Electro-hydraulic braking systems for motor vehicles are known which comprise a brake pedal, a braking device connected to at least one vehicle wheel which is capable of being brought into communication with an electronically controlled valve arrangement in order to apply hydraulic fluid under pressure to the braking device, a hydraulic pump, and a hydraulic pressure accumulator fed by said pump for the provision of hydraulic fluid under pressure which can be passed to the braking device via the electronically controlled valve arrangement in order to apply hydraulic fluid under pressure to the braking device in so called"brake by wire"mode in proportion to the driver's braking demand as sensed at the brake pedal.

In view of the fact that, with an electrically-actuated braking system, the driver's wishes are acquired by sensors at the brake pedal, and conducted to the electronic control system by means of electrical signals, such systems are described as electro-hydraulic braking (EHB) systems.

In the case of an electro-hydraulic braking system of this form, the braking energy required for braking the vehicle is provided in normal use by the electrically-actuated braking system ("brake-by-wire"mode). In order, however, to brake the vehicle in the event of an unexpected failure of the electrically-actuated braking system, the braking system also features an ancillary hydraulic braking system for the immediate actuation of the brakes in which a direct connection can be established between the brake pedal and the brakes by means of switchover valves and hydraulic lines, (this is referred to as the"push-through"mode).

Thus, to provide a redundant hydraulic emergency system, a direct connection can be established in the"push-through"mode between the brake pedal and the brakes by means of switch over valves and hydraulic lines. This conventional system requires a switching device by which, under normal operating conditions, the brake pressure which is produced in the electrical system, and, in the event of a defect in or the failure of the electrical system, the brake pressure produced in the hydraulic ancillary system, is transferred to the brakes.

Of course, the braking system is automatically in the push-through mode if the vehicle is held on the brakes before the system is activated or if the vehicle is free-wheeling under braking before the system is activated. In these and similar circumstances, a problem with known systems is that as soon as the vehicle engine is started and initialisation of the electronic braking system begins, the system instantly changes over from"push-through"braking to"brake by wire"braking, and adopts the braking demand level corresponding to the prevailing push-through brake pedal travel/effort. However, this is much greater than the driver's actual demand under push-through braking due to the effects of rear-axle braking and the electronic boost ratio and the result is an uncomfortable jolt within the system and vehicle due to the sudden increase in braking. The driver has then to compensate for the braking level which is much higher than expected.

In accordance with the present invention, a push-through condition is recognised during the initialisation stage of the EHB and the initial EHB demand is set at the prevailing push-through braking or deceleration level and then adapted smoothly to the desired EHB demand.

In some embodiments, after the initialisation stage of the EHB mode, the initial EHB demand is held for a predefined time interval at the prevailing push- through braking or deceleration level, at least for the front brakes.

In some embodiments, it is preferred that the adaption is arranged not before the brake pedal is first released after the initialisation stage of the EHB mode, at least for the front brakes.

Preferably, the adaption follows a substantially straight line characteristic from the initial demand set at the prevailing push-through braking or deceleration level to the desired EHB demand, at least for the front brakes.

Preferably, initial EHB brake pressure for the rear brakes is set at zero and gradually adapted, either by a curved or straight line, to match the EHB brake pressure for the front brakes.

Preferably, the existence of the push-through condition during EHB initialisation is recognised by any one or combination of the conditions that: (a) both front brake pressures are equal to the master cylinder pressure and are greater than zero, (b) pedal travel is greater than zero and within a range of travel expected for push-through at that pressure, and (c) both rear brake pressures are zero.

Advantageously, the initial EHB demand for the front axle brakes is set at the prevailing master cylinder pressure level, the initial EHB demand for the rear axle brakes is set at zero, and the final EHB demand level for both the front and rear axle brakes being set at a predetermined function of prevailing master cylinder pressure.

Preferably, the maximum rear axle demand rise rate is set to be equal to the ratio of the final EHB demand level and a desired adaption period.

The invention is now described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic diagram of a vehicle braking system to which the present invention can be applied; Fig. 2 is a flow diagram showing the decision process in one embodiment of a system in accordance with the present invention; Figs. 3a and b comprise brake pressure versus time curves illustrating system initialisation for the prior art systems and for an embodiment of a system operating in accordance with the present invention respectively; Fig. 4 comprises brake pressure versus time curves illustrating system initialisation for another embodiment of a system operating in accordance with the present invention; Figs. 5 and 6 are brake pressure versus time curves illustrating system initialisation for further embodiments in accordance with this invention; and Fig. 7 is a flow diagram showing the decision process in embodiments corresponding to Figs. 5 and 6.

The braking system shown in Fig. 1 includes, for the purpose of emergency - actuation and for use when the electronic braking system is not operational for any reason a"push-through"brake circuit 100, which is fed from a brake cylinder 102, actuated by means of the brake pedal 101. The brake pedal 101 has an associated sensor 101a for the acquisition of the driver's braking demand. The driver's demand is transferred to an electronic control unit (ECU), evaluated there, and used as the source for the generation of electrical control signals for actuating valves, described further hereinafter, and a hydraulic pump 110. Switch-over valves 104a, 104b are arranged between the"push-through"brake circuit 100 and the wheel brakes of a vehicle axle 103a, 103b, in order to apply brake fluid to the wheel brakes 103a, 103b, either via the"push-through"brake circuit 100, or via electrically-actuated brake channels 105a, 105b (brake-by-wire).

The switch over valves 104a, 104b in the electrically non-actuated state, i. e. their preferred position, connect the"push-through"brake circuit 100 with the wheel brakes 103a, 103b, in which situation the connection to the electrically actuated brake channels 105a, 105b is blocked. In the event of electrical actuation, the switch-over valves 104a, 104b connect the wheel brakes 103a, 103b, with the electrically-actuated brake channels 105a, 105b, allocated to them, in which context, the connections to the"push-through"brake circuit 100 are blocked. In order to increase safety, for example in the event of a defective valve reset spring 107a, 107b, the switch-over valves 104a, 104b are each capable of being moved into the preferred position corresponding to the"push-through"actuation, by means of pressure control lines 106a, 106b.

In addition to this, elements referred to as de-coupling or separation cylinders 108a, 108b, are connected in the electrically actuated brake channels 105a, 105b, upstream of the switch-over valves 104a, 104b. By means of the cylinders 108a, 108b, hydraulic separation between the"push-through"brake circuit 100 and the electrically-actuated brake channels 105a, 105b, is ensured.

Brake pressure modulation in the electrically actuated brake channels 105a, 105b, and in the electrically-actuated brake channels 105c, 105d, which are allocated to the wheel brakes of the other vehicle axle 103c, 103d, is effected in a known manner by means of control valves 109a, 109b, 109c, 109d, the brake pressure being provided by a pump 110 operated by an electric motor M, and from a pressure accumulator 111.

The system as described thus far is conventional and operates in accordance with well-known techniques.

As explained in the introduction hereto, a problem with the conventional operation of systems of the abovedescribed type is that, if the brake pedal is already being applied when the engine is started and the electronic braking system is powering up and initialising, a jolt occurs in the system because the pedal travel, and master-cylinder pressure ie. the parameters which govern demand in the"brake by wire"mode, are both higher for a given"push through"deceleration, than those needed to give the same deceleration in the"brake-by-wire"mode.

As illustrated in Fig. 3a, upon initialisation at time tl, the known prior art software ramps up the demand pressure substantially instantaneously (dotted line) from zero to a level corresponding to the prevailing pedal travel and/or master cylinder pressure, and controls the pressure at both axles (solid lines) to this level.

This results in a rapid increase in deceleration to a level approximately (in a typical case) six times greater than that achieved with the same pedal effort in the"push- through"mode. As indicated by the dotted undulating curves, achieving an intermediate deceleration by modulation of the pedal effort by the driver is likely to involve a process of over and under-shoot.

One simple solution (not shown) would be to limit the rate at which demand could build-up following new initialisation. This would allow the driver to compensate by reducing the pedal input as the demand (and thus brake pressure) ramped up. However, there would be a delay until the ramp reached the level of braking already established via push-through and, if the demand was still based upon travel, the amount of compensation needed would be considerable.

This solution is therefore still with substantial disadvantages.

In the first system embodiment in accordance with the present invention whose operation is illustrated in Fig. 3b, the demand ramp is arranged to be started, not at zero as in the known systems, but rather from the current level of (push-through) braking. This allows a more gradual ramp to be used, rnaking the driver's task easier without having to extend the adaption period, which would extend stopping distance. An important factor in the application of this technique is to identify the push-through condition during system initialisation (for example that both front brakes = master cylinder pressure = greater than zero; pedal travel = expected pedal travel for push-through at that pressure; both rear brake pressures = 0) so that the start of the ramp is at the current (ie. prevailing) level of braking, and not that which would normally correspond to the observed travel/master cylinder pressure.

The upper end of the ramp (time t2), ie. fully developed demand, should preferably be based upon master cylinder pressure alone, without any travel dependency, such that only effort needs to be modulated by the driver.

In Fig. 3b, the rear brake pressure is not raised immediately to the same level as the front brake but follows a curve gradually increasing the rear brake pressure so that it becomes the same on the front brake pressure at approximately time tb.

In a second embodiment whose operation is illustrated in Fig. 4, the rear brake pressure is raised from zero along a straight line characteristic so as to reach the level of the front brake pressure only at time t2.

In other systems of a type in which the pedal controls vehicle deceleration, rather than brake pressure, the deceleration demand is ramped up, with the push- through deceleration forming the initial ramp value.

If a jolt is to be avoided altogether in systems which control only brake pressure, the ramp can be tailored (in a manner not shown) in order to compensate automatically for the additional brake force of the rear axle, which would have been unbraked in"push-through"at the start of the ramp.

In all cases, the demand needs to remain master-cylinder-pressure based for the remainder of the brake application, irrespective of the demand level.

Reference is now made to the sequence flow diagram of Fig. 2 which illustrates one possible sequence operation of a system embodying the present invention.

The flow diagram of Fig. 2 involves the following sequence steps: 10-Start 11-Is the system in initialisation phase? 12-Are both front brake pressures > zero and = master cylinder pressure? 13-Is pedal travel > zero and within range expected for push-through at this pressure? 14-Are both rear brake pressures = zero? 15-Set initial EHB demand: Front axle = present master cylinder pressure, Rear axle = zero 16-Set final EHB demand: Front axle = rear axle = f (present master cylinder) pressure 17-Set maximum front-axle demand rise rate = (final value-initial value)/adaption period 18-Set maximum rear-axle demand rise rate = final value/adaption period 19-End.

The smooth transition which can be achieved by systems in accordance with the present invention is more comfortable for the vehicle driver and passengers and allows the driver to modulate pedal input during pressure rise, so as to avoid overbraking.

In a third embodiment whose operation is illustrated in Fig. 5, the front axle brakes are arranged to remain in the push-through mode until the brake pedal is first released, only the rear brake being braked with EHB during the start-up phase. This arrangement has an advantage in situations such as that in which fluid displaced into the brakes during push-through could become trapped when EHB is subsequently initialised.

In Fig. 5, the rear brakes are operated at zero pressure during the push- through stage and are raised to the desired level during the EHB stage according to a straight-line characteristic. In Fig. 6, the rear brakes are again operated at zero pressure during the push-through stage but during the EHB stage first rise according to a curved characteristic up to the push-through level of the front brakes, but then rise thereafter according to a straight line characteristic.

Reference is now made to the sequence flow diagram of Fig. 7 which illustrates a possible sequence operation of a system corresponding to Fig. 5 or Fig. 6.

The flow diagram of Fig. 7 involves the following sequence steps: 20-Start 21-Is the system in initialisation phase? 22-Are both front brake pressures > zero and = master cylinder pressure: 23-Is pedal travel > zero and within range expected for push-through at this pressure? 24-Are both rear brake pressures = zero? 25-Set initial EHB demand: Front axle = Zero. Rear axle = Zero 26-Set final EHB demand: Front axle = Zero. Rear axle = f (present m/cylinder) pressure.

27-Set maximum rear axle demand rise rate = (final value-initial value)/adaption period.

28-End.