Method for determining a trigger condition for recalibrating an automatic parking brake

Abstract

A method is provided for determining a trigger condition for recalibration at a predetermined time (t0) during the release process of an automatic parking brake (1) with a brake motor (2) for generating an electromechanical braking force. The object of the invention is to provide a method that unambiguously detects a load-free release process of the brake motor and enables recalibration to be triggered only when necessary. For this purpose, the method is characterized by determining the motor current (i(t0)) expected at the predetermined time (t0).

Description

[0001] The invention relates to a method for determining a trigger condition for recalibration, to a control and / or regulating device and to an automatic parking brake system.

[0002] A method for recalibrating an automatic parking brake in a motor vehicle is known from DE 10 2011 004 763 A1. Recalibration is performed when a malfunction occurs during the release of the parking brake. A malfunction can result, for example, from a voltage drop in the vehicle's electrical system. A malfunction can also occur if the driver attempts to drive off before the brake has fully released and the release process is complete. In the event of a malfunction, the brake motor is typically switched off first. If necessary, hydraulic assistance is requested, and then the brake motor is restarted to continue the release of the brake piston until the clamping force generated by the parking brake is relieved and the parking brake is operating without load.

[0003] In addition to the cases mentioned above, recalibration is also initiated if the automatic parking brake releases under no-load conditions. A release under no-load conditions occurs when the motor current drops to its idle level immediately after the initial peak. In this case, the release process is aborted because the position of the spindle nut can no longer be reliably determined. After the release process is aborted, the recalibration process proceeds in two steps. First, the brake pads are moved against the brake discs by actuating the brake motor. The contact of the brake pads with the brake discs is detected by an increase in the motor current. Subsequently, the automatic parking brake is released in the usual manner. The recalibration process is then complete.After recalibration, the automatic parking brake is in the open position and the closing process can be restarted manually or automatically.

[0004] A disadvantage of known recalibration methods for unloaded release processes is that, at the time of the check (hereinafter referred to as the predetermined time) to determine whether the brake motor is in an unloaded state, the check is performed solely by comparing the motor current present at this predetermined time with an assumed threshold value. The current drawn by the brake motor is thus measured a predetermined time after the start of the release process and compared with the threshold value. If the measured current at the predetermined time is below the threshold value, unloaded release is assumed, and recalibration is initiated. However, this method fails to consider that the resulting current profile is a superposition of the inrush current and the force reduction curve. Furthermore, the force reduction curve is significantly influenced by the hydraulic pre-pressure generated by the driver.Furthermore, the length of the motor's inrush current depends on several parameters, particularly the temperature, the motor's mechanical time constant, and the wiring. For example, a large time constant of the brake motor and a high motor voltage can lead to an incorrect interpretation of a motor current inrush peak as a force reduction curve, even though release occurs without force reduction. To reliably detect the state of unloaded release at the predetermined time even in such situations, the threshold value must be set correspondingly high, which sometimes triggers unnecessary recalibrations.

[0005] The motor current profile after the brake motor is restarted can therefore vary depending on the operating state of the parking brake system. Currently, the operating state of the parking brake system is not considered when triggering recalibration, resulting in frequent and unnecessary recalibrations. In the prior art, the chosen threshold value thus represents a compromise between detecting a load-free release state of the parking brake and unnecessarily initiating a recalibration process.

[0006] The object of the present invention is therefore to provide a method for determining a trigger condition for a recalibration process in the case of a load-free release process, which clearly recognizes such a load-free release process of the brake motor and enables the recalibration to be triggered only when necessary.

[0007] The problem is solved by the features of the independent patent claims. Further developments of the invention are specified in the dependent claims.

[0008] A method according to the invention serves to determine a trigger condition for recalibration at a predetermined time during the release process of an automatic parking brake, which has a brake motor for generating an electromechanical braking force. According to the invention, the method determines or predicts the motor current expected at the predetermined time.

[0009] A particular advantage of the present invention lies in the fact that, at the predetermined time for deciding on recalibration during the release process of an automatic parking brake, no arbitrarily chosen compromise value of a motor current is used as a threshold. Rather, the motor current at the predetermined time is recalculated as the trigger condition for each release process. This calculation takes into account the current operating conditions of the parking brake and, in particular, the brake motor. In this way, the theoretically expected current at the predetermined time in the case of a load-free release process can be calculated. If the actual measured motor current at the predetermined time is within the range of the calculated current, especially plus a safety threshold, it can be assumed that the brake motor is operating under no-load conditions at that time and the recalibration can be started.Otherwise, no recalibration of the parking brake system is performed because it can be assumed that the clamping force has not yet dissipated. The unloaded release situation can thus be clearly identified, preventing erroneous recalibrations. Overall, by calculating the expected current at a predetermined time during an unloaded release state, the trigger condition for recalibration can be advantageously determined largely independently of voltage fluctuations in the vehicle electrical system, variations in motor parameters due to manufacturing tolerances, and temperature.

[0010] Advantageously, the motor current expected at a predetermined time serves as the trigger condition for recalibrating the parking brake. The trigger condition is preferably defined as the current expected at the predetermined time. This requires calculating the current profile during the release process, preferably at the beginning of the release process based on the operating conditions. The current profile is preferably calculated using an exponential function, which approximates the release process and the resulting exponential current drop. Unlike the prior art, no arbitrarily chosen estimated value is used as the trigger condition; instead, the trigger condition is calculated individually and situationally based on the motor parameters. Therefore, unnecessary recalibrations are no longer required, as they are completely eliminated by the method according to the invention.

[0011] A particularly advantageous method is one in which the motor current expected at a predetermined time is determined based on a maximum current at the beginning of an inrush current surge, an idle current, and a time constant. Specifically, to determine the tripping condition or tripping threshold, the maximum current at the beginning of an inrush current surge is advantageously determined based on the motor resistance, the motor voltage measured at the time of switch-on, and the idle current. By using the motor parameters, the idle current, and the applied voltage at the time of motor start, the motor current at the predetermined time, which is present in the case of a load-free tripping state, can be determined.

[0012] Advantageously, the time constant is determined based on the motor resistance, the motor constant, and the inertia of the brake motor, in particular the rotor or armature. Using the brake motor's time constant, the release condition, namely the current present at the predetermined time in the event of unloaded release, can be advantageously calculated.

[0013] To determine whether the parking brake system is actually in a load-free release state at the predetermined time and therefore requires recalibration, the expected motor current at that time is advantageously compared with the actual motor current measured at that time. If the comparison shows that the measured motor current is within the range of the calculated motor current (trigger condition), it can be assumed that the system is indeed in a load-free release state in which the clamping force of the brake piston has been completely relieved.

[0014] Advantageously, the predicted motor current plus a safety threshold is compared with the actual motor current at a predetermined time. This prevents an incorrect decision at the predetermined time if the measured current deviates slightly from the calculated current.

[0015] Accordingly, recalibration is advantageously not triggered if a motor current measured at a predetermined time is above the level of the motor current calculated as the trigger condition, particularly plus a safety threshold. If the measured current at the predetermined time is above the calculated trigger threshold, it can be assumed that the brake pads are still in contact with the brake disc.

[0016] The method according to the invention takes place in a control unit in a motor vehicle, which may be part of an automatic parking brake system (APB).

[0017] 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:

[0018] Fig. 1 A sectional view of an automatic parking brake for a vehicle with an electric brake motor to generate a clamping force that secures the vehicle;

[0019] Fig. 2 a diagram of a typical current flow when releasing a parking brake with force reduction;

[0020] Fig. 3. A diagram of a typical current flow when releasing a parking brake without force reduction at room temperature; and

[0021] Fig. 4 A diagram of a typical current flow when releasing a parking brake without force reduction at an ambient temperature of 90°C.

[0022] Fig. Figure 1 shows a sectional view of an electromechanical parking brake (automatic parking brake) 1 for a vehicle that uses a brake motor 2 , which in this case is designed as a DC motor, can exert a clamping force to secure the vehicle. The brake motor 2drives 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 provided, which are located on an inner end face or a rear face of a brake piston 5 is attached. The spindle nut 4 This occurs during a rotational movement of the brake motor. 2 and a resulting rotational movement of the spindle 3 , displaced in the axial direction. The spindle nut 4 and the brake piston 5 are 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.

[0023] In the event of an engagement of the automatic parking brake 1 , the brake motor rotates 2 and the spindle nut 4The brake piston is then subjected to axial pressure. 5 and the brake disc 7 moved until a predetermined maximum clamping force is reached. During a release process of the automatic parking brake 1 The brake motor rotates 2 in the opposite direction, thus reducing the clamping force. This is in addition to the electromechanical clamping force of the automatic parking brake. 1 Hydraulic assistance may be provided. In this case, the automatic parking brake will be used. 1 through one on the back of the brake piston 5 The acting fluid pressure is relieved. Accordingly, the parking brake releases. 1 in this case with only a slight or no reduction in power. The brake motor 2 It then reaches its unloaded release state more quickly, in which the clamping force of the automatic parking brake 1 has been completely dismantled.

[0024] Fig. Figure 2 shows a diagram of a typical motor current curve during a release process with force reduction at room temperature and an applied voltage of approximately U = 12 V. A release process with force reduction in this context means that there is no or only minimal hydraulic assistance to the automatic parking brake. 1 is the case, so that the clamping force is at least predominantly electromechanical due to the automatic parking brake. 1 is produced and therefore must be broken down during the dissolution process.

[0025] The depicted motor current curve can, in the event of a disturbance of a release process as described above, be followed by the restarting of a brake motor. 2 connect to activate the automatic parking brake. 1 to bring it into a load-free state in which the clamping force is completely relieved. As the diagram illustrates, the motor current increases after the brake motor is switched on. 2The current rises steeply to a negative peak at the start of the motor. This is due to the brake motor being reversed for the release process. 2 The motor current is negative in this case. At the example time t = 80 ms, the current is... Fig. 2. The motor current shown is approximately I = –5.2 A.

[0026] The brake motor 2 The current drawn decreases over time until an idle current I is reached. L adjusts. During the drop in current, the pressure on the brake disc is increased. 7 The applied clamping force is progressively reduced until it is in the idle state of the brake motor. 2 is completely disassembled and the brake pads 8 , 8‘ consequently no longer compatible with the brake disc 7 be in contact.

[0027] Fig. Figure 3 shows another diagram illustrating a typical current flow when releasing the automatic parking brake without force reduction. 1This shows that during the release process without force reduction, the automatic parking brake is hydraulically assisted. 1 such that no clamping force needs to be actively reduced, as it is generated hydraulically. In the case of the Fig. The exemplary current curve shown in Figure 3 is based on a dissolving process at room temperature and an operating voltage of approximately U = 12 V.

[0028] Even in the depicted release without force reduction, the current curve shows an inrush current peak, which, however, due to the lack of a force reduction phase, relatively quickly transitions into the idle phase of the brake motor. 2 transitions. Thus, the motor current drawn at time t = 80 ms is only about I = -0.8 A and is therefore far below the current value of the current curve with force reduction according to Fig. 2.

[0029] To illustrate how the inrush current of the brake motor behaves 2superimposed on the force reduction curve and how the operating conditions affect the current profile of the brake motor 2 The effects show Fig. Figure 4 shows another diagram of the current curve during a loosening process without force reduction, performed at an ambient temperature of 90°C and an operating voltage of U = 15 V. A measurement at t = 80 ms yields a motor current of approximately I = -1.36 A.

[0030] It thus becomes apparent that the high temperature results in a delayed inrush current, which leads to a higher current value at time t = 80 ms, which, unlike the current curve in Fig. 3, above the idle current I L lies and could be interpreted as a high pressure exerted by the driver via the service brake.

[0031] In summary, it is evident that the operating conditions of the parking brake system influence the current profile during the release process. According to the present invention, the current expected in the unloaded release state is determined at the predetermined time t0. The curve of the current driven by the brake motor 2 The current i(t) drawn is determined at the beginning of the release process as a function of the inrush current and the force decay. The current profile of a load-free release process is approximated using an exponential function with the following formula:

[0032] Here, i(t) is the calculated motor current at time t, I Max the maximum motor current at the beginning of the inrush current, τ the mechanical time constant of the brake motor 2 and I L The no-load current that occurs when there is no load. The motor current I MaxThe initial current surge is calculated as follows:

[0033] This is U Mot the voltage of the brake motor measured at the time of switch-on 2 , R Mot the motor resistance and I L the no-load current, which is used to estimate the inrush current I Max must first be subtracted. The mechanical time constant τ is determined using the motor resistance R. Mot , the motor constant K Mot and the moment of inertia J of the brake motor armature is determined as follows:

[0034] In order to calculate the current waveform i(t), the motor resistance R must first be determined. Mot , the motor constant K Mot and the idle current I L These parameters can be determined. The determination of these parameters is known from the prior art. For example, the motor constant K is determined. Mot and the motor resistance R Motdetermined from known relationships based on the current values ​​of the motor voltage U, the motor current I and the motor speed, preferably during motor start-up directly after the brake motor has started. 2 can be determined. The no-load current I L This occurs in the phase following the inrush current of the brake motor. 2 determined in which the speed of the brake motor 2 is constant and the idle current I L only from the load or friction of the brake motor 2 is determined.

[0035] The course of the braking process during a release operation by the brake motor 2 The recorded current i(t) is therefore determined by superposition or by joint consideration of the switch-on peak I. Max and the force reduction, represented by the mechanical time constant τ of the brake motor 2, determined. The course of the motor current i(t) is simulated or predicted using an exponential function.

[0036] The following procedure results for predicting the current at a predetermined time t0: The motor parameters R are Mot and K Mot as well as the idle current I L determined. With the help of the brake motor at the start time. 2 measured motor voltage U Mot can then the current I Max The current must be calculated at the beginning of the inrush current. When determining this current, it is sufficient to simplify by assuming a system. 1 Order. The inductance of the brake motor. 2This can be neglected because the measurement time is significantly longer than the expected time constant τ and due to the fact that at high temperatures the mechanical time constant τ is much larger than the electrical time constant. Using the exponential function, the expected current i at the predetermined time t0, which is the time for determining the unloaded release, is determined. To calculate the current profile i(t), the no-load current I must be determined. L to be added back in.

[0037] At the predetermined time of determining the unloaded release, for example after t0 = 80 ms, the calculated current i(t0) is compared with the current I measured at this predetermined time t0. MotThe measured current is compared. For this purpose, a safety threshold is preferably applied, which is added to or subtracted from the calculated value. If the comparison shows that the measured current is at the level of i(t0), it can be assumed that the automatic parking brake is engaged. 1 The brake pads are in the state of unloaded release at the predetermined time t0, and recalibration can be initiated. However, if the current measured at the predetermined time t0 is above the calculated current value i(t0), it can be assumed that the brake pads are... 8 , 8‘ still on the brake disc 7 are located and the clamping force has therefore not yet been reduced.

[0038] In summary, it is evident that the present invention provides a triggering condition that is individually dependent on the operating conditions of the parking brake. 1The trigger condition is calculated in the form of the current i expected at the predetermined time t0. This makes the trigger condition largely independent of voltage fluctuations in the vehicle electrical system, variations in motor parameters due to manufacturing tolerances, and temperature. Recalibration is thus initiated only when truly necessary.

[0039] Furthermore, a comparison of the measured current with the calculated expected current i(t0) can be performed not only at a predetermined time t0, but at one or more additional subsequent predetermined times t1, t2, ... etc. In particular, a further comparison of the measured current can be performed after a certain period of time, for example at time t1.

[0040] Preferably, the calculation of the expected current profile and the comparison of an expected current at a predetermined time t0 are implemented programmatically in a control unit. Here, the current, voltage, and speed values ​​acquired by a suitable measuring device are preferably evaluated in the manner described above. The control unit can be a standalone control unit of the automatic parking brake system. 1 or it could be an existing control unit of a vehicle dynamics system, such as ESP (Electronic Stability Program) or similar.

[0041] Furthermore, it should be noted that the present invention can be used with various types of parking brake systems. In principle, the method according to the invention can be used for all known automatic parking brakes that include an electromechanical locking mechanism located on the wheel brake and that utilize current and voltage measurement technology at the brake motor. 2 have. QUOTES INCLUDED IN THE DESCRIPTION

[0042] 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

[0043] DE 102011004763 A1

[0002]

Claims

[1] Method for determining a trigger condition for recalibration at a predetermined time (t0) during a release process of an automatic parking brake ( 1 ) with a brake motor ( 2 ) to generate an electromechanical braking force, characterized by that a motor current (i(t0)) expected at the predetermined time (t0) is determined. [2] Method according to claim 1, characterized by that the motor current (i(t0)) expected at the predetermined time (t0) serves as a trigger condition for a recalibration of the parking brake ( 1 ) serves. [3] Method according to claim 1 or 2, characterized by that the motor current (i (t0)) expected at the predetermined time (t0) is determined based on a maximum current (I Max ) at the beginning of an inrush current, an idle current (I L ) and a mechanical time constant (τ) of the brake motor ( 2 ) is determined. [4] Method according to any one of the preceding claims, characterized by that the maximum current (I Max ) at the beginning of an inrush current based on the motor resistance (R Mot ), the motor voltage measured at the time of switch-on (U Mot ) and the idle current (I L ) is determined. [5] Method according to any one of the preceding claims, characterized by that the time constant (τ) is determined based on the motor resistance (R) Mot ), the motor constant (K Mot ) and the moment of inertia (J) of the brake motor ( 2 ), in particular the anchor, is determined. [6] Method according to any one of the preceding claims, characterized by that the motor current (i(t0)) expected at the predetermined time (t0) is compared with the actual motor current measured at the predetermined time (t0). [7] Method according to any one of the preceding claims, characterized bythat the expected motor current (i(t0)) plus a safety threshold is compared with the actual motor current at the predetermined time (t0). [8] Method according to any one of the preceding claims, characterized by that no recalibration is triggered if a motor current measured at the predetermined time (t0) is above the level of the motor current (i(t0)) expected at the predetermined time (t0), in particular plus a safety threshold. [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.