Abstract:
An anti-skid brake control system is described which recognizes a driving condition in which it is not possible to sufficiently brake the vehicle. The braking effect can be improved by switching off the anti-skid brake control system for the front wheels and/or at least reducing the driving torque acting on the driven rear wheels.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an anti-skid brake control system for a motor vehicle having forward and rearward sets of vehicle wheels, at least one of which is driven. The anti-skid brake control system includes speed sensors associated with the vehicle wheels; an evaluation circuit responsive to the wheel speed sensors for producing brake pressure control signals; and brake pressure control means for controlling the brake pressure applied to the vehicle wheels in response to the control signals. 
     At a low friction coefficient, and this applies especially to a rear wheel driven vehicle, there is a situation in which, during the control of a pressure level corresponding to the locking limit of the front wheels, the momentary balance at the rear axle between braking torque and driving torque--especially at a high idle speed during cold operation--results in a positive driving torque. A consequence thereof is that the vehicle does not come to complete stop despite a locking of the front wheels. A stopping of the vehicle is only possible when the brake pressure is increased to a value which exceeds the motor driving torque. This requires in some cases significant brake pressures and hence high pedal forces. 
     This situation can be intensified with the combination of an anti-skid brake control system based on the feedback principle and a vacuum brake power booster. When the energy consumed by the pressure modulation is not sufficiently replaced and when the throttle is partially open, the level control point of the booster continuously decreases. In this case the foot pressure required for reaching a sufficient pressure level to surpass the driving torque increases to values which cannot be reached in borderline cases. 
     The above described problems can result in a situation in which a vehicle--especially with rear drive--cannot be stopped on a slippery road even when great pedal forces are applied unless the driver disengages the clutch or sets the shift lever of an automatic transmission to &#34;neutral&#34;. However, especially in case of an automatic transmission, such a reaction cannot be expected by a driver in a &#34;stress situation&#34;. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to configure an anti-skid brake control system such that the above described problems are avoided. 
     This object, as well as other objects which will become apparent from the description that follows, are achieved, in accordance with the present invention, by providing the anti-skid brake control system with a device for determining whether the speed decrease of a non-driven wheel falls below a prescribed small value within prescribed periods of time during activation of the brake pressure control, and a device responsive to the determining device for effecting at least one of the following: 
     (1) disabling the anti-skid brake control function of at least the non-driven wheel set, and 
     (2) reducing the driving torque on the driven wheel set. 
     For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention and to the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING The only figure illustrates an embodiment of an anti-skid brake control system in accordance with the invention represented as a block diagram. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The anti-skid brake control system includes shown in the figure four speed sensors 1 to 4 for the wheel speeds which are assigned to the front wheels (1 and 2) and the driven rear wheels (3 and 4). The signals thereof are supplied to an evaluation circuit as it is commonly known. From the supplied speed sensor signals this circuit generates brake pressure control signals for the brake pressure control devices 6 to 9, represented as solenoid valves, so as to vary the brake pressure at the front wheels (valves 6 and 7) and the rear wheels (valves 8 and 9). 
     The evaluation circuit 5 includes a block 5a here the reference speed V Ref  (a speed curve approximated to the vehicle speed) required for the slippage control is generated from the wheel speed signals as commonly known. This reference speed V Ref  is brought out from the evaluation circuit 5 and supplied to a comparator 10 to which is also supplied a comparison value (e.g. 25 km/h) through a terminal 10a. The comparator 10 passes signals which correspond to speed values &lt;25 km/h and supplies these signals to a circuit element 11. This circuit receives pulses at intervals T from a pulse generator 12 and checks at each interval T whether the speed decrease Δ v is less than a prescribed vehicle deceleration of, for example, 0.05 g. 
     If this is the case it releases a control signal. However, this signal can only fulfill its control function if it is passed through an AND-gate 13. This is the case if an AND-gate 14 produces an output which, in turn, is the case when it receives an input via line 14a indicating that the control portion which is associated with the non-driven front wheels is activated and when it receives a &#34;0&#34; input via line 14b indicating that the control portion associated with rear wheels is not in operation. 
     The control signal thus reaches the circuit point 15 when the reference speed V Ref  is under 25 km/h, when it decreases by only a relatively small value and only when at least one of the non-driven front wheels is controlled by the anti-skid brake control system and neither of the rear wheels is controlled. 
     In this case either 
     1. the anti-skid function of the non-driven axle, so long as it is the front axle, is switched off via AND-gate 16; 
     2. and/or the fuel cut-off (block 17) is activated while by-passing the motor speed dependence; 3. and/or the engine ignition and/or the fuel injection is interrupted (blocks 18, 19); 
     4. and/or the ignition timing is retarded by the highest possible value (block 20); 
     5. and/or in the case of a conventional clutch and gear shift the motor is automatically decoupled by an electrically/hydraulically actuated clutch (block 21); 
     6. and/or an automatic transmission is electrically or hydraulically set to &#34;neutral&#34; or electrically or hydraulically set to the highest possible gear (block 22) 
     These measures, once taken, remain effective until the vehicle is stopped or the brake is released. 
     The advantage of the invention is the monitoring of a critical driving condition during which the vehicle cannot be stopped by means of the pedal force applied and during which a high amount of energy is consumed by the brake power booster. 
     There has thus been shown and described a novel anti-skid brake control system which fulfills all the objects and advantages sought therefor. Many changes, modifications, variations and other uses and applications of the subject invention will, however, become apparent to those skilled in the art after considering this specification and the accompanying drawing which disclose the preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.