Abstract:
A common problem in wireless charging systems in automotive applications is alignment of the wireless power transfer transmitter and receiver units. Poor alignment leads to poor power transfer capacity and a longer charging time. Costly optical sensors and electromechanical systems may be employed to align the transmitter and receiver units. A solution to this problem is to employ a freely moveable wireless power transmitter in at least one direction that is magnetically attracted to the wireless power receiver unit in the vehicle that does not require electrically powered sensors or mechanical alignment systems.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of Patent Cooperation Treaty (PCT) application No. PCT/CA2015/050623, filed 3 Jul. 2015 and entitled WIRELESS POWER TRANSFER SYSTEMS HAVING GUIDES FOR FOREIGN OBJECT REMOVAL AND METHODS OF FABRICATION AND USE OF SAME, which in turn claims the filing date benefit of U.S. application Ser. No. 62/021084, filed on 4 Jul. 2014. PCT application No. PCT/CA2015/050623 and U.S. application Ser. No. 62/021084 are hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention pertains to wireless power transfer systems which transfer power wirelessly from a wireless power transmitter to a wireless power receiver. Particular embodiments provide wireless power transfer systems having guides for removal of foreign objects from a vicinity (e.g. an air gap) thereof. 
       BACKGROUND 
       [0003]    Power can be wirelessly conveyed from one place to another using the Faraday effect, whereby a changing magnetic field causes an electrical current to flow in an electrically isolated secondary circuit. A form of wireless power transfer (WPT) currently in use involves magnetic inductive charging. One form of magnetic inductive charging is shown in WPT system  10  of  FIG. 1 . The  FIG. 1  WPT system  10  comprises two coils  12 ,  14  in close proximity but separated by an air gap  16 . One coil  12  of WPT system  10  acts as a wireless power transmitter and the other coil  14  acts as the receiver of wireless power. A time-varying current flows in transmitter coil  12 , which produces a time-varying magnetic field (shown as flux lines in  FIG. 1 ). This time-varying magnetic field induces current in the nearby receiver coil  14  (Faraday&#39;s law), which can then be used to charge various devices (not shown) which may be electrically connected to receiver coil  14 . 
         [0004]    In PCT application No. PCT/CA2010/000252 (published under WO/2010/096917), a magnetic-coupling technology has been described to provide a number of viable magnetically-coupled WPT systems that can be used to charge, by way of non-limiting example, batteries generally, electric (e.g. battery operated) vehicles, auxiliary batteries, electric (e.g. battery operated) buses, golf carts, delivery vehicles, boats, drones, trucks and/or the like.  FIG. 2  schematically depicts a WPT system  20  incorporating a magnetic-coupling technology of the type described in PCT/CA2010/000252. WPT system  20  comprises a wireless magnetic power transmitter  22  and a wireless magnetic power receiver  24  separated by an air gap  26 . The power transfer in WPT system  20  is via rotational magnetic coupling rather than via magnetic induction coupling. In the  FIG. 2  WPT system  20 , transmitter  22  comprises a permanent magnet  22 A and receiver  24  comprises a permanent magnet  24 A. Transmitter magnet  22 A is rotated (and/or pivoted) about axis  28 . The magnetically coupled permanent magnets  22 A,  24 A interact with one another (magnetic poles represented by an arrow with notations of “N” for north and “S” for south in  FIG. 2 ), such that movement of transmitter magnet  22 A about axis causes corresponding movement (e.g. rotation and/or pivotal movement) of receiver magnet  24 A about axis  27 . The time-varying magnetic fields generated by rotating/pivoting magnets  22 A,  24 A of WPT system  20  typically have lower frequencies compared to WPT systems based on magnetic induction. The  FIG. 2  WPT system  20  transfers power most effectively when there is strong coupling between rotating/pivoting magnets  22 A,  24 A. 
         [0005]    A challenge faced by WPT systems is the presence of foreign objects in the vicinity of the system (e.g. in the air gap between the WPT transmitter and the WPT receiver. Such foreign objects can cause problems ranging from minor to severe, including fires or explosion. The nature and severity of the problems caused by such foreign objects is typically dependent on the material properties of the foreign object. Foreign objections comprising metals may be particularly problematic because of heat generated by eddy currents produced in the metal-containing foreign object in response to the magnetic fields of the WPT system. The problems associated with foreign objects (particularly metals) are associated with both inductive charging WPT systems (of the type shown in  FIG. 1 ) and magneto-dynamic coupling (MDC) WPT systems (of the type shown in  FIG. 2 ). Although the eddy currents produced by the relatively lower-frequency MDC WPT systems are relatively less likely to lead to severe heating causing fire, such eddy currents can still cause undesirable localized heating, reduced power transmission efficiency, power reduction and/or damage to system components, even in MDC WPT systems. 
         [0006]    There is a general desire to remove foreign objects (e.g. objects containing metal) from a vicinity of WPT systems. There may be a general desire for such foreign object removal to occur without, or with minimal, user intervention. There may be a general desire to move such foreign objects to positions which minimize the heat generation and/or power loss associated with such objects or which reduce the heat generation and/or power loss associated with such foreign objects to minimal levels. 
         [0007]    The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the description and a study of the drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]    Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. 
           [0009]      FIG. 1  is a schematic illustration of a prior art magnetic induction based WPT system comprising a WPT transmitter coil and a WPT receiver coil in close proximity. 
           [0010]      FIG. 2  is a schematic illustration of two magnetically-coupled rotating/pivoting magnets in a prior art magnetically-coupled WPT system. 
           [0011]      FIG. 3  schematically depicts a WPT system comprising a WPT transmitter and a WPT receiver, wherein the WPT transmitter and the WPT receiver each comprise guides for removal of foreign objects from a vicinity of the WPT system according to a particular embodiment. 
           [0012]      FIG. 4  schematically depicts a WPT transmitter or WPT receiver comprising a housing with a guide for removal of foreign objects from a vicinity of the WPT system according to a particular embodiment. 
           [0013]      FIG. 5  schematically depicts a WPT transmitter or WPT receiver comprising a housing with a guide for removal of foreign objects from a vicinity of the WPT system according to a particular embodiment. 
           [0014]      FIG. 6  schematically depicts a WPT transmitter or WPT receiver comprising a housing with a guide for removal of foreign objects from a vicinity of the WPT system according to a particular embodiment. 
           [0015]      FIG. 7  schematically depicts a WPT transmitter or WPT receiver comprising a housing with a guide for removal of foreign objects from a vicinity of the WPT system according to a particular embodiment. 
           [0016]      FIG. 8  schematically depicts a WPT transmitter or WPT receiver comprising a housing with guides for removal of foreign objects from a vicinity of the WPT system according to a particular embodiment. 
           [0017]      FIG. 9  schematically depicts a plan view of a WPT transmitter or WPT receiver comprising a housing with a magnetic sweeper and guides for removal of foreign objects from a vicinity of the WPT system according to a particular embodiment. 
           [0018]      FIG. 10  schematically depicts a side view of a WPT transmitter or WPT receiver comprising a housing with a magnetic sweeper and guides for removal of foreign objects from a vicinity of the WPT system according to a particular embodiment. 
       
    
    
     DESCRIPTION 
       [0019]    Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
         [0020]    One aspect of the invention provides apparatus and methods for removal of foreign metallic objects from the vicinity (e.g. charging region) of a wireless power transfer (WPT) system automatically, which may occur without end user&#39;s intervention or even awareness. Foreign objects only need to be moved to positions which minimize the heat generation and/or power loss associated with such objects or which reduce the heat generation and/or power loss associated with such foreign objects to minimal (e.g. suitably small) levels. A guide comprising a textured pattern may be provided on the outward-facing surface of the WPT transmitter and/or WPT receiver in a MDC wireless charging system. For either of the WPT transmitter or the WPT receiver, the guide and/or its textured pattern may be arranged or oriented in such a way that the net force on the foreign object is based primarily on the sum of the force produced by interaction of the object with the magnetic fields of the WPT system (e.g. the magnetic fields of the permanent magnets enclosed in the WPT transmitter and/or the WPT receiver) and the normal force from a guide surface of the guide (e.g. the surface texturing). Such net force may be oriented to force the foreign object toward an end of the WPT transmitter and/or the WPT receiver and/or toward such other safe location, where the magnetic fields associated with the WPT system are at relatively low levels. Once directed to such safe locations, foreign objects may be removed during scheduled maintenance (e.g. some time when charging is complete or power transfer is not otherwise being effected). In some embodiments or circumstances, foreign objects may fall from such safe locations away from the WPT system (e.g. under the force of gravity). In some embodiments, the textured pattern of the guide may be provided in the form of a helical structure (e.g. a helical guide) on the surface of the WPT transmitter and/or the WPT receiver. 
         [0021]    Another aspect of the invention provides a wireless power transmitter (also referred to as a WPT transmitter, a wireless power transmitting unit and/or a wireless power transmitting device) for transferring power to a wireless power transfer receiver (also referred to as a WPT receiver, a wireless power receiving unit, and/or a wireless power receiving device). The WPT transmitter and WPT receiver may be parts of a wireless power transfer (WPT) system. The WPT transmitter comprises: a transmitter magnetic system for creating a transmitter magnetic field; and a transmitter housing for supporting the transmitter magnetic system in an interior of the transmitter housing. The magnetic field created by the WPT transmitter has a spatial configuration which moves about a transmitter axis (e.g. pivots or rotates), relative to the transmitter housing. The transmitter housing comprises an outer surface and the outer surface comprises a guide. 
         [0022]    The guide may comprise a guide surface which extends in one or more directions such that an orthogonal projection of the one or more extension directions of the guide surface onto a notional plane containing the transmitter axis forms one or more angles, α, with the transmitter axis, wherein 0°&lt;α&lt;90°. The guide may additionally or alternatively comprise a guide surface which extends in one or more directions that are non-parallel with the transmitter axis and non-orthogonal to the transmitter axis. The guide may additionally or alternatively comprise a guide surface which extends in one or more directions which have component directions that are parallel to the transmitter axis and component directions that are orthogonal to the transmitter axis. The guide (and/or its guide surface) may additionally or alternatively extend to wrap around the transmitter axis as the guide (and/or its guide surface) extends in one or more directions aligned with the transmitter axis. The guide may additionally or alternatively comprise at least one of: a flange which is raised relative to a remainder of the outer surface; a groove which is depressed relative to the remainder of the outer surface; and a combination of a flange which is raised relative to the remainder of the outer surface and a groove which is depressed relative to the remainder of the outer surface. 
         [0023]    Another aspect of the invention provides a WPT receiver for receiving power from a WPT transmitter. The WPT transmitter and WPT receiver may be parts of a WPT system. The WPT receiver comprises: a receiver magnetic system for creating a receiver magnetic field; and a receiver housing for supporting the receiver magnetic system in an interior of the receiver housing. The receiver magnetic field created by the receiver magnetic system has a spatial configuration which moves about a receiver axis, relative to the receiver housing. The receiver housing comprises an outer surface and the outer surface comprises a guide. 
         [0024]    The guide may comprise a guide surface which extends in one or more directions such that an orthogonal projection of the one or more extension directions of the guide surface onto a notional plane containing the receiver axis forms one or more angles, β, with the receiver axis, wherein 0°&lt;β&lt;90°. The guide may additionally or alternatively comprise a guide surface which extends in one or more directions that are non-parallel with the receiver axis and non-orthogonal to the receiver axis. The guide may additionally or alternatively comprise a guide surface which extends in one or more directions which have component directions that are parallel to the receiver axis and component directions that are orthogonal to the receiver axis. The guide (and/or its guide surface) may additionally or alternatively extend to wrap around the receiver axis as the guide (and/or its guide surface) extends in one or more directions aligned with the receiver axis. The guide may additionally or alternatively comprise at least one of: a flange which is raised relative to a remainder of the outer surface; a groove which is depressed relative to the remainder of the outer surface; and a combination of a flange which is raised relative to the remainder of the outer surface and a groove which is depressed relative to the remainder of the outer surface. 
         [0025]    Another aspect of the invention provides a method for removing foreign objects from a WPT transmitter (which may form part of a WPT system). The method comprises: providing a transmitter magnetic system for creating a transmitter magnetic field; supporting the transmitter magnetic system in an interior of a transmitter housing; moving a spatial configuration of the transmitter magnetic field about a transmitter axis, relative to the transmitter housing. The transmitter housing comprises an outer surface and the method comprises providing the outer surface with a guide. The method may comprise shaping the guide to provide a guide surface which extends in one or more directions such that an orthogonal projection of the one or more extension directions of the guide surface onto a notional plane containing the transmitter axis forms one or more angles, α, with the transmitter axis, wherein 0°&lt;α&lt;90°. The method may additionally or alternatively comprise shaping the guide to provide a guide surface which extends in one or more directions that are non-parallel with the transmitter axis and non-orthogonal to the transmitter axis. The method may additionally or alternatively comprise shaping the guide to provide a guide surface which extends in one or more directions which have component directions that are parallel to the transmitter axis and component directions that are orthogonal to the transmitter axis. The method may additionally or alternatively comprise shaping the guide (and/or its guide surface) to wrap around the transmitter axis as the guide extends in one or more directions aligned with the transmitter axis. The method may additionally or alternatively comprise shaping the guide, such that the guide comprises at least one of: a flange which is raised relative to a remainder of the outer surface; a groove which is depressed relative to the remainder of the outer surface; and a combination of a flange which is raised relative to the remainder of the outer surface and a groove which is depressed relative to the remainder of the outer surface. 
         [0026]    Another aspect of the invention provides a method for removing foreign objects from a WPT receiver (which may form part of a WPT system). The method comprises: providing a receiver magnetic system for creating a receiver magnetic field; supporting the receiver magnetic system in an interior of a receiver housing; moving a spatial configuration of the receiver magnetic field about a receiver axis, relative to the receiver housing. The receiver housing comprises an outer surface and the method comprises providing the outer surface with a guide. The method may comprise shaping the guide to provide a guide surface which extends in one or more directions such that an orthogonal projection of the one or more extension directions of the guide surface onto a notional plane containing the receiver axis forms one or more angles, β, with the transmitter axis, wherein 0°&lt;β&lt;90° The method may additionally or alternatively comprise shaping the guide to provide a guide surface which extends in one or more directions that are non-parallel with the receiver axis and non-orthogonal to the receiver axis. The method may additionally or alternatively comprise shaping the guide to provide a guide surface which extends in one or more directions which have component directions that are parallel to the receiver axis and component directions that are orthogonal to the receiver axis. The method may additionally or alternatively comprise shaping the guide (and/or its guide surface) to wrap around the receiver axis as the guide extends in one or more directions aligned with the receiver axis. The method may additionally or alternatively comprise shaping the guide, such that the guide comprises at least one of: a flange which is raised relative to a remainder of the outer surface; a groove which is depressed relative to the remainder of the outer surface; and a combination of a flange which is raised relative to the remainder of the outer surface and a groove which is depressed relative to the remainder of the outer surface. 
         [0027]    Another aspect of the invention provides a magnetically coupled WPT system comprising: at least one WPT transmitter comprising a transmitter housing with a raised or depressed or a combination of a raised and depressed pattern on an outward facing surface thereof; at least one WPT receiver comprising a receiver housing with a raised or depressed or a combination of a raised and depressed pattern on the outward surface; an electrical power source connected to provide power to the WPT transmitter, which in turn transfers the power wirelessly to the WPT receiver. 
         [0028]    Another aspect of the invention provides an induction WPT system comprising: at least one WPT transmitter comprising a transmitter housing, a transmitter magnetic system, for creating a transmitter magnetic field, supported within the transmitter housing and a sweeper magnetic system (also referred to herein as a sweeper, for brevity), for creating a sweeper magnetic field, supported within the transmitter housing. The sweeper may comprise one or more permanent magnets and/or one or more suitably configured coils and may be moveable in a sweeper movement direction relative to the transmitter housing. Movement of the sweeper may be linear or rotational. The housing may comprise an outer surface comprising a guide. The guide may comprise a guide surface which extends in one or more directions that form one or more angles, γ, with the sweeper movement direction, wherein 0°&lt;γ&lt;90°. As the sweeper moves, magnetic debris that adhered to the transmitter housing surface is attracted by the sweeper magnetic field and directed toward the guide. As the sweeper continues to move, the guide directs the magnetic debris toward an edge of the transmitter housing for removal. 
         [0029]    Another aspect of the invention provides an induction WPT system comprising: at least one WPT receiver comprising a receiver housing, a receiver magnetic system, for creating a receiver magnetic field, supported within the receiver housing and a sweeper magnetic system (also referred to herein as a sweeper, for brevity), for creating a sweeper magnetic field, supported within the receiver housing. The sweeper may comprise one or more permanent magnets and/or one or more suitably configured coils and may be moveable in a sweeper movement direction relative to the receiver housing. Movement of the sweeper may be linear or rotational. The housing may comprise an outer surface comprising a guide. The guide may comprise a guide surface which extends in one or more directions that form one or more angles, θ, with the sweeper movement direction, wherein 0°&lt;θ&lt;90°. As the sweeper moves, magnetic debris that adhered to the receiver housing surface is attracted by the sweeper magnetic field and directed toward the guide. As the sweeper continues to move, the guide directs the magnetic debris toward an edge of the receiver housing for removal. 
         [0030]      FIG. 3  schematically depicts a wireless power transfer (WPT) system  100  in the form of a wireless charging system  100 . WPT system  100  comprises opposing WPT transmitter  102  and WPT receiver  112 . WPT transmitter  102  comprises a transmitter magnetic system (not visible in  FIG. 3 ) which may comprise current carrying coils surrounding a magnet rotor for use in a magneto-dynamic coupling (MDC) wireless charging technology. The transmitter magnetic system is supported in an interior of transmitter housing  104 . The transmitter magnetic system creates a corresponding transmitter magnetic field which has a spatial configuration which moves about (e.g. pivots and/or rotates) about a transmitter axis  103 . Transmitter housing  104  may be constructed of a plastic, rubber or other non-metallic material. In some embodiments, transmitter housing  104  may comprise a soft ferromagnetic material. Transmitter housing  104  comprises an outer surface  104 A which, in the illustrated embodiment, comprises a generally cylindrical shape with a cylinder axis which may be aligned with and/or coincident with transmitter axis  103 . In the illustrated embodiment, WPT transmitter  102  comprises anchor sites  106  to mount WPT transmitter  102  to a suitable support in a desired location such as, for example, in a wireless charging parking area. WPT transmitter  102  of the  FIG. 3  embodiment also comprises an optional conduit  108  through which a power cable may extend to supply electrical power to the transmitter magnetic system. Such power may be used to move the transmitter magnetic field about transmitter axis  103  (e.g. by rotating a permanent magnet rotor in the transmitter magnetic system which may involve providing electrical power to the coils surrounding the permanent magnet rotor). 
         [0031]    In the illustrated embodiment, outer surface  104 A of transmitter housing  104  also comprises a guide  110  which is raised relative to (i.e. projects outwardly from) a remainder  104 B of outer surface  104 A to provide a guide surface  110 A. Guide  110  of the  FIG. 3  embodiment (and/or its guide surface  110 A) has a generally helical shape. Guide surface  110 A may extend in one or more directions such that an orthogonal projection of the one or more extension directions of guide surface  110 A onto a notional plane containing transmitter axis  103  may form one or more angles, α, with transmitter axis  103 , wherein 0°&lt;α&lt;90°. Guide surface  110 A may extend in one or more directions that are non-parallel with transmitter axis  103  and non-orthogonal to transmitter axis  103 . Guide surface  110 A may extend in one or more directions which have component directions that are parallel to transmitter axis  103  and component directions that are orthogonal to transmitter axis  103 . Guide  110  (and/or its guide surface  110 A) may additionally or alternatively extend to wrap around transmitter axis  103  as guide  110  (and/or its guide surface  110 A) extends in one or more directions aligned with transmitter axis  103 . 
         [0032]    In the illustrated  FIG. 3  embodiment, guide  110  comprises a flange  110 B (e.g. ridges or fins) which is raised relative to (e.g. extends outwardly from) a remainder  104 B of outer surface  104 A. Guide  110  may additionally or alternatively comprise at least one of: a groove or channel which is depressed relative to the remainder  104 B of outer surface  104 A; and a combination of a flange which is raised relative to the remainder  104 B of outer surface  104 A and a groove which is depressed relative to the remainder  104 B of outer surface  104 A. The flange  110 B of guide  110  provides a guide surface  110 A which may be of variable height (relative to the remainder  104 B of outer surface  104 A). Similarly, where guide  110  comprises grooves or channels, such grooves or channels may be of variable depth. Guide  110  may comprise a variable number of flanges and/or grooves that run or wrap around outer surface  104 A of transmitter housing  104  and/or around transmitter axis  103 . In the illustrated  FIG. 3  embodiment, guide  110  comprises a helical shape. Guide  110  need not comprise large number of helical or screw type wraps (about transmitter axis  103 ) and may comprise fewer that one wrap around axis  103 . Guide  110  may be arranged in a right or left-handed direction or a combination of right and left-handed directions in a double-helix type shape. WPT transmitter  102  may comprise a receptacle (not shown) at one or both ends of transmitter housing  104  for collecting foreign objects which may be guided there by guide  110 . 
         [0033]    In the  FIG. 3  WPT system  100 , WPT receiver  112  is substantially opposed to and aligned with WPT transmitter  102 . WPT receiver  112  comprises a receiver magnetic system (not visible in  FIG. 3 ) which may comprise current carrying coils surrounding a magnet rotor for use in a magneto-dynamic coupling (MDC) wireless charging technology. The receiver magnetic system is supported in an interior of receiver housing  114 . The receiver magnetic system interacts with the transmitter magnetic field and in response to such interaction, creates a receiver magnetic field which has a spatial configuration which moves about (e.g. pivots and/or rotates) about a receiver axis  113 . Receiver housing  114  may be constructed of a plastic, rubber or other non-metallic material. In some embodiments, receiver housing  114  may comprise a soft ferromagnetic material. Receiver housing  114  comprises an outer surface  114 A which, in the illustrated embodiment, comprises a generally cylindrical shape with a cylinder axis which may be aligned with and/or coincident with receiver axis  113 . In the illustrated embodiment, WPT receiver  112  comprises anchor sites  106  to mount WPT receiver  112  to a suitable support in a moveable platform such as, for example, in a battery operated vehicle. WPT receiver  112  of the  FIG. 3  embodiment also comprises an optional conduit  118  through which a power cable may extend to extract electrical power from the receiver magnetic system. 
         [0034]    In the illustrated embodiment, outer surface  114 A of receiver housing  114  also comprises a guide  120  which is raised relative to (i.e. projects outwardly from) a remainder  114 B of outer surface  114 A to provide a guide surface  120 A. Guide  120  of the  FIG. 3  embodiment (and/or its guide surface  120 A) has a generally helical shape. Guide surface  120 A may extend in one or more directions such that an orthogonal projection of the one or more extension directions of guide surface  120 A onto a notional plane containing receiver axis  113  may form one or more angles, β, with receiver axis  113 , wherein 0°&lt;β&lt;90°. Guide surface  120 A may extend in one or more directions that are non-parallel with receiver axis  113  and non-orthogonal to receiver axis  113 . Guide surface  120 A may extend in one or more directions which have component directions that are parallel to receiver axis  113  and component directions that are orthogonal to receiver axis  113 . Guide  120  (and/or its guide surface  120 A) may additionally or alternatively extend to wrap around receiver axis  113  as guide  120  (and/or its guide surface  120 A) extends in one or more directions aligned with receiver axis  113 . 
         [0035]    In the illustrated  FIG. 3  embodiment, guide  120  comprises a flange  120 B (e.g. ridges or fins) which is raised relative to (e.g. extends outwardly from) a remainder  114 B of outer surface  114 A. Guide  120  may additionally or alternatively comprise at least one of: a groove or channel which is depressed relative to the remainder  114 B of outer surface  114 A; and a combination of a flange which is raised relative to the remainder  114 B of outer surface  114 A and a groove which is depressed relative to the remainder  114 B of outer surface  114 A. The flange  120 B of guide  120  provides a guide surface  120 A which may be of variable height (relative to the remainder  114 B of outer surface  114 A). Similarly, where guide  120  comprises grooves or channels, such grooves or channels may be of variable depth. Guide  120  may comprise a variable number of flanges and/or grooves that run or wrap around outer surface  114 A of receiver housing  114  and/or around receiver axis  113 . In the illustrated  FIG. 3  embodiment, guide  120  comprises a helical shape. Guide  120  need not comprise large number of helical or screw type turns (about receiver axis  113 ) and may comprise fewer that one wrap around axis  113 . Guide  120  may be arranged in a right or left-handed direction or a combination of right and left-handed directions in a double-helix type shape. WPT receiver  112  may comprise a receptacle (not shown) at one or both ends of receiver housing  114  for collecting foreign objects which may be guided there by guide  120 . 
         [0036]      FIG. 4  depicts a WPT transmitter  102  of a WPT system  100  according to a particular embodiment and is used to illustrate the mechanism by which guide  110  removes metallic foreign objects and other debris from WPT system  100 . It will be appreciated from the discussion that follows that the operation of WPT receiver  112  may be substantially similar to that of WPT transmitter  102  shown in  FIG. 4  and described herein. As discussed above, WPT transmitter  102  comprises a transmitter housing  104  with an outer surface  104  comprising a guide  110  having the features described above in connection with  FIG. 3 . In the  FIG. 4  embodiment, it is assumed that WPT transmitter  102  comprises a transmitter magnetic system which comprises a permanent magnet (not shown) rotating about transmitter axis  103  in a counterclockwise direction  212 , causing a corresponding counterclockwise rotation of the transmitter magnetic field about transmitter axis  103 . 
         [0037]    In the  FIG. 4  illustration, the dotted lines are used to show guide  110  on an opposite side of outer surface  104 A of transmitter housing  104 .  FIG. 4  depicts an unwanted magnetic foreign object  214  adhered to outer surface  104 A of transmitter housing  104  by attraction to the permanent magnet of the transmitter magnet system. As the magnet of transmitter magnetic system rotates in counterclockwise direction  212 , the corresponding transmitter magnetic field also rotates and foreign object  214  moves (e.g. by magnetic interaction with the moving transmitter magnetic field) around outer surface  104 A of housing  104  (e.g. in a circular manner where transmitter housing  104 A has the illustrated cylindrical shape) until foreign object  214  encounters guide  110  (or more particularly, a guide surface  110 A of guide  110 —see  FIG. 3 ). As the transmitter magnet continues to rotate about transmitter axis  103  and relative to transmitter housing  104 , the helical or screw type pattern of guide  110  and its guide surface  110 A directs or forces foreign object  214  in direction  216  (leftward in the illustrated  FIG. 4  view). Guide  110  can be shaped to direct object  214  into a suitably located receptacle (not shown) or channel or safe location until object  214  can be removed (e.g. during a routine maintenance operation). It is not a requirement that the helical shape of guide  110  comprise any number of wraps around transmitter axis  103 . In some embodiments, the helical shape of guide  110  comprises greater than or equal to two wraps around transmitter axis  103 . In some embodiments, the helical shape of guide  110  comprises greater than or equal to three wraps around transmitter axis  103 . In some embodiments, the helical shape of guide  110  comprises fewer than one full wrap around transmitter axis  103 . 
         [0038]      FIG. 5  depicts a WPT transmitter  102  of a WPT system  100  according to a particular embodiment and is used to illustrate the mechanism by which guide  110  removes metallic foreign objects and other debris from WPT system  100 . It will be appreciated from the discussion that follows that the operation of WPT receiver  112  may be substantially similar to that of WPT transmitter  102  shown in  FIG. 4  and described herein. As discussed above, WPT transmitter  102  comprises a transmitter housing  104  with an outer surface  104  comprising a guide  110  having the features described above in connection with  FIG. 3 . In the  FIG. 5  embodiment, it is assumed that WPT transmitter  102  comprises a transmitter magnetic system which comprises a permanent magnet (not shown) rotating about transmitter axis  103  in a clockwise direction  213  (which is opposite to the rotational direction  212  of  FIG. 4 ), causing a corresponding clockwise rotation of the transmitter magnetic field about transmitter axis  103 . 
         [0039]    In the  FIG. 5  illustration, the dotted lines are used to show guide  110  on an opposite side of outer surface  104 A of transmitter housing  104 .  FIG. 5  depicts an unwanted magnetic foreign object  214  adhered to outer surface  104 A of transmitter housing  104  by attraction to the permanent magnet of the transmitter magnet system. As the magnet of transmitter magnetic system rotates in clockwise direction  213 , the corresponding transmitter magnetic field also rotates and foreign object  214  moves (e.g. by magnetic interaction with the moving transmitter magnetic field) around outer surface  104 A of housing  104  (e.g. in a circular manner where transmitter housing  104 A has the illustrated cylindrical shape) until foreign object  214  encounters guide  110  (or more particularly, a guide surface  110 A of guide  110 ). As the transmitter magnet continues to rotate about transmitter axis  103  and relative to transmitter housing  104 , the helical or screw type pattern of guide  110  and its guide surface  110 A directs or forces foreign object  214  in direction  217  (rightward in the illustrated  FIG. 5  view). Guide  110  can be shaped to direct object  214  into a suitably located receptacle (not shown) or channel or safe location until object  214  can be removed (e.g. during a routine maintenance operation). It is not a requirement that the helical shape of guide  110  comprise any number of wraps around transmitter axis  103 . In some embodiments, the helical shape of guide  110  comprises greater than or equal to two wraps around transmitter axis  103 . In some embodiments, the helical shape of guide  110  comprises greater than or equal to three wraps around transmitter axis  103 . In some embodiments, the helical shape of guide  110  comprises fewer than one full wrap around transmitter axis  103 . 
         [0040]      FIG. 6  depicts a WPT transmitter  402  of a WPT system  400  according to a particular embodiment. It will be appreciated from the discussion that follows that the operation of a WPT receiver having features similar to WPT transmitter  402  may be substantially similar to that of WPT transmitter  402  shown in  FIG. 6  and described herein. Like the above-discussed WPT transmitters, WPT transmitter  402  comprises a transmitter housing  404  with an outer surface  404  comprising a guide  410 . In the  FIG. 6  embodiment, it is assumed that WPT transmitter  402  comprises a transmitter magnetic system which comprises a permanent magnet (not shown) rotating about transmitter axis  403  in a counterclockwise direction  212 , causing a corresponding counterclockwise rotation of the transmitter magnetic field about transmitter axis  403 . The  FIG. 6  transmitter  402  is different than transmitter  102  described above in that the guide  410  of transmitter  402  has a different shape than guide  110  of transmitter  102 . In particular, guide  410  of the  FIG. 6  embodiment comprises a double-helical shape having guide component  412  and guide component  411 , which may be broken where guide component  411  crosses guide component  412 . The breaks in guide component  411  provide small spaces through which metal-containing debris can pass. In the  FIG. 6  illustration, the dotted lines are used to show guide  410  on an opposite side of outer surface  404 A of transmitter housing  404 . 
         [0041]      FIG. 6  depicts an unwanted magnetic foreign object  214  adhered to outer surface  404 A of transmitter housing  404  by attraction to the permanent magnet of the transmitter magnet system. As the magnet of transmitter magnetic system rotates in counterclockwise direction  212 , the corresponding transmitter magnetic field also rotates and foreign object  214  moves (e.g. by magnetic interaction with the moving transmitter magnetic field) around outer surface  404 A of housing  404  (e.g. in a circular manner where transmitter housing  404 A has the illustrated cylindrical shape) until foreign object  214  encounters guide  410  (or more particularly, a guide surface of guide  410 ). As the transmitter magnet continues to rotate about transmitter axis  403  and relative to transmitter housing  404 , the helical or screw type pattern of guide  410  and its guide surface  410 A directs or forces foreign object  214  in direction  216  (leftward in the illustrated  FIG. 6  view). Guide  410  can be shaped to direct object  214  into a suitably located receptacle (not shown) or channel or safe location until object  214  can be removed (e.g. during a routine maintenance operation). It is not a requirement that the double-helical shape of guide  410  comprise any number of wraps around transmitter axis  403 . In some embodiments, the helical shape of each guide component  411 ,  412  comprises greater than or equal to two wraps around transmitter axis  403 . In some embodiments, the helical shape of each guide component  411 ,  412  comprises greater than or equal to three wraps around transmitter axis  403 . In some embodiments, the helical shape of each guide component  411 ,  412  comprises fewer than one full wrap around transmitter axis  403 . 
         [0042]    In general, for MDC WPT systems, the transmitter and receiver magnetic fields can be caused to move about their respective transmitter/receiver axes in either angular direction (e.g. by suitable pivotal and/or rotational movement of the permanent magnet(s) in their respective magnetic systems). Guides  110 ,  410  may be shaped to allow for magnetic foreign objects to be forced toward either end of the corresponding transmitter housing  104 ,  404  as illustrated in  FIGS. 4-6 . In the embodiments of  FIG. 4-6 , the magnetic foreign objects will be forced toward either end of the outward surface  104 A,  404 A of the cylindrical transmitter/receiver housing  104 ,  404 , depending on the direction in which the transmitter/receiver magnetic field moves about axis  103 ,  403  and the direction of the helical guide  110 ,  410  wrapping around axis  103 ,  403 . For metallic materials that are non-magnetic (e.g. aluminum and/or the like), the eddy currents produced in the electrically conductive metal by the moving magnetic fields associated with WPT system are beneficial to removal. Such non-magnetic metaling objects will be forcefully expelled from the charging region of WPT system as the field produced by eddy-currents in the metal will oppose the field of the WPT system and produce a net force on the foreign object which causes the object to be expelled. 
         [0043]      FIG. 7  depicts a WPT transmitter  502  according to a particular embodiment and is used to illustrate the mechanism by which guide  510  removes metallic foreign objects and other debris from a WPT system. It will be appreciated from the discussion that follows that the operation of a WPT receiver may be substantially similar to that of WPT transmitter  502  shown in  FIG. 7  and described herein. As discussed above, WPT transmitter  502  comprises a transmitter housing  504  with an outer surface  504 A comprising a guide  510  having many of the features described above in connection with  FIG. 3 . In the  FIG. 7  embodiment, it is assumed that WPT transmitter  502  comprises a transmitter magnetic system which comprises a permanent magnet  550  rotating about transmitter axis  503  in a counterclockwise direction  512 . In the  FIG. 7  illustration, the dotted lines are used to show guide  510  on an opposite side of outer surface  504 A of transmitter housing  504 . 
         [0044]    WPT transmitter  502  comprises a transmitter magnetic system  550  which may comprise current carrying coils surrounding a magnet rotor for use in a magneto-dynamic coupling (MDC) wireless charging technology. The transmitter magnetic system is supported in an interior of transmitter housing  504 . The transmitter magnetic system creates a corresponding transmitter magnetic field which has a spatial configuration which moves about (e.g. pivots and/or rotates) about a transmitter axis  503 . Transmitter housing  504  may be constructed of a plastic, rubber or other non-metallic material. In some embodiments, transmitter housing  504  may comprise a soft ferromagnetic material. Transmitter housing  504  comprises an outer surface  504 A which, in the illustrated embodiment, comprises a generally polyhedral (e.g. tetrahedral or cuboid) shape with a longitudinal axis which may be aligned with and/or coincident with transmitter axis  503 . In some embodiments, the polyhedral shape is a cuboid. In some embodiments, WPT transmitter  502  comprises anchor sites (not shown in  FIG. 7 ) to mount WPT transmitter  502  to a suitable support in a desired location such as, for example, in a wireless charging parking area. WPT transmitter  502  may also comprise an optional conduit (not shown in  FIG. 7 ) through which a power cable may extend to supply electrical power to the transmitter magnetic system. Such power may be used to move the transmitter magnetic field about transmitter axis  503  (e.g. by rotating a permanent magnet rotor in the transmitter magnetic system which may involve providing electrical power to the coils surrounding the permanent magnet rotor). 
         [0045]    In the illustrated embodiment, outer surface  504 A of transmitter housing  504  also comprises a guide  510  which may be raised relative to (i.e. project outwardly from) a remainder  504 B of outer surface  504 A to provide a guide surface  510 A. Guide  510  of the  FIG. 7  embodiment (and/or its guide surface  510 A) has a quasi-helical shape with a non-circular cross-section as depicted in  FIG. 7 . Guide surface  510 A may extend in one or more directions such that an orthogonal projection of the one or more extension directions of guide surface  510 A onto a notional plane containing transmitter axis  503  may form one or more angles, α, with transmitter axis  503 , wherein 0°&lt;α&lt;90°. Guide surface  510 A may extend in one or more directions that are non-parallel with transmitter axis  503  and non-orthogonal to transmitter axis  503 . Guide surface  510 A may extend in one or more directions which have component directions that are parallel to transmitter axis  503  and component directions that are orthogonal to transmitter axis  503 . Guide  510  (and/or its guide surface  510 A) may additionally or alternatively extend to wrap around transmitter axis  503  as guide  510  (and/or its guide surface  510 A) extends in one or more directions aligned with transmitter axis  503 . 
         [0046]    In some embodiments, guide  510  comprises a flange (e.g. ridges or fins) which may be raised relative to (e.g. extends outwardly from) a remainder  504 B of outer surface  504 A. In other embodiments, guide  510  may additionally or alternatively comprise at least one of: a groove or channel which is depressed relative to the remainder  504 B of outer surface  504 A; and a combination of a flange which is raised relative to the remainder  504 B of outer surface  504 A and a groove which is depressed relative to the remainder  504 B of outer surface  504 A. The flange of guide  510  provides a guide surface  510 A which may be of variable height (relative to the remainder  504 B of outer surface  504 A). Similarly, where guide  510  comprises grooves or channels, such grooves or channels may be of variable depth. Guide  510  may comprise a variable number of flanges and/or grooves that run or wrap around outer surface  504 A of transmitter housing  504  and/or around transmitter axis  503 . In the illustrated  FIG. 7  embodiment, guide  510  comprises a generally helical shape. Guide  510  need not comprise a large number of helical or screw type wraps (about transmitter axis  503 ) and may comprise fewer that one wrap around axis  503 . Guide  510  may be arranged in a right or left-handed direction or a combination of right and left-handed directions in a double-helix type shape. WPT transmitter  502  may comprise a receptacle (not shown) at one or both ends of transmitter housing  504  for collecting foreign objects which may be guided there by guide  510 . 
         [0047]      FIG. 7  depicts an unwanted magnetic foreign object  514  adhered to outer surface  504 A of transmitter housing  504  by attraction to the permanent magnet of the transmitter magnetic system. As the magnet of transmitter magnetic system rotates in counterclockwise direction  512 , the corresponding transmitter magnetic field also rotates and foreign object  514  moves (e.g. by magnetic interaction with the moving transmitter magnetic field) around outer surface  504 A of housing  504  (e.g. in a manner corresponding to the cross-sectional shape of transmitter housing  504 A) until foreign object  514  encounters guide  510  (or more particularly, a guide surface  510 A of guide  510 —see  FIG. 7 ). As the transmitter magnet continues to rotate about transmitter axis and relative to transmitter housing  504 , the helical or screw type pattern of guide  510  and its guide surface  510 A directs or forces foreign object  514  in direction  516  (leftward in the illustrated  FIG. 7  view). Unlike in the  FIG. 4  embodiment, foreign object  514  may be guided over corners  504 C of housing  504 . Guide  510  can be shaped to direct object  514  into a suitably located receptacle (not shown) or channel or safe location until object  514  can be removed (e.g. during a routine maintenance operation). In some embodiments, the quasi-helical shape of guide  510  comprise greater than or equal to two wraps around transmit axis  503 . In some embodiments, the quasi-helical shape of guide  510  comprise greater than or equal to three wraps around transmit axis  503 . In some embodiments, the quasi-helical shape of guide  510  comprises fewer than one full wrap around transmitter axis  403 . 
         [0048]      FIG. 8  depicts a WPT transmitter  602  according to a particular embodiment and is used to illustrate the mechanism by which guide  610  removes metallic foreign objects and other debris from a WPT system. It will be appreciated from the discussion that follows that the operation of a WPT receiver may be substantially similar to that of WPT transmitter  602  shown in  FIG. 8  and described herein. As discussed above, WPT transmitter  602  comprises a transmitter housing  604  with an outer surface  604 A comprising a plurality of guides  610  having many of the features described above in connection with  FIG. 3 . In the  FIG. 8  embodiment, it is assumed that WPT transmitter  602  comprises a transmitter magnetic system which comprises a permanent magnet  650  rotating about transmitter axis  603  in a counterclockwise direction  612 . In the  FIG. 8  illustration, the dotted lines are used to show guides  610  on opposite sides of outer surface  604 A of transmitter housing  604 . 
         [0049]    WPT transmitter  602  comprises a transmitter magnetic system  650  which may comprise current carrying coils surrounding a magnet rotor for use in a magneto-dynamic coupling (MDC) wireless charging technology. The transmitter magnetic system is supported in an interior of transmitter housing  604 . The transmitter magnetic system creates a corresponding transmitter magnetic field which has a spatial configuration which moves about (e.g. pivots and/or rotates) about a transmitter axis  603 . Transmitter housing  604  may be constructed of a plastic, rubber or other non-metallic material. In some embodiments, transmitter housing  604  may comprise a soft ferromagnetic material. Transmitter housing  604  comprises an outer surface  604 A which, in the illustrated embodiment, comprises a generally polyhedral (e.g. tetrahedral or cuboid) shape with a longitudinal axis which may be aligned with and/or coincident with transmitter axis  603 . In some embodiments, the polyhedral shape is a cuboid. In some embodiments, WPT transmitter  602  comprises anchor sites (not shown in  FIG. 8 ) to mount WPT transmitter  602  to a suitable support in a desired location such as, for example, in a wireless charging parking area. WPT transmitter  602  may also comprise an optional conduit (not shown in  FIG. 8 ) through which a power cable may extend to supply electrical power to the transmitter magnetic system. Such power may be used to move the transmitter magnetic field about transmitter axis  603  (e.g. by rotating a permanent magnet rotor in the transmitter magnetic system which may involve providing electrical power to the coils surrounding the permanent magnet rotor). 
         [0050]    In the illustrated embodiment, outer surface  604 A of transmitter housing  604  also comprises a plurality of guides  610  which may be raised relative to (i.e. projects outwardly from) a remainder  604 B of outer surface  604 A to provide a guide surface  610 A. Each guide  610  of the  FIG. 8  embodiment (and/or its guide surface  610 A) has a generally linear shape. Unlike the  FIG. 7  embodiment, each guide  610  of the  FIG. 8  embodiment extends across only a single side of housing  604 . Each guide surface  610 A may extend in one or more directions such that an orthogonal projection of the one or more extension directions of each guide surface  610 A onto a notional plane containing transmitter axis  603  may form one or more angles, α, with transmitter axis  603 , wherein 0°&lt;α&lt;90°. Each guide surface  610 A may extend in one or more directions that are non-parallel with transmitter axis  603  and non-orthogonal to transmitter axis  603 . Each guide surface  610 A may extend in one or more directions which have component directions that are parallel to transmitter axis  603  and component directions that are orthogonal to transmitter axis  603 . 
         [0051]    In some embodiments, guide  610  comprises a flange (e.g. ridges or fins) which may be raised relative to (e.g. extends outwardly from) a remainder  604 B of outer surface  604 A. In other embodiments, guide  610  may additionally or alternatively comprise at least one of: a groove or channel which is depressed relative to the remainder  604 B of outer surface  604 A; and a combination of a flange which is raised relative to the remainder  604 B of outer surface  604 A and a groove which is depressed relative to the remainder  604 B of outer surface  604 A. The flange of guide  610  provides a guide surface  610 A which may be of variable height (relative to the remainder  604 B of outer surface  604 A). Similarly, where guide  610  comprises grooves or channels, such grooves or channels may be of variable depth. Guide  610  may comprise a variable number of flanges and/or grooves that run along outer surface  604 A of transmitter housing  604 . In the illustrated  FIG. 8  embodiment, each longitudinal side of housing  604  comprises a guide  610 . Each guide  610  comprises a generally linear shape. In other embodiments, guide  610  may not be linear. WPT transmitter  602  may comprise a receptacle (not shown) at one or both ends of transmitter housing  604  for collecting foreign objects which may be guided there by guide  610 . 
         [0052]      FIG. 8  depicts an unwanted magnetic foreign object  614  adhered to outer surface  604 A of transmitter housing  604  by attraction to the permanent magnet of the transmitter magnet system. As the magnet of transmitter magnetic system rotates in counterclockwise direction  612 , the corresponding transmitter magnetic field also rotates and foreign object  614  moves (e.g. by magnetic interaction with the moving transmitter magnetic field) around outer surface  604 A of housing  604  (e.g. in a non-circular manner where transmitter housing  604 A has the illustrated polyhedral shape) until foreign object  614  encounters a guide  610  (or more particularly, a guide surface  610 A of a guide  610 —see  FIG. 8 ). As the transmitter magnet continues to rotate about transmitter axis and relative to transmitter housing  604 , the linear extension of guide  610  and its guide surface  610 A directs or forces foreign object  614  in direction  616  (rightward in the illustrated  FIG. 8  view). Unlike in the  FIG. 7  embodiment, foreign object  614  is not guided over the corners of housing  604 . Instead, each individual guide  610  directs foreign object  614  from left to right on a single face of housing  604 . Guide  610  can be shaped to direct object  614  into a suitably located receptacle (not shown) or channel or safe location until object  614  can be removed (e.g. during a routine maintenance operation). 
         [0053]    A competitive advantage of the WPT systems with debris removal systems as described here is associated with magneto-dynamic coupling (MDC) WPT systems where the rotating magnetic field in the MDC WPT system naturally produces forces on any foreign metallic objects which can then be directed away from the charging region. The magnetic fields rotate around the axes of the WPT transmitter and WPT receiver and twice per cycle there will be a magnetic pole pointing away from the charging region entirely. In contrast, inductive coupling systems typically operate on a time-varying magnetic field which points predominantly along the axis between transmitter and receiver (e.g. an axis corresponding to the coil(s) associated with the transmitter and receiver magnetic systems) and varies in amplitude rather than direction. In the MDC wireless charging system, the rotation of the field in normal operation is enough to remove small foreign objects in a manner of seconds, while larger ferromagnetic objects could be removed by slowing the charger rotation for a few seconds until they are automatically expelled. 
         [0054]      FIGS. 9 and 10  depict a WPT transmitter  702  of an inductive wireless power transfer system according to a particular embodiment and is used to illustrate the mechanism by which sweeper  775  removes metallic foreign objects and other debris from an inductive wireless power transfer system. It will be appreciated from the discussion that follows that the operation of a WPT receiver may be substantially similar to that of WPT transmitter  702  shown in  FIGS. 9 and 10  and described herein. WPT transmitter  702  comprises a transmitter housing  704  having a sweeper magnetic system  775  therein and an outer surface  704 A comprising a plurality of guides  710 . In the  FIG. 9  embodiment, it is assumed that WPT transmitter  702  comprises a transmitter magnetic system which comprises a plurality of coils  780  energized to create a transmitter magnetic field  780 A that varies with time to thereby transfer power to a WPT receiver (not shown). 
         [0055]    WPT transmitter  702  comprises a transmitter magnetic system comprising a plurality of magnetic field generating coils  780  which may be used for induction power transfer. The transmitter magnetic system is supported in an interior of transmitter housing  704 . The transmitter magnetic system creates a corresponding transmitter magnetic field  780 A which has a spatial configuration which varies with time. Transmitter housing  704  may be constructed of a plastic, rubber or other non-metallic material. In some embodiments, transmitter housing  704  may comprise a soft ferromagnetic material. Transmitter housing  704  comprises an outer surface  704 A which, in the illustrated embodiment, comprises a generally polyhedral (e.g. tetrahedral or cuboid) shape. This is not mandatory. In other embodiments, outer surface  704 A may be rounded (e.g. cylindrical or conical). In some embodiments, WPT transmitter  702  comprises anchor sites (not shown in  FIGS. 9 and 10 ) to mount WPT transmitter  702  to a suitable support in a desired location such as, for example, in a wireless charging parking area. WPT transmitter  702  may also comprise an optional conduit (not shown in  FIGS. 9 and 10 ) through which a power cable may extend to supply electrical power to the transmitter magnetic system. Such power may be used to vary transmitter magnetic field  780 A (e.g. by energizing and/or varying the energization of coils  780 ). 
         [0056]    In the illustrated embodiment, WPT transmitter  702  comprises a sweeper  775 . Sweeper  775  may be supported for movement, in a direction  712 , within housing  704 . Sweeper  775  may comprise one or more magnetic field generating units such as, permanent magnets and/or magnetic field generating coils. The one or more magnetic field generating units of sweeper  775  create sweeper magnetic field  775 A. As can be seen from  FIG. 9 , sweeper  775  is generally elongated in the x-direction and has a width in the y-direction. An x-direction dimension of sweeper  775  may be substantially the same as an x-direction dimension of housing  704 . In some embodiments, the x-direction dimension of sweeper  775  is between 50% and 95% of the length as the x-direction dimension of housing  704 . In other embodiments, the x-direction dimension of sweeper  775  is greater than 95% of the length of the x-direction dimension of housing  704 . In some embodiments, sweeper  775  may have a different geometry, such as a geometry corresponding to the shape of outer surface  704 A of housing  704 . Sweeper  775  may be movable in one or more directions  712 . For example, in the illustrated embodiment, sweeper  775  may be translated back and forth in the y-direction. In other embodiments, sweeper  775  may be pivoted or rotated as desired. Sweeper  775  may be moved by a drive system such as a separate motor (electric or otherwise) and may be supported for movement by one or more of tracks, rails, pulleys, cables, bearings etc. As sweeper  775  is moved in the direction of movement  712 , the sweeper magnetic field  775 A also moves in the direction of movement  712 . In this way, sweeper magnetic field  775 A may be a spatially varying magnetic field. 
         [0057]    In the illustrated embodiment, outer surface  704 A of transmitter housing  704  also comprises a plurality of guides  710  which may be raised relative to (i.e. project outwardly from) a remainder  704 B of outer surface  704 A to provide a plurality of guide surfaces  710 A. Guides  710  of the  FIGS. 9 and 10  embodiment (and/or guide surfaces  710 A) have a substantially linear shape as depicted in  FIG. 9 . In addition to extending in directions aligned with the dominant transmitter magnetic field direction  780 A (e.g. the axial direction  780 A of the transmitter coil(s) used to create the transmitter magnetic field), guide surfaces  710 A may also extend in one or more directions orthogonal to the dominant transmitter magnetic field direction  780 A. Guide surface  710 A may extend in one or more directions that are non-parallel with the direction of movement  712  of sweeper  775  and non-orthogonal to the direction of movement  712  of sweeper  775 . Guide surface  710 A may form one or more angles, γ, with direction of movement  712  of sweeper  775 , wherein 0°&lt;γ&lt;90°. Guide surface  710 A may extend in one or more directions which have component directions that are parallel to direction of movement  712  of sweeper  775  and component directions that are orthogonal to direction of movement  712  of sweeper  775 . 
         [0058]    In some embodiments, guides  710  comprise a flange (e.g. ridges or fins) which may be raised relative to (e.g. extends outwardly from) a remainder  704 B of outer surface  704 A. In other embodiments, guides  710  may additionally or alternatively comprise at least one of: a groove or channel which is depressed relative to the remainder  704 B of outer surface  704 A. The flange of guides  710  provides guide surfaces  710 A which may be of variable height (relative to the remainder  704 B of outer surface  704 A). Similarly, where guides  710  comprise grooves or channels, such grooves or channels may be of variable depth. Guides  710  may comprise a variable number of flanges and/or grooves that run across or around outer surface  704 A of transmitter housing  704 . Guides  710  need not comprise large number of flanges or grooves. WPT transmitter  702  may comprise a receptacle (not shown) at one or both ends of transmitter housing  704  for collecting foreign objects which may be guided there by guides  710 . 
         [0059]      FIG. 9  depicts an unwanted magnetic foreign object  714  adhered to outer surface  704 A of transmitter housing  704  by attraction to transmitter magnetic field  780 A. The presence of foreign object  714  may be detected automatically (e.g. by a loss in efficiency) or may be detected manually. Upon detection of foreign object  714 , transmission of power by transmitter  702  may be shut off to allow for removal of foreign object  714  by sweeper  775  and guides  710 . After transmission of power by transmitter  702  is shut off, sweeper  775  may be energized (if necessary to create sweeper magnetic field  775 A) and may be moved in the direction of movement  712 . As sweeper  775  is moved in the direction of movement  712 , it will pass under foreign object  714  which will be attracted thereto by sweeper magnetic field  775 A. Foreign object  714  will therefore move in the direction of movement  712  until foreign object  714  encounters guide  710  (or more particularly, a guide surface  710 A of guide  710 ). As sweeper  775  continues to move in the direction of movement  712 , guide surface  710 A directs or forces foreign object  714  in direction  716  (rightward in the illustrated  FIG. 9  view). Guide  710  can be shaped to direct object  714  into a suitably located receptacle (not shown) or channel or safe location until object  714  can be removed (e.g. during a routine maintenance operation). In some embodiments, outer surface  704 A of WPT transmitter  702  does not comprise guide  710  and when sweeper  775  is moved in direction of movement  712 , foreign object  714  is directed to an edge of outer surface  704 A without the aid of a guide. 
         [0060]    Aspects of the present invention facilitate the removal of deleterious magnetic and non-magnetic metallic foreign objects from the vicinity of (e.g. the air gap between) the WPT transmitter and WPT receiver in magneto-dynamic coupling MDC WPT and induction WPT (e.g. wireless charging) systems. Aspects of the present invention may be used in mobile applications such as, but not limited to, electric powered automobiles, transit buses, delivery vehicles, golf carts, underwater remote operated vehicles or trucks. 
         [0061]    Embodiments of the invention described herein may be used in any magnetically-coupled wireless charging systems and induction wireless charging systems for, but not limited to, electric powered automobiles, transit buses, delivery vehicles, trucks, drones, boats, golf carts or other consumer devices. Particular embodiments allow for low cost and low maintenance automatic wireless charging stations of simple construction and assembly and further encourage adoption of electric vehicle technology. 
         [0062]    As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example:
       Various embodiments described herein may each include a variety of features. It should be understood that this description and the accompanying claims include additional embodiments that comprise combinations of any of the features of any of the embodiments herein.   In some instances, this description and the accompanying claims use terms generally to describe directions, orientations, shapes, relationships (e.g. equalities) and/or the like. For example, transmitter magnetic system may have a first magnetization direction that is orthogonal to a transmitter magnetization-variation direction. Such directions, orientations, shapes, relationships and/or the like should be considered to accommodate the specified directions, orientations, shapes, relationships and/or the like and/or relatively small deviations (from an operational or engineering perspective) from the specified directions, orientations, shapes, relationships and/or the like.   In some instances, this description and the accompanying claims refer to receiver magnetic systems. Where the receiver magnetic systems comprise coils, the reference to receiver magnetic system is a matter of nomenclature and doesn&#39;t necessarily mean that the receiver magnetic system is driven to generate corresponding magnetic fields. In practice, the receiver magnetic system may instead have currents induced therein, which induced currents may in turn create corresponding magnetic fields.   In this description and the accompanying claims, elements (such as, by way of non-limiting example, WPT transmitters and WPT receivers) are said to overlap or align with one another in a direction or along a direction. For example, a WPT receiver may overlap or be aligned with a WPT receiver along a particular direction. When it is described that two or more objects overlap or are aligned in or along a particular direction, this usage should be understood to mean that line oriented in that particular direction could be drawn to intersect the two or more objects.       
 
         [0067]    While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended aspects or claims and aspects or claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations.