Method for determining a braking torque of an electromechanical drum brake, drum brake

Abstract

The present invention relates to a method for determining a braking torque (Mb) of an electromechanical drum brake (2) comprising a brake drum (3) and two displaceable brake shoes (4, 5), and comprising a controllable actuator (9) comprising an electric motor (10) and an actuator element (11) coupled to the electric motor (10), wherein the actuator element (11) cooperates with the brake shoes (4, 5) to displace them and is held displaceably in a positively guided or almost positively guided manner by an actuator bearing (19). It is provided that only one or at least one force acting on the actuator bearing (19) is detected and a direction of action of the force is determined, and that the braking torque (Mb) is calculated as a function of the detected force and direction of action.

Description

[0001] The present invention relates to a method for determining a braking torque of an electromechanical drum brake having a brake drum and at least two displaceable brake shoes, and having a controllable actuator having an electric motor and an actuator element coupled to the electric motor, wherein the actuator element cooperates with the brake shoes to displace them and is held displaceably in a positively guided or almost positively guided manner by an actuator bearing.

[0002] Furthermore, the invention relates to a drum brake device for a motor vehicle, which has a drum brake with a brake drum and two displaceable brake shoes, with a controllable actuator which has an electric motor and an actuator element coupled to the electric motor, wherein the actuator element cooperates with the brake shoes for their displacement and is held displaceably in a positively guided or almost positively guided manner by an actuator bearing. State of the art

[0003] The use of drum brakes, particularly on the rear axle of a motor vehicle, is already known from the state of the art. With the increasing electrification of motor vehicles, the design of an electromechanically actuated drum brake has become attractive. Due to low top speeds and modified operating strategies, the use of electromechanical drum brakes is now conceivable on both the rear and front axles.

[0004] While hydraulically actuated brakes typically use a pressure sensor in the hydraulic system to determine the brake clamping force, which also allows the braking torque to be estimated and controlled, this is not possible with electromechanically actuated brakes. Uncertainties in the friction pairing between the brake shoes and brake drum represent a significant limitation to the accuracy of braking torque estimation. In drum brakes, the self-reinforcing effect, which may be present, also increases the uncertainty.

[0005] From published patent application WO 2019 / 063279 A1, it is also known to measure a difference in support forces at the bearings of an approaching and a trailing brake shoe and use this to estimate the braking torque. However, to implement this method, the actuator forces acting on the brake shoes must be equal. This can be achieved, for example, by a floating actuator bearing. Disclosure of the invention

[0006] The method according to the invention with the features of claim 1 has the advantage that a braking torque can be precisely detected even in a drum brake that provides a fixed mounting of the actuator. With this fixed mounting, an actuator element that displaces the brake shoes is positively guided or almost positively guided by an actuator bearing, thus allowing no or only minimal bearing play, in particular of no more than 1 mm or no more than 2 mm. For this purpose, the invention provides that only one or at least one force acting on the actuator bearing is detected and a direction of action of the force is determined, and that the braking torque is calculated depending on the detected force and direction of action. The braking torque is thus calculated by detecting the force exerted on the actuator bearing during actuation and determining the braking torque depending on the direction of action of this force.The force acting on the actuator bearing is recorded in only one direction, i.e., in particular, only a one-dimensional force sensor is used, which can record a force in only one direction of action. In addition, the invention determines the direction in which the force acts. The recording of the force as such and the recording of the direction of the force therefore preferably do not take place in the same way. Rather, the direction of action of the force is preferably recorded independently of the force itself. Knowing the acting force and the direction of action of the force, it is possible to calculate which portion of the force acts on which brake shoe and thus which braking torque is provided by the drum brake. An important advantage here is that the determined braking torque is independent of the friction coefficient and / or braking characteristic of the drum brake, which allows a high level of accuracy to be achieved.Furthermore, the method is independent of the type of bearing of the brake shoes, i.e., in particular, regardless of whether the brake shoes have a support bearing or a pivot bearing. Furthermore, the invention can utilize existing sensor signals from the electric motor to detect the direction of action of the force and to detect the force itself. Furthermore, the method according to the invention offers the advantage that the braking torque can be precisely determined with just a single force measurement.

[0007] According to a preferred embodiment of the invention, a rotation angle of the electric motor is monitored, and the effective direction of the force is determined depending on the detected rotation angle. In this case, a single force sensor and a rotor bearing sensor are sufficient to determine the braking torque. Knowing the rotation angle, which is detected in particular by means of a rotor bearing sensor and can therefore also be understood as the rotor position angle, the position of the actuator element and thus that of the brake shoes can be calculated. This makes it possible, in particular, to calculate the contact point between the actuator element and the brake shoe, which results in the effective direction of the detected force on the respective brake shoe. Thus, by monitoring the rotation angle and the force acting on the actuator bearing, the forces acting on the brake shoes and thus the braking torque can be determined.

[0008] Particularly preferably, the effective direction of the force is determined depending on the contact geometry of the actuator element and the mating contact geometry of the brake shoes. Depending on whether the actuator element is designed as an actuator wedge or an actuator cam, different contact geometries result, which interact with the mating contact geometries of the respective brake shoe. However, knowing the respective contact geometry and the mating contact geometry allows the effective direction of the respective force to be precisely calculated.

[0009] Furthermore, it is preferably provided that partial forces acting on the actuator bearing are detected in at least two different directions in order to determine the effective direction of the force acting on the actuator bearing. In particular, a two-dimensional force sensor or two force sensors are used to detect the partial forces acting on the actuator bearing. Knowing the acting partial forces allows the direction in which the force of the brake shoes acting on the actuator to be calculated. Knowing the geometry of the actuator element and the brake shoes allows the braking torque to be advantageously determined.

[0010] Particularly preferably, the effective direction of the force is determined as a function of the angle of rotation and as a function of the partial forces. In particular, the braking torques determined as described above are compared with each other to perform a plausibility check. If the braking torques deviate from each other, for example, beyond a specified tolerance range, it can be determined that a malfunction must exist in one of the sensors.

[0011] Particularly preferably, an actual torque of the electric motor is monitored, and the actuator forces acting on the brake shoes, and thus a braking torque of the drum brake, are estimated based on the actual torque. Thus, a braking torque estimate is additionally performed, which is preferably used to check the plausibility of the previously calculated braking torque. This, in particular, increases the operational reliability of the method. In this respect, it is preferably provided that the calculated and estimated braking torques are compared with each other for plausibility purposes.

[0012] Furthermore, it is preferably provided that a friction coefficient of the drum brake is determined as a function of the calculated and estimated braking torque. By combining the above-mentioned estimation methods, the current braking characteristic is preferably also estimated or determined.

[0013] According to a preferred development of the invention, in order to control the actuator, the point in time at which a force acts on the actuator bearing is recorded, wherein a release clearance of the drum brake is determined as a function of the recorded point in time. Up to the point in time at which the brake shoes hit the brake drum and begin to apply a force to it, no detectable force acts on the actuator bearing. By recording the point in time at which a force is recorded after the actuator has been controlled, the time period required by the actuator element to move from its initial position into a contact position with the brake shoes can be determined. With knowledge of the motor position, in particular the angle of rotation, and / or the operating speed of the electric motor, the so-called release clearance that the actuator element must initially overcome in order to generate any braking torque is calculated.This clearance is then preferably advantageously taken into account in the further process when controlling the actuator, in particular to reduce or avoid it.

[0014] The drum brake device according to the invention with the features of claim 10 is characterized by a control unit that is specifically designed to carry out the method according to the invention when used as intended. This results in the advantages already mentioned above.

[0015] Further advantages and preferred features and combinations of features emerge in particular from the above description and from the claims. The invention will be explained in more detail below with reference to the drawings. Fig. 1 a drum brake device in a simplified representation, Fig. 2 a simplified detailed view of the drum brake, Fig. 3 the drum brake according to a first embodiment, Fig. 4 the drum brake according to a second embodiment and Fig. 5A and B the drum brake according to a third initial example.

[0016] Fig. Figure 1 shows a simplified representation of an advantageous drum-forming device 1 for a motor vehicle, in particular an electric vehicle. The drum brake device 1 comprises a drum brake 2 with a brake drum 3 and two brake shoes 4 and 5 arranged therein. The brake shoes 4, 5 are each pivotably held at one end on a brake carrier 8 by a pivot bearing 6 or 7, respectively. While the brake drum 3 is rotationally fixedly connected to a wheel of the motor vehicle, the brake carrier 8 is fixed to the body.

[0017] The brake shoes 4, 5 are assigned an electrically controllable actuator 9, which has an electric motor 10 and an actuator element 11 coupled to the electric motor 10. According to the present exemplary embodiment, the actuator element 11 is designed as an S-shaped cam, which lies between the brake shoes 4, 5 at the ends facing away from the pivot bearings 6, 7. Advantageously, the brake shoes 4, 5 are subjected to a spring force by at least one spring element 12 such that their ends facing away from the pivot bearings 6, 7 are pressed against the S-shaped cam. At their ends facing the actuator element 11, the brake shoes 4, 5 each have a counter-contact geometry 13, 14, which is designed to interact with the S-shaped contact geometry 15 of the actuator element 11. In the present case, the actuator element 11 is mounted so as to be rotatable about a rotation axis 26.The actuator element 11 is symmetrical in its S-shape and the axis of rotation 26 is arranged centrally.

[0018] Known methods for estimating the braking torque of drum brake 2, which are based on the actuator force, have so far led to inaccurate estimates due to the variation in the friction coefficient of drum brake 2 and the brake characteristic value. Alternative methods based on measuring the difference in the support forces do increase the accuracy, but are limited in their known form to drum brakes with a floating actuator and support of the brake shoes by a support bearing with low friction. The method described below enables precise braking torque determination even with the Fig. This is made possible by the drum brake 2 shown in Figure 1 with a fixed actuator mounting. Since the actuator element 11 is only rotatable about the rotation axis 26, but not displaceable, it is considered to be fixedly mounted.

[0019] The core of the advantageous method is the determination of the braking torque based on the difference between the actuator forces acting on the two brake shoes 4, 5, of which, depending on the direction of rotation of the wheel, one acts as the leading brake shoe 4, 5 and one as the trailing brake shoe 4, 5. This advantageous method determines the braking torque at least substantially independently of the coefficient of friction or the braking characteristic of the drum brake 2, thereby achieving a high level of accuracy. Furthermore, the method presented below is independent of the type of bearing of the brake shoes, i.e., independent of whether the brake shoes 4, 5 are supported by a pivot bearing or a support bearing.

[0020] While the method is explained here with reference to the S-shaped actuator element, it should be noted that the method can equally be carried out with other actuator shapes, in particular with a wedge-shaped actuator element, and is carried out alternatively. Preferably, the method is executed by a control unit 16 that is specifically designed to carry out the method during intended use, for example, at regular intervals or after each commissioning of a motor vehicle having the drum brake device 1. The control unit 16 is, for example, a brake control unit of the motor vehicle.

[0021] According to a first embodiment, the braking torque is determined as a function of signals from a rotor bearing sensor 17 of the electric motor 10 and a force sensor 18, which is associated with an actuator bearing 19 supporting the actuator element 11. In the present case, the actuator bearing 19 is, for example, a rotatable bearing pin 20, which is rotationally fixedly connected to the actuator element 11. The bearing pin 20 is operatively connected to the electric motor 10 for its rotation. Fig. 1 this is shown in a highly simplified manner for reasons of clarity.

[0022] In Fig. 3 to 5 show different variants of the force sensor 18, which will be discussed in more detail later.

[0023] Fig. 2 shows the drum brake 2 from Fig. 1 in an enlarged detailed view in the area of ​​the actuator element 11. The S-cam is actuated by an actuator torque M Actthat is provided by the electric motor 10, is rotated by an angle β. This creates a force between the brake shoes 4, 5 and the actuator element 11, which spreads the brake shoes 4, 5 outwards against the inside of the brake drum 3. Because the actuator element 11 is mounted in its center so that it can rotate but not be moved, the actuator bearing 19 can absorb the acting actuation forces. As a result, the actuation forces are generally unequal, in contrast to a drum brake with floating actuation, so that the actuation force F a1 , which acts on the brake shoe 5, is not equal to the actuating force F a2 which acts on the brake shoes 4.

[0024] In electromechanical actuators, as shown here, the actuator element 11 is typically connected to the electric motor 10 via a gearing. Preferably, the electric motor 10 is a brushless electric motor, which has a rotor position sensor 17 for commutation. The electric motor 10 is optionally coupled to the bearing pin 20 via a gearing with a predetermined gear ratio. The angular position β or the rotational movement of the cam, which is coupled via the gearing to the rotor position or rotor bearing position of the electric motor 10, can be detected and is known by the rotor bearing sensor. From the geometry of the actuator element 11 and the brake shoes 4, 5, the contact point or the contact location between the actuator element 11 and the respective brake shoe 4, 5 or the mating contact geometry 13, 14 is calculated with a known rotation β.Knowing the contact point also allows the angle α to be determined, which indicates the direction of action of the force between actuator element 11 and the respective brake shoe 4, 5. In the case of a symmetrical system or a geometric actuator element 11, as provided here, this angle is the same for both brake shoes 4, 5. However, a transfer to asymmetrical actuator elements is possible using simple calculations.

[0025] Now, any force component in the actuator bearing 19 is measured to determine the difference between the actuator forces FA1 and FA2. For example, the force equilibrium in the X direction is used to derive the following relationship: F1x=cos(α) Δ Fa

[0026] Analogously, any direction of force can be changed. In this context, the force F 1Xby the sensor 18 in the actuator bearing 19, in particular in the form of at least one strain gauge, and the angle α is determined based on the rotor bearing sensor 17 and the geometry of the drum brake 2. The difference in the actuator forces is the torque of the brake M b proportional: Mb=κ Δ Fa

[0027] Preferably, all parameters contained in ĸ depend exclusively on the geometry of the drum brake 2 and in particular are independent of the friction coefficient of the brake.

[0028] Thus, a simple, one-dimensional force sensor 17 on the actuator bearing 19 is sufficient to measure the braking torque M bThe method can be applied to all actuator elements 11 that are fixedly mounted and thus exert unequal actuator forces on the brake shoes 4, 5, and whose force direction can be estimated or calculated based on the geometry of the actuator mechanism.

[0029] Alternatively, the actuator bearing 19 or the actuator bearing is designed to be limitedly flexible, for example in the x-direction, and thus allows a deflection or displacement of, for example, a maximum of 1 to 2 millimeters within a specified tolerance range. This makes such a drum brake 2 a middle ground between the known concepts with force and displacement compensation. The permissible deflection dx is advantageously recorded with a displacement sensor and mapped to F using a characteristic curve. 1X converted. The angle α, the force ΔF a , the braking torque M b are then determined analogously to the procedure described above.

[0030] According to a further embodiment, two force sensors or a two-dimensional force sensor 18 are assigned to the actuator bearing 19, so that the forces F 1X and F 1Y can be directly detected. From this, the effective direction in which the force actually acting on the actuator bearing 19 acts can be determined directly from the force equilibrium conditions, without having to take into account the angle of rotation of the electric motor 10. This variant of the method is particularly preferable if the rotor bearing sensor 17 is not to be included or is not present due to safety considerations.

[0031] According to a further embodiment, the above-mentioned methods are combined to verify the plausibility of the two methods of differential force estimation. This allows errors or malfunctions in one of the sensors to be detected.

[0032] Preferably, in addition to the braking torque estimation, the absolute actuator forces F A1 and F A2 from the actuator torque M act calculated. The actuator torque M act , which acts on the actuator element 11, can be estimated as a function of the motor current of the electric motor 10. The average actuator force depends on the braking force and thus on the braking torque M bvia the braking factor C*. This also allows an estimate of the braking torque MB to be made. This estimate then depends on the friction coefficient or braking characteristic C* and is subject to greater fluctuations if this deviates from the expected value. However, this variant is independent of the sensor technology, in particular independent of the force sensor used on the actuator bearing 19. Therefore, this second estimation option is preferred for checking the plausibility of the calculated braking torque. This allows, for example, the functionality of the force sensor(s) to be verified. Alternatively, the two estimation methods can be combined to precisely estimate the current braking characteristic C*.

[0033] Preferably, the contact point between the brake shoes 4, 5 and the brake drum 3, and thus the so-called air gap, is determined by differential measurement of the actuator forces during operation. When the electric motor 10 is energized while the brake drum 3 is rotating, the bearing force in the actuation is zero until the brake shoes 4, 5 come into contact with the brake drum. From this moment on, a non-zero bearing force is measured at the actuator bearing 19. This allows the rotor bearing sensor 17 to precisely determine the clearance between the brake shoes 4, 5 and the brake drum 3.

[0034] Fig. Figure 3 shows a first embodiment of the drum brake device 1, in which the force sensor 18 is designed as a Hall sensor 21. The Hall sensor 21 is assigned to one end of the bearing pin 20, which is assigned to or supports the actuator element 11. A magnet 22 is arranged on the end face of the bearing pin 20, which is opposite the Hall sensor 21. The bearing pin 20 is rotatably mounted in a rolling element bearing 23 between the magnet 22 and the actuator element 11.

[0035] If an actuator force (ΔF) is now exerted by the contact of the actuator element 11 with the respective brake shoe 4, 5, the shaft end shifts slightly transversely to the rotation axis of the bearing pin 20. This is advantageously determined by the Hall sensor 21 and thus the force acting on the actuator bearing 19, in particular on the rolling element bearing 23, is determined depending on the geometry of the bearing pin 20, which is assumed to be known, as well as its material properties.

[0036] Fig. 4 shows a further embodiment in which the force sensor 18 is designed as an eddy current sensor 24 or as a distance sensor which is radially assigned to the bearing pin 20, wherein in this case the force sensor 18 also detects a displacement or deformation of the bearing pin 20.

[0037] In the examples of the Fig. 3 and Fig. 4, it is assumed that the actuator forces cause an elastic deformation of the bearing pin 20, which can be determined by the respective force sensor 18 in the manner described above. The force acting on the actuator bearing 19 is calculated depending on the deformation.

[0038] Fig. 5A and Fig. 5B shows a further embodiment in which the force sensor 18 is assigned to the rolling element bearing 23 and is designed, for example, as a strain gauge 25. By monitoring the strain, the force acting on the bearing is also determined. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] WO 2019 / 063279 A1

[0005]

Claims

[1] Method for determining a braking torque (M b ) an electromechanical drum brake (2) having a brake drum (3) and two displaceable brake shoes (4, 5), and having a controllable actuator (9) having an electric motor (10) and an actuator element (11) coupled to the electric motor (10), wherein the actuator element (11) cooperates with the brake shoes (4, 5) for their displacement and is held displaceably in a positively guided or almost positively guided manner by an actuator bearing (19), characterized by that only one or at least one force acting on the actuator bearing (19) is detected and a direction of action of the force is determined, and that the braking torque (M b ) is calculated. [2] Method according to claim 1, characterized by that a rotation angle of the electric motor (10) is monitored and the direction of action of the force is determined as a function of the rotation angle. [3] Method according to one of the preceding claims, characterized by that the direction of action of the force is determined as a function of a contact geometry (15) of the actuator element (11) and a counter-contact geometry (13, 14) of the respective brake shoe (4, 5). [4] Method according to one of the preceding claims, characterized by that partial forces acting on the actuator bearing (19) are detected in at least two different directions in order to determine a direction of action of the force acting on the actuator bearing (19). [5] Method according to one of the preceding claims, characterized by that the direction of action of the force is determined depending on the angle of rotation and depending on the partial forces. [6] Method according to one of the preceding claims, characterized bythat an actual torque of the electric motor (10) is monitored and, depending on the actual torque, the actuator forces acting on the brake shoes (4,5) and thus a braking torque (M b ) of the drum brake (2) is estimated. [7] Method according to one of the preceding claims, characterized by that the calculated and estimated braking torque are compared for plausibility purposes. [8] Method according to one of the preceding claims, characterized by that a braking characteristic value of the drum brake (2) is determined as a function of the calculated and estimated braking torque. [9] Method according to one of the preceding claims, characterized by that after the activation of the actuator (9) the time at which a force is detected is recorded, and that a release play of the drum brake (2) is determined as a function of the time. [10] Drum brake device (1) for a motor vehicle, with a drum brake (2) which has a brake drum (3) and two displaceable brake shoes (4, 5), and with a controllable actuator (9) which has an electric motor (10) and an actuator element (11) coupled to the electric motor (10), wherein the actuator element (11) cooperates with the brake shoes (4, 5) for their displacement and is held displaceably in a positively guided or almost positively guided manner by an actuator bearing (19), characterized by a control device (16) which is designed to carry out a method according to one of claims 1 to 9 when used as intended.

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