Abstract:
A system that transfers energy wirelessly includes a transmitter of the energy and a receiver of the energy. A housing made of a material that approximates properties of a perfect magnetic conductor. The housing is arranged to direct a magnetic field from the transmitter to the receiver to improve an efficiency of the energy transfer from the transmitter to the receiver.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to wireless energy transfer, and more particularly to transferring energy using perfect magnetic conductors. 
       BACKGROUND OF THE INVENTION 
       [0002]    Perfect magnetic conductors (PMCs) are a variant on the concept of metamaterials that has an extremely high permeability μ (mu), and an extremely high magnetic field saturation value. Like most metamaterials, PMCs do not occur naturally, but are realized artificially. For example, an approximate band-limited artificial PMC can be constructed by placing periodic elements such as square conductive patches with central conducting vias through an insulating substrate connecting to a conducting backplane, sometimes called the “mushroom array” configuration. The arrays can be modified by adding spirals, inductors, etc., to alter the frequency response. Unmodified, these PMCs typically have bandwidths of five to ten percent of their center frequency, which is entirely adequate for the purposes of wireless energy transfer. 
         [0003]    An efficient PMC can be constructed from a grounded ferrite slab with an appropriate bias voltage. Electromagnetic band gap (EBG) materials can also be used for PMCs. 
       SUMMARY OF THE INVENTION 
       [0004]    The embodiments of the invention improve an efficiency of wireless energy transfer by using perfect magnetic conductors (PMCs) as reflectors and field confinement devices placed adjacent to transmit and receive antennas. 
         [0005]    If the energy transfer system uses an array of resonators, it is possible to improve the energy transfer efficiency by arrange a layer of PMC as a reflective backing adjacent to the array. 
         [0006]    The PMC provides partial confinement of the magnetic field and focuses the magnetic field in the direction from the receive antenna(s). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic of a perfect magnetic conductor (PMC) in the presence of an electric current and associated magnetic field according to embodiments of the invention; 
           [0008]      FIGS. 2-4  are oblique exploded, oblique assembled and side cross-sectional schematic views of a wireless energy transfer device using PMC according to embodiments of the invention; 
           [0009]      FIG. 5  is a cross-sectional schematic view of a wireless energy transfer using PMC according to embodiments of the invention; 
           [0010]      FIG. 6  is an oblique schematic view of a wireless energy transfer array according to embodiments of the invention; and 
           [0011]      FIG. 7  is a sectional schematic view of a wireless energy transfer array according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]    The embodiments of our invention improve an efficiency of wireless energy transfer by using perfect magnetic conductors (PMCs) as reflectors and field confinement devices placed adjacent to transmit and receive antennas. 
         [0013]    In one embodiment, a transmit loop and a transmit resonator are arranged in a housing made of the PMC, with an open side of the housing facing a receive loop, and a receive resonator. The resonator is not required, but can improve the efficiency of the energy transfer. 
         [0014]    For an array of resonators, the PMC is arranged as a flat underlay layer below the array of resonators. The effect of this arrangement is that the energy transfer efficiency is greatly increased, (to a first approximation efficiency is doubled, and losses are halved) and the energy distribution is more uniform. 
         [0015]    As shown in  FIG. 1 , a first electric current  120  induces a first circular magnetic field  130  on one side of a near-field PMC  110 . The PMC causes an equivalent a second electric current  140  and a second magnetic field  150  on the other side of the PMC. 
         [0016]    In contrast with a conventional electrical conductor that generates eddy currents that oppose entry by a magnetic field, the PMC is designed to maximize the entry by the magnetic field. 
         [0017]    To a first approximation, the PMC reflects a mirror image of any current-carrying conductor, measurable on the same side of the PMC conductor as the original current. As a side effect, no magnetic field is measureable on the opposite side of the PMC due to the current-carrying conductor. Just as any other reflector, the current  140  and the magnetic field are above the PMC  110 . 
         [0018]      FIGS. 2-3  shows oblique exploded and assembled views, respectively. Here, a wireless energy transmit loop  210  and transmit resonator  220  are arranged in a first PMC housing including a bottom  231  and sides  232 ,  233 ,  234 , and  235 . The wireless energy receiver loop  260  and receiver resonator  250  are arranged in a second housing including a top  241 , and sides  242 ,  243 ,  244 , and  245 . The open sides of the housings face each other. 
         [0019]    It is understood that during operational use, the transmitter can be connected to a power source while the receiver is connected to a load. 
         [0020]      FIG. 4  shows a cross section of the arrangement. A transmit loop  410  and a transmit resonator  420  are partially enclosed in a PMC housing  430 , with the open side facing a receive resonator  440  and a receiver loop  450 , which are also both partially enclosed by the housing, again with the open sides facing each other. 
         [0021]      FIG. 5  shows another embodiment with a transmit loop  510  and a transmit resonator  520  arranged in a PMC housing  530 . In this embodiment, the PMC housing  530  has a hollow extension of PMC  535  extending within the axis of symmetry to further confine the magnetic field generated by transmit loop antenna  510  and the transmit resonator  520 . Similarly, a receive resonator  540  and a receive antenna loop  550  are arranged in a PMC housing  560 , again with a hollow extension  565  extending within the axis of symmetry of the receive loop antenna  550  and receive resonator  540 . Here, the geometry of the housings are similar to “bundt” baking pans with a hollow core, i.e., hollow half toroids. 
         [0022]    The central extensions  535  and  565  improve the efficiency of the wireless energy transfer system. 
         [0023]      FIG. 6  shows an arrangement using a wireless energy transfer resonator array. As in  FIGS. 2-3 , the transmit loop antenna  610  is enclosed in an open-sided PMC  630  housing. An array of resonators  620  composed of resonators  620   a,    620   b ,  620   c  etc. is arranged above the transmit loop antenna  610 . An extension of the PMC housing  630  is the underlayment and PMC shield  640 . The shield  640  extends beneath the array of resonators  620 , and serves two purposes. The shield prevents any losses due to the magnetic field of the array of resonators  620  into an area below the array  620  to effectively double the field strength above the array of resonators  620  by the PMC reflector effect as shown in  FIG. 1 . Completing the wireless energy transfer system, resonator  650  and receive loop  60  are enclosed in an open-sided PMC housing  670 . 
         [0024]      FIG. 7  shows an arrangement for a wireless energy transfer resonator array. A wireless energy transmitter loop  710  and resonator  720   a,  which is an element of the resonator array  720  are enclosed in an open-sided PMC housing  730 . The housing has both an internal extension  750  and extended underlayment  740  to intensify and confine the magnetic field. A receive resonator  770  and receive loop  780  are enclosed in a PMC housing  790 , which is also equipped with a hollow internal extension  795 , similar to what is shown in  FIG. 5 . As before, the PMC housing effectively doubles the magnetic field strength, and minimize losses due to straying magnetic fields. 
         [0025]    It should be noted that the designations of “transmit” and “receive” loop antennas are entirely arbitrary. In all embodiments, the functionality is identical if the RF energy is applied to the “receive” loop antenna and energy is extracted from the “transmit” loop antenna. Likewise, it is possible to mix and match the use of PMC housings, shields, underlayments, or extensions. Some embodiments do not require the internal PMC extensions such as  535  and  565  in  FIG. 5 . Some PMC fabrication techniques can use solid rather than hollow extensions. Further, it is not necessary that both the receiver and transmitter both use the PMC. Cost, weight, and other design considerations may dictate that only an underlayment of PMC without a full housing is the most effective design, or that only the transmitter or receiver contains the PMC, or no resonators, instead using only loop antennas and PMC to achieve wireless energy transfer. 
         [0026]    Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.