Abstract:
A linear motor ( 30 ), including a mover and a stator, the mover having a cylindrical body ( 33 ) that forms an elongate circular bore ( 31 ) and the stator being an elongate shaft disposed within the bore. The cylindrical body ( 33 ) includes a plurality of electrical windings ( 32 ) and the shaft includes a synchronous or variable reluctance topology, or a plurality of magnets. Electrical energising of the windings ( 32 ) results in relative movement and/or force generation between the cylindrical body ( 33 ) and the shaft. The cylindrical body ( 33 ) being disposed within a housing ( 37 ) with a coolant space ( 35 ) being formed between the cylindrical body ( 33 ) and an internally facing cylindrical surface ( 38 ) of the housing ( 37 ). The coolant space ( 35 ) being formed along at least a major portion of the length of the cylindrical body ( 33 ) and the coolant space ( 3 ) being substantially cylindrical and of substantially constant cross-section.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a linear motor. 
       BACKGROUND OF INVENTION 
       [0002]    Linear motors are widely used in a variety of machines and devices. Forms of linear motors include flat-bed linear motors and tubular linear motors. Linear motors provide direct linear movement, as opposed to linear movement provided by rotary motors that convert rotary movement to linear movement such as though gears or screws, belts or pulleys. Elimination of devices to convert rotary movement to linear movement reduces the complexity and cost of drive arrangements. Linear motors can operate at very high speeds and with high acceleration. Linear motors also are very reliable given that they have few moving parts and are highly accurate and can operate with low vibration. 
         [0003]    Linear motors include a forcer or mover and a stator. The mover is the moving part of the motor and the stator is stationary. The mover includes coils and the stator is magnetic or magnetic field reactive such as including magnets, so that when the coils are energised, relative movement and/or force between the mover and stator takes place. 
         [0004]    A typical linear motor includes a housing that is square in cross-section and that includes a central bore that circular. The housing includes windings wound around the bore. A shaft of circular cross-section extends through the bore and projects out either end. The shaft houses the magnets (if they are used). Either of the housing or the shaft can be fixed so that the other of the housing or the shaft can move or provide force. The resultant movement and/or force is linear. 
         [0005]    Linear motors produce heat as they operate, so that they often include a coolant system to dissipate heat. In some prior art arrangements, the coolant system includes coolant attachments that are applied to one or more surfaces of the housing and coolant passes through the attachments to dissipate heat. In some arrangements, a coolant attachment is applied to one side of the square housing and extends for the length of the housing. In other arrangements, a coolant attachment is applied to two or more sides of the housing and each attachment extends for the length of the housing. It is not normal for a coolant attachment to be applied to all four sides of the housing, given that often the housing has external fittings or connections, such as components for mounting it to machinery or devices and accordingly, often the coolant attachment is applied to only one or two of the housing sides. This means that the coolant attachment or attachments can be less effective in dissipating heat that is generated in sections of the linear motor that are spaced or remote from where the coolant attachment or attachments are applied. 
         [0006]    See for example  FIG. 1 , in which a schematic cross-sectional illustration of a prior art linear motor  10  is shown. The linear motor  10  has a square housing  11  enclosing a circular coil or winding  12 . A coolant attachment  13  is applied to the top wall  14  of the housing  11 . In  FIG. 1 , it can be seen that the bottom wall  15  is spaced furthest from the coolant attachment  13 . It follows that heat generated in the winding  12  adjacent the bottom wall  15  is less easily dissipated than heat generated adjacent the top wall  14 , or adjacent the side walls  16  or  17 . 
         [0007]    Also, even in the sections of a linear motor that are proximate a coolant attachment, the spacing between the coils and the external side walls of the housing varies. For example,  FIG. 1  shows that the winding  12  is spaced from the top, bottom and side walls of the housing  11  a greater amount at the corners of the housing  11  than intermediate the corners shown, so that dissipation of heat even on walls of the housing on which coolant attachments are provided can vary. 
         [0008]    Thus, in some forms of the prior art, heat generated in the linear motor is not evenly dissipated and in addition, there can be large thermal variation in the heat generated within the motor, both of which can affect thermally sensitive components in the immediate vicinity of the linear motor. This is a particular problem where the linear motor is employed in high precision machinery, such as high precision grinding and milling machinery, where even small temperature fluctuations can affect the accuracy of the machinery. 
         [0009]    Linear motors are also more efficient than other actuators such as ball screws and their use would be preferred if the heat they generate can be adequately dissipated. 
         [0010]    Linear motors can also be difficult to mount to machinery and devices. Most linear motors known to the applicant are “face mountable” which means that a face of the housing of the motor mounts against a face of the machinery with which the motor is to be used.  FIG. 2  illustrates such a mounting and shows a linear motor  20  with a rectangular housing  21  of square cross-section and an elongate shaft  22  extending through the housing  21 . The hatched housing surface  23  forms a mounting face and includes four threaded openings  24  for receiving fasteners to fix the linear motor  20  to a machine. Four further threaded openings  26  (only three of which are visible in  FIG. 2 ) are formed on the front surface  27  of the housing  21  for fixing the linear motor  20  to a machine from the front surface. While either of the surfaces  23  or  27  provides secure fixing, ready access to the fasteners that secure the linear motor  20  to the machinery is not often provided, so that installation and removal of the linear motor  20  from the machinery is not easy. 
         [0011]    It is an object of the invention to overcome or at least alleviate one or more of the difficulties associated with prior art arrangements. 
       SUMMARY OF INVENTION 
       [0012]    In one embodiment of the invention there is provided a linear motor that includes a mover and a stator, the mover having a cylindrical body that forms an elongate circular bore and the stator being an elongate shaft disposed within the bore, the cylindrical body including a plurality of electrical windings and the shaft including a synchronous or variable reluctance topology, or including a plurality of magnets, whereby electrical energising of the windings results in relative movement or force generation between the cylindrical body and the shaft, the cylindrical body being disposed within a housing with a coolant space being formed between the cylindrical body and an internally facing cylindrical surface of the housing, the coolant space being formed along at least a major portion of the length of the cylindrical body, the coolant space being substantially cylindrical and of substantially constant cross-section. 
         [0013]    A linear motor of the above kind is envisaged to provide advantages over the prior art because it provides for even heat dissipation about the windings. That is, the cylindrical coolant space encircles the windings in a manner that the spacing between the windings and the coolant space is constant, or in other words, the proximity of the windings to the coolant space is constant or does not vary. Advantageously, this means that all sections of the windings are cooled equally so that the linear motor does not generate greater heat in some parts of the motor than in others. This allows a linear motor according to the invention to be installed more readily in the immediate vicinity of thermally sensitive components either without affecting the operation of those components, or affecting those components in a more predictable manner. Either outcome is advantageous given that if the thermal effect on components in the immediate vicinity of the linear motor is negligible or predictable as a result of employing the present invention, design of machinery or equipment that employs the linear motor can be less difficult. Moreover, the advantages that flow from the use of linear motors can be achieved in machinery or equipment that would otherwise not be able to use linear motors because of the difficulties associated with prior art linear motors. 
         [0014]    A linear motor according to the invention can also be of more compact shape than prior art linear motors because the shape of the housing can be more compact by the absence of coolant attachments of the above described kind. Moreover, the cylindrical body of the linear motor can be arranged to fit into existing actuator housings such as ball screw housings, by sliding the cylindrical body into the housing after suitable modification of the housing as might be required, such as after increasing the internal diameter of the housing. This means that retrofit is possible, thereby allowing the advantages that flow from the use of linear motors to be embodied in machinery or equipment that previously used other forms of drive. 
         [0015]    For example, because of improved dissipation of heat and ease of retrofit, a linear motor according to the invention is expected to enable relatively easy replacement of existing ball screws actuators, for improved performance. 
         [0016]    The coolant arrangement discussed above can form a thermal barrier between the linear motor and surrounding components. By the complete encirclement of the windings by the coolant space (which is substantially cylindrical), thermal transfer from the linear motor can be minimised, or even be negligible. This again differs from the prior art which employs a coolant attachment applied to just one side of a square housing, or even two or three sides of the housing, whereby thermal escape can occur through sides of the housing that do not have a coolant attachment. 
         [0017]    In addition, in a linear motor according to the present invention, an insulation layer can be positioned within the coolant space, such as against the internally facing cylindrical surface of the housing, for the purpose of reducing thermal transfer from the coolant space to outside the linear motor. This is appropriate where the coolant system of the linear motor is capable of removing all, or substantially all of the generated heat which is captured within the coolant space. The insulation layer should be of low thermal conductivity. The insulation layer can be made of rubber or ceramic for example. Other possibilities include plastics, composites (fibre glass, G11, carbon fibre) or epoxy. 
         [0018]    Still further, the opposite ends of the linear motor can be made of a thermal and/or electrically insulated layer or material to form a thermal and/or conductive barrier at each end of the motor and thus to further capture generated heat in the coolant space. The thermal and/or electrically insulated layer or material can be made of the same materials as listed above in relation to the insulation layer. 
         [0019]    The coolant space can be formed in any suitable manner. In some forms of the invention, the windings of the cylindrical body are located within a cylinder that extends for the length of the windings and the coolant space is formed on the opposite side of the cylinder to the windings. In this form of the invention, the cylindrical housing extends about the cylinder and is spaced from the cylinder to form the coolant space. The cylinder can be formed from aluminium or other suitable metal, or other non-magnetic material. The cylinder can be in contact with the outer surface of the windings, or as close to the surface as possible, so that heat from the windings is conducted directly to the cylinder for dissipation into the coolant space. In some forms of the invention, the windings are immersed or embedded in a resin, such as an epoxy resin, and the cylinder can be in contact with the resin coating of the outermost windings. 
         [0020]    In the above form of the invention in which the windings of the cylindrical body are located within a cylinder, the cylindrical body can be provided without a housing for later insertion into a housing. This can be suitable for example where the housing is an integral part of a machine, such as part of a cast part of a machine. This can also be suitable where the linear motor of the invention is being employed to replace a ball screw and the housing of the ball screw is to be used (perhaps with some modification) to house the cylindrical body. The invention therefore extends to a cylindrical body as described herein as a separate component to the housing, but which is configured to interact with a housing in the manner described. The present invention is unique in this respect, in that no linear motor known to the applicant can be inserted into an existing housing in the manner proposed in the present invention. 
         [0021]    The coolant space can have at least one inlet and outlet so that coolant can be introduced into the coolant space through the inlet and discharged through the outlet. The coolant can be cooled before reintroduction into the coolant space via the inlet or the coolant can be of a kind which is not reused, water for example. 
         [0022]    The coolant space can be open throughout its length, or it can include passageways, or disturbances to direct or disrupt the flow of coolant through the coolant space, or to make the flow turbulent. In some forms of the invention, the coolant space can include a spiral or helix so that coolant flows between the inlet and outlet in a spiral or helix path. This increases the time that the coolant will spend in the coolant space before it reaches the outlet. 
         [0023]    Alternatively, the coolant space can include projections that the coolant is required to flow about between the inlet and the outlet. Other structures include fins that extend lengthwise of the linear motor. The fins can direct coolant between a pair of adjacent fins in one direction only, or the fins can be constructed for return movement of the coolant along an adjacent pair of fins. The coolant can be liquid or gas, although liquid is most likely. 
         [0024]    In other embodiments of the invention there is provided a linear motor that includes a mover and a stator, the mover having a cylindrical body that forms an elongate circular bore and the stator being an elongate shaft disposed within the bore, the cylindrical body including a plurality of electrical windings and the shaft including a synchronous or variable reluctance topology, or including a plurality of magnets, whereby electrical energising of the windings results in relative movement and/or force generation between the cylindrical body and the shaft, the cylindrical body being disposed within a housing that has opposite first and second ends, whereby the cylindrical body includes a flange for attachment to one of the first and second ends for mounting the cylindrical body within the housing. 
         [0025]    The use of a flange formed at one of the first and second ends allows the linear motor to be securely fixed in place, but in addition, allows ready access to the fasteners that secure the linear motor to the machinery. This means that installation and removal of the linear motor from the machinery is easier than in prior art linear motor that employ face mountings. 
         [0026]    In the embodiment of the invention in which the housing includes a flange formed at one of the first and second ends for mounting the housing to a machine, the housing can be a cylindrical housing, or it can be a square housing in accordance with linear motors of the prior art which also employ face mountings. In either forms, the benefits of improved access to fasteners for installing and removing the linear motor are provided. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0027]    In order that the invention may be more fully understood, some embodiments will now be described with reference to the figures in which: 
           [0028]      FIG. 1  shows schematically in cross-section, a prior art linear motor arrangement with a coolant attachment. 
           [0029]      FIG. 2  shows a prior art linear motor with a face mounting arrangement. 
           [0030]      FIG. 3  is a cross-sectional view of a linear motor according to one embodiment of the invention. 
           [0031]      FIG. 4  is an exploded view of a linear motor according to one embodiment of the invention for installation in a machine component. 
           [0032]      FIG. 5  is a view of a cylindrical body for use in a linear motor according to the invention. 
           [0033]      FIG. 6  is an alternative view of a cylindrical body for use in a linear motor according to the invention. 
           [0034]      FIG. 6 a    is a detailed view of a portion of the cylindrical body of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    With reference to  FIG. 3 , a cross-sectional view of a linear motor  30  is illustrated where the cross-section is taken perpendicular to the lengthwise axis of the motor. The motor  30  includes an elongate circular bore  31  defined by electrical windings  32  (copper windings for example) and a cylinder or cylindrical body  33  which is shown in touching engagement with the outer face  34  of the windings  32 , but which could, in alternative embodiments, be slightly spaced from the outer surface  34 . 
         [0036]    A coolant space  35  encircles the body  33  and forms a space within which coolant can flow to dissipate heat which is generated by the windings  32 . The coolant can be liquid or gas, although liquid is most likely. The coolant space  35  is defined between the outer surface  36  of the body  33  and the facing inner surface  38  of the cylindrical housing  37 . In  FIG. 3 , the housing  37  is shown as cylindrical on the inner surface  38  as well as the outer surface  39 . However, it should be appreciated that the shape of the housing in respect of the outer surface is not particularly important to the invention, and for example, the housing could be square or rectangular as an example, or otherwise shaped. Likewise, the outer surface could include fins for heat dissipation, mounting lugs or a variety of other fittings such as might be required to fix the housing in place relative to a machine or machine component. 
         [0037]    An insulation layer can be positioned within the coolant space against the inner surface  38  of the housing  37 . The insulation layer can have low thermal conductivity and can be made of rubber or ceramic for example. The insulation layer will reduce thermal transfer from the coolant space  35  through the housing  37  to outside the linear motor  30 . 
         [0038]    The linear motor  30  would also include an elongate shaft which is disposed closely within the bore  31 . In forms of the invention not limited to that illustrated in the drawings, the shaft could be a hollow shaft which is non-magnetic, and which includes a plurality of magnets, such as rare earth magnets, and in some forms of the invention, these can be spaced apart by steel spacers. The shaft can include magnets that are assembled side by side with the magnet polarity reversed. In some arrangements, two or more magnets would be placed side by side with the magnet polarity in the same direction, and then a next set of magnets would be assembled adjacent the first set with the polarity in the opposite direction. Spacers can be interposed between the adjacent magnets or the adjacent sets of magnets. In this arrangement in respect of  FIG. 3 , when the windings  32  are energised, either the shaft will move within the bore  31 , or if the shaft is fixed, the windings  32  and the other components described as extending about the windings  32  would all move relative to the shaft. Control of the energisation of the windings  32  results in control of the relative movement and/or force between the shaft and the windings. 
         [0039]    The coolant space  35  forms a space within which coolant can flow between an inlet and an outlet for the purpose of dissipating heat which is generated within the windings  32 . The cylindrical housing  37  effectively forms a cooling jacket to confine coolant to between the outer surface  36  of the body  33  and the inner surface  38  of the housing  37 . The inlet and outlet that facilitates ingress and egress of coolant from within the coolant space  35  can be placed in any suitable position and take any suitable form. The coolant can be injected into the coolant space  35  through a port under pressure, or it can be gravity fed. 
         [0040]    The coolant space  35  is shown in  FIG. 3  as being an open space. While this is acceptable, a preferred arrangement is illustrated in  FIG. 4 , in which a helix or spiral formation  40  extends along the length of the body  33  and which creates a spiral or helical path along the length within which coolant can flow. This can increase the time taken for coolant to exit the coolant space  35 , and can thus allow the coolant within the space  35  to absorb a greater amount of heat for dissipation. Alternative arrangements to such a helical or spiral formation include a series of parallel and spaced apart cylindrical flanges or fins, that include openings or breaks, to allow coolant to flow through the flanges or fins between opposite ends of the linear motor. These arrangements can be used with liquid or air cooling. Other arrangements could be employed to create a convoluted path within the coolant space  35 , for the purpose of slowing the speed of flow through the coolant space, creating a turbulent flow, or for ensuring that coolant uniformly flows completely about the coolant space  35  and thus about the windings  32 . 
         [0041]    What is important, is that the inner surface  38  be substantially cylindrical, so that the coolant space  35  is also formed to be substantially cylindrical and of substantially constant cross-section throughout the length of the windings  32  despite the existence of a helical or spiral formation or flanges or fins as discussed above. 
         [0042]    With reference to  FIG. 4 , the cylindrical body  33  is illustrated removed from the cylindrical housing  37 , in order to illustrate the spiral  40  which is formed on the outer surface  36  of the cylindrical body  33 . The outer surface  41  of the spiral  40  is at a height which is a very close fit against or close to the inner surface  38  of the housing  37 . This close fit is intended to prevent leakage of coolant fluid past the spiral  40 , over the top of the outer surfaces  41 . While some leakage can be tolerated, the intention is that the majority of the cooling fluid takes a spiral path from one end of the linear motor  30  to the other, along the spiral  40 . 
         [0043]    Not evident in  FIG. 4 , is the windings  32 , which are radially within the cylindrical body  33 . 
         [0044]    Also not evident in  FIG. 4 , is an insulation layer applied to the inner surface  38  of the housing  37 , for the purpose of reducing thermal transfer from the coolant space  35  to outside the linear motor  30 . 
         [0045]      FIG. 4  also illustrates a machine component  45  to which the cylindrical housing  37  has been formed integrally. Inner surface  38  and outer surface  39  of the housing  37  are also identified in  FIG. 4 . 
         [0046]    Alternative to the  FIG. 4  arrangement, the housing  37  could be attached by suitable fasteners to the machine component  45 , such as to an end or underneath surface. 
         [0047]    The other components of the linear motor  30  have been assembled externally of the housing  37  and in  FIG. 4 , are ready for insertion into the housing  37 .  FIG. 4  conveniently illustrates that the outer surface  39  of the housing  37  is not required to be cylindrical, but rather, can include a shape or profile suitable for attachment to the machine component  45  and suitable for the attachment of other components to the housing  37 , such as coolant inlet and outlet ports. 
         [0048]      FIG. 5  illustrates a form of cylindrical body  47  which is very similar to the cylindrical body  33  of  FIG. 4 , but illustrates the use of fins  48  that extend lengthwise of the body  47 . The fins  48  direct coolant between a pair of adjacent fins in one direction only (axially in the embodiment illustrated), but the fins can be constructed for return movement of the coolant along an adjacent pair of fins by terminating some of the fins prior to their illustrated end points. 
         [0049]    Returning to  FIG. 4 , this also illustrates an example of the second embodiment of the invention, in which the linear motor  30  includes a mounting flange  50 , that is attached to one end of the cylindrical body  33  and which includes screw openings  51  for receipt of screws  52  for threaded engagement within threaded openings  53  of the mounting face  54  of the housing  37 . Alternatives to the screws  52  include the use of studs, welding or gluing. The illustrated arrangement enables the secure fixing of the cylindrical body  33  and associated components within and to the housing  37 , and thus to the machine component  45 . It will readily be appreciated, that in the arrangement shown, access to the screws  52  is easily facilitated, as compared to the arrangement of  FIG. 2 , where screw access can be more difficult. 
         [0050]    Clearly the shape of the flange  50  could take other forms and a greater or lesser number of screw openings and screws could be employed. 
         [0051]      FIGS. 6 and 6   a  illustrate a cylindrical body  60  which is very similar to the cylindrical body  33  of  FIG. 4 , but which includes a longitudinal slit or gap G completely through the body  60  between the opposite ends  62  and  63  (see  FIG. 6 a    for better illustrating the gap G). This form of cylindrical body eliminates the formation of electromagnetic induction in the cylindrical body  60 , so that a magnetic field that would otherwise oppose relative movement between the mover and the stator of the linear motor is not developed. In other words, in a linear motor according to the invention, the cylindrical body can be formed circular but be split longitudinally to prevent electromagnetic induction (large eddy current) which advantageously will eliminate large cogging forces for high speed application. 
         [0052]    It will be appreciated from the construction of the linear motor  30  of  FIGS. 3 and 4  that the motor  30  can provide for even heat dissipation about the full circumference of the windings  32 . Moreover, by the arrangement disclosed, the coolant space forms a thermal barrier between the linear motor  30  and other machine components, such as the machine component  45 . Thus, where machine components are thermally sensitive, the heat generated by the linear motor  30  does not build up or remain in place to affect those components. The use of the insulation layer as described above in contact with the inner surface of the housing  37  will assist this, as will the use of a thermal barrier at each end of the motor  30 . Still further, the provision of the spiral  40  formed as an integral part of the cylindrical body  33  (formed by machining or casting for example), permits the coolant space  35  to be easily integrated into the linear motor  30 . This contrasts with the prior art, in which a coolant attachment is attached to a wall of the housing of a linear motor (as shown in  FIG. 1 ), with the consequential disadvantages as described above. 
         [0053]    The linear motor which is disclosed in  FIGS. 3 and 4  is expected to increase the force output for a prior art motor of the same size. This occurs because force output is relative to the amount of current drawn by the motor. As the current and force is increased, so is the heat. If a portion of the heat is removed, the current, can be increased because the difficulties associated heat build-up are not realised. 
         [0054]    Moreover, the disclosed arrangement which employs the mounting flange  50  is also expected to enable the linear motor of the invention to replace ball screws and ball nuts that are also flange mounted, for improved performance. 
         [0055]    The coolant that can be used with a linear motor according to the invention and including according to the embodiments of  FIGS. 3 and 4  can be a cooling liquid of any suitable form, or alternatively, air cooling could be employed. As described above, the coolant path need not be necessarily take a helix or spiral form, but rather, the coolant space can simply be an open cylindrical space, or can include projections, fins or other disruptors or disturbances to alter the direction of flow through the coolant space, or to create turbulence in that flow. 
         [0056]    The invention advantageously integrates a coolant space or jacket into a linear motor and in an alternative form, provides for flange mounting. Each of these improvements is particularly suited to the use of linear motors in the machine tool industry. Linear motors have not been employed in common practice in the machine tooling industry to date, despite the advantages they provide, given that linear motors are disadvantageous in terms of the heat output they give and the difficulty in their mounting. The heat output of linear motors is particularly problematic for high precision machines, particularly where those machines are required to provide highly accurate repeatability. In that type of machine, thermal growth in components of the machine as a result of heat output from a linear motor cannot be tolerated. Where linear motors have been implemented in the machine tooling industry, the poor thermal dissipation provided to date has led to the requirement for separate chiller systems to be employed to minimise heat transfer between the motor and the machine components. Disadvantageously, this adds cost and complexity. 
         [0057]    The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the present disclosure. 
         [0058]    Throughout the description of this specification the word “comprise” and variations of that word, such as “comprises” and “comprising”, are not intended to exclude other additives or components or integers.