Device for dynamically controlling the trim of a vehicle

This device is intended for a vehicle in which the wheels (1) are each equipped with a damper (2) and with a suspension spring (3); it comprises, for each wheel, an actuator (4) interacting with the spring, and provided with a position sensor (5), an electro-hydraulic interface (6) for controlling the actuator, this interface being connected to the actuator, to a hydraulic unit (8) of the vehicle and to a source of electrical energy, and a computer (7) for trim management, connected to the interfaces, and programed to take account of each parameter of the driving of the vehicle, especially its speed and the angle of its steering wheel; on the basis of these parameters, the computer controls the interfaces (6) and the actuators within the context of loops for slaving the latter to reference output travels, corresponding to maintaining the vehicle in predetermined trim stored in the memory of the computer. This trim system makes it possible to prevent rolling and pitching during the phases of acceleration, braking and cornering, without affecting the comfort of the occupants.

BACKGROUND OF THE INVENTION 
The subject of the present invention is a device for controlling the trim 
of a vehicle with four wheels, each equipped with a damper and with a 
suspension spring. 
It is known that the operation and characteristics of the vehicle 
suspension are governed by three key parameters: the trim, the stiffness, 
provided in mechanical systems by a spring, and the damping, provided by a 
hydraulic damper or a friction device. 
In order to stop the vehicle from rolling, especially during cornering, 
most vehicles are currently equipped with anti-roll bars of fixed 
stiffness, with one bar per axle, this bar being connected to the springs 
of the suspension. However, in some cases, these bars encounter siting 
difficulties, and their overall size may be a problem. What is more, as 
soon as the loads are not strictly symmetrical on the two wheels of one 
axle, the stiffness of the anti-roll bar stiffens the suspension, which 
constitutes another significant drawback of this type of embodiment. 
More recently, three types of suspension have been proposed, aiming in 
general to improve the road holding of the vehicle and the comfort of the 
occupants. Thus, a first type of so-called "active" suspension is 
contrived so that it anticipates the dynamics of the connection of the 
vehicle with the ground with regard to the essentially random layout of 
the road, incorporating the parameters relating to speed, turn radius, 
load, acceleration/deceleration, ride height. For this purpose the 
suspension includes a servo valve associated with a hydraulic actuator and 
driven by a control solenoid valve. However, this suspension does not have 
a spring and therefore does not fulfill the stiffening function. What is 
more, it exhibits the drawback of requiring very significant amounts of 
hydraulic and electrical energy in order to fulfill the stiffening 
function despite the absence of the spring, so as to allow the stored-up 
energy to be restored. In effect, the electrical energy required may be of 
the order of 20 to 30 Kw. 
The object of the so-called "semi-active" suspension, while retaining the 
existing elements and especially the dampers, is to vary the reference 
point at which the suspension elements bear, by incorporating all or some 
of the aforementioned parameters. For competition vehicles, the prime 
objective is therefore to maintain the ground effect as far as possible. 
In this "semi-active" suspension, the actuator is coupled to a 
hydropneumatic damper, which fulfills the stiffening function by the 
compression of the gas. The damping function may be fulfilled by a 
restriction between the actuator and the hydropneumatic cylinder, and 
allows the ground clearance to be corrected. 
This device, used in some top-of-the-range touring vehicles, makes it 
possible to manage the rolling and pitching in ways other than with bars 
of fixed stiffness. Depending on the required dynamics, it is then 
possible to assume that the energy required is brought down to 
approximately 1/5 to 1/10 of the energy necessary for an active 
suspension. 
Nevertheless, a "semi-active" suspension does not allow dynamic correction 
of the trim, because it is only possible to adjust it for two or three 
predetermined ground clearance values. Furthermore, its time constants are 
high. 
Finally, a third type of embodiment recently mounted on top-of-the-range 
touring vehicles, is so-called "variable" suspension, which includes 
dampers with a programed damping law, instead of the conventional dampers. 
Such a device is passive, in contrast to the two aforementioned types of 
suspension, because the only external energy required is that which causes 
the orifice plates of the damper to vary, upon a command from a suitably 
programed computer (it being possible for this suspension to be associated 
with anti-roll bars). 
However, the variable suspension does not incorporate the stiffness 
function, because the computer only drives the damping, that is to say the 
flow rate of fluid through a restriction. Thus, this suspension too does 
not make it possible to perform dynamic management of the trim of the 
vehicle, that is to say in fact to monitor the position of the whole of 
the vehicle with respect to the ground (ride height for each axle, angle 
of roll, pitch, ...) in each of the various situations which follow on 
from each other during a road journey. 
SUMMARY OF THE INVENTION 
The object of the invention is to produce a device for dynamically 
controlling the trim of the vehicle during driving, therefore especially 
for controlling its roll and pitch, in order to overcome the shortfalls of 
the suspensions mentioned hereinabove, thus enhancing the comfort of the 
occupants of the vehicle and its road holding ability. 
In accordance with the invention, the device for controlling trim 
comprises, for each wheel, an actuator interacting with the suspension 
spring, and provided with a position sensor, an electro-hydraulic 
interface for controlling the actuator, this interface being connected on 
the one hand to the actuator and on the other hand to a hydraulic unit of 
the vehicle and to a source of electrical energy, and a computer for 
dynamic trim management, connected to the interfaces, and programed to 
take account of each parameter of the driving of the vehicle, especially 
its speed and the angle of its steering wheel, and, on the basis of these 
parameters, to control the electro-hydraulic interfaces and hence the 
actuators within the context of loops for slaving the latter to reference 
output travels, corresponding to maintaining the vehicle in predetermined 
trim for various situations, stored in the memory of the computer. 
Electro-hydraulic interfaces may, for example, be either solenoid valves, 
or servo valves. 
Advantageously, these interfaces are driven by an "all or nothing" signal 
of determined period, pulse-width modulated and supplied by the computer. 
Specially designed software allows this computer to store in memory 
reference values for the positions of the various actuators of each axle, 
therefore especially for the height of the body of the vehicle above the 
ground. In addition, the computer is connected to a line for taking 
account of each parameter of the car during driving, especially 
acceleration, deceleration, speed, angle of the steering wheel, (i.e., 
parameters depending directly upon the driver). On the basis of all this 
data, the trim management software determines the orders which are 
appropriate for the electro-hydraulic interfaces and therefore for the 
actuators, in order to prevent any parasitic and undesirable movement of 
the vehicle with respect to the various reference values provided for the 
various types of situation envisaged (bends, maximum straight-line speed, 
. . . ). 
Other particular features and advantages of the invention will emerge 
during the description which will follow, given with reference to the 
appended drawing which illustrates one embodiment thereof by way of 
non-limiting example.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The block diagram of FIG. 1 illustrates a device for controlling the trim 
of a vehicle with four wheels 1, each equipped with a damper 2 and with a 
suspension spring 3. 
With each wheel 1 there is associated a trim actuator 4 interacting with 
the spring 3 and provided with a position sensor 5 (represented in FIG. 
2). To each actuator 4 there is connected an electro-hydraulic interface 6 
for controlling the corresponding actuator, this interfate 6 itself being 
connected by a control connection to a central computer 7 for dynamic 
management of the trim of the vehicle. The four control interfaces 6 are 
connected together in pairs as well as to a hydraulic unit 8 of the 
vehicle. 
This electro-hydraulic actuating system makes it possible to act between 
the bearing reference of the stiffened function of the suspension, 
provided by the spring 3, and the anchorage to the body, the damping 
function, fulfilled by the damper 2 (passive element) being in parallel. 
In order for this function to satisfy the desired criteria, it must be 
slaved in terms of position between the reference alone and the position 
of the body of the vehicle. 
The most advantageous layout for achieving this result consists in using, 
for each wheel 1, a linear actuator 4 slaved in terms of position via the 
computer 7 which incorporates all the parameters for the dynamics of the 
vehicle, so as to prevent rolling and pitching in the phases of 
acceleration, braking and cornering. 
The source of hydraulic energy constituted by the unit 8 is outside the 
trim management system 9 proper, surrounded by a chain line in FIG. 1. It 
may be either created especially, or taken from the existing installation 
of the vehicle and intended for other functions. The energy required is a 
function of the amplitude which is to be corrected and of the inherent 
dynamics of the vehicle. Of course the trim control device 
diagrammatically represented in FIG. 1 dispenses with anti-roll bars. The 
driver may, depending on the load, the shape of the path and the 
sensitivity which he desires, introduce a weighting of this function at 
the dashboard, connected by a control line 11 to the computer 7 in order 
to allow the latter to receive orders from the driver. Moreover, the 
computer 7 is of course programmed with suitable trim-management software, 
which makes it possible to take account of each parameter of the driving 
of the vehicle, especially its speed and the angle of its steering wheel, 
and, on the basis of these parameters, to control the electro-hydraulic 
interfaces 6 and hence the actuators 4 within the context of loops for 
slaving the latter to reference output travels. These travels correspond 
to maintaining the vehicle in predetermined trim corresponding to various 
situations stored in the memory of the computer 7. 
A concrete embodiment of the trim-dynamics control device according to the 
invention will now be described with reference to FIGS. 2 to 5. 
Each trim actuator 4 includes a sleeve 12 slidably mounted around a 
hydraulic cylinder 13 including a closed internal chamber 14 filled with 
gas and in which a piston 15 secured to an axial rod 16 can slide. The 
latter is mechanically secured to a yoke 17 connected to the body (not 
represented) of the vehicle, and a coil suspension spring 18, coaxial with 
the rod 16 and with the cylinder 13, is interposed between the yoke 17 and 
the U-shaped end 19 of the sleeve 12. The latter bears slidably on the 
external wall of the cylinder 13 via internal collars 20, 22 and, on the 
other side of these, bears slidably on the external wall of a duct 21 for 
supplying with hydraulic fluid the annular space 23 delimited by the 
internal collar 20 and a shoulder 10 of the wall of the cylinder, in which 
the duct 21 is formed. 
The sleeve 12 is provided with a position sensor 5, powered by an 
electrical connection 25 connected to the battery of the vehicle. The 
sensor 5 is supported by a lug 36 of the cylinder 13 and passes slidingly 
through a yoke 37 secured to the sleeve 12. The duct 21 is connected by a 
pipe 26 to an interface which, is constituted by a solenoid valve 27 of a 
type which is, in itself, known and which does not require any 
description. The solenoid valves 27 are connected together in pairs by 
hydraulic fluid pipes 28, 29, the pipes 28 joining up in order to form a 
single pipe 31 for return to tank, while the pipes 29 join up in order to 
form a single pipe 32 for supplying hydraulic liquid under pressure. 
The four solenoid valves 27 are also connected by electrical connections 33 
to the computer 7, itself powered by the battery via a connection 34. 
The sleeve 12 and the cylinder 13 form the trim actuator 4, and the gas 
chamber 14 as well as the piston 15 which slides therein constitute the 
damper. 
Each trim actuator 4 can assume two extreme positions, represented in FIGS. 
3 and 4. In the first, position the extreme bottom position, illustrated 
in FIG. 3, the internal collar 20 comes into abutment against the shoulder 
10 and closes off the inlet of the duct 21, the yoke 17 and the body of 
the vehicle therefore being in a bottom position (the actuators 4 have 
been represented horizontally for convenience of the drawing). In order to 
place the actuator 4 and the body in the extreme top position, the sending 
of pressurized fluid into the duct 21 via the associated solenoid valve 27 
is commanded. The hydraulic pressure exerted on the collar 20 moves the 
sleeve 12 away from the shoulder 10 and frees the annular chamber 23, 
which fills with fluid while the sleeve 12 continues its travel until its 
terminal collar 22 comes into abutment against a projecting flange 35 at 
the end of the cylinder 13. During the travel of the sleeve 12, the spring 
18 is firstly compressed and raises the yoke 17 and the body of the 
vehicle. The travel of the sleeve 12 is damped by the piston 15 sliding in 
the pneumatic chamber 14, and which moves integrally with the yoke 17. 
Finally, during its travel, the sleeve 12 entrains the yoke 37 which 
glides along the sensor 5. 
As already indicated, the trim management software of the computer 7 takes 
account of each parameter of the car: acceleration, braking, ground 
clearance of each wheel 1, speed, angle of the steering wheel during the 
journey . . . ). Each position sensor 5 continuously supplies the position 
of the corresponding actuator 4, and therefore of the wheel 1 with respect 
to the body of the vehicle. This information is entered into the computer 
7 at the same time as the other aforementioned information, then the angle 
of the wheel etc. and the orders from the driver. In particular, knowledge 
of the angle of the steering wheel and of the speed gives the software 
advance notice of what is about to happen, in a time lapse of the order of 
100 milliseconds between the rotation of the steering wheel and that of 
the wheels 1. The computer 7 therefore puts this time to good use in order 
to give correction orders to the solenoid valves 27 and hence to the 
actuators 4, by modulating the flow rate of fluid into the ducts 21 
(injection or removal of the fluid), this being so as to slave the 
position of each actuator 4, and therefore the trim, the rolling and the 
pitching of the body of the vehicle, to the reference values stored in the 
memory of the computer 7. 
Thus, by virtue of the invention, a system is produced for dynamically 
managing the rolling, pitching and therefore the trim of the vehicle, 
without affecting the comfort in terms of stiffness, suspension and 
damping. 
Each solenoid valve 27 is driven by a pulse-width modulated signal S (FIG. 
5) having two different widths t1 and t2. This signal supplies the 
associated actuators 4 with an "ingoing" flow rate q1 during the time t1 
when use is connected to the high pressure fluid pipe 32, and an 
"outgoing" flow rate q2 during the time t2 (t1+t2=T), when use is 
connected to the low pressure fluid pipe 31. 
For a constant reference value xe, the output travel xs of the actuator 4 
is constant, and the operation is governed by the following relationship: 
during an operating period T 
EQU .DELTA.xs=q1 t1-q2 t2 
The flow rate q1 is a function, on the one hand, of the passage 
cross-section .theta. of the fluid, and, on the other hand, of the square 
root of the difference between the high pressure P1 and the pressure 
generated by the antagonistic force PA 
##EQU1## 
The flow rate q2 is a function, on the one hand, of the passage 
cross-section .theta. of the fluid, and, on the other hand, of the square 
root of the difference between the pressure generated by the antagonistic 
force PA and the low pressure P0 
##EQU2## 
These relationships generate, within automatic-control loops, positions for 
each of the wheels which are functions of the values t1, t2 (t2=T-t1). 
The driving of the solenoid valves 27 in "all or nothing" mode, causing the 
times during which oil is admitted and expelled to vary, offers 
significant advantages: on the one hand, simplicity and, on the other 
hand, in the event of a breakdown in the electrical power supply, the 
de-energized solenoid valves 27 all become immobilized in the same 
position. This results in the vehicle regaining a determined trim, whereas 
with proportionally controlled solenoid valves, the trim is differential, 
because it varies from one actuator to another. 
These solenoid valves 27 therefore constitute bistable interfaces, 
advantageously used in place of proportional hydraulic interfaces. 
The solenoid valves 27 may be replaced by servo valves, which are, however, 
more expensive. In general, the electro-hydraulic control interfaces may, 
depending on the type of vehicle, be grouped together into a single 
module. According to another possible variant embodiment, the mechanical 
suspension springs 18 may be replaced by hydropneumatic springs. 
The description of the software corresponding to the system in accordance 
with the invention will be given hereafter, by way of a supplement. 
Drawing-up the source program of this software is within the grasp of the 
person skilled in the art within the context of his normal knowledge on 
the basis of this description and of the foregoing description. 
INPUTS/OUTPUTS 
Hardware Inputs/Outputs 
Sensor supplying information for calculating the speed of the vehicle 
4 LVDT (linear inductive sensors) measurement giving the height at each 
wheel, coded over 10 bits 
4 PWM (pulse-width modulation) controls--having a period of 20 ms--for 
driving the EVPs (driving solenoid valves) (1 per wheel) 
Software Inputs/Outputs 
1 map for each wheel (4 in total) of body height on the same scale as the 
measurement from the associated LVDT, taking account of the min (LVDTmin) 
and max (LVDTmax) values supplied by the LVDT for the car in the top and 
bottom position. The map assuming the form: 
______________________________________ 
Speed V0 V1 V2 V3 V4 V5 V6 
Height L0 L1 L2 L3 L4 L5 L6 
______________________________________ 
Car speed (V) on the same scale as the speeds V0 to V6, this car speed is 
saturated at V6 if it is greater than V6. 
Automatic-control gain (G), common to the 4 wheels. 
Height of each wheel (4 in total) over 10 bits (H). 
The cyclic ratio of the PWM for controlling each wheel (PWM) (4 in total). 
CALCULATION ALGORITHM 
The following description applies to one wheel, it is valid for all 4 
wheels, each wheel having its own inputs/outputs. Only the vehicle speed 
and the automatic-control gain are common to all 4 wheels. 
The value of the height of the body is sampled 4 times (every 5 ms) between 
each body height calculation (every 20 ms). A mean of these 4 values is 
taken, this mean is used to calculate the slaving in the variable H. 
Calculation Pseudocode 
Vehicle speed (V) acquisition 
Body height (H) acquisition 
Search for the markers Vi and Vi+1 in the body height map such that: 
EQU Vi.ltoreq.V.ltoreq.Vi+1 
Calculation of the body height reference value by linear interpolation: 
EQU Ref=(Li+l-Li)/(Vi+1-Vi).times.(V-Vi)+Li 
Calculation of the discrepancy with gain (G): 
EQU Discrepancy=G.times.(Ref-H) 
Calculation of the cyclic ratio of the PWM control (PWM): 
##STR1## 
where Dmax=LVDTmax-LVDTmin. 
The effective control of the PWM is a square signal for which the time 
spent in the high state is 1% of 20 ms for 1% of a cycle and 99% of 20 ms 
for 99% of a cycle.