GB2196076A

GB2196076A - Anti-skid-controlled brake system

Info

Publication number: GB2196076A
Application number: GB08721848A
Authority: GB
Inventors: Helmut Fennel; Johannes Graeber; Ivica Batistic; Otto Determann
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co oHG
Priority date: 1986-09-26
Filing date: 1987-09-17
Publication date: 1988-04-20
Anticipated expiration: 2007-09-17
Also published as: GB2196076B; GB8721848D0; JPS6393656A; DE3632836C2; DE3632836A1; FR2604406A1; FR2604406B1; JP2720898B2

Abstract

A circuit configuration for an anti-skid-controlled automotive vehicle's brake system which is controllable in accordance with a time-division multiplexing method has a resetting-device (5)-equipped master cylinder (3) for a braking pressure generator (2), the wheel brakes (11-14) being connected to the master cylinder (3) via electromagnetically-operable multi- directional valves (15-18). The arrangement is such as to ensure that, during anti-skid control. the sequence of pressure control in the wheel brakes (11-14) and/or the intervals in which the wheel brakes are connected to the master cylinder (3) are dependent on the rotational behaviour of the wheels. According to one embodiment, the braking pressure reduction has priority during anti-skid control as long as the pressure reduction times required for regaining the running stability of a wheel lie under a maximum valve (Tmax). When the maximum value (t6; Tmax) has been reached, priority of pressure reduction is temporarily suspended in favour of a pressure build-up in the wheel brakes of one or more stable-running wheels (VR). <IMAGE>

Description

SPECIFICATION Anti-skid controlled brake system This invention relates to a circuit configuration for an anti-skid-controlled brake system, in particular for an automotive vehicle, of the kind which enables the braking pressure to be controlled in accordance with a time-division multiplexing method in dependence on the rotational behaviour of the vehicle's wheels and which has a master cylinder for a braking pressure generator, which master cylinder is equipped with a controllable resetting device, the wheel brakes being connected to the master cylinder via electromagnetically-operable multi-directional valves which can be switched over to lock the pressure medium passage after the onset of brake slip control, the pressure variation in the wheel brakes of the individual wheels and/or wheel groups being variable consecutively by the control of the pressure in the working chambers of the master cylinder by means of the resetting device as well as by the opening, i.e. by switching-over to passage, of the pressure medium paths from the master cylinder to the respective wheel brake and by the locking of the pressure medium paths to the remaining wheels.

There are already known brake systems where-for anti-skid control or rather for controlling the slip the braking pressure can be controlled in accordance with a time-division multiplexing method. To this end, in a known brake system (German Published Patent Application = DE-OS No. 3317629) electromagnetically-operable multidirectional valves are inserted into the pressure medium lines by means of which the wheel brakes are connected individually and/or in pairs to a braking pressure generator, these multi-directional valves allowing the pressure medium passage to be opened or to be locked closed. The braking pressure generator consists of a master cylinder and of a resetting unit connected upstream thereof.By means of the resetting unit and associated control valves it is possible to generate an auxiliary force which is opposed to the pedal force and by means of which-for the purpose of slip control-the force acting on the pistons in the master cylinder and, hence, the braking pressure generated in the master cylinder can be reduced. If all wheels are stable-running, all the wheel valves, i.e., the valves inserted between the master cylinder and the wheel brakes, are connected for passage. If there appears a lock-up tendency at any one of the wheels, the wheel valves leading to the other wheels are changed over to a closed condition for a short time, thus only the pressure medium circuit of the anti-skid-controlled wheel being connected to the wheel cylinder.Now, by metering pressure into the resetting unit, an auxiliary force is built up which partially or completely compensates the brake pedal pressure, whereby the braking pressure is reduced in the wheel brake of the wheel solely connected in this phase. The pressure in the other wheels remains constant during this phase. After the pressure reduction down to the desired pressure level, by switching the valve over, the reduced pressure is kept constant at this wheel which at first had become instable. As soon as the counterforce has been decreased again and pressure has been built up anew in the master brake cylinder it is possible to continue the build-up of braking pressure in the other wheel brakes. Consecutively or by way of time division, it is possible in this manner to adjust the braking pressure at any wheel to the desired value calculated by the associated electronics.

In such systems controlled in accordance with a time-division multiplexing method and including the master cylinder in the pressure modulation, it is impossible to effect pressure control in the individual control circuits simultaneously and independently. During one pressure reduction phase, at the maximum it is possible to keep the braking pressure constant in the other circuits, yet not to increase the pressure further. In unfavourable situations such as during cornering or in case of varying friction values on the right and left sides of the vehicle, this will lead to underbraking of the stable-running wheels. On the other hand, a continuation of the pressure build-up and a delay in pressure reduction might lead to the lock-up of a wheel and thus endanger the driving stability and the steerability.

It is an object of this invention to overcome these disadvantages and to provide a circuit configuration for a time-division-multiplexingmethod-operated anti-skid-controlled brake system, which also in unfavourable situations ensures a slowing-down with a short stopping distance and the maintenance of the driving stability as well as of steerability.

According to the invention there is provided a circuit configuration for an anti-skid-controlled brake system, in particular for an automotive vehicle, of the kind which enables the braking pressure to be controlled in accordance with a time-division multiplexing method in dependence on the rotational behaviour of the vehicle's wheels and which has a master cylinder for a braking pressure generator, which master cylinder is equipped with a controllable resetting device, the wheel brakes being connected to the master cylinder via electromagnetically-operable multi-directionai valves which can be switched over to lock the pressure medium passage after the onset of brake slip control, the pressure variation in the wheel brakes of the individual wheels and/or wheel groups being variable consecutively by the control of the pressure in the working chambers of the master cylinder by means of the resetting device as well as by the open ing, i.e. by switching-over to passage of the pressure medium paths from the master cylinder to the respective wheel brake and by the locking of the pressure medium paths to the remaining wheels, characterised in that, during brake slip control, the sequence of pressure control in the wheel brakes of the individual wheels and/or the intervals in which the wheel brake is connected to the master cylinder are made dependent on the rotational behaviour of the wheels.

According to an advantageous embodiment of this invention, during anti-skid control, the braking pressure can be modulated in accordance with predetermined patterns which can be varied or changed over in dependence on the rotational behaviour of the wheels.

According to a further embodiment, during anti-skid-control, braking pressure reduction has priority as long as the pressure reduction times required for regaining the running stability of a wheel or rather for terminating the lock-up tendency lie under a predetermined maximum value and priority of pressure reduction can at least temporarily be suspended in favour of a pressure build-up in the wheel brake(s) of one or several stable-running wheels when the maximum value is reached.

According to a further embodiment, there is provided a measuring circuit for establishing the pressure reduction times and a priority allocation circuitXwhich at first gives priority to pressure reduction and which, when a predetermined maximum pressure reduction time is exceeded, keeps the pressure in the wheel brake of the instable-running wheel constant and permits a pressure build-up in the wheel brakes of the remaining wheels.

Thus, the circuit configuration according to the invention is based on the consideration that, for masons of driving stability and of steerability, first priority should be given to pressure reduction. However, for instance, if during cornering-due to the relief of the wheels on the inside ofthe curve or in the case of varying friction values on the right and left sides of the vehicle the pressure reduction takes too much time that the wheel contributes little to slowing-down and to driving stability, pressure control at the wheel brake of this wheel will temporarily be set aside and priority will be given to the pressure build-up at the stable-running wheel(s).Thereby, even in such situations difficult to control, a short stopping distance will be achieved, without the driving stability and the steerability being jeopardized. All that is accepted is that the relieved wheel or rather the wheel running with a low friction value, whose braking pressure is temporarily kept constant on a low value which is not yet an optimum value with regard to slip, goes on running with a relatively high slip for a short time or-in extremely unfavourable and rare situations-will lock. However, this can be tolerated due to the bad contact the wheel has with the road.

By means of this relatively simple measure, however, the overall braking behaviour is decisively improved.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematical, simplified representation of the most important hydraulic and electronic components of a brake system controllable by a circuit configuration according to the invention; Figure 2 is a diagram of the time variation of the wheel velocities and of the pressure in the wheel brakes during a braking operation on a road surface with varying friction coefficients on the right and left sides; and Figure 3 is a block diagram of the essential units of a circuit configuration for carrying out this invention.

Figure 1 shows an anti-skid controlled hydraulic brake system controlled in accordance with a time-division multiplexing method. This system has a braking pressure generator 2 operable by means of a brake pedal 1 and essentially consisting of a tandem master cylinder 3 with a hydraulic brake booster 4 connected upstream and with a resetting device 5 provided for braking pressure modulation.

An auxiliary pressure source 6 is provided for the supply of the brake booster 4 and of the resetting device 5. The auxiliary pressure source 6 is essentially made up of an electricmotor-driven hydraulic pump 7 and of a hydraulic pressure accumulator 8. The auxiliary pressure source 6 is directly connected to the brake booster 4 and, via a valve arrangement 9, it is connected to the resetting device 5. A common pressure compensation and pressure medium storage reservoir 10 supplies pressure medium to both the master cylinder 2 and the resetting device 5 as well as to the auxiliary pressure source 6.

Separate pressure medium reservoirs will, of course, be necessary if different pressure media or hydraulic media are used for the actuation of the resetting device 5 and for the master cylinder 2 and the brake circuits I, II respectively. Instead of using the electric-motor drive M of the pump 7 it is also possible to have the pump driven via the automotive vehicle's engine.

Connected to the two brake circuits 1,11 of the tandem master cylinder 3 are the wheel brakes 11-14 of the vehicie's wheels VR,HL,VL,HR via in each case a multi-directional valves which in the basic position is connected for passage. The multi-directional valves are 2/2-way valves 15-18 which can be electromagnetically switched over to lock closed. In the illustrated embodiment, a diagonal brake circuit allocation was chosen. The right front wheel is designated VR, the left front wheel is marked VL, the right rear wheel is marked HR, the left rear wheel being marked HL.

In the embodiment illustrated, the rotational behaviour of the wheels is detected by means of wheel sensors 19-22 which, for instance, feed a signal with a frequency proportional to the wheel's rotation to an electronic unit 27 via signal lines 23-26. The sensor signals are processed in the control unit 27 by logical combination of the signals by means of hardwired or programmed electronic circuits, valve control signals being generated out of the sensor signals and supplied to the wheel valves 15-18, namely the 2/2-way valves inserted into the pressure medium paths from the master cylinder 2 to the wheel brakes 1114, via signal lines 30. Via signal lines 31, they are supplied to the valve arrangement 9 at the inlet of the resetting device 5.

The valve arrangement 9 comprises a 2/2way valve 32, which in the rest position is connected for passage and which electromagnetically can be switched over to lock closed and which connects a pressure chamber or rather resetting chamber 34 of the resetting device 5 with the reservoir 10, and of a normally locking 2/2-way valve 33 which can be switched over electromagnetically to passage and which serves to supply pressure from- the auxiliary pressure source 6 into the pressure chamber 34 of the resetting device 5.

The brake system illustrated in Figue 1 works as follows: In the rest position and during normal, i.e., uncontrolled braking operations, all valves 1518 and 32,33 are in their illustrated rest positions. The wheel brakes 11-14 thus hydraulically communicate with the working chambers 35 and 36 within the tandem master cylinder 2. A braking force being applied to the brake pedal 1 in the direction of the arrow F, a force boosted in the brake booster 4 is transmitted to a piston 38 of the resetting device 5 via a push rod 37. From the piston 38 it is transmitted to the master cylinder 2 via a second push rod 39. No resetting force is generated as, during normal braking operations, the pressure chamber 34 directly communicates with the pressure compensation reservoir 10 via the valve 32.Via the two pistons 40 and 41 of the tandem master cylinder 2, a braking pressure is generated in the brake circuits 1,11 which is proportional to the pedal force F.

A lock-up tendency being sensed at one or several vehicle's wheels, anti-skid control comes on. By locking of the wheel valves leading to the wheel brakes of the stable-running wheels, the pressure medium is locked in at the latter, the braking pressure being thereby kept constant in said wheels. By changing-over of the two valves 32,33 of the valve arrangement 9, pressure is thereupon supplied from the auxiliary pressure source 6 into the pressure chamber 34 of the resetting device 5. As a result, the piston 38 of the resetting device 5 is moved back contrary to the pedal's actuating direction and finally there is a relief of the master cylinder 2. The pressure in the brake circuits 1,11 is reduced.Via a wheel valve-any one of valves 15 to 18-which had remained in the open position or which was switched back to passage, pressure medium now flows back from the relevant wheel brake into the working chamber 35 or 36 of the master cylinder 2. The braking pressure in this wheel is thus decreased via the master cylinder 2 by means of the actuation of the resetting device 5.

By switching-back of one of the wheel valves 15-18, in each case, into the open position or certain periods established by means of the electronic circuits in the control unit 27 and by simultaneous blocking of the pressure medium paths to the wheel brakes of the remaining vehicle's wheels and by establishing of the desired pressure in the master cylinder 2-namely by pressurisation and relief of the piston 38 of the resetting device 5, consecutively, it is possible to vary, i.e., decrease, keep constant and reincrease, the braking pressure in the wheel brakes of the individual wheels and thus to control the brake slip individually per wheel. This type of anti-skid control is referredCto as an example of a timedivision multiplexing method.

Of course, via several wheel valves 15-18 simultaneously open or switched back to passage, it is also possible and it also makes sense to decrease or reincrease the braking pressure in several wheels at the same time.

The rating and mode of operation of a circuit configuration for controlling such a brake system according to the invention are illustrated by the diagrams of Figure 2 which refer to a braking operation on a road surface with different friction values on the right and left sides.

The same time scale applies to the three diagrams of Figure 2. The illustrated curves represent the braking pressure PvL, PVR, PHR,HL prevailing in the wheel brakes of the individual vehicle's wheels at the same time as well as the momentary wheel velocities v,,,v,,,v,,, and VHL as well as finally the vehicle,s velocity VF.

The upper diagram refers to the left front wheel VL which has the worst road contact during the controlled braking operation under consideration. In the assumed situation, the friction coefficient is considerably less on the vehicle's left side than on the right one. Similar conditions could result in cornering to the left since-as is known-the wheel on the inside of the curve is relieved due to the centrifugal force.

In the braking operation according to Figure 2, anti-skid control comes about at time to. At first, as shown by the upper diagram, the left front wheel VL becomes unstable. Pressure reduction starts after a very short phase wherein the pressure remains constant. In period t1-t2 namely the wheel valve 17-cf. Figure 1-was switched to passage while the other wheel valves 15,16,18 interrupted the pressure medium path. At the same time, by activating the resetting device 5-namely by means of changing-over of the valves 32,33 and by means of supplying pressure into the pressure chamber 34-a counterforce opposing pedal force F was generated and thus a pressure reduction was brought about in the working chambers 35,36 of the master cylinder 2.In the embodiment of the control circuit represented here, pressure modulation is brought about by pulse sequences. The first pressure reduction pulse has terminated at time t2 Valve 17 switches over into the locking position. At time t3-see lower diagram in Figure 2-the first pressure reduction- pulse controlling the pressure at the rear axle has terminated, too.

In the illustrated embodiment of this invention, the braking pressure in the rear wheel brakes 12,14 is controlled concurrently according to the known "selectlow" method wherein the braking pressure is governed by the wheel with less road contact-in the present example-by the left rear wheel. As shown by the broken curve ih the lower diagram of Figure 2, the right rear wheel therefore runs with a slight slip and thus considerably contributes to driving stability. The velocity VHR of the right rear wheel almost coincides with the vehicle's velocity VF.

The pulse pause or rather the pressure reduction pause starting at time t3 can be utilised for supplying braking pressure into the right front wheel VR running on the side with the high friction value or rather friction coefficient. This can be seen from the curve variation PVR in the middle diagram of Figure 2.

At time t4, pressure reduction having priority in the circuit is continued in the wheel brakes of the left front wheel VL and of the rear wheels HR, HL. Therefore, pressure build-up at the right front wheel must be interrupted until time t5. It is only possible to keep the pressure constant until time t5.

The dash-dot curve in the middle diagram of Figure 2 illustrates the ideal pressure variation pj achievable with respect to a complete utilisation of braking capacity in case of individual braking pressure control, yet not in the case of a time-division multiplexing operatrion, in the wheel brake 11 of the right wheel VR running with a higher friction value.If a wheel such as the front wheel VL in the example explained with reference to Figure 2 remains unstable despite continued braking pressure reduction, i.e. if it goes on running with excessive slip due, for instance, to a very strong relief in the curve or to an extremely slippery road surface on that side, without the further development according to the invention there would result the pressure variation P'VR for the right wheel VR, this pressure variation p;, being illustrated by a broken line in the middle diagram. Due to the pressure reduction priority, the pressure supplied into the wheel VR running with a higher friction value would remain considerably behind the ideal pressure variation p illustrated by a dash-dot line.According to this invention, therefore, the pressure reduction time until regaining the running stability of a wheel is measured and priority allocation will be changed if the wheel continues to show a lock-up tendency after a predetermined pressure reduction time.

At time t6, the maximum pressure reduction time of the left front wheel VL is reached in the braking operation illustrated with the aid of the diagrams of Figure 2. The wheel continues to run unstably. Consequently, this wheel contributes little to braking and to driving stability. Therefore, now, priority is no longer given to the pressure reduction but rather to the pressure build-up in the wheel brake of the front wheel VR running with a high friction value.At time te, therefore, as shown by the full line of the pressure variation p,, in the middle diagram of Figure 2, pressure build-up is continued in the wheel brake of the right front wheel until, at time t7, slip control comes on for this wheel, too, as can be seen from the pressure variation PVR and from the velocity drop or rather the short-time deviation of the right front wheel's velocity v,, from the vehicles velocity v VF. The stability limit of the right front wheel VR is reached for the second time at time t8 and for a third time at time t9.

The existing residual pressure in the left front wheel VL is kept constant as of t6.

Thereby it is accepted that this wheel VL continues to run with excessive slip or even locks-as in extremely unfavourable cases. After time t7, the residual pressure in the left front wheel VL is further reduced in the control operation illustrated here. During the short-time phases of increase of pressure pvR in the time between t7 and ts, of course, pressure reduction in the left front wheel VL must be interrupted each time. However, this cannot be seen in curve PVL due to the residual pressure in the left front wheel being extremely small during this period.

As compared with a circuit configuration with permanent pressure reduction priority, the stopping distance is considerably reduced due to the better utilisation of the braking effect of the wheel running with a higher friction value.

By means of the time-division-multiplexingmethod-operated brake system in this way almost the same braking behaviour is achieved as in a brake system with individual wheel slip control.

The embodiment shown in Figure 3 is a circuit configuration for achieving the braking behaviour explained with reference to Figure 2.

By means of the circuits 42-46, at first, the momentary velocity of all wheels vv,, VVR, v,,, vHR is compared with the vehicle's velocity VF or with the vehicle's reference velocity formed in a logic circuit 46. By means of a measuring circuit 47 which can also contain a cornering identification circuit the pressure reduction time T is measured individually per wheel. The pressure reduction time exceeding a predetermined limit value Tmax, a priority allocation circuit 49 is switched over via signal lines 48.

Whereas, previously, priority was given to pressure reduction, now, priority is given to a pressure build-up as explained with reference to Figure 2.

The circuit according to Figure 3 can be realised by a hardwired additional circuit or by programmation of a program-controlled circuit and can be combined with an anti-skid control circuit 50 which has been merely sketched out.

Claims

1. A circuit configuration for an anti-skidcontrolled brake system, in particular for an automotive vehicle, of the kind which enables the braking pressure to be controlled in accordance with a time-division multiplexing method in dependence on the rotational behaviour of the vehicle's wheels and which has a master cylinder for a braking pressure generator, which master cylinder is equipped with a controllable resetting device, the wheel brakes being connected to the master cylinder via electromagnetically-operable multidirectional valves which can be switched over to lock the pressure medium passage after the onset of brake slip control, the pressure variation in the wheel brakes of the individual wheels and/or wheel groups being variable consecutively by the control of the pressure in the working chambers of the master cylinder by means of the resetting device as well as by the opening, i.e. by switching-over to passage, of the pressure medium paths from the master cylinder to the respective wheel brake and by the locking of the pressure medium paths to the remaining wheels, characterised in that, during brake slip control, the sequence of pressure control in the wheel brakes (11-14) of the individual wheels (VR,VL,HR,HL) and/or the intervals in which the wheel brake is connected to the master cylinder (3) are made dependent on the rotational behaviour of the wheels.

2. A circuit configuration as claimed in claim 1, characterised in that, during anti-skid control, the braking pressure can be modulated in accordance with predetermined patterns which can be varied or rather changed over in dependence on the rotational behaviour of the wheels (VR,VL,HR,HL).

3. A circuit configuration as claimed in claim 1 or 2, charcterised in that, during anti-skid control, braking pressure reduction has priority as long as the pressure reduction times required for regaining the running stability of a wheel or rather for terminating the lock-up tendency lie under a predetermined maximum value (TmaX) and in that priority of pressure reduction can at least temporarily be suspended in favour of a prsssure build-up in the wheel brakes of one or several stable-running wheels-i.e., of wheels not revealing any lock-up tendency-when the maximum value (Tmax) is reached.

4. A circuit configuration as claimed in any one of claims 1 to 3, characterised in that it has a measuring circuit (47) for establishing the pressure reduction times and a priority allocation circuit (49) which at first gives priority to pressure reduction and which, when a predetermined maximum pressure reduction time (Tmax) is exceeded, keeps the pressure in the wheel brake of the instable-running wheel constant and permits a pressure build-up in the wheel brakes of the remaining wheels.

5. A circuit configuration for an anti-skidcontrolled brake system substantially as described with reference to the accompanying drawings.