Abstract:
Braking device including a master cylinder ( 2 ), a primary piston ( 3 ), a control member ( 4 ), a booster ( 6 ) coupled to the control member ( 4 ), and an assist valve (VA) defined by a reaction piston ( 17 ) with a bore ( 18 ) therein for receiving a ratio control member ( 24 ). The control member ( 24 ) has a piston ( 29 ) with a head ( 25 ) located in a chamber ( 26 ) between the reaction piston ( 17 ) and a bushing ( 27 ) retained in the bore ( 18 ). The bushing ( 27 ) has an axial passage ( 28 ) that receives a head ( 25 ) of piston ( 29 ) to form a valve ( 36 ) for closing an inlet to a passage ( 30 ) in the piston ( 29 ). The bushing ( 27 ) slides within bore ( 18 ) to limit a pressure difference between the chamber ( 26 ) and a front part ( 18   a ) of the bore ( 18 ) to a value below a pressure difference that causes the valve ( 36 ) to open.

Description:
The invention relates to a boosted braking device, for a motor vehicle, of the type which comprise a master cylinder controlling the pressure in at least one brake circuit, a primary piston mounted to slide in the master cylinder to create a pressure variation therein, this primary piston being subjected to an actuating force made up of an input force exerted by a manual-control member generally consisting of a brake pedal, and a boost force exerted by a booster which is coupled to the manual-control member. 
   BACKGROUND OF THE INVENTION 
   The booster may be pneumatic and comprise a rigid casing divided into two pneumatic chambers by a moving partition which may be subjected to a difference in pressure between the chambers under the action of a valve actuated by the manual-control member. 
   A braking device of this type is known, for example from EP-B-0 662 894. 
   The conditions under which braking is performed may differ. A first scenario corresponds to ordinary conditions when an obstacle is seen from a distance and braking is performed relatively gently; this braking is known as “slow braking”. Another scenario is that of abrupt braking or “emergency braking”, for example when an obstacle appears suddenly in front of the driver who has to bring his vehicle to rest as quickly as possible. 
   In order to satisfy these various braking conditions, a boosted braking device of the above defined type has been supplemented by an emergency assist valve capable of initiating the intervention of at least two boost ratios corresponding respectively to slow braking and to emergency braking; the boost ratio for slow braking is lower and the reaction opposing the advance of the manual-control member is greater. For emergency braking, the boost ratio is higher and the reaction against the manual-control member is weaker, which means that the driver can brake for longer and more heavily. 
   By convention, the term “front” will be used in the remainder of the description to denote a direction directed from the control member toward the master cylinder and the term “rear” or “back” will be used for the opposite direction. 
   The emergency assist valve comprises a reaction piston which slides in a sealed manner at the rear of a bore of the primary piston, the front part of this bore communicating with the interior volume of the master cylinder, the reaction piston comprising an axial passage through which there passes a ratio control which at its front end has a head located in a chamber lying between the reaction piston and a bushing housed further forward in the bore of the primary piston, this bushing having an axial passage in which a rapid piston of cross section smaller than that of the reaction piston can slide, which rapid piston also has an axial passage open at each end, the rear end of the rapid piston and the head of the ratio control forming a valve capable of shutting off the inlet to the passage of the rapid piston. 
   The slow boost ratio involves the reaction piston, which has the larger cross section; emergency braking involves the rapid piston which has a smaller cross section. 
   It has become apparent that such a braking device equipped with an emergency assist valve, while being entirely satisfactory from the operational and braking force point of view, makes noise, particularly at the end of slow braking. 
   The object of the invention is, above all, to provide a braking device of the type defined previously, equipped with an emergency assist valve. In which the aforementioned drawback does not occur, or at least occurs to a lesser extent. It is also desirable for the solution proposed to remain simple and economical. 
   SUMMARY OF THE INVENTION 
   According to the invention, a boosted braking device for a motor vehicle, equipped with an emergency assist valve, of the type defined previously, is characterized in that it comprises a means for limiting the pressure difference between, on the one hand, the chamber lying between the reaction piston and the bushing and, on the other hand, the zone of the bore of the primary piston which is situated forward of the bushing, to a value below the one which can cause the valve formed by the head of the ratio control and the rear end of the rapid piston to open. 
   Before finding the solution, the inventors had to find the source of the noise in question. The inventors were able to determine that this noise was produced under slow-braking conditions beyond the maximum boost force that the booster could provide, that is to say beyond the “saturation” point defined later on, and that it was due to a sudden flow of fluid following the sudden opening of the valve. According to the invention, this undesired opening of the valve is avoided and the noise is eliminated. Furthermore, the valve is acted upon less often, and its life is improved. 
   According to a first possibility, the means for limiting the pressure difference consists in mounting the bushing so that it can slide in a sealed manner in the bore of the primary piston. Advantageously, a fixed stop, in particular a ring anchored in the wall of the bore of the primary piston, is provided to retain the bushing on the side facing toward the reaction piston, while the other side of the bushing is subjected to the action of a spring. 
   According to an alternative form, the bushing is axially immobilized in the bore of the primary piston and the means for limiting the pressure difference comprises a connecting duct connecting between the chamber and the zone forward of the bushing, this connecting duct comprising a non-return valve rated at a value that allows flow without perceptible noise in the chamber-front zone direction. 
   According to another alternative form, a compressible foam ring is placed in the chamber, this ring being capable of reducing in volume once a given limiting pressure is reached, this pressure being lower than the limiting value that is acceptable for avoiding the opening of the valve. 
   Apart from the provisions explained hereinabove, the invention consists in a certain number of other provisions which will be dealt with more fully hereinafter with regard to some exemplary embodiments described in detail with reference to the appended figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a part view in section with partial cutaway of a braking device according to the invention; 
       FIG. 2  is a graphical depiction of the variation in braking pressure plotted on the ordinate axis as a function of the input force exerted on the brake pedal, plotted on the abscissa axis; 
       FIG. 3  is a part view in section, on a larger scale, of elements of  FIG. 1  during slow braking before saturation is reached; 
       FIG. 4  is an axial section, on a larger scale, of elements of  FIG. 3  during slow braking after saturation is reached; and 
       FIG. 5 , finally, schematically depicts an alternative form of embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The overall structure and general operation of a boosted braking device of the type of the invention are known, particularly from patents EP-B-0 652 894 or FR-B-2 658 466, and only a brief reminder thereof will be given. For further details, reference may be made to the two aforementioned patents. 
     FIG. 1  shows a braking device  1  for a motor vehicle which comprises a master cylinder  2 , depicted in part, and a primary piston  3  mounted to slide in the master cylinder  2 . A manual-control member  4  consisting of a linkage coaxial with the primary piston  3  is designed to exert on this piston an input force from back to front, that is to say from right to left in the depiction of FIG.  1 . The forward displacement of the primary piston  3  creates an increase in pressure of the liquid in the interior volume  5  of the master cylinder  2 , connected to at least one hydraulic brake circuit. The control member  4  is generally actuated by a brake pedal  4   a , depicted schematically. 
   A pneumatic booster  6  is coupled to the control member  4 . The booster  6  comprises a rigid casing  7  divided internally in a sealed manner into two pneumatic chambers  7   a ,  7   b  by a moving partition  8  comprising a diaphragm  8   a  made of elastomer and a rigid skirt  8   b . The chamber  7   a  is connected permanently to a source of partial vacuum (not depicted) by a nozzle A. A pneumatic piston  9  in the form of a sleeve coaxial with the primary piston  3  is fixed to the rigid skirt  8   b . The pneumatic piston  9  is mounted to slide on a plunger distributor  10  in a sealed manner by virtue of an O-ring  11 . The plunger distributor  10  comprises, toward the front, a smaller-diameter extension  10   a  and, toward the rear, a blind axial housing  10   b , open toward the rear, that accommodates a ball  4   b  provided at the end of the linkage  4 . 
   On the opposite side to the piston  3 , the plunger distributor  10  has a frustoconical widening  12  which may bear in a sealed manner against an elastomer ring  13  connected in terms of axial translation to the pneumatic piston  9 . The assembly  12 ,  13  constitutes part of a three-way valve B (partially depicted) which either allows the chamber  7   b  to be isolated from the atmosphere and the chambers  7   a ,  7   b  to be placed in communication or allows the chambers  7   a ,  7   b  to be isolated from one another and air at atmospheric pressure to be let into the chamber  7   b  when the widening  12  moves axially away from the ring  13 . 
   The pneumatic piston  9  is returned toward its position of rest, depicted in  FIG. 1 , by a compression spring  14  placed between the piston  9  and the wall opposite of the casing  7  to which the master cylinder  2  is fixed. A cup  15  with a central opening  16  bears axially via its exterior peripheral edge against a shoulder of the pneumatic piston  9 . The spring  14  presses the edge of this cup against the piston  9 . The cup  15  bears axially, via the interior edge surrounding its opening  16 , against the rear end of the primary piston  3 . The extension  10   a  of the plunger distributor  10  can pass freely through the opening  16 . 
   An emergency assist valve VA is provided to initiate the intervention of at least two boost ratios corresponding respectively to slow braking and to emergency braking. 
   The valve VA comprises a reaction piston  17 , of which a smaller-diameter front part  17   a  slides in a sealed manner, by virtue of an O-ring  17   b , in an axial bore  18  of the primary piston  3 . The rear part  17   c  of the reaction piston has a greater diameter and has a flange  19  which can slide in a larger-diameter rear zone of the bore  18  of the primary piston  3 . A ring  20  made of elastic material is placed against the flange  19 , on the same side as the part  17   a . This ring  20  can come to bear axially against a shoulder  21  of the primary piston  3 . On the other side, the flange  19  acts as an axial thrust base for a preloaded, helical compression spring  22  known as the jump spring. At its other rear end the spring  22  bears against a clip-in ring  23  made of plastic anchored to the primary piston  3  and in which the rear part  17   c  of the reaction piston slides. 
   The reaction piston  17  has an axial bore in which a ratio control  24  is mounted to slide in a sealed manner by virtue of an O-ring  24   a . The rear end of the ratio control  24  bears against the extension  10   a  of the plunger distributor  10 . The front end of the ratio control  24  has a head  25  located in a chamber  26  of the bore  18  lying between the reaction piston  17  and a bushing  27  housed further forward in the bore  18 . 
   The bushing  27  has an axial bore  28  which passes completely through it and in which a rapid piston  29  is mounted to slide, in a sealed manner by virtue of an O-ring  29   a . The outside diameter of the rapid piston  29  is smaller than that of the part  17   a  of the reaction piston. The rapid piston  29  has, passing through it axially, a passage  30  which opens at each end. The front end of the rapid piston  29  bears against a ring  31  pushed elastically by a spring  32  against the bushing  27 . The spring  32  bears, at the other end, against an end wall  33  located forward of the bore  18 . The front zone  18   a  of the bore  18 , in which the spring  32  is housed, communicates via a passage  34  with the interior volume  5  of the master cylinder  2 . The spring  32  is subjected to a preload lower than that of the spring  22 . A valve  35  is also provided so that when the volume  5  is no longer under pressure, this volume can be resupplied with liquid from a reservoir (not depicted). 
   The rear end of the rapid piston  29  is located in the chamber  26 , near the head  25  of the ratio control  24 . This rear end of the rapid piston  29  forms, with the head  25 , a valve  36  which, for example, has an annular bearing surface  37  ( FIG. 3 ) on the end face of the rapid piston  29 , and an elastomer stopper  38 , fixed in the head  25 , part of which projects relative to the head, this part having an annular peripheral edge capable of bearing in a sealed manner against the bearing surface  37 . A compression spring  39  is positioned between two shoulders provided respectively on the piston  29  and on the head  25 , so as to open the valve  36  at rest. 
   According to the invention, a means E is provided for limiting the pressure difference between the chamber  26  and the zone  18   a  situated on the opposite side of the bushing  27  to the chamber  26 , to a value such that the valve  36  cannot open in the event of slow braking beyond the saturation point. As a preference, the pressure difference is limited to a value of below 5 bar. 
   Before continuing with the description, reference is made to  FIG. 2  to explain the expression “saturation”. 
   Curve C in  FIG. 2  represents, on the ordinate axis, the braking pressure P or hydraulic pressure in the volume  5 , as a function of the input force F exerted on the control member  4  in the axial direction. Curve C comprises a horizontal first part C 1  of zero value on the ordinate axis, which corresponds to the travel of the linkage  4  for operating the three-way valve B. 
   C 1  is followed by a vertical segment C 2  known as the “Jump” which corresponds to a phase of the spring  22  being compressed by the reaction piston  17 , following the increase in pressure of the liquid in the master cylinder  2  as a result of the boost force exerted by the booster on the primary piston  3 . The reaction piston  17  retreats until it comes into abutment against the extension  10   a.    
   Curve C continues in the form of a segment C 3  of steep gradient which corresponds to the combined action of the control member  4  and of the booster  7  of the primary piston  3 . Saturation occurs when the boost force of the booster  6  has reached its maximum value, that is to say when the pressure difference between the two chambers  8   a ,  8   b  can no longer increase; this situation corresponds to the abscissa value F 4 . Curve C continues in the form of a segment C 4 , the gradient of which is not as steep as that of the segment C 3 , because the increase in pressure P is due only to the increase in the manual input force exerted on the member  4 . 
   According to a first possibility, the means E for limiting the pressure difference between the chamber  26  and the zone  18   a  lies in the bushing  27  being mounted to slide within the bore  18 . The bushing  27  slides in a sealed manner by virtue of an O-ring  27   a . An elastic spilt ring  40  is anchored in a groove of the bore  18 , behind the bushing  27 , to act as an axial stop for this bushing  27  pushed by the spring  32 . 
   Mounting the bushing  27  so that it can slide allows this bushing to move forward, that is to say to move in a direction which increases the volume of the chamber  26  and therefore reduces the pressure in this chamber. 
   According to an alternative form depicted schematically in  FIG. 5 , the bushing  27  is axially immobilized in the bore  18  and the means E for limiting the pressure difference between the chamber  26  and the zone  18   a  comprises a connecting duct  41  connecting these two spaces and equipped with a non-return valve  42  capable of allowing liquid to pass from the chamber  26  to the zone  18   a . The valve  42  is rated at a pressure value which is low enough, preferably below 5 bar, for liquid to flow without perceptible noise. 
   According to another alternative form, not depicted, a precompressed cellular foam ring, capable of crushing only upward of a certain limiting pressure, for example of the order of 80 bar, is provided in the chamber  26  so that, by becoming crushed upward of the limiting pressure, it allows the volume available for the liquid in this chamber  26  to increase. 
   That being the case, the way in which the boosted braking device according to the invention works is as follows, in the case of slow braking. 
   The manual-control member  4  is displaced, relatively slowly, to the left. The valve B is actuated and cuts off the communication between the chambers  7   a  and  7   b  and then the frustoconical widening  12  moves away from the elastic ring  13  which allows air at atmospheric pressure to enter the chamber  7   b  of the pneumatic booster  6 . The moving partition  8  transmits the boost force to the pneumatic piston  9 , which moves forward against the action of the spring  14 . The piston  9  carries with it the cup  15  which pushes the primary piston  3  forward. The pressure of the liquid increases in the volume  5  and in the chamber  26  because the valve  36  is open. The pressure gives rise to a backward force on the reaction piston  17 . When this force reaches and exceeds the preload of the spring  22 , the reaction piston  17  retreats until such time as it comes into abutment against the extension  10   a , as illustrated in FIG.  3 . The jump C 2  ( FIG. 2 ) occurs. 
   Under the action of the input force on the member  4 , the plunger distributor  10  continues its forward movement; the pneumatic piston  9  follows the plunger distributor  10  and pushes the primary piston  3  with the boost force. The pressure of the liquid in the volume  5  continues to increase, and also in the chamber  26 . 
   When saturation is reached, the booster  6  produces its maximum boost force. The primary piston  3  remains stationary for a brief period of time because the force exerted backward by the pressure P balances the boost force. 
   The control member  4  continues to advance under the action of the manual input force. The ratio control  24  advances relative to the primary piston  3  and the head  25  via its plug  38  presses against the bearing surface  37  of the rapid piston  29 . The valve  36  closes. The reaction piston  17  bearing against the extension  10   a  also advances relative to the primary piston  3  until the ring  20  comes into abutment against the shoulder  21  of the primary piston  3  as illustrated in FIG.  4 . Continued input force on the control member  4  allows the primary piston  3  to be driven along, via the reaction piston  17 , and the braking pressure to be increased. 
   During the advancing movement of the reaction piston  17  relative to the primary piston  3 , the volume of the chamber  23  tends to decrease, which causes, in this chamber  26 , an increase in pressure which, if it becomes too great, would cause the valve  36  to open suddenly and would cause the noise mentioned at the start of the description, as a result of sudden flow. 
   This unwanted flow is avoided, according to the invention, because the bushing  27  moves forward, compressing the spring  32  when the pressure increases in the chamber  26 . The pressure in the chamber  26  is thus held at a limited value which does not cause the valve  36  to open; as a preference, the pressure difference between the chamber  26  and the zone  18   a  is limited to a value of below 5 bar. 
   According to the alternative form in  FIG. 5 , the bushing  27  is immobilized in the bore  28  but the valve  42  allows liquid to flow from the chamber  26  to the zone  18   a  under a pressure which low enough for no noise to be perceptible; as a preference, the pressure difference between the chamber  26  and the zone  18   a  is limited to a value of below 5 bar. 
   The operation under rapid braking is unmodified. As soon as rapid braking begins, the ring  20  comes to bear and remains bearing against the shoulder  21  of the primary piston  3 . 
   When the driver releases the brake pedal  4   a  with his foot, the valve B, on the one hand, closes at the level of  12  and  13  and, on the other hand, allows the pressures in the chambers  7   a  and  7   b  to return to a state of equilibrium, thus canceling the action of the booster  6  and causing an end to braking and a return to the position of rest. 
   Although the description has been given using a pneumatic booster, the booster could be hydraulic, but with saturation corresponding to maximum boosting.