Fadingkompensation

Abstract

Method for controlling a hydraulic brake-by-wire braking system, wherein a deviation between the actual volume intake and the desired volume intake is detected as an additional volume requirement by means of a volume intake monitoring system, characterized in that fading that occurs is detected by means of a fading monitoring system and, when fading is detected, the additional volume requirement is taken into account in the pressure control, wherein, when fading is detected, a hydraulic pump (2) is switched on so that pressure is built up jointly by a linear actuator (5) and the hydraulic pump (2).

Description

[0001] The invention relates to a method for controlling a hydraulic brake-by-wire braking system and a hydraulic brake-by-wire braking system according to claim 9, wherein a deviation between the actual volume intake and the target volume intake is detected as an additional volume requirement by means of a volume intake monitoring system.

[0002] Braking systems for motor vehicles must be highly reliable in order to be able to brake the vehicle safely to a standstill at any time, even in the event of isolated faults. This typically involves a hydraulic distribution between two circuits, either diagonally or in a black-and-white distribution. In such a brake-by-wire system, an electric actuator assumes the actual pressure setting for the wheel brakes. The driver enters a simulator via a corresponding hydraulic connection; a brake pressure request is generated based on the brake pedal actuation, and the electric actuator implements this. The brake pressure on the front and rear axles is normally equivalent.

[0003] By decoupling the driver from the wheel brake, the simulator provides them with consistent pedal feel, regardless of the condition and type of wheel brake. With conventional hydraulic wheel brake systems, braking effectiveness decreases under high brake loads, a phenomenon known as fading. The brake's friction coefficient decreases and the brake's volume absorption increases. The driver notices this immediately, as greater pedal force is required to achieve the same level of deceleration. By decoupling the driver from the wheel brake, this feedback is lost. Therefore, it is possible for the driver to unknowingly over-apply the wheel brake due to the lack of direct feedback.

[0004] In racetrack operation, situations arise in which the wheel brakes are not sufficiently cooled or are not sufficiently circulated with cooling air. In some cases, design measures are taken to maintain the brakes at a constant temperature so that they do not cool down and efficiency remains high. This is preferably done with non-metallic brake discs, as the required operating temperatures are higher than with traditional cast iron discs. This can cause the brake fluid in the brake caliper to reach its boiling point at high temperatures. As a result, the brake fluid outgasses, and vapor bubbles or air form in the brake caliper.

[0005] DE102019215418A1 discloses a method for monitoring brake fluid volume. This method determines the effective volume displacement to indicate a leak.

[0006] From DE 10 2019 005 982 A1 a method for detecting brake fading is known in which adaptive reference maps are used that include vehicle deceleration data and brake system fluid volumes at different pressure points in a histogram.

[0007] From DE 10 2012 221 968 A1 a method for operating a brake system is known in which hydraulic fluid is temporarily stored above a predetermined temperature and is returned to the brake circuit when brake fading occurs.

[0008] Furthermore, DE 10 2017 127 519 A1 discloses the provision of a brake fading management system in a brake-by-wire system. This system includes an actuator target arbitration module that transmits commands to an actuator of the braking system. For this purpose, it receives displacement-adjusted actuator target data from a compliance module.

[0009] Despite existing measures, the occurrence of fading and the formation of air in the brake caliper cannot be prevented. Fading is the reduction in braking effect due to the brake heating up. There are several causes for this. At higher temperatures, a drop in the coefficient of friction between the brake pads and the brake disc or drum can be observed. The temperature balance of the brake is determined by the cooling of the caliper and brake disc, the necessary air supply and exhaust, and the design of the brake booster. The brake booster should have sufficient reserves to maintain foot force assistance even when the braking effect decreases during fading, as otherwise the driver is forced to apply the high master cylinder pressures required for high braking forces using foot force. If the driver is unable to do this, the possible braking effect is reduced and the braking distance increases.In extreme cases, this can lead to an accident. With very high volume consumption or a high amount of air in the saddle, monitoring can no longer reliably distinguish between air and leaks within the specified limits. As a result, a monitoring function detects leaks, and the system attempts to isolate them. This results in a circuit breaker or system degradation. The rider's availability of the braking system is limited.

[0010] The task is therefore to improve the safety of a brake-by-wire brake.

[0011] This is achieved according to the invention by a method according to claim 1 using fading monitoring, which detects any fading occurring and, upon detection, takes the additional volume requirement into account during pressure control. Thus, the deceleration desired or required by the driver is achieved quickly and safely, preventing the occurrence of dangerous situations due to under-braking.

[0012] In a preferred embodiment of the invention, the volume intake monitor transmits the additional volume requirement to a pressure regulator, which takes the additional volume requirement into account when controlling a linear actuator to build up pressure. Thus, the actual volume requirement is directly used.

[0013] In a preferred embodiment of the invention, the pressure regulator comprises a pilot control that determines a target volume from the pressure-volume characteristic curve based on a pressure setpoint and an actual pressure value and controls the linear actuator to deliver the target volume. Actual pressure control of the actual pressure value to the pressure setpoint occurs after the pressure pilot control, and the additional volume requirement is taken into account during the pilot control. Such a control can set a desired or required pressure value particularly quickly and precisely. By passing on the additional volume requirement directly to the pilot control, slower readjustment is avoided.

[0014] In a preferred embodiment of the invention, the fading monitor detects fading when a brake disc temperature greater than a threshold is detected. This can, in particular, be derived from a temperature model, thus eliminating the need for sensors. Preferably, the temperature is used as an additional feature, along with deviations from the pressure-volume characteristic curve, for fading detection.

[0015] In a preferred embodiment of the invention, the volume uptake monitoring detects a deviation from a theoretical pressure-volume characteristic curve, wherein the deviation can be detected as a pressure-independent offset and / or as a factor.

[0016] In a preferred embodiment of the invention, the fading monitor detects fading when the deviation, in particular the factor, increases with time and / or temperature.

[0017] In a preferred embodiment of the invention, if an actual pressure-volume characteristic curve deviates from a target pressure-volume characteristic curve by more than a threshold value, the brake system is degraded. This can occur, for example, due to incorrect interpretation as a leak. If fading is detected by the fading monitoring system, the threshold value is increased, preventing the brake system from degrading despite the deviation and thus remaining available for longer.

[0018] According to the invention, when fading is detected, a hydraulic pump is activated, specifically to provide the additional volume required, so that pressure builds up jointly through the linear actuator and the hydraulic pump. Since the volume flows of the two units are summed, a pressure can be set very quickly despite the additional volume required.

[0019] In a preferred embodiment of the invention, fading is only detected when the brake fluid level is not low. This prevents a genuine leak from being mistakenly misidentified and interpreted as fading. The safety of the braking system is thus ensured.

[0020] The object is further achieved by a hydraulic brake-by-wire braking system according to claim 9, comprising a linear actuator and a hydraulic pump and a control unit which is configured to control the linear actuator to build up pressure, characterized in that the control unit is also configured to carry out one of the above methods.

[0021] Further features, advantages, and possible applications of the invention will become apparent from the following description of exemplary embodiments and the drawings. All described and / or illustrated features, both individually and in any combination, are part of the subject matter of the invention, regardless of their summary in the claims or their references.

[0022] Fig. 1 shows schematically a braking system according to the invention;

[0023] In Fig. Figure 1 shows a redundant hydraulic braking system for motor vehicles. For example, the braking system is designed to operate four hydraulically actuated wheel brakes 8; expansion to more wheel brakes is easily possible. For example, the wheel brakes (HL, HR) are assigned to the rear axle and the wheel brakes (VL, VR) to the front axle of the vehicle and are thus arranged in a black-and-white distribution.

[0024] The brake system comprises a first structural unit, which is designed, for example, as a first electro-hydraulic brake control unit with a valve block and a first electronic control device ACTUATOR ECU, and a second structural unit, which is designed, for example, as a second electro-hydraulic brake control unit with a valve block and a second electronic control device MODULATOR ECU.

[0025] A pressure medium reservoir 4 with three chambers is arranged on the first structural unit, wherein a first reservoir connection is assigned to the first chamber, a second reservoir connection is assigned to the second chamber and a third reservoir connection is assigned to the third chamber.

[0026] A first electrically actuated pressure source 5 is arranged in the first structural unit.

[0027] In the second structural unit, a second electrically actuated pressure source 2 and wheel-individual brake pressure modulation valves are arranged, which are designed as an electrically actuated inlet valve 6 and an electrically actuated outlet valve 7 for each wheel brake 8.

[0028] The first pressure source 5 and the second pressure source 2 are connected on the pressure side to a brake supply line to which the four inlet valves 6 are connected. Thus, all four wheel brakes 8 can be actuated by means of the first pressure source 5 and / or the second pressure source 2.

[0029] An electrically actuated circuit isolation valve 40 is arranged in the brake supply line, so that when the circuit isolation valve 40 is closed, the brake supply line is separated into a first line section, to which the inlet valves 6 or the wheel brakes 8 of the rear axle are connected, and a second line section, to which the inlet valves 6 or the wheel brakes 8 of the front axle are connected. The second pressure source 2 is hydraulically connected to the first line section, and the first pressure source 5 is hydraulically connected to the second line section. When the circuit isolation valve 40 is closed, the brake system is thus separated or divided into two hydraulic brake circuits I and II, or a first partial circuit and a second partial circuit.In the first brake circuit I, pressure source 2 is connected (via the first line section) only to the wheel brakes 8 of the rear axle, and in the second brake circuit II, the first pressure source 5 is connected (via the second line section) only to the wheel brakes 8 of the front axle. The circuit separation valve 40 is advantageously designed to be open when de-energized. Such an operating mode can be called circuit separation mode or ACS for "Active Circuit Separation."

[0030] As already mentioned, the braking system comprises an inlet valve 6 and an outlet valve 7 for each hydraulically actuated wheel brake 8, which are hydraulically interconnected in pairs via central connections and each connected to a hydraulic wheel connection of the second structural unit to which the corresponding wheel brake 8 is connected. A check valve opening towards the brake supply line is connected in parallel to each of the inlet valves 6. The output connections of the outlet valves 7 are connected to the pressure fluid reservoir 4 or its third chamber via a common return line. The input connections of all inlet valves 6 can be supplied with a pressure provided by the first pressure source 5 or, for example, if the first pressure source 5 fails, by the second pressure source 2 via the brake supply line (i.e., when the circuit isolation valve 40 is open).

[0031] The first electrically controllable pressure source 5 of the valve block is designed as a hydraulic cylinder-piston arrangement (or a single-circuit electro-hydraulic actuator (linear actuator)), the piston of which can be actuated by a schematically indicated electric motor with the interposition of a rotation-translation gear, also shown schematically, in particular can be moved forwards and backwards in order to build up and reduce pressure in a pressure chamber. The piston delimits the pressure chamber of the pressure source 5. To control the electric motor, a rotor position sensor (only schematically indicated) is provided that detects the rotor position of the electric motor. By means of this sensor, the piston position and speed, and thus the released or absorbed brake fluid volume, can be determined.

[0032] A system pressure line section is connected to the pressure chamber of the first electrically controllable pressure source 5. By means of the line section, the pressure source 5 or its pressure chamber is connected to a hydraulic connection of the first structural unit, which is connected to a hydraulic connection of the second structural unit via a hydraulic connecting element. This connection represents the only hydraulic pressure connection, in particular the only hydraulic connection, between the first and second structural units. It is a hydraulic connection for transmitting brake pressure to actuate the wheel brakes 8. Depending on the installation position of the first and second structural units, this line can encompass a large volume.

[0033] The pressure chamber is connected to the pressure fluid reservoir 4 via a (suction) line, regardless of the piston's actuation state. A check valve 53, closing toward the pressure fluid reservoir 4, is arranged in the line and connected to the second chamber. A further connection is connected to the first chamber of the brake fluid reservoir 4 via an electrically switchable isolating valve 23.

[0034] The second electrically controllable pressure source 2 of the second assembly is designed, for example, as a two-piston pump whose two pressure sides are interconnected. The suction sides are connected to the return line and thus to the pressure fluid reservoir 4. The pressure sides are connected to the first line section of the brake supply line.

[0035] In addition to the pressure source 2 and the brake pressure modulation valves 6, 7, an electrically actuated, preferably normally open, isolation valve 26 is arranged in the second structural unit, for example. Isolation valve 26 is hydraulically arranged between the connection and the second line section of the brake supply line. Thus, the first pressure source 5 is detachably connected to the second line section or the brake supply line via the isolation valve 26.

[0036] For example, the braking system includes a pressure sensor in brake circuit I, which is thus assigned to the second pressure source 2. This is advantageous for burst protection during active circuit separation, i.e., when the circuit separation valve 40 is closed. However, the pressure sensor can also be located in brake circuit II.

[0037] For example, the brake system for leakage monitoring comprises a level measuring device for determining a pressure medium level in the pressure medium reservoir 4.

[0038] Each valve block is assigned an electronic control device. Each electronic control device comprises electrical and / or electronic elements (e.g., microcontrollers, power units, valve drivers, other electronic components, etc.) for controlling the electrically actuated components of the associated valve block and, if applicable, the associated sensors. The valve block and electronic control device are advantageously designed as an electrohydraulic unit, as is known in the art.

[0039] The first electronic control device controls the first pressure source 5. For example, the first pressure source 5 is supplied with energy (from a first electrical energy source) via the first electronic control device.

[0040] The second electronic control device controls the second pressure source 2. For example, the second pressure source 2 is supplied with energy (from a second electrical energy source) via the second electronic control device.

[0041] For example, the first pressure source 5 can be or is controlled exclusively by the first electronic control device and the second pressure source 2 can be or is controlled exclusively by the second electronic control device.

[0042] The brake system has a primary pressure source 5 and a secondary pressure source 2, each electrically operated by an ECU and having a suction port and a pressure port. No brake fluid can flow into the pressure port of the secondary pressure source 2, even when de-energized. For example, the primary pressure source 5 is a linear actuator with a suction check valve 53, and the secondary pressure source 2 is a piston pump. Preferably, the secondary pressure source 2 can generate a higher pressure than the primary pressure source 5.

[0043] The suction sides of the two pressure sources 2, 5 are connected to a pressure medium reservoir 4, preferably each with separate chambers.

[0044] The pressure side of the primary pressure source 5 is connected to a primary circuit node via an electromagnetic valve 26, also called a pressure connection valve or isolation valve.

[0045] The pressure side of secondary pressure source 2 is connected directly (without the interposition of a valve) to a secondary circuit node. The two circuit nodes are connected via an electromagnetic valve 40, also called a circuit dividing valve.

[0046] During normal operation, the pressure in the wheel brakes is built up by primary pressure source 5. The pressure in primary pressure source 5 is released by retracting its piston. The pressure is modulated for each wheel as needed by the inlet and outlet valves. If necessary, isolation valve 26 is closed so that primary pressure source 5 can draw in additional volume.

[0047] If a particularly high flow rate is required, both pressure sources 5 and 2 operate in parallel. If a particularly high pressure is required, the isolation valve 26 is closed, and the secondary pressure source 2 increases the pressure above the pressure of the primary pressure source 5. Outside of braking situations, atmospheric pressure equalization can be permanently ensured via the isolation valve 23 and the isolation valve 26.

[0048] Isolation valve 26 is preferably controlled by the secondary ECU. The following description of operation in the event of a fault refers to this valve assignment.

[0049] If the primary system fails electrically, particularly the primary or its power supply, the secondary ECU closes the isolation valve 26 to build up pressure via the secondary pressure source 2. Pressure is released via the exhaust valves 7. Preferably, the intake and exhaust valves are controlled by the secondary ECU so that the pressure can be modulated for each individual wheel.

[0050] If the secondary system fails electrically, specifically the secondary ECU or its voltage source, the pressure is increased and decreased via the primary pressure source 5, as in normal operation. Individual wheel pressure control is not required, but joint modulation of the wheel pressures remains possible to prevent the vehicle from being destabilized by locking wheels.

[0051] An electrically switchable valve 23 forms a further connection between the brake fluid reservoir 4 and the output port of the linear actuator 5. This isolation valve 23 is designed to be open when de-energized, so that in the de-energized state, the wheel brakes 8 are connected to the brake fluid reservoir 4. The cylinder-piston assembly 5, for example, has no sniffing holes.

[0052] The control devices are configured to implement a pressure buildup using the linear actuator 5 and / or the piston pump 2 based on pressure requests. These can originate from automatic functions of the assistance program and, in particular, from the driver, who transmits the driver's braking request via the brake pedal. In this embodiment, the brake pedal is designed to be dry and thus has no hydraulic connection, nor any switchable hydraulic connection option. The driver's request is transmitted to the control devices as a brake pedal actuation variable. The brake pedal actuation variable can be a brake pedal travel, a brake pedal force, and / or similar variables.

[0053] As soon as a pressure request is present, the isolation valve 23 is closed to separate the pressure side of the linear actuator 5 from the brake fluid reservoir 4 in order to build up braking pressure. After the pressure request is met and the pressure is reduced by the linear actuator 5, the isolation valve 23 is reopened to ensure pressure relief.

[0054] If fading is detected, the system should provide further support to the driver. To achieve this, the linear actuator should provide the actual required air volume during fading, while also being tolerant to larger air volumes and robust against system degradation.

[0055] If fading is detected, the additional volume requirement is communicated to the pressure regulator to more quickly adjust the desired pressure. Based on the pressure-volume characteristic of the wheel brake 8, the pressure regulator controls the linear actuator 5 and moves to a position corresponding to a specific pressure. If the position is insufficient for the desired pressure, the pressure regulator adjusts the linear actuator position. A higher volume requirement due to fading would have to be adjusted without taking this into account.

[0056] If fading is detected, the information about the additional volume requirement from the volume monitoring function (VDM) can be used and added to the volume requirement from the stored pressure-volume characteristic curve. The pressure regulator can then directly adjust to the larger position, which corresponds to the sum of the volumes, and thus build up pressure more quickly. This increases the pressure buildup performance during fading.

[0057] If fading occurs, the volume monitoring function (VDM) can identify the actual pressure-volume characteristic curve based on reference points and calculate a factor to the target pressure-volume characteristic curve. At high brake fluid temperatures, the brake fluid can outgas, significantly increasing the volume absorption. If the differential volume is sufficiently large, which is recorded as an offset to the pressure-volume characteristic curve, this can lead to system degradation. This could, for example, result in switching to a circuit-cutting mode.

[0058] However, when fading occurs, increased volume intake should not lead to such system degradation. The system should remain available to the driver. To achieve this, the short-term change in the actual pressure-volume characteristic curve and the associated increased volume requirement are tolerated during fading. During fading, the VDM switches to a "tolerant mode" and widens the threshold values. This means that a larger volume intake is tolerated before system degradation is initiated. When transitioning to tolerant mode, the brake disc temperature from a temperature model can also be taken into account.

[0059] For systems with two pressure sources 2, 5, as in Fig.1, a further embodiment of the invention can be used alternatively or additionally. Pressure buildup or fading support can be achieved simultaneously by the linear actuator 5 and the hydraulic pump 2. The simultaneous use of pump 2 during fading can improve the overall pressure control performance for pressure buildup.

Claims

[1] Method for controlling a hydraulic brake-by-wire braking system, whereby a volume intake monitoring system detects a deviation between the actual volume intake and the target volume intake as an additional volume requirement, characterized by that fading occurring is detected by means of a fading monitor and, when fading is detected, the additional volume requirement is taken into account in the pressure control, wherein, when fading is detected, a hydraulic pump (2) is switched on so that pressure is built up jointly by a linear actuator (5) and the hydraulic pump (2). [2] Method according to claim 1, characterized by that the volume intake monitoring transmits the additional volume requirement to a pressure regulator, which takes the additional volume requirement into account when controlling a linear actuator (5) to build up pressure. [3] Method according to one of the preceding claims, characterized bythat the pressure regulator comprises a pilot control which determines a target volume from the pressure-volume characteristic curve based on a pressure target value and an actual pressure value and controls the linear actuator (5) to deliver the target volume, wherein an actual pressure control of the actual pressure value to the pressure target value takes place after the pressure pilot control and wherein the additional volume requirement is taken into account in the pilot control. [4] Method according to one of the preceding claims, characterized by that the fading monitoring detects fading when a brake disc temperature greater than a threshold value is detected. [5] Method according to one of the preceding claims, characterized by that the volume uptake monitoring detects a deviation from a theoretical pressure-volume characteristic curve, whereby the deviation can be detected as an offset and / or factor. [6] Method according to one of the preceding claims, characterized bythat the fading monitoring detects fading when the deviation, especially the factor, increases with time and / or temperature. [7] Method according to one of the preceding claims, characterized by that if an actual pressure-volume characteristic curve deviates from a target pressure-volume characteristic curve by more than a threshold value, the braking system degrades, and if fading is detected, the threshold value is increased, making the braking system available for longer. [8] Method according to one of the preceding claims, characterized by that fading is only detected when a brake fluid level does not indicate a low level. [9] Hydraulic brake-by-wire braking system comprising a linear actuator (5) and a control unit which is designed to control the linear actuator (5) to build up pressure, characterized bythat a hydraulic pump (2) is provided and the control unit is also designed to carry out a method according to one of claims 1 to 8.

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