Rear wheel steering apparatus for automobile

A rear wheel steering apparatus for an automobile detects a vehicle speed and turns rear wheels at a rear wheel turning angle ratio according to the detected vehicle speed upon turning of front wheels. The apparatus includes a vehicle speed detection section for detecting a vehicle speed, a setting section for setting a rear wheel turning angle ratio in accordance with the vehicle speed detected by the vehicle speed detection section, a travel detection section detecting a travel state of a vehicle, a change section for changing turning angle ratio control by the setting section to fail-safe control when a detection output from the vehicle speed detection section is zero and the travel detection section detects the travel state of the vehicle, a brake detection section for detecting a brake operation state, and an inhibition section for inhibiting a change from the normal turning angle ratio control to the fail-safe control by the change section when the brake detection section detects a brake operation state.

BACKGROUND OF THE INVENTION 
The present invention relates to a rear wheel steering apparatus for an 
automobile for turning rear wheels together with front wheels upon 
operation of a steering wheel and, more particularly, to a rear wheel 
steering apparatus for an automobile capable of changing a ratio of a rear 
wheel turning angle to a front wheel turning angle (to be referred to 
simply as a rear wheel turning angle ratio hereinafter) in accordance with 
a vehicle speed. 
A known four-wheel steering apparatus comprising a rear wheel steering 
apparatus for an automobile (or vehicle) turns front and rear wheels in 
accordance with an operation of a steering wheel, and changes the rear 
wheel turning angle ratio based on a predetermined rear wheel turning 
angle ratio characteristic in accordance with a vehicle speed, as 
disclosed in Japanese Patent Publication No. 60-44185. In this case, the 
rear wheel turning angle ratio characteristic is set as follows. More 
specifically, the front and rear wheels are turned in an opposite phase 
state at a low vehicle speed lower than a predetermined vehicle speed 
(e.g., 35 km/h) to improve a turning radius of a vehicle. At the 
predetermined vehicle speed, the rear wheels are not turned to set a 
so-called 2WS mode. At a high vehicle speed higher than the predetermined 
vehicle speed, the front and rear wheels are steered in an in-phase state 
to set a so-called 4WS mode, thereby improving travel stability of a 
vehicle (lane change stability). 
In a conventional rear wheel steering apparatus of this type, when a 
vehicle travels along a curved road at a vehicle speed higher than the 
predetermined vehicle speed, since the front and rear wheels are turned in 
the in-phase state, a so-called high-speed cornering characteristic can be 
maintained well. During high-speed cornering, if a driver finds an 
obstacle ahead and quickly depresses a brake pedal, the vehicle is quickly 
decelerated, and the rear wheel turning angle ratio is quickly changed to 
the opposite phase state, accordingly. For this reason, in order to 
maintain a stable vehicle position during cornering and to assure stable 
travel, the front and rear wheels must firmly grip the road surface. 
Nevertheless, the degree of in-phase state is decreased, and finally, the 
rear wheels are turned in the opposite phase state. As a result, an 
inertial force acts in a vehicle body to generate a yaw rate, and 
accidental turning tendency of the vehicle is enhanced. Such a phenomenon 
appears as a so-called tuck-in phenomenon. 
In particular, when the rear wheels are locked and slip by braking, the 
following problems are posed. That is, in this case, although the vehicle 
body travels at a certain speed, a vehicle speed sensor used for 
controlling the rear wheel turning angle ratio detects a wheel speed. For 
this reason, the output from the vehicle speed sensor immediately becomes 
zero upon locking. As a result, the rear wheel turning angle ratio is 
immediately changed to the maximum value of the opposite phase state 
according to the detection result from the vehicle speed sensor. In 
combination with a decrease in gripping force of tires upon locking, a 
very unstable travel state may occur. 
For this reason, as disclosed in Japanese Utility Model Publication No. 
62-5974, in a vehicle-speed sensitive rear wheel steering apparatus for a 
vehicle, when a quick deceleration of a vehicle is detected, the rear 
wheel turning angle ratio is set in an in-phase range to obtain a steering 
angle corresponding to the travel condition of the vehicle (an angle for 
increasing a slip angle), thereby preventing so-called tuck-in. 
As disclosed in Japanese Patent Laid-Open (Kokai) No. 59-81275, in a 
vehicle-speed sensitive rear wheel steering apparatus for a vehicle, when 
a quick deceleration of a vehicle is detected, a change in rear wheel 
turning angle ratio is delayed to prevent the above-mentioned tuck-in 
phenomenon. 
As a known means for detecting the quick deceleration of the vehicle, a 
vehicle speed sensor is used, and the quick deceleration is determined 
when the output from the vehicle speed sensor is quickly decreased. 
Alternatively, both a brake sensor and the vehicle speed sensor are used, 
and the quick deceleration is determined when the output from the vehicle 
speed sensor is quickly decreased after a brake pedal is depressed. 
As disclosed in Japanese Patent Laid-Open (Kokai) No. 59-81274, upon quick 
deceleration, as alternative means of a technique for delaying a change in 
rear wheel turning angle ratio, a means for fixing the rear wheel turning 
angle ratio or a means for setting the rear wheel turning angle ratio to 
"0" to set the 2WS mode is known. 
However, in these prior arts, after the quick deceleration of a vehicle is 
detected, the rear wheel turning angle ratio is controlled to prevent 
tuck-in. For this reason, the following problems are posed. 
More specifically, when a quick deceleration of a vehicle is detected, a 
time of generation of the quick deceleration is very short, e.g., in the 
order of several hundreds of msec. Meanwhile, a sampling time of the 
vehicle speed sensor for controlling the rear wheel turning angle ratio is 
131 msec at present. For this reason, in actual deceleration detection, 
only 2 to 3 detection results can only be obtained every 131 msec, and it 
is impossible to calculate the deceleration per unit time based on such a 
small amount of data so as to accurately determine whether or not the 
present deceleration falls in a quick deceleration range. In this manner, 
the conventional apparatus can only discriminate a deceleration with 
considerable inaccuracy. 
When the rear wheel turning angle ratio is to be changed based on such an 
inaccurate quick deceleration discrimination result, a quick deceleration 
may be erroneously discriminated even if it is not so. If such erroneous 
discrimination is made, although the rear wheel turning angle ratio should 
be set in an opposite phase state as the vehicle speed is decreased, the 
in-phase state is maintained. Although the rear wheel turning angle ratio 
should be, for example, fixed to maintain a safe travel state when a 
vehicle is quickly decelerated in practice, the quick deceleration cannot 
be determined due to inaccuracy of the vehicle speed sensor, and the rear 
wheel turning angle ratio is not fixed but may be changed in an opposite 
phase state. Thus, a travel property of the vehicle is greatly impaired 
due to inaccuracy of the current vehicle speed sensor. 
Since this vehicle speed sensor is conventionally of a contact type, 
so-called chattering often occurs. When chattering occurs in the vehicle 
speed sensor, a control mechanism determines a quick acceleration since an 
input signal is temporarily increased. Meanwhile, when the chattering 
terminates, the normal number of input signals is recovered. Therefore, 
the control mechanism determines a quick deceleration as a reaction of the 
preceding quick acceleration discrimination. 
In this manner, when a quick deceleration is erroneously determined, the 
rear wheel turning angle ratio is fixed in the in-phase state although it 
need not be fixed, and the travel property of the vehicle is impaired. 
In the vehicle-speed sensitive rear wheel steering apparatus using the 
existing vehicle speed sensor, a travel property may be impaired due to 
inaccuracy of the vehicle speed sensor, and since only the quick 
deceleration is taken into consideration, an unnecessary or erroneous 
fixing operation of a rear wheel turning angle ratio is performed, and a 
travel property is spoiled. 
When a quick deceleration of a vehicle is detected or when both the vehicle 
speed sensor and the brake sensor are used, the following problems are 
pointed out in addition to the above-mentioned problems of the vehicle 
speed sensor. 
When a vehicle travels along a road of a bad condition, if a wheel climbs 
over a projecting portion of a three-dimensional pattern on the road 
surface while a brake pedal is being depressed, the rotational speed of 
the wheel is instantaneously decreased, and the detection result of the 
vehicle speed sensor may fall in the quick deceleration range. In this 
case, if this travel state occurs during cornering, the rear wheel 
steering angle ratio is fixed in the in-phase state in the conventional 
apparatus. Therefore, even if a driver intends to perform cornering with 
good turnability in the opposite phase state, the vehicle cannot be turned 
well, and a travel property is considerably impaired. 
In this manner, in the conventional apparatus, when the brake pedal is 
quickly depressed to avoid danger during high-speed cornering, since 
control for fixing the rear wheel turning angle ratio in the in-phase 
state is performed in consideration of only the quick deceleration in the 
vehicle speed sensor, an unnecessary or erroneous fixing operation of the 
rear wheel turning angle ratio is performed, and a travel property is 
spoiled. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of the above problems, 
and has as its first object to provide a rear wheel steering apparatus for 
an automobile which can satisfactorily maintain a travel property matching 
with a travel state during cornering. 
In the prior art, if a deceleration is detected during turning at a 
constant steering angle, a rear wheel turning angle ratio K is changed to 
change the rear wheel turning angle even at the constant steering angle. 
As a result, the tuck-in phenomenon occurs, and a driver feels uneasy and 
cannot easily drive the vehicle. In consideration of this problem, the 
prior art employs a means for fixing the rear wheel turning angle ratio 
during deceleration, and so on. 
That is, it can be considered that the prior art employs a vehicle body 
speed (actual vehicle speed) and a value of a vehicle speed sensor 
(detected vehicle speed) while they coincide with each other. Therefore, 
the prior art adopts an arrangement wherein a rear wheel turning angle 
ratio is fixed at a point corresponding to a deceleration (B) for 
preventing tuck-in or more. As can be understood from this, there is no 
doubt that the prior art corresponds to the generic concept of the present 
invention wherein the rear wheel turning angle ratio is fixed at a point 
corresponding to a deceleration (A) at which wheels are locked. 
However, since the prior art has (B) as an object to be controlled, it only 
detects a deceleration, and in this case, various problems are posed as 
described in this specification. Since often not only (B) but also (A) may 
not be detected, a problem is posed. In this invention, a deceleration 
region of (B) corresponds to a region wherein the vehicle body speed 
coincides with the value of the vehicle speed sensor. Therefore, the rear 
wheel turning angle ratio can be controlled based on the sensor value, 
thus posing no problem. However, in a region of (A), it is dangerous if 
the rear wheel turning angle ratio cannot be reliably fixed. Thus, the 
present invention provides control capable of reliably detecting the 
region of (A). 
In the region of (A), the vehicle body speed is different from the value of 
the vehicle speed sensor, the rear wheel turning angle ratio becomes a 
maximum value in the opposite phase state, and the tires lose a gripping 
force since the wheel are locked. Therefore, the present inventors 
considered that this region must be reliably controlled. 
In order to attain the above first object, according to a first aspect of 
the present invention, there is provided a rear wheel steering apparatus 
for an automobile for detecting a vehicle speed and turning rear wheels 
based on a rear wheel turning angle ratio according to the detected 
vehicle speed in accordance with a turning operation of front wheels, 
comprising: discrimination means for discriminating a travel state; first 
setting means for, when the discrimination means discriminates a normal 
travel state, setting a rear wheel turning angle ratio based on a first 
characteristic according to the detected vehicle speed; and second setting 
means for, when the discrimination means detects that wheels are locked, 
setting the rear wheel turning angle ratio based on a second 
characteristic for stabilization. 
In the first aspect of the rear wheel steering apparatus for an automobile 
with the above arrangement, when locking of wheels is detected, a control 
mode is changed from normal rear wheel turning angle ratio change control 
to rear wheel turning angle ratio change control having a characteristic 
for stabilization. In this manner, even if wheels are locked, a travel 
state of a vehicle can be safely controlled. 
It is a second object of the present invention to provide a rear wheel 
steering apparatus for an automobile, in which when lock control is 
performed upon detection of a lock state of wheels in order to 
satisfactorily maintain a travel property matching with a travel condition 
during cornering, fail-safe control is not erroneously performed and lock 
control can be reliably performed. 
In order to attain the second object, according to a second aspect of the 
present invention, there is provided a rear wheel steering apparatus for 
an automobile for detecting a vehicle speed and turning rear wheels based 
on a rear wheel turning angle ratio according to the detected vehicle 
speed in accordance with a turning operation of front wheels, comprising: 
vehicle speed detection means for detecting a vehicle speed; setting means 
for setting a rear wheel turning angle ratio in accordance with the 
vehicle speed detected by the vehicle speed detection means; abnormality 
discrimination means for discriminating that an output from the vehicle 
speed detection means is quickly decreased over a predetermined value; 
change means for, when the abnormality discrimination means detects that 
the output from the vehicle speed detection means is quickly decreased, 
changing rear wheel turning angle control by said setting means to 
fail-safe control; brake detection means for detecting a brake operation 
state; and inhibition means for, when the brake detection means detects 
the brake operation state, inhibiting a change from the normal rear wheel 
turning angle ratio control to fail-safe control by the change means. 
In the second aspect of the rear wheel steering apparatus for an automobile 
with the above arrangement, a lock state of wheels is detected under the 
condition that the brake detection means detects the brake operation 
state. A quick decrease in output from the vehicle speed detection means 
while the brake operation state is detected is discriminated not as a 
failure but as a lock state of the wheels. In this manner, when the brake 
detection means detects the brake operation state, a change from normal 
rear wheel turning angle ratio control to fail-safe control in the change 
means is inhibited. 
It is a third object of the present invention to provide a rear wheel 
steering apparatus for an automobile, which can reliably prevent rear 
wheels from being turned at a wrong turning angle ratio upon erroneous 
lock detection of wheels. 
In order to attain the above third object, according to a third aspect of 
the present invention, there is provided a rear wheel steering apparatus 
for an automobile for detecting a vehicle speed and turning rear wheels 
based on a rear wheel turning angle ratio according to the detected 
vehicle speed in accordance with a turning operation of front wheels, 
comprising: vehicle speed detection means for detecting a vehicle speed; 
discrimination means for discriminating a travel state; first setting 
means for, when the discrimination means discriminates a normal travel 
state, setting a rear wheel turning angle ratio based on a first 
characteristic according to the detected vehicle speed; second setting 
means for, when the discrimination means detects that wheels are locked, 
setting the rear wheel turning angle ratio based on a second 
characteristic different from the first characteristic; and inhibition 
means for, when a detection output from the vehicle speed detection means 
is quickly increased over a predetermined value, inhibiting setting in the 
second setting means. 
In the third aspect of the rear wheel steering apparatus for an automobile 
with the above arrangement, when a lock state of wheels is detected, a 
control mode is changed from normal turning angle ratio control based on 
the first turning angle ratio characteristic to turning angle ratio 
control based on the second characteristic different from the first 
turning angle ratio characteristic. When the detection output from the 
vehicle speed detection means is quickly increased over the predetermined 
value, the inhibition means inhibits setting in the second setting means. 
In this manner, even if a vehicle speed is quickly increased, the rear 
wheel turning angle ratio is kept to have the first characteristic, and 
the automobile can be controlled with good travel property. 
It is a fourth object of the present invention to provide a rear wheel 
steering apparatus for an automobile, in which when lock control is 
performed upon detection of a lock state of wheels in order to 
satisfactorily maintain a travel property matching with a travel condition 
during cornering, after the lock state of the wheels is released, lock 
control is quickly canceled, and normal rear wheel turning angle ratio 
control is recovered to keep travel stability in a normal travel state and 
to improve product quality. 
The fourth object can be attained by fourth to seventh aspects of a rear 
wheel steering apparatus for an automobile below. 
That is, in order to attain the fourth object described above, according to 
a fourth aspect of the present invention, there is provided a rear wheel 
steering apparatus for an automobile for detecting a vehicle speed and 
turning rear wheels based on a rear wheel turning angle ratio according to 
the detected vehicle speed in accordance with a turning operation of front 
wheels, comprising: vehicle speed detection means for detecting a vehicle 
speed; lock detection means for detecting a lock state of wheels; setting 
means for, when the lock detection means detects the lock state of the 
wheels, changing a rear wheel turning angle ratio from a normal first 
characteristic to a second characteristic for stabilization to set a lock 
control state; and cancel means for, when a predetermined period of time 
has passed from detection of the lock state of the wheels by the lock 
detection means, changing the rear wheel turning angle ratio from the 
second characteristic to the first characteristic to cancel the lock 
control state. 
In the fourth aspect of the rear wheel steering apparatus for an automobile 
with the above arrangement, lock control is canceled after a predetermined 
period of time has passed from detection of the lock state of the wheels. 
In this manner, after the lapse of the predetermined period of time, the 
vehicle speed detected by the vehicle speed detection means coincides with 
an actual vehicle speed. Since cancellation of the lock state of the 
wheels is discriminated in this manner, after the lock state of the wheels 
is actually released, the lock control state can be reliably canceled, and 
a normal rear wheel turning ratio angle control state can be recovered. 
Thus, travel stability in a normal travel state can be maintained, and 
product quality can be improved. 
In order to attain the fourth object, according to a fifth aspect of the 
present invention, there is provided a rear wheel steering apparatus for 
an automobile for detecting a vehicle speed and turning rear wheels based 
on a rear wheel turning angle ratio according to the detected vehicle 
speed in accordance with a turning operation of front wheels, comprising: 
vehicle speed detection means for detecting a vehicle speed; lock 
detection means for detecting a lock state of wheels; setting means for, 
when the lock detection means detects the lock state of the wheels, 
changing a rear wheel turning angle ratio from a normal first 
characteristic to a second characteristic for stabilization to set a lock 
control state; and cancel means for, when the vehicle speed detected by 
the vehicle speed detection means substantially coincides with an actual 
vehicle speed, changing the rear wheel turning angle ratio from the second 
characteristic to the first characteristic to cancel the lock control 
state. 
In the fifth aspect of the rear wheel steering apparatus for an automobile 
with the above arrangement, after the lock state of the wheels is 
detected, release of the lock state of the wheels is discriminated when 
the vehicle speed detected by the vehicle speed detection means 
substantially coincides with an actual vehicle speed. Since release of the 
lock state of the wheels is discriminated in this manner, the lock control 
state can be quickly canceled after the lock state of the wheels is 
actually released, and a normal rear wheel turning angle control state can 
be recovered. Thus, travel stability in a normal travel state can be 
maintained, and product quality can be improved. 
In order to attain the fourth object, according to a sixth aspect of the 
present invention, there is provided a rear wheel steering apparatus for 
an automobile for detecting a vehicle speed and turning rear wheels based 
on a rear wheel turning angle ratio according to the detected vehicle 
speed in accordance with a turning operation of front wheels, comprising: 
vehicle speed detection means for detecting a vehicle speed; lock 
detection means for detecting a lock state of wheels; setting means for, 
when the lock detection means detects the lock state of the wheels, 
changing a rear wheel turning angle ratio from a normal first 
characteristic to a second characteristic for stabilization to set a lock 
control state; brake detection means for detecting a depression state of a 
brake pedal; and cancel means for, when a predetermined period of time has 
passed after detection of depression of the brake pedal by the brake 
detection means is stopped, changing the rear wheel turning angle ratio 
from the second characteristic to the first characteristic in the setting 
means to cancel the lock control state. 
In the sixth aspect of the rear wheel steering apparatus for an automobile 
with the above arrangement, lock control is canceled when the 
predetermined period of time has passed after a braking force to the 
wheels is released. In this manner, after the lapse of the predetermined 
period of time, the wheels grip a road surface, and the vehicle speed 
detected by the vehicle speed detection means substantially coincides with 
an actual vehicle speed. When cancellation of the lock state of the wheels 
is discriminated in this manner, it is determined that no trouble is 
caused if the lock control state is canceled after the lapse of the 
predetermined period of time from detection of release of depression of 
the brake pedal by the brake detection means, and a normal rear wheel 
turning angle ratio control state is recovered. As a result, travel 
stability in a normal travel state can be assured, and product quality can 
be improved. 
In order to attain the fourth object, according to a seventh aspect of the 
present invention, there is provided a rear wheel steering apparatus for 
an automobile for detecting a vehicle speed and turning rear wheels based 
on a rear wheel turning angle ratio according to the detected vehicle 
speed in accordance with a turning operation of front wheels, comprising: 
vehicle speed detection means for detecting a vehicle speed; lock 
detection means for detecting a lock state of wheels; setting means for, 
when the lock detection means detects the lock state of the wheels, 
changing a rear wheel turning angle ratio from a normal first 
characteristic to a second characteristic for stabilization to set a lock 
control state; and cancel means for, when a vehicle speed signal excluding 
a signal indicating that a vehicle speed is zero is continuously input 
from the vehicle speed detection means for a predetermined period of time, 
changing the rear wheel turning angle ratio from the second characteristic 
to the first characteristic to cancel the lock control state. 
In the seventh aspect of the rear wheel steering apparatus for an 
automobile with the above arrangement, when vehicle speed data excluding 
vehicle speed zero data is input from the vehicle speed detection means 
for a predetermined period of time after the lock state of the wheels is 
detected, it is determined that the lock state of the wheels is released 
and the wheels grip a road surface. When release of the lock state of the 
wheels is determined in this manner, the lock control state can be quickly 
canceled, and a normal rear wheel turning angle ratio control state can be 
recovered. As a result, travel stability in a normal travel state can be 
assured, and product quality can be improved. 
It is a fifth object of the present invention to provide a rear wheel 
steering apparatus for an automobile which can quickly determine a lock 
state of wheels, and can reliably perform lock control. 
In order to attain the fifth object, according to an eighth aspect of the 
present invention, there is provided a rear wheel steering apparatus for 
an automobile for detecting a vehicle speed and turning rear wheels based 
on a rear wheel turning angle ratio according to the detected vehicle 
speed in accordance with a turning operation of front wheels, comprising: 
a plurality of vehicle speed detection means for detecting a vehicle 
speed; setting means for setting a rear wheel turning angle ratio in 
accordance with the vehicle speeds detected by the plurality of vehicle 
speed detection means; and discrimination means for discriminating a lock 
state of wheels using the detection output from one of the plurality of 
vehicle speed detection means. 
In the eighth aspect of the rear wheel steering apparatus for an automobile 
with the above arrangement, a detected vehicle speed used for setting a 
rear wheel turning angle ratio is determined based on vehicle speeds 
detected by two vehicle speed detection means, and a detected vehicle 
speed used for discriminating a lock state of the wheels employs a 
detected vehicle speed obtained from one of the two vehicle speed 
detection means. In this manner, the detected vehicle speed for lock 
discrimination is directly input to the discrimination means without being 
subjected to data processing, and lock discrimination can be quickly 
executed. 
It is a sixth object of the present invention to provide a rear wheel 
steering apparatus for an automobile, in which when a lock state of wheels 
is detected to perform lock control in order to satisfactorily maintain 
travel property matching with a travel state during cornering, erroneous 
fail-safe control is prevented from being executed, and lock control can 
be reliably performed in an actual travel state, and to provide a rear 
wheel steering apparatus for an automobile which can prevent fail-safe 
control from being erroneously executed. 
In order to attain the sixth object, according to a ninth aspect of the 
present invention, there is provided a rear wheel steering apparatus for 
an automobile for detecting a vehicle speed and turning rear wheels based 
on a rear wheel turning angle ratio according to the detected vehicle 
speed in accordance with a turning operation of front wheels, comprising: 
vehicle speed detection means for detecting a vehicle speed; setting means 
for setting a rear wheel turning angle ratio in accordance with the 
vehicle speed detected by the vehicle speed detection means; travel 
detection means for detecting a travel state of a vehicle; change means 
for, when a detection output from the vehicle speed detection means is 
zero and the travel detection means detects the travel state of the 
vehicle, changing turning angle ratio control by the setting means to 
fail-safe control; brake detection means for detecting a brake operation 
state; and inhibition means for, when the brake detection means detects 
the brake operation state, inhibiting a change from the normal turning 
angle ratio control to the fail-safe control by the change means. 
In the ninth aspect of the rear wheel steering apparatus for an automobile 
with the above arrangement, the lock state of wheels is detected under the 
condition that the brake detection means detects the brake operation 
state. Thus, an output zero state of the vehicle speed detection means in 
a state wherein the brake operation state is detected is determined not as 
a failure but as a lock state of wheels. An output zero state of the 
vehicle speed means in a state wherein the brake operation state is 
detected while the vehicle stands still is determined not as a failure but 
as a check state. As a result, when the brake operation state is detected 
by the brake detection means, a change from normal turning angle ratio 
control to fail-safe control by the change means is inhibited. 
Other features and advantages of the present invention will be apparent 
from the following description taken in conjunction with the accompanying 
drawings, in which like reference characters designate the same or similar 
parts throughout the figures thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of a rear wheel steering apparatus for an automobile 
according to the present invention will be described hereinafter with 
reference to the accompanying drawings. 
FIGS. 1 and 2 show a four-wheel steering mechanism 12 having a rear wheel 
steering apparatus 10 according to an embodiment of the present invention. 
In FIGS. 1 and 2, reference symbols FL, FR, RL, and RR denote four wheels 
of a vehicle steered by the four-wheel steering mechanism 12. The left and 
right front wheels FL and FR are connected to be turned by a front wheel 
steering apparatus 14, and the left and right rear wheels RL and RR are 
connected to be turned by the rear wheel steering apparatus 10. 
The front wheel steering apparatus 14 is constituted by a pair of left and 
right knuckle arms 16L and 16R, tie rods 18L and 18R, and a relay rod 20 
for coupling the left and right tie rods 18L and 18R. A steering wheel 24 
is connected to the front wheel steering apparatus 14 through a rack and 
pinion steering mechanism 22. The steering mechanism 22 comprises a first 
rack 28 formed on the relay rod 20, and a steering shaft 32 having an 
upper end coupled to the steering wheel 24 and a lower end to which a 
first pinion 30 meshed with the first rack 58 is attached. In this manner, 
the left and right front wheels FL and FR can be turned in accordance with 
an operation (rotation) of the steering wheel 24. 
The rear wheel steering apparatus 10 has left and right knuckle arms 34L 
and 34R, tie rods 36L and 36R, and a relay rod 38 for coupling the tie 
rods 36L and 36R as in the front wheel steering apparatus 14, and further 
has a hydraulic power steering mechanism 26. The power steering mechanism 
26 is fixed to a vehicle body, and comprises a power cylinder 40 using the 
relay rod 38 as a piston rod. 
A piston 40a is disposed in the power cylinder 40 to be in slidable contact 
with its inner surface and to be movable in the axial direction. The 
piston 40a is integrally attached to the relay rod 38. The interior of the 
power cylinder 40 is partitioned into two hydraulic chambers 40b and 40c 
by the relay rod 38 via piston 40a. These hydraulic chambers 40b and 40c 
are coupled to a control valve 46 through pipes 42 and 44, respectively. 
Two pipes, e.g., an oil supply pipe 50 and an oil exhaust pipe 52 
extending to a reservoir tank 48 are connected to the control valve 46. A 
hydraulic pump 54 driven by an engine (not shown) is connected to the oil 
exhaust pipe 52. 
The control valve 46 is of a known spool valve type The control valve 46 
comprises a cylindrical valve casing 46a integrally mounted on the relay 
rod 38 through a coupling member 56, and a spool valve (not shown) fitted 
in the valve casing 46a. In the control valve 46 with this arrangement, a 
compressed oil from the hydraulic pump 54 is supplied to one hydraulic 
chamber 40b (or 40c) of the power cylinder 40 upon movement of the spool 
valve, and as a result, a drive force for the relay rod 38 can be assisted 
by the compressed oil. 
Return springs 40d for biasing the relay rod 38 to a neutral position (a 
position where a turning angle .theta.R of the rear wheels RL and RR 
becomes zero) are respectively mounted in the hydraulic chambers 40b and 
40c of the power cylinder 40 described above. 
A second rack 58 is formed on the relay rod 20 of the front wheel steering 
apparatus 14 at a position different from the first rack 28 constituting 
the steering mechanism 22. A second pinion 62 mounted on the front end of 
a rear steering shaft 60 extending along the back-and-forth direction of 
the vehicle is meshed with the second rack 58. The rear end of the 
steering shaft 60 is coupled to the rear wheel steering apparatus 10 
through a turning angle ratio control mechanism 64. 
The turning angle ratio control mechanism 64 has a control rod 66 which is 
held to be slidable along a moving axis defined along the direction of a 
vehicle width, as shown in FIG. 2. One end of the control rod 66 is 
coupled to the spool valve in the control valve 46. 
The turning angle ratio control mechanism 64 comprises a holder 68 having a 
U-shaped proximal end portion. A support pin 70 is pivotally and axially 
supported between the two ends of the U-shaped portion of the holder 68. A 
swing arm 72 is fixed to the central portion of the support pin 70 to be 
perpendicular thereto. More specifically, the swing arm 72 is swingably 
received by the holder 68 through the support pin 70. A support shaft 74 
is fixed to the central portion of the holder 68. The support shaft 74 is 
supported on a casing (not shown) of the turning angle ratio control 
mechanism 64 fixed to the vehicle body to be pivotal about a pivot axis 
which is determined to be perpendicular to the moving axis of the control 
rod 66 described above. 
In other words, the support pin 70 of the swing arm 72 is located at the 
intersection between both the axes (i.e., the moving axis of the control 
rod 66 and the pivot axis of the support shaft 74), and extends in a 
direction perpendicular to the pivot axis. When the holder 68 is pivoted 
about the pivot axis together with the support shaft 74, an inclination 
angle defined by the support pin 70 at its distal end and the moving axis 
of the control rod 66 is changed. More specifically, an inclination angle 
defined by a swing path plane of the swing arm 72 having the support pin 
70 as the center and a plane perpendicular to the moving axis (to be 
referred to as a reference plane hereinafter) is changed. 
One end of a connecting rod 78 is coupled to the distal end portion of the 
swing arm 72 through a ball joint 76. The other end portion of the 
connecting rod 78 is coupled to the other end portion of the control rod 
60 through a ball joint 80. In this manner, the control rod 66 is 
displaced along the direction of the vehicle width according to the swing 
movement of the distal end of the swing arm 72. 
The connecting rod 78 slidably extends through a through hole formed in an 
eccentric portion of a disk-like rotational force providing member 82 near 
the ball joint 76 through a ball joint 84. A large-diameter bevel gear 88 
pivotally supported through a support shaft 86 located coaxially with the 
moving axis of the control rod 66 is formed integrally with the rotational 
force providing member 82. A small-diameter bevel gear 90 attached to the 
rear end of the rear steering shaft 60 is meshed with the large bevel gear 
88. In this manner, the rotational force of the steering wheel 24 is 
transmitted to the rotational force providing member 82. 
For this reason, the rotational force providing member 82 is rotated by an 
amount corresponding to a rotational angle of the steering wheel 24, and 
as a result, the connecting rod 78 is pivoted about the moving axis of the 
control rod 66. When the swing arm 72 is pivoted about the support pin 70 
and when the axis of the support pin 70 coincides with the moving axis of 
the control rod 66 (at the neutral position), the ball joint 76 at the 
distal end of the swing arm 72 swings in only the reference plane, and the 
control rod 66 is kept in position without being moved along the moving 
axis. 
When the axis of the support pin 70 is inclined with respect to the moving 
axis and when the swing path plane of the swing arm 72 is deviated from 
the reference plane, the ball joint 76 is displaced in the direction of 
the vehicle width upon swing movement of the swing arm 72 about the 
support pin 70. As a result, this displacement is transmitted to the 
control rod 66 through the connecting rod 78. The control rod 66 is then 
moved along the moving axis, and the spool valve of the control valve 46 
is actuated. 
Even if a swing angle of the swing arm 72 about the axis of the support pin 
70 remains the same, the displacement of the control rod 66 in the 
right-and-left direction is changed upon a change in inclination angle of 
the support pin 70, that is, in pivot angle of the holder 68. 
More specifically, if the axis of the support pin 70 forms an angle 
deviated clockwise from the moving axis of the control rod 66 (at the 
opposite phase position), the ball joint 76 at the distal end of the swing 
arm 72 swings obliquely across the reference plane, and the control rod 66 
is deviated to the right in FIG. 2 upon clockwise steering operation of 
the steering wheel 24. When the control rod 66 is deviated to the right, 
the rear wheels RL and RR are steered counterclockwise, as will be 
described later. In other words, in this opposite phase state, the rear 
wheels are turned in a direction opposite to the turning direction of the 
front wheels. 
Meanwhile, if the axis of the support pin 70 forms an angle deviated 
counterclockwise from the moving axis of the control rod 66 (at the 
in-phase position), the ball joint 76 at the distal end of the swing arm 
72 swings obliquely across the reference plane at a side opposite to the 
opposite phase state, and the control rod 66 is deviated to the left in 
FIG. 2 upon clockwise steering operation of the steering wheel 24. When 
the control rod 66 is deviated to the left, the rear wheels RL and RR are 
turned to the right, as will be described later. In other words, in this 
in-phase state, the rear wheels RL and RR are turned in the same direction 
as the turning direction of the front wheels FL and FR. 
In order to change an inclination angle of the support pin 70 with respect 
to the moving axis, i.e., an inclination angle of the holder 68 with 
respect to the reference plane, a sector gear 92 as a worm wheel is 
integrally attached to the support shaft 74 of the holder 68. The sector 
gear 92 is meshed with a worm gear 96 which is rotated together with a 
rotational shaft 94. A bevel gear 98 is integrally attached to the 
rotational shaft 94. The bevel gear 98 is meshed with a bevel gear 102 
which is mounted coaxially with an output shaft 100a of a stepping motor 
100 as a control motor. 
In this manner, when the stepping motor 100 is operated, the sector gear 92 
is pivoted, and the inclination angle of the holder 68 with respect to the 
reference plane is changed. In other words, the in-phase, opposite phase, 
and neutral states of the turning angle .theta.R of the rear wheels RL and 
RR with respect to the front wheels FL and FR are changed and controlled 
in accordance with the rotational amount of the stepping motor 100. 
The neutral position is defined at a position where the central line of the 
sector gear 92 is perpendicular to the central line of the rotational 
shaft 94. When the sector gear 92 is pivoted clockwise when viewed from 
above the vehicle body, the rear wheel turning angle ratio is controlled 
in the opposite phase state wherein the rear wheels RL and RR are directed 
in a direction opposite to the front wheels FL and FR. On the other hand, 
when the sector gear 92 is pivoted counterclockwise, the rear wheel 
turning angle ratio can be controlled in the in-phase state wherein the 
rear wheels RL and RR are directed in the same direction as that of the 
front wheels FL and FR. 
Stopper members 104 and 106 on opposite phase and in-phase sides are 
mounted on a casing (not shown) of the turning angle ratio control 
mechanism 64 at two, i.e., left and right sides of the sector gear 92. The 
stopper members 104 and 106 comprise pins for regulating a pivot range of 
the sector gear 92. In this manner, when the pivot angle from the neutral 
position reaches -17.5 degrees, the sector gear 92 abuts against the 
opposite-phase side stopper member 104, and its pivotal movement is 
regulated. 
Upon pivotal movement of the sector gear 92 to the in-phase side, when the 
pivot angle from the neutral position becomes 20 degrees, the sector gear 
92 abuts against the in-phase side stopper 106, and its movement is 
regulated. The control position of the stepping motor 100 is regulated so 
that the sector gear 92 is located at an opposite phase side position in 
an initial state. 
Note that in order to regulate a maximum moving range of the relay rod 38 
in the rear wheel steering apparatus 10, rod stoppers 108 are arranged at 
both the sides of the coupling member 56. 
As shown in FIG. 1, an oil filter 110 is interposed midway along the oil 
supply pipe 50. A fail-safe oil branch pipe 112 branches from the oil 
filter 110 in addition to the oil supply pipe 50. The distal end portion 
of the oil branch pipe 112 is forked, and two distal ends 112a and 112b 
are respectively connected to input ports of fail-safe solenoid valves 114 
and 116. The output ports of the pair of solenoid valves 114 and 116 are 
respectively connected to return oil branch pipes 118a and 118b. These 
return oil branch pipes 118a and 118b are combined into one return oil 
pipe 118, and the pipe 118 is connected to the reservoir tank 48. 
Upon an ON operation of an ignition key of the vehicle, the solenoid valves 
114 and 116 are energized to close the corresponding oil branch pipes 112a 
and 112b, and are deenergized to open the pipes 112a and 112b. As a 
result, only when the solenoid valves 114 and 116 are closed, an oil 
pressure from the hydraulic pump 54 acts on the control valve 46. If a 
failure is detected during the rear wheel steering operation, as will be 
described later, an energization to the solenoid valves 114 and 116 is cut 
off by a control unit 132 (to be described later), thus executing a 
fail-safe operation. 
As a result, one of the solenoid valves 114 and 116 is opened, so that the 
oil pressure from the hydraulic pump 54 is released to the reservoir tank 
48 without acting on the control valve 46. Therefore, no oil pressure acts 
on the power cylinder 40, and the relay rod 38 is pressed by the pair of 
return springs 40d from two sides, and is mechanically biased to the 
neutral position. That is, a fail-safe function is effected, and the rear 
wheels are fixed at the neutral position, that is, the vehicle is 
mechanically fixed in a two-wheel steering (2WS) state. 
As described above, the rear wheel turning angle ratio is defined by the 
pivot position of the sector gear 92. A turning angle ratio sensor 120 is 
attached to the sector gear 92 so as to detect the pivot angle of the gear 
92, thus detecting the preset rear wheel turning angle ratio. Note that 
the turning angle ratio sensor outputs a zero value at the neutral 
position, outputs a positive value in the in-phase state, and outputs a 
negative value in the opposite phase state. 
As shown in FIG. 3, in this embodiment, in order to achieve the 
vehicle-speed sensitive four-wheel steering system, two, i.e., first and 
second vehicle speed sensors 122 and 124 are arranged. The first vehicle 
speed sensor 122 is connected to a coupling portion of a speedometer shaft 
128 to a transaxle 126, and the second vehicle speed sensor 124 is 
arranged in a speedometer 130. The vehicle speed sensors 122 and 124 are 
of lead switch type, and output a four-pulse output signal per revolution 
of the speedometer shaft 128. 
The stepping motor 100 serves as a turning angle ratio changing means, and 
its operation is controlled by an output from a control unit 132 
incorporating a microcomputer, as shown in FIG. 1. The control unit 132 
executes a fail-safe operation when a predetermined failure occurs in the 
4WS state, and fixes the rear wheel turning angle ratio to a value which 
is different from a preset normal characteristic and is determined 
according to a vehicle speed before lock detection in a wheel lock 
detection state (to be described in detail later). 
The control unit 132 receives a power supply voltage from a battery 
(indicated by +B in FIG. 4) through a relay/timer circuit 134, as shown in 
FIG. 4. The first and second solenoid valves 114 and 116 and the stepping 
motor 100 also receive the power supply voltage from the battery through 
the relay/timer circuit 134. The voltage from the relay/timer circuit 134 
is supplied to the control unit 132 through an oil level switch 136 (to be 
described later). 
The relay/timer circuit 134 is connected to an ignition switch (indicated 
by IG in FIG. 4). Upon an OFF operation of the ignition switch, the 
relay/timer circuit 134 cuts off the power source voltage from the battery 
to respective components connected thereto after a predetermined period of 
time has passed. The relay/timer circuit 134 is connected to an L terminal 
of an alternator 138 for detecting a failure, as will be described later. 
When an input voltage at the L terminal is equal to or lower than a 
predetermined voltage, the control unit 132 disables the relay to stop 
supply of a voltage to the respective components, thereby interrupting 4WS 
control. 
Note that the control unit 132 detects a voltage generated by the 
alternator 138. When the detected voltage exceeds a predetermined voltage, 
a drive state at a predetermined engine speed is detected. 
When an ignition voltage input upon ON operation of the ignition switch is 
not equal to or higher than 9 V, the relay/timer circuit 134 outputs a 
failure signal to the control unit 132. 
As shown in FIG. 4, the control unit 132 is connected to the first and 
second vehicle speed sensors 122 and 124, a brake sensor 140 which is 
turned on when a brake pedal is depressed, and an inhibitor switch 142 
attached to a selector lever. 
The control unit 132 executes the following failure detection as well as 
the above-mentioned two types of failure detection based on outputs from 
the sensors connected thereto. More specifically, the failure signal is 
output to the control unit 132 when the first and second vehicle speed 
sensors 122 and 124 output vehicle speed quick change signals, when 
outputs therefrom are different from each other, when the outputs become 
zero during travel, when the calculated value of the pivot angle of the 
sector gear 92 upon operation of the stepping motor 100 is different from 
the output value from the turning angle ratio sensor 120, when an output 
circuit of the solenoid valves 114 and 116 malfunctions, is disconnected 
or is short-circuited, when an output circuit of the stepping motor 100 
malfunctions, is disconnected, or short-circuited, when the output from 
the turning angle ratio sensor 120 falls outside a setting range, when the 
reference output signal from the turning angle ratio sensor 120 cannot be 
fetched, and when the oil level switch 136 in the tank of the hydraulic 
pump is turned on. 
Note that the control unit 132 receives the above-mentioned failure signal, 
and executes one of three types of fail-safe operations according to the 
content of the failure, that is, such that the stepping motor 100 is 
driven to forcibly set a 2WS mode, that energization to the solenoid 
valves 114 and 116 is cut off so that no oil pressure acts on the control 
valve 46 and the control valve 46 is mechanically fixed by the biasing 
forces of the pair of springs 40d, or that a voltage supply to the rear 
wheel steering apparatus 10 is cut off to interrupt the control operation. 
In this embodiment, when the engine speed is equal to or higher than 2,200 
r.p.m. and when an OFF signal is output from the inhibitor switch 142, 
that is, the selector lever is brought to a travel setting position other 
than an N (neutral) range or a P (parking) range, a travel state is 
determined. In this state, the control unit 132 is set so that when no 
vehicle speed data is output from the first and second vehicle speed 
sensors 122 and 124, an abnormality of the vehicle speed sensors is 
determined as described above, and the failure detection is executed. 
The failure detection operation itself is not a wrong control content. 
However, if the failure detection operation is always executed to induce a 
fail-safe operation, drawbacks are caused in the following cases. For this 
reason, in this embodiment, the failure detection operation based on the 
abnormality of the vehicle speed sensor described above is inhibited. 
More specifically, upon an inspection of a vehicle in an auto repair shop, 
in order to inspect whether or not an engine output can be regularly 
obtained, a test is performed, wherein an acceleration pedal is depressed 
to increase an engine speed while the brake pedal is depressed to forcibly 
stop the vehicle. In this test, if an engine is normal, the engine speed 
is increased to a value exhibiting a maximum torque. In this test, if a 
failure detection is not restricted, the above-mentioned failure detection 
is executed since no vehicle speed data is obtained from the vehicle speed 
sensors 122 and 124 although the travel state is detected. 
However, in this embodiment, when the brake pedal is depressed to turn on 
the brake switch 140, it is defined that the above-mentioned failure state 
is caused by the test, and in this case, inhibition control for 
restricting failure detection is executed. 
In this embodiment, an unnecessary failure detection operation is inhibited 
in the test of the vehicle by utilizing the brake switch, thus improving 
workability of a test operation. 
In a normal travel state, the control unit 132 has a rear wheel turning 
angle ratio according to the present travel state, i.e., the detected 
vehicle speed in accordance with a normal first turning angle ratio 
characteristic indicated by a solid curve I in FIG. 5. In this control 
unit 132, the stepping motor 100 is driven and controlled so as to turn 
the rear wheels RL and RR based on the rear wheel turning angle ratio 
determined according to a higher one of the two vehicle speeds detected by 
the transaxle side first vehicle speed sensor 122 and the speedometer side 
second vehicle speed sensor 124. 
The vehicle speed detection is executed by calculating a moving average 
from the latest six vehicle speed data sampled at 131-msec intervals by 
the vehicle speed sensors 122 and 124. 
In the control unit 132, a lock control operation when the wheels are 
locked during travel of the vehicle is executed as separate control 
independently of the above-mentioned fail-safe operation. 
As discussed in detail in the prior art, in this embodiment, when a vehicle 
travels along a curved road at a speed higher than a predetermined vehicle 
speed (35 km/h), since the front and rear wheels are turned in the 
in-phase state, a so-called high-speed cornering characteristic can be 
maintained well. If a driver finds an obstacle ahead during the high-speed 
cornering, he quickly depresses a brake pedal. When the wheels are locked 
by the quick braking, although an actual speed of the vehicle is not 
quickly decreased, the vehicle speed data output from the first and second 
vehicle speed sensors 122 and 124 become zero. For this reason, the 
detected vehicle speeds become lower than the predetermined vehicle speed, 
and the front and rear wheels are turned in the opposite phase state. 
That is, during cornering, the rear wheels are turned from the in-phase 
state to the opposite phase state. For this reason, although the front and 
rear wheels must firmly grip the road surface in the in-phase state in 
order to assure stable travel while keeping the vehicle position during 
cornering, the rear wheels are turned to the opposite phase state to 
generate a yaw rate, and accidental turning tendency of the vehicle is 
quickly enhanced. As a result, a so-called tuck-in phenomenon occurs, and 
the vehicle may cause a spin, resulting in a dangerous state. 
In order to evade such danger, in this embodiment, when the control unit 
132 detects the lock state of the wheels, danger evasion control for 
setting a rear wheel turning angle ratio at a stable side (to be referred 
to as lock control hereinafter) is executed. 
The control unit 132 executes lock detection as follows. That is, the 
control unit 132 determines that the wheels are locked: 
(1) when the brake pedal is depressed and the brake sensor 140 is turned 
on; 
(2) when the detection result of the vehicle speed from the first vehicle 
speed becomes substantially zero; and 
(3) when the detected vehicle speed quickly before substantial zero 
determination of the vehicle speed is made from the detection result of 
the first vehicle speed sensor is 30 km/h. 
The detection content of the first vehicle speed sensor 122 in the above 
condition (2) is set to be "substantially zero" since the existing lead 
switch type vehicle speed sensor has poor detection accuracy and makes 
zero determination within a detected vehicle speed range of about 10 km/h 
to zero. 
In the condition (3), the detected vehicle speed immediately before 
substantial zero determination of the vehicle speed is made is defined by 
the average value of the six detection results immediately before the last 
sampling time of the six detection results used when the substantial zero 
determination of the vehicle speed is made, i.e., 131 msec before. 
When the three conditions are satisfied, lock determination is made, and 
the control unit executes the following lock control based on this lock 
determination. 
In this lock control, in order to maintain a stable travel state, a fixing 
operation for fixing the rear wheel turning angle ratio to a value at an 
instance when the vehicle speed immediately before lock determination is 
detected is executed. More specifically, as indicated by a broken curve II 
in FIG. 5, when the vehicle is quickly braked at a vehicle speed of 35 
km/h or higher and the wheels are locked, even though the vehicle speed 
detected by the first vehicle speed sensor 122 is substantially zero, the 
following rear wheel turning angle ratio is kept unchanged in the in-phase 
state when the detection is made. With this fixing operation of the rear 
wheel turning angle ratio, the rear wheel turning angle ratio is held in a 
stable side, and the rear wheels can no longer be turned. In this 
embodiment, travel property when the wheels are locked can be safely 
assured. 
As can be understood from the above discussion, when the vehicle speed 
immediately before the lock state is equal to or higher than 30 km/h and 
is lower than 35 km/h, the rear wheel turning angle ratio is kept 
unchanged in the opposite phase state. However, since the rear wheel 
turning angle ratio when the vehicle speed is equal to or higher than 30 
km/h and is lower than 35 km/h is almost zero, if this rear wheel turning 
angle ratio is kept unchanged, no problem is posed. 
As described above, in the normal rear wheel steering control or failure 
detection, the first and second vehicle speed sensors 122 and 124 are used 
as detection means. As a vehicle speed detection means in the lock 
detection operation, only the transaxle side first vehicle speed sensor 
122 is used, and the speedometer side second vehicle speed sensor 124 is 
not used. The first and second vehicle speed sensors 122 and 124 are 
coupled to each other through the speedometer shaft 128. Since the 
speedometer shaft 128 is rigid but long, a change in rotation of a driven 
gear at the transaxle cannot be accurately and quickly transmitted to the 
far second vehicle speed sensor 124 due to a variation in a twist 
direction or an inertia upon transmission of rotation. 
In this embodiment, in the normal rear wheel steering control or in the 
failure detection, the two vehicle speed sensors 122 and 124 are used. 
However, in the lock detection operation, only one vehicle speed sensor, 
in particular, the transaxle side first vehicle speed sensor 122 is used. 
In this manner, in the lock detection operation, a vehicle speed can be 
detected in a short response time. 
In this embodiment, a comparison operation is not executed such that after 
the values of the two vehicle speed sensors are compared, a higher 
detection value is employed. For this reason, a detection operation time 
becomes very short, and the short response time can be assured. 
In this embodiment, as the failure control content described above is 
separately executed from the vehicle speed detection operation in the lock 
detection operation, a coincidence/noncoincidence of the output values 
from the first and second vehicle speed sensors 122 and 124 is detected. 
As long as both the output values coincide with each other, failure 
detection is not made, and the first vehicle speed sensor 122 is employed 
as the vehicle speed detection means. 
However, if the output from the first vehicle speed sensor 122 is quickly 
changed and becomes zero for any cause and if only an output from the 
second vehicle speed sensor 124 is obtained, lock determination is not 
made using the detection result from the second vehicle speed sensor 124 
and the failure detection is made based on the noncoincidence between the 
detection outputs from the vehicle speed sensors 122 and 124, thus 
executing the above-mentioned fail-safe operation. 
More specifically, in the fail-safe operation in this case, energization to 
the first and second solenoid valves 114 and 116 is cut off, and the 
hydraulic circuit is disconnected. Upon disconnection of the hydraulic 
circuit, the rear wheels RL and RR are not turned but are fixed in the 
so-called 2WS state by the biasing force of the pair of coil springs 40d 
even when the front wheels FL and FR are turned. A fail warning lamp 144 
arranged in a meter panel shown in FIG. 4 flashes once. In this manner, 
the fail-safe operation is executed. 
In the control unit 132, failure detection based on a malfunction of the 
first vehicle speed sensor 122 described above is not executed when the 
brake pedal is depressed to turn on the brake switch 140, and the lock 
control described above is executed. The failure detection based on the 
fact that the output from the first vehicle speed sensor 122 is quickly 
changed and becomes zero is executed only when the brake switch 140 is 
kept OFF. 
Of the three conditions of lock determination described above, a state 
wherein the vehicle speed detected by the first vehicle speed sensor 122 
is quickly changed and substantially becomes zero and a state wherein the 
first vehicle speed sensor 122 malfunctions and immediately stops 
outputting data in the condition (2) appear as an identical state. For 
this reason, in this embodiment, data corresponding to a zero output from 
the first vehicle speed sensor 122 when the brake switch 140 is turned on 
is used for lock determination, and data corresponding to a zero output 
from the first vehicle speed sensor 122 when the brake switch 140 is kept 
OFF is used for failure detection. 
In this embodiment, when the wheels are locked by the braking operation 
during travel, even if the output from the first vehicle speed sensor 122 
becomes zero, since this zero state is based on the lock state of the 
wheels, no failure detection is made and the lock detection operation is 
reliably executed. 
As a result, the drawback caused when the failure detection is performed 
when the output from the first vehicle speed sensor 122 becomes zero and 
the steering state is forcibly fixed to the 2WS state, that is, the 
drawback that when the wheels are locked by quick braking during travel at 
a vehicle speed of 35 km/h or higher, the operation for forcibly fixing 
the 2WS mode is executed based on the fail-safe control although the rear 
wheels are fixed at the turning angle ratio for the in-phase state and the 
stable travel state must be maintained, and the rear wheels RL and RR are 
turned from the in-phase positions to the neutral position, can be 
reliably prevented. 
In this embodiment, the fact that the output from the first vehicle speed 
sensor 122 becomes zero while the brake switch 140 is kept OFF during 
travel means that the stop state is detected by the first vehicle speed 
sensor 122. Therefore, in this state, an abnormal state such as a 
malfunction of the first vehicle speed sensor 122 is detected, and the 
above-mentioned failure detection is executed. 
In the control unit 132, the following limitation about the lock detection 
is imposed. That is, in the above-mentioned lock detection operation, when 
the vehicle speed detection result from the first vehicle speed sensor 122 
accompanies a quick increase in vehicle speed over a predetermined value, 
lock detection is not executed, and the rear wheels RL and RR are steered 
in accordance with the first turning angle ratio characteristic indicated 
by the solid curve I in FIG. 5. 
More specifically, the control unit 132 detects a change in vehicle speed 
detection result per second from the first vehicle speed sensor 122. If it 
is determined that the change in vehicle speed per second exceeds a 
predetermined value (40 km/h), i.e., the vehicle speed is quickly 
increased, even if the lock detection state occurs immediately thereafter, 
in other words, the above-mentioned lock determination conditions are 
satisfied, no lock detection is performed, and shift to lock control is 
inhibited. In the lock control inhibition state, the rear wheels RL and RR 
are steered in accordance with the first turning angle ratio 
characteristic described above. 
In the control unit 132, since the limitation is imposed on lock detection, 
when a road surface has a low friction coefficient, in other words, when 
the road surface is frozen, safe travel property of the vehicle can be 
assured when the acceleration pedal is depressed strongly upon starting 
and a so-called wheel spin state occurs. 
More specifically, when the wheel spin state occurs upon starting of the 
vehicle, control of the vehicle is lost and the vehicle may be turned 
sideways. In this case, a driver quickly depresses the brake pedal while 
turning the steering wheel 24 in order to correct the vehicle position. If 
the wheels are locked in this state, the above-mentioned three lock 
determination conditions are satisfied. However, in this embodiment, as 
described above, lock detection immediately after determination of a quick 
increase in vehicle speed is inhibited. Therefore, a drawback that when 
the lock detection is executed, the rear wheel turning angle ratio 
immediately before lock detection is kept unchanged and a normal rear 
wheel turning angle ratio cannot be recovered upon restarting, can be 
reliably prevented. 
In this embodiment, since the limitation is imposed on the lock detection, 
a drawback caused when chattering in the vehicle speed sensor occurs and 
only the detection output indicates a quick increase in vehicle speed 
although an actual vehicle speed is left unchanged, can be reliably 
prevented. 
If the above-mentioned limitation is not imposed, chattering such as engine 
vibration occurs in a stop state, and thereafter, when a vehicle speed 
zero state is detected, the control unit 132 detects the lock state of the 
wheels, and executes a fixing operation of the rear wheel turning angle 
ratio. However, chattering always occurs with a quick increase in vehicle 
speed. In this embodiment, since the lock detection operation is inhibited 
upon detection of the quick increase in vehicle speed, an unnecessary 
fixing operation of the rear wheel turning angle ratio based on the 
erroneous operation of the vehicle speed sensor caused by chattering can 
be prevented. Thus, normal turning angle ratio control based on the first 
turning angle ratio characteristic is executed, and good travel property 
can be assured. 
In the above description, the lock detection operation, the lock control 
operation, and various control operations therefor in the control unit 132 
have been described. 
However, when lock control is executed based on lock detection, the rear 
wheel turning angle ratio is fixed to a ratio according to the vehicle 
speed immediately before lock detection, as described above. Such an 
operation is necessary in view of assurance of safe travel state. However, 
after the lock state of the wheels is released, it is unnecessary. 
Therefore, after the lock state of the wheels is released, the lock 
control operation is canceled as soon as possible to release a state 
wherein the rear wheel turning angle ratio is fixed, and normal turning 
angle ratio control must be executed. For this purpose, in this 
embodiment, the cancel operation of the lock control operation, in other 
words, a recover operation from the lock control state to the normal 
turning angle ratio control operation is performed, as will be described 
later. 
The recover operation (lock cancel operation) will now be described. 
The recover operation is basically executed when it is determined that it 
is safe if the lock control operation is canceled after the lock detection 
operation. The control unit 132 includes a first mode wherein the cancel 
operation is executed when an actual vehicle speed coincides with the 
vehicle speed detected by the first vehicle speed sensor 122, and a second 
mode wherein the cancel operation is executed when the detected vehicle 
speed is lower than the actual vehicle speed. 
In the first mode, when the actual vehicle speed substantially coincides 
with the vehicle speed detected by the first vehicle speed sensor 122 
after the lock detection operation, this means that the wheels perfectly 
grip the road surface. Therefore, it is determined that a dangerous state 
which might cause a slip has been terminated. A substantial coincidence 
between the actual vehicle speed and the vehicle speed detected by the 
first vehicle speed sensor 122 is determined when one of the following 
three conditions is satisfied. 
The first condition is that a time period wherein the locked vehicle can be 
reliably stopped under any condition (travel condition, road surface 
condition) passes. After the time period has passed, a vehicle speed zero 
state (stop state) is established, and as described above, after the lock 
detection operation, the actual vehicle speed substantially coincides with 
the vehicle speed detected by the first vehicle speed sensor 122. 
Therefore, at this timing, the lock control operation is canceled. In this 
manner, the normal turning angle ratio control operation as indicated by 
the solid curve I in FIG. 5 is recovered. 
More specifically, a cancel period T1 from when the lock control operation 
is started after the above-mentioned three conditions are satisfied and 
the lock detection is performed until the lock control operation is 
canceled is defined as follows when the vehicle travels at x km/h 
immediately before wheel lock: 
EQU Cancel Period T1 (sec) =x.multidot.C 
where C : constant 
In this embodiment, the constant C is 0.142. The value "0.142" is 
calculated from 17 sec corresponding to a time period required to stop a 
vehicle when the vehicle which travels along a road having a friction 
coefficient of 0.2 at 120 km/h is quickly braked and keeps traveling along 
the road while the wheels are locked. Note that the friction coefficient 
of 0.2 is experienced in a frozen road surface state during mid-winter in 
North European countries, and is sufficient as a setting condition. 
In this manner, as shown in FIG. 6A, even when the vehicle traveling at 60 
km/h is quickly braked and the wheels are locked, the cancel period T1 is: 
EQU 50.times.0.142=7.1 (sec) 
Therefore, the vehicle can be stopped after 7.1 sec. 
Note that in FIG. 6A, a solid curve indicates the vehicle speed detected by 
the first vehicle speed sensor 122, and an alternate long and short dashed 
curve indicates an actual vehicle speed. As can be apparent from the 
alternate long and short dashed curve in FIG. 6A, the vehicle is stopped 
3.6 sec after the wheels were locked in this embodiment. This setting also 
applies to FIGS. 6B to 6D. 
Since the first condition is set in this manner, after the lapse of the 
cancel period T1, even if the driver continuously depresses the brake 
pedal and the brake switch 140 is kept ON, the vehicle is surely stopped. 
For this reason, after the lapse of the cancel period T1, lock control is 
canceled, and the normal turning angle ratio control operation is 
executed, thus posing no problem. 
In this embodiment, when the driver notices the stop state of the vehicle 
and intends to restart the vehicle, the rear wheel turning angle ratio is 
not fixed to a ratio immediately before the lock state, and the normal 
turning angle ratio control can be executed. Thus, good travel property of 
the vehicle can be maintained. 
A second condition for canceling the lock control operation will be 
described below. The second condition is that the currently detected 
vehicle speed exceeds a vehicle speed immediately before lock detection. 
When the second condition is satisfied, the actual vehicle speed 
substantially coincides with the vehicle speed detected by the first 
vehicle speed sensor at the vehicle speed immediately before lock 
detection. Therefore, at this timing, the lock control operation is 
canceled. In this manner, the normal turning angle ratio control operation 
as indicated by the solid curve I in FIG. 5 can be recovered. 
More specifically, as shown in FIG. 6B, while the vehicle travels at 50 
km/h, when the brake pedal is quickly depressed and the wheels are locked, 
the lock state is detected, and the lock control operation is started. In 
this case, the driver notices that the vehicle is completely stopped, 
releases the brake pedal, and depresses the acceleration pedal to 
accelerate the vehicle. In this case, since the wheels grip the road 
surface, the output from the vehicle speed sensor 122 is increased in 
accordance with depression of the acceleration pedal, as indicated by a 
solid curve in FIG. 6B. When the detected vehicle speed exceeds the 
vehicle speed immediately before lock detection, unless the rear wheel 
turning angle ratio is changed to the in-phase side, travel stability of 
the vehicle cannot be assured. For this reason, the lock control operation 
is canceled, and the rear wheels are steered at the rear wheel turning 
angle ratio according to the present vehicle speed in view of safe travel. 
Under the second condition, the driver may release the brake pedal 
relatively earlier after the wheels were locked, i.e., before the actual 
vehicle speed is not so decreased, and may depress the acceleration pedal. 
In this case, when the wheels grip the road surface, the output from the 
vehicle speed sensor 122 is increased according to depression of the 
acceleration pedal, as indicated by the solid curve in FIG. 6B. When the 
currently detected vehicle speed exceeds the vehicle speed immediately 
before lock detection, unless the rear wheel turning angle ratio is 
changed to the in-phase side, travel stability of the vehicle cannot be 
assured, as described above. For this reason, the lock control operation 
is canceled, and the rear wheels are steered at the rear wheel turning 
angle ratio according to the present vehicle speed in view of safe travel. 
In this embodiment, when the vehicle is accelerated after lock detection, 
when the currently detected vehicle speed coincides with and exceeds the 
vehicle speed immediately before lock detection, the lock control 
operation is canceled, and the normal turning angle ratio control 
operation is started, thus assuring safe travel. 
A third condition for canceling the lock control operation will now be 
described. The third condition is that a constant vehicle speed is 
continuously input for a predetermined period T2 regardless of the ON/OFF 
state of the brake switch. That is, after lock detection, when vehicle 
speed data is input from the first vehicle speed sensor 122 for the 
predetermined period T2, the lock control operation is canceled. In this 
manner, the normal turning angle ratio control operation as indicated by 
the solid curve I in FIG. 5 is recovered. 
More specifically, as shown in the left half in FIG. 6C, while the vehicle 
travels at 50 km/h when the brake pedal is quickly depressed and the 
wheels are locked, the lock detection is performed, and the lock control 
operation is started. A driver may notice that the vehicle is completely 
stopped, release the brake pedal, and depress the acceleration pedal to 
start the vehicle. In this case, since the wheels grip the road surface, 
the output from the first vehicle speed sensor 122 is increased according 
to depression of the acceleration pedal, as shown in FIG. 6C. When the 
vehicle speed is continuously output from the vehicle speed sensor 122 for 
917 msec set as the predetermined period T2, since the wheels grip the 
road surface, unless the rear wheel turning angle ratio is changed 
according to normal turning angle ratio control corresponding to the 
detected vehicle speed, travel stability of the vehicle cannot be assured. 
For this reason, the lock control operation is canceled, and the rear 
wheels are steered at the rear wheel turning angle ratio according to the 
present vehicle speed in view of safe travel. 
Under the third condition, as shown in the right half in FIG. 6C, a driver 
may decrease a depression amount of the brake pedal relatively early, in 
other words, before the actual vehicle speed is not so decreased, so that 
the wheels grip the road surface. In this case, since the driver does not 
release the brake pedal, the brake switch 140 is kept ON. However, when 
the wheels grip the road surface, the output from the first vehicle speed 
sensor 122 is recovered, as indicated by the solid curve in FIG. 6C. As 
described above, when 917 msec have passed after the vehicle speed data 
was output from the first vehicle speed sensor 122, unless the rear wheel 
turning angle ratio is changed according to the detected vehicle speed, 
travel stability of the vehicle cannot be maintained. For this reason, in 
view of safe travel, the lock control operation is canceled, and the rear 
wheels are steered at a rear wheel turning angle ratio according to the 
present vehicle speed. 
In this embodiment, when vehicle speed data is output for the predetermined 
period T2 after lock detection, it is determined that the wheels grip the 
road surface and the detected vehicle speed coincides with the actual 
vehicle speed. In the state wherein the wheels grip the road surface, lock 
control is quickly canceled regardless of the ON/OFF state of the brake 
switch, and the normal turning angle ratio control is started, thus 
assuring travel safety. 
In the first to third conditions, as has been described above, when the 
detected vehicle speed coincides with the actual vehicle speed, this means 
that the wheels grip the road surface. Therefore, the lock control 
operation need not be performed, and is canceled to recover the normal 
turning angle ratio control operation. 
However, a fourth condition is set at a timing at which it can be 
determined that if the lock control operation is canceled, sufficient 
safety can be guaranteed before the detected vehicle speed coincides with 
the actual vehicle speed. More specifically, the fourth condition is 
established when the predetermined period T2 has passed after the brake 
switch 140 is turned off. When the fourth condition is satisfied, the 
wheels are about to grip the road surface after braking is released, and a 
vehicle speed detected by the first vehicle speed sensor 122 will coincide 
with the actual vehicle speed soon. At this timing, the lock control 
operation is canceled, and the normal turning angle ratio control 
operation indicated by the solid curve I in FIG. 5 is recovered. 
More specifically, as shown in the right half in FIG. 6D, while the vehicle 
travels at 50 km/h, a driver erroneously performs quick braking, and the 
wheels are temporarily locked. In this lock state, when the driver notices 
that the wheels are locked, he releases the brake pedal, and the wheels 
are about to grip the road surface In this case, when the driver releases 
the brake pedal, the brake switch 140 is turned off. When the 
predetermined period T2 (in this embodiment, 786 msec) has passed after 
the brake switch 140 was turned off, the lock control operation is 
released, and the normal turning angle ratio control operation is 
recovered. 
The predetermined period "786 msec" is determined in the fourth condition 
such that since the sampling time corresponds to 131 msec, the control 
content is recovered from the lock control to the normal turning angle 
ratio control after six vehicle speed data are input. As described above, 
the vehicle speed is calculated by taking a moving average of six vehicle 
speed data. The fourth condition is satisfied when 786 msec have passed 
for which six vehicle speed data necessary for taking a moving average are 
output after the lock state of the wheels is released by releasing braking 
and the first vehicle speed sensor 122 restarts outputting the vehicle 
speed data. 
A detected vehicle speed when 786 msec have passed after braking was 
released is an average value of the latest six vehicle speed data. In 
practice, if a vehicle speed at this time coincides with the actual 
vehicle speed, the detected vehicle speed does not coincide with the 
actual vehicle speed. However, as described above, the above-mentioned 
behavior of the vehicle is erroneously caused by excessive depression of 
the brake pedal by the driver. Therefore, unlike the above-mentioned three 
conditions, the vehicle travels relatively stably. In this manner, unlike 
the above three conditions, before the actual vehicle speed coincides with 
the detected vehicle speed, the normal turning angle ratio control can be 
recovered without causing a problem. 
In other words, in the fourth condition, before the detected vehicle speed 
coincides with the actual vehicle speed, lock control is positively 
canceled, and the normal turning angle ratio control operation is 
recovered. For this reason, the detected vehicle speed when the normal 
control is recovered is lower than to the actual vehicle speed. In order 
to change the rear wheel turning angle ratio, the stepping motor 100 is 
driven upon an instruction from the control unit 132 so as to attain the 
rear wheel turning angle ratio according to the detected vehicle speed 
data. The operation speed of the stepping motor 100 is set to be changed 
in accordance with the operation range, in other words, a change in 
detected vehicle speed. 
If the vehicle speed data is actually input to the control unit 132 when 
the normal turning angle ratio control is recovered and the control unit 
132 drives the stepping motor 100 based on this data, since the latest 
vehicle speed data is zero, the unit 132 operates the stepping motor 100 
in a short operation time in order to compensate for this difference. In 
this manner, the rear wheels RL and RR are quickly turned, and in the 
worst case, the tuck-in phenomenon described above may occur. 
However, in this embodiment, when the normal turning angle ratio control is 
recovered, vehicle speed data based on the actual vehicle speed is not 
input to the control unit 132, and an average value of six vehicle speed 
data including zero data is employed as detected vehicle speed data. As a 
result, the detected vehicle speed is always lower than the actual vehicle 
speed. Immediately after the normal control is recovered, since the 
stepping motor 100 is driven based on the detected vehicle speed lower 
than the actual vehicle speed, the rear wheels RL and RR are relatively 
slowly turned, and the above-mentioned problem will not be posed. 
The detected vehicle speed gradually approaches the actual vehicle speed as 
the sampling time of 131 msec has passed. As a result, the stepping motor 
100 can be operated to follow a change in vehicle speed data, and is not 
overloaded, thus achieving a good operation state. 
The description of the cancel operation of the lock control operation, that 
is, the resuming operation from the lock control state to the normal 
turning angle ratio control operation is ended, and a control procedure of 
a series of lock control operations in the control unit 132 will be 
schematically described below with reference to the flow charts shown in 
FIGS. 7A to 7C. 
The control sequence shown in FIGS. 7A to 7C is executed every sampling 
time of 131 msec described above. More specifically, when this control 
sequence is started, it is checked in step S1 if a lock flag F is "1". The 
lock flag F is set to be "0" when the control operation is started, and is 
set to be "0" when the lock detection operation (to be described later) is 
executed. If the lock flag F is set to be "0" in step S1, the lock 
detection operation is started. If the lock flag F is set to be "1" in 
step S1, the flow jumps to step S13 to detect a coincidence/noncoincidence 
of the outputs from the first and second vehicle speed sensors 122 and 
124, as will be described later. 
If NO in step S1, i.e., if the lock flag F is "0", a quick increase in 
vehicle speed caused by wheel spin or chattering of the sensor is 
discriminated in steps S2 and S3. That is, it is checked in step S2 if an 
increment D2 of the output from the speedometer side second vehicle speed 
sensor 124, more specifically, a numerical value D2 defined by a 
difference in an increasing direction between the vehicle speed detection 
result detected at the immediately preceding sampling timing and the 
currently detected vehicle speed detection result is equal to or larger 
than a predetermined value (in this embodiment, 7 km/h/131 msec). This 
predetermined value is set to be an acceleration of 7 km/h per sampling 
time, and corresponds to a change of about 53 km/h per second. 
If NO in step S2, i.e., if the increment D2 of the output from the second 
vehicle speed sensor 124 is smaller than the predetermined value, it is 
checked in step S3 if an increment D1 of the output from the transaxle 
side first vehicle speed sensor 122 is equal to or larger than the 
predetermined value, i.e., 7 km/h/131 msec in this embodiment. 
If YES in step S2 or S3, i.e., if the increment D1 of the output from the 
first vehicle speed sensor 122 or the increment D2 of the output from the 
second vehicle speed sensor 124 is larger than the predetermined value, it 
is determined that the vehicle speed is quickly increased, and a flag A 
indicating a vehicle speed quick increase state is set to be "1" in step 
S4. When the vehicle speed quick increase flag A is "1", this indicates 
the vehicle speed quick increase state, and the lock detection operation 
is inhibited. On the other hand, if the vehicle speed quick increase flag 
A is "0", this indicates that the vehicle speed quick increase state is 
not established, and the lock detection operation and the lock control 
operation are executed. 
In the vehicle speed quick increase state, since the lock detection 
operation is not executed as described above, if this vehicle speed quick 
increase state is caused by chattering inherent to the sensor, the control 
sequence returns, and the initial state is recovered. 
That is, after the flag A is set to be "1" in step S4, if the vehicle speed 
quick increase state is caused by chattering, it is checked in step S5 if 
chattering is terminated. In step S5, if the outputs from both the vehicle 
speed sensors 122 and 124 are 56 km/h or lower, it is determined that the 
chattering is terminated. If NO in step S5, that is, if the vehicle speeds 
detected by the sensors 122 and 124 are 56 km/h or higher and it is 
determined that chattering is not yet terminated, the lock detection 
operation must be inhibited from being executed. For this reason, as 
described above, the control sequence returns to the initial state. 
If YES in step S5, that is, if the outputs from both the vehicle speed 
sensors 122 and 124 are 56 km/h or lower, it is determined that chattering 
will be terminated soon. In this case, it is checked in step S6 if a 
predetermined period of time, e.g., 0.786 sec., has passed after the quick 
increase discrimination was made. When the detected vehicle speed is 
lowered below 56 km/h, after the lapse of 0.786 sec, it can be determined 
that chattering is be terminated. Therefore, the above-mentioned judgement 
is made. 
If NO in step S5, that is, if 0.786 sec have not yet passed, since lock 
detection must be inhibited, the control sequence returns to the initial 
state. If YES in step S5, i.e., if 0.786 sec have passed, it is determined 
that the vehicle speed quick increase state is substantially terminated 
regardless of whether it is caused by wheel spin or chattering. In step 
S7, the vehicle speed quick increase flag A is set to be "0", and the 
control sequence returns. 
If NO in step S3, that is, if it is determined that the vehicle speed is 
not quickly increased, it is checked in step S8 if the vehicle speed quick 
increase flag A is "1". If YES in step S8, that is, if the vehicle speed 
quick increase state is determined, since the vehicle speed quick increase 
state is not yet terminated although a quick increase in vehicle speed 
caused by wheel spin is terminated, the flow returns to step S5 to wait 
for termination of chattering. 
If NO in step S8, that is, if it is determined that the vehicle speed quick 
increase state is not established, the lock detection operation and the 
lock control operation are executed. 
If the vehicle speed quick increase flag A is "0", it is checked in step S9 
if the brake switch 140 is ON. If YES in step S9, that is, if the brake 
switch 140 is ON, it is checked in step S10 if the vehicle speed detected 
by the first vehicle speed sensor 122 at the immediately preceding 
sampling time, i.e., 131 msec before is higher than 30 km/h. 
If YES in step S10, that is, if the immediately preceding vehicle speed 
detection value is higher than 30 km/h, it is checked in step S11 if the 
vehicle speed detected by the first vehicle speed sensor 122 during the 
present detection operation is substantially 0 km/h. If YES in step S11, 
that is, if the currently detected vehicle speed is substantially 0 km/h, 
since the three conditions for lock detection are satisfied, as described 
above, lock detection is then executed. 
Based on this lock detection, the lock flag F is set to be "1" in step S12. 
If NO in steps S10 and S11, since lock detection conditions cannot be 
satisfied, the flow returns to step S9, and step S9 is executed. 
After the lock flag F is set to be "1" in step S12, it is checked in step 
S13 if the detection result of the first vehicle speed sensor 122 
coincides with the detection result of the second vehicle speed sensor 
124. If NO in step S13, that is, if the detection outputs from both the 
sensors 122 and 124 do not coincide with each other, this means a failure 
state. Thus, the fail-safe control operation is executed in step S14, and 
the flow returns. 
If YES in step S13, that is, if both the detected vehicle speeds coincide 
with each other, since the lock detection is made in a normal state, the 
lock control operation of the above-mentioned control content is started. 
Discrimination of whether or not lock detection is performed in a normal 
state is necessary for the following reason. As described above, in the 
lock detection, only the vehicle speed detected by the first vehicle speed 
sensor 122 is used. If discrimination of the normal state in step S13 is 
not performed, failure detection cannot be executed although the output 
from the second vehicle speed sensor 124 is different from that from the 
first vehicle speed sensor 122, and a failure should be detected. As a 
result, lock detection may be executed based on the detection result of 
the first vehicle speed sensor 122 which may not be incorrect. 
After the lock control operation is started in step S15, it is checked in 
step S16 if one of the above-mentioned four lock cancel conditions is 
satisfied. If NO in step S16, that is, if none of the lock cancel 
conditions is satisfied, the flow returns to step S15, and the lock 
control operation is continued. 
If YES in step S16, i.e., if one of the lock cancel conditions is 
satisfied, the lock control operation is canceled, and the normal turning 
angle ratio control operation is recovered in step S17. In step S18, the 
vehicle speed quick increase flag A is set to be "0", and the control 
operation is returned to the initial state. 
If NO in step S9, that is, if the brake switch 140 is OFF, the 
above-mentioned lock detection operation is not executed. In steps S19 and 
S20, a quick increase in outputs from the vehicle speed sensors 122 and 
124 is discriminated for failure detection. 
That is, in step S19, a quick increase in output from the first vehicle 
speed sensor 122 is discriminated. In step S19, it is checked if a 
decrement El of the output from the transaxle side first vehicle speed 
sensor 122, more specifically, a numerical value El defined by a 
difference in a decreasing direction between the vehicle speed detection 
result detected at the immediately preceding sampling timing and the 
currently detected vehicle speed detection result is equal to or higher 
than a predetermined value (in this embodiment, 10 km/h/131 msec). This 
predetermined value is set to a deceleration of 10 km/h per sampling time, 
and corresponds to a change of about 76 km/h per second. 
If YES in step S19, that is, if the quick increase in vehicle speed 
detected by the first vehicle speed sensor 122 is determined, a failure is 
detected, and the flow advances to step S14. Thus, the fail-safe control 
operation is executed. 
If NO in step S19, that is, if the quick increase in vehicle speed from the 
first vehicle speed sensor 122 is not determined, a quick increase in 
output from the second vehicle speed sensor 124 is discriminated in step 
S20. In step S20, it is checked if a decrement E2 of the output from the 
speedometer side second vehicle speed sensor 124 is equal to or higher 
than the predetermined value (in this embodiment, 10 km/h/131 msec). 
If YES in step S20, that is, if the quick increase in vehicle speed 
detected by the second vehicle speed sensor 124 is determined, a failure 
is detected, and the flow jumps to step S14 to execute the fail-safe 
control operation. 
If NO in step S20, that is, if no quick increase in vehicle speed of the 
first and second vehicle speed sensors 122 and 124 is determined, it is 
checked in step S21 if the detection result of the first vehicle speed 
sensor 122 coincides with the detection result of the second vehicle speed 
sensor 124 as in step S13. If NO in step S21, that is, if the detection 
outputs from both the sensors 122 and 124 do not coincide with each other, 
since the failure state has occurred as described above, the flow jumps to 
step S14 to execute the fail-safe control operation. Thereafter, the flow 
returns. 
If YES in step S21, that is, if the detection outputs from both the vehicle 
speed sensors 122 and 124 coincide with each other, since no failure state 
is established, the flow returns to the initial state of the control 
operation. 
In this manner, a series of control operations are completed. 
As described above, in this embodiment, the lock state of the wheels is 
detected, and the lock control operation is executed to avoid danger in 
the wheel locked state by the vehicle-speed sensitive four-wheel steering 
apparatus. Therefore, in order to solve the above-mentioned problems, a 
so-called antilock brake system (ABS) for preventing wheels from being 
locked need not be employed. 
The present invention is not limited to the arrangement of the 
above-mentioned embodiment, and various changes and modifications may be 
made within the spirit and scope of the invention. 
For example, in the embodiment, in the lock control operation, normal 
turning angle ratio control indicated by the solid curve I in FIG. 5 is 
interrupted, and the rear wheel turning angle ratio is controlled in 
accordance with a characteristic for maintaining a stable travel state of 
the vehicle. More specifically, the rear turning angle ratio is fixed to a 
ratio according to the vehicle speed immediately before lock detection. 
However, the present invention is not limited to the above-mentioned lock 
control for stabilization unlike the normal turning angle ratio control, 
wherein the rear wheel turning angle ratio is fixed to a ratio according 
to the vehicle speed immediately before lock detection. As shown in a 
first modification indicated by an alternate long and short dashed curve 
III in FIG. 5, as a mode for maintaining a stable travel state in lock 
control, the rear wheel turning angle ratio can be fixed to "0" to 
forcibly set the 2WS mode. 
Although not shown, as a second modification of a mode for setting the 
stable travel state in the lock control, delay control may be executed In 
this second modification, when the lock detection is made, although the 
lock control is performed in accordance with the same characteristic as 
that in normal turning angle ratio control, its change rate is reduced, 
that is, the operation speed of the stepping motor is set to be low. As a 
result, in stabilization lock control in the second embodiment, although 
the rear wheel turning angle ratio is changed in accordance with the 
characteristic indicated by the solid curve I in FIG. 5, a time required 
for changing a rear wheel turning angle ratio to the opposite phase state 
according to the vehicle speed of 0 km/h detected upon lock detection is 
prolonged. 
More specifically, in a state wherein the brake pedal is kept depressed and 
the wheels are slipping, when the actual vehicle speed reaches zero due to 
friction, the rear wheel turning angle ratio is slowly changed from the 
in-phase side to the opposite phase side, so that it is delayed to be 
around zero. 
In this manner, when the stabilization control content in lock control is 
set like the first and second modifications, sufficiently stable control 
can be realized although the control content of the embodiment is stabler. 
In addition, the lock state is released, and the normal control is 
recovered at an earlier timing. 
In the above embodiment, the rear wheel turning angle ratio is fixed to a 
ratio according to the detected vehicle speed immediately before lock 
detection upon lock detection. When the lock state is released and normal 
turning angle ratio control is restarted, the rear wheel turning angle 
ratio is set to be a ratio according to the vehicle speed at this time, 
e.g., a vehicle speed determined to be 0 km/h if the vehicle stands still. 
For this reason, if the rear wheel turning angle ratio fixed according to 
lock detection is in the in-phase side, the stepping motor 100 must be 
driven to change the rear wheel turning angle ratio to the opposite phase 
side. While the stepping motor 100 is operated in this manner to complete 
a setting operation to a predetermined rear wheel turning angle ratio, the 
normal turning angle ratio control operation cannot be started. 
In contrast to this, in the first modification, when the lock control 
operation is canceled, the rear wheel turning angle ratio is fixed to 
zero, and in the second modification, the rear wheel turning angle ratio 
becomes substantially zero. In this manner, in the first and second 
modifications, a time required for recovering normal control can be 
shortened as compared to the above embodiment, and a timing for 
substantially recovering the control can be set earlier. 
As many apparently widely different embodiments of the present invention 
can be made without departing from the spirit and scope thereof, it is to 
be understood that the invention is not limited to the specific 
embodiments thereof except as defined in the appended claims.