Control for the automatic locking of the transfer transmission of a motor vehicle

Control for the automatic locking of the transfer transmission of a motor vehicle equipped with a multi-axle drive, having a wheel slip control system for the wheels of one axle and having an electronic unit that carries out a comparison of a quantity determined from the steering angle and the rotational speed difference of the wheels of the other axle that characterizes the moving condition of the motor vehicle with a limit value assigned to the steering angle and the driving speed. Control signal for the lock of the transfer transmission is generated when a control signal of the wheel slip control system is present or the quantity characterizing the moving condition of the motor vehicle exceeds the limit value.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates to control for the automatic locking of the transfer 
case of a motor vehicle equipped with a multi-axle drive as a function of 
the steering angle and the driving speed of the motor vehicle for example. 
This type of arrangement is known from DE-OS 35 05 455and corresponding 
U.S. Pat. No. 4,681,185. It concerns a vehicle having a multi-axle drive 
with a transfer transmission and locks for the transfer transmission and 
for the differential gears. In this case, the connecting of the transfer 
transmission and of the locks takes place automatically as a function of 
several criteria which characterizes the moving condition of the vehicle 
and include steering angle and driving speed. Among the criterion for 
example, is a reference quantity of the rotational speeds of the wheels of 
one axle. 
In the technical field of drives, concepts that have a multi-axle drive are 
known to be very efficient. A disadvantage in the case of low adhesion 
coefficients is a spinning of the wheels which results in a loss of 
lateral control and impairs the driving stability considerably. As a 
remedy, a wheel slip control of all driven wheels is known and 
corresponding U.S. patent application Ser. No. 912,321 (DE-OS 35 34 443). 
This is very effective but also very costly. It is the objective of the 
present invention to provide for the known, very costly combination of the 
multi-axle drive and a wheel slip control of all driven wheels an 
alternative solution that is just as effective but much less costly. 
According to the invention, this objective is achieved by a control for the 
lock of a transfer transmission on an automatic vehicle with multiple axle 
drive using both the lock control signal generated as a function of 
steering angle and drive speed as well as the control signal from the 
antislip system. A logic is provided for providing the locking control 
when either a lock control signal is generated by the standard transfer 
transmission control lock system or by the control signal of the antislip 
system. The antislip system output control generally include a brake 
torque control and an engine torque control signal. As a result, at least 
the costly differential gear locks and the electronic control system and 
the hydraulic brake system of the wheel slip control can be saved for the 
wheels of one axle. In addition, only one brake circuit must be connected 
with one pressure reservoir. The connecting of the transfer transmission 
lock, in addition, has advantages with respect to driving dynamics that 
apply particularly in the case of obstacle-avoidance maneuvers or when 
driving turns in the case of low adhesion coefficients. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
The drawing, in diagram form, shows the drive train of a motor vehicle with 
all-wheel drive. The vehicle engine 1, including power control element la 
which is represented as a throttle valve, acts on a transfer transmission 
3 that is equipped with an electro-pneumatic lock 3a, via a step gear 2. 
The transfer transmission 3 is connected via a differential gear 4, 5 to 
the front wheels 6,7 and the rear wheels 8, 9 respectively. A rotational 
speed sensor 10, 11, 12 and 13 is assigned to each wheel. Wheel brakes 14 
and 15 for the rear wheels 8 and 9 are shown as disk brakes, for example. 
The vehicle is equipped with a wheelslip control system ASR that is known, 
for example DE OS 35 34 443, the electronic system of which is represented 
diagrammatically as a box 16. The rotational speed signals of the 
rotational speed sensors are fed to the electronic system 16. In the case 
of the occurrence of a slip, the electronic system 16 uses them in a known 
way to form the control signals for the wheel brakes 14, 15 of the rear 
wheels via braking torque regulators 17, 18. The hydraulic part of these 
brakes (the brake pedal, the brake cylinder, the pressure reservoir, the 
brake valves, etc.), for reasons of clarity, are not shown. The effect of 
the brake signal B and of the wheel slip control system on the wheel 
brakes is shown diagrammatically. The electronic system 16 also forms 
control signals which, via a torque regulator 19, affect the power control 
element la in the sense of a reduction of the driving power, when both 
rear wheels tend to spin simultaneously. 
In addition, an electronic unit 20 is provided to which the rotational 
speed signals of the front wheel rotational speed sensors 10 and 11 as 
well as a steering angle signal from a steering angle sensor 21 which is 
proportional to the steering angle of the front wheels. 
The electronic unit 20, that is described in detail in DE-OS 35 05 455, and 
commonly assigned, U.S. patent application Ser. No. 828,275 filed Feb. 11, 
1986 now U.S. Pat. No. 4,681,185, evaluates the rotational speed 
difference of the front wheels and compares it with the driving speed that 
is also derived from the front wheel speeds and limit values assigned to 
the steering angle. When these are exceeded, an output signal is emitted. 
This criterion responds irrespective of slip criterion of the wheel-slip 
controlled rear wheels if, for example, a front wheel spins or if the 
vehicle, without longitudinal acceleration, drives into a correspondingly 
narrow turn and measures, for example, also counter steering motions in 
turns that are driven rapidly. 
The output signals of the electronic unit 20 and of the wheel slip control 
16, the control signals for the braking torque regulator 17 and 18 as well 
as for the engine torque regulator 19, are fed to the four inputs of an 
OR-gate 22, the output signal of which is the control signal for the 
electropneumatic lock 3aof the transfer transmission 3. 
Therefore, the lock of the transfer transmission will always be connected 
when an output signal of the electronic unit 20 is present or the wheel 
slip control system 16 responds. 
If the lock 3a of the transfer transmission 3 is connected, the front axle 
is frictionally connected with the rear axle. The corresponding braking 
torque from the rear axle will then at the same time also affect the front 
wheels. If a front wheel, during the locking of the transfer transmission 
receives a speed deviation with respect to the vehicle speed or the 
reference speed, the torque regulator 19 will be connected via the wheel 
slip control system 16 which results in a reduction of the driving torque. 
Although the present invention has been described and illustrated in 
detail, it is to be clearly understood that the same is by way of 
illustration and example only, and is not to be taken by way of 
limitation. The spirit and scope of the present invention are to be 
limited only by the terms of the appended claims.