Patent Publication Number: US-2015066155-A1

Title: Wireless charging for prosthetic device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/870,704 (Atty Docket No. 2919-32732.PROV), filed on Aug. 27, 2013, which is hereby incorporated by reference in its entirety. This application also claims the benefit of U.S. Provisional Application No. 61/907,975 (Atty Docket No. 54919-03450), filed on Nov. 22, 2013, the contents of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates to prosthetic devices. More particularly, the present disclosure relates to charging prosthetic devices. 
     BACKGROUND 
     Many modern prosthetic devices are electrically powered to provide actuation or damping of the prosthetic device. While such powered prosthetic devices can provide a more natural motion, the mobile nature of prosthetic devices generally requires the use of a power storage unit such as a rechargeable battery to power the prosthetic device. Charging the power storage unit usually involves plugging a power supply into the prosthetic device. While the power storage unit charges, movement of the prosthetic device is restrained by a cable connected to the power supply or the prosthetic device must be removed. Plugging a power supply into the prosthetic device also typically requires a power input jack on the prosthetic device which can compromise the prosthetic device&#39;s resistance to environmental conditions such as dirt, moisture and water. In addition, charging a prosthetic device using a power input jack may require removal of an outer skin or a hole in an outer skin in order to access the power input jack. The outer skin can enclose the prosthetic device to provide a more natural and aesthetic appearance. Removing the outer skin or providing a hole in the outer skin adversely affects the aesthetic appearance of the device or can require additional effort in removing the outer skin. 
     SUMMARY 
     In view of the foregoing, the present disclosure involves wirelessly charging a prosthetic device via magnetic coupling. To further improve the freedom of movement of the prosthetic device while charging, some aspects of the present disclosure involve wirelessly charging a prosthetic device using resonant magnetic coupling. Traditional magnetic induction methods of charging devices typically rely on a tight coupling between transmitter and receiver coils to maintain a power transfer efficiency. Resonant magnetic coupling can allow for a farther distance between transmitter and receiver coils so as to improve the freedom of movement while charging and to allow for the simultaneous charging of multiple prosthetic devices. 
     According to one embodiment, a prosthetic device includes a power storage unit to power the prosthetic device and an electromagnetic receiver including a plurality of coils arranged about a portion of the prosthetic device. The electromagnetic receiver is configured to receive a magnetic field from an electromagnetic transmitter magnetically coupled with the electromagnetic receiver and to generate electric power from the magnetic field. Circuitry of the prosthetic device is configured to store the electric power generated from the magnetic field in the power storage unit. 
     By arranging a plurality of coils about a portion of the prosthetic device, it is ordinarily possible to allow for charging from different angles between the prosthetic device and the electromagnetic transmitter. In some embodiments, the magnetic field is a resonanting magnetic field with a resonant frequency of the electromagnetic receiver. 
     According to another embodiment, the present disclosure includes an electromagnetic transmitter including circuitry configured to receive electric power from a power supply. A plurality of coils of the electromagnetic transmitter is configured to generate a magnet field using the electric power to magnetically couple with an electromagnetic receiver of a prosthetic device. In one aspect, the electromagnetic transmitter is further configured to generate a resonating magnetic field with a resonant frequency of the electromagnetic receiver of the prosthetic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the embodiments of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the disclosure and not to limit the scope of what is claimed. 
         FIG. 1  is a block diagram depicting wireless charging of a prosthetic device according to an embodiment. 
         FIG. 2  illustrates a prosthetic device including an electromagnetic receiver according to an embodiment. 
         FIG. 3  is a front view of an electromagnetic receiver including adjacent coils according to an embodiment. 
         FIG. 4  is a side view of an electromagnetic receiver with overlapping flexible circuits according to an embodiment. 
         FIG. 5  illustrates a prosthetic device charging system with multiple electromagnetic transmitters according to an embodiment. 
         FIG. 6  illustrates a portable electromagnetic transmitter inside a car according to an embodiment. 
         FIG. 7  is a front view of an electromagnetic transmitter with partially overlapping coils according to an embodiment. 
         FIG. 8  is a side view of an electromagnetic transmitter with overlapping flexible circuits according to an embodiment. 
         FIG. 9  is a flowchart for a charging process performed by an electromagnetic transmitter according to an embodiment. 
         FIG. 10  is a flowchart for a charging process performed by a prosthetic device according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, numerous specific details are set forth to provide a full understanding of the present disclosure. It will be apparent, however, to one of ordinary skill in the art that the various embodiments disclosed may be practiced without some of these specific details. In other instances, well-known structures and techniques have not been shown in detail to avoid unnecessarily obscuring the various embodiments. 
       FIG. 1  depicts wireless charging of prosthetic device  106  using electromagnetic (EM) transmitter  104 . Prosthetic device  106  can be, for example, a battery powered prosthetic joint such as a prosthetic ankle or knee, or a prosthetic leg including both a prosthetic ankle and knee. 
     EM transmitter  104  is powered by power supply  102  and is configured to transmit magnetic field  124  to EM receiver  112  of prosthetic device  106 . As will be discussed in more detail below, power supply  102  can be an alternating current (AC) power supply (e.g., from a wall outlet) or a direct current (DC) power supply (e.g., from a battery or wall power adapter). 
     In the example of  FIG. 1 , EM transmitter is further configured to transmit magnetic field  124  as a resonating magnetic field at a resonant frequency of EM receiver  112  of prosthetic device  106 . In some embodiments, such a resonant frequency can be within a range of 100 kHz and 10 MHz. 
     In one implementation, each of EM transmitter  104  and EM receiver  112  can include a plurality of coils or inductors electrically connected to one or more tuning capacitors for tuning to a frequency, f, which can be represented as shown in Equation 1 below: 
     
       
         
           
             
               
                 
                   f 
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                     1 
                     
                       2 
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                        
                       π 
                        
                       
                         LC 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
     where L is an inductance of the plurality of coils at resonance and C is a capacitance of the at least one tuning capacitor for the plurality of coils. Power transfer efficiency through resonance can be improved by reducing resistance in the transmitting or receiving coils. 
     In some implementations, EM transmitter  104  can include different inductors and/or capacitors for generating magnetic fields at different frequencies. In this regard, the tuning capacitor can include a variable capacitor for tuning to different frequencies. In yet other implementations, EM transmitter  104  can include a chipset or integrated circuit for generating a magnetic field. 
     EM transmitter  104  can also include circuitry for communicating with prosthetic device  106  or controlling operation of EM transmitter  104 . Such circuitry can include, for example, a controller, a processor, wireless communication chipset, or an application-specific integrated circuit (ASIC) which executes computer-readable instructions stored in a memory of EM transmitter  104 . 
     As shown in  FIG. 1 , prosthetic device  106  includes EM receiver  112 , battery management system (BMS)  114 , and electronics  118 , each of which is carried by prosthetic device  106 . EM receiver  112  is configured to receive magnetic field  124  from EM transmitter  104  and to generate electric power from magnetic field  124 . The power generated over time is proportional to the strength of the magnetic field. EM receiver  112  includes a plurality of inductors or coils which convert magnetic field  124  into an electric field to generate electric power. The plurality of coils can be electrically connected to one or more tuning capacitors to tune to a frequency used by EM transmitter  104 . In yet other implementations, EM receiver  112  includes a chipset or integrated circuit for receiving magnetic field  124  and converting magnetic field  124  into an electric field to generate electric power. 
     EM receiver  112  can also include circuitry for controlling operation of EM receiver  112 . Such circuitry can include, for example, a controller, a processor, a wireless communication chipset, or an ASIC for executing computer-readable instructions stored in a memory of prosthetic device  106 . 
     Although inductive chargers, such as those used for electric toothbrushes, can provide wireless charging, such inductive charging systems generally require that the power transmitter and the power receiver are spatially aligned with each other. This would require a user of a prosthetic device to remove the prosthetic device for charging or keep the prosthetic device in a fixed position while charging. As with wired charging, keeping the prosthetic device in a fixed position would be cumbersome for the user of the prosthetic device as it limits mobility of the prosthetic device and introduces charge time inefficiencies when the transmitter and the receiver are not properly aligned. 
     By tuning EM transmitter  104  and EM receiver  112  to approximately the same resonant frequency, EM transmitter  104  and EM receiver  112  do not need to be closely aligned and the distance between them can be increased so that EM transmitter  104  can be remote from prosthetic device  106  while still transferring power to prosthetic device  106 . In some implementations, the amount of distance between EM transmitter  104  and EM receiver  112  can vary from several inches to over ten feet. Moreover, it is ordinarily possible to transfer power to prosthetic device  106  without having to remove prosthetic device  106  or restrict a user&#39;s movement of prosthetic device  106 . In addition, EM resonant wireless charging can allow for simultaneous charging of multiple prosthetic devices, which can be especially useful for users with multiple prosthetic devices. 
     In some implementations, circuitry of EM transmitter  104  can adjust an amount of electric power used from power supply  102  to dynamically adjust for changes in the position of prosthetic device  106  or to dynamically adjust to charging additional devices while maintaining a real-time communication link. In another implementation, EM transmitter  104  may use between 10 and 20 Watts from power supply  102  to generate magnetic field  124 . EM transmitter  104  may then vary the amount of power between 10 and 20 Watts based on a reflected power in magnetic field  124  that is not received by EM receiver  112  and is reflected back to EM transmitter  104 . 
     A decrease in the reflected power can indicate that more devices are being charged or that the positioning of prosthetic device  106  has changed such that more of the transmitted power is received by EM receiver  112 . In such an example, EM transmitter  104  may then increase the power used from power supply  102  toward an upper power limit so as to transfer more power via magnetic field  124 . 
     On the other hand, an increase in the reflected power reflected back to EM transmitter  104  can indicate that less of the transmitted power is being received. In one implementation, if the reflected power exceeds a threshold, EM transmitter  104  may first increase the power used from power supply  102  to increase a range of magnetic field  124 . If the proportion of reflected power to transmitted power does not decrease after increasing the power used, EM transmitter  104  may then determine that prosthetic device  106  is no longer within a range to efficiently receive magnetic field  124 . EM transmitter  104  may then stop generating magnetic field  124  and enter a low power or standby state. 
     Adjustments to the power used to generate magnetic field  124  can also be made based on digital communications between EM transmitter  104  and prosthetic device  106  using a wireless communications link such as, for example, a Bluetooth Low Energy or a wireless Ethernet communications link. In this regard, each of EM transmitter  104  and prosthetic device  106  can include a wireless communication module or chipset so that EM transmitter  104  can adjust a frequency or a power used to generate magnetic field  124  based on information received from prosthetic device  106  concerning a location or charging efficiency of EM receiver  112 . 
     In some implementations, EM transmitter  104  and EM receiver  112  may also operate in accordance with a particular wireless charging standard, such as Qualcomm&#39;s WiPower standard, A4WP&#39;s Rezence standard, or the Wireless Power Consortium&#39;s Qi standard. 
     As shown in the example of  FIG. 1 , prosthetic device  106  includes BMS  114  which includes power storage unit  116  that can, for example, include a rechargeable battery or super capacitor capable of storing power. BMS  114  may also include circuitry for storing power generated from magnetic field  124  in power storage unit  116 . Such circuitry can include a full wave rectifier and a regulator circuit to convert AC power generated from magnetic field  124  into DC power for charging power storage unit  116 . 
     Electronics  118  can include controls for actuation and/or damping of prosthetic device  106  and electronics for communication with other devices. In this regard, electronics  118  can include at least one of a motor, a valve, a sensor, or a controller for actuating or damping a movement of prosthetic device  106 . 
     In one implementation, electronics  118  also includes an antenna for receiving a radio frequency (RF) beacon transmitted from EM transmitter  104 . In such an implementation, EM transmitter  104  can periodically transmit beacons and electronics  118  can respond by transmitting device information to EM transmitter  104 . The communication between EM transmitter  104  and electronics  118  may be in accordance with a particular communications protocol such as Bluetooth. The device information can indicate different frequencies at which EM receiver  112  can tune to for receiving power from EM transmitter  104  via magnetic field  124 . EM transmitter  104  may then select a frequency to tune to based on the device information received from prosthetic device  106 . 
     In other implementations, the device information may include information about prosthetic device  106  such as a proximity or alignment indication for EM receiver  112  with respect to EM transmitter  104 , an average power usage rate, or information about BMS  114 , such as at least one of a charging efficiency, a state of charge, a charge capacity, and an average or estimated charge time. EM transmitter  104  may use this device information to adjust the rate at which power is transferred to EM receiver  112  by changing the amount of power used from power supply  102  to generate magnetic field  124 . For example, if the device information indicates that the current charge level is fully charged, EM transmitter  104  may select a lower rate or power at which to transfer power to EM receiver  112 . In another example, if the device information indicates a long estimated charge time, EM transmitter  104  may select a higher rate or power at which to transfer power to EM receiver  112 . 
     In some implementations, the device information may be wirelessly transmitted to a mobile device such as a cellular phone or tablet to allow an application on the mobile device to display prosthetic device information to a user. Such prosthetic device information can include information concerning a proximity or alignment of EM receiver  112  with respect to EM transmitter  104 , an average power usage rate, a charging efficiency, a state of charge, a charge capacity, and an average or estimated charge time. 
       FIG. 2  illustrates an example of a prosthetic device including an electromagnetic receiver according to an embodiment. In the example of  FIG. 2 , prosthetic device  206  includes a prosthetic ankle joint and a prosthetic foot. As discussed above, wireless charging of prosthetic device  206  can reduce the need for a power input which can allow dirt and moisture into prosthetic device  206 . In addition, a substantially uniform outer layer can be placed around prosthetic device  206  for a more natural appearance without requiring any holes for a power input or requiring removal of the outer layer for charging. 
     In the example embodiment of  FIG. 2 , EM receiver  220  is located about a top portion  224  of prosthetic device  206 . In other embodiments, EM receiver  220  may be placed about different portions of prosthetic device  206  such as along a sole portion of the foot or around a portion of prosthetic device  206  closer to the ankle joint. 
     EM receiver  220  includes a plurality or array of coils  222  that are arranged adjacent to one another so that the diameters of coils  222  completely surround portion  224  of prosthetic device  206 . As shown in  FIG. 2 , each coil  222  of the plurality of coils is in physical contact with another coil  222  and forms a ring that completely surrounds portion  224 . 
     By arranging coils  222  about portion  224 , it is ordinarily possible to increase the freedom of motion of prosthetic device  206  while charging since EM receiver  220  is capable of receiving a magnetic field from different angles. In other words, the rotation or angle of prosthetic device  206  may change with respect to an EM transmitter while charging since different coils  222  may be used in varying degrees depending upon the relative position of the coil with respect to the EM transmitter. The use of multiple coils  222  can also increase the amount of electric power generated from the magnetic field by providing for better magnetic coupling with the EM transmitter. 
     As shown in  FIG. 2 , coils  222  partially overlap each other to further improve a power transfer efficiency of EM receiver  220  since coils  222  cover all angles around portion  224 . In other embodiments, coils  222  may only touch on their edges as opposed to overlapping or EM receiver  220  may include small gaps between coils  222 . In yet other embodiments, prosthetic device  106  may include bands of coils  222  at different heights along prosthetic device  206  so as to allow for placement of an EM transmitter at different heights while charging. 
       FIG. 3  provides a front view of EM receiver  320  where coils  322  are arranged substantially in the same plane with each coil  322  adjacent to another coil  322  so that coils  322  touch one another. Each coil  322  can include a printed circuit board (PCB) trace along flexible circuit  324  or a flexible wire mounted on flexible circuit  324 . This can generally allow EM receiver  320  to be flexible enough to wrap around a portion of the prosthetic device. 
     EM receiver  320  also includes circuitry  326  which is configured to store electric power generated by coils  322  in a power storage unit. Circuitry  326  is electrically connected to each of coils  322  via traces  316  and  318 . Electric power generated by coils  322  travels along traces  316  and  318  to circuitry  326 , which can include a summing circuitry to add the electric power generated by coils  322  before storing the electric power in a power storage unit via power output  328 . In some embodiments, circuitry  326  can also include a full wave rectifier or a regulator circuit to convert AC power into DC power for charging a power storage unit. 
       FIG. 4  provides a side view of EM receiver  420  including overlapping flexible circuits  424  and  432  according to an embodiment. As shown in  FIG. 4 , EM receiver  420  includes a top plurality of coils  422  and a bottom plurality of coils  430  each arranged on flexible circuits  424  and  432 , respectively. Other embodiments may include more than the two layers of flexible circuits shown in  FIG. 4 . 
     Although there is a small lateral gap between each coil of coils  422  and each coil of coils  430 , the coils are arranged such that the coils of flexible circuit  422  are laterally offset from the coils of flexible circuit  432  so as to provide increased coverage for receiving a magnetic field. The coils of both flexible circuits may be connected to one another using the same traces on one of the flexible circuits may or may use separate traces or wiring. 
     EM receiver  420  of  FIG. 4  also includes circuitry  426  which may include a summing circuitry for adding the electric power generated by coils  422  and  430  before storing the electric power in a battery storage unit via power output  428 . In some embodiments, circuitry  426  can also include a full wave rectifier or a regulator circuit to convert AC power into DC power for charging a power storage unit. 
       FIG. 5  illustrates prosthetic device charging system  500  including EM transmitters  504  and  508  according to an embodiment. EM transmitters  504  and  508  can have a construction similar to EM transmitter  104  of  FIG. 1  and are powered by power supply  502 , which can be a power distribution system for building  510 . 
     Each of EM transmitters  504  and  508  is constructed to secure to a building structure and placed in relation to a different area of building  510 . In particular, EM transmitter  504  is placed above room  512  of building  510  and transmitter  508  is placed above room  514  of building  510 . By locating EM transmitters  504  and  508  in different areas of building  510 , a user of prosthetic device  506  can continue to charge prosthetic device  506  even when they move from room  512  to room  514 , or vice-versa. In this regard, EM transmitters can be strategically placed within a building to allow for continuous charging of a prosthetic device or devices as a user moves throughout the building. 
     Although the embodiment of  FIG. 5  shows EM transmitters  504  and  508  above a ceiling, other embodiments can include EM transmitters  504  and  508  inside rooms  512  and  514 , such as mounted on an interior wall surface of rooms  512  and  514  or beneath furniture in rooms  512  and  514  such as a chair. The placement of EM transmitters  504  and  508  can be made to improve a power transfer efficiency based on a likely location of an EM receiver in a particular room and EM transmitters  504  and  508  may or may not be visible from within the room. In addition, the locations of EM transmitters  504  and  508  do not need to be over the room as shown in  FIG. 5 . In other embodiments, EM transmitters  504  and  508  can be strategically placed in other locations for power transfer efficiency such as below a floor or within a wall. 
     In addition to prosthetic device charging system  500  including EM transmitters  504  and  508 ,  FIG. 5  also includes portable EM transmitter  522  mounted or secured on chair  518 . Portable EM transmitter  522  may charge prosthetic device  506  in addition to EM transmitter  504  or EM transmitter  508  to provide for quicker charging. Portable EM transmitter  522  can be detachably secured to chair  518  using, for example, Velcro, a magnet, a strap, or a clip, so as to allow portable EM transmitter  522  to be repositioned or located elsewhere, such as on chair  519  in room  514 . In the example of  FIG. 5 , portable EM transmitter  522  includes power supply  516  which may be connected to an outlet in room  512 . 
     In the example of  FIG. 5 , prosthetic device  506  is charged by EM transmitter  504  via resonating magnetic field  524  while also being charged by portable EM transmitter  522  via resonating magnetic field  521 . EM receiver  520  of prosthetic device  506  is magnetically coupled with EM transmitter  504  and portable EM transmitter  522  at a resonant frequency of EM receiver  520  so that EM receiver  520  is not required to be closely aligned with EM transmitter  504  or portable EM transmitter  522  to receive power via magnetic fields  524  and  521 . Accordingly, a user of prosthetic device  506  is able to move prosthetic device  506  while it charges. 
     In the example of  FIG. 5 , EM transmitter  508  is not transmitting a magnetic field. In this regard, charging system  500  may determine by comparing reflected powers received at EM transmitters  504  and  508  that prosthetic device  506  is closer to EM transmitter  504  than to EM transmitter  508 . In other implementations, charging system  500  may use a digital wireless communications link to determine a relative location of EM receiver  520 . Charging system  500  may then place EM transmitter  508  into a low power or standby state where no magnetic field is generated by EM transmitter  508 . In other embodiments, EM transmitters  504  and  508  may each continuously generate magnetic fields regardless of whether the magnetic fields are received by EM receiver  520 . 
       FIG. 6  illustrates portable EM transmitters  604  and  612  according to an embodiment. As shown in  FIG. 6 , portable EM transmitter  604  is in the form of a mat that can be plugged into power supply  602 , which in the example of  FIG. 6 , is a cigarette lighter in the interior of an automobile. EM transmitter  604  is connected to power supply  602  via power cable  608 , which is securely routed with clip  606  to avoid interference with operation of the automobile. In other embodiments, portable EM transmitter  604  can include a wall plug for obtaining power from a power outlet. 
     Portable EM transmitter  612  is secured onto a portion of the car seat and is electrically connected to portable EM transmitter  604  to receive power from power supply  602  via portable EM transmitter  604 . This arrangement of transmitters can provide for charging coverage in both a horizontal direction with EM transmitter  604  and in a vertical direction with EM transmitter  612 . 
     EM transmitter  612  may be detachably secured onto the interior of the automobile using, for example, Velcro, a magnet, a strap, or a clip. Both EM transmitters  604  and  612  can be moved to different locations such as to different areas of the automobile, a different automobile, or to different locations at an office or home. Other embodiments may include only one of portable EM transmitter  604  or  612  without the other. 
     As with EM transmitters  104 ,  504 ,  508 , and  522 , portable EM transmitters  604  and  612  include a plurality or array of coils for generating a magnetic field to magnetically couple with an EM receiver of a prosthetic device. By tuning EM transmitters  604  and  612  to a resonant frequency of the EM receiver, the prosthetic device can wirelessly charge while allowing movement of the prosthetic device. 
     In the example of  FIG. 6 , EM transmitter  604  is located mostly below seat  610  to allow the driver to wirelessly charge a prosthetic device while driving. In other implementations, EM transmitter  604  can be placed in other locations such as on a back of seat  610  to allow for charging by users in different seats such as the back seat. 
       FIG. 7  provides a front view of EM transmitter  702  capable of being secured onto a prosthetic device. EM transmitter  702  includes flexible circuit  708  which can allow for EM transmitter  702  to be wrapped around the prosthetic device. By locating EM transmitter  702  next to an EM transmitter of a prosthetic device, it is ordinarily possible to provide quicker charging of the prosthetic device due to the decreased distance between EM transmitter  702  and the EM receiver. 
     As shown in  FIG. 7 , EM transmitter  702  is configured as a belt that can be wrapped around an exterior portion of a prosthetic device such as prosthetic device  206  in  FIG. 2 . In more detail, attachment portions  710  and  712  allow EM transmitter  702  to form a loop that can be worn around the prosthetic device. Attachment portions  710  and  712  can include Velcro, a magnet, a clip, a strap, a buckle, or other ways of securing EM transmitter  702  onto itself. 
     In one embodiment, one or both of attachment portions  710  and  712  can include a magnet that can be used to secure EM transmitter  702  onto a prosthetic device. The magnet may also be used to properly align EM transmitter  702  laterally or vertically onto the prosthetic device by securing EM transmitter  702  onto a corresponding magnet located near an EM receiver of the prosthetic device. Such alignment of EM transmitter  702  can help to ensure a more efficient alignment of coils  704  with respect to the coils of an EM receiver of the prosthetic device. In other embodiments, EM transmitter  702  can use other alignment indicators to indicate when EM transmitter  702  is properly aligned with respect to an EM receiver of the prosthetic device. Such indicators can include a marking that corresponds to another marking on the prosthetic device, a user application on a cellular phone or other mobile device, or an LED. 
     In the example embodiment of  FIG. 7 , coils  704  are arranged substantially in the same plane with each coil  704  partially overlapping an adjacent coil  704  to provide for better coverage in the transmission of the magnetic field. Each coil  704  can include a printed circuit board (PCB) trace or flexible wire on flexible circuit  708 . Such a construction can generally allow EM transmitter  702  to be flexible enough to wrap around a portion of the prosthetic device. 
     EM transmitter  702  also includes circuitry  722  which is configured to receive power from a power supply via power cord  720 . Circuitry  722  is electrically connected to each of coils  704  via traces  716  and  718  to deliver power to coils  704  for generating a magnetic field. 
       FIG. 8  provides a side view of EM transmitter  802  that is capable of being wrapped around a prosthetic device and includes overlapping flexible circuits  808  and  822  according to an embodiment. As shown in  FIG. 8 , EM transmitter  802  includes attachment portions  810  and  812  for forming a loop with EM transmitter  802  so that EM transmitter  802  can be worn around the prosthetic device. Attachment portions  810  and  812  can include Velcro, a magnet, a clip, a strap, a buckle, or other ways of securing EM transmitter  802  onto itself. 
     In one embodiment, one or both of attachment portions  810  and  812  can include a magnet that can be used to secure EM transmitter  802  onto a prosthetic device. The magnet may also be used to properly align EM transmitter  802  laterally or vertically onto the prosthetic device by securing the attachment portion onto a corresponding magnet located near an EM receiver of the prosthetic device. Such alignment of EM transmitter  802  can help to ensure a more efficient alignment of coils  804  with respect to the coils of an EM receiver of the prosthetic device. Other embodiments may use different alignment indicators such as a marking that corresponds to another marking on the prosthetic device or an LED to indicate when EM transmitter  702  is properly aligned with respect to an EM receiver of the prosthetic device. 
     As shown in  FIG. 8 , EM transmitter  802  includes a top plurality of coils  804  and a bottom plurality of coils  824  each arranged on flexible circuits  808  and  822 , respectively. Other embodiments may include more than the two layers of flexible circuits shown in  FIG. 9 . 
     Although there is a small lateral gap between each coil of coils  804  and each coil of coils  824 , the coils are arranged such that the coils of flexible circuit  808  are laterally offset from the coils of flexible circuit  822  so as to provide increased coverage in transmitting a magnetic field. The coils of both flexible circuits may be connected to one another using the same traces or wiring on one of the flexible circuits or may use separate traces or wiring. EM transmitter  802  also includes circuitry  818  which receives power via power supplying circuit  820  and delivers power to coils  804  and  824 . 
       FIG. 9  is a flowchart for a charging process which can be performed by EM transmitter  104  according to an embodiment. In block  902 , circuitry of EM transmitter  104  transmits a beacon during a low power state to identify any devices such as prosthetic device  106  that can be wirelessly charged. 
     In block  904 , circuitry of EM transmitter  104  receives device information in response to the beacon. As discussed above, the device information can include information about a prosthetic device such as identifying information, particular frequencies that the device can tune to, an average power usage of the device, or information about its power storage unit. After receiving the device information, EM transmitter  104  may exit its low power state and enter a transmission state for charging a prosthetic device such as prosthetic device  206  in  FIG. 2 . 
     In other embodiments, blocks  902  and  904  may be omitted such that EM transmitter  104  does not transmit a beacon or receive device information before generating a magnetic field. In such embodiments, EM transmitter  104  may instead periodically generate a magnetic field and measure a level of reflected power to determine whether there is a device within an effective range that can be charged. In other embodiments, EM transmitter  104  may continuously generate a magnetic field without entering a low power state. 
     In block  906 , coils of EM transmitter  104  generate resonating magnetic field  124  at a frequency that can be based on the device information received in block  904 . In some embodiments, the frequency is within the range of 100 kHz and 10 MHz. Circuitry of EM transmitter  104  may also set in block  906  an initial power used from a power supply for generating the magnetic field. 
     In block  908 , circuitry of EM transmitter  104  adjusts the power used to generate the magnetic field based on a reflected power or updated device information. The reflected power may be expressed as a proportion of the power used to generate the magnetic field. As discussed above, the circuitry of EM transmitter  104  may increase the power used if the reflected power decreases since this may indicate that additional devices are charging with the magnetic field. The circuitry of EM transmitter  104  may also temporarily increase the power used to generate the magnetic field if the reflected power increases since this may indicate that the prosthetic device is farther away from EM transmitter  104 . This temporary increase in power can serve as a test to determine whether the prosthetic device is still within an effective range for charging. 
     EM transmitter  104  may also use updated device information received from prosthetic device  106  via a digital wireless communications link. The updated device information can indicate a position or charging efficiency of prosthetic device  106 . If the updated device information indicates that prosthetic device  106  is far away or is not charging efficiently, EM transmitter  104  may increase the power used to generate the magnetic field. 
     In other embodiments, block  908  may be omitted such that the power used to generate the magnetic field could be a fixed power level. 
     In some implementations, the circuitry of EM transmitter  104  can determine in block  908  to stop generating the magnetic field if a reflected power reaches or exceeds a threshold or if the updated device information indicates that prosthetic device  106  is too far away or no longer charging. For example, a threshold for the reflected power can be a value such as 80% of the power used to generate the magnetic field. A reflected power greater than or equal to the threshold may indicate that prosthetic device  106  is too far away for charging. In other embodiments, block  908  may be omitted such that EM transmitter  104  does not enter a low power state but rather continues to generate a magnetic field regardless of the reflected power or the receipt of any updated device information. 
     In block  910 , the circuitry of EM transmitter  104  optionally receives updated device information from prosthetic device  106  indicating a state of charge for prosthetic device  106 . In this regard, prosthetic device  106  may periodically transmit updated device information indicating a current state of charge. EM transmitter  104  may then stop generating the magnetic field in block  912  in response to receiving device information indicating that prosthetic device  106  is fully charged. 
       FIG. 10  is a flowchart for a charging process which can be performed by prosthetic device  106  of  FIG. 1  according to an embodiment. The process begins in block  1002  when electronics  118  receives a beacon from a remote EM transmitter such as EM transmitter  104  to set up a wireless communications link between the EM transmitter and prosthetic device  106 . 
     In block  1004 , electronics  118  transmits device information to the EM transmitter via a wireless communications link using an antenna of electronics  118 . Electronics  118  may also wirelessly transmit device information to a mobile device running an application for monitoring prosthetic device  106 . The transmitted device information can include information about prosthetic device  106  such as identifying information, a resonant frequency or other frequencies that EM receiver  112  can tune to, an average power usage of prosthetic device  106 , positioning or alignment information for charging, or information about BMS  114 . 
     In other embodiments, blocks  1002  and  1004  may be omitted such that prosthetic device  106  does not receive a beacon from an EM transmitter or does not transmit device information. 
     In block  1006 , coils of EM receiver  112  receive a resonating magnetic field from the remote EM transmitter. As discussed above, coils of EM receiver  112  are magnetically coupled with the EM transmitter at a frequency so as to allow for less alignment between the remote EM transmitter and EM receiver  112 . 
     In block  1008 , coils of EM receiver  112  generate electric power from the resonating magnetic field. In block  1010 , the generated electric power is converted from AC power to DC power using BMS  114  and the converted DC power is stored in power storage unit  116  of BMS  114 . 
     In block  1011 , electronics  118  optionally transmits updated device information to the EM transmitter and a mobile device. The updated device information can indicate a current state of charge, a position or alignment, or a charging efficiency for prosthetic device  106 . 
     In block  1012 , electronics  118  determines whether power storage unit  116  is fully charged. If so, the charging process of  FIG. 10  ends in block  1014 . On the other hand, if it is determined in block  1012  that power storage unit  116  is not fully charged, the charging process of  FIG. 10  returns to block  1006  to continue to receive the resonating magnetic field generated by the remote EM transmitter. 
     By magnetically coupling the EM transmitter with EM receiver  112  at a resonant frequency of EM receiver  112 , it is ordinarily possible to wirelessly charge prosthetic device  106  without maintaining a tight alignment between the EM transmitter and EM receiver  112 . This can generally allow for a user of prosthetic device  106  to freely move prosthetic device  106  while it is charging without having to remove prosthetic device  106 . In addition, such wireless charging ordinarily allows for prosthetic device  106  to be better sealed from environmental conditions by not needing an exterior electrical connection for charging, which may otherwise require removal of an exterior cover while charging. Furthermore, EM resonant, wireless charging can allow for simultaneous charging of multiple prosthetic devices. 
     Those of ordinary skill in the art will appreciate that the various illustrative logical blocks, modules, and processes described in connection with the examples disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. Furthermore, the foregoing processes can be embodied on a computer readable medium which causes a processor or computer to perform or execute certain functions. 
     To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, and modules have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Those of ordinary skill in the art may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure. 
     The various illustrative logical blocks, units, modules, and controllers described in connection with the examples disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an ASIC, a wireless communication chipset, a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The foregoing description of the disclosed example embodiments is provided to enable any person of ordinary skill in the art to make or use the embodiments in the present disclosure. Various modifications to these examples will be readily apparent to those of ordinary skill in the art, and the principles disclosed herein may be applied to other examples without departing from the spirit or scope of the present disclosure. The described embodiments are to be considered in all respects only as illustrative and not restrictive.