Fail-safe vehicle control system

A fail-safe control unit for a vehicle having a first control system for controlling a steer angle and a second control system for controlling a condition of the vehicle, such as a driving force distribution in a 4WD system, or a damping characteristic of a suspension system. When one of the first and second control systems becomes abnormal, and the other remains normal, the fail-safe control unit adjusts the normal control system so as to maintain or improve the directional stability of the vehicle notwithstanding the failure of the abnormal system.

BACKGROUND OF THE INVENTION 
The present invention relates to a system for controlling a vehicle having 
a steer angle control system and another vehicle control system, such as a 
4WD control system or a suspension control system, for controlling a 
condition of the vehicle affecting a handling characteristic of the 
vehicle. 
Recently, many publications have disclosed various 4WS control systems, 4WD 
control systems and suspension control systems. 
Some examples of 4WS control are disclosed in U.S. Pat. No. 4,705,131, 
Japanese Patent Provisional Publication No. 60-229873, and Japanese 
Utility Model Provisional Publication No. 62-23773. 
Some examples of 4WD control are disclosed in U.S. Pat. No. 4,757,870, No. 
4,754,834, No. 4,773,500 and No. 4,776,424, and "Seigyo Riron no Ohyo to 
Jidosha no Seigyo" (Application of Control Theory and Control of 
Automobile), Symposium of Society of Automotive Engineers of Japan, p41, 
published in 1986. The 4WD system disclosed in the last-named Japanese 
document is mounted on a NISSAN CUE-X concept car. The 4WD system of that 
document is an electronically controlled torque split, full-time 4WD 
system having a hydraulic multiple-disc clutch in a drive path for front 
wheels. A controller controls the torque capacity of the multiple disc 
clutch by varying an applied clutch fluid pressure. The driving force 
distribution ratio of the front wheel driving force to the rear wheel 
driving force is approximately Tc: (T-Tc), where Tc is the torque capacity 
of the multiple disc clutch, and T is the transmission output torque. 
Therefore, the controller controls the torque capacity Tc to provide an 
optimum front wheel and rear wheel driving torque distribution in 
accordance with signals of wheel speed sensors and an accelerator position 
sensor. 
The Mitsubishi Galant is an example of a vehicle combining the 4WS system 
and the 4WD system. 
An example of suspension control is disclosed in "T12 NISSAN Sahbisu Shuho" 
(NISSAN Service Bulletin), pages C-7, C-9 and C-10, published by NISSAN 
Motor Co., Ltd. October, 1985. 
Some of the above-described control systems are equipped with a fail-safe 
means for detecting a failure, and electronically or mechanically 
adjusting an abnormal system in which the failure is detected so as to 
maintain the safety of the vehicle. 
However, conventional fail-safe control systems are unsatisfactory in that 
no consideration is given to the influence of a failure of one control 
system on another control system in the same vehicle. In general, a 
vehicle having a second vehicle control system such as a 4WD control 
system or a suspension control system in addition to a 4WS control system, 
is tuned to have optimum vehicle characteristics when both control systems 
are normal and functioning properly. Therefore, the vehicle stability 
becomes worse when one control system is brought to a stop because of its 
failure. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a fail-safe vehicle 
control system which can more reliably protect the stability of a vehicle 
having two different control systems, against failures in the control 
systems. 
It is another object of the present invention to provide a vehicle having a 
steering control system, a second vehicle control system such as a 4WD 
control system or a suspension control system, and a fail-safe means which 
can protect the vehicle stability against failures in the control systems 
by controlling not only the control system which becomes abnormal, but 
also the control system which remains normal. 
According to the present invention, a vehicle comprises a first control 
system, a second control system, and fail-safe means. The first control 
system comprises a steering system for steering the vehicle, and a first 
controlling means for controlling a condition of the steering system. For 
example, the first controlling means is arranged to control a rear wheel 
steer angle of the vehicle. The second control system comprises a drive 
system for driving the vehicle, a suspension system for supporting a 
vehicle body on wheels of the vehicle, and a second controlling means for 
controlling a condition of at least one of the drive system and the 
suspension system. For example, the second controlling means controls a 
driving force distribution between front wheels and rear wheels, or a 
suspension characteristic of the suspension system. The fail-safe means is 
connected with the first and second control systems, and arranged to 
adjust the first control system when a failure is detected in the second 
control system, and to adjust the second control system when a failure is 
detected in the first control system.

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIG. 11, a vehicle of the present invention has a steering 
system 301, a second system 302 such as a 4WD system or a suspension 
system, a first controlling means 303 for controlling the steering system, 
a second controlling means 304 for controlling the second system 302, and 
a fail-safe means 305 for adjusting a first control system constituted by 
the first controlling means 303 and the steering system 301 and a second 
control system constituted by the second controlling means 304 and the 
second system 302 when a failure is detected in the first or second 
control system. The vehicle may further comprise various sensors 306 for 
sensing vehicle operating conditions and providing necessary information 
to the first and second control systems. 
A first embodiment of the present invention is shown in FIGS. 1-5. 
A vehicle shown in FIG. 1 has a drive system and a rear wheel auxiliary 
steering system which are substantially the same as those shown in 
Japanese Patent Application No. 63-33892. As shown in FIG. 1, the vehicle 
has left and right front wheels 1L and 1R, left and right rear wheels 2L 
and 2R, and a steering wheel 3. 
The drive system shown in FIG. 1 includes an engine 4, a transmission 5, a 
rear drive line which always connects the rear wheels 2L and 2R with an 
output member of the transmission 5, and a front drive line capable of 
controlling the amount of driving torque transmitted from the transmission 
5 to the front wheels 1L and 1R. The rear drive line includes a rear 
propeller shaft 6 which is always connected with the transmission 5, and a 
rear differential 7. The front drive line includes a front drive clutch 8, 
a front propeller shaft 9, and a front differential 10. 
The steering system of the vehicle includes a steering gear 11 connected 
between the steering wheel 3 and the front wheels 1L and 1R so that the 
front wheels 1L and 1R can be steered by the steering wheel 3 in a 
conventional manner, and a rear wheel steering actuator 12 for steering 
the rear wheels 2L and 2R. 
The rear wheel steering actuator 12 of this embodiment is a spring center 
type hydraulic actuator. When oil pressure is supplied to a right chamber 
12R of the actuator 12, the actuator 12 steers the rear wheels 2L and 2R 
to the right by an angle proportional to the oil pressure. When the oil 
pressure is supplied to a left chamber 12L, the actuator 12 steers the 
rear wheels 2L and 2R to the left by an angle proportional to the oil 
pressure. 
A rear wheel steering control valve 13 of an electromagnetic proportional 
type is connected with the actuator 12, and arranged to control the oil 
pressures of the left and right chambers 12L and 12R of the actuator 12. 
The control valve 13 has four variable orifices 13a, 13b, 13c and 13d 
which are connected in a bridge circuit as shown in FIG. 1. The four modes 
of the bridge circuit are connected, respectively, with a pump 14, a 
reservoir 15, a left oil passage 16 leading to the left chamber 12L of the 
actuator 12, and a right oil passage 17 leading to the right chamber 12R 
of the actuator 12. The control valve 13 further includes left and right 
solenoids 13L and 13R. When both solenoids are off, one opposite orifice 
pair 13a and 13b and the other opposite orifice pair 13c and 13d are both 
fully opened so that both chambers 12L and 12R are put in a 
non-pressurized state (an equilibrium state). When the solenoid 13L or 13R 
is engergized by a left solenoid current I.sub.L or a right solenoid 
current I.sub.R, then orifice pair 13c and 13d or orifice pair 13a and 13b 
is closed to a reduced opening corresponding to the magnitude of the 
solenoid exciting current I.sub.L or I.sub.R, so that the oil pressure 
corresponding to the current I.sub.L or I.sub.R is supplied to the power 
chamber 12L or 12R of the actuator 12. 
An electromagnetic proportional pressure regulating valve 18 is connected 
with the front wheel drive clutch 18 and controls the clutch pressure Pc 
supplied to the front drive clutch 8. The front drive clutch 8 transmits a 
driving torque corresponding to the clutch pressure Pc to the front wheels 
1L and 1R. The regulating valve 18 normally holds the clutch pressure Pc 
equal to zero. As a current I.sub.C for energizing a clutch control 
solenoid 18a of the valve 18 is increased, the clutch pressure regulating 
valve 18 increases the clutch pressure Pc by admitting the output pressure 
of the pump 14 to the clutch 8. 
A controller 19 is provided for controlling the exciting currents I.sub.L, 
I.sub.R and I.sub.C of the solenoids 13L, 13R and 18a. 
Various sensors are connected with the controller 19. In this embodiment, 
the sensors are a steering angle sensor 20 for sensing a steering angle 
.theta., wheel speed sensors 21L, 21R, 22L and 22R for sensing the 
rotational speeds wf1, wf2, wr1 and wr2 of the wheels 1L, 1R, 2L and 2R, 
respectively, a vehicle speed sensor 23 for sensing the vehicle speed V, 
and a throttle opening sensor 24 for sensing the throttle opening degree 
TH. The steering angle sensor 20 of this embodiment is a sensor for 
sensing the steering wheel angle .theta. of the steering wheel 3. Output 
signals of these sensors are input to the controller 19. 
As shown in FIG. 2, the controller 19 of this embodiment has a 4WS control 
section 19a and a 4WD control section 19b. 
FIG. 3 shows a driving force distribution control procedure of this 
embodiment, and FIG. 4 shows a rear wheel steer angle control procedure. 
The controller 19 periodically repeats each of the control procedures. 
In step 51 of FIG. 3, the controller 19 determines a desired front wheel 
and rear wheel driving force distribution ratio in accordance with a 
predetermined normal control mode. In this embodiment, the distribution 
ratio is determined by using the signals of the wheel speed sensors 21L, 
21R, 22L and 22R and the throttle opening sensor 24, as in the NISSAN 
CUE-X concept car (disclosed in Symposium of Society of Automotive 
Engineers of Japan "Seigyo Riron no Ohyo to Jidosha no Seigyo" 
(Application of Control Theory and Automobile Control), page 41, 1986). In 
the present invention, it is possible to employ any one of various front 
and rear wheel driving force distribution control systems. Some examples 
are disclosed in U.S. Pat. No. 4,757,870, No. 4,754,834, No. 4,773,500 and 
4,776,424. 
In step 52, the controller 19 determines whether a 4WS failure signal is 
present or not. If it is, then the controller 19 proceeds from step 52 to 
step 53, and from step 53 to step 55. In step 53, the controller 19 
corrects the front and rear wheel driving force distribution ratio so as 
to increase the driving force transmitted to the front wheels relative to 
the driving force of the rear wheels. If the 4WS failure signal is not 
present, then the controller 19 proceeds from step 52 to step 55 by way of 
step 54. In step 54, the controller 19 sets a correction equal to zero so 
that the desired front and rear wheel driving force distribution ratio 
determined in step 51 remains unchanged. 
In step 55, the controller 19 determines whether a failure is present in 
the sensors and solenoid used in the 4WD control to determine whether the 
4WD control system is functioning properly. If the 4WD control system is 
not normal, then the controller 19 proceeds from step 55 to step 56 to 
stop the 4WD control, and then to step 57 to deliver a 4WD failure signal 
to the 4WS control section 19a. 
If no failure is detected in the 4WD control system, then the controller 19 
proceeds from the step 55 to step 58. In step 58, the controller 19 
outputs a 4WD control signal to achieve the front and rear wheel driving 
torque distribution ratio determined in step 53 or 54. In this embodiment, 
the 4WD control signal is the solenoid exciting current I.sub.C, which is 
sent to the solenoid 18a of the electromagnetic pressure regulating valve 
18 to perform duty factor control. After step 57 or 58, the controller 19 
returns to step 51 to repeat a sequence of operations shown in FIG. 3. 
When the 4WS control system is abnormal and the 4WD control system is 
normal, the fail-safe control system of this embodiment increases the 
driving force distribution ratio, which is the ratio of the driving force 
transmitted to the front wheels to the driving force transmitted to the 
rear wheels, so that the understeer tendency is increased and the vehicle 
stability is improved. 
The 4 wheel steering control procedure of this embodiment is shown in FIG. 
4. 
In step 71, the controller 19 reads the current values of the steering 
wheel angle .theta. and the vehicle speed V. 
In step 72, the controller 19 determines values of a rear wheel steering 
proportional gain (or constant) K and a rear wheel steering derivative 
gain (or constant) .tau.(tau) by using the current value of the vehicle 
speed V. The proportional gain K and the derivative gain .tau.(tau) are 
functions of the vehicle speed V which are determined so as to provide the 
ideal vehicle dynamic characteristics in which the characteristic of the 
yaw rate gain with respect to the steering frequency is flat so that the 
yaw rate is produced in proportion to the front wheel steering amount 
independently of the steering speed without phase delay. Then, the 
controller 19 determines a rear wheel steering time constant Tr by using 
the following equation (1): 
EQU Tr=(.tau./K)/A (1) 
where A is a constant from 2 to 3. 
In step 73, the controller 19 determines whether the 4WD failure signal is 
present or not to determine whether the 4WD system is abnormal or not. If 
the 4WD system is abnormal, then the controller 19 proceeds from step 73 
to step 74. In step 74, the controller 19 increases the proportional gain 
K by .DELTA.K (Delta K) (i.e., K=K+.DELTA.K), or decreases the derivative 
gain .tau.(tau) by .DELTA..tau.(Delta tau) (i.e., 
.tau.=.tau.-.DELTA..tau.), or decreases the derivative gain .tau.(tau) to 
zero(i.e., .tau.=0). After step 74, the controller 19 proceeds to step 76. 
If the 4WD system is normal, and hence the answer in step 73 is negative, 
then the controller 19 proceeds from step 73 to step 75. In step 75, the 
controller 19 sets a correction quantity equal to zero, so that the 
constants K, .tau.(tau) and Tr remain unchanged. Then, control is 
transferred from step 75 to step 76. 
In step 76, the controller 19 determines a rear wheel steer angle 
.delta..sub.r by using the values of the constants K, .tau., Tr obtained 
in steps 72, 74 and 75. In this embodiment, the rear wheel steer angle is 
calculated by the following equation (2): 
##EQU1## 
where .delta..sub.r (S) is the Laplace transform of the rear wheel steer 
angle and s is a Laplace transform variable. Instead of the equation (2), 
it is possible to employ one of various other equations such as; 
##EQU2## 
where B.sub.f, B.sub.r, .tau..sub.f and .tau..sub.r are functions of the 
vehicle speed and .delta..sub.f (s) is the Laplace transform of the front 
wheel steer angle (the front and rear wheel steer angle ratio is in the 
form of first-order/first-order. 
In step 77, the controller 19 determines whether the 4WS system is abnormal 
or not by determining whether a failure is detected in the sensors and 
solenoids used in the 4WS control. If a failure is detected in the 4WS 
system, then the controller 19 stops the 4WS control in step 78, and 
delivers the 4WS failure signal to the 4WD control section 19b. 
If the 4WS system is normal, the controller 19 performs 4WS control in step 
80 by outputting a 4WS control signal to achieve the rear wheel steer 
angle .delta..sub.r obtained in step 76. In this embodiment, the 
controller 19 delivers the exciting currents I.sub.L and I.sub.R to the 
solenoids 13L and 13R in step 80. After step 79 or 80, the controller 19 
returns to step 71 to repeat the procedure of FIG. 4. 
When the 4WD control system is abnormal and the 4WS control system is 
normal, then the fail-safe control system of this embodiment varies the 
rear wheel steer angle in such a direction as to increase the amount of 
same-direction rear wheel steering by which the rear wheels are steered in 
the same direction as the steering direction of the front wheels, or to 
decrease the amount of opposite-direction rear wheel steering by which the 
rear wheels are steered in the direction opposite to the steering 
direction of the front wheels. Therefore, the control system can maintain 
or improve the vehicle stability by increasing the understeer tendency. 
The directional stability of a vehicle can be expressed by a static 
stability dM/d.beta. (where M is a static restoring yaw moment and .beta. 
is an attitude angle), as disclosed in "Kiso Jidosha Kogaku (second 
volume)", pages 29-39, written by KONDO, published June, 1979. The static 
stability is given by 
EQU dM/d.beta.=(l.sub.1 T+l.sub.2 T).alpha.+C.sub.1 l.sub.1 -C.sub.2 l.sub.2 
-l.sub.2 T (3) 
where T is the total driving force of the vehicle, .alpha. (alpha) is the 
ratio of the front wheel driving force to the total driving force, and 
l.sub.1, l.sub.2, C.sub.1 and C.sub.2 are constants determined by the 
specifications of the vehicle. When the stability dM/d.beta. is greater 
than zero, the vehicle is directionally stable (This characteristic 
corresponds to the understeer tendency.). When the static stability 
dM/d.beta. is smaller than zero, the vehicle is unstable (an oversteer 
tendency). Therefore, when the total driving force T is positive, the 
static stability dM/d.beta. increases, and the understeer tendency 
increases with increase of the share .alpha.(alpha) of the driving force 
alloted to the front wheels. In view of this, the control system of this 
embodiment is arranged to increase the vehicle stability by increasing the 
share of the driving force alloted to the front wheels when the 4WS system 
is abnormal. 
When the 4WD system becomes abnormal and the clutch is disengaged as in the 
above-mentioned NISSAN CUE-X, the share of the front wheel driving force 
is reduced to zero, and the share of the rear wheel driving force is 
increased to 100% (two rear wheel drive mode), so that the vehicle 
stability becomes worse. In contrast to this, the control system of this 
embodiment is arranged to prevent the vehicle stability from being 
decreased by the failure of the 4WD system by utilizing the 4WS control 
system. When the proportional gain K is increased in step 74, the steady 
state gain and phase delay of the yaw rate are changed from the solid line 
characteristic curves to the curves shown with short dashes in FIG. 5, so 
that the damping of the yaw rate is improved, and the stability is 
increased so as to compensate for the influence of the failure of the 4WD 
system. Furthermore, it is possible to improve the vehicle stability by 
decreasing the derivative gain .tau.(tau) (though the steering response 
gain is somewhat decreased). 
When the 4WS system becomes abnormal and the steering mode is reduced to 
the two wheel steering mode, the understeer tendency is decreased and the 
steering response gain is enhanced as shown by the curves in FIG. 5 with 
alternating long and short dashes. As a result, the driver has an 
unnatural feeling. The control system of this embodiment can prevent such 
an unnatural feeling by increasing the share of the driving force alloted 
to the front wheels. 
A second embodiment of the present invention is shown in FIGS. 6-10. 
A vehicle shown in FIG. 6 has a rear wheel auxiliary steering system and a 
suspension system which are substantially the same as those shown in 
Japanese Patent Application No. 63-22686. As shown in FIG. 6, the vehicle 
has front wheels 201L and 201R, rear wheels 202L and 202R, and a steering 
wheel 203. 
The steering system of the vehicle includes a steering gear 204 connected 
between the steering wheel 203 and the front wheels 201L and 201R so that 
the front wheels are steerable through the steering gear 204 in the 
conventional manner. In this steering system, the front wheel steer angle 
.delta..sub.f is determined by the steering wheel angle .theta. and the 
steering gear ratio N, as .delta..sub.f =.theta./N. 
A rear suspension system includes left and right transverse links 205L and 
205R, and left and right upper arms 206L and 206R. The rear wheels 202L 
and 202R are connected with a rear suspension member 207 of the vehicle 
body through the rear suspension system in such a manner that the rear 
wheels are steerable. An actuator 209 for steering the rear wheels is 
provided between knuckle arms 208L and 208R of the left and right rear 
wheels 202L and 202R. Both ends of a piston rod of the rear wheel steering 
actuator 209 are connected with the knuckle arms 208L and 208R through 
left and right side rods 210L and 210R. 
The rear wheel steering actuator 209 of this embodiment is a spring center 
type double acting hydraulic actuator. Left and right power chambers of 
the actuator 209 are connected with an electromagnetic proportional 
pressure control valve 212, respectively, through left and right fluid 
passages 211L and 211R. The control valve 212 is further connected with a 
pressure supply passage 215 and a drain passage 216 of a pressure source 
including a pump 213, and a reservoir 214. There is further provided a 
power steering control valve (P.S. valve). 
The pressure control valve 212 has left and right solenoids 212L and 212R, 
and is connected with a controller 217 which controls exciting currents 
I.sub.1 and I.sub.R of the solenoids 212L and 212R. When the controller 
217 stops supplying the currents I.sub.L and I.sub.R and holds both 
solenoids off, the control valve 212 makes the pressures of the left and 
right power chambers of the actuator 209 equal to each other. Therefore, 
the actuator 9 is held in an equilibrium state and performs no steering 
action. When the left solenoid 212L is energized by the current I.sub.L, 
the control valve 212 increases the pressure of the left power chamber of 
the actuator 209 by an amount proportional to the magnitude of the 
exciting current I.sub.L. Therefore, the actuator 209 steers the rear 
wheels 202L and 202R to the left. When the controller 217 energizes the 
right solenoid 212R by the current I.sub.R, then the control valve 212 
increases the pressure of the right power chamber of the actuator 209 by 
an amount proportional to the magnitude of the current I.sub.R and 
accordingly the actuator 209 steers the rear wheels 202L and 202R to the 
right. 
The controller 217 is connected with a steering angle sensor 218 for 
sensing the steering wheel angle .theta. of the steering wheel 203 and a 
vehicle speed sensor 219 for sensing the vehicle velocity V. 
The suspension system of this vehicle has adjustable shock absorbers 220, 
221, 222 and 223 which are provided, respectively, for the left and right 
front wheels 201L and 201R and the left and right rear wheels 202L and 
202R. The four wheels 201L, 201R, 202L and 202R are independently attached 
to the vehicle body. Each shock absorber has a motor 220a, 221a, 222a or 
223a for adjusting a suspension damping characteristic, and a shock 
absorber sensor 220b, 221b, 222b or 223b, as shown in FIG. 7. The 
controller 217 is connected with each motor, and arranged to control each 
of motor driving currents I.sub.c1, I.sub.c2, I.sub.c3 and I.sub.c4 of the 
damping characteristic adjusting motors 220a-223a. By controlling the 
rotation of the adjusting motor, the controller 217 can adjust the damping 
characteristic, such as a suspension damping force, of each shock 
absorber, individually. 
In this embodiment, each shock absorber is adjusted to one of three damping 
modes, a hard mode (H), a medium mode (M) and a soft mode (S). The 
controller 217 of this embodiment adjusts the shock absorbers to the H 
mode when the vehicle velocity becomes low (when the vehicle is at rest, 
for example), or when the angular speed .theta. of the steering wheel 
becomes high. When the road condition is bad (when the distance L between 
the vehicle body and the road surface is outside a predetermined range), 
then the controller 217 of this embodiment adjusts the front shock 
absorbers to the M mode, and the rear shock absorbers to the S mode. Each 
shock absorber sensor is arranged to sense a condition of the associated 
shock absorber to discriminate the H, M and S damping modes. 
In the embodiment, four road surface sensors 224L 224R and 225L and 225R 
are provided, respectively, near the left and right front wheels 201L and 
201R and the left and right rear wheels 202L and 202R. The road surface 
sensors of this embodiment are of an ultrasonic type. Each road surface 
sensor has an ultrasonic transmitter and receiver, and measures the 
distance L between the vehicle body and the road surface by measuring the 
time interval between transmission of an ultrasonic wave and reception of 
the reflected wave. 
The controller 217 is connected with the steering angle sensor 218, the 
vehicle speed sensor 219, the shock absorber sensors 220b-223b and the 
road surface sensors 224L, 224R, 225L and 225R. The controller 217 
periodically performs a suspension control procedure shown in FIG. 8 for 
controlling the motor driving currents I.sub.c1, I.sub.c2, I.sub.c3, and 
I.sub.c4 of the shock absorbers, and a rear wheel steering control 
procedure shown in FIG. 9. 
As shown in FIG. 7, the controller 217 of this embodiment has a 4WS control 
section 217a, and a suspension control section 217b. 
In step 251 of the suspension control, the controller 217 uses the steering 
angle .theta. (the steering angular speed .theta.), the vehicle velocity 
V, the distances L between the vehicle body and the road surface, and the 
conditions of the shock absorbers, and determines optimum front and rear 
suspension damping forces suited to the driving conditions and road 
conditions. It is possible to employ various suspension control 
strategies. Some examples are disclosed in U.S. Pat. Nos. 4,616,848, 
4,733,883 and 4,717,173. 
In step 252, the controller 217 determines whether a 4WS failure signal is 
present or not to determine whether the 4WS system is abnormal or not. If 
the 4WS failure signal is present, then the controller 217 proceeds from a 
step 252 to step 253 and increases the front wheel suspension damping 
force by one step. For example, the front wheel suspension damping force 
is increased from the level of the medium damping mode to the level of the 
hard damping mode, or from the level of the soft mode to the level of the 
medium mode. Then, the controller 217 proceeds from step 253 to step 255. 
If the 4WS system is normal, then the controller 217 proceeds from step 
252 to step 255 by way of step 254. In step 254, the controller 217 sets a 
correction quantity equal to zero, so that the front and rear wheel 
suspension damping forces remain unchanged at values determined in step 
251. 
In step 255, the controller 217 determines whether there is a failure in 
the suspension control system by detecting a failure in the sensors and 
actuators used for the suspension contrtol. If the suspension system is 
abnormal, then the controller 217 proceeds from step 255 to steps 256 and 
257. The controller 217 stops the suspension control in step 256, and 
delivers a suspension failure signal in 4WS control section 219a at the 
step 257. 
If the suspension system is normal, then the controller 217 proceeds from 
step 255 to step 258 to output a suspension control signal. In this 
embodiment, the controller 217 delivers the motor driving currents 
I.sub.c1, I.sub.c2, I.sub.c3 and I.sub.c4, respectively, in shock absorber 
adjusting motors 220a-223a at the step 258. Therefore, each shock absorber 
is adjusted to the level of the damping force (H or M or S) determined in 
steps 251 and 253 or steps 251 and 254. After step 257 or 258, the 
controller 217 returns to step 251. 
In this way, this control system makes the front suspension damping 
characteristic harder when the 4WS system is abnormal and the suspension 
system is normal. Therefore, this control system maintains or improves the 
vehicle stability in spite of the failure of the 4WS system by increasing 
the understeer tendency by means of the suspension control system. The 
present invention is applicable to a vehicle having a variable stabilizer 
bar or a suspension system adjusting a spring constant. In this case, the 
fail safe system is arranged to increase the roll stiffness distribution 
on the front wheels' side when the 4WS system is abnormal and the 
suspension system is normal. 
The 4WS control procedure shown in FIG. 9 is almost the same as that of the 
first embodiment shown in FIG. 4. Steps 271, 272 and 274-280 are 
substantially the same as steps 71, 72 and 74-80, respectively. In step 
273 of the second embodiment, the controller 217 checks the suspension 
failure signal instead of the 4WD failure signal. If the suspension 
failure signal is present, then the controller 217 proceeds from step 273 
to step 274. If the suspension system is normal, then the controller 217 
proceeds from step 273 to step 275. In step 279, the 4WS failure signal is 
delivered to the suspension control section 217b. 
When the suspension control system is abnormal, and the 4WS system is 
normal the control system of the second embodiment varies the rear wheel 
steer angle in such a direction as to increase the amount of 
same-direction rear wheel steering, or to decrease the opposite-direction 
rear wheel steering in the same manner as in the first embodiment. 
Therefore, the control system of the second embodiment can maintain or 
improve the vehicle directional stability by increasing the understeer 
tendency in spite of the failure of the suspension control system. 
FIGS. 10A and 10B show effects of the control system of the second 
embodiment. 
When the front suspension damping force is increased with respect to the 
rear suspension damping force, the transient wheel load transfer in the 
front wheels is increased relatively during cornering and the front wheel 
cornering force is temporarily decreased so that the understeer tendency 
is increased and the directional stability is improved. Therefore, the 
control system of the second embodiment increases the understeer tendency 
by increasing the front suspension damping force, as shown by with short 
dashes in FIG. 10A, when the 4WS system becomes abnormal. 
When, for example, the suspension control system becomes abnormal and 
unable to increase the damping force during rolling, the roll angle of the 
vehicle tends to increase, and the steering characteristic of the vehicle 
tends to become unstable by the influence of "roll steer" and other 
factors. In such a case, the control system of the second embodiment can 
maintain or improve the vehicle stability by adjusting the control 
characteristic of the 4WS system. When the control system increases the 
proportional gain K of the rear wheel steering, the steady state gain and 
the phase delay of the yaw rate are lowered especially in a high speed 
range from solid line curves to the curves with short dashes shown in FIG. 
10B, and accordingly the vehicle stability is increased. 
When the 4WS system becomes abnormal and the steering mode is reduced to 
two wheel steering mode, the understeer tendency is decreased and the 
steering response gain of the vehicle is enhanced as shown by in FIG. 10B 
so that the driver feels an unnatural feeling. The control system of the 
second embodiment can prevent such an unnatural feeling by hardening the 
front suspension.