Vehicle suspension system

A vehicle suspension system has fluid cylinders connected between the vehicle body and the respective wheels. A fluid control system controls feed and discharge of hydraulic fluid to and from the fluid cylinders and changes the suspension properties of the suspension system. A failure in the fluid control system is detected and when the control system fails, one of first to third measure modes is taken on the basis of the kind of the failure. Warning is just given and the control of feed and discharge of hydraulic fluid to and from the fluid cylinders being continued when the first measure mode is taken, the control of feed and discharge of hydraulic fluid to and from the fluid cylinders is interrupted with the chassis height fixed to the present height when the second measure mode is taken, and the fluid in the fluid cylinders is discharged and the chassis height is lowered when the third measure mode is taken.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a vehicle suspension system, and more 
particularly to a vehicle suspension system which includes a fluid 
cylinder and in which the suspension properties change with change in the 
amount of fluid introduced into the fluid cylinder. 
2. Description of the Prior Art 
As disclosed, for instance, in Japanese Patent Publication No. 59-14365, 
there has been known a socalled hyrdro-pneumatic suspension system which 
comprises fluid cylinders connected between the vehicle body and the 
respective wheels and a gas spring connected to each of the fluid 
cylinders. 
Further, there has been known an active control suspension system in which 
the amount of fluid introduced into the fluid cylinder for each wheel is 
changed separately from the other fluid cylinders and the properties of 
which is changed according to the operating condition of the vehicle. 
When such an active control suspension system is actually incorporated in a 
vehicle, fail-safe measures against a failure in the flow control system 
for controlling the amount of fluid introduced into the fluid cylinders or 
in various sensors should be taken. 
As the fail-safe measures, immediate interruption of the control, warning 
and the like can be employed. However, it is preferred that suitable 
measure be selected according to the kind of the failure. That is, when 
the control is immediately interrupted for a failure which does not affect 
the nature of the control, the effect of the active suspension system, 
e.g., improvement in the driving performance, will be easily lost. On the 
other hand, just a warning is not sufficient in view of safety for a 
failure which affects the nature of the control occurs. 
SUMMARY OF THE INVENTION 
In view of the foregoing observations and description, the primary object 
of the present invention is to provide a suspension system in which, in 
case the control system for controlling feed and discharge of hydraulic 
fluid to and from the fluid cylinders fails, suitable measure is taken 
according to the kind of the failure, thereby obtaining an excellent 
driving performance as long as possible without adversely affecting 
safety. 
In accordance with one aspect of the present invention, there is provided a 
vehicle suspension system comprising fluid cylinders connected between the 
vehicle body and the respective wheels; a fluid control system which 
controls feed and discharge of hydraulic fluid to and from the fluid 
cylinders and changes the suspension properties of the suspension system; 
a failure detecting means which detects a failure in the fluid control 
system; a measure mode determining means which receives a signal from the 
failure detecting means and determines which of first to third measure 
modes is to be taken on the basis of the kind of the failure represented 
by the signal, warning being just given and the control of feed and 
discharge of hydraulic fluid to and from the fluid cylinders being 
continued when the first measure mode is taken, the control of feed and 
discharge of hydraulic fluid to and from the fluid cylinders being 
interrupted with the chassis height fixed to the present height when the 
second measure mode is taken, and the fluid in the fluid cylinders being 
discharged and the chassis height being lowered when the third measure 
mode is taken; and a measure mode performing means which receives a signal 
from the measure mode determining means and performs the measure mode 
determined by the measure mode determining means. 
In accordance with another aspect of the present invention, there is 
provided a vehicle suspension system comprising fluid cylinders connected 
between the vehicle body and the respective wheels; a fluid control system 
which controls feed and discharge of hydraulic fluid to and from the fluid 
cylinders and changes the suspension properties of the suspension system; 
a failure detecting means which detects a failure in the fluid control 
system; a measure mode determining means which receives a signal from the 
failure detecting means and determines which of first to third measure 
modes is to be taken on the basis of the kind of the failure represented 
by the signal, warning being just given and the control of feed and 
discharge of hydraulic fluid to and from the fluid cylinders being 
continued when the first measure mode is taken, the control of feed and 
discharge of hydraulic fluid to and from the fluid cylinders being 
interrupted with the chassis height fixed to the present height when the 
second measure mode is taken, and the fluid in the fluid cylinders being 
discharged and the chassis height being lowered when the third measure 
mode is taken; a measure mode duration determining means which determines 
whether the determination of the measure mode determining means is to be 
canceled when the ignition switch of the vehicle is turned off or to be 
held until the failure is removed; and a measure mode performing means 
which receives a signal from the measure mode determining means and the 
measure mode duration determining means and performs the measure mode 
determined by the measure mode determining means. 
When the failure detected by the failure detecting means is one which can 
sometimes be removed without service, the measure mode duration 
determining means determines, that the determination of the measure mode 
determining means is to be canceled when the ignition switch of the 
vehicle is turned off. On the other hand when the failure detected by the 
failure detecting means is one which can be never removed without service, 
the measure mode duration determining means determines, that the 
determination of the measure mode determining means is to be held until 
the failure is removed. The failures which require the first measure mode 
belong the former and the failures which require the third measure mode 
belong the latter. The failures which require the second measure mode 
include both the former and the latter. 
Generally, when a failure which does not affect the nature of the control 
occurs (e.g., when only a small amount of oil remains in the oil 
reservoir), the first measure mode is taken. When a failure which affects 
the nature of the control occurs (e.g., when a sensor such as a chassis 
height sensor fails), the second measure mode is taken. Further, when the 
failure is such as to cause the chassis heights at the respective wheels 
to differ from each other, the third measure mode is taken and the chassis 
is leveled.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, reference numerals 1, 2F and 2R respectively denote a vehicle 
body, a front wheel and a rear wheel. A fluid cylinder 3 is connected 
between each wheel and the vehicle body 1. The fluid cylinder 3 comprises 
a cylinder body 3a and a piston 3b which is received in the cylinder body 
3a and forms a liquid pressure chamber 3c in the cylinder body 3a. The 
piston 3b is connected to a piston rod 3d the upper end of which is 
connected to the vehicle body 1. The cylinder body 3a is connected to the 
wheel at the lower end thereof. 
A gas spring 5 is connected to the liquid pressure chamber 3c of each fluid 
cylinder 3 by way of a communicating passage 4. The inner space of each 
gas spring 5 is divided into a gas chamber 5f and a liquid pressure 
chamber 5g by a diaphragm 5e, and the liquid pressure chamber 5g is 
communicated with the liquid pressure chamber 3c of the fluid cylinder 3. 
Each fluid cylinder 3 is connected to a hydraulic pump 8 by way of a liquid 
pressure passage (high pressure line) 10. A flow control valve 9 which is 
provided in the liquid pressure passage 10 for each fluid cylinder 3 
controls feed and discharge of hydraulic fluid to and from the fluid 
cylinder 3. 
A main pressure sensor 12 detects the discharge pressure of the hydraulic 
pump 8 (more strictly, the pressure of accumulated oil at accumulators 22a 
and 22b which will be described later), cylinder pressure sensors 13 
detect the liquid pressure in the liquid pressure chambers 3c of the 
respective fluid cylinders 3c, chassis height sensors 14 detect the 
chassis heights at the respective wheels (cylinder stroke), vertical 
acceleration sensors 15 detect the vertical accelerations of the vehicle 
at the respective wheels, a lateral acceleration sensor 16 detects the 
lateral acceleration of the vehicle, a steering angle sensor 17 detects 
the turning angle of the front wheels 2F (as the dirigible wheels), a pair 
of vehicle speed sensors 18 detect the vehicle speed, and valve position 
sensors 19 detect the stroke position of the respective flow control 
valves 9. The detecting signals of these sensors 12 to 11 are input into a 
controller 20 which may be of a CPU, for instance, and the controller 20 
changes the suspension properties on the basis of the detecting signals. 
In FIG. 1, the cylinder pressure sensors 13, the chassis height sensors 14 
and the vertical acceleration sensors 15 for the front wheels 2F are not 
shown. 
The hydraulic pressure circuit which controls feed and discharge of the 
hydraulic fluid to and from the fluid cylinders 3 is shown in FIG. 2. In 
FIG. 2, the hydraulic pump is a variable volume type swash plate piston 
pump and is connected to a hydraulic pump 21 for a power steering system, 
so that they form a two-throw pump. The hydraulic pump 21 is driven by a 
motor 21a. The liquid pressure passage 10 connected to the hydraulic pump 
8 is provided with three accumulators 22a which are connected thereto 
through the same connection. The liquid pressure passage 10 branches into 
a front wheel side passage 10F and a rear wheel side passage 10R at the 
connection of the accumulators 22a. Further, the front wheel side passage 
10F branches into left and right front wheel side passages 10FL and 10FR 
which are respectively communicated with the liquid pressure chambers 3c 
of the fluid cylinders 3FL and 3FR for the left and right front wheels. 
The rear wheel side passage 10R is provided with an accumulator 22b and 
branches into left and right rear wheel side passages 10RL and 10RR at a 
portion downstream of the accumulator 22b. The left and right rear wheel 
side passages 10RL and 10RR are respectively communicated with the liquid 
pressure chambers 3c of the fluid cylinders L and 3RR for the left and 
right rear wheels. 
Gas spring groups 5FL, 5FR, 5RL and 5RR each consisting of four gas springs 
5a to 5d are respectively communicated with the liquid pressure chambers 
3c of the corresponding fluid cylinders 3 by way of communicating passages 
4. Each of the gas springs 5a to 5d are connected to the communicating 
passage 4 by way of an orifice 25. The orifices 25 exhibit attenuating 
effect and the gas in the gas chambers 5f of the gas springs 5a to 5d 
exhibit damping effect. An attenuation changing valve 26 for changing the 
effective cross-section area of the communicating passage 4 is provided in 
the communicating passage 4 between the first and second gas springs 5a 
and 5b. The attenuation changing valve 26 moves between an open position 
where it wide opens the communicating passage 4 and a closed position 
where it substantially narrows the effective cross-sectional area of the 
communicating passage 4. 
An unload valve 27 and a flow control valve 28 are connected to the liquid 
pressure passage 10 upstream of the accumulators 22a. The unload valve 27 
moves between an introducing position where it introduces the hydraulic 
oil discharged from the hydraulic pump 8 into a swash plate actuating 
cylinder 8a so that the oil discharge rate of the hydraulic pump 8 is 
reduced and a discharge position where it discharges the hydraulic oil in 
the cylinder 8a. The unload valve 27 moves from the discharge position to 
the introducing position when the discharge pressure of the hydraulic pump 
8 exceeds a predetermined upper limit (160.+-.10kgf/cm.sup.2), and stays 
in the introducing position until the discharge pressure of the hydraulic 
pump 8 falls below a predetermined lower limit (120.+-.10kgf/cm.sup.2), 
thereby holding the discharge pressure of the hydraulic pump 8 within a 
predetermined range, i.e., 120 to 160kgf/cm.sup.2. The flow control valve 
28 moves between an introducing position where it introduces the hydraulic 
oil discharged from the hydraulic pump 8 into a swash plate actuating 
cylinder 8a through the unload valve 27 and a discharge position where it 
discharges the hydraulic oil in the cylinder 8a to an oil reservoir 29 
through the unload valve 27, thereby holding constant the pressure 
difference between the pressure in the liquid pressure passage 10 upstream 
of a constriction 30 and that downstream of the constriction 30 and 
holding constant the discharge rate of the hydraulic pump 8 while the 
discharge pressure of the hydraulic pump 8 is held in the predetermined 
range by the unload valve 27. To each of the fluid cylinders 3 is fed the 
hydraulic oil accumulated in the accumulators 22a and 22b. The pressure of 
the hydraulic oil accumulated in the accumulators 22a and 22b is referred 
to as the main pressure. 
Four flow control valve assemblies 9, each for one of the wheels, are 
provided in the liquid pressure passage lo downstream of the accumulators 
22a. Since the four flow control valve assemblies 9 are the same in 
structure, only the flow control valve assembly 9 for the left front wheel 
2FL will be described here. The flow control valve assembly 9 comprises a 
feed side flow control valve 35 provided in the left front wheel side 
passage 10FL and a discharge side flow control valve 37 provided in a low 
pressure line 36 through which the hydraulic oil in the left front wheel 
side passage 10FL is discharged to the reservoir 29. Each of the flow 
control valves 35 and 37 moves between an open position and a closed 
position and is provided with a built-in differential valve which holds 
the hydraulic pressure constant at a predetermined value when the flow 
control valve is in the open position. When each of the flow control 
valves 35 and 37 is in the open position, the degree of opening can be 
changed by a control signal. A check valve 38 which operates in response 
to a pilot pressure is provided in the left front wheel side passage 10FL 
between the feed side flow control valve 35 and the fluid cylinder 3FL. To 
the check valve 38 is applied the hydraulic pressure in the liquid 
pressure passage 10 upstream of the feed side flow control valve 35 (i.e., 
the main pressure) as the pilot pressure through a pilot line 39. The 
check valve 38 is closed when the pilot pressure is not higher than 
40kgf/cm.sup.2. That is, feed of the hydraulic oil to the fluid cylinder 3 
and discharge of the hydraulic oil from the same are both possible only 
when the main pressure is higher than 40kgf/cm.sup.2. 
A fail-safe valve 41 is provided in a communicating passage 42 which 
communicates the low pressure line 36 and a portion of the liquid pressure 
passage 10 downstream of the accumulators 22a. When a failure occurs, the 
fail-safe valve 41 opens to return the hydraulic oil in the accumulators 
22a and 22b and lower the pressure in the hydraulic pressure circuit. A 
constriction 43 is provided in the pilot line 39 and delays closure of the 
check valve 38 (e.g., for one second) when the fail-safe valve 41 opens. 
Reference numeral 44 denotes a relief valve which opens and returns the 
hydraulic oil in the fluid cylinders to the low pressure line 36 when the 
hydraulic pressure in the fluid cylinders 3FL and 3FR for the front wheels 
becomes abnormally high. A return accumulator 45 is provided in the low 
pressure line 36 and accumulate pressure when the hydraulic oil is 
discharged from the fluid cylinders 3. 
The controller 20 includes a chassis height control system which causes the 
chassis height to a desired height on the basis of the detecting signals 
of the chassis height sensors 14, a vertical vibration control system 
which reduces the vertical vibration of the vehicle body on the basis of 
detecting signals of the vertical acceleration sensors 15a, a load control 
system which equalizes the loads supported on the left and rear wheels for 
each of the front wheels and rear wheels on the basis of the detecting 
signals of the cylinder pressure sensors 13, and a fluid cylinder control 
system which improves the response of the fluid cylinders 3 during 
cornering on the basis of the detecting signals of lateral acceleration 
sensor 16, the steering angle sensor 17 and the vehicle speed sensor 18. 
When one or more of the components of these control systems fail, the 
controller 20 performs a fail-safe function in the manner shown in FIG. 3. 
The controller 20 first determines whether flag F (the function of which 
will become apparent later) is "1". (step S1) When it is determined in 
step S1 that the flag F is not "1", the detecting signals of the sensors 
12 to 19 are input in step S2 and the controller 20 determines in step S3 
whether a failure has occurred on the basis of the detecting signals. When 
it is determined in step S3 that no failure has occurred, the controller 
20 immediately returns. Otherwise the controller 20 proceeds to step S4, 
and determines the kind of the failure and determines the measure mode to 
be taken according to the kind of the failure. In accordance with the 
present invention, the failures in the control systems are divided into 
three types, A-type, B-type and C-type, and first to third measure modes 
are respectively taken for the A-type, B-type and C-type failures. When 
the first measure mode is taken, warning is just given and the control of 
feed and discharge of hydraulic fluid to and from the fluid cylinders is 
continued. When the second measure mode is taken, the control of feed and 
discharge of hydraulic fluid to and from the fluid cylinders is 
interrupted with the chassis height fixed to the present height. When the 
third measure mode is taken, the fluid in the fluid cylinders being 
discharged and the chassis height being lowered. Further, when it has been 
determined that the failure is of the B-type, the controller 20 subdivides 
the failure into B-1-type and B-2-type according to whether the measures 
for the failure have to be continued until the failure is removed or may 
be interrupted when the ignition switch of the vehicle is turned off. In 
the case of a B-1-type failure, the measure mode is continued until the 
failure is removed, and in the case of a B-2-type failure, the measures 
for the failure is interrupted when the ignition switch of the vehicle is 
turned off. 
When it is determined that the failure is of the C-type, the controller 20 
lights the warning lamp W, opens the fail-safe valve 41, and wide opens 
both the flow control valves 35 and 37. (steps S6 to S8) A predetermined 
time after the opening of the flow control valves 35 and 37, the 
controller 20 closes both the flow control valves 35 and 37. (steps S9 and 
S10) The predetermined time substantially corresponds to the time by which 
the constriction 43 delays closure of the check valve 38, and may be about 
one second, for instance. Thereafter, the controller 20 sets the flag F to 
"1" and returns. (step S11) 
When it is determined in step S5 that the failure is not of the C-type, the 
controller 20 determines in step S12 whether the failure is of B-1-type. 
When it is determined in step S12 that the failure is of B-1-type, the 
controller 20 lights the warning lamp W, closes both the flow control 
valves 35 and 37, and opens the fail-safe valve 41. (steps S13 to S14) 
Thereafter, the controller 20 sets the flag F to "1" and returns. (step 
S16) Otherwise, the controller 20 determines in step S17 whether the 
failure is of B-2-type. When it is determined in step S17 that the failure 
is of B-1-type, the controller 20 lights the warning lamp W, closes both 
the flow control valves 35 and 37, and opens the fail-safe valve 41. 
(steps S18 to S20) Therafter, the controller 20 sets the flag F to "1" and 
returns. (step S21) 
When it is determined in step S17 that the failure is not of B-2-type, the 
controller 20 determines in step S22 whether the failure is of the A-type. 
When it is determined that the failure is of the A-type, the controller 20 
lights the warning lamp W in step S23, and returns after setting the flag 
F to "1" in step S24. When it is determined in step S22 that the failure 
is not of the A-type, that is, when the failure is not of the A-type, the 
B-type or the C-type, the controller 20 returns after interrupting the 
control in step S25. 
When it is determined in step S1 that the flag F is "1", the controller 20 
determines whether the failure is of the B-1-type or the C-type. When it 
is determined that the failure is of the B-1-type or the C-type, the 
controller 20 immediately returns. Otherwise, the controller 20 determines 
in step S27 whether the ignition switch is off. When it is determined that 
the ignition switch is off, the controller 20 returns after setting the 
flag F to "0" in step S28. Otherwise, the controller 20 returns 
immediately. 
For example, the following phenomena indicate occurrence of a failure which 
belongs to the A-type failure. 
1. When the chassis height is lower than the reference value by 30 mm or 
more when the ignition switch is turned on (the fail-safe valve 41 is 
closed at that time). This can be caused when the check valve 38 is 
clogged by dust, which can be removed when the pressurized oil flows 
through the liquid pressure passage 10. 
2. When the vehicle speeds which are calculated on the basis of the outputs 
of the respective vehicle speed sensors 18 differ from each other by a 
predetermined value. Though the vehicle speed is used in the fluid 
cylinder control system which improves the response of the fluid cylinders 
3 during cornering, the vehicle is not so important in the control and 
accordingly the flow control itself need not be stopped. 
The following phenomena indicates occurrence of a failure which belongs to 
the B-1-type failure 
1. When the main pressure is below the reference pressure (30kgf/cm.sup.2) 
when several seconds have elapsed after the ignition switch is turned on. 
This can be caused when the fail-safe valve 41 gets trapped in the closed 
position, when the liquid pressure passage is broken or the main pressure 
sensor 12 fails. These failures are serious and can be never removed 
without service. 
2. When the output signal of the main pressure sensor 12 is at a voltage 
higher than the upper limit voltage (e.g., 4.5v). This can be caused by a 
Vcc short circuit in the main pressure sensor 12. 
3. When the main pressure is higher than 185kgf/cm.sup.2. This can be 
caused when the unload valve 27 fails. When the main pressure falls below 
100kgf/cm.sup.2, the controller 20 temporarily interrupts the control 
until it rises above 110kgf/cm.sup.2 (step S25 in FIG. 3) 
4. When the main pressure does not rise for a predetermined time (e.g., 
five seconds) while the main pressure is below 100kgf/cm.sup.2 and the 
control is being interrupted. This can be caused when the unload valve 27, 
the main pressure sensor 12 or the like fails. 
5. When the state that the change with time of the main pressure P is 
represented by formula .vertline.P(t)-P(t.DELTA.t).vertline.2kgf/cm.sup.2 
wherein .DELTA.t is 1 second continues for 10 minutes. This can be caused 
when the signal of the main pressure sensor 12 is fixed. 
6. When the electric line to the sensors 12 to 19 or the actuators for the 
hydraulic pump 8 or the like is cut. 
7. When the output signal of the oil level sensor (not shown) which detects 
the amount of oil in the reservoir 29 continues to be off for more than 1 
second. This can be caused when the oil line is broken. 
8. When the output signal of the cylinder pressure sensor 13 is at a 
voltage higher than the upper limit voltage (e.g., 4.5v). This can be 
caused by a Vcc short circuit in the cylinder pressure sensor 13. 
9. When the output signal of the cylinder pressure sensor 13 is at a 
voltage lower than the lower limit voltage (e.g., 0.5v). This can be 
caused by a GND short circuit in the cylinder pressure sensor 13. 
10. When the output signal of the chassis height sensors 14 is at a voltage 
higher than the upper limit voltage (e.g., 4.5v). This can be caused by a 
Vcc short circuit in the chassis height sensors 14. 
11. When the output signal of the chassis height sensors 14 is at a voltage 
lower than the lower limit voltage (e.g., 0.5v). This can be caused by a 
GND short circuit in the chassis height sensors 14. 
12. When the output signal of the vertical acceleration sensors 15 is at a 
voltage higher than the upper limit voltage (e.g., 4.5v) for one second or 
more. This can be caused by a Vcc short circuit in the vertical 
acceleration sensors 15. 
13. When the output signal of the vertical acceleration sensors 15 is at a 
voltage lower than the lower limit voltage (e.g., 0.5v) for one second or 
more. This can be caused by a GND short circuit in the vertical 
acceleration sensors 15. 
14. When the output signal of the lateral acceleration sensor 16 is at a 
voltage higher than the upper limit voltage (e.g., 4.5v) for one second or 
more. This can be caused by a Vcc short circuit in the lateral 
acceleration sensor 16. 
15. When the output signal of the lateral acceleration sensor 16 is at a 
voltage lower than the lower limit voltage (e.g., 0.5v) for one second or 
more. This can be caused by a GND short circuit in the lateral 
acceleration sensor 16. 
14. When the output signal of the steering angle sensor 17 is at a voltage 
higher than the upper limit voltage (e.g., 4.5v) for one second or more. 
This can be caused by a Vcc short circuit in the steering angle sensor 17. 
15. When the output signal of the steering sensor 17 is at a voltage lower 
than the lower limit voltage (e.g., 0.5v) for one second or more. This can 
be caused by a GND short circuit in the steering angle sensor 17. 
16. CPU error 
The following phenomena indicate occurrence of a failure which belongs to 
the B-2-type failure. 
1. When the output signal of the main pressure sensor 12 is at a voltage 
lower than the lower limit voltage (e.g., 0.5v). This can be caused by a 
GND short circuit in the steering angle sensor 17, and can be sometimes 
removed without service. 
2. When the state that the change with time of the main pressure P is 
represented by formula 
.vertline.P(t)-P(t-.DELTA.t).vertline..ltoreq.2kgf/cm.sup.2 wherein 
.DELTA.t is 1 second continues for at least 5 seconds after the vertical 
acceleration G(t) and the acceleration of gravity G have come to satisfy 
formula G(t)-1G&lt;-0.1G. This can be caused when the signal of the main 
pressure sensor 12 is fixed but can be sometimes removed without service 
since duration is short 
3. When the state that the change with time of the main pressure P is 
represented by formula 
.vertline.P(t)-P(t.DELTA.t).vertline..ltoreq.2kgf/cm.sup.2 wherein 
.DELTA.t is 1 second continues for at least 5 seconds while the present 
chassis height H(t) and the reference chassis height Ho satisfy formula 
H(t)-Ho when the wheels bump. This can be caused when the signal of the 
cylinder pressure sensor 13 is fixed but can be sometimes removed without 
service since duration is short 
4. When the main pressure falls below 90kgf/cm.sup.2. This can be caused 
when the oil line is broken. 
5. When the vertical acceleration at a certain wheel becomes not higher 
than a predetermined value (0.1G) and the output of the chassis height 
sensor 14 does not change for 3 seconds. This can be caused when the 
signal of the chassis height sensor 14 is fixed and can be sometimes 
removed without service. 
6. When the state that the output of a certain vertical acceleration sensor 
at a certain time is equal to that 100 ms before while the outputs of 
other two or the other three vertical acceleration sensors at the time are 
equal to those 100 ms before continues for 500 ms. This can be caused when 
the signal of the vertical acceleration sensor 15 is fixed and can be 
sometimes removed without service. 
7. When the output of a certain chassis height sensor 14 does not 
approximate to the reference value for ten minutes while the vehicle runs. 
This can be caused when the signal of the chassis height sensor 14 is 
fixed and can be sometimes removed without service. 
The following phenomenon indicates occurrence of a failure which belongs to 
the B-2-type failure. 
When the stroke position of the flow control valve 35 and/or the flow 
control valve 7 for each wheel deviates from the target position commanded 
by the command signal and this state continues for a predetermined time. 
This can be caused when the flow control valve 9 gets jammed. In this 
case, the chassis heights at the respective wheels cannot be equalized but 
to discharge the hydraulic oil in all the fluid cylinders 3. 
When the sum of the cylinder pressure and 10kgf/cm.sup.2 becomes higher 
than the line pressure, the control is interrupted in order to prevent 
reverse flow of the hydraulic oil. 
Though the present invention is applied to the suspension system having 
both the fluid cylinders and the gas springs in the embodiment described 
above, the present invention can be applied to the suspension system which 
is not provided with gas springs.