Abstract:
The invention relates to a preferably bistable magnetic brake, which is intended in particular for locking an actuating device of an electromechanical wheel braking device in its braking position at a given time. To enable releasing the magnetic brake even in the event of a defect, the invention proposes embodying the magnetic brake with two redundant electromagnets for its actuation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an magnetic brake and more particularly to an improved electromechanical braking device especially useful for motor vehicles. 
     2. Description of the Prior Art 
     One such magnetic brake is known from U.S. Pat. No. 5,185,542. The known magnetic brake has one rotatable part and one rotationally fixed part, which are in frictional or positive engagement with one another in a braking position of the magnetic brake, so that the rotatable part is held or at least braked by the rotationally fixed part, and which in a released position of the magnetic brake are free of one another, so that the rotatable part is freely rotatable. For actuation, the known magnetic brake has a spring element, which presses the rotationally fixed part or the rotatable part against the respectively other part, as well as an electromagnet, which by being supplied with current disconnects the rotatable part and the rotationally fixed part from one another counter to the force of a spring element; that is, the spring element puts the magnetic brake in its braking position and keeps it there, and the magnetic brake can be released by means of the electromagnet. It is equally possible to put a magnetic brake into the braking position by supplying current to the electromagnet, while conversely a spring element releases the magnetic brake. 
     The magnetic brake has the disadvantage that in the event of a defect, or in other words if its electromagnet or its power supply fails, it cannot be actuated. 
     ADVANTAGES OF THE INVENTION 
     The magnetic brake of the invention as defined by the characteristics of claim  1  has a second electromagnet, with which it is actuatable. The magnetic brake of this invention is advantageous because it has a magnetic brake that is actuatable selectively by its first or second electromagnet; the two electromagnets are redundant. This has the advantage of high operational reliability of the magnetic brake of the invention; failure is virtually precluded. 
     Preferably, the two electromagnets are each connected to their own, mutually independent power supplies, so that there is also redundance in terms of the power supply for actuating the magnetic brake, which further reduces the likelihood of failure of the magnetic brake (claim 2). 
     In a preferred feature of the electromechanical wheel braking device, the magnetic brake is embodied in bistable form (claim 3); that is, it remains both in the released position and in the braking position without current being supplied to its electromagnets. The electromagnets serve to switch the magnetic brake over from the released position to the braking position and conversely from the braking position into the released position. For the switchover between the two positions, only a brief supply of current selectively to the first or the second electromagnet is necessary. The bistable embodiment of the magnetic brake can be done for instance with the aid of a permanent magnet, which keeps the magnetic brake in one of its two positions counter to the force of a spring element, while conversely, after the switchover by means of one of its two electromagnets, the magnetic brake is kept in the other position counter to the force of the permanent magnet by the spring element; the force of the permanent magnet in this other position of the magnetic brake is weakened by an air gap, caused by the switchover, in its magnetic circuit. 
     In a feature of the invention in accordance with claim  4 , the magnetic brake is part of an electromechanical braking device for a motor vehicle; it serves to lock the electromechanical braking device in the actuated position, so that a braking force generated with the braking device is kept constant, without current being supplied to the electromechanical braking device. Supplying current to the electromechanical braking device is necessary solely to generate or boost the braking force and/or to reduce the braking force, which is understood also to mean a complete release of the electromechanical braking device. The electromechanical braking device can as a result be used as a parking brake, which once a braking force has been brought to bear maintains it without current being supplied. The electromechanical braking device can also be locked during a braking event with constant braking force using the magnetic brake, so that the braking force is maintained without current being supplied to the braking device. Only in order to vary the braking force is the magnetic brake switched into its released position and is current supplied to the braking device in such a way that its braking force varies in the desired way. In a preferred feature, the electromechanical braking device is embodied in non-self-locking fashion; that is, it releases itself when there is no current to the electric motor and the magnetic brake is released, because of a reaction force to the contact pressure force with which its friction brake linings are pressed against a brake body, such a brake disk or a brake drum, except for a negligible residual braking force. This feature of the invention has the advantage that the electromechanical braking device can be released in every case, because of the redundance of the magnetic brake, even if its electric motor or its power supply fails. It is therefore unnecessary to provide a second electric motor to release the electromechanical braking device in the event of a defect. The expense for enabling the release of the electromechanical braking device even in the event of a defect is minimal; it is limited to the provision of a second electromagnet for the magnetic brake. 
    
    
     DRAWING 
     FIG. 1, an axial section through a magnetic brake of the invention; and 
     FIG. 2, an axial section through an electromechanical braking device according to the invention. 
    
    
     The two drawing figures, for the sake of clarity, are schematic illustrations of exemplary embodiments of the invention and are to different scales. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The magnetic brake  10  of the invention, shown in FIG. 1, has a cup-shaped housing  12  of ferromagnetic material. An annular permanent magnet  16  with axial magnetization is mounted concentrically in the housing  12 , on a bottom  14  of the housing  12 . In a continuation of the permanent magnet  16 , a hollow-cylindrical magnet core  18  of ferromagnetic material is mounted concentrically with the housing  12  on the permanent magnet  16 . Two annular magnet coils  20 ,  22  are slipped onto the magnet core  18 , axially adjacent one another. The magnet coils  20 ,  22  are located in an annular interstice between the magnet core  18  and the housing  12 . Each magnet coil  20 ,  22 , together with the magnet core  18 , forms one electromagnet  18 ,  20 ;  18 ,  22 . A helical compression spring  26  is inserted as a spring element into a cylindrical interior  24  inside the hollow-cylindrical magnet core  18  and inside the annular permanent magnet  16 ; this spring is braced against the bottom  14  of the housing  12  and presses against an armature disk  28 , which is disposed on a side, remote from the bottom  14 , of the permanent magnet  16  and of the two electromagnets  18 ,  20 ;  18 ,  22  in the housing  12  of the magnetic brake  10 . The armature disk  28  is joined to the housing  12  in an axially displaceable fashion but fixed against relative rotation by means of preferably a plurality of splines  30 , which are distributed over the circumference of the housing  12  and extend longitudinally of the housing and are integral with the housing  12 , and which protrude inward in the housing  12  and engage complimentary grooves  32  in the circumference edge of the armature disk  28 . Only one pair of splines  30  and grooves  32  can be seen in the drawing. 
     On an end face of the armature disk  28  remote from the permanent magnet  16  and the two electromagnets  18 ,  20 ;  18 ,  22 , a brake lining  34  in the form of an annular disk is fixedly mounted. A coupling disk  36  is disposed on the side of the brake lining  34  in the housing  12 , on its open face end remote from the bottom  14 . The coupling disk  36  is press-fitted for instance onto a shaft  38 , coaxial with the housing  12 , of an electric motor not shown in FIG.  1  and in this way is disposed rotatably in the housing  12  of the electromagnet  10 . 
     The function of the magnetic brake  10  of the invention is as follows: The magnetic brake  10  has two stable positions, namely the braking position, shown in FIG. 1, and a release position, not shown, in which the armature disk  28  rests on an end face, toward it, of the two electromagnets  18 ,  20 ;  18 ,  22 . In other words, the magnetic brake  10  is embodied in bistable form. In the braking position shown, the helical compression spring  26  presses the armature disk  28 , which is axially movable in the interstice between the coupling disk  36  and the two electromagnets  18 ,  20 ;  18 ,  22 , with its brake lining  34  against the coupling disk  36 . The armature and coupling disks  28 ,  36  are joined together in a manner fixed against relative rotation by the contact pressure force of the helical compression spring  26  because of frictional engagement; that is, the armature disk  28  which is fixed against relative rotation in the housing  12  keeps the coupling disk  36  in a manner fixed against relative rotation in the housing  12 . Since in the braking position, there is an axial air gap, between the magnet core  18  and the armature disk  28 , that weakens a magnet field exerted by the permanent magnet  16  onto the armature disk  28  via the magnet core  18 , the force of the helical compression spring  26  is greater than the magnetic force exerted on the armature disk  28  by the permanent magnet  16 ; that is, the helical compression spring  26  presses the armature disk  28  against the coupling disk  36 , counter to the magnetic force of the permanent magnet  16 . 
     For switching the magnetic brake  10  over to the released position, one of the two magnet coils  20 ,  22  is supplied with current in such a way that it increases the magnetic field of the permanent magnet  16 , specifically so markedly that the magnetic force is greater than the force of the helical compression spring  26 , so that the armature disk  28  is attracted to the magnet core  18  counter to the force of the helical compression spring  24 . As a result, the brake lining  34  is lifted from the coupling disk  36 , and the coupling disk  36  is freely rotatable. After the switchover to the released position, the current through the magnet coil  20 ,  22  is turned off again. Since in the released position of the magnetic brake  10  the armature disk  28  rests directly on the face end of the magnet core  18 , so that there is no longer any air gap, the magnetic force exerted by the permanent magnet  16  via the magnet core  18  suffices to keep the armature disk  28  in contact with the magnet core  18 , counter to the force of the helical compression spring  26 . Accordingly, when it is without current, the magnetic brake  10  remains in its released position. The magnetic circuit is closed by the magnet core  18  via the armature disk  28 , contacting in the released position of the magnetic brake  10 , and via the housing  12 . 
     To switch the magnetic brake  10  back into the braking position, one of the two magnet coils  20 ,  22  is supplied with current, now in the opposite direction, so that the magnet field generated by the magnet coil  20 ,  22  that is supplied with current is in the opposite direction from the magnetic field of the permanent magnet  16 . In this way, the magnetic field is weakened, specifically so much that the helical compression spring  26  forces the armature disk  28  away from the permanent magnet  16  and the two electromagnets  18 ,  20 ;  18 ,  22  and presses it with its brake lining  34  against the coupling disk  36 , and as a result the magnetic brake  10  is again in the braking position. The magnetic brake  10  can accordingly be switched over from the braking position into the released position by a brief current pulse through one of its two magnet coils  20 ,  22 , and can be switched back from the released position to the braking position by a current pulse of opposite polarity. When it is without current, the magnetic brake  10  stays either in the braking position or in the released position. 
     The two magnet coils  20 ,  22  are connected to mutually independent power supplies, not shown in the drawing. If one of its two electromagnets  18 ,  20 ;  18 ,  22  or one of the two mutually independent power supplies for the electromagnets  18 ,  20 ;  18 ,  22  fails, the magnetic brake  10  can accordingly still always be switched over; as a consequence, it has high operational reliability. 
     The housing  12 , on its open face end, has a screw flange  40 , which is integral with the housing  12  and has screw holes  42 , and with which the magnetic brake  10  can be flanged, for instance to an electric motor, not shown in FIG. 1, or other device, with a shaft  38  that is meant to be locked intermittently. 
     FIG. 2 shows an electromechanical wheel braking device  44  according to the invention, which is embodied as a disk brake and which can be locked with the magnetic brake  10  shown in FIG.  1  and described above. The wheel braking device  44  has a floating caliper  46 , in which a pair of friction brake linings  48  are mounted on both sides of a brake disk  50  that can be set into rotation between them. 
     For pressing one of the two brake linings  48  against the brake disk  50 , the wheel braking device  44  of the invention has a spindle drive  52 , which is built into its floating caliper  46 . For the sake of low friction and high efficiency, the spindle drive  52  is embodied as a rolling-contact thread drive in the form of a roller thread drive. It has a threaded spindle  56 , resting coaxially in a spindle nut  54 , and eight profile rollers  58 , which are disposed in an interstice between the spindle nut  54  and the threaded spindle  56 . The profile rollers  58  have profiling extending around the circumference, which has a form that is complimentary to a profile of a nut thread  60  of the spindle nut  54  and to a threaded profile  62  of the threaded spindle  56  that matches the threaded profile of the nut thread  60 . The profiling around the circumference of the profile rollers  58  has no pitch. In a departure from the exemplary embodiment shown, however, it is also possible (not shown) to embody the profile rollers  58  with profiling with a pitch, or in other words with a thread. With their profiling, the profile rollers  58  engage both the nut thread  60  and the spindle thread  62 . Driving the spindle nut  54  to rotate drives the profile rollers  58  to execute an orbiting motion about the threaded spindle  56 , like planet wheels of a planetary gear. During their orbiting motion, the profile rollers  58  roll along the spindle thread  62 ; during the orbiting motion about the threaded spindle  56 , they execute a rotational motion about their own axis. By way of the orbiting profile rollers  58 , a rotational drive of the spindle nut  54  brings about a translational motion of the threaded spindle  56  in the axial direction. 
     The spindle drive  52  is embodied in non-self-locking fashion; that is, a thread pitch of the spindle thread  62  and of the nut thread  60  is selected to be so great that a force, acting in the axial direction on the threaded spindle  56 , sets the spindle nut  54  to rotation and displaces the threaded spindle  56  axially. 
     The spindle nut  54  is supported rotatably in the floating caliper  46  by a pair of axial angular roller bearings  70  and is braced axially on the floating caliper  46  via the angular roller bearings  70 . 
     For rotationally driving the spindle nut  54 , the wheel braking device  44  of the invention has an electric motor  64 , which is flanged to the floating caliper  46  at a right angle to the spindle drive  52 . The electric motor  64  drives the spindle nut  64  via a bevel gear system  66 ,  68 , which has a plate gear wheel  66 , press-fitted onto the spindle nut  54  in a manner fixed against relative rotation, meshing with which is a bevel gear wheel  68  that is press-fitted onto a shaft  38  of the electric motor  64  in a manner fixed against relative rotation. The electric motor  64  is embodied as an electronically commutatable motor. 
     The threaded spindle  56  is integral with a brake lining plate  72 , which is embodied on a face end of the threaded spindle  56  toward the brake disk  50 . The brake lining plate  72  has a groove, not visible in the drawing, which is engaged by a spline  74  that is integral with the floating caliper  46 . In this way, the threaded spindle  56  is held in the floating caliper  46  in a manner secure against relative rotation. One of the two friction brake linings  48  is mounted fixedly on the brake lining plate  72  of the threaded spindle  56 . The other friction lining  48  rests in the floating caliper  46  in a manner known per se. 
     The magnetic brake  10  is mounted on the electric motor  64  on a face end remote from the spindle drive  52 . It is screwed to the electric motor  64  by means of screws  76  that are inserted through its screw flange  40 . The shaft  38  of the electric motor  64  protrudes from the electric motor  64  on both sides. On a side of the electric motor  64  remote from the floating caliper  46 , the coupling disk  36  of the magnetic brake  10  is press-fitted onto the shaft  38  of the electric motor  64  in a manner fixed against relative rotation. 
     The function of the wheel braking device  44  of the invention is as follows: For actuation, the spindle nut  54  is driven by the electric motor  64  to rotate in an actuating direction of rotation, so that the threaded spindle  56  is displaced translationally, axially in the direction of the brake disk  50 . The spline  74  of the floating caliper  46  prevents any rotation of the threaded spindle  56 . The threaded spindle  56  presses the friction brake lining  48 , mounted on its brake lining plate  72 , against one side of the brake disk  50 . Via a reaction force, the second wheel brake lining  48  is pressed against the other side of the brake disk  50  in a manner known per se via the floating caliper  46 . The brake disk  50  is braked, and a braking force or braking moment is proportional to the driving moment brought to bear by the electric motor  64 . 
     To release the wheel braking device  44  or to reduce the braking force, the spindle nut  54  is driven in the opposite, restoring direction of rotation, and as a result the threaded spindle  56  is moved translationally away from the brake disk  50 . The friction wheel lining mounted on its brake lining plate  72  is lifted from the brake disk  50 . The threaded spindle  56  is restored far enough that a gap between the friction wheel linings  48  and the brake disk  50 , which gap remains regardless of any wear of the friction brake linings  48 , exists when the wheel braking device  44  is not actuated; the so-called “air play” of the wheel braking device  44  of the invention remains constant. 
     During the actuation and release of the wheel braking device  44 , the magnetic brake  10  is in its released position, so that the shaft  38  of the electric motor  64  is freely rotatable. When the wheel braking device  44  is used as a parking brake, the wheel braking device  44  is actuated, so that the brake disk  50  is held in a manner fixed against relative rotation between the friction brake linings  48 . Next, by supplying current to one of its two electromagnets  18 ,  20 ;  18 ,  22 , the magnetic brake  10  is switched over into its braking position, and in this way the shaft  38  of the electric motor  64  is blocked, and as a result the wheel braking device  44  is locked, and the braking force once brought to bear is maintained while the electric motor  64  and the magnetic brake  10  are without current. Also, when the wheel braking device  44  is used as a service brake, if a braking force exerted on the brake disk  50  is temporarily kept constant, this can be done by providing that after the braking force is brought to bear, the magnetic brake  10  is switched over to its braking position with the electric motor  64 ; all that is required is a brief current pulse to one of its two electromagnets  18 ,  20 ;  18 ,  22 . The braking force is as a result kept constant without supplying current to the electric motor  64  and without supplying current to the magnetic brake  10 . For varying the braking force, the magnetic brake  10  is switched over to its released position. In this way, the electric motor  64  is supplied with current only in order to vary the braking force and in particular in order to increase the braking force. On the one hand, this saves energy and relieves an on-board electrical system of a vehicle that can be braked with the wheel braking device  44 . On the other, hand heating of the electric motor  64  is reduced, since the electric motor is supplied with current only for varying the braking force but when the braking force is being kept constant is currentless. Hence there is less of a load on the electric motor  64 , and accordingly a less powerful and thus smaller, lighter electric motor  64  can be used. 
     In the case of a defect, that is, if an electronic control system of the electric motor  64 , its power supply, or the electric motor  64  itself fails, the magnetic brake  10  is switched to its released position, so that the shaft  38  of the electric motor  64  is freely rotatable. As a result, the threaded nut  54  is also freely rotatable. The threaded spindle  56  is forced axially away from the brake disk  50  by the friction brake lining  48  pressed against the brake disk  50 , and since the spindle drive  52  is non-self-locking, the threaded spindle sets the spindle nut  54  into rotation. The wheel braking device  44  is released, until the contact pressure force of the friction brake linings  48  against the brake disk is so slight that the threaded spindle  56  does not move any further, because of internal friction of the spindle drive  52 , the bevel gear system  66 ,  68 , and the electric motor  64 . The friction brake linings  48  rest on the brake disk  50  with a negligible residual force that is so slight that the brake disk  50  is virtually freely rotatable, and a motor vehicle equipped with the wheel braking device  10  can be driven without causing overheating of the wheel braking device  44 . It is understood that the magnetic brake  10  can be disposed at some other point in the wheel braking device  44  instead, and can for instance lock the spindle nut  54  in a directly releasable way (not shown). 
     The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.