GB1604007A - Anti-skid vehicle brake control system having a safety circuit arrangement for disabling the system in the event of a breakdown of the system - Google Patents

Description

(54) AN ANTI-SKID VEHICLE BRAKE CONTROL SYSTEM HAVING A SAFETY CIRCUIT ARRANGEMENT FOR DISABLING THE SYSTEM IN THE EVENT OF A BREAKDOWN OF THE SYSTEM (71) We, WABCO FAHRZEUGBREMSEN Gmb.B.0200H., formerly Wabco Westinghouse G.m.b.H., a Company organised according to the laws of the Federal Repub lic 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 an anti-skid vehicle brake control system having a safety circuit arrangement for disabling the system in the event of a breakdown of the system.

The anti-skid control system includes at least two sensors for scanning the rotating behaviour of at least two of the vehicle wheels, a circuit for deriving from each sensor a signal representing the rotational speed of the respective wheel, threshold circuits for producing retardation-, acceleration- and slipcontrol signals from the wheel speeds, a circuit for producing a reference speed signal and a logic circuit evaluating the control signals for energising solenoid operated brake valves to regulate the brake pressure in such a way as to reduce skidding. One such brake control system is described in the specification of British Patent No. 1,550,453.

In the event of a breakdown of an antiskid vehicle brake control system the wheels of one axle may lock, although the brakes on the wheels of another axle are correctly regulated, which can lead to directional instability of the vehicle. For example, in socalled diagonal-individual-regulating system (DIR-systems), which consist of only two brake pressure regulating circuits arranged diagonally, in which a sensor is disposed at the front axle on the right wheel and at the rear axle on the left wheel, and in which the other wheel or wheels are respectively controlled by the monitored wheel on the same axle, there is the danger, arising from a particular breakdown of the system that the front wheels may be correctly braked and the wheels of the rear axle may be locked, resulting in the vehicle exhibiting an unstable driving behaviour or even spinning.

It has been proposed to monitor the operating time of the brake valve solenoids in order to detect such malfunctions, which of course has the disadvantage that a relatively long time passes before the malfunction is detected, so that the onset of unstable driving behaviour cannot be prevented.

It has also been proposed to provide a selective disconnection, by which, although a slight reduction of the maximum operating time of the solenoids is achieved, the minimum time for detection of a malfunction is determined by the total slip duration, which means that the time taken to switch off in the event of a malfunction is still so long that directional instability cannot be avoided.

It is therefore an object of the present invention to avoid the above difficulty by detecting and responding to a malfunction when it occurs so quickly that directional instability of the vehicle is safely avoided.

According to the present invention there is provided an anti-skid vehicle brake control system having a safety circuit arrangement for disabling the anti-skid vehicle brake control system in the event of a breakdown of the system, in which the brake control system includes at least two sensors respectively responsive to the rotational behaviour of at least two wheels of the vehicle, means for deriving from each sensor a signal representing the speed of rotation of the respective wheel, a circuit for deriving a wheel reference speed signal, threshold circuits for each wheel responsive to the wheel speed signal and the reference speed signal to produce retardation, acceleration and slip control signals respectively, when the particular wheel speed displays a retardation greater than a threshold value, when the particular wheel speed display an acceleration greater than a threshold valve, and when the particular wheel speed is smaller than the reference speed by more than a threshold value, and a logic circuit responsive to the control signals to produce output control signals for energising one or more solenoid brake control valves to regulate the brake pressure, wherein for each sensor there are provided a further threshold cicuit in which the wheel speed signal is compared with a predetermined threshold value and produces an output signal when the wheel speed signal falls below the threshold value, gating means connected to receive the output of the further threshold circuit and the slip control signal for the particular wheel, the gating means producing an output when both signals are present and means responsive to an output from the gating means persisting for more than a predetermined period of time to disable the brake control system.

The principle of the invention resides in detecting whether in the presence of a slip control signal the wheel speed remains below a particular threshold for longer than a predetermined time, and if so then switching off or otherwise disabling the brake control unit immediately. The predetermined time should be so chosen that even in the case of large changes in the coefficient of friction between the wheels of the vehicle and the road, the locking time of a wheel being braked remains below this time.

By means of the invention, the time taken from the occurrence of a malfunction resulting in locking a wheel or the wheels of one axle to the detection of a malfunction and the disabling of the brake pressure regulating unit, is shortened considerably. As a result of the use of the invention the situation in which the wheels of one axle lock whilst those of another axle are correctly braked is prevented from arising, and consequently the directional stability of the vehicle is retained in the case of a malfunction which otherwise could result in the loss of that stability. The switching off provided by the invention is also effective in the case of malfunctions in the solenoid valve, in the cabling and in the case of defective final amplifiers for the valves.

The invention may be limited in its effectiveness in certain anti-skid brake control systems to the speed of the vehicle being above a particular lower speed, for example approximately 16 km/h because no slip control signal can be generated at such low speeds. Although the circumstance may arise that the rear wheels lock and the front wheels are correctly braked in such units, this is not critical since at such low speeds little danger can arise from directional instability of the vehicle. This even gives the advantage that disabling of the anti-skid control system in error in the lower speed range is avoided, since in such anti-skid systems one wheel or one axle may lock from speeds of 11 km/h without there in fact being any malfunction.

In order that the invention may be fully understood and readily carried into effect it will now be explained in detail with reference to the single figure of the accompanying drawing which shows by way of example a block circuit diagram of a circuit arrangement for use in a system according to the invention.

In the figure a sensor 2 is provided to scan the rotating behaviour of a vehicle wheel (not shown). In known method and manner a signal representing the speed of the wheel, a signal representing a wheel reference speed and a slip control signal are produced respectively by circuits 4, 6 and 8 from the output of the sensor 2. The slip control signals are applied to a logic circuit 10, which also receives retardation- and acceleration-control signals (for the purpose of simplification of the drawing the stages which produce the latter signals from the sensor output and the inputs to the circuit 10 are not shown). The retardation, acceleration and slip signals are usually produced by respective threshold circuits in an anti-skid vehicle brake control unit. The logic circuit 10 is connected to control a solenoid brake valve 12.The antiskid vehicle brake control unit is shown as including a safety circuit 14 for modifying the operation of the brake control unit or disabling it in the event of a breakdown. The safety circuit will not be discussed in detail, but may, for example, be as described in the specification of co-pending Patent Application No. 720/78. Serial No. 1596191.

As so far described the circuit corresponds to a customary design of anti-skid brake control unit. The circuit shows in addition, however, a threshold circuit 16, which is connected to receive from the circuit 4 the wheel speed signal and in which the wheel speed is compared with a predetermined threshold value, for example 4 km/h, the circuit 16 producing a high output when the wheel speed is above the threshold value.

The output of the threshold circuit 16 is combined with the output of the circuit 8, which produces the slip control signals, in an AND-gate 18, the input of the AND-gate to which the threshold circuit 16 is connected being negated.

A delayed response timing element 20 with a predetermined switching delay is connected to the output of the AND-gate 18.

The switching delay may be, for example, 300 msec. This time is a typical value since the time chosen must be such that even in the case of large changes in the value of the coefficient of friction between the wheels and the road the locking time for a wheel would be less than delay time of the element 20.

The output of the element 20 is combined with the output of the safety circuit 14 in an OR-gate 22, by means of which a malfunction indicator 24 can be energised and a switch (not shown) for switching off the brake pressure control unit can be actuated.

The circuit arrangement shown in the drawing operates as follows: The actual wheel speed signal is continuously compared in the threshold circuit 16 with the predetermined threshold, which may be, for example, 4 km/h. If the signal value falls below the threshold, the circuit 16 produces a low output. If, at the same time, a slip control signal appears from the circuit 8, the ANDcondition is fulfilled at the AND gate 18 on account of the negated input for the connection from the circuit 16 so that the timing element 20 is energised and, after the expiry of the predetermined delay period, which as mentioned above may be, for example, 300 msec, it emits a signal if both the low output signal of the circuit 16 and the slip control signal continue to be applied to the ANDgate 18 for a period longer than the delay time.The output signal of the timing element then actuates the malfunction indicator 24 and causes the brake pressure control unit to be switched off.

WHAT WE CLAIM IS: 1. An anti-skid vehicle brake control system having a safety circuit arrangement for disabling the anti-skid vehicle brake control system in the event of breakdown of the system, in which the brake control system includes at least two sensors respectively responsive to the rotational behaviour of at least two wheels of the vehicle, means for deriving from each sensor a signal representing the speed of rotation of the respective wheel, a circuit for deriving a wheel reference speed signal, threshold circuits for each wheel responsive to the wheel speed signal and the reference speed signal to produce retardation, acceleration and slip control signals respectively, when the particular wheel speed displays a retardation greater than a threshold value, when the particular wheel speed displays an acceleration greater than a threshold value, and when the particular wheel speed is smaller than the reference speed by more than a threshold value, and a logic circuit responsive to the control signals to produce output control signals for energising one or more solenoid brake control valves to regulate the brake pressure, wherein for each sensor there are provided a further threshold circuit in which the wheel speed signal is compared with a predetermined threshold value and produces an output signal when the wheel speed signal falls below the threshold value, gating means connected to receive the output of the further threshold circuit and the slip control signal for the particular wheel, the gating means producing an output when both signals are present and means responsive to an output from the gating means persisting for more than a predetermined period of time to disable the brake control system.

2. A system according to claim 1 wherein the gating means is an AND-gate having a first input to which the slip control signal is applied and a second, negated, input to which the output of the further threshold circuit is applied, the further threshold circuit being such as to produce a low output signal when the actual wheel speed is below the predetermined threshold value.

3. A system according to claim 1 or 2 wherein the means reponsive to an output of the gating means includes a delayed response timing element the output of which is connected to actuate a switch for switching off the brake control system.

4. A system according to claim 1, 2 or 3, wherein the predetermined period of time is of the order of 300 ms.

5. An anti-skid vehicle brake control system having a safety circuit arrangement for disabling the system in the event of a breakdown of the system, the arrangement being substantially as described herein with reference to the single figure of the accompanying drawing.

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

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. The output of the element 20 is combined with the output of the safety circuit 14 in an OR-gate 22, by means of which a malfunction indicator 24 can be energised and a switch (not shown) for switching off the brake pressure control unit can be actuated. The circuit arrangement shown in the drawing operates as follows: The actual wheel speed signal is continuously compared in the threshold circuit 16 with the predetermined threshold, which may be, for example, 4 km/h. If the signal value falls below the threshold, the circuit 16 produces a low output. If, at the same time, a slip control signal appears from the circuit 8, the ANDcondition is fulfilled at the AND gate 18 on account of the negated input for the connection from the circuit 16 so that the timing element 20 is energised and, after the expiry of the predetermined delay period, which as mentioned above may be, for example, 300 msec, it emits a signal if both the low output signal of the circuit 16 and the slip control signal continue to be applied to the ANDgate 18 for a period longer than the delay time.The output signal of the timing element then actuates the malfunction indicator 24 and causes the brake pressure control unit to be switched off. WHAT WE CLAIM IS:

1. An anti-skid vehicle brake control system having a safety circuit arrangement for disabling the anti-skid vehicle brake control system in the event of breakdown of the system, in which the brake control system includes at least two sensors respectively responsive to the rotational behaviour of at least two wheels of the vehicle, means for deriving from each sensor a signal representing the speed of rotation of the respective wheel, a circuit for deriving a wheel reference speed signal, threshold circuits for each wheel responsive to the wheel speed signal and the reference speed signal to produce retardation, acceleration and slip control signals respectively, when the particular wheel speed displays a retardation greater than a threshold value, when the particular wheel speed displays an acceleration greater than a threshold value, and when the particular wheel speed is smaller than the reference speed by more than a threshold value, and a logic circuit responsive to the control signals to produce output control signals for energising one or more solenoid brake control valves to regulate the brake pressure, wherein for each sensor there are provided a further threshold circuit in which the wheel speed signal is compared with a predetermined threshold value and produces an output signal when the wheel speed signal falls below the threshold value, gating means connected to receive the output of the further threshold circuit and the slip control signal for the particular wheel, the gating means producing an output when both signals are present and means responsive to an output from the gating means persisting for more than a predetermined period of time to disable the brake control system.

2. A system according to claim 1 wherein the gating means is an AND-gate having a first input to which the slip control signal is applied and a second, negated, input to which the output of the further threshold circuit is applied, the further threshold circuit being such as to produce a low output signal when the actual wheel speed is below the predetermined threshold value.

3. A system according to claim 1 or 2 wherein the means reponsive to an output of the gating means includes a delayed response timing element the output of which is connected to actuate a switch for switching off the brake control system.

4. A system according to claim 1, 2 or 3, wherein the predetermined period of time is of the order of 300 ms.

5. An anti-skid vehicle brake control system having a safety circuit arrangement for disabling the system in the event of a breakdown of the system, the arrangement being substantially as described herein with reference to the single figure of the accompanying drawing.