Installation for the control of the brake force at wheels of motor vehicles

An installation for the control of the brake force at the wheels of motor vehicle combinations, such as of truck-trailer combinations, in which one locking protective brake force control system each is coordinated to the truck and to the trailer which operate independently of one another, and by means of which each individual wheel is controllable within the stable range of the friction value-slippage-curve; a measuring device is thereby arranged in the connection between the truck and the trailer for measuring the tow bar force occurring thereat, whereby this measuring device is connected with the locking protective brake force control system of the truck and is operable to produce a signal when a predetermined, pushing tow-bar force value is exceeded, which blocks an increase of the brake force at the truck for the duration of the presence of the signal from the measuring device.

The present invention relates to an installation for the control of the 
brake force at the wheels of interconnected motor vehicle units, such as 
truck-trailer combinations, whereby one brake force control installation, 
which operates by itself, is coordinated to the pulling vehilce (truck) 
and to the trailer, by means of which each individual wheel is 
controllable within the stable range of the friction-slippage curve. 
Brake force control systems or brake slippage control installations are 
known in the prior art which detect the moving condition of the wheels and 
which during the beginning of a locking operation, so far reduce the brake 
force at one or several wheels of a vehicle, that the corresponding wheel 
or wheels just continue to brake while still rolling on the road, under 
utilization of the maximum possible friction value between the road 
surface and the tire. It becomes possible by these systems to brake a 
vehicle in an optimum manner under all possible road conditions-from dry 
concrete to an icy surface-while maintaining the vehicle's stability and 
steerability and more particularly to do so also in curves. When braking 
in curves, exclusively the prerequisite has to be fulfilled that the 
non-braked vehicle does not already leave the road due to excessive 
velocity as a result of the centrifugal force influence. 
If one equips in a tractor-trailer unit both vehicles according to the 
manner described above, then each vehicle brakes for itself in an optimum 
manner, corresponding to the respectively available friction values, 
whereby also in this case, priority is given to the lateral guidance 
behavior and the directional stability. However, precisely, this desired 
optimum braking of each part of a vehicle combination entails the danger 
for the truck-trailer combination of swerving or breaking-out, if the 
friction value or coefficient of the road changes in the course of the 
braking operation from low values over a very short piece of the road to 
high values. This might be the case, for example, when the truck-trailer 
combination passes over from an icy road stretch again onto the dry road 
stretch. 
Since the braking truck-trailer combination drives at first with the truck 
part onto the road portion having the large friction value, the brake 
force is increased by the brake force control installation initially at 
the front axle and then at the rear axle corresponding to the new 
conditions whereas in contrast thereto, the brake forces at the trailer 
are still correspondingly small. In such a case, the braking of the 
pulling truck vehicle is large whereas that of the trailer is small, which 
has a consequence the feared over-running of the trailer. In such a 
condition, the trailer may swerve or break-out laterally and may do so 
even when the brake force at the trailer itself has been reduced by the 
brake force control installation thereof to the value of zero which may 
readily be the case, for example, on an icy road stretch. Even if the 
entire residual friction is then available at these axles for the lateral 
guidance, the swerving or breaking-out can no longer be prevented as a 
result of the over-running of the trailer. 
The present invention is concerned with the task to avoid this 
disadvantage. An installation is to be provided according to the present 
invention, by means of which a breaking-out or swerving of the trailer can 
be prevented with an operating brake force installation, when the pulling 
vehicle passes over a road surface having a better friction value or 
coefficient. The underlying problems are solved in the installations of 
the aforementioned type in that a measuring member is arranged in the 
connection between the pulling vehicle section or truck and the trailer 
for the tow-bar force occurring thereat, in that this measuring member is 
operatively connected with the brake force control installation of the 
pulling vehicle, and in that this measuring member produces a signal when 
a predetermined pushing two-bar force value is exceeded, and that an 
increase of the brake force at the pulling vehicle is adapted to be 
blocked for the duration of this signal. 
It is already known from the German Offenlegungsschrift No. 2,164,325 to 
determine the two-bar force between a pulling vehicle and a trailer by 
means of a measuring member and to utilize the same for the control of the 
pressure in the brake cylinder of the trailer. A pressure force in the 
tow-bar thereby effects an increase of the brake pressure at the trailer 
and--insofar as the wheels thereof are during the braking operation in the 
stable range of the friction-slippage curve--also an increase of the 
trailer braking action. As a result thereof, the tow-bar force is kept 
within limits and simultaneously the braking action of the truck-trailer 
combination and its trajectoral stability is improved. 
If, however, a road portion having a sudden friction value increase is 
driven over while braking wih a tractor-trailer combination-- having this 
known tow-bar force control system--then a stronger braking action 
initiated by the driver leads to an immediate locking of the wheels of the 
trailer--as soon as the pulling vehicle has passed over the road portion 
with the higher friction value--owing to such a brake force control 
installation at the trailer (and also without the same) and thus to a 
swerving or breaking-out of the trailer. 
The problem underlying the present invention not only cannot be solved with 
this known prior art installation but instead it would even favor a 
swerving or breaking out of the trailer. In the installation according to 
the present invention, in contradistinction thereto, the increase of the 
brake force at the pulling vehicle, whether initiated by the driver or 
regulated by a locking protective installation installed into the vehicle, 
is prevented in an advantageous manner for such length of time during the 
transition from a road surface with a poor friction value to a road 
surface with good friction value, until both vehicle sections are on the 
road surface with the high friction value. A breaking-out or swerving is 
therefore prevented with certainty. The present invention is therefore 
based on the general concept, so to speak of, to preclude an uneven 
braking of a pulling vehicle and of a trailer in case of road surfaces or 
road conditions having strongly differing friction values for such length 
of time until approximately the same friction values or coefficients again 
apply for the entire truck-trailer combination. 
It is additionally proposed by the present invention that a threshold value 
switch or switching circuit is coordinated to the measuring member which 
responds when exceeding a predetermined tow-bar force value, and whose 
digital output signal exists for the duration of the period of time when 
the predetermined tow-bar force value is exceeded, and is operable to act 
as blocking signal in the brake force control system of the pulling 
vehicle. This may take place in a first embodiment in that according to 
the present invention, the threshold value switch is operatively connected 
by way of an output-connected circuit consisting of matching amplifier and 
of a filter or a corresponding circuit with the signal line to the inlet 
valves of the brake force control installation at the pulling vehicle. In 
another embodiment according to the present invention, provision is made 
that a timer circuit having a predetermined running period is started by 
the digital output signal of the threshold value switch or switching 
circuit which, in its turn, blocks the inlet valves for this time duration 
by way of a corresponding circuit of conventional type. It is thereby 
additionally proposed according to the present invention that the running 
period of time duration of the timer circuit is automatically variable in 
dependence on the vehicle velocity in the sense that with an increasing 
vehicle velocity, the time duration is reduced. 
The present invention is applicable analogously also to a semi-trailer or 
five-wheel trailer. In this case, however, the mass of the taken-along 
semi-trailer would have to be determined additionally by a measurement of 
the static semi-trailer or trailer axle pressures. Furthermore, the 
semi-trailer pushing force would have to be compared by the use of 
conventional logic elements with the mass of the semi-trailer and with the 
deceleration of the semi-trailer vehicle and only then would have to be 
controlled correspondingly. Consequently, not as simple a control is 
possible in connection therewith as with a truck-trailer combination so 
that it would be more simple in connection with a semi-trailer motor 
vehicle to delay the braking increase only at the front axle of the 
semi-trailer pulling section in the case of a sudden change of the 
friction value so that the full brake force becomes effective thereat 
only, when also in that case the endangered rear axle has driven onto the 
road part with the large friction value. 
Accordingly, it is an object of the present invention to provide an 
installation for controlling the brake force at the wheels of motor 
vehicle combinations which avoids by simple means the aforementioned 
shortcomings and drawbacks encountered in the prior art. 
Another object of the present invention resides in a brake force control 
installation for controlling the wheels of motor vehicle truck-trailer 
combinations which minimizes the danger of swerving by the trailer. 
A further object of the present invention resides in a control installation 
for controlling the brake force at the wheels of motor vehicle 
truck-trailer combinations which effectively prevents an over-running of 
the trailer when the truck passes from a road section with a low friction 
surface onto a road section with a high friction surface. 
A still further object of the present invention resides in a brake force 
control installation of the type described above which is simple in 
construction, effective in operation and capable of preventing serious 
accidents due to a lateral swerving of the trailer.

Referring now to the drawings, wherein like reference numerals are used 
throughout the various views to designate like parts, according to FIG. 
1a, a truck-trailer combination, consisting of a pulling vehicle or truck 
10 and of the trailer 11 is, for example, on an icy road portion 12 having 
a low friction coefficient. The front axle 13 of the truck vehicle 10 is 
exactly in the transition onto a dry road portion 14 with a high friction 
value or friction coefficient. Up to this point, all of the axles of the 
truck-trailer combination were therefore on a road portion with 
essentially the same friction coefficient. The force at the tow-bar 15-- 
as long as no locking protective control takes place--is thereby a 
function of the individual braking operations of the truck-trailer 
components and of the weight relation thereof. However, if a brake force 
control takes place--for example, during a full braking on the icy road 
portion 12--then the tow-bar force will itself approach a value of zero by 
the locking protective control system which is of conventional 
construction known as such in the prior art. However, at the moment at 
which the front axle 13 of the pulling vehilce 10 passes over onto the dry 
road portion 14, the locking protective brake force control installation 
commences to increase the brake force at this front axle 13. This means, 
the tow-bar force now increases to a higher value. This take place for 
such length of time until according to FIG. 1b, also the rear axle 16 of 
the pulling vehicle 10 passes over onto the good road portion 14. Now the 
brake force is also increased at this rear axle 16 and the tow-bar force 
further increasees suddenly, so to speak of, which according to FIG. 1c, 
leads to a breaking-out or swerving of the trailer 11 since the brakes of 
the trailer 11 not only are not controlled for increased braking action 
but instead as a result of the road condition, cannot be controlled at all 
so as to increase the braking action. The trailer 11 will therefore now 
assume an angular position to the road and an ugly accident is the 
unavoidable consequence. 
According to FIG. 2, the pulling vehicle 10 is equipped with a known 
locking protective brake force control installation of conventional type. 
The latter consists of the electronic unit 17 of conventional 
construction, known as such in the art, which processes the signals of the 
sensors 18 arranged at the front axle 13 and at the rear axle 16 into 
switching pulses, by means of which the valves 19 at the two axles or at 
the respective corresponding wheels are controlled in a known manner. 
Since the brake force control installation, as such, and th valve control 
system is known in the art and does not form any part of the present 
invention, a detailed description thereof is dispensed with herein. 
The trailer 11 includes a locking protective brake force control unit 17', 
constructed in principle in the same manner, which is therefore also not 
described herein in detail. Both brake force control installations 17 and 
17' are completely independent of one another. A measuring member 20 of 
conventional construction is arranged at the tow-bar 15 which detects the 
tow-bar force according to magnitude and sign. In the instant case, the 
two-bar force is primarily of interest if it occurs as pushing or shear 
force. A threshold switch 21 of conventional construction is coordinated 
to this measuring member 20 which feeds a digital output signal to the 
electronic part 17 by way of the line 22 when a predetermined pushing or 
shear value is exceeded. An upward control, i.e., or increase of the brake 
force at the pulling vehicle is prevented in a conventional manner by this 
output signal from the threshold switch 21 for such length of time as this 
digital output signal occurs, i.e., for such a length of time until also 
the trailer 11 is on the road portion 14 with the higher friction value. 
The operative interaction in the brake force control installation of the 
pulling vehicle 10 thereby takes place by conventional means in such a 
manner that the signals to the inlet valves of the wheels are 
correspondingly blocked. Since this can be achieved in a known manner, a 
detailed description thereof is dispensed with herein for the sake of 
simplicity. The pulling vehicle or truck 10 therefore continues to brake 
with a correspondingly small force--even though it is on a road with a 
good friction value--for such length of time until the predetermined 
threshold value is again dropped below at the measuring member 20 because 
in the meantime the trailer 11 has now passed onto the same road portion 
14 with the good friction value and its braking action has been raised 
correspondingly by the locking protective installation. 
FIG. 3 is a graphical representation which illustrates at first at the top 
portion the friction value or friction coefficient curve in the road 
portions 12 and 14. Plotted directly therebelow are the braking at the 
trailer and at the pulling vehicle as well as at the entire truck-trailer 
combination and the resulting tow-bar force. It should be mentioned in 
that connection that the plotted values do not represent absolute values 
but are merely intended to bring out the tendency. The curve applies over 
a predetermined distance or travel and more particularly the four 
indicated positions of the truck-trailer combination have been picked out. 
According to the middle portion of FIG. 3, there is shown that within the 
area 23, a quite considerable tow-bar force occurs. This area 23 
corresponds to FIG. 1c, i.e., within this area, the trailer 11 breaks out 
or swerves. 
According to the lower portion of FIG. 3, this high tow-bar force is 
decreased within the area 23, and mor particularly in that the breaking of 
the pulling vehicle or truck is delayed in the described manner until at 
least the front axle of the trailer has also passed over onto the road 
portion with the high friction value. However, it should be mentioned once 
again that the indicated lines are not intended to represent absolute 
values and also no relations to one another but merely a tendency. 
Since the various electronic and electromechanical control systems are 
known as such in the art, a detailed description thereof is dispensed with 
herein for the sake of simplicity and the same are shown and described 
only schematically. 
As mentioned before, in one embodiment of the present invention as shown in 
FIG. 4 the threshold switch 21 is operatively connected by way of a 
circuit 33 including a matching amplifier 24 and a filter 25 or by way of 
a corresponding circuit with the signal line 22 leading to the inlet 
valves of the brake force control installation 17 at the truck. However, 
according to another embodiment of the present invention as shown in FIG. 
5, it is also possible that the digital output signal of the threshold 
switch 21 is used to start a timing circuit 26 providing a predetermined 
time delay, which timing circuit 27, in turn, blocks by way of a 
corresponding circuit the inlet valves for this time period. Additionally, 
the time delay of the timing circuit which may be a conventional timer 
time delay element or the like, may thereby also be automatically 
adjustable as a function of the vehicle velocity in accordance with 
velocity 28 supplied thereto in such a manner that with an increasing 
vehicle velocity the time period is reduced. Again, since the actual 
components producing such control are well-known in the art, and involve 
only conventional circuit elements and circuits, a detailed description 
thereof is dispensed with herein. 
While I have shown and described only one embodiment in accordance with the 
present invention, it is understood that the same is not limited thereto 
but is susceptible of numerious changes and modifications as known to 
those skilled in the art, and I therefore do not wish to be limited to the 
details shown and described herein but intend to cover all such changes 
and modifications as are encompassed by the scope of the appended claims.