Method for actuating a parking brake with an electrically driven parking brake mechanism

Abstract

The invention relates to a method for actuating a parking brake (1) with an electrically driven parking brake mechanism (4). The method according to the invention is characterized by the following steps: - Applying the parking brake (1) by means of the electric motor (2), - Switching off the electric motor when the parking brake is engaged (1), - Switching on the electric motor (2) after a predetermined time (t postrun ) and retensioning the parking brake mechanism (4) in the tightening direction, - Determining an electrical quantity (I) of the electric motor (2) during retensioning, - Comparing the determined electrical quantity (I) with a reference value (I) Ref ), and - Continuation or termination of the retensioning process depending on whether the electrical quantity or a derived quantity is smaller or larger than the reference value (IRef ).

Description

[0001] The invention relates to a method for actuating a parking brake with an electric motor-driven parking brake mechanism according to the preamble of patent claim 1 and to a control device according to the preamble of patent claim 9.

[0002] Parking brakes are used to hold a vehicle stationary without the driver having to apply the service brake. Various types of parking brakes are known from the prior art, including electrically operated parking brakes, which are also known as automatic parking brakes (APB). Such parking brakes comprise a control element, such as a button, with which the parking brake can be locked or released when the vehicle is stationary. When the control element is operated, a connected control unit detects the parking brake request and accordingly controls an actuator, such as a hydraulic pump and / or an electric motor, in order to build up braking force at the vehicle's wheels and lock the parking brake, or to release the parking brake.

[0003] A known type of parking brake comprises an electric motor for driving the parking brake mechanism, which is located directly on the wheel brake (SUG, "Motor-on-Caliper" MoC) of the rear axle, to apply or release the brake there. For example, DE 10 2009 028 505 discloses an automatic parking brake that comprises an electric motor and a hydraulic actuator that supports the electric motor. The electric motor acts directly on a brake piston of the hydraulic brake system via a gear, such as a spindle drive.

[0004] After applying and locking such a parking brake, the brake may release slightly after a while, causing the vehicle to roll unexpectedly. This problem occurs particularly if the brake has overheated before the vehicle came to a stop and is locked while still hot. The brake then cools down over time, causing the mechanical components of the brake to shrink slightly, thus reducing braking effectiveness.

[0005] To prevent the vehicle from rolling away, conventional parking brakes are retensioned after a certain period of time. According to the state of the art, the parking brake is retensioned in two situations: in the first situation, a brake disc temperature model detects that the brake has overheated. In this case, the parking brake is retensioned after a predetermined period of time, i.e. in a time-controlled manner. In another situation, rolling away of the vehicle is detected, for example, using the wheel speed sensors. In this case, the parking brake is retensioned immediately after movement of the vehicle is detected. However, both parking brake systems with a retensioning function known from the state of the art have certain disadvantages. In the first system, information about the brake disc temperature must be available, which is relatively complex.On the other hand, if the brake disc temperature exceeds a specified limit, the parking brake is always re-tensioned, even if re-tensioning isn't absolutely necessary. This places unnecessary strain on the brake components. The second system mentioned above, on the other hand, only intervenes once the vehicle has already loosened its grip and begun to roll away. In some cases, this may be too late.

[0006] DE 10 212 202 959 A1 discloses a parking brake in which a clamping force is generated by an electromotive braking device. In this case, the re-tensioning process can be interrupted if the motor current exceeds a limit or the motor speed falls below a limit.

[0007] It is therefore the object of the present invention to provide a method for actuating an automatic parking brake which does not require temperature information and which already re-tensions the parking brake before the vehicle has released.

[0008] This object is achieved according to the invention by the features specified in claim 1. Further embodiments of the invention emerge from the subclaims.

[0009] According to the invention, a method for actuating a parking brake with an electric motor-driven parking brake mechanism is proposed, in which the parking brake is first applied by means of the electric motor, and the electric motor is then switched off in the applied state. According to the invention, the electric motor is switched on again after a predetermined time and the parking brake mechanism is driven in the application direction, i.e., the parking brake begins to be re-tensioned. In this process, an electrical variable of the electric motor, such as the current consumed by the electric motor, is determined and compared with a reference value. The electrical variable determined when re-tensioning the parking brake is a measure of the clamping force of the parking brake. The re-tensioning process is then continued or aborted depending on the result of the comparison. According to one embodiment of the invention, for example,It is checked whether the electrical value is smaller or larger than the reference value. If the parking brake clamping force is found to be smaller than a specified value, the retensioning process continues; otherwise, the retensioning process is aborted. Such a process is relatively simple to implement. Furthermore, such a parking brake does not require any information about the brake disc temperature.

[0010] The electrical variable determined when the parking brake is re-applied is preferably the current consumed by the electric motor or a variable derived therefrom. The reference value is preferably a function of the current consumed by the electric motor during application, approximately when the maximum application force is reached, i.e., approximately at the time of switch-off. This current value is a measure of the application force generated by the electric motor. When the electric motor is switched on again after a predetermined switch-off period, a current value comparison can be used to determine whether and to what extent the application force has decreased, and whether or not re-applying the brake is necessary.

[0011] The reference value can also be a function of a hydraulic brake pressure or a value proportional to it, which is present in the brake circuit at approximately the time the electric motor is switched off. If the vehicle has a hydraulic braking system with a pre-pressure sensor, the measured pre-pressure can, for example, be used to determine the reference value. The reason the reference value depends on the hydraulic pressure is that the electrical power generated by the electric motor depends on the assistance provided by the hydraulic brake. In principle, the greater the contribution of the hydraulic brake, the lower the power generated by the electric motor, and vice versa.

[0012] According to a specific embodiment of the invention, the reference value results from the following relationship: lRef=lt4+prges×ηgesAB×kmot where I t4the current consumed by the electric motor (2) at the time t4 of switching off, p a hydraulic brake pressure, r ges a resulting radius from gear and spindle ratio, A B the brake piston surface, and k Mot is a motor constant.

[0013] According to another embodiment of the invention, the reference value I Ref For example, it can also be determined according to the following relationship: lRef=lt4+prges×ηgesAB×kmot−T where T is a tolerance threshold that can be determined empirically, for example.

[0014] The current reference value can be calculated or read from a set of curves or a table stored in the system.

[0015] The time period between switching off the electric motor and the start of the re-tensioning process can be, for example, between 3 minutes and 15 minutes, in particular between 4 minutes and 8 minutes, and preferably about 5 minutes to 6 minutes.

[0016] If the parking brake according to the invention detects that re-tensioning of the brake is necessary, the parking brake is preferably re-tensioned until a predetermined application force is reached. It is clear to those skilled in the art that the criterion of "reaching a predetermined application force" can be monitored in various ways. In addition to measuring the actual application force, the power consumption of the electric motor could also be measured and compared with a reference value, among many other possibilities, because the following applies: F Klemm ∼ M Mot ∼ k Mot × I Mot where F Klemm the application force of the parking brake, M Motthe motor torque of the electric motor and k Mot is a motor constant. As can be seen, there is a proportional relationship between the clamping force F Klemm and the motor current I Mot .

[0017] To determine whether the clamping force has reached a specified value, it can also be determined, for example, whether the current consumed by the electric motor during clamping satisfies the following relationship: l=lt7+(lref−lGrenz) where It 17 a minimum value of the current I consumed by the electric motor when re-tensioning the parking brake after a specified time (T postrun ), I ref a reference value of the current consumed by the electric motor as a function of the current I t4 , and I Grenz is the theoretical minimum value of the current I consumed by the electric motor when re-tensioning the parking brake after the maximum cooling time, ie when the brake is cold.

[0018] The invention is explained in more detail below with reference to the accompanying drawings. They show: Fig. 1 a sectional view through an electromechanical parking brake for a vehicle, with an electric motor operated parking brake mechanism; Fig. 2 the temporal progression of various operating variables of a parking brake during a brake application process; Fig. 3 the temporal progression of various operating variables of a parking brake during a brake retensioning process; Fig. 4 the current required by the electric motor to re-tension the parking brake as a function of the brake cooling time; and Fig. 5 a flowchart with various method steps for controlling a parking brake according to an embodiment of the invention.

[0019] Fig. Figure 1 shows an electromechanical parking brake 1 for holding a vehicle stationary. The parking brake 1 comprises a brake caliper 6 that engages a brake disc 9 in the form of a brake calliper. The brake calliper is part of a hydraulically actuated service brake.

[0020] The parking brake 1 further comprises a parking brake mechanism 4, which is actuated by an electric motor 2. The electric motor 2 drives a spindle 8 via a reduction gear 3. A component designed as a spindle nut 7 is arranged in a rotationally fixed manner on the threaded spindle 8, which component moves axially when the threaded spindle 8 rotates either in the application direction Z or in the release direction L of the brake. The threaded spindle 8 and the spindle nut 7 are arranged within a brake piston 5, on the side of which facing the brake disc 9, a brake pad carrier and a brake pad 10 are fastened. On the opposite side of the brake disc 9, there is another brake pad 10, which is fastened to the brake caliper 6.

[0021] To apply the parking brake, the electric motor 2 is operated in such a way that the spindle nut 7 moves in the application direction Z, pressing against the bottom of the brake piston 5 and moving it in the application direction Z. The application of the parking brake can be assisted by building up hydraulic brake pressure, with the resulting braking force then consisting of an electromotive component and a hydraulic component. The hydraulic brake pressure can be built up automatically, for example, using a hydraulic pump; however, it can also be generated by the driver by pressing the brake pedal.

[0022] As soon as a desired application force is reached, the electric motor 2 is switched off. The brake application force is determined from the motor current I. The electric motor 2 of the parking brake 1 is controlled by a control unit 11, which contains a suitable braking algorithm. Since the parking brake mechanism 4 is self-locking, the parking brake 1 remains stable in this state, and the vehicle is held without power.

[0023] Fig. 2 shows the time course of various operating variables of the parking brake of Fig. 1 during a brake application process. In this case, the motor current I, the motor voltage U, the clamping force F, and the angular velocity ω of the electric motor 2 are shown.

[0024] At the beginning of the application process, at time t0, an application request is detected and the electric motor 2 of the parking brake 1 is switched on. Switching on the electric motor 2 creates a current peak, which subsequently decreases until an idle current is reached at approximately time t1. The motor voltage U increases during this period, as does the motor speed ω. During the idle phase, the brake pads 10 are not yet in contact with the brake disc 9; for this to happen, the brake clearance must first be overcome. At approximately time t3, the brake pads 10 then contact the brake disc 9. As the application process continues, a braking force F builds up, which is essentially proportional to the motor current I. The following applies: F∼MMot∼kMot×l

[0025] Between times t3 and t4, the braking force F increases approximately linearly. Due to the increasing load on the electric motor, the speed ω decreases simultaneously, as does the motor voltage U. In contrast, the motor current I increases. At time t4, a desired application force F is reached, and the electric motor 2 is switched off.

[0026] While the vehicle is stationary, parking brake 1 cools down to ambient temperature. The application force F decreases with temperature, following an exponential characteristic curve, where: F(t)=F0×e−tτ

[0027] To check whether the application force F of the parking brake 1 is still sufficiently high after some time to hold the vehicle securely, the electric motor 2 is switched on again after a predetermined time, e.g., after 5 to 10 minutes, and the parking brake mechanism 4 is driven in the application direction. This means that a re-tensioning process is initiated.

[0028] Fig. Figure 3 shows various operating variables of the parking brake 1 during such a re-tensioning process. In this example, the motor current I, the motor voltage U, the angular velocity ω of the electric motor 2, and the application force F are shown. As can be seen, the switching on of the electric motor 2 again creates a current peak, which in the following curve, approximately at time t7, drops to a minimum value It 17 drops and then rises again. The motor voltage U and the angular velocity ω show approximately the opposite behavior.

[0029] In order to determine whether the clamping force F is still sufficient to keep the vehicle safely stationary, the minimum value It 17 of the motor current I with a reference value I Ref compared. This determines whether the following criterion is met: lt7 <lRef

[0030] If the criterion is met, it is assumed that the brake must be re-tensioned; because a low current value It 17 means that the mechanical load on the electric motor 2 during application is relatively low. The re-tensioning process is therefore continued. If, on the other hand, the criterion is not met, this means that the parking brake is still sufficiently applied. The re-tensioning process is therefore aborted. The threshold value comparison described above therefore has the advantage that brake 1 is only re-tensioned when it is really necessary. This protects the mechanical components of the brake and, on the other hand, can save energy. Instead of comparing the motor current I with a reference value I Ref Another operating variable of the brake, such as a torque or a force, can also be compared with a corresponding reference value.

[0031] The reference value I Ref is preferably a function of the current I t4 , which the electric motor 2 consumes approximately at the time of switching off at the end of the first brake application process and can optionally also be a function of a hydraulic brake pressure. According to a special embodiment of the invention, the reference value I Ref determined according to the following relationship: lRef=lt4+prges×ηgesAB×kmot where I t4 the current consumed by the electric motor (2) at the time t4 of switching off, p a hydraulic brake pressure, r ges a resulting radius from gear and spindle ratio, A B the brake piston surface, and k Mot is a motor constant.

[0032] The reference value I Refcan either be calculated using the formula mentioned above or, if it is already stored in the system, read out from a table or a set of curves, for example.

[0033] The re-tensioning process is preferably carried out until a specified clamping force is reached. A specific clamping force is reached, for example, when the current I consumed by electric motor 2 satisfies the following relationship: l=lt7+(lref−lGrenz) where I t7 a minimum value of the current (I) consumed by the electric motor (2) when re-tensioning the parking brake (1) after a predetermined time (T postrun ), I ref a reference value of the current consumed by the electric motor (2) as a function of the current I t4 , and I Grenz the theoretical minimum value of the current (I) consumed by the electric motor (2) when re-tensioning the parking brake (1) in the cold state.

[0034] It is clear to the person skilled in the art that instead of the current I, another operating variable of the electric motor 2, such as a motor torque M or the clamping force F, can be monitored in order to estimate the clamping force.

[0035] Fig. Figure 4 again shows the minimum motor current I that the electric motor 2 would need to re-tension the parking brake 1, depending on the cooling time t. The longer the parking brake 1 cools down, the smaller the motor current I is in principle, with the characteristic curve asymptotically approaching a limit value I Grenz which represents the current that the electric motor 2 minimum requires when the brake 1 has cooled down to ambient temperature.

[0036] Fig.5 again shows the essential method steps of a method for operating a parking brake 1, wherein step S1 represents the application of the brake; in step S2, the electric motor is switched off and switched on again in step S3 after a predetermined waiting time. In step S4, the retensioning process begins, wherein in step S5 the motor current I is compared with a reference value. If the motor current I is less than the reference value, the retensioning process continues in step S6. If, on the other hand, the motor current I is greater than the reference value, the retensioning process is aborted. Returning to step S6, the retensioning process continues until the application force of the brake 1 exceeds a predetermined threshold value. If this is the case, the retensioning process is aborted and the method ends.

Claims

[1] Method for actuating a parking brake (1) with an electric motor-driven parking brake mechanism (4), the method comprising the following steps: - Applying the parking brake (1) by means of the electric motor (2), - Switching off the electric motor when the parking brake (1) is applied, - Switching on the electric motor (2) after a specified time (t postrun ) and starting a re-tensioning process by driving the parking brake mechanism (4) in the application direction, - Determining an electrical value (I) of the electric motor (2) during re-tensioning, - Comparing the determined electrical quantity (I), or a quantity derived therefrom, with a reference value (I Ref ), and - Continue the re-tensioning process or cancel the re-tensioning process depending on the result of the comparison. characterized by that the reference value (I Ref) is determined according to the following relationship: lRef=lt4+prges×ηgesAB×kmot where I t4 the current consumed by the electric motor (2) at the time t4 of switching off, p a hydraulic brake pressure, r ges a resulting radius from gear and spindle ratio, A B the brake piston surface, and k Mot is a motor constant. [2] Method for actuating a parking brake (1) according to claim 1, characterized by that the reference value (I Ref ) is a function of the current (I) consumed by the electric motor (2) at the time (t4) of switching off, or of a quantity proportional to this current (I). [3] Method for actuating a parking brake (1) according to claim 1 or 2, characterized by that the reference value (I Ref) is a function of a hydraulic brake pressure (p) or a variable dependent thereon, which is applied to the parking brake (1) at the time (t4) when the electric motor (2) is switched off. [4] Method for actuating a parking brake (1) according to one of the preceding claims, characterized by that the reference value is determined according to the following relationship: lRef=lt4+prges×ηgesAB×kmot−T where I t4 the current consumed by the electric motor (2) at the time t4 of switching off, p a hydraulic brake pressure, r ges a resulting radius from a gear and a spindle ratio, A B the brake piston surface, k Mot is a motor constant, and T is a tolerance threshold. [5] Method for actuating a parking brake (1) according to one of the preceding claims, characterized bythat the electric motor (2) is switched on again after a period of 3 minutes to 15 minutes, in particular after 4 minutes to 8 minutes, and the parking brake (1) is re-tensioned. [6] Method for actuating a parking brake (1) according to one of the preceding claims, characterized by that the parking brake (1) is re-tensioned until a specified clamping force is reached. [7] Method for actuating a parking brake (1) according to one of the preceding claims, characterized by that the parking brake (1) is re-tensioned until a predetermined application force is reached, whereby the criterion for reaching the predetermined application force is determined by whether the current (I) consumed by the electric motor (2) satisfies the following relationship: l=lt7+(lref−lGrenz) where It 17 a minimum value of the current (I) consumed by the electric motor (2) when re-tensioning the parking brake (1) after a predetermined time (Tpostrun ), I ref a reference value of the current consumed by the electric motor (2) as a function of the current I t4 , and I Grenz the theoretical minimum value of the current (I) consumed by the electric motor (2) when re-tensioning the parking brake (1) in the cold state. [8] Control device comprising means for carrying out one of the previously claimed methods.

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