Brakes – Wheel – Axially movable brake element or housing therefor
Reexamination Certificate
1998-08-25
2001-11-20
Oberleitner, Robert J. (Department: 3613)
Brakes
Wheel
Axially movable brake element or housing therefor
C188S072200, C188S07000R, C188S368000, C188S156000, C188S162000
Reexamination Certificate
active
06318513
ABSTRACT:
The present invention relates to an electromechanical brake, in particular for vehicles, having an electric actuator which generates an actuation force and acts on at least one frictional element so as to press the latter, in order to bring about a frictional force, against a rotatable component of the brake which is to be braked.
Electromechanical brakes are known per se. DE 195 43 098 C2 describes a vehicle brake which is designed as a disc brake, can be actuated electrically and whose brake linings can be pressed against the brake disc with the aid of an electric motor. The electric motor transmits its actuation force, via a so-called planetary rolling-contact threaded spindle, onto an axially displaceably mounted piston which interacts with the brake lining.
WO 96/03301 discloses a further vehicle brake which is also designed as a disc brake, can be actuated electrically and whose brake linings can in turn be pressed against the brake disc by means of an electric motor serving as an actuator. The electric motor comprises a spindle gear mechanism and, by means of a spindle element which can be of different designs, is connected, in the direction of displacement of the brake linings, to an axially displaceable piston which acts on a brake lining. In this publication, there is optional provision for the use of an additional gear mechanism for converting the torque and rotational speed.
A major problem with conventional brakes with an electric actuator is the high actuator force which has to be applied in order to achieve a sufficient braking effect. The necessary, high actuator force and the resulting large power demand of the actuator make it necessary to use as drive source very large drive units, usually electric motors, which have large torques, and are also heavy and expensive. The result of this is that electromechanical brakes have, to date, not become widespread as vehicle brakes, for example.
The invention is based on the object of making available an electromechanical brake whose electric actuator has to apply an actuation force which is considerably smaller than that of conventional brakes of this design, so that the brake according to the invention is suitable, for example, for use in motor vehicles or else in railbound vehicles. Moreover, the brake according to the invention is intended to have a very good control response, a high dynamic performance, in particular, being important in this context.
This object is achieved, according to the invention, on the basis of an electromechanical brake of the type mentioned at the beginning and by virtue of the fact that between the component to be braked and the electric actuator there is an arrangement which brings about self-energization of the actuation force generated by the electric actuator, and in that there is a device for comparing a setpoint value of a frictional force with the actual value of the frictional force, which device, in the event of a deviation of the actual value from the setpoint value, drives the electric actuator to correspondingly increase or reduce the generated actuation force, and thus approximates the actual value to the setpoint value of the frictional force.
The self-energization arrangement which is used according to the invention and which increases the actuation force generated by the electric actuator in a purely mechanical way without the introduction of auxiliary forces, the proportion of the actuated force which is necessary to bring about a desired frictional force and which is 100% in conventional electromechanical brakes is drastically reduced. The greatest part of the necessary force is, according to the invention, applied by the self-energization arrangement, so that it is, for example, readily possible to compress the actuation force to be generated by the actuator during normal operation of the brake to approximately 2% of the previously customary value. The power demand of a brake according to the invention drops to the same degree. It is thus possible to use significantly smaller, and thus more light-weight and also cheaper electric motors as actuator, which additionally have the advantage of enhanced dynamics.
So-called self-energizing brakes are known from the prior art, in particular from the field of drum brakes for motor vehicles. Self-energizing brakes have, however, the disadvantage that their coefficient of friction increases disproportionately as the actuator force increases. Since, in conventional hydraulic brake systems, the distribution of the brake force at the individual wheel brake cylinders is determined by the pressure in the brake lines and the area of the hydraulic pistons, the different coefficients of friction which are present in reality between the friction linings of the brake and the brake disc or brake drum to be braked inevitably lead, when self-energizing brakes are used, to great differences in the braking forces at the individual wheels of a vehicle. The driver of the vehicle becomes aware of this because his vehicle pulls into a skewed alignment during braking, i.e. it changes its direction of travel in an undesired way. In particular on a slippery road surface this can lead to the respective vehicle skidding. Because of these disadvantages which are associated with them, self-energizing brakes have therefore no longer been used as a service brake in the field of motor vehicles for some time.
The present invention overcomes the prejudice of the specialist field, which has become established in the course of time, and makes use of the fact that, owing to their multi-element, relatively complex drive train, which comprises an engine and downstream transmission elements, including the associated bearings as well as possibly a gear mechanism in addition, electromechanical brakes generally have a sensor system for measuring force or torque and/or position in order to produce a desired distribution of braking force. This sensor system, which is already present, can, according to the invention be used, without modifications, to carry out the already mentioned setpoint/actual value comparison with respect to the frictional force, so that, when a corresponding difference is detected, for example by an electronic control unit which is coupled to the sensor system, the electric actuator can be driven in order to approximate the actual value of the frictional force to its setpoint value. The actual value of the frictional force can be determined here in a known manner by means of direct or indirect measurement.
Owing to its dynamic performance, which is significantly improved in comparison with conventional electromechanical brakes and which results from the considerably lower actuator forces which have to be applied, the electromechanical brake according to the invention makes it possible to exploit the advantage of a self-energizing arrangement without the previously associated disadvantage, by virtue of the fact that fluctuations in frictional force between a frictional element and the component of the brake which is to be braked are compensated for with such speed that, for example, they no longer adversely affect the driving performance of a motor vehicle.
In preferred embodiments of the brake according to the invention, the self-energizing arrangement has at least one wedge with an angle of inclination &agr; which is supported on an associated abutment in a sliding or rolling fashion. According to one embodiment, the angle of inclination &agr; is defined in such a way that an input force which is applied to the self-energizing arrangement by means of the electric actuator is, with respect to the customarily prevailing direction of rotation of the component to be braked, always positive irrespective of a changing coefficient of friction between the frictional element and the component to be braked. In this context, mention is also made of a pressure wedge because the dimension of the self-energization is dimensioned by the selection of the angle of inclination &agr; only to be of such a size that, irrespective of a changing coefficient of friction &
Dietrich Johannes
Gombert Bernd
Grebenstein Markus
Deutsches Zentrum fur Luft - und Raumfahrt E.V.
Kramer Devon
Oberleitner Robert J.
Wood Herron & Evans
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