U.S. patent application number 09/931853 was filed with the patent office on 2002-02-28 for electric power steering control system and method for controlling the electric power steering control system.
This patent application is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Inoue, Noriyuki, Kifuku, Takayuki, Kurishige, Masahiko, Nishiyama, Ryoji, Tsutsumi, Kazumichi.
Application Number | 20020026270 09/931853 |
Document ID | / |
Family ID | 18749546 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020026270 |
Kind Code |
A1 |
Kurishige, Masahiko ; et
al. |
February 28, 2002 |
Electric power steering control system and method for controlling
the electric power steering control system
Abstract
In the conventional electric power steering control system, a
steering force applied by a driver to a steering wheel is detected
and a torque of a motor is determined on the basis of the detected
steering force. In such a prior system, at the time of going round
a gentle curve, going round an intersection at an extremely low
speed or the like, any steering wheel return torque is not
generated unless the driver returns the steering wheel, and drive
feeling is not improved. First, a road surface reaction torque
estimator 15 estimates a road surface reaction torque on the basis
of a steering wheel angle. A neutral point learning unit 24 and a
neutral point compensator 25 learn a neutral point of the steering
wheel on the basis of the road surface reaction torque with respect
to the steering wheel angle. A return torque for returning the
steering wheel to the neutral point is computed from a difference
between a current angle of the motor and the learned neutral point.
As a result, the steering wheel returns to the neutral point under
the current driving condition without application of any force to
the steering wheel by the driver.
Inventors: |
Kurishige, Masahiko; (Tokyo,
JP) ; Inoue, Noriyuki; (Tokyo, JP) ;
Nishiyama, Ryoji; (Tokyo, JP) ; Tsutsumi,
Kazumichi; (Tokyo, JP) ; Kifuku, Takayuki;
(Tokyo, JP) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
700 THIRTEENTH ST. NW
SUITE 300
WASHINGTON
DC
20005-3960
US
|
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha
Tokyo
JP
|
Family ID: |
18749546 |
Appl. No.: |
09/931853 |
Filed: |
August 20, 2001 |
Current U.S.
Class: |
701/41 ;
180/443 |
Current CPC
Class: |
B62D 15/0245 20130101;
B62D 5/0466 20130101 |
Class at
Publication: |
701/41 ;
180/443 |
International
Class: |
B62D 006/00; B62D
011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2000 |
JP |
2000-261758 |
Claims
What is claimed is:
1. An electric power steering control system comprising: a motor
for applying an assisting torque to a steering wheel used for
manipulating running wheels of a vehicle; steering torque detecting
means for detecting a torque applied to said steering wheel by a
driver of said vehicle as a steering torque; motor angle detecting
means for detecting a rotating angle of said motor; road surface
reaction torque estimating means for estimating a road surface
reaction torque of said running wheels using said steering
torque;and neutral point learning means for learning a neutral
point of the steering wheel under the actual driving condition from
said estimated road surface reaction torque and the rotating angle
of said motor.
2. The electric power steering control system according to claim 1
further comprising: steering angle compensation means for computing
are turn torque to return said steering wheel to said neutral point
from a difference between the rotating angle of said motor and said
learned neutral point and for converting said return torque into an
electric current of said motor, whereby said return torque is
applied to said steering wheel whenever said steering wheel is
turned from said neutral point.
3. The electric power steering control system according to claim 1
further comprising: speed detecting means for detecting speed of
the vehicle; and means for stopping operation of the neutral point
learning means when said vehicle speed detected by said speed
detecting means is smaller than a predetermined value.
4. The electric power steering control system according to claim 3
further comprising rotating direction detecting means for
outputting different codes according to the rotating direction of
the motor, wherein the neutral point learning means learns a
neutral point in left steering and a neutral point in right
steering respectively according to the codes outputted by said
rotating direction detecting means at the time of learning the
neutral point and subsequently learns a neutral point by computing
an average of said neutral point in left steering and said neutral
point in right steering.
5. The electric power steering control system according to claim 3
further comprising rotating direction detecting means for
outputting different codes according to the rotating direction of
the motor, wherein the neutral point learning means learns a
neutral point on the basis of a value obtained by subtracting or
adding a preliminarily stored friction torque of a steering
mechanism according to said rotating direction from or to the
estimated road surface reaction torque.
6. The electric power steering control system according to claim 5
further comprising rotating speed detecting means for detecting a
rotating speed of the motor or angular acceleration detecting means
for detecting a rotating angular acceleration of the motor, wherein
the neutral point learning means learns a neutral point when at
least one of said rotating speed and said rotating angular
acceleration of said motor is smaller than a predetermined value
and does not learn the neutral point when the rotating speed or the
rotating angular acceleration of the motor is larger than said
predetermined value.
7. The electric power steering control system according to claim 5,
wherein the steering angle compensating means comprises a limiter
for limiting an output of the steering angle compensating means to
be within a predetermined level.
8. The electric power steering control system according to claim 4
further comprising: speed detecting means for detecting a vehicle
speed; a damping compensator for compensating a damping on the
basis of speed of the motor; and an inertia compensator for
compensating an inertia on the basis of acceleration of said motor;
wherein said damping compensator and said inertia compensator work
when said vehicle speed exceeds a predetermined level, and said
damping compensator and said inertia compensator stop working when
the vehicle speed does not exceed said predetermined level.
9. An electric power steering control system comprising: a motor
for applying an assisting torque to a steering wheel used for
manipulating running wheels of a vehicle; steering torque detecting
means for detecting a torque applied to said steering wheel by a
driver of said vehicle as a steering torque; motor angle detecting
means for detecting a rotating angle of said motor; motor current
setting means for setting a target value of an electric current
flowing in said motor by receiving a torque command signal; motor
current detecting means for detecting a value of the electric
current flowing in said motor; road surface reaction torque
estimating means for estimating a road surface reaction torque of
said running wheels using said steering torque and the current of
said motor; neutral point learning means for learning a neutral
point of said steering wheel under the current driving condition
from said estimated road surface reaction torque and the rotating
angle of said motor; and steering angle compensation means for
computing a return torque to return said steering wheel to said
neutral point from a difference between the rotating angle of said
motor and said learned neutral point and outputting the return
torque to said motor current setting means, whereby the return
torque is applied to said steering wheel whenever said steering
wheel is turned from said neutral point.
10. The electric power steering control system according to claim 9
further comprising: speed detecting means for detecting speed of
the vehicle; and means for stopping operation of the neutral point
learning means when said vehicle speed detected by said speed
detecting means is smaller than a predetermined value.
11. The electric power steering control system according to claim
10 further comprising rotating direction detecting means for
outputting different codes according to the rotating direction of
the motor, wherein the neutral point learning means learns a
neutral point in left steering and a neutral point in right
steering respectively according to the codes outputted by said
rotating direction detecting means at the time of learning the
neutral point and subsequently learns a neutral point by computing
an average of said neutral point in left steering and said neutral
point in right steering.
12. The electric power steering control system according to claim
10 further comprising rotating direction detecting means for
outputting different codes according to the rotating direction of
the motor, wherein the neutral point learning means learns a
neutral point on the basis of a value obtained by subtracting or
adding a preliminarily stored friction torque of a steering
mechanism according to said rotating direction from or to the
estimated road surface reaction torque.
13. The electric power steering control system according to claim
12 further comprising rotating speed detecting means for detecting
a rotating speed of the motor or angular acceleration detecting
means for detecting a rotating angular acceleration of the motor,
wherein the neutral point learning means learns a neutral point
when at least one of said rotating speed and said rotating angular
acceleration of said motor is smaller than a predetermined value
and does not learn the neutral point when the rotating speed or the
rotating angular acceleration of the motor is larger than said
predetermined value.
14. The electric power steering control system according to claim
12, wherein the steering angle compensating means comprises a
limiter for limiting an output of the steering angle compensating
means to be within a predetermined level.
15. The electric power steering control system according to claim
11 further comprising: speed detecting means for detecting a
vehicle speed; a damping compensator for compensating a damping on
the basis of speed of the motor; and an inertia compensator for
compensating an inertia on the basis of acceleration of said motor;
wherein said damping compensator and said inertia compensator work
when said vehicle speed exceeds a predetermined level, and said
damping compensator and said inertia compensator stop working when
the vehicle speed does not exceed said predetermined level.
16. A method for controlling an electric power steering control
system including: a step of detecting a motor angle in which a
rotating angle of a motor for applying an assisting torque to a
steering wheel used for manipulating running wheels of a vehicle is
detected; a step of estimating a road surface reaction torque in
which a road surface reaction torque is estimated using a steering
torque applied to said steering wheel by a driver of said vehicle
and a current signal of said motor; a step of learning a neutral
point in which a neutral point of said steering wheel is learned
from said road surface reaction torque and the rotating angle of
said motor; and a step of compensating a steering angle in which a
torque for returning said steering wheel to said neutral point is
computed from said neutral point and the rotating angle of said
motor.
17. The method for controlling an electric power steering control
system according to claim 16 further including: a step of detecting
a vehicle speed; and a step of stopping an operation of the step of
learning a neutral point when said vehicle speed is lower than a
predetermined value.
18. The method for controlling an electric power steering control
system according to claim 16 further including: a step of detecting
a rotating direction in which different codes are outputted
according to the rotating direction of the motor; and a step of
learning a neutral point by learning a neutral point in left
steering and a neutral point in right steering respectively
according to the codes of the rotating direction outputted at the
time of learning the neutral point and subsequently computing an
average of said neutral point in left steering and said neutral
point in right steering.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention relates to a control system of an
electric power steering apparatus for vehicles in which an electric
motor generates a torque for assisting a steering torque generated
through manipulation of a steering wheel by a driver and to a
method for controlling the system.
[0003] 2. Background Art
[0004] FIG. 9 is a block diagram showing a construction of a
conventional electric power steering control system similar to, for
example, a system described in the Japanese Patent Publication
(unexamined) No. 186994/1995.
[0005] In the drawing, reference numeral 1 is a steering torque
detector for detecting a steering torque generated in a steering
wheel when a driver manipulates the steering wheel. Numeral 2 is a
steering torque controller for computing an assisting (auxiliary)
torque and outputting an assisting torque signal on the basis of an
output of the steering torque detector 1. Numeral 3 is a motor
speed detector for detecting a motor speed. Numeral 4 is a damping
compensator for computing a damping compensation signal on the
basis of the motor speed detected by the motor speed detector 3.
Numeral 5 is a motor acceleration detector for detecting a motor
acceleration using an output of the motor speed detector 3. Numeral
6 is an inertia compensator for computing an inertia compensation
signal on the basis of the motor acceleration detected by the motor
acceleration detector 5.
[0006] Numeral 7 is a judgment (determination) unit for judging
whether or not the direction of the output of the steering torque
detector 1 and that of the output of the motor speed detector 3 are
the same. The result of judgment is outputted to each of the
steering torque controller 2, the damping compensator 4, and the
inertia compensator 6.
[0007] Numeral 8 is a first adder for computing a sum (i.e., this
sum is a target torque) of the assisting torque signal, the damping
compensation signal, and the inertia compensation signal. Numeral 9
is a motor current determiner for computing a target current signal
from the target torque computed by the first adder 8. Numeral 10 is
a motor in which a motor current value corresponding to an applied
voltage is generated. By this motor 10, an assisting torque that is
approximately in proportion to the motor current value is
generated, and a steering mechanism not shown is driven. Numeral 11
is a motor current detector for detecting a current value of the
motor 10. Numeral 12 is a second adder for obtaining a difference
between the target current signal outputted by the motor current
determiner 9 and the motor current value detected by the motor
current detector 11. Numeral 13 is a motor drive for determining a
voltage to be applied to the motor 10 on the basis of the
difference between the target current signal computed by the motor
current determiner 9 and the motor current value detected by the
motor current detector 11. The motor drive 13 applies the
determined voltage to the motor 10. Numeral 14 is a speed detector
for detecting a speed of the vehicle and outputting the detected
speed signal to the steering torque controller 2, the damping
compensator 4, and the inertia compensator 6, respectively.
[0008] Described below is operation of the conventional electric
power steering control system of above construction.
[0009] When a driver of a vehicle manipulates a steering wheel not
shown, the steering torque detector 1 and outputted to the steering
torque controller 2 measures a steering torque applied to the
steering wheel. The steering torque controller 2 computes the
assisting torque signal that is approximately in proportion to the
output signal of the steering torque detector 1. Accordingly, the
motor 10 is driven and an assisting torque is generated on the
basis of the assisting torque signal, and a steering torque of the
driver is assisted to reduce the steering torque that the driver
feels.
[0010] At this time, the judgment unit 7 judges whether or not the
direction of the output of the steering torque detector 1 and the
direction of the output of the motor speed detector 3 are the same.
If they are judged the same, the damping compensator 4 and the
inertia compensator 6 do not work, but only the steering torque
controller 2 works. The steering torque controller 2 determines an
assisting torque signal according to the output of the steering
torque detector 1 and the speed signal from the speed detector 14.
A target torque is determined on the basis of the determined
assisting torque signal, and the motor current determiner 9
determines a motor drive current.
[0011] In the case that the direction of the output of the steering
torque detector 1 and the direction of the output of the motor
speed detector 3 are not the same, the steering torque controller 2
does not work, but the damping compensator 4 and the inertia
compensator 6 work. In this case, the target torque is determined
on the basis of the outputs of the damping compensator 4 and the
inertia compensator 6, and the motor current determiner 9
determines the motor drive current.
[0012] The direction of the target torque is arranged to be the
same as that of rotation of the motor when the vehicle is driven at
a low speed. The direction is arranged to be opposite to that of
rotation of the motor when the vehicle is driven at a high speed.
Therefore, when the driver is turning (steering) the steering
wheel, the steering torque of the driver is assisted so as to
reduce the torque necessary for the steering manipulation. When the
driver is returning the steering wheel, the motor 10 is controlled
so as to assist the steering wheel to return to the starting point
if the vehicle is driven at a low speed and prevents the steering
wheel from returning at an excessive speed of rotation if the
vehicle is driven at a high speed.
[0013] The foregoing flow is shown in FIG. 10. The flow shown in
this drawing is hereinafter referred to as main routine. For better
understanding, FIG. 11 shows the relation between the directions of
the steering torque caused by increase and decrease in speed of the
vehicle and of the motor speed.
[0014] In general, the driver turns the steering wheel when the
vehicle goes round a curve or an intersection. Then the driver
returns the steering wheel utilizing a spontaneous returning force
of the steering wheel due to road surface reaction torque of tires
when the vehicle goes back to a straight running after going round
the curve or the intersection. However, when the vehicle is driven
at a low speed or when the vehicle is driven at a high speed but
the steering wheel is steered a little, the road surface reaction
torque of the tires is weak. Consequently, the road surface
reaction torque is smaller than friction torque in the steering
mechanism, and in many cases the steering wheel does not return
when the vehicle goes back to the straight running. To cope with
this problem, in the prior art shown in FIGS. 9 to 11, whether or
not the output of the steering torque detector 1 and that of the
motor speed detector 3 are the same is judged when the vehicle is
driven at a low speed. If the outputs are not the same, the motor
drive current is established so as to rotate the motor 10 in the
same direction as the motor rotation, thereby the returning
property of the steering wheel in driving at a low speed is
improved. However, in most cases, it is necessary for the driver to
add a torque to the steering wheel in order to return the steering
wheel, which results in a phenomenon that steering sense or feeling
is deteriorated.
[0015] In the prior art, when the steering wheel is manipulated
within a range where the road surface reaction torque of the tires
is small such as going round a curve at a low speed or going round
a gentle curve at a high speed, the steering wheel stops and the
motor 10 will not rotate unless the driver applies a certain torque
in the direction of returning the steering wheel. That is, in this
case, it is not possible for the judgment unit 7 to judge whether
or not the direction of the output of the steering torque detector
1 and that of the output of the motor speed detector 3 are the
same. Therefore it is not possible for the steering torque
controller to establish the torque (motor driving current) so as to
rotate the motor in the same direction as that of the motor
rotation. Consequently, a problem exists in that returning property
of the steering wheel is not improved because any electric current
does not flow in the motor.
SUMMARY OF THE INVENTION
[0016] The present invention was made to resolve the
above-discussed problems and has an object of obtaining an electric
power steering apparatus capable of returning a steering wheel
without application of a torque in returning direction of the
steering wheel when the steering wheel is manipulated within a
range wherein road surface reaction torque of tires is small such
as a case of going round a curve at a low speed or a case of going
round a gentle curve at a high speed, controlling returning
property of the steering wheel by using a target steering angle
(the target steering angle is 0.degree. when the driver wants to
return the steering wheel to the original point) corresponding to
the will of the driver, thereby improving the returning property of
the steering wheel in any driving condition and improving
convergence and damping performance after unhanding the steering
wheel by using the steering angle. The invention also provides a
method for controlling the electric power steering apparatus.
[0017] An electric power steering control system according to the
invention comprises:
[0018] a motor for applying an assisting torque to a steering wheel
used for manipulating running wheels of a vehicle;
[0019] steering torque detecting means for detecting a torque
applied to the steering wheel by a driver as a steering torque;
[0020] motor angle detecting means for detecting a rotating angle
of the motor;
[0021] road surface reaction torque estimating means for estimating
a road surface reaction torque of the running wheels using the
steering torque;
[0022] neutral point learning means for learning a neutral point of
the steering wheel under the actual driving condition from the
estimated road surface reaction torque and the rotating angle of
the motor; and
[0023] steering angle compensation means for computing are turn
torque to return the steering wheel to the neutral point from a
difference between the rotating angle of the motor and the learned
neutral point and for converting the return torque into an electric
current of the motor, whereby the return torque is applied to the
steering wheel whenever the steering wheel is turned from the
neutral point.
[0024] Another electric power steering control system according to
the invention comprises:
[0025] a motor for applying an assisting torque to a steering wheel
used for manipulating running wheels of a vehicle;
[0026] steering torque detecting means for detecting a torque
applied to the steering wheel by a driver as a steering torque;
[0027] motor angle detecting means for detecting a rotating angle
of the motor;
[0028] motor current setting means for setting a target value of an
electric current flowing in the motor by receiving a torque command
signal;
[0029] motor current detecting means for detecting a value of the
electric current flowing in the motor;
[0030] road surface reaction torque estimating means for estimating
a road surface reaction torque of the running wheels using the
steering torque and the current of the motor;
[0031] neutral point learning means for learning a neutral point of
the steering wheel under the current driving condition from the
estimated road surface reaction torque and the rotating angle of
the motor; and
[0032] steering angle compensation means for computing a return
torque to return the steering wheel to the neutral point from a
difference between the rotating angle of the motor and the learned
neutral point and outputting the return torque to the motor current
setting means, whereby the return torque is applied to the steering
wheel whenever the steering wheel is turned from the neutral
point.
[0033] It is preferable that the electric power steering control
system is provided with:
[0034] speed detecting means for detecting speed of the vehicle;
and
[0035] means for stopping operation of the neutral point learning
means when the vehicle speed detected by the speed detecting means
is smaller than a predetermined value.
[0036] It is also preferable that the electric power steering
control system is provided with rotating direction detecting means
for outputting different codes according to the rotating direction
of the motor, and the neutral point learning means learns a neutral
point in left steering and a neutral point in right steering
respectively according to the codes outputted by the rotating
direction detecting means at the time of learning the neutral point
and subsequently learns a neutral point by computing an average of
the neutral point in left steering and the neutral point in right
steering.
[0037] It is also preferable that the electric power steering
control system is provided with rotating direction detecting means
for outputting different codes according to the rotating direction
of the motor, and the neutral point learning means learns a neutral
point on the basis of a value obtained by subtracting or adding a
preliminarily stored friction torque of a steering mechanism
according to the rotating direction from or to the estimated road
surface reaction torque.
[0038] It is preferable that the electric power steering control
system is provided with rotating speed detecting means for
detecting a rotating speed of the motor or angular acceleration
detecting means for detecting a rotating angular acceleration of
the motor, and the neutral point learning means learns a neutral
point when at least one of the rotating speed and the rotating
angular acceleration of the motor is smaller than a predetermined
value and does not learn the neutral point when the rotating speed
or the rotating angular acceleration of the motor is larger than
the predetermined value.
[0039] It is preferable that the steering angle compensating means
is provided with a limiter for limiting an output of the steering
angle compensating means to be within a predetermined level.
[0040] It is preferable that the electric power steering control
system is provided with:
[0041] speed detecting means for detecting a vehicle speed;
[0042] a damping compensator for compensating a damping on the
basis of speed of the motor; and
[0043] an inertia compensator for compensating an inertia on the
basis of acceleration of the motor;
[0044] wherein the damping compensator and the inertia compensator
work when the vehicle speed exceeds a predetermined level, and the
damping compensator and the inertia compensator stop working when
the vehicle speed does not exceed the predetermined level.
[0045] A method for controlling an electric power steering control
system according to the invention includes:
[0046] a step of detecting a motor angle in which a rotating angle
of a motor for applying an assisting torque to a steering wheel
used for manipulating running wheels of a vehicle is detected;
[0047] a step of estimating a road surface reaction torque in which
a road surface reaction torque is estimated using a steering torque
applied to the steering wheel by a driver and a current signal of
the motor;
[0048] a step of learning a neutral point in which a neutral point
of the steering wheel is learned from the road surface reaction
torque of the running wheels and the rotating angle of the motor;
and
[0049] a step of compensating a steering angle in which a torque
for returning the steering wheel to the neutral point is computed
from the neutral point and the rotating angle of the motor.
[0050] It is preferable that the method for controlling an electric
power steering control system includes:
[0051] a step of detecting a vehicle speed; and
[0052] a step of stopping an operation of the step of learning a
neutral point when the vehicle speed is lower than a predetermined
value.
[0053] It is also preferable that the method for controlling an
electric power steering control system includes:
[0054] a step of detecting a rotating direction in which different
codes are outputted according to the rotating direction of the
motor; and
[0055] a step of learning a neutral point by learning a neutral
point in left steering and a neutral point in right steering
respectively according to the codes of the rotating direction
outputted at the time of learning the neutral point and
subsequently computing an average of the neutral point in left
steering and the neutral point in right steering.
[0056] Since the invention is composed as described above,
following advantages are performed.
[0057] The rotating angle of the steering wheel is detected using
the motor angle in which the neutral point has been corrected on
the basis of the signal of estimated road surface reaction torque.
As a result, the neutral point of the steering wheel can be grasped
without using a steering wheel angle sensor. The steering wheel
return torque is generated on the basis of the grasped neutral
point of the steering wheel. As a result, in the case that the
steering wheel does not return such as going round a gentle curve,
the steering wheel spontaneously returns without the driver's
returning operation of the steering wheel, and it is possible to
construct a power steering control system superior in drive
feeling.
[0058] The road surface reaction torque is estimated on the basis
of the actual electric current detected by the detector. As a
result, as compared with the case of using a target electric
current, the neutral point of the steering wheel angle is learned
more accurately even when there is an offset between the target
value and the actual electric current.
[0059] It is arranged that the neutral point is not learned when
the vehicle is driven at a low speed by using the vehicle speed
detecting means. As a result, it is possible to improve accuracy in
detecting the neutral point and generate a favorable and
appropriate assisting torque of the motor.
[0060] The electric power steering control system is provided with
the motor angle speed detecting means. The neutral point of the
steering angle is learned by learning the neutral point in left
steering and that in right steering according to the code of the
motor angle speed outputted at the time of learning the neutral
point and by computing an average of the neutral point in left
steering and that in right steering. As a result, both in left and
right steering, friction components of the steering mechanism of
approximately the same intensity but in the opposite direction are
automatically cancelled. Accuracy in detecting the neutral point is
further improved without preliminarily grasping the intensity of
friction of the steering mechanism, and as a result it is possible
to generate a favorable and appropriate assisting torque of the
motor at all times.
[0061] The neutral point is learned by subtracting the friction
torque in the left and right steering as an offset quantity from
the road surface reaction torque estimated by the road surface
reaction torque estimating means. As a result, it is possible to
further improve accuracy in detecting the neutral point and
generate a favorable and appropriate assisting torque of the motor
at all times.
[0062] The electric power steering control system is constructed so
that the neutral point may be learned within the predetermined
range of the motor angle speed or within the predetermined range of
the motor angle acceleration. Therefore, it is possible to learn
the neutral point only under the steering conditions that the
neutral point can be learned most accurately, therefore accuracy in
detecting the neutral point is not deteriorated. As a result, it is
possible to generate a favorable and appropriate assisting torque
of the motor at all times.
[0063] The electric power steering control system has a
construction in which the limiter limits the output for correcting
and controlling the fundamental target current. As a result, even
if the neutral point of the steering angle is erroneously learned,
it is possible to generate an assisting torque with which the
driver can easily recover the neutral point, thereby improving
safety.
[0064] The electric power steering control system is provided with
the damping compensator and the inertia compensator, and they are
controlled to work or not to work according to the speed of the
vehicle. As a result, it is possible to obtain an electric power
steering control system superior in drive feeling.
[0065] The method for controlling an electric power steering
control system according to the invention includes the steps of
estimating a road surface reaction torque, learning a neutral point
of the steering wheel from the road surface reaction torque,
correcting a rotating angle of the motor on the basis of this
neutral point, and compensating a steering angle by computing a
torque for returning the steering wheel to the neutral point from
the rotating angle. As a result, it is possible to generate a
torque for returning the steering wheel to the neutral point
without using a steering wheel angle sensor and without driver's
returning manipulation of the steering wheel.
[0066] The step of learning a neutral point is stopped when the
speed of the vehicle is lower than the predetermined value. As a
result, it is possible to prevent the neutral point from erroneous
judging and control the electric power steering control system more
accurately.
[0067] The neutral point is learned by learning the neutral point
in left steering and that in right steering according to the
rotating direction of the motor and by using an average of them as
the original neutral point. As a result, it is possible to
recognize the neutral point more accurately.
BRIEF DESCRIPTION OF THE DRAWINGS
[0068] FIG. 1 is a block diagram showing a construction of an
electric power steering control system according to Embodiment 1 of
the present invention.
[0069] FIG. 2 is a flowchart explaining operation of the electric
power steering control system in FIG. 1.
[0070] FIG. 3 is a graph explaining the operation in FIG. 1.
[0071] FIG. 4 is a flowchart explaining operation of an electric
power steering control system according to Embodiment 2 of the
invention.
[0072] FIG. 5 is a characteristic diagram showing the relation
between speed of the vehicle and a learning weighting coefficient
in the flowchart in FIG. 4.
[0073] FIG. 6 is a flowchart explaining operation of an electric
power steering control system according to Embodiment 3 of the
invention.
[0074] FIG. 7 is a block diagram showing a construction of an
electric power steering control system according to Embodiment 6 of
the invention.
[0075] FIG. 8 is a block diagram showing a construction of an
electric power steering control system according to Embodiment 7 of
the invention.
[0076] FIG. 9 is a block diagram showing a construction of a
electric power steering control system according to the prior
art.
[0077] FIG. 10 is a flowchart explaining operation of the electric
power steering control system in FIG. 9.
[0078] FIG. 11 is a characteristic diagram showing the operation of
the electric power steering control system in FIG. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0079] Embodiment 1
[0080] FIG. 1 is a block diagram showing an electric power steering
control system according to Embodiment 1 of the present
invention.
[0081] In FIG. 1, reference numeral 1 is a steering torque detector
(means for detecting steering torque) for detecting a steering
torque (Tsens) applied to the steering wheel when the driver
manipulates the steering wheel. Numeral 2 is a steering torque
controller for computing an assisting torque signal to be given by
a motor on the basis of an output of the steering torque detector
1. Numeral 3 is a motor speed detector (rotating speed detecting
means) for detecting a motor speed and also detecting a rotating
direction thereof (rotating direction detecting means) at the same
time. Numeral 4 is a damping compensator for computing a damping
compensation signal on the basis of the motor speed detected by the
motor speed detector 3. Numeral 5 is a motor acceleration detector
for detecting a motor acceleration using an output of the motor
speed detector 3. Numeral 6 is an inertia compensator for computing
an inertia compensation signal on the basis of the motor
acceleration detected by the motor acceleration detector 5.
[0082] Numeral 7 is a judgment unit for judging whether or not the
direction of the output of the steering torque detector 1 and that
of the output of the motor speed detector 3 are the same. This
judgment unit 7 outputs the result of judgment to each of the
steering torque controller 2, the damping compensator 4, and the
inertia compensator 6.
[0083] Numeral 8 is a first adder for computing the sum (target
torque) of the assisting torque signals outputted by the damping
compensator 4, the inertia compensator 6 and the steering torque
controller 2, and the output signal of a steering angle compensator
26. Numeral 9 is a motor current determiner (motor current setting
means) for computing a target current signal from the target torque
computed by the first adder 8. Numeral 10 is a motor in which a
motor current value corresponding to an applied voltage is
generated and an assisting torque approximately in proportion to
the motor current value is generated. The assisting torque is
applied to the steering wheel used for manipulating the running
wheels of the vehicle. Numeral 11 is a motor current detector
(motor current detecting means) for detecting the current value (I)
of the motor 10.
[0084] Numeral 12 is a second adder for obtaining a difference
between the target current signal outputted by the motor current
determiner 9 and the motor current value detected by the motor
current detector
[0085] Numeral 13 is a motor drive for determining a voltage to be
applied to the motor 10 on the basis of the difference between the
target current signal computed by the motor current determiner 9
and the motor current value detected by the motor current detector
11. The determined voltage is applied to the motor 10. Numeral 14
is a speed detector (speed detecting means) for detecting a vehicle
speed and outputting a signal of the detected speed to the steering
torque controller 2, a neutral point learning unit 24, the damping
compensator 4, the inertia compensator 6, and the steering angle
compensator 26.
[0086] Numeral 15 is a road surface reaction torque estimator (road
surface reaction torque estimating means) for estimating a road
surface reaction torque. The road surface reaction torque functions
to restore a steering angle of front wheels to a starting point,
(this starting point does not indicate a mechanical central angle
but indicates a balance point of rotating force of the steering
wheel changing depending on the condition of the road surface and
on the driving condition). That is, the road surface reaction
torque return the steering wheel using the steering torque detected
by the steering torque detector 1 and the motor current value
detected by the motor current detector 11.
[0087] Numeral 23 is a motor angle detector (motor angle detecting
means) for detecting a rotating angle of the motor, outputs a zero
point of the motor angle and a present angle (hereinafter referred
to as motor angle) and also detects a motor rotating angle
acceleration (motor angle acceleration detecting means). Numeral 24
is a neutral point learning unit for computing a zero point (this
is the neutral point and changes sometimes during driving the
vehicle as described above) of the steering wheel rotation. The
computation of the zero point is performed on the basis of the
motor angle detected by the motor angle detector 23 and the road
surface reaction torque estimated by the road surface reaction
torque estimator 15, thereby learning the neutral point. Numeral 25
is a neutral point corrector (neutral point correcting means) for
computing a link gear ratio with respect to the motor 10 and the
steering wheel not shown using the output of the motor angle
detector 23. This neutral point corrector 25 further computes a
relative value of the steering angle, corrects the result of the
computation using the neutral point learning unit 24, and detects
an absolute value of the steering angle. The neutral point learning
unit 24 and the neutral point corrector 25 are collectively
referred to as neutral point learning means.
[0088] Numeral 26 is a steering angle compensator (steering angle
compensating means) for computing a steering angle compensation
signal (i.e., steering wheel return torque) on the basis of the
absolute value of the steering angle detected by the neutral point
corrector 25.
[0089] A portion surrounded with a one-dot chain line is the
portion introduced according to the invention, and a portion
outside the portion surrounded with the one-dot chain line is the
same as in the conventional electric power steering control system
shown in FIG. 9.
[0090] Concerning the operation of the electric power steering
control system in FIG. 1, operation performed by the characterizing
part of the invention is hereinafter described with reference to
the flowchart shown in FIG. 2.
[0091] Parameters j, k, .DELTA.K shown in the flowchart are
preliminarily established to be reset to zero when the power source
of the control system is switched from OFF to ON and a program
stored in a ROM of the control system starts its operation. The
flow in FIG. 2 is carried out at the predetermined time intervals
in the flow of FIG. 10 showing the mentioned conventional art.
[0092] In step 101, the steering torque is read in and stored.
[0093] In step 102, the motor current is read in and stored.
[0094] In step 103, the road surface reaction torque estimator 15
computes a road surface reaction torque Treact based on the
following Expression (1) using a steering torque signal Tsens and
the motor current signal I (this is hereinafter referred to as a
step of estimating a road surface reaction torque).
Treact=Tsens+Kt.multidot.I (1)
[0095] In this expression, Kt indicates a torque constant of the
motor (steering shaft conversion).
[0096] where: the motor angle (rotating angle of the motor shaft)
is read in and stored as .theta.m (this is called a step of
detecting a motor angle).
[0097] In step 105, whether or not an absolute value of the road
surface reaction torque Treact is smaller than a predetermined
minute value Treact0 is judged. If the result of judgment is YES,
the step proceeds to step 106. If the result of judgment is NO, the
step proceeds to step 113, and a road surface reaction estimating
arithmetical counter K is incremented by 1 in step 113 from K to
K+1, and this processing routine is completed.
[0098] In step 106, whether or not K-K0 is larger than a
predetermined value of .DELTA.K is judged, and if the result of
judgment is YES, the step proceeds to step 107. If the result of
judgment is NO, the step proceeds to step 113. This step is
intended to prevent that judgment of YES are made many times
immediately after a judgment of YES is once made in step 105 due to
noise or the like without steering to left and right at a
predetermined frequency.
[0099] In step 107, if the result of judgment in step 106 is YES,
K0 is reset as expressed by K0=K.
[0100] In step 108, the motor angle .theta.m is stored as a
displacement .DELTA..theta.m(j) from the neutral point at the jth
time.
[0101] In step 109, a neutral point learning counter j is
incremented by 1 from j to j+1.
[0102] In step 110, whether or not the neutral point learning
counter j is equal to 2 is judged. If the result of judgment is
YES, it is judged that steering was carried out in such a manner
that the steering wheel is turned to left and right passing the
neutral point two times, and the step proceeds to step 111. If the
result of judgment is NO, the step proceeds to step S113.
[0103] In step 111, displacement of the motor angle from the
neutral point in the case of passing the neutral point two times is
computed by arithmetic averaging, and the result of computation is
stored as a neutral point learned value .theta.m (0) (this is
hereinafter referred to as a step of learning a neutral point). The
step proceeds to step 112, and the neutral point learning counter j
is reset to zero in step 112.
[0104] Steps 109 to 112 are carried out by the neutral point
learning unit 24.
[0105] In step 113, the road surface reaction estimating
arithmetical counter K is incremented by 1 (from K to K+1), and
this processing routine is finished.
[0106] The neutral point correcting quantity detected in this
processing routine is established as a neutral point offset value
.theta.m(offset).
[0107] Using the neutral point corrector 25, after obtaining a
result of subtracting .theta.moffset from the motor angle .theta.m,
conversion of a link gear rate from the motor to a steering column
shaft is further carried out, thus a steering wheel angle .theta.s
is obtained (this is hereinafter referred to as a step of
correcting a neutral point). Then, a
proportional-differential-integral controlled variable is computed
on the basis of a difference between the Os and a target steering
angle .theta.s0 (this is the learned neutral point.theta.m0, and in
most cases the target is 0.degree. because the target is to return
the steering wheel to the original point). The controlled variable
is added to a fundamental steering torque controlled variable as
the steering wheel return torque (this is hereinafter referred to
as a step of compensating a steering angle). On the basis of the
obtained result, a target motor current value is determined, and
the electric current is controlled so that the motor current may
coincide to the target value.
[0108] It is also possible to obtain a similar result when the
foregoing step of obtaining the steering wheel angle .theta.s is
omitted and the return torque is directly obtained from the
difference between the motor angle .theta.m and the neutral point
learned value .theta.m (0). In this embodiment, the neutral point
learning counter j is immediately reset to zero when the neutral
point learning counter j indicates 2 in steps 110 to 112. However,
it is also preferable to reset the neutral point learning counter j
when it indicates a numeral larger than 2. For example, it is also
preferable that an arithmetic average of the displacement
.DELTA..theta.m from the neutral point is continuously obtained
until the neutral point learning counter j indicates 100. It is
also preferable to use a moving average in 100 times of the
displacement .DELTA..theta.m from the latest neutral point instead
of carrying out the arithmetic averaging.
[0109] Described below is the reason why it is possible to detect
the road surface reaction torque from the foregoing Expression
(1).
[0110] An equation of motion of the steering mechanism can be shown
in the following Expression (3).
J.times.d.omega.s/dt=Thd1+Tmtr-Tfric.multidot.sign(cos)-Treact
(3)
[0111] where: d.omega.s/dt: rotating acceleration of steering wheel
shaft
[0112] Thd1: steering torque
[0113] Tmtr: motor output torque (steering wheel shaft
conversion)
[0114] Tfric: friction torque in the steering mechanism
[0115] Treact: road surface reaction torque (steering wheel shaft
conversion)
[0116] As a result of solving Expression (3) for the road surface
reaction torque Treact, a following Expression (4) is obtained.
Treact=Thd1+Tmtr-J.times.d.omega.s/dt-Tfric.multidot.sign(.omega.s)
(4)
[0117] Accordingly, it is possible to compute the road surface
reaction Treact from the steering torque, motor output torque,
steering wheel shaft rotation acceleration, and friction torque in
the steering mechanism.
[0118] It is possible to substitute the steering torque signal
Tsens for the steering torque Thd1.
[0119] It is possible to substitute a value obtained by multiplying
the motor current detection signal I by the torque constant for the
motor output torque Tmtr.
[0120] It is possible to substitute a motor acceleration signal d
.omega. for the steering wheel shaft rotation acceleration, and the
value of this term is so small that can be ignored in general
unless an extremely sharp steering is conducted.
[0121] The influence Tfric of the friction torque in the steering
mechanism in left steering and that in right steering are
approximately the same in intensity and opposite in direction. It
is therefore possible to cancel the influence of the friction
torque Tfric by computing the displacement of the motor angle from
the neutral point through arithmetic averaging when the steering
wheel is turned left and right and by storing the result of the
computation as the neutral point learned value .theta.m0.
[0122] For better understanding, FIG. 3 shows how the road surface
reaction torque changes when the steering wheel is turned left and
right. The axis of abscissas in FIG. 3 indicates a time axis, and
this axis does not indicate a specific time but simply indicates a
flow of time during the turning of the steering wheel by the
driver. The axis of ordinates indicates a steering angle (300) and
an intensity of road surface reaction torque (301).
[0123] The road surface reaction in the drawing is a road surface
reaction estimated in the foregoing Expression (1). The zero point
of the road surface reaction is not coincident to the zero point of
the steering angle due to influence of the friction torque.
However, it is possible to cancel the influence of the friction
torque by arithmetically averaging a neutral steering angle 1
(shown in the drawing) learned at the road surface reaction zero
point in left steering and a neutral steering angle 2 (shown in the
drawing) learned at the road surface reaction zero point in right
steering and using the obtained result as a neutral learned value.
As a result, it is possible to learn the steering angle neutral
point on the basis of the road surface reaction torque detected in
the foregoing Expression (1).
[0124] Embodiment 2
[0125] An electric power steering control system according to
Embodiment 2 of the invention has the same construction as the
system in FIG. 1, but has a different processing flow.
[0126] The vehicle speed is not taken into consideration when the
neutral point is learned in the flowchart in FIG. 2 in the
foregoing Embodiment 1. But the flow in Embodiment 2 takes the
vehicle speed into consideration as shown in the flowchart of FIG.
4.
[0127] That is, the flow in FIG. 4 is carried out before the
neutral point is learned (for example, between steps 110 and 111)
with reference to the flow in FIG. 2 of the foregoing Embodiment
1.
[0128] More specifically, in step 201, an average speed is read in
at first (this is hereinafter referred to as a step of detecting a
vehicle speed). If it is judged in step 202 that the vehicle speed
is not lower than a predetermined value, the neutral point
correcting quantity.theta.m0 is read-in in step 203, and the
neutral point offset value .theta.moffset is established to be
.theta.moffset=.theta.m0 in step 204.
[0129] If it is judged in step 202 that the vehicle speed is lower
than the predetermined value, this processing routine is closed
without changing the neutral point offset value .theta.moffset.
[0130] The .theta.moffset is preliminarily established to be reset
to an initial value when the power source of the control system is
switched on and the program stored in the ROM in the control system
starts its operation. This processing routine is carried out at
predetermined time intervals in the main routine, description of
which is omitted.
[0131] Computation accuracy is prevented from lowering by carrying
out the foregoing process and not using the neutral point offset
value as .theta.moffset when the neutral point offset value is
learned under the conditions that the road surface reaction force
is small and the computation cannot be carried out with
accuracy.
[0132] In the description of this embodiment, the system is
constructed so that only the neutral point correcting
quantity.theta.m0 learned when the vehicle speed is higher than a
predetermined value may be used. It is also preferable that a
weighting coefficient W to be learned corresponding to the vehicle
speed is predetermined and stored in a memory of the control
system. Then the current neutral point correcting quantity
.theta.m01 and the previous neutral point correcting quantity
.theta.m02 are established to be an expression of
.theta.moffset=(1-W) .theta.m02+W.multidot..theta.m01, and the
learning weight coefficient W is established to be increased as the
vehicle speed increases as shown in FIG. 5. Thus, it is established
that the result of the neutral point learned under the condition
that the vehicle is driven at a low speed may be hardly reflected.
The weight coefficient W herein is 0.ltoreq.W.ltoreq.1.
[0133] Embodiment 3
[0134] In the foregoing Embodiment 1, when the neutral point
learning counter j indicates 2, it is judged that left and right
steering is performed, and the average of .DELTA..theta.m(j) is
obtained. It is also preferable to take a process shown in the flow
of FIG. 6. Steps 101, 102, 103, 105, 106, and 107 in FIG. 6 are the
same as the steps of the same numerals in FIG. 2, and further
description thereof is omitted herein. In step 304, a rotating
speed signal of the motor is read in, and the steering direction is
recognized on the basis of the code of the rotating speed signal of
the motor (this is hereinafter referred to as a step of detecting
the rotating direction). In step 308, a displacement
.DELTA..theta.m_L from the neutral point in left steering and a
displacement .DELTA..theta.m_R from the neutral point in right
steering are distinguished according to the steering direction and
stored. In step 309, a left steering counter and a right steering
counter are incremented according to the respective steering
directions. If both of them comes to not less than 1 in step 310,
the step proceeds to step 311, and a value obtained by computing
(.DELTA..theta.m_L+.DELTA..theta.m_R)/2 on the basis of the
displacement .DELTA..theta.m_L from the neutral point in left
steering and on the basis of the displacement .DELTA..theta.m_R
from the neutral point in right steering is used as the neutral
point offset value .theta.moffset (this is hereinafter referred to
as a second step of correcting the neutral point).
[0135] In this embodiment, the neutral point learning counter is
reset to zero when both of the neutral point learning counters j_L
and j_R come to not less than 1. It is also preferable that the
counter is reset to zero when both of the counters come to not less
than 100, for example. In this case, in step 310, a value obtained
by computing (.DELTA..theta.m_L_ave+.- DELTA..theta.m_R_ave)/2 on
the basis of an arithmetic average .DELTA..theta.m_L_ave of the
displacement .DELTA..theta.m_L from the neutral point in left
steering and on the basis of an arithmetic average
.DELTA..theta.m_R _ave of the displacement .DELTA..theta.m_R from
the neutral point in right steering is used as the neutral point
offset value .theta.moffset.
[0136] Embodiment 4
[0137] In general, it is preferable to construct a steering
mechanism so that steering in bilateral symmetry may be secured
when the steering wheel is turned left and right. However, the
steering wheel is not positioned in the middle of the width
direction of the vehicle in most cases, and therefore the structure
thereof is bilaterally asymmetrical. As a result, in some operation
area subject to an influence of nonlinear element due to friction
or the like, bilaterally symmetrical steering is not secured in
some cases. In order to cope with this problem, it is preferable to
carry out the process of subtracting the friction torque of the
steering left and right as the offset quantity from the road
surface reaction torque estimated by the road surface reaction
torque estimating means in step 103 in FIG. 2 and learn the neutral
point on the basis of this result. The following expression is used
in this process.
Treact=Tsens+Kt.multidot.I-Tfric.multidot.sign(.omega.) (2)
[0138] where: sign(.omega.) is a code of the motor speed .omega..
According to this code, the friction torque Tfric stored in the ROM
(not shown) in the control system is looked up from a table stored
in advance in the system.
[0139] Embodiment 5
[0140] Under actual driving conditions, in specific driving pattern
such as changing lane, the steering wheel is turned left and right
and the neutral point is learned more accurately in a certain
steering pattern. Steering speed and steering acceleration in such
a specific driving pattern remain within a predetermined range in
many cases. In this Embodiment 5, for the purpose of reducing
learning frequency except the steering pattern in which the neutral
point can be learned with accuracy, the rotating angle detecting
means for detecting a rotating angle of a motor is inputted to the
neutral point learning means for learning a neutral point of the
steering angle. And the neutral point is learned when the steering
speed and the steering acceleration remain within a predetermined
range. Alternatively, referring to FIG. 2, it is also preferable
that the rotating angle of the motor is read in and the motor speed
and its acceleration are computed in step 104. And whether or not
the motor speed and acceleration are in a predetermined range is
judged at the same time when intensity of the absolute value of the
road surface reaction torque is judged in step 105.
[0141] Embodiment 6
[0142] In the foregoing Embodiments 1 to 5, it is preferable that
the torque generated by the steering angle compensator 26 does not
exceed a predetermined value even when the neutral point learning
unit 24 learns erroneously the neutral point so that the driver may
easily recover the neutral point. For that purpose, it is preferred
that an output limiter 16 is provided in the latter stage of the
steering angle compensator 26. As a result of this, the output of
the steering angle compensator 26 shown in FIG. 7 does not exceed a
predetermined value, and it is possible to prevent a dangerous
driving situation in the case of malfunction.
[0143] Embodiment 7
[0144] In the construction of FIG. 1 showing Embodiment 1, the
damping compensator 4, the inertia compensator 6, the judgment unit
7, the motor acceleration detector 5, and the motor speed detector
3 have been conventionally used as described in the drawing
explaining the conventional example referring to FIG. 10. These
elements are not always required in the constructions according to
the foregoing Embodiments 1 to 6. FIG. 8 shows a construction
without them. Operation referring to FIG. 8 is the same as that of
the flow in FIG. 2 showing the foregoing Embodiment 1, and further
description thereof is omitted herein.
* * * * *