GB1592932A

GB1592932A - Anti-skid brake control systems

Info

Publication number: GB1592932A
Application number: GB582578A
Authority: GB
Original assignee: Wabco Fahrzeugbremsen GmbH
Current assignee: Wabco Fahrzeugbremsen GmbH
Priority date: 1977-02-14
Filing date: 1978-02-14
Publication date: 1981-07-15
Also published as: DE2706132A1; FR2386433A1; JPS6337745B2; YU39835B; JPS53100382A; SU1032996A3; FR2386433B1; DE2706132C2; YU32678A

Description

(54) IMPROVEMENTS IN OR RELATING TO ANTI-SKID BRAKE CONTROL SYSTEMS (71) We, WABCO FAHRZEUG BREMSEN G.m.b.H, formerly Wabco Westinghouse G.m.b.H., 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 anti-skid brake control systems.

In the control of the brake pressure in an anti-skid braking system it has been proposed to derive a vehicle reference speed by obtaining a speed for each front wheel and to select from the two speeds of the front wheels the maximum value as the reference speed for the entire vehicle. In this known process for the derivation of the vehicle reference speed, the speeds of the front wheels are used to form the reference speed because the speeds of the front wheels cannot exceed the speed of the vehicle, except for brief and negligible periods, the vehicle in question being driven by its rear wheels. The speeds of the rear wheels are not emplqyed in forming the reference value because the rear wheels may spin when accelerating and thus push up the reference speed to a speed above that of the vehicle.

In the case of a diagonal (one front and one rear wheel) or rear axle anti-skid control system, the problem of using the speed of a rear (driven) wheel in the derivation of the vehicle reference speed arises again. Since it is desirable that the reference speed be formed from at least two wheel speeds, a rear wheel speed therefore has to be used that would produce an incorrect and possibly dangerous reference value when the rear wheel spins. Consider, for example, a vehicle that is being driven up a slippery slope, for example one that is snow-covered or icy; the driven wheels of the vehicle spin and it starts to slip downwards.As a reaction, the driver will step on the brake pedal but will find that the vehicle anti-skid system prevents the brake pressure rising from zero because the reference speed has been pushed up as a result of the driven wheels slipping when attempting to drive forwards and produces a slip signal at all the other wheels, and for some time afterwards the anti-skid system prevents the brake pressure in all the wheel (brake) cylinders increasing.

It is an object of the present invention to avoid the above difficulty.

According to one aspect of the present invention there is provided a method of controlling brake pressure in an anti-skid brake control system in which the rotational speeds of at least one driven and at least one undriven wheel of a vehicle are used to provide a reference speed value for detecting the onset of skidding, wherein the maximum of the two wheel speeds is taken as the reference speed value only when the brakes are controlled, and when the brakes are not subject to control the reference speed value is derived from the wheel speeds in such a way as not to be falsified by slipping of a driven wheel due to excessive driving torque.

According to a second aspect of the present invention there is provided an antiskid brake control system including means for measuring the rotational speeds of at least one driven and at least one undriven wheel of a vehicle and producing signals representing the wheel speeds and means for driving a reference speed value from the signals representing the wheel speeds, wherein the means for deriving a reference speed value includes at least one maximum selecting circuit to which the signals representing the wheels speeds are connected and from which the reference speed value is obtained and a switching device connected to prevent an output being obtained from the maximum selecting circuit in response to switching signals indicating that the system is not controlling brake pressure, the deriving means being such that when an output cannot be obtained from the maximum selecting circuit the reference speed value is derived in such a way that it would not be falsified by slipping of a driven wheel due to excessive driving torque.

In one embodiment of the invention the minimum value is selected as the reference value when the brakes are not subject to anti-skid control and the maximum value is selected as the reference value when the brakes are under anti-skid control. In other words, during unbraked driving the reference speed is formed from the wheel having the lowest speed and only after the appearance of switching criteria is the maximum value formation necessary for a brake control process initiated. A switching criterion may, for example, be a store signal which is set when a slip signal appears for the first time, and which is reset when a signal indicating the end of the brake control process is released.

In another embodiment the reference speed circuits of the driven wheels are used for the formation of the overall reference speed only when a brake control process is in progress for all wheels or for a wheel that is not driven.

By virtue of the present invention, the pushing of the reference speed by spinning driven wheels and the resulting disabling of the brake system by the anti-skid brake control system may be reliably avoided.

Moreover, the invention may be embodied in a simple design of anti-skid control system.

In order that the invention may be fully understood and readily carried into effect, it will be explained in more detail with reference to the accompanying drawings in which the embodiments of the invention are illustrated. In the drawings: Figure 1 shows a block circuit diagram of one example of a device, according to the invention in which two wheel speeds are used to form a minimum and a maximum value which are selectively used to form the overall reference speed; Figure 2 shows graphically the operation of the circuit of Figure 1; Figure 3 shows graphically the operation of the circuit of Figure 1 during a vain attempt to drive on the lowest frictional value of a spinning rear wheel; Figure 4 shows a block circuit diagram of another embodiment of the invention for the formation of the overall reference speed; and Figure 5 shows graphically the operation of the circuit of Figure 4.

Reference will now be made to Figure 1.

Reference circuits 1 and 2 derive vehicle speeds from wheel speed values conveyed via lines 4 and 6, wherein it is assumed for the following explanation that circuit 1 is allocated to a front wheel, for example to the right-hand wheel and circuit 2 is allocated to a rear wheel, for example to the left-hand rear wheel of a diagonal control system.

The output signals of the reference circuits 1 and 2 are fed both to minimising (minimum selecting) and maximising (maximum selecting) circuits 8 and 10, respectively, in which the minimum and the maximum value of the vehicle speeds is determined. The outputs of the minimising and maximising circuits are connected via a changeover switch 12 to the input of a reference speed value circuit 14 that derives an overall reference speed for all control channels of the remainder of an anti-skid brake control system (not shown) from the speed values fed to it.

The changeover switch 12 is shown as a relay actuated by switching signals from a store 16. Control signals AVR and AHL from slip threshold stages (not shown) associated with the right-hand front wheel (VR) and the left-hand rear wheel (HL) are fed via an OR-gate 18 are applied to the set inputs of the store 16. A signal indicating the end of a brake control process is fed to the reset input R of the store 16.

When a brake control process has terminated, the store 16 is reset via the rest input and at the same time the changeover switch 12 is switched back to the position shown to connect the circuit 8 to the circuit 14. The minimum value formed in the minimising circuit 8 from the speeds derived by circuits 1 and 2 then forms in this case the reference speed value for the vehicle. In this position, a spinning driven wheel cannot influence the reference speed value.

If a braking action is initiated and a A- signal appears from either the right-hand front wheel as AVR or the left-hand rear wheel as AHL during the braking, then this signal sets the store 16 via the OR-gate 18, and the store 16 delivers a switching signal which throws the changeover switch 12 immediately to connect the circuit 10 to the circuit 14. In this case the maximum value formed in the maximising circuit 10 from the two reference speeds is taken as the reference speed value for the vehicle as is usual in anti-skid control systems.

When the control process comes to an end the store 16 is reset again and the changeover switch 12 is moved to position shown in Figure 1.

The functioning of the circuit shown in Figure 1 at the start of a braking action will be explained in more detail with reference to Figure 2. The diagram shown in Figure 2 shows in particular the behaviour of the circuit during the initial braking phase when a brake control process is in progress.

It is assumed that the driver of the vehicle actuates the motor vehicle pedal brake valve for full braking at time t,. To explain the processes which now follow it will be assumed that one wheel (Vl) starts to slip earlier than the other wheel (V2). After a deceleration signal (-b signal) has appeared, the vehicle speed value VRef l is produced. the vehicle speed VRef 2 is at this time at the same level as the speed V2. As a changeover has not yet occurred, the lower speed of the two speeds VRef, and VRef 2 is selected as the reference speed value. As one or more wheels start to skid, the slip threshold Al is exceeded by the speed V, at a time t2.The appearance of a signal leads to the immediate switching over to the circuit 10 for maximum value formation. From this moment onwards to the end of the brake control process the reference speed value is obtained from the greater of the two vehicle speeds.

Referring to Figure 3, the function of the circuit shown in Figure 1 will be explained during a vain attempt to drive off on a surface of very low coeffient of friction in which a (driven) rear wheel skids under the influence of excessive drive torque. Let the speed of the spinning rear wheel be denoted by V2. The speed VRe, 2 is pushed up by the speed V2 of the spinning rear wheel, and after the futile attempt to start the vehicle moving the speed VRef 2 starts to form in the circuit 2. However, as the switch 12 is switched to select the minimum value the reference speed value is zero, corresponding to the speed of the front wheel, i.e. no A-signal can be produced and switching over to the circuit for selecting the maximum value is prevented.In this case, therefore, the brake pressure of the vehicle cannot be held down by the antiskid control circuit; it is only the wheel brake cylinder of the spinning wheel that would be without pressure for the duration of the deceleration signal as it slows down when the driving torque is reduced.

Figure 4 shows a block circuit diagram of another arrangement according to the invention for diagonal control to prevent the reference speed value being pushed up when the rear wheels spin.

A sensor 20 is coupled to the right front wheel and a sensor 22 is coupled to the left rear wheel. The sensor signals are fed to switching circuits 24 and 26 for ascertaining the corresponding speeds from which vehicle speeds are obtained by reference circuits 28 and 30. From the vehicle speeds there is formed in turn in a maximising stage 32 a reference speed value which is used by all control channels (not shown) of an anti-skid braking system.

Connected to the output of the rear wheel circuit 26 there is a switch 34 which is actuated by the output signals of a store 36 which may be set and reset. The store 36 is set by signals AVVR which are obtained when the anti-lock system commences a controlled braking action. These signals AVVR may, for example, be obtained from the energising signal for the outlet valve of the undriven wheel, for example the front wheel, or may be obtained as a result of a slip threshold of the undriven or driven wheel being exceeded or may be obtained via pressure switches or via brake light switches. When the store is set the switch 34 is switched on and the maximising stage 32 is able to select the maximum value from the two vehicle speeds derived from a front and a rear wheel as the required reference speed value.

The resetting of the store 36 is carried out after termination of the control process, for which purpose, for example, a signal derived from the switching off of the outlet valve (time-delayed switch-off signal (signal AV delayed)), or signals from the release of the pressure operated brake light switch or the termination of the acceleration signal may be used. When the store 36 is reset the switch 34 is switched back again.

The circuit shown in Figure 4 thus has the effect that the vehicle speed derived from the rear wheel is not formed during unbraked travel, as a result of which it is not possible for the reference speed value to be pushed up because the rear wheels are spinning.

The circuit shown in Figure 4 can also be used for rear axle brake control, when two vehicle speeds are derived respectively from the rear wheels and are not switched to the maximising circuit until a braking action occurs, for example in response to a signal taken from a brake light switch. For this purpose a further switch similar to the switch 34 and actuated by the store 36 would be inserted in the line between the circuits 24 and 28.

With reference to the graph shown in Figure 5 the manner in which the circuit shown in Figure 4 functions will be described in greater detail using a practical example, it being assumed that the vehicle is being driven on a slippery road. If the driver suddenly accelerates the vehicle, then the driven wheels, in this example the rear wheels, slip due to excessive torque, and the speed HL measured at the left rear wheel for example, excess the speed VVR measured, for example,, at the right front wheel, see curve a drawn with a broken line in Figure 5, the downwardly sloping portion of this curve occurring as a result of deceleration following closure of the throttle.

In these circumstances the reference speed value VRef still corresponds to the speed VVR of the front wheel, as the changeover switch 34 (Figure 4) is still open.

At the time t1, a braking process is initiated, the corresponding speeds (VHL VVR and VRef) start to fall and the brake force control process is started. The reference speed value is derived only from the speed of the front wheel at this time, and the rear wheels are co-controlled by the referenc espeed value derived from the speed of the front wheels. At the time t2, a deceleration signal, indicating incipient skidding, occurs for the first time which opens the brake cylinder outlet valve AV of the right front wheel.This signal sets the store 36 and the output signal of the store 36 causes the switch 34 to be actuated, as a result of which the reference circuit 30 is connected to the circuit 26 and produces an output which is applied to the maximising stage 32, and thus the speed of the left rear wheel is also used for the formation of the reference speed value and the reference speed value follows the continuing slope of the dot-dash line at b, until, at the end of the control process, the switch 34 is opened again by the AV delayed signal. This switching signal is in this example derived from the release of the outlet valve AV with a delay (signal AV delayed).

The method and apparatus according to the invention may also be used in individual wheel braking control systems in which at least one driven wheel is used for the formation of a reference speed value.

It will be appreciated that modifications can be made to the embodiments described without departing from the invention. In particular, the invention is applicable to anti-skid brake control systems for front wheel driven vehicles and therefore references to rear driven wheels in the description above should be taken as including front driven wheels, although, of course, references to diagonal control systems will still include one front and one rear wheel on opposite sides of the vehicle.

WHAT WE CLAIM IS: 1. A method of controlling brake pressure in an anti-skid brake control system in which the rotational speeds of at least one driven and at least one undriven wheel of a vehicle are used to provide a reference speed value for detecting the onset of skidding, wherein the maximum of the two wheel speeds is taken as the reference speed value only when the brakes are controlled, and when the brakes are not subject to control the reference speed value is derived from the wheel speeds in such a way as not to be falsified by slipping of a driven wheel due to excessive driving torque.

2. A method according to claim 1 wherein when the brakes are not subject to control the reference speed value is derived from the rotational speed of one or more undriven wheels only.

3. A method according to claim 1 wherein when the brakes are not subject to control the reference speed value is derived by taking the minimum of the wheel speeds.

4. A method according to any of claims 1, 2 and 3 wherein a signal indicating whether or not the brakes are controlled is derived from the speed or deceleration of a wheel, a control signal produced by the anti-skid brake control system for operating a brake pressure medium valve, or a brake light switch, the indicating signal being used to effect the changeover from one method to the other of deriving the reference signal value.

5. A method according to claim 4 wherein the indicating signal is terminated when the brake control process or the braking action comes to an end.

6. A method according to any preceding claim wherein the speeds of two wheels, one driven and one not driven, diagonally located on a vehicle are used to provide the reference speed value at least when the brakes are controlled.

7. A method of controlling the brake pressure in an anti-skid brake control system substantially as herein described with reference to Figures 1, 2 and 3 or Figures 4 and 5 of the accompanying drawings.

8. An anti-skid brake control system including means for measuring the rotational speeds of at least one driven and at least one undriven wheel of a vehicle and producing signals representing the wheel speeds and means for deriving a reference speed value from the signals representing the wheel speeds, wherein the means deriving a reference speed value includes at least one maximum selecting circuit to which the signals representing the wheels' speeds are connected and from which the reference speed value is obtained and a switching device connected to prevent an output being obtained from the maximum selecting circuit in response to switching signals indicating that the system is not controlling brake pressure, the deriving means being such that when an output cannot be obtained from the maximum selecting circuit the reference speed value

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. wheel, see curve a drawn with a broken line in Figure 5, the downwardly sloping portion of this curve occurring as a result of deceleration following closure of the throttle. In these circumstances the reference speed value VRef still corresponds to the speed VVR of the front wheel, as the changeover switch 34 (Figure 4) is still open. At the time t1, a braking process is initiated, the corresponding speeds (VHL VVR and VRef) start to fall and the brake force control process is started. The reference speed value is derived only from the speed of the front wheel at this time, and the rear wheels are co-controlled by the referenc espeed value derived from the speed of the front wheels. At the time t2, a deceleration signal, indicating incipient skidding, occurs for the first time which opens the brake cylinder outlet valve AV of the right front wheel.This signal sets the store 36 and the output signal of the store 36 causes the switch 34 to be actuated, as a result of which the reference circuit 30 is connected to the circuit 26 and produces an output which is applied to the maximising stage 32, and thus the speed of the left rear wheel is also used for the formation of the reference speed value and the reference speed value follows the continuing slope of the dot-dash line at b, until, at the end of the control process, the switch 34 is opened again by the AV delayed signal. This switching signal is in this example derived from the release of the outlet valve AV with a delay (signal AV delayed). The method and apparatus according to the invention may also be used in individual wheel braking control systems in which at least one driven wheel is used for the formation of a reference speed value. It will be appreciated that modifications can be made to the embodiments described without departing from the invention. In particular, the invention is applicable to anti-skid brake control systems for front wheel driven vehicles and therefore references to rear driven wheels in the description above should be taken as including front driven wheels, although, of course, references to diagonal control systems will still include one front and one rear wheel on opposite sides of the vehicle. WHAT WE CLAIM IS:

1. A method of controlling brake pressure in an anti-skid brake control system in which the rotational speeds of at least one driven and at least one undriven wheel of a vehicle are used to provide a reference speed value for detecting the onset of skidding, wherein the maximum of the two wheel speeds is taken as the reference speed value only when the brakes are controlled, and when the brakes are not subject to control the reference speed value is derived from the wheel speeds in such a way as not to be falsified by slipping of a driven wheel due to excessive driving torque.

is derived in such a way that it would not be falsified by slipping of a driven wheel due to excessive driving torque.

9. A system according to claim 8 wherein the switching device is arranged to disconnect from the maximum selecting circuit the or each signal representing the speed of a driven wheel in response to the switching signals so that the output of the maximum selecting circuit does not depend on the signal or signals representing the speed of a driven wheel, the reference speed value always being derived from the output of the maximum selecting circuit.

10. A system according to claim 8 wherein the means for deriving a reference speed value also includes at least one minimum selecting circuit to which the signals representing the wheels speeds are connected and the switching device is connected to obtain the reference speed value either from the output of the maximum selecting circuit or from the output of the minimum selecting circuit, so that the reference speed value is the minimum of the wheel speeds when the system is not controlling brake pressure.

11. A system according to claim 8,9 or 10 wherein the means for producing signals representing wheel speeds includes means for limiting the rate of fall of the signals.

12. A system according to any of claims 8 to 11 wherein the switching signals are derived from a wheel speed or deceleration threshold stage, a device such as a brake light switch indicating the start of a braking action, or a control signal for a valve regulating the brake pressure.

13. A system according to any of claims 8 to 12 including means for terminating the switching signals at the end of a brake application or control process.

14. A system according to any of claims 8 to 13 wherein the switching device includes a bistable store which is set to one or another state in response to the switching signals and a relay controlled by the output of the store.

15. An anti-skid brake control system substantially as herein described with reference to Figures 1, 2 and 3 or Figures 4 and 5 of the accompanying drawings.