Abstract:
The torque on at least one vehicle wheel is reduced when a potentially dangerous drive slippage is exceeded. The torque is then increased stepwise, the height of the steps being constant and the maintenance times between increases varying in dependence upon wheel slippage, vehicle acceleration, and/or the number of previous cycles.

Description:
BACKGROUND OF THE INVENTION 
     Drive slip control systems vehicle are known, e.g. from U.S. Pat. No. 3,528,536, where at least one (usually two) driven wheels reduce the engine torque when a preset slippage threshold is exceeded. When the slippage value falls below this threshold, the engine torque is increased, for example, by changing the throttle position stepwise until at least one wheel is unstable again. This procedure is referred to as a control cycle which, in most cases, is repeated several times successively in a control procedure. In known systems, the height of the steps is continuously increased to obtain a progressive curve of increase of the engine torque curve. 
     SUMMARY OF THE INVENTION 
     The inventive control system maintains the height of the increasing steps constant but varies the maintenance times of constant torque between steps. This variation is then made to depend upon various conditions such as the slippage value, the vehicle acceleration and/or the number of cycles which already occurred in a control procedure. The maintenance time is calculated in each computation procedure, i.e. it is constantly adjusted to new road and vehicle conditions. 
     When the drive slip is high, close to the acceptable slippage limit, the maintenance times at the driven axle are extended since optimum slippage will soon be reached and a spinning (instability) of the driven wheels is about to occur. When the vehicle acceleration is high, the maintenance times are reduced (indication of high friction coefficient) since substantially more torque can be transmitted in this case. 
     A high vehicle acceleration is an indication of a high friction coefficient which permits a rapid increase without risking a spinning of the driven wheels. Low vehicle acceleration is caused either by a low friction coefficient or low engine torque. 
     When the engine torque is too low, the drive slippage is very small (close to 0 km/h) and, in this case, there is a rapid increase. A positive result thereof is improved traction when the friction coefficient has changed or when exiting a curve. 
     Allowing for the control cycles which already occurred in the control procedure permits a minute and careful approach to the optimum values of the engine torque so that the control vibrations decay faster and jolting is avoided. If a substantial slippage does not occur at the driven wheels (e.g. at μ-change) over a longer period of time in this condition, the number of control cycles which already occurred can be ignored for further calculation. 
     On the one hand, the vehicle acceleration can be analogously included in the calculation of the maintenance time, on the other hand, the maintenance time can be changed stepwise when discrete acceleration thresholds are exceeded. These discrete acceleration thresholds can be made to depend upon the vehicle speed (small acceleration at a high vehicle speed). It is also possible to change the maintenance time only after a prescribed threshold of the vehicle acceleration has been passed. This threshold too can be made to depend upon the speed of the vehicle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a drive slip control in accordance with the invention, 
     FIG. 2 shows the possible curve of a controlled engine torque. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For reasons of simplicity FIG. 1 shows a drive slip control which acts only on the engine torque in case one of the wheels becomes instable. Only one sensor of a driven wheel is represented. The invention, however, can also be used with drive slip control systems when operating the brakes and acting upon the engine torque, the latter applies in particular when all driven wheels become unstable. 
     FIG. 1 shows a sensor 1 of a driven wheel and a sensor 2 of a non-driven wheel. The difference ΔV of the two wheel speeds is formed in a block 3. If this difference (it corresponds to the wheel slippage) exceeds a positive threshold (drive slippage) preset in comparator 4, a motor-driven actuator 15 changes the position of the throttle 12 and the engine torque is reduced (cf. period t 1  to t 2  in FIG. 2). The possibilities of the pedal acting upon the actuator or the throttle are not represented here. This possibility is interrupted during control. 
     The disappearance of the output signal of the comparator 4 activates, via an invertor 6 and a differentiator 7, a computer 5 which controls all the subsequent increases of the engine torque. The output signal of block 3 (ΔV), the vehicle acceleration &#34;a&#34; of a terminal 8, the counting result of a counter 9, and a signal from comparator 11 are supplied to this computer. The longitudinal acceleration &#34;a&#34; of the vehicle is fed to an element 12 having several different thresholds. 
     Via a multiple line 10, the computer 5 is informed which threshold was just exceeded. The thresholds can also be affected by the vehicle speed (speed of the non-driven wheel) in that the thresholds are reduced with an increasing speed (line 13). 
     The counter 9 determines the number of control cycles which already occurred in the control procedure in that it counts the reducing signals at the output of a comparator 4. The counter 9 is reset by an output signal of the computer 5 when, for example, ΔV is small over a certain period of time or a control procedure has been completed. 
     Using the signal supplied, the computer 5 continuously calculates the maintenance time tH (T 1 , T 2 , T 3 , . . . in line 5) to be inserted between two successive increase signals of the same width (cf. line 5 in FIG. 2). 
     The calculation can be carried out, for example, according to the following or a similar equation: ##EQU1## wherein t H  is the maintenance time, 
     K 1 , K 2  are constant factors, 
     ΔV=(VD-VND) is the difference of the driven axle speed to the non-driven axle speed, 
     n is the number of the control cycles which already occurred in the control procedure (where n=1, if ΔV is small over a certain period of time) and 
     a* is a signal which corresponds to the instantaneously valid vehicle acceleration. 
     The desired effects of the values ΔV, a*, and n have already been explained above. The stepwise torque increase ends when, in case of a new instability, a signal occurs at the output of comparator 4 and the computer 5 is deactivated again. 
     If a vehicle does not move after the vehicle is started (e.g. on a hill or in case of low μ), this information is supplied to computer 5 via a comparator 11 with a very small comparative value. The computer then reduces the height of the steps of increase (hence the width of the pulses of line 5 of FIG. 2), for example in half, and thus supports departing by means of a slowly increasing and minute engine torque.