.delta.=2.25 are made. In this case, from
expression (1), the load torque T, at the time when the operating
quantity is .pi./2rad and the yaw rate is .pi./9rad/sec,
constitutes 5 N.multidot.m.
[0073] FIG. 6 (1) to FIG. 8 (3) are Bode diagrams showing one
example of frequency response simulation of the yaw rate .gamma.
relative to the load torque in the steering device of the
above-mentioned constitution, and Nyquist diagrams, wherein FIG. 6
(1) to FIG. 6 (3) show a case in which vehicle velocity is 20
km/hour, FIG. 7 (1) to FIG. 7 (3) show a case in which vehicle
velocity is 60 km/hour, and FIG. 8 (1) to FIG. 8 (3) show a case in
which vehicle velocity is 100 km/hour.
[0074] Here, it is supposed that the inertial moment Im of the
operating member 1 is Im=0.04 kg.multidot.m.sup.2, the viscuous
resistance Cs of the operating member 1 is Cs=1.0
N.multidot.m.multidot.sec, and the spring constant Ks of the
elastic member 30 is Ks=3.183 N.multidot.m/rad.
[0075] Further, it is supposed that the transfer function
G1=G.delta..multidot.(0.1+5/s), and that G.delta.=0.28 at a vehicle
velocity of 20 km/h, G.delta.=0.19 at a vehicle velocity of 60
km/h, and G.delta.=0.165 at a vehicle velocity of 100 km/h. It is
supposed that the transfer function G3=Kt.multidot.(3+13/s), and
that Kt=0.05 at a vehicle velocity of 20 km/h, Kt=0.6 at a vehicle
velocity of 60 km/h, and Kt=1.0 at a vehicle velocity of 100 km/h.
Furthermore, it is supposed that there are no delays in the
operating actuator 19.
[0076] FIG. 9 (1) to FIG. 9 (3), FIG. 10 (1) to FIG. 10 (3) are
Bode diagrams showing one example of frequency response simulation
of the yaw rate .gamma. relative to the target yaw rate .gamma.* in
the simulation model of the comparative example shown in FIG. 11,
and Nyquist diagrams, wherein FIG. 9 (1) to FIG. 9 (3) show a case
in which vehicle velocity is 60 km/hour, and FIG. 10 (1) to FIG. 10
(3) show a case in which vehicle velocity is 100 km/hour. In the
simulation model, G is the transfer function of the steering angle
.delta. relative to the deviation between the target yaw rate
.gamma.* and the actual vehicle yaw rate .gamma., this transfer
function G is set such that proportional integral (PI) control is
performed, and here it is supposed that G=0.1+5/s.
[0077] If the Bode diagrams of FIG. 6 (1), FIG. 6 (2), FIG. 7 (1),
FIG. 7 (2), FIG. 8 (1), and FIG. 8 (2) are compared with the Bode
diagrams of FIG. 9 (1), FIG. 9 (2), FIG. 10 (1), and FIG. 10 (2),
there is practically no difference between the responsiveness of
the yaw rate .gamma. relative to the operating torque Th in a case,
in which the target yaw rate .gamma.* is set by controlling the
load torque T acting on the operating member 1 on the basis of the
present invention, and the responsiveness of the yaw rate .gamma.
relative to the target yaw rate .gamma.* in a case, in which the
target yaw rate .gamma.* is set without controlling the load torque
T acting on the operating member 1. That is, responsiveness is not
lowered even though the load torque T acting on the operating
member 1 is controlled. Conversely, if the Nyquist diagrams of FIG.
6 (3), FIG. 7 (3) and FIG. 8 (3) are compared with the Nyquist
diagrams of FIG. 9 (3) and FIG. 10 (3), stability is enhanced
greatly by controlling the load torque T acting on the operating
member 1.
[0078] An example of simulation results for the steering device of
the above-mentioned constitution is shown by FIG. 12 (1) and FIG.
12 (2), which show changes over time in the yaw rate .gamma.,
target yaw rate .gamma.*, steering angle .delta. and operating
angle .delta.h relative to the step input of 2.7 N.multidot.m of
operating torque Th at timing t1 to t2 (0.5 to 5 seconds) in a
vehicle travelling at a velocity of 60 km/hour, and the coefficient
of friction between the vehicle and the surface of the road is
regarded as 1 up until t3 (2.5 seconds), and is regarded as 0.1
thereafter. It is shown that, when the yaw rate .gamma. drops in
accordance with a change in the coefficient of friction, revisions
are made such that the operating angle .delta.h becomes smaller by
controlling the control torque Tm so as to do away with the
deviation between the target operating angle .delta.h* and
operating angle .delta.h corresponding to the yaw rate .gamma., and
therefore the steering angle divergence is prevented without
increasing the deviation between the yaw rate .gamma. and the
target yaw rate .gamma.*.
[0079] According to the above-mentioned constitution, the operating
angle .delta.h is generated by the operation of operating member 1
in accordance with the load torque T, which is sum of the control
torque Tm outputted by the operating actuator 19 and the operating
torque Th inputted by the driver. The control torque Tm functions
so as to do away with the deviation between the operating angle
.delta.h and the target operating angle .delta.h*. Accordingly, in
a case in which the operating angle .delta.h does not attain the
target operating angle .delta.h*, the control torque Tm functions
as an auxiliary force for the operation of the operating member 1,
and in a case in which the operating angle .delta.h exceeds the
target operating angle .delta.h*, the control torque Tm functions
as a reactive force against the operation of the operating member
1.
[0080] Further, the steering actuator 2 is controlled such that the
yaw rate .gamma. follows the target yaw rate .gamma.* corresponding
to the operating angle .delta.h and load torque T. The yaw rate
.gamma. and lateral acceleration Gy change by the control of the
steering actuator 2. The target operating angle .delta.h*
corresponds to the yaw rate .gamma. and lateral acceleration Gy,
and the operating angle .delta.h corresponds to the target yaw rate
.gamma.*.
[0081] Accordingly, in a case in which the yaw rate .gamma. does
not attain the target yaw rate .gamma.*, because the operating
angle .delta.h exceeds the target operating angle .delta.h*, the
reactive force against the operation functions as mentioned
hereinabove. In accordance therewith, in a case in which the yaw
rate .gamma. does not increase due to a drop in the coefficient of
friction between the road surface and the wheels, or due to the
tire lateral force having reached its limit even when the operating
torque Th is increased, the reactive force against the operation
can be made to function. At this time, even if the driver further
increases the operating torque Th, the increase in the operating
torque Th can be offset by an increase in the reactive force
against the operation, and therefore the load torque T acting on
the operating member 1 can be maintained approximately constant,
and the target yaw rate .gamma.* corresponding to the load torque T
and operating angle .delta.h does not increase. That is, because
the operating angle .delta.h and load torque T, and in turn the
target yaw rate .gamma.* can be held in check by the reactive force
against the operation, the divergence of the steering angle .delta.
can be prevented, and vehicle behavior can be stabilized. Further,
when a delay occurs in the yaw rate following the target yaw rate
due to a delay in the response of the steering actuator 2, because
the reactive force against the operation functions as before, it is
possible to alleviate the wrong feeling resulting from the delayed
response of this steering actuator 2, thus enabling improved
steering feel.
[0082] Furthermore, the steering device can be controlled with
accommodating to vehicle behavior characteristics, such that the
yaw rate becomes smaller at low vehicle velocity by making the
affect of lateral acceleration greater at low vehicle velocity, and
making the affect of the yaw rate greater pursuant to an increase
in vehicle velocity, in response to the target operating angle
.delta.h*. Furthermore, because the setting value of the vehicle
velocity Vc, at the time when the component corresponding to a
value arrived at by dividing the lateral acceleration Gy by vehicle
velocity V is equal to the component corresponding to a value of
the yaw rate .gamma. in the target operating angle .delta.h*, can
be changed by changing the crossover velocity Vo, it is possible to
control the steering device with accommodating even closer to
vehicle behavior characteristics.
[0083] The present invention is not limited to the above
embodiment. For example, the steering actuator and the operating
actuator can be controlled by setting the target operating angle in
accordance with the yaw rate alone at a speed greater than a set
vehicle velocity, and, in addition, by setting the target steering
angle and target operating angle in accordance with the operating
angle alone at a speed less than a set vehicle velocity at which
the yaw rate is hardly generated.
* * * * *