System for the control of the couplings in the drive train of a motor vehicle

A system for the control of couplings in the locking mechanisms or differentials of a drive train in an all-wheel-drive vehicle is disclosed. The drive train of a vehicle having at least two drive axles (4.times.4 vehicle) comprises, starting from the engine and transmission block 1, a power take-off with switchable coupling 2, a front axle differential with locking coupling 3, a rear axle differential with locking coupling 5, steering angle sensor 10, rotational speed sensors 11, status sensors 12 and actuators 13, 14, 15. The rotational speed sensors 11 are installed in proximity to the wheels 4, 6. To be able to automatically switch all the couplings in keeping with terrain conditions, a control system 18 having individual modules (21,24,25) for each coupling (2,3,5) is provided. The control modules (24,25) of the hierarchically lower couplings (3,5) also emit control signals for one or several hierarchically higher coupling(s) (2,5) before they actuate their own couplings (3,5). The slip signals are slip totals signals formed by integration of the rotational speed differences and are compared with slip totals threshold values to obtain control signals with which the couplings are controlled.

FIELD OF THE INVENTION 
The instant invention relates to a system for the automatic control of the 
couplings in the drive train of an all-wheel-drive off-the-road vehicle. 
Specifically, the invention relates to a system in which slip signals 
formed on the basis of rotational wheel speed signals, are compared with 
threshold values to produce control signals for the couplings. 
BACKGROUND OF THE INVENTION 
Off-the-road vehicles are vehicles with two or three or more driving axles. 
The performance profile of such vehicles is mainly designed for 
off-the-road travel. In these vehicles one of the axles, usually the front 
axle (or front axles, if more than one), can be driven either permanently 
or by being switched into line. Therefore, the corresponding coupling is 
employed either to switch the front axle drive into line or for locking 
the central differential. Furthermore, one coupling is provided in these 
vehicles for the locking of each axle differential in order to achieve 
full off-the-road capability. The couplings may be disk couplings, as well 
as positively engaging couplings. In heavy vehicles the latter are 
preferred because of the high torque to be transmitted and the limited 
availability of installation space. 
Steering an all-wheel vehicle off the road is an art which consists not the 
least in the ability to take the correct action in the drive system 
depending on the driving conditions and the nature of the terrain. These 
actions include connecting the front axle drive and actuating the 
longitudinal (inter-axle) locking mechanism or differential and the 
transverse (intra-axle) locking mechanism or differential. For this 
reason, different actions are taken by the driver in known all-terrain 
vehicles. Therefore, individual switches, or at least one switch with 
several switching positions, are provided. A rigid sequence of actions 
must then be followed. However, this sequence of actions does not help the 
driver in judging the state of the vehicle and in selecting the drive 
mode. 
Automatic locking differentials which lock dependently of the current 
rotational speed differences are also known. However, if several 
differentials are installed in series, a problem arises as described 
through the following example. Differentials installed in series include 
first a longitudinal inter-axle differential and then intra-axle 
differentials connected in the direction of torque flow. 
If the right rear wheel slips in a vehicle having such a series 
differential arrangement, the action of the non-locked differentials 
causes all the rotational speeds in the drive train to change. The 
difference between the rotational speeds of the two rear wheels is 
greatest. The difference between the median rotational speeds on the front 
and rear wheels, i.e., the average produced by the axle differentials, is 
smaller. This difference in speeds leads to a locking of the rear axle 
differential which results in only one driving wheel being available which 
will easily lose road adherence since the right wheel is slipping. The 
longitudinal locking mechanism or differential is released only when it 
slips and three wheels are driving, insofar as the vehicle has not become 
mired by then. Furthermore, when the wheels of an axle are slipping, the 
danger that they may laterally push away exists. 
The correct action in this situation is to lock the longitudinal 
differential first. But if the switching threshold of the longitudinal 
differential is put lower than that of the transverse differential, the 
longitudinal differential will be locked first. However, this will cause 
the rotational speed difference of the rear wheels to be reduced and the 
switching threshold for the locking of the rear axle transverse 
differential will no longer be attained. 
If the vehicle is equipped with a connectable front wheel drive instead of 
a longitudinal locking mechanism, the above described effect becomes even 
more apparent. The individual rotational speed differences will depend on 
the switching state of the couplings. Because the switching state of the 
couplings depends on rotational speed differences while the rotational 
speed differences depend on the switching state, an erratic back and-forth 
switching will arise if individual clutches are automatically operated 
individually. 
Furthermore, automatically locking differentials are as a rule locked, 
although usually only in part, by disk couplings which, aside from their 
known disadvantages, are subject to much greater wear under such 
conditions. 
If the rotational speeds of all the wheels are taken into consideration in 
an automatic transmission and if these speeds are processed centrally, 
very complicated and intricate logical criteria are needed. However, these 
criteria are still unable to cover all possible situations. Since the 
rotational speeds of the wheels depend on the switched state of the 
individual couplings in this case too, the danger of undesirable switching 
still exists, particularly undesirable and dangerous switching back into 
the unlocked state. 
A system is disclosed in DE-C 35 05 455 and the publication 
AUTMOBIL-INDUSTRIE 1/87 (pages 27 to 32, dealing with the 4-MATIC of 
MERCEDES-BENZ). This known system automatically actuates the connection of 
the front axle drive, the central locking mechanism and the rear-axle 
locking mechanism. However, its performance profile strongly emphasizes 
the requirements of a fast road vehicle and is geared for safety. It is 
basically different from a system designed for off-the-road travel. In 
this system, an automatically locking differential is provided for the 
rear axle. The control of the front axle coupling and the longitudinal 
differential is based only on the axle speeds. Furthermore, this known 
system is dependent on the smooth engagement of disk couplings because of 
the safety reasons mentioned earlier. 
A control system for the control of individual couplings is disclosed, for 
example, in the assignee's EP-OS 510 457 (U.S. Pat. No. 5,335,764). In 
this system, for the control of a positively engaging coupling, with all 
of the applications mentioned above being possible, rotational speed 
differences are compared with threshold values. The special disclosed 
systems function with actuating elements that are simple in their action 
and require no uncoupling command, but automatically uncouple when the 
transmitted torque is small enough. The instant invention has the 
coordination of such individual systems as its object and is applicable to 
the systems described therein, but is in no way limited to them. It is 
also suitable for any positively engaging coupling controlled in any 
manner and even for disk couplings. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an automatic control 
system which meets in every way the requirements for the operation even of 
heavy vehicles over difficult terrain. 
This and other objects of the present invention are achieved by providing a 
system for automatic control of couplings in a drive train of an 
all-wheel-drive off-the-road vehicle having a transfer case constituting a 
power take-off for the front axle(s). The system has a plurality of drive 
axles, each of said plurality of drive axles having a pair of wheels 
connected thereto. A plurality of rotational speed sensors are associated 
with the wheels which produce rotational wheel speed signals indicative of 
the rotational speed of the wheels. A plurality of actuable couplings are 
associated with the transfer case and with the drive axles. The actuable 
couplings are arranged in a hierarchy of higher and lower couplings. A 
separate control module is dedicated to each of the actuable couplings. 
Each separate control module receives rotational wheel speed signals from 
the rotational speed sensors and produces control signals based thereon 
for actuation of the couplings. A control module dedicated to a lower 
coupling produces control signals which actuate at least one higher 
coupling before producing control signals which actuate its dedicated 
lower coupling. The control signals are produced by comparing slip total 
signals reflecting total slip of the wheels with slip total threshold 
values. 
In another embodiment of the invention, the plurality of couplings include 
at least one coupling associated with each of the axles. 
In another embodiment of the invention, a first rear axle drive, a 
connectable front axle drive, a coupling for connection of the connectable 
front axle drive, a front axle drive control module dedicated to the 
coupling for connection of the connectable front axle drive, a first 
transverse rear axle locking control module and a transverse front axle 
locking control module are provided. The coupling for the connection of 
the connectable front axle drive can be closed by its dedicated control 
module, by the first transverse rear axle locking control module and by 
the transverse front axle locking control module. The first transverse 
rear axle locking control module holds the coupling for connection of the 
front axle drive in engagement for a given period of time. Furthermore, a 
second rear axle drive, an inter-axle differential, an inter-axle locking 
coupling, a second rear axle transverse locking coupling, an inter-axle 
locking control module dedicated to the inter-axle locking mechanism 
coupling and a second transverse rear axle locking control module can be 
provided. The inter-axle locking coupling can be closed by its dedicated 
module, by the first and second transverse rear axle locking control 
modules, and by front axle drive control module. The first and second 
transverse rear axle locking control modules, the transverse front axle 
locking control module and the front axle drive control module hold the 
inter-axle locking coupling in engagement for a given period of time. 
Further, a transverse front axle locking coupling and a transverse front 
axle locking control module dedicated to the transverse front axle locking 
coupling can be provided. The transverse front axle locking control module 
can only close the transverse front axle locking coupling. 
In still another embodiment of the invention, a first rear axle drive, a 
permanently connected front axle with the transfer case having a 
longitudinal locking mechanism, a coupling for the longitudinal locking 
mechanism and couplings for the transverse locking of the front axle and 
of the rear axle, a longitudinal locking control module dedicated to the 
longitudinal locking coupling, a first transverse rear axle locking 
control module and a transverse front axle locking control module are 
provided. The coupling for the longitudinal locking mechanism can be 
closed by its dedicated control module, the first transverse rear axle 
locking control module and the transverse front axle locking control 
module. The first transverse rear axle locking control module and the 
transverse front axle locking control module hold the coupling of the 
longitudinal locking mechanism in engagement for a given period of time. 
Further, a second rear axle drive, an inter-axle differential, an 
inter-axle locking coupling, a second rear axle transverse locking 
coupling, an inter-axle locking control module dedicated to the inter-axle 
locking coupling and a second transverse rear axle locking control module 
can be provided. The inter-axle locking coupling can be closed by its 
dedicated module, the first and second transverse rear axle locking 
control modules and the longitudinal locking control module. The first and 
second transverse rear axle locking control modules, the transverse front 
axle locking control module and the longitudinal locking control module 
hold the inter-axle locking coupling in engagement for a given period of 
time. 
Additionally, the first transverse rear axle locking coupling and the 
second transverse rear axle locking coupling are associated with a common 
module. 
In another embodiment of the invention, a first rear axle drive, a front 
axle drive, a first transverse rear axle locking coupling, a transverse 
front axle locking coupling, a first transverse rear axle locking control 
module dedicated to the coupling for the first transverse rear axle 
locking mechanism and a transverse front axle locking control module are 
provided. The coupling for the first transverse rear axle locking 
mechanism can be closed by its dedicated control module and the transverse 
front axle locking module. The transverse front axle locking module holds 
the first transverse rear axle locking coupling in engagement for a given 
period of time. 
In still another embodiment of the present invention, a system for 
automatic control of couplings in a drive train of an all-wheel-drive 
off-the-road vehicle is provided. The vehicle has a transfer case with a 
power take-off and a plurality of drive axles. Each of the plurality of 
drive axles has a pair of wheels connected thereto. A plurality of 
rotational speed sensors is associated with the wheels. These speed 
sensors produce rotational wheel speed signals indicative of the 
rotational speed of the wheels. A plurality of actuable couplings are 
associated with the transfer case and the drive axles. The actuable 
couplings are arranged in a hierarchy of higher and lower couplings. A 
control which controls each of said actuable couplings is provided. The 
control receives the rotational wheel speed signals from the rotational 
speed sensors and produces control signals based thereon for actuation of 
the couplings. In order to control a lower coupling, the control produces 
control signals which actuate at least one higher coupling before 
producing control signals which actuate the dedicated lower coupling. The 
control signals are produced by comparing slip total signals reflecting 
total slip of said wheels with slip total threshold values. 
The objects of the present invention are achieved by providing a separate 
control module for each individual coupling. These control modules produce 
signals for the individual couplings. The system is decentralized. An 
intricate logic for the analysis of the rotational wheel speed signals is 
not needed. Depending on the design of the system, the individual modules 
are either program modules, i.e., if the system comprises a 
microprocessor, or hardware modules, i.e., if the system comprises 
hard-wired components. The hierarchical interconnection of the couplings 
is taken into account in the present invention. The control modules of the 
hierarchically lower couplings also transmit control signals to one or 
several hierarchically higher coupling(s) before they trigger their own 
coupling. This produces a "democratic hierarchy" and the different systems 
are able to intervene in another system before they actuate themselves. 
However, in the case of highly changeable rotational speed differences, 
this hierarchy may not be sufficient because one disengagement may pass 
another in view of the final duration of engagement. Furthermore, in the 
case of a rotational wheel speed difference, i.e., sudden loss of ground 
adherence of one wheel during a steep climb, also known as "breaking out", 
timely shifting of a claw or positively engaging coupling may no longer be 
possible. Therefore, the slip signals are signals of total slip formed by 
the integration of the rotational speed differences. These total slip 
signals are compared with threshold total slip values. This means that the 
greater the rotational speed difference, the quicker the slip total 
threshold is reached. Consequently, switching is very rapid in the case of 
a great increase in rotational speed. If the increase in speed is slower 
or the lower rotational speed difference is low, switching takes place 
only after a longer period of time. 
Furthermore, a correct evaluation based on advance-indication signs is 
possible due to the slip totals, even when the slip varies, and 
unnecessary switching is avoided. The slip total has the dimensions of an 
angle or an arc. Therefore, the switching thresholds can also be adapted 
to mechanical conditions, especially to the particularities of a claw or 
positively engaging coupling. Thus, it is also possible to ensure that the 
thresholds of the slip totals only take effect when they are run through 
in ascending order and that the holding function takes effect when the 
values to be added together decrease again. 
The threshold values in the control modules of the hierarchically lower 
couplings for the engagement of hierarchically higher couplings are lower 
than for the engagement of their own couplings. During the actuation of a 
coupling and its holding function, the holding function ensures that the 
coupling remains engaged for a certain period of time. When the rotational 
speed difference between the rear wheels increases, the associated module 
first causes a locking of the longitudinal differential, since the 
threshold is lower than the one which causes transverse locking. The 
rotational speed difference between the rear wheels is processed in the 
module of the locking mechanism of the rear axle. However, provisions are 
also made for the transverse locking to remain engaged for a certain 
period of time, because this locking can lower the rotational speed 
difference to such an extent that a release of the rear axle locking 
mechanism does not occur at all. The period of time may also depend on the 
speed or the speed of change. If the engaged locking does not result in 
any decrease of rotational speed difference, the slip total will continue 
to climb and will reach the next threshold value for the release of the 
coupling. In this instance, the holding function ensures that no 
back-switching, i.e., renewed disengagement of the coupling, ensues. 
If the invention is applied to a vehicle having a permanent rear axle drive 
and a connectable front axle drive, the coupling for the connection of the 
front axle drive can be locked by its associated module, by a rear axle 
transverse-locking module and by a transverse front axle locking module. 
The transverse rear axle locking module keeps the coupling for the 
connection of the front axle drive in engagement for a certain period of 
time. 
If the invention is applied to a vehicle having a permanent rear axle 
driven and a front axle having a connectable longitudinal locking 
mechanism or differential, the coupling for the longitudinal locking 
mechanism can be locked by its own module, by a transverse rear axle 
locking module and by a transverse front axle locking module. The 
transverse rear axle locking module and the transverse front axle locking 
module retain the coupling for the connection of the front axle drive in 
engagement for a certain period of time. 
The holding function according to the present invention decreases the 
switching frequency on the terrain in systems with controlled 
back-switching (according to any criteria whatsoever), as well as in 
systems with automatic back-switching through drop in torque, as in EP-OS 
510 457, and in addition leads to a valuable gain in safety. 
In a further embodiment of the invention, the coupling of the transverse 
rear axle locking mechanism can be locked by its associated module and by 
the module of the transverse front axle locking mechanism. The transverse 
front axle locking module holds the coupling of the transverse rear axle 
locking mechanism or differential in engagement for a certain period of 
time. Consequently, the slip of a front wheel does not immediately result 
in the locking of the front axle differential, which would impair the 
steerability of the vehicle. Instead, the transverse rear axle locking 
mechanism is actuated first, resulting in greater traction gain. The 
advantages of the holding function in this embodiment are the same as 
those mentioned above. For the same reason, the module of the transverse 
front axle locking mechanism is able to lock only its own coupling thereby 
ensuring that it is locked last in order to maintain steerability. 
In another embodiment of the invention, a vehicle having two drive rear 
axles and an inter-axle locking mechanism is provided. Advantageously, the 
coupling of the inter-axle locking mechanism is lockable by its own module 
and by the transverse locking modules of the two rear axles. The 
longitudinal differential module, the transverse rear axle locking module 
and the transverse front axle locking module hold the coupling for the 
locking of the inter-axle differential in engagement for a certain period 
of time. This takes into account that the inter-axle locking mechanism is 
hierarchically higher than the transverse rear axle locking mechanism and 
the central locking mechanism. In a field test, it has further been shown 
that a staggered actuation of the two rear axle transverse locking 
mechanisms provides no further advantages as a rule. Therefore, it is a 
welcome simplification, that the two rear axle transverse locking 
mechanisms are associated with a common module. 
Off-the-road vehicles are also driven on solid roads. For reasons of 
safety, the actuation of the transverse locking mechanism is 
advantageously inhibited. However, the modules are able to trigger a 
hierarchically higher locking action. The possible triggering of 
longitudinal locking as a result of transverse slip provides a 
considerable gain in safety which cannot be attained in any other manner. 
In another embodiment of the invention, the front axle locking module can 
actuate the rear axle locking mechanism, as well as the central locking 
mechanism, by breaking through the hierarchy because the associated 
thresholds are lower in the central locking mechanism then in the front 
axle locking mechanism. If these threshold values are designed so that two 
couplings cannot be switched at the same time, the switching impact is 
decreased. In the in the case of electric or pneumatic actuation there is 
danger of an unacceptable slowing down of the engaging speed as a result 
of simultaneous power consumption by two actuators. 
This can be ensured under all circumstances by connecting the switching 
actuators of these couplings in series as a redundant safety. The second 
actuator can only be actuated when the first one has already switched. The 
holding force is then already considerably lower. This also prevents 
actuation of a hierarchically lower locking mechanism to trigger a 
solenoid valve in case of malfunction in the control device or the cables 
without previous actuation of the higher locking mechanism. 
By contrast with off-the-road passenger cars, it may be advantageous in 
heavy trucks to connect the axles for braking. The coupling for 
longitudinal locking, and possibly also the coupling of the inter-axle 
locking mechanism, are closed during braking for the longitudinal locking 
or connection of the front axle drive. As a result, different braking 
forces between the front axle and rear axle are equalized in such vehicles 
particularly in vehicles without ABS (anti-lock braking system). The 
blocking tendency of the individual axles is prevented by static or 
dynamic axle load shift. Overall, a gain in travel stability is achieved 
for as long as no transverse locking mechanism is triggered. 
The control system also requires signals which indicate the switched state 
of individual couplings. These signals are normally received by sensors at 
the actuators. Defects in such actuators may cause delicate operational 
malfunctions and must, therefore, be recognized. In a further embodiment 
of the invention, signals indicating the switched state of individual 
couplings are acknowledged. These signals are formed from the rotational 
wheel speed signals. These signals are used either directly or for the 
control of other acknowledging signals. 
Another safety function which makes it possible to continue travelling with 
defects, comprises closing the coupling for the longitudinal locking 
mechanism or for connection of the front axle drive and possibly also of 
the inter-axle locking mechanism. It has been shown that this state is 
better for vehicle stability and traction. Furthermore, a malfunction may 
occur while the vehicle is in the field and the vehicle can continue to 
operate. 
In another embodiment, individual rotational wheel speed differences are 
compared with threshold values below which the rotational wheel speed 
differences are not taken into account to calculate the slip totals. The 
slip totals are used for calibration of the system. This prevents more 
than insignificant diameter differences of the tires from causing an 
actuation of a locking mechanism at any time and unexpectedly. 
Furthermore, a criterium for a regular state in which calibration can be 
carried out is thereby created. 
In another embodiment of the invention, positively engaging couplings are 
used which are engaged by single-acting actuators and disengaged by spring 
force when the transmitted torque drops below a given value. Positively 
engaging couplings are very space-saving by comparison with disk couplings 
and are characterized by short shifting distances which can be covered 
very rapidly. The actuators can, thus, be designed very simply and without 
any delay elements. Control based on slip totals which causes a decrease 
in wear with disk couplings is especially favorable for positively 
engaging couplings because engagement of the coupling is ensured under all 
circumstances at the optimal differential rotational speed independent of 
the gradient of the differential speed. The simplification of control 
achieved by eliminating a disengagement control with its known problems is 
not only multiplied by the number of modules but simplifies the entire 
system to a degree going far beyond this. 
In a further embodiment of the invention, it is possible for closed 
couplings to be held in their closed state during gear shifting in the 
main transmission. The holding function already provided can, thus, also 
be used to prevent unwanted opening of individual couplings during gear 
switching in the main transmission. For this reason it is recommended to 
install a contact on the coupling pedal which triggers the holding 
function with a positive coupling as soon and for as long as it is pushed 
down.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows the drive train of a 4.times.4 vehicle having two driving 
axles. The drive train starting at the engine and transmission block 1, 
consists of a power take-off or transfer case with a longitudinal 
differential and switchable coupling 2, a front axle differential with 
locking coupling 3 and a rear axle differential with locking coupling 5. 
The front wheels are indicated by reference numbers 4 and the rear wheels 
are indicated by reference numbers 6. The switchable coupling 2 of the 
power take-off serves either to connect the front axle drive or to lock 
the longitudinal differential of the power take-off. No distinction 
between these will be made below unless mentioned especially, because the 
effect is absolutely similar. 
The drive train furthermore comprises sensors 10,11,12 and actuators 
13,14,15. Sensor 10 is a steering angle sensor. Sensors 11 are, in this 
case, rotational-speed sensors located near the front and rear wheels 4, 
6. However, because the transmission ratios are known, these speed sensors 
11 may also be located at the input or output of the individual 
transmissions or anywhere else. Sensors 12 are status sensors for the 
switched position of the individual elements, such as the actuator, 
coupling, brake pedal, etc. They are not necessary for the switched 
position of the actuators because the status of the actuators can also be 
ascertained by other means. The actuators 13,14,15 are mounted on the 
couplings 2,3,5, respectively, which they actuate. In the case of 
pneumatic actuation, the actuators 13, 14, 15 are air pressure cylinders, 
which are shown greatly enlarged in the drawing. Arrows indicate the 
actual location of the actuators: actuator 13 on the power take-off, 
actuator 14 on the rear axle differential, and actuator 15 on the front 
axle differential. The control system consists of a control device 18 and 
the actuation control 19. In the control device 18 it is possible to 
distinguish, although not necessarily topographically, between a data 
collection part 20 and the modules 21,24,25. The modules 21, 24, 25 each 
respectively comprises a computing part 21',24',25' and a memory part 
21",24",25". The slip-dependent decisions are made and implemented in the 
computing portions 21',24',25' and safety controls, e.g., no engagement of 
the transverse locking differentials beyond a given travelling speed, are 
stored in the memory parts 21",24",25". The modules 21,24,25 supply the 
electric control signals for the solenoid valves 26,28,29, which in the 
simplest case may be two-position valves. The solenoid valves 26,28,29 
control the supply of compressed air from a reservoir 30 to the actuators 
13,14,15. 
FIG. 2 shows a variant for vehicles with three driving axles in which the 
same parts are given the same reference numbers. The additional parts are 
an inter-axle differential with locking coupling 7, a second rear axle 
differential with locking coupling 8, actuators 16,17 and two additional 
rear wheels 9. The control module 18 contains two additional modules: the 
inter-axle locking module 22 and the second transverse rear axle locking 
module 23. 
Furthermore, a control valve 27 for actuator 16 for the inter-axle locking 
mechanism has been added. The actuators 14 and 17 are controlled by a 
common solenoid valve 28. If the front axle differential with locking 
coupling 3, actuator 15, solenoid valve 29 and module 25 were left out, a 
6.times.4 vehicle would result to which the invention can also be applied. 
A further difference exists between FIGS. 1 and 2. In the system of FIG. 1, 
the actuators 13, 14, 15 are supplied through parallel air conduits from 
reservoir 30. Such an arrangement could cause a switching delay in the 
case of simultaneous actuation of several actuators when the volume in the 
reservoir is low. Therefore, the arrangement in FIG. 2 is as follows: the 
compressed-air supply conduit 31 for the solenoid valve 26 and the 
longitudinal locking mechanism bifurcate only downstream of the solenoid 
27 for the inter-axle locking mechanism; the compressed-air supply conduit 
32 for the solenoid valve 28 of the transverse rear axle locking mechanism 
bifurcates only downstream of the solenoid valve 26 of the longitudinal 
locking mechanism; and the conduit 33 going to the solenoid valve 29 of 
the front axle locking mechanism bifurcates only downstream of the 
solenoid valve 28 of the rear axle locking mechanism. 
When electromagnetic actuators are used, this arrangement can be repeated 
by means of appropriate electrical switches. Therefore, not only is 
simultaneous actuation of two couplings excluded but a redundant 
protection of the hierarchy of the locking mechanisms is created. 
This hierarchy of the locking mechanisms is derived from the travel-dynamic 
effect of the individual locking mechanisms. Therefore, as in any 
hierarchy, certain locking mechanisms have more influence than others, 
meaning that direct influence on the travel dynamics, as well as indirect 
influence on the other locking mechanisms, occurs. If the hierarchy is not 
taken into account, an erratic back-and-forth switching of the individual 
locking mechanisms occurs. If the hierarchy is too rigid, the locking 
mechanisms do not fully use their influence. In trucks, the hierarchy, 
starting from the top, is the inter-axle locking mechanism, if the truck 
is 6.times.6, then a longitudinal locking mechanism or connection of the 
front axle drive, the transverse rear axle locking mechanism and, finally, 
the transverse front axle locking mechanism. This hierarchy is used, among 
other reasons, in order to impair the steerability of the vehicle as 
little as possible. 
FIG. 3 schematically shows the actuation of the couplings 2,3,5 of FIG. 1 
over time through the corresponding modules as a function of slip. The 
figure has the modules arranged as follows, from left to right: module 21 
of the longitudinal locking mechanism, module 24 of the transverse rear 
axle locking mechanism and module 25 of the transverse front axle locking 
mechanism. Additional switching criteria have been left off for the time 
being. In module 21, a slip is calculated from the rotational wheel speed 
signals of the sensors 11 or, more precisely, a rotational speed 
difference is calculated from the difference between the mean values of 
the rotational speeds of the front wheels 4 and the rear wheels 6, taking 
into account the steering angle. From this rotational speed difference a 
slip total is formed by integration or addition. 
In the uppermost diagram on the left side, the slip total having the 
physical dimension of an angle is entered on the ordinate and the time is 
entered on the abscissa. A constant rotational speed difference is 
represented by an ascending straight line 40 which reaches the slip total 
threshold 41 as predetermined for longitudinal locking (SSS.sub.-- LSpVG). 
The steeper line 40 is, the faster it reaches the threshold 41. As a 
result, the control signal for the engagement of the locking coupling is 
transmitted at the power take-off 2 to the solenoid valve 26. It is then 
held back for a predetermined time (approximately 2 seconds), during which 
time back-switching is prevented. This is indicated in the curve entitled 
triggering LSP by rectangular signal 42, wherein LSP refers to the 
longitudinal locking mechanism. 
In module 24 of the transverse rear axle locking mechanism, a slip is 
calculated from rotational wheel speed signals of the sensors 11, or, more 
precisely, a rotational speed difference is calculated from the difference 
between the rotational speeds of the rear wheels 6. From this rotational 
speed difference, a slip total, represented by an ascending straight line 
43 in the case of constant slip, is again obtained by integration or 
addition. However, in this case, two thresholds apply to the slip totals, 
a first threshold 44 (SSS.sub.-- LSpVG.sub.-- by.sub.-- QSpHA) and a 
second threshold 45 (SSS.sub.-- QSpHA.sub.-- by.sub.-- QSpHA or in short 
SSS.sub.-- QSpHA). In the transverse rear axle locking module 24, a 
threshold 44 for the actuation of a coupling 2 without the longitudinal 
locking module 21 is below the threshold 45 for the actuation of the 
transverse locking coupling 5. This longitudinal locking mechanism is 
reached at the point in time t.sub.1. The longitudinal locking mechanism 
is triggered and held for a predetermined period of time, as shown by 
rectangular signal 46, which means in this case at the minimum that a 
disengagement of the longitudinal locking mechanism is impossible for the 
predetermined period of time. If this does not reduce the slip at the rear 
wheels, the slip total continues to increase until the threshold 45 is 
reached at time t.sub.2. Now the transverse rear axle locking mechanism 5 
is triggered, as shown by rectangular signal 47. However, the slip is 
often already reduced by the longitudinal locking mechanism 2 to such an 
extent that the transverse locking mechanism 5 is not even triggered. In 
that case, the longitudinal locking mechanism was not engaged by its 
module but by the transverse locking module. 
Module 25 for the transverse front axle locking mechanism 3 uses the 
rotational-speed difference between the front wheels 4 to calculate a slip 
total. The slip total is represented by a straight line 48 which 
intersects three slip-total thresholds. Threshold 49, when reached, 
results in the longitudinal locking mechanism (SSS.sub.-- LSp.sub.-- 
by.sub.-- QSpVA) being connected. Then threshold 50 is intersected which 
in this case equals the preceding threshold 49, (SSS.sub.-- QSpHA.sub.-- 
by.sub.-- QSpVA). The corresponding actuations are again indicated below 
by the rectangular signals 52,53,55. The signals 52,53 are again 
maintained for a certain period of time. In addition a dead time 54 is 
provided, so that the actuation of the transverse front axle locking 
mechanism takes place only after a certain period of time in order to 
maintain the steerability of the vehicle. The steerability could also be 
achieved by an even higher threshold for as long as possible. 
All the slip-total thresholds, which shall be designated hereinafter 
comprehensively by SSS.sub.-- * (the thresholds of non-module couplings as 
SSS.sub.-- *.sub.-- by.sub.-- *), may be constants or may also be 
rotational-speed-dependent variables. The variables could be selected 
differently depending on the chosen gear or depending on whether the 
vehicle is in traction mode (engine drives) or thrust mode (engine 
brakes). 
The time diagram of FIG. 4 is for a 6.times.6 vehicle and has the following 
additional elements: module 22 for the locking mechanism of the inter-axle 
differential 7 and module 23 for the locking mechanism of the second 
transverse rear axle differential 8. The course of events shown in FIG. 4 
is analogous to FIG. 3 and takes into account the fact that the intra-axle 
locking mechanism 7 is the highest in the hierarchy. In anticipation of 
FIG. 5 the designations of the flags which lead to the activation of the 
locking mechanisms according to the program are also entered in FIG. 4. 
FIG. 5 shows as an example the repetitive implementation of the program in 
the computing part 24' of module 24 of coupling 5 of the transverse rear 
axle locking mechanism, starting with the START field 100. In the 
formulas, the value attribution of a variable is designated by "&lt;-" and 
the following designations are used: 
______________________________________ 
N.sub.-- DIFF.sub.-- QSpHA 
Difference between the rotational speeds 
of the two rear wheels; 
N.sub.-- HAR 
Rotational speed of the right rear wheel; 
N.sub.-- HA1 
Rotational speed of the left rear wheel; 
N.sub.-- HA Mean rotational speed of the rear wheels; 
SL.sub.-- Lenk 
Slip caused by the steering angle (slip 
is the rotational speed difference 
rendered dimension-less by one rotational 
speed); 
DN.sub.-- Lenk 
Difference in rotational speed caused by 
the steering angle; 
DN.sub.-- SL 
Dynamic rotational-speed difference; 
DN.sub.-- SL.sub.-- MIN 
Lower threshold of the dynamic 
rotational-speed difference below which 
rotational-speed differences are not 
taken into account; 
SL.sub.-- MIN 
Lower threshold of the dynamic slip below 
which rotational-speed differences are 
not taken into account; 
S.sub.-- GR Tire-specific limit slip depending on the 
transmitted torque, is taken from a 
function table f2; 
DN.sub.-- GR 
Appertaining limit value of the 
rotational difference speed; 
FLAG.sub.-- vor(before) 
Marker which is set when the slip is 
leading (traction slip); 
FLAG.sub.-- nach(after) 
Marker which is set when the slip is 
trailing (thrusting slip). 
______________________________________ 
In field 101, all these magnitudes are used to define the flags for 
traction or thrusting slip. In fields 102,105 the decision is made whether 
traction or thrusting slip has occurred or whether the differential 
rotational speed is below the threshold DN.sub.-- SL.sub.-- MIN. For a 
traction slip, a traction slip total phi.sub.-- vor(before) is added up 
and in field 107 a thrusting slip total phi.sub.-- nach(after) is set 
equal to zero. For a thrusting slip, the thrusting slip total phi 
nach(after) is added up in field 106 and the traction slip total 
phi.sub.-- vor(before) is set equal to zero in field 104. When the 
differential rotational speed is below the threshold DL.sub.-- SL.sub.-- 
MIN, both slip totals are set equal to zero in the fields 104,107 causing 
the entire program to be repeated without external effect, as described 
below. 
In field 108, the program asks whether the marker Flag.sub.-- QSpHA.sub.-- 
locked is still set according to the previous passage. In field 109 the 
differential rotational speed is added up. A control is then made in field 
110 as to whether the angle of rotation has been reached at which it is 
possible to be certain that either the locking mechanism is open or that 
the axle is broken. If yes (Y) the angle has been reached, the locking 
mechanism is open and the total zero is set in field 111 so that a new 
addition can be started with the next passage. If the locking mechanism is 
already open (N) in field 108, the fields 109, 110, 111 are circumvented 
along path 112. 
If the locking mechanism is open according to field 108 or field 111, the 
slip totals phi.sub.-- vor(before) and phi.sub.-- nach(after) are set to 
zero in fields 113 to 117 and an advance indication limitation is carried 
out because the slip thresholds can be determined as a function of advance 
indications and it is necessary to make certain that the slip total curves 
43 do not change their quadrant, or in other words, that phi.sub.-- 
vor(before)&gt;0 and phi.sub.-- nach(after)&lt;0 (fields 114,117). The round 
field 119 only serves for orientation. In field 120 the following 
slip-total thresholds are taken from functions tables f3 and f4 (they 
depend in this case on the rotational speed of the rear axle): 
______________________________________ 
SSS.sub.-- LSpVG.sub.-- by.sub.-- QSpHA 
Slip-total threshold in the trans- 
verse rear axle locking mechanism 
module for actuation of the 
longitudinal locking mechanism; 
SSS.sub.-- QSPHA.sub.-- by.sub.-- QSpHA 
Slip-total threshold in the trans- 
verse rear axle locking mechanism 
module for the actuation of the 
transverse rear axle locking mech- 
anism (the SSS.sub.-- QSPHA for short). 
______________________________________ 
In field 121 the marker for the release of the transverse locking mechanism 
and in field 122 the markers for the release of the longitudinal locking 
mechanism are set when the slip total phi.sub.-- vor or phi.sub.-- 
nach(after) exceeds the applicable slip-total threshold. 
______________________________________ 
Flag.sub.-- QSpHA 
Markers for the release of the 
transverse rear axle locking 
mechanism; 
Flag.sub.-- LSpVG.sub.-- BY.sub.-- QSpHA 
Markers for the release of the lon- 
gitudinal locking mechanism by the 
module of the transverse rear-axle 
locking mechanism. 
______________________________________ 
Field 123 is again used for orientation. In field 124 the markers, 
including those of other modules, are read out and the rotational speed 
differences of field 101 are subjected to a safety control. 
The rotational speed difference may not be too great at the switch of a 
claw coupling. When this control is passed (Y) the program goes through 
the timer in field 125 which carries out the holding function. Upon 
further controls in fields 127, 128, the timer 130 decides whether the 
actuation signal is to be given in field 131 (Y) or not (N) in field 132. 
From there the program returns to the starting field 100. 
The program portion of FIG. 5 is related to the analog program portions of 
module 21 (longitudinal locking mechanism) and module 25 (transverse front 
axle locking mechanism). In module 21, the markers (Flag.sub.-- LSpVG) are 
set in field 150 for the actuation of the longitudinal locking mechanism. 
Three markers, Flag.sub.-- QSpVA in field 152, Flag.sub.-- QSpHA.sub.-- 
by.sub.-- QSpVA in field 153 and Flag.sub.-- LSpVG.sub.-- by.sub.-- QSpVA, 
can be set in module 25 in field 154. All these markers are accessible 
from all three modules and they are queried in module 21 in field 151, in 
module 24 in field 124 and in module 25 in field 155. 
The signals which are thus produced now go into the testing or securing 
portions 21",24",25" and then to the solenoid valves 26,28,29. 
Additional release criteria may be provided for as indicted by the fields 
140,141,142. Thus, field 140 may cause the coupling (2) for the 
longitudinal locking mechanism or the connection of the front axle drive 
and possibly also the coupling (7) of the inter-axle locking mechanism to 
be closed during braking. Furthermore, field 141 can prevent a closed 
coupling (2,3,5;2,3,5,7,8) from opening during gear shifting in the main 
transmission (1). Finally, field 142 can ensure that in case of 
malfunction of one of the rotational-speed sensors (11) or of a 
steering-angle sensor (10) the coupling (2) of the longitudinal locking 
mechanism and, possibly, also the coupling (7) be closed for longitudinal 
locking or connection of the front axle drive. 
While the invention has been described by reference to specific 
embodiments, this was for purposes of illustration only. Numerous 
alternative embodiments will be apparent to those skilled in the art and 
are considered to be within the scope of the invention.