Abstract:
An ABS is described in which the wheel slip is used. For this purpose, a reference signal is required, the gradient of which is obtained from an auxiliary reference signal. In order to achieve an even better matching of the reference to the vehicle speed, the gradient is made dependent on the extent of the pressure reduction in a closed-loop control cycle, specifically the gradient is reduced with increasing pressure reduction.

Description:
BACKGROUND OF THE INVENTION 
     From WO 88/06544 (U.S. Ser. No. 392,932, incorporated herein by reference) FIG. 7 with associated description it is known, in an anti-lock control system which uses the slip of the wheels as controlled variable, to generate a reference speed signal and an auxiliary reference speed signal for the purpose of slip formation. In the case of the auxiliary reference speed signal, the fastest wheel determines its curve. The gradient of this auxiliary reference speed signal serves only for the determination of the gradient of the reference speed signal during the instability of a wheel, the magnitude of the reference otherwise being determined by the second-fastest wheel. 
     SUMMARY OF THE INVENTION 
     The invention provides a closer approximation of the reference speed to the actual vehicle speed and hence an improvement of closed-loop control overall, in particular in extreme conditions such as μ jumps and in the case of initial braking. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An illustrative embodiment of the invention will be described in greater detail with reference to the 
     FIG. 1 shows a block diagram of the illustrative embodiment 
     FIG. 1A illustrates the formation of the gradient of the auxiliary reference signal, 
     FIG. 1B is a graphic illustration of the gradient of the auxiliary reference signal. 
     FIG. 2 shows the behaviour of the signals in the case of a negative μ jump 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, wheel sensors 1-4 associated with the vehicle wheels, generate and transmit their signals to an evaluation circuit 5. On the basis of the signals from the sensors 1-4, the circuit 5 controls 3/3 solenoid valves 6 associated with the wheels and varies the brake pressure at the wheels so as to avoid too large a brake slip. 
     For this purpose, the brake slip can be formed and compared with a setpoint brake slip or the slip can be used or involved in closed-loop pressure control in some other way. Slip should here be taken to mean both the difference between the reference speed and the wheel speed and the difference relative to the reference speed. 
     As shown above, for the purpose of slip formation a reference speed is required. This can be formed in the manner described in WO 88/06544 with reference to FIG. 7. The important point here is that the gradient of an auxiliary reference speed signal is used for its formation. The formation of the reference speed signal, which does not form a subject-matter of this invention, is performed in the evaluation circuit 5 while the formation, according to the invention, of the auxiliary reference speed signal is performed in the lower part 5a of the evaluation circuit. 
     The evaluation circuit 5 supplies a shift register 51 with the values of the speed of the fastest wheel, determined in the evaluation circuit 5 at a predetermined clock rate. The shift register has a 1  places, of which only the last a 2 , e.g. a 2  = a  1/4 are evaluated outside ABS control. 
     Referring to FIG. 1A, each new measured value input brings the longest-stored of the a 2  measured values out of block 51 evaluation. In each clock cycle, those a 2  measured values which have just been stored are added up in a block 521 and divided by a 2  in block 522. This value a 2  &#39; is likewise stored in block 523. The filtered measured value newly obtained is now subtracted from the longest-stored filtered measured value in block 524 and the difference is divided in block 525 by the expression (a 2  -1). T, T being the clock time. The result is a measure of the gradient of the auxiliary reference signal (FIG. 1B). 
     The number of places which can be evaluated in block 52 can be changed over and is changed over to a 1  (e.g. 4a 2 ) at the beginning of closed-loop control (line 53). The possibility of changing over has the advantage that a more rapid and better adaptation is obtained during initial braking. The gradient signal at the output of the block 52 is now fed to an adder/subtracter 54. The evaluation circuit 5 contains a time-measuring device which determines the pressure reduction phases at all wheel brakes during a control cycle of each wheel and, as a function of this measured value (sum of pressure reduction times), subtracts a value from the gradient signal. If, upon the occurrence of a negative μ jump (sheet of ice) at all wheels, there is a virtually simultaneous and pronounced pressure reduction, this measured value is large. The auxiliary reference gradient and hence the reference gradient is thus corrected towards smaller values, this then corresponding of course to reduced vehicle deceleration. The gradient of the auxiliary reference signal V&#39; ref  corrected as a function of the pressure in this way is then fed to the evaluation circuit 5 for the determination of the gradient of the reference signal in phases of a slump in rotational speed. The behaviour in the case of a negative μ jump is shown by FIG. 2, where this μ jump occurs at t 1 . The vehicle speed is denoted by V F , the reference signal, which is identical with this up to t 1 , by V Ref  and the wheel speed signal by V R . 
     As explained above, the pressure reduction time is measured from t 1 . The sum of the time values measured in each case brings about a substraction in the adder/subtractor 54 in each clock cycle. The auxiliary reference signal and hence also the reference signal V Ref  thus has a continuously decreasing (negative) gradient, i.e. from t 1  onwards, the reference signal no longer follows the predetermined V Ref  but makes a transition to a V Ref  which approaches the vehicle speed V F  again much more rapidly, which vehicle speed likewise has a smaller gradient from t 1  onwards because of the underlying surface. 
     In the evaluation circuit 5, it is also established by time measurement whether the initial value prior to pressure reduction is exceeded again during the pressure build-up (end of μ jump). If this is the case, then in each clock cycle a value dependent on the percentage by which the gradient value is exceeded is added on to the gradient value in the adder 54, the gradient thus being matched again to the gradient of the vehicle speed, which gradient is now also greater. 
     An appropriately programmed computer can also carry out the functions described instead of the arrangement in FIG. 1. 
     Outside closed-loop control, low-level filtering is operative since, in the case of hard depression of the brake, a rapid increase in the deceleration is possible. In certain driving conditions, &#34;additional depression&#34; is possible for as long as the fastest-running wheel is not effecting closed-loop control, as a result of which, in turn, deceleration increases (e.g. cornering). It is permissible for filtering to be higher-level here than outside closed-loop control but it must be lower-level than when all wheels are effecting closed-loop control.