Abstract:
A torque delivering apparatus, including: polygonal cross-section stator body having a plurality of exterior side faces of even number extending between opposite axial end faces, the stator including cylindrical bore extending between the opposite axial ends and centred on central axis of the stator body; a rotor assembly having cylindrical cross-section sized for rotation within the cylindrical bore about the central axis with at least one permanent magnet and shaft coupled to the magnet for rotation; and a plurality of solenoid coils, each coil having plurality of windings and routed to have sections extending parallel along opposite ones of the plurality of exterior side faces; wherein each of the plurality of coils is configured to selectively receive current and generate magnetic field in the stator that is applied to the rotor magnet, the rotor being subject to magnetic torque within the cylindrical bore for rotating and aligning the magnetic field of the permanent magnet with the generated magnetic field.

Description:
FIELD OF THE INVENTION 
       [0001]    The present disclosure relates generally DC motors/actuators and more particularly to slotless brushless DC motors/actuators having coils wound outside and along opposing sides of a stator in which a rotor is received. 
       BACKGROUND TO THE INVENTION 
       [0002]    Brushless DC (“BLDC”) motors are known. They include, among other things, a stator and a rotor. The stator is typically made from laminated steel stampings which are stacked to form a cylindrical shape with a central opening for receiving the rotor. The steel laminations in the stator may be slotted or slotless. A slotless stator has lower inductance and can therefore run at very high speeds. The absence of “teeth” that form the slots permit reduced requirements for the cogging torque, thereby making slotless BLDC motors appropriate for low speed use as well. Slotless BLDC motors may be more expensive than slotted BLDC motors, however, because more windings may be necessary to compensate for the larger air gap between the rotor and stator. 
         [0003]    More specifically, many existing slotless motor designs include an outer casing or housing, a stator, a rotor assembled in some fashion with permanent magnets, axially fixed relative to the casing and stator so as to be rotatable within the central opening or bore of the stator, and windings provided with the stator, which energize and magnetize the stator in order to apply a torque to the permanent magnet members affixed to or comprising the rotor. The stator may consist of a hollow steel cylinder, constructed of a solid iron core, steel laminations with a circular cross-section stacked to make a cylinder (as indicated above), or concentric rings of amorphous ferroalloy tape assembled by rolling or successive layering. The windings responsible for the drive and magnetization of the stator are then typically wound onto the stator in one of two ways. In the first approach, the stator is constructed to have external protrusions which serve as arms around which a coil may be wound, placed at a specified series of angular positions around the exterior of the stator. In another approach, the stator is a plain cylinder, with no exterior or internal features beyond those required for interfacing the stator to other components. The windings are attached directly to the inner bore of the stator using a bobbin or adhesive. 
         [0004]    Such slotless motors eliminate the preferential magnetic circuits present in normal slotted, armature-wound motors, and the cogging torques and slot losses typically found in permanent-magnet-rotor based motors. In theory, slotless motors should be able to achieve higher efficiencies over a greater range of operational conditions vs. a typical slotted stator motor design. Moreover, the simplified stator leads to much simpler, and therefore cheaper, manufacturing of the motor. The simplification of the field coil winding process also improves manufacturability. 
         [0005]    Notwithstanding the foregoing advantages of slotless motors, conventional designs are still in need of improvement. The external protrusion design is effective, but creates some preferential magnetization directions through the diameter of the cylinder, which creates some “slot losses” and cogging torque. These designs also increase manufacturing difficulty by adding armatures of a sort back into the manufacturing and assembly process, negating many of the manufacturing benefits of slotless motors. 
         [0006]    However, this design does have the advantage of allowing a very close tolerance within the bore, minimizing the air gap between the stator and rotor, maximizing the efficiency of the slotless motor design and giving such motors a greater amount of torque vs. size. 
         [0007]    The internal coil winding slotless motor design has the opposite set of problems—the stator is extremely easy to design and manufacture vs. traditional slotted stators or external armature slotless stators, and the coils are much easier to wind. However, the inclusion of the coils on the interior of the stator requires the presence of a large air gap between the stator and rotor, greatly reducing efficiency and available power of this slotless design vs. traditional slotted motors by increasing the reluctance of the magnetic circuit formed between the magnetic elements present in the rotor and stator material. There are also obvious reliability and heating issues when considering a coil simply adhered to the wall of a stator, only millimeters away from a rapidly spinning rotor. There are alternative attachment methods; however, none eliminate the above efficiency decrease due to the increased air gap. 
         [0008]    Thus, there is a definite need for a slotless BLDC motor/actuator design which is as easy to make as the internal coils designs, but retains the close tolerances and higher efficiencies of the external coils designs. 
       SUMMARY OF THE INVENTION 
       [0009]    In a first aspect, the present invention provides in more generic terms a torque delivering apparatus, but in particular a slot-less BLDC type motor or rotary actuator, including: a stator having a ferromagnetic body with a plurality of exterior (flat) sides of even number forming a polygonal cross-section between a first and a second axial end, the stator body including a cylindrical bore extending between the first and second axial end and centred on a central axis of the stator; a rotor assembly having a cylindrical cross-section sized for rotation with small gap clearance within the cylindrical bore about the central axis, including at least one magnet and a shaft coupled to the magnet for rotation with the magnet about the central axis; and a plurality of solenoid coils, the number being half that of the even number of exterior sides of the stator, each coil including a plurality of windings extending around the stator along opposite ones of the plurality of exterior sides, each of the plurality of coils being configured to selectively receive current which generates a magnetic field in the stator that is applied to the rotor magnet such that the latter is subject to magnetic torque for rotating the rotor within the cylindrical bore to align with the magnetic field generated by the coils. 
         [0010]    In a preferred embodiment of the above motor/actuator, the rotor is comprised of the shaft and one or more, diametrically polarized cylindrical permanent magnets, preferably of rare earth material type, such as NdFeB or SMCo. Preferably, the magnet(s) includes a keyed central opening configured to receive a key coupled to the shaft, which traverses the magnet(s), to cause the shaft to rotate with rotation of the magnet(s), thereby to provide torque output from the motor/actuator to an appliance mechanically coupled to the shaft outside the stator. 
         [0011]    Preferably, the plurality of coils is configured to be energized (by a controller or otherwise) in sequential order to cause continuous rotary motion of the rotor within the stator. Thus, a BLDC servo motor can be implemented that can be used to deliver regulated continuous torque to an appliance connected to the shaft. Further, the controller can also be configured to selectively energize and fully de-energised (with backward or forward current-flow) by a controller for cogging the rotor into a limited number of positions and thus provide a stepper motor. Furthermore, a more sophisticated driver (controller) may be employed to proportionally control the power to the coils, allowing the rotor to position between the cog points and thereby rotate extremely smoothly. The skilled person will appreciate that motor/rotary actuator embodiments of the invention may be used in a vast area of applications, from small dimension stepper motor applications, to larger, electronically commutated DC motors with single or multi-phase coil windings and precise speed control for electric vehicular applications. 
         [0012]    Advantageously, the motor/actuator further includes a first end cap coupled to the first end of the stator and a second end cap coupled to the second end of the stator, each of the first and second end caps having a polygonal footprint equal or similar to the cross-section of the stator body and including a crenelated polygonal wall with crenels arranged to receive and locate the plurality of coils as these wind about the end caps. Such end caps can be easily machined from suitable non-ferromagnetic stock materials, such as aluminium, or fine cast, and serve to secure the position of the coils on the stator, without separate bobbins. 
         [0013]    The above-mentioned and other features and advantages of this invention, and the manner of implementing it, will become more apparent and the invention itself will be better understood by reference to the following description of a preferred embodiment of the invention provided with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a perspective view of a motor/actuator embodiment of the present invention; 
           [0015]      FIG. 2  is a side elevation of the motor/actuator of  FIG. 1 ; 
           [0016]      FIG. 3  is a top plan view of the motor/actuator of  FIG. 1 ; 
           [0017]      FIG. 4  is a perspective view of a stator and magnet according to one embodiment for use in the motor/actuator of  FIG. 1 ; 
           [0018]      FIG. 5  is an exploded, perspective view of the motor/actuator of  FIG. 1 ; and 
           [0019]      FIG. 6  is cross-sectional view of the motor/actuator of  FIG. 1  taken along line A-A of  FIG. 3 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
       [0020]    Corresponding reference characters indicate corresponding parts throughout the several Figures. The exemplifications set out herein illustrate exemplary embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention. 
         [0021]    Referring now to  FIGS. 1-3 , a motor/actuator  10  according to one embodiment of the present invention generally includes a stator body  12 , a rotor assembly  14  ( FIGS. 4 and 5 ) including a shaft  16  and a permanent magnet  36 , a first end cap  18 , a second end cap  20 , and three solenoid coils  22 . 
         [0022]    Stator  12  is formed as a hexagonal prism, with a regular polygonal cross-section of even order. Stator body  12  can be made from a variety of materials, including stacked laminations of electrical steel or similar material, concentric polygonal shells of a suitable amorphous ferromagnetic alloy, or machined from suitable ferromagnetic stock material. 
         [0023]    As best shown in  FIGS. 4 and 5 , a cylindrical bore  24  is machined axially through stator body  12  from a first axial end face  26  to an opposite, second axial end face  28 , perpendicular to both end faces  26 ,  28  and centred within stator  12 . The inner surface  30  of bore  24  is, in one embodiment, polished to be smooth, and held to a close tolerance. Threaded holes  32  or similar fastening features are machined into both axial end faces  26 ,  28  of stator  12 , located around bore  24 , to provide mounting points for the end caps  18 ,  20 , and optionally an external housing (not illustrated), to seal and protect stator  12  as is further described below. 
         [0024]    In the embodiment shown, stator body  12  has six planar, exterior side faces  34 A-F of equal size, sides for short. As best shown in  FIG. 4 , side  34 A is parallel to and opposite of side  34 D, side  34 B is parallel to and opposite of side  34 E and side  34 C is parallel to and opposite of side  34 F. While the hexagonal cross-section of stator body  12  provides six sides in this embodiment, it should be understood that more (but preferably not fewer) sides may define the periphery of stator body  12 , as long as the overall number of exterior sides is even in number. That is, octagonal or decagonal cross-section stator bodies are also contemplated. 
         [0025]    Referring now to  FIGS. 4 and 5 , rotor assembly  14  includes, apart from shaft  16 , a single, cylindrical, diametrically polarized (dipole) permanent magnet  36 , having an outer peripheral face  38 , a first axial end face  40  and a second axial end face  42 . The term diametrically polarised serves to denote a magnetic body in which a half-cylindrical N pole and a half-cylindrical S pole are separated by a diameter plane (as schematically represented by the chain line on the visible axial end face of magnet  36  in  FIG. 4 ) extending between the opposite axial end faces  40 ,  42  of the magnet  36 ,. A centred, keyed bore  44  extends along a central axis  46  of magnet  36  (and thus shaft  16  and stator  12 ) between the end faces  40 ,  42  of magnet  36 , thereby allowing magnet  36  to be mechanically coupled to shaft  16  through its centre. 
         [0026]    While not illustrated herein, magnet  36  may also be formed from one or more, diametrically magnetized hollow cylindrical shells, or several discrete magnet elements installed into a central rotor body by slots or some other method of direct attachment such as fasteners or adhesion. Equally, rather than having a single cylindrical permanent magnetic material body  36 , a number of discrete cylindrical magnets (each with a keyed bore extending there through) may be mounted in sequence along the shaft  16 . Alternatively, magnet  36  may be a composite body comprised of one (or more) rectangular, active permanent magnetic material body (or bodies) magnetised in a thickness direction thereof, with passive ferromagnetic material pole (extension) elements attached to the opposite mayor faces of the magnet, the pole elements shaped to form a cylinder about the centrally located active permanent magnetic material body (or bodies), whereby such rotor also has a N and a S pole at diametrically opposite sides of the centrally located active magnetic material. It should be understood also that multiple polarity magnets, eg quarto-poles, may be used. 
         [0027]    Other magnet configurations may be employed with the rotor  14 , consistent with the teachings of the present invention. Nonetheless, the preferred embodiment uses a single permanent magnetic material cylindrical body, as the most magnetically efficient arrangement, given that passive ferromagnetic pole extension materials add ‘dead’ weight to the rotor assembly  14  and decrease magnetic efficiency. 
         [0028]    Cylindrical permanent magnet  36  is installed so that shaft  16  lies coaxial with central axis  46 , a minimum air gap being present between the facing exterior cylindrical surface  38  of magnet  36  and inner cylindrical surface  30  of bore  24  of stator body  12 . In one embodiment, magnet  36  lies completely within bore  24 , but is sufficiently long such that its terminal end faces  40 ,  42  terminate very close to the end faces  26 ,  28  of stator  12 , respectively. 
         [0029]    Keyed bore  44  of magnet  36 , in one embodiment, includes a cylindrical opening  46  and a pair of opposed slots  48 ,  50  extending into magnet  36  from terminal end  40 . Slots  48 ,  50  are sized to receive a rectangular key  52  which fits within a slot  54  formed through shaft  16  to retain and fix the rotary position of magnet  36  with shaft  16 . In one embodiment, shaft  16  consists of a non-magnetic material that runs through the centre of magnet  36 , or has the required features to retain individual magnet elements by slots, fastening or adhesion. 
         [0030]    Shaft  16  includes a first end  56 , a second end  58 , and a shoulder  60 . Ends  56 ,  58  include any of a variety of bearing components (not shown) that cooperate with counter-bearing features/components secured in proximity about bore  66  of end caps  18 ,  20 , or an external housing (not shown) to permit rotation of shaft  16  with magnet  36 . Shoulder  60  engages with counter-bearing elements at an inward surface of end cap  18  to limit the extent to which shaft  16  extends through end cap assembly  18 . As will be apparent to those skilled in the art, other configurations may be used to control the extent of shaft  16  as well as its rotation. 
         [0031]    As indicated above, rotor assembly  14  is fixed relative to end caps  18 ,  20  so that magnet  36  may rotate within bore  24  of stator  12  in an axially defined position. Rotor assembly  14  will remain parallel with axis  46  through bore  24  to prevent contact between outer surface  38  of magnet  36  and inner surface  30  of bore  24 . In addition, shaft  16  is appropriately locked against travel parallel, or into and out of bore  24 , so that magnet  36  remains axially positioned within bore  24 . 
         [0032]    End caps  18 ,  20  are similar in construction. The interface between shaft  16  and end cap  18 ,  20  may differ somewhat in various embodiments depending upon how one of ordinary skill in the art may want to implement, in detail, the bearing components for shaft  16  at the respective end caps  18 ,  20 . Nonetheless, given the external similarities, only end cap assembly  18  is described in detail herein. 
         [0033]    End cap  18  comprises a base plate  62 , hexagonal in plan view, with six integrally formed merlons (or cops, protrusions)  64 A-F, extending perpendicularly from a mayor face of base plate  62 , thereby resembling a hexagonal, crenelated wall standing proud from the base plate  62 , with six identical crenels  76  between the cops  64 A-F. As noted, base plate  62  has a central opening  66  configured to receive end  56  of shaft  16 . Further, each protrusion (cop)  64 A-F has a through hole  68 , extending from the terminal top end into and through base plate  62 , which aligns with a corresponding threaded hole  32  in the end face  26  of stator  12  when end cap  18  is mounted to stator  12 . Through holes  68  are sized to receive fasteners  70  and include recesses  72  to receive the heads  74  of fasteners  72 . 
         [0034]    As noted, each pair of adjacent protrusions  64 A-F form between them a crenel (channel)  76  that is positioned centrally relative to a corresponding side  34 A-F of stator  12  when end cap  18  is mounted to stator  12 . The crenels  76  of opposing pairs of protrusions  64 A-F are aligned to facilitate winding and retention of coils  22  at stator body  12  as is further described below. 
         [0035]    End cap base plate  62  and protrusions  64 A-F are, in one embodiment, made of a nonmagnetic material such as aluminium, formed to match the cross section of stator body  12 . Central opening  66  of base plate  62  may include additional counter bores and features on either side of base plate  62  to accommodate counter-bearing components, as has been alluded to above, for rotor shaft  16 . These bearing components interface with the features machined on end  56  of shaft  16 , fixing the axial position of magnet  36  relative to stator  12  but permitting free rotation of magnet  36  as indicated above. 
         [0036]    Crenels  76  of end caps  18 ,  20  act as receptacles and guides for the set  22  of three (solenoid) coils  78 , whereby courses of wire are wound around end caps  18 ,  20  to extend parallel to and in contact with the pairwise opposite sides  34   a ; 34   d,    34   b , 34   e  and  34   c;    34   f  of stator body  12 . Crenels  76  thus serve to secure and retain the coil set  22  on the outside of rotor body  12  with out additional fastening elements. It may be further noted that the six crenels  76  converge towards the centre of each end cap  18 ,  20 . At this convergence location, the windings of coils  78  are routed to define an annular passage for end portion  56  of shaft  16  which protrudes end cap  18 ,  20  beyond central openings  66 , for coupling with a torque receiving appliance or component. 
         [0037]    As best shown in  FIG. 5 , coil set  22  includes, in this embodiment, three individual solenoid coils  78 . As best shown in  FIG. 1 , each coil  78  is wrapped around both end caps  18 ,  20  and stator  12 . Coils  78  are wound repeatedly from end cap  18  to end cap  20 , around one set of parallel side faces  34 A-F of stator  12 . Coils  78  are routed through opposing crenels  76  on end cap  18 , down the centreline of one side face  34 A-F of stator  12 , through the corresponding opposing crenels  76  on the axially opposite end cap  20 , and up the centre of the opposite, parallel side face  34 A-F of stator  12 . By this positioning, a series of three, similar, rectangular coils  78  are wound around stator  12  and end caps  18 ,  20 , radially and angularly symmetric about central axis  46  of stator  12 . While the individual windings may be suitably isolated as in conventional electric motors, the rectangular coils  78  will come into contact with the stator&#39;s external planar faces  34 A-F, thus improving coupling of the induced magnetic (B-) field into stator body  12 , as noted below. 
         [0038]    In order to operate the motor/actuator  10  described above, the skilled person will know that current is selectively applied to coils  78 . The application of current to each of coils  78  induces a magnetic field (denoted B-field) within the volume enclosed by the relevant coil  78 . As coils  78  are wrapped around the entire stator  12 , the B-field also magnetizes stator  12  in a certain direction depending upon which of the three coils  78  is/are energised, and the direction of current flow within the energised coils. The magnetization of stator  12 , in addition to the induced B-field of the coils  78 , creates a magnetic torque on magnet  36  in bore  24 , given that it is free to rotate, as long as the vector of the permanent magnetic field (denoted H-field) of the diametrically magnetised permanent magnet  36  is not aligned with the B-field vector of the energised coil  78 , causing it to rotate with shaft  16  about axis  46  in seeking to align the H-field vector of the rotor  14  with the prevailing B-field vector of the stator coils  78 . 
         [0039]    As magnet  36  initiates rotation from a rest position, peripherally subsequent coil(s)  78  to the one used to initiate rotor rotation, may be energised sequentially, thus also changing direction of magnetisation of stator body  12 . Thus, magnet  36  may be continually pulled along in rotation about axis  46  as the direction of magnetization of stator  12  is changed by charging the coil(s)  78  in a given sequence. 
         [0040]    Coils  78  may be sequenced (ie energised) in a variety of ways—one coil  78  may be charged at a time, in rotational progression, causing continuous motion of magnet  36 . Alternatively, coils  78  may be charged in opposite directions to boost the field within stator  12  and increase force upon magnet  36 . 
         [0041]    The skilled person is cognisant of various modes of energising the solenoid coils  78  in slot-less, brush-less DC motors, as well as controllers that may find use to achieve different modes of operation (eg as a rotary servo actuator, stepper motor, etc) and for further detail the person skilled in the art is directed to relevant standard literature. 
         [0042]    While this invention has been described with reference to an exemplary design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.