FR2855135A1 - Pneumatic servomotor for vehicle, has sensor immobilized between reaction plate and shroud, where predetermined gap separates front end of fitment from back end of sensor, in resting or non-braking position - Google Patents

Abstract

The servomotor has a sensor (62) assembled on front end of a plunger. The plunger is associated to a control rod connected to a brake pedal. A fitment (66) is assembled between the sensor and a shroud (68) of back part of the plunger. The sensor is immovable between a reaction plate and a shroud (64), and a predetermined gap separates front end of the fitment from the back end of the sensor in resting or non-braking position.

Description

I

  The invention relates to a pneumatic brake booster, in particular for a motor vehicle, of the type comprising means for emergency braking assistance.

  A pneumatic brake booster for a motor vehicle comprises a rigid envelope divided into two chambers by a movable wall secured to an axial piston which is mounted between a control rod connected to a brake pedal and a push rod acting 10 on a piston of a master cylinder of the vehicle brake system.

  The control rod ends on the piston side of the actuator by a plunger guided in an axial passage of this piston to come into contact with a reaction disc which is carried by the push rod and which is interposed between it and the aforementioned piston and plunger.

  When the driver of the vehicle presses on the brake pedal, the force which is transmitted by the pedal and the control rod is amplified by the servomotor, the piston of which is moved by pneumatic pressure in the direction of the master cylinder. The reaction disc is compressed by the servomotor piston and transmits the reaction from the braking circuit to the control rod and to the brake pedal, this reaction being felt by the driver and allowing him to modulate the effort he applies to the brake pedal and therefore modulates the braking of the vehicle.

  During emergency braking, the control rod and the plunger are moved in the booster faster than its piston and the driver may have a feeling of maximum braking force while the booster has not yet provided maximum assistance.

  The driver may thus have to prematurely release the force which he applies to the brake pedal even if the maintenance of maximum braking is necessary.

  It has already been proposed to equip these servomotors with emergency braking assistance means, which include a feeler slidingly mounted on the plunger so as to be able to come into contact with the reaction disc, and means for blocking the feeler with respect to to the piston of the servomotor in a support position on the reaction disc, these blocking means being of the unidirectional type and being triggered automatically when the driver after emergency braking slightly releases the force which he exerts on the pedal of brake.

  Thanks to the locking means, the feeler is held by the piston of the servomotor in abutment on the reaction disc to apply a maximum force on the push rod acting on the piston of the master cylinder 10 when the driver has slightly released the force. exerted on the brake pedal.

  The sensor blocking means are released when the driver completely releases the force applied to the brake pedal.

  In a known embodiment, these blocking means comprise a bush mounted to slide on the plunger between the feeler and an axial stop carried by the plunger, and means for axially blocking the bush relative to the piston, these blocking means being constituted by an annular key axially traversed with clearance by the sleeve and pivotally mounted in a transverse housing of the piston of the servomotor.

  Elastic return means urge the key towards a blocking position of the sleeve in which any axial movement of the sleeve in the direction opposite to the reaction disc is prohibited, axial movement of the sleeve towards the reaction disc being authorized.

  When the driver, after having slightly released the force applied to the brake pedal, increases this force again to a maximum value, the plunger is re-moved towards the reaction disc by the control rod without the the driver may feel any resistance to this displacement, the entire reaction of the braking circuit being supported by the probe blocked on the piston of the servomotor.

  During this phase, the driver has no more force feedback at the brake pedal and it is difficult for him to modulate the force he applies to this pedal and consequently to modulate the braking of the vehicle.

  The object of the invention is in particular to provide a simple, economical and effective solution to this problem, without adversely affecting the effectiveness of emergency braking.

  To this end, it provides a pneumatic brake booster for a motor vehicle, comprising a rigid envelope divided into two chambers by a movable wall secured to an axial piston which is mounted between a control rod and a push rod, the control rod 10 terminating in a plunger guided in an axial passage of the piston, a reaction disc carried by the push rod and interposed between the latter and the aforementioned piston and plunger, and braking assistance means d emergency comprising a probe mounted axially sliding on the plunger on the side of the reaction disc and a sliding mounted socket 15 on the plunger between the probe and axial stop means carried by the plunger, as well as means for axially unidirectional locking of the sleeve with respect to the piston, characterized in that the probe is tubular with open ends and is freely traversed by the plunger and in that, in a posi tion of rest or non-braking, the probe is in support, on the side opposite to the reaction disc, on the piston of the servomotor.

  Thanks to this arrangement, if the driver presses the brake pedal again after releasing his force during emergency braking, part of the reaction of the braking circuit is transmitted by the reaction disc to the diver and to the control rod and is felt by the driver, which allows him to modulate the force which he applies to the brake pedal and therefore to modulate the braking of the vehicle.

  In addition, there is at this time an increase in the value of the jump without modification of the slope of the assistance curve provided by the booster, as will be seen in more detail below.

  According to another characteristic of the invention, the probe is in abutment on the reaction disk in the aforementioned rest or non-braking position.

  Preferably, in this position, the feeler is immobilized axially between the reaction disc and a shoulder of the internal cylindrical surface of the axial passage formed in the piston and in which the plunger is guided.

  A predetermined axial clearance separates the feeler from the aforementioned bushing, in the rest or non-braking position, this clearance forming a triggering threshold.

  The invention will be better understood and other characteristics, details and advantages thereof will appear more clearly on reading the description which follows, given by way of example with reference to the appended drawings in which: - Figure 1 is a partial schematic view in axial section of a pneumatic booster of the prior art; - Figure 2 is a schematic half-view in axial section, on a larger scale, of the emergency braking assistance means in the booster according to the invention; - Figure 3 is a graph schematically showing curves of variation of the output force as a function of the input force 20 in the booster according to the invention.

  In the following description, by convention, what is on the left in the drawings will be described as being at the front and what is on the right as being at the rear.

  The pneumatic brake booster for a motor vehicle 25, a part of which is shown diagrammatically in FIG. 1, comprises in known manner a rigid casing 10 which is divided into a front chamber 12 connected to a source of vacuum and a rear chamber 14 by a movable wall secured to an axial piston 16, the rear chamber 14 being connected to the front chamber 12 in the absence of braking and 30 communicating with the ambient atmosphere during braking.

  The piston 16 of the booster is associated on one side with a control rod 18 connected to a brake pedal not shown and, on the other side with a push rod 20 which cooperates with a piston of a master cylinder not shown of the motor vehicle braking circuit, a return spring 22 being mounted in the booster inside the front chamber 12 to constantly urge the piston 16 backwards and bring it back to a rest or non-braking position shown in the drawing.

  The control rod 18 carries at its front end a plunger 24 whose rear end cooperates with an annular valve 26 of a three-way valve which is mounted in the rear tubular end of the piston 16 of the booster around the rod 18 and which selectively supplies atmospheric air to the rear chamber 14 of the booster as a function of the force applied to the control rod 18 by the brake pedal, or to connect the rear chamber 14 to the front chamber 12 in the absence of braking, in a manner well known in the art.

  The push rod 20 carries at its rear end a cup 28 in which is housed a reaction disc 30 of substantially incompressible material such as in particular rubber or an elastomer, to which is applied a front face 32 of the piston 16 of the booster.

  A probe 34 is slidably mounted on the front end of the plunger 24 and is guided in an axial passage of the front end wall of the piston 16 so as to be able to come into contact with the reaction disc 30.

  A cylindrical sleeve 36 is slidably mounted on the plunger 24, between a shoulder 38 of the rear end of the plunger and the feeler 34, 25 and is associated with unidirectional locking means carried by the piston 16 of the booster, these means being constituted by a flat annular key 40 which extends transversely in a suitable housing formed in the piston 16 around the bush 36 and which is crossed axially with clearance by this bush.

  A hooking spout 42 formed at one end of the flat key 40 is intended to cooperate with a shoulder 44 of the front end of the sleeve 36 to block the latter relative to the piston 16 of the booster. Elastic return means formed by a compression spring 46 are mounted parallel to the axis of the booster between the front transverse wall of the piston 16 and the key 40 to urge the latter permanently in tilting backwards in its housing of the piston 16.

  In the rest position shown in Figure 1, the end of the flat key 40 opposite to that comprising the hooking spout 42 carries a stop 48 applied to a fixed part 50 of the actuator and holding the key 40 in an oblique position o the hooking spout 42 is spaced from the shoulder 44 of the front end of the bush 36.

  This known servomotor operates in the following manner: During normal braking, when the driver presses the brake pedal, the control rod 18 and the plunger 24 are moved forward, which has the effect of closing the communication between the 15 chambers 12 and 14 of the booster and connecting the rear chamber 14 to the surrounding atmosphere.

  The pressure increases in the rear chamber 14 and moves the piston 16 forwards. This displacement compresses the reaction disc 30 which is applied to the probe 34 carried by the front end of the plunger 24.

  The probe 34 is at its rear end in abutment on the front end of the sleeve 36 which is in abutment on the shoulder 38 of the plunger 24. In this way, the reaction of the braking circuit is transmitted to the rod control 18 and to the brake pedal, and is felt by the driver who can thus modulate the force which he applies to the brake pedal.

  During emergency braking, the control rod 18 and the plunger 24 are moved forward faster than the piston 16 of the booster. The probe 34 pushed by the sleeve 36 itself bearing on the shoulder 38 of the plunger 24 is applied to the reaction disc 30 and compresses it.

  When the piston 16 of the servomotor is itself moved forward by the increase in pressure in the rear chamber 14, the end of the key 40 carrying the hooking spout 42 pivots forward and the spout 42 engages on the shoulder 44 of the front end of the sleeve 36.

  If the driver then slightly releases the force which he applies to the brake pedal, the control rod 18 and the plunger 24 are moved backwards, but the sleeve 36 cannot follow this movement and is blocked on the piston 16 of the booster by the key 40, the spout 42 of which abuts on the shoulder 44 of the socket 36. The plunger 34 remains in support 10 on the reaction disc 30.

  If the driver presses the brake pedal again before the control rod 18 and the plunger have returned to their initial rest or non-braking positions, the control rod 18 and the plunger 24 are again moved towards the front without any reaction 15 from the braking circuit being applied to them, the entire reaction from the braking circuit being transmitted to the piston 16 and to the probe 34 which is in abutment on the sleeve 36 blocked on the piston 16 by the key 40 and the attachment spout 42. The driver does not, during this phase, force feedback to the brake pedal and it is difficult for him to modulate the force which he applies to this pedal.

  The invention proposes to overcome this drawback by means of the device shown diagrammatically in FIG. 2.

  In this figure, there are a number of components of the device of Figure 1, which are associated with the same references as in Figure 1 to facilitate understanding.

  In FIG. 2, the reference 16 therefore designates the piston of the pneumatic assistance booster, the front part of which bears on the reaction disc 30 carried by the end of the push rod (not shown) cooperating with a piston d 'a master cylinder.

  The control rod 18 connected to the brake pedal carries at its end before the plunger 24 which is guided in an axial passage 60 of the piston 16 leading to the reaction disc 30. A feeler 62 is slidably mounted on the front end of the plunger 24. The rearward displacement of the probe 62 is limited by a shoulder 64 of the internal cylindrical surface of the axial passage 60 of the piston and its displacement towards the front is limited by the reaction disc 30. In the position of rest or non-braking shown in FIG. 2, the probe 62 is immobilized between the reaction disc 30 and the shoulder 64 of the piston 16.

  The probe 62 is of tubular shape with open ends and the front end of the plunger 24 is engaged and guided in the probe and 10 is capable of coming into contact with the reaction disc 30 by sliding in the probe 62.

  A cylindrical bush 66, corresponding to the bush 36 of FIG. 1, is slidably mounted around the plunger 24, between the feeler 62 and a shoulder 68 of the rear part of the plunger 24.

  This socket 66 cooperates with unidirectional locking means 70 which have been shown diagrammatically in FIG. 2 but which may be of the same type as those shown in FIG. 1 or of a similar type and which fulfill the same function.

  In the rest or non-braking position of FIG. 2, a predetermined axial clearance 72 separates the front end of the socket 66 from the rear end of the probe 62. In this rest position, the socket 66 is maintained resting on the shoulder 68 of the plunger 24 by any suitable means, for example by elastic return means mounted between this bushing and a front part of the piston 16.

  The operation of the booster according to the invention will now be explained with reference to FIG. 3, which represents the variations in the output force of the booster as a function of the input force in three cases of normal braking, rapid braking and emergency braking respectively.

  Curve A is a normal braking curve and includes a first part a for catching up dead strokes and a second part b called the jump phase during which the exit force increases without the entry force varying, this jump phase corresponding to the start of the supply of the rear chamber 14 with air at atmospheric pressure and to the start of the forward movement of the piston 16 which begins to compress the reaction disc 30 until the latter comes in contact with the front end of the plunger 24.

  The normal braking curve A comprises a third part c during which any force applied by the driver to the brake pedal generates an increase in the assistance force applied to the piston 16 10 which results in an increase in the force applied to the push rod and by an increase in the reaction exerted on the plunger 24 and transmitted to the brake pedal.

  A fourth part d of curve A is a saturation phase in which the pressure in the rear chamber 14 is equal to atmospheric pressure 15 and can no longer increase. The increase in the output force exerted on the push rod 20 is then due to the increase in the force applied by the driver to the brake pedal.

  Curve B is a rapid braking curve and comprises a first part a for catching up dead strokes and a second part 20 e corresponding to a very rapid forward movement of the control rod 18 and of the plunger 24, the force applied by the driver with the brake pedal being amplified by the booster until the saturation thereof.

  Curve C is an emergency braking curve and comprises a first part a for compensating for dead strokes, a second part e for very rapid movement of the control rod 18 and of the plunger 24 forwards at the start of the aforementioned curve B then a third part f during which the driver slightly releases his force on the brake pedal, a fourth part go the driver reapplies a force on the brake pedal, this fourth part g being a phase of amplification by the servomotor which continues until saturation thereof, the saturation phase in emergency braking being represented by part h of curve C which is in the extension of part d of curve A. In part e of the emergency braking curve C, the sleeve 66 5 is moved forward by the plunger 24 and pushes the probe 62 forward after catching the above-mentioned axial clearance 72. The probe 62 is moved forward relative to the piston 16 and moves away from the shoulder 64, compressing the reaction disc 30.

  When in the next phase f the driver slightly releases the force applied to the brake pedal, the plunger 24 and the control rod 18 are moved backwards but the sleeve 66 is blocked by the means 70 in its extreme position before and maintains the probe 62 spaced from the shoulder 64 of the piston 16. In this position, the plunger 24 having retracted with respect to the piston 16, the jump is increased as shown in b '.

  When, during the following phase g, the driver presses the brake pedal again, the assistance provided by the booster is the same as in normal braking, the slopes of part c of curve A and of part g of curve C being equal in that in both cases, it is the front end of the plunger 24 which acts on the reaction disc 30 to modulate the braking force which transmits a return of brake pedal effort.

  The device of FIG. 2 therefore makes it possible, in the event of emergency braking, to increase the jump while then retaining the same assistance ratio 25 as during normal braking and while retaining force feedback at the brake pedal. 1The

Claims (7)

  1. Pneumatic brake booster for a motor vehicle, comprising a rigid envelope divided into two chambers 5 (12,14) by a movable wall secured to an axial piston (16) which is mounted between a control rod (18 ) and a push rod (20), the control rod (18) ending in a plunger (24) guided in an axial passage (60) of the piston (16), a reaction disc (30) carried by the rod thrust (20) and interposed between the latter, the aforementioned piston (16) and the plunger (24) 10, and emergency braking assistance means comprising a feeler (62) mounted axially sliding on the plunger ( 24) on the side of the reaction disc (30) and a sleeve (66) slidingly mounted on the plunger between the feeler (62) and axial stop means (68) carried by the plunger (24), as well as means ( 70) of axial unidirectional locking of the sleeve (66) relative to the piston (16), characterized in that the feeler (62) is tubular with open ends and is freely traversed by the plunger (24) and in that in a rest or non-braking position, the feeler (62) bears on the side opposite the reaction disc (30) on the piston (16 ) of the servomotor.   2. Booster according to claim, characterized in that in the rest or non-braking position, the feeler (62) is supported on the reaction disc (30).   3. Booster according to claim 1 or 2, characterized in that, in the rest or non-braking position, the feeler (62) is immobilized axially between the reaction disc (30) and the piston (16) of the servomotor.   4. Booster according to one of the preceding claims, characterized in that, in the rest or non-braking position, the feeler (62) is separated from the sleeve (66) by a predetermined axial clearance (72).   5. Booster according to one of the preceding claims, characterized in that, in the rest or non-braking position, the feeler (62) is supported on a shoulder (64) of the internal surface of the axial passage (60 ) in which the diver (24) is guided.   6. Booster according to one of the preceding claims, 5 characterized in that the bush (66) is biased in the direction opposite to the reaction disc (30) by return means bearing on the piston (16) of the booster.   7. Booster according to one of the preceding claims, characterized in that the means (70) for axial locking of the sleeve (66) 10 comprise an annular key (40) axially traversed with play by the sleeve (66) and tilting mounted in a transverse recess of the piston (16), and return means (46) urging the key (40) towards a locking position of the sleeve preventing it from moving axially in the direction opposite to the reaction disc (30 ).