Note: Descriptions are shown in the official language in which they were submitted.
CA 02497729 2007-10-19
1
DESCRIPTION
POWER STEERING APPARATUS
TECHNICAL FIELD
The present invention relates to a power steering
apparatus which applies a steering assist lorce to a steering
mechanism of a motor vehicle by a hydraulic pressure generated
by a pump driven by an electric motor.
BACKGROUND ART
Power steering apparatuses are known which assist
operation of a steering wheel of a motor vehicle by supplying
a working oil from an oil pump to a power cylinder coupled to
a steering mechanism. The oil pump is driven by an electric
motor, and a steering assist force is generated by the power
cylinder in accorda4ce with the revolutions per minute (rpm)
of the electric motor.
Drive control of the electric motor is performed, for
example, on the basis of the steering angle of the steering
wheel. That is, the steering angle is determined on the basis
of an output of a steering angle sensor provided in
association with the steerzng wheel, and the driving of the
electric motor ia controlled on the basis of the steering
angle. More specifically, if the steering angle of the
steering wheel is within a motor stop range defined around a
steering angle midpoint, steering assist is considered to be
unnecessary, so that the electric motor is stopped. On the
other hand, if the steering angle of the steering wheel is out
of the motor stop rarige, the electric motor is driven to
generate a steering assist force.
CA 02497729 2007-10-19
2
Detection of the steering angle midpoint is achieved, for
example, by sampling steering angle data outputted from the
steering angle sensor and regarding the most frequent steering
angle data as corresponding to the steering angle midpoint.
However, the power steering apparatus described above
performs the drive control of the electric motor with the use
of the steering angle sensor, so that the drive control cannot
be performed on the basis of the steering torque. Therefore,
the steering feeling is not satisfactory.
Where a motor vehicle travels along a straight road
inclined transversely of the vehicle, for example, a torque
should be applied to the steering wheel for stabilization of
the vehicle even though the steering angle of the steering
wheel is small enough to be within the motor stop range. In
such a case, the aforesaid power steering apparatus does not
provide the steering assist even under application of the
steering torque. Hence, the.sense of steering control for the
driver is deteriorated.
A torque sensor may be used instead of the steering angle
sensor to perform the drive cQrxtrol in accordance with the
steering torque. However, the use of the torque sensor is not
preferable, because the torque sensor is less reliable than
the steering angle sensor.
DISCLOSURE OP THE INVENTION
An object of the present invention is to overcome or at
least mitigate the disadvantages of known power steering
apparatus or at least provide an altexnative.
According to the present invention, there is provided a
power steering apparatus, for generating a steering assist
force by a hydraulic pressure generated by a pump driven by
an electric motor, comprising: steering angle detecting means
for detecting a steering angle with respect to a steering
angle midpoint, actuation threshold determining means for
CA 02497729 2007-10-19
3
determining an actuation threshold in accordance with the
steering angle detected by the steering angle detecting means
when the electric motor is stoppEd, and actuation control
means for actuating the electric motor when a change amount
of the steering angle detected by the steering angle detecting
means exceeds a predetermined actuation threshold in a motor
stop state, the actuation control means comprising; actuation
steering angle calculating means for determining, on the basis
of the value of the steering angle detected by the steering
angle detecting means and the actuation threshold determined
by the actuation threshold determining meang, an actuation
steering angle which corresponds to a steering angle at which
the electric motor in an off state is actuated, and means for
actuating the electric motor, when the electric motor is off,
when the value of the steering angle detected by the steering
angle detecting means reaches the actuation steering angle
determined by the actuation steering angle calculating means,
wherein the actuation threshold determining means sets the
actuation threshold smaller as the value of the steering angle
detected by the steering angle detecting means increases when
the electric motor is stopped.
Preferably, the steering angle has a value close to the
steering angle midpoint, the steering angle changes by a
relatively large change amount to get out of a play angle
range of the steering wheel. That is, a large amount of the
change in the steering angle is required until the steering
assist becomes necessary. On the contrary, where the steering
angle is relatively large, a great steering assist force is
required immediately after the steering operation is started.
In embodiments of the present inventiori, the actuation
threshold is determined in accordance with the steering angle
detected when the electric motor i$ stopped, and the electric
motor is actuated on condition that the change amount of the
steering angle exceeds the actuation threshold. Therefore, the
CA 02497729 2007-10-19
4
actuation threshold can be set greater, i.e. higher, when the
steering angle at the stop of the motor is close to the
steering angle midpoint, and set smaller, i.e. lower, when the
steering angle (absolute value thereof) at the motor stop is
relatively large. Thus, the electric motor is not actuated
needlessly when the steering angle is close to the steering
angle midpoint, and a great steering assist force can
immediately be generated when the steering angle is large.
Therefore, the enhancement of the energy saving and the
improvement of the sense of steering control for the driver
can be achieved.
The actuation control means may include actuation
steering angle calculating means for determining, on the basis
of the value of the steering angle detected by the steering
angle detecting means and the actuation threshold determined
by the actuation threshold determining means, an actuation
steering angle which corresponds to a steering angle at which
the electric motor in an off state is to be actuated; and
means for actuating the electric motor, when the electric
motor is off, on condition that the value of the steering
angle detected by the steering angle detecting means reaches
the actuation steering angle determined by the actuation
steering angle calculating means.
Further, the actuation threshold determining means is
preferably adapted to set the actuation threshold smaller,
i.e. lower, as the value of the ateering angle detected by the
steering angle detecting means at the motor stop increases.
In accordance with another embodiment of the present
invention, the power steering apparatus further comprises
vehicle speed detecting means (vehicle speed sensor 13)for
detecting a vehicle speed, and the actuation threshold
determining means is adapted to set the actuation threshold
higher as the vehicle speed detected by the vehicle speed
detecting means increases.
CA 02497729 2007-10-19
Thus, immediate motor actuation is ensured at low speed
traveling, and motor actuation sensitivity is reduced at high
speed traveling. Therefore, needless actuation of the electric
motor is prevented at the high speed traveling at which the
5 steering assist is Xees necessary, while the steering assist
is immediately started at a low speed traveling. This enhances
the energy saving as well as the sense of steering control
for the driver.
Power steering apparatus according to the present
invention may be used in combination with the power steering
apparatus disclosed and claimed in corresponding Canadian
patent application No. 2, 268, 856, which is directed to a power
steering apparatus for generating a steering assist force by
a hydraulic pressure generated by
a pump driven by an electric motor, comprising steering
angular speed detecting means for detecting a steering angular
speed, electric current detecting means for detecting a value
of motor current flowing through the electric motor, and stop
controlling means fox stopping the electric motor in response
to duration of a predetermined time period during which the
steering angular speed detected by the steering angular speed
detecting means is kept not greater than a predetermined stop
threahold and the motor current value detected by the electric
current detecting means is kept within a predetermined motor
stop range.
The foregoing and other objects, features and effects of
the present invention will become more apparent from the
following description of the embodiment with reference to the
attached drawings.
BRIEP DESCRIPTION OF THE DRAWINGS
CA 02497729 2007-10-19
6
FIG. X is a conceptual diagram illustrating a basic
construction of a power steering apparatus according to one
embodiment of the present invention;
FIG. 2 is a flow chart for explaining drive control of
a tnotor;
FIG. 3 is a flow chart illustrating an actuation steering
angle calculating process for determining an actuation
steering angle;
FIGS. 4A and 48 are diagrams for explaining exemplary
setting of first and second constants;
FIG. 5 is a diagram showing a relationship between the
steering angle and the actuation steering angle;
FIG. 6 is a diagram for explaining a relationship between
the motor actuation sensitivity and the vehicle speed;
FIG. 7 is a flow chart for explaining a motor stop range
determining process; and
FIG. 8 is a graph showing a relationship between the
motor electric current value and the steering torque.
EMBODIMENT OF THE INVENTION
FIG. 1 is a conceptual diagram illustrating a basic
construction of a power steering apparatus according to one
embodiment of the present invention. This power steering
apparatus is provided in association with a steering mechanism
1 of a motor vehicle for applying a steering assist force to
the steering mechanism 1.
The steering mechanism 1 includes a steering wheel 2 that
tQ be operated by a driver, a steering shaft 3 coupled to the
steering wheel 2, a pinion gear 4 provided at a distal end of
the steering shaft 3, and a rack shaft 5 having a rack gear
5a meshed with the pinion gear 4 and extending transversely
of the motor vehicle. Tie rods 6 are connected to opposite
CA 02497729 2007-10-19
7
ends of the rack shaft 5, and further connected to knuckle
arms 7 which respectively support left and right front wheels,
FL and FR as steerable wheels. The knuckle arms 7 are
respectively provided rotatably about king pins 8.
With this arrangement, when the steering wheel 2 is
operated to rotate the steering shaft 3, the rotational motion
is converted into a linear motion transverse to the motor
vehicle by the pinion gear 4 and the rack shaft 5. The linear
motion is converted into rotational motions of the knuckle
arms 7 about the king pins 8, thereby achieving the steering
of the left and right front wheels FL, FR
A torsion bar 9 which is adapted to be twisted in
accordance with the direction and magnitude of a steering
torque applied to the steering wheel 2 and a hydraulic
pressure control valve 23 which is adapted to change its valve
aperture in accordance with the direction and magnitude of the
torsion of the torsion bar 9 are incorporated in the steering
ehaft 3. The hydraulic pressure control valve 23 is connected
to a power cylinder 20 for applying a steering assiet force
to the steering mechanism 1. The power cylinder 20 includes
a piston 21 provided integrally with the rack shaft 5, and a
pair of cylinder chambers 20a and 20b split by the cylinder
21. The cylinder chambers 20a and 20b are connected to the
hydraulic pressure control valve 23 'via oil supply/return
lines 22a and 22b, respectively.
The hydraulic pressure control valve 23 is diepased in
an oil circulation line 24 which extends through a reservoir
tank 25 and an oil pump 26. The oil pump 26 is driven by an
electric motor 27, so that a working oil contained in the
reservoir tank 25 is pumped up and supplied to the hydraulic
pressure control valve 23. An excess of the working oil is
returned to the reservoir tank 25 from the hydraulic pressure
Control valve 23 via the oil circulation line 24.
CA 02497729 2007-10-19
g
When a torsion is exerted on the torsion bar 9 in one
direction, the hydraulic pressure control valve 23 supplies
the working oil to one of the cylinder chambers 20a, 20b of
the power cylinder 20 via one of the oil supply/return lines
22a, 22b. When a torsion is exerted on the torsion bar 9 in
the other direction, the hydraulic pressure control valve
supplies the working oil to the other of the cylinder chambers
20a, 20b cria the other of the oil supply/return lines 22a,
22b. When virtually no torsion is exerted on the torsion bar
9, the hydraulic pressure control valve 23 is in a so-called
equilibrium state, so that the working oil is not supplied to
the power cylinder 20 but circulated in the oil circulation
line 24.
When the working oil is supplied to either one of the
cylinder chambers of the power cylinder 20, the piston 21
moves transversely of the motor vehicle. Thus, a steering
assist force acts on the rack shaft 5.
An exemplary construction of the hydraulic pressure
control valve is disclosed in detail, for example, in Japanese
Unexamined Patent Publicatzon No. 59-11B577 (1904), to which
the reader is directed for reference.
The driving of the motor 27 is controlled by an
electronic control unit 30. The electronic control unit 30 is
comprised of a microprocessor which includes a CPU 31, a RAM
32 which provides a work area for the CPU 31, a ROM 33 storing
therein operation programs for the CPU 31, and buses 34
interconnecting the CPU 31, the RAM 32 and the ROM 33.
The electronic control unit 30 receives steering angle
data outputted from a steering angle sensor 11. The steering
angle sensor 11 is provided in association with the steering
wheel 2. The steering angle sensor 2 sets at an initial value
"0 a steering angle of the steering wheel 2 observed when an
ignition key switch is actuated for startup of an engine, and
outputs steering angle data which has a value corresponding
CA 02497729 2007-10-19
9
to a steering angle relative to the initial value and a sign
corresponding to a steering direction.
The electronic control unit 30 also receives electric
current data applied from an electric current detecting
circuit 12 which detects electric cu.xrezit flowing through the
motor 27. The electric current data has a value proportional
to the value of a consumed electric current of the motor 27
(motor electric current).
Further, the electronic control unit 30 receives vehicle
speed data outputted from a vehicle speed sensor 13. The
vehicle speed sensor 13 may be adapted to directly detect a
vehicle speed or, alternatively, adapted to calculate the
vehicle speed on the basis of an output pulse of a wheel speed
sensor provided in association with the wheels.
The electronic control unit 30 controla the driving of
the motor 27 on the basis of the steering angle data, the
electric current data and the vehicle speed data outputted
from the steering angle sensor 11, the electric Gurrent
detecting circuit 12 and the vehicle speed sensor 13,
respectively.
FIG. 2 is a flow chart for explaining the drive control
of the motor 27. The CPU 31 first judges whether or not the
motor 27 is off (Step S1), For this judgment, a flag may be
employed, for example, which a.s to be set when the motor 27
is aotuated and reset when the motor 27 is stopped.
It the motor 27 is in an off state (YES in Step S1) , the
CPU 31 calculates an absolute steering angle 8 with respect
to a steering angle midpoint 60 on the basis of the steering
angle data outputted from the steering angle sensor 11 (Step
S2).
The steering angle midpoint 80 is a steering angle of the
steering wheel 2 observed when the motor vehicle travels
straight. The CPU 31, for example, samples steering angle data
CA 02497729 2007-10-19
outputted from the steering angle sensor 11 after the ignition
key switch is actuated, and prepares a histogram of values of
the steering angle data. After a predetermined number of data
are sampled, the CPU 31 determines the most frequent steering
5 angle data, which is regarded as steering angle data
corresponding to the steering angle midpoint 00. The steering
angle data of the steering angle midpoint 00 thus determined
is stored in the RAM 32. In Step S2, the CPU 31 determines the
absolute steering angle 8 on the basis of the steering angle
10 data from the steering angle sensor 11 and the steering angle
data of the steering angle midpoint 60 retained in the RAM 32.
The CPU 31 further judges whether or not the absolute
steering angle 9 thus determined is equal to or greater than
an actuation steering angle 6t stored in the RAM 32 (Step 53) .
The actuation steering angle et corresponds to an absolute
steering angle of the steering wheel 2 at which the motor 27
is to be actuated. The actuation steering angle Ot has been
determined, through an actuation steering angle calculating
proce.as which will be described later, depending on the
absolute steering angle observed at the precedent stop of the
motor 27, and stored in the RAM=32.
The absolute steering angle 0 and the actuation steering
angle et are each provided, for example, with a positive sign
if the angle is formed on the right of the steering angle
midpoint 00 or with a negative sign if the angle is formed on
the left of the steering angle midpoint 60. Strictly speaking,
the judgment in Step S3 should be performed through comparison
of the absolute values of the absalute steering angle @ and
the actuation steering angle Ot. For simplification of
explanation, it is herein assumed that the absolute steering
angle 8 and the actuation steering angle 6t each have a
positive value.
If it is judged that the absolute steering angle e does
not reach the actuation steering angle 8t (NO in Step S3 ), the
CA 02497729 2007-10-19
11
program returns to Step Sl. On the other hand, if the absolute
steering angle 9 reaches the actuation steering angle et (YES
in Step S3), the CPU 31 actuates the motor 27 (Step W.
The rpm of the motor 27 is determined in accordance with
a steering angular speed Ve of the steering wheel 2. More
specifically, the CPU 31 determines, on the basis of the
steering angle data outputted from the steering angle sensor
11, the steering angular speed Ve which ia a time-related
change rate of the eteering angle (Step S5). The CPC7 next
judges whether or not the steering angular $peed Ve thus
determined is equal to or smaller (i.e. lower)than a
predetermined first threshold VT1 (VT].-10 (degree/sec)) (Step
S6). If the steering angular speed V6 is not greater than the
first threshold VT1 (YES in Step S6), the motor 27 is dxiven
so that the motor rpm R is equal to a predetermined first rpm
R1 (e.g., R1-1800 (rpm)) (Step S7). That ie, if the steering
angular speed ve is not greater than the first threshold VTx,
the motor 27 is driven constantly at the first rpm R1
irrespective of the value of the steering angular speed VO.
Tf the steering angular speed VA is greater than the
first threshold VTl (NO in Step S6), the CPU 31 judges whether
or not the steering angular speed ve is smaller than a second
thre$hold VT2 (e.g., VT2.600 (degree/sec)) which is greater
than the firat threshold VT1 (Step S8). If the steering
angular speed VO is smaller than the eecond threshold VT2 (YES
in Step SB), the CPU 31 drives the motor 27 at a motor rpm R
according to the steering angular speed V (Step S9). More
apecifically, if the steering angular speed V6 is within a
range which is greater than the first threshold VT1 and
smaller than the second threshold VT2, the CPU 31 determines
the motor rpm R so that the motor rpm R varies generally
linearly with the steering angular speed ve between the first
rpm R1 and a second rpm R2 (R2>R1).
CA 02497729 2007-10-19
12
If the steering angular speed Ve is not smaller than the
second threshold VT2 (NO in Step S6), the CPU 31 drives the
motor 27 so that the motor rpm R is equal to the predetermined
second rpm R2 (e.g., R2=6000 (rpm)) (Step s10). That is, if
the steering angular speed Ve is nc,t smaller than the second
threshold VT2, the motor 27 is driven constantly at the second
rpm R2 irrespective of the ateering angular speed V6.
If it is judged in Step S1 that the motor 27 is driven,
the CPU 31 determines the steering angular speed VO on the
basis of the steering angle data outputted from the steering
angle sensor 11 (Step S1l), and judges whether or not the
steering angular speed Ve thus determined is equal to or
smaller than a predetermined stop threshold VS (e.g., VS-10
(degree/see)) (Step S12). If the steering angular speed VB is
greater than the stop threshold VS (NO in Step S12), the
program goes to Step S6, and the CPU 31 determines the motor
rpm R on the basis of the value of the steering angular speed
VA, and drives the motor 27 at the motor rpm R thus
determined,
If the steering angular a peed V6 is not greater than the
stop threshold VS (YES in Step S12), the CPU 31 determines a
motor electric current value Im on the basia of the electric
current data outputted from the electric current detecting
circuit 12 (Step S13). Then, it is judged whether or not the
motor electric current value Im thus determined is within a
motor stop range DI (Step S14), The motor stop range aI is a
range of the motor electric current value im where no steering
assist is required, and is determined through a motor stop
range determining process to be described later. If the motor
electric current value Im i+s within the motor stop range 61
(=YES in Step S14), the CPU 31 judges whether or not the motor
electric current value Im is kept within the motor stop range
DI for a predetermined time period (e.g., 1 to 3 seconds)
(step S15). If this judgment i.s positive (YES in Step S15),
CA 02497729 2007-10-19
13
the CPU 31 stops the motor 27 (Step S16) because the steering
wheel 2 is considered to be virtually unoperated. Thereafter,
the CPU 31 perforrms the actuation steering angle calculating
process to determine the actuation steering angle Ot (Step
S17). On the other hand, if the judgments in Steps S14 and S15
are both negated, the CPU 31 performs the process sequence
from Step S6 to determine the motor rpm R and drive the motor
27 at the rpm thus determined.
FIG. 3 is a flow chart showing the actuation steering
angle calculating process. The CPU 31 determines the absolute
steering angle e at the stop of the motor on the basis of the
steering angle data outputted from the steering angle sensor
11 (Step Ti). Then, the CPU obtains a vehicle speed V on the
basis of the vehicle speed data outputted from the vehicle
speed sensor 13, and determines an actuation threshold d6
according to the vehicle speed V (Step T2). The actuation
threshold dO corresponds to a change amount of the steering
angle which serves as a trigger for the actuation of the motor
27, That is, the motor 27 is actuated when the change amount
of the steering angle reaches the actuation threshold d9.
More specifically, the actuation threshold dO is obtained
by substituting into the following equation (1R) or (lL) a
first constant A and a second constant B for the obtained
vehicle speed V. The first constant A and the second constant
B are factors for determining a sensitivity for the actuation
of the motor 27, and a table indicative of a correlation
between the vehicle speed V and the constants A and B is
preliminarily stored in the ROM 33. The conetant A is the
maximum value of the actuation threshold dO (absolute value
thereof), and the constant B corresponds to the number of
steering angle values which take the same actuation threshold
dO. Where the steering angle sensor 11 is adapted to output
a pulse for every turn by a given steering angle, for example,
the steering angle 6 may be expressed by the count value of
CA 02497729 2007-10-19
14
a counter which is counted up or down by the pulse output. In
such a case, the constant B may correspond to the numbez of
count values which take the same actuation threshold d6. It
is noted that the constants A and 8 each have a positive
value.
For right-turn steering (positive steering angle 0)
d6=A--(8/B),__(1R)
For left-turn steering (negative steering angle 0)
de=-A+(8/B)... (1L)
When the vehicle speed V is zero, i.e., the vehicle
stops, the actuation threshold d9 is not determined on the
basis of the above equation (1R) or (IL) but eet at a
predetermined minimum actuation threshold.
The CPU 31 determines a first actuation steering angle
9t1 by adding the actuation threshold de to the absolute
steering angle 9 at the motor stop determined in the aforesaid
manner (Step T3). Where the motor 27 is off, the first
actuation steering angle At1 is an absolute steering angle at
which the motor 27 is to be actuated when the steering wheel
2 is turned in such a direction that the absolute value of the
absolute steering angle e increases.
The CPU 31 determines, in addition to the first actuation
steering angle 6t1, a second actuation steering angle et2
which is to be employed when the steering wheel 2 is turned
in a direction oppoaite to the direction in which the absolute
value of the absolute steering angle 0 increases, i.e., in
such a direction that the absolute value of the absolute
steering angle e decreases(Step T4). More specifically, the
second actuation steering angle is set at the same value as
the maximum actuation threshold A or -A as mhown in the
following equations (2R) and (2L) :
For right-turn steering (negative ateering angle 0)
et2=A ... (2R)
For left-turn steering (positive steering angle 8)
CA 02497729 2007-10-19
9t2=-A ...(2L)
The maxinoum actuation threshold A or -A is equal to the
threshold de at em0, i.e., the threshold with respect to the
steering angle midpoint. Therefore, when the steering
5 operation is started from the steering angle midpoint or
performed in the direction in which the absolute value of the
absolute steering angle e decreases, the motor 27 is aotuated
only when the steering operation is performed to get out of
a so-called play angle range around the steering angle
10 midpoint.
T.he CPU 31 stores the first and second actuation steering
angles et1 and At2 thus determined in the RAM 32 (Step T5).
In FIG. 2, the first and second actuation $teering angles
et1 and 6t2 are generally designated as the actuation steering
15 angle et.
FIGS. 4A and 4B are diagrams for explaining the fiz=st
constant A and the second constant B. The first constant A is
determined for each predetermined vehicle speed range, and
corresponds to the maximum value of the actuation threshold
de to be determined for the corresponding vehicle speed range.
More specifically, as shown in FIG. 4A, where the vehicle
speed V is lower than V1 (e.g., V1=30 (km/h)), the first
constant A is set at Al (e.g., Alal). Where the vehicle speed
is not lower than Vl and lower than V2 (e.g., V2=60 (km/h)),
the first constant A is set at A2 (e.g., A2-3). Further, where
the vehicle speed V is not lower than V2, the first constant
A is set at A3 (e,g., A3,6).
The $econd constant B is determined for each
predetermined vehicle speed range, and corresponds to the
number of the absolute steering angle values which take the
same actuation, threshold de for the corresponding vehicle
speed range. More specifically, as ahown in FIG. 4B, when the
vehicle speed V is lower than Vi, the second constant B ia set
at BZ (e.g., 91=1). When the vehicle speed V is not lower than
CA 02497729 2007-10-19
16
Vi and lower than V2, the second constant B is set at B2
(e.g., B2=2). Further, where the vehicle speed V is not lower
than V2, the second constant B ie set at 33 (e,g,, B3=3).
The first constant A and the second constant B are not
neceesarily set in a stepwise form as shown, in FIG. 4, but may
be set as being linearly variable as indicated by
two-dot-and-dash lines, for example, when the vehicle speed
V is lower than V2.
The actuation threshold d8 is set greater, i.e. higher,
for a higher vehicle speed by setting the first constant A
greater for a higher vehicle speed. Further, the decrease rate
of the absolute value of the actuation threshold d9 with an
increase in the absolute value of the absolute steering angle
0 at the motor stop is reduced by setting the second constant
B greater for a higher vehicle speed. Therefore, even if the
absolute value of the absolute steering angle 0 at the motor
stop is relatively high, a relatively large amount of the
change in the steering angle is required for the actuation of
the motor 27. Thus, needless motor actuation is prevented when
the vehicle speed is high. When the vehicle speed is low, the
actuation sensitivity is increased, so that a steering assist
force can immediately be generated.
FIG. 5 is a diagram showing a relationship between the
absolute steering angle 8 and the first actuation steering
angle Oti, particularly, a relationship existing between the
absolute steering angle e and the first actuation steering
angle 8ti when the first constant A and the second constant
B are "5" and "3", respeCtively. In FIG. 5, the absolute
steering angle 0 at the motor atop is represented by the tail
end of an arrow, the actuation threshold dO is represented by
the length of the arrow, and the first actuation steering
angle eti is represented by the head of the arrow. Further,
vertical lines represent absolute steering angles 0_
CA 02497729 2007-10-19
17
As apparent from FIG. 5, the actuation threshold de
decreases as the value of the absolute steering angle 6 at the
motor stop increases. That is, the sensitivity for the
actuation of the motor 27 becomes higher as the value of the
absolute steering angle 6 at the motor stop increases for the
following reasons,
Where the absolute steering angle 6 has a value close to
the steering angle midpoint e0, the steering assist is
provided only when the steering wheel 2 is operated to get out
of the play angle range of the steering wheel 2. Therefore,
when the steering angle is close to the steering angle
midpoint, excessive steering assist can be suppressed by
setting the actuation threshold d8greater(highex), so that the
energy saving can be enhanced. On the contrary, when the
absolute steering angle e has a great value, a satisfactory
sense of steering control for the driver can be ensured by
immediately providing the steering assist.
FIG. 6 is a diagram for explaining a relationship between
the sensitivity for the actuation of the motor 27 (which
becomes higher as the absolute value of the actuation
threshold dO decreases) and the vehicle speed V. As apparent
from FIG. 6, the sensitivity for the actuation of the motor
27 varies depending upon the vehicle speed V, even if the
absolute steering angle 8 at the motor stop has the same
value. More specifically, the sensitivity for the actuation
of the motor 27 is low at high speed traveling, and high at
low $peed traveling, This is because little steering assist
force is required at the high speed traveling and the steering
assist should immediately be provided at the low speed
traveling.
When the motor vehicle stops with a vehicle speed V of
zero, the actuation threshold de is set at the predetermined
minimum value, so that the sensitivity for the actuation of
the motor 27 is kept constant irrespective of the value of the
CA 02497729 2007-10-19
18
absolute steering angle 8. where a so-called parking steering
operation is performed when the vehicle stops, a greater
steering assist force is required and, therefore, it is
preferred that the steering assist is immediately provided
irrespective of the value of the absolute steering angle e.
FIG. 7 is a flow chart for explaining the motor stop
range oI determining process. The CPU 31 constantly monitors
the motor electric current value Im (Step U1). On the basis
of the motor electric current value Im, the CPU 31 determines
a non-load electric current value 10 which correaponds to a
motor electric current value observed when the motor 27 is in
a non-load state (Step U2). Using the non-load electric
current value io thus determined, the CPU 31 determines the
motor stop range oz (step U3). More specifically, the CPU 31
determines as the motor stop range AI a range defined between
the non-load electric current value IO thus determined and a
value IO+dI resulting from summation of the non-load electric
current value IO and an electric current threshold dI which
is predetermined in accordance with the specifications of the
motor vehicle.
FIG. S is a graph showing a relationship between the
steering torque T and the motor electric current value Im. The
abscissa and the Qrdinate represent the steering torque T and
the motor electric current value Im, respectively. The motdz
electric current value Im in a range around a steering torque
T of zero is expressed by a curve having a local point at T=0.
When the steering torque T is zero, the motor 27 is in the
non-load state and, therefore, the minimum value of the motor
electric current value Im corresponds to the non-load electric
current value 10.
On the other hand, a torque range where no steering
assist force is required to be applied to the steering wheel
2 ie determined by the specifications of the motor vehicle.
Provided that the torque range is defined between torque
CA 02497729 2007-10-19
19
threaholds Ti and -Tl with its midpoint set at zero, a
difference between the non-load electric current value I0 and
an electric current value for these torque thresholds Tl, -TX.
is preliminarily determined which is employed as the electric
current threshold dl. The range defined between the non-load
electric current value XO and the value I0+dI obtained by
adding the electric current threshold dI to the non-load
electric current value IO is considered to be the motor stop
range Al where the steering wheel 2 is not operated. The
electric current threshold dI is preliminarily determined for
each type of motor vehicles, and stored in the ROM 33.
The non-load electric current value I0 varies mainly
depending on the temperature of the working oil. More
speca.fically, when the temperature of the working oil is low,
for example, the working oil has a high viscosity, so that the
load on the motor 27 is greater than when the temperature of
the working oil is high. Therefore, the motor electric current
value Im is high when the temperature of the working oil is
low. That is, the Im-T curve in FIG. 8 is shifted upward with
the non-load electric current value 10 being increased.
In this embodiment, therefore, the non-load electric
current value 10 is calculated, and the range between the
calculated non-load electric current value I0 and the value
10+dl resulting from the summation of the non-load electric
current value 20 and the electric current threshold dz stored
in the ROM 33 is defined as the motor stop range I. The calculation of the non-
load electric current value
IO is achieved, for example, by determining the most frequent
electric current value out of sampled motor electric current
values Im. More specifically, the CPU 31 samples electric
current data outputted from the electric current detecting
circuit 12 over a predetermined time period (e.g., 10 (min)
to 1(hour)) on condition that the motor rpm R is kept
constant. The motor electric current values Im determined on
CA 02497729 2007-10-19
the basis of the electric current data obtained through the
sampling have a normal distributi.on. In this case, a motor
electric current value Im at a steering torque of zero is the
most frequent electric current value, which is employed as the
5 non-load electric current value 10,
Otherwise, the minimum electric current value, which is
determined out of motor electric current valueg Im sampled by
a predetermined number of times or during a predetermined time
period on condition that the motor rpm R is kept constant, may
10 be employed as the non-load electric current value 10.
in accordance with the embodiment described above, it is
judged, on the basis of the motor electric current value Im,
whetheX or not the steering assist is required, and the
judgment result is employed as one condition for stopping the
15 motor 27, in view of the fact that the motor electric current
value Im varies depending upon the steering torque. Therefore,
the drive control of the motor 27 can be performed in
accordance with the steering torque even without the use of
a torque sensor, so that an improved sense of steering control
20 for the driver can be ensured.
As the absolute value of the absolute steering angle 8
at the motor stop increases, the sensitivity for the actuation
of the motor 27 with respect to a change in the steering angle
is increaeed. Therefore, needless motor actuation can be
suppressed when the steering angle is close to the steering
angle midpoint_ In addition, when the steering angle 8 is
large, a steering assist force can immediately be generated.
Thus, the energy saving can be enhanced, and catch-up feeling
of the ateering wheel can be eliminated.
Further, the sensitivity for the actuation of the motor
27 is increased at the low speed traveling which requires a
greater steering assist force, while the actuation sensitivity
is reduced at the high Qpeed traveling. Hence, the enhancement
CA 02497729 2007-10-19
21 of the energy saving and the improvement of the sense of
steering control for the driver can both be ensuxed.
Znduetrial Applicability
As previously described, the power steering apparatus
according to the present invention is used for applying a
steering assist force to a ateerzng mechanism of a motor
vehicle.
