Procedure for releasing an automatic parking brake

Abstract

In a method for releasing an automatic parking brake (1) comprising an electromechanical braking device for generating an electromechanical clamping force, the release travel (Δs) of the automatic parking brake (1) is estimated based on a measured motor voltage (u) and motor current (i), and the estimated release travel (Δs) is compared with a predetermined minimum total release travel (Δsges). The object of the invention is to provide a method for releasing an automatic parking brake in which the negative effects of a current measurement error on the release process of the automatic parking brake are avoided, and sufficient opening of the automatic parking brake is ensured at all times without the need for costly and calibrated measuring chains. To ensure that the minimum total release travel (Δsges) is achieved, at least one additional cut-off criterion (tmax, tmin) is checked.

Description

[0001] The invention relates to a method for releasing an automatic parking brake, a control or regulating device, and an automatic parking brake.

[0002] An automatic parking brake is known, for example, from DE 10 2012 202 962 A1. In such automatic parking brakes, a clamping force is exerted on a brake disc by an electrically driven spindle. The automatic parking brakes of the type discussed here do not require a Hall sensor or other sensors to calculate the motor speed. Instead, the clamping and releasing process of the automatic parking brake is essentially controlled by evaluating the motor voltage and current, which, through the application of a suitable algorithm, provides information about the spindle travel and the clamping force of the brake. Locking systems of this type have the advantage that no sensors are required and the braking force can be estimated within the control electronics, which can therefore be located spatially separate from the brake motors.The challenge with sensorless automatic parking brakes is that a lower degree of accuracy must be accepted during the clamping and releasing processes. The problem arises particularly during the releasing process, as the motor currents are very low due to the decreasing clamping force. When measuring these low motor currents, significant measurement deviations can lead to uncompensable measurement errors that remain undetected despite redundant acquisition of the motor currents and voltages. These measurement errors can thus lead to an incorrect calculation of the spindle nut's travel distance, resulting in insufficient release of the automatic parking brake and a residual grinding torque.

[0003] The object of the present invention is therefore to provide a method for releasing an automatic parking brake in which the negative effects of a current measurement error on the release process of the automatic parking brake are avoided, and sufficient opening of the automatic parking brake is ensured at all times without the provision of costly and calibrated measuring chains.

[0004] The aforementioned problem is solved by the features of the independent claims. Further developments of the invention are specified in the dependent claims.

[0005] The method serves to release an automatic parking brake comprising an electromechanical braking device for generating an electromechanical clamping force. Preferably, the release travel of the automatic parking brake is estimated based on a measured motor voltage and current, and this estimated release travel is compared with a predetermined minimum total release travel. The method preferably dispenses with sensors used for speed measurement. To ensure that the minimum total release travel is achieved, at least one additional shutdown criterion is checked.

[0006] A significant advantage of the invention is that the release process of the automatic parking brake is no longer solely based on calculating an estimated value derived from measured motor currents and voltages. Instead, at least one additional cut-off criterion is used to ensure that the automatic parking brake is sufficiently open and that no residual friction torque exists between the brake pad and the brake disc. This additional cut-off criterion prevents premature interruption of the release process and thus an insufficient release travel of the automatic parking brake due to an (undetected) current measurement error. Rather, the additional cut-off criterion ensures that the automatic parking brake is sufficiently open at all times.

[0007] In the method according to the invention, the at least one additional switch-off criterion can be a time interval measured from the start of the release process, which specifies the minimum runtime of the release process to ensure sufficient opening of the automatic parking brake. This time interval can be a minimum or a maximum time interval. By specifying a minimum time interval after the start of the release process, it is ensured that the specified release travel has actually been fully traversed by the spindle nut if the estimated release travel corresponds to the specified release travel. By specifying a maximum time interval, on the other hand, the release travel can be additionally or alternatively limited. Thus, when the condition of the specified maximum time interval is met or no longer met, the brake motor is switched off and the release process is terminated.The same applies if the specified minimum time period is reached.

[0008] To avoid the negative influence of measurement errors on the value of the additional shutdown criterion to be verified, this criterion is preferably determined independently of a measured motor current. A preferred embodiment of the invention is one in which the determination of the additional shutdown criterion depends on the measured motor voltage. Since the motor voltage typically exhibits a substantially constant value within a range insensitive to measurement errors during the release process of the automatic parking brake, the motor voltage is well suited for determining the minimum time interval.

[0009] By using a simplified model, the time interval corresponding to a given minimum total release path can be calculated. This means that the theoretical time required by the automatic parking brake to move from an engaged state or the start of the release process to the end of a minimum total release path can be calculated. Motor parameters are preferably used to calculate this time interval. These parameters can include, in particular, the motor constant, the gear ratio of the transmission unit, and the motor voltage of the brake motor. It is especially advantageous if the motor constant is determined as a function of temperature information. This temperature information is preferably the temperature of the brake motor, which can either be measured by sensors or determined by other means.This allows the variability of the motor constants to be estimated and the accuracy of the shutdown criterion to be improved.

[0010] To obtain an even more precise determination of the time-based cut-off criterion, information about any existing hydraulic pre-pressure can be considered for determining at least one additional cut-off criterion. Such pre-pressure can be present if the automatic parking brake is assisted by a hydraulic vehicle brake. In this case, the clamping force of the automatic parking brake consists of an electromechanical and a hydraulic component, so that virtually no clamping force needs to be actively reduced electromechanically. However, if such pre-pressure is not present, the clamping force must be actively reduced electromechanically, which can extend the time period by a variable or a fixed amount.

[0011] Another method according to the invention serves to release an automatic parking brake, which includes an electromechanical braking device for generating an electromechanical clamping force. In this method, a motor current is redundantly measured during a release process, and the resulting measured currents are compared. As a switching criterion for the release process, the measured motor currents must fall below a predetermined threshold value after a defined period of time. If the switching criterion is met, the release process can therefore be terminated safely and reliably without the risk of residual drag torque. This method preferably avoids the use of sensors intended for measuring rotational speed.

[0012] The method according to the invention takes place in a control or regulating unit in a motor vehicle, which may be part of the parking brake system.

[0013] 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:

[0014] Fig. 1 A sectional view of an automatic parking brake for a vehicle, with an electric brake motor to generate a clamping force to secure the vehicle;

[0015] Fig. 2 a diagram of the distribution of relative current measurement errors with identification of the typical current range of a switch-on peak and during a tensioning process of an automatic parking brake;

[0016] Fig. 3 a schematic representation of a circuit for the redundant formation of a current measurement path;

[0017] Fig. 4 A schematic diagram of two measuring currents after passing through the current measuring path. Fig. 4 in comparison, and

[0018] Fig. 5 a flowchart of a release process of an automatic parking brake implemented in a control unit according to the invention.

[0019] Fig. Figure 1 shows a sectional view of a known automatic (automated) parking brake. 1 for a vehicle that uses a brake motor 2 can exert a clamping force to secure the vehicle. The brake motor 2 In this case, it is designed as an electric motor which has a spindle mounted in an axial direction. 3 , in particular a threaded spindle. On its brake motor 2 The spindle is located at the far end. 3 with a spindle nut 4 equipped with the automatic parking brake in the engaged state. 1 on an inner end face or a rear face of a brake piston 5 is attached. The spindle 3 , the spindle nut 4 and the brake piston 5are in a brake caliper 6 mounted, which is a brake disc 7 grips like a pincer. On both sides of the brake disc. 7 Each is a brake pad 8 , 8‘ arranged.

[0020] During the engagement of the automatic parking brake 1 , the spindle nut 4 in the axial direction onto the brake piston 5 and the brake disc 7 moved until the brake piston 5 a predetermined maximum clamping force on the brake disc 7 generated. In addition to the electromechanical clamping force, pressure can also be applied to the back or bottom of the brake piston. 5 be provided with a hydraulic fluid that relieves the automatic parking brake 1 serves.

[0021] To calculate the travel distance of the spindle nut 4Without the use of speed sensors, the motor voltage and motor current of the brake motor are used and the spindle travel is calculated using an algorithm as follows:

[0022] Where u is the measured motor voltage, i is the measured motor current, Δs(n + 1) is the distance traveled at the current sampling time, Δs(n) is the distance traveled at the previous sampling time, T A the sampling frequency (approx. every 5 ms), ü the gear ratio of the gearbox including the spindle thread, K the motor constant, u(n + 1) the voltage value at the current time, i(n + 1) the current value at the current time and R the total resistance of the brake motor. 2 including the supply lines from the control unit.

[0023] The sampling times for the motor voltage u and motor current i are generally constant and preferably occur every 5 ms. This means that the current motor current i and the current motor voltage u are averaged over a period of 5 ms. The motor constant K and the resistance R are determined during the turn-on peak of the motor current i. The starting values ​​for the estimation method described above are the nominal values ​​of the automatic parking brake. 1 From the above algorithm for calculating the distance traveled by the spindle nut 4 It follows that the distance traveled per calculation interval increases when the motor constant K is small, the voltage u is high, the resistance R is small and / or the motor current i is low.

[0024] During the release process of the automatic parking brake 1The motor current i changes particularly sharply. This is due to the switching-on process of the brake motor. 2 Initially, there is a very sharp increase in the motor current i, which is typically i > 15 A (inrush peak). After approximately 50 ms, the inrush peak has usually already dropped to the level of an idle current. The clamping force of the automatic parking brake 1 In this state, it is completely degraded. The duration of the no-load current is relatively long and typically lasts approximately 800 ms at an average current i of approximately 0.6–1 A.

[0025] It is known that higher currents can generally be measured more accurately than lower currents. This illustrates Fig. 2, which shows an exemplary diagram of the distribution of relative current measurement errors during an engagement process of the automatic parking brake 1The graph shows the motor current i in the different phases of a clamping process. The abscissa of the diagram plots the motor current i in the various phases of a clamping process, while the ordinate indicates the relative current measurement error. During the clamping process, the motor current i initially exhibits the switch-on peak described above, which is labeled current range A. The typical current range of the clamping process is further represented by B, and the typical current profile during the brake motor's no-load operation is shown. 2 is marked with C.

[0026] It becomes clear that the relative proportion of measurement errors increases sharply during current measurement in the no-load range of the motor current. These current measurement errors result primarily from uncompensable measurement errors and tolerances in the measurement chain. For example, the measurement result depends on the measurement resolution of the analog-to-digital converter (ADC). Depending on the selected measurement resolution, uncompensable components can be of the same order of magnitude as the actual measurement signal. Consequently, a 100% measurement deviation can occur, significantly distorting the measurement result. At higher currents, the influence of the measurement resolution becomes increasingly less significant, as it becomes relatively small compared to the magnitude of the current. Overall, the signal measurement tolerance is very high at low currents. This means that measurement errors in the range of approximately 30% must be tolerated, and faulty monitoring of the measured current must be ruled out.

[0027] To identify current measurement errors, redundant current measurement paths are used, as in Fig. Figure 3 shows that the current is split into two current paths, I1 and I2, which are separated by two shunt resistors connected in series. 9 , 9‘ to be measured. Each current path, I1 and I2, has its own amplifier circuit. 10 and a low-pass filter 11 The currents are assigned to a specific current path before being evaluated by an analog-to-digital converter (ADC) (not shown) and made available to software in a control unit. Both currents, I1 and I2, can then be directly compared. If the difference between the two currents exceeds a predefined threshold, a warning signal can be issued to the driver of a vehicle, alerting them to a potential electrical fault in the automatic parking brake. 1 indicates.

[0028] With regard to Fig. In the redundant measurement of the motor current i described in section 3, the low-pass filter through which each of the partial currents I1 and I2 passes is subject to tolerances. The cutoff frequency of the low-pass filters may, for example, be nominally defined at 200 Hz. However, due to component tolerances, the cutoff frequencies can deviate considerably from their nominal value. In particular, cutoff frequencies of 150 Hz or 300 Hz are possible. In addition to the high dynamics during the switch-on peak, this results in a phase shift between the two measured currents I1 and I2, as described in section 3. Fig. Figure 4 illustrates this. Since the actual effective phase shift is unknown, an apparent measurement deviation can occur with two otherwise identical signals. Overall, it can be concluded that higher currents are favorable with regard to measurement tolerance. At the same time, it is noticeable that a greater dynamic range of current changes occurs precisely in the case of higher currents. This can lead to significant signal distortions, which, due to component tolerances, can introduce a potential phase shift in the signal waveform.

[0029] On the other hand, a measuring range can be chosen to exclude a current measurement error by comparing the two measured currents, which does not exhibit high dynamics and which is in Fig. 4 is marked with the arrows on the right in the image. The distortion of the two currents I1 and I2 due to component tolerances and a resulting phase shift is virtually eliminated in this quasi-stationary measuring range. However, the measured current is usually very low, and the undetected measurement error, as explained above, can therefore be very large.

[0030] According to the present invention, an additional shut-off criterion is therefore used during the loosening process. This ensures a minimal total loosening distance of the spindle nut. 4 was actually achieved, and that an undetected current measurement error did not lead to an incorrect calculation of the solution path.

[0031] The additional shutdown criterion is preferably a time interval calculated using a highly simplified model with constant values ​​for the motor parameters. This makes it possible to calculate the corresponding time for a given overall solution path, during which the spindle nut 4 required to traverse the entire solution path. The specified total solution path is the travel distance of the spindle nut. 4 , which is sufficient to activate the automatic parking brake 1 to open wide enough and thus create a residual grinding torque between the brake disc 7 and the brake pads 8 , 8‘ to avoid.

[0032] In one embodiment of the present invention, the calculation of the time-based shutdown criterion is achieved solely by using the measuring voltage u. The current component (–R·i(n + 1)) is neglected in this variant and can therefore be used particularly for optimally fast brake motors. 2 Devices with a current draw of i ~ 0 A are used. The calculation of the time-based shutdown criterion t ges This can then be done using the following formula: where Δs is the specified total loosening path of the spindle nut. 4 , ü is the gear ratio of the gearbox unit including the spindle thread, K is the motor constant, and u is the motor voltage. If we define the motor parameters and the specified total solution path t ges The following values ​​are used as examples: Δs = 1.4 mm, ü = 5·10 5 1 / m, K min = 0.010 Nm / A and u = 12V, so we get:

[0033] After a 583 ms loosening process, it can therefore be ensured that the spindle nut 4 was proceeded far enough to completely loosen the brake pads 8 , 8‘ from the brake disc 7 to ensure this. In contrast, using a nominal motor constant of K results in nom = 0.014 Nm / A a nominal switching time t ges_nom as a shutdown criterion, which is calculated as follows:

[0034] The use of nominal motor parameters thus leads to a reduced minimum duration of the release process. Using the finding that the automatic parking brake engages after Δs = 1.0 mm... 1 Since the brake system is usually loosened and after a further 0.4 mm it no longer operates mechanically but entirely hydraulically, the following ratio can be assumed in the limiting case: t ges_nom / t ges_min = K nom / K min= 0.014 Nm / A / 0.010 Nm / A and consequently 817 ms / 583 ms = 1.4, where the corresponding solution paths to be achieved Δs nom / Δs min = 1.4 mm / 1.0 mm = 1.4. This means that the automatic parking brake 1 at t ges = 583 ms even in the worst case is opened just wide enough to avoid residual grinding torque.

[0035] In other words, the release process of the automatic parking brake can 1 up to the specified minimum total solution path with an assumed constant voltage of u = 12 V, never faster than t ges = 583 ms. The minimum time for the loosening process calculated in this way thus avoids the need to determine the minimum travel distance of the spindle nut, as this calculation is independent of a measured current i and a consequently incorrectly estimated motor constant K or motor resistance R. 4 The threshold is not met.

[0036] The process in a program to ensure the achievement of the minimum overall solution path Δs ges the spindle nut 4 In this case, it could be as follows:

[0037] In an alternative embodiment of the invention, the current component (–R·i(n + 1)) can be taken into account when calculating the shutdown criterion. In this case, however, the time-based shutdown criterion should be calculated with a minimum possible motor resistance R and a minimum possible no-load current. The minimum no-load current must be determined by measurements on several brake motor prototypes. 2 to be determined in advance. The calculation of the minimum shutdown time t ges The shutdown criterion for the solving process is then calculated as follows: t ges = Δs·ü·K u – R·i

[0038] The formula above clearly shows that the denominator becomes smaller when the measured motor current i is taken into account than when this component is neglected. Furthermore, it is evident that this relationship determines the minimum time for the specified minimum total travel of the spindle nut. 4 increased. This ensures even more reliably that the automatic parking brake is sufficiently open after the release process has finished, in order to avoid any residual drag torque.

[0039] Similarly, a maximum release distance can also be defined using a time criterion. The time required for the automatic parking brake to release sufficiently can thus also be limited. In this case, the corresponding maximum values ​​of the automatic parking brake or the release process must be considered when calculating the shutdown criterion. These are, for example, i max= 4A, R max = 1 Ω and K max = 0.018 Nm / A. When calculating the maximum permissible duration of the dissolving process, the following results are obtained, taking these values ​​into account:

[0040] When using a particularly stiff gearbox and a correspondingly high motor current draw of, for example, i = 4A, as well as correspondingly high line and transmission resistances R and a maximum motor constant K, the maximum permissible time to open the automatic parking brake is therefore t ges = 1575 ms.

[0041] A process in a program for determining whether the maximum total solution path of the spindle nut has been reached. 4 In this case, it could be as follows:

[0042] A corresponding flowchart to ensure that the minimum overall solution path for the spindle nut is achieved. 4 is in Fig. 5 shown. In the Fig. In the embodiment of the invention shown in Figure 5, not only a minimum shutdown criterion is taken into account, but also a maximum time-based shutdown criterion. In step S1, the start signal for the release process of the automatic parking brake is first generated. 1 Given a process initiated by the vehicle's driver, step S2 then compares the calculated (estimated) loosening distance Δs of the spindle nut. 4 with the specified minimum total release path, which in this example is 1.4 mm. The query formula could, for example, be: Δs ≤ 1.4 mm? If the calculated release path is equal to or greater than the specified minimum total release path of the spindle nut. 4In a further step S3, an additional shutdown criterion is checked. The query for the additional minimum shutdown criterion can be as follows: t ≤ 583 ms? In the example shown, the duration t of the release process is checked after its start in step S1 using a minimum time-based shutdown criterion, which, based on the calculation above, is 583 ms. If the check shows that the time t since the start of the release process is still less than the minimum time-based shutdown criterion, the routine continues at point S1' and the criterion is checked again in step S2. However, if the check shows that the time t since the start of the release process has reached or exceeded the minimum time-based shutdown criterion, the routine for releasing the automatic parking brake is terminated in step S5.

[0043] If, however, the calculated loosening path in step S2 is not equal to or greater than (but less than) the specified minimum total loosening path of the spindle nut 4In the illustrated embodiment, a maximum shutdown criterion is additionally checked in step S6. Here, the duration t of the release process after its start in step S1 is checked against a maximum time-based shutdown criterion, which, for example, is 1575 ms according to the calculation above. The query for the additional maximum shutdown criterion can be as follows: t ≥ 1575 ms? If the check shows that the time t since the start of the release process is still less than the maximum time-based shutdown criterion, the routine continues in step S7 or S1', and the criterion is checked again in step S2. If, however, the check shows that the time since the start of the release process has reached or exceeded the maximum time-based shutdown criterion, the routine for releasing the automatic parking brake is executed in step S5. 1completed. In step S5, the automatic parking brake is therefore always sufficiently open so that a residual drag torque is always avoided. It is understood that the selected numerical values ​​are purely exemplary and may vary depending on the application. In an alternative embodiment of the invention, both current curves I1 and I2 (see figure) must be Fig.4) within a defined time after switching on, the current must fall below a defined value. The earliest possible time is defined by the highest expected voltage, for example, 16 V, and the largest motor constant K (at a low temperature of, for example, -40 °C, coinciding with a maximum within the batch variation). The latest possible time is defined by the lowest voltage u, for example, u = 10 V, and the smallest motor constant K (at a high temperature of, for example, +80 °C, coinciding with a minimum within the batch variation). Additionally, the phase shift between the two current curves I1 and I2, as described above and resulting from the low-pass filters, can be taken into account. This embodiment also ensures that measurement errors do not lead to insufficient release of the automatic parking brake. 1 lead.

[0044] Overall, the present invention provides advantageous methods that prevent insufficient opening of an automatic parking brake. 1 to avoid errors in the loosening process. For this purpose, the invention provides at least one additional shutdown criterion, which is checked during the program sequence of the loosening process. The respective shutdown criterion is designed such that, regardless of any measurement error, in particular due to the measurement of the motor current i, it ensures a sufficient travel distance of the spindle nut during every loosening process. 4 This can ensure the persistence of residual drag torque after the release of an automatic parking brake. 1 This reliably avoids the risk.

[0045] A method according to the invention is preferably implemented in the form of a program (software) in a control or regulating device located near the automatic parking brake. 1or may be located in another place in the motor vehicle, and in particular may be part of the automatic parking brake. 1 educates. QUOTES INCLUDED IN THE DESCRIPTION

[0046] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0047] DE 102012202962 A1

[0002]

Claims

[1] Method for releasing an automatic parking brake ( 1 ), comprising an electromechanical braking device for generating an electromechanical clamping force, wherein a release travel (Δs) of the automatic parking brake ( 1 ) is estimated based on a measured motor voltage (u) and motor current (i), and the estimated solution path (Δs) is compared with a given minimum total solution path (Δs ges ) is compared, characterized by that to ensure the achievement of the minimum overall solution path (Δs ges ) at least one additional shutdown criterion (t max , t min ) is queried. [2] Method according to claim 1, characterized by that at least one additional shutdown criterion is a time period (t max , t min ) which is measured from the beginning of the dissolving process, where the time span is a minimum or a maximum time span (t max , t min) is about. [3] Method according to any one of the preceding claims, characterized by that the additional shutdown criterion is independent of a measurement of the motor current (i). [4] Method according to any one of the preceding claims, characterized by that the determination of the additional shutdown criterion (t max , t min ) depends on the motor voltage (u). [5] Method according to any one of the preceding claims, characterized by that the additional shutdown criterion (t max , t min ) depending on a motor constant (K), a given total solution distance (Δs ges ), is determined by the gear ratio of the transmission unit (ü) and the motor voltage (u). [6] Method according to claim 5, characterized by that the motor constant (K) is determined as a function of temperature information. [7] Method according to any one of the preceding claims, characterized bythat information about an existing hydraulic pre-pressure is required for determining at least one additional shutdown criterion (t max , t min ) is being considered. [8] Method for releasing an automatic parking brake ( 1 ), comprising an electromechanical braking device for generating an electromechanical clamping force, wherein a motor current (i) is redundantly measured during a release process, and the resulting measured currents (I1, I2) are compared with each other, characterized by that, as a shutdown criterion for the release process, the measured motor currents (I1, I2) must have fallen below a predetermined threshold after a defined period of time. [9] Control and / or regulating device for carrying out a method according to any one of claims 1 to 8. [10] Automatic parking brake ( 1 ) in a motor vehicle with a control and / or regulating device according to claim 9.