IMPROVEMENTS IN FLUID-PRESSURE OPERATED ANTI-SKID BRAKING SYSTEMS FOR VEHICLES

Abstract: Brakes (1,2; 3,4) on the wheels (5,6; 7,8) at opposite ends of a common axle are each controlled by a separate skid control unit. Each skid control unit incorporates a memory chamber (33, 34) for storing a memory pressure dependent upon the pressure occurring in the brake at the point at which a skid is detected. When a skid is detected at a wheel a latch valve (21) closes and the brake on the skidding wheel is released and re-applied when the skid signal terminates. The brake is re-applied in a fast first stage, due to pressure from the associated memory chamber, and in a slower second stage due to the latch valve being closed and restricting the rate of increase of braking pressure when it is closed. During a skid the pressure in the memory chamber (33, 34) decays through a decay valve (52, 53) from an initial value substantially equal to the brake pressure at the detection of the skid to a lower value dependent upon the duration of the skid. Thus the longer and more severe the skid, the lower will be the memory pressure which re-applies the brake in the first stage of brake re-application, which compensates for a higher build up of pressure between the isolated memory chamber and the source (29) of control fluid which would otherwise cause brake re-application to overshoot its desired re-application pressure.

Description: This invention relates to improvements in fluid-pressure operated braking systems according to the preamble to Claim 1. Such systems will hereinafter be referred to as "of the kind set forth". A system of the kind set forth is disclosed in GB-A-2007787 in which, when after detection of a skid and consequent isolation of the memory chamber from the supply, cessation of the skid is detected, the isolating valve opens to permit of supply of control fluid to re-apply the brake at said relatively high rate by utilising only control fluid from the control fluid supply passage. The memory chamber remains isolated from the supply during this first stage of operation until the control pressure has reached a value equal to that in the memory chamber, whereupon the control fluid is permitted to enter the memory chamber and thus also has to increase the pressure in the memory chamber, causing the above mentioned re-application rate to fall to said reduced rate. Such a system requires the provision of a memory chamber having a relatively large volume which results in a relatively bulky valve and slow brake response time because of the time taken to fill the large volume of the memory valve. The aim of the present invention is to provide a new and improved braking system of the kind set forth which avoids or reduces these shortcomings. This aim is achieved by opening the isolating valve when the skid has ceased to connect the memory chamber to the supply line to provide control fluid from the memory chamber for re-application of the brake at said relatively high rate during the first stage. It was found that occasionally the pressure of the knee point of brake re-application increased during a skid to a level higher than intended. Allowing the memory pressure to decay during a skid ensures that the memory pressure does not increase. In systems of the kind set forth, when the skid control unit recognises a skid signal it isolates the memory chamber from the supply and vents the brake to atmosphere. We have appreciated that there remains a connection portion of the system between the supply and a valve which isolates the memory chamber from the supply which connection portion may continue to be pressurised by the supply of control fluid during the skid. Thus the pressure in the connection portion may rise during a skid. When the skid is over and the connection portion and the memory chamber are re-connected with each other, and with the remainder of the braking system, the effective pressure applied nominally by the memory chamber can be higher than expected because of the build up of pressure in the connection portion. The memory pressure decay means allows us to compensate for the build up of pressure in the connection portion during a skid by reducing the pressure in the memory chamber during the skid so that when the brake is re-applied it is re-applied at a more carefully controlled pressure than hitherto. The memory pressure decay means enables the pressure of the knee point of the control fluid applied in the first stage of brake re-application to be maintained substantially constant independent of the duration of the skid. The memory pressure decay means enables the pressure of the knee point of the control fluid applied in the first stage of brake re-application to be smaller for skids of larger duration. The connection portion is preferably pressurised at the reduced rate of brake re-application. The rate of decay of the memory pressure through the decay means may be related to the relative volumes of the connection portion and the memory chamber, and to the rate of pressure increase in the connection portion after a skid has been detected. The pressure to which the memory pressure decays is preferably dependent on the duration of the skid. A latch valve may be provided to control the flow of control fluid between a source of control fluid and the skid control unit, closure of the latch valve restricting flow of control fluid from the source to the unit to the reduced rate of the second stage of brake re-application. A relay valve may be provided in the skid control unit to control whether the limit applies or releases the brake. The relay valve, and the latch valve, may be solenoid operated. One embodiment of our invention is illustrated in the accompanying drawings in which:- FIGURE 1 is a layout of a two-axle vehicle provided with a relay-operated anti-skid pneumatic braking system; FIGURE 2 is a schematic layout in more detail of the system arrangement for the rear axle; FIGURE 3 is a performance graph; FIGURE 4 is an end view of one practical form of control unit with all the valves and other components incorporated within a common housing; FIGURE 5 is a section on the line 5-5 of Figure 4; FIGURE 6 is a plan of the unit; FIGURE 7 is a section on the line 7-7 of Figure 6; FIGURE 8 is an end elevation of a one-way valve incorporated in the system and the control unit; and FIGURE 9 is a section on the line 9-9 of Figure 8. The anti-skid pneumatic braking system illustrated in Figure 1 of the drawings is similar to that of EP-A-0236085. In the anti-skid pneumatic braking system illustrated in Figure 1 of the drawings brakes 1, 2, 3, 4 are provided on each of two front wheels 5, 6 on a first axle of a vehicle, and on two rear wheels 7, 8 on a second axle of the vehicle, respectively. Each wheel is provided with a respective speed sensor 9, 10, 11 and 12, and the output signals from the sensors are fed into an electronic control module 13 which differentiates the signals and emits energising currents for operating front and rear skid control units 14 and 15. Each skid control unit 14, 15 controls the supply of control pneumatic fluid pressure under the control of a treadle valve 29 to a relay valve 16, 17 for each respective wheel, and each relay valve 16, 17 in turn, controls the supply of brake actuating air under pressure from a reservoir 18, 19 to the brakes 1 and 2, and 3 and 4. The arrangement provided on the front axle is in accordance with EP-A-0236085, but the modified arrangement applied to the rear axle and as illustrated in Figure 2 will be described. As illustrated Figure 2 shows, schematically, the construction of the skid control unit 16 for the brakes 3, 4 on the rear axle. As illustrated, control fluid from the treadle valve 29 is supplied to an inlet passage 20 and through a normally open latch valve 21, operated by a solenoid 22, to the two relay valves 16 and 17 through passages 23 and 24 respectively. A one-way valve 25, 26 respectively, and a solenoid-operated cut-off valve 27, 28, respectively, each incorporating a normally closed exhaust port, are located in each passage 23 and 24 between the latch valve 21 and the respective relay valve 16, 17. Each relay valve 16, 17 is of known construction incorporating a piston 30 working in a bore 31 and operable, in response to the pressure of the control fluid, to control a main valve 32 between the common reservoir 19, the respective brake 3, 4, and a central exhaust passage 39. A memory chamber 33, 34 is disposed between each solenoid valve 27, 28 and a connection in the passage 23, 24 between the one-way valve 25, 26 and the solenoid valve 27, 28, and one-way valves 35, 36 and 37, 38 are provided between the solenoid-operated valves 27, 28 and the chambers 33, 34, and between the memory chambers 33, 34 and the passages 23, 24. Each memory chamber 33, 34 is provided with a restricted passage 52, 53, through which the pressure can decay at a low rate towards the upstream side of the valves 27 and 28 respectively. Finally a one-way valve 40, 41 is provided in a passage 42, 43 leading from the inlet passage 20 to each respective passage 23, 24 on the downstream side of the latch valve 21, between the two one-way valves 25, 37 and 26, 38 respectively. Each one-way valve 40,41 is illustrated on an enlarged scale in Figures 8 and 9 of the drawings. As illustrated each valve comprises a seating 44 of frusto-conical outline, and a valve member 45 in the form of a ball for engagement with seating 44. The movement of the ball 45 away from the seating is limited by three angularly spaced inwardly directed radial fingers 46 and, when the ball 45 is in engagement with the seating 44, a limited flow past the ball is permitted through a notch 47 in the seating 44. As shown in Figures 4 to 7, all the valves 21, 40, 41, 35, 37, 36, 38, 27, 28, 16 and 17 and other components (33, 34) on each axle are housed and defined within a common housing 51. The solenoid 22 of the latch valve and solenoids 47, and 48 of the valve 27 and 28 are adapted to be energised by a current from the control module 13. Specifically the control module 13 is arranged to energise the solenoid 22 and the solenoid 47 in response to a signal from the respective sensor 11, and separately to energise the solenoid 22 and the solenoid 48 in response to a signal from the sensor 12. When any such current is emitted, the solenoid 22 will be energised before, or simultaneously with, the respective solenoid 47 or 48. In a normal inoperative position shown in Figure 2 of the drawings, three solenoids 22, 47, and 48 are de-energised so that the latch valve 21 and the valves 47 and 48 are in the open position. Both relay valves 16, 17 are closed to isolate the brakes 3 and 4 from the reservoir 19. Upon operation of the treadle valve 29 the control pressure is supplied to the inlet passage 20, and through the open latch valve 21, since the one way valves 40, 41 are closed by that pressure. The control pressure flows through the one-way valves 25 and 26, the passages 23 and 24, and the open valves 27 and 28 to act on the pistons 30 of the relay valves 16, 17. In addition the control pressure enters both memory chambers 33 and 34 through the respective one-way valves 35. The whole system is therefore subjected throughout to an equal control pressure. The pressure acting on the pistons 30 causes both relay valves 16, 17 to operate in a known manner to supply air under pressure from the reservoir 19 to the actuators of the brakes in order to apply both brakes 3, 4 simultaneously. The pressure rises with time as shown in the portion A, B of the graph of Figure 3. Upon release of the treadle at the end of the braking cycle, air from the brakes 3, 4 is released to atmosphere through the exhaust passages 39, and the control pressure is released through the one-way valves 40 and 41 and an exhaust port on the treadle valve. When the brakes 3, 4 are applied as described above and both wheels of the vehicle pass onto a surface of a low co-efficient of friction (µ) which is sufficiently low for both sensors 11, 12 to emit skid signals substantially simultaneously, an equivalent energising current is emitted by the control module 13 to energise the three solenoids 22, 47 and 48 and thereby close the latch valve 21, and both valves 27 and 28. Closure of the latch valve 21 isolates the direct flow of control pressure from the inlet passage 20 to the passages 23, and 24, but a restricted flow into the passages 23 and 24 still takes place through the notches 47 in the seatings 44 of the one-way valve 40, 41. Closure of the valves 27, 28 isolates the passages 23 and 24 from the relay valves 16 and 17, which close by the exhaustion to atmosphere through the exhaust port 50 of the control pressures acting on the pistons 30. The pressure then drops from point B as shown in Figure 3. Due to the presence of the one-way valve 35, 36, a memory pressure is trapped in each memory chamber, and that pressure is equal to the value of the control pressure applied to the pistons 30 when the skid signals were emitted by the sensors 11, 12 at the skid point. However, the pressure in each memory chamber 33, 34 is permitted to decay at a rate determined by flow through the respective restricted passage 52, 53, which rate of decay is matched to the rate at which the pressure applied to the relay valves 17, 18 during the first stage of brake re-application would otherwise increase due to the restricted flow of control pressure through the notch 47 increasing the pressure on the passages 23, 42 and 24, 41 upstream of valves 27, 28. When each wheel recovers and the skid signals are cancelled, say when the brake pressure has reduced to point C in Figure 3, indicative that the quality of the road surface has improved sufficiently for the brakes 7, 8 to be re-applied, the control module 13 cuts-off the energising current to the two solenoids 47, 48 so that these two valves open again, but the solenoid 22 remains energised to hold the latch valve 21 in its closed position. Opening the valves 27, 28 enables the pressures trapped in the memory chambers 33, 34 to be released through the one-way valves 37, 38 to act on the pistons 30 and act as application pressures to cause the relay valves 16, 17 to operate again to re-apply the brakes 3, 4 as described above. Since the volumes of the chambers 33, 34 have to fill the remainder of the passages and the spaces in the bores 31 above the pistons 30, known as the application chambers, such volumes will only be sufficient to raise the brake pressure rapidly in a first stage up to a knee or changeover point D, as shown in Figure 3, which is lower than the said point B. Thereafter the brakes are re-applied, in a second stage, but at a rate of pressure increase slower than that of the first, by the flow to the relay valves 16, 17 of a restricted flow of fluid through the notches 47 in the seatings of the one-way valves 40, 41. This is depicted in the graph of Figure 3 by the line D E. The restricted passages 52, 53 ensure that the pressures in the memory chamber 33, 34 have not increased, which otherwise might cause the brakes to be re-applied in the first stage to a point in excess of the desired point D. Should the skid last for a significantly period of time the pressure in passages 42, 43, and 23, 24, will increase due to restricted flow past one way valves 40, 41. The decay passages 52, 53 enable the pressure in the memory chambers to decrease during the skid to compensate for the build up in pressure in the passages upstream of the relay valves 27, 28. Thus any tendency for such "overshoot" to occur is avoided and we maintain the height of point D constant independent of the duration of the skid. Should a further skid condition occur during the application of the brakes in a common cycle, then the sequence of brake release, and brake re-application as described above, will be repeated. When the brakes are applied as described above, and one wheel, say the wheel 7, passes onto a surface of low µ with the other wheel travelling over a surface of high µ, a so-called split µ situation arises. Provided the surface of high µ is sufficiently high for no skid signal to be emitted by the sensor 12, only the sensor 11 will emit a skid signal, in response to which the control module 13 energises the solenoid 22, and the solenoid 47 to close the latch valve 21, and the valve 27. The pressure applied to the brakes 3 is then relieved as described above. Since no skid signal had been emitted by the speed sensor 12, the solenoid 48 remains de-energised and the brake 4 continues to be applied. However any increase in pressure applied to the brake 4 is controlled by a control pressure which can increase only at a rate determined by flow through the notch 47 in the one-way valve 41, due to closure of the latch valve 21, which occurs at the skid point B. Thus, such a rate of pressure increase from B to H is equivalent to that normally experienced in a second stage of brake re-application, as indicated in the graph of Figure 3 by the portion shown from D to E. The control module 13 is arranged to take into account the behaviour of all the wheels, and a latch valve 21 on each axle, when closed during a skid cycle, will remain closed until the control module 13 has demanded, from information from all four speed sensors 9, 10, 11 and 12, that the vehicle has moved onto a sufficiently good surface for the anti-lock system to be de-activated, and for the latch valves 21 to re-open. Naturally the release of the brakes at the end of a braking cycle will de-activate the anti-lock system. The one-way valves 25, 26 are provided to ensure that the two brake circuits on each axle are separate from each other, and there can be no mixing of the air on the two circuits, particularly after the latch valve 21 has closed. The arrangement described above for the brakes on the rear axle may also be provided for the brakes on the front axle We may choose to arrange the memory pressure decay means 52, 53 such that the longer the duration of the skid the lower the knee point D to which the brakes are eventually re-applied in a first stage of brake re-application. This may in any case occur in a very long skid if the pressure in passages 23, 42; 24, 33 reaches the maximum pressure that the supply 29 can achieve whilst the memory pressure continues to decrease.

Claims: A fluid-pressure operated braking system for vehicles comprising a supply (18, 19) of brake-operating fluid for actuating a wheel brake (1, 2, 3, 4), which supplies fluid to the brake through a skid control unit (14, 15), a line (23, 24) which supplies control fluid to the skid control unit (14, 15) from a supply (29), sensors (9, 10, 11, 12) provided to detect wheel skids, means (13) responsive to a skid signal produced by any of the sensors (9, 10, 11, 12) for operating an isolating valve (27, 28) of the skid control unit (14, 15) to relieve the pressure of the control fluid, and thus of the brake operating fluid, at a skid point, the unit (14, 15) incorporating a memory chamber (33, 34) for storing a memory pressure dependent upon the control pressure and thus on the brake operating fluid pressure, occurring at the skid point to provide a datum or knee point between first and second stages of brake re-application, the brake (1, 2, 3, 4) being applied in the first stage of brake re-application at a relatively higher rate than it is re-applied in the second stage of brake re-application, and the rate of brake re-application in said second stage being at a reduced rate in comparison to that at which the brake is applied before a skid point is detected, pressure decay means (52, 53) being associated with the memory chamber to allow the memory pressure stored in the memory chamber to decay when a skid point is detected relating to the wheel associated with the skid control unit from an initial value substantially equal to that of the pressure of the control fluid immediately before the skid point was detected to a lower pressure, the isolating valve (27, 28) being provided in a passage communicating the memory chamber (33, 34) with a supply (29) of control fluid, so that when a skid signal is detected for a wheel associated with the skid control unit (14, 15) the isolating valve (27, 28) is closed to isolate the memory chamber from the supply of control fluid (29), a connection portion (23, 42; 24, 43) of the passage extending between the supply (29) of control fluid and the isolating valve (27, 28) continuing to be pressurised by the control fluid whilst the isolating valve (27, 28) is closed, characterised in that when the skid has ceased the isolating valve (27, 28) is opened to connect the memory chamber (33, 34) to the line (23, 24) to provide control fluid from the memory chamber for re-application of said brake (1, 2, 3, 4) at said relatively high rate during the first stage, the memory pressure decay means (52, 53) allowing the memory pressure to decay to compensate, upon said brake re-application in the first stage, for the pressurisation of the connection portion. A braking system according to Claim 1, characterised in that the pressure to which the memory pressure decays during a skid is dependent on the duration of the skid. A braking system according to Claim 1 or Claim 2, characterised in that after a skid signal has been detected the connection portion (23, 42; 24, 43) is pressurised at the reduced rate of brake re-application. A braking system according to any one of the preceding claims, characterised in that the rate of decay of memory pressure during a skid is substantially equal to the reduced rate of brake re-application. A braking system according to any one of the preceding claims characterised in that the rate of decay of memory pressure during a skid is related to the relative volumes of the memory chamber (33, 34) and the connection portion (23, 42; 24, 43) and the rate of pressure increase in the connection portion after a skid signal has been detected. A braking system according to any one of the preceding claims, characterised in that a latch valve (21) controls the flow of control fluid between the source (29) of control fluid and the skid control unit, closure of the latch valve restricting flow of control fluid from the source to the unit to the reduced rate of the second stage of brake re-application. A braking system according to Claim 6, characterised in that the memory pressure decay means comprises a restricted passage (52, 53) communicating the memory chamber (33, 34) with the downstream side of the latch valve (21). A braking system according to Claim 6 or Claim 7, characterised in that the memory pressure decay means comprises a restricted passage (52, 53) communicating the memory chamber (33, 34) with the upstream side of a relay valve (27, 28) incorporated in the skid control unit. A braking system according to any one of Claims 6 to 8, characterised in that brakes (1, 2; 3, 4) on opposite ends of a common axle are each controlled by a separate skid control unit (16, 17) and a single latch valve (21) common to both units closes at the first skid point to be sensed by a sensor (9, 10; 11, 12) on either wheel, whereafter re-application of the two brakes is controlled in response to signals from the individual sensors, but at rates of pressure increase determined by the memory chambers of the respective units, and by restricted flows of control fluid, and each memory chamber (33, 34) has its own independent memory pressure decay means (52, 53).

Inventors: BROOME, WILLIAM SYDNEY, CARSWELL, EDWIN ROBERT, MCCANN, DENIS JOHN, WILLIAMS, ALLAN
Applicants: GRAU LIMITED
Application year: 1988
Main classification: B60T8
IPC codes: B60T8