Patent Publication Number: US-2023144336-A1

Title: Systems including resonator circuits and methods for wireless charging using same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119 of the earlier filing date of U.S. Provisional Application Ser. No. 62/983,033 filed Feb. 28, 2020, the entire contents of which are hereby incorporated by reference in their entirety for any purpose. 
    
    
     STATEMENT REGARDING RESEARCH &amp; DEVELOPMENT 
     This invention was made with government support under Grant Nos. CNS1305072 and CNS1823148, awarded by the National Science Foundation. The government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     Examples described herein relate generally to wireless power charging. Examples of charging systems with relays of resonator circuits are described. 
     BACKGROUND 
     A growing number of electronic devices are in use to collect, process, communicate, and/or display data or provide other functionality. Electronic devices, particularly those which are portable, may exhaust their energy supply, and may need to be charged. The need to charge an electronic device may be disruptive to the device&#39;s operation and/or usefulness and may cause downtime. Accordingly, convenience mechanisms and systems for charging electronic devices are desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic illustration of a system arranged in accordance with examples described herein. 
         FIG.  2 A  is a schematic illustration of a resonator circuit arranged in accordance with examples described herein. 
         FIG.  2 B  is a schematic illustration of a resonator circuit arranged in accordance with examples described herein. 
         FIG.  3    is a schematic illustration of a resonator circuit arranged in accordance with examples described herein. 
         FIG.  4    is a schematic illustration of a charging system arranged in accordance with examples described herein. 
         FIG.  5    is a schematic illustration of multiple paths of resonator circuits arranged in accordance with examples described herein. 
         FIG.  6    is a schematic illustration of paths selected for charging based on charging location and distance from the charging system. 
         FIG.  7    is a schematic illustration of a charging system arranged in accordance with an implemented example described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Linearly arranged passive relay resonators may be able to extend wireless power transfer range. However, extending this relay concept to a 2-D planar and/or 3-D array with the intention to cover a larger area may present challenges. For example, naively constructing a plane of tessellated relays may result in a poor efficiency of power transfer due to complex interactions between relays as the number of relays increases. Examples described herein include electronically reconfigurable relay transmitter systems, which may allow efficient transfer in large relay arrangements and can track a moving receiver across its coverage area. Examples of receiver tracking methods which can scan the entire coverage area over 2000 times per second in some examples and, once found, can configure the relay array to efficiently deliver power to the receiver (e.g. A mobile device) are described. 
     Magnetic resonance systems are generally a power efficient method of wireless power transfer that does not require close proximity. A magnetic resonance system may include a loop and a coil, where the loop is connected to a power signal and the coil amplifies the magnetic field. A growing demand for charging sensors, medical devices, and robots makes desirable a platform that may deliver power to a moving target in a large space. Passive resonators acting as relays may extend power transfer in one dimension. Examples of reconfigurable relay platforms (e.g.  2 D and/or  3 D platforms) with adaptively routed power based on receiver location are described herein. 
       FIG.  1    is a schematic illustration of a system arranged in accordance with examples described herein.  FIG.  1    includes charging system  102 , mobile device  104 , power source  106 , controller  108 , and resonator circuits  112  including resonator circuits  112 . The charging system  102  includes resonator circuits  112  and controller  108  and may be used to charge mobile device  104 . Additional, fewer, and/or different components may be included in other examples. 
     In the example of  FIG.  1   , mobile device  104  may be powered using the charging system  102 . Resonator circuits  112  may be connected to controller  108 . The controller  108  may be connected to resonator circuits  112  wirelessly or via physical connection. The controller  108  may activate selected resonator circuits—e.g., by setting and/or selecting a resonant frequency of the activated resonator circuit such that the activated resonator circuit transfers power at an operating frequency. The controller  108  may scan through multiple arrangements of activated resonator circuits to identify a presence of the mobile device  104  and a path of resonator circuits which may be used to power the mobile device  104 . When activated, a selected group of the plurality of resonator circuits may deliver power from the power source  106  to the mobile device  104 . 
     Examples of systems described herein may include resonator circuits, such as resonator circuits  112  of  FIG.  1   , including resonator circuit  110 . Resonator circuits may include a resonator and a control circuit. The resonator may be coupled to (e.g., electrically connected to) the control circuit. Resonator circuits may receive power wirelessly, and transfer power to other resonator circuits and/or electronic devices. Twenty-seven resonator circuits are shown in  FIG.  1   , but any number may generally be used in other examples. 
     Resonators generally refer to circuits or circuit elements that have a resonant frequency. A resonator that sympathetically resonates when it is placed in an existing oscillating field which has the same frequency as its existing frequency may be referred to as a passive relay. Resonators can convert electrical energy to magnetic fields, which can be received by a device, such as mobile device  104  (e.g., through a resonator incorporated into and/or connected to mobile device  104 ) and converted back to electrical energy. Resonators may be passive and act as relays which provide for power transfer to another resonator and/or device. Examples of resonators include coils. The coils may be made of a conductive material, such as a metal, such as a metal wire. The coils may contain multiple turns. Any number of turns may generally be used. The coils may have a natural resonant frequency. Resonators may in some examples include a primary and secondary coil. Resonators described herein may in some examples be high Q resonators—e.g., resonators with a high Q value. A high-Q resonator may refer to a resonator with relatively low intrinsic losses and that can store energy for a relatively long time. Since the resonator loses energy at its intrinsic decay rate, 2Γ, its Q, also referred to as its intrinsic Q, nay be given by Q=ω/2Γ. The quality factor may also be related to the number of oscillation periods, T, it takes for the energy in the resonator to decay by a factor of e. In some examples, a high-Q value may be Q&gt;100, although other values may also be used. 
     Generally any size resonator may be used. To cover a fixed array using resonator circuits described herein, the larger each resonator is, generally the fewer number of total relays may be used. In examples where a design goal may be to maximize coverage area and a receiver (e.g., electronic device to be charged) can be close to the charging system, the size of a coil used as a resonator may be selected as a size whose frequency splitting caused by over-coupling starts to disappear with the receiver being very close. If the receiver will be at a distance from the TX plane, the coil size may be chosen in some examples to be the largest that gives the highest energy transfer to the receiver at a given clearance distance. In some examples, coils with the same outer diameter can be made with different Q based on geometry. In some examples, coils may be used having the highest Q value for a given diameter. 
     Examples of resonator circuits described herein may include one or more resonators and one or more control circuits. For example, a resonator circuit may include a resonator coupled to a control circuit. Examples of control circuits include switches, such as semiconductor circuits containing one or more transistors. A parasitic capacitance of the transistor may be used to change the resonant frequency of the resonator circuit (e.g., the combination of the resonator and the control circuit). Control circuits may be controlled using low power, digital control signals which may place the control circuit into a particular state. The control circuit may have one state in which it presents an amount of impedance to the resonator. For example, the control circuit may close one or more transistors such that parasitic capacitance of the one or more transistors is presented to the resonator. In this state, the resonator circuit (e.g., the resonator and the control circuit) may have a resonant frequency determined by a combination of the resonant frequency of the resonator as modified by the impedance provided by the control circuit (e.g., the parasitic capacitance of one or more transistors). The control circuit may have another state in which it presents a different amount of impedance to the resonator than in the first state. For example, the control circuit may open one or more transistors such that the parasitic capacitance of the transistors is presented to the resonator. In this state, the resonator circuit may have a different resonant frequency determined by the resonant frequency of the resonator as modified by the impedance presented by the control circuit in this state (which may be zero in some examples). It is to be understood that the different impedances presented in the different states and/or the different resonant frequencies of the resonator circuits in the different states may not be constant—e.g., they may vary in accordance with power level or other variations occurring in the system. In one of the states, the resonant frequency of the resonator circuit may be such that the resonator may transmit power at an operating frequency of the charging system (e.g., at a power frequency). In the other state, the resonator frequency of the resonator circuit may be such that the resonator may transmit less power at the operating frequency of the charging system (e.g., half as much power, a third as much power, a quarter as much power, and eighth as much power, and/or no power). Other examples of control circuits include switching circuits which may connect and/or disconnect impedance elements (e.g., one or more transistors, capacitors, inductors, and/or resistors) to the resonator. Generally, digitally-controlled switches may be used as control circuits in some examples. 
     Accordingly, examples of resonator circuits described herein may have a resonant frequency. The resonant frequency of the resonator circuit may be selected, adjusted, and/or changed using a controller. For example, the controller  108  of  FIG.  1    may be used to control a resonant frequency of each of the resonator circuits  112 . The resonant frequency of the resonator circuit may be determined by parameters of the resonator and the control circuit. An impedance of the control circuit connected to the resonator may determine in part the resonant frequency of the resonator circuit. The resonator may have a resonant frequency determined in part by its geometry, thickness, material property, or other factors. The control circuit may connect and/or remove impedance from electrical communication with the resonator, which may change a resonant frequency of the resonator circuit. By selecting, adjusting, and/or changing a resonant frequency of each of the resonator circuits, the controller  108  may control which resonator circuits are activated (e.g., able to transmit power at an operating frequency) and which are not (e.g., transmit less or no power at the operating frequency relative to the activated resonator circuits). Accordingly, the controller  108  may activate resonator circuits in generally any pattern, and a pattern may typically be utilized which provides a path from a power source to one or more electronic devices to be charged (e.g., power source  106  to mobile device  104 ). Activating and/or de-activating resonator circuits may also be referred to as spatial tuning. Examples of resonant frequencies that may be used as operating frequencies include 13.56 MHz, 6.78 MHz, and 27.12 MHz. For example, one or more frequencies in an industrial, scientific, and medical (ISM) band may be used. Other bands may be used in other examples. 
     Resonator circuits included in charging systems described herein may be arranged in any of a variety of ways. In the example of  FIG.  1   , the resonator circuits  112  are depicted arranged in generally a grid, although any arrangement may be used. In some examples, resonator circuits may be arranged in a honeycomb pattern (e.g., a hexagonal grid). The arrangement of resonator circuits may be selected to shape and/or extend the resonant field which extends wireless power transfer. The resonator circuits may be arranged in a linear pattern and/or in clusters in some examples. Generally, resonators (e.g., coils) of the resonator circuits may be coplanar in some examples. In some examples, resonators and/or resonator circuits can be oriented in flat, perpendicular, or angled positions, or combinations thereof, relative to each other. The plurality of resonator circuits may be spatially configured in both a 2-D and/or 3-D arrangement. An example of a configuration may be a co-axial configuration, where the resonator circuits (e.g., relays) are on a same axis as a transmitter. This arrangement, which has one path that connects the transmitter and the receiver is referred as 1-leg-n-hop. A plurality of coplanar, electronically reconfigurable resonator circuits may be adaptively switched to the suitable 1-leg-n-hop configuration based on the receiver location (e.g., location of mobile device  104 ). In one example, a charging system includes nineteen hexagonally arranged coils. Generally, the number of resonators used in a system may be the number that can fill the intended area with the selected unit relay, where the relays may be spaced by a distance at which power can be coupled from one to another (e.g., their critical coupling distance) with their neighboring coils. The relays may also be placed outside or inside their critical coupling distance. 
     Examples of charging systems described herein may include one or more controllers, such as controller  108  of  FIG.  1   . Examples of controllers may be implemented using generally any electronics (e.g., circuitry). Examples of controllers include one or more controllers, microcontrollers, processors, and/or custom circuitry such as application specific integrated circuits (ASICs) and/or field programmable gate arrays (FPGAs). The controller may provide control signals to control circuits of resonator circuits. While a single controller  108  is shown in  FIG.  1    connected to all resonator circuits  112 , generally any number of controllers may be provided, which may be coupled to groups of resonator circuits. Generally, the controller may place a control circuit in one of multiple states, such as one of two states (e.g., open and closed). The controller may switch on and/or off each resonator circuit and/or groups of resonator circuits. The impedance presented by a control circuit to the resonator may be different in each state, such that the resonator circuit has a different resonant frequency in each state. Control signals provided by the controller may configure a control circuit or a plurality of control circuits. The controller may be connected to control circuits through a physical connection, wireless connection, or a combination thereof. Accordingly, the controller may select a path of resonator circuits to activate to provide power to an electronic device. The controller may in some examples implement a control algorithm which may be responsible for switching relay resonators on and off in a predetermined and/or controlled manner. In this manner, a controller may be used to search for the presence of one or more electronic devices to charge and/or a preferred path for activated resonator circuits to provide power to one or more electronic devices. The control algorithm may be implemented, for example, using instructions encoded in computer-readable media (e.g., one or more memories or other storage) and executed by the controller. In some examples, the controller circuitry itself may be arranged to implement the control algorithm(s). 
     In some examples, controllers described herein may receive signals from one or more resonator circuits. Signals received from one or more resonator circuits may be used by the controller to detect a presence and/or location of one or more electronic devices to charge and/or to select an arrangement of resonator circuits to activate to supply power to the electronic device(s). In some examples, a directional coupler (not shown in  FIG.  1   ) may be provided between one or more resonator circuits and the controller, such as between resonator circuits  112  and controller  108 . The directional coupler may provide reflections from the resonator circuits  112  to the controller  108 . Reflection coefficients obtained through the directional coupler may be used to detect the presence, absence, and/or location of an electronic device (e.g., mobile device  104  of  FIG.  1   ). In some examples, in addition to or instead of reflection signals, the controller  108  may receive one or more authentication signals from electronic devices, such as from mobile device  104 . The authentication signal may provide an indication to the controller  108  that a particular electronic device may be acceptable to charge, and/or may provide information regarding power requirements for an electronic device. For example, an authentication signal may include an indication that payment had been made and/or payment authorization. In some examples, the authentication signal may include a power level of the electronic device (e.g., fraction of power remaining, amount of power needed, etc.). In some examples, the authentication signal may include power parameters desired by the electronic device (e.g., frequency and/or amount of power). 
     Examples of resonator circuits described herein may be coupled (e.g., electrically connected to) one or more power sources. For example, the resonator circuits  112  of  FIG.  1    may be connected to power source  106 . The power source  106  may be implemented using, for example one or more batteries, energy-harvesting circuitry, wired power, and/or wireless power receiving circuitry. The power source may generally have an operating frequency, such that when one or more activated resonator circuits has the appropriate resonant frequency, power from the power source may be transmitted by the resonator circuit. 
     To apply power to a network of resonator circuits, a driven loop may be provided and connected to the power source  106  and positioned to activate an initial one of the resonator circuits (e.g., central one of the resonator circuits). While examples of a driven loop are described herein, in other examples, other impedance matching structures may be used. The driven loop may generally provide impedance matching between power circuitry (e.g., a power amplifier) and a resonator. In this manner, the driven loop may be driven by the power circuitry, and may cause the associated resonator to resonate power to neighboring resonator(s). A frequency at which the initial one of the resonator circuits resonates responsive to the driven loop may be referred to as the operating frequency. 
     The driven loop may be powered by a power amplifier (which may be coupled to a power source) and the driven loop may be coupled to an initial resonator. The driven loop may be centered among an array of resonators or may be placed at any other location (e.g., on the edge). The driven loop may be implemented using, e.g., a printed circuit board (PCB) trace and/or wire. 
     While a single charging system is shown in  FIG.  1   , any number of additional systems of resonator circuits coupled to controllers may be provided and arranged about an environment. In this manner, 3-D coverage for charging electronic devices may be provided. The additional systems may include their own power source and/or may receive power wirelessly from another power source (e.g., power source  106 ). The controllers of multiple systems may be coordinated to activate resonator circuits in the various systems of resonator circuits—for example, the controllers may be in wireless communication with each other to select a set of resonator circuits for activation in an environment. 
     During operation, systems described herein may provide power to one or more electronic devices. The electronic devices may also be referred to as mobile devices, which may be able to be carried, transported, held, and/or worn by a person. Electronic devices which may be worn by a person and/or may be implanted in or on a person may be referred to as wearable and/or implantable devices. Generally, the electronic devices to be charged may include or be in communication with a resonator to receive power from one or more of the resonators in the charging system. So for example, the mobile device  104  may include a resonator which may receive power from one or more of the resonator circuits  112 . Examples of electronic devices include, but are not limited to mobile phones, personal computers, portable personal computers, wearable smart devices, audio devices, implanted medical devices, powered toys, portable batteries, electrical lighting, appliances, electric vehicles, robots, remote control devices, electrical tools, media streaming devices, gaming devices, or combinations thereof. Wearable devices which may be charged in accordance with techniques described herein include, but are not limited to, watches, glasses, head-mounted displays (e.g., augmented, virtual, and/or mixed reality headsets), rings, socks, earrings, and/or necklaces. Examples of implanted devices (e.g., implanted medical devices) which may be charged in accordance with systems and/or techniques described herein include one or more neural stimulators and/or one or more left ventricular assist devices (LVAD). 
     During operation, charging systems described herein may be used to charge one or more electronic devices (e.g., wholly and/or partially charge the devices). For charging, the mobile device may be placed proximate a charging system—for example, the mobile device  104  may be placed proximate charging system  102 . Proximate generally refers to in contact or at a distance to where power may be transferred—e.g., from one or more resonant circuits of a charging system, such as charging system  102  to a resonator of a mobile device (e.g., mobile device  104 ). Examples of distances include within one centimeter of the charging system in some examples, within four centimeters of the charging system in some examples, within one inch of the charging system in some examples. Proximate may not need an unobstructed space between an electronic device and a charging system. For example, a space between an electronic device and a charging system described herein may be wholly and/or partially filled with intervening material—e.g., one or more substrate material(s), case(s), skin, tissue, clothing, air, and/or other fluid. Devices charged proximate a charging system  102  may be stationary or in motion. Proximate may include generally any spatial directions from the charging system. Examples of spatial directions include the transverse plan in some examples, sagittal plane in some examples, frontal plane in some examples. 
     Multiple paths of resonator circuits may be scanned between a power source and the electronic device. For example, the controller  108  may activate patterns of the resonator circuits  112  in multiple paths between power source  106  and a location of the mobile device  104 . In some examples, the controller  108  may scan through multiple paths of activated resonator circuits to identify a charging location corresponding to the mobile device  104 . Once a charging location is identified in some examples, then a scan of multiple paths between power source  106  and the charging location may be made. Each path refers to a collection of activated resonator circuits which may transport power from a power source to a charging location. A path may be linear (e.g., a straight line), or may be non-linear (e.g., may include groups or clusters of resonator circuits and/or one or more angles or turns). A path may be scanned by activating it—e.g., by applying control signals to the resonator circuits in the path that cause the resonator circuits to be activated (e.g., to have a resonant frequency suitable for transmitting power at an operating frequency). 
     The controller may select a path of the multiple scanned paths to use for charging the electronic device. For example, the controller  108  may select a path based on reflected signals received from one or more of the resonator circuits  112 . A path may be selected, for example, having a minimum amount of reflected signals from among a group of candidate paths. Other metrics may be used to select a path. In some examples, a path and/or individual resonator circuits in a path, may be selected based on an amount of energy transfer between the electronic device and the path (e.g., between mobile device  104  and one or more of resonator circuits  112 ). In some examples, a path and/or individual resonator circuits in a path, may be selected based on an amount of power transfer between individual ones of the resonator circuits (e.g., power transfer within the charging system  102 ). 
     While the charging of a single mobile device is depicted in  FIG.  1   , it is to be understood that multiple electronic devices may be charged (e.g., simultaneously and/or partially simultaneously) in other examples. For example, a controller may activate two paths in some examples—one to each of two charging locations. In some examples, a controller may alternate between activation of a path to a first charging location and activation of a path to a second charging location. In some examples, an amount of time each path may be activated may be determined in part by a charging level of the electronic device at the charging location. For example, a controller may activate one path to an electronic device having a lower charge level (e.g., 20 percent charged) longer than another path to a different electronic device having a higher charge level (e.g., 80 percent charged). Other metrics may be used to determine path activation time—e.g., total power need of each device, payment provided by each device, type and/or identity of each device. 
     Accordingly,  FIG.  1    depicts an example of a charging system which may be used to charge a mobile device. In some examples described herein, a charging system may also be referred to as a transmitter—e.g., it is a system which may transmit power during operation. Examples of mobile devices and/or other electronic devices described herein may also be referred to as receivers—e.g., they may receive power during operation. 
     Examples of charging systems described herein may be designed to provide delivery of power (e.g., efficient delivery of power) to multiple points within an environment and/or to be able to adapt to movement of an electronic device that may be under charge. Examples of systems described herein may be deployed in any number of environments—homes, buildings, streets, hospitals, medical centers, factories, operating rooms, airports, transit centers, etc. 
       FIG.  2 A  is a schematic illustration of a resonator circuit arranged in accordance with examples described herein. The resonator circuit  206  includes resonator  202 , capacitor  218 , and control circuit  210 . The control circuit  210  may receive control signals  214 . Controllers described herein may provide the control signals  214 . The resonator circuit  206  may be used to implement and/or may be implemented using resonator circuits described herein, including resonator circuit  110  of  FIG.  1   . Additional, fewer, and/or different components may be used in other examples. 
     In the example of  FIG.  2 A , resonator  202  is implemented using a coil. The capacitor  218  is coupled to the resonator  202  and serves as a tuning capacitor which may be sized to permit the resonator  202  to resonate (e.g., transmit power) at a particular operating frequency. While a capacitor is shown, generally any impedance element and/or combination of impedance elements may be used (e.g., resistors, capacitors, and/or inductances). 
     The control circuit  210  is provided to add and/or subtract impedance to the resonator circuit  206  such that the resonator circuit  206  has generally two states in this example—with the resonator circuit  206  having a different resonant frequency in each state. In one state, the resonator may resonate at the operating frequency (e.g., may transmit power). In the other state, the resonator may resonate at a different frequency, such that power is not transmitted at the operating frequency, or is transmitted at a reduced efficiency. 
     For example, the control circuit  210  includes two transistors in a series configuration. The transistors may be semiconductor transistors, e.g., semiconductor switches. In the example of  FIG.  2 A , the two transistors share a source and a gate. In a first state, a controller may provide control signals  214  as a low signal, which may turn off the transistors (e.g., open the semiconductor switches), blocking and/or impeding current flow. The parasitic capacitance of the two transistors of the control circuit  210  in this state may be added to the resonator circuit  206  and may change the resonant frequency of the  206  away from the operating frequency. Generally, an output capacitance of one or more of the transistors in this state may be selected to be smaller than the capacitor  218 , such that the resonant frequency may be moved significantly in this state. However, in another state, the controller may provide control signals  214  as a high signal, which may turn on the transistors in the control circuit  210 , which may allow current flow. The on transistors may additionally introduce resistance in that current path in some examples. Generally, transistors having a low on-resistance as possible may be used in some examples. In this state, the resonant frequency of the resonator circuit  206  may be such that the resonator  202  resonates power at the operating frequency. Additionally, a voltage breakdown of the transistors used in the control circuit in some examples may be selected such that the control circuit may tolerate the expected voltage across the transistors when the transistors are switched off. Analogous to the described example, in some examples the controller may provide signals  214  as a low signal, which may turn on the transistors, allowing the current to flow. The controller may also provide signals  214  as a high signal, which may turn off the transistors, blocking and/or impeding current flow. This may occur, for example, in examples using depletion mode MOSFET transistors. 
       FIG.  2 B  is a schematic illustration of a resonator circuit arranged in accordance with examples described herein. The resonator circuit  208  includes resonator  204 , capacitor  220 , and control circuit  212 . The control circuit  212  may receive control signals  216 . Controllers described herein may provide the control signals  216 . The resonator circuit  208  may be used to implement and/or may be implemented using resonator circuits described herein, including resonator circuit  110  of  FIG.  1   . Additional, fewer, and/or different components may be used in other examples. 
     In the example of  FIG.  2 B , resonator  204  is implemented using a coil. The capacitor  220  is coupled to the resonator  204  and serves as a tuning capacitor which may be sized to permit the resonator  204  to resonate (e.g., transmit power) at a particular operating frequency. In the example of  FIG.  2 B , the capacitor  220  is connected in a parallel configuration across the resonator  204  and between the resonator  204  and the control circuit  212 . In the example of  FIG.  2 A , the capacitor  218  had been connected in a series configuration between one end of the resonator  202  and the control circuit  210 . While a capacitor is shown, generally any impedance element and/or combination of impedance elements may be used (e.g., resistors, capacitors, and/or inductances). For example, a resistor may be placed in parallel with the capacitor  220  of  FIG.  2 B  in some examples. 
     The control circuit  212  may operate in an analogous manner to the control circuit  210  described with respect to  FIG.  2 A , and may receive control signals  216  from a controller. 
     Note that examples of control circuits described herein, including control circuit  210  of  FIG.  2 A  and control circuit  212  of  FIG.  2 B  include semiconductor devices which may present parasitic capacitances to the resonator circuit even when the switches provided by the semiconductor devices are off, or open. In this manner, a change of state in the resonator circuit may be achieved by adjusting and/or moving a resonant frequency of the resonator circuit toward or away from an operating frequency. This may be in contrast to use of mechanical switches which may physically disconnect a resonator from a power source. 
       FIG.  3    is a schematic illustration of a resonator circuit arranged in accordance with examples described herein. The resonator circuit  302  includes resonator  304 , capacitor  308 , capacitor  310  and control circuit  306 . The control circuit  306  may receive control signals  312 . Controllers described herein may provide the control signals. The resonator circuit  302  may be used to implement and/or may be implemented using resonator circuits described herein, including resonator circuit  110  of  FIG.  1   . Additional, fewer, and/or different components may be used in other examples. 
     In the example of  FIG.  3   , resonator  304  is implemented using a coil. The capacitor  308  and capacitor  310  are arranged in series with one another, and the series combination of capacitor  308  and capacitor  310  are in parallel with the resonator  304 . The control circuit  306  is arranged in parallel with the capacitor  310 . 
     The capacitor  308  and  310  are coupled to the resonator  304  and serves as a tuning capacitor which may be sized to permit the resonator  304  to resonate (e.g., transmit power) at a particular operating frequency. In the example of  FIG.  3   , the control circuit  306  is connected in a parallel configuration between the cap  310  and control signal. 
     The control circuit  306  includes two transistors in a series configuration. The transistors may be semiconductor transistors, e.g., semiconductor switches. In the example of  FIG.  3   , the two transistors share a source and a gate. In a first state, a controller may provide control signals as a low signal, which may turn off the transistors (e.g., open the semiconductor switches), blocking and/or impeding current flow. In this state, in the example of  FIG.  3   , opening the semiconductor switches of control circuit  306  may have the effect of placing both capacitor  308  and capacitor  310  in parallel with the resonator  304 . Accordingly, a resonant frequency of the resonator circuit  302  may be determined in accordance with a parallel combination of the resonator  304  and capacitor  308  and capacitor  310  (which may be modified by a parasitic capacitance of the switches in the control circuit  306 ). In another state, the controller may provide control signals as a high signal, which may turn on the transistors in the control circuit  306 , which may allow current flow. In this state, the resonator  304  may effectively be in parallel with only the capacitor  308  (e.g., the capacitor  310  may have been shorted out and may not contribute significantly to the resonant frequency of the resonator circuit  302 ). Accordingly, the resonant frequency of the resonator circuit  302  may be determined by a parallel combination of the resonator  304  and the capacitor  308 . The capacitor  308  and capacitor  310  may be sized such that in one state the resonant frequency of the resonator circuit  302  is such that power may be resonated at an operating frequency, but in the other state power is not resonated and/or is resonated to a lesser degree. 
     In this manner, control circuits described herein may be used to add and/or subtract impedances from a resonator circuit. In one state, the resonator circuit may resonate (e.g., transmit power) at an operating frequency (e.g., a frequency configured to transmit power originated from a resonator circuit driven by a driven loop). In other states, the resonator circuit may resonate at a different resonant frequency which may not transmit power, or may transmit less power, from a resonant circuit resonating at the operating frequency. 
       FIG.  4    is a schematic illustration of a charging system arranged in accordance with examples described herein. The charging system  420  includes a number of resonator circuits, including resonator circuit  402 , resonator circuit  406 , resonator circuit  408 , and resonator circuit  410 . An electronic device may be provided for charging at charge location  412 . The charging system  420  includes a directional coupler  404 , power amplifier  414 , detector  416 , and controller  418 . The power amplifier  414  may be coupled to and receive power from one or more power sources and be coupled to and provide an amplified signal to directional coupler  404 . The directional coupler  404  may be coupled to and receive reflections of power signals from the resonator circuits. The directional coupler  404  may be coupled to the detector  416  and may provide reflections to detector  416  which may in turn be coupled to and provide a measure of reflectance to controller  418 . The controller  418  may be coupled to the power source and the resonator circuits, and may activate and/or deactivate the resonator circuits to transmit power along one or more paths. Additional, fewer, and/or different components may be used in other examples. 
     The charging system  420  may be used to implement and/or may be implemented by the charging system  102  described with respect to  FIG.  1   . Each of the resonator circuits in the charging system  420  may be implemented in accordance with any of the resonator circuits described with respect to  FIG.  1   . For example, each of the resonator circuits may include a resonator and a control circuit. Similarly, the controller  418  may be used to implement and/or may be implemented by the controller  108  of  FIG.  1    and may operate in accordance with any description provided with respect to  FIG.  1   . For example, the controller  418  may activate selected ones of the resonator circuits by providing control signals to the resonator circuits which adjust or select a resonant frequency of the resonator circuit to transmit power. 
     In the example of  FIG.  4   , the controller  418  has activated three resonator circuits—resonator circuit  406 , resonator circuit  408 , and resonator circuit  410 . These activated resonator circuits provide a path between a power source and the charge location  412 . The remainder of the resonator circuits shown in  FIG.  4    are not activated, and do not contribute significantly to power transfer between the power source and the charge location  412 . 
     Resonator circuits described herein may also be referred to as relays. Resonator circuits may include resonators, which may also be referred to as relays. Generally, a passive relay is a resonator that may sympathetically resonate when it is placed in an existing oscillating field which has the same frequency as its existing frequency. Relays placed in a particular arrangement may therefore extend the resonant field—e.g., activated resonator circuits described herein may cause power to be transmitted through a resonant field which includes multiple resonator circuits. In some examples, resonator circuits may be arranged in a co-axial arrangement, where the resonators (e.g., relays) are on a same axis as a transmitter and/or power source. However, in some examples, resonator circuits may additionally or instead be arranged in a co-planar arrangement (e.g., with coils positioned in a same plane, as generally depicted in  FIG.  1    and  FIG.  1   ). A linear resonator circuit (e.g., relay) arrangement, which has one path that connects the transmitter and the receiver may be referred as 1-leg-n-hop, where n refers to a number of resonators in a path between a power source and a charging location (e.g., n=2 in the example of  FIG.  4    where the power source is provided at resonator circuit  406 , and must hop through resonator circuit  408  and resonator circuit  410  to reach charge location  412 ). Other n values may be used in other examples, including 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or other numbers of resonator hops. 
     Generally, when an electronic device (e.g., receiver) is positioned proximate a charging location at the end hop, the resonators may form a good power transfer channel at their common resonant frequency. It may be desirable, however, for charging to occur when an electronic device is placed generally at any location proximate the charging system—e.g., the charging location is not predetermined or fixed. Accordingly, to effectively route power throughout a plane, charging systems described herein may use coplanar, electronically reconfigurable resonator circuits (e.g., relays) that may be adaptively switched (e.g., by a controller) to the most suitable 1-leg-n-hop configuration based on the location at which an electronic device (e.g., receiver) is presented for charging. All or portions of resonator circuits (e.g., resonators and/or resonator circuits) may be placed in a hexagonal grid so that spacing between neighboring resonators is generally equal, making each path with the same number of hops generally the same length. In the example of  FIG.  4   , nineteen hexagonally arranged coils are provided in charging system  420 . 
     A power source is provided in the charging system  420  by driving a loop (e.g., a conductive loop) under one of the resonator circuits. In the example of  FIG.  4   , the power amplifier  414  drives a loop (e.g., a driven loop) provided under resonator circuit  406 . That is, a power source is initially coupled to one of the activated resonator circuits, generally a resonator circuit in a central position relative to other resonator circuits in the system. Accordingly, the resonator circuit  406  may provide an initial power source to neighboring resonator circuits. The controller  418  may switch on and off each relay resonator (e.g., by selecting and/or adjusting a resonant frequency of the resonator circuit). The controller  418  may detect the presence or absence of a receiver on a particular candidate end hop by way of a reflection coefficient. The reflection coefficient may be observed via directional coupler  404  positioned at an output port of the power amplifier  414 . In the example of  FIG.  4   , the resonators in the resonator circuits may have identical geometries, such as five turn PCB coils with a Q of 120 and resonant frequencies of 13.47 MHz in some examples. Other resonators, including other materials, Q value, and/or resonant frequencies may be used in other examples. The resonators in a charging system need not all be identical in some examples. 
     The arrangement of resonator circuits in  FIG.  4    presents an input impedance to the power amplifier  414 . The input impedance of the adaptive multi-hop system may be used in predicting and optimizing efficiency, because power amplifier efficiency generally varies with load impedance. 
     During operation, the controller  418  may scan various paths (e.g., groups) of resonator circuits. A variety of scan patterns may be used. For example, the controller may activate resonator circuits for a particular number of hops (e.g., 1, 2, 3, etc.) in a radial pattern rotating around a center resonator circuit. The presence of an electronic device to be charged (e.g., a receiver) may be detected based on reflections detected by the detector  416  through the directional coupler  404 . The detector may be implemented, for example, using a gain and phase detector. The controller  418  may compare reflections of a power signal received from the resonator circuits with a threshold, and based on the comparison, determined that an electronic device to be charged is present at an end hop (e.g., final resonator) of the path. The comparison may be, for example, when the reflections are less than or equal to the threshold value. While scanning in some examples, the input impedance of the relay system may be set at an impedance at which the amplifier dissipates a lower and/or the least power, so reduce any wasted power during scanning to determine the presence of an electronic device. Once a location of the charging device has been detected by the controller  418 , the input impedance of the relay system (e.g., the resonator circuits) with the charging device should conjugate-match the output impedance of the power amplifier for optimal efficiency in some examples. To reduce and/or minimize the power consumed while scanning for a receiver, types of patterns used in scanning may be based on a consideration of the power amplifier characteristics and system input impedance. Amplifiers may exhibit differing power consumption with the number of hops being scanned (e.g., 1-hop and/or 2-hop paths). There is a relationship between input impedance of a multi-hop path and the number of hops between power source and charging location. Accordingly, scan patterns may prioritize paths having a number of paths that may reduce power loss. 
     For example, in an example charging system  420  of  FIG.  4   , where each resonator circuit includes a 5-turn PCB coil having a Q of 120, and impedance is measured at 13.47 MHz, the impedance of the network of resonator circuits increases for 1-hop paths as the receiver is brought closer to the charging system. However, for 2-hop paths, the input impedance of the network of resonator circuits decreases as the receiver is brought closer to the charging system. In one example, when no receiver (e.g., electronic device to be charged) is present, the input impedance of a 1-hop path is around 12Ω, and the input impedance of a 2-hop path is around 180Ω. In some examples, the controller may prioritize paths having a number of hops where the power consumption is lower than another number of hops. In the example of  FIG.  4   , the power consumption of both  12  Ω 1-hop paths and 180Ω 2-hop paths was similar using a Class E power amplifier to implement power amplifier  414 . Accordingly, in one example, 2-hop paths may be prioritized during scanning because they have a longer range. In other examples, if power consumption was lower using a particular number of hops, paths with that number of hops may be prioritized (e.g., scanned first, before other number of hops). In some examples, a preferred distance from the charging system for placing an electronic device may be determined based on a distance at which the input impedance of the network of resonator circuits best matches the impedance of the power amplifier  414 . In the example charging system  420  of  FIG.  4    using the example provided particulars for the resonators, the system may have a 50Ω input impedance, which may match an impedance of a Class E implementation of the power amplifier  414  when an electronic device is placed for charging at a proximate distance of 4 cm. Other distances may be used in other examples. 
     Examples of controllers described herein (e.g., controller  108  of  FIG.  1    and/or controller  418  of  FIG.  4   ) may implement methods for locating a receiver (e.g., an electronic device to be charged) and/or identifying a path of resonator circuits to activate between a power source and the receiver. The algorithms used by the controllers may be encoded in computer readable media in some examples and executed by the controller (e.g., microcontroller code, executable instructions, etc.) Techniques used by the controller may be explained in part by considering the multi-hop system as a passive 2-port system, where port  1  is the transmitter (e.g., charging system) and port  2  is the receiver (e.g., electronic device to be charged). By way of explanation, a goal of the charging system may be to optimize an amount of energy reflected transferred from the charging system to the receiver, which may be represented by the S-parameter notation, e.g., |S 21 | 2 . However, S 21  may not be measured during operation in some examples, as it may utilize measurements at both the transmitter and receiver. Accordingly, examples of controllers described herein may utilize r as an approximation of S 21 . F is notation refers to a reflection coefficient at the charging system (e.g., an amount of energy reflected back to the charging system). Generally r, the reflection coefficient, may change with changes in load conditions (e.g., positions of one or more electronic devices to be charged proximate the charging system). 
     Accordingly, controllers described herein may utilize reflection coefficients to determine the presence of an electronic device to be charged at a location. For example, the controller  418  may receive signals indicative of the reflection coefficient from the directional coupler  404  and detector  416 . The controller  418  may scan paths to different end points (e.g., end resonator circuits), and for each path may detect the reflection coefficient. In some examples, the controller  418  may select a path having a particular reflection coefficient (e.g., minimum reflection coefficients of multiple paths) and/or relationship between the reflection coefficient and a threshold (e.g., reflection coefficient below a threshold) to indicate the presence of an electronic device to be charged at the associated end resonator circuit. However, in some examples, the overall reflectance value may not be used. In some examples, the controller  418  may utilize a change in reflectance value to determine the presence of an electronic device to be charged at the associated end resonator circuit. For example, a characterization of each or multiple resonator circuit endpoint may be stored (e.g., in a memory accessible to controller  418 ). The characterization may indicate a baseline value of reflection coefficient expected when no electronic device to be charged is present at that endpoint. During operation, if the controller  418  detects a change in reflection coefficient corresponding to that endpoint location which exceeds a threshold difference, the controller  418  may determine that an electronic device to be charged is present at that endpoint. 
     The method may be fast during run time operation but may utilize prior calibration of the system. The tracking algorithm may consist of three sequential tasks: calibration, rough-scanning and focused-scanning. The calibration process may take place once after the manufacture of the relay array, or on each system boot. In calibration, the value of all selected relay configurations may be measured one-by-one and saved as system calibration values. During normal operation, the rough-scanning mode may be the way each receiver search cycle starts. In rough-scanning, it may be switched between a prioritized set of possible configurations until a change is detected from the calibration value. The threshold of change detection may be small. The first configuration in which a change may be detected is called the pilot configuration. The configurations that share any one of the unit relays in the pilot configuration are referred as the related configurations. After the pilot configuration has been identified, the focused-scanning mode is entered, where it may be switched between all the related configurations and may select the one with the largest change for the detected receiver. 
     In some examples, the method utilized by the controller, such as controller  418  may include three general tasks: calibration, rough-scanning, and/or focused-scanning. Instructions for performing each task may be encoded, e.g., in computer readable media, such as microcontroller code. In some examples, calibration may be performed at a single time after manufacture of the charging system and/or may occur on each system boot and/or at periodic times during operation or the life of the charging system. During calibration, the reflection coefficient (e.g., Γ) may be measured for each candidate charge location (e.g., resonator circuit endpoint). The reflection coefficient for each selected configuration (e.g., each measured path to each measured resonator circuit endpoint) may be saved as system calibration values (e.g., in a memory accessible to controller  418 ). 
     During normal operation, controllers (e.g., controller  418 ) may implement a rough-scanning procedure. During rough-scanning, the controller may scan through a set of possible configurations (e.g., the controller may activate various candidate paths to candidate endpoint locations). Paths having a particular number of hops expected to have improved impedance matching characteristics to the power amplifier may be prioritized (e.g., scanned first) in some examples. The controller  418  may measure reflection coefficients of each candidate path during operation and compare the measured reflection coefficient to the stored system calibration value for that path and/or charge location. If a change is detected, in some examples a change greater than a threshold, then the controller  418  may determine an electronic device to be charged is present proximate the associated charge location. The configuration where a change was detected may be referred to as a pilot configuration. 
     During a focused-scanning operation, the controller may activate related configurations to the pilot configuration. The related configurations share at least one resonator circuit in common with the pilot configuration. That is, during rough-scanning, the controller may identify a path having an electronic device predicted to be present at the endpoint. During focused-scanning, the controller may identify a preferred path to the endpoint. Accordingly, during focused-scanning, the controller mat scan (e.g., activate) multiple different paths (e.g., related paths) to the endpoint. A path may be selected which produces a largest change in the reflection coefficient measured by the controller. For example, the controller may measure a reflection coefficient obtained for each of a number of candidate paths to an endpoint identified during rough-scanning. For each candidate path, the reflection coefficient may be compared to a baseline coefficient (e.g., a system calibration value and/or average reflection coefficient). A path may be selected based on these comparisons—e.g., a path producing a largest change from the baseline coefficient may be selected. Just as an example of paths, referring to  FIG.  4   , controller  418  may identify the presence of an electronic device at charge location  412  corresponding to resonator circuit  410  as an endpoint. Two candidate paths may be (1) from resonator circuit  406  through resonator circuit  408  to resonator circuit  410 ; and (2) from resonator circuit  406  through resonator circuit  402  to resonator circuit  410 . In the example of  FIG.  4   , the path from resonator circuit  406  through resonator circuit  408  to charge location  412  may have produced a larger change in reflection coefficient (e.g., a lower reflection coefficient) than the path from resonator circuit  406  through resonator circuit  402  to resonator circuit  410 . Accordingly, the path from resonator circuit  406  through resonator circuit  408  to charge location  412  may be activated to provide charging to charge location  412  in practice. 
     Once a path is selected during focused-scanning operation, that path may remain activated and may provide power to an electronic device proximate the charging location. The path may remain continuously activated, and may be periodically activated to provide charging (e.g., the path may be activated and alternated with other activated paths to other charging locations when multiple electronic devices are positioned proximate the charging system). 
     Any pattern or number of paths may be evaluated in rough-scanning and focused-scanning described herein. In some examples, the controller may periodically scan a number of paths to look for electronic devices to charge (e.g., by cycling through multiple activated paths). 
     During charging, the controller  418  may continue to monitor the reflection coefficient. If a change in the reflection coefficient occurs (e.g., a change beyond a threshold), and/or if the reflection coefficient returns to within a threshold of a baseline and/or stored system calibration value, the controller  418  may determine the electronic device may have moved. Accordingly, the controller  418  may return to a rough-scanning mode to look for a new location and/or different electronic device for charging. 
     The design of a relay network (e.g., array of resonator circuits) may proceed taking a variety of factors into consideration. As an example, consider the design of a relay transmitter that targets to deliver power to a receiver that is 2 cm in diameter with a minimum transmitter-to-receiver clearance 2 cm, and the diameter of the transmitter can be no more than 45 cm. Firstly, the relay size options may be calculated based on number of hops. In some examples, resonator size (e.g., coil diameter) may be calculated to be 15 cm, 9 cm, 6.5 cm, 5 cm for 1-hop, 2-hop, 3-hop, and 4-hop networks, respectively. Then the resonator size that has the largest energy transfer to a the given receiver at a given clearance may be used. In one example where the minimum clearance is 2 cm, a coil diameter of 6.5 cm may be used. This results in a 2-hop relay transmitter to cover an area of 45 cm. 
       FIG.  5    is a schematic illustration of multiple paths of resonator circuits arranged in accordance with examples described herein. Controllers described herein may scan and/or utilize multiple paths of resonator circuits. Generally, a path may be a selection of resonator circuits which is activated to provide power to a particular charging location. While, in some examples, the path may include a continuous line, and/or a linear line, of resonator circuits, in other examples, the path may be discontinuous, non-linear, and/or include clusters of resonator circuits (e.g., one or more groups of neighboring resonator circuits). Controllers described herein may utilize reflection coefficients of paths to determine the presence and/or location of an electronic device and/or a preferred path to charge an electronic device. The electromagnetic radiation pattern of the collection of resonator circuits may interact such that a path having a discontinuous, non-linear, and/or clustered grouping of resonator circuits may provide a best electromagnetic strength and/or coupling at the charging location. Moreover, the selection of path may be different based on a distance of a charging location from a surface of the charging system. 
     In the example of  FIG.  5   , twelve configurations (e.g., paths) of resonator circuits are shown. For each configuration, an enabled (e.g., driven) primary coil is indicated. That coil may be a location of a power source entry into an array of resonator circuits (e.g., the resonator circuit having a driven loop placed above and/or below the resonator). For each configuration, the enabled resonator circuits (e.g., relays) are also shown. Those enabled resonator circuits refer to those which have been activated by a controller to adjust a resonant frequency of the resonator circuit to a resonant frequency that is able to transmit power at an operating frequency. Disabled resonator circuits (e.g., relays) are also shown in  FIG.  5   . The disabled resonator circuits are those which have not been activated by a controller to transmit power at the operating frequency. For disabled relays, the controller may have provided a signal to adjust and/or select a resonant frequency of the resonator circuit that does not transmit power at the operating frequency and/or transmits less power at the operating frequency. 
     The paths shown in  FIG.  5    may be implemented by charging systems described herein—e.g., charging system  102  of  FIG.  1    and/or charging system  420  of  FIG.  4   . For example, the controller  108  of  FIG.  1    may control charging system  102  to implement the twelve configurations shown in  FIG.  5   . In another example, the controller  418  of  FIG.  4    may control the resonator circuits shown in  FIG.  4    to implement the configurations (e.g., paths) shown in  FIG.  5   .  FIG.  5    illustrates eleven configurations. In configuration  0 , all relays may be disabled. In configuration  1 , only a resonator circuit associated with a driven loop may be activated (e.g., a center resonator circuit). In configuration  2 , the resonator circuit associated with a driven loop may be activated as well as one neighboring resonator circuit. In configuration  3 , the resonator circuit associated with a driven loop may be activated as well as two neighboring resonator circuits, both on a same side of the initial resonator circuit and extending in a same direction. In configuration  4 , the resonator circuit associated with a driven loop may be activated as well as two neighboring resonator circuits, one on either side of the initial resonator circuit. In configuration  5 , the resonator circuit associated with a driven loop may be activated as well as four neighboring resonator circuits, two on either side of the initial resonator circuit. In configuration  6 , the resonator circuit associated with a driven loop may be activated as well as three neighboring resonator circuits, two on one side of the initial resonator circuit, and one on an opposite side. In configuration  7 , the resonator circuit associated with a driven loop may be activated as well as two additional resonator circuits, one adjacent the initial resonator circuit, and another adjacent that one, at an angle. In configuration  8 , the resonator circuit associated with a driven loop may be activated as well as an additional resonator circuits, disposed at an angle from the initial resonator circuit. In configuration  9 , the resonator circuit associated with a driven loop may be activated as well as three additional resonator circuits, arranged in a cluster. Two of the three additional resonator circuits are adjacent the initial resonator circuit, and one is adjacent those at an angle. In configuration  10 , the resonator circuit associated with a driven loop may be activated as well as three additional resonator circuits, arranged in a cluster. One of the three additional resonator circuits is adjacent the initial resonator circuit, and two are adjacent that one at an angle, but not adjacent the initial resonator circuit. In configuration  11 , the resonator circuit associated with a driven loop may be activated as well as four additional resonator circuits, arranged in a cluster. One of the four additional resonator circuits is adjacent the initial resonator circuit, and two are adjacent that one at an angle, but not adjacent the initial resonator circuit, and one is adjacent those at an angle—a straight line from the first additional resonator circuit. Note that in all configurations, if a charging location is considered to be proximate a furthest resonator circuit from the initial resonator circuit, all 12 configurations shown in  FIG.  5    may be considered ‘1-hop’ or ‘2-hop’ configurations, in that the charging location may be reached in a maximum of two hops, even if more than two resonator circuits are activated. Accordingly, the number of hops used to describe a path may not be equal to the number of resonator circuits activated to form the path. 
       FIG.  6    is a schematic illustration of paths selected for charging based on charging location and distance from the charging system. In some examples, a controller may scan through multiple paths (e.g., in a focused-scanning mode) to identify a path which will be activated to provide charge to a particular charging location. The path selected may be based on an evaluation of the reflection coefficient achieved with each path. The performance may vary with frequency, receiver, charging location, and distance from the charging system. 
     In the example of  FIG.  6   , a controller may have scanned the 12 configurations of  FIG.  5    to identify a preferred configuration (e.g., the configuration having the smallest reflection coefficient and/or greatest change from a baseline reflection coefficient) for charging based on ten different charging locations (shown with an ‘X’ in  FIG.  6   ). Those preferred configurations are reported in  FIG.  6    for a distance of 4 cm and 0 cm (e.g., in contact with the charging system). 
     For charging location  1 , an electronic device to be charged may be positioned above a resonator circuit two circuits away from an initial resonator circuit (e.g., a resonator circuit positioned to resonate responsive to a driven loop). At this location, in this example, the optimal path was configuration  3  of  FIG.  5    when the charging device was positioned 4 cm from the charging system, and configuration  2  of  FIG.  5    when the charging device was in contact with the charging system. 
     For charging location  2 , an electronic device to be charged may be positioned above a midpoint between two resonator circuits extending away from an initial resonator circuit. At this location, in this example, the optimal path was configuration  2  of  FIG.  5    when the charging device was positioned 4 cm from the charging system, and also configuration  2  of  FIG.  5    when the charging device was in contact with the charging system. 
     For charging location  3 , an electronic device to be charged may be positioned above a resonator circuit adjacent an initial resonator circuit. At this location, in this example, the optimal path was configuration  2  of  FIG.  5    when the charging device was positioned 4 cm from the charging system, and configuration  8  of  FIG.  5    when the charging device was in contact with the charging system. 
     For charging location  4 , an electronic device to be charged may be positioned above a midpoint between an initial resonator circuit and an adjacent resonator circuit. At this location, in this example, the optimal path was configuration  11  of  FIG.  5    when the charging device was positioned 4 cm from the charging system, and configuration  8  of  FIG.  5    when the charging device was in contact with the charging system. 
     For charging location  5 , an electronic device to be charged may be positioned above an initial resonator circuit positioned to resonate responsive to a driven loop. At this location, in this example, the optimal path was configuration  1  of  FIG.  5    when the charging device was positioned 4 cm from the charging system, and configuration  0  of  FIG.  5    when the charging device was in contact with the charging system. 
     For charging location  6 , an electronic device to be charged may be positioned above a midpoint between resonator circuits two hops from the initial resonator circuit. At this location, in this example, the optimal path was configuration  7  of  FIG.  5    when the charging device was positioned 4 cm from the charging system, and configuration  2  of  FIG.  5    when the charging device was in contact with the charging system. 
     For charging location  7 , an electronic device to be charged may be positioned above a midpoint between resonator circuits adjacent to one another at an angle, and one being adjacent to the initial resonator circuit. At this location, in this example, the optimal path was configuration  9  of  FIG.  5    when the charging device was positioned 4 cm from the charging system, and configuration  2  of  FIG.  5    when the charging device was in contact with the charging system. 
     For charging location  8 , an electronic device to be charged may be positioned above a midpoint between resonator circuits adjacent to one another at an angle, and each being adjacent to the initial resonator circuit. At this location, in this example, the optimal path was configuration  2  of  FIG.  5    when the charging device was positioned 4 cm from the charging system, and configuration  1  of  FIG.  5    when the charging device was in contact with the charging system. 
     For charging location  9 , an electronic device to be charged may be positioned above a midpoint between an initial resonator circuits and another resonator circuit adjacent to the initial resonator circuit at an angle. At this location, in this example, the optimal path was configuration  11  of  FIG.  5    when the charging device was positioned 4 cm from the charging system, and configuration  2  of  FIG.  5    when the charging device was in contact with the charging system. 
     For charging location  10 , an electronic device to be charged may be positioned above a resonator circuit two hops from an initial resonator circuit—one hop adjacent along a straight line, and another at an angle. At this location, in this example, the optimal path was configuration  9  of  FIG.  5    when the charging device was positioned 4 cm from the charging system, and configuration  2  of  FIG.  5    when the charging device was in contact with the charging system. 
     In this manner, controllers may select paths (e.g., collections of activated resonator circuits) that may vary based on receiver, frequency, distance, and charging location. 
     IMPLEMENTED EXAMPLES 
       FIG.  7    is a schematic illustration of a charging system arranged in accordance with an implemented example described herein. The charging system  702  includes relay array  704 . The relay array  704  includes multiple resonators, including coil  706 . Driven loop  708  is positioned under a central relay of the relay array  704 . The charging system  702  further includes controller  710  coupled to the relay array  704  and power circuitry  712  coupled to the controller  710  and the relay array  704 . Additional, fewer, and/or different components may be included in other examples. 
     The charging system  702  may be used to implement and/or implemented by charging systems described herein, such as charging system  102  of  FIG.  1    and/or charging system  420  of  FIG.  4   . For example, the relay array  704  may include resonators and resonator circuits as described with reference to  FIG.  1    and/or  FIG.  4   . The controller  710  may be implemented by and/or used to implement any controller described herein, including controller  418  of  FIG.  4    and/or controller  108  of  FIG.  1   . 
     Accordingly, charging system  702  includes a reconfigurable relay board, e.g., relay array  704  and a relay control board, e.g., controller  710 . Relay array  704  was implemented on a four-layer PCB, with nineteen hexagonally arranged coils, and a diameter of 31 cm at its widest point. Each coil had five turns and a diameter of 59 mm. The driven loop  708  was mounted under the center coil of the board, and was connected in series with a 91 pF tuning capacitor and a control circuit including a pair of BUK7K134-100E MOSFET switches that share a source and gate. When the gate voltages are high, the series switches allowed current flow and the relay may resonate. When the gate voltages are low, the switches block current flow, pushing the resonant frequency away from the system&#39;s operating frequency and effectively shutting off the relay. The control signals that reconfigure all relays may be generated by the microcontroller, then level shifted to 10 V. The power circuitry  712  includes a signal source and an amplifier. At the output of the power amplifier, a directional coupler is connected to compute the reflection coefficient. 
     A power signal was provided by a power transmitter (e.g., power circuitry  712 ) including a signal source and a Class E amplifier designed for a 50Ω load. At an output of the power amplifier, a SYDC-20-22-HP+ directional coupler was connected in series. The reflection coefficient from the coupler was processed by an AD8302 gain and phase detector. The controller  710  was implemented using a TI MSP432P401R, which had a 1-Msps, 14 bit ADC, and was used to collect samples from an output of the gain and phase detector (e.g, from power circuitry  712 ). The control algorithm may be implemented using a microcontroller. The microcontroller may be used to collect samples from the output of the gain and phase detector. A monolithic RC low pass filter, implemented in this example using an ELK-EV333FA, was placed between the detector and ADC to reduce noise at the carrier frequency. For each reflection coefficient data point, seven consecutive samples were taken and digitally averaged to further reduce the impact of noise. The controller firmware utilized 5.7 kB of MSP432 program memory. 
     The calibration procedure was triggered once on system power-on and used to provide the baseline reflection coefficient value for each configuration. Once a possible receiver has been identified in the rough scanning mode, the algorithm transitioned to a focused scanning mode. At the end of the focused scan the prototype remained in the configuration which best targets the receiver. The prototype continued sampling the reflection coefficient until it observed a significant change, which may indicate that the receiver has moved, at which point it may return to rough scanning mode. 
     An electronic device (e.g., a receiver) was used for charging using the charging system  702 . The receiver used in the experimentation included an eight-turn PCB coil with 2 cm diameter and Q of 90. The receiver was separated by 5 mm and 1.6 cm respectively above the single transmitter and the relay transmitter for the measurements. The locations that are near the center of each relay coil had better efficiency of power delivery to the receiver than locations in between relays. 
     The rough scanning mode of controller  710  scanned the entire search space 2083 times per second. Once a pilot configuration was found, the focused scan took only 120 microseconds. With this fast tracking algorithm, power was automatically routed to the receiver as it moves, at a rate faster than the eye can detect. 
     From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made while remaining with the scope of the claimed technology. 
     Examples described herein may refer to various components as “coupled” or signals as being “provided to” or “received from” certain components. It is to be understood that in some examples the components are directly coupled one to another, while in other examples the components are coupled with intervening components disposed between them. It is also to be understood that the term coupled typically refers to the components being in electrical communication. Just because components are stated as coupled does not mean that electromagnetic coupling (e.g., matching) has occurred. Also, signals may be provided directly to and/or received directly from the recited components without intervening components, but also may be provided to and/or received from the certain components through intervening components.