Apparatus for actively controlling steer angle of front wheels of vehicle

In an apparatus for actively controlling a steer angle of front wheels of a vehicle, a steering angle of a steering wheel is detected and then an active control amount for actively controlling the steer angle of the front wheels including a proportional term and a differential term is calculated. After only the differential term has been delayed, the steer angle of the front wheels is controlled in accordance with the active control amount. Since the differential component in the active control amount has been delayed, the steer angle of the front wheels is changed gradually, so that the variation in the steering force is suppressed. Therefore, the maneuverability and operating properties of the vehicle can be improved.

BACKGROUND OF THE INVENTION 
Field of the Invention and Related Art Statement 
The present invention relates to an apparatus for controlling a steer angle 
of front wheels of a vehicle, and more particularly to an apparatus for 
effecting an active control in which a steer angle of the front wheels is 
increased actively in accordance with a steering wheel angle. 
There has been proposed an apparatus for actively controlling a steer angle 
of front wheels of a vehicle, in which, in addition to a usual control of 
the front wheels in proportion to a steering wheel angle, the steer angle 
of the front wheels is auxiliarily controlled in an active manner. There 
has been also proposed an apparatus for auxiliarily controlling both the 
front wheels and the rear wheels in an active manner. The latter apparatus 
is utilized in a so-called four wheel steering (4WS) vehicle. For 
instance, in Japanese Patent Publication Kokai Sho No. 60-161,266, the 
above mentioned steer angle controlling apparatus is described. 
In the known apparatus for actively controlling the steer angle of front 
wheels, an active control amount for auxiliarily or actively controlling 
the steer angle of the front wheels is calculated in accordance with a 
steering wheel angle and a running velocity of the vehicle, and the steer 
angle of the front wheels is additionally increased by the thus calculated 
active control amount. This improves the stability and handling properties 
of the vehicle. For instance, when the steer angle of the front wheels is 
increased by the active control amount, upon a turn of the vehicle, a 
head-turning characteristic of the vehicle can be improved due to an 
increase in a yaw rate, i.e. an angular velocity of a yawing movement of a 
body of the vehicle. 
In the known apparatus for controlling the steer angle of the front wheels 
in the active manner, the active control amount is derived by effecting 
calculations based on a steering wheel angle and an angular velocity of 
the steering wheel angle. Therefore, although the steering wheel angle is 
the same, a sideslip angle of the front wheels might be changed in 
dependence upon the angular velocity of the steering wheel angle. That is 
to say, the sideslip angle of the front wheels is increased in relation to 
a differential in time of the steering wheel angle. This results in that 
at an initiation of steering wheel operation by a driver, a force 
necessary for manipulating the steering wheel, i.e., a so-called steering 
force, is increased very abruptly, so that the steering force shows a 
large variation as illustrated by a curve A in FIG. 4C. Therefore, the 
known apparatuses for actively controlling the steer angle of the front 
wheels could not achieve optimum stability and maneuverability. 
SUMMARY OF THE INVENTION 
The present invention has for its object to provide a novel and useful 
apparatus for controlling the steer angle of the front wheels of the 
vehicle in an active manner, in which the variation of the steering force 
for operating the steering wheel can be suppressed to improve the 
maneuverability and handling properties, while the merit of the active 
control can be remained. 
According to the invention, an apparatus for controlling a steer angle of 
front wheels of a vehicle comprises: 
sensing means for detecting a steering wheel angle to produce a steering 
wheel angle signal; 
signal processing means for processing said steering wheel angle signal to 
derive an active control amount including at least a delayed first order 
differential of said steering wheel angle signal; 
actuating means for steering the front wheels in accordance with said 
steering wheel angle signal as well as said active control amount. 
According to the invention, by delaying at least the first order 
differential of the steering wheel angle signal, the variation in the 
steering force for manipulating the steering wheel including the initial 
abrupt change can be suppressed to a large extent, so that the feeling of 
operating the steering wheel can be resembled to that which is obtained 
upon driving a vehicle without the active control of the steer angle of 
the front wheels, while the merits of the active control of the steer 
angle of the front wheels can be still obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a schematic view showing the principal construction of the 
apparatus for actively controlling a steer angle of front wheels of a 
vehicle according to the invention. A vehicle 31 is illustrated 
symbolically as having front wheels 32, rear wheels 33 and steering wheel 
34. A steering wheel angle is detected by a steer angle sensor 35 and a 
steering wheel angle signal is parallelly supplied to a main controller 36 
and an auxiliary controller 37. In the main controller 36, the steering 
wheel angle signal is processed in a usual manner to derive a main steer 
angle control signal, which is then supplied to a main front wheel 
actuator 38. In the auxiliary controller 37, an active control amount 
including at least a first order differential in time of the steering 
wheel angle signal is derived, and then only the differential component in 
the active control amount is delayed by a suitable time to produce an 
auxiliary steer angle control signal. This auxiliary steer angle control 
signal is supplied to an auxiliary front wheel actuator 39. In this 
manner, the steer angle of the front wheels 32 is controlled by both the 
main actuator 38 and the auxiliary actuator 39. According to the 
invention, the auxiliary steer angle control signal is obtained by 
delaying the differential of the steering wheel angle signal, so that the 
steer angle of the front wheels 32 is changed gradually and thus the 
variation of the steering force can be suppressed. It should be noted that 
according to the invention, in addition to the first order differential of 
the steering wheel angle signal, one or more higher order differentials 
such as second and third order differentials may be also delayed. In an 
embodiment which will be explained hereinafter, only the first order 
differential of the steering wheel angle signal is delayed. 
FIG. 2 is a schematic view illustrating an embodiment of the steer angle 
active control apparatus according to the invention. Reference numerals 1 
and 2 denote front wheels and rear wheels, respectively. The front wheels 
1 can be steered by transferring a steering input applied to a steering 
wheel 3 to the front wheels 1 via a steering gear 4 so as to perform a 
usual main steering control. In order to perform the auxiliary steer angle 
control for the front wheels 1, a housing of the steering gear 4 is 
linearly moved by an actuator 5 of an auxiliary steer angle control 
apparatus. The front wheels 1 can be steered auxiliarily by the auxiliary 
steer angle control up to the maximum steer angle .alpha.. In this 
embodiment, the steer angle of the rear wheels 2 is also controlled 
actively. That is to say, the steer angle of the rear wheels 2 can be 
actively controlled up to the maximum angle .beta. by means of an actuator 
6 provided in a rear wheel steering apparatus. In the present embodiment, 
it is assumed that .alpha.&gt;.beta.. 
The apparatuses for controlling the steer angles of the front and rear 
wheels comprise, in addition to the respective actuators 5 and 6, a common 
pressure source formed by an oil pump 7, distributing valve 12 and steer 
angle controlling valves 14 and 15. The oil pump 7 sucks an oil in a 
reservoir tank 8 and discharges the sucked oil into a main circuit 9. The 
oil discharged into the main circuit 9 is delivered by the distributing 
valve 12 into a front wheel steer angle auxiliarily controlling circuit 10 
and a rear wheel steer angle auxiliarily controlling circuit 11. 
In the distributing valve 12, a shuttle spool 12a is resiliently supported 
into a neutral position by means of springs 12b and 12c and pressure 
chambers 12d and 12e are formed on both sides of the shuttle spool. These 
pressure chambers 12d and 12e are communicated with the main circuit 9 by 
means of orifices 12f and 12g, respectively which have different diameters 
and are formed in the shuttle spool 12a. The pressure chambers 12d and 12e 
are further communicated with rear and front wheel steer angle auxiliarily 
controlling circuits 11 and 10 by means of openings 12h and 12i, 
respectively formed in the shuttle spool and output ports 12j and 12k, 
respectively formed in a holding of the distributing valve 12. When the 
shuttle spool 12a is moved in accordance with a pressure difference 
between the pressure chambers 12d and 12e, the communication degrees 
between the openings 12h and 12i and the output ports 12j and 12k, 
respectively are changed in opposite senses to perform the adjustable 
distribution of the oil. 
Now a required flow rate Q.sub.f of the oil for the circuit 10 will be 
considered. The required flow rate Q.sub.f may be expressed by a product 
between a pressure receiving surface area SA of a piston of the actuator 5 
which actively controls the steer angle of the front wheels 1 and a moving 
speed v of the piston (Q.sub.f =S.sub.A .times.v). The moving speed v of 
the piston of the actuator 5 can be expressed by v=2 .pi..times.f.times.d, 
wherein f is a frequency of the active control signal for the front wheels 
l and d is a stroke of the piston. Then, Q.sub.f =S.sub.A 
.times.2.pi..times.f.times.d is obtained In a similar manner, a required 
flow rate Q.sub.r of the oil for the circuit 11 for actively controlling 
the steer angle of the rear wheels 2 can be obtained. Since the discharge 
rate Q.sub.0 of the oil pump 7 may be expressed by the following equation: 
Q.sub.O =Q.sub.f +Q.sub.r, a distribution ratio for obtaining the required 
flow rates Q.sub.f and Q.sub.r is achieved by setting the diameters of the 
orifices 12g and 12f in accordance with ratios of Q.sub.f /Q.sub.0 and 
Q.sub.r /Q.sub.0, respectively. In this manner, the distributing valve 12 
can deliver the oil into the circuits 10 and 11 at the required flow rates 
Q.sub.f and Q.sub.r. Further, when the oil pressure in the circuit 10 or 
11 is changed due to the variation of the flow rate, the shuttle spool 12a 
of the valve 12 is moved rightward or leftward so that the opening 12i or 
12h is closed by the inner wall of the housing. In this manner, it is 
possible to prevent the balance of the flow rate ratio from being 
disturbed, and thus the pressure variation in one circuit gives no 
influence upon the other circuit. 
The active steer angle control is carried out by controlling the steer 
angle control valves 14 and 15, while the pressure variation in one 
circuit does not affect the pressure in the other circuit as explained 
above. 
Each of the steer angle control valves 14 and 15 is constructed by a 
pressure control valve which is provided between the auxiliary steer angle 
control circuit 10 and 11 and the actuators 5 and 6, respectively. The 
valves 14 and 15 are further provided between the actuators 5 and 6 and a 
common drain circuit 13. 
When solenoids 14a and 14b in the control valve 14 are not energized, all 
the oil supplied from the circuit 10 is fedback into the drain circuit 13 
and chambers 5a and 5b of the actuator 5 are kept at a non-pressure state. 
Then, the actuator 5 is held in a neutral position by means of springs 5c 
and 5d provided therein, so that the steering gear 4 is driven into a 
condition in which the steer angle of the front wheels 1 is not actively 
controlled. When the solenoid 14a is energized, the valve 14 functions to 
increase the pressure in the chamber 5a and the chamber 5b is drained, so 
that the actuator 5 is extended. Then, the steering gear 4 is moved 
rightward in FIG. 2 and the front wheels 1 are auxiliarily steered 
leftward within said maximum angle .alpha.. When the solenoid 14b is 
energized, the chamber 5b is compressed and the chamber 5a is drained, so 
that the length of the actuator 5 is shortened and the steering gear 4 is 
moved leftward. Then, the front wheels 1 are steered rightward within the 
maximum angle .alpha.. In this manner, the steer angle of the front wheels 
1 is actively controlled. 
The construction and operation of the rear wheel active control apparatus 
including the steer angle control valve 12 and actuator 6 for actively 
controlling the steer angle of the rear wheels 2 are the same as those 
explained above, so that portions similar to those of the steer angle 
control valve 14 and actuator 5 are denoted by applying the same suffixes 
a to d and their detailed explanation is dispensed with. 
The solenoids 14a, 14b of the valve 14 and the solenoids 15a, 15b of the 
valve 15 are on-off controlled by a controller 16, to which are supplied a 
steering wheel angle signal generated from a steering wheel angle sensor 
17 for detecting a steering angle .theta. of the steering wheel 3, a 
running velocity signal produced by a running velocity sensor 18 for 
detecting a running velocity V of the vehicle, and a signal generated by a 
stroke sensor 19 for detecting a stroke of the rear wheel actuator 6. 
The controller 16 comprises input detecting circuit, calculating circuit, 
memory circuit for storing a steer angle controlling program which is 
executed in the calculating circuit, and output circuit for sending 
control signals to the steer angle controlling valves 14 and 15. The 
calculating circuit derives the auxiliary steer angle control signals for 
the front and rear wheels 1 and 2 in accordance with the above mentioned 
parameters detected by the sensors and the output circuit supplies on-off 
control signals I.sub.Fa, I.sub.Fb, I.sub.Ra, and I.sub.Rb to the 
solenoids 14a, 14b, 15a, and 15b, respectively of the steer angle 
controlling valves 14 and 15. 
The auxiliary steer angle control amounts .delta..sub.f and .delta..sub.r 
for the front and rear wheels 1 and 2, respectively, are calculated in 
accordance with the operating conditions of the steering wheel 3 (steering 
wheel angle .theta. and angular velocity .theta.' of the angle .theta.) 
and the running velocity V of the vehicle. In the present embodiment, the 
angular velocity of the steering wheel angle .theta., i.e., a first order 
differential of the steering wheel angle .theta.', is utilized as a 
transient component, and the auxiliary or active steer angle control 
amounts .delta..sub.f and .delta..sub.r are calculated in the following 
manner. 
EQU .delta..sub.f =K.sub.f(V) .times..theta.+T.sub.f(V) .times..theta.'(1) 
EQU .delta..sub.r =K.sub.r(V) .times..theta.+T.sub.r(V) .times..theta.'(1) 
Here, K.sub.f(V) and K.sub.r(V) are coefficients for .theta. in the 
proportional components, said coefficients varying in accordance with the 
running velocity V of the vehicle, and T.sub.f(V) and T.sub.r(V) are 
coefficients for .theta.' in the differential components, said 
coefficients also varying in accordance with the running velocity V of the 
vehicle. 
Each of the above mentioned equations (1) and (2) has the proportional term 
(first term) and the differential term (second term). That is to say, 
during the transient period of the steer angle active control (.theta. is 
small, but .theta.' is large), a sharp characteristic is obtained by the 
differential term and during a steadystate period (.theta. is large, but 
.theta.' is small), the stability is realized by the proportional term. 
The equation (2) performs a so-called phase reverse control. 
According to the invention, the controller 16 is constructed such that when 
the steer angle of the front wheels 1 is actively controlled in accordance 
with the differential of the steering wheel angle, the differential equal 
to or higher than the first order of the auxiliary steer angle control 
signal is delayed such that the initial abrupt change and the succeeding 
oscillatory variation in the steering force for manipulating the steering 
wheel 3 can be suppressed sufficiently. 
That is to say, in the equation (1), the first term represents the 
proportional component .delta..sub.fp of the active steer angle control 
signal and second term shows the differential component .delta..sub.fd 
thereof which is a first order differential of the steering wheel angle 
.theta., and only the differential component .delta..sub.fd is delayed. 
When the transient steer angle control signal .delta..sub.fd is derived by 
the first order differential of the steering wheel angle .theta., i.e., 
the steering wheel angular velocity .theta.', the delay can be given by 
producing a differential by deriving a difference between sample values 
obtained at sampling points which are separated by N sampling period. 
FIG. 3 is a flow chart showing an example of a program for giving the delay 
for the first order differential of the steering wheel angle. This program 
is executed at a regular time interruption. 
In a step 100, the steering wheel angle .theta. detected by the sensor 17 
is entered. In a next step 110, an angular velocity .theta.' of the 
steering wheel angle is calculated. That is to say, when the operation for 
entering the steering wheel angle is carried out every 5 ms, the angular 
velocity of the steering wheel angle can be derived by the following 
equation: 
##EQU1## 
, wherein .theta..sub.0 is a steering wheel angle obtained at an instant 
program cycle and .theta..sub.N is a steering wheel angle entered at a 
program cycle which precedes the instant cycle by five cycles. 
In a next step 120, the front wheel auxiliary steer angle .delta..sub.f is 
calculated by applying the thus derived angular velocity of the steering 
wheel angle in the equation (1). And in a step 130, the steer angle 
controlling process is performed by using the thus calculated front wheel 
auxiliarily control amount .delta..sub.f. 
According to the invention, by delaying only the differential component in 
the active steer angle control amount, it is possible to resemble the 
steering force characteristic to that of the normal vehicle in which the 
active control is not carried out. 
According to the invention, the reason for delaying only the differential 
component of the active steer angle control amount is that the 
proportional component has a less influence upon the variation of the 
steering force than the differential component. 
According to the invention, by delaying the differential component of the 
active steer angle control amount, the steering force characteristic can 
be resembled to that without the active control. This may be explained 
hereinbelow. 
FIG. 4A shows the variation of the steering wheel angle, FIG. 4B the 
variation of the auxiliary steer angle of the front wheels, FIG. 4C the 
variation of the steering force, and FIG. 4D represents the variation of 
the yaw rate. In FIGS. 4B to 4D, the variations in case of the known 
active control apparatus are also shown. 
In a normal vehicle in which the active control is not performed, the steer 
angle of the front wheels is changed in proportion to the steering wheel 
angle, and this generates a slip angle which then produces self-aligning 
torque and cornering force. Due to plays in the column system and steering 
system, the steer angle is delayed with respect to the steering wheel 
angle. Therefore, in the normal vehicle, a phase difference between the 
steer angle and the steering force becomes within a certain range, and 
there is not produced an initial abrupt change in the steering force. 
In the vehicle adopting the active control of the steer angle, the steer 
angle of the front wheels is increased in accordance with the steering 
wheel angle, so that the head-turning characteristic is improved. On the 
other hand, a delay between the steering wheel angle and the steer angle 
is hardly produced. Particularly, the active control amount includes the 
differential component or transient component .delta..sub.fd, when the 
steering wheel is manipulated at a high speed, the front wheels generate a 
relatively large slip angle and thus the steering force is increased 
abruptly as illustrated curves A in FIGS. 4B and 4C. 
When the differential component is delayed, it is possible to decrease the 
above mentioned initial abrupt change and the large decrease succeeding 
thereto can be suppressed and the steering force characteristic can be 
resembled to that of the normal vehicle without the active control for the 
steer angle as depicted by curves B in FIG. 4C. 
As shown by a curve B in FIG. 4D, the response of the yaw rate is somewhat 
made dull by delaying the differential component according to the 
invention as compared with the known apparatus without delay (curve A), 
but it is superior to that of the normal 2 WS vehicle without the active 
control (curve C). That is to say, the property for directing the head 
into a desired direction can be improved by increasing the yaw rate, while 
the large variation of the steering force can be suppressed. In this 
manner, according to the invention, the merits of the active control can 
be substantially attained and the demerit of the know active control 
apparatus can be removed. 
In the above explained embodiment, the transient component of the active 
steer angle control amount is delayed by deriving a difference between 
sample values of the steering wheel angle detected at sampling timings 
which are separated by five sampling periods. When this delaying method is 
used, it is possible to improve the resolution of the differential when 
use is made of the steering wheel angle sensor of the pulse system. That 
is to say, when the steering wheel angle is detected by the pulse type 
sensor, the resolution of the differential can be increased by delaying 
the differential in the above explained manner. 
In the above embodiment, the transient component .delta..sub.fd of the 
steer angle active control amount is formed by the first order 
differential of the steering wheel angle .theta., but according to the 
invention, it is possible to form the transient component by a linear sum 
of high order differentials generally expressed by the following equation, 
wherein T.sub.f1, T.sub.f2, T.sub.f3 are coefficients: 
EQU .delta..sub.fd T.sub.f1.theta. '+T.sub.f2.theta. ''+T.sub.f3.theta. '''+-- 
Then, one or more higher order differentials may be delayed in addition to 
the first order differential. 
According to the invention, the delay of the differential component of the 
active steer angle control amount can be realized by various methods. For 
instance, the differential component may be delayed by a digital filter. 
In case of using the digital filter, the active control amount 
.delta..sub.f of the steer angle of the front wheels may be expressed by 
.delta..sub.f =(K.sub.f +T f s).times..theta., wherein s is a Laplace 
operator. When the active control amount is passed through a digital 
filter, it can be represented by the following Laplace transforming 
equation: 
##EQU2## 
wherein .zeta. and W.sub.n are constants. 
The signal delay by using the digital filter has been well known in the art 
of the automatic control and the first term represents the delaying 
transfer function expressing the transient characteristic. .zeta. and 
W.sub.n denote a damping coefficient and the specific frequency of the 
delaying element, respectively, and T.sub.f represents the gain constant. 
In case of delaying the transient component by means of the digital 
filter, various characteristics of the digital filter such as the specific 
frequency are suitably designed to obtain the desired properties. 
As explained above in detail, according to the invention, by delaying the 
differential component of the active control amount of the steer angle of 
the front wheels, it is possible to suppress the initial abrupt increase 
and the succeeding large oscillatory variation in the steering force, so 
that the maneuverability and handling properties can be improved.