Front axle braking during degradation

Abstract

The invention relates to a method for controlling a hydraulic motor vehicle brake system, wherein, based on a brake request variable, a brake pressure is to be built up at wheel brakes of a front axle and a rear axle by means of an electrical pressure supply device, wherein, in a fault-free case, inlet valves of the wheel brakes on the front axle and rear axle are opened and a brake request variable is implemented by joint pressure build-up on the front axle and rear axle.In order to achieve a high deceleration as quickly as possible with limited delivery capacity of the pressure supply device, in the event of a fault with a degradation of the pressure supply device, the pressure build-up is first implemented on the front axle by closing the inlet valves of the wheel brakes of the rear axle before or at the start of a volume delivery by means of the pressure supply device, so that a pressure build-up is first implemented via open inlet valves only on the wheel brakes of the front axle, and if a condition exists, the wheel brakes on the rear axle are switched on by opening the inlet valves.

Description

[0001] The invention relates to a method for controlling a hydraulic motor vehicle brake system, wherein, based on a brake request variable, a brake pressure is to be built up at wheel brakes of a front axle and a rear axle by means of an electrical pressure supply device, wherein, in a fault-free case, inlet valves of the wheel brakes on the front axle and rear axle are opened and a brake request variable is implemented by joint pressure build-up on the front axle and rear axle.

[0002] The braking request can originate from a brake pedal and thus be initiated by the driver, or be generated by an assistance system or an autonomous system. In such brake-by-wire braking systems, the driver has no hydraulic access to the wheel brakes when a fault is present. Instead, the brake pressure is generated by an electrical pressure supply device. If a fault occurs, a brake pedal-coupled master cylinder is connected to the wheel brakes via appropriate valve circuits, and the driver must apply the brake pressure unaided using muscle power. However, this results in much poorer deceleration values.

[0003] It is therefore an object of the present invention to achieve the shortest possible braking distance even when a fault occurs.

[0004] The object is achieved according to the invention by the features of claim 1, wherein in the event of a fault with a degradation of the pressure supply device, the brake pressure is first applied to the front axle by closing inlet valves of the wheel brakes of the rear axle before or at the start of a volume delivery by means of the pressure supply device, so that a pressure build-up is first implemented via open inlet valves only at the wheel brakes of the front axle, and wherein, if a condition exists, the wheel brakes on the rear axle are activated by opening the inlet valves.

[0005] Based on a braking request variable, target values ​​for the wheel brakes on the front and rear axles are determined, each of which is greater than zero. In particular, the target values ​​can be the same. The braking request variable is therefore implemented in such a way that the front axle is supplied first and only then is the rear axle supplied with brake fluid from the electrical pressure supply device. The system preferably remains completely by-wire, i.e. the driver still has no access to the wheel brakes but is only connected to a simulator. The method according to the invention is particularly advantageous for use with purely electric brake pedals, which do not allow any hydraulic or mechanical access to the wheel brakes.

[0006] If braking is initiated by the driver, the braking request variable can include a brake pedal travel, a brake pedal force, and / or a master cylinder pressure. The braking request variable can also be a variable derived from one or more of these variables, such as a target deceleration of the vehicle. Furthermore, the braking request variable can also originate from an assistance function or a virtual driver or motion system and correspond to or be equivalent to a variable described above.

[0007] In the event of a fault in the braking system, fallback levels are used that involve changes of varying severity depending on the fault, thus limiting functionality. One such reaction to a fault is a degradation of the pressure supply device. This means that the pressure supply device is used with reduced performance characteristics.

[0008] By preferentially building up pressure at the front axle, a higher brake pressure can be generated with the same delivered volume. This takes advantage of the downward gradient of the relationship between pressure and volume of the medium delivered to the wheel brakes. This allows the target value to be reached very quickly at the front axle, which already provides very high deceleration due to the brake force distribution, which is approximately two-thirds distributed across the front axle.

[0009] In a preferred embodiment of the invention, the degradation of the pressure supply device comprises a power limitation, torque limitation, volumetric flow limitation and / or speed limitation.

[0010] In a preferred embodiment of the invention, in a fault-free case, a first pressure setpoint is determined based on the braking request variable and implemented in the wheel brakes of the front axle and the rear axle. In a fault case, based on the same braking request variable, a second pressure setpoint is determined which is greater than the first pressure setpoint and is only implemented on the wheel brakes of the front axle. The second pressure setpoint can be set such that when braking on the front axle it corresponds to the same vehicle deceleration as when braking on both axles with the first pressure setpoint. For this purpose, it can be set to 120%-150% of the first pressure value. The rear axle can then be engaged when the second pressure value is reached, and while the pressure on the rear axle is increasing, the pressure on the front axle can be reduced from the second pressure value to the first pressure value.As a final state, the front axle and rear axle can then again be at the common first pressure value.

[0011] In a further preferred embodiment of the invention, degradation is triggered by a fault in a motor position sensor of the pressure supply device, by an excessively low on-board power supply voltage, or by exceeding a temperature threshold. A synchronous motor drive requires motor angle information to correctly switch the commutation field and achieve the optimum of speed, acceleration, and maximum pressure. Therefore, degradation is performed if the motor position sensor is faulty. If the on-board power supply voltage is low, the motor cannot be supplied with the maximum current, which also reduces the volumetric conveying rate. If the motor overheats, its performance is also degraded to prevent irreversible damage and total failure. This protects motor components such as magnets, coils, and bridge drivers from excessive temperature damage.The temperature can be measured using a sensor or calculated from a model.

[0012] Temperature-dependent current limitation is also known as "derating." When critical temperature thresholds are exceeded, the current consumption or current flow to the motor is limited. This results in thermal protection but also poorer performance.

[0013] In a further preferred embodiment of the invention, the brake pressure is only initially applied to the front axle if a pressure demand gradient and / or deceleration demand gradient exceeds a threshold value. The pressure demand gradient and / or deceleration demand gradient preferably corresponds to a volume flow that can still be delivered by the electric pressure supply device in the degraded state. Thus, depending on the need, only the front axle or both axles are used for braking first, ensuring the best possible braking performance.

[0014] In a further preferred embodiment of the invention, the condition for engaging the rear axle includes reaching a pressure threshold and / or the elapse of a predetermined period of time. In one variant, the rear axle can be engaged when 80% of the target value is reached on the front axle. If the wheel brakes of the front axle are set to the second pressure target value and the pressure threshold is set to this second pressure target value, the engagement of the rear axle can be implemented such that the volume flow via the inlet valves into the wheel brakes of the rear axle exceeds the delivered volume flow of the electric pressure supply device to such an extent that the braking pressure of the inlet valves drops due to cross-flows. This can be controlled so that the overall deceleration of the vehicle remains constant.

[0015] In a further preferred embodiment of the invention, the condition for engaging the rear axle also includes the initiation of ABS control on the front axle. It is thus determined that, due to the friction coefficient on the front axle, a further increase in brake pressure cannot lead to greater deceleration. Accordingly, the available volume flow of the electric pressure supply device is now directed to the rear axle. Even if different brake pressures are requested at the rear axle, the difference between which is greater than a threshold value, for example, 1 bar, the rear axle is engaged.

[0016] In a preferred embodiment of the invention, the pressure supply device is a linear actuator. A linear actuator can, in particular, not only build up pressure quickly and in a controlled manner, but also reduce it without switching exhaust valves, thus avoiding noise and vibration.

[0017] In a preferred embodiment of the invention, the pressure supply device comprises a brushless electric motor. Such a motor is particularly efficient, dynamic, and durable.

[0018] In a preferred embodiment of the invention, the degradation includes a speed limitation to a speed less than 50% of the nominal speed. This allows the motor to cool down to solve thermal problems. In addition,

[0019] In a further preferred embodiment of the invention, the wheel brakes of the rear axle are engaged in such a way that the brake pressure on the front axle does not drop. Accordingly, the inlet valves on the wheel brakes on the rear axle do not open directly into the fully open state. Instead, they are controlled according to a characteristic curve with a coil current such that the volume flow flowing via the inlet valves into the wheel brakes of the rear axle is less than or equal to the volume flow delivered by the electric pressure supply device. This prevents brake fluid from the wheel brakes on the front axle from flowing into the wheel brakes on the rear axle via crossflows, which would cause the pressure in the wheel brakes on the front axle to drop.However, it can also be provided that when a pressure reduction is requested on both wheel brakes of the front axle, the inlet valves of the rear axle are opened further in order to implement this pressure reduction.

[0020] The object is also achieved by a hydraulic motor vehicle brake system according to claim 12, comprising a control device configured to carry out the above method.

[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. Fig. 1 shows schematically a braking system according to the invention, Fig. 2 shows a pV diagram of front axle wheel brakes and rear axle wheel brakes;

[0022] The Fig. The braking system for a motor vehicle shown in Figure 1 comprises four hydraulically actuated wheel brakes 8a-8d. The braking system comprises a master brake cylinder 2 actuated by means of an actuation or brake pedal 1, a travel simulator or simulation device 3 interacting with the master brake cylinder 2, a pressure fluid reservoir 4 under atmospheric pressure, an electrically controllable pressure supply device 5, and wheel valves, i.e., wheel-specific brake pressure modulation valves, which are designed, for example, as inlet valves 6a-6d and outlet valves 7a-7d.

[0023] Furthermore, the braking system comprises at least one electronic control and regulation unit 12 for controlling the electrically actuated components of the braking system. The control and regulation unit 12 has, in particular, at least two separate subunits, each of which controls a portion of the hydraulic units.

[0024] For example, the wheel brake 8a is assigned to the left front wheel (FL), the wheel brake 8b to the right front wheel (FR), the wheel brake 8c to the left rear wheel (RL) and the wheel brake 8d to the right rear wheel (RR).

[0025] The master brake cylinder 2 has a master brake cylinder piston 15 in a housing 16, which defines a hydraulic pressure chamber 17, and constitutes a single-circuit master brake cylinder 2. The pressure chamber 17 accommodates a return spring 9, which positions the piston 15 in an initial position when the master brake cylinder 2 is not actuated. The pressure chamber 17 is connected, on the one hand, to the pressure fluid reservoir 4 via radial bores formed in the piston 15 and a corresponding pressure equalization line 41, which can be shut off by a relative movement of the piston 15 in the housing 16. The pressure chamber 17 is also connected, via a hydraulic line section (also referred to as the first supply line) 22, to a brake supply line 13, to which the inlet connections of the inlet valves 6a-6d are connected. The pressure chamber 17 of the master brake cylinder 2 is thus connected to all inlet valves 6a-6d.

[0026] For example, no hydraulic valve, in particular no electrically or hydraulically actuated valve and no check valve, is arranged in the pressure equalization line 41 or in the connection between the pressure chamber 17 and the pressure medium reservoir 4.

[0027] Alternatively, a diagnostic valve, in particular one that is open when de-energized, can be included in the pressure equalization line 41 or between the master brake cylinder 2 and the pressure medium reservoir 4, preferably a parallel connection of a diagnostic valve that is open when de-energized with a check valve that closes towards the pressure medium reservoir 4.

[0028] A separating valve 23 is arranged between the supply line 22 connected to the pressure chamber 17 and the brake supply line 13, or rather, the pressure chamber 17 is connected to the brake supply line 13 via the first supply line 22 with a separating valve 23. The separating valve 23 is designed as an electrically actuated, preferably normally open (SO), 2 / 2-way valve. The separating valve 23 can shut off the hydraulic connection between the pressure chamber 17 and the brake supply line 13.

[0029] A piston rod 24 couples the pivoting movement of the brake pedal 1 resulting from pedal actuation with the translational movement of the master brake cylinder piston 15, whose actuation travel is detected by a preferably redundant travel sensor 25. The corresponding piston travel signal thus serves as a measure of the brake pedal actuation angle. It represents a driver's braking request and can be used as a braking request variable.

[0030] A pressure sensor 20 connected to the first supply line 22 detects the pressure built up in the pressure chamber 17 by the displacement of the piston 15. This pressure value can also be evaluated to characterize or determine the driver's braking request and can also be used as a braking request variable. As an alternative to a pressure sensor 20, a force sensor 20 can also be used to determine the driver's braking request.

[0031] The simulation device 3 is, for example, hydraulically designed and hydraulically coupled to the master brake cylinder 2. The simulation device 3 essentially has, for example, a simulator chamber 29, a simulator rear chamber 30, and a simulator piston 31 separating the two chambers 29, 30 from each other. The simulator piston 31 is supported on a housing by an elastic element 33 (e.g., a simulator spring) arranged in the (for example, dry) simulator rear chamber 30. The hydraulic simulator chamber 29 is connected, for example, to the pressure chamber 17 of the master brake cylinder 2 by means of a preferably electrically actuated, preferably normally closed simulator release valve 32.

[0032] The braking system comprises an inlet valve 6a-6d and an outlet valve 7a-7d for each hydraulically actuated wheel brake 8a-8d. These valves are hydraulically interconnected in pairs via central connections and connected to the wheel brake 8a-8d. A non-return valve (not specifically designated) opening toward the brake supply line 13 is connected in parallel to each of the inlet valves 6a-6d. The outlet connections of the outlet valves 7a-7d are connected to the pressure fluid reservoir 4 via a common return line 14.

[0033] The electrically controllable pressure supply device 5 is designed as a hydraulic cylinder-piston arrangement or a single-circuit, electro-hydraulic actuator or linear actuator, whose piston 36 can be actuated by a schematically indicated electric motor 35 with the interposition of a rotation-translation gear 39, also shown schematically. The piston 36 delimits the single pressure chamber 37 of the pressure supply device 5. A rotor position sensor, only schematically indicated, serving to detect the rotor position of the electric motor 35 is designated by the reference numeral 44. The electric motor of the linear actuator is designed as a brushless motor and uses the rotor position sensor or motor position sensor 44 for the correct control of the individual motor phases.

[0034] A line section (also referred to as the second supply line) 38 is connected to the pressure chamber 37 of the electrically controllable pressure supply device 5. The supply line 38 is connected to the brake supply line 13 via an electrically actuated, preferably normally closed, connection valve 26. The connection valve 26 can be used to control the hydraulic connection between the pressure chamber 37 of the electrically controllable pressure supply device 5 and the brake supply line 13 (and thus the inlet connections of the inlet valves 6a-6d).

[0035] The actuator pressure generated by the force acting on the pressure medium contained in the pressure chamber 37 is fed into the second supply line 38. In a "brake-by-wire" operating mode, particularly when the braking system is fault-free, the supply line 38 is connected to the brake supply line 13 via the connection valve 26. In this way, during normal braking, wheel brake pressure is increased and decreased for all wheel brakes 8a-8d by moving the piston 36 forwards and backwards. All inlet valves can be open during this process, so that the same brake pressure is generated at the wheel brakes of the front and rear axles.

[0036] When the pressure is reduced by moving the piston 36 back, the pressure medium previously displaced from the pressure chamber 37 of the pressure supply device 5 into the wheel brakes 8a-8d flows back into the pressure chamber 37 in the same way.

[0037] Alternatively, individual wheel brake pressures can be easily adjusted using the inlet and outlet valves 6a-6d, 7a-7d. Upon a corresponding pressure reduction, the pressure fluid portion released via the outlet valves 7a-7d flows via the return line 14 into the pressure fluid reservoir 4.

[0038] A refill of pressure medium into the pressure chamber 37 is possible by moving the piston 36 back when the connection valve 26 is closed, in that pressure medium can flow from the container 4 via the line 42 with a check valve 53 opening in the flow direction to the actuator 5 into the actuator pressure chamber or pressure chamber 37.

[0039] For example, pressure chamber 37 is also connected to pressure fluid reservoir 4 via one or more sniffer holes when piston 36 is not actuated. This connection between pressure chamber 37 and pressure fluid reservoir 4 is severed upon (sufficient) actuation of piston 36 in actuation direction 27.

[0040] An electrically actuated, normally open circuit isolation valve 40 is arranged in the brake supply line 13, which divides the braking system into two hydraulic sub-circuits. The brake supply line 13 is divided into a first line section 13a, which is connected (via the isolation valve 23) to the master brake cylinder 2, and a second line section 13b in the second hydraulic sub-circuit, which is connected (via the connection valve 26) to the pressure supply device 5. The first line section 13a is connected to the inlet valves 6a, 6b of the wheel brakes 8a, 8b, and the second line section 13b is connected to the inlet valves 6c, 6d of the wheel brakes 8c, 8d.

[0041] When the circuit isolation valve 40 is open, the braking system is designed as a single circuit. By closing the circuit isolation valve 40, the braking system can be separated or divided, particularly as required by the situation, into two hydraulic sub-circuits, brake circuits I and II. In the first brake circuit I, the master brake cylinder 2 (via the isolation valve 23) is connected only to the inlet valves 6a, 6b of the wheel brakes 8a, 8b of the front axle VA, and in the second brake circuit II, the pressure supply device 5 (with the activation valve 26 open) is connected only to the wheel brakes 8c and 8d of the rear axle HA.

[0042] When the circuit isolation valve 40 is open, the input ports of all inlet valves 6a-6d can be supplied with a pressure via the brake supply line 13 that, in a first operating mode (e.g., "brake-by-wire" operating mode), corresponds to the brake pressure provided by the pressure supply device 5. In a second operating mode (e.g., in a de-energized fallback operating mode), the brake supply line 13 can be pressurized with the pressure of the pressure chamber 17 of the master brake cylinder 2.

[0043] Advantageously, the brake system comprises a level measuring device 50 for determining a pressure medium level / level in the pressure medium reservoir 4. Advantageously, a situation detection for circuit separation by means of the circuit separation valve 40 takes place via the level measuring device 50.

[0044] For example, the hydraulic components and hydraulic units, namely the master brake cylinder 2, the simulation device 3, the pressure supply device 5, the valves 6a-6d, 7a-7d, 23, 26, 40, and 32, as well as the hydraulic connections including the brake supply line 13, are arranged together in a hydraulic control and regulation unit 60 (HCU). The electronic control and regulation unit (control system) 12 is assigned to the hydraulic control and regulation unit 60. Preferably, the hydraulic and electronic control and regulation units 60, 12 are designed as a single unit (HECU).

[0045] The braking system includes a pressure sensor 19 or system pressure sensor for detecting the pressure provided by the pressure supply device 5. The pressure sensor 19 is arranged behind the connection valve 26, as seen from the pressure chamber 37 of the pressure supply device 5.

[0046] The synchronous motor drive of linear actuator 5 requires motor angle information to correctly switch the commutation field and provide the optimum combination of speed, acceleration, and maximum pressure. If the rotor position sensor 44 fails, the linear actuator cannot reliably maintain synchronous operation. This is degraded; without a sensed angular position, the commutation field operates at a reduced speed and higher torque. Instead of the possible 6000 rpm, the motor is limited to 1500 rpm. This means a reduced maximum flow rate that can be provided by the linear actuator.

[0047] In order to minimize the influence of this degradation on the braking performance, the invention provides for a braking request to be implemented first on the front axle. Fig. Figure 2 shows the volume requirements of the wheel brakes on the front axle 60 and the rear axle 61. It can be seen that even at low pressures, a relatively large volume must be pumped into the wheel brakes. This is due, on the one hand, to the existing clearance between the brake pads and the brake disc, and, on the other hand, to the compression of the brake pads. At higher pressures, these influencing factors are essentially eliminated, so that a further pressure increase can be achieved with comparatively less brake fluid volume.

[0048] In a comparison, a vehicle deceleration dynamic of 100 bar on both axles is to be compared with a front axle braking force of 150 bar. It is assumed that the braking power has an axle distribution of 66 / 34 (66% is applied to the front axle), and thus a front axle braking force of 150 bar is the equivalent of 100 bar on both axles. If the corresponding volume requirement is read from Fig.2, with two front axle wheel brakes and two rear axle wheel brakes, this results in a volume requirement of 10cm 3 The volume requirement of the front axle braking of 150 bar, which as stated above leads to approximately the same vehicle deceleration, is only 8 cm 3 . Neglecting the acceleration phase of the engine, the target deceleration is thus achieved 20% faster by the method according to the invention.

[0049] As soon as the front axle wheel brakes have reached the target value, the rear axle inlet valves 6c, 6d are controlled to open to establish the ideal brake force distribution. These valves are supplied with an electric current, which regulates a flow rate greater than the flow rate of the linear actuator. This creates a crossflow from the front axle wheel brakes to the rear axle wheel brakes, reducing the pressure in the front axle wheel brakes. The inlet valves of the rear axle wheel brakes are controlled in such a way that the pressure reduction in the front axle and the pressure buildup in the rear axle add up to a constant vehicle deceleration.

Claims

[1] Method for controlling a hydraulic motor vehicle brake system, wherein, based on a brake request variable, a brake pressure is to be built up at wheel brakes (8) of a front axle and a rear axle by means of an electrical pressure supply device (5), wherein in a fault-free case, inlet valves (6) of the wheel brakes (8) on the front axle and rear axle are opened and a brake request variable is implemented by common pressure build-up on the front axle and rear axle characterized bythat in the event of a fault with a degradation of the pressure supply device (5), the pressure build-up first takes place on the front axle by closing inlet valves (6) of the wheel brakes (8) of the rear axle before or at the start of a volume delivery by means of the pressure supply device (5), so that a pressure build-up is first implemented via open inlet valves (6) only on the wheel brakes (8) of the front axle, and wherein, if a condition exists, the wheel brakes (8) on the rear axle are switched on by opening the inlet valves (6). [2] Method according to claim 1, characterized by that the degradation of the pressure supply device (5) comprises a power limitation, torque limitation, volumetric flow limitation and / or speed limitation. [3] Method according to claim 1 or 2, characterized bythat in a fault-free case, based on the braking request variable, a first pressure setpoint is determined and implemented in the wheel brakes (8) of the front axle and the rear axle, and in a fault case, based on the same braking request variable, a second pressure setpoint is determined which is greater than the first pressure setpoint and is implemented only at the wheel brakes (8) of the front axle. [4] Method according to one of the preceding claims, characterized by that the degradation is triggered by a fault in a motor position sensor of the pressure supply device (5), by an excessively low vehicle electrical system voltage or by exceeding a temperature threshold value. [5] Method according to one of the preceding claims, characterized by that the brake pressure is only built up first at the front axle if a pressure request gradient and / or deceleration request gradient exceeds a threshold value. [6] Method according to one of the preceding claims, characterized by that the condition for engaging the rear axle includes reaching a pressure limit and / or the expiration of a specified period of time. [7] Method according to one of the preceding claims, characterized by that the condition for engaging the rear axle also includes the initiation of ABS control and / or various requirements for the rear axle wheel brakes on the front axle. [8] Method according to one of the preceding claims, characterized by that the pressure supply device (5) is a linear actuator. [9] Method according to one of the preceding claims, characterized by that the pressure supply device (5) comprises a brushless electric motor. [10] Method according to one of the preceding claims, characterized by that the degradation of the speed is less than 50% of the nominal speed. [11] Method according to one of the preceding claims, characterized by that the wheel brakes (8) of the rear axle are activated in such a way that the brake pressure on the front axle does not drop. [12] Hydraulic motor vehicle brake system, comprising a control device configured to carry out a method according to one of claims 1 to 11.

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