Method for controlling active suspension system on the basis of rotational motion model

In order to cope appropriately with transient changes in the rotational angle of a vehicle body in various traveling states of the vehicle body, the rotational angle of the vehicle body is presumed on the basis of a beforehand stored equation of motion on the vehicle body and a sensed value of an acceleration of the vehicle body and supply of a fluid to suspension cylinders is controlled in accordance with the presumed rotational angle.

BACKGROUND OF THE INVENTION 
The present invention relates to control of a suspension system of an 
automobile and more particularly to an active suspension control method 
for controlling an active suspension system by injection or discharge of a 
fluid into or from suspension cylinders. 
Conventionally, suspension cylinders are controlled by feedback of control 
based on lateral acceleration or vertical acceleration of a sprung mass 
supported above each suspension cylinder or by feed forward control based 
on steering angle or vehicle speed. These types of techniques and related 
techniques are disclosed, for example, in Japanese Patent Un-examined 
Publications JP-A-63-242707 and JP-A-63-269709. 
The conventional active suspension control methods for suspension cylinder 
control have the following problems: 
(i) They do not allow for the transient roll and pitch characteristics of a 
vehicle body during vehicle traveling, and so a control command from a 
controller cannot clearly follow changes in the roll and pitch angles. 
(ii) They do not allow for changes in the attitude of the vehicle body 
caused by different kinds braking, such as engine braking, foot braking or 
hand braking, and so changes in the pitch angle cannot be sufficiently 
suppressed. 
(iii) They do not allow for the information from an acceleration sensor 
when the vehicle body is tilted, and so cannot sufficiently suppress 
changes in the attitude of the vehicle body when the same is tilted, for 
example, on a slope. 
(iv) They do not allow for manual control of active suspensions and cannot 
adjust the lengths of the suspension cylinders when the vehicle is at a 
stop. 
(v) They do not allow for changes in the energy required for operation of 
the suspension cylinders, and the discharge and so injection of fluid from 
and into the suspension cylinders do not closely follow rapid changes in 
the attitude of the vehicle body. 
(vi) They do not allow for the inability to control the suspension 
cylinders caused by the failure of means for feeding fluid to the 
suspension cylinders or breakage of a main fluid tank. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an effective, 
economical high-safety active suspension control method which solves those 
problems which are inherent in the conventional techniques, copes 
appropriately with transient changes in the roll and pitch angles in 
various traveling states of the vehicle body and also provides for manual 
adjustment of the attitude of the vehicle body. 
In order to achieve the above object, the inventive active suspension 
control method has the following features: 
(1) In a vehicle having a fluid supply unit for discharging and injecting 
fluid from and into suspension cylinders disposed at the respective wheels 
of the vehicle, a main tank for storage of fluid, an acceleration sensor 
for sensing an acceleration acting on the vehicle body during traveling 
and a controller for control of the fluid supply unit in accordance with 
the signal from a sensor, an equation of motion representing the rotation 
of the vehicle body is stored beforehand, the rotational angle of the 
vehicle body is presumed on the basis of the equation of motion and an 
acceleration signal from the acceleration sensor, and the fluid supply 
unit is controlled in accordance with the presumed rotational angle to 
thereby control the rotational angle of the vehicle body. 
(2) The presumed rotational angle is corrected with a signal from a 
vertical acceleration sensor. 
(3) A brake pressure sensor is provided which determines whether the 
vehicle is being braked by foot or by the engine when the vehicle is in a 
braked state. The value of the presumed rotational angle is corrected with 
a signal from the brake pressure sensor. 
(4) A roll angle sensor which senses the roll angle of the vehicle body and 
a pitch angle sensor which senses the pitch angle of the vehicle body are 
provided. The presumed rotational angle obtained from the acceleration 
sensor is corrected with signals from the roll and pitch angle sensors. 
(5) The controller calculates the energy required for discharge and 
injection of fluid by the fluid supply unit and changes the capability of 
supply of fluid to the suspension cylinders in accordance with the results 
of the calculation. 
(6) Provided on the vehicle body are a fluid path opening and closing unit 
which controls the discharge and injection from and into the main tank in 
accordance with commands from the controller and a main tank pressure 
sensor which senses the pressure in the main tank. When the pressure in 
the main tank as indicated by the signal from the main tank pressure 
sensor is lower than a predetermined value, the fluid path opening and 
closing unit is closed to stop the discharge and injection of fluid. 
(7) Provided on the vehicle body is an active suspension operating unit 
which is operated by the driver in the vehicle compartment to adjust the 
attitude of the vehicle body arbitrarily when the vehicle is at a stop. 
(8) Vehicle height sensors are provided at the corresponding wheels of the 
vehicle instead of the vertical acceleration sensors to adjust the 
presumed rotational angles with signals from the height sensors. 
(9) In the closing operation of the fluid path opening and closing unit of 
the item (6), the controller returns the vehicle heights at the 
corresponding wheels to the original values and then starts to close the 
fluid path opening and closing unit. 
In the item (1), a presumption equation obtained from the equation of 
motion on the vehicle body rotation presumes a change in the rotational 
angle of the vehicle body from the acceleration acting on the vehicle 
body. Thus, the suspension cylinders act to suppress changes in the 
attitude of the vehicle body and particularly suppresses such changes 
highly even in a transient traveling state. 
In the items (2) and (8), the presumed value of rotational angle of the 
vehicle body is prevented from failure of indication of the actual roll or 
pitch angle due to changes in the characteristics of the vehicle and the 
suspensions. 
The actual roll or pitch angle is sensed from the vehicle heights. Changes 
in the characteristics of the vehicle body are calculated from a deviation 
of the actual roll or pitch angle from the pressured value. The parameters 
of the presumption equation are adjusted with the calculated changes, so 
that changes in the attitude angle of the vehicle body are suppressed even 
if changes in the characteristics, such as a change in the load on the 
vehicle, occur. 
The item (3) allows for the difference between pitching torques occurring 
when the braking methods used at the same longitudinal accelerations 
differ, using a signal from the sensor which senses the braked state. This 
difference prevents the pitch angle from changing. 
In the item (4), the actual acceleration component included in a signal 
from the acceleration sensor is separated using a signal from the attitude 
angle sensor. Thus, the acceleration is sensed correctly to prevent 
possible control based on wrong information. 
In the item (5), energy required for control of the suspension cylinders is 
presumed on the basis of signals from the sensors provided at respective 
required positions in the vehicle body and the extent of operation of the 
actuators. A signal is output which generates an engine output 
corresponding to the presumed required energy, so that a rapid response to 
a rapid change in the attitude of the vehicle body is provided to maintain 
the control capability constant. 
In the items (6) and (9), if a signal from the pressure sensor in the main 
tank indicates a pressure lower than a predetermined value for a given 
interval, the controller determines that an uncontrollable state occurs 
and gives a command to close all the flow control valves to thereby stop 
the functions of the active suspensions and hence to increase safety. 
In the item (9), a signal from the manual operation unit is used for 
calculation of a quantity of operation to control the flow of fluid 
instead of the acceleration signal. Thus, any attitude of the vehicle body 
can be realized when the vehicle is at a stop.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a block diagram of an active suspension control apparatus to 
which the present invention is applied. The apparatus includes a brake 
pressure sensor 1, a transversal acceleration sensor 2 which senses an 
acceleration which acts transversally on the traveling vehicle, a 
longitudinal acceleration sensor 3 which senses an acceleration which acts 
in the longitudinal direction on a traveling vehicle, a manual operation 
unit 4 provided in the vehicle compartment in which the user manipulates 
the attitude of the vehicle body, four vertical acceleration sensors 5a-5d 
for sensing vertical accelerations of the vehicle body at the 
corresponding wheels, a vehicle speed sensor 6 which senses the traveling 
speed of the vehicle, an active suspension controller 7 which controls a 
vehicle body attitude changing unit on the basis of the respective signals 
from the sensors, a fluid supply unit 8 which supplies fluid to suspension 
cylinders to be described in more detail later, valve control units 9a-9d 
which control the passage of fluid, fluid spring chambers 10a-10d, 
suspension cylinders 11a-11d the lengths of which are adjusted by 
injecting and discharging fluid thereinto and therefrom, wheels 12a-12d, 
an engine control unit 13 which controls the output of the engine, etc., a 
main fluid tank 14 which maintains the fluid at high pressure, fluid path 
resistance generators 15a-15d, a vehicle body attitude sensor 16 which 
senses the attitude of the vehicle body, and a main tank pressure sensor 
17 which senses the pressure in the main fluid tank 14. 
The active suspension controller 7 receives from the respective sensors the 
following pieces of information: namely, braked state information S14 from 
the brake pressure sensor 1, transversal acceleration information S15 on 
the transversal acceleration acting on the vehicle body from the 
transversal acceleration sensor 2, longitudinal acceleration information 
S16 from the longitudinal acceleration sensor 3, manual operation commands 
S1-S3 from the manual operation unit 4, vehicle speed information S4 from 
the vehicle speed sensor 6, information S8 on the pressure of fluid in the 
main fluid tank 14 from the main tank pressure sensor 17, pieces of 
information S17a-S17d on vertical acceleration from vertical acceleration 
sensors 5a-5d, and roll angle information S18a and pitch angle information 
S18b relating to the vehicle body from the vehicle body attitude angle 
sensor 16. 
On the basis of the above pieces of information, the active suspension 
controller 7 outputs flow rate designating signals S9a-S9d to the valve 
control units 9a-9d which control the injection and discharge of fluid 
into and from the suspension cylinders 11a-11d and outputs a signal S10 to 
the engine control unit 13 to change the engine output in accordance with 
flow rate. 
The suspension cylinders 11a-11d position the body with respect to the 
wheels 12a-12d vertically in accordance with commands from the active 
suspension controller 7. 
FIG. 2 is a block diagram of the active suspension controller of FIG. 1. 
The active suspension controller 7 includes an acceleration signal 
correction unit 18 which corrects the acceleration signal, an attitude 
angle presuming unit 21 comprising a roll angle presuming unit 22 which 
produces the presumed roll angle value and a pitch angle presuming unit 23 
which produces the presumed pitch angle value, a pitch angle correcting 
unit 24 which corrects the presumed pitch angle according to the braked 
state, and an operation quantity calculating unit 25 which generates and 
supplies a flow designating signal to the valve control units 9a-9d and an 
engine output adjusting signal to the engine control unit 13. 
The engine control unit 13 of FIG. 1 is connected with the fluid supply 
unit 8 and increases the engine output and the output of the fluid supply 
unit 8 in response to an output increase command from the active 
suspension controller 7 so as to supply enough fluid to the suspension 
cylinders 11a-11d. Conversely, in response to an output decrease command 
from the active suspension controller 7, the engine control unit 13 
decreases the engine output and the output of the fluid supply unit 8. 
Control of the roll angle in the active suspension control unit of FIGS. 1 
and 2 is provided as follows: The transversal acceleration information S15 
from the transversal acceleration sensor 2 is corrected with vehicle body 
attitude angle information S18a and S18b in the transversal acceleration 
signal correcting unit 19 of the acceleration signal correcting unit 18, 
the resulting signal is input to the roll angle presuming unit 22 of the 
attitude angle presuming unit 21 to calculate the presumed value S5 of the 
roll angle. 
On the basis of the presumed value S5, the operation quantity calculating 
unit 25 calculates flow and required energy and outputs flow designating 
signals S9a-S9d and an engine output adjusting signal S10. 
Control of the pitch angle is provided as follows: Longitudinal 
acceleration information S16 from the longitudinal acceleration sensor 3 
is corrected with vehicle body attitude angle information S18a and S18b in 
the longitudinal acceleration signal correcting unit 20 of the 
acceleration correcting unit 18 and the resulting signal is input to the 
pitch angle presuming unit 23 of the attitude angle presuming unit 21 for 
calculating the presumed pitch angle value S6, which is then corrected 
with brake pressure information S14 in the pitch angle correcting unit 24 
and the resulting signal is input to the operation quantity calculating 
unit 25 to be subjected to processing similar to that in the case of the 
roll angle. 
FIG. 3 is a block diagram indicative of the operation and structure of the 
acceleration signal correcting unit 18. 
The unit 18 includes transversal acceleration correcting unit 19 and 
longitudinal acceleration correcting unit 20, and is connected to 
transversal acceleration sensor 2, longitudinal acceleration sensor 3, 
vehicle body attitude angle sensor 16, roll angle presuming unit 22 and 
pitch angle presuming unit 23. 
The vehicle attitude angle sensor 16 senses the attitude angles .theta.x 
and .theta.y of the vehicle body in the x and y directions. 
The signals from the transversal and longitudinal acceleration sensors 2 
and 3 contain a gravity component which depend on the attitude angle of 
the vehicle body and is different in direction from the actual 
acceleration acting on the vehicle body. Thus, the signals A'y, A'x, from 
the sensors 2 and 3 are passed through the transversal and longitudinal 
acceleration correcting units 19 and 20 to provide accurate transversal 
and longitudinal accelerations Ay and Ax which are not dependent on the 
attitude of the vehicle. 
FIG. 4 is a block diagram indicative of the operation and structure of the 
roll angle presuming unit 22 of the attitude angle presuming unit 21. The 
roll angle presuming unit 22 is connected to transversal acceleration 
correcting unit 19 and vertical acceleration sensors 5a-5d at the wheels, 
and include relative vehicle height calculating units 27-30 at the wheels, 
actual roll angle calculating unit 32, parameter correcting unit 31, and 
presumed roll angle value correcting unit 26. 
Substituting the transversal acceleration signal Ay from the transversal 
acceleration correcting unit 19 into the following equation of motion, 
obtained from a model of roll motion: 
EQU Ir.phi..sub.2 +Cr.phi..sub.1 +(Kr-Ms.multidot.Hr.multidot.g).phi..sub.0 
=Ms.multidot.Ay.multidot.Hr (1) 
where 
Ir is the roll moment of inertia; 
.phi..sub.0 is the roll angle; 
.phi..sub.1 is the roll angular velocity; 
.phi..sub.2 is the roll angular acceleration; 
Cr is the roll-equivalent damping coefficient; 
Kr is the roll stiffness; 
Ms is the on-sprung weight; 
Hr is the distance between roll axis centroids; 
g is the gravitational acceleration; and 
Ay is the lateral acceleration signal; 
and solving the equation of motion (1) numerically, we obtain the presumed 
roll angle value. 
The actual roll angle calculating unit 32 of the roll angle presuming unit 
22 calculates the actual roll angle Or on the basis of relative vehicle 
heights Z.sub.2fl, Z.sub.2rl, Z.sub.2fr, Z.sub.2rr obtained by passing 
vertical acceleration signals Z.sub.1fl, Z.sub.1rl, Z.sub.1fr, Z.sub.1rr 
from the on-spring vertical acceleration sensors 5a-5d at the wheels 
through the relative vehicle height calculating units 27-30, and the roll 
moment Ir of equation of motion (1) is corrected with the actual roll 
angle .theta.r and the presumed roll angle value .phi.. The function 
fr(.phi., .theta..sub.r) which is the roll moment correcting coefficient 
in FIG. 4 has a characteristic of FIG. 15 where the abscissa is given by 
EQU .intg.T.sub.1 (.phi.-.theta..sub.r)dt-.intg.T.sub.2 (.phi.-.theta..sub.r)dt 
where 
T.sub.1 is an integrating interval comprising a time from Ay=0 to the next 
Ay=0; 
T.sub.2 is an integrating interval comprising a time representing a half 
period of Ay after T.sub.1. 
By this construction, the roll angle is correctly presumed even if the 
motion characteristics of the vehicle body changes. 
Information S5 on the presumed roll angle is output to the operation 
quantity calculating unit 25. 
FIG. 5 is a block diagram indicative of the operation and structure of the 
pitch angle presuming unit 23 of the attitude angle presuming unit 21. 
The pitch angle presuming unit is connected to longitudinal acceleration 
correcting unit 20 and vertical acceleration of sprung sensors 5a-5d, and 
includes wheel relative vehicle height calculating units 34-37, actual 
pitch angle calculating unit 39, parameter correcting unit 38, and pitch 
angle presuming unit 23. 
Inserting the longitudinal acceleration signal Ax from the longitudinal 
acceleration correcting unit 20 into the following equation of motion 
obtained from a model of motion: 
EQU Tp.lambda..sub.2 +Cp.lambda..sub.1 
+(Kp-Ms.multidot.Hp.multidot.g).lambda..sub.0 
=Ms.multidot.Ax.multidot.Hp(2) 
where 
Ip is the pitch moment of inertia; 
.phi..sub.0 is the pitch angle; 
.phi..sub.1 is the pitch angular velocity; 
.phi..sub.2 is the pitch angular acceleration; 
Cp is the pitch-equivalent damping coefficient; 
Kp is the pitch stiffness; 
Hp is the distance between pitch axis centroids; and 
Ms is the sprung weight; 
and solving the equation of motion (2) numerically, we have a presumed 
value .lambda. of the pitch angle. 
Also, in the pitch angle presuming unit 23, the pitch moment Ip of the 
equation of motion (2) is corrected with the presumed value .lambda. and 
the actual pitch angle .theta.p obtained from vertical acceleration 
signals Z1fl, Z1rl, Z1fr, Z1rr as in the roll angle presuming unit 22. A 
function f.sub.p (.lambda., .theta.p) comprising a pitch moment correcting 
coefficient in FIG. 5 has a characteristic shown in FIG. 16 where the 
abscissa of FIG. 16 is given by 
EQU .intg.T.sub.3 (.lambda.-.theta..sub.p)dt-.intg.T.sub.4 
(.lambda.-.theta..sub.p)dt 
where 
T.sub.3 is an integrating interval comprising a time from Ax=0 to the next 
AX=0; and 
T.sub.4 is an integrating interval comprising a time of a half period of Ax 
after T.sub.3. 
The presumed pitch angle information S6 is output to the manipulation 
quantity calculating unit 25. 
FIG. 6 is a block diagram indicative of the operation and structure of the 
pitch angle correcting unit 24. The pitch angle correcting unit 24 is 
connected to pitch angle presuming unit 23 and brake pressure sensor 1, 
and operates as a computing unit which processes received pieces of 
information. 
The pitch angle correcting unit 24 corrects the presumed pitch angle 
information S6 from the pitch angle presuming unit 23 as a function of a 
signal Pb indicative of the intensity of foot braking from the brake 
pressure sensor 1 to provide the resulting signal .lambda..sub.3. 
FIG. 7 is a graph of the relationship between signal Pb indicative of the 
intensity of foot braking and function h(Pb) of the signal Pb. 
FIG. 8 is a block diagram of the operation and structure of the operation 
quantity calculating unit 25. The operation quantity calculating unit 
includes manual valve operation quantity calculating unit 40, valve 
operation quantity calculating unit 41, operation quantity switching unit 
42, fluid output unit 43, and required energy presuming unit 44, and is 
connected to valve control units 9a-9d and engine control unit 13. 
The manual valve operation quantity calculating unit 40 receives roll 
direction movement quantity information S1 and pitch direction movement 
quantity information S2 from the manual operation unit 4 and calculates 
flow designating signals S11a-S11d for manual operation. 
The valve operation quantity calculating unit 41 receives presumed roll 
angle value information S5 and presumed pitch angle value information S7 
instead of the roll direction movement quantity information S1 and pitch 
direction movement quantity information S2 and calculates flow designating 
signals S12a-S12d. 
The operation quantity switching unit 42 receives switching signal S3 and 
vehicle speed information S4 from the manual operation unit 4 and 
determines whether the two kinds of flow designating signals S11, S12 
should be used for valve control. 
The fluid output unit 43 determines whether the signals determined by the 
operation quantity switching unit 42 should be output to the valve control 
units 9a-9d in accordance with a control stop designating signal S8 from 
the main tank pressure sensor 17. 
The required energy presuming unit 44 presumes a required quantity of 
energy on the basis of flow designating signals S9a-S9d from the fluid 
output unit 43 and outputs a signal S10 to instruct the engine control 
unit 13 to increase the output in accordance with the presumed quantity. 
FIG. 9 is a block diagram indicative of the operation of the manual valve 
operation quantity calculating unit 40. The unit 40 calculates valve 
operation quantities S11a-S11d from roll and pitch direction movement 
quantities S1-S2 in accordance with the equations in FIG. 9. Reference 
characters S'11a-S'11d of FIG. 9 denote the values of S11a-S11d obtained 
one preceding sample interval before. Reference characters l and k each 
represent half of the wheel base. 
FIG. 10 is a block diagram of the operation of the valve operation quantity 
calculating unit 41. The unit 41 calculates valve operation quantities 
S12a-S12d using presumed roll and pitch angle information S5, S7 like the 
manual valve operation quantity calculating unit 40. 
FIG. 11 is a flowchart indicative of the processing operation of the 
operation quantity switching unit 42. The manual operation unit 4 is set 
to output a manual switching designating signal S3 other than 0 only when 
the manual operation is performed. If S3 is not 0 (step 450) and vehicle 
speed information S4 is 0 (step 460), S11a-S11d are selected as valve 
operation quantities S9a-S9d (step 480). If otherwise, S12a-S12d are 
selected as valve operation quantities S9a-S9d (step 470) to permit manual 
operation. 
FIG. 12 is a flowchart indicative of the operation of the fluid output unit 
43. The control designating signal S8 from the main tank pressure unit 17 
has a value other than 0 when the main tank pressure is lower than a 
preset value for a given interval (step 490). In this case, all the valve 
operation quantities S9a-S9d are changed to 0 to inhibit valve control 
(step 500). If otherwise (step 490), the valve operation quantities 
S9a-S9d are output to the valve control units 9a-9d (step 510). Before 
valve control is inhibited, vehicle heights may be adjusted beforehand to 
the corresponding set references. 
FIG. 13 is a block diagram indicative of the operation and structure the 
required energy presuming unit 44 which calculates required quantities of 
energy from the valve operation quantities S9a-S9d. The unit 44 calculates 
a signal S10 to the engine control unit 13 as a function of the sum of the 
absolute values of the valve operation quantities S9a-S9d. The engine 
control unit 13 receives a signal 10 indicative of an increment in the 
throttle opening to increase the engine output to thereby increase the 
fluid supply ability of the fluid supply unit 8. 
FIG. 14 is a graph of a function h(x) for calculating the signal S10. 
The active suspension control method which the present invention 
contemplates is realized by using the vehicle height sensors in 
substantially a similar manner to that mentioned above, instead of the 
vertical acceleration sensors of FIG. 1. 
Thus, the present embodiment produces the following effects: 
(1) Since the discharge and injection of fluid from and into the suspension 
cylinders will accurately follow the transient characteristic of the roll, 
a change in the roll angle is greatly reduced; 
(2) Since the parameters of an equation of presumed roll angle are changed 
depending on vehicle heights, the presumed roll angle is corrected with 
suspension characteristic or load change; 
(3) Since the discharge and injection of fluid from and into the suspension 
cylinders will accurately follow the transient pitch characteristic, a 
change in the pitch angle is greatly reduced; 
(4) The presumed pitch angle is corrected with suspension characteristic or 
load change by changing the parameters of the equation of presumed pitch 
angle depending on vehicle height; 
(5) Since the fluid designating signals to the suspension cylinders are 
corrected depending on the braked state, a change in the attitude during 
braking is greatly reduced; 
(6) Even when the vehicle body is tilted, correct acceleration information 
is obtained and for example, a change in the attitude of the vehicle body 
is suppressed; 
(7) Since the engine output changes following a change in the required 
energy for suspension control and the output of the fluid supply unit is 
changed, the control performance is improved when the attitude of the 
vehicle body is changed rapidly; 
(8) Since suspension control is inhibited after vehicle height is adjusted 
when the fluid pressure in the main tank is low, safety is improved and 
engine output is suppressed when the fluid supply unit fails or when the 
main tank is broken; and 
(9) Since the suspension cylinders can be manually operated, the field of 
view is ensured when the vehicle starts forwardly or backwardly, and 
getting out of a snowy road or a muddy place is easy. 
According to the present invention, transient changes in the roll and pitch 
angles in various traveling states of the vehicle are appropriately coped 
with and the attitude angle of the vehicle body can be manually adjusted. 
Thus, effective economical high-safety active suspension control is 
achieved.