GB2307526A - A braking system for a vehicle - Google Patents

Abstract

A braking system for a vehicle having ABS comprises at least one axle where the wheels are subjected to the same brake pressure, and a respective wheel speed sensor 120 at each wheel of that axle. The brake pressure to the axle is subject to a "select-low" control mode, so that the first wheel on that axle to skid causes axle braking pressure reduction. After this brake pressure reduction and detection of split-mu conditions at the axle, the control mode is changed into a "select-high" control mode wherein the buiid-up of axle pressure is continued at a controlled rate above the pressure at which skidding occurred for at least a second time on the first wheel on said axle to skid. The system is also responsive to front and rear axle loads.

Description

DESCRIPTION BRAKING SYSTEMS FOR VEHICLES HAVING ABS The present invention relates to vehicle anti loc (ABS) braking systems in which there is at least one vehicle axle where there is single channel (axle) control of the brakes.

Electronic Braking Systems are conventionally organised into four channels so that each wheel will have a pressure control channel devoted to setting braking pressure individually. Lower cost systems are able to offer the advantages of EBS by providing only two channels of pressure control organised on an axle basis and thereby satisfying the recuirement for a split system and allowing braking pressure distribution to be controlled typically by setting braking pressures in proportion to the axle loads in static or dynamic conditions.

A two channel system is shown in the accompanying Fig. 1 which comprises a single electrically controlled relay valve 110,112 for each axle, respectively which in normal EBS operation set the braking pressures in response to the driver demand which is now electrical and produced by operation ot a brake pedal 114. As shown in Fig. l, some systems provide aie load measurements made from suitable sensors 116,118 associated with each axle anc tns allows braking levels co be made in proportion to axle load values.These are made on at least those selected brake applications where stability may be at risk because of the level of braking demanded or the conditions of the road surface.

However, on slippery roads where the braking demand is higher than the road surface is capable of supporting, the EBS has to provide an ABS anti-lock function in order to control the wheel skidding which will be produced. In a two channel EBS, the ABS function has to be something of a compromise because the individual control of wheel pressures, however desirable, is not possible. Systems are known in which ASS operation is based on axle control and the most common arrangement is known as "select-low whereby the speed of the slowest wheel on an axle is paramount in controlling the axle braking pressures at any instant and therefore the first wheel to skid causes axle pressure all and also the last wheel to recover is selected in the controller to set the brake reapply pressure.Generally this is the safest control strategy since it prevents either wheel from locking or indeed from operating in deep slip, therms treservng vehicle stability and steerabilit.

On split-adhesion surfaces eg where one side o5 e road surface is more slippy than the other, this strategy favours stability at the expense of stopping distance since the axle braking pressure is held down to the low-mu skid level, thereby preventing the axle from reaching the higher braking pressures which the high-mu wheel will stand. Braking regulations require a stopping distance performance on split-adhesion surfaces which a "select-low" strategy cannot provide so that the control algorithm must be made more complex if two channel EBS is to be legal and acceptable.

AlLernative control, wich no increase in complexity, is "select-high', where the wheel on the axle with the highest speed controls the axle pressure. This is good for stopping distance but is not popular because the low-mu wheel is always allowed to lock since the signal therefrom is ignored. In the case of an homogeneous surface, "select-high" will allow the first wheel to skid without taking any action so chat unless brakes and adhesion are well balanced wheel lock will occur as it will when braking in a bend when wheel loading is likely to be diEerent between the left and right hand sides of the vehicle.

It Is an object of the present invention co seek to solve the problem by more íntelligent control using all four wheel speeds and other braking system data, such as pressures, along with other vehicle information, such as axle loadings, which are available in EBS systems but not provided in conventional ABS schemes.

The principle employed is to generate from the wheel speed sensors, corresponding sets of wheel data (speed deceleration, acceleration and slip) by suitable processing in a controller. On homogeneous surfaces, an effective "select-low" regime is employed whereby the lowest wheel speed data on the axle is examined continuously in an algorithm which generates the level of axle pressure appropriate to the maintenance of effective control of the speed of that wheel selected. Thus, the first wheel to skid is instrumental in controlling pressure throughout the cycle and the last wheel to recover effects the ralication of braking pressure to both wheels on the axle.

On split adhesion surface, the tyre to road friction level on one side of the vehicle is considerably lower than on the other side of the vehicle. This condition ideally requires individual control of wheel braking pressures but, even in such a system, independent control of font wheel brakes, purely on the strict basis of controlling wheel speeds during skidding, is never used since the steering pull generated threatens control of the vehicle directional stability.In systems described herein, select-low9t control is employed initially until at least a second skid cycle has commenced when, depending on the output signal of a split-adhesion detector, an internal controller decision may be taken to switch from this Ilselect-low" regime into a "select-high mode in which a slow pressure build up is continued instead of responding to the skid signal from the low-mu wheel.

This action gradually uses more of the adhesion available to the high-mu wheel but in so doing, abandons any further control of the low-mu wheel which loses speed until locking occurs.

It is an important provision that in the case described above, the slow build-up of axle braking pressure is not simply continued until the locking point of the high-mu wheel is reached, but Is restricted with the following aims: (a) Not allowing Wke high-mu wheel to lock or develop high slip; (b) Reducing steering pull and maintaining 3 high level of vehicle stability.

A number of strategies can be associated with the amount of pressure increases to be allowed ovr and above the pressure Psl at which the low-mu wheel skidded for the second time and at which the mode change took place. This pressure build-up is defined as: Pbu = Pc + k.Psl where Pc is a constant pressure increase which is related to vehicle parameters. and k.Psl is a further increase which is proportional to Psl.

The build-up in pressure may be set at a constant limit as Pc is given a fixed value and k is set to zero.

Alternatively, the constant Pc may be zero and the build-up pressure related solely to Psl, where k has a value which is related to selected vehicle parameters.

The values of Pbu are likely to be different on each axle as are the rates of pressure rise from Psl to Psl + Pbu since these are as fast as possible, consistent with limiting the disturbance on the vehicle directional stability. Front axle build-up rate and build-up limit are particularly important as the steering correction needed to maintain a straight vehicle line has to be limited to be acceptable.

In the case of unladen vehicles where the load on the rear axle is very light, adequate stability is maintained only by holding the pressure at Psl for the rear axle and thereby not taking any advantage cf the higher adhesion which exists under the hlgh-mu wheeis.

In these circumstances, the preferred action is to continue with the "select-low" control mode and maintain both wheels in low slip even though the split-adhesion conditions are clearly detected on both axles Above a preset level of rear axle load, stability is not so precarious and the alternative "select-high" strategy may be pursued, but the level of this threshold is not constant over a normal range of commercial vehicles It is largely dependent on the wheelbase o, the vehicle since long wheelbase vehicles arc- not so sensitive to stability disturbances as for example, are short wheelbase tractors. Also, load distribution is a factor which has a further bearing on the stability problem which is being addressed in this decision.Thus, the level of Pbu which is targeted is made dependent on vehicle wheelbase in order to take as much advantage of the high-mu wheel adhesion as stability considerations will allow Ar. alternative pressure build-up strategy which makes use cf stored =feedback signals is attractive in some vehicles and makes use of measurements which were made at the point at which Psl was stored. That point occurred when the second low-mu wheel skid was detected, provided that the slit-nu detector was already indicating the split condition and a reading was also made of the corresponding slip at the high-mu wheel on the axle.The data pair of slip and Psl are readily combined so as no indicate the level of pressure increase which can be taken to increase the slip of the hgh-mu wheel up to a preset threshold on the assumed but reasonable basis of slip being proportional to braking pressure on the leading edge of the mu-slip curve. Thus the braking pressure on the axle will build up to that valuc which will be predicted to produce typically S-10X slip at the highmu wheel.

eg. Suppose at Psl = ibar, and the high-mu slip was 2k, then: Target pressure for 8s slip would be 4bar.

Thus, in accordance with a first aspect of the present invention, there is provided an EBS system having at least 2-channels and which is provided for ABS purposes with four wheel speed sensors which generate signals which are processed in an electronic controller to produce four wheel-sreed control channels, the data in two of which are selected for operation on the basis of the lowest wheel speed on each axle, so as to combine the control data in a select-low control mode, wherein the first wheel to skid causes axle braking pressure reduction and, after this brake release cycle has been completed, the control mode is changed, alter detection of splitadhesion conditions, into a "select-high control mode, and wherein the buld-up of axle pressure is continued at a controlled rate above the pressure (Psl) at which skidding occurred for at least a second time on the first wheel on said axle to skid.

Preferably, in the ABS function of this F.BS system, the pressure build-up (Pbu) at the controlled rate is of limited amplitude above the stored pressure level (Psl), this limit being set by the relationship Pbu = Pe + KPsl wherein Pc is a pressure diference k is pressure increase factor and either value can be zero being designed so as not to exceed the high-mu skid pressure.

Advantageously, the switch-ovr to modified "select high" mode is only allowed after at least one complete "select low" pressure control cycle has tn place, said cycle including the pressure fast rise phase on detection of low-mu wheel recovery and the subsequent slow pressure build up phase in which said low-mu wheel commenced a second skid cycle at which point the axle pressure value was stored for subsequent reference as the low wheel skid pressure, Psl after which this low wheel is allowed to lock as the slow brake pressure build up is maintained.

The value of Pbu can be set individually for each axle on a vehicle and may be different on front and rear axles.

In the unladen condition, the pressure build up is preferably zero and the ABS is arranged to remain in the "select low" condition even though splitadhesion conditions are clearly detected.

Advantageously, detection of split-mu conditions and change over to modified "select high" on one axle causes a similar change over from "select low" on the other axle.

Preferably, Pbu is related to the axle load by making Pc proportional to axle load minus a preset threshold.

In other embodiments, Pbu can be related to axle load but also related to the vehicle wheelbase in that Pbu will be larger in the case of a long wheelbase.

Advancageously, the pressure increase is set to produce a target slip level at the high-mu wheels as predicted from a measurement the slip at that wheel which occurred at the point of determination of Psl as derived above, by allowing for slip at this whecl being proportional to brake pressure.

In some embodiments, the split adhesion surface can be detected from di=erences in slip across t axle, where slip values are calculated from separate left and right wheel speed references which move apart during cornering, and said differences, when sensed over typically 0.5 secs are greater than a preset threshold (typically 4km/h) and where at the same time both the higher wheel speed must be in low slip and the corresponding low-mu wheel on the second axle must be in at least 2km/h slip Advantageously, in the "select-low" to "selecthigh" switching arrangement described hereinbefore, the A3S control, if in the "select-high" mode, will revert back to "select-low" if the previously locked low-mu wheel, re-accelerates.

Alternatively, or in addition, if the high-mu wheel skids yet the low-mu wheel does not recover and if the skid pressure on commencement of the second nigh-mu wheel skid cycle minus the stored Psl reading is not greater than a preset low value, the skid control pressure release period is extended so that the low-mu wheel has an imDroved chance of recovery and if such recovery is detected then the switch back to the "select-low" mode is effected.

In accordance with a second aspect of the present invention, there is provided an EBS system having three channels, in which the front razzle pressure is controlled in a select-low" mode wherein the first wheel to skid causes axle pressure reduction and after a first skid control brake release the control mode is changed on detection of split-adhesion conditions into a modified "select-high" control mode, wherein axle pressure is built up at a preset controlled rate above the pressure Psl at which skidding previously occurred on a second cycle at the first wheel on said front axle to skid, by an amount which is set to be limited at a fixed level below the predicted high-mu wheel skid pressure calculated from (Pslf/Pslr)Pshr, where Pslr is the low-mu wheel second cycle skid pressure on the rear axle and Pshr is the current level of detected skid pressure at the high-mu wheel on the rear axle.

The invention is described further hereinaftex, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 is a diagrammatic illustration of a typical two-channel :9S having axle load sensing on each axle but where, in an optional alternative, axle load can be calculated rather than measured.

Fig. 2 shows the response of a typical "selectlow" ABS control system in which the RS wheel s running on a much higher adhesion (mu) surface than the LHS wheel; Fig; 3 shows the response of a typical simple "select-high" ABS control system where braking pressure is controlled by the RMS wheel on the high-mu surface; Figs. 4 and 5 illustrate the operation of a system in accordance with the present invention on spli-mu surfaces in laden and unladen conditions, respectixel-i and Fig. 6 Is a block diagram of systems in accordance with the present invention, showing thc ASS provisions in detail but the EBS provisions oly in outline.

In Fig. 1, the two axle-based relay valves 110,112 are electrically controlled and along with four wheel speed sensors 120 are mandatory to the system of ABS being described. The relay valves set the axle braking pressures in proportion to the driver's demand as generated by the electrical signal from the pedal 114. This process will be interrupted whenever the wheel speeds, derived by well knows processing of the wheel speed sensor outputs. are interpreted as the commencement of a wheel skid condition and the present invention is concerned, inter alia, with how the pressure output signals ae modified to produce good stability, along with acceptable stopping distances1 on split-mu surface, with such a two channel system.

Fig. 2 shows the response of a select-low ABS control system in which the RS wheel is running on a much higher adhesion surface than the LHS wheel. The skid cycling on the LS wheel is effective in controlling the braking pressure so that the RHS wheel runs in low slip, maintaining excellent stability but not gencrating braking forces that its surface would stand. The stopping distance is poor and not, under European regulations, acceptable.

Fig. 3 shows the response of a simple select-high ABS control system where braking pressure is controlled by the RHS wheel on the high-mu surface.

The skid condition developed at the low-mu wheel is içlored and the wheel is allowed to lock. A prudent select high system will, with EBS, interrupt the fast rise of braking pressure and, on detection ot the lowmu wheel skid, restrict to a more gradual build up to prevent serious directional disturbance. The brake pressure in Fig. 3 is allowed to rise until the HS wheel skids and then this wheel controls the axle braking pressure in a skid cycling mode. This mode generally has some problem achieving good stability with one wheel locked and the high-mu wheel in a skid cling condition even though stopping distance could be expected to be good.

Referring now to Figs. to 6, which illustrate the structure and operation of an EBS system in accordance with the present invention, the wbeel speed sensors provide speed pulses on four channels (see left-hand side of Fig. 6) into a wheel speed converter 1 which produces four speed signals associated respectively with the wheels on each end of front and rear axles of the vehicles. all signals are connected to a vehicle speed reference processor 2 which selectively uses these inputs at all times that the vehicle is in operation, to generate a best estimate of the vehicle speed in ways which are well known to workers in this field. The vehicle reference (steed) signal is used in all speed sensitive sections of the EBS but in particular in slip detectors 3 and o which derive slip level at the wheels during braking.

The wheel speeds on each axle are compared in high/low sensors 5 and 6 which, in addition to speed inputs, have a further command input normally set to "selct-low" but capable of being switched into "select-high" b the output from an appropriate select-high controller 13 or 14. These selectors 13,14 control associated switches, SF for block 5 and SR for block 6, which connect the appropriate wheel speeds to skid detectors 9 and 10 and recovery detectors 11 and 12 so that skid control decisions can be taken using the appropriate wheel speeds on each axle.The selection decisions are made individually in control 13 for the front axle and control 14 for the rear axle but the cross coupling of these control output signals allows switching on one axle to influence the decision on the other axle. It is usual for the front axle split-mu detector to signal first and this detection is morc robust, particularly since a rear axle slip signal is fed from detector 4 into split-mu detector 7.

A typical detection process is to sense that .h2 0.5 sec average slip figures show a difference between the wheels on the front axle to be more than 3km/h whilst the slip on the high-mu wheel is less than 1km/h and slip on the same side rear axle wheel is less than 9Rmjh.

Generally, the split-mu condition will be sensed on the first cycle, at which point the control mode is "select-low" and switching to "sel.ecr:-high" is deferred until the second skid cycle is detected.

This is done at skid cycle counters 19 and 20 which roide master-enable signals to select-high controllers 13 and 14 and the rear selection decision can be made if either the rear or the front split-mu detector is signalling split conditions. This is achieved by feeding the rear decision block 14 to OF gate 41 which has inputs from both split-mu detectors 7 and 8. This rear circuit has an additional master input to enable "select-high" switching and this is fed from axle load threshold sensor 42 which signals the enable only when there is at least a given level of rear axle load. In this way, "select-high" switching is prevented at this axle if the vehicle is lightly loaded. This is especially valuable in the case of a short wheelbase tractor with no trailer coupled.Front and rear axles may make the "selecthigh" decision at different points in time depending on the detection of the second skid cycles at the lowmu wheels. At the switching point on each axle, a pulse is generated which causes two memories to store as follows: a) Psl memory 33 or 34 for the axle which stores the low-mu wheel skid pressure for later reference; and b) A high-mu wheel slip memory 31 or 32 which receives the slip level at Psl so this is available for the target pressure calculation in order to set a limit which should produce a preset level of operating slip.

The normal operation of the EBS is to respond to the electrical brake demand signals from the pedal sensors 15 and 16 and multiply these by the axle load signals in multipliers 21 and 22. Many installations may not have a front load sensor in the interests of economy, in which case the axlc- load is inferred with reasonable accuracy from the stored unladen front axle weight plus a percentage of the rear axle load, excluding the unladen component. This arrangement gives load conscious braking to each axle to give the braking distribution where skidding is last likely.

The apportioned demands are scaled to give pressure commands to each pressure control channel 35,37 and 39 or 3,38 and t0 and fed to the inputs via miter stages 27 and 28 which have dynamic inputs which define the reduced pressure override levels which will be applied so as to create the skid control pressure cycles as dictated by the skid cycle controllers i7 and 18. These blocks regulate axle pressure to dump pressure when a skid occurs and to carefully rappy pressures as the controlled wheel recovers. In the split-mu case, the pressure is not allowed to be reapplied fully to the levels which may be set by the scaling multipliers elements 23 and 24 if the braking demand is high but the blocks 29 and 30 set the limit above Psl unless the split adhesion condition is revoked.Blocks 25 and 26 set the limit on pressure demand to the lower of the signals from blocks 17 or 29 and 18 or 30 and under normal r.on-skidding conditions the output from these blocks is maintained at a level higher than the maximum demand from the scaling multipliers in the fully laden condition, so as to represent no limiting action on the normal signals.

In the "select-high" mode, elements 29/30 which set the pressure limits above the appropriate Psl are programmed or configured to calculate the target pressures according to a strategy selected from a number of alternatives based on Pbu = Pc + K. Psl.

These factors are typically related to the vehicle specification such that lighter loads or short wheelbase give smaller values of Pbu.

Whilst this system is described in terms of a hardware block diagram for ease of understanding, it should be recognised that a practical implementation would be engineered in software residing in an ECU microcontroller. In such a controller, te factors Pc and k are programmed so as to suit the vehicle parameters, at the post-build phase and in these circumstances wither Pc or k may be set to zero.

The aim of Pbu selection is to maintain the axte pressure below the skid pressure of the high-mu wheel on each axle by suitable limiting of pressure abo-;e Psl.

The operation of this arrangement is shown in Figs. 4 and 5 which cover the laden and unladen conditions of a typical vehicle running on split adhesion surfaces such that the left hand wheels are on low-mu and the right hand wheels are on much higher mu. In Fig. , front and rear axle pressures build up to the low mu skid levels and result in pressure dumps to correct the falls of wheel speed in the skids. The split-mu detectors would respond during these first skid cycles, which clearly show substantial slips at the left wheels whilst the right wheels are running in order low slip.Thus at the point of detection of the second skid cycles, the pressure Psl are stored for each axle as the change over to the "select-hih" mode is made and a "select-high" pulse is produced for each channel. The rise in pressures Pbu at each axle is shown after build up to be limited to well below the high-mu skid levels of the right hand side wheels.

The front Pbu is seen to be considerably lower than the rear Pbu and the rate of pressure rise is also much slower on the front axle to keep steering pull at an acceptable level.

The laden case operates with a clear change-over to "selec,-higW but if the vehicle is unladen, stability considerations prevent such a change-over at the rear axle. This situation is shown in Fig. 5 where te front axle does change over in the sam wav as in Fig. 4 afr a first cycle in "select-low', during which the split-mu condition is detected as previously described for both front and rear axles.

However, the select-high controller 14 does not generate a "select-high" command in the unladen condition since the load comparator 42 does not produce the necessary enable signal and the rear channel remains in "select-low". Thus, skid control of the low-mu wheel is maintained throughout the stop as can be seen in Fig. 5. As the rear load is increased at some point, the preset threshold is exceeded and "select-high" switching will take place but the level of Pbu allowed at the rear is severely restricted so as to maintain directional stability when the low-mu wheel. has been allowed to lock.

In this system which suhsequently enters a special control mode upon detection of split-mu conditions, means have to be provided for reverting from this mode should the disappearance of split-mu conditions make the "select-high" no longer appropriate.

A number of possible adhesion level changes have to be considered:- 1) Low-mu Increasing to high-mu is a clear possibility which could cause the low-mu wheel to recover from the locked condition and generate an acceleration signal which would be sensed In clock 3 or 44 for front and rear axles respectively. The limit placed on the braking pressure applied to the high-mu wheel has a clear advantage in allowing the low-mu wheel to recover should there be any significant improvement in adhesion on that side of the vehicle.

The low-mu wheel acceleration is detected in elements 43 or 4t which produce an output signal provided that the channel is operating in the "selecthigh" mode, to cancel the split-mu detection and restore the channel into the normal "select-low" state. The slip detectors 3 and 4, at the end of the lower; wheel recovery phase, will produce slip signals which could, if appropriate differences exist later in the stop, set up the split-mu detector again.

2) The high-mu level falling toward the low-mu surface causes the high-mu wheel to skid as the adhesion falls to the point at which the braking forces can no longer be reacted at the road. The skid detectors produce signals which in the skid cycle control effect sudden pressure command reductions leading to a dump of axle braking pressures so as to allow speed recoveries at the high-mu wheels. This may be accompanied by some similar recoveries at the low-mu wheels but this will depcnd on the pressure drops taken and the adhesion under the low-mu wheels.

If a low-mu wheel spins up then the acceleration detected will cause the "select-high1, mode to be revoked as described above. However, if the low-mu wheel does not spin up after the pressure dump phase has been completed, pressure will be built up again as the high-mu wheel recovers and this controller pressure increase will be maintained until this wheel side again. At this point the skid pressure Psh is sampled in the skid cycle control elements 17 and 18 and compared with the stored values of Psl the low-mu wheel skid pressure.If this difference is small, typically O.Sbar, the skid cycle control will generate a longer dump signal ir. response to this new skid condition and the low-mu wheel will effectively be given a second and more favourable chance to recover.

Low-mu wheel spin up, if it occurs, then causes the split-mu and "select-hig" states to be revoked. If the low-mu wheel does not recover, the "se'ect-high" control mode wil be maintained but the build-up pressure limit Psl + Pbu set by elements 29 or 30 is reduced to lie below Psh by 0.25bar at least so that high-mu wheel skidding is Inhibited.

3) If both surfaces approach a mid-mu condition such that the high-mu wheel will skid, the stragy outlined above will be followed and the "select-high" mode will be revoked if the low-mu wheel rCcov^- r3 or else the pressure limit will be reducec.

4) If the high and low adhesion split changes sides, it is almost certain that both wheels will be affected in that the "high-mu" wheel will skid and the "low-mu" wheel will spin up, giving dual conditions both of which will cause cancellation of the "selectlow regime until the new slip signals are recognised and select-high" mode and reinstatement of the normal "select-high" is reinvoked in the manner described originally. The period of "select-low" operation Dust after the transition, i5 vital in generating the most stable conditions at a point of maximum disturbance which will improve the chance of maintaining good vehicle stability.

The principles embodied herein, whilst primarily applicable to two channel EBS schemes, show the control strategy of "select-low" switching to "selecthigh" which provide, with only a small amoune of modification, an advantageous means or controlling the axle ABS operation in a three channel EES which has individual pressure setting channels for each of the wheels on the other axle. Normally, the front axle is controlled as a single channel and the rear al has wheels with individual pressure control.

Tn such a system, the pressure limit imposed once the "select-high" mode has been entered, is set by reference to the skid pressures in force at the individually controlled rear axle wheels. This Is achieved by measuring the front low-mu wheel skid pressure PsIf as sampled on the second skid cycle when the rate of pressure rise was slow and controlled as described above. The rear low-mu wheel circuit is also sampled to read the skid pressure Psl on the second skid cycle when the pressure rise rate on that wheel is also slow.The ratio Pslf/Pslr is formed to indicate the front/rear skid pressure relationship at the current vehicle loading condition. because the rear axle ABS is individual wheel control, the high-mu wheel either receives the full braking pressure or skids at some lower pressure. In. either case, the pressure in this wheel brake is stored and, if skidding is taking place, the pressure Pshr at the skid detection point is stored and is updated at each new skid cycle. This pressure is the pointer to allow estimation of the front high-mu wheel skid pressure as calculated from: Pshfe = (Pslf/Pslr)Pshr.

The front axle pressure is allowed to build up slowly but must not reach this skid pressure. The pressure therefore is limited to a value of Pshfe Pm, where Pm is a preset margin so that there will be a cotrolled shortfall below the estimated sfe.

Claims (1)

1. An ABS system having at least one axle where there is single channel, axle control and which is provided with respective wheel speed sensors at each vehicle wheel of that axles to generate wheel speed signals which are processed in an electronic controller to producc corresponding wheel sped control channels, the data in which are selected for operation on the basis of the lowest wheel speed on that axle, such as to combine the control data in a "select-low" control mode wherein te first wheel of that axle to skid causes axle braking pressure reduction ard, alter Lhis brake release cycle has been completed, the control mode is changed after detraction of split-adhesion conditions into a "select-high" concrol mode wherein the build-up of axle pressure is continued at a controlled rate above the pressure at which skidding occurred for at least a second time on the first wheel on said axle to skid.

2. An EBS system having at least 2-channels and 'n which is provided for ABS purposes with four wheel speed sensors which generate signals which are processed in an electronic controller to produce four wheel-specd control channels, the data in two o,~ whIch are selected for operation on the basis of the lowest wheel speed on each axle, so as to conbine the control data in a "select-low" control mode, wherein the fIrst wheel to skid causes axle braking pressure reduction and, after this brake release cycle has been completed, the control mode is changed, after detection of split-adhesion conditions, into a "select-high" control mode, and wherein the build-up of ayle pressure is continued at a controlled rate above the pressure (Psl) at which skidding occurred for at least a second time on the first wheel on said axle to skid.

3. An ABS system as claimed in claim 1 or 2, wherein the pressure build up (Pbu) at the controlled rate is of limited amplitude above the stored pressure level Psl, this limit being set by the relationshit:- Pbu = Pc + Ksl where Pc is a pressure difference k is pressure increase factor and either value can be zero being designed so as not to exceed the high-mu skid pressure.

4. An ABS system as claimed in claim l, 2 or wherein the swItch-over to modified "select high" mode is only allowed after at least one complete "selct- low" pressure control cycle has taken place, said cycle including a pressure fast-rise phase on detection of low-mu wheel recovery and a subsequent slow pressure build up phase In which said low-mu wheel commenced a second skid cycle at which point the axle pressure value was stored for subsequent reference as the low wheel skid pressure, Psl after which this low wheel is allowed to lock as the slow brake pressure build up is maintained.

5. An ABS system as claimed in claim 2 or claim 3, when appendant to claim 2, in which the value of Pbu is set individually for each axle on the vehicle.

5. An ABS system as claimed in any of claims 1 to 5, in which for rear axles in the unladen condition the pressure build up is zero and te ABS remains in the "select low" condition even though split-adhesion conditions are clearly detected.

7. An ABS system as claimed in any of claims 1 to 5, wherein detection of split-mu conditions and change over to modified "select high" on one axle causes a similar change over from "select low" on the other axle.

B. An ALS system as claimed in claim 3, in which Pbu is related to the axle load by making Pc proportional to axle load minus a preset threshold.

9. An ABS system as claimed in claim 3 or claim 8, In which Pbu is related to axle load but also related to the vehicle wheelbase in that Pbu will be larger in the case cf a long wheelbase.

10. An ABS system as claimed in claim 1 or 2, in which the pressure increase is set to produce a target slip level at the high-mu wheel as predicted from a measurement the slip at that wheel which occurred at the point of determination of Psl by allowing for slip at this wheel being proportional to brake pressure.

11. An ABS system as claimed in claim 1, wherein the split adhesion surface is detected from differences in slip across the axle where slip values are calculated from separate left and right wheel speed references which move apart during cornering and said differences, when sensed over a preset period are greater than a preset threshold and where at the same time both the higher wheel speed must be in low slip and the corresponding low-mu wheel on the second axle must be in at least 2km/h slip.

12. An ABS system as claimed in any of claims 1 to 11, wherein the ABS control if in the "select high" mode will revert back to "select-low" if the previously locked low-mu wheel re-accelerates.

13. An ABS system as claimed in any of claims 1 to 12, in which if the high-mu wheel skids yet the low-mu wheel does not recover and if the skid pressure on commencemen. of the second high-mu wheel skid cycle minus the stored Psl reading is not greater than a preset low value, the skid control pressure release period is extended so that the low-mu wheel has an improved chance of recovery and if such recovery is detected then the switch back to the "select-low" mode is effected.

14. A three channel EBS in which the front axle pressure is controlled in a "select-low" mode wherein the first wheel to skid causes axle pressure reduction and after a first skid control brake release the control mode is changed on detection of splitadhesion conditions into a modified "select-high" control mode characterised by the action of building up axle pressure at a preset controlled rate above the pressure Psl at which skidding previously occurred on a second cycle at the first wheel on said front axle to skid, by an amount which is set to be limited at a fixed level below the predicted high-mu wheel skid pressure calculated from (Pslf/Pslr)Pshr where Psl is the low-mu wheel second cycle skid pressure on the rear axlc and Pshr is the current level of detected skid pressure at the high-mu wheel an the rear axle.

15. An ABS system substantially as hereinbefore described with reference to and as illustrated in the acompanyig drawings.