Wheel brake for a motor vehicle and method for controlling a wheel brake

Abstract

The invention relates to a wheel brake (RB1) for a motor vehicle, comprising at least one brake body (1a) having a friction surface (1b) which is movable relative to a brake pad (3), wherein the brake pad (3) can be set in a feed movement (Z) in the direction of the friction surface (1b) via an actuator and the actuator has at least one brake piston (6) which can be set in a translational movement for the purpose of the feed movement (Z), and wherein at least one piezo element is additionally arranged in the brake piston (6), which can be enlarged by applying a voltage in the direction of the feed movement (Z). According to the invention, it is proposed that at least two piezo elements are present and arranged in such a way that, when voltage is applied, they can exert a force on a holding device (4) for the brake pad (3) at at least two different locations.

Description

[0001] The invention relates to a wheel brake for a motor vehicle having the features of the preamble of patent claim 1. The invention also relates to a method for controlling a wheel brake having the features of the preamble of patent claim 5.

[0002] As is well known, the purpose of a wheel brake is to convert a clamping force into a braking torque on the wheel. In the unbraked state, however, the residual drag torque should be as low as possible. To achieve this, the wheel brake is given a certain amount of clearance, among other things. However, this has the disadvantage of increasing the response time when braking is requested. In hydraulic wheel brakes, this also requires a higher volume of hydraulic fluid to be absorbed.

[0003] Furthermore, a brake disc and brake pads clamped by a brake caliper can fluctuate in their axial dimension due to temperature or other environmental influences. This leads to a change in the clamping force of the wheel brake in the static clamping state. When the clamping force is removed, the desired clearance only builds up slowly, as the brake piston, which builds up the tension, must be moved in its guide.

[0004] Braking torques also place asymmetrical loads on the brake pads, causing them to wear at an angle. Lateral forces acting on the vehicle, such as those that occur when cornering, cause the wheel bearings to tilt, and thus also the brake disc when using disc brakes. The deflection of the brake disc pushes the brake pistons back into their guides, leading to an unwanted increase in the brake clearance.

[0005] EP 1 141 573 B1 discloses a method for controlling an electromechanical wheel brake actuator in a motor vehicle. Specifically, the wheel brake actuator comprises a tubular brake piston driven by an electric motor via a self-locking thread pair toward a brake disc. A piezoelectric element is housed in the brake piston. The brake piston presses brake pads against or releases them from the brake disc. The brake piston is moved by the electric motor with an application force below a predetermined value. Once the predetermined value is reached, the brake piston is pressed against the brake pads by the piezoelectric element with an application force above the predetermined value.To implement a parking brake function, the piezo element is subjected to an electrical voltage whose polarity is opposite to that required to generate normal brake application forces during service braking. The shortened piezo element is then pressed against the brake pads by the electric motor and a magnetic coupling. The voltage to the piezo element is then switched off, whereupon the piezo element attempts to expand back to its original length, generating a high brake application force.

[0006] EP 1 319 859 B1 describes a braking device with a floating caliper, in which a brake pad is movable by a drive shaft transversely to the direction of movement of a brake disc. A conversion device serves to convert a rotary motion from a drive shaft into a force acting on the brake pad. Furthermore, a drive ring surrounds the drive shaft, which can be set into a circular displacement movement by several piezo actuators. The drive shaft can be set into a rotary motion by frictional engagement with the drive ring.

[0007] The generic US Patent No. 5,090,518 A describes a wheel brake with a pair of brake actuation and release force units. The release force units feature electric motors with non-reversible mechanical output elements to deliver the required brake actuation force up to the limits of the electric motors' performance. Furthermore, piezoelectric elements, which generate high forces with low expansion during rapid changes in the applied voltage, are arranged in series with the motor output elements to transmit the brake actuation force. These elements are alternately energized and de-energized in opposite phases.The piezoelectric expansion effect of each energized element is mechanically captured by the motor unit with the de-energized element during each excitation cycle, so that the forces actually applied to actuate the brake are increased far beyond the maximum power of the electric motors. This is achieved by the alternating excitation of the piezoelectric elements and the alternating follow-on actions of the motors. The non-repulsive assemblies serve to store the resulting mechanical force increases.

[0008] DE 198 58 764 A1 discloses a braking system having an actuating device controlled by a brake pedal via electrical lines. The actuating device is designed as an electromechanical wheel brake actuator mounted on a brake caliper of each wheel. The wheel brake actuator contains a brake piston driven by an electric motor toward a brake disc, which presses two brake pads against the brake disc or releases them from it. A piezoelectric element is connected to the brake piston. The piston is moved by the electric motor with an application force below a predetermined value. Once the predetermined value is reached, the piezoelectric element presses the piston against the brake pads with an application force above this value.

[0009] DE 10 2011 013 153 A1 describes an anti-lock braking system with a brake caliper. A brake piston is driven by hydraulic pressure from a master brake cylinder. Furthermore, a brake disc is present, which can be brought into frictional contact with brake pads. A piezo actuator consisting of piezo elements is arranged on the back of each brake pad. The piezo actuator is arranged on one side between the brake pad and the brake piston, and on the other side between the brake pad and an arm of the brake caliper.

[0010] Finally, EP 1 386 092 B1 discloses a disc brake with an electric motor-driven wear adjustment system. Specifically, the disc brake comprises a brake caliper that spans a brake disc and has an application device arranged in the caliper for applying a brake pad. Furthermore, an adjustment system arranged in the brake caliper compensates for brake pad and / or disc wear by adjusting the distance between the brake pad and the brake disc. The adjustment system comprises an adjuster rotation device actuated by an electric motor drive. In addition to the electric motor drive, the adjustment system comprises an electromechanical release device designed as a piezo system that assists in releasing the adjuster rotation device.

[0011] The present invention is based on the object of proposing a wheel brake for a motor vehicle that enables rapid bridging of a clearance during braking, while simultaneously ensuring even wear of the brake pad. In particular, the forces of the piezo elements used should be able to be precisely adapted to the prevailing load case.

[0012] This object is achieved by a wheel brake having the features of patent claim 1. Advantageous embodiments or further developments of the invention can be found in the respective dependent claims.

[0013] The invention relates to a wheel brake for a motor vehicle, comprising at least one brake body having a friction surface that is movable relative to a brake pad. The brake pad can be set in a feed motion toward the friction surface via an actuator, wherein the actuator has at least one brake piston that can be set in a translational motion for the purpose of the feed motion. At least one piezo element is also arranged in the brake piston, which can be enlarged by applying a voltage in the direction of the feed motion.

[0014] It is also proposed that at least two piezo elements are present and arranged in such a way that, when voltage is applied, they can exert a force on a holding device for the brake pad at at least two different locations.

[0015] With the help of the piezo elements, the brake pad's feed movement toward the friction surface can be supported and accelerated. This allows for very rapid bridging of any existing clearance when a braking action is detected. Because the piezo elements can exert pressure on a brake pad retaining device at at least two points, the force exerted on the brake pad is also made possible. In other words, the arrangement of the piezo elements at two different locations creates the basic prerequisite for regulating the forces at different points on the brake pad.

[0016] Furthermore, tilting of the brake body, particularly of a brake disc, during operation of a motor vehicle, which occurs primarily when cornering, or deformation of the brake caliper due to braking torques, leads to asymmetrical loading of the brake pads. This normally leads to undesirable angular wear of the brake pads. The inventive use of the piezo elements, however, enables a uniformly distributed, symmetrical pressure buildup on the brake pads, thus counteracting angular wear. By appropriately controlling the piezo elements, the forces acting on the brake pad can be evened out.

[0017] According to the invention, at least three piezo elements are provided and arranged in such a way that they can exert a force on the brake pad retaining device at three different locations. This contributes to the fact that the forces of the piezo elements can be adjusted even more precisely to the respective load case.

[0018] In a first embodiment of the invention, it is conceivable that the wheel brake is a disc brake, with the three different locations distributed in a triangular pattern. Thus, by appropriately distributing the piezo elements, precise control of the forces acting on the brake pad can be achieved.

[0019] Due to the aforementioned arrangement of the piezo elements, the magnitude of the voltage distribution and any differences that occur can be evaluated after the voltages across the piezo elements have been recorded using a suitable measuring, evaluation, and control device. Depending on this, the voltage differences can then be compensated for by applying a suitable voltage to the piezo elements. For example, it is conceivable to apply more voltage to a radially inner piezo element than to a radially outer piezo element. During braking, the deformation, in particular the widening, of the brake caliper leads to the brake pad being in tighter contact with the brake disc in the radially outer area than in the radially inner area. The increased voltage applied to the radially inner piezo element can thus counteract the oblique loading of the brake pad.

[0020] According to another refinement, the piezo elements are firmly connected to the brake piston and the brake pad retaining device. This allows the piezo elements to quickly restore clearance after a braking operation. To do this, a previously applied voltage is removed or even an opposite voltage is applied to the piezo elements. The piezo elements thus shrink in size, counter to a previous advance movement, causing the piezo elements to actively retract the brake pad retaining device and thus the brake pad.

[0021] A further embodiment of the inventive concept proposes that several piezo elements are arranged one above the other to form a series stack of piezo elements, with at least two series stacks being arranged next to each other, forming a series-parallel stack. In other words, in a series stack, several piezo elements are connected in series. In a series-parallel stack, at least two series stacks are connected in parallel.

[0022] In this way, the desired effect of piezo elements can be multiplied. On the one hand, connecting the piezo elements in series increases the distances that can be bridged, and on the other hand, connecting them in parallel increases the forces that can be generated.

[0023] As mentioned at the beginning, the present invention also proposes a method for controlling a wheel brake.

[0024] This is based on a method for controlling a wheel brake in a motor vehicle according to one of patent claims 1 to 5. Upon detection of an existing or expected braking action, a voltage is applied to at least one piezo element. This occurs in such a way that the piezo element enlarges and presses against a retaining device for a brake pad. In this way, any existing clearance between the brake pad and a friction surface of a brake body can be quickly bridged.

[0025] An existing braking action can be detected, in particular, by a sensor-detected actuation of the brake pedal. An expected braking action can be detected, in particular, by signals from driver assistance systems. Possible examples include signals from an ACC or ESC system, or even predictive systems in dangerous situations.

[0026] In a further development of the method, after the brake pad has been applied to the friction surface, a voltage is detected which is emitted by the piezo element, whereby the voltage is used as the basis for further control of the piezo element.

[0027] Such a procedure creates the basic prerequisite for suitable control strategies when operating the wheel brake.

[0028] According to a further development, the voltage of at least two piezo elements arranged at different locations is detected. Depending on this, the piezo elements are controlled and voltage is applied.

[0029] For example, it is conceivable to measure the forces in each row stack of piezo elements, allowing the determination of a total clamping force. From this, a force center point can be determined relative to the surface area of ​​the brake pad. If the determined actual force center point deviates from a target force center point, the actual force center point can be adjusted back to the target point by appropriately adjusting the voltage applied to the row stack of piezo elements, thus achieving the desired, symmetrical force applied to a brake pad.

[0030] Furthermore, an unwanted reduction in the clearance, which occurs, for example, when cornering, can be detected by measuring the voltage generated by the piezo elements and compensated for by appropriate control strategies, so that the clearance can be quickly restored, for example, when driving straight ahead.

[0031] A further highly expedient embodiment of the invention proposes that the voltage of at least three piezo elements arranged at different locations is detected and the piezo elements are controlled as a function thereof.

[0032] This enables both a very precise evaluation of the forces acting on a disc brake pad and a very flexible adjustment of the center of force acting on the brake pad. In particular, the triangular arrangement of the piezo elements allows for force to be applied to the brake pad around two perpendicular axes running parallel to the plane of the brake pad. This enables very effective counter-control of brake pad tilting, which can occur during cornering.

[0033] According to another embodiment, the voltage output by at least two piezo elements arranged at different locations is detected, and the detected voltages are compared with each other. If the piezo elements have different voltages, at least one of the piezo elements is controlled in such a way that the output voltages of the piezo elements are equal again. In other words, this can equalize the pressures acting on a brake pad, preventing angular wear of the brake pad.

[0034] Preferred embodiments of the invention are illustrated in the figures and are explained in more detail in the following description with reference to the figures. This also makes further features and advantages of the invention clear. The same reference symbols, even in different figures, refer to the same, comparable, or functionally identical components. Corresponding or comparable properties and advantages are achieved even if a repeated description or reference to them is not made. The figures are not, or at least not always, to scale. In some figures, proportions or distances may be exaggerated in order to emphasize features of an embodiment more clearly.

[0035] They show, schematically Fig. 1 a motor vehicle with wheel brakes according to the invention, Fig. 2a a wheel brake according to section view II from Fig. 1, rotated by 90°, in a first embodiment and according to section Ila from Fig. 3, Fig. 2b a wheel brake according to section view II from Fig. 1, rotated by 90°, in a second embodiment, Fig. 3 a side view of the wheel brake according to view III of Fig. 2a, where the view of the piezo elements is clear, Fig. 4 a view of a wheel brake not covered by the invention, Fig. 5 a view according to view V from Fig. 4, Fig. 6 a sectional view of an application of a series-parallel stack of piezo elements in a brake control system with brake booster, Fig. 7a to 7c the arrangement of piezo elements for generating series stacks and series-parallel stacks and Fig. 8 a signal flow diagram to illustrate the operation of the wheel brake.

[0036] In Fig. Figure 1 shows a motor vehicle K equipped with wheel brakes RB1 according to the invention. The wheel brakes RB1 are designed, in particular, as disc brakes with a disc-shaped brake body 1a and a brake caliper 2a extending over the disc.

[0037] From the Fig. 2a shows that the brake caliper 2a of each disc brake RB1 is designed as a floating caliper brake (also known as a floating caliper brake). During braking, a brake pad 3 mounted on a plate-like brake pad holder 4 is pressed against a friction surface 1b of the brake body 1a. Due to the resulting reaction force, the floating brake caliper 2a moves to the right in the figure, which also presses the brake pad 3 located on the opposite side against the friction surface 1b of the brake body 1a. In the unbraked state, a clearance is formed between the brake pads 3 and the friction surfaces 1b, which clearance is referred to as the air gap LÜ.

[0038] It is based on the Fig. 2a and Fig. 3 shows that the Fig. 2a brake pad holder 4 arranged on the right side of the brake body 1 with three series-parallel stacks 5RPa, 5RPb and 5RPc of piezo elements 5 (cf. Fig. 7c). The series parallel stacks 5RPa, 5RPb and 5RPc are in particular distributed in a triangular shape to the brake pad holder 4, arranged within a brake piston 6 and connected to it. In other words, the series parallel stacks 5RPa, 5RPb and 5RPc form a force application point at three points S1, S2 and S3 with the brake pad holder 4 and the brake piston 6, wherein the three points S1, S2 and S3 are arranged in a triangular shape distributed on the brake pad holder 4 and also the brake piston 6. With the same force from the series parallel stacks 5RPa, 5RPb and 5RPc on the brake pad holder 4, a resulting force center of gravity KS acting on the brake pad holder 4 is established, which approximately corresponds to the area center of gravity of the brake pad holder 4.

[0039] Due to the triangular arrangement of the series-parallel stacks 5RPa, 5RPb and 5RPc, the brake pad holder 4 and thus the brake pad 3 can be bent about two axes a, b that are approximately parallel to the brake body 1a and perpendicular to each other.

[0040] For example, if the parallel stack 5RPa on one side and the parallel stacks 5RPb, 5RPc on the other side are controlled so that they undergo a size change, the brake pad holder 4 can be bent around the axis b. If the parallel stacks 5RPb and 5RPc are controlled, the brake pad holder 4 can be bent around the axis a.

[0041] In the Fig. 2b shows an alternative design of a wheel brake RB2. The wheel brake RB2 also has triangularly arranged series-parallel stacks 5RPa, 5RPb and 5RPc. In contrast to the Fig. In this case, 2a is a fixed-caliper brake with a fixed brake caliper 2b. However, the brake pad 3, located on the left side of the brake body 1a in the figure, can also be advanced toward the brake body 1a via parallel stacks 5RPa', 5RPb' (not visible), and 5RPc' engaging the brake pad holder 4.

[0042] The invention is not only applicable to disc brakes. Fig. Figure 4 shows a wheel brake RB3 designed as a drum brake. The wheel brake RB3 has two brake pads 12a, each connected to a brake pad holder 12b.

[0043] The brake pad holders 12b can be pushed apart by brake pistons 14, which are operatively connected to pressure plates 15, and moved against the friction surface 13b of a drum-like brake body 13a. In this process, the brake pad holders 12b are pushed apart against a spring force of spring elements 18.

[0044] In a lower part of the wheel brake RB3 in the figure, the brake pad holders 12b are supported against a reaction block 16. Here, too, the brake pad holders 12b are held together by a spring element 19.

[0045] Furthermore, it can be seen that series-parallel stacks 5RPe of piezo elements 5 are arranged in the brake piston 14 and are connected to it and to the pressure plates 15.

[0046] Furthermore, parallel stacks 5RPf of piezo elements 5 are arranged in the reaction block 16, such that the parallel stacks 5RPf can push the brake pad retainers 12b apart. By simultaneously actuating the upper parallel stacks 5RPe and the lower parallel stacks 5RPf, an existing clearance LÜ can be quickly bridged. Furthermore, this allows for even contact of the brake pads 12a with the friction surface 13b.

[0047] From the Fig. 5 shows that two parallel stacks 5RPe are arranged next to each other in each brake piston 14. This results in two force application points at two adjacent locations S1, S2. Ideally, this results in a resulting force center KS located between locations S1, S2. This counteracts tilting of the brake pads 12a. The same applies to the lower parallel stacks 5RPf.

[0048] Based on Fig. Figure 6 shows an application of a series-parallel stack 5RPd of piezo elements 5 in a brake control system with a brake booster. A piston 9 can be seen, which is movable via a coupling rod 10 and can thus compress brake fluid in a brake line 8a. The brake line 8a leads to brake pistons (not shown in detail) on the wheel brakes.

[0049] Furthermore, a storage chamber 7 is visible, containing a brake fluid reservoir. Arranged in the storage chamber 7 is a series-parallel stack 5RPd of piezo elements 5, which can press against a pressure plate 8. When voltage is applied to the series-parallel stack 5RPd, the pressure plate 8 is displaced toward the brake line 8a, reducing the volume of brake fluid within the storage chamber 7. This allows additional brake fluid to be quickly introduced into the brake line 8a when needed, thus eliminating, for example, existing clearance in the wheel brakes.

[0050] If a brake pedal 11 connected to the coupling rod 10 is actuated, this is detected by a brake pedal sensor 11a. At the same time, the parallel stack 5RPd is activated, and the pressure plate 8 is moved in the manner described above.

[0051] Based on the Fig. 7a to 7c it is shown that a row stack 5R ( Fig. 7b) consists of several piezo elements 5 arranged one behind the other ( Fig. 7a). This allows a bridgeable path change to be multiplied. If at least two row stacks 5R are arranged next to each other, a row-parallel stack 5RP is created (cf. Fig. 7c). This allows for an increase in the force that can be generated.

[0052] Finally, the Fig. 8 illustrates a possible procedure for operating the wheel brakes RB1. Thus, the brake pedal sensor 11a is connected to an evaluation and control device 20 via a data bus C, which may be embodied, for example, as a CAN bus. Likewise, the series-parallel stacks 5RPa, 5RPb, and 5RPc of each wheel brake RB1 are connected to the evaluation and control device 20 via the data bus C.

[0053] If the brake pedal 11 is now actuated by the driver, this is detected by the brake pedal sensor 11a and a corresponding signal is forwarded to the evaluation and control device 20. The evaluation and control device 20 supplies the series-parallel stacks 5RPa, 5RPb and 5RPc with corresponding voltage signals so that they expand and the existing clearance LÜ (cf. Fig. 2a) bridge.

[0054] Conversely, the pressure exerted on the series-parallel stacks 5RPa, 5RPb, and 5RPc generates voltage signals that are transmitted to the evaluation and control device 20. If the voltage signals deviate too much from one another, this indicates an uneven load on the brake pads 3. Therefore, the evaluation and control device 20 sends signals to the series-parallel stacks 5RPa, 5RPb, and 5RPc, which work toward equalizing the load on the brake pads 3. Preferably, a resulting force center KS should be located at the center of gravity of the brake pad holder 4 (see Fig. 3). List of reference symbols 1a brake body 1b Friction surfaces 2a Brake caliper, floating caliper 2b Brake caliper, fixed caliper 3 brake pad 4 brake pad holder 5 Piezo element 5R piezo elements in series, series stack 5RP piezo elements in series and parallel, series-parallel stack 5RPa series-parallel stack 5RPb row-parallel stack 5RPc row-parallel stack 5RPd row-parallel stack 5RPe row-parallel stack 5RPf row-parallel stack 6 brake pistons 7 Pantry 8 printing plate 8a brake line 9 pistons 10 coupling rod 11 Brake pedal 11a Brake pedal sensor 12a brake pads 12b Brake pad holder 13a Brake body, drum 13b Friction surface 14 brake pistons 15 printing plates 16 Reaction block 17 Brake drum 18 spring elements 19 Spring element 20 Evaluation and control device a, b axes C data bus K Motor vehicle KS resulting center of force LÜ clearance RB1 wheel brake, disc brake RB2 wheel brake, disc brake RB3 wheel brake, drum brake S1, S2, S3 force application points Z feed movement

Claims

[1] Wheel brake (RB1, RB2) for a motor vehicle (K), with at least one brake body (1a) which has a friction surface (1b) which is movable relative to a brake pad (3), wherein the brake pad (3) can be set in a feed movement (Z) in the direction of the friction surface (1b) via an actuator and the actuator has at least one brake piston (6) which can be set in a translational movement for the purpose of the feed movement (Z), and wherein in the brake piston (6) at least one piezo element (5) is additionally arranged, which can be enlarged by applying a voltage in the direction of the feed movement (Z), characterized by that at least three piezo elements (5) are present and arranged in such a way that they can exert a force on a holding device (4) for the brake pad (3) when voltage is applied at three different points (S1, S2, S3). [2] Wheel brake (RB1, RB2) according to claim 1, characterized bythat the wheel brake (RB1, RB2) is a disc brake and the three different locations (S1, S2, S3) are distributed in a triangular shape. [3] Wheel brake (RB1, RB2) according to one of the preceding claims, characterized by that the piezo elements (5) are firmly connected to the brake piston (6) and firmly to the holding device (4) for the brake pad (3). [4] Wheel brake (RB1, RB2, RB3) according to one of the preceding claims, characterized by that a plurality of piezo elements (5) are arranged one above the other in such a way that a series stack (5R) of piezo elements (5) is formed, wherein at least two series stacks (5R) are arranged next to one another, so that a series-parallel stack (5RP) is formed. [5] Method for controlling a wheel brake (RB1, RB2, RB3) according to one of the preceding claims in a motor vehicle (K), wherein after detection of an existing or expected braking operation, at least one piezo element (5) is subjected to a voltage such that the piezo element (5) enlarges and presses against a holding device (4) for a brake pad (3). [6] Method according to claim 5, characterized by that after the brake pad (3) has been applied to the friction surface (1b), a voltage is detected which is emitted by the piezo element (5), the voltage being used as the basis for further control of the piezo element (5). [7] Method according to claim 5 or 6, characterized by that the voltage of at least two piezo elements (5) arranged at different locations (S1, S2) is detected and the piezo elements (5) are controlled as a function thereof. [8] Method according to one of claims 5 to 7, characterized bythat the voltage of at least three piezo elements (5) arranged at different locations (S1, S2, S3) is detected and the piezo elements (5) are controlled as a function thereof. [9] Method according to one of the preceding claims 5 to 8, characterized by that the output voltage is detected by at least two piezo elements (5) arranged at different locations (S1, S2), wherein the detected voltages are compared with one another and, if the voltages of the piezo elements are different, at least one of the piezo elements is controlled in such a way that the output voltages of the piezo elements are again the same.

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