Height control system for automotive suspension system with vehicle driving condition dependent variable target height

A height control system utilizing at least first and second mutually different target height ranges for selectively using in vehicular height control. The first target height range is utilized for detecting the height adjustment initiating timing and the second target height range is utilized for detecting the height adjustment terminating timing. The first and second target height ranges are variable depending upon the vehicle driving condition, such as vehicle driving speed, road surface condition and so forth.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a height control system for an 
automotive suspension system for adjusting or regulating vehicular height 
within a predetermined height range. More specifically, the invention 
relates to a vehicular height control system which can vary a target 
height, about which the predetermined height range is defined, so that 
frequency of height adjustment during vehicular travel on an undulated 
road. 
2. Description of the Background Art 
In the recent years, there have been developed various vehicular height 
control systems for regulating height of a vehicle body relative to road 
surface and whereby for regulating vehicular attitude. In the typical 
construction of prior proposed vehicular height control system, vehicular 
height is monitored by a vehicle height sensor which monitors height of 
the vehicle body relative to the road surface or to a suspension member 
rotatably supporting a road wheel. Height adjustment is performed by 
adjusting suspension force to be exerted between the vehicle body and the 
suspension member to maintain the vehicular height within a predetermined 
height range. In practice, the suspension force is generated by means of a 
pressure chamber which is connected to a pressurized fluid source to 
introduce thereinto and discharge therefrom a pressurized working fluid, 
such as air, viscous oil and so forth. 
Some of the prior proposed height control systems employs variable target 
height for varying the vehicle height range toward which the vehicle 
height is adjusted, depending upon the vehicle driving condition. For 
example, one of the vehicular height control system employing the variable 
target height has been disclosed in the Japanese patent First (unexamined) 
publication (Tokkai) Showa 61-263818. In the disclosed system, two target 
heights are employed. One of the target height is used for detecting the 
vehicular height requiring the height adjustment for starting height 
adjustment. The other target height is used for detecting the timing for 
termination of the height adjustment. 
In the prior proposed height control systems, detects are encountered when 
the vehicle enters into an undulated road from smooth road for temporarily 
stopping. Because the undulation of the road surface tends to cause 
variation of the relative distance between the vehicle body and the road 
wheel to vary the vehicular height out of the predetermined height range. 
If vehicle starts to run immediately after temporary stop at the undulated 
road, height adjustment may not be required despite of the fact that the 
vehicular height as monitored is out of the predetermined height range. 
However, the prior proposed height control system is responsive to the 
vehicular height out of the predetermined height range to perform height 
adjustment to adapt the vehicular height at the undulated road. This 
requires another height adjustment when the vehicle starts to run again on 
the smooth road. 
Furthermore, when the vehicle travels on the undulated road repeating 
temporary stop, height adjustment is too frequency performed to exhaust a 
compressor as a fluid pressure source and to degrade riding comfort. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a height 
control system which can solve the drawback in the prior proposed height 
control systems. 
Another object of the invention is to provide a vehicular height control 
system which can avoid too frequent height control by varying target 
height depending upon the vehicular driving condition. 
In order to accomplish the aforementioned and other objects, a height 
control system, according to the present invention, utilizing at least 
first and second mutually different target heights for selectively using 
in vehicular height control. The first target height is set to be used in 
the vehicular height control while vehicle is running. On the other hand, 
the second target height is selected to perform vehicular height control 
while the vehicle is resting or not running. 
According to one aspect of the invention, a height control system for an 
automotive suspension control system comprises a suspension system 
disposed between a vehicle body and a suspension member rotatably 
supporting a road wheel, the suspension system including means for varying 
suspension force to be exerted between the vehicle body and the suspension 
member, a first sensor monitoring a vehicular height for producing a 
height indicative first sensor signal, a second sensor monitoring a 
vehicular driving condition to produce a vehicular driving condition 
indicative second sensor signal, and a controlling means for receiving the 
first sensor signal, and checking the value of the sensor signal with 
respect to a predetermined target height range indicative values for 
deriving a control signal to operate the varying means to adjust the 
suspension force for maintaining the vehicular height within the target 
height range, the controlling means being defining the target height range 
in relation to a predetermined target height with a given acceptable 
height deviation from the target height, which given acceptable deviation 
is variable depending upon the vehicle driving condition as represented by 
the second sensor signal. 
According to another aspect of the invention, a height control system for 
an automotive suspension control system comprises a suspension system 
disposed between a vehicle body and a suspension member rotatably 
supporting a road wheel, the suspension system including means for varying 
suspension force to be exerted between the vehicle body and the suspension 
member, a first sensor monitoring a vehicular height for producing a 
height indicative first sensor signal, a second sensor monitoring a 
vehicular driving condition to produce a vehicular driving condition 
indicative second sensor signal, and a controlling means for receiving the 
first sensor signal, and checking the value of the sensor signal with 
respect to predetermined first and second target height range indicative 
values, the first and second target range indicative values serving as 
first upper and first lower criteria and second upper and second lower 
criteria for deriving a control signal to operate the varying means to 
adjust the suspension force for maintaining the vehicular height within 
the target height range, the controlling means utilizing the first target 
height range for detecting height adjustment initiating timing and the 
second target height range for detecting height adjustment terminating 
timing, and determining the first and second target height ranges in 
relation to a predetermined target height with a given first and second 
acceptable height deviations from the target height, which first and 
second acceptable deviation is variable depending upon the vehicle driving 
contion as represented by the second sensor signal. 
The controlling means may derive the first upper and first lower criteria 
and second upper and second lower criteria with setting the predetermined 
target height as a center value thereof. 
The vehicular height control system further comprises a timer means 
triggered in response to the first sensor signal indicative of the 
vehicular height out of the first target height range to measure a period 
of time in which the vehicular height is held out of the first target 
height range, and the controlling means is responsive to a timer value 
representative of the measured elapsed time greater than a given value to 
initiate height adjustment operation. The given period may be variable 
depending upon the vehicle driving condition as represented by the second 
sensor signal. 
In the preferred arrangement, the first acceptable height deviation is set 
greater than the second acceptable height deviation. 
In practice, the second sensor monitors vehicle speed to produce a vehicle 
speed indicative second sensor signal and/or a road surface condition to 
produce a road surface condition indicative second sensor signal. The 
first acceptable height is variable depending upon the second sensor 
signal value which represents the vehicle speed and/or the road surface 
condition. Also, the second sensor may be active during vehicle is 
stopping to discriminate between temporarily stopping condition and 
parking condition of the vehicle to produce vehicular stopping condition 
indicative sensor signal having a value variable depending upon 
temporarily stopping condition and parking condition of the vehicle. The 
first acceptable height may further variable depending upon the second 
sensor signal value indicative of the ignition switch position. 
Similarly, the second acceptable height deviation may be variable depending 
upon the vehicle speed, road surface condition and/or the ignition switch 
position. Furthermore, the aforementioned given period of time is also 
variable depending upon the vehicle speed, road surface condition and/or 
the ignition switch position.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, particularly to FIG. 1, the preferred 
embodiment of a height control system in an automotive suspension system, 
according to the present invention, employs suspension system 10.sub.FL, 
10.sub.FR, 10.sub.RL and 10.sub.RR for rotatably supporting front-left, 
front-right, rear-left and rear-right vehicular wheels. The suspension 
systems 10.sub.FL, 10.sub.FR, 10.sub.RL and 10.sub.RR include suspension 
struts 12.sub.FL, 12.sub.FR, 12.sub.RL and 12.sub.RR which include shock 
absorbers and height control actuator means 14.sub.FL, 14.sub.FR, 
14.sub.RL and 14.sub.RR. 
The actuator means 14.sub.FL, 14.sub.FR, 14.sub.RL and 14.sub.RR generally 
comprise pressure chambers filled with a working fluid. In the preferred 
embodiment, the pressure chambers 14.sub.FL, 14.sub.FR, 14.sub.RL and 
14.sub.RR are filled with gas, such as an air. Each of the pressure 
chambers 14.sub.FL, 14.sub.FR, 14.sub.RL and 14.sub.RR is connected to a 
pressurized air source 16 via pressure supply system 18. Height control 
valve means 20.sub.FL, 20.sub.FR, 20.sub.RL and 20.sub.RR are disposed 
within the pressure supply system 18 for controlling air pressure to be 
supplied for respectively corresponding pressure chambers 14.sub.FL, 
14.sub.FR, 14.sub.RL and 14.sub.RR. As seen from FIG. 1, the height 
control valve means 20.sub.R is designed for commonly adjusting the air 
pressure in the pressure chambers 14.sub.RL, 14.sub.RR of the rear 
suspension systems 10.sub.RL and 10.sub.RR. 
Height sensors 24.sub.FL, 24.sub.FR, 24.sub.RL and 24.sub.RR are provided 
at positions where the respective front-left, front-right, rear-left and 
rear-right suspension systems 10.sub.FL, 10.sub.FR, 10.sub.RL and 
10.sub.RR are provided for monitoring relative height between vehicle body 
26 and suspension members 28.sub.FL, 28.sub.FR, 28.sub.RL and 28.sub.RR 
(shown in FIG. 2) which are connected to the vehicular wheels. Respective 
height sensors 24.sub.FL, 24.sub.FR, 24.sub.RL and 24.sub.RR produces 
vehicle height indicative signals S.sub.FL, S.sub.FR, S.sub.RL and 
S.sub.RR. S.sub.RR of the height sensors 24.sub.FL, 24.sub.FR, 24.sub.RL 
and 24.sub.RR are fed to a controller 30. 
The controller 30 includes a discriminator stage and a control signal 
generator stage. The vehicle height indicative signals S.sub.FL, S.sub.FR, 
S.sub.RL and S.sub.RR are input to the discriminator stage of the 
controller. In the discriminator stage, each of the vehicle height 
indicative signals S.sub.FL, S.sub.FR, S.sub.RL and S.sub.RR is compared 
with a maximum height indicative upper criterion H.sub.U and a minimum 
height indicative lower criterion H.sub.L to discriminate whether the 
vehicle height as represented by the corresponding vehicle height 
indicative signal is within a target height range defined by the upper and 
lower criteria H.sub.U and H.sub.L. 
When the vehicle height level out of the target height range, the control 
signal generator stage is triggered for operation. In response to 
initiation of the operation of the control signal generator stage, a 
communication valve control signal is, at first, output to the 
communication valve 22 to open the latter. In response to the height 
control signal of the control signal generator stage, respective height 
control valve means 20.sub.FL, 20.sub.FR, 20.sub.RL and 20.sub.RR are 
operated to adjust the vehicle height level at respectively corresponding 
wheel positions to be within the target height range. 
As shown in FIG. 1, the air source 16 comprises a compressed air supply 
network including a motor driven compressor 40 which is associated with an 
electric motor 42 to be driven by the driving force transmitted therefrom. 
The compressor 40 is connected to the pressure supply passage system 18. 
An air dryer 44 is disposed in the pressure supply passage system 18 and 
downstream of the compressor for drying the air fed from the compressor. 
In addition, a pressure accumulator 46 is provided in a pressure 
accumulation system 48 which is in communication with the pressure supply 
passage system 18 at both ends. An one-way check valve 50 is provided in 
the pressure accumulation system 48 and upstream of the pressure 
accumulator 46. An electromagnetically actuated pressure accumulation 
control valve 52 is provided downstream of the pressure accumulator 46. 
The pressure supply passage system 18 includes a ventilation port 54, at 
which a ventilation control valve 56 is provided for opening and closing 
the port. 
As seen from FIG. 1, the height control valves means 20.sub.FL, 20.sub.FR, 
20.sub.RL and 20.sub.RR comprise electromagnetically actuated valves. The 
height control valves 20.sub.FL, 20.sub.FR, 20.sub.RL and 20.sub.RR, the 
pressure accumulation control valve 52 and the ventilation control valve 
56 are respectively connected to the controller 30 to be controlled 
respective valve positions. The height control valves 20.sub.FL, 
20.sub.FR, 20.sub.RL and 20.sub.RR, employed in the shown embodiment, are 
so designed as to open while they are activated, to establish 
communication between the pressure chambers 14.sub.FL, 14.sub.FR, 
14.sub.RL and 14.sub.RR and the pressure supply passage system 16 and to 
close while they are deactivated, to block the communication. The 
accumulation control valve 52 is so designed as to be activated to open 
for establishing communication between the pressure accumulator 46 and the 
pressure supply passage means 16 and to be deactivated to close for 
blocking the communication. The ventilation control valve 56 is designed 
to be activated to open the valve for exposing the pressure supply passage 
means 16 to the atmosphere for ventilation of the pressurized air through 
the ventilation port 54 and to be deactivated to shut the valve. 
In order to monitor the pressure accumulated on the pressure accumulator 
46, a pressure sensor 60 is provided. The pressure sensor 60 is designed 
for generating an accumulated pressure indicative signal to be input to 
the controller 30 as a compressor control parameter. 
The height sensors 24.sub.FL, 24.sub.FR, 24.sub.RL and 24.sub.RR, employed 
in the shown embodiment, comprise stroke sensors disposed between the 
vehicle body 26 and the suspension members 28.sub.FL, 28.sub.FR, 28.sub.RL 
and 28.sub.RR. Each of the stroke sensors 24.sub.FL, 24.sub.FR, 24.sub.RL 
and 24.sub.RR may comprise an electrostatic capacity-type stroke sensor as 
shown in FIGS. 2 and 3, for example. 
As shown in FIGS. 2 and 3, the preferred example of the electrostatic 
capacity-type stroke sensor is generally represented by the reference 
numeral `110`. The electrostatic capacity-type stroke sensor 110 is 
designed for measuring relative stroke between a cylindrical member 112 
and a rod member 114. In the shown construction, the rod member 114 is 
coaxially arranged through the cylindrical member 112 and thrustingly 
supported by means of supporting plug 116 and a cylindrical support 
section 118 integrally formed with the cylindrical member. 
Both of the cylindrical member 112 and the rod member 114 are made of 
electrically conductive material and, in turn, electrically isolated to 
each other. 
Inner and outer cylinders 120 and 122 are coaxially disposed between the 
cylindrical member 112 and the rod member 114. The cylindrical member 112, 
the outer cylinder 122, the inner cylinder 120 and the rod member 114 are 
coaxially arranged in spaced apart relationship and held in place by means 
of an annular support 124 which is made of an electrically insulative 
material. 
The inner cylinder 120 is electrically connected to the cylindrical member 
112 to constitute therewith a grounding electrode. On the other hand, the 
outer cylinder 122 is connected to a terminal 126 which is, in turn, 
connected to a sensor circuit 128. In the shown embodiment, the sensor 
circuit 128 comprises a RC oscillator. The outer cylinder 122 is designed 
to function as positive electrode. The sensor circuit 128 is also 
connected to the grounding electrode formed by the inner cylinder 120 and 
the cylindrical member 112 to be grounded therethrough. 
With the plug 116, the cylindrical supporting section and the insulative 
support 124 establishes gas tight seal. A gaseous state dielectric 
material which has stable dielectric constant is filled in the enclosed 
space. 
On the other hand, a dielectric member 130 is provided for movement with 
the rod member 114. The dielectric member comprises a radially extending 
disk-shaped section 132 and coaxially arranged inner and outer cylindrical 
sections 134 and 136. The inner diameter .phi.d.sub.2 of the inner 
cylindrical section 134 of the dielectric member 130 is greater than the 
outer diameter .phi.d.sub.1 of the inner cylinder 120 and the outer 
diameter .phi.d.sub.3 is smaller than the inner diameter .phi.d.sub.4 of 
the outer cylinder 122 so that the inner cylindrical section 134 may enter 
into the annular space defined between the inner and outer cylinders 120 
and 122, as shown in FIG. 2. On the other hand, the inner diameter 
.phi.d.sub.6 of the outer cylindrical section 136 is greater than the 
outer diameter .phi.d.sub.5 of the outer cylinder 122 and the outer 
diameter .phi.d.sub.7 is smaller than the inner diameter .phi.d.sub.8 of 
the cylindrical member 112 so that the outer cylindrical section may enter 
into the annular space defined between the outer cylinder 122 and the 
cylindrical member. On the other hand, the disk-shaped section 132 is 
rigidly fixed to the outer periphery of the rod member 114 so that the 
dielectric member 130 may be moved according to movement of the rod member 
114. 
With the construction set forth above, the electrostatic capacity Ct is 
created between the inner and outer cylinders 120 and 122 and the inner 
and outer cylindrical sections 134 and 136 of the dielectric member 130 
while relative displacement between the cylindrical member 112 and the rod 
member 114 occurs. 
Here, assuming the possible maximum relative stroke between the cylindrical 
member 112 and the rod member 114 is as X, shown in FIG. 2, the relative 
dielectric constant of the dielectric member is e.sub.2 ; and the relative 
dielectric constant of the gaseous dielectric material is e.sub.. It is 
further assumed that the electrostatic capacity of overlapping portion 
between the inner and outer cylindrical sections 134 and 136 and the inner 
and outer cylinders 120 and 122 is C.sub.1 ; the electrostatic capacity of 
the portions of the inner and outer cylinders 120 and 122 outside of the 
inner and outer cylindrical sections 134 and 136 is C2; and the 
electrostatic capacity of the portion where the inner and outer cylinders 
120 and 122 are supported is C.sub.3. In this case, the electrostatic 
capacities C.sub.t, C.sub.1 and C.sub.2 can be respectively illustrated by 
the following equations: 
EQU C.sub.t =C.sub.1 +C.sub.2 +C.sub.3 (1) 
EQU C.sub.1 +2 e.sub.0 x [1/{(l/e.sub.2 -l/e.sub.1)ln.multidot.d.sub.3 /d.sub.2 
+l/e.sub.1 .multidot.lnd.sub.4 /d.sub.1 }+{1/(l/e.sub.2 
-l/e.sub.1)ln.multidot.d.sub.7 /d.sub.6 +l/e.sub.1 .multidot.lnd.sub.8 
/d.sub.5 }] (2) 
EQU C.sub.2 =2 e.sub.0 (l-x).multidot.[1/{(l-e.sub.1)ln(d.sub.4 
/d.sub.1){+1/{(l-e.sub.1)ln(d.sub.8 /d.sub.5)}] (3) 
where e.sub.0 is dielectric constant in vacuum 
Here, it is assumed: 
EQU [1/{(l/e.sub.2 -l/e.sub.1)ln.multidot.d.sub.3 /d.sub.2 +l/e.sub.1 
.multidot.lnd.sub.4 /d.sub.1 {+{1/(l/e.sub.2 
-l/e.sub.1)ln.multidot.d.sub.7 d.sub.6 +l/e.sub.1 .multidot.lnd.sub.8 
/d.sub.5 }]=A; and 
EQU [1/{(l-e.sub.1)ln(d.sub.4 /d.sub.1)}+1/{(l-e.sub.1)ln(d.sub.8 /d.sub.5)}]=B 
A and B are both constant. The equations (2) and (3) can be modified 
utilizing A and B, the electrostatic constant Ct can be illustrated by: 
EQU Ct=2 e.sub.0 x(A-B)+2 e.sub.0 1B+C.sub.3 (4) 
As will be appreciated herefrom, the electrostatic constant Ct is variable 
in proportion to the stroke x. 
As set forth, the detector circuit comprises the RC oscillator whose 
oscillation cycle period can be described by: 
EQU T=(1/K)RC (5) 
where K is constant. From this the frequency output characteristics of the 
RC oscillator can be described by: 
EQU T=(R/K)2 e.sub.0 x(A-B)+C.sub.0 (6) 
where C.sub.0 =2 e.sub.0 1B+C.sub.3 
As will be appreciated from the foregoing equations, the frequency 
oscillation cycle period T is proportional to the relative displacement 
stroke x. Therefore, by monitoring the frequency oscillation cycle period 
T, the stroke x can be detected. 
Here, the diameters .phi.d.sub.1 through .phi.d.sub.8 can be varied 
depending upon the temperature of atmosphere due to thermal expansion. 
Among those variable dimensions, the radios .phi.d.sub.3 /.phi.d.sub.2 and 
.phi.d.sub.7 /.phi.d.sub.6 are regarded constant despite of various rate 
of thermal expansions. Similarly, the radios .phi.d.sub.4 /.phi.d.sub.1 
and .phi.d.sub.8 /.phi.d.sub.5 are regarded constant. 
On the other hand, the dielectric constants e.sub.1 and e.sub.2 of the 
gaseous dielectric material filled in the space defined in the cylindrical 
member and the dielectric member 130 can be made substantially in constant 
at any temperature range by appropriately selecting the materials. For 
example, the dielectric material which exhibits stable dielectric constant 
in relation to temperature variation may be selected among air or resin, 
such as polyacetal resin, polypropylene resin. Therefore, as the gaseous 
dielectric material, the air may be used in the preferred embodiment so as 
to avoid influence of the temperature variation and to obtain stable 
dielectric constant thereof. 
However, as the dielectric material for forming the dielectric member 130, 
a material which can provide higher resolution in measurement of the 
relative stroke between the rod member and the cylindrical member, is 
preferred. In order to obtain higher resolution, greater electrostatic 
capacity to be established becomes necessary. This, in turn, requires 
higher dielectric constant of the dielectric material forming the 
dielectric member 130. Satisfactorily high dielectric constant is 
obtainable by utilizing inorganic material, such as mica. Such inorganic 
material is known to have substantially high dielectric constant but is 
know as expensive material and as difficult material for machining and/or 
mass-production. On the other hand, synthetic resins are less expensive 
and easy for forming a desired shape. However, the synthetic resin 
generally have low or insufficient dielectric constant as required for the 
stroke sensor of the type disclosed hereabove. 
In this view, the preferred embodiment of the stroke sensor, according to 
the present invention, employs a composite dielectric material which is a 
composition of the synthetic resin material and inorganic material. As a 
resin material to form the composition, thermoplastic resin, such as 
polypropylene, polyacetal, polybuthylene terephtalate, polyphenylene 
sulfide and so forth are preferred in view of solubility with the 
inorganic material. As inorganic material, ceramics are used. 
Each of the stroke sensors 24.sub.FL, 24.sub.FR, 24.sub.RL and 24.sub.RR 
are connected to the controller 30 via a multiplexer 62 and an 
analog-to-digital (A/D) converter 64. On the other hand, the controller 30 
comprises a microprocessor 66 having an input/output interface 68, an 
arithmetic circuit 70, such as CPU, and a memory 72, such as ROM, RAM, 
register and so forth. The input/output interface 68 is connected to the 
A/D converter to receive one of the height indicative signal from the 
corresponding one of the stroke sensors 24.sub.FL, 24.sub.FR, 24.sub.RL 
and 24.sub.RR as selected by the multiplexer 62. The multiplexer 62, is 
designed to receive a clock signal output from the microprocessor to 
select one of the height indicative signals in a given order and at a 
given timing. 
The input/output interface 68 is also connected to driver circuits 72, 74, 
76, 78, 80, 82, to feed thereto control signals. The driver circuits, 72, 
74, 76 and 78 are respectively connected to the height control valves 
20.sub.FL, 20.sub.FR, 20.sub.RL and 20.sub.RR to feed thereto height 
control driver signals. The driver circuits 72, 74, 76 and 78 output HIGH 
level height control driver signals when the height level of respectively 
corresponding vehicle body portions is out of the target height range and, 
otherwise, output LOW level height control driver signals. Therefore, the 
height control valves 20.sub.FL, 20.sub.FR, 20.sub.RL and 20.sub.RR are 
responsive to the HIGH level height control signal to be activated and to 
the LOW level height control signal to be deactivated. Furthermore, the 
driver circuit 80 is connected to the pressure accumulation control valve 
52 to feed a HIGH level accumulator control signal to open the latter for 
establishing communication between the pressure accumulator 46 and the 
pressure supply system 18 and to feed a LOW level accumulator control 
signal to close the latter for blocking the communication. 
The driver circuit 82 is connected to the ventilation control valve 56 to 
open in order to expose the pressure supply system 18 to the atmosphere by 
HIGH level ventilation control signal and to shut for closing the pressure 
supply system. 
In addition, the microprocessor 66 detects the pressure in the pressure 
accumulator 46 dropping lower than a predetermined pressure to generate a 
compressor control signal. The compressor control signal is fed to a 
driver circuit 84 via the input/output interface 68. The driver circuit 84 
is connected to a power supply control relay 86 which establishes and 
blocks electric connection between the electric motor 42 and a vehicular 
battery 88. In response to the compressor control signal, the driver 
circuit 84 produces a compressor drive signal for energizing the power 
supply control relay 86 for establishing electric connection between the 
battery 88 and the motor 42 to drive the motor. As a result, the 
compressor 40 is driven by the driving force of the motor 42 to supply the 
pressure to the pressure accumulator 46. 
Furthermore, the microprocessor 66 is connected to a vehicle speed sensor 
61, a vehicular door switch 63, a foot brake switch 65, a parking brake 
switch 67 and an ignition switch 69. The vehicle speed sensor 61 monitors 
vehicular travelling speed to produce a vehicle speed indicative signal. 
The vehicle door switch 63 is designed to detect open and close of a 
vehicular door to produce a HIGH level door position indicative signal 
when the door is in open position and to produce a LOW level door position 
indicative signal when the door is in closed position. The foot brake 
switch 63 is designed to detect a foot brake position to produce a LOW 
level foot brake position indicative signal while the a brake is not 
applied and to produce a HIGH level foot brake position indicative signal 
when the foot brake is applied. The parking brake switch 65 is designed to 
output a parking brake position indicative signal while is held LOW level 
while the parking brake is in released condition and is switched into HIGH 
level when the parking brake is applied. On the other hand, the ignition 
switch is variable of the switch position between various switch positions 
e.g. LOCK position in which steering lock device is active to lock a 
vehicular steering, OFF position where electric power supply is shut, ACC 
position in which electric power is supplied to electric equipments in the 
vehicle but is not supplied to an ignition system, IGN position in which 
the electric power is supplied not only to the electric equipments but 
also to the ignition system, and START position in which the electric 
power is supplied to the ignition system and a starter motor and is not 
supplied to the electric equipments. However, in order to simplify the 
following discussion, any ignition switch position in which power supply 
for the microprocessor 66 is established will be referred to as "ON" 
position and any ignition switch position in which power supply for the 
microprocessor 66 is blocked will be referred to as "OFF" position. 
In the shown embodiment of the vehicular height control system, two target 
height ranges are employed, which target height ranges are substantially 
the same as that discussed in the aforementioned Japanese Patent First 
Publication 61-263818. Namely, one of the target height range is used for 
detecting the height adjustment start timing and thus will be hereafter 
referred to as "adjustment starting height range". The other target height 
range is used for detecting the height adjustment terminating timing and 
thus hereafter referred to as "adjustment terminating height range". The 
adjustment starting height range is defined by "upper and lower adjustment 
starting criteria HU.sub.start and HL.sub.start ". Similarly, the 
adjustment terminating height range is defined by "upper and lower 
adjustment terminating criteria HU.sub.end and HL.sub.end ". The upper and 
lower adjustment starting criteria HU.sub.start are determined in relation 
to a predetermined target height H.sub.target which forms the center value 
of the adjustment starting height range with an acceptable height 
difference DB. On the other hand, the upper and lower adjustment 
terminating criteria HU.sub.end and HL.sub.end are determined in relation 
to the predetermined target height H.sub.target with an acceptable height 
difference SB. The acceptable height differences DB and SB are variable 
depending upon the vehicle driving conditions. 
Practical height control operation to be performed by the preferred 
embodiment of the vehicular height control system according to the 
invention will be discussed herebelow with reference to FIGS. 4(A), 4(B) 
and 4(C) which shows a series of height control program to be executed 
every given timing, e.g. every 20 ms. 
At a step 1002 immediately after start execution of the height control 
program, the vehicle speed indicative signal is checked whether the 
vehicle speed is zero. When the vehicle speed indicative signal indicates 
the vehicle speed zero as checked at the step 1002, road surface condition 
is checked whether the road surface is smooth or not, at a step 1004. In 
practice, the road surface condition is discriminated on the basis of the 
vehicle height indicative signal value. Practical process of 
discrimination of the road surface condition has been discussed in the 
Japanese Patent First publication No. 62-184911. 
When the undulation on the road surface is detected at the step 1004, the 
acceptable height difference DB for determining the upper and lower 
adjustment criteria HU.sub.start and HL.sub.start is set at a value of 25 
mm, at a step 1006. 
On the other hand, when the road surface condition as checked at the step 
1004 is smooth, check is performed whether the door position indicative 
signal is LOW and one of the foot brake position indicative signal and the 
parking brake position indicative signal is HIGH or not, at a step 1008. 
When all of the door position indicative signal is LOW, it represents that 
the vehicular door is closed. On the other hand, when one of the foot 
brake position indicative signal and the parking brake position indicative 
signal is HIGH, it represents that the vehicular brake is applied. At this 
condition, the acceptable difference DB is set at a value of 30 mm, at a 
step 1010. When the door position indicative signal is HIGH or both of the 
foot and parking brake position indicative signals are LOW as checked at 
the step 1008, the acceptable difference DB is set at a value of 10 mm, at 
a step 1012. On the other hand, when the vehicle speed as checked at the 
step 1002 is not zero, the acceptable difference DB is set at a value of 
10 mm, at a step 1014. 
After one of the steps 1006, 1010, 1012 or 1014, the vehicle speed 
indicative signal value is again checked at a step 1016. When the vehicle 
speed indicative signal is zero as checked at the step 1016, the road 
surface condition is again checked at a step 1020. If the road surface 
condition is smooth as checked at the step 1020, the foot and paring brake 
position indicative signals are checked at a step 1022. When neither of 
the foot and parking brake position indicative signals are LOW as checked 
at the step 1022, or when the vehicle speed indicative signal as checked 
at the step 1016 is not zero, the acceptable height difference SB for the 
upper and lower adjustment terminating criteria HU.sub.end and HL.sub.end 
is set at a value of 2 mm, at a step 1024. On the other hand, when the 
road surface is undulated as checked at the step 1020 or one of the foot 
and parking brake position indicative signals is HIGH as checked at the 
step 1022, the acceptable height difference SB is set at a value of 8 mm, 
at a step 1026. 
After the step 1024 or 1026, the ignition switch position indicative signal 
which is LOW to represent the ignition switch at OFF position and LOW to 
represent the OFF position, at a step 1028. If OFF position of the 
ignition switch is detected at the step 1028, an ignition switch ON-set 
flag FL.sub.IGN is reset at a step 1030. Then, a discrimination delay time 
value Ts which represents a period of time to perform height adjustment 
therewithin for adjusting the vehicular height in UP and DOWN direction, 
is set at a value of 45 sec., at a step 1032. 
On the other hand, when the ignition switch position indicative signal is 
HIGH as checked at the step 1028, the ignition switch ON-set flag 
FL.sub.IGN is checked at a step 1034. If the ignition switch ON-set flag 
FL.sub.IGN is not set as checked at the step 1034, the flag FL.sub.IGN is 
set at a step 1036. Thereafter, the discrimination delay time value Ts is 
set at a value of 0 sec., at a step 1038. On the other hand, when the 
ignition switch ON-set flag FL.sub.IGN is set as checked at the step 1034, 
the vehicle speed indicative signal value is checked at a step 1040. When 
the vehicle speed indicative signal value is not zero as checked at the 
step 1040, the discrimination delay time value Ts is set at a value of 20 
sec, at a step 1042. When the vehicle speed indicative signal value as 
checked at the step 1040 is zero, the door position indicative signal is 
checked at a step 1044. If the door position indicative signal is LOW as 
checked at the step 1044, the discrimination delay time value Ts is set at 
a value of 10 sec, at a step 1046. On the other hand, when the door 
position indicative signal is HIGH as checked at the step 1044, the 
discrimination delay time value Ts is set at a value of 2 sec., at a step 
1048. 
After one of the steps 1032, 1036, 1042, 1046 and 1048, the upper and lower 
adjustment criteria HU.sub.start and HL.sub.start are calculated by the 
following equations, at a step 1050: 
EQU HU.sub.start =H.sub.target+DB 
EQU HL.sub.start =H.sub.target-DB 
At a step 1052, the upper and lower adjustment criteria HU.sub.end and 
HL.sub.end are calculated by the following equations: 
EQU HU.sub.end =H.sub.target +SB 
EQU HL.sub.end =H.sub.target -SB 
The upper and lower adjustment criteria HU.sub.start and HL.sub.start and 
the upper and lower adjustment criteria HU.sub.end and HL.sub.end are set 
in the memory 72. 
At a step 1054, the vehicle height indicative signal value h is read out. 
Then, a height adjustment flag FL.sub.ADJUST is checked at a step 1056. If 
the height adjustment flag FL.sub.ADJUST is not set as checked at the step 
1056, the vehicle height indicative signal value h is compared with the 
upper adjustment criterion HU.sub.start, at a step 1058. When the vehicle 
height indicative signal value h is greater than the upper adjustment 
criterion HU.sub.start as checked at the step 1058, a value t.sub.UP of an 
UP-timer (not shown) in the microprocessor 66 is reset to zero, at a step 
1060. Thereafter, a value t.sub.DOWN of a DOWN-timer (not shown) in the 
microprocessor 66 is incremented by a value t.sub.cycle corresponding to 
updating interval, at a step 1062. Then, the DOWN-timer value t.sub.DOWN 
is compared with the discrimination delay time value Ts, at a step 1064. 
If the DOWN-timer value t.sub.DOWN is smaller than the discrimination 
delay time value Ts as checked at the step 1064, process returns to the 
step 1002. On the other hand, when the DOWN-timer value t.sub.DOWN is held 
greater than or equal to the discrimination delay time value Ts, the 
height adjustment flag FL.sub.ADJUST is set at a step 1066. Then, downward 
height adjustment is performed at a step 1068. 
In the practical operation of the downward height adjustment, DOWN command 
is produced. The microprocessor 66 is responsive to this DOWN command to 
output control signal to the ventilation control signal to open the 
ventilation control valve 56 to operate the latter into open position. 
Also, control signal is fed to one of the height control valves 20.sub.FL, 
20.sub.FR, 20.sub.RL and 20.sub.RR to open to reduce the fluid pressure in 
corresponding one of the pressure chambers 14.sub.FL, 14.sub.FR, 14.sub.RL 
and 14.sub.RR. Therefore, the fluid pressure in the corresponding one of 
pressure chamber 14.sub.FL, 14.sub.FR, 14.sub.RL and 14.sub.RR is reduced 
to reduce the suspension force to support the vehicle body on the 
suspension member. Therefore, the vehicle body height relative to the 
suspension member is lowered. 
Thereafter, downward adjustment of the vehicular height, check is performed 
whether height adjustment is completed or not, at a step 1070. The 
microprocessor 66 may discriminates height adjustment operation 
precedingly performed and checkes whether height adjustment is completed 
in relation to the precedingly performed height adjustment operation. 
Namely, in case that the step 1070 is carried out after the step 1068, the 
height indicative signal value is again checked against the upper 
adjustment criterion HU.sub.start. When completion of the height 
adjustment is detected at the step 1070, process goes END and otherwise, 
returns to the step 1002. 
On the other hand, when the vehicular height indicative signal value h is 
smaller than or equal to the upper adjustment start criterion 
HU.sub.start, the height indicative signal value h is compared with the 
lower adjustment start criterion HL.sub.start, at a step 1072. When the 
vehicle height indicative signal value h is smaller than the lower 
adjustment criterion HL.sub.start as checked at the step 1072, the value 
t.sub.DOWN of the DOWN-timer in the microprocessor 66 is reset to zero at 
a step 1074. Thereafter, the value t.sub.UP of the UP-timer in the 
microprocessor 66 is incremented by a value t.sub.cycle corresponding to 
updating interval, at a step 1076. Then, the UP-timer value t.sub.Up is 
compared with the discrimination delay time value Ts, at a step 1078. If 
the Up-timer value t.sub.Up is smaller than the discrimination delay time 
value Ts as checked at the step 1078, process returns to the step 1002. On 
the other hand, when the Up-timer value t.sub.Up is held greater than or 
equal to the discrimination delay time value Ts, the height adjustment 
flag FL.sub.ADJUST is set at a step 1080. Then, upward height adjustment 
is performed at a step 1082. 
In the practical process of the upward height adjustment operation, UP 
command is issued in response to detection of the DOWN-timer value 
t.sub.DOWN greater than the discrimination delay time value Ts and in 
response to the vehicular height indicative signal value h smaller than 
the lower adjustment start criterion HL.sub.start. The microprocessor 66 
is then responsive to the UP command to check the pressure in the pressure 
accumulator 46. When the pressure in the pressure accumulator 46 is 
sufficiently high to increase the pressure in the corresponding one of 
pressure chambers 14.sub.FL, 14.sub.FR, 14.sub.RL and 14.sub.RR, the 
control signal is fed to the accumulation control valve 52 to open. On the 
other hand, when the fluid pressure in the pressure accumulator 46 is not 
sufficient, the microprocessor 66 outputs HIGH level control signal to the 
relay 86 to drive the pump motor 42 to drive compressor 40. 
Simultaneously, HIGH level control signals are fed to corresponding one of 
the height control valves 20.sub.FL, 20.sub.FR, 20.sub.RL and 20.sub.RR. 
By this, the fluid pressure in the corresponding one of the pressure 
chambers 14.sub.FL, 14.sub.FR, 14.sub.RL and 14.sub.RR is increased to 
increase the suspension force supporting the vehicle body on the 
suspension member. Consequently, the vehicle body height relative to the 
suspension member is raised. Thereafter, downward adjustment of the 
vehicular height, check is performed whether height adjustment is 
completed or not, at a step 1070. When completion of the height adjustment 
is detected at the step 1070, process goes END and otherwise, returns to 
the step 1002. 
When the height indicative signal value h is greater than or equal to the 
lower adjustment start criterion HL.sub.start as checked at the step 1072, 
which means that the vehicular height is within the adjustment start 
target height range. Therefore, the UP-timer value t.sub.UP and the 
DOWN-timer value T.sub.DOWN are reset at a step 1084. Thereafter process 
goes to the step 1070 to check whether height adjustment is completed. 
On the other hand, when the height adjustment flag FL.sub.ADJUST is set as 
checked at the step 1056, process goes to a step 1086, in which the 
vehicular height indicative signal value h is compared with the upper 
adjustment terminating criterion HU.sub.end. When the vehicular height 
indicative signal value h is greater than the upper adjustment terminating 
criterion HU.sub.end as checked at the step 1086, downward height 
adjustment is performed at a step 1088. Thereafter, process goes to the 
step 1070 to check whether the height adjustment is completed or not. When 
the vehicular height indicative signal value h is smaller than or equal to 
the upper height adjustment terminating criterion HU.sub.end, the height 
indicative signal value h is then compared with the lower adjustment 
terminating criterion HL.sub.end at a step 1090. If the height indicative 
signal value h is smaller than the lower adjustment terminating criterion 
HL.sub.end, upward height adjustment is performed at a step 1092. After 
the step 1092 or when the height indicative signal value h is greater than 
or equal to the lower adjustment terminating criterion HL.sub.end which 
means that the vehicular height is within the height adjustment 
terminating target height range, the height adjustment flag FL.sub.ADJUST 
is reset at a step 1094. Then, the process goes to the step 1070 to check 
whether height adjustment is completed. 
Practical operation of height adjustment in the preferred embodiment of the 
height control system, according to the invention, will be discussed with 
reference to FIG. 5 which shows a chart of vehicle driving condition when 
the vehicle travels a smooth road, stops at the smooth road, again travels 
to enter into undulated road, stopped at undulated road, gets out from the 
undulated road and entering into the smooth road, stops at the smooth road 
and again travels on the smooth road. In the shown chart, vehicle runs on 
the smooth road during a period T.sub.1. At the end of the T.sub.1 period, 
the vehicle stops on the smooth road. The vehiclestops on the smooth road 
during a period T.sub.2. After the T.sub.2 period, the vehicle again runs 
on the smooth road and thereafter get into the undulated road, during a 
period T.sub.3. During a period T.sub.4, the vehicle stops on the 
undulated road. After the T.sub.4 period, the vehicle again runs to get 
out from the undulated road and to return into the smooth road, during the 
period T.sub.5. Then, the vehicle stops on the smooth road during a period 
T.sub.6. After the T.sub.6 period, the vehicle resumes running on the 
smooth road in a period T.sub.7. During the period T.sub.1 through 
T.sub.7, the vehicular height level and the height adjustment criteria 
HU.sub.start, HL.sub.start, HU.sub.end and HL.sub.end vary as shown by the 
solid line A, as controlled by the preferred embodiment of the height 
control system. By varying the height adjustment criteria HU.sub.start, 
HL.sub.start, HU.sub.end and HL.sub.end as shown, the vehicular height is 
held within the target height range throughout of travel. Therefore, no 
height adjustment is taken place as shown in (D). 
This can be compared with the height control pattern possibly taken place 
by the height control system disclosed in the foregoing Japanese Patent 
First Publication No. 61-263818, which is illustrated by broken line. As 
will be seen from (B) and (C), since the height adjustment criteria 
HU.sub.start, HL.sub.start, HU.sub.end and HL.sub.end in the prior 
proposed system are held constant regardless of the vehicle driving 
condition, height adjustment is performed for once lowering the height and 
thereafter rising. Namely, in the shown example, the vehicular height 
becomes increased out of the adjustment start target height range due to 
undulation on the road surface. The vehicular height higher than the upper 
height adjustment start criterion HU.sub.start is detected at a time 
t.sub.1. Since the T.sub.4 period is longer than a fixed period of 
discrimination delay time Ts, downward height adjustment is taken place at 
during a period from a time t.sub.2, at which the discrimination delay 
time Ts expires, to a time t.sub.3. Therefore, the vehicle height is 
lowered. After returning to the smooth road, the vehicle height as lowered 
during the T.sub.4 period becomes lower than the lower adjustment start 
criterion. This is detected at a time t.sub.4. At a time t.sub.5 at which 
the Ts period expires, upward height adjustment is performed. Upward 
height adjustment is terminated at a step t.sub.6 at which the vehicular 
height is resumed within the adjustment terminating target height range. 
As seen from this, the vehicular height control system according to the 
invention, successfully avoid unnecessary height adjustment and thus 
reduces frequency of occurence of height adjustment. This clearly improves 
riding comfort of the vehicle and expands the life of the compressor. 
It should be appreciated that the height control to be performed by the 
microprocessor at the steps 1066, 1076, 1082, and 1086 incorporate various 
control parameters in addition to the vehicular height. Practical height 
control processes employing various height control parameters have been 
disclosed in the patents and co-pending applications, listed herebelow. 
The disclosure of the below-listed references are herein incorporated by 
reference. 
U.S. Pat. No. 4,519,169 
European Patent First publication 01 14 680 
U.S. Pat. No. 4,659,104 
U.S. Pat. No. 4,718,695 
U.S. Ser. No. 906,239, filed on Sept. 12, 1986 
U.S. Ser. No. 056,761, filed on Jun. 2, 1987 
German patent First Publication 37 15 441 
U.S. Ser. No. 120, 964, filed on Nov. 16, 1987