Method for providing a clamping force generated by an automatic parking brake to a vehicle

Abstract

A method is provided for supplying a clamping force generated by an automatic parking brake (1) to a vehicle, wherein the clamping force is generated by a brake piston (5) acting on a brake disc (7) and a brake motor (2). The object of the invention is to provide a method that ensures reliable function of the automatic parking brake even in the event of a loss of clamping force due to a temperature change of the brake disc, and which furthermore minimizes the stress on the components of the automatic parking brake. For this purpose, after the automatic parking brake (1) has been fully applied, hydraulic pressure is built up in a brake circuit of the vehicle and a pressure loss gradient (dp / dt) in the brake circuit of the vehicle is determined, whereby the automatic parking brake (1) is re-tensioned or not, depending on the determined pressure loss gradient (dp / dt).

Description

[0001] The invention relates to a method for providing a clamping force generated by an automatic parking brake, to a control or regulating device and to an automatic parking brake for a vehicle.

[0002] From DE 10 2011 005 842 A1, an automatic parking brake for use in a vehicle is known, which can exert a clamping force on a brake disc to secure a stationary vehicle. The clamping force is generated by a spindle nut and a brake piston, which are moved by a brake motor and clamp the brake disc between two brake pads during a clamping operation. The brake piston can also be actuated by hydraulic brake fluid, which is achieved by the regular vehicle brake (service brake). Due to braking operations performed while driving, the brake disc can heat up. This creates a problem when securing the stationary vehicle using the parking brake, as the increased temperature changes the coefficient of friction and the expansion of the brake disc.If the brake disc cools down after the automatic parking brake has been applied, a loss of clamping force can occur. Therefore, a retensioning process is usually initiated some time after the initial application. This process is calculated and carried out based on the brake disc temperature. If the measured brake disc temperature exceeds a defined value, a significant loss of clamping force in the automatic parking brake is assumed once the brake disc has cooled down. Consequently, a retensioning process is performed to compensate for this loss of clamping force. However, if the brake disc temperature falls below the predetermined temperature value, no retensioning process is performed because no functional impairment is expected.

[0003] The above-described, purely temperature-based parking brake system has the disadvantage that, despite the retensioning process, some of the clamping force can be lost due to thermal relaxation. This is particularly true if prolonged braking maneuvers or several hard braking maneuvers immediately precede the application of the parking brake. During braking maneuvers, the brake disc and brake pads heat up and expand. During the subsequent cooling process, and with the parking brakes engaged, the components contract, resulting in a loss of some clamping force. The brake disc temperature model (BTM) only provides a rough indication of the brake disc temperature. The BTM is inaccurate due to altered airflow characteristics (e.g., caused by a spoiler or wheel covers), the material (steel or aluminum rims), and ambient conditions.Depending on the configuration, "estimation errors" of 50°C to 100°C are possible.

[0004] Conventional braking systems often retighten the parking brake without considering how much retightening is needed or when the optimal time for retightening is. In some cases, retightening occurs multiple times, which necessarily leads to a long overrun time for the control unit and thus to increased component stress, as the electric brake motor and gearbox have to start up against a pre-tensioned system several times.

[0005] The object of the present invention is therefore to provide a method that ensures reliable function of the automatic parking brake even in the event of a loss of clamping force due to a temperature change of the brake disc, and which also minimizes the stress on the components of the automatic parking brakes.

[0006] The problem is solved by the features of the independent patent claims. Further developments of the invention are specified in the dependent claims.

[0007] According to a first embodiment of the invention, a method is provided for supplying a clamping force generated by an automatic parking brake to a vehicle, in which the clamping force is generated by a brake piston acting on a brake disc and a brake motor. After the automatic parking brake has been fully applied, hydraulic pressure is built up in a brake circuit of the vehicle, and a pressure loss gradient in the brake circuit is determined. Depending on the determined pressure loss gradient, the automatic parking brake is either re-tensioned or not. For ease of understanding, the term pressure loss gradient (dp / dt) is used; this term should, of course, also be understood to represent a value that equates to the pressure loss gradient (dp / dt).

[0008] The system according to the invention eliminates the need for preventive retensioning of the parking brake(s). The pressure loss in the vehicle's brake circuit reliably indicates how the clamping force of the automatic parking brake develops due to thermal relaxation. Furthermore, the control unit's overrun time is minimized, and the component stress on the brake system is reduced. Even extremely stiff brake systems, where component stress is particularly high, are thus easier to handle. Such high stiffness is typically sought to improve service braking performance (lower volume absorption) and reduce residual braking torque. The brake piston can, for example, be retracted by a sealing ring, which results in better "free movement" in a stiffer system than in a softer one.

[0009] Advantageously, the automatic parking brake is retensioned depending on the determined pressure loss gradient when a predetermined limit value for the pressure loss gradient is exceeded. In this way, it can be precisely defined which clamping force loss is still within the tolerance range and which is not.

[0010] Advantageously, the expected total clamping force loss at the automatic parking brake is determined using the pressure loss gradient (dp / dt) value and compensated for by retensioning. In this way, the originally defined clamping force is reliably restored.

[0011] The pressure loss gradient can be determined at the brakes of a rear axle of the vehicle, but it can also be determined at the brakes of a front axle to calculate the expected total clamping force loss at the rear axle. This has the advantage that, since the brake pistons of the front axle are not mechanically locked by a parking brake, significantly lower forces can be applied, resulting in a reduction of component stress.

[0012] Advantageously, the vehicle's incline is determined, and the clamping force is adjusted accordingly. This avoids unnecessary retensioning when the vehicle is on level ground, for example, and a reduced clamping force is sufficient to securely fasten the vehicle.

[0013] Advantageously, the clamping force loss on the automatic parking brake is additionally monitored using force measurement technology, which allows for even more precise retensioning.

[0014] Furthermore, a control unit for carrying out the method according to the invention and an automatic parking brake for a vehicle are provided.

[0015] Further features and advantages of the invention will become apparent from the description of exemplary embodiments with reference to the accompanying figures. The figures show: Fig. 1 A sectional view of a conventional automatic parking brake for a vehicle; Fig. 2 a flowchart of a method for providing a clamping force generated by the automatic parking brake and securing the vehicle according to an embodiment of the invention; and Fig. 3 a diagram showing a p (pressure)-V (volume) characteristic curve.

[0016] Fig. Figure 1 shows a section through a known automatic (automated) parking brake 1 for a vehicle, which can exert a clamping force to secure the vehicle by means of a brake motor 2. The brake motor 2 is designed as an electric motor that drives a spindle 3, in particular a threaded spindle, which is mounted axially. At its end facing away from the brake motor 2, the spindle 3 is provided with a spindle nut 4, which bears against an inner end face or rear face of a brake piston 5. The spindle 3, the spindle nut 4, and the brake piston 5 are mounted in a brake caliper 6, which grips a brake disc 7 in a clamping manner. A brake pad 8, 8' is arranged on each side of the brake disc 7.

[0017] In the event of an application of the parking brake 1, the spindle nut 4 is moved axially towards the brake piston 5 and the brake disc 7 by the rotation of the spindle 3 until a predetermined target clamping force is reached. In addition to the electromechanical clamping force, the rear of the brake piston 5 may be supplied with hydraulic fluid, which releases the automatic parking brake and / or initiates braking during normal vehicle operation (service brake).

[0018] Fig. Figure 2 shows a flowchart of a method for providing a clamping force generated by an automatic parking brake 1 that secures a vehicle. The method ensures a retensioning strategy with low stress on the components, which guarantees reliable vehicle securing even in the event of a loss of clamping force due to thermal relaxation of the components involved.

[0019] According to a first embodiment of the invention, a locking process of the parking brake 1 is initially performed (S10). During the execution of the locking process, the result of the BTM is evaluated. In a next step, depending on the result of the BTM, a decision is made as to whether the brake is "hot" or "cold" (S20). If the brake is cold (no or n-path), the locking process is completed (S21), and no further actions are required (S22). If the brake is hot (yes or j-path), the locking process is also completed (S23); however, after the locking control is completed, hydraulic pressure build-up occurs (S24). This pressure build-up takes place only at the rear axle of the vehicle, which means that only minimal volumes need to be displaced, since the brake system is already mechanically pre-tensioned by the parking brake 1, resulting in a very short actuation time for the corresponding hydraulic pumps.Alternatively, hydraulic pressure can also be supplied by an ESP system or, for example, by an iBooster. A mechanical-hydraulic pressure supply is also conceivable.

[0020] If no BTM is provided, the hydraulic pressure build-up (S24) generally takes place after the locking process of the parking brake 1.

[0021] The pressure loss in the brake circuit is then measured (S25) and a pressure loss gradient (pressure reduction gradient) dp / dt is determined. The system is either completely sealed (no valve leakage), in which case the pressure loss correlates with the clamping force loss. Alternatively, there is a minimal leakage, resulting in a pressure loss similar to thermal relaxation. The pressure loss gradient dp / dt is then evaluated (S30) and compared to a limit value. If the pressure loss gradient dp / dt is less than this limit value (j-path), no further action is necessary (S33). The suspicion of a "hot brake" is therefore unfounded. If the pressure loss gradient dp / dt is greater than the limit value (n-path), the retensioning distance (S31) is determined as a function of the pressure loss gradient dp / dt and preferably the current gradient of the vehicle, and the retensioning of the parking brake 1 is carried out accordingly.Advantageously, the retensioning force corresponds to the clamping force loss.

[0022] Since the time constant of the cooling behavior of a brake system is known (a generically determined quantity, provided the BTM is available) and the clamping force loss / pressure loss within a defined time unit (dp / dt) is also known, the total expected clamping force loss can be deduced. Depending on the vehicle's gradient, a decision is then made as to whether the expected clamping force loss is acceptable or at what point retensioning is necessary to safely hold the vehicle. The necessary retensioning distance (given a known clamp stiffness) or the necessary retensioning force can also be estimated and provided for retensioning. No further action is required (S32).

[0023] In a further embodiment of the invention, the process is identical to that of the method according to the first embodiment of the invention. The difference, however, lies in the fact that the hydraulic pressure is built up at the front axle brake in step S24. Since the brake pistons of the front axle are not mechanically locked by a parking brake, significantly lower forces can be used (e.g., < 30 bar). This leads to a further reduction in component stress, as the valves can be actuated with lower flow rates.

[0024] The electronic valves of the rear axle do not need to be actuated in this case, as the rear axle brakes are already pre-tensioned by the automatic parking brake 1. There is virtually no additional fluid volume intake. This also minimizes the risk of misinterpretation (hot brake, cold brake) due to valve leakage.

[0025] The pressure change with the same relaxation behavior is also greater at lower pressures, because the clamp stiffness is softer in this range, which also results in a finer measurement resolution. According to the degressive pV characteristic curve in Fig. 3 and especially the right edge (high forces / pressures), a defined volume change / displacement change ΔV2 (e.g., due to the "shrinking" of the brake disc 7 and the corresponding movement of the brake piston 5) results in only a small pressure loss Δp2. The same applies at a lower pressure level and with a defined volume change / displacement change ΔV1 when ΔV1 = ΔV2 (see left edge of the characteristic curve in Fig. 3), on the other hand, leads to a significantly higher pressure loss Δp1 (Δp1 > Δp2), which is more favorable in terms of measurement accuracy.

[0026] The probability of a leak and the associated unintentional pressure loss in the valves involved is significantly reduced at lower pressures. The described system requires a UPS (uninterruptible power supply). The electronic valves of the rear axle can be controlled, but this is not mandatory.

[0027] Since both the brake force distribution between the front and rear axles of the vehicle and the temperature correlation between the front and rear axles are known, the behavior of the front axle can be used to infer the behavior (clamping force loss) of the rear axle. In other words, by examining the pressure profile at the front axle, one can deduce the force profile of a mechanical system at the rear axle.

[0028] The present invention can be used in conjunction with all common automatic parking brakes. These are in particular so-called "motor-on-caliper" parking brakes, which are used in conjunction with Fig. The method is therefore universally applicable and ensures a safe and targeted provision of clamping force for various types of parking brakes. Furthermore, the method can be easily integrated into a control or regulating device in the form of an algorithm, thus requiring no additional mechanical components. Appropriate pressure sensors are typically already present in the brake circuit.

Claims

[1] Method for providing a clamping force generated by an automatic parking brake (1) to a vehicle, wherein the clamping force is generated by a brake piston (5) acting on a brake disc (7) and a brake motor (2), characterized by , that after a completed application process of the automatic parking brake (1) a hydraulic pressure is built up in a brake circuit of the vehicle, a pressure loss gradient (dp / dt) is determined in the brake circuit of the vehicle, and a retensioning of the automatic parking brake (1) takes place or does not take place depending on the determined pressure loss gradient (dp / dt). [2] Method according to claim 1, characterized by , that the retensioning of the automatic parking brake (1) takes place depending on the determined pressure loss gradient (dp / dt) when a predetermined limit value for the pressure loss gradient (dp / dt) is exceeded. [3] Method according to claim 1 or 2, characterized by, that by means of pressure loss gradients (dp / dt) an expected total clamping force loss at the automatic parking brake (1) is determined and compensated by retensioning. [4] Method according to any one of the preceding claims, characterized by , that the pressure loss gradient (dp / dt) is determined at the brakes of a rear axle of the vehicle. [5] Method according to any of the preceding claims, characterized by , that the pressure loss gradient (dp / dt) at the brakes of a front axle of the vehicle is determined in order to determine the expected total clamping force loss at the rear axle of the vehicle. [6] Method according to any one of the preceding claims, characterized by , that the existing gradient of the vehicle is determined, and the retensioning is carried out depending on the gradient of the vehicle. [7] Method according to any of the preceding claims, characterized by, that the clamping force loss on the automatic parking brake (1) is additionally monitored by means of force measurement technology. [8] Control or regulating device for carrying out the method according to any of the preceding claims. [9] Automatic parking brake (1) for a vehicle with a control or regulating unit according to claim 8.

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