GB2193276A

GB2193276A - Vehicle anti-lock braking

Info

Publication number: GB2193276A
Application number: GB08716585A
Authority: GB
Inventors: Philip Michael Antrobus; Bernard Bollons
Original assignee: Automotive Products PLC
Current assignee: Automotive Products PLC
Priority date: 1986-08-02
Filing date: 1987-07-14
Publication date: 1988-02-03
Anticipated expiration: 2007-07-14
Also published as: GB2193276B; GB8618936D0; GB8716585D0

Abstract

The invention provides a control system for an antilock braking system including a servo brake pressure modulating mechanism (50:50') which may be controlled by two solenoid valves (70 and 78). Solenoid valve (70) when energised causes reduction in the braking effort and when de-energised causes reapplication of the braking effort and solenoid valve (78) permits reduction or reapplication of the braking effort at a slow rate when de-energised and at a fast rate when energised. The control means includes sensing means (11;11') associated with a wheel for producing a signal corresponding to the speed of the wheel; means (103) for deriving there from a signal corresponding to wheel acceleration or deceleration; means (107,114,72) for energising the solenoid valve (70) when the wheel deceleration reaches a first predetermined magnitude; means (108,115,114,72) for de- energising the solenoid valve (70) when wheel deceleration falls to a second predetermined magnitude; means (109,116,117,80) for energising solenoid valve (78) when wheel acceleration reaches a first predetermined magnitude; and means (116,117,80) for de-energising the second solenoid valve (78) after a predetermined time period has elapsed from when the wheel acceleration reached said first predetermined magnitude. The first predetermined magnitude may be increased and permitted to decay to its original valve when the first valve is switched to its reapply mode and the second valve may be switched to its fast mode when wheel deceleration reaches a third deceleration magnitude greater than the first, which third magnitude may also be increased and permitted to decay to its signal value.

Description

SPECIFICATION Vehicle anti-lock braking system This invention relates to vehicle antilock braking systems.

In the vehicle antilock braking system disclosed in British Patent Application No. 86 091 63, servo means is provided for varying the effort applied to the brake on a wheel, in order to prevent the wheel locking during braking. The servo means is controlled by a first solenoid valve in response to which pressure in the braking system may be either reduced or reapplied. A second solenoid valve varies the rate of reapplication, between a fast and slow rate.

The present invention provides a control system by means of which the first and second solenoid valves may be controlled during a braking operation, in response to wheel acceleration or deceleration, to provide an antilock cycle which will reduce the braking effort as wheel locking conditions approach and will reapply the braking effort as the wheel accelerates.

According to one aspect of the present invention a control system for an antilock braking system including servo means for varying the effort applied to a brake on a wheel, a first solenoid valve by means of which said servo means may be controlled to either reduce or reapply the braking effort and a second solenoid valve by means of which said servo means may be controlled to reapply the braking effort at either a fast or slow rate, comprises; sensing means associated with the wheel for producing a signal corresponding to the speed of the wheel; means for converting the wheel speed signal into a signal corresponding to wheel acceleration or deceleration; means for switching said first solenoid valve, so that the servo means will act to reduce the braking effort applied to the wheel, when the wheel deceleration reaches a first predetermined magnitude; means for switching said first solenoid valve, so that the servo means will act to reapply the braking effort, when the wheel deceleration falls to a second predetermined magnitude; means for switching said second solenoid valve, so that the servo means will go from a slow rate of reapply to a fast rate of reapply, when the wheel acceleration ieaches a first predetermined magnitude; and means for switching said second solenoid valve, so that the servo means will go from a fast rate of reapply to a slow rate of reapply, after a predetermined time period has elapsed from when the wheel acceleration reached said first predetermined magnitude.

The actual values at which the solenoid valves are switched will depend of the characteristics of the antilock braking system and upon the vehicle, but generally, the first predetermined magnitude of deceleration will be between 1 g and 69, typically 1.59; the sec ond predetermined magnitude of deceleration is at a point just before the wheel starts to accelerate, typically 0.49; the first predeter mined magnitude of acceleration will be be tween 1 g and 8g, typically 49; and the pre determined time period will be between 15 and 60 milliseconds, typically 20 milliseconds.

Preferably, the second solenoid valve will also be arranged to control the servo means so that braking effort may be reduced at either a fast or slow rate. Further means will be provided in the control system by which the second solenoid valve may be switched, so that the servo means goes from a slow rate of reduction of braking effort to a fast rate of reduction of braking effort, when the wheel deceleration reaches a third predeter mined magnitude, said third predetermined magnitude being in excess of the first predet ermined magnitude of deceleration.Moreover, means may be provided in the control system for converting the wheel speed signal into a rate of change of deceleration or acceleration signal, this signal may be used for switching said second solenoid valve, to cause the servo means to go from a fast rate of reduction of braking effort to a slow rate of reduction of braking effort, when the rate of change of deceleration has a predetermined value which is approximately zero. The third predet ermined magnitude of deceleration must be greater than the first predetermined magnitude and will generally be between Ig and 69, typi cally 2.59.

At the end of an antilock cycle, the slow rate of reapply will continue until the full brak ing effort is re-established or the wheel decel eration reaches the first predetermined magni tude, when a further antilock braking cycle will commence.

During a braking operation, the velocity of the wheel will be below that of the vehicle, due to slippage. Depending on the road sur face, if the slip increases above a given per centage of the vehicle speed, locking of the wheel will be liable to occur. It is conse quently undesirable to commence reapplication of the brakes if the slip is above a predeter mined value, even if deceleration of the vehicle wheel falls to the second predeter mined magnitude.

Where a wheel of the vehicle is unbraked, the speed of this wheel may be taken as the speed of the vehicle. However, normally all the wheels of the vehicle will be braked, so that this will not be possible. However means may be provided in the control system for producing a simulated vehicle speed from the speed of the wheel, when an antilock cycle is commenced.This vehicle speed simulation means may, for example, assume a decay in the vehicle speed of 19. Means may then be provided to give a measure of the slip be tween the actual wheel speed and the simu lated vehicle speed and switch the first solenoid valve, so that the servo means will act to reapply the brake effort, only when the wheel deceleration falls to said second predetermined magnitude and slip between the wheel and simulated vehicle speed is below a predetermined magnitude. The predetermined magnitude of slip may generally be from 10% to 50%, typically 30%.

The control system may also include a low speed disable means, which will make the antilock system inoperable at low wheel speeds, for example up to 3 to 4 miles per hour.

The control system may be powered via the ignition switch of the vehicle. However, power to the solenoid valves is preferably controlled through a switch actuated by movement of the brake pedal, for example the brake light switch of the vehicle, so that an antilock cycle cannot be initiated until the brakes have been applied.

An embodiment of the invention is now described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates diagramatically an antilock braking system formed in accordance with the present invention; Figure 2 is a cross section through a servo means and valve means for use in the vehicle antilock braking system illustrated in figure 1; Figure 3 is a cross section along the line Ill-Ill in figure 2; Figure 4 is a block diagram of the control system used in the antilock system illustrated in figure 1; Figure 5 is a graph showing plots of wheel speed (C); wheel deceleration (D) and brake pressure (E) against time; Figure 6 is a block diagram of part of the control system illustrating modifications to the control system illustrated in figure 4; and Figure 7 is a graph similar to that of figure 5 illustrating a typical antilock cycle of the antilock system under control of the modified control system illustrated in figure 6.

In the antilock braking system illustrated in figure 1, the front wheels of a vehicle are provided with disc brakes 1,1' controlled by actuating calipers 2,2' respectively and rear wheels are provided with drum brakes 3,3' being controlled by hydraulic cylinders 4,4' respectively.

The braking system is a dual system controlled by a servo assisted dual master cylinder 5 of conventional design. The master cylinder 5 has two outlets 6,6' which provide a source of hydraulic pressure for two legs of the dual system, one leg serving caliper 2 of the offside front wheel and cylinder 4 of the nearside rear wheel amd the other leg serving caliper 2' of the nearside front wheel and cylinder 4' of- the offside rear wheel.

The two legs of the system are identical in arrangement and operation and only one leg is described in detail below. Similar components have been identified with similar references.

The outlet 6 of master cylinder 5 is connected to the hydraulic fluid inlet port 13 of an antilock servo mechanism 50 and the outlet port 15 from said servo mechanism 50 is connected via line 7 to the nearside front caliper 2 and offside rear cylinder 4.

As illustrated in greater detail in figures 2 and 3, the servo mechanism 50 comprises a cylinder body 12 having an inlet port 13 and an outlet port 15 opening into a bore 16. A valve seat member 17 is secured in the bore 16 and has a control port 18 which can be closed by ball 19. A control plunger 21 is slidable in the bore 16 and in the position shown in figure 2 is operative to unseat the ball 19 against a light compression spring and open communication between the inlet port 13 and outlet port 15.

The cylinder body 12 is mounted through a closure member 52 of a dish shaped casing 51, so that it projects coaxially into the casing 51. A piston 25 and dished plunger 28 of integral moulded plastic construction are slidingly mounted on the cylinder body 12 so that the plunger 28 engages the end of control plunger 21. A pair of helical compression springs 26 and 27 act between the end wall of the casing 51 and the piston 25 to urge the piston 25 towards the closure member 52. An elastomeric diaphragm 24 is secured at its outer periphery between the closure member 52 and casing 51 in a circular groove 53, and at its inner periphery to the piston 25 in groove 54, thereby separating the casing 51 into two separate chambers 31 and 32.

A valve 55 has valve block 56 formed integrally of the closure member 52. A valve housing 57 is clamped about the valve block 56 to define a pair of chambers 60 and 61 on one side of the valve block 56 and a third chamber 62 on the other side of the valve block 56. The housing also defines a first port 64 which opens into chamber 60 via valve seat formation 65 and a second port 66 which opens into chamber 60 via a bore 67 through the valve block 56, the bore 67 being coaxial with the valve seat formation 65. A valve member 70 is provided between the valve seat formation 65 and bore 67. A spring 71 urges the valve member 70 into engagement with the seat formation 65 to close port 64. A solenoid 72 (solenoid A) is arranged to move the valve member 70 against the spring 71 away from seat formation 65 and into engagement with valve block 56, thereby opening port 64 and closing bore 67.

Chamber 60 is connected to the chamber 62 by means of bore 73 through the valve block 56. A bore 75 with restricter orifice 76 is provided through valve block 56 to connect chamber 62 with chamber 61. A second bore 77 of very much larger diameter is provided in parallel with bore 75 between chambers 62 and 61. The end of bore 77 adjacent chamber 62 is closed by valve member 78 which is urged into engagement with the valve block 56 by a spring 79. A second solenoid 80 (solenoid B) is arranged to move valve member 78 against the spring 79, thereby opening bore 77 to chamber 62. A transverse bore 81 through the valve block 56 connects bore 77 with chamber 32 of servo mechanism 50.

A third bore 82 with restricter orifice 83 is also provided between chambers 62 and 61.

The third bore 82 is provided with a nonreturn valve 84 which seats towards the chamber 62.

Chamber 31 of the servo mechanism 50 and port 66 of the valve 55 are connected via vacuum line 8 to a vacuum reservoir 9 and port 64 of the valve is open to atmosphere.

Solenoids A and B of the valve 55 are controlled by an electric control system 10 which processes signals from a variable reluctance output voltage sensing device 11, associated with the front wheel of the vehicle.

The pickup 100 of the variable reluctance output voltage sensing device 11 produces an output which typically ranges from 40mV at 3 mph to 1500mV at 100 mph. The frequency of the signal, which is proportional to the number of teeth of the toothed wheel 99, is typically set at 17.6Hz per mph. This signal is fed to a pickup amplifier 101, where it is converted to a 10V peak to peak square wave.

The square wave from amplifier 101 is fed to; means 102 for producing an analogue output signal, the voltage of which is proportional to the wheel velocity; means 103 for producing an analogue output signal, the voltage of which is proportional to wheel acceleration or deceleration; and means 104 for producing an analogue output signal, the voltage of which is proportional to the rate of change of acceleration, i.e. da/dt.

The analogue signal of wheel speed from means 102, is fed to a low speed disable circuit 105 which, if the wheel speed is below 3 to 4 mph, will prevent the antilock control system 10 from operating to energise solenoids A or B.

The analogue signal corresponding to wheel speed from means 102, is also fed to a wheel logic circuit 111 which produces a signal simulating vehicle speed, this signal being based upon the wheel speed signal when an antilock cycle is commenced and assuming an average deceleration of 1g. This simulated vehicle speed signal is fed to comparator means 112where it is compared with the actual wheel speed signal from 102 and produces an output signal when the difference (ie slip) is below a predetermined value, for example 30%.

The analogue signal proportional to wheel acceleration or deceleration produced by means 103, is fed to an acceleration logic circuit 106 where; threshold switching means 107 produces an output signal when deceleration of the front wheel reaches a first predetermined magnitude, for example 1.59; threshold switching means 108 produces an output signal when deceleration of the front wheel falls to a second predetermined magnitude, for example 0.49; theshold switching means 109 produces an output signal when the acceleration of the front wheel reaches a first predetermined magnitude, for example 49; and threshold switching means 110 produces an output signal when the deceleration of the front wheel reaches a third predetermined magnitude, for example 2.59.

The analogue signal proportional to rate of change of acceleration produced by means 104, is fed to a threshold switching means 113, which produces an output signal when deceleration starts to fall, i.e. da/dt is approximately zero.

The output of switching means 107 is connected to an input of switching circuit 114, by means of which a signal on the output of the switching means 107 may switch circuit 114 to energise solenoid A. The outputs of switching means 108 and comparator 112 are connected via an AND-gate 115, to a second input of switching circuit 114, so that simultaneous signals on the outputs of switching means 108 and comparator 112 may switch the circuit 114 to de-energise solenoid A.

The output of switching means 109 is connected through timer 116 to a first input of switching circuit 117, so that a signal on the output of switching means 109 may switch circuit 117 to energise solenoid B for a predetermined period set by the timer 116, for example 20 milliseconds. The output of the switching means 110 is connected to a second input of switching circuit 117, so that a signal on switching means 110 may switch circuit 117 to de-energise solenoid B. The output of switching means 113 is connected to a third input on the switching circuit 117, so that a signal on the output of switching means 113 may switch circuit 117 to de-energise solenoid B.

Solenoids A and B are connected through switching circuits 114 and 117 respectively to the vehicle power supply by a brake light switch 118. The control circuit is powered by a 10 volt stabilised supply (not shown) which is connected to the vehicle supply via the ignition switch.

A typical antilock braking cycle is illustrated in figure 5, where curve C represents wheel speed, curve D represents wheel deceleration, curve E represents brake pressure, line F represents simulated vehicle speed obtained from speed logic circuit 111 and line F' represents the wheel speed corresponding to the simulated vehicle speed with 30% slip. The shift in time between wheel speed and deceleration curves and the brake pressure curve is due to delays in the solenoid valve operation and electronics.

Upon application of the brakes, at point G the solenoid switching circuits 114 and 117 are connected through brake light switch 118 to the vehicle power supply. However, both solenoids A and B remain de-energised and both chambers 31 and 32 of servo means 50 are connected to vacuum, so that the master cylinder 5 will be connected to the caliper 2 and wheel cylinder 4 and the braking effort will depend on the force applied to the brake pedal by the operator.

When deceleration of the wheel reaches 1.59 at time H, an output signal is produced by threshold switching means 107 and this switches circuit 114 to energise solenoid A.

As a result, valve member 70 closes bore 67 and opens seat 65, so that chamber 32 is connected to atmosphere through restricted bores 75 and 82. Air will thus be allowed into chamber 32 at a relatively slow rate. The inbalance in pressure across the diaphragm 24 will oppose the load applied by springs 26 and 27 and force piston 25 away from the closure member 52. Movement of the piston 25 will be transmitted to plunger 21, so that the ball 19 will close seat 18, isolating the caliper 2 and cylinder 4 from the master cylinder 5. Further movement of the plunger 21 will permit the brake fluid in caliper 2 and cylinder 4 to flow back into bore 16, thus reducing the braking effort.

When decleration of the wheel reaches 2.59 at time J, an output signal is produced by threshold switching means 110 and this switches circuit 117 to energise solenoid B. As a result, valve member 78 is opened so that air can enter chamber 32 at a fast rate through the relatively large bore 77, with a corresponding increase in the rate of reduction of braking effort.

When the deceleration of the wheel begins to fall, i.e. da/dt is approximately zero at time K, an output signal is produced by threshold switching means 113 and this switches circuit 117 so that solenoid B is de-energised. As a result, valve member 78 closes bore 77 and the servo means 50 reverts to a slow rate of reduction- of braking effort.

When the- deceleration of the wheel falls to 0.49 at time L, an output signal is produced by a threshold switching means 108. However, as slip at this point is above 30%, no signal will appear at the output of comparator 112. It is not until time M when slip falls below 30% that an output signal will be produced at comparator 112 and thus circuit 114 will be switched via AND-gate 115 to de-energise solenoid A. As a result, valve member 70 closes seat 65 and opens bore 67 thus connecting chamber 32 to vacuum via restricted bore 75 and reapplying braking effort at a relatively slow rate.

When wheel acceleration reaches 49 at time N, an output signal is produced at threshold switching means 109 which switches on timer 116. This in turn switches circuit 117 to energise solenoid B and open bore 77 to give a fast rate of reapplication of the brake. At the end of 20 milliseconds, time P, timer 116 switches circuit 117 to de-energise solenoid B, so that the servo means 50 reverts to a slow rate of reapply. This slow rate of reapply continues until full braking effort is re-established or another antilock cycle begins.

Figure 5 represents a full antilock braking cycle. One or more of the stages described may be omitted, if conditions require. For example, if slow reduction of braking effort is sufficient to prevent the wheel decelerating at above the third predetermined magnitude, for example 2.5g, the fast reduction stage will be omitted and the cycle will next go to the slow reapplication stage.

Under hard braking the suspension and tyres of a vehicle are distorted and if the brakes are suddenly released the tyres and suspension revert to their undistorted condition. As a result the wheel speed will, for a short time, exceed the speed of the vehicle giving rise to a corresponding acceleration and deceleration of the wheel. Similar effects are caused when the wheel encounters either a pot hole or hump as it must instantaneously alter its speed to keep in contact with the road surface. With the antilock system described above such transient deceleration signals can trigger an antilock cycle and reduce braking effort when there is no danger of wheel lock.

In the modified control system illustrated in figure 6, the output from AND-gate 115 is connected to means 130 which is connected to the threshold switching means 107 and serves to increase the threshold of switching means 107 from a voltage corresponding to the first predetermined magnitude of deceleration to a voltage correspqnding to an increased magnitude of deceleration, when a signal appears on the output of AND-gate 115. Means 130 also allows the threshold of switching means 107 to revert to the voltage corresponding to the first predetermined magnitude of deceleration in a truncated exponential decay. Typically means 130 will increase the threshold of switching means 107 by a voltage corresponding to an increase in the first predetermined magnitude of deceleration of the order of 2.59, for example from 1 .5g to 49, and decays back to its original value in the order of 200 milliseconds.

The output from switching means 113 is similarly connected to means 131 which is connected to threshold switching means 110 and serves to increase the threshold of switching means 110 from a voltage corresponding to the third predetermined magnitude of deceleration to a voltage corresponding to an increased magnitude of deceleration, when a signal appears at the output of switching means 113. Means 131 also allows the threshold of switching means 110 to revert back to its original value in a truncated exponential decay. Typically means 131 will increase the threshold of switching means 110 by a voltage corresponding to an increase in the third predetermined magnitude of deceleration of the order of 3.5g, for example from 2.59 to 79, and decays back to its original value in the order of 200 milliseconds.

If the wheel accelerates as a result of suspension unwind or travelling over an uneven surface, as illustrated in broken line at X on figure 7, it is possible that the resulting deceleration Y could exceed 1 .5g or even 2.5g and the signal resulting could trigger a reduction in braking effort. However, with the modified control system, when solenoid A is switched off at time M, means 130 increases the threshold of switching means 107, so that a deceleration of 49 is required to initiate reduction in braking effort. Similarly at time K, the threshold of switching means 110 is increased by means 131, so that a deceleration of 7g is required to switch solenoid B.It is unlikely that the transient deceleration will reach 4g and consequently the control system will miss transient deceleration signals resulting in the circumstances described above. The decay in the threshold voltage provided by means 130 and 131 respectively will enable the threshold of switching means 107 and 110 to fall sufficiently between the end of one antilock cycle and the beginning of the next, so that genuine deceleration signals will trigger the antilock cycle in the normal way.

In the modified control system described above, the threshold of switching means 107 is increased when solenoid A is switched to reapply braking effort and that of switching means 110 is increased when solenoid B is switched so that the servo means goes from fast to slow reduction in braking effort. Other triggering points may however be used, means 130 and 131 being connected to appropriate points on the control system.

Various modifications may be made without departing from the invention. For example, while in the above embodiment, the servo means may reduce -braking effort at a fast or slow rate or reapply braking effort at a fast or slow rate and the slow rate of reapply is slower than the slow rate of reduction, the invention is applicable to any system which provides for at least one rate of reduction and at least two rates of reapply. Furthermore, although described with reference to a dual braking system split diagonally, the invention is applicable to single or multi-circuit braking systems.

In the above description, unless stated otherwise the various circuit elements will produce a digital output signal. This may be produced by switching from a low state to a high state or from a high state to a low state.

Alternatively each circuit element could produce a coded signal and switching circuits 114 and 117 could be replaced by a processor which would control solenoids 72 and 80 in accordance with the coded signal received.

Claims

1. A control system for an antilock braking system including servo means for varying the effort applied to a brake on a wheel, a first solenoid valve by means of which said servo means may be controlled to either reduce or reapply the braking effort and a second solenoid valve by means of which said servo means may be controlled to reapply the braking effort at either a fast or slow rate, said control means comprising; sensing means associated with the wheel for producing a signal corresponding to the speed of the wheel; means for converting the wheel speed signal into a signal corresponding to wheel acceleration or deceleration; means for switching said first solenoid valve, so that the servo means will act to reduce the braking effort applied to the wheel, when the wheel deceleration reaches a first predetermined magnitude; means for switching said first solenoid valve, so that the servo means will act to reapply the braking effort, when the wheel deceleration falls to a second predetermined magnitude; means for switching said second solenoid valve, so that the servo means will go from a slow rate of reapply to a fast rate of reapply, when the wheel acceleration reaches a first predetermined magnitude; and means for switching said second solenoid valve, so that the servo means will go from a fast rate of reapply to a slow rate of reapply, after a predetermined time period has elapsed from when the wheel acceleration reached said first predetermined magnitude.

2. A control system according to Claim 1 in which the first predetermined magnitude of deceleration is between Ig and 6g; the second predetermined magnitude of deceleration is at a point just before the wheel starts to accelerate,; the first predetermined magnitude of acceleration is between 1g and 89; and the predetermined time period is between 15 and 60 milliseconds.

3. A control system according to claim 1 or 2 in which means is provided for increasing said first predetermined magnitude of deceleration at some point in the antilock braking cycle after the first solenoid valve has been switched to cause the servo means to reduce braking effort, said meanS permitting said first predetermined magnitude to decay back to its original value over a predetermined time period.

4. A control system according to claim 3 in which the first predetermined magnitude is increased by the order of 2.5g and decays back to its original value in the order of 200 milliseconds.

5. A control systems according to claim 3 or 4 in which the first predetermined magnitude of deceleration is increased when the first solenoid valve is switched to reapply braking effort.

6. A control system according to any one of the preceding claims which said second solenoid valve may be controlled so that reduction in braking effort may be at a fast or slow rate, means being provided by which the second solenoid valve may be switched, so that the servo means will go from a slow rate of reduction of braking effort to a fast rate of reduction of braking effort, when wheel deceleration reaches a third predetermined- magni- tude, said third predetermined magnitude being in excess of the first predetermined magnitude of deceleration.

7. A control system according to Claim 6 in which said third predetermined magnitude of deceleration is between 1g and 69.

8. A control system according to claim 6 or 7- in which means is provided for increasing said third prdetermined magnitude of deceleration at some point in the antilock braking cycle after the servo means has been switched to go to a fast rate of reduction of braking effort, said means permitting said third prdetermined magnitude of deceleration to decay back to its original value over a predetermined time period.

9. A control system according to claim 8 in which the third predetermined magnitude of deceleration is increased by the order of 3.5g and decays back to its original value in, of the order of 200 milliseconds.

10. A control system according to claim 8 or 9 in which said third predetermined magnitude of deceleration is increased when the servo means is switched to go from fast to slow reduction of braking effort.

11. A control system according to any one of claims 6 to 10 in which means is provided for converting the wheel speed signal into a rate of change of deceleration or acceleration signal, and means for switching said second solenoid, so that the servo means will go from a fast to a slqw rate of reduction of braking effort, when the rate of deceleration is approximately zero.

12. A control system according to any one of the preceding claims including means for producing a simulated vehicle speed signal; means for comparing this simulated vehicle speed signal with the actual wheel speed signal and producing an output when slip is less than a predetermined magnitude; and means for switching the first solenoid valve, so that the servo means will act to reapply the braking effort, only when the wheel deceleration falls to said second predetermined magnitude and slip between the wheel and simulated vehicle speed is below said predetermined magnitude.

13. A control system according to Claim 12 in which the predetermined magnitude of slip is from 10% to 50%.

14. A control system according to Claim 12 or 13 in which the means for producing a simulated vehicle speed signal, processes the wheel speed signal occurring when an antilock cycle is commenced.

15. A control system according to any one of the preceding claims in which means is provided to disable the antilock system at wheel speeds below a predetermined magnitude.

16. A control system according to any one of the preceding claims in which means is provided to permit energisation of the solenoid valves only after a braking operation has been commenced.

17. A control system according to Claim 16 in which energisation of the solenoid valves occurs through switching means controlled by operation of the brake actuation means.

18. A control system for an antilock braking system including servo means for varying the effort applied to the brake on a wheel, a first solenoid valve by means of which said servo means may be controlled to either reduce or reapply the braking effort and a second solenoid valve by means of which said servo means may be controlled to reapply the braking effort at either a fast or slow rate, substantially as described herein with reference to, and as shown in, Figures 1 to 5 or figures 6 and 7 of the accompanying drawings.