GB2298013A - ABS control system for road vehicles - Google Patents

Abstract

An anti-skid braking system, for wheeled vehicles having fluid actuated brakes associated with the vehicle wheels, wherein, when a split-mu condition is detected in which the wheels of the vehicle at opposite ends of an axle are running on a high-mu surface and a low-mu surface, respectively, pressure reduction at the high-mu wheel for effecting yaw-moment compensation is allowed to occur at times when the pressure at the low-mu wheel is not being reduced, provided that the measured rate of change of angular acceleration of the low-mu wheel is determined to be exceeding a first predetermined threshold.

Description

DESCRIPTION ABS CONTROL SYSTEM FOR ROAD VEHICLES The present invention relates to anti skid (ABS) braking systems for wheeled vehicles having fluid actuated brakes associated with the vehicle wheels and comprising speed sensors associated with the vehicle wheels, and a scanning control means responsive to speed signals from the speed sensors to actuate a pressure reduction device to periodically release the fluid pressure applied to the brake of any wheel which is determined, by detection of a predetermined level of relative slip between that wheel and the road surface, to be about to lock, and to later re-apply the actuating pressure to that brake when the tendency of that wheel to lock has reduced.The invention is concerned in particular with the control of vehicles when braking on road surfaces with differing grip under the left-hand and right-hand wheels (i.e. a split-mu surface). Maintaining the original course under these conditions requires steering correction. However, under panic conditions it is difficult for normal drivers to gauge the correct amount and to apply it quickly enough. ABS is usually programmed to assist the driver retain stability in this case, but interaction between the brakes and transmission can still cause problems with prior-art systems. The driver's difficulty becomes more acute in small cars, and generally increases with speed.

Many of the decisions described above are taken based on a knowledge of vehicle speed or surface friction. Most electronic braking systems do not measure these variables directly. Vehicle speed is estimated by monitoring and filtering the speed signals from all of the road wheels. When wheels are skidding, this estimate can be improved by using a measurement of vehicle acceleration from a chassis mounted accelerometer if this is fitted as part of the system.

Surface friction can be judged from vehicle-wide behaviour, such as vehicle deceleration, or from the behaviour of an individual wheel, such as reacceleration rate after a wheel skid or skid-pressure if this can be measured. The information used to judge surface friction may be different for different decisions within the ABS logic.

Although it will be described hereinafter with reference to a flow-valve-type ABS, in which pressure rise occurs by default whenever the dump solenoid is not energised, this invention is equally applicable to two solenoid per channel systems in which pressure rise is controlled by a second "hold" or inlet control solenoid.

Anti-lock braking systems have to accommodate many different road conditions, a particularly difficult one of which is the situation where the wheels on one side of the vehicle are running on a surface having a relatively high coefficient of friction (p) eg. tarmac) and the other side of the vehicle is running on a surface having a relatively low coefficient of friction (y) (e.g.ice). Such a surface is referred to as a split-mu surface.

Braking on a split-mu surface generates a yaw moment about the centre of gravity of the vehicle when the drag forces at the high-mu wheels exceed those at the low-mu wheels. Furthermore, depending upon the design details of the vehicle chassis, the difference in drag force across the front axle may unbalance the steering mechanism so as to generate a steering-wheel torque that will tend to accentuate the yaw. In order to maintain his original course, the driver must apply steering correction in two ways. First he must exert sufficient effort to counteract the drag-generated steering-wheel torque. Then he must proceed to balance the drag-generated yaw moment by turning the steering wheel away from the higher-mu surface until tyre side-force generates an equal but opposite moment.

The operation of a conventional anti-lock (ABS) system assists in the abovedescribed conditions by ensuring that the wheels continue to rotate, so that they are able to generate the required steering (i.e.

side-) forces. Also the ABS' normal select-low mode of rear-axle control (ie, the operating mode of the ABS by which the control of both wheels of an axle of the vehicle should be controlled in accordance with the rotational condition of that one of the wheels determined to be operating on the surface having the lower mu) prevents significant differences in drag across the rear axle, so that drag asymmetry is confined to the front axle. However, in a panic situation, the drag force increases very quickly, so that both the yaw moment and the steering-wheel torque will reach their maximum strength in a very short time interval thereby leaving the driver insufficient time to respond effectively. It is therefore usual for ABS logic to include so-called Yaw-Moment-Compensation (YMC) logic to assist the driver in retaining control of the vehicle during the initial braking phase.

This is achieved by slowing-down the rate at which the drag asymmetry builds-up at the front axle.

Conventional Yaw-Moment-Compensation (YMC) operates as follows. Once surface asymmetry has been detected, convention (prior-art) ABS systems effect yaw moment compensation by copying dump-solenoid commands for the low-mu wheel onto the corresponding solenoid for the high-mu wheel. ("Dump solenoids" are solenoids used in flow-valve type ABS systems to control the release/dump of pressure at the wheel brake actuators when a potential wheel lock condition is detected, to thereby release the braking effect and enable wheel recovery to occur). Pressure at the high-mu wheel is thus reduced in approximate synchronism with that at the low-mu wheel, minimising drag asymmetry.Whenever the low-mu wheel stops pressure dumping, or when the number of periods of copy dumping reaches a predetermined maximum, a regime of very slow pressure increase is initiated at the high-mu wheel. This is accomplished by pulsing its solenoid ("dump" solenoid for a flow-valve system, "hold" solenoid for an S/S system) in a predetermined pattern and results in a controlled increase in drag at the wheel. The initial pressure reduction at the high-mu wheel means that significant drag asymmetry exists only for a very short time interval during the critical first few moments; this allows stability to be maintained without the need for prompt steering correction.Ideally the YMC parameters are selected such that no steering correction is required at first, after which the controlled increase in drag should match the rate at which a normal driver is able to gauge and apply the necessary correction.

In practice, the systems operating as described above have problems in maintaining stability under certain conditions. The stability of small cars on split-mu depends critically upon the prompt reduction in dray asymmetry described above. Their short wheelbase and low polar mass-moment of inertia mean that any delay in the pressure-reduction process allows significant yaw and course deviation to develop very quickly. Most small cars are front-wheel-driven and unfortunately, prior-art logic can cause the dragreduction in the high-mu wheel to be delayed or interrupted if such a car is braked "in gear".

Consequent attempts by the driver at rapid steering correction can make the situation worse if drag symmetry is re-established after correction has already been applied. It is usually very difficult to regain stability and course following such destabilisation - especially at higher speeds where short-lived steering errors can cause significant lateral displacement of the vehicle.

In-gear braking constrains the freedom of the low-mu front wheel to rotate independently of that on the opposite side of the car because relative angular movement is influenced by the differential gearing.

Furthermore, the transmission couples the wheels to the engine's inertia. Thus in-gear panic braking can cause the initial locking pressure at the low-mu front wheel to exceed the normal locking-pressure because excessive brake torque at that wheel can be reacted by the high-mu front wheel, as well as by the engine's inertia.

In a rigid mechanical system such contraints would lead to delayed skid-detection at the low-mu wheel, and hence prolonged drag asymmetry. In practice, however, elasticity in the suspension and engine mounting arrangements provides two ways through which the low-mu wheel is still able to decelerate at a relatively high rate during the initial braking phase, whereby the abovedescribed in-gear effect is felt only after a delay.

Firstly, the compliance of the engine mountings permits the complete power-train assembly to rotate within the vehicle chassis in response to the application of brake torque via the transmission and this permits corresponding rotation of the low-mu wheel (with respect to the high-mu wheel) via the differential gearing.

Secondly, the transmission of excess brake torque to the high-mu wheel increases the drag between that wheel and the (high-mu) road surface, forcing the tyre to adopt a higher level of slip as well as causing additional suspension deflection. Moving from a lower slip level to a higher one involves some (small) wheel rotation, and so does the relative rearward movement of the wheel hub due to suspension deflection. The resulting angular movement of the high-mu wheel is then also available at the low-mu wheel.

Ultimately, however, further wheel-deceleration via elastic deflection will be impossible and a rebound effect will occur, causing a momentary increase in wheel-speed before the wheel continues to decelerate at a rate determined by the rotational inertia of the engine and transmission.

The interruption of wheel deceleration caused by the rebound effect usually cancels the dump command for the low-mu wheel. With the prior-art YMC logic described above the copy-dumping at the high-mu wheel is thus also interrupted, leaving a relatively high drag force present at the high-mu wheel. Although the pressure reduction at the low-mu wheel is incomplete its drag force is small because the surface adhesion is low; the result is that the period for which drag asymmetry exists becomes prolonged. Because the instability has already been set in train it is often of little help that the copy-dumping is able to continue after the rebound. In practice, the removal of drag asymmetry after steering correction has already been applied will cause a yaw moment in the opposite sense to the original, thus adding to the complexity of the driver's task.

It is a basic object of the present invention to provide a solution to the aforegoing problems of conventional ABS systems.

The present invention makes use of the appreciation that rapid changes in wheel acceleration occur during the rebound process so-called delta-wheel-acceleration, and these can be combined with other pertinent observations to identify the split-mu in-gear situation so that the pressure reduction at the high-mu wheel need not be delayed or interrupted.

In accordance with the present invention, pressure reduction at the high-mu wheel for effecting yaw-moment compensation is allowed at times when the pressure at the low-mu wheel is not being reduced, provided that the rate of change of angular acceleration of the low-mu wheel exceeds a first predetermined threshold.

In a typical case, said first predetermined threshold is greater than or equal to, +6g per (typically 7ms) scan period.

Preferably, the prevailing angular acceleration of the low-mu wheel should still be negative (i.e.

deceleration) for said pressure reduction at the highmu wheel to be allowed. If the prevailing wheel acceleration is positive, then its value should not exceed a second predetermined value (typically +4g) and, preferably, the delta-wheel-acceleration of the immediately preceding scan period should have exceeded the first predetermined threshold.

Preferably, the slip of the low-mu wheel with respect to the estimated vehicle speed should exceed a third predetermined threshold (typically 25Km/h).

By way of example, the delta-wheel-acceleration figure could represent a change from -13g in the preceding scan period to -5g in the current scan period (a delta-wheel-acceleration value of +8g). If, in the next scan period, a wheel acceleration of +2g is registered, then pressure reduction at the high-mu wheel could be allowed for that period also. However, such pressure reduction is dependent upon the occurrence of a delta-wheel-acceleration-qualified reduction in the preceding scan period. If the change registered in the initially considered scan period (from -13g to -5) had been smaller, e.g. from -10g to -5g, then pressure reduction at the high-mu wheel would not be allowed during that period because the delta-wheel-acceleration value would have been less than the required threshold value of 6g.As a direct consequence of the failure to reach the threshold value the following period would also fail to qualify for pressure reduction because its wheel acceleration value (+2g) is positive.

In practice a further restriction is preferably established to the effect that the aggregate dumpsolenoid energisation time of the high-mu front wheel should not exceed that of the low-mu front wheel.

This prevents delta-wheel-acceleration induced pressure-reduction from reversing the sense of the drag asymmetry.

The invention is described further hereinafter by way of example only, with reference to the accompanying drawings, in which: Fig. 1 comprises a series of actual operational curves for a typical prior art ABS system operating on a split-mu surface; Fig. 2 comprises a similar series of actual operational curves for a system embodying the present invention; and Fig. 3 is a flow-chart illustrating the basic operation of one possible system in accordance with the present invention.

Referring first to Fig. 1, the following operational traces are shown: A = Front right wheel brake pressure B = Front left wheel brake pressure C = Vehicle Deceleration D = Front right wheel speed E = Front left wheel speed F = Steering wheel angle G = Yaw velocity (degrees/sec) H = Front right wheel acceleration (g) I = Brake lamp switch status J = Front right dump solenoid status K = Front left dump solenoid status Thus, in Fig. 1, speed and angular acceleration are shown for the two front wheels. These values are computed by the vehicle electronic control unit (ECU) and are deemed valid for the complete scan period (7ms in this case) of the ECU under consideration.

The brakes are applied in an emergency manner at time t = 0. Shortly afterwards, the pressure in the brakes (traces A and B) begin to rise, followed by the vehicle deceleration (trace C) as the brake torque is reacted by the generation of drag between the tyres and the road. Although the front-left wheel has no difficulty in reacting to the applied torque because its tyre in on a high-mu surface, the tyre of the front-right wheel soon reaches the limits of its lowmu surface. This is marked by the development of significant slip, and by relatively high levels of wheel angular deceleration. Drag asymmetry develops from this point.

At time t = O.083s the dump solenoid of the front-right wheel is energised to effect pressure reduction so as to prevent that wheel from locking.

At the same time, the dump solenoid of the front-left wheel is fired for a short period in order to cause a hydraulic modulator to switch that channel from unrestricted pressure-rise to a controlled mode in which further pressure increase is regulated by a rate at which the flow-valve admits fluid to the brake.

Thus pressure rise continues at the high-mu wheel, but at a slower rate; and the degree of drag asymmetry grows larger.

At this stage, the existence of surface-adhesion asymmetry cannot be reliably discriminated from differences in locking pressure due to manufacturing tolerances or dynamic wheel loading variations.

However, by the time a further 21ms has elapsed (t= 0.104s) it can be observed that there are still no signs of imminent locking at the high-mu wheel. This is the point at which, for a small car, YMC action needs to be started if stability is to be maintained on a real split-mu surface but, at this precise moment the low-mu wheel's dump solenoid is de-energised in response to a sudden reduction in angular deceleration.

Trace (H) shows that this "recovery" is a transient effect typical of the rebound phenomenon discussed above, but the prior art logic has to accept it as genuine, for the time being, because it is unable to distinguish its characteristics from those of a normal skid-control cycle. Thus the de energisation of the low-mu wheel's dump solenoid prevents the start of copy-dumping at the high-mu wheel, and this allows the established drag asymmetry to continue. Even if the copy dumping had been able to start before the rebound occurred, it would have been interrupted part-way through its process, and so similar, but perhaps less severe vehicle/driver responses would be expected.

Once rebound is complete, the wheel continues to decelerate, and the low-mu dump solenoid is energised again at time t = 0.160s. YMC copy-dumping can now begin, but by this stage the yaw velocity and angle (the area under the curve) have both increased considerably. The vehicle is yawing anticlockwise (towards the high-mu surface) and the driver is attempting to counter this with an opposing steering (side-force)-generated moment in order to maintain his original straight-line course.

Since the high-mu wheel is now at an angle to the direction of travel its ability to resist brake torque will have reduced, leading to observable slip.

However, the following brake-torque-reduction created by the YMC action relieves the slip and increases its ability to generate side force (i.e. to respond to steering input). By time t= 0.28s the drag asymmetry and its associated yaw moment are both relatively weak, but the steering-generated moment is still increasing because the driver has not yet realised what is happening. This causes a rapid change in the yaw behaviour, with the yaw velocity reaching zero at 0.43s and then building-up in a clockwise direction to around 5 deg./ s before the driver begins to reduce the steering input. Thus a violent yaw oscillation has begun in which the driver's delayed response can serve to reinforce the instability.

By way of comparison the equivalent traces A' to K' of Fig. 2 show the result of braking under the same road and gear conditions but utilising ABS logic modified in accordance with the present invention.

The traces of Fig. 2 show that the low-mu wheel's dump solenoid is first energised at 0.067s and copy dumping is able to start in the normal manner 2lms later because the low-mu solenoid is still energised at that stage. The rebound phenomenon causes it to be de-energised 7ms later, in the normal conventional manner as per the prior art. In this case, however, the high-mu wheel's solenoid remains energised because of the action of the delta-wheel-acceleration logic.

Thus, in the present case, during the preceding scan period the angular acceleration of the front-right wheel was equivalent to a linear acceleration of -19g.

In the current scan period the corresponding value is -8.5g. The difference (10.5g) is greater than 6g, the direction of change is positive, and the current value is still negative; thus the prescribed conditions required to allow pressure reduction at the high-mu wheel are met without further criteria being needed.

In the next scan period a value of +l.lg is observed. This meets the requirements of magnitude and direction of change, but this time the current value is positive. In this circumstance extra restrictions are set. Because the current value is less than +4g, and because the pressure reduction in the preceding scan period resulted from the deltawheel-acceleration logic, the reduction of pressure at the high-mu wheel is allowed to continue.

Moving on by a further scan period, the requirements for pressure reduction qualified by delta-wheel-acceleration are no longer met. However, at this time, the first of the series of "very-slowpressure-rise" pulses referred to in the initial discussion of low conventional Yaw Moment Compensation works occurs. This is timed to begin a fixed time interval after the first scan period of YMC activity.

However, it should be noted that, for circumstances where prior-art-style copy-dumping has not started (e.g. the traces of Fig. 1 described above) the pulse series remains dormant. Thus the fortuitous timing of this pulse is a by-product of the delta-wheelacceleration logic.

The conditions under which the traces of Fig. 2 were recorded were extreme, with polished ice on the low-mu surface and dry asphalt on the high-mu side.

The slip at the low-mu wheel before the brakes were supplied is due to the effect of the engine's overrun torque and the drag asymmetry thus caused has provoked the driver to apply slight steering correction.

Referring now to Fig. 3, there is shown an example of the logic required to achieve detection of delta-wheel-acceleration in the form of a simplified flow diagram. For interpretation of this flow diagram the following definitions should be noted.

(a) "THIS" side is the side of the vehicle about which the solenoid demand decision is being made.

(b) The "OTHER" side is the side of the vehicle whose acceleration profile is being examined in order to make a solenoid demand decision of "THIS" side.

(c) Each wheel is examined as both "THIS" side and the "OTHER" side in each period. This enables alternating surface adhesion asymmetry to be catered for.

(d) A "delta pulse" is a pressure dump on "THIS" wheel, the requirement for which has been determined by the process described herein (e) A "period" is the process period of the control system determining the operation of the solenoid(s).

The logic steps (boxes) shown in Fig. 3 are as follows: 10 - Delta Wheel Acceleration enhancement of In-Gear GMA Operation.

12 - Was a Delta pulse issued on THIS wheel in the last period? 14 - Is the wheel acceleration on the OTHER wheel currently greater than +Og? 16 - Is the wheel acceleration on the OTHER wheel currently greater than +4g? 18 - Is the change in the OTHER wheel acceleration over the last period greater than +6g? 20 - Is the slip on the OTHER wheel greater than 25kph? 22 - Has THIS side dumped for as long as the OTHER side already? 24 - ALLOW a delta pulse on THIS side this period.

26 - do not allow a delta pulse on THIS side this period 28 - Done.

It is emphasised that the absolute numeric references shown in the flow chart of Fig. 3 are examples extracted from a particular embodiment of the present technique and it is not intended to serve as any limitation on the principles of the present invention.

By virtue of the use of the present invention, interference with GMA function by in-gear transmission coupling effects is reduced significantly and the vehicle operates in a more stable manner.

Claims (10)

1. An anti-skid braking system for wheeled vehicles having fluid actuated brakes associated with the vehicle wheels, comprising speed sensors associated with the vehicle wheels, and a scanning control means responsive to speed signals from the speed sensors to actuate a pressure reduction device to periodically release the fluid pressure applied to the brake of any wheel which is determined, by detection of a predetermined level of relative slip between that wheel and the road surface, to be about to lock, and to later re-apply the actuating pressure to that brake when the tendency of that wheel to lock has reduced, and wherein, when a split-mu condition is detected in which the wheels of the vehicle at opposite ends of an axle are running on a high-mu surface and a low-mu surface, respectively, pressure reduction at the high-mu wheel for effecting yaw-moment compensation is allowed to occur at times when the pressure at the low-mu wheel is not being reduced, provided that the measured rate of change of angular acceleration of the low-mu wheel is determined to be exceeding a first predetermined threshold.

2. An anti-skid braking system as claimed in claim 1, wherein said first predetermined threshold is greater than or equal to + 6g per scan period of said scanning control device.

3. An anti-skid braking system as claimed in claim 1 or 2, wherein pressure reduction at the high-mu wheel is only allowed while the prevailing angular acceleration of the low-mu wheel remains negative.

4. An anti-skid braking system as claimed in claim 1 or 2, wherein pressure reduction at the high-mu wheel is only allowed while the prevailing angular acceleration of the low-mu wheel remains negative or, if positive, not above a second predetermined value of positive acceleration.

5. An anti-skid braking system as claimed in claim 1 or 2, wherein pressure reduction at the high-mu wheel is only allowed while (a) the prevailing angular acceleration of the low-mu wheel remains negative or, if positive, not above a second predetermined value of positive acceleration, and (b) provided that deltawheel-acceleration in the immediately preceding scan period is determined to have exceeded said first predetermined threshold.

6. An anti-skid braking system as claimed in claim 4 or 5, wherein said second predetermined value is approximately + 4g.

7. An anti-skid braking system as claimed in any preceding claim, wherein pressure reduction at the high-mu wheel is only allowed if the measured slip of the low-mu wheel with respect to the estimated vehicle speed exceeds a third predetermined threshold.

8. An anti-skid braking system as claimed in claim 7, wherein said third predetermined threshold is approximately 25Km/h.

9. An anti-skid braking system as claimed in a any preceding claim, wherein pressure reduction at the high-mu wheel is only allowed if the measured aggregate dump-solenoid energisation time of a high-mu front wheel, does not exceed that of a low-mu front wheel, whereby to prevent delta-wheel-acceleration induced pressure-reduction from reversing the sense of drag asymmetry.

10. An anti-skid braking system substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 3 of the accompanying drawing.