Abstract:
An electrically controlled brake or clutch includes a rotatable first mechanical system ( 101, 108 ) and a second mechanical system that is stationary for the case of a brake but rotatable for the clutch case. In the second system windings are wound around two soft magnetic parts ( 102, 103 ) so that electric current flowing in the windings affects magnetic fluxes through the soft magnetic parts to move them in a direction that affects the effective length of an air gap in the closed main magnetic path. A spring ( 401 ) creates a force acting in a direction opposite that of the attraction force. The soft magnetic parts are arranged so that the main magnetic flux path passes along a closed loop about the rotational axis of the first mechanical system, this giving a compact design of the brake or clutch.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority and benefit from Swedish patent applications Nos. 0601229 8, filed May 31, 2006, 06017131-3, filed Aug. 16, 2006, and 0601809-7, filed Aug. 24, 2006, the entire teachings of which are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present, invention is concerned with brakes, in particular holding brakes for servo motors. 
       BACKGROUND 
       [0003]    Servo motors are often used in applications where it is important that they will not move during power off or when there is reason to assume that the control system of the servo motor is not behaving properly, for example when an emergency stop button has been pressed. 
       SUMMARY 
       [0004]    It is an object of the invention to provide a brake or clutch that that at least in some embodiments can have a compact shape. 
         [0005]    It is another object of the invention to provide a brake or clutch that at least in some embodiments can be produced in a cost-efficient way. 
         [0006]    An electrically controlled brake that can also be used or designed as a clutch includes as conventional a rotatable first mechanical system having one or more friction parts/surfaces and a second mechanical system that has one or more friction parts/surfaces. The friction surfaces can made to come in contact with each other, providing a braking or coupling effect, and be withdrawn from each other releasing the brake or clutch. The second mechanical system is stationary for the case of a brake and is rotatable for the clutch case. Electric windings are provided, e.g. wound around two soft magnetic parts, and are arranged so that electric current flowing in the windings affects magnetic fluxes through the soft magnetic parts to move at least one thereof. The movement is in a direction that affects the effective length of one or more air gaps in the closed main magnetic path created by the current and the soft magnetic parts. In particular the electric current gives attraction forces over the air gap or gaps which forces tend to move one of or both the soft magnetic parts to reduce the length of the air gap. One or more springs create forces acting in a direction substantially opposing the attraction forces. In the movement the friction part of the first mechanical system comes in frictional engagement or frictional disengagement with the friction part of the second mechanical system. Frictional disengagement here means that a frictional engagement between the two mechanical system is released. 
         [0007]    The soft magnetic parts are arranged so that the main magnetic flux path passes along a closed loop that passes about the rotational axis of the first mechanical system, this making it possible to e.g. give the brake or clutch a compact design. 
         [0008]    The soft magnetic parts can together have a toroidal shape having e.g. substantially the same axis as the rotational axis. 
         [0009]    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 
         [0010]    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: 
           [0011]      FIG. 1  is a front view of a normally active, asymmetrical, angular movement brake in its braking state, 
           [0012]      FIG. 2  is similar to  FIG. 1  but shows the brake in its non-braking state, 
           [0013]      FIG. 3  is a front view of the magnetically permeable parts of the brake of  FIGS. 1 and 2 , 
           [0014]      FIG. 4  is a front view of a flat spring arrangement for the brake of  FIGS. 1 and 2 , 
           [0015]      FIG. 5  is front view similar to  FIG. 1  of the brake including flat springs as shown in  FIG. 4 , 
           [0016]      FIG. 6  is a cross-sectional view in an axial plane of the brake of  FIGS. 1 and 2  assembled inside the rotor of an electric motor, 
           [0017]      FIG. 7  is a front view of a normally active, symmetrical, angular movement brake in its braking state, 
           [0018]      FIG. 8  is a front view of the magnetically permeable parts of a parallel movement brake, 
           [0019]      FIG. 9  is a front view of a flat spring suitable for the brake parts of  FIG. 8   
           [0020]      FIG. 10  is a cross-sectional view in radial plane of a parallel movement brake having internal linear guides, 
           [0021]      FIG. 11  is a front view of the brake of  FIG. 10  including windings, 
           [0022]      FIG. 12  is a cross-sectional view in a radial plane of a parallel movement brake including air gap mounted springs, 
           [0023]      FIG. 13  is a perspective view of the brake of  FIG. 12 , and 
           [0024]      FIG. 14  is similar to  FIG. 12  showing a brake that is active (braking) when a control current is active. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  is a front view of a normally active brake in the braking state thereof. The brake consists of two groups of components. The first group is connected to a rotating device, for example the rotor of a motor. In the case shown, there are only two components in this group including the hollow circular cylinder or drum,  101  and the central shaft  108 . The other group is normally connected to a non-rotating or stationary part such as a motor frame. There are two main components in this group, the half-arcs  102  and  103 , each of which has the shape of the half of cylindrical ring, i.e. a cylindrical ring segment corresponding to an angle of substantially 180°. These two parts can rotate within a very limited angle around magnetically permeable shafts  104  and  105 , respectively, the shafts located at axially opposite positions near one of the flat axial surfaces of the half-arc.  FIG. 1  shows the two half-arcs where each thereof has been rotated in the clockwise direction around its shaft to take its maximum clockwise position. The movement of the first half-arc  102  is limited to the position where the brake lining pad  106  of high friction material on the envelope surface of the half-arc comes in contact with the interior envelope curved surface of the hollow cylinder  101 . The movement of the second half-arc  103  is limited in the same way. As a consequence of these two movements, there are two air gaps like  107  between the two half-arcs. The force required to move the two half-arcs  102 ,  103  clockwise around their respective shafts  104 ,  105  can be arranged by springs configured e.g. as that shown in  FIG. 5 . 
         [0026]      FIG. 2  shows the two half-arcs  102 ,  103  where each half-arc has been rotated around its shaft in the counter-clockwise direction to take its maximum counter-clockwise position. The movement of the first half-arc  102  is limited to the position where it is pressed against the second half-arc  103 . The movement of the second half-arc  103  is limited in the same way. As a consequence of these two movements, there is practically no gap  107  between the two half-arcs in this state. Consequently, there will appear a gap  201  between the brake lining  106  and the interior cylindrical surface of the brake drum  201 . The force required to move the two half-arcs counter-clockwise around their respective shafts  104 ,  105  can be arranged by an electric current flowing in coils, not show, located in winding slots like  202  provided in the half-arcs, the electric current creating a magnetic field in the magnetically permeable parts shown in  FIG. 3 . The winding slots and the wire turns of the coils therein are located in axial planes, i.e. planes passing through the axis of the brake. 
         [0027]      FIG. 3  shows the magnetically permeable parts of the second (non-rotating) group. They include the two half arcs  102  and  103 , their magnetically permeable shafts  104  and two pins  303  used to permit a path for the braking spring force. In  FIGS. 1 and 2 , the magnetically permeable parts are covered by other parts containing the winding slots like  202 . 
         [0028]    In the closed state shown, the two half arcs are in a position corresponding to a non-active brake caused by current flowing in the coils in the winding slots like  202 . 
         [0029]      FIG. 4  shows a flat spring suitable to provide the force required to create sufficient force between the friction lining  106  and the interior of the hollow cylinder  101 . 
         [0030]      FIG. 5  shows springs like  401  assembled in the brake of  FIG. 2 . 
         [0031]      FIG. 6  shows a brake like that of  FIGS. 1-5  assembled in an electric motor, e.g. a servo motor. The right end of the shaft  104  extending through the soft iron half-arc  301  is rigidly secured in the rear shield  602 . The plastic coil support is visible as  601 . The rotor shaft corresponds to the central shaft  108  of  FIG. 1 , and the motor rotor magnet holding cylinder corresponds to the hollow cylinder  101 . The spring  401  is shown in suitable positions. 
         [0032]    The force available over a magnetic air gap like  107  between the two half-arcs is very dependent on the length of the air gap (parallel with the flux lines). Spring loaded magnetically actuated brakes should have small air gaps to permit a large force from a small current. On the other hand, the air gap must be large enough to ensure that the friction surfaces used when the brake is active will be engaged when the brake is active and disengaged when the brake is passive. The required length of the air gap is therefore dependent of the mechanical tolerances in the parts in the brake force path. To overcome the mechanical tolerances of the parts in the force path, the air gap must be longer than the sum of the mechanical tolerances of these parts. 
         [0033]    An advantage of the azimuthal or circumferential force path of the brake of  FIGS. 1-6  is that the cost to get tight tolerances of cylindrical parts is comparatively low. The inside of a hollow cylinder like  101  with a diameter of 52.5 can easily be made with a tolerance class  6  corresponding to a diameter variation of 19 micrometers, i.e. is a radial uncertainty of 9.5 micrometers. Using similar rotation production technologies for the adjustment of the friction lining  106  on a set of two half-arcs with no air gap in position  107  can give a total uncertainty of for example 19 micrometers measured at the air gap  201  of the brake lining. This would require some 25 micrometer air gap in position  107  of  FIG. 1  to cover the uncertainty of the mechanical dimensions of the parts used (the difference between 19 and 25 micrometers is caused by the distances from the shafts  104  and  105 ). Even after that margins have been added to handle other uncertainties, an air gap  107  of 70 micrometers instead of the 200 micrometers that are normal in spring actuated brakes permit an excitation current of some 35% of the conventional one and a power loss of some 10% of the power loss for the same device using a conventional air gap. 
         [0034]    From this discussion it is obvious that the brake as described herein can be made have its mechanically critical dimension tolerances in the radial direction, utilising the fact that it is less expensive to manufacture radial dimensions with a high precision than axial dimensions. This can make the brake cost-efficient. Also, since short air gaps can be produced at a reasonable cost, the brake can be made to have a high torque to power loss ratio for the braking/releasing operation. 
         [0035]      FIG. 7  shows a slightly different brake. The two parts may rotate slightly around the shafts  701  and  702 . The required spring force is applied between pins  703  and  704 . 
         [0036]    The brake lining  707  will give a higher brake torque for a counter-clockwise movement of the brake drum  708  than for a clock-wise movement of the drum, as the friction force will cause an increase of the force perpendicular to the drum surface, causing a positive feedback. In the brake of  FIG. 7 , this is compensated by the fact that the brake torque from the other lining  711  will give lower brake torque for a counter-clockwise movement of the brake drum  708  than for a clockwise movement of the drum, thus giving a torque that is independent of the rotation direction. The brake shown in  FIG. 1  will have a torque dependent on the rotation direction and may hence be suitable for e.g. robot parts moving against gravity. 
         [0037]    The angle shown as  709  in  FIG. 7  will affect the brake torque obtained for a given spring force. For a given magnetic air gap, there will be a corresponding possible spring force. If the angle  709  is reduced by another design of the position of the brake lining, the friction force and therefore the torque caused by a constant spring force will change. A given magnetic air gap will give different gaps between the brake lining like  711  and the brake drum depending on the angle  709 . 
         [0038]    Large values of the angle  709  combined with high friction coefficients for the lining—drum materials will result in a self-locking brake with a brake torque that is limited only by the breakdown of the weakest components. 
         [0039]      FIG. 8  shows the magnetically active parts of a toroidal brake that has a parallel movement of the two parts. While such a brake can have two moving parts, the embodiment of  FIG. 8  has one arc part  801  fixed to the chassis by screws in holes  803 ,  804 ,  805  and aligned by pins in holes like  806 . The other arc part  802  moves vertically. With no electric current flowing in the coils described under item  903  and  1101  below, the two parts are separated by a spring force, and the lining  807  is pressed by the spring force against the interior surface of a drum. 
         [0040]      FIG. 9  shows a flat spring  902  intended to be connected to the two parts  801 - 802  shown in  FIG. 8 . There are four winding slots like  903  which can be wound using toroidal winding machines. 
         [0041]      FIG. 10  is a radial axial sectional view of a brake that has internal linear bearings and springs. The springs like  1004  are centered around pins like  1003  that are pressed into the moving arc part  1002  but can move smoothly inside bearings like  1005 , that can be PTFE covered steel tubes. 
         [0042]      FIG. 11  shows the winding  1101  of the brake of  FIG. 10 . The brake is shown in its no current, braking state in  FIG. 10  and in its current carrying, not braking state in  FIG. 11 . 
         [0043]      FIG. 12  is a radial axial sectional view of a parallel movement brake having the springs in the magnetic air gap. The brake is shown in its active (braking) state. It contains two half toroids  1202  and  1203 . The half toroids are permitted to move inside a narrow space. Radially the half toroid  1202  is restricted against movements upward as seen in the drawing by the friction block  106  and the brake drum  1208 . In the direction left, downwards and right it is limited by the stationary bars like  1204 . These bars are preferably made of a material having a very low magnetic permeability such as some stainless steels. 
         [0044]      FIG. 12  shows the brake after the drum  1208  has turned in the clockwise direction, thus moving the two half toroids in the clockwise direction. This has caused the half toroids to make a small clockwise rotation, causing a small gap indicated by arrow  1206  between the lower side of the stationary bar  1204  and the half toroid  1203  and a direct contact between the half toroid  1202  and the stationary bar  1204 . Should there appear a counter-clockwise movement of the drum  1208 , the two half toroids would move counter-clockwise and a gap would instead appear on the upper side of the stationary bar  1204 . 
         [0045]    The brake is shown with a hollow shaft  1207 . 
         [0046]    The springs are located in the gap  1205  but are not visible in any detail in  FIG. 12 . 
         [0047]      FIG. 13  shows the brake of  FIG. 12  in another view. The stationary bar  1204  shown only in a section in  FIG. 12  is here shown complete. On its end there is a top part  1301  that restricts the movement of the half toroids in the axial direction. The springs  1302  act to separate the two half toroids. If there is no electric current flowing in the coils, the springs will separate the two half toroids until the friction blocks  106  are pressed against the drum  1208 . In this state, there might be a total air gap between the two half toroids of some 0.4 mm. The springs are preferably made of steel having a high magnetic permeability, and the thickness of the spring material is not included in the air gap. When the coils are connected to a suitable DC voltage, the two half toroids are attracted by a force larger than the separating force from the springs, and the half toroids are then pressing against the stationary parts like  1204 , leaving only a minor air gap in the order of 0-0.1 mm. It might be preferable to make sure that both half toroids press against the stationary parts like  1208  by intentionally designing the system so that a small air gap remains when the half toroids press against the bar  1204 ; this will fix the position of the half toroids so that they cannot vibrate freely. 
         [0048]      FIG. 14  is a front view of a brake that is substantially similar to that of  FIG. 12  except that it acts on the centre shaft  1401  through two friction blocks like  1402 . This brake is active, i.e. is braking, when the controlling current in the coils is active. To limit the movement of the half toroids, blocking bars like  1403  are provided. 
         [0049]    As is obvious for those skilled in the art, the invention shown can be varied in many ways. All embodiments shown have the friction blocks or pads  106  mounted to the outside of the half toroids or half-arcs pressing against the inside surface of a drum when there is no current in the windings. Obviously, if the friction blocks are moved to the inside of the half toroids and then pressing against a central shaft, the braking effect would appear when there is electric current flowing in the windings. Also, the embodiments shown have one drum part rotating while the half toroids and their associate hardware are stationary, thus giving a brake. Obviously, the half toroids and their associated hardware can be assembled on a rotating part, thus creating a clutch. 
         [0050]    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.