U.S. patent application number 13/380942 was filed with the patent office on 2012-05-03 for method and a system for changing a vehicle's trajectory.
This patent application is currently assigned to VOLVO LASTVAGNAR AB. Invention is credited to Jochen Pohl, Sten Ragnhult, Jan-Inge Svensson.
Application Number | 20120109465 13/380942 |
Document ID | / |
Family ID | 43411241 |
Filed Date | 2012-05-03 |
United States Patent
Application |
20120109465 |
Kind Code |
A1 |
Svensson; Jan-Inge ; et
al. |
May 3, 2012 |
METHOD AND A SYSTEM FOR CHANGING A VEHICLE'S TRAJECTORY
Abstract
A method for changing a vehicle's trajectory, wherein the
vehicle includes a steering arrangement including a manual steering
device, at least one pair of ground engaging members and a
mechanical interconnection therebetween, includes the steps of
applying a braking force to at least one of the ground engaging
members so that the vehicle's trajectory is changed, and
simultaneously suppressing steering device disturbances resulting
from the mechanical interconnection.
Inventors: |
Svensson; Jan-Inge;
(Goteborg, SE) ; Pohl; Jochen; (Goteborg, SE)
; Ragnhult; Sten; (Onsala, SE) |
Assignee: |
VOLVO LASTVAGNAR AB
Goteborg
SE
|
Family ID: |
43411241 |
Appl. No.: |
13/380942 |
Filed: |
June 29, 2009 |
PCT Filed: |
June 29, 2009 |
PCT NO: |
PCT/SE2009/000339 |
371 Date: |
December 27, 2011 |
Current U.S.
Class: |
701/42 |
Current CPC
Class: |
B60T 2201/083 20130101;
B60W 10/20 20130101; B60W 10/18 20130101; B62D 15/025 20130101;
B60T 8/1708 20130101; B60T 8/17557 20130101; B60W 30/12 20130101;
B62D 6/008 20130101; B62D 5/0472 20130101 |
Class at
Publication: |
701/42 |
International
Class: |
B62D 6/00 20060101
B62D006/00 |
Claims
1. A method for changing a vehicle's trajectory, wherein the
vehicle comprises a steering arrangement comprising a manual
steering device, at least one pair of ground engaging members and a
mechanical interconnection between the manual steering device and
the ground engaging members, wherein the method includes the step
of applying a braking force to at least one of the ground engaging
members so that the vehicles trajectory is changed, comprising
using a reference generator for steering system control,
simultaneously suppressing steering device disturbances resulting
from the mechanical interconnection, wherein the reference
generator is arranged to continuously receive an input signal
indicative of a steering intent of a driver and based on that
signal output a signal indicative of a desired steering torque,
which desired steering torque is compared to the actual steering
torque of the vehicle, and wherein the output signal is
continuously adapted so that the desired steering torque is
transmitted to the driver.
2. A method according to claim 1, comprising receiving a signal
indicative of a current driving scenario, determining if it is
desired to change the vehicle's trajectory by braking the at least
one ground engaging member based on the driving scenario signal in
order to avoid an undesired situation and if so automatically
applying the braking force to the at least one ground engaging
member.
3. A method according to claim 2, wherein the undesired situation
represents a predicted departure from a predicted desired future
trajectory of the vehicle.
4. A method according to claim 1, characterized by suppressing the
steering device disturbances resulting from the mechanical
interconnection by decoupling a driver steering feel from the
influence of the mechanical connection.
5. A method according to claim 1, characterized by suppressing the
steering device disturbances resulting from the mechanical
interconnection by receiving a signal indicative of a steering
angle and/or steering torque in the steering arrangement,
determining if the steering angle will result in steering device
disturbances, determining if it is desired to apply a force
counteracting the steering device disturbances resulting from the
steering angle and/or steering torque and if so generating a
corresponding signal to an actuator arranged to apply such a
counteracting force to the steering arrangement.
6. A method according to claim 1, comprising providing the driver
of the vehicle with a desired steering feel based on a determined
desired guiding force, which is applied to the steering device.
7. A method according to claim 5, comprising providing the driver
of the vehicle with a desired steering feel based on a determined
desired guiding force, which is applied to the steering device, and
applying the determined guiding force to the steering device via
the actuator.
8. A method according to claim 6, comprising receiving at least one
signal indicative of a vehicle state and determining the guiding
force based on the at least one vehicle state signal,
9. A method according to claim 8, comprising determining a value
and direction of the guiding force for changing the vehicle's
future trajectory.
10. A method according to claim 8, comprising comparing the
determined guiding force with a limit value, and if the determined
guiding force exceeds the limit value applying the braking force to
the at least one ground engaging members so that the vehicle's
trajectory is changed.
11. A method according to claim 10, comprising limiting the guiding
force applied to the steering device to the limit value or
below.
12. A method according to claim 6, comprising determining the
guiding force based on at least one steering device guiding force
operation model.
13. A method according to claim 12, wherein the at least one
guiding force operation model comprises at least one desired
steering characteristic parameter.
14. A method according to claim 13, wherein the at least one
desired steering characteristic parameter comprises at least
lateral acceleration and/or yaw rate and that the method comprises
the step of modifying and/or canceling the effect of the lateral
acceleration and/or yaw rate during the application of the braking
force to the at least one ground engaging member.
15. A method according to claim 1, wherein the steering device
comprises a steering wheel and that the guiding force forms a
guiding torque applied to the steering wheel.
16. A method according to claim 1, comprising applying a brake
pressure to at least one brake configured to brake the at least one
of the ground engaging wheels.
17. A system for changing a vehicle's trajectory, wherein the
vehicle comprises a steering arrangement comprising a manual
steering device, at least one pair of ground engaging members and a
mechanical interconnection between the manual steering device and
the ground engaging members wherein the system comprises an
arrangement for applying a braking force to at least one of the
ground engaging members so that the vehicle's trajectory is
changed, and an arrangement for suppressing steering device
disturbances resulting from the mechanical interconnection, wherein
a reference generator is operatively connected to the arrangement
and arranged to continuously receive an input signal indicative of
a steering intent of a driver and based on that signal output a
signal indicative of a desired steering torque, which desired
steering torque is compared to the actual steering torque of the
vehicle, and wherein the output signal is continuously adapted so
that the desired steering torque is transmitted to the driver.
18. A system according to claim 17, wherein the arrangement for
suppressing steering device disturbances comprises means for
detecting a steering angle and/or steering torque in the steering
arrangement, means for determining if the steering angle and/or
steering torque will result in steering device disturbances and
generating a corresponding signal and an actuator arranged to
receive the signal and responsively apply a force counteracting the
disturbances to the steering arrangement.
19. A system according to claim 17, wherein the system comprises
means for receiving a signal indicative of a current driving
scenario, determining if it is desired to change the vehicle's
trajectory by braking based on the driving scenario signal and
generating a corresponding braking force signal to the means for
applying a braking force.
20. A system according to claim 19, wherein the means for
determining if it is desired to change the vehicle's trajectory is
configured to generate a signal to a means for controlling the
steering arrangement.
21. A system according to claim 17, wherein the system comprises
means for providing the driver of the vehicle with a desired
steering feel based on a determined desired guiding force.
22. A system for changing a vehicle's trajectory, wherein the
vehicle comprises a steering arrangement comprising a manual
steering device, at least one pair of ground engaging members and a
mechanical interconnection between the manual steering device and
the ground engaging members, wherein the system comprises an
arrangement for applying a braking force to at least one of the
ground engaging members so that the vehicle's trajectory is
changed, and an arrangement for suppressing steering device
disturbances resulting from the mechanical interconnection, wherein
a reference generator is operatively connected to the arrangement
and arranged to continuously receive an input signal indicative of
a steering intent of a driver and based on that signal output a
signal indicative of a desired steering torque, which desired
steering torque is compared to the actual steering torque of the
vehicle, and wherein the output signal is continuously adapted so
that the desired steering torque is transmitted to the driver.
23. A system according to claim 22, wherein the arrangement for
suppressing steering device disturbances comprises a control
function for detecting a steering angle and/or steering torque in
the steering arrangement, a control function for determining if the
steering angle and/or steering torque will result in steering
device disturbances and generating a corresponding signal and an
actuator arranged to receive the signal and responsively apply a
force counteracting the disturbances to the steering
arrangement.
24. A system according to claim 22, wherein the system comprises a
control function for receiving a signal indicative of a current
driving scenario, determining if it is desired to change the
vehicle's trajectory by braking based on the driving scenario
signal and generating a corresponding braking force signal to the
arrangement for applying a braking force.
25. A system according to claim 24, wherein the control function
for determining if it is desired to change the vehicle's trajectory
is configured to generate a signal to a controller for controlling
the steering arrangement.
26. A system according to claim 22, wherein the system comprises an
actuator for providing the driver of the vehicle with a desired
steering feel based on a determined desired guiding force.
Description
BACKGROUND AND SUMMARY
[0001] The present invention relates to method and a system for
changing a vehicle's trajectory, wherein the vehicle comprises a
steering arrangement comprising a manual steering device, at least
one pair of ground engaging members and a mechanical
interconnection between the manual steering device and said ground
engaging members.
[0002] A number of active safety functions aim at changing the
vehicles future trajectory, for reasons such as avoiding a
collision, a roadway departure or obtaining a proper lane position.
A change in trajectory can be done through a change in steering
wheel angle, which is exactly what the driver does while driving
the vehicle.
[0003] Thus, the vehicle's trajectory may be changed due to an
undesired situation such as a departure from a desired future
trajectory of the vehicle. In other words, the invention is
applicable for so-called lane keeping of a vehicle during
operation. A current driving scenario that represents an
unintentional lane departure may be determined based on the
vehicle's position, direction and/or orientation with respect to a
traffic lane (or road edge). Further, there are systems known,
which are configured for monitoring the course of the traffic lane
ahead of the vehicle, such as by monitoring lane markings using a
vision system. The lane keeping support system is preferably
configured to provide such guiding force only in situations in
which the supply of such guiding force to the steering device is
deemed to be appropriate after analysis of all input data, such as
the course of the traffic lane ahead of the vehicle, further
vehicles on the road and a predicted driving behaviour of the
vehicle.
[0004] The guiding force exerted onto the steering device is
resistive if counteracting the force applied by the driver onto the
steering device, or supportive if acting in the same direction as
the force applied by the driver onto the steering device, thus for
instance reducing the effect of e.g. frictional forces acting on
the wheels and the like which are experienced by the driver as
resistance when operating the steering device. The steering device
is normally formed by a conventional steering wheel in the case of
a vehicle. However, the invention is applicable to other steering
devices, such as a joystick, a sliding nipple or any other suitable
steering device for steering the vehicle. For instance, in the case
that the steering device is a steering wheel, the guiding force
will appear as a guiding torque exerted onto the steering
wheel.
[0005] In many countries/regions there are legal requirements
limiting the allowable guiding force to be applied to the steering
device. According to a known method, the guiding torque applied to
the steering wheel is automatically limited to the allowed limit
during an intervention. However, such automatic limitation may lead
to that the intervention is unsuccessful since the intervention
could not be carried out to the desired extent.
[0006] It is desirable to achieve a method for changing a vehicle's
trajectory which creates conditions for a further improved safety
during operation, especially in case there is a predefined limit
for the amount of a steering device guiding force.
[0007] A method according to an aspect of the invention includes
applying a braking force to at least one of said ground engaging
members so that the vehicle's trajectory is changed, and
simultaneously suppressing steering device disturbances resulting
from the mechanical interconnection.
[0008] The term "ground engaging members" comprises wheels fitted
with tyres, but may also cover other types of ground engaging
members, such as caterpillar tracks.
[0009] The braking force is preferably applied by means of a
differential braking force to said pair of ground engaging
members.
[0010] Thus, the vehicles trajectory can be changed by differential
braking,--where a longitudinal force is generated by applying a
brake pressure to a single wheel or a wheel pair (such as front and
rear at the same side). The resulting longitudinal force generates
a torque around the vehicles centre of gravity, thereby changing
the vehicle trajectory.
[0011] A problem with prior art solutions for differential braking
on the front axle is that a disturbing torque is introduced into
the steering system which results in a steering wheel angle. As a
consequence, a front axle sideslip angle is introduced, which
results in a lateral force. This force can act so that the
resulting torques of lateral and longitudinal force around the
vehicle's centre of gravity counteract or act in the same
direction. Which of these two alternatives that is the case depend
on the actual design of the front suspension. In any case, the
exact amount of the achieved steering wheel angle is dependent
whether the driver has the hands on the wheel or not, and which
steering wheel angle that is allowed by the driver. In other words,
the loop gain of the transfer function between lateral acceleration
(or yaw rate) and brake pressure (or wheel brake torque) is
dependent on both the actual vehicle front suspension and the
driver.
[0012] By the inventive step of simultaneously suppressing steering
device disturbances resulting from the mechanical interconnection,
the problem of the disturbing torque introduced into the steering
system is relieved.
[0013] According to an example embodiment, the method comprises the
step of receiving a signal indicative of a current driving
scenario, determining if it is desired to change the vehicle's
trajectory by braking said at least one ground engaging member
based on the driving scenario signal in order to avoid an undesired
situation and if so automatically applying the braking force to
said at least one ground engaging member. Thus, changing the
vehicle's trajectory can be performed also in additional ways to
braking said at least one ground engaging member. One such
additional way is to change the vehicle's trajectory by steering
via the steering arrangement. For example a guiding force is
applied to the steering device for changing the vehicle's
trajectory. Preferably, the guiding force is applied during a
driver steering operation. Preferably, the guiding force applied is
only supportive, i.e. it is limited to such an extent that the
driver still has full authority to steer the vehicle. However, the
system may be configured to take control of the vehicle and in the
case of lane keeping, return it to a safe position in the original
lane.
[0014] Preferably, the undesired situation represents a predicted
departure from a predicted desired future trajectory of the
vehicle.
[0015] According to a further example embodiment, the method
comprises the step of suppressing said steering device disturbances
resulting from the mechanical interconnection by decoupling a
driver steering feel from the influence of the mechanical
connection. Thus, this embodiment creates conditions for decoupling
the hardware (mechanical connection) from the steering feel. In
other words, the embodiment creates conditions for an
application-independent (hardware-independent) steering feel.
[0016] According to a further example embodiment, the method
comprises the step of suppressing said steering device disturbances
resulting from the mechanical interconnection by receiving a signal
indicative of a steering angle and/or steering torque in the
steering arrangement, determining if the steering angle will result
in steering device disturbances, determining if it is desired to
apply a force counteracting the steering device disturbances
resulting from the steering angle and/or steering torque and if so
generating a corresponding signal to an actuator arranged to apply
such a counteracting force to the steering arrangement.
[0017] Preferably, a steering torque is determined by measuring the
twist of a torsion bar in the steering arrangement. More precisely,
a first angular sensor is arranged at a first end of the torsion
bar and a second angular sensor is arranged at a second end of the
torsion bar (opposite the first end). The steering torque can be
determined based on the relative angular movement (twist) of the
torsion bar and the stiffness of the torsion bar. According to an
alternative, one or several strain gauges may be used.
[0018] Such a method may be performed via an Electronic Power
Assisted System (EPAS) comprising a controlling function, below
referred to as a reference generator, which is configured to
determine a desired torque to be applied to the steering wheel in
order to provide the driver with a desired steering feel. In other
words, the reference generator describes a nominal vehicle.
[0019] Thus, there is a mechanical connection between the steering
device and the ground but the inherent steering feel resulting from
the mechanical connection during operation is eliminated or at
least suppressed. In other words, the t guiding force is
continuously determined during operation so that the driver
experiences a desired feel in the steering device instead of the
inherent steering feel resulting from the mechanical
connection.
[0020] When a reference generator is used for steering system
control, then the disturbance torque is automatically compensated
for, and thus no front axle slip angle is produced, and thus no
lateral force. The influence of both suspension geometry and driver
is thus effectively removed from the loop. The braking function
therefore becomes a much more predictable process.
[0021] Therefore, according to a further example embodiment, the
method comprises the step of providing the driver of the vehicle
with a desired steering feel based on a determined desired guiding
force, which is applied to the steering device. Preferably, the
method comprises the step of applying the determined guiding force
to the steering device via said actuator. From an implementation
point-of-view it is wise to add guiding torque via the actuator if
the actual torque is lower than the desired torque for a determined
steering feel and to cancel guiding torque via the actuator if the
actual torque is higher than the desired torque for the determined
steering feel.
[0022] According to a further example embodiment, the method
comprises the step of receiving at least one signal indicative of a
vehicle state and determining the guiding force based on said at
least one vehicle state signal. Preferably, the method comprises
the step of determining a value and direction of the guiding force
for changing the vehicle's future trajectory. The value of the
guiding force may be determined for assisting the driver in
changing the vehicle's future trajectory.
[0023] According to a further example embodiment, the method
comprises the step of comparing the determined guiding force with a
limit value, and if the determined guiding force exceeds the limit
value applying the braking force to said at least one ground
engaging members so that the vehicle's trajectory is changed.
[0024] The limit value can represent a maximum allowable torque,
which the EPAS can add due to legal requirements or internal safety
requirements. Preferably, the changing of the vehicle's trajectory
is performed via the steering arrangement and only if such a limit
is reached the brakes are used to increase the torque around the
vehicles centre of gravity, thereby contributing to the change the
vehicle trajectory or alternatively completely taking over the
steering.
[0025] Let's say we would like to steer the vehicle to the right
and the suspension geometry is as in one type of truck. We would
then brake the wheels on the right hand side of the vehicle. We
would get a torque around the centre of gravity turning the vehicle
in the clockwise direction. Without the reference generator
function the front right wheel would get a disturbance angle to the
right. With the reference generator function this disturbance angle
would be removed leading to a much more predictable steering by
braking.
[0026] According to a further example embodiment, the method
comprises the step of limiting the guiding force applied to the
steering device to the limit value or below. Thus, in addition to
the steering by braking, a steering effect is achieved by means of
the steering arrangement.
[0027] According to a further example embodiment, the method
comprises the step of determining the guiding force based on at
least one steering device guiding force operation model.
Preferably, the method comprises the steps of continuously during
operation determining the guiding force, continuously applying the
determined guiding force to the steering device, and
modifying/interrupting the determined guiding force if the limit
value on steering device torque is reached.
[0028] Preferably, said at least one guiding force operation model
comprises at least one desired steering characteristic parameter.
Preferably, said at least one desired steering characteristic
parameter comprises at least one of damping of steering device
movements, tire friction, self alignment of the steering device to
a neutral position and friction in a mechanical connection between
the steering device and the wheels. Especially, said at least one
desired steering characteristic parameter comprises at least
lateral acceleration and/or yaw, rate and the method comprises the
step of modifying and/or canceling the effect of the lateral
acceleration and/or yaw rate.
[0029] A steering by braking will give a lateral acceleration (or
yaw rate) on the vehicle which will give a deviation between the
real vehicle behaviour and the behaviour estimated by the vehicle
model in the reference generator function. >>This deviation
will give a torque in>the steering wheel that will work against
the steering by braking. To avoid this the measured lateral
acceleration (or yaw rate) needs to be decoupled during a steering
by braking. Alternatively this torque in the steering wheel can be
compensated for since we know how much torque around the centre of
gravity the steering by braking will give.
[0030] The desired lateral acceleration (yaw rate) from steering by
braking will give a desired torque around the vehicles centre of
gravity by using the estimated Inertia of the vehicle. From the
desired torque around the vehicles centre of gravity brake forces
can be calculated if the track width is known.
[0031] Further, to be able to know how much torque around, the
vehicles centre of gravity the brakes will contribute with it's
advantageous to have a good estimate of the brake factors of the
brakes. The brake factor relates the brake pressure to the brake
torque and can vary due to e.g. speed, brake pressure, temperature
and contamination.
[0032] It is also desirable to achieve a system for changing a
vehicle's trajectory which creates conditions for a further
improved safety during operation, especially in case there is a
predefined limit for the amount of the guiding force.
[0033] A system for changing a vehicle's trajectory according to an
aspect of the invention is provided, wherein the vehicle comprises
a steering arrangement comprising a manual steering device, at
least one pair of ground engaging members and a mechanical
interconnection therebetween characterized in that the system
comprises an arrangement for applying a braking force to at least
one of said ground engaging members so that the vehicle's
trajectory is changed, and an arrangement for suppressing steering
device disturbances resulting from the mechanical
interconnection.
[0034] According to an example embodiment, said arrangement for
suppressing steering device disturbances comprises means for
detecting a steering angle and/or steering torque in the steering
arrangement, means for determining if the steering angle and/or
steering torque will result in steering device disturbances and
generating a corresponding signal and an actuator arranged to
receive the signal and responsively apply a force counteracting the
disturbances to the steering arrangement. Preferably, a delivered
steering device guiding force is measured and compared with an
estimated desired steering device guiding force, wherein the
delivered steering device guiding force is adapted by use of a
feedback controller to be substantially the same as the desired
steering device guiding force through adapting the amount of said
guiding force.
[0035] According to a further example embodiment, said system
comprises means for receiving a signal indicative of a current
driving scenario, determining if it is desired to change the
vehicle's trajectory by braking based on the driving scenario
signal and generating a corresponding braking force signal to said
means for applying a braking force.
[0036] According to a further example embodiment, said means for
determining if it is desired to change the vehicle's trajectory is
configured to generate a signal to a means for controlling the
steering arrangement.
[0037] According to a further example embodiment, said system
comprises means for providing the driver of the vehicle with a
desired steering feel based on a determined desired guiding
force.
[0038] Further preferred embodiment and advantages thereof emerge
from the description below, the figures and the claims.
BRIEF DESCRIPTION OF FIGURES
[0039] The invention will be described in greater detail below with
reference to the embodiment shown in the accompanying drawings, in
which
[0040] FIG. 1 schematically shows a system for performing the
inventive method according to one embodiment,
[0041] FIG. 2-3 shows two different examples of steering a vehicle
by braking
[0042] FIG. 4 schematically shows a work flow for the inventive
method according to one embodiment, and
[0043] FIG. 5 schematically shows a flow chart for the inventive
method according to one embodiment.
DETAILED DESCRIPTION
[0044] The invention is below described for application in a truck.
However, the invention should not be regarded as limited to trucks,
but it may be applied also in other vehicles, such as cars. FIG. 1
schematically shows a system 1 for performing a control
method--according to one embodiment. The system 1 comprises a
mechanical steering arrangement 2, which may be of a conventional
type. The mechanical stearing arrangement 2 comprises a steering
device 3 in the form of a steering wheel, at least one ground
engagement member 4 in the form of a wheel and a mechanical
connection 5 between the steering wheel 3 and the wheels 4 for
transmission of steering signals from the steering wheel 3 to the
wheels 4.
[0045] The steering wheel 3 is arranged in a vehicle passenger
compartment and manually operated by the driver of the vehicle to
steer the wheels 4. The steering arrangement 2 comprises a steering
linkage means 6 extending from the steering wheel 3 down to a
Hydraulic Power Assisted System (HPAS) 7 for converting angular
rotation in the steering linkage 6 to a linear movement via a
steering member 8. The steering linkage means 6 comprises an
electric steering gear. The HPAS may be of conventional type
comprising a hydraulic cylinder (not shown) and a torsion bar (not
shown). The steering member 8 is coupled on opposite ends to a left
and right wheel 4 and configured to turn the wheels 4 in response
to steering signals from the steering wheel 3.
[0046] The system 1 further comprises an actuator 9 to provide
supported adjustment of the steering angle. The actuator 9 is
preferably formed by an electric motor. The actuator 9 provides a
guiding force, and more specifically a guiding torque, or assist
torque, to the steering assembly for assisting the driver in
steering the steering wheel. The electric motor may be arranged
around a steering column in the steering arrangement 2, wherein the
magnetic field acts directly on the steering column. Alternatively,
the electric motor may be arranged beside the steering column and
act on the steering column via a mechanical linkage/, preferably
via pinion gears.
[0047] The system 1 further comprises a torque-measuring device 10
for measuring a manual torque applied by the driver to the steering
wheel. The torque-measuring device 10 is of elastic constitution
and preferably comprises a torsion bar. In other words, a steering
wheel angle is measured via the torsion bar. More specifically, the
electric steering gear comprises said torsion bar.
[0048] The system 1 further comprises an Electrical Power Assisted
Steering (EPAS) system 11. The EPAS 11 comprises a regulating loop
12, which is configured to achieve a torque-free steering. The
regulating loop 12 is configured to receive an input signal
indicative of a current steering torque in the steering wheel 3.
The input signal is received from the torque-measuring, device 10.
Basically, the regulating loop 12 is configured to output a signal
to the actuator 9 so that said torque free steering is
achieved.
[0049] The regulating loop 12 comprises a controller, or regulator,
27 which comprises a filter function. The filter function may be
based on an inverse model of the steering dynamics of the present
vehicle. Further, the regulator 27 may be configured to reduce
errors in the model and to reduce disturbances and measurement
noise in order to reduce the risk of instability in the system.
[0050] The regulator 27 is configured to receive a signal
indicative of a torque to be applied to the steering arrangement
via the electric motor and, in response thereto produce an output
signal. The regulating loop 12 further comprises an electric motor
control means 28, which is configured to receive the output signal
indicative of a torque from the regulator 27 and produce a signal
with a corresponding current value to the electric motor. According
to an alternative, the regulator 27 and the electric motor control
means 28 are combined in a single controller.
[0051] The EPAS further comprises a controlling function 13, below
referred to as a reference generator, which is configured to
determine a desired torque to be applied to the steering wheel in
order to provide the driver with a desired steering feel. In other
words, the reference generator describes a nominal vehicle."
[0052] Further, the reference generator 13 is operatively connected
to the regulating loop 12 and outputs a signal indicative of a
desired steering torque. The regulating loop is configured to
compare the desired steering torque to the actual, current steering
torque and continuously adapt the output signal to the actuator so
that the desired steering torque is transmitted to the driver. In
other words, the actuator is controlled so that it applies the
difference in torque between the desired torque value from the
reference generator and the current actual torque in the steering
assembly so that the actual torque is controlled to substantially
equal the desired torque.
[0053] The reference generator 13 comprises at least one steering
device guiding force operation model and in the example in FIG. 1 a
plurality of guiding force operation models 14,15,16,17,18. The
guiding force operation model preferably comprises a mathematical
model. The model (s) is designed in a way to achieve a desired
steering feel in the steering device. Thus, the model (s) can be
designed in different ways for different vehicle types and/or for
different desired steering feels.
[0054] Further, the model (s) comprises at least one desired
steering characteristic parameter. More specifically, each model is
configured to produce a guiding torque value T for one desired and
predetermined steering characteristic parameter based on at least
one input 19. In other words, the steering characteristic parameter
is a guiding force influencing operational parameter. Each model
comprises a mathematical function, wherein the torque value is
determined as a function of a value of the input, see illustrated
examples of the functions in FIG. 1.
[0055] The individual torque values resulting from the models are
summed up to a torque sum, which forms an output 20 from the
reference generator. According to the shown embodiment, the
reference generator comprises models for the following steering
characteristic parameters: vehicle lateral acceleration, damping of
steering device movements, tire friction, self alignment of the
steering device to a neutral position and friction in the
mechanical connection between the steering device and the
wheels.
[0056] The signals input to the reference generator comprises a at
least one signal indicative of a steering intent of the driver,
such as a steering wheel angle (.delta.) and a rate of change
of>>the steering wheel angle (d.delta./dt). According to an
alternative to the steering wheel angle, the signal indicative of a
steering intent may be an electric motor angle or a wheel angle.
According to an alternative to the rate of change of the steering
wheel angle, the signal indicative of a steering intent may be a
rate of change of the electric motor angle or a rate of change of
the wheel angle.
[0057] The signals input to the reference generator comprises at
least one signal indicative of a vehicle body motion, such as
lateral acceleration (Ay) and/or yaw rate. Such a vehicle body
motion may be sensed by a sensor arranged in the vehicle.
[0058] The vehicle lateral acceleration model 14 preferably
receives a signal indicative of a current lateral acceleration as
an input signal. According to a preferred example, the vehicle
lateral acceleration is the most important steering characteristic
parameter.
[0059] The damping model 15 represents a predetermined relationship
between a guiding torque value and the current steering wheel speed
for achieving a desired steering feel. Thus, the damping model 15
preferably receives a signal indicative of a steering wheel speed
(rate of change of the steering wheel position).
[0060] According to the example function shown in FIG. 1, the
torque value increases dramatically for small input values of the
steering wheel speed. Further, the torque value increases
substantially less for larger input values of the steering wheel
speed. In other words, the curve flattens out. The damping model 15
is preferably a pure statical mapping. The torque value output from
the damping model is configured to act i in an opposite direction
with regard to the current steering wheel speed. The damping model
is preferably designed so that the resulting torque is smaller for
higher steering wheel speeds and higher for smaller steering wheel
speeds. In this way, the damping torque is proportional to the
steering wheel angle speed during normal driving and limited to a
maximum value during parking or evasive maneuvers.
[0061] Thus, the vehicle lateral acceleration model 14 and the
damping model 15 are linked to each other.
[0062] The self alignment model 17 represents a predetermined
relationship between a guiding torque value and the current
steering wheel angle for achieving a desired steering feel. By self
alignment of the steering device to a neutral position is meant an
active return, i.e. the return of the released steering wheel to a
central setting. The self alignment model 17 preferably receives a
signal indicative of the steering wheel angle and a signal
indicative of vehicle speed as input signals. The purpose of the
vehicle speed input signal is to be able to modulate the desired
aligning torque with the current vehicle speed in a way that the
self alignment torque can be reduced during high speed driving.
[0063] Regarding the friction models 16,18, a certain amount of
friction feel in the steering wheel is desired. For example,
Coulomb friction is desired during on-centre handling in order to
achieve a desired torque build-up for small steering wheel angle
deviations. Further, Coulomb friction is as well desired while
driving long curves, so that the steering forces are reduced,
wherein the driver can "rest" the steering wheel on the
friction.
[0064] The tire model 16 comprises a hysteresis curve, which
represents a tire model. Preferably, the model 16 is a dynamic
model of an unrolling tire with regard to steering torque. The
relation between the steering wheel angle and the torque is given
by a physical relationship, where the deflection of individual
rubber elements is modeled dependent on the differential angle of
the steering wheel and the torsion and relaxation of the rubber
elements due to the rolling tire. The resulting model yields thus a
smaller hysteresis effect with increasing vehicle speed and
constant steering wheel angle frequency.
[0065] The inventive method, creates conditions for canceling the
actual friction effect in the steering wheel resulting from the
actual steering arrangement and instead applying a desired
resistance torque to the steering wheel, which represents a nominal
friction feel for the driver. Thus, the hardware (mechanical
steering arrangement) is decoupled from the friction steering feel.
In other words, the invention creates conditions an
application-independent (hardware-independent) friction steering
feel.
[0066] The tire friction model 16 and the mechanical connection
friction model 18 are in principle similar to each other. The tire
friction model 16 represents the friction between the tire and the
ground while the mechanical connection friction model 18 represents
the friction in the upper steering wheel steering column assembly.
Thus, the friction coefficient in the mechanical connection
friction model 18 is higher than in the tire friction model 16. The
tire friction model 16 preferably receives a signal indicative of a
steering wheel angle and a signal indicative of vehicle speed.
[0067] The mechanical connection friction model 18 preferably
receives a signal indicative of a steering wheel angle.
[0068] According to an example of the friction model 18, the value
of the steering wheel angle .delta. is multiplied by a stiffness K,
which corresponds to a lumped spring stiffness in Nm/Rad. The
resulting value is input to a Laplace operator s. The derived
steering wheel angle signal, i.e. the steering wheel angle speed
multiplied with the stiffness K is used in an integrating function
with anti windup functionality, indicated through the integrational
limits and the inverse of the laplace transformator. The limit
values are chosen in order to limit the frictional torque to the
desired maximum and minimum values. The mentioned anti-windup
functionality is intended to cease integration once the
integrational limits are reached. The relationship between the
steering wheel angle .delta. and the output torque value is
schematically shown in box 18 in FIG. 1.
[0069] The steering characteristic model (s) 14,15,16,17,18 is
preferably designed so that a different steering characteristic
parameter takes precedence over the others in different driving
scenarios. According to one example, the lateral acceleration is
configured to take precedence over the other steering
characteristic parameters during driving in high speed. According
to a further example, steering system friction and tire friction
are configured to take precedence over the other
steering-characteristic parameters during driving in low speed. The
damping torque is equally active regardless of vehicle speed.
According to a further example, the self alignment is configured to
take precedence over the other steering characteristic--parameters
during driving in an intermediate speed interval between the high
speed and the low speed.
[0070] The present invention concerns a method for assisting the
driver of the vehicle during operation. According to a preferred
embodiment, the control method is configured to allow a control of
the steering characteristics experienced by a driver of the vehicle
during traveling. In other words, the control method is configured
to provide the operator with a steering feel (or steering
sensitivity or tactile feedback) through the steering wheel.
[0071] With regard to friction feel, according to an example
embodiment, the method comprises the step of determining the
desired resistance torque based on an input representing a steering
angle. By determining a direction of the actual steering angle
(clockwise or counterclockwise) and instantly applying a torque in
the same direction, the effect of the friction in the steering
arrangement can be effectively cancelled.
[0072] The system 1 further comprises a safety function 21 in the
form of a lane keeping control function. The lane keeping control
function 21 is configured to avoid a departure from an intended
desired future trajectory of the vehicle. Lane keeping functions
are known and will not be described in detail here. The lane
keeping function 21 is configured to predict if a guiding force
to--a vehicle steering device is desired in order to avoid an
unintentional lane departure based on a current driving-scenario.
More specifically, the lane keeping control function 21 receives at
least one input 33 indicative of the current driving scenario and
responsively determines an output torque value 25.
[0073] The lane keeping control function 21 comprises a lane
monitoring system 33, preferably comprising a camera.
[0074] The lane monitoring system 33 produces a signal indicative
of a current lane position. The lane keeping control function 21
further receives a signal (not shown) indicative of a required lane
position and possibly a signal (not shown) indicative of a vehicle
speed.
[0075] The lane keeping control function 21 is configured to
determine (calculate) a lateral acceleration value based on the
required lane position and the vehicle speed. In other words, when
the vehicle approaches a curve a value of the lateral acceleration
is calculated for maintaining the vehicle in the desired lane
during the curve. The lane keeping control function 21 produces an
output signal 25 indicative of a corresponding torque value to the
steering arrangement 2,
[0076] According to one example, the steering arrangement 2 is
controlled in accordance with the output signal 25 from the safety
function 25 so that the vehicle's trajectory is changed accordingly
when the lane keeping function 21 decides that it is necessary.
[0077] According to the example shown in FIG. 1, the output torque
value 25 from the lane keeping control function 21 and the output
torque value 20 from the reference generator 13 are summed up to a
total torque value 26, which is indicative of a desired steering
torque to be applied to the steering wheel 3. The regulating loop
12 receives the total desired torque value 26.
[0078] Thus, the system is configured to decide whether to cancel a
guiding force contribution of at least one of said plurality of
desired steering characteristic parameters so that the predicted
total guiding force is sufficient for avoiding the undesired
situation during said intervention. The frictional effects 16,18
are preferably always cancelled during a lane keeping maneuver. The
effect of the lateral acceleration (in the model 14) and/or yaw
rate is modified and/or canceled during said application of the
braking force to said at least one ground engaging member.
[0079] The invention may be applied in a truck comprising a tractor
and a trailer having a plurality of wheel axles. Heavy motor
vehicles, such as load-carrying commercial vehicles, are normally
designed with different brake arrangements 40, for example wheel
brakes (i.e. disc brakes) 42,43,44,45, a hydraulic retarder and an
engine brake.
[0080] The safety function 21 is further configured for comparing
the determined guiding force with a limit value. If the determined
guiding force exceeds the limit value, a signal is sent to the
brake arrangement 40, which is configured to apply a braking force
to said at least one ground engaging member so that the vehicle's
trajectory is changed. The limit value can represent a maximum
allowable torque, which the EPAS can add due to legal requirements
or internal safety requirements. Thus, if such a limit is reached
the brakes 42,43,44,45 are used to increase the torque around the
vehicles centre of gravity, thereby contributing to the change the
vehicle trajectory or alternatively completely taking over the
steering.
[0081] The braking system 40 comprises a brake controller 41
configured to receive said brake signal and said plurality of brake
devices 42,43,44,45. Each brake device 42,43,44,45 is configured to
brake one of said wheels individually. The brake controller 41 is
configured to generate a brake signal to at least one of said
plurality of brake devices 42,43,44,45 in order to change the
vehicle's trajectory.
[0082] The brake arrangement 40 is preferably an electronically
controlled pneumatic brake arrangement. The brake controller 40
forms a computerized control unit which is arranged so as to
distribute the available brake pressure in a suitable manner
between the brakes of the tractor and the trailer (the brakes of
the trailer are not shown). This distribution function is sometimes
referred to as a "brake adaptation function" or between the tractor
and trailer as a "coupling force control". By means of such a
function, a high degree of brake compatibility, or brake balance,
within the tractor and between the tractor and the trailer can be
obtained. In other words, the aim with a brake adaptation function
is to distribute the brake pressure in an optimum manner within the
tractor and between the tractor and the trailer.
[0083] In order to control the brake balance of the tractor and the
trailer, the so-called "brake factor", or "brake gain", is used as
an input parameter to the brake control unit. The brake factor,
normally designated Bf, can be defined as a relation between the
received brake torque and the applied brake cylinder pressure for a
given wheel axle, i.e. Bf=Tbrake/Pcyi [Nm/bar/axle]
[0084] wherein Tbrake indicates the received brake torque for the
axle in question, and wherein Poyi indicates the applied brake
cylinder pressure for said axle. By determining a value which
represents the brake factor Bf for each axle of a vehicle, the
control unit may be operated so as to achieve the above-brake
adaptation function. More precisely, tests can be carried out for a
certain wheel axle wherein the applied brake pressure Poyi (i.e.
the pressure acting on a wheel brake disc by means of a
corresponding brake pad) is measured while the retardation of the
free rolling axle is also measured during braking. A value
representing the retardation can also be obtained by means of an
accelerometer. By measuring the retardation, a value representing
the brake torque Tbrake can be calculated. When calculating the
brake torque Tbraker certain factors such as the air resistance and
the rolling resistance must be compensated for. By using the values
of the brake torque Tbrake and the brake pressure Poyi, the brake
factor Bf can be calculated using the above-mentioned
relationship.
[0085] The braking of an axle can further be done when the driver
has requested auxiliary braking i.e. braking with an engine brake
or retarder. If each axle is braked in order from front to rear
with the same amount of brake force as the driver has requested
from the auxiliary brakes the brake factor for each axle can be
obtained without the driver feeling a disturbance. By dividing the
brake factor for the axle with two an estimate of the brake factor
for each wheel can be obtained.
[0086] Consequently, the brake factor Bf can be regarded as a value
which represents the efficiency of the brakes. Also, a low brake
factor may for example indicate possible malfunctions in the
brakes. For example, the contact area of the brakes might be
contaminated with dirt or rust, which means that it will need
conditioning.
[0087] FIGS. 2 and 3 shows two different examples of steering a
vehicle by braking. A longitudinal force is generated by applying a
brake pressure to a single wheel or a wheel pair (such as front and
rear at the same side). The resulting longitudinal force generates
a torque around the vehicles centre of gravity, thereby changing
the vehicle trajectory. FIGS. 2 and 3 indicates two situations, in
which we would like to steer the vehicle to the right. We would
then brake the wheels on the right hand side of the vehicle. We
would get a torque around the centre of gravity turning the vehicle
in the clockwise direction. FIG. 2 discloses a situation where the
brake is applied for steering the vehicle, but there is no steering
via the steering arrangement, i.e. the wheels are not angled in
relation to a longitudinal direction of the vehicle. FIG. 2
discloses a situation where the brake is applied for steering the
vehicle, and in addition the steering arrangement is steered, i.e.
the wheels are angled in relation to a longitudinal direction of
the vehicle.
[0088] FIG. 4 discloses a flow chart for an embodiment of the
control method. The method starts in box 401. The method comprises
the step of receiving 403 a signal indicative of a current driving
scenario and determining if it is desired to change the vehicle's
trajectory by braking said at least one ground engaging member
based on the driving scenario signal in order to avoid an undesired
situation. If it is determined that the vehicle's trajectory should
be changed by braking said at least one ground engaging member a
braking force is applied in the next step 405 to said at least one
ground engaging member. Simultaneously, steering device
disturbances resulting from the mechanical interconnection are
suppressed.
[0089] FIG. 5 discloses a flow chart for an embodiment of the
control method. The method starts in box 501. The method comprises
the step of receiving 503 a signal indicative of a current driving
scenario and determining if it is desired to change the vehicle's
trajectory by effecting the steering arrangement based on the
driving scenario signal in order to avoid an undesired situation.
If it is determined that the vehicle's trajectory should be changed
by steering a steering force is applied in the next step 505 to the
steering device. Simultaneously, steering device disturbances
resulting from the mechanical interconnection are suppressed. The
method then moves on to comparing 507 a steering device guiding
force with a limit value, and if the guiding force exceeds the
limit value, a braking force is applied to said at least one ground
engaging member for changing the vehicle's trajectory.
Simultaneously, steering device disturbances resulting from the
mechanical interconnection are suppressed. Especially, the effect
of a lateral acceleration in a model for determining steering feel
(which has been described above) is suppressed and preferably
canceled during said steering by braking.
[0090] If the braking force is predicted not to be desired in step
403 (or 503), the method goes back to start 401 (or 501) directly.
Likewise, if the predicted guiding force in step 507 does not
exceed the limit value, the method goes back to start 501 directly.
Further, the method is continuously repeated.
[0091] The steering angle is preferably determined by measuring a
steering wheel deflection. Alternatively, the steering angle may be
determined by measuring a wheel angle or anywhere in between the
steering wheel and the ground engaging wheel in the mechanical
steering arrangement.
[0092] Although the invention has above been described for lane
keeping, the invention is applicable for other active safety
functions, such as other path correction functions, such as side
wind compensation or collision avoidance (such as Emergency Lane
Assist, ELA). In other words, a lane guidance regulation system is
integrated into the EPAS. In the same way, further functions may be
integrated into an exemplary method according to the present
invention.
[0093] In order to further increase the driving stability of
vehicles, steering systems may include a driving dynamics regulator
that adjusts the setting of the steered wheels independently from
the steering wish of the driver.
[0094] The reference generator 13 and the regulating loop 12
(comprising the controllers 27,28) are preferably implemented in
software.
[0095] A value of the vehicle lateral acceleration may be estimated
from a measured vehicle yaw rate.
* * * * *