Patent Publication Number: US-2011056786-A1

Title: Electrically Controlled Brake

Description:
RELATED APPLICATIONS 
     This application claims priority and benefit from Swedish patent application No. 0800126-5, filed Jan. 19, 2008, the entire teachings of which are incorporated herein by reference. The application is based on technology disclosed in published International patent application WO 2007/139480 and the corresponding U.S. patent application, the entire teachings of which are also incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention is concerned with brakes, in particular holding brakes for servo motors. 
     BACKGROUND 
     Servo motors brakes are often used in applications where it is important that they will not move when the servo motor is disengaged. Servo motors are often used in applications, such as positioning, with a high fraction of the operating time maintaining a fixed position, i.e. staying at a stationary position. To save unnecessary heating of such a motor, a brake used for maintaining the position should be active during such intervals and the servo motor passive. This requires that the brake has a very limited play. 
     Spring loaded brakes are normally designed including a friction disc that is connected to the motor shaft over splines. This gives an unwanted play between the brake friction disc and the motor shaft. Such brakes used in servo motors are normally designed to be normally engaged and permitting the shaft to move only when a release electromagnet is energised. 
     The British patent 989,868 discloses a spring loaded drum brake that has two friction surface carrying shoes 1 and 2 connected to the brake frame over a common pin 3. The play between shoes and pin will give a play in the position of the brake drum when torques of different magnitudes and signs act on the drum. 
     The published International patent application WO 2007/139480 discloses embodiments of a brake assembled inside a hollow rotor and including a toroidal soft magnetic part. These embodiments all have some play between the toroidal soft magnetic parts and the brake stator, permitting the shaft of the brake to have a play against the brake stator when the brake is in an activated state. 
     SUMMARY 
     It is an object of the invention to provide a brake that has its mechanically critical dimension tolerances in the radial direction, thereby utilising that radial dimensions are less expensive to achieve with a high precision than axial dimensions. 
     It is another object of the invention to provide a brake in which the magnetic property of toroidal cores is used to obtain a more efficient brake. 
     It is another object of the invention is to provide a brake having a high torque to power loss ratio. 
     It is yet another object of the invention to provide a brake having negligible play. 
     Generally, an electrically controlled brake for use in a motor, e.g. an electric motor or server motor, includes a first mechanical system or first group of components which is mounted to rotate about an axis and which its main component includes a braking drum that has an inner cylindrical wall or surface. Also, the brake includes a second mechanical system or second group of components. In the second system or group the components can be more or less stationary or fixed, at a maximum performing e.g. only relative small movements. There is at least one winding to which a control electric current can be applied. At least one movable braking shoe has a friction part for acting against or for engagement with the inner cylindrical wall or surface of the braking drum. The braking shoe is made from magnetically soft iron material and is arranged so that electric current flowing in the winding affects magnetic fluxes through the braking shoe. Such magnetic fluxes cause attraction forces over a first air gap and thereby they attempt or tend to move the braking shoe to reduce the width of the air gap. Furthermore, the second system or group includes at least one spring that is mounted to create elastic forces acting on the at least one movable braking shoe. The elastic forces act in a direction so that they substantially oppose the attraction forces over the airgap created by an electrical current flowing in the winding. The elastic forces also attempt or tend to move the braking shoe so that the friction part thereof is moved to come in engagement with the inner wall or surface of the braking drum. The attraction forces instead attempt or tend to move the braking shoe so that the friction part is moved to be free of or relieved from engagement with the inner wall or surface of the braking drum. 
     The spring can be mounted so that in the movement of the braking shoe there is no mechanical play. Mechanical play is here taken to mean that, since the braking shoe must be connected to one of the completely stationary or fixed components in order to perform a braking action when its friction part or braking lining is brought in contact with the inner wall or surface of the braking drum, this connection must be without play. For achieving this the spring can be mounted at an inner end of the braking shoe and then the first airgap can be located at an outer, opposite end of the braking shoe. Furthermore, the spring can be rigidly attached to the inner end of the at least one movable braking shoe so that the spring and the braking shoe forms a unified structure. The spring then has a free portion which is not attached to the braking shoe and not attached to any completely fixed or stationary component and which can be elastically deformed. The free portion can be called a bridge portion and it can be flexed or perform a simple bending movement when the movable braking shoe moves. Such a flexing or simple bending movement means that the bridge portion in deformed in substantially one single plane. 
     Two movable braking shoes can be arranged, both having inner ends at the spring and outer ends at the first airgap. Then the spring acts at said inner ends and can be rigidly attached to them so that the spring and the two braking shoes form a unified, generally C-shaped structure. The first airgap is then located at the free ends of the C-shape and the spring at the central position between the free ends. Then the spring forms a bridge between the inner ends of the two braking shoes where the bridge can include one or more bridge portions that are located between said inner ends and are flexed or perform a simple bending movement when the two movable braking shoes move. 
     The spring can have a substantially flat shape, i.e. have the shape of a plate. Such a flat spring can then have two opposite ends, the inner ends of the braking shoes being attached to regions at the opposite ends at a distance of each other. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the novel features of the invention are set forth with particularly in the appended claims, a complete understanding of the invention, both as to organization and content, and of the above and other features thereof may be gained from and the invention will be better appreciated from a consideration of the following detailed description of non-limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which: 
         FIG. 1  is a front view of the inner parts of a brake in the braking state thereof, the brake being normally active and the outer braking drum and other rotating parts removed, 
         FIG. 2  is similar to  FIG. 1  but also showing rotating parts and the brake in its non-braking state, 
         FIG. 3  is a sectional view of the brake of  FIGS. 1 and 2  in the braking state thereof, the section taken in a plane passing magnetically permeable parts, 
         FIGS. 4 ,  5  and  6  are detail views showing gaps of the brake for the state illustrated in  FIG. 2 , and 
         FIGS. 7 ,  8  and  9  are similar to  FIGS. 4 ,  5  and  6  but showing gaps of the brake for the state illustrated in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     An electrically controlled brake for use in a motor, e.g. an electric motor or server motor, will now be described. The brake is designed so that in its braking action there is no mechanical play. This means that when the braking action is active, the object being braked cannot move or be moved, even over the smallest distance, provided of course that no exaggerated, destructing forces are applied. 
     One embodiment of such a brake is illustrated in the front view of  FIG. 1 , the brake being normally active and the figure showing the brake in its braking state. In the embodiment shown the brake has a symmetrical structure taken about a plane extending horizontally in  FIG. 1  and comprises two mechanical systems or two groups of components. The first mechanical system or group is connected to or partly included in a rotating device, for example the rotor of a motor, and is not shown in the figure. The second group is normally connected to a not rotating part or stationary part, such as a frame of the motor, and has the same function as the components shown in  FIGS. 8 ,  9 ,  10  and  11  in the cited International patent application WO 2007/139480 but with a different design. In the second group there are two movable components, the brake segments or braking shoes  101  and  102 , herein called half arcs, made from magnetically soft iron. Each of the movable half arcs has the general shape of substantially a half of a cylindrical ring, or more precisely a cylindrical ring segment corresponding to an angle somewhat smaller than 180° as illustrated in the figure, in the range of e.g. up to about 175°. Between first ends of the half arcs  101  and  102  a block  103  of magnetically soft iron material is located. The soft iron block has first flat surfaces at the first ends of the half arcs, the flat surfaces arranged adjacent to but generally not in contact with opposite first flat surfaces of the two half arcs. The first flat surfaces can as illustrated be parallel or nearly parallel to the mentioned symmetry plane. 
     A spring  104 , that in the illustrated embodiment is initially flat, is rigidly attached to the three soft iron parts  101 - 103 , i.e. to the two half arcs and the soft iron block  103 , also at the first ends of the half arcs, such as to second flat surfaces located perpendicularly to the first flat surfaces. The spring  104  is also rigidly attached to a bar  105  that in turn is rigidly attached to a frame of the motor via a ring  111  located at a first, axially inner side of the half arcs  101 ,  102  and the block  103 . In this ring one of two bearings, not shown, for the rotor of the motor for which the brake is intended can be mounted. The soft iron block  103  and the bar  105  are located on opposite sides of the spring  104  so that the portions of the spring at the end portions thereof, which are not in contact with the soft iron block but with the inner ends of the half arcs  101 ,  102 , can elastically bend, together with the half arcs, in a direction towards the axis of the brake. The bending movement is accomplished by the fact that the free portions of the spring  104  are elastically deformed, these free portions also called bridge portions and being flexed or performing a simple bending in the movement of half arcs  101 ,  102 . 
     The second, outer ends of the half arcs  101 ,  102  have flat surfaces that are located opposite each other and substantially parallel to each other, generally leaving a gap  107  between the flat surfaces. 
     In the braking state shown in  FIG. 1  the spring  104  tends to press the half arcs  101 ,  102  outwards, away from the rotational axis of the brake, to make them come in contact with the inner cylindrical wall or surface of a rotatable hollow cylinder or rotor drum, not shown, by linings or friction parts such as  110  in a way similar to that of a conventional drum brake. 
     In the magnetic circuit consisting of the soft magnetic parts  101 - 103  there are generally three magnetic air gaps, the first air gap  107  between the second flat surfaces at the second ends of the half arcs and two small second air gaps  108  and  109  between the first flat surfaces at the first ends of the half arcs  101 ,  102  and the first flat surfaces of the soft iron block  103 . When the brake is active and the friction parts  110  are pressing against the inner wall or surface of the rotor drum, all these air gaps are as large as possible. The second two airgaps will be almost closed when a sufficient electrical current is applied to flow in coils  106  wound around the two half arcs  101 ,  102 , forcing the two half arcs to move, tending to close the magnetic circuit, thereby closing the larger first air gap  107 . This will also release the friction parts  110  from the inner wall of the drum, thus eliminating the braking function by releasing the drum. 
     The width of the two second air gaps can be adjusted during assembly of the brake by pressing the two half arcs  101  and  102  together by e.g. applying a force over the two friction parts  110  so that the second air gaps are made as small as possible and the first air gap is eliminated and then tightening screws  112  attaching the first ends of the half arcs to the spring  104 . 
       FIGS. 2 and 3  are views of a brake similar to that of  FIG. 1  where the rotating components of the first group also are visible, these component also being parts of the motor, in this case an electrical motor. A motor rotor magnet holding ring  201  has shallow grooves  203  for rotor magnets, not shown, and forms the hollow drum or braking drum, radially enclosing components of the second group. Thus, it has an inner cylindrical wall against which the friction parts  110  can act. A hollow rotor shaft  202  is located radially inside the components of the second group and is concentric with and rigidly connected to the magnet carrying ring  201 .  FIG. 2  is a front view of the brake, seen from an outer side thereof, and  FIG. 3  is a sectional view, the section taken along a plane through the soft magnetic parts  101 , 102  and  103 . 
     In the view of  FIG. 2  the brake is energised. As is seen in the detail view of  FIG. 6 , the second air gap  107  between the half arcs  101  and  102  is closed, and as a consequence thereof, the brake friction parts  110  are not in contact with the inner wall of the hollow drum  201 , see the detail view of  FIG. 4 . The two second airgaps  108 ,  109  are almost or entirely closed, see the detail view of  FIG. 8 . 
     In the sectional view of  FIG. 3  the brake is not energised. As is seen in the detail view of  FIG. 7 , the first air gap  107  between the second ends of the half arcs  101  and  102  is open. The spring  104  will try to return its original flat shape, but is stopped when the brake friction parts  110  establish contact with the inner wall of the hollow drum  201 , see the detail view of  FIG. 5 . The spring is, as shown in the detail view of  FIG. 9 , a little or marginally more flat than the same spring shown for the energised case in the detail view of  FIG. 8 . 
     As is obvious for those skilled in the art, the invention shown can be varied in many ways. For example, only one movable braking shoe such as  101  may be arranged, the other braking shoe replaced with a stationary component having the same basic design at the braking shoe but without a friction lining. 
     While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous other embodiments may be envisaged and that numerous additional advantages, modifications and changes will readily occur to those skilled in the art without departing from the spirit and scope of the invention. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within a true spirit and scope of the invention. Numerous other embodiments may be envisaged without departing from the spirit and scope of the invention.