GB2125912A - Improvements in anti-skid braking systems - Google Patents

Abstract

A vehicle anti-skid brake control system comprising means for effecting release of a brake in response to a signal indicating an incipient skid, means for sensing the brake pressure, and means for effecting re-application of the brake when the incipient skid signal has ceased. The system incorporates means for reducing the rate of reapplication of brake pressure when the said pressure has reached a level which is a predetermined function of the brake pressure at which a previous incipient skid signal was generated. <IMAGE>

Description

SPECIFICATION Improvements in anti-skid braking systems This invention relates to anti-skid braking systems for vehicles.

Known anti-skid braking systems include means for monitoring the rotational speed of associated wheels and detecting when skidding is imminent and when it has been averted, and means for modulating the braking pressure supplied to the actuators of associated brakes for the avoidance of skidding.

The pressure modulating means is normally a pressure control mechanism having a first operational state in which it freely permits communication of pressure between the driver's control mechanism and the associated brake actuators to enable normal application and release of the brakes, and a second operational state in which it isolates the associated brake actuators from the driver's brake control mechanism and reduces braking pressure in them for the avoidance of skidding of the associated wheels.

The wheel speed monitoring and skid detection functions are normally performed by electronic means which are arranged to change the operational state of the pressure modulating means from the first state to the second state when an incipient skid is detected and to return the pressure modulating means to its first operational state when skidding has been averted.

The latter control function is essential to maintenance of vehicle braking whilst the vehicle is in motion and the driver's demand for braking is sustained. Hence under braking conditions in which skidding of the associated wheels would otherwise occur the associated brakes are cyclically released (partially or wholly) and reapplied to avoid skidding and to maintain braking at or near an optimum level dictated by tyre and road surface conditions.

In systems as described, incipient skid corrective action is taken when loss of wheel speed exceeds some predetermined criteria, such as, for example, detection of wheel deceleration exceeding a fixed threshold rate of deceleration.

Detection of the subsequent state of successful skid avoidance is based upon the subsequent wheel speed rise characteristics satisfying criteria which ensure that wheel rotation is being sustained in a manner which is compatible with the changing speed of the braked vehicle.

The foregoing detection principles give rise to control difficulties. The predetermined criteria forming the basis for incipient skid detection require to be established with respect to ideal tyre-to-road conditions for avoidance of limitation of braking under good conditions for avoidance of limitation of braking under good conditions by premature operation of the anti-skid system. This leads to a situation in which the controlled wheels may be considerably overbraked, prior to taking incipient skid corrective action, when the vehicle is being braked on a slippery surface. For avoidance of skidding, the pressure modulation means is required to reduce pressure rapidly, and consequently it must also be arranged to reapply pressure rapidly so as to avoid untoward loss of braking after skidding has been averted.For the system to perform effectively its essential function of skid avoidance, coupled with maintenance of adequate braking, it is required to cycle rapidly and over a wide pressure range. This leads to excessive energy consumption in the operation of the braking system, which is a serious disadvantage when the vehicle braking system relies for its full effectiveness upon a reserve of stored pressure. Furthermore the system can be harsh in operation under some conditions and give rise to considerable vibration effects.

According to the invention, a vehicle anti-skid brake control system comprises means for effecting release of a brake in response to a signal indicating an incipient skid, means for sensing the brake pressure, and means for effecting reapplication of the brake when the incipient skid signal has ceased, the system incorporating means for reducing the rate of reapplication of brake pressure when the said pressure has reached a level which is a predetermined function of the brake pressure at which a previous incipient skid signal was generated.

By the foregoing means a considerable proportion of the brake pressure required to give another incipient skid condition can be reimposed rapidly upon detection of satisfactory skid avoidance in a skid control cycle, thus reimposing a substantial braking effect upon the vehicle at the earliest possible time in that cycle. Thereafter braking pressure rises relatively slowly to produce another incipient skid signal.

Thus the rate of system cycling is slower than it would otherwise be, and overbraking prior to incipient skid detection is minimised giving a reduced pressure range. These effects combine to give considerable improvement of energy consumption, and the latter effect reduces operational harshness.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating a system in accordance with the invention; Figure 2 is a graph illustrating the operation of the system of Figure 1; Figures 3 and 4 are graphs illustrating alternative modes of operation; Figure 5 is a side elevation, partly in section, of a control unit assembly; and Figure 6 is an end elevation, partly in section, of the control unit assembly shown in Figure 5.

For convenience, the system shown in Figure 1 is discussed as it may be used in conjunction with an air pressure braking system.

The brake pressure modulating mechanism 1 comprises two air pressure control valves 2 and 3.

Valves 2 and 3 are arranged so that the outlet of valve 2 is connected to the inlet of valve 3 by a passageway 4. The inlet 5 of valve 2 is supplied with air pressure from the driver's brake control mechanism (not shown) and the pressure outlet 6 of valve 3 is connected to an air brake actuator 7 by conventional piping 8. Pressurisation of the working chamber 9 of the air brake actuator 7 is converted by conventional piston or diaphragm means into a force acting through the rod 10 to apply an associated brake (not shown) with effectiveness proportional to the air pressure in the working chamber 9.

Valve 2 is a two-way valve having two operational states. These are a first state, which is effective when its operating solenoid 11 is unenergised, in which its inlet 5 is in free communication with its outlet 4, and a second state, effective when a solenoid 11 is energised, in which communication between its inlet 5 and its outlet 4 is restricted.

Valve 3 is a three-way valve also having two operational states. These are a first state, which is effective when its operating solenoid 12 is unenergised, in which its inlet 4 is in free communication with its outlet 6, and a second state, effective when solenoid 12 is energised, in which its outlet 6 is isolated from its inlet 4 and connected to atmosphere via an exhaust port 1 3.

It follows from the foregoing description that, with both solenoids 11 and 12 unenergised, normal application and release of the brake is permitted by virtue of both valves 2 and 3 being in their free communication conditions. If however, solenoid 12 is energised, as in response to an incipient skid signal, then the operational state of valve 3 is changed to isolate the working chamber 9 from the driver's brake control mechanism and connect it to atmosphere at exhaust port 1 3 to reduce its level of pressurisation and the actuation force applied to the associated brake.

De-energisation of the solenoid 12, as in response to a "skid averted" signal will cause the valve 3 to revert to its first operational state so that, provided pressure has been maintained at inlet 5, pressure in the working chamber 9 will rise at a relatively rapid rate until such time that solenoid 11 is energised to change the operational state of valve 2 which then adopts a restrictive mode so that the rate of pressure rise in working chamber 9 becomes relatively slow.

Energisation and de-energisation of solenoids 11 and 12 is controlled by the electronic module 14 to which they are connected by wires 1 5 and 1 6, and 1 7 and 1 8, respectively. Power supply for the solenoids 11 and 1 2 and for the electronic circuits of module 14 are derived from the vehicle's electrical system via wires 1 9 and 20.

The electronic module 1 4 receives wheel speed information from a wheel speed sensor (not shown) via wire 21. This is normally in the form of an A.C. voltage, generated by a magnetic reluctance device in conjunction with an interrupted ferrous surface which rotates with the wheel or a transmission element, such that the frequency is proportional to wheel rotational speed. This information is processed by the electronic circuits of module 14, and is used to derive the "incipient skid" signal and the "skid averted" signal in sympathy with which it energises and de-energises, respectively, solenoid 12.

A pressure transducer 22 is connected to the air brake actuator 7 so as to receive pressure which exists in the working chamber 9. The output of transducer 22 is in the form of a voltage which is proportional to pressure in the working chamber 9. This output is conveyed by a wire 23 to the electronic control module, where it is used, with reference to other data, to control the energisation and de-energisation of solenoid 11.

The control behaviour of the improved system is described with reference to Figure 2, which is a typical graph having a vertical scale representing either air pressure in the working chamber 9 or the voltage representation of this as produced by the transducer 22. Starting from the origin 0, brake pressure rises as brakes are app!ied by the driver as indicated by line 30. At the point 31 where an incipient skid signal is generated and whilst pressure generated by the driver's brake control mechanism may continue to rise, as indicated by dotted line 32, the working chamber 9 is isolated from this pressure and its pressure is reduced, as indicated by line 33, by energisation of solenoid 12 and the consequent change of valve 3 from its first operational state to its second operational state.

Also at point 31 of Figure 2 the voltage level proportional to the brake pressure level which provoked the incipient skid condition is stored within the electronic control module 14 as a reference voltage indicated by chain-dot line 34.

Brake pressure will continue to fall until, at point 35, a skid averted signal is generated.

Solenoid 12 is then de-energised and valve 3 reverts to its first operational state giving rapid brake pressure rise, and hence voltage rise proportional to brake pressure rise, as indicated by line 36. When the voltage, proportional to pressure, has risen to a predetermined proportion of its reference level 34, as represented for example by chain-dot line 37, then solenoid 11 is energised to change valve 2 from its first operational state to its second operational state thus reducing the rate of increase of brake pressure, as indicated by line 38, until another incipient skid signal is generated. Solenoid 12 is then energised and solenoid 11 de-energised enabling the whole process to be repeated.

If another incipient skid signal is not generated, due, say, to improvement of road surface conditions, within a pre-determined slow brake pressure rise time, then solenoid 11 is de-energised to allow a rapid pressure rise up the level selected by the driver, or to initiate skid control again at a higher brake pressure level. The pressure/voltage level at which change of brake pressure rise rate takes place can be established in several ways other than that indicated by Figure 2.

For example, as in Figure 3 which uses the same reference numbers as Figure 2 for comparable features, the voltage reference 34 is allowed to decay at a predetermined rate, and the change of rise rate occurs at the intersection of the fast rise rate curve 36 with voltage reference 34. This approach can be used to make the overall behaviour of the anti-skid system more adaptive to varying road surface conditions. Equally the two approaches can be combined effectively by reducing the rate of decay of reference voltage 34, as shown in Figure 3, and using a larger proportion of it to define the change point than that indicated by Figure 2 in which the reference level is fixed.

Valve 2 has been described as a two-way, twostate valve, the second state of which is restrictive compared with the first state. There are many conventional ways of achieving this object. For example, a valve having a two position spool, one position giving free flow and the other position giving restricted flow, may be used. Alternatively, a poppet valve with a restrictive by-pass, provided, say, in the form of a notch in the poppet seating or an external restricted path in parallel with the valve, such that when the poppet seating is closed a restricted flow path remains. Yet another alternative is to provide the restricted effect by cyclically energising and de-energising solenoid 11 in conjunction with valve 2 in a form which effectively stops flow when its solenoid is energised.The reduced brake pressure rise rate is then effected by small step increments as indicated by line 38 of Figure 4. This method has the advantage that the slow pressure rise rate can be made adaptive by variation of the frequency, or 'mark-space' ratio, of solenoid energisation.

It is an advantage if as many as possible of the control elements of the improved anti-skid system are integrated into one unit; this confers ease of fitment to vehicle and minimises the need for exposed wiring, so improving overall reliability. It is also an advantage in air brake systems to employ solenoid-operated pilot valves for the control of large valves having flow areas compatible with the rapid pressure change requirements: this confers economic and response-speed advantages. A preferred construction for use with air brake systems will now be discussed with reference to Figures 5 and 6.

Figure 5 is a part section through the improved anti-skid system integrated control unit assembly.

The electronic control module 50 is secured to, and forms a cover for, a solenoid valve housing 51. Fastened to the underside of the solenoid valve housing 51 is the main valve assembly 52.

The principal elements of the main valve assembly 52 are an inlet valve seating 53 formed in the main valve body 54, which can be closed by a rubber faced poppet valve 55 in response to pressurisation of a first working chamber 56 formed between the base of the solenoid valve housing 51 and a flexible diaphragm 57, formed integrally with the rubber facing of the poppet valve 55, which is sealingly retained at its outer periphery by the attachment of the main valve body 54 to solenoid valve housing 51.An exhaust valve, coaxial with the inlet valve comprises a seating 58 which is normally held closed by a second rubber faced poppet valve 59 due to the action of a return spring 60, through the intermediary of a stem 61 (which is common to the inlet and exhaust valves) and part-spherical abutments 62, formed on the underside of the poppet valve 59 and a working chamber cover 63 secured to the lower end of the stem 61.

Associated with the second poppet valve 59 is a second working chamber 64 defined by the working chamber cover 63 and a flexible diaphragm 65, formed integrally with the rubber facing of the poppet valve 59, which is sealingly clamped at its outer periphery to the working chamber cover 63 by a clamping ring 66. A rubber sealing ring 67 is provided at the engagement of the poppet valve 59 with the stem 61 to sealingly permit sliding motion of the former with respect to the latter.

The main valve body 54 is provided with two inlet ports 68 and 69 which communicate with the annular space surrounding the inlet valve seat 53 under the flexible diaphragm 57. It is also provided with four outlet ports, two of which, 70 and 71, can be seen in Figure 5, which connect the outlet chamber 72, located within and between the inlet and exhaust valve seatings, 53 and 58, to external ports arranged for pipe connections.

Multiple inlet and outlet ports are provided for installation flexibility; unwanted ports are plugged.

One, or more, inlet port is connected to the driver's brake control mechanism. One, or more, outlet port is connected to associated brake actuator(s).

From the foregoing description it is clear that if working chamber 56 only is pressurised both poppet valves will move downwards as a unit. This movement is limited by the upper poppet valve 55 closing onto the inlet valve seating 53, and the lower poppet valve 59 is opened away from the exhaust valve seating 58 by an amount which is equal to that shown for the inlet valve opening in Figure 5. Thus the outlet ports are isolated from the inlet ports, and connected to exhaust to reduce the pressure in the brake actuators. The exhaust opening of the valve is protected against the ingress of road dirt by the metal cover 73 and rubber flap 74. Holes 75 are provided in the metal cover 73 which are covered by the rubber flap 74 which readily deflects when the main valve is in the exhaust mode to permit relatively free loss of exhaust air to atmosphere.

To control separate supplies of air to the two working chambers 56 and 64 of the main valve assembly, two solenoid operated pilot valves 76 and 77 are located within the solenoid valve housing 51. These are connected, for electrical operation, to the electronic control module 50 by flying leads 78 and 79, respectively. A snap connector 80 is used to connect them to another flying lead 81 emerging from the electronic control module 50 through which the latter issues its commend signals.

The solenoid operated pilot valves 76 and 77 are of conventional design and similar in construction and characteristics. When their solenoids are unenergised their outlet ports 82 and 85, respectively, are connected to their exhaust ports 83 and 86, respectively and isolated from their inlet ports 84 and 87, respectively.

When their solenoids are energised their outlet ports 82 and 85 respectively, are connected to their inlet ports 84 and 87, respectively, and their exhaust ports 83 and 86, respectively, are blanked off.

Each solenoid operated pilot valve takes its inlet supply from the inlet ports 68 and 69 of the main valve assembly 52 via passageways 88 and 89 which sealingly communicate through the main valve body 54 and the base of the solenoid valve housing 51. The outlet port 82 of solenoid valve 76 communicates with the first working chamber 56 via passageway 90. Energisation of its solenoid will therefore cause closure of the inlet seating and opening of the exhaust seating to give a rapid reduction of pressure in the brake actuators in response to an incipient skid signal from the electronic control module.

De-energisation of its solenoid, when the skid is averted, will give rapid pressure rise in the brake actuators.

The outlet port 85 of solenoid valve 77 communicates with a passageway 91 which extends through a spigot 92 which sealingly and slidably engages with a bore in the stem 61 to communicate via drillings 93, 94 and 95 with the second working chamber 64 so that energisation of its solenoid causes pressurisation of the latter to give a main valve condition in which both its inlet seating 53 and its exhaust seating 58 are closed to halt rapid rise of pressure in the brake actuators at the point in time where the rate of rise of pressure is required to be reduced.

The mechanism for slow brake pressure rise will now be described with reference to Figure 6, which is a part section through the integrated control unit at right angles to that taken in Figure 5.

When the solenoid of solenoid valve 77 is energised the passageway 91 is pressurised and hence a passageway 96 is also pressurised. This passageway 96 leads to a one-way by-pass valve 97 a rubber facing of which is urged into sealing contact with a seating 98 by a spring 99. The one-way valve is arranged to open at relatively low pressure difference so that air pressure can communicate, via valve spring chamber 100, drilling 101, and passageway 102, with the outlet chamber 72 and the outlet ports 71 and 71 a. This by-pass route is restricted relative to the flow capacity of the main valve inlet seating and therefore provides a slow pressure rise rate at the brake actuators only when the s solenoid of solenoid valve 77 is energised to close both inlet and exhaust seatings of the main valve assembly.

An important objective of the preferred construction is to safely and reliably house the pressure transducer within the integrated control unit structure. While it is desirable for the voltage proportional to pressure to be representative of pressure in the brake actuator it is not necessary to mount the pressure transducer at the brake actuator as in Figure 1. It can be located remote from the brake in the vicinity of the main valve assembly, where air pressure changes are undamped by piping and actuator volume and apply electrical resistive and capacitive damping to the voltage output from the transducer to modify the signal to relate substantially to air pressure behaviour within the actuator working chamber. Thus, as illustrated in Figure 6, the pressure transducer 103 can take the form of a transistor package.This contains solid state elements which by, for example, piezo, resistive or capacitive means, interpret pressure in electrical terms. The pressure transducer 103 is electrically attached to a printed circuit board 104 in conventional fashion along with other electronic components (not shown). On assembly of the electronic module 50 a sealing ring 105, together with a rigid back-up ring 106, is placed on the outside diameter of the pressure transducer package and inserted for sealing engagement in a bore 107 formed in the electronic control module housing. The module is then filled with an encapsulating medium 108 to protect and support all components mounted on printed circuit board 104 and other circuit boards included in the electronic control module assembly.

A small vent hole 109 is provided in the end face of the pressure transducer package to allow air pressure to communicate with the sensing elements within. Pressure is conveyed to hole 109 from the outlet chamber 72 of the main valve assembly by means of passageways 110, 111 and 112, and the bore 107. In this way the pressure sensitive device is ideally housed and connected.

The only external connections to be made are the unavoidable ones of brake pipes and flying leads for electric power supply 11 3 and wheel speed sensor 114.

Claims (12)

1. A vehicle anti-skid brake control system comprising means for effecting release of a brake in response to a signal indicating an incipient skid, means for sensing the brake pressure, and means for effecting re-application of the brake when the incipient skid signal has ceased, the system incorporating means for reducing the rate of re-application of brake pressure when the said pressure has reached a level which is a predetermined function of the brake pressure at which a previous incipient skid signal was generated.

2. A system according to claim 1 wherein the said level is a predetermined fixed proportion of the brake pressure at which a previous incipient skid signal was generated.

3. A system according to claim 1 wherein the said level is compared with a reference which initially corresponds to the brake pressure at which a previous incipient skid signal was generated but which is allowed to decay at a predetermined rate.

4. A system according to any of claims 1-3 arranged to control a fluid-pressure operated braking system wherein the reduced rate of re-application of brake pressure is achieved by causing the flow of fluid to a brake actuator to pass through a restrictor.

5. A system according to any of claims 1-3 arranged to control a fluid-pressure operated braking system wherein the reduced brake pressure rise rate is effected by small step increments.

6. A system according to claim 4 comprising main inlet and exhaust valve members of the poppet type mounted coaxially so as to be movable as a unit to open the inlet and close the exhaust so as to supply fluid pressure to a brake actuator or to close the inlet and open the exhaust so as to exhaust fluid from a brake actuator, means being provided for closing the main inlet and exhaust valves when the said level is reached.

7. A system according to claim 6 wherein the main inlet and exhaust valve members are mounted on a common stem, one such valve member being movable relative to the stem so as to permit both the inlet and the exhaust valve to be closed at the same time.

8. A system according to claim 7 wherein a first working chamber is provided to operate the inlet valve and the stem provides a passage for fluid which is connected to a second working chamber formed between the exhaust valve member and a cover secured to the stem so as to provide means for moving the exhaust valve member relative to the stem and thus enable the main inlet and exhaust valves to be closed at the same time.

9. A system according to any of claims 6-8 wherein a restricted flow by-pass route is provided through a one-way by-pass valve when the main inlet and exhaust valves are both closed.

10. A system according to any of the preceding claims wherein the wheel speed and deceleration rate and brake pressure are sensed and processed by electronic means to control the operation of the system through solenoid-operated pilot valves.

11. A system according to any of the preceding claims in which the operative components including a brake pressure transducer are mounted in a common control unit structure remote from the brakes.

12. A system according to any of the preceding claims used in conjunction with an air pressure braking system.

1 3. A system constructed and arranged substantially described herein and illustrated in Figures 1, 5 and 6 of the accompanying drawings.