Vehicle suspension system

A vehicle suspension system provided between a vehicle body and each of road wheels. The vehicle suspension system includes damping force varying means (a) for switching extension and compression side damping force characteristics in a plurality of steps. The damping force varying means is operable to provide a 0.1 or less damping coefficient ratio of the compression side damping force with respect to the compression side damping force. The vehicle suspension system comprises a shock absorber (b) for damping vibrations transmitted from the road wheel by movement of a fluid enclosed therein, vehicle behavior detecting means (c) for detecting a vehicle behavior, damping force characteristic control means (e) having ordinary control means (d) for controlling the damping force characteristic of the shock absorber based on a vehicle behavior indicative signal produced from the vehicle behavior detecting means (c), braking application detecting means (f) for detecting the application of braking to the vehicle, correcting means (g) provided in the damping force characteristic control means for switching the damping force characteristic varying means of the shock absorber to provide a 1.0 or less damping coefficient ratio of the extension side damping force with respect to the compression side damping force when the braking application detecting means detects the application of braking to the vehicle. The contact load variations resulting from a vehicle behavior produced in the presence of vehicle braking are suppressed to ensure sufficient braking force.

FIELD OF THE INVENTION 
This invention relates to a suspension system including a spring and a 
shock absorber provided between a vehicle body and a vehicle road wheel 
and, more particularly, to a vehicle suspension system adapted to control 
the shock absorber to have an optimum damping force characteristic in the 
presence of vehicle braking. 
BACKGROUND OF THE INVENTION 
In general, vehicle suspension systems include a plurality of links, a 
spring for absorbing vibrations transmitted from the road surface to the 
vehicle body, and a shock absorber for damping the vibrations. Some 
conventional shock absorbers are of the variable damping force 
characteristic type having a damping force characteristic controllable 
according to vehicle operating conditions that are sensed during vehicle 
operation. For example, Japanese Patent Kokai NO. 4-103420 discloses a 
vehicle suspension system adapted to control the damping force 
characteristic of a shock absorber. 
In the vehicle suspension system disclosed in the Japanese Patent Kokai, 
the time rate of change (actual rate of change) of the damping force 
actually obtained by the shock absorber is calculated. The calculated time 
rate of change of the damping force is compared with a reference time 
rate. Normally, a hard damping characteristic is selected to ensure good 
driving stability. In order to provide a good driving feel to the 
passenger during vehicle driving on a bad road, the shock absorber damping 
force characteristic is switched from the hard characteristic to a soft 
characteristic when the calculated time rate of change of the damping 
force exceeds the reference time rate. The reference time rate is set at a 
greater value in the presence of vehicle braking than in the absence of 
vehicle braking. The reference time rate is increased in response to the 
operation of an antiskid braking system, that is, in response to the 
application of urgent braking to the vehicle. This is effective to 
suppress the vehicle posture changes according to the degree of the 
vehicle braking. 
Although such a conventional system can suppress vehicle posture changes by 
retaining the shock absorber to have a hard damping force characteristic 
during the application of braking to the vehicle, the hard damping force 
characteristic is provided during both of the extension and compression 
strokes of the shock absorber. For this reason, the following difficulties 
are associated with the conventional system. 
If the shock absorber has a hard damping force characteristic during the 
application of braking to the vehicle, the contact load (the force under 
which the tire contacts the road surface) will increase to increase the 
braking force during its compression stroke, whereas the contact load will 
decrease to decrease the braking force during its extension stroke. For 
this reason, the braking distance will increase particularly when the 
vehicle is running on a bad road causing shock absorber movements. This 
tendency will be enhanced since the shock absorber is set to have a 
greater damping force during its extension stroke than during its 
compression stroke with regard to the direction in which the weight of the 
vehicle body acts. 
FIG. 18 shows variations in the contact load. The dotted curve relates to 
the contact load variations measured when the ratio TEN/COM (damping 
coefficient ratio) of the damping force (TEN) provided during its 
extension stroke with respect to the damping force (COM) provided during 
its compression stroke is greater than 1.0. The solid curve relates to the 
contact load variations measured when the damping coefficient ratio is 
equal to or less than 1.0. As can be seen from FIG. 18, the contact load 
is smaller in the former case than in the latter case when the vehicle is 
running on a bad road causing a high frequency road surface input. 
FIGS. 19(a), 19(b) and 19(c) show contact loads variations with respect to 
respective load variation centers at damping coefficient ratios (TEN/COM) 
of 4.0, 1.5 and 0.8. It can be seen from these figures that the deviation 
of the load variation center in the load decreasing direction increases as 
the damping coefficient ratio increases. 
In view of the above considerations, the invention has for its object to 
provide a vehicle suspension system which can ensure sufficient braking 
force by suppressing contact load reductions resulting from vehicle 
behaviors produced in the presence of vehicle braking. 
DISCLOSURE OF THE INVENTION 
According to the invention, the above object can be achieved by a vehicle 
suspension system as shown in the block diagram of FIG. 1. The vehicle 
suspension system is provided between a vehicle body and each of road 
wheels and it includes damping force varying means (a) for switching 
extension and compression side damping force characteristics in a 
plurality of steps, the damping force varying means being operable to 
provide a 0.1 or less damping coefficient ratio of the compression side 
damping force with respect to the compression side damping force, a shock 
absorber (b) for damping vibrations transmit ted from the road wheel by 
movement of a fluid enclosed therein, vehicle behavior detecting means (c) 
for detecting a vehicle behavior, damping force characteristic control 
means (e) having ordinary control means for controlling the damping force 
characteristic of the shock absorber (b) based on a vehicle behavior 
indicative signal produced from the vehicle behavior detecting means (c), 
braking application detecting means (f) for detecting the application of 
braking to the vehicle, correcting means (g) provided in the damping force 
characteristic control means (e) for switching the damping force 
characteristic varying means (a) of the shock absorber (b) to provide a 
1.0 or less damping coefficient ratio of the extension side damping force 
with respect to the compression side damping force when the braking 
application detecting means (f) detects the application of braking to the 
vehicle. 
In the vehicle suspension system of the invention, as described above, the 
correcting means (g) switches the damping force characteristic varying 
means (a) of the shock absorber (b) to a position where the damping 
coefficient ratio of the extension side damping force to the compression 
side damping force is equal to or less than 1.0 in the presence of vehicle 
braking. 
That is, when the vehicle is running on a bad road causing movement of the 
shock absorber (b), the tire contact load increases at a greater 
compression side damping force provided during the compression stroke of 
the shock absorber (b) and the tire contact load increases at a smaller 
extension side damping force provided during the extension stroke of the 
shock absorber. It is, therefore, possible to prevent contact load 
reductions resulting from vehicle behaviors so as to ensure good braking 
force by setting 1.0 or greater damping coefficient ratio of the extension 
side damping force with respect to the compression side damping force 
(that is, the extension side damping force is smaller than the compression 
side damping force). 
It is possible to suppress contact load reductions resulting from vehicle 
behaviors produced in the presence of vehicle braking when the vehicle is 
running on a bad road causing shock absorber movement. This is effective 
to ensure good braking force. 
In the invention as claimed in claim 2, the vehicle suspension system 
includes pitch rate detecting means (h) for detecting a vehicle pitch 
rate. The function of the correcting means (g) is stopped when the pitch 
rate detected by the pitch rate detecting means (h) exceeds a 
predetermined threshold value. 
With the vehicle suspension system arranged in this form, the operation of 
the correcting means (g) is stopped even upon the application of braking 
to the vehicle as long as the vehicle pitch rate detected by the pitch 
rate detecting means (h) exceeds a predetermined threshold valve. It is, 
therefore, possible to suppress vehicle pitching motions by the operation 
of the control means (d).

PREFERRED EMBODIMENTS OF THE INVENTION 
Preferred embodiment of the invention will be described with reference to 
the accompanying drawings. 
First of all, description will be made to a first embodiment of the vehicle 
suspension system of the invention. FIG. 2 is a schematic diagram of the 
first embodiment of the suspension system of the invention applied to an 
automotive vehicle. In this embodiment, the suspension system is of the 
strut type including a spring and a shock absorber SA provided between a 
vehicle body and each of road wheels. Shock absorbers SA.sub.FL, 
SA.sub.FR, SA.sub.RL and SA.sub.RR (the character SA is used throughout 
the invention to designate each of these four shock absorbers) are 
provided for the respective road wheels. Front and rear vertical 
acceleration sensors (referred hereinafter to as vertical G sensors) 
1.sub.FS and 1.sub.RS are provided on the vehicle body at the position 
intermediate the front-left and -right road wheels SA.sub.FL and SA.sub.FR 
and at the position intermediate the rear-left and -right road wheels 
SA.sub.RL and SA.sub.RL for detecting the vertical accelerations at the 
respective intermediate positions. A longitudinal acceleration sensor 
(referred hereinafter to as longitudinal G sensor) 2 is provided on the 
vehicle body at a position intermediate the front-left and -right shock 
absorbers SA.sub.FL and SA.sub.FR for detecting the longitudinal vehicle 
acceleration (or deceleration). This longitudinal G sensor acts as a 
braking application detecting means for detecting the application of 
braking to the vehicle. A road wheel speed sensor 5 is provided for 
detecting the speed of rotation of each of the road wheels. A vehicle 
speed sensor 6 (not shown in FIG. 2) is provided on the vehicle body for 
detecting the vehicle speed. A brake switch BS (not shown in FIG. 2) is 
provided for detecting the application of braking to the vehicle (that is, 
the operator's depression of the brake pedal). The brake switch 
constitutes a braking application detecting means. A control unit 4, which 
is provided near the driver's seat, receives signals fed thereto from the 
vertical G sensors 1.sub.FS and 1.sub.RS, the longitudinal G sensor 2, the 
road wheel speed sensors 5, the vehicle speed sensor 6 and the brake 
switch BS, and it produces a drive control signal to a pulse motor 3 
provided for each of the respective shock absorbers SA. 
As shown in the system block diagram of FIG. 3, the control unit 4 includes 
an interface circuit 4a, a CPU 4b and a drive circuit 4c. The interface 
circuit 4a receives signals fed thereto from the vertical G sensors 
1.sub.FS and 1.sub.RS, the longitudinal G sensor 2, the road wheel speed 
sensors 5, the vehicle speed sensor 6 and the brake switch BS. 
The interface circuit 4a includes three kinds of signal processing circuits 
shown in FIGS. 14(a), 14(b) and 14(c). 
That is, the signal processing circuit as shown in FIG. 14(a) is provided 
for each of the front and left vertical G sensors 1.sub.FS and 1.sub.RS 
for producing a control signal V. This signal processing circuit includes 
a low pass filter LPF1 having a cutoff frequency of 0.05 Hz to convert the 
corresponding one of the sprung mass acceleration signals G fed thereto 
from the vertical G sensors 1 into a sprung mass vertical speed by 
integrating the sprung mass acceleration signal G. The signal processing 
unit also includes a band pass filter BPF comprised of a high pass filter 
HPF and a low pass filter LPF2 for noise removal and phase correction. The 
high pass filter has a cutoff frequency of 0.7 Hz and the low pass filter 
has a cutoff frequency of 1.5 Hz. 
The signal processing circuit as shown in FIG. 14(b) is provided for 
obtaining the vehicle pitch rate V.sub.P. This signal processing circuit 
includes a calculation circuit E for calculating a relative acceleration 
difference of the rear road wheel side vertical acceleration G.sub.RS 
sensed at the position intermediate the rear road wheels by the rear road 
wheel side vertical G sensor 1.sub.RS from the front road wheel side 
vertical acceleration G.sub.FS sensed at the position intermediate the 
front road wheels by the front road wheel side vertical G sensor 1.sub.FS. 
The signal processing circuit also includes a low pass filter LPF1, a high 
pass filter HPF and a low pass filter LPF2 which are substantially the 
same of the filters used in the signal processing circuit of FIG. 14(a). 
That is, these filters are arranged for converting the relative 
acceleration difference to a corresponding vehicle pitch rate V.sub.P. 
The signal processing circuit as shown in FIG. 14(c) is provided for 
converting the vehicle longitudinal acceleration G.sub.FR to a 
corresponding vehicle deceleration G.sub.FR ' for use in determining the 
application of braking to the vehicle. This signal processing circuit 
includes a low pass filter LPF3 for removing noise and extracting the Dc 
component. 
The control unit 4 also includes an antiskid control device (ABS device) 
for preventing the road wheels from being locked in the presence of 
vehicle braking based on the signals fed thereto from the road wheel speed 
sensors 5 and the vehicle speed sensor 6, and an ABS operation detecting 
means for detecting the operation of the antiskid control device. 
FIG. 4 is a sectional view showing the strut arrangement including the 
shock absorber SA. The shock absorber SA includes a cylinder 30, a piston 
31 positioned to divide the cylinder 30 into upper and lower chambers A 
and B, an outer envelope 33 positioned to define a reservoir 32 along with 
the outer periphery of the cylinder 30, a base 34 provided to divide the 
lower chamber B from the reservoir 32, and a guide member 35 provided to 
guide the sliding movement of a piston rod 7 coupled to the piston 31. 
FIG. 5 is an enlarged sectional view showing the piston 31. The piston 31 
is formed with bores 31a and 31b. A compression side damping valve 20 is 
provided in cooperation with the bore 31a, and an extension side damping 
valve 12 is provided in cooperation with the bore 31b. A stud 38, which 
extends through the piston 31, is fixed to a bound stopper 41 threadedly 
engaged on the tip end of the piston rod 7. The stud 38 has a 
communication bore 39 which forms a flow passage bypassing the bores 31a 
and 31b to connect the upper and lower chambers A and B. (This flow 
passage includes an extension side second flow passage E, an extension 
side third flow passage F, a bypass flow passage g and a compression side 
second flow passage J, as will be described in greater detail.) An 
adjuster 40 is provided for rotation within the communication bore 39 to 
vary the effective cross section of the flow passage. The stud 38 is 
provided on its outer periphery with extension and compression side check 
valves 17 and 22 to permit flow through the flow passage including the 
communication bore 39 only in a predetermined direction but inhibit flow 
through the flow passage in the opposite direction. The rotation of the 
adjuster 40 is made through a control rod 70 by a corresponding one of the 
pulse motors 3 (see FIG. 4). The stud 38 has a first port 21, a second 
port 13, a third port 18, a fourth port 14 and a fifth port 25 formed 
therein in this order from above to below in the drawing. 
The adjuster 40 has a hollow portion 19 and first and second lateral holes 
24 and 25 to make a connection between the interior and exterior. The 
adjuster is formed in its outer periphery with a longitudinal groove 23 
extending axially thereof. 
Thus, four flow passages are formed for fluid communication between the 
upper and lower chambers A and B during an extension stroke. These flow 
passages includes an extension side first flow passage D leading through 
the bore 31b and the extension side damping valve 12 to the lower chamber 
B, an extension side second flow passage E leading through the second port 
19, the longitudinal groove 23, the fourth port 14 and the extension side 
damping valve 12 to the lower chamber B, an extension side third flow 
passage F leading through the second port 13, the longitudinal groove 23, 
the fifth port 16 and the extension side check valve 17 to the lower 
chamber B, and a bypass passage G leading through the third port 18, the 
second lateral hole 25 and the hollow portion 19 to the lower chamber B. 
Three flow passages are formed for fluid communication during a 
compression stroke. These flow passages includes a compression side first 
flow passage H leading through the bore 31a and the compression side 
damping valve 20, a compression side second flow passage J leading through 
the hollow portion 19, the first lateral hole 24, the first port 21 and 
the compression side check valve 22 to the upper chamber A, and a 
compression side third flow passage G leading through the hollow portion 
19, the second lateral hole 25 and the third port 18 to the upper chamber 
A. 
That is, the shock absorber SA is arranged to provide extension and 
compression side damping force characteristics variable in a plurality of 
steps with rotation of the adjuster 40, as shown in FIG. 6. When the 
adjuster 40 rotates in the counter-clockwise direction from a region 
(referred hereinafter to as a soft region SS) where soft damping force 
characteristics are provided on both of the extension and compression 
sides, a change is made to a region (referred hereinafter to as an 
extension side hard region HS) where the damping force characteristic can 
vary in a plurality of steps on the extension side, whereas a low damping 
force characteristic is held on the compression side, as shown in FIG. 7. 
When the adjuster 40 rotates in the clockwise direction from the region 
SS, a change is made to a region (referred hereinafter to as a compression 
side hard range SH) where the damping force characteristic can vary in a 
plurality of steps on the compression side, whereas a low damping force 
characteristic is held on the extension side. 
FIGS. 8(a), 8(b) and 8(c) are sectional views taken along the lines K--K of 
FIG. 5 when the adjuster 40 is placed at respective positions 1, 2 and 3 
of FIG. 7. FIG. 9 contains sectional views taken along the lines L--L of 
FIG. 5 when the adjuster 40 is placed at respective positions 1, 2 and 3 
of FIG. 7. FIG. 10 contains sectional views taken along the lines N--N of 
FIG. 5 when the adjuster 40 is placed at respective positions 1, 2 and 3 
of FIG. 7. FIGS. 11, 12 and 13 shows the damping force characteristics at 
the respective positions. 
For the shock absorber SA, the compression side hard region SH where a soft 
damping force characteristic is held on the extension side and the damping 
force characteristic can vary toward the hard side on the compression side 
and the region indicated by the hatched area is a range where the damping 
coefficient ratio TEN/COM of the extension stroke damping force (TEN) with 
respect to the compression stroke damping force (COM) is equal to or less 
than 1.0 (TEN/COM.ltoreq.1.0). 
The braking operation of the control unit will be described with reference 
to the flow diagram of FIG. 15 and the timing chart of FIG. 16. 
In the step 101 of the flow diagram of FIG. 15, a determination is made as 
to whether or not the signal fed from the brake switch BS is on. If the 
brake pedal is depressed (YES), then the control is transferred to the 
step 102. 
In the step 102, a determination is made as to whether or not the control 
flag FLAG2 to be described later is set at 1. If the answer to this 
question is "NO", then the control is transferred to the step 103. 
In the step 103, a determination is made as to whether or not the vehicle 
speed Sv exceeds a predetermined vehicle speed ON threshold valve S.sub.ON 
(for example, a value ranging from 30 to 40 Km/h). If the answer to this 
question is "YES", then the control is transferred to the step 104. 
In the step 104, a determination is made as to whether or not the pitch 
rate V.sub.P exceeds a predetermined posture control ON threshold value 
V.sub.P-ON. If the answer to this question is "NO", then the control is 
transferred to the step 105. 
In the step 105, a determination is made as to whether or not the ABS 
operation is performed. If the answer to this question is "YES", then the 
control is transferred to the step 106. 
In the step 106, a determination is made as to whether or not the vehicle 
speed Sv exceeds a predetermined vehicle speed OFF threshold value 
S.sub.OFF (0 Km/h). If the answer to this question is "YES", then the 
control is transferred to the step 107 where the control flag FLAG2 is set 
at 1. Following this, the control is transferred to the step 108. 
In the step 108, a control suitable in the presence of vehicle braking is 
performed for the suspension system. That is, the pulse motors 3 are 
driven to control the damping force control positions (see FIG. 7) of the 
shock absorbers SA on the front and rear road wheel sides to the adjuster 
positions corresponding to the compression side hard region SH where a 
soft damping force characteristic is provided on the extension side and 
the damping force coefficient ratio TEN/COM of the extension stroke 
damping force (TEN) with respect to the compression stroke damping force 
(COM) is equal to or less than 1.0. Here, one control cycle is terminated. 
If the question inputted in the step 101 is "NO" (the brake switch BS is 
off), then the control is transferred to the step 110 where the control 
flag FLAG2 is cleared to 0. Following this, the control is transferred to 
the step 111 where a control suitable in the absence of vehicle braking is 
performed for the suspension system. That is, in the step 111, each of the 
pulse motors 3 is driven toward a target damping force position P 
calculated as a function of the control signal V from the following 
equation (1): 
EQU P=P.sub.max (v-V.sub.NC)/(V.sub.H -V.sub.NC) (1) 
where P.sub.max is a predetermined maximum damping force position, v.sub.H 
is the control proportional range and V.sub.NC is the control dead zone. 
The maximum damping force position P.sub.max, the control proportional 
range v.sub.H, and the control dead zone V.sub.NC are set at an extension 
side maximum damping force position P.sub.max-T, an extension side control 
proportional range v.sub.H-T, and an extension side control dead zone 
V.sub.NC-T, respectively, when the control signal V has a positive value 
and at a compression side maximum damping force position P.sub.max-C, a 
compression side control proportional range v.sub.H-C, and the control 
dead zone V.sub.NC-C, respectively, when the control signal has a negative 
value. 
If the question inputted in the step 102 is "YES" (the control flag FLAG2 
is set at 1, then the control is transferred to the step 109. In the step 
109, a determination is made as to whether or not the braking application 
decision signal, that is, the vehicle deceleration G.sub.FR ' exceeds a 
threshold value G.sub.-ON. If the answer to this question is "YES", then 
the control is transferred to the step 106. Otherwise, the control is 
transferred to the step 110. 
If the answer to the question inputted in the step 103 is "NO" (the vehicle 
speed Sv is equal to or less than the vehicle speed ON threshold value 
S.sub.ON), then the control is transferred to the step 105. 
If the answer to the question inputted in the step 104 is "YES" (the pitch 
rate V.sub.P exceeds the posture control ON threshold value V.sub.P-ON), 
then the control is transferred to the step 110. 
If the answer to the question inputted in the step 105 is "NO" (the ABS 
control is not performed), then the control is transferred to the step 
109. 
If the answer to the question inputted at the step 106 is "NO" (the vehicle 
speed Sv is equal to or less than the vehicle speed OFF threshold value 
S.sub.OFF), then the control is transferred to the step 110. 
Here, one control cycle is terminated. A similar control cycle is repeated. 
The operation of the control unit 4 will be described with reference to the 
timing chart of FIG. 16. 
(1) Control in the presence of vehicle braking 
A control suitable in the presence of vehicle braking is performed when the 
brake switch is ON and at least one of the following conditions (i) to 
(iii) is fulfilled: 
(i) The vehicle speed Sv exceeds the vehicle speed ON threshold valve 
S.sub.ON, the pitch rate V.sub.P is equal to or less than the posture 
control ON threshold valve V.sub.P-ON, and the ABS operation is performed 
or the deceleration G.sub.FR ' exceeds the threshold value G.sub.-ON even 
when the control flag FLAG2 is cleared to 0. That is, the vehicle speed is 
fast, the vehicle pitch is small and rapid braking is applied to the 
vehicle. 
(ii) The vehicle speed Sv remains greater than the vehicle speed OFF 
threshold value S.sub.OFF and the ABS operation is performed or the 
deceleration G.sub.FR ' exceeds the threshold value G.sub.-ON even when 
the control flag LFAG2 is cleared to 0 and the vehicle speed Sv is equal 
to or less than the vehicle speed ON threshold value S.sub.ON. That is, 
rapid braking is applied to the vehicle even when the vehicle speed is 
slow. 
(iii) The control flag FLAG2 is set at 1 and the deceleration G.sub.FR ' 
exceeds the threshold value G.sub.-ON. That is, in the presence of rapid 
braking to decrease the vehicle speed in a predetermined fashion. 
During the control performed in the presence to vehicle braking, the 
damping force characteristics of the shock absorbers SA provided on the 
front and rear road wheel sides are controlled to the compression side 
hard range SH where a soft damping characteristic is provided on the 
extension side and to an adjuster position (indicated by the hatched area 
of FIG. 7) where the damping coefficient ratio TEN/COM of the extension 
stroke damping force (TEN) with respect to the compression stroke damping 
force (COM) is equal to or less than 1.0. This is effective to suppress 
the contact load variations (reductions) resulting from the vehicle 
behavior when the vehicle is running on a bad road causing a 
high-frequency road surface input so as to ensure sufficient braking 
force. 
That is, FIG. 18 shows contact load variations. The dotted curve relates to 
the conventional system where the contact load variations measured when 
the ratio TEN/COM (damping coefficient ratio) of the damping force (TEN) 
provided during its extension stroke with respect to the damping force 
(COM) provided during its compression stroke is greater than 1.0. The 
solid curve relates to the system of the invention where the contact load 
variations measured when the damping coefficient ratio is equal to or less 
than 1.0. As can be seen from FIG. 18, the contact load is smaller in the 
former case than in the latter case when the vehicle is running on a bad 
road causing a high frequency road surface input. 
FIGS. 19(a), 19(b) and 19(c) shows contact loads variations with respect to 
respective load variation centers at damping coefficient ratios (TEN/COM) 
of 4.0, 1.5 and 0.8. It can be seen from these figures that the deviation 
of the load variation center in the load decreasing direction increases 
when the damping coefficient ratio is equal to or greater than 1.0. 
According to the invention where the damping coefficient ratio (for 
example, 0.8) is equal to or less than 1.0 there is almost no deviation of 
the load variation center in the load decreasing direction. 
(II) Ordinary control 
When the brake switch is OFF, the ordinary control is performed. The 
ordinary control is performed when the conditions (i) to (iii) described 
hereinbefore are not fulfilled even though the brake switch is ON. 
During the ordinary control, each of the pulse motors 3 is driven toward 
the target damping force position P calculated from Equation (1). 
The ordinary control will be described with reference to the timing chart 
of FIG. 17. As shown in the timing chart of FIG. 17, each of the pulse 
motors 3 is driven toward the extension side target damping force position 
P so as to control the corresponding shock absorber SA toward the 
extension side hard range HS when the control signal V exceeds the 
extension side control dead zone V.sub.NC-T. 
When the control signal v is between the extension side control dead zone 
V.sub.NC-T and the compression side control dead zone V.sub.NC-C, each of 
the pulse motors 3 is controlled to control the corresponding shock 
absorber SA to the soft range SS. 
When the control signal is less than the compression side control dead zone 
V.sub.NC-C, each of the pulse motors 3 is driven toward the compression 
side target damping force position P so as to control the corresponding 
shock absorber SA to the compression side hard range SH. 
In the timing chart of FIG. 17, the character (a) indicates a range where 
the control signal V changes from a negative value (downward) to a 
positive value (upward) and the relative speed remains negative (the shock 
absorber SA is in the compression stroke). In this range (a), the shock 
absorber SA is controlled to the extension side hard range HS based on the 
direction of the control signal V so as to provide a soft characteristic 
on the compression side which corresponds to the stroke of the shock 
absorber SA. 
The character (b) indicates a range where the control signal V remains 
positive (upward) and the relative speed changes from a negative value to 
a positive value (the shock absorber SA is in the extension stroke). In 
this range (b), the shock absorber SA is controlled to the extension side 
hard range HS based on the direction of the control signal V so as to 
provide a hard characteristic in proportional to the value of the control 
signal V on the extension side which corresponds to the stroke of the 
shock absorber SA. 
The character (c) indicates a range where the control signal V changes from 
a positive value (upward) to a negative value (downward) and the relative 
speed remains positive (the shock absorber SA is in the extension stroke). 
In this range (c), the shock absorber SA is controlled to the compression 
side hard range SH based on the direction of the control signal V so as to 
provide a soft characteristic on the extension side which corresponds to 
the stroke of the shock absorber SA. 
The character (d) indicates a range where the control signal V remains 
negative (downward) and the relative speed changes from a positive value 
to a negative value (the shock absorber SA is in the extension stroke). In 
this range (d), the shock absorber SA is controlled to the compression 
side hard range SH based on the direction of the control signal V so as to 
provide a hard characteristic in proportional to the value of the control 
signal V on the compression side which corresponds to the stroke of the 
shock absorber SA. 
As described above, the damping force characteristic control of this 
embodiment is the same as the control utilizing the sky hook theory where 
a hard characteristic is provided on the side of the stroke of the shock 
absorber SA when the control signal v calculated based on the sprung mass 
vertical speed and the relative speed between the sprung mass and the 
unsprung mass have the same sign (ranges b, d), and a soft characteristic 
is provided on the side of the stroke of the shock absorber SA when the 
control signal V and the relative speed have opposite signs (ranges a, c). 
Furthermore, in this embodiment, the deceleration force characteristic is 
switched without the operation of the pulse motor 3 upon a change from the 
range (a) to the range (b) and upon a change from the range (c) to the 
range (d). 
As described above, this embodiment of the vehicle suspension system of the 
invention has advantages listed as follows: 
1 Since the damping force characteristic is controlled to the compression 
side hard range SH where the damping coefficient ratio of the extension 
side damping force with respect to the compression side damping force is 
equal to or less than 1.0 (that is, the extension side damping force is 
less than the compression side damping force) in the presence of vehicle 
braking, it is possible to prevent contact load reductions resulting from 
vehicle behaviors so as to ensure sufficient braking force. 
2 Since the damping force characteristic control is switched from the 
control suitable in the presence of vehicle braking to the ordinary 
control when the vehicle pitch rate exceeds a predetermined threshold 
valve even upon the application of braking to the vehicle, it is possible 
to suppress the vehicle pitching motions in the presence of vehicle 
braking. 
3 Since the damping force characteristic is switched at a smaller frequency 
as compared to the damping force characteristic control utilizing the 
conventional sky hook theory, it is possible to increase the control 
response speed and also to improve the durability of the pulse motors 3. 
Description will be made to a second embodiment of the vehicle suspension 
system of the invention. 
The second embodiment is substantially the same as the first embodiment 
except for the mode of the control performed in the control unit 4. Thus 
like reference numerals have been applied with respect to the equivalent 
components and a detailed description therefor will not be repeated here. 
The braking operation of the control unit used in the second embodiment 
will be described with reference to the flow diagram of FIG. 20 and the 
timing chart of FIG. 21. 
In the step 201 of the flow diagram of FIG. 15, a determination is made as 
to whether or not the signal fed from the brake switch BS is on. If the 
answer to this question is "YES", then the control is transferred to the 
step 202. 
In the step 202, a determination is made as to whether or not the vehicle 
speed Sv exceeds a predetermined vehicle speed ON threshold valve S.sub.ON 
(for example, a value ranging from 30 to 40 Km/h). If the answer to this 
question is "YES", then the control is transferred to the step 203. 
In the step 203, a determination is made as to whether or not the ABS 
operation is performed. If the answer to this question is "YES", then the 
control is transferred to the step 204. 
In the step 204, a determination is made as to whether or not the vehicle 
speed Sv exceeds a predetermined vehicle speed OFF threshold value 
S.sub.OFF (0 Km/h). If the answer to this question is "YES", then the 
control is transferred to the step 205. 
In the step 205, a control suitable in the presence of vehicle braking is 
performed for the suspension system. That is, the pulse motors 3 are 
driven to control the damping force control positions of the shock 
absorbers SA on the front and rear road wheel sides to the adjuster 
positions corresponding to a HARD1 where the damping force coefficient 
ratio TEN/COM of the extension stroke damping force (TEN) with respect to 
the compression stroke damping force (COM) is equal to or less than 1.0, 
as indicated by the dotted curve of the damping force characteristic 
diagram of FIG. 22. Here, one control cycle is terminated. 
If the question inputted in the step 201 is "NO" (the brake switch BS is 
off), then the control is transferred to the step 207, where an ordinary 
control suitable in the absence of vehicle braking is performed for the 
suspension system. Here, one control cycle is terminated. 
If the answer to the question inputted in the step 202 or 203 is "NO" (when 
the vehicle speed Sv is equal to or less than the vehicle speed ON 
threshold value S.sub.ON or when the ABS control is not performed), then 
the control is transferred to the step 206. In the step 206, a 
determination is made as to whether or not the braking application 
decision signal, that is, the vehicle deceleration G.sub.FR ' exceeds a 
threshold value G.sub.-ON. If the answer to this question is "YES", then 
the control is transferred to the step 204. Otherwise, the control is 
transferred to the step 207. 
If the answer to the question inputted in the step 204 is "NO" (the vehicle 
speed Sv is equal to or less than the vehicle speed OFF threshold value 
S.sub.OFF), then the control is transferred to the step 207. 
Here, one control cycle is terminated. Thereafter, a similar control cycle 
is repeated. 
The operation of the control unit 4 will be described with reference to the 
timing chart of FIG. 16. 
(1) Control in the presence of vehicle braking 
A control suitable in the presence of vehicle braking is performed when the 
brake switch is ON and at least one of the following conditions (iv) and 
(v) is fulfilled: 
(iv) The vehicle speed Sv exceeds the vehicle speed ON threshold valve 
S.sub.ON, and the ABS operation is performed or the deceleration G.sub.FR 
' exceeds the threshold value G.sub.-ON. That is, the vehicle speed is 
fast, and rapid braking is applied to the vehicle. 
(v) The vehicle speed Sv remains greater than the vehicle speed OFF 
threshold value S.sub.OFF and the deceleration G.sub.FR ' exceeds the 
threshold value G.sub.-ON even when the vehicle speed Sv is equal to or 
less than the vehicle speed ON threshold value S.sub.ON. That is, rapid 
braking is applied to the vehicle even when the vehicle speed is slow. 
(II) Ordinary control 
When the brake switch is OFF, the ordinary control is performed. The 
ordinary control is performed when the conditions (iv) and (v) described 
hereinbefore are not fulfilled even though the brake switch is ON. 
With the second embodiment, thus, the vehicle pitching motions cannot be 
suppressed in the presence of vehicle braking. However, it is possible to 
prevent contact load reductions resulting from vehicle behaviors so as to 
ensure sufficient braking force and also simplify the control as compared 
to the first embodiment so as to reduce the cost. 
Although the invention has been described in connection with specified 
embodiments, it is to be understood that the invention is not limited in 
any way to the illustrated embodiments and it is evident that many 
alternatives, modifications and variations will be apparent to those 
skilled in the art. Accordingly, it is intended to embrace all 
alternatives, modifications and variations that fall within the scope of 
the appended claims. 
For example, in the step 109 of the flow diagram of FIG. 15 and the step 
206 of the flow diagram of FIG. 20, a determination is made as to whether 
or not the braking application decision signal, that is, the vehicle 
deceleration G.sub.FR ' exceeds a threshold value G.sub.-ON. The vehicle 
deceleration G.sub.FR ' may be replaced with a tire slip ratio TS. 
FIG. 24 is a block diagram showing a signal processing circuit used in 
calculating the tire slip ratio TS. As shown in FIG. 24, the road wheel 
speed pulse signal .theta..sub.P is fed from the road wheel speed sensor 5 
to an F/V converter which converts it into a corresponding voltage signal. 
The converted signal is fed to a low pass filter LPF which removes noise 
from the converted signal. This signal is used to calculate the road wheel 
speed V .theta. and the pseudo vehicle speed V.sub.IM. The tire slip ratio 
TS is calculated from the following equation (2): 
EQU TS=(V.sub.IM -V.theta.)/V.sub.IM (2) 
It may be considered that a braking force is produced when the tire slip 
ratio has a positive value and a driving force is produced when the tire 
slip ratio is 0, as shown in FIG. 16. 
It is, therefore, possible to judge the application of braking to the 
vehicle by a determination as to whether or not the tire slip ratio TS 
exceeds a predetermined slip ratio threshold value TS.sub.ON (set 
substantially at 0) in the step 109. 
The slip ratio value calculated for the antiskid control may be used as the 
tire slip ratio TS. 
Although the invention has been described in connection with the case where 
the adjuster position where the damping coefficient ratio TEN/COM of the 
extension stroke damping force (TEN) with respect to the compression 
stroke damping force (COM) is equal to or less than 1.0 is set in the 
compression side hard range SH, it is to be understood that it may be set 
in a wider range extending from the compression side hard range SH to the 
extension side hard range HS, as shown in FIG. 23. 
USEFULNESS IN THE FIELD 
The vehicle suspension system of the invention is useful as a suspension 
system for suspending the front or rear road wheels of a passenger car.