Control element with a rotatably mounted actuating part and an electrically controlled brake with haptic confirmation, as well as associated use and methods.

Abstract

The invention relates to an operating element (1) comprising: a carrier (3); an actuating part (2) rotatably mounted on the carrier (3) about a pivot axis (D) for performing a rotary adjustment by an operator; a position transmitter (4) and a position sensor (5) for position detection; an electrically controlled brake (6, 7, 8) acting between the carrier (3) and the actuating part (2), inhibiting or blocking the rotary adjustment of the actuating part (2); a control unit (15) for position-dependent energizing the electrically controlled brake (6, 7, 8) in order to block the actuating part (2) by means of the electrically controlled brake (6, 7, 8) to generate an end stop or to inhibit it to generate a detent haptic;wherein the electrically controlled brake (6, 7, 8) is an electromagnetic brake comprising an exciter (6, 7) having a coil (8) and a magnetic core (7) and a brake disc (6) which interacts magnetically with the exciter (7, 8), wherein the brake disc (6) and the exciter (6, 7) are rotated relative to each other during rotation adjustment and a maximum contact area (A) exists between the brake disc (6) and the magnetic core (7) at least during blocking or inhibiting. max ) is designed and; wherein the electrically controlled brake (6, 7, 8) and / or the actuating part (2) are mounted in such a way that, when an actuating force (F) acts orthogonally on the axis of rotation (D), the exciter (6, 7) and the brake disc (6) can be tilted relative to each other following the actuating force (F) while providing a relative minimum tilt (14), so that the maximum contact area (A max ) on a minimal installation area (A min) is reduced; and the control unit (15) is designed to reverse the minimum tilting (14) by restoring the maximum contact area (A) for haptic and / or acoustic reinforcement of the end stop or the locking haptic. max ) to effect; as well as related uses and procedures.

Description

[0001] The invention relates to an operating element with a rotatably mounted actuating part, also called a handle, which is designed in particular as a thumbwheel, the latter also referred to as an input roller. The operating element has an electrically controllable, electromagnetic brake for generating an end stop and / or haptic feedback. In motor vehicles today, a multitude of operating functions must be set, with a trend toward performing this multitude of operating functions using a single rotary control in order to set parameters or select between different options according to a selected operating function.While in certain operating situations an input in which the actuating part can be rotated freely without mechanical resistance is preferred, in other operating situations when setting parameters or selecting predefined options it is preferable to experience haptic feedback. This is caused by a mostly periodic sequence of pulse-like braking effects that inhibit the rotary adjustment and is perceptible to the operator on the actuating part in the form of a tangible rotation inhibition, which is referred to as detent haptics. In rotary actuators this is usually achieved by a mechanical contour with an associated spring element. This means that the spring rests against the contour with a mechanical preload and slides up and down when the rotary actuator is operated. This sliding movement is disadvantageously associated with friction and is subject to wear and can also cause undesirable grinding noises.If universal use of the rotary actuator in diverse operating situations is desired, and since disabling this mechanical locking effect is difficult to achieve, electrically controlled, electromagnetic brakes with associated control are used. The electrically controlled, electromagnetic brake is controlled position-dependently based on the position information received from a position sensor, periodically inhibiting the rotary adjustment. Furthermore, this type of brake can even be used to create a disableable end stop by blocking the actuating part.

[0002] From DE 600 08 740 T2 an operating element according to the preamble of claim 1 is known, namely encoder-switch arrangements which have an encoder part which provides electrical output signals which indicate the instantaneous angular position change of a rotary roller or a tuning wheel of the encoder part. An operating element according to the preamble of claim 1 is known from US Pat. No. 6,211,474 B1. The rotationally operated electronic component of a multi-input switch is arranged in one direction of the rotation axis, while the switches are arranged in the direction perpendicular thereto, whereby the entire rotationally operated electronic component does not move. A device for generating tactile force is known from US Pat. No. 6,468,158 B1.

[0003] However, the known solutions generated by an electromagnetic brake have so far lacked a distinct click, i.e., a distinct acoustically or haptically perceptible impulse, for example, when the position-dependent maximum braking effect is reached, such as the maximum effective braking force, or when the end stop is reached with the brake locked. It would be conceivable to provide an additional electrically controlled actuator to mechanically stimulate the actuating part accordingly. However, the provision of an additional actuator fails in most cases due to the limited installation space.

[0004] The invention is therefore based on the object of providing an operating element, in particular for a motor vehicle, with a rotatably mounted actuating part and an electrically controllable, electromagnetic brake, which is capable of providing additional acoustic or haptic feedback in addition to the inhibiting or blocking effect, and in particular is structurally simple and space-saving, is therefore inexpensive to manufacture and in particular requires comparatively little installation space.

[0005] This object is achieved by an operating element according to claim 1, as well as the use and method of the independent claims. Advantageous embodiments can be found in the dependent claims. It should be noted that the features listed individually in the claims can be combined with one another in any technologically expedient manner and demonstrate further embodiments of the invention. The description, particularly in conjunction with the figures, further characterizes and specifies the invention.

[0006] The invention relates to an operating element, in particular for a motor vehicle. The operating element according to the invention has a carrier. The carrier is made, for example, from a plastic, such as a thermoplastic. The term "carrier" is to be interpreted broadly and can have a design directed solely at the attachment of the operating element. However, it can also have a design that protects one or more components, for example, an electronic component, of the operating element. For example, it is designed in some areas as a substantially closed housing.

[0007] The operating element according to the invention further comprises an actuating part mounted on the carrier so as to be rotatable about a rotation axis. The actuating part is made, for example, of a plastic, such as a thermoplastic, and is produced, for example, using an injection molding process. The rotation axis is oriented, for example, orthogonal to a user interface facing the operator. The actuating part is preferably designed as a thumbwheel, i.e., operation is achieved using one-finger operation, and the rotation axis lies, for example, in the plane of the user interface facing the operator or is parallel to this user interface. The user interface is defined, for example, by the dashboard or a steering wheel into which the operating element is integrated.

[0008] According to the invention, a position sensor is further provided, which is fixed to the actuating part and / or moves synchronously with the actuating part during its rotational adjustment, as well as a position sensor, which is fixed to the carrier and interacts with the position sensor. These together form the detection means and serve to detect the position of the actuating part. For example, the detection means is a potentiometer. Contactless detection means, such as optical, capacitive, or inductive detection means, are preferably provided. The detection means are designed, for example, as absolute value encoders or as incremental encoders. For example, the position sensor is rotationally driven by the actuating part via a transmission, in particular a gear transmission.

[0009] According to the invention, an electrically controllable brake is provided which acts between the carrier and the actuating part and inhibits or blocks the rotational adjustment of the actuating part.

[0010] According to the invention, a control unit is further provided, electrically connected to the position sensor and the electrically controllable brake, for position-dependent control of the electrically controllable brake in order to specify at least one end stop for the rotary adjustment of the actuating part in at least one of the rotational directions by blocking the electrically controllable brake and / or to generate a locking haptic during the rotary adjustment by inhibiting it using the electrically controllable brake. For example, the blocking effect is designed such that at least the manual rotary adjustment is counteracted by a corresponding braking effect, which is perceived as insurmountable at least by the operator. For example, the blocking torque in the end stop position is 5 N / m or more.A detent haptic is understood, for example, as an inhibiting effect that counteracts the rotary adjustment, increasing and then decreasing with further rotary adjustment after reaching a maximum. In a preferred embodiment, the corresponding maxima of the inhibiting effect are distributed evenly across the possible adjustment range of the actuating part.

[0011] According to the invention, the electrically controllable brake is an electromagnetic brake comprising an exciter having a coil and a magnetic core, and a brake disc that magnetically interacts with the exciter. The brake disc and the exciter are rotated relative to one another during rotational adjustment. The magnetic core and / or the brake disc are, for example, rotationally symmetrical. The magnetic core comprises, for example, an inner partial body that passes through the coil, as well as a cup-shaped outer partial body located radially outside the coil. For example, the magnetic core is "E"-shaped when viewed in section along the rotational axis.

[0012] According to the invention, a maximum possible contact surface, referred to as the maximum contact surface for short, is formed between the brake disc and the magnetic core, at least during blocking or braking. The magnetic core and the brake disc do not necessarily have to be directly adjacent to one another. Preferably, a thrust washer is provided between the brake disc and the magnetic core to improve the sliding friction between the brake disc and the magnetic core, particularly in the event that there is no operating current at the exciter and no braking effect, with the brake disc and the magnetic core adjoining one another exclusively via the thrust washer.

[0013] Preferably, the maximum contact surface is rotationally symmetrical. For example, the maximum contact surface is arranged concentrically to the axis of rotation.

[0014] According to the invention, the electrically controllable brake and / or the actuating part are mounted in a yielding manner, preferably elastically yielding, such that, when an actuating force acts orthogonally to the axis of rotation, the exciter and the brake disc can be tilted relative to one another following the actuating force, providing a relative minimal tilt, so that the maximum contact surface is reduced to a minimum contact surface by the relative tilt. Thus, for example, a minimal, V-shaped gap is created between the brake disc and the magnetic core, at least during actuation. For example, the gap has a maximum clear dimension between the magnetic core and the brake disc of at most 0.2 mm, preferably at most 0.1 mm.

[0015] According to the invention, the control unit is further designed to select a working current for energizing the exciter for the haptic and / or acoustic amplification of the end stop or the locking haptic such that, counter to the effect of the actuating force, the maximum contact surface is (re-)established by reversing the minimal tilting of the magnetic core and brake disc. For example, the time or course of the energization is selected such that the maximum contact surface is established simultaneously with the reaching of the end position or the reaching of one of the maxima of the inhibiting effect of the locking haptic. When the exciter of the electrically controllable brake designed as an electromagnetic brake is excited, the aforementioned gap closes, for example, which results in minimal movement but significant acceleration of the actuating part, in particular its user interface.The operator perceives this acceleration as haptically equivalent to a hard engagement of a mechanical detent spring in a detent gate.

[0016] Thus, reversing the minimal tilt caused by the actuating force results in a pronounced click, i.e. a pronounced acoustically or haptically perceptible impulse on the actuating part, for example when the position-dependent maximum braking effect is reached, such as the maximum effective inhibition, or when the end stop is reached with the brake locked. This additional impulse, which acoustically or haptically underlines the haptic effect, is possible without the provision of an additional electrically controllable actuator. In this way, in addition to the Ruffini corpuscles (SA-II receptors), which primarily act on stretching, the so-called Vater-Pacini corpuscles (PC receptors) are also stimulated. "Haptic amplification" here refers to a brief acceleration of the operating surface of the actuating part. Furthermore, an improved haptic sensation can also be generated during slow rotary movements.

[0017] Preferably, the brake disc is rigidly attached to the actuating part, and the actuating part is supported on the carrier by a yielding abutment, preferably an elastically yielding abutment, to enable the relative tilting between the brake disc and the magnetic core following the actuating force. For example, the actuating part is supported on the carrier via the magnetic core, with the abutment made of an elastomer or rubber being arranged between the magnetic core and the carrier.

[0018] Preferably, a shaft protrudes from the brake disc, which is connected in a rotationally fixed manner to the carrier or in a rotationally fixed manner to the brake disc. Each shaft engages in a corresponding recess in the magnetic core with a clearance to minimize tilting. For example, the shaft is formed by a bearing journal whose free end engages in a blind hole formed in the magnetic core.

[0019] The electrically controllable brake preferably has a magnetorheological fluid arranged between the magnetic core and the brake disc. A magnetorheological fluid is a suspension of magnetically polarizable particles (carbonyl iron powder) finely dispersed in a carrier fluid. This fluid prevents dirt from penetrating the space between the magnetic core and the brake disc and also reduces an annoying clicking noise when the brake disc and magnetic core come into contact during brake activation. Due to the magnetic properties of this fluid, the braking effect is only slightly altered during braking and locking.

[0020] Preferably, the actuating part defines a hollow volume in which the electrical exciter and the brake disc of the electrically controllable brake are arranged.

[0021] Preferably, the minimum tilt specified by the minimum contact surface and the maximum contact surface corresponds to a tilt angle of 1° or less.

[0022] The invention further relates to the use of the control element in one of the previously described embodiments in a motor vehicle.

[0023] The invention further relates to a method for the haptic and / or acoustic confirmation of an operating input, comprising the following steps. In a provision step, an operating element in one of the previously described embodiments is provided. In an energization step, the exciter is energized by the control unit of the provided operating element in a position-dependent manner. For the haptic and / or acoustic amplification of the end stop or the locking haptics, the control unit selects a working current for energizing the exciter such that, counteracting the effect of the actuating force, a maximum contact surface is established between the brake disc and the magnetic core by reversing the minimal tilt.

[0024] The invention is explained in more detail with reference to the following figures. These figures are to be understood as examples only and represent preferred embodiments. They show: Fig. 1 a perspective view of a first embodiment of the control element 1 according to the invention; Fig. 2 a sectional view orthogonal to the rotation axis D of the Fig. 1 shown embodiment; Fig. 3 a sectional view orthogonal to the rotation axis D of the Fig. 1 embodiment under load of the actuating part 2 with an actuating force F; Fig. 4 shows a sectional view orthogonal to the axis of rotation D of a second embodiment of the operating element 1 according to the invention under load of the actuating part 2 with an actuating force F.

[0025] An embodiment of the control element 1 according to the invention is described below with reference to Fig. 1. It comprises an actuating part 2, also called a handle, which is mounted on a support 3 for rotation about a rotation axis D and has a radially encircling actuating surface 13. In the embodiment shown, the actuating part 2 is designed as a thumbwheel, i.e., operation is achieved by one-finger operation, and the rotation axis D is substantially parallel to a viewing surface facing the operator. For example, the support surface 16 forms this viewing surface or is at least parallel to a panel surface (not shown), the latter forming the viewing surface.

[0026] Furthermore, a position sensor 4, for example a perforated disc, is provided on the actuating part 2, as well as a position sensor 5, such as a light barrier, to enable position detection of the actuating part 2. Alternatively to securing the position sensor 4 to the actuating part 2, a gear can be provided to achieve synchronous movement of the position sensor 4 with the actuating part 2 during its rotational adjustment. The position sensor 5 is secured to the carrier 3. Furthermore, an electrically controllable brake 6, 7, 8, designed as an electromagnetic brake, acting between the carrier 3 and the actuating part 2, inhibiting or blocking the rotational adjustment of the actuating part 2, is provided.

[0027] Furthermore, a control unit 15 is provided, which is electrically connected to the position sensor 5 and the electrically controllable brake 6, 7, 8 designed as an electromagnetic brake, for position-dependent control of the electromagnetic brake 6, 7, 8 in order to specify at least one end stop for the rotary adjustment of the actuating part 2 in at least one of the directions of rotation by blocking the electrically controllable brake 6, 7, 8 and / or to generate a detent haptic during the rotary adjustment by inhibiting the rotary adjustment by means of the electrically controllable brake 6, 7, 8. For example, the blocking effect is designed such that at least the manual rotary adjustment is counteracted by a corresponding braking effect, which is perceived as insurmountable at least by the operator. For example, the blocking torque in the end stop position is 5 N / m or more.A detent haptic is understood, for example, as an inhibiting effect that counteracts the rotary adjustment, increasing and then decreasing with further rotary adjustment after reaching a maximum. In a preferred embodiment, the associated maxima of the inhibiting effect are distributed evenly across the possible adjustment range of the actuating part 2.

[0028] The electrically controllable brake 6, 7, 8 is an electromagnetic brake comprising an exciter 7, 8 having a coil 8 and a magnetic core 7, and a brake disc 6 that magnetically interacts with the exciter 7, 8. The brake disc 6 and the exciter 7, 8 are rotated relative to one another during rotational adjustment. The actuating part 2 defines a hollow volume in which the electrical exciter 7, 8 and the brake disc 6 of the electrically controllable brake 6, 7, 8 are arranged.

[0029] In the embodiments shown, the magnetic core 7 and the brake disc 6 are rotationally symmetrical. The magnetic core 7 comprises an inner body portion that extends through the coil 8, as well as a cup-shaped outer body portion located radially outside the coil. For example, the magnetic core 7 is "E"-shaped in a section along the rotational axis D, as shown in FIG. Fig. 2 shows.

[0030] The brake disc 6 is rigidly fixed to the actuating part 2 and the actuating part 2 is supported by a yielding, preferably elastically yielding abutment 10 via the magnetic core 7 on the carrier in order to enable a relative tilting between the brake disc 6 and the magnetic core 7 following an actuating force F. The abutment 10 made of an elastomer or rubber is arranged between the magnetic core 7 and the carrier 3. A shaft 12, which is connected to the brake disc in a rotationally fixed manner and is designed as a bearing journal, projects from the brake disc 6 and is provided with play to enable a Fig. 3 engages into a corresponding recess in the magnetic core 7 formed as a blind hole.

[0031] As further stated in Fig. 2, between the brake disc 6 and the magnetic core 7, at least during blocking or inhibiting, there is a maximum possible contact surface, briefly referred to as maximum contact surface A maxThe magnetic core 7 and the brake disc 6 do not necessarily have to be directly adjacent to one another. Thus, in the embodiments shown in the figures, a thrust washer 11 is provided between the brake disc 6 and the magnetic core 7 to improve the sliding friction between these two, in particular in the event that there is no working current at the exciter 7, 8 and no braking effect, wherein the brake disc 6 and the magnetic core 7 are exclusively adjacent to one another via the thrust washer 11 and, for example, the common contact surface of the thrust washer 11 and the magnetic core 7 during braking and inhibiting of the electrically controllable brake 6, 7, 8 of the maximum contact surface A max Consequently, the maximum contact area A max arranged rotationally symmetrically and concentrically to the axis of rotation D.

[0032] As in the Fig. 3, the electrically controllable brake 6, 7, 8 and the actuating part 2 are mounted in such a yielding, preferably elastically yielding manner that, when an actuating force F acts orthogonally to the axis of rotation, the exciter 7, 8 and the brake disc 8 can be tilted relative to one another following the actuating force, providing a relative minimum tilt 14, so that the annular maximum contact surface A max on a, here almost point-like minimal contact surface A min is reduced by the relative tilting. The minimum contact area A min and the maximum contact area A maxThe specified minimum tilt 14 has a tilt angle α of 1° or less, and a minimal, V-shaped gap opens away from the operator between the brake disc 6 and the magnetic core 7, at least upon actuation. Here, the gap has a maximum clearance between the magnetic core and the brake disc of no more than 0.2 mm, preferably no more than 0.1 mm.

[0033] The control unit 15 is designed to select such a working current for energizing the exciter 7, 8 for the haptic and / or acoustic amplification of the end stop or the latching haptics that, contrary to the effect of the actuating force F, the maximum contact surface A max is (re-)established. For example, the time and / or course of the current supply is selected such that the maximum contact area A maxsimultaneously with reaching the end position or reaching one of the maximums of the inhibiting effect of the locking haptics. When the exciter 7, 8 of the electrically controllable brake 6, 7, 8, designed as an electromagnetic brake, is activated, the aforementioned gap closes, resulting in a minimal movement but significant acceleration of the actuating part 2, in particular its user interface 13. The operator perceives this acceleration as haptically equivalent to the hard engagement of a mechanical detent spring in a detent gate.

[0034] Thus, reversing the minimal tilt 14 caused by the actuating force F results in a pronounced click, i.e. a pronounced acoustically or haptically perceptible impulse on the actuating part 2, for example when the position-dependent maximum braking effect is reached, such as the maximum effective inhibition, or when the end stop is reached with the brake blocked. This additional impulse, which acoustically or haptically underlines the haptic effect, is possible without the provision of an additional electrically controllable actuator. Thus, in addition to the Ruffini corpuscles (SA-II receptors), which primarily act on stretching, the so-called Vater-Pacini corpuscles (PC receptors) are also stimulated. "Haptic amplification" here means a brief acceleration of the operating surface 13 of the actuating part 2. Furthermore, an improved haptic sensation can also be generated during slow rotary movements.

[0035] Fig. Figure 4 shows a second embodiment, which differs in the rotatable mounting of the actuating part 2. In contrast to the first embodiment, the shaft 12 extends through the magnetic core 7, which in turn forms a clearance-dependent opening allowing the relative tilting of the magnetic core 7 and the brake disc 6.

[0036] In an embodiment not shown, the electrically controllable brake 6, 7, 8 has a magnetorheological fluid arranged between the magnetic core 7 and the brake disc 6. A magnetorheological fluid is a suspension of magnetically polarizable particles (carbonyl iron powder) finely dispersed in a carrier fluid. This fluid prevents dirt from penetrating the space between the magnetic core 7 and the brake disc 6 and also reduces an annoying clicking noise when the brake disc 6 and the magnetic core 7 come into contact during the activation of the electrically controllable brake 6, 7, 8. Due to the magnetic properties of this fluid, the braking effect is only slightly changed during braking and locking.

Claims

[1] Control element (1), comprising: a carrier (3); an actuating part (2) mounted on the support (3) so as to be rotatable about a rotation axis (D) for carrying out a rotational adjustment by an operator; a position sensor (4) fixed to the actuating part (2) and / or moving synchronously with the actuating part (2) during the rotary adjustment; a position sensor (5) fixed to the carrier (3) and cooperating with the position sensor (4); an electrically controllable brake (6, 7, 8) acting between the carrier (3) and the actuating part (2) and inhibiting or blocking the rotational adjustment of the actuating part (2); a control unit (15) electrically connected to the position sensor (5) and the electrically controllable brake (6, 7, 8) for the position-dependent energization of the electrically controllable brake (6, 7, 8) in order to specify at least one end stop for the rotary adjustment of the actuating part (2) in at least one of the directions of rotation by position-dependent blocking of the electrically controllable brake (6, 7, 8) and / or to generate a latching haptic during the rotary adjustment by position-dependent inhibiting of the rotary adjustment by means of the electrically controllable brake (6, 7, 8); wherein the electrically controllable brake (6, 7, 8) is an electromagnetic brake comprising an exciter (6, 7) having a coil (8) and a magnetic core (7) and a brake disc (6) magnetically cooperating with the exciter (7, 8), wherein the brake disc (6) and the exciter (6, 7) are rotated relative to one another during the rotational adjustment and a maximum contact surface (A max ) is trained and; wherein the electrically controllable brake (6, 7, 8) and / or the actuating part (2) are mounted in such a yielding manner that, when an actuating force (F) acts orthogonally on the axis of rotation (D), the exciter (6, 7) and the brake disc (6) can be tilted relative to one another following the actuating force (F) while providing a relative minimum tilt (14), so that the maximum contact surface (A max ) to a minimum contact area (A min) is reduced; and the control unit (15) is designed to select, for the haptic and / or acoustic amplification of the end stop or the latching haptics, such a working current for energizing the exciter (6, 7) that, counter to the effect of the actuating force (F), the maximum contact surface (A max ) is produced. [2] Operating element (1) according to the preceding claim, wherein the brake disc (8) is rigidly fixed to the actuating part (2) and the actuating part (2) is supported by a yielding abutment (10) on the carrier (3) in order to enable the relative tilting between the brake disc (6) and the magnetic core (7) following the actuating force (F). [3] Operating element (1) according to one of the preceding claims, wherein a shaft (12) connected in a rotationally fixed manner to the carrier (3) or in a rotationally fixed manner to the brake disc (6) projects from the brake disc (6), which shaft engages in a corresponding recess of the magnetic core (7) with play for minimal tilting (14). [4] Operating element (1) according to one of the preceding claims, wherein at least during the inhibiting or blocking the maximum contact surface (A max ) is formed by a thrust washer (11) arranged between the brake disc (6) and the magnetic core (7). [5] Operating element (1) according to one of the preceding claims, wherein the maximum contact surface (A max ) is rotationally symmetrical about the axis of rotation (D). [6] Control element (1) according to one of the preceding claims, wherein the electrically controllable brake (6, 7, 8) has a magnetorheological fluid arranged between the exciter (7, 8) and the brake disc (6). [7] Operating element (1) according to one of the preceding claims, wherein the actuating part (2) defines a hollow volume in which the electrical exciter (7, 8) and the brake disc (6) of the electrically controllable brake (6, 7, 8) are arranged. [8] Operating element (1) according to one of the preceding claims, wherein the contact surface defined by the minimum contact surface (A min ) and the maximum contact area (A max ) predetermined minimum tilt (14) corresponds to a tilt angle (α) of 1° or less. [9] Operating element (1) according to one of the preceding claims, wherein the actuating part (2) is designed as a thumbwheel. [10] Use of the control element (1) according to one of the preceding claims in a motor vehicle. [11] Method for haptic and / or acoustic confirmation of an operating input with the following steps: Providing an operating element (1) according to one of the preceding claims; position-dependent energization of the exciter (7, 8) by the control unit (15), wherein for the haptic and / or acoustic amplification of the end stop or the latching haptics, the control unit (15) selects such a working current for energizing the exciter (7, 8) that, counter to the effect of the actuating force (F), a maximum contact surface (A max ) between the brake disc (6) and the magnetic core (7).

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