Rear wheel steering apparatus for an automobile

A rear wheel steering apparatus for a vehicle with hydraulic power steering for the front wheels is disclosed. Pressure sensors detect the hydraulic pressure in the two chambers of a power cylinder for the front wheels. A differential amplifier which is connected to the pressure sensors produces an output signal corresponding to the pressure difference between the chambers. An electric motor for steering the rear wheels is controlled by a control unit in response to the output signal from the differential amplifier and an output signal from a vehicle speed sensor. At high speeds, the rear wheels are steered in the same direction as the front wheels, and at low speeds, they are steered in the opposite direction. When the steering wheel is in a neutral position and a sudden lateral force acts on the vehicle to cause a cornering force to be exerted on the front wheels, the rear wheels are steered in the direction tending to increase the restoring moment acting on the vehicle.

BACKGROUND OF THE INVENTION 
This invention relates to a rear wheel steering apparatus for an 
automobile, and more particularly it relates to a rear wheel steering 
apparatus for an automobile having a hydraulic power steering mechanism 
for the front wheels. 
In recent years, devices for steering not only the front wheels but also 
the rear wheels of automotive vehicles have been developed. In general, 
the direction in which the rear wheels is steered by such devices varies 
depending on the speed of the vehicle. At high speeds, the rear wheels are 
steered in the same direction as the front wheels, as a result of which 
the vehicle responds more quickly to changes in the direction of steering. 
At very low speeds, the rear wheels are steered in the opposite direction 
from the front wheels in order to reduce the turning radius of the 
vehicle, thereby making it easier to turn the vehicle around sharp corners 
and to park it. 
Various types of rear wheel steering devices have been proposed. For 
example, U.S. Pat. No. 4,313,514 discloses a mechanical device for 
steering the rear wheels. The rotation of the steering wheel is 
transmitted to both the front and rear wheels by a mechanical linkage. The 
angle by which the rear wheels are steered is proportional to the angle by 
which the steering wheel is turned. An actuator controls the location of a 
pivot point in the linkage in accordance with the speed of the vehicle. At 
high speeds, the pivot point is positioned such that the front and rear 
wheels are steered in the same direction, and at low speeds. It is 
positioned such that the front and rear wheels are steered in opposite 
directions. 
U.S. Pat. No. 4,586,581 discloses a rear wheel steering apparatus for a 
vehicle having a hydraulic power steering apparatus for the front wheels 
in which the rear wheels are steered by hydraulic actuators. The steering 
shaft contains two control valves disposed in series, one of which 
controls the flow of oil to a power cylinder for the front wheels, and the 
other of which controls the flow of oil to the hydraulic actuators for the 
rear wheels. The angle of steering of the rear wheels is controlled in 
accordance with the torque applied to the steering wheel. 
There are a number of problems with these and other such conventional 
steering devices. When the rear wheels are steered in accordance with the 
angle of steering of the front wheels, a steering angle sensor must be 
provided. This is commonly in the form of a sensor which measures the 
rotational angle of the steering shaft or the linear movement of the rack 
of a rack and pinion steering gear. However, with this arrangement, there 
are problems with the steering shaft from the standpoint of space and 
problems with the rack from the standpoint of its surroundings. 
On the other hand, if the rear wheels are steered in accordance with the 
torque applied to the steering wheel, a device for measuring the steering 
torque must be provided. A torque bar is generally used for this purpose. 
However, if the vehicle is equipped with hydraulic power steering for the 
front wheels, a torsion bar is already housed within a control valve in 
the steering shaft. If another torque bar in the form of a torsion bar is 
employed for use in steering the rear wheels, there end up being two 
torsion bars inserted in the steering shaft in series, resulting in an 
undersirable decrease in the stiffness of the steering shaft. 
It is conceivable to employ the torsion bar of the control valve of the 
hydraulic power steering device for both the front wheels and the rear 
wheels. In this case, a torque sensor which outputs electrical signals to 
the control valve could be used. However, this arrangement has problems 
with respect to space and resistance to oil pressure of the torque sensor. 
There is the further problem that conventional rear wheel steering devices 
do not improve the directional stability of a vehicle. If a vehicle is 
travelling in a straight line, as shown in FIG. 7a, and is subjected to a 
sudden lateral force due to a gust of wind or the like, cornering forces 
F1 and F2 act on the front and rear wheels, respectively, and the vehicle 
veers from a straight course by an angle B. In a vehicle with a 
conventional rear wheel steering apparatus, if the steering wheel of the 
vehicle is maintained in a neutral position, both the front and rear 
wheels remain pointing straight ahead. Therefore, the restoring moment M 
which acts in the direction tending to return the vehicle to its original 
course is small and the vehicle continues to deviate from its original 
course, just as in a vehicle having unsteerable rear wheels. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a rear wheel 
steering apparatus for an automotive vehicle having a front wheel 
hydraulic steering device which can increase the restoring moment of the 
vehicle when a lateral force is applied to it. 
It is another object of the present invention to provide a rear wheel 
steering apparatus which does not require a torsion bar for the rear 
wheels to be implanted in the steering shaft of the vehicle. 
The present invention resides in a rear wheel steering apparatus for a 
vehicle having a hydraulic power steering device for the front wheels of 
the vehicle. The hydraulic steering device has a power cylinder which is 
divided into two chambers by a piston. The piston is connected to the 
steering linkage of the front wheels such that when there is a pressure 
difference between the two chambers of the power cylinder, an auxiliary 
steering force is exerted on the steering linkage. The rear wheels are 
steered in accordance with the speed of the vehicle and the pressure 
difference between the two chambers of the power cylinder. As a pressure 
difference results either when the driver of the vehicle turns the 
steering wheel or when a cornering force acts on the front wheels, the 
rear wheels can be turned to counteract deviations in the course of the 
vehicle which produce cornering forces, even when no steering force is 
applied to the steering wheel. 
A rear wheel steering apparatus in accordance with the present invention 
comprises means for detecting hydraulic pressure differences between the 
two chambers of the power cylinder and producing a corresponding output 
signal, means for measuring the speed of the vehicle and producing a 
corresponding output signal, steering means for steering the rear wheels, 
and control means for controlling the steering means in accordance with 
the output signals from the means for detecting hydraulic pressure 
differences and the means for measuring the vehicle speed. When there is a 
pressure difference in the power cylinder. at high vehicle speeds, the 
rear wheels are steered in the same direction as the front wheels would 
tend to be steered by the resulting auxiliary steering force. At low 
vehicle speeds, the rear wheels are steered in the opposite direction from 
the direction in which the front wheels would tend to be steered by the 
auxiliary steering force. At intermediate vehicle speeds, the rear wheels 
are maintained in a straight-ahead position. 
In a preferred embodiment, the means for detecting pressure differences 
comprises a first pressure sensor which measures the hydraulic pressure in 
the first chamber of the power cylinder and produces a corresponding 
output signal, a second pressure sensor which measures the hydraulic 
pressure in the second chamber of the power cylinder and produces a 
corresponding output signal, and a differential amplifier into which the 
output signals from the pressure sensors are input and which produces an 
electrical output signal corresponding to the difference between the 
pressures measured by the two pressure sensors. 
There is no restriction on the type of device used to steer the rear 
wheels, but in a preferred embodiment, the rear wheels are steered by an 
electric motor, a steering gear which converts the rotation of the 
electric motor into linear motion, and a steering linkage comprising tie 
rods and knuckle arms which are driven by the steering gear.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinbelow, a preferred embodiment of a rear wheel steering apparatus in 
accordance with the present invention will be described while referring to 
the accompanying drawings. FIG. 1 of which is a schematic view of a 
vehicle to which this embodiment is applied. The vehicle is equipped with 
a conventional hydraulic power steering mechanism for the front wheels. 
The steering wheel 1 of the vehicle is connected to a front wheel steering 
gearbox 3 by a steering shaft 2. The gearbox 3 contains an unillustrated, 
conventional steering gear for converting the rotation of the steering 
shaft 2 into linear movement, such as a rack and pinion steering gear. A 
right tie rod 4a and a left tie rod 4b are moved in a straight line by the 
rack. The outer ends of the tie rods 4a and 4b are respectively connected 
to right and left knuckle arms 6a and 6b, which are respectively connected 
to right and left front wheels 5a and 5b in a conventional manner. 
A hydraulic pump 7 is driven by the unillustrated engine of the vehicle. 
The input port of the pump 7 is connected to a reservoir 8 for power 
steering fluid by a first pipe 10. The discharge port of the pump 7 is 
connected to an inlet of a control valve 9 by a second pipe 11. A third 
pipe 12 which is connected to an outlet of the control valve 9 returns 
power steering fluid to the reservoir 8. 
The control valve 9, which is opened and closed in accordance with the 
torque applied to the steering wheel 1, controls the supply of power 
steering fluid to a power cylinder 13 which produces an auxiliary force 
for steering the front wheels. The power cylinder 13 is divided into a 
right chamber 13a and a left chamber 13b by a piston 13c which is 
connected to the left tie rod 4b. The right and left chambers of the power 
cylinder 13 are connected to the control valve 9 by a fourth pipe 14 and a 
fifth pipe 15, respectively. Power steering fluid which is pumped by the 
hydraulic pump 7 can be introduced into either the right or left chamber 
via the fourth and fifth pipes. 
A right hydraulic pressure sensor 16 and a left hydraulic pressure sensor 
17 are disposed so as to detect the pressures in the right and left 
chambers, respectively, of the power cylinder 13. Each of the pressure 
sensors produces an electrical output signal which corresponds to the 
detected pressure and which is input to a differential amplifier 18. The 
differential amplifier 18 produces an electrical output signal which 
corresponds to the difference in the levels of the input signals from the 
pressure sensors and therefore to the difference in pressure between the 
right and left chambers of the power cylinder 13. The output signal from 
the differential amplifier 18 is input to a control unit 24. A speed 
sensor 23 which detects the speed of the vehicle produces an electrical 
output signal which corresponds to the speed and provides it as an input 
signal to the control unit 24. 
The control unit 24 controls an electric motor 25 for steering the rear 
wheels of the vehicle. An electromagnetic brake 26 for braking the motor 
25 is connected to the output shaft of the motor 25. The electromagnetic 
brake 26, which is also controlled by the control unit 24, is of the type 
which is released when a current is supplied thereto and which exerts a 
braking force when the supply of current to the brake 26 is stopped. 
The output shaft of the motor 25 is connected to a reduction gear 27 
comprising a worm and a worm wheel, for example. The output shaft of the 
reduction gear 27 is connected to a rear wheel steering gearbox 19. The 
gearbox 19 contains a steering gear such as a rack and pinion steering 
gear for converting the rotation of the output shaft of the reduction gear 
27 into linear motion, which is transmitted to right and left tie rods 20a 
and 20b, respectively. The outer ends of the tie rods are connected to 
right and left rear wheels, 21a and 21b, through right and left knuckle 
arms 22a and 22b, respectively. 
The angle of rotation of the output shaft of the reduction gear 27 is 
detected by an angle sensor 28 which is mounted on the gearbox 27. The 
angle sensor 28 produces an electrical output signal which corresponds to 
the detected angle and which is input to the control unit 24. A rotational 
speed sensor 29 which is mounted on the motor 25 detects the rotational 
speed of the motor 25 and produces a corresponding electrical output 
signal, which is also input to the control unit 24. 
Based on the input signals from the differential amplifier 18 and the speed 
sensor 23, the control unit 24 calculates the direction in which and the 
angle by which the rear wheels should be steered, and it controls the 
motor 25 so as to achieve the calculated direction and angle of steering. 
The operation of the illustrated embodiment is as follows. When the engine 
of the vehicle is started, the hydraulic pump 7 is driven and pumps power 
steering fluid from the reservoir 8 and circulates it through the second 
pipe 11, the control valve 9, and the third pipe 12 back to the reservoir 
8. When the steering wheel 1 is turned to the right, the control valve 9 
enables power steering fluid under pressure to flow into the right chamber 
13a of the power cylinder 13, and the pressure acting on the piston 13c 
helps push the left tie rod 4b to the left, thereby reducing the steering 
torque which the driver must exert. On the other hand, when the steering 
wheel 1 is turned to the left, the control valve 9 enables power steering 
fluid to flow into the left chamber 13b of the power cylinder 13, and the 
pressure acting on the piston 13c helps push the left tie rod 4b to the 
right. 
FIG. 2 is a graph of the hydraulic pressure in the right or left chamber of 
the power cylinder 13 as a function of steering torque when the steering 
wheel 1 is turned to the left or right, respectively. As can be seen from 
this graph, the hydraulic pressure increases parabolically with increasing 
steering torque, and there is a slight hysteresis in the relationship 
between pressure and steering torque as the steering torque rises and 
falls. 
The pressure sensors 16 and 17 detect the pressures in the right and left 
chambers of the power cylinder 13 and produce corresponding output 
signals. The differential amplifier 18 produces an electrical output 
signal corresponding to the difference between the two input signals. 
Based on the electrical output signal from the differential amplifier 18 
and the output signal from the speed sensor 23, the control unit 24 
computes a target steering angle At for the rear wheels. 
The relationship between the target angle At and the pressure difference 
between the right and left chambers of the power cylinder 13 is 
illustrated in FIG. 5. The pressure difference equals the pressure in the 
right chamber 13a minus the pressure in the left chamber 13b, and a 
positive steering angle corresponds to rightwards steering of the rear 
wheels while a negative steering angle corresponds to leftwards steering. 
At a high vehicle speed of 75 km/hr and above, the target steering angle 
At increases continuously as the pressure difference increases and has the 
same sign as the steering angle for the front wheels, meaning that the 
front and rear wheels are steered in the same direction. At a low vehicle 
speed of less than 10 km/hr, when a prescribed pressure difference 
develops between the chambers of the power cylinder 13, the rear wheels 
are steered by the maximum amount (approximately 2.5 degrees) in the 
direction opposite to the front wheels. In an intermediate speed range of 
10 to 75 km/hr, the rear wheels are not steered, regardless of the 
pressure difference. 
The control unit 24 receives an input signal from the rotational angle 
sensor 28 corresponding to the present steering angle A of the rear 
wheels, and it determines the error Ae(=At-A) between the target steering 
angle At and the present steering angle A. 
The control unit 24 also computes a target rotational speed Nt for the 
motor 25, and determines the error Ne(=Nt-N) between the target rotational 
speed Nt and the actual rotational speed N, which is indicated by the 
output signal from the rotational speed sensor 29. 
Based on the steering angle error Ae and the rotational speed error Ne, the 
control unit 24 controls the direction and speed of rotation of the motor 
25. 
When the vehicle is moving with the steering wheel 1 in the neutral 
(straight ahead) position and no cornering force is acting on the front 
wheels, a hydraulic pressure of approximately 2-3 kgf/square cm will exist 
in both chambers of the power cylinder 13 due to the operation of the pump 
7. As the output signals of the two pressure sensors 16 and 17 will be 
substantially equal, the output of the differential amplifier 18 will be 
substantially zero, and the target steering angle At calculated by the 
control unit 24 will be zero. If the rear wheels should deviate from a 
neutral position, the control unit 24 will drive the motor 25 to return 
the rear wheels to the neutral position. 
Next the case will be described in which the vehicle is traveling at a high 
speed and a torque is applied to the steering wheel 1. If the vehicle 
speed is 150 km/hr and the driver applies a clockwise torque of 0.4 
kgf.times.m to the steering wheel 1, a pressure difference between the two 
chambers of the power cylinder 13 will be produced. For a steering torque 
of 0.4 kgf.times.m, a pressure of approximately 20 kgf per square cm will 
develop in the right chamber 13a, as shown by point a in FIG. 2. 
However, there will still be a hydraulic pressure of 2-3 kg per square cm 
in the left chamber 13b, so the pressure difference between the two 
chambers of the power cylinder 13 will be approximately 17-18 kgf per 
square cm, as shown by point a of FIG. 3. The differential amplifier 18 
will produce an output signal corresponding to this pressure difference. 
Based on the detected pressure difference, the control unit 24 will 
calculate a target steering angle At. For a pressure difference of 17-18 
kgf per square cm, the target steering angle At is approximately 0.7 
degrees, as shown by point b of FIG. 5. If the present steering angle A of 
the rear wheels is 0 degrees, then the error Ae is equal to 0.7 degrees. 
As 0.7 degrees is a large, positive value, the control unit 24 selects a 
high value of approximately 2000 rpm for the target rotational speed Nt of 
the motor 25. If the motor 25 is stationary, the error Ne between the 
target rotational speed Nt and the present rotational speed N is 2000 rpm, 
and the control unit 24 applies a full voltage to the motor 25 to make it 
steer the rear wheels to the right. When the rotational speed N of the 
motor 25 nears the target rotational speed Nt (when the error Ne becomes 
less than approximately 200 rpm), the rotational speed of the electric 
motor 25 is held constant by the control unit 24, and if the actual speed 
N exceeds the target rotational speed Nt by at least 200 rpm, the 
rotational speed of the electric motor 25 is reduced towards the target 
rotational speed Nt. 
The rotation of the electric motor 25 is transmitted to the reduction gear 
27 through the brake 26. The reduction gear 27 steers the rear wheels to 
the right through the steering gear within the steering gearbox 19 and the 
steering linkage comprising the tie rods 20a and 20b and the knuckle arms 
22 and 22a. 
The control unit 24 continuously calculates the error Ae between the actual 
steering angle A and the target steering angle At. As the error Ae 
decreases, the control unit 24 decreases the target rotational speed Nt. 
When the error Ae is near zero (less than or equal to approximately 0.05 
degrees), the target rotational speed Nt is set to zero, and a halt 
command is issued to the electric motor 25. 
When a counterclockwise torque is applied to the steering wheel 1 while the 
vehicle is running at a high speed, the rear wheels are steered to the 
left by a process similar to that described above for a clockwise steering 
torque. 
When the vehicle is traveling at an extremely low speed of less than 10 km 
per hour, if the steering wheel is turned by the maximum amount in either 
direction, the unillustrated rack in the front steering gearbox 3 will hit 
a stopper, and the pressure difference between the two chambers of the 
power cylinder 13 will abruptly rise to a high value of approximately 50 
kg per square cm. In this case, the control unit 24 will control the motor 
25 so as to steer the rear wheels by a maximum angle (approximately 2.5 
degrees) in the opposite direction from the front wheels. Steering the 
rear wheels in this manner will decrease the turning radius of the vehicle 
and increase its turning ability. 
When a cornering force acts on the front wheels, an axial force in the same 
direction as the cornering force is exerted on the rack of the front wheel 
steering gear. This force on the rack produces a pressure difference 
between the two chambers of the power cylinder 13, just as when a torque 
is applied to the steering wheel 1. The relationship between the force 
acting on the rack of the steering gear and the pressure difference in the 
power cylinder 13 is illustrated in FIG. 4. When the cornering force and 
the force on the rack are rightwards, the piston 13c is pushed rightwards. 
The pressure in the right chamber 13a of the power cylinder 13 is 
increased over that in the left chamber 13b, so a positive pressure 
difference is produced. When the cornering force and the force on the rack 
are leftwards, the pressure in the left chamber 13b is increased over that 
in the right chamber 13a, so a negative pressure difference is produced. 
The differential amplifier 18 produces an output signal corresponding to 
this pressure difference, and the control unit 24 controls the motor 25 so 
as to steer the rear wheels in the direction in which the cornering force 
on the front wheels acts. 
When the vehicle is traveling in a straight line with the steering wheel 1 
in the neutral position and a sudden lateral force (such as a strong gust 
of wind) is exerted on the vehicle. the vehicle will be made to deviate 
from its original course by an angle B, as shown in FIG. 7b. As a result, 
cornering forces F1 and F2 corresponding to the angle of deviation B are 
exerted on the front and rear wheels, respectively. If all the wheels were 
maintained in a neutral position, only a small restoring moment M would 
act on the vehicle and it would continue to veer from its original course. 
However, in a vehicle equipped with the present invention, the cornering 
force F1 acting on the front wheels produces a pressure difference in the 
power cylinder 13. Based on the pressure difference, the rear wheels are 
steered in the direction of the front wheel cornering force F1 by an angle 
A, as shown in FIG. 7b. Steering the rear wheels in the direction of the 
front wheel cornering force F1 increases the rear wheel cornering force 
F2, as a result of which a large restoring moment M is generated which 
returns the vehicle to its original course. 
As can be seen from the above description, a rear wheel steering apparatus 
in accordance with the present invention increases the directional 
stability of a vehicle with respect to lateral forces. In addition, it 
does not require a torsion bar for use in steering the rear wheels, so it 
is not necessary to install two torsion bars in series in the steering 
shaft, and the stiffness of the steering shaft is not decreased. 
Furthermore, the present invention does not require a steering angle 
sensor for detecting the steering angle of the front wheels, so it can be 
more compact than conventional rear wheel steering apparatuses which 
employ a steering angle sensor.