Method for actuating a brake system in a vehicle

Abstract

In a method for actuating a brake system in a vehicle in which the brake system has a hydraulic vehicle brake and an electromechanical braking device, the electromechanical braking force is actuated when a brake servo unit of the hydraulic vehicle brake partially or entirely breaks down, the electromechanical braking force being increased in multiple stages.

Description

[0001] Description

[0002] title

[0003] Method for actuating a braking system in a vehicle

[0004] The invention relates to a method for actuating a braking system in a vehicle, wherein the braking system comprises a hydraulic vehicle brake equipped with a brake booster unit and at least one electromechanical braking device with an electric brake motor for generating an electromechanical braking force.

[0005] State of the art

[0006] DE 10 2004004992 A1 describes a parking brake system in a vehicle in which an electric brake motor generates a clamping force that holds the vehicle stationary by moving a brake piston against a brake disc. The parking brake system is integrated into the vehicle's hydraulic brake system. During normal braking, the brake piston is pressed against the brake disc by the hydraulic brake fluid.

[0007] DE 102018210 021 discloses a method for operating a

[0008] A braking system of a motor vehicle is known, which is equipped with a hydraulic braking device, an electromechanical braking device, and a brake pedal. In the event of a failure of the hydraulic braking device, an electromechanical braking force is exerted via the electromechanical braking device to decelerate the motor vehicle.

[0009] In a method known from DE 102018208 877 A1 for operating a motor vehicle braking system which is equipped with a hydraulic braking device, an electromechanical braking device, a brake pedal as the first actuating device and a button as the second actuating device, in the event of a fault in the hydraulic braking device, an electromechanical braking force for decelerating the motor vehicle is generated by means of the electromechanical braking device, regardless of which of the two actuating devices is actuated.

[0010] Disclosure of the invention

[0011] The method according to the invention relates to a vehicle with a hydraulic vehicle brake, via which the vehicle is braked during regular braking operation by applying hydraulic brake pressure to wheel brake devices of the hydraulic vehicle brake, whereby a brake piston in the wheel brake device is adjusted against a brake disc.

[0012] The hydraulic vehicle brake also has at least one brake booster unit to amplify the brake pressure generated by the driver. The brake booster unit is preferably electrically actuated, for example, by means of an electrically controllable actuator, which is advantageously connected downstream of a master brake cylinder of the vehicle brake. The electrically controllable actuator is, for example, a plunger driven by an electric motor. The plunger displaces brake fluid into one or more brake circuits of the hydraulic vehicle brake and builds up hydraulic brake pressure.

[0013] In principle, the brake booster unit can also be designed to be non-electrically actuated, for example hydraulically actuated.

[0014] In addition, at least some of the wheel brake devices of the hydraulic vehicle brake are each equipped with an electromechanical braking device with an electric brake motor, in particular the wheel brake devices on the left and right wheels of the vehicle's rear axle. The electromechanical braking device with the electric brake motor can be actuated when the vehicle is parked to electromechanically generate a parking braking force that permanently immobilizes the vehicle. In a preferred embodiment, the electric brake motor of the electromechanical braking device acts on the same brake piston as the hydraulic vehicle brake.

[0015] In the method according to the invention for actuating the braking system, in the event of a partial or complete failure of the brake booster unit of the hydraulic vehicle brake, the electromechanical braking device is actuated in addition to the hydraulic braking force during braking. This occurs in such a way that the electromechanical braking force increases in several stages, with the electromechanical braking force being kept constant or approximately constant at each stage. The electromechanical braking force thus builds up according to a step function. It has been shown that this approach significantly improves braking force control. Dead times that could impair braking force control are eliminated or at least reduced.The brake force regulators used for the hydraulic vehicle brake and the electromechanical braking device can continue to be used even in the event of failure or partial failure of the brake booster unit of the hydraulic vehicle brake.

[0016] According to an advantageous embodiment, a maximum electromechanical brake force assistance is defined, with the levels of electromechanical brake force each corresponding to a defined percentage of the maximum electromechanical brake force assistance. The maximum electromechanical brake force assistance is, for example, at a deceleration of 4 m / s. 2The value of the maximum electromechanical brake force assistance can be set on a vehicle-dependent basis and / or determined from the current driving situation, for example as a function of current driving state and operating variables such as the current vehicle speed or the vehicle load, etc. The various levels that the electromechanical brake force assumes during the build-up can assume different percentage values ​​of the maximum electromechanical brake force assistance, whereby both equidistant intervals between the levels and intervals of different sizes between the levels are possible.

[0017] Different numbers of stages for the electromechanical braking force are considered during setup. Preferably, at least three or at least four stages are specified, which is sufficient to achieve improved braking force control. If necessary, at least seven or at least ten stages are specified. It is possible to implement a fixed number of stages or to set the number of stages variably, particularly depending on current driving conditions and operating variables such as the current vehicle speed or vehicle load, etc.

[0018] The minimum activation duration for each stage to transition from one stage to the next is advantageously at least 20 ms, preferably at least 50 ms. This ensures that the stages can be precisely adjusted. The minimum activation duration can be fixed or variably set, particularly depending on current driving conditions and operating variables such as the current vehicle speed or vehicle load, etc.

[0019] The current activation duration of each individual stage can depend on the vehicle's state or operating variables, for example, the electrical voltage in the vehicle's electrical system or the speed of the brake motor. Preferably, all stages have the same activation duration. Alternatively, it may be appropriate for different stages to have different activation durations.

[0020] According to a further advantageous embodiment, the maximum electromechanical brake force assistance is set to a value that is lower than the maximum possible mechanical braking force. This ensures that the maximum electromechanical brake force assistance reaches a level at which wheel locking is avoided. The maximum electromechanical brake force assistance represents an upper limit up to which a gradual buildup of electromechanical braking force is possible.

[0021] According to yet another advantageous embodiment, the electromechanical braking force depends on the level of brake pedal actuation. Upon reaching a brake pedal actuation limit, the electromechanical braking force is limited to the maximum electromechanical brake force assistance. For example, a defined pedal force with which the brake pedal is actuated, for example, 200 N, is set as the brake pedal actuation limit. Alternatively, a defined pedal travel can also be set when the brake pedal is actuated.

[0022] In a further preferred embodiment, the electromechanical braking force is only generated when the hydraulic braking pressure in the hydraulic vehicle brake reaches or exceeds a minimum pressure, which is, for example, five bar. Up to the minimum pressure, only the hydraulic vehicle brake is actuated, and only when the minimum pressure is exceeded is the electromechanical braking force generated. However, it is possible for the electric brake motor to be actuated even below the minimum hydraulic pressure without generating electromechanical braking force in order to reduce the free travel until the brake pads contact the brake disc. The free travel reduction can be linked to the additional condition that the hydraulic braking pressure is greater than zero or that the brake pedal travel is greater than zero.

[0023] Alternatively, it is also possible that the electric brake motor remains inactive below the minimum hydraulic pressure.

[0024] In principle, the brake control mechanisms can remain in place even when the hydraulic vehicle brake and the electromechanical braking device are actuated simultaneously. In the event of increased slip on a vehicle wheel, it is expedient to first reduce the electromechanical braking force acting on this wheel and then only if a further reduction in braking force is required to also reduce the hydraulically generated braking force. According to a further advantageous embodiment, not only the build-up but also the reduction of the electromechanical braking force is carried out in several stages, for example, upon completion of a braking operation or upon reduction of the braking force. Even during the reduction, the gradual reduction of the electromechanical braking force is advantageous with regard to braking force control.However, it may be sufficient for the electromechanical braking force to be reduced in one step and without intermediate stages, particularly in cases where the driver completely releases the brake pedal.

[0025] The invention also relates to a control unit containing means configured to carry out the above-described method. The means comprise at least one memory unit, at least one computing unit, a control unit input, and a control unit output. The control unit can be used to control the adjustable components of the hydraulic vehicle brake and / or the electromechanical braking device. Multiple control units may be present, one of which is assigned to the hydraulic vehicle brake and another to the electromechanical braking device.

[0026] The invention further relates to a braking system in a vehicle, wherein the braking system is equipped with a hydraulic vehicle brake and an electromechanical braking device with an electric brake motor and also with at least one previously described control device for controlling the adjustable components of the braking system.

[0027] The invention also relates to a vehicle with a braking system as described above. The vehicle is, in particular, a motor-driven vehicle with at least two axles, for example, a passenger car or a truck.

[0028] The invention also relates to a computer program product with a program code designed to execute the method steps described above. The computer program product runs in the control unit described above.

[0029] Further advantages and practical embodiments can be found in the further claims, the description of the figures, and the drawings. They show:

[0030] Fig. 1 is a schematic representation of a hydraulic vehicle brake with a brake booster forming an actuator, wherein the wheel brake devices of the vehicle brake on the vehicle rear axle are additionally designed as an electromechanical braking device with an electric brake motor,

[0031] Fig. 2 shows a section through an electromechanical braking device with an electric brake motor,

[0032] Fig. 3 is a flow chart showing process steps for building up an additional electromechanical braking force in the event that a brake booster unit of the hydraulic vehicle brake has failed partially or completely,

[0033] Fig. 4 is a flow chart following Fig. 3 with a control loop for limiting the electromechanical braking force.

[0034] In the figures, identical components are provided with identical reference symbols.

[0035] The braking system in a vehicle shown in Fig. 1 comprises a hydraulic vehicle brake 1 with a front axle brake circuit 2 and a rear axle brake circuit 3 for supplying and controlling wheel brake devices 9 on each wheel of the vehicle with brake fluid under hydraulic pressure. In an alternative embodiment, it is also possible for the brake circuits to be designed diagonally. The two brake circuits 2, 3 are connected to a common master brake cylinder 4, which is supplied with brake fluid via a brake fluid reservoir 5. The master brake cylinder piston within the master brake cylinder 4 is actuated by the driver via the brake pedal 6, and the pedal travel exerted by the driver is measured via a pedal travel sensor 7. Between the brake pedal 6 and the master brake cylinder 4 is a brake booster 10, which comprises, for example, an electric motor, which preferably actuates the master brake cylinder 4 via a transmission (iBooster).The brake booster 10 is an electrically controllable actuator for influencing the brake pressure. The iBooster 10 forms a brake boosting unit.

[0036] The actuating movement of the brake pedal 6 measured by the pedal travel sensor 7 is transmitted as a sensor signal to a control unit 11, which generates actuating signals for controlling the brake booster 10. The wheel brake devices 9 are supplied with brake fluid in each brake circuit 2, 3 via various switching valves that are part of a brake hydraulic system 8. The brake hydraulic system 8 also includes a hydraulic pump that is part of an electronic stability program (ESP).

[0037] The brake booster can be carried out additionally or alternatively by means of an electrically controllable actuator which is connected downstream of the master brake cylinder 4 of the vehicle brake 1.

[0038] The vehicle brake 1 is provided with an additional hydraulic supply line 24 which connects the brake fluid reservoir 5 to the outlet valves of wheel brake devices 9.

[0039] Fig. 2 shows a detail of the wheel brake device 9, which is arranged on a wheel on the rear axle of the vehicle. The wheel brake device 9 is part of the hydraulic vehicle brake 1 and is supplied with brake fluid 22 from the rear axle brake circuit. The wheel brake device 9 also has an electromechanical braking device which, like the hydraulic vehicle brake 1, is part of the braking system in the vehicle and is preferably used to immobilize the vehicle when stationary, but can also be used to brake when the vehicle is moving. The electromechanical braking device comprises a brake caliper 12 with a pair of calipers 19 that engages over a brake disc 20. As an actuator, the braking device has a DC electric motor as the brake motor 13, the rotor shaft of which drives a spindle 14 in rotation, on which a spindle nut 15 is mounted. When the spindle 14 rotates, the spindle nut 15 is axially adjusted.The spindle nut 15 moves within a brake piston 16, which supports a brake pad 17, which is pressed by the brake piston 16 against the brake disc 20. On the opposite side of the brake disc 20 is another brake pad 18, which is held stationary on the caliper 19. The brake piston 16 is sealed on its outer side against the receiving housing by a surrounding sealing ring 23.

[0040] Within the brake piston 16, the spindle nut 15 can move axially forward toward the brake disc 20 upon rotation of the spindle 14, or axially backward until it reaches a stop 21 upon rotation of the spindle 14 in the opposite direction. To generate a clamping force, the spindle nut 15 acts on the inner end face of the brake piston 16, whereby the brake piston 16, which is mounted axially displaceably in the braking device, is pressed with the brake pad 17 against the facing end face of the brake disc 20.

[0041] The hydraulic braking force is generated by the hydraulic pressure of the brake fluid 22 from the hydraulic vehicle brake 1 acting on the brake piston 16. The hydraulic pressure can also have a supporting effect when the vehicle is stationary when the electromechanical braking device is actuated, so that the total braking force is composed of the electric motor-driven component and the hydraulic component. While the vehicle is moving, either only the hydraulic vehicle brake is active, or both the hydraulic vehicle brake and the electromechanical braking device, or only the electromechanical braking device, are active to generate braking force. The control signals for controlling both the adjustable components of the hydraulic vehicle brake 1 and the electromechanical wheel braking device 9 are generated in the control unit 11. Fig.Figure 3 shows a flowchart with method steps for generating additional electromechanical braking force in the event that a brake booster unit of a hydraulic vehicle brake has failed. Figure 3 thus refers to a braking situation involving actuation of the hydraulic vehicle brake, but with a complete or at least partial failure of a brake booster unit such as the iBooster 10 or a plunger. In this case, the braking force initiated by the driver via actuation of the brake pedal is not amplified or is only insufficiently amplified, so that the driver must press the brake pedal significantly harder to achieve the same deceleration values ​​compared to an intact hydraulic vehicle brake.

[0042] In the initial situation according to method step 30, the aforementioned partial or complete failure of a brake booster unit in the hydraulic vehicle brake is assumed. In the following method step 31, a query is made as to whether the hydraulic vehicle brake is being actuated, in particular by actuating the brake pedal. If this is not the case, the system returns to the first method step 30 following the "No" branch ("N") and checks are made again at cyclic intervals to determine whether the brake pedal is being actuated. If, on the other hand, the query in step 31 reveals that the brake pedal is actually being actuated by the driver, the system advances to the next method step 32 following the "Yes" branch ("Y").

[0043] In method step 31, as an alternative to actuating the brake pedal, it can also be queried whether a braking force request is made in any other way, in particular via an automatically operating driver assistance system.

[0044] In process step 32, a query is made as to whether the hydraulic brake pressure exceeds a first, lower threshold. If this is not the case, the "No" branch returns to the beginning of the process. However, if the current hydraulic brake pressure exceeds the lower threshold, a minimum requirement must be assumed, which triggers the subsequent actions. In this case, the "Yes" branch leads to the next process step.

[0045] 33, in which the free travel until the brake pads of a brake disc are in contact is reduced by actuating the electromechanical braking device.

[0046] The reduction of the free travel can, if necessary, also be achieved below the lower limit, which represents the minimum pressure. This has the advantage of reducing dead times when activating the electromechanical braking force as the braking demand increases.

[0047] Following the pre-positioning of the electromechanical braking device in process step 33, the next process step

[0048] 34, the further query as to whether a second brake pressure limit value is exceeded, which is higher than the first, lower brake pressure limit value according to method step 32. If this is not the case, the system returns to method step 32 following the "No" branch. However, if the current brake pressure exceeds the limit value according to method step 34, the system proceeds to the next step 35 following the "Yes" branch, in which the electromechanical braking device is actuated by controlling the electric brake motor 13 and an electromechanical braking force is generated in addition to the hydraulic braking force. This can compensate for the partial or complete failure of the brake booster unit in the hydraulic vehicle brake.

[0049] Fig. 4 shows the activation of the electromechanical braking device in method step 35 with a control loop for limiting the maximum generated electromechanical braking force. After the activation of the electromechanical braking force in step 35, the next step 36 queries whether the wheel slip at the relevant vehicle wheel, at which the electromechanical braking force is generated, exceeds a threshold value. If so, the "yes" branch is followed by step 37, according to which the electromechanical braking force is automatically reduced in the electromechanical braking device. The system then returns to step 36 and queries again at cyclical intervals whether the wheel slip at the relevant wheel is exceeded.If, however, the query in step 36 reveals that the wheel slip is still below the threshold value, the system proceeds to step 38 following the "No" branch, where it is queried whether the current braking request, i.e., the braking force requested by the driver, is greater than what is provided by the electromechanical braking device—taking the hydraulic braking force into account. If this is the case, the system returns to step 35 following the "Yes" branch, and the electromechanical braking device is adjusted accordingly to implement the braking request. If, however, the braking request is below the current electromechanical braking force, the system can proceed to step 37 following the "No" branch to reduce the electromechanical braking force. However, a return to process step 35 is also possible.

[0050] In the electromechanical braking force buildup described in Fig. 3 and Fig. 4, the increase occurs in several stages, with the electromechanical braking force being kept constant or at least approximately constant during each stage. The minimum activation time for changing to a higher or lower stage is, for example, 50 ms. Several stages can be specified, for example, three, four, five, or even more stages, with the electromechanical braking force being increased gradually from stage to stage. The reduction of the electromechanical braking force can also be carried out in stages.

Claims

Claims 1. A method for actuating a braking system in a vehicle, the braking system comprising a hydraulic vehicle brake (1) equipped with a brake booster unit (10, 25) and at least one electromechanical braking device having an electric brake motor (13) for generating an electromechanical braking force, wherein in the event of a partial failure or a complete failure of the brake booster unit (10, 25) of the hydraulic vehicle brake (1), the electromechanical braking device is actuated during braking in addition to the hydraulic braking force in such a way that the increase in the electromechanical braking force is carried out in several stages, the electromechanical braking force being kept constant or approximately constant at each stage.

2. Method according to claim 1, characterized in that the brake booster unit (10, 25) in the hydraulic vehicle brake (1) can be actuated electrically.

3. Method according to claim 1 or 2, characterized in that a maximum electromechanical brake force assistance is determined, wherein the stages of the electromechanical brake force each correspond to a defined percentage of the maximum electromechanical brake force assistance.

4. Method according to one of claims 1 to 3, characterized in that at least three stages, preferably at least seven stages, in particular ten stages of the electromechanical braking force are defined.

5. Method according to one of claims 1 to 4, characterized in that to change the electromechanical braking force of the electromechanical brake motor is controlled for a control period of at least 20 ms, preferably for 50 ms, in order to increase or decrease the electromotive braking force component by one step. Method according to claim 5, characterized in that the control period depends on state or operating variables of the vehicle, for example on the electrical voltage in the on-board electrical system or on the engine speed of the brake motor (13). Method according to one of claims 1 to 6, characterized in that the maximum electromechanical braking force assistance is set to a value that is smaller than the maximum possible mechanical braking force. Method according to one of claims 1 to 7, characterized in that the electromechanical braking force depends on the level of brake pedal actuation, wherein when a brake pedal actuation limit value is reached, the electromechanical braking force is limited to the maximum electromechanical braking force assistance.Method according to one of claims 1 to 8, characterized in that the electromechanical braking force is only generated when the hydraulic braking pressure in the hydraulic vehicle brake (1) reaches or exceeds a minimum pressure of, for example, five bar. Method according to claim 9, characterized in that, in the event that the hydraulic braking pressure is below the minimum pressure, the electric brake motor (13) is actuated to reduce the free travel until the brake pads (17, 18) contact the brake disc (20). Method according to one of claims 1 to 10, characterized in that, in the event of increased slip at a wheel of the vehicle, the electromechanical braking force acting on this wheel is reduced. Method according to one of claims 1 to 11, characterized in that, upon termination of a braking operation or reduction of the... Braking force, the reduction of the electromechanical braking force is carried out in several stages. Control unit (11) with means for controlling the adjustable components of the hydraulic vehicle brake (1) and / or the electromechanical braking device, which is set up to carry out the method according to one of claims 1 to 12. Braking system in a vehicle, with a hydraulic vehicle brake (1) and an electromechanical braking device with an electric brake motor (13), and with a control unit (11) according to claim 13 for controlling the adjustable components of the braking system. Vehicle with a braking system according to claim 14. Computer program product with program code that is designed to carry out steps of the method according to one of claims 1 to 12 when the computer program product runs in a control unit (11) according to claim 13.

Images