Drive slip regulating system for a four-wheel drive vehicle

Drive torque determined from the throttle position and engine speed is used to compute the maximum possible instantaneous vehicle acceleration, which in turn is used to limit the increase of the reference value, such as a vehicle reference speed. The reference value is used to generate slippage values, which in turn are used to control braking and engine torque in a four wheel drive vehicle.

BACKGROUND OF THE INVENTION 
The invention relates to a drive slippage control system for a four wheel 
drive vehicle, which system provides slippage values used for controlled 
braking of the wheels and/or controlling the drive torque. The slippage 
values are derived from the speed signals of the wheels and a reference 
value approximated to the vehicle speed derived from the wheel speed 
signals. 
In case all-wheel driven vehicles start on low ground it is possible that 
all wheels are accelerated more rapidly as compared to the vehicle 
acceleration. For a high friction value, however, the wheel acceleration 
will conform with the vehicle acceleration. Based on this situation, it is 
difficult to form a reference value, which is required to determine the 
wheel slippage (drive slippage). 
SUMMARY OF THE INVENTION 
It is hence an object of the invention to form the reference value when all 
four wheels are slipping. 
This object is accomplished by calculating a maximum-possible vehicle 
acceleration which in turn limits the possible increase in the reference 
value. This maximum possible acceleration is determined from the vehicle 
torque, which may be calculated from the throttle position and the engine 
speed.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In a computing block 1 to which the throttle position D.sub.K is supplied 
by a sensor 2 and the engine speed n.sub.mot is supplied by a sensor 3, 
the instantaneous driving torque A is calculated based on the values 
supplied according to the relation 
EQU A=K.sub.1 D.sub.K +K.sub.2 n.sub.mot +K.sub.3 
wherein K.sub.i are constants which are functions of the specific engine 
data and the various engine types (Otto, Diesel, aspirated or charged 
engine). The driving torque can also be determined from a suitably stored 
engine data field. The vehicle has a transmission characterized by a 
transmission multiplication between the engine and wheels. 
From an output of block 1 the value A as determined is, together with a 
reference value (as vehicle speed), supplied to a further computing block 
4 which calculates the maximum-possible vehicle acceleration a.sub.Fmax 
from the instantaneous driving torque and the vehicle speed while allowing 
for instantaneous transmission multiplication, according to the relation: 
EQU a.sub.Fmax =c.sub.1 A+c.sub.2 .omega.+c.sub.3 v.sub.ref +c.sub.4 
wherein c.sub.i are constants which are a function of the various vehicle 
parameters (dynamic radius, air resistance coefficient, weight of vehicle, 
etc.) and the parameters of the drive line (transmission data, moments of 
inertia) and wherein .omega. is the wheel acceleration. The kinetic energy 
in the drive line and the dead times present are allowed for when 
determining a.sub.Fmax. 
This acceleration value is supplied to a block 5 to which signals 
corresponding to the wheel speeds v.sub.ref are also supplied via 
terminals 5' and which forms, based on this data, the reference value 
approximated to the vehicle speed. This is indicated as v.sub.ref in the 
FIGURE. The maximum-possible instantaneous vehicle acceleration a.sub.Fmax 
is used to limit the increase of the vehicle speed; if the wheel speeds 
signal a stronger increase, the latter is not admitted; what is admitted 
is an increase along with the maximum-possible instantaneous vehicle 
acceleration. The reference value formed in block 5 is then supplied to a 
block 6 which generates control signals from the supplied wheel speeds 
v.sub.ref and the reference value so as to brake individual wheels and/or 
reduce the engine momentum (terminals 6'). As already mentioned the 
reference value, such as vehicle speed is, supplied to the block 4 via a 
line 7.