Anti-lock brake system and drive-slip control system
This application is a continuation of application Ser. No. 07/842,143, filed Mar. 23, 1992, now abandoned. 
    
    
     STATE OF THE ART 
     An anti-lock brake system is known from the German Offenlegungsschrift 37 07 235 in which a reference speed curve is derived from the wheel speed signals, where the slope of the speed curve is a measure of a vehicle deceleration. The anti-lock brake system is installed in a vehicle in which all-wheel drive is used for at least part of the time. In the instability phases on low friction coefficients, the signal of a wheel deceleration signal generator supports the slope of the reference speed. 
     ADVANTAGES OF THE INVENTION 
     On flat road surfaces, vehicle deceleration calculated from wheel speeds agrees with the output signal of an acceleration sensor during an anti-lock brake system braking operation. Short-term perturbations of the acceleration sensor output signal--such as vehicle pitch motion--can be removed by appropriate filter design. If the measured and the calculated vehicle deceleration differ, the braking operation is taking place on an uphill or a downhill slope. The acceleration sensors used in the anti-lock brake system are based on the spring/mass principle, and therefore, measure a resultant of the vehicle acceleration and the vehicle attitude relative to the gravitational acceleration. If the vehicle is standing on a hill having, for example, a slope of 20%, the acceleration sensor indicates a measured value A 1  =0.19 g, where A 1  =g . sin arctan P/100 and P=slope in %. 
     In contrast to the normal anti-lock brake system arrangement, it is wise--particularly in the case of a large roadway slope condition--to give braking priority over driving stability. This applies even more as the calculated vehicle deceleration becomes less than the measured vehicle deceleration. A roadway slope can be recognized by an analysis of a difference between the calculated and measured deceleration and anti-lock brake system control strategies can be accordingly adapted. 
     If A 1  is the magnitude of the measured deceleration and A 2  is the magnitude of the deceleration calculated from the wheel speeds, the following relationship is formed: 
     
         ΔA=|A.sub.1 |-A.sub.2 
    
     A roadway slope condition is recognised if A 1 , without brake actuation or ΔA in the case of brake actuation, becomes greater than a threshold value. The controller specifications, or parameters, are then switched, in or adjusted, an insensitive direction to improve the braking. Slip thresholds or deceleration thresholds can, for example, be set at a high level, but also the thresholds can be set, or a combination, of values. It is also, however, possible to include ΔA in an analog manner in the calculation of instability or slip equations, more slip being permitted for a larger ΔA. 
     The teaching of the invention is also applicable to anti-slip control. The braking or traction can be improved by switching over an anti-lock brake system/anti-slip control system control strategy in the situation of large roadway downhill and uphill slopes, particularly in the case of a soft surface. This is particularly important in the case of all-terrain vehicles. Up to now, a switch operated by the driver has been provided for these vehicles. By way of the A sensor, the system is switched over automatically. For the above vehicles, an A sensor is usually necessary in any case because of the all-wheel concept. 
    
    
     The invention is explained in more detail by way of the illustrative example shown in the drawing. 
     In the drawing, four wheel speed measured value signal generators are indicated by 1 to 4. Their signals are supplied to an anti-lock brake system analysis circuit 5 which varies the pressure at the wheel brakes by way of brake pressure control valves 6-8 when there is a tendency to lock. 
     The reference speed V Ref  necessary for the slip calculation is formed in a part 5a of the analysis circuit. The slope of the reference speed curve is a measure of the vehicle deceleration. The part 5a supplies a corresponding vehicle deceleration signal to a comparator 9 to which is also supplied the signal of a vehicle decelerometer 10. In a first embodiment, the comparator 9 emits a signal when ΔA=|A 1  |-A 2  exceeds a certain threshold. This signal then, for example, switches the slip and deceleration thresholds in the analysis circuit 5 to higher values. It is, however, also possible for the comparator to supply an analog signal corresponding to the value ΔA to the analysis circuit; this signal is then included in the slip and acceleration determination, for example, and, with increasing magnitude, decreases slip and acceleration determination signals. This also makes the controller less sensitive. 
     In the anti-slip control case, the reference speed is obtained by averaging the speeds of the non-driven wheels, for example. In this case, the vehicle acceleration measured in 10 is then compared with the slope of the reference speed determined in 5a and the control system is again made less sensitive in the case of a ΔA located above the threshold. In this case, an actuator 11 for changing the engine torque can be additionally included in the control system. 
     Another indication of cross-country travel is provided by the large wheel speed changes initiated by roadway irregularities. The recognition of a so-called poor roadway is within the state of the art. In the presence of a poor roadway, the influence of ΔA on the controller is increased.