Abstract:
The invention relates to a brake arrangement for a land vehicle comprising the following characteristics: an electric motor with a downstream connected (reduction) gear which acts on the friction pads of a brake arrangement, with the gear being designed self-locking, and an actuation mechanism by means of which the self-locking function can be cancelled or established.

Description:
This application is a continuation of PCT/EP98/06850, filed Oct. 29, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a brake arrangement for a land vehicle. In particular, the present invention relates to a brake arrangement for a land vehicle, in which the brake arrangement can be actuated electrically. In such brake arrangements which are also referred to as “brake-by-wire” arrangements, there exists the problem of performing the release and application movements of the friction elements with high dynamics relative to the brake disk or the brake drum. This applies in particular during the driving operation of the land vehicle. In addition, high application forces are to be exerted in order to ensure a substantial deceleration of the land vehicle. 
     Moreover, there is the necessity to provide a parking brake function during standstill of the land vehicle, which prevents the vehicle from automatically starting to move on slopes. 
     Between the service brake function and the parking brake function exists the contradictory requirement that a parking brake must be self-locking while a service brake must not be self-locking. For this reason, the parking brake function in brake-by-wire arrangements has up to now been realised separately from the service brake function. Among others, this requires considerable installation space and is cost-intensive. 
     SUMMARY OF THE INVENTION 
     The present invention is based on the object to provide a brake arrangement in which the parking brake function and the service brake function are integrated in an actuator unit for the friction elements, which simultaneously ensures that the service brake can by no means come into the self-locking condition. 
     This object is solved by the vehicle brake described in the following. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an actuator unit of a vehicle brake system in accordance with the present invention. 
     FIG. 1 shows an actuator unit of a vehicle brake system, which acts upon the brake disk  10  of a wheel  12  (not to scale) non-rotatably connected with the brake disk via a shaft  14 . The brake disk is accommodated between two friction pads  16   a ,  16   b.   
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The release or application movement, respectively, of the friction pads  16   a ,  16   b  relative to the brake disk  10  is effected via a spindle ( 18 )/nut ( 20 ) arrangement. Depending on the sense of rotation of an electric motor  24 , the spindle  18  is brought into an application or release rotation by said electric motor  24  via a gear arrangement  22  described in detail below. The control of the electric motor is effected in a manner not explained in detail as a function of an electric control signal which depends on the pedal position of the brake pedal of the land vehicle or on an actuation of a parking brake lever, respectively. 
     In a preferred manner, the gear arrangement  22  which is designed as a reduction gear has an overall reduction of 200:1 and is a two-stage design. This results in a compact actuator unit, wherein primarily the electric motor  24  can be selected to be of a small size and nevertheless be able to exert a sufficiently high clamping force. For this purpose, via a first reduction stage of 4:1 designed as a toothed belt gear  35 , the electric motor  24  drives a swash plate mechanism  36  which provides a second reduction stage of 50:1. The swash plate mechanism  36  comprises a swash ring  37  which is non-rotatably accommodated in an outer supporting ring  38  and abuts the drive shaft  39  at a chamfered end so that the rotational movement of the drive shaft  39  brings the swash ring  37  into a wobble motion. The swash ring  37  comprises a spur toothing  39  which is in engagement with the spur toothing of an output ring  41 , with the output ring  41  being non-positively connected with the spindle  18  acting as an output shaft. With the outer supporting ring  38  being stationary, the wobble motion of the swash ring  37  causes the consecutive other teeth of the spur toothing  39  of the swash ring  37  to come into engagement with the spur toothing  40  of the output ring  41 . Due to the fact that the number of teeth Z 1  of the spur toothing of the output ring  41  is selected lower than the number of teeth Z 2  of the swash ring  37 , a relative rotational movement of the output ring  41  with respect to the stationary supporting ring  38  results, which has an opposite sense of rotation to that of the drive shaft  39 . Thus, a rotation of the output ring  41  by the difference in the teeth numbers of the spur toothing of the swash ring  37  and the output ring  41  results. If, for example, the output ring  41  has the tooth number Z 1 =98 and the swash ring  37  has the tooth number Z 2 =100, the reduction i of 50:1 resulting therefrom is obtained according to the formula i=Z 2 /(Z 1 −Z 2 ). 
     For the second reduction stage, either a gear mechanism known as “Cyclo gear mechanism” or a gear mechanism known as “Harmonic drive system” which, among others, is known from DE 296 14 738 U1 can be employed in lieu of the swash plate mechanism. This gear type, too, comprises a stationary supporting ring, the input and output senses of rotation are opposite to each other as well, with the reduction being determined from the difference in the numbers of teeth or circumferential length, respectively, of a non-rotable versus a rotatable ring. 
     “Cyclo gear mechanisms” are characterised by their extremely high torque capacity, a very high efficiency, high possible gear ratios and up to a five-fold overload capability which has a favourable effect for overcoming a possibly required break-away torque at the spindle/nut arrangement. “Cyclo gear mechanisms” are eccentric gear mechanisms whose gear outer profile describes a cycloid characteristic. A disk is driven via an eccentric element and rolls around in a ring. If the disk is provided with a closed cycloid characteristic and the ring replaced by pins arranged in a circle, a positive connection is achieved thereby. A “cyclo gear mechanism” has three moving main components, the drive shaft with the eccentric element, the cam disks, and the output shaft. The double eccentric element rotates with drive speed and drives two cam disks which are offset to each other by 180° via roller bearings. Thus, the cam disks simultaneously rotate about two different axes. Rotation with reduced speed in the opposite direction is generated. With a full revolution of the eccentric element, each cam disk revolves by a cam section. Generally, the cam disk has one tooth less than the pins provided in the pin ring. In these cases, the gear ratio is determined by the number of cam sections of a cam disk. For the transmission of the reduced rotation movement to the output shaft, the cam disks are provided with holes arranged in a circle. The output shaft has a coaxial driver disk on which driver pins arranged in a circle are located which engage into the corresponding holes in the cam disk. The driver pins and the outer pins are fitted with rollers which provide for a purely rolling force transmission between the cam disks and the driver pins of the output shaft. 
     Both a swash plate mechanism and a harmonic drive mechanism enable reduction ratios of up to 320:1. Therefore, the first reduction stage can be omitted for achieving a reduction of 200:1. For example, a serial arrangement of electric motor and gear mechanism would then be preferred instead of the parallel arrangement of electric motor and gear mechanism, with the electric motor directly driving gear drive shaft. 
     It is essential for a swash plate mechanism and for the the gear mechanism known from DE 296 14 738 U1 that the gear mechanism in reverse rotation is self-locking, which means no force counteracting the drive, however high it may be, is able to rotate the drive shaft in the reverse direction. It also means, however, that the drive shaft can be changed in the one or the other direction. 
     Due to the fact that the spindle/nut arrangement  18 / 20  is designed self-locking, the position obtained upon clamping of the spindle/nut arrangement  18 / 20  is maintained even after switching off the electric motor  24 . 
     As described above, the supporting ring  38  is non-rotatable in the rest position of the arrangement. By means of the configuration described below it is possible to achieve a reverse rotation and thus a release movement of the friction pads  16   a ,  16   b  (via the spindle/nut arrangement  18 ,  20 ), even with the swash plate mechanism virtually at standstill. 
     At a bearing journal  50  a pinion is arranged rotatably and also axially slidably in the direction of arrow P, which meshes with an outer toothing of the supporting ring  38 . This pinion  52  is rigidly connected with a tension rod  54 . The pinion  52  is biased in the opposite direction of arrow P by means of a coil spring  56  against a wall  58  which is wound about the tension bar  54 . Moreover, a helical spring  60  is wound about the tension bar  54 , which acts as torsion spring accumulator. 
     At the bottom of the bearing journal  50  a saw toothing is formed so that upon an axial movement of the tension bar  54  in the direction of arrow P, the charged spring accumulator  60  can bring the pinion  52  into rotation in the sense of rotation D. In the engaged condition of the pinion  52  (loaded by the coil spring  56 ), the saw toothing causes the pinion  52  to be not rotatable. In the normal case, the supporting ring  38  is therefore non-rotatable. 
     The brake arrangement functions as follows: 
     Upon supplying the electric motor  24  with current, the spindle/nut arrangement  18 ,  20  is brought into rotation via the gear arrangement so that the friction pads  16   a ,  16   b  perform an application movement towards the brake disk  10 . 
     In the opposite current supply of the electric motor  24 , the spindle/nut arrangement  18 ,  20  is brought into a reverse rotation via the gear arrangement so that the friction pads  16   a ,  16   b  perform a release movement away from the brake disk  10 . 
     If the tension bar  54  is operated in the direction of arrow P, the pinion  52  clears the saw toothing at the bottom of the bearing journal, so that the tension bar  54  is brought into rotation by means of the charged spring accumulator  60 . The consequence of this is that the pinion  52  which is rigidly connected with the tension bar  54  brings the supporting ring  38  into rotation. This causes a rotational movement of the spindle  18  so that the nut  20  moves the friction pads  16   a ,  16   b  away from the brake disk  10 . The last described process takes place in the case of a release of the parking brake or upon an emergency actuation. Thus, the parking brake function can be released even without electrical actuation. 
     For charging the spring accumulator  60  the tension bar  54  (e.g. electromagnetically operated) is moved in the direction of arrow P, and simultaneously the electric motor  24  is activated. In this manner, the supporting ring  38  starts to rotate and thereby drives the pinion in the opposite sense of rotation, whereby the spindle/nut arrangement reaches a stop, for example, in that the friction pads come into contact with the brake disk. The consequence of this is that the torque applied by the electric motor is utilised to tighten the spring accumulator  60  in the sense of an energy storage.