Abstract:
A motor rotor assembly that includes multiple motor rotor sections and a rotor bar that extends through the motor rotor sections, such that the rotor bar and the motor rotor sections are configured such that the rotor sections are step-skewed, or continuously skewed, from each other. The assembly may be used in an IPM or Synchronous Reluctance motor; and, the motor rotor sections may be of solid core or laminations. Various assembly components, IPM and Synchronous Reluctance motors, and methods of construction/assembly are also disclosed. The present invention has been described in terms of specific embodiment(s), and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to electric machines and, more particularly, to electric machines, such as interior permanent magnet machines and Synchronous Reluctance motor machines, that have offset rotor sections. 
         [0002]    One general application for electric machines, and interior permanent magnet (IPM) machines in particular, is for use in underground mining vehicles, wherein typically electric wheel motors (e.g., IPM) are connected to the wheels via a gearbox. This application typically requires very high torque at low speeds and yet maintenance of the rated power over a very wide speed range (e.g., on the order of 15:1). 
         [0003]    IPM machines unfortunately suffer from both a manufacturing and a technical (i.e., electro-mechanical) shortcoming. With IPM machines, permanent magnets typically are inserted into slots in the rotor structure and pushed entirely through the entire slot depth in order to fill the entire stack length. Due to small clearances between the magnets and the slots in the laminations, and the unevenness of such slots along the entire length, the magnets and/or laminations may be damaged during this insertion process. 
         [0004]    Further, depending on their magnitude, torque “ripple”, or torque oscillations, of the IPM, may result in damage to the rotor, the gearbox, and/or the mechanical system(s) connected to the IPM (due to fatigue or excessive torque). Additionally, the frequency of the torque ripple might excite resonant modes of the mechanical system(s), further posing an additional threat to the IPM and/or surrounding systems. With regards to torque ripple, similar attributes and shortcomings may also be found, in part, with Synchronous Reluctance motors. 
         [0005]    Various attempts at reducing torque ripple have included modifying the stator, via stator skewing with a continuous skewing arrangement. This methodology suffers from an undesirable increase in manufacturing cost and complexity. For example, this can cause an additional complexity with the inserting of coils into the slots. Another countervailing trend in reducing torque ripple is using an odd number of stator slots per pole pair. While this method has proven effective in helping reduce torque ripple, it suffers from the undesirable tradeoff of increasing core losses, which, in turn, may harm efficiency. 
         [0006]    Accordingly, there is an ongoing need for improving on current electric machine technologies and/or manufacturing thereof that address at least one of complexity, cost, efficiency, and/or performance without some of the current tradeoffs encountered with current methodologies. 
       BRIEF DESCRIPTION 
       [0007]    The present invention addresses at least some of the aforementioned drawbacks by providing improvements to electric machines, such as an interior permanent magnet (IPM) machines and Synchronous Reluctance motors, such that the electric machines may be both manufactured more efficiently and/or operate with more technical efficiency. More specifically, the present invention is directed to an IPM machine or a Synchronous Reluctance motor that includes offset rotor sections. Further aspects of the present invention include components and assemblies that provide for the offset features of these electric machines. In an embodiment, a vehicle, such as an underground mining vehicle, may employ compact traction motors that utilize aspects of the present invention. 
         [0008]    Therefore, in accordance with one aspect of the invention, a component comprises a longitudinal axle, having a plurality of keybars extending outward from a surface of the longitudinal axle, wherein each of the plurality of keybars are disposed axially along and circumferentially around the longitudinal axle, further wherein an axis of the plurality of keybars is parallel to the longitudinal axle. 
         [0009]    In accordance with another aspect of the invention, an assembly comprises a plurality of motor rotor sections; and a rotor bar extending through the plurality of motor rotor sections, wherein the rotor bar and the plurality of motor rotor sections are configured to step-skew the plurality of motor rotor sections from each other. 
         [0010]    In accordance with another aspect of the invention, a method comprises providing a longitudinal axle shaft; and removing material from the longitudinal shaft to define one of: a plurality of recesses configured to receive a plurality of keybar protrusions; and a plurality of keybar protrusions, wherein the plurality of keybar protrusions are disposed circumferentially around the longitudinal axis. 
         [0011]    In accordance with another aspect of the invention, an Interior Permanent Magnet (IPM) machine rotor comprises a plurality of motor rotor sections, wherein the plurality of motor rotor sections are step-skewed. 
         [0012]    In accordance with another aspect of the invention, a Synchronous Reluctance motor rotor comprises a plurality of motor rotor sections, wherein the plurality of motor rotor sections are step-skewed. 
         [0013]    In accordance with another aspect of the invention, a method of assembly comprises providing a plurality of rotor core sections; and assembling each of the plurality of rotor core sections on a keyed axle shaft, said keyed axle shaft includes at least one key thereon, wherein the at least one key accommodates the plurality of rotor core sections, thereby defining a skewed rotor core stack assembly. 
         [0014]    Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0016]      FIG. 1  is a graph illustrating torque over time and the effects in reducing torque ripple in applying aspects of the present invention. 
           [0017]      FIG. 2  is a perspective view of a rotor shaft component, according to an embodiment of the present invention. 
           [0018]      FIG. 3  is a perspective exploded view of the assembling of rotor structure components and the rotor shaft component of  FIG. 2 , according to an embodiment of the present invention. 
           [0019]      FIG. 4  is a perspective view of the completed assembly of  FIG. 3 , according to an embodiment of the present invention. 
           [0020]      FIG. 5A  is a side elevation view of a solid rotor core section, according to an embodiment of the present invention. 
           [0021]      FIG. 5B  is a side elevation view of a rotor core section comprised of a plurality of rotor laminations, according to an embodiment of the present invention. 
           [0022]      FIG. 6  is an end view of a rotor shaft component, according to an embodiment of the present invention. 
           [0023]      FIG. 7  is a top view of a rotor section, according to an embodiment of the present invention. 
           [0024]      FIG. 8  is a perspective view of a rotor shaft component, according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art with respect to the presently disclosed subject matter. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a”, “an”, and “the” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item, and the terms “front”, “back”, “bottom”, and/or “top”, unless otherwise noted, are used for convenience of description only, and are not limited to any one position or spatial orientation. 
         [0026]    If ranges are disclosed, the endpoints of all ranges directed to the same component or property are inclusive and independently combinable (e.g., ranges of “up to about 25 wt. %,” is inclusive of the endpoints and all intermediate values of the ranges of “about 5 wt. % to about 25 wt. %,” etc.). The modified “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes the degree of error associated with measurement of the particular quantity). Accordingly, the value modified by the term “about” is not necessarily limited only to the precise value specified. 
         [0027]    Aspects of the present invention provide a motor design methodology that offers several advantages including both an easier manufacturing process that leads to lower production costs, coupled with a reduction in torque ripple. This design, in turn, translates into less stringent requirement in the design of a gearbox connected between the electric motor employing this design with the wheel. Ultimately, this improvement may further lead to cost savings and/or small dimensions with the gearbox. 
         [0028]    Torque ripple for purposes herein can be estimated by the following equation: 
         [0000]        T   ripple =( T   max   −T   min )/ T   avg    
         [0029]    Aspects of the present invention solves both a manufacturing and electro-mechanical problem with IPM machines in that certain embodiments allow for the use of multiple short rotor sections which facilitates the insertion of permanent magnets into the rotor structure, thereby reducing the risk of damage to magnets and/or rotor sections or rotor laminations. Additionally, from an electro-mechanical point of view, certain embodiments angularly shift multiple rotor sections with respect to adjacent rotor sections (e.g., shift by a certain constant angle along the same direction), which causes a large reduction in the amplitude of torque ripple. The reduction in torque ripple results in a concomitant reduction in gearbox size and/or greater safety factor in the motor/gearbox system.  FIG. 1  depicts a graph showing torque over time. As shown, the amount of torque ripple is greatly decreased when aspects of the present invention (e.g., step-skewed rotor) are applied to a rotor in an electric machine. 
         [0030]    Certain symbols and definitions and concomitant equations are used herein, per the following Table: 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
               
                 symbol 
                 definition 
                 equation 
                   
               
               
                   
               
             
             
               
                 N sect   
                 Number of rotor sections 
                   
                   
               
               
                 θ skew   
                 rotor skew angle between the 
               
               
                   
                 end sections of the entire rotor 
               
               
                   
                 stack, for reduction of torque 
               
               
                   
                 ripple 
               
               
                 θ sect   
                 skew angle between adjacent 
                 θ sect  = θ skew / 
                 Eq. (1) 
               
               
                   
                 rotor sections 
                 (N sect  − 1) 
               
               
                 θ key   
                 constant angle that is required 
                 θ key  = θ sect  + θ 0   
                 Eq. (2) 
               
               
                   
                 to physically separate the key- 
               
               
                   
                 bars 
               
               
                 θ 0   
                 additional mechanical offset that 
                 In one embodiment: 
                 Eq. (3) 
               
               
                   
                 allows for the adjacent rotor 
                 θ 0  = 360/N poles   
               
               
                   
                 sections to physically step skew 
               
               
                   
                 adequately 
               
               
                 N poles   
                 Number of rotor poles 
               
               
                   
               
             
          
         
       
     
         [0031]    In certain embodiments of the present invention the rotor of the IPM machine is divided into N sect  axial sections, wherein each section is offset (or skewed) from its ‘neighboring’, adjacent section with an angle θ skew /(N sect −1), wherein “θ skew ” is the rotor skew angle between the end sections of the entire rotor stack. In this manner, the IPM machine can feature a lower torque ripple than that obtained in the axially straight rotor version. Such torque ripple mitigation will result in lower fatigue on the mechanical parts, thereby improving life of the machine and the various connected mechanical components. Further, because the need to push the magnets through the entire rotor stack is no longer required, the insertion of pre-magnetized magnets is thereby made easier. 
         [0032]    In an embodiment, the rotor assembly may use a number N sect  of separate rotor sections to reduce the length along which the magnets must be pushed, thus reducing the risk of damage. These pre-assembled N sect  sections are then mounted on a motor shaft, resulting in a skewed rotor assembly. In some particular embodiments, compression may be applied to the assembly after compression plates, or other elements, are applied to either end of the rotor assembly. 
         [0033]    Additionally, in an embodiment a small angular rotation between adjacent rotor sections is provided that will also help improve the profile of the electromagnetic torque produced by the motor. In fact, the presence of high order harmonics in both stator and rotor fluxes introduces a series of sinusoidally-varying torque components (with zero average value) superimposed to the constant torque that is required. Shifting the various sections of the rotor all by the same angle and in the same direction, the interaction of stator and rotor fluxes will not be the same along the axial length of machine, yet there will be some phase delay between the various sections. By providing a proper shift angle such phase delay can be used to produce equal and opposite sinusoidal torque components acting on the various sections of the rotor, thus filtering out most of the torque ripple yet with little reduction to the average value of torque. 
         [0034]    This skew angle, or small angular rotation, between adjacent rotor sections is found in equation [1]: 
         [0000]      θ sect =θ skew /( N   sect −1)  [1]
 
         [0035]    The proper value of the shifting between the sections should be carefully evaluated for each machine, depending upon its geometry, winding scheme and supply conditions. In one embodiment of the invention, for example, the rotor is 300 mm long and divided into 5 rotor sections, each 60 mm long and shifted 1.25° (i.e., θ sect ) from its neighbors. In this particular embodiment, the peak-to-peak ripple is found to be only 6% the average torque, compared to the value of 30% obtained in a case of straight rotor. Meanwhile, the average torque is reduced by just 1%. 
         [0036]    Another characteristic of certain embodiments is that in order to accommodate with the shifted rotor sections, the shaft may include many key-bars along the axial length, to lock the rotor sections to the shaft. Such key-bars may be both axially and angularly displaced. In the angular direction such displacement is equal to the required shift between sections, θ sect , to reduce torque ripple plus a constant angle, θ 0 , that may be required to physically separate the key-bars enough so as to accommodate the locking of rotor sections to the shaft. For example, in the above mentioned embodiment, each key-bar is displaced by 61.25°. (e.g., θ key =61.25°; θ sect =1.25°; θ 0 =60°). This total angle between adjacent keybars, θ key , is shown in equation [2]: 
         [0000]      θ key =θ sect +θ 0   [2]
 
         [0037]    θ 0  is an additional mechanical offset that allows for the adjacent rotor sections to more easily physically accommodate the step skewing in relation to each other more easily. In a particular embodiment, θ 0  is related to the quantity of rotor poles N poles  in the rotor assembly. In particular embodiments the value θ 0  is found in equation [3]: 
         [0000]      θ 0 =360/ N   poles   [3]
 
         [0038]    In other embodiments, θ 0  may be virtually any value and wholly unrelated to quantity of poles. In certain embodiments, θ 0  may even have a value of zero (0). 
         [0039]    In another embodiment of the present invention, the rotor sections  60  (e.g., solid core or rotor laminations) may feature, on their inner diameter, a series of equally displaced notches to provide proper mating with the key-bars as well as a guide for the section shifting. 
         [0040]    Referring to  FIG. 2 , a perspective view of a rotor shaft component, according to an embodiment, is shown. The rotor shaft component, or axle, is shown as  10 . As will be discussed herein the axle  10  may be used in coordination with a rotor assembly; a stator; and, thereby in combination be part of an electric machine. The axle  10  may comprise a longitudinal element, or axle, along a longitudinal axis, denoted X. Depending on the embodiment, the axle  10  may further comprise one or more end elements  14  that aid in the use of the axle  10  with the various rotor sections as discussed herein. The length of the axle  10  further comprises a plurality of keybars  12  extending from the body of the axle  10 . 
         [0041]    The quantity of keybars  12  may vary depending on the configuration of the rotor assembly and/or electric machine that it is used in combination with. The quantity may be any quantity from two to virtually infinite, although it is envisioned that a typical quantity of keybars  12  for many, but not all, embodiments is in the magnitude of between three and ten keybars  12  along the axle  10 . The location and configuration of the plurality of keybars  12  is significant in that they aid in providing for the skewing of various rotor sections thereon as discussed herein. The plurality of keybars  12  are configured to match with corresponding plurality of notches on a plurality of rotor sections to provide the step skewing of rotor sections, and, in certain embodiments, continuous skewing of rotor laminations. The plurality of keybars  12  is located so that they are distributed axially along and circumferentially around the shaft of the axle  10 . The plurality of keybars  12  are substantially parallel to the longitudinal axis, X. That is the midpoints of plurality of keybars  12  would define a helical, or helicoidal, pattern around and along the axle  10 . In an embodiment, a portion of each keybar  12  may overlap, or extend partially, in the axial length with another adjacent keybar  12 . Although  FIG. 2  shows straight keybars  12 , in other embodiments, other shapes and configuration of keybars  12  may be used, including for example helical-shaped keybar(s). (See e.g.,  FIG. 8 ). 
         [0042]    Various methods for manufacturing the component  10  may be used in various embodiments. For example, the various elements (e.g.,  12 ,  14 ) of the component  10  may be created by the removal of material from a single, or multiple, ingot elements. In another embodiment, material may be removed along the shaft of the axle  10  so as to define voids, or recesses, configured to receive separate keybar elements, or protrusions,  12  that could be fixedly, or removably, attached to the plurality of voids. In still other embodiments, various elements (e.g.,  12 ,  14 , and the like) may be attached via other means and manners. 
         [0043]    Referring to  FIG. 6  along with  FIG. 2 , an end view of an embodiment of a section of the axle  10  is shown.  FIG. 6  is showing the key bars  12  configured for two adjacent rotor sections (not shown). The offset angle between the adjacent key bars  12  is depicted as θ key , wherein θ key =θ sect +θ 0 , wherein θ key  comprises an electrical offset suitable to reduce torque ripple by at least partially cancelling out ripple components in the adjacent rotor sections, and further wherein θ 0  comprises the additional mechanical offset that allows for the physical accommodation of adjacent rotor sections to step skew adequately. As shown, two keybars for two corresponding rotor sections (not shown) are shown at approximately “12 o&#39;clock” and “2 o&#39;clock”. In the embodiment shown, the keybars for the other rotor sections of the rotor assembly are omitted for purposes of clarity. In the particular embodiment shown, additional balancing keybars  12  are shown and located 180° from the two key bars  12 . Thus, the two balancing keybars  12  are shown at approximately “6 o&#39;clock” and “8 o&#39;clock”. The embodiment shown (along with the omitted keybars) would be a suitable axle  10  for use, for example, in a 6-pole IPM or Synchronous Reluctance Machine. The shaft keybars  12  in the axle  10  of  FIG. 6  are configured to match corresponding keybar notches  64  shown in the inner opening  62  of the rotor section  60  shown in  FIG. 7 . By way of example only, the 6 rotor poles of the rotor section  60  of  FIG. 7  when used with the axle  10  depicted would results in offsets between adjacent rotor sections  60  of 1.25°. 
         [0044]    Referring to  FIGS. 3 and 4 , a plurality of rotor sections  60  are shown being assembled along an axle shaft  10  to form a rotor assembly  50  in  FIG. 3  and shown completely assembled in  FIG. 4 . The rotor stack, or assembly,  50  comprises a plurality of pre-manufacture rotor sections  60 , assembled together on the shaft  10 . In an embodiment, each of the plurality of rotor sections  60  is installed in a step-skewed configuration. Two skewing options include both a 1-slot pitch and half-slot pitch angular displacement between the two ends of the rotor stack  50 . The angular rotation between two consecutive rotor sections  60  can be calculated from Equation [1] stated in the Table above. 
         [0045]    An analysis has been conducted while delivering rated torque, the condition when the absolute value of the torque ripple is largest and, thus, more harmful to the mechanical components connected to the shaft. The rotor assembly  50  has been assumed being made of five (5) rotor sections  60 . 
         [0046]    As shown in  FIGS. 3 and 4 , each rotor section  60  may be premanufactured. The rotor sections  60  are each place in a skewed fashion on the axle  10 . As every rotor section  60  has a corresponding keybar  12 , the plurality of rotor sections  60  comprise a rotor assembly, or stack  50 . Thus, for an electric machine (e.g., IPM or Synchronous Reluctance Machine) the angular offset between consecutive keybars  12  can be found from equation [2], stated above and found in the Table. 
         [0047]    Referring to  FIGS. 5A and 5B , two embodiments of a rotor section  60  are shown according to embodiments of the present invention are shown in elevation views. The first embodiment ( FIG. 5A ) depicts a single rotor section  60  that includes an opening  62  therethrough and further comprises a solid core rotor core section. The second embodiment ( FIG. 5B ) depicts a single rotor section  60  that similarly includes an opening therethrough, but contrastingly further comprises a plurality of rotor laminations  64 . It should be apparent that the quantity of rotor laminations  64  may vary from the embodiment depicted in  FIG. 5B . Further, the rotor laminations  64  may be fixedly attached to each other to form the particular separate rotor sections  60 . Still further, in other embodiments, the rotor laminations  64  may be freely stacked (e.g., non-fixedly attached) with the particular rotor sections  60 . 
         [0048]    Referring to  FIG. 8 , another embodiment of a rotor axle component  10  is shown in perspective view. As depicted, the axle component  10  may include an end element  14  that aids in the keeping of the rotor sections  60  thereon. In the embodiment shown, the axle  10  further comprising a keybar  16  configured in a continuous helical profile, as opposed to the straight keybars  12  shown, for example in  FIG. 2 . Depending on the embodiment, the helical keybar  16  may be a single keybar configured in a continuous helicoidal pattern partially around the shaft of the axle component  10 . In another embodiment, two continuous helical keybars  16  may be located 180° opposite each other on the shaft of the axle component  10 . In this manner, the two helical keybars  16  act as balancing keybars to each other. 
         [0049]    In an embodiment of the present invention the rotor axle component  10  depicted in  FIG. 8  may be used with rotor sections  60  as those depicted in  FIG. 5B . That is the plurality of rotor sections  60  each comprised of a plurality of rotor laminations  64  may be placed on the axle component  10  having at least one continuous helical keybar  16 . In another embodiment, the rotor axle component  10  depicted in  FIG. 8  may be used with rotor sections  60  as those depicted in  FIG. 5A . That is the plurality of rotor sections  60  each comprise solid core rotor sections and may be placed on the axle component  10  having at least one continuous helical keybar  16 . In this manner, the rotor stack, or assembly, will have a continuously skewed configuration amongst the plurality of rotor sections  60 . 
         [0050]    A method of assembling a rotor core assembly may include assembling each of the rotor core sections on a keyed axle shaft, as discussed herein. The keyed axle shaft, depending on the embodiment, may have one or more keys thereon. The key(s) accommodate the multiple rotor sections, thereby defining a skewed (continuous or step-skewed) rotor core stack assembly. The rotor core stack assembly may have a compressive force applied to it. In an embodiment, one or more compression plates may be first adjoined to one, or both, end(s) of the rotor core stack assembly, prior to compression. In the IPM embodiment, a plurality of magnets may be inserted through the rotor core sections and affixed to the rotor core sections. In an embodiment, the affixing of magnets may be done by one of: infusing a resin on the rotor core sections; clamp the magnets with a filler or wedge material; and, shrinking the magnets into the rotor core sections. The method is suitable for IPM or Synchronous Reluctance motor (with exception of magnets; step or continuous skewed configurations; and, solid core or plurality of lamination rotor sections. 
         [0051]    Under aspects of the present invention, the components  10 ,  60  and assemblies  50  and the electric machines  100  discussed herein may be used as a traction motor for virtually any vehicle. A vehicle support frame (not shown) may be connected to the one or more electric machine  100 . Suitable vehicles for use include, but are not limited to, an off-highway vehicle (OHV), a locomotive, a mining vehicle, electric-motorized railcar, automobiles, trucks, construction vehicles, agricultural vehicles, airport ground service vehicles, fork-lifts, non-tactical military vehicles, tactical military vehicles, golf carts, motorcycles, mopeds, all-terrain vehicles, and the like. 
         [0052]    Note that while various embodiments discussed herein describe the improvements to be used in and with IPM, it should be apparent that the various aspects of the present are equally suited for use in and with Synchronous Reluctance machines. 
         [0053]    Therefore, in accordance with one aspect of the invention, a component comprises a longitudinal axle, having a plurality of keybars extending outward from a surface of the longitudinal axle, wherein each of the plurality of keybars are disposed axially along and circumferentially around the longitudinal axle, further wherein an axis of the plurality of keybars is parallel to the longitudinal axle. 
         [0054]    In accordance with another aspect of the invention, an assembly comprises a plurality of motor rotor sections; and a rotor bar extending through the plurality of motor rotor sections, wherein the rotor bar and the plurality of motor rotor sections are configured to step-skew the plurality of motor rotor sections from each other. 
         [0055]    In accordance with another aspect of the invention, a method comprises providing a longitudinal axle shaft; and removing material from the longitudinal shaft to define one of: a plurality of recesses configured to receive a plurality of keybar protrusions; and a plurality of keybar protrusions, wherein the plurality of keybar protrusions are disposed circumferentially around the longitudinal axis. 
         [0056]    In accordance with another aspect of the invention, an Interior Permanent Magnet (IPM) machine rotor comprises a plurality of motor rotor sections, wherein the plurality of motor rotor sections are step-skewed. 
         [0057]    In accordance with another aspect of the invention, a Synchronous Reluctance motor rotor comprises a plurality of motor rotor sections, wherein the plurality of motor rotor sections are step-skewed. 
         [0058]    In accordance with another aspect of the invention, a method of assembly comprises providing a plurality of rotor core sections; and assembling each of the plurality of rotor core sections on a keyed axle shaft, said keyed axle shaft includes at least one key thereon, wherein the at least one key accommodates the plurality of rotor core sections, thereby defining a skewed rotor core stack assembly. 
         [0059]    While only certain features of the invention have been illustrated and/or described herein, many modifications and changes will occur to those skilled in the art. Although individual embodiments are discussed, the present invention covers all combination of all of those embodiments. It is understood that the appended claims are intended to cover all such modification and changes as fall within the intent of the invention.