Steering control system for vehicle

A steering control system for a vehicle is provided. This system includes a front power cylinder which provides steering assistance for front wheels and a rear power cylinder which provides steering assistance for rear wheels. The system further includes a single hydraulic pump which provides a pressurized hydraulic fluid to the front and rear power cylinders and a flow dividing valve which controls a supply rate of the pressurized hydraulic fluid to the front and rear power cylinders based on vehicle speed to steer the front and rear wheels with a preselected steering angle relationship.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates generally to a steering system for a four 
wheel steering vehicle. More particularly, the invention relates to a 
system for controlling the steering angles of front and rear wheels 
according to a steered angle of a steering wheel to enhance traveling 
stability when turning. 
2. Background Art 
A steering control system is well known in the art which includes a front 
power steering unit having a control valve for switching a direction of 
working fluid supplied to one side or another of a piston in a power 
cylinder for turning front wheels synchronously with the rotation of a 
steering shaft to provide assistance in steering the front wheels to the 
right or the left. 
Recently, a steering rear wheels has been developed. This system further 
includes a rear power steering unit, as well as a front power steering 
unit, each having a control valve for controlling pressure of working 
fluid directed to a power cylinder dependent upon the lateral acceleration 
of a vehicle when turning, switching a direction of the working fluid 
applied to one or the other sides of a piston in the power cylinder 
according to vehicle speed. This control valve is operable to control the 
power cylinder for actively steering the rear wheels. The system requires 
separate pumps for building up working fluid pressure for directing 
working fluid to the front and rear power steering units. 
However, the above prior art system requires separate hydraulic control 
circuits with independent pumps therefor for steering the front and rear 
wheels respectively. This results in a bulky system. In a case where such 
pumps are driven by engine power, it imposes an added load on the engine 
causing it to be slowed down. Further, space for accommodating the system 
is limited and thus installation thereof is difficult. 
SUMMARY OF THE INVENTION 
It is accordingly one object of the present invention to avoid the 
disadvantages of the prior art. 
It is another object of the present invention to provide a compact steering 
control system for installation on a vehicle which is operable to control 
steering of front and rear wheels with minimum power consumption. 
According to one aspect of the present invention, there is provided a 
system for controlling steering operations of the front and rear wheels of 
a vehicle comprising; first means for monitoring traveling condition 
parameters of the vehicle to provide signals indicative thereof; a first 
actuator for providing steering assistance for front wheels; a second 
actuator for providing steering assistance for rear wheels; a driving 
source for applying a driving force to the first and second actuators; 
and, second means for controlling distribution of the driving force 
provided by the driving source to the first and second actuators to steer 
the front and rear wheels with a preselected steering angle relationship 
therebetween when turning based on the signals from the first means. 
In addition, the above-described aspect of the invention may also include a 
third actuator for providing assistance in steering the front wheels, the 
second means directing the driving force to the third actuator with 
blocking of the supply of the driving force to the first and second 
actuators below a preselected vehicle speed. 
According to a further aspect of the invention, a system for controlling 
steering operation of front and rear wheels of a vehicle may comprise; 
monitoring means for monitoring traveling condition parameters of the 
vehicle to provide signals indicative thereof; a first actuator for 
steering the front wheels; a second actuator for steering the rear wheels; 
a third actuator for providing assistance in steering the front wheels 
below a preselected vehicle speed so as to reduce effort required by a 
driver to steer the front wheels; a single driving source for applying a 
driving force to the first, second, and third actuators; first means for 
blocking a supply of the driving force to the first and second actuators 
and directing the driving force to the third actuator below the 
preselected vehicle speed to steer the front wheels according to rotation 
of a steering wheel; and, second means for controlling a supply rate of 
the driving force to the first and second actuators above the preselected 
vehicle speed based on the signals from the monitoring means to provide a 
preselected steering angular relationship between the front and rear 
wheels when turning.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, particularly to FIG. 1, there is shown a 
steering control system according to the present invention. This system 
includes a power steering gear 1 (i.e., a power cylinder) which is 
operable to provide steering assistance as necessary for steering front 
wheels F by changing a direction of hydraulic fluid fed from a power 
steering valve 3 to one or the other sides of the power cylinder 1 
according to rotation of a steering shaft 2. In the drawing, for the sake 
of simplicity, the power steering valve 3 is indicated away from the 
steering shaft 2, however actually the valve is disposed on the base of 
the steering shaft. 
The power cylinder 1 includes separate chambers 1b and 1c defined by a 
piston 1a. The power steering valve 3 supplies the hydraulic fluid to 
either of the chambers 1b or 1c forcing a piston rod 1d to move, and the 
movement of the piston rod provides assistance in steering the front 
wheels when turning. The power steering valve 3 is connected to a fluid 
feed line 4 and a drain line 5. 
The power cylinder 1 is installed on a vehicle frame through an insulating 
member (not shown) such as rubber so as to allow the power cylinder to 
move itself in the longitudinal direction thereof. 
Additionally, a cylinder for front wheels 6 is rigidly attached to the 
power cylinder 1 which includes a piston rod 6a slidably disposed 
therewithin for providing major steering assistance to the front wheels. 
The front cylinder 6 is fixed on the vehicle frame. 
Lateral displacement of the piston rod 6a causes the power cylinder 1 to 
move in the axial direction thereof with deformation of the insulator, 
thereby steering the front wheels at an angle corresponding to the 
movement of the power cylinder. The front cylinder 6 includes separate 
chambers defined by a piston 6b which is centered therein by spring force 
exerted by centering springs 6e and 6f. 
The system further includes a cylinder for rear wheels 7 which is adapted 
for providing assistance in steering rear wheels R according to sliding 
motion of a piston rod 7a. The rear cylinder 7 includes separate chambers 
7c and 7d defined by a piston 7b which is centered therein by spring force 
exerted by centering springs 7e and 7f. 
Pressure control of hydraulic fluid and switching of the fluid supply 
direction to either chamber of front and rear cylinders 6 and 7 are 
accomplished by front and rear control valves 8 and 9 respectively. 
The front control valve 8 is connected to a front feed line 10 and a drain 
line 11 with connection to chambers 6c and 6d of the front cylinder 6 
through hydraulic power lines 12 and 13 respectively and is operable to 
supply pressurized hydraulic fluid to either of the chambers 6c and 6d 
according to shifting motion of a valve spool (not shown) dependent upon 
controlled current if1 and if2 applied to solenoids 8a and 8b and to 
control the fluid pressure. 
The rear control valve 9 is connected to a fluid feed line for the front 
wheels 14 and the drain line 11 with connection to the chambers 7c and 7d 
of the rear cylinder 7 through hydraulic power lines 15 and 16 
respectively and is operable to supply pressurized hydraulic fluid to 
either of the chambers 7c and 7d according to shifting motion of a valve 
spool (not shown) according to controlled current ir1 and ir2 applied to 
solenoids 9a and 9b and to control the fluid pressure. 
A pump P is driven by a motor M to pump hydraulic fluid from a reservoir 
tank T, and directs it to a main power hydraulic line 17 for the power 
steering valve 3 and the front and rear control valves 8 and 9. 
The main power hydraulic line 17 communicates with a first flow dividing 
valve 18 which is then connected to the fluid feed line 4 and an active 
control feed line 19. The first flow dividing valve 18 is operable to 
control the flow rate of hydraulic fluid to the fluid feed line 4 and the 
the active control feed line 19 according to operation of a solenoid 18a 
energized by control current ip. 
Additionally, the active control feed line 19 is connected to a second flow 
dividing valve 20 which is operable to control the flow rate of hydraulic 
fluid to the fluid feed line for the front wheels 10 and the fluid feed 
line for the rear wheels 14 according to operation of a solenoid 20a 
energized by control current ih. 
The system includes a steering angle sensor 22, a vehicle speed sensor 23, 
and an acceleration sensor 24. The steering angle sensor 22 is installed 
on the steering shaft 2 for sensing a steered angle of a steering wheel to 
provide a signal .theta. indicative thereof. The vehicle speed sensor 23 
is installed on a drive shaft (not shown) to provide a signal V indicative 
of vehicle speed. The acceleration sensor 24 is installed on the vehicle 
body for sensing lateral acceleration of the vehicle to provide a signal g 
indicative thereof. The system further includes a controller 21 which 
receives the signals .theta., V, and g output from the sensors 22, 23, and 
24 respectively to provide control currents if1, if2, ir1, ir2, ip, and ih 
to the first and second flow dividing valves 18 and 20 and the control 
valves 8 and 9 for controlling operation thereof based on the vehicle 
parameters .theta., V, and g. 
The controller 21 provides control signals to the front and rear control 
valves 8 and 9 so that frequency characteristics of, for example, yaw rate 
and lateral acceleration of a vehicle are made flat to provide highly 
balanced stability and response when turning at a preselected speed or 
higher (this control is described in MOTOR FAN MAGAZINE, September, 1987, 
on pages 30 and 31; published by SANEISHOBO, Japan). In other words, a 
steering angle for the rear wheels R is controlled relative to a steered 
angle of the front wheels F with a preselected steering angle relationship 
therebetween at the preselected speed or higher to obtain driving 
stability. Control of the front and rear control valves will henceforth be 
referred to as `active control`. 
Referring to FIG. 2, control operation of the first flow dividing valve 18 
is shown. In FIG. 2, the vertical axis indicates the flow rate Q (l/m) of 
hydraulic fluid to be applied to corresponding valves, while the 
horizontal axis indicates vehicle speed V (km/h). Qp indicates capacity, 
or the total amount of fluid discharged from the pump P, Qps indicates the 
flow rate supplied to the fluid feed line 4 for the power steering gear 
(i.e., to the power steering valve 3), and Qw indicates the flow rate 
supplied to the active control valve 19 (i.e., to control valves 8 and 9). 
The graph shows that the active control is not operative below the 
preselected low speed V.sub.1 and all pressurized hydraulic fluid 
discharged from the pump is fed to power cylinder 1 via the power steering 
valve 3 to provide assistance in steering the front wheels, as the vehicle 
speed becomes high, the flow rate to the power steering valve 3 is 
decreased, while the flow rate to front and rear cylinders 6 and 7 via the 
control valves 8 and 9 is increased, and above a preselected speed 
V.sub.2, the steering force required to steer the wheels becomes quite 
small and thus steering assistance provided by the power cylinder 1 is 
unnecessary, fixing the flow rates of the Qps and Qw as indicated in the 
graph. When assistance provided by the power cylinder 1 becomes 
unnecessary, steering control is carried out via front cylinder 6 and rear 
cylinder 7. 
Under active control, the second flow dividing valve 20 increases the flow 
rate of the hydraulic fluid to the front feed line 10 when driving force 
to the front cylinder 6 is necessary (at speeds above V.sub.1) and 
increases the flow rate to the rear feed line 14 when driving force to the 
rear cylinder 7 is necessary. 
The control current ih to the second flow dividing valve 20 is proportional 
to the control current if1 and if2 to the front control valve 8 and the 
control current ir1 and ir2 of the rear control valve 9. A flow rate of 
the hydraulic fluid from the second flow dividing valve proportional to 
these current values is provided. 
In operation, when a vehicle is running at low speed, the high degree of 
steering assistance provided by the power cylinder 1 is necessary, while 
active control for the front and rear wheels is unnecessary when turning. 
Therefore, the first flow dividing valve 18 blocks the active control feed 
line 19 completely and fully opens the power steering feed line 4 so that 
all fluid discharged form the pump Qp is supplied to the power cylinder 1 
through the valve 3 with the result that the flow rate Qps becomes 
maximum. 
When the vehicle is running at intermediate speeds, the necessity of 
steering assistance is gradually decreased as the vehicle speed increases 
and active control to the front and rear wheels becomes gradually 
necessary. Thus, the first flow dividing valve 18 decreases the flow rate 
Qps to the fluid feed line 4 for the power cylinder 1 gradually, while the 
flow rate Qw to the active control feed line 19 is increased 
proportionally. 
When the vehicle is running at high speeds, the steering assistance 
provided by the power cylinder 1 is almost completely unnecessary, while 
active control is performed frequently. Therefore, the first flow dividing 
valve 18 increases the flow rate Qw to the active control feed line 19 and 
restricts the flow rate Qps to the power steering feed line 4 down to a 
minimum level so that almost all the fluid discharged from the pump Qp is 
used for active control. 
At intermediate and high speeds under active control, the second flow 
dividing valve 20 is controlled by the control currents if1, if2, ir1, and 
ir2 output to the control valves 8 and 9. As a result of this control, 
similar to that of the first flow dividing valve 18, the flow rate between 
the front feed line 10 and the rear feed line 14 is adjusted so as to 
increase the flow rate of hydraulic fluid to the cylinder (6 or 7) which 
requires more driving force. 
It will be appreciated that control of the power cylinder and steering 
control for the front and rear wheels (i.e., active control), and active 
control between the front and rear wheels are simultaneously accomplished 
with a minimum flow rate from the pump. 
It is thus apparent that the system according to the invention may be made 
compact with low energy consumption and is thus very advantageous for 
installation in a vehicle. It will further be apparent that the present 
invention, although disclosed in terms of the particular arrangement 
described hereinabove, may equivalently be achieved by changing the flow 
rate discharged from the first and second flow dividing valves according 
to control properties when turning. 
Therefore, it should be understood that various changes and modifications 
may be made without departing from the sprit and scope of the invention as 
defined by the following claims. For instance, in place of hydraulic 
fluid, a gas such as air may be used for driving force in the system. 
Further, in the embodiment, steering of the front wheels is accomplished 
by two separate hydraulic cylinders, however, this may also be 
accomplished by a single cylinder.