GB1600050A

GB1600050A - Process and circuit arrangement for anti-skid control of brake pressure of a vehicle

Info

Publication number: GB1600050A
Application number: GB1574778A
Authority: GB
Original assignee: Wabco Fahrzeugbremsen GmbH
Current assignee: Wabco Fahrzeugbremsen GmbH
Priority date: 1977-04-20
Filing date: 1978-04-20
Publication date: 1981-10-14
Also published as: FR2387824A1; DE2717457A1; DE2717457C2; JPS53134186A; JPH0141535B2; SU1245256A3; FR2387824B1

Description

(54) PROCESS AND CIRCUIT ARRANGEMENT FOR ANTI-SKID CONTROL OF BRAKE PRESSURE OF A VEHICLE (71) We, WABCO FAHRZEUG BREMSEN GmbH, formerly WABCO WESTINGHOUSE GmbH., a Company organised according to the laws of the Federal Republic of Germany, of 3000 Hannover 91, Postfach 91 12 80, Federal Republic of Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a process and a circuit arrangement for controlling the brake pressure of a vehicle having an anti-skid control device.

There have been proposed vehicle antiskid system having measures for preventing the overall reference speed used to ascertain whether a wheel is skidding from being pushed up as a result of one or more of the drive wheels spinning.

In one system the overall reference speed is selected in the uncontrolled driving state (i.e. brakes not applied) as the minimum value of the wheel speeds or reference speeds, and in the controlled driving state (i.e. brakes applied) as the maximum value of the wheel speeds or reference speeds. In another system, the reference speeds of the driven wheels have been used to form the overall reference speed when a brake control of all wheels or of the undriven wheels is in progress.

A disadvantage of the first-mentioned process is that owing to the formation of the minimum value in the uncontrolled driving state, until a control process begins, or until switching over to formation of the maximum value occurs, the reference speed is held relatively low, as a result of which for example in the first control cycle, the slip control signal for the rear wheel is reached at a relatively belated stage thus resulting in an initially insensitive control of the brakes.

It is a further disadvantage of the above system that a defective sensor cannot be recognised, and owing to the selection of the minimum value as the reference value, the value obtained corresponds to a stationary vehicle in which slip control signals cannot appear and the time for energising a brake pressure medium solenoid valve would be a maximum. Brake pressure can be controlled only via the operative sensor and as a slip control signal cannot be generated, there is a risk that the wheels controlled in this manner will lock.

A further drawback is the result Ol' the fact that when the service brake is not operated slip control signals cannot be obtained at the driven wheels so that anti skid control of an additional brake, which acts only on the driven wheels, for example the rear wheels, and which may be a conventional eddy-current brake, is impos sible.

With vehicles having several driven axles, there is a disadvantage when for example, at least two driven wheels spin under excessive power, and the wheels are used to form a common reference value, because despite athe selection of a minimum value as the overall reference speed, the reference speed is then pushed up and deviates too greatly from the true speed of the vehicle. The consequence is that brake force is not used to an optimal manner.

In the second system mentioned, ineffi cient brake control can arise when, with vehicles having one driven (rear) axle, an undriven front wheel, for example, starts to slip before the rear driven wheel, because the reference speed, being derived from the rear wheels only, can then likewise deviate too much from the vehicle speed.

Moreover, with vehicles having two driven axles, disadvantages result when a front wheel spins, because the reference speed then increases and can deviate very greatly from the vehicle speed, which in turn has an adverse effect on the control behaviour when initiating a braking action.

It is an object of the present invention to reduce at least some of the disadvantages of the known system.

According to one aspect of the invention there is provided a process for the anti-skid control of the brake pressure in brake circuits of a vehicle, including measuring the rotational speeds of at least two wheels or rotating parts coupled to two wheels of a vehicle, establishing reference speeds related to the rotational speeds, and forming a common overall reference speed by selecting the maximum value of the reference speeds, which overall reference speed is used to detect slipping of a wheel and signals representing such slipping and excessive acceleration or retardation of a wheel are used to regulate a brake pressure control operation so as to reduce skidding, wherein if no brake pressure control operation is taking place, the reference speed or the wheel speed of the driven wheel or axle is temporarily disconnected from, or reduced before being used in, the generation of the overall reference speed or the reference speed or the wheel speed of the driven wheel or axle is replaced by the wheel speed of the undriven wheel, the reference speed or the wheel speed of the driven wheel or axle being reconnected or restored to its unreduced value upon the appearance of a signal indicating slipping of the driven wheel or axle and/or after a predetermined time following the disconnection or reduction.

According to a second aspect of the invention there is provided a circuit arrangement for the anti-skid control of brake pressure of a vehicle including rotation sensors allocated to wheels or rotating parts coupled to wheels of a vehicle, first circuits responsive to signals from the sensors for ascertaining the speeds corresponding to the' sensor signals and producing speed signals, reference speed circuits for deriving from the speed signals reference speed signals allocated to the wheels or parts, acceleration, deceleration and slip threshold stages connected to receive the speed signals and produce corresponding behaviour signals for the wheels or parts, a maximum selecting circuit connected to receive the reference speed signals and derive therefrom an overall reference speed, wherein there are provided sensors for at least one driven wheel and one undriven wheel, and a logic circuit responsive to speed signals and behaviour signals from the threshold stages for generating an output signal in the presence of a signal indicating that no brake pressure control operation is taking place, which output signal is applied to set a switch means connected to receive the wheel speed or reference speed signal of the driven wheel to interrupt or reduce the reference speed for the driven wheel which is applied to the maximum selecting circuit or to substitute the wheel speed signal for the undriven wheel for that of the driven wheel, the output signal of the logic circuit being terminated and the switch means reset on the occurrence of a slip signal from the driven wheel or after a predetermined time following the setting of the switch means.

In one embodiment of the invention in the event of the driven wheels or axles spinning, only the speeds of the undriven wheels are used to form the reference, so that spinning rear driven wheels for example, cannot push up the overall reference speed. When the vehicle is stationary or when a sensor of the undriven wheels is defective, the reference of the driven wheels is not disconnected, and when the vehicle is being driven (and the sensor of the undriven wheels is working) connection is made to the reference speed of the undriven wheels, normally the front wheels. The invention ensures that the circuit arrangement for forming the reference speed for the driven wheels or the driven axle is used to form the overall reference speed again only when it has been ensured that the speed of the driven wheel or wheels or of the driven axle is no longer greater than the speed of the vehicle.If the circuit for forming the reference speed is interrupted then upon bringing it into use again the reference speed increases in jumps. However, if the circuit for forming the wheel speed is interrupted, then the reference speed increases after a delay when this circuit is brought into use again. If the steps to be specified in detail later are implemented, the steps being namely the interrupting and connecting up of the circuit for forming the speed of the driven wheels whilst the circuit for forming the sped of the undriven wheel remains connected continuously to the circuit for forming the reference speed of the driven wheels, then there is obtained the advantage that firstly a delayed increase in the reference speed is produced and secondly, the time for the increase becomes less, because the speed of the undriven wheels is used as the starting point before the increase, whereas in the prior art the reference speed has to increase from zero.

In vehicles having two driven axles, the invention may be applied to detect both the spinning of the wheels of one axle and of the wheels of both axles and to render ineffective the tendency of such spinning to push up the reference speed.

The switching out of the driven wheel's countribution to the reference speed may be effected only when spinning of the driven wheels actually occurs.

In order to be able to recognise a station- ary front wheel, a defective sensor or any other defect, for example a break in cables to the sensor, these circumstances .being indicated by the absence of a speed signal from the particular wheel, according to a further development of the invention, when a speed signal does not appear for an undriven wheel or wheels, the slip control signal for that wheel is cut off or blocked until the appearance of the next speed signal. Thus, it is ensured that the brake cylinder of the undriven front wheel is not deprived of pressure in the event of an immediately following braking action.

Moreover, if a test connection is provided the defect may be detected by the test connection itself with the next test pulse and not only during braking by the monitoring of the operation of the solenoid valves controlling the brake pressure. If a sensor becomes defective at an undriven wheel during normal driving the slip control signal for this wheel is cut off and the failure of the sensor can be established by the next test pulse.

According to another development of the invention, when there is a test connection for the individual function units and on the appearance of speed signals for the undriven wheels, provision is made for the test connection to be disconnected also upon the appearance of a slip control signal at the undriven wheel or upon the appearance of an acceleration control signal at the driven wheel, in order, when the driven wheels spin, to prevent the anti-skid control device from being disconnected prematurely by a test connection.

According to another embodiment of the invention, when a speed signaL for the undriven wheels is not present, provision may be made for a deceleration signal from the driven wheels also to be blocked. By this means the reference speed is not'affected and the production of slip control signals is prevented so that the driven wheels or driven axle is not deprived of brake pressure. By this means also a spurious brake control is avoided and it is ensured that the defect is recognised by the next test signal so that the electronic control circuit can be interrupted.

Alternatively, the reference speed signal of the driven wheels need not be cut off even when wheels spin, but a predetermined fraction of the reference speed of the driven wheels may be used to form the overall reference speed whilst the vehicle is not braked. The full actual reference speed of the driven wheels is used to form the overall reference only when a brake force control is in progress and a slip control signal for the driven wheels has appeared.

With diagonal brake control systems, undesirable brake behaviour can occur if control is effected only in response to the speed and acceleration of the undriven wheels, which then show a tendency to be unstable. The disadvantage can be avoided by use of the invention.

A spinning driven wheel decelerates very strongly, for example, when the vehicle is decelerated. The reference speed adjusts more slowly than the wheel speeds, owing to the way in which it is formed. In order in such as case to obtain a more favourable adjustment of the reference speed, in another example of the invention upon the appearance of a deceleration signal for the driven wheel or the driven axle, switching over to a more rapid adjustment to the reference speed may be effected. The true speed of the vehicle can thereby rapidly be matched by the reference speed, resulting in a better control of braking.

In order that the invention may be fully understood and readily carried into effect it will now be explained in greater detail with reference to the accompanying drawings in which the embodiments are illustrated. In the drawings: Figure 1 shows a circuit arrangement according to an example of the invention:: Figure 2 shows a grahic representation of the progress of the different speeds and reference speeds when a rear driven wheel spins during driving, the time sequence of the slip control signals that may possibly occur also being represented; Figure 3 is a graphic representation of the progress of the different speeds when the rear driven wheel spins when the vehicle is stationary or when a sensor at the undriven front wheel is defective, the time sequence of the control signals also being shown; Figure 4 shows a graphic representation of the progress of the speeds when driving without braking and with a defective sensor at the undriven wheel at the same time; and Figure 5 shows another circuit arrangement for carrying out the process of another example of the invention.

Reference will now be made to Figure 1 which shows schematically a circuit arrangement for carrying out an anti-skid brake control process according to an example of the invention, a diagonal circuit arrangement for obtaining the overall reference speed being assumed, in which the right front wheel (VR) and the left rear (HL) wheel are scanned. In Figure 1, the numeral 2 denotes a toothed wheel coupled to the front wheel VR and the numeral 4 a toothed wheel coupled to the rear wheel HL; 6 and 8 are the sensors scanning the toothed wheels.

A switching circuit 10 for ascertaining the speed of the right front wheel VR and a reference switching circuit 12 for ascertaining the reference speed of the right front wheel VR are connected in series to the output of the sensor 6. A switching circuit 14 for ascertaining the speed of the left rear wheel HL and a reference speed circuit 16 for ascertaining the reference speed of the left rear wheel HL are connected in series to the output of the sensor 8. The outputs of the reference circuits 12 and 16 are connected to a switching circuit 18 for ascertaining the overall reference speed, the switching circuit being a maximum selecting stage.

The switching circuits 10 and 14 allocated to the right front wheel VR and the left rear wheel HL respectively are linked via a logic circuit 20. In the switching circuit 14 for the left rear wheel HL there may be provided as required several switches such as 22, 23 and 24 which are actuated by the output signals of the logic circuit 20. The manner in which these switches function will be described in greater detail below in conjunction with the description of the operation of the circuit.

The logic circuit 20 includes AND-gates 25, 26, 27 and 28, an OR-gate 30 and RS flip-flops 32 and 34, which are all, as shown, interconnected and connected to the reference speed circuits of the right front wheel VR and the left rear wheel HL. In addition, an AND-gate 36 is provided which combines an output signal of the logic circuit 20 and the output signal of a test connection (not shown). Another output (not shown) of the flip-flop 34 may be used to reset the time basis for the generation of the test signals.

The AND-gate 25 receives at an inverting input the output signal of the switching circuit 10 and at a non-inverting input the slip control signal XVR of the right front wheel VR. The slip control signal kVR of the right front wheel VR is also applied as input to the AND-gate 27 and the OR-gate 30.

The AND-gate 26 is energised by the B-output signal of the flip-flop 32 and the deceleration signal bHL of the left rear wheel HL. The A-output signal of the flip-flop 32 is fed to an inverting input of the AND-gate 27. The SET-input of the flipflop 32 is connected to the output of the AND-gate 25 and the RESET-input of the flip-flop 32 is connected to the output of the switching circuit 10. The AND-gate 28 has two non-inverting inputs and one inverting input to which are respectively applied a slip control signal XVR of the right front wheel VR from the output of the OR-gate 30, the output signal of the switching circuit 10 for the right front wheel, and a signal tv4 indicating the start of the brake pressure control.The output of this AND-gate 28 is applied to the SET-input of the flip-flop 34, the RESET-input of which -receives the slip control signal HL of the left rear wheel HL, and the A-output of which, depending on the construction of the circuit, is connected to operate one of the switches 22, 23 and 24 (22 as shown in Figure 1).

The different signals mentioned above, such as the deceleration signal -bHL of the left rear driven wheel, the slip control kVR signal of the undriven, right front wheel, the first acceleration control signal +bHL of the left rear driven wheel, the signal tv4 indicating the start of the brake pressure control, which signal is initiated by the signal -bHL (or -bVR not shown), the slip control signal HL of the left rear driven wheel which may be released by the wheel itself or by a test connection, the TEST SIGNAL of the test connection itself (not shown), and the speed signal VVR of the right front wheel are applied to the circuit via lines 38, 40, 42, 44, 46, 48 and 50 respectively. For the sake of clarity, the references used above have also been used in the Figure 1.

The manner of operation of the circuit of Figure 1 will now be explained in more detail. In the inoperative state a "high" signal is present at the B-outputs of the flip-flops 32 and 34, and at their A-outputs there are "low" signals. The inoperative state corresponds to the reset state so that the same initial conditions apply in the driving state in which the speed signal VvR of the right front wheel (the undriven wheel) is present, and the state in which the slip control signal XHL of the left rear wheel (the driven wheel) is present.

Let it first of all be assumed that the vehicle is being driven and that the rear driven wheels are spinning. The relationships resulting on account of the circuit arrangement according to the invention are shown in Figure 2, it being presupposed for the sake of simplicity that the vehicle speed, and thus the speed of the undriven wheel (here vv), remains constant, although does not constitute a limiting factor.

As the rear wheels spin, in addition to the speed of the left rear wheel (v), the reference speed VRefHL and also the overall reference speed overall reference are pushed up owing to the formation of a maximum value in the switching circuit 18.On the appearance of a slip control signal hVR at the right front wheel at the point in time tl, or on the appearance of an acceleration signal +bHL at the spinning rear wheel (not shown in Figure 2) the speed signal VVR iS present at the one non-inverting input of the ANDgate 28 and to the other non-inverting input there is applied via the OR-gate 30 either the signal /Vr or the signal +bHL, and, in addition, at the inverting input of the AND-gate 28 there is also present a "high" signal, as a control process is not yet in progress and the corresponding signal tv4 is zero.Thus the AND-gate 28 produces an output signal and sets the flip-flop 34, as a result of which a "high" signal appears at the A-output of this flip-flop which opens the switch 22 and interrupts the switching circuit for the formation of the reference speed of the left rear wheel HL. so that only the reference speed of the right front wheel (VRCfVR) from the circuit 12 is used to form the overall reference speed in the circuit 18.

The reference speed of the rear wheel HL falls to () km/h because the switch 22 is open.

Simultaneously. the TEST SIGNAL on the line 48 is interrupted by the AND-gate 36 so that the electronic anti-skid control cannot be interrupted prematurely by a test procedure.

If, for example. at the time t the left rear wheel decelerates again, for example because the vehicle is decelerated or the brakes are applied, there now appears a signalbH, for the left rear wheel. which signal releases a tv4 signal (signal indicating the start of the brake pressure control process) which blocks the AND-gate 28 by way of its inverting input. At the same time, the reference speed of the right front wheel (vRe,R) and the overall reference speed (Vovcr.:ll refcrence) develop in the predetermined manner and both front wheel and the left rear wheel slip.

As soon as the slip signal HL of the left rear wheel appears, at the time t3, the flip-flop 34 is reset and the switch 22 is closed, so that the reference circuit 16 of the left rear wheel is connected to the circuit 18 for the formation of the overall reference again, the overall reference speed being ascertained from the two individual reference speeds by forming a maximum value.

These relationships are represented by the gradient of the curve in Figure 2 from the time t3 onwards. At the same time the inhibition of the test signals is ended by releasing the AND-gate 36 again. At first the overall reference speed continues to be formed from the reference speed of the right hand front wheel VR as a result of the selection of the maximum value, until at the' time t4 the reference speed of the left rear wheel HL exceeds the reference speed of the right front wheel VR and that of the overall reference speed; from this time onwards the reference speeds alternate in their effectiveness in the formation of the maximum value from control cycle to control cycle.

If switch 23 or switch 24 were to be controlled by the output of the flip-flop 34 instead of switch 22, the time and the amount by which the reference speed is pushed up would vary depending on the particular connection. When switch 22 is controlled, the reference speed is pushed up in jumps, as shown in Figure 2 at t3, because the reference value is always available and is effective only as either zero or its full value.

If switch 23 is controlled, then a delayed rise is obtained owing to the fact that in practice the formation of the reference speed from the speed value available is delayed. With this arrangement also, as when the switch 22 is controlled, the output changes over a full range of values from zero to the actual reference value.

If the arrangement with switch 24 controlled is chosen then the advantage is obtained that when the switch is closed, a speed value VVR iS already available, being the speed of the undriven right front wheel VR, so that the change in the reference speed is relatively small for it to attain the actual reference value. This linking with the speed of the right front wheel VR, or of the undriven wheel could alternatively be carried out using the switch 22.

When the vehicle is stationary, i.e. when the front wheel VR is stationary, and when the left rear wheel HL is spinning, the circuit of Figure 1 operates in the following manner, reference being made also to Figure 3, in which the relationships being established are shown in graphical form.

When a front wheel is stationary, the speed of the right front wheel is zero, i.e. the AND-gate 28 is blocked and the flip-flop 34 cannot be flipped from the reset state into the set state, and, moreover, the switch 22 remains closed and the overall reference speed is formed by the reference speed of the left rear wheel HL.

At a specific time, for example tl, a slip control signal XVR of the right front wheel appears. In order to prevent the brake cylinders of the front wheel being deprived of pressure, as already mentioned, the XvR-signal is blocked by the AND-gate 28, until the speed of the right front wheel becomes greater than zero and there is an output from the circuit 10. In addition, the deceleration signal (-bHL) of the left rear wheel is blocked by the AND-gate 26 since, because of v,, being zero, the flip-flop 32 has been reset, which is important when the wheels are spinning whilst the vehicle is stationary because the rear wheel brakes cannot then be deprived of pressure.

Moreover, this is important when a sensor 6 for the right front wheel VR is defective or when there is a break in the cable to the sensor 6; in these last two cases the speed VVR of the right front wheel would likewise be zero, and the -b signal could not interrupt the testing, as otherwise provided, so that the defective sensor or the break in the cable could be recognised by a test signal and the electronic control unit could be switched off in good time. The test connection is not disabled as the AND-gate 36 in this case is open; this had the advantage that the defect can be recognised via the test connection instead of by monitoring of the operation of the solenoid valves.

When driving or when accelerating in normal driving, with a defect occurring simultaneously, for example owing to a defective front wheel sensor or a breka in the calbe, as a result of which the speed of the right front wheel VR is falsely represented as being zero, the slip control signal XVR of the right front wheel or the acceleration signal of the left rear wheel (+bHL) is blocked in order to avoid the brake cylinders of the vehicle being deprived of pressure, cf. Figure 4. As the test connection remains connected up, the defect is detected by the test and the electronic control unit can be disconnected in good time.

Reference will now be made to Figure 5, which shows a circuit arrangement in which the reference speed of the driven wheel (vRefHL), here for example of the left rear wheel, a diagonal control again being assumed, is not interrupted when the rear wheel spins, but is divided in a divider circuit 50, for example by a factor of two.

Thus in this example, half of the reference speed signal of the left rear wheel HL is used to form the overall reference in an overall reference switching circuit 52.

In addition to the divider 50 already mentioned and the switching circuit 52, the circuit arrangement also includes a logic circuit 54, a flip-flop 58 and an inverter 60.

The reference speed signal of the right front wheel (VRefVR) is fed to the switching circuit 52 by way of a line 62, and, either the full reference speed value of the left rear wheel or, as in this example, half the value of that reference speed is fed to the switching circuit 52 by way of the logic circuit 54 and a line 64.

The logic circuit 54 receives not only the reference speed VRefHL of the left rear wheel, but also the Q output signal of the flip-flop 58, which is set by a slip control signal HL for the left rear wheel and is reset by a signal (to4) indicating the control process or by a signal (BS) obtained by way of the brake light switch, which signal is inverted to the inverter 60, so that when braking has ended the divider 50 becomes operative again.

The logic circuit 54 has two AND -gates 68 and 70. The full reference speed value of the rear wheel and the Q-output signal of the flip-flop 58 are fed to the two inputs of the AND-gate 68. The halved reference speed value of the left rear wheel from the divider 50 is fed to a non-inverting input of the AND-gate 70 and the output signal of the flip-flop 58 is fed to an inverting input of this gate.

The outputs of the AND-gate 68 and 70 are combined in a OR-gate 72, the output of which forms the output of the logic circuit 54.

The selected rear wheel reference speed appearing on the line 64 in addition to being applied to the switching circuit 52 is compared with the present rear wheel speed VL in a comparator 56. When the inputs to the comparator 56 are substantially equal it produces an output which may be used to open the gate 36 (Figure 1) to pass a test signal.

The circuit shown in Figure 5 operates as follows: In the ordinary case, provided that a brake pressure control operation is not in progress, this process being indicated by the presence of the signal tv4 or more simply the brake light signal BS could be used, flip-flop 58 is in the reset state, so that the AND-gate 68 is blocked and the AND-gate 70 is open; if a reference speed value for the left rear wheel HL is present, there appears at the output of the logic circuit 54 a signal representing half of that reference speed value, which signal, for the purpose of forming the overall reference speed, is fed to circuit 52.The advantage of this measure is that because of the selection of the maximum value is the overall reference speed, the overall reference speed is formed from the reference speed of the right front wheel VR until half the value of the reference speed of the left rear wheel HL exceeds that of the right front wheel VR, for example when the left rear wheel spins. As a result of the reference speed of the left rear wheel being divided by two, even when it spins, the overall reference speed will not rise too steeply and will remain within reasonable limits.

Switching over to the actual reference speed of the left rear wheel HL is effected as soon a slip control signal HL appears at the left rear wheel after initiation of a braking process, because the AND-gate 70-is then blockedsand AND-gate 68 is opened. The gates 68 and 70 are returned to their original states after termination of the braking process by resetting the store 58. The slip control signal HL of the left rear wheel is only formed when the slip threshold between the reference speed of the right front wheel VR and the speed of the left rear wheel HL is exceeded.

After the appearance of the slip control signal XHL, the undivided reference speed of the left rear wheel HL is used in conjunction with the reference speed of the right front wheel VR to ascertain the overall reference speed.

When the driven rear wheels are spin ning, the reference speed of these wheels is held at 50% of the actual value as described above. When the vehicle's accelerator pedal is released the driven wheel may decelerate sufficiently for a deceleration signal -b to be produced and thus the adjustment of the reference speed value is started. A signal derived from the adjusted reference speed value can now be linked with the output of the store 58 and used to operate a switching circuit for the more rapid adjustment of the rear wheel reference speed. By this means it is possible for the reference speed to assume the correct value even within the reaction time of the driver of the vehicle, so that the system is ready to control braking correctly in good time at all wheels.

The circuit of Figure 5 can be modified or supplemented. If the reduction of the left rear wheel reference speed (50% in the example given) provided is inadequate, then the difficulty remains that as the left rear driven wheel spins a slip control signal may be formed at the right front wheel which remains there for a longer time than the normal monitoring time of the solenoid valves. In order to overcome this a circuit may be provided to block the slip control signal.

Although the examples of the invention described above in conjunction with the Figures all utilise a diagonal control system, it will be understood that the invention may be applied to other control systems having at least one driven axle, for example a rear axle brake control. It would, of course, be necessary to provide a speed sensor on an undriven wheel for the generation of a reference speed even though the brakes on that wheel may not be controlled.

The circuit arrangements described above with reference to Figure 1 or Figure 5 may be modified by the addition of suitable logical circuits to implement the developments of the invention described earlier either separately or in combination.

WHAT WE CLAIM IS: 1. A process for the anti-skid control of the brake pressure in brake circuits of a vehicle, including measuring the' rotational speeds of at least two wheels or rotating parts coupled to two wheels of a vehicle, establishing reference speeds related to the rotational speeds, and forming a common overall reference speed by selecting the maximum value of the reference speeds, which overall reference speed is used to detect slipping of a wheel and signals representing such slipping and excessive acceleration or retardation of a wheel are used to regulate a brake pressure control operation so as to reduce skidding, wherein if no brake pressure control operation is taking place, the reference speed or the wheel speed of the driven wheel or axle is temporarily disconnected from, or reduced before being used in, the generation of the overall reference speed or the reference speed or the wheel speed of the driven wheel or axle is replaced by the wheel speed of the undriven wheel, the reference speed or the wheel speed of the driven wheel or axle being reconnected or restored to its unreduced value upon the appearance of a signal indicating slipping of the driven wheel or axle and/or after a predetermined time following the disconnection or reduction.

2. A process according to claim 1 wherein the disconnection or reduction is also dependent on the appearance of a non-zero signal indicating slipping of the undriven wheel or on the appearance of a signal indicating acceleration above a threshold value of a driven wheel or axle.

3. A process according to claim 2, wherein if the speed signal for the undriven wheel is zero or absent, the slip signal of this wheel is blocked until a non-zero speed signal reappears.

4. A process according to claim 2 or 3, wherein a signal indicating deceleration of the driven wheel or axle is also blocked when the reference speed or wheel speed of that wheel or axle is disconnected or reduced.

5. A process according to claim 2, 3 or 4, wherein a test circuit is provided for the individual function units, and when a nonzero speed signal for the undriven wheel is present, the test circuit is disconnected or temporarily blocked by the appearance of the slip signal for the driven wheel or axle, the test circuit not being disconnected if the speed signal for the undriven wheels is zero or absent.

6. A process according to claim 5, wherein the test circuit is reconnected on the appearance of a slip signal for the driven wheel or axle.

7. A process according to claim 5, wherein the test circuit is reconnected when a comparator detects parity between the speeds of the driven wheel and the undriven wheel.

8. A process according to claim 5, wherein a monitoring device is provided in the test circuit for monitoring the generation of wheel or axle slip signals and is disconnected when a speed signal for the undriven wheel is present and either an acceleration signal for the driven wheel or axle or a slip signal for the undriven wheel is present and a brake pressure control operation is not taking place, and is reconnected on the appearance of a slip signal for the driven wheel or axle or when a comparator establishes parity between the speed of the driven wheel and the speed of the undriven wheel, or after a given time following the disconnection.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. ning, the reference speed of these wheels is held at 50% of the actual value as described above. When the vehicle's accelerator pedal is released the driven wheel may decelerate sufficiently for a deceleration signal -b to be produced and thus the adjustment of the reference speed value is started. A signal derived from the adjusted reference speed value can now be linked with the output of the store 58 and used to operate a switching circuit for the more rapid adjustment of the rear wheel reference speed. By this means it is possible for the reference speed to assume the correct value even within the reaction time of the driver of the vehicle, so that the system is ready to control braking correctly in good time at all wheels. The circuit of Figure 5 can be modified or supplemented. If the reduction of the left rear wheel reference speed (50% in the example given) provided is inadequate, then the difficulty remains that as the left rear driven wheel spins a slip control signal may be formed at the right front wheel which remains there for a longer time than the normal monitoring time of the solenoid valves. In order to overcome this a circuit may be provided to block the slip control signal. Although the examples of the invention described above in conjunction with the Figures all utilise a diagonal control system, it will be understood that the invention may be applied to other control systems having at least one driven axle, for example a rear axle brake control. It would, of course, be necessary to provide a speed sensor on an undriven wheel for the generation of a reference speed even though the brakes on that wheel may not be controlled. The circuit arrangements described above with reference to Figure 1 or Figure 5 may be modified by the addition of suitable logical circuits to implement the developments of the invention described earlier either separately or in combination. WHAT WE CLAIM IS:

1. A process for the anti-skid control of the brake pressure in brake circuits of a vehicle, including measuring the' rotational speeds of at least two wheels or rotating parts coupled to two wheels of a vehicle, establishing reference speeds related to the rotational speeds, and forming a common overall reference speed by selecting the maximum value of the reference speeds, which overall reference speed is used to detect slipping of a wheel and signals representing such slipping and excessive acceleration or retardation of a wheel are used to regulate a brake pressure control operation so as to reduce skidding, wherein if no brake pressure control operation is taking place, the reference speed or the wheel speed of the driven wheel or axle is temporarily disconnected from, or reduced before being used in, the generation of the overall reference speed or the reference speed or the wheel speed of the driven wheel or axle is replaced by the wheel speed of the undriven wheel, the reference speed or the wheel speed of the driven wheel or axle being reconnected or restored to its unreduced value upon the appearance of a signal indicating slipping of the driven wheel or axle and/or after a predetermined time following the disconnection or reduction.

9. A process according to claim 5 or 8,

wherein a store is set to effect the disconnection of the test circuit, and the store is reset to effect the reconnection of the test circuit.

10. A process according to claim 9, wherein on setting the store the time basis for the periodic generation of the test signals of the test circuit is reset.

11. A process according to claim 1, wherein the reference speed or the wheel speed of the driven wheel or axle is reduced by being divided by a fixed number.

12. A process according to claim 11, wherein half of the reference speed is the reduced speed.

13. A process according to claim 11 or 12, wherein on the appearance of a signal representing deceleration of the driven wheel or axle, a more rapid adjustment of the reference speed is brought into effect.

14. A circuit arrangement for the antiskid control of brake pressure of a vehicle including rotation sensors allocated to wheels or rotating parts coupled to wheels of a vehicle, first circuits responsive to signals from the sensors for ascertaining the speeds corresponding to the sensor signals and producing speed signals, reference speed circuits for deriving from the speed signals reference speed signals allocated to the wheels or parts, acceleration, deceleration and slip threshold stages connected to receive the speed signals and produce corresponding behaviour signals for the wheels or parts, a maximum selecting circuit connected to receive the reference speed signals and derive therefrom an overall reference speed, wherein there are provided sensors for at least one driven wheel and one undriven wheel, and a logic circuit responsive to speed signals and behaviour signals from the threshold stages for generating an output signal in the presence of a signal indicating that no brake pressure control operation is taking place, which output signal is applied to set a switch means connected to receive the wheel speed or reference speed signal of the driven wheel to interrupt or reduce the reference speed for the driven wheel which is applied to the maximum selecting circuit or to substitute the wheel speed signal for the undriven wheel for that of the driven wheel, the output signal of the logic circuit being terminated and the switch means reset on the occurrence of a slip signal from the driven wheel or after a predetermined time following the setting of the switch means.

15. A circuit arrangement according to claim 14 wherein the logic circuit also requires a slip signal for the undriven wheel or an acceleration signal for the driven wheel in order to generate the output signal.

16. A circuit arrangement according to claim 15 wherein the logic circuit also requires a non-zero speed signal for the undriven wheel in order to generate the output signal.

17. A circuit arrangement according to claim 16, wherein the logic circuit includes an AND-gate which is arranged to block the slip signal for the undriven wheel when a speed signal for the undriven wheel is not present.

18. A circuit arrangement according to claim 16 or 17 wherein the logic circuit includes a second AND-gate having two non-inverted inputs and one inverted input, the speed signal for the undriven wheel being applied to one of the non-inverted inputs and the slip signal of the undriven wheel and the acceleration signal stage of the driven wheel being applied to the other of the non-inverted inputs, and a signal indicating the start of a brake control process being applied to the inverted input, the output of the second AND-gate being connected to the SET-input of a binary store, the RESET-input of which is connected to receive the slip stage signal for the driven wheel and the output of which is connected to the switch means.

19. A circuit arrangement according to claim 14 including a divider to which the reference signal for the driven wheel is applied, the logic circuit being connected to receive as input the reference signal for the driven wheel and the output of the divider and produce as output signal one or other of the inputs under the control of a flip-flop that may be set by the slip signal of the driven wheel and reset by the inverse of a signal indicating that a braking or a brake controlling operation is taking place, the output signal of the logic circuit being applied to the maximum selecting circuit.

20. A circuit arrangement according to claim 19, wherein the logic circuit has a first AND-gate, one input of which is connected to the reference speed circuit of the driven wheel and the other input of which is connected to the output of the flip-flop, a second AND-gate the one input of which is connected to the output of the divider and the other input of which is an inverted input and is connected to the output of the flip-flop, and an OR-gate to which the outputs. of the AND-gates are applied.

21. A process for the anti-skid control of brake pressure substantially as described herein with reference to Figure 1 in conjunction with Figure 2, 3 or 4, or to Figure 5 of the accompanying drawings, or to those Figures modified as herein described.

22. A circuit arrangement for the antiskid control of brake pressure substantially as described herein with reference to Figure 1 in conjunction with Figure 2, 3 or 4, or to Figure 5 of the accompanying drawings, or to those Figures modified as herein de scribed.