Checking an actual pressure sensor of a motor vehicle brake system

Abstract

Method for checking an actual pressure sensor (13) of a motor vehicle brake system (8), the method comprising the method steps (100): determining a time length of a reaction interval (Δt), (200): Setting a limit pressure (p 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) supplies a brake pressure (p aus ) so that a trailer braking system (7) of the trailer (2) by means of the braking pressure (p aus ) can be operated, (400): Starting the reaction interval (Δt) at the beginning of the braking process and measuring the actual pneumatic brake pressure values ​​(p ist ) by means of an actual pressure sensor (13), wherein a starting brake pressure (p start) of the motor vehicle braking system (8) is measured by means of the actual pressure sensor (13) at the beginning of a braking operation which is carried out in the electronic braking mode for the trailer (2), (500): Measuring pneumatic reaction brake pressures (p re ; p re1 , p re2 , p re3 ) within the reaction interval (Δt) by means of the actual pressure sensor (13), (600): Check whether the reaction brake pressures (p re ; p re1 , p re2 , p re3 ) the limit pressure (p min ), whereby (600a) the operation of the motor vehicle brake system (8) is continued in the electronic operating mode if at least one measured reaction brake pressure (p re3 ) the limit pressure (p min ) and (600b) the operation of the motor vehicle brake system (8) in the electronic operating mode is aborted if none of the measured reaction brake pressures (pre ) the limit pressure (p min ) exceeds.

Description

[0001] The invention relates to a method for checking an actual pressure sensor of a motor vehicle brake system and to a motor vehicle having a control device which is configured to carry out the method.

[0002] The invention particularly relates to so-called off-highway vehicles such as agricultural tractors, which have a motor vehicle braking system with an electronic trailer control system. In order to be able to provide such a trailer control system with as few sensors as possible, the correct functioning of the electronic trailer control system is typically verified by plausibility checks. However, particularly with sensors that permanently measure a pressure of 0 bar, it is difficult for the electronic trailer control system to detect this and maintain the correct functioning of the electronic trailer control system. Thus, if a desired service brake pressure is less than 1 bar and an actual pressure sensor measures an actual brake pressure value of 0 bar, the deviation between the desired service brake pressure and the actually measured value will be less than 1 bar.The natural response of the control system now presents a problem: Since the measured actual brake pressure value is 0 bar, the system typically attempts to reach the target value by opening an inlet valve of a trailer control valve. However, since the actual pressure sensor constantly indicates 0 bar, the inlet valve is also constantly opened. This leads to a sudden increase in brake pressure and an unwanted emergency braking, even though the target pressure is below 1 bar.

[0003] For example, DE 10 2013 110 736 A1 discloses detecting a pressure sensor defect using a plausibility check. It checks whether pressure signals are different from zero or greater than a threshold. If this is the case, the pressure sensor is functional. If, however, the pressure signal is zero or below a threshold, the associated pressure sensor is defective.

[0004] Furthermore, DE 196 33 835 A1 discloses a method and a device for controlling the braking system of a vehicle, wherein, when the parking brake actuation is detected, the electrically influenced control variable of the service brake is controlled to predetermined values.

[0005] The object of the present invention is to improve the control of a motor vehicle braking system with an electronic trailer control system. This object is achieved by the subject matter of the independent patent claims. Advantageous embodiments are the subject matter of the subclaims, the following description, and the figures.

[0006] According to the present invention, a plausibility check for detecting a malfunction of an output pressure sensor (hereinafter: actual pressure sensor) is proposed. The invention describes a special plausibility check for the early detection of a malfunction of the actual pressure sensor. The described plausibility check relates to a sensor or system defect that leads to a pressure measurement of 0 bar at the actual pressure sensor. The idea is to start a specially developed plausibility check each time braking begins that is carried out via an electronic trailer braking mode. In the electronic braking mode, the braking of the motor vehicle and, in this case, the braking of the trailer also occurs only indirectly, thereby enabling extensive electronic control of the braking performance via driver assistance systems such as an anti-lock braking system or a starting control system.In particular, this electronic control system enables precise adjustment of the braking power to achieve effective deceleration of the vehicle in each situation. The actual brake pressure values ​​measured by the actual pressure sensor are an important input variable for the further control of the vehicle's braking system. The plausibility check is intended to confirm whether a minimum output pressure is reached at the actual pressure sensor, particularly as a result of multiple intake valve actuations within a predefined time. If the minimum output pressure is not reached within the predefined time, the system automatically deactivates the electronic trailer brake control and can, in particular, switch to a mechanical redundancy mode to prevent the trailer from overbraking as described above.

[0007] In this sense, according to a first aspect of the invention, a method for checking an actual pressure sensor of a motor vehicle brake system is provided. The method comprises, in particular, the following method steps. (100): Setting a time length of a reaction interval, (200): Setting a limit pressure, in particular a pneumatic or hydraulic limit pressure, (300): Operating a motor vehicle braking system of a motor vehicle in an electronic braking mode, wherein the motor vehicle braking system provides a braking pressure (in particular a pneumatic or hydraulic braking pressure) to a trailer connected to the motor vehicle, so that a trailer braking system of the trailer can be actuated by means of the braking pressure, (400): starting the reaction interval at the beginning of the braking process and measuring actual brake pressure values ​​(in particular pneumatic or hydraulic actual brake pressure values) by means of an actual pressure sensor, wherein a starting brake pressure of the motor vehicle brake system is measured by means of the actual pressure sensor at the beginning of a braking process which is carried out in the electronic braking mode for the trailer, (500): Measuring reaction brake pressures, in particular pneumatic or hydraulic reaction brake pressures, within the reaction interval by means of the actual pressure sensor, (600): Check whether the reaction brake pressures exceed the limit pressure, whereby (600a) the operation of the motor vehicle brake system is continued in the electronic operating mode if at least one measured reaction brake pressure exceeds the limit pressure, and (600b) the operation of the motor vehicle braking system in the electronic operating mode is interrupted if none of the measured reaction braking pressures exceeds the limit pressure.

[0008] The method according to the invention is particularly characterized by the fact that no additional actual pressure sensor is required for plausibility checks. Furthermore, the driver of the motor vehicle does not recognize or notice the plausibility check, as it runs in the background. Furthermore, early detection of an error is enabled to initiate the system response, thus preventing unintentional emergency braking, in particular including a warning to the driver.

[0009] Method steps (400) to (600) can be performed during each braking operation initiated when the motor vehicle braking system is operated in electronic braking mode. This allows a particularly high safety standard to be achieved. On the other hand, computing time can be saved by, in particular, performing method steps (500) and (600) only when the value 0 bar is measured as the starting brake pressure in method step (400). Alternatively, it can be provided that method steps (500) and (600) are only performed when a starting brake pressure between 0 bar and the specified limit pressure is measured in method step (400).

[0010] According to one embodiment, a target value not higher than 1 bar is defined for the pneumatic or hydraulic brake pressure in particular, wherein the brake pressure is regulated based on the defined target value for the brake pressure and based on the actual brake pressure values ​​measured by the actual pressure sensor. Thus, the method according to the invention can be used in particular for gentle braking requests, for which no testing was previously known from the prior art. The limit pressure can in particular be set to a value that lies between 0 bar and the target value of the brake pressure. The response interval can, for example, be set to a time length that lies between 50 milliseconds and 300 milliseconds.

[0011] The motor vehicle braking system can have a trailer control valve, in particular an electro-pneumatic one, with an inlet valve. The output pressure of the motor vehicle braking system can be regulated by means of the inlet valve by setting the inlet valve to an open state in order to increase the output pressure. In this context, one can speak of an impulse to increase the output pressure or the pneumatic brake pressure. The inlet valve can also be set back to a closed state in order to prevent or at least reduce an increase in the output pressure. The intensity of the pressure increase can be further varied by varying the degree of opening of the inlet valve. Thus, a wider opening of the inlet valve will generally lead to a greater pressure increase than a narrower opening.The inlet valve can, in particular, be opened and closed several times in succession within the reaction interval. According to one embodiment, it is provided that the inlet valve is opened at least once within the reaction interval in order to increase the brake pressure of the motor vehicle brake system such that the measured reaction brake pressures exceed the limit pressure. The trailer control valve can be instructed accordingly by an electronic control unit. The inlet valve can also be opened such that not only the limit pressure is exceeded, but instead a service brake pressure requested or desired for the braking process is exceeded.

[0012] In particular, pressure control can be achieved by "carefully" increasing the pneumatic brake pressure to a desired service brake pressure below 1 bar, e.g., to 0.5 bar, without exceeding this target value. To achieve this, the inlet valve may, for example, not be fully opened, but only partially. This careful pressure control could result in the minimum pneumatic pressure not being exceeded within the response interval, resulting in the test failing because the control is too slow. To counteract this, pressure control is set in such a way that an initial opening pulse of the inlet valve of the trailer control valve is artificially extended in order to force the response pressures to reach a certain value, in particular the pneumatic limit pressure, more quickly.In this sense, according to one embodiment, the inlet valve is opened several times within the reaction interval, whereby the inlet valve remains open for a longer period of time when it is opened for the first time than when it is opened again after the first time. This leads to a very small pressure peak in the temporal progression of the pneumatic brake pressure at the beginning of the reaction interval. This pressure peak is small enough not to exceed the desired service brake pressure. Instead, the pressure peak signals to the control unit software within the reaction interval that the actual pressure sensor is not stuck at 0 bar, but is able to deliver correct values.

[0013] Furthermore, it is possible to further increase the diagnostic scope. The plausibility check described above is performed while the driver requests a brake. This leads to a high degree of diagnostic coverage, but only detects a system or sensor malfunction at the beginning of the braking process. To alert the driver to reduced performance, it might be helpful to check the sensor performance before braking. Since the pressure peaks mentioned can be very small (e.g., ∼ 100 mbar), it is possible to trigger a single high inlet valve pulse even when there is no braking request. In this sense, according to one embodiment, the inlet valve is set to an open state once while no braking process is being performed, so that a pressure peak occurs in the temporal progression of the brake pressure while no braking process is being performed.

[0014] In particular, the inlet valve pulse can be followed by an outlet valve pulse, so that this brief pressure increase has no effect on the trailer's braking pressure, but it is still possible to see a value on the actual pressure sensor. In this case, the sensor can be marked as "OK" and the system remains in normal operation. In this sense, according to one embodiment, the trailer control valve, in particular an electro-pneumatic one, has an outlet valve, wherein the inlet valve is placed in a closed state after having previously been placed in the open state once, and the outlet valve is subsequently placed in an open state once while no braking is performed, thus counteracting the pressure peak.The advantage of this strategy lies in the continuous use of these "test pulses" (a slight pressure increase via the intake valve) when the vehicle is stationary or while driving. With this solution, the diagnostic coverage is even higher, and errors are detected even before braking. With this type of testing, it is also advantageous if the actual pressure sensor is located near the source where the pressure is generated. This prevents the sensor from failing to detect the effect of the short pulse. In this sense, one embodiment provides for the actual pressure sensor to be located adjacent to the intake valve.

[0015] According to a second aspect of the invention, a motor vehicle is provided. The motor vehicle comprises a motor vehicle braking system with an electronic control unit. The motor vehicle braking system is configured to be operated in an electronic braking mode, wherein the motor vehicle braking system provides a trailer connected to the motor vehicle with a braking pressure, in particular pneumatic or hydraulic, such that a trailer braking system of the trailer can be actuated by means of the braking pressure. The electronic control unit is configured, in particular, to carry out method steps (400) to (600) according to a method according to the first aspect of the invention. The motor vehicle is, in particular, an off-highway vehicle. An off-highway vehicle can be understood as a vehicle that can be operated at least predominantly off-road.The vehicle can, in principle, be any non-rail-bound vehicle. It can be an articulated or articulated vehicle. The vehicle can have a trailer. For example, such a vehicle with a trailer can be a vehicle combination, a truck combination, or a semi-trailer combination. The vehicle combination can, for example, be an agricultural machine (tractor) with a trailer. The vehicle can also be a dump truck, a tractor, or a truck.

[0016] In the following, embodiments of the invention are explained in more detail with reference to the schematic drawing, wherein identical or similar elements are provided with the same reference numerals. Fig. 1 a motor vehicle to which a trailer is coupled, Fig. 2 Details of a motor vehicle braking system for the motor vehicle according to Fig. 1, Fig. 3 a pressure / time diagram with an output pressure of the motor vehicle brake system increasing over time according to Fig. 2, Fig. 4 a workflow diagram of an embodiment of a method according to the invention for checking an actual pressure sensor of the motor vehicle brake system according to Fig. 2, Fig. 5 a bar chart with opening times of an inlet valve of the motor vehicle brake system according to Fig. 2 to increase its output pressure and Fig. 6 a pressure / time diagram with a pressure drop due to the opening times of the intake valve after Fig. 5 over time increasing output pressure of the motor vehicle brake system according to Fig. 2.

[0017] Fig. 1 shows a motor vehicle 1 and a trailer 2. The motor vehicle 1 is, for example, an agricultural utility vehicle, in particular a tractor. The motor vehicle 1 has an engine 3 for driving wheels 4 of the motor vehicle 1. The trailer 2 does not have an engine. The motor vehicle 1 further 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 exemplary embodiment shown, the trailer 2 rolls on four wheels 6 (or pairs of wheels). At least one of these wheels 6 can be braked and brought to a standstill by a trailer braking system 7 of the trailer 2.

[0018] The motor vehicle 1 has a motor vehicle braking system 8. The motor vehicle braking system 8 is connected to the trailer braking system 7 via a compressed air line 9 (“trailer control line”). The term “connected” is to be understood in particular as meaning that the respective interconnected elements are pneumatically connected to one another, i.e. that a pneumatic pressure medium, in particular compressed air, can be provided under pressure by one element and act on the other element or in the other element. In the exemplary embodiment shown, the motor vehicle braking system 8 generates a pneumatic braking pressure p aus , which is transmitted via the compressed air line 9 to the trailer brake system 7, which generates the pneumatic brake pressure p aus to brake the wheels 6. The compressed air line 9 thus serves to control the pressure of the trailer braking system 7 and can therefore also be referred to as the trailer control line.

[0019] Fig. 2 shows some details of the motor vehicle braking system 8 according to Fig. 1. The motor vehicle braking system 8 comprises an electro-pneumatic trailer control valve 10 (in English: electronic Trailer Control Valve eTCV). A pressure output 11 of the trailer control valve 10 is connected to the compressed air line 9. The pneumatic brake pressure p is present at the pressure output 11. aus which corresponds to a set pressure level of the motor vehicle brake system 8. A measuring line 12 branches off from the compressed air line 9 and leads to an actual pressure sensor 13. Furthermore, the motor vehicle brake system 8 in the illustrated embodiment has a foot pedal 14, a foot brake valve 15 (FBV), a demand pressure sensor 16, and an electronic control unit 17 (ECU).

[0020] A driver of motor vehicle 1 (not shown) can use his or her foot to actuate foot pedal 14, which is connected to foot brake valve 15. Foot brake valve 15 generates a control pressure for motor vehicle brake system 8 depending on the intensity of the actuation of foot pedal 14. In the illustrated embodiment, motor vehicle brake system 8 comprises two independent control circuits 18 and 19. Alternatively, only one control circuit may be provided, which is then connected to trailer control valve 10 and demand pressure sensor 16.

[0021] The demand pressure sensor 16 electronically transmits the value of a desired service brake pressure p soll (Target value of the pneumatic brake pressure p aus ), which is requested by the driver via the foot pedal 14, to the electronic control unit 17. The electronic control unit 17 regulates based on the desired service brake pressure p solland the actual brake pressure values ​​(p ist ) the outlet pressure p aus of the electronically controlled trailer control valve 10. For this purpose, the trailer control valve 10, which operates in a closed control circuit with the actual pressure sensor 13, can have an inlet valve 20a and an outlet valve 20b. The actual brake pressure value p measured by the actual pressure sensor 13 ist is transmitted electronically to the electronic control unit 17. Thus, the electronic control unit 17 can respond to the actual brake pressure value p measured by the actual pressure sensor 13. ist and to the desired service brake pressure p measured by the demand pressure sensor 16 soll The actual brake pressure value p detected by the actual pressure sensor 13 ist can be controlled via a brake map 21 stored in the electronic control unit 17 with the desired service brake pressure p soll be linked.

[0022] The motor vehicle brake system 8 can be operated in an electronic brake mode, according to which a service brake pressure p desired by the driver soll is automatically adjusted so that the output pressure p aus in the compressed air line 9. The driver does not need to intervene in the control. The actual brake pressure value p detected by the actual pressure sensor 13 ist normally corresponds to the outlet pressure p aus . In rare cases, however, it may happen that the actual pressure sensor 13 measures an actual pressure value of 0 bar and / or transmits it to the electronic control unit 17. This may be due to a defect in the actual pressure sensor 13 itself or to a defect in the connection, particularly in the measuring line 12, between the actual pressure sensor 13 and the output pressure p aus be due to.

[0023] The following cases can be distinguished: In the first case, the desired service brake pressure p soll greater than 1 bar. If the actual pressure sensor 13 detects an actual brake pressure value p ist of 0 bar and transmits it to the electronic control unit 17, then the deviation between the desired service brake pressure p soll and the actually measured value is greater than 1 bar. In such a case, known plausibility checks stored in the electronic control unit 17 can detect a malfunction, place the trailer control valve 10 into a mechanical redundancy mode, and deactivate the electronic braking mode.

[0024] In a second case, the pneumatic brake pressure p aus a target value p soll which is not higher than 1 bar. If the actual pressure sensor 13 detects an actual brake pressure value p istof 0 bar and transmits it to the electronic control unit 17, then the deviation between the desired service brake pressure p soll and the actually measured value is less than 1 bar. In this case, no plausibility check is available in state-of-the-art systems. The natural reaction of the control system is now problematic: Since the measured actual brake pressure value p ist 0 bar, the electronic control unit 17 typically attempts to set the target value p by opening the inlet valve 20a of the trailer control valve 10. soll However, since the actual pressure sensor 13 constantly indicates 0 bar, the inlet valve 20a is also constantly opened. This leads to a sudden increase in the pneumatic brake pressure p aus in the trailer control line 9 and an undesirable emergency braking, although the target pressure p soll takes on a value below 1 bar.

[0025] In order to find a solution for the second case group described (p soll <= 1 bar; p ist = 0 bar), a plausibility check is carried out which relates to a defect in the actual pressure sensor 13 or a system defect which leads to an actual brake pressure value p ist = 0 bar at the actual pressure gauge 13. The plausibility check is performed, in particular, at the start of each braking operation performed via the electronic trailer braking mode. The plausibility check is intended to confirm whether a minimum output pressure p min (Limit pressure) at the actual pressure sensor 13 as a result of several actuations of the inlet valve 20a within a predefined time t max is reached. If the minimum outlet pressure p min not within the predefined time t maxis reached, the motor vehicle braking system 8 automatically switches off the electronic trailer brake control and goes into a mechanical redundancy mode in order to avoid the above-mentioned overbraking of the trailer 2.

[0026] Fig. 3 and Fig. 4 shows how a corresponding method for checking the actual pressure sensor 13 of the motor vehicle brake system 8 can be executed. Thus, in a first method step 100, a temporal reaction interval Δt can be defined. For example, the temporal reaction interval Δt can be stored in a memory unit of the electronic control unit 17, e.g., via a human-machine interface configured for this purpose. The reaction interval Δt is a time period that begins at the start time t startbegins, at which a braking process is initiated by means of the motor vehicle braking system 8, wherein the motor vehicle braking system 8 is operated in its electronic braking mode. The start of the braking process can be defined, for example, as the actuation of the foot pedal 14 by the driver. Alternatively, the electronic transmission of the desired service brake pressure p soll by the demand pressure sensor 16 to the electronic control unit 17. The reaction interval Δt ends at a specified end time t max , which is also determined in step 100. Thus, the reaction interval Δt results from the difference between the specified end time t max and the start time t start (Δt = t max - t start ). The reaction interval t maxIn particular, it can be set once and permanently for a large number of braking applications and stored, for example, in the electronic control unit 17. If necessary, the value of the reaction interval Δt can be changed. Furthermore, the reaction interval Δt can have several different values ​​for different braking types. In particular, the reaction interval Δt does not have to be set for every braking application.

[0027] In a second method step 200, a pneumatic limit pressure p min For example, the pneumatic limit pressure p min stored in the memory unit of the electronic control unit 17, e.g., via the human-machine interface mentioned above. The second method step 200 can - as by Fig. 4 - after the first method step 100. Alternatively, the first method step 100 and the second method step 200 can run simultaneously, or the second method step 200 can run before the first method step 100. The threshold for the limit pressure p min can be selected with a very low pressure level to detect the defect as early as possible. The limit pressure p min will not be higher than the desired service brake pressure p soll selected, in particular not higher than 1 bar. Similar to the reaction interval t max the limit pressure p min in particular, it can be set once and permanently for a large number of braking operations and can be stored, for example, in the electronic control unit 17. If necessary, the value of the limit pressure p min Furthermore, the limit pressure p minhave several different values ​​for different braking types. The limit pressure p min In particular, it does not have to be determined every time you brake.

[0028] In a third method step 300, the motor vehicle brake system 8 is operated in the electronic braking mode as described above, wherein the motor vehicle brake system 8 supplies the trailer 2, which is connected to the motor vehicle 1, with the pneumatic brake pressure p aus so that the trailer brake system 7 by means of the pneumatic brake pressure p aus can be actuated or so that the trailer brake system 7 uses the pneumatic brake pressure p aus can use.

[0029] When the driver of the motor vehicle 1 initiates braking by actuating the foot pedal 14, the request pressure sensor 16 transmits the corresponding desired service brake pressure p sollto the electronic control unit 17, which is the starting time t start The desired service brake pressure p soll assumes a value below 1 bar, e.g. 0.7 bar. The limit pressure p min can, for example, be set to a value of 0.2 bar (second method step 200). The reaction interval Δt can, for example, be set to a time length of 150 milliseconds. In a fourth method step 400, at the starting time t start a pneumatic start-brake pressure p start of the motor vehicle brake system 8 by means of the actual pressure sensor 13. Furthermore, the value of the measured pneumatic start brake pressure p start be transmitted electronically to the electronic control unit 17. Furthermore, in the fourth method step 400, the reaction interval Δt at the start time t start started.

[0030] Fig. 3 shows by a first graph 22 (“fault free”; fault-free test, since the pressure p min before time t max is reached) the time course of the pneumatic brake pressure p aus , which is measured by the actual pressure sensor 13. The first graph 22 starts at the zero point of the coordinate system and has an essentially exponential curve. According to the first graph 22, at the starting time t start the value 0 bar as pneumatic start brake pressure p start measured (method step 400). In this case, as a plausibility check, it should be checked whether the value 0 bar is plausible or whether there is a defect, in particular in the actual pressure sensor 13. For this purpose, in a fifth method step 500, a plurality of pneumatic reaction brake pressures are measured within the reaction interval Δt using the actual pressure sensor 13. In Fig. 3 are purely exemplary three such reaction brake pressures p re1 , p re2 , p re3 marked. When the reaction interval Δt is finished, i.e. after time t max , reaction brake pressures no longer need to be measured for the plausibility check.

[0031] In a sixth method step 600, it is checked whether the reaction brake pressures p re1 , p re2 , p re3 the minimum pneumatic pressure p min In the example according to Fig. 3 the measured actual brake pressure value P ist such that a first reaction brake pressure p re1 and a second reaction brake pressure p re2 Values ​​below the minimum pneumatic pressure p min assume.

[0032] The first reaction brake pressure p re1 is higher than the pneumatic start-brake pressure p start(0 bar). The electronic control unit 17 had previously measured the value (0 bar) of the pneumatic brake pressure p measured by the actual pressure sensor 13 and transmitted to the electronic control unit 17. aus with the desired service brake pressure p soll = 0.7 bar. To determine the pneumatic brake pressure p aus to the desired service brake pressure p soll To raise the trailer, the electronic control unit 17 has instructed the trailer control valve 10 to open its inlet valve 20a. By opening the inlet valve 20a, the pneumatic brake pressure p has increased according to the first graph 22. aus increased so that the measured first reaction brake pressure p re1 is higher than the pneumatic start brake pressure p start (0 bar). The minimum pneumatic pressure p min the first reaction brake pressure p re1 but not.

[0033] The second reaction brake pressure p re2 is higher than the first reaction brake pressure p according to the first graph 22 re1 . The electronic control unit 17 had previously measured the value of the first reaction brake pressure p measured by the actual pressure sensor 13 and transmitted to the electronic control unit 17 re1 with the desired service brake pressure p soll = 0.7 bar, which has not yet been reached. To determine the pneumatic brake pressure p aus to the desired service brake pressure p soll To raise the trailer, the electronic control unit 17 has instructed the trailer control valve 10 to open its inlet valve 20a or to leave it open. By opening the inlet valve 20a, the pneumatic brake pressure p has increased according to the first graph 22. aus increased such that the measured second reaction brake pressure p re2 is higher than the first reaction brake pressure pre1 . The minimum pneumatic pressure p min the second reaction brake pressure p re2 but not.

[0034] The third reaction brake pressure p within the reaction interval Δt re3 However, it takes a value that corresponds to the minimum pneumatic pressure p min The electronic control unit 17 had previously measured the value of the second reaction brake pressure p measured by the actual pressure sensor 13 and transmitted to the electronic control unit 17 re2 with the desired service brake pressure p soll = 0.7 bar, which has not yet been reached. To determine the pneumatic brake pressure p aus to the desired service brake pressure p sollTo raise the trailer, the electronic control unit 17 has instructed the trailer control valve 10 to open its inlet valve 20a or to leave it open. By opening the inlet valve 20a, the pneumatic brake pressure p has increased according to the first graph 22. aus increased so that the measured third reaction brake pressure p re3 is now higher than the minimum pneumatic pressure p min . Thus, the check in method step 600 shows that at least one measured reaction brake pressure, namely the third measured reaction brake pressure p re3 , the minimum pneumatic pressure p min Based on this, it can be concluded that the actual pressure sensor 13 has the value 0 bar for the pneumatic start brake pressure p starthas measured correctly and is not defective. Consequently, the operation of the motor vehicle braking system 8 continues in the electronic operating mode according to a method step 600a (Alternative 1).

[0035] Fig. 3 shows by a second graph 23 (“stuck at zero”; the actual pressure sensor 13 permanently shows 0 bar and the system test fails because p min is not achieved, in particular not before time t max ) an alternative time course of the pneumatic brake pressure p aus , which is measured by the actual pressure sensor 13. The second graph 23 starts at the zero point of the coordinate system and has a horizontal course. According to the second graph 23, at the starting time t start the value 0 bar as pneumatic start brake pressure p startmeasured (method step 400). In this case, as a plausibility check, it is to be checked whether the value 0 bar is plausible or whether there is a defect, in particular in the actual pressure sensor 13. For this purpose, in the fifth method step 500, a plurality of pneumatic reaction brake pressures p re within the reaction interval Δt using the actual pressure sensor 13. The second graph 23 runs horizontally such that all measured reaction brake pressures p re assume the value 0. This means that the actual pressure sensor 13 continuously measures a pneumatic brake pressure p aus of 0 bar. When the reaction interval Δt is finished, i.e. after time t max , reaction brake pressures no longer need to be measured for the plausibility check.

[0036] In the sixth method step 600, it is checked whether the reaction brake pressures p re the minimum pneumatic pressure pmin Since none of the reaction brake pressures p re exceeds the value 0 bar, the check shows that none of the measured reaction brake pressures p re the minimum pneumatic pressure p min Based on this, it can be concluded that the actual pressure sensor 13 has the value 0 bar for the pneumatic start brake pressure p start has not measured correctly and is defective. Consequently, the operation of the motor vehicle braking system 8 in the electronic operating mode is aborted according to a method step 600b (2nd alternative). Instead, the motor vehicle braking system 8 can be operated from this point on, for example, in the mechanical redundancy mode already mentioned above.

[0037] For the type of test described above, special requirements and pressure control settings may be required. For a desired service brake pressure p sollof 0.5 bar, the control “carefully” increases the pressure to the desired value p soll and an attempt is made to achieve this setpoint p soll This would normally result in the minimum pneumatic pressure p min will not exceed Δt within the reaction interval and the test fails because the control is too slow. Therefore, the pressure control is adjusted such that the initial opening pulses of the inlet valve 20a of the trailer control valve 10 are artificially extended to force the reaction pressures to reach a certain value, in particular the pneumatic limit pressure p min , to reach faster.

[0038] Fig. 5 shows that a first inlet valve pulse 24 acts for a longer time than the subsequent inlet valve pulses 25, which are not changed. For this purpose, the inlet valve 20a, when opened for the first time within the reaction interval Δt during the first pulse 24, can remain open for a longer period than during the subsequent pulses 25 (eight in the illustrated embodiment) and correspondingly subsequent opening times. The electronic control unit 17 can instruct the trailer control valve 10 accordingly. This leads to a very small peak 26 in the temporal progression of the pneumatic brake pressure p, represented by a third graph 27. aus . Fig. Figure 6 shows that the first pulse 24 within the reaction interval Δt leads to a very small pressure peak 26 right at the beginning of the pressure increase. This peak 26 is small enough not to exceed the desired service brake pressure psoll Instead, the pressure peak within the response interval Δt signals to the control unit software that the actual pressure sensor is not stuck at 0 bar, but is capable of delivering correct values.

[0039] Furthermore, it is possible to further increase the scope of diagnostics. The plausibility check described above is performed while the driver requests braking. This leads to a high degree of diagnostic coverage, but only detects a system or sensor malfunction at the beginning of the braking process. To alert the driver to reduced performance, it can be helpful to check the correct function of the actual pressure sensor 13 in particular before braking. Since the pressure peaks mentioned can be very small (e.g., ∼ 100 mbar), it is possible to trigger a single high inlet valve pulse even when there is no braking request. For this purpose, the inlet valve 20a can be opened once while no braking is being performed. In this way, a further pressure peak (not shown) is generated in the temporal progression of the pneumatic brake pressure while no braking is being performed.In particular, the intake valve pulse can be followed by an exhaust valve pulse, so that this short pressure increase has no effect on the brake pressure p. aus of the trailer 2, but it is still possible to see a value at the actual pressure sensor 13. In this case, the actual pressure sensor 13 can be marked as "OK" and the system 8 remains in normal operation. To do this, the inlet valve 20a can be set to a closed state after having previously been set to the open state once. Subsequently, in particular at a very short interval thereafter, the outlet valve 20b can be set to an open state once while no braking is carried out. In this way, the pressure peak previously generated by the single opening of the inlet valve 20a can be counteracted. Reference symbol p aus pneumatic brake pressure p ist Actual brake pressure value pmin pneumatic limit pressure p re Reaction pressure second graph p re1 first reaction pressure first graph p re2 second reaction pressure first graph p re3 third reaction pressure first graph p start pneumatic start-brake pressure p soll desired service brake pressure Δt reaction interval t start Start time t max End time 1 motor vehicle 2 trailers 3 Engine 4 wheel 5 Trailer hitch 6 wheel 7 Trailer braking system 8 Motor vehicle braking system 9 Compressed air line 10 Trailer control valve 11 Pressure output 12 measuring line 13 Actual pressure sensor 14 Foot pedal 15 Foot brake valve 16 Demand pressure sensor 17 electronic control unit 18 first control circuit 19 second control circuit 20a Inlet valve 20b Exhaust valve 21 Brake map 22 first graph 23 second graph 24 first impulse 25 subsequent pulses 26 pressure peak 27 third graph 100 first procedural step 200 second process step 300 third process step 400 fourth process step 500 fifth process step 600 sixth process step 600a Process step (1st alternative) 600b Process step (2nd alternative)

Claims

[1] Method for checking an actual pressure sensor (13) of a motor vehicle brake system (8), the method comprising the method steps (100): determining a time length of a reaction interval (Δt), (200): Setting a limit pressure (p 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) supplies a brake pressure (p aus ) so that a trailer braking system (7) of the trailer (2) by means of the braking pressure (p aus ) can be operated, (400): Starting the reaction interval (Δt) at the beginning of the braking process and measuring the actual pneumatic brake pressure values ​​(p ist ) by means of an actual pressure sensor (13), wherein a starting brake pressure (p start) of the motor vehicle braking system (8) is measured by means of the actual pressure sensor (13) at the beginning of a braking operation which is carried out in the electronic braking mode for the trailer (2), (500): Measuring pneumatic reaction brake pressures (p re ; p re1 , p re2 , p re3 ) within the reaction interval (Δt) by means of the actual pressure sensor (13), (600): Check whether the reaction brake pressures (p re ; p re1 , p re2 , p re3 ) the limit pressure (p min ), whereby (600a) the operation of the motor vehicle brake system (8) is continued in the electronic operating mode if at least one measured reaction brake pressure (p re3 ) the limit pressure (p min ) and (600b) the operation of the motor vehicle brake system (8) in the electronic operating mode is aborted if none of the measured reaction brake pressures (p re) the limit pressure (p min ) exceeds. [2] Method according to claim 1, wherein the method steps (400) to (600) are carried out during each braking operation which is initiated when the motor vehicle braking system (8) is operated in the electronic braking mode. [3] Method according to claim 1, wherein the method steps (500) and (600) are only carried out if in the method step (400) the value 0 bar is used as the starting brake pressure (p start ) is measured. [4] Method according to one of the preceding claims, wherein - for the brake pressure (p aus ) a target value (p soll ) which is not higher than 1 bar, and - the brake pressure (p aus ) based on the specified target value (p soll ) for the brake pressure (p aus ) and based on the actual brake pressure values ​​(p ist ) is regulated. [5] Method according to claim 4, wherein the limit pressure (p min ) is set to a value between 0 bar and the target value (p soll ) of the brake pressure (p aus ) lies. [6] Method according to one of the preceding claims, wherein the reaction interval (Δt) is set to a time length of 50 milliseconds to 300 milliseconds. [7] Method according to one of the preceding claims, wherein - the motor vehicle (1) has a trailer control valve (10) with an inlet valve (20a), and - the inlet valve (20a) is set into an open state at least once within the reaction interval (Δt) in order to reduce the brake pressure (p aus ) of the motor vehicle braking system (8) such that the measured reaction braking pressures (p re3 ) the limit pressure (p min ) exceed. [8] Method according to claim 7, wherein - the inlet valve (20a) is set to an open state several times within the reaction interval (Δt), and - the inlet valve (20a), when first opened, remains open for a longer period than when it is opened again after the first opening. [9] Method according to claim 7 or 8, wherein the inlet valve (20a) is set once into the open state while no braking operation is carried out, so that a pressure peak (26) in the time course (27) of the brake pressure (p aus ) occurs while no braking is taking place. [10] Method according to claim 9, wherein - the trailer control valve (10) has an outlet valve (20b), and - the inlet valve (20a) is placed in a closed state after having previously been placed in the open state once, and the outlet valve (20b) is subsequently placed in an open state once while no braking operation is carried out, so that the pressure peak (26) is counteracted. [11] Motor vehicle (1) comprising a motor vehicle braking system (8) with an electronic control unit (17), wherein - the motor vehicle braking system (8) is designed to be operated in an electronic braking mode, wherein the motor vehicle braking system (8) provides a braking pressure to a trailer (2) connected to the motor vehicle (1), so that a trailer braking system (7) of the trailer (2) can be actuated by means of the braking pressure, and - the electronic control unit (17) is configured to carry out the method steps (400) to (600) according to a method according to one of the preceding claims.

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