GB2182740A - Control system for an anti-skid braking system - Google Patents

Abstract

The braking system comprises a hydraulic brake actuator (14) arranged to apply a brake of a wheel (11) in response to operation of a driver-controller master cylinder (15). An anti-skid unit (18) interposed between the master cylinder and the actuator is capable of isolating the actuator from the master cylinder in response to a signal indicative of a skid condition and of tntrolling the brake actuation force. A control system (22) comprises to a speed sensor (25) for measuring the rotational speed of the wheel and means for measuring the brake actuation force by measuring pressure in actuator (14) by a pressure sensor (26). The control system derives a value for vehicle deceleration from the wheel speed and brake actuation force values and derives a measure of the vehicle speed during braking. The control system then controls by signals (23, 24) to unit 18 the brake actuation force such as to maintain the wheel speed in a predetermined relationship to vehicle speed. The unit (18) may e.g. incorporate <IMAGE>

Description

SPECIFICATION Control system for an anti-skid braking system The invention relates to control systems for anti-skid braking systems.

A typical anti-skid braking system comprises a brake acting on a vehicle wheel, a brake actuator arranged to apply a brake actuation force to the brake in response to operation of a driver controlled operating means and an anti-skid unit interposed between the operating means and the actuator and capable of isolating the actuator from the operating means in response to a signal indicative of a skid condition and of then controlling the brake actuation force. Normally the brake is operated hydraulically in which case the operating means is a master cylinder and the brake actuator is a wheel cylinder. The antiskid unit then incorporates a valve to isolate the master cylinder from the wheel cylinder whereupon the same valve or another valve causes the pressure in the wheel cylinder to be reduced temporarily.Means is also provided to increase the pressure again in the actuator to re-apply the brakes.

Such an anti-skid system requires a control system to cause the anti-skid unit to operate.

A typical control unit incoporates a wheel speed sensor. The value of wheel deceleration, derived from wheel speed, is normally used as the basic indication of a skid condition which brings the anti-skid system into operation. As the brake is released in response to high wheel deceleration, the braking effort reduces and the wheel accelarates towards its free-rolling speed, that is the speed at which it would free-wheel with movement of the vehicle.

This wheel accelaration is then used as an indication that a skid condition no longer exists and the brake is then re-applied.

It has been established that the effectiveness of a wheel and brake combination in generating a force to retard the vehicle varies with the ratio between the actual wheel speed and the free-rolling wheel speed. Typically the brake is most effective at a ratio of the order of 82.5%.,The effectiveness drops rapidly for ratios above 88% and reduces gradually as the ratio is reduced. For this reason it is desirable to operate the brake during a skid condition to such an extent as to maintain the optimum or nearly optimum ratio of actual wheel speed to free rolling wheel speed. The freerolling wheel speed is in direct proportion to vehicle speed so it is particularly valuable to know the vehicle speed. Measurement of actual vehicle speed is difficult to achieve because the normal road wheels are not freerolling.One attempt to obtain an effective measure of free-rolling wheel speed has been to provide a vehicle with an additional fifth wheel purely to measure vehicle speed independently of the speeds of the braked wheels.

Clearly this is cumbersome. Another possibility which has been considered is to start from actual vehicle speed at the time when the brakes are applied and then assume a given rate of decay of vehicle speed. This arrangement can often lead to inaccuracies.

The present invention is concerned with an improved technique for deriving actual vehicle speed (or free-rolling wheel speed) in a vehicle at a time when the brakes are applied and the actual wheel speed is less than the free-rolling wheel speed and to make use of this actual vehicle speed in a control system for anti-skid braking system.

In accordance with the present invention there is provided a control system for an antiskid braking system, the anti-skid braking system comprising a brake acting on a vehicle wheel, a brake actuator arranged. to apply a brake actuation force to the brake in response to operation of a driver-controlled operating means and an anti-skid unit interposed between the operating means and the actuator and capable of isolating the actuator from the operating means in response to a signal indicative of a skid condition and of then controlling the brake actuation force, the control system comprising means for mesuring the rotational speed of the wheel, means for measuring the brake actuation force, means for deriving a value for vehicle deceleration from said wheel speed and brake actuation force and thus deriving a measure of the vehicle speed during braking and means for controlling the brake actuation force such as to maintain the wheel speed in a predetermined relationship to vehicle speed.

The actuator may be a fluid pressure operated actuator and the brake actuation force may then be derived by measuring fluid pressure. The operating means may constitute a hydraulic master cylinder.

The system may be such that once a skid control condition has been established, the ratio of actual wheel speed to free-rolling wheel speed is maintained between the upper and lower pre-determined limits, one to each side of the ratio providing maximum braking force.

The upper limit may lie in the range 85% to 90% and the lower limit may lie in the range 70% to 80%. These levels are offset to both sides of the typical maximum brake force which is generated at 17.5%. Indication of a skid condition may be provided to the control system by detection of a value of wheel deceleration above a pre-determined value.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of an anti-skid braking system incorporating a control in accordance with the present inven tion; Figure 2 shows the relationship between brake actuation force and the ratio of actual to free-rolling wheel speed with the aid of which the system will be explained; and Figure 3 is a block diagram of an electronic control unit.

Figure 1 shows a braking system for a road wheel 11 in diagrammatic form. In practice the braking system is applied to more than one wheel and some parts of the system would be duplicated for each wheel to be braked in a conventional manner. However, for the purposes of explanation it is simplest to consider the braking of a single wheel. Wheel 11 has a brake constituted by a disc 12, a caliper 13 incorporating friction pads (not shown) which are engaged frictionally against the disc by means of a hydraulic actuator 14. Fluid pressure or the actuator 14 is generated in a hydraulic master cylinder 15 by means of a driver controlled foot pedal 16. The hydraulic line 17 between master cylinder and actuator incorporates an anti-skid unit 18 which is represented only diagrammatically.The antiskid unit incorporates a pressure blocking valve 19 which can be operated to isolate the actuator 14 from pressure generated in the master cylinder 15. A pressure control valve 21 can then be employed to reduce the pressure in the actuator 14. A variety of kinds of anti-skid control units are available, some incorporating a single valve providing the dual function of pressure blocking from the master cylinder and pressure relief from the actuator.

These various systems also employ means for re-building pressure in the actuator either by direct re-pressurisation on the actuator side of the blocking valve or by opening the blocking valve to allow master cylinder pressure again to be supplied to the actuator. Details of the anti-skid hydraulic valve system 1 8 are not important to the present invention and any typical known system may be employed.

The control signals for application to the anti-skid unit are esssentially a blocking signal supplied from a control system 22 on line 23 to the blocking valve 19 and a pressure control signal supplied on line 24 to relieve pressure in the actuator down to a controlled level or for a controlled limited time.

A wheel speed sensor 25 is typically an electro-magnetic pick-up which co-operates with a toothed wheel or with other protuberances on the wheel to provide a train of pulses at a frequency indicative of wheel speed. This actual wheel speed signal is supplied to the control unit 22.

A pressure sensor 26 measures the pressure in the actuator 14 and applies a signal representative of this pressure to the control unit 22.

Control unit 22 is a digital electronic control unit, the nature of which can be briefly explained by reference to its operation. Further details will be described subsequently with reference to the block diagram of Figure 3.

The pressure in actuator 14 is proportional to the force with which the pads are applied against the disc 12 and thus in general is proportional to the braking force or braking torque actually applied to the wheel at any instant. Thus a pressure signal Pb applied to the control unit 22 constitutes a signal representative of not only brake actuation force but also of brake force Fb applied to the wheel. In response to the signals V and Pb including the variation of these signals with time, the control unit 22 senses when a skid condition has developed, applies a blocking signal on line 23 to the blocking valve 19 and applies a pressure control signal on line 24 to cause appropriate variation of the pressure in the actuator.

In a typical case the pressure control signal is not commanding a specific pressure as such but instead commands a release of fluid from or supply of further fluid under pressure to the actuator. For this reason the pressure control signal 24 does not bear a simple and close relationship to the actual pressure signal or brake force signal.

Further requirements of the control system will now be described with reference to Figure 2. Consider a vehicle wheel having a pneumatic tyre running in contact with a road and with a brake force applied to resist rotation of the wheel. In this condition, the actual rotational speed of the wheel is less than the free-rolling wheel speed and figure 2 shows the drag force generated as a proportion of maximum drag force in relation to the ratio of actual wheel speed V to free-rolling wheel speed Vf, this ratio being presented as a percentage of Vf. The free-rolling speed Vf is of course directly proportional to the actual vehicle speed and so is not known unless the actual vehicle speed is known. Vehicle speed is normally measured from actual wheel speed assuming that the wheel is free rolling but of course in a skid condition the wheel is not free-rolling. Figure 2 shows that for a typical tyre on a typical road surface, the drag force increases rapidly and substantially linearally for slip ratios up to about 12% (that is V/Vf between 100% and 88%). With a greater degree of slip up to about 18%, the drag force increases more slowly to a maximum and thereafter the drag force falls off slowly with increasing slip to approximately 0.6 when the wheel has ceased to rotate. From figure 2, it is evident that when maximum deceleration of a vehicle is required during braking, this is best achieved by maintaining V/Vf in the region of 82%. For other road surfaces with higher friction co-effients, maximum drag force might be generated when V/Vf is about 75% and similarly for low friction co-efficient surfaces maximum braking might be obtained at a value of V/Vf equal to 90%.For any friction co-efficient, effective braking is achieved if V/Vf is maintained between 90% and 70%. V can be measured readily. If Vf (or actual vehicle speed) is also known reasonably accurately, it is a relatively simple matter to control the brake actuation force such that this is increased when V/Vf decreases below a lower limit of say 70% or 80% and is increased when V/Vf exceeds an upper value of say 85% or 90%.

As a further refinement, when a measure can be obtained for the friction co-efficient of the road surface, a more specific band of wheel speed can be selected to provide even more effective braking.

In a control system in accordance with the present invention the technique used for calculating Vf is as follows. Immediately prior to brake application, a free-rolling condition exists and Vf equals V which is measured directly.

When the brakes are applied, pressure in the brake actuator 14 is sensed and a measure of the brake force Fb is obtained by measuring this pressure as explained previously. The forces controlling rotation of the wheel are the brake force Fb tending to stop rotation of the wheel and a peripheral ground reaction force Fg in a direction to maintain rotation of the wheel. There is a simple relationship between these forces and wheel acceleration V which can be expresed as follows: Fg-Fb=KV K is a constant dependent on the moment of the inertia of the wheel.

V can be calculated simply because V is being measured continuously. A measure of Fb is being measured continuously. The constant K can be established simply for any wheel. Thus it is possible to calculate Fg.

Fg is the retardation force applied to the vehicle to slow it down under braking. The mass of the vehicle is known. This can either be taken as a constant value or can be measured by static suspension deflection. From this mass and the force Fg it is a simple matter to calculate the deceleration of the vehicle. This deceleration taken in conjunction with the vehicle speed immediately prior to braking gives a measure of the actual vehicle speed and thus of the free-running wheel speed Vf throughout the period while the vehicle is being braked.

All of these calcuiations are carried out in an effectively continuous manner at intervals of no more than a few milliseconds in the control unit 22. When V exceeds a pre-determined negative value indicative that the wheel is decelerating faster than the vehicle can be decelerated, this is taken as a signal that a skid condition has been established. A blocking signal is then produced on line 23 and the blocking valve 19 is closed. A brake actuation control signal is then established on line 24 to control the fluid pressure applied to actuator 14 and thus the brake actuation force in such a manner as to maintain the wheel speed V between its preset limits, for example such that V/Vf lies between 88 and 75%. In this way effective braking is achieved with the wheel still rotating and thus with good directional stability of the vehicle maintained.

A further refinement is possible as follows.

The maximum value achieved for Fg can be identified and the value of V/Vf at which this is achieved can also be recorded. This should be done on each occasion when the brakes are applied in response to establishment of the existence of a skid condition. For the remainder of that particular brake application, the range of values between which V/Vf is maintained can be set around this particular value.

Blocking valve 19 is preferably a one-way valve such that even when it is closed, the pressure on the actuator side of this valve can not exceed the pressure on the master cylinder side. Thus, as the driver releases the pedal 16, the pressure in the system decays and the brake is released. The control unit 22 is then re-set ready for a further brake application.

Figure 3 shows further details of the control system 22 and its relationship to other parts of the brake system. Functions carried out in a micro processor are represented for convenience by individual bjocks although in practice these are software functions. The pressure Pb from brake pressure sensor 26 is converted to a brake force Fb in block 31. This conversion can be effected by calculation when there is a simple mathematical relationship between Pb and Fb but if the relationship is complex, a look-up table may be used.

The signal V from the wheel speed sensor 25 is differentiated in a unit 32 to produce a wheel deceleration (or acceleration) signal V.

The signal V is also supplied to a hold unit 33 which is arranged to hold the wheel speed at an initial value Vo in response to a signal Fb indicating that the brake has been applied. A clock 34 provides a reference and controls repeated sampling operations.

Unit 35 calculates the wheel decelerating force Fg which also represents a vehicle decelerating force from the derived values of Fb and V and from a constant K referred to earlier. The vehicle decelerating force represented by signal Fg and the signal Vo are then employed in unit 36 to calculate the new vehicle speed. For this, the vehicle deceleration can be calculated from the decelerating force in conjunction with the vehicle mass which is known. For a vehicle expected to carry a load which is a significant proportion of its mass, a mass signal may be provided on vehicle start up by measuring static suspension deflection.

In some cases a constant mass value may be stored permanently in the micro processor.

The calculated new vehicle speed gives a measure of the free running wheel speed.

Unit 37 then calculates the slip (1-V/Vf) from the continuously directly measured value of V and the calculated value of Vf. A further unit 38 is programmed to provide brake pressure apply signals and brake pressure relief signals on lines 23 and 24 under the control of the slip percentage signal from unit 37.

An anti-skid control system as described above may be provided for each wheel of a vehicle or one system may be employed for a pair of wheels or for all of the wheels of a vehicle. In either of the latter two cases, the value of V can be measured from one wheel only or from both wheels or in the case of driving wheels of a vehicle from a common drive shaft to the wheels. Arrangements of this kind clearly involve some compromise in the control of the braking because different wheels may arrive at a skid condition at different times, for example to uneven loading of the vehicle or dynamic load transfer during cornering or differing friction co-efficients on different parts of a road. However, even with such a compromise, it is possible to achieve effective braking without the loss of control which would be associated with wheel lock.

Claims (7)

1. A control system for an anti-skid braking system, the anti-skid braking system comprising a brake acting on a vehicle wheel, a brake actuator arranged to apply a brake actuation force to the brake in response to operation of a driver-controlled operating means and an anti-skid unit interposed between the operating means and the actuator and capable of isolating the actuator from the operating means in response to a signal indicative of a skid condition and of then controlling the brake actuation force, the control system comprising means for measuring the rotational speed of the wheel, means for measuring the brake actuation force, means for deriving a value for vehicle deceleration from said wheel speed and brake actuation force and thus deriving a measure of the vehicle speed during braking and means for controlling the brake actuation force such as to maintain the wheel speed in a predetermined relationship to vehicle speed.

2. A control system for an anti-skid vehicle braking system as claimed in claim 1 wherein the actuator is a fluid pressure operated actuator and the brake actuation force is derived by measuring the fluid pressure.

3. A control system for an anti-skid vehicle braking system as claimed in claim 2 wherein the operating means is a hydraulic master cylinder.

4. A control system for an anti-skid vehicle braking system as claimed in any one of the preceding claims wherein once a skid condition has been established the ratio of actual wheel speed to free-rolling wheel speed is maintained between upper and lower predetermined limits one to each side of the ratio providing maximum braking force.

5. A control system for an anti-skid vehicle braking system as claimed in claim 4 wherein said upper limit lies between 85 and 90 per cent and said lower limit lies between 70 and 80 per cent.

6. A control system for an anti-skid vehicle braking system as claimed in claim 4 or claim 5 wherein a skid condition is indicated to the control system by detection of a value of wheel deceleration above a predetermined value.

7. A control system for an anti-skid vehicle braking system substantially as herein described with reference to and as illustrated by the accompanying drawings.