Abstract:
An electrohydraulic servo brake includes a master brake cylinder, which is controlled by a push rod. The push rod is configured to be actuated by a first auxiliary piston via a reaction disk. A plunger piston presses on the reaction disk. An end of the push rod is connected to the first auxiliary piston by a connection apparatus. The connection apparatus includes a rest for a rated compression spring, which rest is rigidly connected to the push rod, and a central piston, which rests against the reaction disk along an axis and is surrounded by a second auxiliary piston, which is placed against the reaction disk and is moved by the spring and is held in an end position by the central piston. The second auxiliary piston is configured to recede in relation to the central piston rigidly connected to the push rod.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2014/058995, filed on May 2, 2014, which claims the benefit of priority to Serial No. FR 1354079, filed on May 3, 2013 in France, the disclosures of which are incorporated herein by reference in their entirety. 
     The present disclosure relates to an electrohydraulic servo brake which has a main brake cylinder which is controlled by the push rod which is actuated by the auxiliary piston by means of the reaction disc, wherein there extends through the auxiliary piston itself a plunger piston which presses on a portion of the reaction disc by being connected to the control rod which is itself connected to the brake pedal, and a detector which detects the movement of the control rod in order to actuate the electric motor of the servo brake, which electric motor drives the auxiliary piston. 
     BACKGROUND 
     Such an electrohydraulic servo brake is known according to the general prior art. However, with such a servo brake, when the ABS system is actuated, the modulation sends a specific quantity of brake fluid into the main brake cylinder. This becomes evident as a backward movement of the brake pedal, together with weak pressure peaks in the main brake cylinder. These pressure fluctuations are in the order of magnitude of ±30 bar. As a result of the mechanical structure thereof, however, the servo brake with electric motor is a rigid transmission system which does not move backward spontaneously under the action of such loads. This results in an increase of the amplitude of the pressure peaks during this maneuver. This increase may exceed 100 bar so that the transmission of the servo brake is subjected to great fatigue at the expense of the reliability thereof. 
     SUMMARY 
     An object of the present disclosure is to develop an electrohydraulic servo brake whose pressure peaks produced during the use of the ABS system are reduced in order to reduce the forces applied to the transmission of the servo brake. 
     To this end, the present disclosure relates to a servo brake of the type defined above, characterized in that the end of the push rod is connected to the auxiliary piston by means of a connection which contains:
         a support which is securely connected to the push rod for a tared pressure spring,   a central piston in abutment in accordance with the axis against the reaction disc,   an auxiliary piston which surrounds the central piston by being placed against the reaction disc in a state pushed by the tared spring and retained in the end position by the central piston, wherein the auxiliary piston can move backward with respect to the central piston which is securely connected to the push rod.       

     According to the disclosure, the pushing action of the tared spring corresponds to a pressure in the main brake cylinder. Advantageously, this pressure is fixed at 130 bar so that below this pressure in the main brake cylinder the servo brake operates normally: the auxiliary piston pushes the push rod by means of the reaction disc without the reaction disc becoming deformed with respect to the central piston and the auxiliary piston which are carried by the end of the push rod. 
     However, if the pressure in the main brake cylinder exceeds this desired pressure which is fixed at 130 bar in this instance, which corresponds to the taring of the pressure spring of the connection portion, the auxiliary piston gives way to the pushing action of the pressure spring so that the reaction applied to the auxiliary piston by the main brake cylinder and the translation thereof (toothed rod, toothed pinion, etc.) is reduced, whilst the pushing action transmitted by the central piston to the plunger piston and to the push rod and then to the brake pedal is increased. 
     This change of the distribution of the loads applied in return by the main brake cylinder to the servo brake improves the operation of the servo brake and reduces the loading connected with the pressure peaks and consequently the fatigue of the transmission, which becomes evident generally as improved efficiency and longer service-life of the transmission. 
     In the region of this desired pressure which corresponds to the tared reaction of the spring, the amplification ratio of the servo brake is changed: it is reduced beyond this critical pressure which, as already stated, reduces the loads applied to the translation and increases the load applied to the brake pedal and consequently the foot of the driver. 
     According to an advantageous feature, the auxiliary piston is in the form of a crown having a shoulder in order to be engaged on the central piston and, by means of the forward movement thereof, to be driven in the direction of the pushing action of the push rod, whilst it can at the same time move forward with respect to the central piston by the spring being compressed. 
     This embodiment has the advantage of being mechanically very simple and nonetheless protecting the reaction disc in order to prevent excessive displacement since this is limited by the displacement travel of the auxiliary piston. 
     According to another feature, the forward movement of the annular auxiliary piston, which is fitted on the end of the push rod, relative to the push rod is limited by a stop. 
     According to another feature, the pressure spring is tared for a desired pressure in the main brake cylinder, and this pressure is in particular fixed at 130 bar. 
     The cylindrical portion which is a simple component or which is a simple form of the push rod has the dual function of guiding and protecting the pressure spring in order to prevent any deflection and, on the other hand, it protects the spring from compression by the reaction disc also thereby being protected from any excessive deformation, which can lead to the central piston beginning to press through the front side of the reaction disc. 
     According to another advantageous feature, the limit pressure which is determined for the desired pressure is 130 bar. 
     The servo brake according to the disclosure has the advantage of taking over a large number of elements of known servo brakes so that the integration thereof in a production or assembly chain does not present any particular difficulty and does not lead to additional costs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is described in greater detail below with reference to an embodiment, which is illustrated schematically in the drawings, in which: 
         FIG. 1  is an axial section of an electrohydraulic servo brake, 
         FIGS. 2A-2B  schematically show the connection between the auxiliary piston and the push rod of the electrohydraulic servo brake according to the disclosure, 
         FIG. 2A  shows the connection in the lower pushing position when the pushing action is below the tared force, 
         FIG. 2B  is a view similar to that of  FIG. 2A  of the connection between the auxiliary piston and the push rod in the position which has exceeded the tared pushing action, 
         FIG. 3  is a graph which sets out the force at the brake pedal and emphasizes the development of this force below and above the tared pushing action, 
         FIG. 4  is a graph similar to that of  FIG. 3  which sets out the reaction force which is applied to the control rod before and after the tared pushing action. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a general diagram of an electrohydraulic servo brake comprising a main brake cylinder  1  (in this instance, a tandem main brake cylinder) which has a primary piston  11  and a secondary piston  12  which delimit a primary chamber  13  and a secondary chamber  14  which are both supplied with hydraulic fluid from a brake fluid container which cannot be seen in  FIG. 1  and which is connected to the primary chamber  13  and to the secondary chamber  14 . The hydraulic fluid which is placed under pressure in these chambers supplies two brake circuits which are also not illustrated. 
     The main brake cylinder  1  is controlled on the basis of the primary piston  11  which itself is actuated by a push rod  15  which is accommodated for a large part in the primary piston and whose end  151  cooperates with the auxiliary piston  16  of the electromechanical portion of the servo brake in the opposite direction to the end in abutment with the primary piston  11  by means of a thrust piston  152 . The auxiliary piston  16  has two toothed rods  161  symmetrically in the plane section. These toothed rods  161  cooperate with toothed wheels  162  which rotate synchronously in opposing directions by being driven by means of a motor which is not illustrated. The toothed wheels  162  are incorporated in the body  3  of the electromechanical portion of the servo brake. The auxiliary piston  16  is pushed back by a restoring spring  17  into the rest position which surrounds the rear portion of the main piston  11  along the movement axis XX. 
     The auxiliary piston  16  transmits its pushing action to the thrust piston  152  of the push rod  15  by means of the interposition of a reaction disc  18  whose front side  181  (side at the side of the main brake cylinder) is in abutment against the auxiliary piston  151  and whose rear side  182  (at the side of the brake pedal) is in abutment with the base of the receiving member  163  which is formed by the auxiliary piston  16 . The auxiliary piston  16  is open at the center thereof about the axis XX and forms the guiding passage  164  of a plunger piston  19  which also moves into abutment against the rear side  182  of the reaction disc  18 . The plunger piston  19  is connected to the control rod  20  which itself is connected to the brake pedal  21 . The details of the control rod  20  and the different devices such as the movement sensors, in order to detect the braking requirement and to evaluate this information, are in this instance known devices which are not described in detail. 
     The electrohydraulic servo brake is controlled when the brake pedal  21  is actuated from the displacement movement of the control rod  20  (arrow F D ) which is placed by means of the plunger piston  19  on the reaction disc  18 . The movement thereof is detected by a movement sensor which is not illustrated in order to actuate the electric motor of the servo brake in order to drive the pinions  162  which move the auxiliary piston  16  forward by means of the toothed rods  161 . The auxiliary piston  16  applies a pushing action against the reaction disc  18  which itself pushes the thrust piston  152  of the push rod  15  in order to push the primary piston  11  and, by means of the fluid compressed in the primary chamber  13 , the secondary piston  12  of the tandem main brake cylinder  1 . 
     In the servo brake according to the prior art, the connection between the push rod  15  and the reaction disc  18  (front side  181 ) is produced by means of the thrust piston  152 , which is mounted so as to be able to be adjusted on the push rod  15 , but which is securely connected to the push rod in the translation direction when the adjustment was carried out (adjustment by means of screwing) so that the entire front side  181  of the reaction disc  18  rests on the piston  152  of the push rod  15 ; this piston  152  is itself introduced into the opening of the receiving member  163  which receives the reaction disc  18  in the auxiliary piston  16 . 
     The electrohydraulic servo brake according to the disclosure differs from this known electrohydraulic servo brake as a result of the connection portion  100  between the push rod  15  and the actuation piston  16 . This connection portion  100  which is surrounded in  FIG. 1  by an ellipse comprises according to the disclosure a connection portion  100 , a connection which is schematically illustrated in  FIGS. 2A and 2B . 
     Conventionally, the front side or “forward direction” is the direction toward the main brake cylinder. The rear side or “rear direction” is the direction toward the brake pedal. 
     The connection portion  100  thus has between the rear end of the push rod  15  and the reaction disc  18  a support  101  in the form of a disc or a crown which is securely connected to the rod  15  and which receives a pressure spring  102  which develops a tared force which corresponds to a desired pressure P C  which is present in the main brake cylinder. This desired pressure is, for example, a pressure of 130 bar. This pressure is evident as a reaction force which is dependent on the geometry of the servo brake and which defines the force to which the pressure spring  102  is tared. As will be seen below, the structure of the connection portion  100  is configured in such a manner that the pressure spring  102  withstands a load which is smaller than or equal to this tared force and it gives way from the time at which it is subjected to a force which is equal to or greater than this tared force. 
     Under these conditions and, as a result of the use of incorrect terminology, the desired pressure in the main brake cylinder, as a result of the use of incorrect terminology, has become synonymous with the reaction force in the connection portion, a force for which the connection portion  100  changes its operating type, that is to say, its amplification mode of the servo brake. 
     The end  151  of the push rod  15  continues in the form of a main piston  103  around the axis XX. This main piston  103  carries an auxiliary piston  104  in the form of an annular crown, provided with a shoulder  1041  around the central opening thereof, in order to be assembled on the end  151  of the push rod  15  upstream of the main piston  103 . This auxiliary piston  104  is pushed by the pressure spring  102  in order to normally be held in abutment against the main piston  103 . In this support position, the front side  1042  of the auxiliary piston  104  and the side  1032  of the central piston  103  are in the same plane, in a state placed against the front side  181  of the reaction disc  18 . 
     The end  151  carries a cylindrical portion  153  having a large diameter in order to accommodate the volume in the pressure spring  102  and to support it. This cylindrical portion  153  also forms a front stop  154  for the auxiliary piston  104  and limits the forward movement thereof in the pushing direction (F MTC ). As can be seen in  FIGS. 2A, 2B , the auxiliary piston  104  can thus be moved relative to the end  151  of the push rod  15  (and the central piston  103 ) between the end position thereof toward the right according to  FIG. 2A , in a state pushed by the spring  102  and in abutment against the central piston  103 , and the other end position toward the left ( FIG. 2B ). The displacement range (e) of the auxiliary piston  104  is limited in order to prevent excessive deformation of the reaction disc  18 , which comprises a resilient material which is deformable, but not compressible (that is to say, without volume change). 
     The other side  182  (rear side) of the reaction disc  18  is in abutment with the base of the receiving member  163  in the auxiliary piston  16  on a face which is in the form of a support crown and which is centered about the axis XX. At the center thereof about the axis XX, the auxiliary piston  16  forms a guiding passage  164  through which the control rod  20  extends and which receives the plunger piston  19  at the front end of the control rod  20 . The plunger piston  19  moves into abutment against the rear side  182  of the reaction disc  18  and against the support crown of the auxiliary piston  16 . At the beginning of a brake actuation, as long as the pushing action which is applied by the control rod  20  and in particular the auxiliary piston  16  to the reaction disc  18 , which itself moves against the auxiliary piston  104  and the main piston  103 , is smaller than the pushing action P C , to which the spring  102  is tared, the load is transmitted to the push rod  15  which itself actuates the main brake cylinder as mentioned above. During this actuation, the side of the central piston  103  and that of the auxiliary piston  104  abut the front side  181  of the reaction disc  18  which remains planar. The pushing action is transmitted to the rod  15  by the central piston  103  and the auxiliary piston  104  and the spring  102  thereof, which is not compressed since the load applied to the pedal  21  and the control rod  20  is lower than the tared reaction of the spring  102 . 
     However, as soon as the load applied exceeds the tared pushing action P C , the spring  102  no longer holds back the auxiliary piston  104  which gives way backward and enables the reaction disc  18  to “creep” around the main piston  103  ( FIG. 2B ), so that the reaction applied to the control rod  20  increases more than provided for by the amplification coefficient of the servo brake. In this instance ( FIG. 2B ), the proportion of the load transmitted by the control rod  20  in the overall load increases and the proportion of the auxiliary piston  16  decreases. The deformation of the reaction disc  18  is in principle limited by the auxiliary piston  104  moving into abutment against the cylindrical portion  153  in order not to damage the reaction disc  18  by a pushing-through action, which the central piston  103  could produce, being initiated. 
       FIGS. 3 and 4  show the transmission of the pushing action from the reaction disc  18  below and above the pushing action in accordance with the tared reaction of the pressure spring  102 . In the graphs, this pushing action is associated with the desired pressure P C  which is present in the main brake cylinder  1  and which is fixed at 130 bar. 
       FIG. 3  shows the reaction R ped  which is applied to the pedal  21  in accordance with the pressure P (force P ped ) in the main brake cylinder  1 , which corresponds to the straight line DR which represents the linear relationship between the force applied to the pedal (or the reaction applied by the servo brake to the pedal) in accordance with the pressure in the reaction disc. 
     The graph of  FIG. 4  illustrates the amplification straight line DR which connects the pushing load B A  applied by the electromechanical servo brake, that is to say, the load applied by the auxiliary piston  16  whose forward movement is controlled by the electric motor with interposition of the toothed pinions  162  and the two toothed rods  161  in accordance with the pressure P to which the reaction disc is subjected. 
     In the two graphs, the desired pressure P C  below the load corresponding to the desired pressure P C  is emphasized, in practice the relationship corresponds to the linear relationship which represents the amplification coefficient of the servo brake, but from this desired pressure the amplification coefficient for each additional load requested by the brake system no longer follows the linear relationship of the straight line DR, but instead changes to a straight line segment DR 1 , DR 2  with another gradient. 
     After passing through the desired pressure P C , the straight line segment DR 2 , on which the force of the amplifier develops, is located below the straight line DR which corresponds to an amplification coefficient which is reduced from the pressure point P C . 
     In contrast, the complement of the pushing action is provided by the actuation of the pedal whose reaction R ped  ( FIG. 3 ) develops in accordance with the straight line segment DR 1  over the segment DR. That is to say, the reaction which is transmitted back to the pedal  21  increases whilst the force applied by the auxiliary piston  16  and consequently by the electromechanical servo brake decreases with respect to this linear line DR.