Abstract:
A master cylinder ( 32 ) in a vehicle hydraulic braking installation uhydraulic-fluid supply device for actuating wheel brake cylinders ( 20 ). The master cylinder ( 32 ) being isolated from the wheel brake cylinders ( 20 ) by at least one shut-off valve ( 30 ). The master cylinder ( 32 ) being associated with a simulator ( 46 ) having a piston ( 50 ) located in a simulator chamber ( 56 ). The piston ( 50 ) being subjected to fluid pressure of brake fluid from the master cylinder ( 32 ) and an elastic element ( 52 ). The master cylinder ( 32 ) being of the having a primary piston ( 44 ) and a secondary piston ( 60 ) slidably located in a bore ( 45 ) of a housing ( 48 ). Bore ( 45 ) has a peripheral groove ( 72 ) which is associated with a land ( 68 ) on the secondary piston ( 60 ) to allow communication with the simulator chamber ( 56 ) when the master cylinder ( 32 ) is in a positon of rest. A sealing element ( 70 ) carried by the secondary piston ( 60 ) engages the housing when the secondary piston moves to interrupt communicaton between the bore ( 45 ) and simulator chamber ( 56 ) to thereafter simulate brake actuation travel.

Description:
The present invention relates to a master cylinder intended in particular to equip a motor vehicle electro-hydraulic braking installation. 
     BACKGROUND OF THE INVENTION 
     Electro-hydraulic braking installations conventionally comprise a service braking system using an external energy source and an emergency braking system using muscle power as its source, these two braking systems being controlled by a brake master cylinder, the actuating pedal of which is situated in the cockpit of the vehicle. 
     The braking system with an external energy source comprises a generator of brake fluid at high pressure, comprising a hydraulic pump associated with a hydraulic pressure accumulator. Upon a braking action performed by this system, the pressure supplied by the hydraulic pressure accumulator is communicated to the wheel brake cylinders via at least one solenoid valve so that the pressure leaving this solenoid valve has a value which is a function of the travel of the brake pedal and of the force with which this pedal is actuated, or a function of the force with which a handbrake lever is actuated, or alternatively a function of the brake fluid pressure produced using the footbrake pedal or the handbrake lever. 
     In such operation in service braking mode, the master cylinder is normally isolated from the braking installation of the vehicle by means of a shut-off solenoid valve. This then means that brake fluid cannot flow back from the master cylinder towards the wheel brake cylinders and that its piston cannot move or can move only by a minimum travel. However, for the brake pedal or handbrake lever to have a normal actuating travel, depending on the force with which they are actuated, a device which simulates the brake actuation travel is used. 
     Such a device that simulates the brake actuation travel is known, for example, from document U.S. Pat. No. 4,462,642. The known brake actuation travel simulator comprises a simulator cylinder with a simulator piston which can move in this cylinder when it is acted upon by the pressure of the brake fluid from the master cylinder, against the action of a spring and which can move inside this cylinder. 
     In the event of failure of the braking system with an external energy source, for service braking mode, the shut-off solenoid valve is switched to allow the master cylinder to actuate the wheel brake cylinders itself, for an emergency braking operating mode using as its energy source muscle power provided by the driver of the vehicle. 
     The brake actuation travel simulator device according to the aforementioned document is hydraulically connected to the master cylinder and to the shut-off solenoid valve and communicates, even in emergency braking mode, with the brake master cylinder. The known brake-actuating simulator therefore has the drawback of absorbing a certain amount of brake fluid in emergency braking mode using muscle power, and this needlessly increases the brake pedal travel and detracts from the effectiveness in emergency braking. 
     SUMMARY OF THE INVENTION 
     The present invention falls within this context in that it proposes, in the known way, a master cylinder for a vehicle hydraulic braking installation, the installation comprising a service braking system using external energy, and an emergency braking system using muscle power for actuating wheel brake cylinders, it being possible for the master cylinder to be isolated from the wheel brake cylinders by at least one shut-off valve for service braking using external energy, the master cylinder being associated with a simulator simulating the brake actuation travel and comprising a simulator piston defining a simulation chamber which can receive brake fluid from the brake master cylinder, an elastic simulator element urging simulator piston against the action of the pressure of the brake fluid in the simulation chamber, the master cylinder being of the tandem type and comprising a bore in which a primary piston and a secondary piston are mounted so that they can slide from respective positions of rest and therein delimit a primary working chamber and a secondary working chamber, respectively. 
     In this context, the object of the present invention is to propose a master cylinder, associated with a brake actuation travel simulation device for a motor vehicle electro-hydraulic braking installation which allows an emergency braking mode using muscle power, in which all of this muscle power is used for the emergency braking without this power being dissipated into other devices, it being necessary for this master cylinder to be reliable under all circumstances, easy to manufacture and low in cost. 
     To this end, the master cylinder of the invention, which in other respects is in accordance with the above preamble, is essentially characterized in that the simulation chamber has an inlet orifice opening into the bore, and in that means of selective communication connect the simulation chamber to the primary working chamber when the secondary piston is in its position of rest, and isolate the simulation chamber from the primary working chamber when the secondary piston is moved away from its position of rest. 
     According to a first embodiment of the invention, the means of selective communication comprise a peripheral groove formed in the bore and a sealing element borne by the secondary piston and capable selectively of shutting off the bore some distance from the peripheral groove, the peripheral groove being arranged in the bore between the primary piston and the inlet orifice of the simulation chamber, and the sealing element being located selectively facing the peripheral groove when the secondary piston is in its position of rest. 
     According to a second possible embodiment of the invention, the means of selective communication comprise: an axial hole made in the secondary piston and having an inlet opening into the primary working chamber; a radial hole made in the secondary piston and having an outlet permanently communicating with the simulation chamber and selectively placed in communication with the inlet of the axial hole; and an elongate plunger which is stationary with respect to the bore, mounted so that it can slide in the axial hole and interacting at least with the axial hole of the secondary piston to form a hydraulic valve which selectively isolates the outlet of the radial hole from the inlet of the axial hole when the secondary piston is moved from its position of rest. 
     In this second embodiment, the plunger rests, for example, on a pin passing across the bore. 
     According to a first possible alternative form of the second embodiment of the invention, the plunger has a blind axial hole and a radial passage communicating with the blind axial hole, the radial passage forming a first seat for the hydraulic valve, being arranged selectively facing the radial hole of the secondary piston when this secondary piston is in its position of rest, and being shut off by the axial hole of the secondary piston which itself forms a second seat for the hydraulic valve, when this secondary piston is moved away from its position of rest. 
     According to a second possible alternative form of the second embodiment of the invention, the inlet of the axial hole in the secondary piston bears an annular seal which forms a first seat for the hydraulic valve and which is selectively shut off by the plunger, which itself forms a second seat for the hydraulic valve, when the secondary piston is moved away from its position of rest. 
     In this second embodiment, the plunger rests, for example, on a pin passing across the bore. 
     Other objects, features and advantages of the present invention will emerge more clearly from the description which follows of one embodiment given by way of an illustration with reference to the appended drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a diagrammatic view of a hydraulic braking installation incorporating a master cylinder in accordance with the present invention; 
     FIG. 2 depicts a longitudinal section of the master cylinder equipping the braking installation of FIG. 1, and corresponding to a first embodiment; 
     FIG. 3 depicts, on a larger scale, part of the master cylinder of FIG. 2, 
     FIG. 4 is a view in enlarged part section of a master cylinder in accordance with a first alternative form of a second embodiment of the invention; and 
     FIG. 5 is a view in enlarged part section of a master cylinder in accordance with a second alternative form of the second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The vehicle braking installation depicted in FIG.  1  and noted overall by the reference  10  is designed in the form of a braking installation using external energy, connected to all the wheel brake cylinders, and of an emergency braking installation using muscle power, connected by two independent brake circuits I and II to the front wheel brake cylinders of the vehicle. 
     In FIG. 1, the brake circuits are depicted diagrammatically using hydraulic circuitry symbols: the external-energy brake circuit has, as its external energy source for service braking, a hydraulic pump  12  the intake of which is connected to a hydraulic fluid supply tank  14 . The hydraulic pump  12  is driven by an electric motor  16 . The delivery side of the hydraulic pump  12  is connected to a hydraulic accumulator  18  which delivers brake fluid under pressure for service braking, it being possible for a pressure-limiting valve (not depicted) to be connected between the intake and delivery pipes of the hydraulic pump  12  to limit the maximum delivery pressure of the hydraulic pump  12 . 
     Wheel brake cylinders  20  are connected to the delivery pipe of the hydraulic pump  12  and to the hydraulic accumulator  18  via an inlet valve  22  allowing pressure in the cylinders  20  to increase. To drop the braking pressure in the wheel brake cylinders  22  [sic], an outlet valve  24  is provided which places the wheel brake cylinder  20  in communication with the supply tank  14 . Any given braking pressure can thus be obtained in the wheel brake cylinders  20  using the inlet valve  22  and the outlet valve  24 , controlled appropriately by a computer (not depicted) which also controls the operation of the pump motor  16  and which receives signals that represent the pressure in the wheel brake cylinders, these signals being delivered by pressure sensors  26 , and signals representing the pressure in the hydraulic accumulator  18 , delivered by a pressure sensor  28 . 
     In the event of failure of the braking installation using external energy, so to obtain emergency braking using muscle power, the brake circuits I and II are connected independently of one another, each by means of a shut-off valve  30 , to a tandem master cylinder  32  on which the feed tank  14  is mounted and with which the brake master cylinder  32  communicates directly. The shut-off valve  30  is a twoway, two-position solenoid valve which is open in the position of rest, and which is also controlled by the computer. 
     During operation in service braking mode using external energy, the shut-off valve  30  is closed, that is to say that, from the hydraulic point of view, the brake master cylinder  32  is isolated from the vehicle braking installation. During service braking using external energy, the brake master cylinder  32  acts as a sensor of the reference value for the hydraulic pressure in the wheel brake cylinders  20 , it being necessary for this pressure to be controlled by the computer. For this, the master cylinder  32  is equipped with a sensor  34  which senses the travel of the pedal  36  which actuates the master cylinder, and with a sensor  38  which detects actuation of this pedal  36 , a sensor  40  being connected to the primary brake circuit to detect the pressure in the master cylinder, the signals provided by the sensors  34 ,  38  and  40  being delivered to the computer. As an alternative, provision could be made for the sensor  38  also to detect the force exerted by the driver of the vehicle on the pedal  36 . 
     The vehicle braking installation  10  is actuated using the brake pedal  36 , which actuates a control rod  42  of the brake master cylinder  32 , this rod itself actuating a primary piston  44  sliding in a bore  45  formed inside the brake master cylinder  32 . 
     Upon a service braking action using external energy caused by actuation of the brake pedal  36 , the shut-off valves  30  are closed, and this means that brake fluid cannot be delivered from the master cylinder into the brake circuits I and II. In order that the driver of the vehicle should, however, experience the usual sensation of actuating the brake pedal  36 , characterized by a given travel of the pedal  36  in relation with the pressure generated in the hydraulic circuit, and therefore with the feeling of slowing the vehicle down, a brake actuation simulator  46  is connected to the primary brake circuit I of the brake master cylinder  32 . 
     As can best be seen in FIG. 2, the brake actuation simulator  46  comprises a simulator body  48  in which there is formed a bore  49  where a simulator piston  50  can slide in a sealed manner. The body  48  may be arranged in the form of a cartridge to be screwed into the master cylinder or, as has been depicted in FIG. 2, be of a single piece with the body of the master cylinder. The simulator piston  50  is subject to the action of a compression spring  52  which also bears on a cap  54  secured to the simulator body  48 , and within the bore  49  it delimits a simulation chamber  56 . 
     The way in which the braking installation just described functions will now be explained briefly, assuming that all the components are operational. Under this assumption, the shut-off valves  30  are energized by the computer each time the sensor  38  detects actuation of the brake pedal  36 , which means that these valves  30  prevent communication between the master cylinder and the rest of the braking installation. 
     When the driver of the vehicle actuates the brake pedal  36 , the control rod  42  actuates the primary piston  44  of the master cylinder which then generates an increase in pressure in the primary working chamber  58  situated between the primary piston  44  and a secondary piston  60  itself also sliding in the bore  45  and delimiting therein a secondary working chamber  59 . This increase in pressure is communicated to the simulation chamber  56  and is exerted on the simulator piston  50 , which then moves against the action of the compression spring  52 . 
     More specifically, and as can best be seen in FIGS. 2 and 3, the secondary piston  60  is formed with a part  62  for sliding and guidance in the bore  45 , for example by means of two lands  64  and  66  fitted with sealing cups. The secondary piston  60  is also formed with a land  68 , of a diameter more or less equal to that of the bore  45 , and fitted with an O-ring seal  70 . The bore  45  is also formed, at the front end of the primary working chamber  58 , with a peripheral groove  72 , so that in the position of rest, the groove  72  lies facing the land  68  of the secondary piston  60 . The simulation chamber  56  also opens out into the bore  45  downstream of the groove  72 , via an opening  74 . 
     When the pressure increases in the primary working chamber  58 , brake fluid can thus be delivered to the simulation chamber  56 , passing over the O-ring seal  70  and through the opening  74 . This then allows the primary piston  44  to move. The stroke  34 , actuation or force  38  and pressure  40  sensors then emit signals which are supplied to the computer which in turn controls the motor  16  of the pump  12  and the solenoid valves  22  and  24  in order to generate, within the wheel brake cylinders  20 , an increase in pressure which corresponds to the signals received from these sensors, and therefore a braking action which is in relation with the action of the driver of the vehicle on the brake pedal. 
     When one of the components of the braking installation experiences a failure, this is detected by the computer which then commands the deenergizing of the shut-off valves  30 , which return to their position of rest depicted in FIG.  1  and therefore allow communication between the master cylinder  32  and the rest of the braking installation. 
     In this failure situation, when the driver of the vehicle actuates the brake pedal  36 , the control rod  42  actuates the primary piston  44  of the master cylinder which then generates an increase in pressure in the primary working chamber  58  situated between the primary piston  44  and the secondary piston  60 . As the shut-off valves  30  are then open, the pressure exerted on the secondary piston  60  generates on the latter a force which makes it move forward. In this movement, the land  68  moves and the O-ring seal  70  comes into contact with the bore  45 , thus closing the communication between the primary working chamber  58  and the simulation chamber  56 . The primary piston  60  then in turn causes an increase in pressure in the secondary working chamber  72  situated between it and the closed end of the bore  45 . This increase in pressure is then communicated to the wheel brake cylinders by the hydraulic circuits I and II. 
     It can therefore indeed be seen that in this failure situation, the simulation chamber is taken out of the circuit, which means that all of the brake fluid from the primary and secondary chambers of the master cylinder is used to effect emergency braking using muscle power. All of the muscle power of the driver of the vehicle is thus used for emergency braking without this power being dissipated into other devices such as the travel simulator  46 . The master cylinder is of a particularly simple design, which guarantees that it will be reliable and ensures a low manufacturing cost. 
     FIGS. 4 and 5 respectively illustrate first and second alternative forms of a second embodiment of the invention. 
     Just like in the first embodiment, the simulation chamber  56  has an inlet orifice  74  which opens into the bore  45 , and means of selective communication are provided for connecting the simulation chamber  56  to the primary working chamber  58  when the secondary piston  60  is in its position of rest and for isolating the simulation chamber  56  from the primary working chamber  58  when the secondary piston  60  is moved away from its position of rest, that is to say if a component of the braking installation should fail. 
     More specifically, these means of selective communication essentially comprise (FIGS. 4 and 5) an axial hole  601  and a radial hole  603 , both made in the secondary piston  60 , and a plunger of elongate shape  80  which rests on a pin  90  passing across the bore  45  so as to remain stationary with respect to the bore  45 . 
     The plunger  80  is mounted so that it can slide in the axial hole  601  of the secondary piston  60 , this axial hole having an inlet  602  which opens into the primary working chamber  58 . 
     The radial hole  603  in the secondary piston  60  has an outlet  604  which permanently communicates with the simulation chamber  56  and which is selectively placed in communication with the inlet  602  of the axial hole  601 . 
     Finally, the plunger  80  interacts with the axial hole  601  to form, at least with it, a hydraulic valve that allows the outlet  604  of the radial hole  603  to be isolated from the inlet  602  of the axial hole  601  when the secondary piston  60  is moved away from its position of rest. 
     In the first alternative form (FIG.  4 ), the plunger  80  has a blind axial hole  801  and a radial passage  802  which communicates with this blind axial hole  801 . 
     The radial passage  802 , which forms a first seat for the hydraulic valve, is placed facing the radial hole  603  of the secondary piston  60  when this piston is in its position of rest. 
     By contrast, when the secondary piston  60  is moved away from its position of rest, the radial passage  802  finds itself shut off by the axial hole  601  of the secondary piston  60 , which itself forms a second seat for the hydraulic valve, thus preventing brake fluid from flowing into the simulation chamber if a component of the braking installation should fail. 
     In the second alternative form (FIG.  5 ), the inlet  602  of the axial hole  601  in the secondary piston  60  bears an annular seal  605  which forms a first seat for the hydraulic valve. 
     Thus, when the secondary piston  60  is moved away from its position of rest, the annular seal  605  is shut off by the plunger  80  which itself forms a second seat for the hydraulic valve, so that any flow of brake fluid into the simulation chamber is prevented if a component of the braking installation should fail.