Process and apparatus for controlling the brake system of a vehicle

The onset and the end of the braking action, especially the braking action of the component vehicles of a combination vehicle, are determined on the basis of the course of the deceleration of the vehicle. On the basis of the values found for the onset and end of the braking action, the hysteresis between the component vehicles is compensated, so that the braking action begins and ends at essentially the same time at all the wheel brakes.

BACKGROUND OF THE INVENTION 
The invention pertains to a process and to apparatus for controlling the 
brake system of a vehicle, especially a combination vehicle, wherein the 
brake tensioning force exerted on the wheel brakes is influenced by an 
electronic control unit. 
A process and device of the type in question are known from DE 41 12 845 
A1. In the compressed air brake system described there, the application 
pressure i.e., the pressure required to initiate braking action of the 
individual wheel brakes, is determined by evaluation of the change in 
pressure over time during the actuation of the brake. The application 
pressure thus determined is used to correct the nominal pressure derived 
from the driver's command for the individual wheel brakes for the purpose 
of producing a braking force which is the same at all of the individual 
wheels of the vehicle or at the wheels of one axle. A corresponding method 
can also be used to determine the separation pressure when the brakes are 
released. It has been found that, when each of the wheel brakes has a 
different hysteresis i.e., differences between the applications and 
releases, the braking forces at the time of contact and release at the 
individual wheel brakes or at the brakes of the individual component 
vehicles of a combination vehicle can be quite far apart. This leads to 
different braking durations at the individual wheel brakes or within the 
individual component vehicles, which lead to nonuniform wear on the wheel 
brakes and to a reduction in driving comfort. This effect is especially 
clear in the case of vehicle combinations with different types of brake 
systems, such as, for example, when one of the component vehicles has a 
conventional pneumatic brake system while the other component vehicle has 
an electrically controlled brake system. 
SUMMARY OF THE INVENTION 
It is the task of the invention to provide measures for compensating for 
this hysteresis of the brake system of a vehicle or vehicle combination. 
This is achieved by determining the onset and the end of braking action by 
evaluating the deceleration of the vehicle. Brake tensioning forces are 
adjusted so that the onset of the braking action and the end of braking 
action occurs at essentially the same time at all the wheel brakes. 
The process according to the invention makes it possible to compensate for 
cases in which the various vehicle brakes have different hystereses in 
response to the application and release forces. As a result, the driving 
comfort and the wear properties are improved. 
The process offers special advantages in the case of combination vehicles, 
because in that case essentially the same application and release forces 
of the component vehicles are arrived at automatically. As a result, in 
addition to an improvement in the driving comfort and in the wear 
properties of the vehicle combination, the forces acting between the 
component vehicles during the braking process are also reduced. 
Particular advantages are derived when at least one component vehicle has 
an electrically controlled brake system. In this case, no additional 
sensors are required. The trailer control module of a tractor vehicle 
serves both as an aid in determining the application and release forces 
and also as an actuator/final control element/servo element for the brake 
system of the trailer. 
It is especially advantageous that the process is able both to recognize 
the application and release points and also to compensate for different 
hystereses in the vehicle brake system.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
FIG. 1 shows a general block circuit diagram of an electronically 
controlled brake system based on the example of an electronically 
controlled compressed air brake system with emergency brake circuit for a 
two-axle tractor vehicle with a trailer coupling. 
FIG. 1 shows a central control unit 10, which includes at least one 
microcomputer. In addition, so-called pressure control modules 12, 14, 16, 
18 are provided at decentralized locations, each being assigned to a wheel 
brake 20, 22, 24, 26 of the tractor vehicle. Central control unit 10 is 
connected by way of a communications system 28, e.g., CAN, to pressure 
control modules 12-18. To detect the brake pressure supplied and to detect 
additional variables such as axle loads, wheel rpm's, brake temperatures, 
etc., lines 34, 36, 38, 40 leading from corresponding measuring 
instruments 42, 44, 46, 48 are connected to pressure control modules 12-18 
as well. Central control unit 10 is also connected by a line 50 to 
electrical section 52 of a brake value sensor 54. In addition, a trailer 
control module 70 is provided to control the brake system of the trailer; 
this module is connected via communications system 28 to central control 
unit 10. In addition, a line 56 leads from central control unit 10 to an 
electrical plug connection 58 leading to the trailer. 
In the system shown in FIG. 1, the pneumatic section of the brake system 
consists of two brake circuits, one for the front axle and one for the 
rear axle. Pressure control modules 12, 14 assigned to wheel brakes 20, 22 
of the front axle are supplied via lines 60 with feed pressure originating 
from a supply tank 62. Control lines 66 extend from pneumatic section 64 
of the brake value sensor to pressure control modules 12, 14. In addition, 
a line 68 leads from line 66 to trailer control module 70 for controlling 
the brake system of the trailer. From trailer control module 70, a 
pneumatic control line 72 leads to a first coupling head 74; a feed line 
76 leads to a second coupling 78. In addition, trailer control module 70 
is also connected by a line 80 to a pressure supply tank 82. The second 
brake circuit consists of second pneumatic section 84 of brake value 
sensor 54, pressure supply tank 86, air feed line system 88, and pneumatic 
control line system 90. Feed lines 88 and control lines 90 lead from brake 
value sensor 54 to pressure control modules 16, 18 assigned to wheel 
brakes 24, 26 of the rear axle. 
In a preferred exemplary embodiment, a measure for the degree of actuation 
of the brake pedal is sent to central control unit 10 by electrical 
section 52 of brake value sensor 54. This signal is processed in the 
central control unit, and nominal pressure, nominal braking moment, 
nominal force, or nominal slip values for the individual wheel brakes or 
for the trailer are determined in accordance with predetermined 
characteristic curves or characteristic fields, possibly under 
consideration of additional operating parameters such as axle loads, 
wheel-specific variables, vehicle deceleration, etc. These nominal values 
are transmitted via communications system 28 to the individual pressure 
control modules and to the trailer control module, which adjust the 
pressure being supplied to the individual wheel brakes or to the brake 
system of the trailer so that it matches the specified nominal value. In 
the event of a defect in the electrical section of the brake system, this 
section or at least the defective part (e.g., the front or rear axle) is 
turned off, and the pneumatic emergency brake circuit or circuits are put 
into service. When the emergency brakes are operating, the driver 
specifies the wheel brake pressure in the individual wheel brakes by means 
of pneumatic sections 64, 84 of brake value sensor 54 via control lines 
66, 90, respectively, and this pressure is supplied to the wheel brakes by 
the pressure control modules in the absence of electric actuation. 
The process according to the invention can be used advantageously not only 
in conjunction with the brake system shown in FIG. 1 but also in brakes of 
systems of other designs such as electro-pneumatic systems, 
electro-hydraulic systems, and brake systems with purely electrical 
closing of the brakes. In addition to the combination, shown in FIG. 1, of 
an electrically controlled tractor vehicle with a conventionally braked 
trailer, a vehicle combination consisting of a conventionally braked 
tractor and a conventionally braked trailer, a conventionally braked 
tractor and an electrically braked trailer, and an electrically braked 
tractor and an electrically braked trailer can also be provided. For the 
brake system of the conventionally braked trailer, a trailer control 
module is to be provided, which can be actuated electrically either by the 
central control unit, or, if the tractor also has conventional brakes, by 
the ABS/ABR control unit, for example. This trailer control module makes 
it possible for the braking pressure or the braking force in the trailer 
brake system to be controlled from a remote location, i.e., from the 
tractor. 
The individual wheel brakes usually make contact at different times; in 
particular, the wheel brakes of the tractor usually make contact at a 
different time than those of the trailer. This means that, when a braking 
process begins, the braking actions at the individual wheel brakes occur 
at different times. A similar type of behavior can be observed upon 
completion of the braking process and the brakes are released. The braking 
behavior is especially unsatisfactory when the hysteresis between the 
force required to initiate the braking action (braking pressure) and the 
force expended when the brakes are released (braking pressure) is 
different between the individual wheel brakes or the component vehicles. 
The process according to the invention provides solutions by means of which 
it is possible to compensate for this hysteresis and to arrive at 
essentially the same application and release forces at the wheel brakes. 
The basic idea of the process according to the invention is that the 
application and release forces of the individual wheel brakes or of the 
component vehicles are determined from the change over time in the 
deceleration of the vehicle and the assignment of the forces produced. The 
hysteresis compensation is then accomplished through appropriate 
correction of the nominal values for the individual wheel brakes or 
component vehicles on the basis of the difference between the application 
forces and between the release forces. 
The behavior of the vehicle or vehicle combination on which the 
determination of the application and release forces is based is described 
on the basis of the diagrams shown in FIGS. 2a and 2b, which illustrate a 
pneumatic brake system by way of example. The deceleration Z of the 
vehicle or of the vehicle combination is plotted versus the actuation 
distance .beta. of the brake pedal for the pressure buildup phase (FIG. 
2a) and also for the pressure reduction phase on release of the brake 
(FIG. 2b). Upon actuation of the brake pedal by the driver, a brake 
tensioning force is built up at the individual wheel brakes. Let us say 
that, after the brake pedal has been actuated in such a way as to travel a 
distance A, a first wheel brake or the brake system of the first component 
vehicle of a vehicle combination exercises its braking function. This 
leads to the deceleration of the vehicle. The curve of the deceleration 
shows an inflection near this point A. This inflection is determined by 
comparison of the deceleration values before and after the contact point 
associated with actuation distance A. The brake tensioning force (braking 
pressure) exerted on the wheel brake is then used as a basis for storing 
the application pressure of the first wheel brake of the first component 
vehicle. Let us say that now an additional wheel brake or the brakes of 
the second component vehicle make contact at actuation distance B, 
after--in terms of both time and distance--contact has been made at point 
A. Here, too, the deceleration curve shows an inflection, which is 
recognized in the same way as described above. And again, after the 
inflection in the deceleration curve is recognized, the application force 
of the second wheel brake or of the second component vehicle is derived 
from the brake tensioning force (braking pressure) just applied. 
A similar curve for the deceleration versus the actuation distance is 
obtained upon release of the brake (FIG. 2b). Let us say the second wheel 
brake or the brakes of the second component vehicle release at an 
actuation distance of C, whereas the first wheel brake or the brakes of 
the first component vehicle release at an actuation distance of D. 
Evaluable inflections in the deceleration curve can be identified at both 
C and at D; and, after the inflection points have been determined, the 
release forces for the wheel brake or component vehicle in question can be 
derived from the corresponding brake tensioning forces (braking forces). 
To compensate for the hysteresis between the application forces and between 
the release forces of the brakes (A to B or D to C), the differences are 
found between the determined application forces and between the release 
forces of the brakes (difference A-B, D-C). The corresponding values are 
then used as offset values for controlling the second wheel brake or 
second component vehicle, the first offset value being intended for the 
pressure buildup phase, the second offset value for the pressure reduction 
phase. This offset value corrects the course of the nominal brake 
tensioning force for the associated wheel brake or component vehicle with 
respect to the actuation distance of the brake pedal in such a way that 
the wheel brakes in question make contact at the same time and separate at 
the same time. When the process according to the invention is applied to 
the brake systems of component vehicles, the correction value represents a 
correction of the nominal value for the associated brake system. From this 
nominal value, the wheel-specific nominal values are then formed, possibly 
by means of wheel-specific corrections (also in relation to the 
application and release forces). 
In certain concrete cases, the following difficulties can occur in 
connection with the use of the process according to the invention. If the 
application forces differ only slightly and the release forces differ only 
slightly, it is more difficult for the system to recognize the inflection 
points on the deceleration curves. It is therefore advantageous to use, 
for example, the trailer control module or the control module of the 
second wheel brake to influence the curve of the brake tensioning force 
being sent to the trailer or to this wheel brake in such a way as to make 
it easier to recognize the inflection points. It is advantageous in this 
case to stagger the buildup or reduction of the force in the component 
vehicles or wheel brakes. As a result, the time interval between the 
application of the wheel brakes or the release of the wheel brakes becomes 
artificially increased, which means that the system can more easily 
recognize and assign the inflection points to the wheel brakes or 
component vehicles. 
It has also been found in a concrete example that, in cases where the 
hystereses are widely different, the order in which the values are 
detected can be turned around in extreme cases. Two contact values are 
determined during the pressure buildup phase, and two release values are 
found during the pressure reduction phase. If the hystereses are different 
enough, the sequence of these values during the buildup and reduction 
phases can be turned around and thus lead to an incorrect assignment of 
the values to the individual wheel brakes or component vehicles. 
To avoid this, it is advantageous for a wheel brake or the brake system of 
the tractor to be measured separately from the other wheel brake or other 
component vehicle. The separate measurement of the application and release 
forces of the brake system of the tractor or of the first wheel brake is 
done on a roller test stand or during a special braking process, where the 
application and release values thus found are stored as vehicle constants. 
When the component vehicle is being operated without a trailer, 
furthermore, the application and release values can be determined by a 
method other than that based on deceleration. Then, during the operation 
of both vehicles together, the application and release values of the 
tractor determined in this way are used to assign the values determined by 
the process described above to the correct component vehicle or to the 
correct wheel brake. The hysteresis is then compensated on the basis of 
the current values determined as described above, which it has been 
possible to assign correctly to the individual component vehicles or wheel 
brakes on the basis of the stored values. 
The process described above for determining the application and release 
pressures is explained by the flow chart shown in FIG. 3 on the basis of a 
preferred exemplary embodiment of an electro-pneumatic brake system. The 
section of the program sketched in the flow chart is initiated during the 
pressure buildup or pressure reduction phase when the conditions for the 
identification of the brake system are present. These conditions can 
include, for example, a time condition, a condition for the length of time 
the vehicle has been operated, a condition for the actuation of the brake 
pedal, etc. Once an identification process has been completed successfully 
during an operating cycle, no further identifications are performed during 
this cycle. 
As soon as this section of the program has started, it checks in an 
advantageous exemplary embodiment (broken line) to see in a first step 100 
whether a counter running concurrently has reached its predetermined 
maximum value T.sub.1max. If this is not the case, then in step 102 the 
buildup or reduction in the pressure in the trailer is blocked. If the 
counter has reached its maximum value, then in step 104 the pressure is 
allowed to build up or decrease in the trailer. As a result of this 
measure, a delay is achieved in the buildup of the braking pressure or in 
the release of the brake between the two component vehicles. In the 
following step 106, the deceleration Z is determined by comparing the 
velocity of the vehicle with a vehicle velocity value determined during a 
preceding run of the program. Then, in question step 108, the program 
checks to see whether there is an inflection in the deceleration curve on 
the basis of the current deceleration and the deceleration value found 
during the preceding run of the program. If this is not the case, the 
program checks to see in step 110 whether the conditions are present for 
cancelling the identification process. The identification is canceled 
when, for example, a specified maximum time interval has been exceeded or 
when the braking process has been ended. Otherwise, the program goes back 
to step 100. If an inflection in the deceleration curve was found in step 
108, then in step 112 a flag is inspected to see if it is has a value of 
1. If this is not the case, then in step 114, the braking pressure P 
prevailing at this moment is stored, possibly under consideration of a 
tolerance value .DELTA.i, as the application/release pressure P.sub.A 
/P.sub.L ; and, in the following step 116, the flag is set to a value of 
1. Then, in step 118, in the preferred exemplary embodiment, the value 
determined in step 114 is assigned to a component vehicle on the basis of 
the stored values P.sub.AZ and P.sub.LZ for the tractor, and the program 
section goes back to step 100. If, when this section of the program runs 
again, question step 108 finds a new inflection, then, because the flag 
has a value of 1, in step 120 another pressure value for the application 
or release pressure is stored on the basis of the prevailing pressure P 
and possibly the tolerance value .DELTA.i, this new pressure value being 
assigned in step 118 to the corresponding component vehicle. 
The assignment to the tractor occurs when the determined values are 
sufficiently close to the stored reference values. Otherwise, the 
determined pressure value is assigned to the second component vehicle. 
As a result of the measures illustrated in FIG. 3, therefore, the 
application/release pressures are stored on the basis of the determination 
of the inflections of the deceleration curve during both the pressure 
buildup phase and the pressure reduction phase. Through the advantageous 
addition of the measures for delaying the braking or delaying the release 
of the braking at the brakes of the component vehicles and for assigning 
the stored values on the basis of previously determined values, it is 
possible to determine the application and release pressures of the brake 
system of a combination vehicle in a reliable, certain, and accurate 
manner. 
The pressure values which have been determined are then used during the 
operation of the brake system to compensate for the hysteresis between the 
application pressures and between the release pressures of the various 
brake systems. This is shown in FIG. 4. After the program starts, the 
actuation distance .beta. of the brake pedal is accepted as input in the 
first step 200. Then, in step 202, the application pressures P.sub.AZ, 
P.sub.AA, and the release pressure P.sub.LZ, P.sub.LA Of the individual 
component vehicles are accepted as input; and the difference between the 
application pressures of the tractor and the trailer (.DELTA..sub.1) and 
the difference between the release pressures of the tractor and the 
trailer (.DELTA..sub.2) are determined in step 204. Then, in step 206, the 
nominal pressure value P.sub.sollz for the tractor is determined on the 
basis of the actuation distance .beta. and additional variables such as 
axle loads, wheel velocities, etc. Next, in step 208, the program checks 
to see whether pressure is being built up. If the answer is yes, step 210 
reads the nominal pressure P.sub.sollA to be produced in the trailer from 
a previously determined characteristic diagram on the basis of the 
actuation distance .beta. and the difference .DELTA..sub.1. If the brake 
system is not in a pressure buildup phase, step 212 of the program checks 
to see whether pressure is being released. If the answer is yes, step 214 
reads the nominal pressure for the trailer from a predetermined 
characteristic diagram based on the actuation distance .beta. and the 
difference .DELTA..sub.2. If pressure is not being released, then in step 
216 the nominal pressure for the trailer remains constant at the current 
value. in final step 218 of this section of the program, the nominal 
values determined for the tractor and the trailer are sent to their proper 
destinations. 
The nominal values are arrived at in steps 210 and 214 in such a way that 
the onset of the braking action and its end occur in the trailer at 
essentially the same time as they do in the tractor. As a result, the 
hysteresis present between the tractor and the trailer is essentially 
compensated. 
In addition to the determination of the application/release pressures and 
the hysteresis compensation in combination vehicles described in 
conjunction with FIGS. 3 and 4, the process according to the invention 
described above can also be used for the individual wheel brakes of a 
vehicle. The vehicle can also be equipped with other types of brake 
systems, in which the operating parameter to be determined will not be the 
braking pressure but rather the braking moment, the brake tensioning 
force, etc.