Automotive suspension control system utilizing variable damping force shock absorber

A suspension control system for an automotive vehicle is provided. This suspension control system comprises front and rear variable damping force shock absorbers and a control unit which controls the shock absorbers to assume damping force characteristics in a range between preselected higher and lower damping coefficients. The control unit controls the front and rear shock absorbers in a manner wherein the damping force characteristics of the front shock absorber in an extension stroke and of the rear shock absorber in a compression stroke are modified to the higher damping coefficients respectively when vehicle speed is substantially zero.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a suspension control system for 
an automotive vehicle which utilizes variable damping force shock 
absorbers for suppressing nose-dive and/or squatting motion of a vehicle 
body when starting or braking as well as vertical vibrations during 
parking. 
2. Description of the Prior Art 
Japanese Patent First Publication No. 61-75007 discloses a suspension 
control system for controlling damping characteristics of shock absorbers. 
This system is operable to monitor depression of an accelerator pedal 
during traveling at a speed below a preselected value by means of vehicle 
speed, selector position, throttle opening, and longitudinal acceleration 
sensors for enhancing damping characteristics of the front shock absorbers 
in an extension or rebounding stroke and of the rear shock absorbers in a 
compression or bounding stroke when the depression of the accelerator 
pedal is over a preselected degree. With this damping characteristic 
modification, squatting motion of the vehicle body which may be caused by 
sudden acceleration of the vehicle while traveling at a relatively low 
speed is restricted. 
However, since the above conventional suspension control system is 
responsive to signals from the various sensors indicative of a degree of 
depression of the accelerator pedal to modify the damping force 
characteristics of the shock absorbers, a time lag occurs between 
detection and a time when the damping force characteristics of the shock 
absorbers are actually modified, resulting in squatting motion of the 
vehicle body not being suppressed sufficiently when the vehicle starts 
quickly after being parked. 
Additionally, Japanese Patent First Publication No. 63-305014 discloses a 
suspension control system which is adapted for suppressing nose-dive of a 
vehicle body during braking. This system projects vertical vibrational 
force acting on the vehicle body based on damping force of shock 
absorbers. When the projected vibrating force is less than a preselected 
threshold value, the shock absorbers are controlled to assume harder 
damping force characteristics, while above the threshold value, softer 
damping force characteristics are provided. Additionally, the system 
monitors braking operation when the shock absorbers are oriented to the 
softer damping force characteristics. If the vehicle is in braking 
operation, the shock absorbers are controlled to exhibit the harder 
damping force characteristics for suppressing nose-dive of the vehicle 
body and reaction therefrom when the vehicle stops. 
However, such prior art suspension control systems encounter a drawback in 
that, during parking of the vehicle after braking operation, the system 
may be responsive to vertical motion of the vehicle body due to passengers 
getting in and/or out of the vehicle so as to switch the damping force 
characteristics of the shock absorbers unnecessarily, thereby causing 
power consumption to be increased as well as causing the shock absorbers 
to become degraded prematurely. 
SUMMARY OF THE INVENTION 
It is therefore a principal object of the present invention to avoid the 
disadvantages of the prior art. 
It is another object of the invention to provide a suspension control 
system for an automotive vehicle which serves to modify a damping 
coefficient of a shock absorber for suppressing vehicle attitude change 
such as nose-dive and/or squatting motion effectively. 
According to one aspect of the present invention, there is provided a 
suspension control system for an automotive vehicle which comprises a 
front shock absorber disposed between a vehicle body and a suspension 
member rotatably supporting a front wheel, the front shock absorber being 
controllable to assume damping force characteristics in a range between 
preselected higher and lower damping coefficients over extension and 
compression strokes, a rear shock absorber disposed between the vehicle 
body and a suspension member rotatably supporting a rear wheel, the rear 
shock absorber being controllable to assume damping force characteristics 
in the range between the preselected higher and lower damping coefficients 
over extension and compression strokes, a vehicle speed sensor means for 
detecting vehicle speed and providing a signal indicative thereof, and a 
control means responsive to the signal from the vehicle speed sensor means 
to provide control signals for controlling the front and rear shock 
absorbers in a manner wherein the damping force characteristics of the 
front shock absorber in the extension stroke and of the rear shock 
absorber in the compression stroke are modified to the higher damping 
coefficients respectively when the vehicle speed is substantially zero. 
In the preferred mode, a vehicle attitude change detecting means may be 
provided for detecting vehicle attitude change and providing a signal 
indicative thereof, the control means providing the signals to control the 
front and rear shock absorbers so that the damping force characteristics 
of the front shock absorber in the extension stroke and of the rear shock 
absorber in the compression stroke are modified to the higher damping 
coefficients respectively when the vehicle speed is substantially zero and 
the vehicle attitude change is below a preselected degree. 
The vehicle attitude change detecting means may be provided with a vertical 
speed determining means for determining vehicle speed of the vehicle body. 
The control means may hold the damping force characteristics at a 
preselected damping coefficient when the vehicle speed is substantially 
zero and the vertical speed remains below a preselected threshold value 
for a preselected period of time. 
According to another aspect of the present invention, there is provided a 
suspension control system for an automotive vehicle which comprises shock 
absorbers disposed between a vehicle body and a suspension member 
rotatably supporting a wheel respectively, each shock absorber being 
controllable to assume damping force characteristics in a range between 
preselected higher and lower damping coefficients over extension and 
compression strokes, a vehicle speed sensor means for detecting vehicle 
speed and providing a signal indicative thereof, a vertical speed 
determining means for determining vertical speed of the vehicle body and 
providing a signal indicative thereof, and a control means responsive to 
the signals from the vehicle speed sensor means and the vertical speed 
determining means to provide control signals for controlling the shock 
absorbers in a manner wherein the damping force characteristics of the 
shock absorbers are maintained at a preselected damping coefficient within 
the range between the preselected higher and lower damping coefficients 
when the vehicle speed is substantially zero and the vertical speed is 
below a preselected threshold value for a preselected period of time.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, particularly to FIGS. 1 and 2, a suspension 
control system according to the present invention is shown. This control 
system comprises generally four suspension units which include 
front-right, front-left, rear-right, and rear-left shock absorbers 
SA.sub.1, SA.sub.2, SA.sub.3, and SA.sub.4, vertical acceleration sensors 
1 (hereinafter, referred to as a vertical G sensor), a vehicle speed 
sensor 5, pulse motors 3 for controlling damping positions of the shock 
absorbers respectively, and a control unit 4. The shock absorber as 
generally referred to will be hereafter represented by "SA". 
The shock absorbers SA is disposed between a vehicle body and a suspension 
member supporting a road wheel rotatably. The vertical G sensors 1 are 
mounted on portions of the vehicle body adjacent the shock absorbers for 
monitoring vertical accelerations acting on the vehicle body respectively 
to provide signals indicative thereof to the control unit 4. The control 
unit 4 is arranged adjacent a driver's seat and is operable to provide 
control signals to the pulse motors 3 for modifying damping force 
characteristics of the shock absorbers SA respectively to suppress vehicle 
attitude change. 
The control unit 4 includes an interface circuit 4a, a CPU (Central 
Processing Unit) 4b, and a drive circuit 4c. Sensor signals from the 
vertical G sensors 1 and the vehicle speed sensor 5 are input to the 
interface circuit 4a. 
Referring to FIG. 3, a cross-sectional structure of the shock absorber SA 
is shown. The shock absorber SA includes an inner cylinder 30, a piston 
assembly 31 for defining an upper chamber A and a lower chamber B, an 
outer cylinder 33 for defining a reservoir chamber 32 between the outer 
cylinder 33 and the inner cylinder 30, a base or bottom member 34 for 
defining the lower chamber B and the reservoir chamber 32, a guide member 
for guiding slide movement of a piston rod 7 linked to the piston assembly 
31, a suspension spring 36 interposed between a flange installed on the 
outer cylinder 33 and the vehicle body, and a bumper rubber member (or 
bushing) 37. 
Referring to FIG. 4, there is shown a cross-sectional structure of the 
piston assembly 31. The piston assembly 31 includes penetrating holes 31a 
and 31b, an extension phase damping valve 12, and a compression phase 
damping valve 20. The extension and compression phase damping valves 12 
and 20 serve to open and close the penetrating holes 31a and 31b 
respectively. 
The piston assembly 31 further includes the piston rod 7, a communication 
hole 39, an adjusting pin 40, an extension phase check valve 17, a 
compression phase check valve 22, and a stud 38. The top end of the piston 
rod 7 is screwed into a bound stopper 41 which, in turn, is screwed into 
the stud 38 passing through the piston assembly 31. The communication hole 
39 is formed in an end portion of the stud 38 which fluidly communicates 
between the upper chamber A and the lower chamber B. The adjusting pin 40 
includes a hollow portion 19, a lateral hole 24, an axial slot 25, and a 
longitudinal groove 23 in an outer periphery thereof. The adjusting pin 40 
is supported by the piston rod 7 so as to be allowed to circumferentially 
rotate for modifying a flow passage cross-sectional area of the 
communication hole 39. The rotational movement of the adjusting pin 40 is, 
as shown in FIG. 3, controlled by the pulse motor 3. The extension phase 
check valve 17 is operable to allow a working fluid to flow from the upper 
to lower chambers A and B, while the compression phase check valve 22 
allows fluid flow from the lower to upper chambers B and A. Additionally, 
provided in an end portion of the piston rod 7 are a first port 21, a 
second port 13, a third port 18, a fourth port 14, and a fifth port 16 (as 
they will be referred to hereinafter). 
Hence, four fluid flow passages are formed between the upper chamber A and 
the lower chamber B as fluid flow communicable passages during an 
extension or rebounding stroke of the shock absorber SA: 
1) a first extension phase passage D directing fluid flow from the 
penetrating hole 31b to the lower chamber B through an inner side of the 
opened extension phase damping valve 12; 
2) a second extension phase passage E directing fluid flow from the second 
port 13, the longitudinal hole 23, and the fourth port 14 to the lower 
chamber B through the outer periphery of the extension phase damping valve 
12; 
3) a third extension phase passage F directing fluid flow from the second 
port 13, the longitudinal hole 23, and the fifth port 16 to the lower 
chamber B via the opened extension phase check valve 17; and 
4) a bypass flow passage G directing fluid flow from the third port 18 to 
the lower chamber B through the axial slot 25 and the hollow portion 19. 
For the compression phase or during a bounding stroke of the shock absorber 
SA, the following three passages are provided as the fluid flow passages: 
1) a first compression phase flow passage H directing fluid flow from the 
penetrating hole 31a to the upper chamber A through the opened compression 
phase damping valve 20; 
2) a second compression phase flow passage J directing fluid flow from the 
hollow portion 19, the lateral hole 24, and the first port 21 to the upper 
chamber A through the opened compression phase check valve 22; and 
3) a bypass flow passage G directing fluid flow from the hollow portion 19, 
the axial slot 25 and the third port 18 to the upper chamber A. 
With the above arrangements, rotation of the adjusting pin 40 causes a 
damping coefficient of the shock absorber SA to vary at multiple stages, 
as shown in FIG. 5, within a range from the lowest damping coefficient 
(hereinafter, referred to as a softer damping position) to the highest 
damping coefficient (hereinafter, referred to as a harder damping 
position) during bounding and rebounding strokes. 
Referring to FIG; 6, a relation between a position of the adjusting pin 40 
and damping force characteristics of the shock absorber SA is shown. When 
the adjusting pin 40 is rotated in a counterclockwise direction from the 
position 2 (in a softer damping range SS wherein the lowest damping 
coefficients are established during both bounding and rebounding strokes) 
to the position 1, the damping coefficient is increased toward the harder 
damping position (in a rebounding harder damping range HS) only in the 
rebounding stroke (i.e., during extension). Alternatively, rotating the 
adjusting pin 40 in a clockwise direction to the position 3 causes the 
damping coefficient to change toward the harder damping position (in a 
bounding harder damping range SH) only in the bounding stroke (i.e., 
during compression) without any change in the rebounding stroke. 
It will be noted that, in FIG. 6, when the adjusting pin 40 is rotated and 
halted at the three positions 1, 2, and 3, the cross-sectional situations 
of the adjusting pin taken along the line K--K, along the line M--M, and 
along the line N--N are shown in FIGS. 7(A), 8(A), and 9(A), FIGS. 7(B), 
8(B), and 9(B), and FIGS. 7(C), 8(C), and 9(C) respectively. 
In addition, the damping force characteristics at the positions 1, 2, 3 of 
the adjusting pin 40 are shown in FIGS. 10, 11, and 12. 
Referring to FIGS. 13 and 14, there is shown a flowchart of a program or 
sequence of the logical steps performed by the control unit 4 for 
controlling a position of the pulse motor 3 to modify a damping position, 
or damping force characteristics of the shock absorbers SA for 
anti-squatting motion control. The damping force characteristics 
modification control, as discussed below, is carried out for every shock 
absorber SA. 
After entering the program, the routine proceeds to step 101 wherein the 
control unit 4 monitors vehicle speed Sp based on a signal from the 
vehicle speed sensor 5. The routine then proceeds to step 102 wherein it 
is determined whether the vehicle speed Sp is zero or not. If a NO answer 
is obtained in step 102, the routine then proceeds to step 104 to perform 
basic control of damping force characteristics, as will be described 
hereinafter in detail. Alternatively, if a YES answer is obtained in step 
102 concluding that the vehicle is parked, the routine proceeds to step 
103 wherein the front shock absorbers SA.sub.1 and SA.sub.2 are controlled 
to assume the damping force characteristics in the rebounding harder 
damping range HS, while the rear shock absorbers SA.sub.3 and SA.sub.4 are 
controlled to exhibit the damping force characteristics in the bounding 
harder damping range SH. After steps 103 or 104, the routine returns to 
the initial step. 
With the above modification of the damping force characteristics, when the 
vehicle has been stopped, the damping force characteristics of the front 
shock absorbers SA.sub.1 and SA.sub.2 are modified to in the rebounding 
harder damping range HS, while the damping force characteristics of the 
rear shock absorbers SA.sub.3 and SA.sub.4 are modified to in the bounding 
harder damping range SH to avoid squatting motion which causes the rear of 
the vehicle body to be lowered and the front thereof to rise as a result 
of acceleration when the vehicle starts. 
Referring to FIG. 14, after step 104, the routine proceeds to step 201 
wherein the control unit 4 monitors vertical acceleration G.sub.1 acting 
on a portion of the vehicle body adjacent a road wheel based on a signal 
from the vertical G sensor 1. The routine then proceeds to step 202 
wherein the vertical acceleration G.sub.1 is integrated to determine 
vertical speed V.sub.n. When the vertical speed V.sub.n is in an upward 
direction, a positive signal is provided, while it is in a downward 
direction, a negative signal is provided. 
The routine then proceeds to step 203 wherein it is determined whether the 
vertical speed V.sub.n is greater than zero or not. If a YES answer is 
obtained concluding that the vertical speed V.sub.n is greater than zero, 
that is, that the vehicle body is rising, the routine then proceeds to 
step 204 wherein the shock absorber SA is controlled to assume the damping 
force characteristics in the rebounding harder damping range HS. 
Alternatively, if a NO answer is obtained in step 203 concluding that the 
vertical speed V.sub.n is less than or equal to zero, the routine then 
proceeds to step 205 wherein it is determined whether the vertical speed 
V.sub.n is substantially zero or not. If a YES answer is obtained, the 
routine then proceeds to step 206 wherein the shock absorber SA is 
controlled to assume the damping force characteristics in the softer 
damping range SS. Alternatively, if a NO answer is obtained in step 205 
concluding that the vertical speed V.sub.n is in the downward direction, 
that is, that the vehicle body is being lowered, the routine then proceeds 
to step 207 wherein the damping force characteristics are modified to in 
the bounding harder damping range SH. 
After steps 204, 206, or 207, the routine returns back to the initial step 
101 as shown in FIG. 13. 
In the above suspension control, the damping force characteristics of the 
shock absorber SA are switched based on the magnitude of the vertical 
speed V.sub.n, however, they may be modified based on determination of 
whether the vertical speed V.sub.n and relative speed between the vehicle 
body and the wheel are in the same direction or opposite directions. 
Referring to FIG. 15, there is show a time-chart which represents the 
operation of the suspension control system. 
Assuming that the vertical speed V.sub.n varies according to a sine curve, 
when the vertical speed V.sub.n is in the upward direction (i.e., a 
positive value), the shock absorber SA is controlled to assume the damping 
force characteristics in the rebounding harder damping range HS wherein a 
damping coefficient C is increased in proportion to a value of the 
vertical speed V.sub.n (C=k.multidot.V.sub.n) only in the rebounding 
stroke while maintaining the lower damping coefficient in the bounding 
stroke. 
When the vertical speed V.sub.n is in the downward direction (i.e., a 
negative value), the shock absorber SA is controlled to exhibit the 
damping force characteristics in the bounding harder damping range SH 
wherein the damping coefficient C is increased in proportion to a value of 
the the vertical speed V.sub.n (C=k.multidot.V.sub.n) only in the bounding 
stroke while maintaining the lower damping coefficient in the rebounding 
stroke. 
With the above damping coefficient modification, the same damping force 
characteristic control as that according to the so-called skyhook theory 
may be carried out without detecting relative speed between the vehicle 
body and the wheel (i.e., between unsprung and sprung portions) so that 
when the vertical speed V.sub.n and the relative speed between the vehicle 
body and the wheel are in the same direction, the damping coefficient of 
the shock absorber SA, as shown in the ranges b and d in FIG. 15, is 
modified to a higher value (i.e., the harder damping characteristics), 
while when the vertical speed V.sub.n and the relative speed are in 
opposite directions, the damping coefficient of the shock absorber SA, as 
shown in the ranges a and c in FIG. 15, is modified to a lower value 
(i.e., the softer damping characteristics). Additionally, it will be noted 
that the damping force characteristics may be changed without driving the 
pulse motor 3 during transition periods between the rages a and b and 
between the ranges c and d. 
This embodiment is, as described above, able to modify the damping force 
characteristics according to the shyhook theory only utilizing the 
vertical G sensors. Therefore, the number of parts which make up the 
system may be decreased as well as reduction in mounting space, weight, 
and assembling processes of the system. Additionally, the number of 
changing operations of the damping force characteristics of the shock 
absorber SA may be reduced as compared with the conventional damping force 
characteristics control, resulting in a control response rate as well as 
durability of the pulse motors 3 being improved. 
Referring to FIGS. 16 and 17, there is shown an alternate embodiment of the 
suspension control according to the present invention. 
In the flowchart as shown in FIG. 16, the routine, after entering the 
program, proceeds to step 301 wherein the control unit 4 monitors the 
vehicle speed Sp. The routine then proceeds to step 302 wherein it is 
determined whether the vehicle speed Sp is zero or not. If a NO answer is 
obtained, the routine then proceeds to step 104 as already discussed in 
FIG. 13 wherein the basic suspension control is carried out. 
Alternatively, if a YES answer is obtained in step 302 concluding that the 
vehicle is stopped, the routine proceeds to step 303 wherein it is 
determined whether the vertical speed V.sub.n is greater than or equal to 
a preselected first threshold value V.sub.A and smaller than or equal to a 
preselected second threshold value V.sub.B or not. If a NO answer is 
obtained, the routine then proceed to step 104. Alternatively, if a YES 
answer is obtained concluding that the vertical speed V.sub.n falls within 
the range of the first to second threshold values V.sub.A and V.sub.B, the 
routine then proceeds to step 304 wherein it is determined whether a 
period of time Ts during which the vertical speed V.sub.n falls within the 
range from the first to second threshold values V.sub.A and V.sub.B 
exceeds a preselected threshold value .DELTA.t or not. If a NO answer is 
obtained, the routine then proceeds to step 104. Alternatively, if a YES 
answer is obtained, the routine then proceeds to step 305 wherein the 
front shock absorbers SA.sub.1 and SA.sub.2 are controlled to assume the 
damping force characteristics in the rebounding harder damping range HS, 
while the rear shock absorbers SA.sub.3 and SA.sub.4 are controlled to 
exhibit the damping force characteristics in the bounding harder damping 
range SH. After steps 305 or 104, the routine returns to the initial step. 
With the above damping force characteristic control, when the vehicle, as 
shown in FIG. 17, is brought to a stop and the vertical speed V.sub.n 
falls within the range from the first to second threshold values V.sub.A 
and V.sub.B so that vertical vibrations of the vehicle body caused by 
longitudinal acceleration as a result of braking operation is suppressed, 
the damping force characteristics of the front shock absorbers SA.sub.1 
and SA.sub.2 are modified to in the rebounding harder damping range HS, 
while the damping force characteristics of the rear shock absorbers 
SA.sub.3 and SA.sub.4 are modified to in the bounding harder damping range 
SH. Thus, the suspension control system of this embodiment is able to 
suppress squatting of a vehicle body which may occur when starting, while 
assuring suitable damping force characteristics during parking. 
Referring to FIGS. 18 and 19, there is shown flowcharts of a third 
embodiment of the invention. 
After entering the program, the routine proceeds to step 401 wherein the 
control unit 4 provides a control signal to the pulse motor 3 to adjust 
same to an initial position. The routine then proceeds to step 402 wherein 
the control unit 4 monitors the vertical speed V.sub.n and the vehicle 
speed Sp. Afterward, the routine proceeds to step 501 in the subprogram as 
shown in FIG. 19, as will be described hereinafter in detail. 
After termination of the logical steps in FIG. 19, the routine returns back 
to step 404 wherein it is determined whether the vehicle speed Sp is zero 
or not. If a NO answer is obtained, the routine then returns to step 402. 
Alternatively, if a YES answer is obtained concluding that the vehicle is 
traveling, the routine then proceeds to step 405 wherein it is determined 
whether an absolute value of the vertical speed V.sub.n is smaller than a 
preselected threshold value Vt or not. If a NO answer is obtained, the 
routine then returns to step 402. If a YES answer is obtained 
(.vertline.V.sub.n .vertline.&lt;Vt), the routine then proceeds to step 406 
wherein a count value T of a timer provided in the control unit 4 is 
incremented. In step 407, it is determined whether the count value T is 
greater than a preselected threshold value Tt or not. If a NO answer is 
obtained, the routine then returns to step 402. If a YES answer is 
obtained (T&gt;Tt) concluding that a predetermined period of time has been 
elapsed after the vehicle stops, the routine then proceeds to step 408 
wherein parking suspension control is initiated so that the pulse motor 3 
is held at a preselected position in the rebounding harder damping range 
HS. 
With the above damping force characteristic control, when the vehicle speed 
Sp becomes substantially zero at the time g in FIG. 20 and the vertical 
speed V.sub.n falls in the range of .+-.Vt over the period of time Tt at 
the time h, that is, when pitching and/or bouncing motion of the vehicle 
body caused by nose dive and reaction therefrom becomes smaller than a 
preselected degree, the shock absorber is held at the preselected position 
within the rebounding harder damping range HS. Therefore, even when the 
vehicle body vibrates due to passengers getting in and/or out of the 
vehicle, the damping force characteristics are maintained constant. 
Referring to FIG. 19, the subroutine carried out after step 402 in FIG. 18 
will be discussed below. 
In step 501, the control unit 4 monitors the vertical acceleration G.sub.1. 
The routine then proceeds to step 502 wherein the vertical acceleration 
G.sub.1 is integrated to determine the vertical speed V.sub.n which 
represents a positive value in an upward direction and a negative value in 
a downward direction. 
Afterward, the routine proceeds to step 503 wherein it is determined 
whether the vertical speed V.sub.n is greater than or equal to zero or 
not. If a YES answer is obtained (V.sub.n .gtoreq.0), the routine then 
proceeds to step 504 wherein it is determined whether the vertical speed 
V.sub.n-1 one program cycle before is smaller than zero or not. This 
determination is made for determining whether a direction of the vertical 
speed V.sub.n has been changed or not. If a NO answer is obtained in step 
504, the routine then proceeds directly to step 507. Alternatively, if a 
YES answer is obtained (V.sub.n-1 &lt;0) concluding that the direction of the 
vertical speed has been changed, the routine then proceeds to step 506 
wherein a threshold value V.sub.S1 for the vertical speed V.sub.n is 
initialized to a preselected value. In step 507, it is determined whether 
the vertical speed V.sub.n is greater than or equal to the threshold value 
V.sub.S1 or not. If a NO answer is obtained, the routine then proceeds 
directly to step 509. If a YES answer is obtained in step 507, the routine 
then proceeds to step 508 wherein the threshold value .sub.S1 is updated 
to the current value of the vertical speed V.sub.n. The routine then 
proceeds to step 509 wherein a target damping position T.sub.P (i.e., a 
damping coefficient) of the shock absorber SA in the rebounding stroke is 
determined according to the relation of T.sub.P =(V.sub.n /V.sub.S1).sup.2 
.times.F.sub.+MAX where F.sub.+MAX is a position where the maximum damping 
force is generated in the rebounding stroke. The squaring a value of 
V.sub.n /V.sub.S1 in the above equation corrects damping force 
characteristics linearly with respect to the vertical speed V.sub.n. 
If a NO answer is obtained in step 503, the routine then proceeds to step 
505 wherein it is determined whether the vertical speed V.sub.n-1 one 
program cycle before is greater than zero or not to determine whether a 
direction of the vertical speed V.sub.n has been changed or not. If a NO 
answer is obtained, the routine then proceeds directly to step 511. 
Alternatively, if a YES answer is obtained (V.sub.n-1 &gt;0) concluding that 
the direction of the vertical speed has not been changed, the routine then 
proceeds to step 510 wherein a threshold value V.sub.S2 for the vertical 
speed V.sub.n is initialized to a preselected value. In step 511, it is 
determined whether the vertical speed V.sub.n is smaller than or equal to 
the threshold value V.sub.S2 or not. If a NO answer is obtained, the 
routine then proceeds directly to step 513. If a YES answer is obtained in 
step 511 concluding that the vertical speed V.sub.n becomes greater than 
the threshold value V.sub.S2 i a negative direction, the routine then 
proceeds to step 512 wherein the threshold value .sub.S2 is updated to the 
current value of the vertical speed V.sub.n. The routine then proceeds to 
step 513 wherein a target damping position T.sub.P (i.e., a damping 
coefficient) of the shock absorber SA in the bounding stroke is determined 
according to the relation of T.sub.P =(V.sub.n /V.sub.S2).sup.2 
.times.F.sub.-MAX where F.sub.-MAX is a position at which the maximum 
damping force is generated in the bounding stroke. 
After steps 509 or 513, the routine proceeds to step 404 as already 
mentioned in FIG. 18. 
Referring to FIGS. 21 and 22, time-charts are shown which represent the 
operation of damping force control performed according to the flowcharts 
as shown in FIGS. 18 and 19. 
FIG. 21 shows variations in the vertical speed V.sub.n, damping force F, a 
stroke direction, relative speed between the vehicle body and the wheel, 
and a damping position (i.e., a damping coefficient) of the shock absorber 
SA. In this time-chart, the vertical speed V.sub.n varies according to a 
sine curve over the range of bounding to rebounding strokes, the peak 
values P.sub.1 and P.sub.2 of the vertical speed V.sub.n are greater than 
the threshold values V.sub.S1 and V.sub.S2 respectively which are 
initialized in steps 506 and 510. 
In the range "a", the vertical speed V.sub.n is directed upward below the 
initial threshold value V.sub.S1. Under these conditions, a damping 
coefficient of the shock absorber in the rebounding stroke is modified in 
proportional to a value of the vertical speed V.sub.n. At this time, the 
shock absorber SA is, however, in the bounding stroke and thus the 
bounding softer damping characteristics (i.e., the lowest damping 
position) are provided to serve to suppress lifting motion of the vehicle 
body caused by vibration input from a road surface. 
In the range "b" which is defined between the initial threshold value 
V.sub.S1 and the peal value P.sub.1, according to steps 507 and 508 as 
shown in FIG. 19, the threshold value V.sub.S1 is sequentially updated to 
agree with a value of the vertical speed V.sub.n. Therefore, the damping 
position or damping coefficient of the shock absorber SA is maintained at 
the maximum rebounding damping position +F.sub.MAX until the peak value 
P.sub.1 is reached. This increase in damping force during the rebounding 
stroke suppresses upward vibrations of the vehicle body effectively. 
In the range "c" between the peak value P.sub.1 and zero (0) of the 
vertical speed V.sub.n, since the threshold value V.sub.S1 has been 
updated to the peak value P.sub.1 when the vertical speed V.sub.n reaches 
the peak value P.sub.1, a value of the square of V.sub.n /V.sub.s1 in the 
equation in step 509 indicates less than one (1) and thus decrease in the 
vertical speed V.sub.n below the peak value P.sub.1 causes the damping 
position or damping coefficient of the shock absorber SA in the rebounding 
stroke to be reduced in proportional to reduction in the vertical speed 
V.sub.n. The damping coefficient of the shock absorber SA may 
alternatively be regulated after a preselected period of time from a time 
when the vertical speed V.sub.n reaches the peak value P.sub.1. 
In the range "d" of the vertical speed V.sub.n from zero (0) to the 
negative threshold value P.sub.S1, a damping coefficient of the shock 
absorber in the bounding stroke is modified in proportional to a value of 
the vertical speed V.sub.n. At this time, the shock absorber SA is, 
however, in the rebounding stroke and thus the rebounding softer damping 
characteristics (i.e., the lowest damping position) are provided to serve 
to suppress lowering motion of the vehicle body caused by vibration input 
from a road surface. 
In the range "e" of the vertical speed V.sub.n from the initial threshold 
value V.sub.S2 to the peal value P.sub.2, according to steps 511 and 512 
as shown in FIG. 19, the threshold value V.sub.S2 is updated to agree with 
a value of the vertical speed V.sub.n. Therefore, the damping position or 
damping coefficient of the shock absorber SA is maintained at the maximum 
bounding damping position -F.sub.MAX until the peak value P.sub.2 is 
reached, thereby increasing the damping coefficient during the bounding 
stroke so as to suppress a downward vibration of the vehicle body 
effectively. 
In the range "f" between the peak value P.sub.2 and zero (0) of the 
vertical speed V.sub.n, since the threshold value V.sub.S2 has already 
been updated to the peak value P.sub.2 when the vertical speed V.sub.n 
reaches the peak value P.sub.2, the square of V.sub.n /V.sub.S2 in the 
equation in step 513 indicates less than one (1) and thus increase in the 
vertical speed V.sub.n above the peak value P.sub.2 causes the damping 
position or damping coefficient of the shock absorber SA in the bounding 
stroke to be reduced in proportional to reduction in the vertical speed 
V.sub.n. 
Referring to FIG. 22, the relation between the vertical speed V.sub.n and 
the damping position (i.e., a damping coefficient) is shown wherein the 
vertical speed V.sub.n varies according to a sine curve over bounding and 
rebounding strokes within the range from the initial threshold values 
V.sub.S1 to V.sub.S2. 
When the vertical speed V.sub.n, as can be seen in the drawing, varies 
within the range from the initial threshold values V.sub.S1 to V.sub.S2, 
the damping position or damping coefficient of the shock absorber SA in 
the same stroke as the vertical speed V.sub.n is modified in proportional 
to a value of the vertical speed V.sub.n. 
With the above damping characteristic control, as already mentioned, within 
the range between the initial threshold values V.sub.S1 and V.sub.S2, the 
shock absorber SA is controlled to assume the damping force 
characteristics according to the vertical speed V.sub.n in the same 
direction of stroke as the vertical speed V.sub.n. After the vertical 
speed V.sub.n exceeds the threshold value V.sub.S1 or V.sub.S2, the 
threshold value is updated to coincide with the vertical speed V.sub.n 
sequentially until the vertical speed V.sub.n falls within the range 
between the threshold values V.sub.S1 and V.sub.S2 so that the shock 
absorber SA is maintained at the preselected maximum damping position in 
the same direction of stroke as the vertical speed V.sub.n. Accordingly, 
vibrations of the vehicle body due to low frequency input from a road 
surface are damped effectively while high frequencies caused by 
protrusions on the road surface are damped with the lower damping force 
characteristics for assuring vehicle riding comfort. 
Additionally, vibrations of the vehicle body caused by nose dive and 
reaction therefrom are suppressed with the harder damping force 
characteristics, in the same direction of stroke as the vertical speed 
V.sub.n, variable according to a value of the vertical speed V.sub.n. 
When the vehicle has passed over protrusions on a road surface, the softer 
damping force characteristics in a stroke of direction opposite the 
vertical speed V.sub.n prevents the vibrations from the road surface from 
being transmitted to the vehicle body. 
While the present invention has been disclosed in terms of the preferred 
embodiment in order to facilitate better understanding thereof, it should 
be appreciated that the invention can be embodied in various ways without 
departing from the principle of the invention. Therefore, the invention 
should be understood to include all possible embodiments and modification 
to the shown embodiments which can be embodied without departing from the 
principle of the invention as set forth in the appended claims. 
For example, while the above mentioned suspension control system switches a 
damping coefficient between the harder and softer damping force 
characteristics based on the vertical speed V.sub.n, the switching 
operation of the damping coefficient may be made based on determination of 
whether the vertical speed V.sub.n and the relative speed between the 
vehicle body and the wheel are in the same direction or not. 
Additionally, in the above suspension control system, when a damping 
coefficient of the shock absorber either in bounding or rebounding strokes 
is modified to a higher damping coefficient to assume the harder damping 
characteristics, while a damping coefficient in the other stroke is 
modified to a lower damping coefficient to exhibit the softer damping 
characteristics. However, the same damping coefficient may be provided 
both in the bounding and rebounding strokes. 
Further, the damping coefficient of the shock absorber SA may be decreased 
according to the vertical speed V.sub.n as well as vehicle speed.