GB2120337A - Modulators of anti-skid hydraulic braking systems for vehicles - Google Patents

Abstract

A modulator of an anti-skid hydraulic braking system enables a brake (1) on a wheel of a vehicle to be re-applied in two successive stages comprising a rapid first stage, followed by a slower second stage. This is achieved by a modulator piston assembly (6) comprising a first piston (7) and a second piston (8) of a greater effective area than the first. The two pistons (7) and (8) are arranged to be movable together as a single unit during the second stage, but the first piston (7) is movable relative to the second (8) during the first stage. The movement of the pistons (7) and (8) in the second stage is determined by the difference between the effective areas of the two pistons (7) and (8), and the size of a restrictor (27) which damps the return movement of the piston assembly (6). <IMAGE>

Description

SPECIFICATION Improvements in anti-skid hydraulic braking systems for vehicles This invention relates to anti-skid hydraulic braking systems for vehicles of the kind in which hydraulic fluid from a source of hydraulic fluid under pressure is supplied to a wheel brake of a vehicle through a modulator assembly which is adapted to modulate the supply of fluid from the source to the brake in response to skid signals from skid sensing means, the modulator assembly comprising a de-boost piston working in a bore in a housing, an expander chamber connected to the brake and defined in the bore between one end of the piston and a first valve, the first valve being disposed between the source and the expander chamber, and a second valve for controlling the application to the de-boost piston of a fluidsustained support force to determine the relative position of the de-boost piston in the bore, the second valve normally being so operative that the support force holds the de-boost piston in an advanced position in which the effective volume of the expander chamber is at a minimum value, the first valve normally being open to provide communication between the source and the brake at least when the de-boost piston is in the said advanced position, and means responsive to a skid signal to manipulate the second valve in order to reduce the support force whereafter the de-boost piston can move away from the advanced position and into a retracted position in which the effective volume of the expander chamber is increased thereby reducing the pressure applied to the brake, the reduction in the support force also permitting the first valve to close.

In known anti-skid hydraulic braking systems of the kind set forth the brake-applying pressure is reduced rapidly in response to a skid signal and, at the termination of the signal, the pressure is reapplied to the brake at a fixed rate by return movement of the de-boost piston towards the advanced position. This means that if the vehicle is travelling over a greasy patch of low co-efficient of friction when the skid signal is generated the rate of brake re-application at the termination of the skid signal, determined by the return movement of the de-boost piston, will be relatively slow. This, in turn means that the vehicle stopping distance will be increased and sufficient advantage cannot therefore be taken of the improved surface conditions.

The ideal cycle for any anti-skid system is to reduce brake-applying pressure rapidly in response to a skid signal in order to prevent a wheel lock. This usually means that the brakeapplying pressure is reduced to a relatively low value. On wheel recovery signified by the termination of the skid signal, the brake pressure is re-applied rapidly to a first value slightly lower than that at which the previous skid signal was produced, and thereafter the brake pressure increases, but at a lower rate of pressure increase.

Such an ideal cycle is weli known and has been used in pneumatically operated braking systems in which a memory of air under pressure, related to the pressure at which a skid signal was emitted, can be retained and referred to.

Proposals have been made to achieve the said ideal cycle in hydraulic braking systems by the use of rapidly oscillating solenoid-operated valves with the power to re-apply the brake being supplied by an electric motor. Although effective, the apparatus necessary to carry out such proposals is complex and expensive, and the apparatus is noisy in operation.

According to our invention in an anti-skid hydraulic braking system of the kind set forth support fluid acts on the de-boost piston through a modulator piston assembly to provide the fluid sustained support force, and the modulator piston includes first and second pistons defining pressure-responsive faces of different effective areas, the two pistons being movable together as a single unit and also being relatively movable axially independently of each other, and the second valve is movable between first and second operable positions, in the first operable position of the second valve in which no signal is operative the support fluid is applied to the first piston to hold the de-boost piston in the advanced position, and, in response to a skid signal, the support fluid is released and the first piston is subjected to a net force of sufficient magnitude to cause it to move relatively away from the abutment on the second piston and in a direction to reduce the support force and enable the de-boost piston to move towards its retracted position, and restrictor means are incorporated for restricting return flow to a reservoir of fluid displaced by movement of the two pistons as a single unit, termination of the skid signal with the second valve returning to the first operable position causing the support fluid to be applied again to the first piston to urge the deboost piston towards the advanced position at a first rate until movement of the first piston is arrested by the engagement with the abutment on second piston whereafter further movement of the de-boost piston towards the advanced position continues at a second rate slower than the first and determined by the difference in areas between a second face of the first piston and a first face of the second piston and the size of the restrictor means.

The restrictor means may comprise a restricted orifice in the second piston or an orifice in a flow regulating valve with which one end of the first piston is in communication.

Our invention has made it possible for an hydraulic modulator to be adapted to provide a two-stage re-apply rate following the correction of a skid. Specifically we provide a fast re-apply to a point which is always a ratio of the pressure reduced. For example, if the chosen ratio determined by the difference in area between the second face of the first piston and the first face of the second piston is 2/3 then, if the brake pressure falls from 1200 p.s.i. to 0 p.s.i., the brakes will be re-applied to 800 p.s.i. quickly, and from 800-1200 p.s.i. more slowly. However, if the pressure falls from 1200 to 600 p.s.i., the brakes will be re-applied to 1000 p.s.i. rapidly and from 1000-1200 p.s.i. more slowly.

Similarly on an icy surface, if the pressure falls from 300 to 0 p.s.i. the brakes will be re-applied to 200 p.s.i. rapidly, and from 200-300 p.s.i. more slowly.

In one construction first and second faces of the first piston are disposed at its opposite ends, the second face being at the inner end which is of greater effective area than the outer end which has the first face, and the inner end includes an axial extension which co-operates with the deboost piston, and the second piston which is of annular outline and is of substantially greater effective area than either of the two ends of the first piston is siidably guided on a third portion of the first piston which is of reduced diameter and which interconnects both ends of the first piston, the support fluid being applied to the first face of the first piston, and the second face on the first piston being in communication with a first face on the second piston when the second valve is in the first operable position, and, in response to a skid signal, the second face of the first piston being isolated from the first face of the second piston, and the first and second faces of the first piston, which are of different effective areas, being interconnected.

When the second valve is in its first operable position, the de-boost piston is held in its advanced position by pressure acting on the first face of the first piston comprising the end remote from the de-boost piston, and the second face of the first piston is in communication with the first face of the second piston, which comprises an annular face on the side of the second piston remote from the outer end of the first piston.

After correction of a skid fluid displaced by the inner end of the first piston is accommodated between that end and the second piston during the first rapid stage of brake re-application with the first piston moving relative to the second piston until the abutment arrests that movement of the first piston whereafter such displaced fluid is accommodated by a return flow to the reservoir through the restricted orifice.

Alternatively the de-boost piston is defined by the outer end of the first piston, and the inner face of the first piston is subjected to the support fluid to hold the de-boost piston in its advanced position and is isolated from an adjacent face of the second piston when the second valve is in its first operable position, movement of the second valve into the second operable position isolating the said inner face of the first piston from the support fluid, and interconnecting the said inner face with the adjacent face of the second piston whereby to urge the modulator piston assembly away from the said advanced position with the first piston moving through a greater retracted distance than the second piston.

The modulator also include a safety device to ensure that the first valve remains open in the event of failure of the supply of support fluid.

The safety device may comprise a spring of which the load is overcome when the piston assembly is subjected to the said net force.

Alternatively the safety device may comprise a flow-regulating valve of which a spool controlling operation of the first valve is urged by a spring into a position to hold the first valve open upon failure of the supply of support fluid.

Two embodiments of our invention are illustrated in the accompanying drawings in which: Figure 1 is a layout of an hydraulic anti-skid braking system including a longitudinal section through a modulator assembly; Figure 2 is similar to Figure 1 but showing a different modulator assembly; Figure 3 is similar to Figure 1 but showing a layout of a different hydraulic braking system; and Figure 4 is a graph of brake pressure plotted against time.

In the braking system shown in the layout of Figure 1 a brake 1 is adapted to be applied by a master cylinder 2 and the supply from the master cylinder to the brake is modulated by a modulator assembly 3.

The modulator assembly 3 comprises a housing 4 provided with a stepped bore 5 in which works a piston assembly 6 comprising a first piston 7 and a second piston 8. The first piston 7 has inner and outer end portions 9 and 10 of different effective areas which work in spaced positions of the bore 5 and the portion 9 carries an axial extension 11 which co-operates with the adjacent end of a de-boost piston 12 working in a second bore 13 which is coaxial with, and spaced from, the first bore 5.In the manner inoperative position shown in the drawing the de-boost piston 11 is held in an advanced position in which the volume of an expansion chamber 14 between the piston and the first one-way valve 1 5 is at a minimum and the valve 1 5 is open, by means of a safety spring 1 6 of which a movable abutment 1 7 for one end of it is carried by the free end of the extension 11. In this position the master cylinder 2 is in free communication with the brake 1.

A solenoid-operated valve 20 responsive to signals from a wheel speed sensor (not shown) controls operation of a valve member 21 which is alternatively engageable with spaced valve seatings 22 and 23, and the valve member 21 is readily movable since it is at all times totally immersed in hydraulic fluid. When the valve member 21 is in engagement with the seating 22 fluid in an hydraulic accumulator 24 is applied to the end portion 10 of the first piston 7 to augment the force of the safety spring 16 holding the first valve 1 5 open. In this position a chamber 25 in the bore 5 between the end portion 9 and the end of the bore 5 is in communication through the seating 23 with a chamber 26 defined in the bore 5 between the end portion 9 and the second annular piston 8.

A restricted orifice 27 in the piston 8 connects the chamber 26 to a reservoir 28 for fluid at ail times.

The effective area of the end portion 9 is greater than that of the end portion 10, and the effective areas of opposite sides of the end portion 9 are equal.

In the inoperative position shown in the drawings, in which the de-boost piston 12 is held in its advanced position as described above, the brake 1 can be applied directly from the master cylinder 2 through the expansion chamber 14 and the open valve 1 5. In this position the second annular piston 8 is retracted by the engagement of a shoulder 29 on an intermediate portion between opposite end portions 9 and 10 of the first piston 7 and on which the second piston 8 is slidable, with an abutment 30 on the second piston 8.

When a skid signal is produced by the skid sensor, the solenoid valve 20 is operated against the load in the spring 31 to cause the valve member 21 to move away from the seating 22 and into engagement with the seating 23. This cuts-off communication between the charnber 25 and 26, and interconnects the opposite end portions 9 and 10 of the first piston 7 so that they are both exposed to pressure in the accumulator 24.

Due to the difference in the effective areas of the end parts 9 and 10, the first piston 7 is subjected to a net force of a magnitude sufficient to urge the first piston 7 rapidly relatively away from the piston 12 which, due to the brake pressure, follows it. This permits the one-way valve 1 5 to close and isolate the master cylinder 2 from the brake 1 , with subsequent movement of the de-boost piston 12 in the same direction increasing the effective volume of the expansion chamber 14 to reduce the brake pressure and release the brake.

The rapid movement of the first piston 7 is accompanied by a movement of the piston 8 in the same direction but through a different distance determined by the ratio of the area of the end piston 9 and the area of the piston 8. For example, if the area ratio is 3:1 and the first piston 7 travels 12 mm, the annular piston 8 will move 4 mm and a gap will open between the abutments 29 and 30 of 8 mm.

At the termination of the skid signal the valve member 21 moves away from the seating 23 and re-engages with the seating 22. Communication between opposite ends of the first piston 7 is cutoff and the chambers 25 and 26 are again interconnected. Fluid passes rapidly from chamber 25 to chamber 26 and the second piston 8 remains stationary with this fluid being accommodated in the chamber 26 until the abutment 30 arrests movement of the first piston 7. Thereafter movement of the piston 7 is accompanied by the second piston 8 but at a rate determined by the return of fluid from the chamber 26 and to the reservoir 28 through the restricted orifice 27. This means that the remainder of the return travel of 4 mm is achieved fairly slowly to reduce the rate of pressure increase of the fluid for re-applying the brake.

If the piston 7 initially moves 3 mm, the piston 8 moves 1 mm so that, when the valve member 21 engages with the seating 22, the piston 7 will move back quickly for 2 mm and then slowly for 1 mm.

As the deflection of hoses and brakes are relatively linear with pressure at pressures exceeding 100 p.s.i., the travel of the piston 7 represents pressure drops and increases.

Should the accumulator fail the bias pressure will disappear and only the spring 1 6 provides a force to oppose the brake-pressure force. This means that the valve 1 5 will close to limit the brakes at a lower pressure, for example 900 p.s.i., but because hydraulic servo will always be fitted with accumulator power available, the driver, must be able to generate 900 p.s.i. unaided by the servo. This would require 170 Ibs pedal effort and would give a deceleration of .55 g.

We can, therefore, take advantage of the driver being unable to overcome the spring without servo assistance to provide a much smaller spring than would be required to guarantee 2000 p.s.i.

for normai braking.

In the construction of Figure 2 the spring 16 and the orifice 27 are omitted and instead of being operated by the de-boost piston 12 the first valve 1 5 is controlled by a flow regulating valve 32 in communication with the chamber 25.

When the chamber 25 is at the same pressure as the reservoir the valve 1 5 is held open by the force in a spring 33 which acts on the spool 34 of the flow regulating valve 32.

When the chamber 25 is connected to the accumulator 24 that pressure also acts on the spool 34 to urge it against the loading on the spring 33 to permit the first valve 1 5 to close.

The construction and operation of the embodiment of Figure 2 is otherwise the same as Figure 1 and corresponding reference numerals have been applied to corresponding parts.

When the chambers 25 and 26 are reconnected the piston 7 returns to abut the annular piston 8. Thereafter fluid flows via the flow regulating valve 32 to the reservoir 28 at a controlled rate.

The braking system shown in the layout of Figure 3 comprises a full-power braking system in which pressure fluid stored in the accumulator 24 is supplied directly to the brake 1 under the control of a pedal-operated brake metering valve 2' which replaces the master cylinder 2. This is in complete contrast to the systems of Figures 1 and 2 described above in which the operating fluid for the brakes 1 comprises the pressurised output of a master cylinder 2 which is entirely separate from the accumulator 24. Specifically, fluid under the control of the metering valve 2', admits regulated pressure fluid from the accumulator 24 on the modulator assembly 3. The fluid passes through the open first valve, into the expansion chamber and directly to the wheel brake 1, and a one-way valve 57 is disposed between the reservoir 28 and the accumulator 24.

The modulator assembly 3 comprises a housing 58 having a first valve 59 which controls communication between a passage 60 connected to the metering valve 2', and an outlet passage 61 connected to the brake 1. The first valve 59 is normally held in an open position by means of a piston assembly 35 comprising a first piston 36 and a second piston 37. As illustrated the first piston 36 works in a bore 38 and is carried by a piston-rod 39 which, in turn, works through an opening at the end of the bore 38 which is remote from the valve 59 and slidably extends through a sealed opening in the second piston 37. The second piston 37 is of substantial diameter and works in a co-axial bore 40, and an enlarged head 41 at the free end of the piston-rod 39 forms an abutment for one end of a safety spring 42.When no pressure is present in the system the spring 42 holds both pistons 36 and 37 in advanced positions in which the valve 59 is held open and the passage 60 communicates with the passage 61 through the valve 32 and an expansion chamber 43 which is defined in the bore 38 between the valve 59 and the adjacent outer face 44 of the first piston 36.

A solenoid-operated valve 45 receives signal form a controller 46 which, in turn, is responsive to signals received from a wheel speed sensor 47.

The valve 45 includes a valve head 48 which is alternativeiy engageable with a pair of valve seats 49 and 50 which are of smaller and greater areas, respectively. When the solenoid-operated valve 45 is de-energised, indicating that no skid signal is present, the head 48 engages with the seating 49 to isolate a face 51 on the inner end of the piston 36, which is of smaller area, from the adjacent inner face 52 of the second piston 37. In this position the head 48 is spaced from the seating 50 so that the face 51 is in communication with the passage 60.

A restricted orifice 53 in the piston 37 connects the face 52 to the reservoir 28 through a chamber 54 in bore 40 on the oppsote side of the piston 37.

In the inoperative position shown in the drawing the valve 59 is held in its open position by a probe 55 projecting from the piston 36.

When the valve 2' is operated fluid from the accumulator 24 is supplied to the brake 1 and also acts on the face 51 of the piston 36 to hold the valve 59 open, and augment the load in the spring 42 which acts to hold the head 41 in abutment with the piston 37, and the piston 37 in engagement with a stop defined by the face 56 at the adjacent, inner, end of the bore 40.

When a skid signal is produced by the skid sensor 47 the solenoid-operated valve 45 is operated to cause the head 48 to move away from the seating 49 and into engagement with the seating 50. This isolates the passage 60 from the face 51 and places the faces 51 and 52 in communication. The pressure acting on the face 51 falls and the brake pressure acting on face 44 of piston 36 subjects to it a force which moves it downwardly against the force in the spring 42.

Movement of the piston 36 displaces fluid from the bore 38 into the bore 40, to move the piston 37 downwardly.

Since the area of the face 52 of the piston 37 is substantially greater than that of the face 51, the piston 36 will move a proportionately greater distance than the piston 37 with the head 41 separating from the piston 37.

The valve 59 closes in response to a relatively small downward movement of the piston 36 which isolates the metering valve 2' from the brake 1, and then further movement of the piston 36 in the same direction causes the effective volume of the expansion chamber 43 to be increased to reduce the brake pressure and release the brake. The wheel then recovers from its skid.

When the wheel has recovered from its skid the sensor 47 signals the wheel re-acceleration to the controller 46 which releases the solenoid valve 45. The valve head 48 is moved in the opposite direction to return to its initial position with the two faces 51 and 52 isolated from each other, and the face 51 exposed to pressure from the valve 2'.

The increase in pressure applied to the face 51 urges the piston 36 relatively towards the valve 59 and with respect to the piston 37, and the brake is re-applied rapidly by fluid displaced from the chamber 43 by the piston 36. This continues until the head 41 re-engages with the piston 37 which arrests movement of the piston 37.

Thereafter movement of the piston is accompanied by the piston 37, but at a rate determined by the return of fluid from a chamber defined in the bore 40 between the two faces 52 and 56 and to the reservoir 28 through the restricted orifice 53. This occurs in a similar manner to that described above with reference to the embodiments of Figures 1 and 2.

The graph of Figure 4 shows a typical brake release and re-application sequence including skid correction for the three constructions described above. The broken line indicates the value of brake pressure at which brake release occurs automatically.

Initial brake release occurs at A when the pressure is released until it decreases to a value B at which the skid signal disappears. Up to point C the brake pressure is re-applied rapidly by one piston operating on its own, and the gradual increase to point D is accomplished by both pistons of the piston assembly moving together.

At D the brake pressure is again released to a point E, higher than point B and indicating that the surface ,u has improved, and at which the skid signal disappears. The brake pressure is then reapplied through two stages F and G and, at G, the pressure is again released to point H, at a similar value to B, indicating that the surface ,u has deteriorated in comparison with that at point H.

Thereafter the re-application and release cycle is repeated as shown.

Claims (12)

1. An anti-skid hydraulic braking system of the kind set forth in which support fluid acts on the de-boost piston through a modulator piston assembly to provide the fluid-sustained support force, and the modulator piston includes first and second pistons defining pressure-responsive faces of different effective areas, the two pistons being movable together as a single unit and also being relatively movable axially independently of each other, and the second valve is movable between first and second operable positions, in the first operable position of the second valve in which no signal is operative the support fluid is applied to the first piston to hold the de-boost piston in the advanced position, and, in response to a skid signal, the support fluid is released and the first piston is subjected to a net force of sufficient magnitude to cause it to move relatively away from an abutment on the second piston and in a direction to reduce the support force and enable the de-boost piston to move towards its retracted position, and restrictor means are incorporated for restricting return flow to a reservoir of fluid displaced by movement of the two pistons as a single unit, termination of the skid signal with the second valve returning to the first operable position causing the support fluid to be applied again to the first piston to urge the de-boost piston towards the advanced position at a first rate until movement of the first piston is arrested by the engagement with the abutment on the second piston whereafter further movement of the de-boost piston towards the advanced position continues at a second rate slower than the first and determined by the difference in areas between a second face of the first piston and a first face of the second piston and the size of the restrictor means.

2. A system as claimed in Claim 1, in which the restrictor means comprises a restricted orifice in the second piston.

3. A system as claimed in Claim 1, in which the restrictor means comprises an orifice in a flowregulating valve with which one end of the first piston is in communication.

4. A system as claimed in any preceding claim, in which the first and second faces of the first piston are disposed at its opposite ends, the second face being at the inner end which is of greater effective area than the outer end, which has the first face, the inner end including an axial extension which co-operates with the de-boost piston, and the second piston which is of annular outline and is of substantially greater effective area than either of the two ends of the first piston is slidably guided on a third portion of the first piston which is of reduced diameter and which interconnects both ends of the first piston, the support fluid being applied to the first face of the first piston, and the second face on the first piston being in communication with a first face on the second piston when the second valve is in the first operable position and, in response to a skid signal, the second face of the first piston being isolated from the first face of the second piston, and the first and second faces of the first piston, which are of different effective areas, being interconnected.

5. A system as claimed in Claim 4, in which the de-boost piston is held in its advanced position by pressure acting on the first face of the first piston comprising the end remote from the de-boost piston when the second valve is in its first operable position, and the second face of the first piston is in communication with the first face of the second piston, which comprises an annular face on the side of the second piston remote from the outer end of the first piston.

6. A system as claimed in Claim 3 or Claim 4, in which fluid displaced by the inner end of the first piston after correction of a skid is accommodated between that end and the second piston during the first rapid stage of brake re-application with the first piston moving relative to the second piston until the abutment arrests that movement of the first piston whereafter such displaced fluid is accommodated by a return flow to the reservoir through the restricted orifice.

7. A system as claimed in any of Claims 1 --3, in which the de-boost piston is defined by the outer face of the first piston, and the inner face of the first piston is subjected to the support fluid to hold to de-boost piston in its advanced position and is isolated from an adjacent face of the second piston when the second valve is in its first operable position, movement of the second valve into the second operable position isolating the said inner face of the first piston from the support fluid, and interconnecting the said inner face with the said adjacent face of the second piston whereby to urge the modulator piston assembly away from the said advanced position with the first piston moving through a greater retracted distance than the second piston, subsequent return of the second valve to the first operable position again isolating the said inner face from the said adjacent face, and subjecting the said inner face to the support fluid, whereby the first piston moves relative to the second during the first rapid stage at brake re-application until the abutment arrests that movement of the first piston with subsequent movement being accompanied by a corresponding movement of the second piston.

8. A system as claimed in any preceding claim in which the modulator assembly also include a safety device to ensure that the first valve remains open in the event of failure of the supply of support fluid.

9. A system as claimed in Claim 8, in which the safety device comprises a spring of which the load is overcome when the piston assembly is subjected to the said net force.

10. A system as claimed in Claim 8, in which the safety device comprises a flow-regulating valve of which a spool controlling operation of the first valve is urged by a spring into a position to hold the first valve open upon failure of the suppiy of support fluid.

11. An anti-skid hydraulic braking system substantially as described herein with reference to and as illustrated in Figures 1 and 4 of the accompanying drawings.

12. An anti-skid hydraulic braking system sybstantially as described herein with reference to and as illustrated in Figures 2 and 4 of the accompanying drawings.

1 3. An anti-skid hydraulic braking system substantially as described herein with reference to and as illustrated in Figures 3 and 4 of the accompanying drawings.