U.S. patent application number 10/951979 was filed with the patent office on 2005-02-24 for active steering for handling/stability enhancement.
This patent application is currently assigned to DELPHI TECHNOLOGIES, INC.. Invention is credited to Brown, Todd D., Chandy, Ashok, Chen, Hsien H., Gryczan, Chester W., Martens, John D..
Application Number | 20050043874 10/951979 |
Document ID | / |
Family ID | 32989730 |
Filed Date | 2005-02-24 |
United States Patent
Application |
20050043874 |
Kind Code |
A1 |
Chen, Hsien H. ; et
al. |
February 24, 2005 |
Active steering for handling/stability enhancement
Abstract
A control system for a steering system in a vehicle comprising:
a reference model responsive to an operator input that computes
desired states of the vehicle; a feedforward controller in operable
communication with the reference model. The feedforward controller
computes a first control value based on input from said reference
model and based on at least one of: a lateral velocity, a rate of
lateral velocity, a lateral acceleration, and a combination,
wherein the combination includes a yaw rate with at least one of a
lateral velocity, a rate of lateral velocity, and a lateral
acceleration of the motor vehicle. The system also includes an
actuator for affecting the steering system based on the first
control value, the actuator in operable communication with the
feedforward controller.
Inventors: |
Chen, Hsien H.; (Troy,
MI) ; Martens, John D.; (New Hudson, MI) ;
Brown, Todd D.; (Brighton, MI) ; Chandy, Ashok;
(Fenton, MI) ; Gryczan, Chester W.; (Brighton,
MI) |
Correspondence
Address: |
MICHAEL D. SMITH
DELPHI TECHNOLOGIES, INC.
Legal Staff, Mail Code: 480-410-202
P.O. Box 5052
Troy
MI
48007-5052
US
|
Assignee: |
DELPHI TECHNOLOGIES, INC.
|
Family ID: |
32989730 |
Appl. No.: |
10/951979 |
Filed: |
September 28, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10951979 |
Sep 28, 2004 |
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10402570 |
Mar 28, 2003 |
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6804594 |
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Current U.S.
Class: |
701/42 |
Current CPC
Class: |
B62D 6/003 20130101 |
Class at
Publication: |
701/042 |
International
Class: |
B62D 012/00; G06F
007/00 |
Claims
1. A control system for a steering system in a vehicle comprising:
a reference model responsive to an operator input and tire
relaxation, which computes desired states of the vehicle; a
feedforward controller in operable communication with said
reference model, said feedforward controller computes a first
control value based on input from said reference model and based on
at least one of: a lateral velocity, a rate of lateral velocity, a
lateral acceleration, and a combination, wherein said combination
includes a yaw rate with at least one of a lateral velocity, a rate
of lateral velocity, and a lateral acceleration of said motor
vehicle; an actuator for affecting the vehicle based on said first
control value, said actuator in operable communication with said
feedforward controller; a state estimator that estimates an actual
state of said vehicle; and a feedback controller that computes a
second control value by comparing said estimate of actual state
with a desired state, wherein said feedback controller computes a
blended solution for said second control value based on a vehicle
velocity and wherein said second control value is based on said yaw
rate if a vehicle velocity is below a selected threshold; and
wherein said second control value is based on said at least one of
a lateral velocity, a rate of lateral velocity, and a lateral
acceleration of said vehicle if said vehicle velocity is above a
selected threshold; otherwise said second control value is based on
a combination of said yaw rate, said at least one of a lateral
velocity, a rate of lateral velocity, and a lateral acceleration of
said vehicle.
2. The control system of claim 1 further comprising a state
estimator, said state estimator estimates an actual state of said
vehicle.
3. The control system of claim 2 further comprising a feedback
controller, said feedback controller computes a second control
value by comparing said estimate of actual state with a desired
state.
4. The control system of claim 3 wherein said feedback controller
computes a blended solution for said second control value based on
a vehicle velocity.
5. The control system of claim 3 wherein: said second control value
is based on said yaw rate if a vehicle velocity is below a selected
threshold; said second control value is based on said at least one
of a lateral velocity, a rate of lateral velocity, and a lateral
acceleration of said vehicle if said vehicle velocity is above a
selected threshold; otherwise said second control value is based on
a combination of said yaw rate said at least one of a lateral
velocity, a rate of lateral velocity, and a lateral acceleration of
said vehicle.
6. A control system for a steering system in a vehicle comprising:
a reference model responsive to an operator input and tire
relaxation, which computes desired states of the vehicle; a
feedforward controller in operable communication with said
reference model, said feedforward controller computes a first
control value based on input from said reference model and based on
at least one of: a lateral velocity, a rate of lateral velocity, a
lateral acceleration, and a combination, wherein said combination
includes a yaw rate with at least one of a lateral velocity, a rate
of lateral velocity, and a lateral acceleration of said motor
vehicle; an actuator for affecting the vehicle based on said first
control value, said actuator in operable communication with said
feedforward controller; wherein said reference model computes said
desired state for the vehicle based on a magnitude of a lateral
acceleration and at least one of a throttle command and a magnitude
of a longitudinal accelerations; wherein said desired state for the
vehicle includes at least one of a desired yaw rate, a desired
lateral velocity, a desired rate of lateral velocity, and a desired
lateral acceleration for said vehicle; and wherein said first
control value is based on at least one of a vehicle speed, a
steering wheel position, and vehicle tire dynamics.
7. The control system of claim 1 wherein said desired state for the
vehicle includes at least one of a desired yaw rate, a desired
lateral velocity, a desired rate of lateral velocity, and a desired
lateral acceleration for said vehicle.
8. The control system of claim 1 wherein said first control value
is based on at least one of a vehicle speed, a steering wheel
position, and vehicle tire dynamics.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to control systems for motor
vehicle steering systems, and more particularly to a system and
method including a dynamic feedforward feature for integrated
control of the motor vehicle steering and brakes.
[0002] Unified or integrated chassis control systems have been
proposed which control the brakes, steering, and suspension of a
motor vehicle. The purpose of unified chassis control is to improve
vehicle performance in all driving conditions by coordinating
control of the chassis subsystems. Unified chassis control systems
typically utilize a supervisory control concept that utilizes three
fundamental blocks: a reference model, a state estimator, and a
vehicle control. The vehicle control element normally incorporates
a feedback control. This element computes control values by
comparing actual states obtained from the state estimator with
desired states from the reference model.
[0003] It is well known that when brakes are applied during a
steering maneuver, a yaw rate error is induced. It such
circumstances, the conventional chassis control systems are
relatively slow to compensate.
SUMMARY OF THE INVENTION
[0004] The foregoing and other features and advantages of the
invention will become further apparent from the following detailed
description of the presently preferred embodiments, read in
conjunction with the accompanying drawings. The detailed
description and drawings are merely illustrative of the invention
rather than limiting, the scope of the invention being defined by
the appended claims and equivalents thereof.
[0005] Disclosed herein in an exemplary embodiment is a control
system for a steering system in a vehicle comprising: a reference
model responsive to an operator input that computes desired states
of the vehicle; a feedforward controller in operable communication
with the reference model. The feedforward controller computes a
first control value based on input from the reference model and
based on at least one of: a lateral velocity, a rate of lateral
velocity, a lateral acceleration, and a combination, wherein the
combination includes a yaw rate with at least one of a lateral
velocity, a rate of lateral velocity, and a lateral acceleration of
the motor vehicle. The system also includes a means for affecting
the steering system based on the first control value, the means in
operable communication with the feedforward controller.
[0006] Also disclosed herein is a method of controlling a steering
system in a vehicle comprising: computing a desired state of the
steering system; and computing a first control value based on the
desired states of the steering system. The first control value is
also computed based on at least one of: a lateral velocity, a rate
of lateral velocity, a lateral acceleration, and a combination,
wherein the combination includes a yaw rate with at least one of a
lateral velocity, a rate of lateral velocity, and a lateral
acceleration of the vehicle. The method further includes affecting
the steering system based on the first control value.
[0007] Further disclosed herein is a control system for a steering
system in a vehicle comprising: a means for computing a desired
state of the steering system; a means for computing a first control
value based on the desired state of the steering system and based
on at least one of: a lateral velocity, a rate of lateral velocity,
a lateral acceleration, and a combination. The combination includes
a yaw rate with at least one of a lateral velocity, a rate of
lateral velocity, and a lateral acceleration of the vehicle. The
system further includes a means for affecting the steering system
based on the first control value.
[0008] Also disclosed herein in yet another embodiment is a storage
medium encoded with a machine-readable computer program code, the
computer program code including instructions for causing a
controller to implement the above mentioned method for controlling
a steering system in a vehicle.
[0009] Further disclosed herein in yet another embodiment is a
computer data signal, the data signal comprising code configured to
cause a controller to implement the abovementioned method for
controlling a steering system in a vehicle.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The present invention will now be described, by way of an
example, with references to the accompanying drawings, wherein like
elements are numbered alike in the several figures in which:
[0011] FIG. 1 is a simplified block diagram of a vehicle control
system according to an exemplary embodiment;
[0012] FIG. 2 is a block diagram a block diagram representation of
a control system including a reference model and the dynamic feed
forward control system using a multi-order reference model; and
[0013] FIG. 3 is a flow chart depicting an exemplary embodiment of
the methodology for computing dynamic feedforward control including
blending;
[0014] FIG. 4 is a flow chart depicting an exemplary embodiment of
the methodology for computing closed loop control with pole
placement;
[0015] FIG. 5 is a flow chart depicting an exemplary embodiment of
the methodology for computing throttle scheduling; and
[0016] FIG. 6 is a flow chart depicting an exemplary embodiment of
the methodology for computing velocity blending of an exemplary
embodiment.
DETAILED DESCRIPTION
[0017] FIG. 1 shows a control system 10 according to an exemplary
embodiment for chassis subsystems 12 of a motor vehicle 14. The
chassis subsystems may include front and/or rear steering and
braking. The chassis subsystems further include sensors, actuation
devices and the like for appling commands to the control system 10
and thereby controlling the vehicle 14. A controller 16 performs
many estimation and control functions conventionally performed by
the subsystems themselves. The controller 16 includes a reference
model 18, an estimator 20 of vehicle state and environment, and a
vehicle level controller including a feedforward controller 22 and
a feedback controller 24.
[0018] The reference model 18 determines the desired vehicle
response, in terms of measureable variables, to driver inputs 25
using measured inputs from sensors 26 and some estimates. For
example, driver inputs may include, but not be limited to steering
angle applied to a steering input device e.g., steering wheel,
throttle, braking, selectable operating mode, e.g. two wheel
steering, four wheel steering, trailering, and the like. In an
exemplary embodiment, a driver input to a steering wheel is
detected by a sensor 25 for utilization in controller 16 at
reference model 18a to ascertain a desired steering command. The
estimator 20 uses measured inputs such as from sensors 26, measured
outputs such as from sensors 28, and some preliminary estimates
from individual modules to generate estimates of vehicle states
which are not measured directly. The estimator 20 may also estimate
variables describing potentially relevant environmental effects
such as surface coefficient of adhesion, bank angle of the road
surface, roughness of the road surface, and others. Also disclosed
herein in another exemplary embodiment is a method of throttle
scheduling of the yaw rate gain in the computation of the desired
yaw rate from the reference model 18a. Moreover, the throttle
scheduling could similarly, be applied for lateral velocity.
[0019] In order to perform the prescribed functions and desired
processing, as well as the computations therefore (e.g., the
dynamic feedforward control, control algorithm(s), and the like),
controller 16 may include, but not be limited to, a processor(s),
computer(s), DSP(s), memory, storage, register(s), timing,
interrupt(s), communication interface(s), and input/output signal
interfaces, and the like, as well as combinations comprising at
least one of the foregoing. For example, controller 16 may include
input signal processing and filtering to enable accurate sampling
and conversion or acquisitions of such signals from communications
interfaces. Additional features of controller 16 and certain
processes therein are thoroughly discussed herein.
[0020] As exemplified herein and disclosed above one such process
may be dynamic feedforward control. Controller 16 receives various
input signals from sensors 26 including, but not limited to, those
identified above, to facilitate such processing and may provide one
or more output signals in response. The controller 16 uses the
desired, measured and estimated values to determine the corrective
action on the vehicle level, for example moments and forces acting
on the vehicle, that will bring the vehicle response into
conformance with the desired response. The controller 16 also
performs partition of control among the subsystems 12. Thus, the
controller 16 decides whether and to what extent to activate any
subsystem(s) in a given situation.
[0021] The feedforward controller 22 computes a control value based
on input from the reference model 18. An exemplary embodiment
optionally provides a driver to chassis subsystem transfer function
30. In the case of steering, this could represent the transfer
function between steering wheel position and road wheel position.
In the case of braking, this could represent the transfer function
between pedal force and caliper forces at each of the braked wheels
of the motor vehicle. This transfer function can be included in the
feedforward and feedback control design. It should be noted that
the lines on the block diagram in FIG. 1 show primary control
paths. In reality, due to the non-linear nature of vehicle
dynamics, the present invention contemplates other links such as
bank angle from the state estimator 20 to the feedforward
controller 22. A control term based on the driver inputs (inputs to
the reference model 18a) are also appropriate and fit the
definition of feedforward control.
[0022] A dynamic feedforward (DFF) control as a function of vehicle
yaw-plane dynamics was introduced in commonly owned copending U.S.
patent application Ser. No. 09/935,274, Attorney Docket No.
DP-305682 filed Aug. 22, 2001, the disclosures of which are
incorporated by reference herein in their entirety. In that dynamic
feed forward (DFF) control the reference model 18 was primarily
responsive to vehicle yaw rate and ignored the effects of tire
dynamics. While well suited for its intended purposes, this
reference model included the potential for mathematical anomalies
under certain conditions, diminishing the benefits of the reference
model 18.
[0023] In an exemplary embodiment a dynamic feedforward control is
introduced which expands the abovementioned reference model 18 and
feedforward control 22 to enhance the control system 10 for rear
wheel steering. The new reference model 18a and feed forward
control 22a provides a solution based on vehicle yaw rate as
described above, and lateral velocity. Additionally tire
characteristics and dynamics have been added to the reference model
18a to refine the model and improve its integrity over a wider
array of operational scenarios. It will be appreciated that as
discussed herein, for continuity, clarity, and simplification of
description, all references herein cite modeling and control based
on lateral velocity. It is noteworthy to appreciate that the
exemplary embodiments disclosed herein are equally applicable
utilizing similar dynamic vehicle parameters including, but not
limited to rate of lateral velocity and lateral acceleration.
[0024] Continuing with the dynamic feed forward control 22, in an
exemplary embodiment, a linear blend of the two solutions within
the feedforward control 22a for vehicle yaw rate and lateral
velocity over a selected range of hand wheel spectral content. The
selected range is defined by a selected lower threshold and a
selected upper threshold. For hand wheel frequencies less than the
lower threshold, the dynamic feedforward control based on yaw rate
is employed, and for hand wheel frequencies above the upper
threshold, the dynamic feedforward control based on later velocity
is used. For frequencies between the selected upper and lower
thresholds a blend to the reference model solutions based on yaw
rate and lateral velocity is employed. It will be appreciated that
at low hand wheel angle frequencies, the vehicle dynamics are more
appropriately modeled based on yaw rate response, while for higher
frequency hand wheel angle inputs the vehicle dynamics are more
appropriately modeled based on lateral velocity.
[0025] In yet another exemplary embodiment, another blending
methodology is disclosed. A linear blend of the output from a
direct front-to-rear wheel angle transfer function to the dynamic
feedforward front-to-rear wheel angle transfer function over a
selected speed range is employed. Finally, throttle scheduling of
the yaw rate gain (and/or the lateral velocity gains) in the
computation of the desired yaw rate from the reference model 18a is
depicted.
[0026] Turning now to FIG. 2 as well, a block diagram
representation of the controls system 10 is depicted
[0027] The yaw rate shown above is given by:
.phi.=P.sub.1.theta.+P.sub.2G.sub.s[G.sub.ff.phi..sub.r+G.sub.fb(.phi.-.ph-
i..sub.r)] (1)
[0028] where
[0029] P.sub.1 is the portion of the vehicle plant modeling the
front wheel steering response;
[0030] P.sub.2 is the portion of the vehicle plant modeling the
rear wheel steering response;
[0031] .theta. is the front wheel angle (proportional to hand wheel
or steering wheel angle);
[0032] G.sub.s is a transfer function modeling actuators;
[0033] G.sub.ff is the dynamic feed forward control; and
[0034] .phi..sub.r is the desired yaw rate from the reference model
18a.
[0035] Let us assume the yaw rate is equal to the desired yaw rate
as:
.phi.=.phi..sub.r (2).
[0036] Substituting (2) into (1) yields:
(1-P.sub.2G.sub.sG.sub.ff).phi..sub.r=P.sub.1.theta. (3).
[0037] Let us assume:
.phi..sub.r=G.sub.ref.theta. (4).
[0038] Substituting (4) into (3) yields:
(1-P.sub.2G.sub.sG.sub.ff)G.sub.ref.theta.=P.sub.1.theta. (5).
[0039] Rearranging (5), the general dynamic feed-forward transfer
function is given by: 1 G ff = P 2 - 1 G S - 1 [ 1 - G ref - 1 P 1
] ( 6 )
[0040] where P.sub.1 represents the transfer function to relate the
yaw rate to the front road wheel and is given by: 2 P 1 = . = P 1 (
z ) P ( p ) . ( 7 )
[0041] The transfer function to relate the yaw rate to the rear
wheel angle is expressed as: 3 P 2 = . r = P 2 ( z ) P 2 ( p ) . (
8 )
[0042] DFF Control Based on Yaw Rate
[0043] Once again, referring back to FIG. 2, the rear wheel steer
command is given by: 4 r_ . = G ref G ff = G ref ( z ) P 1 ( p ) -
G ref ( p ) P 1 ( z ) G ref ( p ) P 2 ( z ) ( 9 )
[0044] where the G.sub.ref represents (in this instance) the
desired yaw rate and is assumed to be a 2 zero, 4 pole (2Z4P)
transfer function expressed as: 5 G ref = G ref ( z ) G ref ( p ) =
k d 1 2 2 2 3 2 ( s 2 + 2 3 3 s + 3 2 ) ( s 2 + 2 1 1 s + 1 2 ) ( s
2 + 2 2 2 s + 2 2 ) . ( 10 )
[0045] The transfer functions P.sub.1(z), P.sub.2(z), P.sub.1(p)
shown in equation 9 may readily be determined from a bicycle model
including tire relaxation at the front and rear tires as a function
of such vehicle parameters as vehicle mass M, vehicle speed
V.sub.x, vehicle yaw inertia I.sub.z, front and rear cornering
stiffness c.sub.f, c.sub.r, and tire relaxation length at the front
and rear axle L.sub.f, L.sub.r, respectively
[0046] The DFF rear wheel control command (based on desired yaw
rate) is determined by substituting equations (10) and (11) into
equation 9, which yields: 6 r_ . = m 1 s 6 + m 2 s 5 + m 3 s 4 + m
4 s 3 + m 5 s 2 + m 6 s + m 7 l 1 s 6 + l 2 s 5 + l 3 s 4 + l 4 s 3
+ l 5 s 2 + l 6 s + l 7 ( 11 )
[0047] where the m.sub.i and l.sub.i are functions of the vehicle
parameters and states.
[0048] DFF Control Based on Lateral Velocity
[0049] Similarly, once again from FIG. 2, the rear wheel steer
command is given by: 7 r_vy = G ref_vy G ff_vy = G ref_vy ( z ) P 1
( p ) - G ref ( p ) P 1 _vy ( z ) G ref ( p ) P 2 _vy ( z ) ( 12
)
[0050] where the G.sub.ref.sub..sub.--.sub.vy represents in this
instance, the desired lateral velocity and is assumed to be (in
this instance) a 2 zero, 4 poles (2Z4P) transfer function expressed
as: 8 G ref_vy = G ref_vy ( z ) G ref ( p ) = k d_vy 1 2 2 2 4 2 (
s 2 + 2 4 4 s + 4 2 ) ( s 2 + 2 1 1 s + 1 2 ) ( s 2 + 2 2 2 s + 2 2
) . ( 13 )
[0051] Once again, the transfer function P.sub.1.sub..sub.--.sub.vy
(z), P.sub.2.sub..sub.--.sub.vy (z) shown in equation 12 may
readily be determined from a bicycle model including tire
relaxation at the front and rear tires as a function of such
vehicle parameters as vehicle mass M, vehicle speed V.sub.x,
vehicle yaw inertia I.sub.z, front and rear cornering stiffness
c.sub.f, c.sub.r, and tire relaxation length at the front and rear
axle L.sub.f, L.sub.r, respectively. Hence, the DFF rear wheel
control command (based on desired lateral velocity) is determined
by substituting equation (13) into equation (12), which yields: 9
r_vy = n 1 s 6 + n 2 s 5 + n 3 s 4 + n 4 s 3 + n 5 s 2 + n 6 s + n
7 q 1 s 6 + q 2 s 5 + q 3 s 4 + q 4 s 3 + q 5 s 2 + q 6 s + q 7 (
14 )
[0052] where n.sub.i and q.sub.i are functions of the vehicle
parameters and states.
[0053] Blend of Dynamic Feedforward Solutions
[0054] As disclosed earlier in exemplary embodiment, a linear blend
of the two dynamic feedforward solutions for vehicle yaw rate and
lateral velocity may be employed. The blend may be over a selected
range of hand wheel spectral content and/or based on selected
weighting. In a first embodiment, the blending is a function of a
weighting applied to each solution formulating a composite. The
dynamic feed forward rear wheel control may be computed as:
.delta..sub.r.sub..sub.--.sub.ff=.epsilon..delta..sub.r+(1-.epsilon.).delt-
a..sub.r.sub..sub.--.sub.vy (15).
[0055] where .epsilon. (range between 0 and 1) is the weighting on
either yaw rate based or lateral velocity based dynamic feedforward
control.
[0056] In yet another exemplary embodiment, the composite dynamic
feedforward rear wheel control may be a computed as a function of
steering frequency input.
.delta..sub.r.sub..sub.--.sub.ff=G.sub.filt.delta..sub.r+(1-G.sub.filt).de-
lta..sub.r.sub..sub.--.sub.vy (16).
[0057] where in one embodiment 10 G filt = 1 s filt + 1 and
[0058] .omega..sub.filt=blend time constant.
[0059] For hand wheel frequencies less than a lower threshold, the
dynamic feedforward control based on yaw rate is employed, and for
hand wheel frequencies above an upper threshold, the dynamic
feedforward control based on lateral velocity is used. For
frequencies between the selected upper and lower thresholds a blend
to the reference model solutions based on yaw rate and lateral
velocity is employed. It will be appreciated that at low hand wheel
angle frequencies, the vehicle dynamics are more appropriately
modeled based on yaw rate response, while for higher frequency hand
wheel angle inputs the vehicle dynamics are more appropriately
modeled based on lateral velocity.
[0060] FIG. 3 shows a dynamic feedforward control method for a rear
steer application including a multi-order transfer function
responsive to both vehicle yaw rate and lateral velocity. The
bicycle model parameters are first obtained at block 100, and then
at block 102 the desired yaw rate gain, natural frequency and
damping are determined according to the desired tuning as discussed
previously. At block 104 the desired lateral velocity gain, natural
frequency and damping are determined according to the desired
tuning. A cornering stiffness adaptation based on the desired
steady state lateral acceleration is then performed at 106,
followed by filtering the steering wheel position, the steering
wheel rate, and the vehicle speed as shown by block 108. The
dynamic feedforward control commands based on vehicle yaw rate and
lateral velocity is then determined at block 110. At block 112 a
blend process based on the frequency content of the hand wheel
angle is employed to provide a blended dynamic feedforward control
command based on either yaw rate lateral velocity, or a blended
combination thereof.
[0061] Vehicle Speed Blending
[0062] Referring now to FIG. 6, in yet another exemplary
embodiment, another blending methodology 400 is disclosed. A linear
blend of the output from a direct front-to-rear wheel angle
transfer function to the dynamic feedforward front-to-rear wheel
angle transfer function over a selected speed range is employed.
The selected speed range is defined by a selected lower threshold,
denoted V.sub.X.sub..sub.--.sub.THRESH.sub..s- ub.--.sub.L and a
selected upper threshold, denoted
V.sub.X.sub..sub.--.sub.THRESH.sub..sub.--.sub.U. For vehicle
speeds, V.sub.x less than the lower threshold,
V.sub.X.sub..sub.--.sub.THRESH.sub- ..sub.--.sub.L the direct
relationship is used, and for vehicle speeds V.sub.x above the
upper threshold, V.sub.X.sub..sub.--.sub.THRESH.sub..su- b.--.sub.U
the dynamic feedforward relationship is used. In calculating the
direct relationship, the desired steady-state yaw gain from the
dynamic feedforward tuning is maintained while the dynamics are
disregarded. It will be appreciated that at low speeds, the
equations describing the dynamics in the reference model 18A are
less robust and may produce unwanted effects in the vehicle.
[0063] The blending process of an exemplary embodiment is
accomplished as follows:
[0064] Denote the front wheel steering angle as .delta..sub.fwa and
note that it may be linearly approximated from the hand wheel
angle, .delta..sub.hwa, as .delta..sub.fwa=.delta..sub.hwa/G.sub.r
where G.sub.r is the gear ratio from hand wheel to road wheel in
the vehicle 14. The rear wheel angle output from the dynamic
feedforward algorithm is given as a function the front wheel angle
.delta..sub.fwa and the desired tuning as suggested by:
.delta..sub.r.sub..sub.--.sub.ff,DFF,Dyn=f.sub.DFF(.delta..sub.fwa,
Tuning, vehicle state) (17)
[0065] where f.sub.DFF may be a 6th order transfer function (in one
implementation) that depends on a vehicle model, vehicle
parameters, vehicle state, and desired tuning.
[0066] More specifically, the dynamic feedforward transfer function
from hand wheel to rear wheel angle is given by a steady-state
relationship:
.delta..sub.r.sub..sub.--.sub.ff,DFF,SS=(1-K.sub.m)*.delta..sub.fwa
(18)
[0067] where is K.sub.m is a yaw-gain multiplier, a variable in the
tuning set. It will be appreciated that such a linear, non-dynamic
transfer function is similar to the ratio employed in existing rear
and four-wheel steering systems.
[0068] Thus, to respect this relationship and yet remove the
dynamics, the feedforward transfer functions are divided. The
steady-state portion of the transfer function from equation (17) is
used at low vehicle speed while the dynamic transfer function (17)
is used at medium and high speeds with blending performed in the
intermediate speeds. That is, for an exemplary embodiment, where
V.sub.x=vehicle speed, LT=lower threshold, and UT=upper threshold,
the following rule is formulated and applied:
.delta..sub.r.sub..sub.--.sub.ff,DFF=.delta..sub.r.sub..sub.--.sub.comp,DF-
F,SS for: V.sub.x<LT; (19a)
.delta..sub.r.sub..sub.--.sub.ff,DFF=(V.sub.x-LT)/(UT-LT)*.delta..sub.r.su-
b..sub.--.sub.ff,DFF,Dyn+
[1-(V.sub.x-LT)/(UT-LT)]*.delta..sub.r.sub..sub.--.sub.ff,DFF,SS
for: LT<V.sub.x<UT; and (19b)
.delta..sub.r.sub..sub.--.sub.ff,DFF=.delta..sub.r.sub..sub.--.sub.ff,DFF,-
Dyn for: V.sub.x>UT (19c).
[0069] Continuing now with FIG. 6, the methodology 400 will now be
described. In an exemplary embodiment, at block 410 the vehicle
speed V.sub.x is determined. As will be appreciated the vehicle
speed may be determined from a variety of sensors including, but
not limited to, wheel speed sensors, drive train sensors,
transmitting sensors, e.g. radar, Doppler, laser, and the like as
well as combinations including at least one of the foregoing. At
decision block 412 a determination is made as to whether the
vehicle speed, V.sub.x is less than the lower threshold,
V.sub.X.sub..sub.--.sub.THRESH.sub..sub.--.sub.L. If so, the rear
wheel control is computed based upon the steady state transfer
function (equation 19a) as depicted at block 414. Otherwise, the
method 400 continues to decision block 416 to determine if the
vehicle speed, V.sub.x exceeds than the upper threshold,
V.sub.X.sub..sub.--.sub.THRESH.- sub..sub.--.sub.U. If so, the rear
wheel control is computed based upon the dynamic transfer function
(equation 21c) as depicted at block 418. Otherwise, the method 400
continues to block 420 to where the rear wheel control is computed
based upon the blended dynamic transfer function (equation 19b). It
will be appreciated, that the blending disclosed herein, while
described in terms of the dynamic feed forward control could also
be utilized in the other control processes. For example, similar
blending may be employed in the closed loop feedback control
24.
[0070] Throttle Scheduling
[0071] Turning now to FIG. 5, in yet another exemplary embodiment,
a methodology 300 of throttle scheduling of the yaw rate gain in
the computation of the desired yaw rate from the reference model
18A is depicted. It will be appreciated that while the embodiment
disclosed herein is described in terms of yaw rate gain, the
methodology is also applicable to lateral velocity. In an exemplary
embodiment the method is initiated by obtaining the nominal yaw
rate gain denoted in the equations as k.sub.d as depicted at block
310. At block 312 the longitudinal acceleration, denoted A.sub.x
and lateral acceleration, denoted A.sub.y are acquired. In an
exemplary embodiment, the accelerations are measured with onboard
sensors and signals therefrom filtered. Continuing with blocks 314
and 316 if the magnitude of the lateral acceleration, A.sub.y, and
longitudinal acceleration, A.sub.x exceed a selected first and
second threshold, then methodology transitions to block 318 for
computation of the throttle gain. Otherwise, if the magnitude of
either the longitudinal, A.sub.x or lateral acceleration A.sub.y is
less than the selected first or second threshold, then the yaw rate
gain is maintained at the nominal value as depicted at block 320.
At block 318 a throttle gain is established as a function of the
longitudinal acceleration A.sub.x. In an exemplary embodiment a
look up table is employed to establish the throttle gain, however
other implementations are readily conceivable. At block 322 the
sign of the longitudinal acceleration A.sub.x is determined. It
will be appreciated that in an exemplary embodiment, a positive
longitudinal acceleration, A.sub.x is indicative of the application
of throttle (depending on sign convention), while a negative
longitudinal acceleration, A.sub.x is indicative of reduction or
removal of throttle. At blocks 324, and 326 the yaw gain is
scheduled as a function of the nominal yaw gain, throttle gain, a
selected threshold, and the longitudinal acceleration, A.sub.13.
Finally, at block 328 the computation of the desired yaw rate is
depicted, in this embodiment including scheduling as a function of
the throttle.
[0072] Closed-Loop Control
[0073] Continuing with FIG. 1 and FIG. 2 and turning now to the
closed loop control and specifically the feedback control 24. In an
exemplary embodiment of the control system 10 the feedback is
enhanced to include closed loop control based on pole placement. A
second order transfer function to relate the yaw rate to the
rear-wheel steer control is given by: 11 P 2 = r = b r1 s + a 12 b
r2 - a 22 b r1 s 2 - ( a 11 + a 12 ) s + a 11 a 22 - a 12 a 21
where a 11 = - ( a 2 C f + b 2 C r ) I z V x a 12 = - aC f + bC r I
z V x a 21 = - aC f + bC r MV x - V x a 22 = - ( C f + C r ) MV x b
r1 = - bC r I z . ( 20 )
[0074] Once again, from FIG. 2, assume the yaw rate feedback
control transfer function .sup.G.sub.fb can express as a
proportional and a derivative term associated with the yaw rate
error
G.sub.fb=K.sub.p+K.sub.ds (21).
[0075] The closed-loop transfer function of the system P.sub.2
subjected to yaw rate feedback control is: 12 C ( s ) R ( s ) = K d
b r1 s 2 + [ K d + K p b r1 ] s + K p ( 1 + K d b r1 ) s 2 + [ - a
11 - a 22 + K d + K p b r1 ] s + + K p . ( 22 )
[0076] Using the pole placement design method, the proportional
gain K.sub.p and derivative control gain K.sub.d are respectively
expressed as: 13 K p = ( 1 + K p b r1 ) n 2 - K d = ( 2 n n + a 11
+ a 22 ) + b r1 ( - n 2 + ) - 2 n n b r1 + + b r1 2 n 2 ( 23 )
[0077] where:
.eta.=a.sub.12b.sub.r2-a.sub.22b.sub.r1
.lambda.=a.sub.11a.sub.22-a.sub.12a.sub.21.
[0078] The tire cornering stiffness at the front and rear are
functions of the desired steady-state lateral acceleration and the
surface friction estimation. The cornering stiffness (y-axis)
reduces as the desired steady-state lateral acceleration increases
(x-axis). Furthermore, the proportional and derivative gains
increase when the surface friction reduces.
[0079] The closed-loop yaw rate feedback control for the steer
control is the summation of the proportional, derivative, and a
"window" integral term with saturation. The final closed-loop
control command includes the contribution from the yaw rate and
lateral velocity feedback.
[0080] Turning now to FIG. 4, a simplified flowchart depicting an
exemplary implementation of a method 200 for closed loop feedback
control in a rear wheel steering system. The method 200 initiates
with a determination of a desired yaw rate and a desired lateral
velocity. In an exemplary embodiment, these are based on the
reference model 18a as described with the equations above as
depicted at block 202. The method continues with the measurement
and processing of the actual vehicle parameters as shown and 204.
Based on the desired and measured parameters values, errors are
computed. In an exemplary embodiment, an error is computed for the
yaw rate as well as the derivative of yaw rate as shown at blocks
206 and 208 respectively. In addition, at block 210 an integral of
the yaw rate is computed employing well-known windowing techniques.
The windowing provides a means of approximating an integral, while
preventing excessive errors from accumulating. Moreover, in yet
another exemplary embodiment, the window duration may be a function
of vehicle speed.
[0081] Continuing with FIG. 4 the lateral velocity error is
calculated at block 212. At block 214 a surface friction estimation
is preformed to determine the coefficient of friction of the road
surface. At block 216 a proportional, and derivative gain are
computed employing a pole placement technique, to compute the
desired gains that will provide desired performance and meet
performance requirements. For example, the gains may be computed to
stabilize the rear wheel control system and provide a selected
response characteristic such as overshoot, steady state error,
settling time and the like, as well as combinations including at
least one of the foregoing. In an exemplary embodiment a pole
placement methodology is employed to solve the control equations
and determine the requisite gains, however, it will be appreciated
that other control system solution methodologies may be
employed.
[0082] Continuing once again with FIG. 4, and turning now to block
218, where the control gains may now be scheduled based on surface
friction. In an exemplary embodiment, the scheduling provides a
means of manipulating the feedback gains as the road surface
adhesion varies. Such an approach facilitates maintaining vehicle
dynamic response and stability over a wide array of operating
conditions. Finally, at block 220 the closed loop steering command,
denoted .DELTA..delta..sub.r is formulated based on the
proportional, integral and derivative yaw rate terms and the rate
of lateral velocity.
[0083] Continuing now with discussion of the characteristics of the
closed loop control 24, it will also be appreciated that the
vehicle's natural frequency and damping ratio are preferably
decreased as the vehicle speed is increased. An advantage of the
multi-order transfer function is that it allows a driver to choose
a desired handling characteristic of a vehicle by tuning
vehicle-speed-dependent parameters of the reference models.
[0084] Continuing now with FIGS. 1 and 2, the composite rear
steering control command denoted .delta..sub.r.sub..sub.--.sub.comp
is the summation of the dynamic feedforward control
.delta..sub.r.sub..sub.--.su- b.ff, and the feedback or closed loop
control denoted .DELTA..delta..sub.r. This composite rear wheel
steering command may be applied to one addition transfer function
denoted G.sub.s. The transfer function G.sub.s is optionally
introduced for modeling actuators separately. In an exemplary
embodiment, it provides a simple gain. Continuing with the figure,
the total rear steer control command denoted
.delta..sub.r.sub..sub.--.sub.tot is applied to the plants as
described above.
[0085] The disclosed invention can be embodied in the form of
computer or controller implemented processes and apparatuses for
practicing those processes. The present invention can also be
embodied in the form of computer program code containing
instructions embodied in tangible media 13, such as floppy
diskettes, CD-ROMs, hard drives, or any other computer-readable
storage medium, wherein, when the computer program code is loaded
into and executed by a computer or controller, the computer becomes
an apparatus for practicing the invention. The present invention
may also be embodied in the form of computer program code as a data
signal 15, for example, whether stored in a storage medium, loaded
into and/or executed by a computer or controller, or transmitted
over some transmission medium, such as over electrical wiring or
cabling, through fiber optics, or via electromagnetic radiation,
wherein, when the computer program code is loaded into and executed
by a computer, the computer becomes an apparatus for practicing the
invention. When implemented on a general-purpose microprocessor,
the computer program code segments configure the microprocessor to
create specific logic circuits.
[0086] It will be appreciated that the use of first and second or
other similar nomenclature for denoting similar items is not
intended to specify or imply any particular order unless otherwise
stated.
[0087] While the invention has been described with reference to an
exemplary embodiment, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments falling within the scope of the appended
claims.
* * * * *