Method and device for operating a parking brake of a motor vehicle

Abstract

The invention relates to a method for operating a parking brake (1) of a motor vehicle, which has at least one wheel brake device (2, 3). The wheel brake device has a controllable electric motor (6, 7) for actuating it, which drives a spindle for moving a brake piston between an engaged position and a release position. To calibrate the parking brake (1), the electric motor is controlled to move the brake piston to the release position in a first step and from the release position to the engaged position in a subsequent second step. It is provided that in the second step, during the movement of the brake piston, the power supply to the electric motor (6, 7) is interrupted in an idle phase (II), and at least one motor parameter of the electric motor (6, 7) is determined as a function of the coasting behavior of the electric motor (6, 7).

Description

[0001] The invention relates to a method for operating a parking brake of a motor vehicle, which has at least one wheel brake device, wherein the wheel brake device has a driveable electric motor for actuating it, which drives a spindle for moving a brake piston of the wheel brake device between a clamping position and a release position, wherein, for calibrating the parking brake, the electric motor is controlled in order to move the brake piston in a first step into the release position and in a subsequent second step from the release position towards the clamping position.

[0002] Furthermore, the invention relates to a corresponding parking brake, as well as a computer program and a computer program product. State of the art

[0003] Methods of the type mentioned above are known from the prior art. Parking brakes for motor vehicles generally have two wheel brake devices, each assigned to a wheel of a rear axle of the motor vehicle, in order to lock it when necessary. The wheel brake devices are usually equipped with an electric motor actuator that moves a brake piston of the respective wheel brake device to generate a braking force. The brake piston is usually actuated by a spindle driven by an electric motor.

[0004] German patent application DE 10 2008 052 845 A1 discloses a parking brake with an actuator that has a DC motor capable of operating in two directions. A hydraulic pre-pressure can be taken into account to adjust the clamping force when applying or releasing the parking brake.

[0005] A precise understanding of the electric motor's parameters allows for a complete and accurate mathematical description of the parking brake's system dynamics. In addition to the motor constant and resistance, the moment of inertia is another motor parameter that describes the parking brake's dynamics and, like the other parameters, is subject to manufacturing tolerances.

[0006] During initial commissioning or ongoing operation of a parking brake, it is advantageous to calibrate it, particularly to determine the position of the spindle or brake piston for actuating the wheel brake mechanism. It is known to calibrate the parking brake by activating the electric motor to first move the brake piston into a release position and then, in a second step, into an engagement position. In the release position, the brake piston is in a non-use position, and in the engagement position, the brake piston applies a braking force, particularly to a brake disc. The brake piston can be moved between these two positions by the electric motor, specifically by means of a spindle driven by the electric motor.By moving to the two aforementioned end positions, which are characterized by the fact that further movement of the brake piston is mechanically impossible, the parking brake and in particular the electromechanical actuator can be calibrated. Disclosure of the invention

[0007] The method according to the invention, with the features of claim 1, has the advantage that at least one motor parameter of the electric motor used to control the parking brake is determined during the calibration process. This allows for compensation of serial variation in the motor parameters and calibration of the parking brake accordingly. Particularly during operation, the motor parameters of the electric motor can change, for example, due to wear. The method according to the invention makes it possible to detect these changes even during operation and to adjust the control of the electric motor accordingly. According to the invention, in the second step, i.e., during the idle phase, the power supply to the electric motor is interrupted while the brake piston is moving towards the applied position, and at least one motor parameter of the electric motor is determined based on the coasting behavior of the electric motor.An additional step is integrated into the standard calibration process, enabling the determination of the electric motor's parameters. This method can also be performed independently of the calibration process. By interrupting the power supply during the idle phase, in which the brake piston is moved without applying any braking force, the electric motor coasts to a stop. The coasting behavior of the electric motor is influenced by its motor parameters. This allows the motor parameters to be determined during the idle phase and used for subsequent control of the electric motor.

[0008] In particular, it is provided that the moment of inertia of the electric motor is determined as a motor parameter as a function of a change in speed, especially as a function of a change in angular velocity, of the electric motor and / or the spindle. When the electric motor is no longer energized, the speed or angular velocity of its output shaft decreases due to its internal frictional torque. The moment of inertia counteracts this frictional torque. Knowing the frictional torque and the change in speed, the moment of inertia of the electric motor can thus be easily determined. It is therefore preferred that the moment of inertia be determined as a function of a frictional torque of the electric motor.

[0009] According to an advantageous embodiment of the invention, the frictional torque during the idling phase, particularly before the power supply is interrupted, is determined as a function of a stored motor constant of the electric motor. This provides a current frictional torque of the electric motor, which can be used to evaluate the moment of inertia.

[0010] Particularly preferred is the determination of the motor constant of the electric motor as a motor parameter, either additionally or alternatively, depending on the coasting behavior of the electric motor, especially as a function of an induced voltage of the electric motor during coasting. This also allows the determination of the motor constant during operation of the parking brake, thus enabling the detection and compensation of any manufacturing variations in this respect as well.

[0011] Furthermore, it is preferably provided that the determined motor constant is compared with the stored motor constant, whereby the determined motor constant is stored if it deviates from the stored motor constant by a predefinable limit. During the first execution of the method, a calculated motor constant, for example, is used as the basis for controlling the electric motor and is stored. If the determined motor constant deviates from the stored motor constant by more than 5%, for example, the determined motor constant is stored as the basis for further operation of the parking brake. Should the motor constant change again due to wear, this is detected by performing the method at a later time and taken into account accordingly by storing a newly determined motor constant.

[0012] Furthermore, it is preferably provided that the determined frictional torque is compared with a previously determined frictional torque, and that the determined frictional torque is discarded if it deviates from the previously determined frictional torque by a predefinable limit. In this context, an excessively large deviation is considered an incorrect measurement. Thus, with regard to the frictional torque, the current effective frictional torque of the electric motor is always recorded and, if necessary, stored when the procedure is carried out repeatedly in order to update the motor parameters of the electric motor. In particular, it is provided that the procedure is carried out regularly during operation of the parking brake, especially at intervals or depending on the number of parking brake activations.

[0013] The parking brake according to the invention, with the features of claim 8, is characterized by a specially designed control unit which has specific means and is configured to carry out the method according to the invention. Further features and advantages will become apparent from what has already been described.

[0014] The computer program according to the invention provides that all steps of the method according to the invention are carried out when it runs on a computer, in particular a control unit.

[0015] The computer program product according to the invention, comprising program code stored on a machine-readable medium, executes the method according to the invention when the program runs on a computer.

[0016] The invention will now be explained in more detail using exemplary embodiments. These will be illustrated as follows: Fig. 1. A parking brake in a simplified representation, Fig. 2 Signal progressions of the parking brake during a clamping operation, Fig. 3 Signal patterns of the parking brake during a power supply interruption, Fig. 4 a method for operating the parking brake and Fig. 5 another method for operating the parking brake.

[0017] Fig. Figure 1 shows a simplified representation of a parking brake 1 for a motor vehicle. The parking brake 1 has two wheel brake devices 2, 3, one of which is assigned to the left wheel of a rear axle of the motor vehicle and the other to the right wheel. Both wheel brake devices 2, 3 have an actuator 4 or 5, respectively, each comprising an electric motor 6, 7. The electric motors 6, 7 are each connected by an electrical line 8, 9 to a control unit 10, which in turn is connected to an electrical system 11 and / or to a vehicle battery associated with the electrical system 11. The control unit 10 controls the electric motors 6, 7 to actuate the wheel brake devices 2, 5. The actuators 4, 5 each clamp a brake disc 12, 13 assigned to the respective wheel between a pair of brake shoes 14, 15 of the respective wheel brake device 2, 3.

[0018] The respective electric motor 6, 7 is connected via a spindle to a respective brake piston, which applies a corresponding actuating force to one of the brake shoes of the respective pair 14, 15. Due to the floating bearing of the other brake shoe of the brake shoe pair, both brake shoes are pressed against the respective brake disc 12, 13 as usual, so that the respective brake disc 12, 13 is clamped between the two brake shoes. In this state, the brake piston is in a clamping position. When the respective brake piston is moved back from the clamping position by the electric motor 6, 7, thus reducing the braking force acting on the brake disc 12, 13, the parking brake 1 is released.For calibrating the parking brake 1, in particular for determining the position of the spindle and / or the brake piston, it is known to move the brake piston into a release position in which the respective brake shoe is spaced apart from the brake disc 12, 13. In particular, the release position is defined by a mechanical end stop against which the brake piston, or in particular a spindle nut associated with the brake piston, is moved. This spindle nut interacts with the spindle to move the brake piston into the clamping position when the spindle is driven by the electric motor 6, 7. Starting from this position, the brake piston is then moved into the clamping position by actuating the electric motor 6, 7. This moves the piston from one end position to another, in each of which further movement of the piston is mechanically prevented.This allows the spindle position to be adjusted for the regular operation of parking brake 1. The described procedure is performed again, particularly during initial commissioning or after replacing components of parking brake 1. During the recalibration of parking brake 1, the spindle position is initially unknown to the control unit 10. To determine this position, the procedure described above is followed: the respective brake piston is first moved to the release position, so that parking brake 1 is completely open, and then to the clamping position. The subsequent defined release process completes the (re-)calibration, and the spindle position is again known to the control unit 10.

[0019] Fig. Figure 2 shows the signal curves of parking brake 1 during the second step of the calibration process, i.e., when the brake piston is moved into the clamping position. This is shown in Figure 2. Fig. The current i, the voltage U, and the rotational speed n of the electric motor 6 are plotted over time t. In the following exemplary embodiment, the method will first be discussed using the left wheel brake assembly 2 as an example. Preferably, the method is of course applied to both wheel brake assemblies 2 and 3. The clamping process is divided into three phases I, II, and III. In phase I, characterized by inrush current peaks, several motor parameters, but at least the motor constant k, are M and the electrical resistance R of the parking brake 1 or the wheel brake device 2 is determined.

[0020] Phase II represents an idle phase. Here, an idle current is established while the motor speed n remains constant. During this phase, the free travel of actuator 4 is overcome, specifically the free travel of a nut up to the piston crown of the brake piston.

[0021] Characteristic of phase III, the so-called force increase phase, is the increase of the current i as a result of the increase in the clamping force by applying the brake pads 14 to the brake disc 12.

[0022] The idle current measured in phase II is recorded or measured and expediently stored in a non-volatile memory of the control unit 10.

[0023] In the following equation (1) the mechanical differential equation for the electric motor 6, which is designed as a DC motor in this case, is given first. Jdwdt=kMi−MReibung−MLast

[0024] In this context, K means m the motor constant of the electric motor 6, i the measured current, M Reibung the frictional torque of the electric motor 6, M LastThe load torque of the electric motor 6 and J*dw / dt represent the change in the angular velocity of the electric motor 6. For the no-load phase II, the change in angular velocity is negligible and therefore vanishes. Since the system is idling, there is also no load torque. Equation (1) therefore simplifies to the following equation (2), from which the frictional torque M can be determined. Reibung The following is calculated: 0=kMi−MRibble

[0025] This equation requires knowledge of the motor constant, which is determined in Phase I. The value of the calculated frictional torque is stored in a non-volatile memory of control unit 10.

[0026] The procedure is now supplemented in idle phase II by interrupting the power supply to the electric motor 6 at a predetermined time during idle phase II. This is described in Fig. Figure 3 shows the signal changes of current i, rotational speed n, and voltage U over time t. At time t1, the current supply is interrupted by the control unit 10. A suitable time is the time of phase II, at which the value for the frictional torque of the electric motor 6, particularly in the time interval Δt, is determined. MR was determined. In principle, however, any other point in time in the idle phase II can also be used as point t1, at which it is ensured that the spindle comes to a complete standstill before overcoming the idle travel and thus before reaching phase III, and that the frictional torque is determined until switching off.

[0027] In the non-volatile memory, in addition to information about the time t1 of the event "switch-off" or "interruption of power supply," the speed information of the electric motor 6 is also stored. As soon as no change in the speed of the electric motor 6 is detected, the time t2 of the occurrence of this event, i.e., the standstill of the electric motor 6, is also stored in the memory. Knowing the times t1 and t2 of the described events contributes to determining the change in speed during the period between the events, as these times define the period under investigation.

[0028] The following equation gives the electrical differential equation of the electric motor: UM=RMi+Ldidt+kMw

[0029] This stands for U m for the motor voltage, R Mfor the motor resistance, w for the speed of the electric motor 6, L for the inductance of the electric motor 6, di / dt for the change of the current i over time t and k M for the motor constant of the electric motor 6.

[0030] After the power is switched off, equation (3) decomposes into equation (4) as follows: UM=kMw

[0031] By measuring the induced voltage U m of the electric motor 6 and with knowledge of the rotational speed w, the motor constant k is obtained. MThis value is stored in memory and compared to the value determined in Phase I. If the comparison shows that the determined value differs from the previously determined value by a predefined limit, the value determined during the coasting phase, i.e., after the power supply is interrupted, is stored and used as the motor constant. In this case, the frictional torque is also recalculated with the updated parameter.

[0032] During the coasting process, current i vanishes in the mechanical differential equation because the circuit is no longer closed. Phase II is characterized by the absence of a load torque. For these reasons, equation (1) simplifies to equation (5): Jdwdt=−MReibung

[0033] The rate of change of the angular velocity dw / dt is preferably determined by conventional methods, in particular by time differentiation of the angular velocity, estimations, or determinations using system identification methods. The frictional torque M Reibung is determined from equation (2). Equation (5) is preferably not evaluated during the entire duration of the time interval explained in the section above. The times at which the velocity signal is not differentiable, for example at time t1 or t2, are not considered, since it is not possible to specify an instantaneous rate of change at these times due to abrupt changes. The moment of inertia calculated in each time step is averaged in a suitable manner.

[0034] The newly determined moment of inertia is advantageously compared with the standard value stored in the control unit. The result is evaluated appropriately to minimize the impact of potential measurement errors. If the value deviates from a tolerated range of the standard value, the newly determined moment of inertia is not trusted. The system assumes a faulty measurement and the value is not used. If the moment of inertia determined by the described method is within the tolerable range, the newly determined value is stored in the non-volatile memory of the control unit 10 and used as an engine parameter for the force estimation algorithm for actuating the parking brake 1.

[0035] Once the electric motor 6 has come to a standstill and the calculation and comparison of the moment of inertia has been completed, the electric motor 6 is energized again in such a way that the described clamping process is completed as part of the (re-)calibration.

[0036] The force required to actuate the parking brake 1 is then based on the calculation of the mechanical differential equation, see equation (1). Therefore, precise knowledge of the moment of inertia is advantageous for an accurate estimation of the clamping force.

[0037] Advantageously, it is provided that datasheet values, for example of motor constant and motor resistance, are used to determine the moment of inertia. Advantageously, the motor parameters motor constant and motor resistance are estimated to determine the moment of inertia during the inrush current peaks (phase I) until the start of the coasting process (t1).

[0038] Fig. 4 and Fig. Figure 5 provides an overview of the described procedure in a flowchart.

[0039] Fig. Figure 4 shows the procedure for determining the moment of inertia. This begins in step S1 with the start of the (re-)calibration process. For this purpose, in step S2, the brake piston is first moved to the release position and then, by reversing the voltage of the electric motor 6, towards the clamping position. In step S3, during the idle phase II, the frictional torque M is measured. Reibung determined according to equation (2).

[0040] Subsequently, in step S4, the electric motor 6 is switched off or the power supply to the electric motor 6 is interrupted.

[0041] The moment of inertia is then determined according to equation (5) as described above in step S5. In a subsequent step S6, the determined moment of inertia is compared with a previously stored value. If the deviation is small, for example, less than 10%, the most recently determined moment of inertia is used as the new value and stored in step S7.1. The stored moment of inertia is weighted if necessary. If the comparison shows that the determined moment of inertia deviates from the previously stored moment of inertia by a predefined value, for example, more than 10%, the newly determined value is discarded in step S7.2, and the previously stored value is used.

[0042] The selected value is then used as the basis for the further operation of the parking brake in step S8.

[0043] Fig.Figure 5 shows the procedure for determining the motor constant k. M The steps S1, S2, and S4 follow each other. In a subsequent step S9, the motor constant k is determined. M determined according to equation (4), as described above.

[0044] Subsequently, in step S10, the determined motor constant is compared with a previously stored motor constant. If the determined motor constant deviates from the previously stored motor constant by a predefined limit, for example, 5%, the determined motor constant is defined as a new parameter in step S11.1. If the determined motor constant does not deviate from the stored motor constant by a predefined limit, the previously stored motor constant continues to be defined and used as the motor parameter for controlling parking brake 1 in step S11.2.

Claims

[1] Method for operating a parking brake (1) of a motor vehicle, which has at least one wheel brake device (2,3), wherein the wheel brake device has a controllable electric motor (6,7) for its actuation, which drives a spindle for moving a brake piston between a clamping position and a release position, wherein, for the purpose of calibrating the parking brake (1), the electric motor (6,7) is controlled in order to move the brake piston in a first step into the release position and in a subsequent second step from the release position towards the clamping position, characterized by , that in the second step, during the relocation of the brake piston, the power supply to the electric motor (6,7) is interrupted in an idle phase (II) and at least one motor parameter of the electric motor (6,7) is determined depending on the coasting behavior of the electric motor (6,7). [2] Method according to claim 1, characterized by, that as a motor parameter a moment of inertia of the electric motor (6,7) is determined as a function of a change in speed of the electric motor (6,7) and / or the spindle. [3] Method according to any one of the preceding claims, characterized by , that the moment of inertia is determined as a function of a frictional torque of the electric motor (6,7). [4] Method according to any one of the preceding claims, characterized by , that the frictional torque in the idling phase (II) is determined as a function of a stored motor constant of the electric motor (6,7). [5] Method according to any one of the preceding claims, characterized by , that the motor constant of the electric motor (6,7) is determined as a motor parameter depending on the coasting behavior of the electric motor (6,7). [6] Method according to any one of the preceding claims, characterized by, that the determined motor constant is compared with the stored motor constant, whereby the determined motor constant is stored if it deviates from the stored motor constant beyond a predefinable limit. [7] Method according to any one of the preceding claims, characterized by , that the determined frictional torque is compared with a previously determined frictional torque, and that the determined frictional torque is discarded if it deviates from the previously determined frictional torque beyond a predefinable limit. [8] Parking brake for a motor vehicle, comprising at least one wheel brake device (2,3), wherein the wheel brake device has a controllable electric motor (6,7) for its actuation, which has a spindle for moving a brake piston between a clamping position and a release position, characterized by a specially designed control unit that performs the method according to one of the preceding claims. [9] Computer program that performs all steps of a method according to any one of claims 1 to 7 when the program is running on a computer. [10] Computer program product comprising program code stored on a machine-readable medium for carrying out the method according to any one of claims 1 to 7 when the program is executed on a computer.

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