Shock absorber having controlled damping force characteristics for use in a suspension system of a vehicle

A suspension system of a vehicle for calculating a product of the absolute speed zs of the sprung member and the relative speed zs-zu between the sprung member and the unsprung member and for controlling the damping force characteristic of the shock absorber to be higher if that product is more than the predetermined value and to be lower if the product is less than that value. An insensible range is formed for restricting switching of the damping force characteristic when .vertline.zs.vertline. and .vertline.zs-zu.vertline. are less than the predetermined value .delta.z, .delta.r, so that the damping force characteristic of the shock absorber is kept lower if .vertline.zs.vertline.>.delta.z and .vertline.zs-zu.vertline. or .vertline.zs-zu.vertline.<.delta.r and it is kept higher if .vertline.zs.vertline.>.delta.z and .vertline.zs-zu.vertline. or .vertline.zs-zu.vertline.<.delta.r. Through the above structure, unnecessary switching of the damping force characteristic of the shock absorber can be prevented without lowering the riding comfort and running stability. Also, a sensor for detecting oscillation frequency is not required, resulting in lesser cost.

BACKGROUND OF THE INVENTION 
This invention relates to a suspension system for a vehicle and more 
particularly to a suspension system which comprises a shock absorber of 
damping force characteristic variable type provided between a sprung 
member and an unsprung member. 
In general, a suspension system for a vehicle comprises a shock absorber, 
for damping the oscillation of a wheel, interposed between a sprung member 
and an unsprung member. There are various types of shock absorbers and 
shock absorbers of damping force characteristic variable type are grouped 
into one which damping force characteristic (characteristic of different 
damping coefficient) is changed into two levels (higher level and lower 
level) and the another which the damping force characteristic is changed 
into many levels or changed without steps. 
A well known shock absorber of damping force characteristic variable type 
is, as disclosed in the Japanese Patent Application Laying Open Gazette 
No. 61-163011, is to detect absolute speed of the sprung member and 
relative speed between the sprung member and the unsprung member by each 
detecting means and check signs of them. If these signs are not same, it 
is observed that the damping force generated by the shock absorber is 
acting on the oscillation-stimulating direction with respect to the 
vertical oscillation of a vehicle and set the damping force characteristic 
of a shock absorber lower (i.e., SOFT side). When these signs are same, it 
is is observed that the damping force is acting on oscillation-restricting 
direction and set the damping force characteristic of the shock absorber 
higher (i.e., HARD side). Thus, passenger riding comfort and running 
stability is improved. 
The Japanese Utility Model Registration Application Laying Open Gazette No. 
61-110412 and 63-40213 disclose a shock absorber in which an insensible 
range is formed near neutral position with respect to the relative 
displacement between the sprung member and the unsprung member in order to 
prevent the frequent switching of the damping force characteristic of the 
shock absorber near the neutral position of that displacement. In this 
insensible range, switching of the damping force characteristic of the 
shock absorber is restricted and lower damping force characteristic is 
maintained. 
Maintaining the lower damping force characteristic of the shock absorber in 
the insensible range is effective when the sprung member is in high 
oscillation frequency region due to the road bumps. However, if lower 
damping force characteristic of the shock absorber is maintained when the 
sprung member is in low oscillation frequency range, running stability is 
not be satisfied. 
In order to solve the above problem, a method may be taken for detecting 
the oscillation frequency of the sprung member by a sensor so that the 
damping force characteristic in the insensible range is set lower during 
high oscillation frequency region and the damping force characteristic in 
the insensible range is set higher during low oscillation frequency 
region. However, this method requires a sensor for detecting the 
oscillation frequency and results in higher cost. 
On the other hand, when considering the absolute value of the absolute 
speed of the sprung member and absolute value of either the relative 
displacement between the sprung member and the unsprung member or the 
relative speed between the sprung member and the unsprung member with 
respect to oscillation frequency, .vertline.zs.vertline. which is the 
absolute value of the absolute speed of the sprung member is, as shown in 
FIG. 6, kept higher during oscillation frequency is lower than the 
resonance point .omega.2 (near 10.about.20 Hz) of the unsprung member and 
decreased during oscillation frequency is higher than the resonance point 
.omega.2 of the unsprung member. Also, each .vertline.zs-zu.vertline. 
which is the absolute value of the relative speed between the sprung 
member and the unsprung member and .vertline.zs-zu.vertline. which is the 
absolute value of the relative displacement between the sprung member and 
the unsprung member is, as shown in FIG. 7, maximum at both the resonance 
point .omega.1 (near 1.0.intg.2.0 Hz) of the sprung member and the 
resonance point .omega.2 of the unsprung member and decreases during the 
oscillation frequency is lower than the resonance point .omega.1 of the 
sprung member and during the oscillation frequency is higher than the 
resonance point .omega.2 of the unsprung member. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a suspension system which 
utilizes characteristics of an absolute value of absolute speed of a 
sprung member, an absolute value of relative displacement between the 
sprung member and an unsprung member, and an absolute value of the 
relative speed between the sprung member and the unsprung member and 
distinguishes high oscillation frequency region and low oscillation 
frequency region by these characteristics, so that the damping force 
characteristic of the shock absorber within the insensible range is kept 
either higher or lower. 
In order to achieve the above object, the suspension system for a vehicle 
has the hereinafter described construction. 
The suspension system for a vehicle comprises a shock absorber of damping 
force characteristic variable type interposed between the sprung member 
and the unsprung member, a sprung-member absolute speed detecting means 
for detecting the absolute speed of the sprung member, a relative speed 
detecting means for detecting the relative speed between the sprung member 
and the unsprung member, and a control means for controlling the damping 
force characteristic of the shock absorber to be higher if a product, 
which is the absolute speed of the sprung member multiplied by the 
relative speed between the sprung member and the unsprung member, is more 
than a predetermined value and to be lower if that product is equal to or 
less than the predetermined value. Moreover, an insensible range setting 
means is provided for forming an insensible range which restricts the 
switching of the damping force characteristic when each the absolute value 
of the absolute speed of the sprung member and the absolute value of 
either the relative speed between the sprung member and the unsprung 
member or the relative displacement between the sprung member and the 
unsprung member is less than each predetermined value. The damping force 
characteristic of the shock absorber is kept lower when each absolute 
speed of the sprung member and either the relative speed between the 
sprung member and the unsprung member or the relative displacement between 
the sprung member and the unsprung member is each within an insensible 
range and the damping force characteristic of the shock absorber is kept 
higher when the absolute speed of the sprung member is out of its 
insensible range and either the relative speed between the sprung member 
and the unsprung member or the relative displacement between the sprung 
member and the unsprung member is within the insensible range. 
By the above structure, the insensible range setting means distinguishes 
high oscillation frequency region and low oscillation frequency region on 
the basis of the absolute speed of the sprung member detected by the 
sprung-member absolute speed detecting means and the relative speed 
between the sprung member and the unsprung member detected by the relative 
speed detecting means or relative displacement between the sprung member 
and the unsprung member, which is an integrated value of the relative 
speed. Then it changes the damping force characteristic of the shock 
absorber in the insensible range according to the oscillation region. That 
is, if absolute speed of the sprung member and either the relative speed 
between the sprung member and the unsprung member or relative displacement 
between the sprung member and the unsprung member are in insensible 
ranges, it is defined as a high oscillation frequency region and the 
damping force characteristic of the shock absorber is kept lower. On the 
contrary, if the absolute speed of the sprung member is out of its 
insensible range and relative speed between the sprung member and the 
unsprung member or relative displacement between the sprung member and the 
unsprung member is in the insensible range, it is defined as the low 
oscillation frequency region and the damping force characteristic of the 
shock absorber is kept higher. Through this, the unnecessary switching of 
the damping force characteristic of the shock absorber is prevented 
without lowering the comfortableness to ride in and running stability, and 
reducing the noise and oscillation can be planned. Moreover, a sensor for 
detecting the oscillation frequency is not required, resulting in lesser 
cost.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment is described below with reference to the 
accompanying drawings. 
FIG. 1 shows a layout of components of a suspension system. In FIG. 1, 
reference numerals 1.about.4 designate four shock absorbers, for damping 
the oscillation of wheels, provided in right and left front wheels (only 
left front wheel 5L is shown in the drawings) and right and left rear 
wheels (only left rear wheel 6L is shown in the drawings). Each shock 
absorber 1.about.4 has an actuator 25 (refer to FIG. 2) for switching 
damping force characteristic of a shock absorber to either higher level or 
lower level and a vehicle height sensor (not shown in the drawings) for 
detecting relative displacement between a sprung member and an unsprung 
member. Reference numeral 7 designates a coil spring provided at outer 
circumference of each shock absorber 1.about.4 at its upper part, and 
reference numeral 8 designates a control unit for controlling the damping 
force characteristic variably by receiving a signal from the above vehicle 
height sensor of each shock absorber 1.about.4 and outputting a signal to 
the actuator provided in each shock absorber 1.about.4. 
Reference numerals 11.about.14 designate four acceleration sensors for 
detecting acceleration in the vertical direction (Z direction) of the 
sprung member of a wheel, 15 designates a vehicle speed sensor for 
detecting a vehicle speed provided in a meter of an instrument panel, 16 
designates a steering angle sensor for detecting a steering angle of the 
front wheels by the rotation of the steering shaft, 17 designates an 
accelerator opening sensor for detecting an opening of an accelerator, 18 
designates a brake pressure switch for checking whether the brake is under 
operation (i.e., whether the vehicle is braking) on the basis of the brake 
fluid pressure, and 19 designates a mode selecting switch by which a 
driver switches the damping force characteristics of the shock absorbers 
1.about.4 to either HARD, SOFT, or CONTROL. Those sensors 11.about.17 and 
switches 18 and 19 output signals to the control unit 8. 
FIGS. 2A and 2B show the structure of each shock absorber 1.about.4, where 
FIG. 2A is illustrating a case that the damping force characteristic of 
each shock absorber 1.about.4 is HARD (high damping force is generated) 
and FIG. 2B is illustrating a case that the damping force characteristic 
of each shock absorber 1.about.4 is SOFT (low damping force is generated). 
The vehicle height sensor in each shock absorber 1.about.4 is not shown in 
this drawing. 
In FIG. 2, reference numeral 21 designates a cylinder, a piston unit 22 
formed by a piston and a piston rod is inserted slidably therein. The 
cylinder 21 and the piston unit 22 are mounted to the axle (unsprung 
member) or a vehicle body (sprung member) through each joint. 
Two orifices 23, 24 are provided in the piston unit 22. The one orifice 23 
is open at all times, and the another orifice 24 is openable/closable by 
an actuator 25. The actuator 25 is consisted of a solenoid 26, a control 
rod 27, and two springs 28a, 28b. The control rod 27 slides vertically in 
the piston unit 22 by magnetic force from the solenoid 26 and force from 
the both springs 28a, 28b so that the orifice 24 is opened/closed. 
An upper chamber 29 and a lower chamber 30 in the cylinder 21 and a hollow 
part, for communicating with both chambers, provided in the piston unit 22 
are filled with fluid having adequate viscosity. This fluid can flow 
between the upper chamber 29 and the lower chamber 30 through either 
orifice 23 or 24. 
The explanation is made below about the action of each shock absorber 1-4. 
When the solenoid 26 is not electrified, the control rod 27 is pushed 
downwardly since the force of the spring 28a, which acts on the control 
rod 27 to push it downwardly, is set larger than the force of the spring 
28b, which acts on the control rod 27 to push it upwardly, then the 
orifice 24 is closed (refer to FIG. 2A). Therefore, the fluid can flow 
only through the orifice 23 and the damping force characteristic of each 
shock absorber 1.about.4 is set HARD (higher damping force). 
When the solenoid 26 is electrified, the control rod 27 is pushed upwardly 
by the magnetic force of the solenoid 26, then the orifice 24 is opened 
(refer to FIG. 2B). Therefore, the fluid can flow through both orifices 
23, 24 and the damping force characteristic of each shock absorber 
1.about.4 is set SOFT (lower damping force). As described above, since the 
damping force characteristic of each shock absorber 1.about.4 is HARD when 
the solenoid 26 is not electrified, each shock absorber 1.about.4 is kept 
HARD even if the control unit 8 is in trouble and worsening steering 
stability is prevented. FIG. 3 shows the oscillation model of the 
suspension system, where ms designates sprung mass, mu designates unsprung 
mass, zs designates displacement of the sprung member, zu designates 
displacement of the unsprung member, ks designates a spring constant of a 
coil spring 7, ri designates a spring constant of a tire, and v(t) 
designates a damping coefficient of a shock absorber. 
FIG. 4 shows the block diagram of a control part of the suspension system. 
In FIG. 4, a first set of a vehicle height sensor 41, an acceleration 
sensor 11, and an actuator 25a corresponds to a front left wheel 5L, so as 
a second set of a vehicle height sensor 42, an acceleration sensor 12, and 
an actuator 25b corresponds to a front right wheel, a third set of a 
vehicle height sensor 43, an acceleration sensor 13, and an actuator 25c 
corresponds to a rear left wheel 6L, and a forth set of a vehicle height 
sensor 44, an acceleration sensor 14, and an actuator 25d corresponds to a 
rear right wheel. The actuators 25a.about.25d are identical with the 
actuator 25 of FIG. 2 and each the vehicle height sensor 41.about.44 is 
provided in each shock absorber 1-4. 
Also, r1.about.r4 designates signals of relative displacement between the 
sprung member and the unsprung member outputted to the control unit 8 from 
the first.about.forth vehicle height sensors 41.about.44, and those 
signals take continuous numbers. When each shock absorber 1.about.4 
extends, the signal is positive and when each shock absorber 1.about.4 is 
compressed, the signal is negative. The relative displacement is expressed 
by the deviation from the relative displacement where a vehicle is not 
moving, which is defined 0, (i.e., zs-zu, the difference between zs which 
is the displacement of the sprung member and zu which is the displacement 
of the unsprung member as shown in FIG. 3). 
Signals of z.sub.G 1.about.z.sub.G 4, vertical (Z direction) absolute 
acceleration of the sprung member, are outputted to the control unit 8 
from the first.about.forth acceleration sensors 11.about.14, and those 
signals take continuous numbers. When the sprung member receives the 
upward acceleration, the signal .theta.H, is positive and when the sprung 
member receives the downward acceleration, the signal is negative. 
Vehicle speed signal VS, steering angle signal accelerator opening signal 
TVO are outputted to the control unit 8 from a vehicle speed sensor 15, a 
steering angle sensor 16, and an accelerator opening sensor 17 
respectively. Those signals take continuous numbers. The vehicle speed 
signal VS is positive when the vehicle moves forwardly and it is negative 
when the vehicle moves rearwardly. The steering angle signal .theta.H is 
positive when a steering wheel turns to counterclockwise from a driver 
(i.e., turning to the left), and it is negative when the steering wheel 
turns to clockwise (i.e., turning to the right). 
Brake pressure signal BP is outputted to the control unit 8 from a brake 
pressure switch 18, and the signal has two alternatives, ON or OFF. "On" 
means that the brake is under operation and "OFF" means that the brake is 
not under operation. 
Actuator control signals v1.about.v4 are outputted to the actuaters 
25a.about.25d from the control unit 8, and those signals have two 
alternatives, [1] or [0]. When it is [1] (refer to FIG. 2), the solenoid 
26 of the actuator 25 is not electrified and the damping force 
characteristic of each shock absorber 1.about.4 is HARD. When it is [0], 
the solenoid 26 of the actuator 25 is electrified and the damping force 
characteristic of each shock absorber 1.about.4 is SOFT. 
Moreover, mode selecting signal is outputted to the control unit 8 from the 
mode selecting switch 19, this signal is a parallel signal and takes 
either HARD, SOFT, or CONTROL in the present invention. "HARD" means that 
a driver selects HARD mode, so as SOFT means that a driver selects SOFT 
mode, and CONTROL means that the a driver selects CONTROL mode. As will be 
described later, when HARD mode is selected, the damping force 
characteristic of all shock absorbers 1.about.4 is set HARD, and when SOFT 
mode is selected, the damping force of all shock absorbers 1.about.4 is 
set SOFT. However, when CONTROL mode is selected, the damping force 
characteristic of each shock absorber 1.about.4 is switched to either HARD 
or SOFT according to the driving condition and the road surface. 
FIG. 5 shows a control flow of the control unit 8. This control is 
processed by the control program of the control unit 8. This control 
program is repeated in a given interval (1.about.10 ms) by a starting 
program. The control is described below with this control flow. 
First, at step S1, the mode selecting signal is checked whether it is HARD. 
If this judgement is YES, which means it is HARD, the all actuator control 
signals v1.about.v4 are set [1] at step S21 and these control signals 
v1.about.v4 are outputted at step S16. By this way, the damping force 
characteristic of all shock absorber 1.about.4 is set HARD. The flow is 
finished here. 
If the mode detecting signal in not HARD, it is checked whether the mode 
selecting signal is SOFT at step S2. If this judgement is YES, which means 
it is SOFT, the all actuator control signals v1.about.v4 are set [0] at 
step S22 and this control signals v1.about.v4 are outputted at step S16. 
By this way, the damping force characteristic of all shock absorbers 
1.about.4 is set SOFT. The flow is finished here. 
If both judgement at steps S1, S2 are NO, which means the mode selecting 
signal is CONTROL, signals r1.about.r4 of relative displacement between 
the sprung member and the unsprung member are inputted at step S3, then 
these signals r1.about.r4 are differentiated by differentiation or so at 
step S4 in order to gain r1.about.r4 which are signals of relative speed 
between the sprung member and the unsprung member. A relative speed 
detecting means 51 for detecting r1.about.r4, the relative speed between 
the sprung member and the unsprung member (i.e., the difference between 
the absolute speed of the sprung member and the absolute speed of the 
unsprung member, (zs1-zu1) .about. (zs4-zu4)), is formed by the above 
steps S3, S4 and the vehicle height sensors 41-44. 
Next, after signals z.sub.G 1.about.z.sub.G 4 which are the absolute 
acceleration of the sprung member are inputted at step S5, these Z.sub.G 
1.about.z.sub.G 4 are integrated by integration or so at step S6 in order 
to gain the vertical absolute vehicle speed Z.sub.G 1.about.z.sub.G 4. 
Since these Z.sub.G 1.about.z.sub.G 4 are vertical absolute speed of the 
sprung member at the acceleration sensors 11.about.14, they are converted 
to zs1.about.zs4 which are vertical absolute speed of the sprung member at 
the shock absorbers 1.about.4 at step S7. These zs1.about.zs4 can be 
gained if three values of Z.sub.G 1.about.z.sub.G 4 are found, so Z.sub.G 
1.about.z.sub.G 3 will be used and z.sub.G 4 is a substitute. Here, as 
shown in FIG. 1, suppose a imaginary horizontal xy coordinate is provided. 
The coordinates for the acceleration sensors 11.about.13 are expressed 
(x.sub.G 1,y.sub.G 1).about.(x.sub.G 3,y.sub.G 3) and the shock absorbers 
1.about.4 are expressed by (xs1,ys1).about.(xs4,ys4), then zs1.about.zs4 
are obtained by the following formula: 
##EQU1## 
where two efficient matrixes and a product of them are predetermined and 
given as a constant. 
A sprung-member absolute speed detecting means 52 for detecting 
zs1.about.zs4, the vertical absolute speed of the sprung member at the 
shock absorber 1.about.4, is formed by the above steps S5.about.S7 and the 
acceleration sensors 11.about.14. 
Thereafter, at step S8, the absolute value of zsi (i=1, 2, 3, 4), the 
absolute speed of the sprung member of each wheel, is checked whether it 
is more than a predetermined value .delta.zi. If this judgement is YES, 
the absolute value of ri, the relative speed between the sprung member and 
the unsprung member for each wheel, is checked whether it is less than a 
predetermined value .delta.ri at step S9. If the judgement at step S8 is 
NO, the absolute value of ri, the relative speed between the sprung member 
and the unsprung member for each wheel, is checked whether it is less than 
the predetermined value .delta.ri at step S10. 
The above .delta.zi is a predetermined value for setting insensible range 
which restricts switching of the damping force characteristic of each 
shock absorber 1.about.4 with respect to zsi, the absolute speed of the 
unsprung member. As shown in FIG. 6, .vertline.zsi.vertline., the absolute 
value of the absolute speed of the sprung member, is set less than the 
predetermined value during the oscillation frequency is higher than the 
resonance point .omega.2 of the unsprung member. The above .delta.ri is a 
predetermined value for setting insensible range which restricts switching 
of the damping force characteristic of each shock absorber 1.about.4 with 
respect to ri(=zsi-zui), the relative speed between the sprung member and 
the unsprung member. As shown in FIG. 7, .vertline.ri.vertline., the 
absolute value of the relative speed between the sprung member and the 
unsprung member, is set lesser than the predetermined value during the 
oscillation frequency is less than the resonance point .omega.1 of the 
sprung member and the oscillation frequency is higher than the resonance 
point .omega.2 of the unsprung member. 
Thereafter, if either judgement at step S9 or S10 is NO, the judging 
function hi is obtained by the following formula at step S11: 
EQU hi=ri.multidot.zsi (i=1, 2, 3, 4) 
This judging function hi is a product which is ri, the relative speed 
between the sprung member and the unsprung member, multiplied by zsi, the 
absolute speed of the sprung member, at each wheel. 
A vehicle speed signal VS and a steering angle signal .theta.H are inputted 
at step S12 and gain value g is set at step S13. The gain value g is a 
product (g=g1.multidot.g2) which is a gain value g1 multiplied by a gain 
value g2, g1 is a gain value with respect to the vehicle speed and g2 is a 
gain value with respect to the steering angle and both gain values are 
obtained by the prememorized maps shown in FIGS. 8 and 9 respectively. The 
gain value g1 decreases while the vehicle speed increases and also the 
gain value g2 decreases while the steering angle increases. At step S14, a 
predetermined value Ki (=g.multidot.ri.sup.2) is set, Ki is the above gain 
value g multiplied by a square of ri of relative speed between the sprung 
member and the unsprung member. 
After setting the predetermined value Ki, at step S15, if the judging 
function hi obtained at step S11 is more than the predetermined value Ki 
(hi&gt;Ki), then vi=1 and if hi is equal to or less than the predetermined 
value Ki (hi.ltoreq.Ki), then vi=0. Thereafter, actuator control signals 
v1.about.v4 are outputted at step S16 and the flow is returned to the 
start. A control means 53, for calculating the judging function hi which 
is ri, the relative speed between the sprung member and the unsprung 
member, multiplied by zsi, the absolute speed of the sprung member, and 
for switching the damping force characteristic of each shock absorber 
1.about.4 to either HARD or SOFT according to whether hi is more than the 
predetermined value Ki, is formed by the above steps S11, S15, and S16. 
Also a predetermined value changing means 54 for changing the 
predetermined value Ki according to the driving condition and road surface 
is formed by the steps S12.about.S14. If the judging function hi is equal 
to the predetermined value Ki(hi=Ki), the actuator control signal v1 can 
be kept in the prior state and accordingly the damping force 
characteristic is not changed. 
On the other hand, if the judgement at step S9 is YES, the actuator control 
signal vi of the shock absorber is set [1], and this control signal vi is 
outputted at step S18. If the judgement at step S10 is YES, the actuator 
control signal vi of the corresponding actuator is set [0] at step S19 and 
outputted at step S20. Insensible range setting means 55 is constructed by 
the steps S8.about.S10 and S17.about.S20. This means is for maintaining 
the damping force characteristic of each shock absorber 1.about.4 in SOFT 
when both absolute speed of the sprung member and either the relative 
speed between the sprung member and the unsprung member or the relative 
displacement between the sprung member and the unsprung member are in 
insensible ranges (the judgement at step S8 is NO, and at step S10 is YES) 
or for maintaining the damping force characteristic of each shock absorber 
1.about.4 in HARD when the absolute speed of the sprung member is out of 
the insensible range and the relative speed between the sprung member and 
the unsprung member is in the insensible range (both judgements at steps 
S8 and S9 are YES). 
According to the above control, if a driver selects the CONTROL mode and 
the judging function hi, which is ri (=zsi-zui), the relative speed 
between the sprung member and the unsprung member, multiplied by zsi, the 
absolute speed of the sprung member, is more than the predetermined value 
Ki (hi&gt;Ki) (i.e., the damping force acts downwardly when the sprung member 
bounces upwardly and each shock absorber 1.about.4 extends, or the damping 
force acts upwardly when the sprung member bounces downwardly and each 
shock absorber 1.about.4 is compressed), it is observed that the damping 
force generated by each shock absorber 1.about.4 acts in the 
oscillation-restraining direction with respect to the vertical oscillation 
of the sprung member, then the damping force characteristic of each shock 
absorbers 1.about.4 is changed to HARD. If the judging function hi is 
equal to or less than Ki (hi.ltoreq.Ki) (the contradictory situation of 
the above), it is observed that the damping force generated by each shock 
absorber 1.about.4 acts in the oscillation-stimulating direction with 
respect to the vertical oscillation of the sprung member, then the damping 
force characteristic of each shock absorber 1.about.4 is changed to SOFT. 
By this way, the oscillation-restraining energy becomes larger than the 
oscillation-stimulating energy which is transmitted to the sprung member 
and accordingly, riding comfort and running stability is improved. 
The predetermined value Ki is a product (=g.multidot.ri) which is the gain 
value g multiplied by a square of ri, the relative speed between the 
sprung member and the unsprung member, and therefore it becomes large in 
the high oscillation frequency region of the sprung member due to road 
bumps. Thus, the damping force characteristic of each shock absorber 
1.about.4 in the high oscillation frequency region is hardly changed to 
HARD, and noise and oscillation caused by unnecessary changing of the 
damping force characteristic can be prevented. Also, unsmooth feeling on 
the sprung member caused by the road bumps is restricted and riding 
comfort is improved. 
Moreover, the predetermined value changing means 54 for changing the 
predetermined value Ki according to the road bumps and the oscillation 
caused by it does not need a detecting means for detecting the road 
surface or vehicle oscillation region. Therefore, the present embodiment 
possesses an advantage such as lesser cost. 
Furthermore, since the gain value g for setting the predetermined value Ki 
is a product which is the gain value g1 multiplied by the gain value g2, 
both g1 and g2 decrease while the vehicle speed increases and while the 
steering angle increases respectively, the damping force characteristic is 
set HARD during high speed driving and high speed cornering where the high 
vehicle stability is required. Therefore the securing the stableness can 
be planned. 
Furthermore, if the sprung member bounces in the insensible range where 
either ri, the relative speed between the sprung member and the unsprung 
member, or zsi, the absolute speed of the sprung member, is less than the 
predetermined value .delta.ri and .delta.zi respectively, switching of the 
damping force characteristic of each corresponding shock absorber 
1.about.4 is restricted. Thus, noise and oscillation caused by unnecessary 
and frequent switching of the damping force characteristic can be 
prevented. In this case, the damping force characteristic of each shock 
absorber 1.about.4 is maintained SOFT during the oscillation frequency is 
higher than the resonance point .omega.2 of the unsprung member or 
maintained HARD during the oscillation frequency region is lower than the 
resonance point .omega.1 of the sprung member. Thus, the improvement of 
riding comfort and running stability can be planned. 
Furthermore, this control does not require a sensor for distinguishing low 
oscillation frequency region and high oscillation frequency region. This 
results in lesser cost since the detecting means for detecting the road 
surface and vehicle oscillation range is also not required. Although, the 
present invention has been shown and described in terms of the preferred 
embodiment thereof, it should not be considered as being particularly 
limited thereby. The details of any particular embodiment could be varied. 
For example, in the above embodiment, high oscillation frequency region 
and low oscillation frequency region are distinguished in the insensible 
range setting means 55 on the basis of ri(=zs-zu), the relative speed 
between the sprung member and the unsprung member, and zsi, the absolute 
speed of the sprung member. However, ri (=zs-zu), the relative 
displacement between the sprung member and the unsprung member can be used 
in stead of ri, the relative speed between the sprung member and the 
unsprung member to distinguish high oscillation frequency region and low 
oscillation frequency region. That is, both the absolute value of 
ri(=zs-zu), the relative displacement between the sprung member and the 
unsprung member, and the absolute value of ri(=zs-zu), the relative speed 
between the sprung member and the unsprung member, are maximum at 
resonance points .omega.1 of the sprung member and .omega.2 of the 
unsprung member and also decrease during the oscillation frequency is 
lower than the resonance point .omega.1 of the sprung member and higher 
than the resonance point .omega.2 of the unsprung member. Therefore, an 
insensible range for restricting switching the damping force 
characteristic is formed if the absolute value of ri, the relative 
displacement between the sprung member and the unsprung member, is less 
than the predetermined value. Thus, the damping force characteristic of 
the shock absorber is maintained lower if both the absolute speed of the 
sprung member and the relative displacement between the sprung member and 
the unsprung member are within the insensible range and the damping force 
characteristic of the shock absorber is maintained higher if the absolute 
speed of the sprung member is out of the insensible range and the relative 
displacement between the sprung member and the unsprung member is within 
the insensible range. 
Although the present invention is applied to a shock absorber of which the 
damping force characteristic is changed into higher level and lower level, 
it is a matter of course that the present invention is applicable to such 
a shock absorber that the damping force characteristic is variable into 
multisteps (more than three steps) or variable with steps.