Hydraulic distributor with a reaction biased control member

The distributor, more particularly for power steering systems, includes two cooperating elements defining two parallel fluid flow circuits (10,11 ) each including three serially arranged restrictor means (1,2,3;1',2',3') actuable during relative displacement of said cooperating elements, and, for each circuit, in parallel with second restriction means (2;2') an external restriction means (EV;EV' ) controlled by external control means so as to selectively vary the position of the transition point between the first and second portions of the characteristic curve, assistance pressure delivered by the distributor/reaction on the input control member of the distributor (14).

The invention concerns hydraulic distributors and in particular, those used 
to control the operation of a power servomotor for assisting various 
mechanisms, for example the steering system of a vehicle, according to the 
position of an input control member, for example the steering wheel of 
said vehicle. 
There are several types of distributors performing this function and which 
are based on the use of two cooperating elements which are displaceable 
one relative to the other, respectively linked to a control member 
reveiving the actuating force and to a controlled mechanism and defining 
therebetween an interface where there are openings and passages 
communicating with input and return motor fluid lines and with the two 
ends of the assistance servomotor, whereby relative displacement of said 
elements creates a modulable pressure differential between both ends of 
the servomotor. 
There are known distributors of this type, in which the interface between 
the two elements is a cylindrical form with longitudinal grooves and 
cavities or in a disc or star-shaped form. In both cases, those 
distributors are designed to comprise at least one circuit establishing 
communication between a pressure fluid source and a return line or a 
reservoir, and including at least two serially arranged valve means which 
are simultaneously actuated by a control device, the output of a first of 
said valve means being connected to one end of an assistance hydraulic 
power motor controlled by a distributor, the output of a second of said 
valve means being connected to a hydraulic reaction device applying a 
reaction to the control unit, so as to define a pressure/torque 
characteristic with an initial portion of the curve corresponding to a 
practically nil pressure, and a following portion of the curve 
corresponding to an actuating assistance pressure approximately 
proportional to the torque (or force) applied to the input control member 
of the distributor. 
In this way, in the particular case of a power steering mechanism, the 
driver receives information concerning the actuating force developed by 
the servomechanism. The distributor is generally designed in such a way 
that the reaction characteristic curve only begins after a certain given 
distance from a zero point or neutral position of the distributor, so as 
to make driving easier. However, experiments have shown that it is useful, 
given driving conditions, to vary the reaction characteristic position in 
the pressure/torque diagram, or in other words, to arrange the system so 
that reaction can begin at any desired distance from the neutral position. 
This is the purpose of this invention, which provides a hydraulic 
distributor, more particularly for a vehicle power steering system, 
including two cooperating elements under action of an input control member 
and defining at least one modulable circuit establishing fluid 
communication between a pressure fluid source and a reservoir, this 
circuit including three serially arranged valve means simultaneously 
actuable during relative displacement between the two elements, the output 
of the first valve being connectable to one end of a hydraulic motor 
controlled by the distributor and the output of the second valve means 
being connectable to a hydraulic reaction system for applying a reaction 
to the control member and furnishing a characteristic curve actuating 
pressure of the hydraulic motor control torque applied to the control 
member having an initial portion of zero-slope and following a second 
portion with a given slope, a fourth modulable valve means in parallel 
with said second valve means and controlled by additional external control 
means so as to selectively modify the transition point position between 
said first and second portions of the characteristic curve. 
It appears also useful to vary, for adaptation to different vehicle driving 
conditions, the slope of the second "proportional reaction" portion of the 
pressure/torque driving conditions. For this purpose, and according to 
another feature of the invention, a fifth valve means is located in series 
between the first valve means output and the second valve means input and 
actuated during relative displacement between the two distributor elements 
so as to determine the slope of a third terminal portion of the 
pressure/torque characteristic curve as from a determined value of the 
force applied to the distributor control device 
There are also known other types of distributors which provide such a third 
zone of predetermined slope of the pressure/torque characteristic curve, 
which is achieved for example, by resilient members of approximate 
dimensions, this being useful when the steering system is operated in 
limit conditions, in other words, in sharp curves, or under parking 
maneuver conditions. For the same reasons as above, in this case it is 
also useful to have the possibility of varying of the position of the 
third portion of the curve. Therefore, according to another feature of the 
invention, a sixth externally controlled external valve means is arranged 
in parallel with the third valve means of the distributor so as to modify 
the slope of the second portion of the distributor pressure/torque 
characteristic curve. 
Obviously, the invention also includes the conjugated use of the above 
mentioned additional valve means which is separately actuated so as to 
simultaneously modify the different portions of any distributor 
characteristic curve as desired. 
In a preferential embodiment of the invention, the hydraulic distributor is 
of the type where both cooperating elements are respectively composed of a 
star-shaped rotor located in a disc-shaped stator cavity, the rotor 
coupled to the control unit, the cooperating edges of recesses formed in 
the facing surfaces of the rotor and of the stator defining variable 
restrictions forming the first, second and third valve means, and is 
characterized in that the rotor includes reaction arms associated to 
discharge openings of the stator, each reaction arm separating two 
reaction chambers, each limited on the opposite side by an intermediate 
arm associated to an intermediate passage opening of the stator, the 
fourth external valve means being inserted in a circuit establishing 
communication between the reaction chamber and the adjacent intermediate 
passage opening. 
In certain constructions of this type of distributors, the stator is 
received for limited rotational movement within a housing attached to the 
steering mechanism and is biased towards a central or neutral position by 
means of resilient members. In such a case, according to the invention, 
the stator has a radial protrusion movably between two stationary stop of 
the housing, and the resilient members comprise a C spring, the free ends 
of which are applied against the opposite sides of the stator protrusion 
and of a centering stop attached to the housing, the spring supplying the 
reaction for the third portion of the servocontrol actuator.

FIG. 1 shows the diagram of a hydraulic distributor of the open center type 
which includes two circuits in parallel, 10 and 11 between a pressure 
fluid source 12 and a return line or reservoir 13, and each including 
three serially arranged valve means 1, 2, 3, and 1', 2', 3', repectively, 
which are simultaneously controlled by a single control member 14, to be 
described later. Branching between the valve means for restrictions 1, 2 
and 1', 2' of each circuit, distribution conduits 15, 16 lead to the 
respective ends of a double action power actuator. 
In parallel to the second restriction 2 and 2' are arranged four variable 
valve means or restrictions EV and EV', respectively in such a way that 
the through fluid flows thereof are added to those of the second 
restrictions 2 and to 2'. Branched between each pair of restrictions 2 and 
3 (respectively 2' and 3'), conduits 18 and 19 lead to two hydraulic 
reaction devices, the purpose of which is to apply to the control member 
14 a reaction proportional to the control force which is applied thereto. 
To facilitate the explanation, we will consider that the distributor is 
part of an automobile power steering system and that it is composed of a 
star-shaped valve, as shown in FIGS. 3 to 7, of the type described in the 
European Patent Application No. 0,021,970 in the name of the applicant. 
The star-shaped distributor valve of FIGS. 3 to 7 comprises a flat 
star-shaped rotor 20, centrally coupled to a triangular shaft 21 
controlled by the steering wheel, and rotatingly received within a 
disc-shaped cavity of a stator conventionally composed of a central 
annular part 22 stacked between two side plates or discs 23 attached to 
the corresponding opposite axial sides of the central annular part. The 
stator assembly is mounted within a stationary housing 24 for limited 
rotation therein, as defined by the two facing surfaces of a radial window 
25 of the stator within which is received with play a radial lug 26 of the 
stator. Rotor 20 includes reaction arms 27 of greater radius received in 
cavities of the stator (central part 22) so as to define in these cavities 
corresponding reaction chambers C and C' on both sides of each reaction 
arm. Various seals 28 are provided for tight hydraulic separation of the 
different functional chambers useful for valve operation, as shown more 
clearly on FIGS. 4 to 7. 
As shown on FIGS. 3 and 4, an input opening E is recessed in the facing 
surfaces of the side plates 23 facing opposite sides of rotor 20, in a 
centered position between two adjacent reaction arms 27, so as to receive 
the fluid from pressure source 12 via a conduit 29. Similarly, two 
substantially identical intermediate openings B and B' are formed 
(recessed in the facing walls of plates 23, on both sides of each input 
opening E, as also, two identical additional discharge openings D and D' 
wherefrom extend return conduits 30 leading to reservoir 13. Cavities are 
formed in the rotor faces between arms of the rotor which also delimitates 
similarly chambers A and A' adjacent to the stator opening E and 
straddling openings E and B (E and B'), respectively, as also rotor 
chambers C and C' straddling openings B and D (respectively B' and D'), 
adjacent chambers A and C (or A' and C') being mutually separated by an 
intermediate arm 50 of the rotor. Conduits 31 and 32 lead respectively 
from opening B and chamber C to the opposite ends of the variable internal 
restriction EV, while similar conduits 33 and 34 lead from opening B' and 
chamber C' to the variable restriction EV'. In FIG. 4 both variable 
restrictions EV and EV' form a unit and are driven by solenoids 35 and 
36', which are respectively controlled by electrical signals from an 
external control device (not shown). 
FIG. 4 clearly shows how the valve means or restrictions 1-3 and 1'-3' 
described in FIG. 1 are formed between parts 20 and 23 (rotor and stator) 
of the star-shaped distributor. Starting from central input stator opening 
E, where the fluid under pressure arrives through conduit 29, which 
opening faces a short input arm 52 of the rotor), and progressing towards 
the right, in other words in the fluid circulation direction of this side 
of the valve, there is first the first restriction 1 formed by square edge 
of opening E and the chamfered edge 37 of the radial adjacent edge of the 
chamber A of the rotor. In rotor chamber C, the left-hand stepped edge 38 
forms with the adjacent edge of opening B of stator 23 the restriction 2, 
and the right hand ramp-shaped edge 39 forms with the adjacent edge of 
stator exhaust opening D the restriction 3. The other not-mentioned edges 
do not form there control restrictions and have no effect on system 
operation in this direction. On the left of input opening E, the relative 
arrangement of the cooperating edges is symmetrical to that which have 
just been described, so that the assembly forms the two parallel circuits 
10 and 11 described above. 
This arrangement is clearly shown on FIG. 3, where there are for example 
three groups of valve means of the same type arranged angularly around the 
periphery of the star-shaped distributor. 
The described distributor enables external modulation of the central 
operating point as will be shown; it offers a maximum reaction which is 
practically constant in the maximum input torque operating zone (FIG. 2) 
supplied by a C-spring 40 located around the valve, the ends of which are 
applied with a predetermined load against the opposite sides of lug 26 of 
the stator 22, and, at the same time, against a centering stop (not shown) 
placed axially immediately behind the above lug and which makes part of 
the stator housing. Assuming that the two variable external restriction EV 
and EV' produce a given constant restriction in their rest condition 
position, and that their electromagnetic control means 35 and 36 are 
maintained in a de-energized condition, the distributor operation is 
illustrated by curve 0-1-2-3of FIG. 2 diagram which shows the relationship 
between effective pressure applied to the power servocontrol mechanism 17 
as a function of the input torque, and where there are three operating 
zones, namely: initial position corresponding to straight line driving, 
represented by portion 0-1 of the characteristic curve; driving position, 
corresponding to normal driving in slight curves, represented by portion 
1-2; and maneuvering position represented by portion 2-3, corresponding to 
driving in very sharp curves or during parking maneuvers. The rest 
position is defined by point 0 of the characteristic curve and is called 
central position. 
In this central position 0 (illustrated on FIG. 4), a fluid pressure from 
pump 12 is proportionally distributed on both sides of the input (opening 
E) and passes through restrictions 1, 2 and 3 (and EV, in parallel with 2) 
in one direction, and restrictions 1', 2' and 3' (and EV', in parallel 
with 2') in opposite direction, in other words to the right or to the left 
of FIG. 4. The pressure of the circulating fluid is applied equally on 
both sides of each reaction branch 27, whereby there is no reaction torque 
applied. 
When the steering wheel is moved, rotor 20 is rotated with respect to the 
stator, for example in the direction of arrows on FIGS. 4 and 5. When, as 
shown in FIG. 5, rotor 20 leaves the rest position, restrictions 3' and 2 
and EV become active so as to maintain equal pressure in chambers A and 
A', so as that there is no assistance furnished by actuator 17. However, 
the pressure in chamber C' increases as compared to that in chamber C. In 
other words, the different reaction arms separating the above-mentioned 
chambers are subjected to a higher fluid pressure on their faces oriented 
in the clockwise direction (in relation to FIG. 5) and therefore an 
anti-clockwise hydraulic reaction is produced on said arms which is 
transmitted to the steering wheel by rotor 20 and shaft 21. This operation 
phase is defined on FIG. 2 by portion 0-1. 
By continuing the steering wheel rotation, chamfer 37 comes closed to the 
adjacent lateral edge of opening E (FIG. 6), so that restriction 1 also 
becomes active or operant. Under these conditions, the pressure begins to 
increase in the area between restrictions 1 and 3' (on the left input 
opening E), said pressure being transmitted by conduit 16 to the left side 
of actuator 17, so that the latter is actuated in the same way. Since this 
pressure increase also exists in chamber C', an increasing reaction Pr, 
directly proportional to the power assisting pressure Pa, acts on each 
reaction arm 27. The increase of control torque displaces the operation 
point of the system along portion 1-2 of the characteristic curve. When 
reaching point 2, the reaction arms 27 come into abutment against the 
stator end of chambers C' wherein they are displaced (FIG. 7). From this 
moment on, spring 40 is actuated, since the lug 26 of the stator 22 begins 
to drive the corresponding branch end of the springs; in other words the 
torque value in position 2 is determined by the spring load, and the 
increase of the torque under these conditions occurs in relation to the 
elasticity of the spring. 
The above-described operation corresponds in practice to the actuation of a 
distributor valve without external modulation when considering 
restrictions 3 and EV constant). If, on the contrary, solenoid 35, which 
controls restriction EV, is actuated to increase the value of the 
restriction passage section in EV, under the same operating conditions, 
the total pressure resulting of the sum of 3 and EV is less, and the 
reaction area of the central point will be more restricted than before. 
Consequently, when the value of the fluid passage section of EV increases, 
curve 0-1-2-3- tends towards curve 0"-1"-2"-3". On the contrary, if the 
value of the fluid passage section in Ev is reduced, there is displacement 
of the characteristic towards curve 0'-1'-2'-3', whereby resulting in a 
widening of the central reaction area. 
Solenoids 35 and 36 can be controlled by any adequate conventional means, 
such as responsive to vehicle operating conditions, for example by an 
in-board processor which controls the vehicle running functions in 
relation to given driving or running circumstances or parameters. 
FIGS. 8 to 15 show another embodiment of the invention. Here spring C 40 is 
suppressed and the reaction corresponding to the third portion 2-3 of the 
characteristic curve is controlled by an additional restriction 4-4', 
located between restrictions 1 and 2 (respectively 1' and 2') of the 
circuit of FIG. 1 and defined by chamfers 41 and 42 at the level of the 
separation between chamber A and cavity B on one hand (A' and B' on the 
other hand), as shown in FIG. 11 (the remaining integers of the 
distributor remaining identical to these of the preceding embodiment). 
Operation in this case (FIG. 9) is identical to that which precedes, as 
concerns the initial and driving areas (FIGS. 11 and 12); at a given 
moment of the driving area 1-2 (point 2), chamfer 42, however, comes 
sufficiently close to the adjacent edge of stator opening B' to enter into 
action, so that from this moment on, the assistance power pressure and the 
reaction pressure (or torque) are differentiated by the pressure drop 
which occurs in the thus formed restriction 4'. For this purpose, the 
chamfer 42 (41) is dimensioned such that the pressure in chamber C' only 
slightly increases, so as to stabilize system operation, while the 
pressure in chamber A' continues to increase, as also, simultaneously the 
assistance pressure in the actuator (FIGS. 13 and 14). When reaching 
position 3, maximum operating pressure of the system is attained: 
restrictions 1 and 3' completely close while restrictions 2 and 4' (as 
well as EV) are held partially open to retain system stability until the 
mechanical stop is reached. 
In the same way as in the arrangement of FIG. 1, control of modulable 
external restrictions EV and EV' enables the distributor characteristic to 
be shifted which, in this case, adopts the forms shown in FIG. 9 which 
requires no additional explanation. 
Other variants are possible according to the same principles. For example, 
as shown on FIG. 16, other external restrictions ED and ED' can be 
inserted in parallel with third (downstream) restrictions 3 and 3' of FIG. 
1, and they can be arranged so that they can be controlled as previously 
described. 
Assuming that the distributor no longer has a spring ensuring reaction in 
the (third) maneuvering portion of the curve, it is possible to obtain, 
without modulation of external restrictions, reactions corresponding to 
portions 0-1-2 of the characteristic curve, as shown on FIGS. 17 and 18. 
However, restrictions ED and ED' in parallel with restrictions 3 and 3' 
enable the slope of portion 1-2 to be varied as shown on FIG. 19, which 
means that, by simultaneously actuating the two external restrictions ED 
and EV (or EB' and EV'), it is possible to obtain different characteristic 
curves in compliance with those of FIG. 20. Thus, by adequately adjusting 
the actions of the external restrictions, any characteristic curve within 
the shaded area of FIG. 21 can be obtained.