Method for the brake control of a vehicle combination

Abstract

A method of brake control of a vehicle combination (2) composed of a tractor vehicle (4) with an electronically controlled braking system (5) and a trailer vehicle (6, 8) with a pneumatically controlled pneumatic braking system (7, 9), involves introducing the brake control pressure (p.sub.BC) of the trailer vehicle (6, 8) into a tractor brake control line (19) extending to a tractor vehicle-side brake coupling head (26) via an electronically controlled trailer control valve (13) of the tractor vehicle (4). At the beginning of a braking operation, a pressure pulse (34) exceeding a target brake control pressure (p.sub.BC_soll) is introduced into the tractor brake control line (19). The volume of a trailer brake control line (28, 29, 32) coupled to the brake coupling head (26) is ascertained, and the absolute value (p.sub.PI) and/or the duration (t.sub.PI) of the pressure pulse (34) introduced are/is established depending on the ascertained volume.

Claims

1. A method for controlling brakes of a vehicle combination (2) composed of a tractor vehicle (4) with an electronically controlled hydraulic or pneumatic braking system (5) and a trailer vehicle (6, 8) with a pneumatically controlled pneumatic braking system (7, 9), the method comprising the following steps: introducing, during a brake operation, via an electronically controlled trailer control valve (13) of the tractor vehicle (4), a brake control pressure (psc) of the trailer vehicle (6, 8) into at least one tractor brake control line (19) extending to a tractor vehicle-side brake coupling head (26); introducing, at the beginning of the braking operation, a pressure pulse (34) exceeding a target brake control pressure (p.sub.BC_soll) to be applied into the tractor brake control line (19); ascertaining determining, by an electronic control unit (ECU), an available internal volume of at least one trailer brake control line (28, 29, 32) coupled to the brake coupling head (26) and in fluid communication with the tractor brake control line (19), wherein the available internal volume depends on a quantity of trailer vehicles coupled to the brake coupling head (26) via the at least one trailer brake control line (28, 29, 32); and determining at least one of an absolute value (p.sub.PI) and a duration (t.sub.PI) of the pressure pulse (34) communicated to the at least one trailer brake control line (28, 29, 32) depending on the volume of the at least one trailer brake control line (28, 29, 32) wherein the volume of the at least one trailer-side brake control line (28, 29, 32) is determined via the following steps: detecting, via sensors, during a pressure drop (p.sub.PD) due to a disengagement of the wheel brakes (35, 36, 37, 38; 40, 41, 42, 43, 44, 45, 46, 47) of the vehicle combination (2), the air pressure prevailing in the tractor brake control line (19) and the at least one trailer brake control line (28, 29, 32) connected to the tractor vehicle-side brake coupling head (26); determining, based on a time profile of the air pressure, at least one of the duration (t.sub.PD) of the pressure drop (p.sub.PD) and the mean pressure gradient ((dp.sub.PD/dt.sub.PD).sub.m) of the pressure drop (p.sub.PD); and determining, on the basis of at least one of the duration of the pressure drop (t.sub.PD) and the mean pressure gradient ((dp.sub.PD/dt.sub.PD).sub.m) of the pressure drop (p.sub.PD), the volume of the at least one trailer brake control line (28, 29, 32) of the at least one trailer vehicle (6, 8).

2. The method as claimed in claim 1, wherein the ascertained determining available volume of the at least one trailer brake control line (28, 29, 32) of the trailer vehicle (6, 8) is ascertaining determining after every start-up of the tractor vehicle (4) and, thereafter, stored in a non-volatile data memory of then electronic control unit (ECU).

3. The method as claimed in claim 1, comprising the further step of inferring, in response to a long duration (t.sub.PD) of the pressure drop (p.sub.PD) exceeding a predetermined time period, a total volume of the at least one trailer brake control line (28, 29, 32) of more than one trailer vehicle (6, 8).

4. The method as claimed in claim 1, wherein, in the event that the mean pressure gradient (|(dp.sub.PD/dt.sub.PD).sub.m|) of the pressure drop (p.sub.PD) falls below a predetermined limit pressure gradient, a total volume of the at least one trailer brake control line (28, 29, 32) of more than one trailer vehicle (6, 8) is inferred.

5. The method as claimed in claim 1, wherein, when a current value of the available internal volume of the at least one trailer brake control line (28, 29, 32) has not been ascertained determined, a value of the available internal volume of the at least one trailer brake control line (28, 29, 32) is utilized that was ascertaining determining, and stored, in a preceding driving operation of the vehicle combination (2).

6. The method as claimed in claim 1, wherein, when a current value of the available internal volume of the at least one trailer brake control line (28, 29, 32) has not been ascertained determined, a predefined standard value of the available internal volume of the at least one brake control line (28, 29, 32) is utilized.

7. The method as claimed in claim 1, wherein the absolute value (p.sub.PI) of the pressure pulse, as compared to the target brake control pressure (p.sub.BC_soll) is greater with increasing volume of the at least one trailer brake control line (28, 29, 32).

8. The method as claimed in claim 1, wherein the absolute value (p.sub.PI) of the pressure pulse, as compared to the target brake control pressure (p.sub.BC_soll) to be applied, is limited by a previously established maximum permissible pressure (p.sub.BC_max) in the at least one trailer brake control line (28, 29, 32) in such a way that the previously established maximum pressure (p.sub.BC_max) in the at least one trailer brake control line (28, 29, 32) is not exceeded during the pressure pulse (p.sub.BC_soll+p.sub.PIp.sub.BC_max).

9. The method as claimed in claim 1, wherein the duration (t.sub.PI) of the pressure pulse is greater with increasing volume of the at least one trailer brake control line (28, 29, 32).

10. A method for controlling brakes of a vehicle combination (2) including a tractor vehicle (4) with an electronically controlled hydraulic or pneumatic braking system (5) and a trailer vehicle (6, 8) with a pneumatically controlled pneumatic braking system (7, 9), the method comprising the following steps: determining, by an electronic control unit (ECU), an available internal volume of at least one trailer brake control line (28, 29, 32) coupled to a tractor vehicle-side brake coupling head (26) of a tractor vehicle (4), the trailer brake control line in fluid communication with at least one tractor brake control line (19) that extends to the brake coupling head, wherein the available internal volume depends on a quantity of trailer vehicles coupled to the brake coupling head (26) via the at least one trailer brake control line (28, 29, 32); and after determining the volume of the at least one trailer brake control line, determining, depending on the determined volume of the at least one trailer brake control line (28, 29, 32), at least one of an absolute value (p.sub.PI) and a duration (t.sub.PI) of a pressure pulse (34) to be applied and communicated to the at least one trailer brake control line (28, 29, 32); wherein the tractor vehicle (4) includes an electronically controlled trailer control valve (13) wherein, during a braking operation, a brake control pressure (p.sub.BC) of the trailer vehicle (6, 8) is introduced via the electronically controlled trailer control valve (13) of the tractor vehicle (4) into the at least one tractor brake control line (19) that extends to the coupling head (26); and introducing, at the beginning of the braking operation, the pressure pulse (34), wherein the pressure pulse exceeds a target brake control pressure (p.sub.BC_soll) to be applied into the tractor brake control line (19); wherein the available internal volume of the at least one trailer-side brake control line (28, 29, 32) is determined via the following steps: detecting, via sensors, during a pressure drop (p.sub.PD) due to a disengagement of the wheel brakes (35, 36, 37, 38; 40, 41, 42, 43, 44, 45, 46, 47) of the vehicle combination (2), the air pressure prevailing in the tractor brake control line (19) and the at least one trailer brake control line (28, 29, 32) connected to the tractor vehicle-side brake coupling head (26); determining, based on a time profile of the air pressure, at least one of the duration (t.sub.PD) of the pressure drop (p.sub.PD) and the mean pressure gradient ((dp.sub.PD/dt.sub.PD).sub.m) of the pressure drop (p.sub.PD); and determining, on the basis of at least one of the duration of the pressure drop (t.sub.PD) and the mean pressure gradient ((dp.sub.PD/dt.sub.PD).sub.m) of the pressure drop (p.sub.PD), the volume of the at least one trailer brake control line (28, 29, 32) of the at least one trailer vehicle (6, 8).

11. The method as claimed in claim 10, wherein the available internal volume of the at least one trailer brake control line (28, 29, 32) of the trailer vehicle (6, 8) is determined after every start-up of the tractor vehicle (4) and, thereafter, stored in a non-volatile data memory of the electronic control unit (ECU).

12. The method as claimed in claim 10, comprising the further step of determining, in response to a long duration (t.sub.PD) of the pressure drop (p.sub.PD) exceeding a predetermined time period, a total available internal volume of the at least one trailer brake control line (28, 29, 32) of more than one trailer vehicle (6, 8).

13. The method as claimed in claim 10, wherein, in the event that the mean pressure gradient (|(dp.sub.PD/dt.sub.PD).sub.m|) of the pressure drop (p.sub.PD) falls below a predetermined limit pressure gradient, a total available internal volume of the at least one trailer brake control line (28, 29, 32) of more than one trailer vehicle (6, 8) is determined.

14. The method as claimed in claim 10, wherein, when a current value of the available internal volume of the at least one trailer brake control line (28, 29, 32) has not been determined, a value of the available internal volume of the at least one trailer brake control line (28, 29, 32) is utilized that was determined, and stored, in a preceding driving operation of the vehicle combination (2) for determining the absolute value or duration of the pressure pulse.

15. The method as claimed in claim 10, wherein, when a current value of the volume of the at least one trailer brake control line (28, 29, 32) has not been determined, a predefined standard value of the available internal volume of the at least one brake control line (28, 29, 32) is utilized to determine the absolute value or duration of the pressure pulse.

16. The method as claimed in claim 10, wherein the absolute value (p.sub.PI) of the pressure pulse, as compared to the target brake control pressure (p.sub.BC_soll) is greater with increasing volume of the at least one trailer brake control line (28, 29, 32).

17. The method as claimed in claim 10, wherein the absolute value (p.sub.PI) of the pressure pulse, as compared to the target brake control pressure (p.sub.BC_soll) to be applied, is limited by a previously established maximum permissible pressure (p.sub.BC_max) in the at least one trailer brake control line (28, 29, 32) in such a way that the previously established maximum pressure (p.sub.BC_max) in the at least one trailer brake control line (28, 29, 32) is not exceeded during the pressure pulse (p.sub.BC_soll+p.sub.PIp.sub.BC_max).

18. The method as claimed in claim 10, wherein the duration (t.sub.PI) of the pressure pulse is greater with increasing volume of the at least one trailer brake control line (28, 29, 32).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 shows a top view of a vehicle combination made up of a tractor vehicle and two trailer vehicles;

(3) FIG. 2 shows profiles, represented in a diagram, of brake control pressures and wheel brake pressures of the vehicle combination according to FIG. 1 over time without an application of a pressure pulse;

(4) FIG. 3 shows profiles, represented in a diagram, of brake control pressures at a coupling head of a brake control line over time for ascertaining the volume of the brake control line, and

(5) FIG. 4 shows profiles, represented in a diagram, of brake control pressures and wheel brake pressures of the vehicle combination according to FIG. 1 over time with an application of a pressure pulse.

DETAILED DESCRIPTION OF THE DRAWINGS

(6) The vehicle combination 2 depicted in a top view in FIG. 1 includes a tractor vehicle 4 designed as an agricultural tractor, a first trailer vehicle 6 coupled onto the tractor vehicle 4, and a second trailer vehicle 8 coupled onto the first trailer vehicle 6. The first trailer vehicle 6 is designed as a tandem axle trailer with two tandem axles 10, 12. The second trailer vehicle 8 is a drawbar trailer with a steerable front axle 14 and a rear axle 16 rigidly secured at a frame. The vehicle combination could also have a reversed trailer sequence or two identical trailer types. Wheel brakes 35, 36, 37, 38; 40, 41, 42, 43; 44, 45, 46, 47 are arranged at the wheels of the tractor vehicle 4 and at the wheels of the two trailer vehicles 6, 8 for decelerating the same.

(7) The agricultural tractor 4 comprises an electronically controlled hydraulic braking system 5 (not completely represented). The first trailer vehicle 6 comprises a pneumatically controlled and pneumatically actuatable first braking system 7 with a first supply line 20 and a first brake control line 28. The second trailer vehicle 8 also comprises a pneumatically controlled and pneumatically actuatable second braking system 9 with a second supply line 24 and a second brake control line 32.

(8) For the brake control of the wheel brakes 40, 41, 42, 43; 44, 45, 46, 47 of the two trailer vehicles 6, 8, the braking system 5 of the agricultural tractor 4 comprises a trailer control valve 13, controlled by an electronic control unit ECU, from which a supply line 17 of the tractor vehicle 4 extends to a tractor vehicle-side supply coupling head (red) 18 and a brake control line 19 of the tractor vehicle 4 extends to a tractor vehicle-side brake coupling head (yellow) 26. The supply line 20 of the first trailer vehicle 6 is connected at the tractor vehicle-side supply coupling head 18, and the brake control line 28 of the first trailer vehicle 6 is connected at the tractor vehicle-side brake coupling head 26.

(9) From the supply line 20 of the first trailer vehicle 6, a branched-off supply line 21 extends to a trailer-side supply coupling head (red) 22 (not readily apparent in FIG. 1) situated at the rear of the first trailer vehicle 6 and at which a second supply line 24 of the second trailer vehicle 8 is connected. Similarly, a brake control line 29 branched-off from the brake control line 28 of the first trailer vehicle 6 also extends to a trailer-side brake coupling head (yellow) 30 (not readily apparent in FIG. 1) situated at the rear of the first trailer vehicle 6 and at which a second brake control line 32 of the second trailer vehicle 8 is connected.

(10) The two trailer-side supply lines 20, 24 and the two trailer-side brake control lines 28, 32 extend to a first trailer brake valve 25 at the first trailer vehicle 6 and to a second trailer brake valve 27 at the second trailer vehicle 8, in which the wheel brake pressures p.sub.B at the wheel brakes 40, 41, 42, 43; 44, 45, 46, 47 of the vehicle axles 10, 12; 14, 16 of the two trailer vehicles 6, 8 are set. Due to the large common volume of the brake control lines 28, 29, 32, in the case of a braking operation without further measures, there is a time-delayed response of the wheel brakes 40, 41, 42, 43; 44, 45, 46, 47 at the vehicle axles 10, 12; 14, 16 of the two trailer vehicles 6, 8. This is illustrated in the diagram depicted in FIG. 2. In this diagram, the profiles of the brake control pressure p.sub.BC1 at the brake coupling head 26 of the tractor vehicle 4, of the wheel brake pressure p.sub.B_RA1 at the wheel brakes 40, 41, 42, 43 of the two vehicle axles 10, 12 of the first trailer vehicle 6, of the brake control pressure p.sub.BC2 at the brake coupling head 30 of the first trailer vehicle 6, of the wheel brake pressure p.sub.B_FA2 at the wheel brakes 44, 45 of the front axle 14 of the second trailer vehicle 8, and of the wheel brake pressure p.sub.B_RA2 at the wheel brakes 46, 47 of the rear axle 16 of the second trailer vehicle 8 during a braking operation are depicted over time t.

(11) The conventionally controlled braking operation represented there begins at the point in time t.sub.1, after which, initially, the brake control pressures p.sub.BC1, p.sub.BC2 at the two brake coupling heads 26, 30 increase. At the point in time t.sub.2, the wheel brake pressures p.sub.B_RA1, p.sub.B_FA2, p.sub.B_RA2 at the wheel brakes 40, 41, 42, 43; 44, 45, 46, 47 of the vehicle axles 10, 12; 14, 16 of the two trailer vehicles 6, 8 also increase. At approximately the point in time t.sub.3, the brake control pressures p.sub.BC1, p.sub.BC2 at the two brake coupling heads 26, 30 and the wheel brake pressures p.sub.B_FA2, p.sub.B_RA2 at the wheel brakes 44, 45, 46, 47 of the vehicle axles 14, 16 of the second trailer vehicle 8 reach their setpoint values, while the maximum value of the wheel brake pressure p.sub.B_RA1 at the wheel brakes 40, 41, 42, 43 of the vehicle axles 10, 12 of the first trailer vehicle 6 is first reached at the point in time t.sub.4. At the point in time t.sub.5, the braking operation is terminated, and the brake control pressures p.sub.BC1, p.sub.BC2 as well as the wheel brake pressures p.sub.B_RA1, p.sub.B_FA2, p.sub.B_RA2 subsequently drop again to values close to zero.

(12) In order to improve the response characteristic of the wheel brakes 40, 41, 42, 43; 44, 45, 46, 47 at the vehicle axles 10, 12; 14, 16 of the two trailer vehicles 6, 8, it is provided that, at the beginning of a braking operation, a pressure pulse exceeding the brake control pressure p.sub.BC1_soll to be applied is applied, by the trailer control valve 13, into the tractor vehicle-side brake control line 19 extending to the brake coupling head 26 of the tractor vehicle 4 and, thereby, also into the connected brake pressure line 28 of the first trailer vehicle 6, into the brake pressure line 29 branched off therefrom, and into the brake control line 32 of the second trailer vehicle 8. The absolute value p.sub.PI of the applied pressure pulse, by which the pressure pulse exceeds the brake control pressure p.sub.BC1_soll actually to be applied, and/or the duration t.sub.PI of the applied pressure pulse are/is established depending on the volume of the connected brake control lines 28, 29, 32 of the coupled trailer vehicles 6, 8. The pressure difference p.sub.PI and/or the duration t.sub.PI of the pressure pulse are/is increased as the volume of the connected brake control lines 28, 29, 32 increases.

(13) The volume of the connected trailer-side brake control lines 28, 29, 32 is ascertained in advance during a disengagement of the wheel brakes 35, 36, 37, 38; 40, 41, 42, 43; 44, 45, 46, 47 or the parking brakes of the vehicle combination 2 in such a way that, during a disengagement of the wheel brakes 35, 36, 37, 38; 40, 41, 42, 43, 44, 45, 46, 47 of the vehicle combination 2, the air pressure difference p.sub.PD setting in, in the brake control lines 19, 28, 29, 32 connected to the tractor vehicle-side brake coupling head 26, is measured during the pressure drop by the electronic control unit ECU by means of a pressure sensor 33, and that, on the basis of the time profile of the air pressure, the duration t.sub.PD of the pressure drop and/or the mean pressure gradient (dp.sub.PD/dt.sub.PD).sub.m of the pressure drop are/is determined, and that, on the basis of the duration of the pressure drop t.sub.PD and/or the mean pressure gradient (dp.sub.PD/dt.sub.PD).sub.m of the pressure drop, the volume of the connected brake control lines 19, 28, 29, 32 of the coupled trailer vehicles 6, 8 is determined.

(14) A corresponding time profile of the brake control pressure p.sub.BC1 present at the brake coupling head 26 of the tractor vehicle 4 during a disengagement of the wheel brakes is depicted in the diagram of FIG. 3. As is readily apparent, the brake control pressure p.sub.BC1 drops degressively along the represented curve profile from an initial value p.sub.PD_1 at a first point in time t.sub.PD_1 before the pressure drop over a time period t.sub.PD up to a second point in time t.sub.PD_2 after the pressure drop to a second value p.sub.PD_2. The pressure gradient dp.sub.PD/dt.sub.PD of the pressure drop p.sub.PD is given at an arbitrary point in time in the time period t.sub.PD by the pressure differential dp.sub.PD with respect to the time differential dt.sub.PD. The mean pressure gradient (dp.sub.PD/dt.sub.PD).sub.m of the pressure drop p.sub.PD is represented by a wide-dashed line.

(15) The effect of the application of a pressure pulse 34 at the beginning of a braking operation is represented in the diagram depicted in FIG. 4, in which the time profiles of the brake control pressure p.sub.BC1 at the brake coupling head 26 of the tractor vehicle 4, of the wheel brake pressure p.sub.B_RA1 at the wheel brakes 40, 41, 42, 43 of the vehicle axles 10, 12 of the first trailer vehicle 6, of the brake control pressure p.sub.BC_2 at the brake coupling head 30 of the first trailer vehicle 6, of the wheel brake pressure p.sub.B_FA2 at the wheel brakes 44, 45 of the front axle 14 of the second trailer vehicle 8, and of the wheel brake pressure p.sub.B_RA2 at the wheel brakes 46, 47 of the rear axle 16 of the second trailer vehicle 8 during a braking operation according to the invention are depicted.

(16) The braking operation begins at the point in time t.sub.1, after which, initially, the brake control pressures p.sub.BC1, p.sub.BC2 at the two brake coupling heads 26, 30 increase. Due to the pressure pulse 34 applied with the absolute value p.sub.PI and the duration t.sub.PI, the wheel brake pressures p.sub.B_RA1, p.sub.B_FA2, p.sub.B_RA2 at the wheel brakes 40, 41, 42, 43; 44, 45, 46, 47 of the vehicle axles 10, 12; 14, 16 of the trailer vehicles 6, 8 now increase already at the point in time t.sub.2. At the point in time t.sub.3, the brake control pressures p.sub.BC1, p.sub.BC2 at the two brake coupling heads 26, 30 reach their setpoint values p.sub.BC1_soll relatively quickly. Shortly thereafter, at approximately the point in time t.sub.4, the wheel brake pressures p.sub.B_RA1, p.sub.B_FA2, p.sub.B_RA2 at the wheel brakes 40, 41, 42, 43; 44, 45, 46, 47 of the vehicle axles 10, 12; 14, 16 of the trailer vehicles 6, 8 also reach their setpoint values. At the point in time t.sub.5, the braking operation is terminated, whereupon the brake control pressures p.sub.BC1, p.sub.BC2 and the wheel brake pressures p.sub.B_RA1, p.sub.B_FA2, p.sub.B_RA2 drop again to values close to zero.

(17) It becomes apparent from the comparison of the diagrams from FIG. 2 and FIG. 4 that the brake control pressure p.sub.BC2 at the brake coupling head 30 of the first trailer vehicle 6 and the wheel brake pressures p.sub.B_RA1, p.sub.B_FA2, p.sub.B_RA2 at the wheel brakes 40, 41, 42, 43; 44, 45, 46, 47 of the vehicle axles 10, 12; 14, 16 of both trailer vehicles 6, 8 increase considerably sooner and reach their setpoint values considerably faster with the application of the pressure pulse 34 at the beginning of the braking operation according to FIG. 4. As a result, an apparently improved braking operation of the vehicle combination 2 is achieved, because the vehicle combination 2 is straight during a braking operation and a jack-knifing about a trailer coupling of one of the trailer vehicles 6, 8 or about the drawbar axle of the first trailer vehicle 6 is prevented. Due to the preceding ascertainment of the volume of the brake control lines 19, 28, 29, 32 and the adaptation of the absolute value p.sub.PI and/or of the duration t.sub.PI of the applied pressure pulse 34 depending on the volume of the brake control lines 19, 28, 29, 32, the pressure pulse 34 is optimally matched to the particular trailer vehicles 6, 8 coupled onto the tractor vehicle 4.

(18) While the above description constitutes the preferred embodiments of the present invention, the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.

LIST OF REFERENCE NUMBERS

(19) 2 vehicle combination 4 tractor vehicle, agricultural tractor 5 braking system of the tractor vehicle 6 first trailer vehicle, tandem axle trailer 7 braking system of the first trailer vehicle; first pneumatic braking system 8 second trailer vehicle, drawbar trailer 9 braking system of the second trailer vehicle; second pneumatic braking system 10 first tandem axle at the first trailer vehicle 12 second tandem axle at the first trailer vehicle 13 trailer control valve 14 front axle of the second trailer vehicle 16 rear axle of the second trailer vehicle 17 supply line of the tractor vehicle 18 tractor vehicle-side supply coupling head (red) 19 tractor vehicle-side brake control line 20 supply line of the first trailer vehicle 21 supply line branched off from supply line 20 22 supply coupling head (red) at the first trailer vehicle 24 supply line of the second trailer vehicle 25 trailer brake valve at the first trailer vehicle 26 tractor vehicle-side brake coupling head (yellow) 27 trailer brake valve at the second trailer vehicle 28 brake control line of the first trailer vehicle 29 brake control line branched off from the brake control line 28 30 brake coupling head (yellow) at the first trailer vehicle 32 brake control line of the second trailer vehicle 33 pressure sensor 34 pressure pulse 35 first wheel brake at the front axle of the tractor vehicle 36 second wheel brake at the front axle of the tractor vehicle 37 first wheel brake at the rear axle of the tractor vehicle 38 second wheel brake at the rear axle of the tractor vehicle 40 first wheel brake at the first tandem axle 41 second wheel brake at the first tandem axle 42 first wheel brake at the second tandem axle 43 second wheel brake at the second tandem axle 44 first wheel brake at the front axle of the second trailer vehicle 45 second wheel brake at the front axle of the second trailer vehicle 46 first wheel brake at the rear axle of the second trailer vehicle 47 second wheel brake at the rear axle of the second trailer vehicle ECU electronic control unit p.sub.B_RA1 wheel brake pressure at the wheel brakes of the first trailer vehicle p.sub.B_FA2 wheel brake pressure at the wheel brakes of the front axle of the second trailer vehicle p.sub.B_RA2 wheel brake pressure at the wheel brakes of the rear axle of the second trailer vehicle p.sub.BC brake control pressure p.sub.BC_max maximum pressure in the brake control line p.sub.BC_soll target brake control pressure (general) to be applied p.sub.BC1 brake control pressure at the brake coupling head 26 of the tractor vehicle p.sub.BC1_soll brake control pressure to be applied p.sub.BC2 brake control pressure at the brake coupling head 30 of the first trailer vehicle p.sub.PD_1 initial brake control pressure before the pressure drop p.sub.PD_2 brake control pressure after the pressure drop dp.sub.PD pressure differential of the pressure drop dp.sub.PD/dt.sub.PD pressure gradient of the pressure drop (dp.sub.PD/dt.sub.PD).sub.m mean pressure gradient of the pressure drop p.sub.PD pressure drop, air pressure difference caused by the pressure drop p.sub.PI absolute value of the pressure pulse t time t.sub.1-t.sub.5 points in time t.sub.1-t.sub.5 points in time t.sub.PD_1 first point in time (before the pressure drop) t.sub.PD_2 second point in time (after the pressure drop) dt.sub.PD time differential of the pressure drop t.sub.PD duration of the pressure drop t.sub.PI duration of the pressure pulse