CHECKING AN ACTUAL-PRESSURE SENSOR OF A MOTOR VEHICLE BRAKE SYSTEM

Abstract

A method for checking an actual-pressure sensor of a motor vehicle brake system includes establishing the duration of a reaction interval, establishing a threshold value, operating the motor vehicle brake system in an electronic braking mode where the brake system delivers a braking pressure for operation of a trailer brake of a trailer connected to the motor vehicle. The reaction interval starts at the beginning of the braking process and pneumatic actual braking pressure values are measured by means of an actual-pressure sensor, where a starting braking pressure is measured by the actual-pressure sensor at the beginning of a braking process carried out in the electronic braking mode for the trailer. Pneumatic reaction braking pressures are measured within the reaction interval and compared to a threshold value. Brake system operation is discontinued in the electronic operating mode if none of the measured reaction braking pressures exceeds the threshold value.

Claims

1. A method for checking an actual-pressure sensor (13) of a motor vehicle brake system (8), the said method comprising: (100): establishing a duration of a reaction interval (?t); (200): establishing a threshold value (p.sub.min); (300) operating a motor vehicle brake system (8) of a motor vehicle (1) in an electronic braking mode, wherein the motor vehicle brake system (8) delivers a braking pressure (p.sub.aus) to a trailer (2) connected to the motor vehicle (1), so that a trailer brake system (7) of the trailer (2) can be actuated by means of the braking pressure (p.sub.aus); (400): starting the reaction interval (?t) at the beginning of a braking process and measuring pneumatic actual braking pressure values (p.sub.ist) by means of an actual-pressure sensor (13), wherein a starting braking pressure (p.sub.start) of the motor vehicle brake system (8) is measured by the actual-pressure sensor (13) at the beginning of the braking process which is carried out in the electronic braking mode for the trailer (2); (500): measuring pneumatic reaction braking pressures (p.sub.re; p.sub.re1, p.sub.re2, p.sub.re3) within the reaction interval (?t) by means of the actual-pressure sensor (13); (600): checking whether the reaction braking pressures (p.sub.re; p.sub.re1, p.sub.re2, p.sub.re3) exceed the threshold value (p.sub.min); and (600a): continuing the operation of the motor vehicle brake system (8) in the electronic operating mode when at least one measured reaction braking pressure (p.sub.re3) exceeds the threshold value (p.sub.min), or (600b): discontinuing the operation of the motor vehicle brake system (8) in the electronic operating mode when none of the measured reaction braking pressures (pre) exceeds the threshold value (p.sub.min).

2. The method according to claim 1, wherein the process steps (400) to (600) are carried out for every braking process initiated when the motor vehicle brake system (8) is operated in the electronic braking mode.

3. The method according to claim 1, wherein the process steps (500) and (600) are only carried out when, in process step (400), the value 0 bar is measured as the starting braking pressure (p.sub.start).

4. The method according to claim 1, comprising: determining a target value (p.sub.soll) for the braking pressure (p.sub.aus), the target value (p.sub.soll) not in excess of 1 bar; and regulating the braking pressure (p.sub.aus) on the basis of the target value (p.sub.soll) for the braking pressure (p.sub.aus) and on the basis of the actual braking pressure values (p.sub.ist) measured by the actual-pressure sensor (13).

5. The method according to claim 4, wherein the threshold value (p.sub.min) is set at a value between 0 bar and the target value (p.sub.soll) of the braking pressure.

6. The method according to claim 1, wherein the reaction interval (?t) is set to a duration of 50 milliseconds to 300 milliseconds.

7. The method according to claim 1, wherein the motor vehicle (1) comprises a trailer control valve system (10) with an inlet valve (20a), and the method comprises moving the inlet valve (20a) to an open condition at least once within the reaction interval (?t) in order to increase the braking pressure (p.sub.aus) of the motor vehicle brake system (8) in such manner that the reaction braking pressures (p.sub.re3) exceed the threshold value (.sub.min).

8. The method according to claim 7, wherein within the reaction interval (?t) the inlet valve (20a) is moved to the open condition several times, and moving the inlet valve (20a) to the open condition for a first time includes holding open the inlet valve (20a) for a longer time than when subsequently moving the inlet valve to the open condition again after the first time.

9. The method according to claim 8, comprising moving the inlet valve (20a) to the open condition while no braking process is being carried out, so that a pressure peak (26) occurs in the time variation (27) of the braking pressure (p.sub.aus) while no braking operation is in progress.

10. The method according to claim 9, wherein the trailer control valve system (10) comprises an outlet valve (20b), and the method comprises moving the inlet valve (20a) to a closed condition after the inlet valve has previously been moved to the open condition, and the outlet valve (20b) is then moved once to the open condition while no braking operation is in progress, so that the pressure peak (26) is counteracted.

11. A motor vehicle (1) comprising a motor vehicle brake system (8) with an electronic control unit (17), wherein the motor vehicle brake system (8) is configured to be operated in an electronic braking mode, and the motor vehicle brake system (8) delivers to the trailer (2) connected to the motor vehicle (1) a braking pressure so that a trailer brake system (7) can be actuated by virtue of the said braking pressure, and the electronic control unit (17) is configured to carry out the process steps (400) to (600) in accordance with the method according to claim 1.

12. The method according to claim 7, comprising moving the inlet valve (20a) to the open condition while no braking process is being carried out, so that a pressure peak (26) occurs in the time variation (27) of the braking pressure (p.sub.aus) while no braking operation is in progress.

13. The method according to claim 12, wherein the trailer control valve system (10) comprises an outlet valve (20b), and the method comprises moving the inlet valve (20a) to a closed condition after the inlet valve has previously been moved to the open condition, and the outlet valve (20b) is then moved once to its open condition while no braking operation is in progress, so that the pressure peak (26) is counteracted.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Below, example embodiments of the invention are explained in greater detail with reference to the schematic drawing, wherein the same or similar elements are denoted by the same indexes and which shows:

[0024] FIG. 1: a motor vehicle to which a trailer is coupled,

[0025] FIG. 2: details of a motor vehicle brake system for the motor vehicle in FIG. 1,

[0026] FIG. 3: a pressure/time diagram with an output pressure of the motor vehicle brake system in FIG. 2, which increases with time,

[0027] FIG. 4: a working sequence diagram of an example embodiment of a method according to the invention for checking an actual-pressure sensor of the motor vehicle brake system in FIG. 2,

[0028] FIG. 5: a bar chart with opening times of an inlet valve of the motor vehicle brake system in FIG. 2, for increasing its output pressure, and

[0029] FIG. 6: a pressure/time diagram with an output pressure of the motor vehicle brake system in FIG. 2 that is increasing over time due to the opening times of the inlet valve as shown in FIG. 5.

DETAILED DESCRIPTION

[0030] FIG. 1 shows a motor vehicle 1 and a trailer 2. The motor vehicle is, for example, an agricultural utility vehicle, in particular a tractor. The motor vehicle has a motor 3 for driving wheels 4 of the motor vehicle 1. The trailer 2 does not have a motor. The motor vehicle 1 also has a trailer coupling 5 by means of which the motor vehicle 1 and the trailer 2 are mechanically coupled to one another, so that the motor vehicle 1 can in particular tow the trailer 2. In the example embodiment shown, the trailer 2 rolls on four wheels 6 (or wheel pairs). At least one of those wheels 6 can be braked and brought to a standstill by a trailer brake system 7 of the trailer 2.

[0031] The motor vehicle 1 has a motor vehicle brake system 8. The motor vehicle brake system 8 is connected to the trailer brake system 7 by way of a compressed-air line 9 (the trailer control line). The term connected is in particular understood to mean that the elements respectively connected to one another are pneumatically linked with one another, i.e., that a pneumatic pressure medium, in particular compressed air, is supplied under pressure by one element and can act on or in the other element. In the example embodiment shown, the motor vehicle brake system 8 produces a pneumatic brake pressure p.sub.aus which is delivered via the compressed-air line 9 to the brake system 7 of the trailer, which uses the pneumatic brake pressure paus to brake the wheels 6. Thus, the compressed-air line 9 serves for the pressure control of the trailer's brake system 7 and can therefore be regarded as the trailer control line.

[0032] FIG. 2 shows some details of the motor vehicle brake system 8 according to FIG. 1. The motor vehicle brake system 8 comprises an electro-pneumatic trailer control valve 10 (eTCV). A pressure outlet 11 of the trailer control valve system 10 is connected to the compressed-air line 9. To the pressure outlet is applied the pneumatic brake pressure paus, which corresponds to a set pressure level of the motor vehicle brake system 8. From the compressed-air line 9 a measurement line 12 branches off, which leads to an actual-pressure sensor 13. In addition, in the example embodiment shown the motor vehicle brake system 8 comprises a foot pedal 14, a foot-brake valve 15 (FBV), a required-pressure sensor 16, and an electronic control unit 17 (ECU).

[0033] A driver of the motor vehicle 1 (not shown) can actuate the foot pedal 14 with his foot, which pedal is connected to the foot-brake valve 15. Depending on the actuation intensity of the foot pedal 14, the foot-brake valve 15 produces a control pressure for the motor vehicle brake system 8. In the example embodiment shown, the motor vehicle brake system 8 comprises two independent control circuits 18 and 19. Alternatively, only one control circuit can be provided, which is then connected to the trailer control valve 10 and the required-pressure sensor 16.

[0034] The required-pressure sensor 16 conveys electronically the value of a desired service brake pressure p.sub.soll (the desired value of the pneumatic brake pressure p.sub.aus) called for by the driver by way of the foot pedal 14, to the electronic control unit 17. On the basis of the service brake pressure p.sub.soll required and the actual brake pressure values (p.sub.ist) measured by the actual-pressure sensor 13, the electronic control unit 17 regulates the output pressure p.sub.aus of the electronically controlled trailer control valve system 10. For that purpose, the trailer control valve system 10 that acts in a closed control circuit with the actual-pressure sensor 13 can comprise an inlet valve 20a and an outlet valve 20b. The actual brake pressure value p.sub.ist measured by the actual-pressure sensor 13 is conveyed electronically to the electronic control unit 17. Thus, the electronic control unit 17 can access the actual brake pressure value p.sub.ist measured by the actual-pressure sensor 13 and the desired brake pressure value p.sub.soll determined by the required-pressure sensor 16. The actual pressure value p.sub.ist measured by the actual-pressure sensor 13 can then be linked with the desired service brake pressure p.sub.soll by way of a braking characteristic diagram 21 stored in the electronic control unit 17.

[0035] The motor vehicle brake system 8 can be operated in an electronic braking mode, in accordance with which a service brake pressure p.sub.soll desired by the driver is automatically regulated, so that the output pressure p.sub.aus is produced in the compressed-air line 9. In that case the driver does not have to intervene in the regulating process. The actual brake pressure value p.sub.ist detected by the actual-pressure sensor 13 normally corresponds to the output pressure p.sub.aus. Now and then, however, it can happen that the actual-pressure sensor 13 measures an actual pressure value of 0 bar and/or conveys it to the electronic control unit 17. This can be attributed to a defect of the actual-pressure sensor 13 itself, or to a defect in the connection, in particular the measurement line 12, between the actual-pressure sensor 13 and the output pressure p.sub.aus.

[0036] In that event the cases described below can be distinguished. In a first case the desired service brake pressure p.sub.soll can be greater than 1 bar. If the actual-pressure sensor 13 measures an actual brake pressure value of 0 bar and conveys that to the electronic control unit 17, then the difference between the desired service brake pressure p.sub.soll and the value actually measured is larger than 1 bar. In such a case, known plausibility checks stored in the electronic control unit 17 can recognize a functional fault, the trailer control valve system 10 is brought to a mechanical redundancy mode, and the electronic braking mode is switched off.

[0037] In a second case, for the pneumatic brake pressure p.sub.aus, a target value p.sub.soll can be established, which is not greater than 1 bar. If the actual-pressure sensor 13 measures an actual brake pressure value of 0 bar and conveys that to the electronic control unit 17, then the difference between the desired service brake pressure p.sub.soll and the value actually measured is less than 1 bar. In that case, in systems known from the prior art, no plausibility check is available. The natural reaction of the regulating system then poses a problem: since the measured actual brake pressure value p.sub.ist is 0 bar, the electronic control unit 17 tries to reach the target value p.sub.soll, typically by opening the inlet valve 20a of the trailer control valve 10. But since the actual-pressure sensor 13 still indicates 0 bar, the inlet valve 20a also remains continually open. This results in a sudden increase of the pneumatic brake pressure p.sub.aus in the trailer control line 9 and an undesired full braking, even though the target pressure p.sub.soll has a value below 1 bar.

[0038] In order to provide a solution for the second case group described, namely (p.sub.soll<=1 bar; p.sub.ist=0 bar), a plausibility check is carried out which relates to a defect of the actual-pressure sensor 13 or a system defect that results in an actual brake pressure value of p.sub.ist=0 bar at the actual-pressure sensor 13. In this case the plausibility check is in particular carried out at each beginning of a braking operation carried out in the electronic trailer braking mode. The plausibility check is intended to confirm whether a minimum output pressure p.sub.min (threshold value) at the actual-pressure sensor 13 has been reached as a result of multiple actuations of the inlet valve 20a within a predetermined time t.sub.max. If the minimum output pressure p.sub.min has not been reached within the predetermined time t.sub.max, then the motor vehicle brake system 8 automatically switches off the electronic trailer control system and changes to a mechanical redundancy mode in order to avoid the above-mentioned over-braking of the trailer 2.

[0039] FIGS. 3 and 4 show how a corresponding method for checking the actual-pressure sensor 13 of the motor vehicle brake system 8 can proceed. In a first process step 100 a timed reaction interval ?t is established. For example, the timed reaction interval ?t is stored in a memory of the electronic control unit 17, for example, by way of a man-machine interface designed for the purpose. The reaction interval ?t is a time period that begins at the starting time point t.sub.start when a braking operation is initiated by the motor vehicle brake system 8 while the motor vehicle brake system 8 is being operated in its electronic braking mode. The beginning of the braking process can be taken to be, for example, the actuation of the foot pedal 14 by the driver. Alternatively, for example, it can also be taken as the electronic conveying of the desired service brake pressure p.sub.soll to the electronic control unit 17 by the required-pressure sensor 16. The reaction interval ?t ends at a set end time point t.sub.max, which is also established during the step 100. Thus, the reaction interval ?t is obtained as the difference between the set end time point t.sub.max and the starting time point t.sub.start (?t=t.sub.max?t.sub.start). The reaction interval ?t can in particular be established once and for all for a number of braking operations and can be stored, for example, in the electronic control unit 17. If needs be, the value of the reaction interval ?t can be changed. Furthermore, the reaction interval ?t can have several different values, for different braking modes. In particular, the reaction interval ?t does not have to be established for each braking operation.

[0040] In a second process step 200, a pneumatic threshold value p.sub.min is established. For example, the pneumatic threshold value p.sub.min can be stored in the memory of the electronic control unit 17, for example, by way of the above-mentioned man-machine interface. As shown in FIG. 4, the second process step 200 can take place after the first process step 100. Alternatively, the first and second process steps 100 and 200 can take place simultaneously or, however, the second process step 200 can take place before the first process step 100. The threshold for the threshold value p.sub.min can be chosen at a very low pressure level, in order to recognize the defect as early as possible. In this case the threshold value p.sub.min is not chosen higher than the desired service brake pressure, i.e., in particular no higher than 1 bar. Similar to the reaction interval ?t, the threshold value p.sub.min can in particular be established once and for all for a plurality of braking operations and, for example, stored in the electronic control unit 17. If necessary, the value of the threshold value p.sub.min can be changed. Furthermore, the threshold value p.sub.min can have several different values for different kinds of braking. In particular, the threshold value p.sub.min does not have to be established for every braking operation.

[0041] In a third process step 300, as described earlier the motor vehicle brake system 8 is operated in the electronic braking mode, wherein the motor vehicle brake system 8 delivers the pneumatic braking pressure p.sub.aus to the trailer 2 connected to the motor vehicle 1, so that the trailer brake system 7 can be actuated by means of the pneumatic braking pressure p.sub.aus, or so that the trailer brake system 7 can use the pneumatic breaking pressure p.sub.aus for its actuation.

[0042] When the driver of the motor vehicle initiates a braking operation by actuating the foot pedal 14, the required-pressure sensor 16 communicates the corresponding desired service brake pressure p.sub.soll to the electronic control unit 17, and this can be regarded as the starting time t.sub.start. The desired service brake pressure p.sub.soll adopts a value which is lower than 1 bar, for example 0.7 bar. The threshold value p.sub.min can be set, for example, at a value of 0.2 bar (second process step 200). The reaction interval ?t can be set, for example, at a duration of 150 milliseconds. In a fourth process step 400, at the starting time t.sub.start a pneumatic starting brake pressure p.sub.start of the motor vehicle brake system 8 is then measured by the actual-pressure sensor 13. Moreover, the value of the measured pneumatic starting brake pressure p.sub.start can be conveyed electronically to the electronic control unit 17. In addition, in the fourth process step 400 the reaction interval ?t is started at the starting time t.sub.start.

[0043] FIG. 3 shows by a first graph 22 (fault-free checking, since the pressure p.sub.min is reached before the time point t.sub.max) the time variation of the pneumatic braking pressure p.sub.aus measured by the actual-pressure sensor 13. This first graph starts at the zero-point of the coordinate system and has an essentially exponential shape. According to the first graph 22, at the starting time t.sub.start the value 0 bar is measured as the pneumatic starting brake pressure p.sub.start (process step 400). In that case, in the sense of a plausibility check it is checked whether the value 0 bar is plausible, or whether there is a defect, particularly in the actual-pressure sensor 13. For that purpose, in a fifth process step 500 a plurality of pneumatic reaction braking pressures within the reaction interval ?t are measured by the actual-pressure sensor 13. In FIG. 3, purely as an example three such reaction pressures p.sub.re1, p.sub.re2, p.sub.re3 are shown. When the reaction interval ?t has ended, i.e., aftertime point t.sub.max, no more reaction braking pressures have to be measured for the plausibility check.

[0044] In a sixth process step 600 it is checked whether the reaction braking pressures p.sub.re1, p.sub.re2, p.sub.re3 exceed the minimum pneumatic pressure p.sub.min. In the example shown in FIG. 3, the measured actual braking pressure p.sub.ist varies in such manner that a first reaction pressure value p.sub.re1 and a second reaction pressure value p.sub.re2 have values lower than the minimum pneumatic pressure p.sub.min.

[0045] In this case the first reaction braking pressure p.sub.re1 is above the pneumatic starting brake pressure p.sub.start (0 bar). The electronic control unit 17 has previously compared the value of the pneumatic braking pressure p.sub.aus (0 bar), measured by the actual-pressure sensor 13 and sent to the electronic control unit 17, with the desired service brake pressure p.sub.soll=0.7 bar. To increase the pneumatic braking pressure p.sub.aus to the desired service brake pressure p.sub.soll, the electronic control unit 17 has directed the trailer control valve 10 system to move its inlet valve 20a to an open condition. As shown by the first graph 22, by this opening of the inlet valve 20a the pneumatic braking pressure p.sub.aus has increased in such manner that the measured first reaction braking pressure p.sub.re1 is higher than the pneumatic starting brake pressure p.sub.start (0 bar). However, the minimum pneumatic pressure p.sub.min does not exceed the first reaction braking pressure p.sub.re1.

[0046] As shown by the first graph 22, the second reaction braking pressure p.sub.re2 is higher than the first reaction braking pressure p.sub.re1. The electronic control unit 17 has previously compared the value of the first reaction braking pressure p.sub.re1, measured by the actual-pressure sensor 13 and sent to the electronic control unit 17, with the desired service brake pressure p.sub.soll, which has not yet been reached. To increase the pneumatic braking pressure p.sub.aus to the desired service brake pressure p.sub.soll, the electronic control unit 17 has directed the trailer control valve system 10 to move its inlet valve 20a to the open condition or to leave it in the open condition. As shown by the first graph 22, by this opening of the inlet valve 20a the pneumatic braking pressure p.sub.aus has increased in such manner that the measured second reaction braking pressure p.sub.re2 is higher than the first reaction braking pressure p.sub.re1. However, the second reaction braking pressure p.sub.re2 still does not exceed the minimum pneumatic pressure p.sub.min.

[0047] However, the third reaction braking pressure p.sub.re3 located within the reaction interval ?t adopts a value that does exceed the minimum pneumatic pressure p.sub.min. The electronic control unit 17 has previously compared the value of the second reaction braking pressure p.sub.re2, measured by the actual-pressure sensor 13 and sent to the electronic control unit 17, with the desired service brake pressure p.sub.soll=0.7 bar, which has not yet been reached. To increase the pneumatic braking pressure p.sub.aus to the desired service brake pressure p.sub.soll, the electronic control unit 17 has directed the trailer control valve system 10 to move its inlet valve 20a to the open condition or to leave it in the open condition. As shown by the first graph 22, by this opening of the inlet valve 20a, the pneumatic braking pressure p.sub.aus has increased in such manner that the measured third reaction braking pressure p.sub.re3 is now higher than the minimum pneumatic pressure p.sub.min. Thus, the check in process step 600 shows that at least one measured reaction braking pressure, namely, the third measured reaction braking pressure p.sub.re3, is higher than the minimum pneumatic pressure p.sub.min. On that basis it can be concluded that the actual-pressure sensor 13 has measured the value 0 bar for the pneumatic starting pressure correctly and is not defective. Consequently, the operation of the motor vehicle brake system 8 in accordance with a process step 600a in the electronic braking mode is continued (Alternative 1).

[0048] FIG. 3 shows by virtue of a second graph 23 (stuck at zero; the actual-pressure sensor 13 constantly shows 0 bar and the system check fails, since p.sub.min is not reached, in particular before time-point t.sub.max), an alternative time variation of the pneumatic braking pressure p.sub.aus measured by the actual-pressure sensor 13. The second graph 23 begins at the zero-point of the coordinate system and remains horizontal. Thus, according to this second graph 23 at the starting time-point t.sub.start the value 0 bar is measured as the pneumatic starting brake pressure p.sub.start (process step 400). In this case, in the sense of a plausibility check it is checked whether the value 0 bar is plausible or whether there is a fault, particularly in the actual-pressure sensor 13. For that purpose, in the fifth process step 500 a plurality of pneumatic reaction braking pressures pre are measured by the actual-pressure sensor 13 within the reaction interval ?t. The second graph 23 runs horizontally, so that all the measured reaction braking pressures pre adopt the value 0 bar. This means that the actual-pressure sensor 13 is continually measuring a pneumatic braking pressure p.sub.aus of 0 bar. When the reaction interval ?t has ended, i.e., after time-point t.sub.max, no more reaction braking pressures have to be measured for the plausibility check.

[0049] In the sixth process step 600 it is checked whether the reaction braking pressures pre exceed the minimum pneumatic pressure p.sub.min. Since none of the reaction braking pressures pre gets above the value 0 bar, the check shows that none of the measured reaction braking pressures pre exceed the minimum pneumatic pressure p.sub.min. On this basis it can be concluded that the actual-pressure sensor 13 has measured the value 0 bar for the pneumatic starting brake pressure p.sub.start incorrectly and is therefore defective. Consequently, in a process step 600b the operation of the motor vehicle brake system 8 in its electronic operating mode is discontinued (Alternative 2). Instead, from that time-point onward the motor vehicle brake system 8 can be operated in the mechanical redundancy mode already mentioned earlier.

[0050] For the above-described type of checking, special prerequisites and settings of the pressure regulation may be required. With a desired service brake pressure p.sub.soll of 0.5 bar, for example, the control system increases the pressure cautiously to the desired value p.sub.soll and the attempt is made not to exceed that desired value p.sub.soll. That would normally result in the minimum pneumatic pressure p.sub.min not being exceeded within the reaction interval ?t and the test would then fail because the regulation process is too slow. Accordingly, the pressure regulation is adjusted in such manner that the initial opening pulse of the inlet valve 20a of the trailer control valve system 10 is artificially prolonged in order to compel the reaction pressures to reach a particular value, specifically the pneumatic threshold value p.sub.min, more quickly.

[0051] FIG. 5 shows that a first inlet valve pulse 24 lasts longer than the next inlet valve pulse 25, which are not changed. For that purpose, when the inlet valve 20a is moved to its open condition for the first pulse 24 for the first time within the reaction interval ?t, it remains open for a longer time than it does for the pulse 25 that comes immediately after it (particularly in the example embodiment shown) and the corresponding subsequent opening times. The electronic control unit 17 can direct the trailer control valve system 10 to do that. This results in a very small peak 26 in the time variation of the pneumatic braking pressure p.sub.aus as shows by a third graph 27. FIG. 6 shows that the first pulse 24 within the reaction interval ?t results in a very small pressure peak 26 at the beginning of the pressure increase. That peak 26 is small enough not to overshoot the desired service brake pressure p.sub.soll. Instead, the pressure peak signals to the control unit software within the reaction interval ?t that the actual-pressure sensor is not stuck at 0 bar, but is capable of providing correct values.

[0052] Moreover, the scope of diagnosis can be increased still more. The above-described plausibility check is carried out during a braking demand by the driver. This leads to a high degree of diagnostic cover, but a system-fault or sensor-defect function is only recognized at the beginning of the braking process. To alert the driver to a reduced performance level, it can be helpful to check the correct functioning, in particular of the actual-pressure sensor 13, already before the braking. Since the aforesaid pressure peak can be very small (for example 100 mbar), it is possible to trigger a single high inlet valve pulse even when no braking demand has been made. For that purpose, the inlet valve 20a is moved to the open condition just once while no braking process is being carried out. In that way a further pressure peak (not shown) is produced in the time variation of the pneumatic braking pressure, while no braking process is being carried out. The inlet valve pulse can in particular be followed by an outlet valve pulse, so that the brief pressure rise has no effect upon the braking pressure p.sub.aus of the trailer 2, although it is still possible to see a value at the actual-pressure sensor 13. In that case the actual-pressure sensor 13 can be classified as OK and the system 8 remains in normal operation. For this, the inlet valve 20a can be moved to a closed condition after it has previously been moved just once to the open condition. Thereafter, in particular a very short time later, the outlet valve 20b can be moved just once to an open condition while no braking process is being carried out. In that way, the pressure peak produced by opening the inlet valve 20a just once can be counteracted.

INDEXES

[0053] p.sub.aus Pneumatic braking pressure [0054] p.sub.ist Actual braking pressure value [0055] p.sub.min Pneumatic threshold value [0056] p.sub.re Reaction pressure in the second graph [0057] p.sub.re1 First reaction pressure in the first graph [0058] p.sub.re2 Second reaction pressure in the first graph [0059] p.sub.re3 Third reaction pressure in the first graph [0060] p.sub.start Pneumatic starting pressure [0061] p.sub.soll Desired service brake pressure [0062] ?t Reaction interval [0063] t.sub.start Starting time [0064] t.sub.max End time [0065] 1 Motor vehicle [0066] 2 Trailer [0067] 3 Motor [0068] 4 Wheel [0069] 5 Trailer coupling [0070] 6 Wheel [0071] 7 Trailer brake system [0072] 8 Motor vehicle brake system [0073] 9 Compressed-air line [0074] 10 Trailer control valve system [0075] 11 Pressure outlet [0076] 12 Measurement line [0077] 13 Actual-pressure sensor [0078] 14 Foot pedal [0079] 15 Foot-brake valve [0080] 16 Required-pressure sensor 16 [0081] 17 Electronic control unit [0082] 18 First control circuit [0083] 19 Second control circuit [0084] 20a Inlet valve [0085] 20b Outlet valve [0086] 21 Braking characteristic diagram [0087] 22 First graph [0088] 23 Second graph [0089] 24 First pulse [0090] 25 Subsequent pulses [0091] 26 Pressure peak [0092] 27 Third graph [0093] 100 First process step [0094] 200 Second process step [0095] 300 Third process step [0096] 400 Fourth process step [0097] 500 Fifth process step [0098] 600 Sixth process step [0099] 600a 1st alternative process step [0100] 600b 2nd alternative process step