PATENT ABSTRACT
The invention relates to a system for the hydraulic control ( 10 ) of a clutch ( 12 ), e.g. a motor vehicle clutch, comprising an upstream master cylinder ( 14 ) which is connected to a down-stream slave cylinder ( 18 ) by means of a conduit ( 16 ). The invention is characterized in that the system comprises a servo cylinder ( 30 ) which is disposed in the conduit ( 16 ) between the master cylinder ( 14 ) and the slave cylinder ( 18 ), said servo cylinder comprising at least one servo piston which can be subjected to an assist force produced by a servo device.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
     1. Field of the Invention. 
     The invention concerns a system for the hydraulic control of a clutch. 
     The invention concerns more particularly a system for the hydraulic control of a clutch, in particular for a motor vehicle, comprising an upstream sending cylinder connected by a conduit to a downstream receiving cylinder so as to form a hydraulic control circuit. 
     2. Description of the Related Art. 
     It is sometimes desirable to equip the hydraulic control system for a clutch with an assistance device so as to minimise the force that the user has to apply to the clutch control pedal during the declutching phase. 
     Such a device is described for example in the document US-B-6.213.271. 
     In this document, the assistance device is mounted on the sending cylinder of the hydraulic clutch control system. 
     This system has the drawback of requiring a specific sending cylinder adapted to the arrangement of the supplementary elements fulfilling the assistance function. 
     The arrangement of the supplementary elements on the sending cylinder poses problems of space requirements and this makes the sending cylinder more complex to produce. 
     SUMMARY OF THE INVENTION 
     The present invention aims to remedy these drawbacks by proposing a simple and economical solution that does not require modifying the sending cylinder or receiving cylinder. 
     For this purpose, the invention proposes a control system of the type described above, characterised in that it comprises an assistance cylinder that is interposed in the conduit, between the sending cylinder and the receiving cylinder, and which comprises at least one assistance piston that is mounted so as to slide axially in the body of the assistance cylinder between an upstream engagement position and a downstream disengagement position, so as to delimit an upstream hydraulic chamber and a downstream hydraulic chamber with variable volumes according to the axial position of the piston, the upstream chamber being connected to the sending cylinder by a portion of hydraulic circuit referred to the upstream circuit and the downstream chamber being connected to the receiving cylinder by a portion of the hydraulic circuit referred to as the downstream circuit, each hydraulic circuit portion comprising a means of relevelling the volume of fluid connected to at least one fluid reservoir, and in that the assistance cylinder comprises an assistance device that applies an assistance force to the assistance piston during the declutching phase. 
     One advantage of the system according to the invention is that it uses a sending cylinder and receiving cylinder of a standard type, which have not been designed to be equipped with an assistance device. 
     In addition, the clutch control system according to the invention can be arranged in a vehicle without its being necessary to modify the area where the sending cylinder is arranged and/or the area where the receiving cylinder is arranged, compared with a similar vehicle not equipped with the assistance device, the space requirement of the sending cylinder and the space requirement of the receiving cylinder not being modified. 
     Another advantage of the control system according to the invention is that the assistance cylinder and its assistance device do not have any influence on the control law linking the movement of the clutch control pedal to the movement to the movement of the clutch diaphragm. The position of the diaphragm is therefore always dependent on the position of the pedal. 
     Yet another advantage of the control system according to the invention is that, as the two upstream and downstream circuits have a means of relevelling the volume of fluid, the system keeps a constant operating point whatever the variations in the position of the clutch, variations which may stem for example from wear on the clutch, heating thereof, or the control of the clutch. 
     According to other characteristics of the invention:
         the assistance device comprises a regulation means which makes the value of the assistance force vary according to the travel of the clutch control pedal in accordance with a predetermined assistance law;   the assistance device comprises a transmission member which transmits the assistance force to the assistance piston;   the transmission member is connected in terms of axial movement to the assistance piston in both directions of sliding of the piston;   the transmission member cooperates by contact with an associated abutment surface of the assistance piston so that, in the case where the speed of the assistance device is less than the speed of the assistance piston, the assistance device does not slow down the sliding of the assistance piston towards the downstream end;   the transmission member is arranged at an axial end of the assistance piston;   the piston comprises an upstream portion that delimits the upstream chamber and a downstream portion that delimits the downstream chamber, the two portions being connected in axial movement by a connecting rod, and the connecting rod constitutes the transmission member of the assistance device;   the hydraulic circuit being connected to a fluid reservoir in the engagement position, the assistance cylinder comprises at least one discharge orifice which makes at least one hydraulic chamber communicate with the fluid reservoir, when the assistance piston is occupying its upstream position, so as to compensate for the variations in hydraulic volume in the hydraulic circuit over time;   the discharge orifice is arranged in the assistance piston and the discharge orifice makes the upstream chamber communicate with the downstream chamber, when the assistance piston is occupying its upstream position;   the discharge orifice comprises a valve that is controlled by the axial movement of the assistance piston;   the assistance device comprises an elastic element which stores energy during the engagement phase and which restores the energy during the disengagement phase in order to produce the assistance force;   the regulation means is a cam mechanism which is driven by the axial movement of the piston and which regulates the assistance force produced by the elastic element during the disengagement phase;   the assistance device is housed in the cylinder body and the cam mechanism comprises at least one control surface that is produced on an internal wall of the cylinder body;   the elastic assistance element is an axial compression elastic element that is interposed axially between a cup and an abutment surface fixed with respect to the assistance cylinder body, the cam mechanism comprises at least one movable roller which travels over a control surface between an upstream position and a downstream position corresponding respectively to the upstream and downstream positions of the assistance piston, and the movable roller is connected by a first connecting rod to the piston by a second connecting rod to the cup;   the axis by which the connecting rods pivot on the movable roller is concurrent with the rotation axis of the roller;   the control surface comprises an upstream portion inclined with respect to the sliding axis, and a downstream portion roughly parallel to the sliding axis so that, during a first part of the disengagement phase, the movable roller moves first of all on the inclined portion towards the axis and in the downstream direction, from its upstream position, transmitting part of the relaxation force of the elastic assistance element to the assistance piston, by a step-down effect, and then, during a second part of the disengagement phase, the movable roller moves on the downstream portion in the downstream direction, in a roughly axial direction, transmitting all the relaxation force of the elastic assistance element to the assistance piston;   the distance between the pivot axes of the second connecting rod is such that, in the upstream position of the movable roller, the roller moves in the upstream direction beyond the point on the control surface where the second connecting rod is perpendicular to the control surface, so that the expansion force of the elastic assistance element biases the movement roller towards its upstream position;   the axial dimension of the elastic assistance force in the relaxed state is less than the axial distance between the cup and the associated fixed abutment surface, when the piston occupies its downstream position, so as to suspend the assistance force during the end of the travel of the piston in the downstream direction;   the assistance device comprises an electrical actuator that controls the relaxation of the elastic element during the disengagement phase;   the means of regulating the assistance device is an electronic control unit that controls the electrical actuator;   the elastic assistance element is a helical compression spring;   the assistance device is connected to an energy source that is external to the control system and that is installed in the vehicle that the control system equips, and the said energy produces the assistance force that is transmitted to the piston;   the assistance device comprises an electrical actuator controlled so as to transmit an assistance force to the piston during the disengagement phase;   the means of regulating the assistance device is an electronic control unit that controls the electrical actuator producing the assistance force;   the assistance device comprises a ram that is connected to a hydraulic or pneumatic pressure source and that transmits an assistance force to the piston during the disengagement phase;   the means of regulating the assistance device comprises at least one control valve interposed between the ram and the hydraulic or pneumatic pressure source;   the regulation means comprises a two-position control valve connected to a pressure source in order to form a charging valve and a two-position control valve connected to a fluid reservoir in order to form a discharge valve, and each control valve is controlled by the hydraulic pressure in the upstream circuit, so that the hydraulic pressure in the upstream circuit tends towards a first constant value during a disengagement travel and tends towards a second constant value, less than the first value, during an engagement travel;   the regulation means comprises a three-position control valve, a charging position that is connected to a pressure source, an intermediate closure position, and a discharge position connected to a fluid reservoir, and the control valve is controlled, on the charging position side, by the hydraulic pressure in the upstream circuit and, on the discharge position side, by the hydraulic pressure in the downstream circuit, so that the assistance force applied to the assistance piston during the disengagement phase is proportional to the hydraulic pressure in the downstream circuit;   the distributor is controlled by an electronic control unit;   the piston comprises at least one elastic element that returns the piston towards its upstream position.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the invention will emerge from a reading of the following detailed description, for an understanding of which reference will be made to the accompanying drawings, in which: 
         FIG. 1  is a diagram representing a hydraulic clutch control system produced in accordance with the teachings of the invention; 
         FIGS. 2 and 3  illustrate schematically the operating principle of the assistance in the control system according to the invention; 
         FIG. 4  is a diagram similar to that of  FIG. 2  that illustrates a variant embodiment of the dumping to reservoir of the hydraulic control circuits; 
         FIG. 5  is a view in axial section depicting schematically the assistance of the control system according to a first embodiment of the invention in which the assistance device comprises a cam mechanism, the piston being shown respectively in its upstream position and in its downstream position; 
         FIGS. 6 and 7  are partial views in axial section which depict a detail of  FIG. 5  and which illustrate two successive intermediate positions of a movable roller equipping the cam mechanism; 
         FIG. 8  is a diagram illustrating the change in hydraulic pressure in the control circuit and the change in the assistance force according to the travel of the clutch pedal; 
         FIG. 9  is a view similar to that of  FIG. 5  depicting schematically an assistance cylinder according to a second embodiment in which the assistance device is arranged at the upstream end of the assistance cylinder; 
         FIG. 10  is a diagram similar to that in  FIG. 2  that depicts schematically a control system according to a third embodiment of the invention in which the relaxation of the assistance spring is controlled by an electrical actuator; 
         FIG. 11  is a diagram similar to that of  FIG. 9  that depicts an assistance cylinder according to a fourth embodiment of the control system according to the invention in which the assistance force is produced by an electric motor; 
         FIG. 12  is a diagram similar to that of  FIG. 2  that depicts schematically a control system according to a fifth embodiment of the invention in which the assistance force is produced by a ram connected to a pressure source; 
         FIG. 13  is a view in axial section that depicts an assistance cylinder adapted to the control system of  FIG. 12  and comprising a membrane ram; 
         FIGS. 14 ,  16  and  18  are diagrams similar to that in  FIG. 2  that illustrate three different solutions for regulating the assistance force adapted to the control system of  FIG. 12 ; 
         FIGS. 15 and 17  are diagrams illustrating the assistance laws associated respectively with the first two solutions depicted in  FIGS. 14 and 16 ; 
         FIG. 19  is a view in axial section depicting a variant of the embodiment in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, identical, similar or analogous elements will be designated by the same reference numbers. 
       FIG. 1  depicts a hydraulic control system  10  of a motor vehicle clutch  12  produced in accordance with the teachings of the invention. 
     The control system  10  comprises an upstream sending cylinder  14  connected by a pipe or conduit  16  to a downstream receiving cylinder  18  with a similar structure to the sending cylinder  14 . 
     The sending cylinder  14 , the receiving cylinder  18  and the conduit  16  form a hydraulic control circuit  19 . 
     Each sending  14  or receiving  18  cylinder comprises a piston (not shown) able to move axially inside a cylinder body in order to delimit a hydraulic chamber of variable volume. A connection orifice, to which the conduit  16  is connected, opens out in the hydraulic chamber. 
     The sending cylinder  14  comprises a piston rod  20  connected here to a clutch pedal  22  on which the driver of the vehicle acts. 
     The piston of the sending cylinder  14  is designed to expel a control fluid or liquid contained in the hydraulic chamber in the direction of the conduit  16 , during a declutching operation. 
     When the clutch  12  is engaged, the volume of the hydraulic chamber of the sending cylinder  14  is at a maximum whilst the volume of the hydraulic chamber of the receiving cylinder  18  is at a minimum. 
     During the declutching operation, the volume of the hydraulic chamber of the sending cylinder  14  decreases, whilst the volume of the hydraulic chamber of the receiving cylinder  18  increases. 
     The piston of the receiving cylinder  18  then causes the movement of a rod  24 , which acts here on a declutching fork  26  actuating a clutch release bearing  28 . 
     When the driver releases his action on the clutch pedal  22 , the piston of the receiving cylinder  18  is returned towards its initial position by a clutch spring such as a diaphragm  13 . 
     In returning to its initial position, the receiving cylinder  18  pushes the column of oil contained in the hydraulic circuit  19 , which causes the return of the piston of the sending cylinder  14  to its initial position. 
     The clutch pedal  22  is returned to its initial position by a return spring and/or by the return of the piston of the sending cylinder  14 . 
     Generally, each sending  14  and receiving  18  cylinder comprises a spring (not shown) which acts between the piston and the bottom of the body of the cylinder, and which guarantees the return of the piston as far as its initial position in abutment. 
     Preferably, the hydraulic chamber of the sending cylinder  14  is able to be connected to a fluid reservoir  29 , so as to compensate for the variations in volume of the hydraulic circuit  19  over time. 
     To this end, the hydraulic chamber of the sending cylinder  14  comprises at least one discharge orifice (not shown) which is open when the sending piston returns completely to its initial position and which makes the hydraulic circuit  19  communicate with the reservoir  29 . 
     It should be noted that, during the declutching phase, the pedal  22  has dead travel at the start of its pivoting, which corresponds to a movement of the sending piston as far as the axial position in which it closes the discharge orifice. 
     During the dead travel, the sending piston pushes the fluid towards the reservoir  29 , without causing the movement of the receiving piston. 
     In accordance with the teachings of the invention, the control system  10  comprises an assistance cylinder  30  interposed in the conduit  16 , between the sending cylinder  14  and the receiving cylinder  18 . 
       FIG. 2  depicts the control system  10  according to the invention in an simplified manner. 
     It should be noted that, in this figure, the rod  24  of the receiving piston acts directly on the diaphragm  13  of the clutch  12  by means of a bearing comprising a ball bearing (not shown). 
     The assistance cylinder  30  comprises an assistance piston  32  mounted so as to slide along a principal axis A 1 , between an upstream position and a downstream position, so as to delimit an upstream hydraulic chamber  34  and a downstream hydraulic chamber  36  with variable volumes according to the axial position of the piston  32 . 
     The upstream chamber  34  communicates with the chamber  38  of the sending cylinder  14  by means of an upstream portion  40  of the hydraulic circuit  19 , and the downstream chamber communicates with the chamber  42  of the receiving cylinder  18  by means of a downstream portion  44  of the hydraulic circuit  19 . 
     The upstream hydraulic circuit  40  is connected to the fluid reservoir  29  at the sending chamber  38 . 
     The downstream hydraulic circuit  44  is connected to a fluid reservoir, for example the same reservoir  29  as the upstream hydraulic circuit  40 , here at the downstream chamber  36  of the assistance cylinder  30 . 
     According to the embodiment illustrated here, the assistance cylinder  30  comprises a spring  46  which is interposed axially between the assistance piston  32  and the bottom of the downstream chamber  36 , and which returns the assistance piston  32  towards its upstream position. 
     In the diagram, the assistance piston  32  comprises a rod  48  which extends towards the outside through the upstream chamber  34 . 
     According to a variant (not shown), the assistance cylinder  30  with its piston  32  can be replaced by a chamber comprising an intermediate membrane separating the upstream  34  and downstream  36  chambers and fulfilling the role of the assistance piston  32 . This membrane in the same way comprises the rod  48 . 
     In accordance with the teachings of the invention, during the declutching phase, which is illustrated by  FIG. 3 , an assistance force F a  is applied to the assistance piston  32 , here by means of the rod  48 , so as to relieve the abutment force F p  of the user on the pedal  22 . 
     The assistance force F a  is produced by an assistance device  50  which will be described hereinafter. 
     According to a variant embodiment depicted in  FIG. 4 , a discharge orifice  52  is produced in the assistance piston  32 , so as to make the upstream hydraulic circuit  40  communicate with the downstream hydraulic circuit  44  when the piston  32  is occupying its upstream position. 
     According to the diagram in  FIG. 4 , the discharge orifice  32  passes axially through the piston  32  and comprises a discharge valve  54  which is biased elastically towards its closure position and which opens mechanically when the assistance piston  32  comes to occupy its upstream position, by the abutment of a rod of the valve  42  on the bottom of the upstream chamber  34 . 
     The arrangement of the discharge orifice  52  in the piston  32  makes it possible in particular to connect the entire hydraulic circuit  19  to the fluid reservoir  29 , with a single connection, arranged here at the sending cylinder  14 . 
     This arrangement also makes it possible not to add additional dead travel to the clutch pedal  22 , which is the case when the assistance cylinder  30  is connected to the reservoir  29  as in  FIG. 2 . 
     The opening of the discharge valve  54  can be calibrated so that an abrupt increase in the hydraulic pressure in the sending chamber  38  causes, almost immediately, a movement of the assistance piston  32  in the downstream direction, the fluid not having the time to flow through the discharge valve  34 , and therefore the closure of the discharge orifice  52 . This calibration can make it possible to choose the value of the first axial travel of the assistance piston  32  in the downstream direction, before the assistance device  50  has begun to apply an assistance force F a . 
     A description is now given of a first embodiment of the control system  10  according to the invention which is shown in  FIGS. 5 to 7 , in which the assistance device  50  comprises an elastic element which stores energy during the engagement phase and which restores the energy, in the form of an assistance force F a , during the disengagement phase. 
     According to the first embodiment, the assistance cylinder  30  comprises a cylinder body  56  which is provided with an inlet orifice  58  and a discharge orifice  60 . 
     The assistance piston  32  is mounted so as to slide, along the principal axis A 1  within the cylinder body  56 , which has roughly a tubular shape of axis A 1 . 
     In the remainder of the description, elements will be termed internal or external with respect to the principal axis A 1  in a radial direction. 
     Considering  FIG. 5 , the piston  32  is shown in the upstream position on the top half section and in the downstream position on the bottom half section. 
     The upstream chamber  34  communicates with the sending cylinder  14  through the inlet orifice  58 , and the downstream chamber  36  communicates with the receiving cylinder  18  through the discharge orifice  60 . 
     According to the embodiment depicted here, the assistance piston  32  is produced in several parts. 
     The piston  32  comprises an upstream portion  62 , which is designed to slide axially in a complementary upstream bore  64 , and a downstream portion  66  which is designed to slide axially in a complementary downstream bore  68 , the two portions  62 ,  66  being connected with respect to axial movement by an axial connecting rod  70 . 
     The connecting rod  70  comprises here an internal rod  72 , for example made from metal, and an external body  74  moulded onto the internal rod  72 . 
     The upstream end  76  and the downstream end  78  of the rod  70  are each here in the form of a spherical head. 
     The two portions  62 ,  66  here have roughly identical shapes. 
     The upstream portion  62  has overall a tubular shape with an H-shaped axial profile, that is to say it has two tubular parts substantially symmetrical with respect to a transverse separation wall  80 . 
     On the side of the upstream face  82  of the transverse wall  80 , the upstream portion  62  forms a jacket  84  which delimits a part of the upstream chamber  34 . 
     On the side of the downstream face  86  of the transverse wall  80 , the upstream portion  62  forms a housing  88  which receives a roughly cylindrical piece  90  forming a receptacle for the connection between the upstream axial end  76  of the connecting rod  70  and the upstream portion  62  of the piston  32 . 
     According to the embodiment depicted here, the downstream portion  66  is substantially similar to the upstream portion  62  and the downstream portion  66  is arranged substantially symmetrically with the upstream portion  62 , with respect to a transverse symmetry plane. 
     Thus the downstream portion  66  has a transverse separation wall  92  and, on the side of the downstream face  94  of this wall  92 , forms a jacket  96  which delimits a part of the downstream chamber  36 . 
     On the side of the upstream face  98  of the transverse wall  92 , the downstream portion  66  receives a cylindrical piece  100 , similar to that of the upstream portion  62 , forming a receptacle for the connection between the downstream axial end  78  of the connecting rod  70  and the downstream portion  66 . 
     In the embodiment shown, the downstream bore  68  has an annular radial groove  102  which communicates with the liquid reservoir  29 , so as to form a discharge orifice. 
     This embodiment therefore corresponds to the embodiment depicted in  FIGS. 2 and 3 , in which the downstream circuit  44  comprises a connection to the reservoir  29  at the downstream chamber  36 . 
     The jacket  96  of the downstream portion  66  comprises here several radial orifices  104  which are substantially aligned circumferentially and which are arranged opposite the radial groove  102 , when the piston  32  occupies its upstream position, as depicted in the top half of  FIG. 5 . 
     The radial orifices  104  make it possible to put the downstream chamber  36  and the reservoir  29  in communication, when the piston  32  is occupying its upstream position, so as to compensate for the variations in hydraulic volume in the downstream circuit  44  over time. 
     In the downstream position of the piston  32 , as depicted in the bottom half of  FIG. 5 , the orifices  104  are offset axially in the downstream direction, with respect to the radial groove  102 , so that the downstream chamber  36  does not communicate with the reservoir  29 . 
     Naturally, as indicated with reference to the variant in  FIG. 4 , the radial groove  102  and the radial orifices  104  can be omitted in favour of a discharge orifice  52  produced axially in the piston  32  and provided with a discharge valve  54 . 
     The assistance device  50  comprises here an elastic element in the form of an axial helical compression spring  106 . 
     The spring  106  is designed to compress during the engagement phase, under the effect of the return of the assistance piston  32  to its upstream position, so as to store energy, and is designed to restore this energy during the disengagement phase, producing an assistance force F a . 
     The spring  106  is interposed axially between an axially movable annular cup  108  and a fixed annular radial abutment surface  110  provided in the cylinder body  56 . 
     The cup  108  comprises an annular radial abutment surface  111  which is oriented in the upstream direction and which faces the fixed abutment surface  110  oriented in the downstream direction. 
     The spring  106  is here mounted around an internal tubular guide portion  112  of the cylinder body  56 . 
     The assistance device  50  comprises a cam mechanism  114  which is driven by the axial movement of the piston  32 , and which forms a regulation means  115  for varying the value of the assistance force F a  according to the travel C p  of the pedal  22  in accordance with a predetermined assistance law. 
     The cam mechanism  114  comprises, for example, two moving rollers  116 ,  118  which each travel over an associated control surface  120 ,  122 . 
     The two rollers  116 ,  118  are here arranged on each side of the piston  32  and are diametrically opposed. 
     The rotation axis A 2  of each roller  116 ,  118  is substantially orthogonal to the sliding axis A 1  of the piston  32 . 
     Each roller  116 ,  118  is connected by a downstream link  124  to the connecting rod  70  and by an upstream link  126  to the cup  108 . 
     It should be noted that the connecting rod  70  here constitutes a transmission member  71  which enables the assistance device  50  to transmit the assistance force F a  to the assistance piston  32 . 
     On the roller  116 ,  118  side, the links  124 ,  126  are mounted for pivoting about the rotation axis A 2  of the roller  116 ,  118 . 
     The downstream link  124  is mounted for pivoting on the free end of an associated transverse arm  128 ,  130  of the connection rod  70 . 
     According to the embodiment depicted here, the cylinder body  56  forms an envelope  132  that is roughly cylindrical around the assistance device  50 . The envelope  132  is stepped in diameter. 
     Advantageously, the control surface  120 ,  122  associated with each roller  116 ,  118  is produced on the internal wall of the envelope  132 . 
     The control surfaces  120 ,  122  are here substantially symmetrical with respect to an axial plane (A 1 ) and extend roughly in the same axial plane. 
     Each control surface  120 ,  122  comprises an upstream portion  134  inclined with respect to the sliding axis A 1  and a downstream portion  136  roughly parallel to the sliding axis A 1 . 
     The upstream portion  134  has here a rounded profile convex towards the axis A 1  and towards the upstream end. 
     According to an advantageous embodiment, the distance between the pivot axes of each upstream link  126 ,  128  is such that, in the upstream position of the associated moving roller  116 ,  118 , the roller goes beyond, towards the upstream end, the point B 1  on the control surface  120 ,  122  where the upstream link  126 ,  128  is perpendicular to the control surface  120 ,  122 , so that the relaxing force of the assistance spring  106  biases the moving roller  116 ,  118  towards its upstream position. 
     In another advantageous embodiment, when the piston  32  is occupying its downstream declutching position, the axial distance between the abutment surface  111  of the cup  108  and the fixed abutment surface  110  is greater than the axial dimension of the spring  106  in the relaxed state, so that the spring  106  does not axially bias the piston  32  towards its downstream position. 
     The functioning of the cam mechanism  114  according to the invention is now explained, considering in particular the partial positions depicted in  FIGS. 6 and 7  and the diagrams depicted in  FIG. 8 . 
     On the top part of  FIG. 8 , the curve C aval  in continuous line represents a change in the hydraulic pressure P h  in the downstream chamber of the assistance cylinder  30 , during the declutching phase, as a function of the travel C p  of the clutch pedal  22 , and the curve C amont  in a broken line represents the change in the hydraulic pressure P h  in the upstream chamber of the assistance cylinder  30  during the declutching phase as a function of the travel C p  of the clutch pedal  22 . 
     In the bottom part of  FIG. 8 , the curve in a continuous line represents the change in the assistance force F a  produced by the assistance spring  106 , during the declutching phase, as a function of the travel C p  of the clutch pedal  22 , and the straight line in a broken line represents the stiffness of the assistance spring  106 . 
     In the upstream position of the assistance piston  32 , which is illustrated by the top part of  FIG. 5 , the spring  106  is compressed and axially biases (A 1 ) each upstream link  126 , as well as the associated roller  116 ,  118 , towards the control surface  120 ,  122  and towards the outside, without causing any movement of the assistance piston  32 . 
     The moving rollers  116 ,  118  are here held by the associated downstream link  124 , which is connected to the assistance piston  32  in the upstream abutment position. 
     At the start of the declutching phase, the user presses on the control pedal  22  of the clutch  12  so as to move the piston of the sending cylinder  14  in the downstream direction. 
     The first part of the movement of the piston of the sending cylinder  14  corresponds to a dead travel, until the discharge orifice connecting the sending chamber  38  to the reservoir  29  closes. 
     Continuing its movement in the downstream direction, the piston of the sending cylinder  14  then causes an increase in the hydraulic pressure P h  in the upstream chamber  34  of the assistance cylinder  30 , which causes an axial movement A 1  of the assistance piston  32  towards the downstream end. 
     The movement of the assistance piston  32  causes a movement of the rollers  116 ,  118  over the associated control surfaces  120 ,  122 , towards the inside and towards the downstream direction. 
     During a first phase P 1  of its axial movement, the assistance piston  32  causes an additional compression of the assistance spring  106 , so that the assistance device  50  produces a resistance force which opposes the movement of the clutch pedal  22 , which corresponds to a negative assistance force F a , illustrated by the bottom part of  FIG. 8 . 
     During this first phase P 1 , the hydraulic pressure P h  in the upstream chamber  34  of the assistance cylinder  30  is greater than the hydraulic pressure P h  in the downstream chamber  36 . 
     The first phase P 1  of the movement of the piston  32  ends when the rollers  116 ,  118  reach the point B 1  on the control surface  120 ,  122  where the upstream link  126 ,  128  is perpendicular to the control surface  120 ,  122 , which is shown in  FIG. 6 . 
     At the moment when the rollers  116 ,  118  reach this point B 1  of the control surface  120 ,  122 , the relaxation force of the assistance spring  106  is cancelled by the reaction force of the control surface  120 ,  122 , so that the hydraulic pressure P h  equalises between the upstream chamber  34  and the downstream chamber  26  of the assistance cylinder  30 . 
     It should be noted that, during the first phase P 1 , the axial movement of the piston  32  causes the closure of the connection  102  of the downstream chamber  36  with the reservoir  29 , by virtue of the axial offset of the radial orifices  104  in the downstream direction. 
     During a second phase P 2  of the axial movement of the assistance piston  32  in the downstream direction the assistance spring  106  commences to relax, producing an assistance force F a  on the assistance piston  32 . 
     The assistance force F a  produced by the spring  106  is stepped down by the cam mechanism  114 , according to the profile of the upstream portion  134  of the control surface  120 ,  122 , which makes it possible to regulate the assistance force F a  according to the travel C p  of the pedal  22 , in accordance with a predetermined assistance law. 
     The second phase P 2  of the movement of the piston  32  ends when the rollers  116 ,  118  reach the downstream end B 2  of the upstream portion  134  of the associated control surface  120 ,  122 , as depicted in  FIG. 7 . 
     The assistance piston  32  then begins a third phase P 3  of its axial movement, during which the rollers  116 ,  118  travel over the downstream portion  136  of the associated control surface,  120 ,  122 . 
     During this third phase P 3 , the assistance spring  106  transmits all its relaxation force to the assistance piston  32  since the links  124 ,  126  are no longer pivoting and the rollers  116 ,  118  are no longer held axially by the control circuit  120 ,  122 . 
     It should be noted that, during the third phase P 3 , the links  124 ,  126  can be close to an aligned position but it is preferable to keep a minimum inclination angle between the links  124 ,  126 , as in  FIG. 7  and on the bottom part of  FIG. 5 , so as to cause the links  124 ,  126  to pivot, during the return of the assistance piston  32  towards its upstream position, in order to prevent locking of the piston  32  in the cylinder  30 . 
     According to the advantageous embodiment provided here, as the axial distance between the abutment surface  111  of the cup  108  and the fixed abutment surface  110  is greater than the axial dimension of the spring  106  in the relaxed state, the third phase P 3  is followed by a fourth phase P 4  during which the assistance piston  32  continues to slide as far as its downstream abutment position, without benefiting from an assistance force F a  since the assistance spring  106  is in the relaxed state. 
     The fourth phase P 4  is useful for minimising the friction forces caused by the assistance device  50  during the movement of the assistance piston  32 , so as to guarantee the return of the piston  32  in the upstream direction, from its downstream position, in particular when the return force of the assistance piston  32  produced, for example, by the diaphragm  13  is small. 
     When the user releases his pressing on the pedal  22 , the return elements of the clutch  12  such as the diaphragm  13  cause the return of the assistance piston  32  in the upstream direction. 
     The return of the piston  32  in the upstream direction causes a return of the cam mechanism  114  into its initial position and a compression of the assistance spring  106 , which enables it to store elastic energy. 
     Preferably, at the end of the travel of the assistance piston  32  in the upstream direction, which corresponds to the first phase P 1  of the movement of the piston  32  in the downstream direction, the assistance spring  106  causes an elastic return of the piston  32  as far as its upstream position, biasing the movable rollers  116 ,  118  towards their upstream idle positions. 
     In the upstream position of the piston  32 , the orifices  104  of its downstream portion  66  come to be positioned opposite the radial groove  102 , which connects the downstream hydraulic circuit  44  to the liquid reservoir  29 . 
       FIG. 9  depicts schematically a second embodiment of the control system  10  according to the invention. 
     It should be noted that the representation of the second embodiment has been simplified with respect to the representation of the first embodiment in  FIG. 5 . 
     This second embodiment is differentiated from the first mainly by the fact that the assistance device  50  is arranged at the upstream axial end of the assistance cylinder  30  rather than between the two hydraulic chambers  34 ,  36 . 
     According to this embodiment, the assistance cylinder  30  comprises a piston  32  whose upstream transverse face  138  delimits the upstream chamber  34  and whose downstream transverse face  140  delimits the downstream chamber  36 . 
     The piston  32  is here produced overall in a single piece. 
     According to the embodiment depicted here, a helical compression spring  142  is interposed axially between a downstream transverse end surface  144  of the piston  32  and the bottom wall  146  of the downstream chamber  36 . This spring  142  serves to guarantee the return of the piston  32  as far as its upstream abutment position. 
     The assistance device  50  is produced in a similar manner to that of the first embodiment. It comprises in particular an assistance spring  106  and a cam mechanism  114 . 
     The assistance device  50  comprises a transmission member  50  in the form of a transmission rod that extends axially towards the upstream transverse face  138  of the assistance piston  32 . 
     According to an advantageous embodiment, depicted here, the transmission rod  71  cooperates solely by contact with the upstream transverse surface  138  of the assistance piston  32 . 
     This configuration of the transmission rod  71  enables the assistance piston  32  to slide independently of the rod  71 . Thus, where the pressure exerted by the assistance device  50  on the transmission rod  71  is less than the pressure exerted on the assistance piston  32  by the fluid contained in the upstream chamber  34 , the assistance piston  32  can slide in the downstream direction without being slowed down by the movement of the assistance device  50 . 
     Such a configuration also mitigates malfunctioning of the assistance device  50  since the control system  10  can function without assistance. 
       FIG. 9  also depicts a variant embodiment of the device for connecting the downstream chamber  36  to the reservoir  29 . 
     Advantageously, the assistance cylinder  30  comprises here a discharge valve  148  which is controlled by the piston  32 , so as to connect to the reservoir  29  when the piston  32  is occupying its upstream position. 
     To this end, the pipe connecting to the reservoir  29  emerges, through a discharge orifice  150 , in an intermediate cylindrical cavity  152  which is arranged at the downstream axial end of the cylinder body  56 . 
     The intermediate cavity  152  communicates with the downstream chamber  36  through an opening  154  that emerges in the bottom wall  146  of the downstream chamber  36 . 
     The valve  148  comprises a rod or tail  156  which is provided, at its downstream axial end, with a head  158  able to close off the communication orifice  150 , and at its upstream axial end with a control collar  160  delimiting a transverse abutment surface  162  oriented in the downstream direction. 
     The valve  148  is biased axially in the downstream direction and therefore towards the closure position of the discharge orifice  150 , by a valve spring  164  interposed axially between the head  158  and a transverse annular rim  166 , oriented in a downstream direction, of the intermediate cavity  152 . 
     The control collar  160  of the valve  148  is designed to cooperate by contact with the upstream transverse surface of a transverse annular rim  168  arranged at the downstream axial end of the piston  32  so that, at the end of travel of the piston  32  in the upstream direction, the annular rim  168  comes into axial abutment against the transverse surface  162  of the control collar  160  in order to cause an axial movement of the valve  148  in the upstream direction, counter to its spring  164 . 
     The movement of the valve  148  in the upstream direction causes the opening of the discharge orifice  150 , which connects the downstream chamber  36  to the reservoir  29  at the end of the travel of the piston  32  in the upstream direction. 
     The functioning of the assistance cylinder  30  according to the second embodiment is similar to that of the assistance cylinder  30  according to the first embodiment. 
     Compared with the first embodiment, the discharge valve  148  according to the second embodiment has the advantage of requiring a shorter axial travel in order to cause the connection of the downstream chamber  36  to the reservoir  29 , with a sufficient cross section of flow of the fluid. 
       FIG. 10  depicts schematically a third embodiment of the control system  10  according to the invention, in which the assistance device  50  comprises an electrical actuator  170  that controls the relaxation of an elastic assistance element  172  during the declutching phase. 
     According to this embodiment, the assistance force F a  is therefore produced by the relaxation of an elastic assistance element  172 , here a helical compression spring, as in the first and second embodiments. 
     The cam mechanism  114  here has been replaced by the electrical actuator  170 . 
     In the diagram in  FIG. 10 , the electrical actuator  170  comprises a lever  174  which is interposed axially between the rod  48  of the assistance piston  32  and the movable axial end of the assistance spring  172 , and which is controlled pivotally by the transmission shaft  176  on the electric motor  178 . 
     The assistance spring  172  stores elastic energy during the engagement phase, in particular under the effect of the elastic return of the clutch  12  towards its engagement position, which pushes the assistance piston  32  towards its upstream position. 
     During the declutching phase, the electrical actuator  170  releases the assistance spring  172  so as to produce the assistance force F a  on the piston  32 . 
     The motor  178  is preferably controlled by an electronic control unit  180  which constitutes a means  115  of regulating the assistance force F a . 
     The control unit  180  controls, for example, the electric motor  178  in accordance with operating parameters such as:
         the hydraulic pressure P h  in the upstream circuit  40 ,   the travel C p  of the clutch pedal  22 ,   data D ext  external to the control system  10 , for example data relating to the functioning of the vehicle engine, the functioning of the vehicle gearbox, the functioning of the clutch  12 , etc.       

     The operating parameters can be supplied to the control unit  180  by sensors (not shown). 
     The value of the travel C p  of the pedal  22  can be supplied to the control unit  189  by the electric motor  178 , in particular in the case where the rotation of its transmission shaft is linked to the sliding of the assistance piston  32 . 
     The control unit  180  can modulate the assistance force F a  according to at least one predetermined assistance law. 
     A fourth and fifth embodiment of the control system  10  according to the invention are now described, in which the assistance device  50  is connected to an energy source  182 ,  184  which is external to the control system  10  and which is installed in the vehicle that the control system  10  equips. 
     According to these embodiments, the assistance force F a  is produced by the external energy source  182 ,  184  and is then transmitted to the assistance piston  32 . 
     In the fourth embodiment, depicted in  FIG. 11 , the external energy source  182  consists of a source of electric current, which may be the system supplying electrical energy to the vehicle. 
     The assistance cylinder  30  of the fourth embodiment is roughly similar to that of the second embodiment, depicted in  FIG. 9 , except that the cam mechanism  114  of the assistance device  50  is replaced by an electrical actuator  186  which acts directly on the transmission rod  71 . 
     According to the embodiment depicted, the transmission rod  71  is equipped, at its upstream axial end, with a threaded portion  188  which is mounted screwed on a threaded shaft  190  able to be driven in rotation about its axis A 1  by an electric motor  192 , which is connected to the source of electric current  182 . 
     Advantageously, the electric motor  154  can be controlled by an electronic control unit (not shown in  FIG. 11 ) in the same way as the third embodiment described above ( FIG. 10 ). 
     In the fifth embodiment, illustrated by  FIGS. 12 to 19 , the external energy source  184  consists of a hydraulic or pneumatic pressure source. 
     In the remainder of the description, non-limitingly, solely a hydraulic pressure source  184  will be considered, although a pneumatic pressure source can also be envisaged. 
       FIG. 12  illustrates the operating principle of the fifth embodiment. 
     In the fifth embodiment, the control rod  48  of the assistance piston  32  is linked in axial movement (A 1 ) to a ram  194  connected to the pressure source  184  by an auxiliary control circuit  185 , so as to transmit an assistance force F a  to the assistance piston  32  during the declutching phase. 
     The ram  194  comprises a so-called auxiliary piston  196  which slides in an auxiliary cylinder  198  and which, upstream, delimits an auxiliary control chamber  200 . 
     The auxiliary piston  194  cooperates, for example, by contact with the control rod  48  of the assistance piston  32 . 
     During the declutching phase, the pressure source  184  causes an increase in the hydraulic pressure P h  in the control chamber  200  of the ram  194 , which produces an assistance force F a  on the assistance piston  32 , by means of the control rod  48 , or transmission rod. 
       FIG. 13  depicts an example of an assistance cylinder  30  equipped with a ram  194  in accordance with the teachings of the invention, and in which the auxiliary piston  196  is replaced by a flexible membrane  197 , or “unwinding” membrane. 
     On the left-hand part of  FIG. 13 , the assistance piston  32  is shown in the downstream position and on the right-hand part the assistance piston  32  is shown in the upstream position. 
     The assistance piston  32  is here produced in a similar manner to that of the second embodiment ( FIG. 9 ), except that the discharge orifice  52  and the discharge valve  54  are produced in an axial orientation in the body of the assistance piston  32 , as on the variant described with reference to  FIG. 4 . 
     The discharge orifice  52  is offset with respect to the axis to allow a centred abutment of the rod  48  on the piston  32 . In the case of an inclined mounting of the assistance cylinder  30 , the orifice  52  is put in the high position in order to guarantee the purging of the air downstream of the piston  32 , at the time of mounting. 
     The auxiliary cylinder  198  of the ram  194  is arranged here at the upstream axial end of the assistance cylinder  30 . The auxiliary cylinder  198  can be formed in an extension of the assistance cylinder body  56 . 
     The membrane  197  is sealed in the auxiliary cylinder so as to delimit, on the same side as its upstream transverse face  202 , the control chamber  200  which is connected to the pressure source  184  by an auxiliary orifice  204 . 
     The membrane  197  is here of the single-acting type since its downstream transverse face  206  is exposed to atmospheric pressure. 
     The control rod  48  of the assistance piston  32 , or transmission road, comprises at its upstream axial end an abutment disc  208  which is in contact with the downstream transverse face  206  of the membrane  197 . 
     When the hydraulic pressure P h  increases in the control chamber  200 , exceeding atmospheric pressure, the membrane  197  “unwinds”, exerting an axial abutment force directed in the downstream direction on the disc  208 , which produces an assistance force F a  on the piston  32 , by means of the transmission rod  48 . 
     A variant of this latter embodiment, illustrated in  FIG. 19 , consists of using the abutment between the rod  480  on the piston  320  in order to control the opening of a discharge orifice  520 . In this case, the orifice  520  is a channel pierced in the piston  320  along the axis of the rod  480 , and the end of this rod  480  has a complementary shape with respect to that of the outlet  540  of the orifice  420  so as to achieve the obstruction of this orifice when the rod is in abutment on the piston. An elastomer seal can be placed between the rod and the piston. 
     When the clutch is engaged and the assistance is not activated, a small clearance is created between this rod  480  and the piston  320 , the orifice  520  is free. This orifice closes when the assistance force occurs. This variant is simple to implement since it does not require any supplementary part and keeps all the advantages of the discharge orifices presented above. 
       FIGS. 14 to 18  illustrate several possible solutions for regulating the assistance force F a  produced by the ram  194 . 
     According to a first solution, illustrated by  FIGS. 14 and 15 , the means  115  of regulating the assistance force F a  consists of two control valves  210 ,  212  which constitute respectively a charging valve  210  and a discharge valve  212  and which are connected by the auxiliary circuit  185  to the control chamber  200  of the ram  194 . 
     In addition, the charging valve  210  is connected to the pressure source  184  and the discharge valve is connected to a fluid reservoir  29 . 
     A charging valve  210  and a discharge valve  212  are controlled here by the hydraulic pressure P h  in the upstream circuit  40  counter to the return force of a spring  214 ,  216  associated with each valve  210 ,  212 . 
     Advantageously, the stiffness of the spring  214  associated with the charging valve  210  is greater than the stiffness of the spring  216  associated with the discharge valve  212 , so that the opening of the charging valve  210  and the closing of the discharge valve  212  are offset in time, during the declutching travel. 
     In  FIG. 14 , the control system  10  is depicted at rest, in the engaged position, which corresponds to an absence of pressure in the upstream hydraulic circuit  40 . 
     In this position, the charging valve  210  is closed and the discharge valve  212  is open, so that the auxiliary circuit  185  is connected to the reservoir  29 . 
     An explanation is now given of the functioning of the control system  10  of the  FIG. 14 , during the declutching phase, considering in particular  FIG. 15 . 
     In  FIG. 15 , the curve C aval1  in a continuous line represents the change in the hydraulic pressure P h  in the downstream chamber  36  of the assistance cylinder  30  during the declutching phase, as a function of the travel C p  of the clutch pedal  22 , when the clutch  12  is worn. 
     The curve C aval2 , in a broken line, represents the same change as the curve C aval1 , when the clutch  12  is new. 
     When the clutch  12  wears, the hydraulic pressure P h  necessary for performing the declutching operation increases. 
     The curve C amont  in a continuous line represents the change in the hydraulic pressure P h  in the upstream chamber  34  of the assistance cylinder  30 , during the declutching phase, as a function of the travel C p  of the clutch pedal  22 . 
     When the clutch pedal  22  is actuated, the hydraulic pressure P h  in the upstream circuit  40  increases. 
     After a first travel C p1  of the pedal  22 , the hydraulic pressure P h  reaches a first threshold value P hs  which is sufficient to cause the movement of the discharge valve  212  counter to its spring  216 , which causes the closure F v212  of the discharge valve  212 . 
     After a second travel C p2  of the pedal  22 , the hydraulic pressure P h  reaches a second threshold value P hr , referred to as the regulated value, which is sufficient to cause the movement of the charging valve  210  counter to its spring  214 , which causes the first opening O v1  of the charging valve  210 . 
     The charging valve  210  being open, the pressure source  184  is connected to the control chamber  200  of the ram  194 , which produces an assistance force F a  on the piston  32 . 
     The assistance force F a  applied to the piston  32  causes a reduction in the hydraulic pressure P h  in the upstream circuit  40  so that, after a given lapse of time, which corresponds to a third travel C p3  of the pedal  22 , the charging valve  210  returns to its idle position, which corresponds to a first closure F v1  of the charging valve  210 . 
     It should be noted that the discharge valve  212  remains closed since the hydraulic pressure P h  in the upstream circuit  40  does not drop as far as the threshold value P hs  associated with this valve  212 . 
     The return of the charging valve  210  to its idle position once again causes an increase in the hydraulic pressure P h  in the upstream circuit  40  since, the pedal  22  continuing its pressing-down travel, there is a drop in pressure in the control chamber  200  due to the descent of the ram  196 . 
     After a fourth travel C p4  of the pedal  22 , the hydraulic pressure P h  once again reaches the regulated value, which causes a second opening O v2  of the charging valve  210 . 
     This succession of openings and closings of the charging valve  210  continues until the assistance piston  32  is occupying its downstream position. 
     In practice, in order to limit these oscillations, use is made of a charging valve  210  which opens and closes progressively for small oscillations from the regulated value P hr  in this way, the openings and closings are not abrupt and the equilibrium position is reached more rapidly. 
     The charging valve  210  therefore allows a closed-loop regulation of the hydraulic pressure P h  in the upstream chamber  34  of the assistance cylinder  30 , which stabilises around the regulated value P hr . 
     Consequently, as the hydraulic pressure P h  in the upstream chamber  34  of the assistance cylinder  30  is linked to the pressure P h  in the sending cylinder  14 , the force that the user must apply to the pedal  22  tends towards a constant value throughout the declutching phase. 
     When the user releases the pedal  22 , the hydraulic pressure P h  decreases in the upstream chamber  34 , which first of all causes the closure of the charging valve  210  and then, having arrived at the first threshold value P hs , the opening of the discharge valve  212  permitting the return of the assistance piston  32  to its upstream position. 
     One advantage of this solution is that the force used by the user on the pedal  22  is not dependent on the wear on the clutch  12 . 
     This advantage is illustrated in  FIG. 15  by the fact that the pressure curve C amont  is identical for the two pressure curves C ava11 , C ava12  associated with the downstream chamber  36 , the one corresponding to a worn clutch  12  and the one corresponding to a new clutch  12 . 
     According to a second solution, which is illustrated by  FIGS. 16 and 17 , the two valves  210 ,  212  provided in the first solution are replaced by a single three-position control valve  218 . 
     A first position of the control valve  218 , referred to as the charging position, causes the connection of the control chamber  200  to the pressure source  184 . 
     A second position, or intermediate position, of the control valve  218  corresponds to a closure position of the control valve  218 . 
     A third position of the control valve  218 , referred to as the discharge position, causes the connection of the control chamber  200  to the fluid reservoir  29 . 
     In  FIG. 16 , the control valve  218  is shown in its discharge position. 
     The control valve  218  has two control pressures P c1 , P c2  which are applied on each side of the control valve  218  with identical abutment surfaces. 
     The first control pressure P c1  corresponds to the hydraulic pressure P h  in the upstream chamber  34  of the assistance cylinder  30 , and is applied at the same side as the first position. 
     The second control pressure P c2  corresponds to the hydraulic pressure P h  in the control chamber  200  of the ram  194  and is applied at the third position. 
     The functioning of the control valve  218  is as follows. 
     When the two control pressures P c1 , P c2 , are equal, that is to say when the hydraulic pressure P h  is equal in the upstream chamber  34  and in the control chamber  200 , the control valve  218  occupies its intermediate closure position. 
     When the first control pressure P c1  is greater than the second control pressure P c2 , that is to say when the pressure in the upstream chamber  34  is greater than the pressure in the control chamber  200 , the control valve  218  occupies its charging position. 
     When the first control pressure P c1  is less than the second control pressure P c2 , that is to say when the pressure in the upstream chamber  34  is less than the pressure in the control chamber  200 , the control valve  218  occupies its discharge position. 
     The embodiment depicted in  FIG. 16  thus makes it possible to effect a so-called proportional regulation of the assistance force F a , which is illustrated by  FIG. 17 . 
     The curve C aval  in a continuous line represents the change in the hydraulic pressure P h  in the downstream chamber  36  of the assistance cylinder  30 , during the declutching phase, as a function of the travel C p  of the clutch pedal  22 . 
     The curve C amont  in a broken line represents the change in the hydraulic pressure P h  in the upstream chamber  34  of the assistance cylinder  30 , during the declutching phase, as a function of the travel C p  of the clutch pedal  22 . 
     It is found that, in the particular case, illustrated here, where the abutment surfaces of the control pressures P c1 , P c2  are identical, the assistance force F a  produces approximately one half of the total force to be supplied on the piston of the receiving cylinder  18 . 
     Naturally, it is possible to modify the ratio between the assistance force F a  and the total source to be supplied by modifying the ratio between the abutment surfaces of the control pressures P c1 , P c2 . 
       FIG. 18  depicts a third solution in which the regulation means  115  is a three-position control valve  218 , as in  FIG. 18 , but which is differentiated from the second solution in that the control valve  218  is controlled by an electronic control unit  220 . 
     The control unit  220  can control the control valve  218  according to control parameters measured by sensors such as the travel of the pedal C p , the hydraulic pressure in the upstream chamber  34  and external data D ext . 
     The control unit  220  can also control the pressure source  184 , which makes it possible to precisely apportion the required assistance force F a . 
     According to the embodiment depicted in  FIG. 18 , by virtue of the control unit  220 , it is possible to precisely choose the required assistance curve. In particular, it is possible to reproduce the assistance curve of the first and second solutions ( FIGS. 15 and 17 ). 
     It should be noted that the embodiments of the control system  10  according to the invention, which are depicted with the discharge valve  54  produced axially in the assistance piston  32  and with a single connection to the fluid reservoir  29 , could have been represented with two connections of the reservoir  29 , as depicted and described in particular with reference to  FIGS. 2 and 3 . 
     According to a variant (not shown) of the various embodiments described above, it is possible to add in the fluid passage, either in the upstream  40  or downstream  44  circuit, or in the control circuit  185  of the assistance ram  196 , a device reducing the cross section of flow of the fluid in the direction of engagement. 
     This device can be a valve that occupies a first position forming a maximum cross section of flow in the direction of disengagement, and a second position forming a reduced cross section of flow in the direction of engagement. 
     Such a device makes it possible in particular to avoid an impact when releasing the pedal  22  too rapidly. 
     More generally, the assistance device  5  can comprise a regulation means  218  which varies the value of the assistance force F a  as a function of the upstream pressure P h  in the upstream chamber  34  of the assistance cylinder  30 , or the downstream pressure P h  in the downstream chamber  36 , or a combination of the two pressures, according to a predetermined assistance law.