Method for operating a hydraulically operated brake system, brake system and control unit

Abstract

Method for operating a hydraulically operated brake system (1) of a vehicle, in particular a motor vehicle, which has at least one brake line (10-13) assigned to at least one wheel brake (6-9) and which is connected to the high-pressure side of a pump unit (15) for brake fluid by means of a switchable valve (16-19), wherein a suspect brake line (10-13) is selected from the several brake lines (10-13) depending on data from a collision detection system of the vehicle, wherein it is checked whether there is an unexpected pressure drop in the suspect brake line (10-13), and wherein, if an unexpected pressure drop is detected in the brake line (10-13), the valve (16-19) is closed.wherein, to detect an unexpected pressure drop, the current actual power consumption of the pump unit (15) is compared with an expected target power consumption, or, in the case of multiple brake lines (10-13), to detect an unexpected pressure drop in one of the brake lines, the valve of that brake line is opened and the valves of the other brake lines (10-13) are closed.

Description

The invention relates to a method for operating a hydraulically operated brake system of a vehicle, in particular a motor vehicle, which has at least one brake line assigned to at least one wheel brake, which is connected to the high-pressure side of a pump unit for brake fluid by a switchable valve. Furthermore, the invention relates to a braking system for a vehicle, in particular a motor vehicle, especially for carrying out the above method, with at least one brake line assigned to at least one wheel brake, which is connected to the high-pressure side of a pump unit for brake fluid by a switchable valve. Furthermore, the invention comprises a control unit with means for carrying out the above-mentioned method. State of the art Methods and braking systems of the type mentioned above are known from the prior art. Hydraulically operated braking systems apply a working pressure to the wheel brakes, which is transmitted via the brake fluid and generated by a pump unit. In this context, a pump unit is understood to be any means that can provide (fluid) pressure, such as, in particular, positive displacement or centrifugal pumps, for example, piston or vane pumps, or also (master) brake cylinders or pressure accumulators. The brake fluid is conveyed to the wheel brake via a brake line. A valve is provided in the brake line between the wheel brake and the pump unit, which is actuated to control the wheel brake in order to actuate the wheel brake by means of the pressure generated by the pump unit. If the brake line is damaged, for example in an accident, brake fluid can leak from the hydraulic system. In the worst case, so much brake fluid leaks out that insufficient pressure can be built up to initiate braking. Publication DE 10 2008 014 798 A1 shows a brake control device for a motor vehicle. The publication DE 4405 672 A 1 shows a hydraulic brake system with slip control. Document DE 10 2007 025 960 A1 discloses a method for operating a hydraulically operated brake system of a vehicle, which has at least one brake line assigned to at least one wheel brake, which can be connected to the high-pressure side of a pump unit for brake fluid by means of a switchable valve, wherein a suspect brake line is selected from the several brake lines depending on data from a collision detection system of the vehicle and wherein the valve is closed in the brake line. Document DE 198 04 077 A1 discloses a method for operating a hydraulically operated braking system in the context of an accident. Disclosure of the invention The invention provides that the valve closes when an unexpected pressure drop is detected in the brake line. This pressure drop serves as a clear indication of damage to the brake line and a resulting leakage of brake fluid. An unexpected pressure drop also includes situations where pressure cannot be built up in the brake line. If the pressure drops unexpectedly sharply, or if pressure cannot be built up, this indicates damage to the brake line, and the valve closes, thus preventing further leakage of brake fluid supplied to the brake line by the pump unit.In a braking system with multiple brake lines leading to different wheel brakes of the vehicle, this has the advantage of preventing a pressure loss that could affect the entire hydraulic system and maintaining the operation of the remaining wheel brakes and brake lines. This means that even if one of the brake lines is damaged, it is still possible to initiate braking, thus increasing the overall safety of the vehicle. The valve can be an existing inlet valve of the braking system or a separately provided emergency valve. According to the invention, to detect an unexpected pressure drop, the current power consumption of the pump unit is compared with an expected target power consumption.Preferably, a backpressure-current consumption characteristic curve is stored in a memory, which represents the typical current consumption of the pump unit during active pressure build-up (with an intact brake line). If a brake line is defective, the pressure in the brake line decreases, and thus the backpressure acting on the pump unit decreases. This results in the pump unit requiring less current. Therefore, an unexpectedly low pump current can indicate a defect in a brake line. According to the invention, if several brake lines are present, to detect an unexpected pressure drop in one of the brake lines, the valve of that brake line is opened and the valves of the other brake lines are closed. By closing the valves of the other pressure lines, the influences acting on the brake line under test are reduced, so that a detected pressure drop can be clearly attributed to that one brake line.In this way, all brake lines in the braking system can be tested sequentially. Of course, several brake lines can also be grouped together in a single brake circuit, in which case the brake lines of that circuit are advantageously switched through a common valve. While this simplifies the setup, it reduces the accuracy of the procedure, as it only allows conclusions to be drawn about the pressure drop in the entire brake circuit and not in an individual brake line. Alternatively, each brake line within the respective brake circuit can be assigned a separate emergency valve that only activates when an unexpected pressure drop is detected. Advantageously, to detect an unexpected pressure drop, the pressure in the brake line is determined, particularly by means of a pressure sensor, and compared with the expected pressure. The pressure sensor can be positioned at any point on the brake line or even in a section of the line belonging to the wheel brake or the pump unit, provided that the pressure in that section of the line corresponds to the pressure in the brake line. Preferably, the pressure sensor is positioned upstream of the valve, as viewed from the pump unit. This allows the pressure between the valve and the high-pressure side of the pump unit to be determined even after the valve has closed, thus ruling out any other cause for the pressure drop besides damage to the brake line, particularly in the area near the wheel brake. The method can, in principle, be carried out at any time and advantageously whenever a braking process is initiated. It is advantageous to determine the expected pressure based on the pump unit's delivery rate. This allows the expected pressure in the intact brake line to be calculated and / or estimated from the amount of brake fluid delivered by the pump unit. By comparing the expected pressure with the actual pressure in the brake line, a pressure drop can be quickly detected. It is advantageous to define threshold values ​​that the determined pressure difference between the expected and actual pressure must exceed before an unexpected pressure drop is diagnosed. In an advantageous further development of the method, it is provided that a suspect brake line is selected from the multiple brake lines (or brake circuits) before an unexpected pressure drop is detected. To accelerate the process, a suspect brake line is first identified, whereby this means that a brake line is selected which is assumed to be potentially defective. This brake line is then tested for functionality first, as described above. The suspected brake line is conveniently selected based on data from the vehicle's collision detection system. This system may be part of a restraint system that includes safety devices such as seatbelt pretensioners, airbags, or even an ESP system. Modern collision detection systems can not only detect a collision but also identify and determine the point of impact on the vehicle. This allows the system to determine, based on the data, whether and, if so, which of the wheel brakes and / or brake lines might be defective. Particularly after an accident, this can help identify, for example, where brake fluid might be leaking from the hydraulic system in the event of a wheel breaking off.Using the procedure described above, the suspicion can then be confirmed in a particularly simple way and, if necessary, the corresponding valve can be closed, thus preventing further leakage of brake fluid. Furthermore, the system is designed to trigger automatic braking when the vehicle is detected in a collision. This ensures that the vehicle comes to a stop as quickly as possible after an accident to prevent secondary collisions. It is particularly important that a defective brake line is detected as quickly as possible, since triggering automatic braking increases the brake pressure in the wheel brakes. If one of the brake lines is defective, the brake fluid will leak from the hydraulic system even more rapidly, preventing pressure build-up in the remaining wheel brakes unless the corresponding valve is closed quickly, as described above. The braking system according to the invention is characterized by means for detecting an unexpected pressure drop in the braking performance and for closing the valve when an unexpected pressure drop is detected. This results in the advantages described above. The valve can be a conventional control valve (inlet valve) for actuating the wheel brakes of the braking system, or it can be provided as a separate emergency valve. Advantageously, the system includes at least one pressure sensor assigned to the brake line or, where applicable, the brake circuit, a device for determining the power consumption of the pump unit, and / or a collision detection system. The collision detection system, which is expediently part of a restraint system including at least one airbag, allows for the localization of which wheel brake or brake line of the braking system may have been damaged in the event of a collision. The pressure sensor assigned to the brake line allows for the simple determination of the brake fluid pressure in the respective brake line and its comparison with the expected pressure in the brake line.In a control unit, the pressure sensor data can be compared, for example, with the expected data, which are preferably based on the pump unit's delivery rate and / or estimated based on a stored model. Alternatively or additionally, the described device determines the actual power consumption of the pump unit and compares it with an expected target power consumption, calculated, for example, from the pump's delivery rate and an expected back pressure. As described above, several brake lines may be provided, which may be grouped together and divided into several different brake circuits. Furthermore, the invention comprises a control unit containing means designed to carry out a method according to the invention. The invention will now be explained in more detail with reference to the drawing. Fig. 1 shows a simplified representation of a motor vehicle's braking system, and Fig. 2 shows an advantageous method for operating the braking system, illustrated by a flowchart. Figure 1 shows a highly simplified representation of a braking system 1 of a motor vehicle (not shown in detail here) which has four wheels 2, 3, 4 and 5, each of which is assigned a wheel brake 6, 7, 8 and 9 of the braking system 1, respectively. The wheel brakes 6 to 9 are each connected to a braking device 14 via a brake line 10, 11, 12 and 13, respectively. The brake unit 14 comprises a pump unit 15, to whose high-pressure side the brake lines 10, 11, 12, and 13 are connected. Each of the brake lines 10 to 13 is assigned a switchable valve 16, 17, 18, or 19, through which the respective brake line 10 to 13, or the respective wheel brake 6 to 9, is connected to the high-pressure side of the pump unit 15. The valves 16 to 19 are control valves (inlet valves) through which the wheel brakes 6 to 9 can be actuated. Return lines, which are advantageously part of the hydraulic system of the brake system 1 shown here, are not shown for clarity. The valves 16 to 19 can alternatively also be provided or designed as additional emergency valves. Furthermore, each of the brake lines 10 to 13 is assigned a pressure sensor 20 to 23 for detecting the pressure of the brake fluid in the respective brake line 10 to 13.The pressure sensors 20 to 23 and the valves 16 to 19 are advantageously arranged in and / or on the brake unit 14 and connected via appropriate wiring to a control unit 24 of the brake unit 14. The control unit 24 is advantageously connected to a collision detection system (not shown in detail here), which is advantageously part of a restraint system of the motor vehicle. In the present embodiment, the collision detection system further comprises an airbag control unit, which outputs the signal "collision has occurred" in the event of a collision. After the signal has been validated by communication tests, the control unit 24 initiates an automatic braking process that decelerates the vehicle to a standstill, for example, to prevent secondary accidents after a collision.The vehicle may also include an ESP system, which can initiate automatic braking without a collision having occurred. In the initial collision, one of the brake lines 10 to 13 may be damaged and / or, in extreme cases, one of the wheels 2 to 5 may break off. In the event of a wheel breakage, the monitoring algorithms of the control unit 24, which is specifically designed as an ESP control unit (ESP = Electronic Stability Program), can detect a so-called implausible wheel based on speed sensor signals from wheels 2 to 5. In contrast to the brake system shown in Fig. 1, it is also possible, for example, to combine two brake lines 10, 11 and 12, 13 into a single brake circuit, which is connected to the high-pressure side of the pump unit 15 via a single valve. Opening the valve results in the wheel no longer being braked. In the event of a wheel failure with damage to the brake line and an implausible wheel being detected, this would cause brake fluid to be pumped from a reservoir of the hydraulic system into the atmosphere, and after a certain time, both wheels of the affected brake circuit could no longer be braked, or only with minimal braking force. The advantageous brake system provides that in the event of a pressure loss in one of the brake lines 10, 11, 12, or 13, the valve 16, 17, 18, or 19 associated with that brake line is closed.This prevents brake fluid from leaking out and reducing the overall performance of the braking system. At the very least, it ensures that the wheels with intact brake lines can continue to brake optimally. For example, if one of brake lines 10 to 13 is damaged, the wheel brakes of the remaining brake lines can still operate optimally. The advantageous method for operating the brake system 1 will now be explained in more detail by way of example using the flow diagram shown in Fig. 2. In a first step (25), a collision is detected, thereby initiating the advantageous procedure. In the next step (26), the function of wheels 2 to 5 is checked using rotation sensor signals, as described above. If an implausible wheel is found, the subsequent step (27) checks whether the implausible wheel was struck in the collision. For this purpose, the collision detection system collects or determines data regarding the impact location in step (28), which is then compared with the data regarding the implausible wheel in step (27). If the implausible wheel matches the determined impact location, the corresponding brake line is identified as suspect. In the subsequent step (29), it is checked whether there is an unexpected pressure drop in the suspect brake line. In one embodiment, the detection of an unexpected pressure drop can be achieved using the pressure sensors 20 to 23 shown in Fig. 1. For this purpose, the sensors 20 to 23 detect the actual pressure in the respective brake lines 11 to 13 and compare it with an expected pressure. This is done in the control unit 24. The expected pressure is expediently determined by the control unit 24 as a function of the pump output of the pump unit 15. If the pressure difference exceeds a predefined threshold, an unexpected pressure drop is diagnosed. In an embodiment of the invention, which can undoubtedly also be combined with the embodiment described above, an unexpected pressure drop is detected by comparing the actual current consumption of the pump unit with an expected target current consumption. The pressure sensors 20 to 23 can be omitted in this case. If one of the brake lines 10 to 13 breaks, the pump unit consumes less current due to the lack of back pressure in the brake line or the corresponding brake circuit. Thus, by comparing the target current consumption with the actual current consumption, an unexpectedly low pump current can be detected, indicating a defect in one of the brake lines 10 to 13 (or the corresponding brake circuit) and / or in the corresponding valve. In step 30, the reference data described above (target power consumption and / or expected pressure) are recorded and in step 29 compared with the actual data (actual power consumption or actual pressure). A defect in brake line 10 to 13 can be detected more readily in the current consumption of the pump unit the fewer other (unknown) influencing factors affect the resistance of the pump unit. For this reason, targeted test sequences are planned. For this purpose, for a short period, only the valve of valves 16 to 19 assigned to the suspected wheel identified in step 27 is opened, and all other valves are closed. If, for example, after the collision, wheel 2 has been identified as implausible and verified using the impact location determined in step 28, valve 16 is opened and valves 17 to 19 are closed, so that only brake line 10 and wheel brake 6 have an influence on the actual current consumption of the pump unit 15.The resistance, measured as the current consumption of the pump unit 15, results from the pressure build-up or lack thereof at the suspected wheel 2. If a brake line defect is suspected for several of the wheels 2 to 5 or wheel brakes 6 to 9, the procedure described above can be carried out successively for each of the individual wheels 2 to 5 (or brake lines 10 to 13). The procedure can, of course, also be carried out additionally or alternatively by detecting the unexpected pressure drop using the pressure sensors 20 to 23. If the current consumption is too low for all wheels or brake lines, the defect is not in the brake lines, but, for example, in the pump unit, with the result that the braking system can advantageously continue to be operated as before. If, in step 29, the procedure described above and the comparison of pressure values ​​or current consumption values ​​indicate an unexpected pressure drop, the valve of the corresponding brake line is closed in step 31. After completion of the described diagnostic procedure, the remaining (intact) valves are reopened so that the remaining wheels or wheel brakes can be used to brake the wheels. If no unexpected pressure drop is detected in step 29, the brake line is assumed to be intact. Accordingly, in step 32, valves 16 to 19 are held open or opened, and the automatic braking process is carried out with all intact wheel brakes 6 to 9. As an alternative to the collision detection system, which uses data from an airbag control unit, other means can of course be used to detect, in particular, the point of impact, such as optical sensors, radar, or ultrasonic sensors. Advantageously, as early as step 27, the wheel identified as implausible and affected by the collision—i.e., the suspected wheel, wheel brake, or brake line—is disconnected from the braking system by closing the corresponding valve to prevent any potential brake fluid loss as quickly as possible. The valve is only reopened when an unexpected pressure drop is detected. Of course, the described procedure can be carried out not only after a detected collision, but at any time. The procedure is particularly advantageous when performed during or after each start of the vehicle or activation of the braking system. Overall, the described method and brake system can thus detect a defect in one of the brake lines, for example as a brake line break, in one of the valves, for example as a leak, or in the pump unit.

Claims

Method for operating a hydraulically operated brake system (1) of a vehicle, in particular a motor vehicle, which has at least one brake line (10-13) assigned to at least one wheel brake (6-9) and which is connected to the high-pressure side of a pump unit (15) for brake fluid by means of a switchable valve (16-19), wherein a suspect brake line (10-13) is selected from the several brake lines (10-13) depending on data from a collision detection system of the vehicle, wherein it is checked whether there is an unexpected pressure drop in the suspect brake line (10-13), and wherein, if an unexpected pressure drop is detected in the brake line (10-13), the valve (16-19) is closed.wherein, to detect an unexpected pressure drop, the current actual power consumption of the pump unit (15) is compared with an expected target power consumption, or, in the case of multiple brake lines (10-13), to detect an unexpected pressure drop in one of the brake lines, the valve of that brake line is opened and the valves of the other brake lines (10-13) are closed. Method according to claim 1, characterized in that, in order to detect an unexpected pressure drop, the pressure in the brake line (10-13) is determined, in particular by means of a pressure sensor (20-23), and compared with an expected pressure. Method according to one of the preceding claims, characterized in that the expected pressure is determined as a function of the delivery capacity of the pump unit (15). Brake system (1) for a vehicle, in particular a motor vehicle, for carrying out the method according to one or more of the preceding claims, with at least one brake line (10-13) assigned to at least one wheel brake (6-9), which is connected to the high-pressure side of a pump unit (15) for brake fluid by a switchable valve (16-19), characterized by means for detecting an unexpected pressure drop in the brake line (10-13) and for closing the valve (16-19) when an unexpected pressure drop is detected, wherein, in order to detect an unexpected pressure drop, the current actual power consumption of the pump unit (15) is compared with an expected target power consumption, or, in the case of several brake lines (10-13), in order to detect an unexpected pressure drop in one of the brake lines, the valve of this brake line is opened and the valves of the other brake lines (10-13) are closed. Brake system according to claim 4, characterized in that the means comprise at least one pressure sensor (20-23) associated with the brake line, a device for determining the current power consumption of the pump unit (15) and / or a collision detection system. Control unit comprising means configured to carry out the methods according to any one of claims 1 to 3.

Images