Described is an ABS for vehicles in which all wheels are driven. Of particular concern is the forming of a final reference speed for the slip control. From the speeds of wheels with different speeds, an auxiliary reference speed, and a reference speed and a final reference speed for the slip control is formed . In the event of instability, however, the increase of the auxiliary reference speed determines the reference speed.

BACKGROUND OF THE INVENTION 
From DE-A1 3706 514, a method is known to determine, in a vehicle with a 
two-wheel drive, the reference speed for the development of a slip signal, 
other than by the control, through the speed of the slower of the 
non-driven wheels. In the event of instability, the increase of an 
auxiliary reference speed, which is determined, other than by the control, 
through the speed of the faster of the non-driven wheels, determines the 
reference speed. 
SUMMARY OF THE INVENTION 
The object of the invention is to develop a method which determines a final 
reference speed signal for development of a slip signal, as described 
above, for all-wheel drive vehicles rather than two wheel drive vehicles. 
This object is achieved, according to one embodiment of the invention, in 
an improved method for obtaining a final reference speed signal in an 
anti-lock brake system having a slip control facility, for which a 
reference speed signal and an auxiliary reference speed signal having a 
time domain slope which determines characteristics of the reference speed 
signal in the case of instability of at least one wheel, are derived from 
wheel speed signals produced by a wheel sensor means for measuring the 
speed of a fastest, second-fastest, third-fastest, and slowest wheel, 
respectively, wherein when used on a vehicle with four driven wheels, the 
auxiliary reference speed signal is increased with the speed signal of the 
third-fastest wheel and decreased with the speed signal of the fastest 
wheel and in that, between the end of the increase and the beginning of 
the decrease, the auxiliary reference speed signal is held at the last 
speed signal value attained during the increase, and in that the reference 
speed signal is increased with the speed signal of the slowest wheel and 
decreased with the speed signal of the third-fastest wheel and in that, 
between the end of the increase and the beginning of the decrease, the 
reference speed signal is held at the last speed signal value attained 
during the increase and wherein the final reference speed signal 
corresponds to the reference speed signal. 
The object is achieved, according to a further embodiment of the invention, 
in an improved method for obtaining a final reference speed signal in an 
anti-lock brake system having a slip control facility, for which a 
reference speed signal and an auxiliary reference speed signal having a 
time domain slope which determines characteristics of the reference speed 
signal in a case of instability of at least one wheel, are derived from 
wheel speed signals produced by a wheel sensor means for measuring the 
speed of a fastest, second-fastest, third-fastest, and slowest wheel, 
respectively, wherein when used on a vehicle with four driven wheels but 
with a rear axle decoupled from a drive during braking, the auxiliary 
reference speed signal is increased with the speed signal of the 
third-fastest wheel and decreased with the speed signal of the fastest 
wheel and in that, between the end of the increase and the beginning of 
the decrease, the auxiliary reference speed signal is held at the last 
speed signal value attained during the increase, and in that the reference 
speed signal is increased with the speed signal of the slowest wheel and 
decreased with the speed signal of the slowest of the wheels decoupled 
from the drive until a predetermined speed difference occurs between the 
reference speed signal and the auxiliary reference speed signal and in 
that the reference speed signal is then maintained parallel to the 
auxiliary reference speed signal at a difference corresponding to the 
predetermined speed difference and wherein the final reference speed 
signal corresponds to the reference speed signal. 
All wheels are now included in forming the reference. The slip caused by 
the wheel speed differences during cornering is thus successfully 
eliminated. This slip would otherwise lead to a sensitive start of 
control. 
The linking of the reference speed, according to another embodiment of the 
invention, to a faster rotating wheel, in particular the fastest turning 
wheel or to the auxiliary reference speed has the effect that too low a 
reference speed is avoided. 
According to a further embodiment of the invention, an undesirable increase 
of the reference speed is prevented by holding the reference speed 
constant when the wheels are spinning. The increase of the auxiliary 
reference speed can also be used as vehicle retardation during the control 
action.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the components of an antilocking control system. Transducers 
1-4 are assigned to the four vehicle wheels for the determination of the 
wheel speeds. 
An evaluation circuit 5, to which the speed signals from transducers 1-4 
are fed, is connected to four solenoid valves 6-9 which are driven by 
brake pressure control signals produced in evaluation circuit 5. 
In evaluation circuit 5, slip signals S.sub.1 -S.sub.4 are generated in 
addition to other signals. Slip signals S.sub.1 -S.sub.4 are used in the 
control logic contained in evaluation circuit 5 during the development of 
control signals for valves 6-9. Slip signals S.sub.1 -S.sub.4 are obtained 
from the speed signals V.sub.1 -V.sub.4 of the transducers 1-4 which are 
assigned to these wheels. The speed signals are numbered in descending 
order of wheel speed with V.sub.1 being the fastest wheel speed and 
V.sub.4 being the slowest wheel speed. 
A possible circuit for the slip development is shown in FIG. 2. 
The four wheel speed signals V.sub.1 -V.sub.4 of the all-wheel driven 
vehicle are fed via terminals 20, as shown in FIG. 2, to blocks 21, 22, 25 
and 27. Blocks 21 and 22 are selection blocks. With rising speed, block 21 
selects the speed signal V.sub.4 of the slowest wheel and outputs this 
signal as a reference speed signal (see FIG. 3, dotted curve is reference 
speed signal V.sub.Ref). 
As shown in FIG. 3, if the wheel speed signal V.sub.4 of the slowest wheel 
is not increasing, as from t.sub.1, then the reference speed signal 
V.sub.Ref retains the maximum speed value of V.sub.4, until at t.sub.2 the 
speed signal V.sub.3 of the third fastest wheel is the same as the 
retained maximum speed value of V.sub.4. From t.sub.2, the reference speed 
signal V.sub.Ref follows the speed signal V.sub.3 of the third fastest 
wheel, until at t.sub.3, the difference .DELTA.V between speed signal 
V.sub.3 and auxiliary reference speed signal V.sub.HR (shown as a dashed 
line in FIG. 3) reaches a predetermined value and the reference speed 
signal V.sub.Ref follows the auxiliary reference speed signal V.sub.HR 
minus the difference .DELTA.V. 
Auxiliary reference speed signal V.sub.HR is formed in block 22 by 
selecting the third fastest wheel speed signal V.sub.3 during 
acceleration, then retaining from t.sub.1 onward the maximum speed value 
of V.sub.3, until at t.sub.4 the speed signal of the fastest wheel V.sub.1 
is the same as the maximum speed value of V.sub.3 and thereafter following 
the fastest wheel speed signal V.sub.1. 
The selection block 21 transmits reference speed signal V.sub.Ref to block 
24. The selection block 22 transmits auxiliary reference speed signal 
V.sub.HR to block 23 which determines the increase of this signal and 
supplies the increase value to block 24. In the case of instability, i.e. 
during a sudden drop of the speed signal supplied to block 21 due to a 
locking tendency, the increase of the auxiliary reference speed signal 
V.sub.HR determines the increase of the reference speed signal V.sub.Ref, 
when the decrease of the reference speed signal V.sub.Ref exceeds the 
decrease of the auxiliary reference speed signal V.sub.HR. 
As shown in FIG. 4, for the case where the rear wheels of a vehicle with 
all-wheel drive are decoupled from the drive during braking, the reference 
speed signal V.sub.Ref in block 21 is equal to the wheel speed signal 
V.sub.4 of the slowest wheel during increasing speed, and during 
decreasing speed (until t.sub.5 in FIG. 4). At time t.sub.5, the 
difference .DELTA.V between the auxiliary reference speed signal V.sub.HR 
and the wheel speed signal V.sub.4 reaches a predetermined value. 
Subsequently, the reference speed signal V.sub.Ref follows the auxiliary 
reference speed signal V.sub.HR minus the difference .DELTA.V. This 
auxiliary reference speed signal V.sub.HR is formed as described above in 
reference to FIG. 3. 
In both cases (as illustrated in FIGS. 3 and 4), a final reference speed 
signal V.sub.FRef formed in block 24 (with the increase of the auxiliary 
reference speed signal V.sub.HR in the event of instability) is supplied 
to block 25 for the purpose of forming slip signals on lines 26. 
In block 27, it is ascertained whether the wheel speeds adopt physically 
impossible vehicle acceleration values. If this is the case, wheel spin is 
recognized and a command is sent to block 24 which then holds the final 
reference speed signal V.sub.FRef constant. This is shown in FIG. 5, in 
which the wheels spin from t.sub.6 to t.sub.7. During this spinning phase 
(t.sub.6 to t.sub.7), the final reference speed signal V.sub.FRef is held 
constant. At t.sub.7, spinning is no longer recognized. In the spinning 
phase (t.sub.6 to t.sub.7), the rise of the auxiliary reference speed 
signal V.sub.HR is limited in block 23 to the physically possible vehicle 
acceleration. At t.sub.7, the auxiliary reference speed signal V.sub.HR 
follows the third fastest wheel speed signal V.sub.3. 
The following applies outside of the control action. 
In each computing cycle, the retardation reference is compared with the 
wheel speeds. If all of the wheel speeds are smaller than the retardation 
reference (as a rule, therefore, only during vehicle retardation), then 
the retardation reference is decreased. If one or two wheel speeds are 
greater than the retardation reference, the retardation reference remains 
unchanged. If three or four wheel speeds are higher than the retardation 
reference, then the retardation reference is increased. 
A special case arises if more than one wheel is on V-Min. Each of these 
wheels (except for the first) is treated as if it were above the reference 
speed. This eliminates the risk of a speed sensor break on two wheels 
causing an unrecognized ABS failure. 
Accordingly, the FZ-REF is increased when all wheel speeds are above it. If 
two or three wheel speeds are above FZ-REF, Ft-REF is held constant. If 
none or only one of the wheels speeds is above FZ-REF, FZ-REF is 
decreased. 
Upward matching will not take place when all wheels are recognized to be 
spinning. This is the case when all wheel speeds are above V-REF, where 
V-REF has been increased to the maximum value of 0.7 g. (With the vehicles 
concerned, accelerations of more than 0.7 g are not possible, which is why 
such driving conditions lead to the conclusion that all wheels are 
spinning). For this period, FZ-REF is held constant.