Abstract:
Wheel slippage and wheel acceleration signals are used to build up and decrease brake pressure stepwise at the vehicle wheels. Combined signals are used to modify the rate of pressure build-up and decrease.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a drive slip control system where the deviations of the speeds of the driven wheels from the vehicle speed and the accelerations of the driven wheels are determined from the measured speeds of the driven and non-driven wheels. When deviation and acceleration thresholds are reached, signals are generated which are used to change the brake pressure at the corresponding wheels in stages. 
     In a drive slip control system, it is known (DE-A1 33 31 297) to determine the deviation of the wheel speed of the driven wheel from the vehicle speed and to generate a first threshold signal when a first threshold is exceeded. The wheel acceleration is then additionally determined, and a second threshold signal is generated when a second threshold is exceeded. These threshold values are then combined and used to change the brake pressure. The pressure can be changed in steps (pulsed). 
     ADVANTAGES OF THE INVENTION 
     The drive slip control system in accordance with the invention is based on this piece of prior art and optimizes the pressure variation. The result is a user-friendly control. 
     The brake pressure is built up at the corresponding wheel when the deviation signal is greater than a first value close to zero or zero and the corresponding acceleration signal is greater than a second value close to zero or zero. The brake pressure is reduced when the deviation is below a first value, when the deviation is equal to zero, or when the deviation is above a first value and the corresponding acceleration is less than a second value or zero. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an electronic unit and 
     FIG. 2 shows the appertaining hydraulic unit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, the speed sensors 1 and 2 for the driven wheels and the speed sensors 3 and 4 for the non-driven wheels are connected to a signal processor 5. The latter uses the supplied signals to form, for the individual driven wheels, the deviations RA R  and RA L  of the wheel speed from the vehicle speed and the signals DRA R  and DRA L  which correspond to the wheel acceleration. The signals RA R  and RA L  are supplied to lines 6 and 7, and the signals corresponding to the acceleration to lines 8 and 9. With the beginning of the control until the end thereof, an output signal is available on a line 10. 
     The deviations RA R  and RA L  for the driven wheels can be formed as follows: ##EQU1## wherein V 1  is the wheel speed of the sensor 1 etc. The prescribed slippage threshold can, for example, be ##EQU2## The signals DRA can be recovered by differentiating the corresponding signals RA. 
     For reasons of simplicity, the control of only one wheel is considered in the following description. Line 6 is connected to three threshold stages 11, 12, 13. The latter supply an output signal if the input signal is greater than zero (13), equal to zero (12) or less than zero (11). Here zero represents the first value. The signal DRA is also supplied to three threshold stages 14, 15, 16 which supply signals when DRA is greater than zero (14), greater than a prescribed positive threshold S (S&gt;0) (15) or less than zero (16). Here zero represents the second value. The threshold S represents a third value, which is greater than the second value. 
     As soon as the threshold stages 13 and 14 are exceeded (RA&gt;0; DRA&gt;0), and AND-gate 17 opens and the output signal thereof activates a pulse generator 19. Via an OR-gate 20, the output signal of this pulse generator activates an inlet valve 21. 
     As seen in FIG. 2, this valve 21 is interposed between a pressure source 30 and the wheel brake 31. A switch-on valve 32 which activates the pressure source 30 only during control via line 10 is also interposed upstream. Further, an outlet valve 22 is provided to release pressure at the brake 31 to a reservoir 33. When the pressure source 30 is activated, the valves 21 and 22 can be used to vary the brake pressure at the brake 31. The valves 21, 22 can also serve the purpose of an anti-lock brake control. In this case, the pressure is supplied via a line 34. 
     When the pressure source 30 is activated, the pulsed actuation of the valve 21 causes a pulsed increase of the brake pressure. The gradient of the increase is hereby determined by the pulse-pause-relation of the pulse generator 19. 
     If, in addition, signal DRA exceeds the thresholds of the threshold stage 15, an AND-gate 18 opens. The output signal thereof changes the pulse-pause-relation of the pulse generator 19 such that the pressure build-up gradient is increased. 
     If then in a positive deviation (RA&gt;0), the signal DRA becomes smaller than zero, an AND-gate 23 opens. The output signal thereof activates a pulse generator 24, the pulses of which actuate an outlet valve 22 via an OR-gate 25. The pressure is thus reduced with a prescribed gradient while being pulsed. If the deviation RA reaches the value zero, another pulse generator 26 is activated independent of DRA which also slowly reduces pressure because of its pulse-pause-relation. If the deviation becomes negative (RA&gt;0), yet another pulse generator 27 serves to faster reduce pressure. The input signals of the pulse generators 24, 26, 27, are supplied to the inlet valve 21 via the OR-gate 20 since the inlet valve must block the pressure supply during pressure reduction phases. 
     When RA&gt;0 and DRA&gt;0, it is also possible to calculate the pulse-pause relation directly from the sum RA+DRA either by decreasing the pause time of the inlet valve, or by increasing the pulse time of the inlet valve, or both, when the sum is larger than a threshold value.